HK1168341A - Process for preparing polyol esters - Google Patents

Description

Process for preparing polyol esters

The invention relates to a method for producing polyol esters from polyols and linear or branched aliphatic monocarboxylic acids having 3 to 20 carbon atoms, wherein the starting compounds are converted in the presence of a Lewis acid containing at least one element of groups 4 to 14 of the periodic Table of the elements as catalyst.

Esters of polyols, also known as polyol esters, can be used in industry on a large scale for a variety of purposes, for example as plasticizers or lubricants. The selection of suitable raw materials allows control of physical properties such as boiling point or viscosity, and takes into account chemical properties such as hydrolysis resistance or stability to oxidative degradation. Polyol esters can also be designed to address specific performance issues. For a detailed review of the polyol ester applications, see, for example, Ullmann's Encyclopaedia of Industrial Chemistry, 5 th edition, 1985, VCHVerlagsgesellschaft, Vol.A 1, p.305-319; 1990, volume A15, page 438-; 1981, Vol.14, pp.496-498.

The use of polyol esters as lubricants is of great industrial importance, in particular for those applications in which the mineral oil-based lubricants do not fully meet the requirements set. Polyol esters are particularly useful as turbine engine oils and instrument oils. Polyol esters for lubricant applications are generally based on 1, 3-propanediol, 1, 3-butanediol, 1, 4-butanediol, 1, 2-hexanediol, 1, 6-hexanediol, neopentyl glycol, trimethylolpropane, pentaerythritol, 2, 4-trimethylpentane-1, 3-diol, glycerol or 3(4), 8(9) -dihydroxymethyltricyclo [5.2.1.02, 6] decane (also known as TCD alcohol DM) as alcohol component.

Polyol esters are also used to a considerable extent as plasticizers. Plasticizers have many uses in plastics, coatings, sealants and rubber products. Preferably, they interact physically with high molecular weight thermoplastics without undergoing chemical reactions, by virtue of their swelling and dissolving capabilities. This results in a homogeneous system, the thermoplastic range of which is shifted to lower temperatures than the original polymer, one result being that its mechanical properties, such as increased deformability, elasticity and strength, and reduced hardness, are optimized.

In order to develop the widest possible field of application as plasticizers, they must meet a series of criteria. They should ideally be odorless, colorless, and resistant to light, cold, and heat. Furthermore, they are expected to be insensitive to water, relatively non-flammable and not very volatile, and not harmful to health. Furthermore, the production of the plasticizer should be simple and, in order to comply with ecological requirements, waste products, such as by-products which cannot be further utilized and waste water containing contaminants, should be avoided.

One particular class of polyol esters (which are referred to simply as G-esters) contain glycols or ether glycols as the alcohol component, such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 1, 2-propanediol, or higher propylene glycols. They can be prepared in different ways. In addition to the reaction of the alcohol and the acid, optionally in the presence of an acidic catalyst, other methods are used in practice to obtain the G ester, including the reaction of a diol with an acid halide, the transesterification of a carboxylic ester with a diol, and the addition of ethylene oxide to a carboxylic acid (ethoxylation). In industrial production, only the direct reaction of a diol with a carboxylic acid and the ethoxylation of a carboxylic acid have been identified as production methods, and the esterification of a diol and an acid is generally preferred. This is because it is possible to carry out the process with conventional chemical apparatuses without particular complexity and to provide a chemically homogeneous product. In contrast, ethoxylation requires bulky and expensive technical equipment. Ethylene oxide is a very reactive chemical. Which can polymerize explosively and form an explosive mixture with air over a very wide mixing range. Ethylene oxide stimulates the eyes and respiratory tract, causes chemical burns and causes liver and kidney damage, and is carcinogenic. Its operation therefore requires a great deal of safety measures. Furthermore, strict cleanliness of the storage and reaction devices must be ensured to avoid the formation of unwanted impurities due to side reactions of ethylene oxide with foreign matter. Finally, the selectivity of the reaction with ethylene oxide is not very high, since it produces a mixture of compounds of different chain length.

The direct esterification of alcohols with carboxylic acids is one of the basic operations in organic chemistry. In order to increase the reaction rate, the conversion is usually carried out in the presence of a catalyst. The use of an excess of one of the reactants and/or the removal of the water formed during the reaction ensures that the equilibrium is shifted towards the side of the reaction product (i.e. the ester) according to the law of mass action, which means that high yields are achieved.

Comprehensive information on the preparation of polyol Esters (also including Esters of ethylene glycol and Fatty Acids) and on the properties of selected representatives of these classes of compounds can be found in Goldsmith, "polyol Esters of Fatty Acids" (polyhydrodic Alcohol Esters of Fatty Acids), chem. Rev.33, p.257 (1943). For example, esters of diethylene glycol, triethylene glycol and polyethylene glycol are prepared at a temperature of 130 ℃ and 230 ℃ over a reaction time of 2.5 to 8 hours. To remove the reaction water, carbon dioxide was used. Suitable catalysts for the esterification of polyols mentioned are mineral acids, acid salts, organic sulfonic acids, acetyl chloride, metals or amphoteric metal oxides. The water of reaction is removed by means of an entrainer, for example toluene or xylene, or by introducing an inert gas, for example carbon dioxide or nitrogen.

