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Dual Role of Water in Heterogeneous Catalytic Hydrolysis of Sarin by Zirconium-based Metal-Organic Frameworks Mohammad R. Momeni, and Christopher J. Cramer ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03544 • Publication Date (Web): 18 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

Dual Role of Water in Heterogeneous Catalytic Hydrolysis of Sarin by Zirconium-based Metal–Organic Frameworks Mohammad R. Momeni* and Christopher J. Cramer Department of Chemistry, Minnesota Supercomputing Institute, and Chemical Theory Center, University of Minnesota, Minneapolis, Minnesota 55455, USA

ABSTRACT: Recent experimental studies on ZrIV-based metalorganic frameworks (MOFs) have shown the extraordinary effectiveness of these porous materials for the detoxification of phosphorus-based chemical warfare agents (CWAs). However, pressing challenges remain with respect to characterizing these catalytic processes both at the molecular and crystalline levels. We here use theory to compare the reactivity of different zirconium-based MOFs for the catalytic hydrolysis of the CWA sarin, using both periodic and cluster modeling. We consider both hydrated and dehydrated secondary building units, as well as linker functionalized MOFs, to more fully understand and rationalize available experimental findings as well as to enable concrete predictions for achieving higher activities for the decomposition of CWAs.

O

Me

H

O Zr

H

F O

H O

O H2O

Zr

O

Displace

H

O

P

O Concerted H2O Addition/ H-transfer Zr Zr O

H

O P

H H O O

O Zr

H HF Elim.

O O

H O

H O

Zr

Support

Support

Support

SBU-GB

S BU-P5

SBU-P

O F

H O O

O Zr

H Sarin Addition

O

Me Me

P O Zr

H O O

Support

O

H OH Addition

Zr

Zr

O

F

P

O

H O

H O O

Support

Zr

HF Elim.

H

Zr

O

Support

Support

High–surface-area porous metal-organic frameworks (MOFs) with repeating metal oxide nodes interconnected by organic linkers are known to be excellent supports and catalysts for a wide variety of applications.1-3 Recently, Zr–MOFs with open metal sites (whether present by design or as defects) have been shown to be highly effective heterogeneous catalysts for the detoxification of organophosphorus-based chemical warfare agents (CWAs) in buffered aqueous media.4-18 The G series nerve agents, and sarin (GB) in particular, have been of central interest.16-19 Recent theoretical calculations,19 as well as in situ spectroscopic measurements,7 have provided mechanistic insights into the remediation of sarin and its simulant dimethyl methylphosphonate. Specifically, the following elementary steps are proposed for hydrolysis of CWAs on Zr–MOF secondary building units (SBUs) (Scheme 1): (1) binding of sarin to an open Lewis acidic Zr site, (2) nucleophilic attack at phosphorus by either an external water molecule, along a ˝hydrated path,˝ or, in the ˝dehydrated path,˝ by the –OH group that terminates the adjacent Zr site of the SBU (H2O/-OH addition step), and (3) scission of the P-F bond (HF elimination step).7 However, theoretical modeling to date19 has not focused upon observed trends in reactivities for different Zr–MOFs. Neither are experimentally observed different hydrolysis rates for linker functionalized Zr–MOFs yet fully understood.9,12,14,16,18 To gain further mechanistic insights into these variations, we report here results from quantum mechanical studies using both periodic and cluster models to investigate the reaction coordinates for a wide variety of Zr–MOFs catalyzing the hydrolysis of sarin.

