Hydration of α-pinene homogenous catalyzed by acidic polyether-modified ammonium salt ionic liquid

Res Chem IntermedDOI10.1007/s11164-013-1355-1Hydration of a-pinene homogenous catalyzed by acidic polyether-modified ammonium salt ionic liquidin‘‘microreactor’’Lu Li•Yue Liu•Shi-Tao Yu•Shi-Wei Liu•Cong-Xia Xie•Fu-Sheng LiuReceived:17March2013/Accepted:29July2013ÓSpringer Science+Business Media Dordrecht2013Abstract Water-soluble ionic liquids(ILs)with the nonionic polyether modified ammonium(Ac18n)as cation and[HSO4]-,[H2PO3]-,[PTSA]-,or[BF4]-as anion were synthesized and the critical solution temperature(CST)of these ILs in a-pinene were studied.The results showed that the new ILs([Ac1820]?[HSO4]-and [Ac1820]?[H2PO4]-)were of CST in a-pinene at80°C.Then,the new ILs was applied in hydration of a-pinene as catalysts.During the process of reaction,catalysts would carry water molecules to phase of a-pinene and therefore to form the micro-group of water-in-oil.The micro-group is regarded as a micro-reactor for its acidic group,in which the catalytic reaction would have taken place homogeneously when the reaction temperature(T)is higher than CST.When the reaction completed(T\CST),the catalyst would precipitate out from the product,and be recycled efficiently.Keywords HydrationÁPineneÁCSTÁMicro-reactorÁIonic liquids Introductiona-Terpineol,an important monoterpenic monocyclic alcohol,has several applica-tions in the perfume and pharmaceutical industries.It is obtained industrially through Electronic supplementary material The online version of this article(doi:10.1007/s11164-013-1355-1) contains supplementary material,which is available to authorized users.L.Li(&)ÁY.LiuÁS.-T.YuÁS.-W.LiuÁF.-S.LiuCollege of Chemical Engineering,Qingdao University of Science and Technology,53Zhengzhou Road,Qingdao266042,People’s Republic of Chinae-mail:zhanglilu@C.-X.XieKey Laboratory of Eco-Chemical Engineering,Ministry of Education,College of Chemistry and Molecular Engineering,Qingdao University of Science and Technology,53Zhengzhou Road,Qingdao266042,People’s Republic of ChinaL.Li et al. the hydration of a-pinene with an aqueous mineral acid,a subsequent cis-terpene hydrate andfinally partial dehydration(Scheme1)[1–3].The influence of several acids(HCl,H2SO4)in the hydration of a-pinene has been previously studied[4]. However,aqueous mineral acid has inevitable shortcomings,such as corrosion of equipment,pollution of the environment,and complexity of post-processing.In order to solve these problems,solid acid has become the primary object to research as catalysts in the last10years[5–10].Although solid acidic catalysts can avoid the shortcomings of mineral acid,they also have some problems,such as long reaction time and low conversion rate.It is worth mentioning that the hydration of a-pinene was carried out at the interphase between oil and water,and therefore the mass transfer across the interphase was the major factor on reaction.However,the research about the mass transfer across the interphase has been small with regard to the hydration of a-pinene.The emulsifier can be added in the reaction to improve the mass transfer at the interphase,but this method brings big problems during the post-processing[11].Therefore,a new catalyst which has excellent catalytic activity and can improve the mass transfer across the interphase and simplify post-processing in the catalytic system would be of benefit in developing the hydration of a-pinene.To achieve separation of products from the catalyst in homogeneous catalysis, liquid–liquid two-phase systems have been developed[12–14].In such systems,the catalyst complex remains in one phase while the product remains in another immiscible phase.According to the above theory,Jinfirstly reported a new idea of thermoregulated phase transfer catalysis[15].The new catalytic system was used in different hydroformylation and hydrogenation of high-carbon olefins,and the results were good[16].Based on the above ideas,wefirst prepare thermoregulated-acidic ionic liquids(ILs)using polyether modified ammonium(Ac18n,n degree of polymerization)as cation to catalyze the hydration of a-pinene.In the new catalytic system,the performance of thermoregulation is up to chain length of polyether. Meanwhile,when heated to T[CST,catalysts would carry water molecule toOHH+H+Scheme1Hydration reaction of a-pineneModified ammonium salt ionic liquidphase of a-pinene and at the same time the micro group of water-in-oil was formed in the phase of a-pinene.Then,each micro-group of water-in-oil was of the acidic group,and therefore the micro-group is regarded as a micro-reactor.In