Metal nanocrystals embedded in single nanocrystals of MOFs give unusual selectivity as heterogeneous catalysts

Article abstract of DOI:10.1021/nl503007h

The growth of nanocrystalline metal-organic frameworks (nMOFs) around metal nanocrystals (NCs) is useful in controlling the chemistry and metric of metal NCs. In this Letter, we show rare examples of nMOFs grown in monocrystalline form around metal NCs. Specifically, Pt NCs were subjected to reactions yielding Zr(IV) nMOFs [Zr₆O₄(OH)₄(fumarate)₆, MOF-801; Zr₆O₄(OH)₄(BDC)₆ (BDC = 1,4-benzenedicarboxylate), UiO-66; Zr₆O₄(OH)₄(BPDC)₆ (BPDC = 4,4′-biphenyldicarboxylate), UiO-67] as a single crystal within which the Pt NCs are embedded. These constructs (Pt?nMOF)_nanocrystal are found to be active in gas-phase hydrogenative conversion of methylcyclopentane (MCP) and give unusual product selectivity. The Pt?nUiO-66 shows selectivity to C₆-cyclic hydrocarbons such as cyclohexane and benzene that takes place with 100 °C lower temperature than the standard reaction (Pt-on-SiO₂). We observe a pore size effect in the nMOF series where the small pore of Pt?nMOF-801 does not produce the same products, while the larger pore Pt?nUiO-67 catalyst provides the same products but with different selectivity. The (Pt?nMOF)_nanocrystal spent catalyst is found to maintain the original crystallinity, and be recyclable without any byproduct residues.

Full text of DOI:10.1021/nl503007h

Letter
pubs.acs.org/NanoLett
Metal Nanocrystals Embedded in Single Nanocrystals of MOFs Give
Unusual Selectivity as Heterogeneous Catalysts
†
,‡,§,#
†
,‡,§,#
†
,‡,§,∥
†,‡,§
Kyungsu Na,
Kyung Min Choi,
Omar M. Yaghi,*,
and Gabor A. Somorjai*,
†
Department of Chemistry, University of California-Berkeley, Berkeley, California 94720, United States
‡Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States
§Kavli Energy NanoSciences Institute, University of California-Berkeley, Berkeley, California 94720, United States
∥Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 34464, Saudi Arabia
S
* Supporting Information
ABSTRACT: The growth of nanocrystalline metal−organic
frameworks (nMOFs) around metal nanocrystals (NCs) is
useful in controlling the chemistry and metric of metal NCs. In
this Letter, we show rare examples of nMOFs grown in
monocrystalline form around metal NCs. Specifically, Pt NCs
were subjected to reactions yielding Zr(IV) nMOFs
[Zr6O4(OH)4(fumarate)6, MOF-801; Zr6O4(OH)4(BDC)6
(BDC
= 1, 4-benzenedicarboxylate), UiO-66;
Zr6O4(OH)4(BPDC)6 (BPDC = 4,4′-biphenyldicarboxylate),
UiO-67] as a single crystal within which the Pt NCs are embedded. These constructs (Pt⊂nMOF)nanocrystal are found to be active
in gas-phase hydrogenative conversion of methylcyclopentane (MCP) and give unusual product selectivity. The Pt⊂nUiO-66
shows selectivity to C6-cyclic hydrocarbons such as cyclohexane and benzene that takes place with 100 °C lower temperature
than the standard reaction (Pt-on-SiO2). We observe a pore size effect in the nMOF series where the small pore of Pt⊂nMOF-
801 does not produce the same products, while the larger pore Pt⊂nUiO-67 catalyst provides the same products but with
different selectivity. The (Pt⊂nMOF)nanocrystal spent catalyst is found to maintain the original crystallinity, and be recyclable
without any byproduct residues.
