FI85137B - FOERFARANDE FOER FRAMSTAELLNING AV N-VINYLFORMAMID. - Google Patents

Abstract

1. A process for the preparation of N-vinylformamide of the formula CH2 =CH-NH-CHO (I) by pyrolysis of formylalaninenitrile of the formula see diagramm : EP0184074,P6,F1 in the presence of a solid as a catalyst under reduced pressure, at from 250 to 650 degrees C and with cooling of the pyrolysis products, which essentially consist of I and hydrogen cyanide, likewise under reduced pressure and with isolation of I, wherein the pyrolysis products are cooled to a temperature of from 200 to -10 degree C under from 10 to 150 mbar so that substantial fractionation into condensed I and gaseous hydrogen cyanide takes place, I is isolated from the condensate, and the gaseous hydrogen cyanide which has not been separated off is then subjected to chemisorption with about an equimolar amount of acetaldehyde at from -20 to +30 degrees C under from 5 to 140 mbar with formation of lactonitrile, and the latter is isolated after the pressure has been brought to atmosphère pressure.

Description

1 85137

Process for the preparation of N-vinylformamide

DE-PS 12 24 304 discloses, inter alia, a process for the preparation of N-vinylformamide of the formula 5 ch2 = ch-nh-cho (I), in which CHO-CH-NH-CHO (II)

3 I CN

The formylalanine nitrile according to claim 1 is pyrolyzed in the presence of solids as a catalyst under reduced pressure at temperatures preferably between 450 and 650 ° C. In pyrolysis, compound II cleaves to form hydrogen cyanide. The pyrolysis products, as well as unreacted formylalanine nitrile, are condensed under reduced pressure and, after equilibration to atmospheric pressure, fractional distillation is carried out. Hydrogen cyanide, obtained as the first fraction in fractional distillation, is used to prepare a new starting material for pyrolysis. However, such a process, which can also be carried out continuously, accumulates significant amounts of hydrogen cyanide, which must be stored and handled in accordance with strict safety measures.

Formylalanine nitrile (II) can be prepared, for example, by reacting alpha-aminopropionitrile with methyl formic acid or by the method of US-PS 3,822,306 in the reaction of acetaldehyde cyanohydrin with formamide with acids as a catalyst.

It is an object of the invention to provide a process for the preparation of 35 N-vinylformamide which does not present the problem of storing and handling quantities of 2 85137 hydrogen cyanides.

The object is solved according to the invention by a process for the preparation of N-vinylformamide of the formula CH2 = CH-NH-CHO (I) by pyrolysis of the CH2-CH-NH-CHO of formula 10

ύ I CN

formylalanine nitrile in the presence of solids as a catalyst under reduced pressure, at temperatures of 250 to 650 ° C and by cooling the pyrolysis products consisting essentially of compound (I) and hydrogen cyanide, as well as under reduced pressure and isolating (I) by cooling the pyrolysis products at 10 to 150 ° C. mbar to a temperature between 200 and 20-10 ° C so that large-scale fractional separation of the condensed compound (I) and gaseous hydrogen cyanide occurs, (I) is separated from the condensate by fractional distillation and the inseparable cyanogen hydrogen is subsequently subjected to a pressure of 5 to 140 m at temperatures of -20 to + 30 ° in a known manner with the formation of lactic acid nitrile and isolating it at atmospheric pressure after equilibration, whereby the lactic acid nitrile can, if desired, be converted into a compound of formula II by reacting it with with formamide in the presence of acids as a catalyst and by pyrolysis.

Pyrolysis of compound (II) takes place on solids. For this purpose, the solids given in DE-PS 12 24 304, sa-35 as well as alkali and alkaline earth carbonates such as sodium carbonate, potassium carbonate, magnesium carbonate, strontium carbonate, barium carbonate, marble, dolomite can be used. , chalk and magnesite. In addition, magnesium oxide, calcium oxide and barium oxide are also suitable.

