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Theoretical Investigations of CO and H Sorption in Robust Molecular Porous Materials Tony Pham, Katherine A. Forrest, KaiJie Chen, Amrit Kumar, Michael J. Zaworotko, and Brian Space Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03161 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 22, 2016

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Theoretical Investigations of CO2 and H2 Sorption in Robust Molecular Porous Materials Tony Pham,†,§ Katherine A. Forrest,†,§ Kai-Jie Chen,‡ Amrit Kumar,‡ Michael J. Zaworotko,‡ and Brian Space∗,† † Department of Chemistry, University of South Florida, 4202 East Fowler Avenue, CHE205, Tampa, FL 33620-5250, United States ‡ Department of Chemical and Environmental Sciences, University of Limerick, Limerick, Republic of Ireland ABSTRACT: Molecular simulations of CO2 and H2 sorption were performed in MPM-1-Cl and MPM-1-TIFSIX, two robust molecular porous materials (MPMs) with the empirical formula [Cu2 (adenine)4 Cl2 ]Cl2 and [Cu2 (adenine)4 (TiF6 )2 ], respectively. Recent experimental studies have shown that MPM-1-TIFSIX displayed higher CO2 uptake and isosteric heat of adsorption (Qst ) than MPM-1-Cl [Nugent, P. S.; et al. J. Am. Chem. Soc. 2013, 135, 10950–10953.]. This was verified through the simulations executed herein, as the presented simulated CO2 sorption isotherms and Qst values are in very good agreement with the corresponding experimental data for both MPMs. We also report experimental H2 sorption data in both MPMs. Experimental studies revealed that MPM-1-TIFSIX exhibits high H2 uptake at low loadings and an initial H2 Qst value of 9.1 kJ mol−1 . This H2 Qst value is greater than that for a number of existing metal–organic frameworks (MOFs) and represents the highest yet reported for a MPM. The remarkable H2 sorption properties for MPM-1-TIFSIX have been confirmed through our simulations. The modeling studies revealed that only one principal sorption site is present for CO2 and H2 in MPM-1-Cl, which is sorption onto the Cl− counterions within the large channels. In contrast, three different sorption sites were discovered for both CO2 and H2 in MPM-1-TIFSIX: (1) between two TIFSIX groups within a small passage connecting the large channels, (2) onto the TIFSIX ions lining the large channels, and (3) within the small channels. This study illustrates the detailed insights that molecular simulations can provide on the CO2 and H2 sorption mechanism in MPMs.

I.

INTRODUCTION

Although fossil fuels represent the predominant source of worldwide energy production, a major drawback to the use of fossil fuels is the release of greenhouse gases into the atmosphere, most notably CO2 . It is estimated that the CO2 concentration in the atmosphere will be nearly double that of pre-industrial levels by 2050.1 In addition, fossil fuels are a finite energy source that will eventually be exhausted. Potential solutions for stabilizing and reducing CO2 concentrations are separating CO2 from combustion emissions or capturing CO2 directly from the atmosphere. However, current methods to CO2 capture and sequestration, e.g., amine scrubbing,2 are proven to be highly cost effective. As such, a cheap and easily implemented method for the selective capture of CO2 is critical to reduce the environmental impact of fossil fuel utilization. While the removal of CO2 from combustion emissions or the atmosphere is a promising avenue for investigation, shifting the energy source to a cleaner energy carrier is a viable alternative. Molecular H2 is a highly attractive candidate for this purpose since it produces a large amount of energy upon combustion3 and it is abundant in nature (obtained from processes such as the electrolysis of water or steam methane reforming).4 In addition, the combustion of H2 releases only H2 O as the chemical product. However, major complications exist that currently prevents widespread use of H2 for practical applications. For instance, a large volume of the gas is required for storage under near-ambient conditions. At 1 atm, H2 liquefies at 20 K5 and such conditions are unfeasible for most energy-related applications. Moreover, due to the highly flammable nature of H2 , implementation of the gas as an energy carrier requires the creation of a safe and compact storage medium. Metal–organic frameworks (MOFs) are a class of porous crystalline materials that have been shown to be promising for CO2 capture and sequestration6 and H2 storage.7 As materials that are constructed from metal ions and organic

