Process for the simultaneous production of methyl methacrylate and methacrylic acid.

Abstract

1. A process for the simultaneous production of methacrylic acid and methyl methacrylate, comprising: a) producing methacrolein from propionaldehyde and formaldehyde; b) dividing 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 technology]

[0002] EP 3144291 discloses a process for the preparation of alkyl methacrylates and methacrylic acid, in which methacrolein is synthesized in a first reactor, the methacrolein is subjected to oxidative esterification in a second reactor to form alkyl methacrylate, and at least a portion 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 would be desirable to produce both methacrylic acid and methyl methacrylate simultaneously without converting one product to the other. Summary of the Invention

[0004] The present invention provides a process for the co-production of methacrylic acid and methyl methacrylate, comprising: a) producing methacrolein from propionaldehyde and formaldehyde; b) dividing the methacrolein into a first methacrolein stream and a second methacrolein stream; c) producing methacrylic acid in an oxidation reaction from water and the first methacrolein stream; d) producing methyl methacrylate from methanol and the second methacrolein stream in an oxidative esterification reaction. DETAILED DESCRIPTION OF THE INVENTION

[0005] Unless otherwise specified, all compositional percentages are in weight percent (wt%) and all temperatures are in °C. Unless otherwise specified, averages are arithmetic means. An "average concentration" is the arithmetic average of the concentration entering a region and the concentration leaving a region, where a region is an individual reactor, reactor system, or zone within a reactor or reactor system. An "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 a 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.

[0006] The noble metals are any 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 sum of all noble metals.

[0007] "Catalyst center" is the center of gravity of the catalyst particle, i.e., the average position of all points in all coordinate directions. Diameter is any linear dimension passing through the catalyst center, and mean diameter is the arithmetic mean of all possible diameters. Aspect ratio is the ratio of the longest diameter to the shortest diameter.

[0008] A reactor system refers to one or more reactors in which a specified reaction takes place. For example, the oxidation reaction of methacrolein to produce methacrylic acid can be a specified reaction carried out in a reactor system. Similarly, the oxidative esterification reaction of methacrolein to produce methyl methacrylate can be a specified reaction carried out in a reactor system. A reactor system can include a single reactor or multiple reactors. In addition, a 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 walls or barriers defining separate sections, or by differences in reaction conditions, such as pressure, temperature, composition or concentration of catalyst, reactants, or other reaction components (inerts, pH adjusters, etc.). For example, a reactor system can include a single reactor containing a single zone, a single reactor containing multiple zones, multiple reactors each containing a single zone, multiple reactors where one or more reactors have a single zone and one or more reactors contain multiple zones, or multiple reactors each containing multiple zones. By definition, a reactor system containing multiple reactors is considered a multi-zone reactor system. An example of a multi-zone reactor is a continuous tubular reactor containing multiple zones, including one or more mixing zones, a cooling zone, and one or more catalytic zones where reactions occur. Another example of a multi-zone single reactor is a stirred-bed reactor containing an inner wall containing a catalyst defining a catalytic zone through which liquid reactants are circulated, and a feed / removal zone outside the catalytic zone through which reactants enter the reactor and products exit the reactor. When referring to an average concentration or any ratio of a reactor system, the average concentration or ratio is calculated based on what enters and what exits the reactor system. The reactor system can include reactors configured as a fluidized-bed reactor, a fixed-bed reactor, a trickle-bed reactor, a packed bubble column reactor, or a stirred-bed reactor. Preferably, the reactor system includes a fixed-bed reactor, a trickle-bed reactor, or a packed bubble column reactor.

[0009] The catalyst may be in the form of a slurry or a fixed bed, depending on the reactor in which it is present. 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.

[0010] The size of the catalyst can be selected based on the type of reactor. For example, a slurry catalyst may have an average diameter of less than 200 μm, such as 10 μm to 200 μm. A fixed-bed catalyst may have an average diameter of 200 μm or more, such as 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 30 mm or less, preferably 20 mm or less, preferably 10 mm or less, preferably 5 mm or less, preferably 4 mm or less, preferably 3 mm or less.

[0011] The present invention relates to a process for the simultaneous production of methacrylic acid and methyl methacrylate. As used herein, the term "simultaneous production" means the simultaneous production of methacrylic acid and methyl methacrylate in separate reactor systems, i.e., the production of methacrylic acid in an oxidation reactor system while the production of methyl methacrylate in an oxidative esterification reaction.

[0012] Methacrylic acid and methyl methacrylate are formed by the oxidation and oxidative esterification reactions of methacrolein, respectively.

[0013] Methacrolein is preferably produced by either aldol condensation or Mannich condensation. Preferably, methacrolein is formed by the Mannich condensation of propionaldehyde and formaldehyde in the presence of a suitable catalyst. The molar ratio of propionaldehyde to formaldehyde may be in the range of 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.

