锶掺杂对氧化镧催化甲烷氧化偶联反应的影响（英文）

发布时间：2018-05-30 17:14

本文选题：甲烷活化 + 甲基自由基产生　；参考：《催化学报》2017年05期

【摘要】：甲烷氧化偶联反应(OCM)是天然气直接转化利用的重要途径之一.该反应通过甲烷和氧气在催化剂作用下一步将甲烷直接转化为乙烯等具有高附加值的产品,避免了涉及高能耗过程的合成气间接路径,不仅有可能减少中间副产物的生成,还有可能大大提升整个过程的能源利用效率.因此,研究OCM反应具有十分重要的实际意义.目前氧化镧基催化剂具有良好的催化活性、产物选择性和热稳定性,但在OCM反应中产品收率仍未能达到工业应用的要求,因而近几十年来高效OCM催化剂的研发一直是研究热点.实验发现,锶掺杂氧化镧催化剂具有更为优异的催化性能,主要表现在具有比纯氧化镧催化剂更高的催化活性和产物选择性,但对于锶掺杂的影响机制仍然缺乏系统的理论研究.目前普遍认为,甲烷活化是OCM反应的第一步,也是决速步,这主要是由于C-H键活化需要越过很高的能垒,因此往往需要很高的温度.本文主要采用团簇模型,通过密度泛函理论计算来研究OCM反应中锶掺杂对氧化镧催化剂上甲烷活化性能的影响及其作用原理.本文构建了八种锶掺杂的氧化镧团簇作为该催化剂模型,可分为没有自由基性质的团簇(LaSrO_2(OH),La_2SrO_4,La_3SrO_5(OH),La_5SrO_8(OH))和具有自由基性质的团簇(LaSrO_3,La_2SrO_4(OH),La_3SrO_6,La_5SrO_9).我们计算了甲烷在这些锶掺杂氧化镧团簇上Sr-O和La-O酸碱对位点以及氧自由基活性位点上的活化机制,以研究锶掺杂对OCM反应活性的影响,并与我们前期计算的纯氧化镧团簇上甲烷活化性能进行了对比.通过计算甲烷在不同锶掺杂氧化镧团簇上的物理和化学吸附能、活化能垒以及甲基自由基的脱附能,发现锶掺杂氧化镧团簇上的甲烷活化在热力学和动力学上都要比纯氧化镧团簇上更为有利.对于具有相同金属原子数目的团簇,甲烷在La-O上活化的能垒大小为:化学计量比的La-Sr-O团簇非化学计量比的La-Sr-O团簇化学计量比的La-O团簇;而甲烷在Sr-O上活化的能垒大小依次是:化学计量比的La-Sr-O团簇非化学计量比的La-Sr-O团簇.给定一个锶掺杂氧化镧团簇,甲烷在不同活化位点上的活化能垒大小通常是:O·Sr-OLa-O,其中无论何种性质的锶掺杂氧化镧团簇,甲烷在Sr-O上的反应活性要高于La-O上的,而对于具有自由基特征的锶掺杂氧化镧团簇,甲烷更容易在氧自由基位点上发生解离.此外,对于没有自由基特征的锶掺杂氧化镧团簇,甲基自由基的脱附如同纯氧化镧团簇一样是强吸热过程.相反,对于具有自由基特性的锶掺杂氧化镧团簇,甲基自由基的脱附则十分容易.由此可见,锶掺杂促进氧化镧催化剂上OCM反应活性主要有以下两个原因:(1)通过掺杂可以提供具有自由基特性的氧活性位点,(2)对于非自由基性质的团簇,可以增强金属.氧对位点的碱性和甲烷反应活性,从而有效降低了甲烷的活化能垒和甲基自由基的脱附能.
[Abstract]:The oxidative coupling reaction of methane (OCM) is one of the important ways of direct conversion and utilization of natural gas. This reaction directly converts methane to high value-added products such as ethylene through methane and oxygen in the catalyst, thus avoiding the indirect pathway of syngas involved in high-energy consumption processes, which may not only reduce the formation of intermediate by-products. It is also possible to greatly improve the energy efficiency of the whole process. Therefore, the study of OCM reaction is of great practical significance. At present, lanthanum oxide catalysts have good catalytic activity, product selectivity and thermal stability, but the yield of products in OCM reaction is still not up to the requirements of industrial application. Therefore, the research and development of high efficiency OCM catalyst has been a hot topic in recent decades. It is found that strontium doped lanthanum oxide catalysts have better catalytic performance, mainly because of their higher catalytic activity and product selectivity than pure lanthanum oxide catalysts. However, the mechanism of strontium doping is still lack of systematic theoretical study. At present, it is generally believed that methane activation is the first step and the fast step of OCM reaction. This is mainly due to the fact that C-H bond activation needs to cross the high barrier, so it often requires a very high temperature. In this paper, the effect of strontium doping on the activation of methane on La _ 2O _ 3 catalyst in OCM reaction was studied by means of cluster model and density functional theory (DFT). In this paper, eight strontium doped lanthanum oxide clusters have been