- Email: [email protected]
The active surface of silica-supported copper in the hydrogenative transformations of cyclohexene: The role and state of carbonaceous deposits
ELSEVIER
Applied Catalysis
A: General
166 (1998) 185-190
The active surface of silica-supported copper in the hydrogenative transformations of cyclohexene: The role and state of carbonaceous deposits And&
F&i, Istvan P&&6,
Mihily Bartbk*
Department of Organic Chemistry and Organic Catalysis Research Group of the Hungarian Academy Ddm te’r 8, Szeged H-6720, Hungary Received 27 February
of Sciences, Jdwef Attila University
1997; received in revised form 28 July 1997; accepted 30 July 1997
Abstract The evolution of the catalytic surface and its main characteristics are investigated in the hydrogenative transformations of cyclohexene over a silica-supported copper catalyst for the first time in a static circulation system and in a flow reactor. The main tools were: (i) restart reactions when aging was studied, (ii) competitive hydrogenation reaction with the cyclopropane molecule known to require very different surface features to be present and (iii) the removal of weakly as well as strongly held
carbonaceous species. It was found that in hydrogenative transformations of cyclohexene the presence of hydrogen-rich carbonaceous residues are necessary. Without their formation there is no reaction. As their quantity grows the catalyst becomes more active. They are product precursors and unreacted cyclohexane covering the surface to such an extent that the remaining metal ensembles are not enough for cyclopropane hydrogenation to occur. In restart reactions the catalyst undergoes coking but only to a small extent at 4.43 K. 0 1998 Elsevier Science B.V. Keywords:
Copper-on-silica
catalyst; Cyclohexene;
Cyclopropane;
1. Introduction Recently, we have studied the transformations of cyclohexene in Dz atmosphere over a silica-supported copper catalyst in static circulation and flow reactors [l]. In the static system dehydrogenation, dehydrogenation and hydrogenation together and hydrogenation reactions proceed depending on D2 pressure at 443 K. At low enough D2 pressure (2.7 kF’a and below) only dehydrogenation takes place, while at 20 kPa and above only hydrogenation occurs. *Corresponding
author.
0926-860X/98/$19.00 80 1998 Elsevier Science B.V. All rights reserved. PII SO926-860X(97)00253-6
Carbonaceous
residues; Hydrogenative
transformations
In-between both reactions proceed with almost the same rate of benzene and cyclohexane formation. Interestingly, at most Dz pressures the reactions start with an induction period. It disappears only above 40 kPa. The presence of this induction period indicates that the real catalyst is not the pure Cu/Si02, but the reactants make the catalytic surface for themselves. Since there was no other reactant in the system than cyclohexene and deuterium, the major player in making the active surface is suspected to be some or perhaps several forms of carbonaceous deposits resembling more or less the starting material and/or the products. Obviously, some forms of carbonaceous
186
A. F&i et al./Applied Catalysis A: General 166 (1998) 185-190
material may not influence the reaction in a beneficial way, hydrogen-poor coke is known to cause severe activation [2]. The phenomenon that the reactants make the catalytic surface for themselves is not unknown. It was hypothesized by Taylor as early as 1925 [3] and was proven for many reactions, most clearly by Somorjai and co-workers [4]. These studies mainly concerned transition metal catalysts (most frequently platinum) and the most often used probe reaction was hydrogenation. Since copper is rarely considered to be an effective hydrogenation catalysts, this metal escaped studies of this kind. To fill this gap it seemed worth while to investigate the evolution of the active surface and also to collect information about the deactivation procedure in the Cu/Si02-cyclohexene-Dz(Hz) system. These phenomena were studied in static and flow reactors with several methods like desorption or hydrogenation off the residues from close to initial or the used catalyst, aging the catalyst by several runs with or without activation between the runs, using competitive reactions known to require very different surface characteristics [5]. Results of this investigation is detailed in the followings.
