In-depth Experimental and Computational Investigations for

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In-depth Experimental and Computational Investigations for Remarkable Gas/Vapor Sorption, Selectivity and Affinity by a Porous Nitrogen-Rich Covalent Organic Framework Prasenjit Das, and Sanjay K. Mandal Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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Chemistry of Materials

In-depth Experimental and Computational Investigations for Remarkable Gas/Vapor Sorption, Selectivity and Affinity by a Porous Nitrogen-Rich Covalent Organic Framework Prasenjit Das and Sanjay K. Mandal* Department of Chemical Sciences, Indian Institute of Science Education and Research Mohali, Sector 81, Manauli PO, S.A.S. Nagar, Mohali, Punjab 140306, India ABSTRACT: Porous nitrogen-rich covalent organic frameworks (COFs) are most challenging materials for selective CO2 capture, separation and conversion for substantive impact on the environment and clean energy application. On the other hand, separation of industrial cyclic congeners (benzene/cyclohexane) by host-guest interaction through -electron rich and deficient centers in a COF is the key. Based on the strategic design, a triazine-based benz-bis(imidazole) bridged COF (TBICOF) has been synthesized under polycondensation conditions and structurally characterized by various analytical techniques. Due to the presence of a benzbis(imidazole) ring, TBICOF exhibits permanent stability and porosity in the presence of acid and base monitored by wide angle xray (WAX) pattern and N2 sorption studies. The enhanced CO2 uptake of 377.14 cm3g-1 (73.4 wt%) at 195 K, confirms its high affinity towards the framework. CO2 sorption is highly selective over N2 and CH4 due to very strong interactions between CO2 and triazine and benz-bis(imidazole) functionalized pore walls of TBICOF as clearly evident from isosteric heat of adsorption and ideal adsorbed solution theory calculation, which is higher than other reported functionalized MOFs or COFs. Interestingly, TBICOF also behaves as a heterogeneous organocatalyst for chemical fixation of CO2 into cyclic carbonate under ambient conditions. The -electron deficient triazine and benz-bis(imidazole) moieties have been utilized for selective sorption and separation of benzene (641.9 cm3g1)

over cyclohexane (186.2 cm3g-1). Computational studies based on density functional theory (DFT) and Grand Canonical Monte

Carlo (GCMC) molecular simulations further support selectivity of CO2 (over N2 and CH4) and of benzene (over cyclohexane).

INTRODUCTION The rapid change in the climate by global warming owing to extensive carbon dioxide emission into the atmosphere by anthropogenic activities such as industrialisation, fossil-fuel power plants, deforestation and automobile toxic emissions has been a serious issue in the present day world.1,2 The industrial revolution and population expansion is creating environmental imbalance leading to scarcity problems for the future generation. The development of efficient technologies or strategies that can capture CO2 efficiently and mitigate the dilemma is more challenging and urgent task in the present situation.3 Under this canvas, carbon capture and sequestration (CCS) technologies play a crucial role to tackle this urgent global issue by adequate capture and storage of pre- and postcombustion CO2 to decrease the emission in an efficient and durable manner.4-6 The CCS is much more essential to sustain climate change for fossil-fuel combustion in humid conditions. Furthermore, post-combustion capture of flue gas (15:85 v/v CO2/N2) and pre-combustion capture of landfill gas (10-50 % CH4 over CO2) are required in order to avoid pipeline corrosion.7 On the other hand, the catalytic conversion of waste CO2 into workable industrial products is a pompous and alternative strategy for CO2 fixation. The cycloaddition of CO2 to cyclic carbonates is an atom economy process, which is one of the basic principles of green chemistry; moreover, the

product obtained is utilized in industrial and pharmaceutical applications.8,9 Utilizing a suitable material serving as both adsorbent and catalyst for CO2 capture and chemical fixation would be highly desirable. Covalent organic frameworks (COFs), an emerging class of new porous crystalline materials, have been exploited as excellent aspirants for CO2 capture and sequestration.10,11 COFs provides almost all the merits of metal-organic frameworks (MOFs), such as structural diversity, easy surface engineering with ordered pore distribution, tailorable functionality, and high thermal and chemical stability. These features make them excellent candidates for high gas uptake, selectivity and reusability.12-14 Various CO2-philic moieties containing heteroatoms have been incorporated into COFs to improve CO2 capture and selectivity through Lewis acid-base interactions.1519 On the other hand, heterocyclic rings were extensively used in porous organic polymers (POPs) only; however, very recently crystalline COFs with such rings have also been reported.20-24 However, a combination of both triazine and benzbis(imidazole) moieties in the same COF with nitrogen rich surfaces has not been reported for CO2 capture, separation and chemical fixation. Furthermore, the incorporation of such moieties within the framework can be utilized for the exploitation of these materials as selective adsorbents for electron-rich benzene (Bz) over cyclohexane (Cy) by favorable 1

