Isomerization of Alkyl Aromatic Hydrocarbons P. M. PITTS, JR., J. E. CONNOR, JR.,
AND
L. N. LEUM
The Atlantic Refining Co., Philadelphia, Pa.
A two-stage process for the isomerization of alkyl aromatics is described which makes possible the interconversion of ethylbenzene and the xylenes and the conversion of isopropylbenzene to the various trimethyl and methylethylbenzenes. The process involves the use of the Catforming catalyst (platinum on deactivated silica-alumina) under hydrogen pressure a t two different temperatures. The first step is carried out at temperatures that promote hydrogenation of the aromatics and the second stage at temperatures that reform the aromatics by dehydrogenation. In this sequential operation, the Catforming catalyst promotes a more general isomerization than can be achieved by a single-step high temperature treatment. This i s especially true of cumene where the two-stage operation produced good yields of the trimethyl and methylethylbenzenes, while a one-stage high temperature treatment gave nearly complete dealkylation.
D
UAL function catalysts, which promote both carbonium ion type reactions and hydrogenation-dehydrogenation reactions, have been introduced in recent years by a large number of petroleum research organizations. These catalysts have been applied commercially to the reforming of straight run naphthas and to a lesser extent to the production of aromatics from selected cuts. The Catforming catalyst, consisting of platinum on deactivated silica-alumina, has been described in detail and its activity for reforming naphthas discussed in previous papers ( I , 8, 6). As in the reforming of naphthas, the production of specific chemicals in the naphtha boiling range utilizing this catalyst requires a proper balance of the major reactions promoted by the catalyst. These reactions are 1. The isomerization of paraffins 2. The isomerization of naphthenps 3 . The hydrogenation of aromatics and the dehydrogenation of cyclohexane derivatives 4. The hydrocracking of paraffins and naphthenes 5 . The dehydrocyclization of paraffins
The production of benzene from a selected cut of naphtha with details of the effects of various operating variables was discussed in a recent paper (S). The present paper is a continuation of the application of the Catforming catalyst to the production of specific chemicals in the naphtha boiling range. Considerable interest has been focused on the eight carbon atom alkyl aromatics in recent years. The demand for these materials as chemiral intermediates has made it necessary that the petroleum industry produce quantities to augment the production from coal tar. Each isomer has its use-ethylbenzene for styrene, o-xylene for phthalic anhydride, m-xylene for isophthalic acid, and p-xylene for terephthalic acid-with a wide variation in the demands for each of the raw materials. Considering this wide variation, a process for interconversion of these isomers of the Cs aromatics is extremely desirable. Various catalysts which promote carbonium ion reactions, such as aluminum chloride and boron trifluoride, have been used to isomerize these materials, particularly the xylenes. Ethylbenzene, however, does not isomerize to the xylenes under these conditions to any appreciable extent but undergoes intermolecular migrations (6). This paper presents a process for interconversion of the Cs alkyl aromatics including ethylbenzene. The process involves a two-stage reaction using platinum-silica-alumina under 770
pressure of hydrogen wherein the first stage is carried out under conditions such that the major reactions are the hydrogenation of aromatics and isomerization of the resulting naphthenes, and the second stage is performed under conditions such that the reactions occurring are dehydrogenation or dehydroisomerization of naphthenes and isomerization of alkyl aromatics. This isomerization of the C8 aromatics coupled with established separation techniques for the individual isomers results in a commercial process for production of any one or group of the desired materials a t the production level desired. T o a lesser extent, but also of great interest to the chemical industry are certain Cs aromatic compounds. Eecent descriptions of methylstyrenes in the literature ( 4 ) indicate the interest in methylethylbenzenes which are progenitors of these polymer intermediates. Also the production of cross-linked polyesters may require a good source of the trimethylbenzenes as a raw material. The same two-step isomerization process for CS aromatic isomerization may also be applied to Cs aromatic isomerizations. This gives rise to the possibility of making the trimethyl- or methylethylbenzenes from easily prepared cumene. The twostep operation is essential with a cumene feed as will be evident from the data to be presented.
