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Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved. Scandium: Compounds, Productions and Applications : Compounds, Productions and Applications, Nova Science Publishers,

Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved. Scandium: Compounds, Productions and Applications : Compounds, Productions and Applications, Nova Science Publishers,

CHEMICAL ENGINEERING METHODS AND TECHNOLOGY

SCANDIUM

Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved.

COMPOUNDS, PRODUCTIONS AND APPLICATIONS

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CHEMICAL ENGINEERING METHODS AND TECHNOLOGY

SCANDIUM

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COMPOUNDS, PRODUCTIONS AND APPLICATIONS

VIKTOR A. GREENE EDITOR

Nova Science Publishers, Inc. New York

Scandium: Compounds, Productions and Applications : Compounds, Productions and Applications, Nova Science Publishers,

Copyright © 2011 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers’ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works.

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Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. Additional color graphics may be available in the e-book version of this book.

LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA Scandium : compounds, productions, and applications / [edited by] Viktor A. Greene. p. cm. Includes index. ISBN 978-1-61761-622-8 (E-Book) 1. Scandium. I. Greene, Viktor A. QD181.S4S267 2010 546'.401--dc22 2010031750

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CONTENTS Preface Chapter 1

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Chapter 2

Chapter 3

Chapter 4

Chapter 5

vii  Chiral Scandium Complexes in Catalytic Asymmetric Reactions Xiaoming Feng and Xiaohua Liu Application of Scandium Oxide in Dispenser Thermionic Cathodes Jinshu Wang Comparison of Scandium Recovery Mechanisms by Phoshporus-Containing Sorbents, Solvent Extractants and Extractants Supported on Porous Carrier V. Korovin, Yu. Shestak and Yu. Pogorelov Clinical Use of Scandium Enriched Power Laser in Sleep Surgery: Comparison of Postoperative Recovery from Laser Assisted Uvulopalatoplasty Using Different Laser Systems Pavelec Vaclav A Rare Earth, Scandium, Activates Secondary Metabolism in Microorganisms Kozo Ochi

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1 

49 

77 

101 

119 

vi Chapter 6

Chapter 7

Contents Scandium: Compounds, Productions, Applications and Health Impact Kan Usuda, Eri Tanida, Keiko Ohnishi and Koichi Kono Scandium Aluminum Nitride Nanowires T. Bohnen, G.W.G. van Dreumel, P.R. Hageman, G.R. Yazdi, R. Yakimova, E. Vlieg, R.E. Algra, M.A. Verheijen and J.H. Edgar

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Index

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129 

135 

153 

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PREFACE Scandium is a chemical element with symbol Sc and atomic number 21. A silvery-white metallic transition metal, it has historically been sometimes classified as a rare earth element. This book presents topical research material in the study of scandium, including the application of chiral scandium complexes in asymmetric catalysis; application of scandium oxide in dispenser thermionic cathodes; comparison of mechanisms of scandium recovery by phosphorous-containing inorganic and organic adsorbents; clinical use of scandium enriched power laser in sleep surgery; the physiology of scandium in relation to strain improvement and novel antibiotic discovery; and scandium aluminum nitride nanowires. Chapter 1 - Catalytic asymmetric reaction is an important subject in modern organic chemistry. The development of high efficient and environmental friendly catalysts is an important effort in the study of asymmetric catalysis to construct potentially useful chiral molecules. Scandium complex has shown extraordinary high catalytic activity in many organic reactions which may be attributed to its small ionic radii compared with other rare-earth metal complexes. Another characteristic feature of Sc(III) complex is that it’s stable and reusable as Lewis acid catalyst which is different from traditional Lewis acids such as AlCl3, TiCl4, Ti(OiPr)4 etc. The combination of magnetic properties with a chiral ligand resulted in the novel applications in asymmetric catalysis with high ee’s and convenient experimental conditions. Many kinds of chiral ligands, such as bipyridine, bis(oxazolinyl)pyridine compound, N,N’-dioxide compound etc., have been evaluated based on their scandium complexes in asymmetric reactions. In this chapter, useful asymmetric transformations employing chiral scandium complex as Lewis-acid catalysts are discussed. The related papers in recent

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viii

Viktor A. Greene

years according to the reaction type were covered focusing upon catalytic mechanism and comparison of these new types of Lewis acids with other conventional Lewis acids. Successful catalytic asymmetric reactions involved Aldol reaction, Mannich-type reaction, Michael reaction, ene reaction, allylation reaction, ring-opening reaction, Diels-Alder reaction, 1,3-dipolar cycloadditions and other cycloaddition reaction, Friedel-Crafts alkylation, Nazarov reaction as well as other miscellaneous reaction et al. This kind of Lewis acid catalyzed reaction could be available not only in many organic solvents but also in aqueous media. These properties will lead to really environmentally friendly chemical processes. The promise of chiral scandium complexes in asymmetric catalysis has not yet been fulfilled and effective chiral ligands are still proving elusive, then this area presents significant challenges for some time to come. Chapter 2 - The addition of scandium oxide to dispenser cathodes (namely Scandate cathodes), which are typically comprised of a porous tungsten matrix impregnated with Ba-Ca-aluminate, improves the emission by a factor of tens to hundreds under the same operating temperature. The fabrication, emission property and emission mechanism of Scandate cathodes have been reviewed. Four types were summarized in this paper: the traditional impregnated Scandate cathode, “top-layer” impregnated Scandate cathode, impregnated mixed matrix Scandate cathode and pressed Scandate cathode. It was found that decreasing the grain size from micrometer to sub-micrometer could enhance the cathode emission performance. Mixed matrix Scandate cathode prepared with scandium oxide doped tungsten powder which has the characteristic of superfine Sc2O3 particles dispersing uniformly over and among sub-micrometer W grains, might be the main type of Scandate cathodes in the future. The emission models for explaining the conspicuous emission performance were summarized. Although the operating mechanism of Scandate cathodes is still unclear, it is accepted that the emission is correlated with a surface multilayer/monolayer of Ba-Sc-O. The layer is formed after proper activation by diffusion of free or ionic Sc together with Ba and O from the interior of the cathode to its surface. The correlation between the emission properties of scandate cathodes and their surface features has been established. Chapter 3 - The present paper deals with comparison of mechanisms of scandium recovery by phosphorus-containing inorganic and organic adsorbents as well as by organophosphorus solvent extractants and ones supported on TVEX porous carrier. Using 31Р NMR method (magic angle rotation), mechanism of Sc recovery by titanium and zirconium phosphates is determined for diluted hydrochloric,

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Preface

ix

sulfuric and nitric solutions. It is revealed that the presence of three types of functional groups results in scandium sorption at the expense of cation exchange, formation of coordinate bonds with P=O groups as well as by both mechanisms with chelate formation. Prevailing of particular recovery mechanism depends upon acid and pH value. It is found that the mechanism of scandium recovery by organic ionexchange resins defines in many ways by the type of functional groups and their spatial distribution in polymer matrix of the resin. Thus, three mechanisms are defined for Sc sorption from hydrochloric solutions by different ion-exchange resins using 31P NMR method (magic angle rotation). According to the first mechanism, scandium is sorbed with formation of coordinated bond with ionite functional group and remaining its hydration shell as [Sc(H2O)6]3+ ion. By the second adsorption mechanism scandium ions are sorbed with formation of one ionic bond while formation of two ionic bonds with two groups is typical for the third mechanism. For the second and third mechanisms the number of water molecules decreases in Sc first coordination sphere. Using 31Р, 34Sс NMR method, comparison is made for mechanisms of scandium extraction by liquid organophosphorus extractants and ones incorporated in TVEX polymer carrier. The difference in scandium extraction by liquid DEHPA (di-2-ethyl hexyl phosphoric acid) and TVEX-DEHPA is established. It is shown that during Sc recovery by TVEX-DEHPA the complexes are formed typical both for solvent extraction and sorption by phosphorus-containing ion-exchange resins. Chapter 4 – Objectives- The aim of this study was to compare the effectiveness of five power lasers (CO2, diode, KTP, ErCrYSGG,and Nd:YVO4) in laser assisted uvulopalatoplasty and find potential role of laser crystal compounds on the patients recovery. Methods- This is a prospective study of 100 patients who were treated for snoring by laser assisted uvulopalatoplasty with either CO2 (n=21), diode (n=19), KTP (n=21), ErCr:YSGG (n=21) or with Nd:YVO4 laser (n=18). Results- The statistical significance (P99% ee N

O R

N H

O

N O

O H

N

12 R = 2,6-iPr2C6H3

R

Figure 21. Asymmetric three-component Mannich-type reaction of aldimines Scandium: Compounds, Productions and Applications : Compounds, Productions and Applications, Nova Science Publishers,

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18

Xiaoming Feng and Xiaohua Liu

Asymmetric three-component vinylogous Mannich reaction of acyclic silyl dienol ester, aldehydes and 2-aminophenol was accomplished using the same chiral N,N′-dioxide 12-scandium(III) complex as the catalyst (Figure 21) [27b]. A variety of aldehydes were found to be suitable substrates for the reaction and the desired δ-amino α,β-unsaturated esters were obtained in a range of 90-99% yields with good to excellent enantioselectivities (80->99% ee) and complete regioselectivities. The reaction could be carried out at 10 mmol scale without loss of the reactivity and enantioselectivity under airtolerant conditions. Taking advantage of the high coordination number of rare earth metal, Shibasaki group used simple amide ligand associating with a rare earth metal to mimic a unitary metalloenzyme to reproduce a highly ordered transition state [28]. A dynamic conglomerate of substrate/ligand/metal mixture generated a defined transition state assembly, performing Mannich-type reaction of α-cyanoketone to imines with high stereoselectivity (Figure 22). The combination of amide 19 with Sc(OiPr)3 was emerged as the ideal catalyst in the reaction of 2-cyanocyclopentanone and N-Boc imine, furnishing the Mannich product with a contiguous quaternary carbon and trisubstituted stereocenter in 90% yield with anti/syn = 94/6 and 94% ee at 0 oC. In the presence of 2-5 mol% catalyst (amide 19/Sc(OiPr)3 = 2/1), six- and sevenmembered cyclic cyanoketones also served as competent nucleophiles, giving the anti products with 80-92% yield, 89/11-95/5 dr and 91-95% ee. Nonpolar or noncoordiantive substituents on the aromatic ring of imines have little influence on stereoselectivity, while stereoselectivity diminished in the reaction with coordinative substituted imines which implies that hydrogen bonding between the ligand and substrate is manifested in the transition state. Preliminary mechanism study showed that the catalysis proceeded through an ordered association of substrate/amide/Sc from a conglomerate mixture, eliminating the need for precomplexation of the ligand/Sc.   O N

CN n

R

ligand 19 (10 mol%) Sc(OiPr)3 (5 mol%)

Boc

CH2Cl2, 0 oC, 12h

H

OH O

H N

N H

OH

O

NHBoc

n

R CN

88-97% yield, 77-96% ee 64/36-95/5 anti/syn

O 19

Figure 22. Asymmetric Mannich-type reaction of α-cyanoketone to imines Scandium: Compounds, Productions and Applications : Compounds, Productions and Applications, Nova Science Publishers,

Chiral Scandium Complexes in Catalytic Asymmetric Reactions

19

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5. ALLYLATION REACTION A catalytic amount of Sc(OTf)3 showed highest efficiency in enantioselective allylation of β,γ-unsaturated aldehydes [29]. Various β,γunsaturated aldehydes, generated by the treatment of 2-vinyloxiranes with Sc(OTf)3 were effectively trapped by chiral allylating agents based on αpinene to prepare chiral bishomoallylic alcohols in 35-96% yield and 95-97% ee (Figure 23). The acceleration effect of Lewis acid, such as Sc(OTf)3 and AlCl3 was found in allylboration of aromatic and aliphatic aldehydes [30a,30b]. The enantioselective reaction was also tested with poor results. Later, diastereoand enantioselective allylation of aldehydes using stable, air-tolerant chiral allylboronates assisted by Sc(OTf)3 was developed. Mixing the aldehyde with Sc(OTf)3 in CH2Cl2 at -78 oC, followed by the allylboronate could afford the desired homoallylic alcohols (Figure 24). The (E)- and (Z)-crotylboronates gave comparably high levels of enantioselectivity (94-97% ee) to provide the respective anti and syn propionate products in good yields (52-74%) and very high diastereoselectivity (>98%) [30c]. The mechanism studies implicated that a closed six-membered chairlike transition state characterized by internal activation of the aldehyde by the boron center was more possible, in which Sc(III) coordinates more accessible to the least hindered pseudoequatorial boronate oxygen to enhance the electrophilic activity of boronate (TS in Figure 24) [30d]. Diastereomerically pure exo-methylene butyrolactones embed stereogenic quaternary carbon centers could be generated from the stereospecific reaction of chiral 2-carboxyester-3,3-disubstituted allylboronates with aldehydes [31]. The catalyst Sc(OTf)3 not only provided a large rate enhancement over the noncatalyzed process, but also broadened the scope of aldehydes with preserved diastereospecificity (Figure 25). The reasonable proposal for the mode of activation was electrophilic activation of boron by coordination of scandium to one of the boronate oxygens directly or mediated by water. The Lewis acidity of the boron atom would be increased which then leads to enhanced allylboration with an aldehyde via the usual Zimmerman-Traxler transition state (Figure 25)

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20

Xiaoming Feng and Xiaohua Liu R3

  1

R

R2

R1 = Ar, CH3(CH2)4, or R1 to R3 =(CH2)4 R2 = H, Ph, R3 = H, Me, Br, R4 = H, Me

O R4

Sc(OTf)3 R3

O

R1

H R2

R3

1) Sc(OTf)3 (7.5 mol%) THF, -78 oC, 6h

B

R

2) NaOH, H2O2, 14h, rt

R4

OH

1

R2

R4

35-96% yield, 95-97% ee

Figure 23. Enantioselective allylation of β,γ-unsaturated aldehydes

 

O R4

R3

R1 H 2

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R R4 = Ph PhCH2CH2 TBDPSOCH2CH2 BnOCH2 TBDMSOCH2 TBDPSOCH2

Ph O B O

OH R3

Sc(OTf)3 (10 mol%)

R4

CH2Cl2, -78 oC

R2

R1

52-90% yield, 77-98% ee

R1 = R2 = R3 = H R1 = R2 = H, R3 = Me R1 = R3 = H, R2 = Me

O B O

H R1

O

4 R2 R

H

Sc(III)

TS

Figure 24. Enantioselective allylboration of aldehydes O

  * RO2C

O B

R1

R3CHO O

R2

Sc(OTf)3 toluene 16-24 h

* RO2C

OH

R1

R2

R3

R1

3 R2 R

Thermal Reaction: 2 weeks with 10% Sc(OTf)3, 12-24 h H

RO 1

R

O

H

R'O Sc(OTf)3

R3 O R2

B OR'

RO 1

R

O Sc(OTf)3

R'O

H O

3

R

O R2

B

H

OR'

Figure 25. Stereospecific reaction of chiral 2-carboxyester allylboronates

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Chiral Scandium Complexes in Catalytic Asymmetric Reactions

21

Chiral Sc(OTf)3-(S,S)-Ph-pybox complex 20 could catalyzed enantioselective Sakurai-Hosomi addition of terminally substituted allylsilanes to N-phenylglyoxamide [32a], affording anti diastereoselection which was complementary to that in glyoxamide-ene reactions [32b]. Both aliphatic and aromatic allylsilanes were effective nucleophiles in additions to the glyoxamide to afford optically active homoallylic alcohol derivatives (64-89% yield, 91-99% ee and 9/1-99/1 anti/syn) which are versatile building blocks for β-substituted α-hydroxyl and α-amino acids (Figure 26). The catalytic asymmetric allylation of imines provides direct access to optically active homoallylic amines which are useful intermediates for the preparation of bioactive natural products and relevant compounds [33]. Sc(OTf)3 could promote the addition of tetraallyltin to (SFC)-2-p-tolylsulfanyl ferrocene imines to synthesis new central and planar chiral ferrocenyl amines with 72-88% yield and >98% de. Simplified and environmentally friendly multicomponent strategy has been successfully adopted in C2-symmetric N,N’-dioxide 12-Sc(III) complexes catalyzed asymmetric allylation of aldimines (Figure 27) [34]. The scandium catalyst exhibited good ability for the activation of allylstanane reagent and 2aminophenol-derived aldimines in situ. The enantioselectivity was closely dependent on the steric effects of the aromatic amide portion and the amino acid backbone. L-ramipril acid and 2,6-diisopropylbenzalamine oriented N,N’dioxide 12 was found to be the best ligand candidate. The direct allylation of aldimine with three-component method exhibited clearly superiority also in the enantioselectivity to that of pure aldimine prepared beforehand. A variety of aldehydes were investigated under the optimal conditions and the corresponding homoallylic amines were provided in good yields (67-89%) with excellent enantioselectivities (71-97% ee). Neither the electronic property of the substitution at the aromatic ring, nor the steric hindrance had obvious influence on the enantioselectivity. Condensed-ring aromatic aldehydes and hetero-aromatic aldehydes were also found to be suitable substrates. The operational simplicity, practicability, and mild reaction conditions render it an attractive approach for the synthesis of chiral homoallylic amines. Coupled with the use of Lewis acids as Sc(OTf)3 or ZrCl4 to enhance the electrophilicity of chiral substituted N-enoly-4-phenyl-1,3-oxazolidinones, diasteroselective conjugate addition of a series of allylic stannanes were realized to efficiently construct the chiral acyclic compounds (Figure 28) [35].

