Research progress in ionic liquids catalyzed isobutane/butene alkylation☆

Panxue Gan,Shengwei Tang

Multi-phase Mass Transfer and Reaction Engineering Lab,College of Chemical Engineering,Sichuan University,Chengdu 610065,China

1.Introduction

Strong acid catalyzed isobutane/butene alkylation is an important petroleum process to produce high octane number gasoline,which has the characteristics of low contents of sulfur,aromatics and alkenes,and is commonly used as an excellent additive in global gasoline pool.Out of total global gasoline consumption,approximately 13%–15%consists of alkylation gasoline.Alkylation gasoline has a production rate of about 100 million tons per year[1].However,with the increase of vehicle ownership,phasing out of methyl tert-butyl ether(MTBE),expanding use of ethanol-gasoline blends and the ever growing environmental requirements of motor gasoline,the production and demand for alkylation gasoline show an increasing trend with every passing year[2,3].

Nowadays,sulfuric acid(H2SO4)and hydrofluoric acid(HF)have widely been used in commercial alkylation process to synthesize high-grade petrol via the addition reaction of light alkanes and alkenes.In such a process,the miscibility of the acid and hydrocarbon and the mass transfer of these two phase shave significantin fluenceson the reaction efficiency,the yield of target product and the catalyst lifetime[4].The acid soluble oil(ASO)and a large quantity ofwaste acid were produced in H2SO4process as byproducts.It is very costly to regenerate the waste acid[5].In contrast to the H2SO4process,the catalyst consumption of hydrogen fluoride process is relatively lower(less than 1 kg·t?1alkylate).However,there has been much concern about the toxicity of HF along with its volatility and corrosiveness.Although the HF units are easier to expand and have lower maintenance and operational costs than the corresponding H2SO4units[6],yet the fundamental problem of being environment-unfriendly still remains unchanged.So,solid acid is thought to be a potential alternative to replace the traditional acids in catalyzed alkylation processes[7,8].The solid acid catalyst shows good catalytic activity and selectivity for alkylate under certain conditions as well as in a commercial process.However,the use of solid acid can only be commercialized unless the problems,such as rapid deactivation and regeneration of the solid acid,could be solved effectively[9,10].

The reaction mechanismofC4alkylation is complicated due to a number of competing reactions simultaneously existing in the process[11,12].The selectivity of alkylate depends on the formation of carbocation and the protonation of the alkenes.The protonation of the alkenes depends on the acidity of the catalyst,while the concentration of the carbocation depends on the solubility of isobutane in the acid phase[1,13].Obviously,these two characteristics play a vital role in the alkylation process.Corma et al.[14]have reported that H0in the range of?8.1 to?12.7 is beneficial to alkylation.The loweracidity favors the alkene polymerization,while higheracidity aggravates the cracking of alkylate.Because of the reaction between H2SO4and the organics[15],the acid strength of H2SO4is decreased gradually,which in turn affects the selectivity and yield ofalkylate and also produces a lot of waste acid as a byproduct.

The solubility of the reactants in the acid phase has significant influence on the reaction direction and reaction extent of alkylation.In the commercial H2SO4or HF process,the difference in polarity of the isobutane and butene leads to the difference of dissolubility in catalystphase.As a result,the local I/O(isobutane to olefin)ratio in the reaction region is significantly lesser than the required for the desired reaction and therefore,a number of side-reactions are favored due to the excess of olefin.As a crucial step in C4alkylation process,the rate of hydride transfer between C8-carbenium ion and isobutane is around 3–4 orders of magnitude lower than the alkene polymerization or addition[16,17].Therefore,in the presence ofexcessive alkenes,the catalystdeactivation and low selectivity caused by the polymerization are difficult to be avoided.

Roomtemperature ionic liquids(RTILs)are molten salts(Tm＜100°C)and consist of larger organic cations and smaller inorganic anions.The asymmetry in the structure results in theirlow melting points.It is widely accepted that the physicochemical properties of the ILs,such as melting point,density,viscosity,solubility,and acid strength,can be adjusted by modifying the structure and combination of the cations/anions[18–21].Due to the possibility of controlling the acidity and the dissolution properties of the catalytic system,acidic ILs are thought to be a potentialalternative to replace the traditional acids[22–24].With these excellent features,it has extensively been investigated and successfully applied in many different chemical reaction processes such as esterification[25],Friedel–Crafts alkylation[26]and is omerization[27].

Acidic ILs have received extensive research attention due to their unique physicochemical properties.This review is designed to summarize and analyze the research progress catalyzed by ILs,which usually are used as catalysts or co-catalysts for C4alkylation.With a deep analysis of the role of ILs,we can better understand the fundamental catalysis principles and indicate the developmental direction of the exploring of novel catalysts,which could commercially be used in C4alkylation process.

2.Pure Ionic Liquid(IL)Systems

2.1.Chloroaluminate(III)ILs

Chloroaluminate ILs have comparative Lewis acid strength with anhydrous AlCl3and are widely investigated as acid catalyst in a lot of important reactions.They also show remarkable catalytic activity in C4alkylation process[28].A strengthened acidity is helpful to increase the hydride transfer rate between the TMP+(TMP represents Trimethylpentanes)and isobutane and to inhibitboth the isomerization of TMP+and the trend to form heavy ends,such as C12and C16.So the catalyst acidity has significant impact on the catalyst life,selectivity and yield for C8-alkylate[29,30].A proper and stable catalyst acidity is crucial for the process to obtain a long catalyst life and a high yield of target product.

Chauvin et al.[29]have reported the formation of stable molten salt mixture[BMIm]Cl/AlCl3([BMIm]Cl,1-butyl-3-methylimidazolium chloride)at room temperature by adding AlCl3to the[BMIm]Cl.The acidity of the molten salt mixture can be adjusted by changing the content of AlCl3in the mixture.As the AlCl3content increases,so does the strength of the acid.Therefore,altering the composition of ILs is an effective way to adjust the acid strength[31,32].This unique characteristic arouses extensive research on the application of chloroaluminate ILs in C4alkylation[31,32].The catalytic activity of[BMIm]Cl/AlCl3in the alkylation ofisobutane with 2-butene was studied by Chauvin etal.[29].A TMP yield of 88.6%was achieved.Yang et al.[33]established an IR spectroscopy method to study the acidity of[BMIm]Cl/AlCl3by using pyridine as IR spectroscopic probe and the catalytic activity of[BMIm]Cl/AlCl3in the isobutane–butene alkylation was also investigated.

The acidity and catalytic activity of Chloroaluminate ILs,Et3NHCl-AlCl3(triethylamine hydrochloride-aluminum chloride),in the alkylation of isobutane with 2-butene was studied by Liu et al.[34].When the molar ratio AlCl3/Et3NHCl of Et3NHCl-AlCl3varies from 1.5 to 2.0,the main existence form of the anion is changed from[AlCl4]?to[Al2Cl7]?,which leads to an increase in the acidity from ?12.1 to?13.1 and an increase in the mass fraction of C8-alkylate from 39.8%to 46.5%[34].It is obvious that the addition of AlCl3significantly improves both the Lewis acidity and the catalytic activity,whereas the quality of alkylate is also improved at the same time.

A number of studies focused on the mechanism of acidity adjustment.The Lewis acid–base properties of these ILs are determined by the chloroaluminate species.With the help of27Al NMR and FT-IR,the main existence forms of the anions in chloroaluminate ILs are:[AlCl4]?,[Al2Cl7]?and[Al3Cl10]?.These forms have been confirmed by a number of published studies[34–36].Cui et al.proved that the Lewis acidity of Et3NHCl-AlCl3is attributed to[AlCl4]?,[Al2Cl7]?and[Al3Cl10]?[36].The AlCl3composition in ILs has a significant effect on the existence form of the anions.When the molar ratio of AlCl3/[EMIm]Cl is less than 0.5,the anion is[AlCl4]?and the IL is basic.With the increasing content of AlCl3,[Al2Cl7]?is the another existence form of anion and the IL is acidic.And when the molarratio of AlCl3/[EMIm]Cl arrives 0.66,one of the existence forms of anion is[Al3Cl10]?(as can be seen in Table 1).

