Selectivity of thiophene/toluene competitive adsorptions onto zeolites. Influence of the alkali metal cation in FAU(Y)

In the framework of research considering adsorption for gasoline desulfurization purposes, dynamic adsorption experiments have been performed to study a comparison between thiophene and toluene affinities toward Y-faujasite adsorbents (Si/Al = 2.4) exchanged with various alkali metal cations (Li+, Na+, K+, Rb+, Cs+). These experiments have been carried out at two concentration ranges in liquid n-heptane solutions: around 25 mmol center dot L-1 for competitive adsorptions and around 1.25 mmol center dot L-1 for single-solute adsorptions. Complementary microcalorimetry studies have been carried out in order to investigate the interaction of nucleophilic compounds toward alkaline Y zeolites, by measuring the adsorption enthalpy of CCl4 onto NaY and CsY sorbents. The results clearly exhibit two opposite trends for the adsorption of thiophene and toluene molecules onto alkaline Y faujasites. The thiophene affinity increases with cation electropositivity, while the toluene affinity decreases as the cations are more electropositive. Thus, from a favorable adsorption for toluene onto the LiY zeolite (alpha(thio/tol) = 0.38 at ca. 26 mmol center dot L-1), the selectivities shift in favor of thiophene so that the alpha(thio/tol) value reaches 3.00 with the CsY zeolite. The calorimetry measurements of CCl4 adsorption have exhibited an increase of the adsorption enthalpy with more electropositive cations, which seems to indicate that thiophene molecules are adsorbed similarly to CCl4, i.e., by strong interaction between the nucleophilic S atom of thiophene and the cationic charge. However, toluene should be mainly stabilized on the adsorption sites through other interactions, as combined attractions between the H atoms of toluene and the O atoms of the zeolite framework surrounding the S-II cation can be suggested.