The Co-MOF-74 modified with N,N″-Dihydroxypyromellitimide for selective, solvent free aerobic oxidation of toluene

Article abstract of DOI:10.1016/j.catcom.2018.03.021

The present research reports on a successful strategy for the modification of a cobalt-containing metal-organic framework (Co-MOF-74 aka Co-CPO-27) with NDHPI for enhanced catalytic activity in the aerobic oxidation of alkyl-benzenes under mild reaction c

Full text of DOI:10.1016/j.catcom.2018.03.021

Accepted Manuscript
The Co-MOF-74 modified with N,N′-Dihydroxypyromellitimide
for selective, solvent free aerobic oxidation of toluene
Oleksii Pliekhov, Olena Pliekhova, Urška Lavrenčič Štangar,
Nataša Zabukovec Logar
PII:
DOI:
Reference:
S1566-7367(18)30113-4
doi:10.1016/j.catcom.2018.03.021
CATCOM 5362
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Catalysis Communications
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Accepted date:
13 December 2017
15 March 2018
19 March 2018
Please cite this article as: Oleksii Pliekhov, Olena Pliekhova, Urška Lavrenčič Štangar,
Nataša Zabukovec Logar The Co-MOF-74 modified with N,N′-
,
Dihydroxypyromellitimide for selective, solvent free aerobic oxidation of toluene. The
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ACCEPTED MANUSCRIPT
The Co-MOF-74 Modified With N,N'-Dihydroxypyromellitimide For Selective, Solvent Free Aerobic Oxidation of
Toluene.
Oleksii Pliekhova,Olena Pliekhovab, Urška Lavrenčič Štangarc,b; Nataša Zabukovec Logara,b
aNational Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia
bUniversity of Nova Gorica, Vipavska 13, 5000 Nova Gorica, Slovenia
cUniversity of Ljubljana, Faculty of Chemistry and Chemical Technology, Vecna pot 113, 1000 Ljubljana, Slovenia
E-mail: oleksii.pliekhov@ki.si

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Abstract
The present research reports on a successful strategy for the modification of a cobalt-containing metal-organic framework
(Co-MOF-74 aka Co-CPO-27) with NDHPI for enhanced catalytic activity in the aerobic oxidation of alkyl-benzenes under
mild reaction conditions. We present here the synthesis and characterization of the Co-MOF-74@NDHPI composite. The
experimental results show the successful incorporation of NDHPI molecules into the Co-MOF-74 metal-organic
framework. The application of the Co-MOF-74@NDHPI composite in the heterogeneous solvent-free oxidation of toluene
with molecular oxygen as primary oxidant showed a high catalytic performance with 16% conversion and more than 30%
selectivity towards benzoic acid in 2 hours at 100 ºC, O2: 1.6 MPa.

