Showing papers in "Journal of Molecular Catalysis A-chemical in 2006"

Calcined Mg–Al hydrotalcites as solid base catalysts for methanolysis of soybean oil

Abstract: Methyl ester of fatty acids, derived from vegetable oils or animal fats and known as biodiesel, is a promising alternative diesel fuel regarding the limited resources of fossil fuel and the environmental concerns. In this work, an environmentally benign process for the methanolysis of soybean oil to methyl esters using calcined Mg–Al hydrotalcites as solid base catalysts in a heterogeneous manner was developed. When the reaction was carried out at reflux of methanol, with a molar ratio of soybean oil to methanol of 15:1, a reaction time 9 h and a catalyst amount 7.5%, the oil conversion was 67%. The calcined hydrotalcite with an Mg/Al ratio of 3.0 derived from calcination at 773 K was found to be the optimum catalyst that can give the highest basicity and the best catalytic activity for this reaction. The catalysts were characterized with SEM, XRD, IR, DTA-TG and Hammett titration method. The activity of the catalysts for the methanolysis reaction was correlated closely with their basicity as determined by the Hammett method.

Comparison of two different processes to synthesize biodiesel by waste cooking oil

Abstract: The traditional acid and the new two-step catalyzed processes for synthesis of biodiesel expressed as fatty acid methyl ester (FAME) were comparatively studied to achieve an economic and practical method for utilization of waste cooking oil (WCO) from Chinese restaurants. WCO samples with the acid value of 75.92 ± 0.04 mgKOH/g mixed with methanol were catalyzed under 95 °C for various reaction time, followed by methanol recovery under vacuum (10 ± 1 mmHg) at 50 °C with a rotational evaporation. FAME analyzed by gas chromatography (GC) was obtained directly from sulfuric acid catalyzed reaction in the traditional acid method, whereas in the two-step method it was produced from ferric sulfate (2.0%) catalyzed reaction followed by alkali (1.0% potassium hydroxide) transesterification. The conversion of free fatty acids of WCO into FAME in the two-step method was 97.22% at the reaction time of 4 h, mole ratio of methanol to TG of 10:1, compared in the acid method with 90%, 10 h, and 20:1, respectively, showing much higher catalyzed activity of ferric sulfate. This new two-step process showed advantages of no acidic wastewater, high efficiency, low equipment cost, and easy recovery of catalyst compared with the limitations of acidic effluent, no reusable catalyst and high cost of equipment in the traditional acid process.

Nano-Ag particles doped TiO2 for efficient photodegradation of Direct azo dyes

Abstract: Silver nanoparticles doped TiO2 has been prepared and characterised by surface analytical methods such as BET surface area, scanning electron micrographs (SEM), X-ray diffraction (XRD), energy dispersive X-ray micro analysis (EDX), electron spin resonance (ESR) and diffuse reflectance spectroscopy (DRS). We have investigated the photocatalytic degradation of two Direct diazo dyes, Direct red 23 (DR 23) and Direct blue 53 (DB 53) in the aqueous suspensions of TiO2 and Ag deposited TiO2 nanoparticles under UV-A light irradiation in order to evaluate the various effects of silver deposition on the photocatalytic activity of TiO2. The presence of silver in TiO2 was found to enhance the photodegradation of DR 23 and DB 53. The higher activity of silver doped TiO2 is due to the enhancement of electron–hole separation by the electron trapping of silver particles.

Alumina-supported potassium iodide as a heterogeneous catalyst for biodiesel production from soybean oil

Abstract: Biodiesel fuel, a promising alternative diesel fuel produced by a catalytic transesterification of vegetable oils, has become more attractive recently because of its environmental concerns and the fact that it is made from renewable resources. In this work, the transesterification of soybean oil with methanol has been studied in a heterogeneous system, using alumina loaded with potassium iodide as a solid base catalyst. After loading KI of 35 wt.% on alumina followed by calcination at 773 K for 3 h, the catalyst gave the highest basicity and the best catalytic activity for this reaction. The catalysts were characterized by means of XRD, IR, SEM and the Hammett indicator method. Moreover, the dependence of the conversion of soybean oil on the reaction variables such as the catalyst loading, the molar ratio of methanol to oil and the reaction time was studied. The conversion of 96% was achieved under the optimum reaction conditions. Besides, a correlation of the catalyst activity for the transesterification reaction with its basicity was proposed.

