The invention relates to a process for the preparation
of a ketone or an aldehyde, in particular an unsaturated ketone or an unsaturated
aldehyde, from a reaction mixture comprising a starting ketone and a starting aldehyde.
Such a process is generally known as an aldol condensation.
An example thereof is disclosed in the
Journal of Catalysis 106, pages 273-279 (1987)
, wherein the production of methyl vinyl ketone from formaldehyde and acetone
is disclosed. The known process is a vapour-phase aldol condensation between formaldehyde
and acetone using various kinds of single and binary metal oxides and heteropoly
compounds as catalysts. The known process preferably uses V2O5-P2O5
(P/V atomic ratio = 1.06) and Fe2O3-P2O5
(P/Fe = 4.6) catalysts.
The known process has as a disadvantage that a significant
deactivation of the catalyst is observed. In figure 4 of the said article in the
Journal of Catalysis, a very significant deactivation is observed after less than
10 hours of operation. The catalyst deactivation as reported is stronger if the
temperature increases from 180°C to 240°C.
It is the objective of the present invention to reduce
the said disadvantage.
The objective is achieved in a process for the preparation
of a ketone or an aldehyde, comprising the steps of:
whereby at the onset of step b) the molar ratio of starting ketone to starting
aldehyde in the reaction mixture lies between 9:1 and 100:1, and whereby step b)
is executed at a temperature lying between 320°C and 380°C.
- a) forming a reaction mixture by bringing a mixture of a starting aldehyde and
a starting ketone into the gaseous phase;
- b) bringing the reaction mixture in contact with a catalyst, whereby the ketone
and/or the aldehyde is/are formed,
The advantage of the process according to the invention
is that the selection of molar ratios in combination with the selection of reaction
temperatures leads to a prolonged period of high catalyst activity, compared to
the known process.
In step a) of the process according to the invention, a
reaction mixture is formed. The reaction mixture should comprise a starting ketone
and a starting aldehyde; moreover, the reaction mixture should be in the gaseous
As starting ketone, in principle any known ketone or mixture
of ketones is suitable, provided that the ketone of mixture of ketones is in the
gaseous phase or can be brought in the gaseous phase. Ketones as such are known;
preferably, a C3 - C10 aliphatic, cyclic or aromatic ketone
is used. Preferred examples of ketones suitable as starting ketone include acetone,
methylethylketone (MEK) and mesityloxide (4-methylpent-3-en-2-one). Within the context
of the present invention, a reference to a starting ketone embraces the reference
to a mixture of different starting ketones; in a preferred embodiment of the invention,
the starting ketone is a mixture of ketones.
As starting aldehyde, in principle any known aldehyde or
mixture of aldehydes is suitable, provided that the aldehyde or mixture of aldehydes
is in the gaseous phase or can be brought in the gaseous phase. Aldehydes as such
are known; preferably, a C1 - C10 aliphatic, cyclic or aromatic
aldehyde is used as starting aldehyde. Preferred examples of suitable starting aldehydes
include formaldehyde and acetaldehyde. Within the context of the present invention,
a reference to a starting aldehyde embraces the reference to a mixture of different
starting aldehydes; in a preferred embodiment of the invention, the starting aldehyde
is a mixture of aldehydes.
The reaction mixture may be formed by taking a desired
amount of starting materials and combining them, while preferably performing a mixing
operation; if the starting materials are not in the gaseous phase, then they should
be brought into the gaseous phase through means that are as such known to the skilled
person, e.g. through a heating step or a decomposition step. An example of such
a decomposition step is the decomposition of paraformaldehyde or trioxan to form
formaldehyde. If the starting materials are already in the gaseous phase, then step
a) of the present invention can already be executed through the mere combining and
preferably mixing of the starting materials. The starting materials may be present
as such (e.g. in pure form), or they may be present in the form of a mixture with
another compound, e.g. as an aqueous solution which is quite common in for example
formaldehyde. As noted above, it may be desirable to introduce a mixing step so
as to homogenise the reaction mixture prior to the execution of step b).
