1994
Znd. Eng. Chem. Res. 1995,34, 1994-2000
Synthesis of Vanadium Phosphorus Oxide Catalysts by Aerosol Processing Peter M. Michalakos,t Harold E. Bellis: Phyllis Brusky,"Harold H. Kung,*?+ Hai Q. Li,"William R. Moser,"Walter Partenheimer? and Larry C. Satek' Ipatieff Laboratory and Department of Chemical Engineering, Northwestern University, Evanston, Illinois 60208, Chemicals & Pigments Department, E. I. duPont de Nemours & Company, Deepwater, New Jersey 08023, Battelle Columbus Division, 505 King Drive, Columbus, Ohio 43201-2693, Department of Chemical Engineering, Worcester Polytechnic Institute, Worcester, Massachusetts 01609, and Amoco Research Center, Amoco Chemical Company, Naperville, Illinois 60566
The use of a n aerosol process to prepare vanadium phosphorus oxide (VPO) catalysts for butane oxidation to maleic anhydride was investigated. Heating a n aerosol created with air of an aqueous solution of NH4V03 and H3P04 with phosphorus to vanadium atomic ratio PN = 1.2 at 700 "C resulted in a material that had an average vanadium oxidation state of 4.7-5.0. After activation in a butane and oxygen mixture, a stable catalyst was obtained which showed selectivities to maleic anhydride (MA) of up to 48% at 475 "C. X-ray diffraction and laser Raman spectroscopic characterization of the catalyst showed that the as-prepared sample was mostly VOP04.2Hz0, which was converted to a1-VOP04 under reaction conditions. The catalyst was much more active than a VPO catalyst with PN = 1.1or 1.2 prepared by the conventional method in an aqueous medium. Thus, in spite of its lower surface area and selectivity for MA, it produced a higher yield of MA even on the basis of the weight of catalyst.
Introduction The commercial catalysts for the oxidation of butane to maleic anhydride (MA) are based on vanadium phosphorus oxide (VPO) (Centi, 1993; Centi et al., 1988; Hutchings, 1991). Because of their commercial importance, these catalysts have been studied extensively with respect t o the effect of the bulk phases and surface composition (Bergeret et al., 1987; Bordes, 1987; Cavani et al., 1985;Centi et al., 1988; Lin et al., 1993;Miyamoto et al., 1988; Moser and Schrader, 1985; Vedrine et al., 19891, vanadium oxidation state and redox properties (Batis et al., 1991; Bordes and Courtine, 1979; Bosch et al., 1987; Curilla et al., 1988; Hodnett and Delmon, 1984a,b; Hodnett, 1985; Pepera et al., 19851, surface condition and bulk phosphorus to vanadium atomic ratio (PN)(Busca et al., 1986b;Cavani et al., 1985; Garbassi et al., 1986; Haas et al., 1988; Hodnett et al., 1983; Okuhara et al., 1990; Partenheimer and Meyers, 1989; Puttock and Rochester, 1986;Vedrine et al., 1989; Zazhigalov et al., 1988a,b, 1989), morphology (Busca et al., 1986a,b; Cavani et al., 1985; Nechiporuk et al., 19861, preparation method and pretreatment conditions (Bordes and Courtine, 1979; Bordes et al., 1984; Buchanan et al., 1985; Busca et al., 1986a,b; Centi et al., 1983; Garbassi et al., 1985; Hodnett et al., 1983; Hodnett and Delmon, 1984a,b;Hodnett, 1985;Poli et al., 1981; Pulvin et al., 1981), and the catalytic behavior (Buchanan and Sundaresan, 1986; Centi et al., 1985, 1988; Hodnett, 1985; Nechiporuk et al., 1986). The results of these studies show that the catalysts that exhibit the highest MA yields are those that contain crystalline vanadyl pyrophosphate ((VO)~PZO~) as the only detectable bulk crystalline phase, with a slightly disordered (010) plane preferentially exposed, an average vanadium oxidation state of 4.1-4.2,and a bulk P N of slightly above unity.
