Investigation on the nature of catalytically active sites in non-crystalline aluminophosphates from 27Al and 31P MAS-NMR studies

Abstract

Amorphous aluminophosphate (AmAlPO4) materials were synthesized by conventional precipitation technique from different phosphate precursors (Na4P2O7, NaH2PO4, Na2HPO4, (NH4)H2PO4, (NH4)2HPO4 and H3PO4) without using any structure directing agents. All the prepared materials were analyzed by 27Al and 31P MAS-NMR, PXRD, FT-IR, XPS, BET, NH3- TPD and SEM-EDS. The catalytic activity of the materials was checked for synthesis of diphenyl urea (DPU). Phosphoric acid (H3PO4) as a precursor was found to be more active catalyst (which yielded 90% of DPU) than the salt forms. 27Al and 31P MAS-NMR studies showed the presence of variable percentages of tetra and penta coordinated Al species, which are attributed to the catalytic activity of AlPO4. AmAlPO4 materials prepared were amorphous having intrinsic mesoporosity, moderate surface area and moderately strong acidic sites. A synergetic combination of all these surface properties play a subtle role in determining the activity of the differently coordinated Al species in the catalysts.