Table 1 The anion forms of ionic liquid[EMIm]Cl-AlCl3 at different AlCl3/[EMIm]Cl ratios(reproduced from ref.[22]).

Viscosity is a fundamental physicochemical property of the liquid catalyst.It has a dramatic influence on the mixing of the reactant and the catalyst phase or the interphase mass transfer,then it plays a vital role on the reaction efficiency and target product yield.Viscosities of the ILs are strongly dependent upon the compositions.Some works are concerned with the relationships between the viscosity of acidic ILs and the structure or compositions.With longer chains in cation,the resulting ILs become more and more viscous.The van der Waals interactions and hydrogen bonding are the sources of the high viscosities[37].For an IL with the form of[AMIm]X-type([AMIm]=1-alkyl-3-methyl imidazolium,X=anion),the effectof anionic species on the viscosity is much stronger than that of the alkyl chain length[38].The viscosity is also strongly dependent upon the mole fraction of AlCl3in the IL.Among the Cl?,[AlCl4]?and[Al2Cl7]?anions,Cl?allows the strongest hydrogen bonding while[Al2Cl7]?results in the lowest[39].So,a higher AlCl3content not only increases the acidity,but also helps to reduce the viscosity.It is beneficial for the hydrocarbon dispersion in the acid phase,which helps to enhance the mass transfer and the reaction rate in this system.This eventually improves the quality of alkylate.

Since the reaction region of the alkylation lies mainly in the interface,therefore the solubility of the reactants in the catalyst phase exhibits a significant impact on the selectivity of the resultant product[40,41].However,because of the differences in the polarities of isobutane and butene,the rate of alkene polymerization is far more than the corresponding hydride transfer between C8+and isobutane,which deactivates the catalyst and reduces the selectivity of the objective product[42].By increasing the solubility of isobutane in the acid phase,it is likely to improve the local I/O ratio.This not only intensifies the formation of C8-alkylate,but also extends the catalyst life time[30].

It has been reported that by increasing the alkyl chain length of the cation,the solubility of isobutane in the acid phase also increases,which in turn increases the local I/O ratio.With a longer alkyl chain,it is possible to modulate the physical properties of ILs from being hydrophilic to hydrophobic[43].As a result,one could predictor tune the dissolution properties by modifying the cation with different alkyl substituents.

Yoo et al.[30]conducted a detailed study of the alkylation of isobutane and 2-butene in IL media using[AMIm]halide–aluminum chloride.The study encompassed various alkyl groups and halides on IL's cation and anion respectively.For the same halide anion,ILs with longer alkyl chains were more reactive(1-octyl-3-methylimidazolium[OMIm]),which could be attributed to the higher solubility of the isobutane in the ILs.But it should be noted that the ratio of TMP/DMH(DMH represents dimethyl hexanes)was kept almost constant by simply changing the alkyl chain length.If cation is kept constant and anion is varied from Cl?to Br?,a strong Lewis acid[Al2Cl6Br]?forms,which could react with hydrogen atom of the imidazolium ring in the 2-position,and hence results in the formation of a Br?nsted acid[44].Compared to the H2SO4process,[OMIm]Br-AlCl3was more active.However,the selectivity to TMPs was almost one third of the H2SO4process[30].

In an earlier study,Evering and Roebuck[45]discovered that the mixtures of aluminum chloride-ether complexes are highly active to C4alkylation process,where the activity was attributed to the higher solubility of isobutane in ether.With this in mind,Lu et al.[46]synthesized a series of acidic chloroaluminate ILs functionalized with ether group,which show a remarkable impact on the alkylation process.With the adjustment of the Lewis acidity and isobutane solubility improvement in ionic liquid by the ether group,1-methoxyethyl-3-methylimidazolium bromide aluminum chloride([MOEMIm]Br/AlCl3)was the best catalyst among all the investigated ones.The selectivity of C8in alkylate reached 66.6%under optimized conditions(xAlCl3=0.75,I/O=10,T=308 K).

Though chloroaluminate ILs have been shown plausibility,the selectivity and alkylate quality was seldomgreaterthan those obtained in the commercial alkylation processes.Additionally,chloroaluminate ILs are extremely oxophilic[47],forming adducts with C–O functionalities and sensitive to traces of water for the release of HCl.Moreover,the change in acidity during alkylation process and the inability to restore its original value has inhibited the commercial application of the chloroaluminate ILs in alkylation processes.

2.2.Modified chloroaluminate ILs

It is difficult to have a balance among the requirements of acid strength,dissolution characteristics and interfacial characteristics in C4alkylation process.In case of chloroaluminate ILs as main catalysts,the addition of different organic or inorganic additives is beneficial to modify the catalyst's physicochemical properties,due to which better performance can be obtained.

A representative work is the composite ILs chloroaluminate ILs+CuCl with a successfully industrial application at a 100 kt·a?1plant.A composite ionic liquid(CIL)developed by China University of Petroleum shows good catalytic performance for isobutane alkylation.The first commercial 100 kt·a?1CILA plant was successfully commissioned in 2013 in Shandong by Deyan Chemical Co.Ltd.,China.This plant includes four sections namely feed treatment,staged reactors,reactor refrigeration,and product treatment.In addition,this plant also has CIL handling facilities to enable the removal of solids and regeneration of CIL.It is said that:the olefin conversion was 100%,the alkylate yield was 80%of the feed,the average RON of alkylate was 96.8,the catalyst consumption per ton alkylate was 5 kg,the energy consumption per ton alkylate was 150 kgeo.It is also said that,in comparison to HF and H2SO4plants,the CILA plant has better safety and environmental benefits and has been running stably for two years.

In an earlier research,Huang et al.[48]have reported that when CuCl is added to Et3NHCl-AlCl3,side reactions such as cracking,polymerization,and isomerization can be inhibited by the interactions between CuCl and[AlCl4]?or[Al2Cl7]?.However,the mechanism of the process is still unknown.With the help of ESI-MS and27Al NMR,Liu et al.[49]con firmed that a composite anion[AlCl4CuCl]?was formed.As an excellent acceptor of carbenium-ion,[AlCl4CuCl]?composite anion should be preferred for increasing the hydride transfer rather than the chain propagation step which causes the formation ofCarbenium-ion chain mechanism showed that most of the light byproducts of alkylation could be considered as a result of β-scissions ofCompared to the H2SO4process,the TMP yield reached 92%in alkylate.After this,the same researchers systematically investigated the changes of composition,acid strength and product distribution due to the addition of different additives in ionic liquids of[BMIm]Cl-AlCl3and Et3NHCl-AlCl3[50].As reported,the C8selectivity is probably determined by the composition of the modified IL catalyst rather than the acid strength.The complexation of transition metal with 2-butene can increase the isobutane-to-butene ratio during the alkylation reaction[51],which results in a better selectivity of objective product.Hence,the composition change of the modified IL should be responsible for these better results.

