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Al2O3Cl coked in a commercial reactor
Applied Catalysis, 52 (1989) 249-262 Elsevier Science Publishers B.V., Amsterdam
249 -
Printed
in The Netherlands
Hydrocarbons reforming on Pt-Re-S/AI,O,-Cl coked in a commercial reactor C.A. QUERINI,
N.S. FIGOLI* and J.M. PARERA
Znstituto de Znvestigaciones en Catcilisisypetroquimica (ZNCAPE) Santiago de1 Ester0 2654, 3000 Santa Fe (Argentina) (Received
1 December
1988, revised manuscript
received 6 March 1989)
ABSTRACT The deactivation of naphtha reforming catalyst under commercial conditions is very slow, and its study on the laboratory scale under similar operating conditions is impractical owing to the length of the experiments. For this reason, samples of coked catalyst withdrawn from a commercial reactor were catalytically tested with several model hydrocarbons (n-hexane, n-heptane and a mixture of n-pentane and cyclohexane). Changes in activity and selectivity due to the coke previously deposited and the selective poisoning of the acid function with n-butylamine were used to determine the contributions of catalytic functions. Samples taken a few days after the beginning of the commercial run showed that the metal function reaches a stationary state of constant activity for cyclohexane dehydrogenation and n-pentane hydrogenolysis. Temperature-programmed oxidation analysis showed that coke on the metal does not increase from that time. On the other hand, coke deposition on the acid function increases during the whole run. Poisoning by n-butylamine showed that the mechanism of n-heptane dehydrocyclization under commercial conditions is controlled by the acid function.
INTRODUCTION
Coke deposition is one of the most important causes of catalyst deactivation during the normal operation of naphtha reforming [l-3]. For this reason, it is important to know how coke affects metallic and acidic functions of the catalyst, and the effect of coke on the main reforming reactions such as dehydrocyclization, isomerization, dehydrogenation and hydrogenolysis. In recent years, several papers have dealt with this subject [ 3-71, most of them reporting data obtained at atmospheric pressure and with catalyst coked in a different way than in industry. It has been demonstrated [8] that the hydrogen pressure is an important factor regulating the metallic fraction, which remains active after a certain time in operation; results obtained at atmospheric pressure cannot easily be extrapolated to those obtained under industrial pressures. When the active metal fraction is too low, the metal may be the controlling function of bifunctional reactions, such as isomerization, dehydrocyclization [ 91 and hy-
0166-9834/89/$03.50
0 1989 Elsevier Science Publishers
B.V.
250
drocracking, which are normally controlled by the acid unction. Probably the previous factor is the reason for discrepancies in the literature concerning important subjects such as which is the function that controls long-term deactivation [ 10,111 or the possible mechanism of n-heptane dehydrocyclization [ 12,13 1. Consequently, it would be very interesting to know the effects produced by coke deposited under commercial conditions (high pressure). We recently presented some results on the controlling function of the long-term deactivation of a catalyst taken from a commercial reactor 1141. The objective of this work was to analyse the effects of coke deposited under industrial conditions on the catalytic activity and selectivity in the reforming of several hydrocarbons, and also to study the active sites involved in several reforming reactions using a specific poison for the acid function. Samples of Pt-Re-S/Al,O,-Cl were taken from an industrial plant during a 7-month run. The samples were used to study the reforming of n-hexane, n-heptane and a mixture of n-pentane and cyclohexane. The following reactions can be studied: paraffin isomerization, hydrogenolysis, hydrocracking and dehydrocyciization and naphthenes dehydrogenation, ring opening and ring contraction. EXPERIMENTAL
Catalyst The catalyst was Pt (0.3% )-Re (0.3% ) /Al,03-Cl coked during its eighth operating cycle in an industrial plant. Samples were taken from the third (last) reactor. The catalyst contained 0.85% Cl, 0.03% S and had a BET specific surface area of 160 m2 g-’ before starting the cycle. The cycle length was 208 days working at 1.5 MPa, and 27.5 barrels of feed per pound of catalyst were passed over the catalyst. The chlorine content of the samples was between 0.77 and 0.93% [ 141. The coke content and time on-stream of the samples are given in Table 1. Catalytic activity determinations Catalytic activity was measured using a fixed-bed reactor as described [ 15,161. Catalysts were reduced in situ by heating from room temperature to TABLE 1 Coke content of catalyst samples Time on-stream (days)
Coke content (%I
Time on-stream
Coke content (%o)
4 26 49 87
1.6 4.7 7.1 8.4
112 161 208
9.2 10.4 14.0
251
773 K under a flow of hydrogen and maintaining the final temperature for 2 h. The reactants and products were analysed chroma~~aphically on-line using a 100-m capillary column coated with squalane and dual flame ionization detectors. Experiments with several feeds (n-hexane, n-heptane and a mixture of n-pentane and cyclohexane) were carried out under the conditions mentioned below. In some experiments the feed was doped with n-butylamine (a poison of the acid function). n-Pentane-cyclohexane
mixture
The mixture contained 70 wt.-% n-pentane and 30 wt.-% cyclohexane, and reforming was carried out at 0.5 MPa, space velocity (WHSV) 2.75 h-l, molar hydrogen-to-hydrocarbon ratio 5.3 and 773 K. Cyclohexane dehydrogenation was 100% using the fresh catalyst at 773 K; consequently, this reaction was studied at 623 K, a temperature at which other reactions of n-C, and CH did not take place. n-Pentane, n-hexane and n-heptane
The experiments were carried out at 773 K, 1.35 MPa, WHSV=2.5 h-’ and a hydrogen-to-hydrocarbon ratio of 8. Carlo Erba RPE reagents were used and the hydrocarbons were dried with molecular sieves 4A before feeding the reactor. RESULTS
Fig. 1 shows the relative activities of the catalyst samples when using the npentane-cyclohexane mixture as the feed as a function of their coke content for the following n-pentane reaction: Isomerization: Hydrogenolysis: Hydrocracking: Dehydrocyclization:
n-C, n-C,+H, n-C5 + H, n-C,
-+ -+ + +
i-C, Cl+n-C4 c,+c, CP+H2
where n-C&= n-pentane; i-C&= isopentane; C, = methane; C3= propane; n-C, = n-butane; CP = cyclopentane. The relative activity is defined as
CZ= ethane;
conversion to a given product using coked catalyst *lo2 conversion to a given product using fresh catalyst Precoked catalysts are stable from the beginning of the run; the activity of the fresh catalyst was measured 30 min after being fed.
252
I
5
Coke on Cbotolysl
,%
Fig. 1. Relative activity of the n-C5 reactions as a function of coke content of the catalyst. Feed: n-C& (70%) +CH (30)%). P=O.5 MPa; T=773 K; WHSV=2.75 h-l; H,/hydrocarbon=5.3. ( A ) Isomerization; (0 ) hydrocracking (conversion to C,); (0 ) dehydrocyclization; (0 ) hydrogenolysis (conversion to C1) .
01”“” 0
5
15
Coke on Csolyst
,%
Fig. 2. Relative activity of the main CH reactions as a function of coke content on the catalyst. Feed: n-C, (70%) +CH (30)%). P=O.5 MPa; WHSV=2.75 h-l; H.Jhydrocarbon=5.3. (A ) Ring contraction, 773 K; (0 ) CH dehydrogenation to B, 773 K; (0 ) CH dehydrogenation to B, 623 K.
