Tailoring the Adsorption and Reaction Chemistry of the Metal–Organic

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Tailoring the Adsorption and Reaction Chemistry of the Metal− Organic Frameworks UiO-66, UiO-66-NH2, and HKUST‑1 via the Incorporation of Molecular Guests Ann M. Ploskonka† and Jared B. DeCoste*,‡ †

Leidos, Inc., Edgewood Chemical Biological Center, P.O. Box 68, Aberdeen Proving Ground, Maryland 21010, United States Edgewood Chemical Biological Center, U.S. Army Research, Development, and Engineering Command, 5183 Blackhawk Road, Aberdeen Proving Ground, Maryland 21010, United States

‡

S Supporting Information *

ABSTRACT: Metal−organic frameworks (MOFs) are versatile materials highly regarded for their porous nature. Depending on the synthetic method, various guest molecules may remain in the pores or can be systematically loaded for various reasons. Herein, we present a study that explores the effect of guest molecules on the adsorption and reactivity of the MOF in both the gas phase and solution. The differences between guest molecule interactions and the subsequent effects on their activity are described for each system. Interestingly, different effects are observed and described in detail for each class of guest molecules studied. We determine that there is a strong effect of alcohols with the secondary building unit of UiO MOFs, while Lewis bases have an effect on the reactivity of the −NH2 group in UiO-66-NH2 and adsorption by the coordinatively unsaturated copper sites in HKUST-1. These effects must be considered when determining synthesis and activation methods of MOFs toward various applications. KEYWORDS: metal−organic frameworks, catalysis, adsorption, chemical warfare agents, toxic industrial chemicals

1. INTRODUCTION Metal−organic frameworks (MOFs) have become materials of interest in recent years due to their modular nature, high porosity, and wide-ranging applicability in areas including gas storage and separations,1−4 molecular sensing,5,6 catalysis,7 and drug delivery.8,9 For many potential applications, MOFs must have high thermal and chemical stability while maintaining their inherent porosity. Zirconium-based MOFs such as those in the UiO-66 series are of particular interested due to their inherent water stability and ability to incorporate reactive moieties that make them ideal for various applications.10−19 Cu-based MOFs such as HKUST-1 have also been utilized for similar purposes.14,18−20 While HKUST-1 is not as robust as zirconium-based MOFs, it contains a higher overall capacity for certain gases of interest due to the coordinatively unsaturated copper sites that can coordinate to Lewis bases. UiO-66, UiO-66-NH2, and HKUST-1 offer various sites that allow for either chemisorption or physisorption through hydrogen bonding or van der Waals interactions (Figure 1). UiO-66-NH2 has been shown to greatly increase the adsorptive capacity of the MOF over the base material, UiO-66, through hydrogen bonding and increased van der Waals forces from the −NH2 group.21 In particular, UiO-66-NH2 shows similar increased adsorptive capacity in the case of carbon dioxide and methane.22 Furthermore, UiO-66-NH2 has been shown to have enhanced capacity for chlorine gas due to the amine functionality inducing reactive removal via an electrophilic © 2017 American Chemical Society

Figure 1. Crystal structures of UiO-66 (left) and HKUST-1 (right) with carbon (gray), hydrogen (white), oxygen (red), copper (brown), and zirconium (blue) shown.

aromatic substitution reaction rather than simply physisorption.23 In this instance, the −NH2 group acts as an electron donor. and the chlorine molecules react with the aromatic ring. HKUST-1 shows high adsorption for many gases, including NO, CO, H2S, H2O, NH3, and pyridine.19,20,24,25 These molecules have been shown to bind HKUST-1 through the coordinatively unsaturated sites (CUSs) on the copper atoms in Received: May 5, 2017 Accepted: June 8, 2017 Published: June 8, 2017 21579

DOI: 10.1021/acsami.7b06274 ACS Appl. Mater. Interfaces 2017, 9, 21579−21585

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ACS Applied Materials & Interfaces

