Diffusion Rates, Energetics, and the Role of Hydrogen Bonding

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Benzene, Toluene, and Xylene Transport through UiO-66: Diffusion Rates, Energetics, and the Role of Hydrogen Bonding Tyler G. Grissom, Conor H Sharp, Pavel M. Usov, Diego Troya, Amanda J Morris, and John R. Morris J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03356 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 6, 2018

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The Journal of Physical Chemistry

Benzene, Toluene, and Xylene Transport through UiO-66: Diffusion Rates, Energetics, and the Role of Hydrogen Bonding

Tyler G. Grissom, Conor H. Sharp, Pavel M. Usov, Diego Troya, Amanda J. Morris, John R. Morris*

Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, United States 

*Corresponding Author Information: Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, USA. E-mail: [email protected]; Tel: +1-540-231-2471

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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: The high-energy demand of benzene, toluene, and xylene (BTX) separation highlights the need for improved non-thermal separation techniques and materials. Due to their high surface areas, tunable structures, and chemical stabilities, metal-organic frameworks (MOFs) are a promising class of materials for use in more energy efficient, adsorption-based separations. In this work, BTX compounds in the pore environment of UiO-66 were systematically examined using in situ infrared (IR) spectroscopy to understand the fundamental interactions that influence molecular transport through the MOF. Isothermal diffusion experiments revealed BTX diffusivities between 10−8 – 10−12 cm2 s−1, where the rate follows the trend: o-xylene < m-xylene < p-xylene. Corresponding activation energies of diffusion (Ediff) were determined to be 44 kJ mol−1 for the xylene isomers and 34 kJ mol−1 for both benzene and toluene with the diffusionlimiting barrier identified to be molecular passage through the small triangular pore apertures of UiO-66. Furthermore, IR spectroscopy and computational methods showed the formation of two types of hydrogen bonds between BTX molecules and the μ3-OH groups located in the tetrahedral cavities of UiO-66, which indicates BTX molecules are capable of fully accessing the inner pore environment of the MOF. The molecular-level insight into the diffusion mechanism and energetics of BTX transport through UiO-66 presented in this work provide rich insight for the design of next-generation MOFs for cost-effective separation processes.

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1. INTRODUCTION Due to its exceptional mechanical and chemical stabilities, lightweight nature, and recyclability, poly(ethylene terephthalate)-based (PET) materials have widespread use as packaging materials for liquids, food, and pharmaceuticals, as well as fibers for clothing and other textile-based applications. PET is primarily made up of ethylene glycol and terephthalic acid, the latter of which utilizes p-xylene as its major precursor. Because p-xylene is obtained from naturally occurring crude oil sources, it must first be isolated and purified from other hydrocarbon-based compounds, specifically benzene, toluene, and xylene isomers (BTX). The similarities in structure and boiling points of BTX compounds (Table 1) result in high energy requirements to effectively separate these compounds, which makes traditional distillation techniques costly. In fact, separation of BTX mixtures requires over 50 GW per year worldwide,1 which substantiates the need for alternative separation techniques. Currently, other methods such as fractional crystallization2-3 and selective adsorption4-8 are used industrially to obtain the pure, individual BTX compounds. Selective adsorption, typically by use of zeolites in simulated moving beds, accounts for over 75% of the market due to the low relative costs.4 However, current separations are carried out at elevated temperatures and pressures, and employ p-xylene-selective sorbents, which require additional energy to desorb and collect the desired p-xylene,9 further underlining the need for improved separation materials. Table 1: Physical Properties of BTX Compounds Compound Boiling Freezing Kinetic Polarizability Dipole Point Point Diameter (Å3) Moment 10 10 (K) (K) (nm) (D) benzene 353.3 278.7 0.5911 10.012 0 11 13 toluene 383.8 178.2 0.59 12.3 0.38 p-xylene 411.4 286.3 0.5910 14.213 0.0 10 13 m-xylene 412.1 225.1 0.68 14.2 0.36 o-xylene 417.4 247.8 0.6810 14.913 0.62

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Recently, metal-organic frameworks (MOFs) have garnered attention as possible BTXseparation materials due to their high surface areas, chemical tunability, and tailorable pore environments. Specifically, many researchers have focused on the zirconium-based MOF, UiO66, due to its high chemical and thermal stabilities, large surface area, and small pore structure.14 UiO-66

is

composed

of

Zr6O4(OH)4

inorganic

nodes

connected

to

twelve

1,5-

benzenedicarboxylate (BDC) organic linkers and has been extensively characterized by a variety of techniques.14-17 This framework exhibits two distinct pore environments – larger, 11 Å diameter octahedral cavities made up of six inorganic nodes, and smaller, 8 Å diameter tetrahedral cavities comprised of four inorganic nodes.18 Four bridging hydroxyl groups (μ3-OH) present in half of the octahedral inorganic node faces, are nominally situated at the corners of the tetrahedral cavities; however, researchers have shown that a small fraction of additional, terminal –OH groups may be present at under-coordinated zirconium sites caused by missing linker defects.16,19 The interface of each octahedral-tetrahedral cavity is a 6.5 Å triangular pore aperture, or window, which extends through the entirety of the structure. For a molecule to traverse through the pores of UiO-66, it must be able to pass through the small triangular windows. The small pore dimensions of UiO-66, which are similar to the critical diameters of many hydrocarbon species, are ideally sized for small hydrocarbon separations. The 6.5 Å triangular apertures can result in a molecular sieving effect where undesired larger molecules are unable to enter the framework interior.20 According to both molecular simulations and experimental results, the small cavities of UiO-66 create a “reverse-shape selectivity” property where the more compact or branched hydrocarbon molecules are preferentially retained over their respective more linear isomers.20-24 Similar behavior has been observed previously in some zeolites including ZSM-5 and MCM-22.25-27 The reverse-shape selectivity property of UiO-66 has been attributed to smaller-

