DOI: 10.1002/cssc.201100446

Multilayered Supported Ionic Liquids as Catalysts forChemical Fixation of Carbon Dioxide: A High-ThroughputStudy in Supercritical ConditionsCarmela Aprile,[a, b] Francesco Giacalone,[c] Paola Agrigento,[a, c] Leonarda F. Liotta,[d]

Johan A. Martens,[a] Paolo P. Pescarmona,*[a] and Michelangelo Gruttadauria*[c]

Introduction

The development of new catalysts with high activity in thesynthesis of useful products from renewable sources is a sub-ject of great interest in the field of sustainable chemistry.Chemical fixation of carbon dioxide is an attractive topic as itallows the conversion of chemical waste into valuable prod-ucts.[1–4] Cyclic carbonates, synthesized through the reactionbetween CO2 and epoxides, are interesting compounds thatcan find applications as aprotic polar solvents, electrolytes forbatteries, sources for reactive polymer synthesis, and precur-sors for pharmaceuticals.[5–8] The use of carbon dioxide as afeedstock is particularly attractive because CO2 gas is producedin large amounts from fuel combustion and, therefore, repre-sents an easily available, abundant, inexpensive, and renewa-ble carbon source.

Recently, simple supported ionic liquids with halide counter-ions have been successfully employed as catalysts for the cy-cloaddition of carbon dioxide to various epoxides.[9] The reac-tion mechanism involves ring opening by the halide followedby CO2 insertion.[5, 9] Usually, these reactions are performedunder supercritical CO2 (scCO2) conditions because scCO2

serves simultaneously as a reactant and solvent, owing to itshigh solvating power towards various epoxides.[10–13] Support-ed ionic liquids are a class of materials attracting growing in-terest, particularly for their application in heterogeneous catal-ysis.[14–25] The advantages of heterogeneous catalytic systemsbased on a supported ionic liquid phase (SILP) compared withtheir corresponding homogeneous analogous include easierseparation from the reaction media, recyclability, and the pos-sibility of using them in fixed-bed reactors. The most used sup-ports in SILP are cross-linked polystyrene and silica, but othersupports have been also investigated (e.g. , magnetite, toyon-ite, alumina, and chitosan).[9] Ionic liquids are generally sup-ported on polymers by copolymerization or a postsynthesis

modification approach, whereas they are supported on silicaby means of adsorption, sol–gel methods, or grafting.[9]

Herein, we report the synthesis of a new class of multilay-ered, covalently supported, SILP (mlc-SILP) materials with ex-cellent catalytic activity in the cycloaddition of carbon dioxideto epoxides to yield cyclic carbonates. Our new synthetic ap-proach is based on grafting of bis-vinylimidazolium salts ontomercaptoalkyl-modified supports. Due to their double vinylfunctionality, bis-vinylimidazolium salts can form covalentbonds with the support and also copolymerize with eachother, thus leading to the formation of a multilayered ionicphase anchored to the support (Scheme 1).

The resulting new class of materials is expected to achieveexceptionally high loadings of supported ionic liquid, while

Multilayered, covalently supported ionic liquid phase (mlc-SILP)materials were synthesized by using a new approach based onthe grafting of bis-vinylimidazolium salts on different types ofsilica or polymeric supports. The obtained materials were char-acterized and tested as catalysts in the reaction of supercriticalcarbon dioxide with various epoxides to produce cyclic carbo-nates. The material prepared by supporting a bromide bis-imi-dazolium salt on the ordered mesoporous silica SBA-15 was

identified as the most active catalyst for the synthesis of cycliccarbonates and displayed improved productivity comparedwith known supported ionic liquid catalysts. The catalyst re-tained its high activity upon reuse in consecutive catalyticruns. This is the first report of the application of mlc-SILP mate-rials as catalysts in a reaction for the fixation of carbon dioxide.Rapid, parallel screening and comparison of the catalysts wasperformed by means of high-throughput experimentation.

[a] Prof. C. Aprile, P. Agrigento, Prof. J. A. Martens, Prof. P. P. PescarmonaCentre for Surface Chemistry and CatalysisK.U. LeuvenKasteelpark Arenberg 23, 3001 Heverlee (Belgium)Fax: (+ 32) 16321998E-mail : paolo.pescarmona@biw.kuleuven.be

[b] Prof. C. AprileD�partement de ChimieUnit� de Chimie des Nanomat�riaux (UCNANO)Facult�s Universitaires Notre-Dame de la PaixRue de Bruxelles 61, 5000 Namur (Belgium)

[c] Dr. F. Giacalone, P. Agrigento, Prof. M. GruttadauriaDipartimento STEMBIO, sez. Chimica Organica ‘‘E. Patern�’’Universit� di PalermoViale delle Scienze, Pad. 17, 90128, Palermo (Italy)Fax: (+ 39) 091 596825E-mail : michelangelo.gruttadauria@unipa.it

[d] Dr. L. F. LiottaISMN-CNR, Via Ugo la Malfa 153, 90146 Palermo (Italy)

Supporting Information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cssc.201100446.

