Heterogeneous Photocatalysis for Selective Formation of High-Value-Added Molecules: Some Chemical and Engineering Aspects

Exploring the Photocatalytic Cleavage Pathway of the β-5 Linkage Lignin Model Compound on Carbon Nitride

As a globally abundant source of biomass, lignocellulosic biomass has been the centre of attention as a potential resource for green energy generation and value-added chemical production. A key component of lignocellulosic biomass, lignin, which is comprised of aromatic monomers, is a potential feedstock for value added chemical production. The cleavage processes of the linkages between monomers to obtain high value products, however, requires significant investigation as it is a complex, non-facile process. This study focuses on the photocatalytic valorization of a β-5 lignin model compound, a key linkage in the lignin structure. It was found that greater yields of aromatic products were obtained from the photocatalytic conversion of β-5 lignin model compound using carbon nitride (CN) when compared to Evonik P25 titanium dioxide (TiO2). Products of the β-5 model compound photocatalytic conversion were determined and C-C bond cleavage was observed. It was also determined that the solvent participated in the reactions with the introduction of a cyano group to one of the products. Radical quenching experiments revealed that superoxide radicals participated in the CN photocatalytic conversion. These results reveal for the first time the products and possible mechanism of the photocatalytic transformation of β-5 model compounds using CN photocatalysis.

Role of Nb2O5 Crystal Phases on the Photocatalytic Conversion of Lignin Model Molecules and Selectivity for Value-Added Products

The photocatalytic conversion in aqueous media of phenol and guaiacol as a lignin model compound using Nb2O5 with different crystal phases was studied. Nb2O5 particles were synthesized using hydrothermal methods, where it was observed that changes in the solvent control their morphology and crystal phase. Different photocatalytic behavior of Nb2O5 was observed with the selected model compounds, indicating that its selection directly impacts the resulting conversion and selectivity rates as well as the reaction pathway, highlighting the relevance of model molecule selection. Photocatalytic conversion of phenol showed conversion rate (C%) up to 25 % after 2 h irradiation and high selectivity (S%) to pyrogallol (up to 50 %). Orthorhombic Nb2O5 spheres favored conversion through free hydroxyl radicals while monoclinic rods did not convert phenol. Guaiacol photocatalytic oxidation showed high conversion rate but lower selectivity. Orthorhombic and monoclinic Nb2O5 favored the formation of resorcinol with S % ~0.43 % (C % ~33 %) and ~13 % (C % ~27 %) respectively. The mixture of both phases enhanced the guaiacol conversion rate to ~55 % with ~17 % of selectivity to salicylaldehyde. The use of radical scavengers provided information to elucidate the reaction pathway for these model compounds, showing that different reaction pathways may be obtained for the same photocatalyst if the model compound is changed.

S-scheme homojunction of 0D cubic/2D hexagonal ZnIn2S4 for efficient photocatalytic reduction of nitroarenes

The targeted conversion of toxic nitroarenes to corresponding aminoarenes presents significant promise in simultaneously addressing environmental pollution concerns and producing value-added fine chemicals. In this study, we synthesize a 0D/2D ZnIn2S4 homojunction (CH-ZnIn2S4) by in situ growth of cubic ZnIn2S4 (C-ZnIn2S4) quantum dots onto the surface of ultrathin hexagonal ZnIn2S4 (H-ZnIn2S4) nanosheets for photocatalytic reduction of nitroarenes to aminoarenes using water as a hydrogen donor. The optimal performance of photocatalytic nitro reduction over the 0D/2D CH-ZnIn2S4 homojunction reaches 96.1% within 20 min of visible light irradiation, which is 2.45 and 1.52 times than that of C-ZnIn2S4 (39.3%) and H-ZnIn2S4 (63.3%), respectively. The improved photocatalytic performance can be attributed to the formation of a step-type S-scheme homojunction, characterized by identity chemical composition and natural lattice matching. The configuration enables continuous band bending and a low energy barrier of charge transportation, benefiting the charge transfer across the interface while maximizing their redox capabilities. Furthermore, the 2D structure of H-ZnIn2S4 nanosheets offers abundant surface sites to immobilize the 0D C-ZnIn2S4 that provides ample exposed active sites with low overpotential for HER, thereby ensuring high hydrogenation reduction activity of nitroarenes. The study is expected to inspire further interest in the reasonable design of homojunction structures for efficient and sustainable photocatalytic redox reactions.

