ACS Symposium Series - ACS Publications - American Chemical

Chapter 14

Activation of n-Pentane on Magnesium—Vanadium Catalysts 1

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Downloaded by NORTH CAROLINA STATE UNIV on October 8, 2012 | http://pubs.acs.org Publication Date: August 13, 1996 | doi: 10.1021/bk-1996-0638.ch014

S. A. Korili , P. Ruiz , and B. Delmon 1

Aristotle University and Chemical Process Engineering Research Institute, P.O. Box 1520, GR-54006 Thessaloniki, Greece U n i t éde Catalyse et Chimie des Matériaux Divisés, Université Catholique de Louvain, Place Croix du Sud 2/17, B-1348 Louvain-la-Neuve, Belgium 2

The activation of n-pentane in the presence of oxygen has been studied over two magnesium-vanadium oxides, magnesium pyrova nadate, Mg V O , and magnesium orthovanadate, Mg V O , prepa red by the citrate method. The catalytic behavior of mixtures consi sting of magnesium vanadate and antimony oxide, 50-50 % wt, was also investigated. The X-ray diffraction patterns of the vanadates have shown that they were of high purity, while XPS measurements revealed a slight surface enrichment in magnesium. At 350-500°C and atmospheric pressure, similar total n-pentane conversions were achieved with the two magnesium vanadate phases; the main products were linear unsaturated C hydrocarbons, i.e. pentenes and pentadienes, and carbon oxides. Magnesium orthovanadate was more selective than the pyrovanadate towards the pure dehydro genation reaction in comparison with the combustion one, and its selectivity was in general increased by the addition of Sb O . The latter had little effect on the behavior of the pyrovanadate phase. Decreasing the oxygen to alkane ratio in the feed led to a decrease in total conversion and an increase in selectivity, while the C hydro carbon product distributions were not influenced appreciably. The extent of the homogeneous reaction between pentane and oxygen was negligible for oxygen to alkane ratios in the feed equal to 0.5 and 1.0, but increased dramatically when the ratio was raised to 2.0. 2

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Vanadium containing catalysts have attracted considerable research interest due to their performance in selective oxidation reactions. Among them, magnesiumvanadium compounds have been repeatedly reported as effective catalysts for the oxidative dehydrogenation of hydrocarbons, such as transformation of propane to propene (1-3), butane to butene and butadiene (2), and even ethylbenzene to styrene (4), producing mainly unsaturated hydrocarbons with negligible formation of oxygenated organic products. In spite of the great extent of research done, the conclusions drawn on the activity and selectivity of these materials are still controversial. Although it is generally accepted that the rate determining step is the breaking of the first C - H bond of the hydrocarbon to form an alkyl species on the catalyst surface, the way 0097-6156/96/0638-0192$15.00/0 © 1996 American Chemical Society In Heterogeneous Hydrocarbon Oxidation; Warren, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


