Metal Organic Frameworks Grafted by Univariate and Multivariate

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Materials and Interfaces

Metal Organic Frameworks Grafted by Univariate and Multivariate Heterocycles for Enhancing CO2 Capture: A Molecular Simulation Study Chenkai Gu, Jing Liu, Jianbo Hu, and Weizhou Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04950 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 21, 2019

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Industrial & Engineering Chemistry Research

Metal Organic Frameworks Grafted by Univariate and Multivariate Heterocycles for Enhancing CO2 Capture: A Molecular Simulation Study Chenkai Gu†, Jing Liu*,†, Jianbo Hu† and Weizhou Wang‡ †

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering,

Huazhong University of Science and Technology, Wuhan, 430074, China ‡

Henan Key Laboratory of Function-Oriented Porous Materials, College of Chemistry and

Chemical Engineering, Luoyang Normal University, Luoyang, 471934, China

ABSTRACT: The metal organic frameworks (MOFs) comprising heterocyclic ligands show great potentials in terms of CO2 capture. In this work, a series of heterocycles characterized by N, O and S elements were screened through comparing CO2/heterocycle binding energies calculated by ab-initio method. The thiophene, furan and triazole were finally selected and the corresponding CO2 interaction energy decompositions were further analyzed using symmetry-adapted perturbation theory (SAPT). The selected heterocycles were grafted to UiO-67 and then GCMC simulations were systematically carried out to evaluate their contributions on CO2 adsorption properties of MOFs. The force field parameters for heteroatoms were obtained by quantum mechanical method. Among these three heterocycle-grafted MOFs, Triazole@UiO-67 shows the best performance in CO2 adsorption process and the volumetric uptake amount is over 4 times higher than the original UiO-67 at 298 K and 100 kPa. This can be mainly attributed to the enhanced electrostatic force by grafting triazole. In order to evaluate the effects of multivariate heterocycles on CO2 1

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adsorption properties of MOFs, the thiazole (characterized by S and N) and oxazole (characterized by O and N) were grafted to UiO-67 and studied by GCMC method. The volumetric CO2 uptake amount in Oxazole@UiO-67 is nearly 5 times higher than the parent UiO-67 at 298 K and 100 kPa.

 INTRODUCTION Carbon dioxide (CO2) emission, which mainly stems from the combustion of coal, oil and natural gas, is one of the most important causes of global warming.1-3 Carbon capture and storage (CCS) technology, which can capture CO2 from many existing emission sources, is considered as an appealing approach to solve this problem.4,5 So far, many techniques have been developed, including aqueous alkanolamine solutions, enzymatic conversion, membrane separation, and sorbent adsorption.6,7 Compared to other strategies, CO2 capture using porous materials has the advantages of large separation capability, low energy consumption, and low capital cost.8-10 Metal organic frameworks (MOFs), as an emerging new class of porous solid material, have attracted remarkable attention in the field of CO2 capture owing to their high porosities, regular structures and adjustable functionalities.11-13 For most MOF materials, physisorption interactions play dominant roles in CO2 adsorption process, which will help to decrease heat consumption in gas desorption and adsorbent regeneration step.14 Working capacity is one of the basic criterions in terms of evaluating the CO2 adsorption capacity in industry processes including temperature swing adsorption (TSA), pressure swing adsorption (PSA) and vacuum pressure swing adsorption (VPSA). In order to enhance working capacity of MOFs in CO2 2


