Comparison of different molecular sieves for the liquid phase separation of linear and branched alkanes

A carbon molecular sieve adsorbent (CMS-IMP12) obtained from the pyrolysis of a poly(vinylidene chloride-co-vinyl chloride) (PVDC-PVC, Saran™) material was tested and compared with other molecular sieve materials for the separation of a multi-component hydrocarbon mixture. Liquid phase experimental adsorption curves at 303 K were obtained in a stirred tank using a model mixture composed by n-heptane, 2-methylheptane, 2,5-dimethylhexane and 2,2,4-trimethylpentane, considering the last one as a non-adsorbing solvent. Materials compared against the CMS-IMP12 were zeolites ZSM-5 (Si/Al = 15), ZSM-5 (Si/Al = 140), ZSM-22, ZSM-23, Silicalite-1 and Silicalite-2. The CMS-IMP12 adsorbed at least three times the amount adsorbed by the other material tested (i.e. 206 versus 79 mg/gads for Silicalite-1) mainly because three components of the mixture (linear, methyl and non-geminal dimethyl alkanes) were adsorbed in higher proportions. None of the other tested materials adsorbed non-geminal dimethyl branched compounds. Regarding the kinetics of adsorption as represented by their second order rate constants, all the materials except the CMS-IMP12 adsorbed the methyl alkane molecule at the same rate as the n-alkane. The slower adsorption rate observed for 2-methylheptane as compared to n-heptane in the CMS-IMP12 may be due to the competition for adsorption sites between the two slow diffusing species: 2-methylheptane and 2,5-dimethylhexane.