* To whom correspondence should be addressed. Northwestern University. = E. I. duPont de Nemours & Company. 4 Battelle Columbus Division. ' I Worcester Polytechnic Institute. - Amoco Chemical Company. A
0888-5885/95/2634-1994$09.00/0
The commercial catalysts, and thus practically all of the catalysts studied in the literature, are prepared by chemical reaction of a phosphorus and a vanadium compound in an organic or aqueous liquid medium. In the organic liquid phase synthesis, the vanadium09 precursor is reduced and dissolved by an organic compound, typically an alcohol, phosphorus is added, and the precipitate is isolated by filtration. In the aqueous liquid phase synthesis, the vanadium(V1 is reduced and dissolved by an acid, and the precipitate is obtained by evaporation. To ensure complete reaction between the compounds, typically the liquid phase reaction is allowed to proceed for many hours. The resulting solid catalyst precursors are activated by heating in a flow of reactants (butane and air). Preparations of the catalyst by high temperature solid state ~ NH4HzP04 powders reaction of well-mixed V Z O and (Moser and Schrader, 1985)and vapor reaction between volatile compounds of vanadium (VOc13) and phosphorus (POC13)(Takita et al., 1988)have also been reported. In a program supported by Battelle's Advanced Material Center, a member of NASA's Centers for the Commercial Development of Space, the use of aerosol pyrolysis (AP)to prepare the VPO catalyst was explored. Since the aerosol process involves rapid drying and thermal reaction of metal salts in a liquid droplet to form a metal oxide, the transformation of vanadium and phosphorus compounds to the catalyst precursor occurs in a few seconds. Thus the aerosol process could be a method of fast catalyst preparation. It also has the potential of producing different crystalline phases other than those obtained with liquid phase preparation. In this paper, the results of the investigation of the feasibility of preparing an effective VPO catalyst by the aerosol process are presented. In addition to the catalytic properties, these samples were characterized with various techniques, including X-ray diffraction (XRD), thermal gravimetric analysis (TGA), scanning electron microscopy (SEM), X-ray photoelectron spec-
0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 6, 1995 1995
THREE ZONE OVEN
AIR FILTER
PULSED INJECTOR SoLUT1oN Figure 1. Schematic drawing of the aerosol pyrolysis apparatus.
troscopy (XPS),and chemical analysis, for their crystallographic phases, morphology, and chemical compositions.
Experimental Section Catalyst Preparation. An upflow high temperature aerosol decomposition (HTAD)process, shown schematically in Figure 1,was used to prepare the catalysts. The furnace housing was a cylinder 4 in. in diameter and 8 ft. in height, which rested upon the spray nozzle assembly. The furnace volume was 9.88 L. The furnace temperature was between 350 and 700 "C and the pressure was close to atmospheric. Details of the operation of such an upflow reactor have been described previously for the synthesis of other catalysts (Moser, 1993; Moser and Cnossen, 1992; Moser and Lennhoff, 1989). In this study, an aerosol was created by spraying a feed of an aqueous solution of ammonium metavanadate and orthophosphoric acid, with a phosphorus to vanadium atomic ratio (PN) between 1.0 and 1.6, cocurrently upward into the furnace with air as the carrier gas. The vanadium concentration was 0.12 M. In the conventional liquid phase preparation, an alcoholic solution is preferred t o an aqueous solution. An aqueous solution was used in this feasibility study to avoid the danger of explosion by an air-organic vapor mixture. A paper filter on a water-cooled housing was used to collect the powder as it emerged from the furnace. Air was injected through the nozzle at a rate of 2.4 Umin. The make-up air feed rate was 52 L/min, and the liquid feed rate was 7.5 mumin. The contact time, defined as the time spent by a particle in the furnace, was measured to be 8 s at room temperature by observing the aerosol moving upward through a Plexiglas flow reactor. It was calculated to be 3 s at 700 "C after taking into account the change in particle drag with temperature, particle density and size, and the degree of particle back-mixing under actual synthesis conditions. Thus, the actual contact time was between 3 and 8 s in the synthesis of this study. The catalysts prepared by this method are referred to as AP catalysts. For the VPO catalyst prepared by the conventional method in an organic medium, the procedure described by Busca et al. (1986b)was employed using 15 g of V205 (Aldrich 99.6+%), 60 mL of benzyl alcohol, 90 mL of 2-methyl-1-propanol,and 16.7 g of orthophosphoric acid crystals (Aldrich 99%). The light-green powder result-
