Catalytic Performance of Zr-Based Metal–Organic Frameworks Zr-abtc and MIP-200 in Selective Oxidations with H2O2

Article abstract of DOI:10.1002/chem.202005152

The catalytic performance of Zr-abtc and MIP-200 metal–organic frameworks consisting of 8-connected Zr₆ clusters and tetratopic linkers was investigated in H₂O₂-based selective oxidations and compared with that of 12-coordinated UiO-66 and UiO-67. Zr-abtc demonstrated advantages in both substrate conversion and product selectivity for epoxidation of electron-deficient C=C bonds in α,β-unsaturated ketones. The significant predominance of 1,2-epoxide in carvone epoxidation, coupled with high sulfone selectivity in thioether oxidation, points to a nucleophilic oxidation mechanism over Zr-abtc. The superior catalytic performance in the epoxidation of unsaturated ketones correlates with a larger amount of weak basic sites in Zr-abtc. Electrophilic activation of H₂O₂ can also be realized, as evidenced by the high activity of Zr-abtc in epoxidation of the electron-rich C=C bond in caryophyllene. XRD and FTIR studies confirmed the retention of the Zr-abtc structure after the catalysis. The low activity of MIP-200 in H₂O₂-based oxidations is most likely related to its specific hydrophilicity, which disfavors adsorption of organic substrates and H₂O₂.

Full text of DOI:10.1002/chem.202005152

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&
Metal–Organic Frameworks
Catalytic Performance of Zr-Based Metal–Organic Frameworks
Zr-abtc and MIP-200 in Selective Oxidations with H2O2
Nataliya V. Maksimchuk,[a] Irina D. Ivanchikova,[a] Kyung Ho Cho,[b] Olga V. Zalomaeva,[a]
Vasiliy Yu. Evtushok,[a, c] Kirill P. Larionov,[a, c] Tatiana S. Glazneva,[a] Jong-San Chang,*[b, d] and
Oxana A. Kholdeeva*[a]
Abstract: The catalytic performance of Zr-abtc and MIP-200
metal–organic frameworks consisting of 8-connected Zr6
clusters and tetratopic linkers was investigated in H2O2-
based selective oxidations and compared with that of 12-co-
ordinated UiO-66 and UiO-67. Zr-abtc demonstrated advan-
tages in both substrate conversion and product selectivity
for epoxidation of electron-deficient C=C bonds in a,b-
unsaturated ketones. The significant predominance of 1,2-
epoxide in carvone epoxidation, coupled with high sulfone
selectivity in thioether oxidation, points to a nucleophilic ox-
idation mechanism over Zr-abtc. The superior catalytic per-
formance in the epoxidation of unsaturated ketones corre-
lates with a larger amount of weak basic sites in Zr-abtc.
Electrophilic activation of H2O2 can also be realized, as evi-
denced by the high activity of Zr-abtc in epoxidation of the
electron-rich C=C bond in caryophyllene. XRD and FTIR stud-
ies confirmed the retention of the Zr-abtc structure after the
catalysis. The low activity of MIP-200 in H2O2-based oxida-
tions is most likely related to its specific hydrophilicity,
which disfavors adsorption of organic substrates and H2O2.
Introduction
MOFs usually contain a certain density of defects, which can
also behave as catalytic sites.[3] Moreover, their number can be
adjusted by the synthesis procedure.
Metal–organic frameworks (MOFs) consisting of metallic nodes
coordinated to rigid organic linkers are becoming increasingly
used as heterogeneous catalysts for liquid-phase reactions.[1,2]
The main reason for the growing research interest is the
unique blend of properties, such as extremely high surface
area and open porosity, the large proportion of transition
metals in the structure, tunable surface properties, easy and re-
liable preparation by using a great variety of ligands and tran-
sition metals. Many MOFs possess coordination positions occu-
pied by solvent molecules or other exchangeable ligands that
can be easily removed upon activation, resulting in coordina-
tively unsaturated sites.[1,2] In addition to exchangeable ligands,
However, in spite of all the positive features exhibited by
MOFs, their poor structural stability is still one of the limiting
factors that prevents the application of MOFs on an industrial
scale. Given that, a significant piece of research work has been
devoted to the elaboration of MOFs with high thermal and
chemical stability.[4] Zr-MOFs constructed from the highly
robust Zr6On(OH)m clusters and various carboxylate linkers are
one of the most attractive classes of MOFs owing to their out-
standing thermal, hydrothermal, chemical, and mechanical sta-
bility as well as a tunable amount of defects.[5] In particular, the
most studied representative of Zr-MOFs, ZrIV terephthalate
UiO-66, has been employed as a leaching-tolerant and recycl-
able catalyst for a range of H2O2-based oxidative transforma-
tions,[6,7] the majority of which were oxidations of S com-
pounds.[6a–c,7b]
[a] Dr. N. V. Maksimchuk, Dr. I. D. Ivanchikova, Dr. O. V. Zalomaeva,
Dr. V. Yu. Evtushok, K. P. Larionov, Dr. T. S. Glazneva, Prof. O. A. Kholdeeva
Boreskov Institute of Catalysis
pr. Lavrentieva 5, Novosibirsk 630090 (Russia)
E-mail: khold@catalysis.ru
Recently, some of us have found a remarkable effect of acid
additives on the selectivity of H2O2-based cyclohexene oxida-
tion over Zr-MOFs.[7a] It was demonstrated that protons facili-
tate heterolytic activation of the oxidant and prevent its un-
productive degradation on Zr-MOFs, thereby leading to selec-
tive oxygenation of the C=C bond without oxidation of allylic
CÀH bonds via a homolytic mechanism. More recently, we in-
vestigated in detail the mechanism of organic sulfides and sulf-
oxides oxidation with aqueous H2O2 over UiO-66 and UiO-67
by using product, kinetic, adsorption, and spectroscopic
tools.[7b] All the results strongly suggested that H2O2 is activat-
ed on a ZrIV site to give a hydroperoxo zirconium species,
“ZrOOH”, which possesses electrophilic properties, but an adja-
[b] Dr. K. H. Cho, Prof. J.-S. Chang
Research Center for Nanocatalysts
Korea Research Institute of Chemical Technology
P.O. Box 107, Yuseong, Daejeon 305–600 (Korea)
E-mail: jschang@krict.re.kr
[c] Dr. V. Yu. Evtushok, K. P. Larionov
Department of Natural Sciences, Novosibirsk State University
Pirgova str. 2, Novosibirsk 630090 (Russia)
[d] Prof. J.-S. Chang
Department of Chemistry, Sungkyunkwan University
Suwon 440-475 (Korea)
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/chem.202005152.
