Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR 'T'~E PREPARATION OF EROXIDP.TION CA.'Z'P..L'YSTS
The present invention relates to the preparation of
an epoxidation catalyst and to the process of preparing
alkylene oxide with the help of such catalyst.
Background of the Invention
An epoxidation catalyst is understood to be a
catalyst which catalyses the manufacture of an epoxy
group containing compound. A well known process comprises
contacting a hydroperoxide and alkene with a
heterogeneous epoxidation catalyst and withdrawing a
product stream comprising alkylene oxide and an alcohol.
Catalysts for the manufacture of an epoxy group
containing compound, are well known. EP-A-345856
describes the preparation of such catalyst comprising
titanium in chemical combination with a solid silica
l5 and/or inorganic silicate. The preparation comprises (a)
impregnating a silicium compound with a stream of gaseous
titanium tetrachloride preferably comprising an inert
gas, (b) calcining the obtained reaction product of
step (a), and (c) hydrolysis of the product of step (b).
The stream of inert gas also has the function of a
carrier for the gaseous titanium tetrachloride. For such
use, the gas is to be present in a relatively large
amount.
There is a continuous interest in improving the
selectivity of epoxidation processes in general, and more
specifically of processes for the preparation of alkylene
oxide. We found a simple and attractive way to achieve
this.
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Summary of the Invention
The present invention relates to a process for the
preparation of an epoxidation catalyst, which process
comprises impregnating a silicon containing carrier with
a gas stream consisting of titanium halide.
A catalyst of improved selectivity was obtained even
though the carrier had been in contact with the same
amount of titanium halide.
I?etailed description of the invention
The catalyst of the present invention is obtained by
impregnating a silicon containing carrier. In principle,
any silicon containing carrier is suitable for use in the
preparation process according to the present invention.
Examples of silicon containing carriers comprise
zeolites. Preferably, the silicon containing carrier is a
silica carrier.
Silica carriers will substantially consist of silicon
dioxide. However, limited amounts of further compounds
such as contaminants can be present as well.
It is known that contaminants can influence the
performance of the final catalyst. The silica carrier for
use in the present invention preferably contains at most
1200 ppm of sodium, more specifically at most 1000 ppm of
sodium. Further, the silica carrier preferably comprises
at most 500 ppm of aluminium, at most 500 ppm of calcium,
at most 200 ppm of potassium, at most 100 ppm of
magnesium and at most 100 ppm of iron. The amounts are
based on amount of carrier.
The silica carrier preferably is a silica gel. The
silica gel carrier for use in the present invention can
in principle be any carrier derived from a silicon
containing gel. In general, silica gels are a solid,
amorphous form of hydrous silicon dioxide distinguished
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from other hydrous silicon dioxides by their micro-
porosity and hydroxylated surface. Silica gels usually
contain three-dimensional networks of aggregated silica
particles of colloidal dimensions. They are typically
prepared by acidifying an aqueous sodium silicate
solution to a pH of less than 1l by combining it with a
strong mineral acid. The acidification causes the
formation of monosilicilic acid (Si(OH)q), which
polymerizes into particles with internal siloxane
~ linkages and external silanol groups. At a certain pH the
polymer particles aggregate, thereby forming chains and
ultimately gel networks. Silicate concentration,
temperature, pH and the addition of coagulants affect
gelling time and final gel characteristics such as
density, strength, hardness, surface area and pore
volume. The resulting hydrogel is typically washed free
of electrolytes, dried and activated. A suitable silica
gel carrier would be silica support V432 and DAVICAT
P-732, which are commercially available from Grace
Davison.
Silica gel carriers for use in the present invention
preferably have a weight average particle size of at most
2 millimetres. Particle sizes which were found to be
especially suitable were weight average particle_sizes of
from 0.2 to 1.8 mm, more specifically of from 0.4 to
1.6 mm, most specifically of from 0.6 to 1.4 mm.
The silicon containing carrier preferably has a low
water content when contacted with the titanium halide. A
low water content can be achieved in any way known to
someone skilled in the art. A preferred way comprises
drying the silicon containing carrier before impregnating
the silicon containing carrier with the gas stream
consisting of titanium halide. A suitable drying method
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comprises subjecting the silicon containing carrier to a
temperature of from 200 to 700 °C. Surprisingly it has
been found that drying under specific circumstances gives
a further improved catalyst. The preferred drying
conditions comprise drying the carrier at a temperature
of from more than 200 to 300 °C. The drying is preferably
carried out during of from 1 to 8 hours, preferably in
the presence of an inert gas such as nitrogen. The
preferred method has been described in more detail in co-
pending patent application claiming priority of European
application 02258294.4.
A further improvement was observed if the silicon
containing carrier had been subjected to a pretreatment
comprising calcining the silicon containing carrier and
subsequently hydrolysing the carrier obtained. Hydrolysis
comprises treating the carrier with water or steam.
