Nano porous structure of resorcinol–formaldehyde xerogels and aerogels: effect of sodium dodecylbenzene sulfonate

In the past two decades, resorcinol?Cformaldehyde (RF) gels have found widespread applications owing to their low density and adjustable pore size. They are usually prepared through sol?Cgel polymerization of the monomers in an aqueous media followed by evaporative or supercritical drying. In this study, RF gels were synthesized via sol?Cgel polymerization in the presence of sodium dodecylbenzene sulfonate (NaDBS) followed by ambient and supercritical drying. Dimensional measurements along with N₂ sorption analysis and Scanning electron microscopy (SEM) micrographs revealed that pore structure of the gel is chiefly affected by NaDBS. In all samples (xerogels and aerogels), maximum densities were observed at a critical NaDBS concentration (~1?w/v%), whereas considerable pore size increments and pore size distribution broadenings were found at higher concentrations of NaDBS (??5?w/v%). The most increased mesopore volumes were detected in xerogels (133% for acetone-dried and 67% for water-dried samples), while concerning aerogels, the pore sizes enlargement to macropore regime was observed at 5?w/v% of NaDBS. SEM micrographs, in agreement with porosity analysis, depicted that very large pore volumes could be obtained when supercritical drying was employed. However, in the case of xerogels, a more dense structure with smaller pores (micro and mesopores) exists which can only be altered slightly when using large amounts of NaDBS. The results showed that the RF gel pore texture, independent of drying technique, was strongly influenced by the addition of NaDBS, which should be taken into account when using this surfactant in the gel formulation for a wide variety of applications.     

Effects of temperature and feed composition on catalytic dehydration of methanol to dimethyl ether over γ-alumina

Catalytic dehydration of methanol to dimethyl ether (DME) is performed in an adiabatic fixed bed heterogeneous reactor by using acidic γ-alumina. By changing the mean average temperature of the catalyst bed (or operating temperature of the reactor) from 233 up to 303 °C, changes in methanol conversion were monitored. The results showed that the conversion of methanol strongly depended on the reactor operating temperature. Also, conversion of pure methanol and mixture of methanol and water versus time were studied and the effect of water on deactivation of the catalyst was investigated. The results revealed that when pure methanol was used as the process feed, the catalyst deactivation occurred very slowly. But, by adding water to the feed methanol, the deactivation of the γ-alumina was increased very rapidly; so much that, by increasing water content to 20 weight percent by weight, the catalyst lost its activity by about 12.5 folds more than in the process with pure methanol. Finally, a temperature dependent model developed to predict pure methanol conversion to DME correlates reasonably well with experimental data.     

The activation mechanism of oxalic acid on γ-alumina and the formation of α-alumina

Converting the γ phase into the α phase completely is necessary in the presintering stage of industrial alumina (Al₂O₃), which requires high temperature and energy consumption. To reduce the presintering temperature, γ-Al₂O₃ was activated by oxalic acid. XRD, ²⁷Al-MAS-NMR and TG-DSC were used to characterize the γ - alumina before and after activation, and the phase transformation was studied. The formation temperature of α-Al₂O₃ decreased to 1029 °C for oxalic acid activated γ-Al₂O₃, and the α-fraction was 100% for activated γ-Al₂O₃ at 1300 °C. After oxalic acid activation, the diffraction peak intensity of γ-Al₂O₃ decreased significantly; the results of ²⁷Al-MAS-NMR suggested that octahedral [AlO₆] in γ-Al₂O₃ was easier than tetrahedral [AlO₄] to be attacked by oxalic acid, and the formation of pentavalent [AlO₅] with higher reaction activity, which was in favour of the lowering formation temperature of α-Al₂O₃. The dissolution concentration of Al increased after oxalic acid activation, and the dissolution process was controlled by surface reactions. Oxalic acid mainly attacked the octahedral aluminium in γ-Al₂O₃ and extracted Al as three complexes of [Al(C₂O₄)]⁺, [Al(C₂O₄)₂]^- and [Al(C₂O₄)₃]^3-. Oxalic acid activated γ - Al₂O₃ with a lower phase transformation temperature has broad application prospects in the alumina industry.     