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cent basic oxygen atom implements deprotonation of
“ZrOOH”, favoring its transformation into a nucleophilic oxi-
dant, “ZrOOÀ”. The unprecedentedly high selectivity toward
sulfones observed at low conversions of thioethers implies
that nucleophilic oxidizing species predominate over electro-
philic ones in Zr-MOFs. The capability of UiO-66 and UiO-67 to
epoxidize electron-deficient C=C bonds in a,b-unsaturated
compounds also supported this conclusion.[7b]
Table 1. Oxidation of a,b-unsaturated carbonyl compounds with H2O2
over various Zr-MOFs.[a]
Substrate
Catalyst
Time Substrate Epoxide
[h]
conv. [%] selectivity[b] [%]
–
Zr-abtc
4
1
1
2
4
4
2
1
1
7
22
62
63
55
22[e]
30[e]
10[e]
60
26
30
22
9
Zr-abtc_small
Zr-abtc[c]
H4abtc[d]
Zr6 cluster[f]
MIP-200
UiO-66
In the present work, we have studied the reactivity of two
recently discovered Zr-MOFs,[8] Zr-abtc and MIP-200, consisting
of 8-connected Zr6 clusters and tetratopic linkers, 3,3’,5,5’-azo-
benzene-tetracarboxylate (abtc) and 5,5’-methylenediisophtha-
late (mdip), respectively, in a range of H2O2-based oxidations
and compared their catalytic performances with that of 12-co-
ordinated UiO-66/67. For the sake of comparison, abtc-based
MOFs with other metal clusters (Al, Fe, and Cr) were also pre-
pared and assessed.
8
15
20
20
UiO-67
55
–
2
1.5
2
4
4
4
12
7
10
8
75
92
57
80
88
Zr-abtc
MIP-200
UiO-66
UiO-67
–
1
1
1
1
0.5
1
4
50 (1:1)[g]
87 (6:1)[g]
80 (9:1)[g]
40 (1:1)[g]
38 (1:2)[g]
64 (3:4)[g]
Zr-abtc
Zr-abtc_small
MIP-200
UiO-66
UiO-67
15
15
5
8
11
Results and Discussion
Catalysts synthesis and characterization
Zr-abtc and MIP-200 were prepared by a hydrothermal reflux
synthesis using ZrOCl2·8H2O and 3,3’,5,5’-azobenzene tetracar-
boxylic acid (H4abtc) and ZrCl4 and 5,5’-methylenediisophthalic
acid, respectively. Zr-abtc_small with a reduced particle size
was obtained with a lower amount of formic acid (see the Sup-
porting Information for details). Fe-, Cr-, and Al-SOC were syn-
thesized according to reported[9] and original procedures by
using H4abtc as the linker and FeCl3·6H2O, Cr(NO3)3·9H2O, and
AlCl3·6H2O as the corresponding metal sources (see the Sup-
porting Information for details). All MOFs were characterized
by XRD, N2 adsorption, SEM, thermogravimetric analysis (TGA),
and FTIR spectroscopy (Figures S1–S12, Tables S1–S2 in the
Supporting Information).
[a] Reaction conditions: substrate 0.1 mmol, H2O2 0.8 mmol, MOF 4–5 mg,
CH3CN 1 mL, 708C. [b] Epoxide yield based on the substrate consumed.
[c] EtOAc was used as solvent. [d] 4 mg. [e] Other products: 4-hydroxy-2-
cyclohexen-1-one, cyclohex-2-ene-1,4-dione, 3-hydroxycyclohexanone, 4-
acetylbutyric acid. [f] [Zr6O4(OH)4(OAc)12]2 5 mg. [g] In parentheses, the
ratio of 1,2- to 7,8-epoxide for carvone oxidation; other products: di-ep-
oxide and various diols.
electron-deficient C=C bonds (Table S3 in the Supporting Infor-
mation).
Catalytic oxidation of unsaturated ketones
The catalytic performance of Zr-abtc and MIP-200 was first as-
sessed in the epoxidation of electron-deficient C=C bonds by
using three representative a,b-unsaturated carbonyl com-
pounds and it compared with that of UiO-66 and UiO-67
(Table 1).
Despite an excess of the oxidant being used for the oxida-
tion of a,b-unsaturated carbonyl compounds, full substrate
conversions could not be attained. This might be caused by
competitive decomposition of H2O2 with the evolution of mo-
lecular oxygen as well as oxidation of the solvent.
