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Synthesis of a Zr-Based Metal−Organic Framework with Spirobifluorenetetrabenzoic Acid for the Effective Removal of Nerve Agent Simulants Hea Jung Park,† Jin Kyu Jang,† Seo-Yul Kim,‡ Jong-Woon Ha,† Dohyun Moon,§ In-Nam Kang,∥ Youn-Sang Bae,‡ Suhkmann Kim,† and Do-Hoon Hwang*,†

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†

Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Republic of Korea ‡ Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea § Beamline Division, Pohang Accelerator Laboratory, Pohang, Kyungbuk 790-784, Republic of Korea ∥ Department of Chemistry, The Catholic University of Korea, Bucheon, Gyeonggi-do 420-743, Republic of Korea S Supporting Information *

necessary; however, the development of a new MOF is another important effort in the further detoxification of nerve agents. When a MOF is used as the catalyst, combination factors, such as the crystal size, pore size, organic functionality, or metal activity, affect the reaction rate.12,13,19 The catalytic properties of MOFs are usually affected by more than one factor, which make it difficult to compare different MOFs as catalysts. Because of the high water stability and thermal stability of Zrbased MOFs,20−23 we focused on a MOF involving the Zr(IV) cluster metal node. In the ligand, a spirobifluorene derivative, which is thermally, chemically, and morphologically stable, was selected to develop a new Zr-based MOF.24 There are a few studies of spirobifluorene-based MOFs,25−28 but a Zr-based MOF with a spirobifluorene derivative is absent from the literature. The pyrene unit of NU-1000 was replaced by spirobifluorene, which has a reduced π conjugation but similar molecular size to that of pyrene. Two fluorene units of spirobifluorene are orthogonally connected by a spiro carbon as the center, which induces separate π conjugation by two parts. Herein, the new Zr-based MOF containing 4,4′,4′′,4′′′-(9,9′spirobi[fluorene]-2,2′,7,7′-tetrayl)tetrabenzoic acid (L), namely Spirof-MOF, was designed and synthesized as a new catalyst for the hydrolysis of DMNP. The metal node of Spirof-MOF is a [Zr6(μ3-O)8(COOH)8(H2O)8] cluster containing eight carboxylate groups from a tetratopic linker (8-connected) and eight water molecules. Spirof-MOF has the same 8-connected Zr6 cluster node as NU-1000, which is an active site for DMNP hydrolysis. L as a tetratopic organic linker was prepared according to a literature procedure, and all spectra matched the previously reported data28 (Scheme S1). Detailed synthetic procedures for L and Spirof-MOF are described in the Supporting Information. The solvothermal reaction of L and ZrOCl2·8H2O in the presence of formic acid (as a modulator) at 130 °C in an oven gave a white crystal powder. The crystal structure was determined by single-crystal X-ray diffraction, and the crystal data, structure refinement, and relevant bond lengths

ABSTRACT: A new microporous Zr(IV)-based metal− organic framework (MOF) containing 4,4′,4″,4‴-(9,9′spirobi[fluorene]-2,2′,7,7′-tetrayl)tetrabenzoic acid (Spirof-MOF) was synthesized, characterized, and sizecontrolled for the adsorption and decomposition of a nerve agent simulant, dimethyl 4-nitrophenylphosphate (DMNP). Spirof-MOF showed a hydrolysis half-life (t1/2) of 7.5 min to DMNP, which was confirmed by using in situ 31 P NMR spectroscopy. Additionally, size-controlled Spirof-MOFb (∼1 μm) exhibited a half-life of 1.8 min and 99% removal within 18 min for DMNP. The results show that Spirof-MOF is a new active material in removing nerve agent simulants by adsorption and hydrolytic decomposition.

