Ind. Eng. Chem. Res. 1990,29, 1143-1146
1143
Nature of Active Coke in the Oxydehydrogenation of Ethylbenzene to Styrene Luis E. Cadiis,* Osvaldo F. Gorriz, and Juan B. Rivarola Instituto d e Inuestigaciones en Tecnologia Quimica, INTEQUI, Casilla de Correo 290, 5700 Sun Luis, Argentina
T h e carbonaceous material formed on A1203in the course of the oxydehydrogenation reaction of ethylbenzene t o styrene has been considered as the true catalytically active substance. The results of the various analyses made (burn-off experiments; temperature-programmed oxidation, ammonia desorption, ESCA, EPR and SIMS Studies) indicate that the groups containing oxygen in the active coke are quinone/hydroquinone and aroxyl/phenol.
Introduction The carbonaceous material formed on A1203 in the course of the oxydehydrogenation reaction of ethylbenzene to styrene has been considered as the true catalytically active substance (Fiedorow et al., 1981; Alkhazov and Lisovskii, 1976; Emig and Hofmann, 1983; Cadds et al., 1988). Research carried out by Cadds et al. (1988) found that the carbonaceous species, called "active coke", contained oxygen. The catalytic activity of pyropolymers in oxydehydrogenation has been reported by Manassen and Wallach (1965), Degannes and Ruthven (1979), and Iwasaw8 et al. (1972, 1973). Cadtis et al. (1989) have found that the carbonaceous species initially formed on the A1203might be attacked by 0, to produce active coke. These results compare with what Boehm and Knozinger (1983) reported about oxygen reacting with carbon a t high temperature to yield functional groups such as carbonyl, lactone, carboxyphenol (hydroxyl), (hydro) quinone, and aldehyde groups. The reaction temperature a t which the present investigation was carried out is within the range of the maximum rate of formation of those groups (Donnet, 1970). In the present paper we are reporting on possible types of functional groups present in active coke in the oxydehydrogenation of ethylbenzene to styrene. This research was carried out by using various analysis techniques, which resulted in the obtention of more detailed information on such functional groups. Experimental Section The active coke formed on an alumina under the reaction conditions already described by Cadds et al. (1988) was submitted to various analysis techniques. The carbonaceous deposit on the alumina was determined and analyzed by burning it with air in an isothermal integral reactor operated a t atmospheric pressure and at 450 "C temperature. Analysis of the gaseous products was carried out by gaseous chromatography in relation to on-stream time. CO, COz, and H 2 0 were detected in the burning reaction. The amount of each of them was determined by integrating their areas in relation to on-stream time. Thus, the amount of active coke and its C / H ratio were determined. In addition, an attempt was made at identifying the active coke by means of the combination of several analysis techniques. Aiming at determining the distribution of elements in relation to penetration depth and the chemical structure of the surface compounds, the differential analytical method, SIMS, was used. This method consists in sputtering material from the surface by means of a primary ion beam and further mass spectrometry analysis of the secondary 0888-5885/90/2629-1143$02.50/0
ions obtained from this sputtering process. The active coke was analyzed by ESCA in order to establish the possible chemical shift due mainly to valence change or to oxidation of the atom analyzed. The group to which the carbon atom is bound would be inferred from the information thus obtained. The depth information lies in the range 1-10 nm. The formation of paramagnetic centers in active coke has been investigated by EPR. A signal has been obtained for g = 2.0031, corresponding to a free radical. In the same way, it has been stated that those free radicals might be responsible for the coke catalytic activity (Cadds et al., 1988) in the oxydehydrogenation of ethylbenzene to styrene. EPR measurements were obtained a t room temperature a t a Klystron frequency of 9.7 GHz and at 100kHz magnetic field modulation. For a standard estimate of the unpaired spin concentration in the active coke sample, an ultramarine sample having a spin concentration of 1 X 1015was used. In order to determine the degree of condensation of active coke as well as its homogeneity, a thermal-programmed oxidation (TPO) analysis was conducted.
