Rhodium Hydrogenation Catalysts Supported in Metal Organic Frameworks: Influence of the Framework on Catalytic Activity and Selectivity

Article abstract of DOI:10.1021/acscatal.6b00404

The cationic rhodium complexes (dppe)Rh(COD)BF₄ and (MeCN)₂Rh(COD)BF₄ have been supported in metal-organic frameworks bearing anionic nodes (ZJU-28) and anionic linkers (MIL-101-SO₃) via ion exchange. These MOF-supported Rh species serve as recyclable catalysts for the hydrogenation of both the terminal alkene substrate 1-octene and the internal alkene substrate 2,3-dimethylbutene. The nature of the MOF support impacts various aspects of catalysis, including: (i) the rate of 1-octene hydrogenation, (ii) the activity and recyclability of the catalyst in 2,3-dimethylbutene hydrogenation, and (iii) the size selectivity of hydrogenation with alkene substrates appended to calixarenes.

Full text of DOI:10.1021/acscatal.6b00404

Research Article
pubs.acs.org/acscatalysis
Rhodium Hydrogenation Catalysts Supported in Metal Organic
Frameworks: Influence of the Framework on Catalytic Activity and
Selectivity
Douglas T. Genna, Laura Y. Pfund, Danielle C. Samblanet, Antek G. Wong-Foy, Adam J. Matzger,
and Melanie S. Sanford*
Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States
S
* Supporting Information
ABSTRACT: The cationic rhodium complexes (dppe)Rh-
(COD)BF4 and (MeCN)2Rh(COD)BF4 have been supported
in metal−organic frameworks bearing anionic nodes (ZJU-28)
and anionic linkers (MIL-101-SO3) via ion exchange. These
MOF-supported Rh species serve as recyclable catalysts for the
hydrogenation of both the terminal alkene substrate 1-octene
and the internal alkene substrate 2,3-dimethylbutene. The
nature of the MOF support impacts various aspects of
catalysis, including: (i) the rate of 1-octene hydrogenation, (ii)
the activity and recyclability of the catalyst in 2,3-dimethylbutene hydrogenation, and (iii) the size selectivity of hydrogenation
with alkene substrates appended to calixarenes.
KEYWORDS: metal organic frameworks, hydrogenation, catalysis, rhodium, size exclusion
We report herein on the synthesis, characterization, and
catalytic alkene hydrogenation reactivity and selectivity of
cationic Rh complexes loaded into two different anionic MOFs:
ZJU-2846 and MIL-101-SO3 (Figure 1).47 We demonstrate that
varying the MOF support impacts various aspects of catalyst
INTRODUCTION
■
The incorporation of transition-metal catalysts into metal
organic frameworks (MOFs) has been the focus of extensive
recent research efforts.1−12 Supporting a catalyst offers the
potential for combining the benefits of homogeneous catalysis
(e.g., mild conditions, well-defined catalyst structures and/or
ligand environments) with those of heterogeneous catalysis
(e.g., recyclability, high TON, site isolation).13−23 MOFs are
particularly attractive supports due to their diverse functional
groups, pore sizes, and pore geometries.24−26 Thus, they offer
opportunities for tuning the secondary coordination environ-
ment around the transition-metal catalyst.
The vast majority of work in the area of MOF catalysis has
focused on recyclability as the major benefit of MOF-supported
catalysts.13−23 However, the modularity of MOFs also offers the
potential for tuning catalyst performance. A grand challenge for
the field is to develop recyclable MOF-supported catalysts in
which variation of the framework structure (e.g., pore size, pore
shape, pore functional groups) enables tuning of catalyst
activity, selectivity, and/or stability. Recent reports have
demonstrated promising examples of enhanced selectiv-
ity1−11,27−29 and/or catalyst stability30−32 using MOF-
supported catalysts, but such efforts remain in their infancy.
We recently reported an ion exchange strategy for supporting
cationic transition-metal complexes in anionic MOFs.33 This
approach avoids the requirement for covalent tethering of the
catalyst to the framework.34 As a result, it offers the potential to
support the same catalyst in structurally different anionic
MOFs.35−45 This would, in turn, enable an assessment of the
impact of MOF structure on catalyst performance.
Figure 1. Catalyst loading via ion exchange using MOFs with anionic
nodes or pendant anionic organic linkers.