Hydrogen-Bond-Mediated Formation of C-N or C=N Bond during Photocatalytic Reductive Coupling Reaction over CdS Nanosheets

Reductive amination of carbonyl compounds and nitro compounds represents a straightforward way to attain imines or secondary amines, but it is difficult to control the product selectivity. Herein, we report the selective formation of C-N or C=N bond readily manipulated through a solvent-induced hydrogen bond bridge, facilitating the swift photocatalytic reductive coupling process. The reductive-coupling of nitro compounds with carbonyl compounds using formic acid and sodium formate as the hydrogen donors over CdS nanosheets selectively generates imines with C=N bonds in acetonitrile solvent; while taking methanol as solvent, the C=N bonds are readily hydrogenated to the C-N bonds via hydrogen-bonding activation. Experimental and theoretical study reveals that the building of the hydrogen-bond bridge between the hydroxyl groups in methanol and the N atoms of the C=N motifs in imines facilitates the transfer of hydrogen atoms from CdS surface to the N atoms in imines upon illumination, resulting in the rapid hydrogenation of the C=N bonds to give rise to the secondary amines with C-N bonds. Our method provides a simple way to control product selectivity by altering the solvents in photocatalytic organic transformations.

Efficient Photocatalytic Partial Oxidation of Aromatic Alcohols by Using ZnIn2S4 under Green Conditions

The ternary chalcogenide ZnIn2S4 (ZIS) has been synthesized by a simple hydrothermal method in which the carcinogen thiacetamide, universally used as a precursor, has been, for the first time, replaced successfully with the harmless thiourea. ZIS has been used as photocatalyst for the partial oxidation of different aromatic alcohols to their corresponding aldehyde in water solution, under ambient conditions and simulated solar light irradiation. The photocatalytic performance of ZnIn2S4 was better than TiO2 P25. In the presence of ZIS for 4-methoxybenzyl alcohol, piperonyl alcohol, and benzyl alcohol, a selectivity towards the corresponding aldehyde of 99 % for a conversion of 46 %, 75 % for a conversion of 81 %, and 87 % for a conversion of 25 %, respectively, was obtained. For the same alcohols a selectivity of 19 % for a conversion of 41 %, 19 % for a conversion of 13 %, and 16 % for a conversion of 26 %, was observed in the presence of TiO2 P25.

Tailored Exfoliation of Polymeric Carbon Nitride for Photocatalytic H2O2 Production and CH4 Valorization Mediated by O2 Activation

The exfoliation of bulk C3N4 (BCN) into ultrathin layered structure is an effective strategy to boost photocatalytic efficiency by exposing interior active sites and accelerating charge separation and transportation. Herein, we report a novel nitrate anion intercalation-decomposition (NID) strategy that is effective in peeling off BCN into few-layer C3N4 (fl-CN) with tailored thickness down to bi-layer. This strategy only involves hydrothermal treatment of BCN in diluted HNO3 aqueous solution, followed by pyrolysis at various temperatures. The decomposition of the nitrate anions not only exfoliates BCN and changes the band structure, but also incorporates oxygen species onto fl-CN, which is conducive to O2 adsorption and hence relevant chemical processes. In photocatalytic O2 reduction under visible light irradiation, the H2O2 production rate over the optimal fl-CN-530 catalyst is 952 μmol g-1 h-1, which is 8.8 times that over BCN. More importantly, under full arc irradiation and in the absence of hole scavenger, CH4 can be photocatalytically oxidized by on-site formed H2O2 and active oxygen species to generate value-added C1 oxygenates with high selectivity of 99.2 % and record-high production rate of 1893 μmol g-1 h-1 among the metal-free C3N4-based photocatalysts.

Estimating Nonradiative Excited-State Lifetimes in Photoactive Semiconducting Nanostructures

The time evolution of the exciton generated by light adsorption in a photocatalyst is an important feature that can be approached from full nonadiabatic molecular dynamics simulations. Here, a crucial parameter is the nonradiative recombination rate between the hole and the electron that form the exciton. In the present work, we explore the performance of a Fermi's golden rule-based approach on predicting the recombination rate in a set of photoactive titania nanostructures, relying solely on the coupling of the ground and first excited state. In this scheme the analysis of the first excited state is carried out by invoking Kasha's rule thus avoiding computationally expensive nonadiabatic molecular dynamics simulations and resulting in an affordable estimate of the recombination rate. Our results show that, compared to previous ones from nonadiabatic molecular dynamics simulations, semiquantitative recombination rates can be predicted for the smaller titania nanostructures, and qualitative values are obtained from the larger ones. The present scheme is expected to be useful in the field of computational heterogeneous photocatalysis whenever a complex and computationally expensive full nonadiabatic molecular dynamics cannot be carried out.