Results and Discussion The results obtained from burning the active coke with air at 500 "C are shown in Figures 1 and 2. Figure 1 corresponds to burning of active coke after 5 h on-stream time whereas Figure 2 corresponds to 30 h of burning an active coke. In agreement with the above reported results, the amount of coke increases with on-stream time and the degree of condensation of the coke changes with on-stream time. The active coke after 5 h of on-stream time contains 5 wt 7% C, and its C/H ratio is 0.54, whereas the active coke after 30 h of on-stream time contains 12.7 w t % C and its C/H ratio is 0.80. Thus, it was found that both the amount of active coke and its degree of condensation depend on the formation time. In Cadds et ai. (1988), it was shown that the A120, surface activity has a marked effect on the amount and condensation of active coke. The chemical composition of the active coke with onstream time was studied by Cad& et al. (1989), and results showed that initially large molecules are formed on the A1203surface and that, by losing hydrogen by condensation, those molecules form a hydrogen-poor coke. The active coke burn-off curves show that, in the first minutes of burning, the maximum CO, formation rate is reached. Other reports have shown similar results (Schraut et al., 1987). Thus, the coke heterogeneity is made evident, and as shown further by TPO, there is a hydrogen-rich coke that burns off more readily than another hydrogen0 1990 American Chemical Society
1144 Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 C
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-
.--.--__-___
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0
60
90
120 time on stream I m i n ,
Figure 1. Products obtained from burning active coke after 5 h of on-stream time. Temperature, OC
Figure 4. Thermogram by thermal-programmed oxidation of active coke at 30 h of on-stream time, performed on DTA equipment at a 20 OC/min heating rate.
time on stream lminj
Figure 2. Products obtained from burning active coke after 30 h of on-stream time.
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200 300 Loo 500 600 temperature, O C Figure 3. Thermogram by thermal-programmed oxidation of active coke of 5 h of on-stream time, performed on DTA equipment at a 20 "C/min heating rate. 100
poor coke (more condensed). The 5-h coke burn-off did not show CO production, but CO was detected in the 30-h coke burn-off. This can be accounted for by the large amount of coke present in the 30-h burn-off sample. Figures 3 and 4 show the thermograms obtained by TPO for the active coke after 5 and 30 h of on-stream time, respectively. In both cases there are two peaks at AT/ W the first for a temperature of 250 "C and the second for 495 "C. The existence of two peaks confirms the abovementioned coke heterogeneity. The 250 "C peaks corresponds to a hydrogen-rich species, whereas the 495 "C peak corresponds to a condensed species that is hydrogen poor. From comparison of both figures, it follows that the coke
Table I. ESCA Results of Catalysts Used in This Study binding energy/ sample signal corresponding to eV 286.3 fresh A1203 c 1s 531.6 0 1s 283.8 coked A1203 c 1s 285.1 c 1s 0 1s 531.0 533.7 0 1s
obtained after 5 h of on-stream is composed of a higher portion of hydrogen-rich coke than the one obtained after 30 h of on-stream time. This indicates that coke composition clearly depends on the time required for its formation. The samples of fresh A1203 and of A 1 2 0 3 with active coke from 30 h of on-stream time were analyzed by ESCA. The results are shown in Table I. Signals corresponding to A1 2p, A1 2s, 0 Is, and 0 Auger were obtained as well as those of C 1s and C Auger from fresh A1203. The C signal shown is typical of samples that have been in contact with air or that have been manipulated. The C 1s peak for the fresh A1203has a binding energy of 286.3 eV. In the same way, the 0 Is peak for the fresh A1203 has a binding energy of 531.6 eV. Table I shows the C 1s peak in detail for the coked A1203. Two peaks can be seen that have binding energies of 283.8 and 285.1 eV. From this it may be inferred that one of the two signals corresponds to active coke. Taking into account the characteristic core level binding energies listed by Clarck (1977), it was found-in agreement with Schraut et al. (1987)-that the 287.8-eV binding energy would correspond to a carbon in the active coke bound in a carbonyl group. From this result it might be drawn that the oxygen would undergo a similar shift, which was confirmed. The results are shown in Table I. This table shows two peaks for the 0 Is signal at 531.0- and 533.7-eV binding energies. According to the mentioned listing, Clarck (1977), the 533.7-eV binding energy may originate in the oxygen from a carbonyl group. Thus, from ESCA results, it might be stated that there would be functional groups containing oxygen in the active coke. The active coke was investigated by ammonia adsorption on its surface. The results showed that the ammonia was not adsorbed on the surface, which confirmed that carboxyl
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1145
r
m Figure 6. SIMS spectrum of anthraquinone. H
Figure 5. EPR spectrum from two samples of active coke with different degrees of oxidation. The upper signal shows the signal from the oxidized active coke, and the lower signal shows the signal from the reduced active coke.
