共查询到20条相似文献,搜索用时 140 毫秒
1.
Hiroshi Hirashima Hiroaki Imai Vladimir Balek 《Journal of Sol-Gel Science and Technology》2000,19(1-3):399-402
Alumina gels made from the metal alkoxide is known to have high catalyst activity for the selective reduction of NO
x
by hydrocarbons. It is also reported that the fine structure of the gels effects the activity. In this study, the effect of the preparation method on the fine structure and catalyst activity of the gels was investigated. Monolithic gels were obtained by hydrolysis of Al(sec-C4H9O)3. The wet gels were dried at 90°C (xerogels), supercritically dried (aerogels), or dried after immersion in an ethanol solution of methyltrimethoxysilane (modified xerogels). The changes in the microstructure during heating were discussed using the results of TG-DTA, ETA and N2 adsorption. The ETA curves show the 220Rn-release rate, E, of the samples, previously labelled with 228Th and 224Ra, during heating. The decrease in E of the xerogel at temperatures higher than 400°C indicates a gradual decrease in the surface area and porosity. A remarkable decrease in the BET surface area of the xerogel was found after heat-treating at 500°C. On the other hand, constant E of the aerogels and modified xerogels above 450°C suggests high thermal stability. The pore radii, estimated by BJH method, and the catalyst activities at 500°C of the aerogels and the modified xerogels were higher than those of the xerogels. The temperature range in which the alumina gels are applicable as catalysts was determined. 相似文献
2.
T. Heinrich W. Tappert W. Lenhard J. Fricke 《Journal of Sol-Gel Science and Technology》1994,2(1-3):921-924
This work deals with the preparation of aluminosilicate aerogels, especially mullite (3Al2O3·2SiO2) and cordierite (2MgO·2Al2O3·5SiO2) aerogels, from the cohydrolysis of tetraethoxysilane and chelated aluminum-secbutylate; in the case of cordierite magnesium nitrate was added. The influence of various preparation conditions on the aerogel synthesis is described. Crystallization and sintering behavior of mullite aerogels supercritically dried in acetone or alcohol differs from that one of mullite aerogels dried in CO2. During non-isothermal heat treatment the former show a drastically reduced shrinkage compared to the latter. This behavior can be explained by a phase separation during the high temperature autoclaving process. In cordierite aerogels the crystallization of tetragonal mullite at about 1000°C is observed, while the correspondent xerogels show the crystallization of - and - cordierite between 1000 and 1100°C. On the other hand sintering is promoted in cordierite aerogels, which is due to the content of MgO. 相似文献
3.
Lightweight polydicyclopentadiene (pDCPD) based aerogels were developed via a simple sol-gel processing and supercritical
drying method. The uniform pDCPD wet gels were first prepared at room temperature and atmospheric pressure through ring opening
metathesis polymerization (ROMP) incorporating homogeneous ruthenium catalyst complexes (Grubbs catalyst). Gelation kinetics
were significantly affected by both catalyst content and target density (i.e., solid content), while gel solvents also played
important role in determining the appearance and uniformity of wet gel and aerogel products. A supercritical carbon dioxide
(CO2) drying method was used to extract solvent from wet pDCPD gels to afford nanoporous aerogel solid. A variety of pDCPD based
aerogels were synthesized by varying target density, catalyst content, and solvent and were compared with their xerogel analogs
(obtained by ambient pressure solvent removal) for linear shrinkage and thermal conductivity value (1 atm air, 38 °C mean
temperature). Target density played a key role in determining porosity and thermal conductivity of the resultant pDCPD aerogel.
Differential scanning calorimetery (DSC) demonstrated that the materials as produced were not fully-crosslinked. The pDCPD
based aerogel monoliths demonstrated high porosities, low thermal conductivity values, and inherent hydrophobicity. These
aerogel materials are very promising candidates for many thermal and acoustic insulation applications including cryogenic
insulation.
相似文献
George L. GouldEmail: |
4.