Catalysts containing alkali and / or alkaline earth carbonates with alpha alumina as support have proved to be particularly advantageous. Preferably, calcium carbonate, dolomite or mixtures of calcium carbonate and magnesium carbonate with alpha alumina are used. Such catalysts are prepared by impregnating alpha alumina with water-soluble salts, e.g., calcium acetate, and converting to the corresponding carbonates or oxides by heat treatment.

The pyrolysis is carried out at temperatures of 250 to 650, preferably 300 to 550 "and at a pressure of 10 to 150, preferably 10 to 50 mbar.

Pyrolysis cleaves hydrogen cyanide from a compound of formula (11) to form N-vinylformamide (I). The pyrolysis products consist essentially of compound (I) and hydrogen cyanide, but still contain small amounts of unreacted (11) and small amounts of impurities.

It has proved particularly advantageous to carry out the pyrolysis of compound (II) in the presence of 0.1 to 2 moles of formamide, based on one mole of the compound of formula (II). Formamide then fulfills several functions. It acts as a diluent in pyrolysis and here leads to an extension of the service life of the catalysts. Moreover, in particular, obtaining pure N-vinylformamide by fractional distillation is facilitated in the presence of formamide.

Depending on the choice of catalyst, formamide decomposes partially to hydrogen cyanide and water in pyrolysis. Further formation of hydrogen cyanide from formamide is desired so that a sufficient amount of 4,85,137 hydrogen cyanides for the preparation of the starting compound (II) is available for the recirculation process. If different catalysts are used in the pyrolysis, the reaction can be controlled by first pyrolyzing the compound of formula (II) and then passing the reaction mixture through a contact which preferably cleaves the formamide. Contacts which preferably cleave formamide are known, for example, from DE-PS 12 09 561. These are mainly iron-zinc oxides. Pyrolysis can be carried out through different crushings of different catalyst types of catalysts as well as through different temperature profiles in the pyrolysis reactor.

The reaction products leaving the pyrolysis reactor are cooled in the pressure range from 10 to 150 mbar to a temperature in the range from 200 to -10, preferably from 60 to -10 ° C, so that a large-scale fractional separation of condensed N-vinylformamide and gaseous hydrogen cyanide takes place. If the pyrolysis is carried out in the presence of formamide, the uncleaved formamide is condensed together with N-vinyl-20-formoform and optionally unchanged compound (II). In addition, if not all of the formylalanine nitrile (II) is pyrolyzed, the condensed N-vinyl liformamide contains the unreacted compound (II). Separation of N-vinylformamide from compound (II) or formamide by distillation does not present any difficulty. The cooling of the pyrolysis gases is preferably carried out in two or more separate coolers, each with separators. For example, the pyrolysis gas is cooled to 60 ° C in the first heat exchanger, to 20 ° C in the second, and to -10 ° C in the next third condenser, whereby the condensates from the individual heat exchangers are collected separately. Such fractional separation of pyrolysis gas is achieved in essential components, namely condensed N-vinylformamide and gaseous

II

5 85137 in hydrogen cyanide. The separation of hydrogen cyanide by cooling, which is carried out in known methods, is omitted from the process according to the invention.

N-vinylformamide can be separated from the condensates by fractional distillation, so that small amounts of cyanic acid dissolved in condensed N-vinylformamide can be reused.

The hydrogen cyanide from the pyrolysis is preferably subjected to chemisorption with acetaldehyde at temperatures of -20 to +30, preferably -5 to + 15 ° C, after condensation of the high-boiling components at a pressure of 5 to 140, preferably 5 to 45 mbar. This forms lactic acid nitrile which can be associated with formamide as a solvent. Either after the chemisorption, formamide is added or, preferably, the chemisorption of hydrogen cyanide is carried out in the presence of formamide as a solvent. By adding small amounts of base, preferably tert. amines such as triethylamine are controlled by the basicity of the kernisoprtiol solution so that the reaction between acetaldehyde and hydrogen cyanide proceeds extremely rapidly. The amount of base is between 0.005 and 0.02 moles per mole of hydrogen cyanide.

