Effective and selective oxidation of 2-butanol over Mn supported catalyst systems

Article abstract of DOI:10.1016/j.apcata.2016.08.005

Oxidation of alcohols to their corresponding aldehydes/ketones is an important reaction in industries as well as in academic perspective. Selective oxidation (Selox) of alcohols like methanol, ethanol and propanol are well studied in literature; however, alcohols like butanol, pentanol, octanol is a challenging task. Selective oxidation of 2-butanol to methyl ethyl ketone (MEK) is an important reaction due to its wide range of applications. Herein, we demonstrated the selective oxidation of 2-butanol to MEK over Mn supported on different oxide supports. A series of Mn_xO_y-Al₂O₃ (MA), Mn_xO_y-CeO₂ (MC), Mn_xO_y-ZrO₂ (MZ) and Mn_xO_y-SiO₂ (MS) catalysts were prepared by co-precipitation followed by hydrothermal method. As synthesised catalysts were characterised by various physico-chemical characterisation techniques. It was found that the presence of Mn₃O₄ species in MA and MZ catalysts is responsible for maximum catalytic activity towards 2-butanol oxidation. MA catalyst conferred a maximum 2-butanol conversion of 51% and 88% selectivity towards MEK. XPS analysis revealed that Mn in MA catalyst exists in +2 and +3 oxidation states and responsible for 2-butanol oxidation. Moreover it was found that the acidity of the catalyst also plays an important role in catalytic activity.

Full text of DOI:10.1016/j.apcata.2016.08.005

Accepted Manuscript
Title: Effective and selective oxidation of 2-butanol over Mn
supported catalyst systems
Author: Kandasamy Prabu Marimuthu Prabu Ashok Kumar
Venugopal W.V.Y. Sai Sandilya Chinnakonda S. Gopinath
Thirumalaiswamy Raja
PII:
S0926-860X(16)30412-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2016.08.005
APCATA 15968
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
9-6-2016
28-7-2016
3-8-2016
Please cite this article as: Kandasamy Prabu, Marimuthu Prabu, Ashok Kumar
Venugopal, W.V.Y.Sai Sandilya, Chinnakonda S.Gopinath, Thirumalaiswamy Raja,
Effective and selective oxidation of 2-butanol over Mn supported catalyst systems,
Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2016.08.005
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Effective and selective oxidation of 2-butanol over Mn supported
catalyst systems
Kandasamy Prabu, Marimuthu Prabu , Ashok Kumar Venugopal, WVY Sai Sandilya,
Chinnakonda S. Gopinath* and Thirumalaiswamy Raja*
Catalysis Division and PSE division , CSIR-National Chemical Laboratory, Dr. Homi
Bhabha Road, Pune- 411 008, India. E-mail: t.raja@ncl.res.in ;Tel: (+) 91-20-25902006;Fax:
(+) 91-20-25902633.
Author Information
Corresponding author
Thirumalaiswamy Raja*
Present addresses
Catalysis Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune-
411 008, Maharashtra, India. E-mail: t.raja@ncl.res.in; Tel: (+) 91-20-25902006, Fax: (+) 91-
20-25902633
1

GRAPHICAL ABSTRACT
Highlights:
 A series of MnxOy‐Al2O3 (MA), MnxOy‐CeO2 (MC), MnxOy‐ZrO2 (MZ) and MnxOy‐SiO2
(MS) catalysts were prepared by co‐precipitation followed by hydrothermal method
 Selective oxidation of 2‐butanol to MEK over Mn supported on different oxide
supports was studied
 Presence of Mn3O4 species in MA and MZ catalysts is responsible for maximum
catalytic activity towards 2‐butanol oxidation
 MA catalyst conferred a maximum 2‐butanol conversion of 51 % and selectivity of 88
% towards MEK
Abstract: Oxidation of alcohols to their corresponding aldehydes/ketones is an important
reaction in industries as well as in academic perspective. Selective oxidation (Selox) of
alcohols like methanol, ethanol and propanol are well studied in literature; however,
alcohols like butanol, pentanol, octanol is a challenging task. Selective oxidation of 2-
butanol to methyl ethyl ketone (MEK) is an important reaction due to its wide range of
applications. Herein, we demonstrated the selective oxidation of 2-butanol to MEK over Mn
supported on different oxide supports. A series of MnxOy-Al2O3 (MA), MnxOy-CeO2 (MC),
MnxOy-ZrO2 (MZ) and MnxOy-SiO2 (MS) catalysts were prepared by co-precipitation
followed by hydrothermal method. As synthesised catalysts were characterised by various
physico-chemical characterisation techniques. It was found that the presence of Mn3O4
2

