Process for the production of 2-ethylhexanol

The present invention provides a process for producing 2-ethylhexanol having a reduced concentration of 2-ethyl-4-methyl pentanol. It may be practiced in its most basic form as a distillation. It may also be practiced as part of a multi-stage continuous process. In either form it begins with a feed stream comprising n-butyraldehyde containing as a contaminant, isobutyraldehyde, complexes of isobutyraldehyde, oligomers of isobutyraldehyde and mixtures thereof, to which is added or introduced, an amount of water effective to hydrolyze the oligomeric contaminants to the monomeric form of isobutyraldehyde during distillation. The water containing aldehyde mixture is introduced to a distillation zone with a residence time and at a temperature sufficient to hydrolyze the oligomeric contaminants to and then distill substantially all of the isobutyraldehyde overhead. In the multi-stage process, the distilled n-butyraldehyde is then subjected to an alkali-catalyzed aldol condensation reaction to produce 2-ethylhex-2-enal. In a third stage, the 2-ethylhex-2-enal is hydrogenated with a catalyst under temperature and pressure conditions conducive to hydrogenation to produce 2-ethylhexanol.

FIELD OF THE INVENTION 
The present invention relates to the preparation of 2-ethylhexanol. More 
particularly, it relates to a process for reducing the 2-ethyl-4-methyl 
pentanol content in 2-ethylhexanol. 
BACKGROUND OF THE INVENTION 
2-Ethylhexanol is used in large quantities as an esterification component, 
e.g. for the preparation of dioctyl phthalate as a plasticizer for PVC. 
2-Ethylhexanol is made by alkali-catalyzed condensation of n-butyraldehyde 
to yield the unsaturated aldehyde, 2-ethyl-hex-2-enal, which is then 
hydrogenated to yield the desired 2-ethylhexanol. 
Various processes for the production of n-butyraldehyde are known. It is 
also known that n-butyraldehyde will contain, as an impurity, 
isobutyraldehyde and that oligomers of isobutyraldehyde will be present 
due to hydroformylation. These impurities will, if not removed, lead to 
the formation of 2-ethyl-4-methyl-pentenal during the preparation of 
2-ethyl-hex-2-enal. The 2-ethyl-4-methyl-pentenal is then hydrogenated to 
the alcohol, 2-ethyl-4-methyl pentanol during the preparation of 
2-ethylhexanol and cannot be economically removed. 
2-Ethyl-hex-2-enal is prepared by the alkali-catalyzed aldol condensation 
of n-butyraldehyde. 
2-Ethylhexanol is prepared by the hydrogenation of 2-ethylhex-2-enal (oxo 
successive product). In this process hydrogenation can occur both in the 
gaseous phase (DE-AS 11 52 393) and in the liquid phase (DE-AS 19 49 296). 
In these processes higher catalyst loads can generally be achieved in the 
liquid phase due to improved dissipation of heat. 
2-Ethylhexanol is used for production of the plasticizer, di-2-ethylhexyl 
phthalate and for many other uses. The presence of even minor amounts of 
contaminants will reduce purity and may affect the end use and even render 
it unacceptable. Accordingly, there exists a need in the art to reduce the 
quantity of contaminants in general and 2-ethyl-4-methyl pentanol in 
particular. 
SUMMARY OF THE INVENTION 
The present invention provides a process for producing 2-ethylhexanol 
having a reduced concentration of 2-ethyl-4-methyl pentanol. It may be 
practiced in its most basic form as a distillation. It may also be 
practiced as part of a multi-stage continuous process. In either form it 
begins with a feed stream comprising n-butyraldehyde containing as a 
contaminant, isobutyraldehyde, complexes of isobutyraldehyde, oligomers of 
isobutyraldehyde and mixtures thereof, to which is added or introduced, an 
amount of water effective to hydrolyze the oligomeric contaminants to the 
monomeric form of isobutyraldehyde during distillation. The water 
containing aldehyde mixture is introduced to a distillation zone with a 
residence time and at a temperature sufficient to hydrolyze the oligomeric 
contaminants to and then distill substantially all of the isobutyraldehyde 
overhead. In the multi-stage process, the distilled n-butyraldehyde is 
then subjected to an alkali-catalyzed aldol condensation reaction to 
produce 2-ethylhex-2-enal. In a third stage, the 2-ethylhex-2-enal is 
hydrogenated with a catalyst under temperature and pressure conditions 
conducive to hydrogenation to produce 2-ethylhexanol. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention may be practiced in a single stage or in multiple 
stages. A first, essential stage comprises the distillation of 
n-butyraldehyde. In a second stage, the distilled n-butyraldehyde from the 
first stage is subjected to an aldol condensation or enalization reaction 
to produce 2-ethylhex-2-enal. In the third stage is, the 2-ethylhex-2-enal 
is hydrogenated to produce 2-ethylhexanol. A fourth stage may be employed 
to distill the 2-ethylhexanol. 