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22

Xiaoming Feng and Xiaohua Liu

 

O

H N

H

OH (E)-

Ph

OH

10-15 mol% 20

O R

4 Å MS, CH2Cl2

SiMe3

R

H N

NHBoc

Et

Ph

O Et

NHBoc

R = alkyl, aryl O

Ph

N TfO

N Sc OTf 20

(Z)-

CO2H

O N OTf

(E)-

CO2H

91-99% ee, 9/1-99/1 dr

(E)- or (Z)-

CO2H

R

Ph

Figure 26. Chiral Sc(III) complex catalyzed enantioselective Sakurai-Hosomi addition

 

OH NH2

RCHO +

SnBu3

+

Sc(OTf)3 (5 mol%) N,N'-dioxied 12 (5 mol%)

HN

CHCl3, 25oC, 4Å MS

R

*

OH

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67-89% yield, 71-97% ee

Figure 27. Three-component asymmetric allylation of aldimines   O O

O R

Ph

O

R1 2

N

SnBu3

Sc(OTf)3 or ZrCl4 CH2Cl2, -78 oC~ -20 oC

O

O N

R1 R2

Ph 75-89% yield, 4.5/1 - 10/1 dr

Figure 28. Diasteroselective conjugate addition of allylic stannanes

6. ASYMMETRIC DIELS-ALDER REACTION The combination of (R)-BINOL, Sc(OTf)3 and 1,2,6-trimethylpiperdine performed an efficient catalyst 21 for the enantioselective Diels-Alder reaction of acyl-1,3-oxazolidin-2-ones with cyclopentadiene [36]. The corresponding Diels-Alder products were obtained with good yield and enantioselectivity (Figure 29). The unique structure of the scandium catalyst was indicated as shown in Figure 29 from the controlled experiments.

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Chiral Scandium Complexes in Catalytic Asymmetric Reactions

23

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Figure 29. (R)-BINOL-Sc(III) complex catalyzed asymmetric Diels-Alder reaction

The amine employed was crucial to the enantioselectivity and a weak interaction between the nitrogen of amine and the phenolic hydrogen of (R)BINOL was observed by 13C NMR spectroscopy. Therefore, the stereoinduction of the reaction was rationalized by assuming an intermediate octahedral Sc(III)-dienophile complex (Figure 29). The axial chirality of (R)BINOL was postulated to be transferred to the amine part, which worked as a “wall” in the transition state to shield the re face of the dienophile, and allowed the diene to approach the dienophile from the free si face to give the (2R,3R)-adduct in high enantioselectivity. Although (S,S)-salen 22-Sc(OTf)3 complex gave the endo adduct with moderate ee value in the asymmetric Diels-Alder reaction of cyclopentadiene [37a], the addition of 2,6-lutidine could dramatically improve the enantioselecivity to 85% ee (Figure 30) [37b]. The Sc(OTf)3/FERRODIOL 23 complex was used for asymmetric Diels-Alder reaction of cyclopentadiene with 3-acyloxazolidin-2-one in the presence of 2,6-lutidine and 4 Å MS with high yield and good selectivity (Figure 30). Later, chiral Sc(OTf)3-iPr-pybox 14 complex was employed in the asymmetric Diels-Alder reaction of cyclopentadiene with acyl-1,3-oxazolidin2-ones using not only organic solvent dichloromethane but also the less toxic benzotrifluoride and supercritical carbon doxide (scCO2) as solvent in terms of green chemistry (Figure 31) [38a]. Other dienes such as isoprene and trans1,3-pentadiene were also surveyed with moderate to high enantioselectivity. 4

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Xiaoming Feng and Xiaohua Liu

Å molecular sieves may promote the coordination of the oxazolidinone group to the scandium metal center. A chelation model as shown in Figure 31 was proposed to illustrate the stereochemical course in which two carbonyl groups in the dienophile coordinate to scandium and the diene directed form less hindered si face where the isopropyl group does not shield the space [38b]. Detailed investigation on the electronic and steric effects of pybox ligand and effects of metal cations has been carried out [39]. The sense of the asymmetric induction was clearly influenced by the substituent in the 4’- and 5’-positions: in the case of pybox 24 and 14, the Sc(III)-based catalysts gave (R)-adducts as preferred enantiomers while Sc(III)-pybox 26 gave the reversed (S)-adduct. Transition state 1 in Figure 32 was proposed to account for the stereoselectivity in which dienophile was bound to the Sc(III) cation with the ozazolidinone C=O group in the apical position, and cyclopentadiene attacked on the less hindered re face to form predominant (S)-product. The bulkly group at the 4’-position clearly benefited the enantioslectivity, such as ligand 14. And the loss of efficiency expected for ligand 24 with smaller alkyl group has to be compensated by the synergistic effect of the phenyl group in the 5’position. The overall results of ligand 14 and 24 were same attack on the si face which gave (R)-adducts [39a]. Furthermore, the electronic effect of substituents on pyridine indicated that more electrophilic Sc(III) center would bind ligand 27 more tightly which could in turn enhance the steric discrimination capability of the ligand, leading to 96% ee under mild reaction conditions [39b].  

O

O

N

R

R

Sc(OTf)3-Ligand 2,6-lutidine 4 Å MS/CH2Cl2

O

O OC N

O

R = H, Me, Ph L = Salen 22 (10 mol%)

81% yield, 85% ee, 89:11 endo/exo

L = FERRODIOL 23 (10 mol%) 44-99% yield, 37-91% ee, 65:35-90:10 endo/exo Ar N But

N

OH HO tBu

But

Salen 22

tBu

NMe2 Ar

Fe OH

FERRODIOL 23

Figure 30. Asymmetric Diels-Alder reaction of cyclopentadiene Scandium: Compounds, Productions and Applications : Compounds, Productions and Applications, Nova Science Publishers,

Chiral Scandium Complexes in Catalytic Asymmetric Reactions  

O R

O

R

Sc(OTf)3-Pybox 14 O

N

R = H, Me, Ph

O

4 Å MS/ solvent 0 oC, 2 h

OC N

solvent = CH2Cl2, BTF, scCO2 O

N O

O O

4 Å MS solvent 0 oC, 48h

exo isomers

O

endo-

O N

Sc(OTf)3-14

25

O

isomers

O N

O

O

O N

O

isomers

N

O Sc

O

N N

Si attack

O

TS

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Figure 31. Asymmetric Diels-Alder reaction of acyl-1,3-oxazolidin-2-ones with dienes

Besides, the sense of enantioselectivity was also shown to vary with the size of metal cations due to the different geometries of the complex catalysts. Sc(III)-pybox 25 produced (S)-product, but reversed (R)-prodcut was found using La(III)-pybox 25. The enantiodivergent results have been interpreted by starting from the X-ray structure of Sc(III) and La(III) complexes. It was suggested that the coordination of dienophile in Sc(III) complexes of 4’-Phsubstituted pybox 25 coocured with the acryloyl C=O group in the apical position (Figure 32, TS2) which was opposite to that proposed in TS1 (Figure 32). As shown in Figure 32, a pentagonal-bipyramidal coordination was observed for Sc(III) in which the two phenyl groups shielded the re face, so the scandium cation preferentially gave (S)-addcut. While La(III) has higher coordination number of nine and two phenyl groups of ligand 25 induced shielding of the si face to form the major (R)-addcut (Figure 32 ,TS3). The complex pybox 18/Sc(OTf)3 was aslo a very efficient stereoselective catalyst of the DA and HDA reactions between cyclopentadiene and methyl (E)-2-oxo-4-aryl-3-butenoates, thus allowing excellent control of both the diastereo-and enantioselectivities both in the DA and HDA reactions (up to 99.5% ee) [40].The DA/HDA ratio could sometimes be altered from a partial equilibration of the primary reaction products induced by the Sc(III) cation. The asymmetric hetero-Diels-Alder reaction of carbonyl compounds with Danishefsky’s diene under homogeneous conditions was accomplished by Sc[(R)-H8-BNP]3 catalyst 28 (Figure 33). Yb[(R)- BNP]3 could also promote the reaction to afford the corresponding cycloadducts with slightly higher enantioselectivity (up to 99% ee) [41]. The Sc[(R)-H8-BNP]3 catalyst 28 could be recovered by filtration and be reused with high yield and slightly decreased enantioselectivity.

Scandium: Compounds, Productions and Applications : Compounds, Productions and Applications, Nova Science Publishers,

26

Xiaoming Feng and Xiaohua Liu X

  O

Ph

O

N N

N

R

5'

Ph

R1

4'

O

O

N N

R

N

R2

24 R = Me 25 R = Ph

5'

Ph

4'

R2

26 X = H, R1 = Ph, R2 = Me 27 X = Cl, R1 = H, R2 = tBu 14 X = H, R1 = H, R2 = iPr

OTf O

N

N OH2

Sc

N

O

Re-attach

O

OTf

O O

NO N

N O

N O

Si attack

Re attack

O

N O

Sc O OH2 OTf

TS1

TS2 Sc(OTf)3-pybox 25 (S)-product

Sc(OTf)3-pybox 26 (S)-product

O ON OH2 N La N O H2O N O OTf

TS3

La(OTf)3-pybox 25 (R)-product

Figure 32. The rationalized transition states of stereoinduction in the DA reaction  

OMe

Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved.

PhCHO TMSO

1) catalyst 28 CH2Cl2, rt 2) CF3CO2H

O

O

O O P O O

Sc

Ph 3

Sc[(R)-H8-BNP]3 28

Figure 33. Sc[(R)-H8-BNP]3 compound catalyzed asymmetric hetero-DA reaction

The asymmetric aza-Diels-Alder reaction of aldimines with Danishefskytype diene could work well in the presence of L-proline-derived N,N’-dioxides 5-scandium(III) triflate [42]. Multisubstituted Danishefsky’ diene derivative was employed to the aza-Diels-Alder reaction with aldimines derived from aldehydes and 2-aminophenol to get various 2,5-disubstituted dihydropyridinones (Figure 34). The addition of 4-aminobenzenesulfonic acid could promote the yield and the enantioselectivity. Aromatic, heteroaromatic, conjugated, and aliphatic aldimines were tolerate well with moderate to good yield (46-92%) and 71-90% ee. Asymmetric inverse electron-demand aza-Diels–Alder (IEDDA) reaction of N-arylimines with electron-rich alkenes (dienophiles) is one of the most powerful strategies for the synthesis of tetrahydroquinoline derivatives. A polymer-supported scandium catalyst (polyallyl)scandium trifylamide ditriflate (PA-Sc-TAD) has been prepared for the construction of racemic

Scandium: Compounds, Productions and Applications : Compounds, Productions and Applications, Nova Science Publishers,

Chiral Scandium Complexes in Catalytic Asymmetric Reactions

27

quinoline library from aldehyde, an aromatic amine and an alkene [43a]. The first catalyst asymmetric IEDDA reaction with cyclopentadiene and vinylethers as dienophiles was reported by Kobayashi group using chiral binaphthol–ytterbium complexes [43b]. Feng group explored chiral N,N’-dioxide 12-scandium catalyst to the asymmetric three-component IEDDA reaction of aldehydes, anilines and cyclopentadiene (Figure 35) [43c]. In the presence of 0.5-5 mol% N,N'-dioxide 12-scandium complex, various ring-fused tetrahydroquinolines with three contiguous stereocenters were afforded in a one-pot manner with good yields, excellent diastereo- and enantioselectivies (62-99% yield, 90:10-99:1 dr, 9099% ee). Different ring-fused tetrahydroqunioline derivatives could be obtained by simple transformation under mild conditions with excellent enantioselectivities. O

 

HO

OMe

TMSO

Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved.

N,N'-dioxied 5 (20 mol %), Sc(OTf)3 (10 mol %)

N

+ R

N

p-NH2C6H4SO3H, THF (3 mL), rt; 1 N HCl

H

*

R

HO

46-92% yield, 71-90% ee

Figure 34. N,N’-dioxide-Sc(III) catalyzed asymmetric aza-Diels-Alder reaction  

R2 1

R CHO

N,N'-dioxide 12-Sc(OTf)3 (0.5-5 mol%)

NH2

+

+

R

3

R2 R3

H

CH2Cl2

OH

H

R1 = Aryl, Alkyl

OH

R2, R3 = Alkyl, Halo

O Ar

N H

N

N

O

O

H N

R1

28 examples 90:10 to >99:1 dr 90 to >99% ee

O

12: Ar = 2,6-iPr2C6H3

N H

Ar

Figure 35. Asymmetric three-component inverse electron-demand aza-DA reaction   Cl

O

TMSQD/Sc(OTf)3 (15 mol%)

O R

H

Ar

1 equiv iPr2NEt CH2Cl2, 0 oC

O

O

Ar

R

O

O R

Figure 36. Asymmetric condensation of acid chlorides with aldehydes

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Ar

28

Xiaoming Feng and Xiaohua Liu

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7. OTHER ASYMMETRIC CYCLOADDITION REACTION Sc(OTf)3 appears to be generally effective in mediating the formation of highly reversible cycloadducts. In the presence of Sc(OTf)3, a [5 + 2] cycloaddition of the pyridinyl π-complex (98% ee) to methyleneoxindole afforded the spirooxindole complex in high enantiomeric purity [44]. Sc(OTf)3 catalyzed intramolecular lactonization were also used as the key step for the asymmetric synthesis of the A-ring of 1α, 25-dihydroxyvitamin D3 [45]. Glycals reacted smoothly with o-hydroxybenzaldehydes in the presence of Sc(OTf)3 under mild reaction conditions to afford the corresponding pyranobenzopyrans in good yields with high diastereoselectivities [46]. The treatment of substituted o-hydroxybenzylideneanilines and 2,2-dimethoxypropane with Sc(OTf)3 gave 3,4-dihydro-4-amino-2H-1-benzopyran derivatives with good diastereoselectivity [47]. A tandem ring-opening– cyclization reaction of cyclopropanes with imines could also catalyzed by 5 mol% of Sc(OTf)3 for the highly diastereoselective synthesis of 2,3,4substituted pyrrolidines (42-98% yield, 8/1-30/1 cis/trans) [48]. Sc(OTf)3 was also demonstrated as the most effective Lewis acid in diastereo- and enantioselective synthesis of β-lactones with alkyl substituents at the αposition. The combination of a catalytic amounts of cinchona alkaloid derivative and Sc(OTf)3 catalyzed the asymmetric condensation of acid chlorides with aromatic aldehydes to produce β-lactones in the presence of stoichiometric amount of Hunig base by way of acylammonium enolate intermediates (Figure 36) [49]. O

  Cl

Ph-p-NO2 Cl

N

O Cl

O

R

O

BQD/Sc(OTf)3 (10 mol%)

Cl

iPr2NEt, THF, -78 oC

R' N

R Sc

Cl

R

O

O

MeOH CAN

Ph-p-NO2 O HN R

3OTf

3OTf Cl

Cl

Ph-p-NO2 N

O

O

BQD

Cl Cl

R' R N O Sc

O

BQD

Figure 37. Asymmetric cycloaddition of o-benzoquinone imides

Scandium: Compounds, Productions and Applications : Compounds, Productions and Applications, Nova Science Publishers,

CO2Me

Chiral Scandium Complexes in Catalytic Asymmetric Reactions  

O O

O R Sc(OTf)3-indo-Pybox 17 (10 mol%)

X X = CH2, O

MeCN, 3 Å MS 0 oC or rt

Sc(OTf)3-Ph-Pybox 20 (10 mol%) OR2

Et3N, CH2Cl2, r.t.