ligands,8 MOFs are capable of sorbing a considerable amount of CO2 and H2 molecules within the pores and have the ability to release the sorbates facilely through changes in thermodynamics conditions. The tunable nature of MOFs allows for the rational design of different structures with variable CO2 and H2 sorption properties. For instance, MOFs that are synthesized to include open-metal sites have been shown to display high affinity for CO2 and H2 .6,7 In contrast to MOFs, molecular porous materials (MPMs) represent a class of molecular solids in which the structure is held together through weak non-covalent interactions, such as hydrogen bonding.9 As a result, the structures of MPMs are typically less stable than those of MOFs. Because the building blocks of molecular solids tend to pack more densely than in MOFs, the synthesis of MPMs with permanent porosity consitutes a greater challenge than in the case of their organic–inorganic counterpart. In addition, the removal of guest molecules within a molecular solid could result in a collapse of the framework.10 Previous experimental studies have shown that MPMs can be synthesized to exhibit permanent porosity and high stability.9,11–26 MPMs can exhibit either intrinsic or extrinsic porosity, defined as porosity within and between the molecules, respectively.20,24 MPM-1-Cl is an example of a MPM that displays extrinsic porosity. This MPM has the empirical formula [Cu2 (adenine)4 Cl2 ]Cl2 and was first synthesized and reported in reference 21. MPM-1-Cl represented the first example of a robust MPM that contains adenine-containing complexes that are connected together through hydrogen bonding interactions. In this material, four adenine linkers combine with two Cu2+ ions to form the well-known dinuclear copper paddlewheel cluster. Next, a Cl− ion is coordinated to the axial position of each Cu2+ ion of the paddlewheel to create saturated metal centers (SMCs). Further, a Cl− counterion is hydrogen-bonded to two adenine groups of two different [Cu2 (adenine)4 Cl2 ]2+ clusters. The paddlewheel complexes self-assemble into an extrinisically porous hydrogen-bonding network as shown in Figure 1(a). The net

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ence 24 revealed negligible differences between the two sets of data for the considered loading range at both 273 and 298 K (see Supporting Information, Figure S1(a)). For the present study, however, we recollected the CO2 sorption isotherms for MPM-1-TIFSIX such that the measured isotherms include more data points at low loadings. This was essential to extract more accurate CO2 Qst values at such loadings through empirical fitting as explained below. As shown in Figure 3(a), MPM-1-TIFSIX sorbs more CO2 than MPM-1-Cl for all pressures considered at both temperatures according to experimental measurements. Indeed, at 1 atm, MPM-1-TIFSIX exhibits an experimental CO2 uptake of roughly 6.00 and 3.87 mmol g−1 at 273 and 298 K, respectively. Further, it can be observed that MPM-1-TIFSIX displays notably higher CO2 uptake than MPM-1-Cl at low pressures (< 0.1 atm) at the respective temperatures. This indicates that the sorption sites for CO2 at low loadings are more preferred in MPM-1-TIFSIX than in MPM-1-Cl. The simulations executed herein in MPM-1-TIFSIX generated CO2 uptakes that are in outstanding agreement with experiment for pressures up to 0.1 atm at both temperatures. This indicates accurate modeling of CO2 at the primary binding site with proper sorption energetics in this MPM. For simulations of CO2 sorption in MPM-1-TIFSIX at 298 K, very good agreement with experiment was obtained for the uptakes within the entire loading range considered. The simulations predict a CO2 uptake of 4.09 mmol g−1 in MPM-1-TIFSIX at 298 K and 1 atm, which is close to the aforementioned experimental value at this state point. For simulations of CO2 sorption in this MPM at 273 K, however, notable deviations from experiment can be observed for the uptakes starting at 0.2 atm. Thus, although the simulations captured excellent agreement with experiment for the CO2 uptakes for pressures up to 0.1 atm at 273 K, the simulations somewhat oversorb experiment at higher pressures at this temperature. The simulations generated a CO2 uptake of 6.43 mmol g−1 at 273 K and 1 atm. In addition, the largest difference in the CO2 uptakes between experiment and simulation at 273 K and a given pressure is about 0.72 mmol g−1 (observed at 0.4 atm). These results suggest that the simulations were able to produce more sorption onto the secondary and weaker sites in this MPM compared to what is captured physically in experimental conditions. This also implies that the energetic model for sorption onto the secondary and tertiary sites, while reasonable compared with experiment, is less excellent than that observed for the primary site, which saturates at pressures below 0.1 atm at 273 K. This generates a moderately greater concavity in the initial loading region for these weaker sorption sites, with the difference between experiment and simulation narrowing toward similar values after 0.4 atm. As with CO2 sorption in MPM-1-Cl, the maximum calculated error for simulations of CO2 sorption in MPM-1-TIFSIX was ±0.09 mmol g−1 . Overall, the simulations confirmed the experimental finding that MPM-1-TIFSIX sorbs more CO2 than MPM-1-Cl for the state points considered.

