NEO-ACID CATALYSTS AND THEIR USE IN ESTERIFICATION REACTIONS

Abstract: Neo-acid compounds can be used as non-reactive catalysts in carrying out esterification reactions. The neo-acids can be isolated by subjecting a neo-acid composition to esterification with an excess of alcohol and recovering the non-reactive neo-acid fraction therefrom. The non-reactive fraction can be used as a non-reactive catalyst in a subsequent esterification.

Description: NEO-ACID CATALYSTS AND THEIR USE IN ESTERIFICATION REACTIONS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to neo-acid catalysts and to their use as non- reactive catalysts in esterification reactions. 2. Description of the Related Art Reacting a carboxylic acid moiety with an alcohol to form an ester is a basic and well known chemical reaction used in a variety of different chemical synthesis schemes. The reaction is catalyzed by acid. Accordingly, the carboxylic acid reactant can serve as both reactant and catalyst. However, as the reaction proceeds, and the acid reactant is consumed, the amount of acid available to serve as a catalyst steadily decreases. Thus, at higher conversion rates, catalytic action is reduced. This causes the reaction rate to be slowed at higher conversion rates resulting in longer reaction times and/or less than complete conversion into the ester. One approach to counter this effect is to run the reaction with an excess of acid reactant. The excess acid insures that even at higher conversion rates there is sufficient acid remaining to serve as a catalyst. However, the use of a sufficient excess of acid is not always convenient, practical or even possible. For example, in forming complex esters made from polybasic acid, polyhydric alcohol and monohydric alcohol, the hydroxyl-containing compounds are frequently required to be used in stoichiometric excess in order to control chain length, hydroxyl number, and/or physical properties of the ester product. The use of an excess amount of reactant acid as a catalyst is not possible in this esterification reaction because it would prevent the formation of the desired product. Another technique to achieve high or complete conversion in shorter time periods is to add a separate catalyst. Traditional esterification catalysts include acids such as p-toluene sulfonic acid, sulfuric acid, phosphorous acid and trichloroacetic acid as well as metal or metal oxides such as titanium tetra butoxide and aluminum powder. These catalysts can be effectively employed in a variety of esterification reactions. However, removing these types of catalysts from the ester product can be difficult, resulting in lower product quality and/or lower process efficiency. Accordingly, there is a need in the art for new, alternative types of esterification catalysts. SUMMARY OF THE INVENTION The present invention relates to non-reactive catalytic neo-acids. In particular, one embodiment of the present invention relates to a process, which comprises reacting a carboxylic acid compound with a hydroxyl compound to form an ester, wherein the reaction is carried out in the presence of a catalytic amount of a non- reactive catalytic neo-acid compound. Another embodiment of the present invention relates to a process which comprises combining a polybasic acid compound or anhydride thereof, a polyhydric alcohol compound, optionally a mono-hydric alcohol, and a non-reactive catalytic neo-acid compound into a mixture; and reacting together carboxylic acid and hydroxyl groups present in the mixture to form esters. A further embodiment of the present invention relates to a process which comprises subjecting an isomeric mixture of neo-acids to an esterification reaction with an excess of alcohol to esterify all of the reactive neo-acid; isolating the non-reactive neo-acid from said composition; and adding said non-reactive neo-acid to a reaction medium containing a carboxylic acid compound and a hydroxyl compound, in an amount sufficient to catalyze an esterification reaction between said carboxylic acid compound and said hydroxyl compound. These and other embodiments as hereinafter described are based on the discovery that certain neo-acids can serve as non-reactive catalysts in esterification reactions. While not wishing to be bound by any theory, it is believed that the stearic hindrance in certain neo-acid isomers prevents the carboxylic acid moiety from being reacted. These carboxylic acid compounds, while essentially unreactive in the esterification reaction, can nonetheless function as a catalyst. Typically, the non- reactive neo-acid catalysts of the present invention are either non-objectionable in the ester product or easily separated from the ester product. Thus these catalysts will generally avoid the separation difficulty experienced by the aforementioned prior art catalysts. