Direct synthesis of hydrogen peroxide from hydrogen and oxygen over palladium catalyst supported on SO<Subscript>3</Subscript>H-functionalized MCF silica: Effect of calcination temperature of mesostructured cellular foam silica

Palladium catalysts supported on SO₃H-functionalized MCF silica (Pd/SO₃H-MCF-T (T=450, 550, 650, 750, 850, and 950)) were prepared with a variation of calcination temperature (T, °C) of MCF silica. They were then applied to the direct synthesis of hydrogen peroxide from hydrogen and oxygen. Conversion of hydrogen, selectivity for hydrogen peroxide, and yield for hydrogen peroxide showed volcano-shaped curves with respect to calcination temperature of MCF silica. Yield for hydrogen peroxide increased with increasing acid density of Pd/SO₃H-MCF-T catalysts. Thus, acid density of Pd/SO₃H-MCF-T catalysts played an important role in determining the catalytic performance in the direct synthesis of hydrogen peroxide. Pd/SO₃H-MCF-T catalysts efficiently served as an acid source and as an active metal catalyst in the direct synthesis of hydrogen peroxide.     

Effect of the Acidity of the Groups of Functionalized Silicas on the Direct Synthesis of H<Subscript>2</Subscript>O<Subscript>2</Subscript>

Three catalysts consisting of palladium supported on functionalized silica with different acid groups (namely aryl sulfonic, alkyl phosphonic and alkyl carboxylic groups) were prepared and tested in the direct synthesis of hydrogen peroxide in neutral methanol medium at an overall pressure of 5.0 MPa. These catalysts can produce hydrogen peroxide. In addition, the activity results indicated correlations among the acid strength of the acidic group on the supports, the proportion of high binding energy palladium species and the selectivity for hydrogen peroxide.     

Cobalt and iron modified activated carbon from coal tar pitch: preparation and application as catalysts for methanol decomposition

Activated carbons were prepared from waste coal treatment by-products by different preparation and activation procedures and applied as a host matrix of nanodispersed cobalt and iron species. Thus obtained composites were characterized by nitrogen physisorption, XRD, UV–Vis, FTIR, TPR and tested as catalysts in methanol decomposition to hydrogen and CO. It was established that the activated carbon, prepared from waste coal treatment by-products could be successfully used for the preparation of highly active catalysts for methanol decomposition. The facilitated effect of surface functionality decrease on the dispersion and catalytic activity of loaded metal species was demonstrated.     

Assessment of Fe2O3/SiO2 catalysts for the continuous treatment of phenol aqueous solutions in a fixed bed reactor

Different iron-containing catalysts have been tested for the oxidation of phenol aqueous solutions in a catalytic fixed bed reactor in the presence of hydrogen peroxide. All the catalysts consist of iron oxide, mainly crystalline hematite particles, over different silica supports (mesostructured SBA-15 silica and non-ordered mesoporous silica). The immobilization of iron species over different silica supports was addressed by direct incorporation of metal during the synthesis or post-synthesis impregnation. The synthesis conditions were tuned up to yield agglomerated catalysts with iron loadings between 10 and 15 wt.%. The influence of the preparation method and the type of silica support was evaluated in a catalytic fixed bed reactor for the continuous oxidation of phenol in terms of catalysts activity (phenol and total organic carbon degradation) as well as their stability (catalyst deactivation by iron leaching). Those catalysts prepared by direct synthesis, either in presence of a structure-directing agent (Fe₂O₃/SBA-15(DS)) or in absence (Fe₂O₃/SiO₂(DS)), achieved high catalytic performances (TOC reduction of 65% and 52%, respectively) with remarkable low iron leaching in comparison with their silica-based iron counterparts prepared by impregnation. Catalytic results have demonstrated that the synthesis method plays a crucial role in the dispersion and stability of active species and hence resulting in superior catalytic performances.     

