On the Synergy Effect in MoO3–Fe2(MoO4)3 Catalysts for Methanol Oxidation to Formaldehyde Methanol oxidation to formaldehyde was studied over a series of Fe–Mo–O catalysts with various MoFe atomic ratio and the end compositions Fe2O3 and MoO3. The activity data show that the specific activity passes through a maximum with increase of the Mo content and is the highest for Fe2(MoO4)3. The selectivity to formaldehyde, on the other hand, increases with the Mo content in the catalyst. A synergy effect is observed in that a catalyst with the MoFe ratio 2.2 is almost as active as Fe2(MoO4)3 and as selective as MoO3. Imaging of a MoO3 Fe2(MoO4)3 catalyst by SEM and TEM shows that the two phases form separate crystals, and HRTEM reveals the presence of an amorphous overlayer on the Fe2(MoO4)3 crystals. EDS linescan analysis in STEM mode demonstrates that the MoFe ratio in the amorphous layer is 2.1 in the fresh catalyst and 1.7 in the aged catalyst.
Trang 1O R I G I N A L P A P E R
for Methanol Oxidation to Formaldehyde
Emma So¨derhjelmÆ Matthew P House Æ Neil Cruise Æ
Johan HolmbergÆ Michael Bowker Æ Jan-Olov Bovin Æ
Arne Andersson
Ó Springer Science+Business Media, LLC 2008
Abstract Methanol oxidation to formaldehyde was
studied over a series of Fe–Mo–O catalysts with various
Mo/Fe atomic ratio and the end compositions Fe2O3 and
MoO3 The activity data show that the specific activity
passes through a maximum with increase of the Mo content
and is the highest for Fe2(MoO4)3 The selectivity to
formaldehyde, on the other hand, increases with the Mo
content in the catalyst A synergy effect is observed in that
a catalyst with the Mo/Fe ratio 2.2 is almost as active as
Fe2(MoO4)3and as selective as MoO3 Imaging of a MoO3/
Fe2(MoO4)3catalyst by SEM and TEM shows that the two
phases form separate crystals, and HRTEM reveals the
presence of an amorphous overlayer on the Fe2(MoO4)3
crystals EDS line-scan analysis in STEM mode
demon-strates that the Mo/Fe ratio in the amorphous layer is *2.1
in the fresh catalyst and *1.7 in the aged catalyst The
enrichment of Mo at the catalyst surface is confirmed by XPS data Raman spectra give evidence for the Mo in the amorphous material being in octahedral coordination, which is in contrast to the crystalline Fe2(MoO4)3 bulk structure where Mo has tetrahedral coordination X-ray diffraction (XRD) analysis gives no support for the for-mation of a defective molybdate bulk structure The results presented give strong support for the Mo rich amorphous structure being observed on the Fe2(MoO4)3 crystal sur-faces being the active phase for methanol oxidation to formaldehyde
Keywords Methanol oxidation Formaldehyde Fe–Mo–O catalysts Iron molybdate XRD Electron microscopy SEM TEM HRTEM STEM–EDS XPS Raman spectroscopy
1 Introduction Formaldehyde is a reactive intermediate, which is used for the production of a large number of products [1,2] The largest amounts of formaldehyde are used to produce resins (con-densates) with urea, phenol and melamine, which are used for the production of adhesives and impregnating resins Another market is the manufacture of molding compounds for surface coating Formaldehyde is also used as an intermediate in the production of a variety of chemicals where the most important are polyacetals, MDI, 1,4-butanediol and polyols The world consumption of formaldehyde in the form of its solution with water (37% HCHO) was about 28 million tons in the year
2005, and the present growth rate has been estimated to be around 3–4% per year [2]
Methanol and air are the raw materials for commercial production of formaldehyde in two competing technologies,
E So¨derhjelm N Cruise
Perstorp Specialty Chemicals AB, Process and Catalyst
Development, 284 80 Perstorp, Sweden
M P House M Bowker
School of Chemistry, Main Building, Cardiff University,
Cardiff CF10 3AT, UK
J Holmberg
Perstorp Specialty Chemicals AB, Perstorp Formox, 284 80
Perstorp, Sweden
J.-O Bovin
Division of Polymer and Materials Chemistry, Department of
Chemistry, Lund University, Chemical Center, P.O Box 124,
221 00 Lund, Sweden
A Andersson (&)
Department of Chemical Engineering, Lund University,
Chemical Center, P.O Box 124, 221 00 Lund, Sweden
e-mail: arne.andersson@chemeng.lth.se
DOI 10.1007/s11244-008-9112-1
Trang 2which are referred as the silver process and the oxide
pro-cess, respectively The silver process operates at
methanol-rich conditions with silver as catalyst, while the oxide
pro-cess uses an iron molybdate catalyst under methanol-lean
conditions The first use of a silver catalyst was patented in
1910 [1] In 1931, Adkins and Peterson [3] reported that
methanol was selectively oxidised to give formaldehyde
over an oxide catalyst with equimolar amounts of
molyb-denum and iron Although a catalyst with excess Mo gave
lower activity, it was more selective to formaldehyde
However, it was not until the 1950s that that the molybdate
catalyst got commercial importance
The basic chemical composition of the oxide catalysts
