EP0973706A1

EP0973706A1 - Method for eliminating inhibitors of polymerisation of monomer mixtures using an optimised alumina

Info

Publication number: EP0973706A1
Application number: EP98902066A
Authority: EP
Inventors: Christophe Nedez
Original assignee: Rhodia Chimie SAS
Current assignee: Rhodia Chimie SAS
Priority date: 1997-01-22
Filing date: 1998-01-13
Publication date: 2000-01-26
Also published as: NO993557L; JP2000510477A; KR20000070330A; CN1248236A; WO1998032715A1; AU5870598A; TW394780B; NO993557D0; FR2758552A1; BR9806980A; US6288299B1; FR2758552B1

Abstract

The invention concerns a method for adsorbing inhibitors of polymerisation of ethylenically unsaturate monomers, which consists in contacting these inhibitors with an alumina, said alumina having a pore volume of diameter greater than 100 ANGSTROM by at least 0.20 ml/g, preferably by at least 0.25 ml/g, still more preferably by at least 0.30 ml/g, and a specific surface area of at least 30 m2/g, preferably of at least 60 m2/g, still more preferably of at least 80 m2/g.

Description

PROCESS FOR THE ELIMINATION OF INHIBITORS OF POLYMERIZATION OF MIXTURES OF MONOMERS USING AN OPTIMIZED ALUMIN

The present invention relates to a new process for eliminating polymerization inhibitors from mixtures of monomers, in particular ethylenically unsaturated monomers.

In the polymerization industry for a large number of ethylenic monomers, an important problem relates to the storage and / or transport of these monomers. Indeed, one can observe an uncontrolled spontaneous polymerization of these monomers over time from free radicals. These unstable ethylenic monomers are especially those having a second unsaturation such as a COOH function, C = O, C≡N, C = C, C≈S, C = N; it may, for example, be the following monomers: styrene, butadiene, isoprene, (meth) acrylic esters, acrylonitrile, acrolein, chloroprene, vinyl acetate, etc.

To avoid this degradation of the monomers, it is known to stabilize them by means of inhibiting substances which prevent polymerization from occurring.

These substances, more generally known by the term "polymerization inhibitors" can be chosen from: picric acid, nitroaromatics, quinone derivatives (hydroquinone, benzoquinone), naphthols, amines (p-phenylenediamine, phenothiazine), phosphites, p-methoxyphenol, p- tertiobutylcatechol, ...

When it is desired to use the inhibited monomers to polymerize them or to engage them in chemical reactions, it is often necessary to remove the polymerization inhibitors. Several means are used for this purpose:

- it is possible to add a large quantity of initiator to the reactor to combat the effect of the inhibitor, however, this technique is not suitable in all cases,

the temperature can be raised considerably to cause thermolysis of the inhibitor, but the monomer must have high thermal stability,

- The charge of monomers and inhibitor can be distilled, but the monomer must have good thermal stability, in addition such an operation is difficult to carry out on an industrial scale and the boiling point of the inhibitor is, in many cases, higher than that of the monomer, - the inhibitor can be eliminated by adding a dilute solution, for example sodium hydroxide, the charge of monomers then being washed with water in order to remove all traces of caustic compounds, however, the treatment of liquid effluents also poses industrial problems, - Finally, the inhibitor can be adsorbed using a compound such as alumina, silica gel, activated carbon, calcium oxide, aluminum silicate, talc, sulfates of calcium, magnesium sulphates, copper sulphates, magnesium silicate clays, a resin ... Adsorption is one of the most advantageous methods, because it does not have any of the disadvantages mentioned above. Among the adsorbents used, activated alumina is preferred.

The object of the present invention is to provide a new alumina for the adsorption of polymerization inhibitors of ethylenically unsaturated monomers having improved adsorption capacities compared to the aluminas of the prior art.

To this end, the invention relates to a method for absorbing inhibitors of polymerization of ethylenically unsaturated monomers, in which these inhibitors are brought into contact with an alumina, said alumina having a pore volume of diameter greater than 100 Å. at least 0.20 ml / g, preferably at least 0.25 ml / g, even more preferably at least 0.30 ml / g, and a specific surface of at least 30 m / g, of preferably at least 60 m ² / g, even more preferably at least 80 nf7g.