[0014] Examples of catalysts that can be used in the Mannich condensation process include, for example, amine-acid catalysts. The acids of the amine-acid catalysts include inorganic acids (e.g., sulfuric acid and phosphoric acid), and organic mono-, di-, or polycarboxylic acids (e.g., aliphatic C1-C 10 Monocarboxylic acids, C2-C 10 Dicarboxylic acids, C2-C 10 The amine of the amine acid catalyst may include, but is not limited to, amines of the formula NHR 1 R 2 (In the formula, R 1 and R 2 are each independently a C1-C optionally substituted with an ether, hydroxyl, secondary amino, or tertiary amino group; 10 alkyl, or R 1 and R 2 can be taken together with the adjacent nitrogen to form a 5-C7 heterocycle, optionally containing additional nitrogen and / or oxygen atoms, and optionally substituted with C1-C4 alkyl or C1-C4 hydroxyalkyl).

[0015] 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 reactor temperature may range from 20° C. to 220° C., preferably from 80° C. to 220° C., more preferably from 120° C. to 220° C. The reactor pressure may range from greater than 1 bar to 120 bar.

[0016] To prevent the formation of unwanted products, an inhibitor may be added to the reactor, for example, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (4-hydroxy-TEMPO) can be added to the reactor.

[0017] Propionaldehyde, which is used to prepare methacrolein, can be prepared by hydroformylation of ethylene. Hydroformylation processes are known in the art and are disclosed, for example, in U.S. Pat. Nos. 4,427,486, 5,087,763, 4,716,250, 4,731,486, and 5,288,916. The hydroformylation of ethylene to propionaldehyde involves contacting ethylene with carbon monoxide and hydrogen in the presence of a hydroformylation catalyst. Examples of hydroformylation catalysts include metal-organophosphorus ligand complexes, such as organophosphines, organophosphites, and organophosphoramidites. The carbon monoxide to hydrogen ratio may be in the range of 1:10 to 100:1, preferably 1:10 to 10:1. The hydroformylation process may be carried out at a temperature ranging from -25°C to 200°C, preferably from 50°C to 120°C.

[0018] The ethylene used to prepare propionaldehyde can be prepared by dehydration of ethanol. For example, ethylene can be prepared by acid-catalyzed dehydration of ethanol. Ethanol dehydration is known in the art and is disclosed, for example, in U.S. Pat. No. 9,249,066. Preferably, the ethanol is sourced from a renewable resource, such as plant material or biomass, as opposed to ethanol prepared from petroleum-based sources. Using only bio-sourced ethanol in the methacrylic acid production process can result in up to 50% of the carbon atoms of the methacrylic acid (i.e., 2 of the 4 carbon atoms in the methacrylic acid) being derived from renewable resources.

[0019] To further increase the renewable carbon content in methacrylic acid, additional starting materials can be prepared from renewable resources. For example, formaldehyde can be prepared from syngas, which 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. The use of these additional biological resources can further increase the amount of renewable carbon.

[0020] 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.

[0021] Preferably, at least 50%, more preferably at least 75%, and even more preferably 100% of the carbon atoms in the methacrylic acid are derived from renewable or recycled content.

[0022] 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 reaction with methanol.

[0023] Preferably, the splitting of methacrolein into the first methacrolein stream and the second methacrolein stream is adjustable so that the weight ratio of the first methacrolein stream to the second methacrolein stream can be changed to control the respective amounts of methacrylic acid and methyl methacrylate formed. Thus, the formation of the two products can be rapidly and conveniently controlled to meet the demand for either product. One method for making the splitting of methacrolein adjustable includes the use of adjustable valves or flow controllers to control the amount of methacrolein entering each process. For example, the weight ratio of the first methacrolein stream to the second methacrolein stream can be in the range of 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.

[0024] The oxidation reaction to form methacrylic acid and the oxidative esterification reaction to form methyl methacrylate are each preferably carried out in the presence of a heterogeneous precious metal-containing catalyst, a first heterogeneous precious metal-containing catalyst and a second heterogeneous precious metal-containing catalyst, respectively. The first heterogeneous precious metal-containing catalyst and the second heterogeneous precious metal-containing catalyst can be the same or different, and are preferably different.

[0025] The first and second precious metal-containing catalysts comprise particles of a precious metal. Preferably, the precious metal comprises palladium or gold, and more preferably, the precious metal comprises gold.

[0026] The noble metal particles 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 using the following formula:

[0027]

number

[0028]

number

[0029] Preferably, the precious metal-containing catalyst further comprises titanium-containing particles and / or nickel oxide particles.

[0030] Titanium-containing particles can be elemental titanium or titanium oxide (TiO). x Preferably, the titanium-containing particles comprise titanium oxide.

[0031] The titanium-containing particles and nickel oxide particles preferably have an average diameter less than 5 times the average diameter of the precious metal-containing particles, more preferably less than 4 times the average diameter of the precious metal-containing particles, even more preferably less than 3 times the average diameter of the precious metal-containing particles, even more preferably less than 2 times the average diameter of the precious metal-containing particles, and even more preferably less than 1.5 times the average diameter of the precious metal-containing particles. Preferably, the titanium-containing particles and nickel oxide nanoparticles have an average diameter at least half the average diameter of the gold particles, and more preferably at least the same as the average diameter of the gold particles.

