Insights Into The Reaction Mechanism Of Methanol To Olefins On ZSM-5 Zeolite Studied With Solid-state NMR

Light olefins such as ethylene and propylene are important industrial chemicals for production of styrene, vinyl chloride, acrylonitrile etc. Conventionally, light olefins are obtained from steam cracking and catalytic craking of crude oil with its cost governed by oil price. Due to the shortage of petroleum resource in the future and the increasing market demand for light olefins, developing new technology for producing olefins independent of petroleum route is required. Methanol to olefins (MTO), draws more and more attention, which enables the transformation of coal, natural gas or bimass to light olefins via methanol.Since ZSM-5 zeolite was first reported for the catalytic MTO reaction, continuous effort has been devoted to the reaction mechanism studies. Focusing on the initial formation of light olefins, more than 20 different routes have been proposed in the past decades but they are still intensively debated. The’hydrocarbon pool’, trapped in the void of zeolites, that undergoes methylation and olefins elimination has been widely accepted as the reaction center for the formation of light olefins. In this dissertation, the formation mechansim of ethene and propene were studied by in situ solid-state NMR spectroscopy in combination with GC-MS analysis.(1) Over zeolite H-ZSM-5, the aromatics-based hydrocarbon pool mechanism of methanol to olefins (MTO) reaction was studied by GC-MS, solid-state NMR spectroscopy and theoretical calculations. Isotopic labelling experimental results demonstrated that polymethylbenzenes (MBs) are intimately correlated with the formation of olefin products in the initial stage. More importantly, three types of cyclopentenyl cations (1,3-dimethylcyclopentenyl,1,2,3-trimethylcyclopentenyl and 1,3,4-trimethylcyclopentenylcations) and pentamethylbenzenium ion were for the first time identified by solid-state NMR spectroscopy and DFT calculations under both co-feeding ([13C6]benzene and methanol) condition and typical MTO working (feeding [13C]methanol alone) condition. The comparable reactivity of the MBs (from xylene to tetramethylbenzene) and the carbocations (trimethylcyclopentenyl and pentamethylbenzium ions) in the MTO reaction was revealed by 13C labelling experiments, evidencing that they work together via a paring mechanism to produce propene. The paring route in a full aromatics-based catalytic cycle was also supported by theoretical DFT calculations.(2) The formation mechansim of ethene in the methanol to olefins reaction over H-ZSM-5 zeolite was investigated. Three types of ethylcyclopentenyl carbocations, 1-methyl-3-ethylcyclopentenyl,1,4-dimethyl-3-ethylcyclopentenyl and 1,5-dimethyl-3-ethylcyclopentenyl cations were unambiguously identified under working conditions by both solid-and liquid-state NMR spectroscopy as well as GC-MS spectrometry. These carbocations were found to be well correlated to ethene and lower methylbenzenes (xylene and trimethylbenzene) and the mechanistic link between ethene and the ethylcyclopentenyl cations was established. An aromatics-based paring route provides rationale for the transformation of lower methylbenzenes to ethene via ethylcyclopentenyl cations as the key hydrocarbon pool intermediates, which is also supported by theoretical calculations.(3) The reactivity of polymethylbenzenes and their selectivity to olefins in the induction period of MTO reaction was studied over zeolite H-ZSM-5. The 12C/13C methanol-switching experiments was conducted to assess the reactivity of the polymethylbenzenes (PMB) residing in the zeolite voids during the reaction. It was found that the higher methylbenzenes such as pentamethylbenzene is more reactive than the lower methylbenzenes and is thus a relevant reaction intermediate for the formation of light olefins especially propene. Moreover, to explain the reaction mechanism in induction period, identification of carbocations was carried out by solid-state NMR as well. Observation and identification of cyclopentenyl cations and pentamethylbenzenium cations in the MTO process allows us to propose a paring route for the formation of propene in induction period, in which the higher methylbenzenes are correlated with olefin products via these carbocations.