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Continuous catalysis on oxides in the presence of spiltover hydrogen
T. Inui et al. (Editors),New Aspects of Spillover Effect in Catalysis 0 1993 Elsevier Science Publishers B.V. All rights reserved.
385
Continuous catalysis on oxldes in the presence of spiltover hydrogen M.S.W.Vong and P.A.Sermon Department of Chemistry, Brunel University, Uxbridge, Mi&.,
UB8 3PH. UK
Abstract Although inactive in ethene hydrogenation in their own right under the conditions used here, a series of oxides when fluidised upon PValumina pellets, such that they saw only gaseous ethene, gaseous hydrogen and spiltover hydrogen, were active on a sustainable basis and over significant periods of time.
1. INTRODUCTION It is often difficult to differentiate the catalytic activity of support sites in a reaction in which the supported metal also contributes major activity and to discern if spillover is significant and by what mechanism it proceeds [l]. Naturally, different oxides will exhibit different potentials for (i) spillover of hydrogen, and (ii) using this to permit catalysis in for example akene hydrogenation. For example, alkene hydrogenation with spilt-over hydrogen on alumina is more likely than with silica [2], although even with SiO, it has recently been reported after activation with Pt and H,(101 Wa, 703K, 18h). after an induction period of 48h. Here we consider the role of the accepting and supporting oxide in defining the contribution of spillover to catalysis on the support on a continuous basis and also the nature of the active sites and the role of carbonaceous deposits on the oxide surfaces in alkene hydrogenation. The reaction chosen is ethene hydrognation. Dual site catalysis of the hydrogenation of ethene could [3] involve two types of site. Certainly, hydrogen spillover is thought to affect Pt catalysis of ethene hydrogenation [4] and amorphous alumina pre-activated by spiltover hydrogen from Walumina (which could then be removed) is active in ethene hydrogenation (H&I-I, = 12:l) at 453K when its activity was measured in a reciculation system [5], even though less than O.lppb of Pt could have been transferred to the amorphous alumina. 2. EXPERIMENTAL
2.1. Materials and characterization
1.7% and OJ9bPValumina pellets (3 mm diameter) were used. Upon these a powdered oxide (i.e. SiO,, Si0,- 13%A&O,, SiOz-25%A1,0,, y-A&O,(a, b,c), Au/SiO,, or Mo,) was fluidised. These oxides were inactive in the catalytic reaction tested here at 473K etc. Samples (0.5g) of the powdered oxides were analysed at selected intervals by temperatureprogrammed oxidation in an 0, stream (8cm3/min)during heating at 5Wmin from 298K to
386
973K. The product CQ was converted to methane and was then analysed by FID gas chromatography. Samples were also analysed by electron spin resonance.
2.2. Apparatus and catalytic method
The catalytic reactor [a] used is shown in Figure 1. he-purified hydrogen and O.l%CJ-I, in N, entered the reactor at separate inlets here labelled 1 and 2. Tabla 1
P o w h fluidircd upon Wdumina pellets Sunplc
Initial ntc of c h n e hydrogenation (moleculu cJ.& x lO'S/g/min) oxide Alone Spillova
With(a)
SiO, SO,- 13%Al@, SiO,--U%Aba yAl203(a) rAU03(b) yAl203(c) 5%Au/Si02@70)
Mo4
0 0 0 0
0 0 0 0
0 55.94 162.33 30.06 159.14
402.82
71.61 90.37
(b)
0 15.03 75.16 108.41 119.35 46.75 26.30
[-I
[27%] [46%]
[-I
[a%] [30%] [a%]
[29%]
* the O v d d d O M ured Uld lcwl h the fluidid bed W C R 88.1kPa &, 0.03Lw cJ.& with a y bairncc to l o l k h and a mtd flow nte of 460cm'lmin. These beds wac fluidid upon 8 Walumina pellets. Activity w u seen (a) when the oxide is activated by 1.79bWalumina pellets and (b)after pellet nmovd. [I denote the pacentages of the initial activity in the presence of Walumina pellets which remain after rcmovd of the pellets.
