Exploring lignin valorisation: the application of photocatalysis for the degradation of the β-5 linkage

Abstract

The valorisation of lignin has gained significant interest in bioenergy, which is driven by the abundance of the material coupled with the potential to form value-added compounds. As a result, the range of technologies deployed for this application has increased and more recently includes advanced oxidation processes such as photocatalysis. The complexity of lignin is challenging however, and therefore model compounds, which represent key linkages in the native structure, have become crucial as both a tool for evaluating novel technologies and for providing an insight into the mechanism of conversion. Previously, the β-O-4 dimer, the most abundant linkage found in native lignin, has been extensively used as a model compound. Described herein, however, is the first report of photocatalytic TiO2 technology for the degradation of a β-5 model dimer. Under low power UV-light emitting diode irradiation, complete degradation of the β-5 compound (6.3 × 10−3 mg ml−1 min−1) was achieved along with formation and subsequent removal of reaction intermediates. Investigation into the mechanism revealed within the first 2 min of irradiation there was the formation of a diol species due to consumption of the alkene sidechain. Although the data presented highlights the complexity of the system, which is underpinned by multiple oxidative reaction pathways, an overview of the key photocatalytic processes are discussed including the impact of acetonitrile and role of reactive oxygen species.

Photocatalytic Reforming of Biomass: What Role Will the Technology Play in Future Energy Systems

“…While the use of raw biomass and lignins represents a clear step forward for photocatalytic reforming of biomass, it also highlights the need for fundamental research into the effect of the technology on different lignin bonding patterns. As such, there is research which has focused on exploring the impact photocatalysis can have on the lignin structure through deploying model compounds which contain the key linkages found in the native material, e.g., β-O-4 [ 74 – 79 ], β-5 [ 80 ] and 5–5’ [ 81 ] linkages. The proportion of these bonding patterns varies within lignin; however, in softwood species the relative proportions of β-O-4, β-5 and 5–5’ biphenyl are 45–50, 9–12 and 19–22%, respectively [ 82 ].…”

“…Similarly, research associated with β-5 linkage has typically focussed on synthesis, as opposed to photocatalytic conversion. Recently, Murnaghan and coworkers demonstrated the use of TiO 2 in the photocatalytic degradation of a β-5 model dimer [ 80 ]. Under low power (~ 1 W) UV-LED irradiation, complete degradation of the β-5 compound was achieved along with formation and subsequent removal of reaction intermediates.…”

“…Under low power (~ 1 W) UV-LED irradiation, complete degradation of the β-5 compound was achieved along with formation and subsequent removal of reaction intermediates. Murnaghan et al [ 90 ] observed the generation of a diol species within 2 mins of irradiation which was determined as the initial step in the conversion of the dimer. Subsequently, further bond cleavage resulted in the formation of several key reaction intermediates, Fig.…”

“…

Fig. 7 Scheme of events leading to the products found in the TiO 2 -mediated photocatalytic degradation of a β-5 model compound [ 90 ]

…”

Valorizing Waste Biomass via Hydrodynamic Cavitation and Anaerobic Digestion

“…The C6 sugars may then be transformed to other value added products either via biochemical routes ,− (for example, lactic acid, succinic acid, butanol) or via catalytic routes , (for example, hydroxymethyl furfural, furan dicarboxylic acid). In addition to these two main transformation routes, photocatalytic/photoelectrocatalytic valorization routes − could also be explored. Such systems are powerful advanced oxidation processes and have the capability to produce high-value products with appropriate band gap engineering and composite photoactive materials.…”