Nanoporous Polymers Incorporating Sterically Confined N

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Nanoporous Polymers Incorporating Sterically Confined N-Heterocyclic Carbenes For Simultaneous CO2 Capture and Conversion at Ambient Pressure Siddulu Naidu Talapaneni, Onur Buyukcakir, Sang Hyun Je, Sampath Srinivasan, Yongbeom Seo, Kyriaki Polychronopoulou, and Ali Coskun Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03104 • Publication Date (Web): 18 Sep 2015 Downloaded from http://pubs.acs.org on September 27, 2015

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Chemistry of Materials

Nanoporous Polymers Incorporating Sterically Confined NHeterocyclic Carbenes For Simultaneous CO2 Capture and Conversion at Ambient Pressure Siddulu Naidu Talapaneni,† Onur Buyukcakir, † Sang hyun Je, † Sampath Srinivasan, † Yongbeom Seo,‡ Kyriaki Polychronopoulou# and Ali Coskun†,§* †

Graduate School of Energy, Environment, Water and Sustainability (EEWS), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea. ‡

Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Daejeon, Republic of Korea.

#

Department of Mechanical Engineering, Khalifa University, Abu Dhabi, UAE.

§

Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea.

ABSTRACT: Post-combustion CO2 capture and the conversion of captured CO2 into value added chemicals are integral part of today`s energy industry mainly due to their economic and environmental benefits arising from the direct utilization of CO2 as a sustainable source. Sterically-confined N-heterocyclic carbenes (NHCs) have a played a significant role in organocatalysis due to their air-stability, super basic nature and strong ability to activate and convert CO2 gas. Here we report a new class of nanoporous polymer incorporating sterically-confined N-heterocyclic carbenes (NP-NHCs) that exhibit exceptional CO2 capture fixation efficiency of 97% at room temperature, which is the highest ever reported for carbene based materials measured in the solid state. The NP-NHC can also function as a highly active, selective, and recyclable heterogeneous nanoporous organocatalyst for the conversion of CO2 into cyclic carbonates at atmospheric pressure with excellent yields up to 98% along with 100% product selectivity through an atom economy reaction by using epoxides. Narrow pore size distribution of NP-NHC also allowed us to introduce a unique substrate selectivity based on size, just like enzymes, for the corresponding epoxides. This metal-free two in one approach for the CO2 gas fixation/release and conversion provides a new direction for the cost-effective, CO2 capture and conversion processes.

1. INTRODUCTION During the last few decades, the release of the greenhouse gases such as CO2 into the atmosphere has been tremendously augmented due to the increasing dependence on fossil fuels to meet our increasing energy demand. Approximately more than 13 billion tons of carbon in the form of CO2 was emitted into the atmosphere each year, and yet it is increasing continuously.1,2 These emissions are widely considered as primary factors in the global warming and climate change.3, 4 Thus, the global attention has been focused on the development of efficient CO2 capture technologies. Current CO2 scrubbing technologies based on aqueous amine solutions, however, results in high-energy penalties mainly due to the significant amount energy needed to desorb CO2 from such liquids.5, 6 In addition, degradation of amines upon repeated cycling along with their corrosive effects at high amine loadings are still important challenges to be met. To overcome these limitations, several highly porous solid adsor-

bents such as zeolites, porous carbons, porous graphene, N-doped porous carbons,7, 8 carbon nitrides,9, 10 MOF,11, 12 ZIF,13 COF,14 CTF,15 COP,16 HCP and CMP,17-19 PAF,20 PPF,21 PIM,22 POF23 and molecular cages24-26 having extremely high specific surface areas, tunable pore topologies and pore chemistries have been synthesized as promising alternatives for efficient CO2 capture and separation. CO2 valorization into useful and valuable chemicals is a key technology for sustainable low carbon society and part of the green chemistry research because of the fact that CO2 is a renewable and environmentally friendly C1 building block for making organic compounds, materials, and carbohydrates (e.g., glucose).27-29 However, the conversion of captured CO2 by artificial photosynthesis is rather inefficient and challenging due to the fact that CO2 reduction is a multi-electron process.30 One of the most successful approaches for CO2 conversion is the catalytic production of cyclic carbonates and polycarbonates from epoxides via 100% atom economy reaction.31-34 Such cyclic