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that this alkyl species is transformed into selective oxidation products is still a matter of debate. Some researchers focus on reactivity and on the effect of the atomic arrangement of the active sites (2, 5), proposing different explanations for various reactants. Redox properties of catalysts have been found to correlate well with selectivity (1), while the environment of the active phase may also play a role through possible cooperation of phases (1) or contamination by residual elements (2). It appears that there is a great number of factors influencing catalyst perfor mance, and what is very important, the nature of the hydrocarbon being activated is probably one of them. It is therefore challenging to test the behavior of the magnesium vanadates in the oxidative dehydrogenation of other hydrocarbons as well, as for example pentane. In fact, surprisingly little research has been done hitherto on C5 - particularly npentane- oxidative dehydrogenation, regardless of the catalyst. Some results were reported in the 70's, most of them included in U.S. patents, where C5 components were a part of a broad range of hydrocarbons investigated (6-9). The investigations dealt mainly with the reactions of branched and unsaturated C5 molecules, such as the production of isoprene from isopentane (6), from isoamylenes (7), and from a mixed feed consisting from isoamylenes, isopentane and n-pentane (8), or the production of straight chain alkadienes from alkenes (9). Manganese ferrites, bismuth molybdates and composite samples combining iron with nickel and antimony, or cobalt and phosphorous, were tested as potential catalysts in those studies. Recent publications on the subject are dealing mainly with the selective oxidation of pentane over vanadium-phosphorous catalysts for the formation of oxygenated products (10,11). During the oxidation reactions, catalysts often undergo reduction and deacti vation. On the other hand, it has been observed that some oxides, such as Sb204, although inactive directly in the catalytic reaction, can improve catalyst behavior by regeneration of active sites through the action of a surface mobile oxygen species. The phenomenon is known as the remote control mechanism, and the oxides providing oxygen are characterized as donors (12). The scope of the present work is the study of the oxidative dehydrogenation reaction of n-pentane over two magnesium-vanadium catalysts, namely magnesium pyrovanadate, Mg2V207, and magnesium orthovanadate, Mg3V208- Mixed catalysts of these phases with antimony oxide have also been prepared and tested, in order to test whether the combination of this oxide with the active phase leads to an improvement of the catalyst performance. Experimental Catalyst Preparation. The catalysts were prepared by the citrate method (13). Mg(N03>2.6H20 (Fluka purum p.a., >99%) was dissolved in distilled water at ambient temperature. The citric acid (Merck GR, >99.5%) first, and then a slurry of N H 4 V O 3 (Merck GR, >99%), were added to this solution, always under agitation at room temperature. A few ml of H N O 3 65% (Merck GR) were also added to the resulting solution, in order to avoid any precipitation. The metal salts were in suitable proportions for the desired catalyst composition, and the quantity of citric acid was such that the anions were in 10% excess compared to those stoichiometrically required for the cations. The final transparent solution was evaporated at 30°C under reduced pressure in a rotavapor, up to the formation of a very dense homoge neous liquid, which was dried in a vacuum oven at 80°C for 24 h. The precursor formed this way, was decomposed in air by heating in an oven at 300°C for 16 h. The solids were subsequently calcined in the same oven in air at 550°C for 20 h. The mechanical mixtures with Sb204 were prepared by mixing the catalysts with the oxide in fine powder form, dispersing the mixtures in n-pentane under vigorous agitation at room temperature, and then evaporating the solvent by over-

In Heterogeneous Hydrocarbon Oxidation; Warren, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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HETEROGENEOUS HYDROCARBON OXIDATION

night drying at 80°C. Sb204 was prepared by calcination of Sb203 (Merck GR, >99%) at 550°C for 20 h. The composition of each mixture was 50% catalyst - 50% Sb204, on a weight basis. The mechanical mixtures were used as prepared, without additional calcination or other treatment


Catalyst Characterization. Characterization of the prepared catalysts and mechanical mixtures included determination of their elemental composition, identification of formed phases, measurement of their specific surface areas and evaluation of their surface composition. Chemical Analysis. The bulk elemental composition of the catalysts was determined by atomic absorption on a Philips PC 8210 spectrometer. Phase Identification. X-ray diffraction (XRD) patterns of the solids were obtained on a Siemens D5000 powder diffractometer with Ni-filtered CuKoc radiation operated at 40 kV and 50 mA. Surface Area. Specific surface areas were measured by Kr adsorption at -196°C on a Micromeritics ASAP 2000 static apparatus using the BET method. Krypton is considered a suitable adsorbate for the determination of relatively small surface areas (< 20 m /g) because of its low saturation vapor pressure which minimizes possible errors in dead volume corrections (14). 2

Surface Composition. The surface composition of the catalysts was evaluated by X-ray photoelectron spectroscopy (XPS) measurements on a Fisons SSI X-probe spectrometer, model SSX 100/206, equipped with a monochromatized microfocus Al X-ray source (1486.6 eV). The sample powders were pressed into small stainless steel troughs of 4 mm diameter, introduced in the spectrometer at room temperature and outgassed to a pressure of 10~ Torr. Analysis was made in high vacuum, -5x10-9 Torr. The spot size was 1.4 mm and the pass energy was set at 50 eV, while a low energy flood gun set at 6 eV with a nickel grid placed 3 mm above the samples was used for compensation of charging effects. The exact binding energies were calculated with respect to the £ - ( C , H) component of the Is adventitious carbon peak which was fixed at 248.8 eV. The peaks recorded were C Is (284.8 eV), O Is (-530 eV), Mg 2s (-89 eV), V 2p3/ (-517.5 eV) and Sb 3^3/2 (-540 eV). The recorded spectra were decomposed to 85/15 Gaussian/Lorentzian curves, after subtraction of the non-linear background, using a least squares fitting routine. Element atomic ratios on the surface of the samples were calculated from the relative intensities of the decomposed peaks, using the sensitivity factors supplied by the manufacturer, that is 1.00 for C Is , 2.49 7

