CA1086779A - Process for isomerizing alkyl aromatic hydrocarbons - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The present invention provides a process for isomerizing alkyl aromatic hydrocarbons which process comprises reacting a batch of said hydrocarbons with hydrogen and a catalyst under isomerising conditions, the process being performed in at least two successive steps the reaction being carried out in at least one step and the catalyst comprising a refractory inorganic oxide support having acid sites, having halogen present in combined form, and having metals on said support comprising:
a) about 0.02 to 2% of at least one platinum metal, b) about 0.02 to 2% of at least one metal belonging to the group consisting of zirconium, titanium and tungsten, c) about 0.02 to 2% of tin; said weight percentages being based on the total weight of the catalyst.

Description

The invention relates to a process for isomeriziny alkyl aromatic hyclrocarbons. It relates more particularly to the use of catalysts described and claimed in our U.K. Patent No.1,464,931 for lsomerizing alkyl aromatic hydrocarbons, the isomeriæation being performed in at least two steps.
The aforementioned describes and claims a catalyst for the substantially non-cracking hydro-treatment of hydrocarbons comprising a substantially non-cracking refractory inorganic oxide carrier having acid sites, said catalyst having halogen present in combined form, and having metals on said carrier in free or combined state comprising:
a) 0.02 to 2% preferably 0.1 to 7% by weight of at least one platinum metal, b) 0.02 to 2~ preferably 0.02 to 0.60% by weight of at least one metal selected from zirconium, titanium and tungsten, c) 0.02 to 2% preferably 0.05 to 1.00% by weight of tin; said weight percentages being based on the total weight of the catalyst.
Preferably the support of the aforementioned catalyst has a specific surface greater than 15 m /g. The support can, for example, be alumina having a specific surface between 100 and 350 m2/mg. It has a specific pore volume greater than 0.1 cm /g.
The halogen content is preferably between 0.4 and

2% and more preerably between 0.5 and 1.6% referred to the total weight of the catalyst. The halogen is preferably chlorineO
Desirably the metals consist essentially of platinum as the only platinum metal and of one other metal.
The aforementioned patent specification also describes a process for preparing the aforementioned catalysts and their use for hydro-reforming a batch of hydrocarbons.

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Our copending ~application No. 253,010 filed May 20, 1976 describes and claims a process for isomerizing a charge of alkyl aromatic hydrocarbons comprising bringing together said hydrocarbon ~harge, hydrogen and a catalyst as claimed in U.K. Patent No. 1,464,931 under isomerization conditions.
Preferably the halogen content o~ the catalyst is from 1 to 2%
by weight and preferably also the halogen is chlorine.
After continuing their work on the use of the afore-mentioned isomerization catalysts, the applicants have now found that the me~ od of isomerization as described in the specification of copending~Application No. 253,010 is particularly useful when performed in at least two successive steps.
According to this inven~ion there is prov1ded a process for isomerizing alkyl aromatic hydrocarbons which process comprises reacting a batch of said hydrocarbons with hydrogen and a catalyst as claimed in any one of claims 1 to 9 or 17 of ~.A'.
Patent No. 1,464,931 under isomerising conditions, the process being performed in at least two successive steps and the reaction ; being carried out in at least one step.
In order to illustrate the invention more clearly, it will be described with reference to the successive steps, each comprising only one reactor. However, this is not a limitation since the method can be worked inbwo successive steps each comprising more than two reactors, which can be disposed in parallel or in series in each step~ Similarly, only two successive steps will be considered but of course the method accordin~ to the invention could be worked in a larger number of steps.
The temperature and pressure conditions can be substantially identical in the two successive steps. Howe~er, the applicants have observed that very good results are `~'`' " :
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obtained when the conditions are not the s~ne in both steps.
In each step the conditions may vary within the following ranges:
Molar ratio of hydrogen to hydrocarbons: from 3 to 15, ~referably from approximately 8 to approximateiy 10.
Temperatures- below approximately 550~C.and above approximately 250C. in general, Pressures: f.rom approximately 3 to approximately 40 bars.
The total houxly volumetric speed, i.e. relative to the ~:
total vol~me of catalyst contained in all the reactors, may vary from 0.4 to 4~ advantageously from 1 to 2.5, Table I shows the conditions under which it has been shown it is advantageous to perform each step, in the case of.
two steps only.