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volume isomers having greater rotational freedom inside of the MOF cavities, which results in a lower entropic penalty during transport through the confined environment. Breakthrough and chromatographic studies suggest the reverse-shape selective property of UiO-66 holds for the xylene isomers, where retention times and selectivity were found to follow the order o-xylene > m-xylene > p-xylene, which is consistent with their relative bulkiness. While breakthrough, adsorption, and modeling studies have shown that UiO-66 exhibits a reverse shape selectivity with hydrocarbons, and as a result, retains o-xylene more favorably than m- and p-xylene, a complete picture of the xylene isomer diffusion mechanism has yet to be elucidated.20-21,24,28 Our objective is to provide a fundamental understanding of how the interactions between BTX compounds and UiO-66 influence transport, and ultimately the ability of UiO-66 to separate these important compounds. The work presented below utilized in situ infrared (IR) spectroscopy within the pristine confines of an ultra-high vacuum (UHV) system to monitor changes in UiO-66 during BTX adsorption, diffusion, and desorption. The results from these experiments provide diffusion coefficients and diffusion energetics for BTX compounds in UiO-66, which allows for the construction of a structure-function relationship for BTX transport through the MOF.

2. EXPERIMENTAL 2.1 Chemicals. For the MOF synthesis: ZrCl4 (Aldrich), terephthalic acid (Acros Organics), concentrated HCl (37% Fisher Chemical), and N,N’-dimethylformamide (DMF) (Fisher Chemical) were used as received. The guest molecules of interest: benzene-d6 (99.6% D, Aldrich), toluene-d8 (99.6% D, Alfa Aesar), o-xylene-d10 (99% D, Aldrich), m-xylene-d10 (98% D, Aldrich), p-xylene-d10 (99% D, Aldrich), 1-tert-butyl-3,5-dimethylbenzene (98%, Aldrich), o-xylene (>

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99%, Aldrich) were placed into stainless steel cylinders affixed to a gas-handling manifold and degassed using three freeze–pump–thaw cycles.29 Fully deuterated compounds were used in this study to allow for easy monitoring of guest molecules due to the unique IR features associated with ν(C–D) vibrations. 2.2 UiO-66 Synthesis and Characterization. The synthesis of UiO-66 was based on an established 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. 37% HCl (0.286 mL, 3.24 mmol) was added and the reaction mixture was stirred at 343 K for 30 min to ensure complete dissolution of starting materials. The resultant solution was transferred into a Teflon-lined Parr reactor, which was heated 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 framework was dried in air at 333 K for 24 h followed by 473 K for 1 h. The structure and phase purity of the synthesized UiO-66 was confirmed using a Rigaku MiniFlex 600 powder X-ray diffractometer with Cu(Kα) radiation (1.5418 Å). The patterns were collected over a 2θ range of 2−50° with a 0.05° step size and 10°/min scan rate. The powdered samples were mounted onto reflective Si(510) disks. Crystal size and morphology was determined using a LEO 1550 field-emission scanning electron microscope (SEM). Thermal stability and defect density were measured using thermogravimetric analysis (TGA) with a TA Q500 analyzer where the UiO-66 sample was loaded in a platinum pan and heated at 2 K/min under a flow of air. The defectivity of UiO-66 was assessed from the TGA data using a previously reported method.14 Briefly, the weight loss was normalized with respect to ZrO2, where the end plateau was set to 100%. The 640 K plateau corresponds to the dehydroxylated UiO-66 with the formula of

6


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[Zr6O6(BDC)6] for an ideal framework. The weight of this structure should be 220% relative to ZrO2. If missing-linker defects are present, the framework would appear lighter and this plateau would deviate from the ideal value. Therefore, by measuring this difference the number of missing linkers per formula unit can be calculated. 2.3 UiO-66 Sample Preparation. The sample preparation procedure was adapted from Sharp et al.30 where approximately 15 mg of UiO-66 was pressed at 6500 psi into the voids of a 50.0 µm thick tungsten mesh (Tech Etch) containing spot-welded, K-type thermocouples. The mesh was mounted to a sample manipulator using nickel support clamps containing copper power leads. The power leads run through a liquid nitrogen compatible dewar and terminate at an external power supply which allowed for precise control of sample temperature through resistive heating and cryogenic cooling. The sample manipulator was placed onto a custom-built vacuum chamber and gases were evacuated from the chamber until a base pressure below 1 × 10−8 Torr was reached. Operation under ultra-high vacuum conditions keeps the sample clean and allows for the probing of gas–MOF interactions free of ambient or contaminant gas contributions. The UiO-66 sample was thermally activated by heating at 448 K for 12 hours to drive off loosely bound, residual DMF remaining from the synthesis and water collected during ambient-condition storage. 2.4 Isothermal Diffusion Studies. Guest-molecule dosing and isothermal diffusion procedures were adapted from previously reported studies.30-32 The UiO-66 sample was loaded with molecules of interest by first reducing the sample temperature to 193 – 243 K, depending on the guest molecule identity, and then exposing the sample to 1 × 10−5 Torr of the vapor of interest for approximately 60 minutes. The chamber was then evacuated until achieving base pressure– about 10 minutes. The MOF sample was mildly heated to remove possible multilayer formation on the exterior of MOF crystallites and cooled back to the dosing temperature for five minutes.

