Process for preparing dialkyl vinylphosphonates

Process for the continuous preparation of dialkyl vinylphosphonates using catalysts at temperatures of from 150.degree. to 270.degree. C. by dissociation of dialkyl acetoxyethanephosphonates at a pressure of from 5 to 500 mbar in contact with a liquid, catalytically active medium while drawing off the dialkyl vinylphosphonates formed and other volatile reaction products as vapors, by conveying the liquid medium in forced circulation via an evaporator while feeding in fresh dialkyl acetoxyethanephosphonate, if desired admixed with catalyst, corresponding to the distillation of dialkyl vinylphosphonates and other volatile compounds, and drawing off non-volatile material formed as byproduct from the liquid circuit to maintain constant conditions.

Dialkyl vinylphosphonates are important as precursors for the preparation 
of pure vinylphosphonic acid and also as monomers for copolymerization for 
the production of adhesives or fire-resistant plastics. They can be 
prepared in different ways (DE-A 30 01 894, EP 281 122). According to a 
process described in EP 281 122, acetoxyethanephosphonic ester is 
thermally dissociated in the liquid phase in vacuo to give acetic acid and 
dialkyl vinylphosphonate, with the acetic acid and dialkyl 
vinyl-phosphonate distilling out of the reaction mixture. According to the 
procedure described, dialkyl acetoxyethanephosphonate containing, for 
example, vinyl-phosphonic acid (as catalyst) is introduced dropwise into a 
reaction flask (fitted with superposed distillation column, vacuum 
connection, distillate receiver, receiver for discharge of bottoms). 
Acetic acid and dialkyl vinylphosphonate formed are distilled off. In 
continuous operation, an equilibrium state is maintained by also taking 
off liquid-phase material according to the formation of high boilers. The 
yields indicated are 80% of the dialkyl acetoxyethanephosphonate reacted. 
It has been found that, using the arrangement described, the yields become 
significantly smaller on going from the laboratory scale to an industrial 
scale, so that, for economic reasons owing to the yield being too small, a 
certain batch size cannot be exceeded using the arrangement described. 
There was therefore a great need for a process in which scale-up to the 
industrial scale is readily possible and which makes it possible to obtain 
dialkyl vinylphosphonates in high yield. 
This object is achieved by a process for the continuous preparation of 
dialkyl vinylphosphonates using catalysts at temperatures of from 
150.degree. to 270.degree. C. by dissociation of dialkyl 
acetoxyethanephosphonates at a pressure of from 5 to 500 mbar in contact 
with a liquid, catalytically active medium while drawing off the dialkyl 
vinylphosphonates formed and other volatile reaction products as vapors, 
which comprises conveying the liquid medium in forced circulation via an 
evaporator while feeding in fresh dialkyl acetoxyethanephosphonate, if 
desired admixed with catalyst, corresponding to the distillation of 
dialkyl vinylphosphonates and other volatile compounds, and drawing off 
non-volatile material formed as byproduct from the liquid circuit to 
maintain constant conditions. 
Various embodiments are possible for carrying out the process. Thus, for 
example, the circulation of the catalyst can be carried out via a normal 
tube exchanger having an auxiliary vessel for maintaining the level and a 
vapor pipe, connected to the evaporator, fitted with condenser and vacuum 
connection. Other experimental arrangements are also possible. Thus, it 
has proven useful in many cases to carry out the circulation of the 
catalyst via the combination of a stirred vessel with a thin-film 
evaporator or downdraft evaporator. 
Suitable catalytically active media are the same ones as are also specified 
in DE-A 31 20 427 and EP 281 122, both acid and basic. Suitable acid media 
are, for example, sulfuric acid, phosphoric acid, halogen-containing 
carboxylic acids such as dichloroacetic and trichloroacetic acid and also 
trifluoroacetic acid, aromatic sulfonic acids such as benzenesulfonic and 
p-toluenesulfonic acid, vinylphosphonic acid, but in particular products 
which are obtained from the byproducts formed in the liquid phase in the 
present reaction, i.e. relatively high-boiling byproducts, by thermal 
treatment with water, the water treatment being able to be carried out, 
for example, by boiling for a period of from 5 minutes to 2 hours. Basic 
media which can be used are, for example, tertiary aliphatic and aromatic 
amines and phosphanes (in the past described as phosphines), as are 
likewise specified in a great number in DE-A 31 20 437. 
