Catalytic Coupling of CO2 with Epoxide Over Supported and

J. Phys. Chem. A 2010, 114, 3863–3872

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Catalytic Coupling of CO2 with Epoxide Over Supported and Unsupported Amines† K. M. Kerry Yu, Igor Curcic, Joseph Gabriel, Henry Morganstewart, and Shik Chi Tsang* Inorganic Chemistry Laboratory, Wolfson Catalysis Centre, UniVersity of Oxford, Oxford, OX1 3QR U.K. ReceiVed: July 6, 2009; ReVised Manuscript ReceiVed: August 5, 2009

Catalytic coupling of carbon dioxide with epoxide to cyclic carbonate is an important reaction that has recently been receiving renewed interest. This route allows the use of carbon dioxide as a greener chemical feedstock, which challenges the current practices for the synthesis of cyclic carbonates and derivatives. The present study is mainly concerned with catalytic coupling reaction between CO2 and propylene oxide using organic amine as catalyst. The structural aspects of amines and the effects of their immobilization on solid surfaces on reaction kinetics are particularly studied. It is found that 1,5,7-triazabicyclo[4,4,0]dec-5-ene (TBD) amine maintains high catalytic activity both with and without solid support, but other primary amines, such as p-phenylenediamine give much reduced activity when placed on a solid surface. It is attributed to the absence of surface hydrogen in the supported TBD, prohibiting the catalyst sites from CO2 poisoning. The coupling of other epoxides, including epichlorohydrin and styrene oxide over the solid supported amine, is also briefly carried out. Reaction mechanisms are proposed to explain the experimental observations. 1. Introduction The annual global anthropogenic carbon dioxide (CO2) emissions were estimated at 29 billion metric tons per year in 2004, up from 2 billion metric tons per year in 1900.1 This rise is principally due to the advent of fossil fuel burning power stations, the increase in cement production, and the dramatic rise in the number of CO-emitting vehicles. The excess emission of CO2 poses a huge risk to the world as a whole because it is a primary greenhouse gas which contributes to global warming. Without action toward stemming CO2 emissions, global warming could pose a massive threat to our modern life: crop failure due to changing climates, increasingly common freak weather conditions, and loss of habitat are all potential consequences of humans’ carbon-rich lifestyle which are already becoming apparent.2 These dramatic consequences make climate change and the reduction of CO2 emissions a top priority on every political agenda. At the most recent G8 summit of 2009, the worldwide target of a 50% cut in CO2 emissions set at 2007 has been reaffirmed.3 The U.K. in its 2007 Energy White Paper has also aimed to cut emissions by 60% by 2050, with “real progress” by 2020.4,5 In principle, three strategies are possible: namely, reduction of the amount of CO2 produced by improving energy conversion efficiency, storage of CO2, and usage of CO2. The former two are more important and technically feasible. However, the potential to reduce CO2 emissions by chemical usage of CO2 by using a high concentration of CO2 generated at fossil-fuel burning power stations to useful chemicals is challenging. CO2 is currently not considered as a chemical feedstock because it is particularly unreactive. By designing a suitable catalyst to activate CO2, the use of CO2 as a chemical feedstock could be realized, and the production of useful chemicals and building materials could present a significant new economical solution. On the other hand, CO2 is a thermodynamic sink. A process that chemically converts CO2 into more reactive/more useful † Part of the special issue “Green Chemistry in Energy Production Symposium”. * To whom correspondence should be addressed. E-mail: [email protected].

products must require an energy input. To avoid a situation that the CO2 emission for the process is more than its capture, the source of energy supply should come from either renewable (solar, wind, hydro, ground heat, biomass, etc.) or integrated heat management systems (nuclear plant or waste heat from industrial process). It has since been highlighted that the activation of CO2 for its use as a chemical feedstock is of great importance for carbon management in the future and that a good understanding of the kinetics and thermodynamics of catalytically relevant CO2 reactions is vital.6 Currently, a wide variety of different research efforts are being pooled together to tackle the excess emission of CO2. For instance, a number of studies were focused on the large scale conversion of CO2 to fuels (or as an energy carrier): that is, CO2 to CO via photo reduction,7 CO2 to higher hydrocarbons via Fischer-Tropsch,8,9 CO2 to methanol,10-12 CO2 to formic acid,13 and CO2 with methanol to dimethyl carbonate14 or methal formate.15 The synthesis of cyclic carbonates from the coupling of CO2 with epoxide can take place on a large industrial scale. Epoxides derived from natural products have been the subject of gathering interest lately. Cyclic carbonates are themselves value-added products that have many uses: they are often found in paint strippers, adhesives, and cosmetics, and propylene carbonate has also become a key component in electrolytes used in lithium batteries. Another potentially large use of cyclic carbonates is as antiknocking agents for gasoline-powered motor vehicles; antiknocking agents help gasoline engines run more efficiently, etc. Cyclic carbonates are also useful chemical intermediates for the synthesis of a range of useful chemical products, such as dialkyl carbonates, glycols, carbamides, polymers, etc. (Figure 1). Although the production of cyclic carbonates from CO2 and epoxides has been industrialized since the 1950s and uses inexpensive catalysts such as KI,16 the current processes still suffer from major disadvantages, such as high reaction temperatures (180-200 °C), high pressures (50-80 atm) and stoichiometric amounts of activating reagents.17 Using CO2 is particularly attractive from an environmental point of view, since