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates the reaction kinetics of esterifying a group of neo-decanoic acid isomers with iso-decyl alcohol in accordance with Example 1. Figure 2 illustrates a gas chromatogram of the neo-decanoic acid composition used in Example 1. Figure 3 illustrates the gas chromatogram for the recovered neo-decanoic acid and unreacted alcohol from Example 1. Figure 4 illustrates a comparison of the kinetics for catalyzed and uncatalyzed esterification as set forth in Example 2. Figure 5 illustrates the gas chromatogram for the uncatalyzed reaction product of Example 2. Figure 6 illustrates the gas chromatogram for the catalyzed reaction product of Example 2. DETAILED DESCRIPTION OF THE INVENTION Neo-acids, also known as trialkyl acetic acids, are generally represented by the formula: wherein R'-R 3 each represent an alkyl group. Neo-acids have been used, albeit with difficulty, to form a variety of ester products. The stearic hindrance of the trialkyl group reduces the reactivity of the carboxylic acid moiety. Thus, the larger or bulkier the alkyl groups, the more difficult the neo-acid would be to esterify. The present invention turns this normally troublesome propensity into an advantageous property. Namely, by selecting neo-acids that do not react with the hydroxyl groups in the reaction medium, the neo-acid can function as a non-reactive catalyst. As used herein, the phrase "non-reactive" means that the neo-acid is essentially not consumed; i.e., less than 5 %, preferably less than 2 % and more preferably less than 1 % of the neo-acid is consumed in the given esterification reaction under the conditions employed. Because the neo-acid is non-reactive, its presence does not alter the molar ratio of the reactant carboxylic acid group to the hydroxyl group. This ability is particularly useful in esterification reactions where excess alcohol is required, such as in the production of complex alcohol esters. The neo-acid catalyst of the present invention can be added to such esterification reactions without adversely affecting the alcohol excess and thereby provide a catalytic effect without disrupting the desired ester product. Furthermore, because the neo-acid catalyst is an organic acid, it is relatively easy to separate from the ester product. Typically, each of three alkyl groups attached to the alpha carbon in the neo- acid has from 1 to 18 carbon atoms, which may be, independently, straight or branched chain. The total number of carbons in the neo-acid is typically at least 7, generally from 9 to 26, more usually 9 tol8, and conveniently 10 to 14. A non-reactive neo-acid having fewer than 7 carbon atoms in total is possible, but uncommon and would most likely be involved in a reaction where the hydroxyl group was stearically hindered or the reaction conditions otherwise do not favor esterification. Having more than 26 total carbon atoms tends to form a solid, waxy substance that is not easy to use in a liquid reaction environment and is thus not normally desired. Preferred neo-acid catalysts are C , Cio, and C 1 neo-acids. Generally at least one and more usually at least two of Ri - R 3 represent an alkyl group containing 3 to 18 carbon atoms, preferably with branching in at least one of the 3 to 18 carbon atom alkyl groups. In some embodiments, one of the alkyl groups, such as R 2 , is a methyl or ethyl group while the remaining alkyl groups, R 1 and R 3 , each have 3 to 18 carbon atoms. In these embodiments, at least one of, and optionally both of, R 1 and 3 are branched. Preferred non-reactive neo-acid catalysts include branched isomers having a total of 10 or 13 carbon atoms, such as a neo- decanoic acid. The non-reactive neo-acids can be used to catalyze an esterification reaction between a carboxylic acid compound and a hydroxyl compound to form an ester. A "hydroxyl compound" means any compound having at least one hydroxyl group (OH) therein and includes mixtures of such compounds. The hydroxyl compound includes compounds represented by the formula R(OH) n wherein R is an aliphatic hydrocarbyl group, a cycloaliphatic hydrocarbyl group, an aryl group or an aralkyl group and n is 1-8. The hydroxyl compound generally contains 3 to 20 carbon atoms, but is not limited thereto. The "hydrocarbyl group" means a group containing 3 to 20 carbon atoms and which may also be substituted with chlorine, nitrogen and/or oxygen atoms. Accordingly compounds containing oxyalkylene groups such as