The vapors formed as reaction product comprise dialkyl vinylphosphonate 
formed, acetic acid formed and also unreacted dialkyl 
acetoxyethanephosphonate which, in accordance with its partial pressure, 
also vaporizes under the reaction conditions. The vapors are 
advantageously introduced into a distillation column in which acetic acid 
and dialkyl vinylphosphonate distill via the top of the column and the 
liquid phase runs back into the reaction system. If the distillation 
column has no stripper section having additional heating, this 
liquid-phase runback also contains dialkyl vinylphosphonate and acetic 
acid in addition to unreacted dialkyl acetoxyethanephosphonate. If the 
downstream distillation column is provided with a stripper section having 
additional liquid-phase heating, the liquid phase recirculated into the 
reaction system can be obtained virtually free of dialkyl 
vinylphosphonate. The circulation of catalyst can be conducted 
differently. It can be conveyed from the stirred reactor via the thin-film 
evaporator back to the stirred reactor cocurrently with the vapors, so 
that the vaporized dissociation products are introduced via the gas space 
of the reactor into the downstream column. However, it is more favorable 
to draw off the vaporized dissociation products in countercurrent from the 
top of the thin-film evaporator and subsequently to introduce them 
together with the vaporized material from the stirred reactor into the 
downstream distillation column. It is also possible to introduce these 
vapor streams into two separate distillation columns. The stirred reactor 
alone without the circulation of catalyst via the thin-film evaporator 
gives, as described, only poor yields. 
With circulation of catalyst via the thin-film evaporator alone without the 
stirred reactor, only very small throughputs are achieved.

The following examples illustrate the process without restricting it to 
them. 
COMATIVE EXAMPLES 
a) Example 1 (corresponds to Example 2 of EP 281 122) 
50 g of crude vinylphosphonic acid are placed in a 1 l stirred flask fitted 
with a drawing-off facility for the bottoms and a distillation column 
(internal diameter 25 mm, length 0.7 m, packed with 6 mm Raschig rings) 
attached onto it and having an automatic runback divider, distillation 
receiver, down-stream cold trap (cooling with dry ice) and connected 
vacuum pump. After heating to 210.degree. C. at a pressure of 10 mbar, a 
mixture of 95% by weight of dimethyl acetoxyethanephosphonate and 5% by 
weight of vinylphosphonic acid is metered in at a rate of about 140 g/h. 
After establishment of constant conditions, the level of the liquid phase 
in the reaction flask is kept constant by continually draining 
liquid-phase material into a vessel which is likewise evacuated. The 
reflux ratio in the column is set to 1. 
Over a period of 40 hours, 5500 g are introduced. This gives 3200 g of 
distillate, 1020 g of product from the cold trap and 1225 g of material 
which is drained from the bottom. 
The bottoms can, after boiling with water and distilling off the water, be 
again added to the starting mixture as catalyst for the dissociation. The 
distillate contains 89% by weight of dimethyl vinylphosphonate and 1.1% by 
weight of methyl acetate. The remainder is essentially acetic acid. 
The product obtained in the cold trap contains 6% by weight of dimethyl 
vinylphosphonate, about 4% by weight of methanol and 3% by weight of 
acetic acid; the remainder is essentially methyl acetate. Based on 
dimethyl acetoxyethanephosphonate used, the yield of dimethyl 
vinylphosphonate is 80%. 
b) Scale-up of stirred vessel 
The experimental arrangement is similar to that in comparative example a). 
The stirred vessel comprises a 60 l stirred reactor having an outside 
jacket. The outside jacket is operated using heat transfer oil in forced 
circulation for heating the reactor. 
A distillation column of glass is fitted on top of the reactor. The column 
is 2 m long and has an internal diameter of 225 mm. It is packed with 12 
mm glass spirals. The column has an automatic runback divider and a 
condenser operated using refrigerated brine at -10.degree. C., distillate 
receiver and vacuum connection. The vacuum equipment has, on the pressure 
side, a brine-operated condenser with separator. The stirred reactor is, 
in the bottom drainage connection, connected to a pump for pumping out 
bottoms. 
