Hydrothermal syntheses,crystal structures and photocatalytic properties of three POM-templated organic–inorganic hybrid compounds

Three new POM-templated organic–inorganic compounds, Cu₂(3,5-bptp)₂[H₂SiW₁₂O₄₀]·4H₂O (1), Ag₂(3,5-bptp)₂[HPW₁₂O₄₀]·4H₂O (2), [Cu₄(OH)₃Cl(H₂O)₃(4-bpo)₃](SiW₁₂O₄₀)·5H₂O (3) (3,5-bptp = 3,5-bis(3-(pyrid-4-yl)-1,2,4-triazolyl)-pyridine, 4-bpo = 2,5-bis(4-pyridyl)-1,3,4-oxadiazole) were hydrothermally synthesized and characterized by elemental analysis and single crystal X-ray diffraction. Crystal structural analysis indicates that compounds 1 and 2 are isostructural and show one dimensional zigzag chains with [H₂SiW₁₂O₄₀]^2 − as guest molecules. Compound 3 is two dimensional sheets constructed from [Cu₄(OH)₃Cl(H₂O)₃] clusters and 4-bpo ligands with [SiW₁₂O₄₀]^4 − as guest molecules. Furthermore, the photocatalytic degradation of methyl orange (MO) with 3 was studied under visible light irradiation, indicating excellent photocatalytic activity.     

A novel 3D POMOF based on dinuclear copper (II)-oxalate complexes and Keggin polyoxoanions with excellent photocatalytic activity

A novel coordination polymer, [Cu₂(btx)₂(C₂O₄)][H₂SiW₁₂O₄₀]·12H₂O (btx = 1,4-bis(triazol-1-ylmethyl)benzene)] (1) has been synthesized by hydrothermal reaction, and characterized by elemental analyses, IR spectroscopy, and single crystal X-ray diffraction. Compound 1 is composed of α-Keggin type [SiW₁₂O₄₀]^4 − polyoxoanions and dinuclear copper (II)-btx complexes. The dinuclear copper (II)-btx complex is a three-dimensional (3D) porous framework, in which the [SiW₁₂O₄₀]^4 − as 4-coordinating guests are encapsulated, forming a novel 3D POMOF with two kinds of open channels along both a and b axes. Additionally, compound 1 shows excellent photocatalytic properties for the degradation of methylene blue (MB) under both visible and ultraviolet (UV) light. The molecular design of 1 not only generates a new POMOF, but also opens a new avenue to produce photocatalytic materials.     

Catalytic decomposition of 2,3-dihydrobenzofuran to monomeric cyclic compounds over Pd/XCs2.5H0.5PW12O40/OMC (ordered mesoporous carbon) (X = 10–30 wt.%) catalysts

A series of Pd/XCs_2.5H_0.5PW₁₂O₄₀/OMC (ordered mesoporous carbon) (X = 10, 15, 20, 25, and 30 wt.%) catalysts with different Cs_2.5H_0.5PW₁₂O₄₀ contents (X, wt%) were prepared by a sequential incipient wetness impregnation method for use in the catalytic decomposition of 2,3-dihydrobenzofuran to monomeric cyclic compounds. 2,3-Dihydrobenzofuran was used as a lignin model compound for representing β-5 linkage of lignin. Acidity of Pd/XCs_2.5H_0.5PW₁₂O₄₀/OMC catalysts served as an important factor determining the catalytic performance in the reaction. Conversion of 2,3-dihydrobenzofuran and total yield for main products (2-ethylphenol and ethylcyclohexane) increased with increasing acidity of Pd/XCs_2.5H_0.5PW₁₂O₄₀/OMC catalysts.     

A novel polyoxometalate-based metal-organic compound constructed from an unprecedented [Ag8(Hpyttz)4(H2pyttz)2] cluster and an in-situ [VW12O40]3 − anion

A novel polyoxometalate (POM)-based metal-organic compound constructed from a multinuclear Ag^I cluster and Keggin-type [VW₁₂O₄₀]^3 − anion, [Ag₄(Hpyttz)₂(H₂pyttz)][HVW₁₂O₄₀]·3H₂O (1) (H₂pyttz = 5′-(pyridin-4-yl)-1H,2′H-3,3′-bi(1,2,4-triazole), was hydrothermally synthesized and characterized by IR spectroscopy, TG analysis, powder and single-crystal X-ray diffraction. In compound 1, [VW₁₂O₄₀]^3 − (VW₁₂) was in-situ transformed from [SiW₁₂O₄₀]^4 − anion. Eight Ag^I ions were connected by six Hpyttz/H₂pyttz ligands with three coordination modes forming an unprecedented [Ag₈(Hpyttz)₄(H₂pyttz)₂] cluster, which connected with six VW₁₂ anions to construct a 2-D double-layer (3,6)-connected network with {4³}₂{4⁶.6⁶.8³} topology. Moreover, the photocatalytic activity of 1 has been investigated.     

