doi.org/10.26434/chemrxiv.12283742.v1

Isolating the Role of the Node-Linker Bond in the Compression of UiO-66Metal-Organic FrameworksLouis Redfern, Maxime Ducamp, Megan C. Wasson, Lee Robison, Florencia Son, François-Xavier Coudert,Omar Farha

Submitted date: 11/05/2020 • Posted date: 13/05/2020Licence: CC BY-NC-ND 4.0Citation information: Redfern, Louis; Ducamp, Maxime; Wasson, Megan C.; Robison, Lee; Son, Florencia;Coudert, François-Xavier; et al. (2020): Isolating the Role of the Node-Linker Bond in the Compression ofUiO-66 Metal-Organic Frameworks. ChemRxiv. Preprint. https://doi.org/10.26434/chemrxiv.12283742.v1

Understanding the mechanical properties of metal–organic frameworks (MOFs) is essential to thefundamental advancement and practical implementations of porous materials. Recent computational andexperimental efforts have revealed correlations between mechanical properties and pore size, topology, anddefect density. These results demonstrate the important role of the organic linker in the response of thesematerials to physical stresses. However, the impact of the coordination bond between the inorganic node andorganic linker on the mechanical stability of MOFs has not been thoroughly studied. Here, we isolate the roleof this node–linker coordination bond to systematically study the effect it plays in the compression of a seriesof isostructural MOFs, M-UiO-66 (M = Zr, Hf, or Ce). The bulk modulus (i.e. the resistance to compressionunder hydrostatic pressure) of each MOF is determined by in situ diamond anvil cell (DAC) powder X-raydiffraction measurements and density functional theory (DFT) simulations. These experiments revealdistinctive behavior of Ce-UiO-66 in response to pressures under one GPa. In situ DAC Raman spectroscopyand DFT calculations support the observed differences in compressibility between Zr-UiO-66 and the Ce-analogue. Monitoring changes in bond lengths as a function of pressure through DFT simulations provides aclear picture of those which shorten more drastically under pressure and those which resist compression. Thisstudy demonstrates that changes to the node–linker bond can have significant ramifications on themechanical properties of MOFs.

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Isolating the Role of the Node-Linker Bond in the Compression of

UiO-66 Metal–Organic Frameworks

Louis R. Redfern,1 Maxime Ducamp,2 Megan C. Wasson,1 Lee Robison,1 Florencia A. Son,1 François-Xavier Coudert,2* and Omar K. Farha1*

1. International Institute of Nanotechnology and Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States.

2. Chimie ParisTech, PSL University, CNRS, Institut de Recherche de chimie Paris, 75005, Paris, France.

KEYWORDS. Metal-Organic Frameworks, High Pressure, Mechanical Properties, Diamond Anvil Cell.

ABSTRACT: Understanding the mechanical properties of metal–organic frameworks (MOFs) is essential to the fundamental advancement and practical implementations of porous materials. Recent computational and experimental efforts have re-vealed correlations between mechanical properties and pore size, topology, and defect density. These results demonstrate the important role of the organic linker in the response of these materials to physical stresses. However, the impact of the coordination bond between the inorganic node and organic linker on the mechanical stability of MOFs has not been thor-oughly studied. Here, we isolate the role of this node–linker coordination bond to systematically study the effect it plays in the compression of a series of isostructural MOFs, M-UiO-66 (M = Zr, Hf, or Ce). The bulk modulus (i.e. the resistance to com-pression under hydrostatic pressure) of each MOF is determined by in situ diamond anvil cell (DAC) powder X-ray diffraction measurements and density functional theory (DFT) simulations. These experiments reveal distinctive behavior of Ce-UiO-66 in response to pressures under one GPa. In situ DAC Raman spectroscopy and DFT calculations support the observed differ-ences in compressibility between Zr-UiO-66 and the Ce- analogue. Monitoring changes in bond lengths as a function of pres-sure through DFT simulations provides a clear picture of those which shorten more drastically under pressure and those which resist compression. This study demonstrates that changes to the node–linker bond can have significant ramifications on the mechanical properties of MOFs.

INTRODUCTION

Over the past two decades, metal –organic frameworks (MOFs) have become a prominent class of porous materials for many applications, including gas adsorption,1-2 chemical separations,3-4 and heterogeneous catalysis.5-7 These scaf-folds consist of inorganic nodes connected by multitopic or-ganic linkers to form 2- and 3-dimensional frameworks with exceptional porosity and tremendous structural diver-sity.8 While several classes of MOFs have demonstrated high chemical and thermal stability in recent years,9 a rigorous understanding of how these materials respond to mechani-cal stress is still emerging. Interesting phenomena, such as pressure-induced amorphization,10 pore collapse,11 and structural transitions,12 have been well documented in MOFs when exposed to pressures on the order of 1 GPa. Elu-cidating structure-property relationships of MOFs based upon their mechanical properties is challenging due to the sheer number of frameworks and the highly specialized equipment required for experimental measurements. On the other hand, high-throughput computational studies of-fer insight into the structural characteristics that relate to mechanical properties,13 though detailed design principles remain elusive. A combination of experimental and theoret-ical work provides a more nuanced picture of the factors that dictate the mechanical properties of MOFs.