The stirred reactor is initially charged with 10 kg of vinylphosphonic acid 
as catalyst for the dissociation. The entire system is evacuated, pressure 
10 mbar. The stirred reactor is subsequently heated to 195.degree. C. and 
10 kg/h of dimethyl acetoxyethanephosphonate, containing 3% of 
vinylphosphonic acid as dissociation catalyst, are fed into the stirred 
vessel. After constant conditions are reached, the following conditions 
are set: Internal reactor temperature 195.degree. C., pressure 10 mbar. 
Reflux ratio of distillation column 1.5, temperature at top 67.degree. C. 
______________________________________ 
Bottoms drawn off: 3.87 kg/h comprising a 
multi-component mixture 
containing, inter alia, 
phosphoric acid, poly- 
phosphoric acids, 
monomethyl vinylphos- 
phonate, phosphonic 
anhydrides 
Distillate: 3.6 kg/h 
Dimethyl vinylphosphonate 
70% 
Acetic acid 27% 
Methyl acetate 3% 
Condensate on pressure side of vacuum 
2.43 kg/h 
equipment 
Methyl acetate 86.5% 
Dimethyl vinylphosphonate 
4% 
Acetic acid 7.4% 
Dimethyl ether 2% 
______________________________________ 
The yield of dimethyl vinylphosphonate, based on dimethyl 
acetoxyethanephosphonate used, is 40%. 
b.sub.1) Alteration of temperature: 
At the same feed amounts, there is virtually no change in amount and 
composition of the individual streams up to a temperature of 210.degree. 
C. in the stirred reactor. On lowering the temperature, the discharge at 
the bottom increases, strongly below 190.degree. C. 
b.sub.2) Throughput amount: 
Under the conditions described, an increase in the feed amount 
preferentially increases the outflow of bottoms without a significant 
increase in the dimethyl vinylphosphonate in the distillate. 
EXAMPLE 2 
The experimental arrangement is shown in FIG. 1. 
Procedure: From the stirred vessel (1) placed on a balance, the bottoms 
containing the dissociation catalyst are pumped in a circuit via a line 
(2) containing a pump (3) via the thin-film evaporator (4) having an 
outflow for liquid phase into the stirred reactor (1). Both the vapors 
from the thin-film evaporator (4) via line (5), and the vapors from the 
stirred reactor (1) via line (6) are introduced into the column (7), 
automatic runback divider (8) and condenser (9) operated with refrigerated 
brine. The distillate from column 7 flows into the receiver (10) and is 
continually pumped away via line (11) by means of the pump (12). The 
runback from column (7) flows back into the stirred reactor (1). Condenser 
(9) and receiver (10) are connected via line (13) with the vacuum 
equipment (14). The pressure side of the vacuum equipment (14) leads to 
the condenser (15) operated with refrigerated brine and having a liquid 
separator (16). Fresh dialkyl acetoxyethanephosphonate is drawn from 
reservoir (17) via line (18) into the catalyst circulation line (2). From 
the catalyst circuit, high-boiling material formed is drawn off by means 
of the pump (19) via line (20) to keep the level in the stirred reactor 
(1) constant. 
EXAMPLE 2a 
The stirred reactor (1) is initially charged with 10 kg of vinylphosphonic 
acid. After heating to 100.degree. C., circulation (amount 120 l/h) is 
started using the pump (3) and the temperature in the thin-film evaporator 
and stirred reactor (1) is taken up to 195.degree. C. The pressure in the 
system is adjusted to 10 mbar using the vacuum equipment (14). From vessel 
(17), 10 kg/h of dimethyl acetoxyethanephosphonate containing 3% of 
vinylphosphonic acid are conveyed via line (18) into line (2) to the 
thin-film evaporator. The reflux ratio in column (7) is set to 2. The 
liquid-phase contents in the stirred reactor (1) are kept constant using 
the pump (19). After establishment of static conditions, the following 
conditions are obtained. 
______________________________________ 
Pressure: from 10 to 12 mbar 
Temperatures: 
Reactor (1) 195.degree. C. 
Outflow from thin-film 
from 193 to 197.degree. C. 
evaporator (4) 
Temperature in the heat 
225.degree. C. 
transfer oil 
Temperature at top of column (7) 
from 68 to 70.degree. C. 
