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Computer-Aided Design of Interpenetrated TetrahydrofuranFunctionalized 3D Covalent Organic Frameworks for CO2 Capture Ravichandar Babarao, Radu Custelcean, Benjamin P. Hay, and De-en Jiang Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 10 Oct 2012 Downloaded from http://pubs.acs.org on October 11, 2012

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Crystal Growth & Design

Computer-Aided Design of Interpenetrated Tetrahydrofuran-Functionalized 3D Covalent Organic Frameworks for CO2 Capture Ravichandar Babarao,† Radu Custelcean, †,* Benjamin P. Hay, † and De-en Jiang†,* †

Chemical Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831

Abstract: Using computer-aided design, several interpenetrated imine-linked 3D covalent organic

frameworks

with

diamondoid

structures

were

assembled

from

tetrakis-4-

formylphenylsilane as the tetrahedral node, and 3R,4R–diaminotetrahydrofuran as the link. Subsequently, the adsorption capacity of CO2 in each framework was predicted using grand canonical Monte Carlo simulations. At ambient conditions, the 4-fold interpenetrated framework, with disrotatory orientation of the tetrahedral nodes and diaxial conformation of the linker, displayed the highest adsorption capacity (~ 4.6 mmol/g). At lower pressure, the more stable 5fold interpenetrated framework showed higher uptake due to stronger interaction of CO2 with the framework. The contribution of framework charges to CO2 uptake was found to increase as the pore size decreases. The effect of functional group was further explored by replacing the ether oxygen with the CH2 group. Although no change was observed in the 1-fold framework, the CO2 capacity at 1 bar decreased by ~ 32 % in the 5-fold interpenetrated framework. This work highlights the need for a synergistic effect of a narrow pore size and a high density of etheroxygen groups for high-capacity CO2 adsorption.

*

Corresponding authors. E-mail: [email protected]; [email protected]

ACS Paragon Plus Environment


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The interest in gas adsorption and separation with microporous frameworks has increased tremendously over the last few years. Along this line, special attention has recently been paid to separation of CO2 from other gas mixtures using metal-organic frameworks (MOFs). Many recent reviews have highlighted the importance of MOFs for this problem, and recommended several measures to improve the performance of these materials.1-4 A number of strategies have been employed to enhance CO2 selectivity, such as the presence of constricted pore size, open metal-sites, post-synthetic modification, and introduction of functional groups.5 Adding functional groups to the organic linkers, particularly the amine group, has been widely studied for CO2 capture. In this respect, several amino-functionalized MOFs have been synthesized and tested for CO2 selectivity.6-11 Recently, we modeled several functionalized porous–aromatic frameworks (PAFs) to evaluate their CO2 affinity and selectivity, and found the tetrahydrofuranfunctionalized PAFs show better CO2 capacity and selectivity when compared to the aminefunctionalized ones, which we attributed to the highly polar nature of the furan group.12 Another desired feature for enhancing CO2 capacity at ambient conditions is a suitable pore size that matches the kinetic diameter of the CO2 molecule. Reduction in the pore size is normally achieved by adding functional groups to the frameworks, with the size of the resulting pores depending on the bulkiness of the functional groups. Another way of obtaining narrow pore sizes is by taking advantage of framework interpenetration, a phenomenon that has been widely employed to improve H2 storage.13-15 Interpenetration of frameworks not only reduces the pore size, but also stabilizes the frameworks and show a higher permanent porosity than their noninterpenetrated counterparts.16-18 Recently, Makal et al. classified the framework isomers found in MOFs as three distinct groups such as interpenetration or catenation isomers, conformational isomers and orientation isomers.19


