Conversion of Furfural as a Bio-Oil Model Compound over Calcium-Based Materials as Sacrificial Low-Cost Catalysts for Bio-Oil Upgrading

The stabilization and upgrading of biomass and waste-derived pyrolysis oils requires development of reliable, active and low-cost upgrading catalysts. Basic natural materials can act as such catalysts and convert reactive oxygenates present in biomass pyrolysis oils. The conversion of furfural as a model compound has been conducted in an autoclave reactor at 200 °C to 300 °C using different calcium-based materials. CaCO3, Ca(OH)2, CaO, cement raw meal (CRM) and calcined cement raw meal (cCRM) were screened for their catalytic activity and characterized using X-ray powder diffraction (XRD) and X-ray fluorescence (XRF), nitrogen physisorption, carbon dioxide temperature programmed desorption (CO2-TPD) and thermogravimetric analysis (TGA). CaCO3 and CRM had low basicity and showed no catalytic activity at 200 to 300 °C. Notably, 90% conversion of furfural was achieved at 200 °C using Ca(OH)2 with products being mostly furfural di- and trimers. For the basic CaO and cCRM, a temperature of 250 °C or above caused rapid polymerization of furfural. The proposed mechanism follows the Cannizzaro reaction of furfural, catalyzed by basic sites, polymerization of furfuryl alcohol, decarboxylation of furoic acid and decarbonylation of furfural, releasing CO, CO2 and H2O. Calcined cement raw meal showed the most promise for application as low-cost, sacrificial, basic catalyst.