- Email: [email protected]
Alumina Catalyst under Propane Dehydrogenation Conditions
Studies in Surface Science and Catalysis, Vol. 139 J.J. Spivey, G.W. Roberts and B.H. Davis (Editors) 9 2001 Elsevier Science B.V. All rights reserved.
Processes Occurring during Deactivation/Regeneration of a Vanadia/Alumina Catalyst under Propane Dehydrogenation Conditions S David Jackson a'b, David Lennon a, Geoffrey Webb a, and Janice Willis b. a. Department of Chemistry, The University, Glasgow G12 8QQ, Scotland. b. Synetix, R&T Group, PO Box 1, Billingham, Cleveland TS23 1LB, U.K. e-mail: sdj @chem.gla.ac.uk The dehydrogenation of propane over a 12.3 % w/w V2Os/alumina catalyst was studied. Two modes of deactivation were identified, one inter-cycle and one intracycle. Similarly two processes were identified that can cause deactivation, support sintering and carbon laydown. Intra-cycle deactivation is caused by carbon deposition, while the inter-cycle deactivation has a more complex derivation. Under a reaction cycle the y-alumina support changed phase from y to a mixture of 8 and ~, and hence lost surface area. The cause of this change was postulated to be the vanadia catalysing the alumina phase change.
1. INTRODUCTION The catalytic dehydrogenation of lower alkanes was first developed more than fifty years ago using chromia/alumina systems [ 1]. Although there has been development of new processes [2 - 6], the catalyst technology has tended to remain with either modified chromia/alumina or modified platinum/alumina catalysts. Therefore it seemed appropriate to re-examine the possibility of using oxide systems other than chromia to effect the alkane to alkene transition. Supported vanadium pentoxide has been extensively studied for the oxidative dehydrogenation of propane to propene [7 - 10] but rarely for the direct dehydrogenation reaction [6].
2. EXPERIMENTAL The catalyst used throughout this study was 12.3% w/w V2OJalumina. The catalyst was prepared by a dipping and firing technique. Ammonium metavanadate
271
272 was dissolved in demineralised water and the alumina extrudates (S.A. 184 m2g1) placed in the solution for 0.5 h. The extrudates were subsequently drained and fired at 823 K for 3 h. This procedure was repeated twice more. Two reactor systems were used in this study. Pulsed reaction studies were performed in a dynamic mode using a pulse-flow microreactor system with on-line GC to examine the reaction under the first few seconds on-line. Continuous flow reaction studies were performed in a 0.101 MPa, continuous flow microreactor with the gas stream exit the reactor being sampled by on-line GC. Using either system the catalysts (typically 0.3 - 0.5 g) could be reduced in situ in flowing 5% H2/N2 by heating to 873 K and holding at this temperature for 0.25 h. In the pulse mode the reaction gases were admitted by injecting pulses of known size (typically 0.18 cm 3 0.101 MPa) into the helium carrier-gas stream and hence to the catalyst. After passage through the catalyst bed the total contents of the pulse were analysed by GC. Carbon deposition was measured in the pulse mode by difference for each pulse and by combustion of the carbon by dioxygen pulses. In the continuous mode, after reduction had ceased the flow was switched to propane and the first analysis taken alter 3 min, subsequent analyses were taken every 9 min. Catalyst regeneration was performed by flushing the reactor with N2 at temperature before switching the flow to 5 % O2/N2. The CO2 produced was continuously analysed giving a measure of carbon deposition. In both reactor configurations the catalyst could be cycled through this process of reduction/reaction/regeneration. We have defined a process occurring while the catalyst is under propane as being "reaction mode", a process that occurs during a reduction/reaction/regeneration phase as "intra-cycle", and a process that reveals a difference between two cycles as "intercycle".
3. RESULTS The surface area of the catalyst before use was determined to be 163 m2g-1 by dinitrogen adsorption. After use the surface area was re-measured and a value of 70 m2g-1 was obtained. The catalyst, before and after use, was also examined by X-ray diffraction. Before use y-alumina and vanadium pentoxide were detected, however after use the. major alumina phases detected were ct and 8, vanadium pentoxide was also observed The degree of vanadia reduction was determined by measuring the extent of reoxidation by pulsing dioxygen over a reduced catalyst. Therefore immediately after reduction at 873 K, a catalyst was subjected to aliquots of dioxygen while the catalyst bed was still at 873 K. Dioxygen was adsorbed (1.526x102~ oxygen atoms per 0.3 g catalyst) and a ratio of O(ads):V2 was calculated at 1.1:1.