[0032] The amount by weight of the precious metal-containing particles relative to the amount of titanium-containing particles or nickel oxide particles may be in the range of 1:1 to 1:20. Preferably, the weight ratio of the precious metal-containing particles to the titanium-containing particles or 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 even more preferably 1:5 to 1:8.

[0033] Preferably, the precious metal particles are uniformly distributed among the titanium-containing particles or nickel oxide particles. As used herein, the term "uniformly distributed" means that the precious metal particles are randomly dispersed among the titanium-containing particles or nickel oxide particles without substantial agglomeration. Preferably, at least 80% of the total number of precious 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 precious 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 precious metal particles are present in particles having an average diameter of less than 15 nm.

[0034] The noble metal particles in the catalyst may be disposed on the surface of a support material. Preferably, the support material is particles of an oxide material, preferably gamma-, delta-, or theta-alumina, silica, magnesia, titania, zirconia, hafnia, vanadia, niobium oxide, tantalum oxide, ceria, yttria, lanthanum oxide, or a combination thereof. Preferably, in the portion of the catalyst containing the noble metal, the support is at least 10 m 2 / g, preferably more than 30m 2 / g, preferably above 50m 2 / g, preferably more than 100m 2 / g, preferably above 120m 2 In the portion of the catalyst containing little or no precious metal, the support has a surface area of ​​more than 50 m 2 / g, preferably less than 20m 2 / g, the average diameter of the support and the average diameter of the final catalyst particles do not differ significantly.

[0035] Preferably, the aspect ratio of the catalyst particles is 10:1 or less, preferably 5:1 or less, preferably 3:1 or less, preferably 2:1 or less, preferably 1.5:1 or less, preferably 1.1:1 or less. Preferred shapes of catalyst particles include spheres, cylinders, cuboids, rings, multi-lobed shapes (e.g., cloverleaf cross-sections), shapes with multiple holes and "wagon wheels," preferably spheres. Irregular shapes may also be used.

[0036] The precious metal particles may be dispersed throughout the catalyst, or may have a variable concentration density, such as a gradient concentration, or a layered structure. Preferably, at least 90% by weight of the precious metal is in the outer 70% of the catalyst volume (i.e., the volume of the average catalyst particle), preferably the outer 60%, preferably the outer 50%, preferably the outer 40%, preferably the outer 35%, preferably the outer 30%, preferably the outer 25% of the catalyst volume. Preferably, the outer volume of any particle shape is calculated relative to the volume having a certain distance from its inner surface to its outer surface (the surface of the particle) measured along a line perpendicular to the outer surface. For example, for a spherical particle, the outer x% of the volume is the spherical shell, the outer surface is the surface of the particle, and the volume is x% of the volume of the entire sphere. Preferably, at least 95% by weight, preferably at least 97% by weight, preferably at least 99% by weight of the precious metal is 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 precious metal is within a distance from the surface of no more than 30%, preferably no more than 25%, preferably no more than 20%, preferably no more than 15%, preferably no more than 10%, preferably no more than 8% of the catalyst diameter, the distance from the surface being measured along a line perpendicular to the surface.

[0037] Preferably, the catalyst comprises gold particles and titanium-containing or nickel oxide particles on a support material comprising silica. Preferably, the gold particles and titanium-containing or nickel oxide particles form an eggshell structure on the support particles. The eggshell layer may have a thickness of 500 microns or less, preferably 250 microns or less, more preferably 100 microns or less.

[0038] Preferably, at least 0.1 wt.% 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 some of the gold particles are not covered by other gold particles, titanium-containing particles, or nickel oxide particles, i.e., reactants can directly contact the gold particles. Thus, the gold particles may be disposed within the pores of the support material and still be exposed to reactants that can directly contact the gold particles within the pores. More preferably, at least 0.25 wt.% of the total weight of the gold particles is exposed on the surface of the catalyst, even more preferably, at least 0.5 wt.% of the total weight of the gold particles is exposed on the surface of the catalyst, and even more preferably, at least 1 wt.% of the total weight of the gold particles is exposed on the surface of the catalyst.

[0039] The catalyst is preferably produced by precipitating the precious metal from an aqueous solution of the metal salt in the presence of a support. Suitable precious metal salts include, but are not limited to, tetrachloroauric acid, gold sodium thiosulfate, gold sodium thiomalate, gold hydroxide, palladium nitrate, palladium chloride, and palladium acetate. In a preferred embodiment, the catalyst is produced by an incipient wetness technique in which an aqueous solution of a suitable precious metal precursor salt is added to a porous inorganic oxide to fill the pores with the solution, followed by removal of the water by drying. The resulting material is then converted to the finished catalyst by calcination, reduction, or other treatment known to those skilled in the art to decompose the precious metal salt to the metal or metal oxide. Preferably, the catalyst is a C2-C hydroxy group containing at least one hydroxyl or carboxylic acid substituent. 18 Thiols are present in solution. Preferably, C2-C thiols containing at least one hydroxyl or carboxylic acid substituent. 18The thiol has 2 to 12, preferably 2 to 8, and preferably 3 to 6 carbon atoms. Preferably, the thiol compound contains a total of four or less, preferably three or less, and preferably two or less hydroxyl and carboxylic acid groups. Preferably, the thiol compound has two or less, preferably one or less thiol groups. When the thiol compound contains carboxylic acid substituents, they may exist in the acid form, the conjugate base form, or a mixture thereof. The thiol component may also exist in either its thiol (acid) form or its conjugate base (thiolate) form. Particularly preferred thiol compounds include thiomalic acid, 3-mercaptopropionic acid, thioglycolic acid, 2-mercaptoethanol, and 1-thioglycerol, including their conjugate bases.