UiO-66 is among the most well-studied MOFs due to its cubic space group, remarkable chemical and thermal stabil-ity.14 The structure consists of 12-connected Zr6(µ3-O)4(µ3-OH)4 nodes joined by linear 1,4-benzenedicarboxylic acid linkers which assemble into the fcu topology (Figure 1).15 Several studies have demonstrated that UiO-66 is more re-sistant to mechanical stress than other prototypical MOFs,16-17 though the presence of missing linker defects can significantly impact the mechanical properties.18-19 In a re-cent study from our group, we demonstrated that system-atic extension of the organic linker in the UiO family of MOFs leads to a decrease in the bulk modulus (that is, the re-sistance to hydrostatic pressure; K0 = -V ∂P/∂V).20 Given the breadth of knowledge regarding this framework, UiO-66 is a well-established platform for investigating the mecha-nism of structural changes at high pressures. While numer-ous reports support the notion that changes to the organic linker21-22 and topology23-24 can impact the compressibility of MOFs, systematic studies into the effect of changes to the metal node are less prevalent.25

Herein, we investigate the role of the metal-carboxylate bond in the compression of UiO-66 by varying the identity of the metal node from Zr-UiO-66 to Hf-UiO-66 and Ce-UiO-66. By keeping the topology, linker, and experimental con-ditions constant, we isolate the coordination bond as the single structural variable that changes in this series of MOFs. While the Zr and Hf analogues exhibit almost identi-cal behavior under pressure, Ce-UiO-66 is found to be much more compressible. Pressure-dependent Raman spectros-copy supports the observations made from high-pressure in situ powder X-ray diffraction (PXRD) measurements. Den-sity Functional Theory (DFT) simulations of Zr- and Ce-UiO-66 are in good agreement with the experimental observa-tions and reveal that the node–linker bond of Ce-UiO-66 compresses more readily than that of the Zr analogue. To-gether, these results indicate that the identity of the metal node in MOFs can have a significant impact on their re-sponse to high pressures by modulating the strength of the coordination bonds that hold these scaffolds together.

EXPERIMENTAL METHODS

All MOFs were synthesized and activated according to modified literature procedures.26-28 The materials were characterized using ambient pressure powder X-ray diffrac-tion (PXRD) and isothermal nitrogen adsorption to verify their crystallinity and porosity, respectively (Figures S1 and S2). X-ray photoelectron spectroscopy (XPS) was con-ducted with an Al Kα source. Five scans lasting 30 seconds each were collected. Ambient pressure Raman spectroscopy was conducted using a 532 nm excitation laser (see Sup-porting Information for details). High-pressure PXRD was conducted at the 17-BM-B beamline (λ = 0.45390 Å) at the Advanced Photon Source (APS) at Argonne National Labor-atory (ANL) using a diamond anvil cell (DAC) sample envi-ronment. In a typical experiment, the MOF sample was gen-tly ground with an internal standard, CaF2, to ensure uni-form mixing and to break up any large crystals. The mixture was loaded into a hole in a pre-indented stainless-steel gas-ket and sealed in the DAC. An ambient pressure diffraction pattern was collected, then a drop of FluorinertTM FC-70 was

added to the cell as a non-penetrating pressure transmitting fluid. In situ PXRD data were then collected as a function of pressure. Unit cell parameters were then extracted from the PXRD patterns using Le Bail refinement. Bulk moduli (K0 = -V ∂P/∂V) were extracted by fitting a plot of unit cell volumes vs. pressure to a second-order Birch-Murnaghan equation of state. Pressure-dependent Raman spectroscopy experi-ments were conducted at the GSECARS offline Raman spec-trometer located at the APS at ANL.29 Spectra were collected using a 532 nm excitation laser, and ruby fluorescence was used to monitor pressure inside the DAC. For further details regarding DAC diffraction and Raman experiments, see Sup-porting Information.

Each MOF was investigated through computational simu-lations using the Density Functional Theory (DFT) as imple-mented in the CRYSTAL14 code.30 All-electron localized ba-sis sets were used for all the atoms except for hafnium and cerium, for which a pseudopotential approach was used. These basis sets can be found on the CRYSTAL online library with the corresponding acronyms and original references: C: C_G-31d1G_gatti_1994;31 O: basis set used by Vlenzano et al.;15 H: H_3-1p1G_1994;31 Zr: Zr_all_electron_dovesi_un-pub;15 Hf: Hf_ECP_stevens_411d31G_munoz_2007;32 Ce: Ce_ECP_Meyer_2009.33

Several different functionals, at different levels of approx-imation, were considered to describe the exchange and cor-relation components of the energy. Among these, PBESOL0 hybrid functional34 was retained as it gives a good accuracy while keeping an acceptable computational cost. The effect of Grimme-type dispersion corrections35 was tested and found to be of a minor impact on the optimized structures and derived properties. Therefore, these corrections were not included in this study.

Mesh sampling of the reciprocal space was performed us-ing the Monkhorst–Pack scheme.36 Given the size of the sys-tems, a k-point mesh of 2 × 2 × 2 was used for each MOF structure to obtain convergence of the properties of inter-est. This generated mesh was then used for all our calcula-tions. Higher convergence criteria than the defaults

Figure 1. The octahedral cages of UiO-66 are comprised of M6 (M = Zr, Hf, or Ce) nodes connected by 1,4-benzenedicarboxylic acid linkers. Hydrogen atoms are omitted for clarity.

proposed by CRYSTAL14 code were used for geometry op-timization (a maximum of 0.0005 a.u. on atomic displace-ments during one optimization step and 0.0001 a.u. on forces). Optimized structures and representative input files for the calculations are available online at https://github.com/fxcoudert/citable-data

RESULTS AND DISCUSSION

Ambient pressure PXRD patterns for each MOF are in good agreement with the simulated powder patterns (Fig-ure S1) from reported crystal structures,27-28 indicating that the UiO-66 structure is retained when synthesized with dif-ferent metals. Nitrogen physisorption isotherms (Figure S2) indicate that porosity of the UiO-66 MOFs is also main-tained.