273
Figure 1. Effect of Operating Cycles on Catalyst Yield. 30 Cycle 1 1 1
25
Cycle 2
-o 20 (1) T-
Cycle 3
15
Cycle 4 X
10 I
0
10
I
I
I
20 30 40 Time on stream (min)
I
50
60
Yield (propene formed/propane fed) versus time on stream plots are shown in Figure 1 for a continuous flow system at 873 K and indicate that, alter the first cycle, the catalyst is in a pseudo steady-state, i.e. after the first cycle the activity/selectivity are reproducible with time-on-stream. Table 1 Initial activity and selectivity for propane dehydrogenation, a Temp Pulse % Product Distribution b (K) Number CH4 C2H4 C2H6 C3H6 1 0.7 0 0 14.4 773 2 0.8 0 0 13.9 3 0 0 0 14.1 4 0.7 0 0 14.3 5 0 0 0 14.2 873
C3H8 73.3 70.9 70.4 70.2 69.0
% Carbon dep. 11.6 14.3 15.5 14.8 16.8
1 5.6 1.6 1.2 11.1 6.6 73.8 2 5.2 1.9 1.4 15.3 8.6 67.7 3 5.3 1.9 1.5 16.2 8.7 66.3 4 5.0 1.9 1.6 18.3 9.8 63.3 5 4.4 1.9 1.6 17.4 9.0 65.7 a) Conditions: 6000 GHSV, 1 atm. b) Product distribution and carbon deposition calculated on a carbon basis.
274 The initial activity/selectivity of the catalyst was investigated at 773 K and 873 K by pulsing aliquots of propane over a freshly reduced catalyst at each temperature. The pulses are equivalent to one second of continuous flow at the relevant space velocity. Hence the results shown in Table 1 represent the activity/selectivity of the catalyst in its first five seconds of life. Interestingly the amount of propene produced does not increase significantly on increasing the temperature from 773 K to 873 K, yet the conversion increases from 30% to 92%. Much of the increase in conversion is taken up by the catalyst in the form of carbon deposition. Fresh catalysts were tested at both 773 K and 873 K in a continuous flow mode. The results are shown in Table 2. Clearly the yield and the extent of carbon laydown have changed dramatically by the time of the first analysis (3 min., 180 s) compared with that measured under pulse conditions (Figure 2).
Table 2. Continuous flow, initial activity/selectivitya. % Product distribution c Temp Sample (K) Number b CH4 C2H4 C2H6 C3H6 1 0.04 0.01 0 2.76 773 2 0.03 0.02 0.01 2.54 3 0.03 0.02 0.01 2.71 4 0.03 0.02 0.01 2.67 5 0.03 0.02 0.01 2.76 6 0.03 0.02 0.01 2.72
C3H8 96.45 97.32 97.17 97.23 97.16 97.20
% Carbon dep. 0.74 0.08 0.06 0.04 0.02 0.02
1 0.74 0.36 0.46 26.58 71.10 0.76 2 0.59 0.31 0.32 22.98 75.35 0.50 3 0.50 0.30 0.26 19.48 79.07 0.39 4 0.45 0.30 0.23 16.41 82.30 0.31 5 0.41 0.31 0.20 13.76 85.06 0.26 6 0.38 0.31 0.18 11.47 87.44 0.22 a) 3000 GHSV b) Time of first analysis, 3 min on stream; time between analysis 9 min. c) Product distribution and carbon deposition calculated on a carbon basis. 873
275
Figure 2. Variation of yield and carbon laydown with time. 30
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
,80
x~ 25
60
20
40
15
20~
rn
o
' 10
1
10
3
4.
