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Coking on Bifunctional Catalysts
B. Delman and G.F. Froment (Editors), Catalyst Deactivation 1987 © 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
135
COKING ON BIFUNCTIONAL CATALYSTS J.N. PARERA, R.J. VERDERONE and C.A. GUERINI Instituto de Investigaciones en Catalisis y Petroquim;ca -INCAPESantiago del Estero 2654 - 3000 Santa Fe (Argentina) ABSTRACT The coking on bifunctional naphtha reforming catalysts was studied to find out which are the coke precursors and the mechanism of their formation. Runs feeding pure hydrocarbons (mainly n-paraffins) and naphtha doped with hydrocarbons were performed at 500°C and various pressures with bifunctional, acidic, and metallic catalysts. Light paraffins produce small amounts of cyclopentanes which are great coke precursors. With heavy paraffins alkyl aromatics able to form a bicyclic hydrocarbon by cyclization of the alkylic chain are produced. If this bicyclic has an indenic structure (one 5 C atoms ring), it will be a great coke precursor, more reactive than in the case of having two aromatic rings. The coke formation is a bifunctional process, being the metallic function necessary to dehydrogenate the cyclopentanic ring to a cyclopentadienic ring and the acidic function necessary to condense the cyclopentadienic rings producing polyring compounds. The monometallic catalyst has a greater coke forming capacity than the bimetallics due to its greater dehydrogenating capacity. INTRODUCTION In the catalytic processing of hydrocarbons (petroleum refining and petrochemical industry) a carbonaceous deposit called coke is formed on the catalyst surface. This deposit produces catalyst deactivation and changes in the selectivity. Coke formation depends on the catalyst, operational conditions and feed composition. In the case of naphtha reforming, this phenomenon is very important for the economy of the plant because it fixes the length of the cycle between catalyst regenerations by coke burning. For this reason, it is important to know the mechanism of coke formation to act on the parameters in order to decrease its formation. The composition of the naphtha fed to a reforming unit is generally in the range 55-75% paraffins, 20-40% naphthenes (cyclopentanes and cyclohexanes), and 4-10% aromatics. In virgin naphthas nearly all the paraffins are normal. The naphtha is previously hydrotreated to eliminate sulfur and nitrogen compounds, and -at the same time- the olefins, if present, are hydrogenated. Coke forming capacity of pure hydrocarbons in the range C6-C10 (refs. 1 and 2) and coking capacity of several naphtha cuts (ref. 3) were studied for mono and bimetallic catalysts. Coking capacity of the main constituents of naphtha on mono and two bimetallic catalysts are compared in Fig. 1. Relative coking capacities of the hydrocarbons are plotted as a function of the number of carbon atoms of the
136
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6 7 8 9 10 NUMBER OF CARBON ATOMS
Fig. 1. Relative coking capacity of hydrocarbons on mono and bimetallic catalysts as a function of their number of carbon atoms. For each catalyst the values are relative to the one of n-heptane. Curve 1, n-paraffins on Pt/A1203; 2, n-paraffins on Pt-Re-S/A1203; 3, n-paraffins on Pt-Ge/A1203; 4, naphthenes of 5 C atoms 6, naphthering on Pt/A1203; 5, naphthenes of 5 C atoms ring on Pt-Re-S/A120~; nes of 5 C atoms ring on Pt-Ge/A1203; 7, naphthenes of 6 C atoms rlng on Pt/ A1203; 8, aromatics on Pt/A1203; A, m-xylene; S, ethyl benzene; C, o-xylene; D, i-propyl benzene; E, n-propylbenzene hydrocarbon fed. Data were taken from (ref. 1) and (ref. 2), and the coke produced by n-C7 is taken as a unity for each catalyst. The operating pressure used in (ref. 2) for each type of catalyst was the commonest in commercial units -30 kg cm- 2 for mono and 15 kg cm- 2 for bimetallic catalysts, and it can be accepted that the trend is the same at both pressures. Since the main components of naphthas are the n-paraffins, their coking capacity is an important clue to study the coking of the process. As shown in
137
Fig. 1, on Pt/A1 Z0 3 n-C5 produces a large amount of coke, whereas a smaller amount is produced with n-C 6 and even a smaller amount with n-C 7. From the minimum in coking capacity of n-C 7 there is an increase with the increase in the n-paraffin length. On the other hand, on the bimetallics coke formation always increases with the n-paraffin length. The more striking difference is observed in n-C5 and n-C6, being very interesting to explain this difference. The objective of this paper is to give the reasons of the different cokin0 capacity of mono and bimetallic catalysts studying the differences in catalytic selectivities, different coking capacity of reactants and products, and thus stating a mechanism for coke formation. EXPERIMENTAL To see the coking capacity of different hydrocarbons on several catalysts, runs feeding pure hydrocarbons or naphtha doped with 10% wt of pure hydrocarbons were performed. Experiments with doped naphtha are less sensitive to the change of doping agent than in the case of pure hydrocarbons, but have the advantage that the chemical composition of reactants is always nearly the same and is similar to the one of naphtha. Catalysts The catalysts used, with weight percentages on sample dried at 1Z0°C indicated between brackets, were the following: - Pt(0.35)/A1 Z03-Cl(0.88), prepared by impregnation of y-alumina CK 300 (Ketjen Cyanamid, Amsterdam, Sg = ZOO mZ g-l, Vg = 0.48 cm 3 g-l) with a solution of HCl and HZPtC1 6 as quoted in (ref. 3). Pt(0.33)-Re(0.3Z)/A1 Z03-Cl(0.90) of commercial orlgln and sulfided as indicated in (ref. Z) to 0.03% S. Sg = 168 mZ g-l, Vg = 0.50 cm 3 g-l. Pt(0.37)-Ge(0.Z4)/A1 Z0 3-Cl(0.81) of commercial origin. Sg = 178 mZ g-l, Vg 0.46 cm 3 g-l A1 Z03-Cl(1.03), CK 300 y-alumina chlorided at 500°C with a stream of HC1water-air according to (ref. 4). SiO Z-A1 Z03 Grace High Alumina, Z4.Z% A1 Z03, Sg = 341 mZ g-l, Vg = 0.618 cm 3 g-l Pt(0.49)/SiO Z' Sg = Z85 mZ g-l, Vg = 1.Z cm 3 g-l, free of chlorine, provided by Prof. J. Butt from Northwestern University and identified in (ref. 5) as 63.5-SiO Z-Ion X-L. Feeds Pure hydrocarbons Carlo Erba RP and hydrodesulfurized naphtha were used as feed. They were dried previous to the run by passage through a bed of 4 A Molecular Sieve. The naphtha had a boiling point range 40-155°C.
