The reactions of nitroalkanes over Cu-MFI and Fe-MFI catalysts under hydrocarbon-selective catalytic reduction conditions

The reactions of 2-nitropropane (2NP) and 2-methyl-2-nitropropane (2M2NP), typical of secondary and tertiary nitroalkanes, respectively, have been investigated over Cu-MFI and Fe-MFI catalysts under conditions relevant to the selective catalytic reduction (SCR) of NO with the use of alkanes. With both catalysts the primary reaction is decomposition to form NO and the alkene, propene from 2NP, and isobutene from 2M2NP. If O₂ is present, then some N₂ is formed by the subsequent SCR reaction between the alkene and NO_x. Production of N₂ is greater when the feed contains NO, but this is largely due to enhancement of the alkene SCR reaction rather than a direct reaction between the nitroalkane and NO_x. The alkene also reacts partly through deposition of carbonaceous material, which causes deactivation. In the absence of O₂ propene is also formed, through cracking of the oligomers derived from isobutene produced by 2M2NP decomposition. The reduction of NO/O₂ by propene and isobutene over Fe-MFI proceeds in a manner similar to that of the reduction with propane and isobutane, except for a lower production of HCN. With mixed alkane/alkene feeds, the alkene reacts in preference to the alkane. Overall the data show that secondary and tertiary nitroalkanes are less reactive than primary nitroalkanes over transition-metal zeolites because of the absence of the two α-hydrogens necessary for tautomerism to an aci form and its subsequent dehydration to a reactive isocyanate. Thus secondary and tertiary nitroalkanes are unlikely to be a major source of N₂ during alkane SCR over these catalysts but could play a part through the generation of alkenes that are more reactive than alkanes.