Article pubs.acs.org/IECR
Facile Synthesis and Direct Activation of Zirconium Based Metal− Organic Frameworks from Acetone Ann M. Ploskonka,† Stephanie E. Marzen,† and Jared B. DeCoste*,‡ †
Leidos, Inc., 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: In recent years much emphasis has been placed on the synthesis of highly novel metal−organic frameworks (MOFs) with general disregard to development of sustainable synthesis techniques. A novel synthesis of UiO-66 and UiO-66-NH2, two highly stable MOFs that have shown much promise in the area of catalysis and reactive removal of small molecules, from acetone is demonstrated here. Using this method, the MOFs can be activated by simple heating under vacuum without the need for solvent exchange, which can be a timely processing step that requires the use of large amounts of solvent. The activity of the series of MOFs synthesized at various temperatures was determined by the rate of hydrolysis of methyl paraoxon and the reactive capacity of UiO-66-NH2 with chlorine gas. Direct correlations were observed between synthesis temperature, crystallinity, BET surface area, and activity of the MOFs.
1. INTRODUCTION Metal organic frameworks (MOFs) are hybrid structures of regularly arrayed metal-containing nodes connected by organic linkers. MOFs have been of particular interest in recent years due to their porous nature, high surface area, and crystallinity, which make them especially useful for a variety of applications, including gas storage and separations,1−4 molecular sensing,5,6 catalysis,7 and drug delivery.8,9 Several MOFs including HKUST-1 and MOF-74 contain well-dispersed coordinatively unsaturated metal sites, giving them high adsorption capacity and reactivity with adsorbates of interest.10,11 However, one of the biggest challenges in utilizing these MOFs in various applications is the stability of the MOF, particularly in aqueous or humid conditions where the metal-carboxylate bonds are prone to hydrolysis.12−18 Zirconium-based MOFs, especially UiO-66 (Figure 1), have garnered particular attention due to their high thermal and chemical stability, especially in the presence of water.19,20 In addition, these zirconium-based MOFs contain a high defect tolerance, which allows for an increased number of coordinatively unsaturated sites in the secondary building unit (SBU), thereby increasing reactivity with analytes of interest.21−23 UiO-66 has a rather small pore size inherent to the structure, which consists of 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.24 This small pore size makes it an ideal MOF for adsorption of small molecular analytes of interest. To increase the adsorption and reactivity of a specific analyte of interest with UiO-66, the functionality on the organic linker can be readily modified through synthetic methods.25,26 For instance, a pendant amine © 2017 American Chemical Society
Figure 1. Simplistic representation of UiO-66 (top-left) and UiO-66NH2 (bottom-left) showing the identical structure with one of the hydrogen atoms on each organic linker replaced by an −NH2 functional group. Ball and stick representation of the crystalline structure of the MOF UiO-66 (top-right), with carbon (gray), hydrogen (white), oxygen (red), and zirconium (blue) atoms clearly labeled, and a zoomed in representation of a tetrahedral cage (bottomright).
Received: Revised: Accepted: Published: 1478
November 9, 2016 January 13, 2017 January 20, 2017 January 20, 2017 DOI: 10.1021/acs.iecr.6b04361 Ind. Eng. Chem. Res. 2017, 56, 1478−1484
Article
Industrial & Engineering Chemistry Research
recorded on a 300 Hz Oxford NMR. MOF samples were digested in 0.75 mL of DMSO-d6 and 10 μL of HF for NMR analysis. 2.3. Hydrolysis of Methyl Paraoxon. The reaction of UiO-66 and UiO-66-NH2 with methyl paraoxon was performed using a previously reported procedure.36 6 mol % (Zr : methyl paraoxon) of either UiO-66 or UiO-66-NH2 (2.6 mg) and 1 mL of 0.45 M N-ethylmorpholine buffer were added to a 4 dram vial. The solution was stirred for 30 min at room temperature. Methyl paraoxon (4 μL, 0.025 mmol) was added to the vial, and the reaction was stirred. Aliquots (10 μL) were removed every 5 min for 1 h, added to a 5 mL volumetric flask, and diluted to the mark with 0.45 M N-ethylmorpholine. The diluted samples were run on the UV−vis spectrometer, and the growth of the product peak at 407 nm was monitored and used for quantification. 2.4. Chlorine Microbreakthrough Experiments.27 A miniaturized breakthrough apparatus was used to evaluate milligram-scale quantities of MOF samples for the adsorption of chlorine as described in-depth elsewhere.27,37 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. Prior to testing, each MOF was regenerated for 1 h at 120 °C under flowing dry air to remove any physisorbed water. A ballast with a predetermined quantity of challenge gas was then mixed with a stream of dry (−40 °C dew point) air at a rate necessary to achieve a challenge concentration of 2,000 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 concentration. The data is reported and plotted as normalized time (the time divided by the mass of the sample used) versus the signal at a given time divided by the signal at saturation (C/ C0). The corresponding breakthrough curve was integrated to determine the loading at saturation.