Upon applying pressure using a DAC, the PXRD peaks shift steadily to higher angles of diffraction, indicating a de-crease in unit cell volume (Figures S4 and S5, Tables S1 and S2). We extracted the unit cell volumes of each MOF at each measured pressure. Plotting the unit cell volumes of Zr-UiO-66, Hf-UiO-66, and Ce-UiO-66 as a function of pres-sure reveals that the Zr and Hf analogues behave nearly identically over a pressure range of 0-0.4 GPa, followed by a slight divergence above 0.4 GPa (Figure 2). Remarkably, the unit cell volume of Ce-UiO-66 decreases much more rapidly than the other MOFs, with noticeable discontinuities around 0.1 GPa and 0.4 GPa. These discontinuities may indicate pos-sible pressure-induced phase transitions, i.e. changes to the MOF structure upon achieving certain pressures that alter the mechanical properties of the material. Unfortunately, the PXRD patterns collected were of insufficient quality to determine the nature of these suspected structural trans-formations.

The initial data points for each MOF were fit to a second-order Birch-Murnaghan equation of state to estimate the bulk modulus (K0 = -V ∂P/∂V) of the material (Figure 2). The bulk modulus describes the pressure required to in-duce a given change in volume and is inversely related to compressibility. Zr-UiO-66 and Hf-UiO-66 exhibit nearly identical bulk moduli (K0 = 37.9 ± 0.6 GPa20 and 37 ± 1 GPa,

respectively), while Ce-UiO-66 has a much lower bulk mod-ulus (K0 = 16.9 ± 0.7 GPa) as evidenced by the steep decline in unit cell volume with pressure. These results indicate that while Zr-UiO-66 and Hf-UiO-66 resist compression to the same degree, the Ce framework is significantly less rigid. Because the three MOFs have similar properties at ambient conditions and share identical organic linker and topology, we attribute the distinct mechanical properties to differ-ences in the bond between the node and linker.

Given the striking difference in the bulk modulus of Ce-UiO-66, we turned to pressure-dependent Raman spectros-copy to probe the coordination bond between the metal node and organic linker. Unfortunately, the node-linker bond cannot be directly monitored using this technique be-cause this vibration is infrared active. To circumvent this challenge, we selected the C-C stretch between the aromatic core of the linker and the carboxylate carbon and the O-C-O symmetric stretch of the carboxylate (Figure S6) as target vibrations which provide insight into the strength of the metal-linker bond and have been assigned previously.37

Ambient pressure Raman spectra were collected for each material to determine the frequencies that correspond to the vibrations of interest. Then, Raman spectra were col-lected within a DAC with incremental increases in pressure for each MOF (Figure 3). In each sample, significant peak broadening is observed with increasing pressure, which is a common phenomenon in high-pressure Raman spectros-copy.38 In Zr-UiO-66, both vibrations of interest exhibit a steady hypsochromic shift as pressure increases from 0-1.20 GPa. Hf-UiO-66 displays a similar behavior over a pres-sure range of 0-0.92 GPa, though the effect in the C-C stretch (starting at 1437.8 ± 0.7 cm-1) is somewhat obscured by peak broadening. The Raman spectrum of Ce-UiO-66 changes dramatically between ambient pressure and the spectrum collected under pressure (0.24 GPa). Immediate broadening of the two peaks results in a single broad signal, precluding meaningful qualitative analysis. This significant change in the shape of the Raman spectrum may result from structural changes occurring at low pressures. The stark dif-ference in behavior between Ce-UiO-66 and the Zr and Hf counterparts supports the observed contrast in compres-sion from the PXRD data.

We then fit the region of the Raman spectra containing the vibrations of interest with two pseudo-Voigt functions in order to analyze the observed changes in a more quanti-tative manner. The frequencies corresponding to the center of each peak are shown in Figure 4 as a function of increas-ing pressure. In all three MOFs, the C-C stretch shifts stead-ily to higher frequencies in general, with a notable disconti-nuity present around 0.4 GPa in Ce-UiO-66 (Figure 4a). This jump in the Raman spectrum coincides with the dramatic change in compressibility observed in the DAC PXRD data. We assume that these discontinuities are a result of a struc-tural change that occurs around 0.4 GPa, though further characterization is required to elucidate the details of such a transformation. For Zr-UiO-66 and Hf-UiO-66, the O-C-O symmetric stretch steadily increases in frequency with in-creasing pressure; however, in Ce-UiO-66 this vibration re-mains almost invariant after an initial increase in frequency upon pressurizing to 0.24 GPa. The consistency of this Ra-man shift indicates that the bond lengths and vibrational

Figure 2. Unit cell compression of M-UiO-66. Solid curves rep-resent the second-order Birch Murnaghan equation of state used to determine the bulk modulus of each MOF. Data for Zr-UiO-66 are reproduced with permission from Ref. 20

frequency of the carboxylate group in Ce-UiO-66 do not change as rapidly as the Zr and Hf analogues. The distinct behavior of the O-C-O vibration in Ce-UiO-66 is evidence of the role of the metal-carboxylate bond in the pressure re-sponse of the material.

