Consequences of acid strength for isomerization and elimination catalysis on solid acids.
Consequences of acid strength for isomerization and elimination catalysis on solid acids.
复制标题
DOI：
10.1021/ja900829x
复制
发表时间：
2009-04
影响因子：
15
通讯作者：
Josef Macht;R. Carr;E. Iglesia
中科院分区：
文献类型：
--
作者：
Josef Macht;R. Carr;E. Iglesia
我们在此讨论酸催化作用感知固体酸强度的方式。Keggin聚氧乙烯酸盐（POM）簇和沸石的酸强度，因为它们的准确已知的结构，严格描述了它们的去质子化能（DPE）。烷烃异构化和烷醇脱水的测量动力学的机械解释被用来获得中间体和过渡态的速率和平衡常数和能量，并将它们与酸强度。正己烷异构化速率受到烷氧基中间体在双官能金属-酸混合物上的异构化的限制，所述双官能金属-酸混合物旨在维持烷烃-烯烃平衡。异构化速率常数通过在催化过程中用2，6-二叔丁基吡啶滴定测量的可接近质子的数目归一化。正己烯质子化形成的醇盐的平衡常数随去质子化能（DPE）的增加而略有增加，而异构化速率常数随DPE的增加而降低，活化能随DPE的增加而增加，这也与烷醇脱水反应的结果一致。这些趋势与异构化和消除步骤中所涉及的过渡态的热化学分析是一致的。对于所有的反应，屏障增加小于在DPE的伴随增加组合物的变化时，因为在相关的过渡态的离子对的静电稳定变得更有效的弱酸，作为其较高的电荷密度的结果在阴离子共轭碱。烷氧基异构化障碍更敏感的DPE比消除从H-键合的烷醇，步骤，限制2-丁醇和1-丁醇脱水速率;后两种反应显示出相似的DPE敏感性，尽管它们的速率和活化屏障存在显著差异，这表明较慢的反应不一定对酸强度更敏感，而是反映了更不稳定的有机阳离子在其过渡态的参与。静电稳定的这些补偿效应取决于这些有机阳离子中的电荷密度与去除的质子中的电荷密度有多相似。具有更多局部电荷的阳离子有利于与阴离子的强静电相互作用，并且比具有更多扩散电荷的阳离子形成更稳定的离子结构。消除过渡态的离子对含有在sp（2）碳上比异构化过渡态具有更高局部电荷密度的阳离子;因此，这些离子对回收了更大部分的去质子化能量，因此，它们的反应变得不那么敏感酸强度。这些概念使我们得出这样的结论，催化反应的能量困难，所施加的气相反应物的质子亲和力在过渡态类似物，并不决定其敏感性的固体催化剂的酸强度。
We address here the manner in which acid catalysis senses the strength of solid acids. Acid strengths for Keggin polyoxometalate (POM) clusters and zeolites, chosen because of their accurately known structures, are described rigorously by their deprotonation energies (DPE). Mechanistic interpretations of the measured dynamics of alkane isomerization and alkanol dehydration are used to obtain rate and equilibrium constants and energies for intermediates and transition states and to relate them to acid strength. n-Hexane isomerization rates were limited by isomerization of alkoxide intermediates on bifunctional metal-acid mixtures designed to maintain alkane-alkene equilibrium. Isomerization rate constants were normalized by the number of accessible protons, measured by titration with 2,6-di-tert-butylpyridine during catalysis. Equilibrium constants for alkoxides formed by protonation of n-hexene increased slightly with deprotonation energies (DPE), while isomerization rate constants decreased and activation barriers increased with increasing DPE, as also shown for alkanol dehydration reactions. These trends are consistent with thermochemical analyses of the transition states involved in isomerization and elimination steps. For all reactions, barriers increased by less than the concomitant increase in DPE upon changes in composition, because electrostatic stabilization of ion-pairs at the relevant transition states becomes more effective for weaker acids, as a result of their higher charge density at the anionic conjugate base. Alkoxide isomerization barriers were more sensitive to DPE than for elimination from H-bonded alkanols, the step that limits 2-butanol and 1-butanol dehydration rates; the latter two reactions showed similar DPE sensitivities, despite significant differences in their rates and activation barriers, indicating that slower reactions are not necessarily more sensitive to acid strength, but instead reflect the involvement of more unstable organic cations at their transition states. These compensating effects from electrostatic stabilization depend on how similar the charge density in these organic cations is to that in the proton removed. Cations with more localized charge favor strong electrostatic interactions with anions and form more stable ionic structures than do cations with more diffuse charges. Ion-pairs at elimination transition states contain cations with higher local charge density at the sp(2) carbon than for isomerization transition states; as a result, these ion-pairs recover a larger fraction of the deprotonation energy, and, consequently, their reactions become less sensitive to acid strength. These concepts lead us to conclude that the energetic difficulty of a catalytic reaction, imposed by gas-phase reactant proton affinities in transition state analogues, does not determine its sensitivity to the acid strength of solid catalysts.