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maintaining the catalytic behaviour of pure ‘bulk’ ionic liquids.To the best of our knowledge, this is the first time that cova-lently grafted multilayers of ionic liquids have been reportedas catalysts for the chemical fixation of carbon dioxide. In gen-eral, reports on the synthesis and application of covalently at-tached multilayers of ionic liquids are still rare.[26]

We prepared a series of catalysts by using imidazolium saltswith different counterions supported on different materials,such as polystyrene and silicates with high specific surfaceareas. These materials were characterized and tested as cata-lysts for the chemical fixation of carbon dioxide on various ep-oxides with CO2 under supercritical conditions. The rapid andefficient parallel screening of the series of catalysts in the se-lected reactions was performed by using a new high-through-put (HT) unit for the study of catalytic systems under scCO2

(Figure S1 in the Supporting Information). HT experimentationis an established and powerful tool for research in cataly-sis.[27, 28] The application of HT techniques to the study of cata-

lytic reactions in scCO2 has, however, been unexplored untilnow.

Results and Discussion

To develop new materials based on multilayered covalentlysupported ionic liquid phase (mlc-SILP) materials, we con-ceived a straightforward approach through which a suitablebis-vinylimidazolium salt is linked to a mercaptopropyl-modi-fied silica by using the thiol–ene reaction,[29]which involves theaddition of a S�H bond to a double bond by a free radicalmechanism and represents a well-established “click” protocolin materials chemistry.[30–32] Since the bis-vinylimidazolium saltis added in excess relative to the moles of �SH groups(3.62 molsalt molSH groups

�1), the double bonds that do not reactin the thiol–ene reaction are expected to undergo a self-addi-tion reaction, leading to the formation of an imidazoliumcross-linked network. We refer to this unstructured, supportedoligomeric network of imidazolium units as a multilayeredionic liquid phase (Scheme 1).

The synthesis of the mlc-SILP materials was performed byemploying three thiol-functionalized supports : two mercapto-propyl-modified silica prepared by reaction of either amor-phous (1) or ordered (2) mesoporous SBA-15 with (3-mercapto-propyl)trimethoxysilane, and the commercially available mer-captomethyl-modified cross-linked polystyrene (3). As the bis-vinylimidazolium salt, we used the 3,3’-[1,4-phenylenebis(me-thylene)]bis(1-vinyl-1H-imidazol-3-ium) dichloride (4) or dibro-mide (5), which were easily obtained by the reaction of 1-vinyl-imidazole with 1,4-bis(chloromethyl)benzene or 1,4-bis(bromo-methyl)benzene, respectively. The grafting of salts 4 and 5 wasperformed in the presence of AIBN as the radical initiator,either under magnetic (a) or mechanical (b) stirring. Thesethiol–ene reactions lead to the almost quantitative anchoringof the employed salt on the surface of each of the supports(1–3) to give the mlc-SILP materials, as shown by the highloading of imidazolium units and by the high ratio betweensupported salt and anchoring sites determined by elementalanalysis (Table 1). As anticipated, these loadings are higher

Scheme 1. Grafting of bis-imidazolium salts on supports 1–3 and a schemat-ic representation of the possible domains of mlc-SILP catalysts. The wavylines symbolize further connections, either by cross-linking to other bis-imi-dazolium units or by anchoring to the support. AIBN = 2,2’-azobisisobutyro-nitrile.

Table 1. Reaction conditions and resulting loadings of imidazolium unitsin the mlc-SILP materials as a function of the type of supports and of bis-vinylimidazolium salts employed in the synthesis.

mlc-SILP[a] Imidazolium units[b]

[mmol g�1]IL/SHmolar ratio[c]

Supported imidazoliumsalt [wt %]

SiO2�Cl(a) 3.23 3.6 59SiO2�Br(a) 2.79 3.4 63SiO2�Br(b) 2.82 3.5 64SBA-15�Cl(a) 3.20 2.5 58SBA-15�Cl(b) 3.11 2.4 56SBA-15�Br(b) 2.87 2.7 65PS�Cl(a) 3.40 2.0 62PS�Br(a) 3.20 2.6 72

[a] In parentheses: (a) magnetic stirring, (b) mechanical stirring. [b] Eachbis-vinylimidazolium salt contains two imidazolium units. [c] Molar ratioof bis-imidazolium salt in the obtained mlc-SILP to the potential anchor-ing sites on the support, i.e. the number of moles of�SH groups.

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than those generally reported for other supported ionic liq-uids.[9] These results indicate that our synthetic method, inwhich we use an excess of salts 4 and 5 with respect to the �SH groups, leads to the formation of the desired mlc-SILP.

