Ind. Eng. C h e m . Res.
1994,33,
493-503
493
Synergism of Pt/yAl203 and Pt/@Zeolite in the Reforming of Naphthenes Panagiotis G. Smirniotis and Eli Ruckenstein' Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, New York 14260
Composite catalysts of Pt supported on both zeolite p and yA1203 are employed for the reforming of methylcyclopentane and methylcyclohexane. Due to its relatively high Pt dispersion, Pt/yA1203 is responsible for dehydrogenation reactions leading to methylcycloolefins. Ptlp zeolite, which has a much higher Bransted acidity than yA1203,is responsible for the bimolecular alkylation reactions leading to aromatics, the transalkylation reactions of aromatics, and the isomerization reactions of methylcycloolefins to c6 cycloolefins which further via dehydrogenation lead to benzene. The residence time plays an important role because the dehydroisomerization reactions are much slower than the bimolecular alkylation reactions. At a high residence time and with methylcyclopentane as feed, the composites generate a synergistic effect which leads to a maximum in the benzene yield at about 40 wt % Pt/p zeolite. With methylcyclohexane as feed, high aromatic (mainly toluene) yields are obtained for zeolite contents below 30 wt % . Bimolecular alkylation reactions, which are stimulated by the zeolite pore structure and the high Bransted acidity of zeolite 6, also occur and lead to aromatics with a larger number of carbon atoms than the feed naphthene and to a better utilization of c -12.0. Hence, the j3 zeolite possesses under the present preparation conditions acidic strengths HO larger than a value between -10.1 and -12.0. As suggested by Tanabe et al. (1989) the value of HO= -12.0 is an upper limit for superacidity and hence catalysts with HOI-12.0 can be considered as superacids. Infrared investigations with pyridine adsorbed on acidic zeolites (Borade and Clearfield, 1992) have shown that P zeolite possesses a higher Brernsted than Lewis acidity. It was found that the Brernsted to Lewis (B/L) acid sites ratio was about 3 for a zeolite sample with a SiOdA1203 ratio similar to that employed in the present study. For alumina, the suspension containing the indicator changed its color for indicators with pK, > -3.0, indicating that the acidic strength of 7-Al2O3 is much lower than that of P zeolite.
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 497 6.0
I
1
I
V 0
440' 470.
C C
0 500.
c
i
0.0
1
0.0
I
0.2
0.4
I
I
0.6
0.8
I 1.0
& Figure 3. Rate of reaction of methylcyclopentane as a function of zeolite content (5 min on stream, WHSV = 1.26 h-l, Pt loading 0.5 w t %).
Earlier investigations (Peri, 1965)have shown that alumina possesses mainly Lewis acid sites. 3.5. Catalytic Results with Composites. The most important reactions occurring during catalytic reforming include dehydroisomerization of naphthenes to aromatics, dehydrocyclization of n-paraffins to naphthenes and aromatics, isomerization of n-paraffins to isoparaffins, and hydrocracking and hydrogenolysis of alkanes and cycloalkanes to lower molecular weight alkanes (Weiszand Prater, 1957; Ciapetta and Wallace, 1971). In the present work the performance of naphthenes is examined. Methylcyclopentane was selected for investigation because it possesses a cyclic ring with five carbon atoms whose enlargement to the c6 cyclic ring, which is a precursor for aromatics, is of high value in reforming (Weisz, 1962). Methylcyclohexane was chosen because it has already a cyclic ring with six carbon atoms, and its simple dehydrogenation can lead to aromatics. It has been shown earlier (Mills et al., 1953;Keulemans and Voge, 1959)that the reforming of methylcyclopentanes over the bifunctional catalyst Ptly-Al~O3occurs in the following sequence: (1) dehydrogenation of the feed molecule over the metal sites to methylcyclopentenes and migration of the latter molecules to the acidic sites of the catalyst; (2) formation of carbenium ions from the methylcyclopentenes over the Bronsted acidic sites, followed by their isomerization-enlargement to a CS cycloolefin; (3) desorption of the latter molecule from the acidic site and further dehydrogenation of the latter molecule over the metallic sites to benzene. The above pathway is summarized in the following reaction scheme:
The slow step for the generation of aromatics was found to be the isomerization of the methylcyclopentenes over the acidic sites of the catalyst; the dehydrogenation steps and the migration of the olefinicintermediates to the acidic sites are much faster (Keulemans and Voge, 1959).
