Preparation of ammonium dihydrogenphosphate supported on alumina ($NH_4H_2PO_4/Al_2O_3$) and its primary application as a solid acid supported heterogeneous catalyst to the synthesis of 1,2,4,5-tetrasubstituted imidazoles by a one-pot, four-component condensation of benzil, aromatic aldehydes, primary amines, and ammonium acetate under thermal solvent-free conditions were described. The results showed that the novel catalyst has high activity and the desired products were obtained in high yields. Furthermore, the products could be separated simply from the catalyst, and the catalyst could be recycled and reused with only slight reduction in its catalytic activity. Characterization of the catalyst was performed by FT-IR spectroscopy, the $N_2$ adsorption/desorption analysis (BET), thermal analysis (TG/DTG), and X-ray diffraction (XRD) techniques.