Process for the production of methacrylic acid

Abstract

A method for producing methacrylic acid, comprising: a) producing methacrolein from propionaldehyde and formaldehyde; and b) producing methacrylic acid in an oxidation reaction from methacrolein produced in step a) and water. Step b) is carried out at a pressure of more than 1 bar. Step c) is carried out in a liquid phase reaction in the presence of a heterogeneous precious metal-containing catalyst in a reactor system, the reactor system containing an oxygen-containing gas. The average concentration of methacrolein in step b) is less than 40 wt. % based on the total weight of water and methacrolein. The reactor system of step b) 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.

Description

[Technical field]

[0001] The present invention relates to a process for preparing methacrylic acid from methacrolein using a heterogeneous catalyst. [Background technology]

[0002] Methods for preparing methacrylic acid are known. For example, WO 2014 / 146961 discloses a method for preparing methacrylic acid or methacrylic acid by gas-phase oxidation of at least one C4 compound.

[0003] 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.

[0004] In the efficient production of methacrylic acid, it is desirable to maximize selectivity and reduce the formation of any by-products. Summary of the Invention

[0005] The present invention provides a method for producing methacrylic acid, comprising: a) producing methacrolein from propionaldehyde and formaldehyde; b) producing methacrylic acid in an oxidation reaction from methacrolein produced in step a) and water; step b) is carried out at a pressure greater than 1 bar, step b) is carried out in a liquid phase reaction in the presence of a heterogeneous precious metal-containing catalyst in a reactor system, the reactor system comprising an oxygen-containing gas; The average concentration of methacrolein in step b) is less than 40% by weight based on the total weight of water and methacrolein; The process is directed to a reactor system of step b) having 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. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0006] Unless otherwise noted, all compositional percentages are weight percent (wt%) and all temperatures are in °C. Unless otherwise noted, averages are arithmetic averages. An "average concentration" is the arithmetic average of the concentration entering and exiting 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.

[0007] The precious metals are any of gold, platinum, iridium, osmium, silver, palladium, rhodium, and ruthenium. More than one precious metal may be present in the catalyst, in which case the limit applies to the sum of all precious metals.

[0008] "Catalyst center" is the center of gravity of the catalyst particle, i.e., the average location of all points in all coordinate directions. Diameter is any linear dimension passing through the catalyst center, and average diameter is the arithmetic mean of all possible diameters. Aspect ratio is the ratio of the longest diameter to the shortest diameter.

[0009] A reactor system refers to one or more reactors in which a specified reaction takes place. For example, the reaction of oxidation of methacrolein to produce methacrylic acid may be a specified reaction that takes place in a reactor system. A reactor system may comprise a single reactor or multiple reactors. Furthermore, a reactor system may be subdivided into multiple zones, i.e., a multi-zone reactor system. The zones may be defined by physical separation, e.g., by walls or barriers that define separate regions, or by differences in reaction conditions, e.g., pressure, temperature, composition, or concentration of catalysts, reactants, or other reaction components, e.g., inert materials, pH adjusters, etc. For example, a reactor system may comprise a single reactor with a single zone, a single reactor with multiple zones, multiple reactors with a single zone in each reactor, multiple reactors with one or more reactors having a single zone and one or more reactors with multiple zones, or multiple reactors each with multiple zones. By definition, a reactor system with multiple reactors may be considered a multi-zone reactor system. An example of a multi-zone reactor may be a continuous tubular reactor with multiple zones including one or more mixing zones, a cooling zone, and one or more catalytic zones where the reaction takes place. Another example of a multi-zone single reactor may be a stirred bed reactor with an inner wall containing a catalyst that defines a catalytic zone through which liquid reactants circulate, and a feed / removal zone outside the catalytic zone where 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 the reactor system and what exits the reactor system.

[0010] The reactor system may comprise a reactor 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 comprises a packed bubble column reactor.

[0011] 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.

[0012] The size of the catalyst can be selected based on the type of reactor. For example, a slurry catalyst can have an average particle size of less than 200 μm, such as 10 μm to 200 μm. A fixed bed catalyst can have an average particle size 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.

[0013] The precious metal-containing catalyst comprises particles of a precious metal. Preferably, the precious metal comprises palladium or gold, more preferably the precious metal comprises gold.

[0014] The precious metal particles preferably have an average particle size of less than 15 nm, preferably less than 12 nm, more preferably less than 10 nm, even more preferably less than 8 nm. The standard deviation of the average diameter of the precious metal particles is + / - 5 nm, preferably + / - 2.5 nm, more preferably + / - 2 nm. As used herein, the standard deviation is calculated according to the formula:

number

[0015] Preferably, the precious metal-containing catalyst further comprises titanium-containing particles.

[0016] The titanium-containing particles may be elemental titanium or titanium oxide (TiO). x Preferably, the titanium-containing particles comprise titanium oxide.

[0017] The titanium-containing particles preferably have an average diameter less than 5 times the average diameter of the precious metal-containing particles, more preferably an average diameter less than 4 times the average diameter of the precious metal-containing particles, even more preferably an average particle size less than 3 times the average diameter of the precious metal-containing particles, even more preferably an average particle size less than 2 times the average diameter of the precious metal-containing particles, and even more preferably an average particle size less than 1.5 times the average diameter of the precious metal-containing particles.

[0018] The amount by weight of the precious metal-containing particles to the amount of the titanium-containing 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 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.

[0019] Preferably, the precious metal particles are uniformly distributed among the titanium-containing particles. As used herein, the term "uniformly distributed" means that the precious metal particles are randomly dispersed among the titanium-containing 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.