polyetherpolyols are included with the meaning of the above hydroxyl compound formula. Typically, R is a branched aliphatic hydrocarbyl group containing 5 to 13 carbon atoms. Examples of suitable hydroxyl compounds, including both mono-hydric and polyhydric compounds, are n-pentyl alcohol, iso-pentyl alcohol, n-heptyl alcohol, iso-heptyl alcohol, n-octyl alcohol, 2-efhylhexyl alcohol, n-octyl alcohol, iso-octyl alcohol, n-nonyl alcohol, iso-nonyl alcohol, n-decyl alcohol, iso-decyl alcohol, neopentyl glycol, trimethyolethane, trimethylolpropane, trimethylolbutane, pentaerythritol, di-pentaerythritol and mixtures thereof. A "carboxylic acid compound" is any compound that contains at least one carboxylic acid moiety (COOH) therein, and includes mixtures of such compounds. The carboxylic acid compound can be a straight or branched chain aliphatic carboxylic acid compound or an aromatic carboxylic acid compound. Typically the carboxylic acid compound contains 2 to 24 carbon atoms, but is not limited thereto. The carboxylic acid compound can be a polybasic acid (polycarboxylic acid) compound or anhydride thereof such as a dibasic acid. Examples of suitable dibasic acids include oxalic acid, succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, dodecanedioic acid, phthalic acid, and anhydrides thereof. A convenient way to identify a suitable non-reactive catalytic neo-acid is to subject a neo-acid composition, generally a mixture of neo-acid isomers, to esterification with an excess of alcohol under conventional conditions. If non-reactive neo-acids are present, the conversion will reach a plateau where essentially no further conversion of the acid reactant into an ester is occurring. At this point all of the reactive neo-acids have been consumed in the reaction and the unreacted neo-acid remaining in the reaction medium is the non-reactive neo-acid catalyst. The non- reactive neo-acid(s) remaining can be isolated and used to catalyze an esterification reaction by adding it to a reaction medium containing a carboxylic acid compound and a hydroxyl compound, in an amount sufficient to catalyze an esterification reaction. It is preferred that the alcohol used to screen for the non-reactive neo-acid is the same as the hydroxyl compound used in the subsequent esterification reaction. In this way, the neo-acid isomers recovered from the first reaction should be non-reactive for purposes of the second reaction. Also, the neo-acids are conveniently isolated from the first reaction as an alcohol solution that can be directly added to the subsequent esterification reaction. If the alcohol is different from the hydroxyl compound used in the second esterification reaction, then the effect of the alcohol on the structure of the ester product would have to be taken into account and may warrant a further isolation of the neo-acid from the alcohol. Generally, the carboxylic acid compound need not be similar to the neo-acid catalyst and usually the non-reactive catalytic neo-acid has a different number of carbon atoms than the acid reactant being esterified. A preferred non-reactive catalytic neo-acid is a neo-acid isomer or mixture thereof that is non- reactive with iso-decyl alcohol. Neo-acids are, in general, commercially available. Non-reactive neo-acids suitable for use as an esterification catalyst can be empirically determined by screening the commercially available neo-acid compositions in the manner described above. Neo-acids can also be synthesized by well known techniques. For example, small olefins such as propylene can be polymerized to yield larger olefins such as C 6 , C 9 , C12, etc. A carboxyl group (COOH) is added to the olefin at the point of unsaturation to make a corresponding C 7 , C10, C 13 , etc., acid. This synthesis scheme, which is commonly employed commercially, leads to a product having a mixture of acids both in terms of carbon number (i.e. a small amount of the C9 and Cπ mixed with the intended C10 acid, for example) and structural isomers. The range of variation in neo- acid and the level of purity depends upon the synthesis scheme and the conditions used. While it is not necessary to reveal the exact structural formula for the non- reactive neo-acid catalyst, given the empirical screening methodology described above, it is generally desired when seeking to synthesize non-reactive catalytic neo-acids to use conditions that favor the formation of long chains in the neo structure. Such structural features will enhance the likelihood of producing and isolating a non-reactive catalytic neo-acid from the neo-acid population produced. Alternatively, targeted synthesis of a non-reactive catalytic