Experimental High pressure dynamic units, operating procedures, and catalyst preparation techniques have been described in previous papers ( 1 ) . The feed stocks used were technical grade materials. The analyses on both the feed stocks and products were carried out on a Consolidated Engineering mass spectrometer in conjunction with a Beckmann Model IR2 infrared spectrometer for aromatic isomer distribution. Catalysts used were produced by impregnation of a deactivated silica-alumina cracking catalyst of prescribed activity with enough chloroplatinic acid to give the desired platinum concentration after reduction. Ca Aromatic Isomerization. The production of p-xylene by the isomerization of a synthetic mixture of the other three isomers was performed with a conventional Catforming catalyst at 175 pounds per square inch gage and a liquid space velocity (L.S.V.) of 1volume per volume per hour. These data, presented in Table I, show that there is a variation in the extent of conversion of the three isomers in the feed.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 4
GASOLINE PROCESSING Ca Isomerization
Table 1. L.S.V. = 1 . 0 ; HJHC Temperature, F. Total recovery. wt. YG Products (no loss), wt. % CI-Cs Paraffins Ce Paraffins Ethylbenzene o-Xylene m-Xylene p-Xylene Other aromatics CE Naphthenes
=
10; P
..
-
175 lh./sq. inch gage 800 850 900 99.9 99.4 99.0
..
Feed 0:5 19.6 31.9 46.1 0.7 0.6 0.6
+
+
1.2 2.6 13.7 24.1 36.8 14.4 2.9 4.3
1.1 0.7 19.2 24.7 35.5 14.8 2.4 1.1
1.4 0.3 17.9 23.9 35.1 16.6 4.3 0.7
In order to show the isomer distribution more clearly, the percentage composition of only the Cg portion in the products was calculated. These values are shown in Table I1 with the equilibria ( 7 ) a t the temperatures of operation. The ethylbenzene contents in the products a t 850" and 900" F. are practically the same as that of the feed while a part of the ethylbenzene was converted a t 800" F. These results indicate the desirability of using a lower temperature to convert ethylbenzene t o the xylene isomers. A t the pressures used, however, a lower temperature results in appreciable production of naphthenes rather than the desired aromatics and lower pressure is not feasible because of deactivation of the catalyst with coke a t low pressure.
to 1.6% per second as the temperature increases from 800' to 950" F. This is not the usual course of reactions. One obvious explanation for this phenomenon is that some intermediate, whose concentration is inversely proportional to temperature, is involved in a consecutive reaction system from feed t o intermediate to final state. 4.5-
n W
0 2
3.0-
a 0
a a
2.5-
ul
a W
5 0
22
2.0-
OD
0
1.5 I .o
Table
II.
Temperature,
7 000
Comparison of Cs Aromatic Concentrations to Equilibrium Values F.
800
%
Feed Ethylbenzene 19.9 0-X lene 32.5 m-gylene 47.0 p-Xylene 0.7
at equil. 8.0 22.5 47.0 22.5
850
@ ?; product 15.4 27.1 41.4 16.2
%
at equil. 8.5 23.0 46.5 22.0
Cain product 20.4 26.2 37 7 15.7
%
at equil. 9.5 23.0 46.0 21.5
TEMP.
Figure 1.
900
% of
850
900
950
(OF.)
Ethylbenzene isomerization rates
% of
Cs in product 19.2 25.6 37 6 17.7
From these data, it was concluded that ethylbenzene was more difficult to convert to its isomers than the other Cs compounds under conditions where the hydrogenation of aromatics was minirnized, so studies on individual isomers were undertaken to determine their isomerization characteristics. Ethylbenzene. The isomerization of ethylbenzene a t a series of temperatures in the presence of the Catforming catalyst is shown in Table 111. The conversion of ethylbenzene and consequently the production of xylenes decreases a8 the temperature of operation increases. T o show this more clearly, the percentage oi xylenes produced per second of contact time in the catalyst zone (bulk volume of catalyst per volume gas feed per second) has been calculated. This shows a decrease from 3.9% per second
It appeared on the basis of the apparent negative temperature coefficient (of conversion of ethylbenzene to the xylenes) that the intermediate was a partially or completely hydrogenated aromatic. Operation to maximize the intermediate hydrogenated state would increase the conversion of ethylbenzene to xylene. This would involve low temperature operation which favors hydrogenation a t a given pressure, followed by a higher temperature operation to convert the hydrogenated compounds back to aromatics. Operation of this type is shown in Table IV. Partial hydrogenation of ethylbenzene was accomplished a t an average bed temperature of 723" F. and a space velocity of two volumes per hour per volume. These conditions were arbitrarily selected and were not necessarily the optimum conditions. The product of this step was then fed to a higher temperature step where the conversion to aromatics was completed to a large extent and the isomerization continued. Here again are shown
Table IV. Ethy'lbenzeneIsomerization-Two-Stage Operation Table 111.