species in MA and MZ catalysts is responsible for maximum catalytic activity towards 2-
butanol oxidation. MA catalyst conferred a maximum 2-butanol conversion of 51 % and 88
% selectivity towards MEK. XPS analysis revealed that Mn in MA catalyst exists in +2 and
+3 oxidation states and responsible for 2-butanol oxidation. Moreover it was found that the
acidity of the catalyst also plays an important role in catalytic activity.
Keywords: Selective Oxidation, 2-butanol, Methyl Ethyl ketone, Mn based catalysts
1. Introduction
Oxidation of alcohols to their corresponding aldehydes/ketones is one of the
important processes in chemical industries, as well as in academia, for the production of
value added chemicals. Conventionally, aldehydes/ketones were prepared by the oxidation of
alcohols using stoichiometric amount of oxidants, such as salts of permanganates [1],
chromates [2,3] and bromates [4]. Due to the generation of enormous quantity of metal
wastes, conventional methods are replaced by catalytic method for the production of
aldehydes/ketones. In homogeneous catalytic method, high catalytic activity towards desired
product was achieved; however, it has a major drawback in separation and reuse of the
catalysts. To overcome these difficulties, heterogeneous catalytic method was used. It plays a
vital role in separation and reuse of the catalysts; but the challenging task was the synthesis of
catalysts with high selectivity towards desired products. Preparing high active catalysts and
selective towards desired products with sustainability is an interesting area of research in
heterogeneous catalysis. Hence, it provides further details about the physical and chemical
properties of the catalysts.
Alcohols such as methanol, ethanol, propanol and butanol are the raw materials for
the production of renewable and sustainable energy sources [5].Compared to lower carbon
chain alcohols such as methanol, ethanol and propanol, butanol shows less hygroscopic, high
energy density, low vapor pressure, and low affinity to water [6-9]. n-Butanol is one of the
promising biofuels, which has received much attention due to its renewability over other
biofuels [5]. Methyl ethyl ketone (2-butanone) (MEK) is one of the important products,
which has wide spectrum of applications in industries. Commercially, MEK is produced by
dehydrogenation of 2-butanol using copper, zinc catalysts but it requires harsh reaction
3

conditions [10-11]. There are some other processes examined for the production of MEK
such as oxidation of 2-butene [12] and oxidation of isobutyl benzene [13]; however, these
processes are not commercialized. Owing to its less impact to environment and mild reaction
conditions, oxidation of 2-butanol using an oxidant is an alternative method for production of
MEK. In such a process alternative oxidant gases are like air, CO2, O2 and N2O is co-fed to
oxidize butanol to 2-butanone. There are seldom a few reports for the production of MEK
using above said oxidants. Mistry et al. reported that catalytic activity of 2-butanol oxidation
depends upon the morphology of ‘Pt’ nanoparticles supported on γ-Al2O3 [13]. Mapa et al
employed a solid solution of ZnO and InN for selective oxidation of 2-butanol to MEK, but at
high temperatures between 350-450 0C [14]. Hou et al. reported that liquid phase oxidation of
2-butanol using oxygen as oxidant over Au:Pd bimetallic nanoparticles shows higher
selectivity towards 2-butanone at 60 °C [15]. Several d-block metals like Pt, Ru, Rh, Au, Co,
Ni, Ag and Ir [16-17] were tested as catalysts for oxidation of alcohols in liquid [17] as well
as in gas phase [16] reactions.
In many oxidation reactions metals like V, Cr, and Mn were used due to their redox
properties, oxygen storage capacity, etc.[18-20]. Apart from redox properties, certain ratio of
different oxidation states in such metals can alter the catalytic activity. Mn supported
catalysts were used in many field of research due to its redox properties. Controlling an active
oxidation state in Mn-based materials is a challenging task in heterogeneous catalysis. In this
paper we stabilized and maintained the oxidation states of Mn during the selective oxidation
of 2-butanol. In eletrocatalytic [21] and photocatalytic [22] reactions, Mn-based mixed oxide
catalysts showed better catalytic activity with suitable supports in many reactions. In this
work, manganese oxide on Al2O3, CeO2, ZrO2 and SiO2 mixed oxide catalysts were prepared
by co-precipitation and sol-gel methods. Catalysts which has certain ratio of Mn+2/Mn+3
shows better catalytic activity towards oxidation of 2-butanol. This reaction was performed in
fixed bed reactor (FBR) system and characterized by several techniques. Powder X-ray
diffraction (PXRD), Raman spectroscopy, N2 adsorption desorption, temperature
programmed desorption (NH3-TPD), transmission electron microscopy (TEM) confirmed the
structural and textural properties. Presence of different oxidation states in the active catalyst
was confirmed by X-ray photoelectron spectroscopy (XPS). Maximum conversion of 51%
and 88% selectivity towards MEK was achieved by MnxOy-Al2O3 catalyst over 12 hours. By
using surface characterization techniques, surface species on the catalysts were confirmed.
4

