Efficient transformation of hemicellulosic biomass into sugar alcohols with non-precious and stable bimetallic support catalyst

The conversion of hemicellulosic biomass to sugar alcohols commonly applies homogeneous acids and noble metals as catalysts, which is neither economical nor environmentally friendly. In order to overcome the deficiencies of the reports, a series of non-precious catalysts were synthesized through the co-precipitation method and incipient wetness impregnation (IWI) method in converting hemicellulosic biomass, i.e., hemicellulose and xylose into sugar alcohols (i.e., xylitol and arabitol). Reaction experiments showed that the Ni8.98Fe1 @ 1.54SiO2 catalyst prepared by the IWI method has excellent catalytic performance for converting xylose to sugar alcohols in the neutral aqueous solution, and can be applied to the direct conversion of hemicellulose as well. Specifically, the optimum yield of xylitol and arabitol could obtain 99.48 mol% and 26.01 mol% for xylose as the substrate and 88.16 mol% and 20.55 mol% for hemicellulose as the substrate, respectively. Characterization techniques such as X-ray diffraction (XRD), N2 adsorption-desorption isotherm, transmission electron microscopy (TEM), scanning electron microscope (SEM), H2 temperature-programmed reduction (H2-TPR), NH3 temperature-programmed desorption (NH3-TPD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy of pyridine (Py-FTIR), and SQUID vibrating sample magnetometry (SQUID -VSM) were performed to study physicochemical properties of the catalyst. N2 adsorption-desorption isotherm and NH3-TPD characterizations indicated that the catalyst with mesoporous properties and proper acidic sites is vital for hydrolyzing hemicellulose to pentose (i.e., xylose and arabinose). TEM, H2-TPR, and XPS techniques revealed that the (111) plane of metallic Ni is the main active phase for the hydrogenation of pentose into sugar alcohols, and Fe2+ and Fe3+ species act as effective promoters to enhance the pentose hydrogenation. Py-FTIR characterization and reaction conditions indicated that the appropriate ratio of Brønsted to Lewis acidic sites could facilitate the conversion of xylose to arabitol. The reaction mechanisms for converting hemicellulosic biomass to sugar alcohols were proposed based on the systematic study of reaction conditions and characterizations. N2 adsorption-desorption, H2-TPR, and XPS indicated that the catalyst with strong metal-support interaction, large specific surface area and pore volume could be beneficial for catalytic stability, which is verified in leaching and reusability tests. The catalyst has superparamagnetic, which can be quickly separated from the reaction system under the external magnetic field, facilitating the recovery and utilization of the catalyst.