An Oxidative Route for the Production of Methyl Methacrylate: A Study Over Iron Phosphate Catalysts

Article abstract of DOI:10.1007/s10562-016-1740-7

Abstract: An iron phosphate catalyst prepared using the ammonia gel method was supported on silica (CAT 1) and a commercial iron phosphate catalyst was supported on silica using the wet impregnation method (CAT 2). The XRD patterns of both the catalysts showed the presence of quartz-like iron phosphate and a tridymite like phase. In situ XRD under a reducing environment and TPR showed the formation of iron pyrophosphate for both the catalysts at 500?°C. The M?ssbauer spectra of the catalysts were similar and showed the presence of the ferric ion only. NH₃-TPD revealed the presence of Lewis and Br?nsted acidic sites on both the catalysts. Oxidative dehydrogenation reactions of methyl iso-burate (MIB) were carried out using a continuous flow fixed bed reactor at contact times of 0.4 and 0.8?s. The conversion of MIB was marginally higher at a contact time of 0.8?s. CAT 2 gave an 11?% yield of methyl methacryalate (MMA) compared to 21?% over CAT 1 at a contact time of 0.8?s. The conversion of MIB increased with co-feeding water. A maximum conversion of MIB (82?%) was obtained at 400?°C, giving a yield of 20?% MMA at a contact time of 0.8?s over CAT 1. However, co-feeding water increased the hydrolysis reaction also, which increased the yield towards iso-butyric acid and methacrylic acid. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1740-7

Catal Lett
DOI 10.1007/s10562-016-1740-7
An Oxidative Route for the Production of Methyl Methacrylate:
A Study Over Iron Phosphate Catalysts
Avinash Harilal1 Venkata D. B. C. Dasireddy1 Holger B. Friedrich1
•
•
Received: 11 March 2016 / Accepted: 25 March 2016
Ó Springer Science+Business Media New York 2016
Abstract An iron phosphate catalyst prepared using the
ammonia gel method was supported on silica (CAT 1) and
a commercial iron phosphate catalyst was supported on
silica using the wet impregnation method (CAT 2). The
XRD patterns of both the catalysts showed the presence of
quartz-like iron phosphate and a tridymite like phase.
In situ XRD under a reducing environment and TPR
showed the formation of iron pyrophosphate for both the
Graphical Abstract
¨
catalysts at 500 °C. The Mossbauer spectra of the catalysts
were similar and showed the presence of the ferric ion
only. NH3-TPD revealed the presence of Lewis and
Brønsted acidic sites on both the catalysts. Oxidative
dehydrogenation reactions of methyl iso-burate (MIB)
were carried out using a continuous flow fixed bed reactor
at contact times of 0.4 and 0.8 s. The conversion of MIB
was marginally higher at a contact time of 0.8 s. CAT 2
gave an 11 % yield of methyl methacryalate (MMA)
compared to 21 % over CAT 1 at a contact time of 0.8 s.
The conversion of MIB increased with co-feeding water. A
maximum conversion of MIB (82 %) was obtained at
400 °C, giving a yield of 20 % MMA at a contact time of
0.8 s over CAT 1. However, co-feeding water increased the
hydrolysis reaction also, which increased the yield towards
iso-butyric acid and methacrylic acid.
Keywords Iron phosphate Á Methyl methacrylate Á
Methyl iso-burate Á Water co-feed Á Oxidative
dehydrogenation
1 Introduction
Methyl methacrylate (MMA) is an a,b-unsaturated com-
pound and exhibits reactivity typical of this class of com-
pound, namely via the vinyl function and the carboxylic
acid or ester group. It is an important monomer in the
chemical industry [1]. The properties of methyl
methacrylate derived polymers include good mechanical
strength, outstanding optical properties and they can be
used as scratch resistant, antistatic and non-drip coatings.
& Holger B. Friedrich
friedric@ukzn.ac.za
1
School of Chemistry and Physics, University of
KwaZulu-Natal, Durban 4000, Republic of South Africa
123

A. Harilal et al.
However, methyl methacrylate is less reactive than its
corresponding acrylates due to the alpha methyl group [2].