2

2

for O Is, 0.64 for Mg 2s , 5.49 for V 2p / and 9.62 for Sb 3^3/23

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Catalyst Testing. Catalytic tests were performed in a continuous flow microreactor operating at near atmospheric pressure. This microreactor was a Pyrex U-shape tube mounted vertically in a tubular furnace. The temperature of the catalytic bed was monitored by a thermocouple entering the bed through a side thermowell. Temperature control was performed with an independent thermocouple located outside the reactor and connected to the furnace, in order to avoid oscillation phenomena. n-Pentane was fed to the reactor from a certified high purity gas tank (Indugaz, special gas mixture) containing 4 % vol. of n-pentane in helium. Other gases used were oxygen and helium, both from Air Liquide and > 99.995% purity, which were used without any further purification. Mass flow controllers monitored and controlled the flow of gases to the reactor, and the flow in the outlet was continuously measured with a bubblemeter located at the exit of the system. The pressure drop in the reactor, which was always negligible, was monitored by a



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pressure gauge connected to the reactor inlet. The gases were mixed before entering the reactor by passing through a 35 cm stainless steel tube full of glass beads and were heated to 100°C before entering the reactor. The outlet of the reactor and the lines further on were also heated to 100°C to avoid any condensation of the products. Products and reactants emerging from the reactor were diverted through a four-way valve to the sampling valves of a gas chromatograph, equipped with a thermal conductivity detector and a double-column system, consisting of a Porapak Q and a Durapak n-Octane on Porasil C column. This GC system was used for the majority of the experiments reported in the present paper, but in the process of the work it was upgraded to a more powerful and flexible one, consisting of two dete ctors, a thermal conductivity and a flame ionization, and three columns permitting full analysis of all reactants and possible product isomers. Fresh catalysts were ground, pelletized, then crushed and sieved, and only the fraction -800+500 |im was used in the tests. In a typical experiment, 500 mg of catalyst were spread on a Pyrex frit fitted in the reactor, between two layers of glass beeds to avoid any fluidization. The gas feed contained 3.5 % vol. n-pentane, oxygen, at an oxygen to alkane molar ratio equal to 0.5 or 1.0, and the balance helium. The total flow was 25 cm /min (ambient conditions), which corresponded to a space velocity of the order of 10 h" . The usual duration of a catalytic run was about lh, during which several injections were made to the gas chromatograph. The results reported here correspond to a time-on-stream of 30-45 min. Existence of hot spots in the catalytic bed during reaction, was not evident in our experiments, since temperature fluctuations were kept to a minimum: catalyst temperature varied no more than ±2°C during runs. 3

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Results Catalyst Properties. The composition and specific surface area values of the fresh catalysts and the mechanical mixtures are shown in Table I. The magnesium to vanadium ratios corresponding to the bulk composition of the samples, as measured by atomic absorption spectroscopy, were very close to the theoretical ones (within experimental error). The BET surface areas of the catalysts were relatively large for these types of materials, due to the low temperatures applied during calcination. Usually, the surface areas of similar materials reported in the literature (2,4,5) axe below 1 m /g, this being probably the result of calcination temperatures as high as 800°C. The BET surface area of the Sb204 used in the preparation of the mecha nical mixtures was 0.5 m /g. The X-ray patterns of the pure fresh catalysts revealed the existence of a single phase in each sample, the one corresponding to the stoichiometry used for prepara tion. The XRD pattern of the fresh magnesium orthovanadate catalyst is presented as an example in Figure la. The structure of the magnesium pyrovanadate catalyst 2

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Table I. Catalyst Compositions and Specific Surface Areas Catalyst Mg2V2C>7 Mg3V208 Mg2V27 - Sb20 Mg3V208 - Sb20

BulkMg/V atom ratio*

4 4

1.0 1.6 1.0 1.6

(1.0) (1.5) (1.0) (1.5)

Surface area (ntlg) 6 16 3 9

a

Measured by atomic absorption spectroscopy. Numbers in parentheses denote the theoretical values.




