Alumina acidity/basicity scale

The most common approach to surface modification with transition aluminas is through control of the thermal treatment. As shown in Figures 5 and 7, it is quite easy to alter the total surface Lewis and Bronsted acidity and basicity via control of the hydroxyl concentration. To some extent this technique has been used since the early 1950s, particularly in chromatographic aluminas. The so-called Brockmann scale for rating aluminas is based on thermal history, and commercial materials are available today in the various Brockmann grades. Only recently has the concept been applied to large scale adsorption systems. 

As for silica, a variety of alumina powders are available commercially, and layers can be prepared as aqueous slurries following manufacturers instructions. Commercial powders are available in different degrees of activity according to the Brockmann scale and can be obtained as acidic (pH 4.0-4.5), neutral (7.0— 8.0), or basic (9.0-10.0). They are also available with or without a gypsum binder or fluorescent indicator. Commercially precoated alumina plates are usually received with approximately 6-8% water content (activity 111-lV). The variations in pH, specific surface area, and pore size among various TLC aluminas lead to differences in separation properties, and the conditions used to obtain certain results must be carefully documented. 

Synthesis of DNP-Alanyl Diazoketone. DNP-Alanyl acid chloride (40 mg) is made up to 100 ml with anhydrous diethyl ether at 0° and added slowly to 10 ml of dry ether containing 0.01 mole of diazomethane at 0°. (For preparation of ethereal diazomethane, see deBoer and Backer. ) The resulting mixture is stored in a moisture-proof container in the dark at 4° for 12 hr. During large-scale preparations, the DNP-AD is precipitated from the ether solution and washed with ether at 4°. For small-scale synthesis of [ H]DNP-AD, the product is purified on a basic alumina (Brockman activity I) column, 1 X 15 cm, which is eluted with dry dioxane. Ultraviolet absorption Xmax was at 255 and 342 nm IR spectra gave a 2115 cm diazo stretch band. Mass spectroscopy gave no molecular ion, but two major peaks at m/e 210 and 69 were consistent with the two halves (DNP-NH CH CHs) and (0=CCH=N=N) ... 

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