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carbonates or polycarbonates have been widely used as the synthetic organic intermediates in the pharmaceutical and fine chemical industries as aprotic polar solvents, electrolytes for the batteries, constituents of oils and paints, precursors for biomedical applications and raw materials for engineering plastics.35 Various catalytic systems such as metal-centered porphyrins, phthalocyanines, salen and Schiff base complexes and also metal-free N-heterocyclic carbene (NHCs) based ionic liquids, covalent triazine frameworks (CTFs), organocatalysts and quaternary ammonium salts have been developed for the production of cyclic carbonates from CO2 and epoxides.36 Among these, metal free NHCs37 are particularly attractive organocatalysts due to NHCs super basic nature, and stability. NHCs could act as nucleophiles to activate CO2 to form corresponding imidazolium carboxylates, which could subsequently release CO2 from the imidazolium carboxylate adducts upon the completion of a catalytic cycle to yield corresponding cyclic carbonate along with free NHC.38, 39 One important requirement for the application of NHCs for catalytic applications is to introduce steric confinement, which impart stability to NHCs and preclude abstraction of acidic protons and intermolecular dimerization of NHCs. Sterically confined NHCs and their corresponding NHC-metal complexes have been explored both theoretically40-43 and experimentally42,44 in the context of homogeneous catalysis to capture, fix and activate CO2. Their corresponding reaction mechanisms are also well-established.45-49 Despite their high efficiency, homogeneous catalysts based on NHCs present known disadvantages such as dissolution in the reaction media, difficulties in product separation from catalyst and finally catalyst recycling. To overcome these drawbacks, it is essential to construct porous frameworks as heterogeneous catalysts incorporating sterically demanding air stable NHCs with the aim of creating NHC-based porous organocatalysts with superior CO2 capture and conversion efficiency and recyclability compared to their homogeneous counterparts. To the best of our knowledge, no such material has been reported to date. The reports concerning the synthesis of porous polymers incorporating NHCs without any steric confinement are also very rare and also there is no proof for the formation of the free NHC within the nanoporous structure.50 There are also few reports on the fabrication of imidazolium polymeric particles51, 52 and their corresponding palladium complexes53, 54 for heterogeneous catalysis. On the contrary, there have been many studies on the preparation of spherical non-porous imidazole based poly ionic liquids (PILs) and their corresponding polymersupported sterically confined NHCs through the freeradical polymerization of ionic liquid monomers, which were evaluated to reversibly capture, release and for conversion of CO2 at various temperatures.53, 55-60 Herein, we introduced a two in one approach, that is the capture and conversion of CO2 into cyclic carbonates using a nanoporous organocatalyst incorporating sterically demanding free NHCs (NP-NHC). The synthesis of NPNHC was achieved (Scheme 1) without any metal-catalyst

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under environmentally benign conditions over three steps. We have proven the formation of free NHCs within the NP-NHC using solid-state 13C NMR analysis, in which we clearly observed the formation of imidazolium carboxylate, thus proving, for the first time, successful integration of NHCs into the porous materials. The resulting NPNHC with Brunauer–Emmett–Teller (BET) surface of 475 m2 g-1 was found to be highly microporous and showed excellent CO2 capturing efficiency of 97% under ambient conditions. We attribute this high capturing efficiency to the increased accessibility of CO2 molecules to NHCs due to the highly porous nature of NP-NHC. We also utilized NP-NHC as a nanoporous organocatalyst in the preparation of cyclic carbonates by using epoxide and CO2 gas. NP-NHC showed superior performance in the atom economy reaction between epoxides and CO2 gas with excellent conversion yields up to 98 % along with 100% product selectivity under atmospheric pressure of CO2 (0.1 MPa) at 120oC with only 5 wt% catalyst loading. Narrow pore size distribution of NP-NHC also allowed us to introduce substrate selectivity based on size, just like in zeolites, biological systems, for the corresponding epoxides.

