Preparation and characterization of mesoporous Zr-WOx/SiO2catalysts for the esterification of 1-butanol with acetic acid

Abstract

Zr-WO_x clusters on WO_x/ZrO₂ catalysts are known to be active sites for the acid catalyzed reactions, such as dehydration of alcohols and alkane isomerization reactions. However, synthetic methods for producing high density of Zr-WO_x clusters with high surface areas are not currently available. Herein, a facile method for preparing mesoporous Zr-WO_x/SiO₂ is proposed and the effect of Zr/W ratio on its structure and acidity was examined. Results showed that the sequential hydrolysis of zirconium and tungsten via soft-templating resulted in the formation of Zr-WO_x clusters with uniform mesopore structures and a high acidity. The prepared Zr-WO_x/SiO₂ was characterized by N₂ physisorption, XRD, TEM, XPS, UV-Vis spectroscopy, NH₃-TPD and in situ FTIR. Catalytic performance for the esterification of 1-butanol with acetic acid was evaluated. The materials had a high surface area of over 500 m² g⁻¹ and ordered cylindrical pores with a uniform size of ca. 5 nm. Below a Zr/W ratio of ∼0.5, the zirconium was primarily associated with tungstate rather than SiO₂, which indicates the formation of Zr-WO_x clusters. The highest density of Zr-WO_x clusters was obtained at a Zr/W ratio of 0.3 with a strong Brønsted acidity. Consequently, Zr-WO_x/SiO₂, as a Zr/W ratio of 0.3, exhibited the highest activity with a significantly improved performance compared to HZSM-5 and WO_x/ZrO₂ catalysts.