Propylene glycol oxidation with tert-butyl hydroperoxide over Cr-containing metal-organic frameworks MIL-101 and MIL-100

Article abstract of DOI:10.1016/j.cattod.2016.04.008

Chromium-based metal-organic frameworks MIL-101 and MIL-100 catalyzed oxidation of propylene glycol (PG) using tert-butyl hydroperoxide (TBHP) as oxidant. The main oxidation product was hydroxyacetone (HA) while minor oxidation products were acetaldehyde and acetic acid. The best results (88% selectivity toward HA at ca. 10% PG conversion) were achieved over MIL-101 using acetonitrile as solvent. In water, C[sbnd]C bond cleavage products predominated. MIL-101 and MIL-100 revealed different behavior toward adsorption of PG from MeCN solution, which affected their catalytic performances. Additives of radical chain scavengers produced a strong rate-inhibiting effect whereas the presence of molecular oxygen accelerated the reaction, suggesting radical chain mechanism of the oxidation process. The oxidation of PG over MIL-101 at 50?°C proceeded without leaching of the active metal into solution and the catalysis had the truly heterogeneous nature. The MIL-101 catalyst could be recycled without significant loss of activity and selectivity and preserved its structure during, at least three, catalytic cycles.

Full text of DOI:10.1016/j.cattod.2016.04.008

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Propylene glycol oxidation with tert-butyl hydroperoxide over
Cr-containing metal-organic frameworks MIL-101 and MIL-100
Viktoriia V. Torbina , Irina D. Ivanchikovaa,b, Oxana A. Kholdeevaa,b, Igor Y. Skobelevb,
a
a,∗
Olga V. Vodyankina
a
Tomsk State University, Lenin Ave. 36, Tomsk 634050, Russia
Boreskov Institute of Catalysis, Lavrentiev Ave. 5, Novosibirsk 630090, Russia
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Chromium-based metal-organic frameworks MIL-101 and MIL-100 catalyzed oxidation of propylene
glycol (PG) using tert-butyl hydroperoxide (TBHP) as oxidant. The main oxidation product was hydrox-
yacetone (HA) while minor oxidation products were acetaldehyde and acetic acid. The best results (88%
selectivity toward HA at ca. 10% PG conversion) were achieved over MIL-101 using acetonitrile as solvent.
In water, C C bond cleavage products predominated. MIL-101 and MIL-100 revealed different behavior
toward adsorption of PG from MeCN solution, which affected their catalytic performances. Additives of
radical chain scavengers produced a strong rate-inhibiting effect whereas the presence of molecular oxy-
gen accelerated the reaction, suggesting radical chain mechanism of the oxidation process. The oxidation
Received 28 December 2015
Received in revised form 12 April 2016
Accepted 15 April 2016
Available online xxx
Keywords:
Propylene glycol
Selective oxidation
Hydroxyacetone
◦
of PG over MIL-101 at 50 C proceeded without leaching of the active metal into solution and the catalysis
Metal-organic frameworks
tert-Butyl hydroperoxide
had the truly heterogeneous nature. The MIL-101 catalyst could be recycled without significant loss of
activity and selectivity and preserved its structure during, at least three, catalytic cycles.
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
find application in the production of fine chemicals and pharma-
ceuticals, can be obtained through oxidation of PG (Scheme 1).
PG oxidation in the gas phase over iron phosphate catalysts
afforded methyl glyoxal as the main product [9,10]. Vapor phase
dehydrogenation of vicinal diols, including PG, to produce ␣-
hydroxyketones was accomplished using various copper catalysts
[11]. PG reacted with molecular oxygen to form HA as the initial
product and methylglyoxal via subsequent oxidation of HA on Ag
catalysts (500–700 K) [12,13]. The majority of publications on PG
oxidation in the liquid phase have been devoted to the production
of lactic acid using Pd-, Pt- and Au-containing catalysts in alkaline
conditions [14–18]. Very few works reported liquid-phase selective
oxidation of PG to HA, pointing out that this reaction is accompa-
nied by C C bond cleavage processes, which decreases selectivity
toward acetol [19,20]. In particular, Sheldon and Dakka oxidized
PG to HA using H O as oxidant and titanium-silicalite TS-1 as het-
The selective oxidation of alcohols and diols to carbonyl com-
pounds plays a key role in organic synthesis [1]. Traditional
methods for such transformations involve the use of stoichiomet-
ric inorganic oxidants, notably, chromium(VI) reagents [2,3]. In
the past decades, a range of homogeneous catalyst systems have
been developed for the selective oxidation of OH functional groups
[
4]. However, from both economic and environmental viewpoints,
there is a growing demand for atom efficient catalytic methods
employing environmentally benign oxidants and truly heteroge-
neous catalysts that can be easily recovered and recycled [5,6].
Conversion of bio-renewable feedstocks to valuable products
remains a formidable challenge [7]. Propylene glycol (PG) is one of
base chemicals that is traditionally produced from propylene oxide
but can also be generated from glycerol, a low-cost by-product in
transesterification of triglycerides for biodiesel production [8]. A
number of value-added products, such as hydroxyacetone (HA, also
called acetol), methylglyoxal, lactic acid and pyruvic acid, which
2
2
erogeneous catalyst [19]. Kholdeeva et al. have demonstrated that
this reaction can be accomplished with H O2 using a Ti-containing
2
sandwich type polyoxometalate as homogeneous catalyst [20].
Although HA is used in medicine, food, textile, cosmetic and
polymer industries, the scope of its application is still limited by its
high cost [21]. The reason for that is employment of non-catalytic
multistep technologies that deliver HA with poor selectivity and
require expansive purifying steps. Therefore, the development of
∗ Corresponding author.
