Towards directional pyrolysis of xylan: Understanding the roles of alkali/alkaline earth metals and pyrolysis temperature

Abstract

To identify the roles of alkali/alkaline earth metals (AAEM) and reaction temperature during pyrolysis of hemicellulose, the effects of AAEM doping on the pyrolysis behaviors, kinetics, and product yields of xylan at different pyrolysis temperatures were assessed. The results demonstrated that the yields of anhydrosugars from pyrolysis of xylan were dependent on the AAEM cations and their doping concentrations. Demineralization achieved the directional conversion of xylan into xylosan at 300 °C. The doping of even 0.05 mmol/g AAEM significantly suppressed their formation. The yields of xylosan from pyrolysis of xylan doped with different AAEM cations decreased in the following order: Mg2+<Ca2+<Na+<K+. The formation of light oxygenates from pyrolysis of xylan was determined by the pyrolysis temperature rather than the doping of AAEM. Elevating pyrolysis temperature significantly improved their yields whilst reducing the yields of anhydrosugars. Aromatics were only observed during the pyrolysis of xylan at 900 °C. Mg and Na exhibited higher catalytic activity for suppressing the formation of aromatics. Pyrolysis kinetic analysis showed that the doping of AAEM increased the activation energy for the pyrolysis of xylan. These findings suggest that pyrolysis of AAEM-free hemicellulose at low temperatures is the key to achieving directional pyrolysis of hemicellulose into anhydrosugars.