Triangle Cl−Ag1−Cl Sites for Superior Photocatalytic Molecular Oxygen Activation and NO Oxidation of BiOCl

BiOCl photocatalysis shows great promise for molecular oxygen activation and NO oxidation, but its selective transformation of NO to immobilized nitrate without toxic NO₂ emission is still a great challenge, because of uncontrollable reaction intermediates and pathways. In this study, we demonstrate that the introduction of triangle Cl−Ag₁−Cl sites on a Cl-terminated, (001) facet-exposed BiOCl can selectively promote one-electron activation of reactant molecular oxygen to intermediate superoxide radicals (⋅O₂⁻), and also shift the adsorption configuration of product NO₃⁻ from the weak monodentate binding mode to a strong bidentate mode to avoid unfavorable photolysis. By simultaneously tuning intermediates and products, the Cl−Ag₁−Cl-landen BiOCl achieved >90 % NO conversion to favorable NO₃⁻ of high selectivity (>97 %) in 10 min under visible light, with the undesired NO₂ concentration below 20 ppb. Both the activity and the selectivity of Cl−Ag₁−Cl sites surpass those of BiOCl surface sites (38 % NO conversion, 67 % NO₃⁻ selectivity) or control O−Ag₁−O sites on a benchmark photocatalyst P25 (67 % NO conversion and 87 % NO₃⁻ selectivity). This study develops new single-atom sites for the performance enhancement of semiconductor photocatalysts, and also provides a facile pathway to manipulate the reactive oxygen species production for efficient pollutant removal.     

Inside Back Cover: Effective Activation of Strong C−Cl Bonds for Highly Selective Photosynthesis of Bibenzyl via Homo-Coupling (Angew. Chem. Int. Ed. 43/2023)

Carbon-carbon (C−C) coupling of organic halides has been successfully achieved in homogeneous catalysis, while the limitation, e.g., the dependence on rare noble metals, complexity of the metal-ligand catalylst and the poor catalyst stability and recyclability, needs to be tackled for a green process. The past few years have witnessed heterogeneous photocatalysis as a green and novel method for organic synthesis processes. However, the study on C−C coupling of chloride substrates is rare due to the extremely high bond energy of C−Cl bond (327 kJ mol⁻¹). Here, we report a robust heterogeneous photocatalyst (Cu/ZnO) to drive the homo-coupling of benzyl chloride with high efficiency, which achieves an unprecedented high selectivity of bibenzyl (93 %) and yield rate of 92 % at room temperature. Moreover, this photocatalytic process has been validated for C−C coupling of 10 benzylic chlorides all with high yields. In addition, the excellent stability has been observed for 8 cycles of reactions. With detailed characterization and DFT calculation, the high selectivity is attributed to the enhanced adsorption of reactants, stabilization of intermediates (benzyl radicals) for the selective coupling by the Cu loading and the moderate oxidation ability of the ZnO support, besides the promoted charge separation and transfer by Cu species.     

Visible-Light-Promoted Switchable Selective Oxidations of Styrene Over Covalent Triazine Frameworks in Water

Using photocatalytic oxidation to convert basic chemicals into high value compounds in environmentally benign reaction media is a current focus in catalytic research. The challenge lies in gaining controllability over product formation selectivity. We design covalent triazine frameworks as heterogeneous, metal-free, and recyclable photocatalysts for visible-light-driven switchable selective oxidation of styrene in pure water. Selectivity in product formation was achieved by activation or deactivation of the specific photogenerated oxygen species. Using the same photocatalyst, by deactivation of photogenerated H₂O₂, benzaldehyde was obtained with over 99 % conversion and over 99 % selectivity as a single product. The highly challenging and sensitive epoxidation of styrene was carried out by creating peroxymonocarbonate as an initial epoxidation agent in the presence of bicarbonate, which led to formation of styrene oxide with a selectivity up to 76 % with near quantitative conversion. This study demonstrates a preliminary yet interesting example for simple control over switchable product formation selectivity for challenging oxidation reactions of organic compounds in pure water.     

Simple Transformation of Hierarchical Hollow Structures by Reduction of Metal–Organic Frameworks and Their Catalytic Activity in the Oxidation of Benzyl Alcohol

Metal–organic framework (MOF)-based derivatives have been found to be promising heterogeneous catalysts for organic transformations. Herein, hollow-structure Cu-MOFs derived by reduction of Cu₃(BTC)₂ (BTC=1,3,5-benzenetricarboxylate; denoted as RCB) were prepared by using hydrazine hydrate as a reducing agent under various conditions. The influence of hydrazine hydrate induced the structure of Cu₃(BTC)₂ and led to dynamic variation in the interior and exterior as well as oxidation states of the Cu ion. The synthesized materials were characterized by SEM, TEM, N₂ sorption isotherms, XRD, and XPS. The product of the catalytic reaction was observed by GC-MS. In addition, the prepared RCBs were found to have excellent catalytic activity and selectivity for benzyl alcohol oxidation when assisted by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO).     