each micro-reactor,the contact of water and a-pinene strengthened,so the reaction could be easily carried out.After the reaction(T\CST),the catalyst precipitates from the organic phase,and can be easily separated from the product and recycled efficiently. In this paper,the catalytic mechanism and the relevant factors that affect the reaction were carefully discussed.The catalyst has the advantages of aqueous and solid acid,it is a good start to develop the hydration of a-pinene.A preliminary report of this work has been presented elsewhere[17].ExperimentalMaterialsAll materials,such as a-pinene,anhydrous magnesium sulfate,octadecylamine polyoxyethylene(1,812,1,815,1,820,1,830),1,3-propanesulfonic acid lactone(1,3-PS),sulfuric acid,phosphoric acid,p-toluenesulfonic acid(PTSA),tetrafluorobo-rate,were all purchased from Aldrich,and all materials were directly used after drying without further purification.Reaction experimentsThe synthetic reaction of[Ac18n]?[HSO4]-was performed in a100-cm3three-necked glass reactor with a refrigerant and a thermocouple.The reactor was submerged in a thermostatic bath with silicone oil and magnetic stirring.In batch experiments,a certain amount of Ac18n and deionized water wasfirst added.After heating to the desired temperature,the equal-mole amount of sulfuric acid was added,and then heated in an oil-bath for2h.The water of the mixture was then removed under vacuum(5–10mmHg)at90°C,then a light yellow solid [Ac18n]?[HSO4]-was obtained,and the product was analysed with FT-IR,1H NMR,and13C NMR.[Ac18n]?[H2PO3]-,[Ac18n]?[PTSA]-,and[Ac18n]?[BF4]-were prepared in the same way(Scheme2).The tests of CST were performed in a200-cm3three-necked glass reactor with a refrigerant and a thermocouple.The reactor was submerged in a thermostatic bath with silicone oil and magnetic stirring.The ratio of a-pinene,ILs,and water was n(a-pinene):n(ILs):n(water)=1:0.05:5.The a-pinene,ILs,and water were added to the reactor and heated.When the solvent system became homogeneous phase,the temperature was CST.The catalytic tests were performed in a200-cm3three-necked glass reactor with a refrigerant and a thermocouple.The reactor was submerged in a thermostatic bath with silicone oil and magnetic stirring.In batch experiments,0.06mol of a-pinene, and5.4mL of water werefirst added.After heating to the desired temperature,the catalyst was added.Aliquots were extracted with a micropipette atfixed times and immediately analyzed with a GC17Shimadzu gas chromatographer equipped withL.Li et al.an FID and a capillary DB1column(60m in length).The reaction products were identified by comparing the retention times of terpene standards or using their Kovats index with confirmation by mass spectroscopy.The ILs can be directly used in the second cyclic utilization(Scheme1).Results and discussionTo realize the idea mentioned above,we designed new acidic ILs using Ac18n as cation that could be dissolved in a-pinene at high temperature and could not be dissolved in a-pinene at low temperature.The dissolving behavior of new catalysts in a-pinene and water was investigated at different temperatures(Table1).From Table1,it can be seen that ILs with different acidic groups as anion were of different solution temperatures in a-pinene.The ILs with[H2PO4]and[HSO4]as anion could be dissolved readily in a-pinene when the temperature was more than 80°C,and could not be dissolved in a-pinene and formed two phases when the temperature was less than80°ing[PTSA]as anion,the ILs could be dissolved in a-pinene when the temperature was more than100°C,and be partly dissolved in a-pinene when the temperature was less than100°C.However,using[BF4]as anion,the ILs could not be dissolved in a-pinene when the temperature was more than140°C.The results showed that the new ILs([Ac1820]?[HSO4]-and [Ac1820]?[H2PO4]-)were of CST in a-pinene at80°C.The dissolving behavior of these ILs in a-pinene and water was investigated using[Ac1820]?[HSO4]-as an example.It suggested that the thermal motion of molecules between [Ac1820]?[HSO4]-and a-pinene would strengthen with a rise of temperature,so that the system became uniform phase.When the temperature decreased,thermal motion of molecules between[Ac1820]?[HSO4]-and pinene would weaken.At the impact of gravity,[Ac1820]?[HSO4]-would move down,and the intermiscibility between[Ac1820]?[HSO4]-and a-pinene reduced,so the system become two phases.At the same time,the effect of degree of polymerization(n)of [Ac18n]?[HSO4]-and[Ac18n]?[H2PO4]-on the dissolving behavior was also studied(Table2).From Table2,it can be seen that n greatly affected the dissolving behavior between ILs and pinene.When n is less than20,with the increasing of temperature,[Ac18n]?