Postassembly Transformation of a Catalytically Active Composite Material, Pt@ZIF-8, via Solvent-Assisted Linker Exchange

Article abstract of DOI:10.1021/acs.inorgchem.5b02880

2-Methylimidazolate linkers of Pt@ZIF-8 are exchanged with imidazolate using solvent-assisted linker exchange (SALE) to expand the apertures of the parent material and create Pt@SALEM-2. Characterization of the material before and after SALE was performed. Both materials are active as catalysts for the hydrogenation of 1-octene, whereas the hydrogenation of cis-cyclohexene occurred only with Pt@SALEM-2, consistent with larger apertures for the daughter material. The largest substrate, β-pinene, proved to be unreactive with H₂ when either material was employed as a candidate catalyst, supporting the contention that substrate molecules, for both composites, must traverse the metal-organic framework component in order to reach the catalytic nanoparticles.

Full text of DOI:10.1021/acs.inorgchem.5b02880

Communication
pubs.acs.org/IC
Postassembly Transformation of a Catalytically Active Composite
Material, Pt@ZIF-8, via Solvent-Assisted Linker Exchange
Casey J. Stephenson,† Joseph T. Hupp,*,† and Omar K. Farha*,†,‡
†Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States
‡Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia
S
* Supporting Information
imidazolate-based ligand.5 Nanoparticle@ZIF-8 composites
ABSTRACT: 2-Methylimidazolate linkers of Pt@ZIF-8
are exchanged with imidazolate using solvent-assisted
linker exchange (SALE) to expand the apertures of the
parent material and create Pt@SALEM-2. Characterization
of the material before and after SALE was performed. Both
materials are active as catalysts for the hydrogenation of 1-
octene, whereas the hydrogenation of cis-cyclohexene
occurred only with Pt@SALEM-2, consistent with larger
apertures for the daughter material. The largest substrate,
β-pinene, proved to be unreactive with H2 when either
material was employed as a candidate catalyst, supporting
the contention that substrate molecules, for both
composites, must traverse the metal−organic framework
component in order to reach the catalytic nanoparticles.
have been shown to be effective for both size-selective and
regioselective catalysis.4a,c,e As a host framework for a nano-
particle composite, ZIF-8 has the advantages of being free of
long-range defects and synthesizable under a range of
conditions.6 However, because of the small size of the ZIF-8’s
apertures (the largest presents a nominal diameter of 3.4 Å,
expandable to ca. Å), only linear organic substrates pass through
its apertures and enter its pores. Opening up the narrow
apertures of a nanoparticle@ZIF-8 composite would allow larger
substrates, potentially with greater complexity, to enter the MOF
and react with encapsulated nanoparticles.
Solvent-assisted linker exchange (SALE) is a versatile
technique for the synthesis and modification of MOF materials.7
Many MOFs that are not accessible de novo have been
synthesized indirectly via SALE. SALE is a single-crystal-to-
single-crystal transformation in which the linkers of a parent
framework are exchanged with a desired linker.7 To achieve high
degrees of exchange, solvothermal conditions and an excess of
replacement linkers are typically used.7c In this work, we describe
the use of SALE to transform a metal nanoparticle@MOF
composite, Pt@ZIF-8, into a new composite material, Pt@
SALEM-2, by substituting 2-methylimidazolate for imidazolate
(Scheme 1). To our knowledge, this work constitutes the first
report of SALE for a nanoparticle@MOF composite. We
anticipate, however, that this approach will prove attractive and
useful for synthesizing a broad range of catalytic nanoparticle@
MOF composites for which the aperture size, or composition of
he encapsulation of catalytically active nanoparticles within
Tporous materials such as metal−organic frameworks, or
MOFs, is of great interest.1 MOFs are crystalline, modular
materials composed of inorganic nodes and organic linkers.2
Nanoparticles can be encapsulated within MOFs by a top-down
approach, where the MOF is formed first and molecular
precursors of the metal nanoparticle precursors are allowed to
permeate the MOF and then are reduced.3 Top-down methods
typically lack control over the nanoparticle size and distribution,
often yielding particles that are larger than the cavities of the host
framework. Nanoparticles can also form on the exterior of the
MOFan undesirable outcome if the composites are targeted at
reactions for which size-selective or regioselective catalysis is
required.3b However, procedures do exist for the top-down
synthesis of composites where the nanoparticles match the pore
diameter of the host framework and remain fully encapsula-
ted.3b−e Alternatives are bottom-up approaches that involve first
formation of the nanoparticles and then growth of the MOF
around them.4 These methods are capable of yielding,
exclusively, fully MOF-enshrouded nanoparticles with no
detectable direct exposure of particles to external atmospheres
or solutions. Thus, contact between the reactants and a catalytic
nanoparticle or between catalytically formed products and an
external environment is achievable only via molecular navigation
of the channels, cavities, and apertures that define the MOF’s
intrinsic porosity.
