Supercritical Drying Method Research Articles

Mn-substituted Ca–La–hexaaluminate nanoparticles for catalytic combustion of methane

Mn-substituted Ca–La–hexaaluminate rod-like nanoparticles (Ca1−xLaxMnAl11O19−α) with high surface area ranging from 47 to 80m2/g for catalytic combustion of methane have been prepared using alumina sol as the (NH4)2CO3 coprecipitation precursor and a supercritical drying (SCD) method. Ca substitution gave rise to the maximum combustion activity (T10%=459°C) at x=0 owing to the highest surface area. Meanwhile, Ca substitution affects the oxygen sorption property and the oxidation state of Mn ions in the hexaaluminate lattice. Ca0.6La0.4MnAl11O19−α catalyst with high catalytic activities was obtained owing to the excellent performance of activating oxygen.

Effect of drying methods on the structure and catalytic combustion activity of Mn-substituted hexaaluminate catalysts

Conventional oven drying (COD) and supercritical drying (SCD) methods were applied to the preparation of Mn-substituted hexaaluminate (BaMnAl11O19-α) catalysts. The effect of drying methods on phase composition, specific surface area, pore structure and combustion activity of the samples was investigated. The samples obtained by SCD have higher surface area, narrower pore size distribution, and higher combustion activity than those obtained by COD.

Effect of Introduction of Ba, Mn on Thermal Stability and Catalytic Activity to Methane Combustion of Al<sub>2</sub>O<sub>3</sub>

A sol-gel method was employed to prepare Mn-substituted barium hexaaluminate catalysts for methane combustion. Supercritical drying (SCD) and conventional oven drying(CD) methods were used to extract the water in the hydrogel. The effects of drying methods,. as well as the incorporation of Ba, Mn, on properties of the catalysts were investigated by means of N-2-adsorption, TG-DTA and X-ray diffraction. The supercritical drying method could maintain a higher specific surface area of the samples and suppress the transformation into alpha-Al2O3 at elevated temperatures. The introduction of Mn oxides could promote the transformation into hexaaluminates during calcination. BaMnAl11O19 catalyst dried by SCD method has a specific surface area of 35.1 m(2 .) g(-1) and an activity with T-10% (temperature for 10% methane conversion) of 500 degreesC under the condition of 0.1 MPa and 40000 h(-1) GHSV.

Synthesis of isobutene from synthesis gas over nanosize zirconia catalysts

Performances of nanosize zirconium oxides prepared by precipitation, supercritical fluid drying (SCFD) and freeze-drying (FD) methods were evaluated for the selective synthesis of isobutene from synthesis gas (syn-gas). The crystal structure, particle size and specific surface area were measured by XRD, TEM and BET. The acidic and basic properties of the catalysts were studied by NH 3- and CO 2-TPD, respectively. The results showed that the crystal phases, acidic and basic properties of nanosize zirconia depend remarkably on the drying conditions. The catalysts over which the formation of isobutene is favored show a higher ratio of basic to acidic sites on their surfaces. The results of adding acidic and basic components into zirconia suggested that both the acidic and basic sites on the catalysts are required for the isosynthesis. The conversion of CO was increased by adding acidic component into zirconia, and the selectivity for i-C 4H 8 was increased greatly by adding basic component into zirconia. It is suggested that the acidic sites are responsible for the activation of reactant molecules and the basic sites of the catalysts are significant for the formation of i-C 4H 8 from CO hydrogenation.

Influence of supercritical and natural drying methods on structure and properties of porous silicon

We have studied the surface morphology, photoluminescence (PL), Raman spectra and optical absorption of the porous silicon (PS) samples prepared under supercritical drying (SD) and natural drying (ND) process. The experimental results of scan electron microphotograph and Raman spectra show that there are obvious differences in microstructure between the SD and ND samples. No crack of the skeleton has been found in the SD sample, but the skeleton of the ND sample has heavily destroyed. Besides, the PL and absorption spectra of the SD sample are not the same as those of the ND sample.

