Kinetics and mechanism of anthracene oxidation with tert-butyl hydroperoxide over metal-organic frameworks Cr-MIL-101 and Cr-MIL-100

Article abstract of DOI:10.1016/j.jorganchem.2015.03.022

The oxidation of anthracene (AN) with tert-butyl hydroperoxide (TBHP) over metal-organic frameworks (MOFs) Cr-MIL-101 and Cr-MIL-100 produced anthraquinone (AQ) in a nearly quantitative (>99%) yield after 1 and 4 h, respectively, at 100 °C in chlorobenzene (ClPh). At initial stages of the reaction, some amounts (<13%) of intermediate products, anthrahydroquinone and oxanthrone, were detected along with traces of 9-hydroxyanthracene and/or anthrone. Kinetic study revealed that the oxidation reaction is unaffected by the presence of molecular oxygen and is zero order in AN and first order in both TBHP and MOF catalyst. Additives of radical chain scavengers produced no effect on the reaction rate and selectivity. The activation energy E_a was found to be similar for MIL-101 and MIL-100 (15 kcal/mol) while pre-exponential factors were different (7·10⁶ and 3·10⁶ L/(mol·min), respectively). The kinetic results indicated that the AN oxidation with TBHP over both MIL-101 and MIL-100 is not controlled by diffusion. No adsorption of AN on the MOFs from ClPh solution was found at the reaction temperature. A mechanism that involves a reversible interaction between TBHP and Cr^III centers within the MOF framework producing an active oxidizing species (rate-limiting step), followed by oxygen atom transfer from the peroxo species to the aromatic substrate to give primary oxygenated product(s), further oxidation of which leads to AQ, has been suggested.

Full text of DOI:10.1016/j.jorganchem.2015.03.022

Accepted Manuscript
Kinetics and mechanism of anthracene oxidation with tert-butyl hydroperoxide over
metal-organic frameworks Cr-MIL-101 and Cr-MIL-100
I.D. Ivanchikova, I.Y. Skobelev, O.A. Kholdeeva
PII:
S0022-328X(15)00181-3
10.1016/j.jorganchem.2015.03.022
JOM 18973
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 17 December 2014
Revised Date: 23 March 2015
Accepted Date: 24 March 2015
Please cite this article as: I.D. Ivanchikova, I.Y. Skobelev, O.A. Kholdeeva, Kinetics and mechanism of
anthracene oxidation with tert-butyl hydroperoxide over metal-organic frameworks Cr-MIL-101 and Cr-
MIL-100, Journal of Organometallic Chemistry (2015), doi: 10.1016/j.jorganchem.2015.03.022.
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ACCEPTED MANUSCRIPT
The kinetic and adsorption studies revealed that anthracene oxidation with TBHP over Metal-
Organic Frameworks MIL-100 and MIL-101 is not controlled by diffusion. The rate-limiting
step of the reaction is interaction of the oxidant with CrIII centers.

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Kinetics and mechanism of anthracene oxidation with tert-butyl
hydroperoxide over metal-organic frameworks
Cr-MIL-101 and Cr-MIL-100
I. D. Ivanchikova,a I. Y. Skobelev,a O. A. Kholdeevaa,b
a Boreskov Institute of Catalysis, Pr. Ac. Lavrentieva 5, Novosibirsk 630090, Russia.
b Novosibirsk State University, Pirogova str. 2, Novosibirsk 630090, Russia
E-mail: khold@catalysis.ru
Dedicated to the memory of Professor A.E. Shilov
The oxidation of anthracene (AN) with tert-butyl hydroperoxide (TBHP) over metal-organic
frameworks (MOFs) Cr-MIL-101 and Cr-MIL-100 produced anthraquinone (AQ) in a nearly
quantitative (>99%) yield after 1 and 4 h, respectively, at 100 ºC in chlorobenzene (ClPh). At
initial stages of the reaction, some amounts (<13%) of intermediate products,
anthrahydroquinone and oxanthrone, were detected along with traces of 9-hydroxyanthracene
and/or anthrone. Kinetic study revealed that the oxidation reaction is unaffected by the
presence of molecular oxygen and is zero order in AN and first order in both TBHP and MOF
catalyst. Additives of radical chain scavengers produced no effect on the reaction rate and
selectivity. The activation energy Ea was found to be similar for MIL-101 and MIL-100 (15
kcal/mol) while pre-exponential factors were different (7·106 and 3·106 L/(mol·min),
respectively). The kinetic results indicated that the AN oxidation with TBHP over both MIL-
101 and MIL-101 is not controlled by diffusion. No adsorption of AN on the MOFs from

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ClPh solution was found at the reaction temperature. A mechanism that involves a reversible
interaction between TBHP and CrIII centers within the MOF framework producing an active
oxidizing species (rate-limiting step), followed by oxygen atom transfer from the peroxo
species to the aromatic substrate to give primary oxygenated product(s), further oxidation of
which leads to AQ, has been suggested.
