One-pot synthesis of secondary amine via photoalkylation of nitroarenes with benzyl alcohol over Pd/monolayer H1.07Ti1.73O4·H2O nanosheets

Competitive photooxidation of small colorless organics controlled by oxygen vacancies under visible light

Visible-light photooxidation sensitized by surface attachment of small colorless organics on semiconductor photocatalysts has emerged as an economical method for photocatalytic synthesis or degradation. In particular, heteroatom (X = N and Cl)-containing substrates could undergo either C-N coupling or dechlorination degradation via sensitizing TiO2, but the mechanism in conducting the competitive visible-light sensitized photooxidations is still vague. Herein, the visible-light photooxidation of colorless 4-chlorobenzene-1,2-diamine (o-CAN) on TiO2 was revealed, contributing to selective C-N coupling rather than dechlorination. Oxygen vacancies (OVs) were in situ generated on the TiO2 surface, which could be dominant in weakening the Cl-Ti adsorption of o-CAN and regulating the activation of O2 for selective C-N coupling. The C-N coupling product, functionalized as the sensitizer, further promoted the visible-light photooxidation upon N-Ti and Cl-Ti coordination. This process was then confirmed by on-line mass spectrometric analysis, and the intermediates as well as their kinetics were determined. Thereby, theoretical calculations were employed to verify the roles of OVs in competitive photooxidation and lowering the energy barriers as well. Based on the comprehensive characterizations of both the catalysts and intermediates, this work has provided insights into competitive photooxidations under visible light.

Remarkably enhanced catalytic performance in CoOx/Bi4Ti3O12 heterostructures for methyl orange degradation via piezocatalysis and piezo-photocatalysis

A novel heterojunction composite of CoOx/Bi4Ti3O12 was synthesized through a combination of molten salt and photodeposition methods. The optimal sample exhibited superior performance in the piezocatalytic degradation of methyl orange (MO) dye with a degradation rate of 1.09 h-1, which was 2.4 times higher than that of pristine Bi4Ti3O12. Various characterizations were conducted to reveal the fundamental nature accountable for the outstanding piezocatalytic performance of CoOx/Bi4Ti3O12. The investigation of the band structure indicated that the CoOx/Bi4Ti3O12 composite formed a type-I p-n heterojunction structure, with CoOx acting as a hole trapper to effectively separate and transfer piezogenerated carriers. Significantly, the MO degradation rate of the best CoOx/Bi4Ti3O12 sample further increased to 2.96 h-1 under combined ultrasonic vibration and simulated sunlight. The synergy between piezocatalysis and photocatalysis can be ascribed to the following factors. The photoexcitation process ensures the sufficient generation of charge carriers in the CoOx/Bi4Ti3O12, while the piezoelectric field within Bi4Ti3O12 promotes the separation of electron-hole pairs in the bulk phase. Furthermore, the heterojunction structure between Bi4Ti3O12 and CoOx significantly facilitates the surface separation of charge carriers. This increased involvement of free electrons and holes in the reaction leads to a remarkable enhancement in catalytic MO degradation. This work contributes to the understanding of the coupling mechanism between the piezoelectric effect and photocatalysis, and also provides a promising strategy for the development of efficient catalysts for wastewater treatment.

Self-Induced Oxygen Vacancies on Carboxyl-Rich MIL-121 Enable Efficient Activation and Oxidation of Benzyl Alcohol under Visible Light

The oxygen vacancies could efficiently drive the photocatalytic oxidation of aromatic alcohol through the activation of reactants. However, the activation between aromatic alcohol and O₂ (oxidant) over oxygen vacancies is rarely studied. Herein, the ZnIn₂S₄/MIL-121 heterostructure with abundant surface uncoordinated carboxyl was rationally designed for benzyl alcohol (BA) oxidation under visible light. Oxygen vacancies on catalysts were self-induced after the surface complexation between the uncoordinated carboxyl and BA molecules. Based on the reaction results, it is concluded that the oxygen vacancy activation to BA is more effective than that to O₂ for BA oxidation over ZnIn₂S₄/MIL-121 hybrids. Specifically, when the activation fully occurs on BA (reaction in N₂), an admirable conversion of 93.9% (corresponding production rate of benzaldehyde: 2348 μmol·g^-1·h^-1) is achieved, which is 3.4 times that of pure ZnIn₂S₄, whereas an inferior conversion of 45.9% is obtained in O₂ due to the competition effect between O₂ and BA for oxygen vacancies.

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.

Solvent/metal-free benzimidazolium-based carboxyl-functionalized porphyrin photocatalysts for the room-temperature alkylation of amines under the irradiation of visible light

The improvement of novel sustainable catalytic methods for green chemical production is an emergent area in chemical science.

