- Email: [email protected]
Sulfonic acid functionalized mesoporous SBA-15 as catalyst for styrene carbonate synthesis from CO2 and styrene oxide at moderate reaction conditions
Journal of CO2 Utilization 10 (2015) 88–94
Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Sulfonic acid functionalized mesoporous SBA-15 as catalyst for styrene carbonate synthesis from CO2 and styrene oxide at moderate reaction conditions Seenu Ravi, Roshith Roshan, Jose Tharun, Amal Cherian Kathalikkattil, Dae Won Park * School of Chemical and Biomolecular Engineering, Pusan National University, Busan 609-735, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 November 2014 Received in revised form 29 December 2014 Accepted 9 January 2015 Available online 3 February 2015
A sulfonic acid tethered mesoporous silica material (SBA-15-SO3H) was successfully synthesized and characterized using SAXS, N2 physisorption studies, TEM, EA and XPS. This metal-free heterogeneous catalyst with terabutyl ammonium bromide (TBAB) co-catalyst was found to be an efficient catalyst system for styrene carbonate synthesis from CO2 and styrene oxide. The synergistic mechanistic pathways of SBA-15-SO3H with TBAB have been explained by hydrogen bond interactions and nucleophilic effects. A turn over number of 920 was obtained at mild reaction temperature of 80 8C. The catalyst was thermally stable and reused for five times without loss of any significant activity. ß 2015 Elsevier Ltd. All rights reserved.
Keywords: Cyclic carbonates Synergism Catalysis Sulfonic acid Mesoporous silica
1. Introduction Carbon dioxide, the ‘C1’ resource whose atmospheric concentration increases infinitely, is renewable, non-toxic, and low-cost concerning global warming [1]. Recently a great deal of attention has been given on the conversion of CO2 into valuable chemicals [2–4]. Moreover, it is necessary to implement feasible carbon dioxide capture and sequestration technologies to reduce the anthropogenic CO2 emissions, especially from industries. One of the viable processes is to synthesis cyclic carbonates by the catalytic insertion of CO2 to oxiranes in the presence of acid supported catalysts [5,6]. A variety of cyclic carbonates (CC) have several purposes in industrial process and they are frequently used in electrochemical process and as intermediates in the synthesis of polycarbonates, etc. [7–10]. Extensive research is being directed worldwide toward the development of catalytic processes that use carbon dioxide as a feedstock for producing CC. Among this, much of the current research has been focused on the employment of heterogeneous catalysts such as mesoporous silica, resins, metal organic framework and porous organic polymer materials for CO2 fixation into cyclic carbonates [11– 15]. Ionizable moieties such as halides in homogeneous or
* Corresponding author. Tel.: +82 515102399; fax: +82 515128563. E-mail address: [email protected] (D.W. Park). http://dx.doi.org/10.1016/j.jcou.2015.01.003 2212-9820/ß 2015 Elsevier Ltd. All rights reserved.
heterogeneous phases are extensively used in the catalytic synthesis of cyclic carbonate. For example, ionic liquids [12,16], salen complexes [17,18], quaternized compounds [19], betaine [20], phophonium robust type catalysts [21], functional polymers [22,23], supported heterogeneous catalyst with alkali halide or onium salts [24–29] etc. have been successfully employed as catalysts for the CO2 epoxide cycloaddition. Among the reported heterogeneous catalysts, mesoporous silica stands attractive due to their abundant free silanol groups which are responsible for their acidic nature and their easy susceptibility for organo functionalization. Moreover they are attractive materials with high surface area, large pore volume and pore diameter, and hence used for green and sustainable applications including drug delivery, adsorption, catalysis and energy-devices [30–35]. Of the different morphologies of mesoporous silicas, 2D materials are more preferable for cyclic carbonate synthesis since it could offer easy diffusion mechanism for relatively large guest species [30]. Improvisations of heterogeneous catalysts having high turnover frequency by economical and easy synthetic procedures at ambient reaction conditions are desirable in the field of cyclic carbonate synthesis. For example silica supported ionic liquids are prominent in CO2 cycloaddition to epoxide; their turn over frequencies are usually low [16,36]. Moreover metal supported silica catalyst can leach the metal ions easily into the reaction mixture and its removal would be a difficult process. Similarly, quaternary ammonium salts like tetra butyl
S. Ravi et al. / Journal of CO2 Utilization 10 (2015) 88–94
ammonium bromide were used as cocatalyst with heterogeneous MOF materials to perform the cycloaddition reaction at lower reaction temperature [27], however a bulk synthesis was difficult in many of the cases. To overcome these issues, easily synthesizable heterogeneous metal-free catalysts with supported acidic group could be a better solution. Sulfonic acid supported catalysts are highly acidic and they are extensively used in several catalytic processes such as oxidation, coupling, conversion and substitution reactions [37–39]. These reaction mechanisms are explained as the resultant of the ionizable property and/or the acidity of active sulfonic moieties. Here, our motivation is to synthesis styrene carbonate (SC) at ambient reaction conditions using a metal-free acidic heterogeneous catalyst along with onium halide as cocatalyst. Thus, a simple metal-free catalyst that operate under mild reaction conditions is of uttermost priority to minimize costs and a viable utilization of CO2 greenhouse gas to address the economic and environmental concerns. Herein we report the synthesis of styrene carbonate at mild reaction conditions using a sulfonic acid supported mesoporous silica/tetra butyl ammonium bromide binary catalyst system for the first time (Scheme 1). The effects of reaction parameters such as temperature, pressure were studied and the optimization of catalyst ratio was performed. 2. Experimental 2.1. Material synthesis The synthesis of 2D SBA-15 mesoporous material was carried out according to the previous report [9] by sol–gel method using Pluronic P123/TEOS/HCl/H2O. Typically Pluronic P123 was dissolved in aqueous HCl solution with magnetic stirring until the clear solution was obtained. This was followed by the addition of TEOS (1 mol) to the solution; and the pH (1.2) was maintained. The resulting mixture was stirred further for 24 h at 30 8C, transferred to Teflon-lined stainless steel autoclave and heated to 100 8C for 24–36 h. The obtained materials were filtered and washed using distilled water followed by ethanol; dried in vacuum at 60 8C for 6 h and then calcined at 550 8C for 6 h to remove the surfactant from the as-synthesized materials. The pristine SBA-15 materials was functionalized with 3-MPS (3-mercaptopropyl trimethoxysilane) by dispersing in toluene at refluxed condition for 6 h and then filtered, washed and dried at oven at 55 8C for 3 h. Then, the obtained SBA-15-SH were dispersed in methanol/50% H2O2 solution and stirred for 6 h under acidic condition (H2SO4). The oxidized material was filtered and dried in vacuum oven at 55 8C for 8 h, and it was denoted as SBA-15-SO3H. Thenceforth, the obtained materials were successfully characterized. 2.2. Characterization techniques Transmission electron microscope (TEM) images measurements were recorded on a JEOL 2010 electron microscope operating at 200 kV. The powder samples were dispersed in ethanol, then deposited and dried on a perforated Cu grid. Images were recorded at magnifications 150,000 and 200,000. Small angle X-ray scattering (SAXS) with Co-Ka radiation (l = 1.608 A˚) in