10.1021/jp906365g  2010 American Chemical Society Published on Web 09/15/2009

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Yu et al. compared with the corresponding free amines to determine the suitability of using them in the form of heterogeneous catalysts. The results will be presented in this paper.

Figure 1. Various synthesis routes using cyclic carbonate.

CO2 is a cheap, nontoxic, nonflammable, and readily available C1 building block that can be incorporated in the epoxide molecule without the formation of any side products. Thus, the direct usage of CO2 for the production of cyclic carbonates is regarded as a greener approach than the existing practices. Therefore, a large number of studies have been devoted in the past decades to the development of new catalytic systems for the fixation of CO2 for synthesis of cyclic carbonates. A number of new catalysts, including phosphonium salts,18 dimetallic aluminum(salen) complexes,19 Mn salen complexes,20 transition metal complexes, metal salts,21 ionic liquids,22-24 oxychlorides,25 metal oxides,26,27 and polymer-supported complexes28 and zeolites,29,30 have been proposed. Generally, a catalyst system comprising either a strong Lewis acid or a Lewis base seems to be necessary to achieve high yields of cyclic carbonates. Organic amine molecules have recently been shown to catalyze the direct reaction between CO2 and epoxides to form cyclic carbonates.31 A number of supported amines have been synthesized and tested for this type of reaction.32-34 One major advantage of this type of nonmetal-based catalyst system is that it would not introduce metal contaminant(s) to products and the environment. However, the reaction mechanism and the role(s) of amine are not yet clear. On the other hand, amines have long been known to adsorb CO2; liquid amines, such as monoethanolamine, are used commercially to scrub CO2 from gas streams. Amine molecules chemically tethered to a silica surface have also been found to adsorb CO2 with excellent regenerability. Such materials have the potential to be used in industrial processes using a temperature swing to facilitate the adsorption/desorption of CO2. Recently, various amine molecules have been studied and applied for carbon dioxide capture and storage technology35-37 and for catalytic activation of carbon dioxide in production of useful chemicals and building materials where carbon dioxide (CO2) was used as a chemical feedstock.38,39 Thus, the aim of our study was to synthesize, characterize, and test a range of amines of different structures as organic catalysts for the coupling of CO2 with epoxides to cyclic carbonates. This would enable one to gain insights into the structural aspects of the organic catalysts and hints on the reaction mechanism. A particular focus was also placed on investigation of the performances of immobilized amines as

