- Email: [email protected]
Effect of Steam on the Coking and on the Regeneration of Metal Catalysts: A Comparative Study of Alumina-Supported Platinum, Rhenium, Iridium and Rhodium Catalysts.
C.H. Bartholomew and J.B. Butt (Editors), Catalyst Deactivation 1991 0 1991 Elsevier Science Publishers B.V., Amsterdam
111
EFFECT OF STEAM ON THE COKING AND ON THE REGENERATION O F METAL CATALYSTS : A COMPARATIVE STUDY O F ALUMINA-SUPPORTED PLATINUM, RHENIUM, IRIDIUM AND RHODIUM CATALYSTS.
D. DUPREZ, M. HADJ-AISSA and J. BARBER Laboratoire de Catalyse en Chimie Organique, URA CNRS 350 Universith de Poitiers, 40 Av. du Recteur Pmeau 86022 POITIERS, France.
ABSTRACT The effect of steam on cokin by cyclopentane (H20 injected during coking) and on the regeneration (H 0 injected aker coking) of alurmna-supported Pt, Re, Ir and Rh catalysts was studied.%he mechanism of coke formation on Ir, Re and Rh is similar to the mechanism proposed for Pt (dehydrogenation on the metal, olyrnerization of cyclopentadiene on the support, desorption of H2 via a reverse s illovery. Nevertheless the large differences observed in the dehydrogenation activities o f t h e four metals parallel significant changes in coking rates (Pt >Rh=Ir> >Re). When injected during coking, steam decreases the amounts of coke deposited on the Regenerative treatments in steam The coke of coked Pt and Re catalysts gaslfy essentially the carbon deposited on the &. produced on Ir and Rh is very reactive with steam which will then gasify a significant part of the carbon deposited on the support. These results can be explained by the differences, on the coked catalyst, of the coke structure, the coke location and tly8mo&jlity of the oxygen species, the latter determined by complementary experiments of O/ 0 isotopic exchange.
m.
INTRODUCTION While the kinetics and mechanisms of coke formation and their effects on catalyst behavior are relatively well-known in the case of processes in dry atmospheres (hydrocarbodH2 or hydrocarbodinert, see for instance Ref. 1-2),the effect of water on the
coking of metal catalysts has been rarely studied, except in the case of the Ni-based steam reforming catalysts. We showed recently that steam could decrease the coking rate of platinum catalysts and we proposed a mechanism for the inhibition of coke (Refs.3-4). At low partial pressure (4Wa), steam inhibits essentially the dehydrogenation activity of platinum , thus decreasing the formation of olefinic and diolefinic compounds which are the main coke precursors. At a higher pressure (>1 Wa), steam begins to poison the acidic centres of the support on which the coke precursors polymerize. The purpose of our paper is to compare the behaviors of platinum,rhenium, iridium and rhodium under the same coking conditions (Cyclopentane, 400'C). Re and Ir were chosen as they are usually added to Pt in reforming bimetallic catalysts. Rh was studied since it is the most active metal in hydrocarbon steam reforming (Refs. 5-7).
112
Two series of experiments were carried out: (i) steam injected together with cyclopentane during coking in order to study the effects of water on the kinetics of coking, and (ii) steam injected alone after coking in order to study the catalyst regeneration by coke gasification. Carbon deposits on Rh/Al2O3 having been found to be particularly reactive in steam, complementary of 180/160 isotopic exchange, were carried out on coked Rh catalysts so as to evaluate the effect of coke on the mobility of oxygen species on the support.