Thierry Woignier G. Pochet H. Doumenc P. Dieudonné L. Duffours 《Journal of Sol-Gel Science and Technology》2007,41(1):25-30
Volcanic (allophanic) soils are interesting in terms of the control of the greenhouse effect and the knowledge of the porous
features is of importance to understand the mechanism of C and N sequestration. These soils contain a peculiar clay: allophane
aggregates quite close to the synthetic mineral gels aggregates. These volcanic materials behave as gels during drying with
a large irreversible shrinkage that can modify the soil physical properties. Consequently, as for silica gels, we use the
CO2 supercritical drying procedure (SD) to control the drying step and to preserve the structural and textural properties of
the soils. The experimental results show that the N and C content in the soils is clearly dependent on the allophane content.
We also show that the textural properties, such as specific surface area, are higher for the supercritically dried samples,
compared to the classically dried samples, and SAXS results confirm the preserving effect of the SD. With these data, we propose
possible effects of the specific surface area on the C and N content of the allophanic soils. 相似文献
5.
Imene Mejri Mohamed Kadri Younes Abdelhamid Ghorbel 《Journal of Sol-Gel Science and Technology》2006,40(1):3-8
Aerogel and xerogel sulphated zirconia with defined atomic ratio S/Zr = 0.5 and molar hydrolysis ratio h = nH2O/nZrO2 = 3 show different textural and structural properties after calcination at high temperatures. The aerogel obtained just after
solvent evacuation develops only the tetragonal phase, whereas the xerogel dried in an oven is amorphous. Heating to a temperature
above 833 K, leads to transition of the tetragonal phase to the monoclinic one for the two solids, due to sulphur loss but
the tetragonal phase remains stable for the aerogel . Raman, Infrared and XPS spectroscopies show that the loss of the sulphur
at high temperatures seems to be easier for the xerogel than for the aerogel. 相似文献
6.
Silvester Tursiloadi Yu Yamanaka Hiroshi Hirashima 《Journal of Sol-Gel Science and Technology》2006,38(1):5-12
Porous anatase is attractive because of its notable photo-electronic properties. Titania wet gel prepared by hydrolysis of
Ti-alkoxide was immersed in the flow of supercritical CO2 at 60°C and the solvent was extracted (aerogel). Mesoporous TiO2 consisting of anatase nano-particles, about 5 nm in diameter, have been obtained. Thermal evolution of the microstructure
of the aerogel was evaluated by TGA-DTA, N2 adsorption, TEM and XRD, and discussed in comparison with that of the corresponding xerogel. The diffraction peaks of anatase
were found for the as-extracted gel while the xerogel dried at 90°C was amorphous. After calcination at 600°C, the average
pore size of the aerogel, about 20 nm in diameter, was 4 times larger than that of the xerogel, and the pore volume, about
0.35 cm3 g−1, and the specific surface area, about 60 m2 g−1, were 2 times larger than those of the xerogel. XRD peaks of rutile have been found after calcination at 600°C. The particle
sizes of anatase and rutile are about 13 and 25 nm in diameter, respectively. The surface morphology of TiO2 nano-particles has been discussed in terms of their surface fractal dimensions estimated from the N2 gas adsorption isotherms. 相似文献
7.
Application of Alumina Aerogels as Catalysts 总被引:2,自引:0,他引:2
Hiroshi Hirashima Chihiro Kojima Hiroaki Imai 《Journal of Sol-Gel Science and Technology》1997,8(1-3):843-846
Al2O3 gels prepared by hydrolysis of Al-alkoxide were applied as catalysts for NO reduction by hydrocarbon. Xerogels were obtained by drying at 90°C, for 24 h, in air. Aerogels were prepared by supercritical drying of the wet gels in ethanol, using an autoclave. The catalyst activity of the gels for NO reduction with C3H6 as determined at 200–600°C in a fixed bed flow reactor. NO conversion to N2 was about 60% at 550°C with both catalysts, and at 600°C it was better with the aerogel than with the xerogel. The specific surface area of the xerogel, which was larger than that of the aerogel as-dried, decreased to 1/3 by annealing at >500°C. However, the surface area of the aerogel hardly changed by annealing at temperatures up to 700°C. The bimodal pore size distribution of the aerogel hardly changed by annealing, too. The microstructure of the aerogels is stable at high temperatures, and they are better catalysts at high temperatures. 相似文献
8.
