Applied Catalysis, 41 (1988) 225-239
Elsevier Science Publishers B.V., Amsterdam -
225
Printed in The Netherlands
Effect of Water Vapor on the Activity and
Selectivity Characteristics of a Vanadium
Phosphate Catalyst towards Butane Oxidation
ERNEST W. ARNOLD, III and SANKARAN SUNDARESAN’
Department of Chemical Engineering, Princeton University, Princeton, NJ 08544 (U.S.A.)
(Received 15 September 1987, accepted 18 February 1988)
ABSTRACT
The kinetics of n-butane oxidation over a vanadium phosphate catalyst with a phosphorus to
vanadium rate of 1.1 were studied under different levels of water vapor in the gas phase, in order
to assess the effects of water vapor on the activity and selectivity characteristics of this catalyst.
The water vapor appears to accelerate the development of the solid structure, in particular the
evolution of the catalyst surface area. Further, it leads to an enhancement in the selectivity towards partial oxidation and a decline in the activity towards butane oxidation, when compared to
corresponding dry feed conditions.
INTRODUCTION
Within the past decade, maleic anhydride (MA) production has switched
from a benzene feedstock to a more economic feedstock. The decreased reactivity of the saturated hydrocarbon required the development of highly active
and selective catalysts. These catalysts are unsupported vanadium phosphates
often promoted with other substances. Hodnett [ 1] has reviewed the use of
vanadium phosphate catalysts for MA production.
Numerous patents exist for catalyst preparations involving precipitation from
organic or aqueous media. Most commercial catalysts are prepared with an
alcoholic medium .as aqueous medium precipitations require unusual processing; high temperature or subsequent acid washings [ 2-41. The precipitated
precursor is transformed into vanadyl pyrophosphate, (VO)2P207, upon calcination at 623-673 K. Further activation is required to bring the catalyst to a
state of high activity and selectivity [5].
A number of different pretreatments have been claimed to speed the activation step which generally requires several days. Heating the catalyst under
reducing {pure n-butane [ 61 or hydrogen [ 71)) oxidizing (air), inert (nitrogen
[ 81) and reactive (n-butane in air [ 4,9] ) environments have been suggested.
0166-9834/88/$03.50
0 1988 Elsevier Science Publishers B.V.
226
Buchanan et al. [ 51 found that none of the pretreatments caused catalyst performance to stabilize in less than 48 h under reaction conditions and that severely reducing or oxidizing pretreatments were deleterious.
Pretreatment in an environment where water vapor was intentionally added
to the feed gases does not appear to have been considered for pure vanadium
phosphate catalysts. Wrobleski et al. [lo] pretreated their zinc-promoted vanadium phosphate catalyst with steam in nitrogen at 553-688 K for 5 h. A MA
yield of 60.5% at 82.0% conversion was reported, and this yield level is typical
of commercial catalysts. The effect of steam in the pretreatment used by Wrobleski et al. [lo] has not been reported.
Two detailed studies of butane oxidation kinetics over vanadium phosphate
catalysts [ 11,121 have observed reaction rate inhibition by the reaction products. A modified redox expression
reaction rate =
k, (UC,
I+&~+&~
0
0
where Cs, Co, and CM denote the concentrations of n-butane, oxygen and MA,
was found to correlate the experimental data satisfactorily. Here, T is temperature, k, is an Arrhenius rate constant, and K, and K2 are inhibition factors.
Any or all of the reaction products (carbon monoxide, carbon dioxide, water)
could be responsible for the rate inhibition. Since all these products are produced in approximately the same ratio at all times, it is not possible to identify
the species responsible for the rate inhibition from reaction rate data obtained
using feeds containing only air and n-butane [ 111.
Our preliminary experiments showed that MA added to the feed stream did
not significantly affect the reaction rate but that water (steam) caused a marked
rate reduction. Water appeared to have a favorable effect on the catalyst selectivity. Lerou [ 131 has reported significant influence upon the addition of water
and no influence for carbon monoxide and carbon dioxide additions. Water
reduced the rate of n-butane oxidation and increased MA selectivity.
Steam has been suggested for reactivation of vanadium phosphate catalysts.
