Synthesis of nanocrystals of Zr-based metal-organic frameworks with csq-net: Significant enhancement in the degradation of a nerve agent simulant

Article abstract of DOI:10.1039/c5cc03398e

The synthesis of nano-sized particles of NU-1000 (length from 75 nm to 1200 nm) and PCN-222/MOF-545 (length from 350 nm to 900 nm) is reported. The catalytic hydrolysis of methyl paraoxon was investigated as a function of NU-1000 crystallite size and a si

Full text of DOI:10.1039/c5cc03398e

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Synthesis of nanocrystals of Zr-based metal–
organic frameworks with csq-net: significant
enhancement in the degradation of a nerve agent
simulant†
Cite this:DOI: 10.1039/c5cc03398e
Received 24th April 2015,
Accepted 22nd May 2015
Peng Li,a Rachel C. Klet,a Su-Young Moon,a Timothy C. Wang,a Pravas Deria,a
Aaron W. Peters,a Benjamin M. Klahr,a Hea-Jung Park,a Salih S. Al-Juaid,b
Joseph T. Hupp*a and Omar K. Farha*ab
DOI: 10.1039/c5cc03398e
www.rsc.org/chemcomm
The synthesis of nano-sized particles of NU-1000 (length from cellular uptake. Combining nano-sized MOFs with the afore-
75 nm to 1200 nm) and PCN-222/MOF-545 (length from 350 nm mentioned advantages of mesoporous MOFs would potentially
to 900 nm) is reported. The catalytic hydrolysis of methyl paraoxon lead to a new generation of MOF materials for a wide variety of
was investigated as a function of NU-1000 crystallite size and a applications.
significant enhancement in the rate was observed for the nano-
sized crystals compared to microcrystals.
In the past decade, tremendous effort has been devoted to
developing effective approaches to precisely control the crystal-
lite size of MOFs in the nano regime. Several microporous
Metal–organic frameworks (MOFs)1 have been widely evaluated MOFs, including HKUST-1,13d,f MOF-5,13g MOF-177,17 UiO-66,13b
for gas storage,2 gas separation,3 sensing,4 and heterogeneous ZIF-7,18 and ZIF-8,13e have been synthesized as nanocrystals by
catalysis5 due to their exceptional tunability, high surface area various synthetic methods, including ultrasound and microwave
and porosity.6 MOFs are known to catalyze many reactions synthesis,19 microemulsion synthesis,20 surfactant mediated
using Lewis acidic sites on the nodes,7 or by incorporation of hydrothermal syntheses,21 and the coordination modulation
transition metals8 or functional groups on linkers9 and/or nodes.8a method.13d In addition, the synthesis of nano-sized mesoporous
However, most of the reported MOFs in the past decade have a MOFs has also been reported,13h indicating the feasibility of
microporous structure,10 which restricts molecular diffusion and preparing nano-sized mesoporous MOFs.
transport through the material, and often limits catalysis to the
Recently, zirconium-based MOFs have been extensively
surface sites of MOF crystallites. Recently, several MOFs featuring studied due to their high thermal, mechanical, and hydrolytic
larger pores (mesopores) have been reported which can signifi- stabilities.11a,22 Our group has investigated the catalytic activity
cantly reduce diffusion limitations.11 Indeed, catalysis with these of several MOFs with Zr6 nodes, namely, UiO-66, its derivatives,
MOFs typically yields improved catalytic efficiencies compared to and NU-1000, for the degradation of nerve agents and/or their
microporous MOFs, presumably due to catalysis occurring at both simulants (Scheme 1).23 While UiO-66 and UiO-67 feature
interior and exterior sites.12
12-connected Zr6 nodes, NU-1000 presents 8-connected Zr6 nodes
Nano-sized MOF particles, with larger relative external surface and contains terminal water and hydroxyl ligands in addition to
areas, have also been recently described.13 These nano-sized m3-bridging hydroxyl and oxido groups. NU-1000 also has the
particles should not only give higher efficiencies for surface- advantage of wide mesoporous channels (31 Å) that allow for
only catalysis, but also promise to open up new applications for diffusion of the substrate throughout the material (and con-
MOFs such as drug delivery,14 biomedical imaging,15 and bio- sequently catalysis at interior sites), which ultimately leads to
logical assays.16 For these biomedical applications, customized superior catalytic activity.23a
nano-sized crystals are necessary for facile transport through
In order to further improve the catalytic activity of NU-1000,
capillary vessels or the lymphatic system, as well as for effective we sought to synthesize nano-sized particles of NU-1000, which
should have larger relative external surface areas, and therefore
a Department of Chemistry, Northwestern University, 2145 Sheridan Road,
Evanston, Illinois 60208, USA. E-mail: o-farha@northwestern.edu,
j-hupp@northwestern.edu; Tel: +1-847-491-3504
b Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah,
Saudi Arabia
† Electronic supplementary information (ESI) available: Experimental details,
PXRD, TGA analysis, and DLS data. See DOI: 10.1039/c5cc03398e
Scheme 1 Hydrolysis of methyl paraoxon using MOF-catalysts.
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faster rates of methyl paraoxon hydrolysis. Herein, we report
the synthesis of NU-1000 nano-sized crystallites ranging in size