2. RESULTS AND DISCUSSION 2.1. Synthesis. Conventional N-heterocyclic carbenes can be prepared by a three-step synthetic protocol (See Supporting Information, SI, Figure S1) reported by Nolan et al.61, 62 Reaction of 2,6-diisopropyl aniline with glyoxal to form bis(2,6-diisopropylphenyl)diazabutadiene, followed by its reaction with paraformaldehyde in the presence of HCl to yield the corresponding imidazolium chloride (IPr.HCl). Finally, sterically confined NHC is generated upon neutralization of the imidazolium chloride with potassium tert-butoxide in THF. In order to prepare NP-NHC, we first designed and synthesized (Figures S2S11) a tetrahedral core based on tetraphenyl methane incorporating 2,6-diisopropyl aniline moieties at the terminal positions to prepare (Scheme 1) the corresponding 3D nanoporous polymer, NP-NHC. The synthesis of NP-NHC was also achieved over three steps, as described above, without any metal-catalysts under environmentally benign conditions. We have identified isopropyl moieties for the steric confinement of free NHC due to decreased stability of NP-NHC-CO2 adduct, which facilitates efficient release of captured CO2 at elevated temperatures while increasing its catalytic activity for the CO2 conversion process.47 In order to demonstrate the importance of steric confinement, we have also prepared (Figure S12) a non-steric porous NHC (PNHC) as a control polymer starting from tetrakis(4-aminophenyl)methane using the same synthetic strategy. 2.2. Spectroscopic Characterization. The formation of corresponding imine, imidazolium salt and free carbene moieties within the nanoporous polymers were revealed by Fourier transform infrared (FT-IR) spectroscopy. The formation of NP-imine was confirmed (Figure 1) by FT-IR spectroscopy, which exhibited a characteristic -C=N-


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Scheme 1. Schematic representation for the synthesis of sterically embedded NP-Imine, NP-Imidazolium and NP-NHC starting from sterically hindered aromatic tetraamine. The wavy black lines at the terminal positions imply the extension of periodic structures.

NP-Imine

NP-NHC

NP-Imidazolium

stretching bands at 1705 and 1750 cm-1 along with the representative bands at 2870 to 3030 cm-1for aromatic C-H stretching bands of phenyl rings.63, 64 The absence of characteristic stretching and bending vibration modes for free amine (-NH2) at 3400 and 1620 cm-1 in the FT-IR spectrum of NP-imine points to its near quantitative formation (Figure 2 and Figure S13). The typical stretching bands at around 1705 and 1750 cm-1, which was assigned to the C=N- units, disappeared (Figure 1 and Figure S14) completely following the formation of imidazolium ring (NPimidazolium) upon reaction of NP-imine with paraformaldehyde in the presence of HCl. Similar FT-IR signatures were also obtained for the conventional imidazolium salt (Figure S15), which proves the formation of imidazolium rings within the nanoporous structure of NPimidazolium. Upon neutralization of NP-imidazolium with potassium tert-butoxide, we observed significant changes in the FT-IR spectrum. In particular, the pres-

ence of strong vibration band at 1250 cm-1 along with the appearance of stretching bands in the range of 1590 to 1650 cm-1 points to the formation of free carbene and the corresponding CO2-adduct (NP-NHC-CO2) within the nanoporous polymer. The strong stretching band at 1621 cm-1 is attributed (Figure 1 and Figure S14) to asymmetric ν(CO2) vibration of the NP-NHC-CO2, which could have formed by the reaction of the highly active NP-NHC with the atmospheric CO2, immediately after its isolation.47 Evidently, these FT-IR data are also in perfect agreement to those of the analogous conventional IPr-carbene (Figure S15) and the non-steric control polymer, PNHC (Figure S16).47 The molecular connectivity and integrity of NP-NHC, NP-imine and -imidazolium were assessed (Figure 2) with cross polarization magic angle spinning (CP/MAS) 13C NMR spectroscopy. The chemical shifts located at 25.0,