Preferably, the hydrolysis is carried out with steam.
Alternatively, the hydrolysis treatment may comprise a
washing treatment using an aqueous solution of a mineral
acid, an aqueous solution of an ammonium salt or a
combination thereof. Any water which might still be
present after the hydrolysis, is preferably removed
before treating the carrier further. Water is preferably
removed by drying. Preferably, the calcination is carried
out at a relatively high temperature. A preferred
calcination treatment comprises (a) calcining a silica
gel carrier at a temperature of at least 400 °C, (b)
hydrolysing the calcined silica gel carrier,
(c) impregnating the hydrolysed carrier obtained in
step (b) with a titanium-containing impregnating agent,
and (d) calcining the impregnated carrier. Preferably,
the calcinatian of step (a) is carried out at a
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temperature of from 950 to 800 °C, more preferably of
from 500 to 700 °C.
The silica gel carrier for use in the present
invention preferably has a surface area of at most
1000 m2/gram, more preferably at most 800 m2/gram, most
preferably at most 500 m2/gram.
Titanium halides which can be used in the process
according to the present invention comprise tri- and
tetra-substituted titanium complexes which have of from 1
to 4 halide substituents with the remainder of the
substituents, if any, being alkoxide or amino groups. The
titanium halide can be either a single titanium halide
compound or can be a mixture of titanium halide
compounds. Preferably, the titanium halide comprises at
least 50 %wt of titanium tetrachloride, more specifically
at least 70 %wt of titanium tetrachloride. Most
preferably, the gas stream consists of titanium
tetrachloride.
The present invention comprises impregnating the
carrier with gas consisting of titanium halide.
Surprisingly, it was found that catalyst having higher
selectivity for the desired alkylene oxide could be
obtained if the silicon containing carrier was
impregnated with gas consisting of titanium halide. The
preparation according to the present invention is carried
out in the absence of a carrier gas. Without wishing to
be bound to any theory, it is thought that the carrier
gas interferes with the impregnation. However, limited
amounts of further gaseous compounds are allowed to be
present during the contact between the silicon containing
carrier and the gaseous titanium halide. The gas in
contact with the carrier during impregnation preferably
consists for at least 70 %wt of titanium halide, more
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specifically at least 80 owt, more specifically at least
90 owt, most specifically at least 95 owt.
Gaseous titanium halide can be prepared in any way
known to someone skilled in the art. A simple and easy
way comprises heating a vessel containing titanium halide
to such temperature that the gaseous titanium halide is
obtained.
Generally, the impregnated carrier will be calcined
and subsequently hydrolysed before being used as a
catalyst. It is believed that calcination removes
hydrogen halide, more specifically hydrogen chloride
which is formed upon reaction of titanium halide and
silicon compounds present on the surface of the silicon
containing carrier.
The optional calcination of the impregnated carrier
generally comprises subjecting the impregnated carrier to
a temperature of at least 500 °C, more specifically at
least 600 °C. Preferably, the calcination is carried out
at a temperature of at least 650 °C. From a practical
point of view, it is preferred that the calcination
temperature applied is at most 1000 °C.
Hydrolysis of the impregnated and calcined carrier
can remove Ti-halide bonds. The hydrolysis of the
impregnated carrier will generally somewhat more severe
than the optional hydrolysis of the carrier before
impregnation. Accordingly, this hydrolysis of the
impregnated carrier is suitably carried out with steam at
a temperature in the range of from 150 to 400 °C.
Preferably, the hydrolysed impregnated carrier is
subsequently silylated for instance by contacting the
hydrolysed impregnated carrier with a silylating agent,
preferably at a temperature of between 100 and 925 °C.
Suitable silylating agents include organosilanes like
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tetra-substituted silanes with C1-C3 hydrocarbyl
substituents. A very suitable silylating agent is
hexamethyldisilazane. Examples of specific suitable
silylating methods and silylating agents are, for
instance, described in US-A-3,829,392 and US-3,923,843
which are referred to in US-A-6,011,162, and in
EP-A-734764.
The amount of titanium (as metallic titanium) will
normally be in the range of from 0.1 to 10o by weight,
suitably of from 1 to 5°s by weight, based on total weight
of the catalyst. Preferably, titanium or a titanium
compound, such as a salt or an oxide, is the only metal
and/or metal compound present.
