Process for the preparation of mixed polyol-carboxylic acid esters

Abstract

The invention relates to a process for the preparation of mixed polyol-carboxylic acid esters having a molecular weight of greater than or equal to 200 g / mol and less than or equal to 1000 g / mol, wherein in a reaction in at least two stages, polyhydric alcohols are reacted with different carboxylic acids which are present in the form of monocarboxylic acids or monocarboxylic anhydrides, wherein the different carboxylic acids are reacted with the polyhydric alcohols in the order of their reactivity in the esterification reaction, starting with the least reactive, wherein the less reactive carboxylic acids are at least partially reacted in the form of monocarboxylic anhydrides, and subsequently the most reactive carboxylic acids are reacted with the polyhydric alcohols in the form of monocarboxylic acids. Furthermore, the invention also relates to the use of the process for the preparation of mixed polyol esters.

Description

Technical Field

[0001] This invention relates to a method for preparing mixed polyol-carboxylic acid esters with a molecular weight greater than or equal to 200 g / mol and less than or equal to 1000 g / mol, wherein in at least two stages of the reaction, the polyol reacts with different monocarboxylic acids, either as monocarboxylic acids or monocarboxylic anhydrides, wherein the different monocarboxylic acids react with the polyol in order of their reactivity in the esterification reaction, starting with the least reactive monocarboxylic acid, wherein the less reactive monocarboxylic acid reacts at least partially as a monocarboxylic anhydride, and subsequently the most reactive monocarboxylic acid reacts with the polyol as a monocarboxylic acid. Furthermore, this invention relates to the use of this method for preparing mixed polyol esters. Background Technology

[0002] Carboxylic acid esters of polyols, also known as polyol esters (POEs), are technically used on a large scale and in many different ways. For example, these substances can be used as plasticizers or lubricants. An important characteristic of using polyol esters is that the material properties of the resulting esters can be controlled by selecting the polyol and the carboxylic acid. Therefore, it is possible to provide substances whose physical properties, such as boiling point, viscosity, and cloud point, as well as chemical properties, such as resistance to hydrolysis or stability against oxidative degradation, can be specifically tailored to the existing requirements of the application.

[0003] The ability to "tailor" the properties of a substance is particularly important for the operation of refrigeration systems. For example, compressors in refrigeration systems using fluorinated hydrocarbons and chlorinated hydrocarbons (CFCs) as refrigerants operate with various lubricants, such as mineral oils, alkylbenzenes, and synthetic hydrocarbons. However, these lubricants cannot be used with pure fluorinated hydrocarbons due to their poor or lack of miscibility with the refrigerant. For this reason, polyol esters or polyalkylene glycols (polyethers) are suitable for use with fluorinated refrigerants to ensure the compressor functions reliably over a wide temperature range. In addition to miscibility, a suitable lubricant must also have a suitable viscosity and be highly compatible with the substances used in the refrigeration equipment under existing pressure and temperature conditions. Due to the diversity of polyol esters, these requirements can, in principle, be met by specifically adapting them to the specific application conditions.

[0004] The flexibility and adaptability of lubricating oil performance is particularly useful for refrigeration systems, as regulatory requirements for refrigerants can change due to environmental factors. Because of the ban on CFC refrigerants (such as R11, R12, and R22) with high ozone depletion and high greenhouse effect potential, pure fluorinated hydrocarbons (such as R32 and R410A) are increasingly used in this field.

[0005]

[0006] Changes in the chemical composition of refrigerants naturally affect their physical properties, so it is desirable to adjust the properties of the lubricants used with them.

[0007] The patent documents also contain various methods for producing polyol esters.

[0008] For example, DE2317276A1 discloses a method for preparing fully esterified polyols from polyol esterification, comprising esterifying the polyol esterification with an anhydride in the presence of a catalytic amount of perfluoroalkyl sulfonic acid or perfluoroalkyl sulfonic anhydride.

[0009] In addition, DE2721260A1 discloses a two-stage method for preparing polyester, wherein in a first stage, an aromatic polycarboxylic acid selected from the group consisting of isophthalic acid and terephthalic acid reacts with a polyol to form a half-ester mixture, and then in a second stage reacts with a saturated or unsaturated aliphatic polycarboxylic acid to form a polyester, wherein (a) in the first stage, a first portion of the polyol is contacted with the aromatic polycarboxylic acid in an amount sufficient to form a stirable mixture with the aromatic polycarboxylic acid, (b) the stirable mixture is heated to a temperature of at least 190°C, and (c) the remaining portion of the polyol is added to the heated stirable mixture, and the temperature of the mixture is maintained at at least 190°C to form a half-ester mixture.

[0010] Finally, DE102012018207A1 discloses a method for preparing polyol esters by reacting a polyol with a straight-chain or branched aliphatic monocarboxylic acid having 3 to 20 carbon atoms, wherein a mixture of starting compounds is allowed to react in the presence of a Lewis acid containing at least one element from columns 4 to 14 of the periodic table as a catalyst and in the presence of an adsorbent to remove the water formed, and then the crude ester is obtained by treating it with the addition of another adsorbent.

[0011] Such solutions known in the prior art still offer potential for further improvement, particularly in terms of the flexibility and controllability of the composition of available esters. Summary of the Invention

[0012] Therefore, the object of the present invention is to at least partially overcome the disadvantages known in the prior art. In particular, the object of the present invention is to provide an improved method capable of reproducibly producing mixed esters of different compositions with high conversion rates.

