PROCESS FOR PREPARING REFINED ACRYLIC ESTERS

Abstract: The invention relates to a process for preparing refined acrylic esters comprising distilling an acrylic ester containing stream in the presence of a phenolic inhibitor and oxygen. The invention relates in particular to purified methyl methacrylate prepared from propyne.

Description: PROCESS FOR PREPARING REFINED ACRYLIC ESTERS The invention relates to a process for preparing re- fined acrylic esters by distillation of an acrylic ester containing stream in the presence of a phenolic inhibi¬ tor. Acrylic esters can be represented by the general formula CH2=CR :L COOR 2 , wherein R 1 represents a hydrogen atom or an optionally substituted hydrocarbyl group- preferably a hydrogen atom or a methyl group and R 2 represents an optionally substituted hydrocarbyl group, typically an alkyl group such as a methyl group. The in- vention relates in particular to refined methyl acrylate and, more particularly, methyl methacrylate (hereinafter MMA) . Acrylic esters are valuable compounds that readily polymerize or copolymerize with a wide variety of other monomers. However, it is difficult to prepare acrylic esters of sufficient purity, e.g., freed from the corre¬ sponding acrylic acid, alcohol or water, without inadver¬ tently converting part of the acrylic esters into oli- gomers, polymers or other undesirable products during their preparation. The various methods for preparing acrylic esters are discussed in detail in volume 1 of the fourth edition of the Kirk-Othmer Encyclopedia of Chemical Technology (pp. 287-314, in particular Fig. 1 on p. 296) . The same refer- ence provides a typical process for preparing acrylic esters by esterification (pp. 301-302, Fig. 4) wherein acrylic acid, alcohol and a catalyst are fed to an esterification reactor. The esterification products, acrylic ester, excess alcohol and water of esterifica- tion, are taken overhead from the distillation column. The process uses additional wash and distillation columns to yield a refined acrylic ester. To minimize formation of the acrylic ester oligomers, polymers, etc., mild con¬ ditions and short residence times are maintained through¬ out the separation sections. In addition, conventional free-radical polymerization inhibitors are fed to each of the distillation columns. Finally, monomethyl ether of hydroquinone (10-100 ppmw, hereinafter MEHQ) is added to the end product as polymerization inhibitor and the es¬ ters are used in this form in most industrial polymeriza- tions. The reference also discusses alternative routes to acrylic esters, based on acetylene, etc., that each in¬ corporate a final (series of) distillation column(s) to separate refined acrylic ester. Indeed, as the alterna¬ tive routes give rise to a multitude of difficult to separate side-products, distillation to separate refined acrylic ester is essential. The Process Economics Program Report No. 11D, enti¬ tled METHACRYLIC ACID AND ESTERS" by SRI International of January 1993 provides a thorough review of the routes to MMA and ethacrylic acid as well. Concerning MMA purifi¬ cation, suggested treatments include distillation, ex¬ traction, and treatment over ion-exchange resins or car¬ bon beds. To avoid polymerization during purification, MMA must not be exposed to high temperatures, and distil- lation is generally done under vacuum and in the presence of polymerization inhibitors (pp. 5-4 to 5-5) . In section 9 of the Report (MMA from propyne) it is proposed to distil under vacuum conditions to keep temperatures low and minimize decomposition or polymerization. In the Report it is suggested to add about 600 ppmw of MEHQ as polymerization inhibitor (p.9-5). US-A-4,518, 62 discloses a 2 column distillation process for removing water, methanol and methyl isobu- tyrate from MMA by using an n-hexane entrainer. In the examples 0.02% of hydroquinone and 0.03% of phenothiazine are used as polymerization inhibitors in the first dis¬ tillation column, and 5% of hydroquinone is used in the second distillation column. In Japan Kokai 2-17151 (1990, abstract) the distilla- tion of impure MMA in the presence of 200 ppmw 4-benzo- quinone plus 800 ppmw phenothiazine is described. Neither of these documents teach or hint at measures to improve the efficiency of phenolic inhibitors, or identify the inhibitors used best in case the acrylic es- ter containing stream further comprises (the residue of) the catalyst system used to prepare the