Investigating H2 Sorption in a ... - ACS Publications

Subscriber access provided by NEW YORK UNIV

Article 2

Investigating H Sorption in a Fluorinated Metal-Organic Framework with Small Pores Through Molecular Simulation and Inelastic Neutron Scattering Katherine A. Forrest, Tony Pham, Peter A Georgiev, Florian Pinzan, Christian R. Cioce, Tobias Unruh, Juergen Eckert, and Brian Space Langmuir, Just Accepted Manuscript • Publication Date (Web): 17 Jun 2015 Downloaded from http://pubs.acs.org on June 18, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 10

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

Langmuir

Investigating H2 Sorption in a Fluorinated Metal–Organic Framework with Small Pores Through Molecular Simulation and Inelastic Neutron Scattering Katherine A. Forrest,†,§ Tony Pham,†,§ Peter A. Georgiev,k,⊥ Florian Pinzan,‡ Christian R. Cioce,† Tobias Unruh,◦ Juergen Eckert,† and Brian Space∗,† † Department of Chemistry, University of South Florida, 4202 East Fowler Avenue, CHE205, Tampa, FL 33620-5250, United States k Department of Structural Chemistry, University of Milan, 21 Via G. Venezian, I-20133 Milan, Italy ⊥ Faculty of Chemistry and Pharmacy, University of Sofia, 1 James Bourchier Blvd., Sofia 1164, Bulgaria ‡ UFR Sciences et Technologies, Université Blaise Pascal, Clermont-Ferrand, 24 Avenue des Landais, 63170 Aubière, France ◦ Physik Department, Friedrich-Alexander-Universit¨ at Erlangen-N¨ urnberg, Staudtstrasse 3, 91058 Erlangen, Germany ABSTRACT: Simulations of H2 sorption were performed in a metal–organic framework (MOF) consisting of Zn2+ ions coordinated to 1,2,4-triazole and tetrafluoroterephthalate ligands (denoted [Zn(trz)(tftph)] in this work). The simulated H2 sorption isotherms reported in this work are consistent with the experimental data for the state points considered. The experimental H2 isosteric heat of adsorption (Qst ) values for this MOF are approximately 8.0 kJ mol−1 for the considered loading range, which is in the proximity of those determined from simulation. The experimental inelastic neutron scattering (INS) spectra for H2 in [Zn(trz)(tftph)] reveal at least two peaks that occur at low energies, which corresponds to high barriers to rotation for the respective sites. The most favorable sorption site in the MOF was identified from the simulations as sorption in the vicinity of a metal–coordinated H2 O molecule, an exposed fluorine atom, and a carboxylate oxygen atom in a confined region in the framework. Secondary sorption was observed between the fluorine atoms of adjacent tetrafluoroterephthalate ligands. The H2 molecule at the primary sorption site in [Zn(trz)(tftph)] exhibits a rotational barrier that exceeds that for most neutral MOFs with openmetal sites according to an empirical phenomenological model and this was further validated by calculating the rotational potential energy surface for H2 at this site. I.

INTRODUCTION

Molecular H2 remains perhaps the most promising alternative energy carrier to replace petroleum-based gasoline and diesel fuels for vehicular applications in this energy economy.1 The use of H2 in very promising fuel cells generates a large amount of energy (ca. 120 kJ g−1 )2 and releases only H2 O and no greenhouse gases into the atmosphere. H2 is also straightforward to obtain as it can be produced from steam reforming of natural gas3 or electrolysis of water.4 However, one of the challenges to use H2 as a fuel source is that it interacts weakly with its environment under nearambient conditions, thus making the transport of neat H2 difficult because there is no extant practical dense hydrogen storage medium. The construction of porous materials such as metal– organic frameworks (MOFs) is one of the most promising routes to achieve a suitable storage mechanism for molecular H2 .5–8 MOFs are crystalline compounds that are synthesized from metal ions and organic ligands.9–12 They are three-dimensional materials containing pores and channels that can be used to sorb and store H2 . MOFs are highly tunable as a number of different structures can be made by changing the metal ion and/or ligand.13 In addition, H2 sorption in MOFs is based on physisorption, where the interaction energy between the H2 molecule and the framework is rather weak, especially compared to that for a chemical bond. The previously described features allow MOFs to have advantages over traditional materials for H2 storage,

such as zeolites, activated carbons, and metal hydrides. A H2 adsorption enthalpy of 15–30 kJ mol−1 has been proposed as aspirational for room temperature H2 storage in porous materials.14,15 The introduction of open-metal sites (or unsaturated metal centers) in MOFs have been shown to be a promising method to increase the adsorption enthalpy for H2 in such materials.16–19 For example, the MOF Ni-DOBDC (also known as CPO-27-Ni or Ni-MOF-74)20 was found to exhibit an initial isosteric heat of adsorption (Qst ) value of 13.0–13.5 kJ mol−1 .18,21 A high Qst value in such MOFs is attributed to the favorable interaction between the H2 molecule and the metal centers. This has been validated through various studies that pinpoint the location of the sorbed H2 molecules in these materials, such as neutron scattering spectroscopy (elastic and inelastic)22–25 and computational modeling.26,27 Tuning the pore sizes in MOFs to allow for optimal interactions between the H2 molecule and the framework is also an encouraging method to increase the Qst for H2 in these materials.28–30 MOFs with small pore sizes often display a higher H2 Qst than those with larger pore sizes because smaller pores permit the H2 molecules to interact with many components of the framework at the same time. The inclusion of polar functionalities in MOFs with small pore sizes would afford an enhancement in the MOF–H2 interaction. An example demonstrating the effect of pore sizes on the Qst for H2 can be observed when comparing SIFSIX2-Cu (or [Cu(dpa)2 SiF6 ]) and its interpenetrated polymorph SIFSIX-2-Cu-i (or [Cu(dpa)2 SiF6 -i]).30,31 Specifically, SIFSIX-2-Cu-i exhibits a H2 Qst of 8.4 kJ mol−1 ,