Sung-Woo Hwang Hae-Hyun Jung Sang-Hoon Hyun Young-Soo Ahn 《Journal of Sol-Gel Science and Technology》2007,41(2):139-146
Effective ambient-drying techniques for synthesizing crack-free silica aerogel bulks from the industrial waterglass have been
developed. Silica wet gels were obtained from aqueous colloidal silica sols prepared by ion-exchange of waterglass solution
(4–10 wt% SiO2). Crack-free monolithic silica aerogel disks (diameter of 22 mm and thickness of 7 mm) were produced via solvent exchange/surface
modification of the wet gels using isopropanol/trimethylchlorosilane/n-Hexane solution, followed by ambient drying. The effects of the silica content in sol and the molar ratio of trimethylchlorosilane/pore
water on the morphology and property of final aerogel products were also investigated. The porosity, density, and specific
surface area of silica aerogels were in the range of 92–94%, 0.13–0.16 g/cm3, and ∼675 m2/g, respectively. The degree of springback during the ambient drying processing of modified silica gels was 94%. 相似文献
9.
László Péter Bakos Joshua Mensah Krisztina László Tamás Igricz Imre Miklós Szilágyi 《Journal of Thermal Analysis and Calorimetry》2018,134(2):933-939
Nitrogen-containing carbon aerogel was prepared from resorcinol–melamine–formaldehyde (R–M–F) polymer gel precursor. The polymer gel was supercritically dried with CO2, and the carbonization of the resulting polymer aerogel under nitrogen atmosphere at 900 °C yielded the carbon aerogel. The polymer and carbon aerogels were characterized with TG/DTA–MS, low-temperature nitrogen adsorption/desorption (??196 °C), FTIR, Raman, powder XRD and SEM–EDX techniques. The thermal decomposition of the polymer aerogel had two major steps. The first step was at 150 °C, where the unreacted monomers and the residual solvent were released, and the second one at 300 °C, where the species belonging to the polymer network decomposition could be detected. The pyrolytic conversion of the polymer aerogel was successful, as 0.89 at.% nitrogen was retained in the carbon matrix. The nitrogen-doped carbon aerogel was amorphous and possessed a hierarchical porous structure. It had a significant specific surface area (890 m2 g?1) and pore volume (4.7 cm3 g?1). TG/DTA–MS measurement revealed that during storage in ambient conditions surface functional groups formed, which were released upon annealing. 相似文献
10.
P. R. Aravind P. Shajesh P. Mukundan K. G. K. Warrier 《Journal of Sol-Gel Science and Technology》2009,52(3):328-334
Ambient pressure drying has been carried out for the synthesis of silica–titania aerogel monoliths. The prepared aerogels
show densities in the range 0.34–0.38 g/cm3. The surface area and pore volume of these mixed oxide aerogels are comparable to those of the supercritically dried ones.
The surface area for 5wt% titania aerogel has been found to be as high as 685 m2/g with a pore volume of 2.34 cm3/g and the 10wt% titania aerogel has a surface area of 620 m2/g with a pore volume of 2.36 cm3/g. Some gels were also made hydrophobic by a surface treatment with methyltrimethoxysilane and trimethylchlorosilane. The
surface modified aerogels possess high surface areas in the range of 540–640 m2/g, and are thermally stable in terms of retaining hydrophobicity up to a temperature of 520 °C. The pore size distribution
of the aerogels clearly indicates the preservation of the aerogel structure. High Resolution Transmission Electron microscopy
has been employed to characterise the aerogels and Fourier Transform infrared spectroscopy to study the effect of titania
addition to silica and the surface modification. X-ray diffraction patterns were recorded to verify the molecular homogeneity
of the aerogel. 相似文献
11.