acrylic ester. The inventors set out to determine the most optimal conditions for distillation of the acrylic ester, allow¬ ing for high troughputs by distilling at elevated tem- peratures, without significant loss of the acrylic ester. In particular, a process was looked for that allows dis¬ tillation of the product stream of a carbonylation reac¬ tion further comprising (i) (residue of) the homogeneous catalyst system used to prepare the acrylic ester and (ii) optionally an alcohol corresponding to the alcohol residue of the acrylic ester. More in particular, a process was looked for that allows the distillation of a product stream of a carbonylation reaction catalyzed by a catalyst system based on: (a) a Group VIII metal, prefer- ably Pd; a ligand, preferably a phosphine mono- or biden- tate ligand, and (c) a source of anions (Reppe Reactions, see chapter 3 of New Syntheses with Carbon Monoxide by J. Falbe, ISBN 3-540-09674-4) . Such distillations are com¬ plicated by the presence of these catalyst components as they may induce formation of the undesired oligomers, polymers etc. The inventors surprisingly found that these objec¬ tives could be achieved by a process for preparing re¬ fined acrylic esters by distillation of an acrylic ester containing stream in the presence of a phenolic inhibi- tor, characterized in that the distillation is carried out in the presence of oxygen. It has been found that the least formation of acrylic ester oligomers, polymers, etc. is obtained if the oxygen is present in an amount of 0.001 to 1.0 %v (on vapour phase) . If less than the preferred amount is present, some oligomerization or polymerization still occurs, whereas using oxygen in a higher concentration is undesirable for safety and economic reasons. Suitable phenolic inhibitors are known in the art and comprise for instance hydroquinone (hereinafter HQ) or 2, 4, 6-trimethylphenol; substituted hydroquinones such as 2, 5-di-tert-butylhydroquinone, 2, 5-dimethylhydroquinone, 2-methylhydroquinone, or alkyl ethers (preferably methyl ethers) of said (substituted) hydroquinone(s) . Other examples are given in JP-A-05, 320, 095, wherein a process for preparing (meth)acrylic acid ester is described, car¬ ried out by reacting (meth) acrylic acid with alcohol in the presence of an acidic catalyst and a solvent, using as polymerization inhibitor an aromatic compound with at least one alkyl group and at least two hydroxyl groups per benzene ring. The suitability of preparing refined acrylic esters comprising distilling an acrylic ester containing stream in the presence of a phenolic inhibitor and oxygen, however, is not disclosed. Benzoquinone or similar (carbonyl) compounds, al¬ though being well-known inhibitors, are not considered phenolic inhibitors within the definition of the present patent application. Indeed, US-A-3, 794, 567 illustrates the use of the polymerization inhibitors benzoquinone and hydroquinone monomethyl ether as such or in admixture in the presence of air (or oxygen) in the distillation of unsaturated acids (acrylic and methacrylic acid) . How¬ ever, the present inventors found benzoquinone to be an unsuitable inhibitor for the distillation of the acrylic esters, in particular in the presence of oxygen (cf. Table 1 supra) , which is in contrast to the findings of this patent. Apparently, acrylic esters respond differ¬ ently from acrylic acids to polymerization inhibitors during their distillation. Preferably, the phenolic inhibitor is used at a con- centration in the range of 200 to 5000 ppmw, more prefer¬ ably in the range of 500 to 1500 ppmw (based on the acrylic ester) . Using phenolic inhibitor outside the pre- ferred range either increases the formation of the unde¬ sirable products, or merely increases the total costs. The process is in particular suitable for preparing refined MMA. MMA and other acrylic esters may advanta¬ geously be prepared from acetocyanohydrin, ethene, propionic acid, propionaldehyde, methyl propionate or propyne using a homogeneous catalyst system, (cf. the SRI Report mentioned before) . Hydroquinone and substituted hydroquinones are the phenolic inhibitor of preference in case the catalyst system is based on: (a) a Group VIII metal, preferably Pd; (b) a ligand, preferably a mono- or bidentate phosphine ligand; and (c) a source of anions (for suit¬ able catalyst systems and processes for preparing such acrylic esters see, for instance, European patent appli- cations Nos. 