Temperature-Programmed Oxidation of Coke Deposited by 1-Octene

Trends, which are observed in the combustion rate-determining steps, indicate increased stability of the polyaromatic coke. Major differences in the T...
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Energy & Fuels 1999, 13, 888-894

Temperature-Programmed Oxidation of Coke Deposited by 1-Octene on Cracking Catalysts Chao’en Li and Trevor C. Brown* Division of Chemistry, University of New England, Armidale, NSW, Australia Received December 9, 1998

The properties of coke deposited by 1-octene on fresh cracking catalysts have been investigated by analyzing CO, CO2, and H2O evolution during temperature-programmed oxidation (TPO). Of particular significance in the analysis is that the combustion mechanism dependence of the CO and CO2 profiles were taken into account. Catalysts were laboratory coked at temperatures ranging from 200 to 600 °C under otherwise identical conditions. Two types of coke are identified, with the quantity of saturated coke decreasing and polyaromatic coke increasing as the coking temperature is raised. Trends, which are observed in the combustion rate-determining steps, indicate increased stability of the polyaromatic coke. Major differences in the TPO profiles of an industrial spent cracking catalyst, when compared with the laboratory-coked samples, suggest differences in the propensity for oxide formation. Reaction orders with respect to oxygen partial pressures indicate that the intrinsic rate of carbon monoxide evolution is independent of oxygen while carbon dioxide formation shows a more complicated dependence. It follows from the isokinetic temperature for the evolution of carbon dioxide from all substrates that the characteristic vibrational frequency is 674 ( 31 cm-1, which corresponds to the bending motion of CO2.

Introduction Coke deposition is generally inevitable when hydrocarbon reactions are catalyzed by solid substrates. Coke properties, such as amount, type, and location, are affected by feed type, substrate type, and reaction conditions. Temperature-programmed oxidation (TPO) combined with either evolved-gas or gravimetric analysis is often employed to investigate these coke properties. Such details are inferred from the area under TPO profiles and the position of TPO peaks. Additional factors, which can affect the shape of these profiles, are coke particle size and morphology1 and the temperature dependence of the coke combustion mechanism.2,3 Deciding upon the governing rate-determining steps, when analyzing TPO profiles, is particularly important at high heating rates and low oxygen partial pressures. In this paper TPO with evolved CO, CO2, and H2O of a series of 1-octene coked cracking catalysts are investigated. Alkenes are effective precursors to coke deposition,4 while coking reactions of 1-octene over HY and H-ZSM-5 zeolites have been reported.5-7 Information on * Author to whom correspondence should be addressed. Telephone: +61 2 6773 2872. Fax: +61 2 6773 3268. E-mail: tbrown3@ metz.une.edu.au. (1) Querini, C. A.; Fung, S. C. Appl. Catal., A: General 1994, 117, 53-74. (2) Le Minh, C.; Jones, R. A.; Craven, I. E.; Brown, T. C. Energy Fuels 1997, 11, 463-469. (3) Li, C.; Le Minh, C.; Brown, T. C. J. Catal. 1998, 178, 275-283. (4) Wolf, E. E.; Alfani, F. Catal. Rev.sSci. Eng. 1982, 24, 329-371. (5) Abbot, J.; Wojciechowski, B. W. J. Catal. 1987, 108, 346-355. (6) Smirniotis, P. G.; Ruckenstein, E. Ind. Eng. Chem. Res. 1994, 33, 800-813. (7) Anderson, J. R.; Chang, Y.-F.; Western, R. J. Appl. Catal. 1991, 75, 87-91.

the temperature dependence of the amount of polyaromatic and saturated coke is determined by simulating the TPO carbon oxide profiles using a single-site kinetic analysis over a range of oxidation conditions.3 The kinetic model employs the following five-step carbon combustion mechanism:

*C + O2 S *C(O2)

(1)

*C(O2) f *C(O) + CO

(2)

*C(O) f CO

(3)

*C(O2)

*C + O2 98 CO2 *C(O)

*C + O2 98 CO2

(4) (5)

