Implications of Transition State Confinement within Small Voids for Acid Catalysis
Implications of Transition State Confinement within Small Voids for Acid Catalysis
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DOI：
10.1021/jp5050095
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发表时间：
2014-08-07
影响因子：
3.7
通讯作者：
Iglesia, Enrique
中科院分区：
文献类型：
--
作者：
Jones, Andrew J.;Zones, Stacey I.;Iglesia, Enrique
微孔铝硅酸盐的催化多样性反映了它们将过渡态限制在分子尺寸的晶内空隙内的独特能力和充当布朗斯台德酸的质子的数量（但不是强度）。甲醇转化为二甲醚的一级反应速率常数反映了相对于气态和氢键键合的甲醇分子的过渡态能量;在沸石上，对于不同的空隙尺寸和形状以及质子位置，这些常数指数地依赖于正己烷的物理吸附能，表明过渡态的货车德瓦尔斯稳定化导致它们不同的反应性，而不伴随空隙结构或质子位置对酸强度的影响。分散的DME，相关的过渡态的形状和大小的代理的吸附量的贡献，计算使用密度泛函理论和Lennard-Jones相互作用FAU，SFH，CRYSTAL，莫尔，MTW，MFI，和MTT沸石和平均在所有质子位置;一阶速率常数也依赖于这些吸附量指数。与此相反，零级速率常数，这反映了相对于质子化的CH 3OH二聚体的大小相似的过渡态的稳定性，弱依赖于分散稳定，无论是从实验或模拟测量，因为分散力的影响物种大小相似，以相同的程度。这些结果，放在一起，证明了卓越的效果，限制沸石的反应性和方式，通过该方式，在不同的晶内环境中举行的质子周围的局部空隙引起的独特的行为，使沸石无处不在的催化实践。相关过渡态的焓稳定优于在低温下限制所引起的熵损失，这让人想起催化口袋和溶剂在分子或酶催化中是如何做到的。
The catalytic diversity of microporous aluminosilicates reflects their unique ability to confine transition states within intracrystalline voids of molecular dimensions and the number (but not the strength) of the protons that act as Bronsted acids. First-order rate constants for CH3OH conversion to dimethyl ether (DME) reflect the energy of transition states relative to those for gaseous and H-bonded CH3OH molecules; on zeolites, these constants depend exponentially on n-hexane physisorption energies for different void size and shape and proton location, indicating that van der Waals stabilization of transition states causes their different reactivity, without concomitant effects of void structure or proton location on acid strength. The dispersive contribution to adsorption enthalpies of DME, a proxy in shape and size for relevant transition states, was calculated using density functional theory and Lennard-Jones interactions on FAU, SFH, BEA, MOR, MTW, MFI, and MTT zeolites and averaged over all proton locations; first-order rate constants also depended exponentially on these enthalpies. In contrast, zero-order rate constants, which reflect the stability of transition states relative to protonated CH3OH dimers similar in size, depended weakly on dispersive stabilization, whether measured from experiment or simulations, because dispersive forces influence species similar in size to the same extent. These results, taken together, demonstrate the preeminent effects of confinement on zeolite reactivity and the manner by which the local voids around protons held within diverse intracrystalline environments give rise to the unique behaviors that have made zeolites ubiquitous in the practice of catalysis. Enthalpic stabilization of relevant transition states prevail over entropic losses caused by confinement at low temperatures in a manner reminiscent of how catalytic pockets and solvents do so in catalysis by molecules or enzymes.