Kwon Ohsung, Zeynep Ayla E, Potts David S, Flaherty David W
Department of Chemical and Biomolecular Engineering, University of Illinois Urbana-Champaign, Urbana, IL 61801, USA.
School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA.
Angew Chem Int Ed Engl. 2024 Jul 22;63(30):e202405950. doi: 10.1002/anie.202405950. Epub 2024 Jun 17.
Vapor-phase propylene (CH) epoxidation kinetics with hydrogen peroxide (HO) strongly reflects the physical properties of Ti-incorporated zeolite catalysts and the presence of spectating molecules ("solvent") near active sites even without a bulk liquid phase. Steady-state turnover rates of CH epoxidation and product selectivities vary by orders of magnitudes, depending on the zeolite silanol ((SiOH)) density, pore topology (MFI, *BEA, FAU), and the quantity of condensed acetonitrile (CHCN) molecules nearby active sites, under identical reaction mechanisms sharing activated HO intermediates on Ti surfaces. Individual kinetic analyses for propylene oxide (PO) ring-opening, homogeneous diol oxidative cleavage, and homogeneous aldehyde oxidation reveal that secondary reaction kinetics following CH epoxidation responds more sensitively to the changes in zeolite physical properties and pore condensation with CHCN. Thus, higher PO selectivities achieved in hydrophilic Ti-MFI at steady-state reflect the preferential stabilization of transition states for CH epoxidation (a primary reaction) relative to PO ring-opening and oxidative cleavage (secondary reactions) that solvation effects that reflect interactions among condensed CHCN within pores and the extended pore structure.
气相丙烯（CH）用过氧化氢（HO）进行环氧化反应的动力学，强烈反映了含钛沸石催化剂的物理性质，以及即使在没有本体液相的情况下，活性位点附近旁观分子（“溶剂”）的存在。在相同的反应机理下，即Ti表面共享活化的HO中间体，丙烯环氧化的稳态转化率和产物选择性会因沸石硅醇（（SiOH））密度、孔拓扑结构（MFI、*BEA、FAU）以及活性位点附近缩合乙腈（CHCN）分子的数量而有几个数量级的差异。对环氧丙烷（PO）开环、均相二醇氧化裂解和均相醛氧化的单独动力学分析表明，丙烯环氧化后的二级反应动力学对沸石物理性质和与CHCN的孔缩合变化更为敏感。因此，在亲水性Ti-MFI中稳态下实现的较高PO选择性反映了相对于PO开环和氧化裂解（二级反应），丙烯环氧化（一级反应）过渡态的优先稳定化，这是反映孔内缩合CHCN与扩展孔结构之间相互作用的溶剂化效应。