Homogeneous-like photocatalysis: covalent immobilization of an iridium(III) complex onto polystyrene brushes grafted on SiO2 nanoparticles as a mass/charge transfer-enhanced platform

Current heterogeneous photocatalysis faces the major bottlenecks of limited mass transfer, charge recombination and tedious immobilization of expensive photocatalysts. In this work, fac-Ir(ppy)3 is directly anchored at a low cost via covalent linkage to poly(4-vinyl benzyl chloride) (PVBC) brushes grafted on SiO2 nanoparticles (PVBC@SiO2 NPs) via Friedel-Crafts alkylation, affording PVBC@SiO2 NP-supported fac-Ir(ppy)3 with high luminous efficacies such as emission lifetime and quantum yield. In the reductive cross-coupling of benzaldehydes/acetophenones with 1,4-dicyanobenzene (1,4-DCB), the as-fabricated photocatalyst affords benzhydrols in the same yields as homogeneous fac-Ir(ppy)3, except for o-substituted benzaldehydes/acetophenones. In terms of the same yields as homogeneous fac-Ir(ppy)3, a new catalytic model, named homogeneous-like photocatalysis, is proposed. In this catalytic model, the open stretching of PVBC brushes in DMSO enables the anchored fac-Ir(ppy)3 to catalyse the reaction in a similar manner as homogeneous fac-Ir(ppy)3, effectively avoiding charge recombination and mass transfer limitation. Furthermore, no significant decrease in yield (<5%) is observed over eight catalytic cycles, due to the good chemical and mechanical stabilities of PVBC@SiO2 NP-supported fac-Ir(ppy)3. Overall, the immobilization of fac-Ir(ppy)3 onto the PVBC brushes grafted on SiO2 NPs provides a mass/charge transfer-enhanced platform for supported photocatalysts.

Efficient Chemical-Free Degradation of Waterborne Micropollutants with an Immobilized Dual-Porous TiO2 Photocatalyst

Photocatalytic advanced oxidation processes (AOPs) promise a chemical-free route to energy-efficient degradation of waterborne micropollutants if long-standing mass transfer and light management issues can be overcome. Herein, we developed a dual-porous photocatalytic system consisting of a mesoporous (i.e., 2-50 nm pores) TiO2 (P25) photocatalyst supported on macroporous (i.e., >50 nm pores) fused quartz fibers (P25/QF). Our reusable photocatalytic AOP reduces chemical consumption and exhibits excellent energy efficiency, demonstrated by degrading various pharmaceutical compounds (acetaminophen, sulfamethoxazole, and carbamazepine) in natural waters with electrical energy per order (EEO) values of 4.07, 0.96, and 1.35 kWh/m3, respectively. Compared to the conventional H2O2/UVC AOP, our photocatalytic AOP can treat water without chemical additives while reducing energy consumption by over 2800%. We examine these improvements based on mass transport and optical (UVA and UVC) transmittance and demonstrate that the enhancements scale with increasing flow rate.

Bandgap tuning of TiO2 by Cu nanoparticles applied in photocatalytic antifouling-coated PES membranes through PAA-plasma grafted adhesive layer

This study developed an antifouling coating for polyethersulfone (PES) membranes by tuning the bandgap of TiO₂ with Cu nanoparticles (NPs) via a polyacrylic acid (PAA)-plasma-grafted intermediate layer. Cu NPs were synthesized at different molar ratios and precipitated onto TiO₂ using the sol-gel method. The resulting Cu@TiO₂ photocatalysts were characterized using various techniques, showing reduced bandgap, particle size range of 100-200 nm, and generation of reactive free radicals under light irradiation. The 25% Cu@TiO₂ photocatalyst displayed the highest catalytic efficiency for Acid Blue 260 (AB260) degradation, achieving 73% and 96% with and without H₂O₂, respectively. Photocatalytic membranes based on this catalyst achieved an AB260 degradation efficiency of 91% and remained stable over five cycles. Additionally, sodium alginate-fouled photocatalytic membranes fully recovered water permeability after undergoing photocatalytic degradation of foulants. The modified membrane displayed a higher surface roughness due to the presence of photocatalyst particles. This study demonstrates the potential application of Cu@TiO₂/PAA/PES photocatalytic membranes for mitigating membrane fouling in practice.

Carbon Dots Based Photoinduced Reactions: Advances and Perspective

Seeking clean energy as an alternative to traditional fossil fuels is the inevitable choice to realize the sustainable development of the society. Photocatalytic technique is considered a promising energy conversion approach to store the abundant solar energy into other wieldy energy carriers like chemical energy. Carbon dots, as a class of fascinating carbon nanomaterials, have already become the hotspots in numerous photoelectric researching fields and particularly drawn keen interests as metal-free photocatalysts owing to strong UV-vis optical absorption, tunable energy-level configuration, superior charge transfer ability, excellent physicochemical stability, facile fabrication, low toxicity, and high solubility. In this review, the classification, microstructures, general synthetic methods, optical and photoelectrical properties of carbon dots are systematically summarized. In addition, recent advances of carbon dots based photoinduced reactions including photodegradation, photocatalytic hydrogen generation, CO2 conversion, N2 fixation, and photochemical synthesis are highlighted in detail, deep insights into the roles of carbon dots in various systems combining with the photocatalytic mechanisms are provided. Finally, several critical issues remaining in photocatalysis field are also proposed.