ACS Paragon Plus Environment

Langmuir

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 ACS Paragon Plus Environment

Page 2 of 10

Page 3 of 10

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

Langmuir 3

(or Cu-BTC).17,39–43 It will be shown that a cooperative interaction from a number of different moieties in a confined region in [Zn(trz)(tftph)] is responsible for the high H2 Qst value in the material as well as the presence of the lowest energy peak in the INS spectra for the MOF. We also calculate the rotational potential energy surface for a H2 molecule sorbed at the primary site in [Zn(trz)(tftph)]. It will be revealed later that the theoretical value is in good agreement with a value that was determined using an empirical phenomenological model.32 Additionally, it will be shown that the rotational barrier for H2 at the most favorable sorption site in [Zn(trz)(tftph)] is among the highest of reported MOF materials based on INS measurements and calculations.

II.

(a)

RESULTS AND DISCUSSION

Simulations of H2 were performed in [Zn(trz)(tftph)] using three different potentials of increasing complexity: single– site van der Waals potential developed by V. Buch (Buch),44 five–site electrostatic (nonpolarizable) potential by Belof et al. (BSS),45 and five–site polarizable potential by Belof et al. (BSSP).45 For simulations using the BSSP model, polarization was included explicitly in the simulations via a TholeApplequist type model.46–48 All simulations were performed in the rigid 2 × 2 × 2 system cell of the MOF as shown in Figure 1. Further, for the simulations of H2 sorption at the temperatures considered in this work, quantum mechanical dispersion effects were included semiclassically through Feynman-Hibbs corrections to the fourth order (see Supporting Information).49 Note, all simulations in [Zn(trz)(tftph)] were performed with a H2 O molecule coordinated to one of the unique Zn2+ ions (see Supporting Information). Experimental studies on [Zn(trz)(tftph)] have shown that these metal–coordinated H2 O molecules are part of the MOF and that the removal of these molecules causes the framework to collapse.33 The simulated H2 sorption isotherms for the three potentials at 77 and 87 K in [Zn(trz)(tftph)] are compared to the matching experimental data in Figure 2(a). All experimental data were estimated from the original experimental reference for this MOF (reference 33). The simulated H2 sorption isotherm for the Buch model in [Zn(trz)(tftph)] is coincident with experiment at 77 K for the entire pressure range considered. This is typically a bad indicator because the model is physically inadequate if the results differ for the more generally applicable and accurate BSS and BSSP models (see below). This does, however, suggest that the H2 sorption observables in this MOF can be captured by considering only van der Waals interactions, at least if the agreement persists at temperatures other than 77 K. Simulations using the BSS and BSSP models generated isotherms that are only slightly higher than the experimental isotherm and the isotherm produced by the Buch model at 77 K, especially at pressures from 0.05 atm (38 mmHg) and above. The slight oversorption for the BSS and BSSP models compared to experiment for nearly all pressures considered at 77 K could be due to the fact that a perfect crystal of the MOF was used for the simulations. Actual synthesized crystals could have minor deformations and such alterations are likely to result in partially blocked pores for MOFs that have very narrow pore sizes. Still, these isotherms for the

(b)

Figure 2. (a) Low-pressure absolute H2 sorption isotherms in [Zn(trz)(tfpth)] for experiment (black) and simulations using the Buch (blue with circles), BSS (green with squares), and BSSP (red with triangles) models at 77 K (solid, open symbols for models) and 87 K (dashed, closed symbols for models). (b) Isosteric heats of adsorption (Qst ) for H2 in [Zn(trz)(tfpth)] plotted against H2 uptakes for experiment (black) and simulations using the Buch (blue with closed circles), BSS (green with closed squares), and BSSP (red with closed triangles) models. The experimental data were estimated from reference 33.