[0040] In one embodiment of the present invention, the catalyst is produced by deposition precipitation by immersing a porous inorganic oxide in an aqueous solution containing a suitable noble metal precursor salt and then adjusting the pH of the solution to cause the salt to interact with the surface of the inorganic oxide. The resulting treated solid is then recovered (e.g., by filtration) and then converted to the finished catalyst by calcination, reduction, or other pretreatment known to those skilled in the art to decompose the noble metal salt to the metal or metal oxide.

[0041] The catalyst bed may further comprise an inert or acidic material. Preferred inert or acidic materials include, for example, alumina, clay, glass, silica carbide, and quartz. Preferably, the inert 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 between 1 mm and 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.

[0042] After splitting the methacrolein into a first methacrolein stream and a second methacrolein stream, methacrylic acid is produced in an oxidation reaction between water and the first methacrolein stream. The oxidation reaction is preferably carried out in the presence of an oxygen-containing gas in an oxidation reactor system including a catalyst bed containing a first heterogeneous noble metal-containing catalyst.

[0043] The catalyst bed, which may comprise a slurry bed or a fixed bed, comprises heterogeneous precious metal-containing catalyst particles. In one embodiment, the oxidation reactor system further comprises a liquid phase comprising methacrolein, water, and methacrylic acid, and a gas phase comprising oxygen. The liquid phase may further comprise by-products, such as methacrolein dimethyl acetal (MDA). In another embodiment, the production of methacrylic acid occurs in the gas phase.

[0044] Preferably, the average concentration of methacrolein in the oxidation reactor system is less than 40 wt.% based on the total weight of water and methacrolein. Preferably, the oxidation reactor system has an average water to methacrolein ratio of less than 40:1 based on the average amounts of water and methacrolein entering and leaving the system.

[0045] Preferably, the oxygen concentration in the gas stream exiting the oxidation reactor system is at least 1 mol%, more preferably at least 2 mol%, even more preferably at least 2.5 mol%, even more preferably at least 3 mol%, even more preferably at least 3.5 mol%, even more preferably at least 4 mol%, and most preferably at least 4.5 mol%, based on the total volume of the gas stream exiting the oxidation reactor system. Preferably, the oxygen concentration in the gas stream exiting the oxidation reactor system is 7.5 mol% or less, preferably 7.25 mol% or less, preferably 7 mol% or less, based on the total volume of the gas stream exiting the oxidation reactor system.

[0046] Preferably, the liquid phase in the oxidation reactor system is at a temperature of 40°C to 120°C, preferably at least 50°C, preferably at least 55°C. The temperature of the liquid phase in the oxidation reactor system is preferably 110°C or less, preferably 100°C or less. When the oxidation reactor system comprises two or more reactors and / or two or more zones, the temperature in each reactor and / or zone may be the same or different. For example, the reaction mixture exiting a reactor or zone may be cooled before entering the next reactor or zone.

[0047] Preferably, the catalyst bed in the oxidation reactor system is at a pressure of 1 bar to 150 bar (100 kPa to 15,000 kPa). The pressure in the catalyst bed of the oxidation reactor 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 oxidation reactor system may be at least 100 bar. When the oxidation reactor system comprises two or more reactors and / or zones, the pressure in each reactor and / or zone may be the same or different.

[0048] The first heterogeneous precious metal-containing catalyst in the oxidation reactor system can be present in an amount ranging from 0.02 kg to 2 kg of catalyst per gram-mole of methacrylic acid exiting the reactor system over one hour. Preferably, the first heterogeneous precious metal-containing catalyst in the oxidation reactor system is present in an amount of at least 0.02 kg to 0.5 kg of catalyst per gram-mole of methacrylic acid exiting the reactor system over one hour. Preferably, the first heterogeneous precious 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, even more preferably less than 0.25 kg of catalyst, and even more preferably less than 0.2 kg of catalyst per gram-mole of methacrylic acid exiting the oxidation reactor system over one hour.