The volume of pores with a diameter greater than 100 Å represents the cumulative volume created by all pores larger than a diameter of 100 Å. These volumes are measured by the mercury penetration technique, in which Kelvin's law is applied. The specific surface indicated is an area measured by the BET method. The term surface measured by the BET method means the specific surface determined by nitrogen adsorption in accordance with standard ASTM D 3663-78 established on the basis of the BRUNAUER - EMMETT - TELLER method described in the periodical "The Journal of the American Society ", 6J2, 309 (1938). The combination of these two characteristics ensures high adsorption of the inhibitors of polymerization by alumina compared to the aluminas used in the prior art.

Alumina can, for example, be in the form of beads, extrudates or monoliths. The processes for the preparation of aluminas having the characteristics of pore volume and specific surface necessary for the implementation of the process according to the invention are known to those skilled in the art. With regard to the specific surface, this can in particular be controlled by the calcination (or activation) temperature of the aluminas following their shaping.

For the pore volume, its control is essentially due to the choice of the starting alumina used for shaping and to the operating conditions for shaping the alumina. Those skilled in the art know these conditions. Some examples are given below.

If the alumina used is in the form of beads, these beads can be obtained by shaping by rotating technology or by coagulation in drops (called oil-drop).

Shaping by rotary technology is an agglomeration of alumina produced by contacting and rotating the alumina on itself. As an apparatus used for this purpose, mention may be made of the rotating bezel, the rotating drum. This type of process makes it possible to obtain beads with controlled dimensions and pore distributions, these dimensions and these distributions being, in general, created during the agglomeration step.

The porosity can be created by different means such as the choice of the particle size of the alumina powder or the agglomeration of several alumina powders of different particle sizes. Another method consists in mixing with the alumina powder, before or during the agglomeration step, a compound, called pore-forming agent, disappearing completely by heating and thus creating a porosity in the beads.

As pore-forming compounds used, there may be mentioned, by way of example, wood flour, charcoal, sulfur, tars, plastics or plastic emulsions such as polyvinyl chloride, polyvinyl alcohols, mothballs or the like. The amount of pore-forming compounds added is determined by the desired pore volume.

The alumina powder used as starting material can be obtained by conventional methods such as the precipitation or gel method, and the method by rapid dehydration of an alumina hydroxide such as Bayer hydrate (hydrargillite).

The latter alumina is obtained in particular by rapid dehydration of hydrargillite using a stream of hot gases, the inlet temperature of the gases into the apparatus generally varying from approximately 400 to 1,200 ° C., the contact time of the alumina with the hot gases generally being between a fraction of a second and 4-5 seconds; such a process for the preparation of alumina powder has been particularly described in patent FR-A-1 108 011. The latter alumina is the preferred one of the invention. Control of the pore volumes of given diameter can also be carried out during this agglomeration step by an adequate adjustment of the rate of introduction of the alumina powder and possibly of water, of the rotation speed of the device or by the introduction of a shaping primer. Following this agglomeration, the beads obtained can be subjected to various operations intended to improve their mechanical resistance such as ripening by maintaining in an atmosphere at a controlled humidity rate, followed by calcination and then impregnation of the beads by a solution of one or more acids and a hydrothermal treatment in a confined atmosphere. Finally, the beads are dried and calcined so as to be activated.

Shaping by coagulation in drops consists in introducing drops of an aqueous solution based on an aluminum compound into a liquid immiscible with water (petroleum, kerosene, ...) in such a way that the drops form substantially spherical particles, these particles are coagulated simultaneously and / or after the spheroidal shaping by a gelling agent. The beads are then collected then dried and calcined.

This type of bead can for example be prepared, according to the process described in patent EP-A-097 539, by coagulation in drops of an aqueous suspension or dispersion of alumina or a solution of a salt basic aluminum in the form of an emulsion consisting of an organic phase, an aqueous phase and a surfactant or an emulsifier. Said organic phase can in particular be a hydrocarbon, the surfactant or emulsifier is for example Galoryl EM 10®.

These beads can also be prepared according to the process described in patent EP-A-015 801 by mixing at a pH below 7.5 an ultrafine boehmite sol and spheroidal alumina particles, then coagulation in drop of this mixing as indicated previously, and finally drying and calcination.

The alumina can also be in the form of alumina extrudates. These are generally obtained by mixing and then extruding an alumina-based material, and finally calcining. The starting material can be of very varied nature: it can result from the partial and rapid dehydration of hydrargillite, according to the teaching of the application FR-A-1.108.011, or from the precipitation of alumina boehmite, pseudo -boehmite, bayerite or a mixture of these aluminas. During mixing, the alumina can be mixed with additives, in particular porogens as defined above.