In order to better understand the compression of Zr-UiO-66 and Ce-UiO-66, we performed DFT calculations of their

structures, their elastic properties, and their vibrational modes in the harmonic approximation. First, the experi-mental structures were optimized with full use of sym-metry, relaxing both atomic positions and unit cell parame-ters. Very good agreement was found with the experimental cell parameters (Table S6). We then performed frequency calculations in the harmonic approximation in order to de-termine the characteristics of the MOFs’ vibration modes, in particular the C–C stretch and O–C–O symmetric stretch. Again, a good agreement with the experimental results was observed for both compounds (Table S7).

We note here a few negative frequencies are observed in the case of Ce-UiO-66; those vibration modes are related to the ligand bowing out of its average plane, also called “gui-tar string” modes. These vibrational modes have been ob-served in UiO-compounds, as well as in other MOFs,39 and arise from the fact that DFT calculations are performed at 0 K on a crystallographic structure with high symmetry, which is an average structure. At finite temperature, the

Figure 3. Raman spectra of Zr-UiO-66, Hf-UiO-66, and Ce-UiO-66 as a function of pressure. Dashed vertical lines are centered on the ambient pressure peak maxima and are in-cluded as a guide to the eye.

Figure 4. Frequencies of the (a) C-C stretch between carbox-ylate group and aromatic core of the UiO-66 linker and (b) O-C-O symmetric stretch of the carboxylate group as a function of pressure.

ligand would bow out of the plane in two directions, in this low-frequency mode, giving an in-plane average position which is the high-symmetry structure observed by X-ray diffraction.

Determination of the stiffness tensor (i.e., the second-or-der elastic constants) through linear response calculations allowed us to derive the bulk modulus for each material. Re-producing the experimental trend, simulations confirm a different compression between Zr-UiO-66 and its Ce ana-logue, with values of 42 and 37 GPa respectively. Although the difference is not as drastic as is observed in the experi-mental measurements, it demonstrates the softer nature of the Ce-UiO-66 framework.

We then conducted calculations of the structural evolu-tion of the frameworks under pressure: geometry optimiza-tions were performed in the pressure range of 0 to 2 GPa. Summary of the cell parameters for each pressure can be found in the supporting information (Table S6), as well as the optimized structures. As found experimentally, we ob-served a linear variation of the volume with respect to the pressure for Zr-UiO-66. However, our DFT calculations can-not reproduce the possible phase transition experimentally observed at 0.4 GPa for Ce-UiO-66 — indicating that it in-volves a symmetry-breaking transition to a lower-sym-metry phase. In the case of Ce-UiO-66, computational re-sults show a linear variation like that of the Zr analogue, though the slope is steeper, confirming the lower bulk mod-ulus measured and calculated for this MOF.

From the stressed structures, we highlight a few struc-tural variations of chemical importance. Considering first the ligand, a few similarities are observed between Zr- and Ce-UiO-66: variation of the O–C–O angle as well as the dihe-dral angle between ligands was identical in both materials, shifting around 1° in each case. However, using the cell pa-rameter as a reference by plotting (l/l0)/(a/a0) (l = bond length, l0 = bond length at ambient pressure, a = unit cell pa-rameter, a0 = unit cell parameter at ambient pressure) (Fig-ure 5), we found that C–O bond decreases more slowly than the lattice parameter in both cases [(l/l0)/(a/a0) > 1 as pres-sure increases] whereas C–C bond decreases almost exactly at the same rate [(l/l0)/(a/a0) ≈ 1 as pressure increases]. Moreover, we saw that this C–O bond shortens faster in the case of Zr supporting the slower variation of the O–C–O

symmetric stretch for Ce. This demonstrates the im-portance of the carboxylate group to understand the com-pression of this material. Metal–carboxylate distances were also investigated in both materials and we found that they show a faster decrease than the lattice parameter [(l/l0)/(a/a0) < 1 as pressure increases]. We also saw that this decrease is even more pronounced in the case of Ce-UiO-66 which could be due to the orbitals of Ce being more diffuse than those of Zr, leading then to a slightly higher flex-ibility of the Ce–carboxylate bond.

Given the apparent role of the metal-carboxylate bond in determining the compressibility of these MOFs, we consid-ered the possibility of Ce3+ forming during the MOF synthe-sis due to the high reduction potential of Ce4+. This Ce3+ spe-cies could then incorporate into the Ce-UiO-66 structure. To quantify the portion of reduced Ce, we conducted XPS ex-periments, revealing ~10% Ce3+ and ~90% Ce4+ present in Ce-UiO-66 (Figure S7). This ratio corresponds to roughly 47% of Ce6 nodes containing at least one Ce3+ atom, assum-ing they are evenly distributed throughout the MOF. Even with a low proportion of Ce3+, a significant number of nodes likely contain a reduced ion. Ce3+ exhibits a longer ionic ra-dius than Ce4+, which is often invoked to rationalize the well-documented unit cell expansion of bulk CeO2 upon re-duction.40-41 The presence of Ce3+ in the nodes of Ce-UiO-66 may lead to distortions of the node structure of Ce-UiO-66, influencing the mechanical properties of the MOF, and may also contribute to the lowering of the elastic moduli of the Ce-UiO-66 due to the softer Ce-carboxylate coordination al-lowing easier compression and shearing deformations. These rationales are in agreement with the difference ob-served between experimental and computational results on the bulk modulus of Ce-UiO-66, as calculations were per-formed on an ideal system with 100% Ce4+. We hypothesize that the prevalence of Ce3+ throughout Ce-UiO-66 contrib-utes to the uniquely distinct behavior of the framework un-der pressure compared to the Zr- and Hf- analogues.