It should be noticed that a change in the mole fraction of AlCl3in the IL only affects the number of acid sites and the acidity of ILs;it does not have any impacts on the properties of the acid sites.For this reason,Zhang et al.did notobtain a significant improvement on the catalytic behavior of the ILs in isobutane/2-butene alkylation just by changing the mole fraction of AlCl3[52].However,when aromatic compounds are added to Et3NHCl-AlCl3IL,the Lewis acid sites could be modified through Π-complexation between the Lewis acid sites and the Π-cloud of the aromatic ring[45,52].Additionally,the alkyl carbenium could also be stabilized by the interaction of carbenium and aromatic ring,which inhibited the further occurrence of the secondary reaction.Furthermore,when a substituent group of the added aromatics(having an electron-donating group)was incorporated,the Lewis acidity was crippled by the Π-complexation of the Lewis acid sites with Π-cloud of the aromatic ring,whereas the existence of the electron withdrawing group functionalized the other way around.

Evering et al.[45]reported that inhibitors,which are aromatics or metal halides added in the AlCl3-ether,are more inclined to behave as basic and the combination of H+with aromatics is strong.In such a case,a carbenium ion with lower acid strength was formed as seen in Fig.1.

Fig.1.Mechanism of inhibition through the formation of less acidic species(adapted from Ref[45]).

The carbenium ion mechanism is widely accepted by researchers,whereas the protonation of the alkene is the initialstep of the alkylation reaction[42,53].With suitable promoters combined with[BMIm]Cl-AlCl3ionic liquid,the yields of TMP and RON may have comparable values to compete with both H2SO4and HF processes[54,55].The active species(tert-butyl cation)of alkylation reaction are directly generated as a result of the introduction of tert-butyl halide as a promoter[55],instead of hydride shift between the protonated 2-butene and isobutane,which implies that the reaction pathway has been changed.Compared to the H2SO4process,when tert-butyl bromide was added after 5 min,the conversion of the 2-butene was almost 100%,while under other conditions it was only slightly higher than 30%.However,because of the limitation of hydride transfer betweenand isobutane,the selectivity for TMP still remains low,even though the apparent reaction rate has been improved by around 1–2 orders of magnitude.There is a great deal of consensus on the factthat the Lewis acid chloroaluminate ILs are extremely sensitive to moisture and traces of water for the release of HCl through an irreversible hydrolyzation process[47].However,there is little clear quantitative understanding of how the hydrolysate influences the composition and physicochemical properties of the chloroaluminate ILs.The released HCl dissolves in IL and interacts with Lewis acid species,and the H+can be released entirely without the solvation effect,so that the IL probably becomes a superacid.It was verified in an acidity measurement of chloroaluminate IL by a UV spectroscopy method.When anhydrous HCl was filled in chloroaluminate IL,the acid strength was stronger than that of 100%H2SO4.It showed a character of superacid[56].Ferdinand P?hlmann et al.[57]reported that the[BMIm]Cl-AlCl3ILkeepsa constant catalytic activity by filling anhydrous HClcontinuously.The deactivated chloroaluminate IL can also be regenerated by the introduction of anhydrous HCl,which currently is a common method used to regenerate and improve the chloroaluminate IL catalyst[57–59].When HCl or trace water was filled in chloroaluminate IL,a Br?nsted acid species[Al2Cl6OH]?was detected.It is probably the reason for the formation of superacid.The plausible mechanism can be described as seen in Fig.2.

Fig.2.Reaction pathway for the generation of superacid in Lewis acidic ILs(adapted from Ref[54]).

Hence,water seems to be a promising additive to tune the acidity of AlCl3-based ILs.In the case of[OMIm]Br-AlCl3and[(HO3SBu)MIm]HSO4(1-(4-sulfbutyl)-3-methylimidazolium hydrogensulfate),Bui et al.[54]investigated the influence of water as additive.Due to the interaction of H2O and[Al2Cl7]?,H+was released.When[(HO3SBu)MIm]HSO4was used,the proton located in–SO3H group also reacted with AlCl3to form a superacid(see Fig.3).But it should be emphasized that the concentration of water cannot exceed 0.12 mg·g?1in the feed stream.Also,for a higher acidity,the formation of much heavier endis favored.

Fig.3.Formation of superacidic species by the reaction of AlCl3 and sulfonic acid group(adapted from Ref[54]).

The selectivity of C8and TMPs can significantly be improved by the use of modified chloroaluminate ILs.In fact,selectivity higher than the traditional H2SO4and HF processes can be achieved.However,the sensitivity to moisture still remains to be solved.Additionally,it is unfavorable for controlling the reaction process,where the catalytic process is complicated due to the additives.In some extreme cases,the catalyst may be deactivated due to the precipitation of the CuCl[60].

3.Br?nsted Acid ILs

The chloroaluminate ILs performed commendably in C4alkylation process.With adjustment of the amount of AlCl3and reasonable choice of the auxiliaries,the selectivity for C8and catalytic activity can significantly be improved.However,in the presence of traces of water,there is a dramatic change in the acidity[54].There are a variety of novel ILs which have emerged with the development of various applications of the ILs.Therefore,it is becoming a new field to develop novel ILs for the application in the C4alkylation,which would have a strong acidic character and would demonstrate stability to moisture.H2SO4or HF used in the traditional alkylation process belongs to Br?nsted acids.The reason for using H2SO4or HF is that the use of Br?nsted acid favors the formation of target alkylate rather than the unsaturated compounds[42,61,62].So,researchers tend to find alternative Br?nsted acid ILs to replace the chloroaluminate ILs that have previously been used in C4alkylation.Cole et al.[63]reported the synthesis of a series of Br?nsted ILs in 2002 for the first time.The cation of the ILswas modified with sulfonic group and proved catalytic activity in several classical acid-promoted organic reactions.These Br?nsted ILs possess not only strong acidity but also enough stability to moisture orwater.Due to these superior characteristics,these Br?nsted ILs have been applied in many chemical processes[64,65].

3.1.Pure Br?nsted IL

Tang et al.[66]synthesized a series of ILs.Some of these included 1-hexyl-3-methylimidazolium bis(trifluomethylsulfonyl)-amide([HMIm][Tf2N]),1-butyl-3-methylimidazolium hydrogen sulfate([BMIm][HSO4]),1-octyl-3-methylimidazolium hydrogen sulfate([OMIm][HSO4]),3-methyl-1-(3-sulfobutyl)-imidazolium hydrogen sulfate([MBSIm][HSO4]),3-methyl-1-(3-sulfobutyl)-imidazolium trifluoro methylsulfonate([MBSIm][OTf])and 3-methyl-1-(3-sulfopropyl)-imidazolium trifluoro methylsulfonate([MPSIm][OTf]).These ILs have been used as C4alkylation catalysts both in pure form and coupled with H2SO4or TFOH(trifluoromethanesulfonic acid).The results showed that pure Br?nsted ionic liquid with dual-acidic group exhibited comparable value of TMP/DMH.However,some of these ILs,which have single–SO3H group,did not provide satisfactory results for the very low conversion.Additionally,in comparison to pure H2SO4or TFOH and IL/acid systems,all of these ILs perform poorly.Liu Ying et al.[67]have synthesized a series of ILs modified by–SO3H group and have also investigated their catalytic activity towards C4alkylation.For the same cation,anions were varied from[Tf2N]?to[HSO4]?and then to[OTf]?.The conversion of 2-butene and selectivity for C8remained below 60%and 40%respectively,while bothappeared in alkylate due to lower acidity(H0＞?8.2).When–SO3H group was grafted onto the cation,the acidity was enhanced and a better selectivity for C8was observed.

Olah and coworkers[68]found that anhydrous HF forms remarkably stable solutions with pyridine and its analogs to give pyridinium poly(hydrogen fluoride)(see Fig.4),which performs as an IL.Two different kinds of amine-(HF)nILs and HF immobilized on solid polymeric amines ionic solid were synthesized with 70%–95%HF contents.The pyridinium poly(hydrogen fluoride)PPHF,poly ethylenimine(hydrogen fluoride)PEIHF and poly(4-vinylpyridinium)poly(hydrogen fluoride)PVPHF systems were successfully used for isobutane-isobutylene/butane alkylation.It is interesting to note that the acidity of the onium-poly(hydrogen fluoride)complexes depends on their composition,while the equilibrium and the catalytic activity are affected by the free HF in the solution.Additionally,these novel catalysts significantly reduce the volatility of HF through complexation.This modification plays an important role in commercial HF process to minimize the environmental hazards without weakening the activity for alkylation[69].

