Effect of the treatment with H3PO4 on the catalytic activity of Nb2O5 supported on Zr-doped mesoporous silica catalyst. Case study: glycerol dehydration.

Abstract

We have previously demonstrated the influence of the niobium species over the glycerol dehydration reaction and how the catalyst regeneration by thermal treatment modified the catalytic performance due to the transformation of superficial niobium species. This experimental conclusion encouraged us to find a way to maintain and even improve the catalytic behavior of these Nb-based catalysts. Thus, herein, it is reported the influence of phosphoric acid treatment on 8 wt% Nb2O5 supported on a zirconium doped mesoporous silica (Si/Zr =5 molar ratio) catalyst, varying the Nb/P molar ratio between 0.1 and 1. Catalysts were full characterized and tested in the glycerol dehydration at 325 °C. This acid treatment modifies the nature of species present on the catalyst surface, as inferred from 31P NMR data, where the presence of zirconium hydrogenphosphate was detected. A comprehensive study of the influence of acid properties on the catalytic activity has been carried out. Thus, the selectivity to acrolein was improved, which was attributed to this hydrogenphosphate phase and the catalyst stability was associated to the existence of acid sites of low and moderate strength. The best catalyst was studied at higher reaction temperatures, showing the highest glycerol conversion and achieving an acrolein selectivity of 74%, at 350 °C. This catalyst was also regenerated, maintaining its catalytic activity.