H O

F

Me

O

Me

O H

SBU-HB

Zr

KEYWORDS: Metal-Organic Framework (MOF), Functionalized MOFs, Heterogeneous Catalysis, Sarin, Chemical Warfare Agent, Hydrolysis

Me H

P

H

Zr

O

O

F

P O

Me P

O Zr

O H O O

Zr

Support

Scheme 1. Mechanistic Scheme for Hydrolysis of Sarin on Hydrated (Top) vs Dehydrated (Bottom) ZrIV-MOFs. Geometry optimizations used the M06-L meta-GGA density functional20,21 together with the pob-TZVP22 basis set for light elements and a Stuttgart/Cologne basis set with associated pseudopotential (ECP28MWB)23 for Zr. To facilitate direct comparison between results for periodic vs cluster computations, some basis function exponents were modified in the latter to be consistent with the former. Single-point energies were computed with the M06-2X24 hybrid meta-GGA functional with solvation effects included employing the SMD continuum solvation model25 for water (Supporting information (SI) Section S1 provides full computational details). Through detailed comparisons of different reaction pathways, we determine how the displacement of terminal Zr-OH2 groups by sarin contributes to overall observed reaction rates and moreover explains experimentally observed rate accelerations upon dehydration of the Zr–SBUs. Moreover, to rationalize recent experimental findings on the reactivity of linker functionalized UiO–66 and UiO–67 Zr–MOFs,9,12,14,16,18 the effects of functionalizing Zr– UiO–66 and Zr–MOF-808 with –NH2 and –F groups as well as perfluorinated linkers are examined and new Zr–MOFs predicted to have higher reactivities are proposed. Our approach is readily extensible to other classes of MOFs and/or CWAs and should be useful for optimizing the catalytic activity of these porous 3D crystalline materials. We first consider MOF–808, which has a [Zr6(µ3–O)8(O)12]22node coordinated to six 1,3,5-benzenetricarboxylate linkers and has been shown to be one of the most active MOFs for CWA hydrolysis.6 However, the proton topology of this MOF has not

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yet been established. Periodic as well as extended cluster calculations for different proton topologies (Section S2, SI) predict, as is the case for NU–1000,26 that the most stable tautomer involves a mixed proton topology having alternating OH and OH2 groups capping adjacent Zr metal sites (Figure 1, panel b), and results reported here are for that tautomer (or its dehydrated analog).

Figure 1. Structure of MOF–808. (a) The tri-topic benzenetricarboxylate linker, (b) metal-oxide node and (c) optimized crystal structure of the most-stable mix-node proton topology. The energetics of sarin hydrolysis along the hydrated and dehydrated reaction coordinates shown in Scheme 1 were computed for MOF–808, NU–1000 (c pore face of SBU), NU–1000 (large pore face of SBU), and trans-bi-defected UiO–66 with 10 linkers (UiO–66–10).27,28 As sarin is chiral, and as the studied Zr–MOFs present alternative environments on either “side” of an open face (µ3-O vs µ3-OH), many local minima were surveyed for each minimum-energy and transition-state (TS) structure. In NU–1000, the open faces of the SBU are also directed into different environments, namely the “c pore” and the “large pore,” and these were separately considered in our calculations. Results for the lowest energy pathways along the hydrated reaction coordinates are shown in Figure 2 (See Figure S2, SI, for the corresponding lowest energy pathways along the dehydrated reaction coordinates; sections S5-S8 list Cartesian coordinates for all species). ∆G (∆H) (in kcal/mol)

F

O Me H O H P H O O Zr O

UiO-66-10 NU-1000 (c pore) NU-1000 (large pore) MOF-808 O

H

Me O F P

Zr

Support

Zr

+ 18.3 (+ 6.1) + 17.9 (+ 6.8) + 17.9 (+ 6.5)

H

O Zr

O

O Zr

Support

H O O

+ 11.4 (-2.4) H O Me H F P O H H O + 2.5 (-9.0) H O + 1.6 (-10.9) Zr O Zr -0.2 (-12.6) O

-0.9 (-13.2)

