ABSTRACT

We have theoretically studied the hydrodesulphurisation (HDS) mechanism of thiophene and its hydrogenated derivatives over sulphided Co–Mo catalysts supported by four zeolites (ZSM-5, FAU, Beta, MCM-22) by a two-layer ONIOM (our Own N-layered Integrated molecular Orbital and molecular Mechanics) method. Due to low energy barriers, the thiophene can be hydrogenated to 2,3-dihydrothiophene (2,3-DHT), 2,5-dihydrothiophene (2,5-DHT) and tetrahydrothiophene (THT). Direct desulphurisation (DDS) and hydrogenolysis desulphurisation (HYD) pathways were investigated in detail. The calculated results show that in DDS mechanism of thiophene, the rate-determining step is the C–S bond cleavage. In HYD mechanisms of 2,5-DHT, 2,3-DHT and THT, the rate-determining step is hydrogen transfer on ZSM-5, FAU and Beta zeolites except MCM-22 zeolite. The HYD of 2,3-DHT and THT should occur by the stepwise pathway. But for the 2,5-DHT, the concerted pathway is probably more favourable than the stepwise pathway. The reduced density gradient (RDG), differential charge density (DCD), and local orbital locator (LOL) maps show that for all transition states (TSs), there are complicated van der Waals (VDW) and electrostatic interactions between the organic fragment and the catalytic centre.

摘要

我们从理论上研究了噻吩及其氢化衍生物在四种沸石（ZSM-5、FAU、Beta、MCM-22）和双层 ONIOM（我们自己的 N-分层综合分子轨道和分子力学）方法。由于能垒低，噻吩可以氢化为 2,3-二氢噻吩 (2,3-DHT)、2,5-二氢噻吩 (2,5-DHT) 和四氢噻吩 (THT)。详细研究了直接脱硫 (DDS) 和氢解脱硫 (HYD) 途径。计算结果表明，在噻吩的DDS机理中，决速步骤是C-S键断裂。在2,5-DHT、2,3-DHT和THT的HYD机理中，除MCM-22沸石外，在ZSM-5、FAU和Beta沸石上的控速步骤是氢转移。2 的 HYD，3-DHT 和 THT 应该通过逐步途径发生。但对于 2,5-DHT，协同途径可能比逐步途径更有利。约化密度梯度 (RDG)、微分电荷密度 (DCD) 和局域轨道定位器 (LOL) 图表明，对于所有过渡态 (TS)，有机片段和有机片段之间存在复杂的范德华力 (VDW) 和静电相互作用催化中心。