In the presence of the Bronsted acidity of the zeolites, bimolecular alkylation reactions which result in the generation of c>6aromatics occur. These reactions take place between methylcyclopentenes (formed via dehydrogenation of methylcyclopentane over the Pt sites) with carbonium ions formed by the protonation of light paraffins generated by hydrocracking and by the ring opening of methylcyclopentane over the acidic sites. The generated c>6intermediates are disproportionated, producing a c>6 aromatic and a lighter paraffin than that involved originally in the alkylation reaction. The above pathway is summarized for the reaction of methylcyclopentane with a Cq paraffin in scheme 11.
c,+($ "+:-" w
H
I
I
isomerization
alkylation
H
H H /
A
This type of reaction is stimulated by the size of the /3 zeolite pores which enhances the effective bimolecular collisions because the pores of the zeolite /3 are large enough to allow the motion of several molecules but also sufficiently small to generate a favorable steric constraint. Such bimolecular alkylations do not occur in A1203,because its "open" structure does not enhance bimolecular collisions. Hence, by combining the two Pt supported catalysts, one can better balance the higher dehydrogenation capabilities of Ptly-A1203 (due to the higher metal dispersion) with the isomerization and bimolecular alkylation capabilities of the Ptlp zeolite. The activity of the composites as a function of the zeolite content e is presented in Figure 3 for methylcyclopentane as feed. All the data have been recorded at 5 min on stream in order to minimize the effect of coke formation on the product selectivities. The rate of transformation increases with increasing t up to about e = 0.35. A further increase in the zeolite content does not essentially change the activity of the composites. This behavior arises because the PtlA1203 component stimulates mainly dehydrogenation reactions, while the zeolite component stimulates the ring opening and some cracking as primary steps. The increase in activity with increasing e for composites poor in zeolite is due to the enhancement of (a)theisomerization of the methylcyclopentene intermediates generated over the Pt sites to c6 cycloolefins and (b) the bimolecular alkylation reactions. Both reactions shift the dehydrogenation reaction of the feed naphthene (which is rapid and can be considered at equilibrium) to the right, having as a result the increase of the conversion of the feed. For values of e larger than about 0.35, the overall dehydrogenation capability of the composites, which provides cycloolefinic intermediates for isomerization and bimolecular alkylation reactions, decreases. The higher rates of ring opening and cracking counterbalance, however,
498 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994
the decrease in the dehydrogenation rate. As a result, the overall rate of transformation of the feed moleculesremains constant. As mentioned earlier, with methylcyclopentane the main reaction which occurs over these bifunctional catalysts is the isomerization over the acidic sites of the methylcycloolefinic intermediates formed over the Pt sites. The catalytic performance of a Pt/y'Al2O3 enriched in chloride, which can enhance its isomerization capabilities, has been examined for the reaction of methylcyclopentane at 470 "C. It was observed that the level of conversion was the same as that of the sample not enriched in C1-. This is as expected, since as shown in Table 1the C1- concentration reaches a low value after the original severe oxidationreduction cycle. Methylcyclohexane has an entirely different behavior (Figure 4) than methylcyclopentane; the activity remains essentially unchanged with the variation of the zeolite content. Composites rich in yA1203 favor the direct dehydrogenation of methylcyclohexane to toluene because of the relatively high Pt dispersion over these catalysts. The increase of the Brernsted acidity of the composites results in the partial transalkylation of the generated toluene to benzene and Ce aromatics. In addition, bimolecular alkylation reactions between toluene and carbenium ions generated from olefins produced by cracking via their protonation over the Brernsted acid sites result in the generation of C>7 alkylbenzenium ions (Weitkamp, 1985). The latter intermediates are disproportionated to a C>, aromatic and a lighter olefin than that involved originally which can participate in other alkylation reactions. This pathway can be summarized as follows:
+ c=c-c-c
Brmsted acidity
+"
alkylabn
+c-f-c-c H disproponbMtbn
:2.