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capture, many strategies have been carried out, including creating strong metal binding sites15-17 and incorporating functional groups18-20. The construction of strong adsorption sites, like coordinatively unsaturated metal (CUM) sites, can profoundly enhance the CO2 uptake amount of MOF materials.21-23 However, it was found that water or solvent molecules would firstly occupy the unsaturated metal sites during synthesis procedure, and MOF structure was usually destroyed upon the removal of these molecules.24,25 In this regard, decorating organic linkers with functional groups or heteroatoms may be a good alternative. Here, MOFs show moderate interactions with solvent and gas molecules, which can benefit the material activation process as well as gas desorption process. The UiO type MOFs including UiO-66, UiO-67 and UiO-68 showed outstanding chemical and thermal stabilities and thus have been widely used to investigate the CO2 adsorption properties of MOFs in recent years.26,27 Regarding functional groups, Cmarik et al.28 reported that the UiO-66 functionalized by amino showed higher CO2 adsorption capacity than nitro, methoxy and naphthyl. Regarding heteroatom, sulfur was incorporated to a new type of Zr (IV) named Zr-BTDC, which showed larger CO2 sorption capacity than the corresponding heteroatom-free UiO-67.29 It should be noted that many aromatic heterocycles show larger CO2 affinity than benzene ring, and thus the incorporation of heterocycles can improve the CO2 sorption capacity of MOFs. So far, the CO2 adsorption in nitrogen-rich MOF materials has been extensively investigated and has been confirmed to be effective.30,31 However, the detailed interaction mechanisms of CO2 molecules with sulfur and oxygen heterocycles were still unknown. In addition, to the best of the authors’ knowledge, no attempts were made to investigate the effects of multivariate heterocycles on CO2 adsorption 3



capacities in MOFs. In this work, ab-initio calculations and GCMC simulations were combined to explore the effect of heterocycles on CO2 capture in MOFs. Various kinds of heterocycles, including both five-membered and six-membered heterocycles, were systematically investigated to screen for heterocycles with great CO2 affinities by comparing the binding energies of ab-initio calculations. The SAPT energy decompositions were then carried out to compare the contributions of different energy components in CO2 interaction systems of three typical heterocycles: thiophene, furan and triazole (characterized by S, O and N, respectively). Furthermore, in order to understand the effects of multivariate heterocycles on CO2 adsorption properties of MOFs, the thiazole (characterized by S and N) and oxazole (characterized by O and N) were grafted to UiO-67 and compared with the univariate heterocycles.

 MODEL AND METHOD MOF Structures. In UiO series, UiO-67 and UiO-68 are more suitable for grafting heterocycles owing to their larger pore size than UiO-66. Therefore, UiO-67 and UiO-68 were used as parent MOF materials in this work. The periodic structures were constructed from their corresponding experimental XRD data32. The heterocycles, including thiophene, furan, triazole, thiazole and oxazole, were screened out and grafted to UiO-67 or UiO-68. The newly created MOFs were named as Thiophene@UiO-67, Furan@UiO-67, Triazole@UiO-67, Thiazole@UiO-67, Oxazole@UiO-67 and Oxazole@UiO-68, respectively. Then, the periodic structure optimizations were carried out with the Forcite code.33 After considering all the 4


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orientations and positions of the grafted heterocycles, the most suitable orientations and positions were used to better reflect the CO2 adsorption in MOFs. The models of organic linkers and MOF crystal structures are given in Figure 1, and the structural parameters of these MOFs are listed in Table 1. Details of Ab-Initio Calculations. The MP2 method34 has shown its advantage in handling dispersion interactions which play a significant role in CO2-heterocycle systems.35 A basis set of 6-31++G(d,p) was chosen in order to include the effects of diffusion and polarization functions on split-valence basis sets.36 Therefore, the MP2/6-31++G(d,p) method was used to optimize the heterocycle-CO2 interacting systems. Vibrational frequency analysis was performed to confirm that the converged results were local minima. The interactions between CO2 and various heterocycles were evaluated by calculating the corresponding interaction energies. All the possible conformations, including ‘‘heaping’’ and ‘‘coplane’’ conformations, were considered during optimization processes. The basis set superposition error (BSSE)37 was applied to correct the results of interaction energies. The ab-initio calculations were performed using Gaussian 09 program package.38 The values for the interaction energies were calculated from:

Eint eraction  E  AB    E  A   E  B    EBSSE

(1)

where the E(AB) is the total energy of the adsorbate/substrate system in the equilibrium state, E(A) is the total energy of the adsorbate, E(B) is the total energy of the substrate, and ∆EBSSE represents the basis set superposition error (BSSE) energy of the system. The interactions between CO2 molecules and heterocycles derive from both dispersion

force and electrostatic force. After geometry optimizations, energy composition analysis of 5