g, 8.19 mmol), and terephthalic acid (127 mg, 0.764 mmol). The starting materials were added to a 23 mL Teflon liner. Acetone (15 mL) was added, and the solution was sonicated for approximately 5 min. The Teflon liner was transferred to a Parr bomb, and the reaction solution was incubated for 24 h at 140 °C. The solution was filtered, and the resulting solid was washed with acetone (3 × 5 mL). The MOF was activated under vacuum at 150 °C for 24 h. 2.1.3. Synthesis of UiO-66-NH2.32 UiO-66-NH2 was synthesized from zirconium(IV) chloride (152 mg, 0.652 mmol) and 2aminoterephthalic acid (130 mg, 0.718 mmol). The starting materials were added to a 23 mL Teflon liner. Acetone (15 mL) was added, and the solution was sonicated for approximately 5 min. The Teflon liner was transferred to a Parr bomb, and the reaction solution was incubated for 24 h at 100 °C. The solution was filtered, and the resulting solid was washed with acetone (3 × 5 mL). The MOF was activated under vacuum at 150 °C for 24 h. 2.2. Guest Molecule Loading on MOFs. Fifty milligrams of the MOF of interest was placed in a 20 mL scintillation vial. A 2 mL vial was placed inside of the scintillation vial. One milliliter of the desired molecular guest was added to the 2 mL vial. The 20 mL vial was capped and allowed to equilibrate for 24 h at room temperature, after which time the 2 mL vial was removed, and the MOF was characterized. For these experiments MeOH, isopropanol (IPA), DMF, triethylamine (Et3N), acetone, benzene, and pentane were used as guest molecules. 2.3. Physical Measurements. Attenuated total reflectanceFourier transform infrared (ATR-FTIR) spectra were measured on a Bruker Tensor 27 spectrometer from 4000 to 400 cm−1 at a resolution of 2 cm−1. Powder X-ray diffraction (PXRD) patterns were measured on a Rigaku MiniFlex 600 diffractometer equipped with a D/teX Ultra detector with Cu Kα radiation (λ = 1.5418 Å) over a range of 2θ = 3− 50° at a scan rate of 5° min−1. Nitrogen gas isotherm measurements were performed at 77 K on a Micromeritics 3Flex 3500 instrument and analyzed using the BET method over the relevant pressure range.33 Samples were degassed under vacuum at 100 °C for 12 h prior to measurement. 1H NMR spectra were recorded on a 300 MHz Oxford NMR. UiO-66 and UiO-66-NH2 samples were digested in 0.75 mL DMSO-d6 and 10 μL of HF. HKUST-1 samples were digested in 0.75 mL DMSO-d6 and 20 μL of HCl. 2.4. MOF Activity Experiments. 2.4.1. Catalytic Hydrolysis of Methyl Paraoxon. The catalytic hydrolysis of methyl paraoxon (dimethyl (4-nitrophenyl) phosphate, DMNP) by UiO-66 and UiO66-NH2 (Scheme 1) prior to and post guest molecule loading was

the nodes via a Lewis acid−base interaction. Additionally, these molecules tend to bind in a cooperative dimeric fashion such that binding of one molecule to a copper atom influences the binding energy to the second copper atom of the dimer within the secondary building unit (SBU). Under ambient conditions, HKUST-1 readily binds water molecules to the CUSs to achieve saturation conditions at the nodes. It was found that several gases of interest (NO, CO, H2S, and C2H2) do not have binding energies strong enough to overcome these interactions with water.24 Access to the pores of the MOF is crucial to obtain maximum adsorption and reactivity of the material. UiO-66 has rather small pore sizes in its structure, which includes narrow triangular windows with a free diameter of ∼6 Å, octahedral cavities with a diameter of ∼11 Å, and tetrahedral cavities with a diameter of ∼8 Å.26 While analogues of UiO-66 such as UiO66-NH2 have shown increased adsorption and reactivity to certain small molecules of interest, the pore size of these MOFs is diminished due to the presence of the functional group on the linker, which may potentially limit diffusion.27 While HKUST-1 contains pores slightly larger than those of UiO-66, the ability of the MOF to chemisorb rather large species that can act as Lewis bases such as pyridine, as well as its affinity for water, can significantly decrease the accessible pore volume of the sample.28 Residual solvent from MOF synthesis may also impact the surface area and pore size measurements. MOF synthesis is typically conducted solvothermally in which the metal and organic linker precursors are dissolved in solution and incubated in an oven at high temperature.29,30 Dimethylformamide (DMF) and diethylformamide (DEF) are commonly used as solvents for the reaction because they have a high dielectric constant facilitating the dissolution of the starting materials, ensuring maximum yields of crystalline material. However, these solvents have high boiling points, and their ability to hydrogen bond and interact with CUSs makes them difficult to remove from the pores of the MOF.31 Therefore, solvent exchange with a more readily removed solvent such as methanol (MeOH) or acetone is employed prior to activation in an attempt to completely remove the solvent and allow for full evacuation of the pores upon activation. This process requires a large amount of solvent, can take days to complete, and may not fully remove the original solvent from the pores of the MOF. In an effort to curtail this effect, we recently reported a novel synthesis of the UiO-66 family in acetone that allows for complete activation of the pores of the MOF directly from the synthesis product without the need for solvent exchange.32 This process allows for a material completely free of guest molecules that can be systematically loaded with the guest molecules of interest. Herein, we effectively examine the effect of guest molecules on the MOF’s adsorptive and catalytic capabilities by systematically loading the pores of the MOFs of interest, UiO66, UiO-66-NH2, and HKUST-1, with common solvents and other small molecules of varying size, polarity, and functionality.