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Several studies have also examined the effect of interpenetration on CO2 adsorption in different MOFs. For example, Ma et al. synthesized the catenated isomers PCN-6 and PCN-6’ (porous coordination networks), in which the catenated PCN-6 showed a 41% increase in Langmuir surface area and a 133% increase in volumetric hydrogen upake when compared to the non-interpenetrated PCN-6.16 Similarly, an increase in both surface area and CO2 capacity is observed in Cu-TATB-n MOFs synthesized by sonochemical method; the catenated Cu-TATB60 considered to be isostructural to PCN-6 shows a high CO2 adsorption capacity (189 mg/g) when compared to Cu-TATB-30 (156 mg/g) at 298 K and 1 bar.20 However, Prasad et al. observed a decrease in BET surface area for the interpenetrated SNU-71’(5290 m2/g) when compared to noninterpenetrated SNU-70’ (1770 m2/g), while a greater gas adsorption capacities for CO2 at 298 K is observed in SNU-71’ (1.05 mmol/g) than in SNU-70’(0.79 mmol/g).21 Yao et al. reported three interpenetrated MOFs composed of Zn4O clusters and rigid dicarboxylate anions namely SUMOF-n (Su=Stockholm University and n = 2, 3, 4) with different pore size and pore volume. SUMOF-2 is similar to interpenetrated MOF-5 which showed a significantly higher CO2 adsorption capacity (4.26 mmol/g) than the noninterpenetrated MOF-5 (1.50 mmol/g) at 273K and 1 bar.22 By appropriate design of organic building blocks, the degree of interpenetration of MOFs can be controlled. The porosity and sorption properties were analyzed in all the frameworks based on CO2 isotherms at 273 K and found to be different for the catenated and noncatenated structures.23 Recently, Dan-Hardi et al. observed that due to interpenetration a highly flexible and nonporous MIL-88D dried structure is changed to highly rigid structure with accessible three dimensional pore.24 Interpenetration not only increases the uptake of CO2 at low pressure but also enhances the CO2 selectivity over other gases based on sieving effect. For instance, a MOF called UHM-6



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(University of Hamburg Materials) containing unsaturated metal sites and isoreticular to 2-fold interpenetrated framework PMOF-3 (polyhedron based MOF) exhibits high CO2 uptake and selectivity when compared to CH4 and H2 at low pressure.25 Park et al. recently synthesized a multi-functional MOF PCN-124 which is self-interpenetrated and isostructural to PMOF-3. PCN-124 shows a CO2 uptake as high as 5.08 mmol/g at 295 K and 1 bar and adsorption selectivity of CO2/CH4 in the 8-20 range at low pressure.26 Similarly, the effects of both interpenetration and functional group on CO2 and H2 storage have been explored in an aminofunctionalized interpenetrated MOF.11 Bastin et al. reported the separation and removal of CO2 from binary CO2 /N2 and CO2/CH4 as well as ternary CO2/ N2/ CH4 mixtures at fixed bed using an interpenetrated MOF, MOF-508b.27 The effect of catenation on CO2/H2 and CH4/H2 separation is predicted in different isoreticular metal-organic frameworks using Monte Carlo simulations. The results showed that CO2 selectivity in catenated MOFs is much higher than their non-catenated counterparts.28,29 Recently, a partial interpenetrated MOF NOTT-202 exhibit highly selective hysteric sorption of CO2 over other gases at 195 K and 60 mbar.30 Although several studies have been reported to address the effect of functionalized ligand and catenation on CO2 capture in various MOFs,11,28,29,31,32 no similar study has been reported yet for covalent organic frameworks (COFs). COFs represent an emerging class of porous crystalline materials composed of light elements such as C, N, O and B, held together solely by covalent bonds, and forming layered two-dimensional (2D) or three-dimensional (3D) network structures. The synthesis of crystalline COFs is achieved by carefully selecting the building blocks and the reaction conditions that favor fast and reversible covalent bond formation. Many 2D and 3D COFs have been assembled using boronate, borosilicate, imine, hydrazone, and triazine linkages.33-42 Recently, internal functionalization of the boronate 3D COF-102 was reported