KEYWORDS: Metal−organic Framework, metal nanocrystal, heterogeneous catalyst, methylcyclopentane, isomerization
mesoporous silica) and also show a MOF pore-dependent
sually, metal nanocrystals are introduced into the pores of
Umetal−organic frameworks (MOFs) by incorporating activity and selectivity to those products.16 In addition, the
performance, crystallinity, and porosity of this catalyst system
are maintained over multiple cycles with no trapped reactants
and products found after reactions.
metal precursors as guests and reducing them to form the metal
nanoparticles.1−3 These materials have shown interesting
catalytic properties but some challenges exist concerning
control of metal nanocrystal size, location, and order.3−8 To
express more control over this process a second approach was
recently reported where the MOF is grown around preformed
metal nanocrystals allowing better control over their
metrics.9−12 However, it remains difficult to control the growth
of the MOF in single nanocrystalline form, and thus the MOF
is usually grown as polycrystalline around metal nanocryst-
als.11−14 This is undesirable for facile diffusion of substrates and
products to and from the catalytic sites and often leads to
deactivation.15 In this Letter, we show how Pt nanocrystals can
be fully embedded within single crystals of nano MOFs
(nMOF-801, nUiO-66, and nUiO-67) with full control over the
size and location of the Pt (ca. 2.5 nm) and MOF (ca. 150 nm)
nanocrystals. We find that this new (Pt⊂nMOF)nanocrystal
construct is capable of carrying out conversion of methyl-
cyclopentane (MCP), a model reaction for selective C−C and
C−H bond activation. These conversions occur at 100 °C
lower and with higher selectivity to C6-cyclic products
compared to the reference catalyst (Pt supported on
Three members of thermally stable Zr(IV) nMOFs were
c h o s e n [ Z r 6 O 4 ( O H ) 4 ( f u m a r a t e ) 6 , M O F - 8 0 1 ;
Zr6O4(OH)4(BDC)6 (BDC = 1,4-benzenedicarboxylate),
UiO-66; Zr6O4(OH)4(BPDC)6 (BPDC = 4,4′-biphenyldicar-
boxylate), UiO-67]17,18 and grown as single nanocrystals
around Pt nanocrystals (NCs) to make (Pt⊂nMOF)nanocrystal
materials. In a typical synthesis, Pt NCs of 2.5 nm size were
synthesized using poly(vinylpyrrolidone) (PVP) as a capping
agent19 and were placed in N,N-dimethylformamide (DMF)
solution mixture containing ZrCl4, the respective organic link
and acetic acid. This mixture was placed at 120 °C for a day to
produce a cloudy solution. This was centrifuged and the nMOF
product was collected, washed with DMF and methanol, and
dried under vacuum (see Supporting Information for synthesis
details). All materials were characterized by powder X-ray
Received: August 5, 2014
Revised: September 3, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/nl503007h | Nano Lett. XXXX, XXX, XXX−XXX

Nano Letters
Letter
diffraction (PXRD), nitrogen gas adsorption, transmission
electron microscopy (TEM), and inductively coupled plasma
atomic emission spectrometry (ICP-AES). These techniques
were used to establish the crystallinity and permanent porosity
of nMOFs and the position and amount of Pt NCs. All spent
catalysts were analyzed again with PXRD, TEM, and NMR to
evaluate their structural and morphological robustness and
detect any residues within the catalyst construct.
ICP-AES revealed that the amounts of Pt for both Pt⊂nUiO-66
and Pt-on-nUiO-66 were found to be around 0.4 wt % (see
Table S1 in Supporting Information).
Figure 2A shows the schematic reaction pathways of
hydrogenative MCP conversion.16,22 In a typical reaction
For each of the nMOFs, TEM images were collected without
P t N C s ( n M O F ) , e m b e d d e d i n t h e n M O F
(Pt⊂nMOF)nanocrystal, and, as a control, with Pt NCs on the
nMOF (Pt-on-nMOF) (see Supporting Information for
synthesis details). Figure 1 shows the results for nUiO-66
Figure 1. Characterization of nUiO-66, Pt⊂nUiO-66 and Pt-on-nUiO-
66: (A−C) TEM images, (D) PXRD patterns in comparison with
simulated pattern of UiO-66, and (E) N2 adsorption isotherms at 77 K
with adsorption and desorption points represented by closed circles
and open circles, respectively (P/P0, relative pressure).