Scheme 1. Catalytic Hydrolysis of DMNP

evaluated using a previously reported procedure.34 In short, to a 4 mL vial, 2.6 mg of MOF (∼6 mol % Zr) and 1 mL of 0.45 M Nethylmorpholine buffer was added. The solution was stirred for 30 min at room temperature. Methyl paraoxon (4 μL, 0.025 mmol) was added, and the reaction was stirred. Aliquots (10 μL) were removed every 5 min for 30 min, added to a 5 mL volumetric flask, and diluted to the mark with 0.45 M N-ethylmorpholine. The buffering capacity of N-ethylmorpholine was confirmed in each case before and after the reaction by measuring a pH of approximately 9. The 4-nitrophenoxide product peak at 407 nm of the diluted sample was monitored on a JASCO V-650 UV−vis spectrophotometer and used to determine the rate of reaction. 2.4.2. Chlorine and Ammonia Breakthrough Experiments.23,35 A miniaturized breakthrough apparatus was used to evaluate milligramscale quantities of MOF samples for the adsorption of chlorine or ammonia. Approximately 10−15 mg of material was loaded into a nominal 4 mm i.d. fritted glass tube that was subsequently loaded into a water bath for isothermal testing at 20 °C. A ballast with a predetermined quantity of challenge gas was then mixed with a stream

2. EXPERIMENTAL SECTION 2.1. MOF Synthesis and Activation. 2.1.1. HKUST-1. HKUST-1 was purchased from Sigma-Aldrich and activated at 200 °C under vacuum for 24 h. 2.1.2. Synthesis of UiO-66.32 UiO-66 was synthesized from zirconium(IV) chloride (152 mg, 0.652 mmol), benzoic acid (1.00 21580

DOI: 10.1021/acsami.7b06274 ACS Appl. Mater. Interfaces 2017, 9, 21579−21585

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ACS Applied Materials & Interfaces of dry air (−40 °C dew point) at a rate necessary to achieve a challenge concentration of 2000 mg m−3. The contaminated air stream was then sent through the fritted glass tube at a flow rate of 20 mL min−1, equivalent to a residence time of approximately 0.10 s. The effluent stream was sent through a photoionization detector with an 11.7 eV argon lamp to monitor the chlorine or ammonia concentration. The data is reported and plotted as the signal at a given time divided by the signal at saturation (C/C0) vs normalized time (the time divided by the mass of the sample used). The corresponding breakthrough curve was integrated to determine the loading at saturation. 2.4.3. Adsorption and Reaction of 2-Chloroethyl Ethyl Sulfide by UiO-66-NH2. To determine the effect of each molecular guest on the adsorption and reaction of 2-chlorethyl ethyl sulfide (CEES), a simulant for the chemical warfare agent bis(2-chloroethyl) sulfide (sulfur mustard, HD), equilibrium vapor phase measurements were performed. Approximately 20 mg of the MOF was placed in the bottom of a 4 dram vial; then, a 2 mL vial was placed inside of the larger vial, and 200 μL of CEES (excess) was placed in the 2 mL vial. The small vial was uncapped, while the large vial was sealed for a known amount of time. Each sample was removed, and approximately 10 mg of the MOF was digested in 0.75 mL of d6-DMSO and 10 μL of HF. The resulting solution was analyzed via 1H NMR to determine the amount of CEES adsorbed and reacted.

their lack of functionality that can interact with the MOF and their larger size. The presence of coordinatively unsaturated copper sites in HKUST-1 causes different interactions of the guest molecules with the MOF. While the alcohols still exhibit high uptakes, likely due to the interaction of the unpaired electrons on the oxygen atom with the electron deficient Cu sites, an enhancement in the uptake of the amines and acetone was observed as well. Interestingly, HKUST-1 also exhibited an uptake of benzene higher than that of the UiO MOFs, which is likely due to the fact that the aromatic ring of the trimesic acid linker is more exposed in HKUST-1 than the terephthalic acid linker in UiO-66, leading to favorable π−π stacking interactions. The structural effect of guest molecules on each MOF was assessed by PXRD and ATR-FTIR. UiO-66 and UiO-66-NH2 showed no changes in the PXRD to indicate perturbation of the structure by any of the guest molecule species studied (Figure 2). ATR-FTIR data show evidence for the presence of guest molecules in the structure with corresponding peaks observed in the spectra (1016 cm−1 for methanol, 1652 cm−1 for DMF, and 1703 cm−1 for acetone). However, it does not show evidence of the structure being perturbed as the original peaks for the MOF itself are unaltered by the presence of guest molecules. HKUST-1, however, exhibits structural degradation in the PXRD pattern when exposed to Et3N (Figure 2). This observation is supported by the ATR-FTIR data, which shows the disappearance of bands at approximately 1600 cm−1, consistent with perturbation of the Cu−carboxylate bonds.14 The ability of the copper atoms in HKUST-1 to bind the nitrogen atom in the amines has been shown previously to lead to the degradation of the crystalline structure in the case of ammonia.20 3.2. Effect of Guest Molecules on the Catalytic Hydrolysis by UiO MOFs. The catalytic hydrolysis of DMNP by UiO-66 and UiO-66-NH2 samples with and without guest molecules was used to determine the effect on solution state reactivity at the SBU. As the highest molecular loadings were approximately 25 wt %, we kept the mass of catalyst used in each experiment constant. As can be seen in Table 2 (Figures S1 and S2), in general, UiO-66-NH2 exhibits a reaction rate faster than that of UiO-66 for each sample studied. Katz et al. attributed this phenomenon to the −NH2 group aiding in the sorption of DMNP and guiding the reaction toward the Zr− OH-Zr sites on the SBU of the MOF.37 Of course, the addition of the −NH2 group to the MOF would constrict the already small pore openings (∼6 Å),12 indicating that much, if not all, of the reaction likely occurs on the MOF surface in UiO-66. The activated UiO-66 and UiO-66-NH2 prepared here from acetone exhibited faster reaction rates (t1/2 = 19 and 5.0 min, respectively) than those of typical samples prepared in DMF (t1/2 = 45 and 10 min, respectively).34,38 For most of the guest molecules studied, there was a minimal effect on the rate of reaction with the MOF of interest, and in some cases, the rate of reaction was even faster. This is likely due to the guest molecules easily diffusing out of the pores in aqueous solution and not inhibiting the catalytic hydrolysis of DMNP. It is interesting to note here that many of the guest molecules studied have carbonyl and amine groups that may aid in the proton transfer and stabilization of intermediates during the hydrolysis of DMNP. However, in the case of both UiO-66 and UiO-66-NH2, there was a significant decrease in the rate of reaction for samples containing MeOH and IPA, consistent

