Molecular-Level Insight into CO2 Adsorption on the

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Molecular-Level Insight into CO Adsorption on the Zirconium-Based Metal-Organic Framework, UiO-66: A Combined Spectroscopic and Computational Approach Tyler G. Grissom, Darren M. Driscoll, Diego Troya, Nicholas Severino Sapienza, Pavel M. Usov, Amanda J Morris, and John R. Morris J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02513 • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 3, 2019

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The Journal of Physical Chemistry

Molecular-Level Insight into CO2 Adsorption on the Zirconium-Based Metal-Organic Framework, UiO-66: A Combined Spectroscopic and Computational Approach Tyler G. Grissom, Darren M. Driscoll, Diego Troya, Nicholas S. Sapienza, Pavel M. Usov, Amanda J. Morris, John R. Morris* Virginia Tech Department of Chemistry, Blacksburg, Virginia, 24061

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The Journal of Physical Chemistry 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

Abstract Carbon dioxide (CO2) capture and separation in sorbent materials relies on the interactions that take place between guest molecules and topological or chemical features of the sorbent. Although researchers have developed an immense library of metal-organic framework (MOF) designs, which range in shape, size, and chemical functionality, a molecular-level understanding of how MOF structure and functionality affect capture and transport has yet to be fully developed. In this work, in situ infrared spectroscopic techniques, coupled with density functional theory, were used to provide insight to the binding locations and energetics of CO2 on the zirconium-based MOF, UiO-66. Two unique CO2 binding motifs in UiO-66 were identified through differing vibrational frequencies of the asymmetric OCO vibrational mode. One configuration is established through hydrogen bonding with μ3-OH groups on the MOF nodes and a second binding mode exists where CO2 is stabilized through dispersive interactions. Variable-temperature infrared spectroscopy (VTIR) revealed adsorption enthalpies of −38.0 ± 1.5 kJ/mol and −30.2 ± 1.3 kJ/mol for the hydrogen-bonded and dispersion-stabilized complexes, respectively. The techniques and results from this study can be extended to other gas–MOF systems to help reveal how topological features affect gas sorption and to provide insight into next-generation sorbent materials design.

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I. Introduction Anthropogenic carbon dioxide (CO2) emissions and impact of CO2 levels on environmental health has long been a central focus of research for many scientists and engineers. The most prominent contributor to CO2 release is the combustion of fossil fuels, namely coal, which accounts for over 30% of anthropogenic contributions.1 Strategies for reducing CO2 emissions from fossil fuels have been extensively investigated and range from transitioning to cleaner energy sources such as solar and wind, to capturing CO2 directly from the emitting source and repurposing it for other applications.2-5 For CO2 capture to develop into a viable alternative, cost-effective materials with high CO2 storage capacity, high selectivity over N2, O2, H2O, and other combustion gasses, and high stability under various thermal and chemical environments are needed.6 Metal-organic frameworks (MOFs), which are known for their high surface areas, tunable pore shapes and sizes, and near endless chemical compositions, have been identified as a promising class of materials for a wide variety of gas-based applications including storage, sensing, catalysis, and separations.7-11 These advantageous properties of MOFs make them prime candidates as CO2 sequestration materials. However, for industrial-based applications where sorbents may be exposed to heat, pressure, and harsh chemical environments, long-term stability of the material is critical.12 For this reason, research has focused extensively on the zirconium-based MOF, UiO-66, which is stable in humid and acidic environments, can withstand temperatures higher than 375 °C, and is mechanically robust.13-15 In addition, a wide variety of functionalized ligands can easily be incorporated into the framework of UiO-66 through pre- or post-synthetic modification, which allows for optimization of these materials.16 The UiO-66 framework is composed of Zr6O4(OH)4 inorganic nodes with an octahedral arrangement of Zr atoms connected to twelve 1,4-benzenedicarboxylate (BDC) organic linkers.

