Process for simultaneous production of methyl methacrylate and methacrylic acid

Abstract

A process for the simultaneous production of methacrylic acid and methyl methacrylate, the process comprising: a) producing methacrolein from propionaldehyde and formaldehyde; b) splitting the methacrolein into a first methacrolein stream and a second methacrolein stream; c) producing methacrylic acid from water and the first methacrolein stream in an oxidation reaction; and d) producing methyl methacrylate from methanol and the second methacrolein stream in an oxidative esterification reaction.

Description

Technical Field

[0001] The present invention relates to a method for simultaneously preparing methyl methacrylate and methacrylic acid. Background Art

[0002] EP 3144291 discloses a method for preparing an alkyl methacrylate and methacrylic acid, wherein methacrolein is synthesized in a first reactor, the methacrolein is subjected to oxidative esterification in a second reactor to form an alkyl methacrylate, and at least part of the alkyl methacrylate is reacted with water in a third reactor to form methacrylic acid.

[0003] Both methacrylic acid and methyl methacrylate are desirable products. It is desired to produce both methacrylic acid and methyl methacrylate simultaneously without converting one product into the other. Summary of the Invention

[0004] The present invention relates to a method for simultaneously producing methacrylic acid and methyl methacrylate, the method comprising:

[0005] a) producing methacrolein from propionaldehyde and formaldehyde;

[0006] b) splitting the methacrolein into a first methacrolein stream and a second methacrolein stream;

[0007] c) producing methacrylic acid from water and the first methacrolein stream in an oxidation reaction; and

[0008] d) producing methyl methacrylate from methanol and the second methacrolein stream in an oxidative esterification reaction. Detailed Description

[0009] Unless otherwise indicated, all percent compositions are by weight (wt%), and all temperatures are in °C. Unless otherwise specified, the average value is the arithmetic mean. "Average concentration" is the arithmetic mean of the concentration entering a region and the concentration leaving that region, where the region is a single reactor, a reactor system, or a zone within a reactor or reactor system. "Average ratio" is the ratio of the average concentration of one component to the average concentration of another component. For example, the average ratio of water to methacrolein in the reactor system is calculated by dividing the average concentration of water entering and leaving the reactor system by the average concentration of methacrolein entering and leaving the reactor system.

[0010] The noble metal is any one of gold, platinum, iridium, osmium, silver, palladium, rhodium, and ruthenium. More than one noble metal may be present in the catalyst, in which case the limit applies to the total amount of all noble metals.

[0011] "Catalyst center" is the centroid of the catalyst particle, i.e., the average position of all points in all coordinate directions. The diameter is any linear dimension passing through the catalyst center, and the average diameter is the arithmetic mean of all possible diameters. The aspect ratio is the ratio of the longest diameter to the shortest diameter.

[0012] "Reactor system" means one or more reactors in which a specified reaction takes place. For example, the oxidation reaction of methacrolein to produce methacrylic acid can be the specified reaction occurring in a reactor system. Similarly, the oxidative esterification reaction of methacrolein to produce methyl methacrylate can be the specified reaction occurring in a reactor system. The reactor system can include a single reactor or multiple reactors. Additionally, the reactor system can be subdivided into multiple zones, i.e., a multi-zone reactor system. The zones can be defined by physical separation, such as by a wall or barrier defining the separation zone, or by differences in reaction conditions, such as pressure, temperature, catalyst, composition or concentration of reactants or other reaction components (such as inert materials, pH regulators, etc.). For example, the reactor system can include: a single reactor including a single zone, a single reactor including multiple zones, multiple reactors each including a single zone in each reactor, multiple reactors in which one or more reactors have a single zone and one or more reactors include multiple zones, or multiple reactors each including multiple zones. By definition, a reactor system including multiple reactors will be considered a multi-zone reactor system. An example of a multi-zone reactor can be a continuous tubular reactor including multiple zones, including one or more mixing zones, cooling zones, and one or more catalyst zones where the reaction occurs. Another example of a multi-zone single reactor can be a stirred bed reactor including inner walls containing a catalyst, which define a catalyst zone and a feed / removal zone outside the catalyst zone, through which the liquid reactants circulate, and in which the reactants enter the reactor and the products leave the reactor. When referring to the average concentration or any ratio of the reactor system, the average concentration or ratio is calculated based on the substances entering the reactor system and the substances leaving the reactor system. The reactor system can include reactors configured as fluidized bed reactors, fixed bed reactors, trickle bed reactors, packed bubble column reactors, or stirred bed reactors. Preferably, the reactor system includes a fixed bed reactor, a trickle bed reactor, or a packed bubble column reactor.

[0013] Depending on the reactor in which the catalyst is present, the catalyst can be in the form of a slurry or a fixed bed. For example, a slurry catalyst can be used in a stirred bed reactor or a fluidized bed reactor, while a fixed bed catalyst can be used in a fixed bed reactor, a trickle bed reactor, or a packed bubble column reactor. Preferably, the catalyst is in the form of a fixed bed reactor.

[0014] The size of the catalyst can be selected based on the type of reactor. For example, the slurry catalyst can have an average particle size of less than 200 μm, e.g., 10 μm to 200 μm. The fixed bed catalyst can have an average particle size of 200 μm or greater, e.g., 200 μm to 30 mm. Preferably, the average diameter of the catalyst particles is at least 60 μm, preferably at least 100 μm, preferably at least 200 μm, preferably at least 300 μm, preferably at least 400 μm, preferably at least 500 μm, preferably at least 600 μm, preferably at least 700 μm, preferably at least 800 μm; preferably not exceeding 30 mm, preferably not exceeding 20 mm, preferably not exceeding 10 mm, preferably not exceeding 5 mm, preferably not exceeding 4 mm, preferably not exceeding 3 mm.

[0015] The present invention relates to a process for simultaneously producing methacrylic acid and methyl methacrylate. As used herein, the term "simultaneously producing" means producing methacrylic acid and methyl methacrylate simultaneously in separate reactor systems, i.e., producing methyl methacrylate in an oxidative esterification reaction while producing methacrylic acid in an oxidation reactor system.

[0016] Methacrylic acid and methyl methacrylate are formed by the oxidation reaction and oxidative esterification reaction of methacrolein, respectively.