Here, *C is a free carbon site, *C(O2) is a dioxygen surface complex, and *C(O) is a stable oxide surface species. This mechanism indicates two pathways for both CO and CO2 evolution. Reaction 1 is the reversible formation of undissociated surface oxide complexes, and for TPO a steady-state concentration of *C(O2) is assumed. Reactions 2 and 3 are unimolecular pathways to CO evolution, and have been shown to be fundamental to the mechanism of carbon combustion.8 Reactions 4 and 5 involve bimolecular *C/O2 collisions to CO2 formation. Note that a requirement for CO2 evolution is either an undissociated or dissociated oxide species (8) Lear, A. E.; Brown, T. C.; Haynes, B. S. Twenty-Third Symposium (International) on Combustion. Orleans, France: The Combustion Institute: Pittsburgh, 1991; pp 1191-1198.

10.1021/ef980265n CCC: $18.00 © 1999 American Chemical Society Published on Web 05/01/1999

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in the vicinity of the active carbon site. Reaction 5 is similar to that postulated by Ahmed et al.,9 while reaction 4 has been previously proposed by Lear et al.8 Others have concluded variations on reactions 1-5 for the carbon-oxygen mechanism (e.g., refs 10,11), and these could also be incorporated in a simulation of the TPO data. However, the chosen steps provide the most accurate simulation with the least number of optimized parameters. Arrhenius rate parameters and orders with respect to oxygen and carbon are determined by simulating TPO data recorded at different heating rates and different partial pressures of O2. Experimental Section The equipment and technique used to deposit coke on cracking catalysts with single component feeds has been described previously.2,12 Briefly, 500 mg of the cracking catalyst is coked, after calcination to 800 °C in an atmosphere of flowing N2, by means of a controlled injection (1 mL/min) of 1 mL of 1-octene (Aldrich) at set temperatures in the range 200-600 °C. The deactivated substrate is then removed and divided into 100 mg batches for TPO, N2 BET surface area and elemental analysis. The apparatus constructed to continuously monitor CO, CO2, and H2O evolution during oxidation has also been previously described in detail.2,3 Carbon oxides are measured using dual flame-ionization detectors (FID) after conversion to methane over a ruthenium catalyst. One FID monitors the combined CO and CO2 signal while the other detects only CO as CO2 is scrubbed from the line by passing through an ascarite packed bed. Both signals are calibrated using CO and CO2 standards (119 ( 2 ppm CO in N2 and 519 ( 10 ppm CO2 in N2). Maximum CO and CO2 pressures in TPO experiments are ca. 1000 ppm. Water evolution is analyzed using a Model 95A Super Dry Hygrometer (Alpha Moisture Systems) which is sensitive to the ambient dew point over the range 0 to -80 °C. This is calibrated by decomposing accurately weighed quantities of Ca(OH)2 (>98% BDH). All evolved gas partial pressures and reactor temperatures are transferred to a personal computer by means of Strawberry Tree’s data acquisition hardware and software. Conversion of partial pressures pFID (ppm) to carbon-oxide and water evolution rates (µmol (g of catalyst)-1 °C-1) involves consideration of the molar flow rate n3 mol min-1 and the weight of catalyst wcat. Further calibration is required to take into account the reaction temperature and effective pumping rate S.13,14 The rate of CO, CO2, and H2O evolution is:

rate[µmol (g of cat.)-1 °C-1] ) pFID(ppm) × ptotal(atm) × n3 × S wcat × β × R × T R is the gas constant and ptotal is the pressure in the vicinity of the coked catalyst, determined to be 1.2 atm for all experiments. Effective pumping speeds were estimated by adjusting S so that total carbon and hydrogen equaled that measured using a Carlo Erba CHNS-O elemental analyzer. (9) Ahmed, S.; Back, M. H.; Roscoe, J. M. Combust. Flame 1987, 70, 1-16. (10) Walker, P. L.; Taylor, R. L.; Ranish, J. M. Carbon 1991, 29, 411-421. (11) Du, Z.; Sarofim, A. F.; Longwell, J. P. Energy Fuels 1991, 5, 214-221. (12) Le Minh, C.; Li, C.; Brown, T. C. Catalyst Deactivation 1997; Bartholomew, C. H., Fuentes, G. A., Eds.; Amsterdam: Elsevier Science, 1997; pp 383-390. (13) Redhead, P. A. Vacuum 1962, 12, 203-211. (14) Thomas, J. M.; Thomas, W. J. Principles and Practice of Heterogeneous Catalysis; Weinheim: VCH, 1997.

