Applied Catalysis, 41 (1988) 225-239 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

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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 to- wards 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 reac- tivity 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 process- ing; high temperature or subsequent acid washings [ 2-41. The precipitated precursor is transformed into vanadyl pyrophosphate, (VO)2P207, upon cal- cination 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 acti- vation 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.

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Buchanan et al. [ 51 found that none of the pretreatments caused catalyst per- formance to stabilize in less than 48 h under reaction conditions and that se- verely 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 van- adium 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 Wrob- leski 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 prod- ucts. 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 temper- ature, 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 pro- duced 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 selec- tivity. 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 van- adium phosphate catalyst with the addition of an alkyl phosphate and water to the butane-air feed, The useful life of vanadium phosphate oxidation cata- lysts in fixed-bed reactors can be extended substantially by treatment with a phosphorous compound followed by steam treatment [ 151. Neither study re- ported 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-

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mercial fixed-bed MA reactors, and (ii) the use of steam to reactivate vana- dium 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 re- versible 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 hy- drogen 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 char- acteristics 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 modera- tion was found to be necessary in an earlier study which had shown an irrever- sible 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 in- tegral 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 me- tered 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 four- way (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

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(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 se- ries: 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 sep- arate oxygen, nitrogen and carbon monoxide. Water vapor levels were deter- mined with the HP 5790 gas chromatograph either before or after the partial oxidation product analysis. Gas sampling by syringe was used for the perma- nent 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 sys- tem 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 con- stant 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 s) as a function of volume percent n-butane. T= 703 K. Low n-butane conversion, 522%.

Fig. 1 shows a reaction rate dependence between zero and first order in n- butane 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 cata- lytic activity towards butane oxidation was nearly double of that obtained be- fore 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 re- peated 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

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p 60 ..

&;;A o

l q,; '

c l 5 55 .-

Sam

1 L 50 ..

d 0-J

45, : : :: : :: :: + 0 10 20 30 40 50 60 70 80 90 100

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.

‘;i *- @ 7

5 6 t . n n

- 0 10 20 30 40 50 60 70 80 90 100

CONVERSION (%)

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.

@.I 0.60 -0

8

;& l .

l * 9.t a- l

+ 0.55 " 8 o

s

? 8 4 a-

0 or, n . n m n ” n

I n =. 0.50 .. . .

0.451 : : : : : : : : 4 0 10 20 30 40 50 60 70 60 90 100

CONVERSION (%)

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.

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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 anal- ysis. Fig. 2 shows the MA selectivity versus conversion for different water lev- els. 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 en- hanced by water vapor, but its selectivity falls quickly with increasing conver- sion. 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 de- creases monotonically with increasing conversion, but no trend could be de- tected 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 concen- tration, 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 treat- ment but no further changes with subsequent dry reactive treatment. The un- treated (activated) catalyst contained approximately equal amounts of cr- VOPO, and (VO )2P3207 crystalline phases. The two treated catalyst samples [steam (4 days), and steam and dry reaction (4 days each) ] were indistin- guishable, 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 incom- plete activation. Upon exposure to n-butane, vanadium phosphates are re- duced, 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

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a diluent. We believe that water vapor affects the characteristics of the vana- dium 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 selec- tivity 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 by water vapor, which may take place through a step such as

S-+S-O+H,O = 2 S-OH

where S- and S-O denote reduced and oxidized sites on the surface of the V- P-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-bu- tane-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 ex- periments, 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 sec- onds. 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 modifi- cation 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

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of adsorbed partially oxygenated intermediates will decrease when the water vapor level is increased. When [S-O] decreases, the likelihood that the ad- sorbed partially oxygenated intermediates will desorb instead of oxidizing fur- ther 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, hydro- carbons upon adsorption form a reactive partially oxygenated intermediate and that adjacent reactive intermediates may dimerize (or more generally po- lymerize) leading to large intermediates that do not desorb easily, but remain on the surface until smaller species are formed by scission. Unselective oxi- dation 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 mon- oxide 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 gener- ally necessary to activate (following the calcination step) the vanadium phos- phate 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 reac- tive feeds (see Table 2 ). This substantial increase in the surface area was ac- companied 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 struc- ture, in particular the evolution of the catalyst surface area and the composi- tion of the crystalline phases. The slow increase in the surface area observed during the week of kinetic experiments using dry reactive feeds following ac- tivation 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 dis- cussed 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 re- turning 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 in- crease 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 prod- ucts. We found that this selectivity continued to increase slowly (upon contin- ued 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-

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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-block- ing 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 treat- ment 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 van- adium (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 to- wards 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 ulti- mately leads to the unselective oxidation products. An increase in the (P- sites/V-sites) ratio will decrease the concentration of adjacent V-site pairs on the surface and hence the tendency of the hydrocarbon intermediates to di- merize. 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 sur- face 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

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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 deter- mined 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 pro- longed exposure to dry reactive (or in general, oxidizing) conditions, the (P- site/V-site) ratio will decline, leading to a decline in the selectivity.

In summary, we speculate that the intermediate-time-scale effect is attrib- utable 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 ben- eficial. 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 deacti- vation, 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

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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 en- hancement 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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