Hydrogen Bonding Promotes Alcohol C-C Coupling

Photocatalytic Coproduction of Diesel Fuel Precursors and H2 Promoted by [HSO4 -] and Water

Photocatalytic CC coupling of 2,5-dimethylfuran (DMF) derived from processing lignocellulosic biomasses coproduces drop-in fuels and green hydrogen with a low-carbon footprint. However, the high reaction barrier for CH bond breaking and uphill overall reaction lead to the slow kinetics of DMF coupling. Here, we reveal that [HSO4 -] and water can collaboratively promote the rate-limiting step of the CH bond breaking on the Ru-ZnIn2S4 catalyst. An in-depth study suggests that water mediates hole transfer to the CH bond while [HSO4 -] facilitates electron extraction, thus promoting electron and proton transfer on the Ru-ZnIn2S4 surface. Consequently, photocatalytic DMF coupling over Ru-ZnIn2S4 produces diesel fuel precursors (DFPs) and H2 with benchmarking formation rates of 1.5 g gcatal. -1 h-1 and 9.7 mmol gcatal. -1 h-1, respectively. Moreover, the selectivity of branched-chain DFPs reaches 55%. This work puts forward new insight and strategy for photocatalytic CC coupling for the synthesis of biofuels.

Artificial Photosynthesis of Formamide via an Oxidant-Free Photoinduced Radical Coupling Route over Pt-CdS

Large-scale manufacturing of formamide is always involved with the use of carbon monoxide, hence developing a series of eco-friendly synthesis routes is of great significance. Alternative feedstock of low-cost methanol is expected to fulfill this breakthrough due to its green and renewable nature; however, the overoxidation of methanol severely inhibits the efficacious formamide synthesis from methanol and ammonia through the conventional catalytic route. Herein, we report the successful development of a direct radical coupling route for converting methanol and ammonia into high-selectivity formamide and hydrogen without extra oxidants under ambient conditions. The optimized Pt-CdS photocatalysts offered an impressive formamide production rate of 1.45 mmol g-1 h-1, as well as an exceptional hole selectivity reaching up to 63.5%. The oxidant-free radical mechanism of high-efficiency formamide generation as revealed by in situ characterizations (e.g., in situ electron paramagnetic resonance and in situ transient absorption spectroscopy), stems from the photogenerated holes oxidizing the methanol to hydroxymethyl radical for subsequently direct C─N coupling with amino radical. This work demonstrates an efficient oxidant-free photoinduced radical coupling strategy with the promise of an acceptable alternative to current technologies for artificial photosynthesis formamide using clean and abundant solar energy.

Photocatalytic synthesis of ethylene glycol and hydrogen from methyl tert-butyl ether

In this work, we have developed a green and sustainable strategy for the synthesis of ethylene glycol, which is a highly valuable compound in chemical industry. In contrast to the currently applied energy-intensive process based on petroleum resources, this work demonstrates the photocatalytic pathway of methanol dehydrogenative coupling to produce ethylene glycol, utilizing methyl tert-butyl ether as the substrate to protect the hydroxyl group against oxidation. Photocatalytic tests reveal efficient C-C coupling of methyl tert-butyl ether with Pt/C-TiO(B)-650 catalyst under light irradiation, with the target product 1,2-di-tert-butoxyethane at a selectivity of 67% and a Pt-based turnover frequency of 2754 h-1. Scale up test demonstrates high stability of the system, reaching an accumulated turnover number of 120 000 as well as isolation of 13 g of the coupling product after 130 h irradiation. The target ethylene glycol is obtained by the hydrolysis of the dimer using the regenerable acidic resin catalyst.

Terpyridine Ni(II) Complex Grafted CdS Nanorods for Cooperative Selective Benzyl Alcohol Oxidation and Hydrogen Production

Efficient utilization of photogenerated charge carriers to realize photocatalytic solar fuel production and oxidative chemical synthesis is a challenging task. Herein, a conventional amidation reaction route is adopted to successfully construct a novel composite photocatalyst composed of a Ni(II)-terpyridine complex with carboxyl groups grafted on CdS nanorods (labeled as CdS@Ni(terpyC)2). Experimental results have unequivocally revealed that the as-fabricated composite catalyst exhibited a remarkable enhancement in photocatalytic activity for the dehydrogenation of benzyl alcohol under visible light, demonstrating superior hydrogen evolution efficiency and benzaldehyde selectivity, surpassing both pristine CdS and the blend of CdS and Ni(terpyC)2. The carrier dynamics study demonstrated that the Ni(terpyC)2 on the surface of CdS could quickly extract the photogenerated electrons of CdS, which reduced the carrier recombination efficiency, further improving the photocatalytic activity of the catalyst. This work illustrates the effect of surface active site engineering on photocatalysis and is expected to shed substantial inspiration on future surface modulation and design of semiconductor photocatalysts.