*
c-i-c / H
c=c-c
5
+
+
(110
A typical product distribution for various composites with methylcyclopentane as feed is given in Table 3. The selectivities for benzene, xylenes, and C3, Cq, and CShydrocarbons increase while that for toluene decreases with increasing zeolite content (Table 3). The activity of the composites is expected to decrease with increasing E , since the Pt dispersion of the composites decreases, and hence the rate of methylcyclohexane dehydrogenation to toluene decreases. However, the increase in the toluene transalkylation and bimolecular alkylation reactions with increasing E shifts the methylcyclohexanedehydrogenation to toluene. This explains why the activity remains almost unchanged with varying E . The yields of the products lumped in two groups, aromatics and C1-C6 paraffinic and olefinic hydrocarbons, are plotted in Figure 5 as a function of the zeolite content
V
40
3.0
440.
0 470. 0 500'
1
C C
C
'I !
+
I
i
0
0.0 0.0
I
I
0.2
0.4
1
I
I
0.6
0.8
1.0
& Figure 4. Rate of reaction of methylcyclohexane a8 a function of zeolite content (5 min on stream, WHSV = 0.49 h-l, Pt loading 0.5 w t 95). Table 3. Product Distribution and Conversion with Methylcyclopentane (WHSV * 1.26 h-l) and Methylcyclohexane (WHSV = 0.49 h-l) at 470 'C for Various Amounts of Zeolite 9, Incorporated into the Composite (6 min on Stream, Pt Loading 0.6 wt %) methylcyclopentane producta (mol %) methane ethene ethane propene propane isobutane 1-butene n-butane 2-methyl-1-propene isopentane n-pentane cyclopentane methylcyclopentenes cyclohexene benzene toluene ethylbenzene o-xylene rn-xylene p-xylene
c
conversion (mol % )
methylcyclohexane
= 0 c = 0.35 c = 0.80 c = 0 c = 0.35 c = 0.80
1.1
0.9
0.7 3.0 1.8 0.9 2.2 0.4 3.1 4.6 3.3 10.5 25.4 32.2 6.7 0.4
0.3 0.6 1.3 1.1 12.4 4.0
0.8 0.9 2.7 2.6 18.0
0.9
0.1 2.5 2.8 50.9 13.5 0.7 1.4 0.5 1.7
54.0
78.7
80.2
0.4
0.6
0.1
2.2 0.9
1.2
8.1
1.6 6.4 0.7 2.6 1.4 0.4 0.5 1.7 30.1 15.7 1.4 0.9 1.6 1.7
2.7
0.5
0.1 0.1
0.9 0.5 0.7 1.1 1.9 2.6
2.7 2.1 0.3 0.2 0.3
1.4 2.2 1.6 1.0 2.3 3.3 3.9 0.9
0.4 0.7 0.7
1.2 96.8
8.9 70.9
0.2
0.9 1.7 3.5
17.4 51.5 0.3 2.6 3.0 6.2
86.8
87.4
90.8
for methylcyclopentane. The yield Yi of speciesi is defined as Yi
= xs;
where X is the conversion of the feed molecule and Si is the selectivity of species i defined as the ratio of the moles of species i generated to the moles of all the products. A broad maximum at about e = 40 w t 5% occurs for the yield of aromatics, while the yields of c& paraffinic and olefinic hydrocarbons and c>6aromatics increase monotonically with e. The maximum in aromatics is mainly due to benzene (Figure 6). As mentioned before, benzene is generated via the dehydrogenation of methylcyclopentane over Pt sites to methylcyclopentenes and the subsequent isomerization-enlargementof the latter molecules over the Brernsted acidic sites of the catalyst followed by dehydrogenation on the metal sites (reaction scheme I) of the generated C6 cycloolefin. The maximum in the benzene yield is a result of the combination of the complementary
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 499
80.0
4
i
I
0
+
V
V 470'
0
500'
440'
C C C
PtlB selectivities
1
60.0
e
I
20.0 0.0
Table 4. Conversion and Selectivities for Methylcyclopentane over Composites Containing Either PtlB or /3 at 470 OC (WHSV= 1.26 h-l, 5 min on Stream, Pt Loading 0.5 wt %)
i
e
Cl-CS
l
LA I
0.0
conversion
0.0 0.5 1.0
54.0 79.1 87.5
33.1 44.3 61.8
0.2
0.4
0.6
0.8
1.0
Figure 5. Yields of lumped products (filled symbols stand for aromatics, open symbols stand for paraffinic and olefinic C1-Cs hydrocarbons)as a function of zeolite contentfor methylcyclopentane (5 min on stream, WHSV = 1.26 h-1, Pt loading 0.5 wt %). The generated methylcyclopentenes and cyclohexenes are included in the estimation of the C6 hydrocarbon selectivity. i
I
I
i
I
0 + 440. C 0 V 470. C 0 e 500' c
3
CS
CZ6
cycloolefins
aromatics
25.4 1.6 0.2
41.5 54.1 38.0
B selectivities
V
E
100.0
c
paraffins and olefins
Cl-CS c
conversion
0.0 0.5 1.0
54.0 72.9 82.1
paraffins and olefins 33.1 37.4 58.7
c6
Cae