R

O

O N TfO

X 65-94% yield 72-97% ee

O

R1

29

R2O

O

O

R1 a b 34-71% yield, 0-91% ee

N Sc OTf 17

HO

R1

O N OTf

O

c

R1

Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved.

Figure 38. Chiral pybox-Sc(III) complex catalyzed asymmetric Nazarov cyclization

The seminal bifunctional catalyst systems also catalyzed the cycloaddition between o-benzoquinone imides and ketene enolates to afford 1,4benzoxazines which are readily converted into α-amino acid derivatives (Figure 37) [50]. The yields of the scandium cocatalyzed the reactions by increasing the average yield up to 81-92% in shorter reaction time with virtually enantiomerically pure products. The Nazarov reaction is a classic electrocyclization mediated by acids to give cyclopentenones. 50-100 mol% of copper-box complexes has been used as Lewis acid for the cyclization of well designed dienone (Figure 38) [51a]. While 10 mol% of pybox-Sc(OTf)3 17 could efficiently promote the asymmetric Nazarov cyclization of 2-alkoxy-1,4-pentadien-3-ones which involves an asymmetric protonation as the key step with moderate to high yield and enantioselectivity (65-94% yield, 72-97% ee) [51b,51c]. The first example for construction of an asymmetric quaternary center by using the monodentate coordinated divinyl ketones via the interrupted Nazarov reaction, which involves a nucleophilic attack of an alcohol on the α-position of the keto function, has also realized by a catalytic amount of Sc(OTf)3-Ph-pybox complex 20 (Figure 38) [52]. Several α-alkoxy cyclopentenones could be afforded as the major products with moderate to good enantioselectivities.

8. ASYMMETRIC 1,3-DIPOLAR CYCLOADDITION Enantioselective 1,3-dipolar cycloadditions of 2-benzopyrylium-4-olate generated from the Rh2(OAc)4-catalyzed decomposition of omethoxycarbonyl-diazoacetophenone could provide epoxy-bridged tetrahydropyran skeleton (Figure 39). Sc(III)-iPr-Pybox 14 complex (10 mol%)

Scandium: Compounds, Productions and Applications : Compounds, Productions and Applications, Nova Science Publishers,

30

Xiaoming Feng and Xiaohua Liu

could effectively catalyze the reactions with several benzyloxyacetaldehyde derivatives to yield endo adducts (67:33-91:9 endo/exo) selectively with high enantioselectivity (81-93% ee) [53]. For the reaction with benzyl pyruvate, and several other α-ketoesters, the Sc(III)-iPr-Pybox 14 complex (10 mol%) catalyzed the reaction effectively in the presence of trifluoroacetic acid (10 mol%) to yield an exo-adduct with both high diastereo- and enantioselectivity (68:32-97:3 exo/endo, 85-95% ee). On the contrary, Yb(III)-Ph-Pybox 15 gave better results in the enantioselective cycloaddition with 3-acryloyl-2oxazolidinone with high exo-selectivity and enantioselectivity [53b]. Sc(OTf)3 exhibited the best outcomes among a variety of Lewis acids in the diastereoselective 1,3-dipolar cycloaddition between diphenyl nitrone and chiral auxiliary-substituted crotonyl amide in terms of reaction time, capable of supporting H2O and stereoselectivity (Figure 40) [54]. The NMR spectroscopic experiments suggested that the octahedral complex occurred in which cronoyl amide behaved as bicoordinated conformation. O

 

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OMe CHN2

Sc(III)-pybox 14 OHCCH2OCH2Ar

OMe O O

OCH2Ar

O

O

Rh2(OAc)4

OMe O O

OMe O

Sc(III)-pybox 14 TFA RCOCO2CH2Ar

81-93% ee endo O 89-95% ee exo

R CO2CH2Ar

O O

Yb(III)-pybox 15 O

O N

N

N

R

R 14 R = iPr 15 R = Ph

O

O

MeO

O

N N

O

O O

96% ee exo

O

Figure 39. Enantioselective 1,3-dipolar cycloadditions of 2-benzopyrylium-4-olate   Ph H

N

O

Ph

O

O Sc(OTf)3

N Ph

O

Bn

Ph

H O N O

Ph

N H O Bn exo-minor H

O

H O N O

Ph

N H O Bn endo-major

O

H

Figure 40. Diastereoselective 1,3-dipolar cycloaddition of diphenyl nitrone

Later, chiral BINOL-Box 29-scandium complex was developed for the asymmetric 1,3-dipolar cycloaddition of nitrones to alkenes to give endoScandium: Compounds, Productions and Applications : Compounds, Productions and Applications, Nova Science Publishers,

Chiral Scandium Complexes in Catalytic Asymmetric Reactions

31

isoxazolidine as the major diastereomer with endo:exo ratio of 97:3 and 87% ee (Figure 41) [55].

9. ASYMMETRIC RING-OPENING REACTION The asymmetric nucleophilic addition of meso-epoxides is an efficient methods to construct chiral 1,2-difunctional compounds, such as 1,2-diol monoether, 1,2-amino alcohols. The enantioselective addition of aliphatic alcohols to meso-epoxides catalyzed by a novel scandium-bipyridine 30 complex proceeded with in part excellent enantioselectivities and generally good yields (44-98% ee) (Figure 42) [56]. The same chiral catalyst improved the enantioselectivity of the aminolysis of aromatic and aliphatic mesoepoxides substantially (76-96% ee) [56b]. Under the same catalytic system, asymmetric addition of phenylselenol to aromatic meso-epoxides furnished 1,2-seleno alcohols in good yields and up to 94% ee [57].  

Ph

O

O Sc(OTf)3 (5mol%) Bn N BINOL-Box 29 (6mol%) Ph additive, rt. 20h

Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved.

N H

Bn O

O N

N

O

N

N

exo-

O

29

Figure 41. Catalytic asymmetric ,3-dipolar cycloaddition of nitrones 30 (12 mol%) Sc(OTf)3 (10 mol%) CH2Cl2, rt. 12-24 h

 R1 O R1

R2OH R1 = aryl R1 = alkyl

72-83% yield, 89-98% ee 25-93% yield, 44-62% ee

R1

OH

R1

OR2

O R1

R2

N H

R3

R1 = aryl, alkyl

30 (12 mol%) Sc(OTf)3 (10 mol%) CH2Cl2, rt. 12-24 h 54-97% yield, 76-96% ee

N

N OH

R1

1

Ph

OH OH

O

endo-major

O

O

O

R

OH

R1

R3 N R2

30

HO

Figure 42. Asymmetric nucleophilic addition of meso-epoxides

Scandium: Compounds, Productions and Applications : Compounds, Productions and Applications, Nova Science Publishers,

Ph

32

Xiaoming Feng and Xiaohua Liu  R1 O

R2

R1

N H

3 (1.2 mol%) Sc(DS)3 (1 mol%)

Ar

water

R1 = aryl, alkyl

R1

R2

O N H R1 = aryl, alkyl R1 O

R1

Ar N R2

3 (6 mol%) Sc(DS)3 (5 mol%)

R1

H2O (1M), rt, 4-6 h

R1

3 (12 mol%) Sc(DS)3 (10 mol%) H2O, rt, 23-27 h

R1 R1 = aryl

N

N OH

3

HO

R3

OH

R2

56-85% yield, 85-93% ee

R2

HS

OH

61-89% yield, 60-96% ee

R3

R1

R1

NH

R1

OH

R1

S

R2

44-76% yield, 85-93% ee

Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved.

Figure 43. Asymmetric ring-opening of meso-epoxides

Subsequently, Kobayashi et al. established that just 1 mol% of this scandium–bipyridine 3 complex with dodecyl sulfate counterions was sufficient for the aminolysis of epoxides in water with comparable levels of enantioselectivity than that in dichloromethane (Figure 43) [58]. This kind of catalyst was also useful for the asymmetric ring-opening of meso-epoxides with aromatic N-heterocycles, indoles and thiols to afford the corresponding products in moderate to good yields (44-85%) with high to excellent enantioselectivities (85-93% ee) in water [59].

10. ASYMMETRIC ENE REACTION The carbonyl-ene reaction is a powerful C-C bond forming reaction. The diastereoselective carbonyl-ene reaction of N-glyoxyloyl camphorpyrazolidinone with variety of 1,1-disubstituted olefins proceeded smoothly in the presence of Sc(OTf)3 to give the corresponding ene products with high stereoselectivity (Figure 44) [60]. The coordination of the Lewis acid with the dicarbonyl groups benefited the shift of conformational equilibrium from strans toward s-cis conformation in which the ene compound attacked the formyl group from the less hindered si-face which resulted in the high diastereoselectivity. Scandium-pybox complexes 17 or 20 were found to be effective promoters of the catalytic asymmetric carbonyl-ene reaction (Figure 45) [61]. Good yields (73-99%) and high enantiomeric excesses (92-94%) were

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Chiral Scandium Complexes in Catalytic Asymmetric Reactions

33

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observed for 1,1-disubstituted olefins (α-methylstyrene, isobutylene, methylenecyclohexane, and methylenecyclopentane) using complex 17. Steric effect seemed to be important to the regioselective discrimination. When unsymmetrical 1,1-disubstituted olefins were used, terminal olefin was preferentially formed over the more highly substituted alkene and regioselectivity increased with bulkier substituents. Sc(OTf)3-(S,S)-Ph-pybox complex 19 showed better results for trisubstituted olefinic substrates to afford products with good syn selectivities (9:1-24:1 dr) and high enantioselectivities (94-99% ee). The results of geometric isomers of 3-methylpent-2-ene indicated that the major products produced in both cases corresponded to proton transfer from the β-cis substituent through an exo transition state as shown in Figure 45. When acyclic allylsilanes were employed in the reaction, the anti diastereomers were afforded in good yield (68-71%) and enantioselectivity (94-99% ee) and anti/syn ratio (15:1-99:1). Chiral Lewis acids such as Sc(OTf)3-(R,R)-pybox and Cu(OTf)2-(S,S)-Phbox were also surveyed in the enantioselective intramolecular carbonyl-ene reactions of unsaturated α-keto esters [62]. Optically active monocyclic cis-1hydroxyl-2-alkyl esters which are the chiral fragments of many natural products could be obtained in good yield and excellent enantioselectivity.   H O

N Ph

N

O

R

O

Sc(OTf)3 (30 mol%) CH2Cl2, rt

R = CH2tBu, n-Pr, CH2OTBDPS, Ph, p-Cl-Ph,

OH O

N Ph

N

R O

64-87% yield, 97:3-87:13 dr

, s-trans O

N

N O R

H Re-attack

s-cis

s-cis

O

H H

O

N

N

H O

O R H

O

N

N

O

O R H Si attack

Figure 44. Diastereoselective carbonyl-ene reaction

Scandium: Compounds, Productions and Applications : Compounds, Productions and Applications, Nova Science Publishers,

ScLn

34

Xiaoming Feng and Xiaohua Liu  

R1

H N

O

2

R3

R

H

n

Ph

O

OH

N H

R1 = iPr, R2 = H R1 = Me, R2 = Me R1 = tBu, R2 = H

R2

O

Me Me 5 mol% 20

Ph O 95% ee, 83% yield, 10/1 syn/anti O LnSc H

O

R

a

R1

R2

OH

Ph

O

b

H N

Ph

O

94% ee, 78% yield, 4:1 a/b 96% ee, 60% yield, 5.3:1 a/b 94% ee, 81% yield, >99:1 a/b Et

OH H N

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R

CH2Cl2, rt

Ph

H N

1

5 mol% 17

R

H N

Ph R3 O 89-98% ee, 58-99% yield, 9:1-24:1 dr

CH2Cl2, 4Å MS, rt

CHO 1

R2

OH

5 mol% 17 or 20

H N

H

Me Ph

O

X H Me H

LnSc

R

OH

Me Et 5 mol% 20

H N

Ph O 99% ee, 75% yield, 20/1 syn/anti

O

X

O

H Me H

Figure 45. Chiral pybox-Sc(III) complex promoted asymmetric carbonyl-ene reaction

Electrostatically immobilized scandium- and copper-base pybox complexes on silica have been studied in asymmetric carbonyl-ene reaction which compared very well with that of their homogeneous equivalents [63]. In the case of scandium complex, a variation in enantioselectivity (up to 60% ee, S-product) was observed on immobilisation compared to its use homogeneously (13% ee, R-product). The immobilized catalysts could be successfully reused a number of times.

11. MISCELLANEOUS REACTION The reaction of allylsilanes and allenylsilanes provides access to important building blocks for homopropargylic alcohols or dihydrofurans [64]. [Sc(S,S)Ph-pybox](OTf)3 complex 20 could promote the addition of trimethylsilylallene containing either linear or branched alkyl substituents to ethyl glyoxylate. In these reactions, allenylsilanes function as propargylic anion equivalents to afford (R)-propargylic alcohols in 63-96% yield and 84-98% ee

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.

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Chiral Scandium Complexes in Catalytic Asymmetric Reactions

35

after desilylation with K2CO3/EtOH. Hexafluoro-2-propanol was added to suppress the formation of oligomeric byproducts. [3+2] cycloaddition product functionalized dihydrofuran were also produced when the steric bulk of the silane substituent was increased. At lower temperature, dihydrofuran was isolated in 32-98% yields and 85-94% ee in scandium complex 20 catalyzed asymmetric reactions between triisopropylallenes and ethyl gloyoxylate. X-ray crystallographic structure of catalyst 20 was used to elucidate the origin of selectivity in the transformations. The catalyst features a pentagonal bipyramidal geometry. A rationalized model was shown in Figure 46 in which aldehyde functionality is bound in the apical position and ester moiety is bound in the equatorial position. Binding of the aldehyde in the apical position in the addition transition state can be explained by the greater trans influence of the pyridine ligand relative to the triflate ligand as well as the steric factors. Therefore, addition of the allenylsilane from the re face is favored as the si face is effectively shielded by the phenyl group of the ligand. Evans group also developed chiral scandium complex catalyzed carbonyl addition reaction of vinylsilane nucleophiles (Figure 47) [65]. In the presence of chiral pybox 26 and Sc(OTf)3, the reaction between a range of 2-arylsubstituted vinylsilanes containing both electron-donation and electron withdrawing substitutents in the para position and glyoxamide could give good yields (83-92%) and enantioselectivities (97-99% ee). 2-Alkyl-substituted vinylsilanes allowed products in moderate yields with excellent enantioselectivities. The addition of 2,2-disubstituted vinylsilanes exhibited increased reactivity to produce the allylic alcolhols in high yields and enantioselectivities at 2.5 mol% catalyst loading. With 2,2-dialkyl substituted vinylsilanes, a catalytic amount of 2,6-ditertbutylpyridine was added as an external additive to reduce the isomerization of alkenylation products into homoallylic alcohols. Besides, optically active propargylic alcohol has also be obtained via scandium complex 17 catalyzed addition between (trimethylsilyl)phenylacetylene and glyoxamide with 60% yield and 89% ee. Feng group applied N,N’-dioxide 12-Sc(OTf)3 in the three-component Kabachnik-Fields reaction of aromatic aldehydes, 2-aminophenol and diphenyl phosphate [66]. The direct reaction proceeded with extremely high reactivity to produce α-amino phosphonates with 80-87% ee within 1 hour (Figure 48).

