Protons Make Possible Heterolytic Activation of Hydrogen Peroxide over Zr-Based Metal-Organic Frameworks

Article abstract of DOI:10.1021/acscatal.9b02941

The catalytic performance of zirconium-based metal-organic frameworks (UiO-66, UiO-67, and MOF-801) in cyclohexene oxidation with aqueous hydrogen peroxide can be greatly improved by adding a source of protons directly into the reaction mixture. A blend of Zr-MOF and protons favors heterolytic activation of H₂O₂ and makes possible selective formation of epoxide and diol with negligible formation of allylic oxidation products. Additives of HClO₄ suppress the rates of H₂O₂ decomposition over Zr-MOF and increase oxidant utilization efficiency. No structural changes occur in the acid-activated Zr-MOF. The catalyst does not suffer metal leaching, reveals a truly heterogeneous nature of the catalysis, and can be recovered and reused.

Full text of DOI:10.1021/acscatal.9b02941

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Letter
Protons make possible heterolytic activation of hydrogen
peroxide over Zr-based metal-organic frameworks
Nataliya V. Maksimchuk, Ji Sun Lee, Marina Solovyeva, Kyung Ho Cho, Aleksandr
N. Shmakov, Yuriy A. Chesalov, Jong-San Chang, and Oxana A. Kholdeeva
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b02941 • Publication Date (Web): 25 Sep 2019
Downloaded from pubs.acs.org on September 25, 2019
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ACS Catalysis
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Protons make possible heterolytic activation of hydrogen peroxide
over Zr-based metal-organic frameworks
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†,‡
§
†,‡
§
Nataliya V. Maksimchuk, Ji Sun Lee, Marina V. Solovyeva, Kyung Ho Cho,
Aleksandr N. Shmakov, Yuriy A. Chesalov, Jong-San Chang, and Oxana A. Kholdeeva*,†,‡
†
†,‡
§,║
†
Boreskov Institute of Catalysis, Pr. Lavrentieva 5, Novosibirsk 630090, Russia
‡
Novosibirsk State University, Pirogova str. 2, Novosibirsk 630090, Russia
§
Research Center for Nanocatalysts, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusung, Daejeon
3
05-600, Korea
║
Department of Chemistry, Sungkyunkwan University, Suwon 440-475, Korea
ABSTRACT: The catalytic performance of zirconium-based metal-organic frameworks (UiO-66, UiO-67, and MOF-801) in
cyclohexene oxidation with aqueous hydrogen peroxide can be greatly improved by adding a source of protons directly into the
reaction mixture. A blend of Zr-MOF and protons favors heterolytic activation of H
epoxide and diol with negligible formation of allylic oxidation products. Additives of HClO
2
O
2
and makes possible selective formation of
suppress the rates of H
4
2 2
O
decomposition over Zr-MOF and increase oxidant utilization efficiency. No structural changes occur in the acid-activated Zr-MOF.
The catalyst does not suffer metal leaching, reveals a truly heterogeneous nature of the catalysis and can be recovered and reused.
KEYWORDS: cyclohexene, epoxidation, heterogeneous catalysis, hydrogen peroxide, metal-organic frameworks, UiO, zirconium
The development of sustainable oxidation processes on the
basis of environmentally benign oxidants is a demanding task
of the modern chemical industry stimulated by increasing
selective oxidation of benzyl alcohol and S-compounds with
H O . Another way to design selective oxidation catalysts on
2 2
15
the basis of UiO-66 and some other stable Zr-MOFs involves
grafting of catalytically active species (Cr, Ti, V, Mo) onto the
1
ecological concerns. Hydrogen peroxide can be the oxidant of
1
6
choice because it contains 47% of potentially active oxygen
MOF surface or encapsulation of noble metal nanoparticles
2
17
and produces water as the sole byproduct. The elaboration of
within the MOF cages. In this case, MOF serves more as a
new methods for the direct synthesis of H
2
O
2
from H
2
and O
2
support rather than a true catalyst. A controlled exchange of
linkers with chiral M(salen) linkers in a Zr-based MOF has led
to asymmetric metallosalen heterogeneous catalysts.
may open up the new prospects for the widespread use of this
3
18
green oxidant.
Metal-organic frameworks (MOFs) possess a unique blend
of properties, such as intrinsic hybrid nature, open crystalline
structures, ultrahigh surface areas, tunable pore dimensions,
Here, we report on a simple tool that enables tuning the
reactivity and selectivity of UiO-66 and other Zr-MOFs in
H O -based oxidation of olefins through enhancing heterolytic
2 2
pathway of hydrogen peroxide activation by means of proton
additives, without evident changes in the MOF crystalline
structure.