Visible-Light-Driven Porphyrin-Based Bimetallic Metal-Organic Frameworks for Selective Photoreduction of Nitro Compounds under Mild Conditions

Selective reduction of nitroaromatics to the corresponding amines generally requires complex conditions, involving pressurized hydrogen, higher temperatures, or organic acids. In this work, we successfully prepared a series of porphyrin-based MOF photocatalysts (Pd-PMOFs, In-PMOFs, and In/Pd-PMOFs) via a facile solvothermal method for the efficient selective reduction of nitroaromatics to corresponding anilines with deionized water as the hydrogen donor. Being a new structured material (monoclinic, C₅₂H₄₀InN₆O₈Pd), on account of the abundant pore channels, strong light absorption capability, well-matched bandgap, as well as the coordination of indium ions and palladium ions, In/Pd-MOFs have excellent migration efficiency of photo-induced electrons and holes. Specifically, the In/Pd-PMOF photocatalyst manifested superior conversion (100%) and selectivity (≥80%) toward the screened nitro compounds under mild conditions. This work avoids the use of strong reductants, organic acids, and pressurized hydrogen gas as hydrogen sources, providing a promising concept for developing green catalytic systems.

Selective aqueous oxidation of aromatic alcohols under solar light in the presence of TiO2 modified with different metal species

A set of metals modified TiO₂ photocatalysts were prepared starting from titanium tetraisopropoxyde and different metal precursors to study the influence of the addition of the various foreign agents on the physico-chemical and photocatalytic properties of the catalysts. The powders were characterized by X-ray diffraction, Raman spectroscopy, specific surface area measurements, scanning electron microscopy, energy dispersive X-ray spectroscopy, UV-Vis diffuse reflectance spectroscopy, photoluminescence, temperature programmed desorption after CO₂ adsorption. The photocatalytic activity was evaluated using as probe reactions the partial oxidation of three aromatic alcohols: benzyl alcohol (BA), 4-methoxy benzyl alcohol (4-MBA), and 4-hydroxy benzyl alcohol (4-HBA) under simulated solar light irradiation. Different oxidation and selectivity values were obtained for the three substrates depending not only on the type of metals but also on the nature and position of the substituent in the phenyl ring of benzyl alcohol. As a general behaviour, the doped samples allowed the achievement of a greater selectivity especially for 4-MBA even if sometimes with minor conversions. The presence of W or Nb was beneficial for both conversion and selectivity for all the substrates with respect to bare TiO₂.

Towards the Sustainable Production of Ultra-Low-Sulfur Fuels through Photocatalytic Oxidation

Nowadays, the sulfur-containing compounds are removed from motor fuels through the traditional hydrodesulfurization technology, which takes place under harsh reaction conditions (temperature of 350–450 °C and pressure of 30–60 atm) in the presence of catalysts based on alumina with impregnated cobalt and molybdenum. According to the principles of green chemistry, energy requirements should be recognized for their environmental and economic impacts and should be minimized, i.e., the chemical processes should be carried out at ambient temperature and atmospheric pressure. This approach could be implemented using photocatalysts that are sensitive to visible light. The creation of highly active photocatalytic systems for the deep purification of fuels from sulfur compounds becomes an important task of modern catalysis science. The present critical review reports recent progress over the last 5 years in heterogeneous photocatalytic desulfurization under visible light irradiation. Specific attention is paid to the methods for boosting the photocatalytic activity of materials, with a focus on the creation of heterojunctions as the most promising approach. This review also discusses the influence of operating parameters (nature of oxidant, molar ratio of oxidant/sulfur-containing compounds, photocatalyst loading, etc.) on the reaction efficiency. Some perspectives and future research directions on photocatalytic desulfurization are also provided.

Triazine-based two dimensional porous materials for visible light-mediated oxidation of sulfides to sulfoxides with O2