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using a new monomer-truncation strategy.43 However, boronate-linked COFs, which represent the most common category of this class of materials, are hydrolytically unstable, and thereby not practical for most CO2 separation applications. On the other hand, imine-linked COFs are significantly more stable, though they are more difficult to obtain in a crystalline form.44 The only example of crystalline imine-linked COF reported to date consists of a 5-fold interpenetrated framework with dia topology.45 This class of COFs appeared to us as an ideal platform for studying CO2 adsorption and separation, and to theoretically evaluate the effect of interpenetration and functionalization on the CO2 affinity and capacity. Interpenetration results in constricted pore size which in turn create high electric gradient and as a result increases the strength the interaction of CO2 with the host framework due to its high quadrupole moment. Here we report the design and theoretical evaluation of diamondoid 3D COFs functionalized with tetrahydrofuran (THF) groups. Specifically, we are interested in understanding the effects of both interpenetration and the presence of the ether functional group on the CO2 capture performance. In addition, we try to understand the relation between CO2 uptake and CO2/N2 selectivity with degree of interpenetration. In this study we targeted COFs with diamondoid topology (dia),46 which can be selfassembled by reversible imine condensation between tetrakisaldehydes as tetrahedral nodes and diamines

as

ditopic

links

(Figure

1).

Specifically,

we

have

selected

tetrakis-4-

formylphenylsilane44 and 3R,4R–diaminotetrahydrofuran47 as our node and link, respectively, which are both synthetically accessible. Furthermore, the THF link was selected based on its favorable interaction with CO2, as predicted by previous calculations in our group.12 To build the frameworks, the tetrahedral nodes were first oriented in space so that their S4 symmetry axes were parallel, as required in a dia framework. Adjacent nodes were allowed to rotate either



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synchronously or asynchronously about their S4 axes, resulting in either conrotatory frameworks, with all the nodes rotated the same way, or disrotatory frameworks, with the nodes alternating their rotation. Another geometric variable was the configuration of the diaminotetrahydrofuran linker, which can adopt two conformations: diaxial and diequatorial. This resulted in four series of structures, with each series consisting of various frameworks with different degrees of interpenetration (from 1-fold to 5-fold). Unit cells for each of the structures were subsequently generated based on the required symmetry of a dia framework, and then the coordinates and the cell parameters of the structures were fully optimized by using FORCITE module of the Material Studio.48 The energies for all structure were calculated and are shown in Table 1. We observed that the energies in the four different series of frameworks tend to decrease and the stability increases with the degree of interpenetration. Comparing the 1-fold structures, the disrotatory framework with the diaxial linker is the lowest in energy. Overall, the lowest energy structure was found to be the 5-fold interpenetrated framework with disrotatory orientation of the nodes and diaxial conformation of the linker. Figure 2 depicts the 1-fold and 5-fold interpenetrated frameworks with disrotatory nodes and diaxial linkers. To further study the effect of interpenetration and functional group on CO2 capacity, we next selected the disrotatory diaxial series of frameworks, 1- to 5-fold interpenetrated, and examined their CO2 uptake, bearing in mind that the 5-fold structure in this series is the most stable among all examined in Table 1. For gas adsorption in various frameworks, the interactions of gas-adsorbent and gas-gas were modeled as a combination of pairwise site-site Lennard-Jones (LJ) and Coulombic potentials

  uij ( r ) = ∑ 4ε αβ α∈i  β∈ j 

σ   αβ   rαβ 

12

  σ αβ  −    rαβ

  

6

 qq + α β  4πε 0 rαβ 


   

(1)

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where ε 0 = 8.8542 x 10-12 C2 N-1 m-2 is the permittivity of the vacuum, σ αβ and ε αβ are the collision diameter and well depth, respectively. The LJ potential parameters of the framework atoms are adopted from Dreiding force field.49 A number of simulation studies have revealed that Dreiding force field can accurately predict gas adsorption in porous materials such as MOFs and COFs.14,50 Atomic partial charges are calculated based on Qeq charge equilibration method.51 These methods have been recently employed for the screening of several hypothetical MOFs for gas storage.52,53 The model for the adsorbates CO2 and N2 are taken from our previous work.12 The adsorption of pure CO2 was simulated by grand canonical Monte Carlo (GCMC) method54 which has been widely used for the simulation of adsorption. The framework atoms are kept frozen during simulation because adsorption involves low-energy equilibrium configurations and the flexibility of framework has a marginal effect, particularly on the adsorption of small gases. The LJ interactions were evaluated with a spherical cutoff equal to half of the simulation box with long-range corrections added; the Coulombic interactions were calculated using the Ewald sum method. The number of trial moves in a typical GCMC simulation was 2 × 107, though additional trial moves were used at high loadings. The first 107 moves were used for equilibration and the subsequent 107 moves for ensemble averages. Five types of trial moves were attempted in GCMC simulation, namely, displacement, rotation, and partial regrowth at a neighboring position, entire regrowth at a new position, and swap with reservoir. Unless otherwise mentioned, the statistical uncertainty was generally smaller than the symbol sizes presented in the figures. The isosteric heat of adsorption at infinite dilution, the free volume for adsorption and the Henry constant were evaluated using the method mentioned in our earlier work.12 We also estimated the accessible surface area of frameworks using a probe molecule with a diameter