N

erve agents [e.g., soman (GD), sarin (GB), and tabun (GA)] are lethal chemical weapons1,2 that inhibit acetylcholinesterase, resulting in asphyxiation at very low exposure levels.3 The efficient detoxification of those volatile phosphonate nerve agents has received great attention because of the threat of terrorism.4 Steady efforts have been devoted to degrading or adsorbing toxic nerve agents.5−10 The most common method to detoxify G-type nerve agents is hydrolysis of the P−X bond (X = F, CN). Metal−organic frameworks (MOFs) are emerging as key catalysts for the hydrolysis of nerve agents and their simulants. Hupp, Farha, and co-workers have carefully studied and applied Zr(IV)-based MOFs to degrade nerve agents and their simulants.11−17 Lewis acidic Zr(IV) nodes bridged by hydroxides are noteworthy for hydrolysis of phosphate esters because the nodes of the MOF Zr cluster resemble the natural enzyme active site (Zn−OH−Zn) of phosphotriesterase.11,18 For hydrolysis of dimethyl 4-nitrophenylphosphate (DMNP, a nerve agent simulant), NU-1000 exhibited half-lives (t1/2) of 15 and 1.5 min for its hydrated and dehydrated Zr cluster nodes, respectively.12 Screening and examination to find the best MOF among those already reported for the hydrolysis of nerve agents and their simulants are © 2017 American Chemical Society

Received: August 9, 2017 Published: October 2, 2017 12098

DOI: 10.1021/acs.inorgchem.7b02022 Inorg. Chem. 2017, 56, 12098−12101

Communication

Inorganic Chemistry

conventional method of evacuating at 130 °C for 18 h) exhibited a BET surface area of 330 m2/g, which is 6 times lower than that of the ScD Spirof-MOF. The PXRD pattern of the conventionally dried Spirof-MOF sample at 130 °C indicated a collapsed MOF architecture after drying under a vacuum at 130 °C29,30 (FigureS4). However, the topology of Spirof-MOF was unchanged after mild drying at 100 °C under evacuation for 16 h, showing a BET surface area of 1895 m2/g (Figure S4). All of the catalytic Spirof-MOF samples were activated by the same conventional mild drying conditions (evacuating at 100 °C for 16 h). The scanning electron microscopy (SEM) image shows a particle size for the bulk Spirof-MOF of approximately 14−16 μm (maximum dimension; Figure S13a,b). Additionally, two size-controlled Spirof-MOF samples (Spirof-MOFa and SpirofMOFb) were prepared by control of the reaction time and showed similar maximum dimensions of 0.8−1 μm (Figure S13c,d). However, Spirof-MOFb contained additional 400−700 nm small particles, which can be expected to have a faster DMNP hydrolysis rate. The particle-diameter distributions were measured for welldispersed samples in an ethanol solution using a laser diffraction particle size analyzer. The particle diameter was obtained as an equivalent sphere diameter (Dsurface), which is the diameter of a sphere with an external surface area equal to that of the measured particle. The particle diameter is less than the maximum dimension of the sample because of the nonspherical shape of Spirof-MOF. Bulk Spirof-MOF, which has a maximum dimension of 14−16 μm (SEM images), showed maximum particle-diameter distributions for Dsurface = 3.2 μm. This indicates that bulk Spirof-MOF provided the same external surface area in the catalytic reaction as that of an equivalent sphere with a diameter of 3.2 μm (Figure S12a). Size-controlled Spirof-MOFa showed a maximum particle-diameter distribution at 258 nm, which is 12 times smaller than that of the bulk sample (Figure S12b). However, Spirof-MOFb had a maximum particlediameter distribution at 76 nm, which is the smallest diameter among the Spirof-MOF samples, and a second distribution at 258 nm. On the basis of the particle-diameter distribution of the synthesized Spirof-MOF samples, Spirof-MOFb (with the smallest particle diameter) will have the largest external surface area per unit weight among the prepared Spirof-MOF samples. The effect of the external surface area per unit weight of the prepared samples was explored in the hydrolysis test using sizecontrolled Spirof-MOF samples as catalysts. We hydrolyzed DMNP using 1.5 μmol of the Spirof-MOF catalyst loading with different particle sizes, which were monitored by in situ 31P NMR spectra at room temperature. The reaction conversion progress was calculated by comparing the integrated 31P NMR peak for DMNP (δ = −4.4 ppm) to that of the dimethyl phosphate anion (δ = 2.8 ppm) hydrolysis product.13,14 The results are summarized in Table 1. Figure 2 shows the hydrolysis reaction of DMNP in an aqueous buffer solution with Spirof-MOF as a catalyst with the percent conversion of DMNP by Spirof-MOF as a function of time at room temperature. The pore diameters of NU-1000 are 12 and 30 Å,21 which are larger than those of Spirof-MOF (6 and 8 Å); therefore, DMNP (ca. 4.8 × 5.5 × 8.6 Å) is expected to move in and out via the NU-1000 mesopores more easily than via the Spirof-MOF micropores. However, bulk Spirof-MOF showed t1/2 = 7.5 min, which is 1.7 times faster than that of NU-1000 (15 min).12,14 While NU-1000 showed 77% DMNP conversion within 60 min, Spirof-MOF exhibited 97% DMNP

