ADSORBATE SHAPE SELECTIVITY - SEPARATION OF THE HF 134A AZEOTROPE OVER CARBOGENIC MOLECULAR-SIEVE/

Experimental evidence is provided for adsorptive shape selectivity in the separation of the azeotrope between HF and 1,1,1,2-tetrafluoroetha ne (134a) over pyrolyzed poly(furfuryl alcohol)derived carbogenic mole cular sieve (PPFA-CMS). The separation can be accomplished over coconu t charcoal or Carbosieve G on the basis of the differences in the exte nt of equilibrium adsorption of HF and 134a. On these adsorbents 134a is more strongly bound than HF, thus it elutes much more slowly from t he bed. The heat of adsorption for 134a in the vicinity of 200 degrees C on Carbosieve G is similar to 8.8 kcal/mol. In contrast, when the s ame azeotropic mixture is separated over PPFA-CMS prepared at 500 degr ees C, 134a is not adsorbed. As a result 134a elutes from the bed firs t, followed by HF. The reversal is brought about by the narrower pore size and pore size distribution of the PPFA-CMS versus that for Carbos ieve G. Thus the separation over PPFA-CMS is an example of adsorbate s hape selectivity and represents a limiting case of kinetic separation.