2. Experimental Section 2.1. Sample Preparation. Amines {arginine, bis-(3-aminopropyl)-amine, o-phenylenediamine, oleylamine, 2-aminobenzylamine, p-phenylenediamine (PPD), 1,5,7-triazabicyclo[4,4,0]dec-5-ene (TBD), monoethanolamine}, which were applied directly as catalyst, were used as purchased from Aldrich. Silica gel (grade 643) (99%), (3-chloropropyl)triethoxysilane (CPTES) (95%), 3-aminopropyl-triethoxysilane (APTMS) (99%) from Aldrich were used as purchased. Silica was dried by heating at 393 K under vacuum overnight before use. The following surface modification of silica by organics was conducted on the basis of a published method.40 Generally, a 2.0 g batch of n-propylamine-modified silica (NPA@silica) was prepared by a one-step method in which 2.0 g of the silica gel (predried by heating at 393 K under vacuum overnight) was weighed and transferred to a 250 mL roundbottomed flask with 100 mL of dry cyclohexane as solvent. A 3.0 mL portion of APTMS (in excess) was then added, and the reaction mixture was heated under reflux for 24 h under nitrogen. The modified silica was then washed using dichloromethane and DCM in Soxhlet equipment for 24 h, rinsed with acetone, and then dried (Figure 2). In addition, a 3.0 g batch of TBD-modified silica was prepared by a two-step method. First, Cl-linker (CPTES) was chemically tethered to the silica surface then this was reacted with the TBD to yield the catalyst (Figure 3). Here, 3.0 g of the silica gel was dried by heating under vacuum at 393 K overnight. A 3.0 mL protion of CPTES (in excess) was then reacted with the surface hydroxyl groups of the silica in 150 mL cyclohexane and the resulting material was refluxed under nitrogen for 24 h. Excess CPTES was then removed to prevent any unwanted reaction between free linker and amine; this was done by washing the silica with DCM in a Soxhlet apparatus overnight, rinsing with acetone, and drying. Then the Cl-linker-modified silica was allowed to react with 1.0 g of TBD (in excess) in a roundbottomed flask with 150 mL of cyclohexane as solvent and refluxed under N2 for 24 h. Finally, the modified silica was washed once again with DCM in a Soxhlet apparatus for 24 h, rinsed with acetone, and dried. The PPD@silica catalyst was made in a 1.5 g batch by a two-step method. First, CPTES was chemically tethered to the silica surface, followed by reaction with PPD to yield the catalyst (Figure 4). Here, the CPTES-modified silica was prepared as above. A 0.60 g portion of PPD (in excess) was dissolved in 150 mL of DCM and heated to 353 K with the CPTES-modified silica in a 300 mL stainless steel autoclave (with glass linear) under N2 for 24 h. It is noted that DCM was used as the solvent instead of cyclohexane due to the poor solubility of PPD in cyclohexane. A high-pressure reactor was used to maintain the reaction temperature as the same as the refluxing temperature in the preparation of TBD@silica catalyst. Finally the PPD@silica catalyst was washed with DCM in a Soxhlet apparatus for 24 h, then washed with acetone in the Soxhlet apparatus for another 24 h, rinsed with acetone, and then dried. 2.2. Material Characterization. Thermogravimetric analysis (TGA) was conducted to determine the amount of chemically tethered amines on the surface of silica. The method used was to ramp from room temperature to 800 °C at a rate of 5 °C/ min, recording any weight changes of the sample through the whole range with N2 used as the purge gas.

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Figure 2. Possible synthetic path to obtain NPA/silica catalyst.

Figure 3. Possible synthetic path to obtain TBD/silica catalyst.

Figure 4. Possible synthetic path to obtain PPD@silica catalyst.

Physical characterizations (porosity, surface area, pore distribution) of dried samples were carried out using the standard N2 Brunauer-Emmett-Teller (BET) method at a range of partial pressures at 77 K, and the Barrett-Joyner-Halenda (BJH) method was used to elucidate the average pore diameter of the bulk silica catalyst before and after the amine modification. This was conducted to confirm also the accessibility of chemically tethered amines. To gain further insight into how the CO2 was adsorbed onto the silica catalyst, Fourier transform infrared spectra (FTIR) of samples were gathered using a Thermo Scientific Nicolet 6700 FT-IR machine fitted with a variable temperature golden gate. A comparison of spectra between CO2 pretreated aminemodified silica catalyst and the pretreated catalyst followed by heat treatment was conducted. 2.3. Catalytic Activity Test. The reactor setup (Figure 5) consisted of a 300 mL stainless steel Parr autoclave reactor with overhead magnetic stirrer, heating mantle, and internal thermocouple, all attached to a Parr series 4842 controller. The reactor was fitted with an inlet valve, outlet valve, and pressure gauge, as well as a safety rupture disk designed to depressurize the reactor in case the pressure exceeded 200 bar. Nonviscous liquid substrates were charged into the reactor via a stainless steel loading tube with a known and fixed volume. This loading tube was connected prior to the inlet valve of the reactor, through which gases were charged, therefore forcing any liquid substrate into the reactor. In a typical reaction, the amine-immobilized catalyst (containing 1 mmol amine) and 15 mL of DCM were loaded into the tightly sealed reactor, then the reactor was flushed five times at room temperature with 5 bar of CO2 to remove air from the

Figure 5. A schematic diagram showing the reactor setup.

vessel before being further charged to 20 bar of CO2 and raised to the reaction temperature (typically 423 K). While the reaction temperature was closely monitored, top up gas was loaded via the standardized stainless steel loading tube (which had been prefilled with 2.15 mL of propylene oxide, PO) to the desired pressure (50 bar) before closing the valves tightly. The stirrer was set to a fixed speed for all reactions, and time zero was counted once all reactants were charged into the heated reactor. After the desired reaction time (24 h), the reactor was first cooled in an ice bath then in a dry ice/acetone bath before the pressure was released. In addition, a number of different soluble amines were tested for their catalytic activity under comparable reaction conditions. Each test used the same reaction conditions, as follows: 50 bar CO2, 423 K, 1 mmol amine, 2.15 mL PO, 15 mL DCM, a reaction time of 24 h, in a 100 mL Parr autoclave. 2.4. Product Analysis. Quantitative analysis of cyclic carbonate was conducted by GC/MS, for which an Agilent

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Figure 6. TGA of (3-chloropropyl)triethoxysilane-modified silica.

