Activated alumina surface chemistry

Modification of alumina surface chemistry to enhance selective adsorption of particular compounds has become more prevalent in recent years. Active aluminas are multifunctional materials with ratios of surface sites. Engineering the alumina to contain advantageous surface functionalities while reducing undesired sites is fast becoming a science and is a powerful tool in the design of selective adsorption units. 

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. 

Aluminium oxide exists in many crystalline modifications, usually designated by Greek letters, some with hexagonal and some with cubic lattices (cf. refs. 11 and 24). The best known and mostly used forms are a- and 7-alumina but practical catalysts are seldom pure crystallographic specimens. This makes the surface chemistry of aluminas rather complicated. Moreover, the catalytic activity of alumina depends very much on impurities. Small amounts of sodium (0.08—0.65%) poison the active centres for isomerisation but do not affect dehydration of alcohols [10]. On the other hand, traces of sulphates and silica may increase the number of strong acidic sites and change the activity pattern. 

Depending on the chemistry of the water, activated alumina is typically used for one to three months before regeneration or disposal is required (Jekel, 1994, 128). Alumina regeneration begins with a wash of 0.25-1.0 normal (N) NaOH (Clifford and Ghurye, 2002, 221, 229). As shown in the following reaction, hydroxides from the base replace arsenic oxyanions adsorbed onto the surface aluminums ... 

As already explained, the as-method of isotherm analysis can be used to derive the external area, a(ext, S), and the pore volume vp(mic, S). Of course the first requirement is to obtain an appropriate standard isotherm on a non-porous alumina. Strictly, the surface chemistry of the reference material should be exactly the same as that of the porous adsorbent, but in practice this is not easy to achieve because of the complex surface structure of active aluminas. As before, standard isotherm data determined on the non-porous Degussa Aluminiumoxid C have been found suitable for the analysis of various isotherms on the porous aluminas (Sing, 1970). 

The interaction of hydrogen chloride with alumina was also investigated with the same instruments. Changes in the surface hydroxyl groups 29b) were observed that were directly correlated with the active sites. The combination of information from the TOSCA and MARI neutron scattering spectrometers to solve problems in surface chemistry is a powerful method that will undoubtedly be exploited further. It is through an improved awareness of reactant-catalyst interactions that increased efficiencies of industrial chemical processes can be recognized and realized. 

The more demanding research topic will be the description of the radiolytic species surface chemistry. Owing to the very high specific surface of nanostructured materials (up to 1000 m g ), even moderate reaction rates between radiolytic species and surface may have a profound impact on the radiolytic schemes. The few studies available deal only with the surface reactivity of hydroxyl radical in gas phase and suggest a HO capture by silica and alumina. This shows that surfaces that are usually considered as inert may become active under irradiation, once more demonstrating the exceptional reactivity of radiolytic species. 

Aluminas are extensively used as catalysis and supports for catalytic materials. It is well known that the surface chemistry of adatoms may be influenced profoundly by addition on the alumina of minute amounts of electropositive or electronegative ions like alkalies or halogens (ref 1), Asa result the catalytic action of the active surface phase appears modified (refs 2-5), Such modiftcation may be due to (1) modified symmetry of the active sites, (2) different degree of dispersion of sites, or (3) altered surface coverage (refs. 6-9). Additives absorbed on the active sites of alumina may act as poisons. Such a poisoning action cun be expressed in formal terms, as a function of the concentration of added modifier. 

We have recently shown [4-5] that an adjustment of macroporosity (between 0.1 and 1 im) of the used alumina beads permits considerable decrease in the diffusional constraints and thus allows attainment of distinctly improved catalytic performance. We have also shown the importance of the chemistry of the alumina surface while a minimal level of Na20 content (> 1500 ppm) is needed for adequate activity, a level over 2500 ppm favors the sulphation of surface sites in the neighborhood of K atoms and effects a particularly fast chemical deactivation. 

A key factor in the development of adsorption technology for the fluid separation has been the availability of appropriate adsorbents. The most frequently used categories include crystalline materials like zeolites, and amorphous materials like activated carbons, silica and alumina gels, polymeric sorbents, and ion-exchange resins. These materials exhibit a large spectrum of pore structures (networks of micro- and mesopores of different shapes and sizes) and surface chemistry (degrees of polarity), which provide a large choice of core adsorptive properties (equilibria, kinetics, and heat) to be utilized in... 

