Hexagonal Mo/V/W mixed oxide as a catalyst for the partial oxidation of methacrolein to methacrylic acid

Article abstract of DOI:10.1016/j.catcom.2020.106016

This work evaluates for the first time the catalytic performance of a hexagonal Mo/V/W mixed oxide, the so-called h-phase, in the partial oxidation of methacrolein to methacrylic acid. Three catalysts with different phase compositions were compared. One catalyst predominantly contained h-phase, a further consisted of the well-known M1 phase, and a third catalyst was composed of both, h-phase and M1, in similar amounts. The selectivity of the h-phase catalyst is comparable to that of M1.

Full text of DOI:10.1016/j.catcom.2020.106016

CatalysisCommunications141(2020)106016
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Hexagonal Mo/V/W mixed oxide as a catalyst for the partial oxidation of
methacrolein to methacrylic acid
Maximilian Sennericha, Peter Weidlerb, Bettina Kraushaar-Czarnetzkia,⁎
a Institute of Chemical Process Engineering CVT, Karlsruhe Institute of Technology, Campus South, 76131 Karlsruhe, Germany
b Institute of Functional Interfaces IFG, Karlsruhe Institute of Technology, Campus North, 76344 Eggenstein-Leopoldshafen, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords:
Methacrolein oxidation
Methacrylic acid
Molybdenum-based mixed oxide
Mixed oxide M1 phase
This work evaluates for the first time the catalytic performance of a hexagonal Mo/V/W mixed oxide, the so-
called h-phase, in the partial oxidation of methacrolein to methacrylic acid. Three catalysts with different phase
compositions were compared. One catalyst predominantly contained h-phase, a further consisted of the well-
known M1 phase, and a third catalyst was composed of both, h-phase and M1, in similar amounts. The selectivity
of the h-phase catalyst is comparable to that of M1.
1. Introduction
2. Experimental
Methacrylic acid (MAA) and several of its esters are important in-
termediates for acrylic polymers which are used in the manufacturing
of transparent plastics, glues, thickeners and lacquers [1]. MAA is
synthesized by selective oxidation of methacrolein (MAC) usually car-
ried out over heteropoly acid (HPA) catalysts, which combine high
activity and selectivity towards MAA [2]. However, the HPA lifetime
under typical reaction conditions with less than one year is un-
satisfactory. The low stability of HPA has triggered the search for al-
ternative catalysts that combine high selectivity to MAA with improved
long-term stability.
In the partial oxidation of acrolein (ACR) to acrylic acid (AA), Mo/
V/W mixed oxide (MO) catalysts are used with great success; in the
corresponding processes, runtimes of more than three years are typical
[2]. Since acrolein and methacrolein are homologue α,β-unsaturated
carbonyl compounds, it is an obvious thought to explore MO-type
catalysts also in the MAC oxidation. So far, there are only a few studies
involving mixed oxide catalyst in the partial oxidation of MAC to MAA
[2–4]. The materials under investigation were nanocrystalline/amor-
phous (Mo,V,W)5O14 and the so-called M1 phase, both being considered
as active phases in the ACR oxidation catalysis [2,3,5–10]. In the oxi-
dation of MAC to MAA, however, these MO-type catalysts exhibited
poor selectivities (SMAA,MAC ≈ 35% at XMAC ≈ 40%) [2–4]. A hex-
agonal Mo/V/W mixed oxide denoted as h-phase is also known as a
highly active ACR oxidation catalyst [11,12] but has not yet been
evaluated in the MAC oxidation.
2.1. Catalyst preparation
2.1.1. Catalyst A
The hexagonal (Mo,V,W)O3 catalyst was prepared by evaporation
crystallization from an aqueous precursor solution. 29.76 g ammonium
heptamolybdate (Alfa Aesar, 99%), 4.93 g ammonium metavanadate
(Merck, 99%) and 5.32 g ammonium metatungstate (Honeywell, 99%)
were dissolved in 1000 mL demineralized water in order to obtain
Mo8V2W1Ox. Nitric acid was added to adjust the pH value to 5. The
solution was stirred at 85 °C for 90 min under reflux. After that, the
water was completely evaporated at 60 °C under atmospheric pressure.
Consequently, the yellow colored solid precursor crystallized. The
precursor was crushed into powder form and calcinated under nitrogen
atmosphere in a calcination oven (Carbolite© HTR 11/150). The
powder was heated to 325 °C with a heating rate of 2 K/min, held for
240 min, heated further to 400 °C with 2 K/min and held for another
10 min until it was rapidly cooled down by shutting off the heater.
