Demonstrating the Critical Role of Solvation in Supported Ti and Nb Epoxidation Catalysts via Vapor-Phase Kinetics

Abstract

Catalytic oxidation of hydrocarbons with hydrogen peroxide (H2O2) has been of the utmost importance for several decades. The vast majority of studies have been performed in the condensed phase, even though condensed phases introduce complex solvent effects and can promote the leaching of active sites. In response, we have built a custom reactor system to understand H2O2 activation and selective oxidation in the vapor-phase. In this report, we study the epoxidation of cyclohexene with H2O2 over four Lewis-acidic metal oxide catalysts: Ti and Nb grafted on SiO2 and on the Zr based metal–organic framework, NU-1000. The M–SiO2 materials are highly selective to the formation of epoxides and diols, as they can be in the condensed phase, while the NU-1000 based materials are far more prone to overoxidation to CO2, which appears to be connected to their strong reactant adsorption. Apparent activation energies are calculated for all materials when operating in the same kinetic regime, and the heats of cyclohexene adsorption into their pores are then used to directly compare intrinsic enthalpies of activation in the vapor vs condensed phase for the M–SiO2 catalysts. Nb–SiO2 catalysts exhibit similar intrinsic enthalpies of activation in the vapor and condensed phases, whereas the condensed phase transition state in Ti–SiO2 is 24 kJ/mol lower in energy than that of the same material in the vapor phase. These experiments establish another methodology for understanding the various roles of solvent in selective oxidation reactions and studying these reactions under conditions that differ significantly from the thousands of prior studies in the condensed phase.

Vapor-Phase Cyclohexene Epoxidation by Single-Ion Fe(III) Sites in Metal–Organic Frameworks

“…Product rate profiles of the two catalysts present different behaviors initially but then show both catalysts eventually reaching steady-state rates (Figure and Figure S7). Interestingly, neither catalyst fully oxidizes the reactant to CO 2 , unlike the case in our previous study, which may be due to low conversion by both catalysts . Additionally, the radical pathway reaction that eventually yields CO 2 may be slower than decomposition of H 2 O 2 to H 2 O over these catalysts.…”

“…Vapor-phase cyclohexene epoxidation with vaporized H 2 O 2 recently developed by our teamwas used to compare the reactivity and selectivity for the two catalysts. Simplified reaction pathways are given in Scheme , presenting two proposed H 2 O 2 activation mechanisms.…”

“…Reaction tests were performed at 120 °C in a 1/1 ratio of reactant (cyclohexene) and oxidant (H 2 O 2 ), with both reactants in the vapor phase. A detailed description of the reactor was reported in prior work from our team . Catalysts (10 mg) were diluted in 200 mg of quartz sand, and the catalyst beds were supported on quartz wool in a quartz tube reactor.…”

“…A detailed description of the reactor was reported in prior work from our team. 35 Catalysts (10 mg) were diluted in 200 mg of quartz sand, and the catalyst beds were supported on quartz wool in a quartz tube reactor. Cyclohexene (Sigma-Aldrich, ≥99.0%) was introduced by flowing He (5 mL/min, 99.999%, Airgas) through a quartz bubbler at 25 °C, giving a saturated vapor pressure of 11.9 kPa.…”

MOF-enabled confinement and related effects for chemical catalyst presentation and utilization

“…Ideally, bridging or terminal hydroxo and/or aqua node ligands can react with metal ions, labile inorganic complexes, or organometallic complexes, either in the condensed phase (often termed solvothermal deposition in MOFs (SIM)) or via molecular volatilization into the vapor phase (often termed atomic layer deposition in MOFs (AIM)) to accomplish chemical attachment. [89][90][91][92][93][94][95][96][97] Subsequent treatment with steam or H 2 S can yield node-grafted oxy-or sulfido-metal species -either as individual metal ions or as few-atom clusters. 98,99 A majority of the transition-metal elements and a handful of main-group metals have now been grafted in this way to Zr-MOF nodes.…”