Solid acidities of SiO2–TiO2/montmorillonite composites synthesized under different pH conditions

SiO₂–TiO₂/montmorillonite composites were prepared under acidic, neutral and basic conditions and the solid acidity of the resulting composites were determined. All the SiO₂/TiO₂ ratio of the colloidal particles was set at 10 but the resulting SiO₂/TiO₂ ratios were significantly richer in TiO₂. The XRD patterns of the acidic composite showed expanding and broadening of the (001) reflection by intercalation of colloidal SiO₂–TiO₂ particles, but the neutral and basic composites showed only broadening of the reflections and no intercalation. The specific surface areas of the acidic, neutral and basic composites (375, 237 and 247 m²/g, respectively) were much larger than of montmorillonite (6 m²/g). The average pore sizes were about 4, 15 and 50 nm, and the amounts of solid acidic sites measured by the NH₃-TPD were 178, 95 and 86 µmol/g for the acidic, neutral and basic composites, respectively. The solid acid amount of the acidic composite was twice that of a commercial catalyst, K-10, (85 µmol/g) and much higher than the guest phase SiO₂–TiO₂ gel (16 µmol/g) or the host phase montmorillonite (6 µmol/g). The TPD peak temperatures reflect the acid strength, and were similar in all the samples, ranging from 175° to 200 °C.     

Preparation of Pdshell–Aucore/SiO2 catalyst and catalytic activity for acetylene hydrogenation

Various Pd shell thicknesses (0.12–1.5 nm) were synthesized on preformed Au particles of 5 nm size by seeded growth technique (15–80 at% Pd) using sodium citrate and tannic acid. The sols were characterized by UV–vis spectroscopy, TEM and high-resolution TEM (HRTEM) measurements. HRTEM confirmed the pseudomorphic growth of the Pd shell on the Au core. The Au/Pd core/shell particles were fixed onto SiO₂ (Aerosil 200) support by PDDA polycation. The catalytic activity in acetylene hydrogenation and selectivity of competition between acetylene and propene were tested after O₂ and H₂ pretreatment. The samples even in “as prepared” state hydrogenated acetylene. The thin Pd layer (15–30 at% Pd) on Au provided higher hydrogenation activity than the thick Pd shell. However, thermal treatment of the samples in H₂ stream causing Au/Pd intermixing shifted the activity maximum to higher Pd concentration (68–80 at% Pd). Comparison of the TOF (1/s) and selectivity values allowed us to conclude that the homogenized particle with 68–80 at% Pd shows better hydrogenation activity and selectivity than the thin Pd shell (15–30 at% Pd) on the Au core.     

Synthesis of TS-2 in the system SiO2–TiO2–H2O2–TBAOH. Influence of the synthesis variables

The present paper describes the synthesis, characterization and catalytic activity of a titanium silicalite (TS-2) prepared by a method based on the formation of peroxytitanates. The samples were characterized by X-ray diffraction (XRD), FT-IR, UV–Vis, SEM and catalytic measurements for n-hexane oxyfunctionalization.

The influence of the synthesis procedure, the crystallization temperature and the SiO₂/TiO₂ molar ratio on the physicochemical properties of the products are discussed. The maximum incorporation of titanium observed was about a Ti/(Si+Ti) ratio of 0.04. TiO₂ was not formed as can be seen by DR–UV–Vis spectroscopy. An increase in the crystallization temperature above 461 K leads to the formation of MFI phase as impurity. Furthermore the effect of the synthesis temperature on the crystallization curves and the kinetic parameters was also investigated.     