(a)

(b)

i JLLJL

(e) "T

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26 (°) Figure 1. X-Ray diffraction patterns of catalysts. (a) Mg3V20g fresh (b) Mg3V20g used (c) Sb2C>4 fresh (d) M g V 2 0 8 - Sb204 fresh (e) Mg3V20g -Sb2C>4 used. 3



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corresponded to the more stable alpha phase. Also the structure of the antimony oxide corresponded to the alpha phase, containing only minor, unidentified impu rities (Figure lc). As expected, the patterns of the freshly prepared mechanical mixtures included only lines attributable to the particular catalyst used and Sb204; the X R D pattern of the mechanical mixture of magnesium orthovanadate with Sb204 is shown in Figure Id. The atom ratios of the main elements on the surface of the samples, calculated from XPS measurements, are shown in Table II. For both pure catalysts, the surface M g / V ratios were higher than the bulk ones. Similar observations for increased magnesium content on the surface of magnesium vanadates, have been made by other researchers (1,15). No significant differences in the Mg/V surface ratios were , observed between the pure catalysts and their mechanical mixtures. In the latter materials, the surface Sb/(V+Mg+Sb) ratios were lower than the ones corresponding to their bulk composition.

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Table II. Atom Ratios on the Catalyst Surface Catalyst

Mg2V207 Mg3V20g Mg2V2C>7 - S b 0 Mg3V20g - S b 0 2

4

2

4

Mg/V atom ratio used fresh 1.39 1.36 2.06 1.91 1.23 1.14 2.23 1.95

Sb/(V+Mg+Sb) atom ratio fresh used -

0.20 0.08

-

0.29 0.14

(0.30) (0.28)

Measured by XPS. Numbers in parentheses denote the theoretical values. In general, no significant changes of physicochemical properties have been observed when comparing the samples before and after the catalytic tests. No formation of new phases was observed either for the pure catalysts, or for the mechanical mixtures. The X R D patterns of the magnesium orthovanadate catalyst and its mechanical mixture, both already used in reaction studies, are shown in Fi gure lb and le, for comparison with the patterns of the fresh samples. Concerning the mechanical mixtures, the proportion of antimony on the surface increased for the samples after testing. When using pure catalysts in some cases where oxygen was totally consumed during the reaction, drastic changes of color from white-like to black, and an increased surface coverage with carbon, have been observed for the samples after testing. Reaction Studies. We checked for the possible occurrence of gas phase homogeneous reaction by performing several series of tests in an empty reactor at temperatures 300-500°C and with oxygen to pentane ratios in the feed equal to 0.5, 1.0 and 2.0. When the ratio was 0.5 or 1.0, the empty reactor showed practically no activity in the temperature range examined. For example, for the oxygen to paraffin ratio equal to one and at 500°C, the total conversion of pentane was just 1.5%, and the products were pentenes and C2 - C3 light hydrocarbons. When the oxygen to paraffin ratio in the feed was changed to 2.0, the increase of the extent of the homo geneous reaction was impressive. Even at 350-400°C the n-pentane conversion was as high as 30%, and the main products were C5 alkenes, C2 and C3 hydrocarbons, carbon oxides, and probably oxygenated organic compounds. As the temperature increased to 450°C, the conversion dropped abruptly to 2%, then increased slowly