Johnson (ed), "Fatty Acids in the Industry" (Fatty Acids in Industry) (1989) chapter 9, "Polyoxyethylene Esters of Fatty Acids" (Polyoxydehtylene Esters of Fatty Acids) discusses the production and properties of Fatty acid Esters of polyethylene glycol and gives a series of preparation lines. Higher diester concentrations are achieved by increasing the carboxylic acid/diol molar ratio. Suitable measures for removing the water of reaction are azeotropic distillation in the presence of a water-immiscible solvent, heating while passing an inert gas, or carrying out the reaction under reduced pressure in the presence of a drying agent. When the addition of the catalyst is omitted, a longer reaction time and a higher reaction temperature are required. The use of a catalyst allows both reaction conditions to be milder. In addition to sulfuric acid, organic acids such as p-toluenesulfonic acid and polystyrene-type cation exchangers are preferred catalysts. The use of metal powders, such as tin or iron, is also described. According to the teaching from US 2,628,249, the color problem in the case of catalysis with sulfuric acid or sulfonic acid can be alleviated when operating in the presence of activated carbon.

Other metal catalysts for the preparation of polyol esters are also titanium, zirconium or tin alkoxylates, carboxylates or chelates, see for example US 5,324,853a 1. These metal catalysts can be considered high temperature catalysts because they can only reach their full activity at high esterification temperatures, typically above 180 ℃. They are usually not added at the beginning of the esterification reaction, but after the reaction mixture has been heated and has partly reacted with elimination of water. Despite the higher reaction temperatures and longer reaction times required compared to conventional sulfuric acid catalysis, crude esters having lower color numbers are obtained with catalysis using these metal compounds. Conventional esterification catalysts are, for example, tetra (isopropyl) orthotitanate, tetra (butyl) zirconate or tin (II) 2-ethylhexanoate.

The catalytic esterification of polyols with carboxylic acids achieves high conversions in a short time based on the insufficiently added components, but requires longer reaction times for the remaining conversion of the desired polyol ester. Although polyol esters having an acceptable residual content of partially esterified products, expressed by the hydroxyl number in mg KOH/g (DIN 53240) or by the content of partially esterified products as determined by gas chromatography, are obtained, long reaction times are economically disadvantageous because of the limited performance of industrial production plants. In addition, US 5,324,853a1 suggests thorough mixing of the reaction mixture in order to promote the remaining conversion.

After the end of the esterification reaction, it is necessary to ensure adequate removal of the metal catalyst, since traces of metal in the purified polyol ester impair its use as plasticizer or lubricant, for example by affecting the electrical conductivity or the stability to atmospheric oxygen. According to the method of US 5,324,853a1, the crude esterification reaction mixture is mixed with an aqueous solution of sodium carbonate and optionally with activated carbon. This procedure hydrolyzes the metal compounds into insoluble solids, which can be filtered out before further work-up of the crude ester compound. According to 4,304,925A1, the crude esterification product is mixed with water and treated under thermal conditions before the base is added. This converts the hydrolysed metal compounds into filterable precipitates.

The prior art of preparing polyol esters under metal catalysis requires either a specific reactor design to complete the esterification reaction in an economically viable time or requires additional treatment with water under thermal conditions to substantially completely remove the metal catalyst after the esterification reaction is complete and form a hydrolysate that can be filtered off.

It is therefore an object of the present invention to improve and optimize the known process by adjusting and simplifying the successive component steps of the overall process and to simplify the process for producing high quality polyol esters so that the polyol esters can have the largest possible range of applications.

The invention therefore relates to a process for preparing polyol esters by reacting polyols with straight-chain or branched aliphatic monocarboxylic acids having 3 to 20 carbon atoms, characterized in that a mixture of starting compounds is reacted in the presence of a Lewis acid containing at least one element of groups 4 to 14 of the periodic Table of the elements as catalyst and the water formed is removed, followed by a steam treatment.

The reaction between the polyol and the aliphatic monocarboxylic acid starting compound is started in the range of about 120-180 ℃ depending on the starting materials and can subsequently be carried out to completion in different ways.

In one variant of the process according to the invention, the mixture is first heated from room temperature to a temperature of at most 280 ℃ and preferably at most 250 ℃ and, with the temperature remaining constant, the pressure is reduced in stages starting from the standard pressure in order to facilitate the removal of the reaction water. The pressure stage, whether one stage, two stages or the selection of more than two stages and the selection of the pressure to be established at a particular stage may vary within wide limits and match specific conditions. For example, in the first stage, the pressure may be reduced from the standard pressure to 600hPa, and then the reaction may be carried out to completion at a pressure of 300 hPa. These pressure values are guiding values and can be suitably observed.

In addition to the pressure change, it is likewise possible to change the temperature in one, two or more stages from room temperature during the esterification reaction, in order to raise the temperature in stages at constant pressure, generally to a maximum temperature of 280 ℃. However, it was found to be appropriate to heat up to 280 ℃ with a stepwise increase in temperature and to reduce the pressure stepwise. For example, the esterification reaction can be carried out in the first stage at a temperature starting from room temperature up to 190 ℃. Reduced pressure down to 600hPa was also applied to accelerate the expulsion of the reaction water. When the temperature stage of 190 ℃ is reached, the pressure is again reduced to 300hPa and the esterification is carried out to completion at a temperature of at most 250 ℃. These temperature and pressure values are indicative values and can be suitably observed. The temperature and pressure conditions to be established in a particular stage, the number of stages and the specific rate of temperature rise or pressure decrease per unit time can vary within wide ranges and are adjusted according to the physical properties of the starting compounds and the reaction products, the temperature and pressure conditions of the first stage established starting from the standard pressure and room temperature. It has been found particularly suitable to increase the temperature in two stages and to reduce the pressure in two stages.

The lower pressure limit to be established depends on the physical properties of the starting compounds and the reaction products formed, such as boiling point and vapor pressure, and also on the apparatus set-up. Starting from the standard pressure, it is possible to operate in stages within these limits with a pressure reduction from stage to stage. An upper temperature limit of typically 280 ℃ should be observed to prevent the formation of decomposition products, some of which may adversely affect the color. The lower limit of the temperature stage depends on the reaction rate, which must still be high enough to complete the esterification reaction within an acceptable time. Within these limits, it is possible to operate in stages with a temperature increase from stage to stage.