E-mail address: vodyankina o@mail.ru (O.V. Vodyankina).
http://dx.doi.org/10.1016/j.cattod.2016.04.008
0
920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: V.V. Torbina, et al., Propylene glycol oxidation with tert-butyl hydroperoxide over Cr-containing
metal-organic frameworks MIL-101 and MIL-100, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.04.008

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2. Experimental
2.1. Materials
Propylene glycol (Component-reactiv, 99%) and ethyl acetate
(Himabsolut, 98%) were distilled under vacuum and atmospheric
pressure, respectively. Other reagents were purchased from Sigma
Aldrich and used without further purification. All organic solvents
were dried and stored over activated 4 A molecular sieves. TBHP
was used either as a 5.5 M solution in decane or as a 70 wt% solu-
tion in water. The concentration of active oxygen before and after
catalytic reactions was determined by iodometric titration.
2.2. Catalysts preparation and characterization
Cr-MIL-101 was synthesized by the solvothermal method fol-
lowing the procedure of Férey et al. [28] with some modifications
reported elsewhere [36]. Synthesis of Cr-MIL-100 was performed
according to the literature [29] following the protocol described
previously [37]. Activation of MOFs was carried out in vacuum at
◦
◦
1
50 C for 5 h and then at 180 C for 2 h.
Scheme 1. Oxidative transformations of PG.
Textural characteristics of the catalysts were determined from
nitrogen adsorption isotherms (77 K; Sorbtometr M instrument).
◦
Before measurements the samples were degassed at 180 C under
vacuum during 6 h. MIL-101 and MIL-100 revealed BET surface area
2
−1
3
−1
effective catalytic systems for the selective oxidation of PG to HA
using environmentally friendly oxidants and easily recyclable het-
erogeneous catalysts remains a demanding task.
of 3100 and 2100 m g and pore volume of 2.1 and 0.9 cm g ,
respectively. The structure of the MIL-101 and MIL-100 materi-
als was confirmed by XRD (Philips APD1700 X-ray diffractometer).
XRD patterns of the samples of Cr-MIL-100 and Cr-MIL-101 versus
simulated X-ray patterns of these MOFs are shown in Fig. S1 in
Supporting Information (SI). The preservation of the MOF struc-
ture was also verified by FT-IR spectroscopy (SCIMITAR FTS 2000
spectrometer). Corresponding FT-IR spectra are given in Fig. S2.
Metal-organic frameworks (MOFs), a new emerging class of
crystalline materials that consist of coordination bonds between
nodes of metal ions and multidentate organic linkers, have
attracted much recent attention as functional materials for
adsorption, gas separation and storage techniques, as well as het-
erogeneous catalysis [22–26]. MOFs possess a unique combination
of properties that includes extremely high surface areas and pore
volumes, tunable pore size and functionalities, and large fraction of
uniform and accessible metal sites. However, relatively low thermal
and hydrothermal stability may constrain their practical applica-
tions, in particular, in gas-phase heterogeneous catalysis [25,27].
Férey et al. reported the synthesis of mesoporous chromium-
based carboxylates MIL-101 ([Cr X(H O) O(BDC) ]; X = F, OH;
2.3. Catalytic experiments and product analysis
Catalytic oxidation tests were carried out in a temperature-
controlled glass vessel at 40–70 C under vigorous stirring
◦
(500 rpm). Typically, the reaction was initiated by the addition
of TBHP to a mixture containing PG (0.5 or 1 mmol), 1 ml of sol-
vent and 3 mg of catalyst. Aliquots of the reaction mixture were
withdrawn periodically by a syringe through a septum and ana-
lyzed by GC (Chromatec Crystal 5000.1, flame ionization detector,
30 m × 0.22 mm × 0.5 ␮m ZB-WAX capillary column, chloroben-
zene as internal standard), GC-MC (Agilent 7000B system with
triple-quadrupole mass-selective detector Agilent 7000 and GC
Agilent 7890B, 30 m × 0.25 mm × 0.25 ␮m ZB-WAX capillary col-
umn), and HPLC (Agilent 1220 instrument, ZORBAX Eclipse Plus
C18 Analytical 4.6 × 150 mm 5-␮m column). Methyl glyoxal was
determined as ortho-phenylenediamine derivative by GC [39]. Each
experiment was reproduced 3–4 times. NMR spectra were collected
using Bruker Avance-400 spectrometer (400.13 MHz).
In recycling experiments, the catalyst was filtered off after the
reaction, washed with acetonitrile, dried at room temperature
overnight, and then reused. The amount of chromium in filtrates
was determined by inductively coupled plasma atomic emission
spectroscopy (Agilent 4100 microwave spectrometer). After catal-
ysis, Cr-MIL-101 was characterized by XRD, FT-IR and DTA/TGA
3
2
2
3
BDC = 1,4-benzenedicarboxylate; MIL stands for Materiaux de
l’Institute Lavosier) [28] and MIL-100 ([Cr X(H O) O(BTC) ]; X = F,
3
2
3
2
OH; BTC = 1,3,5-benzenetricarboxylate) [29], which demonstrated
good resistance to air, water, common solvents and thermal treat-
◦
ment (up to 275 C in air). Both MIL-101 and MIL-100 consist of
´
quasi-spherical mesoporous cages of two types (34 and 29 A˚ in MIL-
´
1
01 versus 29 and 25 A˚ in MIL-100) which are accessible through
´
microporous windows (16 and 12 A˚ in MIL-101 versus 8.6 and
´
5
.5 A˚ in MIL-100). The amount of Cr is 23 and 26 wt.% in MIL-101
and MIL-100, respectively. Coordinatively unsaturated metal sites
CUS) can be easily created in the structure of these MOFs by heat-
(
ing in vacuum [30]. All these make MIL-100 and, especially, larger
pore MIL-101 attractive candidates for catalysis in the liquid phase
[
31,32]. In particular, selective oxidations of cyclohexane [33] and a
range of unsaturated hydrocarbons [34–38] could be accomplished
using tert-butyl hydroperoxide (TBHP) and/or molecular oxygen as
oxidant.
