New Catalytic Polymeric Membranes Incorporating Ti(IV

Chapter 19

Downloaded by UNIV OF TENNESSEE KNOXVILLE on May 26, 2016 | http://pubs.acs.org Publication Date: August 9, 2005 | doi: 10.1021/bk-2005-0914.ch019

New Catalytic Polymeric Membranes Incorporating Ti(IV) Trialkanolamines Complexes: Synthesis, Characterization, and Application in Catalysis 1,

1,2

3,

3,4

M. G. Buonomenna *, E. Drioli , G. Licini *, and P. Scrimin 1

Department of Chemical Engineering and Materials, University of Calabria, I-87030 Arcavacata di Rende (CS), Italy Research Institute on Membrane Technology ITM-CNR c/o University of Calabria, I-87030 Arcavacata di Rende (CS), Italy Department of Chemical Science, University of Padua, I-35131, Padua, Italy Research Institute of Membrane Technology, ITM-CNR Padua Section, Padua, Italy 2

3

4

The incorporation of chiral homogenous catalyst, Ti(IV)/(R,R,R)-tris-(2-phenylethanol)amine, in polymeric membranes provided the first asymmetric catalytic Ti(IV)based membranes, stable and efficient as heterogeneous catalysts for stereoselective sulfoxidations and chemoselective oxidations of secondary amines to nitrones by alkyl hydroperoxides. Polyvinylidene fluoride (PVDF) -based catalytic membranes gave the best results affording products in short reaction times, high yields and high selectivity using as little as 1% of catalyst, comparable with the performances of the corresponding homogeneous system. The P V D F - T i stability is reasonably good and catalyst activity increases with no loss of selectivity in the subsequent uses of the same membrane.

© 2005 American Chemical Society

Wang and Holladay; Microreactor Technology and Process Intensification ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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310 Chemical processes catalysed by homogeneous transition metal complexes are of critical importance for fine and basic chemical industry. The majority of such metal catalysts are very often rather expensive, they are scarcely environmental compatible or even toxic. The current trend towards a sustainable chemistry has given life to a continuously increasing number of new strategies for catalyst immobilization, enabling an easy recovery, re-use and disposal at low costs of the catalyst , even i f this is usually associated with decreased selectivity and lower chemical yields. A possible solution to the problem could be the development of catalytic polymeric membranes, in which a Process Intensification Strategy can be applied: no catalyst recovery is required, catalysts have longer life and the intact structure of the catalyst should be preserved inside the polymeric membranes, providing no loss of activity and selectivity. A s a proof of principle, Ru-BINAP, Rh-DUPHOS and Jacobsen catalyst for epoxidation of simple olefins were entrapped in polydimethylsiloxane matrices affording performances comparable to the analogous homogeneous systems. A s far as oxygen transfer catalysis is concerned, the Sharpless-Katsuki epoxidation system launched the rich field of titanium catalyzed asymmetric oxidations. Since that breakthrough, chiral Ti(IV) alkoxides have been used to catalyze a variety of oxidative transformations affording highly stereoselective processes in the allylic alcohol epoxidation, β-hydroxyamine N-oxidation sulfoxidation, as well as in the Baeyer-Villiger oxidation of cyclobutanones. 1


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We have recently reported that tetradentate alkoxide ligands, namely C3-symmetric Malkanolmines 1, provide very stable titanium(IV) complexes (Scheme l ) . 1 1

OH

OR

ΡΛΗ (R/?,/?M

p h

R* 1

^c? (R,R,R)-2

Scheme L Synthesis of titanatrane complex (R,R,R)-2 and the corresponding peroxocomplex (R,R,R)-3 by reaction of (R,R,R)-tris-(2-phenylethanol)amine 1 with Ti(iPrO) . 4

In the presence of alkyl hydroperoxides, such species are able to catalyze the asymmetric sulfoxidation of alkyl aryl sulfides with ee's up to 84% and with


311 12

unprecedented catalytic efficiency, reaching 1000 turnover numbers ( T O N ) and the oxidation of secondary amines to nitrones with high chemoselectivities, quantitative yields and T O N up to 1400. 13

