One Step Backward Is Two Steps Forward: Enhancing the Hydrolysis Rate of UiO-66 by Decreasing [OH-]

Article abstract of DOI:10.1021/acscatal.5b00785

The rapid destruction of chemical threats, such as phosphate-based nerve agents, is of considerable current interest. The hydrolysis of the nerve-agent simulant methylparaoxon, as catalyzed by UiO-66 and UiO-67, was examined as a function of pH. Surprisingly, even though typical phosphate-ester hydrolysis mechanisms entail nucleophilic attack of the simulant by aqueous hydroxide, the rate of hydrolysis accelerates as the solution pH is lowered. The unexpected behavior is attributed to a pH-dependent composition change followed by ligand substitution at the Zr₆-based node.

Full text of DOI:10.1021/acscatal.5b00785

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Article
One step backwards is two steps forward: enhancing
the hydrolysis rate of UiO-66 by decreasing [OH-]
Michael J. Katz, Rachel C Klet, Su-Young Moon, Joseph E Mondloch, Joseph T. Hupp, and Omar K. Farha
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b00785 • Publication Date (Web): 26 Jun 2015
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ACS Catalysis
One step backwards is two steps forward: enhancing the hy-
drolysis rate of UiO-66 by decreasing [OH-]
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Michael J. Katz,a Rachel C. Klet,a SuꢀYoung Moon,a Joseph E. Mondloch,a Joseph T. Hupp,*a Omar. K.
Farha*a,b
aDepartment of Chemistry and the International Institute for Nanotechnology, Northwestern University, 2145 Sheridan Road,
Evanston, Illinois 60208−3113, United States.
bDepartment of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia
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ABSTRACT: The rapid destruction of chemical threats such as phosphateꢀbased nerve agents is of considerable current interest.
The hydrolysis of the nerveꢀagent simulant methylparaoxon, as catalysed by UiOꢀ66 and UiOꢀ67, was examined as a function of
pH. Surprisingly, even though typical phosphateꢀester hydrolysis mechanisms entail nucleophilic attack of the simulant by aqueous
hydroxide, the rate of hydrolysis accelerates as the solution pH is lowered. The unexpected behavior is attributed to a pHꢀdependent
composition change followed by ligand substitution at the Zr6ꢀbased node.
KEYWORDS: Metal-Organic Frameworks, Hydrolysis, UiO, Phosphate Nerve Agents, Defects, pH.
Introduction
UiOꢀ66. We have attributed the larger of the rate enhanceꢀ
ments to elimination of nodeꢀbased aqua ligand substitution by
the simulant as a rateꢀattenuating step, and secondarily to the
presentation of a greater number of reactant accessible zircoꢀ
nium(IV) sites for NUꢀ1000 versus UiOꢀ66.20
Metal–Organic Frameworks (MOFs) are a class of porous
materials formed via the coordination chemistry between metꢀ
alꢀbased nodes and polytopic organic linkers.1ꢀ3 Owing to the
tunability of the organic linkers and the wide range of coordiꢀ
nation geometry observed across the metals in the periodic
table, MOFs are potentially interesting for a wide range of
applications such as, but not limited to, gasꢀstorage, chemical
separations, sensing, and catalysis.4ꢀ17
Table 1 summarizes the hydrolysis of methylparaoxon as a
function of the MOFꢀbased catalysts we have investigated to
date.19,20,41 Given the remarkable efficacy of these MOFs toꢀ
wards this key chemical transformation, we were interested in
further probing the mechanism of catalytic hydrolysis of
methylparaoxon, in order to rationally design more active verꢀ
sions of these catalysts. Given their ease of synthesis, we hereꢀ
in return to the use of UiOꢀ66 and UiOꢀ67 to probe the catalyꢀ
sis mechanism (vide infra).