neo-acid isomer could be performed. Non-reactive catalytic neo-acids can be used in any esterification reaction that would benefit from an acid catalyst. For example, in conventional esterification reactions that utilize p-toluene sulfonic acid, tetra iso-propoxy titanate, or another conventional esterification catalyst, the non-reactive catalytic neo-acid can be used as a replacement catalyst. Preferably, the non-reactive catalytic neo-acid is the only non- reactive catalyst present during the esterification reaction; i.e., no catalyst other than the reactant carboxylic acid compound, is added or present during the esterification reaction. The esterification reaction can produce a final product, such as a base stock oil or refrigerant lube, or an intermediary product. Indeed, esterification is commonly used in organic synthesis schemes to protect a hydroxyl or carboxylic acid group, and such reactions are specifically contemplated as being within the scope of an esterification reaction of the present invention. The amount of catalyst added should be effective to catalyze the esterification reaction and is normally at least 1 mol %, generally 1 mol % to 50 mol %, especially 2 mol % to 30 mol %, taking the amount of reactant carboxylic acid compound initially charged as being 100%. In some embodiments, 5 mol % to 15 mol % of the non-reactive neo-acid catalyst is added to the reaction medium, based on the amount of reactant carboxylic acid compound initially charged. The use of small amounts of catalyst may not provide sufficient catalytic effect. On the other hand, the use of large amounts of catalyst tends to dilute the concentration of reactants in the reaction vessel, thereby limiting total yield; i.e. less volume in the vessel for reactants means that less product is produced. The non-reactive catalytic neo-acid can be combined with the reactants in a variety of ways. In one embodiment, the non-reactive catalytic neo-acid is added separately from the acid reactant. As mentioned above, because the non-reactive catalytic neo-acid is non-reactive with the hydroxyl compound, it can be supplied as a separate isolated neo-acid composition or as an alcohol solution, or it can be previously combined into the hydroxyl feed stream in either the isolated or alcohol solution form. This ability to have the neo-acid mixed with the hydroxyl compound represents another aspect of the present invention, specifically a composition consisting essentially of a non-reactive catalytic neo-acid and an alcohol. Preferably, the neo-acid contained in the composition comprises 5 to 95 % by weight of the composition, more preferably 5 to 50 wt %. In general, an esterification reaction proceeds as follows. A carboxylic acid compound and a hydroxyl compound are combined with a non-reactive catalytic neo- acid and reacted at an elevated temperature usually greater than 100°C and frequently within the range of about 180°C to 250°C. Lower temperatures can be used but generally at the cost of much longer reaction times and lower yields. Frequently, the reaction temperature and pressure are sufficient to allow the water produced by esterification to be boiled off. Hydroxyl compound that is vaporized with the water is preferably refluxed back into the reaction mixture. The non-reactive catalytic neo-acid can be present at the start of the reaction or it can be added during the reaction, such as after 10 %, or after at least 50 % or even after at least 90 % by mole of the limiting reagent in the esterification reaction has been reacted. Typically esterification reactions are driven to completion or near completion, i.e. conversions of at least 99.8%, more preferably at least 99.9%, although lower conversion rates are also contemplated. The ester can be fully esterified (at least 96 % conversion of the limiting reagent (either hydroxyl group or carboxylic acid groups) or partially esterified (5% to less than 96% of the carboxylic acid groups are esterified). The ester can be simple or complex. Preferably the non-reactive catalytic neo-acid is used to produce esters that are complex and/or have branching. For example, the esters and the esterification conditions disclosed in U.S. Patents 5,750,750, 4,661,662, 4,053,491, 4,263,159, 4,957,649, 5,164,122, 5,665,585, 5,698,502, and5,817,607 and in WO90/12849 are appropriate for using the non-reactive catalytic neo-acid of the present invention and each one is incorporated herein in its entirety. Example 1 Neo-decanoic acid is reacted with excess iso-decyl alcohol. The reaction proceeds to approximately 70% conversion (see Figure 1) at which point no further appreciable progress is