Catalytic activity of vanadyl phosphate supported on

Catalytic activity of vanadyl phosphate supported on TiO, (anatase) and SiO, (silica) M . MARTINEZ-LARA, L. MORENO-REAL, R . POZAS-TORMO, A . JIMENEZ-LOPEZ,' A N D S . BRUQUE Urziversidad de Mcilugu, Facultczd de Ciencias, Departarnento de Quimica Irzorgu'rzica, Apartado 5 9 , 29071 Malaga, Spairl AND

Can. J. Chem. Downloaded from www.nrcresearchpress.com by 222.240.207.148 on 10/21/15 For personal use only.

P. RUIZAND G. PONCELET' Groupe de physico-chirnie rninbrule et de catnlyse, Place Croix du Sud 1 , B-1348 Louvain-la-Neuve, Belgium Received December 1 1 , 1990' M. MARTINEZ-LARA, L. MORENO-REAL, R. POZAS-TORMO, A. JIMENEZ-LOPEZ, S. BRUQUE, P. RUE, and G. PONCELET. Can. J . Chem. 70, 5 (1992). Oxidation of n-butane was investigated on vanadyl phosphate (VP) prepared in the presence of titania (anatase) and silica as supports. In the case of silica, a-VOPO, was synthesized with no sign of interaction between the silica surface and vanadium phosphate. On the contrary, titania was coated with amorphous VP (P/V = l ) , preventing the crystallization of VOPO, at least up to the VP content of 21%. rz-Butane was completely converted into C 0 2 for the VP/Ti02 catalytic system. Upon impregnation with metal sulfates, maleic anhydride (MA) was produced with selectivities depending on the nature of the added metallic species, the best effect being observed with Fez+(V/Fe = 3). Selectivities to MA were influenced by the P/V ratio, with a maximum at P/V = 1.2. Key words: vanadyl phosphate, maleic anhydride, butane oxidation, anatase, rutile. M. MARTINEZ-LARA, L. MORENO-REAL, R. POZAS-TORMO, A. JIMENEZ-LOPEZ, S. BRUQUE, P. RUIZet G. PONCELET. Can. J . Chem. 70, 5 (1992). On a ktudi6 l'oxydation du butane sur de phosphate de vanadyle (PV) prkpare en prksence de titania (anatase) et de silica comme supports. Dans le cas de la silice, on a synthktise du a-VOPO, sans signes d'interactions entre la surface de la silice et le phosphate de vanadium. Au contraire, le titania etait couvert de PV (P/V = 1) et cette situation emp&che la cristallisation du VOP04, au moins jusqu'a un contenu en PV de 21%. Avec le systirme catalytique PV/Ti02, le butane est completement transform6 en COz. Apres imprkgnation sur des sulfates mktalliques, il y a formation d'anhydride malkique (AM) avec des s6lectivitCs qui d6pendent de la nature des especes mktalliques ajoutkes; les meilleurs resultzits sont obtenus avec le Fe" (V/Fe = 3). Les s6lectivitks en AM sont influenckes par le rapport P/V; le maximum se situe a P/V = l , 2 . Mors clbs : phosphate de vanadyle, anhydride malkique, oxydation du butane, anastase, rutile. [Traduit par la rkdaction]

Introduction Two catalytic processes of industrial importance, namely, the production of maleic anhydride (MA) from C, hydrocarbons and phthalic anhydride from o-xylene, rely widely on vanadium-based catalysts. In the first process, C, hydrocarbons are oxidized into M A using vanadium phosphate catalytic systems (VPO catalysts), while in the second, titania-supported V r 0 5 catalysts are preferred. These two types of catalysts generally contain modifiers, promoters, etc. There has been significant evidence that TiOz imparts special properties to V 2 0 5 , which other more classical supports d o not, or d o so to a lesser extent. Several authors proposed that a solid solution forms between v4+and TiO, (1, 2), though this question still remains debatable. The VPO catalysts, in spite of the large number of studies on these compounds and the progress made in recent years, remain complex systems that are not yet understood. Three main aspects are still subject to some disagreement: the exact nature of the active phase (is it a crystalline phase and, if so, which one (3), or is it rather an amorphous phase (4)); the oxidation state of V and its control (5); the influence of the P/V ratio on the selectivity to MA (6, 7). These questions have been considered in reviews by Hodnett et al. (8) and Centi et al. (9). 'Authors to whom correspondence may be addressed. 2~evision received August 1, 1991 .