Characterization and photocatalytic mechanism of nanosized CdS coupled TiO2 nanocrystals under visible light irradiation

Abstract: Nanosized CdS coupled TiO2 nanocrystals were prepared by a microemulsion-mediated solvothermal method at relatively low temperatures The prepared samples were characterized by X-ray photoelectron spectroscopy (XPS), BET surface area analysis, X-ray diffraction (XRD), UV–vis absorption spectroscopy (UV–vis), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) It was found that the CdS coupled TiO2 materials consisted of uniform anatase TiO2 of 6–10 nm with highly dispersed cubic phase CdS nanocrystals The prepared samples exhibit strong visible light absorption at about 550 nm Meanwhile, they have high surface area in the range of 156–263 m2 g−1 and mesoporous character with the average pore diameter of ca 50–65 nm The coupling between the (1 0 1) crystal planes of anatase and (1 1 1) crystal planes of CdS was observed in the HRTEM image Ti3+ signal was observed in the electron paramagnetic resonance (EPR) spectrum of CdS coupled TiO2 nanocrystals under visible light irradiation It provided the evidence of an effective transfer of photo-generated electrons from the conduction band of CdS to that of TiO2 As expected, the nanosized CdS sensitized TiO2 nanocrystal materials showed enhanced activity in the oxidation of methylene blue in water or nitric oxide in air under visible light irradiation The mechanism of photocatalysis on CdS coupled TiO2 nanocrystals under visible light is also discussed

Effects of calcination temperature on the microstructures and photocatalytic activity of titanate nanotubes

Abstract: Titanate nanotubes were prepared via a hydrothermal treatment of TiO2 powders (P25) in a 10 M NaOH solution at 150 °C for 48 h and then calcined at various temperatures. The as-prepared titanate nanotubes before and after calcination were characterized with XRD, TEM, HRTEM, SEM, FESEM, and nitrogen adsorption–desorption isotherms. The photocatalytic activity of the as-prepared samples was evaluated by photocatalytic oxidation of acetone in air. The effects of calcination temperature on the phase structure, crystallite size, morphology, specific surface area, pore structures and photocatalytic activity of the titanate nanotubes were investigated. The results indicated that at 400 to 600 °C, the calcined nanotube samples showed a higher photocatalytic activity than Degussa P25. Especially, at 400 and 500 °C, the photocatalytic activity of the calcined nanotubes exceeded that of P25 by a factor of about 3.0 times. This could be attributed to the fact that the former had a larger specific surface area and pore volume. With further increase in the calcination temperature from 700 to 900 °C, the photocatalytic activity of the calcined nanotube samples greatly decreased due to the formation of rutile phase, the sintering and growth of TiO2 crystallites and the decrease of specific surface area and pore volume.

A sonochemical route to visible-light-driven high-activity BiVO4 photocatalyst

Abstract: Visible-light-induced BiVO 4 photocatalyst with monoclinic scheelite structure has been successfully synthesized via a facile sonochemical method. The as-prepared BiVO 4 photocatalyst exhibited relatively high surface areas, consisting of primary nanocrystals with average size of ca. 50 nm. The BiVO 4 nanocrystals showed a strong absorption in the visible light region and the band gap was estimated to be ca. 2.45 eV, representing an obvious blue-shift compared with that of the bulk sample. The photocatalytic activities were also evaluated by decolorization of methyl orange under visible light ( λ > 400 nm). The results indicate that the photodegradation rate of as-prepared BiVO 4 nanocrystals is quite high, up to 90% in 30 min, which is much better than that of the reference sample prepared by solid-state reaction (ca. 8%) and the standard photocatalyst P25 (ca. 6%) under the same conditions.

