Note: Descriptions are shown in the official language in which they were submitted.
2193~08
I
- Estt~:r~ n of (meth)acrylic acid with an alkanol
The invention relates to a process for the esterification of
(meth)acrylic acid with an alkanol in the pl~sence of an esterification ca-
5 talyst, in which unreacted starting compounds and the (meth)acrylic esterformed are separated off by ~i~till~ion and a bottom product co,~ ising
oxy esters is obtained. The term (meth)acrylic acid here denotes, as is cus-
tomary, acrylic or ,llellla~ lic acid.
The preparation of alkyl esters of (meth)acrylic acid is usually
o carried out by esterification of (meth)acrylic acid with alkanols at elevated
lempe,alul~ in the liquid phase in the p~sellce or absence of solvent and
in the ~l~sel1ce of acid as catalyst (DE-A 23 39 519). A disadvantage of
this method of plepal~tion is the secondary reactions occulling under the
abo~ elltioned esterification conditions, viz. as yet u~ ,acled starting
alcohol adds onto the double bond of alkyl (meth)acrylate already formed
(Michael addition) to give a colllpoul1d of the general formula I below and
as yet umeac~ed (meth)acrylic acid adds onto the double bond of the ester
formed to give a colllpound of the general formula II. Multiple addition is
also possible. In addition, mixed types can occur. These adducts (alkoxy
20 esters and acyloxy esters) are abb~iattd as oxy esters.
RO--(CH2-CH--C02),~--R (I)
R' R'
CH2=C--CO2--(CH2--CH--C02)y--R (II)
2193~D~
- 2 -
where x,y = 1-S
R = alkyl
R' = H or CH3
When R' = H, the esterification is of acrylic acid, when
s R' = CH3, the esterification is of metllaclylic acid.
The problem of oxy ester formation is particularly acute in the
p.eyaralion of esters of acrylic acid, with the main oxy esters formed being
alko~yyroyionic esters and acylor~yylopionic esters where x,y = 1. In the
~,~palalion of esters of ,.,~ clylic acid, oxy ester formation occurs to a
o lesser extent. The formation of oxy esters is described in DE-A 23 39 529.
This indicates that the formation of oxy esters occurs esse~ lly indepen-
dently of the specific esterification conditions. Of very particular importance
is oxy ester formation in the ylepalation of acrylates of Cl-C8-alkanols, in
particular C4-C8-alkanols, very particularly in the preyalation of n-butyl
S acrylate and 2-ethylhexyl acrylate.
It is a characleli~lic of the oxy esters that their boiling point is
above the boiling point of starting acid, starting alcohol, target ester formed
and any organic solvent also used.
The work-up of any such esterification reaction mixture is normally
20 carried out by sepalating ul~l~aeled starting comyuul~ds and the target esterfrom the reaction lllL~tUlC by ~i.ctill~tion, with the acid catalyst used for the
esterification being able to be removed berul~hand, if app,oyliate~ by extrac-
tion with water and/or aqueous alkali (cf. for example, Ullmann's Encyclo-
pedia of Industrial Chemistry, Vol. A1, 5th Ed., VCH, p. 167 ff.). The
25 bottom product rem~inin~ in such a (li~till~tive work-up contains the oxy
esters which result in a considelable loss in yield.
For this reason, various further methods have been Px~minP~I in
order to solve the problems resulting from formation of the oxy esters.
Thus, JP-A-82t62229 describes the ~lk~linP saponification of the high-boiling
30 esterification residues. In this way, part of the alcohol used and acrylic acid
2193408
and ,B-hydro~yl,lopionic acid or- their salts are recovered. A simple and eco-
nomical return of the products to the esterification reaction is therefore not
possible. JP-B-72/15936 describes the preparation of acrylic esters by reac-
ting ,l~-alko~y~ropionic esters with acrylic acid in the presellce of strong
5 acids (lr~nSeSlel iGcation) However, this forms as by-product equimolar
amounts of ~-alkuAy~ropionic acid which cannot be relull.ed to the esterifi-
cation reaction and thererole .el,resellt waste material. JP-A-93/25086 des-
cribes the dissociation of the Michael addition product butyl ,~-butoxypro-
pionate (see formula I, x = 1, R = butyl) at elevated te~ le and in
o the plesel~ce of sulfuric acid and an excess of water. However, the yield is
only about 30% Finally, JP-A-94/65149 describes the dissociation of the
Michael addition products I and II (see above, x = y = 1) in the p,esellce
of ~ ,i...,. alkoxides. In this reaction, the CO~ ion is likewise low
(< 60%) and large amounts of titanate are llecessal~. This process is there-
15 fore ~necollo...ical and e~vilor~ entally unfriendly because of the largeamounts of titanate to be disposed of.