made. Having reached this plateau, the remaining unreacted neo-decanoic acid and the iso-decyl alcohol are recovered by vacuum stripping and condensation. Illustrative gas chromatograms of the original neo-decanoic acid feed and the recovered unreacted species are shown in Figures 2 and 3, respectively. While the amount of neo-decanoic acid is reduced by the esterification, a portion remains that corresponds to the unreacted/non-reactive neo-decanoic acid isomer. Example 2 A complex ester is synthesized by reacting trimethylol propane (TMP), adipic acid (AA), and iso-decyl alcohol (IDA) at 220°C in an inert nitrogen atmosphere. To demonstrate the beneficial effect of the non-reactive neo-acid catalyst recovered in Example 1, two reactions were run side-by-side with TMP/AA/TDA molar feed ratios of 1/ 2.7 / 3.12. Reaction "A" is uncatalyzed while reaction "B" is catalyzed with 2.1 mole % of the neo-acid catalyst, based on the adipic acid. The amount of IDA was adjusted in reaction "B" to take into account the IDA contained in the catalyst charge (i.e. the catalyst was added as an alcohol solution). Figure 4 illustrates the differences in conversion rate. The presence of the non-reactive neo-acid catalyst facilitates full conversion while the uncatalyzed reaction plateau's at approximately 99.6% conversion. Since the quantity of neo-acid is insignificant compared to the initial amount of AA, the catalyst has almost no effect on the reaction rates at low conversions. But, as the reaction proceeds and the AA is consumed, the neo-acid catalytic effect becomes significant and has a noticeable effect on conversion. Figures 5 and 6 illustrate the gas chromatograms for the products of reaction "A" and "B", respectively. Noticeably, Figure 6 shows the presence of the neo-acid peak, indicating that the neo-acid was non-reactive and hence not consumed. Once the reaction is completed, the neo-acid can be removed by stripping. The invention having been thus described, it will be obvious that the same may be varied in many ways without departing from the scope and spirit of the invention as defined by the following claims.

Claims: We claim: 1. A process, which comprises reacting a carboxylic acid compound with a hydroxyl compound to form an ester, wherein said reaction is carried out in the presence of a catalytic amount of a non-reactive catalytic neo-acid compound. 2. The process according to claim 1, wherein said non-reactive catalytic neo-acid compound has 9 to 26 carbon atoms. 3. The process according to claim 2, wherein said non-reactive catalytic neo-acid compound has 9 to 18 carbon atoms. 4. The process according to claim 3, wherein said non-reactive catalytic neo-acid compound has 10 to 14 carbon atoms. 5. The process according to claim 4, wherein said non-reactive catalytic neo-acid compound is an isomer of neo-decanoic acid. 6. The process according to claim 1, wherein said neo-acid compound has a different carbon atom number than said carboxylic acid compound. 7. The process according to claim 1, wherein said non-reactive catalytic neo-acid compound is present in an amount within the range of about 1 to 50 mole %, based on the initial unreacted molar amount of carboxylic acid compound. 8. The process according to claim 1, wherein said hydroxyl compound is represented by the formula: R(OH) n wherein R is an aliphatic hydrocarbyl group, a cycloaliphatic hydrocarbyl group, an aryl group or an aralkyl group and n is 1-8. 9. The process according to claim 8, wherein said hydroxyl compound contains 3 to 20 carbon atoms. 10. The process according to claim 9, wherein R is a branched aliphatic hydrocarbyl group containing 5 to 13 carbon atoms. 11. The process according to claim 10, wherein said hydroxyl compound is selected from the group consisting of iso-pentyl alcohol, iso-heptyl alcohol, iso-octyl alcohol, 2-ethylhexyl alcohol, iso-nonyl alcohol, iso-decyl alcohol, neopentyl glycol, trimethyolethane, trimethylolpropane, trimethylolbutane, pentaerythritol, di- pentaerythritol and mixtures thereof. 12. The process according to claim 1, wherein said hydroxyl compound is present as a mixture of hydroxyl compounds. 13. The process according to claim 1 , wherein said carboxylic acid compound is a straight or branched chain aliphatic carboxylic acid compound or an aromatic carboxylic acid compound. 14. The process according to claim 13, wherein said carboxylic acid compound is a polybasic acid compound or anhydride thereof. 15. The process according to claim 13, wherein said carboxylic acid compound is a branched chain aliphatic carboxylic acid compound. 16. The process according to claim 13, wherein said carboxylic acid compound contains 2 to 24 carbon atoms. 