Dehydration of fructose and sucrose into 5-hydroxymethylfurfural in the presence of 1-H-3-methyl imidazolium chloride acting both as solvent and catalyst

Abstract: The acid-catalyzed dehydration of fructose was performed in a microbatch reactor at 90 °C using 1-H-3-methyl imidazolium chloride, HMIM+Cl−, acting both as solvent and catalyst. A yield in 5-hydroxymethylfurfural (HMF) as high as 92% is achieved within 15–45 min. A productivity of 0.02 mmol of HMF, per min and per mmol of ionic liquid, can then be obtained. A remarkable feature, as compared with other catalytic systems, is the absence of decomposition of 5-hydroxymethylfurfural which can then be separately or continuously extracted in a simple manner with diethyl ether, thus allowing easier recycling of the ionic liquid after water removal. The absence of decomposition of 5-hydroxymethylfurfural is due to the lower free energy of activation calculated in the presence of the ionic liquid compared to other homogeneous or heterogeneous catalysts, which then makes, in a consecutive reaction network, the formation of the 5-hydroxymethylfurfural intermediate nearly quantitative. Under similar operating conditions, sucrose is nearly quantitatively transformed into HMF and unreacted glucose.

A study on the carboxylation of glycerol to glycerol carbonate with carbon dioxide: The role of the catalyst, solvent and reaction conditions

Abstract: Glycerol was reacted with CO 2 (5 MPa) at 450 K in presence of Sn-catalysts ( n -Bu 2 Sn(OMe) 2 1 , n -Bu 2 SnO 2 or Sn(OMe) 2 3 ), using either glycerol or tetraethylene glycol dimethyl ether (tedmg) as reaction medium. 1 was much more active than 2 . 1 was demonstrated to convert into n -Bu 2 Sn(glycerol-2H) 4 upon reaction with glycerol and elimination of MeOH. Monomeric 4 is proposed to be the active species in catalysis. It converted into a polymeric material with time with consequent reduction of its catalytic activity. Also, after the first catalytic cycle 4 was converted into an oligomeric material that did not contain glycerol. This also caused the reduction of the catalytic activity. 3 was able to uptake CO 2 but was not able to promote the carboxylation of glycerol. 1 and 2 also promoted the trans -esterification of dimethylcarbonate (DMC) with glycerol to afford glycerol carbonate, but at a lower rate than the direct carboxylation of glycerol. This fact seems to rule out that the carboxylation of glycerol may proceed through the preliminary formation of DMC and its subsequent trans -esterification.

Self-cleaning cotton textiles surfaces modified by photoactive SiO2/TiO2 coating

Abstract: The photocatalytic activity of TiO 2 –SiO 2 -coated cotton textiles was investigated through the self-cleaning of red wine stains. It was shown that a TiO 2 –SiO 2 species could be produced at temperatures of 100 °C with acceptable photo-activity on non-heat resistant materials. The most suitable Ti-content of the coating was found to be 5.8% and for SiO 2 , the content was 3.9% (w/w). The discoloration of red wine led to CO 2 evolution that was more efficient for TiO 2 –SiO 2 -coated cotton for samples than of TiO 2 -coated ones. The reasons for these results are discussed. The textile surface did not show any change after several consecutive light-induced discoloration cycles of a red wine stain. By high-resolution transmission electron microscopy (HRTEM), the TiO 2 –SiO 2 layer thickness on the cotton fibers was detected to 20–30 nm. The TiO 2 and SiO 2 were both observed to have particle sizes between 4 and 8 nm. Further electron microscopy work coupled with energy dispersive spectroscopy (EDS) showed that the Ti-particles were always surrounded by amorphous SiO 2 and never alone by themselves. Infrared spectroscopy revealed that no modification of the cotton could be detected after photo-discoloration processes with TiO 2 –SiO 2 , taking a wine stain as model compound. The mixed TiO 2 and SiO 2 colloids lead during the dip-coating and subsequent thermal treatment on cotton to an organized structure of highly dispersed TiO 2 particles always surrounded by amorphous silica.