One way to improve the performance of a catalyst is to increase the number of the active sites exposed to the reacting molecules. High surface area catalysts may be obtained from precursors prepared in nonaqueous solutions (10, 11) or, alternatively, by supporting the catalytic phase. This latter approach was preferred. Up to now, very little work has been concerned with the study of supported VP catalytic systems. Chinchen et al. (12) mention patents to BASF wherein it is "suggested" that a titania/steatite is probably used to support VPO. The same authors also refer to patents to Mitsubishi quoting silica and alumina as supports for these catalysts. Cavani et al. (5) prepared a coprecipitated VPO-TiO, catalyst with 10% V205, but have not yet provided results showing how these systems compare with pure VPO catalysts. SiO, was taken as support for VPO by Varma and Saraf (13) for the oxidation of butenes, whereas Nakamura et al. (14) investigated VPO/ a-Al,O, systems. In spite of the already complex nature of vanadium phosphate catalysts, it was challenging, due to the particular interaction between vanadium compounds and titania, to investigate the catalytic performance of supported VP in the oxidation of n-butane. TiO, (anatase) and a microfibrilous silica were chosen as supports, the VPO phase being prepared using a procedure that, in absence of support, yielded crystalline a - V O P 0 , . 2 H 2 0 (15, 16). This paper summarizes the preliminary results obtained with these systems.

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Experimental Preparation of the catalysts Crystalline a-VOPO, was synthesized according to the method described earlier (15). The supported "VOP04" (hereafter VP) systems were prepared using the standard method, the only difference being the addition of the support to the reaction mixture. Two supports were chosen: Ti02 (Eurotitania from Tioxide, batch 1143; So = 61 m'/g) and a microfibrous S i 0 2 (from Redco; So = 550 m2/g). After refluxing, the slurries were washed and freezedried. The amounts of the different reagents in the reaction mixture corresponded to V20S/(VZOS+ Ti02) weight ratios of 5 , 10, 15, and 22% (named T5, T10, T15, and T22 in Table 1) and V205/ (V20S+ Si02) of 6.6, 12.6, 20, and 31%. Metals (Me) supported on VP/TiO, were obtained by the wet impregnation ( H 2 0 or MeOH) method, using the sulfates of Fez+, Co-+, ~ r " , and ~ n " . Molybdenum-impregnated systems were prepared with ammonium heptarnolybdate. In all cases the V/Me atomic ratio was 3. Impregnation and drying were camed out in a rotavapor. Systems with different P/V ratios were obtained by impregnation of the (Me)VP/TiO, material with the amounts of H3P04 necessary to achieve P/V ratios of 1.1, 1.2, 1.3, and 1.5. All the solids prepared were pelleted, crushed, and sieved. The catalytic tests were performed on the 0.2-0.315 mm fraction.

TABLE1. Chemical analysis data of the VP/Ti02 catalysts (in wt.%) Catalyst

(%)

(%)

(%)

(%)

P/v"

" VOP04"' (%)

T5 T10 T15 T22

4.5 8.1 12.0 18.4

3.3 6.1 9.2 14.0

1.1 2.9 5.1 7.9

91.1 82.9 73.7 59.7

0.94 0.96 0.98 0.97

7.5 13.9 21.0 31.9

V20s

P2O5 H,O

"%TiO, = 100 - (%V,O, + %P,O, 'Molar ratio. 'As inferred from P 2 0 5content.

TiOF

+

%H,O).