1650 1621 1250

Transmission (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

NP-NHC

NP-Imidazolium

NP-Imine

4000

3000

2000

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and calculated ones, that of NP-NHC found to be lower compared to the calculated values. This deviation in EA data was attributed to the presence of NP-NHC-CO2 adducts which is also verified by the CP/MAS 13C NMR, FTIR and TGA analysis. All of the polymers showed some limited evidence of crystallinity, as evaluated by the powder X-ray diffraction (PXRD) analysis (Figure S17). The PXRD peaks were found to be too broad to deduce any atomistic structural information. However, there was a slight long-range order presumably due to the reversibility of imine formation reaction. We also believe that continuous azeotropic removal of water during condensation reaction serves as an irreversible kinetic trap for the polymerization reaction and promotes amorphous nature of the framework. No significant changes have been observed in the PXRD spectra after cyclization and subsequent neutralization steps to form NP-NHC. Fieldemission scanning electron microscopy (FESEM) images (Figure S18) of polymers showed assemblies of nearly spherical shaped, irregular-sized agglomerated particles throughout the samples. The thermogravimetric analysis of all of the samples (Figure S19) indicate that they are stable up to 300oC under air. The weight loss in all the samples below 100oC was attributed to the removal of trapped moisture and solvent molecules. In the case of NP-NHC, however, we observed additional weight loss

1000 -1

Wavenumber, cm

N N

15 11-14 4-6 14

Figure 1. FT-IR spectra of NP-NHC (green curve), NPImidazolium (blue curve) and NP-Imine (Red curve). The red lines indicate key FT-IR signatures of free carbene and corresponding CO2-adduct, NP-NHC-CO2.

29.5 and 65 ppm were ascribed to aliphatic methyl, isopropyl and to the quarternary carbon core of tetraphenyl methane, respectively.51,50, 51, 63 The presence of broad chemical shifts in the range of 120 to 160 ppm with the peak maxima located at 126, 140 and 150 ppm confirmed the presence of aromatic subunits within the polymers (Figure 2).51, 53, 59, 61 Interestingly, we have observed a very sharp peak at 165 ppm in 13C NMR spectrum of NP-NHC, which is attributed to the imidazolium carbonate formation, as previously reported for the non-porous polymer-supported NHCs.58 This characteristic peak proves, for the first time, the presence of highly active free carbenes within the nanoporous structure, which readily forms an adduct with the atmospheric CO2 without applying any external pressure.50 This result also further signifies the important role of steric confinement for the stability of NHCs as bulky protecting groups, which are critical for preventing the decomposition, protonation or intermolecular dimerization of carbenes and also for the stabilization of reactive catalytic intermediates.44, 65, 66 We have also carried out elemental analysis to determine the composition of NP-imine, NP-imidazolium and NP-NHC. While EA analysis of both NP-Imine and –imidazolium showed good agreement between the experimental values

13 12

7-10

11 N

N

3 N

200

180

160

140

120

N

100

δ (ppm)

10

1 2 15

9 8

7 6

N

5 N

3

4 2

1

NP-NHC

NP-Imidazolium

NP-Imine

Amine

200

150

100

50

0

-50

Chemical shift, ppm 13

Figure 2. Solid state CP-MAS C NMR spectra of tetraamine, NP-Imine, NP-Imidazolium and NP-NHC along with the corresponding peak assignments. The spectra were recorded with a contact time of 2 ms, a relaxation time of 5 s, and a spinning frequency of 7 kHz. The carbonyl carbon of glycine 13 was used as an external chemical shift reference for the C NMR.


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surface area for the NP-imidazolium compared to that of NP-imine, which could be attributed to the steric size of the imidazolium unit and increased rigidity of the framework upon cyclization to form the corresponding imidazolium ring. A similar trend in surface area was also observed for the non-steric control polymer (PImCl). The specific surface area of NP-NHC was slightly decreased, when compared with the NP-Imidazolium (Table 1). However, the specific surface area and total pore volume of non-steric control polymer, PNHC, decreased (Figure S23) substantially upon neutralization of PImCl presumably due to the partial collapse of porous structure and also decomposition and/or dimerization of carbene moieties, thus indicating critical role of steric confinement for the stability of resulting NHC. NLDFT pore-size distributions of NP-Imine, NP-Imidazolium and NP-NHC are shown in Figure 3b. All the polymers showed a narrow pore size distribution mainly in the micropore range with a pore width maximum located at about 0.4 nm (Figure 3b inset), which indicates that post-functionalization steps are noninvasive and does not alter pore diameter of polymers. Evidently, we have also observed (Table 1) similar pore volumes of 0.19, 0.21 and 0.19 cm3 g-1 for NP-imine, NPimidazolium and NP-NHC, respectively.