[0017] Methacrolein is preferably produced by aldol condensation or Mannich condensation. Preferably, in the presence of a suitable catalyst, methacrolein is formed by the Mannich condensation of propionaldehyde and formaldehyde. The molar ratio of propionaldehyde to formaldehyde can range from 1:20 to 20:1, preferably 1:1.5 to 1.5:1, more preferably 1:1.25 to 1.25:1, and even more preferably 1:1.1 to 1.1:1.

[0018] Examples of catalysts that can be used in the Mannich condensation process include, for example, amine - acid catalysts. The acid of the amine - acid catalyst can include, but is not limited to, inorganic acids (e.g., sulfuric acid and phosphoric acid) and organic monocarboxylic acids, dicarboxylic acids, or polycarboxylic acids (e.g., aliphatic C1 - C 10 monocarboxylic acids, C2 - C 10 dicarboxylic acids, C2 - C 10 polycarboxylic acids). The amine of the amine - acid catalyst can include, but is not limited to, compounds of the formula NHR 1 R 2 where R 1 and R 2 are each independently a C1 - C 10 alkyl group, which is optionally substituted with an ether, hydroxyl, secondary amino, or tertiary amino group, or R 1 and R 2Together with an adjacent nitrogen, it can form a C5-C7 heterocycle, optionally containing additional nitrogen atoms and / or oxygen atoms, and they are optionally substituted by C1-C4 alkyl or C1-C4 hydroxyalkyl.

[0019] The Mannich condensation reaction is preferably carried out in the liquid phase by reacting propionaldehyde, formaldehyde, and methanol in the presence of an amine-acid catalyst in a reactor at a temperature of at least 20 °C and a pressure greater than 1 bar. The temperature of the reactor can be in the range of 20 °C to 220 °C, preferably 80 °C to 220 °C, and more preferably 120 °C to 220 °C. The pressure of the reactor can be in the range of greater than 1 bar to 120 bar.

[0020] An inhibitor can be added to the reactor to prevent the formation of unwanted products. For example, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (4-hydroxy-TEMPO) can be added to the reactor.

[0021] The propionaldehyde used for the preparation of methacrolein can be prepared by the hydroformylation of ethylene. The hydroformylation process is known in the art and is disclosed, for example, in U.S. Patent No. 4,427,486, U.S. Patent No. 5,087,763, U.S. Patent No. 4,716,250, U.S. Patent No. 4,731,486, and U.S. Patent No. 5,288,916. The hydroformylation of ethylene involves contacting ethylene with carbon monoxide and hydrogen in the presence of a hydroformylation catalyst. Examples of hydroformylation catalysts include, for example, metal-organic phosphorus ligand complexes such as organophosphines, organophosphites, and organophosphonamides. The ratio of carbon monoxide to hydrogen can be in the range of 1:10 to 100:1, preferably 1:10 to 10:1. The hydroformylation process can be carried out at a temperature in the range of -25 °C to 200 °C, preferably 50 °C to 120 °C.

[0022] The ethylene used for the preparation of propionaldehyde can be prepared by the dehydration of ethanol. For example, ethylene can be prepared by the acid-catalyzed dehydration of ethanol. Ethanol dehydration is known in the art and is disclosed, for example, in U.S. Patent No. 9,249,066. Preferably, the ethanol is derived from renewable resources such as plant materials or biomass, rather than ethanol prepared from petroleum-based sources. The use of bio-derived ethanol alone in the process for producing methacrylic acid can result in up to 50% of the carbon atoms in methacrylic acid (i.e., 2 out of 4 carbon atoms in methacrylic acid) being from renewable resources.

[0023] To further increase the renewable carbon content in methacrylic acid, additional starting materials can also be prepared from renewable resources. For example, formaldehyde can be prepared from syngas, where the syngas can be prepared from biomass. Carbon monoxide, which can also be used to prepare propionaldehyde, can also be prepared from renewable resources, as disclosed by Li et al., ACS Nano, 2020, 14, 4, 4905-4915. Using these additional bio-sources can further increase the amount of renewable carbon.

[0024] Alternatively, the starting materials for producing methacrylic acid can be prepared from recycled materials. For example, recycled carbon dioxide can be used to produce methanol, and methanol can be used to produce formaldehyde.

[0025] Preferably, at least 50%, more preferably at least 75%, and even more preferably 100% of the carbon atoms in methacrylic acid are derived from renewable or recycled content.

[0026] The methacrolein produced from propionaldehyde and formaldehyde is split into a first methacrolein stream and a second methacrolein stream. The first methacrolein stream is used to produce methacrylic acid in an oxidation reaction with water. The second methacrolein stream is used to produce methyl methacrylate in an oxidative esterification with methanol.

[0027] Preferably, the splitting of methacrolein into a first methacrolein stream and a second methacrolein stream is adjustable such that the weight ratio of the first methacrolein stream to the second methacrolein stream can be varied to control the respective amounts of methacrylic acid and methyl methacrylate formed. Thus, the formation of the two products can be controlled quickly and conveniently to meet the demand for either product. One method of making the splitting of methacrolein adjustable includes using a control valve or a flow controller to control how much methacrolein flows into each process. For example, the weight ratio of the first methacrolein stream to the second methacrolein stream can range from 0.01:1 to 10:1, preferably 0.02:1 to 5:1, more preferably 0.05:1 to 2:1, and even more preferably 0.1:1 to 1:1.

[0028] The oxidation reaction for forming methacrylic acid and the oxidative esterification reaction for forming methyl methacrylate are each preferably carried out in the presence of a heterogeneous noble metal-containing catalyst, namely a first heterogeneous noble metal-containing catalyst and a second heterogeneous noble metal-containing catalyst, respectively. The first heterogeneous noble metal-containing catalyst and the second heterogeneous noble metal-containing catalyst can be the same or different, and are preferably different.

[0029] The first heterogeneous noble metal-containing catalyst and the second heterogeneous noble metal-containing catalyst contain particles of noble metal. Preferably, the noble metal includes palladium or gold, and more preferably, the noble metal includes gold.

[0030] The particles of the noble metal preferably have an average diameter of less than 15 nm, preferably less than 12 nm, more preferably less than 10 nm, and even more preferably less than 8 nm. The standard deviation of the average diameter of the noble metal particles is + / - 5 nm, preferably + / - 2.5 nm, and more preferably + / - 2 nm. As used herein, the standard deviation is calculated by the following equation:

[0031]

[0032] where x is the size of each particle, is the average value of n particles, and n is at least 500.

