Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Process for the preparation of spherical
particles of magnesium alkoxide
The present invention relates to finely divided, spher-
ical magnesium alkoxide, a process for its preparation and its use
for the preparation of highly active catalysts for the polymeriza-
tion of ~-olefins.
Catalysts for olefin polymerization can be prepared by
many processes, by reacting a solid component, which, as a rule,
contains magnesium, titanium and a halogen, preferably chlorine,
with an organoaluminium compound. The activity and the
stereospecificity of such supported catalysts are usually improved
by incorporating an electron donor (Lewis base) in the carrier
component and by complexing the organoaluminium compound with a
further electron donor. It is known that magnesium alkoxides can
be used as starting materials for the preparation of such
supported catalysts. In this case, the magnesium alkoxide
particles are preferably halogenated with the aid of a suitable
halogenating agent, such as benzoyl chloride, thionyl chloride or
titanium tetrachloride, in the presence of an electron donor, such
as, for example, ethyl benzoate. If halogenating agents other
than titanium tetrachloride are initially used, the latter must be
incorporated in the solid component in the required amount by
subsequent reaction with titanium tetrachloride, a content of 2 to
4.5% by weight of titanium usually being required. Regarding the
details of the catalyst preparation, reference may be made to
~uropean Published Patent Applications 0,216,402 and 0,236,082 and
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to the literature cited there.
Since, in the polymerization of ~-olefins, the mor-
phology of the resulting polymer particles is a faithful reproduc-
tion of the morphology of the catalyst and this in turn is an
exact image of that of the magnesium alkoxide used (see for
example J. Rudolph and J. Gross, Die Angewandte Makromolekulare
Chemie _ (1974), 195-197), it is also necessary to use a magnes-
ium alkoxide of defined morphology to achieve defined properties
of the desired polymer.
European Published Patent Application 0,236,082 des-
cribes the preparation of spherical to raisin-shaped particles of
magnesium alkoxides by conventional spray-drying. A preEerably
alcoholic solution of a carboxylated magnesium alkoxide is sprayed
through a nozzle, which is not defined in detail, or via a rota-
ting disk into a hot accompanying gas; this accompanying gas can
be fed concurrent or countercurrent. Although temperatures of 40
to 120C may be used, the range from 50 to 90C is preferred for
ethanolic solutions since it is only in this range that the
desired spherical to raisin-shaped particles can be produced. As
is evident from the Examples, on the other hand, hollow spheres
are obtained at 100-120UC, many of which disintegrate into
nutshell-like fragments.
Virtually all of the particles obtained by the process
of European Published Patent Application 0,236,082 have a diameter
in the range of from 2 to 250 ~m; preferably, 90% b~ weight of the
particles are in the range from 10 to 40~lm, with a weight average
at about 20 ~m~
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After spray drying, the bound CO2 is expelled again by
heating for several days in a stream of nitrogen at temperatures
of, initially, 70C to, finally, 150C. As an alternative to this
expensive process, the carboxylated magnesium alkoxide obtained is
said to be capable of being directly reacted further. The cata-
lyst obtained is said to be capable of being used both for gas-
phase polymerization and for liquid-phase polymerization.
The process described in European Published Patent
Application 0,236,082 for the preparation of spherical particles
10 of magnesium alkoxide is unsatisfactory for several reasons:
- The preparation of more finely divided magnesium
alkoxide having a mean diameter of less than 10 ~m, as
is desirable especially for the preparation of catalysts
for the liquid-phase polymerization, is not possible by
this method.
- The relatively low drying temperature during spray dry-
ing leads to products which have a considerable residual
moisture content, with the danger of agglomeration and
caking on the walls.
- An inconvenient aftertreatment is required to remove the
bound CO2. Alternatively, it is possible to dispense
with this aftertreatment, but this gives a catalyst hav-
ing substantially lower activity.