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equal to 3.681 Å and the Lennard-Jones parameters for the framework atom are taken from the Dreiding force field. We focus on the disrotatory diaxial series of frameworks with increasing interpenetration from 1-fold to 5-fold. According to symmetry, the five frameworks can be divided into two groups: (a) 1-fold, 3-fold, and 5-fold; (b) 2-fold and 4-fold. Table 2 displays physical characteristics for this series of frameworks. With increasing interpenetration, both the porosity and the accessible surface area decrease in the order 1- > 3- > 5-fold and 2- > 4-fold. In contrast, the isosteric heat and Henry’s constant increase with the order of 1- < 3- < 5-fold and 2- < 4-fold, indicating stronger interaction between CO2 and the framework as the pore narrows with increasing interpenetration. The diameter of the channel along the z-direction is calculated using the HOLE program55 and shown in Figure 3. Both 1-fold and 2-fold frameworks contain threedimensional channels, whereas 3-, 4- and 5-fold have one-dimensional channels running along the z-direction due to the packing of the framework atoms with increasing interpenetration. Figure 4 shows the adsorption isotherms of CO2 at 298 K in different interpenetrated imine-linked covalent organic frameworks. Consistent with the heat of the adsorption as shown in Table 2, the adsorption isotherm follows the order 5- > 3- >1-fold and 4- > 2-fold. The CO2 capacity in 4-fold is greater than in 5-fold at 1 bar and this is due to the greater free volume (see Table 2) available in the 4-fold structure. Given its energetic stability (Table 1), we focus our attention on the 5-fold structure’s CO2 adsorption capacity. At 1 bar, the CO2 capacity reaches around 4.0 mmol/g in the 5-fold framework, greater than those predicted earlier in many other 3D COFs.56 Table S1 in the supporting information shows the comparison of CO2 capacity predicted in this work with some of the interpenetrated MOFs reported in the literature. One can see that the CO2 adsorption capacities in our designed COFs are among the highest.


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Since CO2 in the flue gas is usually at a partial pressure below 0.15 bar, we tried to correlate the low-pressure capacity of CO2 adsorption to the framework property. Figure 5a shows the correlation between CO2 uptakes at 0.1 bar with the heat of adsorption for the five frameworks in Table 2. The CO2 uptake increases roughly exponentially as a function of heat of adsorption, indicating the need to further elevate the heat of adsorption for adsorbing more CO2. If one plots the CO2 uptakes at 0.1 bar against the pore size (Figure 5b), one can see that the amount of CO2 adsorbed increases greatly once the pore size decreases below 10 Å and narrows down further. Of course, as the interpenetration increases, the pore size decreases and helps adsorbing more CO2, but the roles of the electrostatic versus the van der Walls interaction and the THF group in the linker are convoluted in this process. To reveal those roles, we next examined the effects of the framework charges and the THF group below. By switching off the electrostatic interactions between the CO2 molecules and the frameworks, we estimated the contribution of framework charges on CO2 capacity in different interpenetrated structures. We calculated the contribution of framework charges to the total uptakes as (N with-elec – N without-elec)/ N with-elec, where N with-elec and N without-elec denotes the amount adsorbed in the presence and absence of electrostatic interaction between the CO2 molecules and the frameworks. From Figure 6, it can be seen that the contribution of framework charges to CO2 uptake increases in the order 1- < 3- < 5-fold, consistent with the decreasing pore size. Similarly, this kind of behavior was found in MOFs with the same topologies and decreasing pore size.57 However, it is interesting to see from Figure 6 that the contribution of framework charges to CO2 uptake is higher in the 2-fold and 4-fold frameworks when compared to the 5-fold framework, even though the pore size is smaller in the latter. To better understand this observation, we plotted the radial distribution functions between CO2 and various atoms in the frameworks.