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with temperature up to 5% at 500°C. In this temperature range, products were pentenes, C2 and C3, while oxygenated molecules disappeared entirely. In the catalytic experiments, reaction products were mainly pentenes (1pentene, trans- and cis-2-pentene), pentadienes (trans- and cis-1,3-pentadiene), carbon oxides and water. Oxygenated organic products have not been detected. In some cases, light hydrocarbons, mainly C2 and C3, were also produced in small amounts due to cracking reactions. With increasing temperature, total conversion of n-pentane, as calculated from the reactant concentration in the inlet and outlet of the reactor, increased for all samples tested. Conversions of n-pentane achieved at various reaction conditions are presented in Figures 2 and 3, along with the corre sponding selectivities. It should be mentioned that at the early stages of this research, analytical system limitations did not permit full analysis of carbon oxides. Therefore, in order to have a common basis for comparing selectivities of all the samples presented here, we use in this paper the selectivity to C5 dehydrogenation products relative to selectivity to CO2 only, that is the ratio of moles of n-pentane converted to C5 unsaturates to the moles of n-pentane converted to CO2. Since the C O 2 proportion in carbon oxides varied with temperature and active phase present in the sample, comparison is better to be restricted to results obtained at one tempe rature with samples containing the same active phase. As can be seen in Figures 2 and 3, the total n-pentane conversions achieved by the two pure magnesium vanadates under the same reaction conditions had very similar values. The extent of the dehydrogenation reaction and the extent of full oxidation to carbon oxides were different, with the orthovanadate phase being in all cases more selective to dehydrogenation compared to the pyrovanadate one. The same observations can be made when comparing the two mechanical mixtures. Total pentane conversion achieved with the mechanical mixtures was in all cases slightly higher than the one corresponding to the amount of active phase contained in the samples. When compared on selectivity terms, the mechanical mixture con taining magnesium pyrovanadate had similar behavior to the pure catalyst, while the mixture containing orthovanadate was better than the pure catalyst. Hydrocarbon product distributions obtained at reaction temperatures 400, 450 and 500°C, and oxygen to paraffin ratios in the feed equal to 1.0 and 0.5, are shown in Figures 4 and 5. The predominant dehydrogenation products were in most cases the 2-pentenes. As the temperature and the conversion increased, the pentadiene fraction also became significant, especially with the pure catalysts. Hydrocarbon distributions obtained with mechanical mixtures did not differ markedly from the ones with the corresponding catalysts for temperatures up to 450°C. The mechani cal mixtures produced in general less alkadienes than the pure phases, the difference coming almost totally from the 1-pentene fraction. At 500°C, the selectivity patterns of the pure and the mixed catalysts were much more different, with the mixtures always producing less alkadienes than the corresponding pure samples. Oxygen to pentane ratio had no major influence on the hydrocarbon product distributions for all samples tested, apart from a decrease of the alkadiene product fraction with increasing oxygen content in the feed. The possible existence of mass and energy transport limitations under our catalytic test conditions has been checked by application of standard literature criteria (16). It was found this way that even the more probable limiting steps, such as intraparticle mass transfer, interparticle heat and mass transfer and interphase heat transfer, do not prevail in most cases, especially at low conversions. At higher conversions, the increased complexity of the reaction scheme makes the calculation of the several parameters included in these criteria rather ambiguous. Therefore, the results of the calculations under these conditions are less accurate, although being still on the safe side. Some individual experiments in the low conversion range, where pure magnesium orthovanadate at a particle size of -500+300 |im was used, confirmed that there were no transport limitations.


14.

Activation of n-Pentane on Mg—V Catalysts

KORILI ET AL.

5.0

50 Mg V 0 O2/nC =1.0 2


40

2

7

- 4.0

5

H3.0

30 20

A O

2.0 * ^

1.0

"

7+Sb204 one (Table II). The apparent antimony proportion on the surface of the mechanical mixtures increased after testing. The formation of a new phase on the sample surface during the catalytic reaction is not very probable, because the binding energies of elemental lines were the same for the samples prior to and after testing. A more plausible explanation is that carbon, deposited preferably on the catalyst active sites, shielded the XPS emissions of the corresponding elements, this resulting in a relative increase of the Sb signal. Both pure magnesium vanadates proved to be effective for the activation of npentane, with the orthovanadate being the most selective for dehydrogenation products, i.e. alkenes and alkadienes. The enhanced selectivity of the ortho phase is in accordance with what has been observed for n-butane (2) and heavier molecules, like ethylbenzene (4), while in the case of propane the selectivity of the pyro phase to propene is higher than that of the ortho phase (1, 5). In the magnesium orthova nadate catalyst, active sites consist of isolated V O 4 tetrahedra, while in the pyrova nadate the active sites are V2O7, which are in fact corner-sharing V O 4 units. It is logical to suppose that an adsorbed alkyl intermediate, formed by the initial acti vation of the n-pentane molecule, can take up more oxygen from V2O7 than from V O 4 . This could result in the pyrovanadate phase being less selective to dehydrogenated molecules than the orthovanadate one, as observed in the results of the present study. The selectivity towards dehydrogenation products, generally appeared to decrease with increasing temperature (Figures 2 and 3). In fact, this decrease was due to the increase in the conversion. This inverse variation of selectivity with conversion, a common trend for catalytic oxidation reactions, is usually not very much influenced by temperature itself. An enhanced dehydrogenation selectivity was observed at 500°C for an oxygen to pentane ratio equal to 0.5, especially for the orthovanadate containing samples (Figures 2 and 3). This was probably a result of