Zr-abtc demonstrated advantages over UiO-66/67 in terms
of the maximum substrate conversion and epoxide selectivity
whereas MIP-200, oppositely, showed inferior results. To check
that the observed catalytic activity of Zr-abtc is not related to
the functional groups of the linker, H4abtc was tested in the
oxidation of 2-cyclohexen-1-one. However, both the linker and
a Zr6 cluster, [Zr6O4(OH)4(OAc)12]2, themselves exhibited low
substrate conversion and epoxide selectivity, similar to those
found in the blank experiment (Table 1), indicating that a
blend of Zr sites with the specific abtc linker is required for the
observed catalysis. The crucial role of Zr nodes in the catalytic
performance of Zr-abtc was also confirmed by the fact that
SOC MOFs based on the same linker, abtc, and other metals
(Al, Fe, and Cr) revealed very low activity in the epoxidation of
Figure 1 shows that H2O2 degradation over Zr-abtc and MIP-
200 in the absence of organic substrate occurs with an appre-
ciable rate. However, given that the activity of all the Zr-MOFs
studied in this work for H2O2 decomposition was similar
(Figure 1), the superior catalytic performance of Zr-abtc in the
epoxidation of electron-deficient C=C bonds cannot be attrib-
uted to the formation of radical species formed upon homolyt-
ic decomposition of the oxidant.
On the other hand, epoxidation of C=C bonds in a,b-unsatu-
rated ketones in the presence of Zr-abtc was accompanied by
the oxidation of acetonitrile solvent, as evidenced by the de-
tection of considerable amounts of acetamide in the reaction
mixture. This could imply that peroxycarboximidic acid,
H3CC(=NH)OOH, the well-known intermediate formed upon
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dominated, similar yields of carvone oxide isomers were found
in ethyl acetate. Alteration of the catalyst and oxidant amount
allowed the substrate conversion to be increased, but slightly
affected the selectivity. The use of more concentrated H2O2
(77% instead of 30%) reduced the regioselectivity (Table 2).
However, if water was added, the yield of 1,2-epoxide in-
creased again, indicating that H2O plays a significant role in
the preferable epoxidation of the electron-poor C=C bond.
To evaluate the possible impact of internal diffusion on the
epoxidation process, a sample of Zr-abtc with a lower size of
crystallites (ca. 50 vs. 200 nm) was prepared and examined.
However, neither substrate conversion (Table 1), nor the reac-
tion rate (Figure 2) was affected by the size of the crystallites.
Therefore, we may conclude that no diffusion limitation takes
place for Zr-abtc.
Figure 1. H2O2 decomposition over Zr-abtc, MIP-200, UiO-66, and UiO-67. Re-
action conditions: 0.4 mmol H2O2, 9–10 mg catalyst (0.04 mmol Zr), 2 mL
CH3CN, 508C.
the interaction of CH3CN and H2O2 under basic conditions, is
involved in the epoxidation process (the so-called Payne oxida-
tion).[10] Nevertheless, the reaction readily proceeded if acetoni-
trile was replaced with ethyl acetate, although in this case
lower substrate conversions were achieved, for example, 22
versus 26% for cyclohexenone (Table 1) or 8 versus 15% for
carvone (Table 2). This suggests that peroxo species different
from peroxycarboximidic acid may also contribute to the epox-
idation of electron-deficient C=C bonds.
A distinctive feature of Zr-abtc in comparison with other Zr-
MOFs was a significant predomination of 1,2-epoxide over 7,8-
epoxide in the oxidation of carvone, which clearly indicates a
nucleophilic oxidation mechanism leading to epoxidation of
the more electron-poor C=C bond in the substrate molecule.[11]
In contrast, MIP-200, UiO-66, and UiO-67 gave 1,2- and 7,8-ep-
oxides in comparable amounts, like in the blank experiment,
which is more typical of radical oxidations.
Figure 2. Cyclohexenone consumption and epoxide accumulation versus
time in the presence of Zr-abtc and Zr-abtc_small. Reaction conditions: sub-
strate 0.1 mmol, H2O2 0.8 mmol, MOF 4 mg, CH3CN 1 mL, 708C.
Zr-abtc catalyst stability was verified under the conditions
employed for catalytic runs. Only traces of zirconium
(<0.1 ppm) were determined in the reaction mixture by induc-
tively coupled plasma optical emission spectroscopy (ICP-OES)
after separation of the catalyst, indicating the absence of
metal leaching during the reaction. The retention of the Zr-
abtc structure after the catalytic reaction was confirmed by
XRD (Figure 3) and FTIR spectroscopy (Figure 4). The N2 ad-
sorption measurements displayed an insignificant decrease in
the specific surface area and pore volume of the reused Zr-
abtc, which may be caused by adsorption of the reaction prod-
ucts within the micropores (Table S1 in the Supporting Infor-
mation).
The solvent turned out to be the main factor that affected
chemo- and regioselectivity of the epoxidation over Zr-abtc
(Table 2). In sharp contrast to acetonitrile where 1,2-epoxide
Table 2. Effect of reaction conditions on carvone oxidation over Zr-
abtc.[a]
Zr-abtc
[mg]
Solvent
[H2O2]
[m]
Time
[h]
Carvone
conv. [%]
Epoxide
selectivity[b] [%]
4.0
16.0
4.0
16.0
4.0
CH3CN
CH3CN
CH3CN
CH3CN
DMC
0.4
0.4
0.8
0.8
1
0.7
1
0.7
1
12
18
15
22
3
83 (9:1)
89 (7:1)
87 (6:1)
86 (9:1)
n.d.
Despite the retention of the Zr-abtc structure, its activity de-
teriorated during reuse (Table 3). We suggest that accumula-
tion of the reaction products within the micropores is the
main reason for the catalyst deactivation. Previously, some of
us demonstrated that oxidation products cause deactivation of
UiO-66 in the oxidation of propylene glycol[6d] and cyclohex-
ene.[7a] Catalyst deactivation and, as a result, incomplete sub-
strate conversions can also be due to the accumulation of
water inside the MOF pores because a high (0.4–0.8m) concen-
tration of aqueous H2O2 was required for these reactions.
0.8
4.0
EtOAc
CH3CN
CH3CN
CH3CN
CH3CN
0.8
1
1
1
1
8
67 (1:1)
82 (3.5:1)
89 (10:1)
80 (9:1)
79 (14:1)
16.0
16.0
4.0[e]
16.0[e]
0.4[c]
0.4[d]
0.8
22
18
15
19
0.8
1
[a] Reaction conditions: carvone 0.1 mmol, solvent 1 mL, 708C. [b] Epox-
ide yield based on the substrate consumed. In parentheses, the ratio of
1,2-epoxide to 7,8-epoxide. [c] 77% H2O2 was used instead of 30% H2O2.
