Theoretical study on the C4 alkylation mechanism catalyzed by Cu-containing chloroaluminate ionic liquids

Abstract

DFT calculations combined with MD simulations have been used to investigate the complicated reaction mechanism of isobutane-isobutylene alkylation catalyzed by the neat chloroaluminate ionic liquid (NIL) and the Cu-containing chloroaluminate ionic liquid (CIL). Transition states of three key elementary reactions were obtained to address the selectivity of the alkylate formation versus the by-product formation in different ionic liquids. The DFT calculations indicate that the Cu species would significantly inhibit the polymerization of C4= olefins and C8+ carbenium ions. Compared to the competitive H-transfer reaction, the reaction rate of polymerization in the NIL was very fast resulting in poor product selectivity. The Mayer bond order (MBO) and electron localization function (ELF) maps reveal that H-transfer from isobutane to C8+ carbenium ion occurs via a concerted H-Cu bond formation between a C8+ and an isobutane to generate the desired isooctane. The Cu complex did engage in the H-transfer of isobutane/C8+, but it did not weaken the catalytic activity in comparison to the neat chloroaluminate anions. The MD simulations show that a combination of high isobutane concentration and low isobutylene concentration at the CIL-hydrocarbon interface can be attributed to the effects of Cu complex, which also leads to the improvement of the alkylation selectivity. An essential role of Cu species in the CIL alkylation is to impede the formation of C12+ carbenium ions and promote the H-transfer between isobutane and C8+ carbenium ion.