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Selective oxidation of n-butane to maleic anhydride with vanadium phosphorus catalysts prepared by comminution in the presence of dispersants
~
A PT PA LE IY DSS CA L I A: GENERAL
ELSEVIER
Applied Catalysis A: General 154 (1997) 103-115
Selective oxidation of n-butane to maleic anhydride with vanadium phosphorus catalysts prepared by comminution in the presence of dispersants •
a
•
G.J. H u t c h m g s ' ,
Raymond Higgins b
~Leverhulme Centre for Innovative Catalysis, Department of Chemistry, Universi~ of Liverpool, PO. Box 147, Liverpool L69 3BX, UK blcI Chemicals and Polymers Ltd., R and T Dept., Wilton, Middlesborough, UK Received 2 May 1996; received in revised form 22 July 1996; accepted 22 July 1996
Abstract A method of preparation of high surface area vanadium phosphorus catalysts for the partial oxidation of n-butane is described that is based on a comminution procedure. The ball milling of catalysts precursors in the presence of dispersants is shown to decrease significantly the mean crystallite size of the VOHPO4.0.5H20 from >5 x 10 6m to ca. 3.5 × 10 -8 m. A range of dispersants and solvents is described and the use of cyclohexane as solvent and the use of a dispersant based on poly-12-hydroxystearic acid is discussed in detail. These catalyst precursors give final catalysts with high surface area following activation (ca. 40 m 2 g-I versus ca. 10 m 2 g in the absence of the comminution). These catalysts are found to be particularly active for use under fuel rich reaction conditions { [butane] > the higher explosion limit} when high maleic anhydride yields can be obtained. The effect of the addition of promoters is also discussed. In general addition of low levels of La, Ce, Cu and Mo all increase the activity of maleic anhydride formation without any significant effect on selectivity when fuel lean conditions are used { [butane] < lower explosion limit}. However, the effect on activity is much less pronounced when fuel rich conditions are used and in this case the addition of these compounds leads to lower maleic anhydride selectivity. Keywords: Butane oxidation; Malein anhydride; Vanadium/phosphorus; Surface area
1. Introduction Vanadium phosphorus catalysts continue to attract considerable research attention since they remain the only example of an industrially operated catalyst for the oxidation of an alkane [1--4]. It is well known that the most active and selective * Corresponding author. 0926-860X/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved, PII S0926-860X(96)00368-7
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catalysts for n-butane partial oxidation are prepared by the activation of a precursor, namely VOHPOa.0.5H20. Considerable effort has been applied to the identification of improved methods of preparation. Initially catalysts were prepared using aqueous HC1 as solvent and reducing agent [5] and subsequently it was observed that catalysts with higher area could be obtained using alcohols as solvent and reducing agent [6]. These methods of preparation are well suited to the use of fuel lean reaction conditions in which the n-butane concentration is maintained below the lower explosive limit and a typical reaction mixture is 1.5 % n-butane in air. However, there are considerable advantages attained from the use of higher n-butane concentrations, for example those above the higher explosive limit e.g. 15% n-butane in air. A major advantage is that as the process is operated at a lower conversion with recycle of the unreacted n-butane and hence the maleic anhydride can be obtained in higher overall yield (typically ca. 80%) when compared to a fuel lean process yield (typically ca. 60%). In addition, since the exit gases from the reactor contain a high concentration of maleic anhydride the greater part of this can be recovered as a solid rather than as dilute maleic acid, due to the use of a water extraction procedure necessitated by the more dilute exit concentrations obtained from fuel lean conditions. However, for the most part the vanadium phosphorus catalysts currently available are insufficiently active to enable their effective use under fuel rich conditions. In this paper we describe a simple comminution procedure that can be used to prepare catalysts with high area that can be used under fuel rich conditions.
2. Experimental 2.1. Standard aqueous HC1 preparation method with additional water washing
V205 (60.6 g) and aqueous HC1 (35%, 790 ml) were refluxed with stirring for 1.5 h during which the solution was dark blue in colour. H3PO4 (88%, 89.1 g) was added and the solution was refluxed for a further 1.5 h. The solution was then evaporated to near dryness by sidearm distillation (rate of solvent removal 100 ml h -1) to a blue green paste which was dried in air (I10°C, 16 h) to give a blue green solid (P:V atomic ratio =1.20-t-0.01). This material (50 g) was refluxed for 2 h with distilled water (1 1) and the suspension was filtered hot to give a blue solid that was dried in air (110°C, 16 h) to give a blue solid. The P:V atomic ratio of the precursor was 1.05-t-0.01. 2.2. Preparation of catalyst precursor using a standard ball milling procedure A portion of the standard water washed catalyst precursor (30 g) was charged to a 500 ml porcelain pot together with high density alumina balls (diameter ---0.61.2 cm, density 3.5 g m1-1, 275 ml including voidage). Initial experiments utilised
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stainless steel balls but it was found that the resultant catalysts was contaminated by iron and this deleteriously affected catalyst performance. For this reason alumina balls were selected since the transfer of alumina to the catalyst was not found to be deleterious. Solvent (110 ml, Analar) and a dispersant (2 weight-% based on the precursor) were added and the pot was sealed and rotated at 80 rpm for 150 h. The comminuted catalyst precursor was then separated from the alumina spheres, dried for 15 h at 90°C in air, mixed with a pelleting agent (3% by weight Sterotex), pelleted, ground and sieved to give particles of the required size range (500-710 ~tm).
2.3. Preparation of catalyst promoted catalyst precursors using impregnation The ball milled catalyst precursor (10 g, 500-710 ~tm particles) was dried in air at 150°C for 16 h, cooled under desiccation and then impregnated using the method of incipient wetness with a solution of a suitable metal salt, typically the nitrate in a suitable solvent (water or isobutanol). Following drying, the particles were sieved to remove any dust and analysed using atomic absorption spectroscopy to determine the vanadium:promoter atomic ratio.
2.4. Preparation of catalyst promoted catalyst precursors using a standard ball milling procedure. A portion of the standard water washed precursor (30 g) and a suitable promoter metal salt (oxide or nitrate) were added to a 500 ml porcelain pot. High density alumina spheres, cyclohexane and dispersant were then added and the mixture was ball milled as described previously.
2.5. Measurement of mean crystallite size of the precursor. The crystallite size of the comminuted VOHPO4.0.5H20 was estimated using the peak width at half peak height for the <100>, <011> and <220> reflections of the powder X-ray diffraction pattern. Mean crystallite sizes between 2-100 nm can be measured by this method. Electron microscopy of VOHPO4-0.5H20 prior to the comminution procedure showed that the precursor crystallites were typically 5 m in size. After comminution most samples exhibited crystallite sizes of 30-50 nm and for this size range the accuracy of this method is probably 4-5-10 nm. Hence this method provides data that can be used on a comparative basis, although subsequent electron microscopy of specific samples confirmed the crystallite size range.
