Structural Characterization And Application In Isobutane Alkylation Of Ionic Liquid Analogues

Isobutane alkylation with 2-butene is a major process to produce high octane and environment-friendly blending component for gasoline.Two commercial catalysts concentrated sulfuric acid and hydrofluoric acid are gradually restricted because of equipment corrosion,potential environmental pollution and safety problems.The chloroaluminate ionic liquids（ILs）are known as a sustainable alternative catalyst,and composite ionic liquid（CIL）developed by our research group successfully realized the industrialization in the isobutane alkylation in 2013.However,these ILs have high cost and are sensitive to moisture.Moreover,the imidazolium and pyridinium chloroaluminate ILs reported widely by many literatures also exhibit high toxicity,poor biodegradability and difficult preparation.Amide-AlCl₃ based IL analogues as a kind of novel liquid catalyst have attracted more attention because of their tunable Lewis acidity,high catalytic activity,low cost,easy preparation and less moisture-sensitive.However,the relationship between the molecular structure,the physico-chemical property and the catalytic performance of these IL analogues can not be clarified clearly.Therefore,several amide-AlCl₃ based IL analogues with different structures and CuCl modification are firstly applied as safer and economically catalysts in isobutane alkylation with 2-butene.In this dissertation,several amide-AlCl₃ based IL analogues were synthesized through a one-step method using three different structure amides as donor molecules.The effects of the steric and inductive effects of the methyl group substituted on N atom on the asymmetric splitting of AlCl₃ and the coordination site of amide were investigated for these IL analogues.Bidentate coordination through both O and N atoms wasdominantintheN-methylacetamide-AlCl₃（NMA-AlCl₃）and N,N-dimethylacetamide-AlCl₃（DMA-AlCl₃）based IL analogues because of the inductive effect of the methyl group.By contrast,the acetamide-AlCl₃（AA-AlCl₃）based IL analogue presented mainly in the form of the monodentate coordiantion via O atom.Bidentate coordination was favorable to the asymmetric splitting of AlCl₃ with the same amide/AlCl₃ molar ratio compared with monodentate coordination.The ionic species percentage in these IL analogues ranked in the following order:NMA-AlCl₃>DMA-AlCl₃>AA-AlCl₃ under the influence of the steric and inductive effects of the methyl group.The above results can provide more characterization methods and theoretical basis for the understanding and development of these IL analogues.Moreover,the viscosity,density,conductivity,acid amount,acidity and isobutane solubility of amide-AlCl₃ based IL analogues were measured,and the effects of amide structure,amide/AlCl₃ molar ratio and CuCl modification on the physico-chemical properties were investigated further.The results showed that the conductivity of amide-AlCl₃ based IL analogues was much lower than that of traditional Et₃NHCl-AlCl₃IL with same ligand/AlCl₃ molar ratio,whereas the density and viscosity of amide-AlCl₃ based IL analogues were slightly higher.The acid amount of amide-AlCl₃based IL analogues measured by in-situ IR using nitrobenzene as indicator was found that the activity index of amide-AlCl₃ based IL analogues with monodentate structure was lower,whereas the activity index of amide-AlCl₃ based IL analogues with bidentate structure was higher than that of Et₃HNCl-AlCl₃ IL with the same ligand/AlCl₃ molar ratio.The acidity of amide-AlCl₃ based IL analogues was investigated by ³¹P NMR using tributylphosphine oxide as molecular probe.The results showed that both anionic Al species[Al₂Cl₇]^ˉand cationic Al species[AlCl₂L_n]⁺were the active Lewis acidic species in amide-AlCl₃ based ILs,and the acidity of[AlCl₂L_n]⁺was stronger than that of[Al₂Cl₇]^ˉ.The solubility experiment indicated that the isobutane solubility in NMA-AlCl₃ based IL analogues was highest.In addition,the conductivity,viscosity and density of amide-AlCl₃ based IL analogues increased after CuCl modification,whereas the acid amount and isobutane solubility decreased.These research results can provide the design foundation for the development of a suitable catalyst for isobutane alkylation.Then,the alkylation of isobutane with 2-butene to produce high-quality alkylate was catalyzed by several amide-AlCl₃-based IL analogues with different structures and CuCl modification.The influences of the amide structure,amide/AlCl₃ molar ratio,and CuCl modification on the catalytic performance were investigated in an autoclave operated in semicontinuous mode.The results showed that the NMA-AlCl₃ based IL analogue（molar ratio of NMA to AlCl₃ was 0.75:1,marked as 0.75NMA-1.0AlCl₃）with bidentate coordination and low viscosity was an efficient catalyst for isobutane alkylation.Furthermore,CuCl modification further enhanced the catalytic performance of 0.75NMA-1.0AlCl₃.The selectivity of C8 increased from 76.18 wt%to 94.65 wt%,the molar ratio of trimethylpentanes to dimethylhexanes（TMPs/DMHs ratio）and the research octane number（RON）of alkylate reached up to 14.98 and 98.46,respectively.The effects of reaction conditions on the alkylation performance were investigated in an autoclave operated in batch mode.The results indicated that the alkylation of isobutane with 2-butene catalyzed by CuCl-modified 0.75NMA-1.0AlCl₃ was a short-time and fast reaction.The optimal reaction temperature,stirrer speed,reaction time,ionic liquid to hydrocarbon volumetric ratio,and isobutane/2-butene molar ratio were 15°C,30 s,1500 r/min,1:3,and 100:1,respectively.Under these optimal reaction conditions,the selectivity of C8,TMPs/DMHs ratio and RON were 84.10 wt%,14.86 and 96.54,respectively.Finally,the long term isobutane alkylation experiment catalyzed by CuCl-modified0.75NMA-1.0AlCl₃ was carried out in an autoclave operated in continuous mode to investigate the distribution of alkylate under different experimental nodes.The structural variation of CuCl-modified 0.75NMA-1.0AlCl₃ in the presence and absence of deactivation was analyzed by ²⁷Al NMR and ¹H NMR.The results indicated that the alkylation process could be divided into three stages:rising region,stable region and descending region.Meanwhile,the C8 selectivity and TMPs/DMHs ratio were kept at the highest level in the stable region,and alkylate RON was as high as 97.The anionic Al species（[Al₂Cl₇]^ˉ,[AlCuCl₅]^ˉ）and cationic Al species（[AlCl₂L]⁺）from the IL analogue as two active lewis acid species played a catalytic role in the long term experiment,whereas the neutral Al species did not participate into the reaction.Compared with the CIL,the catalytic lifetime of the IL analogue was similar,but the IL analogue had a more competitive cost.