Insight into the Dual Cycle Mechanism of Methanol-to-Olefins Reaction over SAPO-34 Molecular Sieve by Isotopic Tracer St
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doi: 10.1007/s40242-020-0216-x
Article
Insight into the Dual Cycle Mechanism of Methanol-to-Olefins Reaction over SAPO-34 Molecular Sieve by Isotopic Tracer Studies YU Bowen1,2, LOU Caiyi1,2, ZHANG Wenna1, XU Shutao1, HAN Jingfeng1, YU Zhengxi1, WEI Yingxu1* and LIU Zhongmin1* 1. National Engineering Laboratory for Methanol to Olefins, Dalian National Laboratory for Clean Energy, iChEM(Collaborative Innovation Center of Chemistry for Energy Materials), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China; 2. University of Chinese Academy of Sciences, Beijing 100039, P. R. China Abstract Methanol-to-olefins(MTO) reaction is one of the important non-petroleum routes to produce light olefins over acidic molecular sieves. In this study, the complete reaction course of MTO on SAPO-34 molecular sieve with retained organics evolution from induction period to deactivation period was investigated systematically at different weight hourly space velocities(WHSV) of methanol. By the aid of 12C/13C-methanol isotopic switch experiment, the dual cycle mechanism involving aromatics-based cycle and alkenes-based cycle was evaluated during the whole reaction process. The detailed reaction route varied with the evolution of the retained organics in the catalyst at different reaction stages. The aromatics-based cycle and alkenes-based cycle alternately dominate the reaction process. In the efficient reaction period, aromatics-based cycle is the main reaction mechanism, while in the induction and deactivation periods, the contribution of alkenes-based cycle mechanism will become more important. Keywords SAPO-34; Methanol-to-olefin(MTO); Polymethylbenzene; Dual-cycle mechanism; Isotopic tracer
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Introduction
Methanol-to-olefins(MTO) reaction over acidic molecular sieves as a non-petroleum route to produce the light olefins, such as ethene and propene, has attracted extensive interests from academy and industry[1―3]. SAPO-34 molecular sieve with 8-membered ring pore opening, chabazite(CHA) topology and relatively moderate acidity has been applied in the commercial MTO process[4]. The mechanism of MTO has been studied extensively over four decades[1,2,5―7]. A lot of studies have been concerned on the direct mechanism[8―17] and the indirect mechanism[18―34]. In recent studies, the first C―C bond formation was deeply understood by different groups using different characterization methods[8―13]. Fan’s group[12] proposed a crucial intermediate of methoxymethyl cations(CH3OCH2+) and its subsequent transformation with dimethyl ether(DME) or methanol to CH3OCH2CH2OR(R=H or CH3) for the first C―C bond formation by in situ infrared spectroscopy. Lercher’s group[13] postulated that acetate species could lead to the formation of
dimethoxymethane(DMM) and the carbon-carbon bond formation was further demonstrated by Weckhuysen’s group[14] using 2D 13C-13C solid state NMR(ssNMR) spectroscopy. Deng’s group[15] proposed that surface methoxy species bonded to an extra-framework aluminum was responsible for th
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