cycloolefins 25.4 9.5 6.2
aromatics 41.5 53.1 35.1
aromatic yields and the conversion were smaller than those obtained when Pt/o zeolite was employed (Table 4). This indicates that the presence of Pt in zeolite contributes to some extent to the performance of the composites despite its low dispersion. The presence of platinum in the /3 zeolite enhances, in spite of its low dispersion, the intimacy of the two catalytic functions. Weisz (1962) developed a general criterion for polystep reaction systems which determines the conditions under which there is cooperation between the acidic and the dehydrogenation functions. Such a cooperation exists when the following inequality is satisfied:
--dN 1 R 2 < 1 dt Be, D
0.0 0.0
0.2
0.4
U.
,
I
0.6
0.8
1.0
E Figure 6. Benzene yields (filled symbols) and selectivities (open symbols) as a function of zeolite content for methylcyclopentane (5 min on stream, WHSV = 1.26 h-1, Pt loading 0.5 wt %).
features of Ptly-AlaOs and the Pt/p zeolite. The alumina enhances the dehydrogenation of the feed naphthene to methylcyclopentenes, but since it lacks Brernsted acidity, it cannot stimulate the isomerization-enlargement of the latter olefins to Cg cycloolefins which are precursors to aromatics. On the other hand, the involvement of zeolite p enhances the isomerization reactions of methylcyclopentenes because of its Brcansted acidity. Hence, composites containing both catalysts can maximize the rates of aromatization of alkylcyclopentanes. Bimolecular alkylation reactions (reaction schemes I1 and 111)lead to aromatics with a larger number of carbon atoms than methylcyclopentane. These reactions are responsible for the increase in the molar yields of C3 and Cq hydrocarbons and of C>e aromatics with increasing content of zeolite and are stimulated by the suitable pore size of zeolite /3 which enhances bimolecular collisions. Because the dispersion of Pt is much higher over A1203 than over the zeolite 8, experiments with a composite containing equal amounts of Ptly-Al203 and /3 zeolite free of Pt were carried out at 470 OC. I t was found that the
where R is the radius of the zeolite particles, Be, is the concentration of methylcyclopentenes, dN/dt is the rate of generation of methylcyclopentenes, and D is the diffusion coefficient of the latter molecules in the medium which provides the acidic function (0 zeolite). If one uses for the diffusivity of the methylcycloolefins in the 0 zeolite crystals the value of le7 cm2/s(Palekar andRajadhyaksha, 1986),for the radius of the zeolite crystals the value of 0.5 pm, and for dN/dt the value of 4 X mol/s (Figure 3), the left side of the above relation at 470 "C becomes 0.23 indicating that cooperation between the two functions of the composites is achieved. If relatively large zeolite crystals or very active zeolite samples are employed, the intimacy criterion for cooperation between the two functions can be violated. Hence, the use of Pt in the 6 zeolite, despite its low dispersion, can enhance the dehydrogenation capability of the composite. An entirely different behavior was found for methylcyclohexane. A high yield of aromatics, mainly toluene, was produced by Pt/ y-A1203and by composites containing less than about 30 wt 5% zeolite p. The increase in the zeolite content decreases the total aromatic yield (Figure 7). The main product of the reaction of methylcyclohexane over any of the present composites is toluene, which is produced by the direct dehydrogenation of the feed naphthene. The effect of the zeolite content on the toluene yield and selectivity is presented in Figure 8 for various temperatures. Higher yields of toluene are obtained at lower temperatures probably because of lower cracking rates. Pt/A1203provides a higher toluene yield than any of the present composites, because of its higher Pt
500 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 Table 5. Conversion and Aromatic Yields for Methylcyclopentane (WHSV = 1.26 h-l) and Methylcyclohexane (WHSV = 0.49 h-l) at 470 O C for Various Pt Loadings (5 rnin on Stream, t = 0.5).