2.

Isosteric Heats of Adsorption

The simulated CO2 Qst values for MPM-1-Cl and MPM-1-TIFSIX are compared with the corresponding experimental values for a range of loadings in Figure 3(b). As

with the experimental CO2 sorption isotherms, the experimental Qst values for MPM-1-Cl were taken from reference 24, while those for MPM-1-TIFSIX were newly determined in this work. The experimental Qst plot for MPM1-Cl was determined by applying the virial method48 to the measured CO2 sorption isotherms at 273 and 298 K.24 In this work, the experimental Qst values for MPM-1-TIFSIX were obtained by directly applying the Clausius–Clapeyron equation49 to the experimental isotherms for the MPM at both temperatures. In contrast to experiment, the theoretical values were determined through a statistical mechanical expression that relates the Qst to fluctuations in the particle number and total potential energy in GCMC simulation.50 The experimental CO2 Qst values for MPM-1-Cl are nearly constant at about 23.8 kJ mol−1 for the considered loading range. The simulated CO2 Qst plot for this MPM shows a similar trend, with values that are near experiment for all loadings considered. The roughly flat shape of the experimental and simulated Qst plots for MPM-1-Cl indicates that there is only one type of CO2 binding site in this MPM. Note, it can be observed that the experimental and simulated CO2 Qst plot for MPM-1-Cl shows slightly increasing Qst as the CO2 uptake increases. This could be due to favorable sorbate–sorbate interactions in the MPM at higher loadings, a phenomenon that has been observed previously in a HUM.51 On the other hand, the shape of the experimental Qst plot for MPM-1-TIFSIX suggests the presence of multiple CO2 binding sites in this MPM; this was verified from the simulation studies as explained later. The experimental CO2 Qst values for MPM-1-TIFSIX are also significantly higher than those for MPM-1-Cl at low loadings. Indeed, the experimental Qst data for both MPMs indicate that the primary CO2 sorption site in MPM-1TIFSIX is exceptionally stronger than that in MPM-1-Cl. Note, the experimental Qst values shown for MPM-1TIFSIX in Figure 3(b) were derived from the isotherms that were collected in this work for the MPM. When comparing this experimental Qst plot to that shown for MPM1-TIFSIX in reference 24, the formers contains higher Qst values at low loadings (see Supporting Information, Figure S1(b)). On the basis of the CO2 sorption data collected in this study for MPM-1-TIFSIX, the experimental initial CO2 Qst was determined to be 47.2 kJ mol−1 through empirical fitting, which is greater than the corresponding value that was determined for this MPM in previous work (44.4 kJ mol−1 ).24 An initial CO2 Qst value of 50.1 kJ mol−1 was calculated for MPM-1-TIFSIX from the simulations performed herein. To the best of our knowledge, such initial CO2 Qst values remain the highest yet reported for a MPM.16–18 The simulated CO2 Qst plot for MPM-1-TIFSIX displays a noticeable inverse sigmoidal shape, which indicates that the energetics for the primary CO2 sorption site in this MPM is much larger than those for the subsequent sorption sites. The simulated Qst values are nearly constant at about 50 kJ mol−1 until a loading of 0.5 mmol g−1 is reached. Afterward, the Qst values decreases readily to a value of roughly 32 kJ mol−1 at 1 mmol g−1 loading. The simulated Qst values decrease further to approximately 28 kJ mol−1 at 1.5 mmol g−1 and stays near this quantity for the remainder of the considered loading range. When comparing the simulated CO2 Qst plot for MPM1-TIFSIX to that determined in reference 24, the latter did not exhibit the inverse sigmoidal shape that the simulations


