The impact of an isoreticular expansion strategy on the performance of iodine catalysts supported in multivariate zirconium and aluminum metal-organic frameworks

Article abstract of DOI:10.1039/c9dt00368a

Iodine functionalized variants of DUT-5 (Al) and UiO-67 (Zr) were prepared as expanded-pore analogues of MIL-53 (Al) and UiO-67 (Zr). They were prepared using a combination of multivariate and isorecticular expansion strategies. Multivariate MOFs with a 25% iodine-containing linker was chosen to achieve an ideal balance between a high density of catalytic sites and sufficient space for efficient diffusion. Changes to the oxidation potential of the catalyst as a result of the pore-expansion strategy led to a decrease in activity with electron rich substrates. On the other hand, these larger frameworks proved to be more efficient catalysts for substrates with higher oxidation potentials. Recyclability tests for these larger MOFs showed sustained catalytic activity over multiple recycles.

Full text of DOI:10.1039/c9dt00368a

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PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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a Zr-metal–organic
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DOI: 10.1039/C9DT00368A
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The impact of an isoreticular expansion strategy on the
performance of iodine catalysts supported in multivariate
zirconium and aluminum metal-organic frameworks.
Received 00th January 20xx,
Accepted 00th January 20xx
Babak Tahmouresilerd, Michael Moody, Louis Agogo, and Anthony F. Cozzolino*
DOI: 10.1039/x0xx00000x
Iodine functionalized variants of DUT-5 (Al) and UiO-67 (Zr) were prepared as expanded-pore analogues of MIL-53 (Al) and
UiO-67 (Zr). They were prepared using a combination of multivariate and isorecticular expansion strategies. Multivariate
MOFs with a 25% iodine-containing linker was chosen to achieve an ideal balance between a high density of catalytic sites
and sufficient space for efficient diffusion. Changes to the oxidation potential of the catalyst as a result of the pore-expansion
strategy led to a decrease in activity with electron rich substrates. On the other hand, these larger frameworks proved to be
more efficient catalysts for substrates with higher oxidation potentials. Recyclability tests for these larger MOFs showed
sustained catalytic activity over multiple recycles.
www.rsc.org/
prepare multivariate MOFs where only one linker per pore (or
less) contains the catalyst, thereby reducing the occupied
space.19,20
Introduction
The crystallinity, structural flexibility and tunability, thermal
stability, and high surface area of metal-organic frameworks
(MOFs) have led to their adoption as heterogeneous supports
for catalysts.1–5 One useful strategy for translating successful
homogeneous catalysts into MOFs is to incorporate them into
the organic linkers.6–10 Reticular design strategies allow these
linkers to be installed in known crystal topologies, either
uniformly, or mixed with a linker of the same length to give a
multivariate MOF. One of the important considerations in the
choice of linker and topology is framework stability. To this end,
MOFs with enhanced chemical and thermal stability are being
developed.11–13 Another critical consideration is pore and pore-
aperture size. This affects the ability of substrates to diffuse
through the pores and reach the catalytic site. It can also
constrain the catalyst which can presumably influence the
stability of transition states. To address the pore and aperture
size concerns, a key strategy that has emerged is to increase the
pore size by increasing the length of the organic linkers, the so-
called isoreticular expansion strategy.14–18 Limitations of this
strategy include increased fragility of the framework and also
the possibility of an interpenetrated structure. Considering that
modification of a linker to include a catalytically active site
usually increases the radius of the linker, and therefore
decreases accessible pore volumes, an alternative strategy is to
Scheme 1. Generic mechanism catalytic oxidation of aromatic diols by heterogeneous
iodine catalyst in presence of mCPBA.
