Encapsulation in zeolites

Metal macrocycles encapsulated in zeolites can be synthesized in different ways.50-53 The flexible ligand51 or intrazeolite complexation53 method involves the diffusion of a ligand into the pores of a metal-exchanged zeolite, where upon complexation with the metal ion, it becomes too large to exit. This... 

Iron N,N -bis(2-pyridinecarboxamide) complexes encaged in zeolite Y were used for the partial oxidation of alkanes.99 Epoxidation with manganese N,N -bis(2-pyridinecarboxamide) complexes encapsulated in zeolite Y was also reported.100... 

Immobilization of chiral complexes in PDMS membranes offers a method for the generation of new chiral catalytic membranes. The heterogenization of the Jacobsen catalyst is difficult because the catalyst loses its enantioselectivity during immobilization on silica or carbon surfaces whereas the encapsulation in zeolites needs large cages. However, the occlusion of this complex in a PDMS matrix was successful.212 The complex is held sterically within the PDMS chains. The Jacobsen catalyst occluded in the membrane has activity and selectivity for the epoxidation of alkenes similar to that of the homogeneous one, but the immobilized catalyst is recyclable and stable. 

When supported complexes are the catalysts, two types of ionic solid were used zeolites and clays. The structures of these solids (microporous and lamellar respectively) help to improve the stability of the complex catalyst under the reaction conditions by preventing the catalytic species from undergoing dimerization or aggregation, both phenomena which are known to be deactivating. In some cases, the pore walls can tune the selectivity of the reaction by steric effects. The strong similarities of zeolites with the protein portion of natural enzymes was emphasized by Herron.20 The protein protects the active site from side reactions, sieves the substrate molecules, and provides a stereochemically demanding void. Metal complexes have been encapsulated in zeolites, successfully mimicking metalloenzymes for oxidation reactions. Two methods of synthesis of such encapsulated/intercalated complexes have been tested, as follows. 

Professor M. R. Maurya is currently heading the Department of Chemistry, IIT Roorkee. He has more than 26 years of teaching and research experience. He had worked in Loyola University of Chicago, USA, Iowa State University, Ames, Iowa, USA, National Chemical Laboratory, Pune, and Pune University Pune, before joining department of Chemistry at IIT Roorkee in 1996 and became full professor in 2008. His current area of research interests include structural and functional models of vanadate-dependent haloperoxidases, coordination polymers and their catalytic study, metal complexes encapsulated in zeolite cages and their catalytic study, polymer-anchored metal complexes and their catalytic study, and medicinal aspects of coordination compounds. So far, he has guided 21 doctoral and 7 Master s theses, co-authored more than 140 research papers in the international refereed journals. 

C. R. Jacob, S. R. Varkey, and R. Ratnasamy, Selective oxidation over copper and manganese salens encapsulated in zeolites, Microporous Mesoporous Mater. 22, 465 74 (1998). 

R. Raja, and R. Ratnasamy, Oxidation of cyclohexane over copper phalocyanines encapsulated in zeolites, Catal. Lett. 48, 1-10 (1998). 

S. Chavan, D.Srinivas, and R. Ratnasamy, Structure and catalytic properties of dimeric copper(II) acetato complexes encapsulated in zeolite-Y, J. Catal. 192, 286-295 (2000). 

A Ni(salen) complex [salen = bis(salicylidene)ethylenediamine] encapsulated in zeolite is highly efficient in the hydrogenation of simple alkenes (cyclohexene, cyclooctene, 1-hexene).451 This method, which is the encapsulation of metal complexes into the cavities of zeolites, offers the unique possibility of shape-selective... 

Selectivities to various isomers are more difficult to predict when metal oxides are used as catalysts. ZnO preferentially produced 79% 1-butene and several percent of a -2-butene [624-64-6] (75). CdO catalyst produced 55% 1-butene and 45% ar-2-butene. It was also reported that while interconversion between 1-butene and or-2-butene was quite facile on CdO, cis—trans isomerization was slow. This was attributed to the presence of a 71-allyl anion intermediate (76). High or-2-butene selectivities were obtained with molybdenum carbonyl encapsulated in zeolites (77). On the other hand, deuteration using Th02 catalyst produced predominandy the 1,4-addition product, trans-l-bu. en.e-d2 with no isotope scrambling (78). 

The use of heterogeneous catalysts in the synthesis of urethanes from aliphatic and aromatic amines, C02 and alkyl halides has been explored only recently. Titanosilicate molecular sieves [60a], metal phthalocyanine complexes encapsulated in zeolite-Y [60a], beta-zeolites and mesoporous silica (MCM-41) containing ammonium cations as the templates [60b, c], and adenine-modified Ti-SBA-15 [60d, e] each function as effective catalysts, even without any additional base. 

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