0,186,228; 0,190,473; 0,194,707; 0,218,283; 0,218,284; 0,386,833; 0,386,834; 0,392,601; 0,441,446; 0,441,447; 0,489,472; 0,495,547; 0,499,329; 0,521,578; 0,565,199; and 0,571,044). It is observed that optimal concentration of phenolic inhibitor depends on the amount of oxygen present, and vice versa. The higher, respectively lower the concentra¬ tion of phenolic inhibitor, the lower, respectively higher is the required amount of oxygen. The distillation may suitably be carried out at vac- uum or (super)atmospheric pressures using a distillation column operating at a bottom temperature of 60-140 °C, preferably at a temperature in the range of 80-110 °C. Preferably the phenolic inhibitor is introduced into the distillation column together with the acrylic ester containing stream. In addition or instead, it may be in¬ troduced at or near the top of the distillation column. The oxygen may be introduced advantageously at or near the bottom of the distillation column, either by a sepa¬ rate inlet or through leakage in case of vacuum distilla- tion columns. In case the acrylic ester containing stream further comprises (i) a homogeneous catalyst system and (ii) an alcohol corresponding to the alcohol residue of the acrylic ester, it is helpful to carry out the process in two steps. For instance, in step (1) an azeotrope of the alcohol (methanol) and acrylic ester is removed over the top of a first distillation coliαmn (to be recycled if the process is part of an integrated process further compris¬ ing a reactor where the acrylic ester is produced) . The first bottom product is introduced in step 2 into a sec¬ ond distillation column, where heavy ends (oligomers and polymers) and (the residue of) the catalyst system is re¬ moved from the refined acrylate ester over the bottom of said distillation column and the refined ester is recov- ered over the top of said distillation column. Other con¬ figurations, changing the order for removal of the azeo¬ trope and the heavy ends, will be apparent to the man skilled in the art. It is within the gist of the invention to use a cas- cade of distillation columns (in series and/or parallel) instead of using a single distillation column. A particu¬ larly suitable process for removing the azeotrope is dis¬ closed in European patent application No. 0,571,044 and (as part of a total process) in Figure 9.1 of the SRI report, page E-25. The invention further relates to a process for pre¬ paring refined acrylic esters by carbonylating an acety- lenically unsaturated compound, preferably ethyne or propyne, with carbon monoxide and the corresponding alco- hoi, preferably methanol, in the presence of a catalyst system based on: (a) a Group VIII metal, preferably Pd; (b) a ligand, preferably a mono- or bidentate phosphine ligand; and (c) a source of anions, characterized in that the product stream of the carbonylation reaction is proc- essed as set out above. Preferably, both processes are integrated such that the azeotrope stream recovered in step 1 in the two-step distillation set-up is reused. Optionally, also the catalyst system recovered from the bottom stream of the second distillation column of the two-step distillation set-up is reused. The invention will be illustrated by the following model examples. Example 1 MMA, to which 500 ppmw inhibitor was added, was kept at 80 °C under a flow of nitrogen gas or air (modelling the distillation conditions) . The MMA was analyzed on the presence of ppmw PMMA (visual detection limit of 500 ppmw PMMA by observation of haziness using 2 %wt MMA in metha¬ nol, determining the number of days required before PMMA could be detected). The results are set out in Table 1. From these results it may be concluded that in the pres¬ ence of oxygen, the phenolic inhibitors showed improved performance, whereas the performance of p-benzoquinone decreased in the presence of oxygen. Most marked improve- ments were found for 2,4,6-trimethylphenol, HQ and MEHQ, although "IONOL-K" and "IONOL-CP" (trademarks for 4- methyl-2, 6-di-tert-butylphenol and 2,4-dimethyl-6-tert- butylphenol respectively) also performed well in the presence of oxygen. - θ - Example 2 A synthetic feed (modelling the crude product ob¬ tained in an MMA from propyne production process) com¬ prising MMA, 100 or 1000 ppmw inhibitor, and a catalyst system comprising, if present, 40 ppmw Pd, Pd/PN/MeSA in molar ratio 1:10:5 or 1:10:20 (PN stands for 2-pyridyl- diphenylphospine; MeSA stands for methanesulphonic