2. Experimental
2.1. Materials Cyclohexene was obtained via the dehydration of cyclohexanol. After distillation its purity was checked by the GC-MS method. It was found 99.9+% pure. In order to destroy possibly formed peroxides, it was passed through a column filled with freshly activated basic alumina (Camag) under inert (Ar) atmosphere before reactions in the flow system or filling it into a closed evacuated vial for further use in the static circulation reactor. Cyclopropane was an Aldrich product with a purity of 99+%. It was further purified through distillation in the vacuum system. Oxygenfree hydrogen was prepared with a Matheson 8326 generator, operating with a palladium membrane. The 6.36% Cu/silica gel (Cu/S-X, was prepared by an ion-exchange method described in the literature [6]. Details are as follows: silica gel (Strem, large pore, 12&230 mesh) was immersed in a tetramminecopper(I1) solution at pH 11. After 24 h the blue product
was washed thoroughly with water, dried at 393 K (24 h) and decomposed at 773 K in air (3 h). The catalyst was reduced in flowing hydrogen (20 cm’/min) at 673 K for 2 h and kept in a vacuum desiccator until use. The percentage of surface copper atoms (14.3%) was determined by N20 decomposition by the pulse method at 363 K [7]. After reduction 7% of the copper remained ionic in the form of Cus determined by temperature-programmed reduction
[81. Samples (50 mg) of the stored prereduced catalyst were activated in the circulation system under 20 kPa H2 at 573 K for 30 min or in the flow reactor under 20 cm3/min H2 flow for 1 h at 573 K. 2.2. Apparatus,
methodology
and analysis
2.2.1. Static circulation reactor The reactions were carried out in a conventional closed circulation reactor similar to which was described in Ref. [9]. The volume of the reactor was 69 cm3, and total volume of the system was 169.4 cm’. The reactor was heated with an air thermostat. The volume of the sampling capillary was 0.05 cm3 and the total sampling volume was 0.15 cm3. A Hewlett-Packard (HP) Model 5890 gaschromatograph equipped with a flame ionization detector (FID) was attached to the system. Data analysis was performed on an HP 3396A integrator. A 50 m long HP-1 capillary column was used for separating the reactants and the products. The column was operated in the isothermal mode (oven temperature 323 K) with helium as carrier gas (flow rate 1.3 cm3/min). The reactants (2.65 kPa of cyclohexene or 2.65 kPa of cyclopropane or a mixture of 2.65 kPa of cyclohexene and cyclopropane and 13.3 or 33.3 kPa of Hz) were premixed in the circulation part of the system before the reaction. (Cyclohexene was subjected to several freeze-thaw-evacuate cycles before preparing the mixture.) The reaction temperature was 443 K in most cases and in certain experiments carbonaceous material was deposited at 473 K. Basically two types of reactions were performed. The first measurements were run over a fresh sample of catalysts, while restart reactions were done over the used catalyst subjected to evacuation (20 min, 1 Pa, 443 K) before the reaction. For determining the quantity of weakly held residues 53.2 kPa of H2 was used. The amount of strongly held
A. F&i et al./Applied Catalysis A: General 166 (1998) 185-190
carbonaceous material was measured in form of CO2 after removing it through oxidation with 02 gas (13.3 kPa) at 673 K for 1 h. 2.2.2. Flow reactor Reactions were carried out over 50 mg of catalyst samples in Hz flow (15 cm3/min). The partial pressure of cyclohexene was 3.2 kPa. The usual temperature was 303 K. Fresh sample of catalyst (activated in 20 cm3/min H2 flow for 1 h at 573 K) was used initially. In restart reactions 3-l 1 h of Hz purge was applied at 303 K or ramping the temperature from 303 to 573 K (20 min at 303 K, 20 min at 373 K, 10 h at 423 K, 20 min at 573 K). Product accumulation was followed by gas chromatography as it is described in the previous paragraph.
3. Results 3.1. Deactivation behavior by restart reactions in the static circulation system At 443 K cyclohexene in the presence of 33.3 kPa H2 gave only cyclohexane with an induction period over a fresh sample of catalyst. In restart reactions with evacuating the system in-between, the disappearance of the induction period and the gradual hydrogenation activity increase could be observed. In the third restart reaction benzene was also detected. Interestingly, at 10 K higher even the first restart reaction produced benzene (over the fresh catalyst only hydrogenation occurred). Relevant data are summarized in Table 1. Table 1 Product accumulation
in aging experiments
Type of run
Fresh catalyst Restart (1) a Restart (2) Restart (3)
’ Repeated
runs over the catalysts
3.2. Deactivation behavior by restart reactions in the flow reactor At 303 K both benzene and cyclohexane formation were observed over the fresh catalyst. The sample was deactivated in 3 h. In the restart reaction after a 3 h hydrogen purge, steady-state activities have been attained in about an hour both in dehydrogenation and hydrogenation. Moreover, the steady-state hydrogenation rate was about an order of magnitude higher than it was over the initial catalyst in its most active state (2 min sampling). Benzene formation was at similar level as over the initial catalyst in its most active state (2 min sampling). Relevant data are summarked in Table 2. After 2 h of reaction the reactor was purged with hydrogen with a temperature program (20 min at 303 K, 20 min at 373 K, 10 h at 423 K and 20 min at 573 K) and at certain time intervals the effluent gas was analyzed. No desorbed product was found up to 423 K. At 423 K cyclohexane and 573 K benzene (an order of magnitude less than cyclohexane at 423 K) were detected. After the hydrogen purge, a new reaction was started and very similar hydrogenation behavior was found as over the initial catalyst. 3.3. The catalyst su$ace studied by the competitive hydrogenation of cyclopropane and cyclohexene in the static system At 443 K cyclopropane ring-opening product over Ring opening was found propane. After removing
in the static system (2.65 kPa cyclohexene
Products
Benzene Cyclohexane Benzene Cyclohexane Benzene Cyclohexane Benzene Cyclohexane with evacuation
187
gives propane as the sole a fresh sample of catalyst. to be zero order in cyclothe reaction mixture by
+ 33.3 kPa H2, 443 K, 50 mg Cu/Si02)
Concentration/mol% 5 min
15 min
30 min
35 min
0 0 0 0.2 0 0.3 0 0.7
0 2.9 0 0.3 0 0.4 0 5.4
0 39.6 0 1.4 0 7.7 0.02 19.1
0 57.6 0 3.2 0 12.3 0.03 24.5
(20 min, 1 Pa, 443 K) in-between.
