Density functional theory investigation for catalytic mechanism of gasoline alkylation desulfurization over NKC-9 ion-exchange resin

The molecular level understanding of the mechanism about the 3-methylthiophene (3MT) alkylation with isobutylene (IB) as well as the side reaction of IB dimerization over NKC-9 cation exchange resin has been investigated using the density functional theory (DFT) of quantum chemical method. A model of benzene sulfonic acid was used to represent the cation-exchange resin catalyst. Two different reaction mechanism types - stepwise scheme and concerted scheme have been evaluated. Activation energies of each reaction path which were obtained from the DFT results have been improved by single-point MP2 calculations. In the stepwise mechanism, both 3MT alkylation and IB dimerization proceed by adsorption and protonation of the IB to form a sulfonic ester intermediate, and then by C-C bond formation between the sulfonic ester intermediate and another 3MT or IB to give the reaction products. The second step is rate-determining and has activation barriers of 148.41 kJ/mol for 3MT alkylation and 160.52 kJ/mol for IB dimerization. In the concerted mechanism, the reaction occurs in one step of simultaneous protonation and C-C bond formation. The activation barrier is calculated to be 169.10 kJ/mol for 3MT alkylation, and that for IB dimerization is 174.02 kJ/mol. The results revealed that the reaction mechanism of 3MT alkylation was very similar to that of IB dimerization, and the stepwise mechanism dominated both the 3MT alkylation and IB dimerization. Moreover, 3MT alkylation is more easily occurs than IB dimerization during gasoline alkylation desulfurization.