[0013] This problem is solved by the features of the independent claims relating to the method and use according to the invention. Preferred embodiments of the invention are indicated in the dependent claims, the description, or the accompanying drawings, according to which further features described or shown in the dependent claims, the description, or the accompanying drawings may, alone or in any combination, constitute the object of the invention, unless the contrary is clearly drawn from the context.

[0014] According to the present invention, this problem is solved by a method for preparing a mixed polyol-carboxylic acid ester with a molecular weight greater than or equal to 200 g / mol and less than or equal to 1000 g / mol, wherein in at least two stages of reaction, the polyol reacts with different monocarboxylic acids in the form of monocarboxylic acids or monocarboxylic anhydrides, wherein the different monocarboxylic acids react with the polyol in order of their reactivity in the esterification reaction, starting with the least reactive monocarboxylic acid, wherein the less reactive monocarboxylic acid reacts with the polyol at least partially in the form of monocarboxylic anhydrides, and subsequently the most reactive monocarboxylic acid reacts with the polyol in the form of monocarboxylic acid.

[0015] Surprisingly, it was found that large quantities of mixed polyol esters can be produced using the above method, thereby allowing for the determination of ester compositions with varying monocarboxylic acid contents over a wide range through the disclosed process control. This is particularly surprising because esterification reactions with monocarboxylic acids of varying reactivity typically yield only a limited number of possible compositions, and the composition of the polyol esters obtainable without further steps is predetermined by the difference in reactivity between each monocarboxylic acid and the polyol. This limitation is especially pronounced when monocarboxylic acids with significantly different inductive effects and / or steric requirements are esterified with the same polyol. More reactive carboxylic acids always add to the polyol more frequently than less reactive ones. In the case of relatively small polyols, as indicated by the molecular weight range, the rapid reaction of more reactive carboxylic acids also hinders the approach of other present carboxylic acids to the remaining alcohol groups of the polyol, thus significantly hindering the overall reaction, especially for less reactive carboxylic acids. The reaction is further complicated by the fact that, in transesterification reactions, ester compounds that have already been linked once are cleaved by exchanging ester residues. Especially in this case, more reactive acid residues can again replace less reactive acid residues in the ester. Complex situations arise particularly when all alcohol groups in the polyol should or must react. Besides low controllability in composition, this typically leads to inefficiency, significantly prolonged reaction times, and unsatisfactory controllability of the ester composition. Through process control according to the invention, the proportions of different monocarboxylic acids in the ester itself can be controlled; furthermore, through process control according to the invention, the reaction time required for complete conversion of the polyol can be significantly reduced. In this respect, polyol esters of different reactivity monocarboxylic acids with controllable, variable proportions can be obtained, and this is achieved under specific reaction conditions and within a specific reaction time, which is unknown and impossible to achieve with prior art process control methods.

[0016] The method described is a method for preparing mixed polyol-carboxylic acid esters with a molecular weight greater than or equal to 200 g / mol and less than or equal to 1000 g / mol. This method involves the preparation of polyol esters, using polyols with relatively low molecular weights. This is derived from the molecular weight order given above. In this respect, the method does not include the esterification reaction of macromolecular polyols. A polyol is a substance having more than one, for example two, three, or more hydroxyl groups. A single hydroxyl group in the molecule is converted to the corresponding ester through esterification with a carboxylic acid. Preferably, in this method, all hydroxyl groups present in the polyol are converted to the corresponding ester groups. Therefore, in particular, the complete ester of the polyol can be obtained. The carboxylic acid used for the esterification reaction is a monocarboxylic acid. At least two different monocarboxylic acids can be used. However, it is also possible to use three or more monocarboxylic acids. In these cases, there is a "most reactive" monocarboxylic acid and a "least reactive" monocarboxylic acid, the former added as a monocarboxylic acid to the esterification reaction, and the latter added at least partially as an anhydride. In the case of three different monocarboxylic acid reactions, a "moderate" carboxylic acid can be added to the reaction solution as a carboxylic acid or anhydride or a mixture thereof. Monocarboxylic acids can contain aliphatic or aromatic groups. For example, the molecular weight of a monocarboxylic acid can be greater than or equal to 30 g / mol and less than or equal to 250 g / mol. The aliphatic or aromatic groups can have functional groups or substituents other than the carboxyl group.

[0017] According to the present invention, possible reactions may involve, for example, esterification of pentaerythritol molecules. For example, this polyol can be esterified with short-chain monocarboxylic acids (iso-C4) and long-chain monocarboxylic acids (iso-C9). In principle, based on the stoichiometry and reactivity of the individual carboxylic acids, five different tetraesters (non-stoichiometric equations) can be formed:

[0018]

[0019] Reaction Equation 1

[0020] These five different tetraesters are examples of polyol esters formed during the reaction process according to the invention. The composition and specific quantity of the esterified carboxylic acids are determined in principle by the reactivity of the individual monocarboxylic acids, and furthermore, according to the invention, by process control. The proportions of the different esterified carboxylic acids are related to the sum of the mixed polyol esters formed in the method according to the invention.

[0021] In this method, the polyol reacts with various monocarboxylic acids, either as a monocarboxylic acid or a monocarboxylic anhydride, in at least two stages of the reaction. Therefore, the esterification of the polyol is not a simple reaction in which the hydroxyl group reacts with only one carboxylic acid or a mixture of carboxylic acids. The reaction involves the use of at least two different monocarboxylic acids, each used either as a carboxylic acid or as a monocarboxylic anhydride.