and angles are listed in Tables S1 and S2. The diffraction lines were indexed with an orthorhombic unit cell with refined parameters a = 17.842(4) Å, b = 37.867(8) Å, c = 33.693(7) Å, α = 90°, β = 90°, and γ = 90°. Spirof-MOF has a binodal 4,8-connected scu net topology with the point symbol {44·62}{416·612}. More specifically, the hexa-Zr(IV) node is connected to eight organic linkers (i.e., L), and L is connected to four Zr6 clusters (Figure S1). The remnant of the Zr6 cluster metal node was occupied by eight water molecules rather than formate anions used as modulators in the reaction (formate-free form). The aperture sizes of the diamondshaped channels through the a and c axes in the microporous Spirof-MOF were calculated using dummy-pillar diameters of 6 and 8 Å, respectively (Figure 1). Therefore, materials with the

Figure 1. Structural features of Spirof-MOF [Zr 6 (μ 3 O)8(C53H28O8)2(H2O)8]. Space-filled representation and 3D channel diameters along the (a) a axis (6 Å) and (b) c axis (8 Å) of Spirof-MOF.

appropriate molecular sizes with respect to the window can freely move in and out through the channels within Spirof-MOF. Additionally, prolate spheroid cages of approximately 28.6 Å (maximum dimension) were observed inside Spirof-MOF (Figure S3). The synthesized bulk Spirof-MOF and simulated powder X-ray diffraction (PXRD) patterns, which were obtained from the single-crystal X-ray data, showed reasonable agreement (Figure S4). The thermal stability of the as-synthesized SpirofMOF was assessed by thermal gravimetric analysis (TGA). The mass decrease of approximately 66% (solvent loss) upon heating to 250 °C suggests a substantial internal surface area (Figure S7). Both the as-synthesized and activated (supercritical CO2 drying, ScD) Spirof-MOF samples were stable up to 511 °C. Spirof-MOF treated by ScD had a Brunauer−Emmett−Teller (BET) surface area of 2020 m2/g and a total pore volume of 0.99 cm3/g, estimated from the N2 sorption isotherm at 77 K (Figure S8). The pore-size distribution was obtained from the N2 isotherm, indicating pore diameters in the expected range of 8.8−10.5 Å (Figure S9). The degassed Spirof-MOF sample (a 12099

DOI: 10.1021/acs.inorgchem.7b02022 Inorg. Chem. 2017, 56, 12098−12101

Communication

Inorganic Chemistry

samples. Stacked in situ 31P NMR spectra showed that the hydrolysis reaction was 99% complete within 18 min, and the DMNP peak at −4.4 ppm disappeared (Figure S19). The other in situ 31P NMR spectra for the hydrolysis of DMNP with the Spirof-MOF samples are summarized in Figures S16−S18. Additionally, Spirof-MOFb showed an 8.3 times shorter half-life and a 9 times greater turnover frequency (TOF) than those of NU-1000 under the same reaction conditions. Moreover, crystal size control of Spirof-MOFb achieved a half-life similar to that of dehydrated NU-1000 (t1/2 = 1.5 min), which possesses more active metal sites than hydrated NU-1000. In conclusion, we developed a new Zr(IV)-based Spirof-MOF with a tetratopic organic linker L as an active material for the effective removal of nerve agents and their simulants. SpirofMOF exhibited a shorter half-life and total DMNP conversion time than NU-1000, which has the same 8-connected Zr6 cluster active site and a similar BET surface area but larger pores than Spirof-MOF. The size-controlled Spirof-MOF, Spirof-MOFb, showed the shortest half-life (1.8 min) among the three SpirofMOF samples and 99% DMNP conversion within 18 min. Spirof-MOFb showed the fastest DMNP hydrolysis rate with a hydrated 8-connected Zr6 cluster metal node reported to date. These results reveal that Spirof-MOF is clearly effective in removing the nerve agent simulant DMNP. Spirof-MOF provides a scaffold for achieving improved hydrolysis rates for the nerve agent and its simulant through the development of a new MOF.