Enhanced photocatalytic activity of TiO2 powder (P25) by hydrothermal treatment

Abstract: TiO2 powder (P25) was hydrothermally treated in pure water at 150 °C for different times. The P25 powders before and after hydrothermal treatment were characterized with XRD, TEM, UV–vis, XPS and nitrogen adsorption–desorption isotherms. The results showed that a small amount of anatase was transformed into rutile phase and more aggregates of the TiO2 crystallites formed after hydrothermal treatment, resulting in the decrease of specific surface area of the TiO2 powders. The photocatalytic activity of the hydrothermally treated P25 powders was obviously higher than that of the P25 powder. This can be attributed to the formation of more hydroxyl groups in the surface of TiO2. The more hydroxyl groups would generate more hydroxyl radicals, enhance the adsorption of O2 molecules and reduce recombination of the photogenerated electrons and holes. Moreover, more hydroxyl was also beneficial to the activation of the rutile phase, leading to an enhanced photocatalytic activity.

Sol-gel synthesis of Au/TiO2 thin films for photocatalytic degradation of phenol in sunlight

Abstract: Thin films of Au/TiO2 on glass substrates have been prepared by simple sol–gel dip coating method using a sol of colloidal gold doped titanium peroxide (Au/TiO2 sol). The sol of colloidal gold and titanium peroxide forms a viscous gel after 1–3 h depending on the concentration of both colloidal gold and titania. The sol of particular viscosity range (140–2800 cps) was used to deposit the Au/TiO2 thin films of uniform thickness on various substrates such as glass slides, glass helix and silica rashig rings. Films deposited on glass plates after drying were transparent, uniform in color as well as thickness. Optical characterization by UV–vis spectrophotometer showed a shift in optical absorption wavelength to visible region may be due to the incorporation of gold nanoparticles into titania structure. The optimum concentration of gold loading was found to be 1–2% (by weight) beyond this the gold particles disturb the gel network resulting the formation of gelatinous precipitate. Thin film photo-catalyst prepared was characterized by various techniques such as UV–vis, TG-DTA, XRD, ICP-OES and TEM. The photo-activity of the thin films was tested in sunlight using phenol as model pollutant. The kinetic study showed that the rate of decomposition of phenol using Au/TiO2 photo-catalyst was improved by 2–2.3 times than undoped TiO2.

Effect of water on sulfuric acid catalyzed esterification

Abstract: This paper reports on an investigation into the impact of water on liquid-phase sulfuric acid catalyzed esterification of acetic acid with methanol at 60 °C. In order to diminish the effect of water on the catalysis as a result of the reverse reaction, initial reaction kinetics were measured using a low concentration of sulfuric acid (1 × 10 −3 M) and different initial water concentrations. It was found that the catalytic activity of sulfuric acid was strongly inhibited by water. The catalysts lost up to 90% activity as the amount of water present increased. The order of water effect on reaction rate was determined to be −0.83. The deactivating effect of water also manifested itself by changes in the activation energy and the pre-exponential kinetic factor. The decreased activity of the catalytic protons is suggested to be caused by preferential solvation of them by water over methanol. A proposed model successfully predicts esterification rate as reaction progresses. The results indicate that, as esterification progresses and byproduct water is produced, deactivation of the sulfuric acid catalyst occurs. Autocatalysis, however, was found to be hardly impacted by the presence of water, probably due to compensation effects of water on the catalytic activity of acetic acid, a weak acid.

Towards understanding the reaction pathway in vapour phase hydrogenation of furfural to 2-methylfuran

Abstract: The reactivity of furfural and its reaction intermediates is separately investigated in a fixed-bed reactor over a commercial catalyst (C 1 : Cu/Zn/Al/Ca/Na = 59:33:6:1:1, atomic ratio) and a self-made multicomponent one (C 2 : Cu/Cr/Ni/Zn/Fe = 43:45:8:3:1, atomic ratio). Some interesting results are obtained in this work. Firstly, both furfural and furfuryl alcohol can eliminate carbon monoxide to produce furan. Secondly, tetrahydrofuran and 2-methyltetrahydrofuran are all very stable, and n -butanol mainly derives from furan rather than tetrahydrofuran. Thirdly, 2-pentanone, 2-methyltetrahydrofuran, 1-pentanol and 2-pentanol are all the hydrogenation products of 2-methylfuran. Finally, with C 2 catalyst containing the Ni element, the main reaction product of tetrahydrofurfuryl alcohol is tetrahydofuran, while δ-valerolactone is the main product with C 1 one. This work has unified some conflicting mechanisms and discussed the origin of some interesting products, which is important to understand the mechanism and reaction pathway of furfural hydrogenation, and to provide an instruction for the design of new catalytic formulations.