An area of MOF catalysis of particular interest to us is the
hydrolysis of phosphateꢀbased nerve agents (e.g., Sarin, VX,
GD, and Soman) and their simulants such as methylparaoxon
(Figure 1).18ꢀ23 The mode of action of these compounds is inꢀ
hibition of acetylcholinesterase, a hydrolase used to terminate
transmission of the neurotransmitter acetylcholine and thereby
control muscle response. The mode of action of Sarin begins
with phosphonylation of the serine residue at the active site of
acetylcholinesterase, upon elimination of the leaving group
(nitrophenoxide for methylparaoxon and fluoride for Sarin),
that generates robust and biologically inactive phosphoester,
which leads to the inhibition of the enzyme and ultimately
causes asphyxiation.24ꢀ33
TABLE 1: Initial rates and half-lives of Zr-based MOFs
used for hydrolysis of the nerve agent simulant
methylparaoxon19,20,41
MOF catalyst
UiOꢀ66
Half-life (min)
35
45
60
1
UiOꢀ66ꢀNO2
UiOꢀ66ꢀ(OH)2
UiOꢀ66ꢀNH2
UiOꢀ67
In our previous work, we demonstrated that UiOꢀ66,34ꢀ40
MOF formed via the coordination of terephthalate dianions to
a
12+
Zr6O4(OH)4
(Figure 1), is capable of hydrolyzing
4.5
2
methylparaoxon with a halfꢀlife of 35–50 minutes at pH
10.18,19 While this halfꢀlife is remarkable given the low active
catalyst loading (limited to surfaceꢀactive catalytic sites only),
it is too long for applications such as realꢀtime decontaminaꢀ
tion. To that end, we have investigated the role of bdc (1,4 –
benzene dicarboxylate) functionalization, linker length, and
linker denticity on the overall hydrolysis rate. We have obꢀ
served that in comparison to UiOꢀ66, UiOꢀ66ꢀNH2 has a 20ꢀ
fold increase in initial rate; a similar enhancement factor was
observed for UiOꢀ67ꢀNH2 vs. the parent UiOꢀ67 compound
(the difference between UiOꢀ66 and UiOꢀ67 is the use of biꢀ
phenyldicarboxylate versus monophenylꢀdicarboxylate as the
linker).19 In addition to this work, we also compared the bidenꢀ
tate BDC linker with the tetradentate pyreneꢀbased ligand of
NUꢀ1000 and observed a 2.3ꢀ and 23ꢀfold increase in initial
rate for the hydrated and dehydrated forms of NUꢀ1000 vs.
UiOꢀ67ꢀNH2
UiOꢀ67ꢀNMe2
NUꢀ1000
2
15
1.5
Dehydrated
NUꢀ1000
One common mechanism for the hydrolysis of
methylparaoxon entails binding to a coordinatively unsaturatꢀ
ed site on a Lewisꢀacidic metal cation, with concomitant
weakening of phosphorous–oxygen bonds (or P–F in the case
of Sarin); Figure 1c.23,42ꢀ50 A free or metalꢀbound hydroxide
anion is then transferred to the phosphate. Subsequently, the
now deꢀactivated (i.e., nonꢀtoxic) product dissociates from the
active site and the catalyst is regenerated. To probe the hyꢀ
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drolysis mechanism we varied the buffered solution OH− conꢀ
Vis measurements (Varian Cari 5000) (Figure 2). Progress of
centration from 1.6 × 10−6 (pH = 8.3) to 2.0 × 10−4 (pH =
10.2), and followed the reaction kinetics. To our surprise, raꢀ
ther than accelerating with increasing [OH−], the rate of the
catalytic reaction decreased. In contrast, the hydrolysis rate in
the absence of a catalyst systematically increases with increasꢀ
ing hydroxide ion concentration. These observations point to
differences in the mechanism for the catalyzed versus uncataꢀ
lyzed reactions, beyond just Lewis acid weakening of subꢀ
strate bonds.
the reaction was monitored by following the pꢀnitrophenoxide
absorbance at 407 nm to avoid overlapping absorptions with
other species. No spectral evidence for the pꢀnitrophenol was
observed at this pH (10.2). All background reactions were
carried out under identical reaction conditions without the
MOF catalyst.