Figure 2. (A) Schematic reaction diagram of hydrogenative conversion
of MCP. (B) Product selectivity obtained at 150 °C over three
catalysts (Pt-on-SiO2, Pt⊂nUiO-66, and Pt-on-nUiO-66).
because its pore size is in the middle of the series (nMOF-801,
5.4 and 7.0 Å; nUiO-66, 6.8 and 7.2 Å; nUiO-67, 9.6 and 12.6
Å). Figure 1A−C clearly shows the single nanocrystallinity of
nUiO-66, the Pt NCs embedded in the center of the nUiO-66,
and the presence and distribution of Pt NCs on the surface of
nUiO-66. No Pt NCs were found to be located on the outside
of the nUiO-66 nanocrystals in the (Pt⊂nMOF)nanocrystal as
confirmed by Figure 1B. In the case of Pt-on-nUiO-66, 2.5 nm
Pt NCs were deposited postsynthetically (i.e., after formation of
nMOFs) to ensure their location on the outside (Figure 1C and
inset).
High crystallinity of nUiO-66, Pt⊂nUiO-66, and Pt-on-
nUiO-66 is evident from the sharp diffraction lines of their
PXRD patterns (Figure 1D), and the coincidence of the
diffraction lines between these samples clearly indicates
preservation of the bulk UiO-66 structure arrangement for
nUiO-66 and upon introduction of Pt NCs in Pt⊂nUiO-66 and
Pt-on-nUiO-66. The permanent porosity of all these samples is
preserved as confirmed by measurement of the N2 gas-
adsorption isotherm, which exhibits a Type I behavior similar
to that observed in bulk UiO-6617 and nUiO-66 (Figure 1E).
The corresponding Langmuir surface areas of nUiO-66,
Pt⊂nUiO-66 and Pt-on-nUiO-66 were calculated to be 2150,
1910, and 1860 m2 g−1, respectively; values that are also similar
to those were found for UiO-66 (1180−2200 m2 g−1).17,20,21
process, MCP can be converted to various hydrocarbon
products via five different reaction pathways. First, MCP can
be converted into the dehydrogenated version of MCP through
reaction pathway (I). Second, MCP can be ring-opened and
isomerized into isomers through reaction pathway (II). The C5-
cyclic ring of MCP can be further enlarged to C6-cyclic
hydrocarbons through ring-enlargement reaction followed with
(III) hydrogenation or (IV) dehydrogenation to produce
cyclohexane or benzene, respectively. The last reaction pathway
is (V) cracking to produce C1−C5-based hydrocarbons that are
undesired products in this reaction.
Figure 2B shows product selectivity data obtained at 150 °C
over nUiO-66 with different locations of Pt NCs in comparison
with Pt-on-SiO2 (MCF-17) as the reference catalyst. The study
with Pt-on-SiO2 shows the sole catalytic role of Pt NCs because
the mesoporous silica support (SiO2) has no catalytic activity.22
The Pt-on-SiO2 catalyst can efficiently catalyze the reaction
pathways of dehydrogenation and isomerization with selectiv-
ities of 18.2 and 81.8%, respectively. No other products were
obtained. Similarly, when Pt NCs were supported at the
external surface of nUiO-66 crystals, the Pt-on-nUiO-66
catalyst can catalyze both dehydrogenation and isomerization
with selectivities of 33.1 and 66.9%, respectively. The pure
B
dx.doi.org/10.1021/nl503007h | Nano Lett. XXXX, XXX, XXX−XXX

Nano Letters
Letter
nUiO-66 without supporting Pt NCs was also tested for the
same reaction, but it showed no catalytic activity. In contrast,
when Pt NCs were encapsulated inside the nMOF crystal
(Pt⊂nUiO-66), C6-cyclic hydrocarbon products were predom-
inantly obtained (>60%) with selectivities of 22.2% for
cyclohexane (red) and 41.2% for benzene (orange) (Figure
2B). Such a production of C6-cyclic hydrocarbons at 150 °C is
noteworthy because benzene can be produced over Pt-on-SiO2
at higher reaction temperature above 250 °C (see Supporting
Information Figure S1 for catalytic results of Pt-on-SiO2).16
Hence, the facile formation of C6-cyclic products over
Pt⊂nUiO-66 at lower reaction temperature (150 °C) indicates
that the embedding Pt NCs in nUiO-66 is contributing to the
decrease of activation energy for the formation of C6-cyclic
hydrocarbons.