Received: February 8, 2016
Revised: April 13, 2016
© XXXX American Chemical Society
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performance, including reaction rates, recyclability, turnover
number, and size selectivity of alkene hydrogenation in these
systems.
Table 2. Incorporation of Rh Complexes 1a and 1b into
ZJU-28 and MIL-101-SO3
a
entry
MOF
wt % [M]
1
2
3
4
5
6
ZJU-28
---
3.5
2.2
---
RESULTS AND DISCUSSION
■
ZJU-28-1a
Cation Exchange. We focused our efforts on ion exchange
of the cationic Rh complexes (dppe)Rh(COD)BF4 (1a) and
(MeCN)2Rh(COD)BF4 (1b) into anionic MOFs (Figure 1), in
order to study alkene hydrogenation catalysis.48−51 Two
anionic MOFs with very different properties (ZJU-28 and
MIL-101-SO3) were selected for this study (Table 1). ZJU-28
ZJU-28-1b
MIL-101-SO3
MIL-101-SO3-1a
MIL-101-SO3-1b
3.2
1.7
a
wt % Rh is calculated on the basis of the ratio of [Rh] to the wt %
metal of the MOF. See Supporting Information for complete details.
Table 1. Comparison of the Properties of ZJU-28 and MIL-
101-SO3
uptake into ZJU-28 and MIL-101-SO3 for each Rh com-
plex.54−56 PXRD analyses of ZJU-28-1a/1b and MIL-101-SO3-
1a/1b were consistent with minimal structural changes during
catalyst loading (see Supporting Information for full details).57
Comparison of ZJU-28-1a and MIL-101-SO3-1a:
Hydrogenation of Terminal Olefins. ZJU-28-1a and MIL-
101-SO3-1a were first examined as catalysts for the hydro-
genation of 1-octene to n-octane. These reactions were carried
out in a batch pressure reactor at 75 °C (internal temperature)
using 10 bar H2 and 0.02 mol % [Rh] in neat 1-octene (6.4
mmol). The turnover numbers were determined via analysis of
the crude reaction mixture by gas chromatography after 24 h.
The two catalysts afforded comparable results, with ∼4000
turnovers of 1-octene to n-octane in each case (Figure 2;
theoretical maximum is 5000). Furthermore, both materials
could be recycled at least 4 times, and the recycled materials
afforded comparable TON (∼4000) after 24 h.
properties
ZJU-28
MIL-101-SO3
anion location
pore window
inorganic node
9 × 9 Å
organic linker
16 × 14 Å
37 × 36 Å
275 °C
pore diameter
thermal stability
9 × 9 Å
120 °C
was targeted due to our previous success at deploying ZJU-28-
1a in catalytic hydrogenation reactions.33 MIL-101-SO3 was
selected on the basis of two criteria: (1) the anionic charge of
MIL-101-SO3 is centered on the organic linker, which is
complementary to ZJU-28, which is anionic at the metal node;
and (2) the high stability profile of the parent MIL-101 is well
documented, and this property is expected to be advantageous
for catalysis applications. ZJU-28 is prepared from InCl3 and
4,4′,4″-benzene-1,3,5-triyl-tribenzoate (BTB).46,52 It is an
interpenetrated MOF containing two unique channels with
pore window diameters of 9 and 7 Å, respectively. The
inorganic nodes of ZJU-28 bear an overall charge of −1, as they
are composed of InIII centers ligated to four carboxylate ligands.
The monoanionic charge of the framework is balanced by the
+
presence of NH2Me2 guests derived from DMF solvent
decomposition during synthesis. ZJU-28 exhibits modest
thermal stability and is reported to decompose at ∼120 °C.
MIL-101-SO3 is synthesized from terephthalic acid sodium
sulfonate and chromium(III) oxide.47 MIL-101-SO3 has a
Figure 2. Recycling of MOF-supported Rh catalysts in the
hydrogenation of 1-octene to 1-octane.
−
neutral inorganic node and a charged SO3 functional group
appended to the organic linker. This monoanionic charge is
balanced by either a proton or a Na cation. MIL-101-SO3 has
large pseudospherical pores (37 × 36 Å) with hexagonal pore
windows (16 × 14 Å). This material has much higher thermal
stability than ZJU-28, as it is reported to be stable to ∼275 °C.
