An alkylaromatic hydrocarbon is isomerized by contacting a feed including the alkylaromatic and hydrogen in the presence of 1.5-150 ppm free chloride, and not more than 10 ppm water, with a catalyst containing platinum, rhenium and more than 1.2 weight percent combined chloride on an alumina support at 650.degree.-950.degree. F and 100-300 psi hydrogen pressure.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for isomerizing alkylaromatic 
hydrocarbons. 
It is often desirable to convert one alkylaromatic hydrocarbon to a more 
valuable isomer thereof. For example, it is often desired to convert 
ethylbenzene and metaxylene into paraxylene and/or orthoxylene. Processes 
for producing particular xylene isomers from C.sub.8 alkylaromatic 
feedstocks are well known. Typically, a selected xylene isomer is 
recovered from a petroleum fraction, such as reformate, which is rich in 
C.sub.8 alkylaromatics, by fractionation, crystallization or a molecular 
sieve-type separation operation. After the selected isomer has been 
removed from the fraction, the C.sub.8 alkylaromatic residue is typically 
treated in a C.sub.8 alkylaromatic isomerization operation in order to 
form additional amounts of the selected isomer. The newly formed amounts 
of the desired isomer are then recovered from the isomerate by the same 
type separation operation used with the original petroleum fraction. In 
converting the various C.sub.8 alkylaromatic isomers, it has been found 
that ethylbenzene is relatively difficult to convert to xylene as compared 
to the relatively easy conversion of one xylen isomer to another. Prior 
art has acknowledged the difficulty of converting ethylbenzene, and the 
problem of ethylbenzene buildup in isomerization-separation systems has 
been an economic and technical drawback in many isomerization operations. 
Various isomerization catalysts and flow schemes have been suggested by the 
art in attempting to provide efficient isomerization and isomer recovery 
systems for producing a selected C.sub.8 alkylaromatic isomer. For 
example, U.S. Pat. No. 25,753 discloses a two-stage process for 
isomerizing xylenes. In the first stage, a xylene, or a nonequilibrium 
mixture of xylenes, is contacted with a hydrogenation-dehydrogenation 
catalyst under hydrogenation conditions to convert a large proportion 
(10-35%) of the xylenes in the feed to naphthenes. In the second stage, 
the naphthenes formed in the first stage are contacted with a 
hydrogenation-dehydrogenation catalyst under dehydrogenation conditions to 
reconvert the naphthenes to xylenes and simultaneously to isomerize the 
xylenes during the dehydrogenation. One catalyst described as useful in 
the process is platinum on alumina or silica-alumina. 
U.S. Pat. No. 3,078,318 describes the isomerization of a xylene or 
nonequilibrium mixture of xylenes with a platinum-halogen-alumina catalyst 
in a hydrogen atmosphere at 700.degree.-1100.degree. F and 1-1500 
atmospheres pressure. A selected xylene isomer is separated from the 
isomerization reactor effluent and the residue from the isomer-separation 
step is recycled to the isomerization step. 
U.S. Pat. No. 3,381,048 describes a process for isomerization of a xylene 
isomer or a nonequilibrium mixture of xylene isomers using a 
platinum-halogen-alumina catalyst. In the process, the water content of 
the hydrocarbon feed to the isomerization reactor is kept at 20-200 ppm. 
U.S. Pat. No. 3,538,173 describes a process for isomerizing xylenes in 
which ethylbenzene in a C.sub.8 alkylaromatic-rich stream is isomerized to 
xylenes by controlling the C.sub.8 napthenes content in the feed 
introduced into the isomerization reactor to keep the C.sub.8 naphthenes 
content of the feed at 2-9 weight percent of the C.sub.8 alkylaromatic 
content of the feed. A platinum-halogen-alumina catalyst is employed in 
the isomerization reactor at a temperature of 700.degree.-840.degree. F 
and a pressure of 3-20 atmospheres. U.S. Pat. No. 3,553,276 describes a 
process for isomerizing xylenes in which, during recovery of a selected 
xylene isomer from the isomerization reactor effluent, loss of C.sub.8 
naphthenes from the system is minimized by maintaining a high 
concentration of diluent toluene in the effluent from the isomerization 
reactor. The retention of C.sub.8 naphthenes is accomplished by 
introducing large amounts of diluent toluene into the isomerization 
reactor in the feed. A platinum-halogen-alumina catalyst is used in the 
isomerization step at a temperature of 30.degree.-1290.degree. F and a 
pressure of 1-100 atmospheres, or more. 