[0033] Preferably, the noble metal-containing catalyst further comprises titanium-containing particles and / or nickel oxide particles.

[0034] The titanium-containing particles may comprise elemental titanium or titanium oxide TiO x . Preferably, the titanium-containing particles comprise titanium oxide.

[0035] The average diameter of the titanium-containing particles and the nickel oxide particles is preferably less than 5 times, more preferably less than 4 times, even more preferably less than 3 times, still more preferably less than 2 times, and yet more preferably less than 1.5 times the average diameter of the noble metal-containing particles. Preferably, the average diameter of the titanium-containing particles and the nickel oxide nanoparticles is at least half of the average diameter of the gold particles, and more preferably at least the same as the average diameter of the gold particles.

[0036] The weight of the noble metal-containing particles by weight relative to the amount of the titanium-containing particles or the nickel oxide particles may be in the range of 1:1 to 1:20. Preferably, the weight ratio of the noble metal-containing particles to the titanium-containing particles or the nickel oxide particles is in the range of 1:2 to 1:15, more preferably 1:3 to 1:10, even more preferably 1:4 to 1:9, and still more preferably 1:5 to 1:8.

[0037] Preferably, the noble metal particles are uniformly distributed among the titanium-containing particles or the nickel oxide particles. As used herein, the term "uniformly distributed" means that the noble metal particles are randomly dispersed among the titanium-containing particles or the nickel oxide particles, with substantially no agglomeration of the noble metal particles. Preferably, at least 80% of the total number of noble metal particles are present in particles having an average diameter of less than 15 nm. More preferably, at least 90% of the total number of noble metal particles are present in particles having an average diameter of less than 15 nm. Even more preferably, at least 95% of the total number of noble metal particles are present in particles having an average diameter of less than 15 nm.

[0038] The noble metal particles in the catalyst can be disposed on the surface of the carrier material. Preferably, the carrier material is particles of an oxide material; preferably γ-alumina, δ-alumina or θ-alumina, silica, magnesia, titania, zirconia, hafnia, vanadia, niobia, tantalum oxide, ceria, yttria, lanthanum oxide or a combination thereof. Preferably, a part of the catalyst contains noble metals, and the carrier has a surface area greater than 10 m 2 / g, preferably greater than 30 m 2 / g, preferably greater than 50 m 2 / g, preferably greater than 100 m 2 / g, preferably greater than 120 m 2 / g. In the catalyst part containing a small amount of noble metals or no noble metals, the carrier may have a surface area of less than 50 m 2 / g, preferably less than 20 m 2 / g. The average diameter of the carrier and the average diameter of the final catalyst particles are not significantly different.

[0039] Preferably, the aspect ratio of the catalyst particles is not greater than 10:1, preferably not greater than 5:1, preferably not greater than 3:1, preferably not greater than 2:1, preferably not greater than 1.5:1, preferably not greater than 1.1:1. Preferred shapes of the catalyst particles include spherical, cylindrical, rectangular solid, annular, multi-lobed (e.g., alfalfa leaf cross-section), shapes with multiple holes, and "wagon wheel"; preferably spherical. Irregular shapes can also be used.

[0040] The noble metal particles may be dispersed throughout the catalyst or have different concentration densities, such as a gradient concentration or a layered structure. Preferably, at least 90% by weight of the noble metal is in the outer 70% of the catalyst volume (i.e., the volume of the average catalyst particle), preferably the outer 60% of the catalyst volume, preferably the outer 50%, preferably the outer 40%, preferably the outer 35%, preferably the outer 30%, preferably the outer 25%. Preferably, for a volume having a constant distance from its inner surface to its outer surface (the surface of the particle) measured along a line perpendicular to the outer surface, the outer volume of any particle shape is calculated. For example, for a spherical particle, the outer x% of the volume is a spherical shell whose outer surface is the surface of the particle and whose volume is x% of the entire spherical volume. Preferably, at least 95% by weight, preferably at least 97% by weight, preferably at least 99% by weight of the noble metal is located in the outer volume of the catalyst. Preferably, at least 90% by weight (preferably at least 95% by weight, preferably at least 97% by weight, preferably at least 99% by weight) of the noble metal is within a distance not exceeding 30%, preferably not exceeding 25%, preferably not exceeding 20%, preferably not exceeding 15%, preferably not exceeding 10%, preferably not exceeding 8% of the catalyst diameter from the surface. The distance from the surface is measured along a line perpendicular to the surface.

[0041] Preferably, the catalyst comprises gold particles and titanium-containing particles or nickel oxide particles on a carrier material comprising silica. Preferably, the gold particles and the titanium-containing particles or nickel oxide particles form an eggshell structure on the carrier particles. The eggshell layer may have a thickness of 500 microns or less, preferably 250 microns or less, more preferably 100 microns or less.

[0042] Preferably, at least 0.1% by weight of the total weight of the gold particles is exposed on the surface of the catalyst, where the surface includes both the outer surface and the pores of the catalyst. As used herein, the term "exposed" means that at least a portion of the gold particles is not covered by another gold particle, a titanium-containing particle, or a nickel oxide particle, i.e., the reactants can directly contact the gold particles. Thus, the gold particles may be disposed within the pores of the carrier material and are still exposed since the reactants can directly contact the gold particles within the pores. More preferably, at least 0.25% by weight of the total weight of the gold particles is exposed on the surface of the catalyst, even more preferably, at least 0.5% by weight of the total weight of the gold particles is exposed on the surface of the catalyst, and still more preferably, at least 1% by weight of the total weight of the gold particles is exposed on the surface of the catalyst.