4
and surface functionality. All these fascinating features allow
one to consider MOFs as prospective functional materials for
gas storage and separation, molecular recognition, drug
delivery, and heterogeneous catalysis.4,5 Zr-based MOFs are of
particular interest for practical applications owing to their
UiO-66 and isoreticular UiO-67 as well as MOF-801 were
19
prepared by the procedure reported in the literature and
6
outstanding stability. The zirconium terephthalate UiO-66
2
characterized by XRD, elemental analysis, N adsorption,
7
first reported in 2008 has attracted great attention due to its
SEM, TGA, FT-IR and Raman spectroscopy (see Supporting
Information (SI) for details). TGA measurements (Figure S5)
showed that the dehydrated and desolvated sample of UiO-66
lost 52.1% of its weight (from 89.2% to 42.7 wt %) in the last
step in contrast to 54.6% of the calculated weight loss based
7
exceptional robustness to thermal treatments (up to 540 °C),
acidic conditions (pH 1), and aqueous H O .
2 2
8
8b
The absence of coordinatively unsaturated metal sites in
UiO-66 and related materials initially caused doubts about the
possibility of using such MOFs in catalysis. However, recent
studies revealed that catalytic activity of UiO-66 and some
other MOFs can be significantly enhanced through the
2
0
6 6 2 6 4 2 6
on the formula of ideal Zr O (CO C H CO ) . Therefore, the
UiO-66 material used in this work could be considered as a
slightly defective one with 11.4 terephthalate ligands per
inorganic Zr O (OH) cluster instead of 12 in the non-
6 4 4
9
generation of defects in their crystalline structure. Several
‘
defect engineering’ methodologies have been suggested to
defective structure (see SI for calculations). This suggestion
agrees with the ICP and CHNO-analyses data (Tables S2 and
S3).
increase the number of potential Lewis acid sites, including (i)
adjusting synthetic conditions, e.g. metal to linker ratios,
temperature
during the preparation,
Recently, correlations between the number of missing-linker
sites and catalytic activity of UiO-66 have been found for the
10
10,11
12
or time of synthesis, (ii) using modulators
Catalytic performance of UiO-66 was first assessed in the
oxidation of cyclohexene (CyH) with equimolar amounts of
9
a,13
14
and (iii) post-synthetic treatments.
3
0% aqueous H
2 2
O using acetonitrile as solvent (Table 1).
Note that CyH has become a conventional test substrate to
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evaluate contributions of homo- and heterolytic oxidation
pathways by analyzing the composition of the reaction
amount of acid led to a progressive increase in the ratio of
allylic oxidation products and decrease in alkene conversion
and TOF (Figure 1). On the other hand, the addition of an
extra portion of acid (1.5 equiv.) favored the epoxide ring
opening and enhanced the ratio of diol along with some
increase in CyH conversion and TOF (Figure 1).
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6c,d,21
products.
UiO-66 promoted CyH oxidation (15 vs 6%
conversion in the ‘blank’ experiment) and produced a mixture
of epoxide, trans-cyclohexane-1,2-diol and allylic oxidation
products, cyclohexenyl hydroperoxide (HP), 2-cyclohexene-1-
ol (enol), and 2-cyclohexene-1-one (enone). A high ratio
(52%) of the allylic oxidation products unambiguously
CyH conversion Selectivity:
TOF
Epoxide
Diol
Allylic
indicated homolytic oxidation mechanism with the
a
participation of radical species capable of H-atom
1
00
22
abstraction. The addition of 1 molar equivalent (relative to
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3
2
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Zr atoms in the MOF) of HClO increased significantly the
8
6
4
2
0
0
0
0
0
reaction rate and attainable CyH conversion and drastically
altered the composition of products, favoring the formation of
epoxide and diol (Scheme 1, Table 1), the products derived
from heterolytic oxygen transfer to the C=C bond.
OOH
OH
O
OH
O +
+
+
+
OH
0
0.1
0.5
0.7
1.0
1.5
UiO-66 (Zr
6
O
4
(OH)
4
6
(BDC) )
HClO (molar equiv.)
4
OH
O +
Figure 1. Effect of the amount of acid on CyH oxidation with
OH
H O in the presence of UiO-66. Reaction conditions: 0.1 mmol
2
2
CyH, 0.1 mmol H
2 2
O , 2 mg UiO-66 (7 µmol Zr), 0.7–10.5 µmol
o
HClO , 1 mL CH CN, 50 C.
