Composite fiber as a multifunctional catalyst support for the upgradation of lignin-based chemicals

Abstract

Efficient catalytic hydrogenation of lignin-based derivative vanillin (VAN) into valuable chemicals with a high selectivity is important yet challenging because of the lack of selective and active catalysts. Herein, a mixture of polyacrylonitrile (PAN) and water-insoluble tannin (Tan) was employed to create composite fibers through the electrospinning process, resulting in PAN–Tan composite fibers. The formation of hydrogen bonds between the –CN group of PAN and the phenolic –OH group of Tan proved advantageous for high dispersion and reduced physical entanglement, resulting in the production of thinner fibers. Leveraging abundant ortho-phenolic –OH groups of Tan, the PAN–Tan fibers were used to adsorb Pd²⁺ ions, which were reduced by in situ reduction process to Pd⁰ particles. Higher solute concentrations in the spinning solution improved the homogeneity of the electrospun PAN–Tan fibers, facilitating the dispersion and reduction of adsorbed Pd²⁺ on the fiber surface. Subsequently, these Pd/PAN–Tan fibers served as catalysts for the selective hydrogenation of the –CHO group of VAN to produce vanillyl alcohol (VAL) in an aqueous phase. Over 99% of VAN could be hydrogenated into VAL at 50 °C for 60 min under 0.5 MPa H₂. Moreover, the catalyst exhibited universal applicability for the hydrogenation of diverse typical lignin-based aromatic aldehydes. The catalytic hydrogenation of VAN was then extended to a fixed-bed reactor for 24 h, and only VAL was detected in the effluent. Our study underscored the low energy consumption for catalyst preparation without additional reducing agents and high temperature carbonization process, the stability and reusability of the Pd/PAN–Tan fiber catalyst and its potential for large-scale industrial applications.