Jyoti L. Gurav A. Venkateswara Rao Digambar Y. Nadargi 《Journal of Sol-Gel Science and Technology》2009,50(3):275-280
The experimental results on the study of thermal conductivity and effect of humidity on HMDZ modified TEOS based aerogels
dried at ambient pressure, are reported. Silica sol was prepared by keeping the MeOH/TEOS molar ratio, Acidic water (Oxalic
acid) and basic water (NH4OH) concentrations constant at 16.5, 0.001 and 1 M, respectively throughout the experiments and the HMDZ/TEOS molar ratio
(h) was varied from 0.34 to 2.1. Finally, the surface modified wet gels were dried at an ambient pressure. The thermal conductivity
of the aerogel samples was measured. Further, the humidity study was carried out in 80% humid surrounding at 30 °C temperature
over 80 days. The best quality aerogels in terms of low bulk density, thermal conductivity and durability (no moisture absorption)
with an only 2% of weight gain were obtained for TEOS: MeOH: Acidic H2O: Basic H2O: HMDZ molar ratio at 1:16.5:0.81:0.50:0.681, respectively. The thermal stability and hydrophobicity of the aerogel have
been confirmed with Thermo gravimetric and Differential Thermal (TG–DT) analyses and Fourier Transform Infrared Spectroscopy
(FTIR), respectively. Microstructural studies were carried out by Scanning Electron microscopy (SEM). 相似文献
12.
Effective adsorption of CO2 at low partial pressures is required for many technical processes, such as gas purification or CO2 removal in closed loop environmental control systems. Since the concentration of CO2 in such applications is rather low, a high adsorption capacity is a required property for the adsorbent. Silica aerogels possessing an open pore structure, a high porosity and a high surface area, have a great potential for utilisation as CO2 adsorbents. Nonetheless in order to reach high adsorption capacities, silica aerogels should be functionalised, for instance by amino functionalisation. In this work, two different functionalisation methods were applied for the generation of amino functionalised aerogels: co-condensation during the sol-gel process and post-treatment of the gel. The co-condensation functionalisation allows the introduction of up to 1.44 wt.% nitrogen into the aerogel structure with minor reductions in surface area, leading however only to minor increases in the adsorption capacity at low partial pressures. The post functionalisation of the gel causes a greater loss in surface area, but the CO2 adsorption capacity increases, due to the introduction of higher amounts of amino groups into the aerogel structure (up to 5.2 wt.% nitrogen). Respectively, 0.523 mmol CO2/g aerogel could be adsorbed at 250 Pa. This value is comparable with the adsorption capacity at this pressure of a standard commercially available adsorbent, Zeolite 13X. 相似文献
13.
Carlos Eduardo Barão Leandro Daniel de Paris João Henrique Dantas Matheus Mendonça Pereira Lucio Cardozo Filho Heizir Ferreira de Castro Gisella Maria Zanin Flavio Faria de Moraes Cleide Mara Faria Soares 《Applied biochemistry and biotechnology》2014,172(1):263-274
The use of lipases in industrial processes can result in products with high levels of purity and at the same time reduce pollutant generation and improve both selectivity and yields. In this work, lipase from Thermomyces lanuginosus was immobilized using two different techniques. The first involves the hydrolysis/polycondensation of a silica precursor (tetramethoxysilane (TMOS)) at neutral pH and ambient temperature, and the second one uses tetraethoxysilane (TEOS) as the silica precursor, involving the hydrolysis and polycondensation of the alkoxide in appropriate solvents. After immobilization, the enzymatic preparations were dried using the aerogel and xerogel techniques and then characterized in terms of their hydrolytic activities using a titrimetric method with olive oil and by the formation of 2-phenylethyl acetate in a transesterification reaction. The morphological properties of the materials were characterized using scanning electron microscopy, measurements of the surface area and pore size and volume, thermogravimetric analysis, and exploratory differential calorimetry. The results of the work indicate that the use of different silica precursors (TEOS or TMOS) and different drying techniques (aerogel or xerogel) can significantly affect the properties of the resulting biocatalyst. Drying with supercritical CO2 provided higher enzymatic activities and pore sizes and was therefore preferable to drying, using the xerogel technique. Thermogravimetric analysis and differential scanning calorimetry analyses revealed differences in behavior between the two biocatalyst preparations due to the compounds present. 相似文献
14.