The cationic Rh complexes (dppe)Rh(COD)BF4 (1a)53 and
(MeCN)2Rh(COD)BF4 (1b) were loaded into ZJU-28 and
MIL-101-SO3 by shaking suspensions of the corresponding
MOF immersed in a 0.015 M DMF solution of 1a or 1b for 3
days. The resulting materials were then washed with DMF,
dried at room temperature under vacuum, and analyzed using
ICP-OES and powder X-ray diffraction (PXRD). As shown in
Table 2, ICP-OES analysis showed comparable levels of Rh
Characterization of Materials after Recycling. PXRD
and ICP-OES analyses were conducted on the spent catalysts
after the four recycles. PXRD analysis showed that both
materials remain crystalline after catalysis. ICP-OES of ZJU-28-
1a showed no detectable loss of Rh, whereas analysis of MIL-
101-SO3-1a showed some leaching of Rh (3.2 wt % at the start
versus 2.1 wt % after 4 recycles). To probe whether the
observed hydrogenation reactivity is the result of soluble
leached Rh, both ZJU-28-1a and MIL-101-SO3-1a were
subjected to a three-phase test with a Merrifield-resin-
supported terminal alkene substrate. In this experiment, the
MOF catalyst and supported substrate were suspended in
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acetone and subjected to 10 bar H2 for 48 h at 75 °C. No
hydrogenation of the supported alkene was detected with either
catalyst (ZJU-28-1a or MIL-101-SO3-1a). This result indicates
that leached soluble Rh is not the active hydrogenation catalyst
in these transformations. For comparison, the homogeneous
complex 1a catalyzes complete hydrogenation of the supported
alkene under otherwise identical conditions.33
To more quantitatively compare the relative reactivity of
these two catalysts, the reaction progress of 1-octene
hydrogenation was monitored in situ via Raman spectroscopy.
These experiments were conducted in a high pressure batch
reactor retrofitted with a Raman probe.58 The reactions were
carried out at 75 °C (internal temperature) using 65 bar H2 and
0.02 mol % of [Rh]. They were monitored based on
disappearance of a Raman resonance at 1643 cm−1 that is
characteristic of 1-octene.
Figure 4. Comparison of the reaction progress of the hydrogenation of
1-octene with recycled ZJU-28-1a and MIL-101-SO3-1a (materials
recycled seven times for a total of eight runs).
The reaction progress data for fresh ZJU-1a and MIL-101-
SO3-1a are overlaid in Figure 3. The hydrogenation reaction
to those of ZJU-28. Both of these features are expected to
facilitate transport of reactants and products during catalysis.
2,3-Dimethylbutene Hydrogenation with MIL-101-
SO3-1a and ZJU-28-1a. Literature reports indicate that
cationic rhodium complexes are generally poor catalysts for
the hydrogenation of tetrasubstituted alkenes.59,60 Thus, we
next selected 2,3-dimethylbutene as a more challenging
substrate than 1-octene. As anticipated, the hydrogenation of
2,3-dimethylbutene required more forcing conditions than that
of 1-octene. However, after some optimization, we identified
reaction conditions that enabled a comparison of the supported
Rh catalysts (0.003 mol % of [Rh] at 100 °C over 24 h under
75 bar of H2).
As shown in Figure 5, ZJU-28-1a afforded low initial
reactivity for the hydrogenation of 2,3-dimethylbutene under
these standard conditions, providing a TON of 1500
(theoretical maximum TON = 30 000). Significant loss of
activity was observed with each subsequent recycle of ZJU-28-
1a, and after four recycles, this catalyst afforded just 289
turnovers. In contrast, MIL-101-SO3-1a was a much more
Figure 3. Comparison of the reaction progress of the hydrogenation of
1-octene with fresh ZJU-28-1a and MIL-101-SO3-1a (as-synthesized
catalysts).
proceeded significantly slower with ZJU-28-1a, requiring ∼14 h
(∼360 TON/h) for complete consumption of 1-octene. In
contrast, complete consumption of 1-octene was observed
within ∼3 h (∼1660 TON/h) with MIL-101-SO3-1a. We next
compared the performance of the two catalysts after seven
recycles (eight total runs). As shown in Figure 4, the recycled
MIL-101-SO3-1a maintains higher hydrogenation activity than
recycled ZJU-28-1a. However, the recycled MIL-101-1a is a
slower catalyst than fresh MIL-101-SO3-1a (complete con-
sumption of starting material requires ∼6 h (∼800 TON/h)
versus 4 h for the fresh material). This is likely due to Rh
leaching, as the ICP results (above) show that the % Rh in
MIL-101-SO3-1a decreases as the material is recycled. In
contrast, the recycled ZJU-28-1a shows slightly higher reactivity
than fresh ZJU-28-1a, with complete conversion of starting
material requiring ∼11 h (∼450 TON/h) versus 14 h,
respectively.