The specific reaction conditions, such as temperature, reaction time, pressure to be applied or catalyst to be used, should be designed according to the specific polyol ester in order to force the coloring component to form in the background and to avoid as far as possible degradation reactions of the polyol ester at a sufficient reaction rate. Especially in the case of polyol esters based on ether glycols, such as triethylene glycol or tetraethylene glycol, an increase in the degradation of the ether skeleton may occur when the reaction conditions, such as temperature, reaction time and type and amount of catalyst, are not adjusted in a controlled manner to the specific polyol ester.

The esterification reaction can be carried out with stoichiometric amounts of a polyol and an aliphatic monocarboxylic acid. However, preference is given to reacting the polyol with an excess of monocarboxylic acid, which is generally a low-boiling component and can be removed in a simple manner by distillation in the subsequent workup of the crude ester. The aliphatic monocarboxylic acids are used in an excess of 10 to 50 mol%, preferably 20 to 40 mol%, per mole of hydroxyl groups of the polyol to be esterified.

The water of reaction formed is distilled off from the reaction vessel during the esterification reaction together with the excess monocarboxylic acid and passed to a downstream phase separator, where the monocarboxylic acid and the water are separated off according to their solubility properties. In some cases, the monocarboxylic acids used may also form azeotropes with water under the reaction conditions and can serve as entrainers to remove the water of reaction. The progress of the reaction can be monitored by the water obtained. The separated water is removed from the process while the monocarboxylic acid from the phase separator is returned to the reaction vessel. The addition of other organic solvents which assume the role of entrainers, for example hexane, 1-hexene, cyclohexane, toluene, xylene or xylene isomer mixtures, is not excluded, but is limited to a few special cases. The entrainer may be added as early as the start of the esterification reaction or after the relatively high temperature is reached. The reaction is terminated by cooling the reaction mixture when the desired theoretical amount of water has been obtained or the hydroxyl number, as measured, for example, according to DIN 53240, has fallen below the set value.

The catalyst for the esterification reaction of a polyol with a monocarboxylic acid is a Lewis acid containing at least one element of groups 4 to 14 of the periodic Table of the elements, which may be used in solid or liquid form. As used herein, the term "Lewis acid" means having a structure such asGeneral definitions of those elements or compounds of the valence bond explained in Chemie-Lexikon, 8 th edition, Franck' sche Verlagshandlung 1983, volume 3, H-L. Particularly suitable Lewis acids which can be used as catalysts in the esterification reaction include titanium, zirconium, iron, zinc, boron, aluminum or tin, which are used in elemental form in finely distributed form or preferably in the form of compounds. Suitable compounds are, for example, tin (II) oxide, tin (IV) oxide, tin carboxylates such as tin (II) 2-ethylhexanoate, tin (II) oxalate, tin (II) acetate or tin (IV) acetate; tin (IV) alkoxides, such as tetra (methyl) stannate, tetra (ethyl) stannate, tetra (propyl) stannate, tetra (isopropyl) stannate or tetra (isobutyl) stannate; or organotin compounds such as butyltin maleate or dibutyltin dilaurate.

Suitable titanium compounds include alkoxides such as tetra (methyl) orthotitanate, tetra (ethyl) orthotitanate, tetra (propyl) orthotitanate, tetra (isopropyl) orthotitanate, tetra (butyl) orthotitanate, tetra (isobutyl) orthotitanate, tetra (pentyl) orthotitanate or tetra (2-ethylhexyl) orthotitanate; acylates such as titanium glycolate, titanium hydroxybutyrate or titanium hydroxyvalerate; or a chelate such as tetraethylene glycol titanate or tetrapropylene glycol titanate. The corresponding zirconium compounds, such as tetra (methyl) ortho zirconate, tetra (ethyl) ortho zirconate, tetra (propyl) ortho zirconate, tetra (isopropyl) ortho zirconate, tetra (butyl) ortho zirconate, tetra (isobutyl) ortho zirconate, tetra (pentyl) ortho zirconate or tetra (2-ethylhexyl) ortho zirconate, may also be used successfully.

Also suitable are boric acids and boric acid esters, such as trimethyl borate, triethyl borate, tripropyl borate, triisopropyl borate, tributyl borate or triisobutyl borate.

Also suitable are aluminum oxides, aluminum hydroxides, aluminum carboxylates, such as aluminum acetate or aluminum stearate, or aluminum alkoxides, such as aluminum tributoxide, aluminum tri-sec-butoxide, aluminum tri-tert-butoxide or aluminum tri-isopropoxide.

It is also possible to use zinc oxide, zinc sulfate and zinc carboxylates, for example zinc acetate dihydrate or zinc stearate, and iron (II) acetate or iron (II) oxyhydroxide as catalysts.

The catalyst may be added to the reaction mixture as early as the start, or only at elevated temperature and subsequently with safety precautions, for example when water has been set to be removed from the reaction.

The amount of the esterification catalyst added was 1X 10^-5To 20 mol%, preferably from 0.01 to 5 mol%, in particular from 0.01 to 2 mol%, based on the starting compounds added in deficient amounts, suitably based on the polyol. With higher amounts of catalyst, a cleavage reaction of the polyol ester is expected to occur.

Especially in the case of the preparation of polyol esters based on ether glycols, for example based on triethylene glycol or tetraethylene glycol, there is an increased risk of ether chain scission in the conversion stage using high catalyst concentrations for the end of the reaction and in the final residue of free hydroxyl groups, so that the reaction temperature or the pressure to be applied should be adjusted in this case. The higher the catalyst concentration selected, the lower the reaction temperature or pressure to be applied should generally be selected, and an optimized temperature and pressure profile should be used. In the case of too low a catalyst concentration, the esterification rate becomes so low that no acceptable conversion can be observed within an acceptable reaction time.