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1. Introduction
Catalysts have been widely applied in the main technologies for the development of the world economy. Among which,
the selective oxidation of carbon-hydrogen bonds using dioxygen as a “green” oxidant is of crucial importance in allowing
the sustainable activation of raw materials to form valuable chemicals and intermediates in many important industrial
processes, one of which is the selective oxidation of hydrocarbons to corresponding oxygenated products[1]. Oxygen is
the most abundant and available oxidizing agent. Although it is of high commercial interest its direct utilization for
industrial applications is highly limited due to the high activation energy and as a result harsh reaction conditions[2], thus
catalysts are typically required. N-hydroxyphthalimide (NHPI) has been introduced as an effective catalyst for C-H
oxidation[3]. NHPI acts as a precursor of phthalimido-N-oxyl (PINO) radical, which is the effective abstracting species in
all of the free radical processes. During the past few decades a combination of NHPI and cobalt salts was reported to be a
valuable catalytic system for aerobic oxidation and functionalization of a large number of organic substrates [5,6].
One of the straightforward approaches in the catalytic oxidation processes is the impregnation of N-
hydroxyphthalimide (NHPI), a cheap and nontoxic organocatalyst, in the pores of metal-organic framework materials [7]
which simultaneously combine properties that are highly desirable for the design of heterogeneous catalysts: a large
surface area, high porosity and high metal content. Such composite materials show superior performance in selective
oxidations of different organic compounds [8–15] Although, the research is focusing mostly on modification of Cu and Fe
MOFs with NHPI, it has been postulated in homogeneous catalysis that cobalt is the most effective initiator of PINO
radical formation [4], moreover the multi‐nitroxy organocatalysts such as N,N'-Dihydroxypyromellitimide (NDHPI) showed
enhanced catalytic oxidation efficiencies in most cases, even if smaller amounts of catalysts are used[16]. On this basis,
Co-MOF-74 was chosen as a carrier and co-catalyst for the NDHPI molecule. It provides 1.1-1.2 nm large pores [17]
enabling appropriate mass transfer and thus it is suitable for modification with large molecules. Herein, we report the
preparation and characterization of Co-MOF-74@NDHPI composite, which can be used as an effective heterogeneous
catalyst in the solvent-free oxidation of toluene.
2. Experimental
2.1. Materials
All the reagents and solvents used were commercially supplied (Sigma, Aldrich, Alfa Aesar). The GC standard used
and the methanol were of high purity.
2.2. Synthesis of Co-MOF-74
Co-MOF-74 was synthesized using the conventional solvothermal method following a procedure reported in the
literature [18]: a solid mixture of H4DHBDC (2,5-dihydroxy benzenedicarboxylic acid, 0.729 mmol) and Co(NO3)2·6H2O
(cobalt nitrate hexahydrate, 2.45 mmol) was added to a 1:1:1 mixture of DMF-ethanol-water (60 mL) in a 100 mL Duran
bottle, and stirred for 1 h until it became homogeneous. The reaction vial was capped tightly and placed in an oven at 100
ºC. After 24 h, the bottle was removed from the oven and allowed to cool to room temperature. The MOF crystals were
separated by filtration and soaked in methanol for 24 hours under stirring, in order to exchange the DMF that remained in
the pores of the material. Methanol was replaced three times during the procedure.
2.3. Preparation of Co-MOF-74@NDHPI
First, a synthesis of NDHPI was performed according to a procedure reported in the literature [19]: Hydroxylamine
hydrochloride (38.4 mmol) and trimethylamine (Et3N) (38 mmol) were dissolved in 120 mL of absolute ethanol. After
stirring for 10 min, a 1,2,4,5-benzenetetracarboxylic anhydride (19.6 mmol) was added to the solution, which was then
refluxed for 8 h. The resulting red solution was poured into ca. 200 mL of H2O. The product precipitated as a yellow
powder, which was filtered and vacuum-dried at 50 ºC.
In the second step, 0.2 g (0.8 mmol) of NDHPI was dissolved in 10 mL of methanol : acetonitrile mixture (1:1). To this
solution, 1 g (3.2 mmol) of Co-MOF-74 was added. Prior the addition, Co-MOF-74 was activated at 200°C for 5h under a
vacuum. The resulting suspension was heated at 50 ºC overnight. After that, the mixture was cooled and the solvent was
evaporated. The resulting material was then dried at 80 ºC.
2.4. Catalyst characterization
Powder X-Ray diffraction (XRD) was performed on a PANalytical X´Pert PRO diffractometer with Cu Kα radiation
(λ=1.5406 Å). in the 2θ range from 5º to 50º (100 s per step of 0.034º) using a fully opened X’Celerator detector.
Thermogravimetric analysis (TGA) was performed using a Q5000IR (TA Instruments, Inc.) device under air flow (25
mL min−1) from 25 to 700 ºC, with a heating rate of 10 ºC min-1.
The IR spectra of the samples dispersed in KBr pellets were recorded using a PerkinElmer FTIR Spectrum 100
spectrometer with a 4 cm-1 resolution in the frequency range of 4000–400 cm-1.
A Zeiss SUPRA 35 VP scanning electron microscope (SEM) was used for microstructural analysis.