These extrudates can have all kinds of shapes: full or hollow cylinders, multilobes, ... In the process according to the invention, essentially an alumina is used which is in the form of beads resulting from shaping by rotary technology.

In general, aluminas with a particle size of between 0.8 and 10 mm, preferably between 1 and 5 mm, are used. The particle size corresponds in the case of shaping by coagulation in drops to the diameter of the beads and in the case of extrudates to the diameter of their cross section.

Preferably, the method according to the invention uses as an adsorbent an alumina comprising at least one compound of an element chosen from alkalis, rare earths and alkaline earths.

This compound can be an oxide, a hydroxide, a salt or a mixture thereof. There may be mentioned, by way of example, in addition to the hydroxides, the sulfates, nitrates, halides, acetates, formates, carbonates and the salts of carboxylic acids.

The elements chosen from sodium, potassium, lithium, lanthanum and cerium are preferably used.

The level of alkaline, alkaline-earth or rare earth element is generally at least 5 mmol per 100 g of alumina, preferably at most 400 mmol, even more preferably between 10 and 400 mmol.

According to a preferred variant, the alkaline element is sodium and its content is between 15 and 300 mmol per 100 g of alumina.

The deposition of the compound of the doping element on or in alumina can be carried out by any method known to those skilled in the art. It can be carried out, for example, by impregnation of the alumina already prepared with the alkaline elements, of rare earths or alkaline-earth or precursors of these elements, or by mixing of the alkaline elements, of rare earths or alkaline-earth or precursors with alumina during the shaping of these materials. These elements can also be introduced into alumina by coprecipitation of alumina and alkaline, rare earth or alkaline-earth elements or their precursors.

Preferably, the alumina used in the process according to the invention is prepared by:

- impregnation of the alumina with a solution of a compound of an element or of a mixture of compounds, - drying of said impregnated alumina,

- heat treatment of said alumina. The impregnation is carried out in a known manner by bringing the alumina into contact with a solution, a sol or a gel comprising at least one alkaline, rare earth or alkaline-earth element in the form of oxide or salt or one of their precursors.

The operation is generally carried out by soaking the alumina in a determined volume of solution of at least one precursor of an alkaline element, of rare earths or alkaline earths. By solution of a precursor of one of these elements is meant a solution of a salt or compound of the element, or at least one, of the alkaline, rare earth or alkaline-earth elements, these salts and compounds which may be thermally decomposable into oxides. The salt concentration of the solution is chosen according to the quantity of element to be deposited on the alumina.

According to a preferred mode, these elements are deposited by dry impregnation, that is to say that the impregnation is carried out with just the volume of solution necessary for said impregnation, without excess. The heat treatment is carried out at a temperature determined as a function either of the temperature of use of the alumina or of the specific surface desired. It may also be possible to carry out a heat treatment to obtain at least partial thermal degradation of the compound, for example in the form of oxide. However, this degradation is not compulsory, and by way of example, it is not necessary in particular when using compounds such as chlorides, nitrates or hydroxides.

The heat treatment can, for example, be carried out at a temperature between 150 and 1000 ° C, preferably between 300 and 800 ° C.

The adsorption process according to the invention is suitable when the polymerization inhibitor is chosen, for example, from: picric acid, nitroaromatics, quinone derivatives (hydroquinone, benzoquinone), naphthols, amines (p - phenylenediamine, phenothiazine), phosphites, p-methoxyphenol, p-tertiobutylcatechol. Particularly good results are obtained for the adsorption of p-tertiobutylcatechol.

When it comes to purifying an ethylenically unsaturated monomer stabilized by a polymerization inhibitor, the alumina is brought into contact with said mixture of ethylenically unsaturated monomer and inhibitor, for example at room temperature. The charge of monomers can be based on any type of ethylenically unsaturated monomer such as in particular: styrene, butadiene, isoprene, vinyl chloride, vinylidene chloride, tetrafluoroethylene, trifluorochloroethylene, chloroprene, allyl alcohol, vinyl ether, vinyl ester (acetate vinyl), acrylates and alkyl methacrylates (methacrylate, butylacrylate, ethylacrylate, 2-ethylhexylacrylate, methylmethacrylate, ethylmethacrylate, 2-ethylhexylmethacrylate, ...), acrolein, acrylonitrile, acrylamide, vinyl amine, ... or their mixtures.