EXPERIMENTAL The catalysts were prepared by impregnating the RhGne-Poulenc GFS C y alumina (210 m2g-', grain size 0.1-0.25 nun, main impurities Si, Fe, Na, Ti < 500 ppm) with aqueous solutions of chloroplatinic acid, rhenium chloride, chloroiridic acid and rhodium chloride. They were dried at 120°C and, except for Ir, calcined in an air flow at 500°C for 6h. The Ir/A1203 catalyst was calcined at 270°C since this metal is particularly sensitive above this temperature, to the sintering in 0 2 atmospheres (Ref.8). To avoid any change of the chlorine content of catalysts during the coking reaction in H20/cyclopentane, the solids were treated, after calcination, in a flow of H20/N2 at 450°C for 48h (partial pressure of water, Pw = 3 kPa). Finally the catalysts were reduced in hydrogen at 500°C for 16h (400 cm3 per cm3 of catalyst and per hour; 4°C min-I). Dispersion measurements were carried out in a pulse chromatographic apparatus described elsewhere (Ref.9). For Pt, Ir and Rh, oxygen titration of chemisorbed hydrogen was used to determine the accessible fraction of metal (OT/Pts = 1.5 at 25"C, OT/Irs = 2.5 and OT/Rhs = 2 at 60°C). The accessible fraction of rhenium was determined by oxygen chemisorption at 25°C (OC/Res = 2). Coking experiments were carried out in a flow reactor (400"C, atm. pressure, nitrogen to cyclopentane molar ratio of 2.3, volume hourly space velocity, 4000 h-'; partial pressure of steam in wet atmosphere, 23 torr or 3 Wa). Gaseous products were analyzed by G.C. on a squalane column. Carbon deposits were characterized by temperature-programmed oxidation ( P O ) at 7°C min-' in a 1% 02/He mixture (Ref.10). with the l60of the support was carried out on coked Rh/Al203 Exchange of 1802 so as to evaluate the effect of coke on the mobility of oxygen species on Al2O3. Rhodium was chosen because it was shown to be the most effective metal for promoting the exchange, via metal particles, between gaseous 0 2 and the oxygen of the support (Ref.11). For this experiment to be conclusive, the exchange had to be carried out under conditions where the surface migration of oxygen (and not the adsorption/desorption on the metal) was the rate determining step of the reaction. At moderate temperature (300°C) this was possible only with rhodium. The reaction was carried out in a recycle reactor described elsewhere (Ref.11). The coked catalyst (0.03g) was first treated in natural O2 at 310°C so
113
as to eliminate the carbon deposit on the metal and then reduced at 300°C. The rates of exchange were measured in the 260-300°C temperature range. The results were compared to those obtained with the fresh catalyst treated in similar conditions. RESULTS AND DISCUSSION The catalysts used are listed in Table 1 which gives for each sample the chemical composition, the percentage of dispersion and the metal surface area (all these characteristics being measured after the steaming at 450°C). TABLE 1 Characteristics of the catalysts. Catalyst
Metal WL-%
PtA Pt4A RhA Rh2A Ir4A Re4 A
1.2 3.42 0.48 1.81 3.55 3.7
Cl wt.40 1.2
0.34 0.6 0.15 0.24 0.37
Dispersion YO
85 79 81 80 79 10
Metal surface area m2g-1 2.8 7.4 1.8 6.7 7.6 0.85
As the rate of coking on Re was remarkably low, a special procedure was necessary for obtaining significant coke contents on this catalyst. These results will be presented separately.
Coking and regerreration of Pt, Rh and Ir cataijsts The coke contents after 3h coking with cyclopentane at 400°C in dry (CPA + N2) and wet (CPA + N2 + 3 kPa of H20) atmospheres are given in Table 2. Also reported in the Table are the coke contents after 3h regeneration in steam (N2 + 3 kPa of HzO) at 460°C of the catalysts coked in dry atmosphere at 400°C. Rhodium and indium behave quite differently from platinum. For similar metal surface areas, Rh and Ir give coking rates much lower than those obtained on Pt (compare PMA, Rh2A and Ir4A). The most conspicuous differences concern the gasification by steam of the carbon deposits, faster on Ir, and still faster on Rh, than on Pt. For example, 3 kPa of steam a t 460°C can eliminate 67% of the carbon deposited on Rh, 42% on Ir and 17%on Pt.