17. The process according to claim 13, wherein said carboxylic acid is selected from the group consisting of oxalic acid, succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, dodecanedioic acid, phthalic acid, and anhydrides thereof 18. The process according to claim 1, wherein no additional non-reactive catalyst is present during said reaction. 19. A process, which comprises: (a) combining a polybasic acid compound or anhydride thereof, a polyhydric alcohol compound, optionally a mono-hydric alcohol, and a non-reactive catalytic neo-acid compound into a mixture; and (b) reacting together carboxylic acid and hydroxyl groups present in said mixture to form esters. 20. The process according to claim 19, wherein said polybasic acid compound contains 2 to 20 carbon atoms. 21. The process according to claim 19, wherein said polybasic acid compound is selected from the group consisting of oxalic acid, succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, dodecanedioic acid, and phthalic acid. 22. The process according to claim 19, wherein said polyhydric alcohol contains 3 to 20 carbon atoms. 23. The process according to claim 22, wherein said polyhydric alcohol is selected from the group consisting of neopentyl glycol, trimethyolethane, tnmethylolpropane, trimethylolbutane, pentaerythritol, di-pentaerythritol and mixtures thereof. 24. The process according to claim 19, wherein said monohydric alcohol is present in said mixture and contains from 3 to 20 carbon atoms. 25 The process according to claim 24, wherein said monohydric alcohol is selected from the group consisting of n-pentyl alcohol, iso-pentyl alcohol, n-heptyl alcohol, iso- heptyl alcohol, n-octyl alcohol, n-octyl alcohol, iso-octyl alcohol, 2-ethylhexyl alcohol, n-nonyl alcohol, iso-nonyl alcohol, n-decyl alcohol, iso-decyl alcohol 26 The process according to claim 19, wherein said non-reactive catalytic neo-acid contains 7 to 24 carbon atoms 27 The process according to claim 26, wherein said non-reactive catalytic neo-acid is a neo-decanoic acid 28 The process according to claim 19, wherein said mixture contains a stoichiometric excess of hydroxyl groups 29 The process according to claim 28, wherein said reacting step esterifies more than 99 9 % of the carboxylic acid groups in said mixture 30 A process which comprises (a) combining a carboxylic acid compound feed stream, a hydroxyl compound feed stream and a non-reactive catalytic neo-acid to form a reaction mixture, wherein said non-reactive catalytic neo-acid is combined into said reaction mixture separately from said carboxylic acid feed stream, and (b) reacting said carboxylic acid compound with said hydroxyl compound to form an ester 31 The process according to claim 30, wherein said non-reactive catalytic neo-acid is combined into said reaction mixture as an alcohol solution 32 The process according to claim 30, wherein said non-reactive catalytic neo-acid is present in said hydroxyl compound feed stream 33. A process which comprises: (a) subjecting a neo-acid composition to an esterification reaction with an excess of alcohol to esterify all of the reactive neo-acid; (b) isolating the non-reactive neo-acid from said composition; and (c) adding said non-reactive neo-acid to a reaction medium containing a carboxylic acid compound and a hydroxyl compound, in an amount sufficient to catalyze an esterification reaction between said carboxylic acid compound and said hydroxyl compound. 34. The process according to claim 33, wherein said hydroxyl compound is the same as said alcohol. 35. The process according to claim 34, wherein said non-reactive neo-acid is isolated as an alcohol solution and added to said reaction medium as an alcohol solution. 36. A composition consisting essentially of a non-reactive catalytic neo-acid and an alcohol. 37. The composition according to claim 36, wherein said neo-acid comprises 5- 95% of said composition. 38. The composition according to claim 37, wherein said neo-acid is a decanoic neo-acid. 39. A process which comprises: (a) subjecting a neo-acid composition to an esterification reaction with an excess of alcohol to esterify all of the reactive neo-acid; and (b) isolating the non-reactive neo-acid from said composition. 40. The process according to claim 39, wherein said alcohol is iso-decyl alcohol.

Inventors: CLAUSI DOMINIC T, TURNER DAVID W, CLAUSI, DOMINIC, T., TURNER, DAVID, W.
Applicants: CLAUSI DOMINIC T, EXXON CHEMICAL PATENTS INC, TURNER DAVID W, CLAUSI, DOMINIC, T., EXXON CHEMICAL PATENTS INC., TURNER, DAVID, W.
Application year: 1999
Main classification: C07C67
IPC codes: C07C67