2. Experimental
2.1. Catalyst synthesis
All the chemicals used to synthesize catalyst were of reagent grade. Manganese (II)
nitrate (Mn(NO3)2.4H2O, 98%), aluminium (III) nitrate (Al(NO3)3.9H2O, ≥ 95%), cerium (III)
nitrate (Ce(NO3)3.6H2O, 99.5%), zirconyl (IV) nitrate (ZrO(NO3)2.H2O), TEOS (Si(OC2H5)2,
98%), sodium hydroxide (NaOH, 98%), ethanol (C2H5OH) and hydrochloric acid (HCl)
were used as precursors for catalyst synthesis. All the chemicals were purchased from Alfa
Aesar and Merck.
2.2. Hydrothermal method
Mn supported catalysts were synthesized by co-precipitation followed by
hydrothermal method [23].The procedure is as follows: Metal nitrates precursors (Mn2+, Al3+,
Ce3+, Zr4+) were dissolved in water and precipitating agent (NaOH) were added
simultaneously drop wise at constant pH (9.5±0.2). The solution was transferred into a teflon
lined autoclave for hydrothermal synthesis kept at 150 °C for 12 hours. Then the solid was
filtered, washed and dried at 110 °C for overnight followed by calcination at 450 °C for 6
hour in air. As-synthesized MnxOy-Al2O3, MnxOy-CeO2 and MnxOy-ZrO2 catalyst were
labelled as MA, MC and MZ, respectively.
2.3. Sol-gel method
Mn supported SiO2 catalyst was synthesized by sol-gel method [24]. Mn (NO3)2.4H2O
and TEOS was diluted using ethanol and the solution was stirred at room temperature to get
homogeneous clear solution. Half of the alcohol was used for this and another half was used
to dilute the water and acid. The mixed solution of EtOH-H2O-HCl was added drop wise to
the above Mn-TEOS-EtOH solution with constant stirring. After complete addition, the
solution was stirred at 40 °C for 12 hours. Then the solution was dried at 110 °C for
overnight and calcined at 450 °C for 6 hours in presence of air. Finally, MnxOy-SiO2 catalyst
was labelled as MS.
2.4. Characterization
PXRD pattern of synthesized catalysts were collected by PANanalyticalX’pert Pro
dual goniometer diffractometer at a step size of 0.008°and a scan rate of 0.5° min-1. X-ray
was generated by using Cu Kα (1.5418Å) with Ni filter and the flat holder in Bragg–
5