Hypervalent iodine has found use as an oxidant in many
chemical transformations, including as facile and green
reagents in organic synthesis.21–24 Some iodine-mediated
transformation can be performed catalytically with appropriate
terminal oxidants.25–28 We have recently reported a recyclable
iodine catalyst supported in a metal-organic framework catalyst
that was competent for the oxidation of hydroquinones using
terminal oxidant metachloroperbenzoic acid (mCPBA) as
depicted in Scheme 1.29 A multivariate approach was used to
maximize pore and aperture size in order to accommodate
reagents and oxidants in the pores of the framework and allow
for catalytic oxidation to occur within the MOF. Frameworks in
which 25% of the linkers contained the catalysts showed an
ideal balance between catalyst loading and catalyst
accessibility. There were, however, some limitations in the
catalytic performance of MOFs with respect to the yields of the
reaction, the scope of substrates, the size of the substrate, and
the recyclability of the catalysts. In the current study, the impact
of a combination of multivariate and isoreticular expansion
Department of Chemistry and Biochemistry, Texas Tech University, Box 1061,
Lubbock, Texas 79409-1061, USA. E-mail: anthony.f.cozzolino@ttu.edu;
Fax: +1 806 742 1289; Tel: +1 806 834 1832Address here.
† Electronic supplementary information (ESI) available: Synthesis of MOFs, catalysis
conditions, NMR spectra, summary of all reactions, or other electronic format see
DOI: 10.1039/c8cy00794b
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strategies on the catalytic performance of iodine supported in
larger zirconium and aluminum-based MOFs is studied (Figure
1). We highlight an often-overlooked consideration with the
isoreticular expansion strategy – namely that the lengthening of
the linker can have unintended consequences on the electronic
structure of the catalyst itself.
View Article Online
DOI: 10.1039/C9DT00368A
Scheme 3. Preparation of multivariate DUT-5 X%-I and UiO-67 X%-I frameworks.
Conditions and reagents: i) AlCl3·6H2O, DMF, 120 °C, 24 h,30,36 ii) ZrCl4, DMF, ultrapure
water, 95 °C, 100 h.37,38
Powder X-ray diffraction (PXRD) analyses were performed upon
isolation of the frameworks and also following activation of the
frameworks. The patterns were compared with simulated
patterns from the crystal structures of the parent MOFs to
verify that the anticipated framework was formed and retained.
All the PXRD patterns for UiO-67 X%-I and DUT-5 X%-I are in the
good agreement those of UiO-67 and DUT-5 (Figure 1). 30,31
Figure 1. The depiction of DUT-5 (Al) (left),30 and the octahedral pore in UiO-67 (Zr)
(right).31 Metal coordination spheres are represented with polyhedra. Hydrogen atoms
have been removed for clarity.
Results and Discussion
Preparation and Characterization of Iodine Containing
Frameworks
Iodine-containing MOFs with larger accessible inner volumes
were prepared using a combination of multivariate and
isoreticular expansion strategies. The iodine containing linker
(H2IBPDC) was prepared by adapting reported literature
procedures as shown in Scheme 2.32–34 The MOFs were
prepared by treating the appropriate mixture of linkers with
zirconium (IV) chloride or aluminum chloride hexahydrate; an
adaption of literature conditions for the parent frameworks. It
should be noted that the Zr frameworks (UiO-67) were
prepared in the presence of HCl as a modulator to reduce or
eliminate defects in the framework.35 This yielded the
multivariate UiO-67 X%-I or DUT-5 X%-I where X represents the
percentage of linkers that are functionalized with I (Scheme
3).30,36–38
Figure 2. PXRD patterns of activated DUT-5 25%-I, DUT-5 0%-I, and simulated DUT-5 0%-
I30 (top), and UiO-67 25%-I, UiO-67 0%-I and simulated UiO-67 0%-I (bottom).31
Thermogravimetric analysis (TGA) was used to establish the
thermal stability and the temperature limits for activation and
reactions. Both activated frameworks demonstrate good
stability up to 400 °C (Figure 3). These values are slightly lower
than the values reported for UiO-67 0%-I37 (500 °C) and DUT-5
0%-I30 (430 °C). It has been previously demonstrated that
multivariate DUT-5 frameworks show a decrease in stability
with increasing functionalization.36 These iodo-functionalized
frameworks show roughly the same stability as the smaller
analogs.29 TGA analysis for both UiO-67 25%-I and DUT-5 25%-I
after activation revealed three distinguishable profiles of
weight loss. The first profile of 1% weight loss (A→B, 50-100 °C)
and the second profile of additional 6% and 3% weight loss
(100–400 °C) for UiO-67 25%-I and DUT-5 25%-I can be assigned
Scheme 2. Preparation of H2IBPDC. Conditions and reagents: i) HNO3, H2SO4, 0-4 °C, 5 h,32
ii) MeOH, HCl, Sn, 80 °C,39 iii) NaNO2, HCl, 0-4 °C, NaI, 22 °C,32 iv) KOH, THF, reflux.34
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to the loss of adsorbed water. The third profile with a weight
loss of 60% and 78% for UiO-67 25%-I and DUT-5 25%-I can be
assigned to the thermal decomposition of the frameworks and
the formation of the appropriate metal oxide, ZrO2, and Al2O3,
respectively.