It was found that a prolonged period of high catalyst activity
can be achieved in case of a significant molar excess of the starting ketone over
the starting aldehyde. According to the invention, therefore, the molar ratio between
the starting ketone and the starting aldehyde is at least 9:1; more preferably,
the said ratio is at least 12:1 or 15:1; in particular, the molar ratio between
the starting ketone and the starting aldehyde in the reaction mixture is at least
18:1 or even 20:1; most preferably, the molar ratio between the starting ketone
and the starting aldehyde in the reaction mixture is at least 24:1 or even 28:1.
For practical reasons, the molar ratio between the starting ketone and the starting
aldehyde in the reaction mixture is at most 100:1 or 75:1. The molar ratios as given
here are applicable to the reaction mixture as it is formed, and/or at the onset
of step b) as the reaction mixture comes in contact with the catalyst. It was found
that an additional advantage related to a significant molar excess of the starting
ketone over the starting aldehyde is that compared to the known process a higher
selectivity towards compounds formed from a fusion of the starting aldehyde with
the starting ketone can be achieved.
The reaction mixture may comprise additional compounds
besides the starting ketone, the starting aldehyde, and water. Examples of such
additional compounds are stabilizing compounds, such as methanol in case formaldehyde
is used, or inert gases such as for example nitrogen, carbon dioxide, or argon.
In a preferred embodiment of the invention, however, the amount of such additional
compounds is limited to about 50 wt.%, 40 wt.%, 25 wt.% or even 20 or 10 or 5 wt.%
of the reaction mixture. Most preferably, the reaction mixture contains essentially
no additional compounds except stabilizing compounds; this has the advantage that
these compounds do not need to be removed from the reaction mixture afterwards in
order to isolate valuable compounds such as the ketone and/or the aldehyde. Within
the context of the present invention, the terms 'contain essentially no' or 'consist
essentially of' are understood to mean that if there are any further compounds present,
then these further compounds have no influence on the functioning of the invention
or on the effects as achieved by the invention.
Subsequent to the forming of the reaction mixture in step
a) according to the invention, step b) is executed. In step b), the gaseous reaction
mixture is brought in contact with a catalyst. Typically, and preferably, the catalyst
will be a solid. The bringing in contact of the reaction mixture with the catalyst
may be done by means known as such; a typical example of such a known means is putting
the catalyst in a reactor and forcing the gaseous reaction mixture to flow through
the reactor. It is necessary hereby that step b) is carried out at an elevated temperature
lying preferably between 320°C and 380°C. More preferably, the temperature
at which step b) is carried out is at least 325°C or 330°C; the temperature
at which step b) is carried out is preferably at most 375°C or 370°C.
It will typically not be very beneficial to carry out the process according to the
invention at significantly elevated or reduced pressure, although this is possible
in a very wide range between 0.005 MPa and 15 MPa.
The catalyst as used in the process according to the invention
may in principle be any catalyst that can achieve the desired aldol condensation
reaction. Such catalysts are as such known. In a preferred embodiment of the invention,
the catalyst is a solid and comprises silica, i.e. oxide of silicon. Silica as such
is widely known. It was found, surprisingly, that the yield of the process according
to the invention can be higher than in the known process if the silica as comprised
in the catalyst has a high purity of at least 95, 96, 97, 98 or 99 wt.%. Preferably,
the purity of the silica is at least 99.1, 99.2 or 99.3 wt.%; more preferably, the
purity of the silica is at least 99.4, 99.5, 99.6, 99.7 or even 99.8 wt.%;. Without
committing to scientific explanation, it is thought that whereas in many other cases
silica merely acts as and is used as a support for catalyst particles, the silica
itself is - surprisingly - in the process according to the invention the catalyst.
It is therefore additionally preferred that the catalyst as used in the process
according to the invention comprises at least 50 or 60 wt.% of high-purity silica;
more preferably, the catalyst as used in the process according to the invention
comprises at least 75, 85 or even 90 or 95 wt.% of high-purity silica; most preferably,
the catalyst as used in the process according to the invention consists essentially
or even only of high-purity silica.