Scandium: Compounds, Productions and Applications : Compounds, Productions and Applications, Nova Science Publishers,

36

Xiaoming Feng and Xiaohua Liu Me3Si

  O EtO

EtO O 63-96% yield, 84-98% ee

cat. 20

H

ButPh2Si

O

O

N

Sc

O

O

R

EtO

SiPh2tBu 32-98% yields, 85-94% ee

SiMe3

C N

O

C CH2

R CH2Cl2, - 48 oC

H O O N

R

OH

C CH2 R K2CO3/EtOH

R re-face attack O

OTf

20 O

OH

R Ph

EtO2C

O N N Sc N OTf Ph TfO TfO

Figure 46. Asymmetric reactions between allenylsilanes and ethyl gloyoxylate O

  H

Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved.

Me3Si R

Cat. 26

H N

Ph O cat. 17

R H

H N

OH Ph

O

55-92% yield, 97-99% ee

R3

R2

SiMe3 Cat. 26

SiMe3

Ph OH

R1

H N

Ph O Ph 60% yield, -89% ee

R2

OH

H N

1

R

Ph R3 O 55-99% yield, 98-99% ee

Figure 47. Catalytic carbonyl addition reaction of vinylsilane nucleophiles   R

OH

O H

+

+ NH2

O P OPh H OPh

N,N'-Dioxide 12 (10 mol%) Sc(OTf)3 ( 5 mol%) THF, -20 oC, < 1 h

OH

H N * R P OPh O OPh

73-96% yield, 80-87% ee

Figure 48. Aymmetric three-component Kabachnik-Fields reaction

Scandium: Compounds, Productions and Applications : Compounds, Productions and Applications, Nova Science Publishers,

Chiral Scandium Complexes in Catalytic Asymmetric Reactions  

N

R1

Bn

TMSCN Sc(III) catalyst 31 (10 mol%) or HCN toluene, -20 oC

R2 CHO

TMSCN

2

R

R1 *

N N Bn H

Li O Sc O

OH * CN

Sc(III) catalyst 31 (10 mol%) toluene, -20 oC

37

O O

31

Figure 49. Asymmetric cyanation of imines and aldehyde  

OTf N+ F O

OR3

O

O

* OR3 F R2 16-97% yield, 47-88% ee

toluene, rt

O

R1

F Sc[(R)-F8BNP]3 32 (10 mol%)

R1

R2

F F F

O P O

F

F F F

O Sc

O 3

32

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Figure 50. Asymmetric α-fluorination of β-keto esters

The new chiral heterobimetallic complex Sc(BINOL)2Li 31 was prepared and used as a catalyst in the enantioselective addition of a cyanide source (HCN or TMSCN) to several imines [67]. The Strecker reaction of ketimine benzyl-(1-phenyl-propyl-idene)amine could gave 95% ee. But a slight decrease in ee value and reaction rate was observed as the reaction proceeded which might be due to the catalyst poisoning or decay. The catalytic cyanosilylation of benzaldehyde gave the corresponding cyanohydrin in quantitative yield and 84% ee (Figure 49). The use of Sc[(R)-F8BNP]3 catalyst in combination with 1fluoropyridinium triflate (NFPY-OTf) 32 as a fluorinating agent was found to be useful for α-fluorination of either cyclic or acyclic β-keto esters (Figure 50), giving the desired α-fluoro-β-keto esters in high chemical yields and enantiomeric excesses (47-88%) under mild conditions [68]. Enantiomerically enriched O-TMS cyanohydrins have been transformed directly into O-acyl-cyanohydrins using various anhydrides or acid chlorides in the presence of catalytic amounts of scandium(III) triflate with full retention of stereochemistry [69]. Sc(OTf)3 has also been used to promote BaeyerVilliger reaction of methyl ether to give lactone with maintained stereochemistry [70]. Prins-type cyclization of aldehydes with the 3-buten-1-ol in the prescence of Sc(OTf)3 gave 2-substituted tetrahydropyran-4-ols and ethers with excellent diastereoselectivities and good yields [71]. Fragmentation of (2S,3S,4S)-2-allyl-3-iodo-1-oxocyclohexan-2,4-carbolactone

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Xiaoming Feng and Xiaohua Liu

to (4S)-2-allyl-4-hydroxycyclohex-2-en-1-one, a chiral building block of (-)platensimycin, proceeded efficiently by using scandium(III) triflate in DMF/H2O (1:3) at 100 oC [72].

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CONCLUSION The scandium has unique reactivity, magnetic and optical properties which are fundamental to most of their applications. This chapter has mainly surveyed chiral scandium complexes prepared either in situ or as isolate forms and their catalytic abilities in many asymmetric transformations. The creation of scandium complexes with chiral ligands maximized the characteristic features of the scandium cation in asymmetric catalysis. In most cases, Sc(OTf)3 was used which is stable and excellent Lewis acid catalyst both in organic solvents an in aqueous solutions. It is possible to reclaim and reuse scandium triflate salt through extraction with water. For further development of asymmetric scandium catalysis, the recyclable use of scandium complexes is important in “green chemistry”. The promise of highly efficient and environmental friendly chiral scandium complexes in asymmetric catalysis has not yet been fulfilled and effective chiral ligands are still proving elusive. The success in asymmetric reaction occurred in water provides a foundation for conducting organic reactions in the environmental friendly mediums, and then this area presents significant challenges for some time to come.

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Substituted Cronoyl Amide. Tetrahderon, 55, 8509-8524. [55] Kodama, H., Ito, J., Hori, K., Ohta, T. & Furukawa, I. (2000). Lanthanide-Catalyzed Asymmetric 1,3-Dipolar Cycloaddition of Nitrones to Alkenes using 3,3’-Bis(2-oxazolyl)-1,1’-bi-2-naphthol (BINOL-Box) ligands. Jouranl of Organometallic Chemistry, 603, 6-12. [56] (a) Schneider, C., Sreekanth, A. R. & Mai, E. (2004). Scandium– Bipyridine-Catalyzed Enantioselective Addition of Alcohols and Amines to meso-Epoxides. Angewandte Chemie International Edition, 43, 56915694. (b) Mai, E. & Schneider, C. (2007). Scandium-BipyridineCatalyzed Enantioselective Aminolysis of meso-Epoxides. Chemistry-A European Journal, 13, 2729-2741. (c) Tschöp, A., Marx, A., Sreekanth, A. R. & Schneider, C. (2007). Scandium-Bipyridine-Catalyzed, Enantioselective Alcoholysis of meso-Epoxides. European Journal of Organic Chemistry, 2318-2327. [57] Tschöp, A., Nandakumar, M. V., Pavlyuk, O. & Schneider, C. (2008). Scandium-Bipyridine-Catalyzed Enantioselective Selenol Addition to Aromatic Meso-epoxides. Tetrahedron Letters, 49, 1030-1033. [58] Azoulay, S., Manabe, K. & Kobayashi S. (2005). Catalytic Asymmetric Ring Opening of meso-Epoxides with Aromatic Amines in Water. Organic Letters, 7, 4593-4595. [59] Boudou, M., Ogawa, C. & Kobayashia, S. (2006). Chiral ScandiumCatalyzed Enantioselective Ring-Opening of meso-Epoxides with NHeterocycle, Alcohol and Thiol Derivatives in Water. Advanced Synthesis & Catalysis, 348, 2585-2589. [60] Pan, J. F., Venkatesham, U. & Chen, K. M. (2004). An Efficient Diastereoselective Glyoxylate-ene Reaction Using N-glyoxyloyl Camphorpyrazolidinone as an Enophile. Tetrahedron Letters, 45, 93459347. [61] Evans, D. A. & Wu, J. (2005). Enantioselective Syn-Selective Scandium-Catalyzed Ene Reactions, Journal of the American Chemical Society, 127, 8006-8007. [62] Yang, D., Yang, M. & Zhu, N. Y. (2003). Chiral Lewis Acid-Catalyzed Enantioselective Intramolecular Carbonyl Ene Reactions of Unsaturated α-Keto Esters. Organic Letters, 5, 3749-3752. [63] McDonagh, C. & O’Leary, P. (2009). Electrostatically Immobilised BOX and PYBOX Metal Catalysts: Application to Ene Reactions. Tetrahedron Letters, 50, 979-982. [64] Evans, D. A., Sweeney, Z. K., Rovis, T. & Tedrow, J. S. (2001). Highly Enantioselective Syntheses of Homopropargylic Alcohols and

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[66]

[67]

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[69]

[70] [71]

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Dihydrofurans Catalyzed by a Bis(oxazolinyl)pyridine-Scandium Triflate Complex. Journal of the American Chemical Society, 123, 12095-12096. Evans, D. A. & Aye, Y. (2006). Enantioselective Scandium-Catalyzed Vinylsilane Additions: A New Approach to the Synthesis of Enantiopure β,γ-Unsaturated α-Hydroxy Acid Derivatives. Journal of the American Chemical Society, 128, 11034-11035. Zhou, X., Shang, D. J., Zhang, Q., Lin, L. L., Liu, X. H. & Feng, X. M. (2009). Enantioselective Three-Component Kabachnik-Fields Reaction Catalyzed by Chiral Scandium(III)-N,N’-Dioxide Complexes. Organic Letters, 11, 1401-1404. Chavarot, M., Byrne, J. J., Chavant, P. Y. & Vallée, Y. (2001). Sc(BINOL)2Li: a New Heterobimetallic Catalyst for the Asymmetric Strecker Reaction. Tetrahedron: Asymmetry, 12, 1147-1150. Suzuki,S., Furuno, H., Yokoyama, Y. & Inanaga, J. (2006). Asymmetric Fluorination of β-keto Esters Catalyzed by Chiral Rare Earth Perfluorinated Organophosphates. Tetrahedron: Asymmetry, 17, 504507. Norsikian, S., Holmes, I., Lagasse, F. & Kagan, H. B. (2002). A One-pot Esterification of Chiral O-trimethylsilylcyanohydrins with Retention of Stereochemistry. Tetrahedron: Letters, 43, 5715-5717. Hanessian, S. & Auzzas, L. (2008). Alternative and Expedient Asymmetric Syntheses of L-(+)-Noviose. Organic Letters, 10, 261-264. Zhang, W. C. & Li, C. J. (2000). Diastereoselective Synthesis of 2,4Disubstituted Tetrahydropyranols and Ethers via a Prins-Type Cyclization Catalyzed by Scandium Triflate. Tetrahedron, 56, 24032411. Matsuo, J., Kawano, M., Takeuchi, K., Tanaka, H. & Ishibashi, H. (2009). Asymmetric Synthesis of 2-Alkyl-4-hydroxycyclohex-2-en-1ones by Scandium(III) Triflate-Catalyzed Fragmentation of 2-Alkyl-3iodo-1-oxocyclohexan-2,4-carbolactones. Tetrahedron Letters, 50, 19171919.

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In: Scandium: Compounds, Productions… ISBN: 978-1-61761-465-1 Editors: Viktor A. Greene © 2011 Nova Science Publishers, Inc.

Chapter 2

APPLICATION OF SCANDIUM OXIDE IN DISPENSER THERMIONIC CATHODES Jinshu Wang*

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School of Materials Science and Engineering, Beijing University of Technology, Beijing, China

ABSTRACT The addition of scandium oxide to dispenser cathodes (namely Scandate cathodes), which are typically comprised of a porous tungsten matrix impregnated with Ba-Ca-aluminate, improves the emission by a factor of tens to hundreds under the same operating temperature. The fabrication, emission property and emission mechanism of Scandate cathodes have been reviewed. Four types were summarized in this paper: the traditional impregnated Scandate cathode, “top-layer” impregnated Scandate cathode, impregnated mixed matrix Scandate cathode and pressed Scandate cathode. It was found that decreasing the grain size from micrometer to sub-micrometer could enhance the cathode emission performance. Mixed matrix Scandate cathode prepared with scandium oxide doped tungsten powder which has the characteristic of superfine Sc2O3 particles dispersing uniformly over and among sub-micrometer W grains, might be the main type of Scandate cathodes in the future. The emission models for explaining the conspicuous emission performance were summarized. Although the operating mechanism of Scandate * Corresponding author: E-mail: [email protected], Tel & Fax: +86-10-67391101.

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Jinshu Wang cathodes is still unclear, it is accepted that the emission is correlated with a surface multilayer/monolayer of Ba-Sc-O. The layer is formed after proper activation by diffusion of free or ionic Sc together with Ba and O from the interior of the cathode to its surface. The correlation between the emission properties of scandate cathodes and their surface features has been established.

Keywords: Scandium oxide, Thermionic cathode, Electron emission

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1. INTRODUCTION Scandium oxide (Sc2O3) is a rare earth oxide with an isometric system. It is used widely as high-efficiency and multifunctional laser crystals, solid electrolytes, transparent ceramic scintillators[1-5], infrared laser windows, high temperature windows, infrared guided missile domes, and photographic lens used at high-temperatures and in strong radiation environments[6,7]. Another important application is thermionic cathodes. Thermionic cathodes are used as electron sources in microwave and power tubes, cathode ray tubes, plasma devices, and electron beam instruments. The addition of scandium oxide to Ba-W dispenser thermionic cathodes can improve the emission by a factor of tens to hundreds at the same operating temperature. With the rapid development of microwave vacuum devices, an increase of electron emission current density at a given operating temperature is one of the main goals of thermionic cathode improvement for different applications in vacuum tubes. The only thermionic cathode candidate for the continuously improved vacuum devices in high emission current is the scandate cathode, which is considered as the next generation high emission cathodes and, has attracted great interest [8-10]. In this paper, the manufacturing, emission performance and emission model of Scandate cathodes have been reviewed.

2. THE TYPES OF SCANDATE CATHODE 2.1. Pressed Scandate Cathode The study of scandate cathode started in the 1960s. Since in thermionic cathodes barium is generally used to obtain a low work function emitter, a barium- containing compound is essential for preparation of a dispenser

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thermionic cathode. Fingner and his coworkers filed a patent for a new cathode in which they used barium scandate as the barium- containing compound and prepared pressed scandate cathode, which was composed of a mixture of grain tungsten powder and barium scandate (Ba3Sc4O9) prepared by calcining the mixed powder of BaO and Sc2O3[11]. They found that the cathode exhibited good emission property, i.e., the zero field emission current density could reach 1.5-4A/cm2at the temperature of 1000–-1100oC. Ostrom et al. made the pressed cathode by using BaCO3 instead of BaO for the synthesis of Ba3Sc4O9 [12]. The mixture of Ba3Sc4O9 and W was pressed into a pellet, sintered in hydrogen at 1570oC, polished and mounted in a diode test assembly as the cathode. Auger ananlysis was utilized to examine surface changes which that occurred through the activation process. They found that at low temperatures the scandate remained stable, but after heating to 950oC the scandate started to decompose. The barium started to diffuse away from the scandate areas, while scandium and oxygen, were still localized. Based on these results, they concluded that high temperature treatment was essential for the improvement of the emission performance. It was found that such cathodes after activation had a current density about 10A/cm2 at 950oC which was due to activated scandium oxide regions in the surface. Recently, we made an improvement by changing the scandium- containing compound preparation technique and cathode fabrication process [13,14]. Scandium oxide and barium calcium aluminates co-doped tungsten oxide powders were prepared by a Sol-Gel technique using scandium nitrite as the starting material. The doped tungsten oxide powders were reduced into metallic tungsten powders in dry hydrogen atmosphere at high temperature. The doped tungsten powders were die-pressed and sintered in hydrogen into porous matrices of about 2–-3mm in diameter and 1mm in thickness. Figure 1 shows Scanning electron Microscope pictures of scandium oxide doped pressed cathode surface, fracture of the cathode and related EDS results. It could be seen that a homogenous, porous sub-micron structure cathode with quasi-spherical grains was formed, as shown in Figure 1(a) and (b). The EDS result in Figure 1 (c) illustrated that elements of Sc, Ba and Ca were found in the spots of A, C and D while the grain at spot B was composed of tungsten, indicating that Scandium oxide and barium calcium aluminates was were mixed homogenously and existed on the tungsten grain surface or in the pores in the cathode. The sub-microstructure of the cathode is favorable for the uniform distribution of active substance and diffusion of these substances from

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the inner part of cathode to cathode surface during the activation and operation periods, thus it is favorable for maintaining stable emission.