Amberlyst-15: An efficient reusable heterogeneous catalyst for the synthesis of 1,8-dioxo-octahydroxanthenes and 1,8-dioxo-decahydroacridines

Abstract: Amberlyst-15 has been found to be an efficient catalyst for the synthesis of 1,8-dioxo-octahydroxanthenes and 1,8-dioxo-decahydroacridines in excellent yields. The former have been synthesized from aromatic aldehydes and 5,5-dimethyl-1,3-cyclohexanedione while the latter from this mixture along with amines. The method is an easy access to functionalized xanthene and acridine derivatives. The catalyst can be reused.

Highly accessible catalytic sites on recyclable organosilane-functionalized magnetic nanoparticles: An alternative to functionalized porous silica catalysts

Abstract: Diaminosilane-functionalized cobalt spinel ferrite (CoFe2O4) magnetic nanoparticles are synthesized and used as efficient heterogeneous base catalysts for the Knoevenagel condensation of aromatic and heteroaromatic aldehydes with malononitrile The magnetic nanoparticle catalyst is characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR), and nitrogen physisorption measurements Quantitative conversion of the reactants is achieved under mild conditions Recovery of the catalyst is easily achieved by magnetic decantation The supported catalyst is reused five times without significant degradation in catalytic activity No contribution from homogeneous catalysis due to active amine species leaching into reaction solution is detected The performance of the magnetic base catalyst in the Knoevenagel reaction is directly compared with diamine-functionalized SBA-15 and MCM-48 Reaction rates over the non-porous, magnetic nanoparticle catalyst are comparable to the large pore mesoporous silica materials and faster than the small pore MCM-48 material with ∼22 A diameter pores A significant effect of the acidity of the magnetic nanoparticle support on catalyst activity in the Knoevenagel condensation is also observed

Preparation, characterization and photocatalytic activity of in situ N,S-codoped TiO2 powders

Abstract: A simple method for preparing highly photoactive nanocrystalline mesoporous N,S-codoped TiO2 powders was developed by hydrolysis of Ti(SO4)2 in a NH3·H2O solution at room temperature. The as-prepared TiO2 powders were characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermalgravimetric and differential thermal analysis (TG–DTA), UV–vis diffuse reflectance spectra, N2 adsorption–desorption measurements, scanning electron microscope (SEM) and transmission electron microscopy (TEM). The photocatalytic activity was evaluated for the photocatalytic oxidation of acetone and formaldehyde under UV light and daylight irradiation in air, respectively. The results showed that the as-prepared TiO2 powders exhibited a stronger absorption in the UV–vis light region and a red shift in the band gap transition due to N,S-codoping. The photocatalytic activity of the as-prepared N,S-codoped TiO2 powders at a temperature range of 400–700 °C were obviously higher than that of commercial Degussa P25. Especially, the daylight-induced photocatalytic activities of the as-prepared N,S-codoped TiO2 powders were about ten times greater than that of Degussa P25. The high activities of the N,S-codoped TiO2 can be attributed to the results of the synergetic effects of strong absorption in the UV–vis light region, red shift in adsorption edge, good crystallization, large surface area and two phase structures of undoped TiO2 and N,S-codoped TiO2.

Synthesis of cyclic carbonates from epoxides and carbon dioxide over silica-supported quaternary ammonium salts under supercritical conditions

Abstract: A silica-supported quaternary ammonium salt is reported for the first time as a recyclable and efficient catalyst for the synthesis of propylene carbonate from propylene oxide and carbon dioxide under supercritical conditions, which requires no additional organic solvents either for the reaction or for the separation of product. Moreover, the catalyst can be easily recovered by a simple filtration and reused over four times without obvious loss of its catalytic activity. The effects of the types of cation and anion of quaternary ammonium salts, and other reaction parameters on the reaction are investigated. This organic solvent-free process presented here could show much potential application in industry due to its simplicity, easy product separation from reaction medium and catalyst recycling. It could be profitably applied to the development of fix-bed continuous flow reactors, avoiding the use of solvent to isolate the products.

