Separation of aldehydes from ketones via acid-catalyzed cyclotrimerization of the aldehyde

This invention is directed to the discovery that a mixture of aldehydes and ketones where the boiling points of the aldehyde and the ketone are very close can be easily separated through the acid-catalyzed cyclotrimerization of the aldehyde and the subsequent distillation of the ketone and unreacted aldehyde from the reaction mixture. The trimerized aldehyde is cracked back to the starting aldehyde in high purity at elevated temperatures in the presence of an acid catalyst.

TECHNICAL FIELD 
This invention relates to the discovery that ketones and aldehydes having 
close boiling points can be readily separated through the acid-catalyzed 
cyclotrimerization of the aldehyde and the subsequent distillative 
separation of the ketone and unreacted aldehyde from the trimerized 
aldehyde. The cyclotrimerized aldehyde can thereafter be "cracked-back" to 
the starting aldehyde easily and efficiently. 
BACKGROUND ART 
Numerous methods have been developed over the years to produce isoprene. 
One method described in U.S. Pat. No. 4,524,233, herein incorporated by 
reference, discloses the dehydration reaction of 2-methylbutanal (2MBA) 
over a dehydration catalyst such as boron phosphate to yield isoprene. As 
disclosed in U.S. Pat. No.4,524,233, a major by-product of the 2MBA 
dehydration is methylisopropylketone (MIPK). For the economical operation 
of a process as described in this U.S. patent, it will be required to 
recycle the 2MBA that is not converted to MIPK or isoprene, back to the 
dehydration reactor. 
The close boiling points of 2MBA (90.degree.-92.degree. C.) and MIPK 
(92.degree.-94.degree. C.) make separation by distillation difficult. A 
separation of two compounds having close boiling points is known to those 
skilled in this art to present extreme difficulties. 
The inventor herein has unexpectedly found that a mixture of aldehyde such 
as 2MBA and a ketone such as MIPK can be separated after acid-catalyzed 
cyclotrimerization of the aldehyde. The ketone and unreacted aldehyde are 
distilled away from the cyclotrimerized aldehyde easily and efficiently 
and thereafter, the cyclotrimerized aldehyde can be catalytically 
reconverted to the starting aldehyde in high purity. None of the prior art 
suggests or discloses this unique approach to the separation of close 
boiling aldehydes and ketones. 
U.S. Pat. No. 4,163,696 is concerned with a distillation process for the 
recovery of methylisobutyl ketone. This patent discloses an azeotropic 
distillation process for separating toluene from methylisobutyl ketone in 
a spent liquor mixture. A toluene azeotrope former, preferably methanol, 
is added to the liquor in an amount sufficient to form an azeotrope with 
all the toluene present in the mixture. 
It is known that when catalyzed by acids, low molecular weight aldehydes 
add to each other to form trimers. See, for example, Bevington, J C, 
Quart. Rev. (London) 6, 141 (1952). Although these cyclic acetals are 
stable to bases, they can be converted back to monomeric aldehydes by 
acids with heat. The following is a structural representation of the 
acid-catalyzed cyclotrimerization of an aldehyde: 
##STR1## 
wherein R can be a hydrocarbon radical of 1 to 16 carbon atoms. 
These acid-catalyzed cyclotrimerizations of the aldehyde do not affect the 
ketone or other nonaldehyde components. The trimers have substantially 
higher boiling points than their monomers. 
A portion of the instant invention resides in the application of this 
unique property of aldehydes to the specific problem of separating close 
boiling mixtures of an aldehyde and a ketone. The unique process of this 
invention provides a means for separating these close boiling components. 
The aldehyde is selectively trimerized to a high boiling trimer at low 
temperatures by an insoluble solid acid catalyst. After removing the 
catalyst by filtration or other means, the aldehyde trimer is separated 
from the ketone and unreacted aldehyde by stripping or distilling the low 
boiling point compounds from the filtrate. The desired aldehyde is then 
recovered by heating its trimer with a solid acid catalyst to convert it 
back to the monomeric aldehyde species. The thus obtained aldehyde is of 
high purity. 