3.2.Composite Br?nsted ILs

Fig.4.Structures of HF equivalent amine-poly-(hydrogen fluoride)complexes.(Adapted from Ref[68]).

As has been mentioned earlier,the acid strength ofpure Br?nsted ILs was too low to catalyze the C4alkylation,which results in a bad quality alkylate.Wasserscheid and coworkers[70]investigated the catalytic activity of IL/H2SO4mixtures,which have been used to catalyze the Friedel–Crafts alkylation of benzene with 1-decene.The results clearly demonstrate an interplay of acidity and solubility effects caused by the IL additive.In some cases,low amounts ofIL additive resultin a dramatic improvement of product yield[66,71].

Tang et al.[66]reported a new process for isobutane/1-butene alkylation by using certain acidic imidazolium ILs coupled with strong acids such as H2SO4and TFOH.The use of different types of substituted 3-methyl-imidazolium ILs and binary mixtures were investigated,and a significant change was observed due to the addition of small amount of IL.Since its discovery,the concept has attracted the attention of many researchers[71–73].

Zhang's group systematically investigated the binary mixtures of strong acids and additives in isobutane alkylation[71,73,74].Xing et al.[72]synthesized a series of imidazolium-based ILs([CnMIm]-SbF6;n=4,6,8)and pyridine-based ILs([CnPy]-SbF6;n=2,4,6,8).The catalytic activity of these ILs coupled with TFOH was investigated for isobutane/1-butene alkylation.These ILs contain[SbF6]?as anion and performed well when coupled with TFOH.However,when[BMIm][OTf]was used,no alkylate was observed.This may be caused by the lower acidity of[BMIm][OTf](50 wt%)/TFOH(50 wt%)system.According to Kim[75],the better catalytic activity of the ILs containing[SbF6]?may be attributed to the formation of strongly acidic salt[CF3SO3H2][SbF6]through anion exchange between[BMIm][SbF6]and TFOH.When the alkyl chain length of[CnPy][SbF6]ILs was changed,the yield of alkylate increased as the length of the alkyl group increased from 2 to 6 and then decreased for n=8.This may be attributed to the higher solubility of isobutane in the ILs or its solution,where higher hydride transfer from the isobutane to C8carbonium was achieved.This resulted in higher generation of desired C8alkylate and a suppression of side reaction.

On the study of[BMIm][SbF6]/H2SO4binary mixtures,it was observed that the deactivation of H2SO4was dramatically inhibited by the addition of traces of ILs(0.5 wt%)[71].The phenomenon was explained by a mild change in the acidity of the[BMIm][SbF6]/H2SO4system.In comparison to pure H2SO4process,the system resulted in inhibited formation of ASO,which was very important to preserve the catalytic activity[59,76].

As has been mentioned earlier,the C4alkylation is not only influenced by the acid strength,but also by the solubility of the hydrocarbon in the acid phase.There are many studies on the solubility of hydrocarbons in different ILs[50,77–79],and the reaction was obviously affected by the solubility parameters.Recently our group has reported the solubility parameters of three different ILs and the values were used to qualitatively explain the difference of the solubility of isobutane in different ILs[79].Therefore,both the acid strength of the ionic liquids and the dissolution characteristic should be taken into account for such systems.

For the same anions,the solubility of isobutane in ILs follows the order of:ammonium＞imidazolium＞phosphonium[80].Anion of ILs also plays an important role in the solubility of hydrocarbons in the ILs[81].Cui et al.[82]synthesized a series of ammonium-based ILs(AMILs)so as to increase the solubility of isobutane in the acid phase.They studied the catalytic activity of TFOH coupled with AMILs in the alkylation of isobutane and 1-butene.The efficiency of TFOHwas dramatically enhanced by the addition of AMILs.The anion of IL also has an obvious impact on the alkylate quality for the solubility differences of reactants in the catalyst phase.Compared with the catalyst of TFOH and the ILs with anion[OTf]?,a higher I/O ratio was probably obtained in the mixture of TFOH coupled with the ILs containing anion[HSO 4]?,then a higher alkylate quality was observed.

Inspired by the discussion mentioned in composite chloroaluminate ILs,Liu et al.synthesized a series of dual acidic ILs,(3-sulfonic acid)-propyl-triethylammonium metal chloride salt[HO3SC3NEt3]Cl-MClx(MClx=AlCl3,FeCl3,CuCl,ZnCl2),which possess both of the Br?nsted and Lewis acid site.These ILs have shown excellent catalytic activity and selectivity in both of the dimerization of fatty acid methyl ester[83]and isobutane/isobutene alkylation[84].With different Lewis acid MClxadded,the conversion of isobutene and selectivity of alkylate was higher than the pure[HO3SC3NEt3]Cl,which may attributed to the synergetic effect of the both Br?nsted acidic sites and stronger acidity of the Lewis acid.The hypothetic mechanism of the synergetic effect of both the Br?nsted and Lewis acid site is illustrated in Fig.5.The IL,[HO3SC3NEt3]Cl-ZnCl2,showed the best catalytic performance of all the investigated ILs,where the TMP/DMH＞75,with conversion of isobutene 100%.Additionally,the existence of air and moisture seems to have no obvious impact on the stability of these ILs,except for the[HO3SC3NEt3]Cl-AlCl3,which is sensitivity to trace of water.

Fig.5.The mechanism of C4 alkylation reaction in the presence of the B-L acidic IL.(Reproduced from Ref[84]).

From the above discussion,it is clear that the Br?nsted ILs are more resistant to moisture in air than chloroaluminate ILs.However,the acid strength is weaker than the chloroaluminate ILs.Therefore,Br?nsted ILs are less likely to catalyze the C4alkylation alone.The Br?nsted ILs are preferred to act as auxiliaries rather than the main catalysts in the mixtures.Since the physicochemical properties of the catalytic system can be adjusted,therefore the improvement in the catalytic properties is one of the main research directions for the Br?nsted ILs for C4alkylation.

3.3.Supported ILs phase catalyst(SILPC)systems

Due to the existing problems,such as higher product costs,limited knowledge of the physicochemical properties,product separation,and effective recovery of the catalyst,the commercial application of ILs is still limited.Moreover,contrary to the expectations,the application of a large number ofILs as solvents or catalysts in many catalytic processes may not be ‘green’.So,the concept of supported ILs in porous solids through impregnation,immobilization and encapsulation is becoming a hot topic for research and application[85,86].

The SILPC combines the advantages of ILs and heterogenous catalysis.With the characteristics of designable and excellent solubility of organic and inorganic metallic salts,ILs have been applied as solvents for many polar or ionic catalysts.In such systems,the ILs are dispersed on a supporter.This new concept has successfully been applied in many areas such as I/O regulation[87],carbonylation[88],hydroformylation[89]and Heck's reaction[90].Furthermore,parameters such as the interaction between the ILs and the species of supporter,active sites,reaction medium,and reactantsare differentfordifferentILs.Therefore,the selectivity and catalytic activity of such systems also vary with different ILs.With the interaction of ILs and hydroxyl group,chloroaluminate IL[BMIm]Cl-AlCl3and 1-(tri-ethoxy-silyl-propyl)-3-methylimidazolium chloride was successfully immobilized on the surface of the MCM-41[91,92].The system performed well in the alkylation of benzene with dodecene.With the same method,Kumar et al.dispersed the Lewis acid ILs on the MCM-41(FK 700)supporter by impregnation and immobilization[93](Fig.6).The catalytic activity of the immobilized one was higher than the impregnated one.It is also higher than the traditional solid acid catalyst SAC-13 and H-Beta.The reason why the modified catalyst can prolong the catalyst's life should be the existence of[Al2Cl7]?,which acts as a strong Lewis acid and accelerates the cracking of the heavy ends,thus reducing the coke deposition.Moreover,it should be pointed out that the lifetime of the SILPC is shorter than that of the pure IL system,while the C8selectivity is also lower than that of pure IL.