Edwards et al. [ 141 reported the reactivation of a molybdenum-promoted vanadium phosphate catalyst with the addition of an alkyl phosphate and water
to the butane-air feed, The useful life of vanadium phosphate oxidation catalysts in fixed-bed reactors can be extended substantially by treatment with a
phosphorous compound followed by steam treatment [ 151. Neither study reported the effect of the addition of steam on the reaction rate.
One can make the following observations from the above discussions: (i) the
tendency of water vapor to increase MA selectivity and decrease reaction rate
suggests that intentional addition of water vapor to the feed gases may have a
beneficial effect on the yield of the partial oxidation products in commercial
MA reactors and may be used to moderate the hot spot temperature in com-
227
mercial fixed-bed MA reactors, and (ii) the use of steam to reactivate vanadium phosphate based catalysts suggests that the addition of steam may also
be beneficial as a catalyst pretreatment.
These potential practical relevances provide the incentive to improve our
understanding of the effect of water vapor on the characteristics of vanadium
phosphate catalysts towards n-butane oxidation. It will be shown that water
vapor plays a very complex role, bringing about some changes which are reversible and some which are not.
EXPERIMENTAL
The vanadium phosphate catalyst (P/V= 1.1) was prepared following the
procedure disclosed by Udovich and Bertolacini [ 161. Briefly, a slurry of 91.3
g vanadium pentoxide (1.0 mol V) in 750 ml of methanol was reduced by hydrogen chloride gas. A mixture of 74.4 g 85% orthophosphoric acid and 32.3 g
phosphorus pentoxide (1.1 mol P) was added along with 250 ml benzene. After
the mixture had refluxed overnight, solvent was removed using a Dean-Stark
trap. The resulting syrup was dried to a porous cake, which was ground, pressed,
broken and sieved to 25-35 mesh.
The catalyst granules were calcined in air at 663 K for 3 h and then stored
under nitrogen in a desiccator. Catalyst samples were activated for at least 4
days at 703 K under a 1.6% n-butane in dry air feed. It was found in an earlier
study that this activation procedure yielded better selectivity and yield characteristics when compared with other activation and pretreatment procedures
151.
The reactor used was a 7 mm I.D. glass U-tube in an aluminum split block.
The catalyst was diluted with glass granules or a 4 mm diameter axial glass rod
was used inside the reactor to eliminate hot spots. This temperature moderation was found to be necessary in an earlier study which had shown an irreversible loss in selectivity for a catalyst subjected to prolonged runs at 723 K and
high conversions [ 51. All experiments were carried out in a once-through integral mode.
CP-grade n-butane and dry air were metered separately and mixed to achieve
the desired compositions. A portion of this mixture was metered to the reactor,
while the remainder was vented. Wet feeds were obtained by bubbling the metered butane-air mixture through water. By adjusting the temperature of the
water in the bubbler, one can alter the volume fraction of water vapor in the
feed gases entering the reactor.
Two separate gas metering systems were used in conjunction with a fourway (selector) valve located close to the reaction inlet to allow step changes
between dry and wet feeds. Catalyst performance responses were studied with
water pulse and step transient experiments.
Steady-state experiments were carried out at 703 K with dry butane feeds
228
(0.7-1.5%), over a wide range of flow-rates (lo-80% conversions). Wet feed
experiments were performed at 703 K with similar butane concentrations and
conversions for water vapor levels of l&15% by volume in the feed.
The effluent stream was analyzed by on-line gas chromatography. A side
stream ran from the heated effluent line to a HP 5790 gas chromatograph
where partial oxidation products [MA and acetic and acrylic acids (ACs) ]
were separated on a Z-m long Porapak QS column. After the reactor effluents
passed through a water bubbler to trap the partial oxidation products, samples
were injected (via a sampling loop) into two chromatographic columns in series: 1 5-m long 30% bis-2-ethoxy ethyl sebacate column to resolve carbon’
dioxide and hydrocarbons, and a 4-m long 13X molecular sieve column to separate oxygen, nitrogen and carbon monoxide. Water vapor levels were determined with the HP 5790 gas chromatograph either before or after the partial
oxidation product analysis. Gas sampling by syringe was used for the permanent gas analysis during transient experiments to avoid mixing and time lags
associated with the on-line sampling.