Synthesis,structure, and photocatalytic property of a new hybrid compound based on unusual bi-silver-capped polyoxometalates

A new compound based on capped polyoxometalates (POMs), [Ag(phen)₂]₂(Agphen)₂SiW₁₂O₄₀ (phen = 1,10-phenanthroline), has been hydrothermally synthesized and characterized by single-crystal X-ray diffraction analysis, infrared spectrum, UV–vis electronic spectrum, elemental analyses, and powder X-ray diffraction pattern. The structural feature of compound 1 is that each [SiW₁₂O₄₀]^4 − (SiW₁₂) cluster capped by two (Agphen)⁺ fragments. To the best of our knowledge, the compound 1 represents the first example of hybrid compound based on bi-silver-capped POMs to date. The photocatalytic properties of compound 1 were also investigated in detail and the results of photocatalytic experiment show that compound 1 can be used as a photocatalyst towards the decomposition of organic pollutant methylene blue (MB).     

Direct preparation of dichloropropanol from glycerol and hydrochloric acid gas using heteropolyacid (HPA) catalyst by heterogeneous gas phase reaction

Direct preparation of dichloropropanol (DCP) from glycerol and hydrochloric acid gas was carried out in a heterogeneous gas phase reactor using H₃PMo_12?XW_XO₄₀ (X = 0, 3, 6, 9, and 12), H₄SiW₁₂O₄₀, and H₄SiMo₁₂O₄₀ heteropolyacid (HPA) catalysts. Acid property of the HPA catalyst was determined by NH₃-TPD measurement in order to correlate the catalytic activity with the acid property of the catalyst. Acid strength of the HPA catalyst played a key role in determining the catalytic performance in the reaction. Yield for DCP increased with increasing acid strength of the catalyst. Among the catalysts tested, H₃PW₁₂O₄₀ with the highest acid strength showed the highest yield for DCP.     

Fabrication of micellar heteropolyacid catalysts for clean production of monosaccharides from polysaccharides

A micellar heteropolyacids (HPAs) catalyst had been prepared using surfactant cetyltrimethyl ammonium bromide (CTAB) and H₃PW₁₂O₄₀ as precursors. These micellar [C₁₆H₃₃N(CH₃)₃]_xH_3 − xPW₁₂O₄₀ had been characterized to be micellar structure and the catalytic activity was evaluated by the hydrolysis of polysaccharides to the reducing sugars. The best catalytic activity was obtained over [C₁₆H₃₃N(CH₃)₃]H₂PW₁₂O₄₀ (abbreviated as (C₁₆TA)H₂PW), which showed 100% conversion and 99.6% selectivity within 60 min at 80 °C for hydrolysis of sucrose. And it was also active for the conversion of starch and cellulose. The leaching test showed that the HPA micellar catalysts have an excellent stability and can be used as heterogeneous catalysts for six times.     

Modulation of selective sites by introduction of N2O,CO2 and H2 as gaseous promoters into the feed during oxidation reactions

Results obtained by adding gaseous promoters (CO₂, N₂O and H₂) into the reaction feed are presented for two different reactions: (i) oxidative dehydrogenation of propane (ODP), and (ii) catalytic combustion of methane (CCM). The ODP is performed on a mixture of NiMoO₄ and CeO₂, by adding 3 vol.% CO₂ into the feed, and on a NiMoO₄/[Si,V]-MCM-41 mesoporous catalyst, in the presence of 1 or 5 vol.% N₂O in the feed. The CCM is carried out (i) on Pd(2 wt.%)/Ce_xZr_1−xO₂ and Pd(2 wt.%)/γ-Al₂O₃ catalysts, on pure CeO₂ and on a mixture of Pd(2 wt.%)/γ-Al₂O₃ and CeO₂ powders, by adding 3 vol.% CO₂ into the feed, and (ii) on a Pd(2 wt.%)/γ-Al₂O₃ catalyst, in the presence of various amounts of H₂ in the feed. It is shown, through all these various examples, that the activity and/or the selectivity of catalysts can be improved by tuning, in a very controlled manner, the oxidation state of active sites via the use of these gaseous promoters.     