Recently, triazine-based two dimensional (2D) porous materials have received increasing attention in photocatalysis. Herein, CTF-1, a covalent triazine framework, was adopted as the blueprint for designing a 2D bespoke photocatalyst. The thiazolo[5,4-d]thiazole (TzTz) linkage was inserted into the framework of CTF-1, affording TzTz-TA, which belongs to conjugated microporous polymers (CMPs). Rather than the direct insertion via the challenging CH activation, TzTz-TA was assembled from 2,4,6-tris(4-formylphenyl)-1,3,5-triazine and dithiooxamide, in which TzTz was formed in situ by a process of catalyst-free solvothermal condensation. Both CTF-1 and TzTz-TA had similar energy gaps (E_g), photocurrents, and charge carrier lifetimes, in line with the similar molecular underpinnings. However, the reduction potential of TzTz-TA is less negative than that of CTF-1 due to the insertion of TzTz linkage, in a more appropriate position for activating O₂ to superoxide (O₂^•-). In return, blue light-mediated oxidation of sulfides to sulfoxides with O₂ over TzTz-TA was accomplished with significantly superior conversions to those over CTF-1. Intriguingly, extensive sulfides could be oxidized to corresponding sulfoxides with outstanding recycling stability of TzTz-TA. Notably, attendance of an induction period was observed during TzTz-TA photocatalysis. This work highlights the vast potential of designing triazine-based porous materials to meet the tailor-made demands, such as the oxidative transformation of organic molecules with O₂.

Sandwich-Structured Hybrid of NiCo Nanoparticles-Embedded Carbon Nanotubes Grafted on C3N4 Nanosheets for Efficient Photodehydrogenative Coupling Reactions

Exploring cheap and efficient hybrid catalysts offers exciting opportunities for enhancing the performance of photocatalysts in the green organic synthesis field. Herein, a facile and effective approach is designed for the synthesis of a sandwich-structured hybrid in which NiCo bimetallic nanoparticles are embedded in the tip of nitrogen-doped carbon nanotubes (N-CNTs) grafted on both sides of a nitrogen deficient C₃N₄ (N_v-C₃N₄) nanosheet for photodehydrogenative coupling reactions. Such a brand-new type of sandwich-structured hybrid comprises N_v-C₃N₄ nanosheets and surrounding N-CNTs embedded with NiCo nanoparticles at their tips. Remarkably, the resultant hybrid exhibits integrated functionalities, abundant active sites, enhanced visible light absorption, and excellent interfacial charge transfer ability. As a result, the optimized NiCo@N-CNTs@N_v-C₃N₄ photocatalyst shows significantly improved photodehydrogenative coupling performance of amines to imines compared to the control single-metal-based catalysts (Ni@N-CNTs@N_v-C₃N₄ and Co@N-CNTs@N_v-C₃N₄). The mechanistic investigation through experimental and computational study demonstrates that, compared with single-metal-based hybrids, the NiCo bimetallic hybrid exhibits stronger amine adsorption and weaker photogenerated hydrogen atom adsorption, thus promoting the dehydrogenative activation of primary amines and fast generation of imines. This work presents a promising insight for designing and preparing efficient photocatalysts to trigger organic synthesis in high yields.

Nano Ag/AgCl wires-photocatalyzed hydrogen production and transfer hydrogenation of Knoevenagel-type products

An investigation of the relationship between the morphology of Ag/AgCl nanostructured composites with their catalytic performance has been reported.

Efficient Combination of G-C3 N4 and CDs for Enhanced Photocatalytic Performance: A Review of Synthesis, Strategies, and Applications

Recently, heterogeneous photocatalysts have achieved much interest on account of their great potential applications in resolving many tough energy and environmental troubles around the world through an ecologically sustainable way. Heterogeneous nanocomposites composed of graphitic carbon nitride (g-C₃ N₄ ) and carbon dots (CDs) possess broad spectrum absorption, appropriate electronic band structures, rapid carrier mobility, abundant reserves, excellent chemical stability, and facile synthesis methods, which make them promising composite photocatalysts for suitable applications such as photocatalytic solar fuels production and contaminant decomposition. With the rapid development in photocatalysis by hybridization of g-C₃ N₄ and CDs, a systematic summary and prospection of performance improvement are urgent and meaningful. This review first focuses on various kinds of effectively synthetic methods of composites. Following, the strategies available for enhanced performance, including morphology optimization, spectral absorption improvement, ternary or quaternary composition hybrid, lateral or vertical heterostructures construction, heteroatom doping, and so forth, are fully discussed. Then, the applications mainly in efficient photocatalytic hydrogen generation, photocatalytic carbon dioxide reduction, and organic pollutants degradation are systematically demonstrated. Finally, the remaining issues and prospect of further development are proposed as some kind of guidance for powerful combination of g-C₃ N₄ and CDs with high efficiency to photocatalysis.

Recent Progress in the Chemical Upcycling of Plastic Wastes

The massive generation of plastic wastes without satisfactory treatment has induced severe environmental problems and gained increasing attentions. In this Minireview, recent progresses in the chemical upcycling of plastic wastes by using various methods (mainly in the past three to five years) is summarized. The chemical upcycling of plastic wastes points out a "plastic-based refinery" concept, which is to use the plastic wastes as platform feedstocks to produce highly valuable monomeric or oligomeric compounds, putting the plastic wastes back into a circular economy. The different chemical methods to upcycle plastic wastes, including hydrogenolysis, photocatalysis, pyrolysis, solvolysis, and others, are introduced in each section to valorize diverse plastic feedstocks into value-added chemicals, materials, or fuels. In addition, other emerging technologies as well as the new generation of plastic thermosets are covered.

