Preparation of silica aerogel from oil shale ash by fluidized bed drying

The silica aerogel with high specific surface area and large pore volume was successfully synthesized using oil shale ash (OSA) via ambient pressure drying. The oil shale ash was burned and leached by sulfuric acid solution, and then was extracted using sodium hydroxide solution to produce a sodium silicate solution. The solution was neutralized with sulfuric acid solution to form a silica gel. After washing with water, the solvent exchange with n-hexane, and the surface modification with hexamethyldisilazane (HMDZ), the aged gel was dried by fluidization technique and also using a furnace to yield silica aerogels. The physical and textural properties of the resultant silica aerogels were investigated and discussed. The results have been compared with silica aerogel powders dried in a furnace. From the results, it is clear that the properties of silica powders obtained in fluidized bed are superior to that of powders dried in the furnace. Using fluidization technique, it could produce silica aerogel powders with low tapping density of 0.0775 g/cm³, high specific surface area (789 m²/g) and cumulative pore volume of 2.77 cm³/g.     

Synthesis of ultrafine silica powders based on oil shale ash by fluidized bed drying of wet-gel slurry

Ultrafine silica powders were synthesized using oil shale ash which is a solid by-product of oil shale processing. Fluidization technique was employed for the drying of wet-gel slurry at an ambient pressure. Box-Behnken statistical design was used to optimize the factors affecting the extraction efficiency of the silica such as concentration of sodium hydroxide, reaction time and reaction temperature, and to determine the optimum conditions for the extraction process. The result of statistical design shows that the extraction efficiency was in an agreement with the generated model and the experimental results. It is observed that the extraction efficiency of silica was increased by increasing the concentration of sodium hydroxide, the reaction time and the reaction temperature. Further, the ultrafine silica powders were characterized by transmission electron microscopy (TEM), X-ray fluorescence spectroscopy (XRF) and nitrogen adsorption (using BET equation). The BET surface area of the ultrafine silica powders obtained in fluidized bed is comparatively higher (652 m²/g) than that of silica powders dried in the furnace (385 m²/g). It is clear that the properties of silica powders obtained in fluidized bed are superior to that of powders dried in the furnace.     

Influence of nanoparticle surface chemistry and size on supercritical carbon dioxide processed nanocomposite foam morphology

Creating polymer foams with controlled pore size and pore density is an important part of controlling foam properties. The addition of nanoparticles has been shown to cause heterogeneous nucleation and can be used to reduce pore size. In the current study, the effects of filler size and filler surface chemistry on pore nucleation in silica/PMMA nanocomposites are investigated. It was found that as the nanofiller size decreased, the pore density increased by a factor of 2-3 decades compared to that of unfilled PMMA (pore cell densities above 10¹² cells/cm³ were obtained). In addition, fluorination of the silica nanoparticle surface led to decreased pore size without changing the degree of silica aggregation and overall density. By monitoring the pore density as a function of pressure, a qualitative comparison was obtained that showed that fluorination of the nanoparticle reduced the critical free energy of nucleation.     

Fabrication of bimodal mesoporous carbons from petroleum pitch by a one-step nanocasting method

A one-step nanocasting method to prepare a bimodal mesoporous carbon from a highly hydrophobic carbon precursor, i.e., petroleum pitch, has been successfully developed by adopting tetrahydrofuran and hydrofluoric acid as solvent and catalyst, respectively, for the gelation reaction of tetraethyl orthosilicate and water. Experimental results show that the introduction of proper amounts of petroleum pitch does not hamper this gelation reaction, thus forming a uniform silica/carbon composite. It was found that the as-prepared nanoporous carbon has a three-dimensional 3.4 nm-sized wormholelike mesoporous network with well-distributed 17.1 nm-sized particlelike mesopores. Such a bimodal mesoporous carbon has a high Brunauer-Emmett-Teller surface area (782 m² g⁻¹) and a very large total pore volume (3.0 cm³ g⁻¹).     