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the fact that oxygen in the gas phase was almost totally consumed in these cases and only the pentane was in contact with the catalyst surface at the exit part of the catalyst bed. This could alter the development of selectivity, due to reaction of surface oxygen only, but no experiments were carried out with n-pentane alone fed to the reactor, in order to check the reactivity at these conditions. The relative amount of carbon monoxide produced could also play a role. If the C O produced under these conditions contained a greater proportion of CO, then the selectivity, as presented in Figures 2 and 3, would appear to be enhanced. However, in expe riments where both carbon oxides were analyzed, the proportion of monoxide in carbon oxides was not higher in all cases of total oxygen consumption (18). The fact that the 2-pentenes were the predominant hydrocarbon products was probably the result of the greater probability of breaking first a methylene C-H bond instead of a methyl C-H bond. The methylene bonds are easier to break because they are usually weaker than the methyl ones (19). This suggestion is consistent with the fact that the light hydrocarbons produced were usually C and C3, and is further supported by results of the literature concerning oxidation reactions of hydrocarbons of various chain lengths (20-22). Under similar reaction conditions, a tendency of increase of the dehydrogenation rate is observed with increasing chain length of the hydrocarbon, that is with increasing number of methylene C-H bonds. The two pure magnesium vanadates examined exhibited similar pentane conversions when tested under the same conditions. It should be pointed out that the specific surface area of M g V 0 was significantly lower than that of Mg3V 08. Therefore, it can be assumed that for the same surface area, the pyrova nadate catalyst could give higher conversions than the orthovanadate one, thus exhibiting higher specific activity (i.e. per unit surface area). This decrease of conversion per surface area with increasing magnesium content, indicates that the vanadium ions are responsible for developing the catalyst activity for n-pentane oxidative dehydrogenation. In our case, this can only be considered as a trend and cannot yet be extended to any quantitative activity conclusions, since the conversion levels were generally rather high. The conversion increased with temperature for all samples (Figures 2 and 3). The conversion levels at the higher temperatures exa mined were sometimes lower than expected, due to total oxygen consumption. The addition of S b 0 to the catalysts had a general positive effect on the conversions achieved. At each temperature, total n-pentane conversion with each one of the mechanical mixtures was somewhat higher than the value corresponding to the amount of active phase in the sample. This observation is true even at the low temperature - low conversion points. Under the assumption of differential reactor operation, this could be extended to relative sample activities, thus reflecting an improvement of the catalysts through a cooperation between magnesium vanadates and antimony oxide. The latter material is inactive for the reaction when tested pure. However, the reaction studied here is in fact a very complicated scheme and its kinetics are unknown for the moment, so any conclusions on catalyst activities based on conversions should be addressed with caution. It is better that the results are interpreted in terms of conversion only. Antimony oxide had little effect on catalyst selectivity for the pyrovanadate phase, but in the case of the orthovanadate, selectivity to dehydrogenation was generally improved with the mechanical mixtures. This improvement cannot be attributed solely to the lower conversions obtained with the mechanical mixtures, since in many cases the increase in selectivity was greater than the one resulting from the decrease in conversion alone. Further investigation is necessary to clarify the role of S b 0 as creator of selective or inhibitor of non-selective reaction sites and to confirm the existence of a remote control mechanism, as it has been observed in the case of other light alkanes such as butane (23). The effect of changing the oxygen to alkane ratio in the feed on the catalyst performance was in general the same for the two phases examined. Decreasing this