The manufacturing routes for methyl methacrylate are
based on C2 to C4 hydrocarbons as the ultimate feedstock
[3, 4]. Ethylene is one of the olefins which can be used to
produce methyl methacrylate via propionaldehyde, propi-
onic acid or methyl propionate intermediates [5]. BASF
used this process to produce methyl methacrylate com-
mercially [5]. However, the process which involved
ethane/ethylene showed low yield towards methyl
methacrylate. To achieve a higher yield towards methyl
methacrylate, Shell developed a process to produce methyl
methacrylate from propyne (methyl acetylene) [6, 7]. The
process involved a reaction between propyne, carbon
monoxide and water in the presence of palladium acetate,
triphenylphosphine and benzene-phosphonic acid. This
resulted in methacrylic acid being formed, which can then
be esterified to methyl methacrylate. This process gave a
high yield (99 %) of methyl methacrylate. The drawback of
this process is the short life time of the catalysts used [7].
In the C2 and C3 routes to methyl methacrylate, a methallyl
carbon frame has to be formed by adding one or two car-
bons using C1 compounds such as CO, HCHO or HCN. C4
routes were also developed [8–10] which involved the
direct esterification of methacrolein or the oxidative
dehydrogenation of methacrylic acid in the presence of
methanol. In the decade of 1990-2000, many commercial
processes were developed to produce methyl methacrylate
through homogenous routes from ethylene, acetone, pro-
pene and propyne [11–13]. But due to the low yield of
methyl methacrylate through the homogenous route, from
the last decade, heterogeneous catalysed reactions to pro-
duce methyl methacrylate again stimulated the interest in
the chemical industry [14, 15]. Among them, the oxidative
dehydrogenation of iso-butyric acid to methyl methacrylate
is one of the commercial processes which use Mo-P-V as a
catalyst [16, 17]. The oxidative dehydrogenation and
esterification of isobutyric acid (IBA) over copper, iron or
nickel oxide based catalysts is also done commercially by
various chemical companies to produce methyl methacry-
late [18].
oxidative dehydrogenation reactions [24]. They exhibit
both redox and acidic characteristics. These characteristics
are very similar to those of heteropolyacids, molybdenum
and vanadium phosphates which were widely used in the
production of methyl methacrylate commercially. But, the
iron phosphates showed low selectivity when compared to
the hetropolyacids in the production of acrylic acid, maleic
anhydride, and methacrylic aldehyde [25]. This is due to
the lack of Fe = O species in the iron phosphates, thus iron
phosphates cannot induce oxygen insertion reactions.
However, iron phosphates showed high selectivity in the
oxidative dehydrogenation of lactic to pyruvic acid and
glycolic acid to glyoxylic acid, which involve an abstrac-
tion of hydrogen from the reactant [26].
In an oxidative dehydrogenation reaction, water is a
final product of the reaction and it can have a negative
effect on the reaction [21, 27, 28]. However, it was
extensively reported in literature [29–31] that in the pres-
ence of water the activity of iron phosphate catalysts
increased. The oxidative dehydrogenation reactions over
iron phosphate catalysts induce a deposit of carbon on the
surface of the catalyst even in the presence of oxygen,
which decreases the activity of the catalyst. It was reported
that the water suppresses the formation of the carbon
deposit and also plays a pivotal role in removing it [30, 31].
It was also reported that water assists in desorption of
products from the catalyst. A similar role of water was
proposed by Watzenberger and his co-workers [32, 33] for
the molydophosphate catalyst used in the oxidative dehy-
drogenation of isobutyric acid to methacrylic acid. In the
present work, a synthesised iron phosphate catalyst was
compared to a commercially obtained iron phosphate cat-
alyst for the oxidative dehydrogenation of methyl isobu-
tyrate to methyl methacrylate. The role of water on the
selectivity towards products and effects of surface acidity
were also investigated.