The aim of this work was to explore the potential of the
chromium-based MOFs, MIL-101 and MIL-100, for the selective
oxidation of PG. The effects of the oxidant nature and reaction con-
ditions (temperature, concentrations of reagents, solvent nature,
and atmosphere) on the product selectivity and yield have been
investigated. Special attention was paid to determine the nature of
catalysis and evaluate catalyst reusability.
(
NETZH STA 449F1 Jupiter coupled mass-spectrometer QMS 403D
Aeolos, heating 10 K/min from 25 to 400 C, Ar, Al O crucible).
◦
2
3
2
.4. Adsorption measurements
All adsorption measurements were carried out at room tem-
perature (25 C). A concentrated (3.3 M) solution of an organic
compound (PG, HA, MeOH or AcOH) was added by portions (30 ␮l)
◦
Please cite this article in press as: V.V. Torbina, et al., Propylene glycol oxidation with tert-butyl hydroperoxide over Cr-containing
metal-organic frameworks MIL-101 and MIL-100, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.04.008

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Table 1
PG oxidation with TBHP over Cr-containing catalysts.
Catalyst
Conversion, % Product yield,a
%
HA selectivity, %
PG TBHP HA
AAc
AA
on PG
on TBHP
53
Cr-MIL-101
Cr-MIL-100
Cr3(Ac)7(OH)2
No catalyst
10
11
20
3
65
4
87
∼0
8.8
7.6
11.4 3.7
2.1 0.1
1.0
1.0
0.2
0.1
0.5
88
b
69
>100
57 (69)c
52
n.d.
d
traces 70
Reaction conditions: 1 mmol PG, 0.25 mmol TBHP, 1 ml MeCN, PG/Cr = 76 (mol/mol),
◦
5
0
C, 7 h for MIL-101, MIL-100 and blank experiment and 5 h for Cr3(Ac)7(OH)2
(
time when PG conversion and HA yield reached their maximum values).
GC yield based on starting PG. Other identified products were formic acid,
a
formaldehyde and acetone.
b
>
100% selectivity formally calculated based on TBHP is due to a significant role
of O2 in the oxidation process.
c
In parentheses, at 10% PG conversion.
Not determined.
d
to a solution containing 2 ml of CH CN, 8 ␮l of n-octane (inter-
3
Fig. 1. PG oxidation with TBHP over Cr-containing catalysts: Cr-MIL-101—filled, Cr-
MIL-100—open, and Cr3(Ac)7(OH)2—half open symbols. Reaction conditions as in
Table 1.
nal standard) and 20 mg of activated Cr-MIL-101(100). After the
addition of each portion, the mixture was stirred for 5 min (this
time is enough to reach the adsorption equilibrium [36]) and then
MOF was separated by centrifugation (6000 rpm, 15 min) and the
concentration of PG (HA, MeOH or AcOH) was determined by GC
(
3–5 measurements) using a gas chromatograph Chromos GC-1000
equipped with a flame ionization detector and a quartz capil-
lary column BPX5 (30 m × 0.25 mm). Parallel, blank experiments
(
without MOF) were performed. The amount of adsorbed PG (HA,
MeOH or AcOH) was determined as a difference between con-
centrations of the organic compound in the solutions with and
without MOF multiplied on the solution volume. Adsorption con-
stants were estimated using Langmuir (PG and HA) or Fowler
(
methanol) adsorption model.
3
. Results and discussion
3.1. Comparison of the catalytic and adsorption behavior of
Cr-MIL-100 and Cr-MIL-101
Fig. 2. Adsorption of PG from MeCN solution on Cr-MIL-100 (᭿), its fitting with
Langmuir model (᭹), and adsorption of PG on Cr-MIL-101 (ꢀ).
To suppress overoxidation processes, we evaluated the cat-
alytic performance of Cr-MIL-100 and Cr-MIL-101 in PG oxidation
under conditions of the oxidant deficiency ([PG]/[TBHP] = 1/0.25).
Similar conditions were employed earlier for the TS-1/H O sys-
selectivity for HA was higher for MIL-101 (88% versus 69% for Cr(III)
acetate). As one can see from Fig. 1, the initial rate of PG consump-
tion over MIL-100 was higher than over MIL-101. The attainable
level of substrate conversion was similar for both MOFs, but MIL-
101 was more selective toward HA formation. The reaction stopped
completely after 6–7 h although the oxidant consumption was not
complete (see data in Table 1).
2
2
tem [19]. Table 1 presents the results on PG oxidation acquired
over the Cr-MOFs in comparison with homogeneous chromium(III)
acetate, Cr (CH COO)7(OH) . In all cases, HA was the principle reac-
3
3
2
tion product while the main byproducts were acetic acid (AAc)
and acetaldehyde (AA) derived from C C bond cleavage. Minor
amounts of formic acid, formaldehyde and acetone were identi-
fied by GC–MS. While the former two products certainly originated
from C C cleavage, the latter could, in principle, be formed from PG
via both oxidation [40] and acid-catalyzed [41] routes. Moreover,
homolytic TBHP decomposition can also result in acetone forma-
tion [42]. Neither methyl glyoxal nor pyruvic acid, which might
arise in the course of HA overoxidation, was found among the reac-
tion products. The oxidation of the primary hydroxyl group of PG
leading to lactaldehyde and lactic acid did not take place either.
