Al2O3 Surface Complexation for Photocatalytic Organic Transformations

Photocatalytic cyclization of nitrogen-centered radicals with carbon nitride through promoting substrate/catalyst interaction

The use of metal-free carbon nitride and light to drive catalytic transformations constitutes a sustainable strategy for organic synthesis. At the moment, enhancing the intrinsic activity of CN catalysts by tuning the interfacial coupling between catalyst and substrate remains challenging. Herein, we demonstrate that urea-derived carbon nitride catalysts with the abundant −NH₂ groups and the relative positive charged surface could effectively complex with the deprotonated anionic intermediate to improve the adsorption of organic reactants on the catalyst surface. The decreased oxidation potential and upshift in its highest occupied molecular orbital position make the electron abstraction kinetics by the catalyst more energetically favorable. The prepared catalyst is thus utilized for the photocatalytic cyclization of nitrogen-centered radicals for the synthesis of diverse pharmaceutical-related compounds (33 examples) with high activity and reusability, which shows competent performance to the homogeneous catalysts.

Carbon nitride catalysts with positively charged surfaces and abundant −NH2 are found to be effective photocatalysts for dihydropyrazole synthesis. A surface-mediated mechanism where deprotonated intermediates interact with the surface is proposed.

Lewis acid-triggered photocatalytic hydrogen peroxide production in an aluminum-based metal–organic framework

Al-based MIL-101-NH2, which was previously regarded as having silent photo-features, exhibits photocatalytic H2O2 production via O2 reduction accompanied by efficient suppression of undesired H2O2 decomposition.

Transforming a Fluorochrome to an Efficient Photocatalyst for Oxidative Hydroxylation: A Supramolecular Dimerization Strategy Based on Host-Enhanced Charge Transfer

The development of non-covalent synthetic strategy to fabricate efficient photocatalysts is of great importance in theranostic and organic materials. Herein, a fluorochrome N,N'-dimethyl 2,5-bis(4-pyridinium)thiazolo[5,4-d]thiazolediiodide (MPT) was transformed into an efficient photocatalyst through supramolecular dimerization in the cavity of cucurbit[8]uril (CB[8]). The host-enhanced charge transfer interaction within the supramolecular dimer 2MPT-CB[8] dramatically promoted intersystem crossing to produce triplet. In addition, the staggered conformation of 2MPT-CB[8] facilitated the energy transfer and electron transfer of the triplet. As a result, 2MPT-CB[8] could serve as a high-efficiency photocatalyst for the oxidative hydroxylation of arylboronic acids. This supramolecular dimerization strategy enriches the supramolecular engineering of functional π-systems. It is anticipated that this strategy can be extended to fabricate various π-systems with tailor-made functions.

Double Insurance of Continuous Band Structure and N-C Layer Induced Prolonging of Carrier Lifetime to Enhance the Long-Wavelength Visible-Light Catalytic Activity of N-Doped In2O3

Nonmetallic doped metal oxides can be broad in their visible-light-response range. However, the half-filled or isolated impurity state can also be the new recombination center for photogenerated electrons/holes, which seriously influence the photocatalytic activity of the catalyst in the visible-light region. Therefore, how to prolong the photogenerated carrier life of nonmetallic doping metal oxides is the difficult and challenging topic in the field of photocatalysis. In this work, the hexagonal nanosheets assembled by N-doped C (N-C)-coated N-doped In₂O₃ (N-In₂O₃) nanoparticles (N-C/N-In₂O₃ HS) was obtained by simply pyrolyzing the In(2,5-PDC) hexagonal sheets. The N-C/N-In₂O₃ HS catalyst exhibit good photocatalytic activity and cycle stability in the long-wavelength region of visible light (λ = 520 and 595 nm). The effective utilization of long-wavelength visible light for N-C/N-In₂O₃ HS was mainly attributed to the acceptor-donor-acceptor compensation mechanism between the oxygen vacancy (V_O) and substitutional N-doping (N_s) sites, which made the N-C/N-In₂O₃ HS possess a continuous band structure, without the half-filled or isolated impurity state in the band gap, and extended its light absorption edge to 733 nm. The compensation mechanism of nitrogen doping on In₂O₃ can promote the photocatalytic activity under longer-wavelength yellow light (595 nm) irradiation. The N-C layer coated on the N-In₂O₃ nanoparticles acted as a good acceptor of photogenerated electrons, facilitating the effective spatial separation of photogenerated carriers and extend photogenerated carrier lifetimes. The comparative photocatalytic experiments (N-In₂O₃ HS and N-C/N-In₂O₃ HS) show that the presence of N-doped C layer can enhance the photocatalytic efficiency by nearly 10-fold. This double-doping and carbon-coating strategy provided a novel research idea to solve the problem that nonmetal atoms doped metal oxides led to the secondary combination of photogenerated electrons/holes.