Activation of C2–C4 alkanes over acid and bifunctional zeolite catalysts

Abstract: This review paper presents the significant advances which were made in the last decade in the understanding of the transformation over acid and bifunctional zeolite catalysts of the cheap and readily available C2–C4 alkanes into more valuable products: mechanism of activation, reaction scheme, nature of the active sites. Both the transformations of pure alkanes: n-butane isomerization, C2–C4 alkane aromatization and of alkanes in mixture with alkenes: isobutane–butene alkylation or with aromatic hydrocarbons: benzene alkylation with ethane or propane are considered.

Photocatalytic degradation and kinetics of Orange G using nano-sized Sn(IV)/TiO2/AC photocatalyst

Abstract: Sn(IV) doped and nano-sized TiO 2 immobilized on active carbon (AC) (Sn(IV)/TiO 2 /AC) were prepared by the sol–gel and dip-calcination method. An azo dye, Orange G (OG), was used as a model compound to study its photocatalytic activity in a fluidized bed photoreactor. The addition of Sn(IV) on TiO 2 could greatly improve the activity of TiO 2 , and the optimal amount of tin was 2.5 at.%. The effects of calcination temperature, pH value, the initial hydrogen peroxide concentration ([H 2 O 2 ] 0 ), the catalyst amount ([TiO 2 ]), the initial OG concentration ([dye] 0 ) and co-existing negative ions on the photocatalytic activity of Sn(IV)/TiO 2 /AC were studied. The optimal conditions were as follows: pH 2.00, [H 2 O 2 ] 0 = 1.5mL/L, [dye] 0 = 50 mg/L, [TiO 2 ] = 12.5 g/L, when the 300 W high pressure mercury light was used as the light source. Under these conditions, the degradation efficiency of OG reached 99.1% after 60 min reaction. The kinetics of the OG degradation was also analyzed. The results showed that the kinetics of this reaction fit the Langmiur–Hinshelwood kinetics model well and the absorption of OG on the Sn(IV)/TiO 2 /AC surface was the controlling step in the whole degradation process. In addition, the catalyst, liquid and gas were separated effectively, and the integrative process of reaction and separation was achieved during the experiment.

Catalytic oxidations mediated by metal ions and nitroxyl radicals

Abstract: The use of nitroxyl radicals, alone or in combination with transition metals, as catalysts in oxidation processes is reviewed, from both a synthetic and a mechanistic viewpoint. Two extremes of reactivity can be distinguished: stable (persistent) dialkylnitroxyls, such as TEMPO, and reactive diacylnitroxyls, derived from N-hydroxy imides, such as N-hydroxyphthalimide (NHPI). NHPI catalyzes a wide variety of free radical autoxidations, improving both activities and selectivities by increasing the rate of chain propagation and/or decreasing the rate of chain termination. In the absence of metal cocatalysts improved conversions and selectivities are obtained in the autoxidation of hydrocarbons to the corresponding alkyl hydroperoxides. In combination with transition metal cocatalysts, notably cobalt, NHPI and related compounds, such as N-hydroxysaccharin (NHS), afford effective catalytic systems for the autoxidation of hydrocarbons, e.g. toluenes to carboxylic acids and cycloalkanes to the corresponding ketones. Stable dialkylnitroxyl radicals, exemplified by TEMPO, catalyze oxidations of, e.g. alcohols, with single oxygen donors such as hypochlorite via the intermediate formation of the corresponding oxoammonium cation. Alternatively, in conjunction with transition metals, notably ruthenium and copper, they catalyze aerobic oxidations of alcohols via metal-centred dehydrogenation. The role of the TEMPO is to facilitate regeneration of the catalyst (Ru and Cu). In contrast, oxoammonium cations are involved in the aerobic oxidation of alcohols catalyzed by the copper-dependent oxidase, laccase, in combination with TEMPO. This different mechanistic pathway is attributed to the much higher redox potential of the copper(II) in the enzyme.

