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Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
Toward Base Heterogenization: A Zirconium Metal−Organic Framework/Dendrimer or Polymer Mixture for Rapid Hydrolysis of a Nerve-Agent Simulant Zhijie Chen, Timur Islamoglu, and Omar K. Farha* Department of Chemistry and International Institute of Nanotechnology, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States
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ABSTRACT: The base heterogenization is crucial for the practical applications of metal−organic frameworks (MOFs) as catalytic filters, such as masks or protective suits, for the deconstruction of chemical warfare agents (CWAs). Here, we performed the hydrolysis of a phosphate-based nerve agent simulant in the presence of different amine-based bases (i.e., a small organic molecule, dendrimers, and linear and branched polymers) using a Zr-MOF, NU-901, with 4,8-connected scu topology. Remarkably, the catalytic performances of NU-901 using the less-volatile branched polymers and dendrimers are comparable to the volatile N-ethylmorpholine solution. KEYWORDS: zirconium MOFs, chemical warfare agents, hydrolysis, dendrimers, polyethylenimine, polymers
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INTRODUCTION Chemical warfare agents (CWAs) are highly toxic organophosphonate compounds that can cause death by asphyxiation due to irreversible binding of organophosphonates to acetylcholinesterase(AChE), an enzyme that catalyzes the breakdown of acetylcholine to prevent muscle contraction.1 Because of the stockpiles and unfortunate recent use of CWAs, there is a pressing need to develop efficient materials for deconstruction of CWAs.2−5 Metal−organic frameworks (MOFs), a class of porous crystalline hybrid materials, are constructed from organic linkers and inorganic nodes. Because of the ultrahigh surface areas,6 adjustable pore sizes,7−10 and diverse and programmable structure,11−13 MOFs and MOF-composites have been studied as promising catalysts for the destruction of nerve agents, as well as for dimethyl 4-nitrophenyl phosphonate (DMNP), a nerve agent simulant which mimics the reactivity of organophosphonate-based CWAs.3,4,14−18 Particularly, MOFs based on zirconium clusters (i.e., Zr6 nodes) have been explored for the hydrolysis of DMNP in the respective of pore sizes, node connectivities, and particle sizes because of their Lewis acidic and enzyme-inspired Zr−OH−Zr sites and high chemical stability.15,19−23 However, an aqueous base solution (i.e., N-ethylmorpholine solution) is generally required to hydrolyze DMNP to control the pH of the reaction and to regenerate the active Zr sites.14,24 While Nethylmorpholine buffered solution holds promise for destruction of stockpiles of CWAs, it is not practical for the protective suit and mask applications due to the high volatility of Nethylmorpholine. To this end, utilization of amines with lower © XXXX American Chemical Society
vapor pressure or amine containing polymers can be instrumental. Here, we evaluated to the performance of polyethylenimine (PEI), branched and linear PEI,25 dendrimers as bases for the hydrolysis of a phosphate-based nerve agent simulant, DMNP, and compared it to N-ethylmorpholine (Scheme 1). NU-901, a pyrene-based Zr-MOF with the 4,8-c scu (square and cube) topology was selected as catalyst platform for this study because of its rhombic shaped onedimensional channels with catalytically active Zr−O−Zr sites pointing into the channels (Figure 1). Thanks to the accessible 8-c Zr6 nodes and nanosized crystals, NU-901 exhibits nearly instantaneous hydrolysis of toxic DMNP into the nontoxic product in the presence of N-ethylmorpholine. Remarkably, the use of less volatile PEI dendrimer and branched polymer bases showed similar activity toward the hydrolysis of DMNP (Figure 2). Although we previously reported Zr-MOF/linear PEI polymer composite for the detoxification of chemical warfare agents,25 to the best of our knowledge, this is the first systematic study comparing different nonvolatile bases for the hydrolysis of DMNP.