3. RESULTS AND DISCUSSION 3.1. Guest Molecule Adsorption on MOFs. The uptake capacity of UiO-66 (SABET = 850 m2 g−1), UiO-66-NH2 (SABET = 675 m2 g−1), and HKUST-1 (SABET = 1640 m2 g−1) for each molecule in the vapor phase was assessed using a vial-in-vial setup with an excess amount of the chemical of interest present. The guest molecule loading for each sample was determined by digesting the MOF and comparing the signal of the guest molecule to that of the MOF linker, as shown in Table 1. It Table 1. Guest molecule loading in UiO-66, UiO-66-NH2, and HKUST-1 guest molecule

UiO-66 (mol kg−1)

UiO-66-NH2 (mol kg−1)

HKUST-1 (mol kg−1)

MeOH isopropanol dimethylformamide triethylamine acetone benzene pentane

6.2 3.8 3.9 2.1 3.1 1.2 0.84

5.7 4.2 3.5 3.6 2.6 1.7 1.1

18 15 15 9.2 16 11 2.6

should be noted here that the surface area measurements for UiO-66 and UiO-66-NH2 are slightly lower than some other reports in the literature. The crystallinity, surface area, and defects in UiO MOFs can be controlled by varying the solvent, ratio of reactants, type of modulator, and synthesis temperature.36 Here, we use UiO-MOFs synthesized from acetone due to their ability to be completely activated and therefore have consistency throughout the samples studied here.32 UiO-66 and UiO-66-NH2 showed very similar uptake capacities for each chemical studied with the exception of Et3N, for which UiO-66-NH2 exhibited a much higher capacity. This is consistent with the primary adsorption site the UiO-66 MOFs being at the SBU and the linker playing only a secondary role in enhancing the uptake. MeOH and IPA had the highest uptake values, likely due to their small size and the ability of the alcohol groups to hydrogen bond with the SBU. Conversely, benzene and pentane showed the lowest uptake, likely due to 21581

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Figure 2. ATR-FTIR spectra (left) and PXRD patterns (right) for (a) UiO-66, (b) UiO-66-NH2, and (c) HKUST-1 upon exposure to various guest molecules.

Table 2. DMNP Half-Lives for UiO-66 and UiO-66-NH2 Loaded with Various Guest Molecules guest molecule activated MeOH isopropanol dimethylformamide triethylamine acetone benzene pentane

UiO-66 (min) 17 100 150 5.5 9.1 5.8 10 4.6

Table 3. Chlorine and Ammonia Uptake for UiO-66-NH2 and HKUST-1 Loaded with Various Guest Molecules

UiO-66-NH2 (min) 4.0 20 12 7.0 10 5.7 8.9 7.6

with the high levels due to hydrogen bonding with the SBU. This is consistent with the much slower reaction rates of these samples, as the active catalytic sites on the SBU are blocked from performing the catalytic hydrolysis of DMNP. 3.3. Effect of Guest Molecules on Electrophilic Aromatic Substitution by UiO-66-NH2. In an effort to understand how the presence of these guests affect solid−gas reactions of the organic linker with an adsorbate, microbreakthrough experiments of chlorine were performed on UiO66-NH2.23,39 All samples loaded with guest molecules showed some loss of the chlorine loading from the 6.6 mol kg−1 observed for the fully activated sample, as seen in Table 3 and

guest molecule

Cl2 on UiO-66-NH2 (mol kg−1)

NH3 on HKUST-1 (mol kg−1)

activated MeOH isopropanol dimethylformamide triethylamine acetone benzene pentane