Effects of Solvent Exchange Period and Heat Treatment on Physical and Chemical Properties of Rice Husk Derived Silica Aerogels

In this study, hydrophobic silica aerogels were synthesized from rice husk ash-derived sodium silicate through sol-gel processing, solvent exchange, surface modification and ambient pressure drying. By volume, 10% of trimethylchlorosilane (TMCS) in 90% of n-hexane was used as a hydrophobic solution in the surface modification process. The physical and chemical properties of silica aerogels were characterized by density and porosity measurements, scanning electron microscopy (SEM), Fourier transforms infrared (FTIR) spectroscopy, Brunauer–Emmett–Teller theory (BET) and dynamic scanning calorimetry (DSC). The hydrogels prepared were in the form of 2.5 ± 0.5 mm beads and then converted into alcogels through solvent exchange with ethanol for repetition of 3, 6 and 9 days. It is found that the optimal quality of silica aerogels with the BET surface area as high as 668.82 m²/g was obtained from the alcogels of the solvent exchange period of 9 days. Depending on the size of the gel’s block, a longer solvent exchange period will ensure adequate removal of pore water. Post heat treatment on silica aerogels obtained from the 9 days of solvent exchange at 200, 300 and 400 °C for 2 h results in slight decreased of aerogel’s density from 0.048 g/cm³ to 0.039 g/cm³ and the hydrophobicity of the aerogels is decreased above 380 °C as confirmed by DSC analysis.

     

Obtaining of silica nanopowders from natural hydrothermal solutions

Obtaining of nanodispersed silica powders from natural hydrothermal solutions is described. Hydrothermal solutions contain colloid silica forming as a result of the polycondensation of the molecules of orthosilicic acid. Via ultrafiltration with a membrane concentration of hydrothermal solutions, silica sols with SiO2 contents up to 600 g/dm³ (43.0 wt %) and particle radii of 29–135 nm are obtained. The silica powders with the specific surface area of 110–400 m²/g, average pore diameter of 3–10 nm, and pore volume of 0.2–0.3 cm³/g are obtained via the cryochemical vacuum-sublimation drying of sols with the use of liquid nitrogen. The particle size in the powders is in the range from 10 to 100 nm.     

Removal of dibenzothiophenes in kerosene by adsorption on rice husk activated carbon

The capacity of rice husk activated carbon (RHAC) to adsorb refractory sulfur compounds of dibenzothiophenes (DBTs) from commercial kerosene was evaluated in terms of their textural and chemical characteristics. Rice husk activated at 850 °C for 1 h showed an acceptable adsorption capacity for DBTs, despite a much lower specific surface area (473 m²/g) and total pore volume (0.267 cm³/g), when compared to micro-porous activated carbon fiber with a large specific surface area (2336 m²/g) and total pore volume (1.052 cm³/g). The volumes of ultramicropores acting as DBTs adsorption sites, and of mesopores leading DBTs into the ultramicropores were closely related to the DBTs adsorption capacity of the RHACs.     

Preparation of monolith SiC aerogel with high surface area and large pore volume and the structural evolution during the preparation

Resorcinol–formaldehyde/silica composite (RF/SiO₂) aerogel was synthesized by sol–gel process followed by supercritical drying (SCD). Monolithic SiC aerogel was obtained from RF/SiO₂ aerogel after carbothermal reduction. The evolution of physical property, crystal structure, morphology and pore structure from RF/SiO₂ to SiC aerogel was investigated by different methods, such as X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and N₂ adsorption/desorption. The as-synthesized SiC aerogel presented typical mesoporous structure and possessed high porosity (91.8%), high surface area (328 m²/g) and large pore volume (2.28 cm³/g). Carbothermal reduction mechanism was also discussed based on the experiment and characterization results.     