Selectivity switch via tuning surface static electric field in photocatalytic alcohol conversion

Photocatalysis has shown great potential in organic reactions, while controlling the selectivity is a long-standing goal and challenge due to the involvement of various radical intermediates. In this study, we have realized selectivity control in the photocatalytic conversion of alcohols via engineering the surface static electric field of the CdS semiconductor. By leveraging the Au-CdS interaction to adjust lattice strain, which influences the intensity of the surface static electric field, we altered the pathways of alcohol conversion. The increased intensity of the surface static electric field changed the activation pathways of the C-H/O-H bond, leading to the selective formation of targeted C/O-based radical intermediates and altering the selectivity from aldehydes to dimers. A wide range of alcohols, such as aromatic alcohol and thiophenol alcohol, were selectively converted into aldehyde or dimer. This work provides an effective strategy for selectively controlling reaction pathways by generating a surface electric field.

Photocatalytic Methanol Dehydrogenation Promoted Synergistically by Atomically Dispersed Pd and Clustered Pd

Supported metal in the form of single atoms, clusters, and particles can individually or jointly affect the activity of supported heterogeneous catalysts. While the individual contribution of the supported metal to the overall activity of supported photocatalysts has been identified, the joint activity of mixed metal species is overlooked because of their different photoelectric properties. Here, atomically dispersed Pd (Pd1) and Pd clusters are loaded onto CdS, serving as oxidation and reduction sites for methanol dehydrogenation. The Pd1 substitutes Cd2+, forming hole-trapping states for methanol oxidation and assisting the dispersion of photodeposited Pd clusters. Therefore, methanol dehydrogenation on CdS with supported Pd1 and Pd clusters exhibits the highest turnover frequency of 1.14 s-1 based on the Pd content and affords H2 and HCHO with a similar apparent quantum yield of 87 ± 1% at 452 nm under optimized reaction conditions. This work highlights the synergistic catalysis of supported metal for improved photocatalytic activity.

Promoting Proton Donation through Hydrogen Bond Breaking on Carbon Nitride for Enhanced H2O2 Photosynthesis

Photocatalytic H2O2 production has attracted much attention as an alternative way to the industrial anthraquinone oxidation process but is limited by the weak interaction between the catalysts and reactants as well as inefficient proton transfer. Herein, we report on a hydrogen-bond-broken strategy in carbon nitride for the enhancement of H2O2 photosynthesis without any sacrificial agent. The H2O2 photosynthesis is promoted by the hydrogen bond formation between the exposed N atoms on hydrogen-bond-broken carbon nitride and H2O molecules, which enhances proton-coupled electron transfer and therefore the photocatalytic activity. The exposed N atoms serve as proton buffering sites for the proton transfer from H2O molecules to carbon nitride. The H2O2 photosynthesis is also enhanced through the enhanced adsorption and reduction of O2 gas toward H2O2 on hydrogen-bond-broken carbon nitride because of the formation of nitrogen vacancies (NVs) and cyano groups after the intralayer hydrogen bond breaking on carbon nitride. A high light-to-chemical conversion efficiency (LCCE) value of 3.85% is achieved. O2 and H2O molecules are found to undergo a one-step two-electron reduction pathway by photogenerated hot electrons and a four-electron oxidation process to produce O2 gas, respectively. Density functional theory (DFT) calculations validate the O2 adsorption and reaction pathways. This study elucidates the significance of the hydrogen bond formation between the catalyst and reactants, which greatly increases the proton tunneling dynamics.