Cooperative Photocatalysis with 4-Amino-TEMPO for Selective Aerobic Oxidation of Amines over TiO2 Nanotubes

Attaching π-conjugated molecules onto TiO₂ can form surface complexes that could capture visible light. However, to make these TiO₂ surface complexes durable, integrating 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) or its analogues as a redox mediator with photocatalysis is the key to constructing selective chemical transformations. Herein, sodium 6,7-dihydroxynaphthalene-2-sulfonate (DHNS) was obtained by extending the π-conjugated system of catechol by adding a benzene ring and a substituent sodium sulfonate (-SO₃ ^- Na⁺ ). The DHNS-TiO₂ showed the best photocatalytic activity towards the blue light-induced selective aerobic oxidation of benzylamine. Compared to TEMPO, 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO) could rise above 70% in conversion of benzylamine over the DHNS-TiO₂ photocatalyst. Eventually, a wide range of amines could be selectively oxidized into imines with atmospheric O₂ by cooperative photocatalysis of DHNS-TiO₂ with 4-amino-TEMPO. Notably, superoxide (O₂ ^•- ) is crucial in coupling the photocatalytic cycle of DHNS-TiO₂ and the redox cycle of 4-amino-TEMPO. This work underscores the design of surface ligands for semiconductors and the selection of a redox mediator in visible light photocatalysis for selective chemical transformations.

Structural and Morphological Engineering of Benzothiadiazole-Based Covalent Organic Frameworks for Visible Light-Driven Oxidative Coupling of Amines

Covalent organic frameworks (COFs) are appealing platforms for photocatalysts because of their structural diversity and adjustable optical band gaps. The construction of efficient COFs for heterogeneous photocatalysis of organic transformations is highly desirable. Herein, we constructed a photoactive COF containing benzothiadiazole and triazine (BTDA-TAPT), for which the morphology and crystallinity might be easily tuned by slight synthetic variation. To unveil the relationship of photocatalytic properties between the structure and morphology, analogous COFs were synthesized by precisely tailoring building blocks. Systematic investigations indicated that tuning the structure and morphology might greatly impact photoelectric properties. The BTDA-TAPT featuring ordered alignment and perfect crystalline nature was more beneficial for promoting charge transfer and separation, which exhibited superior photocatalytic activity for visible light-driven oxidative coupling of amines. Outcomes from this study reveal the intrinsic synergy effects between the structure and morphology of COFs for photocatalysis.

Synthesis of Ni modified Au@CdS core-shell nanostructures for enhancing photocatalytic coproduction of hydrogen and benzaldehyde under visible light

The development of visible light responsive photocatalysts for simultaneous production of hydrogen (H₂) fuel and value-added chemicals is greatly promising to solve the energy and environmental issues by improving the utilization efficiency of solar energy. Herein, the three-component Ni/(Au@CdS) core-shell nanostructures were constructed by the hydrothermal synthesis followed with photodeposition. The intimate integration of plasmonic Au nanospheres and visible-light responsive CdS shells modified with Ni cocatalyst facilitated the generation and separation of electron-hole pairs as well as reduced the overpotential of hydrogen evolution. The Ni/(Au@CdS) photocatalyst exhibited excellent performance toward the selective transformation of benzyl alcohol under anaerobic conditions, and the yields of H₂ and benzaldehyde reached up to 3882 and 4242 μmol·g^-1·h^-1, respectively. The apparent quantum efficiency (AQE) was determined to be 4.09% under the irradiation of 420 nm. The systematic studies have verified the synergy of plasmonic effect and metal cocatalyst on enhancing the photocatalysis. This work highlights the desirable design and potential application of plasmonic photocatalysts for solar-driven coproduction of H₂ fuel and high-value chemicals.

Imposed dynamic irradiation to intensify photocatalytic reactions

Dynamic irradiation is a potent option to influence the interaction between photochemical reactions and mass transport to design high performant and efficient photochemical processes. To systematically investigate the impact of this parameter, the photocatalytic reduction of nitrobenzene was conducted as a test reaction. Dynamic irradiation was realized through provoked secondary flow patterns, multiple spatially distributed light emitting diodes (LEDs) and electrical pulsation of LEDs. A combined experimental and theoretical approach revealed significant potential to enhance photochemical processes. The reaction rate was accelerated by more than 70% and even more important the photonic efficiency was increased by more than a factor of 4. This renders imposed dynamic irradiation an innovative and powerful tool to intensify photoreactions on the avenue to large scale sustainable photochemical processes.