. Operating Operating conditions conditionq First sten ~econd.step Volumetric speed per From 1 to 4 From 1 to 6 hour (partial) Molar ratio of hydrogen 3 to 15 3 to 15 to.hydrocarbons Temperature 100 to 450 , 300 to 550C., preferably 240 preferably 350 : ::
to 450 ~o 500 .~ .
Pressure 10 to 40 bars, 3 to 30 bars, :
preferably 20 to preferably 10 to 25 The applicants have found that the performance of the 30 catalysts aacording to the aforementioned patent specification :~
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in the reaction of isomerl~ing alkyl aromatlc hydrocarbons is improved when the method according to the lnven~ion i~
- worked under the conditions deflned hereinbafore.
The two~step method is particularly suitable when batches of hydrocarbons are treated under the aforementioned conditions and consist mainly of hydrocarbons containing 8 carbon atoms, the aim being to increase the production of paraxylenes by converting the other hydrocarbons in the batchO The method ~s particularly applicable to batches 1~ containing from approximately 5 to approximately 50~ of ethyl-benezene.
In the two steps the catalysts used may have the same composition or a different composition. In the first step, the proportion of halogen (usually chlorine) in the catalyst is greater than in the catalyst used ln the second step, lf the condltlons in the two steps are as in the preceding Table. The reason is that the equilibrium between chlorine and water (in the batch injected into the reactor) is dependent on the aforementioned c~ndition.
In~addition to the chlorine content, the composition of the catalysts may vary with regard to the nature of the àctive metals or promoters deposited on the support For Example, when the isomerization process is performed in two steps only, the first reactor can have a catalyst containing only platinum as the metal, whereas the second reactor can contain a catalyst according to the parent patent.
Alternatively, the following catalyst combinations can be used:
In the first reactor a catalyst according to Yrench ; Patent SpecificatLon No. 2031984, containing a metal from , .

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crude platin~n and tin as the active metals; alternatively - the first reactor can comprise a catalyst containing at least one metal from crude platinum on a refractory oxide support, together if required with one or more promoters adapted to improve a particular property of the catalyst (e.g. its service life, activity, stability or inhibition of.
cracking activity.).
The second reactor can contaln a catalyst according to the parent patent.
The various catalyst formulae can be used in any desired manner in the two steps. For example, the catalyst according to the parent patent can be used in the first step or the second step or both. The applicant~, however, have ~ ;
found that it is more advantageous to dispose the catalyst more particularly in the ~econd step, when there axe only two steps in the isomerization process, and good results have been obtained with a pl~tinum/alumina formula in the first step and platinum ~ tin + zirconium/alumina in the second step. As explained hereinbefore, the platinum/alumina formula i.n the flrst step may, without greatly changing the result, be replaced by platinum ~ tin on alumina.
As previously stated~ the proportion of haloger. (usually chlorine~ in the formulae of the catalysts in the different steps will vary from one step to another, due to the difference between the operating conditions of the steps.
Thus, the proportion of halogen will usually be from 1 to 2~ in each step. However, the proportion of halogen in the catalyst ln the first step wlll be greater than in the ~-second step, the di~ference varying e.g. rom approximately

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i779 0.05 to approximately 0.50~, relative to the weight o~
catalyst.
As is known, the purpose o the halogen is to confer some acidity to the refractory oxide support. Other elements having the same proI>erty may likewise be added to the various catàlyst formulae in each step. For example, the a~plicants have successfully used added silicon, in the proportion of approximately 0.5 to approximately 2~, relative to the total weight of catalyst, in the aforementioned catalyst formulae.
The following examples illustrate the invention, both in the case where the cataIysts according to the parent patent are disposed in both steps and in the case where the catalysts are disposed in only the last step of the method.
These examples, however, are non-limitative.
EXAMPLE I
As previously stated, a batch of alkyl aromatic hydrocarbons was isomerized and had the followLng composition:
Toluene 1~07%
Ethylbenzene27.44%
Paraxylene 1.44%) Metaxy4lene 68.67%) Total: 71.42%
Orthoxylene 1.31~) and an attempt was made to obtain a maximum yield of paraxylene by this reaction~
The followlng catalyst formulae were used:

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CATALYST ACCORDING TO THE PARENT PATENT (A) Platinum 0.35~ by weight Tin 0.20~ ~y weight Zirconium 0.15% by weight S GONTROL CATALYST (B) Platinum 0.35% by weight ~:
The catalyst supports were alumina having conventional properties, as follows~
Specific surface: 185 m2/g Pore volume: 0.48 cm3/g Average pore radium: 40 R
The chlorine content of the catalyst, indicated in Table II hereinafter, was kept substantially constant by injecting 184 ppm of chlorine in the for~ of dichloropropane into the non-dehydrated batch of alkyl aromatic hydrocarbons, so as to compensate dechlorination due to the molsture content of the batch.
A number of tests were made, under the conditiolls set out in Table II~
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Y7'g The results of these various tests are shown in Table III.
The Table gives:
, . The yield by weight of the various products in the effluent;
` The total percentage yield Rl in hydrocarbons . -containing 8 carbon atoms;
The rat:io R - incomin~ xylenes ; 2 xylenes in the effluent ::
.
, 10 ~he percentage conversion of ethylbenzene (CEB).
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. .,, . : ,., . , ': ' The following notes refer to the results shown in this Table:
(1) In a single ste~, the best result for the formula Pt - Sn - Zr was obtained at 15 bars at 475C. (Test No. 3; see the values for the paraxylene yield, ratio R2 and yield Rl).
(2) A comparison between test 3 and test l shows that, in two steps:
The total yield Rl is higher, The R2 ratio is higher, and The paraxylQne yield is higher.
It is ther~fore more advantageous to produce paraxylene in two steps rather ~han in a single step.
; (3) In a single step, the best result for formula Pt ~was obtained at 22.5 bars at 447.5~C. (Test No. 12;
: see the paraxylene yield).
~4~ A comparison between tests 12 and 7 shows that it is advantageous to work in two steps (Test No. 7) since this increases the paraxylene yleld.
However, the paraxylene pro~uction level is lower than with the Pt - Sn - Zr formulae according to the parent patent, and the advantage of going from one to two steps i9 greater in the case of the aforementioned Pt - Sn - Zr formulae.

In this example, a batch of hydrocarbons was isomerized, the batch having a substantially constant .
compositiont consisting mainly of metaxylene and .
12.
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ethylbenzene. Isomerization was performed according to the invention in two reactors under the following conditions;
First reactor:
Pressure: 20 bars Temperature: 280C.
25 cm3 catalyst containing 0.35% by weight of platinum and l.6% chlorine on alumina, Second reactor:
Pressure: 15 bars Temperature: variable between 490 and 530Cy 15 cm3 catalyst (alumina support) having a composition which varied from one test to another (tests 13 - 18;
see Table IV hereinafter).
The total volumetric speed per hour was 1.5; the molar ratio of hydrogen to hydrocarbon was 7.5. In addition, dichloropropane was added to the hydrocarbon batch, in a proportion of 180 ppm/200 ppm of water present in the batch.
At the beginning of the reaction, the catalyst in the reactors were given reducing treatment with hydrogen tat 20 bars) at 500C. for 2 hours.
. .
The catalysts used in the second reactor are described in Table IV hereinafter.
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:'. ' TABLE IV

Test 13 14 15 16 17 18 Catalyst \ Control Control in second reactor \ _ _ Platinum content 0.35 0.35 0.34 0.38 0.37 0.36 (wt. %) . - _ Lead content _ 0.20 0.21 0.22 0.220.20 ...... _ ..
Z rcon ~m con-. _ _ 0.15 0.14 0.15 O. 79 Chlorine con-1.6 61.69 1.80 1.68 1.73 1.67 Batch:content (wt. %) ` :
- Toluene 0 0 0. 03 0.070O 18 0 - Ethylbenzene27.5627.26 27.45 27.3827.47 27.56 - Paraxylene 2.30 2.11 2.09 2.10 2.29 2.30 .:
- Metaxylene 68.0768.82 62.42 62.6463.91 68.07 - orthoxylene2.07 1.81 8.01 7.81 6.15 2.07 - Total xylenes 72.44 72.74 72.5272.55 72.35 72.44 -Table V to X herelnafter show the results obtained in Tests 13 to 18 respectivelyO

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More particularly we have shown the ratio R = paraxylene in the effluen-t paraxylene in the effluent + losses.
The losses consisted of benzene, toluene, light hydrocarbons (containing fewer than 7 carbon atoms, except for benzene and toluene~ and heavy hydrocarbons (containing 9 or more carbon atoms).
R represents the paraxylene yield with respect to the fresh batch, if the hydrocarbons co~taining 8 carbon atoms are recycled after separation of paraxylene and the losses.
C~ denot s ~e con.erqlo~ o E ethylbenze~e.