[d] 77% H2O2 +H2O (1.3 mmol). [e] Zr-abtc_small was used as the catalyst.
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H2O2 utilization efficiency (Table S5 in the Supporting Informa-
tion, entries 11 and 12), whereas both Cr- and Fe-SOC were
almost inactive (Table S5 in the Supporting Information, en-
tries 13–16).
As was previously established for UiO-66,[7a] the addition of
1 mol equivalent (relative to Zr atoms in the MOF) of HClO4 in-
creased significantly the attainable CyH conversion and drasti-
cally altered the composition of products, favoring the forma-
tion of products derived from heterolytic oxygen transfer to
the C=C bond: epoxide, diol, and ketol. Therefore, we con-
clude that the effect of protons on the mechanism of H2O2 ac-
tivation is a general one for all Zr-MOFs, regardless of their
specific linker and structure.
All Zr-MOFs revealed fairly good activity in the epoxidation
of the electron-rich C=C bond of a bulky terpene, caryophyl-
lene (Scheme 1). Zr-abtc revealed superior selectivity in this re-
action (96%), whereas Zr-abtc_small showed the highest activi-
ty in terms of turnover frequency (TOF) among the MOFs stud-
ied, indicating that the reaction is controlled by diffusion
(Table 4). This is not surprising if we take into account that the
size of the caryophyllene molecule (ca. 8 Š) is comparable with
the pore apertures (see Table S1 in the Supporting Informa-
tion).
Figure 3. XRD patterns for Zr-abtc: (A) fresh and (B) after carvone oxidation
with H2O2 (reaction conditions as in Table 2, line 1).
Figure 4. FTIR spectra for Zr-abtc: (A) fresh, (B) and (C) after one and three
runs of carvone oxidation with H2O2, respectively (reaction conditions as in
Table 2, line 1).
Scheme 1. Oxidation of trans-caryophyllene with H2O2 over Zr-MOFs.
Table 4. Oxidation of trans-caryophyllene with H2O2 over various Zr-
MOFs.[a]
Table 3. Reuse of Zr-abtc in carvone oxidation with H2O2.[a]
1,2-Epoxide yield[b] [%]
MOF
Caryophyllene
conv. [%]
TOF[b]
Epoxide
Run
Carvone conv. [%]
[hÀ1
]
selectivity[c] [%]
1st
2nd
3rd
22
17
14
17
13
7
–
9
n.d.
1.5
4.9
1.4
2.5
77
96
92
90
92
Zr-abtc
Zr-abtc_small
MIP-200
UiO-67
27
36
17.5
44
[a] Reaction conditions: carvone 0.1 mmol, H2O2 0.8 mmol, Zr-abtc 16 mg,
CH3CN 1 mL, 0.7 h, 70 8C. [b] The yield of 7,8-epoxide did not exceed 1–
2 % in all runs.
[a] Reaction conditions: substrate 0.1 mmol, H2O2 0.1 mmol, MOF 1.6–
2 mg, CH3CN 1 mL, 4 h, 508C. [b] TOF=(moles of substrate consumed)/
(moles of Zr”time), determined by GC from initial rates of substrate con-
sumption. [c] 4,5-Caryophyllene oxide yield based on the substrate con-
sumed. Other product: 8,13-caryophyllene oxide.
Catalytic oxidation of unfunctionalized alkenes
Oxidation of cyclohexene (CyH), a model substrate that allows
one to differentiate easily between homolytic and heterolytic
activation of H2O2, was then investigated. The maximum
achieved substrate conversion and selectivities over Zr-abtc
and MIP-200 were close to the results obtained by using UiO-
66 and UiO-67,[7a] but in contrast to the UiO-66/67-based oxida-
tion, ketol was formed as well and H2O2 efficiency reached
higher values (Table S5 in the Supporting Information). Inter-
estingly, Al-SOC was also active in CyH oxidation with H2O2
providing up to 82% selectivity to epoxide along with 92%
Catalytic oxidation of S compounds
The catalytic performance of Zr-abtc and MIP-200 was also
evaluated for the oxidation of methyl phenyl sulfide (MPS) and
methyl phenyl sulfoxide (MPSO), organic substrates that pos-
sess nucleophilic and electrophilic character, respectively
(Table 5 and Table S4 in the Supporting Information). In the
MPS oxidation with 1 equivalent of H2O2, both Zr-abtc and
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Table 5. Oxidation of MPS with H2O2 over various MOFs.[a]
MOF
Time
[h]
MPS
conv. [%]
Product
selectivity [%]
H2O2
efficiency [%]
MPSO
MPSO2
–
24
0.5
0.5
2
2
4
24
24
24
14
51
51
52
46
51
14
25
25
79
8
4
8
9
10
71
76
92
21
92
96
92
91
90
29
24
8
n.d.
98
98
100
88
97
11
31
27
UiO-66
UiO-67
Zr-abtc
Zr-abtc_small
MIP-200
Al-SOC
Fe-SOC
Cr-SOC
[a] Reaction conditions: MPS 0.1 mmol, H2O2 0.1 mmol, MOF 1.6–2.0 mg,
278C, CH3CN 1 mL.
Figure 5. MPS conversion versus time in the presence of Zr-abtc and Zr-
abtc_small. Reaction conditions: MPS 0.1 mmol, H2O2 0.1 mmol, MOF 1.6 mg,
278C, CH3CN 1 mL.