European Published Patent Application 0,216,402 des-
cribe~ a multistage process for the preparation of spherical,
mixed maqnesium alkoxide of the formula Mq(OR)2-a(OR')a
(a = 9 to 0.5~ In the preferred case (R = C2Hs; R' = CH3),
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the foll~winq procedure is ~dopted:
1) An ethanolic suspension of magnesium ethoxide is pre-
pared from magnesium and ethanol; the product is iso-
lated and dried.
2) The dried magnesium ethoxide is dissolved in methanol,
the solution is spray-dried at 15 to 200C, preferably
at 30 to 70C, spherical particles having a diameter of
from 5 t~ 30 ~m, preferably lO to 18~m, being formed.
At this point, a mixed alkoxide predominantly containing
methoxide groups is present; it is unsuitable in this
form for the preparation of olefin polymerization
catalysts~
3) The product from 2) is suspended in ethanol, and the
methanol formed at the equilibrium is distilled off.
4) The product having a low content of methoxide groups is
isolated and dried.
Apart from the many process stages, such as, for
example, changing the solvent twice and drying three times, the
low solubility of the magnesium ethoxide in methanol (see European
Published Patent Application 0,216,402, Example 1) and the
resulting necessity to evaporate large amounts of solvent with
corresponding consumption of energy, are found to be a serious
disadvantage. Furthermore, the product still has a residual
content of undesirable methoxide groups.
The primary aim of the present invention is to provide
finely divided, spherical magnesium alkoxide for the preparation
of a catalyst for the ~olymeri~ation of ~-olefins, which permits
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the production of polyolefins, in particular polypropylene, with
high stereospecificity, high bulk density and good fluidizability
during pneumatic conveying. The polymer should have as low an ash
content as possible even without extraction and must not contain
any fish eyes during processing.
Owing to the known dependence of the polyolefin mor-
phology on that of the polymerization catalyst and hence on the
morphology of the magnesium alkoxide used, it was necessary to
provide a process which permits the preparation of spherical, com-
pact particles of magnesium alkoxide having a relatively narrowparticle size distribution and a weight average of the particle
diameter of less than 10 ~m. In particular, in order to ensure
freedom from fish eyes and good fluidizability, no significant
amounts having diameters greater than about 100~ m must be pre-
sent. This necessitates measures to ensure effective atomization
on the one hand and to prevent agglomeration of still moist
particles in the gas space or on the wall on the other hand.
Previously unobtainable finely divided magnesium alkox-
ide free of methoxide groups and prepared in this manner can be
particularly advantageously used for the preparation of catalysts
for liquid-phase polymerization.
In addition to the liquid-phase polymerization of ole-
fins, gas-phase polymerization processes in which the polymeriza-
tion takes place in, for example, a fluidized bed have become es-
tablished worldwide. A catalyst having coarser particles is
required for this purpose; mean particle diameters of, for
example, 20 ~m are typical~ A further object of the present
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invention is therefore to provide a flexible process in which
alternatively the finely divided magnesium alkoxide according to
the invention, having a weight average particle diameter of less
than 10 ~m, or a magnesium alkoxide having coarser particles and a
weight average of 20 ~m or more can be prepared in the same plant
without conversion, merely by suitable variation of parameters,
such as, for example, load range of the nozzle. Such flexibility
of the plant could contribute substantially to its
cost-efficiency.
Furthermore, it was intended to find a simple and rapid
process for final drying and decarboxylation of the initially
formed finely divided carboxylated magnesium alkoxide.
These objects are achieved in the present invention by
the process described below.