Selective alcohol oxidation to aldehydes and ketones over base-promoted gold–palladium clusters as recyclable quasihomogeneous and heterogeneous metal catalysts

Bimetallic gold–palladium clusters, with an average size of 1.9 nm and composed of 80 mol% gold, proved to be highly active and selective metal catalysts for the organic phase oxidation with O₂ of aliphatic, allylic and benzylic alcohols to the corresponding carbonyl products. Polyvinylpyrrolidone stabilized gold–palladium clusters dispersed in N,N-dimethylformamide emerged as promising quasihomogeneous metal catalysts for the oxidation of benzyl alcohol to benzaldehyde with full selectivity; they could be efficiently recycled with unaffected catalytic performance by solvent-resistant nanofiltration. Highly active and durable heterogeneous catalysts for the amide phase or solvent-free alcohol oxidation were prepared by the quantitative immobilization of the optimized gold–palladium clusters on the high surface area basic BaAl₂O₄ spinel support with preservation of the bimetallic clusters’ nanodispersion.     

Pt/ZrO2:An Efficient Catalyst for Aerobic Oxidation of Toluene in Aqueous Solution

The heterogeneous oxidation of toluene in aqueous medium has been investigated. Artificially contaminated water with aromatic compound (toluene) was exposed to a simple platinized zirconia (1% Pt/ZrO₂) catalyst in the presence of molecular oxygen. This selective oxidation of toluene to benzyl alcohol, benzaldehyde and benzoic acid provides a step for removing toluene from wastewater or converting it into less harmful substances. Different parameters, e.g. the reaction time, temperature, pressure, the amount of catalyst and agitation, etc influenced the toluene conversion and selectivity. Typical batch reactor kinetic data were obtained and fitted to the classical Langmuir‐Hinshelwood model, Mars‐van Krevelen model as well as to the Eley‐Rideal model of heterogeneously catalyzed reactions. The Eley‐Rideal model was found to give a better fit. 1% Pt/ZrO₂ was observed to be the most active for oxidation of toluene at 333 K in oxygenated atmosphere [p(O₂) ca. 101 kPa] with a nominal stirring speed ≧900 r/min. It was found that catalytic oxidation may be an effective method for the removal of volatile organic compounds from aqueous solutions and comparable to other advanced oxidation processes.     

Liquid-phase catalytic oxidation of organic substrates by a recyclable polymer-supported copper(II) complex

Chloromethylated polystyrene beads cross-linked with 6.5 % divinylbenzene were functionalized with 2-(2′-pyridyl) benzimidazole (PBIMH) and on subsequent treatment with Cu(OAc)₂ in methanol gave a polymer-supported diacetatobis(2-pyridylbenzimidazole)copper(II) complex [PS-(PBIM)₂Cu(II)], which was characterized by physicochemical techniques. The supported complex showed excellent catalytic activity toward the oxidation of industrially important organic compounds such as phenol, benzyl alcohol, cyclohexanol, styrene, and ethylbenzene. An effective catalytic protocol was developed by varying reaction parameters such as the catalyst and substrate concentrations, reaction time, temperature, and substrate-to-oxidant ratio to obtain maximum selectivity with high yields of products. Possible reaction mechanisms were worked out. The catalyst could be recycled five times without any metal leaching or much loss in activity. This catalyst is truly heterogeneous and allows for easy work up, as well as recyclability and excellent product yields under mild conditions.     

High Catalytic Efficiency of Nanostructured Molybdenum Trioxide in the Benzylation of Arenes and an Investigation of the Reaction Mechanism