Hydrogenolysis (followed by C, formation) is the reaction most affected by coke; a pseudo-stationary state of 15% residual activity is reached. The dehydrocyclization activity also decreases to a pseudo-stationary state, but the residual activity is higher than that of hydrogenolysis. Hydrocracking (followed by the conversion to C,) and isomerization lose activity continuously as the coke content of the catalysts increases, but hydrocracking shows a larger decay of initial activity. Fig. 2 shows the relative activities for cyclohexane reactions using n-pentane-cyclohexane as feed, viz., dehydrogenation to benzene (B ) at 623 and 773 K and ring contraction to methylcyclopentane (MCP) at 773 K. Whereas dehydrogenation reaches a pseudo-stationary state, the ring contraction activity increases with increasing coke content. For the last reaction, the relative ac-
253 TABLE 2 Conversion to the main products using the fresh catalyst Values at 30 min after starting the run. i-C& i-C 7= Isoparaffms with six and seven carbon atoms, respectively; B=benzene; Cracking=sum of the conversions to n-paraffins with fewer carbon atoms than that fed. Feed n-C& (773 K)’
Feed n-C&+ CH
Feed n-C, (773 K)*
Product
Conversion (%)
Product
Conversion (W)
Product
Conversion (So)
G G i-C3 CP Cracking B (623 K) B (773 K) MCP (773 K)
1.4’ 5.0 41.4” 0.5’ 13.5” 55.0d 98.3d 0.7d
C1 G i-C, MCP Cracking B
2.3 9.4 38.9 0.6 30.0 4.2
G C3 i-C, To1 Cracking
2.7 15.0 15.2 28.2 39.5
"P=O.5MPa; WHSV=2.5 h-l; H*/hy~c~bon=5.3. bP= 1.35 MPa; WHSV=2.5 h-l; Hz/hydrocarbon=8. ‘Conversion of n-C& (773 K). dConversion of CH.
01
0
I
I
I
f
1
5
Coke on Co&s+
, %
Fig. 3. Relative activity of the main n-C6 reactions as a function of the coke content of the catalyst. Feed: n-C,. P= 1.35 MPa; WHSVz2.5 h-i; molarH&-C,=8; T=773 K. (a) Totalconversion; (0 ) dehydrocyclization to B; ( A ) conversion to C,-C, paraffins.
tivity is calculated taking as a reference the conversion obtained with the most coked catalyst. Table 2 shows the conversions to the different products obtained with the fresh catalyst. It can be seen that the main reaction of n-C, is isomer&&ion, of CH dehydrogenation, of n-C6 isomerization and cracking and of n-C7 cracking and dehydrocyclization.
254 60
5
Coke on ‘&+olys+ , % Fig. 4. Conver&m of n-C, to isoparaffins and C,-CI paraffins. P= 1.35 MPa; WHSV=2.5 h-‘; molar &/n-i&=8; T=773 K. (m) Total conversion to isoparaffins; (A ) conversion m 2MP + 3MP, ( A ) conversion to 22DMB + 23DMB, ( 0 ) conversion to i-C,; ( 0 ) conversion m iC5; (a) conversion to f&-C, paraffins.
.
I
I
1
5
5
Coke on Cdpalyst , % Fig. 5. Conversion of n-C, to isoparaffins and C,-C, paraffins. P= 1.35 MPa; WHSV=2.5 h-‘; molar Hz/n& = 8; T= 773 K. (II) Total conversion to iaoparaffins; ( A ) conversion to C7 isoparaffins; ( A ) conversion to C, isoparaf~ns; (0 ) conversion to i-C,; (0 ) conversion tc i-C4; ( q ) conversion to C,-C, paraffins; ( X ) conversion to tcluene.
The relative activities for n-C, dehydrocyclization, the total conversion to paraffins with less than six carbon atoms and total n-C6 conversion are shown in Fig. 3. The conversion to B is almost constant (68% relative activity) for coke contents between 4 and 14%. Conversion to lighter paraffins decreases as the catalyst coke content increases. Fig. 4 shows conversion of n-C&to isoparaffins and to C1-CB paraffins. The total isomer production and the conversion to 2MP + 3MP show smooth maxima for 6% coke. Conversions to i-C, and i-C5 are similar, decreasing slowly as coke on the catalyst increases.
255 TABLE 3 Effect of n-butylamine contents
on the main products
of n-C, reforming
for catalysts
with different
coke
i-C,.,=total isoparaffins with 4,5 and 6 carbon atoms. Conditions: P= 1.35 MPa; WHSV=2.5 h-‘; H,/hydrocarbons=8; T=773 K. Cracking: sum of the conversions to n-paraffins with fewer carbon atoms than that fed. Coke on ;Y;lyst 0
N concentration in feed (ppm 1
1.6
“Conversion *Conversion
( W)
To cracking products
To i-C,_,
To To1
0” 12006 3000b
33.0 10.5 9.9
22.3 0.7 0.1
25.0 2.4 1.0
0"
12006 3000b
25.0 9.1 7.8
16.0 0.5 0.1
20.2 1.7 0.6
0 1200b 3000b
20.5 9.0 8.0
10.9 0.6 0.1
13.3 1.3 0.5
7.1
14.0
n-C, conversion
values at 30 min after starting the run. values at 3 h after the start of feeding with the indicated
concentration
of poison.
TABLE 4 Effect of n-butylamine N concentration
on products
n-Cs conversion”
of n-C&, CH and n-C, reforming,
with fresh catalyst
(% )
n-C& conversion’
CH conversion’
(% )
(% )
(ppm) B 0'
3000d “Pc1.35
MCP Cracking i-C6
4.23 0.59 0.69 1.49
30.0 13.3
30.5 7.9
MCP
B
i-C,
C,
G
12.9 2.1
39.0 43.0
45.7 3.7
0.8 0.4
4.5 1.7
MPa; WHSVc2.5
hh’; H,/hydrocarbons=8; T=773 K. hh’; H,/hydrocarbon=3; T=773 K. ‘Conversion values at 5 h after starting the run for n-Cs and n-C, and 30 h for CH (to deactivate the great initial dehydrogenation activity). dConversion values at 3 h after the start of poisoning. Cracking: sum of the conversions to nparaffins with fewer carbon atoms than that fed.
*P=O.5MPa; WHSV=3
The n-heptane conversion to isoparaffins and the distribution according to their carbon atoms number are given in Fig. 5, The conversion to paraffins with six or fewer carbon atoms decreases while coke increases, but the conver-
256 TABLE 5 Molar fraction ratio of iso- and n-paraffins Fresh catalyst. P= 1.35 MPa; T=773 K, WHSV=2.5 Value
Experimental value Equilibrium value
h-‘; H,/hydrocarbon=8.
Feed n-C,
n-C,
i-C, i-C, _---------~-
i-C,
i-Cc,
2MP
3MP
i-C,
i-C&
2MP
3MP
2MH
3MH
n-C,
n-CS
n-C,
n-C:,
n-Cc
n-C,
n-C,
n-C,
n-C&
n-Cs
n-C,
n-C7
0.39
1.17
1.00
1.51
1.05
0.80
1.25
1.72
1.26
0.95
0.99
1.19
0.58
1.83
1.05
0.58
1.18
1.29
n-C,
sion to C, isoparaffins increases. Fig. 5 includes the conversion of n-C, to toluene. The relative activity for this reaction is similar to that of n-C, isomerization, in~cating that the controlling step should be the same for both reactions. Table 3 summarizes the results of experiments with three catalyst samples with different coke contents that were poisoned by doping n-C, with n-butylamine. Table 4 shows results obtained when the three feeds were doped with n-butylamine and Table 5 includes molar fraction ratios for several iso- and n-paraffins, both tables correspon~ng to expe~ments with the fresh catalyst. DISCUSSION
The coke deposited on the catalyst samples was analysed by thermal programmed oxidation (TPO) in order to determine how coke is distributed on the catalyst functions. The thermograms showed two well defined zones, the first corresponding to coke on the metal and the second to coke on the acid [ 17,181. In our catalyst samples the areas in both zones were evaluated using the Simpson discrete method, and the percentage area correspon~ng to each zone was calculated. By multiplying the total coke concentration by the percentage area, the coke concentration corresponding to each function was obtained. These values are shown as a function of time on-stream in Fig. 6. Coke on the metal increases with time in the first and second samples, then remains constant, while coke on the acid sites increases during the whole run. From a qualitative point of view, the method used to discriminate between coke on the metal and on the acid is important. Quantitatively, the coke percentage on each function is only an approximation because it is assumed that the heat of oxidation is the same for both types of coke. Cyclohexane dehydrogenation to benzene is a non-demanding reaction that
METAL
loo
200
Time,
days
Fig. 6. Concentration of coke deposited on each catalytic function as a function of time in commercial operation.
occurs only on the metallic function [191.As the conversion to benzene is constant from the second sample under industrial conditions, the activity of the metallic function decreases only during the first part of the run, then remains constant. The same behaviour was found in Pt/A1203, Pt-Re-S/Al,O,, Pt-Ge/Al,Os coked in the laboratory by means of accelerated deactivation tests
1201.
