Tuning Redistribution of CuOx Nanoparticles on TiO2 Support.

Nanoparticles exhibit unique catalytic performance, depending on their nanoscale size. However, controlling the particle size of the supported catalysts is still challenging. Here, we present a method for tunable redistribution of CuOx nanoparticles on rutile TiO2 support by physically adding pristine TiO2. The redistribution is driven by the work function difference (WFD) between the TiO2 support and the TiO2 additive, both of which exhibit distinct values, as determined through Kelvin probe force microscopy and electron binding energy analysis. Addition of TiO2 with lower work function (rutile) promotes electron transfer toward the CuOx/TiO2 composite, resulting in nanoparticle aggregation, while addition of TiO2 with higher work function (anatase) results in smaller CuOx on TiO2. The increase in particle size and electron density of CuOx, driven by the addition of rutile TiO2, promoted the complete conversion of nitrobenzene (100%) within 5 h. This is 2.7 times that of dispersed and degraded CuOx driven by mixing with anatase TiO2 (36.9%).

Visible-Light-Driven Furfural Oxidation over CuOx /Nb2 O5.

Maleic anhydride (MA) is an important polyester monomer that can be produced from oxidizing renewable furfural derived from biomass. However, MA generation from furfural requires harsh reaction conditions, and suffers from low efficiency and solvent corrosion. Herein, we design a Nb2 O5 photocatalyst loaded of highly dispersed CuOx (CuOx /Nb2 O5 ), which selectively catalyzes furfural oxidation to MA and the precursor (5-hydroxy-2(5H)-furanone, HF). Due to CuOx loading and forming a complex of ligand to metal charge transfer (LMCT) between the Nb2 O5 surface and adsorbed furfural, the CuOx /Nb2 O5 absorbs visible light to activate furfural though Nb2 O5 has a large band-gap energy (3.2 eV). Singlet oxygen (1 O2 ) is the key active species for C-C bond cleavage and CO generation. MA and HF is produced with a combined yield of 59 % under optimized conditions. This work provides a mild way to provide renewable maleic anhydride via oxidative C-C bond cleavage.

Photocatalytic 2-Iodoethanol Coupling to Produce 1,4-Butanediol Mediated by TiO2 and a Catalytic Nickel Complex.

Photocatalytic 2-iodoethanol (IEO) coupling provides 1,4-butanediol (BDO) of particular interest to produce degradable polyesters. However, the reduction potential of IEO is too negative (-1.9 vs NHE) to be satisfied by most of the semiconductors, and the kinetics of transferring one electron for IEO coupling is slow. Here we design a catalytic Ni complex, which works synergistically with TiO2 , realizing reductive coupling of IEO powered by photo-energy. Coordinating by terpyridine stabilizes Ni2+ from being photo-deposited to TiO2 , thereby retaining the steric configuration beneficial for IEO coupling. The Ni complex can rapidly extract electrons from TiO2 , generating a low-valent Ni capable of reducing IEO. The photocatalytic IEO coupling thus provides BDO in 72 % selectivity. By a stepwise procedure, BDO is obtained with 70 % selectivity from ethylene glycol. This work put forward a strategy for the photocatalytic reduction of molecules requiring strong negative potential.

Photocatalytic reductive C-O bond scission promoted by low-work-function Cd single atoms and clusters.

C-O bond scission via photocatalysis is an important step in biomass depolymerization. Here, we demonstrate the scission of strong ether C-O bonds promoted by low-work-function Cd single atoms and clusters. Their loading on ZnS benefits C-H bond scission, thus weakening the C-O bond for chemical bond breaking.

Hydrogen Bonding Promotes Alcohol C-C Coupling.

Photocatalytic C-C bond formation coupled with H2 production provides a sustainable approach to producing carbon-chain-prolonged chemicals and hydrogen energy. However, the involved radical intermediates with open-shell electronic structures are highly reactive, experiencing predominant oxidative or reductive side reactions in semiconductors. Herein, we demonstrate that hydrogen bonding on the catalyst surface and in the bulk solution can inhibit oxidation and reverse reaction of α-hydroxyethyl radicals (αHRs) in photocatalytic dehydrocoupling of ethanol over Au/CdS. Intentionally added water forms surface hydrogen bonds with adsorbed αHRs and strengthens the hydrogen bonding between αHRs and ethanol while maintaining the flexibility of radicals in solution, thereby allowing for αHRs' desorption from the Au/CdS surface and their stabilization by a solvent. The coupling rate of αHR increases by 2.4-fold, and the selectivity of the target product, 2,3-butanediol (BDO), increases from 37 to 57%. This work manifests that nonchemical bonding interactions can steer the reaction paths of radicals for selective photocatalysis.

Radical generation and fate control for photocatalytic biomass conversion.