Some catalytic applications of aerogels for environmental purposes

To protect our environment, catalysts are useful for the treatment of pollutants once they have been generated, or to prevent or at least to minimize their formation. To meet these requirements, very highly divided supports and active phases are necessary for satisfactory results. In particular, nanoparticles such as those obtained by sol–gel chemistry combined with supercritical drying methods (“aerogels”) may develop specific catalytic properties from the points of view of textural and thermal qualities. Some examples of gas and liquid phase catalysis such as SCR, deNO x , hydrocarbon combustion and the elimination of nitrophenols and cyanide wastes in water are presented here.

Effect of Supercritical Drying Method on Microstructure and Properties of Al2O3 Composites Reinforced with Ultrafine SiC Particulate.

Mixtures of 95vol% ultrafine γ-Al2O3 powder and 5vol% ultrafine SiC powder, with average particle size of 40nm, were mixed wet-ball milling and prepared by drying their slurry with a supercritical drying method and a microwave drying method. Al2O3 specimens dispersed with ultrafine-SiC particles as the secondary phase were hot-pressed, their powders being prepared by both the above drying methods. Difference between the superitical drying and the microwave drying method on the microstructure of Al2O3/5vol% SiC composites was investigated. Dense Al2O3/SiC composites were obtained at lower hot-pressing temperatures when the mixture was prepared by supercritical drying method. Influence of the drying method on the γ- to α-Al2O3 transition behavior was also examined. Al2O3/SiC powder dried by the supercritical drying method showed narrow particle size distribution and the formation of soft agglomerates. Al2O3/SiC composite sintered by hot-pressing the powder dried with the supercritical drying method indicated relatively higher strength, whereas the strength of Al2O3/SiC composite through microwave drying method was low and had the large scatter.

Zirconia aerogels: effect of acid-to-alkoxide ratio, alcoholic solvent and supercritical drying method on structural properties

Zirconia aerogels have been prepared by the sol–gel method using tetra- n-butoxy zirconium(IV) and acid catalysis. The nitric acid-to-alkoxide ratio and the alcoholic solvent, as important sol–gel parameters, were varied. After aging, the solvent entrapped in the sol–gel products was removed applying two different methods: supercritical drying at high-temperature and low-temperature extraction with supercritical CO 2. Finally, for gel stabilization and removing of organic residues the aerogels were calcined at 573 and 773 K, respectively. The effects of the varied sol–gel parameters, different supercritical drying methods, and calcination temperature on the structural and textural properties were investigated. The aerogels were characterized by means of N 2 physisorption, X-ray diffraction, thermal analysis, scanning and transmission electron microscopy. Depending on the kind of alcoholic solvent, nitric acid-to-alkoxide ratio and drying method, the specific surface areas of the mesoporous aerogels varied from 55 to 205 m 2 g −1 after calcination in air at 773 K. The aerogels supercritically dried at high-temperature possessed larger pores (17–65 nm) and BET surface areas (143–205 m 2 g −1) compared to the aerogels dried by low temperature extraction (<20 nm and 55–112 m 2 g −1, respectively). The width of the pore size distribution of all high-temperature supercritically dried gels became smaller with increasing amount of acid. All aerogels dried by low temperature extraction with supercritical CO 2 were X-ray amorphous. Upon calcination in air at 773 K, the amorphous aerogels crystallized to tetragonal zirconia. The high-temperature supercritically dried aerogels contained predominantly tetragonal zirconia. The fraction of monoclinic ZrO 2 shares increased with increasing nitric acid-to-alkoxide ratio, the use of a long-chained branched alcoholic solvent, such as t-butanol, and increasing calcination temperature.

Alumina aerogel catalysts prepared by two supercritical drying methods used in methane combustion

Palladium-supported alumina aerogels were prepared by two different supercritical drying methods. In one method, an alumina wet gel was dried under supercritical conditions of ethanol in an autoclave. In the other, the aerogel was supercritically dried by extracting ethanol using carbon dioxide in an extractor. The Pd-supported alumina aerogel prepared in the autoclave exhibited a high specific surface area of 112.8 m2/g after firing at 1200 °C for 5 h, while the other had a specific surface area of only 5.2 m2/g due to α-alumina transformation. Their catalytic properties for methane combustion were measured. The Pd-supported alumina aerogel prepared in the autoclave combusts methane perfectly at 50–60 °C lower temperature than the other. Palladium particles on the alumina aerogel prepared in the autoclave contained palladium oxide, while those prepared in the CO2 extractor contained only palladium metal.