Figure 1. Experimental evolution rates of CO, CO2, and H2O during TPO, in 0.939% O2 with heating rate of 10 °C/min, of a cracking catalyst coked with 1-octene (coking temperature ) 500 °C). Three heating rates (2, 5, and 10 °C min-1) and three oxygen partial pressures (0.939 ( 0.002%, 1.11 ( 0.01%, and 5.0 ( 0.2%, all dilute in N2) were investigated in the TPO experiments. Gases were supplied and analyzed by BOC Gases, while the Ampol Refinery, Lytton, Queensland, Australia, supplied both the fresh and spent cracking catalysts. The fresh catalyst has a surface area of 211 ( 5 m2 g-1. These catalysts consist of a USY zeolite in an active alumina, silica-alumina, clay, and rare-earth oxide matrix.

Experimental Results In Figure 1 TPO profiles of evolved CO, CO2, and H2O are shown for the cracking catalyst coked with 1-octene at 500 °C. These profiles were obtained when the deactivated catalyst was exposed to 0.939% O2 at 200 °C and heated to 1000 °C at a heating rate of 10 °C min-1. Small peaks in the CO and CO2 evolution rates at ca. 320 and 430 °C correspond to nonlinearities in the heating rate at low temperatures and high heating rates. Beyond 500 °C carbon monoxide evolution shows a single symmetric peak. This is observed for all CO profiles and is independent of the heating rate and oxygen partial pressure. Carbon dioxide evolution has a more complicated profile at 10 °C min-1 and 0.939% O2. Rates up to ca. 550 °C are assumed to predominantly correspond to combustion of saturated coke, while beyond this temperature combustion is of highly aromatic or unsaturated coke. In this higher temperature region there is a small shoulder before and another after the peak at 640 °C. These shoulders are not as distinct as the shoulder and peak observed in the analogous TPO profiles of the spent FCC catalyst. Water evolution, as depicted in Figure 1, has a maximum rate within the temperature range which we have associated with saturated coke. Significant rates of water evolution, however, continue beyond the temperatures when carbon oxide evolution has ceased. This indicates that water adsorption onto the internal walls of tubing located between the catalyst bed and the detector affect direct measurement of the intrinsic kinetics. As a consequence, water evolution has not been modeled. Total carbon content, overall H/C ratios, and surface areas of each coked catalyst are listed in Table 1.

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Table 1. Properties of Coke Deposited on the FCC Spent and 1-Octene Coked Cracking Catalysts 1-octene coked catalyst parameter

FCC spent

200 °C

300 °C

400 °C

500 °C

600 °C

Total (g%/g cat.) unsaturated (%) saturated (%) H/C ratio surface area (m2/g cat.)

0.84 ( 0.03 92.0 ( 0.7 8.0 ( 0.7 0.42 ( 0.03 139 ( 5

2.72 ( 0.06 69.4 ( 0.9 30.6 ( 0.9 0.75 ( 0.03 185 ( 5

2.40 ( 0.15 71 ( 2 29 ( 2 0.67 ( 0.08 168 ( 5

2.54 ( 0.13 75.5 ( 1.2 24.5 ( 1.2 0.62 ( 0.06 169 ( 5

2.81 ( 0.11 78 ( 2 22 ( 2 0.41 ( 0.03 194 ( 5

3.1 ( 0.2 86 ( 3 14 ( 3 0.24 ( 0.03 210 ( 5

Figure 2. Temperature-dependent H/C ratios for an industrial spent cracking catalyst and cracking catalysts coked with 1-octene (coking temperature ) 200, 400, 500, and 600 °C).

