Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 540-544
540
Isomerization of Ethylbenzene and Xylenes over Mordenite- and Faujasite-Based Catalysts Leon M. Polinskit Pittsburgh Energy Technology Center (PETC), U.S. Department of Energy, Pittsburgh, Pennsylvania 15236
Michael J. Balrd’ Amoco Oil RBD, Naperville, Illinois 60566
The isomerization rate of ethylbenzene over a Pt/AI,O, i- mordenite catalyst was 4 times greater than the same reaction over a R/AI,O, faujasite catalyst. The higher conversion over the mordenite-based catalyst was attributed to product hydrogen diffusion out of the side pores in the mordenite, which would shift steady-state equilibrium to the right. o-Xylene isomerization over the two catalysts had similar conversion levels. The selectivity for ethylbenzene isomerization to xylenes was changed by dealuminating the mordenite.
+
Introduction The isomerization of C, aromatics with significant ethylbenzene conten$ has been the subject of several recent investigations (Carr et al., 1978; Edison and Boyum, 1979). The process is commercially used for the production of oand p-xylenes. o-Xylene is used in the preparation of phthalic anhydride, while p-xylene is principally used for producing polyesters. Feed streams to the isomerization unit usually contain only C, aromatics, i.e., 0-,m-, and p-xylenes and about 15-65 wt % of undesired ethylbenzene. Ethylbenzene is difficult to separate from the xylenes, since their boiling points are very close; thus, it must be destroyed via either hydrodealkylation or isomerization to xylenes. The latter method is preferred if side reactions that destroy C, aromatics can be inhibited. Catalysts containing Pt/A120, mixed with zeolites have been prepared that isomerize ethylbenzene to xylenes with minimum side reactions (Carr et al., 1978). In this investigation, which was performed at Engelhard Industries, the isomerization of xylenes and ethylbenzene has been studied by using Pt/A120, catalysts containing different types of zeolites. Data are presented which suggest that the higher conversions of ethylbenzene to xylenes over mordenite-based catalysts relative to faujasite-based catalysts may be related to the selective diffusion of product H2 out of the small side pores of the catalyst. This behavior is related to the “diffusion-manipulated steady-state (DMSS) concept (Polinski, 1972). Before discussion of the experimental data, a brief review of the DMSS theory will be presented. The diffusion-manipulated steady state has been defined as a situation wherein the relative ratios of the extents of conversion of selected reactants participating in a set of parallel and consecutive reactions have been appreciably modified by changing the diffusion paths of selected components involved in these reactions. The components may be the reactants themselves or reaction intermediates subsequently converted to products. The diffusion paths are modified by use of an appropriate shape-selective Deceased; before PETC formerly employed at Engelhard Industries. 0 196-4321/85/ 1224-0540$01.50/0
catalyst that can act as a semipermeable membrane. The localized removal of only certain selected components by the semipermeable membrane has the effect of shifting reaction equilibria in a direction toward formation of the selected components so “removed”. An example of this concept is the selected separation of hydrogen during the steam-reforming reaction taking place in a palladium-alloy shell and tube reactor. The shell side of the reactor is packed with nickel catalyst. During the reaction product hydrogen selectively diffuses through the palladium-alloy tube, thus removing itself from the reacting system and shifting the reaction to the right. In such a system, steady-state conversions of methane of 98% were obtained (Setzer and Eggen, 1969). Under similar conditions in an open system without selective H2 diffusion, the maximum CH4 conversion is 14%. The diffusion-manipulated steady-state concept has been developed from fundamental concepts (Weisz, 1962) used to describe polystep catalyzed reactions. Weisz’s model described the dual role of a platinum-alumina catalyst for re-forming reactions. As an example, for a multistep reaction of the type A-B-Ccatalyst X
catalyst Y
(a)
Weisz showed the overall rate to be a function of the equilibrium concentration of B, the effective diffusivity of B, Deff,and the radius, R, of the two catalyst particles, X and Y. Thus, the overall rate of reaction a is given by
Effective diffusivity is defined (Satterfield, 1970) as Dk8
9i’00r,6(T/MB)’i2
7B
7B
D,ff = - -
(2)
where Dk = Knudsen diffusivity, TB = tortuosity of molecule B through a catalyst pore, T = temperature (kelvin), MB = molecular weight of B, 0 = catalyst porosity, and r p = catalyst pore radius. When 1 and 2 are combined, the maximum rate under nonequilibrium conditions is 0 1985 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 4, 1985 541
9700[B],,r,B(T/M~)~/~
(3)
R2rB
The rate of reaction a can be altered by changing T , R, rp, and/or
7B.