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Initial rates were determined using the method of initial
rates.52 Polynomial fits of order 3–5 were used with the lowest
observed correlation coefficient of 0.98.
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Potentiometric titrations were completed with a Metrohm
Titrando 905 equipped with Dosino 800 20 mL and 10 mL
dosing units using procedures similar to those reported for
Zr(OH)4.53,54 Prior to titration, approximately 50 mg of sample
was mixed with approximately 60 mL of 0.01 M aq. NaNO3
for 18 h. Each titration solution was adjusted using 0.1 M aq.
HCl to a pH of 3, and was then titrated with 0.1 M aq. NaOH
to a pH of 10.5. Titrations ere run in triplicate.
Results and Discussions
Figure 2 illustrates the hydrolysis of methylparaoxon as a
function of time and solution pH. Qualitatively, as the pH is
lowered from 10.2 to 8.8, the overall reaction rate increases.55
In contrast, the hydrolysis rate in the absence of a catalyst does
not systematically increase with decreasing hydroxide ion
concentration; see Figures S1 and S2 in the electronic suppleꢀ
mentary information (ESI), suggesting a different mechanism
for the MOFꢀcatalyzed hydrolysis reaction versus the uncataꢀ
lyzed hydrolysis reaction.
Figure 1: (a) Schematic of the node of UiOꢀ66 (left) and the exꢀ
tended structure showing the relative positions of the octahedral
and tetrahedral pores (right). (b) Reaction of methylparaoxon, a
nerveꢀagent simulant with a MOF catalyst. (c) Generic mechaꢀ
nism for the catalytic hydrolysis of methylparaoxon.
Experimental
All reagents were purchased from commercial sources and
used without further purification. UiOꢀ66, dehydrated UiOꢀ66
and UiOꢀ67 were synthesized according to literature proceꢀ
dures.39,51
Figure 2: Hydrolysis rate of methylparaoxon, as measured via the
formation of nitrophenoxide (λmax = 407 nm) as a function of time
and pH for UiOꢀ66; the buffer is 0.45 M Nꢀethylꢀmorpholine adꢀ
justed with acetic acid.
Unless otherwise noted, hydrolysis experiments were carꢀ
ried out at room temperature as described previously.18 Briefꢀ
ly, to a solid sample of UiOꢀ66 (2.5 mg, 6 mol %, 0.0015
mmole; 0.045 mole % of active surface sites) or UiOꢀ67 (1
mg, 2 mol %, 0.0005 mmole) in an Eppendorf tube was added
an aqueous solution of Nꢀethylꢀmorpholine (0.45 N) with addiꢀ
tional acetic acid used to modulate the pH of the solution. The
resulting mixture was stirred for 30 minutes to finely disperse
the MOF particles. To the suspension was then added
methylparaoxon (6.2 mg, 0.025 mmol). Periodic monitoring
via UVꢀVis spectroscopy was carried out by removing a 20 µL
aliquot from the reaction mixture and diluting with an aqueous
solution of Nꢀethylꢀmorpholine (10 mL, 0.45 M) prior to UVꢀ
To quantitatively probe the [H+] dependence for the hyꢀ
drolysis reaction, we applied the method of initial rates52 to
each of the curves in Figure 2 (Figure 3). The rates, in units of
formation of nitrophenoxide concentration per unit time are
summarized in Table 2. Plotting the initial rates as a function
of [H+] demonstrates, over the pH range of 8.8–9.7, that the
hydrolysis reaction is first order in [H+] concentration or,
equivalently, inverseꢀfirstꢀorder in [OHꢀ]. At pH values below
8.8 the rate is independent of the [H+] concentration (Table 2).