C-C terminal bond rupture (hydrogenolysis) in paraffins occurs only on the metallic function [ 11, and its relative activity in n-C5 hydrogenolysis shows a similar variation to CH dehydrogenation. The residual relative activity for nC, hy~ogenolysis and CH dehydrogenation were 15% (Fig. 1) and 41% (Fig. 2 ), respectively. Compared with values obtained with Pt-Re-S/A1203-Cl coked at a lower pressure (4 and 5%) respectively [ 20 ] ) , the effect of hydrogen pressure on the residual activity of the metal function can be analysed. Coke was deposited at 1.5MPa during the commercial operation, while the deposition was at 0.35 MPa and then at 1.35 MPa in the laboratory experiments in the previous study [ 201. The higher hydrogen pressure used in industry keeps a larger fraction of the metal active. This means that the results obtained under certain conditions cannot be extrapolated to other conditions and also can explain different conclusions [ 10,14,21-231 about the nature of the long-term deactivation controlling function. From our results, it can be inferred that when working at a low pressure, e.g., 1 atm or less, the metal may become the controlling function of the main reforming reactions, and that the situation will be different at high pressures. Values of the catalytic activities using the n-pence-cyciohexane mixture as feed are in agreement with those obtained by TPO. The amount of coke on the metallic function is lower on the sample taken at 4 days, and the activities for CH dehy~ogenation and n-C5 hydrogenolysis are higher than in the following samples. After the 4-day sample a pseudo-stationary state in metallic
catalytic activity is reached. This means that not only the amount of coke but also its quality and toxicity for the reactions on the metal remain constant during most of the run in an industrial reactor. Isomerization of n-C, to i-C&can occur on the metallic function only by the “bond-shift” mechanism [ 241. It can also take place by a bifunctional mechanism where the metal dehydrogenates the n-paraffin and the acid function isomerizes the olefin [ 1,251. As the metallic function shows a large decrease in the first two catalyst samples, reaching a constant activity with the second, the decrease in isomerization activity that follows is due only to the loss of activity of the acid function. In all the samples, isomerization of n-C5 decreases linearly with the amount of coke on the catalyst, not showing a larger decrease for the first samples. For this reason, isomerization can be considered to be controlled by the acid function of the catalyst. The hydrocracking of n-C, to produce C3 shows a large decrease for the first catalyst samples, then deactivates almost linearly with increase in coke on the samples, as shown in Fig. 1. One possibility is that C3 formation is simultaneously controlled by the metallic and the acid functions. The part taking place on the metallic component decreases to a low, constant value in the first samples, and the part taking place on or controlled by the acid function decreases during the whole run. Another possibility is that only acid sites are active sites for cracking. In this instance, at the beginning of the run and together with the initial coke deposition on the metallic function, the strongest acid sites are weakened by coke deposition. This weakened acidity is inactive for cracking but still active for isomerization, which does not show a large initial deactivation. The catalyst coked in the commercial reactor shows a high activity for ring contraction in the samples with high coke contents. The ring contraction is a bifunctional reaction controlled by the acid function [ 26,271. Fig. 2 shows that the increment in the activity for this reaction continues mainly until the decrease in the dehydrogenation capacity of the metallic function ends (4.7% C). As explained previously [ 201, this behaviour is due to “indirect control” of the metallic function on MCP production. It is called “indirect” because it is regulated by the rapid consumption of CH when transformed into B, and by the abundance of CH when this reaction is deactivated. The small increment in the conversion to MCP from 4.7% C can be explained by considering the conversion of CH and MCP to non-cyclic compounds (2MP + 3MP + n-C&), which is 0.46 and 0.36% for the samples containing 4.7 and 14.0% C, respectively. This means that the increment in the production of MCP once the metal has reached the stationary state is due to lower CH and MCP hydrocracking produced by the acid function deactivation. The conversion of n-C, to isoparaffins with six or fewer carbon atoms shows a smooth maximum for coke contents between 5 and 7%. This maximum is caused by the conversion of n-C, to 2MP and 3MP, because the conversion to
259
other paraffins is almost constant (22DMB and 23DMB) or shows a small decrease (i-C, and i-C6). The conversion to 2MP and 3MP increases up to 5% coke owing to the large decrease in the C-C bond rupture of n-C&, 2MP and 3MP produced by coke deposition. Above 5% coke, the decrease in the degradation reactions is insufficient to compensate for the loss of catalyst isomerization activity, and a decrease in the conversion to 2MP and 3MP is produced. A similar analysis can be made for n-C,, but in this instance there is more CC bond rupture and less isomerization, similarly to a previous observation [ 281. The conversion to isoparaffins with fewer carbon atoms than in the feed compound decreases with increase in coke on the catalyst. This is due to a decrease in hydrocracking because the lighter isoparaffins are produced either by cracking of heavier isoparaffins or by isomerization of the n-paraffins obtained by cracking of the heavier ones. To determine which is the most important mechanism, the ratio between the molar fraction of the isoparaffins and the corresponding n-paraffins will be analysed. Table 5 shows that ratios increase with increasing number of carbon atoms in the feed n-paraffin. For example, the i-C.&o-n-C, ratio has the lowest value for n-C, and the highest for n-C7 as feed. The i-C&o-n-C, ratio is higher than the equilibrium value calculated with published data [ 29 ] when the feed is either n-CG or n-CT, indicating that the formation of i-C, occurs mainly by the cracking of heavier isoparaffins and not by isomerization of n-C,. The conversion of n-C, to i-C, decreases with increasing coke content of the catalyst, whereas the isomerization of n-C&and n-C, shows a maximum. With n-C&,,hydrocracking is very low compared with isomerization, and the deactivation of hydrocracking does not affect the conversion to X5. On the other hand, runs with n-C, and n-C, were operated at a higher pressure (1.35 MPa instead of 0.5 MPa for the n-C,--CH mixture) and C6 and C, are longer paraffins, two reasons for producing greater hy~ocracking than that of n-C, and similar to or greater than the extent of isomerization. Thus, isomerization of n-Cs and n-C7 is affected by hydrocracking modifications, and changes in i-C, and i-C7 production are not direct evidence of catalyst deactivation. For this reason, i-C, is not included in Table 3. i4& is representative of isomerization activity, and results in Table 3 indicate that the reaction is controlled by the acid function because, irrespective of the initial activity and the higher metallic activity of the 1.6% C sample, the three catalyst samples are deactivated to the same extant and the activity with 3000 ppm N is negligible. Table 3 also shows that n-C, dehydrocyclization almost disappears when the acid function is poisoned. This means that at the pressure in our experiments (1.35 MPa), the proposed [ 301 dehydrocyclization mechanism via dienes and trienes on the metallic function does not contribute significantly to the total aromatization after the metal has reached the pseudo-stationary state, and that long-term deactivation is controlled by the deactivation of the acid function.