Temperature-dependent H/C ratios have been plotted in Figure 2. These values, calculated from 2 × rateH2O/ (rateCO2 + rateCO), are meaningful only over the range 250-650 °C. Beyond these temperatures water readsorption or low partial-pressure of water causes either hydrogen or carbon oxides to dominate, which leads to meaningless extremes in the ratio. Kinetic Model for Carbon Oxide Evolution Recently we have developed a single-site kinetic model which simulates the carbon combustion processes occurring during TPO of highly unsaturated catalytic coke.3 An important assumption inherent in this model is that the only source of carbon oxides is via reactions 2-5. This then leads to the following temperaturedependent rates of CO evolution:

k3 d[CO] k2 n2 n3 + [-C(O)]pO ) [-C]pO 2 2 dT β β

(6)

where β is the heating rate and the rate constant subscripts refer to the respective reactions, shown in eqs 2 and 3. Previous TPO modeling of spent cracking catalyst have indicated zero-order oxygen pressure dependence (n2 ≈ n3 ≈ 0) and hence unimolecular rate coefficients. The total rate of CO2 evolution is given by

k5 d[CO2] k4 n4 n5 ) [-C]pO + [-C(O)]pO 2 2 dT β β

(7)

The orders with respect to oxygen were previously determined to be ca. 0.75 in the simulation of evolved CO2 monitored during TPO, recorded in different partial pressures of O2, of a spent cracking catalyst.

Using eqs 6 and 7, TPO profiles recorded in 0.939% O2 at the different heating rates are simulated by optimizing the total unsaturated carbon, preexponential factors, and activation energies for each of the four reactions. Following this optimization the order of the O2 partial pressure for each of the four reactions (shown in eqs 2-5) was determined by simulating the TPO profiles obtained using the higher partial pressures of oxygen. The best-fit rate parameters calculated for the low O2 pressure profiles were held constant for the 1.11% and 5% O2 data, and only the four orders were optimized. The Solver module within Microsoft Excel 98 was used to optimize all parameters. This module does not automatically calculate errors, but methodology that has been suggested by Billo15 has been adapted to determine the standard deviation for all optimized parameters. Optimized rate parameters for carbon oxide evolution during TPO of 1-octene coke deposited on the cracking catalyst at five coking temperatures are listed in Table 2. Figures 3a, for CO2 evolution, and 3b, for CO evolution show the four simulated rates and the residual curves for the TPO profiles (PO2 ) 0.939% and β ) 5 °C/min) of the catalyst coked with 1-octene at 500 °C. Residual curves correspond to saturated coke. Rate parameters obtained for reaction 3 are not considered to be meaningful as previously discussed.3 The most likely explanation for this peak is that desorption of CO occurs via a continuous distribution of activation energies.16 As these peaks in the TPO profiles are not distinct and are very small a model including an activation energy distribution would not be effective. Discussion Coke Content and H/C Ratio. Total H/C ratios for 1-octene coking, as listed in Table 1, are less than one and decrease with increasing coking temperature. The majority of the coke must therefore consist of polyaromatic species that increase in complexity at higher temperatures. For example, benzene has a H/C ratio of one while ovalene, which has 10 fused benzene rings, has a H/C ratio of 0.44. Magnoux and Guisnet17 reported a similar trend, but the ratios were a little higher for coking a HY zeolite with n-heptane at the same level of coverage. The H/C ratio for the spent FCC catalyst is similar in magnitude to that of the 500 °C 1-octene coked catalyst, which agrees well with the 500-550 °C cracking conditions of the industrial cracking reactor. The high H/C ratios, which are plotted as a function of TPO temperature in Figure 2, are a result of two (15) Billo, E. J. Excel for Chemists; Wiley-VCH: New York, 1997. (16) Brown, T. C.; Lear, A. E.; Haynes, B. S. Twenty-Fourth Symposium (International) on Combustion. Sydney, Australia: The Combustion Institute, 1992; pp 1199-1206. (17) Magnoux, P.; Guisnet, M. Appl. Catal. 1988, 38, 341-352.