Brentango geometry used for data collection. Autosorb 1C Quantachrome USA was used to
collect Nitrogen adsorption/desorption isotherms for the catalysts. The surface area from the
adsorption branch was calculated by using Brunauer–Emmett– Teller (BET) equation. High
resolution transmission electron microscopy (HRTEM) of all catalysts was recorded by FEI
TECNAI F30 electron microscope operating at 300 kV. Samples were prepared by dispersion
powder catalysts in isopropanol before depositing onto a holey carbon grid. Raman spectra
were recorded on a Horiba JY LabRAMHR800 Raman spectrometer coupled with
microscope in reflectance mode with 514nm excitation laser sources and a spectral resolution
of 0.3 cm-1. Oxidation states of manganese and aluminium was studied by ambient pressure
photoelectron spectrometer (APPES) [25]. X-ray was produced by using Mg Kα with the
energy of 1253.6 eV.
2.5. Catalytic activity
The activity of the catalyst was performed in fixed bed continuous down flow reactor
(FBR) over a temperature range of 200-350 °C atmospheric pressure. 1 ml catalyst was
loaded at the centre of Inconel reactor (13mm X 510 mm), packed with ceramic beads on
either sides of the catalyst bed. Temperatures of the catalyst bed were measured by k-type
thermocouple. 2-butanol and oxygen feed flow were monitored by using isocratic pump (Lab
Alliance series II) and Brook’s Mass flow controllers ( 5850 series) respectively. The volume
of the gaseous products was measured by a wet gas flow meter (Ritter Drum-Type Gas
Meter). The reaction liquid products were condensed and analysed by Agilent 6890N gas
chromatography (FID) with HP-5 (5.5% phenyl methyl siloxane) column and gaseous
products were analysed by customised Thermo Fisher 1110 GC with (Molecular Sieve) TCD
detectors.
3. Results and discussion
3.1. Powder x-ray diffraction
PXRD is one of the primary techniques which offer the detailed information about
catalyst nature and structure. Fig.1. shows the PXRD pattern of as-calcined MA, MZ, MC
and MS catalysts. It is clearly seen from Fig.1 (a), MS mixed oxide catalyst shows the
presence of SiO2 phase at 21˚ and manganese oxide in the form of Mn2O3 phase which
matches with JCPDS No.78-0390. MZ mixed oxide catalyst shows peaks at 18˚, 28˚, 32˚,
36˚, 38˚, 50˚ and 60˚ correspond to Mn3O4 (JCPDS No.24-0734). In MC mixed oxide
catalyst, a high intensity peak at around 28° corresponds to ceria (JCPDS No. 75-0151) and
the other peaks correspond to Mn2O3 phase (JCPDS No.78-0390) which reveals that
6

manganese is well dispersed in ceria. Fig.1. (d) shows MA mixed oxide catalyst, which offer
information about presence of Mn3O4 phase. The presence of active sites in MA mixed oxide
catalyst reveals that dispersion of manganese oxide in alumina is very well than other
catalysts. Presence of Mn3O4 phase was further confirmed by JCPDS data -24-0734.
3.2. Surface area
Catalytic activity of the reaction depends upon different parameters, one of important
parameter is BET surface area. All the as-synthesized catalysts were analysed by N2-
adsorption desorption isotherms. Table.1. provides the details about textural properties of as-
synthesized catalysts. Surface area of manganese containing mixed oxide catalysts increased
in the order, MnxOy-Al2O3˂MnxOy-CeO2˂MnxOy-ZrO2˂MnxOy-SiO2 respectively. MnxOy-
Al2O3 mixed oxide catalyst shows the lowest surface area (84 m2/g) compared to other
catalysts due to be bigger crystallite size [26]. MnxOy-SiO2 mixed oxide catalyst exhibit the
highest surface area in comparison with other catalysts (339 m2/g). High surface area of
MnxOy-SiO2 catalyst is due to SiO2 support, where SiO2 support shows 369 m2/g surface area
which decreases to 339 m2/g after Mn was incorporated.
MnxOy-CeO2 and MnxOy-ZrO2 catalysts showed 89 and 103 m2/g surface area
respectively. Both catalysts showed near 5±1.0 nm crystallite size. Compared to other
catalysts, MnxOy-Al2O3 showed high pore volume (0.32 cc/g) and this may be the one of the
reasons for the maximum catalytic activity of 2-butanol.
3.3. X-ray photoelectron spectroscopy (XPS)
To explore the chemical composition of the catalyst surface and the oxidation state of
the elements, XPS was used. Fig.2. shows XPS spectra of Mn 2p core level of pure
manganese oxide, fresh and used MnxOy-Al2O3 catalysts. XPS spectra of Mn containing
mixed oxide and pure manganese oxide catalysts reveal the information about active species
on the surface and oxidation states of manganese oxide. XPS was recorded to know about
surface chemical composition of manganese oxide in Mn containing mixed oxide catalysts.
Multiple peaks corresponding to Mn+2, Mn+3 and Mn+4 were observed which was
deconvoluted by peak fitting method with Shirley background removal in Mn 2p core level
spectrum. Mn 2p3/2 deconvolution shows three peaks at 640.4, 641.7 and 643.3 eV binding
energy which corresponds to Mn(II), Mn(III) and Mn(IV) respectively [27]. It clearly shows
that manganese in pure manganese oxide has multiple oxidation states. Fig.2. (b) shows the
XPS spectra of Mn 2p3/2 core level for fresh MnxOy-Al2O3 catalyst at 641.7 eV [27,28]. The
7