Electrospinning preparation,characterization, and enhanced photocatalytic activity of an Silicotungstic acid (H4SiW12O40)/poly(vinyl alcohol)/poly(methyl methacrylate) composite nanofiber membrane

Silicotungstic acid (H₄SiW₁₂O₄₀)/poly(vinyl alcohol) (PVA)/poly(methyl methacrylate) (PMMA) composite nanofiber membranes were prepared by an electrospinning technique. A PMMA emulsion was mixed with PVA and H₄SiW₁₂O₄₀ evenly in water (electrospinning solvent). The configuration and elemental composition of the membranes were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, and X‐ray photoelectron spectroscopy. The results indicate that H₄SiW₁₂O₄₀ with an intact Keggin structure existed in the composite membrane. The as‐prepared H₄SiW₁₂O₄₀/PVA/PMMA membranes exhibited enhanced photocatalytic efficiency (>84%) in the degradation of methyl orange (MO); it outperformed H₄SiW₁₂O₄₀ powder (4.6%) and the H₄SiW₁₂O₄₀/PVA nanofiber membrane (75.2%) under UV irradiation. More importantly, the H₄SiW₁₂O₄₀/PVA/PMMA membranes could be easily separated from the aqueous MO solution, and the photocatalytic efficiency of the membranes decreased inappreciably after three photocatalytic cycles. This may have been due to the enhanced water tolerance of the membranes and the stability of H₄SiW₁₂O₄₀ in the membranes. The photocatalytic process was driven by the reductive pathway with a much faster degradation rate because of the presence of PVA. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43193.     

Redox behavior and oxidation catalysis of HnXW12O40 (X = Co2 +, B3 +, Si4 +, and P5 +) Keggin heteropolyacid catalysts

Redox behavior and oxidation catalysis of H_nXW₁₂O₄₀ (X = Co^2 +, B^3 +, Si^4 +, and P^5 +) Keggin heteropolyacid catalysts were investigated. Successful formation of H_nXW₁₂O₄₀ catalysts was confirmed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Reduction potentials of H_nXW₁₂O₄₀ catalysts were determined by electrochemical measurements. First electron reduction potential of H_nXW₁₂O₄₀ catalysts decreased with increasing overall negative charge of heteropolyanion. H_nXW₁₂O₄₀ catalysts were then applied to the liquid-phase oxidation of benzaldehyde to benzoic acid. Yield for benzoic acid increased with increasing first electron reduction potential.     

Effect of Pr on copper-based catalysts for ethane oxychlorination

A series of Pr-doped CuCl₂–KCl/MgO-γ-Al₂O₃ catalysts (Cu 5 wt.%, MgO 10 wt.% and Cu/K = 1:2 molar ratio) has been prepared for the oxychlorination of C₂H₆ to C₂H₃Cl and characterized by XRD, H₂-TPR, ICP, TEM and XPS techniques. The modifier Pr improves the surface and catalytic properties of Cu species in the catalysts for oxychlorination. It is found that the addition of Pr species can improve the electronic transfer from Pr to Cu species and promotes the formation of the reduced and active Cu species. In addition, the dopant promotes the dispersion of amorphous CuCl₂ species. The best catalyst is CuKPr-5 with Pr (5 wt.%), which catalyzes the oxychlorination of C₂H₆ to C₂H₃Cl with highest conversion (97.5%) and the best selectivity (52%).     

Selective conversion of dihydroxyacetone–ethanol mixture into ethyl lactate over amphoteric ZrO2–TiO2 catalyst

The conversion of dihydroxyacetone in ethanol solution into ethyl lactate over several acidic and amphoteric oxides at 100–160 °C was studied. The formation of ethyl lactate with 80–90% selectivity was observed on amphoteric ZrO₂–TiO₂ oxide whereas hemiacetal and acetal of pyruval were the main products obtained over the acidic ZrO₂–SiO₂ catalyst and Amberlyst 15. Amphoteric ZrO₂–3TiO₂ oxide provides 89% yield of ethyl lactate at 140 °C and 1.0 MPa under the feed rate of 4 mmol C₃H₆O₃/g_cat/h.     