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OHC O N O NH2 NH2

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Figure 1. Synthesis of TBICOF and model monomer.  interactions. This addresses an industrially important chemical separation of a liquid phase hydrocarbon mixture of Bz/Cy.25 Their separation cannot be feasible by conventional methods due to their markedly similar boiling points (Bz, 353.3 K; Cy, 353.9 K) and very similar kinetic diameters.26 Herein, we report a new porous, highly stable, nitrogen rich triazine based and benz-bis(imidazole) bridged COF, TBICOF, to showcase its applications in (a) highly selective CO2 sorption, (b) separation over N2 (flue gas) and CH4 (landfill gas) followed by ideal adsorbed solution theory (IAST), and (c) chemical fixation of CO2 to make various cyclic carbonates at ambient conditions. As anticipated, TBICOF has been found to be very efficient and selective for the separation of benzene over cyclohexane (about 44 at 298 K for an equimolar Bz/Cy mixture). Both DFT and GCMC molecular simulation have provided excellent support to the experimental results for the high CO2 uptake, selectivity and interaction site with host TBICOF. Similarly, the strong interaction and selectivity of Bz over Cy with TBICOF was preferentially observed.

RESULTS AND DISCUSSION Synthesis and Structural Characterization. TBICOF was synthesized by the polycondensation reaction between tri(4formylphenoxy)cyanurate (TFPC),27 1,2,4,5-tetraminobenzene (TAB) and imidazole at 120 oC in a mesitylene/NMP/dioxane (2:1:1) mixture for 3 days with 88% yield (Figure 1 and Scheme S1). For the synthesis of TBICOF, several solvothermal methods have been screened (WAX patterns and maps are shown in Figure S1). It has been observed that the paramount crystallinity was obtained using the mesitylene/NMP/dioxane (2:1:1) mixture. Thus, the synthesis of crystalline TBICOF via a cascade reaction without any additional catalyst is remarkable compared to the multistep synthesis of amorphous benzimidazole-based organic polymers.20-22 As shown in Scheme S2, the formation of benzimidazole ring in TBICOF include three steps: (i) formation of imine-linked intermediate, (ii) ring closing to form benzimidazoline intermediate, and (iii) 2


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Figure 2. Structure analysis of TBICOF: (a) Experimental (red) and Pawley refinement (blue) showing very minimal difference (black line) with Rp and Rwp of 1.92% and 3.16%, respectively, for hexagonal AA stacking with space group P6/m. (b) Simulated PXRD pattern for AA structure (purple). (c) AB structure (cyan). (d) ABC structure (violet). (e) AA structure of TBICOF. (f) Pawley refinement structure (Color code; carbon: grey, oxygen: red, nitrogen: blue, hydrogen: yellow). (g) AB structure of TBICOF. (h) ABC structure of TBICOF. 3



acceptor-less dehydrogenation of benzimidazoline intermediate (irreversible). The first step is the rate determining step, through which the crystalline COF structure can be formed by reversible formation of imine linkages. It is noteworthy that the imidazole used in the reaction acts as an additional base (a dehydrogenating agent) to facilitate the aromatization in the third step.24,28 For comparison, a model monomer (Yield: 90%) was also synthesized following the same condensation route except 1,2-diaminobenzene (DAB) was used instead of TAB using EtOH/dioxane (4:1) as a solvent at 60 oC (Scheme S3). FTIR spectra of TBICOF and its model monomer exhibit the characteristic stretching frequency for the C=N bond of benzbis(imidazole) moiety at 1608 and 1609 cm-1, respectively, confirming the completion of reaction (Figures S2-S3). The formation of the model compound was further confirmed by HRMS data with the observation of a peak at 706.2235 (cal. [M+H]+, 706.2270) (Figure S4). The elemental analyses of TBICOF and its model monomer confirmed the calculated values of C, H and N close to the corresponding experimental values. The high N value in this analysis indicates TBICOF is nitrogen rich system. The solid-state absorbance spectra of TBICOF and its model compound show two absorption bands at 384 and 316 nm attributing to the * transition for the Ncontaining conjugated -system, respectively (Figure S5). For investigating the thermal stability, the thermogravimetric analysis (TGA) of TBICOF and its model compound was carried out. A major weight loss near 400 oC for TBICOF from this analysis demonstrates that it is highly stable in comparison with its model monomer (Figure S6). The 13C CP-MAS NMR spectrum of TBICOF (Figure S7) confirming its atomic precision construction (exhibits signals from 100-200 ppm for eight different carbon centres). The peaks corresponding to imidazole unit and triazine ring appeared around 153.03 and 173.1 ppm for TBICOF, while 152.8 and 173.4 ppm for its model monomer (Figure S8). Additional signal of TBICOF in the aromatic region are consistent with the building block. The 1H and 13C NMR spectra of model compound in d -DMSO also 6 show characteristic signals at 7.62 ppm and 152.8 ppm, respectively, confirming the formation of a benzimidazole ring. Structure Solution of TBICOF. In order to ascertain the periodical structure of TBICOF, simulation of PXRD was carried out using Reflex module in Accelrys BIOVIA, Materials Studio 2017R2 software package. The powder diffraction pattern of TBICOF exhibits well-defined diffraction peaks, confirming the crystalline nature of the framework (Figure 2a). Most of the COFs are constructed by the three-connected ligand with ditopic linker having hexagonal layered structure.29-31 Considering the hexagonal system for TBICOF, both eclipsed structure (AA) and staggered structures (AB and ABC) were optimized using the DFT-D calculation (CASTEP method) using space group P6/m and simulated PXRD patterns were generated (Figure 2 and Figures S9-S11, Table S1-S3). The unit cell parameters for hexagonal system differ in ‘c’ axis, where eclipsed (AA) and staggered (AB and ABC) structures have a = b = 36 Å, c = 3.4 Å (AA) or 6.8 Å (AB) or 10.2 (ABC),  =  = 90o and  = 120o, which give (100) facet at 2 = 2.5o. The PXRD