From the slope of the plot we obtain an observed rate constant
of 4.6 × 104 s−1. These data suggest that the rateꢀlimiting step,
or a preceding step, involves proton transfer rather than hyꢀ
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ACS Catalysis
droxide attack. These results are reminiscent of the results chargeꢀcompensated on the associated nodes by two coordiꢀ
nated hydroxides (one per node).36,37,39,57 The remaining coorꢀ
dination site on each node appears to be occupied by H2O. The
top right section of Scheme 1 shows a simplified representaꢀ
tion (some linker carboxylates are again omitted for clarity). It
is thus conceivable that surface defects, which are necessarily
present even in the most pristine synthesis of UiOꢀ66, take the
same form as the internal defects. The proposed defectꢀsite
coordination is akin to that for MOFs containing eightꢀ
connected hexaꢀzirconium(IV) nodes.58 We hypothesize that
the coordinated neutral water molecule in Scheme 1 (top right)
can be readily substituted by the neutral nerve agent (Scheme
1 bottom), but that the strongly ionicallyꢀbound hydroxide
ligand cannot. Once aqua substitution has occurred, hydrolysis
should rapidly follow. Calculations with NUꢀ1000 and DMNP
indicate that ligand substitution (DMNP for H2O) followed by
hydrolysis is downhill by 48 kJ molꢀ1.20
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with UiOꢀ66ꢀNH2 and UiOꢀ67ꢀNH2, which show a similar
enhancement due to the presence of a proximal Brønsted base.
We suspect that the amino moiety may serve to modulate the
pH proximal to the active site in a similar way to the experiꢀ
ments herein.56
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Figure 3: Plot of initial rate as a function of [H+] (or, equivalently,
[OH−]−1). The slope of the bestꢀfit line gives an observed rate
constant of 4.6 × 104 s−1.
Table 2: Initial rates and TOF for UiO-66 as a function of
pH.
pH
10.2 0.0040
Initial Rate (mM s−1)
TOFcall (s−1) TOFcsurface(s−1)a
0.0026
0.0063
0.0096
0.020
0.030
0.12
0.35
0.84
1.3
2.7
4.0
16
9.7
9.5
9.3
9.0
8.8
8.6
8.3
0.0095
0.015
0.030
0.05b
0.2b
0.3b
0.2b
Scheme 1: (top left) Idealized node of UiO-66 showing no
defects; eight out of 12 1,4-benzendicarboxylate (BDC)
moieties removed for clarity. (top right) Presumed form of
defect-containing cluster. (bottom) Proposed reaction
mechanism between bound methylparaoxon and UiO-66 to
form the hydrolyzed product, with substrate binding as the
rate-limiting step. Omitted is fast release of the product
from the node, a step that does not occur in the corre-
sponding phosphotriesterase enzyme when the reactant is
a fluorine-containing nerve agent.
0.17
23
0.14
18
aUiOꢀ66 particles are 400 nm as synthesized. Due to the aperture
size of UiOꢀ66 and the relatively larger kinetic diameter of
methylparaoxon, only the surface sites (ca. 0.75 % of the catalyst
loading)
are
catalytically
active.
bA larger error is associated with these numbers due to the limited
number of data points available for data fitting.
cTurnꢀover frequency (TOF) is determined by dividing the initial
rate (in mmole s−1) by the catalyst loading (in mmoles) or external
surface area adjusted catalyst loading (in mmoles).
One potential explanation for the pHꢀdependent reaction kiꢀ
netics is illustrated in Scheme 2; at low H+ concentrations (i.e.,
high pH), ligated water molecules are largely converted to
substitutionꢀresistant hydroxo ligands. Thus, at solution pH
values above the equivalence point value of the aqua ligand,
only a minority of node sites will offer aqua ligands that are
favorable towards ligand substitution. If methylparaoxon subꢀ
stitution for water is the rateꢀlimiting step, then the overall
hydrolysis reaction will become progressively faster with inꢀ
creasing [H+]. The results in Figures 1 and 2 are consistent
with this picture, provided that the aqua titration endpoint valꢀ
ue is between pH 9 or 10.