Study of the deactivation and regeneration of copper chromite on γ-alumina and magnesium chromite on γ-alumina catalysts for fuel combustion

The causes of the deactivation of catalysts for fuel combustion MeCr₂O₄/ -Al₂O₃ (Me = Cu or Mg) have been investigated using a variety of complex physical-chemical methods: IRS, ESDR, XPS, TPD and a pulse microcatalytic method. It has been concluded that the observed deactivation of catalysts during fuel combustion is due to the combined effect of high temperatures and reduction media.     

Pure vs ultra-pure γ-alumina: A spectroscopic study and catalysis of ethanol conversion

Spectroscopic and catalytic properties of “pure” and “ultrapure” γ-Al₂O₃ samples, both from Sasol, are compared. The “pure” sample has additional absorption in the UV region, much stronger luminescence in the Raman scattering experiments, slightly different skeletal surface OH groups IR spectra, and more evident features of weakly adsorbed pyridine. These effects are attributed to small amounts of Fe^3 +, Ti^4 +, surface silanol groups and Na⁺ ions. The “ultrapure” sample is more selective for the production of ethylene from ethanol at total conversion, while in the presence of oxygen is less active in oxidizing ethanol to other oxygenates and to CO_x.     

Microstructure,strength and CO2 separation characteristics of α-alumina supported γ-alumina thin film membrane

Abstract

Abstract

In this work, a mesoporous γ-Al₂O₃ membrane was synthesised on an α-Al₂O₃ substrate by a sol–gel dip coating process. The membranes were characterised using SEM, field emission SEM, X-ray diffractometry and N₂ adsorption/desorption measurements (Brunauer–Emmett–Teller analyses). The characterisation results revealed that a flawless γ-alumina membrane layer with 3·5 μm thickness and 2·1 nm average pore size was achieved. Subsequent separation tests indicated that CO₂ could be separated from N₂ by the mesoporous γ-Al₂O₃ membrane. In spite of the difficulties of the separation, the optimised microstructure achieved for the prepared membrane led to desirable and regular permeation behaviour. It was also observed that separation could be more efficient in high pressure permeation conditions. The prepared substrate fulfilled the required strength and permeability for the thin γ-Al₂O₃ membrane film under these conditions. Accordingly, an optimised completely ceramic membrane structure, applicable for CO₂ separation applications, was achieved.     

Catalytic conversion of chloromethane to methanol and dimethyl ether over mesoporous γ-alumina

Mesoporous γ-alumina, mp-γ-Al₂O₃, possessing surface area around 385 m²/g and total pore volume of 2.0 cm³/g was prepared via template-free sol-gel synthesis. The catalytic activity of the prepared alumina for the conversion of chloromethane to dimethyl ether, DME, in the presence of water or methanol was studied in the temperature range of 170-450 °C employing FTIR spectroscopy. In the absence of water and methanol, the fresh surface of mp-γ-Al₂O₃ showed 100% conversion of chloromethane to DME at temperatures between 250 and 350 °C. However, rapid deactivation of the catalyst resulted in a sharp decrease in the conversion to < 5% within a few minutes of reaction. The catalytic activity was noticeably enhanced by adding water vapor to the gas feed resulting in higher conversions to DME and methanol. The catalytic activity and DME selectivity were further enhanced in the presence of methanol instead of water. In the temperature range of 200-300 °C, complete conversions were obtained at the beginning of reactions before they declined to values between 31 and 45% depending on the reaction temperature. It was proposed that the surface hydroxyl groups are the active sites where chloromethane molecules dissociatively adsorb forming adsorbed methoxy ion intermediates. The adsorption of molecular methanol regenerates the surface hydroxyl groups and enhances the formation of adsorbed methoxy ions on the surface which react further with chloromethane or methanol molecules to produce more DME.     

Fused deposition modeling of mullite structures from a preceramic polymer and γ-alumina

Thermoplastic filaments for Fused Deposition Modeling fabrication of mullite ceramic components were produced from a polymethylsiloxane ceramic precursor, γ-Al₂O₃ powder and ethylene vinyl acetate (EVA) as an organic, elastomeric binder. Two micron-sized γ-Al₂O₃ powders with different particle size distributions were used. Both type of powders could be successfully 3D printed, however γ-Al₂O₃ with finer particle size resulted in better quality of printed parts with much smoother surface.Sintering experiments showed that mullite starts forming already at 1250 °C, but a final sintering of 1550 °C is necessary to achieve pure mullite using the coarse powder, while some residual α-Al₂O₃ was present when using finer powder.Results demostrate that Fused Deposition Modeling can be used to shape preceramic polymers to generate large scale components with a complex shape in which, during firing, the siloxane produces highly reactive silica that allows for the in-situ formation of silicate ceramic phases, such as mullite.     