CONCLUSIONS

This study investigates the influence of metal identity on the compression of a prototypical MOF, UiO-66. We con-ducted in situ DAC PXRD experiments to quantify the bulk modulus for three materials: Zr-UiO-66, Hf-UiO-66, and Ce-UiO-66. These results indicate that Ce-UiO-66 compresses much more readily than the Zr and Hf analogues (Figure 2). We then conducted in situ DAC Raman spectroscopy to probe two vibrations involving the carboxylate group of the organic linker. While the Raman shifts of the C-C stretch be-tween the carboxylate and aromatic core of the linker in-crease in frequency with pressure for all three MOFs, the O-C-O symmetric stretch of Ce-UiO-66 remains nearly con-stant from 0.24-0.84 GPa. This behavior contrasts the steady hypsochromic shift observed for the same vibration in Zr-UiO-66 and Hf-UiO-66 (Figure 4). DFT simulations re-veal the important role of the inorganic node in influencing the compression of individual bond lengths upon exposure to high pressures.

Changing the metal that comprises the nodes of a MOF has been shown previously to influence the electronic proper-ties,42 catalytic activity,43 and chemical stability44 of the ma-terial. Here, we have presented an interesting example in which altering the metal node of a MOF can have significant

Figure 5. (l/l0)/(a/a0) as a function of pressure for bond lengths involving the linker carboxylate: C-C bond (red), C-O bond (blue), and M-OR bond (green, M = Zr or Ce, R = linker). Filled circles represent bond lengths of Zr-UiO-66 and stars represent bond lengths of Ce-UiO-66.

impact on the mechanical properties of the framework. While the response of Zr-UiO-66 and Hf-UiO-66 to pressure is nearly identical, the Ce analogue exhibits drastically dif-ferent behavior despite the isostructural nature of the three MOFs. These results indicate that the coordination bond be-tween the linker and node of UiO-66 plays a role in the com-pression of the materials. This study provides insight into how these porous scaffolds respond to high pressures and the structural properties that can be adjusted to modulate the mechanical response of MOFs.

ASSOCIATED CONTENT

Supporting Information. Materials synthesis, ambient condi-tion characterization, diamond anvil cell experimental details, and computational details. “This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION

Corresponding Authors

* François-Xavier Coudert – Email: fx.coudert@chimieparis-tech.psl.eu * Omar K. Farha – Email: o-farha@northwestern.edu

Author Contributions

The manuscript was written through contributions of all au-thors.

ACKNOWLEDGMENT

O.K.F. gratefully acknowledges support from the Defense Threat Reduction Agency (HDTRA1-19-1-0007). M.D. and F.-X.C. acknowledge financial support from the Agence Nationale de la Recherche under project "MATAREB" (ANR-18-CE29-0009-01) and access to high-performance computing plat-forms provided by GENCI grant A0070807069. Portions of this work were performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation – Earth Sciences (EAR – 1634415) and Department of Energy- GeoSciences (DE-FG02-94ER14466). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Sci-ence User Facility operated for the DOE Office of Science by Ar-gonne National Laboratory under Contract No. DE-AC02-06CH11357. Use of the GSECARS Raman Lab System was sup-ported by the NSF MRI Proposal (EAR-1531583). This work made use of the IMSERC at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Ex-perimental (SHyNE) Resource (NSF ECCS-1542205), the State of Illinois, and the International Institute for Nanotechnology (IIN). This work made use of the SPID facility of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR-1720139) at the Materials Research Center; the Interna-tional Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. This work made use of the Keck-II facility of Northwestern University’s NUANCE Cen-ter, which has received support from the Soft and Hybrid Nan-otechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR-1720139) at the Materials Research Center; the International Institute for Nan-otechnology (IIN); the Keck Foundation; and the State of

Illinois, through the IIN. M.C.W. is supported by the NSF Grad-uate Research Fellowship under grant DGE-1842165.

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S1

Isolating the Role of the Node-Linker Bond in the Compression of a Metal-Organic Framework Louis R. Redfern,1 Maxime Ducamp,2 Megan C. Wasson,1 Lee Robison,1 Florencia A. Son,1 François-Xavier Coudert,2* and Omar K. Farha1* 1. International Institute of Nanotechnology and Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States. 2. Chimie ParisTech, PSL University, CNRS, Institut de Recherche de chimie Paris, 75005, Paris, France.

