Tailoring the Pore Size and Functionality of UiO-Type Metal–Organic

Communication pubs.acs.org/IC

Tailoring the Pore Size and Functionality of UiO-Type Metal−Organic Frameworks for Optimal Nerve Agent Destruction Gregory W. Peterson,*,† Su-Young Moon,‡ George W. Wagner,† Morgan G. Hall,† Jared B. DeCoste,†,§ Joseph T. Hupp,‡ and Omar K. Farha*,‡,⊥ †

Edgewood Chemical Biological Center, 5183 Blackhawk Road, Aberdeen Proving Ground, Maryland 21010, United States Department of Chemistry and the International Institute for Nanotechnology, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States § Leidos, Inc., P.O. Box 68, Gunpowder, Maryland 21010, United States ⊥ Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia ‡

S Supporting Information *

ABSTRACT: Evaluation of UiO-66 and UiO-67 metal− organic framework derivatives as catalysts for the degradation of soman, a chemical warfare agent, showed the importance of both the linker size and functionality. The best catalysts yielded half-lives of less than 1 min. Further testing with a nerve agent simulant established that different rate-assessment techniques yield similar values for degradation half-lives.

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he development of materials active toward supertoxic chemicals such as chemical warfare agents (CWAs) has once again become an important worldwide issue. Of interest are materials capable of decontaminating environments exposed to CWAs as well as ones competent for destroying bulk stores of CWAs, especially nerve and blister agents. When decontamination entails the removal of agents from air, a traditional approach is filtration through a reactive, porous solid. Selected dispersed metal oxides, for example, have been found to be capable of rapidly degrading organophosphate-based nerve agents, such as O-pinacolyl methylphosphonofluoridate (i.e., soman).1−4 Unfortunately, the degradation reactions are stoichiometric in the metal oxide rather than catalytic. Furthermore, the oxides appear to offer only a limited potential for tunability for increasing reaction rates.4,5 Metal−organic frameworks (MOFs) have shown much promise in applications relevant to the protection from or destruction of chemical threats, including catalysis,6,7 sensing,8 filtration,9,10 and separation.11,12 The ability to easily change both the metal-containing secondary building unit and the organic linker allows for the tuning of properties such as the pore volume, surface area, aperture size, and functionality.13 These properties can affect the chemical removal rate by influencing the reaction mechanism, the degree of accessibility of catalytic sites, and/or intramaterial diffusion rates. Here we have synthesized and evaluated, as catalysts for nerve agent (soman) hydrolysis, several zirconium-based MOFs of the UiO family; see Figure 1. Zirconium-based MOFs are unusually hydrolytically stable, and one (NU-1000) has recently been shown to be catalytically active for nerve agent hydrolysis.14 (Others have been shown to be catalytic for hydrolysis of organophosphates that are not CWAs; see © XXXX American Chemical Society

Figure 1. Idealized structures for (a) UiO-66 and (b) UiO-67, together with (c) the linkers used to build the MOFs studied. The real structures are characterized by missing linkers that leave labile coordination sites (catalytically active sites) on the zirconium(IV) nodes.

refs 16 and 17.) As suggested by Figure 1, the UiO template lends itself to a systematic investigation of the effects of (a) altering the pore aperture size through linker extension and (b) introducing linker functional groups on the kinetics of soman and simulant degradation. The required MOFs were synthesized using previously described techniques,15,16 which are also summarized in the Supporting Information. Proper methods of evaluation are paramount to the development of novel materials. Clearly, supertoxic nerve agents cannot be tested at most laboratories because of the need for specialized equipment and trained personnel. Here, we first evaluated the catalytic degradation of dimethyl 4-nitrophenylphosphate (DMNP), a potential simulant for hydrolysis of nerve agents, using five UiO-66- and UiO-67-type MOFs.14,17 Generally, 1.5 μmol of each MOF was mixed in a 1 mL aqueous solution Received: August 21, 2015

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DOI: 10.1021/acs.inorgchem.5b01867 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry buffered at pH 10 with 0.45 M N-ethylmorpholine. At t = 0, 4 μL of DMNP (25 μmol) was added to the solution with stirring. Cleavage of the P−O bond (see Figure S1) results in the formation of p-nitrophenoxide, which absorbs light at 410 nm. Using UV−vis spectrometry, the reaction was followed as a function of time. In parallel, similar buffered solutions were made, and using 31P NMR spectroscopy, conversion was tracked by measuring decreases in the peaks associated with DMNP. Figure 2a summarizes data from both methods for UiO-66, UiO-66-NH2, and UiO-67, while Figure 2b does the same for the

Table 1. Comparison of DMNP Half-Lives Calculated for the NMR and UV Methods in a pH 10 Buffered Solution t1/2 (min) material

NMR

UV

UiO-66 UiO-66-NH2 UiO-67 UiO-67-NH2 UiO-67-N(Me)2

25 0.9 3.5 1.9 1.5

24 0.7 3.8 4.9 9.4

UiO-6618 to between 8 and 11.5 Å in UiO-6715,18 also substantially reduces the DMNP half-life, from 25 to 3.5 min, i.e., by 7-fold. DMNP has kinetic dimensions of ca. 11 × 4.5 Å,19 leading to diffusion resistance, and therefore much of the reactivity may occur on the outer surface of the MOF. With larger pores, such as those of UiO-67, the diffusion resistance is decreased, and increased access to, and reaction with, interior active site structures may occur. Evaluation of each material using the NMR method was conducted against soman. Figure 3 shows the conversion of

Figure 2. DMNP degradation curves for (a) UiO-66, UiO-66-NH2, and UiO-67 and (b) UiO-67, UiO-67-NH2, and UiO-67-N(Me)2.