100
'0 10,000
1,000
30 300 Log (Time on stream, s)
3,000
DISCUSSION
The results show that there are two modes of deactivation occurring within the process, one inter-cycle and one intra-cycle. Similarly two processes can be identified that are likely to cause deactivation, support sintering and carbon laydown. In the following discussion we will show that the intra-cycle deactivation is caused by carbon deposition, while the inter-cycle deactivation has a more complex derivation. The XRD results show that, as prepared, the catalyst has vanadium pentoxide crystallites on the surface. This is in agreement with the literature. In a study involving TPR, XRD, and 5Iv NMR, Koranne et al. [ 11 ] demonstrated that at vanadium loadings above about 10%, a bulk vanadium oxide phase may be detected. Note that this does not imply that all the vanadium is X-ray visible, a significant proportion will still be X-ray amorphous. At lower weight loadings the dispersion of the vanadium species was such that no XRD pattern was observed. Similar results were reported by Lindblad et al. [ 12], although they detected crystalline V205 with vanadium loadings as low as 6%. The results from the reoxidation indicated that treating the catalyst with dihydrogen at 873 K reduced the V205 to V204. This level of reduction is in keeping with the literature [ 11,12 - 14] where the extent of reduction has been shown to be dependent upon vanadia loading and dihydrogen pressure. One study [ 11 ] revealed
276 that an 8.9% V2Os/alumina catalyst reduced from V205 to V6013 at 758 K in 5 % dihydrogen in dinitrogen, and from V6013 to V204 at 913 K. Given that we were using pure dihydrogen, a reduction in the temperature at which the second conversion takes place may be expected. Indeed a gravimetric study [ 15] using pure dihydrogen reported quantitative reduction of V +5 to V +4 at 773 K on an 11.9% V2Os/alumina catalyst. However it should be stressed that these figures are only atomic balances and do necessarily represent the formation of the bulk compound. A~er use, XRD analysis of the catalyst showed that the alumina had been transformed from gamma to a mixture of delta and alpha. This agrees with the dramatic loss in BET surface area. Such a transformation indicates that the alumina support has been subject to temperatures vastly in excess (> 300 K) of the reaction temperature, or that under some aspect of the reaction sequence the vanadia acts as a catalyst for the phase change of alumina. Either or both of these hypotheses may be applicable. This view is reinforced by the effect shown in Figure 1, where the catalyst was put through a series of cycles. Clearly atier the first cycle the catalyst is stable but with much lower yields, as may be expected with a loss in surface area. Therefore the structural instability is likely to be linked to the regeneration process. Under the oxidation conditions for carbon bum-off, if localised hot-spots are generated then these may initiate the phase change, although in other systems we have studied for this reaction on the same support there has been no evidence of an alumina phase change [ 16,17]. Whereas in a study of vanadia supported on A1PO4 [ 18], the vanadia was observed to catalyse the crystallisation of the A1PO4 at temperatures as low as 773 K in air, even though the amorphous form is stable to 1073 K. This suggests that vanadia may indeed be able to catalyse the alumina phase change. As stated above this change in support structure may be the cause of the loss in activity between cycle 1 and cycle 2 (29% drop in activity). The loss in surface area associated with the phase change will alter the pore structure and hence may cause encapsulation of the vanadia or sintering. Either of these would result in a loss in activity and, due to changes in particle topography an altered selectivity may also be observed. However the changes may also be related to carbonaceous residues that are not removed by the oxidation treatment [ 14, 19]. In related systems, where the support has been common but there has been a change in active phase (Pt or chromia), there has been no phase change in the support, but similar behaviour patterns have been observed due to retained carbonaceous residues [ 19]. However we must also consider the effect of the reduction/re-oxidation cycle on the particle size and topography of the active "V204". It is likely that the first reduction/oxidation cycle will have a more significant effect on size and topography, through a relaxation process, than subsequent cycles. An effect such as this has been evidenced by a growth in particle size between first and second cycles when a Pt catalyst has been tested [20].