Hydrogen-Bond-Enhanced Photoreforming of Biomass Furans over a Urea-Incorporated Cu(II) Porphyrin Framework

Solar-driven upgrading of biomass-derived 5-hydroxylmethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) holds great promise for sustainable production of bio-plastics and resins. However, the process is limited by poor selectivity and sluggish kinetics due to the vertical coordination of HMF at relatively strong metal sites. Here, we purposely developed a Cu(II) porphyrin framework featuring side-chain incorporated urea linkages, denoted as TBUPP-Cu MOF, to render HMF a weak hydrogen bond at the urea site and flat adsorption via π-π stacking with the benzene moiety. The unique configuration promotes the approaching of -CHO of HMF to the photoexcited porphyrin ring towards kinetically and thermodynamically favourable intermediate formation and subsequent desorption. The charge localisation and orbital energy alignment enable the selective activation of O2 over the porphyrin to generate ⋅O2 - and 1O2 instead of highly oxidative H2O2 and ⋅OH via spin-flip electron transfer, which drive the ambient oxidation of proximal -CHO. The effective utilisation of redox species and circumvented over-oxidation facilitate a FDCA selectivity of >90 % with a high turnover number of 193 molHMF molCu -1. The facile purification of high-purity FDCA and zero-waste recycling of intermediates and durable catalyst feature TBUPP-Cu MOF a promising photo-oxidation platform towards net-zero biorefining and organic transformations.

Zinc-indium-sulfide favors efficient C - H bond activation by concerted proton-coupled electron transfer

C - H bond activation is a ubiquitous reaction that remains a major challenge in chemistry. Although semiconductor-based photocatalysis is promising, the C - H bond activation mechanism remains elusive. Herein, we report value-added coupling products from a wide variety of biomass and fossil-derived reagents, formed via C - H bond activation over zinc-indium-sulfides (Zn-In-S). Contrary to the commonly accepted stepwise electron-proton transfer pathway (PE-ET) for semiconductors, our experimental and theoretical studies evidence a concerted proton-coupled electron transfer (CPET) pathway. A pioneering microkinetic study, considering the relevant elementary steps of the surface chemistry, reveals a faster C - H activation with Zn-In-S because of circumventing formation of a charged radical, as it happens in PE-ET where it retards the catalysis due to strong site adsorption. For CPET over Zn-In-S, H abstraction, forming a neutral radical, is rate-limiting, but having lower energy barriers than that of PE-ET. The rate expressions derived from the microkinetics provide guidelines to rationally design semiconductor catalysis, e.g., for C - H activation, that is based on the CPET mechanism.

Metal Sulfide S-Scheme Homojunction for Photocatalytic Selective Phenylcarbinol Oxidation

Metal sulfide-based homojunction photocatalysts are extensively explored with improved photocatalytic performance. However, the construction of metal sulfide-based S-scheme homojunction remains a challenge. Herein, the fabrication of 2D CdIn2S4 nanosheets coated 3D CdIn2S4 octahedra (referred to as 2D/3D n-CIS/o-CIS) S-scheme homojunction photocatalyst is reported by simply adjustment of polyvinyl pyrrolidone amount during the solvothermal synthesis. The formation of S-scheme homojunction within n-CIS/o-CIS is systematically investigated via a series of characterizations, which can generate an internal electric field to facilitate the separation and migration of photogenerated electron-hole pairs. The 2D/3D n-CIS/o-CIS composite exhibits significantly improved photocatalytic activity and stability in the selective oxidation of phenylcarbinol (PhCH2OH) to benzaldehyde (PhCHO) when compared to pure n-CIS and o-CIS samples under visible light irradiation. It is hoped that this work can contribute novel insights into the development of metal sulfides S-scheme homojunction photocatalysts for solar energy conversion.

Tailoring the catalytic sites by regulating photogenerated electron/hole pairs separation spatially for simultaneous selective oxidation of benzyl alcohol and hydrogen evolution

Photocatalytic selective oxidation of alcohols into aldehydes and H2 is a green strategy for obtaining both value-added chemicals and clean energy. Herein, a dual-purpose ZnIn2S4@CdS photocatalyst was designed and constructed for efficient catalyzing benzyl alcohol (BA) into benzaldehyde (BAD) with coupled H2 evolution. To address the deep-rooted problems of pure CdS, such as high recombination of photogenerated carriers and severe photo-corrosion, while also preserving its superiority in H2 production, ZnIn2S4 with a suitable band structure and adequate oxidizing capability was chosen to match CdS by constructing a coupled reaction. As designed, the photoexcited holes (electrons) in the CdS (ZnIn2S4) were spatially separated and transferred to the ZnIn2S4 (CdS) by electrostatic pull from the built-in electric field, leading to expected BAD production (12.1 mmol g-1 h-1) at the ZnIn2S4 site and H2 generation (12.2 mmol g-1 h-1) at the CdS site. This composite photocatalyst also exhibited high photostability due to the reasonable hole transfer from CdS to ZnIn2S4. The experimental results suggest that the photocatalytic transform of BA into BAD on ZnIn2S4@CdS is via a carbon-centered radical mechanism. This work may extend the design of advanced photocatalysts for more chemicals by replacing H2 evolution with N2 fixation or CO2 reduction in the coupled reactions.