In the commercial mineral acid alkylation process,a very high I/O ratio(the actual I/O ratio may be as high as 1000 in H2SO4system[4])is used to promote the formation rate of desired products.However,the local I/O ratio in the reaction region on the solid acid strongly depends on the adsorption capacities of reactants.It is difficult to get a high I/O ratio at the acid sites by just increasing the I/O ratio in feed.A preferential adsorption of olefin over alkane on the solid acid catalyst usually takes place in the is oparaffin/olefin alkylation process[94].Actually,polar reactants are readily adsorbed on the solid acid catalysts due to their intrinsic surface polarity.Low I/O ratio leads to excessive butene existing on the surface of acid sites,which is conducive to the oligomerization,polymerization or addition of butene,and the formation of heavy compounds(C9+).Then the plugging of pore openings and the cover of active sites by heavy compounds is unavoidable[9].

The adsorption properties of catalyst not only have a significant influence on the reaction efficiency,product yield and catalyst life,but also decide the reaction direction.Liu et al.[95]discovered that grafting chloroaluminate IL on different supporter probably leads to different reaction product.When the ILs were supported on glass or activated carbon,the alkylation reaction was taken place.When the support was changed as silica,isobutene oligomerization became the main reaction.

This phenomenon can be attributed to the differences of adsorptive capacity of isobutane and butene on the catalyst.Therefore,it is of great significance to seek for an effective way to modulate the adsorption capacities of reactant on the solid acid catalysts.

Generally,the adsorption capacities of reactants on the catalyst active sites mainly depend on the physicochemical property and surface features of the catalysts.The pore size also has been found to affect the adsorption capacity through steric hindrances[96].It is an efficient ways to improve the local I/O ratio by modulating the surface hydrophobicity through surface modification.Grafting ILs is one of the effective ways to modulate the adsorption properties.The surface character of the SILPC can be customized by a rational design of the ILs structure or the alkyl chain length[97].Thus the relative adsorption capacity of alkenes and alkanes on the catalyst can be modulated,and the local I/O ratio and hydride transfer ability could significantly be improved.Tang et al.[87]obtained an almost stoichiometric adsorption ethane/ethylene ratio on MCM-22 by grafting a dual acidic ionic liquid 3-sulfobutyl-1-(3-propyltriethoxysilane)imidazolium hydrogen sulfate.Shen et al.[98–100]studied the catalytic activity of SBA-15/16 supported with ILs in the alkylation of isobutane with 1-butene.They tuned the acid strength by immobilizing propyl-sulfonic,arene-sulfonic or per fluoroalkylsulfonic acid groups and adjust the hydrophilic-hydrophobic nature of the surface by using organosilica or by capping the surface OHs of silica.Acid functionalized mesoporous organosilica and silanized silica has a hydrophobic surface and leads to a higher paraffin concentration in the pore system.They show a better catalysis than the catalysts with hydrophilic surface.A three-dimensional mesopore solid acid is superior to one-dimensional ones both in butene conversion and TMP selectivity for the low diffusion resistance and resisting deactivation by pore plugging.Materials modified with perfluoroalkylsulfonic acid groups exhibit the best catalytic activity for its strong acidity.They suggested that strong acid sites like perfluoroalkylsulfonic acid groups,three-dimension mesopores structure and hydrophobic surface should be desired for an alkylation catalyst.

Traditional solid acid catalysts performed as high efficiency catalysts for alkylation at the initial stage,while rapid deactivation occurred as the reaction proceeded further for the coke deposition.As described above,the adsorption properties of components on the catalyst,the acid strength,the distribution of acid sites with different acidity and the resistance of mass transfer all have a significant influence on the reaction efficiency,the selectivity of target product and catalyst lifetime.As the suggestion of Shen et al.[99]a good catalyst for the C4alkylation process need a balance on the acidity,surface property and transfer property,so as to get a high local I/O ratio,a proper acid strength and a low resistance of transfer,and then to intensify the reaction with a high reaction efficiency,high yield of objective product and a long catalyst lifetime.

Fig.6.(a)Immobilization by impregnation.(b)Immobilization by grafting.(Adapted from Ref[93]).

Only fragmentary researches about the application of SILPC on C4alkylation process were reported[93,95,98–100].A lot of work is needed to explore the catalytic activity,catalysis mechanism,deactivation mechanism and regeneration technology.However,we believe that the ionic liquids will play an important role in the exploring of new solid acid catalyst of C4alkylation process for its unique character.

4.Summary and Future Prospect

The complicated reaction mechanism and the character of competitive reactions lead to a stringent requirement for the catalyst of C4alkylation process.The catalyst should keep a proper acid strength and relatively centralized distribution so as to provide enough acid sites with desired acidity.Lower acid strength favors the butene oligomerization to form heavy compounds while higher acid strength favors the cracking to form light components.A rapid decrease of acidity leads to a short catalyst lifetime.Meanwhile,the catalyst should have excellent interfacial properties and transport characteristics so as to provide an ideal I/O ratio in the reaction region.

Chloroaluminate(III) ILs can provide an adjustable acidity and tunable miscibility with hydrocarbons. However, the oxophilic character and water sensitivity impede its application. Addition of organic or inorganic additives into chloroaluminate ILs brings a synergistic catalytic effect to intensify the hydride transfer and inhibit the side reactions, and then to improve the alkylation process with a higher selectivity ofC8and TMPs than the traditional H2SO4and HF processes.The catalyst can be regenerated and the acidity of catalyst can be kept at a constant by supplementing HCl.It should be a good candidate for the catalyst of C4alkylation.The feasibility of the application in C4alkylation process has been proved in a 100 kt·a?1plantby China University of Petroleum.However,we should pay close attention to the separation of additives,the escape of HCl released from the catalyst deactivation,and the precipitation of addittves,such as CuCl.The results of long-term operation of the CIL in the commercial plant will provide enough information about the application prospect of CIL in C4alkylation process.

The acidity and dissolution property of Br?nsted acidic ionic liquids also are adjustable.Br?nsted acidic ionic liquids are insensitive and stable to water.And it can provide proton H+directly to favor the formation of target alkylate.However,the acid strength is not strong enough to get a good quality alkylate product.The Br?nsted ILs are good auxiliaries of strong Br?nsted acid to get an adjustable dissolubility,an appropriate acidity and an improved mass transfer.So,synergistic catalytic effect should be expected by mixing Br?nsted ILs with strong inorganic acid,such as H2SO4,HF,etc.The IL/acid composite system would be the most potential catalyst to replace the traditional inorganic mineral acid system.Some good research results are obtained in the lab.However,the feasibility needs to be verified in a commercial plant.And further researches are necessary to obtain the fundamental information,such as the catalysis mechanism,deactivation mechanism,regeneration technology,transfer properties,etc.