114
TABLE 2 Coking and regeneration by steam of Pt, Rh and Ir catalysts. % C after
Catalyst
coking at
coking at
regeneration at
400°C in
400°C in N2+
460°C in N2 + 3 kPa H 2 0
dry N2
3 kPa of H30
PtA
3.75
3.08
2.95
Pt4A
7.45
6.75
6.23
RhA
1.06
0.85
0.49
Rh2A
3.60
2.94
1.17
Ir4A
3.59
3.36
2.06
Coking rates are linked to dehydrogenation activity of metals. Table 3 shows that Rh and Ir produce cyclopentane (CPE) and cyclopentadiene (CPD) in much smaller amounts than Pt. However, Pt produces also large amounts of C1C4 and especially nC5. And yet, Pt is not considered to be a hydrogenolysis catalyst. For instance, Maurel and Leclercq have shown that Pt was two or three orders of magnitude less active than Rh and Ir in cyclopentane hydrogenolysis at 150-300°C (Ref.12). The high activity of Rh in CPA hydrogenolysis was confirmed by Fuentes et af. (Ref.13). The most likely hypothesis is that the formation of hydrogenolysis products during the coking on Rh and Ir is limited by the production of H2 from dehydrogenation reactions.
TABLE 3 Yields of the gaseous products after 5 minutes’reaction of cyclopentane at 400°C. Catalyst
% CPE
% CPD
% nC5
% C1-C4
Rh2A
1.03
0.35
0.14
3.0
Ir4A
3.2
1.2
0.15
2.0
Pt4A
10.9
3.2
4.5
3.1
115
Table 4 gives the stability of the carbon deposits in dry N2 : a higher volatility could result in an apparent decrease of the coking rates. It is clear that coke is more stable on Pt than on Rh and Ir. However, this loss of coke intervenes little in the decrease of the coking rates observed between Pt and Rh (or Ir) catalysts. TABLE 4 Effect of a treatment in dry N2 (3h) on the stability of the coke deposited at 400°C. Catalyst
Temperature of treatment ("C)
wt.-%C before
after 3h in dry N2
Rh2A
460
3.60
3.16
Ir4A
460
3.59
3.06
PtA
460
3.75
3.70
PtA
520
3.75
3.60
Temperatureprogrammed oxidation (TPO) of coke deposited on Rh,Ir and Pt TF'O profiles are shown in Fig.1 (Rh compared to Pt) and 2 (Ir compared to Pt). Significant differences between the three metals can be noted. TPO profiles of coke on Pt show three peaks : peak I at 275-32OoC, peak I1 at 380-440°C and peak I11 at 480-550°C. In accordance with previous results obtained on similar catalysts (Refs.3 and 4), peak I is ascribed to carbon deposited on the metal whereas peaks II and 111 correspond to two different forms of coke deposited on the support : (i) heavy aromatic molecules, having a graphite-like structure, resulting from the polymerization on A1203 of the CPD produced on the metal (peak III) and (ii) carbon deposited on the support in the vicinity of the metal particles resulting from a continuous slow metal-support migration of carbonaceous fragments (peak 11). TPO profiles of the coked Pt4A catalyst after regeneration in steam shows that H20 gasifies essentially the carbon deposited on the metal. On rhodium, there is practically no carbon corresponding to peak 111while this peak is significantly reduced on Ir. This is clearly related to the low dehydrogenation activity of these metals. The high reactivity in steam of the coke deposited on Rh and Ir is certainly due to the abundance of carbon 11 on these catalysts.
116
100
300
500
Fig.1 TPO profiles of coke deposited on
100
300
500
Fig2 TPO profiles of coke deposited on
a - Pt4A coked in dry N2 at 400°C for 3h
a - Pt4A coked in dry N2 at 400°C for 3h
a’ - Pt4A coked in dry N2 at 400°C for 3h
b - Ir4A coked in dry N2 at 400°C for 3h
and treated in steam (3kPa) at 460°C for 3h
b - Rh2A coked in dry N2 at 400°C for 3h c - Rh2A coked in wet (3kPa H20) at 400°C for 3h d - Rh2A coked in dry N2 at 400°C for 3h
1
c - Ir4A coked in wet N2 (3Wa H20) at 400°C for 3h d - lr4A coked in dry N2 at 400°C for 3h
and treated in steam (3Wa) at 460°C for 3h
Surface mobikfy of oxygen species on coked alumina l8O/l6O exchange measurements allowed us to compare the surface mobility of 0 species on fresh and on coked alumina, the punctual sources of 0 species being here rhodium particles which act as a transit area between the gas phase and the support. Coke deposited on the metal was eliminated by combustion at moderate temperature. It was verified that (i) this treatment did not change the TPO profile of the carbon deposited on the support and (ii) there was no carbon oxidation during the exchange experiments (no formation of CO,). The results (Fig.3) show that the mobility of oxygen species on alumina (0anions and OH groups) is practically not affected by the presence of coke, which can explain why the carbon deposits on Rh/A1203 are particularly reactive in steam. Coking and regeneration of rhenium catalysts With cyclopentane, the amounts of carbon deposited on Re/Al203 are extremely low (0.1-0.2%). This is certainly due to the very poor dehydrogenating activity of Re. To estimate the effect of steam of the coking rate, the reaction was carried out with a mixture of cyclopentane (82%), cyclopentene (8.7%) and cyclopentadiene (8.4%), the composition being close to the thermodynamic equilibrium at 400°C.