Table of Contents

Materials and Methods …………………………………………….………..……………….…................S2

Syntheses of MOFs ………………………….…………………….…...……….…………….….……..…S3

Powder X-ray diffraction patterns for MOFs……………………..……………………..…........................S4

N2 isotherms of MOFs…..............................................................................................................................S5

Pore size distributions of MOFs...................................................................................................................S6

In situ high pressure powder X-ray diffraction.............................................................................................S7

In situ high pressure Raman spectroscopy..................................................................................................S11

Computational details…………………………………………………...……………………………….S14

X-ray photoelectron spectroscopy...…………………………………………………….…………….….S15

References.……………………………………...………………..............................................................S16

S2

Materials

Zirconium (IV) chloride, hafnium (IV) dichloride oxide octahydrate, cerium (IV) ammonium

nitrate, terephthalic acid (BDC), glacial acetic acid, triethylamine, and FluorinertTM FC-70

(perfluorotripentylamine) were purchased from Sigma Aldrich Chemicals Company, Inc.

(Milwaukee, WI) and were used as received. Acetone, N,N-dimethylformamide (DMF), were

obtained from Fisher Scientific and used without further purification. All gases used for the

adsorption and desorption measurements were Ultra High Purity Grade 5 and were obtained from

Airgas Specialty Gases (Chicago, IL).

Physical Methods and Measurements

Ambient pressure powder X-ray Diffraction (PXRD) data were collected at the IMSERC X-ray

Facility at Northwestern University on a STOE-STADI-MP powder diffractometer equipped with

an asymmetric curved Germanium monochromator (CuKα1 radiation, λ = 1.54056 Å) and one-

dimensional silicon strip detector (MYTHEN2 1K from DECTRIS). The line focused Cu X-ray

tube was operated at 40 kV and 40 mA. Powder was packed in a 3 mm metallic mask and

sandwiched between two layers of polyimide tape. Intensity data from 2 to 30 degrees 2θ were

collected over a period of 10 mins. The instrument was calibrated against a NIST Silicon standard

(640d) prior the measurement.

N2 adsorption and desorption isotherms were measured on a Micromeritics Tristar II 3020

(Micromeritics, Norcross, GA) instrument at 77 K. Pore-size distributions were obtained using

DFT calculations using a carbon slit-pore model with a N2 kernel in Micromeritics MicroActive

software. Before each run, samples were activated at 120 °C for 12–24 h under high vacuum on a

Smart Vacprep from Micromeritics. Approximately round 50 mg of sample was used in each

measurement and Brunauer– Emmett–Teller (BET) areas were calculated in the region P/P0 =

0.005–0.05.

Ambient pressure Raman spectra were collected at the NUANCE facilities at Northwestern

University using the HORIBA LabRAM HR Evolution Confocal RAMAN spectrometer. Powder

samples were placed on a glass slide and excited using a 532 nm excitation laser. Ambient spectra

were collected from 250 cm-1 to 2000 cm-1.

X-ray photoelectron spectroscopy measurements were carried out on a Thermo Scientific

ESCALAB 250 Xi equipped with an electron flood gun and a scanning ion gun. Data analysis was

S3

completed using the Thermo Scientific Avantage Data System software, and C1s peak (284.8 eV)

peak was used as the reference.

Synthesis

Zr-UiO-66: Zr-UiO-66 was synthesized using a modified literature procedure.1 A 200 mL glass

jar was charged with terephthalic acid (45 mg, 0.27 mmol), triethylamine (7.0 L, 0.05 mmol),

glacial acetic acid (65 mL, 1.135 mol), and 63 mL of dimethylformamide (DMF). The mixture

was dissolved by sonication for 15 minutes followed by heating at 100 °C for 15 minutes.

Separately, an 8-dram vial was charged with ZrCl4 (63 mg, 0.27 mmol) and 4.5 mL of DMF. The

mixture was sonicated to dissolve. After cooling the 200 mL jar to room temperature, the ZrCl4

solution was added and heated at 100 °C in an oven for 24 hours. After cooling to room

temperature, the solid was collected by centrifugation and washed with DMF (3 x 10 mL) and

acetone (3 x 10 mL). The product was soaked with acetone overnight before activation at 120 °C

under vacuum for 18 hours.

Hf-UiO-66: Hf-UiO-66 was synthesized using a modified literature procedure.2 A 1 L bottle was

charged with terephthalic acid (450 mg, 2.71 mmol), DMF (630 mL), acetic acid (64.4 mL), and

triethylamine (70 µL). The bottle was sonicated to dissolve, then heated at 100 °C for 15 minutes.

The solution was cooled to room temperature, and a solution of HfOCl28H2O (1.107 g, 2.7 mmol)

in DMF (45 mL) was added. The mixture was heated at 100 °C overnight. The precipitate was

collected by centrifugation and washed with DMF (3 x 50 mL) and acetone (3 x 50 mL). The solids

were soaked in acetone overnight before collecting by centrifugation, drying in a vacuum oven at

80 °C, and activating at 120 °C under vacuum for 18 hours.

Ce-UiO-66: Ce-UiO-66 was synthesized using a modified literature procedure.3 A 4-dram vial

was charged with terephthalic acid (141.6 mg, 0.85 mmol), DMF (4.8 mL), and an aqueous

solution of cerium (IV) ammonium nitrate (1.6 mL, 0.533 M). The vial was sealed and heated to

100 °C using an aluminum heating block with stirring for 15 minutes. The suspension was

centrifuged to isolate the pale yellow precipitate. The solid was washed with 8 mL of DMF twice,

followed by 4 washes with acetone (8 mL). The product was activated at 100 °C under vacuum

for 18 hours.

S4

Characterization of MOF materials

Figure S1. Ambient pressure PXRD patterns of each Hf-UiO-66 and Ce-UiO-66.

S5

Figure S2. N2 isotherms of each MOF. Open circles denote desorption data.

