Organic-functionalized molecular sieve

C.W. Jones, K. Tsuji, and M.E. Davis, Organic-Functionalized Molecular Sieves as Shape-selection catalysts. Nature London), 1998, 393, 52-54. 

R693 Y, Tozuka, Is Crystal Structure Predictable even when a Single Crystal is Not Available , Farumashia, 2000, 36, 995 R694 K. Tsuji, Studies in Organic-Functionalized Molecular Sieves (OFMSs) , Zeoraito, 2000,17, 162... 

De Vos, D. E., Dams, M., Sels, B. F. and Jacobs, P. A. Ordered mesoporous and microporous molecular sieves functionalized with transition metal complexes as catalysts for selective organic transformations, Chem. Rev., 2002, 102, 3615-3640. 

The term molecular sieve describes a material having pores that closely match the dimensions of a specific molecule. The best-known molecular sieves are composites of microcrystalline zeolites embedded in an inert clay binder. Zeolites are composed of regular clusters of tetrahedral aluminosilicates, with varying percentages of bound cations and water molecules, whose crystal structures incorporate small molecule-sized cavities. Because zeolite pore size is different for each of the numerous different crystal structures in this family, the size-selective nature can be tailored for specific applicatimis. Studies of the transport of liquid and gaseous organic species in molecular sieves indicate that the diffusion rate and equilibrium concentration of sorbed analyte are sensitive functions of their molecular dimensions, as well as zeolite pore size and shsqre [110]. 

Molecular sieves can not only act as catalysts themselves, as previously discussed, but are also able to form the matrix for extra lattice phases which are catalytically active. Most of the applications of these guest species in the molecular sieve concern hydrogenation (metal particles) or oxygenation reactions (metal complexes). If neutral molecular sieve lattices (e.g., the aluminumphosphate VPI-5 [193,194]) are used as the matrix, the supported metal particle and/or metal organic complex constitute the only active phase, while bifunctional catalysts are obtained when molecular sieve lattices with acidic functions (e.g, zeoliteY [195,196]) serve as matrix. 

Recently, among synthetic organic chemists, it has been widely recognized that coexistence of an appropriate amount of molecular sieve zeolites with an asymmetric catalyst is indispensable for performing highly enantioselec-tive asymmetric synthesis (50-52), although the functions of the zeolites have not been clearly elucidated yet. In any case it can be expected that the use of zeolites will certainly be developed for versatile organic syntheses of fine chemicals. 

The past 10-15 years have seen major advances in the development and realization of LLC materials for catalytic applications. These advances range from the use of non-catalytically functionalized LLC phases as a means of accelerating reactions via confinement, to the design of polymerized LLC materials containing discrete catalytic groups that act like organic analogues to catalytic molecular sieves. 

Many of the non-silica compositions showed problems with the stability and quality of the structure. Efforts to address these issues have been on going and quite successful in some cases such as all-alumina compositions (see below). Silica-based materials remain dominant as the most versatile and best quality molecular sieves (structure and stability) available by a facile synthesis. These attributes, especially the convenient synthesis made mesoporous silicate attractive for post-synthesis functionalization with other elements as well as organic moieties with active groups/ccnters. Recently the compositional diversity has been extended further to include both silica and organic moieties within the framework. The new class is referred to as periodic mesoporous organosilicas (PMOs). The synthesis involves surfactant-assisted assembly by hydrolysis of organo-silicon compounds. Additional discussion of the PMOs is presented below. 

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