Self-assembled HPW/silica–alumina mesoporous nanocomposite as catalysts for oxidative desulfurization of fuel oil

Mesoporous silica–alumina–polyoxometalate (HPW/SiO₂–Al₂O₃) nanocomposite materials with silica–aluminum molar ratios of 10–80 have been successfully synthesized by evaporation induced self-assembly method with non-ionic surfactant P123 as template agent. The surface areas and pore sizes of the obtained HPW/SiO₂–Al₂O₃ materials are in the range of 509–623 m² g^?1 and 3.6–3.8 nm, respectively, with different silica–aluminum molar ratios. The incorporated polyoxometalate clusters preserve their intact Keggin structure into the mesoporous frameworks. The Py–FTIR investigations indicate that the surface acidity of catalysts gradually increases with an increase in the percentage of aluminium, and the Lewis acidity sites are predominant. The nanocomposites were used as catalysts, and H₂O₂ as oxidant for oxidative desulfurization (ODS) of model fuel, which was composed of benzothiophene (BT), petroleum ether and benzene. The results show that the adsorption capacity and ODS performance of catalysts have close relationship with their surface acidity. An appropriate amount of Lewis acidity sites can contribute to the selective oxidation of the BT due to the preferential adsorption of BT on the catalyst surface, while the Brönsted acidity sites have a negative impact on the selective oxidation of the BT. As a result, the mesoporous HPW/SiO₂–Al₂O₃ with silica–aluminum molar ratio of 50 shows the highest selectivity for BT oxidation in the presence of benzene and has achieved the goal of desulfurization. In addition, the catalyst shows excellent reusing ability, which makes it a promising catalyst in ODS process.     

Synthesis of wormhole mesoporous silica by compressed carbon dioxide and application in catalytic hydrogenation

Mesoporous silica materials were prepared by infusion and selective condensation of silicon alkoxides within self-assembly formed from protonated hexadecylamine by CO₂ in aqueous media. Compared to the conventional synthesis approach, the introduction of CO₂ facilitates the hydrolysis of silicon alkoxide precursor and does not need adding any co-solvent in the course of the synthesis of mesoporous material. Moreover, CO₂ can be used as a swelling agent increasing the average pore size form 3.2–5.4 nm with expansion rate of approximately 69 %. The influence of CO₂ pressure and temperature were carefully investigated. During the preparation, nonionic surfactant hexadecylamine can turn into cationic surfactant in situ as mesostructure-directing template under pressurized CO₂ condition, which is the key factor for the formation of ordered mesoporous silica. In addition, the supported Pd catalysts were prepared and characterized thoroughly by X-ray diffraction, ¹H NMR, ¹³C NMR, ICP-AES analysis, scanning electron microscopy, transmission electron microscopy and nitrogen adsorption measurements. On the basis of the characterization and hydrogenation performance of the catalysts, the roles of different supports were discussed.     

Ethylene polymerization over (nBuCp)2ZrCl2/MAO catalytic system supported on aluminosilicate SBA‐15 mesostructured materials

Heterogeneous metallocene catalysts were prepared by incipient wetness impregnation of AlSBA‐15 (Si/Al = 4.8, 15, 30, 60, and ∞) mesostructured materials with (nBuCp)₂ZrCl₂/MAO. For comparative purposes commercial silica and silica–alumina (Si/Al = 4.8) supports were also impregnated with the MAO/metallocene catalytic system. A combination of X‐ray powder diffraction, nitrogen adsorption–desorption isotherms at 77 K, transmission electron microscopy, ICP‐atomic emission spectroscopy, and UV–vis spectroscopic data, were used to characterize the supports and the heterogeneous catalysts. Ethylene polymerizations were carried out in a schlenk tube at 70 °C and 1.2 bar of ethylene pressure. The polyethylene obtained was characterized by GPC, DSC, and SEM. Catalysts prepared with mesostructured SBA‐15 supports exhibited better catalytic performance than those supported on amorphous silica and silica–alumina. In general, higher ethylene polymerization activity was achieved if (nBuCp)₂ ZrCl₂/MAO catalytic system was heterogenized using supports with lower pore size in the range of the mesopores and lower Si/Al ratio. All catalysts produced high‐density polyethylene, with high crystallinity values and fibrous morphology when SBA‐15 mesostructured materials were used as supports. POLYM. ENG SCI., 2008. © 2008 Society of Plastics Engineers     