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the binding energies were carried out employing the conventional symmetry-adapted perturbation theory (SAPT)39 at the MP2 level with the basis set 6-31++G(d,p). The components of binding energy include the electrostatic energy (Ees), the exchange repulsion energy (Eexch), the effective dispersion energy including the dispersion-induced exchange energy (Edisp* = Edisp + Edisp-exch), and the effective induction energy including the induction-induced exchange energy (Eind* = Eind + Eind-exch). The SAPT calculations were carried out using Q-Chem computational suites.40 Force Field. The non-bond interactions of CO2-CO2 and CO2-MOF consist of the Lennard-Jones (L-J) potentials and the electrostatic potentials, calculated by

  ij U ij  rij   4 ij   rij 

12

   ij      rij

  

6

 qq  i j  4 0 rij 

(2)

where  0 is the vacuum permitivity constant, and rij represents the atomic distance.  ij and  ij are the repulsion distance and the potential well depth between atom i and j, respectively.

qi and q j are atomic charges of atom i and j, respectively. CO2 is treated by a three site rigid molecule model. The C-O bond length is 1.16 Å and the bond angle is 180°. The partial charges of C and O are qO = 0.70 e and qC = -0.35 e, which describe the quadrupole moment of CO2 molecule.41,42 The L-J parameters for CO2 were taken from the TraPPE force field.43 Dreding force field, which has been widely used to calculate the gas adsorption amount in various MOFs, was applied to describe the gas-adsorbent and gas-gas interactions.44,45 Besides, L-J parameters of Zr were obtained from the universal force field (UFF).46 To better describe the CO2 adsorption process in these MOF materials, the L-J parameters of heteroatoms were determined by fitting classical potential to ab initio data based on MP2 method, as shown in Figure S1. All L-J 6


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parameters in Table S1 were mixed by Lorentz−Berthelot rules47:

 ij   i   j  / 2 ,  ij   i j

(3)

For all GCMC simulations, the atomic ChelpG charges of MOF structures were required by DFT calculations using Guassian 09 package.48 The DFT calculations were carried out on fragmental clusters derived from UiO-67, UiO-68 and their heterocycle-grafted forms with a level of B3LYP. In particular, the LANL2DZ basis set was used for Zr atoms, and 6-31G+(d) was used for the other atoms. The atomic charges were presented in Figure S2-S9, and Table S2-S9.

Details of GCMC Simulation. In order to evaluate the CO2 adsorption amount in UiO-67 grafted by the heterocycle, the grand canonical Monte Carlo (GCMC) simulations were carried out with pressure ranging from 0 to 10 bar. To decrease the boundary effect, the periodic boundary conditions were adopted in three dimensions. The box containing 8 (2 × 2 × 2) unit cells of MOFs was built with all edge length greater than twice the cutoff radius. The framework was managed as rigid with atoms frozen at their crystallographic positions. The non-bond interactions were handled with a cutoff radius of 12.8 Å, and the long range electrostatic interactions were calculated using Ewald summation technique with “tin foil” boundary conditions. For each state point of GCMC simulation, 1×107 steps were used to guarantee equilibration, followed by 1×107 steps to sample the required thermodynamics properties. The GCMC calculations were performed using the Sorption code in Materials Studio 2017.33,49 The values of adsorption isosteric heat were calculated by the following equantion:

Qst  RT 

U sf N  U sf N2  N

N N



U ff N  U ff N2  N

N N

(4)

where N represents the amount of adsorbed molecules, R equals to the gas constant, and < > represents the ensemble average. The first term of this formula represents molecular thermal 7



energy. The second and third terms represent the contributions from adsorbent-adsorbate interaction energy Usf and adsorbate-adsorbate interaction energy Uff , respectively.