The structure of vanadium oxide species on γ-alumina; an in situ X-ray absorption study during catalytic oxidation

The mechanism of catalytic oxidation reactions was studied using in situ X-ray absorption spectroscopy (XAFS) over a 17.5 wt% V₂O⁵/Al₂O₃ catalyst, i.e., at reaction temperatures and in the presence of reactants. It was found that X-ray absorption near-edge structure (XANES) is a powerful tool to study changes in the local environment and the oxidation state of the vanadium centres during catalytic oxidation. At 623 K, the catalyst follows the associative mechanism in CO oxidation. XAFS revealed that the Mars–van Krevelen mechanism is operative at 723 K for CO oxidation. The extended X-ray absorption fine structure (EXAFS) results showed that the structure of the supported V₂O⁵ phase consists of monomeric tetrahedral (Al–O)₃–V=O units after dehydration in air at 623 K. However, the residuals of the EXAFS analysis indicate that an extra contribution has to be accounted for. This contribution probably consists of polymeric vanadate species. The structure remains unchanged during steady-state CO oxidation at 623 and 723 K. Furthermore, when oxygen was removed from the feed at 623 K, no changes in the spectra occurred. However, when oxygen is removed from the feed at 723 K, reduction of the vanadium species was observed, i.e., the vanadyl oxygen atom is removed. The V³⁺ ion subsequently migrates into the γ-Al₂O₃ lattice, where it is positioned at an Al³⁺ octahedral position. This migration process appears to be reversible; so the (Al–O)₃–V=O units are thus restored by re-oxidation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of drying conditions on physicochemical properties of epoxide sol−gel derived α-Fe2O3 and NiO: A comparison between xerogels and aerogels

A simple and easily operated supercritical CO₂ dryer was designed and manufactured with the aim of producing high-surface-area mesoporous α-Fe₂O₃ (hematite) and NiO aerogels. The gels were synthesized by a sol?gel method with the aid of propylene oxide (PO), as the gelation agent, and then dried and calcined at different conditions. The effects of drying and calcination conditions on the physicochemical properties of the final aerogels were investigated using X-ray diffraction (XRD), N₂ adsorption-desorption, Fourier-transformed infrared spectroscopy (FT-IR), and field emission scanning electron microscopy (FE-SEM) analyses. It was demonstrated that α-Fe₂O₃ and NiO aerogels with high surface areas and mesoporosities could be successfully synthesized using the home-made supercritical CO₂ dryer. Supercritical drying of the gels resulted in α-Fe₂O₃ (186 m²/g) and NiO (178 m²/g) aerogels with 186% and 34% higher surface areas, respectively, than xerogels obtained via simple drying at 80°C using a laboratory oven. In addition, the results showed that supercritical CO₂ drying could enhance preservation of the porous network of the oxide nanostructures at high calcination temperatures via suppression of sintering phenomenon. Calcination of α-Fe₂O₃ and NiO aerogels at 600°C yielded 225% and 53% higher surface areas than the corresponding xerogel, confirming the significance of drying step in the sol?gel method. Asphaltene adsorption from a model oil with asphaltene concentration of 3000 ppm indicated that the aerogels possessed higher adsorption capacities for the bulky asphaltene molecules than xerogels calcined at the same temperature of 600°C, which was due to their enhanced textural properties.     

X-ray photoelectron spectroscopy of Pd/γ-alumina and Pd foil after catalytic methane oxidation

Samples of palladium supported on-alumina and a palladium foil were used as catalysts for methane oxidation at 550° C. The samples were quenched quickly in the reaction chamber to room temperature in flowing Ar and then transferred in vacuo for XPS analysis. Structure sensitivity was manifest from an increase in PdO stability and a decrease in carbon deposition relative to Pd with increasing particle size. The results were compared with recent ellipsometric data.     