There are two possibilities for the observed favorable
difference in the activity and selectivity of Pt⊂nUiO-66. First,
the nMOF structure may assist Pt NCs in making C6-cyclic
hydrocarbons by having the initial products occurring at surface
of Pt NCs and further converted over the nUiO-66 porous
framework. This is not supported by our findings where pure
nUiO-66 showed no activity in the conversion of 2-
methylpentane. Instead of 2-methylpentane as a model
reactant, we also tested cyclohexene that is known as one of
the possible intermediate species during this reaction.22
However, cyclohexene was not converted over pure nUiO-66
without the presence of Pt NCs. On the basis of these
observations, we conclude that the nMOF alone has no
catalytic contribution in the absence of Pt NCs. This is because
there are no catalytic sites for dissociation of H2 and activation
of hydrocarbon reactant within the nUiO-66 framework. The
second possibility is that the micropores in nUiO-66 may
accelerate the molecular vibration of reactant or potential
transition states, which can lower the activation barrier toward
the formation of C6-cyclic hydrocarbons. Because the kinetic
diameters of cyclohexane (6.0 Å) and benzene (5.9 Å) are
smaller than the micropores of nUiO-66 (6.8 and 7.2 Å), it
would be quite reasonable.23 More importantly, cyclohexane
cannot be formed over Pt-on-SiO2 under the same reaction
condition. Hence, the formation of cyclohexane over Pt⊂nUiO-
66 might be explained by the increase of local concentration of
H2 inside UiO-66 nanocrystals. Because the diffusion of H2 into
the micropores of nUiO-66 is more facile than the much larger
MCP molecule, the H2 gas should be relatively more localized
in the presence of Pt NCs. This in turn would facilitate the
production of hydrogenated products. In effect, the nMOF
nanocrystals may be acting as a confined nanoreactor wherein
the reactants can be localized with H2 to produce cyclohexane
as well as benzene.
In order to investigate the effect of pore size, we also
prepared two additional isoreticular (of the same topology) Zr-
based nMOFs (nMOF-801 and nUiO-67) with different length
of organic links, and thus different pore sizes of the same
topology as nUiO-66.17,18 The micropore sizes progressively
increased from nMOF-801 (5.4 and 7.0 Å) to nUiO-66 (6.8
and 7.2 Å) and nUiO-67 (9.6 and 12.6 Å) (Figure 3A).17,18
Figure 3B shows the selectivity data obtained at 150 °C and
how the micropore size plays an important role in product
selectivity. Among the three catalysts, the Pt⊂nMOF-801 with
the smallest micropore diameter did not give C6-cyclic
hydrocarbons. This catalyst could only make dehydrogenated
MCP or ring-opened isomers as observed over the Pt-on-SiO2
catalyst. This is because the pore size of nMOF-801 is smaller
Figure 3. (A) Crystal structure representation of isoreticular Zr-MOFs
with different micropore sizes (nMOF-801, nUiO-66, and nUiO-67
from top to bottom). (B) Product selectivity and (C) turnover
frequency (TOF, h−1) obtained at 150 °C over Pt⊂nMOF-801,
Pt⊂nUiO-66, and Pt⊂nUiO-67.
than kinetic diameters of cyclohexane (6.0 Å) and benzene (5.9
Å). In addition, the adsorption geometries of transition states
or intermediate species might be unfavorable toward the
formation of C6-cyclic hydrocarbons within the small micro-
pores of nMOF-801.