-
For multiple reactions such as A, s B, C1 A2
s B2
-+
(b)
C2
(C)
over the same catalyst the ratio of the maximum rates of each reaction is
seg I
(4)
which is also a measure of the relative separability of B1 and B2. The dimensionless number in eq 4 is defined as a segregation number. For catalysts that contain pores larger (>30 A) than typical organic molecules (4-6 A) 7B1
hydrocarbon cracking activities over the different zeolite-based catalysts. Polinski postulated that the 2.8-A connecting ports in mordenite can be exceedingly important because they may act as semipermeable membranes for the separation of the H2 molecule. The H2 molecule, being 2.4 A in diameter (Hersh, 1961), is capable of diffusing through the side ports of mordenite and out of the catalyst, thus leaving a H2-leanmixture within the catalyst volume. With H2 as the reaction product removed from a local catalytic area, the steady-state equilibrium in a flow system is shifted to a higher conversion level. A similar explanation is used in this investigation to explain the large differences in the extents of conversion for ethylbenzene over mordenite- and faujasite-based catalysts. Ethylbenzene/Xylene System The common reactions undergone by mixtures of xylene and ethylbenzene under isomerization conditions form a complex array of reaction paths. For purposes of simplifying the C8aromatics reaction scheme we shall consider only the following reactions: (1)For xylene isomerization
-
- ‘B2’
Another application of the DMSS concept (Polinski, 1972) is using a single reaction and comparing its activity over different molecular sieves or zeolite catalysts. For reaction a over two different zeolites, Z1 and 22, the ratio of the maximum rates using Weisz’s model is
(2) Ethylbenzene isomerization to xylenes
(5)
Assuming two molecules, B and C, are involved in diffusion, eq 5 can also be written as seg I P I , [Cl, cat Z1 seg I1 = seg I (B), (C), cat 22
TB,ZZTC,ZZ -cc
(3) Typical reactions illustrating loss of C8 configuration of ethylbenzene via side chain cracking
-0
“ZH4
(g)
(6)
7B,Z17C,Z1
If molecule B diffuses through the large pores in both catalysts, its toruosity path will not be a factor in the determination of the extent of conversion. However, if molecule C is small enough and one catalyst has smaller pores than the other, molecule C may diffuse from the reaction regime and thus cause a shift in the extent of conversion. This concept has been used (Polinski, 1972) to explain the differences in activity for the hydrocracking of n-hexane over different zeolite-based catalysts. Voorhies and Hatcher (1969) reported that for n-C6 the hydrocracking activity over 0.5% Pd on H-mordenite was approximately 100 times greater than the corresponding activity over 0.5% Pd on H-faujasite. These results have been interpreted as being related to the differences in the H2-C6 segregation numbers in mordenite compared to those of faujasite. Y-faujasite is a three-dimensional molecular sieve having 9-8, entry and exit ports in all directions. In contrast, mordenite consists of an m a y of parallel tubes only marginally larger in diameter than a benzene molecule. The main parallel mordenite channels, stacked like parallel straws, are 7 A X 6.7 A ellipses (Meier, 1961). The connecting channels in mordenite are so small (2.8 A) that most molecules are completely excluded (Barrer and Peterson, 1964). For this reason most discussions of diffusion in mordenites have been limited to mass transfer in the main parallel channels. However, Polinski used the DMSS concept to explain the unexpected differences in
(4) Typical disproportionation reaction illustrating loss of C8 configuration of a xylene
2
@
4
+
@
(i)
