Role of Hydrogen Bonding on Transport of Coadsorbed Gases in

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The role of hydrogen bonding on transport of coadsorbed gases in metal organic frameworks materials Kui Tan, Stephanie Jensen, Sebastian Zuluaga, Eric K Chapman, Hao Wang, Rezwanur Rahman, Jeremy Cure, Tae-Hyeon Kim, Jing Li, Timo Thonhauser, and Yves J. Chabal J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b09943 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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Journal of the American Chemical Society

The role of hydrogen bonding on transport of co-adsorbed gases in metal-organic frameworks materials Kui Tan,1 Stephanie Jensen,2,3 Sebastian Zuluaga,2,3 Eric K. Chapman,2,3 Hao Wang,4 Rezwanur Rahman,1 Jérémy Cure,1 Tae-Hyeon Kim,1 Jing Li,4 Timo Thonhauser,2,3 Yves J. Chabal1* 1

Department of Materials Science & Engineering, University of Texas at Dallas, Richardson, Texas 75080 Department of Physics, Wake Forest University, Winston-Salem, North Carolina 27109 3 Center for Functional Materials, Wake Forest University, Winston-Salem, North Carolina 27109 4 Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, New Jersey 08854 2

Supporting Information Placeholder ABSTRACT: Co-adsorption of multi-components in metalorganic framework materials can lead to a number of cooperative effects, such as modification of adsorption sites or during transport. In this work, we explore the incorporation of NH3 and H2O into MOFs preloaded with small molecules such as CO, CO2, and SO2. We find that NH3 (or H2O) first displaces a certain amount of pre-adsorbed molecules in the outer portion of MOF crystallites, and then substantially hinders diffusion. Combining in situ spectroscopy with first-principles calculations, we show that hydrogen bonding between NH3 (or H2O) is responsible for an increase of a factor of 7 in diffusion barrier of CO and CO2 through the MOF channels. Understanding such cooperative effects is important for designing new strategies to enhance adsorption in nanoporous materials.

Understanding co-adsorption in metal-organic framework (MOF) materials is important for most applications since MOFs are rarely used solely for pure gases and they are typically subject to gas contamination.1-5 Co-adsorption, however, leads to a variety of processes that complicate the analysis, such as competitive molecular adsorption and diffusion.6-8 However, these processes remain poorly understood due to a lack of in situ characterization.6,8-10 They cannot be inferred from surface studies because the complexity of the MOFs’ nanopore environment—containing the metal ion or cluster nodes and small organic molecules as the linker—introduces new reaction pathways compared to flat surfaces, often dominated by kinetics and steric hindrance.7 For instance, hydrogen bonding of H2O and NH3 to the ligand can foster exchange reactions with molecules (e.g. CO2 in MOF-74) adsorbed with higher binding energies.7 Previous studies have primarily focused on the competition of different molecules adsorption in porous materials.10-13 Only a few studies have identified such synergistic effects among adsorbates.14-18 For instance, Yazaydin and co-workers reported that CO2 uptake in the MOF Cu-BTC is significantly enhanced (71% more CO2 at 0.1 bar) by the presence of water coordinated to the open metal sites. GCMC simulations indicated that the electrostatic interaction between the quadrupole moment of CO2 and the electric field created by water molecules is responsible for the enhanced CO2 uptake.14 Later Yazaydin’s team investigated the CO2 capture performance of four ZIF materials in the presence of