MIP-200 behaved similar to UiO-66, revealing high oxidant uti-
lization efficiency, and produced sulfone as the main oxidation
product, indicating predomination of nucleophilic activation of
H2O2.[7b] In contrast, SOC MOFs with metals other than Zr re-
vealed much less activity, and H2O2 efficiency, and produced
sulfoxide as the principle product (Table 5), which is typical of
electrophilic oxidation. Therefore, we may conclude that nucle-
ophilic activation of hydrogen peroxide is a general character-
istic feature of all Zr-MOFs, regardless of their structure.
creased in the order Zr-abtc > MIP-200 > UiO-67 > UiO-66
(Table S2, Figures S8–S12 in the Supporting Information). Al-
though MIP-200 had a significant amount of defects due to
missing formates, this MOF revealed the lowest activity in all
the oxidation reactions studied.
We may assume that the low activity of MIP-200 in H2O2-
based oxidations is, most likely, due to its superior water-sorp-
tion properties and the presence of water hydrogen-bonded
to Zr-OH/H2O structural defects.[8b] Figures S17 and S18 (in the
Supporting Information) show water adsorption isotherms for
Zr-abtc and MIP-200, respectively. One can see that MIP-200 re-
veals a much higher uptake of water below P/P0 =0.25. This
may disfavor adsorption of H2O2 and/or organic substrates
with relatively low polarity within the micropores of this MOF.
The activity of Zr-MOFs in the MPS oxidation decreased in
the order UiO-66/67 > Zr-abtc > MIP-200, as evidenced by the
reaction times required to reach the same level of conversion
(Table 5). The difference in catalytic activity could be related to
several factors, including the size and morphology of the cata-
lyst particles, size of pore entrances, amount of defects, and
specific adsorption properties of the MOFs.
In the oxidation of MPSO to the corresponding sulfone, the
activity of Zr-MOFs followed a similar trend as in the MPS oxi-
dation: UiO-66 > UiO-67 > Zr-abtc @ MIP-200 (Table S4 in the
Supporting Information). Again, Zr-based MOFs revealed supe-
rior catalytic performance relative to the SOC MOFs with Al, Cr,
and Fe nodes (Table S4 in the Supporting Information).
Acid–base properties and their relation to catalyst activity
As the presence of both Lewis acid sites (LAS) and Brønsted
acid sites (BAS) can be crucial for catalysis, we employed FTIR
spectroscopy with adsorption of a weakly interacting base, CO,
as a probe molecule to compare the amount and strength of
LAS and BAS in Zr-abtc, MIP-200, and UiO-66 (see the Support-
ing Information for details). Figure S13 (in the Supporting Infor-
mation) shows the FTIR spectra of the Zr-MOFs in the region
of hydroxyl groups before CO adsorption. The absorption
bands in the range 3640–3675 cmÀ1 characterize the stretching
vibrations of isolated OH groups.[12] Multiple bands in this
range may correspond to chemically inequivalent isolated hy-
droxyls.[12] For MIP-200, three types of OH groups can be distin-
guished (bands at 3675, 3660, and 3640 cmÀ1). The broad ab-
sorption band in the region 3200–3600 cmÀ1 refers to hydro-
gen-bonded hydroxyls. This band is the most intense for MIP-
200, indicating a noticeable amount of water still present in
the sample after vacuum activation at 1508C. This is consistent
with the specific water-sorption properties of MIP-200 reported
by Chang, Serre, and co-workers, in particular, the presence of
some water hydrogen-bonded to Zr-OH/H2O defects.[8b] This
may also explain why the peaks of isolated OH groups of this
sample are not sharp. The shift of the bands upon adsorption
SEM images (Figures S4 and S5 in the Supporting Informa-
tion) show that the average particle size decreases in the
series UiO-67 > MIP-200 > Zr-abtc > UiO-66. On the other
hand, the size of the pore entrances follows the order MIP-200
(1.3 nm hexagonal channels) > UiO-67 (0.8 nm windows) > Zr-
abtc (0.7 nm channels) > UiO-66 (0.6 nm windows). Therefore,
the activity of Zr-MOFs in the MPS oxidation correlates with
neither their particle size, nor the size of the pore apertures. In
addition, the samples of Zr-abtc with an average particle size
of 50 and 200 nm demonstrated similar rates in the MPS oxida-
tion (Figure 5), indicating that the reaction is not controlled by
diffusion of the reactants within the micropores.
Finally, we tried to analyze a possible role of defects in the
Zr-MOF structures in the H2O2-based catalytic oxidations. Previ-
ously, two groups concluded that defects play a significant
role in the thioether oxidation over UiO-66.[6c,7b] TGA measure-
ments showed that the concentration of defects correspond-
ing to the total amount of terminal Zr(OH/H2O) groups de-
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of CO (Figure S14 in the Supporting Information) is 71– mary on the acid–base properties of the Zr-MOFs studied is
73 cmÀ1, which corresponds to a proton affinity of 1425– given in Table S6 (in the Supporting Information).
1430 kJmolÀ1 and characterizes the Brønsted acidity of the OH
groups in the Zr-MOFs as very weak.
The number of basic sites in the Zr-MOFs was also evaluated
by using a methodology based on the adsorption of isobutyric
acid (IBA) from n-hexane, which was recently employed for
UiO-66 and UiO-67.[7b] Figure 7 shows the adsorption isotherms
for Zr-abtc and MIP-200 (see the Supporting Information for
details). According to the value of the strong, irreversible ad-
sorption of IBA, the number of basic sites in the MIP-200
sample was approximately 580 mmolgÀ1, which is in good
agreement with the value of 670 mmolgÀ1 determined by FTIR
spectroscopy (Table S6 in the Supporting Information). Adsorp-
tion of IBA on Zr-abtc revealed a higher number of basic sites
in this material than in MIP-200 (Figure 7) and UiO-66 (Fig-
ure S16 in the Supporting Information). The larger adsorption
value for Zr-abtc (ca. 900 mmolgÀ1) agrees well with the
amount of basic sites determined by FTIR technique
(1077 mmolgÀ1).
