Use of Adsorbents for Recovery of Acetic Acid From Aqueous Solutions Part II – Factors Governing Selectivity

Measurements have been made of uptake of acetic acid and water from low-pH aqueous solution onto polymeric adsorbents and activated carbons. In additon to composite isotherms, isotherms were obtained for acetic acid and water individually by use of gas-chromatographic and Karl Fisher techniques for analyses of both bulk solution and the material taken up by the adsorbent. While capacities for acetic acid are determined by the surface area and the chemical nature of the adsorbent, selectivity is governed by the pore volume and the wetting and swelling tendencies of the sorbent, with the latter being particularly important for polymers. Activated carbons and pyrolyzed polymers give better selectivity than do common polymeric adsorbents. Measurements of pore volume by immersion in various liquids were compared with pore volumes computed from nitrogen adsorption-desorption measurements and from mercury-intrusion porosimetry. The nitrogen adsorption-desorption results were interpreted to obtain micropore and mesopore volumes. The results from the different methods agree well, if allowance is made for lack of full wetting and for swelling tendencies. Higher selectivity for acetic acid over water is obtained for adsorbents having a large percentage of the pore volume as micropores. Measurements of competitive adsorption of acetic acid and methyl ethyl ketone from aqueous solution onto different carbons and pyrolyzed polymers showed that a higher surface density of active hydrogen sites, as measured by reaction with LiAlH₄, leads to an improved selectivity for the carboxylic acid.