The Gibbs free energy of ammonia adsorption on the acid function can be calculated from data shown in Table 3 assuming a Lan~uir-tie adsorption isotherm. Ammonia is the product of n-butylamine decomposition under the operating conditions used. The ammonia coverage can be calculated by considering the conversion to toluene as a measure of the acid function activity. The conversion of n-C, to toluene will be propo~ional to the number of active sites if they are homogeneous, i.e., X,, = cxN,, where X 0= conversion to toluene before poisoning, N, = number of active sites before poisoning and ~1is a proportionality constant. After poisoning:
where NP = number of active sites after poisoning. The ammonia coverage can then be calculated using the expression 8
No-hzl
=
No
x,
-X0
The Langmuir adsorption constant is 8
Ic=(l-6)P where P is the ammonia pressure. For each of the three coked catalyst samples used in the poisoning experiments, two values were calculates one using the conversion reached after poisoning with 1200 ppm N and the other with the conversion obtained after poisoning with 3000 ppm N. The values of K calculated in this way are the same, so the assumption that the isotherms are of the Langmuir type is correct. The Gibbs free energies of adso~tion were obtained from the K values, as followings: dG1 = - 6.8 kcal/mol for the sample with 1.6% C, dG,= -7.1 kcal/ mol for the sample with 7.1% C and dGs = - 6.9 kcal/mol for the sample with 14% C. These results show that there are no differences in mean acid strengths of the sites involved in dehydrocy~lization, otherwise dG, a mean value for ammonia adsorption, would change for different coke contents. This explains why the relative activities for n-C, dehydrocyclization and n-pentane isomerization decrease linearly with the catalyst coke content. Cracking (including C1-Cs n-paraffins) represents C-C bond ruptures that can occur partly on the acid and partly on the metal. Taking into account that n-butylamine poisoning decreases the acid function activity to a very low level, the three poisoned catalyst samples should have a small conversion to C,-C, products, after being poisoned with the amine. Table 3 shows that this conversion is nearly lo%, and that the metal is contributing to the total C-C bond
261
rupture. The contribution of the metallic function to cracking justifies the decay in the initial activity of this reaction shown in Figs. 1,3,4 and 5. The dehydrocyclization of n-C6 to benzene is produced by a mechanism with steps controlled by the acid unction, as indicated by the results for n-butylamine poisoning shown in Table 4. The changes in the relative activity for this reaction with coke content of the catalyst shown in Fig. 3 can be explained as a consequence of changes in selectivity induced by coke. The stabilization observed in the relative activity is due to a compensation between the tendency to decrease because of the deactivation of the acid function and the tendency to increase because of the decrease in hydrocracking. Table 4 shows that the effect of n-butylamine on conversion to MCP is different when feeding n-C6 or CH. There is an increase in the former instance and a decrease in conversion to MCP in the latter. CH is converted into benzene (on the metal function) and into MCP (bifunctional reaction, controlled by the acid function). In this way, with poisoning of the acid function there is a decrease in the conversion of CH into MCP and no change in benzene. Otherwise, n-C& is converted sequentially to benzene through the following bifunctional mechanism: n-C,Zn-Cr
A~R~cP~McP=%I-I=L
M are very rapid steps catalysed by the metallic function and Ai and AZ are steps catalysed by the acid function. AZ decreases to a greater extent than A1 on poisoning with n-butylamine, and therefore an accumulation of MCP is observed. This increase in MCP when the acid function is deactivated is used in some commercial plants to follow catalyst deactivation and as an indication of chlorine addition and temperature increase. Table 4 confirms results in Table 3, as the decrease in isomerization due to poisoning of the acid unction is larger than the decrease in cracking. Hydrogenolysis of n-C5 is produced on the metallic function, and the decrease in Ci produced by poisoning of the acid function is due to the decrease in i-C&production, because i-C5 is more easily hydrogenolysed than n-C, [ 201. The ZMP-to-3MP ratio during n-hexane reforming is used to determine the main isomerization mechanism on a bifunctional catalyst [ 5 1. In the mechanisms occurring on the metal, the value of the 2MP-to-3MP ratio is 2 or more; in the “bond-shift” mechanism 2MP is initially obtained and in the mechanism with a five-carbon ring intermediate the 2MP-to-3MP ratio is ‘2. In the bifunctional acid-controlled mechanism, the equilibrium relation is obtained. According to Sujo and Christoffel’s data [31], the value for that ratio is 1.25 at 450°C. This value is very close to those obtained in this paper; the 2MP-to3MP ratio was 1.31 ( -+0.02) for all samples. This indicates that the isomerization on the acid function determines the concentration of isomers during reforming.
262 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
B.C. Gates, J.R. Katzer and G.C.A. Schuit, Chemistry of Catalytic Processes, McGraw-Hill, New York, 1979, p. 184 J.P. Franck and G. Martino, in J.L. Figueiredo (Editor), Progress in Catalyst Deactivation, Martinus Nijhoff, The Hague, 1982, p. 355. J.P. Franck and G. Martino, in J. Oudar and H. Wise (Editors), Deactivation and Poisoning of Catalysts, Marcel Dekker, New York, 1985, p. 205. G. Lietz, J. Viilter, M. Dobrovolszky and Z. Pa& Appl. Catal., 13 (1984) 77. Z. Paal, M. Dobrovolszky, J. Vijlter and G. Lietz, Appl. Catal., 14 (1985) 33. J. Barbier, Appl. Catal., 23 (1986) 225. J. Biswass, P.G. Gray and D.D. Do, Appl. Catal., 32 (1987) 249. E. Churin, Thesis, University of Poitiers, 1986. J.H. Sinfelt, H. Hurwitz and J.C. Rohrer, J. Catal., 1 (1962) 481. J. Margitfalvi and S. Gobolos, Appl. Catal., 36 (1988) 331. V.K. Shum, J.B. Butt and W.M.H. Sachtler, Appl. Catal., 36 (1988) 337. Z. Pa& J. Catal., 105 (1987) 540. J.M. Parera, J. Catal., 105 (1987) 543. J.M. Parera, C.A. Querini and N.S. Ffgoli, Appl. Catal., 44 (1988) Ll. M.R. Sad, N.S. Ffgoli, J.N. Beltramini, E.L. Jablonski, R.A. Lazzaroni and J.M. Parera, J. Chem. Technol. Biotechnol., 30 (1980) 374. N.S. Ffgoli, M.R. Sad, J.N. Beltramini, E.L. Jablonski and J.M. Parera, Ind. Eng. Chem., Prod. Res. Dev., 19 (1980) 545. J. Barbier, P. Marecot, N. Martin, L. Elassal and R. Maurel, in B. Delmon and G.F. Froment (Editors), Catalyst Deactivation, Elsevier, Amsterdam, 1980, p. 53 N.S. Ffgoli, J.N. Beltramini, A.F. Barra, E.E. Martinelli, M.R. Sad and J.M. Parera, ACS Symp. Ser., No. 202 (1982) 239. M. Boudart, Adv. Catal., 20 (1969) 153. C.A. Querini, N.S. Ffgoli and J.M. Parera, Appl. CataI., 53 (1989) 53. W.M.H. Sachtler and P. Biloen, Am. Chem. Sot., Div. Pet. Chem., Prepr., Seattle Meeting, March. 20-25 (1983) 482. J. Vijlter and V. Kiirscher, Appl. Catal., 8 (1983) 171. V.K. Shum, J.B. Butt and W.M.H. Sachtler, Appl. Catal., 36 (1988) 337. Z. Paal and P. Tetenyi, Catalysis, Specialist Periodical Report, Vol. 5, Chemical Society, London, 1981, p. 80. J.H. Sinfelt, H. Hurwitz and J.C. Rohrer, J. Phys. Chem., 64 (1962) 892. P.B. Weisz, Actes PBme.Congr. Int. Catal., Editions Technip, Paris, 1961, Tome 1, p. 937. M. Chow, S.H. Park and W.M.H. Sachtler, Appl. Catal., 19 (1985) 349. J.M. Parera, C.A. Querini, J.N. Beltramini and N.S. Fgoli, Appl. Catal., 32 (1987) 117. D.R. Stull, E.F. Westrum and C.C. Sinker, The Chemical Thermodynamics of Organic Compounds, Wiley, New York, 1969. Z. Pa&, J. Catal., 105 (1987) 540. I. Surjo and E. Christoffel, J. Catal., 60 (1979) 133.