The free energies of activation overall for MOF–808, NU– 1000 c pore, large pore, and UiO–66–10 along the hydrated pathway are 16.2, 18.1, 19.2, and 17.9 kcal/mol, respectively, which agrees with the experiment observation that MOF-808 is the fastest of these catalysts for hydrolysis of sarin (Figure 2).6 For MOF–808, it is the nucleophilic attack step that is rate determining while, as noted above, for the other 3 it is water displacement. The free energies of these two TS structures relative to one another vary by 1.8, –6.7, –4.8, and –1.8 kcal/mol, for MOF–808, NU–1000 c pore, large pore, and UiO–66–10 respectively, indicating substantial influence of the linker on the SBU electronic structure and local environment. To explore the effects of different cluster models on computed reaction energies for sarin decomposition, metaloxide nodes with carboxylate linkers truncated to formate were built from their corresponding periodic optimized structures, and results are reported in Figure S3, SI. The different cluster models were also compared to corresponding periodic data keeping the exchange-correlation functional and basis sets the same (see SI Table S4 for results). It is apparent that reaction profiles for benzoate-capped cluster models are in closer agreement with periodic results than are those for formate-capped clusters, especially for UiO–66–10, with its smaller pore sizes than MOF–808, illustrating the importance of choosing appropriate cluster models for studying reactivity in MOFs. Optimized H2O-displacement and nucleophilic addition TS structures are shown in Figure 3 for MOF– 808 (analogs for the other MOFs may be found in the SI).

H

+ 16.2 (+ 2.7) + 16.1 (+ 3.4) + 14.4 (+ 0.5)

Me P F

HO H O

all of the studied Zr–MOFs except MOF–808. This result helps to rationalize experimental data showing catalytic activity to increase with decreasing pH,5 which is contrary to what is observed for the simple homogeneous aqueous hydrolysis of nerve agents; we posit that at lower pH, increased numbers of OH groups are converted to OH2 groups, thereby increasing the effective concentration of displaceable sites. It is also well established that prior dehydration of the Zr6 node in NU–1000 accelerates catalytic hydrolysis in aqueous solution,4 which is consistent with our prediction that the step following sarin coordination has a lower activation free energy than predicted for water displacement for this MOF.29

H O

H

Zr

Support

+ 14.4 (+ 2.4) O

O O

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+ 4.8 (-7.1) + 4.8 (-7.3) + 4.7 (-8.0) + 2.4 (-9.4)

+ 13.9 (-0.1) + 13.9 (+ 0.4) + 12.4 (+0.1) + 9.4 (-4.1) Me O H O F P H H O H O O Zr Zr O Support

Support

Figure 2. M06–2X//M06–L 298 K free energies and enthalpies for hydrolysis of sarin along different benzoatecapped cluster-model reaction coordinates relative to separated reactants (Scheme 1 top row). Subsequent loss of HF and recycling of the catalyst is predicted to be exergonic (cf. Ref. 19). For the hydrated pathway, the first step is H2O displacement by sarin, and that step is computed to be rate-determining for

Figure 3. Key bond lengths (Å, M06-L) in MOF-808 clustermodel TS structures for water displacement (left) and nucleophilic addition (right). An alternative pathway in which the terminal –OH group capping the adjacent ZrIV site acts as a nucleophile, referred to in Scheme 1 as the ˝dehydrated pathway˝, was also considered (Figure S2, SI). Troya has previously investigated the dehydrated pathway for a UiO–66–like model.19 This path showed substantially higher activation free energies for every case (and is

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ACS Applied Materials & Interfaces non-catalytic in the absence of water), and is not discussed further here. In sum, the terminal hydroxyl group is much more effective as a general base in a hydrated pathway than as a nucleophile. Selected amino-functionalized Zr–UiO–66 and Zr–UiO–67 MOFs have shown superior activities for hydrolysis of CWAs compared to their unfunctionalized parents.9,12,14,16,18 To further explore these effects and possibly identify more effective Zr– MOF-based heterogeneous catalysts, we studied the effects of functionalizing UiO–66–10 and MOF–808 linkers with –NH2 and –F groups on the hydrolysis of sarin (Table 1). We note that perfluorinated MOFs have been shown to be highly stable in aqueous solution and to be useful for gas sorption, storage, and separation.30 In both UiO–66–10 and MOF–808, functionalizing the linkers with ortho amino groups lowers the computed activation free energies relative to the parent for both the water displacement (by 4.8 and 2.8 kcal/mol) and nucleophilic addition (by 7.7 and 1.7 kcal/mol) steps, respectively, which is in qualitative agreement with available experimental data showing hydrolysis rate accelerations for –NH2 functionalized Zr–UiO–66 and Zr-UiO-67.9,12,14,16,18 Table 1. Relative Free Energies (kcal/mol) of Stationary Points Along Sarin Hydrated Pathway Reaction Coordinate.(a) ortho

