Experimental and theoretical investigations on the hydrolysis of dimethyl ether to methanol over H-ZSM-5

S. Namuangruk, K. Faungnawakij
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引用次数: 1

Abstract

The catalytic hydrolysis of dimethyl ether (DME) over H-ZSM-5 was studied by experimental and theoretical studies. We observed from the pack-bed reactor that this reaction yields methanol as product at low temperature (<300°C), but produces other carbon-containing species at higher temperature (>300°C). However, these observations cannot give the details of the reaction mechanisms. To clarify that how the reaction proceeds, the insight into the reaction mechanisms of DME hydrolysis have been investigated by the ONIOM2(M06/6-31G(d,p):UFF) method. Our calculations showed that DME hydrolysis catalyzed by H-ZSM-5 occur via two mechanisms; stepwise and concerted. For the stepwise mechanism the reaction starts from the demethylation of DME to form surface methoxide intermediate and then followed by the hydrolysis between methoxide intermediate and adsorbing water to produce methanol as the product. The activation barriers of this pathway are 194 and 112 kJ mol−1 for the demethylation and the hydrolysis steps, respectively. For the concerted mechanism, the demethylation and hydrolysis take place simultaneously in a single step by using 125 kJ mol−1 as the activation energy. Our results indicate that the rate-determining step of this reaction is the demethylation of DME in the stepwise mechanism of which the calculated apparent barrier is 76 kJ mol−1. This value agrees well with our experimental observation that the hydrolysis of DME to methanol over H-ZSM-5 required energy of 76 kJ mol−1.
H-ZSM-5催化二甲醚水解制甲醇的实验与理论研究
对H-ZSM-5催化二甲醚(DME)水解进行了实验和理论研究。我们从填料床反应器中观察到,该反应在低温(<300°C)下产生甲醇作为产物,但在较高温度(>300°C)下产生其他含碳物质。然而,这些观察结果不能给出反应机制的细节。为了弄清反应是如何进行的,我们利用ONIOM2(M06/6-31G(d,p):UFF)方法研究了二甲醚水解的反应机理。计算结果表明,H-ZSM-5催化二甲醚水解通过两种机制发生;循序渐进,协调一致。分步反应机理是从二甲醚的去甲基化反应开始,形成表面甲氧基中间体,甲氧基中间体与吸附的水水解生成甲醇。该途径去甲基化和水解的激活垒分别为194和112 kJ mol−1。以125 kJ mol−1为活化能,脱甲基和水解在一个步骤中同时进行。结果表明,该反应的速率决定步骤是二甲醚的去甲基化,其计算的表观势垒为76 kJ mol−1。这一数值与我们在H-ZSM-5上将二甲醚水解为甲醇所需的能量为76 kJ mol−1的实验结果吻合。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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