Table 1. Summary of DMNP Degradation by Spirof-MOF and NU-1000 sample

particle size (μm)a

amount of catalyst (μmol)

t1/2 (min)

TOF (s−1)c

ref

NU-1000 NU-1000 NU1000(dehyd) Spirof-MOFb

15

0.37 1.5 1.5

80 15 1.5

0.007 0.009 0.093

19 12, 14 12, 14

16

0.37

48

0.012

Spirof-MOFb

16

1.5

7.5

0.018

Spirof-MOFab

1

1.5

3.5

0.040

Spirof-MOFbb

1

1.5

1.8

0.077

this work this work this work this work

a

Maximum dimension length determined by SEM. bActivated at 100 °C with evacuating for 16 h. cTOF values were calculated at t1/2 [TOF = (1/2 mol of DMNP)/(mol of catalyst)(t1/2)]. Slightly larger values are obtained if the initial rates are used to calculate TOF.

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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/acs.inorgchem.7b02022. Detailed compound synthesis and characterization, 1H and 31P NMR, TGA, PXRD, N2 isotherms, SEM images, and single-crystal X-ray crystallography data (PDF) Figure 2. (a) Hydrolysis scheme of DMNP and (b) hydrolysis rates of DMNP using the Spirof-MOF samples.

Accession Codes

CCDC 1478753 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

conversion within only 35 min. This indicates that DMNP can more easily access the active site of Spirof-MOF than that of NU1000, even though Spirof-MOF has smaller pores than NU-1000. It seemed that the closer distance between the neighbor Zr6 nodes of Spirof-MOF than that of NU-1000 helped DMNP approach the active site. Additionally, DMNP hydrolysis was performed with Spirof-MOF, having similar particle size to NU1000, because the hydrolysis performance of the MOF catalyst is significantly affected by the MOF crystal size.19 To fairly compare the hydrolysis rate between two MOFs, 0.37 μmol of the bulk Spirof-MOF (16 μm) catalyst was used for the DMNP hydrolysis test at the same conditions as those used for NU-1000 (15 μm) in the literature.19 In the similar particle size, SpirofMOF exhibited a 1.7 times shorter half-life (t1/2= 48 min) than that of NU-1000 (t1/2= 80 min) (Table 1). The size-controlled Spirof-MOF samples, Spirof-MOFa and Spirof-MOFb, exhibited 2.1−4.2 times faster hydrolysis rates for DMNP than the bulk Spirof-MOF did (t1/2 = 7.5 min). Spirof-MOFa showed t1/2 = 3.5 min and 97% DMNP conversion after 32 min. As expected from the Dsurface results and SEM images from the particle size analysis, Spirof-MOFb, which showed the smallest particle size, provided the shortest half-life (1.8 min) among the prepared Spirof-MOF

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Do-Hoon Hwang: 0000-0003-4183-0185 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea grant funded by the Korean Government (Grant NRF2017R1A2A2A05001345 and GCRC-SOP No. 2011-0030013). The X-ray crystallography BL2D-SMC beamline at PLS-II was supported, in part, by MSIP and POSTECH. 12100

DOI: 10.1021/acs.inorgchem.7b02022 Inorg. Chem. 2017, 56, 12098−12101

Communication

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b02022 Inorg. Chem. 2017, 56, 12098−12101