Selective Production of Secondary Amine by the Photocatalytic Cascade Reaction Between Nitrobenzene and Benzyl Alcohol over Nanostructured Bi2 MoO6 and Pd Nanoparticles Decorated with Bi2 MoO6

The synthesis of secondary amine by the photoalkylation of nitrobenzene with benzyl alcohol using a simple light source and sunlight is a challenging task. Herein, a one-pot cascade protocol is employed to synthesize secondary amine by the reaction between nitrobenzene and benzyl alcohol. The one-pot cascade protocol involves four reactions: (a) photocatalytic reduction of nitrobenzene to aniline, (b) photocatalytic oxidation of benzyl alcohol to benzaldehyde, (c) reaction between aniline and benzaldehyde to form imine, and (d) photocatalytic reduction of imine to a secondary amine. The cascade protocol to synthesize secondary amine is accomplished using Bi₂ MoO₆ and Pd nanoparticles decorated Bi₂ MoO₆ catalysts. The surface characteristics, oxidation states, and elemental compositions of the materials are characterized by several physicochemical characterization techniques. Optoelectronic and photoelectrochemical measurements are carried out to determine the bandgap, band edge potentials, photocurrents, charge carrier's separation, etc. An excellent yield of secondary amine is achieved with simple household white LED bulbs. The catalyst also exhibits similar or even better activity in sunlight. The structure-activity relationship is established using catalytic activity data, control reactions, physicochemical, optoelectronic characteristics, and scavenging studies. Bi₂ MoO₆ and Pd nanoparticles decorated Bi₂ MoO₆ exhibit excellent photostability and recyclability. The simple catalyst design with a sustainable and economical light source for the synthesis of useful secondary amine from the nitrobenzene and benzyl alcohol would attract the researchers to develop similar catalytic protocols for other industrially important chemicals.

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.

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.

Selective Photocatalytic Oxidation of Thioanisole on DUT-67(Zr) Mediated by Surface Coordination

DUT-67(Zr) was obtained by a solvothermal route and applied to photocatalytic selective synthesis of thioanisole under light illuminating. The conversion of thioanisole is up to 95%, and the selectivity of methyl phenyl sulfoxide is 98%. The activity of DUT-67(Zr) is over 10 times higher than that of UiO-66. This great increased activity is attributed to the high percentages of oxygen vacancies on DUT-67(Zr). The ESR result shows there are more oxygen vacancies that can expose high density unsaturated Zr sites on DUT-67(Zr). The in situ FTIR reveals that unsaturated Zr sites on DUT-67(Zr) possess Lewis acidity which facilitate the adsorption of the substrates to form the coordination species, promoting the activation of thioanisole. The absorption edge of DUT-67(Zr) with coordination species red-shifts to 360 nm, which can be presented by DRS. Furthermore, the oxygen molecules can be activated by excited electrons to form ^•O₂^-. Finally, a possible photocatalytic process of oxidating thioanisole to methyl phenyl sulfoxide based on the coordination effect between DUT-67(Zr) and thioanisole is proposed at a molecular level.

Pd nanoparticles on green support as dip-catalyst: a facile transfer hydrogenation of olefins and N-heteroarenes in water

Chemo- and regioselective hydrogenation methods using highly green sources, particularly from metal nanoparticles on plant stem as support and water, is an intensive research area, which is highly relevant to the development of green chemistry and technology in the 21^st century. Here, the synthesis and activity of a heterogeneous catalytic system (called “dip-catalyst”) for the transfer hydrogenation of a series of styrenyl, unfunctionalized olefins, quinoline and other N-heteroarenes, are presented. It consists of Pd nanoparticles (15–20 nm) anchored on bio-processed jute plant (Corchorus genus) stem as the support. Pd nanoparticles were decorated on the green support (GS) jute stem by the in situ reduction of K₂PdCl₄ in aqueous medium at 70 °C, using formic acid as the reductant. The Pd@GS was characterized using SEM, EDS, XRD, FTIR, XPS and TEM. Elemental mapping revealed uniform distribution of Pd on the cellulose matrix of the jute stem. The catalyst was successfully applied to the chemoselective transfer hydrogenation of numerous styrenyl, unfunctionalized olefins. Its high functional group tolerance was investigated during the olefins hydrogenation in water. Furthermore, Pd@GS was capable of quantitative hydrogenation to selectively produce 1,2,3,4-tetrahydroquinoline (THQ) in water using stoichiometric amounts of tetrahydroxydiboron (THDB) at 60 °C with turn over frequency (TOF) 4938 h⁻¹. This system is stable in water and displays excellent recyclability; it could be used for 32 consecutive cycles, without losing its original crystallinity or requiring replenishment.

A dip catalyst consisting of Pd nanoparticles immobilized on the surface of thin slices of jute-stem was fabricated. Its versatility as a hydrogen transfer agent for styrenyl, cyclic and unfunctionalized olefins and N-heterocycles was investigated.