The esterification catalyst may be added in liquid or solid form. The solid catalyst, for example tin (II) oxide, zinc oxide or iron (III) oxyhydroxide, is filtered off after the end of the esterification reaction before the crude polyol ester is worked up further. When the esterification catalyst is added in the form of a liquid compound, for example tetra (isopropyl) orthotitanate or tetra (butyl) orthotitanate, which remains dissolved in the reaction mixture after the end of the esterification reaction, these compounds are converted during workup, in the course of the steam treatment, into insoluble precipitates which can be removed by filtration.

In one embodiment of the process of the present invention, the esterification reaction will be carried out in the presence of an adsorbent. In this case, porous, high surface area solid materials are used, which are commonly used in chemical practice in laboratories and in industrial settings. Examples of such materials are: high surface area polysilicic acids such as silica gel (silica xerogel), diatomaceous earth, high surface area alumina and hydrated alumina; mineral materials such as clay or carbonate, or activated carbon. Activated carbon has been found to be particularly useful. Generally, the adsorbent is suspended in finely divided form in the reaction solution, which is agitated by strong stirring or by the introduction of an inert gas. This achieves intimate contact between the liquid phase and the adsorbent. The dosage ratio of the adsorbent can be adjusted substantially freely and thus according to the respective requirements. It is useful to use 0.1 to 5 parts by weight, preferably 0.1 to 1.5 parts by weight, of the adsorbent per 100 parts by weight of the liquid reaction mixture.

Due to the quality requirements for the polyols at the outset, the process steps in the esterification stage, which includes removal of the reaction water, and in the work-up of the crude ester are very important process characteristics, since the adjustment of these process steps can influence the organoleptic and optical properties of the end product to a significant extent. More particularly, the optimized process scheme provides ether glycol based polyol esters having high purity, e.g., based on triethylene glycol or tetraethylene glycol, and provides low color number and high color stability. In contrast, the structure of the raw materials, polyols and aliphatic monocarboxylic acids is critical for the mechanical and thermal properties of polymer materials plasticized with polyol esters and affects the hydrolytic and oxidative stability of lubricants.

The reaction mixture obtained after the end of the reaction comprises the polyol ester as the desired reaction product as well as any unconverted starting material, more particularly the aliphatic monocarboxylic acid which is still in excess in case an excess of monocarboxylic acid has been used according to a preferred version of the process of the present invention. Generally, the unconverted excess of starting compound is first distilled off, which is suitably carried out under application of reduced pressure.

Thereafter, the crude ester is subjected to a steam treatment, which can be carried out, for example, in simple form by introducing steam into the crude product. One advantage of water vapor treatment is that: during which the catalyst still present is destroyed and converted into a filterable hydrolysate. When the esterification reaction is carried out in the presence of an adsorbent, the adsorbent still present promotes the deposition of the catalyst conversion product. In addition, it was found to be advantageous to add the adsorbent at the beginning of the water vapor treatment. The presence of the adsorbent during the water vapor treatment also has a beneficial effect on the color and color stability of the polyol ester. However, it is also possible to filter off the adsorbent after the esterification reaction has ended and the excess starting compound has been removed, i.e.before the steam distillation has taken place.

The water vapour treatment is generally carried out at standard pressure, but the use of a slightly reduced pressure, suitably as low as 400hPa, is not excluded. The steam treatment is generally carried out at temperatures of 100 ℃ and 250 ℃, preferably 150 ℃ and 220 ℃, in particular 170 ℃ and 200 ℃ and is also governed by the physical properties of the polyol ester to be prepared in each case.

In the process step of the steam treatment, it was found appropriate to proceed in a very gentle manner during heating up to the working temperature in order to heat the crude ester to the desired steam treatment temperature.

The duration of the water vapour treatment can be determined by routine experimentation and is typically carried out for a period of 0.5 to 5 hours. Too long a water vapor treatment causes an unfavorable increase in the color number of the polyol ester and should therefore be avoided. An increased reaction of the polyol ester to degrade into acidic compounds is also observed, the content of said acidic compounds being manifested, for example, by an increase in the neutralization value or acid value as measured according to DIN EN ISO 3682/ASTM D1613. In case the treatment time is too short, the removal of residual acid and water is incomplete and the desired polyol ester still has too high an unfavourable acid number and too high a water content. In the case of too short a treatment time, only a very small beneficial effect on the color number of the polyol ester is observed.

The conditions in the water vapor treatment, such as temperature, duration and applied pressure, must be precisely adjusted to the particular polyol ester to achieve the best results in terms of polyol ester color number and to minimize as much as possible the residual content of starting compounds, water and catalyst, while at the same time inhibiting degradation reactions. Especially in the case of using relatively large amounts of catalyst and in the case of preparing polyol esters based on ether glycols, for example based on triethylene glycol or tetraethylene glycol, the conditions in the water vapor treatment should be tailored to the specific polyol ester in order to suppress the disadvantageous degradation of the ether chains.

After the water vapor treatment, solid alkaline substances, such as alkaline silica, alkaline alumina or sodium carbonate, sodium bicarbonate, calcium carbonate or sodium hydroxide in solid form, and alkaline minerals are optionally added to further reduce the neutralization number of the polyol ester.