Figure 7. TGA of 1,5,7-triazabicyclo[4,4,0]dec-5-ene-modified silica.

Technologies 6890N GC system running with a HP-5MS capillary column and an Agilent Technologies 5973 mass selective detector were employed with a known amount of p-xylene (PX) as an external standard. 3. Result and Discussion 3.1. Sample Characterization. The Cl-linker-modified silica was first analyzed to ensure the success of immobilization. Here, two stages of weight loss in TGA were observed: the first, from the loss of any solvents present; and the second, from loss of organic moieties. Here, the second weight loss was recorded as 6.537% (Figure 6). Assuming that after modification, the SiO3 groups of the Cl-linkers form part of the silica network and do not come off as organic moisture, the loading of Cl-linker was determined as 8.43 × 10-4 mol/g.

A typical amine-immobilized sample (after the reaction of free amine with Cl-linker-modified silica), such as TBD@silica, was then analyzed using the same TGA method. This time, the weight loss at higher temperature was found to be 10.330% (Figure 7). This weight loss included the amine-modified Cllinker and nonmodified Cl-linker. On the basis of these values, the percentage of Cl-linkers that have reacted with the amine (Figure 8) is found to be 49.5%, giving a typical loading of 4.18 × 10-4 moles of amine per gram of catalyst (IR confirmed the presence of amine; see Figure 20). Subsequent batches of this material gave similar loadings which were calculated as being between 3.94 × 10-4 and 4.18 × 10-4 moles of amine per gram of catalyst. Similarly, the amine loading of different silica samples is thus summarized in Table 1. It is noted that the TGA data showed



Figure 8. Organic moisture broken down into unmodified Cl-linker (A), modified Cl-linker (B), and amine (C).

TABLE 1: Amine Loadings of Different Catalyst Samples a

sample

type of amine applied

loading/ mol g-1

silica NPA/silica TBD/silica PPD/silica

N.A. n-propylamine 1,5,7-triazabicyclo[4,4,0]dec-5-ene p-phenylenediamin

0 1.70 × 10-3 4.18 × 10-4 5.45 × 10-4

a It is noted that all weight losses due to remaining physisorbed water/solvent in the silica, unreactive linker molecules, and surface dehydroxylation of silica at higher temperature were deducted from the amine loading determination.

TABLE 2: BET Surface Areas and BJH Pore Sizes of Different Silica Samples sample

surface area/m2 g-1

pore diameter/nm

silica NPA@silica TBD@silica PPD@silica

288 201 226 243

16 11 13 12

clearly that all the amine-immobilized samples were thermally unstable as the temperature increased above 450 K due to decomposition of the surface-tethered organic moieties. Hence, the materials could be tested only at temperatures no higher than 450 K. Their N2 physisorption surface areas and pore size measurements are summarized in Table 2. It is noted that there was generally a decrease in both surface area and pore size of the bulk silica after the amine modification. Despite the reduced pore diameters, all of the resulting silica samples should still allow access of internal surfaces by the small-size substrates and products. 3.2. Catalytic Performance on CO2 Coupling Reaction with PO. To establish the structural aspects of amine to the catalysis, the coupling reaction between CO2 and PO under homogeneous conditions was conducted using different amines. 3.2.1. Different Free Amines As Catalysts. A number of different soluble amines, as presented in Figure 9, were tested for their catalytic activity. The resulting catalytic activities are presented in Table 3. It was noted that PC was the only product observed, according to GC/MS analysis. It is interesting to note that the catalytic activity for the production of cyclic propylene carbonate undoubtedly depends on the structure of the amine used. However, there is no apparent relationship of the pKa value of the amine with respect to the activity. It is well-known that the extent of CO2 absorption reaction with amine to form carbamate adduct in solution depends on the basicity (or acidity) of the amine in which the lone pair electrons in the nitrogen as a Lewis base bond to the electron deficient carbon in carbon dioxide.41 Thus, the ratelimiting step for the propylene carbonate formation in this