[0022]

[0023] Carboxylic acid anhydrides are produced by the reaction of two identical monocarboxylic acids in the absence of water. The reaction proceeds in two stages, during which the composition of the reaction environment is actively altered at least once by external intervention. Therefore, one or more substances are added to the already occurring reaction. The “normal” process of the reaction, where reactants are converted into products, is not considered an active change in the reaction environment. Furthermore, for a period of time, the reaction solution contains only one of the monocarboxylic acids / anhydrides. Possible monocarboxylic acids can be selected from, for example, the group consisting of straight-chain or branched C3-C25 monocarboxylic acids.

[0024] Different monocarboxylic acids react with polyols in order of their reactivity in esterification, starting with the least reactive. The contact and reaction of polyols with different monocarboxylic acids are not random, but rather based on the individual reactivity of the monocarboxylic acid with the polyol. In this paper, reactivity refers to the rate of esterification of the monocarboxylic acid with the polyol discussed. For this purpose, the reaction rate of a simple reaction between a monocarboxylic acid and a polyol can be determined, for example. Methods for determining the reaction rate of esterification reactions are known to those skilled in the art. For example, the reaction rate can be determined using spectroscopic methods capable of quantitatively monitoring ester group formation. The esterification reaction is carried out independently with one carboxylic acid and another. In two different rate determinations, the reaction conditions, such as the temperature and molar amounts used, are kept constant. Regardless of the chosen esterification reaction conditions, the ratio between the reactivity of the two monocarboxylic acids associated with the esterification reaction can always be determined. From the rate comparison, a relationship can be derived that one monocarboxylic acid is more reactive than the other. If different reactivity occurs depending on the reaction conditions (e.g., under extreme temperature or pressure conditions), the reaction rate measured with the same composition at a temperature range of 20°C to 30°C and at atmospheric pressure is decisive for determining reactivity. In terms of time, in esterification reactions, the less reactive monocarboxylic acid comes into contact with the present polyol, and then the polyol comes into contact with the more reactive monocarboxylic acid. This can be achieved, for example, by adding the more reactive monocarboxylic acid later to the reaction solution containing the polyol. In this respect, according to the invention, in the relevant esterification reaction, the esterification reaction of the polyol with some or all of the least reactive monocarboxylic acid always occurs first; therefore, the esterification reaction does not necessarily lead to the complete reaction of the least reactive monocarboxylic acid before the addition of another carboxylic acid. It is sufficient that the carboxylic acid or another carboxylic acid is added to the reaction solution before the least reactive carboxylic acid has completely reacted.

[0025] The less reactive monocarboxylic acid reacts with the polyol at least partially in the form of a monocarboxylic anhydride, followed by the most reactive monocarboxylic acid reacting with the polyol in its monocarboxylic acid form. To control the composition of the polyol ester, at least some of the less reactive monocarboxylic acids in the esterification reaction are added to the reaction solution in the form of a monocarboxylic anhydride. Thus, the anhydride portion of the least reactive monocarboxylic acid can preferably be greater than or equal to 25 mol%, preferably greater than or equal to 50 mol%, more preferably greater than or equal to 90 mol%. Preferably, this monocarboxylic acid can also be added in the form of 100 mol% anhydride. This addition in the form of anhydride is initially uncommon because we are dealing with a “simple” system consisting only of the polyol and the least reactive monocarboxylic acid. However, it has been found that by using it in the form of anhydride, the stoichiometry of the esterification can be largely controlled. In this respect, not only is it faster, but esters of polyols with different compositions can also be obtained. In the method presented herein, the most reactive monocarboxylic acid is added in a later stage, rather than in the form of anhydride. This process control has proven advantageous, although it can be assumed that the addition of the most reactive monocarboxylic acid should also be in the form of anhydride. However, this has proven to be disadvantageous in this method. If the goal is to produce a mixture of more than three different monocarboxylic acids in this method, then the carboxylic acid with "moderate" reactivity can be added either in its natural form or as an anhydride. Crucially, the least reactive monocarboxylic acid is added at least partially as an anhydride, while the most reactive monocarboxylic acid is added as an acid.

[0026] In a preferred embodiment of this method, two different monocarboxylic acids can react with a polyol, wherein in a first stage, the less reactive monocarboxylic acid reacts with the polyol in the form of a monocarboxylic acid anhydride at a concentration greater than or equal to 70 mol%. The method described herein has proven particularly suitable for preparing mixed polyol esters from two different monocarboxylic acids. In this respect, the resulting mixed polyol esters carry two distinct ester groups. The aforementioned minimum anhydride content has proven particularly suitable for obtaining the fastest reaction and controlling the ester content of the less reactive carboxylic acid.

[0027] In another preferred embodiment of the method, the less reactive monocarboxylic acid can react with the polyol in the form of a monocarboxylic anhydride in the first stage, while the more reactive monocarboxylic acid reacts in the form of a monocarboxylic acid in the second stage. For the fastest overall conversion and precise control of the respective proportions of the different carboxylic acids, it has proven particularly advantageous to add the less reactive monocarboxylic acid only in the form of anhydrides to the reaction. This approach, along with the use of only the more reactive monocarboxylic acid as a monocarboxylic acid without an anhydride moiety, particularly allows for the preparation of ester compositions that cannot be obtained either from monocarboxylic acids alone or from the anhydrides of the more reactive carboxylic acids in the second reaction stage.