It is preferred that if there are any compounds comprised
in the catalyst besides silica, then these are essentially devoid of Al2O3,
ZrO2, P2O5, V2O5, Ce2O3,
TiO2, and Na2O; it was found that the said compounds are among
the compounds that can have a negative influence on the yield and/or selectivity
of the ketone- or aldehyde-forming reaction.
In a preferred embodiment according to the invention, the
silica as comprised in the solid catalyst comprises or even consists essentially
of fumed silica. As is known, fumed silica preferably consists of clusters of primary
non-porous particles, whereby the cluster structure creates pores. This structure
results in the fumed silica having a high specific surface area; preferably, this
specific surface area lies between 25 and 500 m2/g, more preferably between
50 and 450 m2/g. It is furthermore preferred that the fumed silica has
an amount of at most 20% micropores or even at most 15% or 10% or essentially no
micropores at all. Micropores are defined as pores having a diameter of 5 nm or
less. The low amount or even absence of micropores is advantageous as micropores
are difficult to access for the compounds in the reaction mixture, or even not accessible
at all. It is furthermore preferred that the low amount of micropores is combined
with an average pore size lying between 5 and 100 nm, more preferably between 6
nm and 50 nm. Most preferably, the amount of pores having a diameter of more than
100 nm is at most 10%, 5% or even at most 1%; as a consequence of this, the pore
size distribution will be relatively narrow.
The contact of the reaction mixture with the catalyst can
lead to several types of reaction mechanisms, which are embraced within the context
of the present invention. The reactions may take place on the surface of the catalyst
or they may take place in the gaseous phase, e.g. by a reactive intermediate compound
that was formed on the catalyst and then detached from it. The reactions taking
place at the catalyst may start either from a ketone, from an aldehyde, or from
The nature of the starting ketone and the starting aldehyde
will co-determine which ketone or mixture of ketones will be formed. As is the case
in the known process, the presence of acetone as the starting ketone and formaldehyde
as the starting aldehyde can lead to the formation of a.o. methylvinylketone (MVK),
a know compound having formula (I):
Since MVK is a compound having commercial importance, a.o.
as intermediate compound in the synthesis of Vitamin A, astaxanthin and zeaxanthin,
the reaction mixture in a preferred embodiment of the process according to the invention
comprises upon its formation both a starting ketone and a starting aldehyde, whereby
acetone is primarily or even essentially the starting ketone and formaldehyde is
primarily or even essentially the starting aldehyde.
As indicated above, the formation of a reaction mixture
comprising formaldehyde as starting aldehyde may for practical reasons involve the
use of an aqueous solution of formaldehyde; such solutions, e.g. comprising 37 wt.%
formaldehyde in water, are known. Generally speaking, the person skilled in the
art would expect that the presence of water has a negative influence on the overall
yield of the reaction since water is a by-product in many aldol condensation reactions
such as MVK formation from formaldehyde and acetone, and thus could lead to an equilibrium
shift away from end product formation. It was surprisingly found, however, that
the presence of water in the reaction mixture in step a) or at the onset of step
b) does not have a negative effect; by contrast, a positive effect on the yield
of MVK was observed. In a preferred embodiment of the invention, therefore, the
weight ratio of formaldehyde to water in the reaction mixture is in step a) or at
the onset of step b) at most 60:40, 50:50 or 40:60, preferably at most 30:70 or
25:75, most preferably at most 20:80. The reaction mixture as a whole preferably
comprises - at the onset of the execution of step b) - at least 1 wt.% of water,
more preferably at least 5, 10 or even 15 wt.%; the reaction mixture as a whole
preferably comprises - at the onset of the execution of step b) - at most 50 wt.%
water, more preferably at most 40, 30 or 20 wt.%.
