Engineering a metal-organic framework catalyst by using postsynthetic modification

Full text of DOI:10.1002/anie.200903433

Communications
DOI: 10.1002/anie.200903433
MOF-Based Catalysts
Engineering a Metal–Organic Framework Catalyst by Using
Postsynthetic Modification**
Kristine K. Tanabe and Seth M. Cohen*
Metal–organic frameworks (MOFs) are porous, crystalline
materials that have gathered increasing attention owing to
their high surface areas, uniform pores, and chemical tuna-
bility.[1–6] The ability to synthesize a wide range of MOFs has
made them attractive materials for applications in gas
sorption, separations, and catalysis.[7–12] Recently, postsyn-
thetic modification (PSM) of MOFs has been shown to be a
general, practical approach for incorporating a wide range of
functional groups into MOFs.[13] We[14–19] and others[20–31] have
shown that topologically diverse MOFs can be customized by
PSM with many different types of functional groups. Herein,
PSM is shown to be a novel route to obtain active, stable, and
recyclable MOF-based catalysts.
Previous reports have described the use of PSM to
introduce metal-binding sites into MOFs. The treatment of
several different MOFs with succinic anhydride has resulted
in materials containing free (i.e. uncoordinated) carboxylic
acid groups.[19,29] In one of these reports, it was demonstrated
that the carboxylate-bearing MOF could coordinate Cu2+ ions
from solution.[29] In another study, an isoreticular metal–
organic framework (IRMOF-3) was treated with salicylalde-
hyde, generating salicylimine (salen) chelators within the
MOF lattice with approximately 13% conversion.[22] The
salicylimine sites were metallated with [V(O)(acac)2H2O]
(acac = acetylacetonate) and the resulting MOF was shown to
oxidize cyclohexene in the presence of tBuOOH. This prior
study nicely demonstrated that PSM could generate active
metal sites into a MOF; however, the system was limited by
low activity and reusability, probably a result of framework
collapse. In a very recent report, a similar imine condensation
was reported between 2-pyridinecarboxyaldehyde and
UMCM-1-NH2.[31] UMCM-1-NH2, a MOF synthesized from
BTB (4,4’,4’’-benzene-1,3,5-triyl-tribenzoate), NH2-BDC (2-
amino-1,4-benzenedicarboxylic acid), and Zn(NO)3, was
selected because of its large open channels that allow for
facile diffusion of reagents.[18] The modified MOF, containing
iminopyridine moieties was successfully metallated with
[PdCl2(CH3CN)2], but no enhanced chemical functionality
or reactivity was reported for the palladium-containing
material. Herein, a complete demonstration of the introduc-
tion of isolated, reactive metal sites using two different
chelating ligands and two different metal ions is presented.
Furthermore, one of these metallated frameworks is shown to
be an active, robust, and reusable MOF-based catalyst.
3-Hydroxyphthalic anhydride and 2,3-pyrazinedicarbox-
ylic anhydride were selected as reagents for generating
competent metal binding sites within a MOF. As previously
mentioned, UMCM-1-NH2 has large pores and a very high
surface area (BET ca. 3900 m2 gÀ1) capable of accommodating
not only substituents introduced by PSM, but also substrates
for catalytic transformations.[18] Under mild reaction condi-
tions (see Supporting Information) UMCM-1-NH2 was suc-
cessful transformed with 3-hydroxyphthalic anhydride and
2,3-pyrazinedicarboxylic anhydride into two new MOFs
designated UMCM-1-AMsal and UMCM-1-AMpz, respec-
tively (Scheme 1). 1H NMR spectra of digested samples
(dissolved in DMSO/DCl) indicated approximately 35%
modification for UMCM-1-AMsal and approximately 50%
modification in UMCM-1-AMpz (Figure 1, Supporting Infor-
mation Figure S1). The 1H NMR spectroscopy and mass
spectrometry data also confirm that these anhydrides give
products that do not cyclize to give imide products, but rather
form substituents with free carboxylate groups (Supporting
Information, Figure S1); this is consistent with reports of PSM
using other cyclic anhydrides.[19,29] Thermal gravimetric anal-
ysis (TGA) and powder X-ray diffraction (PXRD) confirm
the modified MOFs have comparable thermal stability and
crystallinity to the parent UMCM-1-NH2, thus showing that
metal chelating groups can be introduced without disrupting
the framework (Supporting Information, Figure S2). BET
surface area measurements (dinitrogen) indicated both modi-
fied materials had surface areas of approximately 3600 m2 gÀ1
(Supporting Information, Table S1).