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(a)

(b)

(c)

Figure 1. SEM micrographes of pressed Scandate cathode surface (a), fracture (b) and related EDS result (c)

Figure 2. Emission property of Scandiun oxide doped pressed cathode Scandium: Compounds, Productions and Applications : Compounds, Productions and Applications, Nova Science Publishers,

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The emission measurements of scandia doped pressed cathodes had been done in close-space diode configuration. In the testing system, the cathode was mounted into a Mo sleeve and set in front of a degassed Mo anode with a distance of 0.5mm. It was also found that proper activation was necessary for achievement of good emission. Typical I-V plots at temperatures from 700°Cb to 900°Cb are illustrated in Figure 2 after the cathode was activated at 1150°Cb for about 3h before measurement. The current density of this cathode reached 46 A/cm2 at 850oCb, much higher than the emission property of the other two kinds of pressed scandate cathode mentioned above.

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2.2. The Traditional Impregnated Scandate Cathode The cathodes comprise a porous metal body which that has an emissive surface and the pores of which contain compounds for dispensing when heated at least barium and scandium to the emissive surface [15]. The porous body was manufactured from tungsten which had a density of approximately 80%. Said porous tungsten body impregnated with 5BaO.3Al2O3.2CaO impregnant mixture was covered with scandium oxide by wetting it with a dilute solution of scandium nitrate in water. The quantity of scandium oxide present was approximately 3% by weight of the overall quantity of the dispensing compounds. The cathode could provide a long life of 3000h and a certain emissive properties (5A/cm2 at 1000oC).

2.3. “Top-Layer” Impregnated Scandate Cathode Hasker and colleagues took a different approach in their scandate cathode design[16-18]. Their cathode, the top-layer cathode illustrated in Figure 3, utilized a thick film configuration. The top-layer scandate cathodes consisted of a porous-tungsten top layer, containing Sc2O3 or ScH2, a porous tungsten plug under the top layer. A Sc2O3 and W mixed layer of about 0.5µm was deposited by sputtering. Then they revised their top layer design by using mixed powders of pure tungsten and tungsten partially covered with ScH2 instead of a mixed powder of Sc2O3 and W, since the mobility of Sc2O3 over the surface is too low. The tungsten plug and W+Sc2O3 or W+W/ScH2 layer were impregnated with BaO.Al2O3.CaO impregnant mixture with the mole ratio of 5:3:2 (BCA 532) to obtain the top layer cathodes.

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Figure 3. Schematic diagram of Hasker top layer cathode

Figure 4. Current-voltage characteristic for both a top-layer scandate cathode and OsRu coated cathode in diode configuration with a distance of about 0.29 mm between cathode and anode[17]

Figure 4 shows the current-voltage characteristic for a top-layer scandate cathode and Os-Ru coated cathode. It was found that the cathodes having top layers containing 5%Sc2O3 had good emission property. The emission value of 100A/cm2 at around 1220K could be obtained. The effective work function was slightly above 1.5eV. This value was confirmed by Ginson etalet al. in their work[19]. The ion-bombardment properties of the two kinds of Sc-top

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layer cathodes indicated that the top layer cathode of W+W/ScH2 had higher emission recovery after ion bombardment than W+Sc2O3 top layer. Another kind of top layer cathode was made by Yamamoto etalet al.[20,21], as schematically shown in Figure 5. A standard impregnated cathode, which serves as a Ba source, was fabricated by standard procedures, i.e., porous body manufacturing and impregnation of electron emissive materials. The porous body was made of a W powder which was pressed and sintered in a vacuum. The electron emissive material was a mixture of BaCO3, CaCO3and Al2O3 (BCA) in a mole ratio of 4:1:1 (BCA411), which was impregnated under an H2 atmosphere. The thin sputtered film of tungsten/scandium oxide was formed on the cathode surface. They found that 2.5-6.5% by weight scandium oxide in the film layer yielded the best results. Further investigation of this system showed that partial oxidation or reaction in the film during activation had a beneficial effect on the emission properties. They identified the presence of Sc2W3O12 as a product of this reaction as illustrated by reaction (1).

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Sc2O3+3WO3=Sc2W3O12

(1)

This kind of cathode provided a higher emission property than Os coated cathode but much lower than that prepared by Hasker. The emission current density of Yamamoto’s cathode reached 35A/cm2 at the temperature of 1000oCb(oCb: brightness temperature). Yamamoto made improvement on the top layer cathode by coating a thin W film containing Sc2W3O12 instead of Sc2O3 onto a basic impregnated cathode based on the previous identification of scandium tungstate compound [22]. The emission current density was about three times better than the Os cathode level, i.e., the emission current density was 80A/cm2 at 1030oCb, in addition to requiring less activation time. Activation was achieved by heating in ultra high vacuum environment at 1150oC for 3-5h, as opposed to the 10h necessary for the activation of W-Sc2O3 film. The improvement of the emission performance was attributed to the chemical behavior of Sc2W3O12. During the activation and operation periods, Sc2W3O12 reacted with Ba atoms to form Sc atoms according to the following reaction (2). Sc2W3O12 +3Ba=3BaWO4+2Sc

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(2)

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Figure 5. Schematic diagram of Yamamoto thin film scandate cathode

The surface distribution, especially Sc and O, was smooth and migrated over the entire cathode surface, and thus it increased the electron emission area. He utilized X-ray diffraction evidence to suggest that with prolonged heat treatment the scandium tungstate compound was disappearing, which supported his hypothesis of Sc formation. Sasaki etalet al. studied the effect of scandium concentration and thickness of Sc2O3-W film [23]. It was found that the electron emission from scandateimpregnated cathodes coated with W-scandia films depended on scandium concentration and film thickness. W-scandia cathodes with low Scconcentration films (2.8 at.% Sc) produced high electron emission when film thickness was 670 nm. On the contrary, the cathodes with high Scconcentration films (13, 22, or 33 at.% Sc) showed a tendency that a thinner film (10 nm) provided higher electron emission than a thicker film (30–220 nm). Wang also found that Sc2O3/W coating on the traditional impregnated cathode prepared by pulse laser deposition had better emission performance than traditional impregnated scandate cathode, Ba-dispenser cathode and OsRu coated M type cathode[24]. Under the condition of drawing the same emission current density, the working temperature was about 200oC and 100oC lower than Ba-dispenser cathode and Os-Ru coated M type cathode, respectively. Under the same working condition, the emission stability of this cathode was much better than impregnated scandate cathode. In order to decrease the activation temperature of scandate cathode, a kind of top layer cathode was prepared with a coating film composed of two metal layers on a Sc-type impregnated cathode pellet [25]. The upper film was molybdenum metal and the lower film was metallic scandium. The activation temperature and ion bombardment recovery temperature were 100-150oCb lower than those for traditional scandate cathode.

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Figure 6. Current-coltage characteristics of a LAD-top-layer scandate cathode at decreasing temperature in a close spaced planar diode configuration. The onset of saturation)(i0) is given as 10% deviation from the space-charge limitation[26]

Gäertner etalet al. made the top layer scandate cathodes which exhibited unprecedented emission capability [26-28]. The top-layer scandate cathodes were prepared by laser ablation deposition (LAD) of a thin polycrystalline layer of 100-400nm of Re and Sc2O3 from the respective targets on 4BaO.CaO.Al2O3(411) impregnated cathode bases. The use of Re in the LADtop-layer instead of tungsten improved the emission behavior. Figure 6 shows the current-voltage characteristics of a LAD-top-layer scandate cathode as a function of decreasing temperature in a closed spaced planar diode configuration. In the parallel table the onset of saturation i10% is listed as a function of temperature, with e.g. i10%=0.5A/cm2 at 500oC. i10% is given as 10% deviation from the space-charge limitation, which differs less than 5-10% from zero field emission current density io. It could be seen from Figure 6, extremely high emission current density could be obtained at a relatively low temperature. Furthermore, cold emission results indicated that the top-layer cathode provided a threshold field strength for field emission of 3.2Vum-1 at 20oC. Although this top-layer cathode exhibited excellent emission performance, the preparation procedures for this top-layer cathode are complicated.

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2.4. Sc2O3 Mixed Matrix Impregnated Cathodes As described in the section 2.2, Sc2O3 was mixed with the impregnant to further reduce the work function of the cathode surface and thus improve the emission property. In this part, Sc2O3 was mixed with the body metal tungsten instead of with the impregnant. Yamamoto and his co-workers began their studies on impregnated cathode with a mixed matrix of W and Sc2O3[29,30].In their approach, a specific percentage (1-16% by weight) of scandium oxide powder was mixed with tungsten powder, pressed, and sintered to 80-85% theoretical density to form matrix. This mixed matrix of Sc2O3-W was then impregnated with standard 4BaO.CaO.Al2O3(411) impregnant, as shown in Figure 7. The cathode provided a 10A/cm2 saturated current density at 850900oCb. However, the emission distribution was not uniform. In order to improve the emission property and uniformity, recently, we used scandia doped tungsten powder to produce a matrix for the scandate cathode instead of scandia mixed matrix [31-36]. Several doping techniques were adopted to prepare Scandia doped tungsten powder: a) the powder prepared by mechanical mixing of Sc2O3 and tungsten, b）solid-liquid doping which scandia was added to tungsten oxide in the form of a scandiumcontaining aqueous solution, and c ） liquid-liquid doping using a Sol-Gel method. We found in our previous work that the addition of Re in the doped tungsten could greatly decrease the particle size of doped tungsten [37], so Re and Sc2O3 co-doped tungsten powder was also prepared. These four kinds of powder were assigned as Sc-1, Sc-2, Sc-3 and Sc-4 respectively. The powder described above was pressed and sintered in hydrogen to obtain the Scandia mixed tungsten porous bodies. Then the bodies were impregnated with a mixture of BaCO3, Al2O3 and CaCO3 in a mole ratio of 4:1:1. Typical results of back scattered electron images of these four kinds of matrix are shown in Figure 8. It is clear that the bodies prepared with powder prepared by Sol-Gel method and Re and Sc2O3 co-doped tungsten have submicron microstructure and superfine Sc2O3 particles dispersing uniformly over and among submicrometer W grains whereas the other two bodies have large grain size. Furthermore, it is interesting to find that tungsten grains of Sc3 had quasispherical shape.

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Figure 7. Schematic diagram of Yamamoto mixed matrix scandate cathode

The emission properties of these four cathodes were determined by direct deviation from the linear part of the LogI-LogU plots at 850 °Cb. The LogILogU plots for four cathodes are shown in Figure 9. The related current densities and slopes of each plot are denoted at the top left corner of the Figure It was evident from the figure that space charge limited current densities increased tremendously up to over 30A/cm2 together with an enhancement on slopes for those cathodes with matrices Sc3 and Sc4 in sub-micrometer grain size and with much uniform distributed nanometer- particles of Sc2O3 whereas for cathodes with matrices in micrometer grain size, the current densities kept nearly identical and were similar to common values of about 10-15 A/ cm2 for mixing matrix or thin film scandate cathode [16,20]. Figures 8 and 9 indicate that both an improvement in the uniformity of the distribution of the scandia and decreasing the grain size from micrometer to submicron are critical factors in attaining high performance in cathodes. Especially, the sub-micrometer microstructure of the matrices, with nanometer-size particles of scandia uniformly distributed throughout the interior of matrix is an ideal structure for obtaining good emission capability. The sub micron structure could provide more paths for the diffusion of active substance. Quasi-spherical tungsten grains have low surface energy, so the growth of tungsten grain could be retrained to a certain extent during activation and operation periods and the sub-micron microstructure could be kept.

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20μm 

20μm 

(a)

(b)

5μm 

5μm 

(d)

Figure 8. Microstructures of Sc2O3-W matrices by different procedures in adding Sc2O3 a. Mixed Sc2O3 with W (Sc1), b. Liquid-Solid doping (Sc2), c. Liquid-Liquid doping (Sc3 ) and d. Liquid-Solid doping with Re added(Sc4)

  Type    Slope     J.(A/cm2)  Sc1      1.35        16.6 

J(A/cm2) 

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(c)

Sc2

1 36

16 8

T=850ºCb 

Voltage(V) 

Figure 9. pProperties of different cathodes with matrices symboled as in Figure 8. Scandium: Compounds, Productions and Applications : Compounds, Productions and Applications, Nova Science Publishers,

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3. CHARACTERIZATION OF SCANDIUM AND ITS ROLE IN THE EMISSION The chemical and physical behaviors at tungsten surface have been studied by A. Shih et al. [8,38,39]. They found that, compared with Ba in the widely used Ba-dispenser cathode, Sc-to-W bonds were much stronger than the Ba-to-W bonds. The desorption temperature of Sc from W surface was several hundred higher than that of Ba from W. Furthermore, oxygen enhanced Sc bonding to W tremendously. The weakest binding state of the three scandium oxide states of Sc2O3, Sc2O2 and ScO had a desorption maximum at the temperature of about 160K than Sc desorption from W. At the activation temperatures of scandate cathodes, the loss of Sc was substantial. The evaporation rate for Sc was much higher than the oxidized Sc. At the 1400 and 1600K, Sc2O3 interacted with W and reduced to Sc2O2 while W oxidized. Sc2O2 then broke into ScO, which desorbed above 1650K. The experimental results indicate that the loss of Sc from the cathode surface might be in the form of ScO or free Sc. Emission models for thermionic cathodes such as Ba-W and M type cathodes have (Ba-cathodes in short in this paper) been established. It is widely accepted that the Ba-O dipole monolayer was attributed its emission phenomenon. However, the scandate cathode working mechanism is not understood since the precise nature of the active surface layer-atomic composition, structure and chemical state is not known. At least three different models were proposed for scandate cathodes.

A. Ba-Sc-O Monolayer Model Similar to that of Ba-cathodes, this model was suggested by J. Hasker based upon the observation that substrate W signals were detected in Auger spectra from the surface of scandate cathodes [16] and was approved by many other researchers. Muller made investigations on the electronic structure and charge distribution at the surface of different types of model surfaces including (a) a monolayer of Ba-Sc-O on W(100), (b) Ba or BaO adsorbed on Sc2O3, and (c) BaO on Sc2O3+WO3[40]. Based on the calculation results using the fully relativistic scattered-wave cluster approach, he found that the monolayer of Ba-Sc-O on W(100) produced the lowest work function. The calculated values

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of 1.5-1.7eV were in excellent agreement with experimental results for Hasker’s scandate cathodes. Yamamoto also utilized concepts consistent with a Ba-Sc-O adsorbate monolayer to explain the mechanism responsible for the emission and proposed an important reaction to discuss the generation of scandium in a scandate cathode as shown in equation (1)[41,42]. The barium was envisioned to be supplied from the bulk cathode materials(commonly 411 impregnated porous tungsten) in a manner identical to that known in standard dispenser cathodes. He assumed that some of the barium reacted with scandium tungsten which was usually part of a thin layer on the surface of the cathode to produce barium mono-tungsten and free scandium. The availability of free scandium was vital to the formation of the Ba-Sc-O layer which was assumed to form on the tungsten surface, and to be responsible for the improved emission behavior. This mechanism was analogous to the monolayer Ba-O dipole layer configuration which had been hypothesized to form on tungsten and is widely accepted to account for the work function lowering in dispenser cathodes. Recently, Vlahos etalet al. used ab initio modeling to explore the surface structure and stability of BaxScyOz monolayer cathodes [43]. They made calculations on the modifications in work function Φ of the W(001) surface induced by Ba- and Sc-monomers as well as Ba–O and Sc–O dimers, as a function of surface coverage, and the results is shown in Figure 10. They found that Ba0.25Sc0.25O structure was the most stable structure and had lowest Φ , which was consistent with the experimental result.

