Zeolite Chemistry in Catalysis

Abstract

The importance of molecular sieves catalysts in industrial catalysis has increased significantly over the past two decades. To date, all commercial applications of zeolite catalysts have involved acidic zeolites, particularly the ultra stable rare earth Y-type zeolite used in the catalytic cracking processes. Recent investigations of zeolite chemistry have revealed some particular features of both basic and acidic zeolites and have opened the new field of improved fuel processing. The new chemical evidence has raised the possibility that physico-chemical features play a role in catalysis. A large effort has therefore been devoted to the shape-selectivity properties of such materials. Moreover, increasing interest has been focussed recently on the understanding of crystallization of zeolites during synthesis, in the synthesis of new zeolites and in the chemical modifications of zeolites with the objective of expanding the applications of such materials.The word zeolite stems from the Greek "zeo" (boiling) and "litos" (stone) meaning a material able to eliminate large amounts of water when heated. More than forty different structures of zeolites have been identified while grea progress has been gained in synthetic zeolites, particularly by Linde and Mobil .Zeolites are usually alumino silicates with a general formula M D (Al Si, 0 N 0 o ), mH o 0 χ y x+2y n-(x+2y) 2n ' 2 where M and D cj^esignajje respectively a mono-(H + , Na + , K + , cjt. .) or a divalent (Ba , Mg , Ca ,...) cation. They are formed of A10.

Synthesis, Characterization, and Adsorbate Interactions of CoAPO-41 and CoAPSO-41 Molecular Sieves

“…There, 31 P MAS NMR spectra show multiple signals due to different P sites and also due to interaction with extraframework molecules. 1,24 Thus, the observed multiple NMR peaks for 31 P in as-synthesized AlPO-41 and CoAPO-41 are probably due to the combined effects of crystallographically independent T sites and the interaction of the P atoms with template and/or water molecules to varying extents. Incorporation of silicon in framework phosphorus sites creates a negative charge which is balanced by protonated amines.…”

“…1 Cobalt-substituted aluminophosphate (CoAPO) and silicoaluminophosphate (CoAPSO) molecular sieves are particularly interesting compounds in reactions such as autoxidation of p-cresol and cyclohexane. 2 Framework substitution of cobalt and other divalent metals has been reported in several structure types.…”

“…Transition-metal-incorporated aluminophosphate molecular sieves have gained considerable significance during the past several years due mainly to the bifunctional characteristics of such materials in several catalytic reactions. 1 Cobalt-substituted aluminophosphate (CoAPO) and silicoaluminophosphate (CoAPSO) molecular sieves are particularly interesting compounds in reactions such as autoxidation of p-cresol and cyclohexane. 2 Framework substitution of cobalt and other divalent metals has been reported in several structure types.…”

Crystallization of Sub-Micrometer Sized ZSM-5 Zeolites in SDA-Free Systems

“…Thus, the increase of the catalyst reactivity and thus, its efficiency can be achieved by increase of easily accessible active sites and by decrease of the length of diffusion path of reactants/intermediates/products and eliminating the probability of the occurrence of cokeformation side-reactions. Upon decreasing the crystal size, the diffusion paths of the reactant and product molecules inside the pores becomes shorter, and thus this can result in the reduction or elimination of undesired diffusion limitations of the reaction rate [8][9][10]. To synthesize small-sized ZSM-5 zeolites and achieve the precise control of crystal size, the synthesis mechanism of ZSM-5 and the critical processes occurring during its crystallization should be understood thoroughly.…”

Zeolitic and layered materials

“…The large interior supercage has a body diagonal dimension of up to 30 A and is believed to contain hydroxyl groups in the framework. A single crystal refinement of the structure of cloverite has led to the space group FmZc while the organic template was not yet located.…”