BSS and BSSP models are considered to be representative of the experiment, especially based on the broad success of these hydrogen potentials in MOFs.27,38,50,51 Note, the error bars for all simulated state points considered are extremely small, and thus, they have been omitted for clarity. Consistent with the relative succes of the Buch model, the BSS and BSSP models show that electrostatic interactions have a negligible effect on H2 sorption in this MOF. Indeed, H2 sorption in [Zn(trz)(tftph)] is dominated by van der Waals (repulsion/dispersion) interactions due to the small pores that are present in the framework. Smaller pores would allow the H2 molecules to interact with multiple portions of the framework simultaneously,30 thus causing van der Waals interactions to dominante. This was confirmed by examining the energy decomposition for each model, as it was re-


Langmuir

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

Page 4 of 10 4

vealed that Lennard-Jones interactions contribute to at least 90% of the total energy for the BSS and BSSP models for all state points considered. In addition, the fact that the BSS and BSSP models produced essentially the same isotherm in [Zn(trz)(tftph)] indicates that polarization interactions are insignificant for H2 sorption in this MOF. Although polarization has been shown to be important for simulations of H2 sorption in MOFs that contain open-metal sites,27,38,50,51 these interactions are negligible in MOFs that contain narrow pore sizes and excludes such metal centers. Apparently, the negatively charged constituents like the fluorine atoms in this MOF do not participate in such interactions as well.31 It can be observed in Figure 2(a) that the Buch model undersorbs the experimental isotherm in [Zn(trz)(tftph)] at 87 K for all pressures considered. As for the BSS and BSSP models, these potentials produced isotherms that are in good agreement with experiment at 87 K for pressures up to 0.60 atm (456 mmHg). At higher pressures at 87 K, however, both models undersorb experiment, with the largest difference between experiment and simulation being observed at nearly 1.2 atm (912 mmHg). The isotherms generated by all three models do not capture the shape of the experimental isotherm at 87 K particularly well. The simulated isotherms at 87 K are essentially a scaled version of those generated at 77 K as typically seen in analogous experiments over this relatively cold and narrow temperature range. Such shape differences usually indicate a structural differentiation in either sorbate binding or MOF structure that are not especially likely in this case. The difference in the shape of the 87 K sorption isotherm between experiment and simulation reflects the fact that the H2 uptake is trending toward saturation in the MOF more quickly under experimental conditions at this temperature. Despite this difference, we believe that the simulated results (considering all models) are in reasonable agreement with experiment at 87 K for all pressures considered. Due to the small surface area of [Zn(trz)(tftph)] (Langmuir value is 100 m2 g−1 ),33 the experimental H2 uptake in this MOF can only reach about 43 cm3 g−1 (1.92 mmol g−1 or 0.39 wt %) at 77 K and 1.0 atm. Note, according to our calculations, H2 saturation in the MOF is obtained at approximately 72 cm3 g−1 (3.21 mmol g−1 or 0.64 wt %), which corresponds to 4 H2 /formula unit. However, the sharp increase in the experimental isotherm at low loadings at both 77 and 87 K suggests a high initial H2 Qst value in this MOF. Indeed, as shown in Figure 2(b), the experimental H2 Qst value for [Zn(trz)(tftph)] starts at around 8.0 kJ mol−1 and it decreases slightly with increasing loading where it reaches a value of about 6.5 kJ mol−1 at roughly 30 cm3 g−1 (1.34 mmol g−1 or 0.27 wt %). It will be shown later that this high initial H2 Qst value for [Zn(trz)(tftph)] is attributed to sorption at a highly favorable binding site in the MOF as a result of the small pore sizes. As stated in reference,33 this Qst plot was obtained through a finite difference approximation to the Clausius–Clapeyron equation.52 The calculated Qst values for the three different H2 potentials in [Zn(trz)(tftph)], as determined through GCMC simulation, are shown in Figure 2(b) with the experimental Qst plot. These Qst values were calculated through fluctuations of the sorbate particle number and total potential energy in the MOF–H2 system.53 The Buch model produces Qst values that are in the closest agreement with experiment for the

considered loading range. These values for the Buch model are nearly constant at about 8.0 kJ mol−1 for all loadings considered. The Qst values for the BSS and BSSP models are slightly higher than those for the Buch model within the considered loading range, with values of approximately 8.75 kJ mol−1 for all loadings for both models. Note, both the BSS and BSSP models produce Qst values that are very similar to each other across the loading range in this MOF. This further indicates that polarization interactions contribute minimally to the H2 sorption energetics in this MOF. In addition, this is consistent with what was observed in the simulated isotherms for these two potentials (Figure 2(a)). While the experimental Qst plot shows slightly decreasing values as a function of uptake, the simulated Qst values for all three models are relatively constant for the loading range considered. Thus, the simulations suggest homogeneity in the binding sites for H2 sorption in [Zn(trz)(tftph)], at least for uptakes up to 40 cm3 g−1 . Although this is mostly consistent with the experimental Qst plot when looking at the values at low loadings, a noticeable decrease can be observed in the plot starting at around 22.5 cm3 g−1 . Hence, a different shape is captured in the experimental Qst plot at high loadings. The difference in the shape in the Qst plots between experiment and simulation could be due to the difference in the methodology used to extract the Qst values for the respective techniques. As stated above, the experimental Qst values are calculated through a finite difference approximation to the Clausius–Clapeyron equation, which is dependent on the fitting parameters used to evaluate the required derivatives. In contrast, the simulated Qst values are obtained independently from the isotherm data and directly from GCMC simulation. Nevertheless, we believe that the simulated Qst values in [Zn(trz)(tftph)] are within the vicinity of experiment for all loadings considered and effectively capture the essential physical chemistry. Note, we also simulated H2 sorption in [Zn(trz)(tfpth)] using the widely used potential by Darkrim and Levesque54 at both 77 and 87 K. It was observed that the simulated isotherms generated by this potential more dramatically oversorbed experiment for most of the considered pressure range at both temperatures (see Supporting Information). This model also produced Qst values that are higher than those for any of the models as shown in Figure 2(b). This suggests that this potential is overly attractive for simulations of H2 sorption in MOFs.55 Two distinct H2 sorption sites were identified within [Zn(trz)(tfpth)] from the simulations in this work. The most favorable sorption site in [Zn(trz)(tfpth)] (site 1) corresponded to the sorption of H2 in a small area of the MOF where the sorbate molecule can interact with a number of different (polar) functionalities at the same time. Specifically, the H2 molecule can interact with an exposed fluorine atom of a tftph linker, a carboxylate oxygen atom of another tftph linker, and a H2 O molecule that is coordinated to a Zn2+ ion simultaneously (Figure 3(a)). At this site, one H atom of the H2 molecule interacts with the negatively charged fluorine and carboxylate oxygen atoms while the other H atom interacts with the O atom of the H2 O molecule. This is a highly favorable sorption site in the MOF that corresponds to a high initial H2 Qst value of 8.0 kJ mol−1 in experiment and a Qst value of 8.0–8.75 kJ mol−1 according to our simulations. It will be shown below that this sorption site imposes a sig-


