CS235607B1 - Method of 2,6-xylenol refining - Google Patents

Abstract

The invention relates to the refining of 2,6-xylenol from methylation products of phenol with at least one free ortho-position by reaction with paraformaldehyde or aqueous formaldehyde in the presence of catalysts. The refining is carried out continuously in a cascade of at least two stirred reactors, the first of which is kept below the boiling point of the reaction mixture, the last one at the boiling point of the reaction mixture. Pure 2,6-oxylenol is useful as an intermediate for organic synthesis and for the production of polymers

Description

(54) A process for refining 2,6-xylenol

The invention relates to the refining of 2,6-xylenol from phenol methylation products with at least one free ortho-position by reaction with paraformaldehyde or aqueous formaldehyde in the presence of catalysts. The refining is carried out continuously in a cascade of at least two stirred reactors, the first being kept below the boiling point of the reaction mixture, the last at the boiling point of the reaction mixture. Pure 2,6-xylenol is useful as an intermediate of organic syntheses and for the production of polymers.

«

The invention relates to the continuous chemical refining of 2,6-xylenol, which is an intermediate in organic syntheses and polymerizations, by reaction with paraformaldehyde or an aqueous solution of formaldehyde in the presence of catalysts.

The source of 2,6-xylenol is a phenol methylation reaction mixture containing predominantly methylphenols / o-, m- and p-cresols / di- and trimethylphenols. To a lesser extent, irrelevant from a quantitative point of view, also other phenol methylation products. Depending on the reaction conditions of the phenol methylation, the reaction mixture contains a different ratio of o-cresol and 2,6-xylenol, depending on whether the main methylation product is to be o-cresol or 2,6-xylenol. After o-cresol has been distilled off, 2,6-xylenol remains the major and most important component of the remainder of the reaction mixture. Of the other methyl derivatives of phenol, the o-, m- and β-cresols themselves form over 95% of impurities.

Until now, 2,6-xylenol has been isolated from this mixture and isolated by physical procedures such as rectification, crystallization, and crystallization. fractional crystallization or a combination thereof. All of these processes are characterized by being very energy intensive, time consuming and requiring complex and costly equipment.

It is known that technical 2,6-xylenol can be successfully removed from the above impurities by reaction with paraformaldehyde or aqueous formaldehyde in the presence of catalysts.

The most suitable catalysts are metal oxides (bivalent and polyvalent), salts of aliphatic carboxylic acids of bivalent and polyvalent metals. In the presence of these catalysts, paraformaldehyde or aqueous formaldehyde solution preferably reacts with compounds having at least one free ortho position relative to the phenolic OH group, such as unreacted phenol, and especially methylation products, especially cresols.

To isolate 2,6-xylenol, the reaction water or excess unreacted aqueous formaldehyde solution is distilled off from the reaction mixture and then 2,6-xylenol.

These processes are discontinuous and have all the disadvantages of discontinuous production.

It has now been found that technical 2,6-xylenol can be successfully refined in a continuous manner. The present invention provides a process for refining 2,6-xylenol which comprises as impurities phenol methylation products having at least one free ortho-position relative to the phenolic hydroxyl group by heating with paraformaldehyde or aqueous formaldehyde in the presence of aliphatic carboxylic acid salt catalysts of at least bivalent wherein the refining is carried out in a cascade of at least two stirred reactors, the first being kept at a temperature below the boiling point of the reaction mixture and the last at the boiling point of the reaction mixture. Separately molten 2,6-xylenol and paraformaldehyde or aqueous formaldehyde solution and catalysts are fed to the heated reactor cascade at a rate such that the mean residence time of the reactive batch in the cascade corresponds to the desired reaction time.

When an aqueous formaldehyde solution is used, the reaction mixture is a heterogeneous system of two sparingly soluble components, 2,6-xylenol in aqueous formaldehyde solution and aqueous formaldehyde solution in 2,6-xylenol. When paraformaldehyde is used, the reaction mixture is only a heterogeneous system until boiling. As the temperature of the reaction mixture rises, the gradual depolymerization of paraformaldehyde results in its gradual dissolution in the technical 2,6-xylenol, and it is already completely dissolved when the reaction mixture boils.

While the boiling point of the reaction mixture using an aqueous formaldehyde solution is 94 to 98 ° C, with paraformaldehyde it is 129 to 160 ° C, depending on the impurity content. In both cases, by the time the reaction mixture reaches the boiling point (about 2 hours), more than 2/3 of the impurities have reacted. As a result, in the first part of the stirred reactor cascade, the reaction takes place at temperatures lower than the boiling point of the reaction mixture. After boiling, the specific kinetic parameters decrease substantially (reaction rate and generation of reaction heat).

When paraformaldehyde is used after boiling the reaction mixture, the reaction water formed becomes slightly turbid. When using an aqueous formaldehyde solution, the reaction mixture is a heterogeneous system from the start of mixing the reaction components. From the penultimate stirred reactor cascade, the reaction mixture or slurry is passed through an overflow to the last unmixed reactor, in which the reaction water or unreacted aqueous formaldehyde solution is separated, the bottom of the reactor is discharged and 2,6-xylenol is removed overflow with reacted impurities. Net

The 2,6-xylenol is then isolated by distillation. The reacted impurities do not flow with 2,6-xylenol and remain in the distillation residue.

To better separate the aqueous and xylene phases in the last unmixed cascade reactor, it is appropriate to add to the penultimate stirred reactor an aromatic hydrocarbon or a mixture thereof, such as benzene, toluene, xylene and others. These hydrocarbons are soluble in 2,6-xylene and, after separation of the heavier aqueous phase, form a 2,6-xylenol solution from which they are isolated.