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Figure 7 shows the radial distribution functions between CO2 and various atoms at 10 kPa in 2fold and 5-fold interpenetrated frameworks; it can be seen that CO2 is predominantly interacting with the ether-oxygen when compared to other atoms in both the frameworks. This is primarily due to the electrostatic interaction between CO2 and the negative oxygen present in the framework. In the 2-fold framework (Figure 7a), the CO2 is located in such a way that it interacts with two ether groups at a distance of ~ 2.85 Å whereas in the 5-fold framework the CO2 is located at a distance of ~ 3.2 Å (Figure 7b). A similar phenomenon was observed in the 4-fold framework. Thus, the presence of strong interactions between the CO2 and the ether groups in both 2- and 4-fold frameworks causes a greater increase in the contribution of framework charges to CO2 uptake when compared to the other frameworks. We also plotted the center-of-mass (COM) probability distributions of CO2 in different interpenetrated covalent-organic frameworks at 10 kPa. As shown in Figure 8a, the CO2 is distributed near the tetrahedral nodes interacting with multiple phenyl rings in case of 1-fold framework. In the 2-fold one, the CO2 molecules are preferentially located near the region where it interacts with two ether-oxygen group (Figure 8b). Similarly in the 4-fold framework, the CO2 molecules are preferentially located near the ether-oxygen group and also in the one-dimensional pores (Figure 8d). In the 3-fold framework, the CO2 molecules are adsorbed in the onedimensional pores (Figure 8c) and similarly in the 5-fold framework (Figure 8e). In order to better understand which factor plays a dominant role in the CO2 uptake, either interpenetration or functional group, additional simulations were conducted by replacing the ether-oxygen in the 5-fold and 1-fold frameworks with the CH2 group to test the CO2 capacity of the resulting non-functionalized frameworks. Figure 9 shows the adsorption isotherm for CO2 in the 1-fold and 5-fold frameworks with the ether-oxygen and CH2 groups, and the percentage


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reduction in CO2 uptake by replacing the ether-oxygen with CH2 group in the 5-fold framework. As seen from Figure 9, the CO2 uptake remains almost the same in the 1-fold framework upon substitution of the ether with the CH2 group. This is because the 1-fold framework is highly porous and its density of ether functional groups is very low. This causes the interaction strength of CO2 with the two frameworks to remain almost the same. In direct contrast, in the 5-fold framework the CO2 capacity is decreased when the ether-oxygen is replaced with CH2 group (Figure 9). The percentage reduction in CO2 uptake is about 24 % at low pressure and finally reaches ~ 32 % at 1 bar. We also calculated the heat of adsorption in the 5-fold framework with the CH2 group and found to be ~ 22 kJ/mol, which is slightly less than the value found in the analogous framework with the ether group (Table 2). Thus, the presence of narrow pore size and high density of ether-groups results in the increase of CO2 capacity in the 5-fold compared to the 1-fold framework. Separation of CO2/N2 at low pressure particularly in post-combustion capture is of industrial importance. So, the adsorption selectivity of CO2/N2 is next predicted in different interpenetrated imine-linked covalent organic frameworks. Figure 10 shows the adsorption selectivity for CO2/N2 mixtures with a bulk composition of 15:85 at 298 K in various interpenetrated covalent organic frameworks with disrotatory nodes and diaxial linkers. Consistent with the adsorption isotherm shown in Figure 4, the adsorption selectivity follows the order 5- > 3- >1-fold and 4- > 2-fold. The selectivity remains almost constant in all the interpenetrated framework except 5-fold, where a slight decrease in selectivity is observed at low pressure. The higher selectivity predicted in the 5-fold framework is mainly due to the presence of narrow pore size which in turn increases CO2 adsorption with large quadruple moment compared to N2. The selectivity of CO2/N2 in 5-fold is around 15 at 1 bar which is higher than