group can be incorporated onto the aromatic ring of the organic linker (UiO-66-NH2) (Figure 1), which has been shown to significantly increase the degree of activity toward analytes such as chlorine,27 carbon dioxide,28 methane,28 methyl paraoxon,29 and nitrogen dioxide.30 MOF synthesis is usually conducted in a hydrothermal manner in which the metal node precursor and the organic linker are dissolved in solution and incubated in an oven at an elevated temperature.31 Typical solvents used, such as DMF and DEF, have a high dielectric constant and are basic in nature, which aids in solubilizing the starting materials and deprotonating the organic linker facilitating self-assembly of the MOF. However, these solvents are difficult to directly remove from the pores of the MOF by thermal means due to their high boiling points and ability to hydrogen bond with the SBU and functional groups on the organic linker, such as the amine group in UiO-66-NH2. MOFs with small pore sizes, such as UiO-66, are also difficult to activate due to diffusion hindrance. The inability to effectively remove solvents from the pores of the MOF directly hinders the overall adsorption capacity of the resulting MOF. Solvent exchange is typically used to overcome this by replacing the solvent in the pores with a more readily removed solvent, such as methanol or acetone, prior to activation. This process, however, requires a large amount of solvent and can take days to complete. Supercritical drying has been used to activate MOFs post-synthesis.32 However, MOFs typically still require solvent exchange prior to supercritical drying. Mechanochemical synthesis has also been utilized to reduce synthesis costs by eliminating the solvent; however, the structure and porosity of the MOF is difficult to control. Furthermore, solvents, such as DMF or methanol, are often used to wash the resulting MOF product, which then must be removed via activation.33,34 We describe here the synthesis of UiO-66 and its analogues in acetone, a volatile solvent from which the MOF can be activated directly.
2. EXPERIMENTAL SECTION 2.1. Synthesis of UiO-66 and UiO-66-NH2. UiO-66 was synthesized from zirconium chloride (0.1520 g, 0.652 mmol) and terephthalic acid (0.1084 g, 0.652 mmol). The starting materials were suspended in 15 mL of acetone, and the solution was transferred to the Teflon liner of 40 mL Parr bomb, which was sonicated for approximately 5 min. The liner was then transferred to the Parr bomb and incubated in an oven for 24 h, unless otherwise noted, at various temperatures. UiO-66-NH2, UiO-66-NO2, and UiO-66-Br were synthesized in an analogous way from zirconium chloride (0.652 mmol) and the respective linker acid (0.652 mmol). 2.2. Physical Measurements. IR spectra were recorded on a Bruker Tensor 27 spectrometer from 4000 to 400 cm−1 at a resolution of 2 cm−1. Powder X-ray diffraction (XRD) data were obtained 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 3−50 2θ at a scan rate of 5° min−1. Thermogravimetric analyses were performed on a TGA Q500 analyzer in a flow of air with a heating rate of 5 °C min−1 to 200 °C, 2 °C min−1 to 600 °C, and 5 °C min−1 to 800 °C. N2 gas sorption measurements were performed on a Micromeritics 3Flex 3500 instrument at liquid nitrogen temperature (77 K) and analyzed using the BET method over the relevant pressure range.35 Samples were degassed at 100 °C for 12 h prior to measurement. UV−vis spectra were recorded on a JASCO V-650 spectrophotometer. 1H NMR spectra were
3. RESULTS AND DISCUSSION 3.1. Physical Characterization of UiO-66 and UiO-66NH2. UiO-66 and UiO-66-NH2 were synthesized solvothermally at temperatures ranging from 25 to 160 °C. The PXRD patterns show an increase in the crystallinity of both UiO-66 and UiO-66-NH2 with increasing temperature, with the optimal temperature determined to be 140 and 100 °C, respectively (Figures 2, S1, and S2). UiO-66 and UiO-66-NH2 under the optimal synthesis conditions yielded 0.214 and 0.161 g, respectively. Formation of crystalline UiO-66 is observed at rather low temperatures (as low as 40 °C), and the crystallinity is retained at temperatures up to 160 °C. However, at lower temperatures XRD peaks at 2θ ≈ 17 and 28° are observed, indicating the presence of the unreacted terephthalic acid linker. The XRD patterns of MOFs synthesized at 140 and 160 °C do not exhibit these peaks, indicating the lack of presence of unreacted terephthalic acid in the structure. In contrast, the UiO-66-NH2 spectra show very little formation of MOF until the synthesis temperature reaches 60 °C; however, at higher synthesis temperatures the MOF starts to form, likely due to the increasing solubility of the organic linker. Interestingly, the XRD spectra do not show the presence of unreacted organic linker as was observed for UiO-66 at the low synthesis temperatures. 1479
DOI: 10.1021/acs.iecr.6b04361 Ind. Eng. Chem. Res. 2017, 56, 1478−1484
Article
Industrial & Engineering Chemistry Research
66-NH2; however, those MOFs typically have a large number of missing linker defects.38 The TGA data (Figure 4) for UiO-66 materials synthesized at temperatures below 140 °C exhibit a loss at 260 °C,
Figure 2. PXRD patterns of UiO-66 (top) and UiO-66-NH2 (middle) synthesized at optimal conditions in acetone (140 and 100 °C, respectively) and the simulated pattern of UiO-66 (bottom).
UiO-66 exhibited an increase in surface area (Figure 3) relative to temperature up to 140 °C where a maximum of 822
Figure 4. TGA data for UiO-66 (top) and UiO-66-NH2 (bottom) synthesized at various temperatures.
consistent with the loss of the unreacted terephthalic acid linker (Figure S3). In general, an inverse relationship between the amount of the unreacted linker and synthesis temperature is observed. As the synthesis temperature increases, the amount of the unreacted linker decreases from approximately 50 to