277 The pulse data at 773 K shows that the catalyst is highly selective to propene. At 773 K and 1 atm. pressure the equilibrium conversion of propane to propene is 17%, from the data in Table 1 it can be seen that the yield of propene is close to equilibrium (14%). The other major product is carbon. This comes as no great surprise, if we determine the equilibrium constant (Kp) for the reaction, C3H8 4H2 + 3C, at 773 K and 1 atm. a value of 1.04xl 08 is obtained. Similarly the equilibrium constant for C3H8 ~ 2CH4 + C, is 4.67xl 08. Hence thermodynamically the production of carbon is highly favoured. As the temperature is increased from 773 K to 873 K we find that the propene yield has moved sharply away from the equilibrium value, while the yield of all the byproducts has increased (Table 1). As the amount produced has decreased on increasing temperature and given that the activation energy for carbon deposition is less than that of the principal gas phase products, then we can postulate that there is a secondary reaction of the propene into the carbonaceous deposit. In the continuous flow tests at 773 K the conversion of the propane was found to be 3% after three minutes on line. If we compare this data with that from the pulse data, which gives information on the first few seconds on line, we find that the conversion has decreased from 30% to 3% during the first three minutes (Table 1 and 2). However after this initial deactivation the catalyst stabilises, and a constant activity and selectivity profile is obtained. This behaviour is mirrored by that for the carbon deposition where the percentage drops from 17% to 1% during the first three minutes on line. The yield of propene however, although decreasing, does not reduce to the same extent. These results can be interpreted by a simple geometric argument. If carbon laydown requires a large ensemble whereas dehydrogenation can be performed on a single site then as the carbon covers the surface the conversion decreases but the dehydrogenation is the least effected because of its single site requirement. However this simplistic argument is not sufficient to explain the results at 873 K (Table 1 and 2). At this temperature there is again a dramatic loss in conversion (93% to 29%) and extent of carbon deposition (74% to 0.76%). However the yield ofpropene increases. For this to occur it is necessary that the number of sites available for the dehydrogenation reaction increase as the catalyst is deactivating or that the activity of the dehydrogenation sites has increased. An increase in the number of sites is not impossible as at the higher temperature and in the presence of a reacting gas, the surface may reconstruct to produce more sites of the appropriate geometry and energetics. The possibility that the sites have increased in activity would require that the rate-determining step was enhanced. Again this could be related to surface reconstruction modifying geometry or energetics. Equally the carbonaceous deposit could, in the early stages of reaction, play a role similar to that found in hydrogenation catalysis [21] and act as a more effective hydrogen-transfer agent than the oxide surface. This ability however would be lost as the deposit ages, and subsequently the residue would act as a poison causing a reduction in yield.
278 5.
ACKNOWLEDGEMENTS.
The authors would like to thank Lynn Duffy, Nikki Gent for their help with the experiments.
6. .
2. 171. 3. 4. 5.
10. 11. 369. 12 13. 14. 15. 16. 17. 18. 19.
20. 21.
REFERENCES.
F. E. Frey and W. F. Huppke, Ind. and Eng. Chemistry, 25 (1933) 54. B. V. Vora, P. R. Pujado, and R. F. Anderson, Energy Progress, 6 (1986) U.S. Patent No. 4,677,237 (1987) assigned to UOP. U. S. Patent No. 4,940,826 (1990) assigned to B.P. plc. European Patent application No. 91106432.7, (1991) assigned to Phillips Petroleum Co; U. S. Patent No. 5,220,092 (1993) assigned to Shell Oil Co. M. A. Chaar, D. Patel, and H. H. Kung, J. Catal., 109 (1988) 463. D. Siew Hew Sam, V. Soenen, and J. C. Volta, J. Catal., 123 (1990) 417. S. L. T. Andersson, Appl. Catal., 112 (1994) 209. T. Blasco, J. M. Lopez Nieto, A. Dejoz, and M. I. Vazquez, J. Catal., 157 (1995)271. G. Martra, F. Arena, S. Coluccia, F. Frusteri, and A. Parmaliana, Catalysis Today, 63 (2000) 197. M. M. Koranne, J. G. Goodwin Jr., and G. Marcelin, J. Catal., 148 (1994) F. Roozeboom, M. C. Mittelmeijer-Hazeleger, J. A. Moulijn, J. Medema, V. H. J. de Beer, and P. J. Gellings, J. Phys. Chem., 84 (1980) 2783. A. Haber, A. Kozlowska, and R. Kozlowski, J. Catal., 102 (1986) 52. H. Bosch, B. J. Kip, J. G. van Ommen, and P. J. Gellings, J. Chem. Sot., Faraday Trans.1, 80 (1984) 2479. N. K. Nag and F. E. Massoth, J. Catal., 124 (1990) 127. S. D. Jackson, P.Leeming and J.Grenfell, J.Catal.~ 150 (1994) 170. S. D. Jackson, G.Webb, I.M.Matheson, and J.Grenfell, Am. Chem. Soc., Preprints, Div. of Petroleum Chem., 40(4) (1995) 628. T. Lindblad, B. Rebenstorf, Z.-G. Yan, S. L. T. Andersson, Appl. Catal., 112 (1994) 187. S. D. Jackson, G. Webb, I. M. Matheson, and J. Grenfell, Proceedings of the Vlth International Symposium on Catalyst Deactivation, "Studies in Surface Science and Catalysis", Eds. B.Delmon and G.F.Froment, Elsevier, Amsterdam, Vol. 88, (1994)p297. S. D. Jackson and J. Grenfell, unpublished results. G. Webb, Catalysis Today, 7 (1990) 139.