View Article Online
DOI: 10.1039/C9DT00368A
Figure 3. TGA of activated UiO-67 25%-I (yellow) and DUT-5 25%-I (red) at a ramp rate of
10 °C min-1 under a flow of air.
ATR-FTIR spectroscopy was performed on both UiO-67 25%-I
and DUT-5 25%-I to verify the absence of impurities in the pores
of the activated frameworks. In particular, DMF from the
synthesis or excess neutral linker could be present. Excess
unreacted ligands could leach during catalytic experiments and
perform catalysis homogeneously. The spectra of the activated
MOFs revealed that the bands attributed to impurities in the as-
synthesized frameworks are absent, consistent with full
activation of the materials (Figure 4). The spectrum for DUT-5
25%-I shows vibrational bands in the typical region for
symmetric and asymmetric stretching modes (1421 cm-1 and
1595 cm-1, respectively) of the bridging carboxylate groups
which is in good agreement with the synthesized DUT-5 0%-I
reported in the literature.30 Similarly, the bands at 1404 and
1593 cm-1 in UiO-67 25%-I can be assigned to the symmetric and
asymmetric stretching modes of the carboxylate; an excellent
match with the reported bands values for UiO-67 0%-I.40
Figure 4. Di-ATR FTIR of as-synthesized and activated UiO-67 25%-I (top) and DUT-5 25%-I
(middle), and linkers (bottom). All spectra plotted as attenuation.
The nitrogen adsorption-desorption measurements for
activated UiO-67 25%-I and DUT-5 25%-I were conducted to
establish the specific surface areas, pore size distributions, and
pore volumes. Analysis of the isotherms reveals a type I
isotherm for both UiO-67 25%-I and DUT-5 25%-I confirming the
expected microporosity with narrow pore size distributions
(Figure 5 and S8). The calculated surface area and pore volume
for UiO-67 25%-I (SBET = 1795 m2/g, Vmicro = 0.877 mL/g) and
DUT-5 25%-I (SBET = 1120 m2/g, Vmicro = 0.550 mL/g) are in a good
agreement with the values reported in the literature (Table
1).30,36–38 Nitrogen sorption for DUT-5 25%-I shows a lower
surface area and pore volume than the literature value for the
framework without the iodo-functionalized linker, consistent
with the large iodine occupying space in the pores. The
multivariate strategy prevents the large iodine from occupying
a
significant fraction of the pore, in line with other
functionalized DUT-5 analogues. Importantly, the surface area
and micropore volume for both MOFs are significantly higher
than the previously reported catalysts UiO-66 25%-I and MIL-53
25%-I which should allow for more efficient diffusion of
molecules within the framework. The pore size distribution
analysis for UiO-67 25%-I showed two different pore sizes with
diameters 1.02 nm and 1.96 nm, assigned to the tetrahedral and
octahedral pores, respectively. The pore size distribution for
DUT-5 25%-I indicated a major pore with a width of 1.02 nm.
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Table 1. Molar Brunauer-Emmett-Teller (BET) surface areas and pore volumes for the
UiO-67 (Zr) and DUT-5 (Al) obtained from nitrogen adsorption isotherms at 77 K.
View Article Online
DOI: 10.1039/C9DT00368A
BET surface
area (×103
m2·mol-1)a
Pore volume
(mL/mol)
MOF
Source
41
UiO-67 0%-I
UiO-67 25%-I
UiO-67 100%-I
DUT-5 0%-I
DUT-5 25%-I
DUT-5 100%-I
771
690
171
567
354
317
469
334
113
193
174
153
This work
This work
30
This work
This work
a) Based on idealized formula [Zr6O4(OH)4(L)6] for UiO-67 and [Al4(OH)4(L)4] for
DUT-5, normalized to one Zr or Al, respectively
Figure 6. Top, a) EDS spectrum of UiO-67 25%-I, b) SEM image, and c, d) EDS–mapping
results for Zr and I over UiO-67 25%-I particles coated with Au/Pd after activation,
bottom, a) EDS spectrum of DUT-5 25%-I, b) SEM image, and c, d) EDS–mapping results
for Zr and I over DUT-5 25%-I particles coated with Au/Pd after activation.