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The assembled three-dimensional framework is comprised of two distinct pore environments— larger, 11 Å diameter octahedral cavities, and smaller, 8 Å diameter tetrahedral cavities. The four bridging hydroxyl groups (μ3-OH) of the Zr6O4(OH)4 node are situated in the corners of the tetrahedral cavity.17 While thermal treatment of the MOF to temperatures greater than 573 K has been shown to reversibly remove the μ-hydroxyl groups to convert the Zr6O4(OH)4 node to Zr6O6, the physical properties and geometry of the MOF remain largely unchanged.18 Within MOF materials, factors such as pore and window sizes, undercoordinated metal sites, surface hydroxyls, and functionalized ligands have an enormous impact on the effectiveness and specificity of the material towards gas capture and separations.6,19-20 The energetics of CO2 adsorption into a variety MOFs, including UiO-66, have been previously studied, where enthalpies of adsorption ranging from −10 to −100 kJ/mol have been measured.6,16,21-23 Unsurprisingly, researchers found that CO2 adsorption in MOFs that contain electron-rich groups, specifically amines, as well as exposed metal sites tend to exhibit stronger adsorption enthalpies relative to other MOFs.16,22,24-25 In these studies, however, adsorption enthalpies were determined by use of adsorption isotherms and/or microcalorimetry.6 Both techniques are largely bulk-sensitive and yield only average enthalpies of adsorption, from which makes information pertaining to the location and energies of specific adsorption configurations is challenging. Physically and chemically complex MOFs contain multiple potential adsorption sites such as undercoordinated metals, electronegative linker moieties, and hydroxyl groups, all of which may serve as unique docking sites for CO2 binding. As such, the average bulk enthalpy of adsorption for CO2 depends on the site-specific binding energy, accessibility, and concentration of each docking point with the framework. Furthermore, heats of adsorption for a given MOF as determined by these techniques have been shown to widely vary based on the preparation of the particular MOF.26

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Understanding both the strength and origin of the gas–surface interactions is an important step towards evaluating the effectiveness of a CO2 capture material. In this work, we characterized site-specific binding enthalpies by utilization of in situ infrared spectroscopic methods, combined with electronic structure calculations, to provide direct insight into the nature of each gas–MOF chemical interaction and the corresponding adsorption enthalpy for CO2 at each site. The studies presented herein provide a molecular-level view of the binding geometry and the energetics of CO2 interactions with UiO-66 in both the hydroxylated and dehydroxylated forms. This work not only provides greater insight into CO2–UiO-66 interactions, but also advances the foundation for understanding the influence of topological features and specific docking sites on CO2 separation and capture.

II. Methods Chemicals For the UiO-66 synthesis: ZrCl4 (Aldrich), terephthalic acid (Acros Organics), concentrated HCl (37% Fisher Chemical), and N,N′-dimethylformamide (DMF) (Fisher Chemical) were used as received. The ambient gas of interest, carbon dioxide (CO2, 99.99%) was purchased from Praxair and used as is. Synthesis and Characterization UiO-66 Synthesis: The UiO-66 synthesis was based on a previously reported literature procedure.14 ZrCl4 (378 mg, 1.62 mmol) and terephthalic acid (539 mg, 3.24 mmol) were suspended in DMF (10 mL) inside a 6-dram vial. HCl (37%, 0.286 mL, 3.24 mmol) was added, and the reaction mixture was stirred at 343 K for 30 min for complete dissolution of starting reagents. The resultant solution was transferred into a Teflon-lined Parr reactor, which was heated