has practically been the same over the years It is well
known that the fresh catalyst consists of two crystalline
phases, namely MoO3and Fe2(MoO4)3[4 9] In spite of
this fact, there have been several process improvements
since the early 1960s as shown in Fig.1 [2] The
intro-duction of gas recirculation from the absorber allowed the
methanol inlet concentration to be increased from 6.5 to 7.5
vol.% A few years later exhaust catalyst systems (ECS)
became common, leading to increased steam production
but no increase in productivity Replacement of the
gran-ular catalyst with ring-shaped catalysts allowed the gas
flow rate to be increased and so the productivity Dilution
of the first part of the catalyst with inert rings (mixed layer)
gave better temperature control and allowed increase of
both the gas velocity and the methanol concentration up to
8.5 vol.% A further development was pressurisation of the
plants (0.3 bar g) More recently a refined catalyst activity
profile (CAP) is used in new plants, allowing the heat of
reaction to be properly distributed along the length of the
reactor tube and making possible operation with a metha-nol concentration at the inlet of *10 vol.%
The durability of the catalyst depends on several factors including the methanol and oxygen concentrations, the temperature and the pellet diameter [10] In practice it varies between 8 and 18 months depending on the oper-ating conditions and tolerances By now, it is well established that the catalyst deactivates because it looses
Mo during operation due to the formation of volatile spe-cies [9 13], causing lower activity and selectivity as well
as increased pressure drop as molybdena needles condense
in the lower part of the reactor Another deactivation cause
is sintering of the catalyst in the hot spot [11,14,15] The deactivated catalyst consists mainly of Fe2(MoO4)3, but contains as well the worse performing Fe2O3and FeMoO4 phases [9,11,13,16,17]
It has been reported that the catalyst must contain both MoO3and Fe2(MoO4)3for being active, selective and long-lasting [8, 9, 12,13,18,19] Different explanations have been given in the literature for the observation that both phases are required One explanation is that Fe2(MoO4)3is the active phase [6,7,20] and MoO3is required to secure that no iron rich phase is formed [12,13,21–23], which is also the opinion expressed in a comprehensive review to summarize the present knowledge about the active phase [4] Other investigators have proposed that the active phase
is Fe2(MoO4)3with excess Mo in the structure Fagherazzi and Pernicone [24] suggested from X-ray diffraction (XRD) data that the active ferric molybdate is Fe-defective due to
Mo6? substitutes for some of the Fe3? ions in octahedral coordination and additional oxygen goes into interstitial positions A similar conclusion was drawn by Leroy et al [25] considering combined XRD and Raman results According to Pernicone [8] the active structure for methanol oxidation has the composition Fe2-3x(Mo1?xO4?1.5x)3with
x B 1/9 and usually below 0.05, consisting of nanoregions
of MoO6octahedra included in the Fe2(MoO4)3structure Some researchers have concluded that the active phase is a solid interstitial solution of MoO3in the Fe2(MoO4)3lattice [5,26,27], rather than being a substitution compound Here
it is also worth pointing out that Massarotti et al [28] excluded any solubility of MoO3in Fe2(MoO4)3, because they obtained very similar lattice-constant values for the stoichiometric molybdate and the molybdate in non-stoi-chiometric samples prepared by solid-state synthesis Moreover, considering IR spectra, Sun-Kuo et al [19] draw the conclusion that the active material consists of a dis-persed polymeric amorphous structure, and Aruanno and Wanke [14] have proposed that the activity is due to for-mation of a Mo rich surface on the Fe2(MoO4)3 crystals Briand et al [29] observed that bulk molybdates and supported molybdenum oxide catalysts possess similar activity and turnover frequency, indicating that the surfaces
0
5
10
15
20
25
Time
Gas recirculation
Ring-shaped catalyst Mixed layer Pressurization
Refined activity profile
Fig 1 Gains in the productivity of formaldehyde per reactor tube and
day since the early 1960s
Trang 3of bulk molybdates may be composed only by molybdenum
oxide species in a two-dimensional overlayer Besides
being a dopant entering in small amount the interstial
positions in the Fe2(MoO4)3 structure, Trifiro` et al [17]
have pointed out that Mo in excess is necessary for
improvement of the mechanical properties of the catalyst
and in the process for securing efficient reoxidation of
formed ferrous molybdate
In light of the disagreement in previous literature on the
role of excess Mo for the performance of iron molybdate in
methanol oxidation, the purpose of our work is to present
new findings to clarify the origin of the observed synergy
between the two catalyst constituents MoO3 and
Fe2(MoO4)3
2 Experimental