6.6 5.3 2.8 3.8 2.8 2.5 1.5 1.5

7.7 4.5 4.2 1.0 1.7 1.1 5.6 4.8

Figure S3. The reactive site in UiO-66-NH2 for chlorine are at the carbon atoms ortho- and para- to the −NH2 group, as the reaction proceeds via an electrophilic aromatic substitution.23 Interestingly, the chlorine loadings were not inversely proportional to the guest molecule loadings in all cases as one may hypothesize. The largest drop off in chlorine uptake was observed for pentane and benzene, which had the two lowest guest molecule loadings. This is likely due to these molecules not interacting strongly with the SBU but instead adsorbing to the organic linker, in turn hampering the reaction of chlorine with the linker. On the other hand, MeOH and DMF did not 21582

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ACS Applied Materials & Interfaces show as large of a drop off in loading, which can be attributed to these molecules interacting mainly with the SBU and the −NH2 group on the linker, respectively, neither of which directly participate in the reaction with chlorine. 3.4. Effect of Guest Molecules on the Adsorption and Reaction of 2-Chloroethyl Ethyl Sulfide by UiO-66-NH2. While DMNP hydrolysis and chlorine microbreakthrough experiments were able to analyze the catalytic activity of the SBU and the reactive removal via electrophilic aromatic substitution of the linker, respectively, the adsorption and reaction of UiO-66-NH2 with CEES was used to probe the perturbation of the reactivity of the −NH2 group by guest molecules. Upon digestion of the MOF in HF, the unperturbed 2-amino terephthalic acid linker, unreacted CEES, and the 2-(2(ethylthio)ethylamino)terephthalic acid reaction product were observed, as seen in Scheme 2. The ratios of these species were Scheme 2. Reaction of CEES with the 2-Aminoterephthalic Acid Linker in UiO-66-NH2

compared to determine the amount of CEES that physisorbed into UiO-66-NH2 as well as the amount that reacted with the linker. An initial experiment was performed on activated UiO66-NH2 to determine the time the adsorption and reaction of CEES takes to reach equilibrium, as can be seen in Figure 3. We determined for subsequent experiments with each of the guest molecules that 24 h was an appropriate time for the adsorption to reach equilibrium. When comparing the effect of each of the guest molecules on the reactivity of the 2-amino terephthlic acid linker with CEES, it was interesting that MeOH, IPA, pentane, benzene, and acetone all exhibited an enhancement of 20−30% in the amount of reactivity when compared to the fully activated material. However, both of the bases, DMF and Et3N, showed a significant drop off in the activity of the linker toward CEES. As this does not follow a trend with the guest molecule loading, it is evident that the type of species plays a role in the activity. Interestingly, these two guest species show the greatest presence after exposure to CEES in the digested UiO-66NH2 1H NMR spectra (Figure S4), lending evidence that the bases were not as easily displaced by CEES as the other guest molecules were. It is likely that DMF and Et3N interact with the amphoteric −NH2 group in an acid−base type interaction, preventing the ability of the group to perform a nucleophilic substitution reaction on CEES. However, the other functional groups do not have the same type of interaction with the −NH2 group on the linker, allowing the CEES reaction to take place. In fact, the presence of other guest molecules is actually aiding in this reaction, likely due to either a slight swelling of the pores, reducing steric hindrance, or providing sites that can assist in the proton transfer from the amine group to produce the HCl leaving group. The maximum reaction capacity achieved for any of the guest molecules was approximately 1 CEES molecule reacted for every 3 linkers, assuming there are 12 linkers per SBU corresponding to approximately 2 CEES molecules reacted per octahedral cage.

Figure 3. Kinetic loading of CEES on guest molecule samples (top, lines added as a guide) and amount of reacted and physisorbed CEES on each guest molecule sample (bottom).

When further taking adsorption of CEES into account, the activated material and the material containing MeOH guests achieve the highest capacities. While again UiO-66-NH2 has a higher capacity for MeOH compared to any other guest studied, it is apparent that it is easily displaced by the sorption of CEES. The total capacity 0.9 molecules of CEES per linker for UiO-66-NH2 corresponds to nearly 6 CEES molecules per cage, indicating that, while the reactive capacity of UiO-66-NH2 is high, so is the physisorption capacity. There is approximately a 20−30% drop off in CEES capacity for IPA, pentane, benzene, and acetone, indicating that, while CEES can still favorably be adsorbed by UiO-66-NH2, these guests are not as easily displaced and continue to occupy some of the pore volume. For the bases DMF and Et3N there is greater than a 50% loss in CEES capacity, indicating that, upon adsorption, these guests have a more permanent effect on UiO-66-NH2 than on the other guests studied. 3.5. Effect of Guest Molecules on the Adsorption Ammonia by HKUST-1. The case of HKUST-1 for ammonia adsorption was evaluated to determine the effect of guest molecules on coordinatively unsaturated copper sites. Ammonia binding in MOFs has been extensively studied in recent years, both computationally and experimentally, to elucidate the exact mechanism for binding in HKUST-1 and similar MOF structures.35,40 Interaction with ammonia can occur either via hydrogen bonding with the organic linker of the MOF or Lewis acid−base interaction with metal sites at the node.41 In the case of HKUST-1, ammonia binding primarily occurs due to the 21583

