Kinetics and Mechanism of Dimethyl Ether Oxidation to Formaldehyde on Supported Molybdenum Oxide Domains

Abstract

Kinetic isotope effect and isotopic tracer/exchange methods were combined with in situ infrared spectroscopy and kinetic data to determine the mechanism of dimethyl ether (DME, CH 3 OCH 3 ) oxidation to formaldehyde (HCHO) on MoO x /Al 2 O 3 . The reaction intermediates and elementary steps established a redox mechanism that led to kinetic rate equations that are consistent with observed dependencies of reactant pressures. Methoxide concentrations as detected by in situ infrared spectroscopy correlated directly with formation rates to establish their importance for the formation of HCHO and CH 3 OH. Reactant partial pressure studies showed that rates of HCHO and CH 3 OH formation are first-order in DME and zero-order in O 2 at low DME pressures. At high DME pressures, rates became independent of DME pressure and acquired positive-order O 2 dependencies. H-D kinetic isotope effects indicated that C-H bond activation is not involved in kinetically relevant steps and transient studies involving CH 3 16 OCH 3 -18 O 2 -Mo 16 O x /Al 2 O 3 confirmed the kinetic relevance of DME dissociative adsorption, the step that precedes C-H bond activation. These studies also indicated that mechanisms for HCHO formation do not discriminate between methoxide species formed from DME oxygen and those formed from lattice oxygen.

Mechanistic Details and Reactivity Descriptors in Oxidation and Acid Catalysis of Methanol

“…16 Figure 3). DTBP forms protonated structures that bind irreversibly, thus rendering Brønsted acid sites inaccessible for catalysis; these molecules cannot coordinate to Lewis acid sites, because access to the N-atom is hindered by the vicinal tert-butyl groups.…”

“…Figure 4 shows ODH and DME formation rates (per POM) from CH 3 OH-O 2 reactants on H 3 PMo/SiO 2 (0. 16) before and during titration with DTBP (433 K, 4 kPa CH 3 OH, 20 kPa O 2 , 7 Pa DTBP), measured using procedures described in sections 2.2 and 2.3. Both rates decreased monotonically with increasing DTBP uptakes; saturation uptakes fully suppressed DME formation and led to a marked decrease in ODH rates (∼20% of initial rates at saturation; Figure 4).…”

Structure and Reactivity of Alkyl Ethers Adsorbed on CeO2(111) Model Catalysts

“…The coordination complex decomposes giving a mixture of ethanol, diethyl ether and ethylene below 130°C and mainly ethylene above 130°C. Dimethyl ether (DME), which is of interest as a suitable diesel fuel, has been extensively studied by Iglesia et al [16][17][18][19][20] on supported MoOx and VOx domains and by Solymosi et al [21][22][23] on metal surfaces. Basically these studies show that the C-O-C moiety of DME breaks on the surface leaving methoxide in the case of oxide surfaces and methoxide and methyl in the case of metals.…”

Mechanistic Study of Selective Oxidation of Dimethyl Ether to Formaldehyde over Alumina-supported Molybdenum Oxide Catalyst

“…Our kinetic study showed that the reaction is pseudo-zero order with respect to both dimethyl ether and oxygen molecule, suggesting that the reaction of the surface intermediate CH 3 O* is likely to be the rate-determining step [5]. DME dissociation over Mo-O sites was recently investigated by 18 O and H-D isotopic tracers and it was revealed that the oxy-dehydrogenation of CH 3 O* could not be the rate-determining step [8]. The HCHO/ CH 3 OH ratio in the products was closely related to the concentrations of DME and O 2 in a certain range in the feed stream.…”