Characterization of microporous carbons by flow microcalorimetry

Abstract

Heats of immersion in n-heptane of several microporous carbons and the heat of displacement of n-heptane by n-butanol and n-dotriacontane have been evaluated in a flow microcalorimeter. The heats of n-butanol adsorption were used to estimate polar sites on the surfaces of the carbons accessible to this alcohol. Comparison with the adsorption of tertiary butanol suggested that in some microporous PVDC carbons most of the polar sites are present in pores having diameters not exceeding 6.5 Å units. Other types of microporous carbons adsorbed very similar amounts of the branched and normal alcohols. The heats of adsorption of n-dotriacontane (a measure of the graphitic and hydrophobic surface sites) were relatively high for all the carbons investigated including the microporous carbons having very high surface areas as estimated from the BET (Brunauer-Emmett-Teller) N 2 adsorption method. The latter carbons also gave very high heats of immersion in n-heptane (ca. 200 J/g) and, in some cases, high heats of immersion in water that could be related to the heats of displacement of n-heptane by n-butanol (i.e., the polarity of the carbons). The adsorption of n-butanol and n-dotriacontane was carried out at different flow rates (i.e., at different rates of supply of the solutes to the adsorbents placed in the flow microcalorimeter) and the rates of heat evolution on adsorption and desorption were related to the nature of porosity in individual carbons.