acid) was kept at 80 °C under a flow of 0.1 %v O2 N2 (modelling the distillation conditions) . The feed was analyzed on ppmw PMMA (detection limit of 10 ppmw PMMA by GPC analy¬ sis) . The results are set out in Table 2. From these results it may be concluded that in absence of the cata¬ lyst system, "IONOL-CP" and "IONOL-K" perform similar to HQ and substantially better than MEHQ. However, in the presence of the catalyst system HQ performs substantially better than the other phenolic inhibitors. Table 1 Inhibitor Stability at 80 °C under Stability at 80 °C under nitrogen in days air in days HQ 1 29 4-tert-butylcatechol 6 14 2, 4-di-tert-butylphenol 1 16 4-methyl-2, 6-di-tert-butylphenol 2-4 19 2, 4-dimethyl-6-tert-butylphenol 13 43 2,4, 6-trimethylphenol 1 > 55 MEHQ 1 26 p-benzoquinone 8 2-4 Table 2 inhibitor type Pd Pd/PN/MeSA inhibitor HQ MEHQ "IONOL-CP" * "IONOL-K" * cone, cone, (ppmw) (molar ratio) (ppmw) (days wit h < 10 ppmw or ppmw PMMA after 1 day) 0 no additives 100 > 7 days 2 days > 6 days > 11 days 40 1:10:5 100 4,800 29,000 17,000 12,000 40 1:10:20 1000 640 solid only 16000 19000 trademarks Example 3 A synthetic feed comprising MMA, 100 ppmw HQ, and a catalyst system comprising, if present, 40 ppmw Pd, Pd/PN/MeSA in molar ratio 1:10:5 was kept at 80 °C under a flow of 0.1 %v O2/N2. The feed was again analyzed on ppmw PMMA. The results are set out in Table 3. From these results it may be concluded that the problem of PMMA formation is particular great in the presence of the complete catalyst system. Example 4 A synthetic feed comprising MMA, a variable amount of HQ, and a catalyst system comprising 40 ppmw Pd, Pd/PN/MeSA in molar ratio of 1:30:35 was kept at 80 °C under a flow of 0.1 %v or 20 %v O2. The feed was analyzed on ppmw PMMA (detection limit of 10 ppmw PMMA by GPC analysis) . The results are set out in Table 3. From these results it may be concluded that the presence of both O2 and HQ are required, and that to achieve the same result in PMMA reduction either a (relatively) large amount of O2 or a (relatively) large amount of HQ may be used. Example 5 A synthetic feed was used comprising 83.8 %wt MMA, 15.1 %wt MeOH, 1.0 %wt methyl crotonate, a catalyst sys¬ tem comprising 10 ppmw Pd (fed as palladium(II) acetate), Pd/PN/MeSA in molar ratio 1:30:35 and 500 or 1000 ppmw HQ. The feed was subjected to a two-step distillation set-up comprising two distillation columns in series, operating at the following conditions: 80-100 °C bottoms temperature; air flow in both bottoms approximately 0.3 NL (NL is litre at 1 bar, 20 °C) per hour (0.05-0.1 %v O2 in vapour phase) ; a feed flow of 105 millilitres per hour (comprising azeotrope 25; MMA 70; heavy ends 10) ; and residence times in the bottoms of 3-4 hours in the first distillation column and 25-50 hours in the second distil- - Il ¬ lation column. From the results (Table 5) it may be concluded that PMMA formation may be reduced in a continuous distilla¬ tion to an acceptable level. Higher PMMA levels are found in the second bottoms, due to concentration. There is only a small effect of the temperature on PMMA produc¬ tion. Example 6 Example 5 was repeated, however, with the difference that instead of a synthetic feed, crude product originat¬ ing from an MMA from propyne production process was used. This feed, to which 500 ppmw HQ was added, comprised 72.7 %wt MMA, 25.5 %wt MeOH, 0.8 %wt methyl crotonate, and catalyst system (10 ppmw Pd, Pd/PN/MeSA mol ratio 1:30:30) . The feed flow amounted to 140 millilitres per hour (comprising azeotrope 55; MMA 75; heavy ends 10) . The results (Table 6) illustrate the suitability of the present process. Table 3 presence of: reaction time PMMA cone. Pd PN ligand MeSA (days) (ppmw) - - - 7 < 10 + - - 7 < 10 - + - 7 < 10 - - + 7 < 10 + + - 3 50 - + + 10 < 10 + - + 10 < 10 + + + 1 4800 Table 4 PMMA cone. (ppmw) HQ (ppmw) 0.1 %v 0? 20 %v Op 50 7,100 1,600 100 7,100 40 250 890 < 10 750 80 < 10 1000 100 < 10 1500 < 10 < 10 Table 5 HQ cone. PMMA cone, in lst/2nd column bottoms (ppmw/feed) (ppmw/ppmw) 80 °C 90 °C 100 °C 500 30 3,100 < 10 1,100 30 12,500 1000 20 520 < 10 710 40 740 Table 6 bottoms air flow PMMA cone. in lst/2nd column tempera¬ bottoms (ppmw/ppmw) ture . (°C) (NL per feed 1st 2nd hour) bottom bottom 80 0.5 820 1,460 17,600 100 3.0 820 1,060 8,900 100 1.5 820 1,200 11,000 mass balance* 820 1,250 10,500 * calculated amount of PMMA present in the bottoms if no PMMA is formed, but merely concentrated.