The intermediate in reaction f is compatible with mechanisms of bicyclo-ring formation in xylene isomerization (Corma et al., 1979) and further helps rationalize a greater-than-equilibriumamount of p-xylene formation at reaction conditions reported by Edison and Boyum (1979). Experimental Section The catalysts were prepared by mixing 0.8-1.0 w t % Pt on 7- or y-alumina with either H-mordenite or HY-faujasite. In each preparation, an aqueous solution of H2PtC16 was impregnated onto the alumina support. The wet powder was transferred to a vacuum chamber and evacuated to 27 in of Hg. H2S was bled into the chamber (to 5 in. of Hg) in order to fix the platinum in place. The material was dried and then blended with the molecular sieve component. For some preparations, an alumina
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 4, 1985
binder was added. The mixture was formed into 1/16-in. diameter extrudates which were calcined in air at 500 O C for 2 h. The reference catalyst was a 50/50 mixture of Pt/Al,O, and silica-alumina. Additional details pertaining to catalyst preparations can be found in Carr et al. (1978). The catalyst were tested in a fixed-bed flow reactor for both xylene and ethylbenzene isomerization. All runs were made at 427 or 440 "C, 13.0 atm (1.3 MPa), 8:l H2: hydrocarbon mole ratio, and a range of weight hourly space velocities between 1 and 8. Hydrocarbon feed consisted of 30 wt 70 ethylbenzene + 70 wt % of either o-, m-, or p-xylene. Treatment of Data The activity of each catalyst was defined by the space velocity required for a given approach to equilibrium: the higher the space velocity, the greater the catalyst activity. The approach to equilibrium was calculated from the relationship F. - P.
(7)
where Xi = approach to equilibrium (ATE) for a C8 aromatic isomer, i (Le., ethylbenzene and o-, m-,p-xylenes), Fi = concentration of i in the feed C8 aromatics, Pi = concentration of i in the product C8 aromatics, and, Ei = the equilibrium concentration of i in the C8 aromatics. The equilibrium concentration of each of the four C8 aromatic isomers is a constant at any fixed temperature. The composition of the isomers in the feed was known, and the steady-state product compositions were measured. The rate at which each isomer approaches equilibrium is expressed as a rate constant, which is related t o catalyst activity. For ethylbenzene the rate of isomerization follows first-order kinetics, and the rate constant is kEB = -WHSV In (1 - XEB) (8) where kEB = rate constant for ethylbenzene isomerization, WHSV = weight hourly space velocity, and XEB = fractional approach to equilibrium for ethylbenzene. For the xylenes (0-,m- and p-) the rate is second order and k is expressed as
where kox = rate constant for o-xylene isomerization and Xox = approach to OX equilibrium. (This form of expression also holds for m- and p-xylene.) Therefore, relative activity or seg I1 for o-xylene isomerization over H-mordenite is calculated by the division of kox for the reaction of H-mordenite by kox for the same reaction over the reference catalyst. Thus
The reference catalyst (Pt/A120, + SiO2/Al2O3)is arbitrarily given a value of 100. Higher or lower values for a particular catalyst indicate that a given approach-toequilibrium concentration (ATE) was achieved at a higher or lower space velocity (SV); Le., the catalyst is more active (higher SV) or less active (lower SV) than the reference. Selectivity during the isomerizations is defined as retention of C8aromaticity. This will, in general, be lower as a higher approach to equilibrium is reached, as illustrated in Figure 1. In order t o make valid comparisons, catalysts were examined either at the same ATE or at the same WHSV. All temperatures and pressures remained constant. A catalyst was judged more selective and thus
v)
0 a !-
I
0 p:
a
4)
0
0 J
w>-
I-
t
w
0 p: W
n.