V. Morales-Flórez A. Santos L. Esquivias 《Journal of Sol-Gel Science and Technology》2011,59(3):417-423
Supercritically dried composites have already been analysed and proposed as carbon dioxide sequesters. However, the economical
and energetic costs of the supercritical drying process had to be re-evaluated, and were eventually found not to enhance the
feasibility of the proposed route for CO2 mineral sequestration. Different composites series were synthesised with the only difference being the drying method. The
structures of the porous matrix were characterised as well as their ability to capture CO2. The first results showed that the xerogel matrix is as good a host as the aerogel one, and also avoids expensive procedures
such as supercritical drying for sample preparation without losing CO2 capture capacity and enhancing the efficiency of the whole carbon sequestration process. In this case, the sample preparation
was simplified as much as possible, with the aim of reducing energetic and economic costs. Although good carbonation efficiencies
were obtained with these cheap samples, the first results showed that previous high carbonation efficiencies could not be
repeated. 相似文献
15.
Samir Chakhari Mohamed Kadri Younes Alain Rives Abdelhamid Ghorbel 《Journal of Sol-Gel Science and Technology》2014,69(2):378-385
Zirconia doped by heteropolytungstic acid HPW have been synthesized by sol–gel method using two drying techniques of the solvent evacuation. Samples were analyzed with adsorption–desorption of N2 at 77 K, and the aerogel catalyst was found to exhibit a higher surface area and a higher average pore diameter compared to xerogel. XRD results show that aerogel develops ZrO2 tetragonal phase, whereas xerogel is amorphous. The thermal analysis studies show that the aerogel’s thermal stability is better than the xerogel one. The catalytic behavior of the aerogel and xerogel toward the nature of the isomerization products probably depends on the acidity and the presence of carbide species. This has been explained by XPS and isopropanol dehydration reaction. In fact, the deconvolution aerogel’s Cls bands reveals the presence of four carbon species assigned to C–C, C=O, C–O and carbide species. 相似文献
16.
Maria Giorgia Cutrufello Elisabetta Rombi Italo Ferino Danilo Loche Anna Corrias Maria Francesca Casula 《Journal of Sol-Gel Science and Technology》2011,60(3):324-332
Porous nanocomposites made out of nickel dispersed on silica or alumina matrices were prepared as prospective catalysts for
the nitroxidation of hydrocarbons in the form of aerogel or xerogel by adopting either a supercritical or a conventional gel
drying procedure. The structural and textural features of the materials were investigated by X-ray diffraction, transmission
electron microscopy and N2 physisorption and combined to the acid/base and reducibility data as deduced by adsorption microcalorimetry and temperature
programmed reduction (TPR) profiles. The alumina-based samples are made out of nanocrystalline nickel aluminate and are mesoporous,
although the aerogel has larger pore volumes and surface area than the xerogel. On the other hand, in the silica-based samples
nickel oxide nanocrystals are dispersed on amorphous silica, the size of the nanocrystals being around 5 nm in the microporous
xerogel and 14 nm in the mainly mesoporous aerogel. TPR data point out that the alumina-based samples have similar reducibility,
whereas significant differences were observed in the silica-supported composites, the NiO–SiO2 aerogel exhibiting improved reducibility at low temperature. The NO-catalyst interaction was monitored by temperature programmed
NO reaction coupled to mass spectrometry and preliminary tests on the use of the NiO–SiO2 xerogel and aerogel nanocomposites for the catalytic nitroxidation of 1-methyl-naphthalene to 1-naphthonitrile were obtained
in a fixed-bed continuous-flow reactor. The data indicate that the aerogel exhibits larger selectivity than the corresponding
xerogel, pointing out the importance of tuning the sol–gel parameters in the design of porous composite materials for catalytic
applications. 相似文献
17.
