Silicalite‑1 As Efficient Catalyst for Production of Propene from 1‑Butene Palani Arudra, Tazul Islam Bhuiyan, Muhammad Naseem Akhtar, Abdullah M. Aitani, Sulaiman S. Al-Khattaf,* and Hideshi Hattori Center of Research Excellence in Petroleum Refining and Petrochemicals, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia * S Supporting Information ABSTRACT: Reaction of 1-butene was studied over silicalite-1 and H-ZSM-5 zeolites with different Al contents (SiO 2 /Al 2 O 3 molar ratio (Si/Al 2 ) = 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/Al 2 = 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. KEYWORDS: silicalite-1, H-ZSM-5, 1-butene, propene, alkene cracking, hydrogen transfer, weak acid H ighly selective propene production has received much attention in recent years because of its use in important derivatives of polymers, intermediates, and chemicals. The demand for propene is growing very rapidly, primarily driven by a high growth rate of polypropylene usage. 1 Several propene production technologies have been inves- tigated including methanol-to-olefin process, steam cracking, propane dehydrogenation, catalytic cracking of C 4 -alkenes, and metathesis. 2 Among these methods, the process of butene cracking has attracted much attention because of the availability of large and stable supplies of butene isomers from FCC and stream cracking processes. 3-5 Cracking of C 4 to C 6 alkenes appears to be a promising technology for production of propene and ethylene. 6,7 The catalysts studied so far for the purpose of propene pro- duction by cracking include H-ZSM-5, H-ZSM-48, PITQ-13, and SAPO-34. 8-15 H-ZSM-5 showed a high propene yield of 35% after modification with phosphorus and lanthanum. 8 H-ZSM-48 showed a higher propene selectivity in C 4 -alkene cracking compared with H-ZSM-5, although the reasons for the high selectivity were not certain. 9 Zhu et al. studied the effects of zeolite pore structure and Si/Al 2 ratio on the cracking of C 4 alkenes to propene. 11 A high selectivity to propene was observed for the medium-pore 10-membered ring zeolites and the small-pore SAPO zeolite. The smaller the pore size of the zeolites, the greater the extent of suppression of the hydrogen transfer reaction of alkenes and the higher the propene selectivity. The H-ZSM-5 with an Si/Al 2 ratio of 366 exhibited the best performance. Lin et al. reported that the H-ZSM-5 modified with phosphorus to reduce the number of strong acid sites showed the best performance in the reaction of 1-butene to propene. 14 They suggested that adjusting the acid site distribution is important. Zeng et al. also reported that the phosphorus-modified PITQ-13 yielded a high propene yield of 41.6 C-wt %. 15 They reported that the modification with phosphorus weakened the acid sites to result in the high yield of propene. Recently, Epelde et al. reported that K-modified H-ZSM-5 showed a high yield of propene in the reaction of 1-butene. They explained the role of K to hinder Received: February 27, 2014 Revised: October 8, 2014 Research Article pubs.acs.org/acscatalysis © XXXX American Chemical Society 4205 dx.doi.org/10.1021/cs5009255 | ACS Catal. 2014, 4, 4205-4214