B. Semiconductor Model Wright and Wood[44] presented a model applying semiconductor principle to describe the emission process from oxide cathodes. Raju and Maloney [45] modified Wright and Woods’ results, which were derived for a thick layer oxidizing coating, to obtain an expression for use on a scandate cathode configuration where they believed emission to occur from a thin semiconducting layer (0.4-0.5um). According to Wright’s model, the applied external field penetrated into surface of semiconductor owing to the low concentration of free electrons in its conduction band, forming a layer of space charge. The energy levels were then tilted in semiconductor layer, producing a reduction of δχ for work function φ, so that.

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Figure 10. Work function Φ(eV) vs surface coverage(ML) for Ba-, Sc-monomers, Ba– O, Sc–O vertical dimers and the Ba0.25O and Ba0.25Sc0.25O alloy structures on W(001). Also shown are top W(001) views of the 1×1, c(2×2), 2×2, 2c(2×2), 3×3, and 4X4 configurations with their respective hollow sites occupied[43]

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φ ' = φ − δχ

(3)

δχ is determined as

δχ = 2kT sinh −1

E 1

4(2 K π nkT ) 2

(4)

Where E is applied electric field, n0 is concentration of electrons in conduction band at T and K is related to dielectric constant of the semiconductor. Raju and Maloney modified the Wright’s model by taking finite thickness of scandate layer into account. The reduction of work function δχ’ was determined by

sinh

δχ '

2kT = tanh( d ) δχ LD sinh 2kT

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(5)

64

Jinshu Wang

in which d is the thickness of the semiconductor layer and LD is the Debye length of the semiconductor, given by

kT ε 0 ε r 12 LD = ( ) n0 e2

(6)

where ε0 and εr stand for vacuum and relative dielectric constant, respectively. We made calculation for the temperature limited (TL) and space charge limited (SL) current densities of the scandate cathodes based on the Schottky effect, Maloney’s semiconductor model and Wright’s model and combination of semiconductor models with Schottky effect, respectively [46]. zero-field emission −ϕ 2 kT

J 0 = A0T e

（7）

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taking Schottky effect into consideration

J TL = J 0 e

0.44 E T

（8）

considering Wright’ s model δχ

J TL = J 0e kT

(9)

considering Maloney’ s model δχ '

J TL = J 0e kT

(10)

Combining semiconductor models with Schottky effect δχ

J TL = J 0e kT

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e

4.4 E T

Application of Scandium Oxide in Dispenser Thermionic Cathodes δχ '

or J TL = J 0 e kT e

4.4 E T

65

(11)

The electric fields are calculated according to Child’s law with SL current densities from 12.5 A/cm2 to 100 A/cm2 and practical measurement parameters 3

J SL

V 2 = 2.33 ×10 A/cm2 2 d −6

(12)

Using the above formulae, we found that the theoretical results matched best with the experimental data when the Wright’s semiconductor model together with the Schottky effect.

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C. Thermo Field Emission Bekh etalet al. presented the thermo-field emission mechanism [47,48]. They studied the influence of rhenium and scandium oxide on the mechanism of emission from impregnated tungsten thermionic emitters. They compared the composition and emission property of two impregnated dispenser cathodes possessing pure tungsten matrix and rhenium-tungsten matrix with the scandate impregnated dispenser cathodes. They found that the emission performance of these cathodes was independent of the emitter matrix material, being related to the existence of scandium oxide in the emission-active substance. However, different matrices had different emitter surfaces. If a tungsten matrix was used, the influence of scandium oxide increased the number of crystallites of the active substance components which were responsible for a high emittance and a variation of their shape on the cathode emitting surface, which gave rise to the emergence of conditions favorable for thermo-field emission. If a rhenium-tungsten matrix is used, the presence of scandium oxide stimulated both the growth of the active substance crystallite dimensions and the composition change in the emitting layer, which was also accompanied by the same thermo-field emission effect.

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Figure 11. Emission mechanism classification[49]

Li etalet al. claimed that the scandate cathodes were between the types of “semiconductor” and “monolayer” – a kind of superposition of semiconductor and monolayer models [49]. They explained the emission of the scandate cathode by a way of between “field assistance thermionic emission” and “thermal assistance field emission”, and classified the emission mechanisms for different cathodes, as shown in Figure 11. They also claimed that the scandate cathode emission characteristics on high current density did not follow either the Richardson-Dushman equation or the existing FowlerNordheim equation, but the equation which can be applied for demonstrating the high emission characteristic is still not known.

4. SURFACE STRUCTURE OF SCANDATE CATHODES Although several emission models were used for explaining the high emission performance of scandate cathodes, it is widely accepted the composition and surface structure at the cathode play important roles in the emission. Yamamoto et al. investigated the surface by means of Auger electron spectroscopy [21,22]. They found that the surface of the scandate cathode was covered by atoms of Sc,O and Ba and the atom concentration of these elements changed with the heat treatment of the cathode. The atom concentration of Sc, O and Ba decreased at the initial stage of the heat treatment at 1150oC due to a desorption of the weakly bound component of the surface atoms. The desorption rate of Sc, O and Ba competed with the

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diffusion rate of atoms from inside, thus, a new steady state condition was established at the surface as the heat treatment proceeded. They also studied the ion bombardment on the atom concentrations at the cathode surface. The ion bombardment destroyed the active substance surface layer, resulting in lowering of the emission capability of the cathodes. After reheating the top layer scandate cathode for 5h, the emission property was completely recovered due to the establishment of a surface layer. However, the anti-ion bombardment experiments done by Hasker etalet al. gave a different result[16]. They found that the surface concentration of Sc, Ba and O decreased with the sputtering time, which was consistent with Yamamoto’s result. But after reheating of the cathode, it gave only partial recovery of the surface concentration, resulting in a decrease of the emission property. Hasker summarized the atom concentration ratio of different elements on the surface of top layer of W+5%Sc2O3 and compared their results with Yamamoto’ result, which is shown in Table 1[16]. They found that the Sc/W ratio of their cathode was about 5 times higher than mixed matrix cathode prepared by Yamamoto and scandium distribution after activation was much more homogenous by a comparison of scandium mapping results for these two kinds of cathode. Deckers and Maneschijn studied the mixed matrix type and scandium oxide modified barium-calcium aluminate impregnated cathodes using transmission electron microscope, energy dispersive X-ray analysis, and electron loss spectroscopy on the material in the pores and high resolution scanning Auger spectroscopy around the openings at the emitting surface [50]. Based on their results, they proposed a model for the structure of scandate cathode, suggesting that tungsten and scandium were strongly bounded, perhaps in the form of a surface alloy. Furthermore, barium and oxygen formed a dipole on these sites, where they were more strongly bound that at tungsten sites. Lensy and Forman made surface studies on scandate cathodes and correlate with emission with surface structure [51]. They duplicated the toplayer scandate by depositing multilayer scandium on a tungsten surface, oxidizing the scandium, and then depositing either Ba or BaO on the scandium oxide surface. By combining the emission property of different cathode configurations, evaporation and surface element distribution analysis results, they suggested that the highest emission came from areas in which barium and oxygen were on the top of Sc2O3. They demonstrated that barium on scandium oxide resulted in an unstable surface complex, whereas BaO on scandium oxide led to a stable configuration.

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Jinshu Wang Table 1[16]. Pph ratios for top layers of W+Sc2O3:W(179eV),Sc(333eV),O(510eV),Ba(584eV)

After activation After sputtering After reactivation Ref.[a] after activation

O/Ba 4.5 8.3 3.2

Sc/Ba 2.3 1.7 1.4

W/Ba 0.5 9.4 1.6

ScW 4.8 0.2 0.9

O/Sc 1.9 4.8 2.2

4.7 2.5[b]

2.6 1.5[b]

0.6 1.8 [b]

4.4 0.9[b]

1.8 1.7[b]

[a] J.Hasker ang H.J.H.Stoffelen, Appl.Surf.Sci.24(1985)330. [b]Mixed matrix cathode of ref.[3],after activation

Table 2[52]. Comparison of frequency factor D0,activation energy, and effective diffusion coefficient Deff at 1300k with results in the literature

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Diffusion species α-Sc,self-diffusion β-Sc,Self-diffusion W-Os W-Ir W-Sc2O3

D0 [cm2/S] 0.948 0.858 0.64 0.32 2.1×10-15

ε[eV]

Deff at 1300[cm2/S]a

3.10 2.62 5.58 5.24 0.85

9.41×10-13 1.54×10-22 1.52×10-21 1.06×10-18

a

Deff at 1300k forα-Sc, β-Sc, W-Os and W-Ir are calculated using D0 andε. β-Sc transforms under 1610 K into α -Sc. Thus, Deff at 1300 K of α -Sc is not presented.

Vaughn et al. also made studies on the correlation between surface configuration of Sc/Sc-oxide and Ba/Ba-oxide on W with electron emission by using photoelectron emission microscopy/thermionic emission microscopy [52]. They observed that the configuration of Ba/BaO on Sc2O3 provided the best thermionic emission. In addition, Ba/BaO could diffuse on the Sc2O3 surface due to their high mobility whereas Sc2O3 could not diffuse at the operating temperatures.

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2

APPH

(a) S B

Loca

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(b)

(c) Figure 12. Migration tendencies of Sc and Ba from scandate cathode underneath W film with 2µm in thickness to surface of W grain (a, b) and Auger spectrum taken from W film 2µm in thickness with Sc2O3-W matrix underneath(c). It is expressed by Sc/W, Ba/W Auger peak-peak height (APPH) ratios taken at points 1-6 of SEM image, traversing from the pore mouths across the surface of an analyzed grain

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We studied Sc- concerned diffusing behaviors of Sc2O3 doped W matrix impregnated (SDM-I) dispenser cathode. It was found that the concentration of Ba,Sc and O increased with the temperature and obvious surface diffusion of Sc with Ba and O was apparent at temperatures above 1000°Cb during heating [33]. Then we measured the element distribution and discussed on the diffusion behavior of the active substance at the surface coated with a W film 2µm in thickness on an impregnated SDM-I cathode after water cleaning and Sc2O3 doped W matrix without impregnation. It was found that after water cleaning, the concentration of Ba, Sc and O was almost negligible down to under the surface about several micrometers [32]. The migration aptitudes of Sc and Ba on W grains of the film were observed, after kept at 1150°Cb for 2 hours, by PHI 700 Auger spectrometer with spatial resolution of 7 nm and were displayed as APPH ratios of Sc/W and Ba/W. The SEM image and APPH ratios taken from points 1- 6 along the checked grain are shown in Figure 12a and b. It could be clearly seen that nearly identical migration aptitudes were for both Sc and Ba from gap to surface of the grain. Since the appeared Sc (and Ba) on surface of W film only came from the underneath cathode, it was no doubt a diffusion of Sc with Ba (and O) occurred at this duration. On the other hand, no scandium was found on the surface of tungsten film coated on the Sc2O3-W matrix without impregnation, as shown in Figure 12c. We also found that at the surface of Sc2O3 doped tungsten matrix, Sc2O3 kept stable during heating up to 1150oC or under ion bombardment [33]. We deduced that these phenomena were caused by the different diffusion behavior of Sc and Sc2O3. Uda etalet al. investigated the bulk diffusion of scandium oxide in tungsten [51]. They compared the diffusion coefficient of Sc2O3 with those of α-Sc, β-Sc, which were shown in Table2 [53]. It was found that Sc2O3 had a very small coefficient and high activation energy, leading to poor diffusion ability. They also demonstrated that Sc2O3 diffused to W substrate without chemical reaction and phase transformation. Therefore, we concluded that after proper activation, scandium diffused in the form of free or ionic Sc but not in the form of Sc2O3 together with Ba and O from the interior of cathode to its surface. This highly mobile, free or ionic Sc, might be liberated from constituents produced by reactions between matrix materials and impregnant. We also investigated the surface coverage of active substance on the SDM-I cathode by applying Auger electron microscopy. The Auger peak-to peak height ratio of Ba/W for the checked scandate cathode was almost four times higher than that of M type cathode and Sc/W was about two times of that monolayer of oxidized Sc, indicating a multilayer Ba, Sc and O not a

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monolayer formed on W matrix for SDM-I cathode[33]. Auger depth profile analysis was applied to study the thickness of surface layer. Figure 13 illustrates the depth profiles taken from the cathode (a) and M-type cathode (b) at one of the analyzed points for each cathode. The results brought a clear figure that the surface structure of the cathode was greatly different from that of M-type cathode. Ba, Sc, O covered the W substrate in thicknesses of about 100nm with nearly identical concentration ratios at each analyzed point on outmost surface. The covering depth of surface layer of the cathode is almost 10 times thicker than that of Ba, O on M-type cathode while the latter indeed corresponds to a monolayer. Therefore, a Ba-Sc-O multilayer, instead of a monolayer layer, with certain ratios was formed at the surface of the cathode after activation, and the Ba-Sc-O multilayer led to the high emission property of the cathode.

(a)

(b) Figure 13. Depth profiles of elements on surface of the cathode(a) and Os coated Mtype cathode (b) by PHI 700 Auger spectroscopy

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5. SUMMARY The development of Scandate cathodes was reviewed. Four types of Scandate cathode, namely pressed scandate cathode, traditional scandate impregnated cathode, mixed matrix scandate cathode and top-layer scandate cathode were developed in the past fifty years. Among all these kinds of cathode, Sc2O3 doped tungsten matrix cathode and top-layer cathode exhibited extremely high emission performance, where the first type of cathode, which has a simple manufacturing procedure, might be the main type in the future. Instead of an improvement in the uniformity of the surface distribution of the scandia being the critical factor in attaining high performance in cathodes, the key factor is the sub-micrometer microstructure of the matrices, with nanometer-size particles of scandia uniformly distributed throughout the interior of matrix. The microstructure plays a dominant role in the emission capability. Although at least three kinds of model, namely, Ba-Sc-O monolayer model, semiconductor model and thermo field emission model were used to explain the high emission phenomenon, it is widely accepted that a surface multilayer/monolayer of Ba-Sc-O formed on the cathode surface plays a crucial role. However, the constitution and the configuration of the elements in the surface Ba-Sc-O layer, and the reactions for producing the free or ionic Sc during impregnation or/and activation should be studied in detail in the future work.

ACKNOWLEDGMENT This work is sponsored by Beijing Natural Science Foundation 2102007 and Program for Excellent Talents in Beijing PHR201006101.