(a)

3

Quantity adsorbed, cm g

-1

200

150

100 NP-Imine NP-Imidazolium NP-NHC

50

0 0.0

0.2

0.4

0.6

0.8

1.0

-1

Relative pressure, P/P0

(b)

NP-Imine NP-Imidazolium NP-NHC -1

0.8 0.6

3

Differential pore volume, cm g

3

Differential pore volume, cm g

0.4 0.2

0.8 0.6 0.4 0.2 0.0 0.4

0.0 0

2

0.8 1.2 Pore size, nm

4 6 Pore size, nm

1.6

2.0

8

10

Figure 3. a) Argon adsorption–desorption isotherms and b) NLDFT pore-size distributions of nanoporous imine, imidazolium and NHC (open symbols: desorption, closed symbols: adsorption; squares: NP-Imine, up triangles: NPImidazolium, and diamonds: NP-NHC) measured at 87 K. For Rouquerol and BET linear plots, see SI Figures S20-S22. Inset: expansion of pore volume versus pore size.

In order to probe the affinity of NP-NHC towards CO2, we have carried out thermogravimetric (TG) adsorptiondesorption analysis under controlled temperature programming. This method is effective and commonly used to evaluate reversible CO2 fixation-release capability of NHCs in the solid-state.38, 39, 58, 67 The TG CO2 adsorptiondesorption plot of the NP-NHC at different temperatures are shown in Figure 4. Prior to CO2 sorption, activation step using N2 flow (40 mL min-1) at 160°C for 60 min was

between 100 to 200 C presumably due to the removal of trapped atmospheric CO2. 2.3. Gas Sorption Studies. The textural parameters such as specific surface area, micropore volume, total pore volume of all the polymers were analyzed by argon adsorption-desorption measurements at 87 K. All the polymers showed typical type I reversible argon gas sorption isotherms (Figure 3a), which indicates the presence of well-developed micropores. The presence of slight hysteresis at lower relative pressures can be attributed to the nanoporous networks effects. The surface areas of NPNHC, NP-imine and NP-imidazolium were estimated from the argon adsorption isotherms using the Brunauer−Emmett−Teller (BET) model, in which the pressure ranges were determined according to the Rouquerol plots (Figures S20-22). The BET surface areas of NP-imine, NPimidazolium and NP-NHC were found (Table 1) to be 483, 537 and 475 m2 g-1, respectively. The shape of the isotherms for all the samples were almost identical, suggesting that the imidazolium ring formation and subsequent neutralization steps to form the NHCs did not alter microporosity of the framework. We have observed higher

160oC

160oC

N2

N2 25 / 40 / 80 / 120oC

100

o

CO2Fixing efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25 / 40 / 80 / 120oC

CO2

CO2 25oC

80

40oC

60 80oC

40

120oC

20 0 0

100

200

300

400

500

600

700

Time (min) Figure 4. Reversible CO2 fixation and release performance of NP-NHC. TGA curves on CO2 fixation-release of NPNHC at room temperature and at elevated temperatures. o Pre-activation at 160 C under N2 atmosphere was applied in order to remove captured atmospheric CO2 during the sample preparation step.



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Table 1. Textural parameters of sterically confined nanoporous polymers and the corresponding non-steric control polymer, PNHC.