2 Experimental
2.1 Catalyst Preparation
There are other potentially viable methods to produce
methyl methacrylate. Among them is the oxidative dehy-
drogenation of methyl isoburate (MIB), which is formed by
the carbonylation of propene in the presence of methanol
[19, 20], of which there are very few reports [16, 21].
The redox function of a metal oxide is essential for the
oxygen insertion and oxidative dehydrogenation reaction.
This implies that oxidation activity is dependent on the
reducibility and re-oxidisibility of the metal oxide [22, 23].
It has been proposed that the oxidative reaction can pro-
ceed selectively when the reducibility of the metal oxide is
moderate. Iron phosphate catalysts are widely used for
The commercial iron phosphate catalyst (Batch No.
08924JA) and colloidal silica (40 wt% suspension in
water) were purchased from Aldrich.
2.1.1 Preparation of Iron Phosphate Catalyst
The catalyst used for this investigation was synthesized via
the ammonia gel method using the procedure outlined by
Muneyama et al. [34]. In this procedure, 12.21 g
(0.03 mol) of iron(III) nitrate was dissolved in 0.5 L
deionised water and a 25 % ammonium hydroxide solution
123

An Oxidative Route for the Production of Methyl Methacrylate: A Study Over Iron Phosphate…
was added drop-wise to this solution to precipitate the
iron(III) hydroxide gel. The solution was decanted and the
precipitate was then mixed with 2 g of colloidal silica
before adding 2.93 g of 85 % orthophosphoric acid solu-
tion. This was boiled for 2 h at 80 °C. The resulting light
brown slurry was placed in an oven at 120 °C for 18 h and
then calcined in a stream of air at 500 °C for 4 h. This
catalyst is denoted as CAT1.
A Micromeritics Gemini Instrument was used to deter-
mine the BET surface area of the catalyst samples. The
samples were initially degassed by heating them to 200 °C
under flowing nitrogen for 12 h using a Micromeritics
Flow Prep 060 apparatus. The surface area measurements
were performed using nitrogen as the adsorbate at a tem-
perature of -198 °C.
The TPR experiments were performed using
a
Micromeritics Autochem II 2920 which incorporates a
TCD detector. A gas mixture of 5 % hydrogen in nitrogen
with a flow of 50 ml/min was passed through the sample
with a heating of rate of 10 °C/min from 25 °C to 750 °C.
In the TPD experiments, the catalyst was pretreated at
350 °C under a stream of helium for 60 min. The tem-
perature was then decreased to 80 °C. A mixture of 4.1 %
ammonia in helium was passed over the catalyst at a flow
rate of 30 mL/min for 60 min. The excess ammonia was
removed by purging with helium for 30 min. The tem-
perature was then raised gradually to 550 °C by ramping at
10 °C/min under the flow of helium and desorption data
was recorded.
2.1.2 Preparation of the Silica Supported Commercial
Iron Phosphate
In the wet impregnation method [28], a slurry of commer-
cial grade iron(III) phosphate dihydrate was added to col-
loidal silica. The mixture was heated to remove the excess
water and then the slurry was kept in an oven for 18 h at
120 °C. This product was then calcined under a flow of air
at 500 °C for 4 h. This catalyst is denoted as CAT 2.
2.2 Characterization of Catalysts
Powder X-ray diffraction patterns were acquired using a
Philips X’Pert PRO diffractometer system equipped with
an ANTON PAAR XRK 600 high temperature stage setup
and an X’Celerator detector. The source of radiation was
2.3 Catalytic Testing
Gas phase oxidation reactions were carried out in a con-
tinuous flow fixed bed reactor in down flow mode. A
tubular stainless steel reactor tube with 10 mm internal
diameter and 300 mm length was used to pack the catalyst.