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The sample was then rapidly heated and held at the desired temperature for approximately 90 minutes to allow for gas diffusion out of the UiO-66 sample. At the conclusion of each experiment, the sample was heated at 373 K to drive off remaining guest molecules. 2.5 Infrared Spectroscopy. The UiO-66 sample was monitored with in situ transmission Fourier-transform infrared spectroscopy (FTIR) before, during, and after gas exposure, and during isothermal diffusion experiments to identify adsorbed gas species, track gas transport, and observe changes to the MOF structure. The IR spectra were recorded using a Bruker IFS 66v/S FTIR spectrometer and a liquid-nitrogen-cooled MCT-A detector integrated into a custom-built UHV system. All spectra were collected using 90 scans and 2 cm−1 resolution. A clean, empty spot on the tungsten mesh was employed as the reference background for the spectra reported below. 2.6 Ab initio calculations. The interactions toluene with a tetrahedral cavity of UiO-66 were calculated using density functional theory (DFT). Periodic DFT calculations on the unit cell were performed using the PBE functional with projector augmented-wave potentials and empirical dispersion corrections using VASP.33 The dimensions of the UiO-66 unit cell were first optimized using a 520 Ry cutoff for the plane-wave basis set departing from the crystallographic coordinates. Toluene was then added to a tetrahedral cavity of the optimized unit cell and all atomic coordinates were relaxed within the determined unit cell dimensions using a 400 Ry cutoff. O-H vibrational frequencies were computed via numerical second derivatives of the O and H positions.

3. RESULTS The transport of BTX molecules through UiO-66 has been investigated using in situ infrared spectroscopy under UHV conditions. The overall objective of this work is to build a structure– function relationship for BTX diffusivities and energetics in UiO-66, and to ultimately understand

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the transport mechanism of each compound. Below, we provide evidence that BTX molecules are capable of diffusing into the pores of UiO-66 and that the diffusion-limiting process for each compound is passage through the small triangular apertures located at the interface of the octahedral and tetrahedral cavities. 3.1 UiO-66 Characterization. Prior to experimental measurements, the samples were evaluated for crystallinity, linker defect density, and thermal stability. PXRD and SEM results are indicative of a highly crystalline material with crystallites that average 200 nm in diameter, and are consistent with previously reported literature (Figures S1 and S2).14 Analysis of TGA data (Figure S3) shows the framework exhibited a gradual weight loss between 373 and 623 K, attributed to the removal of trapped solvent molecules, modulator, and finally the dehydroxylation of Zr6 clusters. Above 673 K, the MOF completely loses its structural integrity, as shown in literature reports.14 The defect analysis confirmed that the UiO-66 framework had a near-ideal structure, as evidenced by the close match between the experimental plateau at 643 K and the theoretical mass of [Zr6O6(BDC)6] formula. The infrared spectrum of UiO-66 following thermal activation under UHV conditions, shown in Figure 1, is consistent with previously reported theoretical and experimental results.15-16 Band assignments are listed in the Supporting Information (Table S1). The feature at 3674 cm−1 is assigned to the ν(O–H) vibration of the four free μ3-OH groups found on each inorganic node, which are arranged such that they are positioned in the corners of the small tetrahedral cavities. A second ν(O–H) feature around 3340 cm−1 is likely due to μ3-OH groups hydrogen bonded to strongly adsorbed DMF, residual solvent, which remained within the MOF following the heat treatment. Bands between 3000 and 2800 cm−1 and the shoulder around 1680 cm−1 also support the presence of DMF in the sample. Repeated thermal and chemical cycling of the MOF over the

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course of several months resulted in no observable changes to the infrared spectrum, owning to the high stability of UiO-66 under a variety of conditions30,34-35 and to the high affinity of DMF molecules to the MOF interior (Figure S5).

Figure 1: Infrared spectrum of activated UiO-66 under vacuum. 3.2 Uptake of BTX Molecules into UiO-66. Following characterization, the MOF was exposed to a controlled flux of the compound of interest while uptake was monitored in situ via infrared spectroscopy. Uptake of each deuterated BTX compound into clean UiO-66 was readily observed by the formation of bands between 2000 and 2400 cm−1 assigned to aromatic and aliphatic ν(C–D) vibrations (Figure 2 and Figures S7-S10) consistent with the pure, condensedphase FTIR spectra of the respective BTX compound (Figure S6). Modes associated with UiO-66 remain largely unaffected–with the notable exception of the depletion of the ν(O–H) feature at 3674 cm−1, which coincides with the appearance of new bands between 3625 and 3670 cm−1 attributed to BTX-molecule interaction with μ3-OH groups on the nodes of UiO-66. The number and position of the new ν(O–H) features was found to be dependent on the identity of the specific adsorbed BTX molecule.

