Process for purifying isoprene

A process for purifying isoprene produced by catalytic decomposition of dimethyldioxane from carbonyl compounds and cyclopentadiene which involves passing isoprene at a temperature of 40.degree. to 70.degree. C. through a bed of a solid product such as an anion exchange resin or alkali (the latter may be used with or without an inert carrier), followed by subjecting the isoprene to close fractionation performed with at least 50 theoretical plates and at a reflux ratio of at least 3. The process of the present invention enables practically complete purification of isoprene from carbonyl compounds and cyclopentadiene. The purified isoprene, when used as a monomer in the production of stereospecific isoprene rubber, results in high-quality products; and reduces by a factor of 2-3 the Ziegler catalyst consumption in the isoprene rubber synthesis.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to processes for purifying isoprene produced 
by catalytic decomposition of dimethyldioxane from carbonyl compounds and 
cyclopentadiene. The isoprene thus purified is used as a monomer in the 
production of stereospecific isoprene rubber. 
2. Description of the Prior Art 
Known in the art is a process for purification of isoprene produced by 
catalytic decomposition of dimethyldioxane from carbonyl compounds and 
cyclopentadiene by close fractionation on at least 50 theoretical plates 
of at least 50 and at a reflux ratio of at least 3, followed by washing 
the resulting isoprene with water. The residual content of carbonyl 
compounds and cyclopentadiene in the purified isoprene produced by this 
process is 0.0005 and 0.0005-0.0004 wt.% respectively. A disadvantage of 
this process is the relatively high residual content of micro-impurities 
(carbonyl compounds and cyclopentadiene) in the purified isoprene, which, 
in turn, results in an impaired quality of isoprene rubber and increased 
Ziegler catalyst consumption during the subsequent polymerization of the 
isoprene. 
SUMMARY OF THE INVENTION 
It is an object of the present to provide a fractionation process for 
purifying isoprene produced by catalytic decomposition of dimethyldioxane 
which makes it possible to purify the isoprene to be substantially 
completely free from carbonyl compounds and cyclopentadiene. 
This and other objects of the present invention are accomplished by a 
process wherein isoprene produced by catalytic decomposition of 
dimethyldioxane is passed at a temperature of 40.degree. to 70.degree. C. 
through a bed of a solid product such as an anion exchange resin or 
alkali, and then subjected to close fractionation using at least 50 
theoretical plates and a reflux ratio of at least 3. 
When passing isoprene through a bed of a solid product at the 
above-mentioned temperatures, microimpurities (carbonyl compounds and 
cyclopentadiene) are converted into highmolecular products, namely 
fulvenes and polyesters which are easily separated from isoprene by 
subsequent close fractionation. Anion exchange resin and alkali act as 
catalysts in this conversion. The content of carbonyl compounds in 
isoprene after passing through the bed of said solid products is reduced 
to 0.0005-0.00008 wt.%, while cyclopentadiene content is reduced to 
0.0005-0.0001 wt.%. After the close fractionation, the residual content of 
carbonyl compounds in the purified isoprene is not more than 0.00005 wt.% 
and that of cyclopentadiene does not exceed 0.00005 wt.%. 
As the anion exchange resin it is advisable to use either a reaction 
product of chloromethylated copolymer of styrene and divinyl-benzene with 
trimethylamine or a reaction product of epichlorohydrin with 
polyethylenepolyamine. 
To improve the degree of isoprene purification, it is advisable to use an 
alkali on an inert carrier. As the inert carrier it is preferred to employ 
silica gel or corundum. 
To purify isoprene as extensively as possible, following close 
fractionation it is advisable to continuously recycle 20 to 80% by weight 
of the purified isoprene for intermixing with the unpurified starting 
material isoprene. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The process for purifying isoprene according to the present invention is 
performed in the following manner. 
Isoprene produced by catalytic decomposition of dimethyldioxane and 
containing 0.1 to 0.2 wt.% of carbonyl compounds (acetaldehyde, acrolein, 
acetone, isovaleric aldehyde, and the like) and 0.05 to 0.1 wt.% of 
cyclopentadiene is heated to a temperature within the range of from 
40.degree. to 70.degree. C. and passed through a vertical cylindrical 
reactor filled with a finely divided solid product, viz. an anion exchange 
resin or alkali (the latter may be employed with or without an inert 
carrier. 
As the anion exchange resin various basic polymers may be used 
corresponding to the formula R.sub.4 N.sup.+ OH.sup.-, such as a reaction 
product of a chloro-methylated copolymer of styrene and divinyl benzene 
with trimethyleneamine, a reaction product of epichlorohydrin with 
polyethylenepolyamine, and the like. As the alkali, potassium, sodium or 
lithium hydroxides may be used. As the inert carrier, alumina gel, silica 
gel, corundum, pumice, and carborundum may be used. 
It is advisable that a space velocity of isoprene passed through the 
reactor filled with the solid product be maintained within the range of 
from 0.5 to 2 hr.sup.-1. From the reactor the isoprene is passed into a 
close fractionation column. Purified isoprene is discharged from the 
column top, while high-boiling products of converted microimpurities 
(fulvenes and polyesters) are discharged from the column bottom. 
To perform isoprene purification as extensively as possible, it is 
advisable that 20 to 80 wt.% of purified isoprene containing only 
microamounts of axeotrope-forming carbonyl compounds be continuously 
recycled back to the purification process for mixing with the starting 
isoprene. 
