Co-beta zeolite highly active in propane–SCR-NO in the presence of water vapor: effect of zeolite preparation and Al distribution in the framework

CoNH4-beta and CoH-beta zeolites, with various Co loadings, were prepared by Co ion exchange into NH4- and H-beta parent zeolites, from which the template was removed by calcination of the as-synthesized beta zeolite in a stream of ammonia and oxygen, respectively. It has been found that the CoNH4-beta zeolites in contrast to CoH-beta zeolites exhibit high and stable activity in selective catalytic reduction of NOx with propane in the presence of water vapor (10%). To analyze the reasons for the exception activity of CoNH4-beta zeolites, both zeolite preparations were studied with respect to the state and location of aluminum, character of Al-related acid sites, and Co ion-exchange capacity by means of FTIR spectra of OH groups of dehydrated zeolites and those after adsorption of d3-acetonitrile, and by 29Si, 27Al, and 27Al 3Q MAS NMR spectra. The procedure of template removal using ammonia provides a charge balance of the framework by NH4+ ions and thus preserves framework TO bonds from perturbation and dealumination. The Co-beta zeolite with high concentrations of regularly Td coordinated Al atoms in the framework (CoNH4-BEA) contains besides the “bare” Co ions the Co ion species adjacent to a single framework Al atom, which moreover, do not adsorb base molecules. These Co species are expected to bear extraframework oxygen(s), and they are suggested to be responsible for the high and stable SCR-NOx activity at a high content of water vapor in the reactant stream, as their concentration correlates with the C3H8SCR-NOx activity under water vapor presence.