188
A. Fhi et al. /Applied
Catalysis A. General
Table 2 Product distribution in aging experiments in the flow system (3.2 kPa cyclohexene, 15 cm3/min Hz flow, 303 K, 50mg Cu/SiOz) Time/min
2 18.5 35 51.5 68 84.5 101 134 a Fresh b Used ’ Used at 373
Benzene/mol% 2b
3’
1”
2b
3’
0.03 0.02 0.01 0.01 Traces
0 0 Traces 0.02 0.02 0.02 0.02 -
Traces Traces 0 0 0 0 0 -
1.2 0.6 0.4 0.3 0.2 0.02
0.03 0.02 1.90 11.4 12.1 12.2 12.0 -
1.1 0.3 0.2 0.1 0.04 0.03 0.01
Traces
evacuating the cyclopropane + cyclohexene + H2 mixture, a restart reaction with only cyclohexene and Hz gave the same products (benzene and cyclohexane) as in the previous reaction, however, some deactivation did take place. After removing surface contaminants from a catalyst sample already used in cyclohexene hydrogenation (443 K) by oxidation (673 K) and rereducing it (573 K), the sample became active again in ring opening. Moreover, its activity increased. The transformation remained close to zero order in cyclopropane. Relevant data are summarized in Table 3 In order to gain information on the chemical properties of surface species responsible for the hydrogenative transformations of cyclohexene, a replacement reaction was executed. In it, the surface was deliberately contaminated by depositing carbonaceous species via introducing cyclohexene-HZ mixture onto the catalyst for a prolonged time at 443 K (for the accurate procedure, see the caption of Table 4) and then running cyclopropane hydrogenation on this surface. Although the cyclopropane-hydrogen mixture did not give propane (i.e. desorbed ring-opening product was not found), it induced the desorption of surface species and benzene, cyclohexane (hydrogen was present) and cyclohexene emerged (Table 4). Desorption was not complete, however, since cyclopropane did not undergo ring opening even after 40 min.
Cyclohexane/mol%
la
catalyst. catalyst. catalyst after 11 h purge with H2 (20 min at 303 K, 20 min K. 10 h at 423 K and 20 min at 573 K.
evacuation at 443 K, a new mixture reacted with about the same rate, that is hardly any deactivation took place. The reaction order remained the same as well. When an equimolar mixture of cyclopropane and cyclohexene (2.67 kPa each) was introduced into the reactor in the presence of hydrogen (13.3 kF’a), the cyclopropane molecule did not react at all. However, cyclohexene underwent dehydrogenation (producing benzene) as well as hydrogenation (producing cyclohexane). After Table 3 Product distribution in various types of restart reactions 443 K) in the static system Type of run
Products
Fresh Restart (1) Restart (2)
Propane Propane Propane Benzene Cyclohexane Benzene Cyclohexane Propane
Restart (3) Restart (4)
Cu/SiOZ
166 (1998) 185-190
(50 mg) (cyclopropane:
2.65 kPa, cyclohexene:
2.65 kPa, Hz: 13.3 kPa,
Composition/mol% 5 min
15 min
0.3 0.3 0 0.2 4.9 Traces 1.9 0.6
1.o 1.1 0 1.8 15.9 0.9 6.9 1.8
30 min
35 min
40 min
2.0 2.2 0 4.9 28.8 3.2 17.1 3.6
2.3 2.5 0 5.6 32.1 3.8 19.2
2.5 2.7 0 6.9 35.3 4.5 21.8 4.6
Fresh: cyclopropane + Hz, fresh catalyst. Restart (I): cyclopropane + HZ, used catalyst in the previous reaction. Restart (2): cyclopropane + cyclohexene + Ha, used catalyst in the previous reactions. Restart (3): cyclohexene + H2, used catalyst in the previous reactions. Restart (4): cyclopropane + HZ, the used catalyst was regenerated (oxidation: 13.3 kPa 0 *, 673 K, 573 K, 1 h).
1h, then reduction
26.7 kPa Hz.
189
A. F&i et al./Applied CatalysisA: General 166 (1998) 185-190
Table 4 Accumulation of desorbed compounds by a cyclopropane + HZ mixture (2.65 kF’a cyclopropane + 13.3 kPa HZ, 443 K) following the deposition of carbonaceous material from cyclohexene hydrogenation (deposition: 2.65 kPa cyclohexene + 13.3 kPa HZ. 1 h. 443 K, and repeated one more time) over Cu/SiOZ in the static system Type of reaction
Products
Specific quantity/10-3/mol/g 5 min
15 min
30 min
35 min
40 min
Replacement
Benzene Cyclohexane Cyxlohexene
0.35 2.66 5.86
1.93 9.48 13.0
3.79 15.3 13.7
4.09 11.6 14.5
4.18 19.8 14.1
3.4. Direct observation of carbonaceous
deposits in
the static system
At 473 K carbonaceous material was deposited onto a fresh sample of catalyst from cyclohexene + H2 mixture. After evacuation, 53.2 kPa Hz was introduced into the reactor at the same temperature. After 30 min the gas composition was analyzed and benzene, cyclohexane and even cyclohexene could be observed in 2 : 4: 1 ratio. The concentration of the weakly held residues was 3.3 x lo-’ mmol/mg. After evacuation, that is after the removal of the Hzdesorbed products, the strongly held residues were removed by oxidation in the form of COZ. Its concentration was 6.7 x 10e4 mmol/mg.
