Effect of Water Density on Methanol Oxidation Kinetics in Supercritical Water

Abstract

We oxidized methanol in supercritical water at 500 degrees C to explore the influence of the water concentration (or density) on the kinetics. The rate increased as the water concentration increased from 1.8 to 5.7 mol/L. This effect of water density on the kinetics observed experimentally was quantitatively reproduced by a previously validated mechanism-based, detailed chemical kinetics model. In this model, reactions of OH radicals with methanol were the fastest methanol removal steps. The rates of these removal steps increased with water density at 500 degrees C because the OH radical concentration increased. The OH radical concentration increased with density because the rates of the steps H + H2O = OH + H2 and CH3 + H2O = OH + CH4, which produce OH radicals, increased. Thus, the main role of water in accelerating methanol oxidation kinetics at 500 degrees C is as a hydrogen donor to a radical (R) in steps such as R + H2O = OH + RH. This system provides a striking example of SCW being involved on the molecular level in the free-radical oxidation as a reactant in elementary steps.

Review on Mechanisms and Kinetics for Supercritical Water Oxidation Processes

“…The difference in the sensitivity analysis was caused by the different water density ranges they studied. The water density under Fujii et al's [129] experimental conditions was 3.0-6.6 × 10 2 kg/m 3 , which was higher than 1.0×10 2 kg/m 3 in the Henrikson et al's [128] experiments. Furthermore, Tatsuya Fujii et al [129] analyzed the effect of diffusion restriction on the reaction at high water density.…”

“…Subsequent studies have further improved the predictive power of the DCKM model. Henrikson et al [128] updated kinetic parameters and thermodynamic data on the basis of the DCKM of methanol proposed by Brock et al [66] to analyze the effect of water density on oxidation kinetics, which nicely reproduced the experimental phenomenon of increased reaction rates due to increased water density. Since OH• reaction with methanol is the fastest reaction step for methanol removal, an increase in water density increased the rate of reactions to produce OH• (H•+H 2 O=OH•+H 2 , CH 3 •+H 2 O=OH•+CH 4 ).…”

“…Hence, the concentration of OH• increased with the increase of water density, which led to an increase in degradation reaction rate. Tatsuya Fujii et al [129] combined experiments and the DCKM of methanol improved by Henrikson et al [128] to investigate the effect of density on methanol oxidation at 420 • C and 34-100 MPa. Different from the study by Henrikson et al [128], the sensitivity coefficients of the methanol concentration and the rate of OH• production to the water density indicated that the rate of reaction (HO 2 •+H 2 O=OH•+H 2 O 2 ) to produce OH• accelerated under high pressure, which in turn promoted the decomposition of methanol.…”

Oxidation and Reduction

“…284 The increase in the rate of oxidation of methanol with supercritical water concentration at 500 • C has been attributed to the increase in the concentration of OH radicals in a quasi-stationary-state. 285 The outermost surface compositions and chemical nature of active surface sites present on orthorhombic Mo-V-O and Mo-V-Te-Nb-O phases have been determined using methanol and allyl alcohol as probes. It is suggested that the vastly different catalytic behaviours shown by Mo-V-O and Mo-V-Te-Nb-O phases are due to different surface locations of V 5+ ions.…”

Excited-State Proton Transfer of 5,8-Dicyano-2-naphthol in High-Temperature and High-Pressure Methanol: Effect of Solvent Polarity and Hydrogen Bonding Ability

“…Supercritical fluids (SCFs) have been well-studied because of their unique physicochemical properties such as diffusion coefficient, refractive index, and dielectric constants around the critical point. − Among the many studies on SCFs, the solvent–solvent interaction in a hydrogen bonding (HB) solvent has attracted the interest of many chemists, and experimental − and theoretical − studies on the behavior of HB in water, methanol, ethanol, etc. at high temperature and pressure have accumulated.…”