Process for removing organic oxygen-containing impurities from an organic composition

The amount of oxygen-containing impurities in an organic composition is reduced at an elevated temperature by contact with a supported metal oxide. For example, the amount of oxygen-containing organic impurities in a crude stilbene product obtained by oxidatively dehydrocoupling toluene using a solid oxidant is decreased by contact with zinc oxide supported on alumina at an elevated temperature and the resulting purified stilbene product is reacted with ethylene to form styrene.

FIELD OF THE INVENTION 
This invention relates to a method for reducing the amount of 
oxygen-containing impurities in an organic composition. More particularly, 
the invention relates to the process for preparing styrene and its 
derivatives from toluene and its derivatives and to the purification of 
the intermediate stilbene and its derivatives. In this process toluene or 
a derivative of toluene is oxidatively dehydrocoupled using a solid 
oxidant to form a reaction product containing stilbene or a derivative of 
stilbene and this reaction product is reacted with ethylene over a 
disproportionation catalyst to form styrene or a derivative of styrene. 
Since water and any oxygenated organic compounds produced as by-products 
in the dehydrocoupling reaction will poison the disproportionation 
catalyst, these compounds are substantially eliminated before the 
ethenolysis reaction. 
DESCRIPTION OF THE PRIOR ART 
Catalysis Reviews, 3(1), 44(1969) discloses that polar compounds in feed 
streams to disproportionation catalysts will deactivate the catalysts. 
Specifically mentioned are water, acetone and methanol. 
In U.S. Pat. No. 3,965,206 toluene is converted to styrene in a multi-stage 
procedure including the oxidative dehydrocoupling of toluene and 
terminating with the reaction of stilbene and ethylene by olefin 
metathesis. In this process a stilbene fraction containing some of the 
polar impurities present in the toluene dehydrocoupling effluent is 
removed from this reaction mixture. Polar impurities are separated from 
this stilbene fraction by one of the following specified techniques: by 
passage through a bed of absorbent material, by fractional 
crystallization, by solvent extraction, or by reaction in the absence of 
water with an active metal compound such as sodium metal or disodium 
stilbene. 
SUMMARY OF THE INVENTION 
When toluene is oxidatively dehydrocoupled at an elevated temperature with 
a solid oxidant, the crude reaction product includes unreacted toluene, 
benzene, bibenzyl, stilbene, water, carbon dioxide and trace amounts of 
organic oxygenated compounds in which the oxygen is present predominantly 
as carbonyl and hydroxyl. We have surprisingly discovered that this crude 
reaction product can be treated for removal of the minute amounts of the 
organic oxygenated compounds in the presence of certain supported metal 
oxides if the water present in the crude dehydrocoupling reactor effluent 
is substantially removed prior to the purification treatment. 
The dehydrocoupling of toluene is an oxidative reaction in which the oxygen 
is supplied by a suitable solid oxidant such as a metal oxide, a non-metal 
oxide, or a mixture thereof. U.S. Pat. No. 3,476,747 discloses arsenic 
pentoxide, antimony tetroxide and pentoxide, bismuth trioxide and 
manganese arsenate as the oxidant for the oxidative dehydrocoupling 
reaction, U.S. Pat. No. 3,494,956 discloses lead oxide, cadmium oxide and 
thallium oxide as the oxidant while U.S. Pat. No. 3,965,206 specifies lead 
oxide, cadmium oxide and bismuth oxide as the oxidant. Any suitable metal 
oxide, non-metal oxide or mixture of such oxides which can supply oxygen 
at the elevated temperatures for the oxidative reaction can be used in 
preparing the crude, stilbene-containing reaction product. The solid 
oxidant is preferably supported on a suitable support such as fused 
silica, alumina, a silica-alumina, and the like, or less preferably it can 
be unsupported. 
This oxidative dehydrocoupling reaction is carried out at an elevated 
temperature, suitably between about 500.degree. C. and about 700.degree. 
C., preferably between about 550.degree. C. and about 625.degree. C. In 
carrying out the reaction heated toluene is introduced into the reactor 
preferably together with steam. Although some water is produced by the 
oxidative reactions taking place in the reactor, it is preferred that a 
substantial amount of water be added to the reactor in the form of steam 
to serve as a diluent and to help control the reaction. The mol ratio of 
added steam to toluene can suitably be from 0 to about 30, preferably from 
about 1 to about 15. It is also possible to add molecular oxygen to 
supplement the oxygen provided by the solid oxidant, but it is preferred 
to carry out the reaction without using free oxygen. Molecular oxygen can 
be used in a mol ratio of molecular oxygen to toluene of 0 to about 4, 
preferably 0 to about 1. 