Key words: anthracene, anthraquinone, selective oxidation, tert-butyl hydroperoxide, metal-
organic framework, MIL-101, MIL-100, kinetics, mechanism
1. Introduction
The development of catalysts for the selective oxidation of hydrocarbons with
environmentally benign oxidants under mild conditions is of both academic and industrial
interest [1-3]. Heterogeneous catalysts offer clear advantages of easier catalyst separation and
recycling and thus better meet the requirements of sustainable chemistry [3-5].
Metal-organic frameworks (MOFs), or porous coordination polymers, have attracted
great attention of the scientific community due to a unique combination of properties, such as
crystalline open structures, extremely high surface areas and pore volumes, tunable pore size
and functionality, and a large fraction of accessible metal centers [6-20]. All these allow them
to be considered as prospective materials for potential applications in gas storage [6-9, 12, 14-
18, 20], molecular recognition [11, 15], liquid-phase separations [8, 12, 15], catalysis [8-19],
and other fields. Since their structure is composed of isolated metal atoms or clusters linked
by polydentate organic ligands (linkers) to give a rigid porous network, MOFs meet the
general requirements of single-site catalysts and may potentially combine advantages of both
homogeneous and heterogeneous catalysts [18, 21].
So far, low thermal, chemical and solvolytic stabilities (in comparison with inorganic
zeolites) limited the application of MOFs in catalysis. In 2005, Férey and co-workers reported
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the synthesis of a mesoporous chromium terephthalate, Cr-MIL-101, which possesses good
resistance to air, water, common solvents, and thermal treatment (up to 300 ºC) [22]. Its
framework consists of terephthalate linkers and Cr3O-carboxylate trimers with octahedrally
coordinated metal ions binding one fluoride and two terminal water molecules; the latter can
be easily removed by heating under vacuum, thereby providing coordinatively unsaturated
catalytically active sites [23]. MIL-101 has a rigid zeotype (MTN type) crystal structure,
consisting of quasi-spherical cages of two modes (2.9 and 3.4 nm) accessible through
windows of ca. 1.2 and 1.6 nm. Changing the linker from 1,4-benzenedicarboxylate for 1,3,5-
benzenetricarboxylate results in the formation of another MOF of a similar MTN structure,
MIL-100, with cages of 2.7 and 2.4 nm accessible through microporous windows of 0.9 and
0.6 nm [24]. The structure of MIL-101 and MIL-100 is schematically shown in Figure 1.
Figure 1. Schematic representation of the MIL-101 and MIL-100 structure.
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The catalytic performance of Cr-MIL-101 was evaluated in various liquid-phase
reactions, such as cyanosilylation of benzaldehyde [25], carboxylation of epoxides [26] and a
range of selective oxidations, including benzylic oxidation of tetralin [ 27 ], thioether
sulfoxidation with H2O2 [23], oxidation of cyclohexane [28], and allylic oxidation of alkenes
with tert-butylhydroperoxide (TBHP) [29] or O2 [30-32]. Although many examples of the use
of MIL-101 in catalysis exist in the literature, data on reaction kinetics are generally missing
and the role of diffusion within MOF crystallites is not always clear, which makes difficult
understanding mechanisms of the catalytic action of the MOFs.
Recently, we reported application of Cr-MIL-101 and Cr-MIL-100 (as well as their Fe
analogues) in the oxidation of anthracene (AN) to 9,10-anthraquinone (AQ) using TBHP as
oxidant [32]. The size of the MOF cages (cage entrances) along with the metal nature strongly
affected the catalytic activity. The turnover frequency (TOF) decreased in the order: Cr-MIL-
101 > Fe-MIL-101 > Cr-MIL-100 > Fe-MIL-100. Among these MOFs, Cr-MIL-101 revealed
superior catalytic performance in terms of AN conversion, AQ selectivity (>99%), and TOF
values. No leaching of active metal into solution occurred under the reaction conditions, and
the MOF could be recycled several times without deterioration of the catalytic properties [32].
In this work, we attempt getting insight into the mechanism of this reaction and report a
comparative kinetic study of AN oxidation with TBHP over Cr-MIL-101 and Cr-MIL-100.
2. Experimental
2.1. Materials
Anthracene (99%), anthraquinone (99%) and chromium(III) acetate hydroxide
(Cr3(OAc)7(OH)2) were purchased from Aldrich and used without additional purification.
