Ammoxidation of 2-methyl pyrazine on supported ammonium salt of 12-molybdophosphoric acid catalysts: The influence of nature of support

Abstract

Influence of the nature of support on the formation of catalytically active species was investigated to clarify the key factor for the synthesis of supported ammonium salt of 12-molybdophosphoric acid (AMPA) catalyst which maintains the activity of ammoxidation during 2-methylpyrazine reaction. With this aim, different loadings of niobia-, silica- and alumina-, supported AMPA catalysts were prepared. The AMPA loading was varied in the range of 5–25 wt%. The synthesized solids were characterized by nitrogen adsorption for BET surface area, XRD and 31P MAS NMR techniques. All the AMPA-supported samples are poorly crystalline even after 25 wt% AMPA loading. Investigations using 31P MAS NMR spectroscopy of samples revealed that Keggin ion existed as at least five different species on the supports. The investigated properties were acidity of the support and amount of AMPA loading on the support. Active sites for the ammoxidation of MP on supported AMPA catalysts seem to be the interacted and/or the lacunary species. Maximum catalytic activity could be obtained at lower loadings with AMPA deposited on acidic supports whereas the less acidic supports require higher loading. It was found that in order to efficiently generate the active interactive species, the support must have an acidity which promotes the formation of support-AMPA interactive species. It is possible to enhance the catalytic activity of the supported AMPA catalyst for ammoxidation of 2-methylpyrazine by controlling the acidity of the support and AMPA loading on the support. Ammonium salt of 12-molybdophosphoric acid (AMPA) supported on niobia, silica and alumina was studied for the ammoxidation of 2-methylpyrazine to 2-cyanopyrazine. The Keggin ion existed as five different species on the supports. The interacted and/or the lacunary species were proposed to be the active sites for the ammoxidation.