| As one of the important fine chemical products,1,8-cineole has huge value in medicine and aromatizer fields.Currently,1,8-cineole was achieved via steam distillation of eucalyptus.However,planting eucalyptus has raised environmental issues such as its potential invasiveness,the effect on water,and so on.Therefore,an alternative measure in addressing this challenge is via chemical synthesis.Our group has previously obtained preliminary results through isomerization of a-terpineol to 1,8-cineole over phosphotungstic acid(PW)/SiO2;however,the unstability of the catalyst has not been solved.As a continuous work,this study pays attention to preparation of PW catalyst with high activity and stable performance.PW immobilized on imidazolium poly(ionic liquid)(PIL)via ion-exchange was prepared.The feasibility of isomerization for a-terpineol to 1,8-cineole catalyzed by PW/PIL(poly(l-butyl-3-vinylimizolium bromide))was discussed.Several factors related to the isomerization were explored.Firstly,the dispersion of PW/PIL catalysts was characterized by TQ FTIR,and XRD.Then,the effects of molar ratio of PW to IL monomer,agitation rate,and drying methods for preparing PW/PIL catalysts were investigated.Finally,the products were identified by GC and GC-MS.Results showed that 1,8-cineole,1,4-cineole,2-carene,limonene,y-terpinene,terpinolene,as well as some oligomerization were mainly produced by a-terpineol over the PW/PIL catalyst.Besides the supercritical CO2 drying,the preferred molar ratio of PW to ionic liquid monomer was 1:3 for preparing the PW/PIL catalyst.The PW/PIL(poly(l-butyl-3-vinylimizolium bromide))catalyst prepared was used to catalytic isomerization of a-terpineol.The effects of reaction temperature,mass of catalyst,reaction time,amount of solvent,and agitation rate for the isomerization of a-terpineol were investigated.Besides,the stable nature of PW/PIL catalyst was examined.Results showed that the preferred conditions for isomerization of a-terpineol to 1,8-cineole over PW/PIL catalysts were as follows:the reaction temperature was 50?,the reaction time was 8 h,the mass fraction was 1.12%(based on the total amount of the reaction solution),the amount of solvent should be 43 mL/g a-terpineol,and the agitation rate should be higher than 280 r/min.100%conversion of a-terpineol and 51.7%selectivity to 1,8-cineole were achieved under the optimum conditions.Furthermore,the PW/PIL showed a stable catalytic performance:98.4%conversion with a corresponding selectivity of 50.8%could still be obtained after seven times of the catalyst reuse.The fresh PW/PIL and reused(seven times)PW/PIL catalyst were characterized by TCQ XRD,and FTIR;results illustrated that the PW could disperse well on PIL after reused.The acid strength of PW/PIL catalysts were characterized by NH3-TPD;results revealed that different molar ratios of PW to IL monomers possessed different acid strengths,and a measurable acid strength was needed for this reaction.A series of 1-alkyl-3-vinylimizolium bromide monomers(alkyl from ethyl to hexadecyl)were applied for the synthesis of PILs as the supports of PW5 revealing that the lipophilicity of longer chain alkyl PIL exposed the PW active sites efficiently,which subsequently enhanced the a-terpineol conversion and reduced the reaction time.In the end,a plausible mechanism for isomerization of a-terpineol in pseudoliquid phase of PW was presented. |
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