2.1 Catalyst Preparation
A series of Fe–Mo–O catalysts was prepared with the
atomic Mo/Fe ratios equal to 0.2, 0.5, 1.0, 1.5 and 2.2
starting from solutions of 100 ml ammonium
heptamo-lybdate (BDH C 99%, 0.254 M for the Mo/Fe 1.5 catalyst)
and 50 ml iron nitrate (BDH C 98%, 0.338 M) The
molybdate solutions were acidified to *pH 2 using nitric
acid (Fisher, Laboratory Grade), before drop wise addition
of iron nitrate with stirring at 60°C giving canary yellow
precipitates, which were evaporated to near dryness at
90°C The resulting solids were then dried at 120 °C
overnight before being calcined in air at 500°C for 48 h
The single oxides of Fe2O3(Aldrich C 99%) and MoO3
(BDH C 99.5%) used were commercially sourced
For comparative purposes the pure Fe2(MoO4)3 phase
was prepared by solid-state reaction comprising mixing
and grinding stoichiometric amounts of Fe2O3and MoO3,
heating at 600°C for 16 h, followed by a second grinding
and heating for another 16 h, now at 650°C
Some of the samples being characterized in this work
have been supplied by Perstorp Formox
2.2 Catalyst Characterisation
The surface areas of the catalysts were measured either
using a Micromeritics Gemini 2360 or a CE instruments
QSurf M1 A five or six point BET method was used with
adsorption of nitrogen at liquid nitrogen temperature and
subsequent desorption at room temperature All samples
were degassed at 150°C for 1 h
XRD was performed using either an Enraf Nonus FR590
fitted with a hemispherical analyser, or, a Seifert XRD
3000 TT diffractometer In both cases Ni-filtered Cu Ka
radiation was used
Fourier transform Raman (FT-Raman) spectra were recorded on a Bruker IFS66 FTIR spectrometer equipped with a Bruker FRA106 FT-Raman device, a Nd:YAG-laser and a germanium diode detector The laser power was 100
mW, the resolution was 4 cm-1 and 400 scans were col-lected for each spectrum
XPS analysis was performed on a Kratos XSAM 800 spectrometer using Al Ka X-ray radiation (1486.6 eV) Quantifications were made using a linear background and instrumental sensitivity factors Charging effects were corrected for by adjusting the main C 1s peak to a position
of 285.0 eV The anode was operated at an accelerating voltage of 13 kV and a current of 19 mA The pass energy was 80 eV and the residual pressure in the spectrometer was
10-8torr, or, lower
SEM images were recorded on a JEOL 840A micro-scope using a tungsten filament and a voltage of 20 keV EDS analyses were made using an Oxford instruments analyser with the INCA software
TEM imaging was performed using a JEOL 3000F microscope, operating at an acceleration voltage of 300 keV The used EDS analyser from Oxford instruments was equipped with the INCA software Linescanning was per-formed using EDS in STEM mode Several dots in a line were analysed on each crystal, starting in the vacuum and then gradually moving onto the crystal The typical dis-tance between the dots was between 20 and 30 nm 2.3 Activity Measurements
Activity and selectivity measurements on the laboratory prepared samples were made on a pulse flow micro reactor system [23] The reactor basically consists of a U-tube mounted vertically within a Phillips PU 4500 GC oven with gas continuously flowing over the bed A small amount of the outlet gas stream is monitored by a Hiden Analytical Hal
201 quadruple mass spectrometer, while the rest of the gas
is vented via a Leybold Heraeus Trivac rotary pump In this work, a heated gas with 10 vol.% O2 in He (BOC) was flowed over 0.5 g catalyst at a rate of 30 ml/min (STP) with
1 ll methanol injections being made every 2 min, while the temperature in the furnace was ramped from *150 to
*380 °C Before each run, methanol injections were made over a bypass to account for the daily drift in the mass spectrometer
Comparative activity measurements on fresh and used commercial samples were made under adiabatic conditions
in a stainless-steel micro reactor with a diameter of 21 mm A flow with 1.75 vol.% methanol in dry 25 l/min (STP) air with
an inlet temperature of 260°C was passed over 13 g of 1 mm particles of crushed and sieved catalyst The conversion of methanol was calculated comparing the signal from a Ber-nath Atomic 3006 FID-analyser before and after the reactor
Trang 43 Results
3.1 Influence of the Phase Composition on the
Catalytic Performance
The laboratory prepared Fe–Mo–O samples were tested in
a pulse flow micro reactor system for their performance in
methanol oxidation In Table1are the metal compositions
and the specific surface areas of the prepared samples
lis-ted XRD and Raman spectroscopy showed that the two
single cation catalysts are single phase haematite Fe2O3
(JCPDS file no 33-664) and orthorhombic MoO3(JCPDS
file no 35-609), respectively [30] The catalysts with the
Mo/Fe ratios 0.2 and 0.5 consist of Fe2O3and Fe2(MoO4)3
(JCPDS file no 31-642) [30] In the samples with the Mo/
Fe ratios 1.0 and 1.5 only diffraction peaks from
mono-clinic Fe2(MoO4)3were observed The stoichiometry of the