Low-temperature sintering and microwave dielectric properties of CaWO4 ceramics for LTCC applications

Microwave dielectric properties of CaWO₄ ceramics were investigated as a function of H₃BO₃ and/or Bi₂O₃ content and sintering temperature. For a single addition of H₃BO₃ (1 ≤ x (wt.%) ≤ 5), the density of specimen increased up to 3 wt.% H₃BO₃, and then decreased. The dielectric constant (K) and the quality factor (Q × f) of the specimens sintered at 850 °C showed lower value than those of specimens sintered above 900 °C due to the poor sinterability. With the increase of H₃BO₃ content of 0.5 wt.% Bi₂O₃–yH₃BO₃ (5 ≤ y (wt.%) ≤ 20), the sintering temperature of CaWO₄ ceramics could be effectively reduced from 1100 to 850 °C without degradation of dielectric properties. For the specimens sintered at 850 °C for 30 min, K was not changed remarkably with Bi₂O₃–H₃BO₃ content; however, Q × f value increased up to 9 wt.% H₃BO₃ of 0.5 wt.% Bi₂O₃–yH₃BO₃, and then decreased. The temperature coefficient of resonant frequency (TCF) shifted to the positive value with increasing Bi₂O₃–H₃BO₃ content. Typically, K of 8.7, Q × f of 70,220 GHz and TCF of −15 ppm/°C were obtained for the specimens with 0.5 wt.% Bi₂O₃–9 wt.% H₃BO₃ sintered at 850 °C for 30 min.     

Liquid-phase oxidation of benzene to phenol by molecular oxygen over transition metal substituted polyoxometalate compounds

Transition metal substituted polyoxometalate (TMSP) compounds were used as catalysts for the liquid-phase oxidation of benzene to phenol by molecular oxygen with ascorbic acid as a reducing agent in an acetone/sulfolane/water mixed solvent. [(C₄H₉)₄N]₅[PW₁₁CuO₃₉(H₂O)] is the best catalyst tested in this study. It showed 9.2% benzene conversion (TON = 25.8) and 91.8% selectivity to phenol for the oxidation of benzene at 323 K for 12 h. The effect of the substituted transition metal in the TMSP compounds on the benzene conversion is in the order: Cu > V > Fe ≫ Mn > Ti > Cr > Co > Ni > Zn. The effect of the central atom in the TMSP compounds on the benzene conversion is in the order: P ≈ Si > Ge > B. [SiW₁₁O₃₉]⁸⁻ is a good polyoxometalate ligand for transition metal ions as [PW₁₁O₃₉]⁷⁻ for the oxidation of benzene. The selectivity to phenol was dramatically improved by adding the sulfolane solvent, but the benzene conversion decreased when a large amount of sulfolane was used in the mixed solvent. Ascorbic acid is indispensable for forming phenol from benzene oxidation by O₂ over TMSP compounds. Higher O₂ pressure in the catalytic system ensured more oxygen molecules solving in the solvent, and promoted the benzene conversion.     

Synthesis and photocatalytic hydrogen evolution activity of three Keggin-type polyoxometalates with different central atoms

A series of new complexes based on Keggin-type polyoxoanions (HTEA)₂{[Na(TEA)₂]H[SiW₁₂O₄₀]}∙5H₂O (1), (HTEA)₂{[Na(TEA)₂][PW₁₂O₄₀]}∙4H₂O (2) and (HTEA)₂{[Na(TEA)₂]H[GeW₁₂O₄₀]}∙4H₂O (3) (TEA = triethanolamine) have been designed and synthesized. Single-crystal X-ray diffraction analyses reveal that compounds 1, 2 and 3 are isostructural, which exhibit the 1D chain structures consisting of Keggin-type polyoxoanions and {Na(TEA)₂} linkers. Their structures were further characterized by XRD, IR spectroscopy, UV spectroscopy, elemental analyses, and thermogravimetric analysis. Moreover, compounds 1–3 present good the photocatalytic hydrogen evolution activities, which may be ascribed to the introduction of the sacrificial electron donors TEA to their structures, further, their photocatalytic hydrogen evolution activities are compared with its homologous Keggin-type heteropolyacid to verify the effect of TEA.     

Vapor-phase hydrogenolysis of glycerol to 1,2-propanediol using a chromium-free Ni-Cu-SiO2 nanocomposite catalyst

A Ni-Cu-SiO₂ nanocomposite was studied as a catalyst for vapor-phase glycerol hydrogenation to produce 1,2-propanediol (1,2-PDO). Substitution of a small amount (3 wt%) of Ni for Cu is beneficial for decreasing the Cu particle size, which would be advantageous for attaining higher 1,2-PDO selectivity and higher glycerol conversion. 92% 1,2-PDO selectivity and 100% glycerol conversion were obtained at 220 °C, 30 bar, and a weight hourly space velocity of 0.5 h^− 1 over Ni(3)-Cu(77)-SiO_2, which were nearly identical to those obtained with the conventional copper chromite (CuO-Cr₂O₃) catalyst. Therefore, the present Ni(3)-Cu(77)-SiO₂ nanocomposite is regarded as a green and efficient catalyst for glycerol conversion into more valuable 1,2-PDO.     