89
the energy range 4–16 keV (energy resolution: DE/E = 5 10 4; photon flux: 1010–1011 ph./s, Beam size: <1 mm2) over the scan range 0.4 nm 1 < q < 5.0 nm 1. Nitrogen adsorption and desorption isotherms were measured using an ASAP 2020 surface-area and pore-size analyzer. Prior to the measurement, the samples were dehydrated at 90 8C for 5 h. The BET (Brunauer–Emmett– Teller) method was used to calculate the specific surface area. The pore size distribution was calculated from the analysis of the isotherm desorption branch by the BJH (Barret–Joyner–Halenda) method coupled with the apparatus software. Elemental analyses were measured on vario MICRO CHNS analysis technic. The infrared spectra were recorded on a Nicolet 6700 FT-IR spectrometer. XPS analysis was performed using an X-ray photoelectron spectrometer (VG, ESCALAB 250) with monochromatic Al Ka radiation (hn = 1486.6 eV). 2.3. Cycloaddition of CO2 with epoxides The synthesis of SC from styrene oxide (SO) and CO2 (Scheme 1) using the SBA-15-SO3H catalyst was performed in a 50 mL stainless steel autoclave equipped with a magnetic stirrer. For each typical batch operation, SBA-15-SO3H catalyst 0.11 g (0.0187 mmol based on elemental analysis), 0.026 g TBAB (0.081 mmol) and SO (20 mmol) were charged into the reactor without solvent. The reactor was then pressurized with CO2 to a preset pressure at room temperature. The reactor was heated to the desired temperature, and then the reaction was started by stirring the reaction mixture at 450 rpm. The reactor pressure increased by about 0.1–3 MPa depending on the reaction temperature. After the completion of the reaction time, the cycloaddition was stopped followed by cooling the reaction mixture in ice and vented off the remaining CO2. The product (4-phenyl-1,3-dioxolan-2-one) was dissolved in dichloromethane and filtered to remove the catalyst. Product analysis was carried out via gas chromatography/mass spectrometry (GC–MS, Micromass, UK) analysis. The conversion of SC was obtained from gas chromatography (GC, HP 6890, Agilent Technologies, Santa Clara, CA, USA) data. 3. Results and discussion 3.1. Characterization The SBA-15-SO3H was obtained through functionalization and subsequent oxidation of 3-MPS into SBA-15 mesoporous silica by post grafting method. Fig. 1 depicts the SAXS pattern of the synthesized SBA-15 and supported catalysts. SBA-15 mesoporous material (a) showed three well resolved characteristic SAXS pattern such as (1 0 0), (1 1 0) and (2 0 0). The functionalized materials SBA-15-SH (b) and SBA-15-SO3H (c) also exhibited similar peak patterns, which represent the highly ordered mesoporous SBA-15 with a 2D hexagonal p6mm symmetry. Moreover, the decrease in relative peak intensities from a–c suggests that the organosilanes are successfully functionalized into the SBA-15 material. In addition, the d-spacing value for SBA15 (9.3 nm) is increased to 9.5 nm after the 3-MPS loading, as shown in Table 1. Nitrogen physisorption experiments were performed to study the changes in the textural properties of the
Scheme 1. Styrene carbonate synthesis.
S. Ravi et al. / Journal of CO2 Utilization 10 (2015) 88–94
90
Fig. 1. SAXS patterns of SBA-15 and supported catalysts.
Table 1 Structural properties of SBA-15 materials. Materials
d (nm)
SBETa (m2/g)
Dpa (nm)
Vp
SBA-15 SBA-15-SH SBA-15-SO3H
9.3 9.5 9.6
530 316 355
4.2 3.9 3.9
0.89 0.54 0.57
a
(cm3/g)
Sb (mmol/g) 0 0.18 0.17
d, spacing value; SBET, surface area; Dp, Pore diameter; Vp, pore volume; S, sulfur value. a Calculated from N2 physisorption studies. b Calculated from elemental analysis.
functionalized mesoporous materials. The nitrogen adsorption isothermal plots of thiol and sulfonic acid functionalized materials showed type IV isotherms (Fig. 2), similar to that of SBA-15 [40]. These observations were associated with the capillary condensation in hexagonal cylindrical mesopores; at relative pressures of P/P0 0.4–0.8, large porous mesostructures with specific BET surface areas of 530, 316 and 355 m2/g were obtained for pristine SBA-15, SBA-15-SH and SBA-15-SO3H, respectively.
The observed hysteresis loops show that they belong to same types of pore structures. The obtained H1 hysteresis loop and the pore diameter and pore volume values varies from SBA-15 to SBA-15SO3H (Table 1), implying the versatile cylindrical pore entrances. The quantitative determinations of sulfur in the SBA-15-SH and SBA-15-SO3H materials were carried out by elemental analysis and the corresponding sulfur values are given in Table 1. Further evidences for the existence of thiol group in SBA-15-SH and sulfonic group in SBA-15-SO3H were provided through XPS analysis (Fig. 3). In this case a sole characteristic thiol (–SH) sulfur 2p binding energy peak at 163.57 eV and the SO3 was observed at 168.22 eV. Since the peak at 163.57 eV was almost invisible in SBA15-SO3H, it could be surmised that the oxidation of –SH groups to – SO3H group was almost complete. The TEM image in Fig. 4 shows that the channeling pores are well-arranged in 2D-hexagonal p6mm symmetry for the SBA-15 (SO3H). It is particularly noteworthy that the channeling pores are larger enough for the conventional reaction synthesis. As molecular diffusions are a factor while applying SBA-15 in catalysis, this SBA-15-SO3H
Fig. 2. N2 – adsorption and desorption isotherm of SBA-15 and supported catalysts.
S. Ravi et al. / Journal of CO2 Utilization 10 (2015) 88–94
91
heterogeneous supported silica materials have been reported for the synthesis of SC; but unfortunately most of them were performed in drastic reaction conditions such as high pressure and temperatures. Contrastingly, we performed the cycloaddition reaction of styrene oxide (SO) and CO2 at moderate conditions of 0.1 MPa and 80 8C for 8 h with 0.1 mol% of SBA-15-SO3H and 0.4 mol% of TBAB co-catalyst (Table 2). In the absence of any catalyst, no SC was yielded. Cycloaddition with SBA-15-SO3H alone gave a SO conversion of 8% with 90% selectivity for SC, whereas cycloaddition with TBAB alone showed 46.0% conversion and over 99% selectivity (entries 2 and 3) under the employed reaction conditions. Interestingly, with SBA-15-SO3H/TBAB catalyst system, 92% SC yield at 98% selectivity (entry 4) was observed; indicating a synergistic involvement of the catalytic units in the reaction. Based on the experimental outputs, the mechanism involved behind this synergism could be attributed to the hydrogen bonding interaction of sulfonic acid with the epoxy oxygen. SBA-15-SO3H/TBAB catalyst system exhibited higher TON than silica-supported ionic liquid catalysts [13,25,26] and metal organic framework with sulfate groups [6]. Very interestingly, a SC yield of 89% was observed even at near-atmospheric pressure of CO2 (0.1 MPa CO2, Entry 5). To date, there are no reports of silica materials employed as catalyst at ambient reaction conditions. The comparison of the catalytic activities of the reported silica based and sulfonate based catalysts are shown in Table 3.
Fig. 3. XPS S2p region of SBA-15-SH and SBA-15-SO3H catalyst.