The catalyst bed (1 mL) was located at the center of the
reactor with 24-grit carborundum packed in the spaces on
either side of the bed. K-type thermocouples were used to
monitor the temperature of the catalyst bed and the reactor
furnace. The thermocouples were controlled by CB-100
RK temperature control units. The methyl iso burate
(Merck, 99 %) was delivered into the system by a high
precision isocratic pump (Lab Alliance Series II). The co-
feeding of water was done using another high precision
isocratic pump. Methyl isobutyrate was passed through the
reactor with a flow of 5.24 ml/min at contact times of 0.4 s
and 0.8 s with an MIB to oxygen molar ratio of 1:1. Air
(Afrox, 79 % N2, 21 % O2) was used as an oxidant and
nitrogen (Afrox, 98 %) was used as the make-up gas. The
amount of water and nitrogen was varied in order to
maintain the desired contact time. The volume of the
gaseous products was measured by a wet gas flow meter
(Ritter Drum-Type Gas Meter). The gaseous and liquid
products were analysed offline using Perkin Elmer Clarus
400 FID and TCD gas chromotographs. The FID was used
for analysis of organic products in both gaseous and liquid
phases, whereas the TCD was used for carbon oxides. The
amount of water in the liquid products was determined by
using a Metrohm Karl Fischer analyser. Carbon balances
˚
Co K a using a wavelength of 1.788 A. The calcined
sample was pre-treated, at atmospheric pressure, in situ, by
flushing with hydrogen, nitrogen or air with flow rates of
50 ml/min for 1 h. The catalysts were scanned at temper-
atures of 25 °C, and at 50 °C steps between 200 and
550 °C. The heating rate was 10 °C/min. The sample was
allowed to stabilize at the desired temperature for 5 min,
followed by a scan in the 2 theta range of 5° to 105°.
For SEM, the samples were viewed using a Philips
XL30 ESEM. The X-ray spectra were collected using a
EDAX Phoenix Sapphir Detector and the spectra was
analyzed using the ZAF standardless analysis package
version 3.2.
The amount of iron and phosphate in the catalysts was
determined using a Perkin Elmer Optical Emission spec-
trometer, Optima 5300 DV and the data was collected and
processed using the Winlab 32 software. The standards
were purchased (1000 ppm Fe and P) from Fluka.
The FT-IR spectra of the samples were obtained using
the KBr disc method and the data collected in air using a
Nicolet 400 D Infrared spectrometer. An appropriate
amount of this mixture was placed in a 13 mm stainless
steel KBr die apparatus, which was then pressurized to 5
bars to form a pellet.
57
¨
Fe Mossbauer measurements on the prepared catalysts
were made at room temperature (RT) in conventional
transmission geometry with a 57CoRh source.
123

A. Harilal et al.
ranged between 98 and 100 % and all data points were
obtained in duplicate with an error of 0.5 %.
tridymite phase (JCPDS No.50-0302) [31]. CAT 1, which
was calcined at 500 °C in air, showed a mixture of the
quartz and tridymite type phases. Contrary to what is sug-
gested in the literature, in situ calcination in air at 550 °C in
our hands didn’t lead to the formation of the quartz phase
only, nor did it enhance the intensity of the quartz phase.
CAT 2 showed a similar XRD pattern to CAT 1. CAT 1 and
CAT 2 were also heated under nitrogen and the acquired
XRD patterns showed that the catalysts consisted of the
quartz and tridymite phases in the temperature range from
25 to 450 °C. This was consistent with what was observed
in air.
3 Results and Discussion
3.1 Characterization Results
The X-ray diffraction patterns of CAT 1, which was heated
in air from 25 to 550 °C, are shown in Fig. 1a. The 2 theta
values at 23°, 24°, 31°, 40° and 43° matched the quartz type
(JCPDS no. 84-0875) and tridymite type iron phosphate
phases. Although there is no crystallographic information
available for the tridymite phase of FePO4, this phase was
identified based on the isotypic AlPO4 analogue of the
Heating CAT 1 and CAT 2 in situ under the reducing gas,
hydrogen, showed that the structure transformed completely
with heating. The XRD patterns show that from 25° to
Fig. 1 In situ XRD of a CAT 1
and b CAT 2 under air
123

An Oxidative Route for the Production of Methyl Methacrylate: A Study Over Iron Phosphate…
450 °C the phases also observed under air and nitrogen
persist, namely a mixture of the tridymite and quartz type
phases (Fig. 2). At 500 °C, the iron phosphate loses the
crystallinity it exhibited at the lower temperatures. The for-
mation of the iron pyrophosphate phase (Fe2P2O7) (JCPDS
No. 76-1762), which is obtained by the reduction of the iron
phosphate phase, was observed from this temperature [16].
