A porous material is one of the commonly used catalyst supports. Silica gel is also a commonly used desiccant and adsorbent in industry. The chemical composition of silica gel is SiO2 · xH2O, which belongs to amorphous form. It is composed of Si-O tetrahedrons connected to form a skeleton structure, and the space in the skeleton is the pore of silica gel. Structural water is connected with silicon atoms in the form of hydroxyl groups and covers the surface of silica gel. The surface of silica gel is weakly acidic, and it also catalyzes some reactions, such as the reaction of ethylene oxide to form acetaldehyde, the dehydration of formic acid and alcohol, the production of carbon disulfide from methane and sulfur or hydrogen sulfide, etc. Copper chloride silica gel catalyst is used in the hydrolysis of chlorobenzene to phenol. At this time, silica gel not only has the function of support, but also provides activity. The content of sodium oxide in commercial silica gel will affect the performance of silica gel in some occasions. Silica gel generally begins to appear obvious sintering phenomenon at 600 ~ 700 ℃, the pore structure is damaged and the specific surface area decreases, especially in the presence of water vapor.
Silica gel industry is based on water glass (sodium silicate) as raw material, hydrolyzed in acidic medium to form gel, and then through aging, washing, drying and other processes to make silica gel, according to the different water content, is translucent or white solid. The commercial commodities include irregular granular, spherical and microsphere silica gel, which has good wear resistance and is often used as catalyst carrier in fluidized bed operation.
When used as catalyst carrier, silica gel is usually immersed in the solution containing catalytic active components to make the solution absorbed in the pores of silica gel, and the active components are distributed on the surface of silica gel through drying, activation and other procedures. The pore structure of silica gel has an important influence on the properties of supported catalysts, such as pore volume and pore size distribution of silica gel. Conventionally, silica gel with an average pore diameter of less than 15 ~ 20 μ is called fine pore silica gel; The average pore diameter is greater than 40 ~ 50 Ω, which is called coarse pore silica gel. Silica gel with fine pore structure has high specific surface area, which is conducive to the dispersion of catalytic active components. However, the fine pore structure is not conducive to the diffusion of reactant molecules, that is, it is not easy to touch the catalytic active components deep in the pores, so it will reduce the utilization rate of the inner surface of the catalyst, and the product molecules generated in the pores are not easy to escape out of the pores, which is easy to cause deep side reactions. The pore structure of silica gel is related to the manufacturing method and conditions, such as gelation, aging, pH, temperature, time, etc. The commercially available silica gel can be reamed by reaming treatment. The common method is to put it in a autoclave and add water or salt containing aqueous solution (such as sodium carbonate and sodium acetate) for hot pressing treatment. For example, at 320 ℃ and 10MPa, the silica gel with specific surface and average pore diameter of 135m2 / g and 123 Å can be changed into 26.9m2/g and 508 Å respectively.
In addition to solid silica gel, liquid silica sol is also an important carrier material. In industry, ion exchange method is usually used to remove sodium from water glass to prepare silica sol. It is a translucent milky white liquid. The content of silica in commercial commodities is generally 20% ~ 30%, and the higher is up to 50%. The diameter of rubber particles is 100 ~ 200 Å, and some commodities are up to 1300 Å. High concentration silica sol usually maintains its stability with the help of a small amount of sodium oxide. After drying, the silica sol becomes a porous solid. For example, when producing acrylonitrile (phospho Mo bismuth oxy) / silica catalyst from acrylonitrile oxidation, the solution containing active components is mixed with silica sol and microsphere catalyst is prepared by spray drying.