During the calcination process, the powder turned black. To increase
the catalyst surface area and the catalytic performance, the catalyst was
brought into suspension with demineralized water with a ratio of 10 g
catalyst per 100 mL of water. The suspension was evaporated under
stirring at a temperature of 60 °C. Afterwards the catalyst was crushed
into powder and calcinated again as previously described.
2.1.2. Catalyst B
The catalyst containing M1 phase and hexagonal phase was
⁎ Corresponding author.
E-mail address: kraushaar@kit.edu (B. Kraushaar-Czarnetzki).
https://doi.org/10.1016/j.catcom.2020.106016
Received 4 February 2020; Received in revised form 10 April 2020
Availableonline27April2020
1566-7367/©2020ElsevierB.V.Allrightsreserved.

M. Sennerich, et al.
CatalysisCommunications141(2020)106016
Notation
CMAC,s
CMAC,g
Fi
kF,i
km,I
MAC concentration on the catalyst surface (mol∙m−3
)
MAC concentration in the gas phase (mol∙m−3
integrated area in XRD diffractogram (−)
)
Abbreviations
surface specific reaction rate coefficient (m∙s−1
)
FID
flame ionization detector
acetic acid
heteropoly acid
methacrylic acid
methacrolein
mass specific reaction rate coefficient (m3∙kg−1∙s−1
molar flow of component i (mol∙s−1
molar flow of component i at the inlet (mol∙s−1
)
̇
HAc
HPA
MAA
MAC
MO
ni
)
̇
ni,0
)
RSi
carbon-based reactor selectivity to component i (−)
TOS
time on stream (h)
̇
mixed oxide
thermal conductivity detector
V
total volumetric inlet flow (m3∙s−1
)
TCD
Wz
Xi
Weisz number for inner mass transfer limitation (−)
conversion of component i (−)
Symbols
zi
τmod
carbon number of component i (−)
modified residence time at standard conditions (kg∙s∙m−3
)
ABET
specific surface area (m2∙g−1
)
obtained by means of spray drying according to example 1) in [13]. An
aqueous precursor containing 29.76 g ammonium heptamolybdate
(Alfa Aesar, 99%), 4.93 g ammonium metavanadate (Merck, 99%) and
2.66 g ammonium metatungstate (Honeywell, 99%) per 1000 mL of
demineralized water was spray dried. The sprayed precursor powder
was then kneaded with added water and calcined similar to catalyst A.
demineralized water was prepared (solution B). Solution A was slowly
added to solution B. The pH of solution AB was adjusted to 2.2 with
sulfuric acid before it was thermally treated in an autoclave at 175 °C
for 24 h. The calcination was also conducted under nitrogen atmo-
sphere. The catalyst was heated to 500 °C with a heating rate of 2 K/
min and held for 120 min until it was cooled down by shutting off the
heaters.
All catalysts were shaped in the same way. They were grinded into a
fine powder before they were pressed into cylindrical pellets (length
15 mm, diameter 11 mm), crushed and sieved. Catalytic tests were
performed with a particle fraction of 0.71 mm to 1.25 mm diameter. To
check on external mass transfer limitation effects, the concentration
gradient of MAC between gas phase and catalyst surface was calculated
(Supplementary Material A.1). Possible mass transfer limitations in the
2.1.3. Catalyst C
The M1 phase was acquired through hydrothermal synthesis in an
autoclave following the procedure of example 2) in [14]. 11.61 g am-
monium heptamolybdate (Alfa Aesar, 99%) and 1.01 g ammonium
metatungstate (Honeywell, 99%) were dissolved in 120 mL of demi-
neralized water (solution A). Another solution containing 3.97 g va-
nadium sulfate oxide hydrate (Alfa Aesar, 99.9%) in 120 mL
Fig. 1. Simplified flow scheme of the lab scale plant.
2

M. Sennerich, et al.
CatalysisCommunications141(2020)106016
inter-crystalline voids (mesopores) were inspected by calculating the
Weisz number Wz (Supplementary Material A.2). Both, internal and
external mass transfer limitations could be ruled out.
300 °C and a pressure of 1.5 bar(a) with a total volumetric inlet flow
between 300 and 1200 mLN/min and a modified residence time be-
tween 500 and 4000 kg·s/m3. The inlet stream composition was 3.4% v/
v MAC, 6.8% v/v O2, 21% v/v H2O and 68.8% v/v N2. The modified
residence time is defined as the quotient of the total mass of active
component and the volumetric inlet flow (Eq. 1) at standard conditions;
values were varied such that a conversion range of from 30% to 80%
was covered.