4
3
Scheme 1. Effect of protons on selectivity of CyH oxidation with
catalyzed by UiO-66 (violet – Zr atoms, red – O atoms).
2 2
H O
With 1 equiv. of the oxidant, the maximal CyH conversion
varied from 20 to 31%, depending on the reaction temperature,
without a significant alteration in the product selectivity
Table 1. Effect of acid on CyH oxidation with H
UiO-66
2
O
2
over
a
(Figure S7). The incomplete substrate conversion might be
catalytic
system
TOFb
(min )
Xc
(%)
product selectivity (%)
d
caused by competitive decomposition of H O with the
2
2
-1
evolution of molecular oxygen. Indeed, the addition of an
extra portion of the oxidant after the reaction stopped allowed
the CyH conversion to be increased from 31 to 41%, but also
augmented the yield of diol (Figure S8b). Interestingly,
epoxide
16
diol
16
allylice
65
– f
-
6
H+ g
-
8
25
26
47
increase in the amount of H
the ratio of homolytic oxidation products and improved
epoxidation selectivity (Figure S8a), although H utilization
efficiency decreased (31.5% for 0.2 M H vs 45% for 0.1 M
). Such behavior is not typical in homogeneous catalysis,
where epoxidation selectivity is normally decreased with
increasing H concentration, but has been precedented for
MOF catalysts.
2
Figure 2 shows that H O degradation over UiO-66 in the
2 2
O in the absence of acid reduced
UiO-66
UiO-66 + H+
0.1
1.3
15h
31i,j
36
11
52
77
21
2
2
O
2
aReaction conditions: 0.1 mmol CyH, 0.1 mmol 30% H
2 2
O
2
O
2
, 2 mg
CN, 50
C, 1 h. TOF (turnover frequency) = (moles of substrate
consumed)/(moles of Zr × time), determined by GC from initial
2 2
H O
4 3
UiO-66 (7 µmol Zr), 7 µmol HClO (if any), 1 mL CH
o
b
2
O
2
c
d
5a,23
rates of substrate consumption. CyH conversion. GC yield based
e
on CyH consumed. Sum of allylic oxidation products (HP + enol
+ enone). No catalyst was present. 7 µmol HClO . H O
4 2 2
conversion 35%. Reaction time 20 min. H
2
f
g
h
absence of organic substrate occurs with an appreciable rate,
i
j
2
O
2
conversion 50%.
24
which is quite typical of Zr-containing catalysts. It has been
3,25
documented in the literature,
that acid additives (H
3
PO
4
,
A similar trend was observed with the larger cage UiO-67
as well as another microporous Zr-MOF, MOF-801 (Table
S4). The nature of acid produced no effect on the ratio of
homo- to heterolytic oxidation products; however, use of
H
2
SO HNO and some others) may retard
4
,
3
H
2
O
2
decomposition over various catalysts. However, the role of
acid is not always evident and may depend on the specific
catalyst system and the nature of the acid. For example,
anhydrous CF
3 3
SO H increased the ratio of epoxide relative to
4 2 4
HClO , in contrast to H SO , produced no effect on the
diol (Table S5).
hydrogen peroxide decomposition over a Pd-containing zeolite
In the absence of Zr-MOF, acid produced only a minor
effect on the product distribution and CyH conversion (see
Table 1).The positive effect of protons could be observed in
different reaction media, but the best solvent, in terms of
attainable CyH conversion and heterolytic oxidation
selectivity, was acetonitrile (Figure S6). Variations in the
2
6
2 2
catalyst. On the other hand, H O degradation catalyzed by
2
7
Keggin Zr-substituted polyoxometalates (Zr-POMs) or Zr-
silicate (Figure S9) revealed an acid-accelerated behavior, and
28
a similar trend was observed for Ti-POMs.
The effect of acid on the rate of H
2
O
2
degradation over Zr-
MOFs was then investigated. Even small additives of HClO
resulted in a significant suppression of H decomposition
4
4
concentration of HClO showed that the optimal amount of
2
O
2
acid to achieve the highest total selectivity toward epoxide and
diol (98%) and 77% selectivity toward epoxide is close to 1
equiv. relative to the amount of Zr (Figure 1). Reduction in the
over UiO-66 (Figure 2) and UiO-67 (Figure S10). Therefore,
we can conclude that acid produced opposite effects on the
2
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To understand the origin of the enhanced heterolytic
rates of the target alkene oxidation reaction and the side
reaction of H degradation, which may imply that different
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O
pathway selectivity of UiO-66 in the presence of small acid
additives, the catalyst stability was verified under the
conditions employed for catalytic runs. Hot filtration tests
unambiguously proved that the observed catalysis has the truly
heterogeneous nature (Figures 3 and S12). Only traces of
zirconium (< 0.1 ppm) were determined in the filtrate by ICP-
AES, which also corroborates the absence of the metal
leaching during the reaction course. The retention of the UiO-
66 structure after the catalytic reaction was confirmed by XRD
2
2
active sites are involved in these two reactions. Given that,
one might anticipate that acid additives would increase
oxidant utilization efficiency in Zr-MOF-catalyzed oxidations.