FTIR spectra of CO adsorbed on the samples at À1968C and
a pressure of 0.1–10 torr are shown in Figure S15 (in the Sup-
porting Information). The absorption band at 2149–2153 cmÀ1
refers to CO adsorbed on OH groups, whereas the absorption
band at 2135 cmÀ1 characterizes the vibrations of physically
adsorbed CO. In turn, the absorption band at 2173–2180 cmÀ1
characterizes CO complexes with zirconium cations (LAS).[12b,13]
The LAS concentration determined from the integrated intensi-
ty of the absorption band at 2173–2180 cmÀ1 was 39, 42, and
78 mmolgÀ1 for UiO-66, Zr-abtc, and MIP-200, respectively. The
estimated strength of the LAS in terms of the CO adsorption
heat was 27–29 kJmolÀ1, which corresponds to relatively weak
Lewis acidity.[13,14]
Recently, some of us suggested that a combination of LAS
with weak basic sites is responsible for the nucleophilic activa-
tion of H2O2 over UiO-66/67.[7b] Given that both Zr-abtc and
MIP-200 revealed the characteristic sign of nucleophilic oxida-
tion, viz. high sulfone selectivity, we evaluated the amount and
strength of basic sites in these materials by using FTIR spec-
troscopy of deuterochloroform as a probe molecule (see the
Supporting Information for details). Figure 6 shows the FTIR
spectra of CDCl3 adsorbed on Zr-abtc and MIP-200. An absorp-
tion band appeared in the range 2200–2300 cmÀ1, was decom-
posed into two individual Gaussian components at 2253 for
Zr-abtc (2247 for MIP-200) and 2268 cmÀ1, corresponding to
hydrogen-bonded CDCl3 complexes with basic sites in the Zr-
MOFs and CDCl3 in the gas phase, respectively.[12a,14] The con-
centration of the basic sites determined from the integrated in-
tensity of the absorption band at 2253 (2247) cmÀ1 was 1077
and 670 mmolgÀ1 for Zr-abtc and MIP-200, respectively. The
strength of the basic sites in the scale of proton affinity (PA)
was 839 and 860 kJmolÀ1 for Zr-abtc and MIP-200, respectively,
which characterizes the basic sites as the weak ones. Similar
values of PA (839–843 kJmolÀ1) were previously reported for
UiO-66.[7b,12a] Most likely, they are represented by terminal hy-
droxyls of ZrOH groups created in the MOF defects.[7b] A sum-
Although the Zr-abtc sample was just slightly defective and
Zr-abtc_small was non-defective at all according to TGA
(Table S2 in the Supporting Information), Zr sites in this MOF
contain terminal H2O and OH groups because the Zr6 clusters
are eight-coordinated.[8a] Note that the maximum content of
Zr(OH/H2O) calculated for defect-free hydrated Zr-abtc by
using the formula of Zr6(m3-O)4(m3-OH)4(abtc)2(OH)4(H2O)4 is
2618 mmolgÀ1. Therefore, we may suggest that the superior
catalytic performance of Zr-abtc in the epoxidation of electron-
deficient C=C bonds correlates with the larger amount of weak
basic sites in this material.
Figure 7. Adsorption of isobutyric acid on Zr-abtc_small and MIP-200 from
n-hexane at 258C.
Conclusion
This work demonstrated that the metal–organic frameworks
Zr-abtc and MIP-200 consisting of eight-connected Zr6-oxohy-
droxoclusters and the tetratopic linkers abtc and mdip differ
greatly in their catalytic activity in selective oxidations when
using hydrogen peroxide as the oxidant. Although the catalytic
performance of Zr-abtc, in terms of attainable substrate con-
versions and product selectivity, is often superior or, at least,
comparable with that of 12-connected UiO-66 and UiO-67,
Figure 6. FTIR spectra of CDCl3 adsorbed on Zr-abtc and MIP-200 samples.
Deconvolution curves are shown as dashed lines.
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spectrometer in the range 400–6000 cmÀ1 with a resolution of
4 cmÀ1 and accumulation of 200 scans. Self-supporting pellets (ca.
20 mgcmÀ2) were prepared from the sample powder, placed in an
IR cell, and evacuated at 1508C for 1 h. CO was adsorbed at
À1968C, and the pressure varied from 0.1 to 10 torr. CDCl3 was ad-
sorbed at 208C by injecting a portion of 5000 mmolgÀ1 into the
cell. A full description of the methodology can be found in the
Supporting Information.
MIP-200 reveals very low activity. Analysis of various physico-
chemical characteristics of the Zr-MOFs, including the size of
the pore entrances, average particle size, concentration of de-
fects, amount of acid and basic sites, as well as water-sorption
properties, allowed us to suggest that the low activity of MIP-
200 is most likely related to its high and specific hydrophilicity
(high water uptake at low P/P0), which disfavors adsorption of
organic substrates and H2O2. The superior activity of Zr-abtc in
the epoxidation of a,b-unsaturated ketones, predomination of
1,2-epoxide over 7,8-epoxide in the epoxidation of carvone, to-
gether with the highly selective formation of sulfone in the
thioether oxidation with an equimolar amount of H2O2, all
point to nucleophilic activation of the oxidant, which can be
realized owing to the presence of a high concentration of
weakly basic sites (presumably, ZrOH) in the Zr-MOF structure.
On the other hand, the capability of Zr-abtc to accomplish se-
lective epoxidation of the electron-rich C=C bond in caryophyl-
lene implies that electrophilic oxidizing species may co-exist
with nucleophilic ones when Zr-MOF is employed as a catalyst
for the activation of hydrogen peroxide. Further spectroscopic
and computational studies may shed light on the structure of
the active species operating in Zr-abtc and other Zr-MOFs.