U.S. Pat. No. 3,879,484 describes a process for isomerizing C.sub.8 
alkylaromatic hydrocarbons, such as xylenes, by contacting the C.sub.8 
alkylaromatic hydrocarbons with a platinum-rhenium-halogen alumina 
catalyst at a temperature of 30.degree.-1112.degree. F and a pressure of 
1-100 atmospheres; see also U.S. Pat. No. 3,577,475. 
Activity and stability are important properties of an isomerization 
catalyst. One measure of activity is the capacity of a catalyst to provide 
sufficient conversion at any given operating temperature to achieve a 
close approach to equilibrium concentrations of isomers in the product. 
Stability refers to the ability of a catalyst to maintain a desired level 
of activity over an extended period of use without the need for 
excessively increasing the operating temperature. Typically, when a 
catalyst begins to lose activity, the operating temperature of the 
isomerization process is increased to maintain the desired activity level. 
A stable catalyst requires only a relatively slow temperature increase, 
while a relatively less stable catalyst requires a more rapid increase in 
temperature to maintain the same activity level. 
In a C.sub.8 alkylaromatics isomerization system for producing paraxylene 
and/or orthoxylene with a catalyst containing platinum and halogen, the 
temperature is typically raised at a constant rate, or stepwise, to 
maintain catalyst activity at a given level. It has been found necessary, 
when the temperature is thus raised, to likewise raise the hydrogen 
pressure in the isomerization system simultaneously to maintain an 
acceptable level of conversion of ethylbenzene to xylenes. An increase in 
hydrogen pressure in the isomerization system causes an increase in 
saturation of C.sub.8 alkylaromatics in the feed to form C.sub.8 
naphthenes, i.e., the selectivity of the catalyst for isomerization is 
reduced by increasing the hydrogen pressure. The formation of excessive 
amounts of C.sub.8 naphthenes is undesirable because it (1) consumes 
hydrogen and (2) consumes C.sub.8 alkylaromatic hydrocarbons. This 
necessitates addition of undesirably large amounts of expensive hydrogen 
to the system and also reduces the potential C.sub.8 alkylaromatic product 
isomer yield. Thus, it is apparent that the stability of an isomerization 
catalyst is important to economical operation of an isomerization system 
because it allows the system to operate at a lower temperature for a 
longer time, thereby providing greater overall catalyst selectivity. 
In an embodiment, the present invention relates to an improved process for 
isomerizing an alkylaromatic hydrocarbon by contacting a feed including 
the hydrocarbon and hydrogen with a catalyst including 0.01-3 weight 
percent platinum and 0.01-3 weight percent rhenium on an alumina support 
at isomerization conditions including a temperature of 700.degree. F to 
900.degree. F and a hydrogen pressure between 100 psi and 300 psi, the 
improvement comprising increasing the activity and selectivity of the 
catalyst by the method comprising: including in the catalyst greater than 
1.2 weight percent combined chloride and contacting the feed with the 
catalyst in the presence of between 1.5 and 150 ppm, by volume, of free 
chloride and not more than 10 ppm, by volume of water, based on the volume 
of the feed. 
I have found that a particularly effective process for isomerizing 
alkylaromatic hydrocarbons is obtained by employing particular 
isomerization conditions in combination with a particular isomerization 
catalyst. The feed is contacted with the catalyst in the presence of 
1.5-150 ppm free chloride under very dry conditions, in the presence of 
not more than 10 ppm, and preferably below 1 ppm, water. The catalyst 
employed in the process of the invention is a platinum-rhenium-alumina 
composition which has a combined chloride content adjusted to above 1.2 
weight percent and preferably about 1.5 weight percent. When the 
above-described isomerization conditions are used in conjunction with the 
high cloride platinum-rhenium catalyst, a particularly active, selective 
and stable isomerization system is achieved. 
