Highly Selective Photocatalytic Valorization of Lignin Model Compounds Using Ultrathin Metal/CdS

Abstract

Depolymerization of recalcitrant lignin is a crucial step in realizing the full potential of transforming biomass to value-added small organic molecules. Herein, we report a photocatalytic system consisting of ultrathin CdS nanosheets decorated with first-row transition metals (M/CdS) for the direct photocleavage of lignin model compounds to small aromatic products. In the meantime, the reducing power of M/CdS is utilized to simultaneously generate another valuable product, H2, hence eliminating the necessity of sacrificial reagents and maximizing the energy conversion efficiency. We further demonstrate that, by judiciously modulating the photocatalytic conditions, it is feasible to yield different products with very high selectivity using the same catalyst of Ni/CdS. A series of control experiments were performed to investigate the mechanistic steps of each reaction and highlight the important roles played by both solvent and base in the photocleavage of the β-O-4 bond in lignin valorization.

Recent advances in the valorization of plant biomass

“…Photocatalytic lignin depolymerization has attracted increasing attention in recent years because of its mild reaction conditions, simple reaction process, and great environmental friendliness [ 80 , 81 ], making it a potential method for replacing the traditional process of lignin depolymerization. The electrons in the photocatalyst are brought into their excited state by light, which may substitute reductants, and each electron transition will produce a hole that may substitute oxidants [ 82 , 83 ].…”

Photocatalytic Conversion of Lignin into Chemicals and Fuels

“…The spatially ordered heterojunction structure formed between TiO 2 and γ‐Fe 2 O 3 , as well as an incorporation of Fe 3+ into TiO 2 , enhanced the photocatalytic activity and selectivity of TiO 2 by increasing its light absorption and decreasing its band gap energy . Sun and co‐workers synthesized a series of first‐row transition metal/CdS nanosheets, including Mn, Fe, Co, Ni, and Cu supported on CdS, for the photocatalytic oxidation and degradation of lignin models . They revealed that Ni/CdS not only showed excellent catalytic performance for conversion of lignin into aromatic products in high yields but also simultaneously promoted H 2 evolution under visible light irradiation …”

“…Sun and co‐workers synthesized a series of first‐row transition metal/CdS nanosheets, including Mn, Fe, Co, Ni, and Cu supported on CdS, for the photocatalytic oxidation and degradation of lignin models . They revealed that Ni/CdS not only showed excellent catalytic performance for conversion of lignin into aromatic products in high yields but also simultaneously promoted H 2 evolution under visible light irradiation …”

Metal Sulfide Photocatalysts for Lignocellulose Valorization

“…One-pot photocatalytic breaking of β-O-4 linkage through first oxidation of C α HOH by holes to produce ketone intermediates, followed by reductive cleavage of C β O bond was realized over ZnIn 2 S 4 [51] and Ni/CdS nanosheet. [52] Mechanistic studies revealed that the oxidation of the alcoholic group (C α HOH) by holes delivered ketone intermediates and protons, which were then reduced by electrons to hydrogen adsorbed on the surface of the catalyst. The hydrogen adsorbed on the surface could reductively break the C β O bond in the β-O-4 ketone intermediate.…”