Photocatalytic Pretreatment of Commercial Lignin Using TiO2-ZnO Nanocomposite-Derived Advanced Oxidation Processes for Methane Production Synergy in Lab Scale Continuous Reactors

Abstract

The photocatalytic pretreatment of lignocellulosic biomass to oxidize lignin and increase biomass stability has gained attention during the last few years. Conventional pretreatment methods are limited by the fact that they are expensive, non-renewable and contaminate the anaerobic digestate later on. The present study was focused to develop a metal-derived photocatalyst that can work with visible electromagnetic spectra light and oxidize commercial lignin liquor. During this project the advanced photocatalytic oxidation of lignin was achieved by using a quartz cube tungsten T3 Halogen 100 W lamp with a laboratory manufactured TiO2-ZnO nanoparticle (nanocomposite) in a self-designed apparatus. The products of lignin oxidation were confirmed to be vanillic acid (9.71 ± 0.23 mg/L), ferrulic acid (7.34 ± 0.16 mg/L), benzoic acid (6.12 ± 0.17 mg/L) and p-coumaric acid (3.80 ± 0.13 mg/L). These all products corresponded to 85% of the lignin oxidation products that were detectable, which is significantly more than any previously reported lignin pretreatment with even more intensity. Furthermore, all the pretreatment samples were supplemented in the form of feedstock diluent in uniformly operating continuously stirred tank reactors (CSTRs). The results of pretreatment revealed 85% lignin oxidation and later on these products did not hinder the CSTR performance at any stage. Moreover, the synergistic effects of pretreated lignin diluent were seen that resulted in 39% significant increase in the methane yield of the CSTR with constant operation. Finally, the visible light and nanoparticles alone could not pretreat lignin and when used as diluent, halted and reduced the methane yield by 37% during 4th HRT.

ZnO–SnO2–Sn nanocomposite as photocatalyst in ultraviolet and visible light

“…Zinc oxide is a well-known inorganic nanomaterial and widely used due to its photocatalytic properties. Due to its photocatalytic, antimicrobial and optical properties, it can be used as an additive in industrial and consumer products (Mirzaei and Darroudi 2017;Chu et al 2021). By adjusting the method to synthesize ZnO nanoparticles, it is possible to obtain rod-shaped, flower-shaped, cube-shaped, spherical nanoparticles, and such materials exhibit different properties (Singh and Dutta 2019).…”

Review on the preparation of fuels and chemicals based on lignin

“…Kärkäs et al 89 developed a catalytic scheme to oxidize lignin model system by combining photooxidation and palladium catalysis, which can effectively oxidize lignin model substrate at room temperature and obtain high yield of oxidation products. Chu et al 90 prepared a metal derived photocatalyst that interacted with visible electromagnetic spectrum light to oxidize commercial lignin. The obtained oxidation products contain vanillic acid, ferulic acid, benzoic acid and p -coumaric acid, accounting for about 85.0% of the detectable lignin oxidation products, and the pretreatment effect is excellent.…”

Toward the Photocatalytic Valorization of Lignin: Conversion of a Model Lignin Hexamer with Multiple Functionalities

“…18 The pretreatment and subsequent photocatalytic oxidation of commercial lignin liquor have also been reported using ZnO−TiO 2 nanoparticles, with the authors observing the formation of four main organic acids: ferulic acid, benzoic acid, p-coumaric acid, and vanillic acid with reported yields in a range of 3.8−9.71 mg L −1 . 11 While these examples demonstrate the feasibility of the process, the low yields reported suggest that photocatalysis was incapable of cleaving all the linkages present within lignin. The bonding patterns and associated bond enthalpies within lignin give rise to the structure being considered as the relatively unreactive portion of biomass.…”

“…This is evident from both observing the trends being reported in the literature and the number of excellent review articles that have been published. , Initial reports often focused on biomass fingerprint compounds such as sugars − and cellulose ,− for the generation of H 2 and/or value-added compounds. Subsequently, this provided the platform for more substantial developments including novel material synthesis, pretreatment processes, − solar activation, , and the valorization of raw biomass feedstocks such as woody materials, grass, and rice husks . While these reports represent the advancement of the technology, they also highlighted key challenges which must be addressed, especially in relation to the chemical composition of lignocellulosic materials.…”