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until 0.95 atm at this temperature. The notable overestimation of the theoretical H2 uptakes compared to experiment at 77 K and pressures ≥ 0.2 atm suggest that there are more H2 molecules sorbing onto the secondary and tertiary sites in MPM-1-TIFSIX in simulation compared to what is obtained in experimental measurements. The maximum calculated error for simulations of H2 sorption in MPM-1TIFSIX was ±0.08 mmol g−1 .

2.

Isosteric Heats of Adsorption

Applying the virial method48 to the experimental H2 sorption isotherms that were measured for MPM-1-Cl in this work resulted in an initial H2 Qst value of only 3.7 kJ mol−1 (see Supporting Information, Figure S3(a)). Thus, the experimental isotherms that were collected herein for MPM1-Cl as well as the empirical fitting method that was employed to extract the Qst suggest that the H2 Qst for the MPM is lower than that for activated carbons.55 This value is not expected to be representative of the actual H2 Qst for MPM-1-Cl, especially since we should account for proper diffusion of H2 into the material in experimental measurements in order to make conclusions about the experimental Qst . GCMC simulations of H2 sorption in MPM-1-Cl suggest that the Qst values, as calculated using fluctuation theory,50 are around 6 kJ mol−1 for all loadings considered, with an initial H2 Qst value of 6.3 kJ mol−1 (see Supporting Information, Figure S16(b)). The experimental H2 Qst values that were determined for MPM-1-TIFSIX through empirical fitting of the measured H2 sorption isotherms at 77 and 87 K are presented in Figure 7(b). The virial method was used to determine the H2 Qst for both MPMs because it appears to be the most widely used method to obtain the experimental H2 Qst in porous materials.7 The experimental initial H2 Qst value for MPM-1-TIFSIX was determined to be 9.1 kJ mol−1 . This value is impressive for a material that is connected together through weak hydrogen bonding interactions and possesses SMCs. To the best of our knowledge, this quantity for the H2 adsorption enthalpy is the highest yet reported for a MPM.9 Further, this initial H2 Qst is higher than that for a number of existing MOFs, especially those that contain open-metal sites.7 This suggests that there is a sorption site present in MPM-1-TIFSIX that is more energetically favorable for the H2 molecules than most types of open-metal sites, e.g., the exposed Cu2+ ions of the copper paddlewheel units as observed in HKUST-1.34–39 In addition, the aforementioned initial H2 Qst value for MPM-1-TIFSIX is comparable to that for Zn-MOF-74 (8.8 kJ mol−1 ),56 Y-FTZB (9.2 kJ mol−1 ),33 and rht-MOF-1 (9.5 kJ mol−1 ),57 three well-known MOFs that contain open-metal sites. Further, this H2 Qst value is slightly higher than that for HUMs that consist of two-dimensional square grids connected together through SiF6 2− (SIFSIX) pillars,29,31 particularly SIFSIX2-Cu-i (8.7 kJ mol−1 ) and SIFSIX-3-Ni (8.3 kJ mol−1 ) (see Supporting Information, Figure S26). Unlike what was observed for CO2 sorption, the experimental H2 Qst plot for MPM-1-TIFSIX does not display an inverse sigmoidal shape. This suggests that the different H2 sorption sites in the MPM have similar energetics. The simulated H2 Qst values that were produced herein for MPM-1-TIFSIX shows a slight inverse sigmoidal shape (Figure 7(b)), which also implies that the energetics associ-

