Activated alumina properties

Traditional adsorbents such as sihca [7631 -86-9] Si02 activated alumina [1318-23-6] AI2O2 and activated carbon [7440-44-0], C, exhibit large surface areas and micropore volumes. The surface chemical properties of these adsorbents make them potentially useful for separations by molecular class. However, the micropore size distribution is fairly broad for these materials (45). This characteristic makes them unsuitable for use in separations in which steric hindrance can potentially be exploited (see Aluminum compounds, aluminum oxide (ALUMINA) Silicon compounds, synthetic inorganic silicates). 

The surface of activated alumina is a complex mixture of aluminum, oxygen, and hydroxyl ions which combine in specific ways to produce both acid and base sites. These sites are the cause of surface activity and so are important in adsorption, chromatographic, and catalytic appHcations. Models have been developed to help explain the evolution of these sites on activation (19). Other ions present on the surface can alter the surface chemistry and this approach is commonly used to manipulate properties for various appHcations. 

The large majority of activated alumina products are derived from activation of aluminum hydroxide, rehydrated alumina, or pseudoboehmite gel. Other commerical methods to produce specialty activated aluminas are roasting of aluminum chloride [7446-70-0], AIQ calcination of precursors such as ammonium alum [7784-25-0], AlH2NOgS2. Processing is tailored to optimize one or more of the product properties such as surface area, purity, pore size distribution, particle size, shape, or strength. 

The gel-based products have traditionally been the most expensive and highest performance activated alumina products. They have very good mechanical properties, high surface area, and their purity and ganima-aluniina stmcture make them somewhat resistant to thermal degradation. On the other hand, they are the most difficult to manufacture and disposal of by-product salts can present an environmental problem. 

High Surface Sodium. Liquid sodium readily wets many soHd surfaces. This property may be used to provide a highly reactive form of sodium without contamination by hydrocarbons. Powdered soHds having a high surface area per unit volume, eg, completely dehydrated activated alumina powder, provide a suitable base for high surface sodium. Other powders, eg, sodium chloride, hydride, monoxide, or carbonate, can also be used. 

The properties of 1,1-dichloroethane are Hsted ia Table 1. 1,1-Dichloroethane decomposes at 356—453°C by a homogeneous first-order dehydrochlofination, giving vinyl chloride and hydrogen chloride (1,2). Dehydrochlofination can also occur on activated alumina (3,4), magnesium sulfate, or potassium carbonate (5). Dehydrochlofination ia the presence of anhydrous aluminum chloride (6) proceeds readily. The 48-h accelerated oxidation test with 1,1-dichloroethane at reflux temperatures gives a 0.025% yield of hydrogen chloride as compared to 0.4% HCl for trichloroethylene and 0.6% HCl for tetrachloroethylene. Reaction with an amine gives low yields of chloride ion and the dimer 2,3-dichlorobutane, CH CHCICHCICH. 2-Methyl-l,3-dioxaindan [14046-39-0] can be prepared by a reaction of catechol [120-80-9] with 1,1-dichloroethane (7). 

Property Grade A activated alumina Sdica gel Type 4A molecular sieve... 

Adsorption is the property of certain extremely porous materials to hold vapors in the pores until the desiccant is either heated or exposed to a drier gas. The material is a solid at all times and operates alternately through drying and reactivation cycles with no change in composition. Adsorbing materials in principal use are activated Alumina and silica gel. Molecular sieves are also used. Atmospheric dew points of minus 1000°F are readily obtained using adsorption. 

We postulated a reaction mechanism with participation of an aromatic radical cation which was formed by one electron transfer from an aromatic hydrocarbon to copper(II) chloride. Activated alumina has electron-acceptor properties, and formation of a radical cation of an aromatic hydrocarbon adsorbed on alumina has been observed by ESR (ref. 13). Therefore, it seemed to us that alumina as a support facilitates the generation of the radical cation of the aromatic hydrocarbon. 

Table 2 summaries overall attrition rate constants (Ka) and physical properties for each dry sorbent. As shown in Table 2, Ka of activated alumina was the lower than any other sorbent, but was similar to activated carbon. However, we used activated carbon as dry sorbent to control CO2 because it is the most cost-effective among others.

Applications PSA cycles are used primarily for purification of wet gases and of hydrogen. One of the earliest applications was the original Skarstrom two-bed cycle (adsorption, countercurrent blowdown, countercurrent purge, and cocurrent repressurization) to dry air stream to less than 1 ppm H20 [Skarstrom, ibid.]. Instrument-air dryers still use a PSA cycle similar to Skarstrom s with activated alumina or silica gel [Armond, in Townsend, The Properties and Applications of Zeolites, The Chemical Society, London, pp. 92-102 (1980)]. 

Occurs in nature in abundance the principal forms are bauxites and lat-erites. The mineral corundum is used to produce precious gems, such as ruhy and sapphire. Activated aluminas are used extensively as adsorbents because of their affinity for water and other polar molecules and as catalysts because of their large surface area and appropriate pore sturcture. As adsorbents, they are used for drying gases and liquids and in adsorption chromatography. Catalytic properties may be attributed to the presence of surface active sites (primarily OFT, 02, and AF+ ions). Such catalytic applications include sulfur recovery from H2S (Clauss catalysis) dehydration of alcohols, isomerization of olefins and as a catalyst support in petroleum refining. 

To achieve a significant adsorptive capacity an adsorbent must have a high specific area, which implies a highly porous structure with very small micropores. Such microporous solids can be produced in several different ways. Adsorbents such as silica gel and activated alumina are made by precipitation of colloidal particles, followed by dehydration. Carbon adsorbents are prepared by controlled burn-out of carbonaceous materials such as coal, lignite, and coconut shells. The crystalline adsorbents (zeolite and zeolite analogues are different in that the dimensions of the micropores are determined by the crystal structure and there is therefore virtually no distribution of micropore size. Although structurally very different from the crystalline adsorbents, carbon molecular sieves also have a very narrow distribution of pore size. The adsorptive properties depend on the pore size and the pore size distribution as well as on the nature of the solid surface. 

Corundum is aluminum oxide, q -A1203, which has a hexagonal crystalline structure that is analogous to hematite. However, water treatment systems most often use activated alumina, which is typically produced by thermally dehydrating aluminum (oxy)(hydr)oxides to form amorphous, cubic (y), and/or other polymorphs of corundum (Clifford and Ghurye, 2002, 220 Hlavay and Poly k, 2005 Mohan and Pittman, 2007). When compared with corundum, amorphous alumina tends to have higher surface areas, greater numbers of sorption sites, and better sorption properties. 

Alumina and /-alumina are also called active aluminas. These materials are seldom phase pure and contain other transition aluminas as impurities. Their properties strongly depend on the type of starting materials, the procedure chosen for thermal treatment, and the operation parameters such as temperature and pressure. Highly active y-alumina has been prepared by shock calcination followed by rehydration [43],... 

Table 2. Properties of different precursors used for active alumina preparation.

Before the mid-1940s few systematic attempts appear to have been made to optimize the properties of such common adsorbents as activated charcoal, silica gel and activated alumina. The increase in research and development activity after the Second World War was largely due to the demand for improved catalysts and adsorbents. It was already known that the extent of the accessible surface was of fundamental importance (Rideal, 1932). In the 1940s the interest in gas adsorption was undoubtedly stimulated by the perceived success of the BET method for the determination of the surface area. 

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