Liquid phase oxidation of n-octane to C8 oxygenates over modified Fe-MOF-5 catalysts

Article abstract of DOI:10.1016/j.catcom.2014.08.002

The Metal Organic Framework materials Fe₄-MOF-5, Fe ₂-Zn₂-MOF-5 and Fe_0.9-Zn₃. ₁-MOF-5 were prepared. The XRD results showed the expected pattern of MOF-5 with a very sharp peak at a 2θ value below 10° which indicate that the material is highly crystalline. The SEM and TEM images showed that the catalysts are cubic in shape. The synthesised Fe₄-MOF-5, Fe ₂-Zn₂-MOF-5 and Fe_0.9-Zn₃. ₁-MOF-5 were tested in the oxidation of n-octane using H ₂O₂ as an oxidant in acetonitrile as solvent. Conversions of 10.5, 4.2 and 3.6% were obtained for Fe₄-MOF-5, Fe ₂-Zn₂-MOF-5 and Fe_0.9-Zn₃. ₁-MOF-5 respectively. It was observed that primary carbon activation increased with decreasing Fe content, as C-1 selectivities of 9.5, 12.9 and 19.8% were achieved for Fe₄-MOF-5, Fe₂-Zn ₂-MOF-5 and Fe_0.9-Zn₃.₁-MOF-5 respectively. Only C8 oxygenate products were observed, which include 1-octanol, 2-octanol, 3-octanol, 4-octanol, octanal, 2-octanone, 3-octanone, 4-octanone and octanoic acid. Furthermore, these catalysts were tested in the oxidation of cyclohexane using H₂O₂ in acetonitrile. Selectivities of 48.3% for cyclohexanol, 47.1% for cyclohexanone and 4.6% for hydroperoxycyclohexane were recorded at a conversion of 40% using Fe ₄-MOF-5 as a catalyst.

Full text of DOI:10.1016/j.catcom.2014.08.002

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Liquid phase oxidation of n-octane to C8 oxygenates over modified Fe-MOF-5
catalysts
Mduduzi N. Cele, Holger B. Friedrich, Muhammad D. Bala
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DOI:
Reference:
S1566-7367(14)00306-9
doi: 10.1016/j.catcom.2014.08.002
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Catalysis Communications
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2 June 2014
29 July 2014
1 August 2014
Please cite this article as: Mduduzi N. Cele, Holger B. Friedrich, Muhammad D. Bala,
Liquid phase oxidation of n-octane to C8 oxygenates over modified Fe-MOF-5 catalysts,
Catalysis Communications (2014), doi: 10.1016/j.catcom.2014.08.002
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Liquid phase oxidation of n-octane to C8 oxygenates over modified Fe-
MOF-5 catalysts
Mduduzi N. Cele, Holger B. Friedrich* and Muhammad D. Bala
School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X 54001,
Durban, 4001, South Africa
Abstract
The Metal Organic Framework materials Fe4-MOF-5, Fe2-Zn2-MOF-5 and Fe0.9-Zn3.1-MOF-
5 were prepared. The XRD results showed the expected pattern of MOF-5 with a very sharp
peak at a 2θ value below 10° which indicate that the material are highly crystalline. The
SEM and TEM images showed that the catalysts are cubic in shape. The synthesised Fe4-
MOF-5, Fe2-Zn2-MOF-5 and Fe0.9-Zn3.1-MOF-5 were testing in the oxidation of n-octane
using H2O2 as an oxidant in acetonitrile as solvent. Conversions of 10.5, 4.2 and 3.6 % were
obtained for Fe4-MOF-5, Fe2-Zn2-MOF-5 and Fe0.9-Zn3.1-MOF-5 respectively. It was
observed that primary carbon activation increased with decreasing Fe content, as C-1
selectivities of 9.5, 12.9 and 19.8 % were achieved for Fe4-MOF-5, Fe2-Zn2-MOF-5 and
Fe0.9-Zn3.1-MOF-5 respectively. Only C8 oxygenate products were observed, which include
1-octanol, 2-octanol, 3-octanol, 4-octanol, octanal, 2-octanone, 3-octanone, 4-octanone and
octanoic acid. Furthermore, these catalysts were tested in the oxidation of cyclohexane using
H2O2 in acetonitrile. Selectivities of 48.3 % for cyclohexanol, 47.1 % for cyclohexanone and
4.6 % for hydroperoxycyclohexane were recorded at a conversion of 40 % using Fe4-MOF-5
as a catalyst.
Keywords: n-octane; cyclohexane; MOF-5; octanol; octanone; oxidation
1. Introduction
Recently, there has been much interest in MOF (metal organic framework) materials. These
types of materials come in different shapes and types and possess special and important
physical and chemical properties [1-7]. They tend to be softer that the traditional
aluminosilicate zeolite materials like ZSM-5, mordenite and faujasite, hence they resemble
* Address correspondence to fax: +27 31 260 3091; e-mails: friedric@ukzn.ac.za (HBF)