catalysis is unlikely akin to those of CO2 absorption. From the table, one could see that the primary nonconjugated amines, such as monoethanolamine and bis-(3-aminopropyl)-amine, gave the lowest activity, whereas the amines with availability of unsaturation close to the N atom gave higher activity. Since catalysis reaction is not only concerned with substrate adsorption but also related to desorption of product to free up active sites, it is likely that the conjugation such as (CdN sp2 bonding) effectively weakens the dative bond between the nitrogen atom in the amine and the carbon atom in CO2 by removing electron density from the nitrogen, which leads to rapid product formation within the experimental time. Notice that an R-benzene ring (i.e. PPD providing such a strong electron-withdrawing effect) gave the second-best activity. It is very interesting that the presence of conjugated “NdC-N” structure in the cases of TBD and arginine are also very effective, rendering them among the best active catalysts. It is seen, however, that for both o-phenylenediamine and 2-aminobenzylamine, their measured activities are lower than expected. The existence of intramolecular hydrogen bonding is likely to reduce the electronwithdrawing effect and, hence, to decrease their overall catalytic activities. With the realization of the superior catalytic activity of PPD and TBD, their supported versions were made and tested for comparison. 3.2.2. Testing Solid Supports. Silica and alumina were attempted as possible candidates to carry these amines. Thus, pure supports before and after tethering of the amines were tested. Table 4 shows that the silica-supported TBD catalyst exhibited excellent catalytic activity, giving 100% yield of PC, the same result as for the free TBD under the same reaction conditions. Also of note is the insignificant activity of pure, dry silica but the appreciable activity of pure, dry alumina (with more acid and base sites). Therefore, silica was used as the amine carrier in the following studies to see the catalytic effect of amines for a CO2 coupling reaction with PO. 3.2.3. Optimizing the Reaction Conditions. Following the successful trial of using TBD@silica as a catalyst for the coupling reaction between PO and CO2, the reaction conditions were optimized by finding the optimal pressure of CO2. The pressure of CO2 was varied from 20 to 140 bar, and the total pressure was kept constant at 140 bar using N2 to balance the pressure. A volcano-shaped activity plot was obtained, as presented in Figure 10. It is known that accessibility of substrate is very important to sustain high activity. If there is a mass transfer limitation, a slow rate will be encountered. It is clearly evident from the figure that at low applied CO2 pressure, the rate of reaction is low (low solubility of CO2), indicative of the mass transfer limitation. The rate reaches its optimum at about 80

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Figure 9. Amines tested as catalysts for CO2 coupling reaction with propylene oxide.

TABLE 3: Catalytic Activity of a Variety of Different Unsupported Aminesa amine

pKa value

PO conversion/%

monoethanolamine bis-(3-aminopropyl)-amine o-phenylenediamine oleylamine 2-aminobenzylamine arginine PPD TBD

9.5 10.7 4.5 10.7 9.5 13.6 6.2 14.5

48.8 55.8 56.6 67.1 70.2 79.9 93.8 100

Figure 11. A plot of PO conversion against reaction time. (Reaction conditions: 0.20 g TBD@silica, 85 bar CO2, 55 bar N2, 423 K.)

a

Reaction conditions: 50 bar CO2 423 K, 1 mmol amine, 2.15 mL PO, 15 mL DCM, a fixed reaction time of 24 h in a 300 mL Parr autoclave.

TABLE 4: Catalytic Activity of Pure and TBD-Modified Alumina and Silicaa catalyst used

amine loading (mol g-1)

amount of catalyst/g

PO conversion/%

pure, dry alumina pure, dry silica TBD@alumina TBD@silica

N/A N/A 4.2 × 10-4 4.18 × 10-4

0.20 0.20 0.20 0.20

19.8 0.6 84.2 100

Conditions: 50 bar CO2, 423 K, 2.15 mL PO, 15 mL DCM, a reaction time of 24 h in a 300 mL stainless steel autoclave.

Figure 12. A plot of PO conversion against reaction temperature with free form amine as the catalyst. (Reaction conditions: 1 mmol amine, 15 mL DCM as solvent, 85 bar CO2 at 423 K, 55 bar N2 at 423 K, 5 h.)

Figure 10. A plot of PO conversion against CO2 partial pressure. The solid data point indicates a test conducted with pure CO2 at 85 bar. (Reaction conditions: 0.20 g TBD@silica; 423 K; 2.15 mL PO; 140 bar total pressure with N2 as diluent; 300 mL autoclave, 5 h.)