Surprisingly, it can be seen that reacting substances with such high vapor pressures, such as hydrogen cyanide (b.p. 27 ° C at normal pressure) and acetal dehyde (bp at 21 ° C normal pressure), can be reacted practically quantitatively at reduced pressures up to 5 mbar. . The addition of acetaldehyde can take place via the silver electrode potential 30 and is adjusted so that the hydrogen cyanide content of the formaldehyde solution is preferably between 0.05 and 0.1% by weight. The amine addition can be controlled via the glass electrode potential. However, vacuum-controlled amine addition is also possible because at a lower 35 tert. in the amine concentration in the formamide solution, the reaction rate of cyanic acid and 6 85137 acetaldehyde decreases And therefore the pressure would increase so that the vacuum could break.

The chemisorption of hydrogen cyanide can be carried out, for example, in a column which can always be cooled or heated as required. Preferably, a solution containing formamide, acetaldehyde and a tertiary base is poured onto the end of the column, and hydrogen cyanide is also introduced into the column from this point. A formamide solution of lactic acid nitrile then accumulates in the separator, from which the lactic acid-10 nitrile can be isolated at atmospheric pressure after equilibration. In a continuous run, the formamide solution containing lactic acid nitrile is recycled to which formamide, acetaldehyde and tert are fed before chemisorption of hydrogen cyanide. amine and, after chemisorption, the lactic acid nitrile and formamide are removed. The chemisorption cycle is preferably carried out using such amounts of formamide as to obtain a solution of lactic acid nitrile in formamide in a molar ratio of 1: 1.5 to 2, preferably 1: 1.8. This solution can be condensed according to method 20 of US-PS 3,822,306 to give the starting product (II), which can then be subjected to pyrolysis to prepare compound (I).

A particularly interesting aspect of this method can be seen in the fact that the hourly amounts of hydrogen cyanide present in the technical plant - at a charge of 10 kmol / hour this means 270 kg of hydrogen cyanide - react in fractions of seconds to produce a immediately recoverable solution of lactic acid nitrile in formamide. percent by weight. The method according to the invention thus represents a considerable step forward for safety-related areas.

7 85137

Example 1 Fractional separation of N-vinylformamide and hydrogen cyanide

The pyrolysis reactor consists of a vertical 5 precious metal tube with a diameter of 40 mm and a length of 650 mm. The reactor can be heated by three independent heating zones. A thin film evaporator is connected in front of the reactor, the outlet of the reactor is connected to three condensers in which the decomposition gases are gradually cooled, namely in the first condenser to 50 ° C, in the second to -5 ° C and in the third to -80 ° C. (Fractional cooling of the decomposition gas to 50 and -5 ° C is an essential feature of the present invention.) For the experiments a) to d), the noble metal tube is filled with the catalysts given in Table I in each case. The apparatus is then degassed so that a pressure of 30 mbar is maintained in the thin-film evaporator. 10 N / l of carbon monoxide per hour are passed through the apparatus and the catalyst is heated to 380-420 ° C.

Over a period of 12 hours, 331 g (3.38 moles) of compound (II) and 134 g (2.98 moles) of formamide are then evaporated every hour. The vapors are passed through catalyst catalyst layers to the above temperature range and then condensed in condensers to 50, -5 or -80 ° C to -25. Fractions 1 and 2 contain N-vinylformamide, while condensate 3 (obtained by cooling to -80 ° C) consists of almost pure hydrogen cyanide. The reaction batches of the compound of formula (II) and formamide or the yields of formamide and formic acid are given in the table.