Ethylbenzene Isomerization-Once-Through Operation P = 175 lb./sq. inch gage: Hz/HC = 10; L.S.V.= 1.0
Temperature F. Total recove&, wt. % Products (no loss), wt. yo C1-G Paraffins CI+ Paraffins Nsphthenes p-X lene na-slene o-Xylene Ethylbenzene Other aromatics Conversion of ethylbenzene, wt. yo Xylenes produced (wt. %) Contact time, sec. Xylenes produced/sec.
April 1955
800 850 900 $50 94.4 95.9 95.7 90.7 Feed
..
.. 0:7 1.2 1.0 97.0 0.1
.. ..
..
..
2.2 1.3 2.2 1.6 0 . 5 0 . 1 . . ,. 3.2 0.9 7.5 5,9 5:2 4:o 14.9 11.3 8.8 5.8 11.5 9.2 7.5 4.5 57.3 69.3 73.6 76.5 3.0 2.0 2.9 7.5 40.9 28.6 24.0 21.1 32.0 24.2 19.2 11.8 8.2 7.9 7.6 7.4 3.9 3.1 2.5 1.6
P = 175 lb./sq. inch gage;
Hz/HC
-
10
First Stage Second Stage 2.0 1.0 1.0 1.0 1.0 723 793 842 894 943 99.8 95.6 95.8 96.5 95.1
Liquid apace velocity Temperature, O F. Total recovery, wt. 7% Products (no loss), wt. 7G Feed Ci-Ca Paraffins . . 0.5 0.9 1.1 2.2 1.3 CE+ Paraffins .. 2.1 1.0 0.4 Naphthenes 30.0 4.8 1.5 0.2 p-Xylene 0:7 12.2 11.1 9.6 8:4 m-Xylene 24.2 21.5 18.7 16.0 o-Xylene 16.6 15.7 13.1 11.4 Ethylbenzene 97.0 66.1 37.1 45.9 48.9 55.2 Other aromatics 0.1 3.4 2.3 2.3 7.1 7.9 Conversion of ethylbenzene, wt. Yo . 61.8 52.7 49.6 43.1 Xylenes produced, wt. % 51.8 46.9 39.8 34.0 Contact time, sec. .. 4:5 8.2 7.9 7.6 7.4 Xylenes produced/total sec. .. .. 4.1 3.8 3.3 2.9
INDUSTRIAL AND ENGINEERING CHEMISTRY
..
:,E]
..
..
.
771
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT Table V. P
=
rn-Xylene Isomerization 175 lb./sq. inch gage; L.S.V.= 1 . 0
Temperature, F. Total recovery, wt. % Products (no loss), wt. % Ci-Ca Paraffins Ce + Paraffins Ca Naphthenes p - X lene m-&lene o-Xylene Ethylbenzene Other aromatios Conversion of nz-xylene Ca Aromatic isomers Total contact time, seo. Cs Isomers/sec. +
Two-Stage Single Pass Second Stage 850 900 850 900 97.5 99.0 95.4 97.2 Feed
2.5 1.4 n4 22:4 44.6 23.9 0.2 4.2 54.5 44.7 7.9 5.7
.,
.. i .'o
98.1 0.8
..
..
.., . ....