Microporous Silica Xerogel Membrane with High Selectivity and High Permeance for Carbon Dioxide Separation 总被引:1,自引:0,他引:1
The asymmetric microporous silica xerogel membrane was prepared by sol-gel method. A multi-layer (asymmetric) structure was obtained by silica xerogel layer on top of mesoporous silica layer (mean pore diameter of 4 nm), supported by porous alumina tube (mean pore diameter of 0.1 m). The ratio of the permeances, CO2/N2 for this membrane attained more than 60 at 298 K. This value is considerably higher than the theoretical Knudsen value (CO2/N2 = 0.8). The gas permeances of this membrane were about 5–10 times higher than the previous membrane. Application of the membrane to an enrichment of CO2 from air was also investigated. CO2 in the air (about 300 ppm) was concentrated to more than 1000 ppm by using the membrane at 298 K. 相似文献
18.
Keysar S. Cohen Y. Shagal S. Slobodiansky S. Grader G.S. 《Journal of Sol-Gel Science and Technology》1999,14(2):131-136
Alumina gels were synthesized by catalyzed gelation of aluminum sec-butoxide (ASB) via the Yoldas process. The gels were aged for up to 6 months and then supercritically dried (SCD) with CO2. The molar ratio of acid to ASB was in the range of 0.01–0.6. Viscosity measurements of the gels showed a shear thinning and plastic behavior with no response up to a limiting yield stress. The gel rheology obeys the Casson model. Analysis of the viscosity as a function of the acid to alkoxide molar ratio, showed that the average molecular weight of the gels is inversely proportional to the acid to alkoxide molar ratio.The viscosity of all the gels increased with aging time for a period of about 6 months reaching an asymptotic value after 1–2 weeks. The viscosity is shown to correlate with the microstructure of these nanomaterials during aging. Aging gives rise to a nearly constant surface area of 350 m2/g regardless of acid to alkoxide ratio in an aging period of about 6 months. 相似文献
19.
Parakkulam Ramaswamy Aravind Lorenz Ratke Matthias Kolbe Gian Domenico Soraru 《Journal of Sol-Gel Science and Technology》2013,67(3):592-600
In present work, we have prepared gels with various compositions of methyltrimethoxysilane—3-(2,3-epoxypropoxy) propyltrimethoxysilane (MTMS-GPTMS) using a two-step acid base sol–gel process. To make a comparative study between the two common drying routes, we prepared gels under supercritical and also under ambient conditions. The density of the supercritically dried hybrid aerogels lies between 0.18 and 0.31 gcm?3, while the density of the ambient dried ones ranges between 0.35 and 0.42 gcm?3. The surface area of MTMS-0.25 GPTMS aerogel dried under supercritical conditions, has been found to be 464 m2 g?1 with a pore volume and average pore diameter of 1.24 cm3 g?1 and 11 nm respectively. The same composition dried under ambient conditions is found to have similar properties i.e. a BET surface area of 439 m2 g?1, pore volume of 1.22 cm3 g?1 and average pore diameter of 11 nm. The aerogels were later pyrolyzed yielding silica/carbon composite aerogels. The pyrolized aerogels possessed a surface area as high as 207 m2 g?1 with a total pore volume of 0.98 cm3 g?1. The pyrolysed aerogels were also calcined to yield carbon free materials. 相似文献
20.
Summary: SAXS patterns of a Ca-alginate hydrogel and of the derived alcogel and aerogel have been recorded. All patterns correspond to disordered arrays of rod-like fibrils. The calculated size of the fibrils of the aerogel, 8 nm, is in good agreement with the results of scanning electron microscopy and N2 adsorption. The results suggest that ethanol exchange and CO2 supercritical drying do not affect the spatial organisation of the secondary structures of the gel and that characterisations of the aerogel can provide information on the organisation of the parent hydrogel. 相似文献