The infra-red spectra of CAT 1 and CAT 2 showed a wide
medium intensity band at 3350 cm-1 which is attributed to
physisorbed water on the catalysts. A low intensity band
observed at 1600 cm-1 was assigned to the bending
vibration of water. A wide, strong band at 1110 cm-1 was
due to the stretching modes of the PO3 group and narrow
medium intensity bands at 640 and 480 cm-1 were attributed
to the bending modes of the PO3 group. The absence of bands
between 740 and 930 cm-1, which would be assigned to the
symmetrical and assymetrical vibrations of P-O-P groups,
suggest that these groups are not present in the crystal
structure. CAT 1 and CAT 2 showed similar UV Raman
spectra. Both spectra were dominated by an intense band at
1001 cm-1 arising from the tetrahedral PO4 group. This and
the absence of the P-O-P band in the IR spectra suggest that
Fig. 2 In situ XRD of a CAT 1
and b CAT 2 under reducing
environment
123

A. Harilal et al.
the PO4 group is alternatively connected to FeO4 groups in a
tetrahedral co-ordination [35]. A band at 440 cm-1 was
attributed to the stretching and bending vibrations of the
phosphate groups.
Fe3? species corresponding to FePO4 [36, 37]. These these
two doublets are not due to the existence of Fe3? atoms in
two distinct crystal sites, but rather to a distribution of the
quadrapole doublet. The large quadrapole splitting
observed is probably an indication that the environment
¨
The Mossbauer spectrum of CAT 1 was fitted with two
paramagnetic doublets with parameters characteristic of
around the Fe3? ion is distorted [37]. The Mossbauer
¨
spectrum of CAT 2 was similar to the spectrum of CAT 1
and showed the presence of the ferric ion only (Fig. 3). The
hyperfine parameters for the catalysts are similar to those
reported for iron in trigonal bi-pyramid coordination [21],
and suggest that interaction with the support increases the
number of oxygen atoms in the iron coordination sphere for
a fraction of the supported phase.
The ammonia TPD profile of CAT 1 revealed two types
of acidic sites which are Lewis (weak) and Brønsted
(strong) acidic sites (Fig. 4). The first peak at 190 °C can
be ascribed to physisorbed ammonia on the surface of the
catalyst [28]. The second peak at 660 °C was due to des-
orption of ammonia at acidic sites and therefore indicates
the Brønsted acid sites on the iron phosphate catalyst. The
presence of adsorbed water on the surface of the catalyst is
responsible for the formation of Brønsted acid sites [38].
The ammonia desorption profile of CAT 2 also showed two
desorption peaks. The first, at 250 °C, is due to the des-
orption of physisorbed ammonia [28]. The second,
assigned as for the synthesized iron phosphate, was due to
desorption due to the Brønsted acidity. CAT 2 showed a
lower Brønsted acidity when compared to CAT 1 (Fig. 4).