Phase transformation during the sintering of γ-alumina and the simulated Ni-laden waste sludge

This study investigated the fundamental phase transformation process when incorporating simulated nickel-laden waste sludge by alumina-based ceramic system. To match the required sintering conditions for commercial ceramic products as a development of waste-to-resource strategy, the investigation focused on the nickel incorporation behavior of γ-alumina in moderate to low temperature sintering environments (1250–750 °C) under a 3 h sintering process. X-ray diffraction (XRD) and Rietveld refinement were used to show the quantitative distribution of phases. The results demonstrated that γ-alumina interacted intensively with nickel oxide at sintering temperatures above 1000 °C. Furthermore, the densification effect of the sintering process is also illustrated, as this is crucial for developing the needed mechanical strength for practical applications. The findings in this study reveal that there is good potential for achieving the stabilization of nickel by thermal treatment with a γ-alumina containing precursor via moderate ceramic sintering temperatures.     

Optimization and sensitivity analysis of rheological properties of high concentration γ-alumina/water suspension

In this study, a high concentration γ-alumina/water suspension containing 75% (wt) γ-alumina nanoparticle was prepared by using a 3D-mixer. TEM imaging, Dynamic Light Scattering (DLS), and X-Ray Diffraction (XRD) analyses were used to investigate the stability of nanoparticles in the base fluid, purity, and crystallinity as well as chemical structure of nanoparticles, respectively. Then, the variation of dynamic viscosity and rheological behavior of the suspension and their dependency on pH, sonication durations, dispersing agent concentrations, and ratios of ceramic balls to nanoparticles weight was studied. The results showed that the rheological behavior of the suspension was similar to non-Newtonian fluids, and a power-law model with yield stress was able to justify this behavior. Moreover a correlation, including pH, sonication time, dispersing agent concentration, and the ratio of ceramic balls to nanoparticles weight as independent parameters, was developed to predict the power of the power-law model. The model showed a low deviation, about 5%, from the experimental values and revealed the significant effect of pH, sonication time, and the ratio of mixing ceramic balls to nanoparticles on the rheological behavior of the suspension. More importantly, by implementing the method used in this work, a very low dynamic viscosity of 11.4 mPa.s was achieved.     

The study of hydrargillite and γ-alumina conversion process in boehmite in different hydrothermal media

The transition of hydrargillite and γ-alumina in boehmite was investigated under hydrothermal conditions at a temperature of 200°C. The methods of X-ray fluorescnce (XRF) analysis, infrared spectroscopy (IR spectroscopy) DTA, TGA, DSC, scanning electron microscopy (SEM), and thermal desorption were used to study the samples subjected to autoclave treatment in different media. It was shown that, at the hydrothermal treatment of hydrargillite and γ-alumina at the initial stages of treatment, regardless of the conditions of the process and the reaction medium, the formed boehmite is characterized by a high specific surface and low value of the heat effect of the formation and decomposition, which is determined by the weak ordering of its structure.     

Rheological characterisation of hydroxyapatite filled polyethylene composites. Part 1 – Shear and extensional behaviour

Abstract

The shear and extensional properties of injection moulding grade hydroxyapatite–polyethylene composites developed for orthopaedic applications have been studied. The composite was prepared without processing aids owing to concerns over the potential biological responses to such additives. The composite containing 20 vol.-% hydroxyapatite filler showed typical pseudoplastic behaviour. However, that containing 40 vol.-% hydroxyapatite filler tended to exhibit yield. The Maron–Pierce equation was found to be useful in predicting the viscosities of the composite systems. The activation energy of the composite and the unfilled polymer were equal, indicating that the 20 vol.-% system exhibits the same flow mechanism as the unfilled polymer. A qualitative assessment of extensional properties was made following Cogswell's method. The extensional stress of the unfilled polymer decreases with increasing temperature whereas the composites behave in a complex manner. For all the systems the Trouton ratios tend to increase with apparent shear rates. The Trouton ratio also indicates that at higher temperatures the flow of these composites is dominated by extensional properties.     