After the water vapor treatment, optionally after filtration of the adsorbent, any added solid alkaline material and other solids obtained, the polyol ester is dried, for example by passing an inert gas over the product at elevated temperature. It is also possible to simultaneously apply reduced pressure and optionally inert gas over the product at elevated temperature. Even without the action of an inert gas, it is possible to operate only at elevated temperatures or only under reduced pressure. The specific drying conditions, such as temperature, pressure and time, can be determined by simple preliminary tests. Temperatures of from 80 to 250 ℃ and preferably from 100 to 180 ℃ and pressures of from 0.2 to 500hPa, preferably from 1 to 200hPa and in particular from 1 to 20hPa are generally employed. The crude ester is then filtered, if not already filtered, to remove solids, any added solid caustic, hydrolysis products of the catalyst, and adsorbent (if added in the esterification stage or prior to steam treatment). Filtration is carried out in conventional filtration units at standard temperature or at temperatures of up to 120 ℃. Filtration may be assisted by common filter aids such as cellulose, silica gel, diatomaceous earth, wood flour. However, their use is limited to special cases.

Upon completion of the filtration, a light-colored polyol ester is obtained which also meets the remaining specifications, such as water content, residual acid content, residual content of catalyst ingredients and residual content of monoester.

The polyols used as starting materials for the process according to the invention correspond to the general formula (I):

R(OH)_n (I)

wherein R is an aliphatic or alicyclic hydrocarbon group having 2 to 20, preferably 2 to 10, carbon atoms and n is an integer of 2 to 8, preferably 2, 3, 4, 5 or 6.

Suitable polyols are likewise compounds of the general formula (II):

H-(-O-[-CR¹R²-]_m-)_o-OH (II)

wherein R is¹And R²Each independently hydrogen, alkyl having 1 to 5 carbon atoms, preferably methyl, ethyl or propyl, or hydroxyalkyl having 1 to 5 carbon atoms, preferably hydroxymethyl; m is an integer from 1 to 10, preferably from 1 to 8, in particular 1, 2, 3 or 4; o is an integer from 2 to 15, preferably from 2 to 8, in particular 2, 3, 4 or 5.

Suitable polyols which can be converted into the light-coloured polyol esters by the process according to the invention are, for example, 1, 3-propanediol, 1, 3-butanediol, 1, 4-butanediol, neopentyl glycol, 2-dimethylolbutane, trimethylolethane, trimethylolpropane, ditrimethylolpropane, trimethylolbutane, 2, 4-trimethylpentane-1, 3-diol, 1, 2-hexanediol, 1, 6-hexanediolPentaerythritol or dipentaerythritol or 3(4), 8(9) -dihydroxymethyltricyclo [5.2.1.0^2，6]Decane.

Other polyols which may be used include ethylene glycol and 1, 2-propylene glycol, and oligomers thereof, especially diethylene glycol, triethylene glycol and tetraethylene glycol which are ether glycols, or dipropylene glycol, tripropylene glycol or tetrapropylene glycol. Ethylene glycol and propylene glycol are industrially produced chemicals. The base materials used for their preparation are ethylene oxide and propylene oxide, from which 1, 2-ethanediol and 1, 2-propanediol are obtained by heating under pressure with water. Diethylene glycol is obtained by ethoxylation of ethylene glycol. Triethylene glycol is obtained as a by-product in the hydrolysis of ethylene oxide to ethylene glycol, as is tetraethylene glycol. Both compounds can also be synthesized by reacting ethylene glycol with ethylene oxide. Dipropylene glycol, tripropylene glycol, tetrapropylene glycol and higher propoxylated products are obtained by multiple additions of propylene oxide to 1, 2-propanediol.

In order to obtain light-colored polyol esters by the process of the present invention, linear or branched aliphatic monocarboxylic acids having 3 to 20 carbon atoms in the molecule are used. Although saturated acids are preferred in many cases, it is also possible to use unsaturated carboxylic acids as reaction components for the ester synthesis, depending on the particular field of application of the plasticizer or lubricant. Examples of monocarboxylic acids as components of the polyol esters are propionic acid, n-butyric acid, isobutyric acid, n-valeric acid, 2-methylbutyric acid, 3-methylbutyric acid, 2-methylpentanoic acid, n-hexanoic acid, 2-ethylbutyric acid, n-heptanoic acid, 2-methylhexanoic acid, cyclohexanecarboxylic acid, 2-ethylhexanoic acid, n-nonanoic acid, 2-methyloctanoic acid, isononanoic acid, 3, 5, 5-trimethylhexanoic acid, 2-propylheptanoic acid, 2-methylundecanoic acid, isoundecanecarboxylic acid, tricyclodecanecarboxylic acid and isotridecanoic acid. The novel process has been found to be particularly useful for the preparation of monoethylene glycol or oligoethylene glycol and 1, 2-propylene glycol or oligopropylene glycol with C₄-to C₁₃-or C₅-to C₁₀Polyol esters of monocarboxylic acids and their use for the preparation of catalysts based on 1, 3-butanediol, neopentyl glycol, 2, 4-trimethyl-pentane-1, 3-diol, trimethylolpropane, ditrimethylolpropane, pentaerythritolOr 3(4), 8(9) -dihydroxymethyltricyclo [5.2.1.0^2，6]Polyol ester of decane.

Polyol esters of ethylene glycol and its oligomers are outstandingly suitable as plasticizers for all customary high molecular weight thermoplastic materials. They find particular utility as additives to polyvinyl butyral which, in combination with glycol esters, is used as an interlayer in the manufacture of multilayer or composite glass. They can likewise be used as coalescents or film-forming assistants in aqueous polymer dispersions which have various uses as coatings. The preparation process of the present invention makes it possible to prepare polyol esters in a simple manner which have excellent color properties and which also meet further quality requirements such as low odor or low acid number. The process of the invention is particularly suitable for the preparation of triethylene glycol di-2-ethylhexanoate (3G8 ester), tetraethylene glycol di-n-heptanoate (4G7 ester), triethylene glycol di-2-ethylbutyrate (3G6 ester), triethylene glycol di-n-heptanoate (3G7 ester) or tetraethylene glycol di-2-ethylhexanoate (4G8 ester).