Immobilized ionic liquid/zinc chloride: Heterogeneous catalyst for synthesis of cyclic carbonates from carbon dioxide and epoxides

Abstract: Chemical fixation of carbon dioxide with epoxides to form cyclic carbonates proceeds very effectively under mild conditions by using immobilized ionic liquid catalyst in conjunction with zinc chloride without any organic solvents. The reaction temperature, carbon dioxide pressure, effects of different metallic complexes and the amount of immobilized ionic liquid were investigated. The optimum reaction conditions were 110 °C and 1.5 MPa, and the catalyst system was recycled and reused.

Effects of noble metal modification on surface oxygen composition, charge separation and photocatalytic activity of ZnO nanoparticles

Abstract: In this paper, ZnO nanoparticles were modified by depositing different amount of noble metal Ag or Pd on their surfaces with a photoreduction method. The as-prepared ZnO samples were principally characterized by X-ray photoelectron spectroscopy (XPS) and surface photovoltage spectroscopy (SPS), and their activity was evaluated by a photocatalytic degradation reaction of phenol solution. The effects of noble metal modification on surface composition, photoinduced charge transfer behavior and photocatalytic activity of ZnO samples were mainly investigated. The results show that, after an appropriate amount of Ag or Pd is deposited on the ZnO surfaces, the O 1s XPS spectrum shifts to higher binding energy, indicating that the content of surface hydroxyl oxygen (O H ) increases. And the SPS intensity greatly decreases, indicating that the photoinduced electrons are easily captured by adsorbed oxygen via noble metal clusters so that the recombination of photoinduced electron and hole pairs is effectively inhibited. These are responsible for the enhancement in the photocatalytic activity. Moreover, the effects of Pd modification on photocatalytic activity is greater than that of Ag, which can be explained by means of surface hydroxyl content as well as charge separation situation.

Effects of pH on the microstructures and photocatalytic activity of mesoporous nanocrystalline titania powders prepared via hydrothermal method

Abstract: Highly photoactive nano-sized TiO2 powder photocatalyst was prepared by a hydrothermal method at 180 °C for 5 h using tetrabutyl titanate (TBOT) as the precursor. The pH values of the starting suspensions were adjusted from 1 to 11 using a 1.0 M HCl or 1.0 M NH3·H2O solution. The obtained TiO2 powders were characterized with X-ray diffraction (XRD), N2 adsorption–desorption measurement, transmission electron microscopy (TEM), X-ray photoelectron spectra, Fourier transform infrared (FTIR), Raman spectra and UV–vis spectrophotometry. The photocatalytic activity was evaluated by photocatalytic oxidation decomposition of acetone in air. The results showed that the pH values of the solutions obviously influenced the microstructures and photocatalytic activity of the as-prepared TiO2 powders. With increasing pH values, the crystallization enhanced, crystallite size increased and BET specific surface areas decreased. The photocatalytic activity of TiO2 powders prepared at a pH range of 6–9 exceeded that of Degussa P-25 by a factor of more than two times.

Effect of catalyst pore size on the catalytic performance of silica supported cobalt Fischer–Tropsch catalysts

Abstract: A series of cobalt catalysts supported on silica with different pore size were prepared by incipient wetness impregnation method. N 2 physisorption, XRD, H 2 -TPR, H 2 -TPD, DRIFTS and O 2 pulse reoxidation were used to characterize the catalysts. The results showed that the pore size of the support had a significant influence on the Co 3 O 4 crystallite diameter, catalyst reducibility and Fischer–Tropsch activity. The larger pore could cause the Co/SiO 2 to form larger Co 3 O 4 crystallite and decreased its dispersion. Catalysts with different pore size showed different CO adsorption property. The catalysts with pore size of 6–10 nm displayed higher Fischer–Tropsch activity and higher C 5+ selectivity, due to the moderate particle size and moderate CO adsorption on the catalysts.

A convenient one-pot synthesis of 2-substituted benzimidazoles

Abstract: 2-Substituted benzimidazoles have been synthesized in excellent yields in a single pot under solvent-free conditions from o -phenylenediamine and aldehydes in the presence of a catalytic amount of In(OTf) 3 at room temperature.