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data for simulated pattern (blue line) were indexed and well fitted by implementing the X-cell program of Pawley refinement and matched with the experimental PXRD pattern (red dots) with a minimum difference (black line) (Figure 2a). An excellent fit was obtained with good Rp (1.92) and Rwp (3.16) and cell parameters a = b = 36.0252 Å and c = 3.4032 Å, which are obtained from the simulated pattern. Moreover, in the experimental WAX profile, the strong peak at 2.5o (100), indicates that the highly periodic structure is present in the COF. These diffraction peaks are at 2 = 2.5o, 5.3o, 10.76, 14.3, 17.5o and 25.1o with d values 33.9, 15.8, 7.15, 6.18, 5.1 and 3.45 Å, corresponding to the (100), (200), (210), (330), (430) and (001) facets, respectively (Bragg’s equation, 2dsin = n, where n = 1 and  = 0.154 nm). Also, the strong - stacking of the 2D sheets (001 facet) was observed with an interlayer distance of 3.4 Å at 2 = 25o. This confirms that TBICOF has the AA stacking; however, the simulated PXRD patterns of AB and ABC were inconsistent with the experimental result. The model monomer shows the crystallinity in WAX patterns and WAX 2D map (Figure S12). Microscopic Analysis and Stability Test. Several microscopic techniques, such as field emission scanning electron microscopy (FESEM), scanning transmission electron microscopy (STEM), high resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM), have been utilized to ascertain the surface property of TBICOF. FESEM and STEM images of TBICOF showcased that the condensation polymerization of TFPC and TAB afforded selfassembly of nanosheets to form microflowers with uniform thickness of 160 nm (Figures 3a,b and S13). Similarly, TEM image of TBICOF depicted the similar self-assembly of nanosheets, and the HRTEM image displayed uniformly ordered porous hexagonal arrangement (Figures 3c-f and S14). These exfoliated nanosheets make a ring-like assembly in mica plate with a height of 12-15 nm and an average diameter of 150200 nm. This result is highly correlated with FESEM and HRTEM data (Figures 3g-i and S15). Furthermore, an AFM image provided the height thickness corresponding to 30-35 unit cell on the basis of their optimized AA stacking model. Overall, the lateral sizes of TBICOF obtained from the SEM, TEM and AFM data indicate that TBICOF has a high aspect ratio in the 2D layers. On the other hand, the model monomer displayed rod like morphology with thickness of 161 nm which is correlating with the TBICOF diameter (Figure S16). The high chemical stability of TBICOF was examined by immersing in 3 N NaOH and 3 N HCl for 7 days and monitored by the WAX and FESEM (Figures S17-S19). Gas Sorption Studies. The successful incorporation of triazine and benz-bis(imidazole) functionalized pore sites in TBICOF and its stability based on TGA and PXRD were the keys to exploring its potential gas sorption studies. Thus, its sorption studies at different temperatures (N2 at 77 K; CO2, N2, CH4 at 263, 273, 298 and 313 K) have been performed. A mixture of type I and type IV reversible isotherm was observed for N2 at 77 K. The BrunnauerEmmet-Teller (BET) and Langmuir surface areas were

4


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Figure 3. (a) FESEM image focussing on micro-flower areas in TBICOF. (b) STEM image of TBICOF. (c) TEM image of TBICOF depicting of microflower. (d, e, f) HRTEM images of TBICOF. (g) 2D AFM image of TBICOF revealing its porous spherical nature. (h) 3D AFM height profile ruminating the uniform nature. (i) Height profile.

found to be 1424 m2g-1 and 1702 m2g-1, respectively (Figure 4a and S20). The Non-Local Density Functional Theory (NLDFT) was utilised to compute the pore size distribution of TBICOF and was found to be ~0.8-4.5 nm (with the highest peak at 2.5 nm compared to the same from simulated at 2.9 nm), which is attributed to mixture of micro or macro porous nature32 (Figure 4b and S21). This broad pore size is due to the presence of flexible TFPC linker in TBICOF (Figure 4b, inset). The chemical stability was not only checked by WAX pattern and FESEM but also followed by N2 sorption isotherm (Figure 4a). N2 sorption isotherms indicate that the porosity was

perfectly maintained in the presence of acid and base. Additionally, it was observed that the BET surface area and pore diameter were almost remain constant. The CO2 sorption curves exhibit fully reversible isotherms at 313 K, 298 K, 283 K, 273 K and 263 K. The CO2 uptake at 313, 298 K, 283 K, 273 K and 263 K was 23.96, 39.04, 54.07, 68.89, and 84.5 cm3g-1 (STP, 4.6, 7.48, 10.62, 13.53 and 16.59 wt%), respectively (Figure 5a). Interestingly, this uptake of CO2 is higher or comparable with triazine and benz-bis(imidazole) based COFs and other well-known MOFs or POPs such as COF-1, COF-5, COF-6, COF-8, COF-10, COF-102-103, TpPa-series, 5



