Photocatalytic synthesis of N-benzyleneamine from benzylamine on ultrathin BiOCl nanosheets under visible light

Reticular framework materials for photocatalytic organic reactions

Photocatalytic organic reactions, harvesting solar energy to produce high value-added organic chemicals, have attracted increasing attention as a sustainable approach to address the global energy crisis and environmental issues. Reticular framework materials, including metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), are widely considered as promising candidates for photocatalysis owing to their high crystallinity, tailorable pore environment and extensive structural diversity. Although the design and synthesis of MOFs and COFs have been intensively developed in the last 20 years, their applications in photocatalytic organic transformations are still in the preliminary stage, making their systematic summary necessary. Thus, this review aims to provide a comprehensive understanding and useful guidelines for the exploration of suitable MOF and COF photocatalysts towards appropriate photocatalytic organic reactions. The commonly used reactions are categorized to facilitate the identification of suitable reaction types. From a practical viewpoint, the fundamentals of experimental design, including active species, performance evaluation and external reaction conditions, are discussed in detail for easy experimentation. Furthermore, the latest advances in photocatalytic organic reactions of MOFs and COFs, including their composites, are comprehensively summarized according to the actual active sites, together with the discussion of their structure-property relationship. We believe that this study will be helpful for researchers to design novel reticular framework photocatalysts for various organic synthetic applications.

Salt-templated porous melamine-based conjugated polymers for selective oxidation of amines into imines under visible light

Conjugated polymers have a broad application foreground in the field of photocatalytic organic synthesis to produce value-added chemicals due to their functional diversity, broad light responsive ability, high thermal and chemical stability, and tunable band structure. Herein, using mixed chloride salts (i.e., NaCl/LiCl) as building template, a series of porous conjugated polymers constructed by melamine and terephthalaldehyde monomers were obtained through a Schiff-base reaction in the absence of any external solvent. Melamine-terephthalaldehyde polymer (i.e., PMTPA-x, x represents the mass ratio of salt-mixture to mixed precursors of PMTPA) materials displayed porous morphologies and possessed different energy band structures via regulating the mass ratio of mixed-salt to monomers. Specifically, PMTPA-20 has a larger specific surface area and more suitable redox potential towards the photocatalytic oxidative coupling of amines to imines. Under visible light, with molecular oxygen as oxidant, PMTPA-20 achieves 97% conversion of benzylamine in 8 h which is 3.9 times higher than that of pristine PMTPA (25% conversion in 8 h). In addition, PMTPA-20 catalyst has good structure stability and reusability performance for photocatalytic reactions.

Crystal facet and Na-doping dual engineering ultrathin BiOCl nanosheets with efficient oxygen activation for enhanced photocatalytic performance

Photocatalytic oxidation (PCO) based on semiconductors offers a sustainable and promising way for environmental remediation. However, the photocatalytic performance currently suffers from weak light-harvesting ability, rapid charge combination and a lack of accessible reactive sites. Ultrathin two-dimensional (2D) materials are ideal candidates to overcome these problems and become hotpots in the research fields. Herein, we demonstrate an ultrathin (<4 nm thick) Na-doped BiOCl nanosheets with {001} facets (Na-BOC-001) fabricated via a facile bottom-up approach. Because of the synergistic effect of highly exposed active facets and optimal Na doping on the electronic and crystal structure, the Na-BOC-001 showed an upshifted conduction band (CB) with stronger reduction potential for O₂ activation, more defective surface for enhanced O₂ adsorption, as well as the highest visible-light driven charge separation and transfer ability. Compared with the bulk counterparts (BOC-010 and BOC-001), the largest amount of active species and the best photocatalytic performance for the tetracycline hydrochloride (TC) degradation were achieved for the Na-BOC-001 under visible-light irradiation, even though it had slightly weaker visible-light absorption ability. Moreover, the effect of the Na doping and crystal facet on the possible pathways for TC degradation was investigated. This work offers a feasible and economic strategy for the construction of highly efficient ultrathin 2D materials.

Molybdenum Carbide-Based Photocatalysts: Synthesis, Functionalization, and Applications

As an effective non-noble, molybdenum carbide (Mo_xC: MoC or Mo₂C) has attracted extensive attention and is regarded as a promising research area in the near future owing to its good biocompatibility, high stability, band gap adjustability, rich valence states, and excellent catalytic activity. This Perspective summarizes the recent progress and achievements for the molybdenum carbide-based catalysts. First, the crystal and band structures of molybdenum carbides are generally presented. Second, various modifying strategies for molybdenum carbides are outlined to enhance the photocatalytic performance, including doping engineering, vacancy engineering, morphology and structure engineering, and the establishment of molybdenum carbide-based composite catalysts. Finally, potential applications in the photocatalysis area of molybdenum carbide-based photocatalyst are generalized. Future development trends and perspective for this promising material are also discussed.

