Efficient dehydration of methyl lactate to acrylic acid using Ca3(PO4)2–SiO2 catalyst

A series of catalysts consisting of Ca3(PO4)2 supported on SiO2 (silicate, colloidal silica and fumed silica) and Ca3(PO4)2–SiO2 (silicate) with different loadings of Ca3(PO4)2 (70 to 95 wt.%) was prepared by sol–gel and wet-impregnation methods. These catalysts were characterized by N2 isotherm for specific surface area, XRD for crystallinity, TPD-NH3 for total acidity and TPR-CO2 for basicity and SEM for morphological features of the active phases over the support. All the catalysts were found to be active in the vapor phase dehydration of methyl lactate (ML) to give mainly acrylic acid (AA), methyl acrylate (MA). Among the catalysts, Ca3(PO4)2–SiO2(silicate) of (80:20 wt.%) was found to be an efficient catalyst in the dehydration of ML, which gave 73.6% conversion of ML with selectivity for AA and MA together (77.1%).

Research highlights
► Ca3(PO4)2–SiO2* (silicate form) with a ratio of 80:20 was the most active catalyst for conversions of ML and selectivity for AA and MA in the vapor phase dehydration of ML. The optimum conditions with Ca3(PO4)2:SiO2* (80:20 wt.%) system for higher conversion of ML and AA and MA selectivities were as; 50% aqueous feed of ML, LHSV = 0.35 h− 1, 370 °C at 20 h. Silica (SiO2) prepared from sodium silicate solution was the best support. Dispersed Ca3(PO4)2 on SiO2 support giving balanced acid–base strength of the catalyst system is necessary to obtain higher conversion of ML and selectivity for AA and MA in the dehydration of ML. Ca3(PO4)2:SiO2* (80:20 wt.%) gave constant conversion of ML (73.6%) without loss in its activity and selectivity for AA and MA together as 77.1% in 100 h continuous reaction carried under optimized conditions.