Chlorobenzene (Fluka) was dried and stored over activated 4 Ǻ molecular sieves. TBHP was
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used as a solution in decane (5.5 M, Aldrich) and determined iodometrically prior to use. All
the other compounds were the best available reagent grade and used without further
purification.
2.2. Catalyst preparation and characterization
Cr-MIL-101 was prepared by the solvothermal method following the protocol reported by
Férey and co-workers [22]. Cr-MIL-100 was synthesized according to the literature [24] with
some modifications that had been described previously [32]. Activation of both MOFs was
carried out by sample evacuation at 150 ºC for 6 h and then at 180 ºC for 2 h. The elemental
analysis data, XRD patterns, IR spectra and textural characteristics of the materials were in
accordance with the literature [22, 24, 30, 32]. The content of Cr was ca. 23 and 26 wt. % in
activated MIL-101 and MIL-100, respectively. The specific BET surface area was 3900 m2·g-1
for MIL-101 and 2100 m2·g-1 for MIL-100.
2.3. Kinetic measurements and product analysis
Catalytic reactions were carried out in temperature-controlled glass vessels, typically at 100
ºC under vigorous stirring (500 rpm). Concentrations of the reactants were varied in the range
of [AN] = 0.02–0.15 M and [TBHP] = 0.1–0.9 M. The amount of catalyst was altered in the
range of 1.3–12 mg, which corresponds to 1.8–53 ꢀmol of CrIII. The reactions were initiated
by the addition of oxidant, TBHP, to a mixture containing substrate and MOF catalyst in
chlorobenzene (1–4 mL). Samples were taken periodically by a syringe during the reaction
course and analyzed by GC. The AN conversions and AQ yields were quantified by GC using
biphenyl as the internal standard. Each experiment was reproduced at least three times. The
experimental error in the determination of the substrate conversions and product yields
normally did not exceed 2%. The temperature dependences of the reaction rate were studied in
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the range of 60–120 ºC. The error in determination of activation energies (Ea) did not exceed
1–1.5 kcal/mol.
9,10-Anthraquinone (AQ). GC–MS (EI): m/z (relative int.): 208 (100) [M]+, 180 (80) [M –
CO]+, 152 (70) [M – 2 CO]+.
9,10-Anthracenediol (anthrahydroquinone, AHQ). GC–MS (EI): m/z (relative int.): 210 (85)
[M]+, 193 (5) [M – OH]+, 181 (15) [M – COH]+, 165 (35) [M – COH, O]+, 152 (15) [M – 2
COH]+, 105 (100) [M – M/2]+.
9-Hydroxyanthron-10-one (oxanthrone, OAT). GC–MS (EI): m/z (relative int.): 210 (60) [M]+,
192 (3) [M – H, OH]+, 181 (100) [M – COH]+, 165 (5) [M – COH, O]+, 152 (25) [M – COH,
CO, H]+.
10H-Anthracen-9-one (anthrone, AT) and/or 9-hydroxyanthracene (HAN). GC–MS (EI): m/z
(relative int.): 194 (100) [M]+, 165 (80) [M – COH or M – CO, -H ]+, 82 (20)+
[(M – COH, -H or M – CO, -2H)/2 ]+.
2.4. Adsorption measurements
Study of AN adsorption on MOFs was carried out at 80 ºC following the methodology
described elsewhere [30]. An appropriate amount (15–25 mg) of activated (see 2.2) MIL-100
or MIL-101 was stirred in ClPh during 1 h at 80 ºC. Then the MOF sample was disposed onto
a thermostated nylon filter (0.45 nm) and a hot solution of AN in ClPh (0.028 M, 3 mL) was
circulated through the filter with the MIL-100(101) sample. Concentration of AN in the
solution was determined after each pass by GC using dodecane as internal standard.
2.5. Instrumentation
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GC analyses were performed using a gas chromatograph Chromos GC-1000 equipped with a
flame ionization detector and a 30 m × 0.25 mm BPX-5 capillary column. GC–MS analyses
were carried out using an Agilent 7000B system with the triple-quadrupole mass-selective
detector Agilent 7000 (HP-5ms quartz capillary column 30 m × 0.25 mm). XRD
measurements were performed on Shimadzu XRD 7000S or DRON-3M. Nitrogen adsorption
measurements at 77 K were carried out using an ASAP-2020 instrument (Micrometrics)
within a partial pressure range of 10−6–1.0. FT-IR spectra were run on a Varian 660
spectrometer. SEM images were obtained using a JEOL JSM-6460 LV instrument.