117 n
r,
(at.
ii n” g-’ )
1.9
1.8
1 0 ~ (K) 1 ~ Fig3 Arrhenius plot of the rates of isotopic exchange between 1802 and the l60of the support on fresh and on coked RhA catalyst samples. Metal carbon is eliminated from the coked sample before exchange measurements. The results obtained on Re4A, as well as on PtA (coked under the same conditions), are given in Table 5.
TABLE 5 Amounts of carbon deposited on Re4A and PtA after coking at 400°C for l h with a mixture of cyclopentane, cyclopentene and cyclopentadiene
70C after coking in N2 + 3kPa of H20 ~~
Catalyst
Re4A PtA
coking in dry N2 5.49 9.38
5.10 9.07
~~~~
~
~~
~
~
regeneration at 460°C inN2 + 3kPaH20 4.89 8.80
Platinum remains more active than rhenium even in the presence of CPE and CPD, which confirms that the metals play a dual role in the formation of coke : dehydrogenation giving coke precursors (non operating here since CPE and CPD are already present in the
reactant) and consolidation of the coke deposited on the support by continuous elimination of hydrogen via a reverse spillover phenomenon. It is clear that Pt remains more effective than Re in coke consolidation. TPO profiles on Re (Fig.?; show a small
118
Fig.4 TPO profiles of coke deposited from a cyclopentane+ cyclopentene+ cyclopentadiene on PtA and Re4A. a - PtA coked in dry N2 at 400°C for l h b - Re4A coked in dry N2 at 400°C for lh c - Re4A coked in wet N2 (3kPa H20) at 400°C for l h d - Re4A coked in dry N2 at 400°C for lh and treated in steam (3kPa) at 460°C for 3h
mixture
of
peak on the metal and a very significant peak corresponding to the peak III of Pt. There is apparently no peak I1 on Re. Steam introduced during coking decreases the amount of carbon deposited on the support while steam introduced after coking oxidizes essentially all the coke deposited on the metal. REFERENCES
1 2 3 4 5 6 7 8 9 10 11
12 13
J. Barbier, A pl. Catal., 23 (1986) 225. J. Biswas, G.L. Bickle, D.D. Do and J. Barbier, Catal. Rev. Sci. Eng. 30 (1988) 161. D. Duprez, M. Hadj-Aha and J. Barbier, Appl. Catal., 49 D. Duprez, M. Hadj-Aissa and J. Barbier, Appl. Catal., 49 E. Kikuchi, K. Ito, T. Ino and Y. Morita, J. Catal., 46 (1977) 382. D.C. Grenoble, J. Catal., 51 (1978) 212. D. Duprez, R. Maurel, A. Miloudi and P. Pereira, J. Catal., 75 (1982) 151. G.B. Mc Vicker, R.L. Garten and R.T.K. Baker, J. Catal., 54 (1978) 129. D. Duprez, J. Chim. Phys., 80 (1983) 487. J. Barbier, E. Churin, J.M. Parera and J. Riviere, React. Kinet. Catal. Lett., 28 (1985) 245. H. Abderrahim and D. Duprez in "Stud. Surf. Sci. Catal." vo1.30 p.359, Elsevier, Amsterdam (1987). R. Maurel and G. Leclercq, Bull. SOC.Chim. Fr. 1234 (1971). S. Fuentes, F. Figueras and R. Gomez, J. Catal., 68 (1981) 419.
