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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the preparation of methacrylic esters,
and, more especially, to the preparation of such methacrylates by
transesterification.
2. Description of the Prior Art
French Pat. No. 2,405,237 describes a process for the preparation of
methacrylic esters by transesterification; it is stated therein that the
process has been carried out in the presence of a transesterification
catalyst, which consists of lithium hydroxide used in a proportion of 0.01
to 2% by weight with respect to the total weight of the reagents, as well
as in the presence of a known methacrylate polymerization inhibitor. This
'237 patent also notes that the reaction mixture is heated, under total
reflux, until that point in time whereat the head of the column the
temperature of the methyl methacrylate/methanol azeotrope is attained,
such azeotrope enabling the gradual elimination of the methanol present in
the reaction medium.
A systematic study of the transesterification of methyl methacrylate with
higher alcohols, utilizing a lithium compound (preferably the hydroxide or
carbonate) as the catalyst, has shown that it is possible and industrially
attractive to only use amounts of catalyst less than those specified in
said French Pat. No. 2,405,237, under such conditions that, on the one
hand, the catalyst is gradually introduced into the reaction medium, and,
on the other, the reaction is carried out in the presence of an inert
azeotrope-forming compound which produces, with the methanol present in
the reaction medium, an azeotrope boiling at a temperature of less than
approximately 60.degree. C. (at atmospheric pressure).
SUMMARY OF THE INVENTION
Accordingly, a major object of the present invention is the provision of an
improved process for the transesterification of methyl methacrylate with a
heavier alcohol, in the presence of a lithium catalyst and wherein the
amount of the lithium catalyst employed ranges from 6 to 30 ppm (expressed
as lithium) of lithium hydroxide or carbonate, the lithium catalyst is
gradually added to the reaction medium, and the reaction is carried out in
the presence of an azeotrope-former which gives rise, together with the
methanol, to an azeotrope boiling at a temperature of less than
approximately 60.degree. C.
Consistent herewith, the amount of the catalyst is expressed relative to
the total weight of the reagents, i.e., the sum of the weights of the
methyl methacrylate and the alcohol.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
More particularly, according to the present invention, and as would be
apparent to one skilled in this art, the subject transesterification is
advantageously carried out in the presence of a methacrylate
polymerization inhibitor. Exemplary of such polymerization inhibitors,
representative are hydroquinone monoethyl ether, hydroquinone,
2,6-di-t-butylpara-cresol, 3,5-di-t-butyl-4-hydroxyanisole,
2,5-di-t-butylhydroxyanisole, N,N'-dinaphthyl-para-phenylenediamine, and
2-t-butyl-4-methoxyphenol, together with any other radical polymerization
inhibitor, the amounts of such inhibitors typically ranging from 100 to
1,400 ppm (by weight relative to the expected theoretical amount of the
ester).
The alcohols that may be used for the transesterification of methyl
methacrylate according to the invention are quite varied and numerous. The
following are exemplary of such heavy alcohols: aliphatic linear or
branched chain monoalcohols, such as n-butanol, n-propanol, lauryl
alcohol, stearyl alcohol, 2-ethylhexanol; cycloaliphatic alcohols, such as
cyclohexanol; aromatic alcohols, such as benzyl alcohol; alcohols bearing
other functional groups, e.g., ethylene glycol monomethylether, ethylene
glycol monoisopropylether; unsaturated alcohols, such as allyl alcohol;
aliphatic polyols, such as monoethylene glycol, diethylene glycol,
butanediol, trimethylol propane, with the transesterifying alcohols being
either natural or synthetic in origin.
The catalyst used comprises lithium, advantageously in the form of lithia
(calcined or monohydrate) or lithium carbonate.
As regards the azeotrope-former, any inert compound (for example, and
preferably, a saturated hydrocarbon, such as hexane) producing an
azeotrope with methanol and boiling at a temperature of less than about
60.degree. C., may be used. In actual fact, the use of such
azeotrope-former enables the elimination of the methanol present (or
formed) in the reaction medium, but at the same time avoiding the
formation of the methyl methacrylate/methanal azeotrope which evaporates
at about 65.degree. C.