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Figure 4. C5 hydrocarbon product distribution for oxygen to n-pentane ratio in the feed equal to 1.0. Total n-pentane conversions are shown in Figures 2 and 3. A:Mg2V2Q7 B:Mg3V2p8 C: M g a V ^ - S b ^ D : Mg V p -Sb2p4 M 1-pentene H 2-pentenes H pentadienes 1

7

7




204

Figure 5. C5 hydrocarbon product distribution for oxygen to n-pentane ratio in the feed equal to 0.5. Total n-pentane conversions are shown in Figures 2 and 3. A:Mg2V2p7 B:Mg3V2p8 C : Mg2V2Q7-Sb2p D: Mg2V207-Sb2Q SI 1-pentene H 2-pentenes U pentadienes 4


4


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ratio from 1.0 to 0.5 resulted in a decrease of the total n-pentane conversion and an increase of the dehydrogenation selectivity. In the case of the samples containing the magnesium pyrovanadate phase, both pure and in the mechanical mixture, the increase in selectivity could be attributed almost totally to the decrease in conver sion. In the case of the samples containing the ortho phase and at low oxygen to alkane ratio, the dehydrogenation selectivity was higher than the one accounted for solely by the decrease of n-pentane conversion. Analogous trends of decreasing conversion and increasing selectivity with decreasing oxygen content in the feed have been reported for similar oxidation reactions of ethane (24), propane and bu tane (20) and ethylbenzene (4) on vanadium containing catalysts. In most of these cases, the effect on selectivity was more pronounced than the one on conversion. The distribution of the dehydrogenation products, pentenes and pentadienes, was not much affected by the oxygen to n-pentane ratio in the feed (Figures 4 and 5). This supports the claim that the dehydrogenation and the combustion reactions usually proceed via parallel paths (21, 22). This is especially true with short resi dence times, while at longer contact times, where a parallel-consecutive scheme of reaction is more probable, it is possible that the dehydrogenation reaction steps depend in the same way on the oxygen partial pressure. The oxygen to n-pentane ratio in the feed affected appreciably the extent of the homogeneous reaction in the empty reactor. At low ratios, the oxygen was insuf ficient to activate the alkane molecule in the absence of catalyst. At higher oxygen content in the feed, homogeneous reaction took place, but then it seemed that there were two possible ways to proceed, depending on the temperature. Low tempe rature homogeneous reaction must be an exothermic process, possibly autocatalytic and leading to formation of both dehydrogenated and oxygenated products. High temperature homogeneous reaction has such a mechanism that the conversion rises very slowly with temperature and the products are only hydrocarbons. These results are in accordance with previous observations on hydrocarbon oxidation and combustion reactions (25) and the turnover temperature, in our case around 400°C, depends on reaction conditions and the reactor type. Conclusions Using the citrate method, high purity magnesium vanadates can be prepared, their actual bulk composition corresponding to the element stoichiometry used for preparation. Magnesium ortho and pyrovanadate prepared in this way proved to be effective for the activation of n-pentane, with the ortho phase being the most selective towards dehydrogenation products. Addition of antimony oxide to the ortho phase had in general a positive effect on catalyst selectivity. No major changes in catalyst performance were observed when antimony oxide was combined with the pyro phase. The homogeneous reaction at the temperature range examined was strongly influenced by the oxygen to alkane ratio, being favored by higher oxygen content in the reactor feed. Dehydrogenation product distributions varied mainly with the active phase present and with conversion and temperature. Acknowledgments. S. A. Korili gratefully acknowledges the financial support of the European Union (Human Capital and Mobility Project ERBCHBI-CT94-1102). She also wishes to thank M. Genet for his help in XPS analyses and Dr A. Gil for valuable discussions. Literature Cited 1. 2. 3.

Gao, X.; Ruiz, P.; Xin, Q.; Guo, X.; Delmon, B. J. Catal. 1994, 148, 56. Kung, M. C.; Kung, H. H. J. Catal. 1992, 134, 668. Burch, R.; Crabb, E. M. Appl. Catal. A 1993, 100, 111.


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