Supporting acidic ILs on solid acid can effectively modulate the surface hydrophobicity and then to adjust the adsorption property.A stoichiometric I/O ratio on the acid sites is probably obtained by tailoring the structure of ionic liquids.SILPC is probably a potential substitute catalyst of the mineral acid.However,the supporting of IL leads to a decrease in specific surface area,pore volume and pore size.A mesoporous or macroporous solid acid is an ideal support to prepare SILPC.The acidity of acidic ILs is different with that of solid acid.The SILPC have at least two kinds of acid sites with different acid strengths.It probably has a significant influence on the catalytic activity and product selectivity.Now,only few works on the research of SILPC in C4alkylation were reported.An in-depth and systematic theoretical research on the fundamental physicochemical properties of these systems is necessary to explore new SILPC.

[1]L.F.Albright,Present and future alkylation processes in refineries,Ind.Eng.Chem.Res.48(3)(2009)1409–1413.

[2]A.E.Wheals,L.C.Basso,D.M.Alves,H.V.Amorim,Fuel ethanol after 25 years,Trends Biotechnol.17(12)(1999)482–487.

[3]J.Anderson,D.DiCicco,J.Ginder,U.Kramer,T.Leone,H.Raney-Pablo,T.Wallington,High octane number ethanol–gasoline blends:Quantifying the potential benefits in the United States,Fuel 97(2012)585–594.

[4]A.Corma,A.Martinez,Chemistry,catalysts,and processes for isoparaffin–olefin alkylation:Actual situation and future trends,Catal.Rev.35(4)(1993)483–570.

[5]E.Furimsky,Spent refinery catalysts:environment,safety and utilization,Catal.Today 30(4)(1996)223–286.

[6]D.Dunham,Upgrade alkylation for refining environment,Hydrocarb.Process.84(9)(2005)93?+.

[7]Y.Traa,J.Weitkamp,Alkylation of isobutane with light alkenes on solid catalysts,Handbook of heterogeneous catalysis,second ed.Wiley-VCH,Weinheim,2008(Chapt.13).

[8]K.Tanabe,W.F.H?lderich,Industrial application of solid acid–base catalysts,Appl.Catal.A 181(2)(1999)399–434.

[9]C.Querini,E.Roa,Deactivation of solid acid catalysts during isobutane alkylation with C4 ole fins,Appl.Catal.A Gen.163(1)(1997)199–215.

[10]A.Feller,A.Guzman,I.Zuazo,J.A.Lercher,On the mechanism of catalyzed isobutane/butene alkylation by zeolites,J.Catal.224(1)(2004)80–93.

[11]W.Sun,Y.Shi,J.Chen,Z.Xi,L.Zhao,Alkylation kinetics of isobutane by C4 ole fins using sulfuric acid as catalyst,Ind.Eng.Chem.Res.52(44)(2013)15262–15269.

[12]K.Patrylak,L.Patrylak,I.Repetskyi,Mechanisms of alkylation of isobutane by butenes and H/D exchange in isobutane molecules on acid zeolites,Theor.Exp.Chem.49(3)(2013)143–157.

[13]A.Feller,J.A.Lercher,Chemistry and technology of isobutane/alkene alkylation catalyzed by liquid and solid acids,Adv.Catal.48(2004)229–295.

[14]A.Corma,A.Martinez,C.Martinez,The effect of sulfation conditions and activation temperature of sulfate-doped ZrO2,TiO2 and SnO2 catalysts during isobutane/2-butene alkylation,Appl.Catal.A Gen.144(1)(1996)249–268.

[15]L.F.Albright,M.A.Spalding,J.A.Nowinski,R.M.Ybarra,R.E.Eckert,Alkylation of isobutane with C4 olefins.1.First-step reactions using sulfuric acid catalyst,Ind.Eng.Chem.Res.27(3)(1988)381–386.

[16]T.Hamzehlouyan,M.Kazemeini,F.Khorasheh,Modeling of catalyst deactivation in zeolite-catalyzed alkylation of isobutane with 2-butene,Chem.Eng.Sci.65(2)(2010)645–650.

[17]M.F.Simpson,J.Wei,S.Sundaresan,Kinetic analysis of isobutane/butene alkylation over ultrastable HY zeolite,Ind.Eng.Chem.Res.35(11)(1996)3861–3873.

[18]G.Chatel,J.F.B.Pereira,V.Debbeti,H.Wang,R.D.Rogers,Mixing ionic liquids—“Simple mixtures”or “double salts”?Green Chem.16(4)(2014)2051–2083.

[19]P.M.Dean,J.M.Pringle,D.R.MacFarlane,Structural analysis of low melting organic salts:Perspectives on ionic liquids,Phys.Chem.Chem.Phys.12(32)(2010)9144–9153.

[20]R.D.Rogers,K.R.Seddon,Ionic liquids–solvents of the future?Science 302(5646)(2003)792–793.

[21]R.Giernoth,Task-specific ionic liquids,Angew.Chem.Int.Ed.49(16)(2010)2834–2849.

[22]K.R.Seddon,Ionic liquids for clean technology,J.Chem.Technol.Biotechnol.68(4)(1997)351–356.

[23]H.Olivier-Bourbigou,L.Magna,Ionic liquids:Perspectives for organic and catalytic reactions,J.Mol.Catal.A Chem.182(2002)419–437.

[24]D.Freudenmann,S.Wolf,M.Wolff,C.Feldmann,Ionic liquids:New perspectives for inorganic synthesis?Angew.Chem.Int.Ed.50(47)(2011)11050–11060.

[25]H.-P.Zhu,F.Yang,J.Tang,M.-Y.He,Br?nsted acidic ionic liquid 1-methylimidazolium tetra fluoroborate:A green catalyst and recyclable medium for esterification,Green Chem.5(1)(2003)38–39.

[26]K.Qiao,C.Yokoyama,Novel acidic ionic liquids catalytic systems for Friedel–Crafts alkylation of aromatic compounds with alkenes,Chem.Lett.33(4)(2004)472–473.

[27]R.Zhang,X.Meng,Z.Liu,J.Meng,C.Xu,Isomerization of n-pentane catalyzed by acidic chloroaluminate ionic liquids,Ind.Eng.Chem.Res.47(21)(2008)8205–8210.

[28]K.S.Yoo,H.Kim,D.J.Moon,P.G.Smirniotis,Preparation and characterization of acidic ionic liquids for alkylation of isobutene with 2-butene,The 2005 Annual Meeting,2005.

[29]Y.Chauvin,A.Hirschauer,H.Olivier,Alkylation of isobutane with 2-butene using 1-butyl-3-methylimidazolium chloride—Aluminium chloride molten salts as catalysts,J.Mol.Catal.92(2)(1994)155–165.

[30]K.Yoo,V.V.Namboodiri,R.S.Varma,P.G.Smirniotis,Ionic liquid-catalyzed alkylation of isobutane with 2-butene,J.Catal.222(2)(2004)511–519.

[31]N.V.Plechkova,K.R.Seddon,Applications of ionic liquids in the chemical industry,Chem.Soc.Rev.37(1)(2008)123–150.

[32]T.Welton,Room-temperature ionic liquids.Solvents for synthesis and catalysis,Chem.Rev.99(8)(1999)2071–2084.

[33]Y.Yang,X.Wang,Y.Kou,Determination of acidity of ionic liquids and alkylation of isobutane with butene by chloroaluminate ionic liquids,Chin.J.Catal.25(1)(2004)60–64.

[34]L.Ying,L.Zhichang,H.Chongpin,X.Chunming,Alkylation of isobutane/butene with AlCl～3/Et～3NHCl ionic liquid catalyets,Chem.React.Eng.Technol.China 20(3)(2004)229–234.

[35]Y.L.Yang,Y.Kou,Determination of the Lewis acidity of ionic liquids by means of an IR spectroscopic probe,Chem.Commun.2(2004)226–227.

[36]J.Cui,J.de With,P.A.Klusener,X.Su,X.Meng,R.Zhang,Z.Liu,C.Xu,H.Liu,Identification of acidic species in chloroaluminate ionic liquid catalysts,J.Catal.320(2014)26–32.