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at 493 K for 24 h. After cooling to room temperature, a white powder was isolated by centrifugation, washed with fresh DMF (4 × 10 mL), and then soaked in DMF (10 mL) for 4 days, the solvent was replaced every 24 h. The resultant material was calcined using a multi-step heating procedure under air to remove residual organic species. The MOF was heated with a 5 K min-1 ramp to 473 K, where the temperature was held for 10 minutes, followed by a 0.5 K min-1 ramp to 543 K, where the temperature was held for 70 hours. The sample was then allowed to cool to room temperature prior to CO2 exposure. MOF Characterization: The structure and phase purity of the MOF before and after vacuum chamber-based experiments were verified using a Micromeritics 3Flex powder X-ray diffractometer with Cu(Kα) radiation (1.5418 Å). The patterns were collected over a 2θ range of 3−50° with a 0.1° step size and 2°/min scan rate. The powdered samples were mounted onto reflective Si(510) disks. Crystal size and morphology were determined using a LEO 1550 field-emission scanning electron microscopy (SEM) system. Thermal stability and defect density were measured using thermogravimetric analysis (TGA) with a TA Q500 analyzer, where the MOF samples were loaded in a platinum pan and heated at 2 K/min under a flow of air. The defectivity of the MOF was assessed from the TGA data using a previously reported method.14 Nitrogen sorption experiments were conducted using a Quantachrome Autosorb-1, where the sample was degassed under vacuum for 12 h at 373 K prior to adsorption and desorption measurements. The surface areas of the samples were determined by fitting the N2 adsorption data at 77 K within a relative pressure (P/P0) range of 0.05 to 0.25 to the Brunauer–Emmett–Teller (BET) equation. Full characterization details can be found in the Supporting Information. Ambient Pressure CO2 Exposure: Ambient pressure, CO2 adsorption studies were carried out using a Thermo Nicolet Nexus 670 infrared spectrometer with a Pike Technologies HC-900 diffuse

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reflectance (DRIFTS) cell. The MOF powered was loaded into the DRIFTS cup and was purged with Ar for one hour at 298 K. The MOF was then exposed to a 10 sccm stream of 1% CO2 in Ar for 30 minutes, followed by 30 minutes of an Ar-only purge. The sample was then activated at 373 K for one hour under a 10 sccm Ar flow to remove residual water collected during storage, and the CO2 exposure study was repeated with the activated MOF. Infrared spectra were collected during CO2 adsorption/desorption with a spectral resolution of 4 cm-1 and an average of 64 scans. Variable Pressure and Temperature Studies: Infrared spectroscopic experiments were carried out in a stainless-steel vacuum chamber with a base pressure of 1 × 10−8 Torr. The MOF sample was prepared for vacuum studies according to previously reported literature.20 Briefly, 15 mg of UiO-66 powder was pressed at 6500 psi into the voids of 50.0 μm thick tungsten mesh (Tech Etch), which was secured onto a nickel support clamp mounted on a custom-built vacuum chamber. Sample temperature was monitored by a K-type thermocouple spot-welded adjacent to the pressed sample, and precisely controlled by an external power supply. Following installation, background gases were evacuated from the vacuum chamber until base pressure was attained. Operation under high vacuum conditions keeps the sample clean and allows for the probing of gas−MOF interactions, free of background or contaminant gas contributions. Once evacuated, the MOF was thermally activated by heating at 448 K for two hours to remove loosely-bound water. For the dehydroxylated UiO-66 studies, the MOF was heated at 573 K for 3 h and allowed to return to room temperature before beginning CO2 exposure studies. For the variable pressure CO2 exposure studies, 50 mTorr to 20 Torr of CO2 was introduced into a sealed vacuum chamber which contained the mounted UiO-66 sampled. Infrared spectra were collected following an equilibration period, upon reaching each desired pressure. For the variable temperature studies, the sample was heated to temperatures ranging from 297 – 363 K