249 -
Printed
in The Netherlands
Hydrocarbons reforming on Pt-Re-S/AI,O,-Cl coked in a commercial reactor C.A. QUERINI,
N.S. FIGOLI* and J.M. PARERA
Znstituto de Znvestigaciones en Catcilisisypetroquimica (ZNCAPE) Santiago de1 Ester0 2654, 3000 Santa Fe (Argentina) (Received
1 December
1988, revised manuscript
received 6 March 1989)
ABSTRACT The deactivation of naphtha reforming catalyst under commercial conditions is very slow, and its study on the laboratory scale under similar operating conditions is impractical owing to the length of the experiments. For this reason, samples of coked catalyst withdrawn from a commercial reactor were catalytically tested with several model hydrocarbons (n-hexane, n-heptane and a mixture of n-pentane and cyclohexane). Changes in activity and selectivity due to the coke previously deposited and the selective poisoning of the acid function with n-butylamine were used to determine the contributions of catalytic functions. Samples taken a few days after the beginning of the commercial run showed that the metal function reaches a stationary state of constant activity for cyclohexane dehydrogenation and n-pentane hydrogenolysis. Temperature-programmed oxidation analysis showed that coke on the metal does not increase from that time. On the other hand, coke deposition on the acid function increases during the whole run. Poisoning by n-butylamine showed that the mechanism of n-heptane dehydrocyclization under commercial conditions is controlled by the acid function.
INTRODUCTION
Coke deposition is one of the most important causes of catalyst deactivation during the normal operation of naphtha reforming [l-3]. For this reason, it is important to know how coke affects metallic and acidic functions of the catalyst, and the effect of coke on the main reforming reactions such as dehydrocyclization, isomerization, dehydrogenation and hydrogenolysis. In recent years, several papers have dealt with this subject [ 3-71, most of them reporting data obtained at atmospheric pressure and with catalyst coked in a different way than in industry. It has been demonstrated [8] that the hydrogen pressure is an important factor regulating the metallic fraction, which remains active after a certain time in operation; results obtained at atmospheric pressure cannot easily be extrapolated to those obtained under industrial pressures. When the active metal fraction is too low, the metal may be the controlling function of bifunctional reactions, such as isomerization, dehydrocyclization [ 91 and hy-
0166-9834/89/$03.50
0 1989 Elsevier Science Publishers
B.V.
250
drocracking, which are normally controlled by the acid unction. Probably the previous factor is the reason for discrepancies in the literature concerning important subjects such as which is the function that controls long-term deactivation [ 10,111 or the possible mechanism of n-heptane dehydrocyclization [ 12,13 1. Consequently, it would be very interesting to know the effects produced by coke deposited under commercial conditions (high pressure). We recently presented some results on the controlling function of the long-term deactivation of a catalyst taken from a commercial reactor 1141. The objective of this work was to analyse the effects of coke deposited under industrial conditions on the catalytic activity and selectivity in the reforming of several hydrocarbons, and also to study the active sites involved in several reforming reactions using a specific poison for the acid function. Samples of Pt-Re-S/Al,O,-Cl were taken from an industrial plant during a 7-month run. The samples were used to study the reforming of n-hexane, n-heptane and a mixture of n-pentane and cyclohexane. The following reactions can be studied: paraffin isomerization, hydrogenolysis, hydrocracking and dehydrocyciization and naphthenes dehydrogenation, ring opening and ring contraction. EXPERIMENTAL
Catalyst The catalyst was Pt (0.3% )-Re (0.3% ) /Al,03-Cl coked during its eighth operating cycle in an industrial plant. Samples were taken from the third (last) reactor. The catalyst contained 0.85% Cl, 0.03% S and had a BET specific surface area of 160 m2 g-’ before starting the cycle. The cycle length was 208 days working at 1.5 MPa, and 27.5 barrels of feed per pound of catalyst were passed over the catalyst. The chlorine content of the samples was between 0.77 and 0.93% [ 141. The coke content and time on-stream of the samples are given in Table 1. Catalytic activity determinations Catalytic activity was measured using a fixed-bed reactor as described [ 15,161. Catalysts were reduced in situ by heating from room temperature to TABLE 1 Coke content of catalyst samples Time on-stream (days)
Coke content (%I
Time on-stream
Coke content (%o)
4 26 49 87
1.6 4.7 7.1 8.4
112 161 208
9.2 10.4 14.0
251
773 K under a flow of hydrogen and maintaining the final temperature for 2 h. The reactants and products were analysed chroma~~aphically on-line using a 100-m capillary column coated with squalane and dual flame ionization detectors. Experiments with several feeds (n-hexane, n-heptane and a mixture of n-pentane and cyclohexane) were carried out under the conditions mentioned below. In some experiments the feed was doped with n-butylamine (a poison of the acid function). n-Pentane-cyclohexane
mixture
The mixture contained 70 wt.-% n-pentane and 30 wt.-% cyclohexane, and reforming was carried out at 0.5 MPa, space velocity (WHSV) 2.75 h-l, molar hydrogen-to-hydrocarbon ratio 5.3 and 773 K. Cyclohexane dehydrogenation was 100% using the fresh catalyst at 773 K; consequently, this reaction was studied at 623 K, a temperature at which other reactions of n-C, and CH did not take place. n-Pentane, n-hexane and n-heptane
The experiments were carried out at 773 K, 1.35 MPa, WHSV=2.5 h-’ and a hydrogen-to-hydrocarbon ratio of 8. Carlo Erba RPE reagents were used and the hydrocarbons were dried with molecular sieves 4A before feeding the reactor. RESULTS
Fig. 1 shows the relative activities of the catalyst samples when using the npentane-cyclohexane mixture as the feed as a function of their coke content for the following n-pentane reaction: Isomerization: Hydrogenolysis: Hydrocracking: Dehydrocyclization:
n-C, n-C,+H, n-C5 + H, n-C,
-+ -+ + +
i-C, Cl+n-C4 c,+c, CP+H2
where n-C&= n-pentane; i-C&= isopentane; C, = methane; C3= propane; n-C, = n-butane; CP = cyclopentane. The relative activity is defined as
CZ= ethane;
conversion to a given product using coked catalyst *lo2 conversion to a given product using fresh catalyst Precoked catalysts are stable from the beginning of the run; the activity of the fresh catalyst was measured 30 min after being fed.
252
I
5
Coke on Cbotolysl
,%
Fig. 1. Relative activity of the n-C5 reactions as a function of coke content of the catalyst. Feed: n-C& (70%) +CH (30)%). P=O.5 MPa; T=773 K; WHSV=2.75 h-l; H,/hydrocarbon=5.3. ( A ) Isomerization; (0 ) hydrocracking (conversion to C,); (0 ) dehydrocyclization; (0 ) hydrogenolysis (conversion to C1) .
01”“” 0
5
15
Coke on Csolyst
,%
Fig. 2. Relative activity of the main CH reactions as a function of coke content on the catalyst. Feed: n-C, (70%) +CH (30)%). P=O.5 MPa; WHSV=2.75 h-l; H.Jhydrocarbon=5.3. (A ) Ring contraction, 773 K; (0 ) CH dehydrogenation to B, 773 K; (0 ) CH dehydrogenation to B, 623 K.
