The catalytic diversity of zeolites: confinement and solvation effects within voids of molecular dimensions
The catalytic diversity of zeolites: confinement and solvation effects within voids of molecular dimensions
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DOI：
10.1039/c3cc40731d
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发表时间：
2013-01-01
影响因子：
4.9
通讯作者：
Iglesia, Enrique
中科院分区：
文献类型：
--
作者：
Gounder, Rajamani;Iglesia, Enrique
分子筛控制某些反应物和产物的进入和排出以及优先含有某些过渡态而基于尺寸排除其他过渡态的能力在沸石催化历史的早期被捕获为形状选择性概念。反应性和选择性的显著后果，特别是在酸催化中，已经激发和维持了许多新的硅酸盐框架的发现，并推动了分级结构和空隙尺寸的工程化以影响催化。微孔空隙的催化多样性的探索和扩展在这里的上下文中，其溶剂化的环境，其中空隙作为主机和稳定的客人，无论是反应中间体或过渡态，通过货车的范德华力。我们使用酸催化的具体例子，包括烷烃中C-C和C-H键的活化，烯烃的烷基化和氢化，二甲醚的羰基化，以及烷醇和醚的消除和同系化反应，这些反应涉及过渡态和不同大小和组成的吸附前体。测量周转率的机械解释，使我们能够分配精确的化学起源速率方程中的动力学和热力学常数，反过来，以确定具体的步骤和中间体，确定负责化学反应性和选择性的自由能差异。这些自由能差异反映了过渡态及其相关前体通过依赖于酸强度的静电相互作用和依赖于空隙内限制的货车范德华相互作用的稳定性。通过考虑合理的过渡态和相关的前体，由Born-Haber热化学循环研究它们各自对活化自由能的贡献。这些实施例表明，沸石空隙不同地溶剂化过渡态和前体，并且对于不同尺寸和化学组成的客体部分明显如此，从而使得给定尺寸和形状的空隙能够为给定的基本步骤提供“合适的配合”，定义为使活化的吉布斯自由能最小化。较紧的约束在低温下是优选的，因为熵增益优于伴随的熵损失，而较松的配合在高温下是有利的，因为熵增益抵消了熵稳定的损失。通过货车德瓦尔斯力的限制和溶剂化不直接涉及强化学键的形成或断裂;然而，它们赋予沸石显著的多样性，尽管它们具有结构刚性和它们的常见铝硅酸盐组成。单一沸石本身可以包含一系列局部空隙环境，每一个具有不同的反应性和选择性;因此，改变给定骨架内这些位置之间的质子分布或通过空隙空间的部分闭塞来改变给定位置可以扩展沸石的催化机会范围。结合精确描述沸石空隙和受限客体之间的货车德瓦尔斯相互作用的理论工具，以及将质子或空间填充部分放置在特定位置的合成方案，这些概念有望扩大微孔固体已经显示出的显著影响和催化多样性。
The ability of molecular sieves to control the access and egress of certain reactants and products and to preferentially contain certain transition states while excluding others based on size were captured as shape selectivity concepts early in the history of zeolite catalysis. The marked consequences for reactivity and selectivity, specifically in acid catalysis, have since inspired and sustained many discoveries of novel silicate frameworks and driven the engineering of hierarchical structures and void size to influence catalysis. The catalytic diversity of microporous voids is explored and extended here in the context of their solvating environments, wherein voids act as hosts and stabilize guests, whether reactive intermediates or transition states, by van der Waals forces. We use specific examples from acid catalysis, including activation of C-C and C-H bonds in alkanes, alkylation and hydrogenation of alkenes, carbonylation of dimethyl ether, and elimination and homologation reactions of alkanols and ethers, which involve transition states and adsorbed precursors of varying size and composition. Mechanistic interpretations of measured turnover rates enable us to assign precise chemical origins to kinetic and thermodynamic constants in rate equations and, in turn, to identify specific steps and intermediates that determine the free energy differences responsible for chemical reactivity and selectivity. These free energy differences reflect the stabilization of transition states and their relevant precursors via electrostatic interactions that depend on acid strength and van der Waals interactions that depend on confinement within voids. Their respective contributions to activation free energies are examined by Born-Haber thermochemical cycles by considering plausible transition states and the relevant precursors. These examples show that zeolite voids solvate transition states and precursors differently, and markedly so for guest moieties of different size and chemical composition, thus enabling voids of a given size and shape to provide the "right fit" for a given elementary step, defined as that which minimizes Gibbs free energies of activation. Tighter confinement is preferred at low temperatures because enthalpic gains prevail over concomitant entropic losses, while looser fits are favored at high temperatures because entropy gains offset losses in enthalpic stabilization. Confinement and solvation by van der Waals forces are not directly involved in the making or breaking of strong chemical bonds; yet, they confer remarkable diversity to zeolites, in spite of their structural rigidity and their common aluminosilicate composition. A single zeolite can itself contain a range of local void environments, each with distinct reactivity and selectivity; as a result, varying the distribution of protons among these locations within a given framework or modifying a given location by partial occlusion of the void space can extend the range of catalytic opportunities for zeolites. Taken together with theoretical tools that accurately describe van der Waals interactions between zeolite voids and confined guests and with synthetic protocols that place protons or space-filling moieties at specific locations, these concepts promise to broaden the significant impact and catalytic diversity already shown by microporous solids.