2.6. Catalyst testing Butane oxidation was carried out using fixed bed laboratory microreactors. Butane and air, in appropriate ratios, were fed to a reactor containing 5 ml catalyst
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(0.6 cm i.d., 18 cm bed length). Exit lines from the reactor were heated to prevent deposition of maleic anhydride prior to analysis. Both feed and product compositions were monitored by on-line gas chromatography. Test experiments showed that in the absence of a catalyst, blank thermal reactions were negligible below 500°C and in all experimental work mass balances typically between 97-103% were obtained. Experiments using standard techniques [7] showed that no diffusion limitations were present under these conditions. As a further check on the analysis of maleic anhydride the effluent gas from the microreactor was scrubbed with acetone for a known time and the resulting solution was analysed specifically for maleic anhydride by polarography. Excellent agreement was obtained between the gc and polarographic methods. For all catalysts tested in this study the transformation of the precursor to the final catalyst was carried out in situ in the reactor by heating in 1.5% n-butane in air at 385°C for 100 h. The data presented in this paper were collected typically between 100-300 h time on line. Representative data obtained from the testing of over 100 catalysts are reported.
2.7. Performance characterisation under fuel lean conditions using a simplified kinetic approach The data obtained from the microreactor tests enable the n-butane conversion and product selectivity to be determined for the different experimental conditions investigated. However, it is necessary to compare different catalyst formulations using a measure of specific activity and selectivity and in this paper the following method is used. When using inlet butane concentrations of 1-2 mol%, it is found that CO and CO2 are the only significant by-products (acetic and acrylic acids are only formed at <1% selectivity). Under these experimental conditions, for the catalyst used in this study, butane loss is found to be of first order [8]. The reaction data can be satisfactorily modelled by a simplified kinetic scheme involving a series-consecutive reaction scheme of pseudo first order reactions:
C4H10
k~ , C4H203
CO + CO 2
Using this simplified model and assuming plug flow conditions, Eq. (1) can be derived at constant temperature. kl + k2 = GHSVln[1/(I - x)]
(1)
where x is the fractional conversion of butane and GHSV is gas hourly space velocity measured at STP. From the plot of I n ( I / I - x ) versus 1/GHSV the value of kl + k2 can be obtained from the slope. The rate constant kl can be
G.J. Hutchings, R. Higgins/Applied Catalysis A." General 154 (1997) 103-I15
107
obtained from Eq. (2). kl = S0(kl + k2)
(2)
where So is the primary selectivity which is defined as the selectivity to maleic anhydride at zero conversion of n-butane. So is determined by measuring the variation in selectivity with conversion as the space velocity is varied at constant temperature and extrapolating the plot of selectivity versus conversion to zero conversion. Using this simplified kinetic scheme it is possible to use two factors to compare catalyst performance (a) the rate constant for butane conversion corrected for surface area (kl + k2)/sa which is a measure of the specific activity, (b) the primary selectivity So which is a measure of the selectivity of the catalyst.
2.8. Performance characterisation for fuel rich conditions
When the catalysts are operated under fuel rich conditions the simple kinetic scheme described previously is not valid since the conversion of n-butane is no longer pseudo first order. In addition the increase in molar volume of the products is significant even for low conversions and hence this effect must be allowed for in the calculation of the true selectivity and conversion. Using the microreactor experimental data the apparent conversion (Capp) and the apparent selectivity (Sapp) are obtainable: [butane]i n - [butane]out [butane]in
Capp ---~
[maleic anhydride]out S,pp = [butane]in -[butane]out The increase in volume on reaction has the effect of making Capp slightly higher than the true conversion (Creal) and Sapp slightly lower than the true selectivity (Srea0. Capp and Sapp are corrected as follows: V [1 -- Capp] Creal ~ 1 - Voo
(3)
Sreal = V0 [maleicanhydride]out V [butane]in -[butane]out
(4)
where V -
-
v0
= Fractional change in volume on reaction to conversion x = 1 + ZaX
(5)
G.J. Hutchings, R. Higgins/Applied Catalysis A: General 154 (1997) 103-115
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Sa = fractional change in volume between zero and complete conversion
r
- -
[CO]out [co2] out -
-
x = true conversion Creal (initially Capp is taken as the best value) b = best value for true selectivity Steal (initially Sapp) In most cases for the conditions used in this study V/Vo =1.02. Eqs. (3), (4) and (5) are solved in an iterative manner (usually 10-20 times) to obtain Creal and Srea~. The selectivity can also be obtained in terms of the products formed, i.e. normalised selectivity S and normalised conversion Coorm. S = [maleic anhydride] Eproducts Eproducts Cno~m = Eproducts + [butane]out where E products = [maleic anhydride]out +0.25[CO] +0.25[CO2] +0.5[acetic acid] +0.75[acrylic acid]. Within experimental error values of S and Cnorm are found to agree very well with Sre~a and Creal. Since calculation of S and Cno~mdoes not involve calculating the difference between the inlet and exit n-butane concentrations, which can incur significant error, the results are quoted as S and Cnorm.
3. Results
3.1. Effect of ball milling on the catalyst precursor Catalyst precursors when prepared using aqueous HC1 as the reducing agent and solvent typically have very low surface areas (ca. 1 m 2 g-l). Pretreatment in situ in the reactor gives a final catalyst with a significantly higher surface area but this can be variable (ca. 6-15 m 2 g-l) depending on the precise activation conditions employed. It has been shown [8,9] that the final surface area of these catalysts largely controls the catalytic performance and therefore, it is important to find an improved method of attaining high reproducible surface areas. In view of these low variable surface areas that were obtained with the present preparation procedure a study was made of comminution of the catalyst precursor using ball milling. The catalyst precursor was ball milled in a range of solvents and dispersants and the results concerning the determination of the mean crystallite size are shown in Table 1. It is apparent that the ball milling process decreased the mean crystallite
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Table 1 Effect of ball milling conditions on mean crystallite size a Solvent
Dispersant b
cyclohexane ethanol hexane CC14 p-xylene cyclohexane cyclohexane cyclohexane cyclohexane cyclohexane
PHSMa/EA/DIMAEMc PHSMa/EA/DIMAEMc PHSMa/EA/DIMAEMc PHSMa/EA/DIMAEMc PHSMa/EA/DIMAEMc PHS octaylamine decylamine CTMAB none
Mean crystallite size <100>
<011>
<220>
35 35 30 >60 35 45 60 >60 30 50
35 40 30 >60 35 40 40 40 30 20
40 60 30 >60 50 55 35 45 30 30
PHSMa poly-12-hydroxystearic acid reacted with glycidylmethacrylate; EA ethylacrylate; DIMAEM dimethyl amino ethyl methacrylate; PHS poly-12-hydroxystearic acid; CTMAB cetyl trimethyl ammonium bromide. a Water washed precursor ball milled for 150 h with solvent and dispersant specified. b 2% by weight dispersant used. c PHSMa/EA/DIMAEM=50/45/5.