peaks at 640.4 and 641.7 eV clearly indicate the presence of Mn in Mn+2 and Mn+3oxidation
states. This satellite structure, noticeable on the higher binding energy of Mn 2p3/2 core-level
in the XPS spectrum of the manganese compounds originates from the charge transfer from
oxygen ligand to unfilled 3d orbital of Mn during the final state of photoelectron emission
process [29]. It is further confirmed that fresh MnxOy-Al2O3 catalyst shows both Mn+2 and
Mn+3oxidation states. The catalytic activity of MxOy-Al2O3 catalyst was evaluated for 2-
butanol oxidation, and it shows maximum conversion (51 %) and selectivity (88 %) than
other tested catalysts. To study the surface chemical composition of the catalyst, the spent
MnxOy-Al2O3 catalyst was analysed by XPS measurement. Spent catalyst analysis reveals the
presence of peaks at 640.4 and 641.7 eV proving that there is no significant change in the
catalyst after reaction. It is also confirmed that the catalyst possess stable oxidation state even
after 12 hour on time on stream under optimum reaction conditions.
O 1s spectra of all fresh and used MA catalyst were recorded. Fresh MnxOy-Al2O3
catalyst showed peaks around 530, and 531.5 eV corresponds to lattice oxygen, surface
hydroxyl species [30, 31]. On the surface of spent catalyst, presence of lattice oxygen and
surface hydroxyl species was confirmed by peaks at 530 and 531.5 eV respectively.
Catalytic activity of MnxOy-Al2O3 catalyst showed maximum yield due to surface oxygen
species which favours selective oxidation of 2-butanol. In spent catalyst O 1s spectrum, it is
clear that the intensity of surface oxygen species is more which confirms the role of these
species.
3.4. Raman spectroscopy
Fig. 4 shows the Raman spectral data of MnxOy-Al2O3, MnxOy-CeO2, MnxOy-ZrO2
and MnxOy-SiO2 catalysts. Raman spectroscopy provides detailed information about vibration
modes of catalysts/materials. Characteristic peaks for MnxOy-ZrO2 catalyst is in the range of
300-700 cm-1. High intensity peak is at 660 cm-1 and two small intensity peaks at 317 and 372
cm-1corresponds to Mn3O4. These characteristic Raman bands support the results obtained in
PXRD. In MnxOy-Al2O3 catalyst presence of broad peak at 645 cm-1 corresponds to Mn3O4
[32].The characteristic Raman band at 630 and 680 cm-1 is not seen which confirms that there
is no formation of Mn2O3. There was no characteristic peaks observed for MnxOy-SiO2
catalyst. MnxOy-CeO2 catalyst showed four characteristic peaks in the range of 300-700 cm-1.
The high intensity peak at 457 cm-1 corresponds to vibration mode of cubic fluorite (F2g)
CeO2 lattice [33, 34] and other peaks corresponds to Mn(III)-O and Mn(IV)-O bonds.
8

