(A) study of model Pt catalysts supported on mesoporous molecular sieve MCM-41

Abstract

MCM-41 is a new mesoporous molecular sieve which consists of linear channels of a uniform pore diameter packed with a hexagonal structure. It attracts much attention in the study of adsorption and catalysis due to very large pore size, compared with conventional crystalline zeolites. In addition, the pore diameter can be tailored in the range of 2-10 nm. The large pore size is important for the application of large molecules too large to enter the zeolite pores. The present work was undertaken to investigate the physicochemical characteristics of MCM-41 for supporting catalytically active metal clusters inside the mesopore. Aluminum containing MCM-41 (AlMCM-41) with a Si/Al ratio of 35 was synthesized by hydrothermal crystallization using a gel mixture with a molar composition of 6 $SiO_2$ : 0.1 $Al_2O_3$ : 1.0 hexadecyltrimethylammonium chloride : 0.25 dodecyltrimethylammonium bromide : 0.25 tetrapropyl ammonium bromide : 0.15 $(NH_3)_2O$ : 1.5 $Na_2O$ : 300 $H_2O$. The AlMCM-41 sample was ion exchanged with $Pt(NH_3)_4^{2+}$. The supported Pt species was activated in $O_2$ flow at 593 K and subsequently reduced with $H_2$ flow at 573 K. The size and location of the resulting platinum clusters supported on the MCM-41 have been investigated by hydrogen chemisorption, $^{129}Xe$ NMR spectroscopy and extended X-ray absorption fine structure (EXAFS). Ethane hydrogenolysis was used as a probe reaction to determine catalytic activity of the Pt clusters. The catalytic activity of the Pt clusters obtained by the ion exchange technique have been compared with those prepared by an impregnation technique using $H_2PtCl_6$. It has been found that the Pt clusters prepared by ion exchange were about 1 nm in size and located inside the AlMCM-41 channel without pore blockage. On the contrary, results of the present work indicate that the impregnation method led to the formation of much larger Pt cluster causing pore blockage. This finding will be useful for design of practica...