Photocatalytic Cleavage of β-O-4 Ether Bonds in Lignin over Ni/TiO2

Changzhou Chen,Jiaping Zhao,Minghao Zhou,Jianchun Jiang,Haihong Xia,Brajendra K Sharma,Peng Liu

doi:10.3390/molecules25092109

Abstract

It is of great importance to explore the selective hydrogenolysis of β-O-4 linkages, which account for 45–60% of all linkages in native lignin, to produce valued-added chemicals and fuels from biomass employing UV light as catalyst. TiO2 exhibited satisfactory catalytic performances in various photochemical reactions, due to its versatile advantages involving high catalytic activity, low cost and non-toxicity. In this work, 20 wt.% Ni/TiO2 and oxidant PCC (Pyridinium chlorochromate) were employed to promote the cleavage of β-O-4 alcohol to obtain high value chemicals under UV irradiation at room temperature. The Ni/TiO2 photocatalyst can be magnetically recovered and efficiently reused in the following four consecutive recycling tests in the cleavage of β-O-4 ether bond in lignin. Mechanism studies suggested that the oxidation of β-O-4 alcohol to β-O-4 ketone by oxidant PCC first occurred during the reaction, and was followed by the photocatalysis of the obtained β-O-4 ketone to corresponding acetophenone and phenol derivates. Furthermore, the system was tested on a variety of lignin model substrates containing β-O-4 linkage for the generation of fragmentation products in good to excellent results.

Highlights

While fossil fuels were widely regarded as the primary source for chemicals and energy, the fraction of chemicals and fuels obtained from renewable resources, such as biomass, can be expected to be good alternatives in the future [1,2,3,4,5]
The trial for the one-step β-O-4 alcohol reduction was a failure, a two-step process was employed for the photocatalytic cleavage of β-O-4 alcohols: PCC oxidation followed by photoreduction
Experimental employed for the photocatalytic cleavage of β-O-4 alcohols: PCC oxidation followed by photoreduction

Summary

Introduction

While fossil fuels were widely regarded as the primary source for chemicals and energy, the fraction of chemicals and fuels obtained from renewable resources, such as biomass, can be expected to be good alternatives in the future [1,2,3,4,5]. Lignin contains complex natural aromatic subunits (sinapyl, coniferyl, coumaryl), and diverse types of linkages (β-O-4, α-O-4 and 4-O-5) [6,7]. Among these types of ether bonds, β-O-4 is the most abundant linkage in lignin, resulting in a variety of studies focused on the cleavage of the β-O-4 bond employing lignin dimeric model compounds (Figure 1a) [8,9]. Tremendous efforts have been devoted into the degradation of lignin by Hartwig [10], Baker [11], Dyson [12], Ellman [13], Barta [14], etc.

Methods

Results

Discussion

Conclusion