Comparison of commercial solid acid catalysts for the esterification of acetic acid with butanol Original Research Article

Esterification of acetic acid with butanol has been studied in a heterogeneous reaction system, using a variety of solid acid catalysts. Comparative esterification experiments have been carried out using the homogeneous catalysts sulphuric acid, p-toluenesulphuric acid and a heteropolyacid. The catalysts have been characterised using gas adsorption analysis (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM) and temperature-programmed decomposition (TPD) techniques. The weight-based activity of the heterogeneous catalysts decreases in the following order: Smopex-101 > Amberlyst 15 > sulphated ZrO2 > H-USY-20 > H-BETA-12.5 > H-MOR-45 > Nb2O5 > H-ZSM-5-12.5. The low activity of ZSM-5 is a result of internal diffusion limitations in the medium sized pores of this zeolite type material. For H-MOR the low activity can be explained by pore blocking in the one-dimensional H-MOR channels. For the H-USY-type zeolites, the influence of the Si/Al ratio on catalytic activity has been examined. Although the amount of acid sites decreases with an increase in the Si/Al ratio, an optimum Si/Al ratio of 20 has been found. The activity of sulphated zirconia shows an optimum calcination temperature, even though both the amount and acidity of the acid sites increase monotonically with calcination temperature. Most likely, at higher calcination temperatures Lewis acid sites are formed and Brønsted acid sites are removed. As Brønsted acid sites are essential for catalysis of esterification reactions this explains the decrease in activity.