Discovery of a New Family of Polyoxometalate-Based Hybrids with Improved Catalytic Performances for Selective Sulfoxidation: The Synergy between Classic Heptamolybdate Anions and Complex Cations

A series of functional cation-regulated isopolymolybdate-based organic-inorganic hybrid compounds, Na₂H₂[Mo₄O₁₂(C₈H₁₇O₅N)₂]·10H₂O (1), Na₂[M(Bis-tris)(H₂O)]₂[Mo₇O₂₄]·10H₂O [M = Cu, 2; Ni, 3; Co, 4; Zn, 5; Bis-tris = 2,2-Bis(hydroxymethyl)-2,2',2″-nitrilotriethanol], and (NH₄)₂[M(Bis-tris)(H₂O)]₂[Mo₇O₂₄]·6H₂O (M = Zn, 6; Cu, 7), were synthesized and characterized toward advanced molecular catalyst design. Compound 1 is a covalently bonded adduct, and its self-assembly process can be probed by electrospray ionization mass spectrometry (ESI-MS). Compounds 2-7 are polyoxometalate (POM)-based hybrids containing classic heptamolybdate anions and complex cations with Bis-tris ligands. All of these compounds showed remarkable catalytic effects for selective sulfide oxidation. To the best of our knowledge, compound 5 presents the best catalytic activity so far among the reported hybrid materials with common easily synthesized small-molecule POM clusters and also exhibits outstanding reliability. The conclusion of the catalytic effect is drawn from the results that Zn-based compounds have better catalytic effects than other transition-metal-containing compounds and the compound constructed by Na⁺ has higher catalytic activity than that constructed by NH₄⁺. The mechanism studies show that the improvements of the catalytic performance are caused by the synergy between classic heptamolybdate anions and complex cations. ESI-MS data and UV-vis spectra revealed that the POM anions can form intermediate peroxomolybdenum units during catalytic reaction. Further, the combination of the substrate thioanisole with complex cations was characterized by NMR experiments and UV-vis spectra. Thus, a new synergistic mechanism of anions and cations is proposed in which the activated thioanisole is used as a nucleophile to attack the peroxomolybdenum bonds, and this provides a new strategy in the design of reliable POM-based catalysts.

Carbon nitride photocatalyzes regioselective aminium radical addition to the carbonyl bond and yields N-fused pyrroles

Addition of N-centered radicals to C=C bonds or insertion into C-H bonds is well represented in the literature. These reactions have a tremendous significance, because they afford polyfunctionalized organic molecules. Despite the tetrahydroisoquinoline (THIQ) moiety widely occurring in natural biologically active compounds, N-unsubstituted THIQs as a source of N-centered radicals are not studied. Herein, we report a photocatalytic reaction between tetrahydroisoquinoline and chalcones that gives N-fused pyrroles-1,3-disubstituted-5,6-dihydropyrrolo[2,1-a]isoquinolines (DHPIQ). The mechanism includes at least two photocatalytic events in one pot: (1) C-N bond formation; (2) C-C bond formation. In this process potassium poly(heptazine imide) is used as a visible light active heterogeneous and recyclable photocatalyst. Fifteen N-fused pyrroles are reported with 65-90% isolated yield. DHPIQs are characterized by UV-vis and fluorescence spectroscopy, while the fluorescence quantum efficiency of fluorinated DHPIQs reaches 24%.

Facile synthesis of bimodal macroporous g-C3N4/SnO2 nanohybrids with enhanced photocatalytic activity

It is of vital importance to construct highly interconnected, macroporous photocatalyst to improve its efficiency and applicability in solar energy conversion and environment remediation. Graphitic-like C₃N₄ (g-C₃N₄), as an analogy to two-dimensional (2D) graphene, is highly identified as a visible-light-responsive polymeric semiconductor. Moreover, the feasibility of g-C₃N₄ in making porous structures has been well established. However, the preparation of macroporous g-C₃N₄ with abundant porous networks and exposure surface, still constitutes a difficulty. To solve it, we report a first facile preparation of bimodal macroporous g-C₃N₄ hybrids with abundant in-plane holes, which is simply enabled by in-situ modification through thermally treating the mixture of thiourea and SnCl₄ (pore modifier) after rotary evaporation. For one hand, the formed in-plane macropores endow the g-C₃N₄ system with plentiful active sites and short, cross-plane diffusion channels that can greatly speed up mass transport and transfer. For another, the heterojunctions founded between g-C₃N₄ and SnO₂ consolidate the electron transfer reaction to greatly reduce the recombination probability. As a consequence, the resulted macroporous g-C₃N₄/SnO₂ nanohybrid had a high specific surface area (SSA) of 44.3 m²/g that was quite comparable to most nano/mesoporous g-C₃N₄ reported. The interconnected porous network also rendered a highly intensified light absorption by strengthening the light penetration. Together with the improved mass transport and electron transfer, the macroporous g-C₃N₄/SnO₂ hybrid exhibited about 2.4-fold increment in the photoactivity compared with pure g-C₃N₄. Additionally, the recyclability of such hybrid could be guaranteed after eight successive uses.