Heterogeneous Photocatalytic Strategies for C(sp3 )-H Activation

Activation of ubiquitous C(sp3 )-H bonds is extremely attractive but remains a great challenge. Heterogeneous photocatalysis offers a promising and sustainable approach for C(sp3 )-H activation and has been fast developing in the past decade. This Minireview focuses on mechanism and strategies for heterogeneous photocatalytic C(sp3 )-H activation. After introducing mechanistic insights, heterogeneous photocatalytic strategies for C(sp3 )-H activation including precise design of active sites, regulation of reactive radical species, improving charge separation and reactor innovations are discussed. In addition, recent advances in C(sp3 )-H activation of hydrocarbons, alcohols, ethers, amines and amides by heterogeneous photocatalysis are summarized. Lastly, challenges and opportunities are outlined to encourage more efforts for the development of this exciting and promising field.

Tailoring Charge Separation in ZnIn2S4@CdS Hollow Nanocages for Simultaneous Alcohol Oxidation and CO2 Reduction under Visible Light

Artificial photosynthesis provides a sustainable strategy for producing usable fuels and fine chemicals and attracts broad research interest. However, conventional approaches suffer from low reactivity or low selectivity. Herein, we demonstrate that photocatalytic reduction of CO2 coupled with selective oxidation of aromatic alcohol into corresponding syngas and aromatic aldehydes can be processed efficiently and fantastically over the designed S-scheme ZnIn2S4@CdS core-shell hollow nanocage under visible light. In the ZnIn2S4@CdS heterostructure, the photoexcited electrons and holes with weak redox capacities are eliminated, while the photoexcited electrons and holes with powder redox capacities are separated spatially and preserved on the desired active sites. Therefore, even if there are no cocatalysts and no vacancies, ZnIn2S4@CdS exhibits high reactivity. For instance, the CO production of ZnIn2S4@CdS is about 3.2 and 3.4 times higher than that of pure CdS and ZnIn2S4, respectively. More importantly, ZnIn2S4@CdS exhibits general applicability and high photocatalytic stability. Trapping agent experiments, 13CO2 isotopic tracing, in situ characterizations, and theoretical calculations reveal the photocatalytic mechanism. This study provides a new strategy to design efficient and selective photocatalysts for dual-function redox reactions by tailoring the active sites and regulating vector separation of photoexcited charge carriers.

Roles of extracellular polymeric substances in the adsorption and removal of norfloxacin during hydrothermal treatment of sewage sludge

Hydrothermal treatment (HT) is promising to remove antimicrobials from sewage sludge (SS); however, the mechanism of antimicrobial degradation during the HT of SS is not fully understood. In this study, the roles of extracellular polymeric substances (EPS) in the removal and transformation of norfloxacin (NOR) during the HT of SS at temperatures of 100 and 160 °C were investigated. The results indicated that the degradation of NOR increased with increasing HT temperature, with maximum NOR removal (52%) achieved at 160 °C. Furthermore, the NOR in sludge showed higher degradation efficiencies than the control as HT temperature was higher than 120 °C. Evident promotion effects of bound-EPS (B-EPS) in sludge were observed on the NOR degradation as HT temperature was higher than 120 °C, leading to the mineralization and deamination of protein-like components in EPS during HT. Beside, the adsorption capacity of NOR during the HT of SS decreased at temperatures higher than 120 °C. The evolution of the spatial structure of B-EPS was predominantly responsible for the adsorption of antimicrobials, a spontaneous process driven mainly by hydrophilic interactions. With the hydrothermal conversion of B-EPS, the electron transfer, and reactive species (3EPS* and ·OH) derived from B-EPS could facilitate the degradation of NOR. In particular, hydrogen bonds between B-EPS and NOR increased the apparent yield of ·OH and accelerated the decarboxylation of NOR during HT at temperatures higher than 120 °C. A toxicity evaluation suggested that HT for NOR degradation could attenuate toxicity, whereas deep oxidation or mineralization would be needed to promote ecosystem safety. These findings provide new insights into the hydrothermal activation of EPS and the interrelated hydrothermal fate of antimicrobials and other toxic pollutants in sludge.