4. Discussion Restart reactions either in the static system or in the flow reactor clearly showed that the real catalyst of the hydrogenative transformations of cyclohexene is not the freshly reduced copper catalyst. The reactants start to create the actual active surface at the first contact. The induction period observed over the fresh catalyst indicates that the active surface should be very different from the reduced Cu/SiO,. In restart reactions gradual activation was registered in the static system and steady-state activity was attained in the flow reactor. The induction period disappeared in restart reaction in the static system, which means that the reaction mixture now met the already transformed surface. One may suspect two main reasons for the change of the catalytic surface, such as (i) surface reconstruction and/or (ii) deposition of selectivity and activity determining or at least influencing surface
-
species like carbonaceous deposits with a variety of hydrogen content. The first possibility cannot be ruled out, moreover, it probably contributed to the observed effects, the second possibility is thought to be the major factor on the following grounds: - Cyclopropane known to require clean metal surface for its hydrogenation did not react either on the catalyst already used for the hydrogenative transformations of cyclohexene or in competitive hydrogenation. After oxidative-reductive regeneration of the catalyst the cyclopropane molecule underwent hydrogenative ring opening with similar kinetic characteristics as over the clean copper surface. - Upon investigating the surface of the used catalyst, a substantial amount of hydrogen-rich carbonaceous residues could be hydrogenated off or displaced from the surface under mild conditions. They were product precursors and unreacted cyclohexene. Of course, over the surface they were product- and reactant-like surface species. Their surface composition was not exactly like that of the gas phase. Coke formation also takes place. However, its extent is not serious under these mild conditions, therefore their deactivation effect was minor compared to the activating behavior of the hydrogen-rich species.
5. Conclusion With the help of relatively simple chemical tools we were able to show that hydrogen-rich residues have major role in forming the catalytically active surface in the hydrogenative transformations of cyclohexene over Cu/SiO*. To the best of our knowledge this is the
190
A. Fdsi et al. /Applied
Catalysis A: General 166 (1998) 185-190
first attempt to gain insight into the evolution of a copper surface in hydrocarbon hydrogenation reaction.
Acknowledgements This work was sponsored by the National Science Foundation of Hungary through grants OTKA TO14315 and T016109. The financial help is greatly appreciated.
References [l] A. F&i, I. Pblinko, T. Katona, M. Bartok, J. Catal. 167 (1997) 215. [2] E Zaera, A.J. Gellman, GA. Somorjai, Act. Chem. Res. 19 (1986) 24; TX Beebe, J.T. Yates, J. Am. Chem. Sot. 108 (1986) 663; J.M. Cogen, W.F. Maier, J. Am. Chem. Sot. 108 (1986) 7752; F. Garin, 0. Maire, S. Zyade, M. Zauwen, A. Frennet, P. Zielinski, J. Mol. Catal. 58 (1990) 185; J. Barbier, E. Churin, P. Marecot, .I. Catal. 126 (1990) 228; D. Espinat, D. Freund, H. Dexpert, G. Martino, .I. Catal. 126 (1990) 496;
F.J. Rivera-Latas, R.A. Dalla Betta, M. Boudart, AIChe .I. 38 (1992) 771; A. Erdohelyi, J. Cserenyi, E Solymosi, J. Catal. 141 (1993) 287; C.A. Querini, S.C. Fung, J. Catal. 141 (1993) 389; G.A. Somorjai, in: Introduction to surface chemistry and catalysis, pp. 505-507, Wiley, New York, Chichester, Brisbane, Toronto, Singapore, 1994; J. Houzvicka, R. Pestman, V. Ponec, Catal. Lett. 30 (1995) 289. [31 H.S. Taylor, Proc. Roy. Sot. (London) A108 (1925) 105. 141 G.A. Somorjai, in Proceedings, 8th International Congress on Catalysis, Berlin, 1984, Vol.1, p. 113, Dechema, Frankfurt am-Main, 1984; G.A. Somorjai, React. Kinet. Catal. Lett. 35 (1987) 37; G.A. Somorjai, J. Phys. Chem. 94 (1990) 1013; G.A. Somorjai, Catal. Lett. 9 (1991) 311; G.A. Somorjai, Catal. Lett. 12 (1992) 17. [51 I. Palink& E Notheisz, M. Bartok, Catal. Lett. 1 (1988) 127; I. Palinko, F. Notheisz, M. Bartok, Stud. Surf. Sci. Catal. 48 (1989) 729. [61 H. Kobayashi, N. Takezawa, C. Minochi, K. Takahashi, Chem. Lett. (1980) 1197. ]71 J.W. Evans, MS. Wainwright, A.J. Bridgewater, D.J. Young, Appl. Catal. 7 (1983)75; B. Denise, R.P.A. Sneeden, B. Beguin, 0. Cheriti, Appl. Catal. 30 (1987) 353. PI A. Molt&, I. Palinko, M. Bartok, J. Catal. 114 (1988) 478; I. Palinko, A. Molt&, J.T. Kiss, M. Bartok, J. Catal. 121 (1990) 396. [91 M. Bartok, F. Notheisz, A.G. Zsigmond, G.V. Smith, J. Catal. 100 (1986) 39.