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Figure 4. Gas adsorption-desorption curves for TBICOF: (a) N2 at 77 K. (b) Pore-size distribution; (inset) simulated poresize of TBICOF including van der Waal radii.

TpBD-series, DhaTpH, ACOF, azoCOP-1-3, PECONF-1, TBILP-1, ZIF-100, Cd-ATAIA, Co-MOF and IISERPMOF20 (see Table S4 for the full list).15-22,33-44 It is noteworthy to observe the high uptakes of CO2 at 0.5 bar (2.54 mmolg-1 at 263 K, 2.01 mmolg-1 at 273 K, 1.61 mmolg-1 at 283 K, 1.01 mmolg-1 at 298 K and 0.632 mmolg-1 at 313 K), which is the partial pressure of CO2 in landfill gas mixture. This steep uptake of CO2 at lowpressure region indicates strong dipole-quadrupole interactions of CO2 molecules with N and O centers present in the triazine and benz-bis(imidazole) moieties of TBICOF. These uptakes are pretty less due to higher pore size of TBICOF compared to BILPs organic polymers.20-22 Surprisingly, when the temperature was lowered to 195 K, a drastic improvement of high CO2 uptake (377.14 cm3g-1 or 73.4 wt% at 1 bar) was obtained, confirming its high affinity towards the framework (Figure 5b). The CH4 sorption curves exhibit fully reversible isotherms with an uptake of 7.59, 13.7, 23.37, and 29.09 cm3g-1 (STP, 0.54, 0.98, 1.67 and 2.08 wt%) at 313, 298, 273 and 263 K, respectively (Figure 5c, Table S4). On the other hand, the uptake of N2 is lower (5.74 cm3g-1, STP, at 263 K; 4.31 cm3g-1, STP, at 273 K and 2.04 cm3g-1, STP, at 298 K) (Figure S22). The quantitative binding affinity was evaluated from coverage-dependent adsorption enthalpies of CO2, CH4 and N2 gas molecules in TBICOF calculated from their adsorption isotherm at 263, 273 and 298 K by

Figure 5. (a) CO2 sorption at 313, 298, 283, 273 and 263 K. (b) CO2 sorption at 195 K. (c) CH4 sorption at 313, 298, 273 and 263 K. (d) Recycling efficiency of TBICOF for CO2 capture.


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Figure 6. Selectivity of CO2 over N2 and CH4 for TBICOF at 313, 298, 273 and 263 K: (Column 1) uptake of CO2, N2 and CH4; (Column 2) change of adsorption selectivity with increasing CO2 mol% from 5 to 30 % at interval of 5% mixture of CO2/N2; (Column 3) change of adsorption selectivity with increasing CO2 mol% from 10 to 50% at interval of 10% in a binary CO2/CH4.

using Clausius-Clapeyron equation and their fitting by virial45 method (Figures S23-S28, Table S5). These two methods showed excellent agreement with each other. The isosteric heat of adsorption (Qst) at zero loading are 42.8, 21.3 and 16.2 kJmol-1 for CO2, CH4 and N2, respectively, which further confirms a decrease of binding affinity in the order of CO2 > CH4 > N2. The Qst for CO2 of TBICOF at zero coverage (42.8 kJmol-1) reflect the very strong interactions between CO2 and triazine and benzbis(imidazole) functionalized pore walls of TBICOF (Figure S24). Moreover, its Qst value is exceptionally higher than other functionalized MOFs, COFs and POPs listed in Table S7.14,15,16 On the other hand, the higher Qst

value for CH4 than that of N2 indicates the stronger adsorbate-absorbent interaction of CH4 compared to N2. Also, the Qst of CH4 is comparable and higher than other MOF, COF and POP based systems (Table S7).15-22,33-43 Recyclability and Efficiency of CO2 Capture. To utilize for practical application in gas uptake and separation, the compatibility and recyclability of material are urgent. For this purpose, TBICOF was implicated as an adsorbent for CO2 uptake cycles at 298, 273 and 263 K up to 1 bar pressure. Remarkably, TBICOF almost reclaimed its CO2 uptake capability up to three consecutive cycles and also even after all gas or vapor sorption (Figure 5d and S29). To perform the sorption measurement, the