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2.5. Adsorption measurements
Nitrogen adsorption was performed using a Micromeritics Tristar 3000 automated gas adsorption analyzer at −196 °C
before the measurement samples were degassed in vacuum at 200 ºC for 5h. The specific surface area of the solid
samples was calculated using the multiple-point Brunauer–Emmett–Teller (BET) method in the relative pressure range of
P/P0 = 0.05–0.25. The total pore volumes were determined as the volume of liquid nitrogen adsorbed at a relative
pressure of 0.95.
Toluene and benzene sorption analysis was performed using an IGA-100 gravimetric analyzer (Hiden Isochema Ltd.).
Toluene sorption isotherms were measured at 25 °C in a relative pressure range from 0 to 0.9. Prior to the
measurements, the samples were degassed at 200 °C in a vacuum.
2.6. Typical procedure of toluene oxidation
Toluene (5 g) and the pre-prepared catalyst (0.4 g) were placed into a 100 mL Parr mini benchtop reactor (model 4565).
The reactions were carried out at 100 ºC in an oxygen atmosphere (1.6 MPa) for 2 hours. After that, the catalyst was
filtered and washed with chloroform. The reaction mixture was analysed using a Varian 3900 gas chromatograph
equipped with a Varian CP-Porabound U column with a diameter of 0.32 mm and a length of 25 m. Quantification was
performed by using chlorobenzene as the GC standard. A given amount of chlorobenzene was added to a known aliquot
of the sample and the solution was injected into the GC machine to determine the percentage selectivity and the
percentage conversion.
3. Results and discussion
3.1. Catalyst characterization
The Co-MOF-74@NDHPI composite was obtained via the wet impregnation technique and along with the starting Co-
MOF-74 material, it was characterized by XRD, FT-IR and TGA, Figures 1, 2, and 3 respectively.
The XRD patterns of Co-MOF-74 and Co-MOF-74@NDHPI are very similar and are in good agreement with the
pattern reported for Co-MOF-74 elsewhere in the literature [20,21]. Both samples show good crystallinity with strong
characteristic peaks at 2 theta: 6.8; 11.9; 16.8; 18.1; 21.6 degree, representing a single phase of Co-MOF-74. It is clear
that the impregnation of NDHPI into Co-MOF-74 did not change the crystal structure.
Co-MOF-74
Co-MOF-74@NDHPI
5
10
15
20
25
30
35
40
2 theta, degree
Figure 1. XRD patterns of the Co-MOF-74 and Co-MOF-74@NDHPI samples.
The FTIR spectra of the Co-MOF-74 and Co-MOF-74@NDHPI samples shows typical bands of carboxyl groups
located at 1650-1550 cm-1 and 1400-1350 cm-1 corresponding to asymmetric and symmetric stretching, respectively. The
broad band with the maximum at 3280 is related to OH groups of coordinated water molecules. The absence of the
characteristic peak at around 2970 cm-1 indicates that the DMF solvent was effectively removed. The appearance of the
new peaks with maximums at 1780, 1710, 900, 700 and 550 cm-1 at Co-MOF-74@NDHPI spectrum indicates that the
NDHPI was successfully impregnated.

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Co-MOF-74
Co-MOF-74@NDHPI
4000
2800
1600
400
Wavenumber, cm-1
Figure 2. FTIR spectra of the Co-MOF-74 and Co-MOF-74@NDHPI samples.
Figure 3 compares the thermogravimetric profiles of the Co-MOF-74 and Co-MOF-74@NDHPI samples. The
thermogram of the Co-MOF-74 sample shows two regions matching those published elsewhere[22] for Co-MOF-74
material in terms of the magnitude of the weight loss and the order of thermal stability. The first weight loss (22 %) region
between 30 and 150 ºC indicates the loss of moisture and perfectly coincides with the first weight loss of the Co-MOF-
74@NDHPI sample. The second weight loss region starts at 250 ºC, where the structure of Co-MOF-74 collapses and the
organic linker is burned. The maximum temperature of the linker decomposition at Co-MOF-74 is in good agreement with
the literature [22]. Modification with NDHPI slightly increases the maximum temperature of the ligand decomposition from
290 ºC in Co-MOF-74 to 315 ºC due to the higher maximum temperature of the NDHPI decomposition (320 ºC) Fig.S1
(supporting information). The TGA results indicate that the impregnated amount of NDHPI in the modified Co-MOF-74
is ~5%, which is in agreement with the calculated loading.
100
Co-MOF-74
80
Co-MOF-74@NDHPI
60
40
20
0
0
200
400
T , °C
600
Figure3. TGA curves of the Co-MOF-74 and Co-MOF-74@NDHPI samples.
The toluene adsorption isotherms (Figure 5) show a 10 wt.% decrease in the total uptake of toluene on Co-MOF-74
sample after modification with NDHPI, from 34 wt.% for Co-MOF-74 to 24 wt% for Co-MOF-74@NDHPI. Further, the