Pre-reduced alumina-supported Pt pellets (e.g. 0.4g) were placed at 3 and upon these fluidised the bed of oxide powder (e.g. 3g) at 4. Products of the reaction left at 5 and were analysed via a constant volume gas sampling valve on a gas chromatograph. After N, flushing for 30min, €& was introduced at the base of the reactor (88.lkPa and 400cm3/min);this fluidised the powder but left the pellets unchanged in position. The reactor temperature was raised to 473K or 573K, held there for lh, and then cooled in flowing & to the selected reaction temperature (e.g. 473K). At this point the reactant stream (31.2Pa C$€, in N, (1:999) at aoCm3/min) was inaoduced simultaneously to give a &€ concentration I, of 130 ppm. The concentration profile of ethene in the reactor was measured by replacing the temperature probe with a sampling tube of the same diameter. From Figure 1 it is clear that once the flow rate of €& exceeds 150cm3/min a constant reactant concentration (0.4 ppm CJ-I,/C.&) was seen; the pellets contributed 4 . 5 % of the activity observed. 3. RESULTS AND DISCUSSION
Figure 2 shows that the activity Seen for the fluidising beds of the powdered oxides is largely independent of reaction time. We have already seen in Figure 1 that an insignificant part of this activity is attributable to direct hydrogenation of the Walumina pellets alone. Figure 2 and Table 1 show how the activity of the oxide k raised by contact with the Walumina pellets in the presence of hydrogen. These observations could be defined by (i) spiltover hydrogen on the oxide becoming involved in catalysis
(1)
I'
387
@)
..... .. ,: .. I
..:.: . . d
4
0
3
L
-
8
u
t (h)
~W~/*pClleo
Figure 1. (a) Reactor system (where 1 is Hz inlet, 2 is CzH4inlet, 3 is Pt/Alz03 pellets, 4 is the fluidised bed of the powdered oxide, 5 is the product exit and 6 is the temperature sensor. (b) The concentration of C z h in position 3 as a function of H2flow rate.
Figure 2. Rates of hydrogenation of CzH4at 483K over 3 g unused SiO2-A12O3 (25%) (a), SiOz-AIzO3 fluidised upon 8 pellets of 1.7% Pt/Alz03 (b), and after removal of Pt/AI& pellets (c).
(ii) hydrogen spillover on the oxides inducing allcene adsorption and the generation of some sort of activity on these novel carbonaceous residues generated by spillover, or (iii) Pt being transferred from the pellets to the oxide powders. Which then is the dominant phenomenon? With regard to the fmt point, as the temperature of the bed in the reactor in Figure 1 was raised from say 373K to 498K the rate of ethene hydrogenation was found to increase (although thereafter it decreased at higher temperatures). The activation energy derived (18kJ/mol) suggests that diffusion (or spillover) is controlling the overall rates of the observed hydrogenation and this transport might involve hydrogen species from the Pt/alumina to the fluidising oxide particles. However, it was found that increasing the concentration of water vapour in the reactor did not change the overall rate of reaction and hence the role of water vapour is far more complicated than simply facilitating spillover alone. With regard to the second point, it is certainly true that carbonaceous material was built up on the oxides which then exhibited activity in their own right. Figure 3 shows that, as this built up, the activity of one of the oxides did increase. During these catalytic runs there was indeed an accumulation of carbonaceous material and its extent of formation does correlate
r I
A
0
I 10
I 2l
30
c awnd(x 10'' i l ) Figure 3. Relationship to catalytic activity in alkene hydrogenation to the extent of carbonaceous material accumulated on SiOz-Al@3 (25%).
