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

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 B eta and MCM-41 for their structure, acidity and catalytic activity by means of XRD, SEM, Xe-129 NMR, DTG, Nc adsorption, IR spectra of pyridine and ca talytic cracking reaction. The XRD result shows that the Beta/MCM-41 compos ite possesses a well-ordered mesoporous MCM-41 phase and a zeolite Beta pha se. The result of N-2 adsorption indicates that the composite contains a bi modal mesopore system and the microporous structure of zeolite Beta. Moreov er, the results of Xe-129 NMR and DTG suggest that the mesopore wall of the composite is constructed by a lot of secondary building units characterist ic of zeolite Beta, which is quite different from the amorphous character o f 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 comp osite possesses more strong Bronsted acid sites due to the initially crysta llized mesostructure and shows higher catalytic activity for n-heptane crac king compared with the mechanical mixture. (C) 2001 Elsevier Science B.V. A ll rights reserved.