To understand the pH dependence, we turned to the strucꢀ
ture of UiOꢀ66. Scheme 1 (top left) shows a UiOꢀ66 node in
idealized form. Given 12 linkers (some omitted for clarity), 4
µ3ꢀbridging hydroxides, and 4 µ3ꢀbridging oxo moieties, it is
not possible for an organophosphate moiety to bind directly to
a zirconium(IV)ꢀcenter. For this smallꢀaperture material, caꢀ
talysis necessarily occurs on surface defects. It has been
demonstrated elsewhere that conventionally synthesized UiOꢀ
66 contains only ~85% of the anticipated total amount of terꢀ
ephthalate linker.36,37,39,57 Further, each missing linker is
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When the solution pH becomes low enough for the equilibꢀ be seen in Figure 4, catalysis with the dehydrated UiOꢀ66 is
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rium in Scheme 2 to shift largely to the left, the hydrolysis
reaction should become pH independent, and remain so for as
long as methylparaoxon substitution remains the rateꢀ
determining step. The approximate invariance of reaction rates
in Table 2 between pH 8.8 and 8.3 is consistent with this sugꢀ
gestion. Conversely, at very high pH, the fraction of available
aqua ligands may become so low that the hydrolysis reaction
proceeds via rateꢀlimiting displacement of coordinated OH−
rather than H2O, resulting in pHꢀindependent kinetics. The rate
entries in Table 2 for pH 9.7 and 10.2 suggest that at pH 10.2
the reaction is in or near this regime. Unfortunately, catalytic
rate measurements at yet higher pH were not feasible.
roughly 10× faster than normal UiOꢀ66; this finding is conꢀ
sistent with the mechanism in Scheme 2 and with our recent
reported observations with NUꢀ1000.20
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Figure 4: Hydrolysis rate of methylparaoxon using UiOꢀ66 and
dehydrated UiOꢀ66 as the catalyst as measured via the formation
of nitrophenoxide (λmax = 407 nm) as a function of time; the buffꢀ
er is 0.45 M Nꢀethylꢀmorpholine.
Scheme 2: (left) Presumed composition of cluster in the
absence of one BDC unit. Terminal hydroxide and aqua
ligands fill the resulting open Zr(IV) coordination sites. At
high pH values the aqua ligand converts to a substitution-
resistant hydroxide, making the framework overall anion-
ic.
In order to further probe the proposed mechanism shown in
scheme 2, we turned our attention to potentiometric titration of
UiOꢀ66. Figure 5 presents the change in pH of a solution conꢀ
taining UiOꢀ66 as a function of NaOH addition. Notably, the
curve reveals an equivalence point at 9.34, i.e., about the value
expected for the proposed reaction scheme.59
Scheme 3: upon heating UiO-66 to 250 °C, a condensation
reaction occurs between the hydroxides on the node. The
new node structure of the perfect UiO-66 node is shown on
the top,39 and the proposed dehydrated defect-UiO-66 is
shown on the right.
Figure 5: Potentiometric titration of UiOꢀ66. The darkꢀgreen and
darkꢀred cross lines the pH values of 10.2 and 8.8, respectively,
which corresponds to the pH range in which there is a linear corꢀ
relation between [H+] and initial rate. The pink line (pH 8.3) indiꢀ
cates the lowest pH examined for the hydrolysis of UiOꢀ66.