388
with the activity seen in ethene hydrogenation. Interestingly, an esr signal increased in ~ intensity at the s 8 time. Turning now to the third point, the overall rate increased as the number or concentration of the Pt initiating centres was increased and this is not surprising. However, no significant Pt could be detected upon the oxide powder after use in the nactor upon the Walumina pellets (i.e. the Pt levels detected by AA was c 10 ppm) and so (iii) can be discounted as being insignificant. 4. CONCLUSIONS A sustained reaction has occurred on oxides which is supported by spillover. This activity is stable with time and is not attributable to direct hydrogenation of the Walumina pellets alone. Although activity on the fluidising oxides is directly related to the number of Ptinitiating sites, no significant Pt was found on the oxides after fluidisation on the pellets. Spiltover hydrogen on the oxide could on the other hand become involved in catalysis. Hydrogen spillover on the oxides could induce alkene adsorption and the generation of activity on these novel carbonaceous residues generated by spillover. Certainly the oxides darkened in colour during the naction and the extent of accumulation of carbonaceous residues on the oxides was directly related to the activity seen in ethene hydrogenation. It nmains difficult to indicate which of these then is the dominant phenomenon in a catalytic context and how such phenomena could be used in a full catalytic process. Further work is in hand using a range of different alkenes. It is however interesting that Pt accelerates spillover-enhanced hydrogenation of coke on alumina [7]. It is also interesting that SiO, does not have the activity as stcn by others [2], but since Au/SiO, does, the surface of the Au or the Au-silica interface is the active site. 5. REFERENCES
1 E. Baumgarten and E. Denecke J. Catal. 95 (1985) 296; E. Baumgarten. C. LentesWagner and R. Wagner J. Catal. 117 (1989) 533; ibid 126 (1990) 314; M.S. Spencer, R. Burch and S.E. Golunski J. Catal. 126 (1990) 31 1 2 J.H. Sinfelt and P.J. Luchesi, J. Amer. Chem. Soc. 85 0963) 3365; D. Bianchi, M. Lacroix, G. Pajonk and S.J. Teichner 3. Catal. 59 0979) 467; M. Lacroix Docteur D’Etat ES Sciences Thesis L’Universite Claude Bernard De Lyon (Oct.1980); R.J. Wiley, S.J. Teichner and G.M. Pajonk J. Molec. Catal. 77 (1992) 201 3 W.M.H. Sachtler and L.J. Bostelaar p.207 Stud. Sur. Sci.Catal. 17 (1983) G.M.Pajonk, S.J.Teichner and G.E.Germain (eds). 4 H. Saltzburg and M.E. Mullins p.295 Stud. Sur. Sci. Catal. 17 (1983) G.M .Pajonk, S.J. Teichner and G.E. Germain (eds.) 5 D. M a t , G.M. Pajonk and S.J. Teichner p.215 Stud. Sur. Sci. Catal. 17 (1983) G.M. Pajonk, S.J. Teichner and G.E. G e d (eds.); M. Lacroix, G.M. Pajonk and S.J. Teichner, Proc. 7th Int. Cong. Catal. p.279 (1981); Bull. Soc. Chim. France 87, 94, 101, 258, 265,(1981). 6 MAW. Lau and P.A. Sermon J. Chem. Soc. Chem Commun. 891, (1978) 7 J.M. Parera, E.M. Traffano, J.C. Muss0 and C.L. Pieck p.101 Stud. Sur.Sci. Catal. 17 (1983) G.M. Pajonk, S.J. Teichner and G.E. Gemain (4s.)
385
Continuous catalysis on oxldes in the presence of spiltover hydrogen M.S.W.Vong and P.A.Sermon Department of Chemistry, Brunel University, Uxbridge, Mi&.,
UB8 3PH. UK
Abstract Although inactive in ethene hydrogenation in their own right under the conditions used here, a series of oxides when fluidised upon PValumina pellets, such that they saw only gaseous ethene, gaseous hydrogen and spiltover hydrogen, were active on a sustainable basis and over significant periods of time.
1. INTRODUCTION It is often difficult to differentiate the catalytic activity of support sites in a reaction in which the supported metal also contributes major activity and to discern if spillover is significant and by what mechanism it proceeds [l]. Naturally, different oxides will exhibit different potentials for (i) spillover of hydrogen, and (ii) using this to permit catalysis in for example akene hydrogenation. For example, alkene hydrogenation with spilt-over hydrogen on alumina is more likely than with silica [2], although even with SiO, it has recently been reported after activation with Pt and H,(101 Wa, 703K, 18h). after an induction period of 48h. Here we consider the role of the accepting and supporting oxide in defining the contribution of spillover to catalysis on the support on a continuous basis and also the nature of the active sites and the role of carbonaceous deposits on the oxide surfaces in alkene hydrogenation. The reaction chosen is ethene hydrognation. Dual site catalysis of the hydrogenation of ethene could [3] involve two types of site. Certainly, hydrogen spillover is thought to affect Pt catalysis of ethene hydrogenation [4] and amorphous alumina pre-activated by spiltover hydrogen from Walumina (which could then be removed) is active in ethene hydrogenation (H&I-I, = 12:l) at 453K when its activity was measured in a reciculation system [5], even though less than O.lppb of Pt could have been transferred to the amorphous alumina. 2. EXPERIMENTAL
2.1. Materials and characterization
1.7% and OJ9bPValumina pellets (3 mm diameter) were used. Upon these a powdered oxide (i.e. SiO,, Si0,- 13%A&O,, SiOz-25%A1,0,, y-A&O,(a, b,c), Au/SiO,, or Mo,) was fluidised. These oxides were inactive in the catalytic reaction tested here at 473K etc. Samples (0.5g) of the powdered oxides were analysed at selected intervals by temperatureprogrammed oxidation in an 0, stream (8cm3/min)during heating at 5Wmin from 298K to
386
973K. The product CQ was converted to methane and was then analysed by FID gas chromatography. Samples were also analysed by electron spin resonance.
