Synthesis of silica xerogel–poly(styrene sulphonic acid) nanocomposites as acid catalysts: effects of temperature and polymer concentration on their textural and chemical properties

Abstract

A series of silica–poly(styrene sulphonic acid) (SiO2–PSSA) nanocomposites were prepared using the sol–gel route. Tetraethoxysilane and (3-aminopropyl)-triethoxysilane (APTES) were used as the SiO2 xerogel sources. The latter was used to provide anchoring sites for retaining the PSSA polymer via acid–base reactions between the amino groups and sulphonic acid sites (electrostatic interactions). During the synthesis, the Si/N atomic ratio was maintained constant (close to 11), whereas the temperature and the PSSA concentration (PSSA/APTES mol ratio, that is, the S/N at. ratio) were varied. The prepared organic–inorganic nanohybrids were characterised by chemical analysis, thermogravimetric analysis, N2 adsorption isotherms and DRIFT and 29Si and 13C MAS-NMR spectroscopic analyses. These materials were applied as catalysts in the dehydration of xylose to furfural and compared with a conventional sulphonic resin, with specific attention to the reusability properties of the catalysts. A synthesis temperature of 348 K and an initial S/N at. ratio between 4 and 8 provided the best balance between the textural, chemical and reutilisation properties of the nanocomposite. Regardless of the temperature and S/N at. ratio initially used for the synthesis, the catalysts deactivated during the first reuse cycle but reached a stable state in successive runs. In contrast, the sulphonic resin continuously and progressively deactivates.