SO2 using molecular simulations and found that a difference in binding energies at the adsorption sites for SO2 and CO2 molecules led to a small (or negligible) effect of SO2 on CO2 adsorption.15 We report here an unexpected synergistic effect that controls molecular transport of a guest molecule in the presence of coadsorbed NH3, without modifying the binding energy at the adsorption site. Combining in situ infrared spectroscopy and firstprinciples calculations, we examine the transport of a number of molecules such as CO2, CO, SO2 and C2H4, C3H6, in MOF-74, upon co-adsorption of NH3. MOF-74 [M2(dobdc), M=Mg2+, Zn2+, Ni2+, Co2+ and dobdc=2,5-dihydroxybenzenedicarboxylic acid] is one of the best-studied MOFs with a high density of coordinatively unsaturated metal centers arranged in metal−oxide pyramidal clusters,19-21 where guest molecules preferentially adsorb. We find that diffusion of CO2, CO, SO2 inside the 1D MOFs channels, providing the only diffusion pathway for small molecules, is dramatically reduced upon NH3 co-adsorption.22 We show that hydrogen bonding between adsorbed NH3 and the guest molecules (CO, CO2, and SO2) directly controls the diffusion barrier. In contrast, the diffusion of C2H4 and C3H6 molecules is much less affected by NH3 because H-bonding is negligible. This understanding provides a mechanistic description of basic adsorption processes and the synergy between adsorbates, which is useful for many applications. The initial experiment is performed by loading CO in Ni-MOF74 at ~298 K, selected because its stretch vibration is very sensitive to the local cationic environment.23,24 Its interaction with the exposed (i.e. activated) metal site Ni2+ involves not only electrostatic but also σ and π orbital interactions,25 leading to a notable binding energy ( ~52.7 kJ/mol) as determined by isotherm measurements.25 Upon loading ~40 Torr CO into the activated NiMOF-74, the strong CO stretch frequency slowly increases to 2170 cm-1 at saturation,25,26 which is clearly distinct from the gasphase band center at 2143 cm-1 (see Fig. 1a bottom spectrum and S2, S3). Isotherm measurements (see Fig. S4) establish the CO occupancy at ~0.7 molecules per metal site Ni2+ at ~40 Torr and 298 K.25 At this point, the system is either evacuated to remove the CO gas or NH3 is added (co-adsorbed). The latter is achieved by introducing pure NH3 into the reactor from a small volume at pressure higher than 40 Torr without removing CO, thus establishing a

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a

0.1

2170

Post-load NH3

2136

Absorbance

~2.5 min ~1.5 min

~0.5 min

β (NH3)

NH3 gas phase

ν(CO)ad 2000 1400 -1 Wavenumber cm

1200

1000

b 0.1

2170 2136

Desorption

Without NH3

0.8

0.6

0.4

10

20 Time (min)

30

40

50

Figure 2. Evolution of the normalized integrated areas of (a) ν(CO) as a function of evacuation time (< 20 mTorr). Red dots indicate the pristine Ni-MOF-74 results without NH3 (see Fig. S7), whereas black diamonds show results for MOF-74 with postloaded NH3. The integrated areas of the ν(CO) bands within the NH3-loaded samples are normalized to the value before desorption, obtained at t = 0 (top pink spectrum in Fig. 1b). The integrated areas of the ν(CO) bands within fresh samples are normalized to the value when the peak is reduced by ~50% under evacuation (see Fig.S9 for the whole desorption range). The error bars are dominated by the change in measuring the (normalized) integrated areas and did not exceed 0.02.

0 min ~13 min ~26 min ~33 min

ν(CO) 2400

With NH3

ν(CO) band

0

ν(CO)g 2143

2200

1.0

0.2

~1.0 min

2400

cm-1 weakens while the band at 2170 cm-1 strengthens, as highlighted in the differential spectrum (top of Fig. 1b), suggesting that some of the weakly bound CO molecules (located in the channel) move back to empty primary binding sites. The band at 1150 cm-1 remains unchanged, consistent with the stability of both NH3 and the MOF crystalline structure (see Fig. S12). Similar experiments were also performed with CO2-loaded MOF-74 and Figure S8 shows that the CO2 (~50%) left after NH3 exchange remains stable (Fig. S8d), in contrast to its behavior in the pristine sample.

Normalized integrated area

mixture of ~ 40 Torr CO/4 Torr NH3. Figure 1(a) shows the evolution of the spectra as a function of time in this CO/NH3 mixture. The intensity of the CO band at 2170 cm-1 decreases by ~50% upon loading NH3 within ~2.5 min. At the same time, a new band appears and grows at 2136 cm-1, which is most likely associated with CO molecules (i.e. is not observed for pure NH3, see Fig. S10) displaced by NH3 from the primary Ni2+ adsorption sites to the secondary sites in the middle of the channel, see Fig. S5 and Table S1.7 In contrast, NH3 adsorption is characterized by an absorption band around ~1125 cm-1,7 associated with the β(NH3) vibration, which is distinguished from the sharp gas-phase NH3 vibrations between 800 and 1150 cm-1 and overlapped with the perturbation of MOF's β(CH)ip mode (see Fig. S6). The concentration of NH3 is estimated at ~0.35/ Ni2+ noting that ~50% of preadsorbed CO molecules are exchanged with NH3.