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those reported in interpenetrated MOFs such as MOF-508b (4)27, NOTT-202a (4.25)30 and slightly lower than those observed in CuTATB-60 (~20).20 In summary, we designed several interpenetrated and THF-functionalized covalent organic frameworks with the diamondoid topology. Their CO2 loading capacities are predicted using GCMC simulations. Among the different interpenetrated frameworks, the 5-fold disrotatory diaxial covalent-organic framework shows the highest stability, the greatest heat of CO2 adsorption, and the greatest adsorption capacity for CO2 and CO2/N2 at low pressure, due to the presence of smaller pore size and stronger interaction with the framework. The polar THF group also plays a significant role, as replacing the ether–oxygen with the –CH2 group shows a decrease in CO2 capacity of ~32 % in the 5-fold framework. This study reveals that the presence of narrow pore size and high density of functional group can increase the CO2 capacity particularly at low pressure which is of industrial importance for post-combustion CO2 capture.

Acknowledgment. This work was sponsored by the Laboratory Directed Research and Development Program at Oak Ridge National Laboratory, managed by UT-Battelle, LLC for the U.S. Department of Energy. B.P.H. was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy.

Supporting Information: Comparison of CO2 uptake in selected interpenetrated and noninterpenetrated MOFs with the present results. This information is available free of charge via the Internet at http://pubs.acs.org/.


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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


Table 1. Energies based on molecular mechanics optimization, and space groups of different interpenetrated structures from dis- and con-rotatory tetrahedral nodes and axial and equatorial linkers.

Dis-rotatory Cell formula

n-Fold

Axial

Equatorial

(kcal/mol)

(kcal/mol)

Space group

C144H128N16O8Si4

1-fold

-52.1923

-12.4029

I-42d

C72H64N8O4Si2

2-fold

-53.6294

-79.406

P-4n2

C144H128N16O8Si4

3-fold

-125.0007

-111.525

I-42d

C72H64N8O4Si2

4-fold

-125.0682

–a

P-4b2

C144H128N16O8Si4

5-fold

-149.3363

–a

I-42d

Con-rotatory Cell formula

n-Fold

Axial

Equatorial

(kcal/mol)

(kcal/mol)

Space group

C144H128N16O8Si4

1-fold

-43.8592

–b

I41

C72H64N8O4Si2

2-fold

-45.9070

-93.881

P42

C144H128N16O8Si4

3-fold

-

-133.5376

I41

C72H64N8O4Si2

4-fold

-126.7280

–a

P4

C144H128N16O8Si4

5-fold

-130.6059

–a

Pc

a

Maximum degree of interpenetration possible for this type of framework is 3

b

Structure converted to the axial analogue upon optimization



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Table 2. Framework density ρf, free volume Vfree, porosity Ø, accessible surface area Asurf, heat of adsorption q st0 and Henry’s constant KH at infinite dilution calculated for the disrotatory diaxial series of frameworks.

ρf Space group

Vfree (cm3/g)

Ø

Asurf (m2/g)

q st0 (kJ/mol)

KH (mmol/g/kPa)

n-fold

(g/cm3)

I-42d

1-fold

0.138

6.59

0.91

8524

9.71

0.009

P-4n2

2-fold

0.254

3.31

0.84

8275

14.20

0.012

I-42d

3-fold

0.760

1.09

0.43

1176

24.19

0.034

P-4b2

4-fold

0.633

0.57

0.52

2325

18.19

0.019

I-42d

5-fold

0.809

0.44

0.36

807

25.09

0.052


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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


Figure 1. Schematic representation of COF self-assembly from the tetrakis-4-formylphenylsilane node and the 3R,4R–diaminotetrahydrofuran link. The link can take two conformations: diaxial and diequatorial. Color code: C, gray; O, red; N, blue; H, white.

diaxial link

diequatorial link



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

Figure 2. A Schematic view of a) 1-fold and b) 5-fold interpenetrated framework with disrotatory nodes and diaxial linkers. Color code: a) C, gray; O, red; N, blue; H, white, and yellow; Si. b) Space filling model showing the 5-fold interpenetration, with the five frameworks displayed in green, purple, blue, yellow, and red.

