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The influence of acid strength was evaluated toward the selectivity to propene on conversion of 1-pentene. For the catalytic cracking of 1-pentene, the main reaction pathways and the molar ratio of propene to ethene (P/E ratio) were controlled by acid strength with the appropriate amount of total acid sites. The results showed that the P/E ratio increased with decreasing amounts of strong acid sites, since the activation energies of individual reaction pathways were influenced by acid strength to a different extent. Strong acid sites could promote pathway I′ (C 5 2– → C 2 2– + C 3 2– ) and pathway II′-1 (C 6 2– → C 2 2– + C 4 2– ), while weak acid sites preferred pathway II′ (2C 5 2– → C 10 + → C 4 2– + C 6 2– ) and pathway II′-2 (C 6 2– → 2C 3 2– ), since pathways II′ and II′-2 underwent some energetically favorable routes (tertiary–secondary, secondary–secondary) of carbenium ion intermediates. By manipulation of the acid strength distribution on ZSM-5, the P/E ratio and selectivity of propene could be significantly improved, suggesting that this can provide an important guideline for improving such a process. In addition, we also designed a coupled process combing butene and pentene coconversion, as pentene and butene could be produced during C 4 2– and C 5 2– catalytic cracking. The coupled process could offer a promising solution to gain high selectivity of propene from C 4 and C 5 olefin cracking.
Lin et al. (Tue,) studied this question.