Effect of grinding time on bismuth oxyhalides optical and morphological properties influence on photocatalytic removal of organic dye

Herein, we reported the synthesis of BiOX (X = Cl, Br) with different grinding time like 15 min and 30 min to analyze the evolution of physiochemical properties and the morphological evolution. The structural, optical, vibrational properties were examined by standard characterization studies. The formation of bismuth oxyhalides were confirmed by XRD and Raman studies. The crystallite size was decreased as in 30 min grinded sample whereas there is an influence of crystal structure. BiOCl (15 and 30 min) samples expelled the nanoflake like structure with the flakes arranged to form a nanoflower morphology. On comparing BiOCl (15 min), there is high orientation of nanoflakes on BiOCl (30 min) sample. As explored in BiOBr (15 and 30 min) samples, the development of nanoplates were found. The growth of nanoplates was enhanced in the better way in BiOBr (30 min) than BiOBr (15 min). The grinding time has explored a great influence on morphology. The photocatalyst test for prepared photocatalysts was performed to reduce the RhB dye. The photocatalysts showed 74%, 97%, 98% and 99.8% for BiOCl (15 min), BiOCl (30 min), BiOBr (15 min) and BiOBr (30 min). The rate constant value obtained was 0.008, 0.011, 0.021, 0.033 and 0.068 min^-1. BiOBr (30 min) sample achieved higher rate constant value. The hierarchical nanostructures and narrow bandgap has made the samples to be a potential candidate to reduce the toxic pollutants with complete efficiency.

Modulating the Acidic Properties of Mesoporous Mox-Ni0.8Cu0.2O Nanowires for Enhanced Catalytic Performance toward the Methanolysis of Ammonia Borane for Hydrogen Production

Rational construction of inexpensive, highly efficient, and stable catalysts for ammonia borane (AB) methanolysis is in high demand but still remains a great challenge. In this work, we have successfully fabricated uniform Mo_x-Ni_0.8Cu_0.2O nanowires using a simple hydrothermal method followed by a post-calcination treatment and flexibly modulated the acidity of their surface by changing the amount of Mo introduced into Ni_0.8Cu_0.2O. The Mo_0.1-Ni_0.8Cu_0.2O catalyst displayed strong catalytic activity toward AB methanolysis with an ultrahigh turnover frequency of 46.9 mol_H2 mol_cat.^-1 min^-1, which is even higher than some noble metal catalysts. In this work, an equation regarding the relationship between the quantity of moderated acid sites of catalysts and its corresponding activity toward AB methanolysis was first determined. A plausible mechanism for AB methanolysis catalyzed by Mo_x-Ni_0.8Cu_0.2O was proposed, and the benefits of the introduction of MoO₃ to Ni_0.8Cu_0.2O for enhancing the catalytic performance were also discussed. These findings can form a basis for the rational construction of inexpensive catalysts with robust performance toward AB methanolysis for hydrogen production.

Enhanced photocatalytic benzyl alcohol oxidation over Bi4Ti3O12 ultrathin nanosheets

Ultrathin Bi₄Ti₃O₁₂ nanosheets (NS) with the thickness about 3.9 nm were successfully synthesized by a hydrothermal method and were used as a photocatalyst for the oxidation of benzyl alcohol (BA) to benzaldehyde (BAD). The photocatalytic performance of NS is about 8 times higher than that of bulk Bi₄Ti₃O₁₂. In-situ FTIR of pyridine adsorption and NH₃-TPD reveal that NS has more surface Lewis acid sites (Ti⁴⁺) for the adsorption and activation of BA. The photogenerated electrons (e^-) and holes (h⁺) of NS can be fully used to produce the superoxide radicals and carbon-centered radicals, respectively. The monolayer nanosheet structure of NS not only greatly promotes the separation of photogenerated carriers, but also achieves the efficient activation of BA molecules via the CO⋯Ti coordination. This work successfully reveals the surface/interface interactions between the surface active sites of a photocatalyst and the reactive molecules via using ultrathin nanosheet as a molecular platform.

In situ synthesis of novel type Ⅱ BiOCl/CAU-17 2D/2D heterostructures with enhanced photocatalytic activity

A novel type Ⅱ BiOCl/CAU-17 2D/2D heterostructures photocatalyst was synthesized by in-situ growth of ultrathin BiOCl on the surface of CAU-17 nanorods through a solvothermal process. The 2D/2D heterostructures endow...