 RESULTS AND DISCUSSIONS Interactions between Various Heterocycles and CO2 Molecules. Ab initio calculations were performed in order to evaluate the interactions between various heterocycles and CO2 molecules. The results are given in Figure 2 and Table 2. Subscripts ‘‘-h’’ and ‘‘-c’’ represent ‘‘heaping’’ and ‘‘coplane’’ conformations, respectively. The benzene, pyrrole, and thiophene have no ‘‘coplane’’ conformations with CO2 molecules. For triazole, 1,2,4-triazine and 1,3,5-triazine, there are no ‘‘heaping’’ conformations. In these cases, the CO2 molecules finally moved to the ‘‘coplane’’ adsorption sites after geometry optimizations although they were firstly placed at ‘‘heaping’’ sites. Compared with benzene-CO2 interaction system (BE = -5.42 kJ/mol), the binding energies of heterocycle-CO2 in ‘‘heaping’’ conformations showed fluctuations. Some are marginally lower, such as pyrimidine-h (-4.38 kJ/mol), tetrazole-h (-4.70 kJ/mol), pyridine-h (-4.24 kJ/mol) and pyrazine-h (-4.95 kJ/mol). Others are higher including pyrrole-h (-7.11 kJ/mol), furan-h (-5.59 kJ/mol), thiophene-h (-5.65kJ/mol), imidazole-h (-9.07 kJ/mol), pyridazine-h (-8.75 kJ/mol) and 1,2,3-triazine-h (-9.07 kJ/mol). Generally, the ‘‘coplane’’ conformations are much more stable than their “heaping” conformations when interacting with CO2. In CO2-oxazole system, the CO2 interaction with oxazole was stronger at the site around N atom than the site around O atom, which was confirmed by the higher binding energy of 11.79 kJ/mol. In CO2-thiazole system, the CO2 molecule cannot stabilize at the site around S atom. Therefore, the CO2 binding site of 8


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thiazole was less than oxazole. Among these heterocycles, triazole showed the largest CO2 affinity with a binding energy of -15.22 kJ/mol which is nearly 3 times higher than benzene. Therefore, triazole was used to represent N-containing heterocycles in the following work. To further understand the CO2-interactions of univariate and multivariate heterocycles, SAPT calculations were performed on thiophene, furan, triazole, thiazole and oxazole which are characterized by S, O, N, S&N and O&N, respectively. The detailed interaction energy decompositions are summarized in Table 3. Generally, for ‘‘heaping’’ conformation, the

dispersion force and electrostatic interactions made contributions to CO2 adsorption with the same magnitude. While for ‘‘coplane’’ conformation, the contributions mainly came from electrostatic interactions. In conformations of furan, the CO2 molecules have two possible interaction structures. Compared to the ‘‘heaping’’ conformation (Ees = -10.27 kJ/mol), the

electrostatic interactions in ‘‘coplane’’ conformation increased to -19.24 kJ/mol, and played a more dominant role. In the ‘‘heaping’’ conformations of furan and thiophene, the values of each SAPT energy component showed no significant difference to benzene. This resulted in their similar total interaction energies with benzene. Among all heterocycles studied in this work, the triazole shows the largest binding energy with CO2 molecules. Through SAPT energy decomposition analysis, this can be mainly attributed to the rapidly-increased electrostatic interactions (Ees = -32.23 kJ/mol), as well as the higher induction energy (Eind* = -4.4 kJ/mol) of CO2 with triazole. CO2 Adsorption in UiO-67 Grafted by Univariate Heterocycles. The above ab-initio calculations screened out three heterocycles named thiophene, furan and triazole which were characterized by S, O and N, respectively. To further evaluate their contributions on CO2 9



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uptakes of MOFs, GCMC simulations were carried out to heterocycle-grafted UiO-67. Firstly, the force field and the method for calculating atomic charges were confirmed by comparing the simulated CO2 adsorption isotherm with the available experiment data.[29] As shown in Figure 3, the simulated CO2 isotherm was in good agreement with the experimental data, which indicated that the force field parameters as well as the atomic charges were reliable to describe CO2 adsorption processes. Besides, in order to improve the accuracy of results, the force field parameters for heteroatoms (including S, O and N) were obtained by ab-initio method, which has been successfully performed to obtain force field parameters in previous work.50-52 Our developed force fields reproduce the ab-initio energies well, as shown in

Figure S1, which indicates that the ab-initio derived force is suitable for simulations of CO2 adsorption in MOFs.