Page 5 of 10

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

Langmuir 5

(a) Site 1

Figure 4. Inelastic neutron scattering (INS) spectra for H2 in [Zn(trz)(tftph)] at different loadings: 1.8 H2 /formula unit (black), 2.7 H2 /formula unit (blue), and 4.3 H2 /formula unit (red). The spectra were collected on the TOFTOF spectrometer at the FRMII with a temperature of 5 K and an incident wavelength of 3.0 ˚ A.

(b) Site 2

Figure 3. Molecular illustration of a sorbed H2 molecule about (a) site 1 and (b) site 2 in [Zn(trz)(tftph)] as determined from simulation. The sorbate molecule is shown in orange. Note, orientational constraints are not imposed on the H2 molecule at the binding sites. Atom colors: C = cyan, H = white, N = blue, O = red, F = pink, Zn = silver.

nificantly high rotational barrier on the sorbed H2 molecule. In this MOF, the H2 molecules can also sorb in the compact region between the fluorine atoms of adjacent tftph ligands (Figure 3(b)). Here, each positively charged H atom of the H2 molecule can interact with an electronegative fluorine atom. This is the secondary sorption site in [Zn(trz)(tfpth)] (site 2) and it has weaker energetics compared to the primary sorption site in the MOF. A single point energy calculation of a saturated system cell with all site 1 H2 molecules (via the BSS or BSSP model) removed revealed that the H2 binding energy associated with the secondary site in this MOF is approximately 7.2 kJ mol−1 . At H2 saturation, the relative distribution of H2 molecules sorbed at site 1 to site 2 is 1:1. The INS spectra for H2 at different loadings in [Zn(trz)(tftph)] are shown in Figure 4. The spectra were collected at the cold neutron time-of-flight spectrometer TOFTOF (at the FRM-II, Munich, Germany) with a temperature of 5 K and an incident wavelength of 3.0 ˚ A. Note, A neutrons in this the INS spectra were also taken with 2.0 ˚ MOF and the results are provided in the Supporting Information. The negative and positive numbers on the x-axis correspond to neutron energy gain and loss, respectively.

The INS spectra for H2 in [Zn(trz)(tftph)] reveal three peaks occurring at approximately 5.4, 6.2, and 7.8 meV. A lower energy rotational tunneling peak in the spectra represents a sorption site that causes a high barrier to rotation on the sorbed H2 , which allows the H2 molecule to exhibit a stronger interaction with the framework. Because the site depicted in Figure 3(a) is the most favorable sorption site within the material, it makes sense that this site corresponds to the 5.4 meV peak in the INS spectra. At the lowest loading measured (1.8 H2 /formula unit), the peak at 6.2 meV is the most prominent of any of the three peaks in the spectra, while the peak at 5.4 meV exists as a low energy shoulder of the 6.2 meV peak. This indicates that, at low loadings, the most energetically favorable site in the MOF is not fully populated first. At a loading of 2.7 H2 /formula unit, the intensity of the 5.4 meV peak increases significantly, while the 6.2 meV peak virtually disappears. In addition, the peak occurring at 7.8 meV becomes more noticeable at this loading. This 7.8 meV peak corresponds to the weakest sorption site in the MOF. In this case, it is the secondary sorption site in [Zn(trz)(tftph)] as shown in Figure 3(b). Although we have identified only two noticeable sorption sites for H2 in [Zn(trz)(tftph)], the INS spectra suggests that there are perhaps three sorption sites in the MOF. We have already assigned the 5.4 and 7.8 meV peaks as the primary and secondary sites in the MOF, respectively. Further calculations and inspection of the modeled structure for H2 in [Zn(trz)(tftph)] reveals that the sorption site that is associated with the 6.2 meV peak does not correspond to a new sorption site outside of those shown in Figure 3, but rather a slightly shifted version of a H2 molecule that is sorbed at the primary site in the MOF. Indeed, it can be deduced in the spectra that the 6.2 meV peak represents a shifted version of the 5.4 meV peak; that is, the peak at 6.2 meV shifts to lower energies to 5.4 meV at higher loadings. The reason for this is because when more H2 molecules enter the MOF,


Langmuir


Page 6 of 10

Page 7 of 10

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

Langmuir 7 III.