Figure 11 shows a typical plot of PO conversion against time, the linearity of which suggests strongly that the reaction is of zero order. Here, the GC/MS data showed only trace amounts of propane-1,2-diol (∼0.2 mol % of PO), which was most likely due to the presence of water from the silica-based catalyst. No other side products were observed. 3.2.4.2. Effect of Reaction Temperature. The activation energies of the reaction for both supported and unsupported PPD and TBD were determined by studying the effects of temperature using the Arrhenius equation. Here, kinetic studies of both nonsupported (Figure 12) and supported (Figure 13) PPD and TBD were preformed. Activation energy of the reaction with TBD@silica was determined to be 74.0 kJ mol-1 according to its Arrhenius plot. This value is sufficiently high to dispel any suspicion of diffusion control and to confirm activation control of the reaction. The nonsupported soluble TBD was also tested over the same temperature range, with the same number of moles of amine (1 mmol), which was presented in 0.20 g of TBD@silica. Here, the activation energy for the nonsupported TBD was determined to be 80.4 kJ mol-1, a slightly higher activation barrier for the reaction.

a

bar; further enhancement in pressure is expected to cause a dilution effect, hence, reducing the reaction rate. 3.2.4. Kinetic Studies of the TBD@silica Catalyzed Reaction. 3.2.4.1. Effect of Reaction Time. To investigate the effect of reaction time and the effect on the amount of PC formed/ accumulated over the reaction, experiments were conducted over 2, 5, 9, and 16 h.


Figure 13. A plot of PO conversion against reaction temperature with supported amine as the catalyst. (Reaction conditions: 0.20 g TBD@silica, 85 bar CO2 at 423 K, 55 bar N2 at 423 K, 5 h.)


Figure 16. A comparison of catalytic activities with respect to free PPD and PPD@silica at different temperatures is made. The activity of PPD@silica is arbitrarily taken as 100%, as shown in the dashed line, as compared to the relative activity of supported PPD at same temperature.

Figure 14. A comparison of catalytic activities with respect to free TBD and TBD@silica at different temperatures is made. The activity of TBD@silica is arbitrarily taken as 100%, as shown in the dashed line, as compared to the relative activity of supported TBD at the same temperature.

Figure 17. FT-IR spectra for free CO2 (bottom), CO2-pretreated PPD@silica at room temperature (middle), and CO2-pretreated PPD/ silica at 323 K after 10 min. Figure 15. A plot of TON against reaction temperature. (Reaction conditions: 0.20 g amine@silica, 85 bar CO2 at 423 K, 55 bar N2 at 423 K, 5 h, TON ) (mol PC)/(mol amine).)

However, it is very interesting to note from Figure 14 that the supported TBD amine (TBD@silica) consistently outperformed the soluble form at the equivalent concentration under the moisture-free testing conditions. The role of the support has not been studied in detail, but surface silanol groups on the silica support might have played a role of (co)activating propylene oxide.34 On the other hand, we do not think this was the main reason because excess (3-chloropropyl)triethoxysilane was used in the immobilization, which should have captured all the reactive silanol groups accordingly.40 No evidence indicating the presence of silanol groups of the supported sample was obtained from FTIR and TGA. As the second most active homogeneous catalyst, PPD was also tested for its potential as a heterogeneous catalyst by its immobilization on silica as PPD@silica. In addition, a primary amine-modified silica catalyst, NPA@silica, was tested, as well, to compare the catalytic activity. The resulting catalytic activities are presented in Figure 15. Activation energies for both of these amine-immobilized catalysts were determined according to their Arrhenius plots from Figure 15. It is, however, surprising that low activation energy values were obtained as 10.5 and 17.3 kJ mol-1 for PPD@silica and NPA@silica, respectively, since the amine

modified silica did not display any diffusion properties, according to their surface area and pore size studies. On the other hand, the unsupported amines at equivalent concentration were far more active than the solid counterparts. It was suspected that the amine catalyst must have been poisoned by CO2 during the reaction in the solid versions, which led to a lower number of active amines on the surface for the coupling reaction. The comparison of the relative activity of the supported (taken arbitrarily as 100%) and unsupported PPD, as shown in Figure 16, clearly shows the inert supported amine. 3.2.5. FT-IR Studies of CO2 Adsorption on Amine-Modified Silica Catalyst. To verify the poisoning effect of adsorbed CO2 on the solid amines, a spectrum of a sample of PPD@silica pretreated with 50 bar of CO2 for 2 h was first collected to identify the type of adsorbed CO2 species present. Then this sample was heated to 323 K for 10 min before a second spectrum was collected to compare the free CO2 spectrum (Figure 17). Figure 17 clearly shows that the typical fundamental ν3 peaks (asymmetric stretches) of the adsorbed CO2 species are displaced toward lower wavenumbers than the free form of CO2, suggesting a weakening of the CdO bonds, possibly via a strong electron donation from the lone pair on nitrogen to the π* antibonding orbital on carbon. The same species was observed in the spectrum collected at 323 K, suggesting a strong chemisorption of CO2.

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Figure 20. Comparison of the FT-IR spectra in the region of 1670-1350 cm-1 for CO2-pretreated PPD@silica and CO2-pretreated TBD@silica at room temperature.