The bifunctional Lewis acid site improved reactive oxygen species production: a detailed study of surface acid site modulation of TiO2 using ethanol and Br−

Modulation of surface acid sites (SAS) can effectively enhance the efficiency of reactive oxygen species (ROS) production.

Facet-engineering palladium nanocrystals for remarkable photocatalytic dechlorination of polychlorinated biphenyls

Rationally constructing functionalized cocatalysts for removing chemically inert polychlorinated biphenyls is significant and challenging.

Unsaturated NiII Centers Mediated the Coordination Activation of Benzylamine for Enhancing Photocatalytic Activity over Ultrathin Ni MOF-74 Nanosheets

Creating accessible unsaturated active sites in metal-organic frameworks (MOFs) holds great promise for developing highly efficient catalysts. Herein, ultrathin Ni MOF-74 nanosheets (NMNs) with high-density coordinatively unsaturated Ni^II centers are prepared as a photocatalyst. The results of in situ ATR-IR, Raman, UV-vis DRS, and XPS suggest that abundant Ni^II centers can act as the active sites for boosting benzylamine (BA) activation via forming -Ni-NH₂- coordination intermediates. The generation of coordination intermediates assists the transfer of photo-generated holes to BA molecules for producing BA cation free radicals, better impelling the breaking of N-H bonds and the photooxidation of BA molecules. The photo-generated electrons further activate O₂ molecules to O₂^•- radicals for triggering the reaction. The experiments reveal that the coordination activation of BA molecules may be a rate-determining step on NMNs rather than the adsorption and activation of O₂ molecules. Moreover, NMNs possess a better ability for the separation of photo-generated carriers in comparison with bulk Ni MOF-74 (NMBs). As a result, NMNs achieve a kinetic rate constant of 0.538 h^-1 for the photocatalytic oxidative coupling of BA under visible light, about 50 times higher than that of NMBs (0.0011 h^-1). Finally, a probable synergetic catalytic mechanism with coordination activation and photocatalysis is discussed on a molecular level. This study not only highlights the importance of coordination activation for heterogeneous photocatalysis but also affords an inspiration for building ultrathin MOF nanosheets.

Thermo-enhanced photocatalytic oxidation of amines to imines over MIL-125-NH2@Ag@COF hybrids under visible light

Thermo-enhanced photocatalysis combines the advantages of thermocatalysis and photocatalysis and provides a very promising approach for the selective oxidation of organic compounds to value-added chemicals. In this work, the amino group in MIL-125-NH₂ first reacts with formaldehyde to form the reducing group (-NH-CH₂OH), which can in situ auto reduce the introduced Ag⁺ ions to Ag clusters/nanoparticles in the cavities. Then the formed MIL-125-NH-CH₂OH@Ag was further coated with a covalent organic framework (COF) through imine bonds to form a series of MIL-125-NH-CH₂OH@Ag@COF hybrids. Oxidative coupling of amines was selected to evaluate the photocatalytic performance of these materials under visible light at set temperatures (20-60 °C). With an optimized composition, MIL-125-NH-CH₂OH@Ag-0.5@COF-2 not only improves the optical properties, but also exhibits the highest conversion (almost 100%) of benzylamine under visible light at 60 °C and good stability for at least three cycles. Free radical capture experiments and electron spin resonance detection demonstrated that holes (h⁺), hydroxyl (˙OH) and superoxide radicals (O₂˙^-) were the active species. The results prove that the MIL-125-NH-CH₂OH@Ag@COF hybrid possessed higher photocatalytic performance than individual MIL-125-NH₂, Ag and COF on account of the efficient separation and transfer of photoinduced electrons and holes. Moreover, the promotion of the reaction temperature on the photocatalytic oxidation of amines has been reported, revealing that the conversion of benzylamine over MIL-125-NH-CH₂OH@Ag-0.5@COF-2 at 60 °C is nearly twice as high as that at 20 °C under visible light irradiation. Therefore, the thermo-enhanced photocatalytic oxidation performance of the MOF@Ag@COF hybrid demonstrates the great potential of thermal energy for further improving the photocatalytic selective oxidation performance.

