Exploring the Photocatalytic Cleavage Pathway of the β-5 Linkage Lignin Model Compound on Carbon Nitride

Abstract

As a globally abundant source of biomass, lignocellulosic biomass has been the centre of attention as a potential resource for green energy generation and value-added chemical production. A key component of lignocellulosic biomass, lignin, which is comprised of aromatic monomers, is a potential feedstock for value added chemical production. The cleavage processes of the linkages between monomers to obtain high value products, however, requires significant investigation as it is a complex, non-facile process. This study focuses on the photocatalytic valorization of a β-5 lignin model compound, a key linkage in the lignin structure. It was found that greater yields of aromatic products were obtained from the photocatalytic conversion of β-5 lignin model compound using carbon nitride (CN) when compared to Evonik P25 titanium dioxide (TiO₂). Products of the β-5 model compound photocatalytic conversion were determined and C-C bond cleavage was observed. It was also determined that the solvent participated in the reactions with the introduction of a cyano group to one of the products. Radical quenching experiments revealed that superoxide radicals participated in the CN photocatalytic conversion. These results reveal for the first time the products and possible mechanism of the photocatalytic transformation of β-5 model compounds using CN photocatalysis.

Keywords: Biomass; Carbon nitride; Lignin model compound; Photocatalysis; Valorization.

Figure 1

Schematic showing linkages present between lignin monomers.

Figure 2

FT‐IR spectrum of CN.

Figure 3

A) XRD pattern and B) TEM images of prepared CN.

Figure 4

A) UV‐vis DRS spectra and B) plots of (αhυ)1/2 versus photon energy of CN and P25.

Figure 5

β‐5 photocatalytic degradation with CN and P25 TiO₂ under different light wavelengths and solvents(a) and plot ln(C/C0) verse time(b). (AN: pure acetonitrile, AW: acetonitrile/water mixture (50/50 v/v)).

Figure 6

Stack HPLC chromatograms of extracted substrate across reaction time periods with P25 (a) and CN (b).

Figure 7

Potential products from β‐5 conversion on CN analyzed by GC‐MS and detailed structure of β‐5 substrate.

Figure 8

Results of radical quenching experiments for P25 and CN (a) in acetonitrile/water (v/v 50 : 50) and acetonitrile (b). Hydroxyl radical yield comparison between P25 and CN in acetonitrile/water (v/v 50 : 50) and CN in acetonitrile with and without β‐5 (c).

Figure 9

Schematic of proposed β‐5 conversion mechanism on CN.