2.2. Apparatus and catalytic method
The catalytic reactor [a] used is shown in Figure 1. he-purified hydrogen and O.l%CJ-I, in N, entered the reactor at separate inlets here labelled 1 and 2. Tabla 1
P o w h fluidircd upon Wdumina pellets Sunplc
Initial ntc of c h n e hydrogenation (moleculu cJ.& x lO'S/g/min) oxide Alone Spillova
With(a)
SiO, SO,- 13%Al@, SiO,--U%Aba yAl203(a) rAU03(b) yAl203(c) 5%Au/Si02@70)
Mo4
0 0 0 0
0 0 0 0
0 55.94 162.33 30.06 159.14
402.82
71.61 90.37
(b)
0 15.03 75.16 108.41 119.35 46.75 26.30
[-I
[27%] [46%]
[-I
[a%] [30%] [a%]
[29%]
* the O v d d d O M ured Uld lcwl h the fluidid bed W C R 88.1kPa &, 0.03Lw cJ.& with a y bairncc to l o l k h and a mtd flow nte of 460cm'lmin. These beds wac fluidid upon 8 Walumina pellets. Activity w u seen (a) when the oxide is activated by 1.79bWalumina pellets and (b)after pellet nmovd. [I denote the pacentages of the initial activity in the presence of Walumina pellets which remain after rcmovd of the pellets.
Pre-reduced alumina-supported Pt pellets (e.g. 0.4g) were placed at 3 and upon these fluidised the bed of oxide powder (e.g. 3g) at 4. Products of the reaction left at 5 and were analysed via a constant volume gas sampling valve on a gas chromatograph. After N, flushing for 30min, €& was introduced at the base of the reactor (88.lkPa and 400cm3/min);this fluidised the powder but left the pellets unchanged in position. The reactor temperature was raised to 473K or 573K, held there for lh, and then cooled in flowing & to the selected reaction temperature (e.g. 473K). At this point the reactant stream (31.2Pa C$€, in N, (1:999) at aoCm3/min) was inaoduced simultaneously to give a &€ concentration I, of 130 ppm. The concentration profile of ethene in the reactor was measured by replacing the temperature probe with a sampling tube of the same diameter. From Figure 1 it is clear that once the flow rate of €& exceeds 150cm3/min a constant reactant concentration (0.4 ppm CJ-I,/C.&) was seen; the pellets contributed 4 . 5 % of the activity observed. 3. RESULTS AND DISCUSSION
Figure 2 shows that the activity Seen for the fluidising beds of the powdered oxides is largely independent of reaction time. We have already seen in Figure 1 that an insignificant part of this activity is attributable to direct hydrogenation of the Walumina pellets alone. Figure 2 and Table 1 show how the activity of the oxide k raised by contact with the Walumina pellets in the presence of hydrogen. These observations could be defined by (i) spiltover hydrogen on the oxide becoming involved in catalysis
(1)
I'
387
@)
..... .. ,: .. I
..:.: . . d
4
0
3
L
-
8
u
t (h)
~W~/*pClleo
Figure 1. (a) Reactor system (where 1 is Hz inlet, 2 is CzH4inlet, 3 is Pt/Alz03 pellets, 4 is the fluidised bed of the powdered oxide, 5 is the product exit and 6 is the temperature sensor. (b) The concentration of C z h in position 3 as a function of H2flow rate.
Figure 2. Rates of hydrogenation of CzH4at 483K over 3 g unused SiO2-A12O3 (25%) (a), SiOz-AIzO3 fluidised upon 8 pellets of 1.7% Pt/Alz03 (b), and after removal of Pt/AI& pellets (c).