ated with H2 sorbing onto the primary and succeeding sites are relatively close to each other (since there is not a big drop in the Qst after the inflection). It can be observed that the theoretical Qst values are in excellent agreement with experiment for loadings up to 5 mmol g−1 . This is consistent with the observed remarkable agreement between experiment and theory for the H2 sorption isotherms for loadings up to 5 mmol g−1 (Figure 7(a)). On the other hand, the simulated H2 Qst values overestimate experiment for loadings beyond 5 mmol g−1 , which is in accordance with the notable oversorption seen for the simulated sorption isotherms compared with experiment past this loading. The initial H2 Qst value was calculated to be 8.8 kJ mol−1 , which is close to that determined through empirical fitting. In essence, the simulations confirmed that the zerocoverage Qst value for H2 in MPM-1-TIFSIX is particularly high for a MPM. While the experimental H2 Qst drops to 5.6 kJ mol−1 at 9 mmol g−1 loading, the simulated Qst values fall to only 6.8 kJ mol−1 near this loading.

3.

Sorption Sites

Although MPM-1-Cl displays negligible uptake of H2 at 77 and 87 K under experimental conditions, our GCMC simulations show that the MPM can sorb a sensible amount of H2 within the pores at the state points considered, assuming that are no hindrances to diffusion (see Supporting Information, Figure S16(a)). As with CO2 sorption, the H2 molecules can only enter the large channels of MPM-1-Cl. Only one distinct sorption site was observed for H2 in this MPM and that is sorption onto the Cl− counterions (those hydrogen-bonded to two different adenine linkers) within the large channels in the structure (Figure 8). The H2 molecule is aligned such that a positively charged H atom of the sorbate can interact with the electronegative Cl− ions. This sorption site corresponds to a H2 Qst value that is near 6 kJ mol−1 according to our theoretical calculations (see Supporting Information, Figure S16(b)). As in the case of CO2 sorption, the Cl− ions that are coordinated to the Cu2+ ions of the copper paddlewheels are not well-defined sites for H2 according to our simulations. This is possibly due to the lower calculated partial negative charge for this ion compared to the Cl− ion that is hydrogen-bonded to two different adenine linkers (see Supporting Information, Table S3). Although it is slightly noticeable, the shape of the simulated H2 Qst plots suggests that there are multiple H2 sorption sites in MPM-1-TIFSIX (Figure 7(b)). It was observed from the simulations that the H2 molecules sorb initially into the small passages that connect the large channels in the structure (site 1) (Figure 9(a)). This site corresponds to the initial H2 Qst value for MPM-1-TIFSIX. This finding was confirmed through canonical Monte Carlo (CMC) simulation studies for a single H2 molecule within the MPM-1-TIFSIX system cell. In this region, the H2 molecules can interact with the equatorial fluorine atoms of the two surrounding TIFSIX groups concurrently. The energetics for H2 sorbed in this confined region is greater than or comparable to that for sorption onto an open-metal site. As with CO2 sorption, the H2 molecules sorb onto all the primary binding sites first before filling the less favorable sorption sites in the MPM. At about 0.5 mmol g−1 loading, all sites within the confined passages are occupied with H2 . The simulated Qst plot for H2 in MPM-1-TIFSIX shows


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served for simulations of H2 sorption in MOFs that contain open-metal sites.52–54,59

IV.