Sample name

[a]

SBET

2 -1

Langmuir 2 -1

[b]

Vmicro

3 -1

[c]

dmicro

Sext 2 -1

[d]

Smicro

2 -1

[e]

Vtotal

3 -1

(m g )

(m g )

(cm g )

(nm)

(m g )

(m g )

(cm g )

NP-Imine

483

623

0.13

0.38

98.5

384.5

0.19

NP-Imidazolium

537

670

0.14

0.38

93.0

444

0.21

NP-NHC

475

625

0.11

0.37

110

365

0.19

PNHC

100

393

--

--

92

--

0.10

[a] Brunauer–Emmett–Teller (BET) surface area calculated over the pressure range (P/P0) 0.01–0.12. [b] Micropore volume calculated using the t-plot method. [c] Micropore diameter calculated from NLDFT method. [d] Micropore surface area calculated from the adsorption isotherm using the t-plot method. [e] Total pore volume obtained at P/P0 = 0.99.

applied in order to fully remove captured CO2 during the sample preparation. When NP-NHC was exposed to CO2 atmosphere with a CO2 flow (10 mL min-1) at 25°C for 5 h, 97% CO2 (9.55 wt%) fixing efficiency (mol of CO2 / mol of NHC) was observed on the basis of the weight increase, which is the highest ever reported for carbene based materials measured in the solid-state.38, 39 This result is mainly due to the increased accessibility of sterically hindered super basic air-stable NHCs within the NP-NHC. Meanwhile, the CO2 fixing efficiency of non-steric control polymer was found to be only about 20% (3.5 wt%) at 25°C for 5 h, (Figure S24) which further verifies the importance of steric confinement in the NHC system for CO2 fixation. Captured CO2 was removed by N2 flow (40 mL min-1) at 160°C for 60 min and around 83% (8.2 wt%) CO2 fixing efficiency was obtained for the second cycle (Figure 4). The CO2 fixing efficiency of the NP-NHC was also measured at 40°C under same flow rate, which was found to be 82% (8.1 wt%) and 78% (7.7 wt%) during the first and second cycles, respectively. The decrease in the CO2 fixing efficiency during second cycle could be due to either incomplete removal of CO2 molecules during the desorption process after first cycle or absorption of trace quantities of water on the NP-NHC. In order to mimic postcombustion CO2 capture conditions, we have also investigated the performance of NP-NHC at elevated temperatures. NP-NHC still exhibits good capability for the fixation of CO2 even at 80 and 120oC under same flow rate as shown in Figure 4. When NP-NHC was exposed to CO2 atmosphere with a CO2 flow (10 mL min-1) for 5 h, the CO2 fixing efficiencies at 80 and 120°C are found to be 52% (5.0 wt%) and 31% (2.95 wt%) for the first cycle, respectively, which slightly decreased to 48% (4.7 wt%) and 26% (2.5 wt%) in the second cycle. We have also investigated (Figure S25) CO2 uptake properties of the NP-Imine and NP-Imidazolium at 273, 298 and 323 K up to 1 bar. NP-Imidazolium showed slightly higher CO2 uptake of 1.74 mmol g-1 compared to that of NP-Imine (1.45 mmol g-1) presumably due to the presence of charged units and its higher surface area. As expected,

both NP-Imidazolium and –Imine showed decrease in CO2 uptake capacity with increasing temperature. 2.4. Organocatalytic CO2 Conversion. Sterically confined NHCs have already proven to be excellent organocatalysts to activate and transform of CO2 into methanol and other valued added chemicals.37-51 In order to demonstrate the performance of the NP-NHC as a nanoporous organocatalyst, we have tested its catalytic activity for the conversion of CO2 into cyclic carbonates Table 2. Cycloaddition of CO2 to epoxides and influence of substrate structure on cyclic carbonate yields. Reaction of CO2 with various epoxides catalyzed by NP-NHC for the preparation of respective cyclic carbonate via atom economy reaction under ambient pressure

1

[a] The products were characterized by H NMR and GC. [b] Yields refer to isolated products.