ing from the liquid phase reaction was dried for 24 h a t 157 "C and activated by heating t o 400 "C in a flow of 2% butane, 21% oxygen, and the balance helium for 24 h. The XRD pattern of this sample showed only vanadyl pyrophosphate as the crystalline phase. The BET surface area was 28 m2/g and the bulk PN ratio was 1.1as determined by inductively coupled plasma atomic emission spectroscopy (ICP). This catalyst is denoted VPO(or). The catalysts prepared by the aqueous method were prepared in the manner described by Poli et al. (1981) using 20 g of V205 in 250 mL of 37% HC1 and 27.9 g of 85%H3P04. The resulting solid precursor was dried in air at 157 "C and activated in a flow of 2% butane, 22% oxygen, and the balance helium for 8 h at 475 "C. Two catalysts, labeled VPO(aq-1.2) with a P N ratio of 1.2 and VPO(aq-1.1)with a PN ratio of 1.1,were prepared. Their XRD patterns were similar to that of VPO(or), except for narrower peaks and different peak intensities (Michalakos, 1994). Their surface areas were 10 m2/g. Catalytic Studies. The catalytic activities in the oxidation of butane were measured with a fused silica tubular reactor of 14 mm 0.d. The temperature of the catalyst bed was monitored by a thermocouple placed in a 3 mm fused silica thermowell located in the center of the catalyst bed. The powdered catalyst was sandwiched between quartz wool, and the parts of the reactor before and after the catalyst bed were filled with quartz chips. Without a catalyst, there was no detectable conversion of butane. The feed was 3% butane, 22% oxygen, and the balance helium. The total flow rate and the weight of catalyst used were adjusted so as to obtain the desired conversion. The standard testing procedure involved heating a fresh catalyst to 400 "C in the reaction mixture, measuring the catalytic activity and selectivity after 2 h, and then measuring the catalytic properties again after 2 h each at 425,450, and 475 "C. This cycle was repeated two more times to allow for activation t o steady state behavior, and to check the stability of the sample. In most cases, the selectivity for maleic anhydride was higher in the second and third cycles than in the first cycle. Unless otherwise indicated, the reaction data for the third cycle are reported. The products were analyzed by on-line gas chromatography using a combination of columns. A Graphpac GB column (Alltech, 1/8 in. x 10 ft) was used to analyze for oxygenates (maleic anhydride, acetic acid, and acrylic acid) with the following temperature program: 2 min at 120 "C, 15 "C/min to 190 "C, and 14 min at 190 "C. Carbon dioxide and hydrocarbons were detected with a VZ-7 column (Alltech, 1/8 in. x 20 ft) at room temperature. In the data presented, conversion was defined as the percent of butane reacted, selectivity for a certain product was the percent of that product in the total products, and yield was the product of conversion and selectivity. Catalyst Characterization. XRD were collected with a Rigaku powder diffractometer or a Phillips diffractometer using Cu Kal,z radiation with a Ni filter. The BET surface areas were measured by nitrogen adsorption using an Omnisorp 360. SEM was performed on a Hitachi ,9470 scanning electron microscope. TGA was carried out using a Perkin Elmer 7 Series thermal analysis system with a flow of 50 mumin argon and a heating rate of 5 "C/min from 25 t o 900 "C. The average vanadium oxidation state was determined by chemical titration using KMn04. Raman spectra were
1996 Ind. Eng. Chem. Res., Vol. 34,No. 6, 1995
collected using the 514.5 nm line of a Coherent INOVA 70-2 Ar ion laser a t 25 mW.