CONCLUSION

In summary, simulations of CO2 and H2 were performed in two robust MPMs, MPM-1-Cl and MPM-1-TIFSIX. In contrast to most types of MOFs, the structure of these materials are connected through weak hydrogen bonding interactions and do not contain open-metal sites. The difference between the two MPMs is simply the axial ligand that is used in the construction of the Kagomé lattice. The utilization of different axial ligands have resulted in distinct gas sorption properties between MPM-1-Cl and MPM-1-TIFSIX as demonstrated through experimental measurements. Such experimental observations have been confirmed through the theoretical studies presented herein. For example, the simulated CO2 sorption isotherms and Qst values for MPM-1Cl and MPM-1-TIFSIX were in very good agreement with the corresponding experimental measurements. The simulations therefore verified the experimental finding that MPM1-TIFSIX exhibits higher CO2 uptake and adsorption enthalpy than MPM-1-Cl. MPM-1-TIFSIX was shown to display exceptional H2 sorption characteristics as revealed by our experimental measurements and molecular simulations. In particular, both experimental and theoretical studies have shown that the MPM can sorb a considerable amount of H2 at 77 and 87 K and pressures lower than 0.1 atm. Moreover, the experimental initial H2 Qst value for MPM-1-TIFSIX was determined to be 9.1 kJ mol −1 , which is close to that calculated through GCMC simulations of H2 sorption in this MPM (8.8 kJ mol−1 ). The experimental zero-loading Qst value for H2 in MPM-1-TIFSIX is greater than that for a number of existing MOFs, especially those that contain open-metal sites, such as HKUST-134–39 and PCN-14.60 The differences in the gas sorption properties between MPM-1-Cl and MPM-1-TIFSIX can be attributed to the distinct structural features for the two MPMs, which are direct consequences of the nature of the ligand used to synthesize the respective materials. In MPM-1-Cl, only the large channels are accessible for the sorbates molecules. According to our simulations, the CO2 and H2 molecules sorbed mainly onto the Cl− counterions that decorate the large hexagonal channels in the structure of MPM-1-Cl. In MPM-1-TIFSIX, both the confined passages adjoining the large channels as well as the small trigonal channels are accessible for the sorbate molecules as a result of using a relatively bulky TIFSIX ligand in the synthesis of the MPM. The primary binding site for both CO2 and H2 in MPM-1TIFSIX is located within the small passages that connect the large channels, where the sorbate molecules can interact with the equatorial fluorine atoms in a constricted area. The secondary binding site for both sorbates corresponds to sorption onto the TIFSIX anions that line the large channels in the structure; this site is analogous to the principle binding site that was observed in MPM-1-Cl for the respective sorbates. Sorption of CO2 and H2 was also observed within the small channels of MPM-1-TIFSIX, which represents the tertiary binding site for both sorbates in this MPM. MPMs could represent a promising alternative to traditional MOFs for gas sorption and separation applications. Like MOFs, MPMs can be modular as various structures

can be synthesized or envisioned by incorporating different types of organic molecules or ligands. Thus, the rational design of a family of MPMs (i.e., platforms) is possible, as demonstrated in the context of MPM-1-X (X = axial ligand).21,24,26 MPMs with extrinsic porosity have the potential to display remarkable gas sorption properties as shown in the case of MPM-1-TIFSIX. MPMs can also be constructed to exhibit high thermal and water stability, which is needed for practical applications in gas sorption and separation. Indeed, according to experimental measurements, MPM-1-Cl and MPM-1-TIFSIX have been shown to display high thermal stability.24 In addition, experimenal studies revealed that MPM-1-TIFSIX retains its crystallinity after exposure to water for 24 hours. The presence of the accessible confined passages connecting the large channels and the small channels in MPM-1TIFSIX suggests for the potential utility of this material in molecular sieving applications. This phenomenon will likely be investigated through future experimental and theoretical studies. We will also attempt to explore the effects of utilizing other different axial ligands on the gas sorption properties in the MPM-1-X platform through such studies. ASSOCIATED CONTENT

Supporting Information. Details of experimental procedures, parametrization, electronic structure calculations, and grand canonical Monte Carlo methods, pictures of MPM fragments, tables of properties, and additional simulation results. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Author ∗ E-mail: [email protected] Author Contributions § Authors contributed equally Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS

We thank Patrick S. Nugent for providing us with the raw data for the experimental CO2 sorption isotherms for MPM-1-Cl and MPM-1-TIFSIX as reported in reference 24. B.S. acknowledges the National Science Foundation (Award No. CHE-1152362), including support from the Major Research Instrumentation Program (Award No. CHE1531590), the computational resources that were made available by a XSEDE Grant (No. TG-DMR090028), and the use of the services provided by Research Computing at the University of South Florida.


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