Claims: C L A I M S 1. A process for preparing refined acrylic esters by distillation of an acrylic ester containing stream in the presence of a phenolic inhibitor, characterized in that the distillation is carried out in the presence of oxygen. 2. A process as claimed in claim 1, wherein the oxygen is present in an amount of 0.001 to 1.0 %v (on vapour phase) . 3. A process as claimed in claim 1 or 2, wherein the phenolic inhibitor is used at a concentration in the range of 200 to 5000 ppmw, preferably in the range of 500 to 1500 ppmw (based on the acrylic ester) . 4. A process as claimed in any one of claims 1 to 3, wherein the phenolic inhibitor is hydroquinone or an optionally substituted hydroquinone. 5. A process as claimed in any one of claims 1 to 4, wherein the acrylic ester containing stream further comprises (i) a homogeneous catalyst system for making methyl methacrylate from acetocyanohydrin, ethene, propionic acid, propionaldehyde, methyl propionate or propyne and (ii) optionally an alcohol corresponding to the alcohol residue of the acrylic ester. 6. A process as claimed in claim 5, wherein the catalyst system is based on: (a) a Group VIII metal, preferably Pd; (b) a ligand, preferably a mono- or bidentate phosphine ligand; and (c) a source of anions. 7. A process as claimed in any one of claims 1 to 6, wherein the distillation is carried out in a distillation colimn operating at a temperature in the range of 60 to 140 °C, preferably at a temperature in the range of 80 to 110 °C. 8. A process as claimed in claim 7, wherein the phenolic inhibitor is introduced into the distillation columns together with the acrylic ester containing stream. 9. A process as claimed in claim 7 or 8, wherein the phenolic inhibitor is introduced at or near the top of the distillation column. 10. A process as claimed in any one of claims 7 to 9, wherein the oxygen is introduced at or near the bottom of the distillation column. 11. A process as claimed in any one of claims 1 to 10, wherein the acrylic ester containing stream further comprises (i) a homogeneous catalyst system for making methyl methacrylate from acetocyanohydrin, ethene, propionic acid, propionaldehyde, methyl propionate or propyne and (ii) optionally an alcohol corresponding to the alcohol residue of the acrylic ester, and the distil¬ lation is carried out in two steps whereby in step (1) an azeotrope of the alcohol and acrylic ester is removed over the top of a first distillation column, the first bottom product is introduced into a second distillation column where heavy ends and the (residue of the) catalyst system is removed from the refined acrylic esters over the bottom of said distillation column and the refined acrylate ester is recovered over the top of said distil- lation column. 12. A process as claimed in any one of claims 1 to 11, wherein the distillation or each step of a two step distillation set-up is carried out in a cascade of distillation columns. 13. A process as claimed in any one of claims 1 to 12, wherein the acrylic ester is methyl methacrylate or methyl acrylate, preferably methyl methacrylate. 14. A process for preparing refined acrylic esters by carbonylating an acetylenically unsaturated compound, preferably ethyne or propyne, with carbon monoxide and an alcohol, preferably methanol, in the presence of a cata¬ lyst system based on: (a) a Group VIII metal, preferably Pd; (b) a ligand, preferably a mono- or bidentate phosphine ligand; and (c) a source of anions, character¬ ized in that the product stream of the carbonylation reaction is processed as claimed in any one of claims 1 to 13.

Inventors: ARNOLDY PETER, KRAGTWIJK ERIC, KREMERS ANTOON PAUL MICHAEL, ARNOLDY, PETER, KRAGTWIJK, ERIC, KREMERS, ANTOON, PAUL, MICHAEL
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1995
Main classification: C07C67
IPC codes: C07C67, C07C69