Note that PG conversion did not exceed 3% in the absence of any
catalyst (see Table 1).
The corresponding kinetic curves for PG consumption and accu-
mulation of the main products over the MOFs and homogeneous
Cr(III) catalyst are given in Fig. 1. Although homogeneous Cr(III)
acetate was more active than both Cr-MOFs and led to higher
PG conversion (20% versus 10–11% with MOFs), it gave increased
amounts of the C C cleavage products, thereby resulting in lower
HA selectivity. Even at a similar level of PG conversion (ca. 10%),
The oxidant utilization efficiency, i.e. HA selectivity based on
TBHP consumed, was similar for MIL-101 and Cr(III) acetate and
attained 52–53%. In this respect, the catalytic performance of MIL-
100 was different: a very low conversion of TBHP was observed
in the presence of this MOF and PG (see Table 1), leading formally
to >100% HA selectivity based on TBHP. This indicates that TBHP
was not the sole oxidant in the system and molecular oxygen con-
tributed significantly into the oxidation process (vide infra).
To rationalize the different catalytic performance of MIL-100
and MIL-101, we investigated adsorption of PG on these MOFs. The
corresponding adsorption isotherms are shown in Fig. 2. Surpris-
ingly, only MIL-100 adsorbs PG from acetonitrile solution while
MIL-101 does not. As one can judge from Fig. 2, Langmuir model
provides a fairly good description of the experimental isotherm
−
1
◦
for MIL-100 (Kads. = 80 ± 23 M at 25 C). The reasons for the dif-
ferent adsorption behavior of MIL-100 and MIL-101 are not yet
completely clear. The structure of the primary chromium trimers
and the mode of the arrangement of super tetrahedra (MTN struc-
Please cite this article in press as: V.V. Torbina, et al., Propylene glycol oxidation with tert-butyl hydroperoxide over Cr-containing
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Table 3
4
Table 2
Effect of solvent nature on PG oxidation with TBHP over MIL-101.
Effect of PG/TBHP molar ratio on PG oxidation over Cr-MIL-101.
Solvent
Time,a
h
Conversion, %
Product yield,b
%
[TBHP], M
Conversion, %
Yield on PG,a
%
HA selectivity, %
HA
AAc
AA
PG
TBHP
HA
AAc
AA
on PG
on TBHP
CH3CN
CH3COOC2H5
CH3OH
7
5.5
2
7
7
10
14
5
8
9
8.8
7.2
3.1
1.6
7.5
7.2
1.6
1.0
4.5
0.3
3.0
1.1
1.2
1.5
0.2
0.8
traces
0.3
0.1
0.125
0.25
0.5
7
86
65
48
26
6.0
8.8
8.2
9.3
0.8
1.0
1.2
1.3
0.05
0.2
0.4
86
88
68
85
57
53
34
>100
10
12
11
b
H2O
CH3CN
0.25
0.3
c
◦
Reaction conditions: 1 mmol PG, 1 ml MeCN, 3 mg Cr-MIL-101, 50 C, 7 h.
d
CH3CN/H2O
7
7
9
8
0.1
0.2
a
CH3CN/H2Oe
GC yield based on starting PG.
b
6
mg Cr-MIL-101.
Reaction conditions: 1 mmol PG, 0.25 mmol TBHP, 1 ml solvent, 3 mg Cr-MIL-101,
◦
5
0
a
C.
Time when PG conversion and HA yield reached their maximum values.
GC yield based on starting PG.
TBHP was used as 70% aqueous solution (0.5 M H2O in the reaction mixture).
concentration of water in the system was 0.5 M) produced insignif-
icant increase in the yield of C C cleavage products (Table 2). The
augmentation of water concentration up to 3.9 M resulted in only
slight reduction of PG conversion and HA yield, indicating that rel-
atively small amounts of water are not detrimental for the title
reaction. On the other hand, large amounts of water in MeCN
(1:1 vol/vol) led to a drastic decrease of HA yield, increase of AcOH
yield and general deterioration of the carbon mass balance (see
Table 2).
b
c
d
e
3
.9 M H2O in CH3CN.
MeCN/H2O 1:1 vol/vol.
ture) are the same for both MIL-100 and MIL-101, but the specific
linker affects the structure and size of the super tetrahedron. This
results in the different geometrical parameters of the MOF struc-
ture, which in turn, might be responsible for the different affinity
toward PG molecules (or their associates [43]). In contrast to PG,
no adsorption of HA from acetonitrile was found on both MIL-100
3.3. Effect of PG/TBHP molar ratio
◦
and MIL-101 at 25 C. Yet, no adsorption of acetic acid was estab-
To verify the effect of PG to TBHP molar ratio on the PG oxi-
lished. These facts allowed us to suggest that neither adsorption of
the principal oxidation product (HA) nor adsorption of the main
detected by-product (AcOH) can be responsible for the catalyst
deactivation.
On the other hand, a significant adsorption of methanol from
MeCN was detected for Cr-MIL-101. The corresponding isotherm
dation over MIL-101, the oxidant concentration in the reaction
mixture was varied. The results are shown in Table 3. Selectivity
for HA based on both PG and TBHP remained quasi constant in the
range of 0.125–0.25 M of the oxidant but then it tended to decrease,
most likely because HA overoxidation became significant. This was
confirmed by the experiment where HA was used as the substrate
instead of PG. In this case, acetic acid and acetaldehyde were the
main products which formed with the yields of 8 and 1.4%, respec-
tively, at HA conversion of 11% after the characteristic time of 7 h.
Like in the oxidation of PG, no methyl glyoxal was found in the
reaction mixture.