3.3. Influence of reaction parameters
Fig. 4. TEM image of SBA-15-SO3H. Table 2 Cycloaddition of SO and CO2 using SBA-15-SO3H/TBAB catalyst. Entry
Catalyst
Co-catalyst
Yield (%)
Sel. (%)
1 2 3 4 5a
– SBA-15-SO3H – SBA-15-SO3H SBA-15-SO3H
– – TBAB TBAB TBAB
None 8 46 92 89
None 90 99 98 98
Reaction conditions: SO-20 mmol, 80 8C, 1 MPa and 8 h. a CO2 pressure = 0.1 MPa, semi-batch.
material with its long channeled and open pore network, are expected to produce efficient conversions. 3.2. Cycloaddition of CO2 with styrene oxide The styrene carbonate (SC) synthesis from styrene oxide and CO2 is an atom-economical process. To date, various
To assay the activity of SBA-15-SO3H/TBAB catalyst in depth, the effect of variation in catalyst amount was carried (Table 4). As can be seen from entries 1 to 3, the catalytic activity was found to increase gradually with increasing the amount of TBAB, yielding 92% at 0.1 mol% of SBA-15-SO3H and 0.4 mol% TBAB. However, when the amount of SBA-15-SO3H was increased from 0.1 to 0.4 mol% by keeping TBAB constant at 0.4 mol% (entries 5 and 6), the yield of SC did not increase prominently. Therefore we investigated the catalytic activity at still lower mol% of SBA-15SO3H (0.05 mol%) by keeping TBAB mol% constant at 0.4. It was observed that the SC yield decreased to 83% with 97% selectivity (entry 4). Rationalizing the results of catalyst ratio study, the optimum amount of SBA-15-SO3H and TBAB was chosen as 0.1 mol% and 0.4 mol% respectively, for the following parameter studies. Thereafter, we investigated the effects of temperature, CO2 pressure and time on the SO–CO2 cycloaddition over SBA-15-SO3H/ TBAB catalytic system. The temperature study in the range 40– 100 8C using the optimized catalyst ratio at the conditions of 1 MPa CO2 pressure and 8 h is shown in Fig. 5(a). It is clear that the reaction temperature has a strong influence on the catalytic activity under these conditions. At lower temperatures (40 8C), the conversion rate of SO was less than 50%. However, as the higher temperatures were applied, SO conversions gradually increased. A SO conversion of 93% was attained at 80 8C, indicating most of the
Table 3 Comparative studies of styrene carbonate synthesis at milder reaction conditions. Catalyst a
Silica gel/nBu4NBr Silica gel/[C4-IM]+X a CILBr-Sia 2D-CCB/TBABb 2D-CCB/TBABb SBA-15-SO3H/TBABc SBA-15-SO3H/TBABc a b c
Catalyst (mol%)
Temp. (8 C)
Pressure (MPa)
Time (h)
Yield (%)
Sel. (%)
TON
Ref.
1 1.8 2.2 0.4 0.4 0.1 0.1
150 160 115 100 100 80 80
8 8 1.62 2 0.1 1 0.1
10 4 5 4 12 8 8
97 96 96 85 91 92 89
99 98 99 99 99 98 98
97 54 217 213 228 920 890
[25] [26] [13] [6]
Silica based catalyst system. sulfonate based catalyst system. Reaction conditions: SO-20 mmol, TBAB-0.4 mol%. TON (turn over number) was calculated based on sulfur mole ratio.
This work This work
S. Ravi et al. / Journal of CO2 Utilization 10 (2015) 88–94
92 Table 4 Effect of catalyst amount on the SC synthesis. Entry
Cat. (mol%)
TBAB (mol%)
Cat/TBAB ratio
Yield (%)
Sel. (%)
1 2 3 4 5 6
0.1 0.1 0.1 0.05 0.2 0.4
0.1 0.2 0.4 0.4 0.4 0.4
1:1 1:2 1:4 1:8 1:2 1:1
40 65 92 83 93 93
97 98 98 97 99 98
Reaction conditions: SO-20 mmol, 80 8C, 1 MPa and 8 h.
reactant molecules attained the activation energy for the ring opened cycloaddition reaction. With further increase in temperatures up to 100 8C, the conversion increased to 98% (with the SC selectivity of 98%) demonstrating that 100 8C was high enough for this catalyst system. The effect of CO2 pressure (batch conditions in the range 0.5– 3 MPa) on the reaction was studied further, and the results are represented in Fig. 5(b). The SO conversion was 93% at 0.5 MPa. As the reactions pressures increased until 1 MPa, the SO conversion also increased slightly, recording a value of 94%. However the conversion still rested at 94% when the pressure was increased
Fig. 5. Effect of conversion of styrene oxide under different reaction parameters such as temperature, pressure and time; reaction conditions: SO-20 mmol, cat – 0.1 mol%, TBAB – 0.4 mol% (a) 1 MPa CO2 pressure, 8 h (b) 80 8C, 8 h (c) 80 8C, 1 MPa CO2 pressure, 80 8C.
S. Ravi et al. / Journal of CO2 Utilization 10 (2015) 88–94
93
Table 5 Synthesis of other epoxide. Entry
Epoxide
1
Product
O
O
Yield (%)
Sel. (%)
93
99
82
99
84
98
11
98
O O
2
O
O
Cl
O O Cl
3
O
O
O
O O
O
4a
O
O
O O
Reaction conditions: epoxide-20 mmol, SBA-15-SO3H-0.1 mol%, TBAB-0.4 mol%, 80 8C, 1 MPa, 8 h. a 15 h.
further to 2 MPa. A further increase in pressure (3 MPa) in fact gave a slight reduced conversion. Too high CO2 pressures are reported to exhibit reduction in the catalytic conversions [41], and it is explained to be the resultant of dilution effect (interaction between SO and the catalyst is reduced at higher CO2 concentrations). It is noteworthy that at 0.1 MPa CO2 itself, the SO conversion was 91% under semi-batch conditions. The time dependent catalytic activity of catalyst at milder reaction condition was also studied (Fig. 5c). By employing the cycloaddition reaction at 80 8C and 1 MPa pressure, the SO conversion was less than 45% in the first 2 h. An increase in conversion was observed as the duration was increased, and a yield of 92% was achieved in 8 h. Prolonged reaction time did not provide higher yields of SC. Thus, the optimum time for the SO–CO2 cycloaddition using SBA-15-SO3H/TBAB system was estimated as 8 h under the aforementioned conditions. 3.4. Scope of other epoxides The verification of catalytic ability of SBA-15-SO3H/TBAB system was extended to a few other commercially available representative epoxides, such as propylene oxide (1a), epichlorohydrin (1b), allyl glycedyl ether (1c), and cyclohexene oxide (1d) at moderate temperature of 80 8C and 1 MPa CO2 pressure. The terminal epoxides got converted to their corresponding cyclic carbonates with appreciable yield as illustrated in Table 5. However, the internal epoxide, viz., cyclohexene oxide formed only lesser yield of cyclic carbonate than other epoxides. This could be ascribed to the high steric hindrance of cyclohexene oxide, which would hinder the nucleophilic attack of anion on to the epoxide.
Scheme 2. A plausible reaction mechanism of SO and CO2 cycloaddition over SBA15-SO3H and TBAB catalyst system.
3.5. Reaction mechanism Based on the experimental observations and previous reports [6], a plausible synergistic reaction mechanism for the cycloaddition of CO2 and SO with sulfonic acid functionalized mesoporous SBA-15 and bromide ions of TBAB catalysts was proposed as follows, and is schematically illustrated in Scheme 2. The acidic hydrogen atom of sulfonic acid moiety (–SO3H) involves in hydrogen bonding interactions with the oxygen atom of the epoxide, thereby activating the epoxide for ring opening reactions. Subsequently, the bromide anion belonging to the TBAB cocatalyst attacks the b-carbon atom of the epoxide, resulting in the
Fig. 6. Recyclability of SBA-15-SO3H/TBAB catalyst. Reaction condition: 20 mmol SO at 80 8C, 1 MPa CO2 pressure, 8 h. Catalyst amounts: 0.110 g/0.026 g of SBA-15SO3H/TBAB.
94
S. Ravi et al. / Journal of CO2 Utilization 10 (2015) 88–94
employed reaction conditions of 80 8C and 1 MPa CO2 pressure. Effect of reaction parameters were studied to find the optimal reaction conditions and the catalyst was reused for four times. Acknowledgement This work was supported by the National Research Foundation (NRF 2014-2055412 and GFHIM-043321) of Korea.