Characterization of all of the catalysts by solid-state 13C NMRspectroscopy confirmed the anchoring of the imidazolium saltsto the support. This is exemplified by the 13C NMR spectrum ofSBA-15�Br(b), which shows similar signals to those of 5 (seethe Experimental Section) except for the disappearance of thesignal corresponding to Ca carbon atoms of the double bondat d= 110 ppm (Figure 1). The disappearance of this signal re-

veals the complete reaction of the terminal double bonds ofthe supported imidazolium salts. The correspondence betweenthe signal at d= 110 ppm and the terminal Ca carbon atomswas confirmed by an HMQC NMR spectroscopy experimentperformed on precursor 5. These results prove that completereaction of the double bonds occurs both through reactionswith thiol groups of the functionalized supports and by self-addition with other vinyl groups. These data also demonstratethat the main structure of 5 is maintained after the graftingprocess. Although the elemental analysis and 13C NMR spec-troscopy data indicate that the synthesis of the mlc-SILPs wassuccessfully achieved, it was still difficult to define the exactstructure of the mlc-SILP materials : a tentative description ofthe domains formed, based on the characterization data, isgiven in Scheme 1. Moreover, at this stage it is not possible toexclude the possibility that a fraction of the bis-vinylimidazoli-um salt copolymerized without reacting with the supportedspecies, thus generating a small amount of unanchored oligo-meric species.

The morphological properties of the mlc-SILP materials interms of specific surface area (Brunauer–Emmett–Teller), cumu-lative pore volume, and mean pore size distribution (Barrett–Joyner–Halenda) were determined by N2 adsorption/desorp-tion measurements (Table S1 in the Supporting Information).Both the amorphous silica and the ordered mesoporous SBA-15 silica show high surface areas (750 and 735 m2 g�1, respec-tively). The ordered mesoporous material SBA-15 is character-ized by the expected narrow pore size distribution, centred at6.8 nm. The surface areas of these materials significantly de-creased after grafting of the ionic liquid phase to about

120 m2 g�1 in the case of amorphous SiO2 and to less than100 m2 g�1 for SBA-15. At the same time, a drastic decrease inthe cumulative pore volume was observed. This dramatic de-crease in surface area and pore volume confirmed the success-ful grafting of the bis-vinylimidazolium salt on the support andindicated that the imidazolium salts were also anchored insidethe pores of both materials. The higher decrease of specificsurface area observed for SBA-15-based mlc-SILP materials canbe ascribed to a higher degree of filling of the narrower poresof SBA-15 and/or to the formation of a more compact poly-meric network.

It is known that imidazolium ionic liquids with halide coun-terions are active catalysts for the chemical fixation of carbondioxide through reaction with epoxides, yielding cyclic carbo-nates. The proposed mechanism for this reaction involves theopening of the epoxide ring by the halide anion, followed bythe insertion of carbon dioxide, and finally by ring closure toyield the cyclic carbonate (Scheme 2).[3] Recently, supported

ionic liquids were reported as heterogeneous catalysts for thisreaction.[9] Our mlc-SILP materials are expected to display im-proved catalytic productivity (grams of product per gram ofcatalyst) compared with other supported ionic liquids as aresult of their multilayered ionic liquid phase, which provides ahigher loading of ionic liquid units per gram of catalyst. Atfirst, we investigated the synthesis of cyclic carbonates bycomparing the mlc-SILP heterogeneous catalysts in which theimidazolium moieties were covalently anchored to a mesopo-rous silica support, such as SBA-15, to those in which a poly-styrene matrix was used as a support. To investigate the versa-tility of our catalysts, three different epoxides were selected assubstrates for the reaction with CO2 : a terminal aliphatic epox-ide, propylene oxide; a terminal aromatic epoxide, styreneoxide; and an internal epoxide, cyclohexene oxide (Scheme 3).The catalytic activity of the series of mlc-SILP catalysts wasstudied by performing the reaction at 100 bar and 150 8C for

Figure 1. Solid-state 13C NMR spectrum of SBA-15�Br(b) ; * denotes spinningside bands.

Scheme 2. Reaction mechanism for carbon dioxide fixation.

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3 h in a new HT unit equipped with 24 parallel batch reactors(Figure 2 a). To the best of our knowledge, this is the first timein which an HT approach has been used to study catalytic reac-tions with CO2 under supercritical conditions.

The advantages of HT techniques include rapid and efficientcomparison of samples and high reliability of results as simul-taneous reactions can be performed in equivalent reactorsunder the same reaction conditions.[28] This is particularly im-portant if working with systems under high pressures and tem-peratures, for which our HT setup guarantees that the pressuri-zation, reaction, and depressurization occur under exactly thesame conditions for all samples in a same experiment, thus al-lowing meaningful comparison of the results. The use of theHT unit coupled with fast GC analysis allows the rapid screen-ing of up to 24 catalysts in 24 h (including both the catalytictest and GC analysis).

The activity trends among the catalysts are similar with eachsubstrate tested: the highest conversion is always achievedwith SBA-15�Br(b) (Figure 3). From a general comparison be-tween the different supports bearing imidazolium salts withthe same counterion, it can be concluded that higher carbon-ate yields are obtained if SBA-15 is used as support. The bettercatalytic performances with SBA-15 than with the polymericsupport (PS), that is SBA-15�Cl @ PS�Cl and SBA-15�Br @ PS�Br, see Table 2, can be explained in terms of the higher surfacearea of the silica support,[30]which is envisaged to favour theformation of a more extended layer of anchored ionic liquid.As expected on the basis of the accepted reaction mecha-nism,[5, 9] supported ionic liquids in which the counterion is bro-

mide provide a higher epoxide conversion than those contain-ing chloride.

