Preparation, Texture And Hydrogen Adsorption Properities Of Mesoporous Silica Aerogels And Microporous Activated Carbons

Porous materials possess several characteristics such as low density, high porosity and surface area, selective permeation for gases, and as a result, porous materials are widely used in many areas such as excellent catalysts and catalyst supports, adsorption and separation, assemblage of nanomaterials and biochemistry and so on. As important material for adsorption and separation, the textural properties of porous materials are the key factor for their application. In the present study, tow typical porous materials, i.e. mesoporous silica aerogels and microporous activated carbons are prepared, and the influence of preparation conditions on textural properties, the relationship between textural properties and hydrogen adsorption behavior are studied.Monolithic silica aerogels were prepared by means of sol-gel method followed by CO2 supercritical drying (SCD). By changing the cycle pressure of liquid CO2 which was used to replace ethanol in the autoclave before supercritical state of CO2 was obtained from 8.5 MPa to 4.5 MPa, a series of silica aerogels with different ratio of micropore, mesopore and macropore volume, having a BET surface area of 678-949 m2/g were obtained. N2 adsorption isotherm was used to characterize the porous texture of the silica aerogels as prepared. Surface energy distributions (SEDs) were calculated by Density Functional Theory (DFT) method deduced from N2 adsorption data. Infrared spectroscopy (IR) was used to determine the surface groups of these samples. All samples exhibited multimodal pore size distributions (PSDs) with maxima in the micropore and meso/macropore regions. The PSD of CO2-SCD silica aerogels was narrowed by increasing the cycle pressure of liquid CO2, and the position of maximum also shifted from large pore width region to small pore width region. Maximum in pore volume was attained at 6.5 MPa for mesopores with little decrease of surface area. SEDs depended not only on the pore size but also on the PSDs and surface properties. Different surface groups lead to significant change of SEDs between EtOH-SCD and CO2-SCD silica aerogels. The relationship between SEDs and pore size was introduced.The behavior of liquid adsorption of methylene blue (MB) is affected by temperature, contact time and porous structure of silica aerogels. The equilibrium adsorption amount may be dominated by the textural properties such as surface area and pore volume at low temperature while the adsorption temperature may play the major role in the case of high adsorption temperature. These materials exhibit significant difference in dye adsorption behavior depending on the different porous structure. The MB adsorption capacity and adsorption rate for sample prepared at 4.5 MPa is higher than samples prepared at 6.5 and 8.5 MPa, and the silica aerogel prepared at 8.5 MPa possesses the lowest adsorption capacity and adsorption rate. All samples are best fitted by Langmuir equation, but the fitting of Freundlich equation shows that the adsorption is also favorable. The adsorption on macroporous silica aerogel prepared at 4.5 MPa is best described by the pseudo-first-order model, while mesoporous silica aerogels prepared at 6.5 and 8.5 MPa follow the pseudo-second-order kinetics model. Surface energy distribution obtained by using DFT method indicated that the surface energy distributions of all samples are very wide, and the surface heterogeneity not only affects the dye adsorption capacity but also the adsorption rate, i.e., both the adsorption capacity and adsorption rate are higher for the sample with more heterogeneous surface.The comparison of activated conditions shows that the surface areas of activated carbons derived from hemp stem, bamboo, coconut shell and walnut shell all increase firstly then decrease with the increase of activation temperature, activation time and the amount of KOH. The best activation temperature, activation time and weight ratio of KOH for hemp stem are 800℃,3.5 h and 4.5, respectively, and results in the highest surface area of 3241 m2/g. The optimum activation conditions are 800℃,3.5 h and KOH weight ratio of 5 for other raw materials including bamboo, coconut shell and walnut shell, and the highest surface areas are 3181,2006和1865 m2/g, respectively.The activated carbons prepared by all raw materials in the current study are microporous materials with little mesopores and macropores. Bamboo and hemp were carbonized to study their microstructure, especially along the longitudinal direction, correlated with their performance on strength, electric and textural properties. KOH activation activated carbons were also prepared to understand their adsorption behavior thoroughly. SEM, XRD and N2 adsorption were performed to investigate the microstructure and textual features. Compressive test, electrical resistivity, density, shrinkage ratio and carbonization yield were also measured. Compared with hemp, bamboo has an attractive structure of dense fiber bundles with transversal diaphragm exists in the microscopic pore channels, and this node-shaped transversal diaphragm reinforced microstructure along the lengthwise direction acts as a reinforcing rib. The charcoals are amorphous carbons with a small amount of graphite crystallite, and the higher graphitization level of bamboo charcoal is a factor of its lower electrical resistivity. The microscopic diaphragm not only prevents shrinkage during carbonization, but also increases the electric flowing path to lower electrical resistivity for bamboo.High surface area microporous activated carbons are prepared from chemical activation of hemp stem with KOH for the first time. A series of activated carbons are prepared at different activation temperatures and activation times. Ethanol was also used as solvent of KOH to synthesis hemp based activated carbons. Ball milling mixture method was performed to prepare activated carbon for comparison. When the activation temperature increases beyond 700℃, the surface chemical properties tend to be stable. The oxidation treatment by nitric acid not only leads to the narrow PSD of activated carbon but also the change of chemical properties of carbon surface and increases the amount of oxygen functional groups, whereas using ethanol to resolve KOH leads to a decrease of the amount of oxygen functional groups. The PSDs are narrowed by using ethanol as solvent of KOH and ball milling methods, results in a decrease of surface area.Nitrogen-doped activated carbons derived from hemp stem are synthesized by using melamine as nitrogen source, and several samples are obtained by changing the weight ratio of melamine. The surface area of as prepared nitrogen-doped activated carbons ranges from 1928 to 3419 m2/g, and reaches maximum when the weight ratio of melamine is 1, then decreases with further increase of melamine amount. The PSDs of activated carbons are narrowed by the increase of melamine amount first, and then widened by the further increase of melamine amount, attributing to the reticle of melamine pyrolysis products NH3 on micropores. When the impregnated amount of melamine is beyond 1 (weight ratio), the surface area decreases due to the over reticle of pores.Silica xerogel processes the highest hydrogen adsorption capacity of 0.73wt% at-196℃,1.0 bar among the gel materials. Hydrogen adsorption capacities for silica aerogels are obtained in the range of 0.46wt%～0.7wt%. The poor hydrogen adsorption capacity of silica gels are attributed to the wide pore size distributions, large pore size and lack of suitable micropores for hydrogen storage. Hydrogen uptakes of activated carbons are linear function of porosities such as specific surface area, micropore surface area, total pore volume, and micropore volume. At-196℃, the hydrogen adsorption mainly takes place in micropores but not absolutely, mesopores in the range 2-5 nm also make important contribution. The gas adsorption amount dominated by ultramicropores at lower pressure, then lager micropores and mesopores make major contributions to the adsorption capacity at higher pressure. The maximum hydrogen uptakes were measured to be 3.28wt% at-196℃,1.0 bar among these materials. The oxidation treatment by nitric acid results in the decrease of hydrogen storage capacity due to the increase of oxygen groups, whereas the hydrogen adsorption is enhanced due to the decrease of oxygen groups of ethanol solvent samples. The adsorption of hydrogen is a stepwise process with the increase of hydrogen pressure, and the introduction of nitrogen makes little contribution to the adsorption of hydrogen at low temperature.