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Absorbance

2300 2200 2100 -1 Wavenumber (cm )

2000

Absorbance

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


Absorbance

Page 11 of 47

3700

3680 3660 3640 -1 Wavenumber (cm )

3600

3620

3200

2800 2400 2000 -1 Wavenumber (cm )

1600

1200

800

Figure 2: Infrared spectra recorded during p-xylene-d10 exposure onto UiO-66 over time (black to blue). Insets show close-up view of ν(O–H) (bottom left) and ν(C–D) (top center) regions.

3.3 Isothermal Diffusion Studies. After exposure to the BTX compound of interest, the MOF sample was rapidly heated (0.5 K/s) and held at a desired temperature where the concentration of MOF-bound species was monitored in real time using IR spectroscopy. The area of the ν(C–D) features between 2000-2400 cm−1 was tracked as BTX molecules diffused out of the sample and into the vacuum chamber, where they were rapidly evacuated (Figure 3a). Five diffusion curves, each for a different sample temperature, for p-xylene-d10 are shown in Figure 3b, which displays the peak areas for the infrared spectra (over the range shown in Figure 3a) with respect to time. The loss of integrated IR signal in Figure 3b, is due to removal of p-xylene-d10 molecules from the MOF sample. In addition, a reduction of the ν(O–H) features between 3625 and 3670 cm−1 (attributed to hydrogen-bonding interactions) was also observed during the BTX diffusion experiments.

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1.0

a

Time

Time

b Relative Peak Area

Time Δ Absorbance

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

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0.8 0.6 0.4

v

0.2

iv ii

2120

2100

2080

2060 -1

Wavenumber (cm

2040

iii

i

0.0 0

2000

)

4000 Time (s)

6000

8000

Figure 3: (a) 298 K isothermal diffusion experiment of p-xylene-d10 showing IR intensity loss of the ν(C–D) region over time (blue to red). (b) Relative IR area as a function of time: i) 323 K ii) 313 K iii) 298 K iv) 283 K v) 273 K. Areas are normalized to the spectrum of the maximum loaded MOF for each temperature. Colored dots are experimental data points and solid black lines represent Fickian diffusion best fit curves. As expected, the rate at which BTX molecules diffused out of the MOF was found to depend on the sample temperature, where diffusion was slower at lower temperatures but increased with increasing temperature. Complete removal of BTX molecules from UiO-66 was observed either at the conclusion of the isothermal diffusion experiment or immediately after heating to 373 K for each compound type, which suggests BTX molecules remain only weakly bound to UiO-66 throughout the diffusion process. BTX diffusion out of UiO-66 was found to be consistent with Fick’s second law of diffusion, whereby transport is driven by a concentration gradient (Eqn. 1): 𝜕𝐶(𝑡, 𝑥) 𝜕 2 𝐶(𝑡, 𝑥) =𝐷 𝜕𝑡 𝜕𝑥 2

(1)

where C is concentration, t is time, x is the length position, and D is the diffusion coefficient. Fick’s second law has been previously used to model diffusion of gases through porous media such as

12


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metal oxides and MOFs under vacuum.30,32 A solution to Fick’s second law, shown in Equation 2, provides the functional form for the time dependence of the relative IR area as molecules diffuse through the sample, reach the MOF−vacuum interface, and desorb into the vacuum: ∞

𝐴𝑡 8 2 2 2 =𝜃∑ 2 𝑒 [−𝐷(2𝑛+1) 𝜋 𝑡⁄𝑙 ] 2 𝐴0 𝜋 (2𝑛 + 1)

(2)

𝑛=0

where At is the integrated IR absorbance at time t, A0 is the integrated IR absorbance at maximum vapor loading, θ is the initial, normalized area, D is the diffusion coefficient, and l is the sample thickness (50.0 μm). Isothermal diffusion coefficients for each BTX compound were determined using a twoparameter, θ and D, least-squares fit of the time-dependent absorbance data.36 Best-fit curves for the p-xylene-d10 data is shown in Figure 3b (black lines) and the corresponding calculated diffusion coefficients, which range from 5.80 × 10−10 cm2 s−1 at 273 K up to 9.24 × 10−9 cm2 s−1 at 323 K, are listed in Table 2. Tabulated diffusion coefficients for the other BTX molecules can be found in the Supporting Information (Table S2). Table 2: Diffusion coefficients of p-xylene-d10 in UiO-66 Determined Using a Fickian Diffusion Model T (K) D (10−9 cm2 s−1) 273 0.58 ± 0.01 283 0.93 ± 0.02 283 1.12 ± 0.02 298 3.13 ± 0.08 313 6.81 ± 0.33 323 9.24 ± 0.51 The dependence of D on temperature suggests that diffusion through UiO-66 is an activated process; as such, an Arrhenius-type analysis was conducted to determine the apparent activation energy of diffusion (Ediff) for each BTX compound (Eqn. 3): 𝐷(𝑇) = 𝐷0 𝑒 (−𝐸𝑑𝑖𝑓𝑓 ⁄𝑅𝑇)

13


(3)


where D(T) is the experimentally determined diffusion coefficient, D0 is a pre-factor fitting parameter, T is the sample temperature, R is the ideal gas constant, and Ediff is a fitting parameter that corresponds to the activation energy of diffusion. Ediff values for BTX transport in UiO-66 are tabulated in Table 3 based on the Arrhenius fits found in Figure 4. Both benzene-d6 and toluened8 were found to have an Ediff value around 34 kJ/mol, while the three deuterated xylene isomers displayed similar but higher values in the range of Ediff = 43 – 44 kJ mol−1. Table 3: Activation Energy of Diffusion Values of BTX Molecules in UiO-66 Determined Using a Fickian Diffusion Model Ediff (kJ mol−1) Compound benzene-d6

32.9 ± 1.9

toluene-d8

34.7 ± 1.1

p-xylene-d10

42.7 ± 2.2

m-xylene-d10

44.2 ± 2.5

o-xylene-d10

44.2 ± 2.4

Figure 4: Arrhenius plot for benzene-d6 (black), toluene-d8 (orange), p-xylene-d10 (blue), mxylene-d10 (green), and o-xylene-d10 (red) diffusion through UiO-66 based on a Fickian diffusion model.

