Journal of Surface Science and Technology

M. A. Rahman ¹, M. A. K. Azad ¹, S. Ahsan ¹, S. Islam ¹, M. A. Motin ¹, M. Asadullah ¹

Affiliations
1 Department of Applied Chemistry and Chemical Technology, University of Rajshahi, Rajshahi 6205, BD

Abstract

Silica-Alumina catalysts having three different Si/Al ratios were prepared. Surface acidity of these catalyst samples was measured by the base exchange method using aqueous ammonium acetate, CH₃COONH₄ solution and the results were compared with those measured by the base exchange method using KOH and NH₄OH as adsorbates from aqueous solution. The amount of K⁴ and NH₄⁺ adsorbed was determined titrimetrically and spectrophotometrically, respectively for measuring the concentration of K⁺ and NH₄⁺ in the solution before and after adsorption. It was found that the surface acidity was more pronounced when ammonium acetate solution was used as an adsorbate. With the use of NH₄OH and KOH as bases considerable dealumination takes place with a consequent decrease of surface acidity of the catalyst samples. In all cases, the surface acidity increased with decrease of sample size. It is seen that surface acidity measured with NH₄OH becomes closer to that measured with KOH at the same(?) Si/Al ratio, but contrary to this, surface acidity measured with NH₄OH becomes closer to that measured with CH₃COONH₄ at high Si/Al ratio. This phenomenon is attributed to dealumination of silica-alumina catalyst samples by the strong base.

Keywords

Surface Acidity, Silica-Alumina Catalysts, Bronsted Acidity, Base Exchange.

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M. A. Rahman ¹, M. Asadullah ¹, M. M. Haque ¹, M. A. Motin ¹, M. B. Sultan ¹, M. A. K. Azad ¹

Affiliations
1 Department of Applied Chemistry and Chemical Technology, University of Rajshahi, BD

Abstract

Charcoal was prepared from coconut shell and activated at three temperatures 300, 350 and 400°C in constant flow of air or nitrogen. Adsorption of oxalic acid and maleic acid from their aqueous solution on charcoal was studied. Oxalic acid gave Langmuir type of adsorption isotherm indicating monolayer formation but maleic acid gave sigmoid shaped isotherm, BET (Brunauer, Emmett and Teller) type II isotherm. These contrasting results are explained as follows: activated charcoal, like graphite has both polar and basal sites and the polar heads of the carboxylic groups in oxalic and maleic acids are chemisorbed on the polar sites of activated charcoal but in addition to this the unsaturated hydrocarbon chain in maleic acid opens up to form weak bonds with the basal sites of the activated charcoal. It has been observed that the amount of adsorption of both oxalic and maleic acid increases with the increase of temperature of activation.

Keywords

Active Carbon, Coconut Shell, Surface Area, Adsorption.

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M. A. Rahman ¹, M. A. K. Azad ¹, M. S. R. Sarkar ¹

Affiliations
1 Department of Applied Chemistry and Chemical Technology, University of Rajshahi, Rajshahi 6205, BD

Abstract

Amorphous silica-alumina characterized by their surface acidity are good cracking catalysts. The active sites of these catalysts are considered to be the acid centres. Amorphous silica-alumina were prepared by the coprecipitation and gelation method and iron(III) dispersed on silica was prepared by the incipient wetness method. The samples were calcined and their surface acidity was measured by the base exchange method using ammonium acetate as an adsorbate. The integral heats of preferential adsorption of pyridine from n-heptane solution on the silica-alumina (amorphous aluminosilicate) and iron dispersed on silica (ferrisilicate) were measured by a flow microcalorimeter. For comparison, surface acidities and integral heats of adsorption of a standard sample of HZSM-5 and chromatographic alumina and silica have also been measured. From the heat of adsorption data it has been found that HZSM-5 gives BET type I (monolayer adsorption) and silica-alumina samples give BET type II (multilayer adsorption) isotherms. Comparison of the integral heat of adsorption with heat of desorption showed that pyridine molecules are mostly physisorped on both HZSM-5 and amorphous siliea-alumina samples. It has also been found that there is a good correlation between the surface acidity and integral heat of adsorption of the silica-alumina samples.

Keywords

Heat of Adsorption, Surface Acidity, Silica-Alumina Catalysts, Microcalorimetry.

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