Flowerlike BiOCl nanospheres fabricated by an in situ self-assembly strategy for efficiently enhancing photocatalysis

For semiconductor-based photocatalytic reactions, defect engineering has been proven as an efficient approach to enhance the photocatalytic performance. In this work, a synergistically PVP/EG-assisted in situ self-assembly strategy has been successfully developed for preparing flowerlike BiOCl nanospheres (NSP) assembled by ultrathin nanosheets (thickness of 3.8 nm) with abundant oxygen vacancies (OVs). During the hydrothermal process, PVP plays a template role in controlling the orientation of the crystallite growth, leading to the forming of nanosheets. Meanwhlie, ethylene glycol would induce the self-assembly of nanosheets into a loose hierarchical architecture duo to its stereo-hindrance effect. NSP achieves a twice higher photocatalytic conversion of benzylamine than BiOCl nanosheets (NST) under visible light. XPS, ESR, NH₃-TPD results manifest that NSP possesses more active sites including OVs and unsaturated Bi atoms than NST, because of avoiding the accumulation of ultrathin nanosheets. In situ FTIR reveals that benzylamine molecules can be chemisorbed and activated on BiOCl interfaces via forming -N^…Bi- species. The OVs can facilitate the forming of superoxide radicals (•O₂^-), achieving the selective photooxidation. Finally, a possible synergetic mechanism based on the interaction of reactants and catalyst interfaces was proposed to illustrate the photocatalytic process at the molecular level.

Aromatic Dendrimers Bearing 2,4,6-Triphenyl-1,3,5-triazine Cores and Their Photocatalytic Performance

The synthesis of two novel aromatic dendrimers structurally derived from 1,3,5-tri[1,3-diphenyl(phenyl-5-yl)phenyl-4′-yl]benzene and bearing 2,4,6-triphenyl-1,3,5-triazine cores is reported. The obtained dendrimers were used for the OLEDs construction, as well as in the role of innovative photocatalysts for the very efficient and selective oxidation of various benzylamines to respective N-benzylidene benzylamines under mild conditions.

Efficient and Selective Visible-Light-Driven Oxidative Coupling of Amines to Imines in Air over CdS@Zr-MOFs

Construction of porous photoactive MOF-based composite systems is regarded as one of the most effective strategies to improve light harvesting, increase the surface area, provide plenty of exposed active sites, and promote the reduction and oxidation abilities of some organic photocatalytic reactions. Herein, we synthesized porous CdS@Zr-MOF photocatalysts based on the representative photocatalyst CdS and crystalline Zr-MOFs, such as MOF-808, NU-1000, and PCN-222, to illustrate their excellent photocatalytic performance for the synthesis of imines in air. The morphology and composition of these photocatalysts were investigated by X-ray powder diffraction (XRD), inductively coupled plasma-atomic emission spectrometry (ICP-AES), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), indicating their crystallinity, high porosity, and interfacial interaction between constituents. Compared with individual components, these porous CdS@Zr-MOF composites could remarkably promote photocatalytic activity for the oxidative coupling of amines under air and visible-light conditions. The photocatalytic reaction showed broad substrate suitability. More importantly, the conversion yield reached up to 95% for the inactive aliphatic amines, and imines were formed as the single product. The improvement of the photocatalytic performance of CdS@Zr-MOF composites can be mainly ascribed to their high surface areas, more exposed active sites, excellent dispersion of CdS, and special porous photocatalytic systems, which tune the band gap, broaden the light response range, and facilitate the carrier separation.

Ultrathin ZnTi-LDH nanosheets for photocatalytic aerobic oxidation of aniline based on coordination activation

Aniline is chemisorbed and activated on ultrathin ZnTi-LDH nanosheets, facilitating the photocatalytic aerobic oxidation of aniline under visible light.

Photoreductive BiOCl Ultrathin Nanosheets for Highly Efficient Photocatalytic Color Switching

The reversible photocatalytic color switching systems (PCSSs) driven by semiconductor nanoparticles have attracted considerable attention because of their wide applications. However, the developed semiconductor nanoparticles with photoreductive activity are mainly limited to TiO₂-based photocatalysts, which greatly hinder their broad applications. Here we report a cocapping ligand-assisted strategy for the development of photoreductive BiOCl ultrathin nanosheets with abundant oxygen vacancies. Both the cocapping ligands and oxygen vacancies in BiOCl ultrathin nanosheets act as sacrificial electron donors to efficiently scavenge the photogenerated holes, endowing the BiOCl ultrathin nanosheets high photoreductive activity and thus enabling the photocatalytic color switching of redox dyes, such as methylene blue (MB) and neutral red. By successfully integrating the BiOCl ultrathin nanosheet/MB/H₂O color switching system with poly(vinyl alcohol) hydrogel to fabricate a twistable gel film and simultaneously solving the dye-leaching issue of the gel film in a water environment, we further demonstrate its application in a colorimetric oxygen indicator for food packaging, exhibiting high sensitivity to monitor oxygen leakage by the naked eye. We believe the work opens a new avenue for designing photoreductive semiconductor nanomaterials to enrich the PCSSs and their applications.