[0043] The catalyst is preferably produced by precipitating a noble metal from an aqueous solution of a metal salt in the presence of a support. Suitable noble metal salts can include, but are not limited to, chloroauric acid, sodium thiosulfatoaurate, sodium thiomalatoaurate, gold hydroxide, palladium nitrate, palladium chloride, and palladium acetate. In a preferred embodiment, the catalyst is produced by the incipient wetness technique, in which an aqueous solution of a suitable noble metal precursor salt is added to a porous inorganic oxide such that the pores are filled with the solution, and then the water is removed by drying. The resulting material is then converted into the final catalyst by calcination, reduction, or other treatments known to those skilled in the art to decompose the noble metal salt into a metal or metal oxide. Preferably, a C2-C 18 thiol containing at least one hydroxyl or carboxylic acid substituent is present in the solution. Preferably, a C2-C 18 thiol has 2 to 12 carbon atoms, preferably 2 to 8 carbon atoms, preferably 3 to 6 carbon atoms. Preferably, the thiol compound contains no more than 4, preferably no more than 3, preferably no more than 2 total hydroxyl and carboxylic acid groups. Preferably, the thiol compound has no more than 2, preferably no more than 1 thiol group. If the thiol compound contains a carboxylic acid substituent, they can exist in the acid form, the conjugate base form, or a mixture thereof. The thiol component can also exist in its thiol(acid) form or its conjugate base (thiolate) form. Particularly preferred thiol compounds include thiomalic acid, 3-mercaptopropionic acid, mercaptoacetic acid, 2-mercaptoethanol, and 1-thioglycerol, including their conjugate bases.

[0044] In one embodiment of the present invention, the catalyst is produced by deposition precipitation, in which a porous inorganic oxide is immersed in an aqueous solution containing a suitable noble metal precursor salt, and then the salt is made to interact with the surface of the inorganic oxide by adjusting the pH of the solution. The resulting treated solid is then recovered (e.g., by filtration) and then converted into the final catalyst by calcination, reduction, or other pretreatment known to those skilled in the art to decompose the noble metal salt into a metal or metal oxide.

[0045] The catalyst bed can also contain an inert material or an acidic material. Preferred inert materials or acidic materials include, for example, alumina, clay, glass, silicon carbide, and quartz. Preferably, the inert material or acidic material located before and / or after the catalyst bed has an average diameter equal to or greater than the average diameter of the catalyst, preferably 1 mm to 30 mm; preferably at least 2 mm; preferably not greater than 30 mm, preferably not greater than 10 mm, preferably not greater than 7 mm.

[0046] After splitting methacrolein into a first methacrolein stream and a second methacrolein stream, methacrylic acid is produced in the oxidation reaction of water and the first methacrolein stream. The oxidation reaction is preferably carried out in an oxidation reactor system in the presence of an oxygen-containing gas, which oxidation reactor system contains a catalyst bed comprising a first heterogeneous noble metal-containing catalyst.

[0047] The catalyst bed, which may comprise a slurry bed or a fixed bed, contains first heterogeneous noble metal-containing catalyst particles. In one embodiment, the oxidation reactor system further comprises a liquid phase and a gas phase, the liquid phase comprising methacrolein, water and methacrylic acid, and the gas phase comprising oxygen. The liquid phase may also contain by-products, for example, methacrolein dimethyl acetal (MDA). In an alternative embodiment, the production of methacrylic acid is carried out in the gas phase.

[0048] Preferably, based on the total weight of water and methacrolein, the average concentration of methacrolein in the oxidation reactor system is less than 40% by weight. Preferably, based on the average amounts of water and methacrolein entering and leaving the system, the oxidation reactor system has an average ratio of water to methacrolein of less than 40:1.

[0049] Preferably, based on the total volume of the gas stream leaving the oxidation reactor system, the oxygen concentration in the gas stream leaving the oxidation reactor system is at least 1 mol%, more preferably at least 2 mol%, even more preferably at least 2.5 mol%, still more preferably at least 3 mol%, yet more preferably at least 3.5 mol%, even yet more preferably at least 4 mol%, and most preferably at least 4.5 mol%. Preferably, based on the total amount of the gas stream leaving the oxidation reactor system, the oxygen concentration in the gas stream leaving the oxidation reactor system is not greater than 7.5 mol%, preferably not greater than 7.25 mol%, preferably not greater than 7 mol%.

[0050] Preferably, the liquid phase in the oxidation reactor system is at a temperature of 40°C to 120°C; preferably at least 50°C and preferably at least 55°C. The temperature of the liquid phase in the oxidation reactor system is preferably not more than 110°C, and preferably not more than 100°C. When the oxidation reactor system comprises more than one reactor and / or more than one zone, the temperature in each reactor and / or zone may be the same or different. For example, the reaction mixture leaving a reactor or zone may be cooled before entering the next reactor or zone.

[0051] Preferably, the pressure of the catalyst bed in the oxidation reactor system is from 1 bar to 150 bar (100 kPa to 15000 kPa). The pressure of the catalyst bed in the oxidation reactor system can be at least 10 bar, preferably at least 20 bar, preferably at least 30 bar, preferably at least 40 bar or preferably at least 60 bar. For example, the pressure in the catalyst bed of the oxidation reactor system can be at least 100 bar. When the oxidation reactor system includes more than one reactor and / or zone, the pressure in each reactor and / or zone can be the same or different.

[0052] For each gram mole of methacrylic acid leaving the oxidation reactor system over a 1-hour period, the first heterogeneous noble metal-containing catalyst in the reactor system can be present in an amount in the range of 0.02 kg to 2 kg of catalyst. Preferably, for each gram mole of methacrylic acid leaving the oxidation reactor system over a 1-hour period, the first heterogeneous noble metal-containing catalyst in the reactor system is present in an amount of at least 0.02 kg to 0.5 kg of catalyst. Preferably, for each gram mole of methacrylic acid leaving the oxidation reactor system over a 1-hour period, the first heterogeneous noble metal-containing catalyst in the oxidation reactor system is present in an amount of less than 0.4 kg of catalyst, more preferably less than 0.3 kg of catalyst, still more preferably less than 0.25 kg of catalyst, and even more preferably less than 0.2 kg of catalyst.

[0053] The amount of methacrylic acid leaving the reactor depends on the conversion of methacrolein in the oxidation reactor system. For example, when the conversion of methacrolein entering the oxidation reactor system is 50%, 2 moles of methacrolein are required to produce 1 mole of methacrylic acid. In this example, for each gram mole of methacrolein entering the oxidation reactor system over a 1-hour period, the heterogeneous noble metal-containing catalyst in the reactor system can be present in an amount in the range of 0.01 kg to 1 kg of catalyst. When the conversion of methacrolein entering the oxidation reactor system is 25%, 4 moles of methacrolein are required to produce 1 mole of methacrylic acid, and for each gram mole of methacrolein entering the oxidation reactor system over a 1-hour period, the heterogeneous noble metal-containing catalyst in the reactor system can be present in an amount in the range of 0.005 kg to 0.5 kg of catalyst. When the conversion of methacrolein entering the oxidation reactor system is 75%, 1.33 moles of methacrolein are required to produce 1 mole of methacrylic acid, and for each gram mole of methacrolein entering the oxidation reactor system over a 1-hour period, the heterogeneous noble metal-containing catalyst in the reactor system can be present in an amount in the range of 0.015 kg to 1.5 kg of catalyst. Without considering any external recycle streams, the oxidation reactor system preferably exhibits a conversion of methacrolein to methacrylic acid of at least 25% in the oxidation reactor system, more preferably at least 35%, and even more preferably at least 40% of methacrolein to methacrylic acid. The addition of an external recycle stream that recycles unreacted methacrolein to the oxidation reactor system can also be used to improve the overall conversion efficiency of the process.