former sample, however, dictates that there must be an
additional iron phase, presumably Fe2O3, which is either
amorphous or highly dispersed The catalyst with Mo/Fe
equal to 2.2 is a mixture of Fe2(MoO4)3 and MoO3 The
catalysts below the stoichiometric level (Mo/Fe \ 1.5)
presented a brown colour, the Mo/Fe 1.5 catalyst presented
a yellow colour, while the Mo/Fe 2.2 catalyst was light
green
For comparing the catalytic activity of the samples,
methanol conversions at 180°C are given in Table 1 It is
seen that the samples with 0.2 and 0.5 Mo/Fe ratios give
the highest conversions, mainly due to their high surface
areas For a better comparison, the conversions should be
normalised with respect to surface area The normalised
values in Table1 indicate that the two samples with the
Mo/Fe ratios 1.5 and 2.2 are the most active preparations
per unit surface area To better account for the differences
in methanol conversion, first order surface area normalised
rate constants were calculated The values clearly show
that the activity increases when the Mo/Fe ratio is
increased from 0 (Fe2O3) up to 1.5 (Fe2(MoO4)3) With further increase of the Mo/Fe ratio, the activity declines Concerning the selectivity to formaldehyde at high meth-anol conversion the data in Table1 for 90% conversion, show that the selectivity steadily increases with the Mo content in the catalyst Of the pure phases, Fe2O3does not produce any formaldehyde, whereas the selectivity on
Fe2(MoO4)3 and MoO3 is 73% and 90%, respectively Considering both activity and selectivity, the sample with Mo/Fe = 2.2 is outstanding, being almost as active as the pure Fe2(MoO4)3and as selective as the pure MoO3 Also in industrial operation of Fe–Mo–O catalysts there
is an optimal Mo/Fe ratio for the catalyst to perform well with good activity, selectivity and durability [9] In Fig.2
are the activities of used commercial catalysts compared with data for the corresponding freshly prepared catalysts The used catalyst samples had been collected from the
Table 1 Specific surface area, activity and selectivity of prepared Fe–Mo–O catalystsa
Catalyst
Mo/Fe ratio
Specific surface
area (m 2 /g)
Conversion at
180 °C (%)
Activity (conversion/m 2
surface area)
First order rate constant k (cm 3 /min/m 2 )
Selectivity (%) to formaldehyde
at 90% conversion b
a Activity and selectivity measured in a pulse flow reactor injecting 1 ll liquid methanol every 2 min into a 30 ml/min gas flow with 10 vol.% O2
in He, passing over 0.5 g catalyst while ramping the temperature from *150 to *380 °C (see Section ‘‘ Experimental ’’)
b The temperature giving 90% methanol conversion on 0.5 g catalyst is given within brackets
0 5 10 15 20 25 30 35 40 45
Inlet layer Fresh catalyst
Outlet layer Fresh catalyst
Outlet layer Used catalyst
Inlet layer Used catalyst
Fig 2 Conversion of methanol as measured in an adiabatic micro reactor (21 mm in diameter) over freshly prepared commercial catalysts and the corresponding used samples The used samples had been collected from the inlet and outlet parts of an industrial multitube reactor after a full lifetime cycle Reaction conditions: 1.75 vol.% methanol in dry 25 Nl/min air with an inlet temperature of
260 °C The amount of catalyst in the micro reactor was 13 g of 1 mm particles of the crushed and sieved catalyst
Trang 5upper and lower part of the reactor, respectively, after the
operation of a catalyst load in an industrial reactor had
been terminated due to normal ageing According to XRD
the inlet fraction consisted mainly of Fe2(MoO4)3, and
elemental analysis indicated the presence of another 0.55
wt.% of Fe2O3in agreement with the catalyst surface being
covered with a thin reddish brown layer The fraction from
the outlet consisted of the MoO3and Fe2(MoO4)3phases,
however, the Mo/Fe ratio was higher than in the unused
catalyst and needle-like MoO3 crystals were observed on
the catalyst rings As previously has been explained, Mo in
the upper part of the catalyst forms volatile species with
methanol [9,10], which species decompose and condense
as needles in the lower part of the bed [9, 11] Thus, the
activity data in Fig.2for industrial catalysts are in general
agreement with the data in Table1 for the laboratory
prepared samples The deactivation of the catalyst at inlet
conditions is due to the surface being coated with a thin
layer of Fe2O3, which according to Table 1 has lower
activity and selectivity for methanol oxidation A
deacti-vation cause for the catalyst at the outlet of the reactor is
that it contains more MoO3compared to the fresh catalyst
The data in Table1shows that the pure MoO3is less active
than the composition Mo/Fe = 2.2 with both MoO3 and
Fe2(MoO4)3
3.2 Catalyst Characterisation with Electron
Microscopy and XPS
MoO3/Fe2(MoO4)3catalysts were characterised with SEM,
TEM, HRTEM and STEM–EDS before and after use in an
industrial reactor SEM shows the presence of large
plate-like MoO3crystals being about 10 lm in size (Fig.3) The