Figure 4 shows the simulated excess CO2 adsorption isotherms in UiO-67, Triazole@UiO-67, Furan@UiO-67 and Thiophene@UiO-67 at 298 K. By grafting different heterocycles to the organic linkers of UiO-67, the CO2 adsorption capacities were profoundly improved especially at low pressure. Specifically, the triazole showed the greatest positive impact on CO2 adsorption of MOFs. At 100 kPa, the CO2 uptake amount of Triazole@UiO-67 was 113.4 cm3(STP)/cm3, which was over 4 times higher than that of the original UiO-67 (27.5 cm3(STP)/cm3). This value was much higher than MOF-177 (4.0 cm3(STP)/cm3)53 and IRMOF-1 (9.5 cm3(STP)/cm3)54, comparable to HKUST-1 (109.0 cm3(STP)/cm3)55,

Bio-MOF-11

(110.8

cm3(STP)/cm3)56

and

MAF-X25

(129.1

cm3(STP)/cm3)57, less than Mg-MOF-74 (160.0 cm3(STP)/cm3)58 at the same conditions. The adsorption amount in Furan@UiO-67 was 91.9 cm3(STP)/cm3, indicating that the CO2 10


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adsorption capacity of Furan@UiO-67 was nearly 3.5 times higher than UiO-67. This data was also comparable to mmen-Cu-BTTri with a volumetric CO2 uptake amount of 99.4 (cm3(STP)/cm3).59 At the whole pressure range (0 - 1000 kPa), the rank of CO2 uptake amounts was Triazole@UiO-67 > Furan@UiO-67 > Thiophene@UiO-67. At 1000 kPa, the adsorption amounts of Triazole@UiO-67, Furan@UiO-67, Thiophene@UiO-67 at 298 K were 182.0 cm3(STP)/cm3, 167.3 cm3(STP)/cm3, and 142.8 cm3(STP)/cm3, respectively. Figure 5 shows the isosteric heats with CO2 uptake amount in UiO-67 grafted by different heterocycles. For Triazole@UiO-67, Furan@UiO-67 and Thiophene@UiO-67, the isosteric heats at initial dilution were 37.34 kJ/mol, 35.18 kJ/mol, and 32.12 kJ/mol, respectively. As CO2 adsorption amount increased, the Qst declined firstly and then increased again. In comparison to UiO-67 (Qst = 19.93 kJ/mol), the Qst of CO2 was profoundly improved in post-grafted MOFs. This means that the affinities of CO2 to MOFs were significantly improved by grafting these heterocycles, resulting in higher CO2 adsorption amounts. The underlying reasons can be summarized as following: (i) The interactions between CO2 molecules and organic linkers were considerably enhanced owing to the stronger CO2-heterocycle dispersion force or electrostatic interactions; (ii) the incorporations of heterocycles result in smaller pore size, which can further strengthen the interactions of CO2 with the frameworks. In addition, the order of Qst was Triazole@UiO-67 > Furan@UiO-67 > Thiophene@UiO-67 > UiO-67 which was consistent with the sequence of CO2 uptakes at low pressure, because Qst played a dominant role at low pressure. At high pressure, pore volume and suface area played more significant role and thus the CO2 adsorption amount of UiO-67 showed a trend of exceeding Triazole@UiO-67, 11



Furan@UiO-67 and Thiophene@UiO-67 with increasing pressure. In order to directly illustrate CO2 adsorption in MOFs, the center of mass (COM) probability distributions of CO2 at different pressures were given in Figure 6. For UiO-67, the CO2 molecules were mainly adsorbed in the tetrahedral cages because of its smaller pore size than octahedral cage, which has been observed in UiO-66(Zr) as well.32,60 After grafting by heterocycles, some pore volume in UiO-67 was occupied by heterocycles, resulting in smaller pore size of octahedral cage. Consequently, the CO2 molecules were adsorbed not only in tetrahedral cages but also in octahedral cages of grafted UiO-67 even at low pressure. To compare the contribution of electrostatic and dispersion forces to CO2 adsorption, the corresponding proportions at 10 kPa were calculated and given in Figure 7. The contribution of dispersion force equals to N nonelec / N total 100% and the contribution of electrostatic interaction is defined as (1  N nonelec / N total ) 100% . Nnonelec is the adsorption amount with all atomic charges of zero. Ntotal represents the total adsorption amount under both dispersion and electrostatic interactions. For CO2 adsorption in UiO-67, the dispersion force played a dominant role, with a percentage of 70%. With the grafting of heterocycles, the contributions of dispersion force declined, and the contributions of electrostatic interactions increased. In comparison to UiO-67, the contributions of dispersion force in Thiophene@UiO-67 and Furan@UiO-67 were much smaller, only accounting for 57% and 53%, respectively. Especially, for Triazole@UiO-67, the electrostatic interactions played a leading role (about 70%) in CO2 adsorption process, contrary to the case of UiO-67. That is to say, the improvement of CO2 uptakes in Triazole@UiO-67 can be mainly attributed to electrostatic interactions. This can also be supported by the results of SAPT interaction energy 12