Characterization methods To determine the chemical composition of the solids, they were dissolved by melting with NaOH. Vanadium was determined by atomic absorption spectrometry and the content of phosphorus was obtained by colorimetric analysis as the blue molybdophosphate complex; previous precipitation of titanium and vanadium was with cupferron. The catalytic systems were also characterized using various techniques: differential thermal and thermogravimetric analyses (Rigaku Thermoflex), infrared spectroscopy (IFS 88 Brucker), X-ray diffraction (Cu K a radiation, Siemens D500 equipment with DACO-MP setup), scanning electron microscopy (Jeol Temscan lOOCX), and electron microprobe analysis EDS (Kevex SC 100). Catalytic measurements The catalytic oxidation of n-butane (n-C,) was carried out in a flow reactor operated at atmospheric pressure. A detailed description of the apparatus and measurement procedure has been given elsewhere (7). The tests were done on a constant catalyst volume basis (height of catalyst bed: 14 cm; internal diameter of the tubular reactor: 3.5 mm). In these conditions, the sample weight was between 1.2 and 1.8 g. The reaction was performed in temperature-programmed mode with n-C, flowing at 0.5 mL/min and air at 35 mL/min. These conditions define a molar concentration of n-C, of 1.4%. The tubing and valves beyond the reactor were heated to prevent condensation of the reaction products. The analyses were performed by gas chromatography.

Results Catalyst characterization Chemical analyses The chemical analysis data of solids before catalytic performances given in Table 1 indicate that, in the VP/TiO, systems, the molar ratio V/P is near 1. The total vanadium content of these catalysts represents 80% of the amount of V introduced in the synthesis mixture. For the VP/SiO, catalysts, only 60% of the starting vanadium is found in the final product. X-ray diffractionand SE microscopy The X-ray diagrams of the VP/TiO, series with different VP contents and of the pure a-VOPO,. 2H 0 phase are shown in Fig. 1. The 001 (7.4 A), 002 (3.7 102 (3.16

A),

FIG. 1 . X-ray powder diagrams of unsupported a-VOPO, (a), T i 0 2 (b), and VP/Ti02 containing 7.5%VP (c), 13.9%VP (d), 21%VP (c), and 31.9%VP (d).

MARTINEZ-LARA E T AL.


,

7

FIG2. SEM photomicrograph of: (a) VP/TiO, and (b) VP/Si02. The bar in the lower right corresponds to 0.42 km.

A), and 200 (3.09 A) reflections were taken to identify the crystalline ci-VOPO,. 2 H 2 0 phase. As can be seen, these diffraction lines are absent in the systems containing less than 21% VP. On the S E micrographs the anatase particles appear as homogeneously coated with VP (Fig. 2(a)). EDX studies verified this situation. Both analyses of aggregates and at various positions on individual particles show similar composition of Ti, V, and P. Only the samples with 31.9% VP contain, in addition, segregated particles of ciVOPO,, thus in agreement with the X-ray diffraction analysis. This situation was also observed by Fierro et al. (17) in the V,O,/TiO, system; X-ray diffraction data and Raman spectra showed that V,05 was highly dispersed over TiO, in V/(V Ti) 5 0.1 1 catalyst; in more vanadium-rich systems the presence of tridimensional V205 crystallites was observed. The picture is very different for the VP phases prepared in the presence of silica. Indeed, the X-ray diffraction spectra, given in Fig. 3, clearly show that crystalline ci-VOPO, is present in all the samples, with peak intensities increasing with the VP content. Trace b in Fig. 3 indicates that the starting silica contains a small amount of residual quartz. The VP/SiO, with 4.1 and 14.6% VP contain, in addition to ci-VOPO,, small amounts of another phase characterized by reflections near 15"(20). This phase has not yet been identified. Examining these samples with SE microscopy confirms the existence of segregated a-VOPO,, particles, the SiO, fibrils being uncoated. This fact is deduced from the microprobe analysis on the fibrous and lamellar materials in