crystal composition was verified by EDS analysis and,
moreover, the observed crystal habit is typical of
orthorhombic MoO3 [31] Generally it was observed that the MoO3crystals are more frequent in fresh catalyst than
in used catalyst Besides the plate-like crystals, the major part of the catalysts consists of smaller crystals, which appear to be randomly close packed as Fig.3 shows
To gain information about the small crystals, samples were investigated by TEM and EDS Most of the observed crystals are Fe2(MoO4)3but also some are MoO3 The two types of crystals are in the same size range as Fig.4shows
Fe2(MoO4)3presents crystals which mostly are rectangular/ elliptic, while the MoO3 crystals are thin and plate-like HRTEM imaging of the Fe2(MoO4)3 crystals revealed a 5–10 nm thick amorphous surface structure, extending around the crystals The amorphous structure, which was observed both in fresh and aged samples, can be seen on the edges of the crystal in Fig.5
Fig 3 SEM image of a fresh MoO3/Fe2(MoO4)3catalyst
Fig 4 TEM image of a fresh MoO3/Fe2(MoO4)3catalyst
Fig 5 A HRTEM image of a typical Fe2(MoO4)3crystal with an amorphous layer extending around the edge of the crystal
Trang 6The very surface of the Fe2(MoO4)3crystals in a freshly
prepared and an aged fraction of the same catalyst were
analysed using EDS in STEM mode Line scans were
col-lected starting in the vacuum outside the edge of the crystal
and then gradually moving in onto the bulkier parts of the
material By using this technique, it is expected to get
information about any difference in elemental composition
between the surface and the bulk of the crystal For the
wedge-like crystal terminations, the surface composition
should be analysed at the edge of the crystal and with
increasing distance from the edge, the analyses should show
an increasing contribution from the bulk as the thickness of
the amorphous layer being only about 5–10 nm The
ana-lysed Mo/Fe ratios are plotted in Fig.6against the distance
from the edge of the analysed crystal For the unused cat-alyst the Mo/Fe ratio is the highest at the edge, where the ratio is *2.1 With increase of the distance from the edge, the Mo/Fe ratio drops to a value of about 1.5 Compared with the fresh catalyst, the data for the corresponding aged sample shows a considerably lower Mo/Fe ratio on the edge (*1.7) and an identical value 1.5 further from the edge The measured ratio 1.5 for the bulk is in perfect agreement with the stoichiometry of the Fe2(MoO4)3crystal
As a complement to the EDS point analyses performed in STEM mode on separate Fe2(MoO4)3crystals, XPS analy-ses were performed on a number of MoO3/Fe2(MoO4)3 preparations to give information about the average Mo/Fe ratio in the surface region Figure7shows a comparison of
Fresh Catalyst
1 1,2 1,4 1,6 1,8 2 2,2 2,4
Distance from crystal edge (m)
crystal 1 crystal 2 crystal 3 crystal 4 crystal 5 crystal 6 crystal 7 crystal 8
Aged Catalyst
1,00 1,20 1,40 1,60 1,80 2,00 2,20 2,40
Distance from crystal edge (m)
crystal 1 crystal 2 crystal 3 crystal 4 crystal 5 crystal 6 crystal 7
Fig 6 The Mo/Fe ratios as
determined by STEM–EDS line
scan analysis on a Fe2(MoO4)3
crystal in (upper figure) a
freshly prepared MoO3/
Fe2(MoO4)3catalyst and (lower
figure) the corresponding
catalyst after ageing in an
industrial reactor
Trang 7the bulk ratio with the corresponding surface ratio as
determined by XPS In agreement with the EDS analyses,
the XPS data clearly shows a general trend, namely that the
catalyst surface is richer than the bulk in Mo
3.3 XRD and Raman Characterisation of the
Fe2(MoO4)3phase
Figure8a shows an overlay of the XRD patterns of a
MoO3/Fe2(MoO4)3catalyst and a phase pure Fe2(MoO4)3
prepared by solid-state reaction from stoichiometric
amounts of MoO3 and Fe2O3 The difference pattern in
Fig.8b clearly exhibit peaks only from orthorhombic
MoO3, indicating no difference in unit cell between the
phase pure Fe2(MoO4)3and the molybdate in the catalyst
with excess MoO3 Included in the figure is a spectrum
recorded for a pure MoO3 sample, showing very intense
(0 k 0) peaks due to the preferred orientation of the
plate-like crystals in the sample holder The most intense peak
is (040), while in the difference pattern the (021) peak is
the most intense for the MoO3 in the catalyst matrix
According to data calculated from the unit cell (JPDS file
no 35-609) [30], the (021) peak should be the most intense
peak for a non-oriented sample, which obviously is the case
for the catalyst sample where the MoO3crystals have no
preferential orientation in a matrix of Fe2(MoO4)3
The Raman spectrum of a deactivated molybdate catalyst
from a full-scale reactor is shown in Fig.9together with the
spectrum of pure MoO3 According to XRD and atomic
absorption the used catalyst consists of Fe2(MoO4), and the
recorded Raman spectrum is in perfect agreement with
spectra reported in the literature for the pure Fe2(MoO4)3
phase [29,32,33] The spectrum in Fig.9of the pure MoO3
