Palladium(II) containing hydrotalcite as an efficient heterogeneous catalyst for Heck reaction

Palladium(II) containing hydrotalcite (Pd-HT) has been found to be an efficient and reusable catalyst in Heck reaction between aryl halides (X = Br, I) and olefins to give carbon–carbon coupled products in good to moderate yields.     

Amino groups immobilized on MCM-48: an efficient heterogeneous catalyst for the Knoevenagel reaction

Aminopropyl was grafted onto MCM-48 via post-synthesis methods. The aminopropyl-functionalized MCM-48 was found to be an effective base catalyst for Knoevenagel condensation.     

A readily prepared,highly reusable and active polymer-supported molybdenum carbonyl Schiff base complex as epoxidation catalyst

Polymer-bound Schiff base ligand 2 was prepared by oxidation of chloromethylated polystyrene to aldehydic polystyrene 1 and then reaction with ethylene diamine. The functionalized polystyrene 2 was used to immobilize Mo(CO)₆ and polymer-bound molybdenum carbonyl Schiff base catalyst 3 was produced. This supported catalyst shows high activity in epoxidation of various alkenes in the presence of tert-butylhydroperoxide (TBHP). The supported molybdenum catalyst can be recovered and reused for eight times without loss in its activity.     

Covalent anchoring of Mo(VI) Schiff base complex into SBA-15 as a novel heterogeneous catalyst for enhanced alkene epoxidation

Molybdenum(VI) Schiff base complexes modified mesoporous SBA-15 hybrid heterogeneous catalysts were synthesized by the reaction of MoO₂(acac)₂ with mesoporous SBA-15 functionalized by grafting procedures of 3-aminopropyl-triethoxysilane and salicylaldehyde, respectively. The physico-chemical properties of the as-synthesized catalysts were analyzed by ICP-AES, XRD, N₂ adsorption–desorption, FT-IR, SEM, TEM and EDX. The as-synthesized catalysts were effective in the catalytic epoxidation of cyclohexene. The catalytic activity can be further enhanced by silylation of the residual Si–OH groups using Me₃SiCl, which was largely due to the higher content of Mo active sites. The conversion and selectivity reached to 97.78 and 93.99 % using tert-butyl hydroperoxide as oxidant for Mo–CH₃–SA–NH₂–SBA-15, while 81.97 and 89.41 % in conversion and selectivity for Mo–SA–NH₂–SBA-15. At the same time, the catalytic performances of the hybrid materials were further systematically investigated under various reaction conditions (solvent, oxidants and alkenes, etc.). Mo–CH₃–SA–NH₂–SBA-15 catalyst can be recycled effectively and reused four cycles with little loss in activity. In addition, the results from hot filtration demonstrated that the catalytic activity mostly resulted from the heterogeneous catalytic process.     

Modified polyacrylonitrile fiber as a renewable heterogeneous base catalyst for Henry reaction and Gewald reaction in water

A polyacrylonitrile fiber catalyst with high polar surface micro‐environment (C‐PANF‐Na) was conveniently synthesized and characterized by elemental analysis, Fourier‐transfer infrared spectroscopy, UV–vis spectroscopy, scanning electron microscopy, X‐ray diffraction, and breaking strength test. The catalytic performance of the fiber was tested by Henry reaction and Gewald reaction in water and favorable results were obtained in terms of high yields, mild conditions, extensive applicability, superior catalytic recyclability, and renewability. It also has advantages of easy preparation, high functional degree, good stability in acid and base, high strength, and good flexibility, and so on. In addition, a high solvent selectivity phenomenon was found in this catalytic system and the reason was explained by proposing a high polar micro‐environment effect. Furthermore, the fiber catalyst performed well in scaled‐up experiment and flow chemistry experiment, which shows tremendous potential in industrial catalyst as an ideal material for packed bed. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45992.     

Immobilized iron Schiff base histidine complexes on Al-MCM-41 and zeolite Y as catalyst for oxidation of cycloalkanes

Iron complexes of N-salicylidene-l-histidine with or without bipyridine ligand immobilized on Al-MCM-41 and zeolite Y designated as Fe(sal-l-his)(bipy)complex/Al-MCM-41 or Fe(sal-l-his)complex/Al-MCM-41 and Fe(sal-l-his)(bpy)complex/Y or Fe(sal-l-his)complex/Y respectively, were prepared and characterized by X-ray powder diffraction (XRD), FT-IR, N₂ adsorption/desorption and chemical analysis techniques. Fe(sal-l-his)/Al-MCM-41 and Fe(sal-l-his)(bipy)complex/Al-MCM-41 were found to successfully catalyze the oxidation of cyclohexane, methyl cyclohexane, cyclooctane and adamantane with H₂O₂. The oxidation results and promising catalytic behavior of Fe(sal-l-his)(bipy)complex/Al-MCM-41 for oxidation of cyclooctane with 90 % conversion and excellent selectivity toward the formation of cyclooctanone will be discussed in this presentation.     