meta

O

O

O

O

O

O

O

O

perfluoro O

O

NH2 F

MOF–808, and as the acceleration is predicted to be smaller for the nucleophilic substitution step, the net effect is to suggest more influence on the activity of UiO–66–10 than on MOF–808. The effect of o–F substitution is predicted to be modest compared to the o–NH2 substitution (decelerating by 1.1 kcal/mol and 0.8 kcal/mol for UiO–66–10 and MOF–808, respectively (for the respective rate determining steps)), which is likely within the error of the computational model. We note that substitution of the 1,4-benzenedicarboxylate linkers of Zr–UiO–66–10 with one –NH2 functional group (or one F group) perforce leads to some defect sites seeing ortho substitution and others meta as the linker is ditopic. Theory, at least with a cluster model, permits the two to be distinguished however, and we predict a decelerating effect in each case. We note that this is in contrast to recent results18 reported for amino-functionalized UiO–67, where the nature of the 4,4´biphenyldicarboxylic acid linker makes distinct site functionalization possible. However, that linker also has a torsion about the biphenyl linkage, making it difficult to compare without a calculation on the much larger UiO–67 system. Perfluorination of UiO–66–10-H and MOF–808-H is also predicted to significantly decelerate hydrolysis, with the free energies of activation for both the water displacement and nucleophilic addition steps increasing in both MOFs by from 2.5 to 6.5 kcal/mol. It is evident that the substantial enhancement of the Zr Lewis acidity that is expected with linker perfluorination should render water displacement more difficult, but it is perhaps less obvious that the nucleophilic attack step should be decelerated. However, that step is facilitated by the Zr–OH group acting as a local general base, and that basicity is substantially reduced by the more Lewis acidic Zr to which the OH group is coordinated, leading to a net reduction in activity for this step in each case.

See Scheme 1 for stationary-point nomenclature. Separated reactants define the zero of free energy. Rate-determining step TS structure free energies are highlighted in bold.

The most salient difference between UiO–66–10, NU–1000, and MOF–808 is the number of carboxylate linkers supporting each Zr6 SBU. Key geometrical (Zr–Zr and Zr–OH2 bond distances, as well as Mayer bond orders for the latter) and energetic (H2O binding energies and electrophilicity indices) data for the MOFs studied in this work are presented in Tables S2 and S3 of the SI. With decreasing number of supporting carboxylate groups, from UiO–66–12 with 12 carboxylate linkers to MOF–808 with only six linkers, there is an increase in the Zr–Zr bond distances from 3.573 Å to 3.626 Å and the Zr– OH2 bond distances from 2.312 Å to 2.368 Å, respectively. Additionally, the computed Mayer bond orders for the Zr–OH2 bonds decrease from 0.367 in UiO-66-11 to 0.351 in MOF808, and the corresponding electrophilicity indices decrease from 8.5 to 8.1. These data all suggest that binding of water is less effective with fewer supporting carboxylates, and the computed dehydration energies (∆Gdehy) for UiO–66–11, UiO– 66–10 and MOF–808 of +1.3, +0.8, and –3.2 kcal/mol, respectively, are in accord with these trends. Computed sequential dehydration free energies for the various MOFs (Figure S4, SI) in general show only small variations, i.e., there is little cooperativity, whether positive or negative, suggesting that multiple faces for a single SBU may simultaneously be active in catalysis.