material was degassed at 120 oC for 24 h under high vacuum. Also it provided very similar result after gas (N2, CO2, CH4) and vapor (Bz, Cy and water) sorption. CO2/N2 and CO2/CH4 Selectivity by IAST Method. Motivated from the aforementioned results, we further investigated the potential of TBICOF for gas separation. To emulate the separation behaviour of TBICOF under a more genuine world setting, binary mixture selectivity at four different temperatures (313, 298, 273 and 263 K) were enumerated by employing the IAST method developed by Myers and Prausnitz.46 The binary mixture of CO2/N2 (15:85, flue gas composition) and CO2/CH4 (50:50, landfill gas composition) with the pressure range up to 100 kPa were used for calculation. The dual-site LangmuirFreundlich (DSLF) isotherm model has been applied for the fitting the unary isotherms of CO2, N2 and CH4 at 263, 273, 298 and 313 K with very good R2 value (Figures S30S41). Using this fitting, the saturation capacities (qm), affinity coefficients (b) and deviation from an ideal homogeneous surface (n) for different gases such as CO2, N2 and CH4 at 313, 298, 273 and 263 K were obtained (Table S6). The calculated adsorption selectivity of TBICOF is 26.6, 27.38, 38.9 and 40.8 (zero coverage) and 44.03, 40.3, 46.7 and 37.3 (higher pressure, 1 atm) at 313, 298, 273 and 263 K for CO2/N2 (15:85), respectively (Figure 6). On the other hand, the CO2/CH4 (50:50) selectivity of TBICOF is 7.74, 8.78, 9.73 and 10.9 (zero coverage) and 5.96, 7.72, 11 and 9.5 (higher pressure (1 atm)) at 313, 298, 273 and 263 K, respectively. It was observed that the selectivity increased slightly from 0 to 12 kPa pressure and decreased up to 35 kPa pressure, followed by a sharp increase at higher pressure at 298 K for CO2/N2 (Figure 6). There is an increase in selectivity with increase in pressure at 313, 273 and 263 K for CO2/N2. On the other hand, there is a sharp decrease in selectivity at lower pressure followed by increase in selectivity at higher pressure at 313, 298, 273 and 263 K for CO2/CH4 (50:50), respectively. This result demonstrates that the adsorption selectivity of CO2/N2 is better than that of CO2/CH4. We also tested and demonstrated the selectivity by increasing the molar ratio of CO2 from 5 to 30% with the increment of 5 mol% for a mixture of CO2/N2 and 10 to 50% with increment of 10 mol% for a mixture of CO2/CH4. The calculated result shows that the predicted selectivity for TBICOF was increasing with an increase in CO2 mole fraction. It was observed that the selectivity increased from 41.13 to 46.09, 33 to 47.7, 32.5 to 78.8 and 28.8 to 55.6 with the increased molar ratio of CO2 from 5 to 30% for a mixture of CO2/N2 at 313, 298, 273 and 263 K, respectively. Also, the selectivity increased from 5.36 to 5.96, 7.3 to 7.8, 10.2 to 11 and 7.9 to 9.5 with increased molar ratio of CO2 from 10 to 50% for a mixture of CO2/CH4 at 313, 298, 273 and 263 K, respectively. Such an enhancement of CO2 in both the cases at all temperatures is remarkable (Figure 6). All these selectivity values are significantly higher and comparable with many well- known MOF based systems such as PCN-88 (CO2/N2: 18/298 K, CO2/CH4: 5/296 K),36 PCN-61 (CO2/N2: 15/298 K),37 ZIF-100 (CO2/N2: 25/298 K and CO2/CH4: 5.9/298 K),40 Cd-ATAIA (CO2/N2: 54.08/298 K and CO2/N2: 46.98/273 K),42 NOTT-202 (CO2/N2: 26.7/273 K and CO2/CH4: 4.3/293 K, CO2/CH4: 2.9/273 K),43 Co-MOF (CO2/CH4: 16.9/313 K)44