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Jinshu Wang Electron emission properties and surface atom behavior of an impregnated cathode coated with Tungsten Thin Film containing Sc2O3. Jpn. J. Appl. Phys., 1988, 28, 490-494. Yamamoto, S; Taguchi, S; Watanaba, I; Kawase, S. Electron emission properties and surface atom behavior of an impregnated cathode coated with tungsten thin film containing Sc2O3, Jpn. J. Appl. Phys., 1986, 25(7) , 971-975. Yamamoto, S; Watanabe, I; Taguchi, S; Sasaki, S. Formation mechanism of a monoatomic order surface layer on a Sc-type impregnated, Jpn. J. Appl. Phys., 1989, 28(3), 490-494. Sasaki, S; Yaguchi, T; Nonaka, Y; Taguchi, S; Shibata, M. Surfce coating influence on scandate cathode performance, Appl. Surf. Sci., 2002, 195, 214221. Wang, Y; Pan, T. Investigation of pulsed laser deposition Sc-coated cathode. applied surface science, 1999, 146, 62-68. Sasaki, S; Amano, I; Yaguchi, T; Matsuzaki, N; Yamada, E; Taguchi, S; Shibata, M. Scandate cathode coated with Mo and Sc films, Appl.Surf. Sci., 1997, 111, 18-23. Gäertner, G; Geittner, P; Lydtin, H; et al. Emission properties of toplayer scandate cathodes prepared by LAD, Appl.Surf. Sci., 1997, 111, 1117. Gärtner, G; Geittner, P; Raasch, D; Wiechert, DU. Supply and loss mechanisms of Ba sispenser cathodes, Appl.Surf. Sci., 1999, 146, 22-30. Gärtner, G; Geittner, P; Raasch, D. Low temperature and cold emission of scandate cathodes. Appl.Surf. Sci., 2002, 201, 61-68. Yamamoto, S; Taguchi, S; Aida, T. Study of metal film coating on Sc2O3 mixed matrix impregnated cathodes, Appl. Surf. Sci., 1984, 17 504-517. Yamamoto, S; Taguchi, S. Electron emission properties and surface atom behavior of impregnated cathodes with rare earth oxide mixed matrix base metals. Appl. Surf. Sci., 1984, 20, 69-83. Yuan, H; Gu, X; Pan, K; Wang, Y; Liu, W; Zhang, K; Wang, J; Zhou, M; Li, J. Characteristics of scandate impregnated cathodes with submicron scandia doped matrices. Appl. Surf. Sci., 2005, 251, 106-113. Liu, W; Zhang, K; Wang, Y; Pen, K; Gu, X; Wang, J; Li, J; Zhou, M. Operating model for scandia doped matrix scandate cathodes. Appl. Surf. Sci., 2005, 251, 80-88. Wang, J; Liu, W; Wang, Y; Li, L; Wang, Y; Zhou, M. Sc2O3–W matrix impregnated cathode with spherical grains. J. Phys. & Chem. Solid,

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2008, 69(8), 2103-2108. [34] Wang, J; Wang, Y; Liu, W; Li, L; Wang, Y; Zhou, M. Emission property of scandia and re doped tungsten matrix dispenser cathode. J. Alloy. & Comp., 2008, 459 (1-2), 302-306. [35] Wang, J; Liu, W; Wang, Y; Li, L; Zhou, M. Scandia doped tungsten matrix for impregnated cathode. Rare Metals, 2008, 27(1), 9-12. [36] Wang, Y; Wang, J; Liu, W; Zhang, K; Li, J. Development of high current-density cathodes with scandia-doped tungsten powders. IEEE Trans. Electron Dev., 2007, 54(5), 1061-1070. [37] Wang, J; Lu, H; Liu, W; Wang, Y; Li, L. Zhou Meiling. A study of scandia doped tungsten nano-powders. J. Rare Earths, 2007, 25, 194198. [38] Shi, A; Yater, JE; Hor, C; Abrams, R. Oxidation of thin scandium films. Appl. Surf. Sci., 2003, 211, 136-145. [39] Shi, A; Yater, JE; Hor, C; Abrams, R. Interaction of Sc and O on W. Appl. Surf. Sci., 2002, 191, 44-51. [40] Müler, W. Work functions for models of scandate surface. Appl. Surf. Sci., 1997, 111, 30-34. [41] Yamamoto, S. Electron emission properties and emission mechanism of a Sc-type impregnated cathode. Shiku, 1988, 31(10), 1-8. [42] Yamamoto, S; Taguchi, S; Watanaba, I; Sasaki, S. Electron emission enhancement of a (W-Sc2O3)-coated impregnated cathode by oxidation of the coated film. Jpn. J. Appl. Phys., 1988, 27(8), 1411-1414. [43] Vlahos, V; Lee, YL; Booske, JH; Morgan, D; Turek, L; Kirshner, M; Kowalczyk, R; Wilsen, C. Ab initio investigation of the surface properties of dispenser B-type and scandate thermionic emission cathodes. Appl. Phys. Lett, 2009, 94, 102-184. [44] Wright, DA; Woods, J. The emission from oxide-coated cathodes in an accelerating Field. J, Proc. Phys. Soc., 1952, 65, 65-134. [45] Raju, RS; Maloney, CE. Characterization of an impregnated scandate cathode using a semiconductor model. IEEE Trans. Electron Dev., 1994, 41, 2460-2467. [46] Wang, Y; Wang, J; Liu, W; Li, L; Wang, Y; Zhang, X. Correlation between emission behavior and surface features of scandate cathodes. IEEE Trans. Electron Dev., 2009, 56(5), 776-785. [47] Bekh, II; Il'chenko, VV; Lushkin, AE. The influencing of the surface structure on the emission properties of the Sc-Ba dispensed cathodes. In Pro.IVEC,Kitakyushu, Japan, May 15-17, 2007, 197-198. [48] Bekh, II; Getman1, OI; Il’Chenko, VV; Lushkin, AE; Panichkina1, VV;

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rakitin, SP. influence of matrix material on the mechanism of emission from Sc–Ba impregnated thermionic emitters. Ukr. J. Phys., 2009, 54(3), 297-302. Li, J; Wang, H; Yu, Z; Gao, Y; Chen, Q; Zhang, K. Emission Mechanism of high current density thermionic cathodes. In Pro. IVEC, Kitakyushu, Japan, May, 2007, 15-17, 143-144. Deckers, S; Manenschijn, A. Materials research for scandate cathodes. presented at 1992 Tri-service/NASA cathode workshop, March 17-19, Greenbelt, Md. USA. Lesny, G; Forman, R. Surface studies on scandate cathodes and synthesized scandates. IEEE. Trans. Electron Dev., 1990, 37, 25952603. Vaughn, JM; Jamison, KD; Kordesch, ME. In situ Emission mocroscopy of scandium/scandium-oxide and barium/barium-oxide thin films on tungsten, IEEE trans. Electron Dev., 2009, 56(6), 794-798. Uda, E; Nakamura, O; Matsumoto, S; Higuchi, T. Emission and life characteristics of thin film top-layer scandate cathode and diffusion Sc2O3 and W. Appl. Surf. Sci., 1999, 149, 31-38.

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[53]

Jinshu Wang

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In: Scandium: Compounds, Productions… ISBN: 978-1-61761-465-1 Editors: Viktor A. Greene © 2011 Nova Science Publishers, Inc.

Chapter 3

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COMPARISON OF SCANDIUM RECOVERY MECHANISMS BY PHOSHPORUSCONTAINING SORBENTS, SOLVENT EXTRACTANTS AND EXTRACTANTS SUPPORTED ON POROUS CARRIER V. Korovin1, Yu. Shestak1 and Yu. Pogorelov2 1

Institute for Geotechical Mechanics, Ukrainian National Academy of Sciences, Dnepropetrovsk, Ukraine; 2 Dneprodzerzhinsk State Technical University, Dneprodzerzhinsl, Ukraine.

ABSTRACT The present paper deals with comparison of mechanisms of scandium recovery by phosphorus-containing inorganic and organic adsorbents as well as by organophosphorus solvent extractants and ones supported on TVEX porous carrier. Using 31Р NMR method (magic angle rotation), mechanism of Sc recovery by titanium and zirconium phosphates is determined for diluted hydrochloric, sulfuric and nitric solutions. It is revealed that the presence of three types of functional groups results in scandium sorption at the expense of cation exchange, formation of coordinate bonds with P=O groups as well as by both mechanisms with chelate formation. Prevailing of particular recovery mechanism depends upon acid and pH value.

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V. Korovin, Yu. Shestak and Yu. Pogorelov

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It is found that the mechanism of scandium recovery by organic ionexchange resins defines in many ways by the type of functional groups and their spatial distribution in polymer matrix of the resin. Thus, three mechanisms are defined for Sc sorption from hydrochloric solutions by different ion-exchange resins using 31P NMR method (magic angle rotation). According to the first mechanism, scandium is sorbed with formation of coordinated bond with ionite functional group and remaining its hydration shell as [Sc(H2O)6]3+ ion. By the second adsorption mechanism scandium ions are sorbed with formation of one ionic bond while formation of two ionic bonds with two groups is typical for the third mechanism. For the second and third mechanisms the number of water molecules decreases in Sc first coordination sphere. Using 31Р, 34Sс NMR method, comparison is made for mechanisms of scandium extraction by liquid organophosphorus extractants and ones incorporated in TVEX polymer carrier. The difference in scandium extraction by liquid DEHPA (di-2-ethyl hexyl phosphoric acid) and TVEX-DEHPA is established. It is shown that during Sc recovery by TVEX-DEHPA the complexes are formed typical both for solvent extraction and sorption by phosphorus-containing ion-exchange resins.

INTRODUCTION The main problem at scandium manufacturing is its selective recovery from solutions during processing of polymetallic raw using sorption and extraction [1]. In this connection knowledge of the mechanisms of scandium sorption and extraction is of both theoretical and practical importance. The present paper makes a comparison of scandium recovery mechanisms by inorganic sorbents based on titanium and zirconium phosphate, phosphorus-containing organic ion-exchange resins as well as by liquid extractants and ones supported on solid extractant (TVEX) matrix.

SCANDIUM RECOVERY BY INORGANIC SORBENTS BASED ON TITANIUM AND ZIRCONIUM PHOSPHATE Inorganic sorbents based on titanium and zirconium phosphates have significant chemical, thermal and radiation resistance as well as high selectivity.

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We have studied scandium sorption by titanium phosphate (1), zirconium phosphate (2) and titanium phosphate in Н+ (4) and Na+ (5) form from hydrochloric, nitric and sulfuric solutions within pH 1.5 – 4. The above samples were preliminary converted into H+ form using 1 mol/L nitric acid. Scandium sorption was studied by static method at phase ratio solid phase / liquid phase = 1/20 (0.5 g of sorbent) from the solutions containing 1 g/L Sc (contact time is 24 h at room temperature). Sorption capacity was defined by decrease of scandium concentration in aqueous phase. Figures 1-3 illustrate dependence of sorption capacity (mg Sc per 1 g of sorbent) of the studied materials upon pH of aqueous phase. It is obvious that sorption capacity of inorganic sorbents depends both on acid type and its concentration in aqueous phase. Thus, samples of modified titanium phosphate (4, 5) have the highest capacity, sorption properties of titanium phosphate (1) are considerably lower while zirconium phosphate (2) virtually does not recover scandium. During scandium sorption from sulfuric solutions sorption capacity negligibly depends upon acid concentration in aqueous phase (Figure 1); it notably decreases for hydrochloric media with pH increase (Figure 2). One may observe maximum of sorption capacity within pH 2.5-3.0 for nitric solutions (Figure 3). The observed dependence of sorption capacity upon pH of aqueous phase for different acids may be explained from scandium state in aqueous solutions and the structure of sorbent function groups.

Figure 1. Dependence of sorption capacity of inorganic sorbents upon initial pH for sulphuric solutions: 1 – titanium phosphate, 2- zirconium phosphate, 3 – titanium phosphate in H+-form; 4 – titanium phosphate in Na+-form Scandium: Compounds, Productions and Applications : Compounds, Productions and Applications, Nova Science Publishers,

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Figure 2. Dependence of sorption capacity of inorganic sorbents upon initial pH for hydrochloric solutions: 1 – titanium phosphate, 2- zirconium phosphate, 3 – titanium phosphate in H+-form; 4 – titanium phosphate in Na+-form

Figure 3. Dependence of sorption capacity of inorganic sorbents upon initial pH for nitric solutions: 1 – titanium phosphate, 2- zirconium phosphate, 3 – titanium phosphate in H+-form; 4 – titanium phosphate in Na+-form

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According to the data we had received using 31P NMR method [2], the following basic groups are present in the sorbents based on titanium and zirconium phosphate:

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Scandium may be recovered from aqueous phase both at the expense of cation exchange with protons of phosphate groups (1 and 2) and via formation of coordination bond with phosphoryl group Р=О (3) as well as by simultaneous mechanisms forming the following chelates:

Scandium exists mainly as [Sc(H2O)6]+3 cation complex in nitric and hydrochloric solutions at pH 1-3 [2]. Most probably, it is recovered from these media by cation-exchange mechanism. In this case reaction equilibrium shifts to Sc sorption with decrease of Н+ ions concentration, and sorbents capacity increases. From the other side, pH increase results in formation of [Sc(H2O)5OH]+2 and [Sc(H2O)4(OH)2]+ hydroxocomplexes, capable for polymerization, that complicates scandium sorption. Scandium hydoxocomplexes form in nitric solutions at рН 2-3; that’s why one may observe increase of sorption capacity up to the indicated pH values with further notable decrease (Figure 3). Hydrolytic processes in hydrochloric solutions start at lower pH values as compared with nitric ones (рН 1-1.5), that’s why sorption capacity in these media monotonously decreases with acidity increase (Figure 2). For the considered pH range of sulfuric solutions Sc forms hydrated anion complexes [Sc(SO4)2]-. These complexes are adsorbed by titanium phosphate, probably, substituting SO42--group by phosphoric group, which has higher affinity to Sc3+-ion. Since scandium state in sulfuric solutions is the same at different pH, sorption capacity virtually does not depend on aqueous phase acidity.

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SCANDIUM SORPTION BY POLYMER PHOSPHORUSCONTAINING ION-EXCHANGE RESINS Polymer ionites are widely used for scandium recovery and purification [1]. Phosphorus-containing ion-exchange resins are the most promising materials for its recovery from industrial solutions due to high affinity of their function groups to Sc. We have studied scandium sorption by phosphorus-containing ionexchange resins (Table 1) from hydrochloric media with Sc content 1 g/L and HCl concentration 0.8-1.0 mole/L. Scandium sorption was carried out by static method at phase ratio solid phase : liquid phase = 1:50 (0.5 g or resin contacted with Sc-containing solution during 24 hours at room temperature). Sorption capacity was determined by decrease of scandium concentration in aqueous phase. Table 1. Function groups structure for the studied ion-exchange resins

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Ion-exchange resin

Function group

Ion-exchange resin

SF-5

AFI-24

KMDF-1

AFI-22

KMDF-3

NFOS

Function group OH CH2 CH2 O P O OH N OH CH2 CH2 O P O OH OH O P N PO(OH)2 N P(OH)3

AFI-5

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Experimental data are given in Figure 4. One may see that the studied resins may be divided into two groups according to the change of equilibrium capacity depending on aqueous phase acidity. Soprtion capacity of NFOS and AFI-22 resins is considerably lower as compared with other ones, it monotonously increases with aqueous phase acidity increase and achieves 17 mg/g at 8 mole/L HCl. The same change of equilibrium capacity is observed for AFI-24 resin, at the same time this value is almost 2.5 times higher. Dependence of equilibrium capacity upon aqueous phase acidity has extreme point for KMDF-1, KMDF-3 and SF-5 resins with maximum sorption capacity 40-45 mg/g. Scandium sorption was studied by KMDF-1 cationite from acidic solutions with low acid (pH 1-4.5) and Sc (0.1 g/L) concentration. 0.1 g of the resin contacted with 50 ml of Sc-containing solution during 24 hours at room temperature. The data obtained are shown in Figure 5. It is evident that distribution coefficient (DSc) within the studied range has an extremum (hydrochloric solution – minimum near рН = 2.7-3.3, sulfuric solutions – maximum near рН = 2.0-2.3, and nitric solutions- maximum at approx. 2.0); however, in all cases DSc is relatively high and changes insignificantly, as for solutions with high acidity (Figure 4).