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more closely the behaviour of enzymes. There are porous materials with 3D structures of
interconnected metal and different inorganic linkers and have a very wide range of
applications, which include gas storage, catalysis etc. [8-12].
One of the most promising MOFs is MOF-5, which type of MOF materials have been
reported to have a high potential in the catalytic field [13]. An iron copper MOF-5 has been
reported in the oxidation of xanthene using t-butylhydroperoxide and the catalyst was found
to be stable and reusable under reaction conditions [14]. In this study we now report the
synthesis of Fe-MOF-5 and Fe-Zn-MOF-5 with different iron content and the testing of them
in the oxidation of n-octane and cyclohexane using H2O2 in acetonitrile solvent. The
activation of medium chain length alkanes is becoming increasingly important as these
paraffins are major low value by-products from gas and coal to liquid plants, the number of
which is increasing [15], and paraffins can offer green routes to valuable oxygenates.
Terminally functionalised alcohols are high value products and are difficult to achieve, since
secondary carbon atoms are more reactive that the primary ones. Systems reported that show
high selectivity to terminal carbon activation usually achieve this with very low conversion,
typically less than 1% [16].
2. Experimental
2.1 Materials
All the reagents and solvents used were commercially supplied (Sigma, Aldrich, Fluka and
Merck). The GC standards used and acetonitrile were of high purity. The acetonitrile was
purged with nitrogen before any catalytic testing to remove oxygen.
2.2 Synthesis of Fe-MOF-5
The synthesis of Fe-MOF-5 was achieved following the reported method [17] by mixing 1.2
g Fe(NO3)3 and 0.334 g terephthalic acid in 40 ml DMF under vigorous stirring followed by
addition of 2.2 ml TEA drop wise. The mixture was stirred for 2 hour when a brownish
precipitate was formed. The precipitate was filtered, washed and added to chloroform and
stood overnight. Thereafter, the precipitate was filtered and dried. The general formula of
MOF-5 is Zn4O13C24H12, and the coefficient for the metals must be kept at 4 molar
equivalents. Zn was added to balance the value of metals when the amount of Fe used was

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less than 4. The synthesis of the Fe-Zn-MOF-5 materials was conducted using the above
mentioned procedure with the inclusion of Zn(NO3)2 and the appropriate amount of Fe(NO)3.
2.3 Catalysts characterization
Fourier Transform-Infrared (FT-IR) spectroscopy data was obtained within the range of 4000
– 400 cm-1 using a Nicolet 400D spectrophotometer. Powder X-Ray Diffraction (XRD) was
performed using a Philips PW 1730/10 diffractometer, using Co Kα radiation, equipped with
a long line focus operating with amperage of 20 mA and voltage of 40 kV. Data was
collected in the range of 2 to 50° (2θ). Inductively Coupled Plasma - Optical Emission
Spectroscopy (ICP-OES) data was collected on a PerkinElmer (Optima 5300 DV) instrument.
Samples were digested in HCl and the analyses were done in triplicate Scanning Electron
Microscope (SEM) data was obtained using a Philips XL30 ESEM at 20 kV operating at a
low vacuum mode of 1 Torr. Transmission Electron Microscope (TEM) images were
obtained from a JEOL JEM 1010. Brunauer-Emmet-Teller (BET) surface area measurements
were obtained using a Micromeritics Gemini instrument. The catalysts were degassed
overnight under nitrogen at 200 °C.
2.4 Catalytic testing
All the reactions were performed under nitrogen. In a typical test run, 13 ml of acetonitrile
was added into a two-neck round bottom flask (100 ml) fitted with a stopper and a condensor.
The oxidation was performed by adding a mass of 0.1 g catalyst, 1 mL cyclohexane and 1
mL H2O2 (30 wt%) as an oxidant. The products were analysed by a PerkinElmer Auto
System GC equipped with Flame Ionisation Detector using a Pona 50 x 0.20 mm x 0.5 μm
column. The residual H2O2 was determined by titrating 1 mL of the reaction solution
acidified with 6M H2SO4 with 0.1 M KMnO4. The carbon balance for all oxidation reactions
was found to be +/- 98 % (above 98 % of the reacted n-octane corresponded to the total moles
of products formed on a carbon basis).
3. Results and discussion
3.1 Catalyst characterisation
The FT-IR results (Table 1) show a peak around 1570 cm-1 which is due to carboxylate
anions in the catalyst material. The peak of the –COOH group of the terephthalic acid