ZnNb2O6 fibre surface as an efficiently product-selective controller for the near-UV-light-induced nitrobenzene reduction reaction

A high aniline yield was achieved by the combination of near-UV light as the driving force of nitrobenzene reduction and ZnNb₂O₆ surface as the product-selective controller.

Enhancing the catalytic activity of the alkaline hydrogen evolution reaction by tuning the S/Se ratio in the Mo(SxSe1-x)2 catalyst

The alkaline hydrogen evolution reaction (HER) plays a key role in photo(electro)catalytic water splitting technologies, particularly in water-alkali electrolyzers. Unfortunately, although transition metal dichalcogenide (TMD) materials, especially MoS2 and MoSe2, are considered efficient, Earth-abundant catalysts for the HER in an acidic electrolyte, they are much less effective under high pH conditions due to a sluggish water dissociation process (Volmer-step) and strong adsorption of the OH- intermediate on their surfaces. Herein we show a novel synergetic effect obtained by tailoring the S/Se ratio in Mo(SxSe1-x)2 alloys. We were able to influence the metal oxidation state and d-band to optimize the catalytic sites for HOH dissociation and OH- desorption while maintaining favourable M-H energetics. The Mo(SxSe1-x)2 (x = 0.58) catalyst exhibited high performance with an onset potential of -43 mV in 0.5 M KOH, i.e., ∼3 and ∼4-fold less than that for MoSe2 and MoS2, respectively. The results obtained in the current study have implications for the rational design of cost-effective electro-catalysts for water reduction based on TMD alloys.

In Situ Growth of Pd Nanosheets on g-C3 N4 Nanosheets with Well-Contacted Interface and Enhanced Catalytic Performance for 4-Nitrophenol Reduction

Loading novel metal nanosheets onto nanosheet support can improve their catalytic performance, but the morphological incompatibility makes it difficult to construct a well-contacted interface, which is of particular interest in supported catalysts. Herein, Pd nanosheets (Pd NSs) are supported onto graphitic carbon nitride nanosheets (CNNSs) with intimate face-to-face contact through an in situ growth method. This method overcomes the limitations of the morphological incompatibility and ensures the intimate interfacial contact between Pd NSs and CNNSs. The nitrogen-rich nature of CNNSs endows Pd NSs with abundant anchoring sites, which optimizes the electronic structure and improves the chemical and morphological stability of Pd NSs. The supported Pd NSs demonstrate high dispersion and exhibit largely enhanced activity toward the reduction of 4-nitrophenol. The concentration-normalized rate constant is up to 3052 min-1 g-1 L, which is 5.4 times higher than that obtained by unsupported Pd NSs. No obvious deactivation is observed after six runs of the recycling experiments. It is believed that the supported novel metal nanosheets with the intimately contacted interface may show promising applications in catalysis.

Calcinable Polymer Membrane with Revivability for Efficient Oily-Water Remediation

Fouling of polymeric membranes remains a major challenge for long-term operation of oily-water remediation. The common reclamation methods to recycle fouled membranes have the issues of either incomplete degradation of organic pollutants or damage to filter membranes. Here, a calcinable polymer membrane with effective reclamation after fouling is reported, which shows full recovery of the original oil/water separation efficiency. The membrane is made of polysulfonamide/polyacrylonitrile fibers by emulsion electrospinning, followed by hydrothermal decoration of TiO₂ nanoparticles. The bonding structured fibrous membrane displays outstanding thermal stability in air (400 °C), strong acid/alkali resistance (at the pH range from 1 to 13), and robust tensile strength. As a result, the chemically fouled polymeric membrane can be easily reclaimed without decreasing in separation performance and mechanical properties by annealing treatment. As a proof-of-concept, the as-prepared membrane is integrated into a wastewater separation tank, which achieves a high water flux over 3000 L m^-2 h^-1 and oil rejection efficiency of 99.6% for various oil-in-water emulsions. The presented strategy on membrane fabrication is believed to be an effective remedy for membrane fouling, and should apply in a wider field of filtration industry.