Used MnxOy-Al2O3 was analysed to observe the nature of catalyst after reaction and to
see what type of coke is formed during reaction. The Raman characteristic peaks of used-
MnxOy-Al2O3 catalyst was compared with fresh MnxOy-Al2O3 catalyst, noticeably strong
vibration band at 650cm-1which corresponds to Mn3O4 and smaller band at 313 and 367 cm-1
corresponds to high surface area manganese oxide were observed [35]. No peaks
corresponding to D and G bands [36] were observed which suggests that there is no graphitic
coke formation after long time on stream (12 and 30 hours) reaction conditions.
3.5. Temperature programmed desorption (NH3-TPD)
Acidity is one of the parameter which plays an important role in the catalytic activity
of 2-butanol oxidation. To find the acidity of the catalysts, NH3-temperature programmed
desorption was carried out for all calcined catalysts and the results are given in Fig. 6.
MnxOy-CeO2 and MnxOy-ZrO2 catalysts showed no peaks which confirmed that both the
catalysts has no acidity. In MnxOy-SiO2 catalyst the peak at 650 °C corresponds to strong
acidic centres and very little amount of weak acid sites (200-300 °C) from the SiO2 were also
observed. MnxOy-Al2O3 catalyst showed two major peaks in which the peak observed at
lower temperature (
̴
200 °C) is due to weak acidic centres present in the catalyst and peak
650 °C) is due to strong acid site. MA exhibits more
observed at a higher temperature (
̴
number of weak as well as strong acid sites in comparison to MS emphasizes that the former
possess more acidic centres. This important feature associated with MA helps in enhancing
the catalytic activity of MA catalyst, which showed maximum and stable catalytic activity
over a period of 30 hours.
3.6. Transmission electron microscopy (TEM)
Fig. 7. shows the TEM images of fresh and used MnxOy-Al2O3 catalyst. TEM images
reveal the information about the morphology and lattice of the catalysts. Fig.7. (a), and (b)
and (c) displays TEM images of fresh MnxOy-Al2O3 catalyst in low and high resolutions,
respectively. Fig.7. (a) and (b) TEM images clearly confirmed the dispersion of manganese
oxide on Al2O3. Indeed, a nice decoration of alumina by nano MnxOy has been directly
demonstrated from the TEM results. The d-spacing value of MnxOy-Al2O3 catalyst is 3.0 Å
which is in agreement with the value obtained from PXRD results. Used MnxOy-Al2O3
catalysts TEM image (Fig. 7 (d)) shows a thin layer of coke deposited on the surface of the
catalyst and there was no other changes in the morphology of the catalyst. Highly dispersed
9

manganese oxide on Al2O3 showed maximum yield on 2-butanol oxidation using molecular
oxygen.
3.7. Effect of supports
Support has an important role in any catalytic activity of a reaction. Choosing a
suitable support is a critical part in heterogeneous catalysis. To study the influence of the
support for 2-butanol to MEK reaction, manganese oxide catalyst was loaded on four
different supports . Manganese oxide on Al2O3, CeO2, ZrO2 and SiO2 mixed oxide catalysts
were subjected to oxidation of 2-butanol at optimized reaction conditions and the results are
given in Fig. 8 and Table 2. MC catalyst shows maximum 2-butanol conversion and MEK
selectivity of 41 and 81 %, respectively. MS catalyst shows better conversion (54 %), but
selectivity towards MEK is very low due to complete oxidation of 2-butanol to undesired
products. Minimum selectivity (33 %) towards MEK was observed with MS catalyst. MA
and MZ catalysts shows about 50 % conversion and 88 % selectivity towards MEK. MA and
MZ catalysts show similar pattern up to certain time on stream. However after certain time on
stream, MA catalyst shows slight increase in catalytic activity but in MZ the catalyst activity
decreases at the end of 12 h. This decrease in catalytic activity may be due to coke formation
which might block the catalytic active centres. MA catalysts shows better activity than other
tested catalysts. There is no conversion of 2-butanol to MEK was observed over alumina,
which confirms that bare alumina is not active for 2-butanol oxidation. 2-butanol conversion
on MA increases dramatically to 51 %. MA shows high conversion as well as yield of MEK,
which is attributed to synergistic effect of manganese and alumina. Conversion of 2-butanol
and yield towards MEK is stable up to 30 hours. Catalytic activity of 2-butanol is further
carried out using MA catalyst for optimization of different parameters.
3.8. Effect of temperature
Temperature is one of the main important factors which affect the catalytic activity.
To examine the influence of temperature, oxidation of 2-butanol was carried out at three
different temperatures ranging from 250, 300 to 350 °C in FBR and the results are shown in
Fig. 9. Oxidation of 2-butanol was carried out at LHSV = 1.8 h-1 with respect to 2-butanol,
and GHSV = 1200 h-1 with respect to oxygen at atmospheric pressure. At low temperature
(250 °C), conversion towards 2-butanol and selectivity towards MEK was observed to be 30
and 80 %, respectively. Further increase in temperature to 300 °C, conversion and selectivity
increased to 51 and 88% respectively and remains constant for 30 hours time on stream. At
10