Enhanced photocatalytic activity of TiO2 by doping with Ag for degradation of 2,4,6-trichlorophenol in aqueous suspension

Abstract: A series of Ag-TiO2 nanocatalysts were synthesized by a sol–gel method with a doping content up to 2 wt%-Ag. The physico-chemical characteristics of the synthesized catalysts were characterized by X-ray diffraction, X-ray photoelectron emission spectroscopy, transmission electron microscopy, UV–vis absorption spectrometer, and optical ellipsometry to study the influence of the Ag content on the surface properties, optical absorption and other characteristics of the photocatalysts. The photocatalytic activity of the Ag-TiO2 was evaluated in the 2,4,6-trichlorophenol (TCP) degradation and mineralization in aqueous solution under UV-A illumination. The experiments demonstrated that TCP was effectively degraded by more than 95% within 120 min. It was confirmed that the presence of Ag on TiO2 catalysts could enhance the photocatalytic oxidation of TCP in aqueous suspension and the experimental results showed that the kinetics of TCP degradation follows a pseudo-first-order kinetic model. It was found that an optimal dosage of 0.5 wt% Ag in TiO2 achieved the fastest TCP degradation under the experimental conditions. The experimental results of TCP mineralization indicated while total organic carbon was reduced by a high portion of up to 80% within 120 min, most chlorine on TCP was more quickly converted to chloride within the first 40 min. On the basis of various characterizations of the photocatalysts, the reactions involved to explain the photocatalytic activity enhancement due to Ag doping include a better separation of photogenerated charge carriers and improved oxygen reduction inducing a higher extent of degradation of aromatics.

Toluene decomposition using a wire-plate dielectric barrier discharge reactor with manganese oxide catalyst in situ

Abstract: Laboratory-scale experiments were performed to evaluate the efficiency of toluene decomposition by using a wire-plate dielectric barrier discharge (DBD) reactor with manganese oxide/alumina/nickel foam catalyst in the discharge area at room temperature and atmospheric pressure. The effects of oxygen content and gas flow rate were investigated. Under the optimal oxygen content and gas flow rate conditions, the combination effect of DBD and catalyst was observed, and the catalyst before/after discharge was structurally characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform-infrared spectroscopy (FT-IR). It has been found that combining DBD with catalyst in situ could improve the toluene removal efficiency, increase carbon dioxide selectivity and suppress byproducts formation. Whether the catalyst existed or not, the major products were carbon dioxide and carbon monoxide when oxygen was enough. The characterization of the catalyst suggested that DBD enhanced the dispersion of the active species, increased the stability as well as the activity of the catalyst, and strengthened the oxidation capability of the catalyst, therefore the removal of toluene was promoted.

Design and study of homogeneous catalysts for the selective, low temperature oxidation of hydrocarbons

Abstract: The direct, low temperature conversion of hydrocarbons to functionalized products using novel, single site catalysts could lead to technological advances that redefine the landscape of the current materials and energy industries. Natural gas continues to represent a vast source of untapped hydrocarbons around the globe that has the potential to replace or augment petroleum as the raw material for materials and energy. Its abundance has garnered much interest in the scientific community as groups have focused on the catalytic conversion of its major component, methane, to functionalized products. The key requirements is to design new catalysts for the oxidative functionalization of methane that operate at lower temperatures and that also meet the basic requirements of selectivity, rate, and lifetime that characterize useful catalysts. Recent advances in the field of hydrocarbon CH activation have shown the potential for transition metal based coordination catalysts to meet these requirements. Described herein are recent advances in designing catalysts based on the CH activation reaction that address the basic requirements for practical systems with emphasis on the issues that have prevented promising reported systems from becoming commercially viable.

Acidic ionic liquid modified silica gel as novel solid catalysts for esterification and nitration reactions

Abstract: Covalent bond immobilized ionic liquids are prepared via radical chain transfer reaction of 1-allylimidazolium based ionic liquids on modified silica gel and proved to be recyclable heterogeneous acidic catalysts for esterification and nitration reactions.