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and POP based systems such as CTFs (CO2/N2: 31.2/273 K),33 TBILP-1 (CO2/N2: 62/298 K, CO2/CH4: 9/298 K),22 and AzoCOP-1-3 (CO2/N2: 96-130.6/298 K, CO2/N2: 63.7-109.6/273 K and CO2/N2: 27.4-44/263 K)34. Literature survey on both CO2/N2 and CO2/CH4 selectivity at different pressure, temperature, concentration is tabulated in Table S7. These selectivity and gas uptakes are tremendous by a porous TBICOF with remarkable physical and chemical stability making these materials very promising for CO2 separation from flue gas as well as landfill gas mixture and also for future use in gas storage. Quantum Computational Studies. To understand the high CO2 capture selectivity compared to N2 and CH4, quantum computational studies based on DFT47 using the Gaussian 09 program and GCMC48 molecular simulation using Material studio program were carried out. For DFT analysis, crucial benz-bis(imidazole) repeating unit of TBICOF in the presence of CO2, N2 and CH4 were optimized using B3LYP/6-31G+(d,p) basis set (Figures 7a-c and S42-S44). It has been observed that the binding affinity of benz-bis(imidazole) is more towards CO2 in comparison to N2 and CH4. In particular, the binding energy was calculated for CO2 (BE = -3.6 (N-H…O) or -3.1 (N…C=O) kcal/mol), N2 (BE = -1.88 kcal/mol) and CH4 (BE = -2.08 kcal/mol) which revealed that CO2 is more stabilized than N2 and CH4. TBICOF adsorbent consisting of (2 x 2 x 1) unit cells for CO2, N2 and CH4 was utilised to perform GCMC simulation at 298 K up to 10 bar (Figures S45a-d). It is noteworthy to observe that the probability distribution of CO2 is very high as depicted by the increase in probability density (illustrated in red color) from 1 bar to 10 bar pressure (blue to red depicting high probability). Based on the loading vs pressure plot it is more clear that the CO2 uptake and selectivity is exclusively high compared to that of N2 and CH4 (Figure S45e). This is in accordance with the obtain experimental results. In addition to that, the very strong interaction of TBICOF with CO2 was also established via GCMC simulation by evaluating the binding energy of CO2 (-35.6 kJ/mol), N2 (-8 kJ/mol) and CH4 (-8.7 kJ/mol) (Figure S45f). This result was well correlated with the isosteric heat of adsorption (Qst) which was obtained from thermodynamic of adsorption process (vide supra). From fixed pressure metropolis GCMC method, the number of CO2 molecules for (2 x 2 x 1) unit cells were calculated to be 9 (3 CO2 molecules per unit cell) in a pore which is well corroborated with the experimental result (2.95 CO2) (Figures 7d and S46). It was observed that there was strong adsorbateadsorbent and adsorbate-adsorbate interactions. The intermolecular distance between CO2 and TBICOF was validated with the DFT calculation (d = 2.27 Å (GCMC)/2.3 Å (DFT) (N-H…O)) or 3.31 Å (GCMC)/3.1 Å (DFT (N…C=O) Å and the bond angle of CO2 is reduced to 174.9o. A very close intermolecular distance between CO2 and BILPs (2.72-2.89 Å using M06/6-311+G* level) and similar angle deformation of CO2 were reported in the literature.49 It is also observed that there is an interaction of CO2 with the phenyl ring of the central benz-bis(imidazole) and the phenyl ring attached to the triazine moiety and thus the overall system becomes highly polar. In the channel, three CO2 molecules arrange themselves in a triangle, where the + carbon atom of one CO2 interacts with - oxygen atom of another CO2 molecule (Figure 7e). This electrostatic


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Figure 7. DFT optimization at B3LYP/6-31G+(d,p) level of benz-bis(imidazole) repeating unit of TBICOF with CO2 (a), N2 (b) and CH4 (c). GCMC molecular simulation: (d) CO2 position in TBICOF. (e) Interaction and distance between CO2 and TBICOF. (f) CH4 position in TBICOF. (g) GCMC simulation of CO2/CH4 (50:50) mixture at 298 K, (h) their binding energies and (i) loading amount in mixed gas phase (simulated and experimental).

interaction between CO2 molecules and the framework attributes to the dipole-quadrupole interaction.49 Furthermore, it was observed that the number of CH4 molecules is 0.67 per unit cell which is also similar to the experimental result (0.72 per unit cell) (Figure 7f). Moreover, GCMC simulation was accomplished for mix gas CO2/CH4 (50:50) at 298 K up to 1 bar (Figure 7g). The very high selectivity, BE and loading were observed for CO2 over CH4 (Figure 7h). The mix gas loading was well validated with GCMC (simulated) and IAST (experimental) results (Figure 7i). Similarly, GCMC simulation was also utilized for mixed gases CO2/CH4 and CO2/N2 (50:50) at 313 K and the high selectivity of CO2 was observed over N2 and CH4 (Figure S47). Chemical Fixation of CO2 Under Ambient Conditions. The inherent CO2-adsorbing property and embedded Lewis basic N and O sites in the framework indicate that TBICOF should act as the favorable heterogeneous catalyst for CO2 related reactions. One of the chemical reactions for CO2 conversion involves epoxides, which upon reaction with CO2 lead to the formation of cyclic carbonates. These cycloaddition products have been intensively scrutinized due to vast applications in pharmaceutical and electrochemical industries.50-52 In this regard, catalytic performance of TBICOF (in presence of tetra-n-butylamonium bromide (TBAB) under CO2 balloon pressure at room temperature for 24 h) in the cycloaddition of CO2 with various aromatic and nonaromatic epoxides to produce various cyclic carbonates were explored.

However, very limited COFs have been explored for heterogeneous chemical fixation of CO2.53,54 The obtained TBICOF was commenced as a heterogeneous catalyst for CO2 cycloaddition without any prior treatment. Initial optimization conditions were carried out for the cycloaddition of CO2 with epichlorohydrin under various catalyst concentration and time. As shown in Table S8, TBICOF shows the efficient catalytic activity for the model reaction with yields 70-95% at room temperature and ambient condition in presence of the co-catalyst TBAB (0.1-0.5 mol%) and CO2 balloon pressure for 24 h. Moreover, the cycloaddition reaction was carried out in the presence of only catalyst, and no product was obtained for the model reaction (entry 7, Table S8). However, only 28% product was obtained from the cycloaddition of CO2 in the presence of TBAB (0.5 mol %) only (entry 8, Table S8). These results depicted that the cycloaddition of CO2 with epichlorohydrin can be obtained in high yield by synergistic utilization of TBICOF and co-catalyst TBAB under ambient condition.54-62 After optimization, the substrate scope was explored for the cycloaddition reaction flourished above (0.1 mol% of TBICOF, 0.5 mol% TBAB and CO2 balloon at ambient condition). For the first time, we developed the effect of halobenzene for the cycloaddition reaction of CO2. The cycloaddition reaction of 2-(4-bromophenyl)oxirane with CO2 provided better yield compared to other 3- or 4-chloro substituted epoxide and styrene epoxide (Table1, entry 4).