Processes Occurring during Deactivation/Regeneration of a Vanadia/Alumina Catalyst under Propane Dehydrogenation Conditions S David Jackson a'b, David Lennon a, Geoffrey Webb a, and Janice Willis b. a. Department of Chemistry, The University, Glasgow G12 8QQ, Scotland. b. Synetix, R&T Group, PO Box 1, Billingham, Cleveland TS23 1LB, U.K. e-mail: sdj @chem.gla.ac.uk The dehydrogenation of propane over a 12.3 % w/w V2Os/alumina catalyst was studied. Two modes of deactivation were identified, one inter-cycle and one intracycle. Similarly two processes were identified that can cause deactivation, support sintering and carbon laydown. Intra-cycle deactivation is caused by carbon deposition, while the inter-cycle deactivation has a more complex derivation. Under a reaction cycle the y-alumina support changed phase from y to a mixture of 8 and ~, and hence lost surface area. The cause of this change was postulated to be the vanadia catalysing the alumina phase change.
1. INTRODUCTION The catalytic dehydrogenation of lower alkanes was first developed more than fifty years ago using chromia/alumina systems [ 1]. Although there has been development of new processes [2 - 6], the catalyst technology has tended to remain with either modified chromia/alumina or modified platinum/alumina catalysts. Therefore it seemed appropriate to re-examine the possibility of using oxide systems other than chromia to effect the alkane to alkene transition. Supported vanadium pentoxide has been extensively studied for the oxidative dehydrogenation of propane to propene [7 - 10] but rarely for the direct dehydrogenation reaction [6].
2. EXPERIMENTAL The catalyst used throughout this study was 12.3% w/w V2OJalumina. The catalyst was prepared by a dipping and firing technique. Ammonium metavanadate
271
272 was dissolved in demineralised water and the alumina extrudates (S.A. 184 m2g1) placed in the solution for 0.5 h. The extrudates were subsequently drained and fired at 823 K for 3 h. This procedure was repeated twice more. Two reactor systems were used in this study. Pulsed reaction studies were performed in a dynamic mode using a pulse-flow microreactor system with on-line GC to examine the reaction under the first few seconds on-line. Continuous flow reaction studies were performed in a 0.101 MPa, continuous flow microreactor with the gas stream exit the reactor being sampled by on-line GC. Using either system the catalysts (typically 0.3 - 0.5 g) could be reduced in situ in flowing 5% H2/N2 by heating to 873 K and holding at this temperature for 0.25 h. In the pulse mode the reaction gases were admitted by injecting pulses of known size (typically 0.18 cm 3 0.101 MPa) into the helium carrier-gas stream and hence to the catalyst. After passage through the catalyst bed the total contents of the pulse were analysed by GC. Carbon deposition was measured in the pulse mode by difference for each pulse and by combustion of the carbon by dioxygen pulses. In the continuous mode, after reduction had ceased the flow was switched to propane and the first analysis taken alter 3 min, subsequent analyses were taken every 9 min. Catalyst regeneration was performed by flushing the reactor with N2 at temperature before switching the flow to 5 % O2/N2. The CO2 produced was continuously analysed giving a measure of carbon deposition. In both reactor configurations the catalyst could be cycled through this process of reduction/reaction/regeneration. We have defined a process occurring while the catalyst is under propane as being "reaction mode", a process that occurs during a reduction/reaction/regeneration phase as "intra-cycle", and a process that reveals a difference between two cycles as "intercycle".
3. RESULTS The surface area of the catalyst before use was determined to be 163 m2g-1 by dinitrogen adsorption. After use the surface area was re-measured and a value of 70 m2g-1 was obtained. The catalyst, before and after use, was also examined by X-ray diffraction. Before use y-alumina and vanadium pentoxide were detected, however after use the. major alumina phases detected were ct and 8, vanadium pentoxide was also observed The degree of vanadia reduction was determined by measuring the extent of reoxidation by pulsing dioxygen over a reduced catalyst. Therefore immediately after reduction at 873 K, a catalyst was subjected to aliquots of dioxygen while the catalyst bed was still at 873 K. Dioxygen was adsorbed (1.526x102~ oxygen atoms per 0.3 g catalyst) and a ratio of O(ads):V2 was calculated at 1.1:1.