[37]P.Bonhote,A.P.Dias,N.Papageorgiou,K.Kalyanasundaram,M.Gr?tzel,Hydrophobic,highly conductive ambient-temperature molten salts,Inorg.Chem.35(5)(1996)1168–1178.

[38]S.V.Dzyuba,R.A.Bartsch,Influence of structural variations in 1-alkyl(aralkyl)-3-methylimidazolium hexa fluorophosphates and bis(trifluoromethylsulfonyl)imides on physical properties of the ionic liquids,Chem Phys Chem 3(2)(2002)161–165.

[39]Z.C.Zhang,Catalysis in ionic liquids,Adv.Catal.49(2006)153–237.

[40]L.Schilder,S.Maass,A.Jess,Effective and intrinsic kinetics of liquid-phase isobutane/2-butene alkylation catalyzed by chloroaluminate ionic liquids,Ind.Eng.Chem.Res.52(5)(2013)1877–1885.

[41]D.J.Ende,R.E.Eckert,L.F.Albright,Interfacial area of dispersions of sulfuric-acid and hydrocarbons,Ind.Eng.Chem.Res.34(12)(1995)4343–4350.

[42]A.Feller,I.Zuazo,A.Guzman,J.O.Barth,J.A.Lercher,Common mechanistic aspects of liquid and solid acid catalyzed alkylation of isobutane with n-butene,J.Catal.216(1–2)(2003)313–323.

[43]J.G.Huddleston,A.E.Visser,W.M.Reichert,H.D.Willauer,G.A.Broker,R.D.Rogers,Characterization and comparison of hydrophilic and hydrophobic room temperature ionic liquids incorporating the imidazolium cation,Green Chem.3(4)(2001)156–164.

[44]A.J.Arduengo III,R.L.Harlow,M.Kline,A stable crystalline carbene,J.Am.Chem.Soc.113(1)(1991)361–363.

[45]A.Roebuck,B.Evering,Isobutane-ole fin alklation with inhibited aluminum chloride catalysts,Ind.Eng.Chem.Prod.Res.Dev.9(1)(1970)76–82.

[46]L.Dan,Z.Guoying,R.Baozeng,J.Zhenxi,Z.Suojiang,Isobutane alkylation catalyzed by ether functionalized ionic liquids,CIESC J.66(7)(2015)2481–2487.

[47]A.S.Berenblyum,E.A.Katsman,Y.Z.Karasev,The nature of catalytic activity and deactivation of chloroaluminate ionic liquid,Appl.Catal.A Gen.315(2006)128–134.

[48]C.P.Huang,Z.C.Liu,C.M.Xu,B.H.Chen,Y.F.Liu,Effects of additives on the properties of chloroaluminate ionic liquids catalyst for alkylation of isobutane and butene,Appl.Catal.A Gen.277(1)(2004)41–43.

[49]Y.Liu,R.Hu,C.Xu,H.Su,Alkylation of isobutene with 2-butene using composite ionic liquid catalysts,Appl.Catal.A 346(1)(2008)189–193.

[50]Y.Liu,R.Li,H.Sun,R.Hu,Effects of catalyst composition on the ionic liquid catalyzed isobutane/2-butene alkylation,J.Mol.Catal.A Chem.398(2015)133–139.

[51]D.J.Safarik,R.B.Eldridge,olefin/paraffin separations by reactive absorption:A review,Ind.Eng.Chem.Res.37(7)(1998)2571–2581.

[52]J.Zhang,C.Huang,B.Chen,P.Ren,M.Pu,Isobutane/2-butene alkylation catalyzed by chloroaluminate ionic liquids in the presence of aromatic additives,J.Catal.249(2)(2007)261–268.

[53]L.F.Albright,Mechanism for alkylation of isobutane with light olefins,Abstr.Pap.Am.Chem.Soc.173(MAR20)(1977)9-9.

[54]T.L.T.Bui,W.Korth,S.Aschauer,A.Jess,Alkylation of isobutane with 2-butene using ionic liquids as catalyst,Green Chem.11(12)(2009)1961–1967.

[55]S.Aschauer,L.Schilder,W.Korth,S.Fritschi,A.Jess,Liquid-phase isobutane/butene-alkylation using promoted lewis-acidic IL-catalysts,Catal.Lett.141(10)(2011)1405–1409.

[56]G.P.Smith,A.S.Dworkin,R.M.Pagni,S.P.Zingg,Br?nsted superacidity of hydrochloric acid in a liquid chloroaluminate.AlCl3-1-ethyl-3-methyl-1H-imidazolium chloride,J.Am.Chem.Soc.111(2)(1989)525–530.

[57]F.P?hlmann,L.Schilder,W.Korth,A.Jess,Liquid phase isobutane/2-butene alkylation promoted by hydrogen chloride using Lewisacidic ionic liquids,Chem Plus Chem 78(6)(2013)570–577.

[58]Harris,T.V.,Elomari,S.,Regeneration of ionic liquid catalysts,Patents.,US 7678727,2010.

[59]A.Berenblyum,L.Ovsyannikova,E.Katsman,J.Zavilla,S.Hommeltoft,Y.Z.Karasev,Acid soluble oil,by-productformed in isobutane alkylation with alkene in the presence of trifluoro methane sulfonic acid:Part I acid soluble oil composition and its poisoning effect,Appl.Catal.A Gen.232(1)(2002)51–58.

[60]H.Ma,R.Zhang,X.Meng,Z.Liu,H.Liu,C.Xu,R.Chen,P.A.Klusener,J.de With,Solid formation during composite-ionic-liquid-catalyzed Isobutane alkylation,Energy Fuel 28(8)(2014)5389–5395.

[61]C.Flego,I.Kiricsi,W.O.Parker,M.G.Clerici,Spectroscopic studies of Lahy–Fau catalyst deactivation in the alkylation of isobutane with 1-butene,Appl.Catal.A Gen.124(1)(1995)107–119.

[62]F.A.Diaz-Mendoza,L.Pernett-Bolano,N.Cardona-Martinez,Effect of catalyst deactivation on the acid properties of zeolites used for isobutane/butene alkylation,Thermochim.Acta 312(1–2)(1998)47–61.

[63]A.C.Cole,J.L.Jensen,I.Ntai,K.L.T.Tran,K.J.Weaver,D.C.Forbes,J.H.Davis,Novel Br?nsted acidic ionic liquids and their use as dual solvent-catalysts,J.Am.Chem.Soc.124(21)(2002)5962–5963.

[64]R.Kore,R.Srivastava,Synthesis and applications of highly efficient,reusable,sulfonic acid group functionalized Br?nsted acidic ionic liquid catalysts,Catal.Commun.12(15)(2011)1420–1424.

[65]T.Akiyama,K.Mori,Stronger Bronsted acids:Recent progress,Chem.Rev.115(17)(2015)9277–9306.

[66]S.Tang,A.M.Scurto,B.Subramaniam,Improved 1-butene/isobutane alkylation with acidic ionic liquids and tunable acid/ionic liquid mixtures,J.Catal.268(2)(2009)243–250.

[67]L.Ying,H.Ruisheng,L.Guili,X.Chunming,Study on the alkylatin of Isobutane/butene in acidic ionic liquids,J.Mol.Catal.China 24(2010)217–221.

[68]G.A.Olah,T.Mathew,A.Goeppert,B.T?r?k,I.Bucsi,X.-Y.Li,Q.Wang,E.R.Marinez,P.Batamack,R.Aniszfeld,Ionic liquid and solid HF equivalent amine-poly(hydrogen fluoride)complexes effecting efficient environmentally friendly isobutaneisobutylene alkylation,J.Am.Chem.Soc.127(16)(2005)5964–5969.