Indeed, while CyH conversion was 31 and 15% in the
experiments with and without acid, respectively, the
corresponding H
2
O
2
conversion reached the values of 50 and
utilization efficiency
3
5% (Table 1). As a result, H O
2 2
improved from 45% to 63% due to the addition of protons.
(
Figure 4), FT-IR (Figure S3), and Raman (Figure S4)
spectroscopy. Moreover, the IR and Raman spectra of the
/acid-treated UiO-66 revealed no changes that might
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0.20
2 2
H O
arise due to the formation of a peroxy acid from an
uncoordinated carboxylate linker, which allowed us to exclude
its participation in the epoxidation process.
0
0
0
0
.15
.10
.05
.00
UiO-66
+
+
UiO-66 + 0.1 eq H
UiO-66 + 0.5 eq H
+
C
B
UiO-66 + 1 eq H
0
30
60
90
120
Time (min)
Figure 2. Effect of acid on H
Reaction conditions: 0.4 mmol H
Zr), 0.004–0.04 mmol HClO
2
O
2
2
decomposition over UiO-66.
2
, 10 mg UiO-66 (0.04 mmol
O
o
4
(if any), 2 mL CH
3
CN, 50 C.
A
5
10
15
20
25
30
Since the reaction stopped before all H
2
O
2
was consumed,
2
we may suggest that accumulation of the reaction products
within the micropores of UiO-66 is the main reason for the
catalyst deactivation. Previously, oxidation products caused
the catalyst deactivation in UiO-66-catalyzed oxidation of
Figure 4. XRD patterns for UiO-66: (A) simulated, (B) fresh, and
C) after CyH oxidation with H in the presence of 1 equiv. of
HClO (reaction conditions as in Table 1).
(
2 2
O
2
9
4
propylene glycol. In fact, the catalyst separated after the
CyH oxidation revealed some decrease in the specific surface
area and pore volume (Table S1). Moreover, if the principal
products, epoxide and water, were added during the reaction
course in the amount close to that formed at the end of the
catalytic reaction, the CyH oxidation stopped without reaching
the expected substrate conversion (Figure S11). Washing the
spent catalyst with methanol under elevated temperature
allowed the catalyst to be regenerated completely. Plots of
CyH conversion vs time (Figure 3) show that the catalyst
retained its activity during, at least, three consecutive runs.
Given that defects can play a significant role in catalysis by
9,15
Zr-MOFs,
we compared TG profiles for the fresh and
reused UiO-66 samples (Figure S5) and found that the latter
was insignificantly more defective: the weight loss in the last
step was 51.4% in contrast to 52.1% determined for the fresh
sample, which corresponds to 11.3 and 11.4 linkers,
respectively. Therefore, the positive effect of small additives
of acid on the CyH epoxidation selectivity cannot be explained
by the formation of a significant portion of new defects.
Indeed, stability of UiO-66 under mild acidic conditions has
8
been well-documented.
In homogeneous catalysis, protonation is known to trigger
the electrophilic reactivity of some peroxo complexes as
3
0
2
1,30
oxygen-transfer agents.
metal-substituted
(
DFT calculations on transition-
Keggin-type POMs
4 n
Bu N) [PM(OH)W11O39] (M = Ti(IV), Zr(IV), Nb(V), V(V),
Runs:
I
II
III
20
10
0
Mo(VI), W(VI), and Re(VII)) showed that the formation of
epoxide via a hydroperoxo intermediate is energetically more
favorable than through peroxo intermediates for such metals
30c
Hot catalyst filtration
as Ti(IV), Zr(IV) and Nb(V). More recently, the same trend
was found for Ti- and Nb-monosubstituted tungstates of the
Lindqvist
(Bu N) [(CH
structure, (Bu
4
N)
3
[(CH
3
O)TiW
5
O
18
]
and
4
2
3
O)NbW O
5
18].30d We could tentatively assume
0
10
20
30
40
50
that protons may facilitate the formation of an active
zirconium hydroperoxo species ‘ZrOOH’ upon interaction of a
Time (min)
terminal Zr-OH bond (such sites might be present in the
Figure 3. Reuse of UiO-66 (solid symbols) and hot catalyst
filtration test (open squares) for CyH oxidation over UiO-66 in
the presence of 1 equiv. HClO . Reaction conditions as in Table
4
9b,c,31
defects of UiO-66)
2 2
with H O (Scheme 2, route A) and
this species is responsible for oxygen atom transfer to the
alkene. Note that feasibility of the formation of ‘ZrOOH’
1
.
