Mechanistic origins of the high-pressure inhibition of methanol dehydration rates in small-pore acidic zeolites
Mechanistic origins of the high-pressure inhibition of methanol dehydration rates in small-pore acidic zeolites
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DOI：
10.1016/j.jcat.2019.10.012
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发表时间：
2019-12-01
影响因子：
7.3
通讯作者：
Gounder, Rajamani
中科院分区：
文献类型：
--
作者：
Di Iorio, John R.;Hoffman, Alexander J.;Gounder, Rajamani
在高甲醇压力（>10 kPa，415 K）下，Bronsted酸催化甲醇脱水生成二甲醚的转换速率在小孔沸石（CHA，AEI，LTA，LEV）上受到抑制，而与骨架Al及其伴随的H+位置的分布无关，但在中孔或大孔沸石上则不受抑制。高压动力学抑制伴随着高阶甲醇簇（例如，三聚体，四聚体）实验观察到的物理吸附的液体状甲醇和振动模式的甲醇簇在红外光谱中的外观，与衰减的这种抑制在较高的温度（>450 K），导致降低甲醇覆盖。DFT预测的甲醇覆盖相图证实，高阶甲醇簇形成的压力和温度范围对应的实验观察到的动力学抑制的发病，和高阶甲醇簇是反应性的，但过量的甲醇增加了表观障碍，形成动力学相关的过渡态，消除二甲醚，从而抑制周转率。这种结合的实验和理论研究提供了精确的机理解释的高压抑制甲醇脱水周转率的小孔布朗斯台德酸沸石。这种严格的分析，使动力学模型的发展，以考虑不同的结构的甲醇前体，甲醇转化形成二甲醚，和方法，以评估的流行程度较高的顺序集群作为反应性和抑制性的中间体内的小孔沸石在甲醇转化。(C)2019爱思唯尔公司All rights reserved.
Turnover rates of Bronsted acid-catalyzed methanol dehydration to dimethyl ether become inhibited at high methanol pressures (>10 kPa, 415 K) on small-pore zeolites (CHA, AEI, LTA, LEV), irrespective of the distribution of framework Al and their attendant H+ sites, but not on medium-pore or large-pore zeolites. High-pressure kinetic inhibition occurs concomitantly with the stabilization of higher-order methanol clusters (e.g., trimers, tetramers) observed experimentally by physisorption of liquid-like methanol and the appearance of vibrational modes for methanol clusters in IR spectra, consistent with the attenuation of such inhibition at higher temperatures (>450 K) that result in decreased methanol coverage. DFT-predicted methanol coverage phase diagrams confirm that higher-order methanol clusters form in pressure and temperature ranges corresponding to the onset of kinetic inhibition observed experimentally, and that higher-order methanol clusters are reactive but that excess methanol increases the apparent barriers to form kinetically relevant transition states that eliminate dimethyl ether and thus inhibit turnover rates. This combined experimental and theoretical investigation provides precise mechanistic interpretation of the high-pressure inhibition of methanol dehydration turnover rates on small-pore Bronsted acid zeolites. This rigorous analysis enables the development of kinetic models to account for the diverse structures of methanol precursors that dehydrate to form dimethyl ether, and methods to assess the prevalence of higher-order clusters that serve as reactive and inhibitory intermediates within small-pore zeolites during methanol conversion. (C) 2019 Elsevier Inc. All rights reserved.