The ICP-OES results give a calculated P/Fe ratio of 1.2
for CAT 1, which supports the formation of the tridymite
Fig. 3 Mossbauer spectra of a CAT 1 and b CAT 2
Fig. 4 TPD-NH3 profile of
CAT 1 and CAT 2
CAT 1
CAT 2
0
100
200
300
400
500
600
700
Temperature (°C)
123

An Oxidative Route for the Production of Methyl Methacrylate: A Study Over Iron Phosphate…
Fig. 5 ODH of MIB over
50
45
40
35
30
25
20
15
10
5
(a)
a CAT 1 and b CAT 2 at a
contact time of 0.4 s with
varying temperature
MMA
MAA
IBA
Cracked products
Carbon oxides
Conversion
0
300
350
400
450
500
Temperature (°C)
60
50
40
30
20
10
0
(b)
MAA
IBA
Cracked products
Carbon oxides
MMA
Conversion
300
350
400
Temperature (°C)
450
500
phase at lower temperatures and validates the observation
and assignments of the XRD patterns respectively. ICP
results of CAT 2 gave a P/Fe ratio of 1.0. CAT 2 showed a
BET surface area of 18 m2/g which was marginally higher
than the surface area of CAT1 (15 m2/g), obtained via the
ammonia gel method.
the first stage and the reduction of the latter phase occurs in
the second stage. This is consistent with the observation
based on the in situ XRD data under H2, where the tridy-
mite phase reduced at relatively low temperatures and was
gradually transformed to the quartz phase at temperatures
below 500 °C.
The TPR profiles of CAT 1 and CAT 2 were similar and
showed an asymmetric reduction peak due to the trans-
formation of the FePO4 phase to the reduced Fe2P2O7
phase [16]. This reduction was also seen in the XRD pat-
terns of the catalysts, when the catalysts were heated under
hydrogen. From the TPR profile, there is an indication of
two reductions occurring which can be attributed to the
presence of the two phases in the iron phosphate catalyst,
namely the tridymite and quartz type phases. It can be
deduced that the reduction of the former phase occurs in
3.2 Catalytic Testing
The products formed in the oxidative dehydrogenation
reactions of methyl isobutyrate (MIB) were methyl
methacrylate (MMA), methacrylic acid (MAA), isobutyric
acid (IBA), cracked products (acetone, methanol and pro-
pene) and carbon oxides (CO and CO2).
Catalytic testing in a blank reactor tube packed with 24
gritt carborundum showed a MIB conversion of 2–5 % in
123

A. Harilal et al.
the temperature range of 300–500 °C. The maximum
selectivity to methyl methacrylate was found at 500 °C
(18 %) and the carbon oxides were the dominant products
at all temperatures.
22 % (300 °C) to a maximum of 30 % (400 °C), subse-
quently the conversion decreased to 27 % as the temper-
ature is increased to a maximum of 500 °C. The
conversion of MIB was marginally higher at a contact
time of 0.8 s when compared to the contact time of 0.4 s.
The yield of MMA also increased with increase in the
contact time over CAT 1. CAT 2 showed the same pro-
duct profile as seen for CAT 1. The conversion of MIB, as
well as the yield of MMA, was lower over CAT 2 at both
contact times. Thus, CAT 2 showed a 11 % maximum
yield of MMA at a contact time of 0.8 s (Fig. 6), whereas
CAT 1 gave a 21 % maximum yield of MMA, at a tem-
perature of 400 °C.
3.2.1 Effect of Contact Time
The results in Fig. 5 showed that CAT 1 was effective in
the oxidative dehydrogenation (ODH) of MIB to the
desired product MMA. This was expected because the
literature [29, 30, 36] shows that iron phosphate catalysts
are effective for a similar reaction, namely the ODH of
IBA to MMA. The conversion of MIB increases from
60
50
40
30
20
10
0
(a)
MMA
MAA
IBA
Cracked products
Carbon oxides
Conversion
300
350
IBA
400
450
500
Temperature (°C)
45
40
35
30
25
20
15
10
5
(b)
MMA
MAA
Cracked products
Carbon oxides
Conversion
0
300
350
400
450
500
Temperature (°C)
Fig. 6 ODH of MIB over a CAT 1 and b CAT 2 at a contact time of 0.8 s with varying temperature
123

An Oxidative Route for the Production of Methyl Methacrylate: A Study Over Iron Phosphate…
Fig. 7 ODH of MIB in the
presence of water over a CAT 1
and b CAT 2 at a contact time
of 0.4 s with varying
temperature
60
(a)
MMA
MAA
IBA
Cracked products
Carbon oxides
Conversion
50
40
30
20
10
0
300
350
IBA
400
450
500
Temperature (°C)
50
40
30
20
10
0
(b)
MMA
MAA
Cracked products
Carbon oxides
Conversion
300
350
400
450
500
Temperature (°C)
3.2.2 Effect of Water
MMA. The amounts of MAA, IBA and MeOH that had
formed also increased (Fig. 7). The increased quantities of
IBA and MAA were again due to hydrolysis caused by
water in the reaction medium [39].