3
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intermediates in UiO-66 was recently confirmed by DFT
Page 4 of 7
the Global Frontier R&D Program and the Institutional
Collaboration Research Program (SK-1301).
1
5b
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1
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1
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calculations.
Alternatively, we may suggest the formation of a bridging
μ-η :η -peroxo species (Scheme 2, route B) via a mechanism
REFERENCES
2
2
similar to that proposed by Mizuno and co-workers for a
divanadium-substituted
γ-Keggin
polyoxotungstate,
3
2
(Bu N) (μ-O)(μ-OH)]. Note that Zr peroxo
4
4 38 2
[γ-PW10O V
2
2
complexes with the μ-η :η -arrangement of the peroxo ligand
have been reported in the literature.33
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Kholdeeva, O. A., Eds.; Wiley: Hoboken, 2013; (b) Handbook of
Advanced Methods and Processes in Oxidation Catalysis; Duprez, D.,
Cavani, F., Eds.; Imperial College Press: London, 2014.
(2) Jones, C. W. Application of Hydrogen Peroxide and
Derivatives, Royal Society of Chemistry, Cambridge, 1999.
(3) (a) Campos-Martin, J. M.; Blanco-Brieva, G.; Fierro, J. L. G.
Hydrogen Peroxide Synthesis: An Outlook beyond the Anthraquinone
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K.; Freakley, S. J.; Lewis, R. J.; Pritchard, J. C.; Hutchings, G. J.
Advances in the Direct Synthesis of Hydrogen Peroxide from
Hydrogen and Oxygen. Catal. Today 2015, 248, 3–9; (c) Menegazzo,
F.; Signoretto, M.; Ghedini, E.; Strukul, G. Looking for the “Dream
Catalyst” for Hydrogen Peroxide Production from Hydrogen and
Oxygen. Catalysts 2019, 9, 251.
H
H2O
Zr
OOH
Zr
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
O
Route A
H2O2
H2O2
H
O
H
O
O
H
H2O
Zr
OH
Zr
H2O
Zr
H+
OH
Zr
OH
O
O
O
H
H O
OH
Zr
2
H O
2
OH
Zr
- H2O
O
O
Zr
Zr
O
H
Route B
Scheme 2. Alternative mechanisms of the formation of active Zr-
peroxo species.
So far, attempts to identify spectroscopically any Zr peroxo
species in Zr-MOFs failed, most likely, because of their
extremely high reactivity. Further experimental and
computational studies on soluble molecular models, Zr-POMs,
are in progress in our group to gain insights into the intimate
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In summary, we demonstrated that both catalytic activity
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2 2
greatly enhanced by small acid additives. Protons disfavor
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oppositely facilitate heterolytic activation of H
makes possible selective oxygenation of the C=C bond,
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AUTHOR INFORMATION
Corresponding Author
H.-C. Zr-Based Metal–Organic Frameworks: Design, Synthesis,
Structure, and Applications. Chem. Soc. Rev. 2016, 45, 2327–2367;
(b) Howarth, A. J.; Liu, Y.; Li, P.; Li, Z.; Wang, T. C.; Hupp, J. T.;
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*
E-mail: khold@catalysis.ru
Author Contributions
(7) Cavka, J. H.; Jakobsen, S.; Olsbye, U.; Guillou, N.; Lamberti,
The manuscript was written through contributions of all authors. /
All authors have given approval to the final version of the
manuscript.
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Notes
The authors declare no competing financial interests.
ASSOCIATED CONTENT
Supporting Information. Full experimental procedures;
characterizing data for UiO-66, UiO-67 and MOF-801; N
adsorption, ICP, EA, TGA, FT-IR and Raman data; SEM images;
catalytic experiments (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
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ACKNOWLEDGMENT
This work was supported by the Ministry of Science and Higher
Education of the Russian Federation (project АААА-А17-
17041710080-4) and partially supported by the Russian
Foundation for Basic Research (grant № 18-29-04022). J.S.C. and
J.S.L. are grateful to Global Frontier Center for Hybrid Interface
Materials (GFHIM) and ISTK for their financial support through
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SYNOPSIS TOC
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