111
EFFECT OF STEAM ON THE COKING AND ON THE REGENERATION O F METAL CATALYSTS : A COMPARATIVE STUDY O F ALUMINA-SUPPORTED PLATINUM, RHENIUM, IRIDIUM AND RHODIUM CATALYSTS.
D. DUPREZ, M. HADJ-AISSA and J. BARBER Laboratoire de Catalyse en Chimie Organique, URA CNRS 350 Universith de Poitiers, 40 Av. du Recteur Pmeau 86022 POITIERS, France.
ABSTRACT The effect of steam on cokin by cyclopentane (H20 injected during coking) and on the regeneration (H 0 injected aker coking) of alurmna-supported Pt, Re, Ir and Rh catalysts was studied.%he mechanism of coke formation on Ir, Re and Rh is similar to the mechanism proposed for Pt (dehydrogenation on the metal, olyrnerization of cyclopentadiene on the support, desorption of H2 via a reverse s illovery. Nevertheless the large differences observed in the dehydrogenation activities o f t h e four metals parallel significant changes in coking rates (Pt >Rh=Ir> >Re). When injected during coking, steam decreases the amounts of coke deposited on the Regenerative treatments in steam The coke of coked Pt and Re catalysts gaslfy essentially the carbon deposited on the &. produced on Ir and Rh is very reactive with steam which will then gasify a significant part of the carbon deposited on the support. These results can be explained by the differences, on the coked catalyst, of the coke structure, the coke location and tly8mo&jlity of the oxygen species, the latter determined by complementary experiments of O/ 0 isotopic exchange.
m.
INTRODUCTION While the kinetics and mechanisms of coke formation and their effects on catalyst behavior are relatively well-known in the case of processes in dry atmospheres (hydrocarbodH2 or hydrocarbodinert, see for instance Ref. 1-2),the effect of water on the
coking of metal catalysts has been rarely studied, except in the case of the Ni-based steam reforming catalysts. We showed recently that steam could decrease the coking rate of platinum catalysts and we proposed a mechanism for the inhibition of coke (Refs.3-4). At low partial pressure (4Wa), steam inhibits essentially the dehydrogenation activity of platinum , thus decreasing the formation of olefinic and diolefinic compounds which are the main coke precursors. At a higher pressure (>1 Wa), steam begins to poison the acidic centres of the support on which the coke precursors polymerize. The purpose of our paper is to compare the behaviors of platinum,rhenium, iridium and rhodium under the same coking conditions (Cyclopentane, 400'C). Re and Ir were chosen as they are usually added to Pt in reforming bimetallic catalysts. Rh was studied since it is the most active metal in hydrocarbon steam reforming (Refs. 5-7).
112
Two series of experiments were carried out: (i) steam injected together with cyclopentane during coking in order to study the effects of water on the kinetics of coking, and (ii) steam injected alone after coking in order to study the catalyst regeneration by coke gasification. Carbon deposits on Rh/Al2O3 having been found to be particularly reactive in steam, complementary of 180/160 isotopic exchange, were carried out on coked Rh catalysts so as to evaluate the effect of coke on the mobility of oxygen species on the support.