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to use during the synthetic procedures and treatments, as certain molecular guests can be difficult to activate from, as well as have an adverse effect toward, a desired application.

interaction of the lone pair on the nitrogen in the ammonia molecule with the coordinatively unsaturated copper sites in the SBU of the MOF.20 The capacities were found to be diminished in each of the samples containing guest molecules in the pores when compared to the activated material (7.7 mol kg−1), as seen in Table 3 and Figure S5. In particular, acetone, DMF, and Et3N exhibited the largest effects on the ability of the MOF to adsorb ammonia. Acetone and DMF contain similar carbonyl groups, while DMF and Et3N contain similar nitrogen atoms, both of which can act as Lewis bases. These functional groups allow these guest molecules all to have the ability to bind the coordinatively unsaturated copper sites on the MOF. This interaction is quite strong, and therefore it is difficult for ammonia molecules to displace these Lewis bases and adsorb to the copper atoms of the SBU. Furthermore, the addition of Et3N to HKUST-1 exhibited some structural degradation in the PXRD (Figure 2), which may have an adverse effect on the adsorption of ammonia as well. Conversely, MeOH and IPA exhibit relatively high guest molecule loadings; however, their ammonia capacities are much higher than those of DMF, ET3N, and acetone, indicating that alcohols are more easily displaced by ammonia. The effect of benzene and pentane on the adsorption of ammonia is not as great as that observed with the other guest molecules. While pentane had a low loading on HKUST-1, this was not the case for benzene. However, because benzene interacts with the organic linker through a π−π interaction, the copper sites are not perturbed, leaving them free to adsorb ammonia.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06274. UV−vis kinetics data plots, chlorine microbreakthrough traces, 1H NMR of digested UiO-66-NH2 exposed to CEES, and ammonia microbreakthrough traces (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jared B. DeCoste: 0000-0003-0345-2697 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge support by the Joint Science and Technology Office for Chemical and Biological Defense (JSTO-CBD) for support under Project BA13PHM210, Dr. Trenton M. Tovar for collection of nitrogen isotherm data, and Mr. Matthew A. Browe for conducting microbreakthrough experiments.

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4. CONCLUSIONS Increased knowledge of how guest molecules interact with MOFs of interest can help to direct research toward application and tailor syntheses such that adsorption and reaction capacities and kinetics are maximized for various systems. Herein, we explored the effect of guest molecules on both gas phase and solution reactivity. Overall, the guest molecules were found to affect the reactivity of the MOF by interacting with either the SBU or the linker. UiO-66 and UiO-66-NH2 show similar trends in both the uptake of the various guest molecules of interest and reactivity of the SBU in the solution phase, as assessed by the catalytic hydrolysis of DMNP. While most guest molecules had little impact on the kinetics of hydrolysis, there was a large adverse effect by alcohols, indicating they interact strongly with the SBU and can perturb the reaction. However, the gas phase reactivity for UiO-66-NH2 with chlorine gas showed the largest reaction inhibition by alkane and aromatic guest molecules, indicating that the guest molecules in this case affect the reactivity by interacting with the linker on the MOF rather than the SBU. The case of a nucleophilic reaction of the −NH2 group of UiO-66-NH2 with CEES showed that the bases (DMF and Et3N) had the greatest adverse effect on reactivity. Ammonia adsorption by HKUST-1 was used as a model system to determine the effect of the guest molecules on coordinatively unsaturated sites. DMF, Et3N, and acetone had the largest effect on the adsorption of ammonia due to the unpaired electrons of these species binding strongly to the copper sites. In summary, it is important to understand the potential interactions of various guest molecules for an application of interest. The tailorability of MOFs allows the user to present various types of sites that can be utilized in multiple ways. Understanding the interactions of these guest molecules with a MOF of interest allows one to make decisions on the solvents

