A density functional theory study on the mechanism of Dimethyl ether carbonylation over heteropolyacids catalyst
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RESEARCH ARTICLE
A density functional theory study on the mechanism of Dimethyl ether carbonylation over heteropolyacids catalyst Kai Cai, Ying Li, Hongbao Shen, Zaizhe Cheng, Shouying Huang, Yue Wang, Xinbin Ma (✉) Key Laboratory for Green Chemical Technology of Ministry of Education, Collaborative Innovation Center of Chemical Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China
© Higher Education Press 2020
Abstract Dimethyl ether (DME) carbonylation is considered as a key step for a promising route to produce ethanol from syngas. Heteropolyacids (HPAs) are proved to be efficient catalysts for DME carbonylation. In this work, the reaction mechanism of DME carbonylation was studied theoretically by using density functional theory calculations on two typical HPA models (HPW, HSiW). The whole process consists of three stages: DME dissociative adsorption, insertion of CO into methoxyl group and formation of product methyl acetate. The activation barriers of all possible elementary steps, especially two possible paths for CO insertion were calculated to obtain the most favorable reaction mechanism and rate-limiting step. Furthermore, the effect of the acid strength of Brønsted acid sites on reactivity was studied by comparing the activation barriers over HPW and HSiW with different acid strength, which was determined by calculating the deprotonation energy, Mulliken population analyses and adsorption energies of pyridine. Keywords dimethyl ether, carbonylation, mechanism, heteropolyacids, density functional theory
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Introduction
Ethanol as promising alternative energy, is capable of relieving the reliance of our society on dwindling crude oil reserves. Owing to the high oxygen content and blending octane, using ethanol can significantly reduce the emission of CO, hydrocarbons and other pollutants in automobile exhaust [1], thus minimizing the negative impacts on the environment. Recently, a two-stage ethanol production technology, which integrates dimethyl ether (DME) Received March 27, 2020; accepted May 11, 2020 E-mail: [email protected]
carbonylation to methyl acetate (MA) and MA hydrogenation has been proposed, owing to flexible feedstock (syngas from various resources), high atom economy, unprecedented selectivity and mild reaction conditions [2,3]. Moreover, the absence of ethanol-water binary azeotrope in products significantly reduces the energy consumption of products separation and purification. Solid acids, such as H-form aluminosilicate zeolites and heteropoly acids (HPAs), have been reported as efficient heterogeneous catalysts for halide-free carbonylation of DME, which is beneficial for the manufacture and environment. These advantages with regard to the green chemistry make this process more promising in industry. HPAs (HnMX12O40) and their salts with super-acidity have been proved active and selective for DME carbonylation, in which Brønsted acid sites (BASs) function as the active sites. HPAs catalyst system without halide promoter exhibits
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