Optimising H2 production from model biogas via combined steam reforming and CO shift reactions

H₂ production was studied through steam reforming of a clean model biogas in a fluidized-bed reactor followed by two stages of CO shift reactions (fixed-bed reactors). The steam reforming of biogas was performed over 11.5 wt.% Ni/Al₂O₃ and a molar CH₄/CO₂ ratio of 1.5 was employed as clean model biogas. Excess steam resulted in strong inhibition of carbon formation and an almost complete CH₄ (>98%) conversion was achieved.To optimise H₂ production, CO shift reactions were carried out at high (523–723 K) and low temperatures (423–523 K) using commercial catalysts, based on Cu/Fe/Cr and Cu/Zn, respectively. Increasing steam concentrations led to a lean CO, high H₂ product. The final product compositions following low temperature CO shift reaction (steam to dry gas ratio of 1.5 at 483 K) yielded H₂ at 68% and a CO concentration of 0.2% (equivalent to CO conversion of >99%).     

Preparation,characterisation and catalytic evaluation of MgF2 supported V2O5 catalysts for ammoxidation of 3-picoline

Non-conventional MgF₂ supported V₂O₅ catalysts with different vanadia contents (2–15 wt.%) were prepared by impregnation (using NH₄VO₃), characterised and catalytically evaluated for selective ammoxidation of 3-picoline to nicotinonitrile. Oxygen and ammonia chemisorption uptakes increased continuously from 60 to over 600 μmol g^?1 and 275 to >750 μmol g^?1, respectively, with rise in V₂O₅ proportion indicating that the redox as well as acidic sites are increasing with increase in V₂O₅ content. Thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) analysis revealed endothermic as well as exothermic thermal effects mainly due to liberation of water and ammonia, and also due to structural changes. XRD patterns showed the formation of crystalline V₂O₅ in the fresh solids having 8 wt.% V₂O₅ and above and NH₄VO₃ phase in the spent samples. The conversion of 3-picoline is observed to increase continuously with increase in V₂O₅ loading. However, the selectivity of nicotinonitrile is found to be independent on conversion of 3-picoline. The catalyst with the highest V₂O₅ loading (15.7 wt.%) displayed the best activity (X > 90%) and selectivity (S > 95%) compared to all other catalysts of this series. The 3-picoline conversion of 10% at 548 K is increased to almost 100% with rise in temperature to 663 K. Increase in 3-picoline feed rate and NH₃: 3-picoline ratio exhibited an inhibiting effect on the conversion, while an increase in air: 3-picoline ratio has no significant influence on the performance.     

Preparation of highly active nickel catalysts supported on mesoporous nanocrystalline γ-Al2O3 for CO2 methanation

In this study, a group of Ni catalysts supported on mesoporous nanocrystalline γ-Al₂O₃ with high surface area and with different Ni contents was prepared and employed in carbon dioxide methanation reaction. The obtained results showed increasing in nickel content from 10 to 25 wt.% decreased the specific surface area of catalysts from 177 to 130 m²/g and increased the nickel crystallite size from 12 to 24 nm. In addition, increasing in nickel content increased the reducibility of catalyst. The catalytic results revealed that the catalyst with 20 wt.% of Ni possessed high activity and stability in CO₂ methanation reaction.     

Low temperature pressureless sintering of dense silicon nitride using BaO-Al2O3-SiO2 glass as sintering aid

Dense Si₃N₄ ceramic with BaO-Al₂O₃-SiO₂ low temperature glass powders as sintering aids were prepared by pressureless sintering techniques at a relatively low temperature (1550 °C). Four kinds of glass powders of compositions melting at 1120 °C, 1300 °C, 1400 °C and 1500 °C, respectively, have been introduced as sintering aids. XRD results demonstrate that the BaO-Al₂O₃-SiO₂ glass powders reacted with BaAl₂O₄ and converted into hexagonal celsian, which is a high-temperature phase with melting point of 1760 °C, so being beneficial to the high temperature properties of the materials. In addition, a portion of α-Si₃N₄ transformed to rod like β-Si₃N₄ with high aspect ratio as shown by XRD and SEM analysis. The bulk density increased with the rise of the melting temperature of the BaO-Al₂O₃-SiO₂ glass powders, the sample obtained with the BaO-Al₂O₃-SiO₂ glass powder melting at 1500 °C reaching a maximum density of 98.8%, an high flexural strength (373 MPa) and a fracture toughness (4.8 MPa m^1/2).