Supported heteropoly acids offering strong option for efficient and cleaner processing for the synthesis of imidazole derivatives under solvent-free condition

Abstract

A series of 12-phosphotungstic acid (PWA) supported on various porous carriers, such as silica, alumina, titania, clay, and carbon were prepared, and their catalytic performance evaluated in the synthesis of imidazoles in solvent-free condition. It was found that PWA supported on silica (PWA/SiO(2)) showed higher activity compared to other catalysts. The observed behavior has more or less correlated with the acidic characteristic found through the potentiometrically titrated acidic sites and proton availability. The catalyst was characterized by FTIR, XRD, TGA/DSC, BET, and SEM. The presence of the Keggin structure can be followed by the above techniques, eliminating any doubt about the collapse of the supported anion. It can be observed that the process tolerates both electron donating and electron withdrawing substituents on the aldehyde with both benzil and benzoin. The general applicability of the method is demonstrated by using both benzylic and aromatic amines. The yields obtained were excellent without forming any side products such as trisubstituted imidazoles, which are normally produced in the presence of strong acids. The protocol developed using PWA/SiO(2) is superior in terms of process simplicity, reusable catalyst, high yields, short reaction time, and preclusion of toxic solvent.