EXPERIMENTAL The catalysts were prepared by impregnating the RhGne-Poulenc GFS C y alumina (210 m2g-', grain size 0.1-0.25 nun, main impurities Si, Fe, Na, Ti < 500 ppm) with aqueous solutions of chloroplatinic acid, rhenium chloride, chloroiridic acid and rhodium chloride. They were dried at 120°C and, except for Ir, calcined in an air flow at 500°C for 6h. The Ir/A1203 catalyst was calcined at 270°C since this metal is particularly sensitive above this temperature, to the sintering in 0 2 atmospheres (Ref.8). To avoid any change of the chlorine content of catalysts during the coking reaction in H20/cyclopentane, the solids were treated, after calcination, in a flow of H20/N2 at 450°C for 48h (partial pressure of water, Pw = 3 kPa). Finally the catalysts were reduced in hydrogen at 500°C for 16h (400 cm3 per cm3 of catalyst and per hour; 4°C min-I). Dispersion measurements were carried out in a pulse chromatographic apparatus described elsewhere (Ref.9). For Pt, Ir and Rh, oxygen titration of chemisorbed hydrogen was used to determine the accessible fraction of metal (OT/Pts = 1.5 at 25"C, OT/Irs = 2.5 and OT/Rhs = 2 at 60°C). The accessible fraction of rhenium was determined by oxygen chemisorption at 25°C (OC/Res = 2). Coking experiments were carried out in a flow reactor (400"C, atm. pressure, nitrogen to cyclopentane molar ratio of 2.3, volume hourly space velocity, 4000 h-'; partial pressure of steam in wet atmosphere, 23 torr or 3 Wa). Gaseous products were analyzed by G.C. on a squalane column. Carbon deposits were characterized by temperature-programmed oxidation ( P O ) at 7°C min-' in a 1% 02/He mixture (Ref.10). with the l60of the support was carried out on coked Rh/Al203 Exchange of 1802 so as to evaluate the effect of coke on the mobility of oxygen species on Al2O3. Rhodium was chosen because it was shown to be the most effective metal for promoting the exchange, via metal particles, between gaseous 0 2 and the oxygen of the support (Ref.11). For this experiment to be conclusive, the exchange had to be carried out under conditions where the surface migration of oxygen (and not the adsorption/desorption on the metal) was the rate determining step of the reaction. At moderate temperature (300°C) this was possible only with rhodium. The reaction was carried out in a recycle reactor described elsewhere (Ref.11). The coked catalyst (0.03g) was first treated in natural O2 at 310°C so
113
as to eliminate the carbon deposit on the metal and then reduced at 300°C. The rates of exchange were measured in the 260-300°C temperature range. The results were compared to those obtained with the fresh catalyst treated in similar conditions. RESULTS AND DISCUSSION The catalysts used are listed in Table 1 which gives for each sample the chemical composition, the percentage of dispersion and the metal surface area (all these characteristics being measured after the steaming at 450°C). TABLE 1 Characteristics of the catalysts. Catalyst
Metal WL-%
PtA Pt4A RhA Rh2A Ir4A Re4 A
1.2 3.42 0.48 1.81 3.55 3.7
Cl wt.40 1.2
0.34 0.6 0.15 0.24 0.37
Dispersion YO
85 79 81 80 79 10
Metal surface area m2g-1 2.8 7.4 1.8 6.7 7.6 0.85
As the rate of coking on Re was remarkably low, a special procedure was necessary for obtaining significant coke contents on this catalyst. These results will be presented separately.
Coking and regerreration of Pt, Rh and Ir cataijsts The coke contents after 3h coking with cyclopentane at 400°C in dry (CPA + N2) and wet (CPA + N2 + 3 kPa of H20) atmospheres are given in Table 2. Also reported in the Table are the coke contents after 3h regeneration in steam (N2 + 3 kPa of HzO) at 460°C of the catalysts coked in dry atmosphere at 400°C. Rhodium and indium behave quite differently from platinum. For similar metal surface areas, Rh and Ir give coking rates much lower than those obtained on Pt (compare PMA, Rh2A and Ir4A). The most conspicuous differences concern the gasification by steam of the carbon deposits, faster on Ir, and still faster on Rh, than on Pt. For example, 3 kPa of steam a t 460°C can eliminate 67% of the carbon deposited on Rh, 42% on Ir and 17%on Pt.