Photocatalysis is an emerging approach for sustainable chemical production from renewable biomass under mild conditions. Active radicals are always generated as key intermediates, in which their high reactivity renders them versatile for various upgrading processes. However, controlling their reaction is a challenge, especially in highly functionalized biomass frameworks. In this Review, we summarize recent advanced photocatalytic systems for selective biomass valorization, with an emphasis on their distinct radical-mediated reaction patterns. The strategies for generating a specific radical intermediate and controlling its subsequent conversion towards desired chemicals are also highlighted, aiming to provide guidance for future studies. We believe that taking full advantage of the unique reactivity of radical intermediates would provide great opportunities to develop more efficient photocatalytic systems for biomass valorization.

Photocatalytic transformations of lignocellulosic biomass into chemicals.

As the largest renewable carbon resource, lignocellulosic biomass has great potential to replace fossil resources for the production of high-value chemicals, in particular organic oxygenates. Catalytic transformations of lignocellulosic biomass using solar energy have attracted much recent attention, because of unique reactive species and reaction patterns induced by photo-excited charge carriers or photo-generated reactive species as well as the mild reaction conditions, which may enable the precise cleavage of target chemical bonds or selective functionalisation of specific functional groups with other functional groups kept intact. Here, we present a critical review on recent advances in the photocatalytic transformation of lignocellulosic biomass with an emphasis on photocatalytic cleavage of C-O and C-C bonds in major components of lignocellulosic biomass, including polysaccharides and lignin, and the photocatalytic valorisation of some key platform molecules. The key issues that control the reaction paths and the reaction mechanism will be discussed to offer insights to guide the design of active and selective photocatalytic systems for biomass valorisation under mild conditions. The challenges and future opportunities in photocatalytic transformations of lignocellulosic biomass are also analysed.

Amine-Mediated Bond Cleavage in Oxidized Lignin Models.

Introducing amines/ammonia into lignin cracking will allow novel bond cleavage pathways. Herein, a method of amines/ammonia-mediated bond cleavage in oxidized lignin ß-O-4 models was studied using a copper catalyst at room temperature, demonstrating the effect of the amine source on the selectivity of products. For primary and secondary aliphatic amines, lignin ketone models underwent oxidative Cα -Cß bond cleavage and Cα -N bond formation to generate aromatic amides. For ammonia, the competition between oxygen and ammonia determined the selectivity between Cα -N and Cß -N bond formation, generating amides and α-keto amides, respectively. For tertiary amines, the lignin models underwent oxidative Cα -Cß bond cleavage to benzoic acids. Control experiments indicated that amines act as nucleophiles attacking at the Cα or Cß position of the oxidized ß-O-4 linkage to be cleaved. This study represents a novel example that the breakage of oxidized lignin model can be regulated by amines with a copper catalyst.

Generation and Confinement of Long-Lived N-Oxyl Radical and Its Photocatalysis.

Generation of controllable carbon radical under the assistance of N-oxyl radical is an efficient method for the activation of C-H bonds in hydrocarbons. We herein report that irradiation of α-Fe2O3 and N-hydroxyphthalimide (NHPI) under 455 nm light generates phthalimide-N-oxyl radical (PINO*), which after being formed by oxidation with holes, is confined on α-Fe2O3 surface. The half-life time of the confined radical reaches 22 s as measured by in situ electron paramagnetic resonance (EPR) after the light being turned off. This allows the long-lived N-oxyl radical to abstract the H from C-H bond to form a carbon radical that reacts with molecular oxygen to form R3C-OO· species, decomposition of which leads to oxygenated products.

Redox inactive metal ion triggered N-dealkylation by an iron catalyst with dioxygen activation: a lesson from lipoxygenases.

Utilization of dioxygen as the terminal oxidant at ambient temperature is always a challenge in redox chemistry, because it is hard to oxidize a stable redox metal ion like iron(III) to its high oxidation state to initialize the catalytic cycle. Inspired by the dioxygenation and co-oxidase activity of lipoxygenases, herein, we introduce an alternative protocol to activate the sluggish iron(III) species with non-redox metal ions, which can promote its oxidizing power to facilitate substrate oxidation with dioxygen, thus initializing the catalytic cycle. In oxidations of N,N-dimethylaniline and its analogues, adding Zn(OTf)2 to the [Fe(TPA)Cl2]Cl catalyst can trigger the amine oxidation with dioxygen, whereas [Fe(TPA)Cl2]Cl alone is very sluggish. In stoichiometric oxidations, it has also been confirmed that the presence of Zn(OTf)2 can apparently improve the electron transfer capability of the [Fe(TPA)Cl2]Cl complex. Experiments using different types of substrates as trapping reagents disclosed that the iron(IV) species does not occur in the catalytic cycle, suggesting that oxidation of amines is initialized by electron transfer rather than hydrogen abstraction. Combined experiments from UV-Vis, high resolution mass spectrometry, electrochemistry, EPR and oxidation kinetics support that the improved electron transfer ability of iron(III) species originates from its interaction with added Lewis acids like Zn(2+) through a plausible chloride or OTf(-) bridge, which has promoted the redox potential of iron(III) species. The amine oxidation mechanism was also discussed based on the available data, which resembles the co-oxidase activity of lipoxygenases in oxidative dealkylation of xenobiotic metabolisms where an external electron donor is not essential for dioxygen activation.