In the first stage, which may be referred to as a distillation zone and 
which typically employs one or more conventional distillation columns or 
towers, n-butyraldehyde containing isobutyraldehyde, complexes of 
isobutyraldehyde, oligomers of isobutyraldehyde and mixtures thereof is 
distilled. Typically, this distillation will be carried out in a single 
distillation tower where the distillation operates at a temperature and 
for a residence time sufficient to distill isobutyraldehyde overhead; 
preferably at a head temperature in the range of from about 70.degree. to 
about 90.degree. C., which as one skilled in the art will appreciate, will 
be selected and will vary depending on column pressure. 
The actual temperatures and number of distillation columns employed may 
vary depending on the crude n-butyraldehyde composition and the specific 
equipment used in the process. 
It is known that isobutyraldehyde will complex or trimerize resulting in a 
product or oligomer which is difficult to remove during distillation of 
n-butyraldehyde. It is also known that these oligomers will, when 
subjected to heat, hydrolyze to the monomeric form during the enalization 
stage. In this stage the monomeric form is converted to 
2-ethyl-4-methyl-pentenal which in the third or hydrogenation stage is 
converted to 2-ethyl-4-methyl-pentanol. 
Accordingly, it has been discovered that if water is added to the 
n-butyraldehyde feed to the distillation column in an amount effective to 
hydrolyze the isobutyraldehyde oligomers to the monomeric form during 
distillation, they can be stripped from the n-butyraldehyde, thereby 
reducing the amount of isobutyraldehyde which reacts to form 
2-ethyl-4-methyl-pentenal during aldol condensation and subsequent 
hydrogenation of the 2-ethyl-4-methyl-pentenal to 2-ethyl-4-methyl 
pentanol during the hydrogenation stage. By this method the content of 
2-ethyl-4-methyl-pentanol in the final product can be substantially 
reduced; typically from a range of from about 0.2 to about 0.4 wt. % 
without the water addition to less than 0.2 wt. %, preferably to within a 
range of from about 0.08 to about 0.1 wt. % with the water addition, based 
on 2-ethylhexanol. The degree of improvement will vary with the amount of 
water added, with increasing amounts of water tending to give increasing 
reductions in the amount of 2-ethyl-4-methyl-pentanol in the 
2-ethylhexanol. 
Preferably, water is added in the range of from about 0.05 to about 2 wt. 
%, more preferably from about 0.5 to about 1.5 wt. %, and even more 
preferably from about 0.7 to about 1 wt. %, based on the crude 
n-/iso-butyraldehyde mixture. The water will typically be added as a 
component of the aldehyde feed but it may also be added to the tower 
reflux or to the upper portion or lower portion of the distillation tower 
or towers as may be determined by one skilled in the art. 
In the absence of water addition, the isobutyraldehyde and oligomers of 
isobutyraldehyde present in the crude n-/iso-butyraldehyde mixture both 
contribute to the ultimate formation of 2-ethyl-4-methyl pentanol, with 
the oligomers having a potentially greater contribution because they, 
unlike the monomeric isobutyraldehyde, are not readily removed during 
distillation. When the process of the present invention is employed, the 
oligomers are converted to isobutyraldehyde which is substantially removed 
through distillation. While some isobutyraldehyde and some oligomer remain 
after distillation and are ultimately converted to 2-ethyl-4-methyl 
pentanol, the contribution to the formation of 2-ethyl-4-methyl pentanol 
represented by the oligomers substantially reduced which, in turn, results 
in an overall reduction in the amount of 2-ethyl-4-methyl pentanol in the 
2-ethylhexanol. 
In the second stage, any conventional enalization reaction typically used 
to produce 2-ethylhex-2-enal from n-butyraldehyde may be employed. The 
enalization reaction is typically conducted in a counter-current (or 
co-current) reactor in the presence of an alkali catalyst which is 
preferably selected from the group consisting of alkali metal hydroxides, 
more preferably sodium hydroxide, in an amount sufficient to catalyze the 
aldol condensation of n-butyraldehyde to 2-ethylhex-2-enal. This is a 
conventional reaction well known to those skilled in the art. 
