Theoretical Studies Of The Acidity Of Cation-exchanged Y And Its Adsorption Desulfurization

Due to increasingly stringent regulations and fuels specifications forenvironmental protection, deep desulfurization of fuels has drawn extensive attentionworldwide. Selective adsorption desulfurization has been considered one of the mostpromising deep desulfurization methods at ambient temperature and pressure.In therecent years, the selective adsorption thiophenic compounds by using metalcation-exchanged Y zeolites have been widely investigated. Cu(I)Y, Ni(II)Y, Ce(III)Yas adsorbents have been reported to have capable of ultra-deep desulfurization withhigh selectivity for the removal of thiophenic compounds from fuel oils.Density functional theory has been employed to investigate the Lewis aciditystrength of cation-exchanged Cu (I) Y, Ni (II) Y, Ce (III) Y zeolites. All of calculationsare carried out by the generalized gradient approximation (GGA) with the BLYPexchange-correlation functional and DNP basis set. A fragment of zeolite frameworkwhich contains SII site was modeled by the6T cluster (Si6O18H12) and12T cluster(Si12O33H18). Fukui function based reactivity descriptors were used to monitor thestrength of Lewis acidity, and corresponding order followed: Ce3+> Ni2+>Cu+. Theresults indicate the Lewis acidity strength sequence derived using relativeelectrophilicity are not depending on population analysis methods and zeolite clusters.A density functional theory (DFT) method has been applied to study theadsorption of thiophenic compounds such as thiophene(TP), benzothiophene(BT),dibenzothiophene(DBT) on cation-exchanged Cu(I)Y, Ni(II)Y, Ce(III)Y zeolites. Allof calculations were carried out by the generalized gradient approximation (GGA)with the BLYP exchange-correlation functional and DNP basis set. The calculatedresults indicate that the stable adsorption configuration of TP molecule adsorbed onthe Cu (I) Y is the η5adsorption mode, whereas for BT and DBT, the η1S adsorptionmode was found for the both molecules. Only the η1S adsorption mode can beobtained for the three thiophenic compounds adsorbed on the Ni (II) Y. The η5 adsorption mode can be ascribed to the adsorption of thiophene molecules on the Ce(III) Y, but the competition of different adsorption modes could possibly occur duringthe adsorption process of BT and DBT on the Ce (III) Y. Moreover, the selectivity ofTP adsorbed on the adsorbents are in the order of Cu (I) Y> Ce (III) Y> Ni (II) Y,while for BT and DBT, the order is Cu (I) Y>Ni (II) Y> Ce (III) Y.The adsorption of thiophenic compounds and the mixture of thiophene and olefinon cation-exchanged Cu(I)Y zeolite have been investigated by the grand canonicalensemble Monte Carlo simulation. The adsorption isotherms of thiophene,benzenethiophene and dibenzenethiophene on Cu(I)Y zeolite were obtained atdifferent temperatures (298K,363K and393K). The calculated results showed that theadsorbed amounts increased with increasing pressure at low pressure and decreasedwith increasing pressure, as well as the order of saturated adsorption amounts wasthiophene> benzenethiophene> dibenzenethiophene. Thiophene molecules weremainly adsorbed with Cu+in supercage, and the cluster-like structure amongthiophene molecules has been found from the distribution of thiophene on Cu(I)Yzeolite at298K. For the mixture of thiophene and olefin, the simulated resultsindicated: olefin have effect the adsorption of thiophene on Cu(I)Y zeolite, the orderof effect was cyclohexene>1,5-dihexene>1-hexene>1-octene. From the adsorptionconfiguration, we found that thiophene and cyclohexene together adsorbed with Cu+.