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~emperature ~f ~ ~
~Yiel ~ ond reactor ~ 490 s S 1 510 ~ 520 ~ 53C~5 in weight ~ ' s ~I s s 2 S ~
~.s-light products s o,gls 1~16 t 1~05 ~ 1,43 t 1,74 ;
s J J J
'; t ~ . 8 ~ ~ :
. 1- benzene - I 0,43 : 1,05 : 1,11 s 1,29 ~ 1,52 s s s s s- toluene ~ 1,80 s 2,28 s 3,16 8 3,70 s 4,24 :
s ~ 2 tI t t saturatéd hydrocarbon6 7~ 4,44 t3, 10 .2,17 s 1,42 containing 8 c~rbon t t .l atoms s ~ ~s - I t I 7~ ~ s 5~ I
t~ethyl~enzene ~ 15,0 ~ 15,58 ~ 16~58 ~ 1S,25 l 16,0t s . t ~~~ paraxylene ~ 16~73 s 17~05 : 16,43 ~ 16~81 s 16,32 :
t s ~
t ~ s ~ s s s t- metaxylene . 2 39~14 s 39,38 : 39,28 ~ 38,30 s 37,78 s t ~ ~ s ~ s ~
~ s s s ~ t orthoxylene t 14,96 1 15,46 ~ 14~83 s 1S,67 ~ 15,04 s S I ~ ' ~ t I
S
I I- heavy products ~ 0,31 : 0,43 ~ o,66 1 0,90 s 0~70 ~
t S t ~ I

0;~29 : o,776 ~ 0~733 5 0,697 1 o~666 , I I t I : s '~ s ~ t I ~ :
. ! CEEI . ~ 43~4 5 43,5 : 39,8 s 44,6 s 41,9 s ! t t t t t ~
t~ 5 s 2 1 ~ :
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Temperature ~f : I t I . S
econd reactor: 490 t 500 s 510 t 520 s 530 , s Yield ~`~_ (C) s s t I t in weight ~ _ : 2 ~ ~ ~ I
l l ~; t ~ S
~ t - light products s 0,50 s 0,50 : 0~85 t 0, 76 ~ 0, 93 s I t I S S
t 8 I S
: t- benzene 1 0,76 s0,42 : 0,80 ~ 0,87 s 1,11 1 ~ / s l ~~ toluene ~ 0,94 ~. 1917 ~ 1956 ~ 2,07 a 2,67 :
l l ~ S St S S , :
~ saturated hydro~arbo~s4~32 3~57 t 2,20 11,20 s 0774 t containing 8 carbon : : 2 ~ ~ .
: atoms s ~ t ~~ :
i t t s I t~ . ; :
, ~ ethy~benzene 1S,37 16,04 s 16,20 15~93 s 16,31 1 ¦ t s~ a t 1- purBxylbne ~ 17$62 t 17,52 s 17,62 ~ 17~80 s 17,60 s .1 ~ 1 s s s i ~ ':î ~ s ~ ~ ~ t ' 1- mdtnxyl~no 1 40~38 s 40~64 t 40~47 ~ 4,3 t 39D78 S
I S t I ~ ~.
_ orth~xyl~ne 1 16,88 s 16~78 t 16,88 s 17,28 17,08 s s t . 1 5 :
t ~ t t 8 - heavy produc~s ~ 0~22 0,34 0,42 s 0,81 . 0,80 ~ s ~ s ~ t : : 0 ,j I R s 0~879: 0,878; 07846 s 0,798 t 0,762 :
s ~ ~ :
3 I s E13 43$6 S41,2 s40,6 41~6 40,2 ~
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Temperature of ~ t second reactor 1 500 t ~10 y 520 8 53 ~
~ Yield in ~ (~`C) t ~ ~ t s .-~, t weight . ~ s t t ! , t ~ 2 1~ s , ~ - light products t 0,S8 s o,68 ~0~77 t 0,98 'j t ~ ~ s. s I
~ ~ - benzene ~ - ~ 0,45 t 0~61 s1 ,o6 :
: : . t t t~ t l s t t t8 ~
I ~ - toluene t OD~5 ~ S~ 1 ~59 a 2~ 12 t . t t t ~ - s . ' ~ ' ~ s~: S
t - saturated hydrocarbons s 7,62 : 3,98 s 2362 t 2J58 s s containing 8 carbon ' t I . 3~ s î atoms t ~ t s i ~ t . ~ t ~
ethylbonz~ne ~ 16~23 ~ 17,02 s16,80s 16~56 s I ~ t ' 8 S ~ ~ ~
- par~xyleno a 16~46 s 16~51 116,62t 16~79 s s ~ t t 3 ~ t 5 j! ~ - metaxylene ~ 39,20 s 40,18 s40,30~ 39,14 2 . .
:
t I ~ ¦ t -- orthoxyl~no ~15~69 S 16~78 : 17~29~ 17,01 :
I I t ~ t ~ I
t i ~ ~ heavy products ~ 0~35 s 0~25 1o,40t ~:0,74 s s . ~ ~ ! t , R : 0,902 s o,867 s 0~83~.0,774 ~
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s t : Y
EB ~4~9 38~0 t 38~8 39~7 s ,, , _ _~ _ _ _ :