[0049] The amount of methacrylic acid exiting the reactor depends on the conversion rate of methacrolein in the oxidation reactor system. For example, at a 50% conversion rate of methacrolein entering the oxidation reactor system, 2 moles of methacrolein are required for every mole of methacrylic acid produced. In this example, the heterogeneous noble metal-containing catalyst in the oxidation reactor system may be present in an amount ranging from 0.01 kg to 1 kg of catalyst for every gram-mole of methacrolein entering the reactor system over one hour. At a 25% conversion rate of methacrolein entering the oxidation reactor system, 4 moles of methacrolein are required for every mole of methacrylic acid produced, and the heterogeneous noble metal-containing catalyst in the oxidation reactor system may be present in an amount ranging from 0.005 kg to 0.5 kg of catalyst for every gram-mole of methacrolein entering the reactor system over one hour. At a 75% conversion rate of methacrolein entering the oxidation reactor system, 1.33 moles of methacrolein are required for every mole of methacrylic acid produced, and the heterogeneous precious metal-containing catalyst in the oxidation reactor system may be present in an amount ranging from 0.015 kg to 1.5 kg of catalyst per gram-mole of methacrolein entering the reactor system over one hour. Disregarding any external recycle stream, the oxidation reactor system preferably exhibits at least a 25% conversion rate of methacrolein to methacrylic acid, more preferably at least a 35% conversion rate, and even more preferably at least a 40% conversion rate of methacrolein to methacrylic acid in the oxidation reactor system. The addition of an external recycle stream, which recycles unreacted methacrolein back to the oxidation reactor system, can also be used to improve the overall conversion efficiency of the process.

[0050] When the first precious metal-containing catalyst comprises gold, the gold may be present in an amount ranging from 0.0001 kg to 0.1 kg per gram-mole of methacrylic acid exiting the reactor system over one hour. Preferably, the gold is present in an amount of at least 0.0001 kg to 0.005 kg per gram-mole of methacrylic acid exiting the reactor system over one hour. Preferably, the gold is present in an amount of less than 0.004 kg per gram-mole of methacrylic acid exiting the reactor system over one hour.

[0051] With respect to the amount of first heterogeneous precious metal-containing catalyst in the oxidation reactor system relative to the amount of methacrolein entering the oxidation reactor system, at a 50% conversion rate of methacrolein entering the oxidation reactor system, the gold in the first heterogeneous precious metal-containing catalyst in the oxidation reactor system can be present in an amount ranging from 0.00005 kg to 0.05 kg of gold per gram-mole of methacrolein entering the reactor system over one hour. At a 25% conversion rate of methacrolein entering the oxidation reactor system, the gold in the first heterogeneous precious metal-containing catalyst in the oxidation reactor system can be present in an amount ranging from 0.000025 kg to 0.025 kg of catalyst per gram-mole of methacrolein entering the reactor system over one hour. At a 75% conversion rate of methacrolein entering the oxidation reactor system, the gold in the first heterogeneous precious metal-containing catalyst in the oxidation reactor system may be present in an amount ranging from 0.000075 kg to 0.075 kg of catalyst per gram-mole of methacrolein entering the reactor system over one hour.

[0052] The pH of the catalyst bed of the oxidation reactor system can range from 2 to 10. Some catalysts can be deactivated under acidic conditions. Therefore, if the catalyst is not acid tolerant, the pH of the catalyst bed is 4 to 10, preferably at least 5, preferably at least 5.5, preferably 9 or less, preferably 8 or less, preferably 7.5 or less.

[0053] A basic material may be added to increase the pH of the reactor system. The basic material may include an Arrhenius base (i.e., a compound that dissociates in water to form hydroxide ions), a Lewis base (i.e., a compound that can donate an electron pair), or a Bronsted-Lowry base (i.e., a compound that can accept 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, and the like. Ammonia can be either a Lewis base or a Bronsted-Lowry base.

[0054] The inventors have discovered that high local concentrations of base material in an oxidation reactor system can result in the formation of Michael adducts as unwanted by-products. Therefore, to help minimize the formation of Michael adducts, the base material is preferably mixed with at least one other material before entering the oxidation reactor system. Preferably, the base material is introduced at a location external to the oxidation reactor system and mixed with one or more reactants or diluents to form a base-containing stream. For example, the base material may be mixed with water or a non-reactive solvent, i.e., a solvent that does not adversely affect the formation of methacrylic acid in the reactor system. The location external to the reactor system may be a mixing vessel. Alternatively, the location external to the reactor may be a line through which components pass into the reactor system, such as a feed line or recycle line, in which sufficient mixing is achieved, such as by turbulence, baffles, jet mixers, or other mixing methods.

[0055] Preferably, the amount of base material in the base-containing 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, based on the total weight of the base-containing stream. The base material is preferably diluted by a factor of less than 1:2, e.g., 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 base-containing stream prior to entering the reactor system.

[0056] Preferably, the base-containing stream is thoroughly mixed before it is added to the reactor system to avoid local spikes in the concentration of the base material in the base-containing stream. For example, it is preferred that the base-containing stream reach at least 95% homogeneity, i.e., the fluctuation in the concentration of the base material deviates within + / - 5% of the average concentration of the base material in the base-containing stream before entering the reactor system. Preferably, the base-containing stream reaches 95% homogeneity within 4 minutes of the introduction of the base material, more preferably within 2 minutes, and even more preferably within 1 minute of the introduction of the base material.