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IV. DISCUSSION Determination of relative diffusivities and activation energies of diffusion, coupled with key spectroscopic signatures, provides insight into the diffusion mechanism of BTX through UiO-66. By systematically examining diffusion of each BTX compound in UiO-66, a structure–function relationship that elucidates the role that the chemical structure of aromatic compounds has on the diffusion mechanism is presented below. 4.1 MOF Pore Accessibility. While the infrared spectra recorded during molecular adsorption indicate that the BTX compounds occupy hydroxyl groups within the MOF (Figure 2), the data does not directly reveal the precise location of the adsorption site. That is, molecules either diffuse into the MOF-pore structure or simply bind to hydroxyl groups that may reside at the crystallite surfaces. For a molecule to access the inner pore structure of UiO-66, it must pass through the small, 6.5 Å diameter apertures located between each octahedral and tetrahedral cavity.18,37 Kolokolov, Khudozhitkov, and coworkers found, using solid-state 2H NMR, that benzene, the smallest of the BTX compounds, can access the pore environment of UiO-66, but that the mobility of benzene is severely hindered in the smaller tetrahedral cavities and that the diffusion-limiting step is passage though the triangular apertures.11,38-39 However, prior experimental evidence of the larger BTX molecules entering the pores of UiO-66 is lacking. Computational work suggests that many MOFs, including UiO-66, exhibit some structural flexibility, and as a result, are capable of allowing molecules larger than their aperture to pass through.40-43 Other computational studies, performed by Lennox and Düren, suggest that a methyl group on a xylene isomer may distort the aperture of UiO-66 by 1 Å, thereby sufficiently opening a window for diffusion into the MOF.44 Our results provide the first experimental evidence that toluene and all of the xylene isomers gain full access to the entire pore structure of UiO-66.

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Intra-crystallite BTX diffusion was verified by an experiment in which the impinging flux of o-xylene was increased to the point where 100% of the hydroxyl groups were occupied (Figure 5). The band at 3674 cm−1, attributed to the free MOF μ3-O–H stretch, completely disappeared upon high o-xylene loading. Concurrent growth of a broad feature centered around 3640 cm−1 suggests the electronic nature of the μ3-OH groups is altered by the presence of nearby o-xylene molecules. The red-shift of the μ3-O–H stretch is consistent with other spectroscopic studies where OH–π hydrogen bonding has been observed.30 While traditional descriptions of hydrogen-bond formation requires the hydrogen bond acceptor to have available electron lone pairs,45 evidence of hydrogen bonding that involves any molecule with electron rich regions, including aromatic46-50 and aliphatic30,51-53 compounds have been observed and modeled. Computational studies have shown that hydrogen bonding between –OH groups and aliphatic and aromatic groups is accompanied by transfer of electron density from the hydrogen-bond acceptor to the σ*-antibonding orbital of the donor O–H bond. Occupation of the O–H σ*-antibonding orbital weakens the bond, which results in an elongation and a red-shift of the ν(O–H) vibration that correlates with the extent of charge transfer.52-57

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3690

3680

Time

Time

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


Absorbance

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3670

3660 3650 3640 -1 Wavenumber (cm )

3630

3620

Figure 5: Infrared spectra of the of ν(O–H) of UiO-66 recorded during exposure to o-xylene at 298 K and 170 mTorr (black to red). The complete depletion of the feature at 3674 cm−1 and the concurrent formation of the band around 3640 cm−1 suggests o-xylene is capable of accessing the internal tetrahedral cavities of UiO-66. Full occupation of the hydroxyl groups, both at the crystallite surfaces and within the MOF, with o-xylene molecules, strongly suggests that the molecules diffuse into the MOF to fill the tetrahedral cavities, where μ3-OH groups predominantly reside. In addition, the lower flux studies revealed hydrogen-bond formation between each of the BTX compounds and internal μ3-OH groups as evidenced by a decrease in the intensity of the 3674 cm−1 ν(O–H) band and subsequent growth of infrared absorbance features between 3625 and 3670 cm−1 (Figure 6). We therefore conclude that o-xylene, along with toluene and the smaller m- and p-xylene isomers, are capable of fully accessing the inner-pore environment of UiO-66. Additionally, due to the pristine nature of the MOF crystals used in these studies (see the TGA analysis in Figure S3), the diffusion through defect sites is expected to be negligible. The role that μ3-OH–BTX interactions play in molecular diffusion within the MOF will be discussed in more detail below.

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Figure 6: Difference spectra of hydroxyl region of BTX-MOF systems showing BTXmolecule hydrogen bonding with the μ3-OH groups of UiO-66. Benzene-d6 (black), toluened8 (orange), p-xylene-d10 (blue), m-xylene-d10 (green), and o-xylene-d10 (red). The unexposed UiO-66 sample was used as the background for each spectrum.