References
Fig. 7. FT-IR spectra of SBA-15-SO3H catalyst before and after the reusability.
C–Br bond formation and the ring opening of epoxide, so that the reaction intermediate is formed. The alkoxide anion (nucleophile) of the intermediate interacts with the electrophilic carbon atom of CO2, and the carbonate complex is formed. Finally, the O of the carbonate moiety bites the carbon atom which in turn causes the C–Br bond to break and the ring closure takes place. Thus the cyclic carbonate is generated along with the regeneration of TBAB, thereby preparing the catalytic system for the fresh molecule of SO. 3.6. Catalyst recyclability Reusability is one of the important criteria for an efficient heterogeneous catalyst. Fig. 6 depicts the activities of recycled SBA-15-SO3H catalyst. Catalyst recyclability in the cycloaddition of SO and CO2 was tested for four consecutive cycles. The catalytic activity was maintained throughout the reuse experiments as displayed in Fig. 6. FT-IR analysis was conducted on the recycled SBA-15-SO3H catalyst after the use of fresh catalyst as well as for the residue of fourth cycle. The catalyst maintained its chemical integrity throughout the recycling process, as determined by its comparison with the fresh catalyst (Fig. 7). 4. Conclusion Synthesis of styrene carbonate (SC) from CO2 and styrene oxide (SO) was achieved under mild reaction condition using sulfonic acid functionalized mesoporous SBA-15 as catalyst. The cycloaddition of CO2 and SO using sulfonic acid tethered mesoporous silica (SBA-15) materials with TBAB co-catalyst was found to be efficient even at low CO2 pressure of 0.1 MPa under semi-batch conditions at 80 8C, and resulted in 89% SC yield. The SBA-15-SO3H based catalyst system may serve as a viable industrial scale catalyst. The catalytic mechanism was explained by hydrogen bonding interactions and nucleophilic effects. The obtained turn over number of 920 was the highest in comparison with the previous silica based reports for SC synthesis by SO–CO2 cycloaddition. The scope of CO2 cycloaddition for other epoxides was tested and found efficient at
[1] S. Klaus, M.W. Lehenmeier, C.E. Anderson, B. Rieger, Coord. Chem. Rev. 255 (2011) 1460. [2] M. Mikkelsen, M. Jorgensen, F.C. Krebs, Energy Environ. Sci. 3 (2010) 43. [3] A.J. Hunt, E.H.K. Sin, R. Marriott, J.H. Clark, ChemSusChem 3 (2010) 306. [4] M. Aresta, A. Dibenedetto, Dalton Trans. (2007) 2975. [5] J. Tharun, G. Mathai, A.C. Kathalikkattil, R. Roshan, J.-Y. Kwak, D.-W. Park, Green Chem. 15 (2013) 1673. [6] A.C. Kathalikkattil, R. Roshan, J. Tharun, H.G. Soek, H.S. Ryu, D.W. Park, ChemCatChem 6 (2014) 284. [7] J.H. Clements, Ind. Eng. Chem. Res. 42 (2003) 663. [8] Y. Ono, Catal. Today 35 (1997) 15. [9] A.G. Shaikh, S. Sivaram, Chem. Rev. 96 (1996) 951. [10] T. Sakakura, J.C. Choi, H. Yasuda, Chem. Rev. 107 (2007) 2365. [11] Z. Wang, S.M. Cohen, J. Am. Chem. Soc. 129 (2007) 12368. [12] Z. Yanga, J. Sun, X. Liu, Q. Su, Y. Liu, Q. Li, S. Zhang, Tetrahedron Lett. 55 (2014) 3239. [13] L. Han, H. Choi, S. Choi, B. Liu, D.-W. Park, Green Chem. 13 (2011) 1023. [14] R. Srivastava, D. Srinivas, P. Ratnasamy, Microporous Mesoporous Mater. 90 (2006) 314. [15] Y. Du, F. Cai, D.-L. Kong, L.-N. He, Green Chem. 7 (2005) 518. [16] W. Cheng, X. Chena, J. Sun, J. Wang, S. Zhang, Catal. Today 200 (2013) 117. [17] A. Decortes, A.M. Castilla, A.W. Kleij, Angew. Chem. Int. Ed. 49 (2010) 9822. [18] J. Melendez, M. North, P. Villuendas, Chem. Commun. 18 (2009) 2577. [19] J. Tharun, Y. Hwang, R. Roshan, S. Ahn, A.C. Kathalikkattil, D.W. Park, Catal. Sci. Technol. 2 (2012) 1674. [20] Y. Tsutsumi, K. Yamakawa, M. Yoshida, T. Ema, T. Sakai, Org. Lett. 12 (2010) 5728. [21] J. Sun, L. Wang, S. Zhang, Z. Li, X. Zhang, W. Dai, R. Mori, J. Mol. Catal. A: Chem. 256 (2006) 295. [22] Y. Xie, Z. Zhang, T. Jiang, J. He, B. Han, T. Wu, K. Ding, Angew. Chem. Int. Ed. 46 (2007) 255. [23] J. He, T. Wu, Z. Zhang, K. Ding, B. Han, Y. Xie, T. Jiang, Z. Liu, Chem. Eur. J. 13 (2007) 6992. [24] K. Motokura, S. Itagaki, Y. Iwasawa, A. Miyaji, T. Baba, Green Chem. 11 (2009) 1876. [25] J.Q. Wang, D.L. Kong, J.Y. Chen, F. Cai, L.N. He, J. Mol. Catal. A: Chem. 249 (2006) 143. [26] J.Q. Wang, X.D. Yue, F. Cai, L.N. He, Catal. Commun. 8 (2006) 167. [27] W.-Y. Gao, Y. Chen, Y. Niu, K. Williams, L. Cash, P.J. Perez, L. Wojtas, J. Cai, Y.-S. Chen, S. Ma, Angew. Chem. Int. Ed. 53 (2014) 2615. [28] O.V. Zalomaeva, A.M. Chibiryaev, K.A. Kovalenko, O.A. Kholdeeva, B.S. Balzhinimaev, V.P. Fedin, J. Catal. 298 (2013) 179. [29] J. Song, Z. Zhang, S. Hu, T. Wu, T. Jiang, B. Han, Green Chem. 11 (2009) 1031. [30] V. Brahmkhatri, A. Patel, Appl. Catal. A: Gen. 403 (2011) 161. [31] S. Ravi, M. Selvaraj, Dalton Trans. 43 (2014) 5299. [32] P. Yang, S. Gai, J. Lin, Chem. Soc. Rev. 41 (2012) 3679. [33] C. Jiang, K. Hara, A. Fukuoka, Angew. Chem. Int. Ed. 52 (2013) 6265. [34] O.A. Kholdeeva, Catal. Sci. Technol. 4 (2014) 1869. [35] A. Walcarius, Chem. Soc. Rev. 42 (2013) 4098. [36] L. Han, S.W. Park, D.W. Park, Energy Environ. Sci. 2 (2009) 1286. [37] A.T. Dickschat, F. Behrends, M. Bhner, J. Ren, M. Weib, H. Eckert, A. Studer, Chem. Eur. J. 18 (2012) 16689. [38] Y. Dong, G. Liu, Chem. Commun. 49 (2013) 8066. [39] P.A. Russo, M.M. Antunes, P. Neves, P.V. Wiper, E. Fazio, F. Neri, F. Barreca, L. Mafra, M. Pillinger, N. Pinna, A.A. Valente, J. Mater. Chem. A 2 (2014) 11813. [40] K. Sing, D. Everett, R. Haul, L. Moscou, R. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603. [41] A.C. Kathalikkattil, D.W. Kim, J. Tharun, H.G. Soek, R. Roshan, D.W. Park, Green Chem. 16 (2014) 1607.
Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Sulfonic acid functionalized mesoporous SBA-15 as catalyst for styrene carbonate synthesis from CO2 and styrene oxide at moderate reaction conditions Seenu Ravi, Roshith Roshan, Jose Tharun, Amal Cherian Kathalikkattil, Dae Won Park * School of Chemical and Biomolecular Engineering, Pusan National University, Busan 609-735, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 November 2014 Received in revised form 29 December 2014 Accepted 9 January 2015 Available online 3 February 2015
A sulfonic acid tethered mesoporous silica material (SBA-15-SO3H) was successfully synthesized and characterized using SAXS, N2 physisorption studies, TEM, EA and XPS. This metal-free heterogeneous catalyst with terabutyl ammonium bromide (TBAB) co-catalyst was found to be an efficient catalyst system for styrene carbonate synthesis from CO2 and styrene oxide. The synergistic mechanistic pathways of SBA-15-SO3H with TBAB have been explained by hydrogen bond interactions and nucleophilic effects. A turn over number of 920 was obtained at mild reaction temperature of 80 8C. The catalyst was thermally stable and reused for five times without loss of any significant activity. ß 2015 Elsevier Ltd. All rights reserved.
Keywords: Cyclic carbonates Synergism Catalysis Sulfonic acid Mesoporous silica
1. Introduction Carbon dioxide, the ‘C1’ resource whose atmospheric concentration increases infinitely, is renewable, non-toxic, and low-cost concerning global warming [1]. Recently a great deal of attention has been given on the conversion of CO2 into valuable chemicals [2–4]. Moreover, it is necessary to implement feasible carbon dioxide capture and sequestration technologies to reduce the anthropogenic CO2 emissions, especially from industries. One of the viable processes is to synthesis cyclic carbonates by the catalytic insertion of CO2 to oxiranes in the presence of acid supported catalysts [5,6]. A variety of cyclic carbonates (CC) have several purposes in industrial process and they are frequently used in electrochemical process and as intermediates in the synthesis of polycarbonates, etc. [7–10]. Extensive research is being directed worldwide toward the development of catalytic processes that use carbon dioxide as a feedstock for producing CC. Among this, much of the current research has been focused on the employment of heterogeneous catalysts such as mesoporous silica, resins, metal organic framework and porous organic polymer materials for CO2 fixation into cyclic carbonates [11– 15]. Ionizable moieties such as halides in homogeneous or
* Corresponding author. Tel.: +82 515102399; fax: +82 515128563. E-mail address: [email protected] (D.W. Park). http://dx.doi.org/10.1016/j.jcou.2015.01.003 2212-9820/ß 2015 Elsevier Ltd. All rights reserved.
heterogeneous phases are extensively used in the catalytic synthesis of cyclic carbonate. For example, ionic liquids [12,16], salen complexes [17,18], quaternized compounds [19], betaine [20], phophonium robust type catalysts [21], functional polymers [22,23], supported heterogeneous catalyst with alkali halide or onium salts [24–29] etc. have been successfully employed as catalysts for the CO2 epoxide cycloaddition. Among the reported heterogeneous catalysts, mesoporous silica stands attractive due to their abundant free silanol groups which are responsible for their acidic nature and their easy susceptibility for organo functionalization. Moreover they are attractive materials with high surface area, large pore volume and pore diameter, and hence used for green and sustainable applications including drug delivery, adsorption, catalysis and energy-devices [30–35]. Of the different morphologies of mesoporous silicas, 2D materials are more preferable for cyclic carbonate synthesis since it could offer easy diffusion mechanism for relatively large guest species [30]. Improvisations of heterogeneous catalysts having high turnover frequency by economical and easy synthetic procedures at ambient reaction conditions are desirable in the field of cyclic carbonate synthesis. For example silica supported ionic liquids are prominent in CO2 cycloaddition to epoxide; their turn over frequencies are usually low [16,36]. Moreover metal supported silica catalyst can leach the metal ions easily into the reaction mixture and its removal would be a difficult process. Similarly, quaternary ammonium salts like tetra butyl
S. Ravi et al. / Journal of CO2 Utilization 10 (2015) 88–94
ammonium bromide were used as cocatalyst with heterogeneous MOF materials to perform the cycloaddition reaction at lower reaction temperature [27], however a bulk synthesis was difficult in many of the cases. To overcome these issues, easily synthesizable heterogeneous metal-free catalysts with supported acidic group could be a better solution. Sulfonic acid supported catalysts are highly acidic and they are extensively used in several catalytic processes such as oxidation, coupling, conversion and substitution reactions [37–39]. These reaction mechanisms are explained as the resultant of the ionizable property and/or the acidity of active sulfonic moieties. Here, our motivation is to synthesis styrene carbonate (SC) at ambient reaction conditions using a metal-free acidic heterogeneous catalyst along with onium halide as cocatalyst. Thus, a simple metal-free catalyst that operate under mild reaction conditions is of uttermost priority to minimize costs and a viable utilization of CO2 greenhouse gas to address the economic and environmental concerns. Herein we report the synthesis of styrene carbonate at mild reaction conditions using a sulfonic acid supported mesoporous silica/tetra butyl ammonium bromide binary catalyst system for the first time (Scheme 1). The effects of reaction parameters such as temperature, pressure were studied and the optimization of catalyst ratio was performed. 2. Experimental 2.1. Material synthesis The synthesis of 2D SBA-15 mesoporous material was carried out according to the previous report [9] by sol–gel method using Pluronic P123/TEOS/HCl/H2O. Typically Pluronic P123 was dissolved in aqueous HCl solution with magnetic stirring until the clear solution was obtained. This was followed by the addition of TEOS (1 mol) to the solution; and the pH (1.2) was maintained. The resulting mixture was stirred further for 24 h at 30 8C, transferred to Teflon-lined stainless steel autoclave and heated to 100 8C for 24–36 h. The obtained materials were filtered and washed using distilled water followed by ethanol; dried in vacuum at 60 8C for 6 h and then calcined at 550 8C for 6 h to remove the surfactant from the as-synthesized materials. The pristine SBA-15 materials was functionalized with 3-MPS (3-mercaptopropyl trimethoxysilane) by dispersing in toluene at refluxed condition for 6 h and then filtered, washed and dried at oven at 55 8C for 3 h. Then, the obtained SBA-15-SH were dispersed in methanol/50% H2O2 solution and stirred for 6 h under acidic condition (H2SO4). The oxidized material was filtered and dried in vacuum oven at 55 8C for 8 h, and it was denoted as SBA-15-SO3H. Thenceforth, the obtained materials were successfully characterized. 2.2. Characterization techniques Transmission electron microscope (TEM) images measurements were recorded on a JEOL 2010 electron microscope operating at 200 kV. The powder samples were dispersed in ethanol, then deposited and dried on a perforated Cu grid. Images were recorded at magnifications 150,000 and 200,000. Small angle X-ray scattering (SAXS) with Co-Ka radiation (l = 1.608 A˚) in