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and exposed to 1.5 Torr of CO2. Infrared spectra were collected following an equilibration period, upon reaching each desired temperature.27 An FTIR spectrometer (Thermo, Nicolet Nexus 470 FTIR) mounted on the side of the vacuum chamber with an external MCT-A detector set to a spectral resolution of 2 cm−1 and a 128-scan average was used for the collection of all infrared data. A clean, empty spot on the tungsten mesh was employed as the reference for the spectra reported below. Electronic Structure Calculations: The interactions of CO2 with a cluster model UiO-66 were calculated using density functional theory (DFT). The model consists of a Zr6O4(OH)4 secondary building unit and 12 benzoate capping ligands that act as surrogates for the terephthalate linkers in the periodic structure. The tetrahedral cavities of UiO-66 contain both hydroxylated and non-hydroxylated corners, and the interactions of CO2 in both environments were probed with the calculations. Missing-linker calculations employed the same model with a benzoic acid unit removed to generate two adjacent undercoordinated zirconium sites. Geometry optimizations and harmonic frequency calculations were conducted with the M06-L28 functional utilizing an ultrafine integration grid as implemented in Gaussian09.29 The 6-31G** basis set was employed for non-metal atoms and the Lanl2dz basis set and pseudopotentials were used for zirconium. During optimization, the aromatic moieties were held to their crystallographic coordinates and all the secondary building unit atoms (including O atoms of the benzoate capping ligands) were relaxed.

III. Results and Discussion The interactions of CO2 with the zirconium-based MOF, UiO-66, have been investigated using infrared spectroscopic techniques and DFT calculations. The objective of this work is to determine the location and energetics associated with CO2 adsorption on one of the most stable and

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well-studied MOFs. Results show that CO2 exhibits two main binding geometries within the cavities of the MOF, one where CO2 hydrogen bonds to μ3-OH groups located within the tetrahedral pores of the MOF and the other where CO2 is stabilized by dispersion forces within the confined pore environment. CO2 Uptake within UiO-66 Under Ambient Conditions. Studies performed under ambient pressure revealed that CO2 adsorbs molecularly onto UiO-66 through weak interactions. The time-resolved DRIFTS spectra of CO2 adsorption onto activated UiO-66 (473 K in He flow for 1 hour), shown in Figure 1, reveals several key changes to infrared bands that can be correlated to physisorbed CO2. The prominent features near 2339 cm-1 are due to νas(CO2) vibrations and are indicative of linearly-adsorbed CO2 in the MOF,26,30-31 where the position is 10 cm-1 redshifted relative to gas phase32 and slightly redshifted relative to adsorbed CO2 on other metal oxide surfaces.33-34 Evidence of weakly bound CO2 is further supported by the emergence of a small band at 2274 cm-1, which is assigned to the νas(CO2) mode of trace amounts of surface-bound 13CO2.35 The feature at 2328 cm-1 is consistent with a combination band of the asymmetric stretch with two non-degenerate bending vibrations.36 The shoulder at 2349 cm-1 that appears at higher loadings suggests a possible secondary adsorption site or CO2 dimer formation.30 Additional features were observed at 3618 and 3641 cm-1 (Figure 1a) and have been assigned to hydrogen bond formation between CO2 and hydroxyl groups present on the MOF nodes. The free μ3-OH groups of UiO-66 are well documented to be available for hydrogen bond formation with guest molecules.20,37-39 The 33 and 56 cm-1 shifts to lower frequencies of the band associated with the free ν(μ3-O–H) vibration in UiO-66 (which exists as a sharp feature at 3674 cm-1)38 are due to a decrease in the force constant of the O–H stretch as CO2 donates electron density to the σ* antibonding orbital of the O–H bond.40-42 The 33 cm-1 redshift is similar to the shift observed for hydrogen bonding within MIL-

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53(Al) and for OH···CH3 hydrogen bonding in UiO-66.20,30,40 The small redshift observed for CO2 is in sharp contrast to the much larger shift when UiO-66 hydrogen bonds to polar molecules such as water,18,43 alcohols,43 DMF,18 and organophosphonates.37 Upon removal of CO2 from the gas stream, the bands associated with CO2 quickly depleted as the MOF returned to its original state. The immediate desorption of CO2 at room temperature indicates that the interactions are weak, which is expected based on the lack of undercoordinated metal sites or reactive sites on the linkers in the low-defect-density UiO-66 sample.44