[0054] When the first noble metal-containing catalyst contains gold, for each gram mole of methacrylic acid leaving the reactor system over a 1-hour period, gold can be present in an amount in the range of 0.0001 kg to 0.1 kg. Preferably, for each gram mole of methacrylic acid leaving the reactor system over a 1-hour period, gold is present in an amount of at least 0.0001 kg to 0.005 kg. Preferably, for each gram mole of methacrylic acid leaving the reactor system over a 1-hour period, gold is present in an amount less than 0.004 kg.

[0055] Regarding the amount of the first heterogeneous noble metal-containing catalyst in the oxidation reactor system relative to the amount of methacrolein entering the oxidation reactor system, when the conversion rate of methacrolein entering the oxidation reactor system is 50%, for each gram-mole of methacrolein entering the oxidation reactor system within a time period of 1 hour, the gold in the first heterogeneous noble metal-containing catalyst in the reactor system can be present in an amount within the range of 0.00005 kg to 0.05 kg of gold. When the conversion rate of methacrolein entering the oxidation reactor system is 25%, for each gram-mole of methacrolein entering the oxidation reactor system within a time period of 1 hour, the gold in the first heterogeneous noble metal-containing catalyst in the reactor system can be present in an amount within the range of 0.000025 kg to 0.025 kg of catalyst. When the conversion rate of methacrolein entering the oxidation reactor system is 75%, for each gram-mole of methacrolein entering the oxidation reactor system within a time period of 1 hour, the gold in the first heterogeneous noble metal-containing catalyst in the reactor system can be present in an amount within the range of 0.000075 kg to 0.075 kg of catalyst.

[0056] The pH in the catalyst bed of the oxidation reactor system can be within the range of 2 to 10. Some catalysts can be deactivated under acidic conditions. Therefore, when the catalyst is not acid-resistant, the pH in the catalyst bed is 4 to 10; preferably at least 5, preferably at least 5.5; preferably not greater than 9, preferably not greater than 8, preferably not greater than 7.5.

[0057] To increase the pH in the reactor system, an alkaline material can be added. The alkaline material can include an Arrhenius base (i.e., a compound that dissociates in water to form hydroxide ions), a Lewis base (i.e., a compound capable of donating a pair of electrons), or a Bronsted-Lowry base (i.e., a compound capable of accepting a proton). Examples of Arrhenius bases include, but are not limited to, hydroxides of alkali metals and alkaline earth metals. Examples of Lewis bases include, but are not limited to, amines, sulfates, and phosphines. Examples of Bronsted-Lowry bases include, but are not limited to, halides, nitrates, nitrites, chlorites, chlorates, etc. Ammonia can be a Lewis base or a Bronsted-Lowry base.

[0058] The present inventors have found that high localized concentrations of basic materials in an oxidation reactor system can lead to the formation of undesirable Michael adducts as by-products. Thus, to help minimize the formation of Michael adducts, the basic material is preferably mixed with at least one other material prior to entering the oxidation reactor system. Preferably, the basic material is introduced at a location external to the oxidation reactor system and mixed with one or more reactants or diluents to form a basic-containing feed stream. For example, the basic material can be mixed with water or a non-reactive solvent (i.e., a solvent that does not negatively impact the formation of methacrylic acid in the reactor system). The location external to the reactor system can be a mixing vessel. Alternatively, the location external to the reactor can be a pipeline through which components travel to the reactor system, such as a feed pipeline or a recycle pipeline, where sufficient mixing occurs, such as by turbulence, baffles, jet mixers, or other mixing methods.

[0059] Preferably, based on the total weight of the basic-containing feed stream, the amount of basic material in the basic-containing feed stream is 50 wt% or less, preferably 25 wt% or less, preferably 20 wt% or less, preferably 15 wt% or less, preferably 10 wt% or less, preferably 5 wt% or less, or preferably 1 wt% or less. Prior to entering the reactor system, the basic material is preferably diluted by a factor of less than 1:2, such as less than 1:3, less than 1:4, less than 1:5, less than 1:10, less than 1:20, or less than 1:100 relative to the total weight of the basic-containing feed stream.

[0060] Preferably, the basic-containing feed stream is thoroughly mixed to avoid localized peaks in the concentration of the basic material within the basic-containing feed stream prior to adding the basic-containing feed stream to the reactor system. For example, it is preferred that the basic-containing feed stream reaches a uniformity of at least 95%, i.e., prior to entering the reactor system, the variation in the concentration of the basic material deviates within + / -5% of the average concentration of the basic material in the basic-containing feed stream. Preferably, the basic-containing feed stream reaches 95% uniformity within 4 minutes of introducing the basic material, more preferably within 2 minutes of introducing the basic material, and even more preferably within 1 minute of introducing the basic material.

[0061] For a mixing vessel, the time required for the additive to reach 95% uniformity is defined as Θ 95 , which can be calculated by the method disclosed by Grenville and Nienow, The Handbook of Industrial Mixing, pages 507 - 509, which gives the following expression for a stirred tank in turbulence:

[0062]

[0063] where T is the tank diameter, H is the liquid level, D is the impeller diameter, N p is the characteristic power number of the impeller, and N is the impeller speed. Similar expressions exist for static mixers, jet mixing vessels, etc.

[0064] Preferably, no basic material is added to the oxidation reactor system, whether inside or outside the reactor system. Preferably, when no basic material is added to the reactor system, the noble metal-containing catalyst comprises an acid-resistant catalyst such as a catalyst comprising gold and titanium-containing particles. Operating the oxidation reactor system in the absence of basic material provides several advantages. One advantage is an increase in selectivity and space-time yield (STY) due to a lower production of Michael adducts. Another advantage is a cost reduction due to a reduced cost of treating aqueous waste. Aqueous waste leaving the oxidation process using basic material can produce large amounts of inorganic salts, which may be difficult or impossible to treat with biological water treatment processes. This may in turn require the use of other waste treatment methods such as incineration.