Table 2 Effect of time of ball milling on mean crystallite size ~ Time(h)
72 150 192 216
Mean c~stallite size(nm) <100>
<011>
<220>
40 35 35 35
40 35 30 35
55 40 40 >35
Water washed precursor ball milled for 150 h with cyclohexane and 2% by weight PHSMa/EA/DIMAEM = 50/45/5, key as in Table 1.
size from 5 x 10 -6 m to 30-50 nm. The effect of time of ball milling is given in Table 2 and it is apparent that after 150 h there is very little benefit from further ball milling. When a dispersant is not used it is apparent that very small mean crystallite sizes for the precursor can still be achieved (Table 1). In general the ball milling procedure increased the surface area of the precursor from 1 to ca. 10 m 2 g - l , however, on subsequent calcination of the precursor ballmilled in cyclohexane alone, i.e. in the absence of the dispersant, no further increase in the surface area was observed. In contrast, catalysts prepared by ball milling with a dispersant in cyclohexane showed significant increase in surface area on calcination (Table 3). The X-ray diffraction analysis showed that the ball milled precursors had not undergone any phase change on ball milling and all the diffraction lines could be indexed to VOHPO4-0.SH20. On the basis of these results cyclohexane was selected as a suitable solvent and the dispersant PHSMa/EA/ DIMAEM 50/45/5 was found to give the high surface area of the calcined precursor.
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Table 3 Effect of ball milling on the surface area of the calcined catalyst a Dispersant
Surface area after ball millingC (mz g ~)
Surface area after calcinationd (mz g-i)
none PHSMa/EA/DIMAEM PHS/EA/GMa PHS/MMA/DIMAEM
7 2e 14 2~
9 20 16 15
a Water washed precursor,surface area before ball milling 1 m 2 g-1. b Key as Table 1, except; Mma methylmethacrylate; Gma glycidyl methacrylate. c After drying at 90°C, ball milling in cyclohexane for 150 h. d After calcination at 400°C for 12 h. e Initial temperature of 90°C; this is too low to remove the dispersant.
3.2. Catalytic performance of ball milled catalysts under fuel lean conditions A series of catalyst precursors were prepared using the ball milling procedure and these were converted into active catalysts for the oxidation of n-butane by pretreatment in situ in the reactor (1.5% butane in air, GHSV = 1000 h -1, 385°C). It is apparent that the ball milled catalysts are very active under fuel lean conditions (Table 4), which is due almost exclusively to the enhanced surface area that can be achieved with this preparation method since the specific activity as determined by the ratio (kl + kz)/sa. The surface area is found to increase on use to >40 m 2 g-t and with CTMAB as the dispersant a surface area as high as 46 m 2 g-1 could be achieved. However, with CTMAB the primary selectivity at 420°C was found to be much lower than that observed using other dispersants for which the primary selectivity was 75-80%. Table 4 Catalytic performance of ball milled catalysts under fuel lean conditionsa Precursor preparation
Final area (m 2 g 1)
Temp (°C)
First order rate constants (h -1) kl+k2
So (%)
PHS/EA/DIMAEMb
36
octylamineb
42
CTMAB b
46
oleylamineb
34
non ball milled ~
9
385 420 385 420 385 420 385 420 385 420
2389 4680 3175 5830 3150 8990 2650 3980 555 810
80 80 75 75 75 55 75 75 85 80
(kl+kz)/sa (g m 2 h 1)
66 130 76 139 68 195 78 117 62 90
a 1.5% butane in air, GHSV ---- 1000 h -1, following stabilisation for 100 h. b Standard ball milling procedure using 2 weight-% of dispersant in cyclohexane, key for dispersants as in Table 1. c Water washed catalyst not ball milled.
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Table 5 Catalytic performance of promoted ball milled catalysts under fuel lean conditions~ Promoter
V:P:M
Final area (m 2 g- I)
Temp CC)
none
1: 1.05:0
36
Ce
1:1.05:0.005
27
Cu
1:1.05:0.005
35
La Mo
1:1.05:0.01 1:1.05:0.025
39 32
385 420 385 420 385 420 385 385 420
First order rate constants (h t) kl+k2 2389 4680 3170 6850 3620 10 000 3190 3735 8600
So (%)
80 80 80 75 80 75 80 80 75
(k~+kz)/Sa ( g m 2h 1)
66 130 117 254 103 286 82 117 269
Catalyst prepared using standard ball milling procedure using 2 weight-% PHS/EA/DIMAEM for 150 h. Tested 1.5% butane in air, GHSV = 1000 h -1, following stabilisation for 100 h. b Promoters added with the precursor during ball milling, Ce,Cu, La added as nitrates and Mo as the oxide.
The catalyst were analysed following use under fuel lean conditions using powder X-ray diffraction and the final catalysts were found to comprise mainly of (VO)2P207 and ctliVOPO 4. It was found that these high area final catalysts contained ca. 20% otuVOPO 4 whereas similar low area catalysts prepared from the equivalent water washed precursor in the absence of ball milling gave higher Gt1IVOPO4 content of ca. 30-40%. A series of promoted catalysts was prepared by adding the promoter compound to the precursor during the ball milling step (Table 5). Ce, Cu, La and Mo were investigated since it had been previously shown that these additives were beneficial under fuel lean conditions [4,8]. The additives were not found to affect the selectivity to maleic anhydride but did significantly enhance the specific activity and in particular Ce, Cu and Mo increased the specific activity by over a factor of two at 420°C. In particular, the selectivity to maleic anhydride at high conversion with these catalysts was not significantly less than So and in general pass yields of 60% at 90% n-butane conversion could be readily attained.
3.3. Catalytic performance of ball milled catalysts under fuel rich conditions A catalyst prepared using the standard ball milling method with cyclohexane as solvent and PHSMa/EA/DIMAEM as dispersant was converted into the active catalyst by in situ pretreatment in the reactor using fuel lean conditions for ca. 200h (1.5% butane in air, GHSV = 1 0 0 0 h -1, 385°C) and then the reaction conditions were changed to fuel rich conditions (n-butane concentration >15%) and the catalyst performance was allowed to restabilise. The results are shown in Table 6. At 360°C an activity of 0.8 tool maleic anhydride 1-1 catalyst h - ~ was observed with a maleic anhydride modivity of 78-80% and this performance was stable for several hundred hours. At this condition the exit reactor maleic anhydride
G.J. Hutchings, R. Higgins/Applied Catalysis A: General 154 (1997) 103-115
112
~5
~5~5~5
~6
rz~6e6
;>
k)
"r~
"0 o
o
e~
.~
~
. ~
.z
•
.~.~-..