GB 923 595 describes the recovery of monomers from the residue
of the esterification of acrylic acid with alkanols in the absence of molecu-
lar oxygen. It reCol~....el~, inter alia, the removal of all volatile monomers
20 prior to the dissociation, dissocialion in the l,resellce of sulfuric acid and
removal of the dissociation products by means of a stream of inert gas.
According to the examples, the dissociation is always carried out at at least
300C. Coke is formed as residue (17~0%) and this has to be dug out of
the reactor. This process is therefore neither economical nor can it be
25 carried out on an industrial scale. A further disadvantage is the need to
exclude oxygen.
CN-A 1,063,678 describes the dissociation of the alko~y~,opionic
ester present in the esterification residue in the l~,esence of sulfuric acid ina c~scade, with temperature and catalyst concelllr~tion (0.8-1.5%) being dif-
30 ferent in each reactor. Coupled with the dissociation is a ~ till~ion to
2193408
separate alkanol and acrylate. The process is very cumbersomeand does not achieve high conversions.
Finally, CN-A 1,058,390 describes the dissociation
of alkoxypropionic esters in the presence of sulfuric acid,
etc., into alkanols and acrylic esters. This is carried out
stepwise. The dissociation is first carried out under reflux
and the reaction products are subsequently distilled off. The
dissociation of the acrylic acid-containing ester residues
from the preparation of ethyl/methyl acrylate (ethyl
10 ethoxypropionate, methyl methoxypropionate) is carried out in
the presence of ethanol or methanol. Here too, the process is
complicated and does not achieve high conversions.
It is an object of the present invention to carry
out the redissociation of the oxy esters present in this
bottom product and to reuse the starting acid, starting
alcohol and target ester obtained thereby for the purposes of
the esterification without the disadvantages of the processes
of the prior art.
We have found that this object is achieved by a
20 process for esterifying (meth)acrylic acid with an alkanol in
the presence of an esterification catalyst in which unreacted
starting compounds and the (meth)acrylic ester formed are
separated off by distillation and a bottom product containing
oxy ester is obtained, wherein the bottom product is separated
off and either (a) the bottom product is admixed directly with
oligomeric (meth)acrylic acid and the oxy esters present in
the bottom product are dissociated at an elevated temperature
in the presence of acid catalysts different from oligomeric
(meth)acrylic acid or (b) the oxy esters are first separated
by distillation from the bottom product, the distillate is
admixed with oligomeric (meth)acrylic acid and is dissociated
at an elevated temperature in the presence of acid catalysts
different from oligomeric (meth)acrylic acid. The alkanol is
preferably n-butanol or 2-ethylhexanol. In general, from 10
to 50% by weight, preferably from 10 to 40% by weight, based
on the amount of bottom product in (a) or distillate in (b),
219340g
4a
of oligomeric (meth)acrylic acid is added. Oligomeric acrylic
acid is normally employed in a form known per se, stabilized
by means of polymerization inhibitors. Advantageously, the
oligomeric (meth)acrylic acid used for this purpose is the
b ~
2193408
distillative purification of raw acrylic acid; this botlom product comprises
mainly compounds of forn1ula III below (see, for example, DE 22 35 326):
CH2= CH-C02-(CH2 CH2 C02) -H (III)
x = 1 - 5
The (meth)actylic acid oligomers can be added to the mixture to
be dissociated prior to the dissociation. They can also be fed separately into
the dissociation reactor.
o These oligomers are not free-radical oligomers, but Michael adducts
of the acid with itself, as are obtained as by-products in, for example, the
distillation of (meth)acrylic acid. These oligomers are usually burnt as
nu~uc ~clable by-pl~lucls from (meth)acrylic acid p~uduction. Under the
redissociation conditions, these oligolnelic (meth)acrylic acids are also
15 redissocialed, continuously generating free (meth)acrylic acid in statu nascen-
di.
Col"pa~d with a prior addition of (meth)acrylic acid, this has the
advantage that the (meth)actylic acid added does not immediately distill off
together with the dissociation products, but the dissociation proceeds continu-
20 ously in the plesence of (meth)acrylic acid, which results in led~ced forma-
tion of by-products (diallcyl ethers, olefins). According to an advantageous
embodiment of the invention, the process of dissociation is carried out in
the p~sence of molecular oxygen.