constructed as the catalyst model, which can be divided into LaSrO2O2OHHHHO _ 4 and La5SrO _ 5O _ 5O _ (5 / O) and La5SrO _ (5o _ 5O _ (5) O _ (HH) and LaSrO _ (3) / La _ (2SrO _ (4) O _ (4) O _ (H) / La3SrO _ (6) La-5SrO _ (9T). We have calculated the activation mechanism of methane on the sites of Sr-O and La-O acid-base pairs and the active sites of oxygen free radicals on these strontium doped lanthanum oxide clusters, in order to study the effect of strontium doping on the activity of OCM reaction. The activation properties of methane on pure lanthanum oxide clusters calculated by our previous calculations were compared. The physical and chemical adsorption energy, activation energy barrier and desorption energy of methyl radical on different strontium doped lanthanum oxide clusters were calculated. It is found that the activation of methane on strontium doped lanthanum oxide clusters is more advantageous in thermodynamics and kinetics than in pure lanthanum oxide clusters. For clusters with the same number of metal atoms, the energy barrier for methane activation on La-O is as follows: La-Sr-O cluster with stoichiometric ratio, La-Sr-O cluster with non-stoichiometric ratio, La-O cluster with stoichiometric ratio; The energy barrier of methane activation on Sr-O is the La-Sr-O cluster with stoichiometric ratio and the non-stoichiometric La-Sr-O cluster with stoichiometric ratio. Given a strontium doped lanthanum oxide cluster, the activation energy barrier of methane at different activation sites is usually O: O Sr-O La-O, in which no matter what kind of strontium doped lanthanum oxide cluster, methane reactivity on Sr-O is higher than that on La-O. For strontium doped lanthanum oxide clusters with free radicals, methane dissociates more easily at oxygen free radical sites. In addition, for strontium doped lanthanum oxide clusters without free radical characteristics, the desorption of methyl radical is a strong endothermic process as pure lanthanum oxide clusters. On the contrary, for strontium doped lanthanum oxide clusters with free radical properties, it is very easy to remove methyl radical from lanthanum oxide clusters. It can be seen that strontium doping can promote the activity of OCM reaction on lanthanum oxide catalyst for the following two reasons: (1) the oxygen active site with free radical property can be provided by doping) the metal can be enhanced for the clusters with non-free radical properties. The activation energy barrier of methane and the desorption energy of methyl radical can be effectively reduced by the alkalinity of oxygen to sites and the activity of methane.
【作者单位】：中国科学院上海高等研究院低碳转化科学与工程中心;中国科学院大学;上海科技大学物质科学与技术学院;
【基金】：supported by the National Natural Science Foundation of China(21473233,21403277) the Frontier Science Program of Shell Global Solutions International B.V.(PT32281) the Ministry of Science and Technology of China(2016YFA0202802) the Shanghai Municipal Science and Technology Commission(14ZR1444600)~~
【分类号】：O621.251

【相似文献】