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Test for catalytic activity, selectivity and cokina A stainless steel bench-scale flow equipment with chromatographic analysis on line was used. The range of pressure was 1-30 kg cm -2 , temperature 500°C, hydrogen:hydrocarbon molar ratio = 2, and weight hourly space velocity = 3. Carbon content of catalysts after the runs was determined by combustion-volumetry. In runs at 15 and 30 kg cm- 2, during an intermediate period the pressure was decreased to 3.5 kg cm- 2 ~nd 10 kg cm- 2, respectively, in order to increase the coke deposition (accelerated coking test). RESULTS Pure hydrocarbons n-Pentane. Runs feeding n-C5 at various pressures and measurina the conversion to CP and the coke deposited on the catalyst were performed on mono and bimetallic catalysts. The results are shown in Fig. 2. For a pressure of 30 kg cm- 2 the conversion to CP was negligible on the three catalysts. The conversion to aromatics of 8 and 9 C atoms was also measured and the results are shown in Table 1.
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15
Fig. 2. Conversion of n-C5 to CP on mono and bimetallic catalysts as a function of pressure. Numbers over the points are the wt% of carbon deposited on the catalyst after the 5 h run
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TABLE 1 n-Pentane reformi ng. Conversion to heavy aromatics Pt/A1 Z03
Pressure kg cm -1
ArCS
1 3.5 5
2.59 0.6Z 0.33 0.31 0.15
10
15 a ArCS
a
Pt-Re-S/A1 Z03
Pt-Ge/A1 Z03
ArC 9
ArCS
ArC 9
ArCS
ArC 9
3.35 0.31 0.17 0.11 O.OS
4.01 0.59 0.43 0.17 0.06
Z.07 0.26 0.16 0.10 O.OOS
0.31 0.3Z O.lS 0.04 0.00
0.06 0.00 0.00 0.00 0.00
aromatic hydrocarbons of S C atoms, xylenes and ethyl benzene
n-Hexane. The reforming of n-C 6 produces MCP as an intermediate in the bifunctional mechanism leading to benzene. Then, the amount of MCP measured at the reactor outlet is the difference between the amount of MCP produced and the amount transformed into benzene and other products. Table Z shows the conversion of n-C6 to MCP and the coke deposited on the catalysts at various pressures. To increase coke deposition in the runs at 15 and 30 kg cm- Z an intermediate period at lower pressure was intercalated at 3.5 and 10 kg cm- Z, respectively. For this reason, the amount of coke can not be compared for different pressures. For each catalyst the conversion of n-C6 into MCP decreases with the increase in pressure, similarly to the case of n-C5. For each pressure, the highest amount of MCP and coke correspond to Pt/A1 Z03, the intermediate amount to Pt-Ge/A1 Z03, and the lowest amount to Pt-Re-S/A1 Z03. The relative position of the bimetallics is opposite to the position with n-C5. TABLE Z n-Hexane reforming, conversion to MCP (%), and coke deposited on the catalyst (wt%)
Catalyst Pt/A1Z 03 Pt-Re-S/ A1Z03 Pt-Ge/ Al e03
b
5 kg cm -Z a
15 kg cm -Z
MCP
%C
MCP
%C
1. 90 LOS 1.11
1.06 0.65 0.S6
1. 30 0.4Z 0.57
4.S5 Z.25 3.90
30 kg cm -Z c t~CP
%C
0.55
0.44
aRun length 7 h. b7 h at 15 kg cm-Z-ZO h at 3.5 kg cm- Z-7 h at 15 kg cm- Z. c7 h at 30 kg cm-Z-ZO h at 10 kg cm- Z-7 h at 30 kg cm- Z
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n-Heptane. In the reforming of n-C 7 the 5 C atoms ring naphthenes produced are MCP and DMCP. Table 3 shows the conversion of n-C7 to these naphthenes, and the coke deposited on the catalyst. Pt-Ge/A1 203 produces the highest conversion to these naphthenes (l.Sl%) and Pt/A1 203 the lowest (1.20%). The relative position of the bimetallics is the same to that with n-C 6. The production of heavy aromatics is small. TABLE 3 n-Heptane reforming. Conversion to cyclopentanes and coke deposited on the catalyst on accelerated coking tests Catalyst
Pressure 15 kg cm
Pt/ A1203 Pt-Re-S/ A1203 Pt-Ge/ A1203
-2 a
30 kg cm -2
b
MCP
DMCP
%C
1'1CP
Df1CP
%C
0.20 O.SO 0.71
1.00 0.53 1.10
4.00 3.95 5.15
°
0.44
0.25
aSimilar conditions to b in Table 2.
bSimilar conditions to c in Table 2
n-Octane and n-Nonane. From n-C7 to n-Cg, increasing the length of the paraffin increases its transformation into aromatics. With n-CS and n-Cg nearly all the intermediates 5 C atoms ring naphthenes are transformed into aromatics. With these paraffins the corresponding aromatics (ArCS and ArC 9) are formed, and also bicyclic aromatics -as indene and naphthalene, can be detected. The amount of bicyclics and coke deposition is higher with Pt/A1 203 Methylcyclopentane. The coke formation at 5 kg cm -2 in 7 h runs with MCP as feed is the following: 4.60% on Pt/A1 203, 3.70% on Pt-Re-S/A1 203 and 3.95% on Pt-Ge/A1 203. The relative order of coking is the same that the one of n-C6 (Table 2). Naphtha doped with hydrocarbons To see the coking capacity of several hydrocarbons, a naphtha doped with 10% by weight of hydrocarbons was used as feed at 1 kg cm -2 The coke formation after 2 h run is shown in Table 4. DISCUSSION Normal paraffins, from n-C5 to n-C10, are the main components of the naphthas to be reformed and their transformation into aromatics is the objective of the process. Then, the reforming of the n-paraffins and of their products is a useful guide to study the coking produced by naphthas over different catalysts.