The synthesis and characterization of nanostructured MoO₃ with a thickness of about 30 nm and a width of about 450 nm are reported. The composition formula of the MP (precipitation method) precursor was estimated to be [(NH₄)₂O]_0.169?MoO₃? (H₂O)_0.239. The calcination of the precursor in air afforded nanostructured pellets of the α‐MoO₃ phase. The nanostructured MoO₃ catalyst exhibited high efficiency in catalyzing the benzylation of various arenes with substituted benzyl alcohols, which were strikingly different to common bulk MoO₃. Most reactions offered >99 % conversion and >99 % selectivity to monoalkylated compounds. MoO₃ is a typical acid catalyst. However, the benzylation reaction over nanostructured MoO₃ does not belong to the acid‐catalyzed type or defect site‐catalyzed type, since the catalyst has no acidity and defect site on surface. Characterization with thermal, spectroscopic, and electronic techniques reveal that the catalyst contains fully oxygen‐coordinated MoO₆ octahedrons on the surface but partially reduced species (Mo⁵⁺) within the bulk phase. The terminal oxygen atoms of Mo?O bonds on the (010) basal plane resemble oxygen anion radicals and act as active sites for the adsorption and activation of benzyl alcohols by electrophilic attack. Such sites are indispensable for catalytic reactions since the blocking of these sites by electron acceptors, such as tetracyanoethylene (TCNE), can greatly decrease catalytic activity. This work represents a successful example of combining a heterogeneous catalysis study with nanomaterial synthesis.     

Remarkable Enhancement of O2 Activation on Yttrium‐Stabilized Zirconia Surface in a Dual Catalyst Bed

Yttrium‐stabilized zirconia (YSZ) has been extensively studied as an electrolyte material for solid oxide fuel cells (SOFC) but its performance in heterogeneous catalysis is also the object of a growing number of publications. In both applications, oxygen activation on the YSZ surface remains the step that hinders utilization at moderate temperature. It was demonstrated by oxygen isotope exchange that a dual catalyst bed system consisting of two successive LaMnO₃ and YSZ beds without intimate contact drastically enhances oxygen activation on the YSZ surface at 698 K. It can be concluded that LaMnO₃ activates the triplet ground‐state of molecular oxygen into a low‐lying singlet state, thereby facilitating the activation of the O₂ molecule on the YSZ oxygen vacancy sites. This phenomenon is shown to improve the catalytic activity of the LaMnO₃‐Pd/YSZ system for the partial oxidation of methane.     

TiO2光催化反应机理及动力学研究进展

光催化处理环境污染物是基于催化反应过程中的一些自由基对污染物的氧化或还原作用，反应途径通常是HO·攻击或穴直接攻击，对可见光敏感的化合物也可能通过激发态来分解。动力学的表述多数符合L－H模式，广泛研究了L－H模式下的吸附与催化活性的关系，对动力学的研究也是了解其反应机理的重要途径。     

Simple synthesis of the novel Cu-MOF catalysts for the selective alcohol oxidation and the oxidative cross-coupling of amines and alcohols

A novel porous metal–organic framework {Cu₂(bbda)_0.5(Hbbda)_1.5(OAc)_1.5.8H₂O} (UoB-5) was synthesized under ultrasound irradiation by employing a new Schiff base ligand H₂bbda (4,4′(1,4-phenylene bis (azanylylidene)) bis (methanylylidene))dibenzoic acid) and was fully characterized. The microporous nature of UoB-5 was confirmed by gas-sorption measurements. This framework acted as a highly effective heterogeneous catalyst for the alcohol oxidation reaction with tert-butyl hydroperoxide (t-BuOOH) as an oxidant. The presence of coordinatively unsaturated metal sites in UoB-5 could be the reason for high performance in this reaction. Furthermore, using the long linker with the free -NC group and uncoordinated -N atom on the wall of the pores created UoB-5 an excellent candidate for the catalytic activities without activation of the framework. It was confirmed with the heterogeneous catalytic experiments on the one-pot tandem synthesis of imines from benzyl alcohols and anilines. Eventually, the new Cu-MOF (UoB-5) could be an alternative catalyst as a more economically favorable and environmentally friendly in the catalysis field.     

Tungstate supported mesoporous silica SBA‐15 with imidazolium framework as a hybrid nanocatalyst for selective oxidation of sulfides in the presence of hydrogen peroxide

In this work, a new heterogeneous catalyst (SBA‐15/Im/WO₄^2?) was prepared, and then its performance in the oxidation of organic sulfides was studied (using 30% H₂O₂ as green oxidant under neutral reaction conditions). This organic–inorganic hybrid mesoporous material was characterized by various techniques, such as FT‐IR, inductively coupled plasma, X‐ray powder diffraction, high‐resolution‐transmission electron microscopy, N₂ adsorption–desorption and thermogravimetric analysis. The catalyst was also applied to the selective oxidation of various sulfides. The hybrid catalyst was easily recovered, and was very stable and retained good activity for at least five successive runs without any additional activation. Moreover, there was no remarkable decrease in the activity and selectivity of the catalyst. The products could be easily isolated by just removing the solvent after filtering the catalyst. The yields of the catalytic productions through this catalyst were in the range from 75% to 97%.     

