Process for the catalytic isomerization of o-cresol

Isomerization of o-cresol to m-cresol by contact with a crystalline aluminosilicate zeolite of the ZSM-type, having a silica to alumina ratio greater than 5, at a temperature of about 350.degree. to 450.degree. C.

The invention relates to a process for the catalytic isomerisation of 
o-cresol using a special zeolite. 
BACKGROUND OF THE INVENTION 
During the catalytic methylation of phenol, the o-isomers frequently occur 
in a predominant proportion while the other isomers are obtained in a 
smaller quantity. However, a higher production of m-cresol may be desired 
and processes have therefore been developed for the isomerisation of the 
o-compounds into the corresponding m-compound. These processes are 
generally carried out in the presence of aluminium oxides (German Patent 
application No. 19 56 383) or aluminium silicates (German Patent 
application No. 20 05 153) as catalysts, but then disproportionation which 
is generally undesirable invariably occurs to a considerable extent in 
addition to isomerisation. 
THE INVENTION 
In accordance with this invention, we have now found that o-cresol can be 
isomerised without substantial disproportionation if the process is 
carried out in the presence of certain special crystalline zeolites as 
catalyst. The invention accordingly relates to a process for the catalytic 
isomerisation of o-cresol which comprises reacting o-cresol at 
temperatures of about 350.degree. to 450.degree. C. in the presence of an 
acidly acting crystalline aluminosilicate zeolite of the ZSM-type with a 
composition of oxides and water in the molar ratio of 1.+-.0.4 M.sub.2 
O/n:Al.sub.2 O.sub.3 :5-100 SiO.sub.2 :0-60 H.sub.2 O, M representing a 
cation having a valence of n. For example, M may be an alkali metal, 
especially sodium, tetramethylammonium, hydrogen, metals of Group II and 
VIII of the Periodic Table, or mixtures of the same. An important 
characteristic of the crystal structure of the zeolites useful as 
catalysts in this invention is that they have a pore dimension greater 
than about 5 Angstroms and pore windows of about a size such as would be 
provided by 10-membered rings of oxygen atoms. The zeolites freely sorb 
normal hexane. In addition, their structure must provide constrained 
access to some larger molecules. A determination of the "constraint index" 
may be made by continuously passing a mixture of equal weight of normal 
hexane and 3 -methylpentane over a small sample of zeolite at atmospheric 
pressure according to the following procedure. A sample of the zeolite is 
crushed to a particule size about that of coarse sand and mounted in a 
glass tube. The zeolite is treated with a stream of air at 1000.degree. F. 
for at least 15 minutes. The zeolite is then flushed with helium and the 
temperature adjusted between 550.degree. F. and 950.degree. F. to give an 
overall conversion between 10% and 60%. The mixture of hydrocarbons is 
passed at 1 liquid hourly spaced velocity over the zeolite with a helium 
dilution to give a helium to total hydrocarbon mole ratio of 4:1. After 20 
minutes on stream, a sample of the effluent is taken and analyzed to 
determine the fraction remaining unchanged for each of the two 
hydrocarbons. The constraint index is calculated as follows: 
##EQU1## 
Catalysts suitable for the present invention are those which employ a 
zeolite having a constraint index from 1.0 to 12.0. Constraint Index 
values for some typical zeolites within the scope of this invention are: 
______________________________________ 
catalyst Constraint Index 
______________________________________ 
ZSM-5 8.3 
ZSM-11 8.7 
ZSM-35 4.5 
ZSM-12 2 
ZSM-38 2 
______________________________________ 
The said catalysts are described, for example, in U.S. Pat. No. 3,702,886 
(ZSM-5), U.S. Pat. No. 3,709,979 (ZSM-11), U.S. Pat. No. 3,832,449 
(ZSM-12), U.S. application Ser. No. 358,192 filed May 7, 1973 (ZSM-21), 
U.S. application Ser. No. 528,061 filed Nov. 29, 1974 (ZSM-35), U.S. 
application Ser. No. 528,060 filed Nov. 29, 1974 (ZSM-38), the entire 
contents of which are incorporated herein by reference. It is particularly 
advantageous to use ZSM-5-catalysts, of the type described in U.S. Pat. 
No. 3,702,886. 
The methyl phenols can be isomered both in the gaseous phase and in the 
liquid phase. The catalysts are slowly deactivated by carbon deposition in 
the course of the reaction. This is delayed when working in the liquid 
phase so that this type of operation will generally be preferred. It also 
allows substantially higher throughput rates than a process carried out in 
the gaseous phase. The process is preferably carried out under elevated 
pressures of about 45 to 80 bar, in particular 50 to 60 bar. If necessary, 
the catalyst is regenerated by burning the deposited carbon with air at 
controlled temperatures, for example 500.degree. C. It then exhibits its 
complete activity again. 
The isomerisation treatments are carried out at temperatures of 350.degree. 
to 450.degree. C., advantageously 380.degree. to 420.degree. C., it being 
possible to compensate the abatement of the catalyst activity by 
increasing the reaction temperature in the course of the reaction. 
Temperatures above about 450.degree. C. are not beneficial since the 
selectivity of the reaction decreases there and the catalysts are 
deactivated relatively quickly, particularly when working in the gaseous 
phase. A selectivity of more than 95% can generally be achieved. The 
hourly flow rate can amount to up to about 5 l/l catalyst volume, 
advantageously about 2.5 to 2.8 l/l when working in the liquid phase.