2,6-xylenol by distillation.

The continuous batch cascade process of the present invention has a number of advantages over an analogous batch process of chemical refining of technical 2,6-xylenol in a stirred batch reactor.

In essence, the productivity of the reactor unit increases. This effect is achieved by eliminating the so-called handling times of the batch reactor (emptying, charge loading, purging and purging), which are a waste time in terms of reactor utilization.

The yield and selectivity of the reaction has been virtually identical to the batch process since the four-member cascade, and the 2,6-xylenol obtained is identical to the product obtained in the batch process.

Continuous cascade refining allows for the specialization of individual equipment / installation, mixing equipment, collection of volatile components or their eventual return to the same or another cascade reactor / and specialization of each member's regime. D of the batch process, each reactor must have universal properties covering the full range of operating conditions from start to end of refining.

In contrast to the batch process, each reactor operates cascade in a stationary mode and with a constant feed. The problem of temperature control is simplified, that is, keeping the required temperature regime of each reactor. Operational capacities save labor for operating continual process equipment.

The 2,6-xylenol chemical refining reactor cascade of the present invention may consist of two to six members, preferably of the same size. In each cascade reactor member, there are overflow connecting tubes. All members are equipped at the top (above the level of the reaction mixture) with a reflux condenser (for returning the volatiles) or a device for collecting them together.

It is advantageous to use shell-shaped, cylindrical, vertical bottom-mounted reactors equipped with stirrers and steam or electrically heated. Stirring must be effective, especially when using an aqueous formaldehyde solution where the reaction mixture is an emulsion system. High speed stirrers with an axial outlet flow directed to the bottom of the vessel are most suitable, for example propellers with a constant helix pitch or six- or possibly three-blade blades with straight, obliquely inclined blades, in all cases with stops on the internal walls of the reactors.

Stirring must ensure heat and mass transfer and homogenization of temperature and concentration according to the batch, and must also keep the emulsion floating and prevent the reaction mixture from being separated into a water (heavier) and 2,6-xylenol phase.

The last member of the at least three-membered cascade need not be equipped with a stirrer and serves as a separator, separating heavier reaction water or unreacted aqueous formaldehyde solution from 2,6-xylenol with reacted impurities. The reaction water or unreacted formaldehyde solution and 2,6-xylenol overflow with the reacted impurities, or their solutions in aromatic hydrocarbons, are removed via the lower effluent to recover 2,6-xylenol by distillation.

The cascade of reactors is connected to a system of dosing pumps or preferably to a multi-component dosing machine. Separately led to the first cascade member:

a) molten 2,6-xylenol, b) paraformaldehyde powder or aqueous formaldehyde solution, c) powdered catalyst salt, aliphatic carboxylic acids of divalent and polyvalent metals or divalent and polyvalent metal oxide.

It is naturally possible to lead the above components together (from the stirred tank) to the first cascade member.

d) an aromatic hydrocarbon, or a mixture thereof into the penultimate reactor (prior to separation).

The streams a /, b / and c) must match the molar and weight ratios of the active ingredients of the refining process and the resulting concentration of these components in the reaction mixture. The ratio of system stationary retention (when the mixing mode is set in each member) to total feed rate (streams a / and b) gives a mean retention time that, together with the concentration ratios of the active reaction components and the reaction temperature, determines the resulting purity of the reaction mixture.

The invention is illustrated by the following examples (% by weight).

Example 1

In a five-member cascade of stirred reactors (except the last one) which comply with the bases in the general section, molten technical 2,6-xylenol, aqueous formaldehyde solution and zinc stearate are metered in such that the final concentration in the reaction mixture is: 1,000 g 2,6- xylenol (containing 4.63% impurities), 263 g of a 37% aqueous solution of formaldehyde and 10 g of zinc stearate, and a dosing rate such that the mean residence time was 2 hours. The temperature in the first cascade reactor is 80 ° C, in the second, third and fourth reactions the reaction mixture is kept boiling. After isolation by distillation, 2,6-xylenol is obtained in a yield of 90 to 91% (for the industrial 2,6-xylenol used) with an impurity content of 0.2%.

Example 2

The six-member cascade of the stirred reactors (except the last one) are fed molten

2,6-xylenol, paraformaldehyde and zinc stearate such that the final concentration in the reaction mixture was: 1,000 g of technical 2,6-xylenol (containing 4.63% impurities), 22.5 g of paraformaldehyde and 10 g of zinc stearate and 900 g of toluene fed to the fourth outlet and a feed rate such that the mean residence time in each of the first four reactors was 2 hours. The temperature in the first cascade reactor is 90 ° C, in the second to fourth the reaction mixture is kept boiling. After isolation by distillation, 91-93% of 2,6-xylenol (for the industrial 2,6-xylenol used) containing 0.1% impurities, or a toluene solution thereof, is obtained.

Claims (1)

SUBJECT OF THE INVENTION Process for refining 2,6-xylenol, containing as impurities phenol methylation products with at least one free ortho position relative to the phenolic hydroxyl group, by heating with paraformaldehyde or an aqueous formaldehyde solution in the presence of aliphatic carboxylic acid salt catalysts of at least divalent metals, The process according to claim 1, wherein the refining is carried out continuously in a cascade of at least two stirred reactors, the first being kept at a temperature below the boiling point of the reaction mixture and the last at the boiling point of the reaction mixture.