Figure 1: Time-resolved DRIFTS difference spectra of UiO-66 exposed to 1% CO2 in He (black to blue). Gas phase CO2 has been subtracted from the spectra. CO2 Binding Energies and Adsorption Sites. Following the ambient-pressure work described above, which demonstrated clear uptake of CO2 within the MOF, systematic infrared spectroscopic studies conducted under high-vacuum conditions were employed to gain additional insight into the nature and energetics of CO2 binding on UiO-66. Spectra for CO2 uptake onto UiO-66 at 300 K (Figure 2) are consistent with the ambient-pressure DRIFTS spectra shown in Figure 1, where evidence of hydrogen bonding and weak adsorption were observed. The small decrease of the 3674 cm-1 band originally present in the clean MOF, and the concurrent growth of the 3641 cm-1

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feature are again indicative of CO2 hydrogen bonding with μ3-OH groups in UiO-66 (Figure 2a). The minimal depletion of the 3674 cm-1 band suggests low CO2 coverage of μ3-OH groups. Given the small kinetic diameter of CO2 (3.30 Å) and the open-pore environment of UiO-66,6,10,45 it is likely that CO2 molecules can access the four μ3-OH groups located on each MOF node. However, because those interactions are weak, limited perturbation of the O–H bond was observed. In addition to changes in the hydroxyl-stretching region, CO2 exposure under vacuum conditions resulted in the formation of a νas(CO2) band at 2340 cm-1 (Figure 2b), consistent with physiosorbed CO2 molecules. Further details regarding the adsorption sites and energetics for CO2 binding have been obtained through variable temperature studies.

Figure 2: Variable pressure IR spectra of UiO-66 exposure to CO2 from 50 mTorr to 20 Torr (black to blue). (a) ν(O–H) region and (b) νas(CO2) region. Gas phase CO2 has been subtracted from the spectra. Variable Temperature Infrared Spectroscopy. A systematic study of CO2 adsorption and the corresponding binding energetics on the hydroxylated and dehydroxylated forms of UiO-66 was conducted using variable-temperature infrared (VTIR) spectroscopy. VTIR is a powerful tool that can be used to determine adsorption enthalpies at discrete binding sites in gas-solid systems. The

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determination of adsorption enthalpies with VTIR has been described in detail in previous reports.46 Briefly, infrared spectra were recorded for adsorbed species under a static pressure of gas within the sample cell. Once equilibrium was established between the adsorbed and gas-phase species, spectra were recorded at a series of sample temperatures. Key infrared absorbance bands were tracked at discrete sample temperatures to determine the relative number of adsorbed molecules. As these studies are conducted under equilibrium conditions, a van’t Hoff analysis relates the equilibrium constant for adsorption, Keq, to the standard enthalpy of adsorption, ΔH˚ads, as shown in Eqn. 1: 𝑙𝑛#𝐾%& ' = )

∘ ∘ −∆𝐻-./ ∆𝑆-./ 3+) 3 𝑅𝑇 𝑅

(1)

where ΔS˚ads is the standard entropy of adsorption, R is the gas constant, and T is the temperature. Under the assumptions that CO2 coverage on the MOF is low and that ΔH˚ads is constant over the temperature range examined, the integrated absorbance, A, at each temperature can be related to the adsorption enthalpy according Eqn. 2.46 ∘ 𝐴 −∆𝐻-./ 𝑙𝑛 ) 3 ∝ ) 3 𝑇 𝑅𝑇

(2)

VTIR spectra of the hydroxylated UiO-66 reveal the presence of two key spectroscopic features upon exposure to CO2 that we hypothesize are due to distinct binding sites in the pore environment of the MOF. A two-Lorentzian-function fit of the νas(CO2) region highlights a narrow band centered at 2340 cm-1 and a lower-intensity broad band at 2336 cm-1 (Figure 3a). In contrast, CO2 exposure on the dehydroxylated MOF yielded only a single feature at 2336 cm-1 (Figure 4a), which was consistent in width and intensity with the 2336 cm-1 band observed in the hydroxylated MOF (Figure 3a). The absence of the band at 2340 cm-1 following exposure of the dehydroxylated sample to CO2 indicates that the feature is related to μ3-OH…OCO type hydrogen bonding at the