Oxovanadium (IV) Schiff base complex immobilized on CPS microspheres as heterogeneous catalyst for aerobic selective oxidation of ethyl benzene to acetophenone

In this work, chloromethylated crosslinked polystyrene microspheres (CMCPS microspheres) were used as starting carrier, and a new immobilized bidentate Schiff base oxovanadium (IV) complex catalyst was prepared. The chloromethyl groups of CMCPS microspheres were first transformed into aminomethyl groups via Delepine reaction with hexamethylene tetramine (HMTA) as reagent, obtaining aminomethylated (AM) microspheres AMCPS. And then the Schiff base reaction between the primary amino group of AMCPS microspheres and salicylaldehyde (SA) was allowed to be conducted, resulting in Schiff base-type resin microspheres, SAAM-CPS, on which bidentate Schiff base ligand SAAM were chemically anchored. Subsequently, the coordination reaction between the ligand SAAM of SAAM-CPS microspheres and vanadyl sulfate (VOSO₄) was carried out, obtaining a new immobilized bidentate Schiff base-type oxovanadium (IV) complex, CPS-[VO(SAAM)₂] microspheres, namely a new heterogeneous oxovanadium (IV) complex catalyst was prepared. On the basis of fully characterizing the microspheres CPS-[VO(SAAM)₂], they were used in the catalytic oxidation of ethyl benzene with molecular oxygen. The experimental results show that in the oxidation reaction of ethylbenzene with molecular oxygen as oxidant, the heterogeneous oxovanadium (IV) complex catalyst, CPS-[VO(SAAM)₂], has high catalytic activity and excellent catalytic selectivity. Under the mild conditions such as at ordinary pressure of dioxygen and at a relatively low temperature of 110 °C, ethyl benzene can be transformed to acetophenone as a single product with a yield of 43%. It was found that the reaction temperature and the used amount of the solid catalyst effect on the oxidation reaction greatly. The catalyst CPS-[VO(SAAM)₂] has excellent recycle and reuse property.     

Zeolite-encapsulated Ru(III) tetrahydro-Schiff base complex: An efficient heterogeneous catalyst for the hydrogenation of benzene under mild conditions

A series of Ru(III) tetrahydro-Schiff base complexes (denoted as Ru[H₄]-Schiff base with Schiff base = salen, salpn and salcn, see Scheme 1) were encapsulated in the supercages of zeolite Y by flexible ligand method. The prepared catalysts were characterized by X-ray diffraction, diffuse reflectance UV–Vis spectroscopy, Infrared spectroscopy, elemental analysis, as well as N₂ adsorption techniques. It was shown that upon encapsulation in zeolite Y, Ru(III) tetrahydro-Schiff base complexes exhibited higher activity for the hydrogenation of benzene than the corresponding Ru(III)-Schiff base complexes. This indicates that hydrogenation of the CN bond of the Schiff base ligands led to a modification of the coordination environment of the central Ru(III) cations. The stability of the prepared catalysts has also been confirmed against leaching of the complex molecule from the zeolite cavities, as revealed by the result that no loss of catalytic activity was observed within three successive runs with regeneration.     

Comments on the use of buckminsterfullerene encapsulated in zeolite Y as a potential catalyst

Buckminsterfullerene encapsulated within zeolite Y is found not to be a reactive radical initiator for the formation of dimethoxyethane from dimethyl ether below 200° C. Above this temperature the expected acid catalysed conversion of dimethyl ether to hydrocarbons is observed. Dimethoxyethane formation is observed when bibenzoyl peroxide is used as a radical initiator. These studies indicate that any radical formed by the encapsulated buckminsterfullerene is relatively unreactive.     

