Laboratory for Computational and Applied Chemistry, Department of Chemistry, Faculty of Science and Center of Nanotechnology, Kasetsart University Research and Development Institute, Kasetsart University, Bangkok 10900 (Thailand), Fax: (+66) 2-562-5555; NANOTEC Center for Nanoscale Materials for Green Nanotechnology, Kasetsart University, Bangkok 10900 (Thailand); Center for Advanced Studies in Nanotechnology and its Applications in Chemical, Food, and Agricultural Industries, Kasetsart University, Bangkok 10900 (Thailand).
Chemphyschem. 2014 Feb 24;15(3):514-20. doi: 10.1002/cphc.201300931. Epub 2013 Dec 20.
Finding novel catalysts for the direct conversion of CO2 to fuels and chemicals is a primary goal in energy and environmental research. In this work, density functional theory (DFT) is used to study possible reaction mechanisms for the conversion of CO2 and C2H6 to propanoic acid over a gold-exchanged MCM-22 zeolite catalyst. The reaction begins with the activation of ethane to produce a gold ethyl hydride intermediate. Hydrogen transfers to the framework oxygen leads then to gold ethyl adsorbed on the Brønsted-acid site. The energy barriers for these steps of ethane activation are 9.3 and 16.3 kcal mol(-1), respectively. Two mechanisms of propanoic acid formation are investigated. In the first one, the insertion of CO2 into the Au-H bond of the first intermediate yields gold carboxyl ethyl as subsequent intermediate. This is then converted to propanoic acid by forming the relevant C-C bond. The activation energy of the rate-determining step of this pathway is 48.2 kcal mol(-1). In the second mechanism, CO2 interacts with gold ethyl adsorbed on the Brønsted-acid site. Propanoic acid is formed via protonation of CO2 by the Brønsted acid and the simultaneous formation of a bond between CO2 and the ethyl group. The activation energy there is 44.2 kcal mol(-1), favoring this second pathway at least at low temperatures. Gold-exchanged MCM-22 zeolite can therefore, at least in principle, be used as the catalyst for producing propanoic acid from CO2 and ethane.
将二氧化碳直接转化为燃料和化学品的新型催化剂的寻找是能源和环境研究的主要目标。在这项工作中，使用密度泛函理论（DFT）研究了在金交换的 MCM-22 沸石催化剂上，将二氧化碳和乙烷转化为丙酸的可能反应机制。反应从乙烷的活化开始，生成金乙基氢化物中间体。然后，氢转移到骨架氧上，导致金乙基吸附在布朗斯台德酸位上。这些乙烷活化步骤的能垒分别为 9.3 和 16.3 kcal/mol。研究了两种丙酸形成的机制。在第一种机制中，CO2插入第一个中间体的 Au-H 键中，生成随后的中间体金羧基乙基。然后通过形成相关的 C-C 键将其转化为丙酸。该途径的速率决定步骤的活化能为 48.2 kcal/mol。在第二种机制中，CO2与吸附在布朗斯台德酸位上的金乙基相互作用。通过布朗斯台德酸质子化 CO2和 CO2 与乙基基团之间同时形成键，形成丙酸。那里的活化能为 44.2 kcal/mol，至少在低温下有利于第二种途径。因此，至少在理论上，金交换的 MCM-22 沸石可以用作从 CO2 和乙烷生产丙酸的催化剂。