REFERENCES

(1) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M. Systematic Design of Pore Size and Functionality in Isoreticular MOFs and Their Application in Methane Storage. Science 2002, 295, 469−472. (2) Suh, M. P.; Park, H. J.; Prasad, T. K.; Lim, D.-W. Hydrogen Storage in Metal−Organic Frameworks. Chem. Rev. 2012, 112, 782− 835. (3) Li, J.-R.; Sculley, J.; Zhou, H.-C. Metal−Organic Frameworks for Separations. Chem. Rev. 2012, 112, 869−932. (4) Sumida, K.; Rogow, D. L.; Mason, J. A.; McDonald, T. M.; Bloch, E. D.; Herm, Z. R.; Bae, T.-H.; Long, J. R. Carbon Dioxide Capture in Metal−Organic Frameworks. Chem. Rev. 2012, 112, 724−781. (5) Cui, Y.; Yue, Y.; Qian, G.; Chen, B. Luminescent Functional Metal−Organic Frameworks. Chem. Rev. 2012, 112, 1126−1162. (6) Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne, R. P.; Hupp, J. T. Metal−Organic Framework Materials as Chemical Sensors. Chem. Rev. 2012, 112, 1105−1125. (7) Song, F.; Wang, C.; Lin, W. A chiral metal-organic framework for sequential asymmetric catalysis. Chem. Commun. 2011, 47, 8256− 8258. (8) Horcajada, P.; Chalati, T.; Serre, C.; Gillet, B.; Sebrie, C.; Baati, T.; Eubank, J. F.; Heurtaux, D.; Clayette, P.; Kreuz, C.; Chang, J.-S.; Hwang, Y. K.; Marsaud, V.; Bories, P.-N.; Cynober, L.; Gil, S.; Ferey, G.; Couvreur, P.; Gref, R. Porous metal-organic-framework nanoscale carriers as a potential platform for drug delivery and imaging. Nat. Mater. 2010, 9, 172−178. (9) Rodrigues, M. O.; de Paula, M. V.; Wanderley, K. A.; Vasconcelos, I. B.; Alves, S.; Soares, T. A. Metal organic frameworks for drug delivery and environmental remediation: A molecular docking approach. Int. J. Quantum Chem. 2012, 112, 3346−3355. (10) Schoenecker, P. M.; Carson, C. G.; Jasuja, H.; Flemming, C. J. J.; Walton, K. S. Effect of Water Adsorption on Retention of Structure and Surface Area of Metal−Organic Frameworks. Ind. Eng. Chem. Res. 2012, 51, 6513−6519. 21584

DOI: 10.1021/acsami.7b06274 ACS Appl. Mater. Interfaces 2017, 9, 21579−21585

Research Article

ACS Applied Materials & Interfaces (11) Burtch, N. C.; Jasuja, H.; Walton, K. S. Water Stability and Adsorption in Metal−Organic Frameworks. Chem. Rev. 2014, 114, 10575−10612. (12) Cavka, J. H.; Jakobsen, S.; Olsbye, U.; Guillou, N.; Lamberti, C.; Bordiga, S.; Lillerud, K. P. A New Zirconium Inorganic Building Brick Forming Metal Organic Frameworks with Exceptional Stability. J. Am. Chem. Soc. 2008, 130, 13850−13851. (13) DeCoste, J. B.; Peterson, G. W.; Jasuja, H.; Glover, T. G.; Huang, Y.-g.; Walton, K. S. Stability and degradation mechanisms of metal-organic frameworks containing the Zr6O4(OH)4 secondary building unit. J. Mater. Chem. A 2013, 1, 5642−5650. (14) DeCoste, J. B.; Peterson, G. W.; Schindler, B. J.; Killops, K. L.; Browe, M. A.; Mahle, J. J. The effect of water adsorption on the structure of the carboxylate containing metal-organic frameworks CuBTC, Mg-MOF-74, and UiO-66. J. Mater. Chem. A 2013, 1, 11922− 11932. (15) Küsgens, P.; Rose, M.; Senkovska, I.; Fröde, H.; Henschel, A.; Siegle, S.; Kaskel, S. Characterization of metal-organic frameworks by water adsorption. Microporous Mesoporous Mater. 2009, 120, 325−330. (16) Greathouse, J. A.; Allendorf, M. D. The Interaction of Water with MOF-5 Simulated by Molecular Dynamics. J. Am. Chem. Soc. 2006, 128, 10678−10679. (17) Jasuja, H.; Burtch, N. C.; Huang, Y.-g.; Cai, Y.; Walton, K. S. Kinetic Water Stability of an Isostructural Family of Zinc-Based Pillared Metal−Organic Frameworks. Langmuir 2013, 29, 633−642. (18) Peterson, G. W.; Wagner, G. W. Detoxification of chemical warfare agents by CuBTC. J. Porous Mater. 2014, 21, 121−126. (19) Min Wang, Q.; Shen, D.; Bülow, M.; Ling Lau, M.; Deng, S.; Fitch, F. R.; Lemcoff, N. O.; Semanscin, J. Metallo-organic molecular sieve for gas separation and purification. Microporous Mesoporous Mater. 2002, 55, 217−230. (20) Peterson, G. W.; Wagner, G. W.; Balboa, A.; Mahle, J.; Sewell, T.; Karwacki, C. J. Ammonia Vapor Removal by Cu3(BTC)2 and Its Characterization by MAS NMR. J. Phys. Chem. C 2009, 113, 13906− 13917. (21) Schoenecker, P. M.; Belancik, G. A.; Grabicka, B. E.; Walton, K. S. Kinetics study and crystallization process design for scale-up of UiO66-NH2 synthesis. AIChE J. 2013, 59, 1255−1262. (22) Luu, C. L.; Nguyen, T. T. V.; Nguyen, T.; Hoang, T. C. Synthesis, characterization and adsorption ability of UiO-66-NH2. Advances in Natural Sciences. Adv. Nat. Sci.: Nanosci. Nanotechnol. 2015, 6, 025004. (23) DeCoste, J. B.; Browe, M. A.; Wagner, G. W.; Rossin, J. A.; Peterson, G. W. Removal of chlorine gas by an amine functionalized metal-organic framework via electrophilic aromatic substitution. Chem. Commun. 2015, 51, 12474−12477. (24) Watanabe, T.; Sholl, D. S. Molecular chemisorption on open metal sites in Cu3(benzenetricarboxylate)2: A spatially periodic density functional theory study. J. Chem. Phys. 2010, 133, 12. (25) Petit, C.; Mendoza, B.; Bandosz, T. J. Hydrogen Sulfide Adsorption on MOFs and MOF/Graphite Oxide Composites. ChemPhysChem 2010, 11, 3678−3684. (26) Katz, M. J.; Brown, Z. J.; Colon, Y. J.; Siu, P. W.; Scheidt, K. A.; Snurr, R. Q.; Hupp, J. T.; Farha, O. K. A facile synthesis of UiO-66, UiO-67 and their derivatives. Chem. Commun. 2013, 49, 9449−9451. (27) Peterson, G. W.; DeCoste, J. B.; Fatollahi-Fard, F.; Britt, D. K. Engineering UiO-66-NH2 for Toxic Gas Removal. Ind. Eng. Chem. Res. 2014, 53, 701−707. (28) Azhar, M. R.; Abid, H. R.; Sun, H.; Periasamy, V.; Tadé, M. O.; Wang, S. One-pot synthesis of binary metal organic frameworks (HKUST-1 and UiO-66) for enhanced adsorptive removal of water contaminants. J. Colloid Interface Sci. 2017, 490, 685−694. (29) Hu, Z.; Peng, Y.; Kang, Z.; Qian, Y.; Zhao, D. A Modulated Hydrothermal (MHT) Approach for the Facile Synthesis of UiO-66Type MOFs. Inorg. Chem. 2015, 54, 4862−4868. (30) Hu, Z.; Zhao, D. De facto methodologies toward the synthesis and scale-up production of UiO-66-type metal-organic frameworks and membrane materials. Dalton Trans. 2015, 44, 19018−19040.