As mentioned above, it is well known in the art to
produce alkylene oxides, such as propylene oxide, by
epoxidation of the corresponding olefin using a
hydroperoxide such as hydrogen peroxide or an organic
hydroperoxide as the source of oxygen. The hydroperoxide
can be hydrogen peroxide or any organic hydroperoxide
such as tert-butyl hydroperoxide, cumene hydroperoxide
and ethylbenzene hydroperoxide. The alkene will generally
be propene which gives as alkylene oxide, propylene
oxide. The catalyst prepared according to the present
invention has been found to give especially good results
in such process. Therefore, the present invention further
relates to a process for the preparation of alkylene
oxide which process comprises contacting a hydroperoxide
and alkene with a heterogeneous epoxidation catalyst and
withdrawing a product stream comprising alkylene oxide
and an alcohol and/or water, in which process the
catalyst has been prepared according to the present
invention.
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_ g _
A specific organic hydroperoxide is ethylbenzene
hydroperoxide, in which case the alcohol obtained is
1-phenyl ethanol. The 1-phenylethanol usually is
converted further by dehydration to obtain styrene.
Another method for producing propylene oxide is the
co-production of propylene oxide and methyl tert-butyl
ether (MTBE) starting from isobutane and propene. This
process is well known in the art and involves similar
reaction steps as the styrenelpropylene oxide production
process described in the previous paragraph. In the
epoxidation step tent-butyl hydroperoxide is reacted with
propene forming propylene oxide and tert-butanol. Tert-
butanol is subsequently etherified into MTBE.
A further method comprises the manufacture of
propylene oxide with the help of cumene. In this process,
cumene is reacted with oxygen or air to form cumene
hydroperoxide. Cumene hydroperoxide thus obtained is
reacted with propene in the presence of an epoxidation
catalyst to yield propylene oxide and 2-phenyl propanol.
The latter can be converted into cumene with the help of
a heterogeneous catalyst and hydrogen. Suitable processes
are described for example in WO 02!48126.
The conditions for the epoxidation reaction according
to the present invention are those conventionally
applied. For propene epoxidation reactions with the help
of ethylbenzene hydroperoxide, typical reaction
conditions include temperatures of 50 to 140 °C, suitably
75 to 125 °C, and pressures up to 80 bar with the
reaction medium being in the liquid phase.
The invention is further illustrated by the following
Examples.
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Examples
The silica gel carrier used in the examples had a
surface area of 300 m2/g and a weight average particle
size of about 1 mm. Substantially all particles had a
particle size between 0.6 and 1.4 mm.
75 grams of silica gel carrier was dried at 250 °C
during 2 hours.
The dried silica gel carrier was contacted with a gas
stream containing titaniumtetrachloride. The gas stream
was obtained by heating titaniumtetrachloride to 200 °C
with the help of an electrical heating system. Different
gas streams were obtained by adding different amounts of
nitrogen. At the end of each experiment, each silica
carrier had been in contact with the same amount of
titaniumtetrachloride.
The impregnated catalysts thus obtained were calcined
at 600 °C during 7 hours. The calcined catalysts were
subsequently contacted with steam at 325 °C during
6 hours. The steam flow consisted of 3 grams of water per
hour and 8 Nl of nitrogen per hour. Finally, the
catalysts were silylated at 185 °C during 2 hours by
being contacted with 18 grams of hexamethyldisilazane per
hour in a nitrogen flow of 1.4 N1 per hour.
The catalysts obtained were analysed for the amount
of titanium deposited on the carrier.
The selectivity of the catalysts was tested in a
continuous epoxidation bench scale unit containing a
number of vessels on automatic weight balances containing
respectively the ethylbenzene hydroperoxide and propene
feed streams, two high pressure pumps, a fixed bed
reactor, a third pump for pumping a recycle stream over
the reactor, means to maintain the reactor continuously
at temperatures between 60 and 120 °C, a stripper to
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remove light boiling components like propene, a cooler
and a vessel for receiving the product.
The feeds were supplied to the reactor via the two
high pressure pumps and mixed together before entering
the reactor. The reactor was operated liquid full at
40 tiara pressure and 90 °C. A large recycle stream was
maintained over the reactor to have isothermal operation
of the reactor bed and to ensure that the catalyst to be
re-activated is contacted with epoxidation reaction
product. The feed was mixed with the recycle stream prior
to introduction into the reactor.
The feed consisted of 40 %wt of propene, 20 owt of
ethylbenzene hydroperoxide and 40 owt of ethylbenzene.
The results obtained are given in Table 1. The
selectivity is the molar ratio of propylene oxide formed
to ethylbenzene hydroperoxide converted.
Example 2
Further catalysts were prepared in a way similar to
the one described in Example 1. However, the impregnated
~0 catalysts were calcined for 6 hours (instead of 7 hours)
while the steam flow during the subsequent hydrolysis
contained 5 grams of water per hour (instead of 3 grams
of water per hour). The results of these experiments are
shown in Table 2.
CA 02508043 2005-05-31
WO 2004/050233 PCT/EP2003/050875
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CA 02508043 2005-05-31
WO 2004/050233 PCT/EP2003/050875
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