[0065] A second methacrolein stream is used to produce methyl methacrylate. Methyl methacrylate is produced in an oxidative esterification reaction (OER) of methanol and the second methacrolein stream. The oxidative esterification reaction is preferably carried out in an oxidative esterification reactor system comprising a catalyst bed in the presence of an oxygen-containing gas.

[0066] The catalyst bed, which may comprise a slurry bed or a fixed bed, comprises second heterogeneous noble metal-containing catalyst particles. The OER system also comprises a liquid phase and a gas phase, the liquid phase comprising methacrolein, methanol and MMA, and the gas phase comprising oxygen. The liquid phase may also comprise by-products such as methacrolein dimethyl acetal (MDA) and methyl isobutyrate (MIB). In the absence of steps to control its formation, MIB can be present in the MMA product stream in an amount exceeding 1 wt% (10,000 ppm) relative to the total weight of MMA, methacrolein and methanol in the product stream leaving the OER system. MIB can be difficult to separate from MMA. Therefore, the present invention seeks to limit the amount of MIB formed such that the amount of MIB in the product stream is in the range of 0.1 ppm to 5000 ppm, preferably 0.1 ppm to 4000 ppm, more preferably 0.1 ppm to 3000 ppm, even more preferably 0.1 ppm to 2500 ppm, and still more preferably 0.1 ppm to 2000 ppm.

[0067] Preferably, based on the total weight of methanol and methacrolein entering the reactor system, the concentration of methanol entering the OER system is greater than 32% by weight. More preferably, based on the total weight of methanol and methacrolein entering the OER system, the concentration of methanol entering the OER system is greater than 35% by weight, and even more preferably greater than 40% by weight. Preferably, based on the total weight of methanol and methacrolein entering the reactor system, the concentration of methanol entering the OER system is less than 75% by weight. More preferably, based on the total weight of methanol and methacrolein entering the OER reactor system, the concentration of methanol entering the OER system is less than 60% by weight, and even more preferably less than 50% by weight.

[0068] Preferably, based on the total weight of methanol and methacrolein in the liquid-phase product stream leaving the OER system, the concentration of methanol in the liquid-phase product stream leaving the OER system is at least 65% by weight. More preferably, based on the total weight of methanol and methacrolein in the liquid-phase product stream leaving the OER system, the concentration of methanol in the liquid-phase product stream leaving the OER system is at least 70% by weight. Preferably, based on the total weight of methanol and methacrolein in the liquid-phase product stream leaving the OER system, the concentration of methanol in the liquid-phase product stream leaving the OER system is less than 100% by weight. Preferably, based on the average total weight of methanol and methacrolein entering and leaving the OER system (i.e., the arithmetic mean of the total weight of methanol and methacrolein entering the OER system and the total weight of methanol and methacrolein leaving the OER system), the average concentration of methanol in the OER system (i.e., the arithmetic mean of the concentrations of methanol entering and leaving the OER system) is greater than 70% by weight. More preferably, based on the average total weight of methanol and methacrolein entering and leaving the OER system, the average concentration of methanol in the OER system is greater than 75% by weight.

[0069] The preferred average weight ratio of methanol to methacrolein in the OER system is from 20:1 to 2:1, where the average weight ratio is based on the average concentrations of methanol entering and leaving the OER system and the average concentrations of methacrolein entering and leaving the OER system.

[0070] One example of an OER system includes a multi-zone or multi-reactor system. In the first zone or reactor, based on the average total amount of methanol and methacrolein entering and leaving the first zone or reactor, the average concentration of methanol in the first zone or reactor is from 50% to 80% by weight. Based on the average total amount of methanol and methacrolein entering and leaving the final zone or reactor, the final zone or reactor has an average methanol concentration in the range of 80% to 100% by weight. Between the first zone or reactor and the final zone or reactor, the reactor mixture can be cooled and / or additional oxygen can be added, such as by adding air to the gas phase entering the final zone or reactor.

[0071] Preferably, based on the total volume of the gas stream leaving the OER system, the oxygen concentration in the gas stream leaving the OER system is at least 1 mol%, more preferably at least 2 mol%, even more preferably at least 2.5 mol%, still more preferably at least 3 mol%, yet more preferably at least 3.5 mol%, even yet more preferably at least 4 mol%, and most preferably at least 4.5 mol%. Preferably, based on the total amount of the gas stream leaving the OER system, the oxygen concentration in the gas stream leaving the OER system is not greater than 7.5 mol%, preferably not greater than 7.25 mol%, and preferably not greater than 7 mol%.

[0072] Preferably, the liquid phase in the OER system is at a temperature of 40 °C to 120 °C; preferably at least 50 °C and preferably at least 55 °C. The temperature of the liquid phase in the OER system is preferably not greater than 110 °C and preferably not greater than 100 °C. When the OER system includes more than one reactor and / or more than one zone, the temperature in each reactor and / or zone may be the same or different. For example, the reaction mixture leaving a reactor or zone may be cooled before entering the next reactor or zone.

[0073] Preferably, the catalyst bed in the OER system is under a pressure of 1 bar to 150 bar (100 kPa to 15,000 kPa). Without wishing to be bound by theory, it is believed that operating the OER system at an increased pressure will reduce the amount of MIB present in the product stream by increasing the amount of oxygen present in the liquid phase. Thus, the pressure in the catalyst bed of the OER system may be at least 10 bar, preferably at least 20 bar, preferably at least 30 bar, preferably at least 40 bar, or preferably at least 60 bar. For example, the pressure in the catalyst bed of the OER system may be at least 100 bar. When the OER system includes more than one reactor and / or more than one zone, the pressure in each reactor and / or zone may be the same or different.

[0074] For each gram-mole of methyl methacrylate leaving the reactor system over a one-hour period, the second heterogeneous noble metal-containing catalyst in the OER system may be present in an amount in the range of 0.02 kg to 2 kg of catalyst. Preferably, for each gram-mole of methyl methacrylate leaving the OER reactor system over a one-hour period, the second heterogeneous noble metal-containing catalyst in the OER system is present in an amount of at least 0.02 kg to 0.5 kg of catalyst. Preferably, for each gram-mole of methyl methacrylate leaving the OER reactor system over a one-hour period, the second heterogeneous noble metal-containing catalyst in the OER system is present in an amount of less than 0.4 kg of catalyst, more preferably less than 0.3 kg of catalyst, still more preferably less than 0.25 kg of catalyst, and even more preferably less than 0.2 kg of catalyst.