Furthermore, in case of sterically hindered epoxide substrate like butyl glycidyl ether, the yield decreased to 72% under the same condition (Table 1, entry 9). This occurrence may be due to limited diffusion of large substrate into the 1D pore channels of TBICOF.55-62 In the endeavor, the cycloaddition reaction of CO2 with cyclohexene oxide gave 70% yield with TON of 700 (Table 1, entry 10). To the best of our knowledge, these TON are comparable with other previously reported MOF based catalyst under similar condition.55-62 In order to examine the stability and durability of TBICOF, the catalyst was isolated after the reaction by centrifugation, filtration and washed with acetone and dried in vacuum. To determine the reusability of TBICOF during the catalytic process, epichlorohydrin has been taken as an example to produce 4-(chloromethyl)-1,3dioxolan-2-one. It was recycled in five consecutive experiments and its catalytic activity was monitored. There is no significant change in the original catalytic activity of TBICOF (Figure 8a) with yield preserved up to > 90%. The recovered framework was characterized by the WAX and FESEM images (Figures 8b,c). Based on the current results and previous reports on related systems52,53,63,64, a possible mechanism for the TBICOF catalyzed cycloaddition reaction of CO2 with epoxides is proposed in Figure 8d. In addition to understanding the interaction of epoxide with TBICOF, GCMC simulation was performed between TBICOF and propylene oxide (PO). It is observed that there is a strong H-bonding interaction between C-O of PO and N-H of TBICOF layers (N-H…O = 2.14-3.6 Å) (Figure S48). Based on the above results, the C-O bond of the epoxide ring is activated by the N-H group of benz-bis(imidazole) ring on the pore wall of the COF via H-bonding. Concomitantly, the less sterically hindered carbon atom of the epoxide is attacked by the Br- of TBAB to form the intermediate (I). This step is followed by the opening of the epoxide to form an oxyanion which is stabilized by the benz-bis(imidazole) N group on the adjacent layer (II). Next, the oxyanion intermediate undergoes a nucleophilic addition to the CO2 molecule to form an alkylcarbonate anion (III). Finally, the ring closure of the intermediate III followed by removal of Br- produces the cyclic carbonate and rejuvenates the COF for further cycles. Therefore, TBICOF is an excellent candidate as a heterogeneous catalyst marked by its high activity and recyclability for chemical fixation via CO2. Vapor Sorption: Separation of Benzene Over Cyclohexane. To establish the selective sorption behaviour of Bz/Cy by TBICOF, the vapor sorption experiment of Bz and Cy was measured at 298 K (Figure 9). It is observed that the uptake amount of Bz (STP, 641.9 cm3g-1 at p/p0 ~ 1 was significantly higher than that of Cy (186.2 cm3g-1, at p/p0 ~ 1) (Figure 9a). Both Bz and Cy show type IV sorption isotherm with very large hysteresis in case of Bz compared to Cy indicating strong interaction with the host. TBICOF adsorption for Bz and Cy are surprisingly higher than those reported for COFs and MOFs, suggesting it is a suitable candidate for industrial

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application (Table S9).27,65,66 For further detailed study, the key separation of Bz/Cy was enumerated by IAST calculation. The unary isotherm of Bz and Cy was fitted with R2 = 0.991 and 0.996, respectively, by employing DSLF isotherm model (Figure S49-S50). The adsorption selectivity (Sads) for an equimolar Bz/Cy mixture by TBICOF was analyzed by the IAST method to obtain a value of >44 at 298 K (Figure 9b). Catalytic Cycloaddition of CO2 with Epoxide to Produce Cyclic Carbonates O

CO2

+

R

O

TBICOF catalyst/ TBAB Room temperature 24 h

O

O

R

Table 1. Substrate scope for cycloaddition of CO2 reaction catalysed by TBICOFa Entry

R

Product

Conversionb (%)

TONc

54

540

67

671

70

703

86

862

64

638

98

981

95

952

82

819

72

724

70

700

O O

1

O

2

O

O O

Cl

Cl

O O O

3 Cl

Cl O O O

4 Br

Br O

O

5 H3C

7

Cl

9

O

O

6

8

O

O

O O O O

Cl

O O O

H3C

O O

H3C

10

O H3C

O

O O

O O

O O

Reaction conditions: 0.1 mol% of TBICOF, TBAB (0.5 mmol), epoxide (1.5 mmol), CO2 (balloon pressure), solvent-free at 26-28 ◦C for 24 h. bCalculated using 1H NMR spectroscopy. cNumber of moles of product per mole of the catalyst. a