Figure 5. Nitrogen sorption isotherm (filled triangles or circles absorption, open triangles
or circles desorption) of UiO-67 25 and 100%-I (top) and DUT-5 25 and 100%-I (bottom)
(77 K).
Catalytic Evaluation
The catalytic oxidation of hydroquinone to benzoquinone
(Scheme 4) was chosen as a model reaction to evaluate the
performance of these larger frameworks. Catalysis was
performed in nitromethane with 2.9 eq. (mCPBA) as co-oxidant,
20 mol% I catalyst, 60 min and 50 °C. mCPBA was chosen as it is
well-established as a suitable terminal oxidant for catalytic
oxidations with iodine.42,43 MOFs with 0% (the same number of
moles of MOF were used as with 25%-I), 25% and 100% iodine
functionalized ligands were tested against these conditions.
Almost no difference was observed (Table 2, entry 1) in the yield
of the reaction from the two control conditions (no MOF and
MOF with no I), indicating that the framework itself is inactive
and that iodine is necessary for catalysis to occur. UiO-67 25%-I
and DUT-5 25%-I were the most competent catalysts for the
oxidation of hydroquinone to benzoquinone with 75% and 73%
yield (Table 2, entry 1), respectively, beyond the background
reaction without MOF (8%). Consistent with expectation, MOFs
with 100% I functionalized linker performed worse than those
with 25% iodine functionalized linker. This behaviour aligns with
the smaller pore volumes in the MOFs with higher iodine
loading (Table 1). Notably, this effect is not as dramatic as it was
The ratio of iodo-functionalized linker to unsubstituted linker
was verified through NMR analysis of the digested frameworks.
For both frameworks, the anticipated ratio of linkers, based on
reaction stoichiometry, was approximately observed (28%-I in
UiO-67 25%-I and 21% for DUT-5 25%-I, see Figures S6 and S7).
It is conceivable that the material is a mixture of two pure-phase
MOFs containing either 0% IBPDC2- or 100% IBPDC2-. To ensure
the even distribution of iodo-functionalized linker in the
frameworks, energy-dispersive X-ray spectroscopy (EDS)
analyses were performed. EDS–mapping results for activated
UiO-67 25%-I and DUT-5 25%-I particles coated with Au/Pd
confirm that the iodo-functionalized linker is consistently
distributed throughout both frameworks (Figure 6).
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with the smaller MOFs, UiO-66 and MiL-53.29 Similarly higher
yields were observed in acetonitrile for the MOFs with 25%
iodine-containing linker. With the much larger substrate, 2,5-di-
tert-butylhydroquinone, higher yields were again observed in
the frameworks with less iodine per pore. Due to the superior
performance, UiO-67 25%-I and DUT-5 25%-I were used for
subsequent reactions. A split test was performed with both
iodo-functionalized MOFs to verify that catalysis occurs
heterogeneously, rather than as a result of catalyst leaching.
After the separation of catalyst from the reaction mixture, no
additional conversion was observed (Figure S14), confirming
heterogeneous catalysis.
Table 2. The yield of catalytic oxidation of hydroquinone and catechol derivatives with
View Article Online
iodo-functionalized MOFs.a
DOI: 10.1039/C9DT00368A
Yield (%)a, b
Temp.
