PROCESS FOR PRODUCING METHACRYLATE ESTERS BY DEHYDRATION OF 2-HYDROXY-2-METHYLPROPIONATE ESTERS

Abstract: A process in which esters of 2-hydroxy-2-­ methylpropionate can be dehydrated in nearly quantitative yields to produce methacrylate esters, which process com­ bines careful control of temperature, mole ratios of reac­ tants, contact times, and sulfuric acid dehydrating agent strength, to obtain quantitative yields of methacrylate esters.

Description: TITLE PROCESS FOR PRODUCING METHACRYLATE ESTERS BY DE)liDRkTION OF 2-HYDROXY-2-MEAHYLPROPIONATE ESTERS FIELD OF THE INVENTION This invention relates to an improvement in a process for making methacrylate esters. BACKGROUND OF THE INVENTION Sulfuric acid has long been known as a dehydrating agent for hydroxy esters, converting them to the corresponding unsaturated esters. However, as pointed out by Volker, et al., in U.S. Patent 3,487,101, at column 1, lines 33 et seq., dehydration proceeds smoothly only with f hydroxy compounds where conventional dehydrating agents, such as H2SO4, can be used. In contrast, the dehydration of a-hydroxy esters meets with considerable difficulties, for decomposition generally occurs and, thus, yield of desired methacrylate ester is ordinarily very low. Sennewald, et al., in U.S. Patent 3,022,336, in column 1, lines 20 et seq., teaches that dehydrating hydroxy esters such as 2-hydroxy-2-metbylpropionates is very difficult since the molecule is unstable and prone to uncontrollable decomposition. SUMMARY OF THE INVENTION Surprisingly, in this invention conditions have been found which, contrary to past teachings, give a high yield dehydration of a hydroxy ester. A process has now been discovered in which 2-hydroxy-2-methylpropionate esters can be dehydrated in nearly quantitative yields to produce methacrylate esters. The new process combines careful control of temperature, mole ratios of reactants, contact times and sulfuric acid dehydrating agent strength, to obtain quantitative yields of methacrylate esters. Specifically, the process of this invention is a process for preparing a methacrylate ester which comprises contacting a lower alkyl (C1-C8) 2-hydroxy-2-methylpropionate ester with sulfuric acid of 90% strength or more in water at a temperature of O to 500C for 5 minutes or less, wherein the mole ratio of sulfuric acid to 2-hydroxy-2-methylpropionate ester is between 5-50/1, followed by recovering the methacrylate ester formed. DETAILED DESCRIPTION OF THE INVENTION Dehydration yield losses increase as temperature is raised at a given reaction time and ratio of sulfuric acid to 2-hydroxy-2-methyl propitiate ester. Thus contact time of ester and sulfuric acid should be short and the reaction quenched as temperature approaches the upper range. Conversely, if desired, the temperature can be lowered and the contact times lengthened. Mole ratio of H2SO4 to 2-hyo-roxy-2-methylpropionate ester should be between 5-50:1, preferably 5-12/1. Again an interrelationship exists and the higher the ratio the shorter the time required for dehydration, all other factors being equivalent. Dehydration of 2-hydroxy-2-methylpropionate esters can occur at acid strengths of 90% H2SO4 or higher. As acid strength decreases below 100% H2SO4, progressively more stringent conditions are necessary for dehydration. These conditions of longer time and/or higher temperature lower yield. At acid strengths significantly above 1008 reaction time is significantly reduced at a given temperature and such strengths are preferred. EXAMPLES EXAMPLE 1 16.96g H2SO4 (101.8%, 176.2 m moles H2SO4) was added to a 25 ml flask equipped with a fluoropolymer coated stirring bar. 2.02g methyl 2-hydroxy-2-methylpropionate (MHiB) (also known as methyl a-hydroxyisobutyrate) 17.1 m moles, was then added at room temperature ( 220C). The temperature rose to 51.50C after one minute of mixing and a sample was taken after 1.4 minutes. Conversion after 1.4 minutes on a mole basis was 58.4% methyl methacrylate (MMA) ) Total ) Yield 24.5% methacrylic acid (MAA) ) 82.9% Conversion after 5 minutes of stirring was 52.28 MMA ) Total ) Yield 44.5% MAA ) 96.78 Conversion after 105 minutes at room temperature showed more MAA present but the total yield remained at 95+%. Note the nearly quantitative yield. Mole ratio H2S04/MHiB was 10.3 Strength of H2SO4 was 101.8% Time of contact was 5 minutes to maximum yield Comparison A Lower Yield, with Lower Ratio H2S04/MHiB 3.40g H2SO4 (101.8%, 35.3 m moles H2SO4) was added to a 15 ml 2 neck flask equipped with a fluoropolymer coated stirring bar. 2.02g MHiB (17.1 m moles) was then added at room temperature ( 220C). The temperature rose to 480C after 1.3 minutes. Conversion after 0.5 minutes on a mole basis was 21.8% MMA ) Total ) Yield 0.48 MAA ) 22.2% Conversion after 5 minutes was 32.48 MMA ) Total ) Yield 1.2% MAA ) 33.5% Conversion after 105 minutes was unchanged 32.88 MMA ) Total ) Yield 1.78 MAA ) 34.5% Temperature was 480C Mole ratio H2SO4/MHiB was 2.06. Strength of H2S04 was 101.8% Time was about 5 minutes to maximum yield Note that yield was much' lower than in Example 1, primarily because the mole ratio H2S04/MHiB was much lower. Comparison B Lower Yield Using Lower Strength H2 SO4 and Lower Temperatures 52.65g H2SO4 (94.2%, 506.1 m moles) was added to 5.96g MHiB (49.5 m moles) at ambient (about 22 C) temperature in a vessel described as in Example 1. Conversion at 5 minutes was 11.58 MMA ) Total ) Yield 1.2E MAA ) 12.7% Conversion after 60 minutes was 30.9% MMA ) Total ) Yield 3.28 MAA ) 34.18 Temperature was ambient. The Mole ratio H2SO4/MHiB was 10.2. The Strength of H2SO4 was 94.28. EXAMPLE 2 - Dehydration using butyl 2-hydroxy 2-methy lpr opionate 4.879 H2SO4 (100.21%, 49.8 m moles) was - added to a 15 ml 2 neck flask equipped with a teflon coated stirring bar. Then 0.81g (5.06 m moles) of butyl 2-hydroxy-2-methylpropionate (BHiB) was added at room temperature ( 260C). The temperature rose to 41.5 C in one minute. Conversion after 5 minutes (temperature was 350C) on a mole basis was 66.1% butyl methacrylate (BMA), 30.8% BHiB unreacted

Claims: CLAIMS: 1. A process for preparing methacrylate esters in at least 95% yield which comprises contacting a lower alkyl 2-hydroxy-2-methylpropionate ester with sulfuric acid of 90-100% strength in water at a temperature of between 0 and 500C for 5 minutes or less, wherein the mole ratio of sulfuric acid to the lower alkyl 2-hydroxy-2-methylpropionate is between 5-50/1.

Inventors: DE COLIBUS, RAYMOND LEW
Applicants: E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1985
Main classification: C07C69
IPC codes: B01J27, C07C69, C07C67, C07B61