Basicity measurements
Evaluation of the number of basic sites in Zr-MOFs was performed
by following the previously reported methodology,[7b] using isobu-
tyric acid (IBA) as acidic probe and n-hexane as solvent. Prior to
measurements, the samples were activated in vacuum at 1508C for
3 h. The number of basic sites was determined from adsorption
isotherms as the end of strong adsorption of IBA, that is, at the
point where a detectable concentration of IBA appeared in the so-
lution.
Acknowledgments
The authors thank Dr M. V. Shashkov for GC-MS measurements.
This work was partially supported by the Ministry of Science
and Higher Education of the Russian Federation within the
governmental order for Boreskov Institute of Catalysis (project
AAAA-A21-121011390008-4) and the Russian Foundation for
Basic Research (grant N 18-29-04022). The studies were carried
out by using facilities of the shared research center “National
center of investigation of catalysts” at Boreskov Institute of
Catalysis.
Experimental Section
MOF synthesis and characterization
Zr-abtc and MIP-200 were prepared by a hydrothermal reflux syn-
thesis according to procedures reported in the literature[8] using
ZrOCl2·8H2O and 3,3’,5,5’-azobenzene tetracarboxylic acid or ZrCl4
and 5,5’-methylenediisophthalic acid and characterized by XRD, N2
adsorption, SEM, TGA, and FTIR spectroscopy (see the Supporting
Information for details). Zr-abtc_small with a reduced particle size
was obtained by using a smaller amount of formic acid (see the
Supporting Information). Before measurements and catalytic ex-
periments, the MOF samples were pre-treated at 1508C under
vacuum for 3 h.
Conflict of interest
The authors declare no conflict of interest.
Keywords: alpha,beta-unsaturated ketones · epoxidation
hydrogen peroxide · metal–organic frameworks · Zr
·
Catalytic oxidations
Catalytic reactions were performed under vigorous stirring
(500 rpm) in thermostat-equipped glass vessels. Each experiment
was reproduced 2–3 times. The reaction products were identified
by GC-MS. Substrate conversions and product yields were deter-
mined by GC using biphenyl as internal standard.
[1] a) Metal–Organic Frameworks: Applications from Catalysis to Gas Storage
(Ed.: D. Farrusseng), Wiley, Weinheim, 2011; b) Metal–Organic Frame-
works as Heterogeneous Catalysts, (Eds.: F. LlabrØs i Xamena, J. Gascon),
RCS Publishing, Cambridge, 2013; c) Metal–Organic Frameworks: Appli-
cations in Separations and Catalysis (Eds.: H. García, S. Navalón), Wiley-
VCH, Weinheim, 2018; d) Elaboration and Applications of Metal–Organic
Frameworks (Eds.: S. Ma, J. A. Perman), World Scientific, Singapore,
2018.
[2] a) Y. K. Hwang, G. FØrey, U.-H. Lee, J.-S. Chang, Liquid Phase Oxidation
via Heterogeneous Catalysis: Organic Synthesis and Industrial Applications
(Eds.: M. G. Clerici, O. A. Kholdeeva), Wiley, Hoboken, 2013, p.371;
b) O. A. Kholdeeva, Catal. Today 2016, 278, 22–29; c) S. Kramer, N. R.
Bennedsen, S. Kegnæ s, ACS Catal. 2018, 8, 6961–6982; d) A. Dhakshina-
moorthy, A. M. Asiri, M. Alvaro, H. García, Green Chem. 2018, 20, 86–
107; e) L. Jiao, Y. Wang, H.-L. Jiang, Q. Xu, Adv. Mater. 2018, 30,
1703663; f) D. Yang, B. C. Gates, ACS Catal. 2019, 9, 1779–1798; g) A.
Dhakshinamoorthy, S. Navalon, A. M. Asiri, H. Garcia, Chem. Commun.
2020, 56, 26–45; h) A. Bavykina, N. Kolobov, S. Khan, J. A. Bau, A. Ramir-
ez, J. Gascon, Chem. Rev. 2020, 120, 8468–8535.
Oxidation of unsaturated carbonyl compounds was carried out at
708C. H2O2 (0.8 mmol) was added to a mixture containing sub-
strate (0.1 mmol), MOF (4–5 mg, 0.02 mmol of active metal), and
solvent (1 mL, CH3CN, EtOAc, or DMC). After the reactions, the cata-
lyst was separated by filtration, washed with acetone, dried in air
at room temperature, and then reused.
Oxidations of S compounds (MPS and MPSO) and unfunctionalized
alkenes (cyclohexene and caryophyllene) were initiated by the ad-
dition of H2O2 (0.1 mmol) to a solution of substrate (0.1 mmol) in
CH3CN solvent (1 mL) at 278C (S compounds) or 508C (alkenes)
containing MOF (1.6–2.0 mg, 7 mmol of active metal).
[3] a) F. Vermoortele, B. Bueken, G. Le Bars, B. Van de Voorde, M. Vandichel,
K. Houthoofd, A. Vimont, M. Daturi, M. Waroquier, V. Van Speybroeck, C.
Kirschhock, D. E. De Vos, J. Am. Chem. Soc. 2013, 135, 11465–11468;
b) J. Canivet, M. Vandichel, D. Farrusseng, Dalton Trans. 2016, 45, 4090–
IR spectroscopy with adsorption of probe molecules
FTIR spectra before and after the adsorption of probe molecules
(CO, CDCl3) were registered by using a Shimadzu IRTracer-100
Chem. Eur. J. 2021, 27, 6985 –6992
www.chemeurj.org
6991
ꢀ 2021 Wiley-VCH GmbH

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doi.org/10.1002/chem.202005152
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4099; c) C. Caratelli, J. Hajek, F. G. Cirujano, M. Waroquier, F. X. Llabre-
s i Xamena, V. Van Speybroeck, J. Catal. 2017, 352, 401–414; d) M.
Taddei, Coord. Chem. Rev. 2017, 343, 1–24; e) A. Dhakshinamoorthy, A.