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: 1 ~ Temperature of second reacto~ 490 ~ 500 s 510 s 520 ~ 530 ~
~Yield i ~ ( C) s s t~ s s . ' weight ' s I s s I li t _ ~ ~ J S ~ ~ t S I t ! so light products s O.S1 s o9~6 s o,64 s 0,57 ~ o,84 I I t t S 5 ~ t S
I ~ ~ ~ t S ~ ~ .
I ; I b~nzon~ 0,45 s 0~78 ~ 0~70 :
1 ~ s s s t :
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l s- tolueno : 0~91 t 1,13 2~ p41 s 1 ,41 . 1 j78 , 2 t ' S J t 2 ~
I I ~ 8 S ~ : ..
S-saturated hydrocarbon~ 8~69 : 6~37 : 3~78 s 2~92s 2,33 :
: containing 8 carbon t S S S :
s atoms : s : ~ ~ s . t ~ I ~ t ~ . ~
s ~ : s ~ :
s- ~thylbenzane t15~54 16~02 . 17~49 17~76 ~ 17,67 .
~ `; 2 ~ : ~ I . s i ! I- p~ra~tylene . sl6,31 tl6,49 s 16,43t16,Sl : 16,73 :
l l ~ t ~ S 8 ~ t j ! , met~xyl~ne s3~,35 s38,95 s 39,60 s39,72 s 39,41 s :
I t S t ~ S '-1' ~:
s- orthoxylone t 16~49 :16,95 2 16,92 :16,98 s 17,14 :
: : s s : s :
t s : t ~ ~ :
I ~ h d s 0~23 0~35 s 0,29 0,35 0~40 . -~ I I s ~
! 1 ~ R ~0~908 sO,8~1 s 0,855 :0,842 ~ ~818 :
~' ~ . s :, s s ~ ~
I I t I : t s ~ ' :
CEB s43,2 s41~5 s 36D6 s35, 1 ~ ~594 ~ ~ :
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¦ ' ~ second reactor ~490 2 500 ~ S10 s S20 S3 Yield i~ .(C) : . : 8 t ~
, weight ~ '` ~ ' J ~ ~ .
'I I :s I I t s I ~
~: :z : s ~ :
It~ light products s 0~45 s o,63 t o,86 t o,as s Oa96 t : ~
3 s 8 s :
t - bonzbne t -- s -- s 0~ 46 t 0~ 94 s 1 ~ 09 s S ~ ~ ~ s jt t ~ t ~ ~ :
- ~ tolueno . 1,02 ~ 1,26 : 1"59 s2,28 s 2r73 :
s ~ ' :
': : : I I z t :- saturated hydrocarbons 8,57 t 5,82 s 4320 s 2,65 ~ 1j88 s : containing 8 carbon : t s s ~ :
atoms . : : s i t :
t ~ ~ S 2 8 ~
s- ethylbenzene : 15,~8 s 16,13 : 16~41 s 15,59 ~15,65 :
S t ~ S ~
t ~ S : ' : t I
:- paraxylene s 16,32 1 16,74 s 16,84 ~ 17~33 s17,23 s t ~ I t s ' s . s : t s :
t- metaxylene s 38,51 ~ 39,20 : 39,33 ~ 39,33 s39,09 :
` S ~ ~ 2 t t : , S ' t :- orthoxylene s 16,28 ~ 16,85 2 17,01 ~ 17,50 ~17,45:
, t s~ S ~
t s~ : ~ s ~ :
s- heavy products 10~18 ~o,39 10,30 10,54 8 0,74 s t ~z 2 1 ~ 8 . tt t I ~ ~
t R . s0~908 s 0,880 t o,840 1 0~79 ~ 0~757 :
- s ~
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CE~ ~ 42,9 2 41,3 t 40~3 s 43,~ t 43,0 :
t ~
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i SYield i ~ (C) ~ s s I s s ¦ 2 weight -- ~ S ~ ~ t : 5 ~ S ~
S ,: . :
; s ; t S g ~ ;
s- light products ois6 . 0,94 s 0,93 C,86 ~ 0~92 .
j : S : S t Z :
,:-- benz~n~ . s -- s o,3o , o,67 , o,76 s 0,~?2 :
I t S S ~ ~ t , I t- toluon~ s o,g6 s 1,25 ~ 1,70 s l~9B s 2,54 ~
' ' ~ I t ~ I t 3 t ~ ~ ~ 3 ~
! j t- saturated hydrocarbo~s6,08 s 4,43 s 2,53 ~ 1,71 : 0,99 t il ~ containing 8 carbon ~ ~ ~ t ~ S
il ~ atoms t S t t 2 t s thylbenzan~ s 15,56 t 15983s 16~20 t 16,18 s 15,95 ~
s s : t ~ t 8 :
P~raXY1~ne : 17~32 ~ 17~44t 17~S8 S 17"68 : 17D68 S
;' . S ~
t ~ S ~ ~ t ;- m~taxylon~ s 39~14 s 39~292 39~64 s 39~86 s 39~41 :
s : s s .....
s- orthoxylollo s 16~99 s 17~27s 17~48 s l7t49 s 17~6~
t ~ s ~ ~
t ~ ~ . ., :- heavy products I0,38 s0,24 s0,27 s0,49 t 0,93 s ~ ~ s ~ .
s s : ~ s ~ . .
2 .. S 0,901 : o,86S~ 0,831 2 0,812 : o~769 t S t ~ 2 ~ :
t : S S S ~ ' : : ' ~ EB s 43,5 : 42,6 : 41,2 s 41,3 t 42,1 .
2 t ~ S : : :
j, .. ,~

