Characterization of Beta/MCM-41 composite molecular sieve compared with the mechanical mixture

Abstract

A Beta/MCM-41 composite was prepared for the first time through a two-step crystallization process combining a low crystallized zeolite Beta synthesis gel with a surfactant cetyltrimethylammonium bromide (CTAB) solution. The composite has been investigated and compared with a mechanical mixture of Beta and MCM-41 for their structure, acidity and catalytic activity by means of XRD, SEM, 129Xe NMR, DTG, N2 adsorption, IR spectra of pyridine and catalytic cracking reaction. The XRD result shows that the Beta/MCM-41 composite possesses a well-ordered mesoporous MCM-41 phase and a zeolite Beta phase. The result of N2 adsorption indicates that the composite contains a bimodal mesopore system and the microporous structure of zeolite Beta. Moreover, the results of 129Xe NMR and DTG suggest that the mesopore wall of the composite is constructed by a lot of secondary building units characteristic of zeolite Beta, which is quite different from the amorphous character of MCM-41 in the mechanical mixture. Although the total number of acid sites of the composite is comparable to that of the mechanical mixture, the composite possesses more strong Brønsted acid sites due to the initially crystallized mesostructure and shows higher catalytic activity for n-heptane cracking compared with the mechanical mixture.