S6

Figure S3. Pore size distributions for each MOF.

S7

In Situ Powder X-ray Diffraction

Each MOF was mixed with CaF2 (~10% by volume) which was used as an internal standard. The

powders were ground gently in a mortar and pestle to ensure even mixing and break up larger

crystals and clumps. The mixture was then loaded into a 250 µm hole in a 250 µm thick stainless-

steel gasket pre-indented to 100 µm thickness using a membrane-driven diamond anvil cell (DAC)

equipped with 500 µm culet anvils. The cell was sealed without applying pressure to measure the

diffraction pattern for the sample under ambient conditions. The cell was opened and a drop of

FluorinertTM FC-70 was added as a non-penetrating pressure transmitting fluid (hydrostatic limit:

~1.0 GPa). The cell was sealed and in situ powder X-ray diffraction data were collected using the

monochromatic X-rays (λ = 0.45390 Å, 100 µm beam size) at the 17-BM-B beamline at the

Advanced Photon Source, Argonne National Laboratory in combination with a VAREX 4343

amorphous silica area detector with a graphite window. Data were typically collected with 6 s

exposures (1 min total exposure per image) as the pressure was varied from 0-1.5 GPa. After

completion of a pressure campaign, the pressure within the cell was released and a final

measurement was made at ambient pressure. Raw images were processed with GSAS-II, with

sample-to-detector distance and tilt parameters based on data obtained for a LaB6 standard.4 The

integrated powder patterns for Hf-UiO-66 and Ce-UiO-66 are shown in Figure S4 and Figure S5,

respectively. The pressure-dependent lattice parameters were extracted from Le Bail fits of

reported structural models to the diffraction data using GSAS-II.5 The unit cell volumes of the

MOF and CaF2 internal standard are listed in Table S1 and Table S2, respectively. Equations of

state were fit to the P vs. V0/V data using EOS-FIT7c and EOS-FIT7-GUI.6-7 A 2nd-order Birch-

Murnaghan equation of state provided the best fit for the data. While in situ diffraction data were

collected to pressures of ~1.5 GPa, only the low pressure regime presented could be effectively fit

by a Birch-Murnaghan equation of state. This can be attributed to possible phase transitions in the

materials and/or to deviatoric stress induced as the pressure approaches the hydrostatic limit of the

pressure transmitting media.

S8

Figure S4. PXRD patterns of Hf-UiO-66 from 0-0.78 GPa.

S9

Figure S5. PXRD patterns of Ce-UiO-66 from 0-0.688 GPa.

S10

Table S1. Parameters from Le Bail fits to variable pressure powder diffraction data for Hf-UiO-66.

P (GPa) V CaF2 (Å3) a UiO-66 (Å) V UiO-66 (Å3) Rwp (%)

0.000 163.32(1) 20.7279(2) 8905.7(3) 10.13

0.29(1) 162.74(2) 20.6776(3) 8841.0(4) 6.82

0.36(2) 162.61(3) 20.6625(3) 8821.6(4) 7.12

0.44(2) 162.46(3) 20.6455(4) 8799.9(5) 9.60

0.51(2) 162.32(4) 20.6262(4) 8775.2(5) 7.89

0.56(2) 162.23(3) 20.6105(5) 8755.3(6) 9.26

0.59(1) 162.16(2) 20.5999(6) 8741.7(8) 11.60

0.64(1) 162.08(2) 20.5912(8) 8731(1) 14.17

0.69(1) 161.98(2) 20.583(1) 8719(1) 16.61

0.72(1) 161.92(2) 20.570(1) 8703(1) 18.91

0.78(1) 161.82(2) 20.558(1) 8688(2) 21.25

Table S2. Parameters from Le Bail fits to variable pressure powder diffraction data for Ce-UiO-66.

P (GPa) V CaF2 (Å3) a DUT-52 (Å) V DUT-52 (Å3) Rwp (%)

0.000 163.0592 21.488(1) 9922(2) 4.56

0.023(3) 163.0075 21.478(1) 9909(2) 4.48

0.051(3) 162.9532 21.465(1) 9891(2) 4.33

0.080(3) 162.8971 21.448(1) 9867(2) 4.32

0.121(3) 162.8165 21.427(1) 9837(2) 4.02

0.131(3) 162.7978 21.418(1) 9824(2) 4.06

0.145(3) 162.7705 21.409(2) 9813(2) 4.35

0.165(3) 162.7313 21.400(2) 9801(3) 4.55

0.186(3) 162.6917 21.389(2) 9785(3) 4.83

0.192(3) 162.6803 21.378(2) 9770(3) 4.97

0.232(3) 162.6035 21.355(3) 9739(4) 5.47

0.272(3) 162.5263 21.308(4) 9675(5) 5.87

0.328(3) 162.4186 21.260(5) 9609(6) 6.14

0.425(3) 162.2324 21.124(5) 9426(6) 6.14

0.523(3) 162.0426 20.986(6) 9241(8) 7.05

0.579(3) 161.9376 20.965(6) 9215(8) 6.67

0.632(4) 161.8361 20.937(6) 9178(7) 6.50

0.688(4) 161.7287 20.934(6) 9174(7) 6.47

S11

In situ Raman Spectroscopy

High pressure Raman spectroscopy experiments were conducted at the GSECARS laser

laboratory at Argonne National Laboratory using a 532 nm excitation laser.8 The sample was

loaded into a 250 µm diameter hole in a 250 µm thick stainless steel gasket that was pre-indented

using the DAC to a thickness of 100 µm. A small spherical ruby (used to determine the pressure

within the cell) was placed roughly 100 µm from the center of the diamond culet to ensure that it

would be visible within the gasket hole. FluorinertTM FC-70 was then added to the sample and the