3. Results and discussion
The oxidation of AN with TBHP smoothly proceeds in the presence of both Cr-MIL-101 and
Cr-MIL-100 as catalysts. A nearly quantitative yield of AQ (>99% AN conversion and >99%
AQ selectivity) can be obtained after 1 and 4 h with MIL-101 and MIL-100, respectively, at
100 ºC in chlorobenzene as solvent.
O
Cr-MIL-101(100)/TBHP
ClPh, 100oC
O
AQ
AN
Our preliminary results showed that in the initial oxidation rate expressed in TOF
values decreased ca. 4 times when Cr-MIL-100 was used as catalyst instead of Cr-MIL-101
[32]. This allowed us to suggest that diffusion limitation may take place for bulky AN and AQ
molecules within MOF particles. The size of AN molecule in two dimensions is 9.2×5.0 Å
while the size of AQ is 9.3×5.4 Å. In turn, the size of the TBHP molecule was estimated as
4.4×3.7×4.0 Å. Given in mind that the average diameter of cage entrances in MIL-100 is 9
and 6 Å for the large and small windows, respectively [24], it was reasonable to suppose that
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AN and AQ molecules can diffuse through the large windows only and their diffusion within
the pores may limit the reaction rate.
Study by SEM revealed that the average size of MOF crystallites was ca. 0.7 and 7 ꢀm
for MIL-101 and MIL-100, respectively (Figure 2a and b). Hence, at a first glance, MIL-100
possesses larger crystals and smaller cages/windows. However, we found that after stirring
MIL-100 in a solvent for 1 h, the size of the MOF particles decreased and became comparable
with that of MIL-101 (Figure 2c).
Figure 2. SEM images of (a) MIL-101, (b) MIL-100, and (c) MIL-100 after stirring in
cyclohexane at 60 ºC during 1 h.
Nevertheless, smaller pores could be responsible for the lower reaction rate over MIL-
100 if the reaction were controlled by diffusion. Hence, we expected some differences in the
reaction kinetics for AN oxidation over MIL-101 and MIL-100.
Typical kinetic curves for AN consumption over MIL-101 and MIL-100 are shown in
Figure 3. It is noteworthy that the reaction rate with a homogeneous chromium catalyst,
Cr3(OAc)7(OH)2, is lower relative to MIL-101 but higher than with MIL-100 (Figure 3).
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Cr-MIL-101
Cr-MIL-100
100
80
60
40
20
0
Cr (OAc) (OH)
2
3
7
0
20 40 60 80 100 120 140 160
Time, min
Figure 3. Kinetic curves for AN consumption over MIL-101, MIL-100 and Cr3(OAc)7(OH)2.
Reaction conditions: AN 0.1 M, TBHP 0.45 M, CrIII 6.6 ꢀmol, ClPh 1 mL, 100 ºC.
While the kinetic curves obtained for MIL-101 and MIL-100 revealed no induction
period, the kinetic curve for chromium acetate showed an S-type behavior that could originate
from a rearrangement of the labile Cr(III) coordination sphere, e.g., ligand exchange, under
the reaction conditions. In all cases, the reaction rate was not affected by light, which
discarded any photochemical process. Additives of the reaction product, AQ (20 mol%),
produced no impact on the reaction rate.
The AN consumption rate over MOFs was not influenced by the rate of stirring of the
reaction mixture in the range of 500–1000 rpm, which implies that external diffusion of the
reactants to the catalyst particles does not limit the overall oxidation rate. In agreement with
this, the reaction was found to be first order in the catalyst (Figure 4).
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Cr-MIL-101
0.006
0.005
0.004
0.003
0.002
0.001
0.000
Cr-MIL-100
0
2
4
6
8
10
12
Cr, µg/mL
Figure 4. Plots of the rate of AN oxidation versus the amount of chromium catalyst. Reaction
conditions: AN 0.1 M, TBHP 0.45 M, ClPh 1–4 mL, 100 ºC.
For both MIL-101 and MIL-100, the reaction was first order in the concentration of the
oxidant, TBHP (Figure 5).
Cr-MIL-101
0.005
0.004
0.003
0.002
Cr-MIL-100
0.001
0.000
0.0
0.2
0.4
0.6
0.8
1.0
TBHP, M
Figure 5. Plots of the rate of AN oxidation versus concentration of TBHP. Reaction conditions:
AN 0.1 M, catalyst 1.5 mg (6.6 ꢀmol CrIII), ClPh 1 mL, 100 ºC
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The character of the kinetic curves shown in Figure 3 implies that the reaction is zero
order in the concentration of AN up to 80–90% conversion. Indeed, plots given in Figure 6
confirm that variation in the initial concentration of AN has practically no impact on the
reaction rate. This finding argues against the preliminary suggestion [32] that the reaction is
controlled by diffusion of the reactants within MOF crystallites.