Overall, these results show that ZJU-28-1a and MIL-101-
SO3-1a are active and recyclable catalysts for the hydrogenation
of 1-octene. Furthermore, hydrogenation catalysis is signifi-
cantly faster with MIL-101-SO3-1a as compared to that with
ZJU-28-1a, both before and after recycling. We hypothesize
that this is due, at least in part, to the significantly larger pore
sizes and pore windows of the MIL-101-SO3 support compared
Figure 5. Recycling of MOF-supported Rh catalysts ZJU-28-1a and
MIL-101-SO3-1a in the hydrogenation of 2,3-dimethylbutene to 2,3-
dimethylbutane.
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effective catalyst for the hydrogenation of 2,3-dimethylbutene,
affording 8800 turnovers after 24 h. Furthermore, upon
recycling, the activity of MIL-101-SO3-1a nearly doubled to
13 500 turnovers.61 This enhanced activity was largely
maintained over three additional recycles.62
(Table 3) were employed for these investigations. These two
substrates have different kinetic diameters (9.5 and 16 Å,
Table 3. Size Selective Hydrogenation of Calixarene
Substrates 2a and 2b
PXRD and ICP-OES analyses were conducted on the spent
catalysts after four recycles. In both cases, the PXRD
diffractograms showed that the crystallinity of the material
was maintained (Figures S4 and S6). In addition, the ICP-OES
analysis showed minimal change in the Rh content of either
material. For example, the recycled ZJU-28-1a contained 3.5 wt
% [Rh, while the recycled MIL-101-SO3-1a contained 3.2 wt %
[Rh]. These latter results indicate that the observed differences
in reactivity between the two materials in this transformation
are not due to Rh leaching.
2,3-Dimethylbutene Hydrogenation with MIL-101-
SO3-1b and ZJU-28-1b. We next sought to probe whether
the support-dependent activity and recyclability of these Rh
hydrogenation catalysts is specific to Rh complex 1a [(dppe)-
Rh(COD)BF4]. As such, we examined the performance of
MIL-101-SO3-1b and ZJU-28-1b for the hydrogenation of 2,3-
dimethylbutene [1b = (MeCN)2Rh(COD)BF4]. As shown in
Figure 6, both ZJU-28-1b and MIL-101-SO3-1b afforded
catalyst
pore window substrate kinetic diameter TON
1a
na
2a
2a
2a
2b
2b
2b
9.5 Å
9.5 Å
9.5 Å
16 Å
16 Å
16 Å
200
200
0
MIL-101-SO3-1a
ZJU-28-1a
1a
16 × 14 Å
9 × 9 Å
na
200
52
0
MIL-101-SO3-1a
ZJU-28-1a
16 × 14 Å
9 × 9 Å
respectively).63 Because the pore windows of ZJU-28 and MIL-
101-SO3 are 9 × 9 Å and 16 × 14 Å, respectively, we
anticipated that these materials should exhibit size-selective
catalysis. Importantly, the observation of selectivity differences
as a function of support would also provide evidence that
catalysis occurs in the pores rather than on the surface of the
MOF or on leached insoluble Rh particles.
The hydrogenation of 2a/b was conducted using 0.54 mol %
of [Rh] in benzene at 35 °C for 24 h. C6D6 was utilized as the
solvent in order to fully dissolve the calixarene substrates. The
1
extent of hydrogenation of 2a/b was determined via H NMR
spectroscopic analysis of the crude reaction mixtures. The
soluble catalyst 1a efficiently catalyzed the hydrogenation of 2a
and 2b, providing full conversion under the reaction conditions
(TON = 185). With MIL-101-SO3-1a, full conversion (TON =
185) was obtained with the smaller calixarene substrate 2a.
However, the larger substrate 2b, which has a kinetic diameter
close to the pore window of the MOF, afforded only partial
hydrogenation (TON = 52) after 24 h. Finally, with ZJU-28-1a
as the catalyst (which has a pore window that is smaller than
the kinetic diameter of both of the calixarenes), no hydro-
genation of either 2a or 2b was detected. Collectively, these
results demonstrate that size-selective catalysis can be achieved
by supporting the same catalyst in two structurally different
metal organic frameworks.