89
the energy range 4–16 keV (energy resolution: DE/E = 5 10 4; photon flux: 1010–1011 ph./s, Beam size: <1 mm2) over the scan range 0.4 nm 1 < q < 5.0 nm 1. Nitrogen adsorption and desorption isotherms were measured using an ASAP 2020 surface-area and pore-size analyzer. Prior to the measurement, the samples were dehydrated at 90 8C for 5 h. The BET (Brunauer–Emmett– Teller) method was used to calculate the specific surface area. The pore size distribution was calculated from the analysis of the isotherm desorption branch by the BJH (Barret–Joyner–Halenda) method coupled with the apparatus software. Elemental analyses were measured on vario MICRO CHNS analysis technic. The infrared spectra were recorded on a Nicolet 6700 FT-IR spectrometer. XPS analysis was performed using an X-ray photoelectron spectrometer (VG, ESCALAB 250) with monochromatic Al Ka radiation (hn = 1486.6 eV). 2.3. Cycloaddition of CO2 with epoxides The synthesis of SC from styrene oxide (SO) and CO2 (Scheme 1) using the SBA-15-SO3H catalyst was performed in a 50 mL stainless steel autoclave equipped with a magnetic stirrer. For each typical batch operation, SBA-15-SO3H catalyst 0.11 g (0.0187 mmol based on elemental analysis), 0.026 g TBAB (0.081 mmol) and SO (20 mmol) were charged into the reactor without solvent. The reactor was then pressurized with CO2 to a preset pressure at room temperature. The reactor was heated to the desired temperature, and then the reaction was started by stirring the reaction mixture at 450 rpm. The reactor pressure increased by about 0.1–3 MPa depending on the reaction temperature. After the completion of the reaction time, the cycloaddition was stopped followed by cooling the reaction mixture in ice and vented off the remaining CO2. The product (4-phenyl-1,3-dioxolan-2-one) was dissolved in dichloromethane and filtered to remove the catalyst. Product analysis was carried out via gas chromatography/mass spectrometry (GC–MS, Micromass, UK) analysis. The conversion of SC was obtained from gas chromatography (GC, HP 6890, Agilent Technologies, Santa Clara, CA, USA) data. 3. Results and discussion 3.1. Characterization The SBA-15-SO3H was obtained through functionalization and subsequent oxidation of 3-MPS into SBA-15 mesoporous silica by post grafting method. Fig. 1 depicts the SAXS pattern of the synthesized SBA-15 and supported catalysts. SBA-15 mesoporous material (a) showed three well resolved characteristic SAXS pattern such as (1 0 0), (1 1 0) and (2 0 0). The functionalized materials SBA-15-SH (b) and SBA-15-SO3H (c) also exhibited similar peak patterns, which represent the highly ordered mesoporous SBA-15 with a 2D hexagonal p6mm symmetry. Moreover, the decrease in relative peak intensities from a–c suggests that the organosilanes are successfully functionalized into the SBA-15 material. In addition, the d-spacing value for SBA15 (9.3 nm) is increased to 9.5 nm after the 3-MPS loading, as shown in Table 1. Nitrogen physisorption experiments were performed to study the changes in the textural properties of the
Scheme 1. Styrene carbonate synthesis.
S. Ravi et al. / Journal of CO2 Utilization 10 (2015) 88–94
90
Fig. 1. SAXS patterns of SBA-15 and supported catalysts.
Table 1 Structural properties of SBA-15 materials. Materials
d (nm)
SBETa (m2/g)
Dpa (nm)
Vp
SBA-15 SBA-15-SH SBA-15-SO3H
9.3 9.5 9.6
530 316 355
4.2 3.9 3.9
0.89 0.54 0.57
a
(cm3/g)
Sb (mmol/g) 0 0.18 0.17
d, spacing value; SBET, surface area; Dp, Pore diameter; Vp, pore volume; S, sulfur value. a Calculated from N2 physisorption studies. b Calculated from elemental analysis.
functionalized mesoporous materials. The nitrogen adsorption isothermal plots of thiol and sulfonic acid functionalized materials showed type IV isotherms (Fig. 2), similar to that of SBA-15 [40]. These observations were associated with the capillary condensation in hexagonal cylindrical mesopores; at relative pressures of P/P0 0.4–0.8, large porous mesostructures with specific BET surface areas of 530, 316 and 355 m2/g were obtained for pristine SBA-15, SBA-15-SH and SBA-15-SO3H, respectively.
The observed hysteresis loops show that they belong to same types of pore structures. The obtained H1 hysteresis loop and the pore diameter and pore volume values varies from SBA-15 to SBA-15SO3H (Table 1), implying the versatile cylindrical pore entrances. The quantitative determinations of sulfur in the SBA-15-SH and SBA-15-SO3H materials were carried out by elemental analysis and the corresponding sulfur values are given in Table 1. Further evidences for the existence of thiol group in SBA-15-SH and sulfonic group in SBA-15-SO3H were provided through XPS analysis (Fig. 3). In this case a sole characteristic thiol (–SH) sulfur 2p binding energy peak at 163.57 eV and the SO3 was observed at 168.22 eV. Since the peak at 163.57 eV was almost invisible in SBA15-SO3H, it could be surmised that the oxidation of –SH groups to – SO3H group was almost complete. The TEM image in Fig. 4 shows that the channeling pores are well-arranged in 2D-hexagonal p6mm symmetry for the SBA-15 (SO3H). It is particularly noteworthy that the channeling pores are larger enough for the conventional reaction synthesis. As molecular diffusions are a factor while applying SBA-15 in catalysis, this SBA-15-SO3H
Fig. 2. N2 – adsorption and desorption isotherm of SBA-15 and supported catalysts.
S. Ravi et al. / Journal of CO2 Utilization 10 (2015) 88–94
91
heterogeneous supported silica materials have been reported for the synthesis of SC; but unfortunately most of them were performed in drastic reaction conditions such as high pressure and temperatures. Contrastingly, we performed the cycloaddition reaction of styrene oxide (SO) and CO2 at moderate conditions of 0.1 MPa and 80 8C for 8 h with 0.1 mol% of SBA-15-SO3H and 0.4 mol% of TBAB co-catalyst (Table 2). In the absence of any catalyst, no SC was yielded. Cycloaddition with SBA-15-SO3H alone gave a SO conversion of 8% with 90% selectivity for SC, whereas cycloaddition with TBAB alone showed 46.0% conversion and over 99% selectivity (entries 2 and 3) under the employed reaction conditions. Interestingly, with SBA-15-SO3H/TBAB catalyst system, 92% SC yield at 98% selectivity (entry 4) was observed; indicating a synergistic involvement of the catalytic units in the reaction. Based on the experimental outputs, the mechanism involved behind this synergism could be attributed to the hydrogen bonding interaction of sulfonic acid with the epoxy oxygen. SBA-15-SO3H/TBAB catalyst system exhibited higher TON than silica-supported ionic liquid catalysts [13,25,26] and metal organic framework with sulfate groups [6]. Very interestingly, a SC yield of 89% was observed even at near-atmospheric pressure of CO2 (0.1 MPa CO2, Entry 5). To date, there are no reports of silica materials employed as catalyst at ambient reaction conditions. The comparison of the catalytic activities of the reported silica based and sulfonate based catalysts are shown in Table 3.
Fig. 3. XPS S2p region of SBA-15-SH and SBA-15-SO3H catalyst.