Baeyer–Villiger oxidation of ketones with hydrogen peroxide catalyzed by silica supported aluminum chloride

Silica supported AlCl₃ catalyst was prepared by ion-exchange technique. Ketones are oxidized by 30% hydrogen peroxide in ethanol and transformed into the corresponding lactones with very high selectivity catalyzed by silica–AlCl₃. The catalyst was easily prepared in large scale with inexpensive materials and recyclable.     

Direct synthesis of hydrogen peroxide from H2 and O2 and in situ oxidation using zeolite-supported catalysts

Four microporous materials, zeolites HZSM-5, Y, Beta and TS-1, were used as the supports to prepare supported gold catalysts using impregnation or deposition precipitation. The gold catalysts were tested in the direct synthesis of hydrogen peroxide from H₂ and O₂ and for CO oxidation. The effect on the catalytic activity of different metal (e.g., Pd, Pt, Cu, Ag, Rh or Ru) on the synthesis of hydrogen peroxide was also tested. Organic substrates, such as cyclohexane or cyclooctene, were introduced to investigate the possibility of in situ H₂O₂ oxidation with these catalysts.     

Effects of in situ and ex situ formations of silica nanoparticles on polyethersulfone membranes

This study deals with the observed changes in the structure and performance of polyethersulfone (PES) membranes due to in situ formation and ex situ addition of silica particles (SiO₂). Hydrolysis and condensation of tetraethyl orthosilicate (TEOS) inside the PES polymer matrix and the reaction of TEOS with ammonium hydroxide were chosen to form in situ and ex situ SiO₂ formations, respectively. The resultant structure confirmed by X-ray diffraction for the composite PES membranes showed the retention of the amorphous nature even after the addition of SiO₂. The FTIR study revealed the functional groups corresponding to silica networks with enhanced OH signatures on the surface of the composite membranes. Field emission scanning electron microscopic images showed the variation in the surface and cross-sectional structures for the pure and composite membranes. Considerable reduction in the thickness of the skin, difference in the pore structure and ‘finger-like’ cross-sectional morphology with the presence of SiO₂ was observed in PES membranes. Both SiO₂/PES composite membranes were showed a minor change in their glass transition temperature (T _g). The ex situ methodically formed composite membrane displayed an increase in the pure water flux and decrease in bovine serum albumin rejection as compared to in situ and pure PES membranes. These kinds of composite membranes can be utilized for water treatment applications demanding higher water flux.     

Guaiacol hydrodeoxygenation in the presence of Ni-containing catalysts

A series of Ni-containing catalysts supported on different materials has been tested in the hydrodeoxygenation of guaiacol, a compound modeling the products of biomass fast pyrolysis. The reaction has been carried out in an autoclave at 320°C and a hydrogen pressure of 17 MPa. The main guaiacol hydrodeoxygenation products are cyclohexane, 1-methylcyclohexane-1,2-diol, and cyclohexanone (which result from aromatic ring reduction). A guaiacol conversion scheme explaining the formation of the main products is suggested. The highest activity is shown by the Ni-containing catalysts on SiO₂ and SiO₂-ZrO₂ supports prepared by the sol-gel method. According to X-ray diffraction and electron microscopic data, the high activity of these catalysts is due to the high concentration of dispersed nickel as reduced films on the surface of the silicate structures. The catalysts offer promise for refining the biomass fast pyrolysis products (bio-oil) into hydrocarbon fuel.     