114
TABLE 2 Coking and regeneration by steam of Pt, Rh and Ir catalysts. % C after
Catalyst
coking at
coking at
regeneration at
400°C in
400°C in N2+
460°C in N2 + 3 kPa H 2 0
dry N2
3 kPa of H30
PtA
3.75
3.08
2.95
Pt4A
7.45
6.75
6.23
RhA
1.06
0.85
0.49
Rh2A
3.60
2.94
1.17
Ir4A
3.59
3.36
2.06
Coking rates are linked to dehydrogenation activity of metals. Table 3 shows that Rh and Ir produce cyclopentane (CPE) and cyclopentadiene (CPD) in much smaller amounts than Pt. However, Pt produces also large amounts of C1C4 and especially nC5. And yet, Pt is not considered to be a hydrogenolysis catalyst. For instance, Maurel and Leclercq have shown that Pt was two or three orders of magnitude less active than Rh and Ir in cyclopentane hydrogenolysis at 150-300°C (Ref.12). The high activity of Rh in CPA hydrogenolysis was confirmed by Fuentes et af. (Ref.13). The most likely hypothesis is that the formation of hydrogenolysis products during the coking on Rh and Ir is limited by the production of H2 from dehydrogenation reactions.
TABLE 3 Yields of the gaseous products after 5 minutes’reaction of cyclopentane at 400°C. Catalyst
% CPE
% CPD
% nC5
% C1-C4
Rh2A
1.03
0.35
0.14
3.0
Ir4A
3.2
1.2
0.15
2.0
Pt4A
10.9
3.2
4.5
3.1
115
Table 4 gives the stability of the carbon deposits in dry N2 : a higher volatility could result in an apparent decrease of the coking rates. It is clear that coke is more stable on Pt than on Rh and Ir. However, this loss of coke intervenes little in the decrease of the coking rates observed between Pt and Rh (or Ir) catalysts. TABLE 4 Effect of a treatment in dry N2 (3h) on the stability of the coke deposited at 400°C. Catalyst
Temperature of treatment ("C)
wt.-%C before
after 3h in dry N2
Rh2A
460
3.60
3.16
Ir4A
460
3.59
3.06
PtA
460
3.75
3.70
PtA
520
3.75
3.60
Temperatureprogrammed oxidation (TPO) of coke deposited on Rh,Ir and Pt TF'O profiles are shown in Fig.1 (Rh compared to Pt) and 2 (Ir compared to Pt). Significant differences between the three metals can be noted. TPO profiles of coke on Pt show three peaks : peak I at 275-32OoC, peak I1 at 380-440°C and peak I11 at 480-550°C. In accordance with previous results obtained on similar catalysts (Refs.3 and 4), peak I is ascribed to carbon deposited on the metal whereas peaks II and 111 correspond to two different forms of coke deposited on the support : (i) heavy aromatic molecules, having a graphite-like structure, resulting from the polymerization on A1203 of the CPD produced on the metal (peak III) and (ii) carbon deposited on the support in the vicinity of the metal particles resulting from a continuous slow metal-support migration of carbonaceous fragments (peak 11). TPO profiles of the coked Pt4A catalyst after regeneration in steam shows that H20 gasifies essentially the carbon deposited on the metal. On rhodium, there is practically no carbon corresponding to peak 111while this peak is significantly reduced on Ir. This is clearly related to the low dehydrogenation activity of these metals. The high reactivity in steam of the coke deposited on Rh and Ir is certainly due to the abundance of carbon 11 on these catalysts.
116
100
300
500
Fig.1 TPO profiles of coke deposited on
100
300
500
Fig2 TPO profiles of coke deposited on
a - Pt4A coked in dry N2 at 400°C for 3h
a - Pt4A coked in dry N2 at 400°C for 3h
a’ - Pt4A coked in dry N2 at 400°C for 3h
b - Ir4A coked in dry N2 at 400°C for 3h
and treated in steam (3kPa) at 460°C for 3h
b - Rh2A coked in dry N2 at 400°C for 3h c - Rh2A coked in wet (3kPa H20) at 400°C for 3h d - Rh2A coked in dry N2 at 400°C for 3h
1
c - Ir4A coked in wet N2 (3Wa H20) at 400°C for 3h d - lr4A coked in dry N2 at 400°C for 3h
and treated in steam (3Wa) at 460°C for 3h
Surface mobikfy of oxygen species on coked alumina l8O/l6O exchange measurements allowed us to compare the surface mobility of 0 species on fresh and on coked alumina, the punctual sources of 0 species being here rhodium particles which act as a transit area between the gas phase and the support. Coke deposited on the metal was eliminated by combustion at moderate temperature. It was verified that (i) this treatment did not change the TPO profile of the carbon deposited on the support and (ii) there was no carbon oxidation during the exchange experiments (no formation of CO,). The results (Fig.3) show that the mobility of oxygen species on alumina (0anions and OH groups) is practically not affected by the presence of coke, which can explain why the carbon deposits on Rh/A1203 are particularly reactive in steam. Coking and regeneration of rhenium catalysts With cyclopentane, the amounts of carbon deposited on Re/Al203 are extremely low (0.1-0.2%). This is certainly due to the very poor dehydrogenating activity of Re. To estimate the effect of steam of the coking rate, the reaction was carried out with a mixture of cyclopentane (82%), cyclopentene (8.7%) and cyclopentadiene (8.4%), the composition being close to the thermodynamic equilibrium at 400°C.