The steps a) and b) according to the invention may be carried
out in batch operation or in continuous operation. In any case, it is preferred
that the residence time of the reaction mixture in step b), in particular the contact
time between catalyst and reaction mixture, is chosen such that an adequate reaction
progression is combined with an economical mode of operation. In achieving this,
it is helpful to consider the residence time parameter W/F; this parameter is defined
as the amount of catalyst present (in grams) divided by the flow of the reaction
mixture (in total amount of moles of all compounds in the reaction mixture per hour).
In the process according to the invention, it is preferred that W/F lies between
1 and 1000 g.h/mol, more preferably between 10 and 750 g.h/mol, in particular between
20 and 500 g.h/mol, and most preferably between 40 and 150 g.h/mol.
Since the reaction mixture comprises - at least in step
a) and at the onset of step b) - an excess of the starting ketone, the situation
can arise that the reaction mixture comprises a high amount of acetone as starting
ketone subsequent to the execution of step b). It is then typically desirable to
recover the starting ketone and/or the starting aldehyde from the reaction mixture.
The invention therefore further also relates to a process comprising after step
b) the steps of:
- c) feeding the reaction mixture to a rectification apparatus;
- d) separating a top stream comprising the starting ketone and/or the starting
aldehyde from the reaction mixture and removing it from the rectification apparatus;
- e) optionally recycling at least a portion of the top stream to step a) or step
In step c) according to the invention, the reaction mixture
is fed to a rectification apparatus. Rectification apparatuses are as such known;
an example of such an apparatus is a distillation column. It is preferred to feed
the reaction mixture to the rectification apparatus with the reaction mixture being
in the gaseous phase, preferably without having undergone phase changes subsequent
to the completion of step b). This has, in combination with the subsequent steps
d) and e), the advantage that hardly any energy related to changing of phase is
lost in the recovery of the starting ketone and/or the starting aldehyde.
In the rectification apparatus, step d) according to the
invention takes place: a - preferably gaseous - stream comprising the starting ketone
and/or the starting aldehyde is separated from the reaction mixture, and subsequently
removed from the rectification apparatus. Since this stream will often be removed
from the upper half of the apparatus, it is hereby named the top stream. The forming
of the top stream by separating it from the reaction mixture may be achieved by
means known as such to the skilled person, such as distillation techniques. Also
the removal of the top stream from the rectification apparatus may be achieved by
means known as such to the skilled person, such as pumping or pressure reduction.
The top stream may now, after having been removed from
the rectification apparatus, be advantageously re-used in the process according
to the invention. The invention therefore preferably comprises - subsequent to step
d) - a step e), in which at least a portion of the top stream is recycled to step
a) or step b). The said portion is preferably at least 25 wt.% of the top stream,
more preferably at least 50 wt.% or even at least 75 wt.%. The separating off of
a portion from the top stream and the subsequent recycling may be achieved by means
as such known to the skilled person.
It may be additionally advantageous to separate a second
portion off from the top stream, condense this second portion and feed it back to
the rectification apparatus as a reflux stream. As is know to the skilled person,
such reflux streams are often beneficial or even necessary in achieving a satisfactory
composition of the top stream and in achieving a satisfactory energy efficiency
of operation of the rectification apparatus.
The starting ketone and/or starting aldehyde as comprised
in the top stream can - as described above - be beneficially recycled to step a)
or b) of the process according to the invention so as to aid in forming the reaction
mixture. Depending on the nature of the top stream, it may be necessary to implement
one or more purification steps on the top stream so as to remove any unwanted compounds.
With the top stream separated off from the reaction mixture,
the reaction mixture may be removed from the rectification apparatus as well - preferably
after the reaction mixture has been brought into liquid form. Depending on the circumstances,
subsequent steps may be desirable in order to isolate the ketone and/or the aldehyde
from the reaction mixture. Examples of such steps are rectification, filtration,
extraction, crystallisation, and the like.