CONCLUSION

In summary, simulations of H2 sorption in [Zn(trz)(tftph)] were performed; it is a MOF with small pore sizes containing exposed fluorine and carboxylate oxygen atoms and accessible metal–coordinated H2 O molecules. The simulated H2 sorption isotherms and associated H2 Qst values in this MOF were in good agreement with the matching experimental data. INS studies were also performed for H2 in this MOF and the resulting spectra revealed at least two peaks that occur at low energies, which corresponds to high barriers to rotation for the respective sites. The transition energy for the lowest energy peak in [Zn(trz)(tftph)] (5.4 meV) is lower than that for most neutral MOFs that contain openmetal sites and comparable to that for charged MOFs that contain counterions. The most favorable sorption site identified from the simulations corresponded to sorption into a confined region where the H2 molecule can interact with a coordinated H2 O molecule, the fluorine atom of a tftph ligand, and the carboxylate oxygen atom of another tftph ligand simultaneously. The rotational barrier for H2 sorbed at this site was calculated to be 50.0 meV using a theoretical potential energy surface. The theoretical value is in good agreement to the value that was determined using an empirical phenomenological model, which is 45.8 meV. The rotational barrier for the primary sorption site in [Zn(trz)(tftph)] is notably higher than those for MOFs that contain open-metal sites. Sorption was also observed between the fluorine atoms of two neighboring tftph ligands in [Zn(trz)(tftph)]. This site has weaker energetics compared to those for the primary site, as indicated by the higher transition energy for this site in the INS spectra (7.8 meV). It was shown that the combination of a water molecule, a fluorine atom, and a carboxylate oxygen atom in a highly compact environment gives rise to a high rotational barrier on the sorbed H2 molecule. Thus, it appears that synthesizing MOFs with small pore sizes, coupled with the inclusion of favorable polar functionalities, is a promising method for increasing the sorption energetics for H2 in porous materials. Indeed, the synergistic interaction from the different moieties at the primary sorption site in [Zn(trz)(tftph)] results in a H2 Qst of 8.0 kJ mol−1 and a rotational barrer of close to 50.0 meV. We speculate that the introduction of open-metal sites and/or counterions in this or like MOF systems in some fashion would afford higher Qst values and rotational barriers for the sorbed H2 . [Zn(trz)(tftph)] is one of few MOFs that contain a high Qst for H2 in the absence of open-metal sites. As observed in this work, this is due to the different moieties at the primary sorption site in the MOF as a result of the small pore sizes. Current work is underway to investigate the Qst values and rotational barriers for H2 in other MOFs with small pore sizes through molecular simulation and INS studies. It is also planned to study CO2 sorption in [Zn(trz)(tftph)] by means of experiment and simulation.

tron scattering studies, tables of properties, pictures of the MOF and components, and additional H2 sorption 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

The authors thank Zeric Hulvey for preparing a sample of [Zn(trz)(tftph)] and for his illuminating discussions on the INS spectra. B.S. acknowledges the National Science Foundation (Award No. CHE-1152362), 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. This publication is also based on work supported by Award No. FIC/2010/06, made by King Abdullah University of Science and Technology (KAUST). The authors also thank the Space Foundation (Basic and Applied Research) for partial support. This work is based in part on experiments performed on the TOFTOF instrument operated by FRMII at the Heinz Maier-Leibnitz Zentrum (MLZ), Garching, Germany. P.A.G. acknowledges support from the Project “Beyond Everest” under EU programme REGPOT-2011-1. This research project was also supported by the European Commission under the 7th Framework Programme through the ‘Research Infrastructures’ action of the ‘Capacities’ Programme, NMI3-II Grant No. 283883.

ASSOCIATED CONTENT

Supporting Information. Details of parametrization, grand canonical Monte Carlo methods, and inelastic neu-