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decompositions. CO2 Adsorption in UiO-67 Grafted by Multivariate Heterocycles. The above GCMC simulations investigated the CO2 adsorption properties in three univariate heterocycles characterized by S, O and N, respectively. In order to further understand the effects of multivariate heterocycles on CO2 adsorption properties of MOFs, the thiazole (characterized by S and N) and oxazole (characterized by O and N) were grafted to UiO-67 and compared with univariate heterocycles. Figure 8 shows the calculated CO2 adsorption isotherms of UiO-67 grafted by univariate as well as the multivariate heterocycles at 298 K. At 10 kPa, the sequence of CO2 uptakes was UiO-67 < Thiophene@UiO-67 ≈ Thiazole@UiO-67 < Furan@UiO-67 < Oxazole@UiO-67 < Triazole@UiO-67, as shown in Figure 8a. The Oxazole@UiO-67 has lower CO2 uptakes than Triazole @UiO-67, which is in agreement with the lower CO2 binding energy of oxazole than triazole in ab-initio calculations. Moreover, the polarizations of Furan@UiO-67 framework is higher than Thiazole@UiO-67 ( as shown in Figure S4 and Table S4) which would lead to more CO2 adsorption in Furan@UiO-67. As pressure increased, the CO2 uptake of Oxazole@UiO-67 exceeded that of Triazole@UiO-67. Besides, the CO2 uptake amount of Thiazole@UiO-67 (characterized by S and N) became larger than Thiophene@UiO-67(only characterized by S), with an increase of 9.1 cm3(STP)/cm3 at 100 kPa. Among all heterocycle-grafted MOFs in this work, the Oxazole@UiO-67 showed the greatest CO2 affinity with a CO2 uptake amount of 120.7 cm3(STP)/cm3 at 100 kPa. This value was nearly 5 times higher than that of UiO-67 (27.5 cm3(STP)/cm3). At 1000 kPa, the CO2 uptakes in Oxazole@UiO-67 was still the largest, reaching up to 192.3 cm3(STP)/cm3, as shown in 13



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Figure 8b. Compared with nitrogen-doped porous aromatic frameworks,61 this value is higher than that of NPAF-1 (136.5 cm3(STP)/cm3), but lower than NPAF-2 (225.2 cm3(STP)/cm3). The rank of heterocycle-grafted UiO-67 at 1000 kPa was Thiophene@UiO-67 < Thiazole@UiO-67 < Furan@UiO-67 < Triazole@UiO-67 < Oxazole@UiO-67, in line with the trend of accessible areas of these MOF materials. In order to evaluate the effect of temperature on CO2 capture, the adsorption amount at 273 K, 298 K, 313 K and 333 K at 100 kPa and 1000 kPa was calculated. The results are presented in Figure 9. From Figure 9a, it can be observed that the CO2 uptakes at low pressure were significantly enhanced by grafting heterocycles. The CO2 molecules were not packed closely in MOFs at 100 kPa and thus the enhancements in Figure 9a can be mainly attributed to the interactions between CO2 molecules and the frameworks of grafted UiO-67. As shown in Figure 9b, when the pressure increased to 1000 kPa, the sequence of CO2 uptakes was Thiophene@UiO-67 < Thiazole@UiO-67 < Furan@UiO-67 < Triazole@UiO-67 < Oxazole@UiO-67 < UiO-67 at low temperature. However, at higher temperature, the order was

UiO-67

Metal Organic Frameworks Grafted by Univariate and Multivariate

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