UiO-67 derivatives. Solid points represent data collected using NMR, while open points represent data collected using UV−vis spectroscopy. Baseline UiO-66 and UiO-67, as well as UiO-66NH2, show an excellent correlation between the NMR and UV− vis methods. Modest but detectable deviations in the methods, however, are apparent for the UiO-67 derivatives, especially at high percent conversion; see Figures 2b and S2. We believe that these differences reflect differences in the sampling methods. Whereas NMR is a bulk method, the UV−vis method requires that aliquots be taken from the reaction mixture. Variations in the homogeneity of the samples, along with a potential optical disturbance (light scattering) due to suspended MOFs, are likely responsible for these deviations. As such, we consider the NMR results to be more accurate. In all cases, at early times the methods show good agreement. Regardless of the method, several trends emerge from the data collected, as shown by the calculated half-lives in Table 1. We see that both the functional group and pore size are important for DMNP elimination. The amine functionality dramatically increases the catalytic activity of UiO-66, reducing t1/2 by 25fold. Increasing the pore aperture size from approximately 6 Å in

Figure 3. Soman degradation by UiO-66 and UiO-67 derivatives measured by NMR at (a) longer and (b) shorter time scales. Lines show pseudo-first-order fits of the data.

soman to pinacolyl methylphosphonate as a function of time. (The degradation mechanism is shown in Figure S1b, which the NMR spectra confirm in Figures S3−S7.) The percent conversion at 3.5 min, which is the first available data point due to agent handling, number of half-lives at 3.5 min, and rate constants assuming pseudo-first-order kinetics are shown in Table 2. In general, the overall rates are faster than the final rates because the reaction is quick to occur initially due to mixing as well as potential inhibition from products as the reaction B

DOI: 10.1021/acs.inorgchem.5b01867 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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Table 2. Soman Overall and Final Half-Lives for the MOFs Studied, as Well as the Pseudo-First-Order Rate Constants in a pH 10 Buffered Solution material UiO-66 UiO-66-NH2 UiO-67 UiO-67-NH2 UiO-67N(Me)2

% conversion at 3.5 min

no. of half-lives at 3.5 mina

52 82 97 92 95

∼1 ∼2 ∼5 ∼3 ∼5

Communication

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01867. Materials synthesis, experimetnal section, hydrolysis mechanisms and plots, NMR spectra, and natural log plots (PDF)

−1

k′ (M s−1) 0.2 0.5 1.0 0.7 1.0

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AUTHOR INFORMATION

Corresponding Authors

a

Estimated from the percent conversion at 3.5 min by assuming pseudo-first-order reaction kinetics. Obviously, more complex decay behavior will yield different values for half-lives, but the listed estimates may facilitate qualitative comparisons.

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.W.P. and J.B.D. (Program BA13PHM210) and O.K.F. and J.T.H. (Grant HDTRA-1-10-0023) gratefully acknowledge support by the Defense Threat Reduction Agency. We thank Mr. McGuirk for sharing the synthesis procedure which we used to synthesize some of the organic linkers.

progresses. In any case, soman is hydrolyzed catalytically in these conditions. The trends seen for the soman data are interesting when compared to DMNP. For both chemicals, adding the amine functionality to UiO-66 increases the reaction rate, although this is much more evident for DMNP compared to soman. This might be due to the difference in the reaction chemistry. Whereas soman is hydrolyzed first at the P−F bond, DMNP is hydrolyzed at the P−O bond. The relative ease of cleavage of the P−F bond allows baseline UiO-66 to be an effective catalyst, and only modest improvements are seen by functionalizing with the amine group in UiO-66-NH2. As with DMNP, increasing the pore size also accelerates soman degradation, again likely because of increased access to active sites within the pore structure. Unlike with DMNP, functionalizing the linker with the −NH2 and −N(Me)2 moieties does not drastically affect the half-life of soman, and UiO-67-NH2 actually has a slightly longer half-life. It is difficult to discern the true effect of the linker functionality for UiO-67 derivatives because the catalytic reaction rates are incredibly fast for the hydrolysis of soman, and the effect of functional groups is less pronounced than those seen for the simulant DMNP. In summary, MOFs, specifically UiO-based materials, are extraordinarily effective catalysts for degradation of the nerve agent soman as well as the agent simulant DMNP. Two methods were used for monitoring the degradation of DMNP as a function of time in a buffered solution, NMR and UV−vis, which provide excellent agreement for UiO-66, UiO-66-NH2, and UiO67. Both the functional group and pore size were found to be important parameters for DMNP degradation. Soman is hydrolyzed as fast as or faster than the simulant. Whereas the functional group identity was found to be important for DMNP degradation, the effect was less evident for soman because hydrolysis was extremely fast, i.e., at the limits of our ability to measure and, therefore, differentiate. The reaction was generally found to follow pseudo-first-order kinetics for a few half-lives, but deviations, perhaps indicative of partial product inhibition, are evident at later times. These important findings, along with the ability to monitor conversion of simulants utilizing multiple techniques, will allow us to conduct research on a wider range of materials, functional groups, and pore sizes, in an effort to find even more reactive materials without the need for a buffered solution.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.5b01867 Inorg. Chem. XXXX, XXX, XXX−XXX