0.003
Cr-MIL-101
0.002
Cr-MIL-100
0.001
0.000
0.00
0.04
0.08
0.12
0.16
[AN], M
Figure 6. Plots of the rate of AN oxidation versus AN concentration. Reaction conditions:
TBHP 0.45 M, catalyst 1.5 mg (6.6 ꢀmol CrIII), ClPh 1 mL, 100 ºC.
Finally, a strong argument against the reaction control by diffusion has come from the
study on the temperature dependence of the reaction rate. For both MIL-101 and MIL-100, the
AN oxidation rate exhibits a typical Arrhenius dependence; the corresponding plots are shown
in Figure 7. The temperature dependence acquired using the homogeneous Cr3(OAc)7(OH)2
catalyst is given for comparison.
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-8
-9
Ea = 15 kcal/mol
Cr-MIL-101
-10
-11
-12
-13
-14
Ea = 18 kcal/mol
Cr3(OAc)7(OH)2
Ea = 15 kcal/mol
Cr-MIL-100
0.0026
0.0028
0.0030
1/T, K-1
0.0032
Figure 7. Arrhenius plots for AN oxidation with TBHP using MIL-101, MIL-100 and
Cr3(OAc)7(OH)2. Reaction conditions: AN 0.1 M, TBHP 0.45 M, CrIII 6.6 ꢀmol, ClPh 1 mL.
Surprisingly, both MOFs revealed similar values of the activation energy (15
kcal/mol). Such value is typical for reactions controlled by a chemical interaction itself and
not by diffusion of reactants, because for the latter, activation energies typically lie below 4
kcal/mol [33]. As one can judge from Figure 7, the lower rate of AN oxidation over MIL-100
is related to the lower pre-exponential factor (3·106 versus 7·106 L/(mol·min) for MIL-101).
We may tentatively suppose that larger cages of MIL-101 provide better conditions for
realization of an activated complex, which is necessary for efficient transformation of the
reactants to the products. In addition, one might expect better accessibility of the Cr(III) sites
in MIL-101 relative to MIL-100 due to the linker difference. Although the structure of the
primary chromium trimers and the mode of arrangement of supertetrahedra are the same in
both MIL-100 and MIL-101 (MTN structure), the structure of the supertetrahedron itself is
determined by the specific linker, which results in the different size of the supertetrahedra and,
therefore, different geometrical parameters of the MOF structure (Figure 1).
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The activation energy and pre-exponential factor estimated for the homogeneous
chromium catalyst, Cr3(OAc)7(OH)2, differ significantly (18 kcal/mol and 4·108 L/(mol·min)).
A possible reason for this can be a rather flexible and labile structure of the Cr(III) centers in
the soluble acetate as compared with the rigid structure of the terephthalate-linked chromium
centers within the framework of MIL-101(100).
Finally, we studied the dynamics of AN adsorption from ClPh solution at 80 ºC on
both MIL-100 and MIL-101 (see Experimental for details). Surprisingly, no changes of AN
concentration was found during circulation of the AN solution through either MIL-100 or
MIL-101 (Figure 8).
0,040
0,035
0,030
0,025
0,020
0,015
0,010
0,040
0,035
0,030
0,025
0,020
0,015
0,010
a
b
0,0
1,0
2,0
Time, min
3,0
4,0
5,0
0,0 0,5 1,0 1,5 2,0 2,5 3,0
Time, min
Figure 8. AN concentration versus circulation time: (a) MIL-100 and (b) MIL-101.
Experimental conditions: 17 mg of MIL-100(101), 3 mL of AN solution in ClPh, 80 ºC.
It was documented in the literature that benzene-like molecules have high affinity to
the MIL-101 surface while being adsorbed from the gas phase [34-36]. This was rationalized
by interactions of benzene rings with aromatic system of 1,4-benzenedicarboxilate, which
serves as the MOF linker. However, the situation may change dramatically when one move
from the gas phase to the liquid phase. For example, Henschel et al. demonstrated that
adsorption of styrene on MIL-101 is greatly affected by the solvent nature [37]. Whereas the
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amount of adsorbed styrene was comparable for mesitylene and toluene, no adsorption of the
aromatic compound from dichloromethane took place. Furthermore, raising temperature
would shift the adsorption–desorption equilibrium towards desorption in case of physical
nature of the sorption process.