Reflux ratio of column (7) 
2 
Amounts: 
Distillate discharge via 
7 kg/h 
pump (12) 
Dimethyl vinylphosphonate 
73% 
Acetic acid 25% 
Methyl acetate and unknowns 
remainder 
Condensate, pressure side 
1.1 kg/h 
of vacuum equipment (14) 
Separator (16) 
Methyl acetate 77% 
Acetic acid 15% 
Dimethyl ether 5% 
Dimethyl vinylphosphonate 
3% 
Bottoms discharge pump (19) 
1.8 kg/h 
Vinylphosphonic acid 17% 
Monomethyl vinylphosphonate 
30% 
Phosphoric acid, polyphosphoric 
53% 
acids, phosphonic anhydrides 
______________________________________ 
The yield of dimethyl vinylphosphonate, based on dimethyl 
acetoxyethanephosphonate reacted, is 76%. 
EXAMPLE 2b 
Increase in amount: The feed amount of dimethyl acetoxyethanephosphonate 
can be increased with a corresponding increase in the temperature of the 
heat transfer oil. 
At an oil temperature of 240.degree. C., 195.degree. C. can be maintained 
in the stirred reactor (1) up to a feed rate of 25 kg/h of dimethyl 
acetoxyethanephosphonate (3% of vinylphosphonic acid). At a reflux ratio 
of 1.5 in column (7), the individual streams in comparison with Example 1 
increase corresponding to the increase in the feed of dimethyl 
acetoxyethanephosphonate. If the feed rate is further increased, constant 
conditions can no longer be established in the stirred reactor (1) and the 
thin-film evaporator (4), even by means of a further increase in the 
temperature of the heat transfer oil. 
EXAMPLE 3 
The experimental arrangement is shown in FIG. 2. The experimental 
arrangement is as in Example 1. However, the liquid-phase outlet of column 
(7) does not flow back into the stirred reactor (1), but is introduced 
into a second column (21). The inlet is in the middle, and the column is 
fitted with a circulation evaporator (22), a runback divider (23), a 
condenser (24), a receiver (25), and a vacuum connection to the vacuum 
equipment (14). 
To keep the liquid level of column (21) constant, bottoms are pumped back 
to reactor (1) using the pump (26). This arrangement allows the throughput 
in comparison with Example 1a to be increased to 33 kg/h of dimethyl 
acetoxyethanephosphonate feed. The static conditions which are established 
are: 
______________________________________ 
Pressure 10 mbar 
Temperatures 
Reactor (1) 195.degree. C. 
Outflow from thin-film 
from 193 to 198.degree. C. 
evaporator (4) 
Temperature at top of column (7) 
from 68 to 70.degree. C. 
Temperature at top of column (21) 
71.degree. C. 
Bottom temperature of 140.degree. C. 
column (21) 
Feed from vessel (17) 32 kg/h 
Dimethyl acetoxy-ethanephosphonate 
Vinylphosphonic acid 1 kg/h 
Reflux ratio column (7) 
1.5 
Reflux ratio column (21) 
1 
Distillate from column (7), 
18.1 kg/h 
discharge pump (12) 
Dimethyl vinylphosphonate 
66.2% 
Acetic acid 33.7% 
Methyl acetate, others 
0.1% 
Distillate from column (21) 
5.9 kg/h 
Dimethyl vinylphosphonate 
85% 
Acetic acid 15% 
Condensate on pressure side of 
3.7 kg/h 
vacuum equipment (14) 
Methyl acetate 82% 
Acetic acid 14% 
Dimethyl vinylphosphonate 
0.3% 
Dimethyl ether 0.1% 
Liquid-phase discharge from 
5.2 kg/h 
reactor (1), pump (19) 
Vinylphosphonic acid 19% 
Monomethyl vinylphosphonate 
27% 
Polymeric phosphonic acid and 
54% 
polyphosphoric acids 
Recirculation of bottoms from 
21 kg/h 
column (21) to reactor (1) 
Dimethyl acetoxyethane-phosphonate 
90% 
Liquid phase from reactor (1) 
10% 
entrained as mist 
______________________________________ 
The yield of dimethyl vinylphosphonate based on dimethyl 
acetoxyethanephosphonate used is 77%.