[0075] The amount of methyl methacrylate leaving the reactor depends on the conversion of methacrolein in the OER system. For example, at a conversion of methacrolein entering the OER system of 50%, 2 moles of methacrolein are required to produce 1 mole of methyl methacrylate. In this example, for each gram-mole of methacrolein entering the reactor system over a 1-hour period, the heterogeneous noble-metal-containing catalyst in the OER system can be present in an amount in the range of 0.01 kg to 1 kg of catalyst. At a conversion of methacrolein entering the OER system of 25%, 4 moles of methacrolein are required to produce 1 mole of methyl methacrylate, and for each gram-mole of methacrolein entering the reactor system over a 1-hour period, the second heterogeneous noble-metal-containing catalyst in the OER system can be present in an amount in the range of 0.005 kg to 0.5 kg of catalyst. At a conversion of methacrolein entering the OER system of 75%, 1.33 moles of methacrolein are required to produce 1 mole of methyl methacrylate, and for each gram-mole of methacrolein entering the reactor system over a 1-hour period, the second heterogeneous noble-metal-containing catalyst in the OER system can be present in an amount in the range of 0.015 kg to 1.5 kg of catalyst. Without considering any external recycle streams, the OER system preferably exhibits a conversion of methacrolein to methyl methacrylate of at least 25% in the OER system, more preferably at least 35% conversion, and even more preferably at least 40% conversion of methacrolein to methyl methacrylate. The addition of an external recycle stream that recycles unreacted methacrolein to the OER system can also be used to improve the overall conversion efficiency of the process.

[0076] When the second noble-metal-containing catalyst comprises gold, gold can be present in an amount in the range of 0.0001 kg to 0.1 kg for each gram-mole of MMA leaving the reactor system over a 1-hour period. Preferably, gold is present in an amount of at least 0.0001 kg to 0.005 kg for each gram-mole of MMA leaving the reactor system over a 1-hour period. Preferably, gold is present in an amount of less than 0.004 kg for each gram-mole of MMA leaving the reactor system over a 1-hour period.

[0077] Regarding the amount of the second heterogeneous noble metal-containing catalyst in the OER system relative to the amount of methacrolein entering the reactor system, when the conversion of methacrolein entering the OER system is 50%, for each gram mole of methacrolein entering the reactor system over a period of 1 hour, the amount of gold in the second heterogeneous noble metal-containing catalyst in the OER system can be present in an amount ranging from 0.00005 kg to 0.05 kg of gold. When the conversion of methacrolein entering the OER system is 25%, for each gram mole of methacrolein entering the reactor system over a period of 1 hour, the amount of gold in the second heterogeneous noble metal-containing catalyst in the OER system can be present in an amount ranging from 0.000025 kg to 0.025 kg of catalyst. When the conversion of methacrolein entering the OER system is 75%, for each gram mole of methacrolein entering the reactor system over a period of 1 hour, the amount of gold in the second heterogeneous noble metal-containing catalyst in the OER system can be present in an amount ranging from 0.000075 kg to 0.075 kg of catalyst.

[0078] The pH in the OER catalyst bed can be in the range of 2 to 10. Some catalysts can be deactivated under acidic conditions. Thus, when the catalyst is acid-intolerant, the pH in the catalyst bed is 4 to 10; preferably at least 5, preferably at least 5.5; preferably not greater than 9, preferably not greater than 8, preferably not greater than 7.5.

[0079] To increase the pH in the OER reactor system, an alkaline material can be added. The alkaline material can include an Arrhenius base (i.e., a compound that dissociates in water to form hydroxide ions), a Lewis base (i.e., a compound capable of donating a pair of electrons), or a Bronsted-Lowry base (i.e., a compound capable of accepting a proton). Examples of Arrhenius bases include, but are not limited to, hydroxides of alkali metals and alkaline earth metals. Examples of Lewis bases include, but are not limited to, amines, sulfates, and phosphines. Examples of Bronsted-Lowry bases include, but are not limited to, halides, nitrates, nitrites, chlorites, chlorates, etc. Ammonia can be a Lewis base or a Bronsted-Lowry base.

[0080] The present inventors have found that high local concentrations of basic materials in a reactor system can lead to the formation of unwanted Michael adducts as by-products. Thus, to help minimize the formation of Michael adducts, the basic material is preferably mixed with at least one other material prior to entering the reactor system. Preferably, the basic material is introduced at a location external to the reactor system and mixed with one or more reactants or diluents to form a basic-containing feed stream. For example, the basic material can be mixed with methanol, water, or a non-reactive solvent (i.e., a solvent that does not negatively impact the formation of methyl methacrylate in the reactor system). The location external to the reactor system can be a mixing vessel. Alternatively, the location external to the reactor can be a pipeline through which components travel to the reactor system, such as a feed pipeline or a recycle pipeline, where sufficient mixing occurs, such as by turbulence, baffles, jet mixers, or other mixing methods.

[0081] Preferably, based on the total weight of the basic-containing feed stream, the amount of basic material in the basic-containing feed stream is 50 wt% or less, preferably 25 wt% or less, preferably 20 wt% or less, preferably 15 wt% or less, preferably 10 wt% or less, preferably 5 wt% or less, or preferably 1 wt% or less. Prior to entering the reactor system, the basic material is preferably diluted by a factor of less than 1:2, such as less than 1:3, less than 1:4, less than 1:5, less than 1:10, less than 1:20, or less than 1:100 relative to the total weight of the basic-containing feed stream.

[0082] Preferably, the basic-containing feed stream is thoroughly mixed to avoid localized peaks in the concentration of the basic material within the basic-containing feed stream prior to adding the basic-containing feed stream to the reactor system. For example, it is preferred that the basic-containing feed stream reaches a level of uniformity of at least 95%, i.e., prior to entering the reactor system, the variation in the concentration of the basic material deviates within + / - 5% of the average concentration of the basic material in the basic-containing feed stream. Preferably, the basic-containing feed stream reaches a level of uniformity of 95% within 4 minutes of introducing the basic material, more preferably within 2 minutes of introducing the basic material, and even more preferably within 1 minute of introducing the basic material.