Hydrogenolysis (followed by C, formation) is the reaction most affected by coke; a pseudo-stationary state of 15% residual activity is reached. The dehydrocyclization activity also decreases to a pseudo-stationary state, but the residual activity is higher than that of hydrogenolysis. Hydrocracking (followed by the conversion to C,) and isomerization lose activity continuously as the coke content of the catalysts increases, but hydrocracking shows a larger decay of initial activity. Fig. 2 shows the relative activities for cyclohexane reactions using n-pentane-cyclohexane as feed, viz., dehydrogenation to benzene (B ) at 623 and 773 K and ring contraction to methylcyclopentane (MCP) at 773 K. Whereas dehydrogenation reaches a pseudo-stationary state, the ring contraction activity increases with increasing coke content. For the last reaction, the relative ac-
253 TABLE 2 Conversion to the main products using the fresh catalyst Values at 30 min after starting the run. i-C& i-C 7= Isoparaffms with six and seven carbon atoms, respectively; B=benzene; Cracking=sum of the conversions to n-paraffins with fewer carbon atoms than that fed. Feed n-C& (773 K)’
Feed n-C&+ CH
Feed n-C, (773 K)*
Product
Conversion (%)
Product
Conversion (W)
Product
Conversion (So)
G G i-C3 CP Cracking B (623 K) B (773 K) MCP (773 K)
1.4’ 5.0 41.4” 0.5’ 13.5” 55.0d 98.3d 0.7d
C1 G i-C, MCP Cracking B
2.3 9.4 38.9 0.6 30.0 4.2
G C3 i-C, To1 Cracking
2.7 15.0 15.2 28.2 39.5
"P=O.5MPa; WHSV=2.5 h-l; H*/hy~c~bon=5.3. bP= 1.35 MPa; WHSV=2.5 h-l; Hz/hydrocarbon=8. ‘Conversion of n-C& (773 K). dConversion of CH.
01
0
I
I
I
f
1
5
Coke on Co&s+
, %
Fig. 3. Relative activity of the main n-C6 reactions as a function of the coke content of the catalyst. Feed: n-C,. P= 1.35 MPa; WHSVz2.5 h-i; molarH&-C,=8; T=773 K. (a) Totalconversion; (0 ) dehydrocyclization to B; ( A ) conversion to C,-C, paraffins.
tivity is calculated taking as a reference the conversion obtained with the most coked catalyst. Table 2 shows the conversions to the different products obtained with the fresh catalyst. It can be seen that the main reaction of n-C, is isomer&&ion, of CH dehydrogenation, of n-C6 isomerization and cracking and of n-C7 cracking and dehydrocyclization.
254 60
5
Coke on ‘&+olys+ , % Fig. 4. Conver&m of n-C, to isoparaffins and C,-CI paraffins. P= 1.35 MPa; WHSV=2.5 h-‘; molar &/n-i&=8; T=773 K. (m) Total conversion to isoparaffins; (A ) conversion m 2MP + 3MP, ( A ) conversion to 22DMB + 23DMB, ( 0 ) conversion to i-C,; ( 0 ) conversion m iC5; (a) conversion to f&-C, paraffins.
.
I
I
1
5
5
Coke on Cdpalyst , % Fig. 5. Conversion of n-C, to isoparaffins and C,-C, paraffins. P= 1.35 MPa; WHSV=2.5 h-‘; molar Hz/n& = 8; T= 773 K. (II) Total conversion to iaoparaffins; ( A ) conversion to C7 isoparaffins; ( A ) conversion to C, isoparaf~ns; (0 ) conversion to i-C,; (0 ) conversion tc i-C4; ( q ) conversion to C,-C, paraffins; ( X ) conversion to tcluene.
The relative activities for n-C, dehydrocyclization, the total conversion to paraffins with less than six carbon atoms and total n-C6 conversion are shown in Fig. 3. The conversion to B is almost constant (68% relative activity) for coke contents between 4 and 14%. Conversion to lighter paraffins decreases as the catalyst coke content increases. Fig. 4 shows conversion of n-C&to isoparaffins and to C1-CB paraffins. The total isomer production and the conversion to 2MP + 3MP show smooth maxima for 6% coke. Conversions to i-C, and i-C5 are similar, decreasing slowly as coke on the catalyst increases.
255 TABLE 3 Effect of n-butylamine contents
on the main products
of n-C, reforming
for catalysts
with different
coke
i-C,.,=total isoparaffins with 4,5 and 6 carbon atoms. Conditions: P= 1.35 MPa; WHSV=2.5 h-‘; H,/hydrocarbons=8; T=773 K. Cracking: sum of the conversions to n-paraffins with fewer carbon atoms than that fed. Coke on ;Y;lyst 0
N concentration in feed (ppm 1
1.6
“Conversion *Conversion
( W)
To cracking products
To i-C,_,
To To1
0” 12006 3000b
33.0 10.5 9.9
22.3 0.7 0.1
25.0 2.4 1.0
0"
12006 3000b
25.0 9.1 7.8
16.0 0.5 0.1
20.2 1.7 0.6
0 1200b 3000b
20.5 9.0 8.0
10.9 0.6 0.1
13.3 1.3 0.5
7.1
14.0
n-C, conversion
values at 30 min after starting the run. values at 3 h after the start of feeding with the indicated
concentration
of poison.
TABLE 4 Effect of n-butylamine N concentration
on products
n-Cs conversion”
of n-C&, CH and n-C, reforming,
with fresh catalyst
(% )
n-C& conversion’
CH conversion’
(% )
(% )
(ppm) B 0'
3000d “Pc1.35
MCP Cracking i-C6
4.23 0.59 0.69 1.49
30.0 13.3
30.5 7.9
MCP
B
i-C,
C,
G
12.9 2.1
39.0 43.0
45.7 3.7
0.8 0.4
4.5 1.7
MPa; WHSVc2.5
hh’; H,/hydrocarbons=8; T=773 K. hh’; H,/hydrocarbon=3; T=773 K. ‘Conversion values at 5 h after starting the run for n-Cs and n-C, and 30 h for CH (to deactivate the great initial dehydrogenation activity). dConversion values at 3 h after the start of poisoning. Cracking: sum of the conversions to nparaffins with fewer carbon atoms than that fed.
*P=O.5MPa; WHSV=3
The n-heptane conversion to isoparaffins and the distribution according to their carbon atoms number are given in Fig. 5, The conversion to paraffins with six or fewer carbon atoms decreases while coke increases, but the conver-
256 TABLE 5 Molar fraction ratio of iso- and n-paraffins Fresh catalyst. P= 1.35 MPa; T=773 K, WHSV=2.5 Value
Experimental value Equilibrium value
h-‘; H,/hydrocarbon=8.
Feed n-C,
n-C,
i-C, i-C, _---------~-
i-C,
i-Cc,
2MP
3MP
i-C,
i-C&
2MP
3MP
2MH
3MH
n-C,
n-CS
n-C,
n-C:,
n-Cc
n-C,
n-C,
n-C,
n-C&
n-Cs
n-C,
n-C7
0.39
1.17
1.00
1.51
1.05
0.80
1.25
1.72
1.26
0.95
0.99
1.19
0.58
1.83
1.05
0.58
1.18
1.29
n-C,
sion to C, isoparaffins increases. Fig. 5 includes the conversion of n-C, to toluene. The relative activity for this reaction is similar to that of n-C, isomerization, in~cating that the controlling step should be the same for both reactions. Table 3 summarizes the results of experiments with three catalyst samples with different coke contents that were poisoned by doping n-C, with n-butylamine. Table 4 shows results obtained when the three feeds were doped with n-butylamine and Table 5 includes molar fraction ratios for several iso- and n-paraffins, both tables correspon~ng to expe~ments with the fresh catalyst. DISCUSSION
The coke deposited on the catalyst samples was analysed by thermal programmed oxidation (TPO) in order to determine how coke is distributed on the catalyst functions. The thermograms showed two well defined zones, the first corresponding to coke on the metal and the second to coke on the acid [ 17,181. In our catalyst samples the areas in both zones were evaluated using the Simpson discrete method, and the percentage area correspon~ng to each zone was calculated. By multiplying the total coke concentration by the percentage area, the coke concentration corresponding to each function was obtained. These values are shown as a function of time on-stream in Fig. 6. Coke on the metal increases with time in the first and second samples, then remains constant, while coke on the acid sites increases during the whole run. From a qualitative point of view, the method used to discriminate between coke on the metal and on the acid is important. Quantitatively, the coke percentage on each function is only an approximation because it is assumed that the heat of oxidation is the same for both types of coke. Cyclohexane dehydrogenation to benzene is a non-demanding reaction that
METAL
loo
200
Time,
days
Fig. 6. Concentration of coke deposited on each catalytic function as a function of time in commercial operation.
occurs only on the metallic function [191.As the conversion to benzene is constant from the second sample under industrial conditions, the activity of the metallic function decreases only during the first part of the run, then remains constant. The same behaviour was found in Pt/A1203, Pt-Re-S/Al,O,, Pt-Ge/Al,Os coked in the laboratory by means of accelerated deactivation tests
1201.