1.61
Results and Discussion
1.4
Catalyst Preparation. Results of preliminary experiments indicated that the process variables that had noticeable effects on the catalytic properties were the PN ratio of the feed solution, furnace temperature, and postsynthesis treatment in nitrogen. These effects will be described below. Other variables that were studied but showed small effects were the residence time of the droplet in the furnace, feed solution concentration, and synthesis atmosphere. In the preliminary experiments, the first series of samples were prepared by varying the PN ratio in the feed solution from 1.0 to 1.6 and with a furnace temperature of 650 "C. It was found that the sample prepared with a feed solution of PN = 1.1was the most active (the highest activity per unit surface area of catalyst) and showed the highest maleic anhydride (MA) selectivity. In the third cycle in the screening test, a butane conversion of 26% and a MA selectivity of 40% at 475 "C were obtained at a W/F of 0.023 g of cabmid mL. Analysis of this catalyst by ICP showed that the PN ratio of the solid was 1.23. (That this value was different from the PN ratio in the feed solution was because of recycling of the condensed solution. For the rest of this study, the samples were prepared without recycling, and the compositions of the feed solution and the final solid were the same.) Samples of PN = 1.2 in the solid were then used for further studies. The effect of furnace temperature on the catalytic properties of AP catalysts (PN = 1.2) was studied over the range of 350-700 "C. Figure 2 shows the activities for butane conversion and the MA yields obtained for these samples. Broadly speaking, catalysts prepared a t the higher furnace temperatures were more active. The MA selectivity ranged from 33 to 48%, but did not show any obvious trend with furnace temperature. Treatment of these samples by heating in flowing NZa t 450 "C for 3 h resulted in poorer catalysts, as shown in Figure 2. The average vanadium oxidation state of the N2-treated samples were about 4, compared t o 4.7-5 for those without the N2 treatment. On the basis of these results, the catalyst prepared a t 700 "C furnace temperature and without subsequent nitrogen treatment was chosen for more detailed characterization. This sample was labeled AP1.2. The surface area of this sample was 3.7 m2/g. The catalytic performance of AP1.2 was compared with those of VPO(aq-1.1) and VPO(aq-1.2) and the results are shown in Table 1. Because of the substantially higher areal activity of the AP1.2 catalyst than those of VPO(aq-1.2) and VPO(aq-l.l), the yield of MA was higher for this catalyst in spite of its lower selectivity for MA. For this catalyst, besides MA, about 40% CO, 13%C02, and less than 2% each of propene, ethene, acetic acid, and acrylic acid were detected. The activity of the catalyst could be maintained for a t least 100 h. Catalyst Characterization. The specific surface area of the AP catalysts were low, generally below 10 m2/g. Within the large uncertainties in determining low surface areas, no obvious trends could be identified with respect to the various processing variables. The used AF' catalysts were hygroscopic and claylike in appearance. The XRD patterns of the Ap1.2 sample are shown in Figure 3. For comparison, the XRD pattern of the VPO-
-1
' 650 1
I
x
700
x 500
s i F 0.8
500 0 0
0.6
550
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0.41
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350
Oa21 1
450
700 450
550
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Activity (pmol/m2-min) Figure 2. MA yields and activities of AP catalysts (PN =1.2) prepared a t different furnace temperatures. *, untreated catalysts; 0 , catalysts posttreated with Nz. Furnace temperature indicated next to data point. Reaction temperature = 475 "C (except posttreated catalyst prepared at 700 "C for which the reaction temperature was 450 "C); C1Hlo:On:He = 3:22:75. Table 1. Comparison of AP1.2 Catalysts with VPO Catalysts Prepared in Aqueous Mediuma
T, C4 activity, MA, MA yield, "C conv, % ymoL'(m2.min) sel % ymol/(m2.min) AP1.2 425 25 0.82 47 0.39 450 39 1.41 47 0.67 46 475 64 2.11 0.97 67 WO(aq-1.1) 400 19 0.19 0.13 425 32 0.32 66 0.21 450 49 0.48 64 0.31 475 68 0.67 61 0.41 WO(aq-1.2) 400 13 0.14 60 0.085 425 23 0.24 60 0.15 450 36 0.39 58 0.23 475 55 0.59 55 0.31 catalyst
a
Butane:oxygen:helium = 3:22:75.
