Silicalite-1 as efficient catalyst for production of propene from 1-butene

Reaction of 1-butene was studied over silicalite-1 and H-ZSM-5 zeolites with different Al contents (SiO₂/Al₂O₃ molar ratio (Si/Al₂) = 23, 80, and 280) to explore an efficient catalyst for the formation of propene as well as to elucidate the reaction scheme and the relevant acid sites involved in the reaction. The formation of alkenes, including propene, increased and those of alkanes and aromatics decreased with decreasing Al content. The percentage of alkenes other than n-butene isomers was 60 C-wt % over silicalite-1 at 550 °C with 34.1 C-wt % propene. Over H-ZSM-5 with Si/Al₂ = 23, the formation of alkenes was negligible, and the main products were alkanes and aromatics, the sum of alkanes and aromatics being 65.4 C-wt % at 550 °C. These product distributions are consistently interpreted by the successive reactions of oligomerization, cracking, and hydrogen transfer. For oligomerization and cracking, in addition to strong acid sites on H-ZSM-5 zeolites, weak acid sites present on silicalite-1 act as active sites. For the hydrogen transfer reaction of alkenes to form alkanes and aromatics, strong acid sites are required. The scheme can also be applicable to the reactions of 1-pentene and 1-hexene. The weak acid sites on silicalite-1 are assumed to be the silanol groups that act as Brønsted acid above 300 °C. The presence of strong acid sites on H-ZSM-5 catalysts, which are the OH groups bridging to Si and Al, results in the consumption of alkenes by hydrogen transfer. Removal of a part of Al contained in silicalite-1 as an impurity and enrichment of surface silanol groups on silicalite-1 resulted in the improvement of the propene yield. It is concluded that silicalite-1 is an efficient catalyst for the formation of propene by the reactions of light alkenes because of the absence of strong acid sites and the presence of weak acid sites.