2.2. Reaction unit
Catalytic experiments were carried out in a lab scale plant with two
tubular fixed-bed reactors in series (Fig. 1). All gases were dosed with
thermal mass flow controllers (Brooks GF40, Brooks SLA5850). Pre-
heated nitrogen (Air Liquide, 99.999%) entered the water evaporator.
Water was fed by a HPLC pump (Bischoff 3350) through a capillary for
additional back-pressure and entered the water evaporator at 80 °C.
Downstream the MAC evaporation was accomplished with an identical
evaporator, which was fed by another HPLC pump (Bischoff 3350) at a
temperature of 125 °C. In contrast to the water evaporation, the back-
pressure generation was achieved with a HPLC back pressure regulator
(Vici JR-BPR2) at pressures > 30 bar(a). Oxygen (Air Liquide, 99.95%)
was added after the MAC evaporator to prevent the occurrence of an
explosive atmosphere during undesired evaporation instabilities. Sub-
sequently the gas mix entered the PFR reactors, which could be in-
dividually bypassed. The feed stream was measured with both reactors
in bypassed position. The two reactors in this setup allowed measuring
two modified residence times (eq. 1) with the same feed stream. Each
reactor was equipped with a blow off valve in case the pressure ex-
ceeded 4 bar(a). After the reactors, the gas stream was mixed with
ethane (Air Liquide, 99.95%) as an internal standard for the online gas
chromatograph (GC, Agilent 6890 N) equipped with a FID – and TCD
detector. A needle valve in the main exhaust pipe allowed for pressure
control and ensured a constant flow rate through the GC sampling loop,
which was connected to the process with a 1/16″ bypass tube. After the
GC all organic components were completely oxidized in a catalytic total
oxidation reactor (CTOR, 0.25% m/m Pd on γ-Al2O3) followed by on-
line IR and paramagnetic detectors for carbon oxides and oxygen (ABB,
AO2020, Uras 26, Magnos 206) in the exhaust gases. An upstream
cooling trap removed water from the gas stream, which could damage
the IR detector. This setup ensured a toxic free gas release into the
environment and a constant monitoring of the plant carbon balance.
Both reactors (stainless steel, length 350 mm, and inner diameter
16 mm) were heated with three heating zones. To ensure ideal plug-
flow and an evenly distributed temperature profile, coarse silicon car-
bide particles of 1 mm were placed above and below the catalyst bed.
The catalyst was embedded in fine silicon carbide particles with 0.2 mm
diameter. Isothermal conditions (ΔTmax = 2 K) were ensured in each
experiment by a coaxially displaceable thermocouple (Ni/Cr–Ni).
Catalytic performance tests were conducted at a temperature of
mKat
V (pN , TN )
τmod
=
̇
(1)
Before taking steady-state measurements, each catalyst was held for
72 h TOS under reaction conditions. All experiments were conducted
for at least 2 h to have a sufficiently large number of GC measurements.
For the evaluation of the measurements, the MAC conversion and the
carbon balanced reactor selectivity of the products were calculated. The
conversion is defined in Eq. 2 as:
nMAC,0
̇
− nMAC
̇
XMAC
=
nMAC,0
̇
(2)
where ṅi is the molar flow and zi the number of carbon atoms of com-
ponent i. The reactor selectivity to the product i (Eq. 3) was calculated
as:
(n˙i − n˙i,0)∙zi
RSi =
(n˙MAC,0 − n˙MAC )∙zMAC
(3)
The surface specific rate coefficient of the catalysts was estimated by
assuming a first order reaction of methacrolein. Integration and trans-
formation of the PFR balance leads to the mass specific reaction rate
coefficient km (Eq. 4):
− ln(1 − XMAC) = km∙τmod
(4)
The surface specific rate coefficient kF (Eq. 5) is obtained as follows:
km
kF
=
ABET
(5)
In addition to the surface specific rate coefficient, the crystallinity
corrected mass specific rate coefficient kC can be calculated according
to Eq.6:
km
kC
=
crystallinity
(6)
Fig. 2. XRD patterns of catalysts A (red), B (blue) and C (black) together with the characteristic XRD reflections of “amorphous” M1, trigonal M1 and h-phase
(bottom). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3

M. Sennerich, et al.
CatalysisCommunications141(2020)106016
2.3. Catalyst characterization
structure, which is characteristic for M1 phases [8,18], catalyst A shows
an irregular surface with no visible rod crystallites. Catalyst B, con-
taining both h- and M1-phase, combines the two morphologies.