The results acquired in this work (Figure 8) allowed us to conclude that adsorption of
AN on the MIL-101(100) surface is negligible under the reaction conditions employed for AN
oxidation with TBHP (80 ºC). Hence, the observed zero-order dependence of the oxidation
rate on the concentration of AN cannot be explained by strong adsorption of the aromatic
substrate on the entire catalyst surface. On the other hand, the lack of AN adsorption does not
necessarily means that it does not penetrate into the MIL cages. Recently, we demonstrated
that even much bulkier molecules of phthalocyanine are able to pass through the MIL-101
cage entrances and this process is rather fast [38].
All the results collectively indicate that AN oxidation with TBHP over both MIL-101
and MIL-101 is not controlled by diffusion. Furthermore, the observed zero order dependence
in AN concentration allows one to exclude any mechanism where the aromatic substrate
participates in the rate-limiting step. Hence, mechanisms that involve electron transfer steps
and formation of intermediate radical cation [39, 40] or formation of σ-complex between AN
and active form of catalyst [41] as rate-determining steps can be ruled out.
Importantly, the reaction proceeds with the same rate under air and inert atmosphere
(Ar), i.e. molecular oxygen is not involved, at least, in the rate-limiting step. This might
indicate a non radical oxidation mechanism, although autoxidation initiated by homolytic
decomposition of TBHP on chromium centers could not be completely ruled out. Indeed, we
found that TBHP easily decomposes on MIL-101 in the absence of AN. To probe further the
AN oxidation mechanism, we studied effects of two conventional radical scavengers, 2,6-di-
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tert-butyl-4-methyl-phenol (ionol) and hydroquinone. For both scavengers, neither AN
consumption nor AQ accumulation rate was changed. Figure 9 shows kinetic curves for AN
decay in the presence of ionol that was added either at the beginning of the reaction or after
ca. 50% conversion was attained. This result allows one to exclude unambiguously any
autoxidation mechanism with long radical chains, i.e., when propagation steps dominate
product formation [42]. Note that autoxidation is independent of O2 pressure when the chains
are reasonably long [42]. On the other hand, 100% selectivity of oxidation is not typical of
radical chain oxidation of hydrocarbons, including autoxidation of AN [39]. In fact, if chains
were short, C–C coupling products would contribute significantly to the overall product
formation as a result of chain termination. However, in the AN oxidation with TBHP over
MIL-101 and MIL-100 we observe a carbon mass balance close to 100% between AN
consumed and AQ produced. All these facts together allowed us to discard autoxidation
mechanism (with both long and short chains) for the title reaction.
100
80
60
40
20
0
0
10
20
30
40
50
60
70
Time, min
Figure 9. Kinetic curves for AN consumption over MIL-101: • – typical reaction (AN 0.1 M,
TBHP 0.45 M, Cr-MIL-101 1.5 mg (CrIII 6.6 ꢀmol), ClPh 1 mL, 100 ºC), – ionol (13.0
ꢀmol) added at the beginning of the reaction, and – ionol (13.0 ꢀmol) added after 20 min of
the reaction.
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On the other hand, the observed kinetic behavior is consistent with a heterolytic
mechanism where the first step is reversible interaction between hydroperoxide and CrIII
centers in the MOF producing an active oxidizing species (Equation 1), most likely an
alkylperoxo complex, and the second step is oxygen atom transfer from the peroxo species to
the aromatic substrate (S) to give oxygenated product (SO) (Equation 2).
k1
CrIII-OOR
CrIII + ROOH
CrIII-OOR + S
(1)
k-1
k2
(2)
CrIII + ROH + SO
Heterolytic type of mechanism was previously suggested for AN oxidation with acidic
dichromate and peracids [39].
Under the assumption that the formation of active form of the oxidant is rate limiting,
the reaction rate law can be described by Equation (3)
d[S]
dt
k1k2[S][ROOH][CrIII]
= −
(3)
k−1+ k2[S]
where [CrIII] is 2/3 of the amount of chromium centers on the catalyst surface normalized by
the reaction volume unit (as was mentioned in Introduction, two of three coordination
positions of Cr(III) are occupied by labile water molecules while one is occupied by F– which
is not labile and, hence, cannot be replaced with TBHP [23]). One can see that, depending on
the ratio of k-1 and k2[S], the reaction order in substrate may vary from 1 to 0. Under the
assumption that k2[S] >> k-1, we can simplify Equation (3) to Equation (4):
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d[S]
dt
= −k1[ROOH][CrIII]
(4)
Equation (4) is fully consistent with the kinetic regularities observed experimentally.