273
Figure 1. Effect of Operating Cycles on Catalyst Yield. 30 Cycle 1 1 1
25
Cycle 2
-o 20 (1) T-
Cycle 3
15
Cycle 4 X
10 I
0
10
I
I
I
20 30 40 Time on stream (min)
I
50
60
Yield (propene formed/propane fed) versus time on stream plots are shown in Figure 1 for a continuous flow system at 873 K and indicate that, alter the first cycle, the catalyst is in a pseudo steady-state, i.e. after the first cycle the activity/selectivity are reproducible with time-on-stream. Table 1 Initial activity and selectivity for propane dehydrogenation, a Temp Pulse % Product Distribution b (K) Number CH4 C2H4 C2H6 C3H6 1 0.7 0 0 14.4 773 2 0.8 0 0 13.9 3 0 0 0 14.1 4 0.7 0 0 14.3 5 0 0 0 14.2 873
C3H8 73.3 70.9 70.4 70.2 69.0
% Carbon dep. 11.6 14.3 15.5 14.8 16.8
1 5.6 1.6 1.2 11.1 6.6 73.8 2 5.2 1.9 1.4 15.3 8.6 67.7 3 5.3 1.9 1.5 16.2 8.7 66.3 4 5.0 1.9 1.6 18.3 9.8 63.3 5 4.4 1.9 1.6 17.4 9.0 65.7 a) Conditions: 6000 GHSV, 1 atm. b) Product distribution and carbon deposition calculated on a carbon basis.
274 The initial activity/selectivity of the catalyst was investigated at 773 K and 873 K by pulsing aliquots of propane over a freshly reduced catalyst at each temperature. The pulses are equivalent to one second of continuous flow at the relevant space velocity. Hence the results shown in Table 1 represent the activity/selectivity of the catalyst in its first five seconds of life. Interestingly the amount of propene produced does not increase significantly on increasing the temperature from 773 K to 873 K, yet the conversion increases from 30% to 92%. Much of the increase in conversion is taken up by the catalyst in the form of carbon deposition. Fresh catalysts were tested at both 773 K and 873 K in a continuous flow mode. The results are shown in Table 2. Clearly the yield and the extent of carbon laydown have changed dramatically by the time of the first analysis (3 min., 180 s) compared with that measured under pulse conditions (Figure 2).
Table 2. Continuous flow, initial activity/selectivitya. % Product distribution c Temp Sample (K) Number b CH4 C2H4 C2H6 C3H6 1 0.04 0.01 0 2.76 773 2 0.03 0.02 0.01 2.54 3 0.03 0.02 0.01 2.71 4 0.03 0.02 0.01 2.67 5 0.03 0.02 0.01 2.76 6 0.03 0.02 0.01 2.72
C3H8 96.45 97.32 97.17 97.23 97.16 97.20
% Carbon dep. 0.74 0.08 0.06 0.04 0.02 0.02
1 0.74 0.36 0.46 26.58 71.10 0.76 2 0.59 0.31 0.32 22.98 75.35 0.50 3 0.50 0.30 0.26 19.48 79.07 0.39 4 0.45 0.30 0.23 16.41 82.30 0.31 5 0.41 0.31 0.20 13.76 85.06 0.26 6 0.38 0.31 0.18 11.47 87.44 0.22 a) 3000 GHSV b) Time of first analysis, 3 min on stream; time between analysis 9 min. c) Product distribution and carbon deposition calculated on a carbon basis. 873
275
Figure 2. Variation of yield and carbon laydown with time. 30
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
,80
x~ 25
60
20
40
15
20~
rn
o
' 10
1
10
3
4.
