Mechanistic Studies of Methanol Oxidation to Formaldehyde on Isolated Vanadate Sites Supported on Mcm-48

Journal Article · Tue Jul 03 04:00:00 EDT 2007 · J.Phys.Chem.C111:420-430,2007

DOI:https://doi.org/10.1021/jp0653149· OSTI ID:909558

Bronkema, J L; Bell, A T

The mechanism of methanol oxidation to formaldehyde catalyzed by isolated vanadate species supported on silica has been investigated by in situ Raman and TPD/TPO experiments. Raman, XANES, and EXAFS were used to characterize the V-MCM-48 sample, prepared with a loading of 0.3 V/nm{sup 2}, and it is concluded that the oxidized form of the vanadium is isolated VO{sub 4} units. The VO{sub 4} species consist of one V=O bond and three V-O-Si bonds in a distorted tetrahedral geometry. Methanol reacts reversibly, at a ratio of approximately 1 methanol per V, with one V-O-Si to produce both V-OCH{sub 3}/Si-OH and V-OH/Si-OCH{sub 3} group pairs in roughly equivalent concentrations. Formaldehyde is formed from the methyl group of V-OCH{sub 3}, most likely by the transfer of one H atom to the V=O bond of the vanadium containing the methoxide group. Formaldehyde is formed in nearly equal concentrations both in the presence and in the absence of gas-phase oxygen. CO and H{sub 2} are produced by the decomposition of CH{sub 2}O at higher temperature. In the absence of O{sub 2}, Si-OCH{sub 3} groups undergo hydrogenation to form CH{sub 4}, and in the presence of O{sub 2}, these groups are oxidized to COx (x = 1, 2) and H{sub 2}O above 650 K. Under steady-state reaction conditions, CH{sub 2}O is produced as the dominant product of methanol oxidation at temperatures below 650 K with an apparent activation energy of 23 kcal/mol. Schemes for the product flows during both TPD and TPO experiments, along with proposed surface intermediates, are presented.

• Research Organization: Stanford Linear Accelerator Center (SLAC)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC02-76SF00515
• OSTI ID: 909558
• Report Number(s): SLAC-REPRINT-2007-057
• Journal Information: J.Phys.Chem.C111:420-430,2007, Journal Name: J.Phys.Chem.C111:420-430,2007 Journal Issue: 1 Vol. 111
• Country of Publication: United States
• Language: English

Related Subjects

10 SYNTHETIC FUELS
ACTIVATION ENERGY
ATOMS
FORMALDEHYDE
GEOMETRY
HYDROGENATION
METHANOL
OXIDATION
OXYGEN
Other
OTHER
SILICA
VANADATES
VANADIUM