Direct Observation of Surface Intermediates Formed by Selective Oxidation of Alcohols on Silica-Supported Molybdenum Oxide

Abstract

The behavior of surface species, formed from the adsorption of methanol and isobutyl alcohol (2-methyl-1-propanol) on MoO3/SiO2, was studied in an oxygen-containing atmosphere by in situ Fourier transform infrared spectroscopy (FTIR). At 250 °C, methanol adsorption resulted in the formation of surface methoxide species (CH3O−), whereas isobutyl alcohol adsorption resulted in surface isobutoxide species ((CH3)2CHCH2O−). Kinetic studies indicated that the rate-determining step for the alcohol oxidation was the cleavage of the α-C−H bond in the alkoxide intermediates. Although the α-C is a primary carbon in the case of methanol (CH3OH) and a secondary carbon in the case of isobutyl alcohol ((CH3)2CHCH2OH), the activation energies for the rate-determining step were found to be similar. Theoretical molecular orbital calculations confirmed these experimental results by also predicting similar activation energies for both alkoxide species. During oxidation of the alkoxide species in the absence of alcohol in the gas phase, new bands were observed, which were assigned to the CO bands of adsorbed acyl species. In the case of these species, further reaction with oxygen produced CO2. The acyl species are therefore likely not to be intermediates that lead to aldehyde products in alcohol oxidation.

Synthesis, characterization and study of catalytic activity of Silver doped ZnO nanocomposite as an efficient catalyst for selective oxidation of benzyl alcohol

“…Furthermore, heterogeneous oxidations of alcohols over metal oxide catalysts have also been studied. The studied catalysts are the oxides of magnesium, [28][29][30] copper, 31 cerium, [32][33][34] titanium, [35][36][37][38][39] tungsten, 40 cobalt, 41 vanadium, 42 molybdenum, 43 niobium, 44,45 and zinc. 6,46,47 Recently semiconductor metal oxides nanoparticles have been widely used for heterogeneous oxidation of alcohols to the corresponding carbonyl compounds.…”

Oxidant‐Free Dehydrogenation of Alcohols Heterogeneously Catalyzed by Cooperation of Silver Clusters and Acid–Base Sites on Alumina

“…[49,50,56,57] It is established that the initial step of the dissociative adsorption of alcohols on metal oxides is proton abstraction by a surface basic site. The surface of silica does not have such sites and hence cannot form 2-propoxide groups.…”

Using DFT in Search for Support Effects During Methanol Oxidation on Supported Molybdenum Oxides

“…Isotopic substitution studies in the 1980s already suggested that the OÀH bond breaking was an equilibrated process, while the CÀH bond breaking was the rate-limiting step for the reaction kinetics. [9][10][11] More recent work on the oxidation of methanol confirmed the existence of methoxy groups (ÀOCH 3 ) on the surface prior to oxidation. [12] These observations together with various kinetic studies, principally conducted through the combined efforts of research groups at Virginia Tech and Tokyo Institute of Technology, led to the proposal of a schematic reaction mechanism [Eqs.…”

“…Computational work has therefore always discussed both structures simultaneously giving a slight advantage to di-oxo structures. [9,16,18,20,21] When deduction of the reaction mechanism was the central goal, molybdenum supported on SiO 2 has received most attention. [15,18] The latest contribution of Gregoriades et al used a cluster model where the SiO 2 support was modeled as a polyhedral oligomeric silsequioxane (POSS).…”