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MOF node. Evidence of CO2 hydrogen bonding to the μ3-OH groups of UiO-66 is further supported by the presence of an infrared band at 3641 cm-1 associated with vibrational excitation of a μ3-OH group that is acting as a hydrogen-bond donor (Figure 2a). Upon partial rehydroxylation of the dehydroxylated UiO-66, in which 58 % of the original μ3-OH groups were recovered (Figure 5a), subsequent CO2 exposure resulted in the reemergence of the 2340 cm-1 feature (Figure 5b). We therefore assign the 2340 cm-1 feature to the νas(CO2) mode of CO2 hydrogen bonded to μ3-OH groups on the node of UiO-66. Previous computational studies report that, under low pressure conditions, CO2 preferentially resides in the smaller, tetrahedral cavities of UiO-66 at a distance of 2.36 Å away from the μ3-OH group.26 The calculated location of CO2 molecules within the pore environment26 suggests that weak interactions with the μ3-OH and organic linkers stabilize CO2 within the MOF. Our VTIR measurements are consistent with these findings.

Figure 3: VTIR analysis of 1.5 Torr CO2 on UiO-66 between 297 – 363 K. (a) FTIR spectra of adsorbed CO2 (gas phase CO2 removed). The dashed green and blue traces were determined from a least squares analysis of a two-Lorentzian-function fit of the 297 K spectrum (top black

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trace). (b and c) van’t Hoff plots based on the fitted 2340 and 2336 cm-1 features, where the slope of the best fit lines yields the corresponding −ΔH˚ads. The 2336 cm-1 feature, present in both the hydroxylated and dehydroxylated forms of UiO-66, is most likely associated with the νas(CO2) mode of CO2 confined to the pore environment of UiO66 through purely dispersive interactions with the π-electrons of the BDC linkers. While an alternative assignment for the 2336 cm-1 band may be CO2 binding at defect sites, we find this unlikely given that the TGA results for the UiO-66 samples used in this study indicated that the sample contained a negligible level of missing linker defects, thus very few undercoordinated zirconium sites. Additionally, one would expect a significantly larger effect on the CO2 vibrational frequency if the molecule forms a coordinate covalent bond at coordinatively unsaturated Zr Lewis acid sites.

Figure 4: VTIR analysis of 1.5 Torr CO2 on dehydroxylated UiO-66 between 297 – 343 K. (a) FTIR spectra of adsorbed CO2 (gas phase CO2 removed). The dashed green and blue traces were determined from a least squares analysis of a two-Lorentzian-function fit of the 297 K spectrum (top black trace). (b) van’t Hoff plots based on the fitted 2336 cm-1 feature, where the slope of the best fit line yields −ΔH˚ads.

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Adsorption enthalpy values elucidated from the VTIR studies are consistent with the hydrogen bonded and dispersion-force stabilized CO2 band assignments. As expected, the amount of adsorbed CO2, based on the areas of the two νas(CO2) features, decreased as the sample temperature increased (Figure 3a). As described above, the energetics of the two binding sites were determined through analysis of the temperature dependence to surface coverage. That is, the integrated absorbances of each feature were plotted versus 1/RT (Figure 3b and 3c). The slopes of the van’t Hoff plots for CO2 on hydroxylated UiO-66 reveal ΔH˚ads to be −38.0 ± 1.5 kJ/mol and −30.2 ± 1.3 kJ/mol for the binding sites associated with the 2340 cm-1 and 2336 cm-1 features respectively. Likewise, ΔH˚ads for the binding site associated with the 2336 cm-1 feature on dehydroxylated UiO-66 was determined to be −32.3 ± 1.2 kJ/mol, consistent with the results for the same spectral feature in the hydroxylated MOF.