and lactone functional groups are not present (Ekaterinina et al., 1980), at least in appreciable amounts. Paramagnetic centers were detected in the active coke formed on the A1203by means of EPR. Those paramagnetic centers were attributed to the possibility of being the active sites of the oxydehydrogenation of ethylbenzene to styrene (Fiedorow et al., 1981; Cadiis et al., 1988). The signal obtained by EPR corresponds to a free radical with a value of g = 2.0031. When the fresh A1203sample was analyzed, no EPR signal was observed. The same result was obtained for the totally burned off A1203 sample. In CadGs et al. (1988), the variation in the concentration of spins/milligram of the active coke in relation to on-stream time and to alumina surface acidity was observed by EPR. Now, a thickening of the line width can be expected if the spins/milligram concentration is decreased by reduction of a coked A1203 from 30 h on-stream time with ethylbenzene. Figure 5 shows the EPR spectra for two samples of active coke with different degrees of oxidation. The upper spectrum shows the signal obtained from the analysis of the active coke formed on the A1203 from 30 h of on-stream time, whereas the lower spectrum shows the signal obtained from an active coke from 30 h of onstream time that has undergone a reduction in ethylbenzene/nitrogen stream of 450 OC for 10 h. The difference in line width between the two signals is 1.1G (active coke, 4.2 G; reduced active coke, 5.3 G). Thus, no marked reduction was observed in the concentration of paramagnetic centers when the active coke surface was reduced. According to what has been already put forward and to existing published information, it might be inferred that this is a quinonoid-type structure. Now, as an organic quinone does not yield an EPR signal, for which there must be an unpaired electron, highly reactive species with unpaired electrons might be expected, which would be located in operating sites adequate for the mechanism proposed by Emig and Hofmann (1983) and Cadds et al. (1988). A reoxidation of the ethylbenzene-reduced active coke allows us to recover, for the active coke, an EPR signal having a line width equal to that of the unreduced active coke. Taking into account the possible functional groups that coke oxidation may yield under the reaction conditions already mentioned in the Introduction, it can be concluded that the groups containing oxygen in the active coke are quinone/hydroquinone- or aroxyl/phenol-type complexes. The results of the various analyses made and the already mentioned reports lead to the notion that there are
Figure 7. SIMS spectrum of fresh A1203.
m Figure 8. SIMS spectrum of coked A1203. Table 11. Explanation of SIMS Spectra mass alumina anthraauinone alumina with coke 12 C+ C+ 13 CH+ CH+ 14 CH2+ CH2+ 23 Na+ Na+ Na+ 24 CZ+ c2+ Al+ and/or CzH3' 21 Al+ CH+ 28 CiH) or CO+ C2H4+or CO+ 39 K+ K+ K+
functional C=O groups in the active coke. By SIMS, it was attempted to obtain information about the secondary ions derived from this type of structure. An anthraquinone sample was analyzed as the reference substance. Fresh A1203 and coked A1203 samples were also analyzed by SIMS. The corresponding positive ion spectra are shown in Figures 6-8. Fresh A1203and coked A1203spectra, Figures 7 and 8, gave peaks whose origin may be attributed to impurities. Peaks were observed at 23 (Na+),39 (K+), 52 (Cr'), 56 (Fe+),and 63 (Cu+),and, in addition, at 40 (Ar') from the primary Ar+ current. The anthraquinone showed the secondary ion spectra of Figure 6. Taking into account that the anthraquinone sample analyzed was high purity and that it only contains C, H, and 0, the resulting peaks can be explained, according to the intensities observed, as a combination of those elements in Table 11.
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Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990
Disregarding the impurities, which are present in very small amounts, there is a clear similitude between the anthraquinone and the coked alumina spectra. This similitude shows that the active coke may have a molecular structure similar to that of anthraquinone.
Conclusions None of the techniques used individually offer precise information on the research performed. Considering the results altogether, it can be concluded that there are functional groups containing oxygen in the active coke structure. In addition, according to Boehm and Knozinger (1983) and Donnet (1970), the functional groups mentioned in the Introduction would be formed under the reaction conditions of this investigation. The suggestion of a redox mechanism (Cadds et al., 1988) and the evidence given by EPR results indicate that, among the functional groups mentioned, only the quinone/hydroquinone and aroxyl/phenol can play that kind of role. In the same sense, the ammonia desorption results allow us to discard the presence of the carboxyl and ladone functional groups, at least in appreciable amounts. It can be concluded that the groups containing oxygen in the active coke are quinone/hydroquinone- or aroxyl/ phenol-type complexes. Even though the active coke is heterogeneous, from it C/H/O content its structure can be conceived as consisting of a system of condensed aromatic rings. The functional groups might form part of the condensed structure as delocalized free radicals, showing a great variety of resonant structures in equilibrium. The evidence given by SIMS results indicates that the active coke may have a molecular structure similar to that of anthraquinone. A representative form would be
catalytic centers depend on on-stream time. In the already mentioned report, the effect of alumina surface acidity on the degree of condensation of the active coke was evidenced.
