Metal-free nitrogen and boron co-doped carbon catalysts for transfer hydrodeoxygenation of biomass: a model compound study

Abstract

Due to the polyoxygenated nature, hydrodeoxygenation (HDO) is generally employed to obtain bio-based chemicals from lignocellulosic biomass. Herein, a metal-free N and B co-doped porous carbon catalyst was prepared by a polymerization and carbonization strategy and applied to the catalytic transfer hydrodeoxygenation (CTHDO) of lignin-derived vanillin (VAN) to 2-methoxy-4-methylphenol (MMP). Among all the catalysts, N and B co-doped porous carbon (NSBCC-0.3, with 0.3% B content) showed the best performance, with a VAN conversion of 52.1% and a MMP selectivity of 68.6% at 240 °C under 1 MPa N₂ pressure in 2 h. Catalyst characterizations showed that B doping can lead to the formation of N–B pair structures, resulting in positively charged B sites and negatively charged adjunct N sites. The B sites with positive charges enhance the adsorption of CO from VAN, while adjunct N sites can capture the active H* from isopropanol. To gain insights into such effects, density functional theory (DFT) calculations were carried out. Seven possible N–B sites were constructed, and the VAN and H* adsorption energies on these sites were calculated. The optimal site configuration was 3-PyN-2-GaN-2; the highest valence state of B at this site was +2.01, and the VAN and H* adsorption energies were −0.3850 eV and −5.9782 eV, respectively. NH₃-TPD and CO₂-TPD analyses demonstrated that boron doping significantly modulates the acid–base site concentration, creating a favorable microenvironment for the CTHDO reaction. This work is expected to provide an efficient and environmentally friendly method for the preparation of heteroatom-doped carbon-based catalysts for biomass conversion processes.