Figure 5: IR spectra of UiO-66 (black), dehydroxylated UiO-66 (red), and partially rehydroxylated UiO-66 (blue). (a) μ3-OH stretching region, (b) νas(CO2) region with 1.5 Torr CO2 at 297 K. The measured adsorption enthalpies are similar to previous reports that utilized microcalorimetry

and

adsorption

isotherm

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techniques


to

provide

values

between


−26 and −35 kJ/mol.16-17,26 Furthermore, adsorption enthalpies between −30 and −34 kJ/mol for CO2 hydrogen-bonded to μ3-OH groups have been previously calculated using DFT, which is similar to the −38 kJ/mol measured in this work.47-49 The slight discrepancy of the ΔH˚ads values determined in this study compared to those reported in previous works is likely due to the specific condition of the MOFs used in each study. The temperature and pressure of MOF activation, a process designed to remove residual water and solvent following synthesis, has been shown to significantly affect the resulting ΔH˚ads. In fact, overall heats of adsorption for CO2 on UiO-66 have been shown to vary by 9 kJ/mol on the same set of samples.26 Similarly, there are variations in published results on the effect of dehydroxylation of UiO-66. In recent work, both minimal changes to ΔH˚ads and an increase of 6 kJ/mol have been reported,26,49 which suggests small topological differences in the MOFs greatly affect CO2–MOF interactions. Computational Studies of CO2 Binding Geometry and Energetics. Electronic structure calculations have been employed to further test our hypothesis (based on the above experimental results) that two CO2 binding environments exist within UiO-66. Figures 6a and 6b show the two calculated optimum structures for CO2 on UiO-66, (a) CO2 stabilized through dispersive interactions and (b) CO2 hydrogen bonded to a μ3-OH. The frequencies of the two νas(CO2) modes, following application of a scaling factor of 0.9367 that is obtained from the ratio of the calculated gas νas(CO2) frequency to the experimental one, were determined to be 2339 cm-1 and 2344 cm-1 for modes (a) and (b) respectively. The calculated wavenumbers are consistent with the 2336 cm-1 and 2340 cm-1 values measured experimentally. Furthermore, the calculated ΔH˚ads values (298 K) were determined to be −32.5 and −42.5 kJ/mol for the dispersion-stabilized and hydrogen-bonded species respectively, in good agreement with the experimental values, both in the trend (the hydrogen-bonded complex exhibiting a larger adsorption enthalpy) and in absolute value.

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Interestingly, despite the stronger adsorption enthalpy for the hydrogen-bonded CO2, a smaller redshift of the νas(CO2) stretch was observed both experimentally and computationally relative to the weaker, dispersion-stabilized CO2 species. While many gas–solid systems show a positive correlation between binding energy and the extent of the redshift in the fundamental vibration of the gas species, there are several examples of small-molecule systems that do not follow this trend.50-52 The change in vibrational frequencies when a molecule binds to a surface is affected by several factors, each of which may impact the total binding strength to different degrees. For example, a CO2 study on M-MOF-74 (Zn, Mg, Co, Ni) reported the extent of charge transfer from CO2 to each metal was largely responsible for the differences in binding energies. However, the vibrational frequency of the asymmetric CO2 stretch was influenced by changes in bond lengths, asymmetric distortion of the carbon atom position relative to the two oxygens, and the electronic environment of the metal.36 For the CO2–UiO-66 system examined in this study, when CO2 resides in the non-hydroxylated corner of the tetrahedral cages, it is closer to the Zr6O4(OH)4 core than when it forms a hydrogen bond with the μ3-OH group. For instance, the average distances between the closest O atom of CO2 and the 3 Zr atoms defining the octahedral face of the node on which CO2 is binding are 4.28 Å in the non-hydroxylated environment, and 4.44 Å when CO2 forms a hydrogen bond. CO2 is also closer to the carboxylate moiety of the linkers as it sits more deeply in the non-hydroxylated corners (the average OCO—C atom of the carboxylate linkers distance is 3.01 Å in the complex held solely by dispersion interactions and 3.11 Å in the hydrogen-bonded complex). Thus, both of the interactions experienced by CO2 and its location relative to the node and linkers are different for the two binding geometries revealed in this work. These two factors combine to provide distinct electronic environments that subtly affect the force constant for the asymmetric stretching motion of CO2 within in the two environments.