[69]S.I.Hommeltoft,Isobutane alkylation—Recent developments,and future perspectives,Appl.Catal.A Gen.221(1–2)(2001)421–428.

[70]P.Wasserscheid,M.Sesing,W.Korth,Hydrogensulfate and tetrakis(hydrogensulfato)borate ionic liquids:Synthesis and catalytic application in highly Br?nsted-acidic systems for Friedel–Crafts alkylation,Green Chem.4(2)(2002)134–138.

[71]Q.Huang,G.Zhao,S.Zhang,F.Yang,Improved catalytic lifetime of H2SO4for isobutane alkylation with trace amount of ionic liquids buffer,Ind.Eng.Chem.Res.54(5)(2015)1464–1469.

[72]Y.Cong,Y.Liu,R.Hu,Isobutane/2-butene alkylation catalyzed by strong acids in the presence of ionic liquid additives,Pet.Sci.Technol.32(16)(2014)1981–1987.

[73]X.Xing,G.Zhao,J.Cui,S.Zhang,Isobutane alkylation using acidic ionic liquid catalysts,Catal.Commun.26(2012)68–71.

[74]H.Ren,G.Zhao,S.Zhang,P.Cui,J.Huang,Triflic acid catalyzed isobutane alkylation with trifluoroethanol as a promoter,Catal.Commun.18(2012)85–88.

[75]J.H.Kim,J.W.Lee,U.S.Shin,J.Y.Lee,L.B.Sang-Gi,C.E.Song,Activation of Lewis acid catalysts in the presence of an organic salt containing a non-coordinating anion:Its origin and application potential,Chem.Commun.44(2007)4683–4685.

[76]L.F.Albright,M.A.Spalding,C.G.Kopser,R.E.Eckert,Alkylation of isobutane with C4 olefins.2.Production and characterization of conjunct polymers,Ind.Eng.Chem.Res.27(3)(1988)386–391.

[77]W.S.Chen,Solubility measurements of isobutane/alkenes in sulfuric acid:Applications to alkylation,Appl.Catal.A Gen.255(2)(2003)231–237.

[78]L.Ferguson,P.Scovazzo,Solubility,diffusivity,and permeability of gases in phosphonium-based room temperature ionic liquids:Data and correlations,Ind.Eng.Chem.Res.46(4)(2007)1369–1374.

[79]Y.Zhang,T.Zhang,P.Gan,H.Li,M.Zhang,K.Jin,S.Tang,Solubility of isobutane in ionic liquids BMIm PF6,BMIm BF4,and BMIm Tf2N,J.Chem.Eng.Data 60(6)(2015)1706–1714.

[80]R.Condemarin,P.Scovazzo,Gas permeabilities,solubilities,diffusivities,and diffusivity correlations for ammonium-based room temperature ionic liquids with comparison to imidazolium and phosphonium RTIL data,Chem.Eng.J.147(1)(2009)51–57.

[81]U.Domańska,Solubilities and thermophysical properties of ionic liquids,Pure Appl.Chem.77(3)(2005)543–557.

[82]P.Cui,G.Zhao,H.Ren,J.Huang,S.Zhang,Ionic liquid enhanced alkylation of isobutane and 1-butene,Catal.Today 200(2013)30–35.

[83]L.Shiwei,Z.Hongxia,Y.Shitao,X.Congxia,L.Fusheng,S.Zhanqian,Dimerization of fatty acid methyl ester using Bronsted–Lewis acidic ionic liquid as catalyst,Chem.Eng.J.174(1)(2011)396–399.

[84]S.Liu,C.Chen,F.Yu,L.Li,Z.Liu,S.Yu,C.Xie,F.Liu,Alkylation of isobutane/isobutene using Bronsted–Lewis acidic ionic liquids as catalysts,Fuel 159(2015)803–809.

[85]P.Munnik,P.E.de Jongh,K.P.de Jong,Recent developments in the synthesis of supported catalysts,Chem.Rev.115(14)(2015)6687–6718.

[86]C.P.Mehnert,R.A.Cook,N.C.Dispenziere,M.Afeworki,Supported ionic liquid catalysis—A new concept for homogeneous hydro formylation catalysis,J.Am.Chem.Soc.124(44)(2002)12932–12933.

[87]K.Jin,T.Zhang,J.Ji,M.Zhang,Y.Zhang,S.Tang,Functionalization of MCM-22 by dual acidic ionic liquid and its paraffin absorption modulation properties,Ind.Eng.Chem.Res.54(1)(2015)164–170.

[88]H.Wang,B.Wang,C.L.Liu,W.S.Dong,Oxidative carbonylation of methanol over copper ion-containing ionic liquids immobilized on SBA-15,Microporous Mesoporous Mater.134(1)(2010)51–57.

[89]A.Riisager,R.Fehrmann,S.Flicker,R.van Hal,M.Haumann,P.Wasserscheid,Very stable and highly regioselective supported ionic-liquid-phase(SILP)catalysis:Continuous flow fixed-bed hydroformylation of propene,Angew.Chem.Int.Ed.44(5)(2005)815–819.

[90]W.Liu,D.Wang,Y.Duan,Y.Zhang,F.Bian,Palladium supported on poly(ionic liquid)entrapped magnetic nanoparticles as a highly efficient and reusable catalyst for the solvent-free Heck reaction,Tetrahedron Lett.56(14)(2015)1784–1789.

[91]M.Valkenberg,W.H?lderich,Immobilisation of chloroaluminate ionic liquids on silica materials,Top.Catal.14(1–4)(2000)139–144.

[92]M.Valkenberg,W.H?lderich,Immobilisation of ionic liquids on solid supports,Green Chem.4(2)(2002)88–93.

[93]P.Kumar,W.Vermeiren,J.-P.Dath,W.F.Hoelderich,Production of alkylated gasoline using ionic liquids and immobilized ionic liquids,Appl.Catal.A Gen.304(2006)131–141.

[94]M.A.Granato,N.Lamia,T.J.Vlugt,A.R.E.Rodrigues,Adsorption equilibrium of isobutane and 1-butene in zeolite 13×by molecular simulation,Ind.Eng.Chem.Res.47(16)(2008)6166–6174.

[95]S.Liu,J.Shang,S.Zhang,B.Yang,Y.Deng,Highly efficient trimerization of isobutene over silica supported chloroaluminate ionic liquid using C4 feed,Catal.Today 200(2013)41–48.

[96]W.Zhu,F.Kapteijn,J.A.Moulijn,M.C.den Exter,J.C.Jansen,Shape selectivity in adsorption on the all-silica DD3R,Langmuir 16(7)(2000)3322–3329.

[97]H.Xing,X.Zhao,R.Li,Q.Yang,B.Su,Z.Bao,Y.Yang,Q.Ren,Improved efficiency of ethylene/ethane separation using a symmetrical dual nitrile-functionalized ionic liquid,ACS Sustain.Chem.Eng.1(11)(2013)1357–1363.

[98]W.Shen,Y.Gu,H.Xu,D.Dubé,S.Kaliaguine,Alkylation of isobutane/1-butene on methyl-modified Nafion/SBA-15 materials,Appl.Catal.A Gen.377(1–2)(2010)1–8.

[99]W.Shen,Y.Gu,H.Xu,R.Che,D.Dubé,S.Kaliaguine,Alkylation of isobutane/1-butene on methyl-modified nafion/SBA-16 materials,Ind.Eng.Chem.Res.49(16)(2010)7201–7209.

[100]W.Shen,D.Dubé,S.Kaliaguine,Alkylation of isobutane/1-butene over periodic mesoporous or ganosilica functionalized with per fluoroalkylsulfonic acid group,Catal.Commun.10(3)(2008)291–294.