[0028] In a preferred feature of this method, the polyol may have two or more and eight or fewer OH groups. The method described herein may be particularly suitable for polyols with a small to moderate number of hydroxyl groups. The composition of different carboxylic acids on these relatively small polyols is particularly challenging due to steric constraints during the conversion process. Particular difficulty arises in the conversion of monocarboxylic acids, especially when these molecules have relatively few hydroxyl groups that are also close together, particularly when they have different reactivity. These differences may be based on the structure of the monocarboxylic acid, such as α-branched or non-α-branched carboxylic acids, or due to inductive effects, such as the magnitude of the +i effect of the alkyl chain. Without theoretical constraints, this is particularly likely due to the fact that the proximity of other monocarboxylic acids to the hydroxyl groups of the polyol is significantly hindered by the pre-existing esterification groups. In these cases, the proposed process can yield compositions that would be impossible to obtain without the proposed process control, or only possible under very harsh reaction conditions that readily form byproducts. Preferably, the polyol having the above-mentioned number of OH groups has a molecular weight greater than or equal to 80 g / mol and less than or equal to 700 g / mol, more preferably greater than or equal to 90 g / mol and less than or equal to 600 g / mol. Polyols within the aforementioned molecular weight range preferably have three, four, five, or even six OH groups.

[0029] According to a preferred embodiment of the method, the polyol can be an aliphatic polyol with a molecular weight greater than or equal to 50 g / mol and less than or equal to 400 g / mol. For aliphatic polyols that correspondingly lack aromatic groups, particularly flexible esters with a wide range of ester compositions can be obtained by the method according to the invention. This is difficult to achieve with prior art methods for aliphatic polyols in the aforementioned molecular weight range because these polyols are relatively small, and the esterification reaction with several monocarboxylic acids leads to a significant change in the steric stress around the polyol. In particular, complete esterification of such polyols is difficult to achieve because the available space around the polyol is significantly reduced with the esterification of additional hydroxyl groups, making it more difficult for additional carboxylic acids to enter the esterification reaction. Preferably, the molecular weight of the polyol can be greater than or equal to 100 g / mol and less than or equal to 300 g / mol, and more preferably, the molecular weight can be greater than or equal to 120 g / mol and less than or equal to 250 g / mol. For these polyols, the proposed method can lead to particularly rapid reactions, especially producing esters with a high proportion of less reactive carboxylic acids.

[0030] In a preferred aspect of this method, different reaction stages can be carried out in a single reaction solution without further processing. For rapid and efficient control of the reaction, it has been found that this method is particularly suitable for a "one-pot" reaction in a single reaction solution. Through the process control shown, the desired ester group composition on the polyol can be determined with only slight deviations. Furthermore, this approach avoids time- and cost-intensive processing of the reaction solution. In this case, being in only one reaction solution means that some reactants can also be added to this single reaction solution at later times. Not all reactants must be present in the reaction solution at the start of the reaction.

[0031] According to another preferred embodiment of the method, the esterification reaction can be carried out without the addition of an esterification catalyst. Surprisingly, it has been found that in the process according to the invention, by delaying the addition of the second carboxylic acid in the form of a carboxylic acid and using the carboxylic acid in the form of a less reactive anhydride, an esterification catalyst is completely unnecessary. Compared to the process without a catalyst, the rate of reaction may not be significantly accelerated by an esterification catalyst, thus making the disadvantage of having to separate the catalyst from the desired product at the end of the reaction more significant.

[0032] In a preferred aspect of this method, the less reactive monocarboxylic acid can be present in the form of anhydrides and react initially only with a portion of the polyol, followed by the addition of the remaining polyol along with the more reactive monocarboxylic acid. For rapid and reproducible process control, it has proven particularly suitable to introduce only a portion of the polyol at the start of the reaction and react it with the more reactive anhydride form of the reactant. Only in the second stage, after a certain reaction time, is the remaining polyol added to the portion of the solution from the first stage, along with the less reactive monocarboxylic acid in anhydride form and the more reactive carboxylic acid in acid form. Without being bound by theory, it is likely that maintaining a particularly low viscosity of the reaction solution, especially at the beginning, will allow the esterification reaction to occur more uniformly. The mixing of the solution can be improved by influencing the rheology of the solution.

[0033] According to a preferred embodiment of the method, the less reactive monocarboxylic acid can be a branched monocarboxylic acid with an alkyl group at the α-position of the carboxylic acid group, while the more reactive monocarboxylic acid can be a branched or unbranched C4-C18 monocarboxylic acid without an alkyl group at the α-position of the carboxylic acid group. In particular, according to prior art methods, esterifying relatively small polyols with monocarboxylic acids exhibiting the aforementioned structural differences can be particularly challenging. Substitution at the α-position can result in extremely poor and slow ester bond formation between the monocarboxylic acid and the polyol. This also occurs if the monocarboxylic acid has only a very small aliphatic backbone. The α-position hindrance leads to significant differences in reactivity, even for larger aliphatic monocarboxylic acids with up to 12 carbon atoms. Regardless of reaction control, according to the prior art, under these prerequisites, the proportion of individual carboxylic acids can only be achieved very slowly, and the individual proportions of different carboxylic acids cannot be controlled. With the method according to the invention, the proportion of carboxylic acids with α-position hindrance can also be significantly increased, particularly. Furthermore, for a given target composition, the reaction proceeds significantly faster with the method according to the invention compared to using only monocarboxylic acids without the anhydride component.