A comparison of the results for SBA-15�Cl(a) and SBA-15�Cl(b) [prepared using (a): magnetic stirring or (b): mechanicalstirring, Table 2] showed that there was almost no differencebetween the catalytic performances of the two solids (lessthan 4 %), indicating that the type of stirring (magnetic or me-chanical) used during the synthesis of the mlc-SILP materialshad negligible influence on their catalytic properties.

When comparing the activity of each catalyst with the threesubstrates, a clear trend can be observed (Figure 3): under theconditions used, the reaction performed with propylene oxideas the substrate gives the highest yield of cyclic carbonate; fol-lowed by styrene oxide and finally by cyclohexene oxide, forwhich only low conversions were achieved. This trend is inagreement with the results reported in the literature and con-firms that terminal epoxides are easier to convert into cycliccarbonates.[33–37] It is worth noting that all of the catalysts pres-ent very high selectivity towards the formation of cyclic carbo-nates (>99 % with styrene oxide and cyclohexene oxide). Onlyif propylene oxide was used as starting material were slightlylower selectivities observed (Table 2). The lower selectivity with

Scheme 3. Cycloaddition of carbon dioxide to epoxides. The amount of mlc-SILP was adjusted in function of the imidazolium content to have 1 mol % ofactive catalyst in each batch reactor.

Figure 2. a) Block with 24 parallel batch reactors and b) visualization reactorof the scCO2 HT unit ; the magnification of the visualization window showsthe phase behaviour during the cycloaddition of carbon dioxide to styreneoxide performed at 80 bar.

Figure 3. Cycloaddition of carbon dioxide to &) styrene oxide, *) propyleneoxide, and ~) cyclohexene oxide, catalyzed by mlc-SILP materials based onSBA-15 and polystyrene as supports for the ionic liquid phase.

Table 2. Addition of CO2 to epoxides in the presence of mlc-SILPcatalysts.[a]

mlc-SILP catalysts Styrene oxide Propylene oxide Cyclohexene oxideX [%] S [%] X [%] S [%] X [%] S [%]

PS�Cl(a) 11 >99 34 89 0.0 >99PS�Br(a) 33 >99 48 92 1.3 >99SBA-15�Cl(a) 24 >99 49 92 1.1 >99SBA-15�Cl(b) 28 >99 44 91 1.6 >99SBA-15�Br(b) 55 >99 84 95 6.4 >99SiO2�Cl(a) 28 >99 – – – –SiO2�Br(a) 51 >99 – – – –SiO2�Br(b) 47 >99 – – – –

[a] Reaction conditions: epoxide (45.8 mmol), catalyst (1 mol %), CO2

(100 bar), 150 8C, 3 h; X = epoxide conversion, S = selectivity towards thecarbonate.

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propylene oxide can be attributed to its high tendency to un-dergo polymerization reactions. Indeed, GC–MS analysis per-formed on a representative sample of the propylene oxideseries showed that propylene oxide tri- or tetramers wereformed as by-products during the reaction. The formation ofpolymers of higher molecular weight (not visible by GC) aswell as the possible leaching of imidazolium salts during thereaction were excluded by the absence of signals relative tothese species in the liquid 1H NMR spectra of the reaction solu-tion of selected samples, recorded after the removal of the cat-alyst.

To understand whether the uniform porosity, high specificsurface area, and long-range order typical of SBA-15-like solidshad a positive influence on the catalytic activity, mlc-SILP ma-terials in which amorphous silica was used as the support in-stead of SBA-15 were tested in the cycloaddition of CO2 to sty-rene oxide and the results were compared to those obtainedwith catalysts with SBA-15 as support (Table 2). Interestingly,the activity of the catalyst in which the inorganic support iscomposed of SBA-15 is only slightly higher than the corre-sponding materials in which amorphous SiO2 is used as sup-port. This shows that a high specific surface area (>700 m2 g�1)is the crucial feature to achieve a multilayered, supported ionicliquid phase with a high catalytic activity. The presence of or-dered mesoporosity, such as that in SBA-15, plays only a minor,if any, role on the catalytic activity of the supported imidazoli-um salts. It can be concluded that the use of high-surface-areasilica as the support, either SiO2 or SBA-15, has a beneficialeffect on the activity of the mlc-SILP catalysts because highercarbonates yields than those with the organic polymeric sup-port are obtained with all of the epoxides used in this work.Since the activities achieved with SiO2 and SBA-15 are verysimilar and considering the lower cost of SiO2 relative to SBA-15, the former material can be considered the most advanta-geous choice for supporting multilayered ionic liquid phases infuture studies. The catalytic results with SiO2 as the supportalso confirmed that the type of stirring employed in the an-choring of the imidazolium salts had no influence on the cata-lytic activity of the mlc-SILP materials.