Scheme 1. Representation of Pt@ZIF-8 Undergoing
Exchange of 2-Methylimidazolate with Imidazolate To Form
Pt@SALEM-2
Our group reported a bottom-up procedure for the
encapsulation of nanoparticles within zeolitic imidazolate
framework 8 or ZIF-8.4a ZIFs are a subset of MOFs with the
general formula M(im)2, where M is a metal and im is an
Received: December 15, 2015
© XXXX American Chemical Society
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DOI: 10.1021/acs.inorgchem.5b02880
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Inorganic Chemistry
Communication
Figure 1. TEM images of (a) a single crystallite and (b) multiple crystallites of Pt@ZIF-8. TEM images of (c) a single crystallite and (d) multiple
crystallites of Pt@SALEM-2.
the local chemical environment, is a variable that, if modulated,
can impart desirable substrate and/or product selectivity.
Table 1. Catalytic Results for the Hydrogenation of Substrates
Using Pt@ZIF-8 and Pt@SALEM-2
The Pt@ZIF-8 composite with 3.0 nm platinum nanoparticles
was synthesized as previously reported (see the Supporting
Information for details).4d Pt@SALEM-2 was obtained by
adapting the method reported by Karagiardi et al.7b for ZIF-8
alone. Briefly, 100 mg of Pt@ZIF-8 and 200 mg of imidazole
were suspended in 20 mL of n-butanol and heated at 100 °C for 4
days. The solid was isolated by centrifugation and washed several
times with methanol. Between washings, the sample was soaked
for several hours in methanol. Finally, the dark-gray solid was
isolated via centrifugation and dried under vacuum overnight at
120 °C. The sample was characterized by powder X-ray
diffraction (PXRD), 1H NMR spectroscopy, inductively coupled
plasma atomic-emission spectroscopy (ICP-AES), and scanning
transmission electron microscopy (STEM). The PXRD pattern
(Figure S1) is in agreement with the pattern reported by our
group for SALEM-2 alone. N2 adsorption measurements indicate
a
Performed in duplicate using 2000 equiv of substrate in 3.6 mL of
1
ethyl acetate with 0.200 mL of undecane as an internal standard. Prior
to each reaction, the catalyst was heated under vacuum at 150 °C for 2
h followed by reduction at 150 °C under H2 for an additional 2 h.
that Pt@SALEM-2 retained its porosity as well. H NMR
spectroscopy of digested samples indicates that approximately
90% of the original linker was exchanged, while ICP-AES
performed on the same sample both before and after SALE
indicates that no leaching of platinum occurred over the course of
the reaction. STEM analysis performed on the same sample
before and after SALE indicates that the platinum nanoparticles
remain essentially unchanged (3.0 0.7 nm for Pt@ZIF-8 vs 2.8
0.4 nm for Pt@SALEM-2; Figure 1).
b
Conversion determined by time-of-flight gas chromatography.
cyclohexene proved unreactive with hydrogen in the presence of
Pt@ZIF-8 as a candidate catalyst (Table 1, entry 3). The absence
of reactivity also serves as confirmation that the platinum
particles are fully MOF-enshrouded. In contrast, with Pt@
SALEM-2 as the candidate catalyst (Table 1, entry 4), cis-
cyclohexene is hydrogenated in 7% yield. (The extent of
conversion is a function of the reaction time; we intentionally
chose 24 h so that the extent of conversion for all reactions would
be far from complete, i.e., conditions where differences in the
reactivity should be readily observable.) The observed activity
supports the contention that SALE indeed does expand the
apertures of the composite material and open continuous
pathways from the external solution to catalytic particles, where
the pathways are navigable by substrates that are too large to
permeate the parent material.
Finally, β-pinene, a substrate with a larger kinetic diameter
than cis-cyclohexene, was found to resist hydrogenation in the
presence of either potential catalyst (Table 1, entries 5 and 6).
This finding is consistent with a molecular gate-keeping role for
the apertures of both composites. Control experiments were run
with a candidate catalyst consisting of platinum nanoparticles on
the exterior surface of ZIF-8. Here β-pinene hydrogenation was
obtained with 15% conversion (Table 1, entry 7), thus
confirming that the particles, when accessible, are catalytically
competent for hydrogenation of the large substrate.