Photocatalytic Production of Ethanolamines and Ethylenediamines from Bio-Polyols over a Cu/TiO2 Catalyst

Valorization of biomass-derived polyols into high-value-added ethanolamines and ethylenediamines is highly attractive. Herein, we report a one-step photocatalytic protocol to convert bio-polyols into a 60 % yield of ethanolamines and ethylenediamines over a multifunctional Cu/TiO2 catalyst. This catalyst enables a tandem process of photocatalytic polyol C-C bond cleavage and reductive amination in one pot at room temperature, and also allows the selective conversion of various bio-polyols and amines. Mechanistic studies revealed that photogenerated holes in TiO2 promote the retro-aldol C-C bond cleavage or oxidative dehydrogenation of polyols, and photogenerated electrons accumulate on small-sized Cu clusters, which facilitate the reductive amination via hydrogen transfer and prevent the H2 generation. This strategy provides new opportunities for the development of non-noble metal photocatalysts and methods of biomass conversion under mild conditions.

Free-Electron Inversive Modulation to Charge Antibonding Orbital of ReS2 Cocatalyst for Efficient Photocatalytic Hydrogen Generation

The free electron transfer between cocatalyst and photocatalyst has a great effect on the bonding strength between the active site and adsorbed hydrogen atom (Hads ), but there is still a lack of effective means to purposely manipulate the electron transfer in a beneficial direction of H adsorption/desorption activity. Herein, when ReSx cocatalyst is loaded on TiO2 surface, a spontaneous free-electron transfer from ReSx to TiO2 happens due to the smaller work function of ReSx , causing an over-strong S-Hads bond. To prevent the over-strong S-Hads bonds of ReSx in the ReSx /TiO2 , a free-electron reversal transfer strategy is developed to weaken the strong S-Hads bonds via increasing the work function of ReSx by incorporating O to produce ReOSx cocatalyst. Research results attest that a larger work function of ReOSx than that of TiO2 can induce reversal transfer of electrons from TiO2 to ReOSx to produce electron-rich S(2+δ)- , causing the increased antibonding-orbital occupancy of S-Hads in ReOSx /TiO2 . Accordingly, the stability of adsorbed H on S sites is availably decreased, thus weakening the S-Hads of ReOSx . In this case, an electron-rich S(2+δ)- -mediated "capture-hybridization-conversion" mechanism is raised . Benefiting from such property, the resultant ReOSx /TiO2 photocatalyst exhibits a superior H2 -evolution rate of 7168 µmol h-1 g-1 .

Synthesis of Hierarchical CdS/NiS Photocatalysts Using Ni-MOF-74 as Template for Efficient Ethanol Conversion and Hydrogen Production under Visible Light

The conversion of ethanol into high-valuable chemicals and H2 by photocatalytic process provides a sustainable approach to produce carbon-chain-prolonged chemicals and hydrogen energy. In this article, Ni-MOF-74 was added to fabricate the hierarchical CdS/NiS-N composites with an elevated specific surface area during the hydrothermal synthesis of CdS microsphere, and the Ni-MOF-74 facilitate the self-assemble growth of CdS and provide a source of Ni for the formation of NiS. The as-prepared photocatalyst was subjected to photocatalytic ethanol conversion, and the hierarchical composite material CdS/NiS-N (100) formed by adding 100 mg of Ni-MOF-74 exhibits the highest photocatalytic activity and stability in an ethanol aqueous solution with a water content of 10 %. Under visible light irradiation, the conversion rate of ethanol reached 15.2 % at the photocatalytic reaction of 5 h. The selectivity of 2,3-butanediol(2,3-BDO) was 25 %, and the selectivity of acetaldehyde(AA) was 63 %. Through various characterizations, it has been proven that a large specific surface area and the coupling interface between CdS and NiS are key factors in improving photocatalytic performance. This work provides an effective strategy for constructing photocatalysts with coupled cocatalysts/semiconductors and large specific surface areas.