Physical and surface characteristics of the mechanically alloyed SiBCN powder

A powder mixture of cubic silicon, hexagonal boron nitride and graphite, with the molar ratio of Si:BN:C=2:1:3, was high-energy ball milled for 40 h, under argon atmosphere. The physical and surface characteristics, the microstructures and the behavior on heating of the as-milled SiBCN powder were carefully studied by SEM, nitrogen adsorption–desorption isotherms, XPS, FT-IR, XRD, TEM and thermogravimetry-differential thermal analysis-mass spectrometry-infrared spectroscopy (TG–DTA-MS-IR). Results show that the as-milled powder is amorphous and mainly consists of near-spherical agglomerates, 6.6±5.3 μm in size deriving from the hard agglomeration of nano-primary particles. The specific surface area, the specific pore volume and the average pore diameter of the powder are 24.5 m²/g, 0.136207 cm³/g and 20.3 nm, respectively. The as-milled powder adsorbs water vapor, CO and CO₂, and it is easy to oxidize. When heated in helium atmosphere, the powder desorbs water vapor, CO and CO₂ at lower temperature, and rapidly degasifies CO and CO₂ at temperatures approximately between 1350 °C and 1500 °C.     

Influence of aging conditions on textural properties of water-glass-based silica aerogels prepared at ambient pressure

The experimental results of aging time and temperature on the textural properties of water-glass (sodium silicate)-based silica aerogels are reported and discussed. Aging of the hydrogel for different times and temperatures led to an ability to increase the stiffness and strength of the networks. These improvements enabled the gel to withstand ambient pressure drying (APD) and, consequently, preserve the highly porous silica network without collapse. The pore size and volume increased with increasing aging temperature and time, while the specific surface area decreased. Monolithic aerogels with extremely low bulk density (～0.069 g/cm³), high specific surface area (820 m²g^?1), large cumulative pore volume (3.8 cm³g^?1), and high porosity (～96%) were obtained by aging at 60 °C for 18 hours. Therefore, easy synthesis of monolithic silica aerogels at ambient pressure is achievable using a relatively inexpensive silica precursor (sodium silicate).     

Rapid synthesis of homogeneous titania-silica composite with high-BET surface area

We rapidly synthesized a homogeneous titania-silica composite with properties desired by the paper and paint industries by the sol-gel method in a controlled manner. The composite was synthesized by impregnating TiOCl₂ (titania precursor) into preformed silica networks (SiO₂ trimers). The first step before the formation of high molecular weight polymers in this rapid, versatile, and reproducible method involves the generation of trimers of SiO₂ after the rapid condensation of silicic acid monomers. The latter were formed as a product of the reaction of aqueous sodium silicate solution (SiO₂/Na₂O = 3.24) with 2 N HCl. In the second step, TiOCl₂ was added to the SiO₂ trimers at 50 °C. The structure of the composite was characterized by FE-SEM, EDS, TEM, XRD, FTIR, and nitrogen physisorption studies. The results demonstrated the homogeneous incorporation of titania into silica, which is normally difficult to perform because of the significant differences between the hydrolysis rates of the precursors. The maximum BET surface area, average diameter of the pores, and the maximum pore volume obtained were 739 m²/g, 27.4 Å, and 0.29 cm³/g respectively. The composite has superior oil absorption (240 ml/100 g) compared to that of the conventional pure TiO₂ (100 ml/100 g) filler. It also shows significant photocatalytic ability. The materials prepared via the proposed method are potential candidates for large scale commercial production.     

Highly mesoporous carbonaceous material of activated carbon beads for electric double layer capacitor

The activated carbon beads (ACB) are prepared by a new preparation method, which is proposed by mixing the coal tar pitch and fumed silica powder at a certain weight ratio and activation by KOH at different weight ratios and different temperatures. The BET surface area, pore volume and average pore size are obtained based on the nitrogen adsorption isotherms at 77 K by using ASAP 2010 apparatus. The results show that our samples have much high specific surface area (SSA) of 3537 m² g⁻¹and high pore volume value of 3.05 cm³ g⁻¹. The percentage of mesopore volume increases with the weight ratio of KOH/ACB ranging from 4% to 72%. The electrochemical double layer capacitors (EDLCs) are assembled with resultant carbon electrode and electrolyte of 1 mol L⁻¹ Et₄NBF₄/PC. The specific capacitance of the ACB sample could be as high as 191.7 F g⁻¹ by constant current charge/discharge technique, indicating that the ACB presents good characteristics prepared by the method proposed in this work. The investigation of influence of carbon porosity structure on capacitance indicates that the SSA plays an important role on the capacitance and all the pore sizes of less than 1 nm, from 1 to 2 nm and larger than 2 nm contribute to the capacitance. Mesopore structure is beneficial for the performance at high current density.     

