Understanding trends in methane oxidation to formaldehyde: statistical analysis of literature data and based hereon experiments

Abstract

A regression tree analysis on selective oxidation of methane to methanol/formaldehyde was applied to identify fundamentals affecting catalyst performance. The electronegativity correlates with methane activation energy and formaldehyde selectivity.

Direct conversion of methane to formaldehyde and CO on B2O3 catalysts

“…Because the desired C 1 partial oxidation products (i.e. CH 3 OH, HCHO, and CO) have much higher electrophilicity than CH 4 , they are kinetically more favored in oxidation, leading to dominant formation of undesired CO 2 at methane conversions even less than 5% on conventional catalysts [7][8][9][10][11][12] (e.g., V 2 O 5 13 , MoO 3 14 , and Fe 2 O 3 15 ). Lattice O anions exposed on these oxide surfaces act as the nucleophilic and oxidative centers that can be regenerated fast via dissociative adsorption of gaseous O 2 during catalytic cycles (also described as the Mars van Krevelen mechanism 16 ).…”

“…Lattice O anions exposed on these oxide surfaces act as the nucleophilic and oxidative centers that can be regenerated fast via dissociative adsorption of gaseous O 2 during catalytic cycles (also described as the Mars van Krevelen mechanism 16 ). To our best knowledge, high selectivities to the desired C 1 partial oxidation products on these traditional metal oxide catalysts are able be obtained merely at low methane conversions (<2%) or low O 2 /CH 4 ratios (<0.5) [7][8][9][10][11][12][13][14][15] , making such oxidation processes impractical.…”

(Ag)Pd-Fe3O4 Nanocomposites as Novel Catalysts for Methane Partial Oxidation at Low Temperature

“…As in mild reaction conditions, the reaction rates are low, and in harder conditions the carbon oxide formation is predominant; an intermediate trade-off working seems to be the best option [2]. In any case, the direct oxidation of methane into methanol, formaldehyde, and other oxygenated products is still very far from being competitive for commercial implementation [2,3]. Among the catalysts employed for direct oxidation of methane to formaldehyde using molecular oxygen, those based on vanadium, molybdenum [4][5][6][7], and especially iron [3,[8][9][10][11][12][13][14][15][16][17] have shown the most promising performance.…”

“…In any case, the direct oxidation of methane into methanol, formaldehyde, and other oxygenated products is still very far from being competitive for commercial implementation [2,3]. Among the catalysts employed for direct oxidation of methane to formaldehyde using molecular oxygen, those based on vanadium, molybdenum [4][5][6][7], and especially iron [3,[8][9][10][11][12][13][14][15][16][17] have shown the most promising performance. Nevertheless, the yield of formaldehyde reported with these catalysts does not exceed 5% [2,4,8,[18][19][20], although higher yields can be found in the literature for other catalysts.…”

“…It has been proposed that isolated tetrahedral Fe 3+ sites are the most selective sites [22] whereas other authors have suggested that Fe with higher aggregation (2D FeOx oligonuclear sites) is more effective than isolated Fe 3+ sites [16]. In any case, the reaction temperatures employed in these articles exceed 400 • C and are often over 500 • C [3,8,12]. Interestingly, recent work by Zhao et al showed that iron species supported on different zeolites (ZSM-5, beta, and ferrierite) activate methane at 350 • C using N 2 O as an oxidant, reaching methane conversions until 3%, with the formation of different partial oxidation products (methanol, formaldehyde, dimethyl ether) but also carbon oxides, obtaining up to 10% selectivity to formaldehyde [9].…”