G.Z Hutchings, R. Higgins/Applied Catalysis A: General 154 (1997) 103-115
113
concentration was 2 mol-%, and this is much higher than exit concentrations observed under fuel lean conditions. Under fuel rich conditions the main products were maleic anhydride, CO and CO2. Acetic acid and acrylic acid were observed as minor byproducts as was the case under fuel lean conditions (for example 0.35 tool-% acetic acid and 0.09 mol-% acrylic acid were formed with 15% butane in air, Cnorm --17 mol-%, GHSV =1000 h -1, 360°C) and no hydrocarbons were observed as byproducts. Powder X-ray diffraction analysis indicated that the final catalysts were found to comprise mainly of (VO)2P207 and ~nVOPO4. It was found that these final catalysts with large area contained ca. 10-20% OtuVOPO4 and was slightly lower when compared with the equivalent catalyst used under fuel lean conditions. The effect of low levels of La was investigated and the results are shown in Table 6. Two methods of addition of La were investigated (a) impregnation of the ball milled precursor with a solution of lanthanum nitrate in isobutanol and (b) ball milling the catalyst precursor with lanthanum nitrate. A blank experiment for method (a) was carded out in which a water washed ball milled precursor was impregnated with isobutanol in the absence of the lanthanum nitrate and this showed significantly lower selectivity to maleic anhydride. This decreased selectivity was also observed in the presence of lanthanum. The addition of the additive in the ball milling step was found to give enhanced activity and so this method was selected for the investigation of the relative effect of Ce, Cu, La and Mo under fuel rich conditions (Table 6). Under comparable conditions these additives had enhanced catalyst activity but the effect was much less pronounced than under fuel lean conditions. In addition the promoted catalysts demonstrated much lower maleic anhydride selectivities when compared to the unpromoted catalyst a feature that was not observed under fuel lean conditions. The ball milled catalysts were found to be particularly stable when operated under fuel lean conditions. For example, the Mo promoted catalyst (V:P:Mo = 1:1.05:0.025) demonstrated a stable activity of 1.05 mol maleic anhydride 1-1 catalyst h-1 with a maleic anhydride selectivity of 72 mol-% for several hundred hours operation at 360°C (15% nbutane in air, GHSV = 1000 h - l ) after prior stabilisation under fuel lean conditions for 200 h. As a fuel rich process would necessitate the recycle of unreacted nbutane it is considered that a fuel rich process would use oxygen rather than air as the oxidant and CO and CO2 as the inerts. Initial experiments showed that CO was not oxidised over the catalyst at temperatures up to 420°C and use of CO and CO2 as inerts was not found to affect the catalyst performance under fuel rich conditions.
4. Discussion The use of a simple ball milling procedure in the presence of a solvent has been shown to provide a reliable method for the preparation of high area catalysts (typically >40 m 2 g - 1). Although increased surface areas could be achieved by ball
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G.J. Hutchings, R. Higgins/Applied Catalysis A: General 154 (1997) 103-115
milling the precursor in the presence of a solvent, much improved materials were obtained when dispersants were added. It is considered that the dispersant molecules are adsorbed on the freshly cleaved catalyst surfaces thereby stopping reagglomeration occurring. The thermal decomposition of the dispersant during the pretreatment stage also probably adds to the attainment of the very high surface areas observed. These high area materials are found to be active and selective for the formation of maleic anhydride under fuel lean conditions. However, the most significant advantage is the observation that the ball milled catalysts can be used under fuel rich conditions at low temperature (<360°C) thereby giving very high yields of maleic anhydride. Lower surface area catalysts derived from standard preparation procedures do not exhibit sufficient activity to be used under fuel rich conditions unless temperature >400°C are used and at this temperature the selectivity to maleic anhydride is greatly decreased. The main advantages of a fuel rich process are twofold. First, the use of partial butane conversion with recycle enables much higher efficiencies for butane utilisation to be achieved. Second, the maleic anhydride is formed at high exit concentrations. This means that a significant amount can be recovered from the reactor effluent by condensation rather than by a water washing procedure needed to treat the more dilute maleic anhydride streams resulting from a fuel lean process. Hence, the separation costs associated with such a process are significantly lower than a conventional fuel lean process. In addition the improved product recovery together with decreased butane losses and improved maleic anhydride yield (e.g. the yield obtained from a fuel rich process is ca. 80 mol-% whereas the yield from a conventional fuel lean process is <70 mol-% and typically ca. 60 mol-%) mean that the operation of a fuel rich process could derive significant benefit concerning the environmental impact and waste management. There are two further comments that should be made concerning the operation of the ball milled catalysts under fuel rich conditions. First, although ca. 100 different formulations were tested in obtaining the data presented in this paper it was observed that under all conditions that the only products formed were maleic anhydride, CO, CO2, acetic and acrylic acids. Furan, crotonaldehyde and unsaturated C4 hydrocarbons were not observed even in trace amounts. This is in contrast to the earlier report of Centi et al. [10] who indicated that significant amounts of C4 hydrocarbons were observed as byproducts when using fuel rich conditions. This may be due to the catalysts used in the present study exhibiting a higher average Voxidation state than the catalysts used by Centi et al. [10]. This is related to the observation that water washed precursors prepared using the aqueous HC1 method lead to the formation of final catalysts containing OtligOPO 4 in addition to (VO)2PzO 7 [8] and this component remains present in the catalyst even after many hundreds of hours of testing under fuel lean or fuel rich conditions. It is not considered that the apparent difference is solely related to the use of the high area catalysts derived from the ball milling technique since the non ball milled catalyst tested under fuel rich conditions (surface area 7 m 2 g - l ) also gave maleic
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115
anhydride, CO, CO2, acetic and acrylic acids as products. The second observation is that the promotional effect of Ce, Cu and Mo is much lower under fuel rich conditions than the effect observed on activity for the same catalysts under fuel lean conditions. It has been noted previously [8] that for catalysts containing ~IIVOPO4 in addition to (VO)2P207, significant promotional effects on activity can be observed and this effect is related to the ~IIVOPO4 content. The decreased promotional effect could therefore be, in part, due to the decreased o,IIVOPO4 content observed for catalysts tested under fuel rich conditions. However, the effect is more pronounced than that could be expected from the small decrease in ~nVOPO4 content observed. It is therefore considered that under the more highly reducing atmosphere of fuel rich conditions when compared to fuel lean conditions, the catalyst promoter becomes less efficient. This indicates that a main role of such promoters is the maintenance of the oxidation state of the active catalyst surface, possibly aiding the retention of the vanadium atoms at the required oxidation level. Under fuel rich conditions the maintenance of the surface oxidation state may be more facile than when the more oxidising fuel lean conditions thereby negating part of the role played by such promoters. However, it must be noted that La promoted catalysts are capable of very high yields under fuel rich conditions and although the overall maleic anhydride yield is a little lower than the non promoted formulation (71.5 mol-%) the activity (1.85 mol maleic anhydride 1-I catalyst h -1) and the exit maleic anhydride concentration (2 mol-%) are much higher under comparable conditions (360°C, GHSV =2000 h -~) and therefore the promotional effect cannot solely be associated with the retention of the surface oxidation state.