DETAILED DESCRIPTION OF THE INVENTION 
The isomerizable alkylaromatic hydrocarbons which can be isomerized 
according to the present invention include orthoxylene, metaxylene, 
paraxylene, ethylbenzene, orthomethylethylbenzene, metamethylethylbenzene, 
paramethylethylbenzene, trimethylbenzenes, diethylbenzenes, 
propylbenzenes, methylpropylbenzenes, etc., and nonequilibrium mixtures 
thereof. The preferred isomerizable hydrocarbons are the C.sub.8 
alkylaromatics, i.e., the xylenes and ethylbenzene. Mixtures of C.sub.8 
alkylaromatics containing a less than equilibrium concentration of a 
desired C.sub.8 aromatic isomer are also preferred. For example, a 
hydrocarbon mixture containing greater than equilibrium concentrations of 
ethylbenzene and metaxylene and less than equilibrium concentrations of 
orthoxylene and/or paraxylene is preferred for use. A source of the 
isomerizable hydrocarbon may be a petroleum fraction or refinery stream 
containing a high or low, but greater than equilibrium, concentration of 
the isomerizable hydrocarbon, such as a C.sub.8 reformate fraction from 
which all or a part of a desired isomer has been removed. The isomerizable 
hydrocarbon may be employed diluted by hydrocarbons including aromatics, 
paraffins and naphthenes, etc. 
The catalyst employed in the present process includes 0.01-3 weight percent 
platinum and 0.01-3 weight percent rhenium on an alumina carrier. The 
catalyst also includes at least 1.2 weight percent combined chloride. 
Combined chloride is chloride chemically bound to the catalyst, as by 
substitution for hydroxyl groups in the alumina carrier. The catalyst can 
be prepared by suitable known methods, such as by aqueous impregnation of 
particulate alumina with the platinum, rhenium and chloride, followed by 
drying and calcination. For example, an aqueous solution of chloroplatinic 
acid, perrhenic acid and hydrochloric acid may suitably be used for 
impregnation of an alumina carrier. The preferred alumina carrier is 
preferably prepared by treating an alpha-alumina monohydrate with a 
monobasic acid, neutralizing the acid with a nitrogen base, such as 
ammonia, shaping the resulting mass into the desired particle form, and 
then drying and calcining. The catalyst used in the process must contain 
at least 1.2 weight percent combined chloride, and preferably contains at 
least 1.5 weight percent combined chloride, based on the total weight of 
the catalyst. Preferably, the combined chloride component is added to the 
catalyst at the same time as the platinum component. The platinum and 
rhenium components each preferably make between 0.1 and 1 weight percent 
of the total catalyst weight. 
The isomerization process may be carried out in any suitable, conventional 
reaction vessel or in a plurality of such reaction vessels connected in 
series or in parallel, and the process may be performed as a batch-type 
operation or a continuous-type operation. The catalyst may be used in a 
fixed bed or a moving bed system. A continuous-type operation using a 
fixed bed of the catalyst is preferably employed, with the feed being 
passed continuously through the catalyst bed. 
Isomerization conditions employed in the process include a temperature 
between 700.degree. F and 900.degree. F, preferably between 750.degree. F 
and 850.degree. F. A hydrogen pressure of 100 psi to 300 psi is used, with 
a hydrogen pressure between 150 psi and 250 psi being preferred. 
The feed which is contacted with the isomerization catalyst in the process 
includes the isomerizable hydrocarbons and hydrogen. The amount of 
hydrogen needed is sufficient to supply the required hydrogen pressure in 
the system and to provide a hydrogen-hydrocarbon of a ratio of from about 
2 to about 20. The feed is preferably continuously passed in contact with 
the catalyst at a liquid hourly space velocity (LHSV) between 0.1 and 10, 
with a LHSV of about 0.5 to 3 being preferred. 
The feed is contacted with the catalyst in the presence of free chloride in 
an amount between 1.5 ppm and 150 ppm, by volume, based on the volume of 
the feed, with the preferred free chloride concentration being between 5 
ppm and 100 ppm, volume, calculated on the feed volume. Free chloride is 
all chloride not in chemical combination in the catalyst. Free chloride or 
a substance which forms free chloride may be added to the feed, when 
necessary, by any conventional means, such as in the form of molecular 
chloride or an organic chloride, e.g., carbon tetrachloride. Free chloride 
or a chloride-forming substance may also be added directly to an 
isomerization reactor. 