(a)

(b)


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Page 21 of 29

Figure 3. Diameter of the channels along the z-direction in various interpenetrated covalent organic frameworks with disrotatory nodes and diaxial linkers.

25 20 Diameter (Å)

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


1-fold

15 2-fold

10 4-fold

5

3-fold 5-fold

0 10

20

30

40

50

Coordinate (Å)


60


Figure 4. Adsorption isotherms of CO2 at 298 K and low pressure in various interpenetrated covalent organic frameworks with disrotatory nodes and diaxial linkers.

5 5-fold 4-fold 3-fold 2-fold 1-fold

4

Loading (mmol/g)

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

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3

2

1

0 0

20

40

60

80

P (kPa)


100

Page 23 of 29

Figure 5. Correlation between CO2 adsorption at 0.1 bar and 298 K with (a) heat of adsorption and (b) minimum pore diameter in different interpenetrated covalent organic frameworks with disrotatory nodes and diaxial linkers. Points are the simulation data and the lines are the fit.

0.6

0.6

(a) Loading (mmol/g)

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


(b)

0.5

0.5

0.4

0.4

0.3

0.3

0.2

0.2

0.1

0.1

0.0

0.0 8

10

12

14

16

18

20

qst ( kJ/mol)

22

24

26

4

6

8

10

12

14

pore size (Å)


16

18

20

22


Figure 6. Contribution of framework charges to CO2 adsorption at 298 K and low pressure in various interpenetrated covalent organic frameworks with disrotatory nodes and diaxial linkers.

Contribution of framework charges (%)

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

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50 4-fold

40

2-fold

30

5-fold

20

3-fold

10

1-fold

0 0

20

40

60

80

P (kPa)


100

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Figure 7. Radial distribution functions between CO2 and various atoms in a) 2-fold and b) 5-fold interpenetrated covalent organic frameworks with disrotatory nodes and diaxial linkers at 10 kPa respectively.

18 16

4 Si - CO2

(a)

(b)

N - CO2

Si - CO2 N - CO2

O - CO2

14

3

O - CO2

12 10

g (r)

g (r)

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


8

2

6 1

4 2

0

0 2

4

6

8

10

12

14

2

4

r(Å)

6

8

r(Å)


10

12

14


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

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Figure 8. Density distributions of CO2 in a) 1-fold, b) 2-fold, c) 3-fold, d) 4-fold and e) 5-fold interpenetrated covalent organic frameworks with disrotatory nodes and diaxial linkers at 10 kPa.

(a)

(b)

(d)

(c)

(e)


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Figure 9. Adsorption isotherms of CO2 at 298 K and low pressure by replacing the ether oxygen with CH2 group in 1-fold and 5-fold interpenetrated covalent organic frameworks with disrotatory nodes and diaxial linkers.

5 5-fold 5-fold_CH2

4

Loading (mmol/g)

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


1-fold 1-fold_CH2

3

2

1

0 0

20

40

60

80

P (kPa)


100


Figure 10. Adsorption selectivity of CO2/N2 mixtures at 298 K in various interpenetrated covalent organic frameworks with disrotatory nodes and diaxial linkers.

20

15 Selectivity CO2/N2

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

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5-fold 4-fold 3-fold 2-fold 1-fold

10

5

0 20

40

60

80

P (kPa)


100

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For Table of Contents Use Only

Computer-Aided Design of Interpenetrated Tetrahydrofuran-Functionalized 3D Covalent Organic Frameworks for CO2 Capture

Ravichandar Babarao, Radu Custelcean, Benjamin P. Hay, and De-en Jiang

TABLE OF CONTENT GRAPHICS node

+

link

Synopsis Interpenetrated imine-linked 3D covalent-organic frameworks with diamondoid structures were designed

from

tetrakis-4-formylphenylsilane

as

the

tetrahedral

node

and

3R,4R–

diaminotetrahydrofuran as the link. Adsorption capacity of CO2 was predicted from grand canonical Monte Carlo simulations for different degrees of interpenetration. The 4-fold interpenetrated framework displayed highest adsorption capacity (~4.6 mmol/g) at 298 K and 1bar due to greater interaction with CO2.


Computer-Aided Design of Interpenetrated ... - ACS Publications

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