Catalyst samples were characterized by powder X-ray diffraction and BET
surface area measurements after reaction. A Quantochrome Quantasorb system was used for the surface area measurements with nitrogen as the adsorbate.
RESULTS
A fresh batch of catalyst was activated under reaction conditions, using only
dry feed. Fig. 1 shows the dependence of the butane consumption rate on the
volume % of n-butane in the feed obtained in the week following the activation.
In these experiments, the feed gases were dried before entering the reactor.
The activity of the catalyst increased by about 25% during this week. A constant rate of increase in the activity was assumed to scale the data in Fig. 1.
The selectivity to MA ranged from 52 to 59% for conversions in the range of
5.5 to 17%, and increased by about 3% over this week.
VOLUME
% N-BUTANE
Fig. 1. Butane consumption rate (mol/g-cat
K. Low n-butane conversion, 522%.
s) as a function of volume percent n-butane. T= 703
Fig. 1 shows a reaction rate dependence between zero and first order in nbutane concentration which agrees with the redox model. A modified redox
representation is often chosen when describing partial oxidation kinetics over
vanadium phosphate catalysts [ 11,121.
Following the dry feed experiments whose results are shown in Fig. 1, kinetic
experiments in which water vapor (6-12% by volume) was added to the feed
were carried out. It was found that upon addition of water vapor, the reaction
rate decreased; the selectivity to MA increased, the production (and hence
selectivity) of acetic and acrylic acids increased severalfold, and the
CO/ (CO + CO*) ratio decreased.
After 24 h of exposure to the wet butane-air feed, kinetic experiments were
carried out using dry butane-air feed. The observed selectivities were about
5% higher than those seen prior to any water vapor exposure, while the catalytic activity towards butane oxidation was nearly double of that obtained before any water vapor exposure.
Subsequent periods (lo-30 h) of exposure to wet feed conditions tended to
enhance both selectivity and activity even further. After prolonged and repeated exposure to wet feed, the catalyst seemed to stabilize at the following
performance: wet feed selectivities of 68-72% for MA and 5.5-8.5% (total) for
ACs at 14-18% conversion levels, and dry feed selectivities of 64-66% for MA
and 1.7-2.0% for ACs at 15-25% conversion levels. The final activity under
dry feed conditions was about five times that observed prior to any exposure
to wet feed conditions.
A second batch of catalyst was activated to monitor the effects of water vapor
more closely. After the dry activation, MA selectivity was 49-51% for 18-24%
conversions. ACs selectivity was near the detection limit of about 1.7% at these
conversions. The catalyst was exposed to a series of short (1.5 or 2.5 min)
pulses of wet (15% water vapor) butane-air feed, while the feed gases consisted
of dry butane-air mixtures during the remainder of the time (before, between
and after the pulses). Selectivities and activities were monitored during the
pulse, approximately 1 h after the pulse and finally, four or more hours after
the pulse.
Table 1 shows the selectivities and reaction rate during and after the three
pulses of wet feed. The selectivities increased during the wet feed pulse and
decreased after returning to dry feeds, while the reaction rate did the opposite.
Thus, the effect of water vapor is at least in part readily reversible. However,
the fact that the catalyst sample was subjected to the water vapor pulses had
apparently brought about increases in the selectivities (ca. 4% for MA) and
reaction rate (ca. 15%) under dry feed conditions. Thus, the exposure of the
catalyst to wet feed has also brought about changes in its characteristics that
are not readily reversible. Additional water vapor exposure resulted in further
increases in the selectivities (to roughly those observed with the first batch of
catalyst, about 65% for MA production with a dry feed) and in the reaction
231
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CONVERSION (%)
Fig. 2. Selectivity to maleic anhydride as a function of n-butane conversion at three different water
vapor levels. T = 703 K. n-Butane = 1.00 k 0.05% by volume in wet or dry air. ( n ) 9-14% water;
(0 ) 5-7% water; (0 )O-4% water.