UMCM-1-AMsal and UMCM-1-AMpz were examined
for their ability to bind metal ions. [Fe(acac)3] and [Cu(acac)2]
were chosen as metal sources because of their solubility in
CHCl3 and for the distinct color changes expected upon
chelation with salicylate and pyrazine carboxylate ligands,
respectively. Addition of [Fe(acac)3] to UMCM-1-AMsal
resulted in an immediate color change from pale yellow to
dark red. Likewise, UMCM-1-AMpz instantly became bluish
green in the presence of [Cu(acac)2]. TGA and PXRD
analysis of the iron and copper containing products UMCM-
1-AMFesal and UMCM-1-AMCupz, respectively, confirmed
that metallation did not compromise the thermal stability or
structural integrity of the MOFs (Figure 2, Supporting
Information, Figure S2). BET surface area measurements
[*] K. K. Tanabe, S. M. Cohen
Department of Chemistry and Biochemistry
University of California, San Diego
9500 Gilman Drive, La Jolla, CA 92093 (USA)
Fax: (+1)858-822-5598
E-mail: scohen@ucsd.edu
[**] We thank Dr. Zhenqiang Wang and Sergio J. Garibay for helpful
discussions, and Dr. Y. Su for performing the mass spectrometry
experiments. This work was supported by U.C.S.D., the NSF (CHE-
0546531; instrumentation grants CHE-9709183, CHE-0116662 and
CHE-0741968), and the DOE (BES Grant No. DE-FG02-08ER46519).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200903433.
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Chemie
Figure 1. 1H NMR spectra of digested UMCM-1-AMsal (ca. 35%
modified). Unmodified NH2-BDC (*), modified NH2-BDC (&). An
additional resonance for the modified ligand is obscured by the large
BTB peaks.
on crystalline samples of UMCM-1-NH2, UMCM-1-AMsal,
and UMCM-1-AMpz did not show any significant transitions
beyond 400 nm. Treatment of UMCM-1-NH2 with either
[Fe(acac)3] or [Cu(acac)2] generated negligible changes in the
reflectance spectra (Figure 3, Supporting Information, Fig-
ure S3); however, UMCM-1-AMFesal showed a distinct band
centered around 500 nm (Figure 3), while UMCM-1-
AMCupz had a band at approximately 700 nm (Supporting
Information, Figure S3). These spectroscopic features are
consistent with Fe3+ salicylate and Cu2+ pyrazine carboxylate
compounds.[32,33] Selective metallation of the modified MOFs
was further confirmed by using atomic absorption (AA)
analysis. Analysis of UMCM-1-NH2 confirmed that uptake of
Scheme 1. Synthesis of UMCM-1-AMFesal (left) and UMCM-1-AMCupz
(right).
also showed that the metallated MOFs remained highly
porous (ca. 3600 m2 gÀ1 for UMCM-1-AMFesal and ca.
3400 m2 gÀ1 for UMCM-1-AMCupz, Supporting Information,
Table S1).
The metallation detected between the chelator-modified
MOFs and Fe3+ or Cu2+ was not detected with unmodified
UMCM-1-NH2. Diffuse reflectance electronic spectroscopy
Figure 2. PXRD analysis of UMCM-1-NH2 (bottom), UMCM-1-AMsal
(middle), and UMCM-1-AMFesal (top).
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Communications
catalyst.[35] Control reactions were prepared under identical
conditions, but included UMCM-1-NH2, UMCM-1-NH2
treated with [Fe(acac)3], UMCM-1-AMsal, or no MOF at
all, instead of UMCM-1-AMFesal. To determine the reaction
yield, the supernatant solution of each reaction mixture was
analyzed by 1H NMR spectroscopy (Supporting Information,
Figure S4, Figure S5). After 24 h, the control reactions all
gave less than 10% conversion and in the presence of
UMCM-1-NH2 treated with [Fe(acac)3], approximately 27%
conversion was achieved in the first round of catalysis, but this
sharply dropped off upon recycling (Table 1). In contrast,
Table 1: Results from Mukaiyama-aldol reactions with UMCM-1-
AMFesal catalyst and control reactions (after 24 h).