It is known that the temperature and the pressure of CO2 atwhich the reaction takes place play a relevant role in the re-sults of the synthesis of cyclic carbonates catalyzed by ionicliquids. Starting from the good results obtained at 100 bar, wedecided to investigate the effect of decreasing the CO2 pres-sure, while remaining under supercritical conditions for carbondioxide. For this purpose, we performed a catalytic test at80 bar using styrene oxide as the substrate in the presence ofSBA-15�Br(b) as the catalyst. This experiment was first per-formed in the 24-HT parallel reactor and then repeated, withthe same result, in a single batch reactor with large, high-pres-sure viewing windows that enabled the direct visualization ofthe phases in the reaction mixture (Figure 2 b). Under these re-action conditions, an almost complete conversion of styreneoxide (90 %) was obtained after 3 h (Figure 4). This result canbe explained on the basis of the structure of the catalyst andthe presence of a multilayered ionic liquid phase. As clearlyseen in Figure 2 b, two phases are always present in our

system, even if working with CO2 under supercritical condi-tions: the upper phase is richer in carbon dioxide, while thebottom phase, which also contains the catalyst, is richer in theepoxide. Taking into account the structure of the catalyst, wepropose that the reaction takes place in the multilayered, cova-lently anchored, ionic liquid phase. An increase in CO2 pressurewill cause two opposite phenomena: the concentration ofcarbon dioxide in the bottom phase, and consequently, in themlc-SILP, will increase while the concentration of the organicreactant will decrease. Since both styrene oxide and CO2 areessential for the formation of the corresponding cyclic carbon-ate, it is not surprising that a decrease in CO2 pressure canlead to a higher yield of carbonate. This behaviour was previ-ously described for other supported ionic liquid heterogeneouscatalysts.[33–36, 38–39]

Notably, the best of our catalysts, SBA-15�Br(b), shows veryhigh catalytic activity: after 1 h reaction at 80 bar it gives avery good epoxide conversion (75 %) and reaches 90 % after3 h reaction, in all cases with a complete selectivity towardsthe cyclic carbonate product (Figure 4). A direct comparisonwith previously reported catalysts is hindered by the differentloadings of ionic liquid on the various catalysts and by the vari-ety of reaction conditions employed in the literature (Table S2in the Supporting Information).[33–38, 40–41] Nevertheless, the ex-cellent catalytic performance of SBA-15�Br(b) is demonstratedby its very high values of productivity and of turnover frequen-cy (45 gstyrene carbonate gcatalyst

�1 and 30 molstyrene carbonate molIL�1 h�1

after 3 h at 80 bar and 150 8C; 25 gpropylene carbonate gcatalyst�1 and

26.5 molpropylene carbonate molIL�1 h�1 after 3 h at 100 bar and

150 8C). Remarkably, these productivities are much higher thanthose of supported ionic liquid catalysts from the literaturestudied at similar high levels of conversion and with similarmolar percentage of ionic liquid units: for example, silica-sup-ported 4-pyrrolidinopyridinium iodide gives19 gstyrene carbonate gcatalyst

�1 after 20.5 h at 1.01 bar and 100 8C;[40]

1-butyl-3-methylimidazolium tetrafluoroborate supported onsilica reaches 8 gpropylene carbonate gcatalyst

�1 after 4 h at 100 bar and160 8C.[38] This improved productivity is a direct consequenceof the higher ionic liquid loading of our catalyst, which impliesthat a much smaller quantity of the solid is needed to performa reaction with the desired molar percentage of ionic liquidunits. The productivity of our catalyst is also superior to thatachieved with other supported ionic liquids with similar load-ings, but tested with higher molar percentages of ionic liquids[e.g. , 1-(2-hydroxy-ethyl)imidazolium bromide on a highly

Figure 4. Catalytic cycloaddition of carbon dioxide to styrene oxide after 1and 3 h of reaction at 80 bar and 150 8C; &) carbonate yield, &) selectivity.

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P. P. Pescarmona, M. Gruttadauria et al.

cross-linked polystyrene with an ionic liquid loading of3.2 mmol g�1 gives 31 gstyrene carbonate gcatalyst

�1 after 6 h at 25 barand 120 8C].[35] The observed high productivity is an importantfeature from an industrial point of view because a reducedamount of catalyst in a process would result in easier workupprocedures and lower costs.

Although activity and selectivity are the first and most im-portant parameters in the evaluation of the performance of acatalyst, another crucial feature is represented by the stabilityof the catalyst under reaction conditions and by its reusability.Heterogeneous catalysts are clearly advantageous comparedwith homogeneous ones in terms of separability from the reac-tion mixture. Ideally, after separation, the solid catalyst shouldbe reusable in consecutive cycles without losing its catalyticproperties. Therefore, we performed recycling tests on themost active supported ionic liquid catalyst, SBA-15�Br(b),under the most favourable reaction conditions identified inthis work (3 h at 80 bar and 150 8C). At the end of each cycle,the catalyst was easily recovered by filtration, washed with tol-uene and ethanol, dried, and reused. The recycling tests per-formed on the reaction with styrene oxide showed that thecatalyst was stable and could be used for at least four cycleswithout loss in catalytic activity (Figure S3 in the SupportingInformation). Characterization of the catalyst by solid-state13C NMR spectroscopy at the end of the fourth cycle providedfurther confirmation that the morphology of the catalyst aswell as the content of the imidazolium salt remained un-changed during the catalytic tests. To assess the reusability ofour catalysts under more stringent conditions, we performed arecycling test of SiO2�Br(b) by employing a fourfold lower cata-lyst loading (0.25 mol %). In the first run, the catalyst gave 22 %yield of styrene carbonate. Upon recycling, the yield graduallyincreased until the third run (Figure 5), and then stabilized inthe following runs (46 % yield in the fifth run). These resultsconfirmed the reusability of the mlc-SILP catalysts. The gradualincrease in activity observed after the first two recycling testscan be ascribed to swelling and deformation (plasticization) ofthe oligomeric network of supported ionic liquids caused bycarbon dioxide,[42] which would lead to enhanced diffusion ofthe reagents. No leaching of imidazolium salt was detected by1H NMR spectroscopy analysis of the reaction solution at theend of any of the catalytic tests. Future research efforts will try