[0083] Preferably, no basic material is added to the OER reactor system, whether inside or outside the reactor system. Preferably, when no basic material is added to the reactor system, the noble metal-containing catalyst comprises an acid-resistant catalyst, such as a catalyst comprising gold and titanium-containing particles. Operating the OER system in the absence of basic material provides several advantages. One advantage is increased selectivity and space-time yield (STY) due to lower production of Michael adducts. Another advantage is cost reduction due to reduced cost of treating aqueous waste. The aqueous waste leaving the oxidative esterification process using basic material can produce large amounts of inorganic salts, which may be difficult or impossible to treat with biological water treatment processes. This may in turn require the use of other waste treatment methods, such as incineration.

[0084] OER generally produces a liquid product stream containing MMA as well as methacrylic acid and unreacted methanol. Preferably, the reaction product is fed to a methanol recovery distillation column, which provides an overhead stream rich in methanol and methacrolein; preferably, this stream is recycled back to the OER. The bottoms stream from the methanol recovery distillation column contains MMA, MIB, MDA, methacrylic acid, salts and water. MDA is preferably hydrolyzed in a medium containing MMA, MDA, methacrylic acid, salts and water. MDA can be hydrolyzed in the bottoms stream from the methanol recovery distillation column. This hydrolysis can be carried out in the methanol recovery column. The bottoms stream from the methanol recovery distillation column can be sent to a separate acetal hydrolysis reactor for additional MDA hydrolysis. Alternatively, MDA can be hydrolyzed in a separate acetal hydrolysis reactor after the organic phase is separated from the methanol recovery column bottoms stream. Water may need to be added to the organic phase to ensure sufficient water is present for MDA hydrolysis; these amounts can be readily determined from the composition of the organic phase. An acid stream can also be added to the hydrolysis reactor to ensure sufficient MDA removal. The product of the MDA hydrolysis reactor is phase separated, and the organic phase passes through one or more distillation columns to produce an MMA product and light and / or heavy by-products.

Claims

1. A method for simultaneously producing methacrylic acid and methyl methacrylate, the method comprising: a) producing methacrolein from propionaldehyde and formaldehyde; b) splitting the methacrolein into a first methacrolein stream and a second methacrolein stream; c) producing methacrylic acid from water and the first methacrolein stream in an oxidation reaction; and d) producing methyl methacrylate from methanol and the second methacrolein stream in an oxidative esterification reaction.

2. The method according to claim 1, wherein the weight ratio of the first methacrolein stream to the second methacrolein stream is in the range of 0.01:1 to 10:

1.

3. The method according to claim 2, wherein the weight ratio of the first methacrolein stream to the second methacrolein stream is in the range of 0.05:1 to 2:

1.

4. The method according to any one of the preceding claims, wherein splitting the methacrolein into the first methacrolein stream and the second methacrolein stream comprises passing the methacrolein through a control valve.

5. The method according to any one of the preceding claims, the method further comprising producing the propionaldehyde from ethylene.

6. The method according to any one of the preceding claims, wherein: step c) is carried out at a pressure in the range of 1 bar to 150 bar; step c) is carried out in a liquid phase reaction in a reactor system in the presence of a first heterogeneous noble metal-containing catalyst, wherein the reactor system comprises an oxygen-containing gas; based on the total weight of water and methacrolein, the average concentration of methacrolein in step c) is less than 40% by weight; and based on the average amounts of water and methacrolein entering and leaving the reactor system of step c), the average ratio of water to methacrolein in the system is less than 40:

1.

7. The method according to any one of claims 1 to 5, wherein: step c) is carried out in a gas phase reaction in an oxidation reactor system in the presence of a first heterogeneous noble metal-containing catalyst, wherein the oxidation reactor system comprises an oxygen-containing gas; based on the total weight of water and methacrolein, the average concentration of methacrolein in step c) is less than 40% by weight; and based on the average amounts of water and methacrolein entering and leaving the oxidation reactor system of step c), the average ratio of water to methacrolein in the oxidation reactor system is less than 40:

1.

8. The method according to claim 6 or 7, wherein based on the total amount of the gas phase leaving the reactor system of step c), oxygen in the gas phase is present in an amount in the range of 1 mol% to 7.5 mol% oxygen.

9. The method according to any one of claims 6 to 8, wherein the first heterogeneous noble metal-containing catalyst is in the form of a slurry or a fixed bed.

10. The method according to any one of claims 6 to 9, wherein the first heterogeneous noble metal-containing catalyst comprises gold.

11. The method according to any one of claims 6 to 10, wherein for each gram mole of methacrylic acid leaving the oxidation reactor system over a period of 1 hour, the first heterogeneous noble metal-containing catalyst is present in an amount in the range of 0.02 kg to 2 kg of catalyst.

12. The method according to any one of the preceding claims, wherein: step d) is carried out at a pressure in the range of 1 bar to 150 bar; step d) is carried out in a liquid phase reaction in an oxidative esterification reactor system in the presence of a second heterogeneous noble metal-containing catalyst, wherein the oxidative esterification reactor system comprises an oxygen-containing gas; based on the total weight of the liquid phase stream leaving the oxidative esterification reactor system, the liquid phase stream contains at least 30% by weight of methanol; based on the total weight of the liquid phase stream leaving the oxidative esterification reactor system, the liquid phase stream contains less than 30% by weight of methacrolein; the liquid phase stream leaving the oxidative esterification reactor system contains more than 0.1 ppm and less than 5000 ppm of methyl isobutyrate; and based on the total amount of the gas phase stream leaving the oxidative esterification reactor system, the gas phase stream contains between 1 mol% and 7.5 mol% of oxygen.

13. The method according to claim 12, wherein for each gram mole of methyl methacrylate leaving the oxidative esterification reactor system over a period of 1 hour, the second heterogeneous noble metal-containing catalyst is present in an amount in the range of 0.02 kg to 2 kg of catalyst.

14. The method according to claim 12 or 13, wherein the liquid phase stream leaving the oxidative esterification reactor system contains more than 0.1 ppm and less than 4000 ppm of methyl isobutyrate.

15. The method according to any one of claims 12 to 14, wherein the second heterogeneous noble metal-containing catalyst is in the form of a slurry or a fixed bed.

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