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Figure 6: Optimum structures of CO2 interactions with UiO-66: (a) stabilized through dispersive interactions, (b) hydrogen bonded, and (c) bound to an undercoordinated zirconium at a missing linker defect site. Color code: Zr: cyan, O: red, H: white, C: brown. Electronic structure calculations provide additional insight into other possible binding geometries and energetics of CO2 within UiO-66. DFT calculations indicate that CO2 is capable of binding to defect sites on the node of UiO-66 with an adsorption enthalpy of −57 kJ/mol (Figure 6c). The calculated enthalpy is consistent with adsorption enthalpies found in several other MOF systems that contain high degrees on undercoordinated metal sites including M-MOF-74,21,53 MIL-101,54 and M-BTT.55 The strength of the metal-bound CO2 configuration suggests CO2 molecules would preferentially bind to defect sites over hydrogen bonding and purely dispersive stabilization. As such, one would anticipate the observation of a third infrared feature associated with strongly-bound CO2 at undercoordinated zirconium sites. However, as shown in Figure 3a, only two features were detected under the temperature and pressure conditions examined in our experimental work, which further demonstrates the low-defect density of the MOF employed in our studies.

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IV. Conclusions Systematic infrared spectroscopic studies supported by DFT calculations reveal CO2 adsorption within the pore environment of UiO-66 relies on two main CO2 binding processes within the pore environment UiO-66. CO2 hydrogen bonds to μ3-OH groups within the tetrahedral pores as evidenced by the perturbation of the free μ3-OH stretching vibration at 3674 cm-1 and by the disappearance of the 2340 cm-1 feature following dehydroxylation of the MOF. Additionally, a comparison of CO2 exposures onto hydroxylated and dehydroxylated UiO-66 revealed that CO2 binding also occurs through solely dispersive interactions. A van’t Hoff analysis of the νas(CO2) modes at 2336 and 2340 cm-1 identified the enthalpy of adsorption to be −38.0 ± 1.5 and −30.2 ± 1.3 kJ/mol for the hydrogen bonded and dispersion-stabilized CO2 species respectively. The adsorption enthalpy for dispersion-force stabilized CO2 was unaffected by the absence of μ3-OH groups, providing further confidence of the 2340 cm-1 band assignment. The molecular-level insights into binding locations and energetics presented in this work highlight the advantages of in situ infrared spectroscopic studies conducted under high-vacuum conditions, and provides the groundwork for understanding how topological features in metal-organic frameworks impact gas sorption and storage.

Associated Content Supporting Information Detailed description of the synthesis procedure for the UiO-66 sample and MOF characterization data including: PXRD, SEM, TGA, N2 sorption data, and IR spectroscopy. XYZ coordinates of CO2—UiO-66 complexes. Author Information Corresponding Author: *E-mail: [email protected]. Phone: +1-540-231-2471 (J.R.M.)

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ORCID Tyler G. Grissom: 0000-0001-7337-006X Darren M. Driscoll: 0000-0001-8859-8016 Diego Troya: 0000-0003-4971-4998 Nicholas S. Sapienza: 0000-0002-9876-3860 Amanda J. Morris: 0000-0002-3512-0366 John R. Morris: 0000-0001-9140-5211 Notes The authors declare no competing financial interest. Acknowledgements This material is based upon work supported by the U.S. Army Research Laboratory and the U.S. Army Research Office under grant no. W911NF-15-2-0107. We are grateful for support of the Defense Threat Reduction Agency under program no. BB11PHM156. The work of A.J.M. and P.M.U. was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, under award number DE-SC0012445. The in situ DRIFTS analysis of CO2 exposure was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. The authors acknowledge Advanced Research Computing at Virginia Tech for providing computational resources and technical support that have contributed to the results reported within this paper. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the ARO, DOE, or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright annotation thereon.

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Molecular-Level Insight into CO2 Adsorption on the

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