Comparison of nitric and oxalic acid in the dealumination of mordenite

Abstract

The dealumination of mordenite was carried out at 100°C with nitric and oxalic acid; after calcination, the samples were characterised using various techniques. The bulk composition showed that both acids were very effective in removing aluminium, but that aluminium was extracted at a higher rate by oxalic acid even at low concentrations. Nitrogen adsorption isotherms and scanning electron microscopy (SEM) images showed the formation of a secondary pore system in the zeolite after treatment with nitric acid but not with oxalic acid. NMR analysis revealed that, at the same Si/Al ratio, the mordenite was almost free of extra-framework aluminium (EFAl) after nitric acid extraction, whereas after extraction with oxalic acid half of the aluminium was octahedrally co-ordinated. The Si(0Al) peak in the 29Si NMR was found to be split when dealumination with nitric acid was carried out due to the presence of less EFAl. Treatment with oxalic acid led to the formation of a larger number of lattice defects.