[0057] For a mixing vessel, the time required for the additive to reach 95% homogeneity is Θ 95 which can be calculated by the method disclosed in Grenville and Nienow, The Handbook of Industrial Mixing, Pages 507-509, which gives the following equation for a stirred vessel in turbulent flow:

[0058]

number

[0059] Preferably, no basic material is added to the oxidation reactor system, either internally or externally. Preferably, if no basic material is added to the reactor system, the precious metal-containing catalyst comprises an acid-resistant catalyst, such as a catalyst composed of gold and titanium-containing particles. Operating the oxidation reactor system in the absence of basic material can provide several advantages. One advantage is increased selectivity and space-time yield (STY) due to reduced Michael adduct formation. Another advantage is cost savings due to reduced costs for treating aqueous waste. Aqueous waste exiting an oxidation process in which basic materials are used can produce large amounts of inorganic salts, which can be difficult or impossible to treat in biological water treatment processes. This, in turn, can necessitate the use of other waste treatment methods, such as incineration.

[0060] The second methacrolein stream is used to produce methyl methacrylate. Methyl methacrylate is produced in an oxidative esterification reaction (OER) with methanol and the second methacrolein stream. The oxidative esterification reaction is preferably carried out in the presence of an oxygen-containing gas in an oxidative esterification reactor system containing a catalyst bed.

[0061] The catalyst bed, which may comprise a slurry bed or a fixed bed, contains second heterogeneous precious metal-containing catalyst particles. The OER system further comprises a liquid phase containing methacrolein, methanol, and MMA, and a gas phase containing oxygen. The liquid phase may further comprise by-products, such as methacrolein dimethyl acetal (MDA) and methyl isobutyrate (MIB). If steps are not taken to control its formation, MIB may be present in the MMA product stream in an amount greater than 1 wt. % (10,000 ppm), based on the total weight of MMA, methacrolein, and methanol in the product stream exiting the OER system. MIB may be difficult to separate from MMA. Therefore, the present invention seeks to limit the amount of MIB formed so 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 even more preferably 0.1 ppm to 2000 ppm.

[0062] Preferably, the concentration of methanol entering the OER system is greater than 32 wt.%, based on the total weight of methanol and methacrolein entering the reactor system. More preferably, the concentration of methanol entering the OER system is greater than 35 wt.%, and even more preferably greater than 40 wt.%, based on the total weight of methanol and methacrolein entering the OER system. Preferably, the concentration of methanol entering the OER system is less than 75 wt.%, based on the total weight of methanol and methacrolein entering the reactor system. More preferably, the concentration of methanol entering the OER system is less than 60 wt.%, and even more preferably less than 50 wt.%, based on the total weight of methanol and methacrolein entering the OER reactor system.

[0063] Preferably, the concentration of methanol in the liquid-phase product stream exiting the OER system is at least 65 wt.%, based on the total weight of methanol and methacrolein in the liquid-phase product stream exiting the OER system. More preferably, the concentration of methanol in the liquid-phase product stream exiting the OER system is at least 70 wt.%, based on the total weight of methanol and methacrolein in the liquid-phase product stream exiting the OER system. Preferably, the concentration of methanol in the liquid-phase product stream exiting the OER system is less than 100 wt.%, based on the total weight of methanol and methacrolein in the liquid-phase product stream exiting the OER system. Preferably, 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 wt.%, based on the average total weight of methanol and methacrolein entering and leaving the OER system (i.e., the arithmetic mean of the total weights of methanol and methacrolein entering and leaving the OER system). More preferably, the average concentration of methanol in the OER system is greater than 75 wt.%, based on the average total weight of methanol and methacrolein entering and leaving the OER system.

[0064] It is preferred that the average weight ratio of methanol to methacrolein in the OER system be in the range of 20:1 to 2:1, where this 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.

[0065] An example of an OER system includes a multi-zone or multi-reactor system. In a first zone or reactor, the average concentration of methanol in the first zone or reactor ranges from 50% to 80% by weight, based on the average total amount of methanol and methacrolein entering and exiting the first zone or reactor. The final zone or reactor has an average methanol concentration ranging from 80% to 100% by weight, based on the average total amount of methanol and methacrolein entering and exiting the final zone or reactor. Between the first and final zones or reactors, the reactor mixture may be cooled and / or additional oxygen may be added, for example, by adding air to the gas phase entering the final zone or reactor.

[0066] Preferably, the oxygen concentration in the gas stream exiting the OER system is at least 1 mol%, more preferably at least 2 mol%, even more preferably at least 2.5 mol%, even more preferably at least 3 mol%, even more preferably at least 3.5 mol%, even more preferably at least 4 mol%, and most preferably at least 4.5 mol%, based on the total volume of the gas stream exiting the OER system. Preferably, the oxygen concentration in the gas stream exiting the OER system is no more than 7.5 mol%, preferably no more than 7.25 mol%, preferably no more than 7 mol%, based on the total volume of the gas stream exiting the OER system.