[0034] In another preferred aspect of this method, the less reactive monocarboxylic acid can be isobutyric acid, and the more reactive monocarboxylic acid can be isononanoic acid. In particular, under existing reaction conditions, the esterification of isobutyric acid and isononanoic acid can only be used to produce extremely limited quantities of different polyol esters. The difference in reaction rates between isobutyric acid and isononanoic acid on polyols, especially on relatively "small" polyols with a molecular weight of 200-300 g / mol, can only lead to unsatisfactory results, because in most cases, isononanoic acid, rather than isobutyric acid, is added to the polyol. Different process controls using different temperatures and different proportions of the individual carboxylic acids do not significantly improve this result. The conversion takes a long time, especially when complete conversion of the polyol is required. Furthermore, the fraction of isobutyric acid, and thus indirectly the fraction of isononanoic acid, cannot be changed. This is not the case with the method according to the invention. The reaction rate is significantly increased, and in particular, the isobutyrate content in the polyol can also be significantly increased.

[0035] In a preferred embodiment of the method, the molar ratio of the polyol to the less reactive monocarboxylic acid and the molar ratio of the polyol to the most reactive monocarboxylic acid, expressed in each case as the moles of the respective component / moles of the polyol, can be greater than or equal to 1 and less than or equal to 3.5. For reproducible control of the reaction and to obtain the shortest possible overall reaction time, it has proven particularly advantageous to use different carboxylic acids in approximately equal total molar concentration ranges relative to the polyol in the reaction solution. In these cases, the different compositions can be particularly advantageously controlled by adjusting the timing and temperature of their addition during the reaction process. This ratio applies after the full amount of the different carboxylic acids has been added to the reaction solution. Surprisingly, a wide range of different polyol esters can be achieved using approximately equal amounts of different carboxylic acids. This is undesirable because, despite different reaction rates, the concentration of the individual monocarboxylic acid is proportional to the reaction rate of ester formation. In the case of using monocarboxylic acids in the form of anhydrides according to the invention, one mole of anhydride corresponds to two moles of monocarboxylic acid.

[0036] In a preferred aspect of this method, the temperatures in different reaction stages can vary, with the temperature in the first esterification stage of the less reactive monocarboxylic acid in anhydride form being greater than or equal to 50°C and less than or equal to 100°C, lower than the temperature in the final esterification stage of the monocarboxylic acid. For most efficient process control, it has been found particularly suitable that the temperature in the first process stage, i.e., the temperature at which the less reactive monocarboxylic acid in anhydride form is added, is lower than the temperature in the second process stage. Despite the lower temperature, a significantly increased reaction rate is still achieved for the overall reaction; therefore, this process control results in a faster overall conversion, for example, of polyols, compared to methods described in the prior art. The temperature range specified above also applies to significantly reducing the proportion of "undesirable" esters, such as the ratio of polyol esters to other undesirable ester components. This makes the esterification reaction well-controllable and efficient.

[0037] Furthermore, the method according to the invention is used to prepare mixed polyol esters comprising at least two different ester groups and having hydroxyl values ​​greater than or equal to 0 and less than or equal to 10. The method according to the invention may be particularly suitable for obtaining polyol esters carrying only a very small proportion of still free hydroxyl groups. Thus, the mixed polyol esters carry ester groups of different monocarboxylic acids, one of which is a less reactive component and the other a more reactive component. Preparing mixed polyesters with two different ester groups can be challenging in the prior art because esterification becomes slower due to higher steric stress during the reaction of polyols, and the possibility of transesterification reactions increases. For embodiments of this prior art, producing polyol esters within a reasonable reaction time is not feasible. This is especially true for relatively small polyols with molecular weights greater than or equal to 80 g / mol and less than or equal to 400 g / mol. These relatively small polyols may carry, for example, three to five hydroxyl groups. In particular, quantitative esterification reactions with correspondingly small hydroxyl values ​​are extremely difficult to achieve for this group of polyols. The hydroxyl value of the ester can be determined by methods known to those skilled in the art, for example according to DIN 53240-2.

[0038] In a preferred embodiment of the application, the mixed polyol ester can be isobutyric acid / isononanoic acid ester of pentaerythritol. Particularly in the case of pentaerythritol esters or pentaerythritol tetraesters, according to the prior art, the esterification reaction with isobutyric acid and isononanoic acid can only be carried out within a narrow, finite range and over a long reaction time. It is impossible to predetermine the number of different ester groups on the polyol, and primarily results in esters with a significant excess of isononanoic acid. This may be undesirable for some applications because the physical properties of the ester, such as viscosity, are not suited to the required specifications. By implementing the method of the present invention, the isobutyric acid content in the polyol ester can be increased, thus allowing for a wide range of control over the physical and chemical properties of the available ester, including, of course, the physical and chemical properties of the complete ester, and obtaining it within a shorter reaction time.

[0039] In another preferred feature of this application, the polyol ester may have greater than or equal to 20 mol% and less than or equal to 50 mol% isobutyrate groups and greater than or equal to 50 mol% and less than or equal to 80 mol% isononanoate groups. Due to the different reactivity of the aforementioned carboxylic acids, the mixed polyol ester typically has a higher isononanoate group content. By means of the method and application according to the invention, the proportion of isobutyrate groups in the mixed polyol ester can be significantly increased. Furthermore, an increase in the proportion of isobutyrate groups can be obtained in a very short reaction time. A further advantage is that the amount of isobutyrate groups within the aforementioned range can be controlled with particular precision, especially for the full ester. Adding an anhydride of a more reactive monocarboxylic acid in the first stage results in a different distribution of the mixed polyol ester compared to using a less reactive monocarboxylic acid in acid form. By using an anhydride, the resulting mixed polyol ester has a desired higher proportion of less reactive esterified monocarboxylic acids. In this respect, mixed polyol esters with different chemical and physical properties can be obtained, thereby achieving a significantly shortened reaction time by using the less reactive carboxylic acid in anhydride form according to the invention. In particular, the polyol ester can be a pentaerythritol tetraester.