2.5 0.8 1.1 22:s 2 2 . 4 44.0 43.8 24.2 24.1 3.3 0.1 2.1 5.9 55.1 55.3 45.3 4 8 . 0 7 . 6 12.4 3.9 6.0
4.0 0.1 0.2 21.6 42.2 24.9 3.3 3.7 56.8 48.0 12.1 4.0
2.1 0.6
0.2
conversions of ethylbenzene and xylenes produced based on the initial feed. Also shown are the per cent xylenes produced per second of contact time (cumulative times for the two steps). The same temperature effect appears here as in the single pass isomerization, but values are higher in this case. Figure 1 gives a comparison of the two methods of operationsingle-step and two-stage. I n this graph the plotted temperatures are the temperatures of the furnace for each point. All temperatures in the single-step runs were within 4' F. of the furnace, while greater than 80% of the bed in the two-step process was within the same limits. Since the endothermic character of the dehydrogenation reaction in the two-step process gives some uncertainty as to the actual temperature to use for graphical representation, the isomer yields per second as a function of the lowest temperatures in the bed are shown in the dotted line. Despite the use of the lowest temperature and the obvious disadvantage of using cumulative times for the two stage process, the greater rate of isomerization for the two-stage process is evident. The slopes of the lines are decidedly different and the production per unit time is much greater a t the high temperatures. This substantiates the theory that an intermediate favored by low temperature is involved in the reaction. m-Xylene. m-Xylene was selected as typical of the xylenes and its isomerization characteristics compared to ethylbenzene. Data from operation with this feed, which duplicated conditions for ethylbenzene in Tables 111 and IV, are shown in Table V. Two facts are important in considering the data. First of all, the conversion per second of m-xylene t o other xylenes is greater than the conversion per second for the ethylbenzene. The xylenes in the products are effectively a t equilibrium for all the operations because of this increased conversion rate. This makes the isomer yields per second of contact time meaningless for the xylene conversion and a comparison of time efficiencies for xylene conversion is not allowable under these conditions. T h e second significant fact is the difference in the ethylbenzene yields of the single-step as compared to the two-stage operation. I n
Table VI. Isomerization of Cumene-Once-Through 0perat ion P = 350 lb./sq. inch gage; Hz/HC Temperature O F. Recovery, wi. 3 '% Feed Products (no loss), wt. % .. CI-Cz Paraffins .. Propane .. C4-CS Paraffins Ce+ Paraffins .. Naphthenes Benzene Trimethylbenzenes Methylethylbenzenes Cumene 95: 8 4.0 Other aromatics
772
=
4 ; L.S.V.= 1 .O 750 850 950 97.0 92.0 96.4
l7:6 43.0 7.9
0.2 24.0 3.5 2.3 4.8 53.3 5.5
0.5 28.6 5.0 1.1 0.9 58.2 1.1
5.6
6.4
4.6
0.4 21.9 3.6
the former case, the yields of ethylbenzene are small enough to be considered insignificant while the yields of ethylbenzene in the two-step operation are about 3%. An intermediate favored by low temperature is essential for conversion of a xylene to ethylbenzene. Cg Aromatic Isomerization (Cumene). A logical continuation of the isomerization of alkyl aromatics is the isomerization of cumene. When one attempts to carry out the isomerization in a single step as shown in Table VI, the major reaction is dealkylation to produce propane and benzene. The yields of C Saromatic isomers are very small. The reaction desired is CsHdi)
CHa I
I
CH3 I
The reaction actually obtained is CaHdi)
Thus, the formation of trimethylbenzenes and methylethylbenzenes from cumene is further complicated by dealkylation. If the rate of hydrogenation and isomerization to materials which are resistant to dealkylation is greater than the dealkylation rate, cumene could be hydrogenated and isomerized preferentially. Subsequent dehydrogenation would produce the desired isomers of cumene Cd&
C?Hdi)
I
l
Data from an operation of this kind are shown in Table VII. The product from the first stage low temperature operation was fed to the second stage as in the previously described operations with the Cg aromatics. A decided improvement is evident, since yields of Cg isomers are as high as 65% whereas a maximum of 8% was obtained in the single-step isomerization.
D i s c u s s i o n of R e s u l t s The isomerization of ethylbenzene t o the xylenes as evidenced from the data does not proceed over platinum-silica-alumina catalysts under conditions where aromatic hydrogenation is forbidden. As temperature is lowered and naphthene concentrations become significant, the conversion of ethylbenzene increases. A hydrogenated intermediate appears to be required. For example, complete hydrogenation to ethylcyclohexane and subsequent isomerization would lead to a mixture of CSand CS ring naphthenes ( I , 3 ) . These would in turn be dehydrogenated or isomerized and dehydrogenated to produce the xylenes. Isolable materials do not include the partially hydrogenated
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 4
GASOLINE PROCESSING
Table VII.