The process of the invention can be carried out continuously or batchwise in reaction apparatuses which are typically used in chemical technology. It has been found that useful devices are stirred tanks or reaction tubes, with batch reaction modes being preferred.

The process of the present invention is illustrated in detail in the following examples, but it is not limited to the embodiments.

Example (b):

example 1:

preparation of triethylene glycol di-2-ethylhexanoate (3G8 ester)

The esterification of triethylene glycol with 2-ethylhexanoic acid was carried out in a heatable 2 l four-necked flask equipped with a stirrer, an internal thermometer and a water separator.

The flask was initially charged with triethylene glycol and a 20 mole% excess of 2-ethylhexanoic acid based on the hydroxyl groups to be esterified and 1.8 mole% tetra (isopropyl) orthotitanate based on triethylene glycol. While stirring and applying a reduced pressure of 600hPa, the mixture was heated to 190 ℃ and the reaction water formed was removed on a water separator. In this example and the following examples, the time at which the water of reaction first appeared was selected as the starting point for determining the reaction time. After a reaction time of 2 hours had been reached in this stage, the pressure was reduced to 400hPa and the temperature was increased to 220 ℃. The progress of the reaction was monitored by continuously weighing the amount of reaction water discharged via the water separator and by taking a sample and subjecting the sample to gas chromatography. The reaction is stopped by cooling the mixture when the triethylene glycol di-2-ethylhexanoate content (% by weight) has reached at least 97% and the residual hydroxyl value does not exceed 5.0mg KOH/g (according to DIN 53240) as determined by gas chromatography. The esterification time was 6 hours.

Example 2:

preparation of triethylene glycol di-2-ethylhexanoate (3G8 ester) with addition of activated carbon during esterification

Example 2 was carried out in the same manner as in example 1, with the only difference that 0.3% by weight of activated carbon, based on the entire reaction mixture, was added at the beginning of the esterification reaction. The reaction was terminated when the index characterizing the degree of esterification described in example 1 was reached. The esterification time was 6 hours.

Example 3:

preparation of triethylene glycol di-2-ethylhexanoate (3G8 ester) with addition of activated carbon during esterification

In a heatable 2 l four-necked flask equipped with stirrer, internal thermometer and water separator, triethylene glycol and a 30 mol% excess of 2-ethylhexanoic acid, based on the hydroxyl groups to be esterified, and 0.018 mol% tetra (isopropyl) orthotitanate, based on triethylene glycol, were initially charged and mixed with 1% by weight of activated carbon, based on the entire reaction mixture. While stirring and applying a reduced pressure of 600hPa, the mixture was heated to 220 ℃ and the reaction water formed was removed on a water separator. After 1 hour of reaction time in this stage, the pressure was reduced to 400hPa and the temperature was maintained at 220 ℃. After a further reaction time of 3 hours, the pressure was further reduced to 300 hPa. The progress of the reaction was monitored by continuously weighing the amount of reaction water discharged via the water separator and by taking a sample and subjecting the sample to gas chromatography. The reaction is stopped by cooling the mixture when the triethylene glycol di-2-ethylhexanoate content (% by weight) has reached at least 97% and the residual hydroxyl value does not exceed 5.0mg KOH/g (according to DIN 53240) as determined by gas chromatography. The esterification time was 8 hours.

Example 4:

preparation of triethylene glycol di-2-ethylhexanoate (3G8 ester) with addition of activated carbon during esterification; tin catalysis

In a heatable 2 l four-necked flask equipped with stirrer, internal thermometer and water separator, triethylene glycol and a 30 mol% excess of 2-ethylhexanoic acid based on the hydroxyl groups to be esterified and 0.36 mol% tin (II) 2-ethylhexanoate based on triethylene glycol were initially charged and mixed with 0.3% by weight of activated carbon based on the entire reaction mixture. While stirring and applying a reduced pressure of 600hPa, the mixture was heated to 220 ℃ and the reaction water formed was removed on a water separator. After a reaction time of 2 hours had been reached in this stage, the pressure was reduced to 400hPa and the temperature was maintained at 220 ℃. After a further reaction time of 4 hours, the pressure was further reduced to 300 hPa. The progress of the reaction was monitored by continuously weighing the amount of reaction water discharged via the water separator and by taking a sample and subjecting the sample to gas chromatography. The reaction is stopped by cooling the mixture when the triethylene glycol di-2-ethylhexanoate content (% by weight) has reached at least 97% and the residual hydroxyl value does not exceed 5.0mg KOH/g (according to DIN 53240) as determined by gas chromatography. The esterification time was 6 hours.

Example 5:

preparation of triethylene glycol di-2-ethylhexanoate (3G8 ester) with addition of activated carbon during esterification; zinc catalysis

In a heatable 2 l four-necked flask equipped with stirrer, internal thermometer and water separator, triethylene glycol and a 30 mol% excess of 2-ethylhexanoic acid based on the hydroxyl groups to be esterified and 0.36 mol% zinc (II) hexanoate dihydrate based on triethylene glycol were initially charged and mixed with 1.0% by weight of activated carbon based on the entire reaction mixture. While stirring and applying a reduced pressure of 600hPa, the mixture was heated to 220 ℃ and the reaction water formed was removed on a water separator. After a reaction time of 2 hours had been reached in this stage, the pressure was reduced to 400hPa and the temperature was maintained at 220 ℃. The progress of the reaction was monitored by continuously weighing the amount of reaction water discharged via the water separator and by taking a sample and subjecting the sample to gas chromatography. The esterification time was 7 hours.