Acknowledgements We thank ICI PLC for permission to publish this work.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
B.K. Hodnett, Catal. Rev.-Sci. Eng., 27 (1985) 373. E. Bordes, Catal. Today, 1 (1987) 499. G. Centi, E Triferr, J.R. Ebner and V.M. Franchetti, Chem. Rev., 88 (1988) 55. G.J. Hutchings, Appl. Catal., 72 (1991) 1. R. Higgins and G.J. Hutchings, US Patent 4222945 (1980), assigned to Imperial Chemical Industries; US Patent 4147661 (1979), assigned to Imperial Chemical Industries. J.W, Johnson, D.C. Johnston, A. J Jacobson and J.E Brady, J. Am. Chem. Soc., 106 (1984) 8123. G.C. Bond, Heterogeneous Catalysis, Principals and Applications, Oxford Chem. Ser., 1987, p. 58. G.J. Hutchings and R. Higgins, in press. G.J. Hutchings, Catal. Today, 16 (1993) 139. G. Centi, G. Fornasari and E Trifiro, J. Catal., 89 (1984) 44.
A PT PA LE IY DSS CA L I A: GENERAL
ELSEVIER
Applied Catalysis A: General 154 (1997) 103-115
Selective oxidation of n-butane to maleic anhydride with vanadium phosphorus catalysts prepared by comminution in the presence of dispersants •
a
•
G.J. H u t c h m g s ' ,
Raymond Higgins b
~Leverhulme Centre for Innovative Catalysis, Department of Chemistry, Universi~ of Liverpool, PO. Box 147, Liverpool L69 3BX, UK blcI Chemicals and Polymers Ltd., R and T Dept., Wilton, Middlesborough, UK Received 2 May 1996; received in revised form 22 July 1996; accepted 22 July 1996
Abstract A method of preparation of high surface area vanadium phosphorus catalysts for the partial oxidation of n-butane is described that is based on a comminution procedure. The ball milling of catalysts precursors in the presence of dispersants is shown to decrease significantly the mean crystallite size of the VOHPO4.0.5H20 from >5 x 10 6m to ca. 3.5 × 10 -8 m. A range of dispersants and solvents is described and the use of cyclohexane as solvent and the use of a dispersant based on poly-12-hydroxystearic acid is discussed in detail. These catalyst precursors give final catalysts with high surface area following activation (ca. 40 m 2 g-I versus ca. 10 m 2 g in the absence of the comminution). These catalysts are found to be particularly active for use under fuel rich reaction conditions { [butane] > the higher explosion limit} when high maleic anhydride yields can be obtained. The effect of the addition of promoters is also discussed. In general addition of low levels of La, Ce, Cu and Mo all increase the activity of maleic anhydride formation without any significant effect on selectivity when fuel lean conditions are used { [butane] < lower explosion limit}. However, the effect on activity is much less pronounced when fuel rich conditions are used and in this case the addition of these compounds leads to lower maleic anhydride selectivity. Keywords: Butane oxidation; Malein anhydride; Vanadium/phosphorus; Surface area
1. Introduction Vanadium phosphorus catalysts continue to attract considerable research attention since they remain the only example of an industrially operated catalyst for the oxidation of an alkane [1--4]. It is well known that the most active and selective * Corresponding author. 0926-860X/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved, PII S0926-860X(96)00368-7
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catalysts for n-butane partial oxidation are prepared by the activation of a precursor, namely VOHPOa.0.5H20. Considerable effort has been applied to the identification of improved methods of preparation. Initially catalysts were prepared using aqueous HC1 as solvent and reducing agent [5] and subsequently it was observed that catalysts with higher area could be obtained using alcohols as solvent and reducing agent [6]. These methods of preparation are well suited to the use of fuel lean reaction conditions in which the n-butane concentration is maintained below the lower explosive limit and a typical reaction mixture is 1.5 % n-butane in air. However, there are considerable advantages attained from the use of higher n-butane concentrations, for example those above the higher explosive limit e.g. 15% n-butane in air. A major advantage is that as the process is operated at a lower conversion with recycle of the unreacted n-butane and hence the maleic anhydride can be obtained in higher overall yield (typically ca. 80%) when compared to a fuel lean process yield (typically ca. 60%). In addition, since the exit gases from the reactor contain a high concentration of maleic anhydride the greater part of this can be recovered as a solid rather than as dilute maleic acid, due to the use of a water extraction procedure necessitated by the more dilute exit concentrations obtained from fuel lean conditions. However, for the most part the vanadium phosphorus catalysts currently available are insufficiently active to enable their effective use under fuel rich conditions. In this paper we describe a simple comminution procedure that can be used to prepare catalysts with high area that can be used under fuel rich conditions.
2. Experimental 2.1. Standard aqueous HC1 preparation method with additional water washing
V205 (60.6 g) and aqueous HC1 (35%, 790 ml) were refluxed with stirring for 1.5 h during which the solution was dark blue in colour. H3PO4 (88%, 89.1 g) was added and the solution was refluxed for a further 1.5 h. The solution was then evaporated to near dryness by sidearm distillation (rate of solvent removal 100 ml h -1) to a blue green paste which was dried in air (I10°C, 16 h) to give a blue green solid (P:V atomic ratio =1.20-t-0.01). This material (50 g) was refluxed for 2 h with distilled water (1 1) and the suspension was filtered hot to give a blue solid that was dried in air (110°C, 16 h) to give a blue solid. The P:V atomic ratio of the precursor was 1.05-t-0.01. 2.2. Preparation of catalyst precursor using a standard ball milling procedure A portion of the standard water washed catalyst precursor (30 g) was charged to a 500 ml porcelain pot together with high density alumina balls (diameter ---0.61.2 cm, density 3.5 g m1-1, 275 ml including voidage). Initial experiments utilised
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stainless steel balls but it was found that the resultant catalysts was contaminated by iron and this deleteriously affected catalyst performance. For this reason alumina balls were selected since the transfer of alumina to the catalyst was not found to be deleterious. Solvent (110 ml, Analar) and a dispersant (2 weight-% based on the precursor) were added and the pot was sealed and rotated at 80 rpm for 150 h. The comminuted catalyst precursor was then separated from the alumina spheres, dried for 15 h at 90°C in air, mixed with a pelleting agent (3% by weight Sterotex), pelleted, ground and sieved to give particles of the required size range (500-710 ~tm).
2.3. Preparation of catalyst promoted catalyst precursors using impregnation The ball milled catalyst precursor (10 g, 500-710 ~tm particles) was dried in air at 150°C for 16 h, cooled under desiccation and then impregnated using the method of incipient wetness with a solution of a suitable metal salt, typically the nitrate in a suitable solvent (water or isobutanol). Following drying, the particles were sieved to remove any dust and analysed using atomic absorption spectroscopy to determine the vanadium:promoter atomic ratio.
2.4. Preparation of catalyst promoted catalyst precursors using a standard ball milling procedure. A portion of the standard water washed precursor (30 g) and a suitable promoter metal salt (oxide or nitrate) were added to a 500 ml porcelain pot. High density alumina spheres, cyclohexane and dispersant were then added and the mixture was ball milled as described previously.