However, in the case of Pt⊂nUiO-67 catalyst having the
largest micropores, C6-cyclic hydrocarbons were produced
easily as observed over the Pt⊂nUiO-66 catalyst (Figure 3B).
The different product selectivity between Pt⊂nUiO-66 and
Pt⊂nUiO-67 might originate from the local concentrations of
reactant and H2 inside the nMOF nanocrystals. The catalytic
activity data based on the number of Pt sites is shown in Figure
3C. The height of the bar in Figure 3C is the conversion rate of
MCP (h−1), which increased progressively with the increase of
micropore size. Each bar can be deconvoluted to the product
formation rate by multiplying the product selectivity with total
TOF of MCP. Among the three catalysts, Pt⊂nMOF-801,
Pt⊂nUiO-66, and Pt⊂nUiO-67 produced the largest yield to
isomers (57.3 h−1), cyclohexane (19.1 h−1), and benzene (51.4
h−1), respectively.
The harsh reaction conditions and temperature employed to
study these reactions prompted us to examine the recyclability
of the catalysts and its performance. The recyclability of
Pt⊂nUiO-66 is shown in Figure 4A for three consecutive runs
under the same reaction conditions expressed above. It is clear
from the similarity in TOF numbers that this catalyst construct
is stable. The spent catalysts were also evaluated by
characterization using PXRD, TEM, and digested NMR.
Again, we find that the catalyst is robust and maintains its
mononanocrystallinity and morphology after the reaction
(Figure 4B,C). The spent catalysts were digested by dissolving
it in aqueous HF and analyzed by solution 1H NMR in DMSO-
d6 (Figure 4D). It is clear from the NMR that only the acidified
organic link (1,4-benzenedicarboxylic acid) and terminating
acetic acid units are shown with no evidence of any other
C
dx.doi.org/10.1021/nl503007h | Nano Lett. XXXX, XXX, XXX−XXX

Nano Letters
Letter
Figure 4. (A) Recyclability of Pt⊂nUiO-66 (no significant changes in product TOF are shown over three times catalytic runs). (B) PXRD patterns
1
of Pt⊂nUiO-66 before (blue) and after reaction (red). (C) TEM image of Pt⊂nUiO-66 after reaction. (D) H NMR spectrum of digested
Pt⊂nUiO-66 after reaction.
residues that may have deposited inside the catalyst structure
after the reaction.
ACKNOWLEDGMENTS
■
G.A.S. acknowledges support by The Chevron Energy
Technology Company and from the Director, Office of
Science, Office of Basic Energy Sciences, Division of Chemical
Sciences, Geological, and Biosciences of the U.S. DOE under
contract DE-AC02-05CH11231 for catalytic studies. The
material synthesis and characterization were supported by
BASF SE (Ludwigshafen, Germany) and U.S. Department of
Defense, Defense Threat Reduction Agency (HDTRA 1-12-1-
0053), respectively. We thank Professor Peidong Yang for use
of the TEM instrument.
The principal findings of this study clearly indicate that it is
possible to make MOF nanocrystals around Pt NCs with the
MOF being in monocrystalline form, (Pt⊂nMOF)nanocrystal, and
that such a construct can be used as a catalyst capable of
lowering the temperature of C−C and C−H bond activation
reactions by 100 °C and favorably altering product selectivity.
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthesis of catalytic materials, their characterization, and
supplementary catalytic reaction results. This material is
available free of charge via the Internet at http://pubs.acs.org.
REFERENCES
■
(1) Hermes, S.; Schroter, M.-K.; Schmid, R.; Khodeir, L.; Muhler, M.;
̈
Tissler, A.; Fischer, R. W.; Fischer, R. A. Angew. Chem., Int. Ed. 2005,
AUTHOR INFORMATION
■
44, 6237.
Corresponding Authors
*E-mail: yaghi@berkeley.edu.
*E-mail: somorjai@berkeley.edu.