+

Fig. 2(b). Summarizing, the type of support plays an important role in the formation of the VPO phase. Anatase prevents the crystallization of a-VOPO,, at least for VP contents up to 21.0%, and the T i 0 2 particles are completely covered with VP. On the contrary, the silica used as support has no effect on the crystallization of the VPO phase, and there is no evidence of vanadium phosphate interacting with the surface of silica. When V205 is supported on TiO, (anatase) a simultaneous reduction of V205 and transformation of TiO, (anatase) into TiO, (rutile) has been observed (1, 2). This polymorphic transformation is catalyzed by V" and is strongly dependent upon the calcination temperature, specially above 800 K. Taking into account that our VP/TiO, systems were air-dried and that the catalytic reactions were carried out up to 673 K, the catalysts will not undergo substantial phase transformation. The aforementioned authors (17) have not found any differences in the percentages of anatase and rutile in the supported catalysts after calcination at 773 K with respect to the unsupported anatase. The addition of reducing metal ions (like ~ e , ' , ~ n , ' ) provokes a gradual diminution of the 001 reflections of ciVOPO, .2H,O, while a new peak appears at 13.4"(20) (6.6 A) whose intensity increases with the amount of the reducing species. Such a change of the X-ray diagram had already been reported by Martinez et al. (15) in the case of redox intercalation in vanadyl phosphate for which the coexistence of a vanadium (4+) phosphate phase containing interlamellar cations and vanadium (5+) phosphate was proposed.

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CAN. J. CHEM. VOL. 70, 1992

Infrared spectroscopy The IR spectra of the VP/Si02 and VP/Ti02 systems exhibit, without important modifications, the absorption band characteristic of the support and of the active phase. The spectral region between 900 and 1200 cm-' is most sensitive to the modifications of the catalyst. Indeed, this is the (1 170, 1080, frequency domain of the vibrations of PO:1030 cm-I) and V-0 groups (1010, 990, 964 cm-') (15, ~ + 16). The spectrum of VP/Ti02 impre nated with ~ e (Fig. . is . assigned to 4B) clearly shows a band at 990 cm- , which the v"-0 vibration as a consequence of a partial reduc. to the spectrum of the irontion of vVby ~ e ~As+ compared free sample (Fig. 4A), the intensity of the band at 964 cm-' of the Fe2+-containingcatalyst is markedly diminished. This band may be attributed to the P-0 stretching vibration of group within an asymmetrical environment, althe PO:though Pulvin et al. (18) proposed to assign it to the pseudosymmetrical vibration of 0-V=O. The IR spectrum of the Fe2+-containingVP/Ti02 sample impregnated with phosphoric acid (Fig. 4C) shows a broad absorption band centered at around 1050 cm-I, which envelopes all the other bands due to the phosphate groups. EPR measurements The EPR spectra of the VP/Ti02 samples (not shown here) are characteristic of vanadyl phosphate, with the signal of trace amounts of V(4+) centers diluted magnetically (19). ~ + rise The sample of VP/Ti02 partially reduced with ~ e gives to EPR signals that denote the presence of V(4+) and Fe(3+) species, with a broad signal due to large amounts of paramagnetic centers. Catalytic activity of the VP/Ti02 systems Influence of the "VOP04" content on the reaction of n-butane The conversion of n-C, for the catalysts containing 7.5, 13.9, 2 1.O, and 3 1.9% VP depends upon the temperature. Thus, total conversion (C) of n-C, is achieved at around 380°C for the two catalysts containing the highest amounts of vanadium, and at temperatures above 400°C for those with lesser VP contents. For the four systems, there is a steep increase in activity between 250 and 350°C. In these runs, total oxidation of n-C, occurred, CO, being the sole reaction product. No maleic anhydride could be detected. For the four catalysts, the relationship between In C and 1/T does not yield a single straight line but, rather, two linear portions with different slopes. The corresponding activation energies calculated from Arrhenius' equation are 88 kJ/mol between 200 and 300°C, and 15 kJ/mol between 300 and 400°C, respectively. This last value is, of course, typical of a reaction limited by the diffusional process. Influence of different metal species on the selectivity to maleic anhydride The oxidizing power of VP/Ti02 being too strong, it was necessary to temper it by adjoining another metal. This was done, as indicated above, by wet impregnations of VP/TiO, catalyst with the sulfates of different metals. The impregnations were canied out on the system containing 3 1.9% VP. In a first series of preparations, the base catalyst was impregnated with FeSO, in order to deposit 2, 4, and 8 wt.% Fe2+. The catalytic tests were performed under the same conditions as those in the preceding section. Figure 5 compares the course of the conversion vs. reaction temperature obtained for the Fez+-loadedcatalysts with


8

FIG3. X-ray powder diagrams of unsupported a-VOPO, (a), SiO, showing the reflections of residual quartz (b), and with different amounts of VP: (c) 4.1%; (d) 14.6%; (e) 21.8%, and (f) 33.0%.