to unravel the nature of these modifications. Notably, in the re-cycling tests in which intermediate levels of epoxide conver-sion were achieved the selectivity towards the carbonate wasslightly lower (90–95 %) than at a higher catalyst loading(Table 2), owing to the formation of styrene oxide dimer as by-product.

Conclusions

Multilayered covalently supported ionic liquid phase (mlc-SILP)materials were synthesized by using the thiol–ene coupling re-action to covalently anchor bis-vinylimidazolium salts to differ-ent types of support : amorphous and ordered mesoporousSBA-15 silica functionalized with mercaptopropyl groups andcross-linked polystyrene with mercaptomethyl groups. Charac-terization of the prepared materials demonstrated the success-ful anchoring of the ionic liquid phase and indicated the for-mation of multilayers generated by the self-addition reactionof the double bonds of the bis-vinylimidazolium salts. The mlc-SILP materials were tested as heterogeneous catalysts for theaddition of carbon dioxide to various epoxides to yield cycliccarbonates. The catalytic tests were performed under supercrit-ical carbon dioxide in a high-throughput unit that allowed 24reactions to be performed simultaneously in parallel batch re-actors. The best catalyst identified in this work was preparedby supporting a bromide bis-imidazolium salt on SBA-15. Thiscatalyst achieved very high conversion and selectivity in thesynthesis of cyclic carbonates by cycloaddition of CO2 to pro-pylene oxide or styrene oxide, and displayed improved produc-tivity relative to known supported ionic liquid catalysts. Thecatalyst could be easily recovered and recycled in consecutivecatalytic runs without loss of activity.

Experimental Section

Synthesis of bis-vinylimidazolium salts 4 and 5

A solution of 1,4-bis(chloromethyl)benzene or 1,4-bis(bromome-thyl)benzene (0.01 mol) and 1-vinylimidazole (0.021 mol) in chloro-form (5 mL) was heated to 50 8C under stirring overnight. Aftercooling at room temperature, the mixture was filtered and washedseveral times with diethyl ether and the solid product dried at40 8C. Carbon and nitrogen contents were determined by combus-tion analysis with a Fisons EA 1108 elemental analyzer. The NMRspectra were recorded on a Bruker 300 MHz spectrometer usingCD3OD as solvent. FTIR spectra were recorded by use of a Shimad-zu FTIR 8300 infrared spectrophotometer.

Dichloride salt 4 : White powder; yield >95 %; m.p. >250 8C;1H NMR (300 MHz, CD3OD): d= 5.47 (dd, J = 8.7, 2.7 Hz, 2 H; cis-CH=CH2), 5.58 (s, 4 H; CH2Ph), 5.99 (dd, J = 15.6, 2.7 Hz, 2 H; trans-CH=CH2), 7.33 (dd, J = 15.6, 8.7 Hz, 2 H; CH=CH2), 7.63 (s, 4 H; Ph), 7.81(d, J = 1.8 Hz, 2 H; 4-H or 5-H), 8.09 (d, J = 1.8 Hz, 2 H; 4-H or 5-H),9.57 ppm (s, 2 H; 2-H); 13C NMR (300 MHz, CD3OD): d= 53.8, 110.1,121.0. 124.4, 129.8, 130.9, 136.1, 136.7 ppm; IR (nujol): nmax = 724,750, 930, 962, 1162, 1547 cm�1; elemental analysis calcd (%) forC18H20Cl2N4 : C 59.5, H 5.5, Cl 19.5, N 15.4; found: C 59.6, H 5.4, Cl19.6, N 15.5.

Figure 5. Recycling experiments with SiO2�Br(b) as the catalyst for the cyclo-addition of carbon dioxide to styrene oxide. Conditions: epoxide(58.4 mmol), mesitylene (5.84 mmol, internal standard), catalyst (0.25 mol %),3 h at 80 bar and 150 8C; carbonate yields were calculated by use of 1H NMRspectroscopy and confirmed by GC analysis.