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Energy & Fuels, Vol. 13, No. 4, 1999 891

Table 2. Optimized Rate Coefficient Parameters, O2 Reaction Orders for Reactions 2-5 Observed during the TPO of a FCC Spent and 1-Octene Coked Cracking Catalystsa 1-Octene Coked Catalyst parameter CO evolution: log A2 E2 (kJ mol-1) n2 log A3 E3 (kJ mol-1) n3 CO2 evolution: log A4 E4 (kJ mol-1) n4 log A5 E5 (kJ mol-1) n5

FCC spent

200 °C

300 °C

400 °C

500 °C

600 °C

6.4 ( 0.2 131 ( 5 0.16 ( 0.06 0.4 ( 0.2 36 ( 3 0.2 ( 0.2

5.2 ( 0.5 110 ( 8 0.4 ( 0.2 2.7 ( 1.0 70 ( 20 0.1 ( 0.2

4.8 ( 0.5 103 ( 8 0.4 ( 0.2 3.1 ( 0.8 76 ( 15 0.2 ( 0.2

5.3 ( 0.4 111 ( 6 0.47 ( 0.13 3.1 ( 0.7 76 ( 13 0.2 ( 0.2

6.0 ( 0.4 125 ( 6 0.5 ( 0.2 3.6 ( 1.1 85 ( 19 0.3 ( 0.2

6.6 ( 0.2 138 ( 3 0.52 ( 0.12 0(2 20 ( 30 0.0 ( 0.3

4.2 ( 0.3 98 ( 5 0.80 ( 0.06 7.0 ( 0.2 151 ( 3 0.7 ( 0.1

3.5 ( 0.3 83 ( 4 0.9 ( 0.2 5.8 ( 1.1 120 ( 20 0.7 ( 0.2

3.1 ( 0.4 75 ( 6 0.8 ( 0.2 5.3 ( 0.9 120 ( 20 0.9 ( 0.3

3.1 ( 0.3 77 ( 5 0.87 ( 0.14 6.2 ( 0.8 133 ( 15 0.9 ( 0.2

4.1 ( 0.3 94 ( 4 0.90 ( 0.13 6.9 ( 1.0 150 ( 20 0.9 ( 0.3

4.4 ( 0.3 101 ( 4 0.86 ( 0.11 7.4 ( 1.3 160 ( 20 1.0 ( 0.3

a The range of TPO conditions employed in the analysis were oxygen partial pressures of 0.939% to 5% and heating rates of 2 to 10 °C min-1.

possible processes. One is that the hydrogen atoms in the coke are preferentially removed. That is, the rate of hydrogen combustion is higher than the rate of carbon combustion. This is particularly demonstrated for the 200 °C coking where the maximum H/C ratio is greater than the ratio for 1-octene (2:1). Also, the low pressures of hydrogen at higher TPO temperatures lead to H/C ratios that are unrealistically low. Others have previously reported this conclusion.18,19 The second reason for high H/C ratios is that as the saturated coke or adsorbed species, which have a high hydrogen content, are removed at these lower temperatures, then the amount of evolved hydrogen should be higher. Furthermore hydrogen may play an integral part in the removal of the coke.20-22 This is perhaps demonstrated by the correlation between highly saturated carbon content and high H/C ratio at low temperature. It is, however, not possible to determine the extent of the second process with the current TPO data. Total coke content increases from 2.40 ( 0.15 g%/g cat. for the catalyst coked at 300 °C to 3.1 ( 0.2 g%/g cat for the 600 °C coked catalyst, while the 200 °C coked catalyst has a higher coke content than either the 300 or 400 °C coked substrates (Table 1). Increased coke content is simply a consequence of the increasing rate of coke deposition with increasing temperature. The higher coke content at the lowest temperature is due to the formation of nonvolatile oligomers, which become blocked in the pores, but which are not thermally stable at higher temperatures. Also the total pore volume may be totally filled as accessibility is not limited by pore blockage, as can occur with increasing temperature.23 The kinetic model fit to the TPO data separates unsaturated and saturated coke quantities. Such a separation implies that there is a sharp distinction between the two types of coke. The distinction lies in the change in combustion rate determining steps, due (18) Haldeman, R. G.; Botty, M. C. J. Phys. Chem. 1959, 63, 489496. (19) Massoth, F. E. I&EC Process Des. Dev. 1967, 6, 200-207. (20) Massoth, F. E.; Menon, P. G. I&EC Process Design and Development 1969, 8, 383-385. (21) Calo, J. M.; Cazorla-Amoro´s, D.; Linares-Solano, A.; Roma´nMartinez, M. C.; Lecea, C. S.-M. D. Carbon 1997, 35, 543-554. (22) Hayhurst, A. N.; Parmar, M. S. Chem. Eng. Sci. 1998, 53, 427438. (23) Dimon, B.; Cartraud, P.; Magnoux, P.; Guisnet, M. Appl. Catal., A: General 1993, 101, 351-369.