Isomerization of Cumene-Two-Stage
Temperature, e F. Pressure Liquid .~space velocity Total recovery, wt. Yo Products (no loss), wt. % CI-Cz Paraffins Propane C 4 4 6 Paraffins Ce Paraffins Cs-Cs Naphthenes Cs Xaohthenes Benzene Trimethylbenzenes hdethylethylbenaenes Other aromatics
First Stage 587 500 1.0 101.9 0.1 4.4 0.4 4.8 27.7 61.8 0.3 0.3 0.1 0.1
+
800 350 1.0 96.1 0.8 2.9 5.8 10.6
16.1 3.2
1.0 38.5 11.0 7.3
Operation
Second Stage 850 900 350 350 1.0 1.0 90.7 89.8 0.9 4.0 6.4 5.4
1.7 3.6 8.4 2.4 2.8
3.8 48.6 11.2 11.1
2.9 48.7 16.5 13.1
;:I
species but these could also be the desired intermediates without going to complete hydrogenation. Since hydrogenated intermediates are indicated for xyleneethylbenzene interconversions, isomerizations should be performed under different conditions dependent on the feed stock and the desired product. If ethylbenzene is a major reactant or desired product, operation in two stages is desired. The first stage is carried out under conditions of temperature and pressure where hydrogenation of the aromatics and isomerization are accomplished while the second stage proceeds at a higher temperature or lower pressure so that dehydrogenation and further isomerization are accomplished. These two stages may be two distinct operations or simply two temperature zones in the same reactor. If ethylbenzene is neither a major reactant or desired product, the isomerization should be performed under pressure and temperature conditions favoring aromatics to minimize ethylbenzene formation. Xylene isomerizations probably do not require naphthenes (or partially hydrogenated aromatics) as intermediates, but may isomerize by methyl transfers around the aromatic ring by conventional carbonium ion reactions as well as b y the route indicated for ethylbenzene. Isomerization of cumene to other Cg aromatics falls’ into the same pattern as the ethylbenzene isomerization. Here, though, is the additional problem of dealkylation which makes i t essential that the two-step process be performed if reasonable yields are to be attained. The isomerization of cumene must involve a
naphthene intermediate. The most likely course of the reaction is through a methylisopropylcyclopentane as the first step followed by continued isomerization to give a mixture of cycloparaffins approaching thermodynamic equilibrium. The cyclohexane derivatives are directly dehydrogenated in the second step while the cyclopentane derivatives are first isome-ized to cyclohexane compounds and then dehydrogenated. Conclusions
This paper presents a method for interconversion of the Cs aromatics. It also describes a method for the production of methylethylbenzenes and trimethylbenzenes from cumene. These isomerizations coupled with appropriate separation techniques for the desired compounds and recycle of the remainder offer promise as a means of producing useful chemical intermediates a t desired levels. Some insight is also provided into the mechanics of the isomerization reactions. Acknowledgment
The authors wish to express their appreciation to The Atlantic Refining Co. for permission to publish these results. We are also indebted to many of our associates who contributed to this work, especially the members of the analytical division who developed new procedures for our use. literature Cited
(1) Ciapetta, F. G., and Hunter, J. B., IND. ENG.CHEM.,45, 147-64 (1953). (2) Ciapetta, F. G., Pitts, P. M., and Leum, L. N., presented at 122nd Meeting, ACS, Atlantic City, Sept. 14-19, 1952. (3) Canner, J. E., Jr., Ciapetta, F. G., Leum, L. N., and Fowle, RI. J., IND.Exa. CHEM.,47, 152 (1955). (4) Dixon, J. K., and Saunders, K. W., Ibid., 46, 652 (1954). (5) Fowle, M. J., Bent, R. D., Ciapetta, F. G., Pitts, P. M., and Leum, L. N., Advances in Chem. Sei-., ACS, No. 5, p. 76, 1951. (6) Lien, A. P., presented at 125th Meeting, ACS, Kansas City, March 29 to April 1, 1954. (7) Taylor, W. J., Wagmen, D. D., Williams, M. G., Pitzer, K. S., and Rossini, F. D., J . Research Natl. Bur. Standards, 37, 95 (1946). RECEIVEDfor review December 2, 1954.
ACCEPTED January 21, 1955.
END OF SYMPOSIUM
Night
photo
of thlee
cat
crackers at Esso Standard refinery at Baton Rouge, La.
April 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
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