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RESULTS AND DISCUSSION NU-901 is a MOF assembled from 1,3,6,8-tetrakis(pbenzoate)pyrene (TBAPy4−) and 8-c Zr6 nodes.26,27 The phase purity of the bulk NU-901 powders was confirmed from powder X-ray diffraction (PXRD) patterns (Figure 1A). The Received: December 18, 2018 Accepted: January 28, 2019
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DOI: 10.1021/acsanm.8b02292 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Nano Materials Scheme 1. (A) Schematic Illustration of Amine-Containing Dendrimers or Polymers As Bases for the Hydrolysis of Dimethyl 4-Nitrophenyl Phosphonate (DMNP) and (B) 31P NMR Spectra of the Reaction without the Catalyst (Top) and with the Catalyst after Full Conversion (Bottom) Indicating Selective Hydrolysis of DMNP (δ = −4.4 ppm) to Dimethoxy Phosphate Anion (δ = 2.8 ppm)
experimental total pore volume at P/P0 = 0.9 of NU-901 obtained from the N2 adsorption isotherm was estimated to be 0.91 cm3·g−1 (Figure 1C). The pore size distribution calculated using density functional theory (DFT) indicated a major peak centered at ∼1.2 nm, which is attributed to the 1-D channels. Additionally the peak around 2.7 nm is attributed to the missing cluster defects, which is in line with our previous observations (Figure 1C, inset).26,27 The apparent Brunauer− Emmett−Teller (BET) surface area of NU-901 was estimated to be 2060 m2·g−1. The scanning electron microscope (SEM) image of NU-901 show oval-shaped aggregated crystallites, with sizes ranging from 300 nm to about 1 μm (Figure S1). We hypothesize that relatively large pore channels and 8-c Zr6 nodes allow NU-901 to be a good platform for the hydrolysis of DMNP using various bases. Indeed, the initial half-life for hydrolysis of DMNP in the presence of N-ethylmorpholine with NU-901 (6 mol % catalyst loading) was found to be very fast (t1/2 = ∼1 min) with turnover frequency (TOF) about 0.14 s−1 (Table 1 and Figure 3). To explore the base heterogenization, we chose a series of amine-rich bases with various molecular weights, such as linear PEI, branched PEI, and PEI dendrimers for the hydrolysis of DMNP (Figure 2B). The amount of base used is determined based on the nitrogen content which is kept the same as Nethylmorpholine (0.395 mmol). Our initial assessment was focused on the performance of PEI dendrimers. The half-life for the hydrolysis of DMNP in the presence of PAMAM 1.0 dendrimer (ethylenediamine core, generation 1.0 solution) and PAMAM 0.0 dendrimer (ethylenediamine core, generation 0.0 solution) was found to be 1.1 and 1.7 min, respectively. Notably, the hydrolysis performance with PAMAM 1.0 is similar to that of Nethylmorpholine. Because of the much higher molecular weights of dendrimers and therefore the lower vapor pressure compared to small organic base (1430 g/mol (PAMAM 1.0)
Figure 1. (A) Structural representation of NU-901 constructed from 8-c Zr6 nodes and 4-c pyrene-based ligands. (B) Powder X-ray diffraction (PXRD) patterns of the simulated and as-synthesized samples are similar. (C) N2 sorption isotherms of NU-901 at 77 K and pore-size distributions based on a density functional theory (DFT) model (inset).
versus 115 g/mol (N-ethylmorpholine)) the former base holds promise for the protective suit applications. We have also compared the catalytic activity using branched PEI and linear PEI and revealed that the hydrolysis rate using branched PEI is faster than linear PEI which can be attributed to the higher percentage of tertiary amine inside the branched PEI, which is a stronger base than secondary amine (Table S1). The half-lives for hydrolysis with branched PEI with molecular weight (MW) = 600 and 1200 g/mol are 1.9 and 1.2 min, respectively, which are faster than that of linear PEI (MW = 2500 g/mol) with 5.3 min (Figure S2). Comparison of B
DOI: 10.1021/acsanm.8b02292 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Nano Materials
Figure 2. (A) Hydrolysis reaction of DMNP, a nerve agent simulant. (B) Chemical structures and molecular weights of different bases (i.e., linear polyethylenimine (PEI), branched PEI, PEI dendrimers, and Nethylmorpholine.
Figure 4. Comparison of hydrolysis profile of DMNP with NU-901 using 6 mol % catalyst loading in the presence of different bases: Nethylmorpholine, PEI dendrimer, branched PEI, and linear PEI.
Table 1. Hydrolysis rate (t1/2) and Turnover Frequencies (TOFs) for Hydrolysis of DMNP with NU-901 in Various Basesa bases
t1/2 (min)
TOFb (s−1)
N-ethylmorpholine PAMAM 1.0 PAMAM 0.0 branched PEI, MW600c branched PEI, MW1200c linear PEI, MW2500c
1 1.1 1.7 1.9 1.2 5.3
0.14 0.12 0.08 0.07 0.11 0.03
Finally, we conducted pH measurements of the hydrolysis process using the best performed base, dendrimer (PAMAM 1.0, Table S2), to further illustrate the effectiveness of the bases in the hydrolysis of DMNP. The results showed that PAMAM 1.0 was able to maintain the initial pH without a significant drop upon completion of the hydrolysis reaction. In conclusion, we have successfully evaluated the different bases for the hydrolysis of a phosphate-based nerve agent simulant, DMNP, using a 4,8-connected scu Zr-MOF, NU901, which showed