Although TBHP in all cases was used in deficiency relative to PG,
its conversion did not reach 100% and decreased with increasing
initial hydroperoxide concentration. In turn, PG conversion did not
exceed 56% relative to the theoretical value that could be achieved if
TBHP were completely used for PG oxidation. Since the adsorption
study showed that no evident adsorption of HA occurs on the MOFs
from MeCN solution (Fig. 2), we could assume that the incomplete
conversion of the reagents might be caused by strong adsorption of
by-products, for example, carboxylic acids. However, the adsorp-
tion study showed that no significant adsorption of AcOH takes
place on MIL-101 from MeCN solution. Moreover, additives of AcOH
into the reaction mixture produced no rate-retarding effects. On
the contrary, additives of formic acid led to decreasing the reaction
rate and attainable level of PG conversion (Fig. S6 in SI). Conse-
quently, adsorption of HCOOH on Cr-MIL-101 can be responsible
for the observed catalyst deactivation. The DTA/TGA data coupled
with mass-spectrometry acquired for the sample of MIL-101 sepa-
rated after the oxidation reaction corroborated desorption of formic
acid together with formaldehyde and acetaldehyde.
(
Fig. S3 in SI) has an S-type shape, which is typical of Fowler model.
This may indicate that, with increasing coverage of MIL-101 surface
with methanol, its adsorption becomes easier, most likely, due to
formation of hydrogen bonds between methanol molecules in the
adsorbed layer. The consequences of the adsorption peculiarities
for the catalytic performance of the MOFs will be discussed in the
following sections.
3.2. Effect of solvent nature
Given that higher selectivity toward HA was attained over Cr-
MIL-101, this MOF was preferably used in further experiments.
Three organic solvents, acetonitrile, ethyl acetate and methanol, as
well as water were used to evaluate the effect of solvent nature
on PG oxidation over Cr-MIL-101. The results are presented in
Table 2. The highest conversion of PG (14%) was attained in ethyl
acetate. However, larger amounts of the C C bond cleavage prod-
ucts formed in this solvent, resulting in lower selectivity for HA. In
methanol, the reaction stopped after 2 h and PG conversion did not
exceed 5%. A possible reason for that could be strong adsorption of
methanol on Cr-MIL-101. Indeed, we found that this phenomenon
does take place (see Fig. S3). Consequently, when methanol is used
as solvent, its adsorption may prevent interaction of PG or TBHP
with the catalyst surface. At the same time, methanol can act as
inhibitor in radical chain processes.
The reaction took place in water and attained 8% conversion
after 7 h, but HA selectivity and total carbon mass balance were
poor. Acetic acid predominated among the identified products (see
3.4. Effect of reaction atmosphere and radical scavengers
The presence of molecular oxygen can be essential for oxida-
tive transformations over MOFs when TBHP is used as oxidant
[33]. Indeed, involvement of O2 in the PG oxidation process was
unambiguously indicated by the >100% selectivity based on TBHP
that had been achieved over MIL-100 (see Table 1). Participation
of molecular oxygen in the PG oxidation was further confirmed
by experiments carried out under inert atmosphere using MIL-101
Table 2). We may suggest that the presence of water facilitates C
C
bond cleavage and formation of unidentified condensation prod-
ucts. Importantly, the structure of MIL-101 remained intact after
the reaction in water, as evidenced by XRD and FT-IR techniques
(
Figs. S4 and S5, respectively, in SI). On the other hand, use of aque-
ous solution of TBHP instead of anhydrous one (in this case the
Please cite this article in press as: V.V. Torbina, et al., Propylene glycol oxidation with tert-butyl hydroperoxide over Cr-containing
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Table 4
Effect of reaction atmosphere on PG oxidation over Cr-MIL-101.
Atmosphere Conversion, % Yield on PG,a
%
HA selectivity, %
PG
TBHP HA
AAc
AA
on PG
on TBHP
Air
Ar
10
4
65
15
8.8
3.5
1
0.3
0.2
0.05
traces n.d.
traces 77
88 (77)b 53
88
93
c
d
Air
Air
∼0.5 n.d.
traces traces
2.3 0.2
n.d.
n.d.
e
3
n.d.
Reaction conditions: 1 mmol PG, 0.25 mmol TBHP (if any), 1 ml MeCN, 3 mg Cr-MIL-
◦
1
01, 50 C, 7 h.
a
GC yield based on starting PG.
In parentheses, at 4% PG conversion.
No TBHP was added.
Not determined.
b
c
d
e
0
.05 mmol TBHP.
Fig. 4. PG oxidation over Cr-MIL-101 with (open) and without (filled symbols) addi-
tives of ionol (6.6 ␮mol). Reaction conditions: 0.5 mmol PG, 0.125 mmol TBHP, 3 mg
Cr-MIL-101, 1 ml MeCN.
the rate of decomposition being just somewhat higher for MIL-101
[
38]. In this work, we also measured the rate of TBHP decomposi-
tion in MeCN in the absence of PG and found a similar activity for
both MIL-100 and MIL-101. Therefore, we may tentatively assume
that the pronounced adsorption of PG on MIL-100 depicted in Fig. 2
disfavors adsorption of TBHP and blocks its access to the active Cr
centers, which suppresses TBHP activation through its decompo-
sition and enhances the contribution of molecular oxygen into the
overall oxidation rate over MIL-100.
The product distribution under argon was close to that real-
ized in air, indicating that the oxidation mechanism does not
change drastically in the absence of molecular oxygen. Dissimilar
to what has recently been observed for aerobic indene oxidation
[
44], no reaction occurred in the absence of TBHP. Moreover, if we
introduced small additives of TBHP (0.05 mmol) into the reaction,
conversion of PG in this case did not exceed 3% over both MIL-100
and MIL-101 (for the latter, see data in Table 4), which might indi-
cate a more complicated role of TBHP than to be a simple initiator.