Alumina-silica aerogel catalysts prepared by two supercritical drying methods for methane combustion

Palladium-supported alumina-silica (10 mol%) aerogels were prepared by two different supercritical drying methods. In one method, an alumina-silica wet gel was dried under supercritical conditions of ethanol in an autoclave at a temperature of 270‡C and pressure of 26.5 MPa. In the other, ethanol was supercritically extracted by carbon dioxide in an extractor at 80‡C and 15.7 MPa. Their catalytic properties for methane combustion were measured after being fired at 1200‡C for 5 h. The palladium-supported alumina-silica aerogels prepared in the autoclave showed higher activity (methane conversion) than the one prepared in the extractor. This was attributed to the autoclave-dried aerogel having a larger pore size and better palladium dispersion.

Preparation of an alumina aerogel with SiC whisker inclusion

Abstract Alumina aerogels containing silicon carbide (SiC) whiskers were prepared using supercritical drying methods, and their thermal properties were examined. The effects of the SiC whiskers on the alumina aerogel were evaluated. The composite aerogel prepared by the supercritical extraction showed a higher specific surface area than an alumina aerogel without whiskers. Many cracks were observed in the composite aerogel fired at 1200 and 1400°C for 5 h. This is due to the difference of thermal shrinkage between the alumina aerogel and the SiC whiskers. The specific surface area of the composite aerogel was 26·1 m 2 /g, and that of the aerogel without whiskers 5·6 m 2 /g. The same effect was observed in a composite aerogel prepared by a different supercritical drying method at a higher temperature and pressure.

Properties of alumina aerogels prepared under different conditions

Alumina aerogels were prepared by the sol-gel method and two different supercritical drying methods. In one method, alumina wet gel was supercritically dried under the supercritical condition of carbon dioxide and ethanol mixture in a CO 2 extractor (80°C, 15.7 MPa). In the other method, alumina wet gel was supercritically dried under the supercritical condition of ethanol itself in an autoclave (270°C, 26.5 MPa). The physical properties of the alumina aerogels were measured for the different drying methods. Fibrillar crystals of boehmite were observed in the alumina aerogel supercritically dried in the autoclave. The properties of the alumina aerogels were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, infrared spectra, and 27Al magic angle spinning nuclear magnetic resonance. The alumina aerogels had differing properties and differing behaviors in the firing process according to the supercritical drying conditions.

Preparation of heat-resistant alumina aerogels

Alumina aerogels were prepared using supercritical drying methods, and their thermal properties were examined. The effects of several additives to the alumina aerogel and supercritical drying methods were examined in order to improve heat resistance. Silica, phosphoric oxide, barium oxide, lanthanum oxide, and SiC whisker were effective for maintaining a high specific surface area of the alumina aerogel at a temperature over 1200 °C. Silica was found to be the most effective among the additives. The addition of 10 mol % silica resulted in an alumina aerogel with the highest specific surface area, 114.3 m2/g at 1200 °C, and increased the transformation temperature to α alumina. Barium oxide and lanthanum oxide formed smaller crystals within the alumina, compared with those of alumina alone. SiC whisker caused many cracks in the alumina aerogel. When supercritically dried, the alumina aerogel was strengthened by treatment at higher temperature and pressure.

Boron Nitride and Composite Aerogels from Borazine Based Polymers

ABSTRACTPoly(borazinylamines) have been processed in THF and toluene solvents and aerogel forms produced by supercritical drying methods. The polymer aerogels have been pyrolyzed and porous amorphous and crystalline BN monoliths have been produced. These have been characterized and effects of processing factors on surface area and pore structure are described.