Figure 6. Recycling of MOF-supported Rh catalysts ZJU-28-1b and
MIL-101-SO3-1b in the hydrogenation of 2,3-dimethylbutene to 2,3-
dimethylbutane.
significantly lower initial activity than ZJU-28-1a and MIL-
101-SO3-1a. For instance, the TON for the first run with MIL-
101-SO3-1a was 8800 while that with MIL-101-SO3-1b was
5800. Furthermore, both catalysts exhibited poor recyclability,
with precipitous drops in TON after five recycles. However,
despite the lower initial reactivity and recyclability of 1b-based
catalysts, these materials show similar trends in activity as a
function of the MOF support. In other words, MIL-101-SO3-1b
affords much higher initial activity than the ZJU-28-supported
Rh catalyst (initial TON = 5800 and 620, respectively). These
data further demonstrate that the MOF support dramatically
impacts catalyst activity.
CONCLUSIONS
■
This report describes the synthesis, characterization, and
hydrogenation reactivity and selectivity of cationic Rh
complexes supported in two different anionic MOFs: ZJU-28
and MIL-101-SO3. This study reveals several important features
of the MOF-supported catalysts. First, we demonstrate that ion
exchange can be used to support transition-metal catalysts in
two very different MOFs: ZJU-28 (which bears a negative
charged at the node) and MIL-101-SO3 (which bears a negative
charge on the organic linker). Second, we show that these
MOF-supported Rh species serve as recyclable catalysts for the
hydrogenation of both terminal and internal alkenes. Third, we
demonstrate that the nature of the MOF support impacts
various aspects of catalysis, including: (a) the rate of 1-octene
Hydrogenation Selectivity for MIL-101-SO3-1a and
ZJU-28-1a. A final set of experiments was conducted to
explore the impact of the MOF support on hydrogenation
selectivity. The calixarene-tethered alkene substrates 2a and 2b
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.6b00404.
■
S
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AUTHOR INFORMATION
Corresponding Author
*E-mail: mssanfor@umich.edu.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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DOI: 10.1021/acscatal.6b00404
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ACS Catalysis
Research Article
(53) Complex 1a was formed in situ by the combination of 1 equiv of
dppe and 1 equiv of (COD)Rh(CH3CN)2BF4 in DMF.
(54) Notably, these values are below the theoretical maxima for Rh
uptake, 18 wt % in MIL-101-SO3 and 14 wt % in ZJU-28, respectively.
(55) No Rh incorporation was observed when the neutral MOF
MIL-101 was subjected to DMF solutions of 1a or 1b under otherwise
analogous conditions. In both cases, less than 0.01 wt % incorporation
of Rh was detected by ICP-OES.
(56) As a control, we also subjected ZJU-28 and MIL-101-SO3 to
neutral metal complexes (e.g., (COT)Fe(CO)3 and trans-
Pd(PPh3)2Cl2). In both cases, less than 0.01 wt % loading of the
corresponding metal was observed.
(57) Our samples of MIL-101-SO3 and MIL-101-SO3-1a have BET
surface areas of 1215 m2/g and 1100 m2/g, respectively. ZJU-28 and
ZJU-28-1a both have measured surface areas of < 50 m2/g.
(58) These transformations were conducted on a 10-fold larger scale
than the experiments in Figure 2, due to the nature of the reactor. See
Supporting Information for full details about the reactor.
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(61) This unexpected increase in reactivity upon recycling may be
due to some type of activation event of the catalyst that occurs during
the first run. Ongoing work is focused on establishing the mechanistic
origin of this effect.
(62) These results compare favorably to those obtained by Lin in the
hydrogenation of 2,3-dimethylbutene catalyzed by mBPP-MOF-Co. A
maximum TON of 170 was obtained in this previously reported
system (although the conditions were considerably less forcing than
ours, as the reaction was conducted in THF at 50 °C under 40 bar
H2). See ref 1 for complete details.
(63) Kinetic diameters were determined on the basis of the lowest
energy conformation as calculated by the Spartan ’04 program by
Wave function.
3574
DOI: 10.1021/acscatal.6b00404
ACS Catal. 2016, 6, 3569−3574