00
85
90
95
PERCENT APPROACH TO EQUILIBRIUM
Figure 1. Percent yield C8 aromatics (selectivity) as a function of percent approach to equilibrium feed mixture: 30% ethylbenzene, 70% m-xylene, 440 "C, 12.9 atm, catalyst 0.4% Pt-on-Al,O, with 20% H-mordenite. Table I. Comparison of Relative Activities for o-Xylene and Ethylbenzene Isomerization over Mordenite and Faujasite Catalysts' o-xylene ethylbenzene isomerization isomerization C8 C8 selectselectRA ivitv. % RA ivity. % 100 5 100 13
run catalvsts descriution A 0.8% P t o n 7-Al,03 (50%) + SiO2-Al2O3(50%) B 0.8% Pt on v-Alz03(50%) + 320 HY faujasite (50%) C 0.8% Pton7-Alz03 (45%) + 450 H-mordenite (45%) + 10% inert binder D repeat of run C 460
23
78
40
16
312
48
12
325
46
"Reaction conditions: 427 "C, 13 atm, H,/:HC ratio 8:1, 1-8 WHSV; run duration was 120 h.
superior when, for a given ATE, it isomerized xylenes and ethylbenzene with a higher yield of C8 components when compared to other catalysts.
Results and Discussion The relative activities and selectivities for the isomerization of o-xylene and ethylbenzene over Pt/A1203mixed with either mordenite or faujasite are summarized in Table I. The relative activity for o-xylene isomerization over the two zeolite-based catalysts was similar in magnitude. However, a 4-fold increase in relative activity was obtained for ethylbenzene isomerization over the mordenite-based catalyst (312, 325 vs. 78). Since the relative activity for o-xylene isomerization over the two catalysts was about the same, the availability of acidity for both catalysts at reaction conditions is similar. Data illustrated in Figure 2 indicates that both xylene isomerization rates and ethylbenzene isomerization rates have the same relative dependence on acidity-function concentration. Thus, it appears that acidity alone cannot account for the large activity difference for ethylbenzene isomerization over mordenite relative to faujasite. Thus, it is postulated for ethylbenzene isomerization that mordenite selectively removes H, via the 2.8-A side channels, creating a diffusion-manipulated rate change by shifting reactions e and f to the right. The C8 selectivity for ethylbenzene isomerization, while similar for the two zeolite-based catalysts, was approximately 2-3 times the value as that for o-xylene isomerization.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 4, 1985 543 350
I
0
A
I
I
€ 8 isomerization to xylenes MX isomerization to PX and OX
z
0 $
300
e a W
5
s
a
c>-
250
t
-I>-
:: W
2 200
W
0
a
I
2
3
4
5
NUMBER OF DEALUMINIZATION TREATMENTS OF H-MORDENITE WITH CONCENTRATED HCI, 70°C, 2 HR, ORIGINAL SiOp/A1203, 14:l I50
I
I
I
1
I
IO
20
30
40
50
Figure 2. Dependency of apparent rates of reaction on amount of acid function (H-mordenite) present. Table 11. Effects of Hydrogenation-Dehydrogenation (HD) Function on the Isomerization of C8 Alkyl Aromatics ATE at WHSV = 4.7 H-morcatalyst Pt, y-A1203, denite, ethylno. wt % wt % wt % benzene p-xylene 0.0
2
0.4
60 70
40
16
30
40
,
Figure 3. Effect of A1203 removal in changing NserI for the set reactions (j + k) (j) ethylbenzene F= m-xylene, (k)ethylbenzene benzene + C2H4 [suspected mordenite side-channel widening].