(2) Meilikhov, M.; Yusenko, K.; Esken, D.; Turner, S.; Tendeloo, G.
V.; Fischer, R. A. Eur. J. Inorg. Chem. 2010, 24, 3701.
(3) Moon, H. R.; Lim, D.; Suh, M. P. Chem. Soc. Rev. 2013, 42, 1807.
(4) 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.
(5) 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.
(6) Zhao, M.; Deng, K.; He, L.; Liu, Y.; Li, G.; Zhao, H.; Tang, Z. J.
Am. Chem. Soc. 2014, 136, 1738.
Author Contributions
#K.N. and K.M.C. contributed equally.
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
D
dx.doi.org/10.1021/nl503007h | Nano Lett. XXXX, XXX, XXX−XXX

Nano Letters
Letter
(7) 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.
(8) Li, X.; Guo, Z.; Xiao, C.; Goh, T. W.; Tesfagaber, D.; Huang, W.
ACS Catal. 2014, DOI: 10.1021/cs5006635.
(9) Tsuruoka, T.; Kawasaki, H.; Nawafune, H.; Akamatsu, K. ACS
Appl. Mater. Interfaces 2011, 3, 3788.
(10) Falcaro, P.; Hill, A. J.; Nairn, K. M.; Jasieniak, J.; Mardel, J. I.;
Bastow, T. J.; Mayo, S. C.; Gimona, M.; Gomez, D.; Whitfield, H. J.;
et al. Nat. Commun. 2011, 2, 237.
(11) Liu, Y.; Tang, Z. Adv. Mater. 2013, 25, 5819.
(12) Sugikawa, K.; Furukawa, Y.; Sada, K. Chem. Mater. 2011, 23,
3132.
(13) Silvestre, M. E.; Franzreb, M.; Weidler, P. G.; Shekhah, O.;
Woll, C. Adv. Funct. Mater. 2013, 23, 1210.
̈
(14) Ke, F.; Qiu, L. G.; Yuan, Y. P.; Jiang, X.; Zhu, J. F. J. Mater.
Chem. 2012, 22, 9497.
(15) Dhakshinamoorthy, A.; Garcia, H. Chem. Soc. Rev. 2012, 41,
5262.
(16) Alayoglu, S.; Pushkarev, V. V.; Musselwhite, N.; An, K.;
Beaumont, S. K.; Somorjai, G. A. Top. Catal. 2012, 55, 723.
(17) Cavka, J. H.; Jakobsen, S.; Olsbye, U.; Guillou, N.; Lamberti, C.;
Bordiga, S.; Lillerud, K. P. J. Am. Chem. Soc. 2008, 130, 13850.
(18) Wißmann, G.; Schaate, A.; Lilienthal, S.; Bremer, I.; Schneider,
A. M.; Behrens, P. Microporous Mesoporous Mater. 2012, 152, 64.
(19) An, K.; Alayoglu, S.; Musselwhite, N.; Plamthottam, S.; Melaet,
G.; Lindeman, A. E.; Somorjai, G. A. J. Am. Chem. Soc. 2013, 135,
16689.
(20) Choi, K. M.; Jeong, H. M.; Park, J. H.; Zhang, Y.; Kang, J. K.;
Yaghi, O. M. ACS Nano 2014, 8, 7451.
(21) Furukawa, H.; Gandara, F.; Zhang, Y.-B.; Jiang, J.; Queen, W. L.;
Hudson, M. R.; Yaghi, O. M. J. Am. Chem. Soc. 2014, 136, 4369.
(22) Na, K.; Musselwhite, N.; Cai, X.; Alayoglu, S.; Somorjai, G. A. J.
Phys. Chem. A 2014, DOI: 10.1021/jp501775q.
(23) Weitkamp, J.; Puppe, L. Catalysis and Zeolites: Fundamentals and
Applications; Springer: New York, 1999; p 332.
E
dx.doi.org/10.1021/nl503007h | Nano Lett. XXXX, XXX, XXX−XXX