Ab Initio Study for Selective Oxidation of Methane with NO_x(x= 1, 2)

Abstract

A reaction model for the conversion of methane to C1-oxygenates (methanol and formaldehyde) with NO x (x = 1, 2) has been proposed theoretically using the ab initio molecular orbital method. The geometric and electronic structures for all the present molecules have been calculated by means of the MP2 (frozen core)/6-311++G(2d,p) level of theory. On the basis of the optimized structures, the single point calculations of the energies are carried out at the CCSD(T) level with the same basis sets. Through the theoretical analysis of the simplified CH4−NO x system instead of the experimental CH4−O2−NO system, we found the possible reaction path leading to C1-oxygenates within all the barriers of less than 40 kcal/mol via CH3O at 800 K. NO2 has a higher activity for the hydrogen abstraction from methane than NO and O2, though the calculated rate constants at 800 K indicate that this reaction is the rate-determining step in the conversion of methane to C1-oxygenates. It is also found that increasing the concentration of NO promotes the yield of formaldehyde with the decreasing formation of methanol, which is consistent with recent experimental results in the CH4−O2−NO system.

Sensitizing effects of NOx on CH4 oxidation at high pressure

“…Yamaguchi and co-workers [14,15] conducted a theoretical study of the selective oxidation of CH 4 by NO x at 800 K and 1 bar, with particular emphasis on NO x -induced Habstraction from CH 4 based on ab initio molecular orbital theory. These calculations confirmed that NO 2 + CH 4 has two important product channels forming CH 3 and either of the two isomers HONO (R11) and HNO 2 (R12), where the latter is favored by a slightly lower energy barrier.…”

“…A comparison with the corresponding initiation reaction CH 4 + O 2 CH 3 + HO 2 indicates that the energy barrier is lowered by about 20-25 kcal/mol when NO 2 is the reactant rather than O 2 , making NO 2 + CH 4 an important initiation reaction in the CH 4 /O 2 /NO x system. The reaction NO + CH 4 HNO + CH 3 (−R28) has a barrier height of magnitude similar to that of CH 4 + O 2 [14] and is not important for initiation. Chan et al [46] presented ab initio calculations of the NO 2 -induced initiation of different aliphatic hydrocarbon fuels at 600-1100 K, including CH 4 and C 2 H 6 , based on a level of theory similar to that of Yamaguchi and co-workers.…”

“…A comparison between the proposed rate coefficients from Yamaguchi and co-workers [14,15] and Chan et al [46] shows excellent agreement above 700 K for the reaction path leading to HNO 2 , whereas the rate coefficient from Chan et al for the formation of trans-HONO is higher than the rate constant from Yamaguchi and co-workers by a factor of 23 at 600 K, falling to a factor 3 at 1100 K. trans-HONO is only slightly more energetically favorable than cis-HONO, by about 0.3 kcal/mol [48], and there is a relatively high cis/trans interconversion barrier of 11.7 kcal/mol. This means that once cis-HONO is formed, e.g., via NO 2 + CH 4 , it is likely to remain in the reaction mixture.…”

“…This interaction has been investigated for a wide range of fuels, including H 2 [3,4], CO [4][5][6][7], CH 2 O [3,8], CH 4 /natural gas [3,4,[9][10][11][12][13][14][15][16][17][18][19][20][21], C 2 hydrocarbons [4,12,[22][23][24], higher hydrocarbons [4,12,[25][26][27][28], and oxygenated hydrocarbons [29][30][31][32][33][34][35]. The sensitizing behavior is a result of interactions with the radical pool as well as direct reactions between NO x and stable species, e.g., H 2 , O 2 , CH 2 O, CH 4 , and C 2 H 6 .…”

NOx-catalyzed partial oxidation of methane and ethane to formaldehyde by dioxygen

“…***Yield: Total conversion · Selectivity. significantly lower barrier for H-atom abstraction from alkanes by NO 2 [20,21]. Scheme 1 outlines the mechanism of the NO x -mediated oxidation of alkanes by dioxygen.…”