Nonane preadsorption

Linares-Solano, A., et al., The n-nonane preadsorption method applied to activated carbons, Adsorpt. Sci. Technol., 1(2), 123-132(1984). 

Bohra, J.N., and Sing, K.S.W., Micropore structure of carbonized rayon yarn by nitrogen adsorption and nonane preadsorption, Adsorpt. Sci. Technol., 2(2), 89-96 (1985). 

Activated carbons are adsorbents with a wide pore size distribution and consequently the precise determination of their porous structure is a rather difficult task. Since activated carbons are es.sentially microporous, most work devoted to their characterization is centred around the determination of the microporosity. Several methods have been extensively used to analyze the adsorption isotherms of nitrogen and other ad.sorptives. The micropore volume filling theory of Dubinin has been successfully used but there are well-known problems when the micropore size distribution is heterogeneous (refs. 1,2). The n-nonane preadsorption technique has also been used in the last few years but it provides information only on narrow microporosity and the results are conditioned by interconnectivity network of the porosity (refs. 3,4). The t- and a-plot methods have also been widely applied using different non-porous reference materials, the selection of which may be critical (refs. 5,6). 

The main objetive of the work described here is to evaluate the microporosity in a series of activated carbons with increa.sing burn-off and ash (CrjOj) content using the isotherms on non-porous carbon and Cr Oj. samples as references in the< -plot method and the comparison with results obtained from the Dubinin theory and n-nonane preadsorption. 

Fig. 4J1 Adsorption isotherms on ammonium phosphomolybdate powder. (1), (2), before pre-adsorption of nonane (3), (4) after preadsorption of nonane. (1), (4), nitrogen (77 K) (2), (3), carbon tetrachloride (298 K). Adsorption is expressed in mm (liquid).

Fig. 4J2 Adsorption isotherms of carbon tetrachloride (at 298 K) on ammonium phosphotungstate compact, (1) before, (2) after preadsorption of n-nonane. (3) is the isotherm of nitrogen, after preadsorption, for reference. Open symbols, adsorption solid symbols,...

Another procedure which may be used for the assessment of microporosity is the preadsorption method. In this approach the micropores are filled with large molecules (e.g. nonane), which are not removed by pumping the adsorbent at ambient temperature. In the most straightforward case, this procedure can provide an effective way of isolating the micropores and leaving the external surface available for the adsorption of nitrogen, or another suitable adsorptive. 

Series P of activated carbons was prepared by carbonization in Nj (1123K) of chromium-tanned leather waste followed by activation in CO (1098K) for different periods of time to cover the 7-70% burn-off range (burn-off is included in the nomenclature of the samples). Adsorption of Nj (77K), CO2 (273K) and n-butane (273K) and preadsorption of n-nonane were determined in conventional gravimetric systems in order to characterize the carbons. 

Preadsorption of n-nonane may help to evaluate the applicability of the -plot method. Table 3 includes the data for samples P-39, P-58 and P-70, selected because their wide micropore size distribution. The V, values (given by the difference at P/P =0.80 between the isotherms without and with n-nonane adsorbed) are very similar to those given in Table 2 for COj (273K), these values giving then the volume of narrow micropores. The relatively lower values of (volume of n-nonane retained by the samples) for P39 and P53 are indicative of porosity interconectivity typical of carbons with medium burn-off as shown elsewhere (ref. 11). 

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