(°C)
#
1
Product
DUT-5
25%-I
UiO-67
25%-I
Backgroundd
24
50
50
19 (22)
0 (8) c
73 (81)
33 (36)
0 (8)c
75 (83)
3
8
8
O
O
O
O
O
O
2
3
24
24
21 (50)
8 (39)
25 (54)
11 (42)
29
31
tBu
tBu
OH
O
O
O
O
4
24
29 (70)
24 (65)
41
R1
R1
O
O
O
20 mol% catalyst
2.9 equiv. mCPBA
tBu
R2
Nitromethane, 1 h
R2
5
6
50
50
72 (96)
85 (92)
52 (76)
73 (80)
24
7
Cl
OH
O
5) R1: Cl, R2: H
1) R1: H, R2: H
2) R1: Me, R2: H
3) R1: tBu, R2: H
4) R1: tBu, R2: tBu
6) R1: Br, R2: H
7) R1: Cl, R2: Cl
8) R1: Br, R2: Br
Br
Cl
O
O
OH
O
7
8
50
50
90 (97)
89 (94)
79 (86)
77 (82)
7
5
O
O
OH
R1
O
Cl
Br
20 mol% catalyst
2.9 equiv. mCPBA
Nitromethane, 1 h
R2
R1
R2
9) R1: H, R2: H
10) R1: H, R2: Me
11) R1: H, R2: tBu
12) R1: tBu, R2: tBu
Br
24
50
13 (13)
42 (42)
48 (48)
56 (56)
0
0
9
O
O
Scheme 4. The oxidation of aromatic diols shown with typical catalytic conditions.
10
24
24
26 (47)
58 (63)
41 (62)
85 (90)
21
5
O
O
tBu
11
12
O
O
tBu
24
29 (79)
27 (77)
50
tBu
O
O
a) Values were determined by 1H NMR using methylsulfonylmethane (MSM) as
an internal standard.
b) The yield of reaction beyond the background reaction is provided, the total
yield is in parentheses.
c) Performed with UiO-67 0%-I or DUT-5 0%-I.
d) Direct oxidation of substrate with mCPBA
The effect of catalyst loading (5, 10, and 20 mol%) on the
oxidation of hydroquinone under the conditions listed in
Scheme 4 was studied. The turnover numbers (TONs) for the
yields beyond the background reactions are provided in Figure
7. The TONs at the lowest loadings are 5 and 10 for DUT-5 25%-I
and UiO-67 25%-I, respectively. This indicates that UiO-67 is a
faster catalyst. As the loading is increased to 10%, no change is
observed for DUT-5 25%-I, however a decrease in TON is
observed for UiO-67 25%-I. The yield of the reaction does not
change for UiO-67 25%-I upon further doubling of the catalyst
loading suggesting that the reaction has slowed significantly. A
similar decrease in TON is observed upon increasing the loading
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of DUT-5 25%-I to 20 mol%. The slowing of the reaction can be, the change to the catalytic site. Using DUT-5 25%-IVfioewrAorxticidleaOtniloinne
DOI: 10.1039/C9DT00368A
in part, attributed to the decrease in the concentration of the of hydroquinone at 24 and 50 °C, a yield of 19 and 73% were
substrate. An additional factor that was suggested by Harned observed when compared to the MIL-53 25%-I (10 and 37%).
for homogeneous systems is that the concentration of
metachlorobenzoic acid is increasing. This, in turn can allow for
the metachlorobenzoate to functionalize the hypervalent
iodine.43 While this is presumed to be beneficial with
homogeneous
catalysts,
the
presence
of
two
metachlorobenzoate groups on the iodine in the MOF could
readily block access to the pores or even the catalyst itself. Two
possible remedies for this would be to utilize flow chemistry in
order to reduce the build-up of the organic acid byproduct
and/or to find a suitable alternative terminal oxidant that is
smaller such as peracetic acid.
Figure 8. Cyclic Voltammogram (CV) of dimethyl 2-iodoterephthalate and dimethyl 2-
iodo-[1,1'-biphenyl]-4,4'-dicarboxylate in acetonitrile in 0.1 M Bu4NBF4. At 100 mV/s vs.
Fc/Fc+.
Figure 7. The effect of different catalyst loading (5, 10, and 20 mol%) on the yield of
quinone for UiO-67 25%-I and DUT-5 25%-I. Shaded areas show extent of background
reaction, TONs beyond background presented at the top of each column.