Figure 3. Evolution rates of (a) CO2 and (b) CO during TPO of the 1-octene coked (coking temperature ) 500 °C) cracking catalyst in 0.939% O2 with heating rate of 10 °C/min. Lines under the TPO profiles represent simulated peaks associated with reaction 2 (*C(O2) f *C(O) + CO), reaction 3 (*C(O) f *C(O2)

CO), reaction 4 (*C + O2 98 CO2), and reaction 5 (*C + O2 *C(O)

98 CO2). Hashed lines are the difference between experimental and model data, which correspond to saturated coke.

to a change in the balance of the four competing oxidation reactions by, for example, a change in propensity to form oxides; unsaturated coke may more readily form stable oxides than saturated coke. Relative percentages of the two types of coke are shown in Figure 4 as a function of coking temperature. As expected, unsaturated coke increases and saturated coke decreases with increasing temperature. For 1-octene reaction over H-ZSM-5, Anderson et al.7 reported a decline in alkanes retained on the zeolite and an increase in the amount of retained alkylbenzenes as the reaction (24) Magnoux, P.; Roger, P.; Canaff, C.; Fouche, V.; Gnep, N. S.; Guisnet, M. Catalyst Deactivation 1987; Delmon, B., Froment, G. F., Eds.; Amsterdam: Elsevier Science, 1987; Vol. 34, pp 317-330.

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Figure 4. Percentage of saturated (O) and unsaturated coke (b) for the 1-octene coked cracking catalysts as a function of coking temperature.

temperature is increased. Magnoux et al.24 have shown using proton NMR that the coke deposition during propene transformations over a USHY zeolite consist of aromatic, alkylaromatic, and aliphatic protons. At low coke coverage the percentage of aromatic and alkylaromatic protons increase while aliphatic protons decrease with increasing coking temperature. At higher coverages, pore blockage and steric limitations affect this trend. More recently a detailed analysis of coke deposited by propane on H-gallosilicate has shown a clear distinction between low- and high- temperature coke.25 The spent FCC catalyst has the highest unsaturated coke content (Table 1). H/C ratios suggest that the 600 °C 1-octene coked catalyst has the highest polyaromatic content, but the spent FCC catalyst has a higher percentage of unsaturated coke. This is probably a consequence of the nature of the hydrocarbon materials employed in the deposition.26 Table 1 also lists the surface areas of the substrates, which are directly proportional to the unsaturated coke content. Alternatively, it may be stated that the higher the percentage of saturated coke the lower the surface area. This suggests that the saturated coke is more effective at blocking access of N2, used in the BET surface area measurements, to the internal surface area of the catalyst. The unsaturated coke may be more evenly distributed over the entire catalyst, while the saturated coke is located only in the micropores of the USY zeolite,.23 The zeolite micropores contribute substantially to the overall surface area of the catalyst. Rate Parameters. Simulated rates using the parameters listed in Table 2 for three coking temperatures and the spent cracking catalyst are plotted in Figure 5a-d for each of the four reactions shown in eqs 2-5. The conditions of the TPO for the simulation are a heating rate of 5 °C min-1 and oxygen partial pressure of 1%. The temperature corresponding to the peak rate in the 1-octene coked catalyst profiles increase with increasing coking temperature for reactions 2, 4, and (25) Choudhary, V. R.; Sivadinarayana, C.; Devadas, P.; Sansare, S. D.; Magnoux, P.; Guisnet, M. Microporous Mesoporous Mater. 1998, 21, 91-101. (26) Snape, C. E.; McGhee, B. J.; Martin, S. C.; Anderson, J. M. Catal. Today 1997, 37, 285-293.