MARTINEZ-LARA ET AL

FIG4. Infrared spectra of VP/Ti02 (A); VP/Ti02 impregnated with FeSO, (B), and the same as (B) with a P/V ratio of 1.2 (C).

that of the base VP/Ti02 catalyst. Obviously, the addition of FeSO, lowers markedly the catalytic performance as compared with the reference catalyst. The catalysts containing 4 and 8% Fe2' exhibit almost similar activities, slightly lower than that of the 2% Fe2+-impregnated catalyst. The effect of adding Fe2' on the production of maleic an-

hydride is important. Whereas the base VP/Ti02 produces only CO,, upon addition of Fe2' maleic anhydride is obtained, in addition to CO, as the main reaction product. In accordance with the IR and EPR results for the ~ e , -im+ pregnated catalysts, these systems have V(4+) and V(5+) centers. The selectivity to MA can be related to the pres-


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C A N . J. CHEM. VOL. 70. 1992

20

LO

60

80

700

C /%/ FIG.6. Evolution of the selectivity to MA vs. conversion for the FeS0,-impregnated VP/TiO?: 0, 0, A:2 , 4 , and 8% Fe respectively.

FIG.5 . Conversion of n-butane vs. temperature for: VP/Ti02 (31.9% VP) and for the same catalyst impregnated with, respec4% (A),and 8% (0) FeSO,. tively, 2% (O), ence of both oxidation states as has been established from EXAFS by other authors (20). A similar conclusion has been proposed by Batis et al. (21) with a catalyst based on (VO),P,O, and v f 5 , where high selectivity to MA was found when specific VOPO, phases (a,, a,,, P, y, etc) are simultaneously present with (VO),P,O,. Figure 6 compares, for the three iron-impregnated catalysts, the selectivities to MA as a function of the conversions obtained at the temperatures indicated in Fig. 5 . At 50% conversion, the system containing 4% Fe2+ is twice as selective as the one with 2% ~ e ' +and almost four times as selective as that for 8% Fe2+. For the three catalysts, the maximum selectivity is attained at around 300°C. In another series of preparations, VP/TiO, (31.9% VP) was impregnated with the sulfates of different metals in order to see the influence of the nature of the metal on the selectivity to MA. Except for Mo, which was introduced as ammonium molybdate, the sulfates were used in the amounts required to load the parent catalyst with 4% Me. This amount corresponds to a V/Me atomic ratio of 3. Figure 7 compares the course of the conversion vs. temperature for the different systems investigated. Although the activity of these catalysts is lower than that of the metal-free catalyst, c o Z f and ~ n "appear, among the different met-

als, to have the most effect on the activity, whereas the M O ~- + impregnated system is the least active. The c o 2 +and ~ n ' + modified catalysts transform approximately twice as much ed at any temperature ben-C, as the ~ o + ~ - l o a d catalyst, tween 250 and 350°C. The c r 3 +system exhibits intermediate behaviour. Considering the selectivities to maleic anhydride, for some of the conversion values obtained from Fig. 7 (Fig. 8), three systems are not very selective, namely those containing ~ n , + c, r 3 + , and M O ~ +The . other, the CO'+-impregnated catalyst, appears with a higher selectivity to MA although slightly lower than those developed by the Fez+-impregnated catalyst. As observed for the system containing Fez+, the maximum selectivity is reached near 30% conversion, which occurs at around 280-300°C. Influence of the P / V ratio on the selectivity to MA The influence of the P/V ratio of unsupported VPO catalysts on the selectivity to MA is still debated. According to different authors (6, 7), the optimal ratio may vary significantly. It was interesting to consider this parameter in our systems, inasmuch as, as mentioned previously, Ti0,-supported VP catalysts are X-ray amorphous. VP/TiO, catalysts containing 4 % Fe2+ were thus prepared with P/V ratios between 1 and 1.5, additional P being supplied by impregnation with the required amounts of phosphoric acid. Figure 9 shows the evolution of the total conversion of n-C, as a function of reaction temperature. Obviously, P/V ratios higher than unity depress the catalytic activity as compared with the catalyst where P/V = 1. The catalyst with the highest P/V (1.5) is the least active. Considering the selectivities to MA, on increasing the P/V ratio there is an influence on the production of MA at least for conversions that do not exceed 40%. This is in accordance with the suggestion of some authors (20) who report that an excess of phosphorus prevents the complete oxida-


MARTINEZ-LARA ET AL.