In the case of UiO-67 25%-I, a yield of 33% and 75% was
observed for the oxidation of hydroquinone at 24 °C and 50 °C,
respectively. These values are roughly the same as the yields
obtained with UiO-66 25%-I under similar conditions. Due to the
larger pore size in UiO-67 25%-I, an increase in the yield of the
reaction was expected; however, a slight decrease in yield for
the catalytic oxidation of hydroquinone was observed. To
rationalize this behaviour, the effect of the linker elongation
strategy on the catalytic site itself was investigated. The
oxidation potential of both linkers (BDC2- and BPDC2-) was
evaluated homogeneously using the methyl esters of the two
linkers, Me2IBPDC and Me2IBDC, as model systems. Cyclic
voltammetry revealed that the shorter linker is more readily
oxidized by 0.2 V (see Figure 8). This contrasts with the DFT
calculated energies of the HOMOs, iodine lone pairs, which are
the same in both molecules within 0.04 eV (see Tables S7 and
S8). One reason for the difference in the experimentally
determined oxidation potential is the ability of the ester oxygen
to engage in an intramolecular halogen bond with the oxidized
iodine,44 thus stabilizing the higher oxidation state. This is only
possible in the shorter linker where the iodine is ortho to the
ester. In the MOFs, the same interaction would occur with the
carboxylate groups that are bound to the metals (See Figure 9).
Examples of this behaviour are observed with a variety of
hypervalent iodine systems with neighbouring nucleophilic
sites.45–48 As a result, the catalyst in the larger pore framework
is less readily oxidized and therefore, less efficient even though
there is more space. This effect is not observed with MIL-
53/DUT-5 as the increase in accessible space appears to offset
Figure 9. Proposed in situ generated hypervalent iodine(III) supported by nucleophile Nu-
in the pores (highlighted in blue for DUT-5 and UiO-67) of a) DUT-5 25%-I or UiO-67 25%-I
or b) MIL-53 25%-I or UiO-66 25%-I.
Despite having good catalytic performance, UiO-66 25%-I was
not able to be recycled with retained activity towards the
oxidation of hydroquinone.29 With the larger UiO-67 25%-I,
however, the catalytic conversion remained constant over four
runs (Figure 10, top). In the case of DUT-5 25%-I, for the first
and second runs, a conversion of 85% was observed. For the
third and fourth runs, the catalytic conversion dropped to 84%
and 82%, respectively. This small loss in conversion for the third
and the fourth run are attributed to catalyst attrition from
manipulations between recycles. This series of conversions for
DUT-5 25%-I shows improvement when compared to the same
family of MOF with the smaller linker (Figure 10, bottom).29
PXRD patterns for the catalysts after the first and fourth runs
were obtained and both frameworks retain their crystallinity
(Figure 11). SEM images of UiO-67 25%-I before catalysis and
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after the fourth run reveal that the overall sizes and shapes of with the smaller MOFs, but it has been shown that they can
View Article Online
the crystallites are retained, but there appears to be selective accelerate the yield of catalytic reactiDoOnsI: 1w0.1it0h39h/Cy9pDeTr0v0a3l6e8nAt
etching of the crystallites which leads to broader peaks in the iodine.49 The catalytic oxidation of hydroquinone to quinone
PXRD. This may have the added benefit of allowing substrates was examined over 1 h in the presence of ethyl alcohol or 2,2,2-
and oxidants to more readily move into the pores of the MOF. trifluoroethanol (TFE) under typical catalytic conditions. With
In the smaller MOFs, restricted access to the catalyst as a result ethyl alcohol as a solvent, a yield of 12% was observed over 1 h.
of accumulation of organic groups bound to the hypervalent The yield of catalytic reaction in the presence of TEF reached
iodine species may be a limiting factor in the ability to recycle 61% after just 10 minutes, but the reaction proceeded
the frameworks. With the larger frameworks, this appears to be sluggishly after that. PXRD following the reaction revealed that
overcome.
the crystallinity of the UiO-67 25% had been drastically reduced.