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TABLE 4 Coke formation on several catalysts fed with naphtha and naphtha doped with 10% of different hydrocarbons Doping agent None CP CPe CPde MCP MCPe MCPde CH CHe CHde Bz n-C5 I-n- C5e 1,3-nC6 de n-Propyl Bz 1,2,4-TM Bz Indene Indane Naphthalene
Catalysts Pt/A1 203 Pt-Re-S/A1 203 Pt-Ge/A1 203 A1203-Cl
Si0 2-A1 203 Pt/ Si02
3.36(-) 2.55(-) 3.75(+) 3.08(-) 9.20(++) 5.68(+) 19.12(+++)17.10(++) 3.50(+) 3.52(+) 7.10(++) 5.13(+) 18.20(+++)17.06(++) 3.31 2.87 3.40 3.70 3.40 3.84 3.20 1. 94 4.09 3.15 4.35 3.15 13.10 9.75(++) 8.80(+) 3.80 3.75 29.78 22.34 9.30 4.45 4.24 4.94
0.70(-) 0.68(-) 1.88(-) 20.28(+) 1. 65 3.15 17.38(+) 1.47 4.10 6.35 0.80 0.80 0.88 9.21 1.09 1. 00 24.12 2.76 4.37
2.60(-) 3.20(-) 5.96(+) 18.30(++) 3.50(+) 5.94(+) 18.61(+) 2.49 4.35 2.61 1. 58 2.10 2.58
0.20(-) 0.20(-) 0.54(-) 11.50(+) 0.20 1. 09 8.04 0.43 0.99 4.83 0.36 0.20 0.22 6.00
1.01(-) 1.20(-) 2.50(-) 5.83(-) 1.10 1.10
CP: cyclopentane, CPe: cyclopentene, CPde: cyclopentadiene, t1: methyl, CH: cyclohexane, CHe: cyclohexene, CHde: cyclohexadiene, Bz: benzene, n-C5: normal pentane, n-C5e: normal pentene, n-C6de: normal hexadiene, TN Bz: trimethyl benzene (+) or (-): positive or negative detection of heavy aromatics (indene, ArCI0+) Most of the other constituents of naphtha are also products of the n-para~fins reforming. The coking capacity of n-paraffins and other hydrocarbons on mono and bimetallic catalysts was already shown in Fig. 1. For each catalyst, the values are relative to the coking capacity of n-C7 and the great difference is in the coking capacity of the n-paraffins (curves 1, 2 and 3). It is our aim to explain the different behavior of the catalysts based on a mechanism of coke formation and the different metallic function activity of the catalysts. Possible reactiqns of n-pentane on the bifunctional catalysts are the following: hydrogenolysis producing methane and n-butane, hydrocracking producing ethane and propane, isomerization producing iso-pentane, and cyclization producing cyclopentane. Possible types of reactions of n-C 6 are the same to those of n-C 5 with the addition of aromatization leading to benzene. In former papers it was shown that -on mono as well as on bimetallic catalysts, naphthenes with rings of 5 C atoms (CP, MCP) are the greatest coke precursors among the hydrocarbons of the same number of C atoms (refs. 1 and 2), as shown in Fig. 1.
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Barbier et al. (refs. 6 and 7), and Parera et al. (ref. S) showed that coke formation results from the transformation of cyclopentadiene (CPde) produced by dehydrogenation of CP on the metallic function of the catalyst. Then, it is possible to suppose that the difference in coke formation from n-C 5 and n-C6 on the different catalysts could be due to the different selectivities to CP and MCP and the different dehydrogenation capacities to transform them into CPde and MCPde, respectively. Data in Fig. 2 and Table 2 are in accordance with this supposition: in the reforming of the light paraffins (n-C5, n-C 6) small amounts of CP and MCP are formed and the coke deposited on the catalysts is related to these amounts, and their dehydrogenation by the catalyst. For instance the amount of CP at 10 and 15 kg cm- 2 are not much different (Fig. 2), but at the higher pressure the amount of coke is many times smaller because the dehydrogenation is smaller due to the higher hydrogen pressure. Pt/A1 203 is the greatest CP and MCP producer and also the greatest coke producer. -2 . On the three catalysts, the amount of MCP obtained from n-C6 at 5 kg cm 1S higher than the amount of CP obtained from n-CS' The decrease in coke formation on Pt/A1 203 passing from n-C 5 to n-C6 shown in Fig. 1 can be ascribed to the lower coking capacity of MCP compared to the one of CP on this catalyst, as shown in the same Figure. The Figure also shows that on the bimetallics n-C6 produces more coke than n-C S' which can be justified by the greater coking capacity of MCP (curves 5 and 6) and its greater production compared to CP. The different coking capacity of the catalysts with compounds having a cyclopentanic ring depends on the dehydrogenating capacity of the catalyst to form a cyclopentadienic (CPde) ring (refs. 6-S). This ring has a conjugate double bond which is very reactive and can condense with other CPde ring by a dienedienophile Diels-Alder type condensation giving an indenic structure (ref. 8). When feeding CP in the case of Pt/A1 203, some indene can be detected (Table 4), and when feeding n-C S some aromatics of Sand 9 C atoms are produced (Table 1). The ArCS are not great coke producers compared to the ArC g which coke production are similar to the ones of cyclopentanes on Pt/A1 203 (Fig. 1). The am~unt of ArC g is small and always the production on Pt/A1 203 is higher. Feeding with n-C7 the production of 5 C atoms ring naphthenes on Pt/A1 203 is smaller than the production of MCP from n-C6, and the coke produced is also smaller. The opposite occurs with the bimetallics (Tables 2 and 3 at 15 kg cm- 2), explaining the difference in mono and bimetallics when passing from n-C6 to n-C 7 in Fig. l. Table 4 shows that increasing the dehydrogenation of the ring in the order CP, CPe, CPde, and MCP, MCPe, MCPde, increases the coking on all the catalysts. When the doping agent is CP, because of thermodynamic limitations only a small amount of CPe and even a smaller one of CPde are produced on the metallic function. Doping with CPe, a bit more CPde is produced and also more coke is formed
143
than in the case of CP. When doping with CPde, the greatest production of coke is obtained. Comparing the coke formation on the metal-acid catalysts (Pt/A1 Z0 3, pt-Re-S/A1 Z03, Pt-Ge/A1 Z0 3) with the one on the acid catalysts (A1 Z03-Cl, SiO ZA1 Z03), it can be seen that the presence of the metallic function able to dehydrogenate CP and CPe is necessary for the production of a great amount of coke. The catalyst with only the acid function produced little coke with CP and CPe, meanwhile the coke is as high as on the metal-acid catalysts when feeding MCPde. SiOZ-A1 Z03 has a stronger acidity than A1 Z0 3-Cl, and produces more coke. Comparing the coking capacity of Pt/SiO Z' it is seen that even the metallic function is able to dehydrogenate CP and CPe, the coke formation is not high because of the absence of acidic function where coke precursors can condense. Then, both functions are necessary to produce coke from CP and MCP: the metallic function is necessary to dehydrogenate up to