(ii) hydrogen spillover on the oxides inducing allcene adsorption and the generation of some sort of activity on these novel carbonaceous residues generated by spillover, or (iii) Pt being transferred from the pellets to the oxide powders. Which then is the dominant phenomenon? With regard to the fmt point, as the temperature of the bed in the reactor in Figure 1 was raised from say 373K to 498K the rate of ethene hydrogenation was found to increase (although thereafter it decreased at higher temperatures). The activation energy derived (18kJ/mol) suggests that diffusion (or spillover) is controlling the overall rates of the observed hydrogenation and this transport might involve hydrogen species from the Pt/alumina to the fluidising oxide particles. However, it was found that increasing the concentration of water vapour in the reactor did not change the overall rate of reaction and hence the role of water vapour is far more complicated than simply facilitating spillover alone. With regard to the second point, it is certainly true that carbonaceous material was built up on the oxides which then exhibited activity in their own right. Figure 3 shows that, as this built up, the activity of one of the oxides did increase. During these catalytic runs there was indeed an accumulation of carbonaceous material and its extent of formation does correlate
r I
A
0
I 10
I 2l
30
c awnd(x 10'' i l ) Figure 3. Relationship to catalytic activity in alkene hydrogenation to the extent of carbonaceous material accumulated on SiOz-Al@3 (25%).
388
with the activity seen in ethene hydrogenation. Interestingly, an esr signal increased in ~ intensity at the s 8 time. Turning now to the third point, the overall rate increased as the number or concentration of the Pt initiating centres was increased and this is not surprising. However, no significant Pt could be detected upon the oxide powder after use in the nactor upon the Walumina pellets (i.e. the Pt levels detected by AA was c 10 ppm) and so (iii) can be discounted as being insignificant. 4. CONCLUSIONS A sustained reaction has occurred on oxides which is supported by spillover. This activity is stable with time and is not attributable to direct hydrogenation of the Walumina pellets alone. Although activity on the fluidising oxides is directly related to the number of Ptinitiating sites, no significant Pt was found on the oxides after fluidisation on the pellets. Spiltover hydrogen on the oxide could on the other hand become involved in catalysis. Hydrogen spillover on the oxides could induce alkene adsorption and the generation of activity on these novel carbonaceous residues generated by spillover. Certainly the oxides darkened in colour during the naction and the extent of accumulation of carbonaceous residues on the oxides was directly related to the activity seen in ethene hydrogenation. It nmains difficult to indicate which of these then is the dominant phenomenon in a catalytic context and how such phenomena could be used in a full catalytic process. Further work is in hand using a range of different alkenes. It is however interesting that Pt accelerates spillover-enhanced hydrogenation of coke on alumina [7]. It is also interesting that SiO, does not have the activity as stcn by others [2], but since Au/SiO, does, the surface of the Au or the Au-silica interface is the active site. 5. REFERENCES
1 E. Baumgarten and E. Denecke J. Catal. 95 (1985) 296; E. Baumgarten. C. LentesWagner and R. Wagner J. Catal. 117 (1989) 533; ibid 126 (1990) 314; M.S. Spencer, R. Burch and S.E. Golunski J. Catal. 126 (1990) 31 1 2 J.H. Sinfelt and P.J. Luchesi, J. Amer. Chem. Soc. 85 0963) 3365; D. Bianchi, M. Lacroix, G. Pajonk and S.J. Teichner 3. Catal. 59 0979) 467; M. Lacroix Docteur D’Etat ES Sciences Thesis L’Universite Claude Bernard De Lyon (Oct.1980); R.J. Wiley, S.J. Teichner and G.M. Pajonk J. Molec. Catal. 77 (1992) 201 3 W.M.H. Sachtler and L.J. Bostelaar p.207 Stud. Sur. Sci.Catal. 17 (1983) G.M.Pajonk, S.J.Teichner and G.E.Germain (eds). 4 H. Saltzburg and M.E. Mullins p.295 Stud. Sur. Sci. Catal. 17 (1983) G.M .Pajonk, S.J. Teichner and G.E. Germain (eds.) 5 D. M a t , G.M. Pajonk and S.J. Teichner p.215 Stud. Sur. Sci. Catal. 17 (1983) G.M. Pajonk, S.J. Teichner and G.E. G e d (eds.); M. Lacroix, G.M. Pajonk and S.J. Teichner, Proc. 7th Int. Cong. Catal. p.279 (1981); Bull. Soc. Chim. France 87, 94, 101, 258, 265,(1981). 6 MAW. Lau and P.A. Sermon J. Chem. Soc. Chem Commun. 891, (1978) 7 J.M. Parera, E.M. Traffano, J.C. Muss0 and C.L. Pieck p.101 Stud. Sur.Sci. Catal. 17 (1983) G.M. Pajonk, S.J. Teichner and G.E. Gemain (4s.)