Preparation and characterization of TiO2–SiO2 nano-composite thin films

TiO₂ nanocrystalline particles dispersed in SiO₂ have been prepared by the sol-gel method using titanium- and silicon-alkoxides as precursors. Nano-composite thin films were formed on the glass substrates by dip-coating technique and heat treated at temperatures up to 500 °C for 1 h. The size of the TiO₂ nanocrystalline particles in the TiO₂–SiO₂ solution ranged from 5 to 8 nm. The crystalline structure of TiO₂ powders was identified as the anatase phase. As the content of SiO₂ increased, the anatase phase tended to be stabilized to higher temperature. TEM results revealed the presence of spherical TiO₂ particles dispersed in a disk-shaped glassy matrix. Photocatalytic activity of the TiO₂–SiO₂ (1:1) thin films showed decomposition of 95% of methylene blue solution in 2 h and a contact angle of 10°. The photocatalytic decomposition of methylene blue increased and the contact angle decreased with the content of TiO₂ phase. TiO₂–SiO₂ with the molar ratio of 1:1 showed a reasonable combination of adhesion, film strength, and the photocatalytic activity.     

Structural changes and activation treatment in a Co/SiO2 catalyst for Fischer–Tropsch synthesis

We examined the effect of the activation process on the structural and morphological characteristics of a cobalt-based catalyst for Fischer–Tropsch synthesis. A 10 wt.% Co/SiO₂ catalyst prepared by wet impregnation was separately activated under H₂, CO or a H₂/CO mixture. The structural changes during activation from 298 to 773 K were studied by in situ X-ray diffraction. Catalysts were examined by SEM, TEM, XPS and in situ DRIFT-MS. The H₂/CO activation produced redispersion of cobalt particles and simultaneous carbon nanostructures formation. The catalyst showed the highest performance in the Fischer–Tropsch synthesis after the H₂/CO activation.     

Sol–gel synthesis of a novel melon–SiO2 nanocomposite with photocatalytic activity

A simple sol–gel method, starting from thiourea, was used to prepare a SiO₂-based nanocomposite that absorbs across the visible region after calcination at 500 °C. The UV–visible spectrum, elemental (N) analysis, thermogravimetric data, and FTIR indicate that the coloration derives from an overlayer of melon, a thermally-stable precursor of graphitic carbon nitride. XPS and solid-state ²⁹Si NMR showed no evidence for surface or bulk doping by N or S. The nanocomposite has unexpectedly high photocatalytic activity in methylene blue degradation under visible light and is probably due to a synergism between melon and SiO₂.     

Densification and phase transformation of Si3N4 in the presence of mechanically activated BaCO3–Al2O3–SiO2 mixture

Densification as well as the →β phase transformation of Si₃N₄ were monitored as a function of activation time of the BaCO₃–Al₂O₃–SiO₂ additive mixture. The composition of the ternary mixture corresponded to celsian (BaAl₂Si₂O₈—BAS). Previously, mechanically activated powder mixtures for various lengths of time were added to Si₃N₄ in the amount of 10–30%. Sintering was performed at 1650–1700°C in nitrogen atmosphere up to 8 h. The changes in densification degree, as well as phase composition, were followed as a function of heating time and the time of mechanical activation of the additive mixture. The results obtained showed that mechanical activation retarded densification in samples heated up to 1700°C. On the other hand, for the constant sintering time, →β transformation of Si₃N₄ was enhanced with increasing activation time, and the amount of additives.     

Sinterability, crystallization and properties of glass–ceramic tiles belonging to CaF2–CaO–MgO–Al2O3–SiO2 system

The effect of CaO substitution by different amounts of MgO on crystallization and various properties of CaF₂–CaO–Al₂O₃–SiO₂ glass system were investigated. It was shown that the properties of obtained glass–ceramics are greatly influenced by MgO contents. The Avrami exponent and activation energy for crystallization of the most promising and the MgO-free specimens were determined. Results suggested that surface crystallization was the main precipitation mechanism of both samples and while the activation energy for crystallization of MgO containing sample was less than MgO-free one, its ability to crystallize was diminished. SEM results confirm occurrence of surface crystallization of samples and depicts the phenomenon of microstructure coarsening by increasing MgO content and also reduced densification of specimen with magnesium oxide more than 9 wt.%.     