Absorbance

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2200

~41 min

2000

β(NH3) 1400 -1

1200

1000

Wavenumber cm

Figure 1. (a) The spectrum on the bottom shows the initial spectrum of CO in Ni-MOF-74 established after 30 min at ~40 Torr, ~298 K. The spectra above are a series of spectra showing the evolution after introducing NH3, establishing a mixture of CO and NH3 (~40 Torr CO and ~4 Torr NH3), as a function of time. (b) Evolution of infrared spectra of Ni-MOF-74 with pre-loaded CO and post-loaded NH3 upon subsequent evacuation (< 20 mTorr) as a function of evacuation time up to 41 min. All the spectra are referenced to the activated MOFs in vacuum. If the cell is evacuated instead, the intensity of the band at 2170 cm-1 remains very stable (see Fig.1b), in sharp contrast to its behavior in Ni-MOF-74 without any NH3 co-adsorbed, as illustrated in Figure 2 (black vs red plots). Note that the band at 2136

The IR absorption data show that both CO and CO2 remain inside the MOF in the presence of NH3 without measurable shifts from their center frequencies of 2170 cm-1 and 2341 cm-1, respectively. Furthermore, there is no clear spectroscopic evidence of the formation of formamide and carbamate (see Fig.S10 and S11), indicating no direct strong interactions between NH3 with CO or CO2. To examine this conclusion, DFT calculations were performed to evaluate both the vibrational frequency and binding energy of CO molecules bound to Ni-MOF-74 when NH3 is adsorbed at nearby four sites, as depicted in Figure 4 with results in Table S127,28. The ν(CO) shifts by 1–6 cm-1 are consistent with previous observations that the frequency of adsorbed molecules within MOF-74 is sensitive to the chemical environment.7,29 In this case, no shift is observed in ν(CO), confirming that NH3 and CO are not co-adsorbed in the same unit cell. Even if co-existence did occur, the CO binding energy would not change by more than 5 kJ/mol, which is not enough to affect the kinetics of desorption. The above observations lead us to conclude that pre-adsorbed CO and post-loaded NH3 molecules are spatially separated, with NH3 residing in the outer region of the MOF microcrystals and acting as a shell layer that confines preloaded CO molecules.

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Figure 3. Co-adsorption of NH3 and CO at first (i), second (ii), and third (iii) nearest neighbor Ni2+ sites, respectively in NiMOF-74. A neighboring site along the c-axis (iv) is also investigated. Black (grey), red (pink), white, blue and light blue spheres represent C, O, H, N, and Ni atoms, respectively. To understand the mechanism by which NH3 hinders the release of CO, we have modeled CO diffusion with and without NH3 present using a transition-state search algorithm, i.e., the climbing-image nudged-elastic-band (NEB) method.30 We first calculate the adsorption geometry of NH3 within the MOF-74 unit cell and then consider how the diffusion barrier is affected for CO molecules to desorb from the structure if the NH3 molecules are adsorbed on the periphery of the MOF narrowing the entrance of the 1D channels. We start by simulating the motion of the CO molecules longitudinally (along the c axis, see Figure 3) through regions of the MOF where the most active adsorption site i.e. metal centers are saturated with CO and NH3 molecules. The structures of two systems are relaxed: one with all the metal centers saturated with CO molecule and a second one where the metal centers are saturated with NH3 molecules (see Fig. 4a and Fig. S13). It is reasonable to assume that all the Ni2+ site on the outer surface are occupied with NH3 (see model in Fig. 4a) since the atomic ratio of N:Ni derived from X-ray photoelectron spectroscopy on the surface region is ~0.9 (see Fig. S14). Note that NH3 does not penetrate through the entire frameworks so that Figure S13 represents the highest CO diffusion barrier when metal sites are fully occupied with CO. In both cases, a CO molecule is placed in the middle of the channel. Figure 4b shows that CO penetrates through the 1D channel of the MOF fully loaded with the same type of molecules by overcoming a diffusion barrier of only 0.03 eV, similar to energy barriers encountered by other small molecules such as CO2 with 0.04 eV.22 On the other hand, if the metal centers are now saturated with NH3 molecules (scenario depicted in Figure. 4a), the CO encounters an energy barrier of 0.21 eV (black line of Figure. 4b), i.e. 7 times higher. The interaction of a CO molecule with NH3 diffusing across the channel where NH3 molecules are adsorbed (at metal centers) is clearly much stronger than with adsorbed CO. At the transition state, where the diffusion barrier is the highest, the CO is located exactly between the two neighboring NH3 molecules as shown in Fig. 4a, experiencing the largest hydrogen bonding. For CO2 diffusion through NH3 terminated channels, the barrier is even higher than that of CO (0.32 eV; see Fig. 4b). Assuming that the pre-factor in the Arrhenius equation is the same, the significantly higher barrier for the CO-NH3 and CO2-NH3 diffusion process compared to CO and CO2 alone explains the decreased desorption rate (over 500 times, derived by theoretical calculation).