2.5. Measurement of mean crystallite size of the precursor. The crystallite size of the comminuted VOHPO4.0.5H20 was estimated using the peak width at half peak height for the <100>, <011> and <220> reflections of the powder X-ray diffraction pattern. Mean crystallite sizes between 2-100 nm can be measured by this method. Electron microscopy of VOHPO4-0.5H20 prior to the comminution procedure showed that the precursor crystallites were typically 5 m in size. After comminution most samples exhibited crystallite sizes of 30-50 nm and for this size range the accuracy of this method is probably 4-5-10 nm. Hence this method provides data that can be used on a comparative basis, although subsequent electron microscopy of specific samples confirmed the crystallite size range.
2.6. Catalyst testing Butane oxidation was carried out using fixed bed laboratory microreactors. Butane and air, in appropriate ratios, were fed to a reactor containing 5 ml catalyst
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(0.6 cm i.d., 18 cm bed length). Exit lines from the reactor were heated to prevent deposition of maleic anhydride prior to analysis. Both feed and product compositions were monitored by on-line gas chromatography. Test experiments showed that in the absence of a catalyst, blank thermal reactions were negligible below 500°C and in all experimental work mass balances typically between 97-103% were obtained. Experiments using standard techniques [7] showed that no diffusion limitations were present under these conditions. As a further check on the analysis of maleic anhydride the effluent gas from the microreactor was scrubbed with acetone for a known time and the resulting solution was analysed specifically for maleic anhydride by polarography. Excellent agreement was obtained between the gc and polarographic methods. For all catalysts tested in this study the transformation of the precursor to the final catalyst was carried out in situ in the reactor by heating in 1.5% n-butane in air at 385°C for 100 h. The data presented in this paper were collected typically between 100-300 h time on line. Representative data obtained from the testing of over 100 catalysts are reported.
2.7. Performance characterisation under fuel lean conditions using a simplified kinetic approach The data obtained from the microreactor tests enable the n-butane conversion and product selectivity to be determined for the different experimental conditions investigated. However, it is necessary to compare different catalyst formulations using a measure of specific activity and selectivity and in this paper the following method is used. When using inlet butane concentrations of 1-2 mol%, it is found that CO and CO2 are the only significant by-products (acetic and acrylic acids are only formed at <1% selectivity). Under these experimental conditions, for the catalyst used in this study, butane loss is found to be of first order [8]. The reaction data can be satisfactorily modelled by a simplified kinetic scheme involving a series-consecutive reaction scheme of pseudo first order reactions:
C4H10
k~ , C4H203
CO + CO 2
Using this simplified model and assuming plug flow conditions, Eq. (1) can be derived at constant temperature. kl + k2 = GHSVln[1/(I - x)]
(1)
where x is the fractional conversion of butane and GHSV is gas hourly space velocity measured at STP. From the plot of I n ( I / I - x ) versus 1/GHSV the value of kl + k2 can be obtained from the slope. The rate constant kl can be
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obtained from Eq. (2). kl = S0(kl + k2)
(2)
where So is the primary selectivity which is defined as the selectivity to maleic anhydride at zero conversion of n-butane. So is determined by measuring the variation in selectivity with conversion as the space velocity is varied at constant temperature and extrapolating the plot of selectivity versus conversion to zero conversion. Using this simplified kinetic scheme it is possible to use two factors to compare catalyst performance (a) the rate constant for butane conversion corrected for surface area (kl + k2)/sa which is a measure of the specific activity, (b) the primary selectivity So which is a measure of the selectivity of the catalyst.
2.8. Performance characterisation for fuel rich conditions
When the catalysts are operated under fuel rich conditions the simple kinetic scheme described previously is not valid since the conversion of n-butane is no longer pseudo first order. In addition the increase in molar volume of the products is significant even for low conversions and hence this effect must be allowed for in the calculation of the true selectivity and conversion. Using the microreactor experimental data the apparent conversion (Capp) and the apparent selectivity (Sapp) are obtainable: [butane]i n - [butane]out [butane]in
Capp ---~
[maleic anhydride]out S,pp = [butane]in -[butane]out The increase in volume on reaction has the effect of making Capp slightly higher than the true conversion (Creal) and Sapp slightly lower than the true selectivity (Srea0. Capp and Sapp are corrected as follows: V [1 -- Capp] Creal ~ 1 - Voo
(3)
Sreal = V0 [maleicanhydride]out V [butane]in -[butane]out
(4)
where V -
-
v0
= Fractional change in volume on reaction to conversion x = 1 + ZaX
(5)
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Sa = fractional change in volume between zero and complete conversion
r
- -
[CO]out [co2] out -
-
x = true conversion Creal (initially Capp is taken as the best value) b = best value for true selectivity Steal (initially Sapp) In most cases for the conditions used in this study V/Vo =1.02. Eqs. (3), (4) and (5) are solved in an iterative manner (usually 10-20 times) to obtain Creal and Srea~. The selectivity can also be obtained in terms of the products formed, i.e. normalised selectivity S and normalised conversion Coorm. S = [maleic anhydride] Eproducts Eproducts Cno~m = Eproducts + [butane]out where E products = [maleic anhydride]out +0.25[CO] +0.25[CO2] +0.5[acetic acid] +0.75[acrylic acid]. Within experimental error values of S and Cnorm are found to agree very well with Sre~a and Creal. Since calculation of S and Cno~mdoes not involve calculating the difference between the inlet and exit n-butane concentrations, which can incur significant error, the results are quoted as S and Cnorm.
3. Results
3.1. Effect of ball milling on the catalyst precursor Catalyst precursors when prepared using aqueous HC1 as the reducing agent and solvent typically have very low surface areas (ca. 1 m 2 g-l). Pretreatment in situ in the reactor gives a final catalyst with a significantly higher surface area but this can be variable (ca. 6-15 m 2 g-l) depending on the precise activation conditions employed. It has been shown [8,9] that the final surface area of these catalysts largely controls the catalytic performance and therefore, it is important to find an improved method of attaining high reproducible surface areas. In view of these low variable surface areas that were obtained with the present preparation procedure a study was made of comminution of the catalyst precursor using ball milling. The catalyst precursor was ball milled in a range of solvents and dispersants and the results concerning the determination of the mean crystallite size are shown in Table 1. It is apparent that the ball milling process decreased the mean crystallite
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Table 1 Effect of ball milling conditions on mean crystallite size a Solvent
Dispersant b
cyclohexane ethanol hexane CC14 p-xylene cyclohexane cyclohexane cyclohexane cyclohexane cyclohexane
PHSMa/EA/DIMAEMc PHSMa/EA/DIMAEMc PHSMa/EA/DIMAEMc PHSMa/EA/DIMAEMc PHSMa/EA/DIMAEMc PHS octaylamine decylamine CTMAB none
Mean crystallite size <100>
<011>
<220>
35 35 30 >60 35 45 60 >60 30 50
35 40 30 >60 35 40 40 40 30 20
40 60 30 >60 50 55 35 45 30 30
PHSMa poly-12-hydroxystearic acid reacted with glycidylmethacrylate; EA ethylacrylate; DIMAEM dimethyl amino ethyl methacrylate; PHS poly-12-hydroxystearic acid; CTMAB cetyl trimethyl ammonium bromide. a Water washed precursor ball milled for 150 h with solvent and dispersant specified. b 2% by weight dispersant used. c PHSMa/EA/DIMAEM=50/45/5.
