Method and catalyst for preparing styrene

A method and catalyst for preparing styrene which comprises dehydrodimerization of toluene, followed by conversion of the resulting dehydrodimerization products to styrene. The conversion is conducted in the presence of ethylene using a catalyst consisting of chromium oxide, tungsten oxide and an alkali metal oxide supported by a carrier. The method according to the present invention makes it possible to increase the yield of styrene and simplify the process technology.

FIELD OF THE INVENTION 
The present invention relates to the art of petrochemistry and, more 
specifically, to a method for preparing styrene. 
Styrene is one of most important monomers in the modern industry of rubber 
and plastics. The world production of styrene is now over 6 mln. tons. 
BACKGROUND OF THE INVENTION 
Known in the art is a method for preparing styrene from toluene which 
comprises passing a mixture of toluene, air and water over bismuth oxide 
at a temperature within the range of from 500.degree. to 580.degree. C. to 
produce a mixture of dibenzyl and stilbene. The dehydrodimerization 
products (dibenzyl and stilbene) are subjected to a catalytic 
hydrocracking to ethylbenzene and benzene. Thus, in hydrocracking 
performed at a temperature of 450.degree. C. under a pressure of 28 atm on 
a catalyst containing 16% by weight of Cr.sub.2 O.sub.3 on alumosilicate, 
conversion of dehydrodimerization products is 26%, the yield of 
ethylbenzene is 31.8% by weight. The resulting ethylbenzene is subjected 
to dehydration to styrene. 
Also known in the art is a process for producing styrene consisting in that 
dibenzyl resulting from dehydrodimerization of toluene is subjected to 
conversion directly to styrene and benzene. Thus, in the presence of a 
catalyst containing, percent by weight: SiO.sub.2 73; Al.sub.2 O.sub.3 
--17; Fe.sub.2 O.sub.3 --1.5; MgO--4; CaO--2, at the temperature of 
550.degree. C. and space velocity of 18.3 hr.sup.-1, conversion of 
dibenzyl is 18.7%; the yield of styrene is 22.4% by weight. 
Upon dehydrodimerization of toluene there is formed a mixture of dibenzyl 
and stilbene, wherein the dehydrodimerization products can be subjected to 
conversion to styrene only after a preliminary hydrogenation of stilbene. 
The prior art methods have certain disadvantages residing in a low yield of 
styrene and multi-stage character of the process. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to increase the desired product 
yield and simplify the process technology. 
This and other objects of the present invention are accomplished by a 
method for preparing styrene by dehydrodimerization of toluene with a 
subsequent catalytic conversion of the resulting dehydrodimerization 
products to styrene at a temperature within the range of from 300.degree. 
to 600.degree. C. The conversion of the dehydrodimerization products is 
conducted in the presence of ethylene with the use of a catalyst 
consisting of the following components, percent by weight: 
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chromium oxide 1.5 to 5.2 
tungsten oxide 4.0 to 6.1 
oxide of an alkali metal 
0.1 to 0.7 
silica or alumosilicate 
the balance. 
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DETAILED DESCRIPTION OF THE INVENTION 
The method for preparing styrene according to the present invention is 
performed in the following manner. 
Toluene is subjected to dehydrodimerization in the presence of oxygen (air) 
on a catalyst containing bismuth oxide at a temperature of from 
400.degree. to 600.degree. C. 
The dehydrodimerization product consists by more than 80% of dibenzyl, the 
balance being stilbene. 
The mixture of dibenzyl and stilbene is subjected to conversion with 
ethylene in the presence of the above-mentioned catalyst consisting of 
chromium oxide, tungsten oxide, an oxide of an alkali metal and silica or 
alumosilicate and possessing dehydrogenating and disproportioning 
properties. Conversion is conducted at an elevated temperature 
(300.degree. to 600.degree. C.) and under atmospheric or overatmospheric 
pressure of up to 5 atm.g. Proportions of the dehydrodimerization products 
of toluene and ethylene may be varied within a wide range. Technologically 
optimal is a ratio within the interval of from 1:3 to 1:10, which is 
associated with recycle of ethylene. 
The conversion catalyst is produced by depositing oxides of chromium, 
tungsten and an alkali metal in an appropriate ratio onto a carrier such 
as silica or alumosilicate. The best results are obtained with the 
following proportions of the components, percent by weight: 
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chromium oxide 1.5 to 5.2 
tungsten oxide 4.0 to 6.1 
oxide of an alkali metal 
0.1 to 0.7 
silica or alumosilicate 
the balance. 
______________________________________ 
The yield of styrene in the method according to the present invention 
comprises 78 to 80% by weight of styrene per the converted products of 
dehydrodimerization of toluene and ethylene. 
The method according to the present invention does not imply separation or 
hydrogenation of stilbene and, thereby, it involves a lesser number of 
process stages, i.e. has a more simplified technology. 
For a better understanding of the present invention, given hereinbelow are 
some specific examples illustrating the method for preparing styrene.