Acknowledgment We thank the CONICET from Argentina and the Universidad Nacional de San Luis for financial aid. Registry No. A1203, 1344-28-1;ethylbenzene, 100-41-4;styrene, 100-42-5.
Literature Cited Alkhazov, T. G.; Lisovskii, A. E. Importance of Condensation Products in Oxidative Ethylbenzene Dehydrogenation over an Aluminum Oxide Catalyst. Kinet. Katal. 1976, 17,434-39. Boehm, H. P.; Knozinger, H. In Catalysis Science and Technology; Anderson, J. R., Boudard, M., Eds.; Springer Verlag: Berlin, 1983; VOl. 4, p 39. Cadiis, L. E.; Arriia, L. A,; Gorriz, 0. F.; Rivarola, J. B. Action of Activated Coke as a Catalyst: Oxydehydrogenation of Ethylbenzene to Styrene. Ind. Eng. Chem. Res. 1988, 27, 2241-46. Cadiis, L. E.; Gorriz, 0. F.; Rivarola, J. B. Active Coke formation in Oxydehydrogenation of Ethylbenzene to Styrene. React. Kinet. Catal. Lett. 1989, in press. Clarck, D. T. Handbook of X-ray and Ultrauiolet Photoelectron Spectroscopy; Heyden: London, 1977. Degannes, P. N.; Ruthven, D. M. The Oxidative Dehydrogenation of Ethylbenzene to Styrene. Can. J.Chem. Eng. 1979,57,627-30. Donnet, J. B. Surface Chemical Groups (on carbon, silica and titanium dioxide). Bull. SOC.Chim. Fr. 1970, 3353-66. Ekaterinina, L. N.; Khrenkova, T. M.; Motovilova, L. V.; Dolmatova, A. G.; Goldenko, N. L.; Yashina, T. N.; Zharova, M. N. Effect of Forms of Carbon Hydrogen and Oxygen Bonding in the Composition of Coals on their Tendency to Undergo Reduction. Khim. Tuerd. Top!. (Moscow) 1980, 14, 45-51. Emig, G.; Hofmann, T. Action of Zirconium Phosphate as a Catalyst for the Oxydehydrogenation of Ethylbenzene to Styrene. J. Catal. 1983,84, 15.
Fiedorow, R.; Przystajko, W.; Sopa, M.; Dalla Lana, J. G. The Nature and Catalytic Influence of Coke Formed on Alumina: Oxidative Dehydrogenation of Ethylbenzene. J. Catal. 1981, 68, 33. Iwasawa, Y.; Soma, M.; Onishi, T.; Tamaru, K. Catalytic activities of Polynaphthoquinone, Containing Metal Halides. Dehydrogenation of Formic Acid, Cyclohexene, Ammonia, and Alcohols, and Isomerization of Butene. J. Chem. SOC.,Faraday Trans. 1 1972, 68, 1617-25.
Formation of active coke was described by Cadiis et al. (1989) as follows: ethylbenzene is adsorbed on the alumina acid center, which facilitates the formation of large molecules. These molecules are partially dehydrogenated and are further attacked by oxygen to produce COz and active coke, while they continue losing hydrogen by condensation until reaching a condensed species. In addition, the burn-off and TPO results together with the EPR results show that the degree of condensation and the number of
Iwasawa, Y.; Nobe, H.; Ogasawara,S. Reaction Mechanism for Styrene Synthesis over Polynaphthoquinone. J.Catal. 1973,31,444. Manassen, J.; Wallach, J. Organic Polymers. Correlation between their Structure and Catalytic Activity in Heterogeneous System. I. Pyrolyzed Polyacrylonitrile and Poly(cyanoacety1ene). J. Am. Chem. SOC.1965,87, 2671-17. Schraut, A.; Emig, G.; Sockel, H. G. Composition and Structure of Active Coke in the Oxydehydrogenation of Ethylbenzene. Appl. Catal. 1987, 29, 311-26.
Received for review June 22, 1989 Accepted January 12, 1990