CPde or MCPde and the acidic function is necessary to condense these coke precursors. It is interesting to compare the coking capacity of the 6 C atoms ring naphthenes on bifunctional and acidic catalysts. It is known (ref. 9) that cyclohexane is rapidly dehydrogenated on the metallic function producing benzene without desorption of the unsaturated intermediates (CHe, CHde). When feeding these intermediates they are also rapidly dehydrogenated in the first part of the catalyst bed, flowing benzene along the bed. Since benzene produces a small amount of coke because of the high stability of the aromatic ring, CH, CHe and CHde produce no much coke on the bifunctional catalysts. On the other hand, the acidic catalysts are not able to dehydrogenate these hydrocarbons. They flow along the catalyst bed and the one more unsaturated, CHde, is able to condense on the acidic function producing an amount of coke greater than that on the bifunctional catalysts. Pt/A1 Z03 is a more dehydrogenating catalyst than the bimetallics and transform more rapidly the cyclohexanes producing less coke. Olefins are poor coke producers. Table 4 shows that I-pentene (l-n-CSe) has a coking capacity Similar to the one of the corresponding paraffin. Diolefins with conjugated double bonds, as l,3-n-C 6de, have greater coking capacity than paraffins and olefins, but still is smaller than the one of CPde and MCPde. All these results confirm that the coke formation in hydrocarbons of S or 6 C atoms is mainly produced by the naphthenes with S C atoms ring which are dehydrogenated to cyclopentadienic rings. The conjugate double bond in a S C atoms ring is more reactive than in an open chain. From n-C7 up, the transformation of n-paraffins into cyclopentanes is negligible and the production of aromatics increases with the length of the chain on all the catalysts. Among the possible products of heavy paraffins reforming, alkyl benzenes and bicyclic aromatics are the greatest coke producers. Then, the selectivity to these products influences the coking capacity of the catalysts. Although the total aromatics production is higher with Pt-Re-S/A1 Z03 than with
144
Pt/A1 203 (ref. 2), the amount of heavy and bicyclic aromatics is greater and greater the coke deposition on the last catalyst. Doping with n-propylbenzene, the coke formation is ~reater on Pt/A1 203 than on Pt-Re-S/A1 Z03 (Table 4), and a higher amount of indene is detected in the reformate with Ft/A1 Z03. This can be assigned to the greater dehydro~enating capacity of Pt/A1 203: the alkylic chain of n-propylbenzene can be dehydrogenated and cyclizated producing indane which on Pt can be dehydrogenated to indene which have the cyclopentadienic ring and is very reactive for condensation to coke. Table 4 shows that indane produces more coke on Pt/A1 Z0 3 than on Pt-Re-S/ A1 Z03, and this could be a consequence of the higher dehydrogenation capacity of Pt/A1 Z0 3 to transform indane to indene. On SiO Z-A1 Z03, indane produces little coke compared to the one produced by indene. This is due to the lack of dehydrogenating capacity to produce the reactive indenic structure from indane. Naphthalene -though being a bicyclic aromatic, has a smaller coking capacity than indene because of the high stability of the aromatic rings compared to the great reactivity of the cyclopentadienic ring. CONCLUSIONS r~echanisms of coke formation on mono (Pt/A1 Z03) and bimetallic (Pt-Re-S/A1 Z03 and Pt-Ge/A1 203) catalysts are similar. The higher coking capacity of Pt/A1 Z03 is due to its higher dehydrogenating capacity producing more cyclopentadienes from n-C5 and n-C6 and more indenic bicyclics from n-C9 and n..C10. Cyclopentadienic rings are important coke precursors due to the great reactivity of the conjugate double bond. This reactivity is greater than the one of the aromatic ring allowing condensation and leading to polycyclic hydrocarbons. The proposed mechanism for coke formation from n-paraffins on the reforming catalysts is shown in Fig. 3. The intermediates are produced from the paraffins and also when using naphtha can enter in the feed. Most part of the monocyclic intermediates leaves the reactor with the reformate, while only a small fraction of the bicyclics leaves it and the other part remains irreversibly adsorbed on the catalyst and is transformed into coke components. The coke formation is a bifunctional reaction requiring the dehydrogenating capacity of the metallic function and the condensating capacity of the acidic function. The dehydrogenating capacity of the bimetallic catalysts is lower than the one of the monometallic. The bimetallics produce smaller amounts of CP rings and have smaller dehydrogenation capacity to produce CPde rings; nevertheless, these catalysts still have the dehydrogenation activity necessary to produce olefins, which are intermediate in the transformation of n-paraffins into aromatics or i-paraffins. In this way, the second metal improves -regarding coke
145
formation, the catalyst metallic function. Also it is interesting to study changes in the acidic function in order to decrease its condensation capacity keeping unaltered its isomerization capacity. FEED n-C 5 -
INTERf1EDIATES DHC
n-C 6 ~
CP ~ 1·1CP ~
CPde f1CPde
C (Diels-Alder) DH of ri ngs Bicyclic hydrocarbons C (indenes, ~ naphthalene)
-
DHC
COKE
Al kyl aromatics
Polycyclic hydrocarbons
C! DH
Turbostratic structure C ~ DH Graphit i c structure
Fig. 3. Mechanism of coke formation from n-paraffins. DHC: dehydrocyclization (metal-acid function), DH: dehydrogenation (on the metallic function) or hydrogen transfer (on the acidic function), C: condensation (acid function) REFERENCES 2 3 4 5 6 7 8 9
J.N. Beltramini, E.E. Martinelli, E.J. Churfn, N.S. Ffgoli and J.M. Parera, Appl. Catal., 7 (1983) 43. J.M. Parera, C.A. Querini, J.N. Beltramini and N.S. Ffgoli, submitted for publication. C.A. Querini, N.S. Ffgoli and J.M. Parera, submitted for publication A.A. Castro, O.A. Scelza, E.R. Benvenuto, G.T. Baronetti and J.M. Parera, J. Catal., 69 (1981) 222. T. Uchiyima, J.H. Hermann, Y. Inoue, R.L. Burwell, J.B. Butt and J.B. Cohen, J. Catal., 50 (1977) 464. J. Barbier, L. Elassal, N.S. Gnep, M. Guisnet and W. Molina, Bull. Soc. Chim. Fr., I (1984) 245. J. Barbier, L. Elassal, N.S. Gnep, M. Guisnet and W. Molina, Bull. Soc. Chim. Fr., I (1984) 250. J.M. Parera, N.S. Ffgoli, J.N. Beltramini, E.J. Churfn and R.A. Cabrol, Proc. 8th Int. Congr. Catal., Berlin, July 1984, Verlag Chemie, Berlin, 1984, p. 593. J.E. Germain, Catalytic Conversion of Hydrocarbons, Academic Press, New York/ London, 1969, p. 81.