., . .
., ' . .
21. ~ ~:

.
.: ~.~ .

, ' , ' . ' .' .' ;7~5~

The results in ~he Tablesshow that the process . according to the invention is very suitable ~or isomerizing aromatic hydrocarbons containing 8 carbon atoms. More particularly, lt is very advantageous compared with a method using catalysts containing only platinum. Comparison with the platinum-tin fonmulae used ~n the second reactor is to the advantage of the platinum + tin + zirconium formulae for Tests 15 and 16, whereas the results are substantially equivalent in Tests 17, 18 and 14.
In order to facllitate this comparison, the single accompanying Figure shows curves whic~, in dependence on the temperature T, give the values of the ratio R for each of the catalyst combinations used in Tests 13 to 18. ~.
:.
~'.

22.
.
, ' C~.

~ 3 .. .. ~ ` ., , .. ., . - . .. . . .

Claims (27)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for isomerizing alkyl aromatic hydrocarbons which process comprises reacting a batch of said hydrocarbons with hydrogen and a catalyst under isomerising conditions, the process being performed in at least two successive steps the reaction being carried out in at least one step and the catalyst comprising a refractory inorganic oxide support having acid sites, having halogen present in combined form, and having metals on said support comprising:
a) about 0.02 to 2% of at least one platinum metal, b) about 0.02 to 2% of at least one metal belonging to the group consisting of zirconium, titanium and tungsten, c) about 0.02 to 2% of tin; said weight percentages being based on the total weight of the catalyst.