According to a further advantageous embodiment of the invention,
25 the product to be dissociated has added to it, in addilion to the acid esteri-
fication catalyst which is different from oligomeric (meth)acrylic acid
and may already be present, further acids selected from the group collsi~ g
of mineral acids such as sulfuric acid or phosphoric acid, and organic acids
different from oligonlelic (meth)acrylic acid, for example alkyl- or arylsul-
30 fonic acids such as methanesulfonic acid or p-toluenesulfonic acid. The totalamount of acid different ftom oligomeric (meth)acrylic acid which is then
2193408
present can be from 1 to 20% by weight, preferably from 5 to 15 % by
weight, based on the amount of the bottom product in (a) or distillate in
(b). It is particularly useful if a ~llipping gas preferably cont~ining molecu-
lar oxygen is passed through the bottom product in (a) or the ~ictill~te in
(b) as an entrainer for the dissociation products. Air or mixtures of air with
inert gas (e.g. nitrogen) are advantageously used as sl~ipping gas.
The advantages of the process of the present invention are, in
particular, that the dissociation proceeds more quickly and that a smaller
amount of by-products such as ethers or olefins is formed. Thus, among
o other things, smaller losses of starting materials, particularly of alcohols,
occur than in known processes. In addition, high dissociation yields can be
achieved. The direct return of the dissociation mixture does not adversely
affect the purity of the (meth)acrylic ester and leads to a low ether content.
For this reason, no complicated separation of the ether from the easily
poly~ i~ble (meth)acrylic ester is ll~cess~-~. All in all, this also means
reduced environmental pollution, since smaller amounts of residue are
obtained.
In the distillative separation of the oxy esters from the bottom
product, the (li.ctill~tion conditions depend on the type of alcohol col.,pon~n
used in the esterification. In general, a l~l~.pc~al~lre of from 100 to 300C
and a ple~i~UlC of from 1 to 50 mbar are employed. Any con~el-lional
dictill~ion appa,~lus is suitable for the distillation process. Since only a
sirnple sep~tion task is to be pclr~ .cd, a simple splash guard is gener-
ally sufflcient, i.e. a column is not normally required.
For the dissociation of the oxy esters sepal~led o~ by tlic~illation
or present in the bottom product, a simple heatable stirred reactor with
jacket heating or heating coil, or else a forced-circulation evaporator, for
example a falling-film evaporator or flash evaporator, coupled with a resi-
dence time vessel, can be used. To achieve better sepalation of the dis-
sociation products, it may be advantageous to use a rectification attachment
2193408
-
superposed on the dissociation apparatus, for example a packed or tray
column. This rectification attachment is generally operated using stabilization
by pol~ c~i~ation inhibitors (e.g. phenothi~7.ine, hydroquinone monomethyl
ether, etc.).
The conditions for ca"ying out the process of the present invention
for dissociating oxy esters formed in the bottom product in the esterification
or sepaldled from the bottom product are as follows:
Catalyst: at least one acid selected from the group consi-
sting of mineral acids, for example sulfuric acid
o and phosphoric acid, and organic acids dilIcl~nl
from oligomeric (meth)acrylic acid, for example
alkyl- or arylsulfonic acids such as
fonic or p-toluenesulfonic acid
Amount of catalyst: 1-20% by weight, preferably 5-15% by weight,
based on the amount of bottom product in (a)
or of the oxy ester (li.ctill~te sepalaled from the
bottom product in (b)
Amount of oligo,neric
(meth)acrylic acid: 5-50% by weight, preferably 10-40% by weight,
based on the amount of bottom product in (a)
or of the oxy ester ~i.still~te separated from the
bottom product in (b)
Te"lpendl~,e: 150-250C, preferably 180-230C
Pressure: preferably at atmospheric p,c~u,e or under
2S ~cduced ~IcS~ulc (so that the dissociation pro-
ducts immediately vaporize) ( < 1 atm)
Sl.ipphlg gas,
if used: amount: 1-100 I/h
Reaction time: 1-10 hours
30 Ccsll~ ioll: 2 90%
2193~08
. ~
- 8 -
The reaction is carried out, for example, by the bottom product
to be dissociated being taken continuously from the dictillative work-up of
the esterification mixture and fed together with the dissociation catalyst to
the dissociation reactor. However, the reaction can also be carried out
5 batchwise. It is also possible to use a semicontinuous reaction procedure in
which the product to be dissociated is continuously fed to the dissociation
reactor (which contains the dissociation catalyst), and the bottom product is
removed batchwise from the dissociation reactor only after the dissociation
is complete. The dissociation products are scpalated off continuously by
lO di~till~tion.
The applicability of the dissociation process described is not
l~lliclcd to a specific type of esterification process from which the oxy
esters, i.e. the addition compounds I and II, are obtained as by-products.
In general, the esters are p~ cd by customary methods (see Ullmann's
~5 Encyclopedia of InJu~llial Ch.,lni~lly, Vol. A1, 5th Ed., VCH, p. 167 fl.).
