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.
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Influence of Ti-incorporated Zeolite Topology and Pore Condensation on Vapor Phase Propylene Epoxidation Kinetics with Gaseous HO.
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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.
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