4.2 Structure-Dependent Diffusion

Rates

and

Pathways.

Isothermal diffusion

measurements revealed that the relative diffusion rates of BTX compounds depend strongly on molecular structure, even across the xylene isomers. We found the relative rates to follow the trend: benzene-d6 ≈ toluene-d8 > p-xylene-d10 > m-xylene-d10 > o-xylene-d10. The ability of benzene-d6 and toluene-d8 to diffuse faster than the xylene isomers is not surprising given their smaller sizes and polarizabilities relative to the xylene isomers. However, one may not have anticipated that benzene-d6 and toluene-d8 have nearly identical diffusivities within UiO-66. While benzene-d6 and toluene-d8 share similar kinetic diameters (Table 1), the methyl group on toluened8 results in a larger polarizability and dipole moment, which in general allows for stronger dispersive interactions. However, the Arrhenius analysis revealed similar Ediff values for both compounds: 32.9 ± 1.9 and 34.7 ± 1.1 kJ mol−1 for benzene-d6 and toluene-d8, respectively. The similar activation energies of diffusion for benzene-d6 and toluene-d8, suggests the differences in dispersive interactions between the guest molecules and MOF have little effect on the rate-limiting

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process; instead, we suggest that passage through the 6.5 Å triangular apertures is the rate-limiting barrier for migration of both compounds within the MOF. If passage through the triangular apertures of UiO-66 is the diffusion-limiting step, then the additional methyl group on the xylene isomers is likely to result in added hindrance for xylene molecules to migrate through the pore windows. This hypothesis is consistent with the diffusivities and Ediff values determined for the three xylene isomers. Overall, they diffuse much slower than benzene-d6 and toluene-d8, and have Ediff values between 7 – 10 kJ mol−1 higher than those determined for benzene-d6 and toluene-d8 (Table 3). The larger activation energies for the xylene isomers can be attributed to the additional distortion of the triangular apertures required for xylene isomers to pass through. Further investigations into the diffusion-limiting step for BTX transport involved the much larger molecule, 1-tert-butyl-3,5-dimethylbenzene (t-BDMB). The bulky structure of t-BDMB likely precludes the molecule from passage through the triangular apertures. Upon dosing, under the conditions described in the Experimental section, ν(C–H) modes consistent with condensed phase t-BDMB appeared, which signified adsorption onto UiO-66 (Figure S11). The formation of a small feature at 3647 cm−1 and a corresponding small depletion of the ν(O–H) band at 3674 cm−1 was also observed, which indicates a small degree of interaction with -OH groups. However, we speculate that the small decrease in the 3674 cm−1 band is due to interactions with terminal –OH groups located on crystallite surfaces, not the μ3-OH located in the internal tetrahedral cavities.58 Not surprisingly, due to the size of the molecule, the diffusion rate for t-BDMB was found to be considerably slower than the BTX molecules (Figure 7).

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Figure 7: Loss of IR area as a function of time for t-BDMB and p-xylene-d10 in UiO-66 at 273 K. Colored dots are experimental data points and solid black lines represent Fickian diffusion best fit curves. Peak areas were normalized to the ν(C–H) and ν(C–D) bands in the maximum-loaded MOF spectrum for t-BDMB and p-xylene-d10 respectively.

Despite the relatively slow rate of diffusion for t-BDMB, we found the activation energy of diffusion to be only 28.6 ± 0.8 kJ mol−1 (Figure S12). The low Ediff value relative to the xylene isomers suggests that t-BDMB experiences only inter-crystallite surface diffusion, rather than diffusion through the MOF pore structure. We attribute the slow diffusion of t-BDMB to dispersive interactions at the surface of the MOF crystallites, and not due to passage though the pore windows. We therefore suggest that the larger Ediff values for BTX compounds (relative to t-BDMB) is a result of the energies required for the BTX compounds to penetrate the small pore apertures as they traverse the MOF structure. 4.3 Role of Hydrogen Bonding. Despite exhibiting similar activation energies of diffusion, the three xylene isomers moved through UiO-66 at very different rates, which indicates that factors beyond passage through pore windows influence molecular transport. Previously reported measurements of the relative rates for both hexane and xylene isomers to breakthrough UiO-66

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showed a reverse-shape selectivity for hydrocarbons, where smaller, bulkier compounds (oxylene) are retained more favorably than larger, more linear compounds (p-xylene),21 consistent with the relative diffusion rates observed in our work. This selectivity is likely due to the entropic advantages gained where the smaller-volume o-xylene molecules are capable of packing more efficiently inside the cavities of UiO-66 compared to the less compact m- and p-xylene isomers. Additionally, more compact molecules have enhanced rotational freedom in the small tetrahedral cavities, which allows the molecules to interact more strongly with the μ3-OH groups of UiO-66 during transport. Hydrogen-bonding energies between –OH groups and hydrocarbons have been previously calculated to be between 2 and 17 kJ mol−1,47,59 which suggests BTX-molecule interactions with μ3-OH groups may play an important role in the diffusion rates and mechanisms. As described above, exposure of UiO-66 to each BTX molecule resulted in a decrease in the intensity of the 3674 cm−1 band associated with the stretching vibration of the μ3-OH groups in the MOF, and a subsequent growth of features between 3625 and 3670 cm−1. Assignment of the new infrared absorbance bands is aided by the observation that benzene uptake, which binds to the μ3OH groups only through OH---π interactions,47-48 results in a single feature that is red-shifted from the original free μ3-OH band by 6 cm−1. The other molecules investigated exhibit new bands upon uptake that match this frequency; therefore, we attribute the 3668 cm−1 features to interactions between the π-electrons from the aromatic rings and the μ3-OH groups of UiO-66. The observed behavior of ν(O–H) red-shifts is consistent with other spectroscopic studies that involve hydrogen bond formation with hydroxyl groups.30,47,60-61 As shown in the difference spectra of the –OH stretching region in Figure 6, each of the five BTX compounds resulted in the formation of a band 6 cm−1 red-shifted from the free –OH band at 3674 cm−1; however, the appearance of additional features further red-shifted from the 3668 cm−1 band were only observed for toluene-d8 and the