Working up of the reaction mixture of examples 1 to 5, comprising distillative removal of 2-ethylhexanoic acid, steam distillation, drying and subsequent filtration

A) The excess 2-ethylhexanoic acid is distilled off until the residual acid content in the crude ester is < 1mgKOH/g (according to DIN EN ISO 3682/ASTM D1613)

B) Steam distillation (duration 1 hour in each case)

Crude ester according to the examples	Bottom temperature (°)	Distillation pressure (hPa)
			1	180	Standard pressure
2	180	Standard pressure
			3	180	Standard pressure
4	200	Standard pressure
			5	180	Standard pressure/time: 1.5 hours

C) Drying (for 0.5 hour in each case)

Crude ester according to the examples	Bottom temperature (°)	Pressure (hPa)
			1	140	2
2	140	2
			3	140	10
4	160	10/duration: 1.5 hours
			5	160	10/duration: 1.5 hours

D) After filtering off the separated solid and the activated carbon added at standard pressure and room temperature, the residue obtained is a light-colored polyol ester with the following index:

gas chromatography analysis (% by weight):

index:

example 6:

preparation of neopentyl glycol di-2-ethylhexanoate by addition of activated carbon during esterification

In a heatable 2 l four-necked flask equipped with stirrer, internal thermometer and water separator, neopentyl glycol and a 30 mol% excess of 2-ethylhexanoic acid based on the hydroxyl groups to be esterified and 0.36 mol% tetra (isopropyl) orthotitanate based on neopentyl glycol were initially charged and mixed with 1% by weight of activated carbon based on the entire reaction mixture. While stirring and applying a reduced pressure of 600hPa, the mixture was heated to 200 ℃ and the reaction water formed was removed on a water separator. After a reaction time of 2 hours had been reached in this stage, the pressure was reduced to 450hPa and the temperature was maintained at 200 ℃. The progress of the reaction was monitored by continuously weighing the amount of reaction water discharged via the water separator and by taking a sample and subjecting the sample to gas chromatography. When the content (% by weight) of neopentyl glycol di-2-ethylhexanoate determined by gas chromatography reaches at least 97% and the residual hydroxyl value does not exceed 5.0mg KOH/g (according to DIN 53240), the reaction is stopped by cooling the mixture. The esterification time was 7 hours.

Subsequently, the excess 2-ethylhexanoic acid is first distilled off at a temperature of 150 ℃ and a pressure of 5 hPa. Thereafter, steam distillation was carried out at a bottom temperature of 200 ℃ for 1.5 hours, followed by drying at a temperature of 160 ℃ and a pressure of 10hPa for 2 hours. After the solids separated off and the adsorbent added have been filtered off, the residue obtained is a light-colored polyester having the following indices:

gas chromatography analysis (% by weight):

neopentyl glycol di-2-ethylhexanoate	96.3
		Neopentyl glycol mono-2-ethylhexanoate	2.5
The rest(s)	1.2

Index:

the inventive measure of simultaneous adjustment of the esterification stage and the work-up stage in a controlled manner makes it possible to obtain polyol esters of outstanding quality which can be used in a wide variety of applications.

Example 7 (comparative):

preparation of triethylene glycol di-2-ethylhexanoate (3G8 ester), tin catalysis

The esterification of triethylene glycol with 2-ethylhexanoate was carried out in a heatable 2 l four-necked flask equipped with stirrer, internal thermometer and water separator.

The flask was initially charged with triethylene glycol and a 30 mole% excess of 2-ethylhexanoic acid based on the hydroxyl groups to be esterified and 1.8 mole% tin (II) 2-ethylhexanoate based on triethylene glycol. While stirring and applying a reduced pressure of 600hPa, the mixture was heated to 220 ℃ and the reaction water formed was removed on a water separator. After a reaction time of 2 hours had been reached in this stage, the pressure was reduced to 400 hPa. The progress of the reaction was monitored by continuously weighing the amount of reaction water discharged via the water separator and by taking a sample and subjecting the sample to gas chromatography. The esterification reaction was stopped after 5 hours.

In this example, the maximum value of triethylene glycol di-2-ethylhexanoate was reached as early as after 2 hours, and the crude reaction mixture had the following composition (calculated without 2-ethylhexanoic acid) as determined by gas chromatography:

triethylene glycol di-2-ethylhexanoate	93.5% by weight
		Triethylene glycol mono-2-ethylhexanoate	2.3% by weight
Diethylene glycol di-2-ethylhexanoate	0.6% by weight
		The rest(s)	3.6% by weight

After 5 hours, the reaction was complete. After removal of the excess 2-ethylhexanoic acid, the crude polyol ester had the following composition as determined by gas chromatography:

triethylene glycol di-2-ethylhexanoate	86.2% by weight
		Triethylene glycol mono-2-ethylhexanoate	0.6% by weight
Diethylene glycol di-2-ethylhexanoate	3.3% by weight
		The rest(s)	9.9% by weight

The remaining indices were not tested.

The results of example 7 show that the maximum value for triethylene glycol di-2-ethylhexanoate is reached as early as after an esterification time of 2 hours. In the subsequent remaining conversion stage, cleavage of the ether chain predominantly takes place, with an increase in the content of diethylene glycol di-2-ethylhexanoate and residual components.

This comparison demonstrates, in comparison to example 4, that at high catalyst concentrations an optimized temperature and pressure profile must be used in order to obtain polyol esters of sufficient quality.

Claims (27)

1. A process for preparing polyol esters by reacting polyols with linear or branched aliphatic monocarboxylic acids having 3 to 20 carbon atoms, characterized in that a mixture of starting compounds is reacted in the presence of a Lewis acid containing at least one element of groups 4 to 14 of the periodic Table of the elements as catalyst and the water formed is removed, followed by a steam treatment.

2. The process according to claim 1, characterized in that the mixture of starting compounds is heated in the presence of the catalyst to a temperature of at most 280 ℃, preferably at most 250 ℃, and the pressure is reduced in stages while keeping the temperature constant.