Langmuir

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

Page 8 of 10 8

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

Turner, J. A. Sustainable Hydrogen Production. Science 2004, 305, 972–974. Haryanto, A.; Fernando, S.; Murali, N.; Adhikari, S. Current status of hydrogen production techniques by steam reforming of ethanol: a review. Energy Fuels 2005, 19, 2098–2106. Oertel, M.; Schmitz, J.; Weirich, W.; JendryssekNeumann, D.; Schulten, R. Steam reforming of natural gas with intergrated hydrogen separation for hydrogen production. Chem. Eng. Technol. 1987, 10, 248–255. Doenitz, W.; Schmidberger, R.; Steinheil, E.; Streicher, R. Hydrogen production by high temperature electrolysis of water vapour. Int. J. Hydrogen Energy 1980, 5, 55–63. Collins, D. J.; Zhou, H.-C. Hydrogen storage in metal–organic frameworks. J. Mater. Chem. 2007, 17, 3154–3160. Zhao, D.; Yuan, D.; Zhou, H.-C. The current status of hydrogen storage in metal–organic frameworks. Energy Environ. Sci. 2008, 1, 222–235. Collins, D. J.; Ma, S.; Zhou, H.-C. Metal-Organic Frameworks: Design and Application; John Wiley & Sons, Inc.: Hoboken, NJ, 2010; pp 249–266. Suh, M. P.; Park, H. J.; Prasad, T. K.; Lim, D.-W. Hydrogen Storage in Metal–Organic Frameworks. Chem. Rev. 2012, 112, 782–835. Yaghi, O. M.; Li, H.; Davis, C.; Richardson, D.; Groy, T. L. Synthetic strategies, structure patterns, and emerging properties in the chemistry of modular porous solids. Acc. Chem. Res. 1998, 31, 474–484. James, S. L. Metal–organic frameworks. Chem. Soc. Rev. 2003, 32, 276–288. Yaghi, O. M.; O’Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J. Reticular synthesis and the design of new materials. Nature 2003, 423, 705–714. Furukawa, H.; Cordova, K. E.; OKeeffe, M.; Yaghi, O. M. The Chemistry and Applications of Metal–Organic Frameworks. Science 2013, 341, 1230444. Eddaoudi, M.; Eubank, J. F. Metal–Organic Frameworks: Design and Application; John Wiley & Sons, Inc.: Hoboken, NJ, 2010; pp 37–89. Bhatia, S. K.; Myers, A. L. Optimum Conditions for Adsorptive Storage. Langmuir 2006, 22, 1688–1700. Ma, S. Gas adsorption applications of porous metal–organic frameworks. Pure Appl. Chem. 2009, 81, 2235–2251. Chen, B.; Ockwig, N. W.; Millward, A. R.; Contreras, D. S.; Yaghi, O. M. High H2 Adsorption in a Microporous Metal– Organic Framework with Open Metal Sites. Angew. Chem. Int. Ed. 2005, 117, 4823–4827. Rowsell, J. L. C.; Yaghi, O. M. Effects of Functionalization, Catenation, and Variation of the Metal Oxide and Organic Linking Units on the Low-Pressure Hydrogen Adsorption Properties of Metal–Organic Frameworks. J. Am. Chem. Soc. 2006, 128, 1304–1315, PMID: 16433549. Dietzel, P. D. C.; Georgiev, P. A.; Eckert, J.; Blom, R.; Strassle, T.; Unruh, T. Interaction of hydrogen with accessible metal sites in the metal-organic frameworks M2 (dhtp) (CPO27-M; M = Ni, Co, Mg). Chem. Commun. 2010, 46, 4962– 4964. Li, B.; Zhang, Z.; Li, Y.; Yao, K.; Zhu, Y.; Deng, Z.; Yang, F.; Zhou, X.; Li, G.; Wu, H.; Nijem, N.; Chabal, Y. J.; Lai, Z.; Han, Y.; Shi, Z.; Feng, S.; Li, J. Enhanced Binding Affinity, Remarkable Selectivity, and High Capacity of CO2 by Dual Functionalization of a rht-Type Metal–Organic Framework. Angew. Chem. Int. Ed. 2012, 51, 1412–1415. Dietzel, P. D.; Panella, B.; Hirscher, M.; Blom, R.; Fjellv˚ ag, H. Hydrogen adsorption in a nickel based coordination polymer with open metal sites in the cylindrical cavities of the desol-