Figure 4. Sorption capacity of ion-exchange resins vs initial acidity of hydrochloric solutions: 1- KMDF-3; 2 – KMDF-1; 3 – AFI-24; 4 – SF-5; 5 – NFOS; 6 – AFI-22

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Figure 5. Dependence of scandium distribution coefficient on pH of aqueous phase for KMDF ion-exchange resin: 1 – hydrochloric solution, 2- sulfuric solution, 3- nitric solution

Mechanism of Scandium sorption by Phosphorus-containing IonExchange Resins According to 31P NMR Data. Using 31P NMR method with magic angle rotation, scandium complex formation with phosphoruscontaining groups was studied for ion-exchange resins after Sc sorption from hydrochloric solution [3]. Scandium sorption was carried out by the above resins from 0.02-0.03 mol/L Sc solutions in 0.1 mol/L HCl during 24 h at room temperature and phase ration solid phase : liquid phase = 1:40. 31 Р NMR spectra (81.04 MHz) were recorded by Bruker CXP-200 impulse spectrometer using magic angle rotation technique. Chemical shifts were measured relatively 85 % phosphoric acid as an external standard. Chemical shift decrease corresponds to increase of magnetic field strength. Spectra with poor resolution were resolved into components using especially designed software. Figure 6 shows 31P NMR (magic angle rotation) spectra of ion-exchange resins before and after sorption, which contain signals of basin function groups (Table 1). One should note that spectra of KMDF-1 and KMDF-3 ionites contain resonance lines with low intensity aside from signals of function groups. In our opinion, these lines may be attributed to phosphoric acid, which

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is captured by resin matrix, and partially hydrolyzed compound C(Cl)(PO(OH)2)2, which were also formed during the resin synthesis. After Sc sorption additional signals appear in 31P NMR (magic angle rotation) spectra for the studied ionites except NFOS. They are caused by phosphorus-containing groups bonded with Sc ions. Average solvate number (ASN), which corresponds to the number of phosphorus atoms bonded with one Sc atom, was calculated using the ratio between signals of phosphorus free and bonded with scandium. ASN value, equaled to 0.63 and 0.43 for SF-5 and KMDF-1 resins correspondingly, means that either one phosphorus-containing group is bonded simultaneously with two scandium atoms or there is a mechanism of Sc sorption where these groups do not enter scandium first coordination sphere. The first assumption, to our mind, is unlikely due to steric reasons. Most possible, scandium is adsorbed by two following mechanisms. Formation of direct bond between function group and scandium ion is typical for the first mechanism; at the same time formation of additional coordination bonds is possible:

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(1)

During realization of the second mechanism scandium is adsorbed remaining its hydrate cover as hexaaquacations that mainly presents in aqueous solution [4]: (2)

Separate signals from scandium, bonded and un-bonded with function groups, are resent in 31P NMR spectra for the first mechanism of scandium sorption. This signal separation does not observe for the second mechanism resulting in low ASN value. ASN value is about 1.5 for KMDF-3 cationite. It may be explained, probably, by the structure of this resin that contains oxyethylenediphosphonic groups capable to form chelate complexes with metals [5]. Most probably, scandium is adsorbed in this case according to the mechanism:

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Sorption capacity of NFOS resin is considerably lower as compared with other ionites; and there are no signals in spectra after scandium sorption. These data means that phosphine oxide groups contained in this resin are not capable to recover scandium from weak acid media, similar to solvent extraction.

Figure 6. 31P NMR spectra (magic angle rotation) of ion-exchange resins: (a) KMDF-3, (b) KMDF-1, (c) AFI-5 and (d) SF-5 before (1) and after scandium adsorption

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Despite NFOS, AFI-5 ampholite contains acidic function groups besides neutral ones. Signals from both types of groups are observed in the initial spectrum. After resin saturation by scandium the third signal, attributed to phosphorus coordinated with Sc, appears. At the same time relative intensity of acidic group decreases while the signal attributed to neutral group does not change. (Figure 6, d). Theses data indicate that scandium is adsorbed only by acidic groups. ASN value 0.53 means that the mechanism of Sc recovery by acidic groups of AFI-5 resin, most probably, is identical to the mechanism of scandium sorption by SF-5 and KMDF-1 ionites. The data obtained show that mechanism of scandium sorption depends in many instances on ionite structure. For KMDF-1 and SF-5 ionites scandium ion is bonded with 1 phosphorus atom; besides, scandium can remain its hydrate cover. This fact may be explained by spatial arrangement of active groups in ionite. In our opinion, phosphorus- containing groups in these resins are fixed in matrix and located from each other by the distance that allows only one group to approach Sc ion and to form its coordination sphere. Besides, approach of hydrated scandium ion is impeded not allowing these groups to enter Sc first coordination sphere atom and resulting in second sorption mechanism. Structure of KMDF-3 resin allows two phosphoric groups to enter scandium first coordination sphere since the resin contains oxyethylenediphosphonic groups. However, as for SF-5 and KMDF-1 ionites, approach of two function groups to one scandium ion is impossible. Besides, approach of hydrated scandium ion to certain oxyethylenediphosphonic groups is impeded resulting in ASN value less than 2.

SCANDIUM EXTRACTION BY TVEX Scandium Extraction by TVEX Containing Neutral Organophosphorus Extractants Solvent-impregnated resins are widely used in hydrometallurgical processing. They are produced either by impregnation of selective extractants onto polymer beads [6, 7] or by polymerization of styrene and divinylbenzene in the presence of extractant. Materials manufactured by the second method are known as Levextrel-type resins [8, 9] and as “solid extractants, TVEX”

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(TVEX, the first letters from Russian words that mean "solid extractants”) [10] and combine advantages of solid extraction and ion exchange. Scandium extraction by neutral organophosphorus extractants supported on TVEX porous matrix was studied by us in details [10-14]. It was found that TVEX porous matrix influences both extractant capacity and Sc complex formation in organic phase depending on extraction mechanism. Thus, scandium is extracted [10] by tri-butyphosphate (TBP) and TVEX containing 50 % TBP from sulfuric solutions by hydrate-solvate mechanism (without extractant enter to the first coordination sphere of scandium ion), and extraction mechanism may be presented by the equations: [ScSO4(H2O)4]+aq + 3H+aq + 2SO42-aq + (3m-4)H2Oaq + TBPo→[TBP][H5O2(H2O)m-2]3+[Sc(SO4)3]3-o

(4)

[Sc(SO4)2(H2О)2]-aq + 3H+aq + SO42-aq + (3m-2)H2Oaq + TBPo → [TBP][H5O2(H2O)m-2]3+[Sc(SO4)3]3-o

(5)

One should note that extraction capacity of TBP supported on porous matrix is approx. 2 times higher as compared with solvent extraction. Using 45Sc, 31P NMR method, it was revealed [10, 11, 12] that during Sc extraction by TBP, 50 % TBP solution in CCl4 and TVEX-50%TBP (Figure 19) a set of complexes with different ligands is formed: Sc(TBP)3Cl3, [Sc(H2O)(TBP)3C12]+, [Sc(TBP)4Cl2]+, [Sc(H2O)2(TBP)2Cl2]+, 2+ 2+ [ScCl(H2O)(TBP)4] cis, [ScCl(H2O)(TBP)4] trans, [Sc(H2O)3(TBP)3]3+; and Sc extraction takes place by solvate mechanism: [Sc(H2O)3Cl3]aq + 3TBPo = [Sc(TBP)3Cl3]o + 3H2Oaq

(6)

[Sc(H2O)4Cl2]+aq + 2TBPo = [Sc(H2O)2(TBP)2C12]+o + 2H2Oaq

(7)

[Sc(H2O)4Cl2]+aq + 3TBPo = [Sc(H2O)(TBP)3Cl2]+o + 3H2Oaq

(8)

[Sc(H2O)4Cl2]+aq + 4TBPo = [Sc(TBP)4Cl2]+o + 4H2Oaq

(9)

[Sc(H2O)4Cl2]+aq + 4TBPo = [ScCl(H2O)(TBP)4]2+o + 3H2Oaq + Cl-aq (10) [Sc(H2O)4Cl2]+aq + 3TBPo = [Sc(H2O)3(TBP)3]3+o + H2Oaq + 2Cl-aq

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(11)

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The presence of solvent and TVEX polymer matrix influences the ratio between these complexes in organic phase. During Sc extraction by TBP solution in CCl4, neutral complex [ScCl3(TBP)3] are formed as well as other complexes with low charge and minimal amount of water molecules in Sc coordination sphere. While extracting by undiluted TBP, the number of extracted scandium complexes is considerably bigger. The complexes with charge from +1 to +3 and presence of H2O molecules in Sc coordination sphere are formed. To our mind, there are the following reasons for the observed differences. Unpolar hydrophobic solvent (ССl4) prevents transfer of water molecules to organic phase together with Sc+3 ions, and low dielectric permeability of TBP solution in ССl4 shifts equilibrium to the formation of neutral scandium complexes or ones with low charge. Higher dielectric permeability of undiluted TBP increases the ratio of scandium complexes bearing the charge more than +1. Transfer of water molecules to organic phase also increases due to their solvatation by tri-butylphosphate molecules. Considerable number of scandium complexes may be explained by high ligand lability in Sc coordination sphere. In case of scandium extraction by TVEX-TBP distribution of scandium complexes in organic phase is intermediate between extraction by TBP and TBP solution in CCl4. Most likely, TVEX matrix influences Sc extraction by tri-butylphosphate like unpolar carbon tetrachloride shifting the equilibrium to the compounds with the smaller ionic charge and lower number of H2O molecules in Sc coordination sphere. Scandium extraction capacity by TBP supported on TVEX porous matrix is higher as compared with corresponding solvent extractant capacity: isotherms of Sc extraction from hydrochloric solutions by TBP, TBP 50% solution in CCl4 and TVEX-TBP almost coincide at low equilibrium Sc concentration; however, TBP in TVEX has higher capacity as compared with 100% TBP at high Sc equilibrium content. Based on 31P, 45Sc NMR data it was defined [13, 14] that scandium is extracted by TVEX containing 50 % of diisooctyl methyl phosphonate (DIOMP) as [ScCl(DIOMP)2(H2O)3]2− complex from 4 mol/L HCl; and as [ScCl(DIOMP)3(H2O)2]2− and [ScCl2(DIOMP)3(H2O)]− complexes from 6 M and 8 M HCl. Therefore scandium extraction from 4 mol/L HCl may be represented by the equation [ScCl(H2O)5]2+aq + 2DIOMPo = [ScCl(DIOMP)2(H2O)3]2+o + 2H2Oaq (12) and for scandium extraction from 6 M and 8 M HCl.

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V. Korovin, Yu. Shestak and Yu. Pogorelov [ScCl(H2O)5]2+aq + 3DIOMPo = [ScCl(DIOMP)3(H2O)2]2+o + 3H2Oaq (13) [ScCl2(H2O)4]+aq + 3DIOMPo = [ScCl2(DIOMP)3(H2O)]+o + 3H2Oaq (14)

Increase of hydrochloric acid concentration in aqueous phase promotes substitution of water molecules by Cl− ions in [Sc(H2O)6]3+ complex forming more extractable chloro-aqua complexes. Extraction capacity of DIOMP in TVEX matrix is approx. 1.5 times higher as compared with solvent extraction.

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Mechanism of Scandium Extraction by Di-2-Ethyl Hexyl Phosphoric Acid and TVEX on Its Basis from Hydrochloric Solutions According to the Data of 31P and 45Sc NMR Spectroscopy Di-2-ethyl hexyl phosphoric acid (DEHPA, HA) is widely used as an extractant for the recovery and separation of rare, rare-earth and radioactive elements, including scandium [1]. The present section presents the comparative study of scandium extraction from hydrochloric solutions by DEHPA solution in CCl4 and TVEX-DEHPA using 31Р and 45Sc NMR method. Scandium extraction was carried out from the solutions containing 0.05 mol/L scandium in 0.2-7.9 mol/L HCl by 0.3 mol/L HA solutions in ССl4 at phase ration organic/aqueous = 1/1 during 30 minutes (Table 2, experiments 16) and by TVEX-HA at phase ratio solid/liquid=2.7:10 during 40 hours (Table 2, experiments 12-17). Phase volume was chosen in such a way that the ratio of aqueous solution per HA mass unit will be the same for liquid extractant and TVEX. HA concentration in TVEX was defined equaled 36.5 %. To obtain extracts with higher Sc content extraction was carried out by HA solution from the solution containing 0.221 mol/L Sc in 1.0 and 7.4 mol/L HCl at phase ratio organic/aqueous = 1/5 (Table 2, experiments 7 and 8). Extracts (Table 2, experiments 2, 4, 6) were washed by water during 5 minutes at intensive agitation and phase ratio organic/aqueous = 1/5. 31 Р (81.04 MHz) and 45Sс (48.62 MHz) hi-res NMR spectra were recorded using Bruker CХР-200 impulse spectrometer. Chemical shifts were measured relatively external references - 85% orthophosphoric acid and 0.1 % acidified Sc(ClO4)3 solution correspondingly.

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Figure 7. Dependence of Sc extraction degree by upon aqueous phase acidity: (1) – TVEX-DEHPA, (2) - 0.3 mol/L DEHPA solution in CCl4

Scandium recovery degree at solvent extraction by DEHPA solution has a minimum within 2-6 mole/L НСl while TVEX-DEHPA almost completely extracts scandium within the whole studied acidity range (Figure 7). 45 Sс NMR spectra both of liquid and solid Sc-containing extracts are broad singlet signals. Spectra of liquid signals have permanent chemical shift 9.6 ppm and half-width W1/2= 535-560 Hz. Chemical shifts of 45Sс NMR signals for TVEX-DEHPA have the following values for different aqueous phase acidity: -7.5 ppm and -7.09 ppm for 0.17 mole/L (Table 2, experiment 12) and 1.06 mole/L HCl (Table 2, experiment 13) correspondingly and -10.4 ppm for all other samples (W1/2= 2420-2480 Hz). Chemical shift values allow to conclude that Cl--ions are not present in scandium first coordination sphere both for liquid and solid extracts. Actually, it had been founded earlier [15] that chemical shift changes from 0 ppm for [Sс(Н2О)6]3+ to -26.5 ppm for [Sс(TBP)6]3+ awhile inclusion of every chlorine-ion into scandium coordination sphere results in chemical shift approx. 45 ppm toward weak field up to +249 ppm for [SсCl6]3-. 31 Р NMR spectra of both liquid extracts and TVEX-DEHPA contain two or three signals (Figure 9). Signal А (+1.15 … +1.7 ppm), which was observed in the spectra of initial DEHPA solutions in CCl4 and TVEX-DEHPA, was attributed to non-coordinated DEHPA molecules. Signal В (-4.5 … -4.7 ppm), which was observed in spectra of all Sc-containing samples, corresponds to

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extractant molecules bonded with scandium Besides, spectra of certain liquid extracts contain third signal С (Figure 9, spectra 2, 4, 5). Its chemical shift exceeds about two times the chemical shift of signal B. Signal B was attributed by us to HA molecules bonded with on scandium ion, and signal C corresponds to extractant molecules bonded with two ions (i.e. to “bridge” molecules). One should note that signal width in TVEX-HA spectra is several times higher as compared with liquid extracts; probably, such difference is caused by decrease of mobility of DEHPA molecules in porous carrier.

Figure 8. 31P NMR spectra of initial (1) and Sc-containing HA samples: 2 – exp. 2 (1.08 mol/L HCl); 3- exp. 4 (3.97 mol/L HCL); 4- exp. 6 (7.94 mol/L HCl); 5 – exp. 7 (1.04 mol/L HCl) Scandium: Compounds, Productions and Applications : Compounds, Productions and Applications, Nova Science Publishers,

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Figure 9. 31P NMR spectra of Sc-containing TVEX-HA samples: 1 – exp. 12 (0.17 mol/L HCl); 2- exp. 13 (1.06 mol/L HCL); 3- exp. 14 (2.34 mol/L HCl); 4 – exp. 15 (4.15 mol/L HCl); 5 – exp. 17 (7.60 mol/L HCl); 6 – exp. 17 heated up to 80 0C

Table 2 presents average solvate number (ASN) values of scandium upon HA calculated from the known mole ratio DEHPA:Sc in organic phase and signal intensity for the extractant free and coordinated with scandium. ASN values near 6 for DEHPA solutions in ССl4 (except experiment No 4) indicate that Sс(А2Н)3 is the main extracted form. The fact that ASN