3. The process according to claim 1, characterized in that the mixture of starting compounds is heated in stages to a maximum temperature of 280 ℃ under constant pressure in the presence of the catalyst.

4. The process according to claim 1, characterized in that the mixture of starting compounds is heated in the presence of the catalyst at temperatures which are raised stage by stage to at most 280 ℃ and the pressure is also reduced stage by stage.

5. A process according to claim 4, characterized in that the mixture of starting compounds is reacted in the first stage at a temperature of at most 190 ℃ and a pressure of at most 600hPa in the presence of the catalyst, and in the second stage the reaction is brought to completion by raising the temperature to at most 250 ℃ and at a pressure of at most 300 hPa.

6. Process according to one or more of claims 1 to 5, characterized in that the amount of catalyst used is 1X 10, based on the starting compound used in insufficient amount^-5To 20 mole%.

7. Process according to claim 6, characterized in that the catalyst is used in an amount of 0.01 to 5 mol%, preferably 0.01 to 2 mol%, based on the starting compound used in an insufficient amount.

8. A process according to one or more of claims 1 to 7, characterized in that the catalyst used is titanium, zirconium, iron, zinc, boron, aluminum or tin, either in elemental form or in the form of a compound thereof.

9. A process according to claim 8, characterized in that the tin compound used is tin (II) oxide, tin (II) oxalate, tin (II) carboxylate, tin (IV) alkoxide or an organotin compound.

10. A process according to claim 8, characterized in that the titanium compound used is an alkoxide, acylate or chelate.

11. A process according to claim 8, characterized in that the boron compound used is boric acid or a borate ester.

12. A process according to claim 8, characterized in that the aluminum compound used is aluminum oxide, aluminum hydroxide, aluminum carboxylates or aluminum alkoxides.

13. A process according to claim 8, characterized in that the zinc compound used is zinc oxide, zinc sulphate or a zinc carboxylate.

14. Process according to one or more of claims 1 to 13, characterized in that the starting compound is converted in the presence of an adsorbent.

15. The process according to claim 14, characterized in that 0.1 to 5 parts by weight, preferably 0.1 to 1.5 parts by weight, of adsorbent is used per 100 parts by weight of reaction mixture.

16. A process according to claim 14 or 15, characterized in that the adsorbent used is silica gel, diatomaceous earth, alumina, hydrated alumina, clay, carbonate or activated carbon.

17. The process according to one or more of claims 1 to 16, characterized in that the water vapor treatment is carried out at a temperature of 100-.

18. The method according to claim 17, characterized in that the solid alkaline substance is added after the water vapor treatment.

19. The method according to claim 18, characterized in that the solid alkaline substance added is alkaline silica, alkaline alumina, sodium carbonate, sodium bicarbonate, calcium carbonate, sodium hydroxide or an alkaline mineral.

20. Process according to one or more of claims 1 to 19, characterized in that the polyol ester is dried after the steam treatment at a temperature of 80 to 250 ℃, preferably 100 and 180 ℃ and a pressure of 0.2 to 500hPa, preferably 1 to 200hPa, especially 1 to 20 hPa.

21. A process according to claim 20, characterized in that the polyol ester is dried in the presence of an inert gas.

22. Process according to one or more of claims 1 to 21, characterized in that the polyol used is a compound of the general formula (I):

R(OH)_n (I)

wherein R is an aliphatic or alicyclic hydrocarbon group having 2 to 20, preferably 2 to 10, carbon atoms and n is an integer of 2 to 8, preferably 2, 3, 4, 5 or 6.

23. Process according to one or more of claims 1 to 21, characterized in that the polyol used is a compound of the general formula (II):

H-(-O-[-CR¹R²-]_m-)_o-OH (II)

wherein R is¹And R²Each independently hydrogen, alkyl having 1 to 5 carbon atoms, preferably methyl, ethyl or propyl, or hydroxyalkyl having 1 to 5 carbon atoms, preferably hydroxymethyl; m is an integer from 1 to 10, preferably from 1 to 8, in particular 1, 2, 3 or 4; o is an integer from 2 to 15, preferably from 2 to 8, in particular 2, 3, 4 or 5.

24. A process according to claim 22, characterized in that the polyol used is 1, 2-propanediol, 1, 3-butanediol, 1, 4-butanediol, neopentyl glycol, 2-dimethylolbutane, trimethylolethane, trimethylolpropane, trimethylolbutane, 2, 4-trimethylpentane-1, 3-diol, 1, 2-hexanediol, 1, 6-hexanediol, pentaerythritol, ethylene glycol or 3(4), 8(9) -dihydroxymethyltricyclo [5.2.1.0 ]^2，6]Decane.

25. A process according to claim 23, characterized in that the polyol used is di-trimethylolpropane, dipentaerythritol, diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, tripropylene glycol or tetrapropylene glycol.

26. A process according to one or more of claims 1 to 25, characterised in that the aliphatic monocarboxylic acid converted is propionic acid, n-butyric acid, isobutyric acid, n-valeric acid, 2-methylbutyric acid, 3-methylbutyric acid, 2-methylpentanoic acid, n-hexanoic acid, 2-ethylbutyric acid, n-heptanoic acid, 2-methylhexanoic acid, 2-ethylhexanoic acid, n-nonanoic acid, 2-methyloctanoic acid, isononanoic acid, 3, 5, 5-trimethylhexanoic acid or 2-propylheptanoic acid.

27. The process according to one or more of claims 1 to 26 for preparing triethylene glycol di-2-ethylhexanoate, tetraethylene glycol di-n-heptanoate, triethylene glycol di-2-ethylbutyrate, triethylene glycol di-n-heptanoate or tetraethylene glycol di-2-ethylhexanoate.