Preparation of polystyrene-based activated carbon spheres with high surface area and their adsorption to dibenzothiophene

Polystyrene-based activated carbon spheres (PACS_K) with high surface area were prepared through KOH activation. Effects of the carbonization temperature and the ratio of KOH to carbon spheres (CS) on the textural structure, hardness and yield of the resultant PACS_K were studied, and their adsorption to dibenzothiophene (DBT) were investigated. The as-prepared PACS_K exhibited a high surface area (up to 2022 m²/g), large total pore volume (≥ 0.78 cm³/g), superior mechanical hardness and high adsorption capacity (ca. 153 mg/g). With the increase of the KOH/CS ratio from 2:1 to 4:1, the surface area, total pore volume, volume of micropores, and volume of mesopores, increase, whereas the volume of small-micropores (< 0.8 nm) decreases from 0.36 to 0.31 cm³/g. The adsorption capacity has a good linear correlation with the volume of small-micropores rather than the surface area. In addition, the large quantity of acidic oxygen-containing groups of PACS_K may also be responsible for their higher adsorption capacity and selectivity of DBT. The PACS_K saturated by DBT can be regenerated by a washing process in a shaking bath or using ultrasonic with toluene at 80 °C.     

The importance of surface hydration and particle shape on the rheological property of silica-based suspensions

Two types of ball milling methods, wet planetary ball milling and dry tumbling ball milling, were used to grind fused silica powders for the preparation of silica-based suspensions in this experiment. The effect of surface hydration and particle shape caused by the two milling methods on the slurry rheology was investigated. The results showed that, with similar particle size distribution, the viscosity of the suspensions prepared from the powders milled by wet ball milling ranged from 275 mPa s to 311 mPa s within 20 min and the suspension exhibited a continuous shear thickening behaviour whereas the viscosity of the suspensions prepared from the powders milled by dry ball milling ranged from 69 mPa s to 74 mPa s and the suspension exhibited a shear thinning behaviour first, and then exhibited a slightly shear thickening behaviour. The reasons were attributed to the differences in surface hydration and particle shape. It was postulated that the two factors affected the slurry rheology through the modification of particle interactions during the flow of high concentration suspensions.     

Synthesis of high surface area mesoporous silica SBA-15 for hydrogen storage application

In the present report, mesoporous silica SBA-15 was synthesized by different methods such as room temperature aging, hydrothermal, and sonication-mediated hydrothermal methods. The effect of different time and temperature on the synthesis method was investigated by conventional characterization techniques. The powders synthesized by different methods showed different properties, mainly in morphology and pore volume. In comparison to others, the powder synthesized by the hydrothermal method at 100°C for 48 hours showed exceptionally high surface area of 3274 m²/g. To date, as per our knowledge, no such value was reported in literature. Finally, the powders were characterized by H₂ storage capacity by the adsorption-desorption method using 99.999% H₂. The hydrogen adsorption capacity of the same powder was observed at 6.02 wt%. Also, this value seems to be the highest H₂ adsorption capacity in comparison to other powders described in literature.     

常压干燥法制备疏水性SiO2气凝胶

利用正硅酸乙酯做先躯体,盐酸与氨水做催化剂,通过二步法制备了二氧化硅气凝胶,并利用三甲基氯硅烷和正己烷做表面改性剂采用共沸法逐级对湿凝胶进行表面改性.测试分析了气凝胶的密度、表观、疏水性、比表面积、孔径分布和热稳定性.结果表明:气凝胶的物理特性及其疏水性受到三甲基氯硅烷与正己烷的体积比(ψ)的影响,其密度随着ψ增大而减小,当ψ=3%时,气凝胶的疏水性最好;所制得的SiO2在350℃仍具有良好的热稳定性,并且具有高比表面积(838.6 m2/g)和孔体积(2.10cm3/g).     