Characterization of nickel catalysts by chemisorption techniques,x-ray diffraction and magnetic measurements: Effects of support,precursor and hydrogen pretreatment

Two series of supported nickel catalysts were prepared by using alumina, silica and titania as supports and nickel nitrate or nickel chloride as impregnating salts. The catalysts, prereduced with hydrogen in the range 300–700°C, were characterized by adsorption of hydrogen and oxygen, X-ray diffraction (XRD) and magnetic methods. Strong effects of the nature of the support, of the nickel salt precursor and, in a few instances, of the reduction temperature on the adsorptive and textural properties of nickel catalysts were observed. For the series prepared from nickel nitrate, alumina support gave the highest dispersions of nickel, which varied only slightly with the reduction temperature, whereas the dispersion of titania-supported catalysts decreased significantly when the reduction temperature was increased. In contrast, the series prepared with nickel chloride always exhibited low metal dispersions which were nearly independent of the nature of the support and the reduction temperature. A strong decrease in hydrogen adsorption was observed on all samples prepared from nickel chloride. This decrease was recorded, for the nitrate preparation series, only on Ni/TiO₂ reduced at 500 and 700°C and on Ni/SiO₂ reduced at 700°C, which, in this instance, may be related to a strong metal-support interaction. On the other hand, oxygen chemisorption took place on all catalysts, allowing the determination of their metallic dispersion. Nickel crystallite sizes calculated from oxygen chemisorption were in good agreement with those determined from XRD and magnetic measurements, provided that the adsorption stoichiometry O/Ni_s=2 is assumed for typical catalysts whereas O/Ni_s = 1 should be applied to Ni/TiO₂ under the strong metal-support interaction state.     

Mesoporous silica based catalysts for the oxidation of azodyes in waste water

Iron-containing catalysts supported on mesoporous silica samples that differ in the methods of their preparation (commercial KSS silica gel, MSM-41 mesoporous silicalite, and silica gels obtained under laboratory conditions by means of spray drying and drying in supercritical СО₂) are synthesized. Their porous structure is studied via low-temperature nitrogen adsorption. Their catalytic activity and stability in the oxidation of carmoisine (an anionic dye) with a 3% hydrogen peroxide solution in water at 60°C and рН 3 are compared. The initial carmoisine and catalyst concentrations in solution are 20 mg/L and 3 g/L, respectively, and the molar Н₂О₂/carmoisine ratio is 459/1. The KSS-based catalyst demonstrates the highest activity and stability with respect to the outwashing of an active component into a solution: the conversion of carmoisine on this catalyst is as high as 99% for 30 min, while the concentration of iron ions in solution (0.27 mg/L) does not exceed the maximum allowable concentration. After the support is preimpregnated with aluminum, the degree of iron outwashing into a solution is more than halved. The synthesized catalysts are of interest with regard to the purification of waste water containing organic admixtures.     

Direct synthesis of hydrogen peroxide from H2/O2 and oxidation of thiophene over supported gold catalysts

Direct synthesis of hydrogen peroxide from H₂ and O₂ was performed over supported gold catalysts. The catalysts were characterized by means of UV–vis, H₂-TPR, TEM and XPS. Based on the results we conclude that metallic Au is the active species in the direct synthesis of hydrogen peroxide from H₂ and O₂. During preparation process of catalyst by deposition–precipitation with urea, the pH value increased and the gold particle size decreased with increasing the urea concentration. The catalyst prepared with higher urea concentration showed a higher activity and its stability also was efficiently improved. Gold nanoparticles, supported on TiO₂ or Ti contained supports, gave a higher catalytic activity. Thiophene can be efficiently oxidized by hydrogen peroxide synthesized in situ from H₂ and O₂ over Au/TS-1.     

Silica nanotubes and hollow silica nanofibers: Gas phase mineralization,polymerization catalysis and in-situ polyethylene nanocomposites

