Vanillin derived from lignin liquefaction: a sustainability evaluation

Abstract

Hydrothermal liquefaction of lignin has been demonstrated as a successful process for the synthesis of value-added phenolic chemical compounds such as vanillin. Vanillin has commercial value as a flavor and fragrance ingredient. This study performs a comparative process simulation and life cycle assessment (LCA) of synthesis of vanillin from depolymerization of lignin, one of the most abundant natural polymers on Earth. Laboratory-scale scenarios for alkali lignin treatment were analyzed using LCA (TRACI 2.1) and green design metrics (process and energy efficiency, waste prevention, renewability, and hazard/pollution avoidance); scenarios included temperature, residence time, lignin loading, gas presence, and catalyst variants. Results show that models which adhere better to green design metrics also result in environmental impact reductions, demonstrating a positive correlation between both sustainability metrics. Vanillin yield increased ~ 7% when reaction time increased from 10 to 20 min; however, the energy used for maintaining operational conditions during process increased between 10 and 50%. Catalyst selection was found to be a deterministic factor affecting results. A catalytic system comprised of a heterogeneous catalyst (nickel oxide) and acidic homogeneous catalyst (supercritical carbon dioxide) was identified as the best option; the catalyst reduced carcinogenic and ecotoxicity impacts by ~ 80 and 90%, respectively when compared to molybdenum oxide. Use of energy and dichloromethane were found to be significant overall environmental impact contributors. Laboratory results can be used and evaluated via LCA to identify sustainable pathways for commercial chemical processing development.