The catalyst was prepared by mixing silica gel (1.5 g, Merck grade 60, 230–400 mesh) with a solution of NH4H2PO4 (0.6 g,5 mmol) in distilled water (10 ml). The resulting mixture was stirred for 30 min to adsorb the ammonium dihydrogen phosphate on the surface of silica gel (Scheme 2).