PERCENT H-MORDENITE IN Pt -Al203-MORDENITE COMPOSITE
1
6
Table 111. Effect of Dealuminization of Mordenite on Extent of Ethylbenzene Conversion seg I number of reaction j/ catalvst dealuminations reaction k H-mordenite 0 1.20 H-mordenite 6 0.14
performed. When this occurred, a change in seg I for the reaction set
80 84
The requirement that the hydrogenation-dehydrogenation (HD) function is necessary for high conversion of ethylbenzene and good selectivity is seen by the following relative comparison of catalyst formulation with and without platinum (Table 11). The effect of Pt on p-xylene conversion, as expected, is not significant. The observation that some HD-function activity in mordenite occurs even without addition of Pt is in line with results by other researchers. Beecher and Voorhies (1969) isomerized npentane and n-hexane successfully with and without Pt metal. The ability of H-mordenite to hydroisomerize paraffins in the absence of group 8-10 metal addition was also demonstrated by Minachev and collaborators (1971). One possible explanation of this phenomenon may be found in the examination of trace metals in commercially available mordenite. The inclusion of 0.2-0.3 wt 70iron in the crystallized commercial material is quite common. If utilized completely, this would be equivalent to about 4 % of the ion-exchangeable sites being occupied by Fe ions. Under reducing conditions the iron would revert to the metal and could act as a source of hydrogenationdehydrogenation functionality. Having postulated the DMSS model as a reasonable mechanism, it was of interest to determine the effect of widening the 2.8-A feeder pores on selectivity of ethylbenzene conversion to xylenes. This procedure amounts to varying the tortuosity, 7,in the mordenite. The change of 7 values in the mordenite catalyst was accomplished by dealumination of the SiOZ/AlzO3structure. This was accomplished by treating the zeolite with concentrated HCl for 2 h at 70 "C. Six treatments were
was observed, as illustrated by data in Figure 3 and Table 111. Reaction k became favored over reaction j as the channels became wider. The suspected change is that, as Al atoms are removed, the side channels and not the main channels are the ones that become larger. This allows C2H4 or C2H6to remove itself from the main channels selectively (as well as Hz). It is well established that dealumination of zeolites enhances acidity. However, increasing acidity would increase the rate of both reactions and would not explain the higher rate of reaction k relative to reaction j. Conclusions The extent of conversion or approach to equilibrium for the isomerization of ethylbenzene over a Pt/A1203+ mordenite catalyst was 4 times the magnitude as for the same reaction over a Pt/Al2O3+ faujasite catalyst. Isomerization of o-xylene over the mordenite- and faujasitebased catalysts had similar conversion levels. The diffusion of Hz out of the side pores of mordenite during the reaction in a flow system was postulated as the driving force for the increased levels of ethylbenzene conversion over the mordenite-based catalyst. Selectivity, or the retention of C8aromatics, was higher for ethylbenzene than for xylene isomerization.
Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 544-549
544
The small isomerization activity for ethylbenzene conversion over A1203+ mordenite (containing no Pt) was attributed to iron impurities in the mordenite. Treatment of mordenite by dealuminating the zeolite resulted in a "postulated" widening of the side pores. This resulted in a change in the selectivity ratio of xylenes/ benzene (derived from ethylbenzene) from 1.2 to 0.14 as C6 and/or C2 migration through the side channels became possible. Acknowledgment We wish to express appreciation to William Carr and Jim Kosco of Engelhard Industries for preparation of many of the catalysts tested. Robert M. Yarrington and Antonio Eleazar, also of Engelhard, are to be thanked for obtaining and calculating the kinetic data. Registry No. EtPh, 100-41-4; o-MeC6H,Me, 95-47-6; p MeC6H4Me,106-42-3;Me2C6H4,1330-20-7;Pt, 7440-06-4;faujasite,
12173-28-3; mordenite, 12173-98-7. Literature Cited Barrer, R. M.; Peterson, D. L. Proc. R . SOC. London, A 1964, 280, 466. Beecher, R.; Voorhies, A. Ind. Eng. Chem. Prod.Res. Dev. 1969, 8 , 366. Carr, W.; Polinski, L.; Hindin. S. G.; Kosko, J. U.S. Patent 4 128591 (to Engelhard Minerals and Chemical Co.), Dec 5, 1978. Corma, A.; Cortes, A.; Nebot, I.; Tomas, F. J . Catal. 1979, 57,444. Edison, R. R.; Boyum, A. A. Oil Gas J. 1979, 77, 140. Hersh, C. K. "Molecular Sieves"; Reinhold: New York, 1961; p 44. Meier, N. M. Z . Krista//ogr. 1961, 115,439. Minachev, K.; Garanin, V.; Isokova, T.; Kharlamov, V.; Bogomolob, V. Adv. Chem. Ser. 1971, No. 102, 441. Polinski, L. Ind. Eng. Chem. Prod. Res. Dev. 1972, 7 1 , 107. Satterfield, C. N. "Mass Transfer in Heterogeneous Catalysis"; MIT Press: Cambridge, MA, 1970; p 42. Setzer, H. J.; Eggen, A. C. W. U.S.Patent 3450500 (to United Aircraft Corp.), June 17, 1969. Voorhies, A,; Hatcher, W. J., Jr. Ind. Eng. Chem. Prod. Res. Dev. 1969, 8 , 361. Weisz, P. B. "Advances in Catalysis and Related Subjects"; Academic Press: New York, 1962; Vol. 13, p 137.