Applied Catalysis
A: General
166 (1998) 185-190
The active surface of silica-supported copper in the hydrogenative transformations of cyclohexene: The role and state of carbonaceous deposits And&
F&i, Istvan P&&6,
Mihily Bartbk*
Department of Organic Chemistry and Organic Catalysis Research Group of the Hungarian Academy Ddm te’r 8, Szeged H-6720, Hungary Received 27 February
of Sciences, Jdwef Attila University
1997; received in revised form 28 July 1997; accepted 30 July 1997
Abstract The evolution of the catalytic surface and its main characteristics are investigated in the hydrogenative transformations of cyclohexene over a silica-supported copper catalyst for the first time in a static circulation system and in a flow reactor. The main tools were: (i) restart reactions when aging was studied, (ii) competitive hydrogenation reaction with the cyclopropane molecule known to require very different surface features to be present and (iii) the removal of weakly as well as strongly held
carbonaceous species. It was found that in hydrogenative transformations of cyclohexene the presence of hydrogen-rich carbonaceous residues are necessary. Without their formation there is no reaction. As their quantity grows the catalyst becomes more active. They are product precursors and unreacted cyclohexane covering the surface to such an extent that the remaining metal ensembles are not enough for cyclopropane hydrogenation to occur. In restart reactions the catalyst undergoes coking but only to a small extent at 4.43 K. 0 1998 Elsevier Science B.V. Keywords:
Copper-on-silica
catalyst; Cyclohexene;
Cyclopropane;
1. Introduction Recently, we have studied the transformations of cyclohexene in Dz atmosphere over a silica-supported copper catalyst in static circulation and flow reactors [l]. In the static system dehydrogenation, dehydrogenation and hydrogenation together and hydrogenation reactions proceed depending on D2 pressure at 443 K. At low enough D2 pressure (2.7 kF’a and below) only dehydrogenation takes place, while at 20 kPa and above only hydrogenation occurs. *Corresponding
author.
0926-860X/98/$19.00 80 1998 Elsevier Science B.V. All rights reserved. PII SO926-860X(97)00253-6
Carbonaceous
residues; Hydrogenative
transformations
In-between both reactions proceed with almost the same rate of benzene and cyclohexane formation. Interestingly, at most Dz pressures the reactions start with an induction period. It disappears only above 40 kPa. The presence of this induction period indicates that the real catalyst is not the pure Cu/Si02, but the reactants make the catalytic surface for themselves. Since there was no other reactant in the system than cyclohexene and deuterium, the major player in making the active surface is suspected to be some or perhaps several forms of carbonaceous deposits resembling more or less the starting material and/or the products. Obviously, some forms of carbonaceous
186
A. F&i et al./Applied Catalysis A: General 166 (1998) 185-190
material may not influence the reaction in a beneficial way, hydrogen-poor coke is known to cause severe activation [2]. The phenomenon that the reactants make the catalytic surface for themselves is not unknown. It was hypothesized by Taylor as early as 1925 [3] and was proven for many reactions, most clearly by Somorjai and co-workers [4]. These studies mainly concerned transition metal catalysts (most frequently platinum) and the most often used probe reaction was hydrogenation. Since copper is rarely considered to be an effective hydrogenation catalysts, this metal escaped studies of this kind. To fill this gap it seemed worth while to investigate the evolution of the active surface and also to collect information about the deactivation procedure in the Cu/Si02-cyclohexene-Dz(Hz) system. These phenomena were studied in static and flow reactors with several methods like desorption or hydrogenation off the residues from close to initial or the used catalyst, aging the catalyst by several runs with or without activation between the runs, using competitive reactions known to require very different surface characteristics [5]. Results of this investigation is detailed in the followings.