21



xylene-d10 isomers. The additional bands that emerge for molecules other than benzene are 22-29 cm−1 red-shifted from the free μ3-OH vibration at 3674 cm−1. These bands are likely due to hydrogen bonding between the methyl groups on the toluene-d8 and xylene-d10 molecules and the μ3-OH groups of UiO-66. These shifts are consistent with C-H moieties on butane hydrogen bonding in UiO-66 observed by Sharp et al.30 and between a series of short-chain alkanes and – OH groups on zeolites observed by Eder et al.51 The band assignments of the two bonded μ3-OH vibrations were further aided by electronic structure calculations. Figure 8 shows the two calculated optimum structures for toluene suggested from the experiment, one exhibiting an OH---π interaction (left panel), and another one characterized by an OH---CH3 hydrogen bond (right panel). Corroborating the experimental suggestion, we find the size of the ν(O–H) redshift for the OH---π interaction (12 cm−1) to be significantly smaller than the redshift from the OH---CH3 band (41 cm−1). Analogous results were obtained with p-xylene (5 cm-1 and 45 cm-1 O-H redshifts, respectively). OH---CH3

OH---π

Figure 8: Optimum structures of toluene in a tetrahedral cavity of UiO-66 exhibiting an OH--π (left) and an OH---CH3 hydrogen bond (right). The shown atoms have been culled from the full unit cell for clarity. Color code: Zr: yellow, O: red, H: white, C(BDC): cyan, C(toluene): magenta.

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Interestingly, the hydrogen-bond red-shifts observed in this study are much smaller than the 140 – 170 cm−1 red-shift measure by Abelard et al. for benzene, toluene, and p-xylene hydrogen bonded to isolated hydroxyl groups on silica.47 The extent of charge transfer from the aromaticmolecule hydrogen bond acceptor and the O-H donor responsible for the O-H vibrational redshift depends on the energetic overlap of the orbitals involved and the distances between donor and acceptor. Since the donor and acceptor moieties are identical, the much lower O-H redshift in UiO66 relative to silica is likely due to a larger separation between the hydrogen bond donor and acceptor. Indeed, while the access of the aromatics to isolated silanols in silica is largely unrestricted, steric repulsion from the BDC linkers in the 8 Å tetrahedral cavities of UiO-66 hinder full access of the aromatic molecules to the μ3-OH. Our computational results show a OH---π hydrogen bond length of in UiO-66 (4.3 Å) much longer than in silica (2.5 Å, calculated by Abelard et al.), lending confidence to the idea that steric hindrance in UiO-66 limits charge transfer in the OH---π hydrogen bond.47 The identification of only a weak OH---π interaction suggests that hydrogen-bond breakage is not the diffusion-limiting step of BTX transport through UiO-66 and supports the hypothesis that passage through the triangular apertures is the rate limiting process. To further probe how formation of hydrogen bonds affects gas transport and to evaluate whether or not hydrogen-bond breakage is diffusion rate limiting, we repeated the isothermal diffusion experiments for dehydroxylated UiO-66. The hydroxyl groups on the inorganic node of UiO-66 were removed by heating the sample to 573 K for 12 hours under vacuum. Dehydroxylation of UiO-66 by thermal treatment at 573 K has been shown to be a reversible process by which the Zr6O4(OH)4 node is converted to Zr6O6 via a dehydration reaction where two water molecules per node are removed.16 Upon dehydroxylation of UiO-66, some node and pore distortion has been reported,15 however effects on the overall surface area and crystallinity are

23



minimal.15 The dehydroxylation of UiO-66 was confirmed spectroscopically by the disappearance of the ν(μ3-O–H) band at 3674 cm−1 and is consistent with previously published literature.15 Isothermal diffusion experiments were performed in the dehydroxylated UiO-66 sample with p-xylene-d10 to compare diffusivities and activation energies of diffusion for both forms of the MOF. In the absence of μ3-OH groups, p-xylene-d10 was found to diffuse through the sample much faster than through the hydroxylated UiO-66, revealing the importance hydrogen bond formation has on diffusion rates (Figure 9a).

Figure 9: p-Xylene-d10 diffusion through hydroxylated (solid blue) and dehydroxylated (outlined blue) UiO-66. (a) Loss of IR area as a function of time (273 K). (b) Arrhenius plot based on a Fickian diffusion model. The activation energy of diffusion for p-xylene-d10 in dehydroxylated UiO-66 was determined to be 46.9 ± 3.5 kJ mol−1, similar to p-xylene-d10 in hydroxylated UiO-66.