The elemental analysis was accomplished by means of wavelength
dispersive X-ray Fluorescence (WDXRF) spectroscopy (Pioneer S4,
Bruker AXS).
3.2. Catalytic performance
X-ray diffraction (XRD) measurements were conducted with a
Bruker D8 Advance diffractometer equipped with a LynxEye XD de-
tector. Cu kα radiation with a step with of 0.015° in a range from 5 to
95° was used. XRD data evaluation was performed with the software
DIFFRAC.SUITE EVA from Bruker. Catalyst crystallinity was estimated
using Eq. (7), where F represents the respective integrated XRD peak
area.
Conversion and selectivity plots of the three catalysts are shown in
Fig. 4. The main products of the partial oxidation of MAC are MAA, CO,
CO2 and acetic acid (HAc). Since the production ratio of CO and CO2
was similar for all measurements, the carbon oxides were lumped to-
gether as COx. The COx and HAc selectivities can be found in the
Supplement. Other mentionable byproducts are acetone and acrylic
acid, which are not shown since their combined selectivity is below 5%.
Both the h-phase catalyst A and the M1 phase catalyst C exceed catalyst
B with a higher selectivity towards MAA, while catalyst B features the
highest selectivity towards COx. All catalysts show roughly the same
selectivity to acetic acid (supplementary Material A.6).
Remarkably, h-phase (catalyst A) and M1 (catalyst C) exhibit similar
selectivity patterns. This is interesting since selective sites for the par-
tial oxidation of unsaturated aldehydes have been ascribed to nano-
crystalline/amorphous M1 and Mo5O14-phases so far [2,9,19]. The
amorphous M1 phase seems to be less selective for MAA, since catalyst
B, containing h-phase and amorphous M1 phase, performs worse than
the h-phase catalyst A. Because of the lack of knowledge about the
coordination of the metal ions and their precise location in both, M1
and h-phase, we refrain from speculations about possible structural si-
milarities between M1 and h-phase as a cause for the similar catalytic
performance. Also, the existence of micropores in both mixed oxides
does not provide a straightforward explanation as these micropores are
far too narrow to participate in diffusional mass transfer.
All catalysts showed a run-in behavior accompanied by a minor
decrease in surface area and a slight drop in conversion within the first
72 h on stream. After the run-in period, the catalysts' activity and se-
lectivity remained unchanged throughout the experiments. Catalyst
activities after completion of the run-in period, i.e. in the steady state,
are reported in Table 2 in terms of first order rated coefficients, which
have been related either to the catalyst mass, to the specific surface area
or to the mass and corrected for the crystallinity. While the mass spe-
cific catalyst activity coefficient increases from catalyst A to C, the
surface specific activity kF of catalyst A lies between catalyst B and C,
with catalyst C also featuring the highest kF value. When relating the
catalyst activity to the crystallinity of the probes, catalyst A shows the
lowest catalytic activity coefficient. Since it is unclear, how the amor-
phous and crystalline catalyst share contribute to the total specific
surface area, the crystallinity and specific surface area cannot be used
for a combined specific rate coefficient.
F
− F
total
amorphous
crystallinity =
F
(7)
total
The catalysts' specific surface areas (ABET) were determined by
means of nitrogen physisorption according to the BET method using a
Micromeritics PhysiSporption ASAP 2020. Scanning electron micro-
graphs (SEM) were taken on a LEO1530. All samples were coated with a
thin carbon film (5 nm) to prevent static charging.
3. Results and discussion
3.1. Catalyst characterization
XRD patterns of the three catalysts are shown in Fig. 2. The phase
composition of the probes estimated by integration of the diffracto-
grams is displayed in Table 1. The phase compositions reported in
Table 1 refer only to the crystalline portions of the catalysts. Catalyst A
is mostly composed of the hexagonal (Mo,V,W)O3 (h-phase) and mat-
ches XRD patterns as reported by Kunert et al. [12,15]. The catalyst also
contains a minor amount of a so-called amorphous M1 phase, to which
the widened peak at 2θ ≈ 22.2° can be assigned. This amorphous M1
phase is reportedly not truly XRD amorphous, but rather lacks far-order
in the a-b planes of the unit cell [5]. Furthermore, catalyst A contains a
minor amount of orthorhombic MoO3 (< 2% m/m). Catalyst B contains
significant amounts of M1 besides h-phase. Catalyst C consists of
amorphous and trigonal M1 phases. The trigonal M1 phase – in contrast
to the amorphous M1 phase – features a far order in a-, b- and c-plane
and exhibits a higher catalytic activity in the partial oxidation of ac-
rolein with similar selectivity towards acrylic acid [5,9,10]. Impurities
in catalyst C are minor amounts of hexagonal (Mo,V,W)O3 (< 2% m/m)
and of a (Mo,V,W)5O14 phase (< 3% m/m). The comparison of XRD
patterns taken before and after catalytic testing for runtimes of at least
120 h indicated no phase changes.