the high temperature (350 °C), very less conversion but high selectivity (90 %) towards MEK
was observed, which is higher than other two temperatures. Decrease in conversion and yield
may be due to complete oxidation of 2-butanol to undesired products like COx, methanol,
acetone and water at higher temperature. An appropriate and suitable reaction condition was
always required for partial oxidation. At 300 °C maximum conversion and selectivity was
observed due to appropriate temperature and active centres to activate the reactants.
Maximum catalytic activity was observed at 300 °C and it is optimised for oxidation of 2-
butanol.
3.9. Effect of 2-butanol flow
To study the effect of contact time between reactants and catalytic active centres,
reaction was performed at different 2-butanol flow and the results are shown in Fig. 10.
Oxidation was carried out at three different 2-butanol flow (LHSV 1.8, 3.6 and 5.4 h-1 with
respect to 2-butanol), at 300 °C, GHSV = 1200 h-1 respect to oxygen at atmospheric pressure.
At LHSV 1.8 h-1, 51% conversion of 2-butanol was observed with maximum selectivity (88
%) of MEK. At this flow, 2-butanol has optimum contact time to react with adsorbed oxygen
on the surface of the catalyst. While increasing the 2-butanol flow to LHSV 3.6 h-1
conversion of 2-butanol was decreased to 26 % but the selectivity of MEK was increased to
90 %. At LHSV 5.4 h-1 conversion as well as selectivity decreased to 20 and 89 %,
respectively. Due to decreasing contact time between reactants and active centres a decrease
in yield was observed at higher LHSV of 2-butanol viz. 3.6 and 5.4h-1. The maximum
conversion of 2-butanol (51 %) and selectivity towards MEK (88 %) was obtained at
appropriate flow rate (LHSV 1.8 h-1), which favours selective oxidation.
3.10. Effect of oxygen flow
To understand the effect of oxidant on catalytic activity, this reaction was carried out
at three different molecular oxygen flows and the results are shown in Fig. 11. In any
oxidation reaction, the change in catalytic activity depends upon the flow of oxidants. In this
work, reaction was carried with three different oxygen flow rates (GHSV 600, 1200 and 1800
h-1). At GHSV = 600 h-1 the conversion towards 2-butanol was 35 %, and 70 % MEK
selectivity was observed. Further increase in oxidant flow to GHSV = 1200 h-1 conversion
attained the maximum (51%) and selectivity (88%) which favours the partial oxidation of 2-
butanol to MEK. Further increase in GHSV to 1800 h-1 relatively low conversion and
11

selectivity was obtained which may be due to complete oxidation of 2-butanol to COx and
undesired side products. Based on the above results GHSV 1200 h-1 (O2) was optimized for
oxidation of 2-butanol.
3.11. Catalyst stability and reaction sustainability
To understand the stability and durability of the MA catalyst, the optimization study was
carried out with long time on stream (TOS) with optimised reaction parameters. The reaction
conditions are GHSV 1.8 h-1 with respect to 2-butanol and GHSV 1200 h-1 with respect to
molecular oxygen at 300 °C .The activity of the catalyst remains same for longer duration (30
h) the result shown in Fig.12.Gradual increase in catalyst activity in the first few hours under
wide variety of reaction conditions demonstrated a change in the nature of the catalyst under
reaction conditions towards higher active form for this particular selective oxidation. Both
conversion and selectivity linearly increases, till the reaction reaches a steady state. When
comparing the oxidation ability of CeO2 alone as catalyst for this reaction, it gives very less
conversion and selectivity (Fig.13.).
4. Conclusion
MA, MC, and MZ catalysts were prepared by co-precipitation followed by
hydrothermal method; however, MS was prepared by sol-gel method. All the catalysts were
characterized by various analytical techniques like Raman spectroscopy, N2-adsorption, XPS,
TEM and NH3-TPD. Presence of Mn3O4 and Mn2O3 phases was revealed by PXRD. Presence
of different oxidation states were confirmed by XPS spectra which shows that MA catalyst
has Mn+2 and Mn+3oxidation states. All the catalysts were subjected to 2-butanol oxidation
using molecular oxygen in continuous flow fixed bed reactor. MA catalyst shows maximum
catalytic activity of 51% conversion of 2-butanol and 88% selectivity towards MEK. Active
centres in the catalysts were confirmed by XPS and Raman spectroscopy. Raman
spectroscopy shows the presence of Mn3O4 which was correlated with PXRD. Nature of
acidity was confirmed by NH3-TPD. When CeO2 alone is used as catalyst for this reaction it
is less active and deactivates fast. Synergetic effect of appropriate acidity along with active
Mn species is responsible for this reaction. Using oxidants like air in oxidation reaction leads
to environmental friendly and green reaction. More work is in progress with CO2 as oxidant
for this reaction.
12

