Bhan Aditya, Iglesia Enrique
Department of Chemical Engineering, University of California at Berkeley, Berkeley, California 94720, USA.
Acc Chem Res. 2008 Apr;41(4):559-67. doi: 10.1021/ar700181t. Epub 2008 Feb 16.
The extent to which spatial constraints influence rates and pathways in catalysis depends on the structure of intermediates, transition states, and active sites involved. We aim to answer, as we seek insights into catalytic mechanisms and site requirements, persistent questions about the potential for controlling rates and selectivities by rational design of spatial constraints around active sites within inorganic structures useful as catalysts. This Account addresses these matters for the specific case of reactions on zeolites that contain Brønsted acid sites encapsulated within subnanometer channels. We compare and contrast here the effects of local zeolite structure on the dynamics of the carbonylation of surface methyl groups and of the isotopic exchange of CD4 with surface OH groups on zeolites. Methyl and hydroxyl groups are the smallest monovalent cations relevant in catalysis by zeolites. Their small size, taken together with their inability to desorb except via reactions with other species, allowed us to discriminate between stabilization of cationic transition states and stabilization of adsorbed reactants and products by spatial constraints. We show that apparent effects of proton density and of zeolite channel structure on dimethyl ether carbonylation turnover rates reflect instead the remarkable specificity of eight-membered ring zeolite channels in accelerating kinetically relevant steps that form *COCH3 species via CO insertion into methyl groups. This specificity reflects the selective stabilization of cationic transition states via interactions with framework oxygen anions. These findings for carbonylation catalysts contrast sharply the weak effects of channel structure on the rate of exchange of CD4 with OH groups. This latter reaction involves concerted symmetric transition states with much lower charge than that required for CH3 carbonylation. Our Account extends the scope of shape selectivity concepts beyond those reflecting size exclusion and preferential adsorption. Our ability to discriminate among various effects of spatial constraints depends critically on dissecting chemical conversions into elementary steps of kinetic relevance and on eliminating secondary reactions and accounting for the concomitant effects of zeolite structure on the stability of adsorbed reactants and intermediates.
空间限制对催化速率和路径的影响程度取决于所涉及的中间体、过渡态和活性位点的结构。我们旨在回答一些长期存在的问题，即通过合理设计用作催化剂的无机结构中活性位点周围的空间限制来控制反应速率和选择性的可能性，以此深入了解催化机理和位点要求。本综述针对含有封装在亚纳米通道内的布朗斯台德酸位点的沸石上的反应这一具体情况来探讨这些问题。我们在此比较并对比了局部沸石结构对沸石表面甲基羰基化动力学以及CD4与沸石表面OH基团同位素交换动力学的影响。甲基和羟基是沸石催化中相关的最小单价阳离子。它们的尺寸小，且除了通过与其他物种反应外无法解吸，这使我们能够区分阳离子过渡态的稳定化以及空间限制对吸附反应物和产物的稳定化作用。我们表明，质子密度和沸石通道结构对二甲醚羰基化转化率的明显影响，实际上反映了八元环沸石通道在加速通过CO插入甲基形成*COCH3物种的动力学相关步骤方面的显著特异性。这种特异性反映了通过与骨架氧阴离子相互作用对阳离子过渡态的选择性稳定。这些羰基化催化剂的研究结果与通道结构对CD4与OH基团交换速率的微弱影响形成鲜明对比。后一反应涉及电荷比CH3羰基化所需电荷低得多的协同对称过渡态。本综述将形状选择性概念的范围扩展到了超出反映尺寸排阻和优先吸附的概念。我们区分空间限制各种影响的能力关键取决于将化学转化分解为动力学相关的基本步骤，消除副反应，并考虑沸石结构对吸附反应物和中间体稳定性的伴随影响。