Gas phase mineralization and mesoscopic replication of polyvinyl alcohol (PVA) nanofibers represents an attractive route to the preparation of silica nanotubes and hollow fibers with independent control of pore diameter and wall size. In the sol/gel gas phase process, PVA nanofibers, produced by electrospinning of aqueous PVA, were encapsulated in a thin silica shell by repeated sequenced feed of SiCl₄ and H₂O vapors, followed by thermal degradation of the PVA core at 550 °C. The hollow fiber wall thickness was governed by the number of SiCl₄/H₂O cycles with an average increase of the wall size of 0.7 nm per cycle. In contrast to conventional sol/gel electrospinning and wet sol/gel dip coating, shearing of such hollow silicate nanofibers afforded single silica nanotubes with an average length of a few microns. Aqueous silica sols added together with PVA gave control of the inner pore architectures. Methylalumoxane (MAO) activated silica nanotubes were used as supports for half sandwich chromium (III) (Cr) and post metallocene (Fe) catalysts for ethylene polymerization and in-situ nanocomposite formation with uniform dispersion of silica nanotubes within the polyethylene matrix. A blend of Cr and Fe was supported on silica nanotubes to produce melt processable polyethylene nanocomposites with bimodal molecular weight distributions.     

Gas separation properties of polyvinylchloride (PVC)-silica nanocomposite membrane

Researchers have focused on improving the performance of polymeric membranes through various methods, such as adding inorganic nanoparticles into the matrix of the membranes. In the present study, the separation of oxygen, nitrogen, methane and carbon dioxide gases by PVC/silica nanocomposite membranes was investigated. Silica nanoparticles were prepared via sol-gel method. Membranes were prepared by thermal phase inversion method and characterized using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and thermal gravimetry (TGA) analyses. The FTIR and SEM analyses demonstrated a nano-scale dispersion and good distribution of silica particles in the polymer matrix. According to TGA results, thermal properties of PVC membranes were improved and DSC analysis showed that glass transition temperature of nanocomposite membranes increased by adding silica particles. We concluded that the permeability of carbon dioxide and oxygen increased significantly (about two times) in the composite PVC/silica membrane (containing 30 wt% silica particles), while that of nitrogen and methane increased only 40 to 60 percent. Introducing 30 wt% silica nanoparticles into the PVC matrix, increased the selectivity of CO₂/CH₄ and CO₂/N₂ from 15.9 and 21 to 18.2 and 27.3, respectively. The diffusion and solubility coefficients were determined by the time lag method. Increasing the silica mass fraction in the membrane increased the diffusion coefficients of gases considered in the current study.     

Oxidative dehydrogenation of ethane over vanadium-based hexagonal mesoporous silica catalysts

Vanadium-containing hexagonal mesoporous silica catalysts were tested in oxidative dehydrogenation of ethane. V-HMS catalysts (0.3–9.0 wt.% V) were prepared by impregnation with solution of vanadyl acetylacetonate, and by incorporation of vanadium in the synthesis process. The prepared catalysts achieved a different distribution of vanadium species (isolated monomeric units with tetrahedral coordination, oligomeric units connected by VOV bonds up to distorted tetrahedral coordination, two-dimensional polymeric units in octahedral coordination, and bulk vanadium oxides). The contribution deals with the understanding of the relationship between the distribution of vanadium species and their activity in ODH of ethane. It has been found that both monomeric and oligomeric vanadium species play important role in ODH of ethane. The activity correlated with the population of oligomeric tetrahedrally coordinated vanadium species, which were evidenced by the UV–vis band at 315 nm. To analyze this effect, V-HMS catalysts were characterized by means of UV–vis spectroscopy, H₂-TPR and N₂-adsorption.     

Selective oxidation of methane with air over silica catalysts

Partial oxidation of methane by oxygen to form formaldehyde, carbon oxides, and C₂ products (ethane and ethene) has been studied over silica catalyst supports (fumed Cabosil and Grace 636 silica gel) in the 630–780 °C temperature range under ambient pressure. The silica catalysts exhibit high space time yields (at low conversions) for methane partial oxidation to formaldehyde, and the C₂ hydrocarbons were found to be parallel products with formaldehyde. Short residence times enhanced both the C₂ hydrocarbons and formaldehyde selectivities over the carbon oxides even within the differential reactor regime at 780 °C. This suggests that the formaldehyde did not originate from methyl radicals, but rather from methoxy complexes formed upon the direct chemisorption of methane at the silica surface at high temperature. Very high formaldehyde space time yields (e.g., 812 g/kg cat h at the gas hourly space velocity = 560 000 (NTP)/kg cat h) could be obtained over the silica gel catalyst at 780 °C with a methane/air mixture of 1.5/1. These yields greatly surpass those reported for silicas earlier, as well as those over many other catalysts. Low CO₂ yields were observed under these reaction conditions, and the selectivities to formaldehyde and C₂ hydrocarbons were 28.0 and 38.8%, respectively, at a methane conversion of 0.7%. A reaction mechanism was proposed for the methane activation over the silica surface based on the present studies, which can explain the product distribution patterns (specifically the parallel formation of formaldehyde and C₂ hydrocarbons).     

