Brønsted酸性與裂解反應性之探討：中孔結構與 骨架外鋁的影響

Abstract

Y型沸石經過水熱處理或脫鋁等後處理程序後，常伴隨中孔結構（mesoporosity）與骨架外鋁物種（extra-framework aluminum, EFAL）的形成，這些結構上的改變對流體化催化裂解（fluid catalytic cracking, FCC）的催化性能具有顯著影響。然而，它們對Brønsted酸性與本徵催化活性(intrinsic catalytic activity)所造成的個別效應仍未被充分理解。本研究運用密度泛函理論（density functional theory, DFT）以及混合量子力學/分子力學（QM/MM）方法，系統性探討中孔與骨架外鋁對FAU型沸石酸強度與裂解反應活性的影響。我們建立三種代表性模型：傳統微孔HY、含骨架外鋁的HY，以及具中孔結構的HY。質子親和力計算顯示，中孔洞的形成會略微削弱Brønsted酸性，而骨架外鋁的影響則依其類型與構型而異——其中Al(OH)₂⁺會增強酸性，而Al(OH)₃則呈現多樣化效應。為進一步釐清中孔的影響，我們模擬n-hexane與1,3,5-triisopropylbenzene（1,3,5-TIPB）在HY與meso-HY模型上的裂解反應。結果顯示，中孔對小分子n-hexane的反應能量影響極小；然而對於體積較大的1,3,5-TIPB，meso-HY模型顯示較高的反應自由能，主因在於其骨架作用力減弱。此研究提供了結構與組成修飾如何影響酸性與催化功能的分子層級探討，並可作為設計多層化沸石催化劑(hierarchical zeolite catalysts)的參考依據。

Mesoporosity and extra-framework aluminum (EFAL) species often form during post-treatment of Y-type zeolites, such as steaming or dealumination, and can significantly influence catalytic performance in fluid catalytic cracking (FCC). However, their individual effects on Brønsted acidity and intrinsic catalytic activity remain incompletely understood. In this study, we employ density functional theory (DFT) and hybrid quantum mechanical/molecular mechanical (QM/MM) methods to systematically investigate the influence of mesopores and EFAL on acid strength and cracking reactivity in FAU-type zeolites. Three representative models were constructed: a conventional microporous HY, an EFAL-containing HY, and a mesoporous HY. Proton affinity calculations reveal that mesopore formation slightly weakens Brønsted acidity, while the impact of EFAL depends on its type and configuration—Al(OH)₂⁺ enhances acidity, whereas Al(OH)₃ exhibits variable effects. To isolate the effect of mesoporosity, we simulate the cracking of n-hexane and 1,3,5-triisopropylbenzene (1,3,5-TIPB) on HY and meso-HY models. For n-hexane, mesoporosity has minimal influence on reaction energetics. In contrast, for bulky molecules such as 1,3,5-TIPB, higher free energy profiles are observed in meso-HY due to reduced framework interactions. These results provide molecular-level insight into how structural and compositional modifications affect acidity and catalytic function, offering guidance for the design of hierarchical zeolite catalysts.