135
COKING ON BIFUNCTIONAL CATALYSTS J.N. PARERA, R.J. VERDERONE and C.A. GUERINI Instituto de Investigaciones en Catalisis y Petroquim;ca -INCAPESantiago del Estero 2654 - 3000 Santa Fe (Argentina) ABSTRACT The coking on bifunctional naphtha reforming catalysts was studied to find out which are the coke precursors and the mechanism of their formation. Runs feeding pure hydrocarbons (mainly n-paraffins) and naphtha doped with hydrocarbons were performed at 500°C and various pressures with bifunctional, acidic, and metallic catalysts. Light paraffins produce small amounts of cyclopentanes which are great coke precursors. With heavy paraffins alkyl aromatics able to form a bicyclic hydrocarbon by cyclization of the alkylic chain are produced. If this bicyclic has an indenic structure (one 5 C atoms ring), it will be a great coke precursor, more reactive than in the case of having two aromatic rings. The coke formation is a bifunctional process, being the metallic function necessary to dehydrogenate the cyclopentanic ring to a cyclopentadienic ring and the acidic function necessary to condense the cyclopentadienic rings producing polyring compounds. The monometallic catalyst has a greater coke forming capacity than the bimetallics due to its greater dehydrogenating capacity. INTRODUCTION In the catalytic processing of hydrocarbons (petroleum refining and petrochemical industry) a carbonaceous deposit called coke is formed on the catalyst surface. This deposit produces catalyst deactivation and changes in the selectivity. Coke formation depends on the catalyst, operational conditions and feed composition. In the case of naphtha reforming, this phenomenon is very important for the economy of the plant because it fixes the length of the cycle between catalyst regenerations by coke burning. For this reason, it is important to know the mechanism of coke formation to act on the parameters in order to decrease its formation. The composition of the naphtha fed to a reforming unit is generally in the range 55-75% paraffins, 20-40% naphthenes (cyclopentanes and cyclohexanes), and 4-10% aromatics. In virgin naphthas nearly all the paraffins are normal. The naphtha is previously hydrotreated to eliminate sulfur and nitrogen compounds, and -at the same time- the olefins, if present, are hydrogenated. Coke forming capacity of pure hydrocarbons in the range C6-C10 (refs. 1 and 2) and coking capacity of several naphtha cuts (ref. 3) were studied for mono and bimetallic catalysts. Coking capacity of the main constituents of naphtha on mono and two bimetallic catalysts are compared in Fig. 1. Relative coking capacities of the hydrocarbons are plotted as a function of the number of carbon atoms of the
136
15.76
4~12.36
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5
6 7 8 9 10 NUMBER OF CARBON ATOMS
Fig. 1. Relative coking capacity of hydrocarbons on mono and bimetallic catalysts as a function of their number of carbon atoms. For each catalyst the values are relative to the one of n-heptane. Curve 1, n-paraffins on Pt/A1203; 2, n-paraffins on Pt-Re-S/A1203; 3, n-paraffins on Pt-Ge/A1203; 4, naphthenes of 5 C atoms 6, naphthering on Pt/A1203; 5, naphthenes of 5 C atoms ring on Pt-Re-S/A120~; nes of 5 C atoms ring on Pt-Ge/A1203; 7, naphthenes of 6 C atoms rlng on Pt/ A1203; 8, aromatics on Pt/A1203; A, m-xylene; S, ethyl benzene; C, o-xylene; D, i-propyl benzene; E, n-propylbenzene hydrocarbon fed. Data were taken from (ref. 1) and (ref. 2), and the coke produced by n-C7 is taken as a unity for each catalyst. The operating pressure used in (ref. 2) for each type of catalyst was the commonest in commercial units -30 kg cm- 2 for mono and 15 kg cm- 2 for bimetallic catalysts, and it can be accepted that the trend is the same at both pressures. Since the main components of naphthas are the n-paraffins, their coking capacity is an important clue to study the coking of the process. As shown in
137
Fig. 1, on Pt/A1 Z0 3 n-C5 produces a large amount of coke, whereas a smaller amount is produced with n-C 6 and even a smaller amount with n-C 7. From the minimum in coking capacity of n-C 7 there is an increase with the increase in the n-paraffin length. On the other hand, on the bimetallics coke formation always increases with the n-paraffin length. The more striking difference is observed in n-C5 and n-C6, being very interesting to explain this difference. The objective of this paper is to give the reasons of the different cokin0 capacity of mono and bimetallic catalysts studying the differences in catalytic selectivities, different coking capacity of reactants and products, and thus stating a mechanism for coke formation. EXPERIMENTAL To see the coking capacity of different hydrocarbons on several catalysts, runs feeding pure hydrocarbons or naphtha doped with 10% wt of pure hydrocarbons were performed. Experiments with doped naphtha are less sensitive to the change of doping agent than in the case of pure hydrocarbons, but have the advantage that the chemical composition of reactants is always nearly the same and is similar to the one of naphtha. Catalysts The catalysts used, with weight percentages on sample dried at 1Z0°C indicated between brackets, were the following: - Pt(0.35)/A1 Z03-Cl(0.88), prepared by impregnation of y-alumina CK 300 (Ketjen Cyanamid, Amsterdam, Sg = ZOO mZ g-l, Vg = 0.48 cm 3 g-l) with a solution of HCl and HZPtC1 6 as quoted in (ref. 3). Pt(0.33)-Re(0.3Z)/A1 Z03-Cl(0.90) of commercial orlgln and sulfided as indicated in (ref. Z) to 0.03% S. Sg = 168 mZ g-l, Vg = 0.50 cm 3 g-l. Pt(0.37)-Ge(0.Z4)/A1 Z0 3-Cl(0.81) of commercial origin. Sg = 178 mZ g-l, Vg 0.46 cm 3 g-l A1 Z03-Cl(1.03), CK 300 y-alumina chlorided at 500°C with a stream of HC1water-air according to (ref. 4). SiO Z-A1 Z03 Grace High Alumina, Z4.Z% A1 Z03, Sg = 341 mZ g-l, Vg = 0.618 cm 3 g-l Pt(0.49)/SiO Z' Sg = Z85 mZ g-l, Vg = 1.Z cm 3 g-l, free of chlorine, provided by Prof. J. Butt from Northwestern University and identified in (ref. 5) as 63.5-SiO Z-Ion X-L. Feeds Pure hydrocarbons Carlo Erba RP and hydrodesulfurized naphtha were used as feed. They were dried previous to the run by passage through a bed of 4 A Molecular Sieve. The naphtha had a boiling point range 40-155°C.