The high sorption and selectivity of benzene over cyclohexane was observed due to the strong - interaction between -electron deficient triazine ring and benzbis(imidazole) moieties and the -electron rich benzene. To further understand the good selectivity of Bz over Cy, GCMC molecular simulation was performed on TBICOF (Figure 9c). Interestingly, it was observed that the uptake of Bz is very high and negligible uptake for Cy with a very high BE of Bz (-47 kJ/mol) compared to Cy (Figure 9c-e). From the fixed pressure metropolis GCMC method, the number and position of Bz molecules for a (1 x 1 x 1) unit cells of TBICOF were observed (Figure S51). Water sorption is very much important for many


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Figure 8. (a) Catalytic recyclability of TBICOF for CO2 cycloaddition reaction with epichlorohydrin. (Yield: violet color and selectivity: brown color). (b) PXRD profiles of TBICOF before and after five runs of chemical fixation. (c) FESEM images of TBICOF before and after five runs of chemical fixation. (d) Plausible mechanism for CO2 cycloaddition reaction catalysed by TBICOF.

applications and the pore size hold an important role in sorption.67,68 TBICOF shows limited water uptake at lower pressure, indicating the affinity of water towards hydrophobic organic COF surface is low (Figure S52). At higher vapor pressure (P/P0 = 0.7), the uptake started and reached a maximum of 310 cm3g-1, indicating pore filling at higher pressure due to large pore size. Moreover, the water sorption exhibits a type IV sorption isotherm accompanied with a large desorption hysteresis due to its strong interaction with TBICOF at higher pressure. This high water uptake capacity at higher pressure anticipates a good correlation with respect to porosity and this result is well correlated with PIZOF-2 reported by Yaghi et al.68 The number and position of H2O molecules for a (2 x 2 x 1) unit cells of TBICOF has been shown through GCMC simulation and its corresponding binding energy also calculated as 17.6 kcal/mol (Figure S53).

CONCLUSION In summary, a new functional triazine based benzbis(imidazole) bridged nitrogen-rich COF, TBICOF, was synthesized under solvothermal conditions and fully characterized by various analytical techniques. The surface properties were characterized by FESEM, HRTEM and AFM analyses. TBICOF possesses hexagonal 2D honeycomb layers with an AA stacking for high thermal and chemo stability. It has high surface area, large pore volume and heteroatom in the pore

wall to permit potential small gas/vapor storage and separation. The incorporation of polar Lewis basic nitrogen-rich benzbis(imidazole) functional group paved a way for significant CO2 sorption. The highly selective separation of CO2 over N2 and CH4 by TBICOF shows its potential toward flue gas and landfill gas separation under the relevant conditions. The high Qst value of CO2 attributes to the strong quadruple moment of CO2 and its interaction with the nitrogen-rich centers embedded in the pore wall. DFT and GCMC molecular simulations matched well with the experimental results. The development of thermo- and chemo-stable porous COFs embedded with multiple functional sites to procreate a synergistic effect and amplify their interactions with CO2 can be a new path forward for practical applications with regard to CO2 separation. Importantly, TBICOF behave not only selective to sorption and separation of CO2, also it acts as a promising metal-free organocatalyst for chemical fixation of CO2 with epoxide under solvent free ambient conditions. The remarkable efficiency from this result shed light on the affinity of CO2 towards the framework. The high sorption and selectivity of Bz over Cy was attributed to the strong - interaction with the -electron deficient triazine ring and benz-bis(imidazole) moieties and the -electron rich benzene, which was further proved by GCMC molecular simulation. Utilizing this approach, the separation of Bz/Cy is an advanced key from the energy-economy standpoint of industrial separation. This high selectivity and gas/vapor uptake by TBICOF among other porous COFs is remarkable. Finally due to its remarkable chemical and physical stability, it serves as a class of promising materials for future use in environmental clean-up and industrial gas/vapor storage.



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EXPERIMENTAL SECTION Materials and methods have been described in SI.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publication website at DOI:10.1021/ Synthesis and characterization details of TBICOF, gas sorption study, Qst and selectivity, comparable table, DFT and GCMC simulation, catalysis, Bz/Cy sorption, water sorption.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This work was funded by IISER Mohali. P. D. is thankful to MHRD, India for a research fellowship. Authors acknowledge the use of departmental and central (NMR, X-ray, SAX, AFM and FESEM) facilities at IISER Mohali and HRTEM analysis by NIPER Mohali.

REFERENCES

Figure 9. Sorption and separation of Bz over Cy by TBICOF: (a) Adsorption-desorption isotherms of Bz (red) and Cy (blue) at 298 K, (b) Uptake capacity of an equimolar Bz (cyan)/Cy (purple) mixture and selectivity (blue) of Bz over Cy calculated by IAST. GCMC molecular simulation: (c) Probability distribution plot of Bz on TBICOF at 10 bar. (d) Loading of Bz and Cy per unit cell of TBICOF. (e) BE of Bz.

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