[0040] Further details, features, and advantages of the subject matter of this invention will become apparent from the dependent claims and the following description of the accompanying drawings and related embodiments. Example

[0041] Esterification reactions are typically carried out under reflux at higher temperatures, and a water separator is used to at least partially remove the reaction water produced during the reaction from the reaction solution.

[0042] The embodiments of the present invention are based on the reaction of isobutyric acid (i-C4) and isononanoic acid (i-C9), the former being a less reactive acid with a branch at the α-position of the carboxyl group, and the latter being a more reactive carboxylic acid with no branch at the α-position of the carboxyl group. In particular, the esterification of i-C4 poses a challenge to ester production according to existing techniques because of its high solubility in water (approximately 210-265 g / L at 20°C) and relatively low boiling point (154°C, 1013 mbar). i-C4 is completely miscible with water above 26°C and forms an azeotrope with water (approximately 72%-79% water) with a boiling point of approximately 99°C. Therefore, the reflux of the reaction mixture begins early, with i-C4 returning to the reaction zone from the water separator along with water, thus shifting the equilibrium partially to the reactant side again. Structurally, i-C4 has a methyl group at the α-position, which stereochemically hinders esterification, thus requiring more time for complete conversion of the polyol, or even completely preventing such conversion.

[0043] The polyol used in the examples is pentaerythritol (PE), a low molecular weight aliphatic polyol with four hydroxyl groups.

[0044] I. Esterification reaction based on existing technology

[0045] Ia Based on existing technology, a one-pot esterification reaction using two carboxylic acids simultaneously

[0046] Figure 1 and Figure 2 The variation in ester composition with the amount of acid precipitate when using a mixture of i-C4 / i-C9 carboxylic acids is shown (conditions: 20 mol% excess acid, up to 250 °C, 30 h esterification). In the region studied, less i-C4 was found in the tetraester than the expected amount of reactants in the acid mixture (approximately 3.4 mol%). Possible reasons include the loss of carboxylic acids with the reaction water and the lower overall reactivity of i-C4 compared to i-C9. Furthermore, the reaction time for complete conversion of all OH groups in the polyol was quite long, 30 h.

[0047] The loss of i-C4 with the reaction water poses a problem in these transformations because the organic components must be removed from the wastewater stream at a significant cost. Furthermore, as mentioned above, removing reactants from equilibrium is detrimental to the reaction rate and the composition of the final product.

[0048] Ib, based on existing technology, involves a one-pot esterification reaction using both carboxylic acids and organic entrainers.

[0049] The esterification reaction was carried out in a multi-necked round-bottom flask connected to a Dean-Stark apparatus for water separation. The mass of the aqueous phase after the reaction was 12% by weight higher than the theoretical value. The water content was found to be 86% by weight, which corresponds to only 96% reaction water in the aqueous phase. The remaining water either escaped through the gas flow or remained dissolved in the organic phase. The remaining 14% by weight of the aqueous phase consisted mainly of isobutyric acid and a small amount of isononanoic acid.

[0050] To avoid excessive reactant content in the reaction water and thus accelerate the reaction, the use of an entrainer is conceivable. The above experiment was repeated using different entrainers (duene, naphthalene, and tetrahydronaphthalene). Despite the use of entrainers, the water content in the aqueous phase could not be further increased to 85% by weight. Therefore, using an entrainer is not a substitute for accelerating and simplifying the reaction.

[0051] According to existing technology, the transesterification reaction of the whole ester is a one-pot esterification reaction.

[0052] The transesterification reaction was also tested to avoid water accumulation and associated acid loss. After 4 hours at 92°C, the reaction between methyl isobutyrate and pentaerythritol was not observed.

[0053]

[0054] The transesterification experiment using pentaerythritol iso-C9 ester and i-C4 at a temperature of about 130 °C and a reaction time of 6 hours showed that the composition of the polyol ester changed only slightly.

[0055]

[0056] Transesterification experiments using pentaerythritol-based iso-C9 esters and i-C4 esters showed that acid exchange occurred. However, the transesterification reaction with i-C4 showed only slight compositional changes at 168 °C and 20 h, even with 50 mol% i-C4 added relative to the OH groups of the polyol. Furthermore, the use of different Lewis acid catalysts or sodium acetate showed no significant difference from the reference reaction without a catalyst.

[0057]

[0058] Under reasonable reaction time and conditions, transesterification is unlikely to significantly alter the composition of esters.

[0059] Id follows the sequential esterification reaction according to existing technology

[0060] Because the simultaneous esterification of i-C4 and i-C9 with pentaerythritol has a relatively long reaction time, and due to the lower tendency for transesterification, experiments were conducted on sequential esterification reactions. In these experiments, the less reactive i-C4 reacted first with pentaerythritol, and then in a second stage, the remaining hydroxyl groups of the polyol were esterified with the more reactive i-C9 at a higher temperature.

[0061]

[0062] This experiment aims to adjust the ester composition of polyols to have an i-C4 content exceeding 30 mol%.