The Evolution of Photocatalytic Membrane Reactors over the Last 20 Years: A State of the Art Perspective

The research on photocatalytic membrane reactors (PMRs) started around the year 2000 with the study of wastewater treatment by degradation reactions of recalcitrant organic pollutants, and since then the evolution of our scientific knowledge has increased significantly, broadening interest in reactions such as the synthesis of organic chemicals. In this paper, we focus on some initial problems and how they have been solved/reduced over time to improve the performance of processes in PMRs. Some know-how gained during these last two decades of research concerns decreasing/avoiding the degradation of the polymeric membranes, improving photocatalyst reuse, decreasing membrane fouling, enhancing visible light photocatalysts, and improving selectivity towards the reaction product(s) in synthesis reactions (partial oxidation and reduction). All these aspects are discussed in detail in this review. This technology seems quite mature in the case of water and wastewater treatment using submerged photocatalytic membrane reactors (SPMRs), while for applications concerning synthesis reactions, additional knowledge is required.

Coupling Strategy for CO2 Valorization Integrated with Organic Synthesis by Heterogeneous Photocatalysis

Photocatalytic reduction of CO₂ to solar fuels and/or fine chemicals is a promising way to increase the energy supply and reduce greenhouse gas emissions. However, the conventional reaction system for CO₂ photoreduction with pure H₂ O or sacrificial agents usually suffers from low catalytic efficiency, poor stability, or cost-ineffective atom economy. A recent surge of developments, in which photocatalytic CO₂ valorization is integrated with selective organic synthesis into one reaction system, indicates an efficient modus operandi that enables sufficient utilization of photogenerated electrons and holes to achieve the goals for sustainable economic and social development. In this Review we discuss current advances in cooperative photoredox reaction systems that integrate CO₂ valorization with organics upgrading based on heterogeneous photocatalysis. The applications and virtues of this strategy and the underlying reaction mechanisms are discussed. The ongoing challenges and prospects in this area are critically discussed.

Photocatalytic Degradation of Sulfolane Using a LED-Based Photocatalytic Treatment System

Sulfolane is an emerging industrial pollutant detected in the environments near many oil and gas plants in North America. So far, numerous advanced oxidation processes have been investigated to treat sulfolane in aqueous media. However, there is only a few papers that discuss the degradation of sulfolane using photocatalysis. In this study, photocatalytic degradation of sulfolane using titanium dioxide (TiO2) and reduced graphene oxide TiO2 composite (RGO-TiO2) in a light-emitting diode (LED) photoreactor was investigated. The impact of different waters (ultrapure water, tap water, and groundwater) and type of irradiation (UVA-LED and mercury lamp) on photocatalytic degradation of sulfolane were also studied. In addition, a reusability test was conducted for the photocatalyst to examine the degradation of sulfolane in three consecutive cycles with new batches of sulfolane-contaminated water. The results show that LED-based photocatalysis was effective in degrading sulfolane in waters even after three photocatalytic cycles. UVA-LEDs displayed more efficient use of photon energy when compared with the mercury lamps as they have a narrow emission spectrum coinciding with the absorption of TiO2. The combination of UVA-LED and TiO2 yielded better performance than UVA-LED and RGO-TiO2 for the degradation of sulfolane. Much lower sulfolane degradation rates were observed in tap water and groundwater than ultrapure water.

Recovery/Reuse of Heterogeneous Supported Spent Catalysts

The rapid separation and efficient recycling of catalysts after a catalytic reaction are considered important requirements along with the high catalytic performances. In this view, although heterogeneous catalysis is generally less efficient if compared to the homogeneous type, it is generally preferred since it benefits from the easy recovery of the catalyst. Recycling of heterogeneous catalysts using traditional methods of separation such as extraction, filtration, vacuum distillation, or centrifugation is tedious and time-consuming. They are uneconomic processes and, hence, they cannot be carried out in the industrial scale. For these limitations, today, the research is devoted to the development of new methods that allow a good separation and recycling of catalysts. The separation process should follow a procedure economically and technically feasible with a minimal loss of the solid catalyst. The aim of this work is to provide an overview about the current trends in the methods of separation/recycling used in the heterogeneous catalysis.

