晶格限域的Fe?SiO2催化剂在甲烷无氧直接转化生成乙烯的反应中表现出优异的性能.但由于反应条件苛刻,对该反应的分子机理研究一直存在较大的挑战.本文采用反应力场的方法模拟近反应条件下甲烷无氧直接转化气相机理,发现当气相只有甲基自由基存在时,很难产生高选择性乙烯产物.当在气相中加入氢自由基时,虽能在一定程度上增强甲烷的活化,但同样较难生成乙烯.高温下热裂解C10H12分子能同时产生氢自由基和乙烯分子,能合理地解释实验中加入C10H12分子可以在一定程度上提高乙烯选择性和甲烷转化率的现象.总之,甲烷无氧直接转化高选择性生成乙烯很难通过单纯的气相反应机理来实现,进而推断催化剂表面在甲烷活化和转化的整个过程中起着至关重要的作用.
With the rapid consumption of petrochemical resources and massive exploitation of shale gas,the use of natural gas instead of petroleum to produce chemical raw materials has attracted significant attention.While converting methane to chemicals,it has long seemed impossible to avoid its oxidation into O-containing species,followed by de-oxygenation.A breakthrough in the nonoxidative conversion of methane was reported by Guo et al.(Science 2014,344,616),who found that Fe?SiO2 catalysts exhibited an outstanding performance in the conversion of methane to ethylene and aromatics.However,the reaction mechanism is still not clear owing to the complex experimental reaction conditions.One view of the reaction mechanism is that methane molecules are first activated on the Fe?SiC2 active center to form methyl radicals,which then desorb into the gas phase to form the ethylene and aromatics.In this study,ReaxFF methods are applied to five model systems to study the gas-phase reaction mechanism under near-experimental conditions.For the pure gas-phase methyl radical system,the main simulation product is ethane after 10 ns simulation,which is produced by the combination of methyl radicals.Although a small amount of ethylene produced by C2H6 dehydrogenation can be detected,it is difficult to explain the high selectivity for ethylene in the experiment.When the methyl radicals are mixed with hydrogen and methane molecules,ethane remains the main product,together with some methane produced by the collision of hydrogen with methyl radicals,while ethylene is still difficult to produce.With the addition of hydrogen radicals to the methane atmosphere,methane activation can be enhanced by hydrogen radical collisions,which produce some methyl radicals and hydrogen molecules,but the methyl radicals eventually combine with the hydrogen species to produce methane molecules again.If some hydrogen molecules and methyl radicals are added to the CH4/H· system,the activation of methane molecules by hydrogen radicals will be weakened.Hydrogen radicals are more likely to combine with themselves or with methyl radicals to form hydrogen and methane molecules,and the high selectivity for ethylene remains difficult to achieve.Thermal cracking of C10H12 at high temperature can produce hydrogen radicals and ethylene at the same time,which can partially explain the enhanced methane conversion and ethylene selectivity in the experiment of Hao et al.(ACS Catal.2019,9,9045).Overall,the selective production of ethylene by nonoxidative conversion of methane over Fe?SiO2 catalyst appears hard to achieve via a gas-phase mechanism.The catalyst surface may play a key role in the entire process of methane transformation.