In general, the mixture of ethylenically unsaturated monomer and inhibitor comprises 2 to 2000 ppm by weight per volume of inhibitor, preferably 5 to 1500 ppm.

The following examples illustrate the invention without, however, limiting its scope.

EXAMPLES

Example 1

The alumina samples tested are pretreated under a stream of nitrogen air at 300 ° C for 2 hours in order to remove any trace of moisture following their storage and in order to be able to compare their effectiveness under identical conditions.

1 g of alumina (dry extract) thus pretreated is introduced into 200 ml of a cyclohexane solution containing 500 ppm (w / v) of p-tertiobutylcatechol (TBC). The temperature is maintained at 25 ° C. The level of TBC present in the solution is analyzed over time by UV-visible. We deduce the adsorption rate of TBC.

The characteristics of the aluminas used are collated in Table 1.

Table 1

Alumina *: Alumina 1: Activated Alumina 1, 5/3 marketed by Procatalyse

Alumina 2: Activated Alumina 2/5 grade A marketed by Procatalyse Alumina 4: Spherite 512 marketed by Procatalyse Alumina 8: Spherite 569 marketed by Procatalyse

Example 2

The alumina samples tested are pretreated under a stream of nitrogen air to

300 ° C for 2 hours in order to eliminate all traces of humidity after their storage and in order to be able to compare their effectiveness under identical conditions.

1 g of alumina (dry extract) thus pretreated is introduced into 200 ml of a styrene solution containing 500 ppm (w / v) of p-tertiobutylcatechol (TBC). The temperature is maintained at 25 ° C. The level of TBC present in the solution is analyzed after 21 hours by UV-visible.

We deduce the adsorption rate of TBC.

The characteristics of the aluminas used are collated in Table 2.

Table 2

Example 3

The alumina 6 of Example 1 is treated with a potassium compound.

The impregnation is carried out dry using a solution of potassium carbonate. The impregnated alumina is then dried overnight at 130 ° C, then calcined at 470 ° C for 1 h 30. The alumina samples tested are pretreated under a stream of nitrogen air at 300 ° C for 2 hours in order to remove any trace of moisture after their storage and in order to be able to compare their effectiveness under identical conditions.

1 g of alumina (dry extract) thus pretreated is introduced into 200 ml of a cyclohexane solution containing 500 ppm (w / v) of p-tertiobutylcatechol (TBC). The temperature is maintained at 25 ° C. The level of TBC present in the solution is analyzed after 160 hours by UV-visible.

We deduce the adsorption rate of TBC.

The characteristics of the alumina are collected in Table 3.

Table 3

Claims

1. A method of absorbing inhibitors of polymerization of ethylenically unsaturated monomers, in which these inhibitors are brought into contact with an alumina, characterized in that said alumina has a pore volume of diameter greater than 100 Å of at least 0 , 20 ml / g, preferably at least 0.25 ml / g, even more preferably at least 0.30 ml / g, and a specific surface of at least 30 m / g, preferably at least 60 m ² / g, even more preferably at least 80 m ² / g.

2. Method according to claim 1, characterized in that the alumina is in the form of beads resulting from a shaping by rotary technology.

3. Method according to claim 2, characterized in that the balls have a particle size between 0.8 and 10 mm, preferably between 1 and 5 mm.

4. Method according to any one of the preceding claims, characterized in that the alumina comprises at least one compound of an element chosen from alkali, alkaline-earth or rare earths.

5. Method according to the preceding claim, characterized in that the alumina comprises at least one compound of one of the following elements: sodium, potassium, lithium, lanthanum and cerium.

6. Method according to claim 4 or 5, characterized in that the alumina comprises at least 5 mmol, preferably at most 400 mmol of at least one alkaline, alkaline-earth or rare earth element per 100 g of alumina .

7. Method according to one of the preceding claims, characterized in that the inhibitor is picric acid, a nitroaromatic, a quinone derivative (hydroquinone, benzoquinone), a naphthol, an amine, a phosphite, p- methoxyphenol or p-tertiobutylcatechol.

8. Method according to one of the preceding claims, characterized in that the alumina is brought into contact with a mixture of ethylenically unsaturated monomer and inhibitor.

9. Method according to the preceding claim, characterized in that the mixture of ethylenically unsaturated monomer and inhibitor comprises 2 to 2000 ppm by weight per volume of inhibitor, preferably 5 to 1500 ppm.