The difference in the pore structure and surface chemistry of different activated aluminas is manifested by significantly different characteristics for adsorption of water vapour as pure gas or from gas mixtures. These properties are illustrated below. 

The activity and selectivity of any highly porous solid catalyst depends, first, upon the chemical nature of its surface, and second, upon what has been called the surface texture, that is, the detailed geometry of the pore system. While neither of these aspects of a surface can be completely defined at the present time, techniques are available which do allow one to gain some reasonable idea of what the surface of such a catalyst is like. In the present section, the surface texture and surface chemistry of the chromia-alumina catalyst system will be discussed as a prelude to a more detailed consideration of catalyst structure. 

The chemical reactivity of the catalyst support may make important contributions to the catalytic chemistry of the material. We noted earlier that the catalyst support contains acidic and basic hydroxyls. The chemical nature of these hydroxyls will be described in detail in Chapter 5. Whereas the number of basic hydroxyls dominates in alumina, the few highly acidic hydroxyl groups also present on the alumina surface can also dramatically affect catalytic reactions. An example is the selective oxidation of ethylene catalyzed by silver supported by alumina. The epoxide, which is produced by the catalytic reaction of oxygen and ethylene over Ag, can be isomerized to acetaldehyde via the acidic protons present on the surface of the alumina support. The acetaldehyde can then be rapidly oxidized over Ag to COg and H2O. This total combustion reaction system is an example of bifunctional catalysis. This example provides an opportunity to describe the role of promoting compounds added in small amounts to a catalyst to enhance its selectivity or activity by altering the properties of the catalyst support. To suppress the total combustion reaction of ethylene, alkali metal ions such as Cs+ or K+ are typically added to the catalyst support. The alkali metal ions can exchange with the acidic support protons, thus suppressing the isomerization reaction of epoxide to acetaldehyde. This decreases the total combustion and improves the overall catalytic selectivity. 

Activated alumina is also widely used as a desiccant because of the same advantages for which silica gel is used. Unlike silica gel, which is amorphous, activated alumina is crystalline. Oxygen vacancies (defects) are easily formed on its surfaces, thus alumina has both Lewis and Brpnsted acid sites. The surface chemistry, as well as the pore structure of activated alumina, can be modified, for example, by treatment with acid (HCl or HF) or alkaline (to alter the acidity) and controlled thermal treatment (to tailor the pore structure). As a result, activated alumina is more versatile than sihca gel and has been applied more often as a sorbent. 

From the above discussion, it can be seen that both pore structure and surface chemistry of the activated alumina can be manipulated and controlled. As a result, activated alumina is a very versatile sorbent and can be tailored for specific applications. 

Activated alumina is a versatile sorbent that can be tailored for many special applications. New applications continue to be developed, mainly by the aluminum companies. Little is disclosed on the details of their modifications. However, the modifications follow simple general principles of surface chemistry, such as acid-base chemistry. Two methods are used for tailoring (1) variation of the activation process, and (2) use of dopants. The following are proven applications of various tailored aluminas ... 

In addition to surface effects, there are other ramifications of changing activation temperature in design of the adsorbent. As seen in Fig. 7, total surface area is a sensitive fimction of temperature, as well as pore distribution and total pore volume. In gas phase separation of small molecules with fast pore transport, surface area is critical for good uptake capacities. Viscous liquids such as large hydrocarbons, on the other hand, tend to require more macroporosity, i.e., inherently lower surface areas, with well designed surface chemistries. Optimum selection of an alumina will depend on the relative importance of these factors to the end user. 

The alumina surface is an extremely versatile and widely used support for studies in many areas of chemistry. To complete the review of the literature in the past two years, Lefondeur et al used EPR to study the paramagnetic properties of nickel nanoparticles deposited on alumina, while Konovalova et al used ID and 2D ESEEM and pulsed ENDOR to study the nature of the adsorbed canthaxanthin and 8 -apo-P-caroten-8 -al radical cations on an activated silica-alumina surface. Both of these excellent and thorough papers describe in detail the interpretations of the EMR data in relation to the role of the surface. 

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