[HSO4]–and[Ac18n]?[H2PO4]-cannot be dissolved in a-pinene,and the same phenomenon can be observed when n is30.Only when n is20,[Ac1820]?[HSO4]-and[Ac1820]?[H2PO4]-can be dissolved in a-pinene at a temperature more than80°C.From the above result,80°C is the CST ofModified ammonium salt ionic liquid[Ac1820]?[HSO4]-and[Ac1820]?[H2PO4]-in a-pinene.For the hydration of a-pinene,80°C was a suitable temperature[1].Next,the catalytic activity of [Ac1820]?[HSO4]-and[Ac1820]?[H2PO4]-on the hydration of a-pinene was researched.It can be seen from Table3that the[Ac1820]?[HSO4]-is of good catalytic activity,and its selectivity to a-terpineol is more than aqueous acid.That result must be relative to the thermoregulated performance of[Ac1820]?[HSO4]-.According to the dissolving mechanism,the phenomenon of the reaction can be explained as Fig.1.At room temperature,the hydrogen bond between catalyst and water makes the catalyst disperse uniformly in water.On heating to the temperature,the hydrogen bond between the catalyst and water would weaken and the catalyst would dissolve in a-pinene.In the process of dissolving,the catalysts would carry water molecular to the phase of a-pinene and at the same time the micro-group of water-in-oil was formed in the phase of a-pinene[19].Then,every micro-group was regarded as a micro-reactor[20].In every micro-reaction,a-pinene could easily be catalyzed to the aimed production.When the reaction completed,catalysts precipitate out from the compound of the product,and could be easily separated from the product and recycled efficiently.For the two ILs,[Ac1820]?[HSO4]-has the better catalytic activity and selectivity to a-terpineol,which can be explained from their acidity.FT-py spectra is an effective tool to verify the type of acid,as Table1The dissolving behavior of different ILs in a-pinene at different temperatureT(°C)[Ac1820]?[H2PO4]-[Ac1820]?[HSO4]-[Ac1820]?[PTSA]-[Ac1820]?[BF4]-40i i ps ps60i i ps ps80s s ps ps100s s s ps120s s–ps140–––pss Solubilization,ps part solubilization,i insolubilization;n(a-pinene):n(ILs):n(water)1:0.05:5Table2The dissolving behavior of different degree of polymerization(n)ionic liquids in a-pinene at different temperaturesT(°C)[Ac18n]?[H2PO4]-[Ac18n]?[HSO4]-121520301215203040i i i i i i i i 60i i i i i i i i 80i i s i i i s i 100i i s i i i s i 120i i s i i i s i 140i i–i i i–is Solubilization,i insolubilization;n(a-pinene):n(ILs):n(water)1:0.05:5L.Li et al.Table3Effect of the type of catalysts on the results of reactionNo.Catalysts Components of product(%)X(%)S(%)a-Pinene Camphene Limonene Terpinolene a-Terpinol1H2SO4(33%)0.34 5.4415.637.7911.2699.613.1 2H3PO40.57 4.5422.0232.5615.1499.317.7 3[Ac1820]?[H2PO4]-9.88 1.1912.411.2129.8765.237.3 4[Ac1820]?[HSO4]- 1.82 5.3613.3014.6245.9197.957.4Reaction conditions:n(a-pinene):n(catalyst)n(water)1:0.05:5,a-pinene0.06mol,reaction temperature80°C, reaction time8hX conversion of a-pinene,S selectivity of a-terpinolModified ammonium salt ionic liquidthere is a absorption peak at1,460cm-1that means that the catalysts is of Lewis acid,and there is a absorption peak in1,537cm-1that means that the catalyst is of Bronsted acid[18].From Fig.2,it can be seen that the two catalysts are all of Lewis acid,and at the same time[Ac1820]?[HSO4]-is of Bronsted acid.It is suggested that the synergistic effect of Lewis and Bronsted acid for[Ac1820]?[HSO4]-can improve its catalytic activity.L.Li et al.The reusability of the[Ac1820]?[HSO4]-catalyst was tested at80°C.As shown in Fig.3,the conversion of a-pinene and selectivity to a-terpineol remained virtually identical during the recycle runs,the results indicated that[Ac1820]?[HSO4]-had good stability under reaction conditions.ConclusionThe concept of CST for the Ac18n cationic ILs has been primarily applied to the hydration of a-pinene.It has been found that hydration of a-pinene catalyzed by [Ac1820]?[HSO4]-proceeds efficiently,and high conversion of a-pinene and yield of a-terpineol are obtained under the optimum reaction conditions.The [Ac1820]?[HSO4]-catalyst could be recycled up to four times and almost no loss in activity was observed.The new type ILs we developed based on the concept of CST provide not only a novel approach for the hydration of a-pinene but also a method for the water–organic matter reactions.Acknowledgments Thefinancial support provided for this research by the National Natural Science Foundation of China(31000275)and Postdoctoral Science Foundation of China(201104582)is 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