shows strong bands at 996, 818, 666 and 284 cm-1together
with a number of less intense bands in the region below
500 cm-1 In the spectrum of the Fe2(MoO4)3phase, bands
are seen at 988, 967, 934, 818, 781 and 348 cm-1 Thus, it is
clearly seen that the molybdate spectrum does not include any contribution from crystalline MoO3because there are
no bands at 666 and 284 cm-1and, moreover, the bands at
988 and 818 cm-1are of equal size, whereas in MoO3the band at 818 cm-1is more than twice as large as the band at
996 cm-1 It has been suggested for Fe2(MoO4)3that the two bands at 988 and 818 cm-1can be from some minor octahedrally coordinated Mo species [32] in contrast to the dominant Mo species, which is tetrahedral in a perfect
Fe2(MoO4)3lattice [24]
4 Discussion The catalytic data in Table 1reveal a pronounced synergy effect in that an atomic Mo/Fe ratio above the stoichiom-etric ratio for Fe2(MoO4)3 is needed for a catalyst to be
0
0.5
1
1.5
2
2.5
3
3.5
Catalyst #
Bulk Surface
Fig 7 Comparison of the Mo/Fe bulk ratios with the corresponding
ratios determined by XPS for a number of MoO3/Fe2(MoO4)3
preparations
0 20 40 60 80 100 120
Theta ( ˚ )
Pure iron molybdate Commercial fresh catalyst
*
*
*
* MoO 3 [0k0] peaks
(a)
-20 0 20 40 60 80 100 120
Theta ( ˚ )
Difference MoO3
(020) (110)
(040) (021) (111) (060)
(200)(061)
(002) (081) (112) (211)
(b)
Fig 8 A comparison of the XRD patterns recorded for a MoO3/
Fe2(MoO4)3 catalyst and the pure Fe2(MoO4)3 phase prepared by solid state reaction (see Section ‘‘ Experimental ’’) The upper figure (a) shows an overlay of the two diffractograms In the lower figure (b)
is shown the resulting difference pattern obtained by subtracting the XRD for the pure phase from that of the catalyst Here it is seen that the remaining peaks in the difference pattern correspond well with the XRD recorded for orthorhombic MoO3 The difference peaks have been indexed according to the JPDS file no 35-609 [ 30 ]
Trang 8both active and selective to formaldehyde formation This
result is in general agreement with previous reports,
although these in most cases have not considered both
activity and the selectivity at high methanol conversion in
the whole range of compositions from iron to molybdenum
oxide A surface area normalised activity maximum for
methanol oxidation has been reported for the ratio Mo/
Fe = 1.7 [18,19], which is not in conflict with the data in
Table1 showing that the catalysts with the Mo/Fe ratios
1.5 and 2.2 present similar activity Moreover, in another
report [22] is described that the highest specific activity
was observed for the stoichiometric composition
Fe2(MoO4)3 It has also been reported that too high Mo
content in the Fe–Mo–O catalyst results in lower activity
[11], which is in agreement with the data in Fig.2 for a
catalyst collected from the outlet of an industrial reactor
This catalyst with excess molybdena needles on the surface
is less active than the corresponding unused catalyst The
observation in Table1 that the normalised activity
(k value) of the pure MoO3is almost a factor three lower
than that for the sample with the Mo/Fe ratio 2.2 is in
perfect agreement with a previous comparison of a
Har-shaw MoO3/Fe2(MoO4)3 catalyst with a MoO3 catalyst
[34] Concerning the selectivity to formaldehyde, the
val-ues in Table1show a steady increase with the Mo content
up to the Mo/Fe ratio 2.2 with a selectivity of 90% at 90%
methanol conversion, which is identical with the value for
the pure MoO3 A similar trend has been reported
previ-ously [18, 23] For instance, Kolovertnov et al [18]
reported that the selectivity was low in Fe rich samples,
whereas samples with Mo/Fe ratios above 1.5 and the pure
MoO3are highly selective to formaldehyde at high meth-anol conversion
In our work, we have chosen first order rate constants as
a means to account for the differences in surface area and conversion among the catalysts (see Table 1) The same method has been adopted by others [18] According to Machiels and Sleight [35], the kinetics for a large number
of molybdates and MoO3follows a power-law rate model with about 0.5 order in the methanol concentration Therefore, we also calculated the half order rate constants for the catalysts in Table 1 The values, however, are not shown as they perfectly confirmed the trend indicated by the first order rate constants
In general, several possibilities have to be considered to explain an observed synergy between phases One of the possibilities is dual phase catalysis, where some of the reaction steps occur on one phase while following steps take part on another phase An example here is the ammoxidation of propane over the Mo–V–Nb–Te–O system with the two phases designated M1 and M2, where propane reacts to give propene on M1 and the formed propene then readsorbs on M2, which is more selective than M1 for the transformation of propene to acrylonitrile [36] In the case of methanol oxidation on MoO3/Fe2(MoO4)3, such type of mechanism is unlikely since the reaction pathway to form-aldehyde involves no intermediate gaseous product and, moreover, the methoxy intermediate does not migrate over the surface as it is relatively strongly bound [23]