100
'0 10,000
1,000
30 300 Log (Time on stream, s)
3,000
DISCUSSION
The results show that there are two modes of deactivation occurring within the process, one inter-cycle and one intra-cycle. Similarly two processes can be identified that are likely to cause deactivation, support sintering and carbon laydown. In the following discussion we will show that the intra-cycle deactivation is caused by carbon deposition, while the inter-cycle deactivation has a more complex derivation. The XRD results show that, as prepared, the catalyst has vanadium pentoxide crystallites on the surface. This is in agreement with the literature. In a study involving TPR, XRD, and 5Iv NMR, Koranne et al. [ 11 ] demonstrated that at vanadium loadings above about 10%, a bulk vanadium oxide phase may be detected. Note that this does not imply that all the vanadium is X-ray visible, a significant proportion will still be X-ray amorphous. At lower weight loadings the dispersion of the vanadium species was such that no XRD pattern was observed. Similar results were reported by Lindblad et al. [ 12], although they detected crystalline V205 with vanadium loadings as low as 6%. The results from the reoxidation indicated that treating the catalyst with dihydrogen at 873 K reduced the V205 to V204. This level of reduction is in keeping with the literature [ 11,12 - 14] where the extent of reduction has been shown to be dependent upon vanadia loading and dihydrogen pressure. One study [ 11 ] revealed
276 that an 8.9% V2Os/alumina catalyst reduced from V205 to V6013 at 758 K in 5 % dihydrogen in dinitrogen, and from V6013 to V204 at 913 K. Given that we were using pure dihydrogen, a reduction in the temperature at which the second conversion takes place may be expected. Indeed a gravimetric study [ 15] using pure dihydrogen reported quantitative reduction of V +5 to V +4 at 773 K on an 11.9% V2Os/alumina catalyst. However it should be stressed that these figures are only atomic balances and do necessarily represent the formation of the bulk compound. A~er use, XRD analysis of the catalyst showed that the alumina had been transformed from gamma to a mixture of delta and alpha. This agrees with the dramatic loss in BET surface area. Such a transformation indicates that the alumina support has been subject to temperatures vastly in excess (> 300 K) of the reaction temperature, or that under some aspect of the reaction sequence the vanadia acts as a catalyst for the phase change of alumina. Either or both of these hypotheses may be applicable. This view is reinforced by the effect shown in Figure 1, where the catalyst was put through a series of cycles. Clearly atier the first cycle the catalyst is stable but with much lower yields, as may be expected with a loss in surface area. Therefore the structural instability is likely to be linked to the regeneration process. Under the oxidation conditions for carbon bum-off, if localised hot-spots are generated then these may initiate the phase change, although in other systems we have studied for this reaction on the same support there has been no evidence of an alumina phase change [ 16,17]. Whereas in a study of vanadia supported on A1PO4 [ 18], the vanadia was observed to catalyse the crystallisation of the A1PO4 at temperatures as low as 773 K in air, even though the amorphous form is stable to 1073 K. This suggests that vanadia may indeed be able to catalyse the alumina phase change. As stated above this change in support structure may be the cause of the loss in activity between cycle 1 and cycle 2 (29% drop in activity). The loss in surface area associated with the phase change will alter the pore structure and hence may cause encapsulation of the vanadia or sintering. Either of these would result in a loss in activity and, due to changes in particle topography an altered selectivity may also be observed. However the changes may also be related to carbonaceous residues that are not removed by the oxidation treatment [ 14, 19]. In related systems, where the support has been common but there has been a change in active phase (Pt or chromia), there has been no phase change in the support, but similar behaviour patterns have been observed due to retained carbonaceous residues [ 19]. However we must also consider the effect of the reduction/re-oxidation cycle on the particle size and topography of the active "V204". It is likely that the first reduction/oxidation cycle will have a more significant effect on size and topography, through a relaxation process, than subsequent cycles. An effect such as this has been evidenced by a growth in particle size between first and second cycles when a Pt catalyst has been tested [20].