Na/SiO2新型固体碱催化制备生物柴油的研究

以NaOH、正硅酸乙酯和乙醇为原料,经溶胶-凝胶法制备新型固体碱催化剂（Na/SiO₂）,用于催化大豆油与甲醇的酯交换反应制备生物柴油,研究催化剂焙烧温度、n(NaOH)∶n(SiO₂)、n(甲醇)∶n(大豆油)、催化剂用量和反应时间对产率的影响以及催化剂的稳定性。结果表明,固体碱催化剂Na/SiO₂在大豆油与甲醇的酯交换反应中具有较高的催化活性,在催化剂焙烧温度600 ℃、n(NaOH)∶n(SiO₂)=2∶1、n(甲醇)∶n(大豆油)=15∶1、催化剂用量为大豆油质量的7%和反应时间3 h的条件下,脂肪酸甲酯产率可达97.42%,催化剂在稳定性试验中呈现出优良的稳定性。     

Utilization of carbon dioxide as soft oxidant for oxydehydrogenation of ethylbenzene to styrene over V2O5–CeO2/TiO2–ZrO2 catalyst

Vanadium oxide and cerium oxide doped titania–zirconia mixed oxides were explored for oxidative dehydrogenation of ethylbenzene to styrene utilizing carbon dioxide as a soft oxidant. The investigated TiO₂–ZrO₂ mixed oxide support with high specific surface area (207 m² g⁻¹) was synthesized by a coprecipitation method. Over the calcined support (550 °C), a monolayer equivalent (15 wt.%) of V₂O₅, CeO₂ or a combination of both were deposited by using wet-impregnation or co-impregnation methods to make the V₂O₅/TiO₂–ZrO₂, CeO₂/TiO₂–ZrO₂ and V₂O₅–CeO₂/TiO₂–ZrO₂ combination catalysts, respectively. These catalysts were characterized using X-ray diffraction (XRD), Raman, scanning electron microscopy (SEM), transmission electron microscopy (TEM), temperature preprogrammed reduction (TPR), CO₂ temperature preprogrammed desorption (TPD) and BET surface area methods. All characterization studies revealed that the deposited promoter oxides are in a highly dispersed form over the support, and the combined acid–base and redox properties of the catalysts play a major role in this reaction. The V₂O₅–CeO₂/TiO₂–ZrO₂ catalyst exhibited a better conversion and product selectivity than other combinations. In particular, the addition of CeO₂ to V₂O₅/TiO₂–ZrO₂ prevented catalyst deactivation and helped to maintain a high and stable catalytic activity.     

Direct synthesis of acetic acid from CH4 and CO2 by a step-wise route over Pd/SiO2 and Rh/SiO2 catalysts

The theoretical and experimental feasibility of direct conversion of CH₄ and CO₂ to acetic acid by an isothermal step-wise route over Pd/SiO₂ and Rh/SiO₂ catalysts was investigated. The methyl radical formation from CH₄ dissociation and CO₂ inserting into the intermediate are regarded as two limiting steps. Preliminary experimental results have shown that the following step-wise route can circumvent the thermodynamic limitation of this direct synthesis at low temperatures. Pd catalysts are more active than Rh catalysts at 170 °C and 200 °C, while formic acid is only produced on Pd catalysts. The optimum contact time of CH₄ and CO₂ with catalysts is 1 min under the experimental conditions. And there is no apparent deactivation resulting from carbon deposition for catalysts during the successive reaction cycles.     

Hydroisomerization and hydrocracking of long chain n-alkanes on Pt/amorphous SiO2–Al2O3 catalyst

The hydroisomerization and hydrocracking of n-hexadecane, n-octacosane and n-hexatriacontane on a 0.3% platinum/amorphous silica–alumina (MSA/E) catalyst was investigated in a stirred microautoclave at 345, 360 and 380°C and between 2 and 13.1 MPa hydrogen pressure. For each n-paraffin, the reaction pathway and the kinetic parameters were determined. The results were used to elucidate the effect of chain length and operating conditions on isomerization and cracking selectivity. The conversion of the n-paraffins lead to the formation of a mixture of the respective isomers, as the main product, together with cracking products. At every temperature, the iso-alkane/n-alkane ratio of cracking products increased considerably with increasing conversion degree. At the same conversion level, higher reaction temperatures lead to cracking products characterized by a lower iso-alkane/n-alkane ratio. The conversion rate constants showed a considerable increase between n-C₁₆ and n-C₂₈, whereas a slight decrease between n-C₂₈ and n-C₃₆ was observed. The hydroisomerization selectivities showed a decrease as a function of chain length and with increasing conversion levels. The increase in reaction temperature leads to a small decrease in the isomerization selectivities only at low-medium conversion degrees and at the highest temperature investigated, while the effect of this parameter on the maximum yields achievable in iso-C16, iso-C28 and iso-C36 was negligible. The results indicate that the conversion of the n-paraffins follows a first-order kinetic in hydrocarbon while the order in hydrogen pressure was −1.1 ± 0.21 for n-C₁₆ and −0.66 ± 0.15 for n-C₂₈. Furthermore, an increase in hydroisomerization selectivity at higher hydrogen pressure for n-C₂₈ conversion was observed.     

