Performance And Properties Of Ionic Liquid And Analogs For C₄ Alkylation Catalysis

Isobutane(C₄)alkylation product is an ideal motor gasoline blending component.Composite ionic liquids(CILs),amide-Al Cl₃-based ionic liquids(LCC)modified with transition metal chlorides(Cu Cl,Ag Cl),can catalyze the alkylation of isobutane/2-butene to generate high-quality alkylates.Compared with traditional strong acid catalysts(H₂SO₄,HF),these new catalysts have the many advantages,such as low corrosiveness,room-temperature reaction,no waste acid,and no harmful emissions.In recent years,novel alkylation catalysts have been extensively studied.Researchers have carried out experimental and model studies on the alkylation reaction mechanism of new alkylation catalysts in recent years.However,the mechanism research based on ionic liquids,especially on the composite ionic liquids,has not been clearly discussed,and the electron configuration of the catalysts was vague.In-depth studies on the Lewis acidity of ionic liquids are even rarer.Therefore,this paper discuss the performance of ionic liquids and analogs for C₄ alkylation by experimental and theoretical methods.In addition,the relative Lewis acidity of ionic liquids was also quantitatively analyzed by theoretical calculations.The results show:1.Using traditional sulfuric acid to catalyze the alkylation of isobutane/2-butene,the product is very complex.The catalyst has no directional selectivity,and the content of desired trimethylpentane is not high.Similarly,the catalytic selectivity of pure chloroaluminate ionic liquids and analogs is not high,too.Only composite ionic liquids(CILs)and analogs were modified by transition metal chlorides(Cu Cl,Ag Cl),they can exhibit high selectivity to trimethylpentane.2.The electronic structure of the composite ionic liquids modified by Cu Cl was systematically analyzed.The complex species,such as[Cu Al Cl₅]^-,Cu Cl-C₄H₈ and[Cu Al Cl₅-C₄H₈]),were detected during the alkylation process.When the chloroaluminate ionic liquid/Cu Cl was mixed with the reactants,the charge transfer from 2-butene to the ionic liquid was significantly increased,and Cu Cl provided a new channel for charge transfer.In addition,the Cu-containing species could modulate the acidity of the ionic liquid,making the ionic liquid have a wider acidity gradient,and activating the C₄H₈ molecules.In the DOS spectra,Cu Cl was observed to form different structures in the chloroaluminate ionic liquid and fill the HOMO orbitals to different degrees.3.The complex reaction process was investigated by using density functional(DFT)calculations and molecular dynamics(MD)simulations.On the basis of transition state theory,three elementary reactions of isobutane-isobutene alkylation are discussed in detail.Through transition state studies,the formation mechanism of the target product and by-products was analyzed.It is found that the hydride transfer occured directly between isobutane and C₈⁺carbanions in pure chloroaluminate ionic liquids.Although the Cu species in the composite ionic liquid did not significantly enhance the reaction rate of hydride transfer,they strongly inhibited the polymerization of C₈⁺carboions and isobutene.The high selectivity of the composite ionic liquid-catalyzed alkylation reaction is the result of the intense competition between hydride transfer and polymerization.The Cu species is essential factor for the improvement of the C₄ alkylation selectivity.4.The simulation results of ionic liquid infrared spectroscopy are also discussed.The infrared spectroscopy of ionic liquids simulated by ab initio computational molecular dynamics(AIMD)method is highly comparable to the experimental spectrum.Moreover,simulated infrared spectroscopy can specifically analyze the spectral peaks corresponding to anions and cations,and then determine the structure and composition of ionic liquids.The order of Lewis acidity of ionic liquids can also be analyzed by simulation calculation,and reveals the main reason of the catalytic activity.