Light-driven transformation of biomass into chemicals using photocatalysts - Vistas and challenges

Lignocellulosic biomass has become an important sustainable resource for fuels, chemicals and energy. It is an attractive source for alternative fuels and green chemicals because it is non-edible and widely available in the planet in huge volumes. The use of biomass as starting material to produce fuels and chemicals leads to closed carbon cycle and promotes circular economy. Although there are many thermo-chemical methods such as pyrolysis, liquefaction and gasification close at hand for processing lignocellulosic biomass and transforming the derived compounds into valuable chemicals and fuels, the photocatalytic method is more advantageous as it utilizes light and ambient conditions for reforming the said compounds. Appraisal of recent literature indicates a variety of photocatalytic systems involving different catalysts, reactors and conditions studied for this purpose. This article reviews the recent developments on the photocatalytic oxidation of biomass and its derivatives into value-added chemicals. The nature of the biomass and derived molecules, nature of the photocatalysts, efficiency of the photocatalysts in terms of conversion and selectivity, influence of reaction conditions and light sources, effect of additives and mechanistic pathways are discussed. Importance has been given also to discuss the complementary technologies that could be coupled with photocatalysis for better conversion of biomass and biomass-derived molecules to value-added chemicals. A summary of these aspects, conclusions and future prospects are given in the end.

Long-Lived Triplet Excited State in a Heterogeneous Modified Carbon Nitride Photocatalyst

Heterogeneous carbon nitrides have numerous advantages as photocatalysts, including strong light absorption, tunable band edges, and scalability, but their performance and continued development are limited by fast charge recombination and an under-developed mechanistic understanding of photodriven interfacial electron transfer. These shortcomings are a result of complex photophysics, leading to rate asynchrony between oxidation and reduction, as well as redox processes driven out of electronic trap states rather than excited states. We show that a well-defined triplet excited state in cyanamide-modified carbon nitride is realized with appropriately sized particles. The utility of this long-lived excited state is demonstrated by its ability to drive a hydroamidation photoredox cycle. By the tuning of the particle size of CN_x, the oxidation-reduction photochemistry of carbon nitride may be balanced to achieve a redox-neutral closed photocatalytic cycle. These results uncover a triplet excited state chemistry for appropriately sized CN_x particles that preludes a rich energy and electron transfer photochemistry for these materials.

Metal-free porous phosphorus-doped g-C3N4 photocatalyst achieving efficient synthesis of benzoin

Photocatalytic organic synthesis is mostly limited by the shortcomings of insufficient light absorption, high photogenerated electron–hole recombination rate and inadequate reactive sites of photocatalysts. To solve these problems, phosphorus-doped g-C₃N₄ with a porous structure was constructed. Benefiting from enhanced light absorption and electron–hole separation efficiency, PCNT has intensive oxygen activation ability to generate superoxide radicals, and is highly active in organic synthesis. In addition, PCNT has enhanced surface nucleophilicity, which is conducive to the carbon–carbon coupling process of the intermediate product benzaldehyde molecules and benzyl alcohol molecules in the benzoin condensation reaction. Metal-free PCNT is expected to replace the previously used highly toxic cyanide catalysts and provide a new way for the low-cost and efficient photocatalytic synthesis of benzoin.

Porous phosphorus-doped g-C₃N₄ (PCNT) has intensive oxygen activation ability to generate superoxide radicals, and can efficiently catalyze synthesis of benzoin from benzyl alcohol, with conversion rate and selectivity near to 100%.

A powerful azomethine ylide route mediated by TiO2 photocatalysis for the preparation of polysubstituted imidazolidines

Lewis- and Brønsted-acid catalyzed 1,3-dipolar cycloaddition between azomethine ylides and unsaturated compounds is an important strategy to construct five-membered N-heterocycles. However, such a catalytic route usually demands substrates with an electron-withdrawing group (EWG) to facilitate the reactivity. Herein, we report a TiO2 photocatalysis strategy that can conveniently prepare five-membered N-heterocyclic imidazolidines from a common imine (N-benzylidenebenzylamine) and alcohols along the route of 1,3-dipolaron azomethine ylide but without pre-installed EWG substituents on the substrates. Our EPR results uncovered the previously unknown mutual interdependence between an azomethine ylide and TiO2 photo-induced hvb+/ecb- pair. This transformation exhibited a broad scope with 21 successful examples and could be scaled up to the gram level.

Tuning Crystallinity and Surface Hydrophobicity of a Cobalt Phosphide Cocatalyst to Boost CO2 Photoreduction Performance

Photocatalytic CO₂ conversion is a promising method to yield carbon fuels, but it remains challenging to regulate catalytic materials for enhanced reaction efficiency and tunable product selectivity. This study concerns the development of a facile and efficient thermal post-treatment method to improve the crystallinity and surface hydrophobicity of a cobalt phosphide (CoP) cocatalyst, which promotes the separation and transfer of photoexcited charge carriers, reinforces CO₂ chemisorption, and weakens the H₂ O affinity. Compared with pristine CoP, the optimal CoP-600 cocatalyst displays a 3.5-fold enhancement in activity and a 2.3-fold increase in selectivity for the reduction of CO₂ to CO with a high rate of 68.1 μmol h^-1 .