11

FIG.8. Evolution of the selectivity to MA vs. conversion for the same catalysts as in Fig. 6: MO~';0Cr3+;0~ n " ; Co2+.

FIG.7. Conversion of n-butane vs. temperature for the VP/Ti02 (31.9% VP) impregnated with the sulfate (4%) of different metals (Mo from ammonium heptamolybdate). tion of vanadium (v4' to v5+) by calcination in air. Beyond this value (40%), the effect is much less obvious (see Fig. 10). The system with P/V = 1.5 that is the least active is also the least selective and that with P/V = 1.2 is the most selective. At higher conversions, i.e., at higher temperatures, this effect vanishes. As for the other catalytic systems investigated, the maximum selectivity is obtained near 25% conversion. Catalytic activity of unsupported crystalline a-VOP04 Oxidation tests of n-C, were performed, as well, under similar experimental conditions on bulk, well-crystallized aVOPO, synthesized according to the method described in ref. 15. These solids were found to be completely inactive in the temperature range investigated. Catalytic activity of a-VOP04/Si0, systems As shown previously, these systems always exhibit segregated crystalline a-VOPO, adjacent to silica whatever the VP content introduced in the preparation. The catalytic runs carried out on these solids showed only partial conversion, which in no case was higher than 25% at 450°C. No maleic anhydride was produced. VP/SiO, (21.8% VP) was then impregnated, first with FeSO, (4% ~ e " ) and subsequently with H3P04,to realize a P/V ratio of 1.2, and tested. The conversion of n-C, in-

creased slightly, to yield 30% at 450°C, 53% of the reaction product being maleic anhydride. The activity could be further improved when silica was precalcined at 700°C. Indeed, at 450°C, 55% of the hydrocarbon was converted, but lower selectivities to MA (1 1%) were obtained (with a maximum of 22% at 400°C). These results agree with those found by Do et al. (22) for V20, supported on TiOz and Al,03, where the Ti02-supported catalyst was found to exhibit better selectivities for MA than the A1203-supportedcatalysts. Although the source of the support effect on the selectivity to MA of V,O,/P,O, catalysts is not clear, Do et al. (23) related selectivity to MA to the presence of OH groups on the surface. Lopez Nieto et al. (24), in studies of the selective oxidation of propene on A1203, SiO,, and TiO, (anatase)-supported V205 catalysts, found that V,O,/TiO, was the most active and selective phase because the V205is highly dispersed on non-porous TiO,, whereas for SiO, and Al,03 part of the V20, species was deposited within the catalyst pores.

Discussion The use of titania as support to VPO compounds makes difficult the comparison between the data in literature and the present results. Indeed, unsupported VPO catalysts used as starting materials are often, if not generally, well-defined crystalline phases. Nevertheless, as seen in this work, the presence of titania in the synthesis medium prevents the formation of a crystalline phase, up to VP contents near 21%. For this reason. this discussion will be restricted to the comparison of sbme of the main tendencies observed in this study with the results already published. First of all, crystalline a-VOPO, is a much less active catalyst for the oxidation of n-butane (in the presence of SiO, as well as unsupported) than is its amorphous counterpart. This difference might possibly be only a surface effect, the fraction of the catalyst surface available to the reagents being


12

CAN. J . CHEM. VOL. 70, 1992

FIG. 10. Effect of the P/V ratio on the selectivity to MA (same symbols as in Fig. 9).

