Highly Efficient and Stable Vanadia–Titania–Sulfate Catalysts for Methanol Oxidation to Methyl Formate: Synthesis and Mechanistic Study

Abstract

Vanadia−titania−sulfate nanocatalysts for methanol oxidation to methyl formate (MF) were prepared by coprecipitation. When calcinated at 400 °C, both methanol conversion and MF selectivity reached ∼98.5% at reaction temperatures of 140−145 °C. Characterizations with several experimental techniques revealed the catalysts as highly dispersed vanadia supported by anatase titania with acidic sites of significant strength and density. The catalysts also showed very high stability with lifetime exceeding 4500 h. Extensive density functional theory calculations using both cluster and surface models revealed MF to form via the hemiacetal mechanism, involving the condensation of methanol and formaldehyde at acidic sites and methanol and hemiacetal oxidations at redox sites. The alternative formyl mechanism was predicted kinetically much less favorable, showing these catalysts to work in a distinct mechanism from the rutile titania photocatalyst. Interestingly, methanol chemisorption at redox sites leads to the formation of acidic sites capable of catalyzing the condensation reaction.

Insights into Redox Dynamics of Vanadium Species Impregnated in Layered Siliceous Zeolitic Structures during Methanol Oxidation Reactions

“…During reaction steps using the [5 V]Si‐ITQ‐6 catalyst a very fast activation of O 2 and regeneration of reduced sites occurs, since no contribution of V 4+ species were observed in the total spectra. By monitoring the gases exiting the XANES cell during the reaction steps (Figure S13), formaldehyde and carbon monoxide were identified as major products, with small amounts of dimethyl ether, due to the low acidity present in the vanadium sites . Under the oxidizing atmosphere, formaldehyde formation is predominant in relation to carbon monoxide.…”

“…By monitoring the gases exiting the XANES cell during the reaction steps ( Figure S13), formaldehyde and carbon monoxide were identified as major products, with small amounts of dimethyl ether, due to the low acidity present in the vanadium sites. [53][54][55] Under the oxidizing atmosphere, formaldehyde formation is predominant in relation to carbon monoxide. This is explained by the use of a limited O 2 /CH 3 OH ratio (0.71), since oxidation to CO and CO 2 are favored in oxygen rich conditions (Table S4).…”

Selective Oxidation of Methanol to Dimethoxymethane at Low Temperatures through Size‐controlled VTiOx Nanoparticles

“…Therefore, a decrease in the reaction temperature is beneficial not only to suppress the formation of methanol downstream byproducts but also to avoid the deep oxidation of methanol to CO x . According to the mechanism of the selective oxidation of methanol, methanol oxidation occurs through a rate‐determining C−H bond‐activation step of the methoxy group to form FA by redox cycles, and then FA reacts with methanol to form a hemiacetal intermediate (CH 3 OCH 2 OH). The hemiacetal is further condensed with methanol to form DMM on the acidic sites or is oxidized to form MF on the redox sites .…”

“…At 150 8C, as malla mount of CO 2 with 1.0-1.3 %s electivity for these catalysts is detected. Therefore, ad ecrease in the reactiont emperature is beneficial not only to suppress the formation of methanol downstream byproducts but also to avoid the deepo xidation of methanol to CO x .A ccording to the mechanism of the selectiveo xidation of methanol, [8,27] methanolo xidation occurs through ar ate-determining CÀHb ond-activation step of the methoxy group to form FA by redox cycles, and then FA reacts with methanolt o form ah emiacetal intermediate (CH 3 OCH 2 OH).T he hemiacetal is further condensed with methanol to form DMM on the acidic sites or is oxidized to form MF on the redox sites. [28] The active sites for this reactiona re the highlyd ispersed vanadia speciesV O x on TiO 2 .W ith ad ecrease in the particles size, the redox and acidity properties of the VTiS-CTAB catalysts are significantly improved.…”

Remarkable Promotion Effect of Sulfation over the SiO2‐Supported Vanadium‐Oxide‐Based Catalysts for UHMWPE

“…While introducing solid superacids consisting of sulfated metal oxides is more preferred, as these compounds are endowed with fairly high acidity, which could be maintained even under temperature higher than 500 °C, and the green nature and low corrosion also make them very attractive . The catalytic applications in alkylation, isomerization, esterification, and reduction of NO are extensively studied, and the enhancement effect in vanadium‐oxide‐based catalysts for methanol oxidation are widely reported . Although it is well known that the acidity has a great influence on olefin polymerization, the effects of sulfated metal oxides on the performance of the catalysts for olefin polymerization are scarcely reported, and as far as we know, no work have been devoted to investigating their effects on the vanadium‐oxide‐based catalysts for ethylene polymerization .…”