While AQ was the only observable reaction product at the end of the reaction (with
carbon mass balance of >99%), some minor amounts of anthrahydroquinone (AHQ, 6-8 %)
and oxanthrone (OAT, 4-5 %) were found by GC at the initial stages of the reaction along with
traces of compound (FW 194) that could be identified as 9-hydroxyanthracene (HAN) or
anthrone (AT). Hence, we may suppose a subsequent transfer of two oxygen atoms from the
active peroxo species to the aromatic substrate followed by dehydrogenation to produce the
final product, AQ (Scheme 1).
OH
O
and / or
AN
HAN
O
AT
O
OH
+
H
OH
OH
O
OAT
AQ
AHQ
Scheme 1. Possible route of transformation of AN to AQ
4. Conclusions
The kinetic and adsorption study performed in this work revealed that anthracene oxidation
with TBHP over both Cr-MIL-101 and MIL-100 is not controlled by diffusion of the reactants.
The established rate law and the values of activation energy (15 kcal/mol), coupled with the
fact that AN is not adsorbed on the MOF surface from ClPh solution at the reaction
temperature strongly support oxidation mechanism that involves rate-limiting interaction of
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TBHP with CrIII centers on the MOF surface to produce active oxidizing species (most likely,
alkylperoxo complex) followed by fast oxygen atom transfer from the oxidizing species to the
aromatic substrate to give primary oxygenated product(s) (9-hydroxyanthracene and/or
anthrone), further oxidation of which leads first to anthrahydroquinone and oxanthrone and
then to the target AQ.
Acknowledgments
We thank Dr. K.A. Kovalenko for providing samples of MIL-101 and MIL-100 and Dr. A.N.
Serkova for SEM measurements. Discussions of Dr. A.B. Sorokin and Dr. A.M. Khenkin are
greatly appreciated. The research was supported by the Russian Foundation for Basic
Research (grant 13-03-00413).
References
[1] A.E. Shilov, G.B. Shul’pin, Chem. Rev. 97 (1997) 2879–2932.
[2] A.E. Shilov, G.B. Shul’pin, Activation and Catalytic Reactions of Saturated Hydrocarbons
in the Presence of Metal Complexes, Kluwer Academic Publishers, Dordrecht, 2000.
[3] Sustainable Industrial Processes, F. Cavani, G. Centi, S. Perathoner and F. Trifiro (Eds.),
Wiley-VCH, Weinheim, 2009.
[4] Modern Heterogeneous Oxidation Catalysis: Design, Reactions and Characterization, N.
Mizuno (Ed.), Wiley-VCH, Weinheim, 2009.
[5] Liquid Phase Oxidation via Heterogeneous Catalysis: Organic Synthesis and Industrial
Applications, M.G. Clerici, O.A. Kholdeeva (Eds.), Wiley, Hoboken, New Jersey, 2013.
[6] J.L.C. Rowsell, O.M. Yaghi, Microporous Mesoporous Mater. 73 (2004) 3–14.
18

ACCEPTED MANUSCRIPT
[7] A.K. Cheetham, C.N.R. Rao, R.K. Feller, Chem. Commun. (2006) 4780–4795.
[8] G. Férey, Chem. Soc. Rev. 37 (2008) 191–214.
[9] A.U. Czaja, N. Trukhan, U. Muller, Chem. Soc. Rev. 38 (2009) 1284–1293.
[10] J.Y. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen, J. T. Hupp, Chem. Soc.
Rev. 38 (2009) 1450–1459.
[11] D. Farrusseng, S. Aguado, C. Pinel, Angew. Chem. Int. Ed. 48 (2009) 7502–7513.
[12] A. Corma, H. García, F.X. Llabrés i Xamena, Chem. Rev. 110 (2010) 4606–4655.
[13] M. Yoon, R. Srirambalaji, K. Kim, Chem. Rev. 112 (2011) 1196–1231.
[14] A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Catal. Sci. Technol. 1 (2011) 856–867.
[15] Thematic issue on metal-organic frameworks, Chem. Rev. 112 (2012) 673–1268.
[16] Y.K. Hwang, G. Férey, U-H. Lee, J.-S. Chang, in: M.G. Clerici, O.A. Kholdeeva (Eds.),
Liquid Phase Oxidation via Heterogeneous Catalysis: Organic Synthesis and Industrial
Applications, Wiley, Hoboken, NJ, 2013, pp. 371–409.
[17] M. Zhao, S. Ou, C.-D. Wu, Acc. Chem. Res. 47 (2014) 1199−1207.
[18] C. Wang, D. Liu, W. Lin, J. Am. Chem. Soc. 135 (2013) 13222−13234.
[19] J. Gascon, A. Corma, F. Kapteijn, F. X. Llabrés i Xamena, ACS Catal. 4 (2014) 361−378.