The conversion of MIB when co-feeding water was higher
when compared to feeding MIB only, at a contact time of
0.4 s (Fig. 7). An optimum conversion of MIB was
obtained at 400 °C. Although higher conversion was
achieved, the yield of MMA (13 %) was lower than in the
case without water (15 %). This could be attributed to the
increase in the formation of the hydrolysis products MAA
and IBA. The expected increase in the selectivity to
methanol was also evident. The effect of feeding water
with MIB at the longer contact time (0.8 s) significantly
increased the conversion of MIB. A maximum conversion
of 82 % was obtained at 400 °C, giving a yield of 20 %
The effect of the water/feed ratio was investigated at a
fixed contact time of 0.8 s. Figure 8 shows that conversion
of MIB increases with increasing water to MIB ratio.
Although the increase in conversion is desired it was also
observed that as the conversion increased beyond a water/
feed ratio of 19, the yield of MMA decreased, again due to
the hydrolysis process. Due to the longer contact time and
the presence of the water in the feed there is also the
possibility of the feed hydrolyzing on the surface of the
catalyst [21, 32, 40].
123

A. Harilal et al.
100
80
60
40
20
0
The addition of water to MIB clearly increased the
conversion of MIB across the temperature range (Figs. 6
and 9) which is most evident for the 0.8 s contact time.
Previous reports [32, 39, 40] have shown that all iron
phosphate catalysts need large amounts of water to be
active and selective in the ODH of IBA, isobutyraldehyde
and isobutronitrile. Studies have also shown that increasing
the ratio or partial pressure of water significantly increases
the conversion of the feed [31, 32, 39, 41]. It was proposed
that water plays a role in maintaining the hydroxylated
form of the active phase. This was reported to account for
the need for the large quantities of water and accounted for
the deactivation observed in the catalysts when the water
Conversion
Yield
0
5
10
15
20
25
water/feed raꢀo
Fig. 8 Effect of water to feed ratio on the conversion of MIB and the
yield of MMA over CAT 1 at a contact time of 0.8 s
Fig. 9 ODH of MIB in the
100
90
80
70
60
50
40
30
20
10
0
(a)
presence of water over a CAT 1
and b CAT 2 at a contact time
of 0.8 s with varying
MMA
MAA
IBA
Cracked products
Carbon oxides
Conversion
temperature
300
350
IBA
400
450
500
Temperature (°C)
80
70
60
50
40
30
20
10
0
(b)
MMA
MAA
Cracked products
Carbon oxides
Conversion
300
350
400
450
500
Temperature (°C)
123

An Oxidative Route for the Production of Methyl Methacrylate: A Study Over Iron Phosphate…
co-feed is stopped [40]. Water was also reported to prevent
the formation of phosphorus rich layers on the surface of
the catalyst [42]. These phosphorus rich sites were reported
to be less active in the ODH reaction. Therefore, water
plays a pivotal role in the catalytic reactions involving iron
phosphates [21, 34, 41].
molybdenum have metal–oxygen double bond species,
while iron phosphate has no metal–oxygen double bond
species [31, 34]. The catalyst with metal–oxygen double
bond species can liberate oxygen atoms, while that without
metal–oxygen double bond species cannot liberate any
oxygen atoms. As a result, it is likely that the former has a
function to insert an oxygen atom into the substrate, while
the latter has no such function.
CAT 2 showed lower conversion of MIB and yields of
MMA than those obtained over CAT 1 in the presence of
water (Fig. 9). The formation of the unwanted hydrolysis
products are also more prevalent using the CAT 2 catalyst.
