Separation of Monobranched and Dibranched Isomers of n-Hexane on Zeolitic Molecular Sieves: A Thermodynamic Study

A thermodynamic study was realized by competitive adsorption over zeolitic adsorbents to determine the efficiency of these solids for the separation of monobranched and dibranched isomers of n-hexane. The effect of the zeolite structure was studied. The medium-pores ZSM-5-type zeolites were better than the large-pores BEA and MOR zeolites. The size and number of the extraframework cations had an important influence on the efficiency of the separation over ZSM-5 zeolites. The sodic Na(6)ZSM-5 sample was found to be the better adsorbent for the separation of the studied mixture because of steric hindrance induced by the presence of Na+ cations in the zeolite structure. The initial composition of the mixture also had an important influence on the separation. In fact, when the initial mixture was equimolar the monobranched isomer was preferentially adsorbed, whereas when the molar percentages of the isomers were different in the initial mixture the adsorption of the majority isomer was favored. The temperature of the adsorption was another important parameter influencing the separation. Indeed, when the temperature of adsorption was low the separation was more effective. At an adsorption temperature of 333 K the Na(6)ZSM-5 sample was the most efficient by adsorbing 65% of the monobranched isomer and only 35% of the dibranched isomer.