2. Experimental
2.1. Materials Cyclohexene was obtained via the dehydration of cyclohexanol. After distillation its purity was checked by the GC-MS method. It was found 99.9+% pure. In order to destroy possibly formed peroxides, it was passed through a column filled with freshly activated basic alumina (Camag) under inert (Ar) atmosphere before reactions in the flow system or filling it into a closed evacuated vial for further use in the static circulation reactor. Cyclopropane was an Aldrich product with a purity of 99+%. It was further purified through distillation in the vacuum system. Oxygenfree hydrogen was prepared with a Matheson 8326 generator, operating with a palladium membrane. The 6.36% Cu/silica gel (Cu/S-X, was prepared by an ion-exchange method described in the literature [6]. Details are as follows: silica gel (Strem, large pore, 12&230 mesh) was immersed in a tetramminecopper(I1) solution at pH 11. After 24 h the blue product
was washed thoroughly with water, dried at 393 K (24 h) and decomposed at 773 K in air (3 h). The catalyst was reduced in flowing hydrogen (20 cm’/min) at 673 K for 2 h and kept in a vacuum desiccator until use. The percentage of surface copper atoms (14.3%) was determined by N20 decomposition by the pulse method at 363 K [7]. After reduction 7% of the copper remained ionic in the form of Cus determined by temperature-programmed reduction
[81. Samples (50 mg) of the stored prereduced catalyst were activated in the circulation system under 20 kPa H2 at 573 K for 30 min or in the flow reactor under 20 cm3/min H2 flow for 1 h at 573 K. 2.2. Apparatus,
methodology
and analysis
2.2.1. Static circulation reactor The reactions were carried out in a conventional closed circulation reactor similar to which was described in Ref. [9]. The volume of the reactor was 69 cm3, and total volume of the system was 169.4 cm’. The reactor was heated with an air thermostat. The volume of the sampling capillary was 0.05 cm3 and the total sampling volume was 0.15 cm3. A Hewlett-Packard (HP) Model 5890 gaschromatograph equipped with a flame ionization detector (FID) was attached to the system. Data analysis was performed on an HP 3396A integrator. A 50 m long HP-1 capillary column was used for separating the reactants and the products. The column was operated in the isothermal mode (oven temperature 323 K) with helium as carrier gas (flow rate 1.3 cm3/min). The reactants (2.65 kPa of cyclohexene or 2.65 kPa of cyclopropane or a mixture of 2.65 kPa of cyclohexene and cyclopropane and 13.3 or 33.3 kPa of Hz) were premixed in the circulation part of the system before the reaction. (Cyclohexene was subjected to several freeze-thaw-evacuate cycles before preparing the mixture.) The reaction temperature was 443 K in most cases and in certain experiments carbonaceous material was deposited at 473 K. Basically two types of reactions were performed. The first measurements were run over a fresh sample of catalysts, while restart reactions were done over the used catalyst subjected to evacuation (20 min, 1 Pa, 443 K) before the reaction. For determining the quantity of weakly held residues 53.2 kPa of H2 was used. The amount of strongly held
A. F&i et al./Applied Catalysis A: General 166 (1998) 185-190
carbonaceous material was measured in form of CO2 after removing it through oxidation with 02 gas (13.3 kPa) at 673 K for 1 h. 2.2.2. Flow reactor Reactions were carried out over 50 mg of catalyst samples in Hz flow (15 cm3/min). The partial pressure of cyclohexene was 3.2 kPa. The usual temperature was 303 K. Fresh sample of catalyst (activated in 20 cm3/min H2 flow for 1 h at 573 K) was used initially. In restart reactions 3-l 1 h of Hz purge was applied at 303 K or ramping the temperature from 303 to 573 K (20 min at 303 K, 20 min at 373 K, 10 h at 423 K, 20 min at 573 K). Product accumulation was followed by gas chromatography as it is described in the previous paragraph.
3. Results 3.1. Deactivation behavior by restart reactions in the static circulation system At 443 K cyclohexene in the presence of 33.3 kPa H2 gave only cyclohexane with an induction period over a fresh sample of catalyst. In restart reactions with evacuating the system in-between, the disappearance of the induction period and the gradual hydrogenation activity increase could be observed. In the third restart reaction benzene was also detected. Interestingly, at 10 K higher even the first restart reaction produced benzene (over the fresh catalyst only hydrogenation occurred). Relevant data are summarized in Table 1. Table 1 Product accumulation
in aging experiments
Type of run
Fresh catalyst Restart (1) a Restart (2) Restart (3)
’ Repeated
runs over the catalysts
3.2. Deactivation behavior by restart reactions in the flow reactor At 303 K both benzene and cyclohexane formation were observed over the fresh catalyst. The sample was deactivated in 3 h. In the restart reaction after a 3 h hydrogen purge, steady-state activities have been attained in about an hour both in dehydrogenation and hydrogenation. Moreover, the steady-state hydrogenation rate was about an order of magnitude higher than it was over the initial catalyst in its most active state (2 min sampling). Benzene formation was at similar level as over the initial catalyst in its most active state (2 min sampling). Relevant data are summarked in Table 2. After 2 h of reaction the reactor was purged with hydrogen with a temperature program (20 min at 303 K, 20 min at 373 K, 10 h at 423 K and 20 min at 573 K) and at certain time intervals the effluent gas was analyzed. No desorbed product was found up to 423 K. At 423 K cyclohexane and 573 K benzene (an order of magnitude less than cyclohexane at 423 K) were detected. After the hydrogen purge, a new reaction was started and very similar hydrogenation behavior was found as over the initial catalyst. 3.3. The catalyst su$ace studied by the competitive hydrogenation of cyclopropane and cyclohexene in the static system At 443 K cyclopropane ring-opening product over Ring opening was found propane. After removing
in the static system (2.65 kPa cyclohexene
Products
Benzene Cyclohexane Benzene Cyclohexane Benzene Cyclohexane Benzene Cyclohexane with evacuation
187
gives propane as the sole a fresh sample of catalyst. to be zero order in cyclothe reaction mixture by
+ 33.3 kPa H2, 443 K, 50 mg Cu/Si02)
Concentration/mol% 5 min
15 min
30 min
35 min
0 0 0 0.2 0 0.3 0 0.7
0 2.9 0 0.3 0 0.4 0 5.4
0 39.6 0 1.4 0 7.7 0.02 19.1
0 57.6 0 3.2 0 12.3 0.03 24.5
(20 min, 1 Pa, 443 K) in-between.