Properties and catalytic mechanism of γ-glutamyltranspeptidase from B.subtilis NX-2

Since γ-glutamyltranspeptidase (GGT) especially catalyses the transfer of the γ-glutamyl moiety to a variety of amino acids and short peptides, GGT has important practical value for enzymatic synthesis of γ-glutamyl compounds. In this paper, the GGT produced from Bacillus subtilis NX-2 was purified by a combination of ammonium sulfate fractionation and ion exchange chromatography, and the properties of purified GGT were investigated. At the conditions of pH 10.0, D-glutamine (D-Gln)/L-tryptophan (L-Trp) with a molar ratio of 5: 7, a temperature 40°C and a reaction time of 4 h, a higher conversion rate of 42% was obtained. According to the time course, the catalytic mechanism of enzymatic synthesis of γ-D-glutamyl-L-tryptophan (γ-D-Gln-L-Trp) was discussed. It was demonstrated that the GGT can catalyze not only the reaction of transpeptidation, but also the irreversible hydrolysis of the products which results in the decrease of the yield of the products. The affinity parameter of GGT to D-Gln (Km) was 5.08 mmol·L⁻¹ and the maximum reaction rate of transpeptidation (r _max) was determined as 0.034 mmol·min⁻¹·L⁻¹, while the affinity parameter of GGT to γ-D-Gln-L-Trp (K’_m) was 2.267 mmol·L⁻¹, and the maximum reaction rate of hydrolysis (r’_max) was 0.012 mmol·min⁻¹·L⁻¹. __________ Translated from Journal of Chemical Engineering of Chinese Universities, 2008, 22(2): 288–293 [译自: 高校化学工程学报]     

Preparation of γ-alumina powder from kaolin derived aluminium sulphate solution and its stability by BaO addition

Abstract

The synthesis of γ-alumina powder from kaolin derived aluminium sulphate solution is described and results are reported on the effect of BaO additions made by two different methods, precipitation mixing and mechanical mixing of BaCl₂ .2H₂O, on the thermal stability of γ-alumina powder at high temperatures. The powders were characterised by XRD, SEM, TEM, and measurements of surface area by the BET method. It was found that powder prepared by precipitation mixing was more thermally stable at high temperatures than powder prepared by mechanical mixing, indicating that the precipitation mixing was more effective in retarding the γ-to α-alumina phase transformation at high temperature.     

Fabrication and photoluminescence of γ-AlON:Sm and Yb

Transparent γ-AlON undoped and doped with one of eight rare earth elements were fabricated by hot pressing at 1850 °C. After characterizing the fundamental physical properties of sintered γ-AlON, the photoluminescence spectra of γ-AlON:Sm and γ-AlON:Yb were examined further because only these samples showed a color under visible light because of the absorbance of a certain wavelength. γ-AlON:Sm exhibited a red emission with emission peaks at 688, 701, and 729 nm, while γ-AlON:Yb exhibited a white emission due to the two broad emission peaks at 438 and 575 nm. γ-AlON:Yb revealed a possible application to UV-excited phosphor-converted white-light-emitting diodes because of its emission characteristics. The schematic crystal structure of the rare-earth-doped γ-AlON and the CIE chromaticity diagram were provided to understand the photoluminescence behavior of γ-AlON:Sm and γ-AlON:Yb.     

Reduction of lean NO2 with diesel soot over metal-exchanged ZSM5, perovskite and γ-alumina catalysts

The catalytic performances of metal-exchanged ZSM5, perovskite and γ-alumina catalysts for the reduction of nitrogen dioxide (NO₂) by diesel soot were investigated. The reaction tests were performed through temperature-programmed reaction (TPR), in which NO₂ and O₂ were passed through a fixed bed of catalyst-soot mixture. On the three types of catalyst, NO₂ was reduced to N₂ by model soot (Printex-U) and most of the soot was converted into CO₂. Pt-, Cu- and Co-exchanged ZSM5 catalysts exhibited reduction activities with conversions of NO₂ into N₂ of about 20%. Among the perovskite catalysts tested, La_0.9K_0.1FeO₃ showed a 32% conversion of NO₂ into N₂. The catalytic activities of the perovskite catalysts were largely influenced by the number and stability of oxygen vacancies. For the γ-alumina catalyst, the peak reduction activity appeared at a relatively high temperature of around 500 °C, but the NO₂ reduction was more effective than the NO reduction, in contrast to the results of the ZSM-5 and perovskite catalysts.