使用非耦合近似方法模擬反應中非諧振性及其在沸石系統中的應用

Abstract

沸石已被廣泛應用於許多不同的領域，尤其是在石油精煉工業中。對具有不同孔隙結構和組成的沸石進行催化性能預測將具有很高的價值。然而，傳統的諧振子模型（Harmonic oscillator，HO）常常無法提供準確的熵或動力學性質計算，因為它忽略了非諧振性。為了解決這些問題，我們使用了考慮獨立一維非谐位能曲面總和的非耦合模式（Uncoupled mode，UM）方法。我們廣泛研究了包括UM-N、UM-VT、UM-T、E-optimized和E'-optimized等五種不同的位能曲面近似方法，發現UM-VT和UM-T在預測氣相分子系統的熱力學和動力學性質和實驗值有很好的一致性。我們還檢查了內座標系統的影響，發現相對於常見的內坐標（Redundant internal coordinate，RIC），使用移動-轉動-內坐標（Translation-rotation-internal coordinate，TRIC）系統和混合內坐標（Hybrid internal coordinate，HIC）系統可以更正確地計算動力學性質。將TRIC內部座標搭配UM方法應用於H-MFI沸石上的烷烴裂解反應，可以略微提高對內在活化熵（Intrinsic activation entropy）的估計，但還需要進一步的改進才能達到化學準確性。

Zeolites have been widely applied to many different fields, especially in oil refining industry. Prediction of catalytic performance for zeolites with different pore topologies and structure formations would be highly valuable. However, the conventional harmonic oscillator model often fails to provide accurate entropy or kinetic properties calculations due to the ignorance of anharmonic effects. To address these issues, we employed uncoupled mode (UM) methods that consider the sum of independent one-dimensional anharmonic potential energy surfaces. We extensively investigated five different potential energy surface sampling schemes including UM-N, UM-VT, UM-T, E-optimized, E’-optimized and found that UM-VT, and UM-T perform well in predicting thermodynamic and kinetic properties in gas phase molecular systems. We also examined the impact of internal coordinate systems, finding that translation-rotation-internal coordinate (TRIC) system and hybrid internal coordinate (HIC) system outperform redundant internal coordinates (RIC) for kinetic property calculations. Applying UM methods with TRIC internal coordinates to alkane cracking reactions over H-MFI zeolite showed slight improvements in estimating intrinsic activation entropy, but further refinements are necessary to achieve chemical accuracy.