[20] C.M. Doherty, D. Buso, A.J. Hill, S. Furukawa, S. Kitagawa, P. Falcaro, Acc. Chem. Res.
47 (2014) 96–405.
[
21
]
N. V. Maksimchuk, O. V. Zalomaeva, I. Y. Skobelev, K. A. Kovalenko, V. P. Fedin, O.
468 (2012) 2017–2034.
A. Kholdeeva, Proc. R. Soc. A
,
[22] G. Férey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surblé, I. Margiolaki,
Science 309 (2005) 2040–2042.
[23] Y.K. Hwang, D.-Y.Hong, J.-S. Chang, H. Seo, M. Yoon, J. Kim, S. H. Jhung, C. Serre, G.
Férey, Appl. Catal. A: General 358 (2009) 249–253.
19

ACCEPTED MANUSCRIPT
[24] G. Ferey, C. Serre, C. Mellot-Draznieks, F. Millange, S. Surble, J.D.I. Margiolaki,
Angew. Chem. 116 (2004) 6456–6461.
[25] A. Henschel, K. Gedrich, R. Kraehnert, S. Kaskel, Chem. Commun. (2008) 4192–4194.
[26] O.V. Zalomaeva, A.M. Chibiryaev, K.A. Kovalenko, O.A. Kholdeeva, B.S.
Balzhinimaev, V.P. Fedin, J. Catal. 298 (2013) 179–185.
[27] J. Kim, S. Bhattacharjee, K.-E. Jeong, S.-Y. Jeong, W.-S. Ahn, Chem. Commun. (2009)
3904–3906.
[28] N.V. Maksimchuk, K.A. Kovalenko, V.P. Fedin, O.A. Kholdeeva, Chem. Commun
(2012) 6812–6814.
. 48
[29] N.V. Maksimchuk, K.A. Kovalenko, V.P. Fedin, O.A. Kholdeeva, Adv. Synth. Catal.
352 (2010) 2943–2948.
[30] I.Y. Skobelev, A.B. Sorokin, K.A. Kovalenko, V.P. Fedin, O.A. Kholdeeva, J. Catal. 298
(2013) 61–69.
[31] I.Y. Skobelev, K.A. Kovalenko, V.P. Fedin, A.B. Sorokin, O.A. Kholdeeva, Kinet. Catal.
54 (2013) 607–614.
[32] O. A. Kholdeeva, I. Y. Skobelev, I. D. Ivanchikova, K. A. Kovalenko, V. P. Fedin, A. B.
Sorokin, Catal. Today, 238 (2014) 54–61.
[33] P. Atkins, J. De Paula, Elements of Physical Chemistry, sixth edition, Oxford University
Press, 2013.
[34] S. H. Jhung, J.-H. Lee, J. W. Yoon, C. Serre, G. Férey, J.-S. Chang, Adv. Mater. 19
(2007), 121–124.
[35] D.-Y. Hong, Y. K. Hwang, C. Serre, G. Férey, J.-S. Chang, Adv. Funct. Mater
. 19 (2009)
1537–1552.
[36] K. Yang, Q. Sun, F. Xue, D. Lin, J. Hazard. Mater. 195 (2011) 124–131.
[37] A. Henschel, I. Senkovska, S. Kaskel, Adsorption, 17 (2011) 219–226.
20

ACCEPTED MANUSCRIPT
[38] O.V. Zalomaeva, K.A. Kovalenko, Yu.A. Chesalov, M.S. Mel’gunov, V.I. Zaikovskii,
V.V. Kaichev, A.B. Sorokin, O.A. Kholdeeva, V.P. Fedin, Dalton Trans. 40 (2011) 1441–
1444.
[39] C.K. Das, N. S. Das, J . Chem. Tech. Biotechnol. 39 (1987) 183–199.
[40] A. Khenkin, L. Weiner, Y. Wang, R. Neumann, J. Am. Chem. Soc.123 (2001) 8531–
8542.
[41] C. Pérollier, C. Pergrale-Mejean, A.B. Sorokin, New J. Chem. 29 (2005) 1400–1403.
[42] E.T. Denisov, N.I. Mitskevich, V.E. Agabekov, Liquid-Phase Oxidation of Oxygen-
Containing Compounds, Consultants Press, New York, 1977, Chapter 1.
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Kinetic of anthracene oxidation with TBHP over MIL-101(100) has been studied.
The reaction is zero order in substrate and first order in both oxidant and catalyst.
The activation energy is similar for MIL-101 and MIL-100.
The oxidation process is not controlled by diffusion of the reactants.
The rate-limiting step is interaction of TBHP with Cr(III) centers of MIL-101(100).