Aldehyde
MOF
Cycle 1 Cycle 2 Cycle 3 Overall
(UMCM-1)
[%]
[%]
[%]
[%]
naphthaldehyde no MOF
<1
n.d.
n.d.
n.d.
15
n.d.
n.d.
n.d.
14
n.d.
n.d.
n.d.
19Æ7
-NH2
-AMsal
NH2 + [Fe-
<10
<10
27
Figure 3. Diffuse reflectance solid-state UV/Vis spectra of UMCM-1-
NH2 (c), UMCM-1-NH2 treated with [Fe(acac)3] (b), UMCM-1-
AMsal (d), and UMCM-1-AMFesal (g).
(acac)3]
-AMFesal[a]
70Æ11 56Æ12 52Æ12 58Æ14
mesitaldehyde no MOF
-NH2
<1
<5
<5
<5
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
either Fe3+ or Cu2+ ions was less than 0.06 wt%. In contrast,
UMCM-1-AMFesal was found to contain 0.77 wt% of Fe3+
,
-AMsal
NH2 + [Fe-
(acac)3]
and UMCM-1-AMCupz was found to have 1.76 wt% of Cu2+
ions (Supporting Information, Table S2). Based on the
number of modified sites (see above), about 50% of the
chelator sites are metallated in UMCM-1-AMFesal and
UMCM-1-AMCupz (assuming a 1:1 metal:ligand ratio).
These experiments clearly show that metallation was selective
for the MOFs that had undergone PSM with chelating
moieties.
-AMFesal[b]
53Æ18 55Æ5
65Æ8
58Æ12
[a] Based on four trials. [b] Based on three trials. n.d. =not determined.
UMCM-1-AMFesal showed approximately 58% conversion
in 24 h at room temperature with both aldehydes over three
catalytic cycles. By comparison, a recently reported MOF-
based catalyst reported conversions as high as 63% with
benzaldehyde and 24% with tert-butylbenzaldehyde, but after
99 h in the presence of 0.2 mmol Mn2+ ions.[35] The results
reported herein give comparable yields in less than one-third
the time and approximately 2000-fold less metal loading. Size
selectivity was observed with UMCM-1-AMFesal, as the
reaction of either aldehyde with a larger silyl enol, namely (1-
tert-butylvinyloxy)trimethylsilane, showed no turnover after
24 h (Supporting Information, Figure S6), providing evidence
that catalysis takes place within the pores of the MOF.
It is important to note that removal of UMCM-1-
AMFesal by filtration resulted in no further reaction,
confirming that the MOF and not some soluble species was
responsible for the observed catalysis. In addition, UMCM-1-
AMFesal is quite robust as a catalyst, retaining full activity
over three cycles (Table 1). Stability of the catalyst was
further verified by PXRD, which showed that UMCM-1-
AMFesal remained crystalline (Supporting Information,
Figure S7). AA analysis also indicated very little leaching
of the catalytic metal ions from the structure after three
catalytic cycles as well, with over 80% of the Fe3+ ions
retained (Supporting Information, Table S2).
Having successfully introduced Lewis acidic Fe3+ sites into
the MOF, the ability of this material to act as a solid-state
catalyst was explored. UMCM-1-AMFesal was used to
catalyze the Mukaiyama-aldol reaction, an important
carbon–carbon bond forming reaction. The Mukaiyama-
aldol reaction can be conducted under mild conditions and
facilitated by Lewis acids at low temperatures and in solvents
compatible with the UMCM family of MOFs.[34] Reaction
mixtures contained 0.014 mmol (0.0001 mmol of Fe3+) of
UMCM-1-AMFesal, 0.1 mmol of aldehyde (mesitaldehyde or
1-naphthaldehyde), and 0.2 mmol 1-methoxy-2-methyl-1-(tri-
methylsiloxy)propene (Scheme 2). Catalysis with UMCM-1-
AMFesal was performed at room temperature in CH2Cl2 for
24 h, in order to compare our results with a reported MOF
The findings detailed herein show that PSM can be used
to introduce chelating sites into a MOF, that these sites can
be metallated with divalent or trivalent transition metals,
and that the resulting materials can be used as robust
Scheme 2. Mukaiyama-aldol reactions with 1-naphthaldehyde (top) and
mesitaldehyde (bottom) that were catalyzed by UMCM-1-AMFesal.
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Published online: September 1, 2009
Keywords: chelation · heterogeneous catalysis · metal–
.
organic frameworks · postsynthetic modification · zinc
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