100.0
I 80.0 'is
7
0
v
L
:
b
e
+
440'
C
7 470.
C C
00
$
e
l
.
500'
*
T
60.0
T
e
-
i
40.0
aromatic yield 21.2 42.8 46.6
P t (wt % ) 0.1 0.5 1.0
Methylcyclohexane conversion toluene yield 81.2 23.4 83.4 46.1 92.5 43.7
aromatic yield 31.3 64.9 61.3
e
a
l
r
0.1 0.5 1.0
Methvlcvclopentane conversion benzene vield 65.9 19.6 79.1 30.3 87.8 36.8
Pt (wt % 1
0 0
B
0.0
0.0
0.2
0.4
0.6
0.8
The metal loading is the same for both y-Al,Os and zeolite 8.
1.0
E Figure 7. Yields of lumped products (filled symbols stand for aromatics, open symbols stand for C1-Ce paraffinic and olefinic hydrocarbons) as a function of zeolite content for methylcyclohexane (5 min on stream, WHSV = 0.49 h-1, Pt loading 0.5 wt %).
c
100.0 r
R a,
80.0
I
I
I
1
0
440'
C
v v
470.
C
0 e 500. c
1
0
u
.-
i
r'
0.0
2.0
4.0 W H S V , h-1
6.0
8.0
Figure 9. Conversion of methylcyclopentane (open symbols) and methylcyclohexane (filled symbols) as a function of WHSV for a composite of c = 0.5 at 470 OC and 5 min on stream.
u
E
20.0
3
20.0
I
0.0 0.0
0.2
0.4
0.6
0.8
1.0
E Figure 8. Toluene yields (filled symbols) and selectivities (open symbols) as a function of zeolite content for methylcyclohexane (5 min on stream, WHSV = 0.49 h-l, Pt loading 0.5 wt %).