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

vated framework. Chem. Commun. 2006, 959–961. Vitillo, J. G.; Regli, L.; Chavan, S.; Ricchiardi, G.; Spoto, G.; Dietzel, P. D. C.; Bordiga, S.; Zecchina, A. Role of Exposed Metal Sites in Hydrogen Storage in MOFs. J. Am. Chem. Soc. 2008, 130, 8386–8396. Liu, Y.; Kabbour, H.; Brown, C. M.; Neumann, D. A.; Ahn, C. C. Increasing the Density of Adsorbed Hydrogen with Coordinatively Unsaturated Metal Centers in Metal–Organic Frameworks. Langmuir 2008, 24, 4772–4777. Peterson, V. K.; Liu, Y.; Brown, C. M.; Kepert, C. J. Neutron Powder Diffraction Study of D2 Sorption in Cu3 (1,3,5benzenetricarboxylate)2 . J. Am. Chem. Soc. 2006, 128, 15578–15579. Brown, C. M.; Liu, Y.; Yildirim, T.; Peterson, V. K.; Kepert, C. J. Hydrogen adsorption in HKUST-1: a combined inelastic neutron scattering and first-principles study. Nanotechnology 2009, 20, 204025. Peterson, V. K.; Brown, C. M.; Liu, Y.; Kepert, C. J. Structural Study of D2 within the Trimodal Pore System of a Metal Organic Framework. J. Phys. Chem. C 2011, 115, 8851–8857. Fischer, M.; Kuchta, B.; Firlej, L.; Hoffmann, F.; Fr¨ oba, M. Accurate Prediction of Hydrogen Adsorption in Metal– Organic Frameworks with Unsaturated Metal Sites via a Combined Density-Functional Theory and Molecular Mechanics Approach. J. Phys. Chem. C 2010, 114, 19116–19126. Forrest, K. A.; Pham, T.; McLaughlin, K.; Belof, J. L.; Stern, A. C.; Zaworotko, M. J.; Space, B. Simulation of the Mechanism of Gas Sorption in a Metal–Organic Framework with Open Metal Sites: Molecular Hydrogen in PCN-61. J. Phys. Chem. C 2012, 116, 15538–15549. Kesanli, B.; Cui, Y.; Smith, M. R.; Bittner, E. W.; Bockrath, B. C.; Lin, W. Highly Interpenetrated Metal–Organic Frameworks for Hydrogen Storage. Angew. Chem. Int. Ed. 2005, 44, 72–75. Ma, S.; Sun, D.; Ambrogio, M.; Fillinger, J. A.; Parkin, S.; Zhou, H.-C. Framework-Catenation Isomerism in Metal– Organic Frameworks and Its Impact on Hydrogen Uptake. J. Am. Chem. Soc. 2007, 129, 1858–1859, PMID: 17256862. Nugent, P.; Pham, T.; McLaughlin, K.; Georgiev, P. A.; Lohstroh, W.; Embs, J. P.; Zaworotko, M. J.; Space, B.; Eckert, J. Dramatic effect of pore size reduction on the dynamics of hydrogen adsorbed in metal–organic materials. J. Mater. Chem. A 2014, 2, 13884–13891. Pham, T.; Forrest, K. A.; McLaughlin, K.; Tudor, B.; Nugent, P.; Hogan, A.; Mullen, A.; Cioce, C. R.; Zaworotko, M. J.; Space, B. Theoretical Investigations of CO2 and H2 Sorption in an Interpenetrated Square-Pillared Metal– Organic Material. J. Phys. Chem. C 2013, 117, 9970–9982. Nicol, J. M.; Eckert, J.; Howard, J. Dynamics of molecular hydrogen adsorbed in CoNa-A zeolite. J. Phys. Chem. 1988, 92, 7117–7121. Hulvey, Z.; Falcao, E. H. L.; Eckert, J.; Cheetham, A. K. Enhanced H2 adsorption enthalpy in the low-surface area, partially fluorinated coordination polymer Zn5 (triazole)6 (tetrafluoroterephthalate)2 (H2 O)2 ·4H2 O. J. Mater. Chem. 2009, 19, 4307–4309. Wang, X.-S.; Ma, S.; Forster, P.; Yuan, D.; Eckert, J.; L´ opez, J.; Murphy, B.; Parise, J.; Zhou, H.-C. Enhancing H2 Uptake by “Close-Packing” Alignment of Open Copper Sites in Metal–Organic Frameworks. Angew. Chem. Int. Ed. 2008, 47, 7263–7266. Pham, T.; Forrest, K. A.; Georgiev, P. A.; Lohstroh, W.; Xue, D.-X.; Hogan, A.; Eddaoudi, M.; Space, B.; Eckert, J. A high rotational barrier for physisorbed hydrogen in an fcumetal–organic framework. Chem. Commun. 2014, 50, 14109–