As a proof-of-concept test of aperture expansion, we studied
the behavior of Pt@ZIF-8 versus Pt@SALEM-2 as potential
catalysts for the hydrogenation of a series of substrates of
progressively larger kinetic diameter: 1-octene, cyclohexene, and
β-pinene. Catalytic trials were conducted at room temperature
using 2000 equiv of substrate under a constant 1 bar of H2 using
0.200 mL of undecane as an internal standard in 3.6 mL of ethyl
acetate as the solvent. To determine whether the composite
materials are inherently active for hydrogenation, we used a linear
substrate with a terminal CC unsaturation, 1-octene, whose
hydrogenation is known to be catalyzed by Pt@ZIF-8.4a,d Using
Pt@ZIF-8 as the catalyst, n-octane was produced in 20%
conversion (Table 1, entry 1). With Pt@SALEM-2 as the
catalyst, 1-octene is hydrogenated to n-octene in 30% conversion
(Table 1, entry 2), indicating that the platinum nanoparticles of
Pt@SALEM-2 are active for catalytic hydrogenation. Given that
the platinum nanoparticle size and distribution are essentially
identical in the two composites, differences in conversion might
reflect faster transport of the substrate through the ca. 6 Å
aperture of Pt@SALEM-2.8
Next, we examined cis-cyclohexene. Consistent with previous
work and with modeling of substrate versus aperture size, cis-
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Morabito, J. V.; Tsung, C.-K. ACS Catal. 2014, 4, 4409−4419.
(e) Rosler, C.; Fischer, R. A. CrystEngComm 2015, 17, 199−217.
(2) (a) Ferey, G. Chem. Soc. Rev. 2008, 37, 191−214. (b) Horike, S.;
Shimomura, S.; Kitagawa, S. Nat. Chem. 2009, 1, 695−704. (c) Zhou,
H.-C.; Long, J. R.; Yaghi, O. M. Chem. Rev. 2012, 112, 673−674.
(d) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. Science
2013, 34110.1126/science.1230444. (e) Farha, O. K.; Hupp, J. T. Acc.
Chem. Res. 2010, 43, 1166−1175. (f) Allendorf, M. D.; Stavila, V.
CrystEngComm 2015, 17, 229−246.
In summary, we report the use of SALE with a nanoparticle@
MOF composite material to synthesize a de novo unattainable
composite material, Pt@SALEM-2, that features larger pore
apertures than the parent compound. PXRD data are in
agreement with previously reported results, while N2 isotherms
1
indicate that the porosity is retained. H NMR spectroscopy
indicates 90% linker exchange, while size-selective reactions
confirm that the gate-keeping apertures of the MOF@nano-
particle composite are expanded by SALE and that the
nanoparticles remain fully enshrouded. STEM of Pt@SALEM-
2 indicates that the platinum nanoparticle size and distribution
are unchanged by SALE. This initial demonstration of catalysis-
relevant linker exchange for an archetypal nanoparticle@MOF
composite suggests extension to other linkers, especially linkers
that may function as cocatalysts but are too delicate for, or are
otherwise incompatible with, the reagents or conditions used for
direct synthesis.
(3) (a) Hwang, Y. K.; Hong, D.-Y.; Chang, J.-S.; Jhung, S. H.; Seo, Y.-
K.; Kim, J.; Vimont, A.; Daturi, M.; Serre, C.; Fer
Ed. 2008, 47, 4144−4148. (b) Proch, S.; Herrmannsdorfer, J.; Kempe,
́
ey, G. Angew. Chem., Int.
̈
R.; Kern, C.; Jess, A.; Seyfarth, L.; Senker, J. Chem. - Eur. J. 2008, 14,
8204−8212. (c) Esken, D.; Zhang, X.; Lebedev, O. I.; Schroder, F.;
Fischer, R. A. J. Mater. Chem. 2009, 19, 1314−1319. (d) Jiang, H.-L.; Liu,
B.; Akita, T.; Haruta, M.; Sakurai, H.; Xu, Q. J. Am. Chem. Soc. 2009, 131,
11302−11303. (e) Esken, D.; Turner, S.; Lebedev, O. I.; Van Tendeloo,
G.; Fischer, R. A. Chem. Mater. 2010, 22, 6393−6401. (f) Guo, Z.; Xiao,
C.; Maligal-Ganesh, R. V.; Zhou, L.; Goh, T. W.; Li, X.; Tesfagaber, D.;
Thiel, A.; Huang, W. ACS Catal. 2014, 4, 1340−1348 For additional
references, see the Supporting Information..
ASSOCIATED CONTENT
* Supporting Information
■
(4) (a) Lu, G.; Li, S.; Guo, Z.; Farha, O. K.; Hauser, B. G.; Qi, X.; Wang,
Y.; Wang, X.; Han, S.; Liu, X.; DuChene, J. S.; Zhang, H.; Zhang, Q.;
Chen, X.; Ma, J.; Loo, S. C. J.; Wei, W. D.; Yang, Y.; Hupp, J. T.; Huo, F.
Nat. Chem. 2012, 4, 310−316. (b) Kuo, C.-H.; Tang, Y.; Chou, L.-Y.;
Sneed, B. T.; Brodsky, C. N.; Zhao, Z.; Tsung, C.-K. J. Am. Chem. Soc.