Characterization and reactivity of pure TiO2–SO4 SCR catalyst: influence of SO4 content

The kinetics of the reaction of NO, N₂O and CO₂ with activated carbon without catalyst and impregnated with a precursor salt of vanadium (ammonium monovanadate) was investigated. The conversion of NO, N₂O and CO₂ was studied (450–900°C) using a TGA apparatus and a fixed bed reactor. The reactor effluents were analysed using a GC/MS on line. The addition of vanadium increased carbon reactivity and adsorption at lower temperatures. For NO and N₂O conversion the main products obtained were N₂, N₂O, CO and CO₂ but for CO₂ conversion only CO was detected. In situ XRD was a useful tool for interpreting catalyst behaviour and identifying phases present during reaction conditions. The catalytic effect of vanadium can be explained by the occurrence of redox processes in which the catalyst is reduced to lower oxidation states such as V₂O₅/V₆O₁₃.     

A novel sol–gel synthesis of mesoporous ZrO2–MoO3/WO3 mixed oxides

Mesoporous ZrO₂–MoO₃/WO₃ mixed oxides have been synthesized through a novel, convenient one step sol–gel technique. Water soluble molybdate/tungstate and zirconium (IV) carbonate complex have been employed in presence of cationic surfactant, tetradecyltrimethylammonium bromide under basic condition. The synthesized materials have shown high specific surface areas and narrow pore-size distributions which were achieved after optimization of the amount of surfactant. Mesoporous ZrO₂–MoO₃ and ZrO₂–WO₃ mixed oxides have shown specific surface areas of 228 and 275 m² g⁻¹ and pore sizes of 3.65 nm and 4.33 nm, respectively. FTIR and Raman studies prove the formation of hetero bonding in mixed oxides.     

Preparation, characterization and catalytic properties of Co–Nb2O5–SiO2 catalysts

Co–Nb₂O₅–SiO₂ catalysts were prepared using three different sol–gel procedures: (i) the colloidal sol–gel method using NbCl₅ and SiCl₄ as precursors; (ii) the polymeric sol–gel method using niobium ethoxide and tetraethyl-orthosilicate (TEOS); (iii) an intermediate procedure between the colloidal and polymeric sol–gel method in which the precursors were those utilized in the CSG but dissolved in a mixture of anhydrous ethanol and CCl₄. In all procedures, the elimination of the solvent carried out between 80 and 110°C was followed by a reduction in hydrogen flow (30 ml min⁻¹) at 773 K. Following these procedures, samples containing 10 wt.% Co and 15 wt.% niobium oxide (expressed as Nb₂O₅) were obtained. The characterization of the catalysts was performed using various techniques: N₂ adsorption and desorption curves at 77 K, NH₃- and H₂-chemisorption, TPO, XPS, XRD, and solid state ¹H MAS-NMR. Hydrogenolysis of butane was evaluated. The low reaction rates are assigned to the effect of the metal size, whereas the isobutane selectivity as well as the relatively high stability is due to the acidity of the support.     