Additives of the conventional radical chain scavenger, ionol,
produced a strong rate-retarding effect (Fig. 4). Coupled with the
disclosed role of molecular oxygen, this strongly supports radi-
cal chain mechanism for PG oxidation over MIL-101 and MIL-100.
According to the results of the adsorption experiment (Fig. 2), no PG
adsorption occurs on Cr-MIL-101 from MeCN, which excludes acti-
vation of the substrate molecule on the active sites of the catalyst.
All these facts collectively allowed us to suggest that PG oxidation,
at least over MIL-101, involves TBHP homolytic decomposition on
Cr(III) centers, giving rise to radicals which further react with sub-
strate molecules through preferable abstraction of H atom bound
to C2 atom.
Fig. 3. PG oxidation with TBHP over MIL-101 (a) and MIL-100 (b) in air (filled sym-
bols) and under Ar (open symbols). In (a) half-open symbols correspond to reaction
in air without TBHP.
3.5. Effect of temperature and nature of catalysis
and MIL-100. The results of these experiments are presented in
Table 4 (MIL-101) and Fig. 3 (MIL-101 in comparison with MIL-
The title reaction is very slow at room temperature (only 2%
PG conversion after 7 h). The increase in the reaction temperature
◦
1
00). Both substrate and TBHP conversions differed significantly
from 40 to 70 C accelerated the reaction and increased the attain-
from the experiment under air. The difference in the initial reaction
rates under Ar and air was more pronounced for MIL-100 than for
MIL-101 (compare kinetic curves in Fig. 3a and b). Furthermore, the
reaction in inert atmosphere over MIL-100 revealed an induction
period while no induction period was observed in air (see Fig. 1).
These results agree with the low TBHP conversion observed over
MIL-100 in the presence of PG (see Table 1). Previously, we found
that MIL-100 and MIL-101 are active in TBHP decomposition in
chlorobenzene in the absence of any oxidizable organic substrate,
able PG conversion; however, it had very little effect on the product
selectivity. The initial rate of PG oxidation exhibited a typical Arrhe-
nius dependence (Fig. 5). The activation energy (Ea) estimated from
the corresponding plot equals to 67 kJ/mol. Such value is typical
for reactions controlled by chemical interaction, because for reac-
tions that are controlled by diffusion of reactants, the value of Ea
is typically below 17 kJ/mol [45]. It is noteworthy that similar Ea
(63 kJ/mol) was previously found for anthracene oxidation with
TBHP over Cr-MIL-100 and Cr-MIL-101, where interaction between
Please cite this article in press as: V.V. Torbina, et al., Propylene glycol oxidation with tert-butyl hydroperoxide over Cr-containing
metal-organic frameworks MIL-101 and MIL-100, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.04.008

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CATTOD-10156; No. of Pages7
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6
Fig. 5. Arrhenius plot for PG oxidation with TBHP over MIL-101.
Fig. 7. Recycling of Cr-MIL-101 in PG oxidation with TBHP. Reaction conditions as
in Table 1.
all the catalytic studies presented in this work (Tables 1–4 were
◦
carried out at 50 C.
3.6. Catalyst stability and reusability
Cr-MIL-101 features fairly good thermal [28] and hydrothermal
[
46] stability and can act as a truly heterogeneous catalyst under
oxidative conditions at elevated temperatures [31–38]. However,
the polarity of substrate and products as well as their concentra-
tions and the reaction temperature can be critical for the catalyst
stability. As was mentioned above leached chromium species
◦
(
34 ppm) were found in the filtrate during PG oxidation at 70 C
and significant contribution of homogeneous catalysis was estab-
lished in this case by the hot filtration test. For this reason, we
carried out recycling experiments at 50 C, under the conditions of
◦
Fig. 6. Hot catalyst filtration test at 50 C. Filled and open symbols correspond to
◦
kinetic curves acquired without and with catalyst separation, respectively.
the truly heterogeneous catalysis. As one can judge from Fig. 7, the
MIL-101 catalyst could be recycled, at least 3 times, without loss of
the catalytic activity and selectivity. It is noteworthy that no cata-
lyst reactivation was needed before reuse. The catalyst was simply
separated by filtration after PG conversion reached its maximum
value (ca. 10%), then washed, dried in air and used in the next run
with a new portion of the reagents. The retention of the MIL-101
structure was confirmed by FT-IR and XRD techniques. The XRD
patterns of the catalyst before and after three operation cycles are
shown in Fig. 8 while Fig. S2 in the SI presents the corresponding
IR spectra.
TBHP and Cr(III) centers within the MOF framework to produce
an active oxidizing species was suggested as the rate-limiting step
[
38]. This fact coupled with the results of the adsorption experi-
ments that had been discussed in Section 3.1 allowed us to make
an assumption that activation of TBHP may also determine the rate
of the overall oxidation process. Further kinetic studies would be
useful to verify this hypothesis.
Previously, some of us demonstrated that reaction temperature
may strongly affect the nature of catalysis over MOFs [36]. Indeed,
we found that the PG oxidation proceeded with the same rate after
◦
catalyst removal by hot filtration at 70 C. The amount chromium
4. Conclusions
in the filtrate determined by atomic emission spectroscopy was
3
4 ppm, indicating substantial leaching of active Cr species under
In this work, we demonstrated that the chromium-based metal-
organic frameworks MIL-101 and MIL-100 are able to catalyze
selectively oxidation of propylene glycol using TBHP as oxidant.
