Research

Lignin, the most abundant aromatic polymer in nature, enables sustainable supply of miscellaneous aromatics as green fuels and chemicals. Obtaining the value-added aromatics from lignin, though subjected to enormous research efforts, mainly relies on depolymerization induced by activated hydrogen species or oxygen species, delivering hydrocarbons and oxygenates. The future bio-refinery demands a broad spectrum of fine chemicals, especially those containing elements other than C, H and O. Heteroatom-containing compounds have emerged as powerful reagents to participate in the bond cleavage in lignin; meanwhile, the obtained heteroatom-containing aromatics, which could be used as dye precursors, pharmaceutical precursors, hydrogen storage materials, etc., extend the application of lignin-derived products...

A mild method of photocatalytic deoxygenation of aromatic ketones to alkyl arenes was developed, which utilized alcohols as green hydrogen donors. No hydrogen evolution during this transformation suggested a mechanism of direct hydrogen transfer from alcohols. Control experiments with additives indicated the role of acid in transfer hydrogenolysis, and catalyst characterization confirmed a larger number of Lewis acidic sites on the optimal Pd/TiO2 photocatalyst. Hence, a combination of hydrogen transfer sites and acidic sites may be responsible for efficient deoxygenation without additives. The photocatalyst showed reusability and achieved selective reduction in a variety of aromatic ketones.

We report the conversion of biomass-derived polyols and sugars into methanol and syngas (CO+H2) via UV light irradiation under room temperature, and the bio-syngas can be further used for the synthesis of methanol. The cellulose and even raw wood sawdust could be converted into methanol or syngas after hydrogenolysis or hydrolysis pretreatment. We find Cu dispersed on titanium oxide nanorod (TNR) rich in defects is effective for the selective C−C bond cleavage to methanol. Methanol is obtained from glycerol with a co-production of H2. A syngas with CO selectivity up to 90% in the gas phase is obtained via controlling the energy band structure of Cu/TNR.

Here we demonstrate that photogenerated radicals can be rapidly terminated by surface hydrogen species during photocatalytic decarboxylation of fatty acids on a hydrogen-rich surface that is constructed by the interactions between H2 and Pt/TiO2 catalyst, thereby greatly inhibiting oligomerization; Cn–1 alkanes can therefore be obtained from bio-derived C12–C18 fatty acids in high yields (≥90%) under mild conditions (30 °C, H2 pressure ≤0.2 MPa) and 365 nm light-emitting dode irradiation. Industrial low-value fatty acid mixtures (namely, soybean and tall oil fatty acids) can be transformed into alkane products in high yields (up to 95%). Our research introduces an efficient biomass-upgrading approach that is enabled by subtle control of the radical intermediate conversion on a heterogeneous surface.

Photocatalytic selective oxidation of hydrocarbons to oxygenated chemicals greatly relies on catalytic materials that show high efficiency of photogenerated holes and electrons separation and visible light absorption capacity. We, herein, report one facile calcination approach with ammonium chloride and melamine as the template to synthesize nitrogen-modified Nb2O5 nanomeshes material (Nb2O5-N), which exhibits a 37-fold reaction rate larger than its commercial counterpart in photocatalytic oxidation of toluene into benzaldehyde under visible light irradiation. The reactivity is ascribed to an extended absorption spectrum within 700 nm by nitrogen modification. In addition, photocurrent response results suggest that a relaxation effect induced by nanomesh structure is beneficial for the separation of charge carriers for enhanced reactivity.

Photocatalytic H2 evolution from organic feedstocks with simultaneous utilization of photogenerated holes achieves solar energy storage and coproduces value-added chemicals. Here we show visible-light H2 production from benzyl alcohol (BAL) with controllable generation of deoxybenzoin (DOB) or benzoin (BZ) through tandem redox reactions. Particularly, DOB synthesis circumvents the use of expensive feedstocks and environmentally unfriendly catalysts that are required previously. Under the irradiation of blue LEDs, the key of steering the major product to DOB rather than BZ is to decrease the conduction band bottom potentials of the ZnIn sulfide catalysts by increasing the Zn/In ratio, which results in the dehydration of intermediate hydrobenzoin (HB) to DOB proceeding in a redox-neutral mechanism and consuming an electron–hole pair. As a proof of concept, this method is used to synthesize DOB derivatives in gram scale.

We present our recent studies on lignin's catalytic conversion to aromatic chemicals. First, we introduce our research on protolignin depolymerization via a fragmentation–hydrogenolysis process in alcohol solvents. Then, focusing on the catalytic cleavage of lignin C–C and C–O bonds, we shed light on a recapitulative adjacent functional group modification (AFGM) strategy for the conversion of lignin models. AFGM strategy begins with the adjacent functional group modification of the target C–C or C–O bond to directly decrease the bond dissociation enthalpy (BDE) of targeted bonds or generate new substrate sites to introduce the cleavage reagent for further conversion. Subsequently, on the basis of these two concepts from AFGM, we summarize our strategies on lignin depolymerization, which highlight the effects of lignin structure, catalyst character, and reaction conditions on the efficiency of strategies.

Using vanadium catalysts under visible light, we selectively cleave the C–C bonds in β-1 and β-O-4 interlinkages occluded in lignin models and extracts by an oxidative protocol. Visible light irradiation triggered single electron transfer between the substrate and the catalyst, which further induced the selective Cα–Cβ bond cleavage and generated the final aromatic products through radical intermediates. Using this photocatalytic chemistry, the reactivity of lignin models and the selectivity of Cα–Cβ bond cleavage were significantly improved. More importantly, this protocol affords aromatic monomers through the fragmentation of organosolv lignins even at room temperature, indicating the potential of photocatalytic C–C bond cleavage of lignin linkages under ambient conditions.

Depolymerization of lignin meets the difficulty in cleaving the robust aryl ether bond. Herein, through installing an internal nucleophile in the β-O-4′ linkage, the selective cleavage of aryl ether was realized by the intramolecular substitution on aryl rings affording non-phenolic arylamine products. In particular, nitrogen-modified lignin models and lignin samples were employed to generate the iminyl radical under photocatalytic reduction, which acted as the internal nucleophile inducing aryl migration from O to the N atom. The following hydrolysis released primary arylamines and α-hydroxy ketones. Mechanism studies including electron spin resonance (ESR), fluorescence quenching experiments, and density functional theory (DFT) calculations proved the aryl migration pathway. This method enables access to non-phenolic arylamine products from lignin conversion.

We demonstrate the coproduction of H2 and diesel fuel precursors from lignocellulose-derived methylfurans via acceptorless dehydrogenative C−C coupling, using a Ru-doped ZnIn2S4 catalyst a​nd driven by visible light. With this chemistry, up to 1.04 g gcatalyst−1 h−1 of diesel fuel precursors (~41% of which are precursors of branched-chain alkanes) are produced with selectivity higher than 96%, together with 6.0  mmol gcatalyst−1 h−1 of H2. Subsequent hydrodeoxygenation reactions yield the desired diesel fuels comprising straight- and branched-chain alkanes. We suggest that Ru dopants, substituted in the position of indium ions in the ZnIn2S4 matrix, improve charge separation efficiency, thereby accelerating C−H activation for the coproduction of H2 and diesel fuel precursors.

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