(or) is shown in curve A, which corresponded to the pattern of (VO)zP207. The as-prepared sample showed strong diffraction peaks at 12.5" and 29.3" 28 (curve B). After the sample was used in reaction, the pattern changed to curve C, with strong diffraction peaks a t 21.3" and 28.9" and 29.8" 28. The major peaks in this pattern can be assigned to ~ I - V O P O (Bordes, ~ 19871, which can be formed by dehydration of VOP04*2H20 upon heating. Such a transformation has been reported by Ladwig (1965). In that study, it was shown that the most intense XRD peak of the sample shifted from about 11.5" 26 in VOP04-2H20, to 13.8" in VOPOgH20, to 20.8" in ar-VOP04 upon dehydration due to collapse of the interlayer spacing in these layered compounds. Such a transformation could be used to interpret the changes in the XRD data of curves B and C.
Ind. Eng. Chem. Res., Vol. 34, No. 6, 1995 1997 o x
A
20
60
40
Degrees T W O Theta
0
20
40 Degrees Two Theta
60
Figure 5. XRD pattern of used AP after heating to 900 "C in He: 0 , ( ~ ~ ) zX , ~vo(po3)z. z ~ ~ ;
Figure 3. XRD patterns of (A) VPO(or); (B)fresh AP1.2; (C) used AP1.2.
1 GOO
800
600
400
200
Wavenumbers (cm- 1 )
20
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Degrces l w o T h d n
Figure 4. XRD patterns of samples aRer storage in air. (A) Fresh AP1.2; (B) used AP1.2. Peaks marked are those of VO(HzP04)z.
This interpretation was also consistent with changes in the XRD patterns of the sample upon storage. The pattern of the used sample changed to one resembling that of a fresh sample upon storage in air for 9 months (Figure 4B) except that the diffraction lines were sharper and more intense, indicating increased crystallinity of the sample, and that small peaks corresponding to vanadyl hydrogen phosphate, VO(HzPOd2, were observed. Thus, upon exposure to moisture in the air, apVOP04 was converted back to VOP04.nH20 with the associated changes in the XRD pattern. A fresh sample after months of storage also showed the appearance of VO(HzPO& diffraction pattern (Figure 4A).
Figure 6. Raman spectra of AP1.2 catalysts: (A) fresh; (B) used.