The product stream from this oxidative dehydrocoupling stage includes water 
as steam, and trace amounts of oxygen-containing containing organic 
compounds in addition to the hydrocarbon components. These 
oxygen-containing impurities are primarily aromatic compounds in which the 
oxygen is present as carbonyl and hydroxyl, with carbonyl predominating. 
These organic polar compounds will decompose in the presence of the 
disproportionation catalyst producing, in part, water. Since water causes 
the rapid deactivation of disproportionation catalysts, the water and the 
oxygen-containing organic compounds must be substantially eliminated from 
the feed stream to the ethenolysis reaction. 
In accordance with our process the organic portion from the oxidative 
dehydrocoupling reactor is subjected to a purification treatment in the 
presence of a suitable metal oxide catalyst to convert the oxygen in these 
oxygen-containing impurities to water which is readily removable. In 
carrying out this purification the hot, gaseous stream from the 
dehydrocoupling reactor is first cooled and the water substantially 
removed by a suitable technique such as by condensation, by selective 
absorption, by molecular sieves, by azeotroping, or any other suitable 
water removing procedure to prevent water inactivation of the metal oxide. 
This dewatered dehydrocoupling reactor effluent will contain no more than 
about one weight percent water, preferably no more than about 0.5 percent 
water. The carbon dioxide in the stream is also removed at this stage. The 
organic portion of the dehydrocoupling reactor effluent is then reheated 
for the purification treatment. 
Metal oxides which can be used in our purification procedure include the 
oxides of Group IIb metals including zinc, cadmium and mercury; Group Vb 
metals including vanadium, niobium and tantalum; Group VIb metals 
including chromium, molybdenum and tungsten; Group VIIb metals including 
manganese and rhenium; and oxides of titanium, iron and nickel. These 
metal oxides are supported on a carrier, preferably alumina. Also useful 
as supports for these metal oxides are silica, silica-alumina, magnesium 
aluminate, zeolites, clays, and other materials commonly used as supports 
in the art. These supports will generally have a surface area of between 
about 50 and about 350 M.sup.2 /g. 
The supported metal oxide catalyst functions as a purification agent by 
aiding in the removal of the oxygen from the organic composition 
undergoing treatment. In preparing these purification agents a compound of 
the desired metal can be deposited on the support or mixed with the 
support by any suitable method commonly known in the art for such purpose, 
such as impregnation from solution, and the like. The resulting material 
is dried and then heated at an elevated temperature in an air atmosphere. 
Combinations of two or more metal oxides can also be used. The resultant 
material which is useful to decrease polar impurities in the 
dehydrocoupling reactor effluent will contain from about 0.1 to about 50 
percent of the metal oxide on the support, preferably from about 2 to 
about 20 percent. 
The supported metal oxide can effect a substantial reduction in the amount 
of the oxygen-containing organic compounds when used in the purification 
of the oxidative dehydrocoupling effluent. The amount of the 
oxygen-containing organic impurities is determined by analyzing for 
carbonyl content. It is believed that using the carbonyl content as the 
indicator of the presence of oxygen-containing organic impurities and as a 
measure of the degree of purification is reliable both because carbonyl in 
the form of aromatic aldehydes and ketones is considered to be the primary 
organic oxygen-containing impurity and for the further reason that this 
primary carbonyl impurity is believed to occur in a generally constant 
proportion with respect to the other organic, oxygen-containing 
impurities. We have found that it is desirable to reduce the amount of 
oxygen-containing compound in this purification step down to 50 parts per 
million (ppm.) or less measured as carbonyl, preferably down to 35 ppm. or 
less carbonyl and most preferably down to 20 ppm. or less carbonyl as 
determined by the following test for measuring trace carbonyl content. 
A 0.8 to 5.0 g. sample of the dehydrocoupling effluent (the amount 
inversely adjusted to the anticipated concentration of carbonyl) is placed 
in the first of two 25 ml. flasks and 1.0 ml. of a saturated solution of 
2,4-dinitrophenylhydrazine (100 ml. of water and 2.0 ml. of hydrochloric 
acid (sp. gr. 1.19) saturated with 2,4-dinitrophenylhydrazine) is added to 
both flasks. One drop of hydrochloric acid (sp. gr. 1.19) is added to both 
flasks with swirling. The flasks are heated at 55.degree. C. in a water 
bath for 30 minutes with swirling every five minutes. The flasks are 
removed from the bath and allowed to stand at room temperature (25.degree. 
C.) for 30 minutes with swirling every five minutes. Five ml. of alcohol 
potassium hydroxide solution (60 g.KOH/1. of methanol/water solution 
containing 11.2 percent water) is added to each flask with swirling and 
allowed to stand for five minutes. Following this each flask is diluted to 
volume with absolute methanol and mixed. Within 10 minutes of the 
addition of the alcoholic potassium hydroxide solution a portion of the 
wine-red colored solution containing the sample is placed in a cuvette and 
measured for absorbance at 430 m.mu. against the prepared blank (second 
flask). The concentration of carbonyl is determined from a calibration 
chart prepared from similar solutions containing a known amount of 
carbonyl and analyzed by the above technique. 