188
A. Fhi et al. /Applied
Catalysis A. General
Table 2 Product distribution in aging experiments in the flow system (3.2 kPa cyclohexene, 15 cm3/min Hz flow, 303 K, 50mg Cu/SiOz) Time/min
2 18.5 35 51.5 68 84.5 101 134 a Fresh b Used ’ Used at 373
Benzene/mol% 2b
3’
1”
2b
3’
0.03 0.02 0.01 0.01 Traces
0 0 Traces 0.02 0.02 0.02 0.02 -
Traces Traces 0 0 0 0 0 -
1.2 0.6 0.4 0.3 0.2 0.02
0.03 0.02 1.90 11.4 12.1 12.2 12.0 -
1.1 0.3 0.2 0.1 0.04 0.03 0.01
Traces
evacuating the cyclopropane + cyclohexene + H2 mixture, a restart reaction with only cyclohexene and Hz gave the same products (benzene and cyclohexane) as in the previous reaction, however, some deactivation did take place. After removing surface contaminants from a catalyst sample already used in cyclohexene hydrogenation (443 K) by oxidation (673 K) and rereducing it (573 K), the sample became active again in ring opening. Moreover, its activity increased. The transformation remained close to zero order in cyclopropane. Relevant data are summarized in Table 3 In order to gain information on the chemical properties of surface species responsible for the hydrogenative transformations of cyclohexene, a replacement reaction was executed. In it, the surface was deliberately contaminated by depositing carbonaceous species via introducing cyclohexene-HZ mixture onto the catalyst for a prolonged time at 443 K (for the accurate procedure, see the caption of Table 4) and then running cyclopropane hydrogenation on this surface. Although the cyclopropane-hydrogen mixture did not give propane (i.e. desorbed ring-opening product was not found), it induced the desorption of surface species and benzene, cyclohexane (hydrogen was present) and cyclohexene emerged (Table 4). Desorption was not complete, however, since cyclopropane did not undergo ring opening even after 40 min.
Cyclohexane/mol%
la
catalyst. catalyst. catalyst after 11 h purge with H2 (20 min at 303 K, 20 min K. 10 h at 423 K and 20 min at 573 K.
evacuation at 443 K, a new mixture reacted with about the same rate, that is hardly any deactivation took place. The reaction order remained the same as well. When an equimolar mixture of cyclopropane and cyclohexene (2.67 kPa each) was introduced into the reactor in the presence of hydrogen (13.3 kF’a), the cyclopropane molecule did not react at all. However, cyclohexene underwent dehydrogenation (producing benzene) as well as hydrogenation (producing cyclohexane). After Table 3 Product distribution in various types of restart reactions 443 K) in the static system Type of run
Products
Fresh Restart (1) Restart (2)
Propane Propane Propane Benzene Cyclohexane Benzene Cyclohexane Propane
Restart (3) Restart (4)
Cu/SiOZ
166 (1998) 185-190
(50 mg) (cyclopropane:
2.65 kPa, cyclohexene:
2.65 kPa, Hz: 13.3 kPa,
Composition/mol% 5 min
15 min
0.3 0.3 0 0.2 4.9 Traces 1.9 0.6
1.o 1.1 0 1.8 15.9 0.9 6.9 1.8
30 min
35 min
40 min
2.0 2.2 0 4.9 28.8 3.2 17.1 3.6
2.3 2.5 0 5.6 32.1 3.8 19.2
2.5 2.7 0 6.9 35.3 4.5 21.8 4.6
Fresh: cyclopropane + Hz, fresh catalyst. Restart (I): cyclopropane + HZ, used catalyst in the previous reaction. Restart (2): cyclopropane + cyclohexene + Ha, used catalyst in the previous reactions. Restart (3): cyclohexene + H2, used catalyst in the previous reactions. Restart (4): cyclopropane + HZ, the used catalyst was regenerated (oxidation: 13.3 kPa 0 *, 673 K, 573 K, 1 h).
1h, then reduction
26.7 kPa Hz.
189
A. F&i et al./Applied CatalysisA: General 166 (1998) 185-190
Table 4 Accumulation of desorbed compounds by a cyclopropane + HZ mixture (2.65 kF’a cyclopropane + 13.3 kPa HZ, 443 K) following the deposition of carbonaceous material from cyclohexene hydrogenation (deposition: 2.65 kPa cyclohexene + 13.3 kPa HZ. 1 h. 443 K, and repeated one more time) over Cu/SiOZ in the static system Type of reaction
Products
Specific quantity/10-3/mol/g 5 min
15 min
30 min
35 min
40 min
Replacement
Benzene Cyclohexane Cyxlohexene
0.35 2.66 5.86
1.93 9.48 13.0
3.79 15.3 13.7
4.09 11.6 14.5
4.18 19.8 14.1
3.4. Direct observation of carbonaceous
deposits in
the static system
At 473 K carbonaceous material was deposited onto a fresh sample of catalyst from cyclohexene + H2 mixture. After evacuation, 53.2 kPa Hz was introduced into the reactor at the same temperature. After 30 min the gas composition was analyzed and benzene, cyclohexane and even cyclohexene could be observed in 2 : 4: 1 ratio. The concentration of the weakly held residues was 3.3 x lo-’ mmol/mg. After evacuation, that is after the removal of the Hzdesorbed products, the strongly held residues were removed by oxidation in the form of COZ. Its concentration was 6.7 x 10e4 mmol/mg.