dispersion. The presence of zeolite decreases the toluene yields since the dehydrogenation capability of the composites decreases. However, the yields of benzene and xylenes increase with the content of zeolite as a result of the toluene transalkylation reaction and bimolecular alkylation reactions. As shown in Table 3, Ptly-AlPOs produces only a small amount of benzene and no xylenes. This indicates that the latter catalyst does not stimulate alkylation reactions, because of its lower Brplnsted acidity and its larger pores which do not enhance the bimolecular collisions. 3.6. Effect of Pt Loading. The effect of Pt loading is presented in Table 5 for both naphthenes, for a composite containing equal metal content of y-Al203 and ,6 zeolite. The increase of the loading plays a beneficial role in the aromatization of methylcyclopentane. The conversion increasesmonotonicallywith Pt loading,more significantly below 0.5 wt % Pt. Increased Pt loadings enhance the dehydrogenation capability of the composite, generating larger amounts of methylcycloolefins which can be further isomerized-enlarged to cyclohexenes. Indeed, the yield of aromatics (mainly benzene) increases with Pt loading
(Table 5). The increases in conversion, and in benzene and aromatic yields, are attenuated by increasing the Pt loading. For methylcyclohexane as feed, higher conversions are observed than for methylcyclopentane. The main product is toluene, which results from the direct dehydrogenation of the feed molecule. The increase in Pt loading from 0.1 to 0.5 wt % increases significantlythe toluene and aromatic yields, due to the higher rates of dehydrogenation. Aromatics with a carbon atom number different than C7 are also produced as a result of toluene transalkylation and of bimolecular alkylation reactions stimulated by the /3 zeolite. The increase of the Pt loading above 0.5 wt 7% does not enhance the aromatization performance of the composites, probably because of increased sintering. The yield of aromatics decreases slightly at any temperature for composites with 1.0 wt % Pt in comparison with 0.5 wt % Pt (Table 51, despite the fact that the conversion is maximum at the former metal loading. 3.7. Effect of WHSV. The effect of WHSV on conversion is presented in Figure 9. Relatively high conversions are achieved at small flow rates for both feed molecules; for methylcyclohexane the conversion is almost complete at any temperature. An increase in WHSV results in an abrupt reduction in conversion, while at large values of WHSV the conversion keeps decreasing. For methylcyclopentane, the benzene selectivity (Table 6) is the highest at small values of WHSV, but decreases significantly as WHSV increases. The selectivities for other aromatics, which are produced by bimolecular alkylation reactions, do not decrease in such a significant manner. Moreover, at any value of WHSV except the
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 601 Table 6. Aromatic Product Distribution (mol %) for Methylcyclopentane and Methylcyclohexane as a Function of WHSV at Various Temperatures (5 min on stream, c = 0.5, Pt Loading 0.5 wt 9%)
440 "C WHSV (h-l)
benzene
c>&3 aromatics
1.26 1.71 3.78 5.04 7.55 10.07
34.7 1.8 2.1
9.0 7.5 4.4 4.6 4.2 2.7
1.1
0.9 1.2
WHSV (h-l)
toluene
0.49 0.66 1.46 1.94 2.92
70.5 38.4 35.9 31.4 21.7
.
100.0
30.3 5.5 3.6 2.4 1.6 2.8
440 "C cg, C>7 aromatics
I
0.0
46.3 46.5 23.9 23.1 22.7 I
I
1
0.0
I
2.0
I
4.0 6.0 Time, h
I
8.0
14.5 12.7 11.8 10.2 6.6 3.4
Methylcyclohexane 470 "C toluene cg, C>7 aromatics
14.8 12.1 9.7 9.2 8.1
lI\
0
Methylcyclopentane 470 "C benzene c > g aromatics
10.0
Figure 10. Conversion as a function of time on stream of methylcyclopentane over ( 0 )y-AlzOs, (v)composite with c = 0.5, ( 0 )@ zeolite, and of methylcyclohexane over (*) y-Al203, (W composite with c = 0.5, ( 0 )@ zeolite as a function of time on stream at 470 "C (Pt loading 0.5 w t %, WHSV = 1.26 h-l for methylcyclopentane and WHSV = 0.49 h-l for methylcyclohexane).