[0067] Preferably, the liquid phase in the OER system is at a temperature of 40°C to 120°C, preferably at least 50°C, preferably at least 55°C. The temperature of the liquid phase in the OER system is preferably 110°C or less, preferably 100°C or less. When the OER system comprises multiple reactors and / or multiple zones, the temperature within each reactor and / or zone may be the same or different. For example, the reaction mixture exiting a reactor or zone may be cooled before entering the next reactor or zone.

[0068] 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 limited by theory, operating the OER system under elevated 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 multiple reactors and / or zones, the pressure in each reactor and / or zone may be the same or different.

[0069] The second heterogeneous precious metal-containing catalyst in the OER system can be present in an amount ranging from 0.02 kg to 2 kg of catalyst per gram-mole of methyl methacrylate exiting the reactor system over one hour. Preferably, the second heterogeneous precious metal-containing catalyst in the OER system is present in an amount of at least 0.02 kg to 0.5 kg of catalyst per gram-mole of methyl methacrylate exiting the OER reactor system over one hour. Preferably, the second heterogeneous precious 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, even more preferably less than 0.25 kg of catalyst, and even more preferably less than 0.2 kg of catalyst per gram-mole of methyl methacrylate exiting the OER reactor system over one hour.

[0070] The amount of methyl methacrylate exiting the reactor depends on the conversion rate of methacrolein in the OER system. For example, at a 50% conversion rate of methacrolein entering the OER system, 2 moles of methacrolein would be required for every mole of methyl methacrylate produced. In this example, the heterogeneous noble metal-containing catalyst in the OER system may be present in an amount ranging from 0.01 kg to 1 kg of catalyst for every 1 gram-mole of methacrolein entering the reactor system over an hour. At a 25% conversion rate of methacrolein entering the OER system, 4 moles of methacrolein would be required for every mole of methyl methacrylate produced, and the second heterogeneous noble metal-containing catalyst in the OER system may be present in an amount ranging from 0.005 kg to 0.5 kg of catalyst for every gram-mole of methacrolein entering the reactor system over an hour. At a 75% conversion of methacrolein entering the OER system, 1.33 moles of methacrolein would be required for every mole of methyl methacrylate produced, and the second heterogeneous precious metal-containing catalyst in the OER system may be present in an amount ranging from 0.015 kg to 1.5 kg of catalyst per gram-mole of methacrolein entering the reactor system over one hour. Disregarding any external recycle stream, the OER system preferably exhibits a conversion of methacrolein to methyl methacrylate in the OER system of at least 25%, more preferably at least 35%, and even more preferably at least 40%. The addition of an external recycle stream, which recycles unreacted methacrolein back to the OER system, can also be used to improve the overall conversion efficiency of the process.

[0071] When the second precious metal-containing catalyst comprises gold, the gold may be present in an amount ranging from 0.0001 kg to 0.1 kg per gram-mole of MMA that exits the reactor system over one hour. Preferably, the gold is present in an amount of at least 0.0001 kg to 0.005 kg per gram-mole of MMA that exits the reactor system over one hour. Preferably, the gold is present in an amount of less than 0.004 kg per gram-mole of MMA that exits the reactor system over one hour.

[0072] With respect to the amount of second heterogeneous precious metal-containing catalyst in the OER system relative to the amount of methacrolein entering the reactor system, at a 50% conversion rate of methacrolein entering the OER system, the gold in the second heterogeneous precious metal-containing catalyst in the OER system may be present in an amount ranging from 0.00005 kg to 0.05 kg of gold per gram-mole of methacrolein entering the reactor system per hour. At a 25% conversion rate of methacrolein entering the OER system, the gold in the second heterogeneous precious metal-containing catalyst in the OER system may be present in an amount ranging from 0.000025 kg to 0.025 kg of catalyst per gram-mole of methacrolein entering the reactor system per hour. At a 75% conversion rate of methacrolein entering the OER system, the gold in the second heterogeneous precious metal-containing catalyst in the OER system may be present in an amount ranging from 0.000075 kg to 0.075 kg of catalyst per gram-mole of methacrolein entering the reactor system per hour.

[0073] The pH in the OER catalyst bed may range from 2 to 10. Some catalysts may be deactivated under acidic conditions. Therefore, if the catalyst is not acid tolerant, the pH of the catalyst bed is 4 to 10, preferably at least 5, preferably at least 5.5, preferably 9 or less, preferably 8 or less, preferably 7.5 or less.

[0074] A basic material may be added to increase the pH in the OER reactor system. The basic material may include an Arrhenius base (i.e., a compound that dissociates in water to form hydroxide ions), a Lewis base (i.e., a compound that can donate an electron pair), or a Bronsted-Lowry base (i.e., a compound that can accept 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, and the like. Ammonia can be either a Lewis base or a Bronsted-Lowry base.

[0075] The inventors have discovered that high local concentrations of base material in a reactor system can cause the formation of unwanted Michael adducts. Therefore, to help minimize the formation of Michael adducts, the base material is preferably mixed with at least one other material before entering the reactor system. Preferably, the base material is introduced at a location external to the reactor system and mixed with one or more reactants or diluents to form a base-containing stream. For example, the base material may be mixed with methanol, water, or a non-reactive solvent, i.e., a solvent that does not adversely affect the formation of methyl methacrylate in the reactor system. The location external to the reactor system may be a mixing vessel. Alternatively, the location external to the reactor may be a line through which components pass into the reactor system, such as a feed line or recycle line, in which sufficient mixing is achieved, such as by turbulence, baffles, jet mixers, or other mixing methods.