117 n
r,
(at.
ii n” g-’ )
1.9
1.8
1 0 ~ (K) 1 ~ Fig3 Arrhenius plot of the rates of isotopic exchange between 1802 and the l60of the support on fresh and on coked RhA catalyst samples. Metal carbon is eliminated from the coked sample before exchange measurements. The results obtained on Re4A, as well as on PtA (coked under the same conditions), are given in Table 5.
TABLE 5 Amounts of carbon deposited on Re4A and PtA after coking at 400°C for l h with a mixture of cyclopentane, cyclopentene and cyclopentadiene
70C after coking in N2 + 3kPa of H20 ~~
Catalyst
Re4A PtA
coking in dry N2 5.49 9.38
5.10 9.07
~~~~
~
~~
~
~
regeneration at 460°C inN2 + 3kPaH20 4.89 8.80
Platinum remains more active than rhenium even in the presence of CPE and CPD, which confirms that the metals play a dual role in the formation of coke : dehydrogenation giving coke precursors (non operating here since CPE and CPD are already present in the
reactant) and consolidation of the coke deposited on the support by continuous elimination of hydrogen via a reverse spillover phenomenon. It is clear that Pt remains more effective than Re in coke consolidation. TPO profiles on Re (Fig.?; show a small
118
Fig.4 TPO profiles of coke deposited from a cyclopentane+ cyclopentene+ cyclopentadiene on PtA and Re4A. a - PtA coked in dry N2 at 400°C for l h b - Re4A coked in dry N2 at 400°C for lh c - Re4A coked in wet N2 (3kPa H20) at 400°C for l h d - Re4A coked in dry N2 at 400°C for lh and treated in steam (3kPa) at 460°C for 3h
mixture
of
peak on the metal and a very significant peak corresponding to the peak III of Pt. There is apparently no peak I1 on Re. Steam introduced during coking decreases the amount of carbon deposited on the support while steam introduced after coking oxidizes essentially all the coke deposited on the metal. REFERENCES
1 2 3 4 5 6 7 8 9 10 11
12 13
J. Barbier, A pl. Catal., 23 (1986) 225. J. Biswas, G.L. Bickle, D.D. Do and J. Barbier, Catal. Rev. Sci. Eng. 30 (1988) 161. D. Duprez, M. Hadj-Aha and J. Barbier, Appl. Catal., 49 D. Duprez, M. Hadj-Aissa and J. Barbier, Appl. Catal., 49 E. Kikuchi, K. Ito, T. Ino and Y. Morita, J. Catal., 46 (1977) 382. D.C. Grenoble, J. Catal., 51 (1978) 212. D. Duprez, R. Maurel, A. Miloudi and P. Pereira, J. Catal., 75 (1982) 151. G.B. Mc Vicker, R.L. Garten and R.T.K. Baker, J. Catal., 54 (1978) 129. D. Duprez, J. Chim. Phys., 80 (1983) 487. J. Barbier, E. Churin, J.M. Parera and J. Riviere, React. Kinet. Catal. Lett., 28 (1985) 245. H. Abderrahim and D. Duprez in "Stud. Surf. Sci. Catal." vo1.30 p.359, Elsevier, Amsterdam (1987). R. Maurel and G. Leclercq, Bull. SOC.Chim. Fr. 1234 (1971). S. Fuentes, F. Figueras and R. Gomez, J. Catal., 68 (1981) 419.