Figure 4. (a) CO interaction with post-loaded NH3 molecules during the diffusion process viewed along the c axis. (b) Diffusion barriers of CO and CO2 molecules, as they diffuse through the adsorbed NH3 molecules. Black (grey in panel a), red, white, light blue, and blue spheres represent C, O, H, N, and Ni atoms, respectively. These results are essential to understand the role of the NH3 molecules: these post-loaded NH3 molecules are adsorbed within Ni-MOF-74 and can establish hydrogen bonding interaction with incoming molecules through the channel, adding an additional barrier to molecules to desorb from the framework. By controlling the amount of post-loaded NH3, the desorption rate can be tuned (see Fig. S15) and if NH3 is loaded first—forming a barrier near the surface region—both adsorption and desorption of molecules are significantly delayed (see Fig. S16 and S17). The measurement is also extended to Co-MOF-74, which has a large crystal size, but exhibits the same effect (see Fig. S18 and S19). Similarly, H2O can also slow down the desorption (e.g. CO2 in Fig. S20, S21 and S22) through increasing the diffusion barrier by a factor of 3 to 4, not as much as NH3 but still very significant. To validate this hydrogen bonding effect, similar measurements have been performed on SO2, C2H4, and C3H6 molecules with postloaded NH3 (see Fig. S23, S24 and S25). SO2 shows a similar effect as CO and CO2 upon post-loading NH3. However, the hydrocarbon molecules C2H4 and C3H6 are well-known nonpolar molecules and therefore cannot form strong hydrogen bonding interaction with NH3. Experimentally we observe that C2H4 and C3H6 cannot be effectively blocked inside the framework by NH3, which is consistent with our above analysis. In summary, we have shown that a variety of weakly adsorbed small molecules such as CO and CO2 can be significantly stabi-


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lized inside MOFs by post-loading NH3 molecules. The effective trapping of these small molecules is not induced by local interaction between NH3 and these molecules—since two adsorbates are spatially isolated—but by the increase of the diffusion barrier due to hydrogen bonding interactions, as a result of which the weakly bonded molecules are confined within the MOFs materials. This understanding opens up new avenues to trap molecules inside MOFs.

ASSOCIATED CONTENT Supporting Information Sample preparation and activation methods: Experimental and calculation methods; IR spectra of pure CO adsorption and coadsorption of NH3 and CO2 in Ni-MOF-74; Isotherm measurement of CO adsorption in Ni-MOF-74; Powder X-ray diffraction X-ray photoelectron spectroscopy, Scanning electron microscopy and Raman spectroscopic measurement of MOFs sample; Energy barriers for the diffusion of CO and CO2 molecules along NH3 and H2O decorated channels of Ni-MOF-74; Post-loading H2O into CO2 loaded Ni-MOF-74; Post-loading NH3 into SO2, C2H4, C3H6 loaded Ni-MOF-74. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author Yves J Chabal: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was entirely supported by the Department of Energy Grant No. DE-FG02-08ER46491.

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Role of Hydrogen Bonding on Transport of Coadsorbed Gases in

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