We predict o–NH2 substitution to facilitate water displacement by sarin in both UiO–66–10 and MOF–808 by about the same amount. However, as that step is not rate-determining for

Extended-cluster and periodic modeling of sarin hydrolysis by Zr–MOFs reveals the dual role of water as a functional group blocking Lewis acidic Zr sites as well as a nucleophile activat-

Functionalized ZrMOFs

SBU-HB

TSdis

SBU-GB

TSadd

UiO–66–10–H

+2.5

+17.9

+4.8

+16.1

perfluoro-UiO–66–10

-0.4

+22.3

+5.0

+18.6

o–F–UiO–66–10

-1.6

+17.4

+1.1

+11.4

m–F–UiO–66–10

+3.3

+20.5

+2.3

+18.7

o–NH2–UiO–66–10

-1.7

+11.4

0.0

+8.4

m–NH2–UiO–66–10

+1.0

+21.3

+3.8

+15.9

MOF–808–H

+1.6

+14.4

+2.4

+16.2

perfluoro–MOF–808

+5.4

+19.5

+7.4

+22.7

o–F–MOF–808

-1.5

+10.8

+0.3

+15.5

o–NH2–MOF–808

-2.8

+8.8

+5.0

+14.5

(a)

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ed by additional node functionality. The activation free energies for the water displacement and nucleophilic attack steps along the hydrolysis reaction coordinate are sensitive to the number of carboxylate linkers coordinating the MOF SBUs, as well as to substitution of those linkers, at least in part because of the effect they have on the underlying Lewis acidity of the metal, although local structural perturbations associated with nearby substituents may certainly also be important when larger groups are employed. Computational prediction of simple reactant properties like Zr–OH2 bond orders and electrophilicities, which are much more readily computed than full reaction coordinates, show promise for future screening of hypothetical MOFs designed to have further increased activity.

ASSOCIATED CONTENT Supporting Information Details of the computations, supporting data including comparisons between different periodic and cluster models, rationalization of the different reactivity trends observed for different studied ZrIV–MOFs as well as computed Cartesian coordinates of all molecules reported in this work (PDF). The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The Authors gratefully acknowledge the Defense Threat Reduction Agency (HDTRA1-18-1-0003) for the financial support. The authors also acknowledge the Minnesota Supercomputing Institute (MSI) for providing resources that contributed to the research results reported within this paper. MRM is grateful for helpful discussions with Omar Farha, Timur Islamoglu, Manuel Ortuño, Dale Pahls, Hung Pham and Debmalya Ray.

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ACS Applied Materials & Interfaces (25) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. Universal Solvation Model Based on Solute Electron Density and on a Continuum Model of the Solvent Defined by the Bulk Dielectric Constant and Atomic Surface Tensions. J. Phys. Chem. B 2009, 113, 6378-6396. (26) Planas, N.; Mondloch, J. E.; Tussupbayev, S.; Borycz, J.; Gagliardi, L.; Hupp, J. T.; Farha, O. K.; Cramer, C. J. Defining the Proton Topology of the Zr6-Based Metal–Organic Framework NU1000. J. Phys. Chem. Lett., 2014, 5, 3716-3723. (27) Rogge, S. M. J.; Wieme, J.; Vanduyfhuys, L.; Vandenbrande, S.; Maurin, G.; Verstraelen, T.; Waroquire, M.; Van Speybroeck, V. Thermodynamic Insight in the High-Pressure Behavior of UiO-66: Effect of Linker Defects and Linker Expansion. Chem. Mater. 2016, 28, 5721-5732. (28) De Vos, A.; Hendrickx, K.; Van Der Voort, P.; Van Speybroeck, V.; Lejaeghere, K. Missing Linkers: An Alternative Pathway to UiO66 Electronic Structure Engineering. Chem. Mater. 2017, 29, 30063019. (29) Why Recoordination of H2O to a Vacant Zr Site Is Slow Once Dehydrated Material Is Exposed to Water Remains an Open Question. The Modeling of that Process Would Require a Molecular Dynamics Approach Beyond the Scope of this Article. (30) Cadiau, A.; Belmabkhout, Y.; Adil, K.; Bhatt, P. M.; Pillai, R. J.; Shhurenko, A.; Martineau-Corcos, C.; Maurin, G.; Eddaoudi, M. Hydrolytically Stable Fluorinated Metal-Organic Frameworks for Energy-Efficient Dehydration. Science, 2017, 356, 731–735.

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