Given our hypothesis, we further examined the hydrolysis
using dehydrated defectꢀUiOꢀ66 as the catalyst. In dehydrated
defectꢀUiOꢀ66, terminal water and hydroxide ligands are reꢀ
moved. Following the behavior of NUꢀ1000, the terminal hyꢀ
droxide can be removed a a neutral water molecule by recruitꢀ
ing a proton from a bridging hydroxide; the cluster now has
Finally, we extended the hydrolysis rate studies to reactions
catalyzed by UiOꢀ67 (Figures S3–S7 and Table S1 in the ESI),
a close structural analogue of UiOꢀ66. We have recently obꢀ
served that UiOꢀ67 elicits significantly faster methylparaoxon
hydrolysis than does UiOꢀ66. With identical catalyst loading,
UiOꢀ67 engenders a 6ꢀfold faster initial rate (t1/2 = 4.5 min)
than does UiOꢀ66. When used at half the catalyst loading of
UiOꢀ66, it induces a ca. 2ꢀfold faster initial rate (t1/2 = 15 min).
+11
the formula Zr6O5(OH)3 and further contains two open sites
on the zirconium node (Scheme 3). We anticipate that the
dehydrated node will more readily bind the substrate. As can
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The manuscript was written through contributions of all authors. /
Given the faster hydrolysis, we examined the pH dependence
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All authors have given approval to the final version of the manuꢀ
script.
of the reaction rate at 0.5 µmole catalyst loading (vs. 1.5
µmole for UiOꢀ66). As shown in Figures S3–S4 and Table S1
(see ESI), and like the behavior with UiOꢀ66, the UiOꢀ67ꢀ
catalyzed hydrolysis rate increases with decreasing pH, yieldꢀ
ing an apparent rate constant essentially identical to that for
UiOꢀ66 (but at oneꢀthird the loading of UiOꢀ66).55 Furtherꢀ
more, although it is has been reported that UiOꢀ67 is unstable
in water, in our hands, UiOꢀ67 showed no such instability.60
Figures S5ꢀS6 in the ESI shows the powder Xꢀray diffractoꢀ
grams and nitrogen sorption isotherms of UiOꢀ67 preꢀ and
postꢀcatalysis. The diffractograms look identical to one anothꢀ
er and the decrease in surface area (2300 m2/g to 1020 m2/g) is
attributed to pore clogging from the hydrolysis products; simiꢀ
lar decreases in surface area have been observed for phosphate
modified Zr6ꢀbased nodes.61,62 Furthermore, filtration of the
MOF midꢀcatalysis ceased the catalytic reaction. In addition,
inductively coupled plasma mass spectrometry of the superꢀ
nate showed no evidence of leached zirconium. Thus, the acꢀ
tive catalyst is UiOꢀ67.
Funding Sources
O.K.F. and J.T.H. gratefully acknowledge DTRA for financial
support (grant HDTRAꢀ1ꢀ10ꢀ0023).
SUPPORTING INFORMATION
Supporting Information Available: Uncatalyzed reaction rate and
pH dependence, hydrolysis rate and pH dependence using UiOꢀ
67, powder xꢀray diffractograms and surface area analysis of UiOꢀ
67 pre/post catalysis, and potentiometric titration of UiOꢀ67. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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The hydroxide driven hydrolysis of the nerveꢀagent simuꢀ
lant, methylparaoxon, has recently been shown to be catalyzed
by UiOꢀ66 and ꢀ67 in buffered solutions at pH = 10. In conꢀ
trast to the uncatalyzed reaction, and to what might otherwise
be expected for a hydroxideꢀconsuming reaction, the catalyzed
reaction accelerates as the OH− concentration is lowered or,
equivalently, the hydronium concentration is increased. The
unexpected behavior can be understood in terms of rateꢀ
limiting simulant displacement of a zirconiumꢀligated water
molecule at a defect site (i.e., missingꢀlinker site) on the MOF
node. Once bound to a highly Lewis acidic zirconium(IV)
node site, the phosphorous–oxygen bonds of the substrate
weaken, with the weakest of the bonds succumbing to nucleoꢀ
philic attack by hydroxide in a step that is fast compared with
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At sufficiently high pH, the majority of the MOF aqua ligꢀ
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