Also, according to the present invention, the catalyst is gradually
introduced into the reaction medium. In fact, the consumption of the
catalyst by the reaction of the lithium with one of the reagents present
must be prevented by maintaining the concentration of the catalyst at a
very low level. The period of time during which the catalyst is introduced
into the reaction medium varies with the experimental conditions and the
kinetics of the reaction. Typically, the catalyst should be gradually
introduced into the reaction medium over a period of from 1/10 to 1/2 of
the overall period of reaction.
It has also been discovered, and this constitutes another object of the
invention, that the catalytic activity of the lithium may be enhanced by
the use of crown ethers and/or cryptants. Exemplary of such cocatalysts,
the following are respresentative: 1,4,7,10-tetraoxocyclodecane (crown
2-4) or tris(3,6-dioxaheptyl)amine (TDA1); the amounts to be used,
expressed in moles per mole of the Li.sup.+ ion, advantageously range
from 0.01 to 2 moles per mole. For further description of such
cocatalysts, see U.S. Pat. No. 4,417,081.
The subject transesterification is preferably carried out in the following
manner: the transesterifying alcohol, the polymerization inhibitor and an
azeotrope-forming solvent are introduced and heated for that period of
time required to dehydrate the reaction medium.
When the medium is sufficiently dehydrated and the temperature of the
reaction mass ranges from 100.degree. to 140.degree. C., and after the
residual acidities of the reagents have been neutralized, the methyl
methacrylate and a methanolic solution of lithium hydroxide or a
methanolic suspension of lithium carbonate are simultaneously introduced
therein. The methanol formed is extracted from the reaction medium by
azeotropic distillation under atmospheric pressure.
Air may be bubbled through the reaction medium to reinforce the
effectiveness of the polymerization inhibitor. Similarly, the addition of
the cryptants or crown ethers enhances the catalytic effect of the
catalyst employed.
The compounds obtained according to the process of the invention are
readily isolated in the following manner:
Subsequent to distillation under reduced pressure to eliminate, e.g., the
hexane and the excess of light compounds, it is possible either to purify
the product ester by distillation, if it has adequate volatility, or,
contrariwise, it may be subjected to a specific treatment which narrowly
depends upon the type of inhibitor used.
Treatment 1
This treatment is adapted for inhibitors of radical polymerization which
contain a nitrogen atom. The treatment consists of passing the reaction
product over acidic clay or charcoal, followed by filtration; the passage
over acid clay may optionally be preceded by acidulation.
Treatment 2
This treatment is particularly suitable for inhibitors of radical
polymerization of the quinone and phenol type and consists of passing the
reaction product over basic clay or charcoal, followed by filtration; it
may advantageously be preceded by basification.
In order to further illustrate the present invention and the advantages
thereof, the following specific examples are given, it being understood
that same are intended only as illustrative and in nowise limitative.
EXAMPLE 1
Into a 6 liter flask having seven necks, equipped with a thermometer, an
agitator, a funnel for the introduction of hexane, a funnel for the
introduction of the catalyst, a funnel for the introduction of methyl
methacrylate, an air bubbler, and which was surmounted by a distillation
column (internal diameter, 40 mm; height 1.50 m; 18 actual plates), a
column head and a downstream condenser and a horizontal decanter,
maintained at 10.degree. C., and also containing a temperature sensor, an
outlet for the bottom layer and means to recycle the top layer to the head
of the column, the following materials were introduced: 1573 g stearyl
alcohol (6 moles), 600 ml hexane and 1 g
N,N'-dinaphthyl-para-phenylenediamine, 5.36 ml of a solution of 2.49%
calcined lithium hydroxide in methanol. After drying for 45 min and when
the column was in equilibrium, methyl methacrylate (690.9 g in 0 hr, 45
min) and a methanolic solution of lithia (10.24 ml in 1 hr, 30 min), were
simultaneously poured therein.
The temperature of the reaction mass was maintained from 115.degree. C. to
120.degree. C. by the optional addition of hexane.
The heteroazeotrope was cooled to 10.degree. C. such that the lower layer
could be decanted. After a reaction time of 4 hr, a conversion yield of
the alcohol of 98% was obtained.