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Fig. 3. Total selectivity to acetic and acrylic acids as a function of n-butane conversion at three
different levels of added water vapor. T= 703 K. n-Butane = 1.00 i 0.05% by volume in wet or dry
air. Key as in Fig. 2.
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Fig. 4. Carbon monoxide fraction of carbon oxides as a function of n-butane conversion at three
different levels of added water vapor. T = 703 K. n-Butane = 1.00 f 0.05% by volume in wet or dry
air. Key as in Fig. 2.
232
treatment. Overnight (15 h) exposure to water vapor restored the 60 + % MA
selectivity for dry feeds.
Catalyst performance for several water levels were determined over a wide
range of butane conversions (15-80% ). All the results presented earlier were
at low conversions (less than 25% ) in order to simplify reaction rate data analysis. Fig. 2 shows the MA selectivity versus conversion for different water levels. The addition of water vapor results in an enhancement of the selectivity
towards MA. This enhancement increases with increasing water level, in the
range of water vapor levels studied. In contrast to the study of Lerou [ 131
which reports that beyond 50% conversion the addition of water vapor led to
little change in the product selectivity, our catalyst appeared to benefit from
the presence of water vapor even at the highest conversions.
Fig. 3 shows the ACs selectivity versus conversion at the different water
levels and Fig. 4 the CO/ (CO+COz) ratio. ACs production is sharply enhanced by water vapor, but its selectivity falls quickly with increasing conversion. Trace amounts of ACs for the lower conversion dry feeds often fall at or
below the detection limit. Although the CO/ (CO + CO*) ratio for wet (9-14%
water vapor) feeds if highly scattered ( + 0.03)) it is always significantly lower
than that for dry feeds. For the dry feeds, the carbon monoxide fraction decreases monotonically with increasing conversion, but no trend could be detected for the wet feeds.
Addition of water vapor to the feed decreases the volume fraction of oxygen
in the feed. In order to test whether the differences between the wet and dry
feeds presented above were simply due to the decrease in the oxygen concentration, experiments were carried out using nitrogen as a diluent instead of
water vapor. Nitrogen was added to some dry feeds to reduce the oxygen levels
from 21 to 16%. Less than 1% change in the MA selectivity, and a slight (if
any at all) decrease in the ACs selectivity were observed.
The X-ray diffraction analysis of the vanadium phosphate catalyst samples
revealed a significant change in the bulk structure due to the water vapor treatment but no further changes with subsequent dry reactive treatment. The untreated (activated) catalyst contained approximately equal amounts of crVOPO, and (VO )2P3207crystalline phases. The two treated catalyst samples
[steam (4 days), and steam and dry reaction (4 days each) ] were indistinguishable, showing only the (VO)zPz07 phase and having about three times
the crystallinity of the untreated sample.
The presence of cw-VOPO, in the untreated catalyst sample indicates incomplete activation. Upon exposure to n-butane, vanadium phosphates are reduced, such as from cr-VOPO, to (VO)zPz07. Increases in surface area and
crystallinity also occur during the activation of vanadium phosphate catalysts.
DISCUSSION
The lack of significant changes in catalyst performance with the addition of
nitrogen to n-butane-air feeds proves that water vapor does not simply act as
233
a diluent. We believe that water vapor affects the characteristics of the vanadium phosphate catalyst in a very complex way. On the basis of our results,
one can identify at least three effects of water vapor on the activity and selectivity characteristics of the V-P-O catalyst towards n-butane oxidation. The
time scales of these effects are vastly different.
Site-blocking effect
The effect occurring at the fastest time scale is the site-occupancy
vapor, which may take place through a step such as
S-+S-O+H,O
=
by water
2 S-OH
where S- and S-O denote reduced and oxidized sites on the surface of the VP-O catalyst respectively. Since water vapor is also a reaction product, some
of the surface sites will be occupied by hydroxyl groups even with dry n-butane-air feeds. At 100% conversion, a 1.5% n-butane-dry air feed can produce
an outlet water fraction of over 6%. However, lower conversions and n-butane
levels gave maximum water levels of about 3% for our dry feed experiments,
which is significantly lower than the water vapor levels encountered in our wet
feed experiments.
