Tungstophosphoric acid/zirconia composites prepared by the sol–gel method: An efficient and recyclable green catalyst for the one-pot synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes

Abstract

Samples of zirconia modified with different contents of tungstophosphoric acid (TPA) were synthesized from zirconium propoxide via sol-gel reactions using polyethylene glycol as template and were characterized by different physicochemical techniques (BET, XRD, FT-IR, and 31P MAS-NMR). The S BET of the solids decreases and the microporosity increases with the increase of the TPA content. According to FT-IR and 31P MAS-NMR studies, the main species present in the samples is [PW 12O 40] 3- anion, which was partially transformed into [P 2W 21O 71] 6- and [PW 11O 39] 7- anions during the synthesis and drying steps. The XRD patterns of the modified samples exhibit neither the characteristic peaks of TPA nor those attributed to its decomposition products. Aryl-14H-dibenzo[a,j]xanthenes have been synthesized by a one-pot condensation of 2-naphthol and aryl aldehydes, catalyzed by tungstophosphoric acid/zirconia composites in a solvent-free medium using conventional heating. The present approach offers the advantages of clean reaction, simple methodology, short reaction time, and high yield. The reaction work-up is very simple and the catalyst can be easily separated from the reaction mixture and reused several times in subsequent reactions without appreciable loss of the catalytic activity.