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adsorption of benzene, which has similar properties to toluene and is one of the common sorbates used for determining
the specific surface area, was applied to determine the BET surface area and the total volume of the pores of the
samples. The obtained results are listed in Table 1. It can be seen that after the incorporation of NDHPI, the surface area
and total volume of the pores in Co-MOF-74@NDHPI sample decreased sharply in comparison with unmodified Co-MOF-
74. Moreover, the results on pore size distribution Fig. S2 (supporting information) indicate decrease in the relative pore
filling for composite material in the range of average pore diameter 10 – 20 Å. These findings were further supported by
the N2 sorption measurements (Table 1). The results show the same trend in the decrease of surface characteristics of
the samples and provide significant evidence of the internal location of the NDHPI in composite material.
40
30
20
Co-MOF-74
10
Co-MOF-74@NDHPI
0
0
0.2
0.4
0.6
0.8
1
P/P0
Figure 4. Toluene adsorption isotherms of the samples.
Table 1: BET surface area and pore volume of the samples.
SBET, m2/g
N2
1197
736
Vp, cm3/g
N2
0,4
0,2
Sample
Benzene
347
190
Benzene
0,2
0,1
Co-MOF-74
Co-MOF-74@NDHPI
3.2. Aerobic oxidation of toluene
The results of the solvent-free aerobic oxidation of toluene are presented in Figure 5. Initially, a blank run in the
absence of the catalyst was performed, which showed no activity in the solvent-free aerobic oxidation of the toluene.
When Co-MOF-74 or NDHPI separately were employed as the catalysts, the reaction proceeded and resulted in 4% and
2% toluene conversion, respectively. The reaction products in those cases were only benzyl alcohol (Bz.OH) and
benzaldehyde (Bz.CHO) with no signs of benzoic acid (Bz.COOH). The conversion increased remarkably when Co-MOF-
74@NDHPI composite material was applied as a catalyst, the toluene conversion value was 16% along with suitable
selectivity on benzoic alcohol and benzaldehyde (18 and 48%) as well as 34% selectivity to benz oic acid, which is
significantly higher in comparison with the previously published [12] experimental results on the solvent-free aerobic
oxidation of toluene using the Cu-BTC@NHPI catalytic system under similar reaction conditions. In order to evaluate the
effect of NDHPI impregnation into Co-MOF-74 on catalytic activity, a control experiment was performed using a physical
mixture of Co-MOF-74 and NHPI with the same stoichiometry as in the composite material. The results of the catalytic
performance showed a 10% conversion of toluene and similar to the experiments with a single NDHPI and Co-MOF-74 as
catalyst product distribution with no traces of benzoic acid. Such a result in the conversion and selectivity can be
attributed to the sum of the catalytic effects of the mixture components and indicate the absence of interaction between
the NDHPI and Co-MOF-74 in contrast with the composite material Co-MOF-74@NDHPI where the synergistic effect is
clearly evidenced. This effect can be explained by the formation of catalytically active complexes between NDHPI and

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open metal sites in activated Co-MOF-74. The crystal structure of the MOF-74 represents a 1-D honeycomb topology,
with pores of 1.1–1.2 nm in diameter. The metal coordination sphere is occupied by six oxygen atoms, one of which
comes from a solvent water molecule[21]. By applying heat under a vacuum, one can successfully remove water from the
coordination sphere developing coordinatively unsaturated (open) metal sites[23], which can be utilized for further post-
synthetic modifications [24]. NDHPI as an analogue of NHPI is able to enter the coordination sphere of cobalt as a ligand,
creating catalytically active complexes [25,26]. Thus it is reasonable to speculate that in the case of activated Co-MOF-74,
NDHPI with a molecular size of around 0.5x1.1 nm can be accommodated inside the intracrystalline space of the MOF,
which favours the increasing oxidation properties of Co-MOF-74.
28
Bz.OH
Bz.CHO
Bz.COOH
Toluene
18
%
34
%
24
20
16
12
8
48
%
29
%
71
%
28
%
30
%
72
%
70
%
4
0
NDHPI
Co-MOF-74
Co-MOF-74/NDHPI
(mechanical mixture)
Co-MOF-74@NDHPI
(impregnation)
Figure 5. Toluene conversion and product selectivity (circle histograms) obtained after 2 h of oxidation at 100ºC, w ith O2 :1.6 MPa using various
catalysts.
The structural stability of the recovered Co-MOF-74@NDHPI catalyst was checked. After the first reaction cycle, the
catalyst was filtered off, washed with chloroform and oven-dryed at 60ºC for 12h. Obtained sample was investigated with
XRD and FTIR techniques. The results are shown in Fig S3 and S4 (supporting information). All characteristic peaks of
the used catalysts are well consistent with the fresh ones, indicating excellent stability of the used Co-MOF-74@NDHPI
catalyst under reaction conditions.
Conclusions
We have demonstrated that the composite materials based on cobalt metal-organic frameworks containing a guest
molecule of multi‐nitroxy organocatalyst NDHPI in the intracrystalline space is an effective catalyst for the process of the
controlled oxidation of toluene in the absence of additional solvents under mild reaction conditions. The use of cobalt and
NDHPI leads to a remarkable enhancement in process efficiency in comparison with previously reported analogues.
Acknowledgement
This research was supported by the Slovenian Research Agency, research program P1-0021.

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Graphical abstract

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Highlights
 Cobalt containing MOF-74 modified with N,N'-Dihydroxypyromellitimide (NDHPI) was
synthesized.
 Co-MOF-74@NDHPI was used as catalyst for aerobic oxidation of toluene.
 Impregnation of NDHPI leads to enhancement in oxidation efficiency.
 Impregnation of NDHPI favours the increasing oxidation properties of Co-MOF-74.