Changing the level of water vapor in the feed will result in a change in the
fraction of surface sites occupied by the hydroxyl groups, [S-OH].
Step experiments, in which the water vapor level in the feed is either increased or
decreased, indicate that this response is fast and completed within tens of seconds. An increase in [S-OH]
will necessarily decrease the number of surface
sites available for oxygen and hydrocarbon to occupy, and therefore a decrease
in the rate of butane oxidation when the level of water vapor in the feed is
increased is hardly surprising.
However, it is more difficult to pinpoint exactly the manner in which the
water vapor alters the selectivity characteristics of the catalyst. One can adapt
a viewpoint that the water vapor is not only an overall reaction product, but
also a reactant in some of the elementary steps involved. For example, one of
the steps leading to the selective oxidation products may involve the reaction
of a partially oxygenated hydrocarbon intermediate with S-OH on the surface.
If this is indeed the case, it is entirely possible that an increase in [S-OH] will
favor the selective pathway over the unselective pathway. Although we cannot
rule out such an active participation of water vapor in the reaction sequence,
it is not necessary to invoke such an active participation of water vapor in order
to explain the observed selectivity changes.
The manner in which the water vapor brings about the selectivity modification may very well be an indirect one, as described below. An increase in the
water vapor level will result in an increase in [S-OH].
This necessarily implies
that the concentration of oxidized surface sites, [S-O 1, and the concentration
234
of adsorbed partially oxygenated intermediates will decrease when the water
vapor level is increased. When [S-O] decreases, the likelihood that the adsorbed partially oxygenated intermediates will desorb instead of oxidizing further increases and this is manifested as an increase in the selectivity towards
incomplete (i.e. partial) oxidation.
It has been hypothesized by some previous researchers that the C, hydrocarbons upon adsorption form a reactive partially oxygenated intermediate
and that adjacent reactive intermediates may dimerize (or more generally polymerize) leading to large intermediates that do not desorb easily, but remain
on the surface until smaller species are formed by scission. Unselective oxidation products (carbon oxides, acetic acid and acrylic acid) are postulated to
arise from these scission products.
As per the above mechanism, an increase in [S-OH] would decrease the
probability of two reactive partially oxygenated intermediates being next to
each other and therefore the dimerization rate. This implies that the selectivity
towards MA should increase upon addition of water vapor which is borne out
by experiments. Even within the unselective pathway, the acetic and acrylic
acids result from incomplete oxidation. Therefore, a decrease in [S-O] caused
by an increase in the water vapor level should increase the ACs/carbon oxides
ratio in the product, which is also borne out by the experiments.
An increase in water vapor level was seen to favor the formation of carbon
dioxide over that of carbon monoxide. This appears contrary to what one would
expect based upon decreased [S-O]. In a separate set of experiments, we found
that our catalyst exhibited slight activity towards carbon monoxide oxidation.
The addition of water vapor to a reactor feed containing carbon monoxide,
oxygen and nitrogen resulted in a decrease in the rate of carbon monoxide
oxidation. No carbon dioxide production was observed when only carbon monoxide water and nitrogen were fed to the reactor, indicating the absence of any
activity towards water-gas shift reaction. These observations suggest that there
are probably different pathways for the formation of carbon monoxide and
carbon dioxide (although a portion of the carbon dioxide may be produced by
the oxidation of carbon monoxide) and that an increase in [S-OH]
favors the
pathway leading to carbon dioxide over that leading to carbon monoxide.
In summary, all the observed effects of water vapor on the selectivity and
activity characteristics occurring on a fast time scale, other than the carbon
oxides ratio, can be qualitatively explained by simply assuming that the S-OH
formed on the catalyst surface is passive and site blocking.
This reversible effect occuring in a fast time scale was observed both before
and after the slow irreversible effects described in the next section had taken
place. Hence, we believe that this reversible effect is not dependent on whether
the catalyst has attained an equilibrium structure or not.