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Example 2

The pyrolysis is carried out in a precious metal tube with a diameter of 53 mm and a length of 1000 m, which is filled with the catalyst E described in Table 1. A thin-film evaporator is connected in front of the pyrolysis tube 5. The pyrolysis gases are cooled in two condensers, namely in the 1st condenser to 50 ° C and in the 2nd condenser to -5 ° C. The non-condensed components, mainly hydrogen cyanide, are fed to a Kemi-sorption circulating reactor. This device consists of a filler-10 bar with a separator. About 5 kg of formamide is pumped into the circulation through a nozzle at the end of a column connected to a pyrolysis device. In order to reach a constant state, formamide, acetaldehyde and triethylamine are additionally introduced into the recycled formamide stream as a catalyst. Hydrogen cyanide is then chemisorbed in the packed column. Inert gases formed as by-products are discharged from the end of the separator by means of a vacuum pump. Maintain a pressure of 20 mbar. The temperature of the circulating chemisorption solution is 5 ° C. In addition, a condenser is built into the circuit between the separator 20 and the circulation pump, by means of which the formamide solution introduced into the circuit is kept at a temperature of 5 ° C.

The apparatus described above is used continuously while evaporating 1478 g of formylalanine nitrile (II) per hour in a thin-film evaporator and passing over catalyst E 25, the upper third of which is heated to 400 and the remainder to 420 ° C. Both condensers separate 74 g of unreacted (II) and a small amount of by-products such as formamide, hydrogen cyanide and lactic acid nitrile per hour, together with 895 g of N-vinylformamide, which is removed from the condensers and subjected to fractional distillation. The yield of N - vinylformamide is 88%, calculated on the reacted compound.

The non-condensable hydrogen cyanide at the end of the packing column is contacted with a formamide solution which is sprayed through a nozzle onto the packing column and to which 594 g of acetaldehyde, 1094 g of formamide and 10 g of triethylamine are continuously fed from the nozzle in each hour. At the same time, 2063 g of a reaction solution containing 46.2% by weight of lactic acid nitrile dissolved in formamide in addition to very small amounts of triethylamine and water are taken per hour at the bottom of the separator. The solution of lactic acid nitrile in the formamide taken from the cycle contains less than 0.1% by weight of free cyanic acid. Formic acid 10 is added to stabilize it and can be condensed to compound II and subsequently pyrolyzed according to the invention.

I;

Claims (6)

11 85137 A process for the preparation of N-vinylformamide of formula 5 ch2 = ch-nh-cho (I) by pyrolysis of formylalanine nitrile of formula 10 CH2-CH-NH-CHO (II) 3 | CN solids in the presence of a catalyst under reduced pressure, at temperatures between 250 and 650 ° C and 15 by cooling the pyrolysis products consisting essentially of compound I and hydrogen cyanide, as well as under reduced pressure and isolation I, characterized in that the pyrolysis products are cooled to a temperature between 10 and 150 mbar. - -10 ° C so that large-scale fractional separation of condensed compound I and gaseous hydrogen cyanide occurs, I is isolated from the condensate by fractional distillation and the inseparable hydrogen cyanide is subsequently subjected to a molar ratio of 5 to 140 mbar at about -20 ° C. in a known manner with the formation of lactic acid nitrile and isolating it at atmospheric pressure after equilibration, whereby, if desired, the lactic acid nitrile can be converted into a compound of formula II by reacting it in an emotional manner; with formamide in the presence of acids as a catalyst and by pyrolysis. Process according to Claim 1, characterized in that the pyrolysis is carried out at a pressure of between 10 and 50 mbar and at a temperature of between 35 300 and 550 ° C and the chemisorption of hydrogen cyanide at a pressure of between 5 and 45 l in the presence of base amounts. Process according to Claim 1 or 2, characterized in that II is pyrolyzed in the presence of 0.1 to 5 moles of formamide, based on one mole of II. Process according to Claims 1 to 3, characterized in that alpha-alumina is used as support for the catalysts. Process according to Claims 1 to 4, characterized in that the chemisorption of hydrogen cyanide is carried out in the presence of formamide, the subsequent mixture of lactic acid nitrile and formamide being reacted in a known manner to give the product 15 II in the presence of acids as catalyst. Process according to Claim 5, characterized in that the pyrolysis of compound II, the fractional separation of compound I and hydrogen cyanide, the chemisorption of hydrogen cyanide, the reaction of lactic acid nitrile and formamide into compound II are carried out continuously and the compound II thus obtained is subjected to pyrolysis. li 13 851 37