In a preferred embodiment of the process, hydrogen is continuously recycled 
to form a part of the feed, after having been separated from the 
isomerized hydrocarbon product. In such an operation, the free chloride 
contained in the recycled hydrogen (primarily as hydrogen chloride) is at 
a fairly high concentration and may provide at least a portion of the 
1.5-150 ppm free chloride concentration which is needed. Generally, 
recycled hydrogen can supply 65-85% of the total amount of free chloride 
required during isomerization. Thus, the total amount of free chloride 
which is added to the system continuously or intermittently may be small 
in relation to the total free chloride contacted with the isomerization 
catalyst, when recycled hydrogen is used. 
According to the invention, the feed is contacted with the isomerization 
catalyst in the presence of not more than 10 ppm, by volume of water, 
based on the volume of the feed. The hydrocarbon charge used in the 
process preferably has a water content of less than 1 ppm (vol.). 
Normally, the supplies of isomerizable alkylaromatic hydrocarbons which 
can be obtained from readily accessable sources, such as petroleum 
refineries, contain a greater amount of water than is permissible in feeds 
which can be used in the present process. Accordingly, the hydrocarbons 
must normally be dried before use in the process to provide the dry 
conditions required. The alkylaromatic hydrocarbon may be dried by, for 
example, distillation drying, contact with a drying agent, such as a 
molecular sieve, or another conventional drying procedure, capable of 
removing sufficient water from the hydrocarbon. Hydrogen is preferably 
conserved in the isomerization system by separating it from hydrocarbon 
products and recycling it continuously to form part of the feed. Only a 
small amount of makeup hydrogen is normally needed in the process when 
such a recycle is practiced. In such cases, it may not be necessary to 
subject makeup hydrogen to a drying procedure, unless the make-up hydrogen 
contains more than 50 ppm, by volume, of water. Thus, in some embodiments 
only the hydrocarbon component of the isomerization feed need be dried, 
because the recycled hydrogen component of the feed is already 
sufficiently dried. 
After carrying out the isomerization operation, the desired alkylaromatic 
isomer may be recovered in a conventional manner. In a C.sub.8 
alkylaromatic isomerization operation, the product isomer is usually 
paraxylene or orthoxylene. In the case of paraxylene recovery, hydrogen is 
normally first separated from the reactor effluent and recycled. Then, a 
C.sub.8 fraction is formed and processed to recover the paraxylene, as by 
paraxylene crystallization or molecular sieve isomer separation in a 
manner known to those skilled in the art. In the case of orthoxylene 
recovery, hydrogen is also separated and recylcled in a preferred 
embodiment; however, orthoxylene has a boiling point sufficiently 
different from other C.sub.8 alkylaromatic isomers to allow it to be 
separated from the other isomers by fractional distillation. 
One preferred method for separating paraxylene is by fractional 
crystallization of paraxylene from a C.sub.8 alkylaromatic fraction. 
Generally, the C.sub.8 alkylaromatic fraction is cooled to a low 
temperature, e.g., -100.degree. F. The cooling of the fraction results in 
crystallization of part of the fraction, with the crystals being rich in 
paraxylene. The crystals are then separated from the paraxylene-lean 
mother liquor by, for example centrifugation. The paraxylene concentration 
of the crystals which are recovered can be increased by serial 
crystallization procedures, by the use of other solvents, and by other 
known methods. Further details of crystallization procedures which are 
suitable may be obtained from U.S. Pat. Nos. 2,985,694 and 3,467,724, the 
teachings of which are incorporated herein by specific reference. 
The residue left after recovery of the desired alkylaromatic hydrocarbon 
isomer may be recycled to form a part of the feed which is contacted with 
the isomerization catalyst. Thus, the hydrocarbons which are in the feed 
preferably include partly fresh feed hydrocarbons and partly recycled 
hydrocarbons. 
The following examples illustrate a preferred embodiment of the present 
invention.