Catalyst surface area
Irrespective of the preparation procedure (aqueous or organic), it is generally necessary to activate (following the calcination step) the vanadium phosphate catalyst for several days before its activity and selectivity characteristics
evolve to a reasonable steady state. When the catalyst is activated using a dry
butane-air feed under typical reaction temperatures, the surface area of the
catalyst increases rapidly during the first few days and at a much slower rate
(about 25% in one week) subsequently.
After activating the catalyst using a dry butane-air mixture for four days
and then carrying out kinetics experiments using dry reactive feeds for a week,
we exposed the catalyst to wet reactive feeds. Our experiments indicate that
the catalyst surface area began to increase rapidly after exposure to wet reactive feeds (see Table 2 ). This substantial increase in the surface area was accompanied by a large increase in the rate of butane oxidation. This suggests
that water vapor plays a significant role in the development of the solid structure, in particular the evolution of the catalyst surface area and the composition of the crystalline phases. The slow increase in the surface area observed
during the week of kinetic experiments using dry reactive feeds following activation may also be due to the water vapor present in the reactor (which is
produced as a reaction product ) .
The activation of the catalyst using wet reactive feed may be desirable as it
accelerates the evolution of the solid structure. The role of water vapor in the
evolution of the catalyst surface area differs from the site-blocking effect discussed earlier in two ways: (i) the time scales required for the manifestation
of these effects are widely different (days as opposed to seconds) and (ii) the
effects of water vapor on the evolution of the surface area and crystalline phase
composition are irreversible while its site-blocking effect disappears upon returning to dry feed conditions.
Intermediate time-scale effect on selectivity
It is now clear from the discussions in the previous two sections that when
a V-P-O catalyst that has been exposed only to dry reactive feeds is exposed
to a wet reactive feed, the activity declines at first (in a matter of seconds) due
to the site-blocking effect of the water vapor, but increases slowly over a period
of several days by a substantial amount. This increase is attributed to the increase in the catalyst surface area. The water vapor also led to a rapid increase
(in a matter of seconds) in the selectivity towards partially oxygenated products. We found that this selectivity continued to increase slowly (upon continued exposure to wet reactive feeds) over a time scale of hours (which is
intermediate to the time scales discussed in the previous two sections) and
then stabilized at a new level (see Results). At various stages during this se-
236
lectivity evolution process, if one returns to a dry reactive feed, the selectivity
will drop in a matter of seconds (while the activity will rise) as the site-blocking effect of water vapor is substantially reduced (some water produced as a
reaction product still remains in the system). But the selectivity does not drop
to the level observed prior to any exposure to wet feeds (see Table 1) . However,
the selectivity can indeed be brought down to the level observed prior to any
exposure to wet feeds by a prolonged exposure to dry feeds (see Results). This
loss of selectivity could be restored by an overnight exposure to a wet reactive
feed. Thus we believe that the water vapor alters the selectivity characteristics
of the catalyst in an almost reversible manner at a time scale of the order of
hours and that this effect is different from the site-blocking effect discussed
earlier.
The X-ray diffraction analysis revealed that the bulk structure of the steam
treated, and steam and dry treated catalyst samples were indistinguishable.
The catalyst reduction and crystallinity increase observed upon steam treatment are not reversed by removal of the water vapor from the feed stream.
These observed changes were simply part of the slow activation process which
would have occurred even without the steam treatment, although at a slower
rate. It has been argued previously [ 71 that for catalysts prepared in an organic
medium most of the changes are confined to a near surface region and hence
it is hardly surprising that the X-ray diffraction studies did not reveal any
effect of the water vapor beyond catalyst activation.
We propose the following speculation for the intermediate-time-scale
effect
of water vapor. The surface of a V-P-O catalyst clearly will contain both vanadium (V-sites) and phosphorous (P-sites). It is generally believed that the
V-sites are active towards butane oxidation while the P-sites are essentially
passive. When the (P-sites/V-sites)
ratio on the catalyst surface is increased,
one may expect the following: (i) the rate of butane oxidation will decline as
the concentration of V-sites per unit area declines, and (ii) the selectivity towards partial oxidation will increase, as explained below. It was hypothesized
in the section Site-blocking effect that the dimerization of partially oxygenated
hydrocarbon intermediates adsorbed on adjacent V-sites on the surface ultimately leads to the unselective oxidation products. An increase in the (Psites/V-sites) ratio will decrease the concentration of adjacent V-site pairs on
the surface and hence the tendency of the hydrocarbon intermediates to dimerize. Thus one may expect an increase in the selectivity towards partial
oxidation products.