Table 2 Effect of time of ball milling on mean crystallite size ~ Time(h)
72 150 192 216
Mean c~stallite size(nm) <100>
<011>
<220>
40 35 35 35
40 35 30 35
55 40 40 >35
Water washed precursor ball milled for 150 h with cyclohexane and 2% by weight PHSMa/EA/DIMAEM = 50/45/5, key as in Table 1.
size from 5 x 10 -6 m to 30-50 nm. The effect of time of ball milling is given in Table 2 and it is apparent that after 150 h there is very little benefit from further ball milling. When a dispersant is not used it is apparent that very small mean crystallite sizes for the precursor can still be achieved (Table 1). In general the ball milling procedure increased the surface area of the precursor from 1 to ca. 10 m 2 g - l , however, on subsequent calcination of the precursor ballmilled in cyclohexane alone, i.e. in the absence of the dispersant, no further increase in the surface area was observed. In contrast, catalysts prepared by ball milling with a dispersant in cyclohexane showed significant increase in surface area on calcination (Table 3). The X-ray diffraction analysis showed that the ball milled precursors had not undergone any phase change on ball milling and all the diffraction lines could be indexed to VOHPO4-0.SH20. On the basis of these results cyclohexane was selected as a suitable solvent and the dispersant PHSMa/EA/ DIMAEM 50/45/5 was found to give the high surface area of the calcined precursor.
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Table 3 Effect of ball milling on the surface area of the calcined catalyst a Dispersant
Surface area after ball millingC (mz g ~)
Surface area after calcinationd (mz g-i)
none PHSMa/EA/DIMAEM PHS/EA/GMa PHS/MMA/DIMAEM
7 2e 14 2~
9 20 16 15
a Water washed precursor,surface area before ball milling 1 m 2 g-1. b Key as Table 1, except; Mma methylmethacrylate; Gma glycidyl methacrylate. c After drying at 90°C, ball milling in cyclohexane for 150 h. d After calcination at 400°C for 12 h. e Initial temperature of 90°C; this is too low to remove the dispersant.
3.2. Catalytic performance of ball milled catalysts under fuel lean conditions A series of catalyst precursors were prepared using the ball milling procedure and these were converted into active catalysts for the oxidation of n-butane by pretreatment in situ in the reactor (1.5% butane in air, GHSV = 1000 h -1, 385°C). It is apparent that the ball milled catalysts are very active under fuel lean conditions (Table 4), which is due almost exclusively to the enhanced surface area that can be achieved with this preparation method since the specific activity as determined by the ratio (kl + kz)/sa. The surface area is found to increase on use to >40 m 2 g-t and with CTMAB as the dispersant a surface area as high as 46 m 2 g-1 could be achieved. However, with CTMAB the primary selectivity at 420°C was found to be much lower than that observed using other dispersants for which the primary selectivity was 75-80%. Table 4 Catalytic performance of ball milled catalysts under fuel lean conditionsa Precursor preparation
Final area (m 2 g 1)
Temp (°C)
First order rate constants (h -1) kl+k2
So (%)
PHS/EA/DIMAEMb
36
octylamineb
42
CTMAB b
46
oleylamineb
34
non ball milled ~
9
385 420 385 420 385 420 385 420 385 420
2389 4680 3175 5830 3150 8990 2650 3980 555 810
80 80 75 75 75 55 75 75 85 80
(kl+kz)/sa (g m 2 h 1)
66 130 76 139 68 195 78 117 62 90
a 1.5% butane in air, GHSV ---- 1000 h -1, following stabilisation for 100 h. b Standard ball milling procedure using 2 weight-% of dispersant in cyclohexane, key for dispersants as in Table 1. c Water washed catalyst not ball milled.
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Table 5 Catalytic performance of promoted ball milled catalysts under fuel lean conditions~ Promoter
V:P:M
Final area (m 2 g- I)
Temp CC)
none
1: 1.05:0
36
Ce
1:1.05:0.005
27
Cu
1:1.05:0.005
35
La Mo
1:1.05:0.01 1:1.05:0.025
39 32
385 420 385 420 385 420 385 385 420
First order rate constants (h t) kl+k2 2389 4680 3170 6850 3620 10 000 3190 3735 8600
So (%)
80 80 80 75 80 75 80 80 75
(k~+kz)/Sa ( g m 2h 1)
66 130 117 254 103 286 82 117 269
Catalyst prepared using standard ball milling procedure using 2 weight-% PHS/EA/DIMAEM for 150 h. Tested 1.5% butane in air, GHSV = 1000 h -1, following stabilisation for 100 h. b Promoters added with the precursor during ball milling, Ce,Cu, La added as nitrates and Mo as the oxide.
The catalyst were analysed following use under fuel lean conditions using powder X-ray diffraction and the final catalysts were found to comprise mainly of (VO)2P207 and ctliVOPO 4. It was found that these high area final catalysts contained ca. 20% otuVOPO 4 whereas similar low area catalysts prepared from the equivalent water washed precursor in the absence of ball milling gave higher Gt1IVOPO4 content of ca. 30-40%. A series of promoted catalysts was prepared by adding the promoter compound to the precursor during the ball milling step (Table 5). Ce, Cu, La and Mo were investigated since it had been previously shown that these additives were beneficial under fuel lean conditions [4,8]. The additives were not found to affect the selectivity to maleic anhydride but did significantly enhance the specific activity and in particular Ce, Cu and Mo increased the specific activity by over a factor of two at 420°C. In particular, the selectivity to maleic anhydride at high conversion with these catalysts was not significantly less than So and in general pass yields of 60% at 90% n-butane conversion could be readily attained.
3.3. Catalytic performance of ball milled catalysts under fuel rich conditions A catalyst prepared using the standard ball milling method with cyclohexane as solvent and PHSMa/EA/DIMAEM as dispersant was converted into the active catalyst by in situ pretreatment in the reactor using fuel lean conditions for ca. 200h (1.5% butane in air, GHSV = 1 0 0 0 h -1, 385°C) and then the reaction conditions were changed to fuel rich conditions (n-butane concentration >15%) and the catalyst performance was allowed to restabilise. The results are shown in Table 6. At 360°C an activity of 0.8 tool maleic anhydride 1-1 catalyst h - ~ was observed with a maleic anhydride modivity of 78-80% and this performance was stable for several hundred hours. At this condition the exit reactor maleic anhydride
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112
~5
~5~5~5
~6
rz~6e6
;>
k)
"r~
"0 o
o
e~
.~
~
. ~
.z
•
.~.~-..
