A Density Functional Theory Study of the Oxidation of Methanol to Formaldehyde over Vanadia Supported on Silica, Titania, and Zirconia

Abstract

Density functional theory was used to investigate the mechanism and kinetics of methanol oxidation to formaldehyde over vanadia supported on silica, titania, and zirconia. The catalytically active site was modeled as an isolated VO 4 unit attached to the support. The calculated geometry and vibrational frequencies of the active site are in good agreement with experimental measurements both for model compounds and oxidesupported vanadia. Methanol adsorption is found to occur preferentially with the rupture of a V-O-M bond (M = Si, Ti, Zr) and with preferential attachment of a methoxy group to V. The vibrational frequencies of the methoxy group are in good agreement with those observed experimentally as are the calculated isobars. The formation of formaldehyde is assumed to occur via the transfer of an H atom of a methoxy group to the O atom of the V=O group. The activation energy for this process is found to be in the range of 199-214 kJ/mol and apparent activation energies for the overall oxidation of methanol to formaldehyde are predicted to lie in the range of 112-123 kJ/mol, which is significantly higher than that found experimentally. Moreover, the predicted turnover frequency (TOF) for methanol oxidation is found to be essentially independent of support composition, whereas experiments show that the TOF is 10 3 greater for titania-and zirconia-supported vanadia than for silica-supported vanadia. Based on these findings, it is proposed that the formation of formaldehyde from methoxy groups may require pairs of adjacent VO 4 groups or V 2 O 7 dimer structures.3

Density functional theory study of the oxidation of methanol to formaldehyde on a hydrated vanadia cluster

“…8,9,28,30,32,56,57,[68][69][70][71][72][73] Sauer and co-workers have studied the oxidation of methanol to formaldehyde by means of a O¼ ¼V(OCH 3 ) 3 model and the corresponding activation energy for the CÀ ÀH bond breaking process is 35.1 kcal/mol. 32 In the case of the oxidation of methane to methanol which involves similar steps, the activation energy of CÀ ÀH bond breaking is calculated to be of 36.6 kcal/mol with the same O¼ ¼V(OH) 3 model cluster.…”

Chemie mit Methan: Studieren geht über Probieren!

“…Viele der Experimente in der Gasphase sind darauf angewiesen, dass es sich bei den reaktiven Spezies um geladene 104] Weitere Beispiele dafür, dass Gasphasenexperimente mit "isolierten" Spezies als Modellsysteme für Reaktionen in der kondensierten Phase angesehen werden kçnnen, sind: 1) Der DEGUSSA-Prozess (die platinvermittelte Kupplung von Methan und Ammoniak zu HCN), [5] 2) das mechanistische Verständnis der Cytochrom-P-450-vermittelten Oxygenierung von C-H-Bindungen, [62,63] das durch die detaillierte Analyse des denkbar einfachsten Modellsystems FeO + /H 2 gewonnen wurde, [63,96] und den außergewçhnlichen Eigenschaften von hochvalenten Platinoxiden, [106] um nur einige zu nennen. [107] hat sich die Situation deutlich verändert -und zwar zum Besseren!…”

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

“…The structural model used herein is inspired by earlier works of Bell et al [15,20,40] A ball-and-stick representation of the monoand di-oxo molybdenum models on a SiO 2 surface are shown in Figure 1. In both cases the SiO 2 support is represented by a ring of four Si atoms.…”