CATALYTIC DEPOLYMERIZATION OF ALKALI LIGNIN INTO VALUABLES USING LANTHANIDE OXIDES-BASED CATALYSTS

Abstract

A heterogeneous nanocatalyst, composed of cerium oxide nanoparticles supported on nitrogen-functionalized graphene oxide sheets (FGCe), was synthesized using an in-situ reflux method. This catalyst was developed for the oxidative pyrolysis of alkali lignin in an ethanol-water system. The catalytic performance of FGCe catalysts was optimized through control of synthesis variables (e.g., graphene oxide (GO) content ranging from 0.2 to 2wt.%) and process variable (e.g., reaction temperatures (393-433 K), catalyst’s mass (10-75 mg), and reaction period (up to 90 min). The reusability of FGCe catalysts has been investigated up to 4 test cycles without employing any catalyst regeneration protocol. Among various nanocatalyst samples, the best lignin conversion and bio-oil yield were achieved using FGCe nanocatalyst prepared with 0.5wt.% GO content. Under optimized reaction conditions, the maximum bio-oil yield of 86%, corresponding to a total lignin conversion of 93%, was observed. Kinetic studies revealed that the activation energy for lignin conversion using the FGCe nanocatalyst was estimated as 24.36 kJ mol-1 at 423K. GC-MS and 1HNMR analyses were used to identify major lignin conversion products, including 2-pentanone-4-hydroxy-4-methyl, 2-methoxyphenol, nonylcyclopropane, vanillin, apocynin, homovanollic acid, and benzoic acid. Density functional theory (DFT) analysis further revealed that the breakdown of lignin structure primarily occurs at oxygen bonds, producing aromatic products. The research’s overall conclusions are anticipated to contribute to developing advanced catalytic materials for transforming lignin into valuable compounds.