Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Hydrogenation catalyst comprising nickel on carbon
Description
The invention relates to a hydrogenation catalyst comprising nickel on a
carbon
support, a process for producing the hydrogenation catalyst and its use for
the
hydrogenation of sorbitol to glycols or the hydrogenation of glucose to
sorbitol.
The preparation of chemical starting materials from renewable sources is
gaining ever
greater importance. Thus, for example, glycols such as propylene glycol and
ethylene
glycol can be produced from maize, with starch firstly being obtained from the
maize
and subsequently being converted into glucose, then sorbitol and subsequently
glycols
such as propylene glycol and ethylene glycol. These are important starting
materials in
the preparation of polymer resins such as polyurethanes or for the preparation
of
polymer crosslinkers and other chemical compounds.
The hydrogenation of sorbitol to glycols such as ethylene glycol and propylene
glycol is
carried out at high temperatures and pressures and also high pH values in an
aqueous
medium. Inorganic supports customarily used for hydrogenation catalysts
generally
withstand these conditions for only a short time, if at all, so that such
catalysts are
unsuitable for the hydrogenation of sorbitol.
As an alternative, catalysts comprising nickel and rhenium on a carbon support
have
been proposed. US 6,841,085 describes the hydrogenation of sugars such as
sorbitol
to predominantly ethylene glycol and propylene glycol using a catalyst having
2.5% by
weight of nickel and 2.5% by weight of rhenium on a coconut carbon support. In
the
production of the catalyst, the support is firstly impregnated with metal salt
solutions of
the active metals, subsequently dried and reduced at 280 C for 16 hours.
A similar process is described in US 7,038,094, in which a catalyst comprising
rhenium
and nickel on a coconut carbon support is likewise used.
It is an object of the present invention to provide hydrogenation catalysts,
in particular
for the hydrogenation of sorbitol to glycols, which withstand high
temperatures and
pressures and an aqueous environment having high pH values and can be produced
simply and inexpensively. They should display preferential selectivity to
propylene
glycol and optionally ethylene glycol.
The object is achieved according to the invention by a hydrogenation catalyst
comprising from 1 to 50% by weight, based on the total catalyst, of nickel on
a carbon
support, wherein the hydrogenation catalyst does not comprise any rhenium.
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It has been found according to the invention that hydrogenation catalysts
comprising
nickel but no rhenium as active metal on a carbon support are suitable for the
hydrogenation of sorbitol to glycols.
The catalysts can be obtained in a simple manner since only impregnation with
an
active metal is necessary. In addition, they are significantly cheaper than
known
catalysts since they dispense with the use of costly rhenium and use
inexpensive
carbon supports.
The catalyst of the invention does not contain any rhenium. This means that no
technically effective amounts of rhenium are comprised in the catalyst and
rhenium
thus has no importance as active metal.
Preference is given to catalysts according to the invention which comprise
only nickel
as active metal. However, it is also possible for further active metals such
as
molybdenum, vanadium or tin or mixtures thereof to be present in addition to
nickel.
The catalyst of the invention comprises nickel in an amount of from 1 to 50%
by weight,
preferably from 5 to 40% by weight, in particular from 10 to 30% by weight,
based on
the total catalyst. The proportion of further metals is from 0 to 25% by
weight,
preferably from 0 to 15% by weight, in particular from 0 to 5% by weight. If
such metals
are present, their minimum amount is preferably 0.5% by weight. Particular
preference
is given to no further active metals apart from nickel, iron, molybdenum,
vanadium
and/or tin being present on the catalyst support. Particular preference is
given to only
nickel being present as active metal on the catalyst support. In particular,
the carbon
support is impregnated only with nickel as metal.
According to the invention, any suitable carbon supports can be used. For
example, it
is possible to use coconut shells, olive stones or peat charcoal as carbon
support. It is
also possible to use synthetic carbon supports. Particular preference is given
to using
coconut shell carbon as support.
The carbon support can be used in untreated form or pretreated form for
producing the
catalyst. Pretreatment of the carbon can be carried out, for example, by means
of heat,
steam, acids or chemical pretreatment. A steam pretreatment of the coconut
shell
carbon with water vapor is particularly preferably carried out.
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The support can have any suitable particle size. The support preferably has an
average
particle diameter in the range from 0.5 to 5 mm, particularly preferably from
0.75 to
3.5 mm, in particular from 1 to 2 mm.
The hydrogenation catalyst used according to the invention can be produced by
any
suitable processes. It is usually produced by impregnation of the support with
a nickel
salt solution, subsequent drying and subsequent reduction. The reduction is
preferably
carried out at a temperature above 300 C, particularly preferably in the range
from
> 300 C to 700 C, in particular in the range from 400 to 600 C, especially in
the range
from 400 to 500 C. For example, the reduction treatment can be carried out at
about
500 C.
An increased reduction temperature leads to more active catalysts which allow
a higher
sorbitol conversion. Particularly good results are obtained at a hydrogenation
temperature of 500 C. However, the selectivity of the catalyst is not reduced
by the
increased activity. The reduction can be followed by stabilization in air,
preferably at
room temperature. The invention also provides a catalyst which can be produced
by
the above process.
The invention also provides a process for producing the above catalyst by
impregnation
of the carbon support with a nickel salt solution, subsequent drying of the
impregnated
support and subsequent reduction of the dried support at a temperature above
300 C.
The abovementioned reduction temperatures are preferably employed here.
Impregnation can be carried out by any suitable impregnation methods.
Preference is
given to carrying out vacuum impregnation. Any suitable nickel salts can be
employed
here. Preference is given to using nickel nitrate as aqueous solution.