Figure 18. FT-IR spectra for free CO2 (bottom), CO2-pretreated NPA@silica at room temperature (middle), and CO2-pretreated NPA@silica at 323 K after 10 min.

Figure 19. FT-IR spectra for free CO2 (bottom), CO2-pretreated TBD@silica at room temperature (middle), and CO2-pretreated TBD@silica at 323 K after 10 min.

To identify the adsorbed CO2 species in both the NPA@silica and TBD@silica, FTIR studies were conducted with the same procedure as described above. Similar to the PPD@silica, a shift in the ν3 vibrations of CO2 to lower wavenumbers of the NPA@silica sample was also observed in the room temperature sample, again suggesting a weakening of the CdO bond via strong electron donation from the nonbonding lone pair of the amine nitrogen to the π* antibonding orbital on carbon. Not surprisingly, the same species was observed in the spectrum collected at 323 K (Figure 18), implying a strong CO2 chemisorption on this solid amine surface. The peaks observed from pretreated PPD@silica and NPA@silica match up to within 0.5 cm-1, suggesting that the species formed in either case are nearly identical. These results clearly confirm that a very stable chemisorbed adduct was formed over the PPD@silica and NPA@silica, likely poisoning the catalysis. In sharp contrast, it is clearly seen from Figure 19 that the peaks in the TBD@silica spectrum line up exactly with the ν3 peaks of free CO2, identifying the species as physisorbed CO2.

The spectrum collected at 323 K shows no peaks in this region, confirming that the weakly bound physisorbed CO2 was driven off from the TBD@silica. Thus, this supported TBD amine does not seem to create strong adsorption of CO2 under identical conditions. Figure 20 shows the detailed comparison of the spectra in the low-frequency region of CO2-pretreated PPD@silica with the CO2-pretreated TBD@silica. A clear, large absorption band of 1625 cm-1 could be assigned to the formation of NH3+ as the CO2 is adsorbed onto the PPD@silica; a small band at 1440 cm-1 due to the presence of adsorbed molecular water; and characteristic bands at 1517 cm-1 (doublet carbamate-bicarbonate), 1420 cm-1 (carbamate C-O bending), and 1371 cm-1 (bicarbonate C-O bending). This spectrum is very similar to those reported earlier with CO2 adsorbed on silica gel containing 3-aminopropyl groups bonded to surface atoms of silicon.41 This evidence clearly confirms that the carbon dioxide is strongly adsorbed as a bidentate mode on the material surface with two surface amino groups (through surface hydrogen bonding interaction) to form ammonium carbamate species.42 When moisture is adsorbed, ammonia bicarbonate surface species can also be formed. Figure 21 shows a tentative model describing how CO2 adsorbed onto the surface by forming H-bonds with the tethered amine. It is noted that the negative charge density in the oxygen atoms of activated CO2 will be greatly neutralized by the ammonium species in the close vicinity. In contrast, the absence of prominent absorption bands at characteristic regions near 1625, 1517, 1420, and 1371 cm-1 in the case of TBD@silica clearly suggests no equivalent strong adsorption of CO2 (Figure 20). In catalysis, it is undesirable for a substrate (CO2) to be too strongly adsorbed onto the catalyst surface. Therefore, it is believed that the CO2 itself acts as a poison, which strongly binds to the amine surface with hydrogen atoms as bidenate species. This accounts for the dramatic attenuation in catalytic activity of the PPD@silica catalyst. The poisoning effect is also thought to be significant on a solid surface, since the surface species are relatively immobile, as compared to solution species (hydrogen bonding interactions can be constantly broken and reformed in solution). On the other hand, the absence of surface hydrogen atoms from immobilized TBD would not allow any charge neutralization on the O atoms in the activated CO2. These unstable but reactive charges on oxygen atoms will surely induce cycloaddition with the incoming propylene oxide, and the weak N-C dative bond due to conjugated N will also desorb the product readily, hence, accounting for high activity (Figure 22). Thus, the key impor-



Figure 21. Formation of stable adsorbed CO2 species on the surface of primary amine-modified silica, with hydrogen bonding formation rendering the adsorbed molecule (surface poisoned) unreactive toward cycloaddition with incoming epoxide.

