Department of Chemical Engineering and Materials Science , University of Minnesota , Minneapolis , Minnesota 55455 , United States.
Acc Chem Res. 2019 Sep 17;52(9):2647-2656. doi: 10.1021/acs.accounts.9b00204. Epub 2019 Aug 12.
Solid catalysts deployed in industrial processes often undergo deactivation, requiring frequent replacement or regeneration to recover the loss in activity. Regeneration occurs under conditions distinct from, and typically more harsh than, the catalysis, placing strict requirements on physicochemical material properties that divert catalyst optimization toward addressing regenerability over high activity and selectivity. Deactivation arises from mechanical, structural, or chemical modifications to active sites, promoters, and their surrounding matrices, and the prevailing mechanism for deactivation varies with the reaction, the catalyst, and the reaction conditions. Methanol-to-hydrocarbons processes utilize zeolites and zeotypes-crystalline, microporous oxides widely deployed as catalysts in the refining and petrochemical industries-as solid acid catalysts. Deposition and growth of highly unsaturated carbonaceous residues within the micropores congest molecular transport and block active sites, resulting in deactivation. In this Account, we describe studies probing the underlying mechanisms of deactivation in methanol-to-hydrocarbons catalysis and discuss examples of leveraging the acquired mechanistic insights to mitigate deactivation and prolong catalyst lifetime. These fundamental principles governing carbon deposition within zeolites and zeotypes provide opportunity to broaden versatility of processes for C valorization and to relax constraints imposed by hydrothermal catalyst stability considerations to achieve more active and more selective catalysis. Methanol-to-hydrocarbons catalysis occurs via a chain carrier mechanism. A zeolite/zeotype cavity hosts an unsaturated hydrocarbon guest to together constitute the supramolecular chain carrier that engages in a complex network of reactions for chain carrier propagation. Productive propagation reactions include olefin methylation, aromatic methylation, and aromatic dealkylation. Methanol undergoes unproductive dehydrogenation to formaldehyde via methanol disproportionation and olefin transfer hydrogenation. Subsequent alkylation reactions between formaldehyde and active olefinic/aromatic cocatalysts instigate cascades for dehydrocyclization, resulting in the formation of inactive polycyclic aromatic hydrocarbons and termination of the chain carrier. Addition of a distinct catalytic function that selectively decomposes formaldehyde mitigates chain carrier termination without disrupting the high selectivity to ethylene and propylene in methanol-to-hydrocarbons catalysis on small-pore zeolites and zeotypes. The efficacy of this bifunctional strategy to prolong catalyst lifetime increases with increasing proximity between the active sites for formaldehyde decomposition and the H sites of the zeolite/zeotype. Coprocessing sacrifical hydrogen donors mitigates chain carrier termination by intercepting, via saturation, intermediates along dehydrocyclization cascades. This strategy increases in efficacy with increasing concentration of the hydrogen donor and provides opportunity to realize steady-state methanol-to-hydrocarbons catalysis on small-pore zeolites and zeotypes.
在工业过程中使用的固体催化剂经常会失活，需要频繁更换或再生以恢复活性损失。再生发生在与催化作用不同且通常更为苛刻的条件下，这对物理化学材料特性提出了严格的要求，促使催化剂优化转向解决高活性和选择性下的可再生性问题。失活是由于活性位、促进剂及其周围基质的机械、结构或化学修饰引起的，失活的主要机制因反应、催化剂和反应条件而异。甲醇制烃过程利用沸石和沸石型-广泛用作炼油和石化工业催化剂的微孔氧化物-作为固体酸催化剂。高度不饱和的碳质残留物在微孔中的沉积和生长会阻碍分子传输并阻塞活性位，从而导致失活。在本报告中，我们描述了探究甲醇制烃催化中失活动力学的研究，并讨论了利用获得的机理见解来减轻失活和延长催化剂寿命的实例。这些控制沸石和沸石型中碳沉积的基本原理为碳的增值过程提供了更广泛的多功能性，并放宽了由水热催化剂稳定性考虑因素施加的限制，以实现更活跃和更具选择性的催化。甲醇制烃反应通过链载体机制进行。沸石/沸石型空腔中容纳不饱和烃客体，共同构成超分子链载体，该载体参与链载体传播的复杂反应网络。有生产性的传播反应包括烯烃甲基化、芳烃甲基化和芳烃脱烷基化。甲醇通过甲醇歧化和烯烃转移加氢反应无生产性地脱氢生成甲醛。随后，甲醛与活性烯烃/芳烃共催化剂之间的烷基化反应引发脱氢环化的级联反应，导致不活跃的多环芳烃的形成和链载体的终止。添加选择性分解甲醛的独特催化功能可以减轻链载体的终止，而不会破坏小孔沸石和沸石型上甲醇制烃反应对乙烯和丙烯的高选择性。这种双功能策略延长催化剂寿命的效果随着甲醛分解的活性位与沸石/沸石型的 H 位之间的接近度的增加而增加。共处理牺牲氢供体通过饱和拦截脱氢环化级联反应中的中间体来减轻链载体的终止。该策略的效率随着氢供体浓度的增加而增加，并为在小孔沸石和沸石型上实现稳态甲醇制烃反应提供了机会。