Our purification procedure can suitably be carried out at a temperature 
between about 150.degree. C. and about 600.degree. C. and preferably at a 
temperature between about 210.degree. C. to about 500.degree. C. Pressure 
is not a critical factor in the purification reactor. The pressure can 
conveniently range from about atmospheric to about 3,500 kPa. and most 
generally will be atmospheric or slightly higher. The crude product stream 
is passed over the catalyst at a liquid hourly space velocity, LHSV, of 
between about 0.1 and about 50, preferably between about 0.5 and about 15 
for most effective conversion and removal of the oxygenated organic 
impurity. 
The product leaving the purification reactor contains toluene, benzene, 
stilbene, bibenzyl, a small amount of water and no more than a residual 
amount of the undesired oxygen-containing organic compound which can be 
tolerated by the disproportionation catalyst. Since the water in this 
effluent stream from the purification stage will significantly reduce the 
activity of the disproportionation catalyst, this water is removed prior 
to the metathesis reactor. Any conventional means for removing water such 
as by condensation, selective absorption by molcular sieves, azeotroping, 
and the like can suitably be used. These various water removal procedures 
involve a cooling of the product stream. Water is removed at this stage 
down to an amount of 20 ppm. or less, preferably to an amount of 10 ppm. 
or less. 
This oxidative dehydrocoupling reactor product stream with the water and 
oxygenated organic compounds substantially removed is now suitable for the 
ethenolysis reaction. The ethylene can be added to this purified product 
stream prior to its introduction into the metathesis reactor or the 
ethylene can be added directly to the reactor. Although one mol of 
ethylene will react by metathesis with one mol of stilbene to form two 
mols of styrene, it is preferred that an excess of ethylene be used to 
help drive the reaction to completion. Therefore, a mol ratio of ethylene 
to stilbene of between about 1:1 to about 100:1 can suitably be used, but 
it is preferred that a mol ratio of about 2:1 to about 20:1 be used. 
In this olefin metathesis reaction between the stilbene and the ethylene, 
any olefin disproportionation catalyst can be used. Examples of suitable 
catalysts include the oxides of tungsten, molybdenum, rhenium, uranium, 
vanadium, niobium, and tantalum and the sulfides and carbonyls of tungsten 
and molybdenum. These catalysts are carried on a suitable support such as 
alumina, silica, silica-alumina, a spinel such as zinc aluminate and 
magnesium aluminate, alumina-aluminum phosphate, and the like. The 
disproportionation catalyst can desirably be modified by addition of a 
suitable compound of an alkali metal, alkaline earth metal, thallium, 
cuprous copper, silver and the like to control surface acidity. Other 
metals such as cobalt and nickel can be associated with such catalysts. 
The conditions for methathesis depend, in part, on the specific catalyst 
which is used. These conditions are well known in the art. The temperature 
for carrying out metathetic reactions is broadly within the range of about 
150.degree. C. to about 650.degree. C. and usually it is between about 
450.degree. C. to about 650.degree. C. The optimum temperature is 
generally between about 500.degree. C. and about 600.degree. C. In our 
process the methathesis reaction is preferably carried out under 
conditions to maximize the conversion of stilbene to styrene. The pressure 
in the reactor will generally be about atmospheric pressure for enhanced 
selectivity. 
The product stream from the metathesis reactor will contain toluene, 
styrene, benzene, ethylene, bibenzyl and minor amounts of stilbene and 
impurities. This stream is fractionated to recover the ethylene, toluene 
and bibenzyl for recycle and to recover styrene and benzene as product. 
The bibenzyl is recycled to the oxidative dehydrocoupling reactor for 
reaction to stilbene. 
Although the above procedure as described provides for the purification of 
the total product stream from the oxidative dehydrocoupling reactor with 
only water and carbon dioxide removed and use of this stream in the 
metathesis reaction, this procedure is not critical. Thus, the crude 
stream from the dehydrocoupling reactor can be fractionated into various 
organic fractions and the recovered stilbene-containing fraction can then 
be treated in accordance with the purification procedure of the present 
invention. 
In carrying out the purification procedure described herein, it may be 
desirable to add a small amount of free hydrogen to the feed gas stream to 
enhance the removal of carbonyl and hydroxyl contamination. Such added 
hydrogen can comprise up to about three volume percent of the feed stream 
to the purification reactor to accomplish a suitable reduction in analyzed 
carbonyl content.