from 75 nm up to 1200 nm (length) and their respective rates
of hydrolysis for methyl paraoxon; 15 000 nm-sized NU-1000
particles were also synthesized based on previous literature
methods and tested for hydrolysis for comparison.11a In addi-
tion, the synthesis of nano-sized PCN-222/MOF-545,11b a related
mesoporous zirconium MOF with same csq-net to NU-1000, is
also described.
Previous reports have relied primarily on the use of modu-
lators (non-bridging ligands that compete with linkers for node
coordination) to control nucleation and obtain crystallites in a
desired size range, in addition to other factors contributing
to crystal growth such as temperature and concentration.13b,d
Extension of these methods to Zr6-based MOFs with tetratopic
linkers is not straightforward since different nets can be formed
including fcu, ftw, scu, and, of course, csq.24 For example, the
linker in NU-1000, 1,3,6,8-tetrakis(p-benzoic acid)pyrene, with a
Zr(IV) salt (ZrCl4 or ZrOCl2Á8H2O) can form either NU-1000 or
NU-901.11a,25 Similarly, at least three distinct phases (PCN-221/
MOF-525,11c,26 PCN-222/MOF-545,11b,c and PCN-22327) are accessible
with the tetracarboxylate porphyrin linker used for PCN-222/MOF-
545. As the realization of these phases is often very sensitive to the
concentration of the modulator, it can be challenging to obtain the
desired pure phase, in the desired crystallite size regime, solely by
altering the modulator concentration. Accordingly, we found that
for NU-1000 lowering the concentration of benzoic acid (BA)—the
typical modulator used to promote formation of Zr6 clusters to
obtain the desired phase—while adding a secondary mono-
carboxylic acid modulator, trifluoroacetic acid (TFA), resulted
in formation of nano-sized crystallites. Similarly, with PCN-222/
MOF-545 we found that lowering the concentration of the
modulator BA, while also lowering the total concentration of
the solution resulted in MOF particles in the nano regime.
Notably, in our hands, other phases of PCN-222/MOF-545-type
MOFs seem more accessible compared to NU-1000 analogues,
and thus this reaction appears to be more sensitive to modulator
concentration.
Fig. 1 Scanning electron microscopy (SEM) images of NU-1000 and PCN-
222/MOF-545 samples with a range of nano-sized particles. (a) NU-1000
(50–100 nm); (b) NU-1000 (100–200 nm); (c) NU-1000 (300–700 nm);
(d) NU-1000 (800–1600 nm); (e) PCN-222/MOF-545 (300–400 nm);
(f) PCN-222/MOF-545 (450–500 nm); (g) PCN-222/MOF-545 (600–700 nm);
(h) PCN-222/MOF-545 (850–1000 nm).
Using these procedures, we were able to obtain NU-1000
nanocrystals with mean sizes ranging from 75 nm (Fig. 1a) up to
1200 nm (Fig. 1d), and PCN-222/MOF-545 nanocrystals with mean
sizes ranging from 350 nm (Fig. 1e) up to 900 nm (Fig. 1h). Other
PCN-222/MOF-545 particles with sizes larger than 1000 nm have
also been obtained by systematically tuning the reaction condi-
tions (see ESI†). Particle sizes were determined by both scanning
electron microscopy (SEM) and dynamic light scattering (DLS)
measurements (Fig. S1, ESI†) for NU-1000 materials.
Powder X-ray diffraction (PXRD) patterns of nano-sized
NU-1000 and PCN-222/MOF-545 indicate that the MOF particles
are one phase and crystalline (Fig. S2 and S3, ESI†). Additionally,
thermogravimetric analyses (TGA) of the NU-1000 samples show
similar behavior to that reported previously for a microcrystal-
line version of the material (Fig. S4, ESI†).
Fig. 2 Nitrogen adsorption (solid) and desorption (open) isotherms at 77 K
for NU-1000 samples with different sizes: 75 nm (black), 150 nm (red),
500 nm (green), 1200 nm (blue), and 15 000 nm (pink).
the previously reported surface area for NU-1000 (Fig. 2).11a In
Nitrogen sorption isotherms of the activated nano-sized NU- addition, density functional theory (DFT) pore size distribution
1000 crystallites show a characteristic Type IV sorption profile, with (PSD) calculations indicate that the ratio of mesopores (31 Å) and
BET surface areas ranging from 2000–2200 m2 gÀ1, which matches micropores (10 Å) increases as the size of NU-1000 nanocrystals
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This observed rate enhancement is attributed to the larger
relative external surface areas and/or faster diffusion through
the nano-sized MOF crystallites compared to microcrystalline
material. We hope that this work will inspire further investiga-
tions into the fabrication of nano-sized mesoporous zirconium
MOF crystallites for desired applications.
J.T.H. gratefully acknowledges DTRA for financial support
(HDTRA1-14-1-0014 for the nano-MOF synthesis and HDTRA1-
10-1-0023 for the catalytic study). O.K.F. and S.S.A. contribution
to this project was funded by the Deanship of scientific Research
(DSR), King Abdulaziz University. Jeddah, under grant no. 23-130-
36-HiCi. The authors, therefore, acknowledge with thanks DSR
technical and financial support. The authors thank Dr Brian Pate
for helpful discussions.
Fig. 3 Rates of hydrolysis of methyl paraoxon using NU-1000 nanocrystals
with mean sizes ranging from 75 nm (black), 150 nm (red), 500 nm (green),
1200 nm (blue), to 15 000 nm (pink).
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