Evaluation of Other Substrates
The scope of the substrates was expanded to other aromatic
diols. The first set of para-hydroquinone substrates increase in
size through the incorporation of electron rich alkyl groups
(Table 2, entries 1-4). Based on size estimates from MM2
optimized structures, all of the substrates have a minimum
diameter orientation that should allow them to fit into DUT-5
(pore apertures of ~11 Å). UiO-67 has smaller pore apertures
(~7 Å, see Figure S16) which may prohibit many of the larger
substrates from entering the pores (See Figure S15). These
more electron-rich substrates react readily with mCPBA at 50
°C, so a lower temperature was used to evaluate the catalytic
activity. At 24 °C, the observed trend was an increase in the yield
of the background reaction, from 3% for p-hydroquinone to
41% for 2,5-di-tert-butylhydroquinone. This increase in
background reactivity mirrors the decrease in oxidation
potential characterized by cyclic voltammetry (Figure 12a, 0.97
eV to 0.84 eV). A comparison with the average calculated bond
dissociation free energies (BDFEs)50 mirrors this trend. In fact,
there is a strong correlation between the experimental
oxidation potentials measured here and the reported BDFEs.50
In the presence of the catalyst, a similar trend in the reaction
yield was not observed. For DUT-5, a yield beyond the
background reaction was observed in all cases, but the yield was
not proportional to the increase in background reactivity. In
other words, enhanced activity was not observed for more
easily oxidized substrates. This is consistent with the larger
substrates not diffusing into the pores as readily. This is more
pronounced with UiO-67 25%-I which has a more restricted
pore aperture. In addition to this, the direct reaction of mCPBA
with the substrate is sufficiently favorable given that higher
oxidation potential of the catalyst and the lower oxidation
potentials of these substrates that the catalytic reaction may be
hampered under these conditions. This is consistent with the
catalytic activity observed with the more readily oxidized UiO-
66 25%-I.29
Figure 10. Catalyst recyclability for oxidation of hydroquinone to benzoquinone in the
presence of UiO-67 25%-I compared to UiO-66 25%-I (top),29 and DUT-5 25%-I compared
1 h, 20 mol% catalyst, 2.9 eq. mCPBA,
nitromethane, 50 °C. Values were determined by 1H NMR in the presence of MSM as an
internal standard.
to MIL-53 25%-I (bottom).29 Conditions:
Hydroquinone derivatives containing electron withdrawing
groups (Table 2, entries 5-8) were probed. The chlorinated and
brominated derivatives have a similar size profile to the methyl
substituted hydroquinone but are not as readily oxidized (Figure
12b). With these substrates, a high yield of reaction beyond the
background reaction was observed at 50 °C. The yields of the
catalytic reactions for oxidation of chlorohydroquinone and 2,5-
dibromohydroquinone (Table 2, entries 5, 8) were reported to
be 40 and 67% for UiO-66 25%-I and 20 and 10% for MIL-53
25%-I.29 For both substrates, the larger-pore catalysts proved to
Figure 11. PXRD patterns of UiO-67 25%-I before the catalytic reaction, and the fourth
run.
The choice of solvent can play a significant role in mediating
these oxidation reactions. Nitromethane was previously found
to promote both high reactivity and selectivity with the smaller
frameworks.29 Halogenated solvents were not previously tested
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be more efficient catalysts. This enhanced performance is having a limiting effect on diffusion into the pores. Consistent
View Article Online
attributed to the increase in pore size associated with the larger with expectations, raising the reactionDtOeI:m10p.e10r3a9t/uCr9eDTfo00r36th8Ae
MOFs. Additional substrates 2-bromohydroquinone and 2,5- oxidation of catechol from 24 to 50 °C improved the yields with
dichlorohydroquinone (Table 2, entries 6 & 7) were tested with both frameworks.
all four catalysts. In the presence of the smaller MOFs (Table Selectivity was evaluated by contrasting the total yield of the
S10), the yields were significantly lower than with the larger desired product with the overall conversion. In addition to
frameworks. This is, again, consistent with the combination of higher yields, these isoreticularly expanded MOF catalysts lead
larger pores and a catalyst that is a more potent oxidant.