The process of the present invention enables substantially complete 
purification of isoprene from carbonyl compounds and cyclopentadiene. When 
thus-purified isoprene is employed as a monomer in the production of 
stereospecific isoprene rubber, the resulting product is of high quality 
and the Ziegler catalyst consumption is reduced by 2-3 times.

For a better understanding of the present invention, the following examples 
illustrating the process of isoprene purification are given hereinbelow. 
EXAMPLE 1 
Isoprene produced by catalytic decomposition of dimethyldioxane and 
containing 0.2 wt.% of carbonyl compounds and 0.05 wt.% of cyclopentadiene 
was heated to 60.degree. C. and passed at the space velocity of 1 
hr..sup.-1 through a vertical cylindrical reactor filled with solid 
potassium hydroxide having a particle size of 5 to 30 mm. Isoprene 
discharged from the reactor contained 0.0005 wt.% of carbonyl compounds 
and 0.0004 wt.% of cyclopentadiene. From the reactor the isoprene was 
delivered into a close-fractionation column having 50 theoretical plates, 
the reflux ratio being equal to 3. Purified isoprene was discharged from 
the column top. The content of carbonyl compounds in the purified isoprene 
was 0.00005 wt.% and that of cyclopentadiene was 0.00005 wt.%. 
EXAMPLE 2 
Isoprene produced by catalytic decomposition of dimethyldioxane and 
containing 0.15 wt.% of carbonyl compounds and 0.1 wt.% of cyclopentadiene 
was heated to 50.degree. C. and passed at the space velocity of 1.5 
hr..sup.-1 through a vertical cylindric reactor filled with a finely 
divided anion exchange resin (reaction product of chloromethylated 
copolymer of styrene and divinylbenzene with trimethylamine) having a 
particle size of 1-3 mm. The content of the carbonyl compounds in the 
isoprene discharged from the reactor was 0.0003 wt.%, and the 
cyclopentadiene content in the discharged isoprene was 0.0005 wt.%. From 
the reactor the isoprene was delivered into a close-fractionation column 
having 55 theoretical plates and a reflux ratio of 4. Purified isoprene 
was discharged from the column 20 wt.% of the purified isoprene was 
continuously recycled for mixing with the starting isoprene fed into the 
the anion exchange resin filled reactor. In the isoprene thus purified 
there were practically no microimpurities (carbonyl compounds and 
cyclopentadiene). 
EXAMPLE 3 
Isoprene produced by catalytic decomposition of dimethyldioxane and 
containing 0.1 wt.% of carbonyl compounds and 0.08 wt.% of cyclopentadiene 
was heated to 40.degree. C. and passed at the space velocity of 2 
hr.sup.-1 through a vertical cylindrical reactor filled with solid sodium 
hydroxide deposited on corundum having a particle size of 2 to 5 mm. The 
content of carbonyl compounds in the isoprene discharged from the reactor 
was 0.00008 wt.% and that of cyclopentadiene was 0.00012 wt.%. From the 
reactor the isoprene was fed into a close-fractionation column having 60 
theoretical plates and a reflux ratio of 3.2. Purified isoprene was 
discharged from the column top. The content of carbonyl compounds in the 
thus-purified isoprene was less than 0.00002 wt.% and that of 
cyclopentadiene was also less than 0.00002 wt.%. 
EXAMPLE 4 
Isoprene produced by catalytic decomposition of dimethyldioxane and 
containing 0.15 wt.% of carbonyl compounds and 0.1 wt.% of cyclopentadiene 
was heated to 70.degree. C. and passed at the space velocity of 0.8 
hr.sup.-1 through a vertical cylindrical reactor filled with potassium 
hydroxide deposited on silica gel having a particle size of 2 to 5 mm. The 
content of carbonyl compounds in the isoprene discharged from the reactor 
was 0.0005 wt.%. The content of cyclopentadiene in the discharged isoprene 
was 0.0002 wt.%. From the reactor the isoprene was fed into a 
close-fractionation column having 60 theoretical plates and a reflux ratio 
of 5. The thus-purified isoprene was discharged from the column top. 80% 
of the purified isoprene was continuously recycled for mixing with the 
starting isoprene fed into the reactor filled with potassium hydroxide 
deposited on said carrier. Isoprene purified by this process contained 
0.00005 wt.% of carbonyl compounds and 0.0005 wt.% of cyclopentadiene. 
EXAMPLE 5 
Isoprene produced by catalytic decomposition of dimethyldioxane and 
containing 0.2 wt.% of carbonyl compounds and 0.08 wt.% of cyclopentadiene 
was heated to 65.degree. C. and passed at the space velocity of 2 
hr.sup.-1 through a vertical cylindrical reactor filled with a finely 
divided anion exchange resin (reaction product of epichlorohydrin and 
polyethylenepolyamine) having a particle size 1 to 3 mm. The content of 
carbonyl compounds in the isoprene discharged from the reactor was 0.0004 
wt.% and that of cyclopentadiene was 0.0003 wt.%. From the reactor the 
isoprene was fed into a close-fractionation column having 50 theoretical 
plates and a reflux ratio of 5. The purified isoprene was discharged from 
the column top. The purified product contained practically no 
microimpurities (carbonyl compounds and cyclopentadiene).