Silica gel

Silica gel, also known as silicic acid gel or silica gel, has the chemical formula mSiO2·nH2O, a white transparent solid substance. It is an amorphous form. Silica gel is a highly active, porous material with a large internal surface area and an internal surface area of more than 450 m2 per g of silica gel. Therefore, it has a strong adsorption property to liquid or gas. Silica gel is insoluble in water and is also insoluble in solvents other than hydrofluoric acid and strong bases. Commercially available solid silica gels are available in both spherical and irregular shapes. According to the pore size, it can be divided into fine pore silica gel (average pore diameter of less than 1.5 to 2.0 nm) and coarse pore silica gel (average pore diameter of more than 4.0 to 5.0 nm). Another color-changing silica gel contains cobalt chloride. When anhydrous, the cobalt chloride is blue. After water absorption, the hydrated cobalt chloride is pink, and it is still blue after drying. According to the change of color when the cobalt chloride absorbs water and dehydrates, the moisture absorption of the silica gel can be indicated. Pink silica gel no longer has the ability to absorb moisture, but after heating and dehydration, the color can be blue again and the moisture absorption capacity can be restored. Silica gel can be prepared by water glass and sulfuric acid, and gelled, washed, desalted, ammoniated, dried and screened. The reaction formula is as follows: nNa2O·mSiO2+nH2SO4→ mSiO2·nH2O+nNa2SO4. General industrial silica gel, mechanical strength 85 ~ 95%, water content less than 2%, moisture absorption rate of 25 ~ 30%. Silica gel is mainly used for gas drying or absorption, liquid dehydration and chromatographic analysis, as a catalyst or catalyst carrier, and as a sizing agent in the textile industry. Fine pore silica gel is mainly used for drying and purifying gases, dehydration and refining of organic products, and can also be used as a carrier for catalysts. Crude silica gel is mainly used to treat high molecular protein in beer, as well as drying of gas and liquid, defogging and dehumidification of gas under high humidity, as catalyst or catalyst carrier in petrochemical industry. Color-changing silica gel is mainly used to indicate the relative humidity of closed packaging, precision instruments, space inside the instrument, and moisture-proof maintenance for precision instruments. Silica gel chromatography for chromatography is a porous substance, which can be represented by the general formula SiO2·xH2O. The molecule has a crosslinked structure of siloxane (—Si—O—Si), and at the same time, there are many silanol groups on the surface of the particles. The strength of silica gel adsorption is related to the content of silanol groups. The silanol group can adsorb moisture by the formation of a hydrogen bond, and therefore the adsorption force of the silica gel decreases as the absorbed moisture increases. If the water absorption exceeds 17%, the adsorption force is extremely weak and cannot be used as an adsorbent, but it can be used as a support agent in the distribution chromatography. For the activation of silica gel, when the silica gel is heated to 100 to 110 ° C, the water adsorbed by the hydrogen bond on the surface of the silica gel can be removed. When the temperature is raised to 500 ° C, the silanol group on the surface of the silica gel can also be dehydrated and condensed into a siloxane bond, thereby losing the activity of adsorbing moisture due to hydrogen bonding, and there is no longer the property of the adsorbent, although it is treated with water. The adsorption activity cannot be restored, so the activation of silica gel should not be carried out at a higher temperature (generally, a small amount of bound water is lost above 170 ° C). Silica gel is an acidic adsorbent suitable for the chromatography of neutral or acidic components. At the same time, silica gel is a weakly acidic cation exchanger, and the silanol group on the surface can release weakly acidic hydrogen ions. When a strong basic compound is encountered, the basic compound can be adsorbed by the ion exchange reaction. Silica gel classification is classified according to use. Silica gel can be divided into desiccant silica gel, adsorbent silica gel, analytical silica gel, packaging drying silica gel, catalyst carrier silica gel and special special silica gel. Silica gel has the characteristics of chemical inertness, large specific surface area, high internal porosity and strong adsorption capacity, and is widely used as a desiccant desiccant, a dehydrating agent, an adsorbent, a filler and a catalyst carrier. It is easy to adsorb polar substances and is difficult to adsorb non-polar organic substances. The moisture in the adsorption gas of the silica gel can reach 50% of its own mass, and in the air stream with a humidity of 60%, the amount of moisture adsorbed by the microporous silica gel can reach 24% of the mass of the silica gel. Therefore, it is often used for drying high-humidity gases. The following is a brief introduction to the main commercially available silicone products and their uses. (1) Spherical silica gel. Spherical silica gel is a spherical particle with a particle size of 2.0 to 8.0 mm. The spherical silica gel produced by various regions has certain differences in physical properties such as specific surface area, pore volume and bulk density. They are mainly used for gas drying, gas adsorption, air humidity regulation, and petroleum. Product refining and catalyst carrier. (2) Granular silica gel. The random particles having a size of 6 to 20 mm have a specific surface area of 700 to 800 m 2 /g. Mainly used for natural gas drying, nitrogen drying, recycling hydrogen drying and natural gas hydrocarbon separation. (3) Water resistant silica gel. Mostly spherical, the specific surface area is 300 ~ 400m2 / g, which is characterized by water does not burst. Mainly used for various gas drying, liquid saturated hydrocarbon dehydration and catalyst carrier. (4) Micronized silica gel. It has an average particle size of 2 to 10 μm and is amorphous. The oil absorption value is 60-300 g/100 g, which is mainly used as an additive for food, medicine and cosmetics. Non-toxic, the amount added in foods and medicines does not exceed 2%. (5) Select adsorption silica gel. It is a finished silica gel product, also known as "chromatographic adsorption silica gel". The "column chromatography silica gel" on the domestic market belongs to this type of silica gel. The shape has a block shape and a microsphere shape, and the particle size is 60 to 300 mesh, and the specific surface area is 300 to 750 m 2 /g. Mainly used for adsorption separation and purification of substances. (6) Blue indicator type silica gel. A type of silica gel that exhibits a different color depending on the relative humidity of the environment, and has a block shape and a spherical shape. It is mainly used as a relative humidity indicator for moisture-proof packaging and an indicator for air drying equipment. Indicates the degree of moisture absorption of the desiccant to determine the relative humidity of the environment. For example, “Ocean Brand” blue glue indicator is light blue, purple and red at 20%, 40% and 60% relative humidity respectively. (7) Silicone desiccant for packaging. Depending on the application and packaging conditions, there are many types and types of silicone desiccant for packaging, such as bagged, special-shaped and special-purpose types. Bags are available in paper bags, cotton bags, non-woven bags, nylon bags, polyethylene bags, etc., which are the most popular types of silicone packaging. The special-shaped class includes a silica gel desiccant such as a plate shape, a column shape, a bar shape, and a plug shape. The special type is a silica gel desiccant specially designed and manufactured according to the requirements of the user. References: Zhu Hongfa, Zhu Yuxia, editor. Fine chemical product manufacturing technology. Beijing: Jindun Press. 2002. p. 168. Lin Chongde, Jiang Yan, Wang Desheng editor; Wang Desheng, editor-in-chief. China Adult Education Encyclopedia, Chemistry·Chemicals.