[0076] Preferably, the amount of base material in the base-containing 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, based on the total weight of the base-containing stream. The base material is preferably diluted by a factor of less than 1:2, e.g., 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 base-containing stream prior to entering the reactor system.

[0077] Preferably, the base-containing stream is thoroughly mixed before it is added to the reactor system to avoid local spikes in the concentration of the base material in the base-containing stream. For example, it is preferred that the base-containing stream reach at least 95% homogeneity, i.e., the fluctuation in the concentration of the base material deviates within + / - 5% of the average concentration of the base material in the base-containing stream before entering the reactor system. Preferably, the base-containing stream reaches 95% homogeneity within 4 minutes of the introduction of the base material, more preferably within 2 minutes, and even more preferably within 1 minute of the introduction of the base material.

[0078] Preferably, no basic material is added to the OER reactor system, either internally or externally. Preferably, if no basic material is added to the reactor system, the precious metal-containing catalyst comprises an acid-resistant catalyst, such as a catalyst composed of gold and titanium-containing particles. Operating an OER system in the absence of basic material can provide several advantages. One advantage is increased selectivity and space-time yield (STY) due to reduced Michael adduct formation. Another advantage is cost savings due to reduced costs for treating aqueous waste. Aqueous waste from an oxidative esterification process in which basic material is used can produce large amounts of inorganic salts, which can be difficult or impossible to treat using biological water treatment processes. This, in turn, can necessitate the use of other waste treatment methods, such as incineration.

[0079] The OER typically produces a liquid product stream containing MMA along with methacrylic acid and unreacted methanol. Preferably, the reaction product is fed to a methanol recovery distillation column, which provides a methanol- and methacrolein-rich overhead stream, which is preferably 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 occur within 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 bottoms stream. Water may need to be added to the organic phase to ensure sufficient water for MDA hydrolysis, the amount of which can be readily determined from the composition of the organic phase. An acid stream may also be added to the hydrolysis reactor to ensure adequate MDA removal. The product of the MDA hydrolysis reactor is phase separated and the organic phase is passed through one or more distillation columns to produce the MMA product and light and / or heavy by-products.

Claims

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

2. 10. The process of claim 1, wherein the weight ratio of the first methacrolein stream to the second methacrolein stream ranges from 0.01:1 to 10:

1.

3. 3. The process of claim 2, wherein the weight ratio of the first methacrolein stream to the second methacrolein stream ranges from 0.05:1 to 2:

1.

4. 4. The process of claim 1, wherein splitting the first methacrolein stream into the second methacrolein stream comprises passing the methacrolein through an adjustable valve.

5. 5. The process of any one of claims 1 to 4, further comprising producing the propionaldehyde from ethylene.

6. Step c) is carried out at a pressure ranging from 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, said reactor system containing an oxygen-containing gas; the average concentration of methacrolein in step c) is less than 40 wt. % based on the total weight of water and methacrolein; The reactor system of step c) has an average water to methacrolein ratio of less than 40:1, based on the average amounts of water and methacrolein entering and exiting the system. The process according to any one of claims 1 to 5.

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

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

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

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

11. 11. The process of any one of claims 6 to 10, wherein the first heterogeneous precious metal-containing catalyst is present in an amount ranging from 0.02 kg to 2 kg of catalyst per gram-mole of methacrylic acid exiting the oxidation reactor system over one hour.

12. Step d) is carried out at a pressure ranging from 1 bar to 150 bar, step d) is carried out in a liquid phase reaction in the presence of a second heterogeneous noble metal-containing catalyst in an oxidative esterification reactor system, said oxidative esterification reactor system comprising an oxygen-containing gas; the liquid-phase stream exiting the oxidative esterification reactor system contains at least 30 wt. % methanol, based on the total weight of the liquid-phase stream; the liquid phase stream exiting the oxidative esterification reactor system contains less than 30 wt. % methacrolein, based on the total weight of the liquid phase stream; the liquid-phase stream exiting the oxidative esterification reactor system comprises greater than 0.1 ppm and less than 5000 ppm methyl isobutyrate; the gas phase stream exiting the oxidative esterification reactor system comprises 1 mol % to 7.5 mol % oxygen based on the total amount of the gas phase stream; The process according to any one of claims 1 to 11.

13. 13. The process of claim 12, wherein the second heterogeneous precious metal-containing catalyst is present in an amount ranging from 0.02 kg to 2 kg of catalyst per gram-mole of methyl methacrylate exiting the oxidative esterification reactor system over one hour.

14. 14. The process of claim 12 or 13, wherein the liquid phase stream exiting the oxidative esterification reactor system comprises greater than 0.1 ppm and less than 4000 ppm methyl isobutyrate.

15. The process of 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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