[0063] In the experiment, the polyol (pentaerythritol) reacted with the i-C4 acid in the first 5 hours. To avoid acid loss in the reaction water (Dean-Stark apparatus), a reaction temperature below the boiling point of i-C4 was chosen. After 5 hours, the i-C9 acid was added and the mixture was heated to 250°C and maintained for another 20 hours. The amount of water formed then decreased over the next 5 hours. The amount of water formed in the first 5 hours was only 29% of the theoretical amount, indicating that the i-C4 conversion was very slow. After the addition of i-C9, 69% of the theoretical amount of water was formed. The missing 2% of the theoretical amount of water was found in the organic phase in the Dean-Stark apparatus. Analysis of the esters after main stripping showed an OH value of 0.5 mg KOH / g, and the contents of isobutyrate and isononanoate were 33 mol% and 67 mol%, respectively.

[0064] A mixed full ester can be obtained through a relatively long reaction time, meaning that the desired proportions cannot be achieved within a given reaction time and temperature using appropriate amounts of reactants. The less reactive i-C4 component undergoes substoichiometric esterification only with the OH groups of the polyol.

[0065] Ie uses the acid anhydride of a less reactive component to simultaneously carry out a one-step esterification reaction.

[0066] A one-pot experiment was conducted using isobutyric anhydride and i-C9. After 30 minutes, GC analysis at a reaction temperature of 150°C showed that a mixture of different anhydrides had formed in the reaction solution.

[0067]

[0068]

[0069] Therefore, it is not advisable to use isobutyric anhydride and isononanoic acid at the same time, because isononanoic acid is highly reactive and will form mixed anhydrides.

[0070] II. According to the present invention, a two-stage esterification reaction using acid anhydrides in the first stage.

[0071] Preliminary experiments using isobutyric anhydride and pentaerythritol revealed high reactivity, necessitating careful increases in reaction temperature. Isobutyric anhydride has a boiling point of 183°C, while the isobutyric acid released during esterification has a boiling point of 156°C. The reaction of carboxylic anhydride with pentaerythritol may suddenly produce a large amount of isobutyric acid. If the reaction temperature exceeds 156°C at this point, boiling deformation may occur. Due to the rapid reaction, temperatures below 156°C can quickly rise above the boiling point of the short-chain acid due to energy release.

[0072] According to the present invention, during the esterification reaction, pentaerythritol first reacts with isobutyric anhydride. The amounts of anhydride and pentaerythritol are used in a molar ratio, as in the hypothetical ester synthesis, with the aim of achieving an iso-C4 content in the ester exceeding 30 mol%, i.e., acid excess. The reaction is carried out at 150°C for 4 hours. Then, isononanoic acid is added, and the esterification reaction is carried out for another 16 hours at a maximum temperature of 250°C. The hydroxyl values ​​of the obtained esters are 3.5 mg KOH / g and 3.8 mg KOH / g, respectively, thus achieving almost complete esterification of the polyol. Furthermore, infrared spectroscopy shows that, at temperatures as high as 150°C, more than 30 minutes after the start of the first stage of the experiment, the spectra recorded during the reaction show almost no difference. This indicates that the esterification reaction of the polyol is almost complete. This shows that the esterification reaction of the i-C4 component, present in the form of anhydride, has reached a high reaction progress at this point. After 4 hours, isobutyric anhydride is no longer found in the mixture by gas chromatography.

[0073] The two-stage method according to the invention, which uses an anhydride in the first stage, compared with a method not according to the invention that does not use an anhydride, shows that anhydride has a rate advantage over acid in the sequential conversion of pentaerythritol. Besides the different OH values, the iso-C4 content of the ester also differs. Due to the higher reactivity of the anhydride, the product obtained after esterification has a higher iso-C4 content (approximately 5 mol%) compared to using a mixture of pure acids.

[0074] At 150°C for 30 minutes + at a maximum temperature of 250°C for 20 hours Only acid According to the present invention Pentaerythritol [g] 100 100 Pentaerythritol [mol] 0.735 0.735 <![CDATA[iso-C4 acid [g]]]> 115 - <![CDATA[iso-C4 acid [mol]]]> 1.305 - <![CDATA[iso-C4 anhydride [g]]]> - 103.2 <![CDATA[iso-C4 anhydride [mol]]]> - 0.652 <![CDATA[iso-C9 acid [g]]]> 351.4 351.4 <![CDATA[iso-C9 acid [mol]]]> 2.221 2.221 <![CDATA[Isobutene content (ester) [mol%]]]> 29 34 Esterification reaction time [hours] 20.5 20.5 Reaction temperature [°C] 150-250 150-250 OH value [mg KOH / g] 11 4

[0075] As can be seen from the table, within the same reaction time and temperature range, not only was a more complete conversion achieved (identifiable by a lower OH value), but a significant increase in the iso-C4 content of the ester was also obtained. Surprisingly, these advantages were maintained even with longer temperatures and reaction times for i-C9 esters, suggesting that strong transesterification reactions should preferentially proceed, which should significantly reduce or completely eliminate the influence of the first reaction stage.

[0076] III. Kinetic considerations for a two-stage esterification reaction using an anhydride in the first stage according to the present invention.

