Reactivity of Pentamethylcyclopentenyl Cations toward Olefin Formation in the Methanol-to-Olefin (MTO) Process

Polymethylcyclopentenyl cations are frequently observed experimentally in zeolite catalysis but are investigated to a lesser extent than aromatics or acyclic olefins. Here, the reactivity of the pentamethylcyclopentenyl cation (pentaMCP+) for the formation of olefins in H-SSZ-13 is investigated by using density functional theory (DFT). We find that pentaMCP+ can react similarly to aromatics in the side-chain mechanism with a selectivity for ethylene. The selectivity is due to a specific transition state that favors the elimination of ethylene over the further methylation steps. Generally, our calculations indicate that methylation of the unsaturated side chain is rate-limiting, as also observed for the aromatic cycle. This is because the formation of the neutral species with an unsaturated side chain from the more stable cationic resting state of the hydrocarbon is already unfavorable and adds to the intrinsic barrier for methylation. To estimate the catalytic activity, microkinetic modeling is performed based on the computed Gibbs free energies. Analogous to the aromatic cycle, we find that a second accessible acid site can lower the overall barriers by making stepwise methylation more favorable. Based on our findings, we suggest that there is a third catalytic cycle operating during MTO conversion that is located between the olefin and aromatic cycle and produces primarily ethylene with overall free energy barriers comparable to those of the olefin and aromatic cycle.

聚甲基环戊烯正离子（polymethylcyclopentenyl cations）在沸石催化实验中被频繁观测到，但相较于芳烃与无环烯烃，其相关研究仍较为匮乏。本研究通过密度泛函理论（DFT），探究了氢型SSZ-13分子筛中五甲基环戊烯正离子（pentaMCP+）生成烯烃的反应活性。研究发现，五甲基环戊烯正离子可通过侧链机理发生反应，反应路径与芳烃类似，且对乙烯具有较高选择性。该选择性源于特定的过渡态：相较于后续甲基化步骤，该过渡态更有利于乙烯的消去反应。总体而言，计算结果表明，不饱和侧链的甲基化反应为限速步骤，这一结论与芳烃循环的相关研究结果一致。究其原因，从烃类更稳定的阳离子驻态生成带有不饱和侧链的中性物种本身就存在热力学劣势，进一步提升了甲基化反应的本征能垒。为评估催化活性，本研究基于计算得到的吉布斯自由能（Gibbs free energies）开展了微观动力学建模。与芳烃循环类似，研究发现第二个可及的酸性位点可通过使逐步甲基化过程更易进行，从而降低总反应能垒。基于上述研究结果，我们提出在甲醇制烯烃（Methanol to Olefins, MTO）转化过程中存在第三条催化循环：该循环介于烯烃循环与芳烃循环之间，主要产物为乙烯，其总吉布斯自由能能垒与烯烃循环及芳烃循环相当。