Figure 5. Simulated evolution rates for CO [(a) reaction 2 (*C(O2) f *C(O) + CO), (b) reaction 3 (*C(O) f CO)] and CO2 *C(O2)

[(c) reaction 4 (*C + O2 98 CO2), (d) reaction 5 (*C + O2 *C(O)

98 CO2)], during TPO of coked cracking catalysts in 0.939% O2 and with a heating rate of 10 °C/min.

5. Furthermore Figure 6 shows that, apart from the 200 °C 1-octene coking, activation energies for the same three reactions increase with increasing coking temperature. The simulated TPO reaction profiles and this activation energy trend demonstrate that the unsaturated coke becomes more stable at higher coking temperatures.

Temperature-Programmed Oxidation of Coke

Figure 6. Activation energies for reactions 2 (O), 4 ([), and 5 (b) for the 1-octene coked cracking catalysts as a function of coking temperature.

Areas under the simulated reaction rate profiles, plotted in Figure 5a-d, show weak trends. For reactions 2 and 5, areas decrease with increasing coking temperature while the opposite is apparent for reactions 3 and 4. These trends may be explained by the changing structure of the coke altering the propensity to form stable oxides, which in turn alters the balance of the competing combustion reactions. However, the trends are weak and more sensitive experiments are required to definitively determine these rate changes as a function of coking temperature. Individual reaction rate TPO profiles for the FCC spent catalyst are distinctly different from the 1-octene coked catalysts. Figure 5a-d shows that for the FCC catalyst reactions 2 and 5 dominate, while for the 1-octene coked catalysts reactions 3 and 4 dominate. This suggests that the structure of the FCC spent coke is such that *C(O) more readily forms and these oxides are more stable than for the 1-octene coked stable oxides. The spent *C(O) is most effectively removed via reaction 5. The difference in coke structure between the substrates is likely to be due to the FCC spent being more graphitic than the laboratory coked samples. Reaction Orders. Mean TPO reaction orders with respect to oxygen for the 1-octene coked catalysts are 0.46 ( 0.17 for reaction 2, 0.2 ( 0.2 for reaction 3, 0.87 ( 0.16 for reaction 4, and 0.9 ( 0.3 for reaction 5. There is no discernible trend in the orders with increasing coking temperature. A comparison with the FCC spent catalyst shows agreement, within experimental error, for reactions 3-5. The mean order for reaction 2, however, is significantly higher for the 1-octene coked catalysts. A change in order from zero-order in the case of the FCC spent to half-order is indicative of a strong pore resistance for the 1-octene coked catalysts.27 This may be due to coke pore blocking in the case of the 1-octene coked catalyst, which has significantly higher coke coverages than the spent FCC catalyst. Alternatively the structure of the USY zeolite in the spent catalyst may have been destroyed, from many months cycling between the industrial cracker and regenerator, and hence only a minor pore resistance is experienced during oxidation. (27) Levenspiel, O. Chemical Reaction Engineering; New York: Wiley, 1972.

Energy & Fuels, Vol. 13, No. 4, 1999 893

Figure 7. Isokinetic relationship for CO2 evolution rate constants (reactions 4 and 5) for all coked cracking catalysts.