C-C terminal bond rupture (hydrogenolysis) in paraffins occurs only on the metallic function [ 11, and its relative activity in n-C5 hydrogenolysis shows a similar variation to CH dehydrogenation. The residual relative activity for nC, hy~ogenolysis and CH dehydrogenation were 15% (Fig. 1) and 41% (Fig. 2 ), respectively. Compared with values obtained with Pt-Re-S/A1203-Cl coked at a lower pressure (4 and 5%) respectively [ 20 ] ) , the effect of hydrogen pressure on the residual activity of the metal function can be analysed. Coke was deposited at 1.5MPa during the commercial operation, while the deposition was at 0.35 MPa and then at 1.35 MPa in the laboratory experiments in the previous study [ 201. The higher hydrogen pressure used in industry keeps a larger fraction of the metal active. This means that the results obtained under certain conditions cannot be extrapolated to other conditions and also can explain different conclusions [ 10,14,21-231 about the nature of the long-term deactivation controlling function. From our results, it can be inferred that when working at a low pressure, e.g., 1 atm or less, the metal may become the controlling function of the main reforming reactions, and that the situation will be different at high pressures. Values of the catalytic activities using the n-pence-cyciohexane mixture as feed are in agreement with those obtained by TPO. The amount of coke on the metallic function is lower on the sample taken at 4 days, and the activities for CH dehy~ogenation and n-C5 hydrogenolysis are higher than in the following samples. After the 4-day sample a pseudo-stationary state in metallic
catalytic activity is reached. This means that not only the amount of coke but also its quality and toxicity for the reactions on the metal remain constant during most of the run in an industrial reactor. Isomerization of n-C, to i-C&can occur on the metallic function only by the “bond-shift” mechanism [ 241. It can also take place by a bifunctional mechanism where the metal dehydrogenates the n-paraffin and the acid function isomerizes the olefin [ 1,251. As the metallic function shows a large decrease in the first two catalyst samples, reaching a constant activity with the second, the decrease in isomerization activity that follows is due only to the loss of activity of the acid function. In all the samples, isomerization of n-C5 decreases linearly with the amount of coke on the catalyst, not showing a larger decrease for the first samples. For this reason, isomerization can be considered to be controlled by the acid function of the catalyst. The hydrocracking of n-C, to produce C3 shows a large decrease for the first catalyst samples, then deactivates almost linearly with increase in coke on the samples, as shown in Fig. 1. One possibility is that C3 formation is simultaneously controlled by the metallic and the acid functions. The part taking place on the metallic component decreases to a low, constant value in the first samples, and the part taking place on or controlled by the acid function decreases during the whole run. Another possibility is that only acid sites are active sites for cracking. In this instance, at the beginning of the run and together with the initial coke deposition on the metallic function, the strongest acid sites are weakened by coke deposition. This weakened acidity is inactive for cracking but still active for isomerization, which does not show a large initial deactivation. The catalyst coked in the commercial reactor shows a high activity for ring contraction in the samples with high coke contents. The ring contraction is a bifunctional reaction controlled by the acid function [ 26,271. Fig. 2 shows that the increment in the activity for this reaction continues mainly until the decrease in the dehydrogenation capacity of the metallic function ends (4.7% C). As explained previously [ 201, this behaviour is due to “indirect control” of the metallic function on MCP production. It is called “indirect” because it is regulated by the rapid consumption of CH when transformed into B, and by the abundance of CH when this reaction is deactivated. The small increment in the conversion to MCP from 4.7% C can be explained by considering the conversion of CH and MCP to non-cyclic compounds (2MP + 3MP + n-C&), which is 0.46 and 0.36% for the samples containing 4.7 and 14.0% C, respectively. This means that the increment in the production of MCP once the metal has reached the stationary state is due to lower CH and MCP hydrocracking produced by the acid function deactivation. The conversion of n-C, to isoparaffins with six or fewer carbon atoms shows a smooth maximum for coke contents between 5 and 7%. This maximum is caused by the conversion of n-C, to 2MP and 3MP, because the conversion to
259
other paraffins is almost constant (22DMB and 23DMB) or shows a small decrease (i-C, and i-C6). The conversion to 2MP and 3MP increases up to 5% coke owing to the large decrease in the C-C bond rupture of n-C&, 2MP and 3MP produced by coke deposition. Above 5% coke, the decrease in the degradation reactions is insufficient to compensate for the loss of catalyst isomerization activity, and a decrease in the conversion to 2MP and 3MP is produced. A similar analysis can be made for n-C,, but in this instance there is more CC bond rupture and less isomerization, similarly to a previous observation [ 281. The conversion to isoparaffins with fewer carbon atoms than in the feed compound decreases with increase in coke on the catalyst. This is due to a decrease in hydrocracking because the lighter isoparaffins are produced either by cracking of heavier isoparaffins or by isomerization of the n-paraffins obtained by cracking of the heavier ones. To determine which is the most important mechanism, the ratio between the molar fraction of the isoparaffins and the corresponding n-paraffins will be analysed. Table 5 shows that ratios increase with increasing number of carbon atoms in the feed n-paraffin. For example, the i-C.&o-n-C, ratio has the lowest value for n-C, and the highest for n-C7 as feed. The i-C&o-n-C, ratio is higher than the equilibrium value calculated with published data [ 29 ] when the feed is either n-CG or n-CT, indicating that the formation of i-C, occurs mainly by the cracking of heavier isoparaffins and not by isomerization of n-C,. The conversion of n-C, to i-C, decreases with increasing coke content of the catalyst, whereas the isomerization of n-C&and n-C, shows a maximum. With n-C&,,hydrocracking is very low compared with isomerization, and the deactivation of hydrocracking does not affect the conversion to X5. On the other hand, runs with n-C, and n-C, were operated at a higher pressure (1.35 MPa instead of 0.5 MPa for the n-C,--CH mixture) and C6 and C, are longer paraffins, two reasons for producing greater hy~ocracking than that of n-C, and similar to or greater than the extent of isomerization. Thus, isomerization of n-Cs and n-C7 is affected by hydrocracking modifications, and changes in i-C, and i-C7 production are not direct evidence of catalyst deactivation. For this reason, i-C, is not included in Table 3. i4& is representative of isomerization activity, and results in Table 3 indicate that the reaction is controlled by the acid function because, irrespective of the initial activity and the higher metallic activity of the 1.6% C sample, the three catalyst samples are deactivated to the same extant and the activity with 3000 ppm N is negligible. Table 3 also shows that n-C, dehydrocyclization almost disappears when the acid function is poisoned. This means that at the pressure in our experiments (1.35 MPa), the proposed [ 301 dehydrocyclization mechanism via dienes and trienes on the metallic function does not contribute significantly to the total aromatization after the metal has reached the pseudo-stationary state, and that long-term deactivation is controlled by the deactivation of the acid function.
