Preparation Of Porous Materials With Two New Technologies Of Supercritical Drying

Porous materials are widely used in catalysis, environmental protection, energy and many other fields because of their good adsorption performance, superior catalytic performance and good stability and durability. During the synthesis of porous materials, drying has a significant impact on the structure and properties of the final products. Compared with conventional drying methods (such as ambient pressure drying, freeze drying, spray drying, etc), supercritical drying process can avoid the crossing of vapor-liquid phase boundaries, then the pore structure of materials can be well preserved and the porous materials with high performance can be obtained. In this thesis, based on the supercritical gas drying and SCF-assisted spray-drying, two new drying technologies were developed and applied to the preparation of superior matting agent (silica), aluminum hydroxide and aluminum with high pore volumes.A self-circulation supercritical drying technology was proposed and established; using silica hydrogel as the raw material, silica matting agents were prepared with the technology. The effects of heating temperature, cooling temperature, the ratio of ethanol to water, and organic solvent used on the drying process and the properties of silica were investigated. Results show that the drying rate increases with the rising of heating temperature, and increases first and then decreases with the rising of cooling temperature; the bulk density and DBP absorption of the resulted products are better than ambient pressure drying, and close to the product by supercritical mixture solvent drying. As the ratio of ethanol to water increases, the drying rate increases and both of the bulk density and DBP absorption of the resulting products become better, which indicates that higher ratio of ethanol to water is better for the products. When acetone is used, the resulted products has higher drying rate, lower bulk density and greater DBP absorption comparing with the products using ethanol. But the gap is not big. Moreover, the resulted product is amorphous. Silica with the BET specific surface area of 234 m2/g, pore volume of 1.86 cm3/g, average particle diameter of 16.60μm, and the extinction ratio of 45.0% could be obtained under optimum conditions.A continuous supercritical drying technology was proposed and constructed; using silica hydrogel as the raw material, silica matting agents were prepared by the technoogy. The effects of pre-expansion pressure, drying pressure, drying temperature, the ratio of ethanol to water, solid content, and flow rate on the properties of silica were investigated. Results show that the pore volumes of silica decreases with the increase of pre-expansion pressure and increases with the rising of drying pressure. The pore structure of the products can be better protected with a smaller gap between the pre-expansion pressure and drying pressure. Moreover, the pore volumes increases with the increase of the ratio of ethanol to water and the decrease of flow rate. However, in the investigated range, the drying temperature and solid contents show little effect on the BET specific surface area, pore volumes, and the average pore size of the as-preapred silica. The resulted product is amorphous. Silica with BET specific surface area of 243 m2/g, pore volume of 2.38cm3/g, average particle diameter of 11.20μm, and the extinction ratio of 47.4%(close to OK-520) could be obtained under optimum conditions.Using aluminum nitrate as the aluminum source, aluminum hydroxide and alumina with high pore volumes were prepared by the combination of precipitation and the continuous supercritical drying. The effects of pre-expansion pressure, drying pressure, and flow rate on the properties of aluminum hydroxide were investigated. Results show that the pore volumes and average pore diameter of aluminum hydroxide decreases with the increase of pre-expansion pressure and increases with the rising of drying pressure. The pore structure of aluminum hydroxide can be better protected with a smaller gap between the pre-expansion pressure and the drying pressure. Moreover, the pore volumes and average pore diameter of the prepared aluminum hydroxide decreases obviously with the increase of the flow rate. Aluminum hydroxide with BET specific surface area of 653 m2/g, pore volume of 3.61 cm3/g, and average pore diameter of 22.1nm could be obtained under optimum conditions. XRD analysis shows that all the resulted products are boehmite. After calcination of the boehmite, amorphous Al2O3 at 500℃ and crystal γ-Al2O3 at 800℃ were obtained respectively. Both of the BET specific surface area and the pore volumes decrease after the calcination. Compared with those reported in literature, the pore volumes of obtained amorphous Al2O3 and crystal γ-Al2O3 (2.86 and 1.84cm3/g, respectively) are quite high. Thus they have great potential for applying in the fields of catalyst and adsorbent.