In the context of bimetallic catalysts, it is well known that the presence of
an adsorbed species can change the composition of the two metals in the surface layer of the catalyst. To our knowledge a systematic study of the effect of
adsorbates on the surface composition of mixed metal oxide catalysts has not
been performed. We speculate that the water vapor leads to a change in the
(P-site/V-site) ratio of the surface layer. The phosphate group has a greater
237
affinity towards water than does the vanadyl group. Hence, it is plausible that
the presence of water vapor draws phosphate groups to the surface layer, thereby
increasing the (P-site/V-site) ratio on the surface which in turn leads to an
enhancement in the selectivity. Hodnett et al. [ 17 ] have shown that a decrease
in selectivity coincided with a marked drop in the surface P/V ratio as determined by X-ray photoelectron spectroscopy.
Under dry reactive conditions, however, the exact opposite will happen. This
is because the phosphorus is always in a 5 + valence state and hence it has no
special preference to whether it is in the bulk or in the surface layer. But the
vanadium which is typically in a less than fully oxidized state is drawn to the
surface layer where the oxygen source (the gas phase) is. Thus under prolonged exposure to dry reactive (or in general, oxidizing) conditions, the (Psite/V-site) ratio will decline, leading to a decline in the selectivity.
In summary, we speculate that the intermediate-time-scale effect is attributable to a reversible change in the (P-site/V-site) ratio on the catalyst surface.
Relevance to reactor operation
Every effect of water vapor observed in our present study seems to be beneficial. It is inexpensive and is often used instead of or along with nitrogen as
a diluent for partial oxidation processes [ 181, and it poses very little additional
separation problems downstream of the reactor.
The water vapor enhances the selectivity towards maleic anhydride. The
unselective oxidation products produced with a dry feed are largely carbon
oxides which have little market value. However, with water vapor present, a
substantial fraction of the so-called unselective products is acetic and acrylic
acids which do indeed have market value. Thus, the addition of water vapor to
the reactor feed should improve the economics of the overall process. Further,
as the heat release associated with the formation of partial oxidation products
is considerably smaller than that for the formation of total oxidation products,
the presence of water vapor mitigates the hot spot problem which is a serious
consideration in the design and operation of these reactors.
Although we have touched upon only the beneficial effects of water vapor, it
is possible that it may have some detrimental effects as well. For example, it is
not known at this time as to how the water vapor affects the useful life of the
catalyst. If the presence of water vapor accelerates the rate of catalyst deactivation, then its beneficial effects may be nullified. A study of the effect of water
vapor on the deactivation of V-P-O catalysts needs to be done to resolve this
issue. It should also be noted that commercial catalysts, which often have other
238
substances added as promoters, may respond differently in the presence of
water vapor that the catalyst used in our studies.
CONCLUSIONS
One can identify at least three effects of water vapor on the activity and
selectivity characteristics of the V-P-O catalyst towards n-butane oxidation.
The time scales of these effects are vastly different.
The site-blocking effect, occurring within tens of seconds, leads to an enhancement in the selectivity towards partial oxidation and a decline in the
activity towards n-butane oxidation when compared to corresponding dry feed
conditions. This effect is readily reversible.
The water vapor plays a significant role in the development of the solid
structure, in particular, the evolution of the catalyst surface area. This effect
is irreversible and takes several days to manifest itself completely.
In addition to the site blocking effect, the water vapor alters the selectivity
characteristics of the catalyst in a beneficial and almost completely reversible
manner at a time scale on the order of hours. It is speculated that this effect is
due to an alteration of the P/V ratio in the surface layer by the water vapor.
ACKNOWLEDGEMENTS
We thank J.S. Buchanan and N. Goeke (Mobil Research and Development
Corporation) for obtaining X-ray diffractograms. Financial support for this
work by the National Science Foundation (CPE-8405132) is gratefully
acknowledged.
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