DISCLOSURE OF THE INVENTION 
There is disclosed a process for the separation of an aldehyde of 3 to 16 
carbon atoms from a close boiling mixture, said process comprising (1) 
contacting the aldehyde containing mixture with a solid acid catalyst at a 
temperature from 0 to 35.degree. C. to cyclotrimerize a major portion of 
the aldehyde contained in the mixture; (2) distill the unreacted aldehyde 
and other components of the mixture from the high boiling cyclotrimerized 
aldehyde in the absence of the catalyst: and (3) contacting the thus 
isolated cyclotrimerized aldehyde with an acid catalyst at elevated 
temperatures to produce essentially pure aldehyde. 
There is also disclosed a process for the separation of 2-methylbutanal 
from a mixture comprising 5 to 85 parts by weight of methylisopropylketone 
and from 5-95 parts by weight of 2-methylbutanal, said process comprising 
contacting the mixture with a solid acid catalyst at a temperature from 
0.degree.-35.degree. C. so that the aldehyde is cyclotrimerized; 
distillative separation of the methylisopropylketone and unreacted 
aldehyde from the cyclotrimerized 2-methylbutanal; contacting the 
cyclotrimerized aldehyde with an acid catalyst at elevated temperatures to 
produce essentially pure aldehyde. 
The composition of the effluent from the 2MBA to diene dehydration reaction 
contains isoprene, MIPK, 2MBA and water in addition to other minor 
by-products. A typical single pass effluent analysis from the dehydration 
reaction of 2MBA to isoprene contains about 2 to 3% by weight MIPK, 
however, as the 2MBA/MIPK is recycled, the level of MIPK increases to a 
point where the yield of isoprene is greatly diminished per pass over the 
catalyst. In order to increase the yield of isoprene per pass, the level 
of MIPK in the recycle must be kept to a minimum. 
In a simple fractionation of the effluent from the dehydration reactor, the 
isoprene is easily removed leaving essentially a mixture of 2MBA and MIPK 
to be recycled to the dehydration reactor. It is advantageous in the 
aldehyde to diene dehydration that the aldehyde be as pure as possible 
thus insuring the highest level of isoprene production, since the presence 
of ketone in the aldehyde feed has been shown to be detrimental to the 
efficient production of the diene. 
Products obtained from 2-methylbutanal (2MBA) dehydration include isoprene; 
light by-products, such as 2-methyl-1-butene, 2-methyl-2-butene and the 
like; with methylisopropyl ketone (MIPK) as the major by-product. Isoprene 
and the light by-products are easily separated by distillation. MIPK is 
known to be more reluctant to dehydrate than 2MBA and to recycle the 
unreacted 2MBA, MIPK should be removed to avoid the dilution of the 
aldehyde feed. It should be appreciated that it would be very costly to 
physically separate the ketone from the aldehyde by distillation due to 
their very similar boiling points of 92.degree.-93.degree. C. for 2MBA and 
94.degree.-95.degree. C. for MIPK. Through the process of this invention, 
essentially pure 2MBA is separated from the mixture by selective 
cyclotrimerization of the 2MBA. The process described and claimed herein, 
when incorporated into the aldehyde to diene process can be depicted as 
follows: 
##STR2## 
The process of this invention provides a novel and unobvious method which 
allows for the efficient and facile separation of an aldehyde from a close 
boiling ketone. The process of this invention is unique and efficient 
since the aldehyde trimerizes at low temperature (room temperature or 
lower) in the presence of an acid catalyst, preferably a solid acid 
catalyst. In contrast, the ketone, such as MIPK, will not react under the 
same conditions. The trimerization of the aldehyde results in a compound 
such as 2,4,6-tri-sec-butyl-1,3,5-trioxane and these cyclic compounds 
usually have very high boiling points (sometimes in excess of 300.degree. 
C.) and are thermally stable in the absence of an acid catalyst. After 
distillative separation of high boiling timer from the mixture the trimer 
can be easily and quantitatively cracked back to the starting aldehyde at 
an elevated temperature in the presence of an acid catalyst. 
Representative of the acid catalysts that are useful in the process of the 
invention are any acidic catalyst; however, insoluble solid acid 
catalysts, such as Nafion (polymer bound perfluorosulfonic acid), 
BPO.sub.4, H.sub.3 PO.sub.4 on SiO.sub.2, Amberlyst 15 and the like are 
both useful and adequate. The solid acid catalysts are preferred in the 
process of this invention due to the ease of separation by filtration, 
their recyclability and suitability for a continuous process. 
