Efficient and Selective C–C Bond Cleavage of a Lignin Model Using a Polyimide Photocatalyst with High Photooxidation Capability

Abstract

Lignin represents the most abundant and sustainable aromatic resource to produce value-added aromatics. However, an efficient and selective cleavage of recalcitrant C–C bonds in lignin under mild conditions remains challenging. Photocatalysis has emerged as a promising strategy for such a C–C bond cleavage under ambient conditions, although the activity and selectivity need to be further improved. Herein, using polyimide as a photocatalyst, we report an efficient and selective C–C bond cleavage in a β-O-4 lignin model under visible light at room temperature. The lignin model was converted into aromatic products with >99% substrate conversion and >99% C–C bond cleavage selectivity, which are superior to previously reported photocatalytic systems. Experimental investigations together with theoretical calculations indicated that the superior performance of the polyimide photocatalyst was attributed to its strong photooxidation capability and efficient charge carrier separation efficiency. Mechanistic studies revealed that the dehydrogenation of the lignin model driven by photogenerated holes was the rate-determining step. This work provides useful guidance for the design of high-performance photocatalysts for selective C–C bond cleavage of lignin.

Photoreforming of Waste Polymers for Sustainable Hydrogen Fuel and Chemicals Feedstock: Waste to Energy

“…324,329,330 should activate C β −H bond selectively to suppress the oxidation of hydroxyl FGs. 321,324,325,330 The three-component photocatalyst CuO x /CeO 2 /TiO 2 for the efficient selective cleavage of the C−C bond rather than β-1 oxi-dehydrogenation was also reported. 331 Likewise, synthetic polymers (plastics) are prepared by condensation polymerization or addition polymerization reactions.…”

“…Thus, we categorized the substrates as natural polymer lignocellulosic biomass (lignin, cellulose, hemicellulose) and synthetic polymers (plastic) (Figure ) based on the bond nature present in these polymers. The targeted cleavage of C–O or C–C bonds present in these polymers and the selective photocatalytic transformation of specific functional groups (FGs) with other FGs kept intact are significant challenging tasks for the selective PR and upgrading of these biomasses. , In lignocellulosic biomass, the selective conversion, and utilization of cellulosic and hemicellulosic products have been widely explored in literature due to their comparatively simple structure. , But among lignocellulosic biomass components, lignin is the largest resource of renewable aromatic compounds. However, due to its complex structure and poor utilization in the world, it is mostly dumped as waste or burned for the generation of power .…”

“…The targeted cleavage of C−O or C−C bonds present in these polymers and the selective photocatalytic transformation of specific functional groups (FGs) with other FGs kept intact are significant challenging tasks for the selective PR and upgrading of these biomasses. 320,321 In lignocellulosic biomass, the selective conversion, and utilization of cellulosic and hemicellulosic products have been widely explored in literature due to their comparatively simple structure. 157,322 But among lignocellulosic biomass components, lignin is the largest resource of renewable aromatic compounds.…”

“…The selective cleavage of C−O motifs in lignin can afford products with less than 50% theoretical yield. 321,327,328 Therefore, achieving the higher yields required selective cleaving of interlinking of C−C bonds. Actually, due to its robust and nonpolar nature, it is difficult to cleave or split C−C bonds in lignin.…”

Band structure engineering of a polyimide photocatalyst towards enhanced water splitting

“…27 However, the photocatalytic efficiency of pristine PI is still moderate because of the severe photogenerated charge recombination, unfavorable electronic band structure and poor surface reaction kinetics. To increase the photocatalytic efficiency, a number of different approaches have been investigated to date, such as doping, [32][33][34] comonomer tuning, [35][36][37][38][39] morphology engineering, [40][41][42] and composite structure design. [43][44][45][46][47][48][49] The most fundamental property of a semiconductor photocatalyst that directly affects its photocatalytic performance is its band structure.…”