H-phase and M1 phase share hexagonal channels as a morphologic
similarity, and M1 exhibits additional heptagonal channels (Fig. 3).
However, these narrow micropores are not accessible to feed molecules.
BET surface areas (Table 1) and the isotherms of nitrogen ad- and
desorption (Supplement Material) clearly show that these micropores
are too narrow for nitrogen adsorption. Hence, the catalytic MAC
conversion must take place on the external crystal surfaces. It is not
exactly known so far how the metal ions are distributed over the lattice
sites. This lack of knowledge is also reflected in Fig. 3 where all metal
ions (Mo, V and W) are represented by blue balls.
In this regard, the catalyst activity is not simply a function of spe-
cific surface area. As for the selectivity, the higher activity of the M1
phase catalyst C could be related to the trigonal M1 phase, which cat-
alyst B does not contain.
4. Conclusions
The present work compared a hexagonal Mo/V/W phase with the
well-known M1 phase in the selective oxidation of methacrolein to
methacrylic acid. Three catalysts containing different amounts of hex-
agonal phase and M1 phase were synthesized. So far, the hexagonal
Scanning electron micrographs of the catalysts have been enclosed
in the Supplement. While catalyst C crystals exhibit a rod/needle like
Table 1
Phase compositions, specific surface areas and elemental compositions of catalysts A, B, and C.*) figures refer to the crystalline portion of the catalyst.
Catalyst
Crystallinity (% m/m)
h-phase (% m/m)*
M1 phase (% m/m)*
ABET (m2/g)
Composition
A
B
C
≈ 79
≈ 68
≈ 58
> 95
≈ 55
< 5
< 5
≈ 45
> 95
13.9
20.6
25.8
Mo8.00V2.02W1.02
Mo8.00V2.01W0.48
Mo8.00V2.34W0.77
4

M. Sennerich, et al.
CatalysisCommunications141(2020)106016
Fig. 3. Crystal structure of h-Phase [16] (left) and M1 phase [8] (right) from ICSD Database [17]. Blue: Mo,V,W; red: O. (For interpretation of the references to colour
in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Catalytic performances of the three catalysts. MAC conversion over the modified residence time (left), MAA selectivity over MAC conversion (right).
Credit author statement
Table 2
Mass specific, surface specific and crystallinity corrected mass specific catalyst
activity coefficients for catalysts A, B and C.
Maximilian Sennerich: Development of Concepts, Methodology,
Catalytic Experiments and Evaluation, Writing.
Catalyst
k (m3/kg/s)
kF(m/s)
kC(m3/kg/s)
Peter Weidler: Catalyst Characterization XRD.
A
B
C
5.17·10−4
5.55·10−4
1.17·10−3
3.71·10−8
2.69·10−8
4.52·10−8
6.55·10−4
8.16·10−4
2.01·10−3
Bettina
Kraushaar-Czarnetzki:
Development
of
Concepts,
Supervision, Writing - Reviewing and Editing.
Declaration of Competing Interest
phase was only known for its catalytic activity in the partial oxidation
of acrolein to acrylic acid. Interestingly, all the tested catalyst exceed
the typical performance of Mo/V/W mixed oxides in the partial oxi-
dation of methacrolein as reported by literature [2]. The hexagonal
(Mo,V,W)O3-phase is stable under typical reaction conditions and
shows a similar selectivity towards methacrylic acid compared to the
M1 phase. These findings are in contradiction to the current literature
knowledge, which considers nanocrystalline/amorphous M1- and
Mo5O14-phases as the selective species in the partial oxidation of un-
saturated aldehydes such as acrolein and methacrolein [2,9,19]. Fur-
ther research will be required to identify relationships between struc-
ture and catalytic performance.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgements
This research did not receive any specific grant from funding
agencies in the public, commercial, or not-for-profit sectors.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
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CatalysisCommunications141(2020)106016
doi.org/10.1016/j.catcom.2020.106016.
crystalline Mo–V–W–O complex oxides with orthorhombic and trigonal structures
and their application as catalysts, Catalysis, Structure & Reactivity 1 (2015) 71–77,
https://doi.org/10.1179/2055075814Y.0000000009.
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