Copper(II) Schiff Base Complex Immobilized on Superparamagnetic Fe3O4@SiO2 as a Magnetically Separable Nanocatalyst for Oxidation of Alkenes and Alcohols

A new heterogeneous catalyst containing a copper(II) Schiff base complex covalently immobilized on the surface of silica‐coated Fe₃O₄ nanoparticles (Fe₃O₄@SiO₂‐Schiff base‐Cu(II)) was synthesized. Characterization of this catalyst was performed using various techniques. The catalytic potential of the catalyst was investigated for the oxidation of various alkenes (styrene, α‐methylstyrene, cyclooctene, cyclohexene and norbornene) and alcohols (benzyl alcohol, 3‐methoxybenzyl alcohol, 3‐chlorobenzyl alcohol, benzhydrol and n ‐butanol) using tert ‐butyl hydroperoxide as oxidant. The catalytic investigations revealed that Fe₃O₄@SiO₂‐Schiff base‐Cu(II) was especially efficient for the oxidation of norbornene and benzyl alcohol. The results showed that norbornene epoxide and benzoic acid were obtained with 100 and 87% selectivity, respectively. Moreover, simple magnetic recovery from the reaction mixture and reuse for several times with no significant loss in catalytic activity were other advantages of this catalyst     

Effect of mechanicochemical treatment on the activity of K<Subscript>2</Subscript>PdCl<Subscript>4</Subscript> in the heterogeneous catalytic hydrochlorination of acetylene

A heterogeneous catalyst for the hydrochlorination of acetylene using gaseous HCl was obtained by prior mechanical activation of K₂PdCl₄ powder in an atmosphere of acetylene or propylene. Active sites are formed during the mechanical treatment in the surface layers of the catalyst, which are Pd(II) complexes with a coordination vacancy.     

Beyond Reduction Cocatalysts: Critical Role of Metal Cocatalysts in Photocatalytic Oxidation of Methane with Water**

Environmentally sustainable and selective conversion of methane to valuable chemicals under ambient conditions is pivotal for the development of next-generation photocatalytic technology. However, due to the lack of microscopic knowledge about non-thermal methane conversion, controlling and modulating photocatalytic oxidation processes driven by photogenerated holes remain a challenge. Here, we report novel function of metal cocatalysts to accept photogenerated holes and dominate selectivity of methane oxidation, which is clearly beyond the conventional concept in photocatalysis that the metal cocatalysts loaded on the surfaces of semiconductor photocatalysts mostly capture photogenerated electrons and dominate reduction reactions exclusively. The novel photocatalytic role of metal cocatalysts was verified by operando molecular spectroscopy combined with real-time mass spectrometry for metal-loaded Ga₂O₃ model photocatalysts under methane and water vapor at ambient temperature and pressure. Our concept of metal cocatalysts that work as active sites for both photocatalytic oxidation and reduction provides a new understanding of photocatalysis and a solid basis for controlling non-thermal redox reactions by metal-cocatalyst engineering.     

Manganese vacancy-confined single-atom Ag in cryptomelane nanorods for efficient Wacker oxidation of styrene derivatives

Single-atom catalysts provide a pathway to elucidate the nature of catalytically active sites. However, keeping them stabilized during operation proves to be challenging. Herein, we employ cryptomelane-type octahedral molecular sieve nanorods featuring abundant manganese vacancy defects as a support, to periodically anchor single-atom Ag. The doped Ag atoms with tetrahedral coordination are found to locate at cation substitution sites rather than being supported on the catalyst surface, thus effectively tuning the electronic structure of adjacent manganese atoms. The resulting unique Ag–O–MnO_x unit functions as the active site. Its turnover frequency reaches 1038 h⁻¹, one order of magnitude higher than for previously reported catalysts, with 90% selectivity for anti-Markovnikov phenylacetaldehyde. Mechanistic studies reveal that the activation of styrene on the ensemble site of Ag–O–MnO_x is significantly promoted, which can accelerate the oxidation of styrene and, in particular, the rate-determining step of forming the epoxide intermediate. Such an extraordinary electronic promotion can be extended to other single-atom catalysts and paves the way for their practical applications.

Manganese vacancy-confined single-atom Ag in cryptomelane nanorods efficiently catalyses Wacker oxidation of styrene derivatives.     