[0077] The method according to the invention aims to produce i-C4 / i-C9 mixed esters with high i-C4 content. The first stage reaction was carried out using i-C4 anhydrides at a reaction temperature of 170°C. Five experiments were conducted in each case. The average and standard deviation of the C4 ratio in a series of experiments are given:

[0078] The i-C4 ratio leads to the conclusion that a particularly high i-C4 ratio can be achieved in the ester by using anhydrides in the first stage. It is equally evident that the i-C4 content in the resulting ester decreases again with prolonged reaction time in the second stage due to the competitive reaction with the i-C9 acid. Nevertheless, esterification via anhydrides appears suitable for providing mixed esters with relatively high i-C4 content under shorter reaction times and milder reaction conditions. Even after 8 hours of reaction with the i-C9 acid, i.e., a total reaction time of 9 hours, these ratios remain in the range of i-C4 content greater than 30 mol%. This ratio is significantly higher than that of esters not treated according to the invention. This significantly higher ratio also allows for significantly longer reaction times for the i-C9 acid to achieve more complete conversion, without inducing strong transesterification reactions.

[0079] To determine the kinetic effects of the esterification reaction, the method according to the invention using i-C4 anhydrides was compared with a method not according to the invention using i-C4 acids in the esterification reaction with PE. In both cases, after the i-C4 components reacted for one hour in the first stage, the i-C9 acid was added in the second stage, and esterification was carried out for 3 hours. The percentages of the mixed PE esters (names are given in reaction equation 1) and the percentages of i-C4 in the total mixture of polyol esters are given.

[0080]

[0081] Comparative studies show that using i-C4 anhydrides in the first stage results in a higher proportion of i-C4 in the resulting mixed polyol esters compared to using i-C4 acids. Furthermore, using i-C4 anhydrides in the first stage leads to a higher degree of esterification after 4 hours of reaction compared to using i-C4 acids, and the reaction is more complete. Notably, despite the longer reaction time in the second stage, using i-C4 anhydrides in the first stage results in a higher proportion of esters such as PE4444 and PE4449. This has a positive impact on the i-C4 content in the total mixed polyol esters.

[0082] In further determining the kinetic effects of the esterification reaction, the method according to the invention was again compared with the i-C4 acid in the form of anhydrides and the subsequent i-C9 esterification reaction stage. With the use of anhydrides, the reaction time for the first stage was maintained at 1 hour. In the non-invention method, with the use of an acid in the first stage, the reaction time was doubled to 2 hours. The reaction time in the second stage varied (2-8 hours). The percentages of the mixed PE esters (see reaction equation 1) and the percentage of i-C4 in the total polyol ester mixture are given.

[0083] According to the present invention:

[0084]

[0085] Not based on the present invention:

[0086]

[0087] It can be seen that by using the anhydride of a less reactive acid, the process time in the first process stage is significantly shortened (1 hour for i-C4 anhydride versus 2 hours for i-C4 acid), resulting in a mixed polyol ester with a fairly high proportion of less reactive acids. This is unusual given the long reaction time and high temperature required for using a more reactive acid in the second process stage.

Claims

1. A method for preparing a mixed polyol-carboxylic acid ester, wherein the mixed polyol-carboxylic acid ester has a molecular weight greater than or equal to 200 g / mol and less than or equal to 1000 g / mol, characterized in that, In at least two stages of the reaction, the polyol reacts with different monocarboxylic acids, either as monocarboxylic acids or monocarboxylic anhydrides, wherein the different monocarboxylic acids react with the polyol in order of their reactivity in the esterification reaction, starting with the least reactive. In the first stage, the less reactive monocarboxylic acid reacts with the polyol in the form of a monocarboxylic anhydride, and in the second stage, the more reactive monocarboxylic acid reacts with the polyol in the form of a monocarboxylic acid. The less reactive monocarboxylic acid is a branched monocarboxylic acid with an alkyl group at the α-position of the carboxyl group, and the more reactive monocarboxylic acid is a branched or unbranched C4-C18 monocarboxylic acid without an alkyl group at the α-position of the carboxyl group.

2. The method according to claim 1, wherein the polyol has two or more and eight or fewer OH groups.

3. The method according to claim 1 or 2, wherein the polyol is an aliphatic polyol having a molecular weight greater than or equal to 50 g / mol and less than or equal to 400 g / mol.

4. The method according to claim 1 or 2, wherein the different reaction stages are carried out in only one reaction solution and no further processing is required.

5. The method according to claim 1 or 2, wherein the esterification reaction is carried out without the addition of an esterification catalyst.

6. The method according to claim 1 or 2, wherein the less reactive monocarboxylic acid is isobutyric acid, and the more reactive monocarboxylic acid is isononanoic acid.

7. The method according to claim 1 or 2, wherein the molar ratio of the less reactive monocarboxylic acid to the polyol and the molar ratio of the more reactive monocarboxylic acid to the polyol are greater than or equal to 1 and less than or equal to 3.

5.

8. The method according to claim 1 or 2, wherein the temperature is different in different reaction stages, wherein the temperature of the less reactive monocarboxylic acid in the first esterification reaction stage is greater than or equal to 50°C and less than or equal to 100°C, which is lower than the temperature in the final esterification reaction stage of the monocarboxylic acid.

9. Use of the method according to any one of claims 1-8 for preparing a mixed polyol ester comprising at least two different ester groups and having a hydroxyl value greater than or equal to 0 and less than or equal to 10.

10. The use according to claim 9, wherein the mixed polyol ester is isobutyric acid / isonononate of pentaerythritol.

11. The use according to claim 10, wherein the polyol ester has greater than or equal to 20 mol% and less than or equal to 50 mol% isobutyric acid groups and greater than or equal to 50 mol% and less than or equal to 80 mol% isononate groups.

Images