The Gibbs free energy of ammonia adsorption on the acid function can be calculated from data shown in Table 3 assuming a Lan~uir-tie adsorption isotherm. Ammonia is the product of n-butylamine decomposition under the operating conditions used. The ammonia coverage can be calculated by considering the conversion to toluene as a measure of the acid function activity. The conversion of n-C, to toluene will be propo~ional to the number of active sites if they are homogeneous, i.e., X,, = cxN,, where X 0= conversion to toluene before poisoning, N, = number of active sites before poisoning and ~1is a proportionality constant. After poisoning:
where NP = number of active sites after poisoning. The ammonia coverage can then be calculated using the expression 8
No-hzl
=
No
x,
-X0
The Langmuir adsorption constant is 8
Ic=(l-6)P where P is the ammonia pressure. For each of the three coked catalyst samples used in the poisoning experiments, two values were calculates one using the conversion reached after poisoning with 1200 ppm N and the other with the conversion obtained after poisoning with 3000 ppm N. The values of K calculated in this way are the same, so the assumption that the isotherms are of the Langmuir type is correct. The Gibbs free energies of adso~tion were obtained from the K values, as followings: dG1 = - 6.8 kcal/mol for the sample with 1.6% C, dG,= -7.1 kcal/ mol for the sample with 7.1% C and dGs = - 6.9 kcal/mol for the sample with 14% C. These results show that there are no differences in mean acid strengths of the sites involved in dehydrocy~lization, otherwise dG, a mean value for ammonia adsorption, would change for different coke contents. This explains why the relative activities for n-C, dehydrocyclization and n-pentane isomerization decrease linearly with the catalyst coke content. Cracking (including C1-Cs n-paraffins) represents C-C bond ruptures that can occur partly on the acid and partly on the metal. Taking into account that n-butylamine poisoning decreases the acid function activity to a very low level, the three poisoned catalyst samples should have a small conversion to C,-C, products, after being poisoned with the amine. Table 3 shows that this conversion is nearly lo%, and that the metal is contributing to the total C-C bond
261
rupture. The contribution of the metallic function to cracking justifies the decay in the initial activity of this reaction shown in Figs. 1,3,4 and 5. The dehydrocyclization of n-C6 to benzene is produced by a mechanism with steps controlled by the acid unction, as indicated by the results for n-butylamine poisoning shown in Table 4. The changes in the relative activity for this reaction with coke content of the catalyst shown in Fig. 3 can be explained as a consequence of changes in selectivity induced by coke. The stabilization observed in the relative activity is due to a compensation between the tendency to decrease because of the deactivation of the acid function and the tendency to increase because of the decrease in hydrocracking. Table 4 shows that the effect of n-butylamine on conversion to MCP is different when feeding n-C6 or CH. There is an increase in the former instance and a decrease in conversion to MCP in the latter. CH is converted into benzene (on the metal function) and into MCP (bifunctional reaction, controlled by the acid function). In this way, with poisoning of the acid function there is a decrease in the conversion of CH into MCP and no change in benzene. Otherwise, n-C& is converted sequentially to benzene through the following bifunctional mechanism: n-C,Zn-Cr
A~R~cP~McP=%I-I=L
M are very rapid steps catalysed by the metallic function and Ai and AZ are steps catalysed by the acid function. AZ decreases to a greater extent than A1 on poisoning with n-butylamine, and therefore an accumulation of MCP is observed. This increase in MCP when the acid function is deactivated is used in some commercial plants to follow catalyst deactivation and as an indication of chlorine addition and temperature increase. Table 4 confirms results in Table 3, as the decrease in isomerization due to poisoning of the acid unction is larger than the decrease in cracking. Hydrogenolysis of n-C5 is produced on the metallic function, and the decrease in Ci produced by poisoning of the acid function is due to the decrease in i-C&production, because i-C5 is more easily hydrogenolysed than n-C, [ 201. The ZMP-to-3MP ratio during n-hexane reforming is used to determine the main isomerization mechanism on a bifunctional catalyst [ 5 1. In the mechanisms occurring on the metal, the value of the 2MP-to-3MP ratio is 2 or more; in the “bond-shift” mechanism 2MP is initially obtained and in the mechanism with a five-carbon ring intermediate the 2MP-to-3MP ratio is ‘2. In the bifunctional acid-controlled mechanism, the equilibrium relation is obtained. According to Sujo and Christoffel’s data [31], the value for that ratio is 1.25 at 450°C. This value is very close to those obtained in this paper; the 2MP-to3MP ratio was 1.31 ( -+0.02) for all samples. This indicates that the isomerization on the acid function determines the concentration of isomers during reforming.
262 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
B.C. Gates, J.R. Katzer and G.C.A. Schuit, Chemistry of Catalytic Processes, McGraw-Hill, New York, 1979, p. 184 J.P. Franck and G. Martino, in J.L. Figueiredo (Editor), Progress in Catalyst Deactivation, Martinus Nijhoff, The Hague, 1982, p. 355. J.P. Franck and G. Martino, in J. Oudar and H. Wise (Editors), Deactivation and Poisoning of Catalysts, Marcel Dekker, New York, 1985, p. 205. G. Lietz, J. Viilter, M. Dobrovolszky and Z. Pa& Appl. Catal., 13 (1984) 77. Z. Paal, M. Dobrovolszky, J. Vijlter and G. Lietz, Appl. Catal., 14 (1985) 33. J. Barbier, Appl. Catal., 23 (1986) 225. J. Biswass, P.G. Gray and D.D. Do, Appl. Catal., 32 (1987) 249. E. Churin, Thesis, University of Poitiers, 1986. J.H. Sinfelt, H. Hurwitz and J.C. Rohrer, J. Catal., 1 (1962) 481. J. Margitfalvi and S. Gobolos, Appl. Catal., 36 (1988) 331. V.K. Shum, J.B. Butt and W.M.H. Sachtler, Appl. Catal., 36 (1988) 337. Z. Pa& J. Catal., 105 (1987) 540. J.M. Parera, J. Catal., 105 (1987) 543. J.M. Parera, C.A. Querini and N.S. Ffgoli, Appl. Catal., 44 (1988) Ll. M.R. Sad, N.S. Ffgoli, J.N. Beltramini, E.L. Jablonski, R.A. Lazzaroni and J.M. Parera, J. Chem. Technol. Biotechnol., 30 (1980) 374. N.S. Ffgoli, M.R. Sad, J.N. Beltramini, E.L. Jablonski and J.M. Parera, Ind. Eng. Chem., Prod. Res. Dev., 19 (1980) 545. J. Barbier, P. Marecot, N. Martin, L. Elassal and R. Maurel, in B. Delmon and G.F. Froment (Editors), Catalyst Deactivation, Elsevier, Amsterdam, 1980, p. 53 N.S. Ffgoli, J.N. Beltramini, A.F. Barra, E.E. Martinelli, M.R. Sad and J.M. Parera, ACS Symp. Ser., No. 202 (1982) 239. M. Boudart, Adv. Catal., 20 (1969) 153. C.A. Querini, N.S. Ffgoli and J.M. Parera, Appl. CataI., 53 (1989) 53. W.M.H. Sachtler and P. Biloen, Am. Chem. Sot., Div. Pet. Chem., Prepr., Seattle Meeting, March. 20-25 (1983) 482. J. Vijlter and V. Kiirscher, Appl. Catal., 8 (1983) 171. V.K. Shum, J.B. Butt and W.M.H. Sachtler, Appl. Catal., 36 (1988) 337. Z. Paal and P. Tetenyi, Catalysis, Specialist Periodical Report, Vol. 5, Chemical Society, London, 1981, p. 80. J.H. Sinfelt, H. Hurwitz and J.C. Rohrer, J. Phys. Chem., 64 (1962) 892. P.B. Weisz, Actes PBme.Congr. Int. Catal., Editions Technip, Paris, 1961, Tome 1, p. 937. M. Chow, S.H. Park and W.M.H. Sachtler, Appl. Catal., 19 (1985) 349. J.M. Parera, C.A. Querini, J.N. Beltramini and N.S. Fgoli, Appl. Catal., 32 (1987) 117. D.R. Stull, E.F. Westrum and C.C. Sinker, The Chemical Thermodynamics of Organic Compounds, Wiley, New York, 1969. Z. Pa&, J. Catal., 105 (1987) 540. I. Surjo and E. Christoffel, J. Catal., 60 (1979) 133.