本文選題：甲烷活化 + 甲基自由基產(chǎn)生��； 參考：《催化學(xué)報(bào)》2017年05期

【摘要】：甲烷氧化偶聯(lián)反應(yīng)(OCM)是天然氣直接轉(zhuǎn)化利用的重要途徑之一.該反應(yīng)通過甲烷和氧氣在催化劑作用下一步將甲烷直接轉(zhuǎn)化為乙烯等具有高附加值的產(chǎn)品,避免了涉及高能耗過程的合成氣間接路徑,不僅有可能減少中間副產(chǎn)物的生成,還有可能大大提升整個(gè)過程的能源利用效率.因此,研究OCM反應(yīng)具有十分重要的實(shí)際意義.目前氧化鑭基催化劑具有良好的催化活性、產(chǎn)物選擇性和熱穩(wěn)定性,但在OCM反應(yīng)中產(chǎn)品收率仍未能達(dá)到工業(yè)應(yīng)用的要求,因而近幾十年來高效OCM催化劑的研發(fā)一直是研究熱點(diǎn).實(shí)驗(yàn)發(fā)現(xiàn),鍶摻雜氧化鑭催化劑具有更為優(yōu)異的催化性能,主要表現(xiàn)在具有比純氧化鑭催化劑更高的催化活性和產(chǎn)物選擇性,但對(duì)于鍶摻雜的影響機(jī)制仍然缺乏系統(tǒng)的理論研究.目前普遍認(rèn)為,甲烷活化是OCM反應(yīng)的第一步,也是決速步,這主要是由于C-H鍵活化需要越過很高的能壘,因此往往需要很高的溫度.本文主要采用團(tuán)簇模型,通過密度泛函理論計(jì)算來研究OCM反應(yīng)中鍶摻雜對(duì)氧化鑭催化劑上甲烷活化性能的影響及其作用原理.本文構(gòu)建了八種鍶摻雜的氧化鑭團(tuán)簇作為該催化劑模型,可分為沒有自由基性質(zhì)的團(tuán)簇(LaSrO_2(OH),La_2SrO_4,La_3SrO_5(OH),La_5SrO_8(OH))和具有自由基性質(zhì)的團(tuán)簇(LaSrO_3,La_2SrO_4(OH),La_3SrO_6,La_5SrO_9).我們計(jì)算了甲烷在這些鍶摻雜氧化鑭團(tuán)簇上Sr-O和La-O酸堿對(duì)位點(diǎn)以及氧自由基活性位點(diǎn)上的活化機(jī)制,以研究鍶摻雜對(duì)OCM反應(yīng)活性的影響,并與我們前期計(jì)算的純氧化鑭團(tuán)簇上甲烷活化性能進(jìn)行了對(duì)比.通過計(jì)算甲烷在不同鍶摻雜氧化鑭團(tuán)簇上的物理和化學(xué)吸附能、活化能壘以及甲基自由基的脫附能,發(fā)現(xiàn)鍶摻雜氧化鑭團(tuán)簇上的甲烷活化在熱力學(xué)和動(dòng)力學(xué)上都要比純氧化鑭團(tuán)簇上更為有利.對(duì)于具有相同金屬原子數(shù)目的團(tuán)簇,甲烷在La-O上活化的能壘大小為:化學(xué)計(jì)量比的La-Sr-O團(tuán)簇非化學(xué)計(jì)量比的La-Sr-O團(tuán)簇化學(xué)計(jì)量比的La-O團(tuán)簇;而甲烷在Sr-O上活化的能壘大小依次是:化學(xué)計(jì)量比的La-Sr-O團(tuán)簇非化學(xué)計(jì)量比的La-Sr-O團(tuán)簇.給定一個(gè)鍶摻雜氧化鑭團(tuán)簇,甲烷在不同活化位點(diǎn)上的活化能壘大小通常是:O·Sr-OLa-O,其中無論何種性質(zhì)的鍶摻雜氧化鑭團(tuán)簇,甲烷在Sr-O上的反應(yīng)活性要高于La-O上的,而對(duì)于具有自由基特征的鍶摻雜氧化鑭團(tuán)簇,甲烷更容易在氧自由基位點(diǎn)上發(fā)生解離.此外,對(duì)于沒有自由基特征的鍶摻雜氧化鑭團(tuán)簇,甲基自由基的脫附如同純氧化鑭團(tuán)簇一樣是強(qiáng)吸熱過程.相反,對(duì)于具有自由基特性的鍶摻雜氧化鑭團(tuán)簇,甲基自由基的脫附則十分容易.由此可見,鍶摻雜促進(jìn)氧化鑭催化劑上OCM反應(yīng)活性主要有以下兩個(gè)原因:(1)通過摻雜可以提供具有自由基特性的氧活性位點(diǎn),(2)對(duì)于非自由基性質(zhì)的團(tuán)簇,可以增強(qiáng)金屬.氧對(duì)位點(diǎn)的堿性和甲烷反應(yīng)活性,從而有效降低了甲烷的活化能壘和甲基自由基的脫附能.
[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.
【作者單位】： 中國科學(xué)院上海高等研究院低碳轉(zhuǎn)化科學(xué)與工程中心;中國科學(xué)院大學(xué);上�？萍即髮W(xué)物質(zhì)科學(xué)與技術(shù)學(xué)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)~~
【分類號(hào)】：O621.251

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