Page 9 of 10

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

Langmuir 9

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

14112. Nouar, F.; Eckert, J.; Eubank, J. F.; Forster, P.; Eddaoudi, M. Zeolite-like Metal–Organic Frameworks (ZMOFs) as Hydrogen Storage Platform: Lithium and Magnesium Ion-Exchange and H2 -(rho-ZMOF) Interaction Studies. J. Am. Chem. Soc. 2009, 131, 2864–2870, PMID: 19206515. Liu, Y.; Eubank, J. F.; Cairns, A. J.; Eckert, J.; Kravtsov, V. C.; Luebke, R.; Eddaoudi, M. Assembly of Metal– Organic Frameworks (MOFs) Based on Indium–Trimer Building Blocks: A Porous MOF with soc Topology and High Hydrogen Storage. Angew. Chem. Int. Ed. 2007, 46, 3278–3283. Pham, T.; Forrest, K. A.; Hogan, A.; McLaughlin, K.; Belof, J. L.; Eckert, J.; Space, B. Simulations of Hydrogen Sorption in rht-MOF-1: Identifying the Binding Sites Through Explicit Polarization and Quantum Rotation Calculations. J. Mater. Chem. A 2014, 2, 2088–2100. Lee, J.; Li, J.; Jagiello, J. Gas sorption properties of microporous metal organic frameworks. J. Solid State Chem. 2005, 178, 2527–2532. Krawiec, P.; Kramer, M.; Sabo, M.; Kunschke, R.; Fr¨ ode, H.; Kaskel, S. Improved Hydrogen Storage in the Metal–Organic Framework Cu3 (BTC)2 . Adv. Eng. Mater. 2006, 8, 293–296. Panella, B.; Hirscher, M.; P¨ utter, H.; M¨ uller, U. Hydrogen Adsorption in Metal–Organic Frameworks: Cu-MOFs and ZnMOFs Compared. Adv. Funct. Mater. 2006, 16, 520–524. Wong-Foy, A. G.; Matzger, A. J.; Yaghi, O. M. Exceptional H2 saturation uptake in microporous metal–organic frameworks. J. Am. Chem. Soc. 2006, 128, 3494–3495. Xiao, B.; Wheatley, P. S.; Zhao, X.; Fletcher, A. J.; Fox, S.; Rossi, A. G.; Megson, I. L.; Bordiga, S.; Regli, L.; Thomas, K. M.; Morris, R. E. High-Capacity Hydrogen and Nitric Oxide Adsorption and Storage in a Metal–Organic Framework. J. Am. Chem. Soc. 2007, 129, 1203–1209, PMID: 17263402. Buch, V. Path integral simulations of mixed para-D2 and ortho-D2 clusters: The orientational effects. J. Chem. Phys. 1994, 100, 7610–7629. Belof, J. L.; Stern, A. C.; Space, B. An Accurate and Transferable Intermolecular Diatomic Hydrogen Potential for Condensed Phase Simulation. J. Chem. Theory Comput. 2008, 4, 1332–1337. Applequist, J.; Carl, J. R.; Fung, K.-K. Atom dipole interaction model for molecular polarizability. Application to polyatomic molecules and determination of atom polarizabilities. J. Am. Chem. Soc. 1972, 94, 2952–2960. Thole, B. Molecular polarizabilities calculated with a modified dipole interaction. Chem. Phys. 1981, 59, 341 – 350. Bode, K. A.; Applequist, J. A New Optimization of Atom Polarizabilities in Halomethanes, Aldehydes, Ketones, and Amides by Way of the Atom Dipole Interaction Model. J. Phys. Chem. 1996, 100, 17820–17824. Feynman, R. P.; Hibbs, A. R. Quantum Mechanics and Path Integrals; McGraw-Hill: New York, 1965; pp. 281. Pham, T.; Forrest, K. A.; Nugent, P.; Belmabkhout, Y.; Luebke, R.; Eddaoudi, M.; Zaworotko, M. J.; Space, B. Understanding Hydrogen Sorption in a Metal–Organic Framework with Open-Metal Sites and Amide Functional Groups. J. Phys. Chem. C 2013, 117, 9340–9354. Pham, T.; Forrest, K. A.; Eckert, J.; Georgiev, P. A.; Mullen, A.; Luebke, R.; Cairns, A. J.; Belmabkhout, Y.; Eubank, J. F.; McLaughlin, K.; Lohstroh, W.; Eddaoudi, M.; Space, B. Investigating the Gas Sorption Mechanism in an rhtMetal–Organic Framework through Computational Studies. J. Phys. Chem. C 2014, 118, 439–456. Pan, H.; Ritter, J. A.; Balbuena, P. B. Examination of the Approximations Used in Determining the Isosteric Heat of Adsorption from the Clausius–Clapeyron Equation. Langmuir 1998, 14, 6323–6327.

53

54

55

56

57

58

59

60

Nicholson, D.; Parsonage, N. G. Computer Simulation and the Statistical Mechanics of Adsorption; Academic Press: London, 1982; pp. 97. Darkrim, F.; Levesque, D. Monte Carlo simulations of hydrogen adsorption in single-walled carbon nanotubes. J. Chem. Phys. 1998, 109, 4981–4984. Garberoglio, G.; Skoulidas, A. I.; Johnson, J. K. Adsorption of Gases in Metal Organic Materials: Comparison of Simulations and Experiments. J. Phys. Chem. B 2005, 109, 13094–13103, PMID: 16852629. Sumida, K.; Brown, C. M.; Herm, Z. R.; Chavan, S.; Bordiga, S.; Long, J. R. Hydrogen storage properties and neutron scattering studies of Mg2 (dobdc)–a metal–organic framework with open Mg2+ adsorption sites. Chem. Commun. 2011, 47, 1157–1159. Abramowitz, M. Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables; Dover Publications, Inc.: Mineola, NY, 1965; Chapt. 25, Sect. 4, pp. 887. Pham, T.; Forrest, K. A.; Hogan, A.; Tudor, B.; McLaughlin, K.; Belof, J. L.; Eckert, J.; Space, B. Understanding Hydrogen Sorption in In-soc-MOF: A Charged Metal–Organic Framework with Open-Metal Sites, Narrow Channels, and Counterions. Cryst. Growth Des. 2015, 15, 1460–1471. Kong, L.; Rom´ an-Pérez, G.; Soler, J. M.; Langreth, D. C. Energetics and Dynamics of H2 Adsorbed in a Nanoporous Material at Low Temperature. Phys. Rev. Lett. 2009, 103, 096103. Xue, D.-X.; Cairns, A. J.; Belmabkhout, Y.; Wojtas, L.; Liu, Y.; Alkordi, M. H.; Eddaoudi, M. Tunable Rare-Earth fcu-MOFs: A Platform for Systematic Enhancement of CO2 Adsorption Energetics and Uptake. J. Am. Chem. Soc. 2013, 135, 7660–7667.


Langmuir


Page 10 of 10

Investigating H2 Sorption in a ... - ACS Publications

Recommend Documents