DAC was sealed without applying pressure. The cell was then loaded into the Raman spectrometer

and aligned with the excitation beam. The position of the ruby was determined and recorded to

ensure consistent pressure readings within the cell. For each recorded spectrum, the screws that

hold the DAC together were tightened very slightly, then the ruby fluorescence was measured (33

mW, 0.02 sec exposure time, one exposure) to determine if a sufficient increase in pressure had

occurred. Then, the DAC was moved to excite the MOF sample without exciting the ruby, and the

Raman spectrum was collected (105 mW, 60 sec exposure time, 5 exposures for Hf-UiO-66, and

105 mW, 60 sec exposure time, 6 exposures for Ce-UiO-66) from ~1372 cm-1 to ~2480 cm-1 in

order to avoid the diamond fluorescence peak at 1335 cm-1. The peaks of interest (Figure S6) were

fit using two pseudo-Voigt functions and refined to determine the peak centers (Tables S3-S5).

S12

Table S3. Raman peak centers for Zr-UiO-66.

Pressure (GPa) C-C stretch frequency (cm-1) O-C-O symmetric stretch frequency (cm-1) R2

0 1430.2(2) 1450.5(2) 0.995

0.25 1436.7(2) 1453.9(1) 0.994

0.29 1437.7(3) 1454.9(2) 0.993

0.37 1439.0(2) 1456.25(9) 0.998

0.46 1440.4(2) 1457.41(8) 0.999

0.63 1442.4(4) 1459.0(1) 0.997

0.85 1444.0(4) 1460.2(1) 0.998

1.20 1446.8(6) 1461.7(1) 0.998

Figure S6. Representative images of the vibrations investigated by variable pressure

Raman Spectroscopy: (left) the C-CO2 stretch and (right) the O-C-O symmetric stretch.

S13

Table S4. Raman peak centers for Hf-UiO-66.

Pressure (GPa) C-C stretch frequency (cm-1) O-C-O symmetric stretch frequency (cm-1) R2

0 1437.8(7) 1457.0(3) 0.995

0.10 1438.4(7) 1457.8(3) 0.993

0.20 1440.9(7) 1460.0(2) 0.996

0.31 1441(1) 1460.5(3) 0.994

0.45 1443(1) 1461.6(3) 0.993

0.54 1444(2) 1463.0(2) 0.995

0.64 1445(3) 1463.3(3) 0.994

0.73 1446(3) 1464.3(3) 0.996

0.92 1447(1) 1465.3(3) 0.995

Table S5. Raman peak centers for Ce-UiO-66.

Pressure (GPa) C-C stretch frequency (cm-1) O-C-O symmetric stretch frequency (cm-1) R2

0 1422.0(1) 1437.3(3) 0.996

0.24 1425(1) 1443.6(8) 0.993

0.30 1424(3) 1443(2) 0.990

0.37 1425(1) 1443.0(8) 0.988

0.44 1430(2) 1444.3(8) 0.993

0.53 1429(1) 1444.8(6) 0.993

0.64 1431(1) 1446.5(7) 0.983

0.73 1430(3) 1445(1) 0.977

0.84 1433(3) 1446(1) 0.959

S14

Computational Details

Table S6. Calculated cell parameters for Zr-UiO-66 and Ce-UiO-66.

Pressure (GPa) a Zr-UiO-66 (Å) a Ce-UiO-66

0 20.7212 21.3944

0.2 20.6884 21.3567

0.4 20.6563 21.3198

0.6 20.6246 21.2834

0.8 20.5927 21.2479

1 20.5612 21.2132

1.2 20.5343 21.1782

1.4 20.5041 21.1437

1.6 20.4740 21.1091

1.8 20.4432 21.0743

2 20.4151 21.0379

Table S7. Calculated vibrational frequencies for Zr-UiO-66 and Ce-UiO-66.

Zr-UiO-66 Ce-UiO-66

Pressure (GPa) C-C stretch

frequency (cm-1)

O-C-O symmetric

stretch frequency

(cm-1)

C-C stretch

frequency (cm-1)

O-C-O symmetric

stretch frequency

(cm-1)

0 1428.4 1446.4 1424.9 1447.6

0.2 1431.5 1455.2 1427.7 1450.1

0.4 1433.2 1450.4 1431.1 1452.9

0.6 1437.1 1452.9 1434.9 1455.5

0.8 1440.2 1454.8 1437.8 1457.9

1 1445.2 1458.3 1440.3 1460.4

1.2 1449.4 1462.0 1442.4 1463.4

1.4 1448.9 1461.3 1445.3 1466.1

1.6 1454.9 1466.7 1448.1 1468.4

1.8 1455.3 1468.1 1424.9 1447.6

2 1456.6 1468.0 1427.7 1450.1

S15

X-ray Photoelectron Spectroscopy (XPS)

Figure S7. XPS spectrum of Ce-UiO-66 (black). Features attributed to Ce (IV) (blue) and Ce (III) (red) are shown, which together produce the overall fit (purple, dashed).

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References

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