Another alternative to be considered is that the catalysis may occur on the grain boundaries between the phases as has been suggested for 3-picoline ammoxidation on vanadia phases [37] However, the electron microscopy results in Figs 3 and 4 show separate crystals of the MoO3 and
Fe2(MoO4)3phases Although in contact with each other, no intergrowth is observed of the crystals from the two phases
In a study of the reduction behaviours of MoO3,
Fe2(MoO4)3and their mixtures by in situ electron micros-copy in a CH3OH/He atmosphere [38], it was noted that the crystals in the mixtures remain as distinct phases with the same reduction behaviour as the individual phases In view
of the above results, it seems that the observed synergy effect is not primarily related to the grain boundaries In other cases an observed synergy between two phases has been explained by oriented growth of one phase on another, e.g in the case of Sb–Sn–O mixed oxides for propene oxidation [39] However, our microscopic investigation showed no oriented growth of MoO3on Fe2(MoO4)3, see Figs 3and4
In some catalyst systems an activation process occurs under influence of the catalytic reaction leading to the for-mation of new phases, which may be more active and selective than the original composition [40] This occur-rence is not the case in methanol oxidation on MoO3/
100 300
500 700
900 1100
Fe 2 (MoO 4 ) 3
Fig 9 The Raman spectra recorded for MoO3and a used catalyst
sample consisting of Fe2(MoO4)3
Trang 9Fe2(MoO4)3catalysts On the contrary, in practice a steady
deactivation of the catalyst is observed with time-on-stream
as the data in Fig.2 confirms, which also concurs with
previous results [9,11] During the deactivation, Fe2O3and
FeMoO4 may form [6, 9, 11] The iron oxide has low
activity and is unselective (Table1), and FeMoO4has been
reported to be selective with an activity comparable to that
of MoO3[35]
HRTEM imaging of the Fe2(MoO4)3phase in the catalysts
revealed an amorphous surface structure on the edges of the
crystals as shown in Fig.5 In fact the same type of structure
was observed by Gai and Labun [38], although they just
briefly mention it as a note in their article when referring to a
small area in one of the images The investigators focused in
their work on the bulk structures and their reduction The
EDS data in Fig.6shows that the amorphous structure on the
fresh catalyst is rich in Mo (Mo/Fe *2.1) while the
amor-phous material on the aged catalyst has a lower content
(Mo/Fe *1.7) Considering there being a link between the
ageing and the composition and the performance of
the catalyst (Fig.2), we believe that the active material is the
amorphous structure with excess Mo as compared to the bulk
Fe2(MoO4)3structure The fact that the Mo/Fe ratio is higher
on the molybdate surface than in the bulk, moreover, is
confirmed by the XPS analyses in Fig.7 A similar trend has
been observed by other investigators using EDS and XPS
[12,13,27] The Raman spectrum of the Fe2(MoO4)3phase
in Fig 9confirms the existence of an additional structure on the molybdate In agreement with published spectra of the pure phase [29,32,33], Fig.9shows two bands at 988 and
818 cm-1, respectively, appearing as shoulders on the strong bands at 967 and 781 cm-1from the tetrahedrally coordi-nated Mo in the bulk [33] In agreement with a previous assignment [32], the two shoulder bands can be from Mo in octahedral environment considering that MoO3with octa-hedrally coordinated Mo gives two bands at similar wavenumbers i.e 996 and 818 cm-1 (Fig.9) Also in IR,
Fe2(MoO4)3gives a weak band at 990 cm-1[27,32], which has been assigned to Mo in octahedral coordination [32] Thus, from these facts it can be proposed that the Mo in the amorphous layer may be in octahedral coordination sur-rounded by six oxygen atoms
The formation of an amorphous surface layer on
Fe2(MoO4)3can be understood considering its crystalline bulk structure, which in idealized form is illustrated in Fig.10 as built up by regular tetrahedra and octahedra Looking at the structure it is seen that it is very open Therefore, the formation of an amorphous overlayer can be
a means for stabilizing the surface It is not clear whether excess Mo is needed in the synthesis only to give an amorphous layer with a ratio Mo/Fe[2 in the catalyst, or, if the MoO3crystalline phase in the finished catalyst has an additional role to sustain the desired Mo/Fe ratio in the active structure during operation in methanol oxidation
Fig 10 The ferric molybdate
structure with Mo in tetrahedral
and Fe in octahedral
coordination shown in idealized
form as built up by regular
polyhedra The red and yellow
polyhedra have Mo and Fe,
respectively, in the center, and
oxygen in the corner positions.