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Dibromide salt 5 : White powder; yield >95 %; m.p. >250 8C;1H NMR (300 MHz, CD3OD): d= 5.46 (dd, J = 7.3, 3.2 Hz, 2 H; cis-CH=CH2), 5.55 (s, 4 H; CH2Ph), 5.96 (dd, J = 18.8, 3.2 Hz, 2 H; trans-CH=CH2), 7.30 (dd, J = 18.8, 7.3 Hz, 2 H; CH=CH2), 7.61 (s, 4 H; Ph), 7.78(d, J = 2.1 Hz, 2 H; 4-H or 5-H), 8.05 (d, J = 2.1 Hz, 2 H; 4-H or 5-H),9.50 ppm (s, 2 H; 2-H); 13C NMR (300 MHz, CD3OD): d= 53.8, 110.3,121.1, 124.5, 129.8, 130.9, 136.1, 136.7 ppm; IR (nujol): nmax = 730,766, 927, 966, 1161, 1550 cm�1; elemental analysis calcd (%) forC18H20Br2N4: C 47.8, H 4.4, Br 35.3, N 12.4; found: C 47.9, H 4.5, Br35.3, N 12.4.

Modified silica, SBA-15, and polystyrene supports

Mesoporous SBA-15 was prepared starting from tetraethylorthosili-cate (TEOS; Aldrich 98 %) as the silica source and by using the tri-block copolymer poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (EO20PO70EO20, Pluronic P123; Aldrich) asthe template.[43] In a typical preparation, Pluronic P123 (8.1 g) wasdissolved in deionized water (146.8 g) and concentrated aqueousHCl (4.4 g, 37 %). The solution was stirred overnight at 35 8C in a250 mL flask. TEOS (16 g) was quickly added to this solution, whichwas then stirred for 24 h at 35 8C. The milky suspension washeated to 100 8C for 24 h in a closed polypropylene bottle. Thesolid product was filtered, washed with an HCl/water mixture, andcalcined at 550 8C for 5 h in air. Reaction of SBA-15 (1.5 g) with 3-(mercaptopropyl)trimethoxysilane (2 mL) in toluene (10 mL) underreflux gave 3-mercaptopropyl-modified SBA-15 (2 ; 1.87 g,1.5 mmolSH groups g�1). 3-Mercaptopropyl-modified silica (1; 1.0 g,1.1 mmolSH groups g�1) was prepared similarly to 2 by using Sigma–Al-drich silica (403563) as starting material. Mercaptomethyl-modifiedcross-linked polystyrene 3 (2.2 mmolSH groups g�1, cross-linked with2 % divinylbenzene) was purchased from Sigma–Aldrich.

mlc-SILP materials

The mlc-SILP materials were synthesized by using an excess ofsalts 4 and 5 with respect to the �SH groups: 3.62 mol of salt permol of �SH, which corresponded to 7.24 mol of vinyl groups permol of �SH. Salts 4 and 5 had different solubilities in ethanol;therefore, the thiol–ene reactions were performed by employing adifferent concentration for each salt. The more soluble chloride salt4 was used with a concentration of 200 (with supports 1 and 2)and 400 mm (with support 3). The less soluble bromide salt 5 wasemployed at a concentration of 130 mm. At this concentration, salt5 was soluble at 78 8C and only poorly soluble at room tempera-ture. More dilute solutions did not result in the anchorage reactionproceeding. In the case of polystyrene support 3, the chloride salt4 was used in a 400 mm solution, whereas no reaction occurred ifit was employed at the same concentration of the bromide salt 5(130 mm). In a typical synthesis, the support (1, 2, or 3), a bis-vinyli-midazolium salt (4 or 5, 4 mmol), AIBN (60 mg), and ethanol wereplaced in a three-necked round-bottomed flask. Argon was bub-bled for 10 min and the reaction mixture was mechanically or mag-netically stirred under argon. The flask was heated to 78 8C tofavour the dissolution of the bis-vinylimidazolium salt. The mixturewas stirred for 20 h. After cooling to room temperature, the solidwas filtered and washed with hot methanol, diethyl ether, andthen dried. The imidazolium salt content was determined by chem-ical combustion analysis performed on EAS Vario Max CN equip-ment. The specific surface areas and pore volumes of the silica-based materials, before and after supporting the ionic liquidphase, were determined from N2 adsorption/desorption isothermsmeasured at �196 8C on a Sorptomatic 1900 (Carlo Erba) instru-

ment. The specific surface areas were determined by applying theBET method[44] to the nitrogen adsorption isotherm. The cumula-tive pore volume was calculated by BJH method[45] applied to thedesorption curve in the range p/p0 : 0.2–0.98 (Table S1 in the Sup-porting Information). Solid-state 13C CP MAS NMR spectra of themlc-SILP materials were recorded on a Bruker Avance DSX400spectrometer (9.4 T; Figure S2 in the Supporting Information).14 000 to 22 000 scans were accumulated with a recycle delay of10 s. The CP contact time was 4 ms. Samples were packed in 4 mmzirconia rotors. The spinning frequency of the rotor was 6000 Hz.Tetramethylsilane was used as a chemical shift reference.