Whereas disproportionation to phenol, obtionally cresol and higher 
alkylated products is invariably found to a considerable extent in the 
processes described hitherto for the isomerisation of alkyl phenols with 
the aid of aluminium oxide or aluminium silicate catalysts, virtually no 
disproportionation, but instead only the desired isomerisation, 
surprisingly takes place in the process according to the invention. 
EXAMPLE 1 
100 g of o-cresol were introduced together with 50 g of pulverulent 
catalyst ZSM-11 into an autoclave provided with a stirring mechanism. The 
mixture was heated to 360.degree. C. and was maintained at this 
temperature for one hour. 
The reaction mixture obtained after cooling had the following composition: 
______________________________________ 
phenol 4.4% by weight 
o-cresol 33.3% by weight 
m-cresol 42.3% by weight 
p-cresol 15.6% by weight 
2,5-xylenol 1.5% by weight 
2,4-xylenol 1.2% by weight 
2,6-xylenol 0.4% by weight 
3,4-xylenol 0.9% by weight 
trimethylphenols 
0.4% by weight 
______________________________________ 
EXAMPLE 2 
150 ml of the zeolite ZSM-5 processed to 4 mm pellets were introduced into 
an electrically heatable tube reactor. Once the reaction zone had been 
heated to 440.degree. C. in a nitrogen stream, o-cresol was passed over 
the catalyst at a volumetric flow rate of 0.2 l/l catalyst.multidot.h. The 
reaction mixture obtained after cooling had the following composition: 
______________________________________ 
phenol 1.5% by weight 
o-cresol 47.5% by weight 
m-cresol 35.1% by weight 
p-cresol 14.4% by weight 
2,5-xylenol 0.7% by weight 
2,4-xylenol 0.4% by weight 
2,6-xylenol 0.2% by weight 
2,3-xylenol 0.1% by weight 
3,4-xylenol 0.1% by weight 
______________________________________ 
Space-time yield m/p-cresol: 0.1 kg/l.multidot.h. 
EXAMPLE 3 
50 ml of a ZSM-5-catalyst were introduced in the form of 4 mm pellets into 
a vertically arranged, electrically heatable pressure-resistant tube 
reactor. In order to ensure that the reactor was uniformly filled with 
liquid, the starting material was pumped through the reactor from the 
bottom to the top. Somme o-cresol was reacted with a volumetric flow rate 
of 2.8 l/l catalyst.multidot.h at a temperature of 380.degree. C. and 
under a pressure of 60 bar. 
The reaction product obtained had the following composition: 
______________________________________ 
phenol 1.3% by weight 
o-cresol 59.0% by weight 
m-cresol 28.3% by weight 
p-cresol 10.1% by weight 
2,5-xylenol 0.3% by weight 
2,4-xylenol 0.2% by weight 
2,6-xylenol 0.4% by weight 
3,4-xylenol 0.4% by weight 
______________________________________ 
Space-time yield m/p-cresol: 1.08 kg/l.multidot.h. 
EXAMPLE 4 
Some o-cresol was reacted in the apparatus described in Example 3 with a 
volumetric flow rate of 2.5 l/l catalyst.multidot.h at 420.degree. C. and 
60 bar over a zeolite of the ZSM-12 type. The reaction mixture obtained 
consisted of: 
______________________________________ 
phenol 3.9% by weight 
o-cresol 53.4% by weight 
m-cresol 27.7% by weight 
p-cresol 11.1% by weight 
2,5-xylenol 1.1% by weight 
2,4-xylenol 1.1% by weight 
2,6-xylenol 0.6% by weight 
2,3-xylenol 0.5% by weight 
3,4 xylenol 0.4% by weight 
trimethylphenols 
0.2% by weight 
______________________________________ 
Space-time yield m/p-cresol: 0.97 kg/l.multidot.h. 
EXAMPLE 5 
In comparison, o-cresol was reacted as described in Example 3, but in the 
presence of a zeolite X and a zeolite A, respectively. In both cases the 
disproportionation of the o-cresol to phenol and higher alkylated phenols 
precedes preferred compared to isomerisation.