4. Discussion Restart reactions either in the static system or in the flow reactor clearly showed that the real catalyst of the hydrogenative transformations of cyclohexene is not the freshly reduced copper catalyst. The reactants start to create the actual active surface at the first contact. The induction period observed over the fresh catalyst indicates that the active surface should be very different from the reduced Cu/SiO,. In restart reactions gradual activation was registered in the static system and steady-state activity was attained in the flow reactor. The induction period disappeared in restart reaction in the static system, which means that the reaction mixture now met the already transformed surface. One may suspect two main reasons for the change of the catalytic surface, such as (i) surface reconstruction and/or (ii) deposition of selectivity and activity determining or at least influencing surface
-
species like carbonaceous deposits with a variety of hydrogen content. The first possibility cannot be ruled out, moreover, it probably contributed to the observed effects, the second possibility is thought to be the major factor on the following grounds: - Cyclopropane known to require clean metal surface for its hydrogenation did not react either on the catalyst already used for the hydrogenative transformations of cyclohexene or in competitive hydrogenation. After oxidative-reductive regeneration of the catalyst the cyclopropane molecule underwent hydrogenative ring opening with similar kinetic characteristics as over the clean copper surface. - Upon investigating the surface of the used catalyst, a substantial amount of hydrogen-rich carbonaceous residues could be hydrogenated off or displaced from the surface under mild conditions. They were product precursors and unreacted cyclohexene. Of course, over the surface they were product- and reactant-like surface species. Their surface composition was not exactly like that of the gas phase. Coke formation also takes place. However, its extent is not serious under these mild conditions, therefore their deactivation effect was minor compared to the activating behavior of the hydrogen-rich species.
5. Conclusion With the help of relatively simple chemical tools we were able to show that hydrogen-rich residues have major role in forming the catalytically active surface in the hydrogenative transformations of cyclohexene over Cu/SiO*. To the best of our knowledge this is the
190
A. Fdsi et al. /Applied
Catalysis A: General 166 (1998) 185-190
first attempt to gain insight into the evolution of a copper surface in hydrocarbon hydrogenation reaction.
Acknowledgements This work was sponsored by the National Science Foundation of Hungary through grants OTKA TO14315 and T016109. The financial help is greatly appreciated.
References [l] A. F&i, I. Pblinko, T. Katona, M. Bartok, J. Catal. 167 (1997) 215. [2] E Zaera, A.J. Gellman, GA. Somorjai, Act. Chem. Res. 19 (1986) 24; TX Beebe, J.T. Yates, J. Am. Chem. Sot. 108 (1986) 663; J.M. Cogen, W.F. Maier, J. Am. Chem. Sot. 108 (1986) 7752; F. Garin, 0. Maire, S. Zyade, M. Zauwen, A. Frennet, P. Zielinski, J. Mol. Catal. 58 (1990) 185; J. Barbier, E. Churin, P. Marecot, .I. Catal. 126 (1990) 228; D. Espinat, D. Freund, H. Dexpert, G. Martino, .I. Catal. 126 (1990) 496;
F.J. Rivera-Latas, R.A. Dalla Betta, M. Boudart, AIChe .I. 38 (1992) 771; A. Erdohelyi, J. Cserenyi, E Solymosi, J. Catal. 141 (1993) 287; C.A. Querini, S.C. Fung, J. Catal. 141 (1993) 389; G.A. Somorjai, in: Introduction to surface chemistry and catalysis, pp. 505-507, Wiley, New York, Chichester, Brisbane, Toronto, Singapore, 1994; J. Houzvicka, R. Pestman, V. Ponec, Catal. Lett. 30 (1995) 289. [31 H.S. Taylor, Proc. Roy. Sot. (London) A108 (1925) 105. 141 G.A. Somorjai, in Proceedings, 8th International Congress on Catalysis, Berlin, 1984, Vol.1, p. 113, Dechema, Frankfurt am-Main, 1984; G.A. Somorjai, React. Kinet. Catal. Lett. 35 (1987) 37; G.A. Somorjai, J. Phys. Chem. 94 (1990) 1013; G.A. Somorjai, Catal. Lett. 9 (1991) 311; G.A. Somorjai, Catal. Lett. 12 (1992) 17. [51 I. Palink& E Notheisz, M. Bartok, Catal. Lett. 1 (1988) 127; I. Palinko, F. Notheisz, M. Bartok, Stud. Surf. Sci. Catal. 48 (1989) 729. [61 H. Kobayashi, N. Takezawa, C. Minochi, K. Takahashi, Chem. Lett. (1980) 1197. ]71 J.W. Evans, MS. Wainwright, A.J. Bridgewater, D.J. Young, Appl. Catal. 7 (1983)75; B. Denise, R.P.A. Sneeden, B. Beguin, 0. Cheriti, Appl. Catal. 30 (1987) 353. PI A. Molt&, I. Palinko, M. Bartok, J. Catal. 114 (1988) 478; I. Palinko, A. Molt&, J.T. Kiss, M. Bartok, J. Catal. 121 (1990) 396. [91 M. Bartok, F. Notheisz, A.G. Zsigmond, G.V. Smith, J. Catal. 100 (1986) 39.