Oxidative Cleavage of α-O-4, ß-O-4, and 4-O-5 Linkages in Lignin Model Compounds over P, N Co-doped Carbon Catalyst: A Metal-free Approach.

Developing efficient metal-free catalysts for lignin valorization is essential but challenging. In this study, a cost-effective strategy is employed to synthesize a P, N co-doped carbon catalyst through hydrothermal and carbonization processes. This catalyst effectively cleaved α-O-4, ß-O-4, and 4-O-5 lignin linkages, as demonstrated with model compounds. Various catalysts were prepared at different carbonization temperatures and thoroughly characterized using techniques such as XRD, RAMAN, FTIR, XPS, NH3-TPD, and HRTEM. Attributed to higher acidity, the P5NC-500 catalyst exhibited the best catalytic activity, employing H2O2 as the oxidant in water. Additionally, this metal-free technique efficiently converted simulated lignin bio-oil, containing all three linkages, into valuable monomers. Density Functional Theory calculations provided insight into the reaction mechanism, suggesting substrate and oxidant activation by P-O-H sites in the P5NC-500, and by N-C-O-H in the CN catalyst. Moreover, the catalyst's recyclability and water utilization enhance its environmental compatibility, offering a highly sustainable approach to lignin valorization with potential applications in various industries.

Linear Scaling Relationships for Furan Hydrodeoxygenation over Transition Metal and Bimetallic Surfaces.

Brønsted-Evans-Polanyi (BEP) and transition-state-scaling (TSS) relationships have become valuable tools for the rational design of catalysts for complex reactions like hydrodeoxygenation (HDO) of bio-oil (containing heterocyclic and homocyclic molecules). In this work, BEP and TSS relationships are developed for all the elementary steps of furan activation (C and O hydrogenation and CHx -OHy scission, for both ring and open-ring intermediates) to oxygenates, ring-saturated compounds and deoxygenated products on the most stable facets of Ni, Co, Rh, Ru, Pt, Pd, Fe and Ir surfaces using Density Functional Theory (DFT) calculations. Furan ring opening barriers were found to be facile and strongly dependent on carbon and oxygen binding strength on the investigated surfaces. Our calculations suggest linear chain oxygenates form on Ir, Pt, Pd and Rh surfaces due to their low hydrogenation and high CHx -OHy scission barriers, while deoxygenated linear products are favoured on Fe and Ni surfaces due to their low CHx -OHy scission and moderate hydrogenation barriers. Bimetallic alloy catalysts were also screened for their potential HDO activity and PtFe catalysts were found to significantly lower the ring opening and deoxygenation barriers relative to the corresponding pure metals. The developed BEPs for monometallic surfaces can be extended to estimate the barriers on bimetallic surfaces for ring opening and ring hydrogenation reactions but fails to predict the barriers for open-ring activation reactions due to the change in transition state binding sites on the bimetallic surface. The obtained BEP and TSS relationships can be used to develop microkinetic models for facilitating accelerated catalyst discovery for HDO.