FeNi doped porous carbon as an efficient catalyst for oxygen evolution reaction

Polymer-derived porous carbon was used as a support of iron and nickel species with an objective to obtain an efficient oxygen reduction reaction(OER)catalyst.The surface features were extensively characterized using X-ray diffraction,X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy.On FeNi-modified carbon the overpotential for OER was very low(280 mV)and comparable to that on noble metal catalyst IrO₂.The electrochemical properties have been investigated to reveal the difference between the binary alloy-and single metal-doped carbons.This work demonstrates a significant step for the development of low-cost,environmentally-friendly and highly-efficient OER catalysts.     

One-pot synthesized multi-functional graphene oxide as a water-tolerant and efficient metal-free heterogeneous catalyst for cycloaddition reaction

A series of functional graphene oxide (F-GO) materials were prepared by silylanization of graphene oxide (GO) with chlorine-terminal silanes and subsequent nucleophilic substitution reaction with tertiary amine in a one-pot approach. The quaternary ammonium salt was generated and immobilized simultaneously in situ, and the primary amine is beneficial to improve the number of surface functionalities on GO. This is the first time that a multi-functional GO material was designed, prepared and used for the synthesis of cyclic carbonates through the cycloaddition of carbon dioxide (CO₂) to epoxides. The F-GO catalyst can be easily separated and reused for at least five times without significant loss of activity (TOF = 46.4 h⁻¹). The excellent performance is attributed to the synergetic effect of silanol group and halide anion for ring opening of epoxide as well as the role of amine for CO₂ adsorption and activation. A possible mechanism is proposed for the cycloaddition reaction over F-GO.     

Modification of a heterogeneous catalyst: Sulfonated graphene oxide coated by SiO2 as an efficient catalyst for Beckmann rearrangement

In this work and in continuation of our strategy for improving the performance of heterogeneous catalysts, the SiO₂@GO-OSO₃H catalyst was applied as a very efficient solid acid catalyst for the Beckmann rearrangement. This catalyst effectively catalyzed the liquid-phase Beckmann rearrangement of cyclohexanone oxime to caprolactam in high yield under very mild conditions. The improvement of GO-OSO₃H catalyst in regard to its separation from reaction mixture is achieved by coating it with a mesoporous SiO₂ layer. The influence of the operating parameters on the performance of the catalyst namely solvent, temperature, amount of catalyst, and time was studied.     

Palladium complex embedded crosslinked polystyrene nanofibers as a green and efficient heterogeneous catalyst for coupling reactions

A simple and green chemical modification coupled with electrospinning technique has been developed to incorporate tetrakis(triphenylphosphine)palladium [Pd(PPh₃)₄] inside crosslinked polystyrene nanofibers (Pd@CPS) as an efficient and stable heterogeneous palladium catalyst. The catalytic activities and recyclabilities of the prepared Pd@CPS catalyst have been evaluated by using Suzuki and Heck reactions of various aromatic halides separately with phenylboronic acid and alkenes. The Pd@CPS exhibited high-catalytic activities for the Suzuki and Heck reactions of aromatic iodides to afford the products in excellent yields (coupling yields >88%). The catalytic activities and the nanofiber structure remained essentially unchanged even after recycling for five times. The high activities and stabilities of the prepared Pd@CPS catalyst can be attributed to the ultrafine fiber and embedment of palladium active species inside the nanofibers.     

Schiff base complex of Cu immobilized on MCM-41 as reusable catalyst for efficient oxidative coupling of thiols and oxidation of sulfides using hydrogen peroxide

Schiff base complex of copper-functionalized MCM-41 (Cu-complex@MCM-41) was synthesized and used as an efficient and novel heterogeneous catalyst for the oxidative coupling of thiols into corresponding disulfides and oxidation of sulfides to sulfoxides using hydrogen peroxide (H₂O₂) as the oxidant. An aliphatic and aromatic series of sulfides and thiols including various functional groups were successfully converted into corresponding products. The all products were obtained in good to excellent yields. The mesoporous catalyst is characterized by FT-IR spectroscopy, BET, XRD, SEM, EDS and TGA. Recovery of the catalyst is easily achieved by simple filtration and reused for several consecutive runs without significant loss of its catalytic efficiency.     

Zeolitic imidazole framework-67 as an efficient heterogeneous catalyst for the synthesis of ethyl methyl carbonate

Zeolitic imidazole framework (ZIF)-67, a novel environmentally benign catalyst, was developed for the preparation of ethyl methyl carbonate (EMC) from dimethyl carbonate and diethyl carbonate. EMC was obtained in 83.39% yield using ZIF-67 as the catalyst when compared to that obtained using ZIF-8 catalyst. NH₃ and CO₂ temperature-programmed desorption methods were used to evaluate the presence of both acidic and basic sites in ZIF-67 catalyst. The lager surface area of ZIF-67 favors for the adsorption of reactants over the solid surface of the catalyst, facilitating the formation of EMC. Moreover, ZIF-67 catalyst exhibited excellent reusability without significant loss in its catalytic activity.     