138
Test for catalytic activity, selectivity and cokina A stainless steel bench-scale flow equipment with chromatographic analysis on line was used. The range of pressure was 1-30 kg cm -2 , temperature 500°C, hydrogen:hydrocarbon molar ratio = 2, and weight hourly space velocity = 3. Carbon content of catalysts after the runs was determined by combustion-volumetry. In runs at 15 and 30 kg cm- 2, during an intermediate period the pressure was decreased to 3.5 kg cm- 2 ~nd 10 kg cm- 2, respectively, in order to increase the coke deposition (accelerated coking test). RESULTS Pure hydrocarbons n-Pentane. Runs feeding n-C5 at various pressures and measurina the conversion to CP and the coke deposited on the catalyst were performed on mono and bimetallic catalysts. The results are shown in Fig. 2. For a pressure of 30 kg cm- 2 the conversion to CP was negligible on the three catalysts. The conversion to aromatics of 8 and 9 C atoms was also measured and the results are shown in Table 1.
• PtlAl203
0 ;;-.. 2
6.
(L'"
Pt-Re-S/AI203
o Pt-Ge/Al203
U
~ Z
0
(/)
fr::
W
> Z
0
u
a a
0.25
5
PRESSURE
10 j
Kg cm-2
15
Fig. 2. Conversion of n-C5 to CP on mono and bimetallic catalysts as a function of pressure. Numbers over the points are the wt% of carbon deposited on the catalyst after the 5 h run
139
TABLE 1 n-Pentane reformi ng. Conversion to heavy aromatics Pt/A1 Z03
Pressure kg cm -1
ArCS
1 3.5 5
2.59 0.6Z 0.33 0.31 0.15
10
15 a ArCS
a
Pt-Re-S/A1 Z03
Pt-Ge/A1 Z03
ArC 9
ArCS
ArC 9
ArCS
ArC 9
3.35 0.31 0.17 0.11 O.OS
4.01 0.59 0.43 0.17 0.06
Z.07 0.26 0.16 0.10 O.OOS
0.31 0.3Z O.lS 0.04 0.00
0.06 0.00 0.00 0.00 0.00
aromatic hydrocarbons of S C atoms, xylenes and ethyl benzene
n-Hexane. The reforming of n-C 6 produces MCP as an intermediate in the bifunctional mechanism leading to benzene. Then, the amount of MCP measured at the reactor outlet is the difference between the amount of MCP produced and the amount transformed into benzene and other products. Table Z shows the conversion of n-C6 to MCP and the coke deposited on the catalysts at various pressures. To increase coke deposition in the runs at 15 and 30 kg cm- Z an intermediate period at lower pressure was intercalated at 3.5 and 10 kg cm- Z, respectively. For this reason, the amount of coke can not be compared for different pressures. For each catalyst the conversion of n-C6 into MCP decreases with the increase in pressure, similarly to the case of n-C5. For each pressure, the highest amount of MCP and coke correspond to Pt/A1 Z03, the intermediate amount to Pt-Ge/A1 Z03, and the lowest amount to Pt-Re-S/A1 Z03. The relative position of the bimetallics is opposite to the position with n-C5. TABLE Z n-Hexane reforming, conversion to MCP (%), and coke deposited on the catalyst (wt%)
Catalyst Pt/A1Z 03 Pt-Re-S/ A1Z03 Pt-Ge/ Al e03
b
5 kg cm -Z a
15 kg cm -Z
MCP
%C
MCP
%C
1. 90 LOS 1.11
1.06 0.65 0.S6
1. 30 0.4Z 0.57
4.S5 Z.25 3.90
30 kg cm -Z c t~CP
%C
0.55
0.44
aRun length 7 h. b7 h at 15 kg cm-Z-ZO h at 3.5 kg cm- Z-7 h at 15 kg cm- Z. c7 h at 30 kg cm-Z-ZO h at 10 kg cm- Z-7 h at 30 kg cm- Z
140
n-Heptane. In the reforming of n-C 7 the 5 C atoms ring naphthenes produced are MCP and DMCP. Table 3 shows the conversion of n-C7 to these naphthenes, and the coke deposited on the catalyst. Pt-Ge/A1 203 produces the highest conversion to these naphthenes (l.Sl%) and Pt/A1 203 the lowest (1.20%). The relative position of the bimetallics is the same to that with n-C 6. The production of heavy aromatics is small. TABLE 3 n-Heptane reforming. Conversion to cyclopentanes and coke deposited on the catalyst on accelerated coking tests Catalyst
Pressure 15 kg cm
Pt/ A1203 Pt-Re-S/ A1203 Pt-Ge/ A1203
-2 a
30 kg cm -2
b
MCP
DMCP
%C
1'1CP
Df1CP
%C
0.20 O.SO 0.71
1.00 0.53 1.10
4.00 3.95 5.15
°
0.44
0.25
aSimilar conditions to b in Table 2.
bSimilar conditions to c in Table 2
n-Octane and n-Nonane. From n-C7 to n-Cg, increasing the length of the paraffin increases its transformation into aromatics. With n-CS and n-Cg nearly all the intermediates 5 C atoms ring naphthenes are transformed into aromatics. With these paraffins the corresponding aromatics (ArCS and ArC 9) are formed, and also bicyclic aromatics -as indene and naphthalene, can be detected. The amount of bicyclics and coke deposition is higher with Pt/A1 203 Methylcyclopentane. The coke formation at 5 kg cm -2 in 7 h runs with MCP as feed is the following: 4.60% on Pt/A1 203, 3.70% on Pt-Re-S/A1 203 and 3.95% on Pt-Ge/A1 203. The relative order of coking is the same that the one of n-C6 (Table 2). Naphtha doped with hydrocarbons To see the coking capacity of several hydrocarbons, a naphtha doped with 10% by weight of hydrocarbons was used as feed at 1 kg cm -2 The coke formation after 2 h run is shown in Table 4. DISCUSSION Normal paraffins, from n-C5 to n-C10, are the main components of the naphthas to be reformed and their transformation into aromatics is the objective of the process. Then, the reforming of the n-paraffins and of their products is a useful guide to study the coking produced by naphthas over different catalysts.