The process of this invention may be conducted on a batch basis or in a 
continuous process. 
BEST MODE 
The following Examples are intended to illustrate and not to limit the 
scope of the present invention.

EXAMPLES 1 and 2 
To a reaction flask fitted with a stirrer, a condenser and a nitrogen inlet 
was charged 200 ml of a 7 to 1 mixture by volume of 2-methylbutanal and 
MIPK and 5 g of the solid acid catalyst Nafion. The mixture was stirred 
under an atmosphere of nitrogen for 48 hours at room temperature or 
45.degree. C. (as indicated). Samples of the reaction mixture were taken 
at different times and submitted for analysis. The reaction mixture was 
then filtered to remove the acid catalyst. The filtrate was returned to 
the reaction vessel fitted with a distillation column and heated. 
The low boiling MIPK and unreacted 2MBA was taken overhead to yield 
essentially pure cyclotrimerized 2MBA in the pot. After all the MIPK and 
2MBA had distilled off, 5 gms of Nafion acid catalyst was charged to the 
reactor. The quantitative decomposition of the trimer to free aldehyde 
began immediately. Pure 2MBA was collected overhead as the pot temperature 
was kept at about 100.degree. C. 
Table I sets out the temperature, hours the 2MBA/MIPK was heated to produce 
the trimer and the percentage of 2MBA that had trimerized. The % trimer 
was determined by NMR spectroscopy. 
TA8LE I 
______________________________________ 
2MBA Trimer Formation from a 7:1 2MBA/MIPK 
Mixture Catalyzed by a Solid Acid Catalyst 
Reaction Time 
% of 2MBA in 
(Hours) Mixture Trimerized 
______________________________________ 
Ex 1 - Room Temp. 
1 37 
3 53 
5 62 
19 77 
22 75 
24 76 
28 76 
44 77 
48 77 
Ex 2 - 45.degree. C. 
1 28 
3 37 
5 37 
19 32 
22 35 
24 34 
28 33 
44 32 
48 33 
______________________________________ 
From the data in Table I it is apparent that room termperature 
trimerization of the aldehyde is favored over the 45.degree. C. the timer 
is less stable in the presence of the catalyst and reverts back to the 
starting aldehyde. 
As described above, the catalyst was separated, the MIPK and unreacted 
2MBA, was flashed off and the cyclotrimerized aldehyde cracked back to the 
starting aldehyde in the presence of the acid catalyst at temperatures 
from 90.degree.-100.degree. C. 
EXAMPLE 3 
Using the apparatus and procedure of Examples 1 and 2, except that the 
solid acid catalyst Amberlyst 15.TM. (Trademark for Rohm & Haas' 
sulfonated styrene-divinylbenzene resin) was used and the volume ratio of 
2MBA to MIPK was 3:1. After 27 hours at room temperature 76% by weight of 
the 2MBA had trimerized. 
EXAMPLE 4 
Using the apparatus and procedure of Example 1, pure MIPK was contacted 
with Nafion and Amerlyst-15.TM. to determine if any reaction took place. 
After 24 hours at room temperature no reaction was detected with either 
catalyst. 
EXAMPLE 5 
A down flow continuous reactor was constructed so that liquid 2MBA was 
passed through a bed of Nafion catalyst at room temperature and at LHSV's 
(liquid hourly space velocities) of 2.25 and 4.5. 70-75% of the 2MBA was 
trimerized using this procedure which simulates a continuous reaction 
system. 
EXAMPLE 6 
Using trimerized 2MBA that was prepared previously, Nafion and BPO.sub.4 
were used to decompose the trimer to 2MBA. In both cases 100% conversion 
of the trimer to 2MBA was realized in about 2 hours at 100.degree. C. 
INDUSTRIAL APPLICABILITY 
The process of this invention provides for the efficient and economical 
separation of a close boiling aldehyde from a ketone. The trimerization of 
aldehydes is known; however, the instant invention provides a 
nondestructive process for the separation of aldehydes from ketones via 
trimerization of the aldehyde. The utilization of an insoluble acid 
catalyst to facilitate product separation is both unique and unobvious. 
Those skilled in this art will readily appreciate the chemical processing 
advantages and economic advantages that can be realized through 
utilization of the process described and claimed herein.