Acknowledgment
TR thanks CSC0125 12 FYP- Network project for funding and KP thanks industry funded
SSP294726 project for his fellowship.
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14

Figures
Fig.1. Powder X-ray Diffraction pattern of as-calcined Mn containing mixed oxide catalysts
(a) MnxOy-SiO2, (b) MnxOy-ZrO2, (c) MnxOy-CeO2, and (d) MnxOy-Al2O3
15

Fig.2. XPS spectra of Mn 2p core levels for (a) Manganese oxide, (b) Fresh-MnxOy-
Al2O3 and (c) Used-MnxOy-Al2O3
16

Fig. 3. XPS spectra of O 1s (a) Fresh-MnxOy-Al2O3 and (b) Used-MnxOy-Al2O3
Fig. 4. Raman spectrum of all as-synthesized catalysts calcined in air at 450 °C
17

Fig. 5. Raman spectrum of fresh and used MnxOy-Al2O3 catalysts.
18

Fig.6. NH3-Temperature Programmed Desorption profile of as-synthesised catalysts calcined
in air at 450 °C
19

Fig.7. TEM images of MnxOy-Al2O3 fresh and spent catalyst (a-c) Fresh MnxOy-Al2O3 and
(d) Spent MnxOy-Al2O3.
20

Fig.8. Conversion of 2-butanol and selectivity of MEK at different Mn supported catalysts
Reaction conditions: 300 °C, LHSV 1.8 h-1 with respect to 2-butanol and GHSV 1200 h-1
with respect to oxygen at atmosphere pressure.
21

Fig.9. Conversion of 2-butanol and selectivity of MEK at various reaction temperatures over
MA. Reaction conditions: LHSV 1.8 h-1 with respect to 2-butanol and GHSV 1200 h-1 with
respect to oxygen at atmosphere pressure.
22

Fig. 10. Conversion of 2-butanol and selectivity of MEK at various 2-butanol flow rates over
MA. Reaction conditions: 300 °C, GHSV 1200 h-1 with respect to oxygen at atmosphere
pressure.
23

Fig. 11. Conversion of 2-butanol and selectivity of MEK at different oxygen flow rates over
MA. Reaction conditions: 300 °C, LHSV 1.8 h-1 with respect to 2-butanol at atmosphere
pressure.
24

Fig. 12. 30 hours TOS study using MA catalyst. Reaction conditions: 300 °C, LHSV 1.8 h-1
with respect to 2-butanol and GHSV 1200 h-1 with respect to oxygen at atmosphere pressure.
25

Fig. 13. Conversion of 2-butanol and selectivity of MEK for (i) MA (ii) CeO2. Reaction
conditions: 300 °C, LHSV 1.8 h-1 with respect to 2-butanol and GHSV 1200 h-1 with respect
to oxygen at atmosphere pressure.
26

Table.1. Textural properties of as-synthesised catalysts calcined in air at 450 °C
S.No Catalysts
Surface area
(m2/g)a
Total pore volume
(cc/g)b
Crystallite size
(nm)c
1
2
3
MnxOy-Al2O3 84
0.32
0.18
0.20
0.25
24.7±1.0
5.5±1.0
4.6±1.0
19.8±1.0
MnxOy-CeO2
MnxOy-ZrO2
MnxOy-SiO2
89
103
339
4
a
c
Calculated from BET Surface area analysis. b Obtain from BJH analysis. Calculated from
PXRD by using Scherer formula
Table. 2. Catalytic activity of MnxOy loaded on different supports
Yield of
MEK
Selectivity (mol %)
S. No. Catalysts
Conversion of 2-
b
butanol (mol %)a
(mol %)
35
MEK Acetone CH3OH COx
1
2
3
4
MnxOy/CeO2
MnxOy/ZrO2
MnxOy/Al2O3
MnxOy/SiO2
41
51
51
55
81
84
88
33
12
08
05
17
02
07
05
14
05
01
02
36
42
45
18
a
Conversion at steady state. Reaction conditions: 300 °C, LHSV 1.8 h-1 with respect to 2-
butanol and GHSV 1200 h-1 with respect to oxygen at atmosphere pressure. b CO and CO2.
27