277 The pulse data at 773 K shows that the catalyst is highly selective to propene. At 773 K and 1 atm. pressure the equilibrium conversion of propane to propene is 17%, from the data in Table 1 it can be seen that the yield of propene is close to equilibrium (14%). The other major product is carbon. This comes as no great surprise, if we determine the equilibrium constant (Kp) for the reaction, C3H8 4H2 + 3C, at 773 K and 1 atm. a value of 1.04xl 08 is obtained. Similarly the equilibrium constant for C3H8 ~ 2CH4 + C, is 4.67xl 08. Hence thermodynamically the production of carbon is highly favoured. As the temperature is increased from 773 K to 873 K we find that the propene yield has moved sharply away from the equilibrium value, while the yield of all the byproducts has increased (Table 1). As the amount produced has decreased on increasing temperature and given that the activation energy for carbon deposition is less than that of the principal gas phase products, then we can postulate that there is a secondary reaction of the propene into the carbonaceous deposit. In the continuous flow tests at 773 K the conversion of the propane was found to be 3% after three minutes on line. If we compare this data with that from the pulse data, which gives information on the first few seconds on line, we find that the conversion has decreased from 30% to 3% during the first three minutes (Table 1 and 2). However after this initial deactivation the catalyst stabilises, and a constant activity and selectivity profile is obtained. This behaviour is mirrored by that for the carbon deposition where the percentage drops from 17% to 1% during the first three minutes on line. The yield of propene however, although decreasing, does not reduce to the same extent. These results can be interpreted by a simple geometric argument. If carbon laydown requires a large ensemble whereas dehydrogenation can be performed on a single site then as the carbon covers the surface the conversion decreases but the dehydrogenation is the least effected because of its single site requirement. However this simplistic argument is not sufficient to explain the results at 873 K (Table 1 and 2). At this temperature there is again a dramatic loss in conversion (93% to 29%) and extent of carbon deposition (74% to 0.76%). However the yield ofpropene increases. For this to occur it is necessary that the number of sites available for the dehydrogenation reaction increase as the catalyst is deactivating or that the activity of the dehydrogenation sites has increased. An increase in the number of sites is not impossible as at the higher temperature and in the presence of a reacting gas, the surface may reconstruct to produce more sites of the appropriate geometry and energetics. The possibility that the sites have increased in activity would require that the rate-determining step was enhanced. Again this could be related to surface reconstruction modifying geometry or energetics. Equally the carbonaceous deposit could, in the early stages of reaction, play a role similar to that found in hydrogenation catalysis [21] and act as a more effective hydrogen-transfer agent than the oxide surface. This ability however would be lost as the deposit ages, and subsequently the residue would act as a poison causing a reduction in yield.
278 5.
ACKNOWLEDGEMENTS.
The authors would like to thank Lynn Duffy, Nikki Gent for their help with the experiments.
6. .
2. 171. 3. 4. 5.
10. 11. 369. 12 13. 14. 15. 16. 17. 18. 19.
20. 21.
REFERENCES.
F. E. Frey and W. F. Huppke, Ind. and Eng. Chemistry, 25 (1933) 54. B. V. Vora, P. R. Pujado, and R. F. Anderson, Energy Progress, 6 (1986) U.S. Patent No. 4,677,237 (1987) assigned to UOP. U. S. Patent No. 4,940,826 (1990) assigned to B.P. plc. European Patent application No. 91106432.7, (1991) assigned to Phillips Petroleum Co; U. S. Patent No. 5,220,092 (1993) assigned to Shell Oil Co. M. A. Chaar, D. Patel, and H. H. Kung, J. Catal., 109 (1988) 463. D. Siew Hew Sam, V. Soenen, and J. C. Volta, J. Catal., 123 (1990) 417. S. L. T. Andersson, Appl. Catal., 112 (1994) 209. T. Blasco, J. M. Lopez Nieto, A. Dejoz, and M. I. Vazquez, J. Catal., 157 (1995)271. G. Martra, F. Arena, S. Coluccia, F. Frusteri, and A. Parmaliana, Catalysis Today, 63 (2000) 197. M. M. Koranne, J. G. Goodwin Jr., and G. Marcelin, J. Catal., 148 (1994) F. Roozeboom, M. C. Mittelmeijer-Hazeleger, J. A. Moulijn, J. Medema, V. H. J. de Beer, and P. J. Gellings, J. Phys. Chem., 84 (1980) 2783. A. Haber, A. Kozlowska, and R. Kozlowski, J. Catal., 102 (1986) 52. H. Bosch, B. J. Kip, J. G. van Ommen, and P. J. Gellings, J. Chem. Sot., Faraday Trans.1, 80 (1984) 2479. N. K. Nag and F. E. Massoth, J. Catal., 124 (1990) 127. S. D. Jackson, P.Leeming and J.Grenfell, J.Catal.~ 150 (1994) 170. S. D. Jackson, G.Webb, I.M.Matheson, and J.Grenfell, Am. Chem. Soc., Preprints, Div. of Petroleum Chem., 40(4) (1995) 628. T. Lindblad, B. Rebenstorf, Z.-G. Yan, S. L. T. Andersson, Appl. Catal., 112 (1994) 187. S. D. Jackson, G. Webb, I. M. Matheson, and J. Grenfell, Proceedings of the Vlth International Symposium on Catalyst Deactivation, "Studies in Surface Science and Catalysis", Eds. B.Delmon and G.F.Froment, Elsevier, Amsterdam, Vol. 88, (1994)p297. S. D. Jackson and J. Grenfell, unpublished results. G. Webb, Catalysis Today, 7 (1990) 139.