Mesoporous metal(IV) phosphates as high performance acid catalysts for the synthesis of photochromic bis-naphthopyran via Claisen rearrangement

Abstract

In this work, a facile and efficient one-step method for the synthesis of highly active mesoporous zirconium and tin(IV) phosphate that exhibit excellent catalytic activity for the synthesis of photochromic bis-naphthopyran via the Claisen rearrangement has been established. The synthesized catalysts were characterized by different textural and spectroscopic techniques such as XRD, FTIR spectra, SEM, TEM and N2 adsorption–desorption at −196 °C. The surface acidity of the prepared catalysts were determined by nonaqueous potentiometric titration with n-butylamine and FTIR of chemisorbed pyridine. XRD and N2 adsorption–desorption measurements showed that, the samples have some mesoporous structure. The surface acidity measurement showed that the samples contain mixture of Lewis and Bronsted acid sites. The results showed that zirconium phosphate samples have higher surface acidities than tin phosphate samples, and the samples with Zr:P ratio 1:3 and Sn:P ratio 1:2 showed the highest surface and Bronsted acidities. The prepared catalysts showed excellent catalytic activity for the synthesis of photochromic bis-naphthopyran via the Claisen rearrangement. High yield (~99 %) with 100 % selectivity of photochromic bis-naphthopyran was obtained in shorter reaction time at room temperature. These acid catalysts may solve the problematic low yield and purification difficulties of the photochromic chromenes reported previously.