Receiued for reuiew January 30, 1985 Accepted May 31, 1985
Coke Tolerance of Catalytic Reforming Catalysts In-Slk Nam, John W. Eldrldge," and J. R. Kittrellt Department of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts 0 1003
Bimetallic naphtha reforming catalysts are noteworthy in their ability to tolerate up to 20% coke before reaching end-of-run activity. A multilayer coke model is presented that adequately fits accelerated coking rate data on reforming catalysts. The model is used to interpret the possible effects of hydrogen partial pressure and catalyst compositional variables on the coke tolerance of t h e catalyst.
Catalytic reforming is one of the major refinery processes for upgrading gasoline octane numbers and for producing aromatic hydrocarbons. Reactions that produce the desirable aromatic and isoparaffinic products with high octane numbers include dehydrogenation, dehydrocyclization, and isomerization. Reforming reaction kinetics and mechanisms have been discussed in detail elsewhere (Smith, 1959; Mayers et al., 1961; Selman and Voorhies, 1975; DePauw and Froment, 1975). Older reformers were typically high-pressure units operated with a platinum on alumina catalyst. The development of new, stable multimetallic catalysts has permitted the use of significantly lower hydrogen partial pressure (Cecil et al., 1972). While the addition of rhenium does not greatly affect initial catalyst activity, it significantly enhances activity maintenance and increases the tolerance of the catlyst to high coke levels, thus permitting reduced operating pressure. Since Chevron (Kluksdahl, 1968) introduced the platinum-rhenium catalyst, Johnson and LeRoy (1974) have suggested that rhenium removes coking precursors by hydrogenation. This view is supported by Ermakov et al. (1977), whose work indicates that rhenium increases the degree of dispersion of platinum. This was deduced both from the 2- or %fold increase in the H2:Pt ratio attained by H2 adsorption and from the degree of dispersion re'KSE, Inc., Amherst, MA 01004. 0196-4321/85/1224-0544$01.50/0
vealed by X-ray studies of Pt and Re-Pt/Si02 catalysts. They also suggest that rhenium modifies the alumina support so as to maximize the platinum surface area or to resist coke accumulation. The sequential tests of Bertolacini and Pellet (1980) involving comparisons of the cosupported platinum-rhenium catalyst with a mixture or a layered two-catalyst system also indicated that rhenium removes coke precursors during the dehydrogenation reaction. The role of rhenium in the catalyst can thus perhaps be explained by a coking precursor removal mechanism with hydrogen. It is well-known that catalyst coking represents one important mechanism of deactivation for reforming catalysts. The coke formation may occur by several mechanisms, producing the various forms of coke broadly classified as noncatalytic and catalytic coke (Trimm, 1982). Catalytic coke formation on metals is quite complex. It can cause encapsulation of the metal or the growth of carbon deposits forming filamentous or platelet coke a t a metallic site. Although it is clear that the stability of reforming catalysts largely depends on the carbon deposition rate, it is not certain whether coke is formed on metallic sites, acid sites, or both. Figoli et al. (1982) observed an initial rapid deposition of coke that mainly affected the metallic function of the catalyst. When the deposition rate was slower, the deposition of coke was observed to affect the acidic function. The influence of the total pressure and hydrogen to hydrocarbon mole ratio has also been widely investigated, since hydrogen partial pressure is a key variable in 0 1985 American Chemical Society