Hydrogenation of 2-Ethylanthraquinone over Pd/SiO2 and Pd/Al2O3 in the Fixed-Bed Reactor. The Effect of the Type of Support

The effects of the type of support and Pd concentration profile in alumina and silica supported egg-shell catalysts and their performance in the hydrogenation of 2-ethylanthraquinone (eAQ) were studied in 'Anthra' (AQ) and 'All-Tetra' systems. The activity and deactivation of catalysts were determined in the fixed-bed reactor. Solution saturated with hydrogen, (concentration of active quinones 60g/dm³, eAQ in the AQ system, 30% of eAQ and 70% of H₄eAQ–2-ethlytetrahydroanthraquinone, in the All-Tetra system) was circulated through the catalyst bed at temperature 50°C and pressure 5bar. The contents of eAQ, active quinones, H₄eAQ and degradation products were determined in the course of hydrogenation by GC method. The egg-shell palladium catalysts (1–2wt% Pd) prepared by the precipitation of palladium hydroxide onto alumina and silica supports pre-impregnated with various alkaline (NaHCO₃, NaH₂PO₄, Na₂SiO₃) solutions were used in the hydrogenation experiments. Pd concentration profile inside the grains of catalysts was characterized by scanning electron microscopy. A difference between alumina and silica carriers with respect to the course of side reactions producing degradation products was found. Degradation of quinones in the hydrogenolytic reactions predominated on alumina supported catalysts, while the catalysts with silica favoured the hydrogenation of aromatic rings resulting in H₄eAQ-active quinone. As a crucial factor for the decrease in the activity during the hydrogenation run, the reactivity of catalyst in the hydrogenolytic reactions was established. Alumina supported catalysts exhibited much higher deactivation than those of silica supported ones. Silica carrier as well as silica species introduced onto alumina under pre-impregnation with Na₂SiO₃ exhibited an advantageous role in the catalyst performance, in terms of activity and deactivation.     

Enhanced selectivity to benzaldehyde in the liquid phase oxidation of benzyl alcohol using nanocrystalline ZSM-5 zeolite catalyst

In the present paper, nanocrystalline hierarchical ZSM-5 zeolites were successfully synthesized by the hydrothermal method in the presence of tetrapropylammonium hydroxide as a single template with the gel composition of 58SiO₂:Al₂O₃:20TPAOH:1,500H₂O. The prepared zeolite catalysts were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Nitrogen adsorption–desorption (BET), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HR-TEM) techniques. The formation of pure and highly crystalline ZSM-5 zeolite phase is confirmed by XRD. The IR vibration band at 550 cm^?1 is assigned to the double 5-rings of MFI-type zeolites. N₂ adsorption–desorption isotherms showed that the synthesized product had high BET surface area and possessed composite pore structures with both micro and mesopores. The catalytic performance of hierarchical ZSM-5 zeolite was investigated in the selective oxidation of benzyl alcohol (BzOH) with hydrogen peroxide (H₂O₂) under mild conditions. The results showed that the conversion of BzOH and the selectivity to benzaldehyde were about 94 and about 99 % respectively, when using 0.08 g ZSM-5 catalyst with acetonitrile as the solvent and H₂O₂ as the oxidant at 90 °C. This catalyst can be retrieved and reprocessed for five times without a significant loss in its activity and selectivity.