| Methanol is currently the preferred feedstock to light olefins and oxygenated chemicals because it provides a thermodynamically and kinetically feasible, although indirect, route for methane conversion. Dimethyl ether (DME, CH3OCH3) is an attractive alternate feedstock because its conversion from synthesis gas is more economical and thermodynamically favorable than methanol and its lack of hydroxyl groups lessens its potential to form water by-products. The catalyst site requirements, kinetics, and mechanism of oxidation and carbonylation reactions of methanol and DME are examined here.; Oxidative DME and methanol reactions proceed in similar pathways on supported MoOx. C-H bonds in methoxide groups, formed initially from DME or methanol dissociation, are cleaved to form formaldehyde, hydroxyl groups, and lattice oxygen vacancies. Hydroxyl groups combine and desorb as water and oxygen vacancies are re-oxidized by O2. No kinetic isotope effect was detected in separate reactions of CH3OCH3 and CD3OCD3 indicating that C-H bond cleavage is not involved in kinetically-relevant steps and transient studies involving CH 316OCH3-18O2-Mo 16Ox confirmed, instead, the kinetic relevance of C-O dissociation to form methoxide groups. First-order rate constants are about 10 times larger for methanol than for DME at 493 K because C-O dissociation steps precede C-H bond activation steps, which limit rates for methanol oxidation.; Ru-, Ru-Na-, and Ru-Na-Sn/USY catalyzed the reaction of methanol (in the absence of CO) to acetic acid, detected as methyl acetate due to rapid esterification reactions, but not without the presence of methanol dehydration, decomposition, and methanation side reactions. DME, CO, H2, CO 2, and CH4 dominated the product stream leading to low (<2%) methyl acetate selectivities. Methyl acetate selectivities improved significantly in reactions of DME and CO on acidic zeolites. Acid mordenite and ferrierite catalyzed DME carbonylation with >99% methyl acetate selectivities at low temperatures. Rates are first-order in CO, zero-order in DME, and strongly inhibited by water. Methanol carbonylation reactions do not occur at similar reaction conditions (423-463 K, 100-930 kPa CO) because water, formed in dehydration reactions, competitively adsorbs onto CO binding sites or inhibits the kinetically-relevant reaction of CO with methyl groups to form acetyl intermediates. |