(31) Stock, N.; Biswas, S. Synthesis of Metal-Organic Frameworks (MOFs): Routes to Various MOF Topologies, Morphologies, and Composites. Chem. Rev. 2012, 112, 933−969. (32) Ploskonka, A. M.; Marzen, S. E.; DeCoste, J. B. Facile Synthesis and Direct Activation of Zirconium Based Metal−Organic Frameworks from Acetone. Ind. Eng. Chem. Res. 2017, 56, 1478−1484. (33) Walton, K. S.; Snurr, R. Q. Applicability of the BET Method for Determining Surface Areas of Microporous Metal−Organic Frameworks. J. Am. Chem. Soc. 2007, 129, 8552−8556. (34) Katz, M. J.; Mondloch, J. E.; Totten, R. K.; Park, J. K.; Nguyen, S. T.; Farha, O. K.; Hupp, J. T. Simple and Compelling Biomimetic Metal−Organic Framework Catalyst for the Degradation of Nerve Agent Simulants. Angew. Chem., Int. Ed. 2014, 53, 497−501. (35) Jasuja, H.; Peterson, G. W.; Decoste, J. B.; Browe, M. A.; Walton, K. S. Evaluation of MOFs for air purification and air quality control applications: Ammonia removal from air. Chem. Eng. Sci. 2015, 124, 118−124. (36) Shearer, G. C.; Chavan, S.; Ethiraj, J.; Vitillo, J. G.; Svelle, S.; Olsbye, U.; Lamberti, C.; Bordiga, S.; Lillerud, K. P. Tuned to Perfection: Ironing Out the Defects in Metal−Organic Framework UiO-66. Chem. Mater. 2014, 26, 4068−4071. (37) Katz, M. J.; Klet, R. C.; Moon, S.-Y.; Mondloch, J. E.; Hupp, J. T.; Farha, O. K. One Step Backward Is Two Steps Forward: Enhancing the Hydrolysis Rate of UiO-66 by Decreasing [OH−]. ACS Catal. 2015, 5, 4637−4642. (38) Peterson, G. W.; Moon, S.-Y.; Wagner, G. W.; Hall, M. G.; DeCoste, J. B.; Hupp, J. T.; Farha, O. K. Tailoring the Pore Size and Functionality of UiO-Type Metal−Organic Frameworks for Optimal Nerve Agent Destruction. Inorg. Chem. 2015, 54, 9684−9686. (39) Browe, M. A.; Napolitano, A.; DeCoste, J. B.; Peterson, G. W. Filtration of chlorine and hydrogen chloride gas by engineered UiO66-NH2 metal-organic framework. J. Hazard. Mater. 2017, 332, 162− 167. (40) Kim, K. C.; Yu, D.; Snurr, R. Q. Computational Screening of Functional Groups for Ammonia Capture in Metal−Organic Frameworks. Langmuir 2013, 29, 1446−1456. (41) Yu, D.; Ghosh, P.; Snurr, R. Q. Hierarchical modeling of ammonia adsorption in functionalized metal-organic frameworks. Dalton Trans. 2012, 41, 3962−3973.

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DOI: 10.1021/acsami.7b06274 ACS Appl. Mater. Interfaces 2017, 9, 21579−21585