Synthesis and characterisation of nanoporous carbide-derived carbon by chlorination of vanadium carbide

Nanoporous carbide-derived carbon (CDC) was synthesised from vanadium carbide (VC) powder via gas phase chlorination in the temperature range from 500 to 1100 °C. The XRD analysis of nanoporous carbon powder samples was carried out to investigate the structural changes (graphitisation) of nanoporous carbons synthesised. The first-order Raman spectra showed the absorption peak at ∼1582 cm⁻¹ and the disorder-induced (D) peak at ∼1345 cm⁻¹. The low-temperature N₂ adsorption experiments were performed and a specific surface area up to 1305 m² g⁻¹ and total pore volume up to 0.66 cm³ g⁻¹ were obtained.     

Polymer electrolyte fuel cells employing electrodes with gas-diffusion layers of mesoporous carbon derived from a sol-gel route

Sol-gel derived mesoporous carbon (MC) for the preparation of gas-diffusion layer (GDL) and its ameliorating effect on the performance of polymer electrolyte fuel cells (PEFCs) is reported. MC with a specific surface area of 370 m²/g, pore diameter of 6.7 nm and pore volume of 0.45 cm³/g has been synthesized by co-assembly of a tri-block copolymer, namely pluronic-F127, as a structure directing agent, and a mixture of phloroglucinol and formaldehyde as carbon precursor. X-ray diffraction and transmission electron microscopy have been employed to examine the structural properties of the MC. Surface morphology of the GDL comprising MC has been studied by scanning electron microscopy. A peak power density of 0.53 W/cm² at a load current-density of 1.1 A/cm² is achieved for the PEFC employing electrodes with GDL of MC compared to the peak power density of 0.47 W/cm² at a load current-density of 0.93 A/cm² for the PEFC employing electrodes with GDL of commercial Vulcan XC-72R carbon, while operating at 70 ^oC with H₂ and air feeds at atmospheric pressure.     

Preparation of activated carbon from Jatropha hull with microwave heating: Optimization using response surface methodology

Preparation of activated carbon has been attempted using steam as the activating agent by microwave heating from Jatropha hull. The response surface methodology (RSM) technique is utilized to optimize the process conditions. The influences of the three major parameters, activation temperature, activation time and steam flow rate on the properties of activated carbon are investigated using analysis of variance (ANOVA), to identify the significant parameters. The optimum conditions for the preparation of activated carbon has been identified to be an activation temperature of 900 °C, activation time of 19 min and steam flow rate of 5 g/min. The optimum conditions resulted in an activated carbon with an iodine number of 988 mg/g and a yield of 16.56% respectively, while the BET surface area evaluated using nitrogen adsorption isotherm correspond to 1350 m²/g, with the pore volume of 1.07 cm³/g. The activated carbon is hetero porous with the micropore volume contributing to 40.8%.     

A green process for the synthesis of controllable mesoporous silica materials

The synthesis, characterization, and application of mesoporous silicas have attracted a lot of attention for over two decades, which stems from their fascinating structures, formation mechanisms and prospects of their applications. Various methods have been developed for the synthesis of these silicas with a tunable pore diameter and a narrow pore size distribution. In this paper, mesoporous silica materials with controllable pore diameters (3-9 nm), narrow pore size distributions, high surface area (>700 m² g⁻¹) and pore volume (>1 cm³ g⁻¹) were prepared by a green template, amphiphilic dendritic polyamidoamine. The resulting silica materials were characterized by ¹H, ¹³C NMR spectroscopy; thermogravimetic analysis; nitrogen adsorption; transmission electron microscope. It was shown that the template could be completely removed and recycled from the silica in an environmentally friendly way by means of a simple water extraction. Furthermore, it was shown that the pore diameter of these materials could be controlled by dendritic polyamidoamine with different generations and functional groups. Meanwhile, the porous framework showed strong thermal stability. Thus, a new environmentally friendly pathway for the controllable synthesis of this fascinating silicas has been proposed.