FIG. 9. Evolution of the conversion of n-butane vs. temperaa t e d with different P/V ratios: ture for the ~ e ~ + - i m ~ r e ~ nVP/Ti02 0: 1.0; . : 1.1; A: 1.2; V: 1.3; and 0: 1.5.

higher for the VP/TiO, systems. Nevertheless, independent of the crystalline or amorphous character of the VP phase, both systems yield only CO, as reaction product. These observations are consistent with the results reported by Bergeret et al. (4), Hodnett (8), and VCdrine et a / . (25), as well as the linear relationship between n-butane conversion and VP content (26). The results obtained with such catalytic systems are a clear illustration that it is not enough to reach high conversions if the oxidizing power of these catalysts is not under control in agreement with Cavani et al. (5). In bulk VPO catalysts, the nature of the phase responsible for the formation of MA is not well identified. (VO),P207 has been advocated by Cavani et al. (27), Contractor et a1. (3), and VCdrine et al. (25), among other authors, as being the active phase, while no quantitative correlation between catalytic activity and selectivity and the content of vanadium pyrophosphate could be found by Van Geem et al. (6) and Hodnett et al. (7, 28). To other authors, it turns out rather that the best catalysts appear to be poorly crystallized or even amorphous VPO phases (25), while for Bordes and Courtine (29) and Morselli et a1. (30) activity and selectivity are associated with the simultaneous presence of p-VOPO, and (VO),P207. Bergeret et al. (4) obtained the best results with a mixture of (V0)2P20, and an amorphous phase produced upon activation. In this respect, our data show that com-

pletely amorphous supported VP with a P/V = 1 does not lead to MA. Maleic anhydride was formed only when the supported VP's were impregnated with a metal sulfate. The promotional effect of a series of elements on the selectivity to MA was investigated by Brutovsky and Gerej (31) and Brutovsky et al. (32) in the case of (VO)2P207,the beneficial effect being in the following sequence: Mn > Co .= Sn > Fe > Cu > Li > Zn > Ce > Ni. Patent literature mentions catalysts containing promoters such as alkalis or alkaline earths (33) and Zn (34), which appear to prolong the life of the catalysts by diminishing the loss of phosphorus. In our catalytic systems prepared by impregnation with metal sulfates, the improvement of the selectivity to MA was in the following order: ~ e , ' > co2' > ~ n >~cr3+ ' >~ 0 ~ ' It was established for the Fe2' -impregnated VP/TiO, that the best selectivities to MA were obtained when the V/Fe atomic ratio was 3. If this sequence is different from the one previously reported, the catalysts are also not similar. These different elements do not simply improve the yield of MA, but in their absence no MA is formed. Their effect is merely to buffer the oxidizing power of VP. Since data on the oxidation state of V and on a possible change in that of the cometal are not yet available, the way these catalytic systems act and favor the formation of MA cannot be discussed in more detail, although, as has been seen in the 1R and EPR discussion of the Fe2'-impregnated system, the higher value for selectivity may be related to the presence in the VP of v4+/v5+ions. The influence of the P/V ratio on the reaction of n-butane has also been extensively investigated. If there seems to be a general consensus (from literature) that excess P with respect to a P/V ratio of 1 improves the selectivity to MA

MARTINEZ-LARA ET AL.

with a parallel decrease in activity ( 3 3 , the results reported by some authors are, at the very least, somewhat confusing. For instance, Van G e e m and Nobel (6) observe a decrease in activity as the P / V ratio increases but with a maximum selectivity for P / V ratios between 0.58 and 1.06. These authors conclude, however, that selectivity to M A is only determined by the degree of conversion of the hydrocarbon. Nevertheless, it has been shown that low P / V ratios favor V P O phases (P-VOPO,) with a vanadium oxidation number close to 5 + , whereas higher P / V ratios stabilize vanadium as V" in p or B phases (3), or (VO)2P207(5, 7, 14, 28, 29, 36, 37) yielding higher selectivities to M A . The influence of the P / V ratio on the selectivities to M A obtained for the Fe/VP/TiO, with a maximum at a value of 1.2 is hence consistent with the results of others. Further investigation is needed to characterize the oxidation state of vanadium in those systems, with particular attention to the effect of adding a co-metal o n the oxidation state of vanadium, and to its distribution o n the solid surface.


+,

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