In the third stage, any of the conventional hydrogenation processes for the 
production of 2-ethylhexanol such as the medium pressure process, or the 
processes disclosed in U.S. Pat. Nos. 4,960,960 or 4,626,604 may be 
employed. This reaction is a conventional one well known to those skilled 
in the art. 
The process is not specific to any particular hydrogenation reaction or to 
any particular catalyst composition. Although cobalt catalysts may be 
used, more recently the use of rhodium complex catalysts has been 
preferred since these offer the advantages of lower operating pressure, 
ease of product recovery, and high n-/iso-aldehyde molar ratios. Typical 
operating conditions for such rhodium complex hydroformylation catalysts 
can be found in U.S. Pat. Nos. 3,527,809, 4,148,830, EP-A-Nos. 0096986, 
0096987, and 0096988. For example, 2-ethylhex-2-enal can be made by 
condensation of 2 moles of n-butyraldehyde. The aldehyde hydrogenation 
reaction then produces 2-ethylhexanol from 2-ethylhex-2-enal. However, in 
such aldehyde hydrogenation reactions there can be used any of the 
conventionally used metal catalysts. 
Typically, the hydrogenation will employ first and second hydrogenation 
zones. The first hydrogenation zone may comprise an adiabatic reactor, a 
reactor with an internal cooling coil, or a shell and tube reactor. In the 
case of a shell and tube reactor the catalyst may be packed in the tubes 
with coolant passing through the shell or it may be the shell that is 
packed with catalyst with coolant flow through the tubes. The first 
hydrogenation zone is generally operated as a trickle bed reactor. In this 
case the hydrogen containing gas of step (b) is generally admixed with the 
liquid phase upstream from the first hydrogenation zone and is partly 
dissolved therein. At the upper end of the first hydrogenation zone the 
concentration of unsaturated organic compound is at its highest in the 
liquid phase; hence the rate of hydrogenation is greatest at the upper end 
of the first hydrogenation zone. 
As the liquid phase passes downwardly through the first hydrogenation zone 
co-currently with the hydrogen it becomes depleted in respect of 
hydrogenatable material and to some extent in respect of dissolved 
hydrogen and the partial pressure of any inert gas or gases present rises 
and the partial pressure of hydrogen falls as the hydrogen is consumed by 
the chemical reactions taking place in the first hydrogenation zone. Hence 
at the lower end of the first hydrogenation zone the driving force for the 
hydrogenation reaction is relatively low. The intermediate reaction 
product exiting the lower end of the first hydrogenation zone accordingly 
usually still contains a minor amount of chemically unsaturated 
hydrogenatable material. 
Generally speaking the hydrogenation conditions in the first hydrogenation 
zone are selected so as to effect hydrogenation of from about 75% to about 
99% or more of the hydrogenatable unsaturated groups present in the 
unsaturated organic material supplied to the first hydrogenation zone. 
Typically the hydrogenation is completed to an extent of from about 85% to 
about 99.5% in the first hydrogenation zone. In some cases, however, the 
extent of hydrogenation may be higher than this, e.g. about 99.8% or even 
up to about 99.99%, in the first hydrogenation zone. 
In the second hydrogenation zone the intermediate reaction product from the 
first hydrogenation zone is fed in liquid form in co-current with a 
downward flow of the hydrogen-containing feed gas. The second 
hydrogenation zone can be operated on a once-through basis; alternatively 
the intermediate reaction can be admixed with recycled product, recovered 
from the lower end of the second hydrogenation zone so that the second 
hydrogenation zone is operated on a partial recycle basis. This may be 
desirable from the standpoint of fluid bed dynamics so as to ensure that 
the or each bed of catalyst is adequately wetted.

The following examples are provided in order to further illustrate the 
invention without limiting its scope. 
EXAMPLES 
Comparative Example 
2-Ethylhexanol was produced using the multi-stage process described above 
in the specification. Water was not added during the aldehyde 
distillation. The 2-ethyl-4-methyl-pentanol content in the 2-ethylhexanol 
product was about 0.35 wt. %. The system was operated at the preferred 
conditions described in the specification. 
Example 1 
2-Ethylhexanol was produced as set forth in the Comparative Example with 
the exception that water was added to the feed of the aldehyde 
distillation tower in an amount equalling 0.9 wt. %. The 
2-ethyl-4-methyl-pentanol content in the 2-ethylhexanol product to about 
0.10 wt. %. 
Example 2 
2-Ethylhexanol was produced as set forth in the Comparative Example with 
the exception that water was added to the feed of the aldehyde 
distillation tower in an amount equalling 0.45 wt. %. The 
2-ethyl4-methyl-pentanol content in the 2-ethylhexanol product was reduced 
to about 0.2 wt. %.