Zn-VCO3 hydrotalcite: A highly efficient and reusable heterogeneous catalyst for the Hantzsch dihydropyridine reaction

The Zn-VCO₃ hydrotalcite was found to be an excellent solid catalyst for the one-pot synthesis of triphenylpyridine-3,5-dicarboxamide via Hantzsch reaction of acetoacetanilide, ammonium hydroxide and various aromatic aldehydes. The combinatorial syntheses were achieved for the first time using hydrotalcite as a heterogeneous catalyst. The catalyst was active for the Hantzsch reaction in water at 60 °C. The products were isolated in good yields (85–93%) with short reaction times (2–3 h). The resulting substituted dihydropyridines were characterized and confirmed by ¹H and ¹³C NMR, FTIR, and HRMS spectral data and the solid catalyst was characterized by XRD, BET, SEM and TEM. The newly synthesized heterogeneous solid catalyst offers simple means for recovery and the isolated catalyst was reused for five rounds for the synthesis of compound 4a, without significant loss of catalytic activity. For all the other reactions carried out with the recycled catalyst, results were similar to with the fresh catalyst.     

Microwave-assisted one-pot three-component synthesis of thiazolidinones using KSF@Ni as an efficient heterogeneous catalyst

Convenient, one-pot three-component reaction for the synthesis of thiazolidinones from aldehydes, thiosemicarbazide, and maleic anhydride in the presence of KSF@Ni as heterogeneous catalyst under microwave irradiation was developed. Products were obtained in reasonable yield in short reaction time.     

Sodium borohydride reduction of aldehydes catalyzed by an oxovanadium(IV) Schiff base complex encapsulated in the nanocavity of zeolite-Y

Selective reduction of a wide range of aromatic and aliphatic aldehydes to the corresponding alcohols with NaBH₄ was achieved in the presence of an oxovanadium(IV) Schiff base complex encapsulated in the nanopores of zeolite Y (VOL-Y). For most of the aldehydes, a conversion of 100% was observed after less than 5 min. High chemoselectivity of the reduction for aldehydes over ketones was verified by a competitive reduction between 4-methylbenzaldehyde and acetophenone.     

Pd nanoparticles supported on ZIF-8 as an efficient heterogeneous catalyst for the selective hydrogenation of cinnamaldehyde

Palladium nanoparticles supported on ZIF-8 (Pd/ZIF-8) were prepared by a facile impregnation method and characterized by XRD, TEM, ICP and N₂ adsorption. The prepared Pd/ZIF-8 catalyst exhibited excellent catalytic performance for the selective hydrogenation of cinnamaldehyde. The reaction solvents had great influences on the catalytic performance of the prepared Pd/ZIF-8 catalyst. Pd nanoparticles supported on ZIF-8 gave much higher catalytic activity and hydrocinnamaldehyde selectivity than those supported on other MOFs or the conventional inorganic supports. The prepared Pd/ZIF-8 catalyst could be reused at least four times without significant loss in activity and selectivity.     

Palladium nanoparticles embedded chitosan/poly(vinyl alcohol) composite nanofibers as an efficient and stable heterogeneous catalyst for Heck reaction

In this study, palladium nanoparticles were successfully embedded into modified chitosan/poly(vinyl alcohol) composite nanofibers (Pd-CS/PVA nanofibers) by electrospinning. Then, the Pd-CS/PVA nanofibers were treated at evaluated temperature to improve its solvent resistance and in situ reduce Pd²⁺ cations into Pd⁰ active species. The incorporated palladium nanoparticles with ultra small mean diameter of 3.73 ± 1.04 nm are evenly distributed inside the Pd-CS/PVA nanofiber. The resulting Pd-CS/PVA nanofiber mat exhibits high catalytic activity for Heck reaction of aromatic iodides with alkenes and can be recycled for 18 times without loss of initial activity. The high catalytic activity and stability of Pd-CS/PVA nanofiber mat can be attributed to the ultra small diameter nanofibers, strong chelating ability of chitosan, and fine embedment of palladium species inside the nanofiber. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48026.