to higher selectivities for the desired products (Table S10) in the
reactions with the hydroquinones. For hydroquinone, the other
observed products of oxidation include 2,5-dihydroxy-1,4-
benzoquinone and maleic acid, representing an over-oxidation
of hydroquinone (Figures S9 and S11).29,51 In the absence of
MOF, these side-produces account for 47% of the reaction
products. This number is reduced to less than 5% in the
presence of either UiO-67 25%-I or DUT-5 25%-I. Interestingly,
in the presence of the UiO-67 or DUT-5 with 0% I, the
selectivities were nearly identical to those without any
framework. This indicates that the simple presence of the
framework is not sufficient to induce selectivity. The MOFs with
100%-I have selectivities between those of the controls and the
frameworks with 25%-I. Given the limited accessibility of the
pores in the MOFs with 100%-I linkers, a possibility is that the
observed oxidations are limited to the surface. This would imply
that it is the catalytic sites within the framework that are
responsible for the observed enhancement of selectivity. In
further support of this, a reaction run with an esterified version
the linker, under otherwise identical conditions, gave a yield of
38% (30% above background) for the desired product. The total
conversion, however, was 58%, so side products account for
34% of the product mixture. While this is lower than the
percentage observed with just mCPBA, it represents a much less
selective reaction than is observed with the MOFs containing
25% iodine. The larger electron-rich substrates, such as 3,5-di-
tert-butylcatechol and 2,5-di-tert-butylhydroquinone, have low
Figure 12. CVs of a) hydroquinone and hydroquinone derivatives containing electron selectivities in the MOFs with 25% I which is likely due to their
donation groups, b) hydroquinone derivatives containing electron withdrawing groups,
c) catechol and catechol derivatives containing electron donation groups. All obtained in
acetonitrile in 0.1 M Bu4NPF6 at 100 mV/s vs. Fc/Fc+.
inability to readily diffuse into the pores. This contrasts with the
high yields and selectivities for the electron-poor
hydroquinones and catechols that were studied. The precise
origin of the selectivity observed in these MOFs is not known
Given the success of these extended frameworks with
and represents an interesting line of future inquiry. Given the
substrates that have higher oxidation potentials the substrate
poorer selectivities of the homogeneous analogues, one
scope was expanded to include catechol. Catechol itself has an
possible model that can be used to rationalize this behaviour is
oxidation potential 0.14 V higher than hydroquinone and a
that the site isolation of the catalyst in the framework prevents
BDFE that is calculated to be 16 kJ/mol higher than
the formation of small amounts of bimolecular catalytically
hydroquinone.50 This increase in oxidation potential leads to a
active species, such as µ-oxo-bridged hypervalent iodine
low yield of the desired product with the smaller MOFs, MIL-53
molecules, which are known to be potent oxidants.52,53
25%-I or UiO-66 25%I (6-7%, see Table S10), at 50 °C. The
oxidation of catechol, 4-methylcatechol, 4-tert-butylcatechol,
and 2,5-di-tert-butylhydroquinone at 24 °C using DUT-5 25%-I
Conclusions
(Table 2, entries 9-12) gave the yields of 13%, 26%, 58%, and
Two new expanded-pore iodine-functionalized robust
29%. The same reactions with UiO-67 25%-I as the catalyst gave
frameworks, based on DUT-5 (Al) and UiO-67 (Zr), were
yields of 48%, 41%, 85%, and 27%, demonstrating that these
prepared as oxidization catalysts. Their performance was
larger MOFs can accommodate larger and more difficult to
inferior to the analogous smaller-pore MOFs with electron-rich
substrates. This resulted from an unintended change to the
electronic structure as a result of the pore-expansion strategy
and should serve as a cautionary note for linker elongation as a
blanket strategy for creating more void volume in a MOF. This
oxidize substrates. Notably, the yields appear to drop off with
the
most
readily
oxidized
substrate,
2,5-di-tert-
butylhydroquinone. This can be attributed to the larger size of
the substrate (minimum diameter of ~9.4 Å, See Figure S15)
8 | Dalton Tans., 2019, 00, 1-3
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18 H. Furukawa, N. Ko, Y. B. Go, N. Aratani, S. B. Choi, E. Choi, A.
slightly poorer performance was offset by greater stability
towards recycling of the catalyst. With substrates that were
more difficult to oxidize, the expanded-pore frameworks were
superior catalysts as compared to their smaller analogues. This
is likely a result of the combination of more space for diffusion
within the framework and the higher oxidation potential of the
iodine catalysts. In some cases, there was higher selectivity
associated with the MOF 25%-I catalysts than in the uncatalyzed
reaction of even the reaction with an analogous homogeneous
catalyst.
View Article Online
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There are no conflicts to declare.
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The present work was support by Texas Tech University and the
National Science Foundation (NMR instrument grant CHE-
1048553). We are grateful for assistance from the Hope-Weeks
group at Texas Tech for assistance with TGA measurements.
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View Article Online
DOI: 10.1039/C9DT00368A
A new iodine catalyst was supported in two different MOFs and the
catalytic activity for the oxidation of hydroquinones and catechols was
evaluated.