The reaction proceeds with high regioselectivity for the secondary
alcohol group. No oxidation of the primary hydroxyl group of PG
leading to lactaldehyde and lactic acid occurs. Selectivity toward
hydroxyacetone attains 88% at ca. 10% PG conversion with MIL-101
as catalyst and acetonitrile as solvent; the oxidant utilization effi-
ciency is 53–57%. Water facilitates C C bond cleavage processes.
With homogeneous chromium acetate, PG conversion can reach
20% under similar reaction conditions, but selectivity for HA was
lower than with MIL-101 during the overall course of the reaction.
The pronounced impacts of radical chain scavengers and molecu-
lar oxygen on the reaction rate suggest radical chain mechanism of
the oxidation process. The adsorption study revealed that MIL-101
◦
the reaction conditions. After catalyst separation at 60 C, the reac-
tion did not stop completely in the filtrate but it continued with a
lower rate, although the chromium amount in the filtrate did not
exceed 0.1 ppm. At the same time, a blank, catalyst-free experiment
at 60 C revealed some PG oxidation to HA under the conditions
employed. Hence, the observed continuation of the reaction after
catalyst filtration at 60 C may be caused by thermal activation of
the oxidant. A similar conclusion was drawn for tetralin oxidation
with TBHP over MIL-101 [34]. In contrast, the hot catalyst filtration
◦
◦
◦
test performed at 50 C (Fig. 6) showed that both PG consumption
and HA accumulation stopped completely after separation of the
catalyst, which confirmed the truly heterogeneous nature of the
catalysis over Cr-MIL-101 at this temperature. Again, the amount
of chromium in the filtrate did not exceed 0.1 ppm. For this reason,
Please cite this article in press as: V.V. Torbina, et al., Propylene glycol oxidation with tert-butyl hydroperoxide over Cr-containing
metal-organic frameworks MIL-101 and MIL-100, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.04.008

G Model
CATTOD-10156; No. of Pages7
ARTICLE IN PRESS
V.V. Torbina et al. / Catalysis Today xxx (2016) xxx–xxx
7
[
[
[
7] P.Y. Dapens, C. Mondelli, Renez-Ramirez, J. Chem. Soc. Rev. 44 (2015)
025–7043.
8] G. Knothe, J. Krahl, J.V. Gerpen, The Biodisel Handbook, AOCS, Champaign,
005.
9] M. Ai, K. Ohdan, J. Mol. Catal. 159 (2000) 19–24.
7
2
[
[
10] M. Ai, A. Motohashi, S. Abe, Appl. Catal. A: Gen. 246 (2003) 97–102.
11] S. Sato, R. Takahashi, T. Sodesawa, H. Fukuda, T. Sekine, E. Tsukuda, Catal.
Commun. 6 (2005) 607–610.
[
[
12] J. Shen, W. Shan, Y. Zhang, J. Du, H. Xu, K. Fan, W. Shen, Y. Tang, J. Catal. 237
(
2006) 94–101.
13] J. Shen, W. Shan, Y. Zhang, J. Du, H. Xu, K. Fan, W. Shen, Y. Tang, Chem.
Commun. (2004) 2880–2881.
[
[
14] L. Prati, M. Rossi, J. Catal. 176 (1998) 552–560.
15] N. Dimitratos, J.A. Lopez-Sanchez, S. Meenakshisundararam, J.M.
Anthonykutty, G. Brett, A.F. Carley, S.H. Taylor, D.W. Khight, G.J. Hutchings,
Green Chem. 11 (2009) 1209–1216.
[
[
16] M.B. Griffin, A.A. Rodriguez, M.M. Montemore, G.R. Monnier, Ch. T. Williams,
J.W. Meldin, J. Catal. 307 (2013) 111–120.
17] Y. Ryabenkova, Q. He, P.J. Miedziak, N.F. Dummer, S.H. Taylor, A.F. Carley, D.J.
Morgan, N. Dimitratos, D.J. Willock, D. Bethell, D.W. Knight, D. Chadwick, Ch.
Kiely, G.J. Hutchings, Catal. Today 203 (2013) 139–145.
[
[
[
18] M. Besson, P. Gallezot, C. Pinel, Chem. Rev. 114 (2014) 1827–1870.
19] R.A. Sheldon, J. Dakka, Erdöel Erdgas Kohle 109 (1993) 520–522.
20] O.A. Kholdeeva, B.G. Donoeva, T.A. Trubitsina, G. Al-Kadamany, U. Kortz, Eur. J.
Inorg. Chem. 34 (2009) 5134–5141.
Fig. 8. XRD patterns of MIL-101 before and after 3 catalytic runs (reaction conditions
as in Table 1).
[
21] N.H. Mohamad, R. Awang, W.M.Z.W. Yunus, Am. J. Appl. Sci. 8 (2011)
1135–1139.
[
[
22] J. Long, O. Yaghi, Chem. Soc. Rev. 38 (2009) 1257–1283.
23] Functional metal-organic frameworks: gas storage, separation and catalysis,
in: M. Schr o¨ der (Ed.), Top. Curr. Chem., vol. 293, Springer, 2010.
and MIL-100 possess different affinity toward PG, and this has con-
sequences for their catalytic performances. The strong adsorption
of PG on MIL-100, most likely, hinders adsorption of the oxidant,
TBHP, on the chromium active centers. The nature of catalysis
[24] Metal-Organic Frameworks: Applications from Catalysis to Gas Storage, in: D.
Farrusseng (Ed.), Wiley, Weinheim, 2011.
[
25] Metal Organic Frameworks as Heterogeneous Catalysts, in: F. Llabrés i
Xamena, J. Gascon (Eds.), RCS Publishing, 2013.
◦
depends on the reaction temperature. At 50 C, the nature of the
catalysis over MIL-101 is truly heterogeneous and no metal leach-
ing into solution occurs. The MIL-101 catalyst can be recycled with
maintenance of the catalytic performance and the MOF structure
during, at least, three operation cycles.