ZnBr2–Ph4PI as highly efficient catalyst for cyclic carbonates synthesis from terminal epoxides and carbon dioxide

The catalyst systems composed of ZnBr₂ and different phosphonium salts were examined for solvent-free synthesis of cyclic carbonates from CO₂ and terminal epoxides under mild conditions. Among the catalysts investigated, ZnBr₂–Ph₄PI was found to be the best while those of ZnBr₂–phosphine oxide (Bu₃PO or Ph₃PO) show no catalytic effect. It is apparent that the halide ions of phosphonium salts have an essential role to play in the reaction. The catalytic activity of ZnBr₂–Ph₄PI increases with a rise of Ph₄PI to ZnBr₂ molar ratio up to 6, above which there is little change in catalytic activity. We observed that with a rise in ZnBr₂ to Ph₄PI molar ratio, there is increase in epoxide conversion but decline in TOF_PO (estimated based on the site number of Zn²⁺). The effect of water on the reaction was investigated for the first time. We found that the presence of even a trace amount of water would result in a marked decline in reactivity, and the observation provides a valid explanation for why reproducibility of results is poor among researchers so far. The influences of other parameters such as reaction temperature and CO₂ pressure on the catalytic performance of ZnBr₂–PPh₄I were also studied. It is shown that the catalyst is sensitive to reaction temperature, and a rise of reaction temperature up to 130 °C favors the formation of cyclic carbonates. We observed that activity increases with rise in CO₂ pressure and reaches a maximum at an initial CO₂ pressure of 2.5 MPa. Moreover, a plausible reaction mechanism has been proposed.     

Fischer–Tropsch synthesis over Co–SiO2 catalysts prepared by the sol–gel method

Fischer–Tropsch synthesis was carried out in slurry phase over uniformly dispersed Co–SiO₂ catalysts prepared by the sol–gel method. When 0.01–1 wt.% of noble metals were added to the Co–SiO₂ catalysts, a high and stable catalytic activity was obtained over 60 h of the reaction at 503 K and 1 MPa. The addition of noble metals increased the reducibility of surface Co on the catalysts, without changing the particle size of Co metal significantly. High dispersion of metallic Co species stabilized on SiO₂ was responsible for stable activity. The uniform pore size of the catalysts was enlarged by varying the preparation conditions and by adding organic compounds such as N,N-dimethylformamide and formamide. Increased pore size resulted in decrease in CO conversion and selectivity for CO₂, a byproduct, and an increase in the olefin/paraffin ratio of the products. By modifying the surface of wide pore silica with Co–SiO₂ prepared by the sol–gel method, a bimodal pore structured catalyst was prepared. The bimodal catalyst showed high catalytic performance with reducing the amount of the expensive sol–gel Co–SiO₂.     

Strong rhodium–niobia interaction in Rh/Nb2O5, Nb2O5–Rh/SiO2 and RhNbO4/SiO2 catalysts: Application to selective CO oxidation and CO hydrogenation

The extent of Rh–niobia interaction in niobia-supported Rh (Rh/Nb₂O₅), niobia-promoted Rh/SiO₂ (Nb₂O₅–Rh/SiO₂) and RhNbO₄/SiO₂ catalyst after H₂ reduction has been investigated by H₂ and CO chemisorption measurements. These catalysts have been applied to selective CO oxidation in H₂ (CO+H₂+O₂) and CO hydrogenation (CO+H₂), and the results are compared with those of unpromoted Rh/SiO₂ catalysts. It has been found that niobia (NbO_x) increases the activity and selectivity for both the reactions.     

Fischer–Tropsch synthesis over Re-promoted Co supported on Al2O3, SiO2 and TiO2: Effect of water

The activity and selectivity of rhenium promoted cobalt Fischer–Tropsch catalysts supported on Al₂O₃, TiO₂ and SiO₂ have been studied in a fixed-bed reactor at 483 K and 20 bar. Exposure of the catalysts to water added to the feed deactivates the Al₂O₃ supported catalyst, while the activity of the TiO₂ and SiO₂ supported catalysts increased. However, at high concentrations of water both the SiO₂ and TiO₂ supported catalyst deactivated. Common for all catalysts was an increase in C₅₊ selectivity and a decrease in the CH₄ selectivity by increasing the water partial pressure. The catalysts have been characterized by scanning transmission electron microscope (STEM), BET, H₂ chemisorption and X-ray diffraction (XRD).