Santiago-Portillo, A. M. Asiri, H. Garcia, ChemCatChem 2019, 11, 899–
923.
[7] a) N. V. Maksimchuk, J. S. Lee, M. V. Solovyeva, K. H. Cho, A. N. Shmakov,
Y. A. Chesalov, J.-S. Chang, O. A. Kholdeeva, ACS Catal. 2019, 9, 9699–
9704; b) O. V. Zalomaeva, V. Yu. Evtushok, I. D. Ivanchikova, T. S. Glazne-
va, Yu. A. Chesalov, K. P. Larionov, O. A. Kholdeeva, Inorg. Chem. 2020,
59, 10634–10649.
[8] a) H. Wang, X. Dong, J. Lin, S. J. Teat, S. Jensen, J. Cure, E. V. Alexandrov,
Q. Xia, K. Tan, Q. Wang, D. H. Olson, D. M. Proserpio, Y. J. Chabal, T.
Thonhauser, J. Sun, Y. Han, J. Li, Nat. Commun. 2018, 9, 1745; b) S.
Wang, J. S. Lee, M. Wahiduzzaman , J. Park, M. Muschi, C. Martineau-
Corcos, A. Tissot, K. H. Cho, J. Marrot, W. Shepard, G. Maurin, J.-S.
Chang, C. Serre, Nat. Energy 2018, 3, 985–993.
[9] H. Chevreau, A. Permyakova, F. Nouar, P. Fabry, C. Livage, F. Ragon, A.
Garcia-Marquez, T. Devic, N. Steunou, C. Serre, P. Horcajada, CrystEng-
Comm 2016, 18, 4094–4101.
[4] a) G. Ferey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surble,
I. A. Margiolaki, Science 2005, 309, 2040–2042; b) X.-C. Huang, Y.-Y. Lin,
J.-P. Zhang, X.-M. Chen, Angew. Chem. Int. Ed. 2006, 45, 1557–1559;
Angew. Chem. 2006, 118, 1587–1589; c) M. Dan-Hardi, C. Serre, T. Frot,
L. Rozes, G. Maurin, C. Sanchez, G. Ferey, J. Am. Chem. Soc. 2009, 131,
10857–10859; d) K. Leus, T. Bogaerts, J. De Decker, H. Depauw, K. Hen-
drickx, H. Vrielinck, V. Van Speybroeck, P. Van Der Voort, Micropor. Meso-
por. Mater. 2016, 226, 110–116; e) X.-L. Lv, K. Wang, B. Wang, J. Su, X.
Zou, Y. Xie, J.-R. Li, H.-C. Zhou, J. Am. Chem. Soc. 2017, 139, 211 –217;
f) Y. Dou, H. Zhang, A. Zhou, F. Yang, L. Shu, Y. She, J.-R. Li, Ind. Eng.
Chem. Res. 2018, 57, 8388–8395; g) S. Yuan, L. Feng, K. Wang, J. Pang,
M. Bosch, C. Lollar, Y. Sun, J. Qin, X. Yang, P. Zhang, Q. Wang, L. Zou, Y.
Zhang, L. Zhang, Y. Fang, J. Li, H.-C. Zhou, Adv. Mater. 2018, 30,
1704303.
[10] G. B. Payne, P. H. Deming, P. H. Williams, J. Org. Chem. 1961, 26, 659–
663.
[11] K. K. W. Mak, Y. M. Lai, Y.-H. Siu, J. Chem. Educ. 2006, 83, 1058–1061.
[12] a) M. N. Timofeeva, V. N. Panchenko, J. W. Jun, Z. Hasan, M. M. Matroso-
va, S. H. Jhung, Appl. Catal. A 2014, 471, 91–97; b) K. I. Hadjiivanov,
D. A. Panayotov, M. Y. Mihaylov, E. Z. Ivanova, K. K. Chakarova, S. M. An-
donova, N. L. Drenchev, Chem. Rev. 2021, 121, 1286–1424; c) G. C.
Shearer, S. Forselv, S. Chavan, S. Bordiga, K. Mathisen, M. Bjørgen, S.
Svelle, K. P. Lillerud, Top Catal. 2013, 56, 770–782.
[5] a) T. Devic, C. Serre, Chem. Soc. Rev. 2014, 43, 6097–6115; b) Y. Bai, Y.
Dou, L.-H. Xie, W. Rutledge, J.-R. Li, H.-C. Zhou, Chem. Soc. Rev. 2016, 45,
2327–2367; c) A. J. Howarth, Y. Liu, P. Li, Z. Li, T. C. Wang, J. T. Hupp,
O. K. Farha, Nature Rev. Mater. 2016, 1, 15018.
[6] a) C. M. Granadeiro, S. O. Ribeiro, M. Karmaoui, R. Valenca, J. C. Ribeiro,
B. de Castro, L. Cunha-Silva, S. S. Balula, Chem. Commun. 2015, 51,
13818–13821; b) X. Zhang, P. Huang, A. Liu, M. A. Zhu, Fuel 2017, 209,
417–423; c) R. Limvorapitux, H. Chen, M. L. Mendonca, M. Liu, R. Q.
Snurr, S. B. T. Nguyen, Catal. Sci. Technol. 2019, 9, 327–335; d) V. V. Tor-
bina, N. S. Nedoseykina, I. D. Ivanchikova, O. A. Kholdeeva, O. V. Vodyan-
kina, Catal. Today 2019, 333, 47–53.
[13] K. Hadjiivanov, J. C. Lavalley, Catal. Commun. 2001, 2, 129–133.
[14] E. A. Paukshtis, E. N. Yurchenko, Russ. Chem. Rev. 1983, 52, 242–258.
Manuscript received: December 1, 2020
Revised manuscript received: February 3, 2021
Accepted manuscript online: February 8, 2021
Version of record online: March 15, 2021
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