3.3. Influence of reaction parameters
Fig. 4. TEM image of SBA-15-SO3H. Table 2 Cycloaddition of SO and CO2 using SBA-15-SO3H/TBAB catalyst. Entry
Catalyst
Co-catalyst
Yield (%)
Sel. (%)
1 2 3 4 5a
– SBA-15-SO3H – SBA-15-SO3H SBA-15-SO3H
– – TBAB TBAB TBAB
None 8 46 92 89
None 90 99 98 98
Reaction conditions: SO-20 mmol, 80 8C, 1 MPa and 8 h. a CO2 pressure = 0.1 MPa, semi-batch.
material with its long channeled and open pore network, are expected to produce efficient conversions. 3.2. Cycloaddition of CO2 with styrene oxide The styrene carbonate (SC) synthesis from styrene oxide and CO2 is an atom-economical process. To date, various
To assay the activity of SBA-15-SO3H/TBAB catalyst in depth, the effect of variation in catalyst amount was carried (Table 4). As can be seen from entries 1 to 3, the catalytic activity was found to increase gradually with increasing the amount of TBAB, yielding 92% at 0.1 mol% of SBA-15-SO3H and 0.4 mol% TBAB. However, when the amount of SBA-15-SO3H was increased from 0.1 to 0.4 mol% by keeping TBAB constant at 0.4 mol% (entries 5 and 6), the yield of SC did not increase prominently. Therefore we investigated the catalytic activity at still lower mol% of SBA-15SO3H (0.05 mol%) by keeping TBAB mol% constant at 0.4. It was observed that the SC yield decreased to 83% with 97% selectivity (entry 4). Rationalizing the results of catalyst ratio study, the optimum amount of SBA-15-SO3H and TBAB was chosen as 0.1 mol% and 0.4 mol% respectively, for the following parameter studies. Thereafter, we investigated the effects of temperature, CO2 pressure and time on the SO–CO2 cycloaddition over SBA-15-SO3H/ TBAB catalytic system. The temperature study in the range 40– 100 8C using the optimized catalyst ratio at the conditions of 1 MPa CO2 pressure and 8 h is shown in Fig. 5(a). It is clear that the reaction temperature has a strong influence on the catalytic activity under these conditions. At lower temperatures (40 8C), the conversion rate of SO was less than 50%. However, as the higher temperatures were applied, SO conversions gradually increased. A SO conversion of 93% was attained at 80 8C, indicating most of the
Table 3 Comparative studies of styrene carbonate synthesis at milder reaction conditions. Catalyst a
Silica gel/nBu4NBr Silica gel/[C4-IM]+X a CILBr-Sia 2D-CCB/TBABb 2D-CCB/TBABb SBA-15-SO3H/TBABc SBA-15-SO3H/TBABc a b c
Catalyst (mol%)
Temp. (8 C)
Pressure (MPa)
Time (h)
Yield (%)
Sel. (%)
TON
Ref.
1 1.8 2.2 0.4 0.4 0.1 0.1
150 160 115 100 100 80 80
8 8 1.62 2 0.1 1 0.1
10 4 5 4 12 8 8
97 96 96 85 91 92 89
99 98 99 99 99 98 98
97 54 217 213 228 920 890
[25] [26] [13] [6]
Silica based catalyst system. sulfonate based catalyst system. Reaction conditions: SO-20 mmol, TBAB-0.4 mol%. TON (turn over number) was calculated based on sulfur mole ratio.
This work This work
S. Ravi et al. / Journal of CO2 Utilization 10 (2015) 88–94
92 Table 4 Effect of catalyst amount on the SC synthesis. Entry
Cat. (mol%)
TBAB (mol%)
Cat/TBAB ratio
Yield (%)
Sel. (%)
1 2 3 4 5 6
0.1 0.1 0.1 0.05 0.2 0.4
0.1 0.2 0.4 0.4 0.4 0.4
1:1 1:2 1:4 1:8 1:2 1:1
40 65 92 83 93 93
97 98 98 97 99 98
Reaction conditions: SO-20 mmol, 80 8C, 1 MPa and 8 h.
reactant molecules attained the activation energy for the ring opened cycloaddition reaction. With further increase in temperatures up to 100 8C, the conversion increased to 98% (with the SC selectivity of 98%) demonstrating that 100 8C was high enough for this catalyst system. The effect of CO2 pressure (batch conditions in the range 0.5– 3 MPa) on the reaction was studied further, and the results are represented in Fig. 5(b). The SO conversion was 93% at 0.5 MPa. As the reactions pressures increased until 1 MPa, the SO conversion also increased slightly, recording a value of 94%. However the conversion still rested at 94% when the pressure was increased
Fig. 5. Effect of conversion of styrene oxide under different reaction parameters such as temperature, pressure and time; reaction conditions: SO-20 mmol, cat – 0.1 mol%, TBAB – 0.4 mol% (a) 1 MPa CO2 pressure, 8 h (b) 80 8C, 8 h (c) 80 8C, 1 MPa CO2 pressure, 80 8C.
S. Ravi et al. / Journal of CO2 Utilization 10 (2015) 88–94
93
Table 5 Synthesis of other epoxide. Entry
Epoxide
1
Product
O
O
Yield (%)
Sel. (%)
93
99
82
99
84
98
11
98
O O
2
O
O
Cl
O O Cl
3
O
O
O
O O
O
4a
O
O
O O
Reaction conditions: epoxide-20 mmol, SBA-15-SO3H-0.1 mol%, TBAB-0.4 mol%, 80 8C, 1 MPa, 8 h. a 15 h.
further to 2 MPa. A further increase in pressure (3 MPa) in fact gave a slight reduced conversion. Too high CO2 pressures are reported to exhibit reduction in the catalytic conversions [41], and it is explained to be the resultant of dilution effect (interaction between SO and the catalyst is reduced at higher CO2 concentrations). It is noteworthy that at 0.1 MPa CO2 itself, the SO conversion was 91% under semi-batch conditions. The time dependent catalytic activity of catalyst at milder reaction condition was also studied (Fig. 5c). By employing the cycloaddition reaction at 80 8C and 1 MPa pressure, the SO conversion was less than 45% in the first 2 h. An increase in conversion was observed as the duration was increased, and a yield of 92% was achieved in 8 h. Prolonged reaction time did not provide higher yields of SC. Thus, the optimum time for the SO–CO2 cycloaddition using SBA-15-SO3H/TBAB system was estimated as 8 h under the aforementioned conditions. 3.4. Scope of other epoxides The verification of catalytic ability of SBA-15-SO3H/TBAB system was extended to a few other commercially available representative epoxides, such as propylene oxide (1a), epichlorohydrin (1b), allyl glycedyl ether (1c), and cyclohexene oxide (1d) at moderate temperature of 80 8C and 1 MPa CO2 pressure. The terminal epoxides got converted to their corresponding cyclic carbonates with appreciable yield as illustrated in Table 5. However, the internal epoxide, viz., cyclohexene oxide formed only lesser yield of cyclic carbonate than other epoxides. This could be ascribed to the high steric hindrance of cyclohexene oxide, which would hinder the nucleophilic attack of anion on to the epoxide.
Scheme 2. A plausible reaction mechanism of SO and CO2 cycloaddition over SBA15-SO3H and TBAB catalyst system.
3.5. Reaction mechanism Based on the experimental observations and previous reports [6], a plausible synergistic reaction mechanism for the cycloaddition of CO2 and SO with sulfonic acid functionalized mesoporous SBA-15 and bromide ions of TBAB catalysts was proposed as follows, and is schematically illustrated in Scheme 2. The acidic hydrogen atom of sulfonic acid moiety (–SO3H) involves in hydrogen bonding interactions with the oxygen atom of the epoxide, thereby activating the epoxide for ring opening reactions. Subsequently, the bromide anion belonging to the TBAB cocatalyst attacks the b-carbon atom of the epoxide, resulting in the
Fig. 6. Recyclability of SBA-15-SO3H/TBAB catalyst. Reaction condition: 20 mmol SO at 80 8C, 1 MPa CO2 pressure, 8 h. Catalyst amounts: 0.110 g/0.026 g of SBA-15SO3H/TBAB.
94
S. Ravi et al. / Journal of CO2 Utilization 10 (2015) 88–94
employed reaction conditions of 80 8C and 1 MPa CO2 pressure. Effect of reaction parameters were studied to find the optimal reaction conditions and the catalyst was reused for four times. Acknowledgement This work was supported by the National Research Foundation (NRF 2014-2055412 and GFHIM-043321) of Korea.