The process according to the invention is characterized
in that a solution of carboxylated magnesium alkoxide in the same
alcohol on which the alkoxide is also based
a) is sprayed via a two-material nozzle with inner atomiza-
tion of the type described in, for example, German
Patent 2,627,880, and which is operated in the part-load
range at 5 to 30~, preferably 10 to 25%, of capacity,
b) into an inert accompanying gas which is under a pressure
of 1.0 to 1.2 bar, preferably 1.01 to 1.05 bar, is fed
concurrent and has been preheated to a relatively high
temperature of 100 to 140C, preferably 105 to 120C, it
being advantageous if the spray drier is in the form of
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a long tube tapering conically downwards,
c) after which the resulting finely divided carboxylated
magnesium alkoxide is dried and is decarboxylated.
The carboxylated magnesium alkoxide used has the formula
Mg(OR)2 x C2
where R = C2 to C4, such as, for example, n-propyl, n-butyl,
isobutyl or preferably ethyl and x = 0.2 to 2Ø It is prepared
in a known manner by dissolving magnesium turnings in the corre-
sponding alcohol and redissolving the initially formed particles
of magnesium alkoxide by passing in dry CO2. Here, the C02 is
merely an auxiliary for increasing the solubility of the alkoxide;
its reaction with the alkoxide is reversible, and it can therefore
be removed again by heating. The larger x, the easier CO2 is
released again on heating; it is advisable here to choose x to be
only as large as is absolutely necessary, in order to avoid exces-
sive C02 contamination of the accompanying gas, which is circu-
lated after the alcohol has been condensed out. Where R = ethyl,
a value for x of 1.0 to 1.25 is therefore particularly preferred.
The solution may have a solids content of 2 to 40% by
weight. Concentrations between 20 and 40% by weight, particularly
preferably 25 to 30% by weight, based on dry substance, are
usually used.
The nozzle used is a two-material nozzle with inner
atomization. Liquid pressure and gas pressure must be the same.
Spraying is carried out at pressures between 10 and ~0 bar,
preferably between 35 and 45 bar.
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Any inert gas may be used as the propellant and as the
accompanying gas. For practical reasons, dry and oxygen-free
nitrogen is used. When carboxylated magnesium ethoxide in ethanol
is used, the temperature of the accompanying gas is particularly
preferably between 105 and 115C.
Under these conditions, single-material nozzles give
particles which are too coarse. Rotating-disk atomizers, which in
principle are more suitable, also give a material which is too
coarse. However, very good results are obtained using the two-
material nozzles described.
The spray drier is generally in the form of a slim tower
which consists of an upper cylindrical part having a diameter-to-
length ratio of 0.3 to 0.9, preferably 0.6 to 0.8, and a
subsequent concial part. It is preferable if this concial part is
longer than the cylindrical part. The accompanying gas is fed in
and removed by the concurrent method, the particles formed being
deposited in a suitable apparatus, such as a cyclone or a filter.
By means of this procedure, it is possible to obtain,
alternatively, a very finely divided magnesium alkoxide or, if
desired, also magnesium alkoxide having coarser particles.
Because of the nozzle used, which has a small spray angle, and the
geometry of the tower, caking on the walls is greatly suppressed.
Relatively stringent conditions are possible for the
final drying and decarboxylation, since, owing to the good pre-
liminary drying compared with the process disclosed in EuroPean
Published Patent Application 0,236,082, the danger of caking is
substantially lower. Any suitable drying apparatus which is
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preEerably suitable for vacuum operation, such as, for example,
paddle driers, tumble driers, rotary kilns, etc., may be used. In
general, drying is carried out at temperatures of about 110 to
150C, preferably 125 to 145C, for a period of 3 to 10 hours,
preferably 4 to 8 hours, with application of sliqhtlv reduced
pressure, preferably 950 to 980 hPa. Under these conditions, the
bound CO2 is effectively removed. A paddle drier having
paddles which pass close to the edge or an apparatus which intro~
duces the heat directly into the product is particularly suitable.
The magnesium alkoxide powder obtained in this manner
can be converted by all conventional methods of the prior art, as
described in, for example, European Published Patent Application
0,236,082, into a catalyst for the polymerization of ~-olefins.
