Experimental Verification of Theoretically Calculated Transition Barriers of the Reactions in a Gaseous Selective Oxidation of CH₄−O₂−NO₂

Abstract

The selective oxidation of methane with oxygen to C 1 oxygenates (methanol and formaldehyde) is an important process. However, the precise reaction mechanism in a gaseous chain reaction has not been clarified. Methane activation and the selectivity of C 1 oxygenates (CH 3 OH, CH 2 O) in a gaseous selective oxidation of CH 4 -O 2 -NO 2 have been examined under atmospheric pressure with both theoretical and experimental approaches. Theoretically calculated transition barrier of hydrogen abstraction from CH 4 of the reaction CH 4 + NO 2 f CH 3 + HNO 2 was lower than that of the reaction with O 2 , i.e., CH 4 + O 2 f CH 3 + O 2 H. This decrease of the transition barrier was experimentally verified by the linear enhancement of CH 4 conversion with NO 2 concentration in CH 4 -O 2 -NO 2 . The experimental varied results of selectivity of C 1 oxygenates on various reaction conditions (NO 2 concentration, CH 4 /O 2 ratio, space velocity) showed the appropriateness of the consideration on the selectivity with the calculated values of transition barriers and rate constants of the selected reaction routes from CH 3 O to CH 3 OH and CH 2 O. After considering the transition barriers and rate constants of each elemental reaction route, we attained ca. 7% yield of C 1 oxygenates.

Recent Advances in Catalytic/Biocatalytic Conversion of Greenhouse Methane and Carbon Dioxide to Methanol and Other Oxygenates

“…Thus, initiators and sensitizers have been incorporated into the reaction mixture for the purpose of lowering the energy barrier of this H-abstraction (Navarro et al, 2006;Alvarez-Galvan et al, 2011). In particular, nitrogen oxides have been used to promote gas-phase reactions with methane (Otsuka et al, 1999;Tabata et al, 2000;Babero et al, 2002;Tabata et al, 2002). The presence of higher hydrocarbons, especially ethane, in small quantities has also been observed to lower the initiation temperature and increase methanol selectivity and yield (Gesser et al, 1985;Fujimoto, 1994).…”

Effect of nitrogen oxides NOx on gas-phase oxidation of methane by oxygen

“…In the presence of such additives, methane oxidation is significantly accelerated, and the selectivity of the reaction with respect to methanol and formaldehyde is also substantially increased [2][3][4][5][6]. According to the data in [2,5], the overall yield with respect to oxygenates has been as high as 4%-6% in the presence of 0.5-2 vol.% NO in the initial reaction mixture.…”

“…According to the data in [2,5], the overall yield with respect to oxygenates has been as high as 4%-6% in the presence of 0.5-2 vol.% NO in the initial reaction mixture. An even higher yield of C 1 oxygenates (up to 16%) is achieved by combined treatment with gas-phase (NO) and heterogeneous (V 2 O 5 /SiO 2 ) catalysts [6].In [2][3][4][5], ideas are discussed and substantiated about the mechanism of action of nitrogen oxides NO x on the process of oxidation of methane by oxygen; these ideas are summarized in the recent review [7]. It is also noted there that without kinetic modeling, it is difficult to establish the effect of the process conditions on the yield of products.…”

“…In [2][3][4][5], ideas are discussed and substantiated about the mechanism of action of nitrogen oxides NO x on the process of oxidation of methane by oxygen; these ideas are summarized in the recent review [7]. It is also noted there that without kinetic modeling, it is difficult to establish the effect of the process conditions on the yield of products.…”

“…The calculations were performed for the conditions under which the homogeneous gas-phase reaction of methane oxidation was studied in [3,4]: NO [3] or NO 2 [4] were added to the initial reaction mixture at atmospheric pressure in flow-through quartz reactors.…”