A supramolecular radical cation: folding-enhanced electrostatic effect for promoting radical-mediated oxidation

We report a supramolecular strategy to promote radical-mediated Fenton oxidation by the rational design of a folded host-guest complex based on cucurbit[8]uril (CB[8]). In the supramolecular complex between CB[8] and a derivative of 1,4-diketopyrrolo[3,4-c]pyrrole (DPP), the carbonyl groups of CB[8] and the DPP moiety are brought together through the formation of a folded conformation. In this way, the electrostatic effect of the carbonyl groups of CB[8] is fully applied to highly improve the reactivity of the DPP radical cation, which is the key intermediate of Fenton oxidation. As a result, the Fenton oxidation is extraordinarily accelerated by over 100 times. It is anticipated that this strategy could be applied to other radical reactions and enrich the field of supramolecular radical chemistry in radical polymerization, photocatalysis, and organic radical battery and holds potential in supramolecular catalysis and biocatalysis.

Highly Efficient Supramolecular Catalysis by Endowing the Reaction Intermediate with Adaptive Reactivity

A new strategy of highly efficient supramolecular catalysis is developed by endowing the reaction intermediate with adaptive reactivity. The supramolecular catalyst, prepared by host-guest complexation between 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) and cucurbit[7]uril (CB[7]), was used for biphasic oxidation of alcohols. Cationic TEMPO⁺ , the key intermediate, was stabilized by the electrostatic effect of CB[7] in aqueous phase, thus promoting the formation of TEMPO⁺ and inhibiting side reactions. Moreover, through the migration into the organic phase, TEMPO⁺ was separated from CB[7] and recovered the high reactivity to drive a fast oxidation of substrates. The adaptive reactivity of TEMPO⁺ induced an integral optimization of the catalytic cycle and greatly improved the conversion of the reaction. This work highlights the unique advantages of dynamic noncovalent interactions on modulating the activity of reaction intermediates, which may open new horizons for supramolecular catalysis.

Ru nanoparticles dispersed on magnetic yolk-shell nanoarchitectures with Fe3O4 core and sulfoacid-containing periodic mesoporous organosilica shell as bifunctional catalysts for direct conversion of cellulose to isosorbide

A green and sustainable approach for biorefining involves the development of bifunctional catalysts for the one-pot conversion of cellulosic biomass to isosorbide. This requires highly efficient, easily separated and versatile metal-acid catalysts for hydrolysis-hydrogenation-dehydration cascade reactions. Herein, we report a new type of metal-acid bifunctional catalyst by dispersing Ru nanoparticles (NPs) on magnetic yolk-shell nanoarchitectures comprising an Fe₃O₄ core and a sulfoacid (SO₃H)-containing periodic mesoporous organosilica shell. The resultant magnetic Ru-SO₃H nanoreactors are highly porous and have large surface areas (>350 m² g^-1), uniform mesopores (∼3.8 nm), well-dispersed Ru NPs (<1.5 wt%) and superior magnetization. Tailoring the size of the Ru NPs and the amount of SO₃H moieties produced a highly efficient Ru-SO₃H nanocatalyst, which delivered a high yield of isosorbide of 58.1% with almost complete conversion of cellulose in 2 h and achieved maximum productivity of 2.19 mol_Isosor h^-1 g_Ru^-1, which was one order of magnitude higher than that achieved using other Ru-containing acidic catalysts. Moreover, the elaborately fabricated Ru-SO₃H nanocatalyst can be easily separated by applying an external magnetic field and can be cycled four times. This work reveals new possibilities for the fabrication of highly efficient, easily separated metal-acid catalysts in virtue of the concept of nanoreactor design.

Visible-Light Photoinduced Electron Transfer Promoted by Cucurbit[8]uril-Enhanced Charge Transfer Interaction: Toward Improved Activity of Photocatalysis

Visible-light photoinduced electron transfer (Vis-PET) is highly important for optoelectronic devices and photoredox catalysis. Herein, we propose a supramolecular strategy to promote the Vis-PET process by charge transfer (CT) interactions. As a proof of concept, a molecular system containing 1,5-alkoxy-substituted naphthalene and viologen moieties was designed to form a CT complex in water. The HOMO/LUMO energy gap was lowered by CT interaction, which turned on the Vis-PET process to generate viologen radical cations. Moreover, when CT interaction was enhanced by cucurbit[8]uril, the efficiency of the Vis-PET process was further promoted and the required irradiation wavelength could be further red-shifted by 100 nm. The Vis-PET system exhibited an improved activity of photocatalysis, as supported by the fast photoreduction of Cytochrome c. This study represents a facile supramolecular way to fabricate radicals with maintained activity under mild conditions, which holds potential to enrich the scope of visible-light photoredox catalysis by the rational utilization of CT systems.

Merging visible light photocatalysis of dye-sensitized TiO2 with TEMPO: the selective aerobic oxidation of alcohols

Efficient visible-light-induced selective aerobic oxidation of alcohols was accomplished by merging the photocatalysis of dye-sensitized TiO₂ with TEMPO catalysis.