The transformation was also consistent with the weight loss measurement in the TGA analysis. There was about 13% weight loss between 50 and 360 "C, which could be roughly divided equally into two stages: one between 50 and 150 "C, and one between 150 and 360 "C. These weight changes corresponded to the loss of the two water molecules. The XRD patterns of the AP catalysts prepared at furnace temperatures lower than 700 "C showed features similar to those of AP1.2, except for broader peaks and the presence of a broad hump between 20" and 30" 28 that indicated the presence of a poorly crystalline phase, which increased in intensity with decrease in furnace temperature. These patterns also changed with time of storage in air. The broad hump indicative of amorphous material decreased in intensity, and the pattern became much closer to that of AP1.2 after
1998 Ind. Eng. Chem. Res., Vol. 34, No. 6, 1995 storage in air. Increased crystallization on storage was observed on all samples whether they had been used in reaction or not (Michalakos, 1994). Heating of the sample in argon above 700 "C caused an irreversible change. The sample lost about 3% of its weight ate600 "C. It changed from a rich green to a gray color and became very hard. The XRD pattern of a sample heated in helium a t 900 "C revealed that it had transformed into a mixture of vanadyl pyrophosphate ((V0)2P207)and vanadyl metaphosphate (VO(P03)2) (Figure 5). The Raman spectra of the fresh AP1.2 and the used AP1.2 catalysts are shown in Figure 6. The spectrum for the used sample was that of UI-VOPO~ (Ben Abdelouahab et al., 1992), consistent with the XRD results. The spectrum of a fresh sample was similar, except for the much broader peaks, suggesting poorer crystallinity, and the presence of a small peak a t 988 cm-l. It could be interpreted as due to a mixture of a1VOP04 and VOP04.2H20 (Ben Abdelouahab et al., 1992). It is possible that dehydration of VOP04.2H20 had occurred by the laser light radiation. The surface elemental composition for the fresh AP1.2 catalyst as determined by XPS indicated that the PN ratio was about 10, and reduced to slightly over 3 after use in reaction, probably due to volatilization of phosphorus species a t elevated temperatures. Enrichment of phosphorus on the surface has been reported in the literature (Partenheimer and Meyers, 1989; Zazhigalov et al., 1988a,b, 1989; Garbassi et al., 1986; Haas et al., 1988; Hodnett et al., 1983), although the phenomenon may be exaggerated by incorrect sensitivity factors (Okuhara et al., 1990). SEM photographs of the AP1.2 catalyst are shown in Figure 7. The fresh AP1.2 catalyst consisted of highly dispersed, smooth, hollow spheres of diameters between 1 and 10 pm. This is typical of powders prepared by spray pyrolysis (Moser and Lennholf, 1989; Moser and Cnossen, 1992). f i r 24 h of reaction, the particles had agglomerated and the surfaces had become rougher. After an additional 100 h of reaction, most spheres had broken into pieces, and the morphology began to resemble the disordered platelet structure of VPO catalysts prepared in the liquid phase. Upon storage, the platelet morphology became more evident.
I
Conclusion It was demonstrated that the aerosol pyrolysis (AP) technique could be used to produce VPO catalysts that were active, selective, and stable for the oxidation of butane to maleic anhydride. The AP catalyst thus produced at high temperatures was primarily VOP04.nH20, which was converted to UI-VOPO~ upon heating. Small amounts of other phases were also present. As prepared, the crystallinity of the sample was not high, but it increased with time of storage. The AP catalysts with the highest yields of MA were synthesized by processing a solution containing a PN of 1.2 at 650-700 "C. The fresh catalyst had an average vanadium oxidation state of 4.7-5.0. The AP catalysts could be converted into a mixture of vanadyl metaphosphate and vanadyl pyrophosphate by heating in an inert atmosphere. The used AP catalysts were hygroscopic and unstable, and returned to the fresh phase upon storage in air.
Acknowledgment This work was supported by Battelle's Advanced Materials Center for Commercial Development of Space under NASA Grant NAGW-811. The contributions of
Figure 7. SEM micrographs of AP1.2 catalyst: (A, top) fresh; (B, middle) after 24 h reaction; (C, bottom) used and aRer storage.
the late Jack Knox of Amoco toward the initiation of the Commercial Mixed Oxide Program, and the as-
Ind.Eng. Chem. Res., Vol. 34, No. 6 , 1995 1999 sistance of Jim Kaduk of Amoco in the interpretation of the X-ray data are gratefully acknowledged.
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Zazhigalov, V. A.; Belousov, V. M.; Pyatnitskaya, A. I.; Komashko, G. A.; Konovalova, N. D.; Goldenberg, G. I. Study of the Distribution of Elements in the Depth of V-P-Co Catalyst Grain. React. Kinet. Catal. Lett. 1989,39, 311. Received for review May 10, 1994 Revised manuscript received November 29, 1994 Accepted March 10, 1995@ IE9403028
Abstract published in Advance ACS Abstracts, April 15, 1995. @