smallest one (Table 6), the selectivity for C>Saromatics is higher than that for benzene. The fact that the reduction of the residence time in the reactor decreases more significantly the benzene generation than that of C>6 aromatics indicates that the isomerization reactions of methylcyclopentenes over the acidic sites to cyclohexene and further to benzene are much slower than the bimolecular alkylation reactions. The transalkylation of the Cb7 aromatics over the Brernsted sites may have a small contribution to the formation of benzene. For methylcyclohexane, the generation of toluene is larger than that of any other aromatics at any flow rate (Table 6). Hence, the dehydrogenation of the feed naphthene dominates the bimolecular alkylation reactions. In Table 6 it is shown that both types of reactions have comparable rates of decrease as a function of the residence time through the bed. 3.8. Time on Stream Behavior. The effect of deactivation on the activity of Ptly-AlnOa, Pt/P zeolite, and a composite containing equal weights of the above catalysts is given in Figure 10 for both naphthenes at 470 "C. For methylcyclopentane, one can observe that the conversion decreases significantly with time on stream due to the high deactivation rate. The conversion reaches a constant
20.1 13.3 10.4 8.9 7.9
benzene 37.7 8.2 3.8 2.9 2.7 2.3
toluene 59.4 22.7 21.5 20.8 15.6
500 "C C>e aromatics 14.5 12.2 11.3 12.1 11.1 7.3 500 O C cg, C>7 aromatics 25.2 21.4 13.2 10.0 5.4
value after 4 h on stream, indicating that there are no further changes in the catalyst condition. Other researchers (Beltramini et al., 1983; Parera et al., 1987) have emphasized that naphthenes with a Cg ring (cyclopentane, methylcyclopentane) are the most effectivecoke precursors among hydrocarbons with the same number of carbon atoms. High hydrogen pressures can decrease significantly the rate of coke formation. The role of hydrogen is to keep the Pt and the acidic sites of the catalyst free of hydrogen-deficient hydrocarbon residues which act as poisons by decreasing the activities of both functions of the catalyst (Myers et al., 1961). The inclusion of PtlP zeolite in the composite leads to a higher initial activity and better time on stream behavior than that of PtlyA1203. All the data presented in the present paper, with the exception of Figure 10, are after 5 min on stream in order to minimize the extent at which coke deactivation alters the intrinsic properties of the composites. For methylcyclohexane, the conversion is higher than for methylcyclopentane for small time on stream, because its dehydrogenation to toluene, which constitutes the main reaction, is more rapid than the dehydroisomerization of methylcyclopentane. A slight decrease in conversion occurs at short times on stream, while for times on stream greater than 4 h the conversion remains practically unchanged. The initial rate of decrease of conversion with time on stream is significantly smaller for methylcyclohexane than for methylcyclopentane. The reaction of methylcyclohexane, which involves mainly the Pt sites, exhibits a small deactivation. In contrast, the reaction of methylcyclopentane, which involves in addition the isomerization of methylcycloolefins over the acidic sites, is expected to be more severelyaltered by the coke deposition which takes place with preference on the acidic sites. 4. Conclusions
It is demonstrated that composite catalysts of Pt/zeolite
8 and Ptly-AlnOapossess beneficial effects in the reforming of naphthenes. The advantageous behavior of the composites can be attributed to the synergism between Pt/ 8-A1203and Pt/o zeolite due to the complementary features that each of them provides. Alumina, because of its higher Pt dispersion, favors dehydrogenation reactions which generate methylcyclopentene intermediates, while zeolite 8 favors, due to its Brernsted acidity, the bimolecular alkylation reactions, the transalkylation reaction, and the
502 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994
isomerization-enlargement reactions of methylcyclopentenes to CScycloolefins. Since the latter reactions are slow, they occur at sufficiently large residence times. At large residence times, the yield of aromatization (mainly benzene) of methylcyclopentane passes through a maximum at about 40wt 5% fl zeolite. With methylcyclohexane, a large amount of toluene is generated for a fl zeolite content below 30 wt 5% . A decrease in the toluene yield associated with increasing yields of benzene and xylenes was observed with further increases in zeolite content, due to the transalkylation of the former molecule. The increase of the zeolite content in the composites results in an increase of bimolecular alkylation reactions for both feeds. It was found that the dehydroisomerization reaction of methylcyclopentane is slower than the dehydrogenation and bimolecular alkylation reactions; the latter two reactions are comparably fast. Coke is formed mainly over the acidic sites of the catalyst.
Nomenclature Be, = equilibrium concentration of methylcyclopentenes D = diffusion coefficient of methylcyclopentenesin the acidic function (6zeolite) HO= Hammett acidity function W / d t = rate of generation of methylcyclopentenes R = radius of the zeolite particles Si = selectivity of i lumped species X = conversion of the feed hydrocarbon Yi = yield of i lumped species Greek Symbols t = weight fraction of zeolite /3 in the composites
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Abstract published in Advance ACS Abstracts, February 15, 1994. @
