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

Abstract

We propose a kinetic model for gas-phase oxidation of methane by oxygen in the presence of nitrogen oxides NO x . The model calculations agree satisfactorily with experimental kinetic data provided in the literature. We consider the basic principles for the effect of nitrogen oxides on the rate of the process and the selectivity with respect to methanol and formaldehyde.Recently some success has been achieved in carrying out the process of direct gas-phase oxidation of methane by oxygen to form C 1 oxygenates (methanol, formaldehyde) on heterogeneous catalysts (V 2 O 5 /SiO 2 , MoO 3 /SiO 2 , FePO 4 /SiO 2 ). However, the maximum yield of oxygenates usually is no greater than 2% [1].An increased yield of oxygenates can be achieved by introducing small amounts of nitrogen oxide NO x additives into the CH 4 -O 2 reaction mixture. 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. 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.The aim of this work was to develop a sufficiently detailed kinetic model for the reactions occurring in a methane-oxygen mixture with NO x additives, to use this model to calculate the dependences of the yield of methanol and formaldehyde on the actual process conditions, and to determine the effect of individual elementary reactions involving participation of nitrogen oxides (and their reaction products) on the methane conversion rate and the selectivity.A kinetic model consisting of 88 elementary homogeneous reactions has been used previously [8,9] to study the reaction of oxidative coupling of methane. In this work, this model was supplemented with steps resulting from the formation of methanol and additions of nitrogen oxides (Table 1). As in [8,9], the rate constants for the additional steps were taken from the database of the National Institute of Standards and Technology (USA) [10,11]. After such an expansion, the kinetic model includes 115 elementary gas-phase reactions.The calculated kinetic dependences were obtained based on a mathematical model consisting of a set of ordinary differential equations of general form dC dt r r j ij i ij i = − ∑ ∑ form cons 126 0040-5760/06/4202-0126