Each oxygen is shared by two
polyhedra and therefore
2-coordinated As indicated in
the three figures, the structure is
viewed along the [100], [010]
and [001] directions,
respectively
Trang 10Previously, it has been proposed that surplus Mo in the
catalyst is needed for avoiding the formation of Fe2O3in the
reoxidation of formed ferrous molybdate FeMoO4 to the
desired ferric molybdate Fe2(MoO4)3with a higher Mo/Fe
ratio [17, 21] In view of the fact that SEM and TEM
imaging shows separate MoO3 and Fe2(MoO4)3 crystals
with only physical contact (Figs.3and4), it seems doubtful
whether MoO3should have such a role It is true that iron
oxide is not formed until the Mo/Fe ratio in the catalyst
approaches the value 1.5 and the catalyst is free from any
crystalline MoO3[9] However, a kinetic study of the Mo
loss from MoO3/Fe2(MoO4)3 preparations has shown that
the loss from the MoO3crystals occurs considerably faster
than from the remaining Fe2(MoO4)3[41], which is
sup-ported by results in our previous deactivation study [9]
In several cases, it has been concluded that Fe2(MoO4)3
is the active phase in MoO3/Fe2(MoO4)3catalysts [4,6,7,
20–22] According to our results this is correct though
incomplete A better description is that the molybdate is a
support for the active structure, which is an amorphous
surface layer with a higher Mo/Fe ratio compared to the
bulk and with Mo in octahedral coordination This finding
is in partial agreement with some of the earlier proposals,
which however were based on indirect evidence In these
works the active material has been described as a Mo rich
molybdate surface [14], a dispersed amorphous structure
[19] and molybdenum oxide species forming a monolayer
[29] Concerning the previous indications of a defective
molybdate bulk structure being formed when prepared in
excess of molybdenum, we have found no evidence for the
formation of either a solid interstial solution of MoO3 in
the Fe2(MoO4)3lattice [5] or Mo6? substituting for some
Fe3?and extra oxygen entering the interstial positions [24]
The XRD patterns in Fig.8 of a MoO3/Fe2(MoO4)3
cata-lyst and the stoichiometric Fe2(MoO4)3phase, indicate no
difference between the crystalline bulk structure of the
ferric molybdate in the catalyst and the corresponding
phase prepared without excess molybdenum This finding
is in agreement with previous reports [6,7,21,28]
5 Conclusions
The present study of methanol oxidation to formaldehyde
on Fe–Mo–O catalysts has demonstrated that the best
performing catalyst is a mixture of the crystalline phases
MoO3 and Fe2(MoO4)3 A synergy effect is observed in
that the catalyst is almost as active as the pure Fe2(MoO4)3,
which is less selective, and as selective as the pure MoO3,
which is less active
HRTEM imaging of a fresh MoO3/Fe2(MoO4)3catalyst
and a corresponding aged catalyst discloses that the active
structure may be an amorphous overlayer on the surface of
the crystalline ferric molybdate Line-scan EDS analysis reveals that the Mo/Fe ratio in the amorphous surface is higher in the fresh catalyst than in the used catalyst The Mo/Fe surface ratio for the latter approaches the value 1.5, although the surface still is amorphous The Raman spec-trum of a catalyst consisting of Fe2(MoO4)3 shows bands indicating that in the amorphous layer the Mo is in octa-hedral coordination
A comparison of the XRD pattern of a MoO3/
Fe2(MoO4)3 catalyst with that for the pure Fe2(MoO4)3 phase evidences that the crystalline bulk structure of ferric molybdate is the same in both samples, ruling out previous proposals of the formation of a defective molybdate with excess Mo in the crystal lattice
Acknowledgements Perstorp Specialty Chemicals AB and the EPSRC in the UK are acknowledged for support of a studentship to M.P.H.
References
1 Reuss G, Disteldorf W, Grundler O, Hilt A (1988) In: Ullmann’s encyclopedia of industrial chemistry, 5th edn, vol A11, VCH Publishers, Weinheim, pp 619–651
2 Chrichton B (2006) In: Informally speaking (newsletter from Perstorp Formox), spring/summer 2006, pp 2–8 http://www perstorpformox.com
3 Adkins H, Peterson WR (1931) J Am Chem Soc 53:1512
4 Soares APV, Portela MF, Kiennemann A (2004) Catal Rev 47:125
5 Abaulina LI, Kustova GN, Klevtsova RF, Popov BI, Bibin VN, Melekhina VA, Kolomiichuk VN, Boreskov GK (1976) Kinet Katal 17:1307
6 Carbucicchio M, Trifiro` F (1976) J Catal 45:77
7 Alessandrini G, Cairati L, Forzatti P, Villa PL, Trifiro` F (1977) J Less-Comm Met 54:373
8 Pernicone N (2003) CATTECH 7:196
9 Andersson A, Hernelind M, Augustsson O (2006) Catal Today 112:40
10 Popov BI, Bibin VN, Boreskov GK (1976) Kinet Katal 17:371
11 Burriesci N, Garbassi F, Petrera M, Petrini G, Pernicone N (1980) In: Delmon B, Froment GF (eds) Catalyst deactivation, Stud Surf Sci Catal, vol 6, Elsevier, Amsterdam, pp 115–126
12 Soares APV, Portela MF, Kiennemann A (2001) Catal Commun 2:159
13 Soares APV, Portela MF, Kiennemann A, Hilaire L (2003) Chem Eng Sci 58:1315
14 Aruanno J, Wanke S (1975) Can J Chem Eng 53:301
15 Aruanno J, Wanke S (1977) Can J Chem Eng 55:93
16 Ma YH, Kmiotek SJ (1988) J Catal 109:132
17 Trifiro` F, Carbucicchio M, Villa PL (1998) Hyperfine Interact 111:17
18 Kolovertnov GD, Boreskov GK, Dzisko VA, Popov BI, Tarasova
DV, Belugina GG (1965) Kinet Katal 6:1052
19 Sun-Kou MR, Mendioroz S, Fierro JLG, Palacios JM, Guerro-Ruiz A (1995) J Mater Sci 30:496
20 Kim TH, Ramachandra B, Choi JS, Saidutta MB, Choo KY, Song S-D, Rhee Y-W (2004) Catal Lett 98:161
21 Boreskov GK, Kolovertnov GD, Kefeli LM, Plyasova LM, Karakchiev LG, Mastikhin VN, Popov VI, Dzisko VA, Tarasova
DV (1966) Kinet Katal 7:144