Previously the 0.75 order with respect to O2 for reactions 4 and 5 were explained by a Langmuir isotherm involving reversible dissociative adsorption of oxygen within the micropores of the catalyst.3 That is, *C(O) intermediate complexes form in the vicinity of either *C(O2) or *C(O), and the coverage θ is given by28

θ)

(bO2pO2)0.5 1 + (bO2pO2)0.5

(8)

Here bO2 is the ratio of the rate constants for dissociative adsorption and associative desorption of O2. If bO2pO20.5 , 1 then a 0.5 order dependence on oxygen is apparent, which becomes a 0.75 order dependence if reaction occurs within the micropores. Such pore resistance was proposed above, on the basis of reaction 2 orders, for the 1-octene coked catalysts, but not the spent FCC catalyst. It may be that interpretation of these orders requires a combination of first-order oxygen adsorption and the dissociative adsorption. Isokinetic Relationship. The isokinetic relationship (IKR)29 or compensation effect30 is frequently observed for related families of catalysts and reactants. Linert31 has shown that IKR is the more accurate method of reporting such correlations. That is, the natural log of the rate constant is plotted against the inverse of the temperature, and a IKR occurs if a family of these plots intersect at a common point. Such a plot is shown in Figure 7 for rate constants calculated for reactions 4 and 5 from all the 1-octene coked and spent TPO profiles. These straight lines, for CO2 evolution, intersect at 697 ( 44 °C. The isokinetic temperature may be related to a frequency of the transition state or resonance frequency of the solid substrate. The relationship between temperature and frequency has been derived using stochastic models,29 transition-state theory,32 and a dynamic model:33 (28) Cheng, A.; Harriott, P. Carbon 1986, 24, 143-150. (29) Linert, W.; Jameson, R. F. Chem. Soc. Rev. 1989, 18, 477-505. (30) Galwey, A. K. Adv. Catal. 1977, 26, 247-322. (31) Linert, W. Chem. Soc. Rev. 1994, 23, 429-438. (32) Rooney, J. J. Catal. Lett. 1998, 50, 15. (33) Larsson, R. Appl. Catal., A: General 1998, 167, N12-N13.

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ω)

RTiso Nhc

Li and Brown

(9)

Here Tiso is the isokinetic temperature and cω the characteristic frequency, while all other symbols have their usual significance. For the IKR plotted in Figure 7, the characteristic frequency occurs at 674 ( 31 cm-1. The carbon dioxide molecule has a very strong band at 667.3 cm-1 in its infrared absorption spectrum, corresponding to a bending motion. The similarity in these values suggests an important role for this bending motion in the dynamics of CO2 evolution during oxidation. Conclusion TPO with evolved gas analysis is a useful technique for investigating properties of coke deposited on catalysts. By taking into account the combustion mechanism, fine details of coke properties can be determined. For the TPO of 1-octene coked cracking catalysts and an industrial spent cracking catalyst the following information has been extracted: (i) Total H/C ratios decrease with increasing coking temperature, indicating that the polyaromatic content of the coke increases. (ii) Temperature-dependent H/C ratios show that hydrogen atoms are preferentially oxidized during TPO.

(iii) The fraction of saturated coke decreases as the fraction of unsaturated coke increases with increasing coking temperature. (iv) A general trend of increasing maximum in evolution rates and combustion activation energies with increasing coking temperature indicates that the unsaturated coke is becoming more stable. (v) The spent FCC catalyst simulated TPO profiles, for individual combustion reactions, are significantly different from the 1-octene simulated TPO profiles. This indicates different propensities for oxide formation. (vi) Reaction orders with respect to oxygen partial pressure for the 1-octene coked catalysts are half-order for CO evolution, which indicates an intrinsic zero-order reaction within the pores of the catalyst. For the spent FCC catalyst the pores do not affect CO evolution. The formation of CO2 is accompanied by oxygen orders varying from 0.7 to 1.0. These are more difficult to explain. (vii) The isokinetic relationship between CO2 evolution rate constant gives an isokinetic temperature, which corresponds to a vibrational frequency of 674 ( 31 cm-1. This magnitude is similar to the CO2 bending motion frequency of 667.3 cm-1. Acknowledgment. Financial support from the Australian Research Council is gratefully acknowledged. C.L. is also appreciative of UNERS and OPRS awards. EF980265N