Catalytic tests

The catalytic tests were performed in the 24-HT reactor block andin the visualization reactor of a scCO2 HT unit (Figure S1 in the Sup-porting Information) manufactured by Integrated Lab Solutions(ILS) in collaboration with Premex and controlled with software de-veloped by ProControl. The 24-HT reactor block consisted of 24parallel batch reactors with individual magnetic stirring, whichwere designed to operate with glass or Teflon liners that couldeasily be removed from the reactor to facilitate workup of the reac-tion or separation of the catalyst (Figure 2 a). CO2 was added simul-taneously to the 24 reactors through parallel lines. To preventcross-contamination between the different reactors, each entranceand exit line of each of the 24 reactors was equipped with acheck-valve, which limited backflow into the reactor. The tempera-ture of the 24-HT block was regulated by using a Huber Thermo-stat. The pressurization of the reactors was achieved by means oftwo ISCO pumps. The risk of overpressure in the unit was prevent-ed by automated depressurization protocols and by the presenceof bursting disks. In a typical experiment, epoxide (45.8 mmol), cat-alyst (0.458 mmol, 1 mol %), and biphenyl (0.41 mmol, as a GC in-ternal standard) were introduced into the glass liner. The reactionblock, either the 24-HT reactor block or the visualization reactor,was closed and the lines and the reactor were purged with N2 for5 min. Each reactor was pressurized with CO2 at 40 bar, while keep-ing the block at room temperature, then the temperature was in-creased to 150 8C at a rate of 10 8C min�1. During this process, thepressure inside the reactor increased to 60 bar. Once a stable pres-sure value was reached, the reactor was filled with CO2 until theselected pressure (80 or 100 bar). The reactor was kept under theselected conditions for 3 h with a stirring speed of 900 rpm. After3 h (tend) the stirring speed was decreased to 100 rpm and the reac-tors were cooled. Depressurization was started once the carbon di-oxide was no longer under supercritical conditions (T = 40 8C, p =

69 bar; these conditions were reached 4 h after tend if the 24-HT re-actor was used and 3 h after tend if the visualization reactor wasused). The system was allowed to depressurize until the reactorswere at room temperature with an internal pressure <2 bar (theseconditions were reached in a further 90 min). It should be notedthat the reaction of carbon dioxide with the epoxide was expectedto continue, although at a lower rate, during the cooling and de-pressurization phases. This did not affect the comparison betweenthe catalysts tested in the 24-HT reactor block, in which each reac-tor experienced the same temperature and pressure conditionsthroughout the whole experiment. For experiments in the visuali-zation reactor, care was taken to maintain the same conditions inthe cooling and depressurization phases in the various tests. Oncedepressurization was complete, the reactor block was opened andthe glass liners were directly removed from the block and centri-fuged. The samples were diluted with toluene (in the case of sty-rene oxide) and with ethyl acetate (in the case of cyclohexene

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P. P. Pescarmona, M. Gruttadauria et al.

oxide and propylene oxide) and analyzed by GC on a Trace GCUltra from Interscience (RTX-5 column, 5 m, 0.1 mm), equippedwith an ultrafast module allowing the rapid analysis of the samples(less than 2.5 min per sample). The unknown product peaks wereassigned by analysis of GC–MS on an Agilent 6890N gas chromato-graph (WCOT fused silica column, 30 m, 0.25 mm) coupled to anAgilent 5973 MSD mass spectrometer. The reaction mixtures at theend of the catalytic test of selected samples were analyzed by1H NMR spectroscopy to confirm the absence of imidazolium saltsin the solution and to determine the carbonate/epoxide ratio,which was used to calculate the GC response factor for com-pounds that were not available commercially (styrene carbonateand cyclohexene carbonate). The 1H NMR spectra were recordedon a 300 MHz-spectrometer (7.0 T). 32 scans were accumulatedwith a recycle delay of 1 s. The pulse length was 8.0 ms and thepower level was 0.0 dB.

Acknowledgements

C. A. , P. A. , J. A. M., and P. P. P. acknowledge sponsorship in theframe of the following research programs : START1, Methusalemand CECAT (K.U. Leuven, Flemish Government), IAP-PAI (BELSPO,Federal Government), NoE IDECAT (EU), and GOA (Flemish Gov-ernment). We thank Stef Us� for technical support with the super-critical CO2 high-throughput unit, Gina Vanbutsele for assistancein the measurements of the N2 isotherms, Jacqueline Vancluysenfor the elemental analyses, and Dr. Kristof Houthoofd for themeasurement of the solid-state NMR spectra. Dr. Anton Nagy,Tobias Brose, and Dr. Robert Cowan from ILS are acknowledgedfor technical support and fruitful discussions on the use of the su-percritical CO2 high-throughput unit. Financial support from theUniversity of Palermo (Funds for selected topics) and the ItalianMIUR within the national project “Catalizzatori, metodologie eprocessi innovativi per il regio e stereocontrollo delle sintesi or-ganiche” is gratefully acknowledged.

Keywords: carbon dioxide fixation · high-throughputscreening · heterogeneous catalysis · ionic liquids · renewableresources

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Received: August 10, 2011Revised: September 21, 2011Published online on November 23, 2011

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Multilayered Supported Ionic Liquids