Beside the fact that the oxygen transfer process affords in both cases the corresponding products with high chemical yields and good selectivities, the Ti(IV) catalyst showed to be rather robust under the reaction conditions, which require the presence of large quantities of alkylperoxide and, i n the case of secondary amine oxidation, affords stoichiometric amount of water. The structure in solution of the Ti(IV) catalyst 2 and of the corresponding peroxo active species 3 has been elucidated via the combined use of H N M R and mass spectrometry. They consist of stable titanatrane units in which the apical alkoxy ligand can easily exchange with other hydroxyl derivatives present in solution (for example iso-propanol, alkyl hydroperoxides or the trialkanolamine itself). In the presence of an excess of alkyl hydroperoxide they afford quantitatively the monomelic peroxocomplex 3, which is the active species in the oxygen transfer process. For all these reasons, titanatrane complexe 2 seemed to us good candidates for preparing Ti(IV) polymeric catalytic membranes to be used in selective oxygen transfer reactions. In fact the catalysts should be robust enough to survive the conditions required for membrane preparation. Once in the polymeric matrix, their stability should improve because of minimized secondary hydrolysis reactions and therefore extending the life of the catalysts. Obviously, the recycling of the catalytic membrane will increase the total T O N of the process. Here we report on the preparation and characterization of different catalytic membranes incorporating Ti(IV)/(R,R,R)-tris(2-phenylethanolamine)-2 and their application for heterogeneous oxidation of a model sulfide, benzyl phenyl sulfide, and a model secondary amine, dibenzyl amine. 12


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Catalytic Membranes Synthesis and Characterization

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The preformed Ti(IV) (i*,i^)-Ms(2-etoanol)amine complex (16%/w) has been entrapped in different polymeric membranes using a phase inversion


312 technique induced by a nonsolvent: D M A and water were respectively employed as solvent and nonsolvent. Three polymers, all characterized by a high thermal, chemical and mechanical stability, were used for the preparation of catalytic membranes: polyvinylidene difluoride ( P V D F ) , a modified polyetheretherketone ( P E E K W C ) and polyacrylonitrile ( P A N ) (Scheme 2). 15


16

17

PVDF-TI

+ T!(OiPr)N(CH CHPhO) 2

(16%)

3

•

PEEK-WC-TI

PAN-TI

Scheme 2. Synthesis and structural unit ofpolymeric membranes incorporating Ti(IV) complex. The top surfaces and the cross section morphology of the membranes have been examined by scanning electron microscopy (SEM). According to the synthetic procedure, in all the cases the catalyst embedding results in the formation of asymmetric hybrid membranes (Figure 1). S E M analysis shows a strong difference i n the membrane morphology by changing the polymer in the casting solution. Figure 1 shows that the morphology o f membranes varies from porous finger-like with elongated macrovoids for P E E K W C - T i and highly asymmetric structure with a dense skin layer for P V D F - T i to an asymmetric dense for P A N - T i . A smooth surface for P E E K W C - T i and P V D F - T i membranes is obtained whereas a large number of small pores is observed on the top surface of P A N - T i membrane.



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Figure 1. Cross sections (left) and top surface (right) morphology of the catalytic membranes, examined by scanning electron microscopy (SEM)


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Catalytic Membrane Reactivity

The screening of activity of the different catalytic membranes [PVDF-Ti, P E E K W C - T i , PAN-Ti] has been assessed in the oxidation of benzyl phenyl sulfide and dibenzylamine, chosen as model substrates. The membrane performances were investigated under batch conditions using an equal active surface and comparable amount of total catalyst loading. In order to have a better comparison o f the results, the reactions were carried out under the best conditions selected for the homogeneous reactions. Due to the solubility of P E E K W C in chlorinated solvents (chloroform or 1,2-dichloroethane, D C E ) , which are the most suitable ones for die homogeneous reactions, the reactivity as been also explored in acetonitrile. Initially the reactivity o f the catalytic membranes has been tested in the sulfoxidation reaction in order to have also indirect evidence on the nature of the catalyst and active species embedded in the different membranes (Scheme 3).

ο p^S^Ph P

h

+CHP

•

? S^Ph

+ CHP

•

P h

New Catalytic Polymeric Membranes Incorporating Ti(IV

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