The catalytic chemistry of nitromethane over Co-ZSM5 and other catalysts in connection with the methane-NOx SCR reaction

The reaction of nitromethane over Co-ZSM5 has been studied with the aim of establishing the paths by which it can be converted to N-2 under the conditions of the methane-SCR reaction over this type of catalyst, when reacted alone it readily decomposes above 250 degrees C to give CO2 and NH3 but the latter is further converted to N-2 at temperatures above 360 degrees C if NO and O-2 are also present. The reaction of ammonia with NO and O-2 is sufficiently fast to account for all N2 formed. The nitromethane reaction systems are stable above 300 degrees C but below that deactivation sets in after a few hours with isocyanic acid (HNCO) eventually becoming the major nitrogen-containing product. If water is then added to the feed the HNCO is largely hydrolysed to NH3 and CO2 and conversion stabilises. Reaction in the presence of water at higher temperature restores the original activity. It is believed that HNCO is the initial decomposition product of nitromethane and deactivation under dry conditions is due to its deposition, possibly as a polymer such as cyanuric acid. Nitromethane also decomposes to CO2 and NH3 over alumina, H-ZSM5, and Na-ZSM5 but only the latter shows deactivation. There is even some conversion over silica but with HNCO observable from the beginning and produced in larger amounts than NH3 below 340 degrees C. Experiments with deuterated nitromethane show that the reaction over Co-ZSM5 has only a small kinetic isotope effect. However, there is rapid H/D exchange between methyl groups and water, most likely via the hydroxyl groups of aci-nitromethane, the enol tautomer of nitromethane. Hydrogen cyanide is a significant minor product over all three zeolite systems at temperatures around 300 degrees C. With Co-ZSM5 its concentration tracks that of ammonia during the course of deactivation and the subsequent enhancement of HNCO hydrolysis when water is added. Nitromethane reacts much faster with NO2 than with NO and O-2 over all three zeolites with complete conversion at approximate to 220 degrees C to give almost entirely N-2 and CO2 with Co-ZSM5 and Na-ZSM5 but with CO and N2O formed as well over H-ZSM5. NO2 may act by removal of strongly adsorbed decomposition products (NH3 or HNCO) since the reaction of NH3 with NO2 is very fast with Co-ZSM5 and gives N-2 as the dominant product. The overall findings can be explained in terms of a scheme involving dehydration/hydrolysis reactions, largely on the aluminosilicate surface, followed by conversion of nitrogen-containing species to N-2 on the transition metal. A reaction scheme which can explain the observations has been developed and its implications with respect to the possible involvement of nitrocompounds in the corresponding SCR reactions of higher hydrocarbons over other catalysts is discussed. (C) 1998 Academic Press.