Janda Amber, Vlaisavljevich Bess, Lin Li-Chiang, Smit Berend, Bell Alexis T
Department of Chemical and Biomolecular Engineering, University of California-Berkeley , Berkeley, California 94720, United States.
Department of Process and Energy, Delft University of Technology , Leeghwaterstraat 39, 2628CB Delft, The Netherlands.
J Am Chem Soc. 2016 Apr 13;138(14):4739-56. doi: 10.1021/jacs.5b11355. Epub 2016 Mar 29.
The effects of zeolite structure on the kinetics of n-butane monomolecular cracking and dehydrogenation are investigated for eight zeolites differing in the topology of channels and cages. Monte Carlo simulations are used to calculate enthalpy and entropy changes for adsorption (ΔHads-H+ and ΔSads-H+) of gas-phase alkanes onto Brønsted protons. These parameters are used to extract intrinsic activation enthalpies (ΔHint‡), entropies (ΔSint‡), and rate coefficients (kint) from measured data. As ΔSads-H+ decreases (i.e., as confinement increases), ΔHint‡ and ΔSint‡ for terminal cracking and dehydrogenation decrease for a given channel topology. These results, together with positive values observed for ΔSint‡, indicate that the transition states for these reactions resemble products. For central cracking (an earlier transition state), ΔHint‡ is relatively constant, while ΔSint‡ increases as ΔSads-H+ decreases because less entropy is lost upon protonation of the alkane. Concurrently, selectivities to terminal cracking and dehydrogenation decrease relative to central cracking because ΔSint‡ decreases for the former reactions. Depending on channel topology, changes in the measured rate coefficients (kapp) with confinement are driven by changes in kint or by changes in the adsorption equilibrium constant (Kads-H+). Values of ΔSint‡ and ΔHint‡ are positively correlated, consistent with weaker interactions between the zeolite and transition state and with the greater freedom of movement of product fragments within more spacious pores. These results differ from earlier reports that ΔHint‡ and ΔSint‡ are structure-insensitive and that kapp is dominated by Kads-H+. They also suggest that ΔSads-H+ is a meaningful descriptor of confinement for zeolites having similar channel topologies.
研究了八种具有不同孔道和笼拓扑结构的沸石对正丁烷单分子裂化和脱氢动力学的影响。采用蒙特卡罗模拟计算气相烷烃在布朗斯台德质子上吸附的焓变和熵变（ΔHads-H+和ΔSads-H+）。利用这些参数从测量数据中提取本征活化焓（ΔHint‡）、熵（ΔSint‡）和速率系数（kint）。随着ΔSads-H+减小（即限制增加），对于给定的孔道拓扑结构，末端裂化和脱氢的ΔHint‡和ΔSint‡减小。这些结果与ΔSint‡的正值一起表明，这些反应的过渡态类似于产物。对于中心裂化（较早的过渡态），ΔHint‡相对恒定，而随着ΔSads-H+减小，ΔSint‡增加，因为烷烃质子化时熵损失较少。同时，相对于中心裂化，末端裂化和脱氢的选择性降低，因为前一个反应的ΔSint‡减小。根据孔道拓扑结构，测量的速率系数（kapp）随限制的变化是由kint的变化或吸附平衡常数（Kads-H+）的变化驱动的。ΔSint‡和ΔHint‡的值呈正相关，这与沸石和过渡态之间较弱的相互作用以及产物碎片在更宽敞孔内更大的运动自由度一致。这些结果与早期报道不同，早期报道称ΔHint‡和ΔSint‡对结构不敏感，且kapp由Kads-H+主导。它们还表明，对于具有相似孔道拓扑结构的沸石，ΔSads-H+是限制的一个有意义的描述符。