G.Z Hutchings, R. Higgins/Applied Catalysis A: General 154 (1997) 103-115
113
concentration was 2 mol-%, and this is much higher than exit concentrations observed under fuel lean conditions. Under fuel rich conditions the main products were maleic anhydride, CO and CO2. Acetic acid and acrylic acid were observed as minor byproducts as was the case under fuel lean conditions (for example 0.35 tool-% acetic acid and 0.09 mol-% acrylic acid were formed with 15% butane in air, Cnorm --17 mol-%, GHSV =1000 h -1, 360°C) and no hydrocarbons were observed as byproducts. Powder X-ray diffraction analysis indicated that the final catalysts were found to comprise mainly of (VO)2P207 and ~nVOPO4. It was found that these final catalysts with large area contained ca. 10-20% OtuVOPO4 and was slightly lower when compared with the equivalent catalyst used under fuel lean conditions. The effect of low levels of La was investigated and the results are shown in Table 6. Two methods of addition of La were investigated (a) impregnation of the ball milled precursor with a solution of lanthanum nitrate in isobutanol and (b) ball milling the catalyst precursor with lanthanum nitrate. A blank experiment for method (a) was carded out in which a water washed ball milled precursor was impregnated with isobutanol in the absence of the lanthanum nitrate and this showed significantly lower selectivity to maleic anhydride. This decreased selectivity was also observed in the presence of lanthanum. The addition of the additive in the ball milling step was found to give enhanced activity and so this method was selected for the investigation of the relative effect of Ce, Cu, La and Mo under fuel rich conditions (Table 6). Under comparable conditions these additives had enhanced catalyst activity but the effect was much less pronounced than under fuel lean conditions. In addition the promoted catalysts demonstrated much lower maleic anhydride selectivities when compared to the unpromoted catalyst a feature that was not observed under fuel lean conditions. The ball milled catalysts were found to be particularly stable when operated under fuel lean conditions. For example, the Mo promoted catalyst (V:P:Mo = 1:1.05:0.025) demonstrated a stable activity of 1.05 mol maleic anhydride 1-1 catalyst h-1 with a maleic anhydride selectivity of 72 mol-% for several hundred hours operation at 360°C (15% nbutane in air, GHSV = 1000 h - l ) after prior stabilisation under fuel lean conditions for 200 h. As a fuel rich process would necessitate the recycle of unreacted nbutane it is considered that a fuel rich process would use oxygen rather than air as the oxidant and CO and CO2 as the inerts. Initial experiments showed that CO was not oxidised over the catalyst at temperatures up to 420°C and use of CO and CO2 as inerts was not found to affect the catalyst performance under fuel rich conditions.
4. Discussion The use of a simple ball milling procedure in the presence of a solvent has been shown to provide a reliable method for the preparation of high area catalysts (typically >40 m 2 g - 1). Although increased surface areas could be achieved by ball
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milling the precursor in the presence of a solvent, much improved materials were obtained when dispersants were added. It is considered that the dispersant molecules are adsorbed on the freshly cleaved catalyst surfaces thereby stopping reagglomeration occurring. The thermal decomposition of the dispersant during the pretreatment stage also probably adds to the attainment of the very high surface areas observed. These high area materials are found to be active and selective for the formation of maleic anhydride under fuel lean conditions. However, the most significant advantage is the observation that the ball milled catalysts can be used under fuel rich conditions at low temperature (<360°C) thereby giving very high yields of maleic anhydride. Lower surface area catalysts derived from standard preparation procedures do not exhibit sufficient activity to be used under fuel rich conditions unless temperature >400°C are used and at this temperature the selectivity to maleic anhydride is greatly decreased. The main advantages of a fuel rich process are twofold. First, the use of partial butane conversion with recycle enables much higher efficiencies for butane utilisation to be achieved. Second, the maleic anhydride is formed at high exit concentrations. This means that a significant amount can be recovered from the reactor effluent by condensation rather than by a water washing procedure needed to treat the more dilute maleic anhydride streams resulting from a fuel lean process. Hence, the separation costs associated with such a process are significantly lower than a conventional fuel lean process. In addition the improved product recovery together with decreased butane losses and improved maleic anhydride yield (e.g. the yield obtained from a fuel rich process is ca. 80 mol-% whereas the yield from a conventional fuel lean process is <70 mol-% and typically ca. 60 mol-%) mean that the operation of a fuel rich process could derive significant benefit concerning the environmental impact and waste management. There are two further comments that should be made concerning the operation of the ball milled catalysts under fuel rich conditions. First, although ca. 100 different formulations were tested in obtaining the data presented in this paper it was observed that under all conditions that the only products formed were maleic anhydride, CO, CO2, acetic and acrylic acids. Furan, crotonaldehyde and unsaturated C4 hydrocarbons were not observed even in trace amounts. This is in contrast to the earlier report of Centi et al. [10] who indicated that significant amounts of C4 hydrocarbons were observed as byproducts when using fuel rich conditions. This may be due to the catalysts used in the present study exhibiting a higher average Voxidation state than the catalysts used by Centi et al. [10]. This is related to the observation that water washed precursors prepared using the aqueous HC1 method lead to the formation of final catalysts containing OtligOPO 4 in addition to (VO)2PzO 7 [8] and this component remains present in the catalyst even after many hundreds of hours of testing under fuel lean or fuel rich conditions. It is not considered that the apparent difference is solely related to the use of the high area catalysts derived from the ball milling technique since the non ball milled catalyst tested under fuel rich conditions (surface area 7 m 2 g - l ) also gave maleic
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anhydride, CO, CO2, acetic and acrylic acids as products. The second observation is that the promotional effect of Ce, Cu and Mo is much lower under fuel rich conditions than the effect observed on activity for the same catalysts under fuel lean conditions. It has been noted previously [8] that for catalysts containing ~IIVOPO4 in addition to (VO)2P207, significant promotional effects on activity can be observed and this effect is related to the ~IIVOPO4 content. The decreased promotional effect could therefore be, in part, due to the decreased o,IIVOPO4 content observed for catalysts tested under fuel rich conditions. However, the effect is more pronounced than that could be expected from the small decrease in ~nVOPO4 content observed. It is therefore considered that under the more highly reducing atmosphere of fuel rich conditions when compared to fuel lean conditions, the catalyst promoter becomes less efficient. This indicates that a main role of such promoters is the maintenance of the oxidation state of the active catalyst surface, possibly aiding the retention of the vanadium atoms at the required oxidation level. Under fuel rich conditions the maintenance of the surface oxidation state may be more facile than when the more oxidising fuel lean conditions thereby negating part of the role played by such promoters. However, it must be noted that La promoted catalysts are capable of very high yields under fuel rich conditions and although the overall maleic anhydride yield is a little lower than the non promoted formulation (71.5 mol-%) the activity (1.85 mol maleic anhydride 1-I catalyst h -1) and the exit maleic anhydride concentration (2 mol-%) are much higher under comparable conditions (360°C, GHSV =2000 h -~) and therefore the promotional effect cannot solely be associated with the retention of the surface oxidation state.
Acknowledgements We thank ICI PLC for permission to publish this work.
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