Abstract
The hierarchical structure can significantly improve the diffusion efficiency of the catalyst and regulate the product distribution. Therefore, the preparation of hierarchical SAPO-34 molecular sieve has been a hot research topic. With Cetyltrimethyl Ammonium Bromide (CTAB) and Diethylamine (DEA) as templates, a two-step crystallization process was employed to synthesize hierarchical SAPO-34 molecular sieves. We found that the aging process is vital for the formation of pure phase SAPO-34. It was investigated the relationship of crystallinity trend and mesoporous content with the crystallization time. The results showed that the prolongation of crystallization time was beneficial to enhance the crystallinity of the molecular sieve, but unfavourable to the retention of mesoporous structure. The formation process of hierarchical SAPO-34 molecular sieve involved agglomeration, disintegration, crystallization, re-agglomeration and growth. The hierarchical SAPO-34 molecular sieve with a satisfactory crystallinity and considerable mesoporous structure could be obtained after 36 hours of crystallization. Moreover, the sample had the most suitable acid strength as well as acid amount. The catalytic activity was investigated by catalytic dimethyl ether (DME) to olefin (DTO) reaction. It revealed that the conversion of DME and the selectivity to olefins over the hierarchical SAPO-34 molecular sieve were significantly enhanced with comparison to that over microporous SAPO-34 molecular sieve. The amount of coke deposition of the hierarchical SAPO-34 molecular sieve (14.2%) was lower than that over the microporous molecular sieve (16.5%). Meanwhile, the propylene selectivity of hierarchical SAPO-34 was higher than that of microporous SAPO-34 in the whole reaction. In a word, the hierarchical SAPO-34 molecular sieve synthesized in this study showed a longer catalytic life, higher coke deposition resistance and higher propylene selectivity.
Highlights
Light olefins such as ethylene and propylene have become the backbone of the modern chemical industry
Hierarchical SAPO-34 zeolite was successfully synthesized from conventional raw materials and templates
The characterization results showing the crystallization process of SAPO-34 synthesized by this method were as follows: The raw materials formed relatively stable spherical agglomerates with hierarchical structure under the function of Cetyltrimethyl Ammonium Bromide (CTAB)
Summary
Light olefins such as ethylene and propylene have become the backbone of the modern chemical industry. Light olefins are mainly produced by catalytic cracking of petroleum. Due to the decrease of petroleum resources, finding a broad route to prepare light olefins is very urgent. SAPO-34 has attracted the attention of researchers because of its unique pore structure and suitable acid performance, etc. This unique structure brings high selectivity to olefins, it brings serious cook deposition [2]. How to improve the catalytic performance of SAPO-34 molecular sieves has become a hot topic
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