References
Fig. 7. FT-IR spectra of SBA-15-SO3H catalyst before and after the reusability.
C–Br bond formation and the ring opening of epoxide, so that the reaction intermediate is formed. The alkoxide anion (nucleophile) of the intermediate interacts with the electrophilic carbon atom of CO2, and the carbonate complex is formed. Finally, the O of the carbonate moiety bites the carbon atom which in turn causes the C–Br bond to break and the ring closure takes place. Thus the cyclic carbonate is generated along with the regeneration of TBAB, thereby preparing the catalytic system for the fresh molecule of SO. 3.6. Catalyst recyclability Reusability is one of the important criteria for an efficient heterogeneous catalyst. Fig. 6 depicts the activities of recycled SBA-15-SO3H catalyst. Catalyst recyclability in the cycloaddition of SO and CO2 was tested for four consecutive cycles. The catalytic activity was maintained throughout the reuse experiments as displayed in Fig. 6. FT-IR analysis was conducted on the recycled SBA-15-SO3H catalyst after the use of fresh catalyst as well as for the residue of fourth cycle. The catalyst maintained its chemical integrity throughout the recycling process, as determined by its comparison with the fresh catalyst (Fig. 7). 4. Conclusion Synthesis of styrene carbonate (SC) from CO2 and styrene oxide (SO) was achieved under mild reaction condition using sulfonic acid functionalized mesoporous SBA-15 as catalyst. The cycloaddition of CO2 and SO using sulfonic acid tethered mesoporous silica (SBA-15) materials with TBAB co-catalyst was found to be efficient even at low CO2 pressure of 0.1 MPa under semi-batch conditions at 80 8C, and resulted in 89% SC yield. The SBA-15-SO3H based catalyst system may serve as a viable industrial scale catalyst. The catalytic mechanism was explained by hydrogen bonding interactions and nucleophilic effects. The obtained turn over number of 920 was the highest in comparison with the previous silica based reports for SC synthesis by SO–CO2 cycloaddition. The scope of CO2 cycloaddition for other epoxides was tested and found efficient at
[1] S. Klaus, M.W. Lehenmeier, C.E. Anderson, B. Rieger, Coord. Chem. Rev. 255 (2011) 1460. [2] M. Mikkelsen, M. Jorgensen, F.C. Krebs, Energy Environ. Sci. 3 (2010) 43. [3] A.J. Hunt, E.H.K. Sin, R. Marriott, J.H. Clark, ChemSusChem 3 (2010) 306. [4] M. Aresta, A. Dibenedetto, Dalton Trans. (2007) 2975. [5] J. Tharun, G. Mathai, A.C. Kathalikkattil, R. Roshan, J.-Y. Kwak, D.-W. Park, Green Chem. 15 (2013) 1673. [6] A.C. Kathalikkattil, R. Roshan, J. Tharun, H.G. Soek, H.S. Ryu, D.W. Park, ChemCatChem 6 (2014) 284. [7] J.H. Clements, Ind. Eng. Chem. Res. 42 (2003) 663. [8] Y. Ono, Catal. Today 35 (1997) 15. [9] A.G. Shaikh, S. Sivaram, Chem. Rev. 96 (1996) 951. [10] T. Sakakura, J.C. Choi, H. Yasuda, Chem. Rev. 107 (2007) 2365. [11] Z. Wang, S.M. Cohen, J. Am. Chem. Soc. 129 (2007) 12368. [12] Z. Yanga, J. Sun, X. Liu, Q. Su, Y. Liu, Q. Li, S. Zhang, Tetrahedron Lett. 55 (2014) 3239. [13] L. Han, H. Choi, S. Choi, B. Liu, D.-W. Park, Green Chem. 13 (2011) 1023. [14] R. Srivastava, D. Srinivas, P. Ratnasamy, Microporous Mesoporous Mater. 90 (2006) 314. [15] Y. Du, F. Cai, D.-L. Kong, L.-N. He, Green Chem. 7 (2005) 518. [16] W. Cheng, X. Chena, J. Sun, J. Wang, S. Zhang, Catal. Today 200 (2013) 117. [17] A. Decortes, A.M. Castilla, A.W. Kleij, Angew. Chem. Int. Ed. 49 (2010) 9822. [18] J. Melendez, M. North, P. Villuendas, Chem. Commun. 18 (2009) 2577. [19] J. Tharun, Y. Hwang, R. Roshan, S. Ahn, A.C. Kathalikkattil, D.W. Park, Catal. Sci. Technol. 2 (2012) 1674. [20] Y. Tsutsumi, K. Yamakawa, M. Yoshida, T. Ema, T. Sakai, Org. Lett. 12 (2010) 5728. [21] J. Sun, L. Wang, S. Zhang, Z. Li, X. Zhang, W. Dai, R. Mori, J. Mol. Catal. A: Chem. 256 (2006) 295. [22] Y. Xie, Z. Zhang, T. Jiang, J. He, B. Han, T. Wu, K. Ding, Angew. Chem. Int. Ed. 46 (2007) 255. [23] J. He, T. Wu, Z. Zhang, K. Ding, B. Han, Y. Xie, T. Jiang, Z. Liu, Chem. Eur. J. 13 (2007) 6992. [24] K. Motokura, S. Itagaki, Y. Iwasawa, A. Miyaji, T. Baba, Green Chem. 11 (2009) 1876. [25] J.Q. Wang, D.L. Kong, J.Y. Chen, F. Cai, L.N. He, J. Mol. Catal. A: Chem. 249 (2006) 143. [26] J.Q. Wang, X.D. Yue, F. Cai, L.N. He, Catal. Commun. 8 (2006) 167. [27] W.-Y. Gao, Y. Chen, Y. Niu, K. Williams, L. Cash, P.J. Perez, L. Wojtas, J. Cai, Y.-S. Chen, S. Ma, Angew. Chem. Int. Ed. 53 (2014) 2615. [28] O.V. Zalomaeva, A.M. Chibiryaev, K.A. Kovalenko, O.A. Kholdeeva, B.S. Balzhinimaev, V.P. Fedin, J. Catal. 298 (2013) 179. [29] J. Song, Z. Zhang, S. Hu, T. Wu, T. Jiang, B. Han, Green Chem. 11 (2009) 1031. [30] V. Brahmkhatri, A. Patel, Appl. Catal. A: Gen. 403 (2011) 161. [31] S. Ravi, M. Selvaraj, Dalton Trans. 43 (2014) 5299. [32] P. Yang, S. Gai, J. Lin, Chem. Soc. Rev. 41 (2012) 3679. [33] C. Jiang, K. Hara, A. Fukuoka, Angew. Chem. Int. Ed. 52 (2013) 6265. [34] O.A. Kholdeeva, Catal. Sci. Technol. 4 (2014) 1869. [35] A. Walcarius, Chem. Soc. Rev. 42 (2013) 4098. [36] L. Han, S.W. Park, D.W. Park, Energy Environ. Sci. 2 (2009) 1286. [37] A.T. Dickschat, F. Behrends, M. Bhner, J. Ren, M. Weib, H. Eckert, A. Studer, Chem. Eur. J. 18 (2012) 16689. [38] Y. Dong, G. Liu, Chem. Commun. 49 (2013) 8066. [39] P.A. Russo, M.M. Antunes, P. Neves, P.V. Wiper, E. Fazio, F. Neri, F. Barreca, L. Mafra, M. Pillinger, N. Pinna, A.A. Valente, J. Mater. Chem. A 2 (2014) 11813. [40] K. Sing, D. Everett, R. Haul, L. Moscou, R. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603. [41] A.C. Kathalikkattil, D.W. Kim, J. Tharun, H.G. Soek, R. Roshan, D.W. Park, Green Chem. 16 (2014) 1607.