2012, 134, 14345−14348. (c) Zhang, W.; Lu, G.; Cui, C.; Liu, Y.; Li, S.;
Yan, W.; Xing, C.; Chi, Y. R.; Yang, Y.; Huo, F. Adv. Mater. 2014, 26,
4056−4060. (d) Stephenson, C. J.; Hupp, J. T.; Farha, O. K. Inorg. Chem.
Front. 2015, 2, 448−452. (e) Choi, K. M.; Na, K.; Somorjai, G. A.; Yaghi,
O. M. J. Am. Chem. Soc. 2015, 137, 7810−7816 For additional
references, see the Supporting Information..
(5) (a) Banerjee, R.; Phan, A.; Wang, B.; Knobler, C.; Furukawa, H.;
O’Keeffe, M.; Yaghi, O. M. Science 2008, 319, 939−943. (b) Banerjee,
R.; Furukawa, H.; Britt, D.; Knobler, C.; O’Keeffe, M.; Yaghi, O. M. J.
Am. Chem. Soc. 2009, 131, 3875−3877.
(6) (a) Chizallet, C.; Lazare, S.; Bazer-Bachi, D.; Bonnier, F.; Lecocq,
V.; Soyer, E.; Quoineaud, A.-A.; Bats, N. J. Am. Chem. Soc. 2010, 132,
12365−12377. (b) Pan, Y.; Wang, B.; Lai, Z. J. Membr. Sci. 2012, 421−
422, 292−298. (c) Tao, K.; Kong, C.; Chen, L. Chem. Eng. J. 2013, 220,
1−5. (d) Liu, X.; Li, Y.; Ban, Y.; Peng, Y.; Jin, H.; Yang, W.; Li, K. Mater.
Lett. 2014, 136, 341−344. (e) Shamsaei, E.; Low, Z.-X.; Lin, X.; Mayahi,
A.; Liu, H.; Zhang, X.; Zhe Liu, J.; Wang, H. Chem. Commun. 2015, 51,
11474−11477.
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02880.
Details of the catalytic experiments, nanoparticle statistics,
and experimental spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: j-hupp@northwestern.edu.
*E-mail: o-farha@northwestern.edu.
■
Author Contributions
All authors have given approval to the final version of the
manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the National
Science Foundation (NSF; Grant DMR-1334928). Acquisition
of the data on trace analysis and gas chromatography instruments
used in the IMSERC facility of Northwestern University was
made possible by support from Northwestern University and
NSF Grant CHE-0923236, respectively. This work made use of
TEM and STEM instruments located in the EPIC facility
(NUANCE Center-Northwestern University), which has
received support from the MRSEC program (NSF Grant
DMR-1121262) at the Materials Research Center, the Nanoscale
Science and Engineering Center (NSF Grant EEC-0647560) at
the International Institute for Nanotechnology, and the State of
Illinois, through the International Institute for Nanotechnology.
PXRD was recorded at the J. B. Cohen X-ray Facility at
Northwestern University supported by the MRSEC program of
the NSF (Grant DMR-1121262) at the Materials Research
Center of Northwestern University. We thank Cassandra
Whitford for assistance with STEM.
(7) (a) Cohen, S. M. Chem. Rev. 2012, 112, 970−1000. (b) Karagiaridi,
O.; Lalonde, M. B.; Bury, W.; Sarjeant, A. A.; Farha, O. K.; Hupp, J. T. J.
Am. Chem. Soc. 2012, 134, 18790−18796. (c) Deria, P.; Mondloch, J. E.;
Karagiaridi, O.; Bury, W.; Hupp, J. T.; Farha, O. K. Chem. Soc. Rev. 2014,
43, 5896−5912. (d) Zhang, W.; Liu, Y.; Lu, G.; Wang, Y.; Li, S.; Cui, C.;
Wu, J.; Xu, Z.; Tian, D.; Huang, W.; DuCheneu, J. S.; Wei, W. D.; Chen,
H.; Yang, Y.; Huo, F. Adv. Mater. 2015, 27, 2923−2929 For additional
references, see the Supporting Information..
(8) The given interpretation is reasonable, but stronger confirmation
will have to come from more extensive comparative studies, which is
beyond the scope of this Communication.
REFERENCES
■
(1) (a) Dhakshinamoorthy, A.; Garcia, H. Chem. Soc. Rev. 2012, 41,
5262−5284. (b) Aijaz, A.; Xu, Q. J. Phys. Chem. Lett. 2014, 5, 1400−
1411. (c) Tian, Y.; Wang, W.; Chai, Y.; Cong, J.; Shen, S.; Yan, L.; Wang,
S.; Han, X.; Sun, Y. Phys. Rev. Lett. 2014, 112, 017202. (d) Hu, P.;
C
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