The polymerization catalyzed therewith can be carried out in any
conventional manner.
The resulting finelv divided catalyst powder is particu-
larly suitable for liquid-phase polymerization of ~-olefins, such
as, for example, ethylene, propylene, but-1-ene, hex-1-ene or mix-
tures thereof, either the liquefied monomer or an inert hydro-
carbon being used as a liquid medium. The polymerization can becarried out continuously or batch-wise.
It should be noted that both the pulverulent magnesium
alkoxide described here and the polymerization catalyst prepared
therefrom have different sensitivities to oxvgen, moisture, carbon
dioxide, acetylenes and sulfur compounds; such catalyst poisons
must therefore be carefully excluded during the process.
Polypropylene prepared using such catalysts has a high
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stereospecificity and a high bulk density and is easily fluidiz-
able during pneumatic conveying, with fluidization beginning at
less than 7 cm/sec, typically less than 3 cm/sec and under suit-
able conditions less than 1 cm/sec. ~ecause of the small particle
size, according to the invention, of the catalyst and the absence
of coarse particles, the polyolefin has a very low ash content and
can be processed without fish eyes.
The invention will be further described by way of
non-limiting examples and with references to the accompanying
drawings in which:
Figure 1 represents a scanning electron micrograph of
the product of Example 3.
Example 1: Ethanolic solution of carboxvlated magnesium ethoxide
.
In a 3 m3 stirred container, 300 kg of magnesium
ethoxide are added to 1,000 kg of ethanol while stirring, and then
145 kg of CO2 are metered in via a dip tube extending below the
liquid surface, in such a way that no gas bubbles appear at the
liquid surface. The magnesium ethoxide particles, which are
virtually insoluble in ethanol, gradually dissolve with reaction
with the CO2. A slightly cloudy opalescent solution of 30.8~ by
weight of carboxylated magnesium ethoxide Mg(OC2H5)2 1.25 C02
is formed.
Example 2 Spraying the solution
In a spray tower in which the cylindrical part has a
diameter-to-len~th ratio of 0.7, 12.3 kg/h of the solution from
Example 1 are sprayed by means of a two-material nozzle with inner
atomization in accordance with German Patent 2,627,880 (load: 7%
of capacity) into an accompanying gas stream (dry N2) heated to
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110C. The drier outlet temperature is about 90C. The particles
are deposited in a double cyclone, the yield is 89.4%, the process
being carried out without fine dust filters downstream of the
cyclone.
The resulting particles of carboxylated magnesium ethox-
ide have a weight average particle diameter of 8.4 ~m (determined
by laser diffraction using a "Microtrac"* apparatus from Leeds &
Northrup). They have the following particle size distribution:
d1o = 3-0 ~m; d50 = 6.9 ~m; dgo = 15.0~ m;
i.e. 10~ by weight of the particles have a diameter of up to 3 m,
etc. The largest diameter measured is 38 ~m.
The residual ethano] content is 4.9% by weight and the
C2 is still bound.
Example 3: Drying and decarboxylation
300 g of the product from Example 2 are subsequently
dried for 5 hours at a pressure of 980 mbar and an internal
temperature of 131C in a stream of nitrogen (about 10 1/h) in a
2 1 laboratory paddle drier having PTFE paddles which pass close
to the edge, a constant weight being achieved.
The primary particle sizes do not change during this
procedure; however, small agglomerates form, these being broken up
again by stirring during further catalyst preparation. By means
of scanning electron micrographs, it is possible to show that the
individual spherical particles are not destroyed by the treatment
in the paddle drier (see Fig. 1).
The magnesium ethoxide thus obtained is used for the
preparation of a catalyst for the polymerization of ~-olefins
*Trade-mark
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according to European Published Patent Application 0,236,082. The
activity and stereospecificity of the catalyst obtained are good;
in liquid-phase polymerization of propylene, a polypropvlene
having high bulk density and good fluidizability is obtained.