EXAMPLE 1 
92 g of toluene are passed, in a mixture with air at a molar ratio between 
oxygen and toluene of 0.8:1, at a temperature of 490.degree. C. and 
contact time of 1.2 sec, over a catalyst containing 37% by weight of 
bismuth oxide on corundum. The resulting liquid product contains 0.8 g 
(0.9% by weight) of benzene, 2.8 g (3.1% by weight) of a mixture of 
dibenzyl and stilbene, the balance being represented by toluene. 
The mixture of dibenzyl and stilbene containing at least 87% by weight of 
dibenzyl is separated by rectification. 
100 g of the mixture of products of toluene dehydrodimerization produced 
under the above-mentioned conditions and 92 g of ethylene are passed at a 
temperature of 490.degree. C. at the space velocity of 200 hr.sup.-1 over 
a catalyst containing 5.2% by weight of Cr.sub.2 O.sub.3, 6.1% by weight 
of WO.sub.3, 0.15% by weight of Na.sub.2 O on silica. 
As a result of the conversion there are obtained (g): H.sub.2 --0.1; 
CH.sub.4 --0.4; C.sub.2 H.sub.4 --71.7; C.sub.2 H.sub.6 --8.0; C.sub.2 
--C.sub.5 hydrocarbons--1.8; benzene 0.8; toluene--7--1; 
ethylbenzene--9.2; styrene 72.4; stilbene--0.6; dibenzyl--17.6; 
high-boiling products--0.8. 
Conversion of dibenzyl is 82.4%. 
The yield of styrene as calculated for the converted stilbene, dibenzyl and 
ethylene is 72% by weight. 
From the reaction products a 99.9% purity styrene is recovered by 
rectification. 
EXAMPLE 2 
92 g of toluene are passed through the reactor at a temperature of 
900.degree. C. and contact time of 0.005 sec. The pyrolysis products 
contain hydrogen and light hydrocarbons in an amount of 0.02 g, benzene 
0.04 g, toluene 90.34 g, xylenes 0.16 g, dibenzyl 1.41 g, high-boiling 
hydrocarbons 0.03 g. Conversion of toluene is 1.8%, selectivity of the 
formation of dibenzyl is 84.7% by weight. 
70 g of dibenzyl produced under the above-described conditions and 158 g of 
ethylene are passed at a space velocity of 180 hr.sup.-1 at a temperature 
of 325.degree. C. over a catalyst consisting of 1.5% by weight of Cr.sub.2 
O.sub.3, 4.0% by weight of WO.sub.3, 0.7% by weight of Na.sub.2 O, 8.8% by 
weight of Al.sub.2 O.sub.3 and 85% by weight of SiO.sub.2. 
As a result of conversion there are obtained (g): hydrogen--0.1; CH.sub.4 
--0.2; C.sub.2 H.sub.4 --137.8; C.sub.2 H.sub.6 --6.4; C.sub.3 --C.sub.5 
hydrocarbons--0.6; benzene 0.4, toluene 1.2; ethylbenzene--4.2; styrene 
50.1; stilbene--0.2; dibenzyl--26.5; high-boiling products--0.3. 
Conversion of dibenzyl is 62.0%. 
The yield of styrene as calculated for the converted dibenzyl and ethylene 
is 80.0% by weight. 
Styrene with the basic compound content of 99.9% by weight is separated 
from the reaction products by rectification. 
EXAMPLE 3 
A mixture of 74 g of dibenzyl and 26 g of stilbene prepared upon 
dehydrodimerization of toluene under the conditions described in the 
foregoing Example 1 is mixed with 136 g of ethylene and passed over a 
catalyst consisting of 3.3% by weight of Cr.sub.2 O.sub.3, 4.5% by weight 
of WO.sub.3, 0.7% by weight of K.sub.2 O; 33.3% by weight of Al.sub.2 
O.sub.3 ; 58.2% by weight of SiO.sub.2, at a temperature of 569.degree. C. 
at the space velocity of 165 hr.sup.-1. 
As a result of the conversion there are obtained, (g): hydrogen--0.1; 
C.sub.2 H.sub.4 --116.2; CH.sub.4 --0.2; C.sub.2 H.sub.6 --5.4; C.sub.3 
--C.sub.5 hydrocarbons--0.3; benzene--1.2; toluene--4.2; 
ethylbenzene--6.8; styrene--76.6; stilbene--0.9; dibenzyl--20.9; 
high-boiling products--3.2. 
Conversion of the mixture of dibenzyl and stilbene is 78.2%. 
The yield of styrene as calculated for the converted mixture of dibenzyl, 
stilbene and ethylene is 78.2% by weight. 
Styrene with the principal compound content of 99.9% is recovered from the 
conversion products rectification.