2. A process according to Claim 1, wherein said metals comprise:
a) 0.1 to 0.7% of at least one platinum metal, b) 0.02 to 0.60% of at least one metal belonging to the group consisting of zirconium, titanium, and tungsten, and c) 0.05 to 1.00% of tin; said weight percentages being based on the total weight of the catalyst.

3. A process according to Claim 1, wherein said support has a specific surface of more than 15 m2/g and a specific pore volume of more than 0.1 cm3/g.

4. A process according to Claim 1, wherein the halogen content is between about 0.4 and 2%, referred to the total weight of the catalyst.

5. A process according to Claim 2, wherein the halogen content is between 0.5 and 1.6%, referred to the total weight of the catalyst.

6. A process according to Claim 1, wherein said metals consist essentially of platinum as the only platinum metal and of one other metal.

7. A process according to Claim 4 wherein said metals consist essentially of platinum as the only platinum metal and of one other metal.

8. A process according to Claim 4, wherein the halogen is chlorine.

9. A process according to Claim 7, wherein the halogen is chlorine.

10. A process according to Claim 3, characterized furthermore by the fact that the support is an alumina whose specific surface is between 100 and 350 m2/g.

11. A process according to Claim 9, characterized furthermore by the fact that the support is an alumina whose specific surface is between 100 and 350 m2/g.

12. A process as claimed in Claim 1, wherein the operating conditions in the steps are as follows:
Pressure: from 3 to 40 bars, Temperature: from 250 to 550°C, Molar ratio of from 3 to 15, hydrogen to hydrocarbons:
Total volumetric speed per hour, measured in the liquid state: from 0.5 to 4.

13. A process as claimed in Claim 12, wherein the volumetric speed is from 1 to 2.5.

14. A process as claimed in Claim 1, wherein the number of steps is two and the operating conditions in the first step are as follows:
Pressure: from 10 to 40 bars, Temperature: from 100 to 450°C, Volumetric speed per hour, measured in the liquid state:
from 1 to 4:

Molar ratio of hydrogen to hydrocarbons: from 3 to 15.

15. A process as claimed in Claim 14, in which the pressure is from 20 to 35 bars and the temperature from 250°C
to 450°C.

16. A process as claimed in Claim 1, wherein the number of steps is two and the operating conditions for the second step are as follows:
Pressure: from 3 to 30 bars, Temperature: from 30 to 550°C
Volumetric speed per hour, measured in the liquid state: from 1 to 6:
Roller ratio of hydrogen to hydrocarbons: from 3 to 15.

17. A process as claimed in Claim 1, 12 or 13, wherein the operating conditions are substantially the same in both steps.

18. A process as claimed in Claim 16, in which the pressure is from 10 to 25 bars and the temperature from 350°C to 500°C.

19. A process as claimed in Claim 1, 12 or 13, wherein the operating conditions are substantially different in the two steps.

20. A process as claimed in Claim 1, 12 or 13, wherein the catalysts used are identical in the successive steps.

21. A process as claimed in claim 1, 12 or 13, wherein the catalysts used in the successive steps are different.

22. A process as claimed in Claim 14, 15 or 16, wherein the proportion of halogen, preferably chlorine, in the catalyst varies from one step to the next, the catalyst used in the first step containing approximately from 0.05 to 0.50% more halogen (relative to the total weight of catalysts) than the catalyst in the second step.

23. A process as claimed in Claim 14, 15 or 16, wherein the catalyst used in the first step contains only platinum as the active metal and if required silicon.

24. A process as claimed in Claim 14, 15 or 16, wherein the catalyst used in the first step contains platinum and tin as the active metals and if required silicon.

25. A process as claimed in Claim 1, wherein the catalyst also contains from 0.1 to 2% of silicon.

26. A process as claimed in Claim 1, 12 or 13, wherein the alkyl aromatic hydrocarbons contain 8 carbon atoms.

27. A process as claimed in Claim 1, 12 or 13, wherein the batch of alkyl aromatic hydrocarbons contains from approximately 5% to approximately 35*% by weight of ethylbenzene.