Figure 22. Weakly adsorbed CO2 species on the surface of TBD-modified silica with no terminal hydrogen on the surface, rendering the adsorbed molecule (activated surface) reactive toward cycloaddition with incoming epoxide.

tance is the absence of surface hydrogen atoms in the TBD@silica after immobilization (the only proton in the secondary amine is eliminated by forming HCl upon immobilization with the surface Cl-linker). From our experimental evidence, this molecular catalyst structure appears to outperform those of the soluble counterpart. Although the recyclability of the TBD@silica catalyst has not been investigated in this work, a related study by Zhang et al. showing high stability of this supported amine is established.34 To gain further insights into the reaction mechanism, two different types of epoxides, namely, epichlorohydrin and styrene oxide, were attempted for the cycloaddition reaction using TBD@silica as the catalyst. By applying the same reaction conditions, reaction kinetics can be directly compared, as presented in Figure 23. Here, cyclic carbonates were formed as the main products, with selectivity >98% in both cases. The corresponding dialcohols were also observed from GC/MS analysis as a minor product due to the existence of trace water in the silica catalyst

Figure 23. A plot of TON against reaction temperature. (Reaction conditions: 0.20 g TBD@silica, 85 bar CO2 at 423 K, 55 bar N2 at 423 K, 5 h.)

used. The activation energy for the cycloaddition of CO2 with epichlorohydrin was determined to be 62.9 kJ mol-1; the activation energy of the styrene oxide was 78.8 kJ mol-1.

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Following the previous discussion on mechanism, the σ electron withdrawing group (-Cl) in epichlorohydrin should render the carbon atoms of epichlorohydrin with a higher positive character than those of PO and styrene oxide. Thus, according to the previous tentative mechanism, it is highly likely that the carbon atoms would be more readily attacked by the partially negatively charged oxygen atom from the activated carbon dioxide. This effect was shown clearly by a lower kick-off reaction temperature and the apparent lower activation energy for the cycloaddition of CO2 with epichlorohydrin. On the other hand, an electron-rich functional group, a benzyl group, in styrene oxide would make it more difficult for the cycloaddition reaction of the activated CO2 to take place. Here, this accounts for the slight enhancement in the activation energy of the styrene oxide as compared with the PO. It is worth noting that epichlorohydrin can be prepared from glycerol (a byproduct in biodiesel production from biomass).43,44 Thus, the uses of carbon dioxide and epichlorohydrin as “green” reagents for the production of cyclic carbonate could have commercial interest. 4. Conclusion It is demonstrated that the TBD amine-modified silica is an active solid catalyst for the cycloadditatioin of CO2 with epoxides such as PO, styrene oxide, and epichlorohydrin, giving high yields of the corresponding cyclic carbonates. This organic amine-based catalyst would not introduce any metal contamination to the products and the environment, as compared to those using metal catalysts. In contrast, for the same reaction, primary amines immobilized on silica have been shown to give much poorer activity as compared to their soluble amine analogues. This is attributed to the formation of stable surface ammonium cabamate and carbonate species from strong and bidentate CO2 adsorption on the amine surface, which deny further cylcoaddition with the epoxide co-substrate. The strong dative bonding between the N-C with no conjugation also leads a slow catalytic activity. It is clearly shown in this study that the supported TBD@silica catalyst gives a superior catalytic performance. Acknowledgment. We are grateful to the EPSRC of U.K. for financial support. References and Notes (1) Marland, G.; Andres, B.; Boden, T.; Carbon Dioxide Information Analysis Center: Oak Ridge, TN, 2007. (2) Walther, G. R.; Post, E.; Convey, P.; Menzel, A.; Parmesan, C.; Beebee, T. J. C.; Fromentin, J. M.; Hoegh-Guldberg, O.; Bairlein, F. Nature 2002, 416, 389–395. (3) G8 Summit 2009, L’Aquila, Italy 2009; G8 Summit 2007, Heiligendamm, 2007. (4) 2007 Energy White Paper, Department of Trade and Industry; U.K. Government: London, 2007. (5) Fulkerson, W.; Judkins, R. R.; Sanghvi, M. K. Sci. Am. 1990, 263, 128–135. (6) Arakawa, H.; Aresta, M.; Armor, J. N.; Barteau, M. A.; Beckman, E. J.; Bell, A. T.; Bercaw, J. E.; Creutz, C.; Dinjus, E.; Dixon, D. A.; Domen, K.; DuBois, D. L.; Eckert, J.; Fujita, E.; Gibson, D. H.; Goddard, W. A.; Goodman, D. W.; Keller, J.; Kubas, G. J.; Kung, H. H.; Lyons, J. E.; Manzer, L. E.; Marks, T. J.; Morokuma, K.; Nicholas, K. M.; Periana, R.; Que, L.; Rostrup-Nielson, J.; Sachtler, W. M. H.; Schmidt, L. D.; Sen, A.; Somorjai, G. A.; Stair, P. C.; Stults, B. R.; Tumas, W. Chem. ReV. 2001, 101, 953.

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JP906365G

Catalytic Coupling of CO2 with Epoxide Over Supported and

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