In case the reaction mixture comprises acetone as starting
ketone and formaldehyde as starting aldehyde, then - as indicated above - MVK can
be prepared. When a gaseous reaction mixture comprising MVK is then according to
step c) fed to a rectification apparatus and brought into liquid form, it was found
that it is important to ensure that undesirable subsequent reactions of MVK such
as polymerisation are minimised. It is therefore a further object of the invention
to provide a method for reducing or even preventing the said subsequent reactions
of MVK. The said method comprises the step of mixing an MVK-stabilizing agent with
MVK when the MVK is transferred from the gaseous phase into the liquid phase or
immediately thereafter. The MVK-stabilizing agent preferably comprises water; more
preferably, the MVK-stabilizing agent comprises a mixture of water with an acid.
Preferably, the acid is acetic acid. In particular, the MVK-stabilizing agent comprises
besides water arid an acid such as acetic acid also hydroquinone. If the MVK-stabilizing
agent comprises hydroquinone, then it is preferred that it also comprises acetonitrile.
In a preferred embodiment of the invention, the MVK-stabilizing agent has the following
whereby the sum of these four compounds adds to 100%.
- between 30 and 95 wt.% acetic acid, preferably between 50 and 90 wt.%;
- between 1 and 40 wt.% water, preferable between 10 and 30 wt.%;
- between 0 and 15 wt.% hydroquinone, preferably between 0.1 and 10 wt.%;
- between 0 and 15 wt.% acetonitrile, preferably between 0.1 and 10 wt.%,
The amount of MVK-stabilizing agent as added to the reaction
mixture, or to the MVK itself after having been further purified, may vary between
wide limits and lies preferably between 0.1 and 10 wt.% relative to the amount of
The minimisation of undesirable subsequent reactions of
ketones such as MVK or of aldehydes may also be supported by choosing favourable
process conditions. It was found that the speed of said undesirable subsequent reactions
is reduced with reducing temperature; this may be achieved by for example carrying
out the process in the rectification apparatus at reduced pressure. Furthermore,
it was found that the absence of light also contributes to the control of undesirable
The preparation of MVK constitutes one example of the industrial
applicability of the process according to the invention. Another example of an unsaturated
ketone that may be formed according to the invention is the preparation of 3-methylcyclohexenon
from a reaction mixture comprising formaldehyde as starting aldehyde and mesityloxide
as starting ketone. The compound 3-methylcyclohexenon is as such known and has formula
Generally speaking, it will be advantageous to choose the
starting material(s) such that the formation of those ketones and/or aldehydes is
targeted that permit handling in the gaseous phase at the prevailing temperatures
in step b). A quick or even immediate phase transfer of the ketones and/or aldehydes
as formed into the liquid or solid phase would be more likely to cause processing
problems and is thus less preferred.
The invention will be illustrated by the following examples
and comparative experiments, without being limited thereto.
Examples 1 - 6
The preparation of MVK according to steps a) and b) was
executed in a unit comprising a vaporiser and a reactor. The vaporiser comprised
two sections: in the first section acetone as starting ketone was vaporised by means
of heating; in the second section a 37 wt.% aqueous solution of the starting aldehyde
formaldehyde was vaporised by means of spraying it into the gaseous acetone stream
and heating; hereby the reaction mixture was formed. The heating was done such that
the reaction mixture had a temperature equalling the reactor temperature. The reaction
mixture was then fed to a reactor containing a catalyst. The temperature in the
reactor was as indicated in the table. The catalyst type is as given in the table
below. As the reaction mixture exited the reactor, a sample was taken and analysed.
Each sample was taken after the catalyst had been in operation for a number of hours
as indicated in the table. The residence time of the reaction mixture in the reactor
is given in the table by means of the residence parameter W/F.
From the data in the table, it is evident that yields in
MVK formation are higher than as reported in the prior art and, furthermore, that
these high yields are achieved over much longer periods of time that the up to 8
hours as reported in the Journal of Catalysis.
Additionally, it can be seen that an increase of the molar
ratio between acetone and formaldehyde - within the range according to the invention
- is beneficial, as is a raise in temperature.
Example / Comparative Experiment
Residence time in reactor W/F (g.h/mol)
Molar ratio acetone / formaldehyde
Reactor Temperature (°C)
Catalyst Operational Hours
Conversion of Formaldehyde (%)
Yield of MVK (%)