Bhan Aditya, Allian Ayman D, Sunley Glenn J, Law David J, Iglesia Enrique
Department of Chemical Engineering, University of California at Berkeley, Berkeley, California 94720, USA.
J Am Chem Soc. 2007 Apr 25;129(16):4919-24. doi: 10.1021/ja070094d. Epub 2007 Mar 31.
The acid-catalyzed formation of carbon-carbon bonds from C1 precursors via CO insertion into chemisorbed methyl groups occurs selectively within eight-membered ring (8-MR) zeolite channels. This elementary step controls catalytic carbonylation rates of dimethyl ether (DME) to methyl acetate. The number of O-H groups within 8-MR channels was measured by rigorous deconvolution of the infrared bands for O-H groups in cation-exchanged and acid forms of mordenite (M,H-MOR) and ferrierite (H-FER) after adsorption of basic probe molecules of varying size. DME carbonylation rates are proportional to the number of O-H groups within 8-MR channels. Na+ cations selectively replaced protons within 8-MR channels and led to a disproportionate decrease in carbonylation turnover rates (per total H+). These conclusions are consistent with the low or undetectable rates of carbonylation on zeolites without 8-MR channels (H-BEA, H-FAU, H-MFI). Such specificity of methyl reactivity upon confinement within small channels appears to be unprecedented in catalysis by microporous solids, which typically select reactions by size exclusion of bulkier transition states.
通过将CO插入化学吸附的甲基中，由C1前体酸催化形成碳 - 碳键的过程选择性地发生在八元环（8-MR）沸石通道内。这一基本步骤控制了二甲醚（DME）羰基化生成乙酸甲酯的催化速率。在吸附了不同大小的碱性探针分子后，通过对丝光沸石（M，H-MOR）和镁碱沸石（H-FER）阳离子交换形式和酸形式中O-H基团的红外波段进行严格的去卷积，测量了8-MR通道内O-H基团的数量。DME羰基化速率与8-MR通道内O-H基团的数量成正比。Na+阳离子选择性地取代了8-MR通道内的质子，并导致羰基化周转速率（每总H+）不成比例地降低。这些结论与没有8-MR通道的沸石（H-BEA、H-FAU、H-MFI）上羰基化速率低或无法检测一致。在小通道内受限的情况下，甲基反应性的这种特异性在微孔固体催化中似乎是前所未有的，微孔固体通常通过排除较大过渡态的尺寸来选择反应。