A typical example of the conditions under which the esterification
prece~lin~ the dissociation of the oxy esters can take place can be briefly
~lcsenlcd as follows:
Alcohol: (meth)acrylic acid 1:0.7-1.2 (molar)
20 Catalyst: sulfuric acid or sulfonic acids
Amount of catalyst: 0.1-10% by weight (preferably 0.5-5%
by weight) based on starting material
Stabili_ation: 200-2,000 ppm of phellotlli~7in~ (based
on the weight of the starting
materials)
Reaction t~ ,: 80-160C, preferably 90-130C
Reaction time: 1-10 hours, plcrclably 1-6 hours
If desired, an entrainer (e.g. cyclohexane or toluene) may be used
to remove the water of esterification. The esterification can be carried out
2193~08
g
at atmospheric ples~u.~, under superatmospheric pressure or subatmospheric
pressure, either continually or batchwise.
In the acid-catalyzed esterification of acrylic acid with alkanols,
the bottom product obtained after separating off the acid esterification
5 catalyst, the unreacted starting materials and the acrylic ester generally has the following composition:
1-20% by weight of acrylic ester
50-80% by weight of alkv~yL,Ivpionates (see formula I)
5-30% by weight of acylo~y~lupionates (see formula II)
l0 re~ n~er: mainly stabili~ers (phenoll~iA~i~u) and polymers
Further details and advantages of the process of the present
invention may be taken from the examples described below.
Firstly, a result achieved using a process not acco~ g to the
present invention will be described by means of a con~al~ e example.
COMPARATIVE EXAMPLE
A glass circulation reactor (volume: 1 1) heated by means of a
heating plug was chalged with 500 g of an oxy ester ~ till~te obtained
from an esterification residue from n-butyl acrylate production which has
been freed of the acid esterification catalyst, together with 40 g of p-tolue-
20 nesulfonic acid. The oxy ester ~ till~t~ co-~p-ised
11.0% by weight of butyl acrylate,
64.8% by weight of butoxy ester I ( R = C4Hg)
20.5% by weight of acyloxy ester II ( R = C4Hg).
The dissociation ~e~ e.alu.e was 195C and the opel~ g ples~llre
25 was 1 atm.
During the dissociation, the esterification residue to be dissociated
was co,.ti...lv~lsly fed to the dissociation reactor, regulated by the level in
the reactor.
The dissociation products were taken off in vapor form and
30 condensed at the top of the column (50 cm x 2.8 cm, empty) superposed
21934 08
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on the dissociation reactor. Over a period of 119.5 hours, 7,401 g of
mixture were fed to the dissociation reactor and 7,080 g of dissociation
products were condensed.
According to analysis by gas chromatography, the condencate
s complised:
72.0% by weight of butyl acrylate
13 .9% by weight of butanol
4.8% by weight of acrylic acid
1.4% by weight of dibutyl ether
o 6.6% by weight of butenes
0.2% by weight of butyl bulo~yl~ol)iollate
Conversion: 96% by weight based on oxy esters.
The dissociation bollollls were still readily handleable (pumpable)
at 25C and contained no solids.
15 EXAMPLE OF THE PROCESS OF THE PRESENT INVENTION
A glass circulation reactor (volume: 1 1) heated by means of a
heating plug was chal~ed with 500 g of the oxy ester dictillqte from the
Col~ slative Exarnple, with addition of 40 g of p-toluenesulfonic acid and
10 g of a ~li.ctillqtion residue obtained in the production of a pure acrylic
20 acid ~ic~illqte and having the following co",~osilion:
5.5% by weight of acrylic acid
54.0% by weight of diacrylic acid
14.5% by weight of dodecylbe.~el-e~.llfonic acid
remqin~er: mainly polymers of acrylic acid and phenothiq7ine.
The dissocialion l~ lul~; was 195C and the ope~ g pl`es~ul'e
was 1 atm.
The oxy ester dictillq-te and the cGllespollding amount of acrylic
acid dic~illqtion residue (20% by weight) were continuously fed to the
reactor, regulated by the level in the reactor. 10% by weight of the feed
30 flow was continuously removed from the reactor.
11 2193~û8
Over a period of 302 hours, 23,985 g of the mixture of oxy
ester ~ictill~te and acrylic acid distillation residue was fed to the dissociation
reactor and 21,580 g of product mixture were conl1el1.ced. According to
analysis by gas c~umatography, the condensate cûlll~ ed:
569.5% by weight of butyl acrylate
6.1% by weight of butanol
19.5% by weight of acrylic acid
0.6% by weight of dibutyl ether
3.1% by weight of butenes
l0 Conversion: 96% by weight based on oxy esters.
It can be seen from the above example of the process of the
present invention that this process is able to achieve higher conversions and
results in smaller losses of starting material than in known processes.