However, the Arrhenius analysis, provided in Figure 9b, reveals an activation energy of diffusion of 46.9 ± 3.5 kJ mol−1, which is within experimental uncertainty to the 42.7 ± 2.2 kJ mol−1 value found for hydroxylated UiO-66. If hydrogen-bond breakage was the diffusion-limiting step, a decrease in the Ediff would be expected upon removal of the hydroxyl groups. Differences

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in diffusivities but similar Ediff values confirm that while hydrogen-bond breakage affects the rate of diffusion, it is not the diffusion-limiting step.

5. CONCLUSIONS The interactions between BTX compounds and UiO-66 have been systematically investigated to elucidate the diffusion rates, energetics, and mechanisms of transport. BTX molecules were shown to access the inner-pore environment of UiO-66 and form hydrogen bonds with the μ3-OH groups located in the small, tetrahedral cavities. Relative diffusion rates of BTX compounds were found to follow the trend: benzene-d6 ≈ toluene-d8 > p-xylene-d10 > m-xylene-d10 > o-xylene-d10, which suggests steric effects and entropic contributions have a significant effect on molecular transport. Hydrogen-bond formation and breakage was also shown to have a major effect on diffusion rates, but these interactions are not the diffusion-rate limiting step. Instead, as noted by the differences in the activation energies of diffusion between the xylene isomers (42-44 kJ mol−1) and the bulky 1-tert-butyl-3,5-dimethylbenzene (29 kJ mol−1), as well as the similarities in the activation energies of diffusion through the hydroxylated (42.7 kJ mol−1) and dehydroxylated (46.9 kJ mol−1) UiO-66, passage through the small triangular windows of UiO-66 is the diffusion limiting barrier for BTX transport.

SUPPORTING INFORMATION AVAILABLE: Included in the supporting information are: a detailed description of the materials and synthesis procedure for the UiO-66 samples, MOF characterization data including: powder x-ray diffraction (PXRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), N2 sorption data, and infrared spectroscopy (IR), Clean BTX compounds characterized by attenuated total reflectance-Fourier Transform infrared

25



spectroscopy (ATR-FTIR), FTIR spectra of UiO-66 exposed to each BTX compound, and a summary of each calculated diffusion coefficient.

Author Information: Corresponding Author: *E-mail: [email protected] ORCID:  Tyler G. Grissom: 0000-0001-7337-006X [email protected] Conor H. Sharp: 0000-0002-5313-0760 [email protected] Pavel M. Usov: [email protected] Diego Troya: 0000-0003-4971-4998 [email protected] Amanda J. Morris: 0000-0002-3512-0366 [email protected] John R. Morris: 0000-0001-9140-5211 [email protected] Notes:  The authors declare no competing financial interest. Acknowledgments: 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. Morris and Usov was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, under Award Number DE-SC0012445. 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. The authors

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acknowledge Advanced Research Computing at Virginia Tech for providing computational resources and technical support that have contributed to the results reported within this paper.

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TOC Graphic

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Absorbance

1 2 3 4 5 6 7 8

2300 2200 2100 -1 Wavenumber (cm )

2000

Absorbance

Absorbance

Page 39 ofThe 47 Journal of Physical Chemistry

3700


3680 3660 3640 -1 Wavenumber (cm )

3600

3200

3620

2800 2400 2000 -1 Wavenumber (cm )

1600

1200

800

1.0

Δ Absorbance

1 2 3 4 5 6 7 8 9 10

Time

Time

Relative Peak Area

The Journalaof Physical Chemistry

Time

Page 40 b of 47

0.8 0.6 0.4

v

0.2

iv

ii ACS Paragon Plus Environment 2120

2100

2080

Wavenumber (cm

2060 -1

)

2040

0.0

i

0

2000

iii 4000 Time (s)

6000

8000

Temperature (K)

370 350 300 of280 260Chemistry 240 Page 41 ofThe 47 Journal Physical 24

220

23

-ln(D)

1 222 3 421 5 20 6 719 8 918 10

ACS Paragon Plus 0.36

0.40

o-xylene-d10 m-xylene-d10 p-xylene-d Environmenttoluene-d8 10 benzene-d6

0.44

1/RT (mol/kJ)

0.48

0.52

Time

Absorbance

1 2 3 4 5 6 7 3690 8

Time

The Journal of Physical Chemistry Page 42 of 47

ACS Paragon Plus Environment 3680

3670

3660 3650 -1 Wavenumber (cm )

3640

3630

3620

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OH---CH OH---π 3 Page 45 ofThe 47 Journal of Physical Chemistry 1 2 3 4 5 6


Temperature (K) b The Journal of Physical Chemistry of 47 320 300 280 260 Page 46240

a 1.0

Relative Peak Area

10.8 2 30.6 4 50.4 6 70.2 8 273 K 90.0 0 1000 10

Hydroxylated UiO-66 Deydroxylated UiO-66

22

Hydroxylated UiO-66 Deydroxylated UiO-66

-ln(D)

21 20 19

ACS Paragon Plus Environment 2000

3000 Time (s)

4000

5000

18 0.38

0.40

0.42 0.44 0.46 1/RT (mol/kJ)

0.48

0.50

Page 47 ofThe 47 Journal of Physical Chemistry 1 2 3 4


Diffusion Rates, Energetics, and the Role of Hydrogen Bonding

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