Thermocatalytic Hydrodeoxygenation and Depolymerization of Waste Lignin to Oxygenates and Biofuels in a Continuous Flow Reactor at Atmospheric Pressure
Swathi Mukundan, Daria C. Boffito, Abhijit Shrotri, Luqman Atanda, Jorge Beltramini, Gregory S. Patience
Abstract
Lignin is the only biomolecule with aromatics that could serve as a feedstock to displace some petroleum for specialty chemicals. However, catalysts that are active and selective on model compounds for lignin fail when applied to real lignin with respect to performance and reaction mechanism. Here, we report kinetic data based on atomizing aqueous solutions of waste lignin, guaiacol, and syringaldehyde in a continuous catalytic fixed bed reactor operating at atmospheric pressure, a 5 s residence time, and a 30 mL min–1 (1:2 Ar:H2) volumetric flow rate (STP). The catalyst, NiMoS2 supported on activated carbon, was synthesized by a microemulsion technique and exhibited a combination of weak, strong, and very strong acid sites. Syringaldehyde reacted mostly to liquid products, and conversion increased with time-on-stream from 42% to 72% after 5 h. The main products were 2,6-dimethoxy-4-methylphenol and 1,6 dimethoxyphenol through hydrodeoxygenation and decarbonylation, respectively. Guaiacol conversion decreased with time-on-stream and ranged from 76% to 62% after 5 h. The main product was toluene via catechol, cresol, and phenol as intermediates. We propose reaction pathways for both syringaldehyde and guaiacol. The liquid fraction produced from the conversion of waste lignin contained 16 compounds that were mostly organic acids, followed by aldehydes, alcohols, and ketones.