Drying is preferably carried out at a temperature in the range from 50 to 150
C and
atmospheric pressure or preferably under reduced pressure. Drying is
particularly
preferably carried out under vacuum or reduced pressure.
The reduction is preferably carried out in the presence of a gas comprising
free
hydrogen, in particular in a hydrogen atmosphere.
The reduction can be followed by stabilization of the nickel-comprising
catalyst, for
example in air at room temperature, in order to obtain a storable, stable
nickel catalyst.
Vacuum impregnation leads to a very well dispersed nickel catalyst comprising
nickel
crystallites having an average crystallite size in the range from 2 to 15 nm.
Very
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uniform crystallites which do not undergo appreciable agglomeration, if any,
and do not
form relatively large clusters even after prolonged use of the catalyst in the
hydrogenation of sorbitol are present here.
Typical hydrogenation conditions in the hydrogenation of sorbitol are a
temperature in
the range from 150 to 350 C, preferably from 200 to 300 C, in particular about
250 C,
a hydrogen pressure in the range from 50 to 300 bar, in particular about 150
bar, a
sorbitol concentration of from 10 to 40% by weight in water, in particular
about 20% by
weight in water, an initial pH in the range from 12 to 13, for example set by
addition of
KOH.
The hydrogenation can also be carried out under the reaction conditions as are
described in US 6,841,085 and US 6,479,713.
To determine the effectiveness and strength of the catalysts, the sorbitol
hydrogenation
is generally carried out at a temperature of 250 C, a hydrogen pressure of 150
bar, a
pH of from 12 to 13 on a 20% strength by weight aqueous sorbitol solution.
The degree of reduction of the sorbitol is preferably in the range from 50 to
99%.
After hydrogenation for a period of about 300 minutes, the strength of the
catalyst is
determined. No reduction in the strength as a result of the hydrogenation is
found for
the carbon supports, in particular the coconut shell carbon supports.
The catalysts of the invention are thus preferably used for the hydrogenation
of sorbitol
to glycols, in particular propylene glycol and ethylene glycol, with small
amounts of
glycerol, or for the hydrogenation of glucose to sorbitol.
The invention therefore also provides a process for preparing glycols by
hydrogenation
of sorbitol, in which the hydrogenation is carried out over a catalyst as
described
above.
In addition, the invention provides a process for preparing sorbitol by
hydrogenation of
glucose, wherein the hydrogenation is carried out over a catalyst as described
above.
The hydrogenation is preferably carried out continuously, with the
hydrogenation
catalyst being present as a fixed bed.
The hydrogenation of sorbitol gives propylene glycol as main product, and also
ethylene glycol in a significantly smaller amount and even significantly
smaller amounts
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of glycerol. Xylitol, butanediol and methanol and also lactic acid are
typically formed as
by-products.
The formation of methane, as occurs in the case of the known catalysts, does
not occur
5 to an appreciable extent according to the invention.
Compared to known catalysts, the catalysts of the invention display improved
selectivity in respect of the preparation of propylene glycol. In particular,
the selectivity
is very high in the case of nickel catalysts having coconut shell carbon
supports.
Conversion and selectivity to propylene glycol and ethylene glycol are
significantly
better in the case of the nickel-comprising hydrogenation catalyst of the
invention than
in the case of a comparative catalyst which additionally comprises rhenium.
Both the
conversion and the propylene glycol selectivity were significantly better in
the case of a
catalyst comprising 10% by weight of nickel on a carbon support than in the
case of a
catalyst comprising 10% by weight of nickel and 1% by weight of rhenium on the
same
carbon support.
In the process for the hydrogenation of glucose for preparing sorbitol, the
reaction is
preferably carried out at a temperature in the range from 50 to 250 C,
particularly
preferably from 90 to 140 C, a pressure in the range from 30 to 250 bar,
particularly
preferably from 60 to 150 bar, and a glucose concentration in the preferably
aqueous
glucose solution in the range from 30 to 70% by weight, particularly
preferably from 40
to 60% by weight. In a continuous process, the space velocity is preferably
from 0.15 to
2 I/I- h.
An addition of base is typically not necessary. After about 300 hours, the
strength of
the catalyst in the fixed bed had not changed.
Compared to known catalysts, the catalysts of the invention display improved
selectivity and activity in respect of the preparation of sorbitol. In
particular, the
selectivity is very high in the case of nickel catalysts and coconut shell
carbon
supports.
The invention is illustrated by the following example.
Example 1: Production of the catalyst
Carbon extrudates or granulated carbon were used as starting materials.
However,
coconut shell carbon as can be obtained from Japan EnviroChemicals Ltd. under
the
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trade name SHIRASAGI C2X8/12 was preferably used. This carbon has a bulk
density
of about 0.5 g/ml and an average particle size of 1.8 mm.
An aqueous solution comprising nickel nitrate in deionized water having, for
example, a
nickel concentration of 14.4% by weight was firstly produced. For example,
53.3 g of
Ni(NO3)2.6 H20 in 22.0 g of water was used for the impregnation of 50 g of
carbon
extrudates.
A vacuum impregnation was carried out as impregnation. The carbon support was
maintained under reduced pressure for 30 minutes, after which it was spray
impregnated with the above solution comprising nickel nitrate. Heating and
drying of
the impregnated support followed. The vacuum was then broken and air was
allowed to
flow in.
To reduce the impregnated catalyst support, this was heated to a reduction
temperature of 410 C or 450 C or 500 C at a heating rate of 60 C/h under
nitrogen
(100 ml/h). Hydrogen was then slowly introduced. After addition of 100% of the
hydrogen, the catalyst was maintained at this temperature for 4 hours. It was
then
quickly cooled to 50 C under nitrogen (100 ml/h). Air was then slowly admitted
in order
to carry out stabilization of the catalyst.
