Solid-state NMR Studies On The Activation And Conversion Of Light Alkanes And CO Over Zeolites

Light alkanes such as methane and ethane are the principle components of natural gas. As clean chemical feedstock, they may be used more efficiently in the chemical industry as alternative of petroleum and coal to meet the increasing energy demands of modern society. However, catalytic activation and conversion of light alkanes requires rigorous conditions because of their chemical inertness. In order to reduce the thermodynamic limits, liquid super-acids are generally employed as efficient homogeneous catalysts, which however cause equipment corrosion and environmental pollution. Therefore, the development of environmental friendly catalysts such as zeolites is desirable for light alkane activation and conversion. Zeolites catalysts are widely used in the petroleum industry for olefin or alcohol conversion. However, the utilization for catalytic conversion of light alkanes with higher chemical inertness remains to be a great challenge.Herein, we studied the co-conversion of light alkanes and CO with other co-reactants over a Zn modified H-ZSM-5zeolite (denoted as ZnZSM-5) by in situ solid-state NMR spectroscopy combined with GC-MS analysis. The reaction mechanism was clarified and the results would be helpful for the rational design of heterogeneous catalysts used for the direct conversion of light alkanes and CO.(1) Carbonylation of methane with CO over ZnZSM-5zeolite catalysts was studied by in situ solid-state NMR spectroscopy. It was found for the first time that acetic acid could be generated directly under mild reaction conditions (573-623K) through two parallel reaction pathways. Namely, CO was activated into methoxy intermediates, which can further interact with residual CO to generate acetic acid (Koch-type mechanism), while methane was activated into zinc methyl intermediates that can be consequently transformed into methyl groups of acetic acid with CO2through a typical organometallic reaction. Importantly, the two pathways are selectively controllable by varying the redox conditions.(2) Co-conversion of ethane and CO over ZnZSM-5zeolite catalysts was investigated by using in situ solid-state NMR spectroscopy. Propionic acid, acetic acid and aromatics could be generated under mild reaction conditions (523～623K). The transformation was traced by alternatively C isotope labeled reactants. Ethane was first activated into zinc ethyl species at lower temperatures, then it could be trapped by CO2derived from oxidation of CO, forming propionic acid through a typical organometallic reaction. Meanwhile, zinc ethyl species was transformed into ethene, and further into aromatics such as benzene and methyl substituted aromatics via dehydrogenation and oligomerization reaction. CO could be activated into methoxy intermediates and further interacts with residual CO to generate acetic acid.(3) Alkylation of benzene with methane was studied under oxidization condition over ZnZSM-5zeolites by using in situ solid-state NMR spectroscopy and GC-MS analysis. The experimental results indicated that the alkylation reaction occurs with selective formation of toluene at temperatures of523～623K using O2or N2O as the oxidant. Using13C isotope labeled reactants, the conversions of methane and benzene were independently monitored, and their respective role in the reaction was determined. It was found by NMR spectroscopy that methane was first activated into methoxy species and zinc methyl intermediates. As an electrophilic agent, the methyl group of methoxy species could directly attack phenyl ring to produce toluene via electrophilic substitution reaction, while the zinc methyl species was not directly involved in the alkylation reaction. However, the similar nature of zinc methyl species (Zn-CH3) to organozinc compounds allowed the facile oxidization of zinc methyl species (Zn-CH3) into methoxy species, and further intereacted with benzene to yield toluene indirectly. As confirmed by GC-MS experiments, methane exclusively provided the methyl group of toluene product while benzene afforded the phenyl ring. Experimental results also indicated that neither methane nor benzene alone could generate toluene.(4) Alkylation of benzene with CO over ZnZSM-5zeolites was studied by using in situ solid-state NMR spectroscopy and GC-MS analysis. We give the first experimental evidence that alkylation of benzene with CO as the alkyl agent proceeds to give toluene accompanied by trace amounts of other substituted aromatics such as ethylbenzene, dimethylbenzene and diphenylmethane at temperatures of523-623K. Using13C isotope labeled experiments, the conversion of CO and benzene were independently monitored, and their respective role in the reaction was determined. It was found by NMR spectroscopy that carbon monoxide is firstly oxidized into carbonate species, then transformed into formate species through hydrogenation at elevated temperature, and further hydrogenated into methoxy species. As an electrophilic agent, the methyl group of methoxy species can directly attack benzene ring to produce toluene via electrophilic substitution reaction. And the toluene could further interact with another benzene molecular to yield diphenylmethane. The addition of trace H2molecular can promote hydrogenation of CO into methoxy species markedly and further favor the alkylation reaction. As confirmed by GC-MS experiments, CO exclusively provided the methyl group of toluene product or other side-chain groups of corresponding substituted aromatics while benzene afforded the phenyl ring. Experimental results also indicated that without the presence of benzene the reaction of CO with H2could not generate any aromatics under identical conditions.