A Heterogenized Molecular Catalyst for the Gas-Phase Cyclotrimerisation of Acetylene to Benzene

Facilitating the cyclotrimerisation of acetylene to benzene in the gas-phase on the surface of solid catalysts has captivated researchers, both theoretically and experimentally, for several decades. Coupled with acetylene production from renewable feedstocks, this reaction offers a promising pathway for the direct, low-temperature synthesis of renewable benzene. Recog-nizing the physical limitations of typical solid catalysts, which adsorb benzene too strongly, this study investigates various high-valent early transition metal chlorides – potent molecular cyclotrimerisation catalysts – as solid catalysts in this gas-phase conversion. Alongside catalytic assessment under industrially relevant conditions, various analytical techniques such as 93Nb solid-state NMR spectroscopy, X-ray emission spectroscopy and quasi in situ X-ray photoelectron spectroscopy were applied to reveal that the catalytic function proceeds through in situ formation of catalytically active reduced species as re-ported for a homogeneous reaction environment. Investigating deactivation phenomena, various catalyst precursors and reac-tion conditions led to the design of an immobilized molecular catalyst via chemical grafting, dispersion and spatial isolation of active catalytic species on a mesoporous silica gel support. The resulting NbClx–silica gel catalyst exhibits an average benzene selectivity of up to 70% at an acetylene conversion level between 30-70% and a lifetime at ≥90% acetylene conver-sion of up to 8.5 h under industrially relevant conditions (10 vol.% C2H2, 3 bar, 180 °C), outperforming previously reported solid catalysts by orders of magnitude.

数十年来，通过固体催化剂表面气相催化实现乙炔环三聚生成苯的反应，一直是理论与实验领域研究者关注的核心课题。结合可再生原料制备乙炔的技术路径，该反应为可再生苯的直接低温合成提供了极具潜力的方案。
鉴于传统固体催化剂存在苯吸附过强的固有缺陷，本研究选取多种高氧化态前过渡金属氯化物——一类高效的分子型环三聚催化剂——作为该气相转化反应的固体催化剂展开探究。本研究在工业相关反应条件下开展催化性能评价的同时，借助固态铌-93（⁹³Nb）核磁共振光谱、X射线发射光谱及准原位X射线光电子能谱等多种分析手段，揭示出该催化过程通过原位生成催化活性还原物种进行——这一机制与均相反应体系中已报道的催化路径一致。
针对催化失活现象展开研究后，本研究通过化学接枝、活性物种分散与空间隔离的策略，在介孔硅胶载体上制备得到固定化分子催化剂，该过程优化了多种催化剂前驱体与反应条件。所制备的NbClx-硅胶催化剂在工业相关反应条件（10%体积乙炔、3 bar、180 ℃）下，当乙炔转化率处于30%~70%区间时，苯平均选择性可达70%；在乙炔转化率≥90%的工况下，催化剂寿命最长可达8.5小时，其催化性能较此前报道的固体催化剂提升了数个数量级。