Solid Base Bi24O31Br10(OH)δ with Active Lattice Oxygen for the Efficient Photo‐Oxidation of Primary Alcohols to Aldehydes

The selective oxidation of primary alcohols to aldehydes by O₂ instead of stoichiometric oxidants (for example, Mn^VII, Cr^VI, and Os^IV) is an important but challenging process. Most heterogeneous catalytic systems (thermal and photocatalysis) require noble metals or harsh reaction conditions. Here we show that the Bi₂₄O₃₁Br₁₀(OH)_δ photocatalyst is very efficient in the selective oxidation of a series of aliphatic (carbon chain from C₁ to C₁₀) and aromatic alcohols to their corresponding aldehydes/ketones under visible‐light irradiation in air at room temperature, which would be challenging for conventional thermal and light‐driven processes. High quantum efficiencies (71 % and 55 % under 410 and 450 nm irradiation) are reached in a representative reaction, the oxidation of isopropanol. We propose that the outstanding performance of the Bi₂₄O₃₁Br₁₀(OH)_δ photocatalyst is associated with basic surface sites and active lattice oxygen that boost the dehydrogenation step in the photo‐oxidation of alcohols.     

Catalytic Performance of Zr-Based Metal–Organic Frameworks Zr-abtc and MIP-200 in Selective Oxidations with H2O2

The catalytic performance of Zr-abtc and MIP-200 metal–organic frameworks consisting of 8-connected Zr₆ clusters and tetratopic linkers was investigated in H₂O₂-based selective oxidations and compared with that of 12-coordinated UiO-66 and UiO-67. Zr-abtc demonstrated advantages in both substrate conversion and product selectivity for epoxidation of electron-deficient C=C bonds in α,β-unsaturated ketones. The significant predominance of 1,2-epoxide in carvone epoxidation, coupled with high sulfone selectivity in thioether oxidation, points to a nucleophilic oxidation mechanism over Zr-abtc. The superior catalytic performance in the epoxidation of unsaturated ketones correlates with a larger amount of weak basic sites in Zr-abtc. Electrophilic activation of H₂O₂ can also be realized, as evidenced by the high activity of Zr-abtc in epoxidation of the electron-rich C=C bond in caryophyllene. XRD and FTIR studies confirmed the retention of the Zr-abtc structure after the catalysis. The low activity of MIP-200 in H₂O₂-based oxidations is most likely related to its specific hydrophilicity, which disfavors adsorption of organic substrates and H₂O₂.     

Holey Lamellar High-Entropy Oxide as an Ultra-High-Activity Heterogeneous Catalyst for Solvent-free Aerobic Oxidation of Benzyl Alcohol

The development of noble-metal-free heterogeneous catalysts is promising for selective oxidation of aromatic alcohols; however, the relatively low conversion of non-noble metal catalysts under solvent-free atmospheric conditions hinders their industrial application. Now, a holey lamellar high entropy oxide (HEO) Co_0.2Ni_0.2Cu_0.2Mg_0.2Zn_0.2O material with mesoporous structure is prepared by an anchoring and merging process. The HEO has ultra-high catalytic activity for the solvent-free aerobic oxidation of benzyl alcohol. Up to 98 % conversion can be achieved in only 2 h, to our knowledge, the highest conversion of benzyl alcohol by oxidation to date. By regulating the catalytic reaction parameters, benzoic acid or benzaldehyde can be selectively optimized as the main product. Analytical characterizations and calculations provide a deeper insight into the catalysis mechanism, revealing abundant oxygen vacancies and holey lamellar framework contribute to the ultra-high catalytic activity.     

Synthesis,characterization and catalytic activity of novel Co(II) and Pd(II)‐perfluoroalkylphthalocyanine in fluorous biphasic system; benzyl alcohol oxidation

Tetrakis[heptadecafluorononyl] substituted phthalocyanine complexes were prepared by template synthesis from 4‐(heptadecafluorononyloxy)phthalonitrile with Co(CH₃COO)^·₂H₂O or PdCl₂ in 2‐N, N‐dimethylaminoethanol. The corresponding phthalonitrile was obtained from heptadecafluorononan‐1‐ol and 4‐nitrophthalonitrile with K₂CO₃ in DMF at 50 °C. The structures of the compounds were characterized by elemental analysis, FTIR, UV–vis and MALDI‐TOF MS spectroscopic methods. Metallophthalocyanines are soluble in fluoroalkanes such as perfluoromethylcyclohexane (PFMCH). The complexes were tested as catalysts for benzyl alcohol oxidation with tert‐butylhydroperoxide (TBHP) in an organic–fluorous biphasic system (n‐hexane–PFMCH). The oxidation of benzyl alcohol was also tested with different oxidants, such as hydrogen peroxide, m‐chloroperoxybenzoic acid, molecular oxygen and oxone in n‐hexane–PFMCH. TBHP was found to be the best oxidant for benzyl alcohol oxidation since higher conversion and selectivity were observed when this oxidant was used. Copyright © 2008 John Wiley & Sons, Ltd.