141
TABLE 4 Coke formation on several catalysts fed with naphtha and naphtha doped with 10% of different hydrocarbons Doping agent None CP CPe CPde MCP MCPe MCPde CH CHe CHde Bz n-C5 I-n- C5e 1,3-nC6 de n-Propyl Bz 1,2,4-TM Bz Indene Indane Naphthalene
Catalysts Pt/A1 203 Pt-Re-S/A1 203 Pt-Ge/A1 203 A1203-Cl
Si0 2-A1 203 Pt/ Si02
3.36(-) 2.55(-) 3.75(+) 3.08(-) 9.20(++) 5.68(+) 19.12(+++)17.10(++) 3.50(+) 3.52(+) 7.10(++) 5.13(+) 18.20(+++)17.06(++) 3.31 2.87 3.40 3.70 3.40 3.84 3.20 1. 94 4.09 3.15 4.35 3.15 13.10 9.75(++) 8.80(+) 3.80 3.75 29.78 22.34 9.30 4.45 4.24 4.94
0.70(-) 0.68(-) 1.88(-) 20.28(+) 1. 65 3.15 17.38(+) 1.47 4.10 6.35 0.80 0.80 0.88 9.21 1.09 1. 00 24.12 2.76 4.37
2.60(-) 3.20(-) 5.96(+) 18.30(++) 3.50(+) 5.94(+) 18.61(+) 2.49 4.35 2.61 1. 58 2.10 2.58
0.20(-) 0.20(-) 0.54(-) 11.50(+) 0.20 1. 09 8.04 0.43 0.99 4.83 0.36 0.20 0.22 6.00
1.01(-) 1.20(-) 2.50(-) 5.83(-) 1.10 1.10
CP: cyclopentane, CPe: cyclopentene, CPde: cyclopentadiene, t1: methyl, CH: cyclohexane, CHe: cyclohexene, CHde: cyclohexadiene, Bz: benzene, n-C5: normal pentane, n-C5e: normal pentene, n-C6de: normal hexadiene, TN Bz: trimethyl benzene (+) or (-): positive or negative detection of heavy aromatics (indene, ArCI0+) Most of the other constituents of naphtha are also products of the n-para~fins reforming. The coking capacity of n-paraffins and other hydrocarbons on mono and bimetallic catalysts was already shown in Fig. 1. For each catalyst, the values are relative to the coking capacity of n-C7 and the great difference is in the coking capacity of the n-paraffins (curves 1, 2 and 3). It is our aim to explain the different behavior of the catalysts based on a mechanism of coke formation and the different metallic function activity of the catalysts. Possible reactiqns of n-pentane on the bifunctional catalysts are the following: hydrogenolysis producing methane and n-butane, hydrocracking producing ethane and propane, isomerization producing iso-pentane, and cyclization producing cyclopentane. Possible types of reactions of n-C 6 are the same to those of n-C 5 with the addition of aromatization leading to benzene. In former papers it was shown that -on mono as well as on bimetallic catalysts, naphthenes with rings of 5 C atoms (CP, MCP) are the greatest coke precursors among the hydrocarbons of the same number of C atoms (refs. 1 and 2), as shown in Fig. 1.
142
Barbier et al. (refs. 6 and 7), and Parera et al. (ref. S) showed that coke formation results from the transformation of cyclopentadiene (CPde) produced by dehydrogenation of CP on the metallic function of the catalyst. Then, it is possible to suppose that the difference in coke formation from n-C 5 and n-C6 on the different catalysts could be due to the different selectivities to CP and MCP and the different dehydrogenation capacities to transform them into CPde and MCPde, respectively. Data in Fig. 2 and Table 2 are in accordance with this supposition: in the reforming of the light paraffins (n-C5, n-C 6) small amounts of CP and MCP are formed and the coke deposited on the catalysts is related to these amounts, and their dehydrogenation by the catalyst. For instance the amount of CP at 10 and 15 kg cm- 2 are not much different (Fig. 2), but at the higher pressure the amount of coke is many times smaller because the dehydrogenation is smaller due to the higher hydrogen pressure. Pt/A1 203 is the greatest CP and MCP producer and also the greatest coke producer. -2 . On the three catalysts, the amount of MCP obtained from n-C6 at 5 kg cm 1S higher than the amount of CP obtained from n-CS' The decrease in coke formation on Pt/A1 203 passing from n-C 5 to n-C6 shown in Fig. 1 can be ascribed to the lower coking capacity of MCP compared to the one of CP on this catalyst, as shown in the same Figure. The Figure also shows that on the bimetallics n-C6 produces more coke than n-C S' which can be justified by the greater coking capacity of MCP (curves 5 and 6) and its greater production compared to CP. The different coking capacity of the catalysts with compounds having a cyclopentanic ring depends on the dehydrogenating capacity of the catalyst to form a cyclopentadienic (CPde) ring (refs. 6-S). This ring has a conjugate double bond which is very reactive and can condense with other CPde ring by a dienedienophile Diels-Alder type condensation giving an indenic structure (ref. 8). When feeding CP in the case of Pt/A1 203, some indene can be detected (Table 4), and when feeding n-C S some aromatics of Sand 9 C atoms are produced (Table 1). The ArCS are not great coke producers compared to the ArC g which coke production are similar to the ones of cyclopentanes on Pt/A1 203 (Fig. 1). The am~unt of ArC g is small and always the production on Pt/A1 203 is higher. Feeding with n-C7 the production of 5 C atoms ring naphthenes on Pt/A1 203 is smaller than the production of MCP from n-C6, and the coke produced is also smaller. The opposite occurs with the bimetallics (Tables 2 and 3 at 15 kg cm- 2), explaining the difference in mono and bimetallics when passing from n-C6 to n-C 7 in Fig. l. Table 4 shows that increasing the dehydrogenation of the ring in the order CP, CPe, CPde, and MCP, MCPe, MCPde, increases the coking on all the catalysts. When the doping agent is CP, because of thermodynamic limitations only a small amount of CPe and even a smaller one of CPde are produced on the metallic function. Doping with CPe, a bit more CPde is produced and also more coke is formed
143
than in the case of CP. When doping with CPde, the greatest production of coke is obtained. Comparing the coke formation on the metal-acid catalysts (Pt/A1 Z0 3, pt-Re-S/A1 Z03, Pt-Ge/A1 Z0 3) with the one on the acid catalysts (A1 Z03-Cl, SiO ZA1 Z03), it can be seen that the presence of the metallic function able to dehydrogenate CP and CPe is necessary for the production of a great amount of coke. The catalyst with only the acid function produced little coke with CP and CPe, meanwhile the coke is as high as on the metal-acid catalysts when feeding MCPde. SiOZ-A1 Z03 has a stronger acidity than A1 Z0 3-Cl, and produces more coke. Comparing the coking capacity of Pt/SiO Z' it is seen that even the metallic function is able to dehydrogenate CP and CPe, the coke formation is not high because of the absence of acidic function where coke precursors can condense. Then, both functions are necessary to produce coke from CP and MCP: the metallic function is necessary to dehydrogenate up to CPde or MCPde and the acidic function is necessary