[26] H.-C. Zhou, S. Kitagawa, Chem. Soc. Rev. 43 (2014) 5415–5418.
[
[
27] A. Corma, H. García, F.X. Llabrés i Xamena, Chem. Rev. 110 (2010) 4606–4655.
28] G. Férey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surble, I.
Margiolaki, Science 309 (2005) 2040–2042.
[29] G. Férey, C. Serre, C. Mellot-Drazniecs, F. Millange, S. Surble, J.D.I. Margiolaki,
Angew. Chem. 116 (2004) 6456–6461.
[
30] 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: Gen. 358 (2009) 249–253.
Acknowledgements
[
31] Y.K. Hwang, G. Férey, U.-H. Lee, J.-S. Chang, Liquid phase oxidation of organic
compounds by metal-organic frameworks, in: M.G. Clerici, O.A. Kholdeeva
The authors thank Dr. K. A. Kovalenko for the synthesis of MIL-
00 and Dr. A. N. Shmakov and V. A. Utkin for XRD and GC–MS
measurements, respectively. The research was supported by “The
Tomsk State University Academic D.I. Mendeleev Fund Program”
grant (8.2.07.2015) in 2015–2016 and partially (I.D.I., I.Y.S. and
O.A.K.) supported by the Russian Foundation for Basic Research
(
Eds.), Liquid Phase Oxidation via Heterogeneous Catalysis: Organic Synthesis
1
and Industrial Applications, Wiley, New Jersey, 2013, pp. 371–410.
32] N.V. Maksimchuk, O.V. Zalomaeva, I.Y. Skobelev, K.A. Kovalenko, V.P. Fedin,
O.A. Kholdeeva, Proc. R. Soc. A 468 (2012) 2017–2034.
33] N.V. Maksimchuk, K.A. Kovalenko, V.P. Fedin, O.A. Kholdeeva, Chem.
Commun. 48 (2012) 6812–6814.
34] J. Kim, S. Bhattacharjee, K.-E. Jeong, S.-Y. Jeong, W.-S. Ahn, Chem. Commun.
(2009) 3904–3906.
35] N.V. Maksimchuk, K.A. Kovalenko, M.S. Melgunov, V.P. Fedin, O.A. Kholdeeva,
Adv. Synth. Catal. 352 (2010) 2943–2948.
[
[
[
[
[
[
[
[
(
grant No. 13-03-00413).
Appendix A. Supplementary data
36] I.Y. Skobelev, A.B. Sorokin, K.A. Kovalenko, V.P. Fedin, O.A. Kholdeeva, J. Catal.
2
98 (2013) 61–69.
37] O.A. Kholdeeva, I.Y. Skobelev, I.D. Ivanchikova, K.A. Kovalenko, V.P. Fedin, A.B.
Sorokin, Catal. Today 238 (2014) 54–61.
38] I.D. Ivanchikova, I.Y. Skobelev, O.A. Kholdeeva, J. Organomet. Chem. 793
(2015) 175–181.
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.04.
08.
0
39] G.V. Mamontov, O.V. Magaev, A.S. Knyazev, O.V. Vodyankina, Catal. Today 203
(
2013) 122–126.
References
[
[
40] E. Diaz, M.E. Sad, E. Iglesia, ChemSusChem 3 (2010) 1063–1070.
41] T.D. Courtney, V. Nikolakis, G. Mpourmpakis, J.G. Chen, D.G. Vlachos, Appl.
Catal. A: Gen. 449 (2012) 59–68.
[42] N.M. Emanuel, E.T. Denisov, Z.K. Maizus, Liquid Phase Oxidation of
Hydrocarbons, Plenum, New York, 1967.
[43] L.H. Thomas, R. Meatyard, J. Chem. Soc. A: Inorg. Phys. Theor. (1966) 92–96.
[44] A. Santiago-Portillo, S. Navalo ı´ n, F.G. Cirujano, F.X. Llabre ı´ s i Xamena, M.
Alvaro, H. Garcia, ACS Catal. 5 (2015) 3216–3224.
[45] P. Atkins, J. De Paula, Elements of Physical Chemistry, 6th ed., University
Press, Oxford, 2013.
[
[
[
[
[
[
1] R.A. Sheldon, J.K. Kochi, Metal-Catalyzed Oxidations of Organic Compounds,
Academic Press, New York, 1981.
2] G. Cainelli, G. Cardillo, Chromium Oxidations in Organic Chemistry, Springer,
Berlin, 1984.
3] G. Tojo, M. Fernández, Oxidation of Alcohols to Aldehydes and Ketones,
Springer, New York, 2006.
4] R. Sheldon, I.W.C.E. Arends, U. Hanefeld, Green Chemistry and Catalysis,
Wiley-VCH, Weinheim, 2007.
5] Fine Chemicals through Heterogeneous Catalysis, in: R.A. Sheldon, H. van
Bekkum (Eds.), Wiley-VCH, Weinheim, 2001.
6] Liquid Phase Oxidation via Heterogeneous Catalysis: Organic Synthesis and
Industrial Applications, in: M.G. Clerici, O.A. Kholdeeva (Eds.), Wiley, New
Jersey, 2013.
[46] J.J. Low, A.I. Benin, P. Jakubczak, J.F. Abrahamian, S.A. Faheem, R.R. Willis, J.
Am. Chem. Soc. 131 (2009) 15834–15842.
Please cite this article in press as: V.V. Torbina, et al., Propylene glycol oxidation with tert-butyl hydroperoxide over Cr-containing
metal-organic frameworks MIL-101 and MIL-100, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.04.008