EXAMPLE 2
The procedure of Example 1 was repeated, but the transesterifying alcohol
was an isodecanol fraction (average molecular weight: 160.3 g). A
conversion of 100% was obtained after 4 hours of reaction.
EXAMPLE 3
Into a 300 liter stainless steel reactor equipped in a manner similar to
the 6 liter flask described in Example 1, the following materials were
charged: 80 kg stearyl alcohol (average molecular weight: 262.15 g), 25 kg
hexane and 50 g N,N'-dinaphthyl-para-phenylenediamine and 5.41 g lithium
hydroxide monohydrate
After the dehydration of the reaction medium, over 0 hr, 15 min, 36.6 kg
methyl methacrylate (over 0 hr, 35 min) were simultaneously introduced
therein. Following a reaction time of 5 hr, 5 g lithium hydroxide
monohydrate dissolved in 200 g methanol were introduced. After a reaction
time of 6 hr, the degree of conversion of the stearyl alcohol was 98.2%
(determined by chromatography).
EXAMPLE 4
The procedure of Example 1 was repeated, but 0.021 g TDA1 (marketed by
Rhone-Poulenc) were mixed into the methyl methacrylate to be introduced.
After a reaction time of 4 hr, an alcohol conversion of 99.2% was
obtained.
EXAMPLE 5
The procedure was similar to that of Example 1, but the transesterifying
alcohol was a lauryl alcohol fraction having an average molecular weight
of 207 g and the polymerization inhibitor was 2,6-di-t-butyl-para-cresol.
After a 4 hr reaction, a conversion of 98.5% was obtained.
EXAMPLE 6
The procedure of Example 1 was repeated, but the transesterifying alcohol
was a lauryl alcohol fraction having an average molecular weight of 207 g
and the polymerization inhibitor was 2-t-butyl-4-methoxyphenol. After a
reaction time of 4 hr, an alcohol conversion of 98.7% was obtained.
EXAMPLE 7
500 g crude stearyl methacrylate obtained as in Example 1 and containing
500 ppm N,N'-dinaphthyl-para-phenylenediamine, were introduced into a 1
liter five-necked flask, equipped with an agitator and surmounted by a
direct condenser, a thermometer, an immersion device for air bubbling and
means for the introduction of solids. The temperature of the reaction mass
was raised to 80.degree.-90.degree. C. under agitation and air bubbling,
after which 6 g Fulcat B were rapidly introduced. The temperature was then
maintained at 80.degree.-90.degree. C. for 0 hr, 30 min, whereupon the
reaction medium was cooled to 50.degree. C. and filtered through a layer
of DIC clarcel. 492 g stearyl methacrylate containing 75 ppm
N,N'-dinaphthyl-para-phenylenediamine were obtained. The filter cake may
advantageously be rinsed with hexane to recycle the polymerization
inhibitor and the retained stearyl methacrylate.
EXAMPLE 8
The procedure of Example 6 was repeated, with 1242 g lauryl alcohol (6
moles), but the polymerization inhibitor was hydroquinone monomethylether
(250 ppm relative to the reagents). After 3 hr, 30 min, a conversion of
98% was obtained.
The top fraction of the reaction medium was removed continuously under
vacuum at 155.degree.-165.degree. C., 70 to 130 mm Hg, in an apparatus
equipped with an agitator, a thermometer, an immersion device for bubbling
air, a Claisen head and a condenser.
The reaction mass, the top fraction of which had been removed, was then
treated in a 4 liter flask equipped with an agitator, a thermometer and a
condenser; the temperature of the reaction mass was then raised to
65.degree. C., 2.7 g sodium hydroxide and 1.5 g Norit D 10 charcoal were
introduced, the temperature was maintained at 65.degree. C. for 0 hr, 20
min, the medium was treated with 3 g sawdust and filtered on DIC.
1532 g lauryl methacrylate containing 50 ppm of hydroquinone
monomethylether were obtained.
While the invention has been described in terms of various preferred
embodiments, the skilled artisan wil appreciate that various
modifications, substitutions, omissions, and changes may be made without
departing from the spirit thereof. Accordingly, it is intended that the
scope of the present invention be limited solely by the scope of the
following claims, including equivalents thereof.
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