to condense these coke precursors. It is interesting to compare the coking capacity of the 6 C atoms ring naphthenes on bifunctional and acidic catalysts. It is known (ref. 9) that cyclohexane is rapidly dehydrogenated on the metallic function producing benzene without desorption of the unsaturated intermediates (CHe, CHde). When feeding these intermediates they are also rapidly dehydrogenated in the first part of the catalyst bed, flowing benzene along the bed. Since benzene produces a small amount of coke because of the high stability of the aromatic ring, CH, CHe and CHde produce no much coke on the bifunctional catalysts. On the other hand, the acidic catalysts are not able to dehydrogenate these hydrocarbons. They flow along the catalyst bed and the one more unsaturated, CHde, is able to condense on the acidic function producing an amount of coke greater than that on the bifunctional catalysts. Pt/A1 Z03 is a more dehydrogenating catalyst than the bimetallics and transform more rapidly the cyclohexanes producing less coke. Olefins are poor coke producers. Table 4 shows that I-pentene (l-n-CSe) has a coking capacity Similar to the one of the corresponding paraffin. Diolefins with conjugated double bonds, as l,3-n-C 6de, have greater coking capacity than paraffins and olefins, but still is smaller than the one of CPde and MCPde. All these results confirm that the coke formation in hydrocarbons of S or 6 C atoms is mainly produced by the naphthenes with S C atoms ring which are dehydrogenated to cyclopentadienic rings. The conjugate double bond in a S C atoms ring is more reactive than in an open chain. From n-C7 up, the transformation of n-paraffins into cyclopentanes is negligible and the production of aromatics increases with the length of the chain on all the catalysts. Among the possible products of heavy paraffins reforming, alkyl benzenes and bicyclic aromatics are the greatest coke producers. Then, the selectivity to these products influences the coking capacity of the catalysts. Although the total aromatics production is higher with Pt-Re-S/A1 Z03 than with
144
Pt/A1 203 (ref. 2), the amount of heavy and bicyclic aromatics is greater and greater the coke deposition on the last catalyst. Doping with n-propylbenzene, the coke formation is ~reater on Pt/A1 203 than on Pt-Re-S/A1 Z03 (Table 4), and a higher amount of indene is detected in the reformate with Ft/A1 Z03. This can be assigned to the greater dehydro~enating capacity of Pt/A1 203: the alkylic chain of n-propylbenzene can be dehydrogenated and cyclizated producing indane which on Pt can be dehydrogenated to indene which have the cyclopentadienic ring and is very reactive for condensation to coke. Table 4 shows that indane produces more coke on Pt/A1 Z0 3 than on Pt-Re-S/ A1 Z03, and this could be a consequence of the higher dehydrogenation capacity of Pt/A1 Z0 3 to transform indane to indene. On SiO Z-A1 Z03, indane produces little coke compared to the one produced by indene. This is due to the lack of dehydrogenating capacity to produce the reactive indenic structure from indane. Naphthalene -though being a bicyclic aromatic, has a smaller coking capacity than indene because of the high stability of the aromatic rings compared to the great reactivity of the cyclopentadienic ring. CONCLUSIONS r~echanisms of coke formation on mono (Pt/A1 Z03) and bimetallic (Pt-Re-S/A1 Z03 and Pt-Ge/A1 203) catalysts are similar. The higher coking capacity of Pt/A1 Z03 is due to its higher dehydrogenating capacity producing more cyclopentadienes from n-C5 and n-C6 and more indenic bicyclics from n-C9 and n..C10. Cyclopentadienic rings are important coke precursors due to the great reactivity of the conjugate double bond. This reactivity is greater than the one of the aromatic ring allowing condensation and leading to polycyclic hydrocarbons. The proposed mechanism for coke formation from n-paraffins on the reforming catalysts is shown in Fig. 3. The intermediates are produced from the paraffins and also when using naphtha can enter in the feed. Most part of the monocyclic intermediates leaves the reactor with the reformate, while only a small fraction of the bicyclics leaves it and the other part remains irreversibly adsorbed on the catalyst and is transformed into coke components. The coke formation is a bifunctional reaction requiring the dehydrogenating capacity of the metallic function and the condensating capacity of the acidic function. The dehydrogenating capacity of the bimetallic catalysts is lower than the one of the monometallic. The bimetallics produce smaller amounts of CP rings and have smaller dehydrogenation capacity to produce CPde rings; nevertheless, these catalysts still have the dehydrogenation activity necessary to produce olefins, which are intermediate in the transformation of n-paraffins into aromatics or i-paraffins. In this way, the second metal improves -regarding coke
145
formation, the catalyst metallic function. Also it is interesting to study changes in the acidic function in order to decrease its condensation capacity keeping unaltered its isomerization capacity. FEED n-C 5 -
INTERf1EDIATES DHC
n-C 6 ~
CP ~ 1·1CP ~
CPde f1CPde
C (Diels-Alder) DH of ri ngs Bicyclic hydrocarbons C (indenes, ~ naphthalene)
-
DHC
COKE
Al kyl aromatics
Polycyclic hydrocarbons
C! DH
Turbostratic structure C ~ DH Graphit i c structure
Fig. 3. Mechanism of coke formation from n-paraffins. DHC: dehydrocyclization (metal-acid function), DH: dehydrogenation (on the metallic function) or hydrogen transfer (on the acidic function), C: condensation (acid function) REFERENCES 2 3 4 5 6 7 8 9
J.N. Beltramini, E.E. Martinelli, E.J. Churfn, N.S. Ffgoli and J.M. Parera, Appl. Catal., 7 (1983) 43. J.M. Parera, C.A. Querini, J.N. Beltramini and N.S. Ffgoli, submitted for publication. C.A. Querini, N.S. Ffgoli and J.M. Parera, submitted for publication A.A. Castro, O.A. Scelza, E.R. Benvenuto, G.T. Baronetti and J.M. Parera, J. Catal., 69 (1981) 222. T. Uchiyima, J.H. Hermann, Y. Inoue, R.L. Burwell, J.B. Butt and J.B. Cohen, J. Catal., 50 (1977) 464. J. Barbier, L. Elassal, N.S. Gnep, M. Guisnet and W. Molina, Bull. Soc. Chim. Fr., I (1984) 245. J. Barbier, L. Elassal, N.S. Gnep, M. Guisnet and W. Molina, Bull. Soc. Chim. Fr., I (1984) 250. J.M. Parera, N.S. Ffgoli, J.N. Beltramini, E.J. Churfn and R.A. Cabrol, Proc. 8th Int. Congr. Catal., Berlin, July 1984, Verlag Chemie, Berlin, 1984, p. 593. J.E. Germain, Catalytic Conversion of Hydrocarbons, Academic Press, New York/ London, 1969, p. 81.