The silica supported commercial catalyst, CAT 2, was
clearly different when compared to the synthesized cata-
lyst, CAT 1. The total selectivity and yield of hydrolysis
products are higher than the yield and selectivity of
oxidative dehydrogenation products over both the catalysts.
Thus it shows hydrolysis is strongly favoured over the iron
phosphate catalysts in the presence of water, which is also
evidenced in the literature [16, 30, 41]. It can be assumed
that MMA is the precursor to MAA. Indeed, it is known
that MAA is formed by the hydrolysis of MMA over iron
phosphate catalysts in the presence of water as co-feed
[39]. Hence, the real (or initial) yield of MMA could be
represented by a total yield of MMA and MAA. Thus, CAT
1 showed a 36 % maximum initial yield of MMA at a
contact time of 0.8 s, whereas CAT 2 gave a 23 % maxi-
mum initial yield of MMA, both at a temperature of
400 °C. This, however, is only really important, should a
re-esterification step be viable.
The mechanisms of formation of the products from MIB
over iron phosphate catalysts can be proposed to be similar
to those of the IBA reaction [30]. For the iron phosphate
catalysts, it is the iron that is susceptible to valance state
changes to form the Fe2?/Fe3? redox couple. This redox
couple is involved in the proposed Mars and Van Krevelen
mechanism [43]. In a manner similar to the activation of
IBA, it is proposed that MMA formation results from the
interaction of the MIB molecule with a surface Fe3? ion
(Lewis acid site) at the site of the nucleophilic oxygen part
of the molecule. The reaction could then proceed in two
steps, which may be concerted.
In the first step (Scheme 1), the hydrogen in the beta
position of the ester molecule reacts with a (PO4)3- ion,
yielding the (PO3OH)2- group with the reduction of Fe3?
to Fe2? [36]. The second step would involve the removal of
hydrogen and the opening of the double bond formed in the
first step. It is very probable that the degrading side-reac-
tions due to C–C bond fission are promoted by the presence
of water. Co-feeding water showed an increase in conver-
sion of MIB over both the catalysts. Along with the higher
conversion of MIB, the selectivity towards MMA was
higher than in the case without co-feeding of water over
both the catalysts especially at the temperatures above
400 °C, due to the formation of the hydrolysis products of
Based on the XRD patterns, CAT 2 appeared almost
amorphous, except for when it was heated to 500 °C under
nitrogen where it exhibited a quartz phase. CAT 1 showed
greater yield towards propene when compared to CAT 2.
The formation of propene is believed to be due to an acidic
decomposition reaction [42]. It is known that the acidity of
the iron phosphates increases with increasing phosphorus
content. Thus, Ai et al. [29] reported that a cut off of
oxygen or an increasing P/Fe ratio leads to an increase the
formation rates of propene. CAT 2 had a P/Fe ratio of 1
while CAT 1 had a P/Fe ratio of 1.2, which implies a higher
concentration of the tridymite phase [16, 21, 37].
3.2.3 Mechanism of the Reaction
The characteristics of iron phosphate catalysts seem to be
unique [21]. In the cases where catalysts consist of oxides
or phosphates, they possess two functions as partial oxi-
dation catalysts. One is the function of promoting the
oxygen insertion reaction. The other is oxidative dehy-
drogenation [31]. Attention was drawn to the presence of
metal–oxygen double bond species in catalysts in order to
understand the difference in catalytic function between iron
phosphate and phosphates of other metals. It is generally
known that oxides and phosphates of vanadium and
Scheme 1 Oxidative dehydrogenation of MIB over iron phosphate
catalysts
123

A. Harilal et al.
MMA and IBA. Therefore, the presence of the oxygen
insertion function along with the water co-feed is not
essential, especially in the case where only oxidative
dehydrogenation is desired [40]. This may be the reason
why iron phosphate, which has no oxygen insertion func-
tion, promotes selectively the oxidative dehydrogenation
reactions in the presence of water.
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