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with epoxides. We have, first of all, studied the catalytic property of NP-NHC in the conversion of CO2 using (±)Propylene oxide to form the corresponding cyclic carbonate at 120°C under atmospheric pressure of CO2 (Table 2). The effect of amount of catalyst on the synthesis of the propylene cyclic carbonate was also investigated. It was found that the yield of the final cyclic carbonate increases from 65 to 98% upon increasing catalyst loading from 2.5 to 5 wt% in the reaction mixture. It is important to note that all the established homogenous systems require very high CO2 pressures of 2 to 4 MPa to convert CO2 into cyclic carbonates by using epoxides,47, 48 which clearly shows the importance of CO2-philic nanoporous structure. The formation of cyclic carbonates from epoxides by using the sterically demanding NPNHC as an organocatalyst can be explained (Figure S26) by a mechanism involving the formation of NP-NHCCO2 adduct upon introduction of CO2 onto NP-NHC followed by the nucleophilic addition of the imidazolium-2-carboxylate to the strained epoxide ring and subsequent intramolecular cyclization of alkoxide intermediates.47, 48 Once establishing the NP-NHC as an excellent catalyst for the atom economy reaction between CO2 and propylene oxide under atmospheric pressure of CO2, the capability of NP-NHC was further investigated by using variety of challenging epoxide substrates to synthesize the corresponding cyclic carbonate derivatives in the presence of 5 wt% nanoporous organocatalyst under identical conditions. The conversion and purity of the cyclic carbonates were determined by 1H NMR spectroscopy and gas chromatography analyses and the corresponding data was shown in supporting information (See SI, Figures S27-S36). In addition, the catalyst was recycled four times without any change in activity and product yields. This indicates that the catalyst is highly stable and can be reused for several cycles. As shown in Table 2 (entries 1-3), NP-NHC can effectively catalyze the coupling reaction of mono aliphatic substituted terminal epoxides with CO2 to afford corresponding cyclic carbonates in excellent yields (92-98%) with 100% product selectivity. Unlike homogeneous systems,47, 48 however, we observed traces or no conversion for the phenyl (1%) and benzyl substituted epoxides (entries 4 and 5), which could be due to the molecular sieving property of the NP-NHC as the pore diameter of NPNHC (~0.4 nm) is less than that of kinetic diameter of the aromatic epoxides, thus introducing substrate selectivity, just like in biological systems, to the organocatalysts. It is important to note that such substrate selectivity is not observed in homogeneous systems.47, 48 Furthermore, the application of this new nanoporous organocatalyst can be extended to other organic transformations such as CO2 to MeOH conversion due to its high specific surface area along with the significant number of air stable carbenes located within the pores of the NP-NHC.

3. CONCLUSION In summary, we have demonstrated, for the first time, the design and preparation of a nanoporous polymer incorporating sterically confined air-stable NHCs. Our structural analysis indicates that the NP-NHC possesses built-in air stable NHCs entangled between two diisopropylbenzene functionalities with an excellent 3D microporous structure, relatively high specific surface area and large pore volume. The super basic nature of NPNHC facilitates its reaction with CO2 to afford corresponding carbonate adducts with an exceptional 97% CO2 fixing efficiency at room temperature. NP-NHCs also showed good CO2 uptake performance at elevated temperatures. This result demonstrates the potential of NP-NHC as a highly efficient adsorbent for reversible fixation-release of CO2 with an excellent fixing efficiency and good capture rate. The CO2 sequestration process on NP-NHC is reversible and provides a convenient method to mask and unmask NP-NHC, thus paving the way toward a practical use of NP-NHC as a porous organocatalyst. NP-NHC showed excellent catalytic performance for the conversion of epoxides into cyclic carbonates by using CO2 under ambient pressure. The catalyst was found to be highly stable and could be recycled several times without any loss in catalytic activity and product yields, which is a major advantage compared to the metal-containing systems. NP-NHC also exhibited unique substrate selectivity due to its narrow pore size distribution. These findings provide a new metal-free approach for the simultaneous capture and conversion of CO2 into value added chemicals. We also believe that the formation of porous organocatalysts and their corresponding metal complexes will offer new directions for the application these materials in several different catalytic transformations including catalytic transformation of CO2 to methanol.

ASSOCIATED CONTENT Experimental methods, synthetic procedures, and additional structural and spectroscopic data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]

ACKNOWLEDGMENT This research was supported by the KUSTAR-KAIST Institute, Korea, under the R&D program supervised by the KAIST. We also acknowledge the support from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013R1A1A1012282) and BK21 PLUS program. The authors wish to thank Prof. Ryong Ryoo for access to the solid-state NMR spectroscopy (IBS, Daejeon, Korea).


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Nanoporous Polymers Incorporating Sterically Confined N

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