Research On Ionic Liquid Catalyst For Alkylation Of Aromatic Hydrocarbons:Preparation And Performance

The detergent containing linear alkyl benzene sulfonate（LAS）with high environmental safety has good solubility in any water quality,excellent washing performance,and high biodegradability.As a fundamental intermediate product of LAS,linear alkyl benzene（LAB）has attracted more attention and investigated extensively.Traditional LAB synthesis processes usually use toxic and corrosive catalysts（hydrofluoric acid,concentrated sulfuric acid and aluminum trichloride）.However,there are some common problems in each process,such as serious environmental pollution,complicated product recovery and purification,moreover,hardly recyclability because of no recrystallization of catalyst.Compared to traditional catalysts,ionic liquids meet the requirements of green chemistry and show unique advantages as catalysts,green solvents and extractants.Using acidic ionic liquid（AILs）as alkylation catalyst can achieve high reaction rate at lower reaction temperature and good selectivity to LAB.The cations of AILs are mainly divided into imidazolium-,pyridinium-,and quaternary ammonium-based.On the one hand,1-methylimidazole as a Lewis base with strong coordination properties is difficult to remove from the ionic liquid,which will cause catalyst poisoning.On the other hand,the raw material of pyridinium-based ionic liquid is harmful to human health.To avoid the disadvantages mentioned above,piperidinium-based AILs with weak basicity and low toxicity were selected as the research object.In this paper,a series of piperidinium-based ionic liquid catalysts were sysnthesised,and Br（？）nsted-Lewis diacidic ionic liquid catalysts were prepared by functionalizing the carboxyl group（-COOH）of the piperidine cation.Then the above catalysts were used for the alkylation of xylene and long-chain olefins（C14 olefins-C18olefins）.The full text of the study is as follows:（1）Preparation and alkylation performance of monoacidic piperidinium-based ionic liquid catalystsThe ionic liquid precursors[Pip_1,x]Br were prepared by the quaternization reaction of 1-methylpiperidine with bromoalkanes.Then the[Pip_1,x]Br was directly combined with Al Cl₃ to prepare a series of piperidinium-based ionic liquids with Lewis acidity[Pip_1,x]Br-n Al Cl₃（x=4,6,and 8,n=1,2,and 3）.Using those monoacid catalysts to catalyze the alkylation of p-xylene and 1-hexadecene.The aluminum nuclear magnetic resonance spectrum(²⁷Al NMR)of[Pip_1,8]Br-n Al Cl₃（n=1,2,and 3）proved that the bromochloroaluminate anions were[Al Cl₃Br]^-,[Al₂Cl₆Br]^-,and[Al₃Cl₉Br]^-when n=2.In particular,the acid anions[Al₂Cl₆Br]^-and[Al₃Cl₉Br]^-with strong catalytic alkylation performance were widely present in the catalytic system,and it can be concluded that[Pip_1,x]Br-2Al Cl₃ was suitable for catalytic alkylation reaction.Furthermore,the FT-IR spectra of[Pip_1,8]Br-2Al Cl₃ with acetonitrile（ACN）as probe confirmed that[[Pip_1,8]Br-2Al Cl₃ had a stronger Lewis acid and excellent catalytic performance for alkylation of long-chain olefins.The conversion rate of 1-hexadecene and the selectivity of 1,4-dimethyl-2-（1-methyl-pentadecyl）benzene（2-LAB）were 99.70%and37.51%,respectively.In addition,the number of cycles was 14 times.（2）Preparation and performance of Br（？）nsted-Lewis diacidic ionic liquid catalystsThrough the quaternization of the halogenated acid with 1-methylpiperidine,the-COOH （Br（？）nsted acid）functional group was bonded to the piperidine cation to realize the functionalization of the ionic liquid.Then the precursor was directly combine with Al Cl₃ to obtain[COOHPip_1,x]X-2Al Cl₃（x=2,4;X=Cl,Br）ionic liquid catalysts with Br（？）nsted and Lewis diacidic,and the order of the acid strength was[COOHPip_1,4]Br-3Al Cl₃,[COOHPip_1,2]Cl-3Al Cl₃,[COOHPip_1,2]Br-3Al Cl_3.The results of ²⁷Al NMR showed that whenχ=0.75,the peak of acidic[Al₃Cl₉Br]^-ion or[Al₃Cl₁₀]^-ion was more obvious,and shifted to the higher field compared with the monoacid piperidinium-based ionic liquid catalyst.The catalytic performance of diacidic ionic liquids in the catalytic alkylation of xylene with long-chain olefins（C14 olefins-C18 olefins）were studied.The conversion rate of 1-hexadecane can reach 100%and the selectivity of 2-LAB was 39.77%when catalyzed by[COOHPip_1,2]Cl-3Al Cl_3.In addition,the conversion rate of 1-hexadecane was 100%and the selectivity of 2-LAB was 38.19%when catalyzed by[COOHPip_1,4]Br-3Al Cl₃.Furthermore,the optimal reaction system for the alkylation of long-chain olefins catalyzed by Br（？）nsted-Lewis diacidic ionic liquid catalyst was:[COOHPip_1,2]Cl-3Al Cl₃ catalyzed the alkylation of o-xylene with 1-tetradecene,among which the molar ratio of catalyst/1-tetradecene was 0.2:1.Besides,the experimental results of the cycle stability demonstrated that the-COOH functionalized piperidinium-based ionic liquids had good catalytic activity and excellent stability,and the cycle number was 17 times.