[0020] The precious metal particles in the catalyst may be disposed on the surface of a support material. Preferably, the support material is a particle 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 combinations thereof. Preferably, in the portion of the catalyst containing the precious metal, the support is at least 10 m 2 / g, preferably 30m 2 / g, preferably 50m 2 / g, preferably 100m 2 / g, preferably 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.

[0021] 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.

[0022] The precious metal particles may be dispersed throughout the catalyst or may have various concentration densities, such as, for example, gradient concentrations or layered structures. Preferably, at least 90% by weight of the precious metal(s) 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% of the catalyst volume, preferably the outer 40% of the catalyst volume, preferably the outer 35% of the catalyst volume, preferably the outer 30% of the catalyst volume, preferably the outer 25% of the catalyst volume. Preferably, the outer volume of any particle shape is calculated for a 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%, preferably at least 97%, preferably at least 99%) of the precious metal(s) 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.

[0023] Preferably, the catalyst comprises gold particles and titanium-containing particles on a support material comprising silica. Preferably, the gold particles and titanium-containing 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.

[0024] Preferably, at least 0.1% by weight of the total weight of the gold particles is exposed on the surface of the catalyst, the surface including 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 or a titanium-containing particle, i.e., the reactants can directly contact the gold particles. Thus, the gold particles may be located in the pores of the support material and still be exposed by the reactants that can directly contact the gold particles in 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 even more preferably, at least 1% by weight of the total weight of the gold particles is exposed on the surface of the catalyst.

[0025] The catalyst is preferably prepared by precipitating the precious metal from an aqueous solution of the metal salt in the presence of the support. Preferred precious metal salts include, but are not limited to, tetrachloroauric acid, sodium gold thiosulfate, sodium gold thiomalate, gold hydroxide, palladium nitrate, palladium chloride, and palladium acetate. In a preferred embodiment, the catalyst is prepared by the 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, and the water is then removed by drying. The resulting material is then converted to the finished catalyst by calcination, reduction, or other treatments known to those skilled in the art to decompose the precious metal salt to the metal or metal oxide. Preferably, the catalyst is prepared by the 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, and the water is then removed by drying. The resulting material is then converted to the finished catalyst by calcination, reduction, or other treatments known to those skilled in the art to decompose the precious metal salt to the metal or metal oxide. 18 Thiols are present in the solution. Preferably, they are C2-C thiols containing at least one hydroxyl or carboxylic acid substituent. 18The thiol has 2 to 12, preferably 2 to 8, preferably 3 to 6 carbon atoms. Preferably, the thiol compound contains a total of hydroxyl and carboxylic acid groups of 4 or less, preferably 3 or less, preferably 2 or less. Preferably, the thiol compound has 2 or less, preferably 1 or less thiol groups. When the thiol compounds contain 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.

[0026] In one embodiment of the present invention, the catalyst is prepared by deposition precipitation, where a porous inorganic oxide is immersed in an aqueous solution containing a suitable precious metal precursor salt, which is then interacted 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 to the finished catalyst by calcination, reduction, or other pretreatment known to those skilled in the art to decompose the precious metal salt to the metal or metal oxide.

[0027] The catalyst bed may further comprise an inert or acidic material. Preferred inert or acidic materials include, for example, alumina, clay, glass, silicon 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 equal to or less than 30 mm, preferably equal to or less than 10 mm, preferably equal to or less than 7 mm.

[0028] The present invention is useful in a process for the production of methacrylic acid which involves reacting methacrolein with water in the presence of an oxygen-containing gas in an oxidation reactor system containing a catalyst bed.

[0029] The catalyst bed, which may comprise a slurry bed or a fixed bed, comprises catalyst particles. 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).

[0030] Preferably, the average concentration of methacrolein in the oxidation reactor system is less than 40% by weight 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.

[0031] 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.

[0032] Preferably, the liquid phase in the oxidation reactor system is at a temperature between 40° C. and 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 not more than 110° C., preferably not more than 100° C. 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 leaving a reactor or zone may be cooled before entering the next reactor or zone.

[0033] Preferably, the catalyst bed in the oxidation reactor system is at a pressure of 1 bar to 150 bar (100 kPa to 15000 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.

[0034] The heterogeneous precious metal-containing catalyst in the oxidation reactor system may be present in an amount ranging from 0.02 kg to 2 kg of catalyst per gram-mole of methacrylic acid that exits the reactor system over one hour. Preferably, the 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 that exits the reactor system over one hour. Preferably, the 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 that exits the reactor system over one hour.

[0035] 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 precious 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 an 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 precious 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 an hour. At a 75% conversion 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 for every gram-mole of methacrolein entering the reactor system over an hour. Ignoring any external recycle stream, the oxidation reactor system preferably exhibits at least 25% conversion of methacrolein to methacrylic acid in the oxidation reactor system, more preferably at least 35% conversion, and even more preferably at least 40% conversion 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.

[0036] When the 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 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 methacrylic acid 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 methacrylic acid that exits the reactor system over one hour.

[0037] With respect to the amount of heterogeneous precious metal-containing catalyst in the oxidation reactor system relative to the amount of methacrolein entering the reactor system, at a 50% conversion rate of methacrolein entering the oxidation reactor system, the gold in the heterogeneous precious metal-containing catalyst in the oxidation reactor 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 over an hour. At a 25% conversion rate of methacrolein entering the oxidation reactor system, the gold in the heterogeneous precious metal-containing catalyst in the oxidation reactor 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 over an hour. At a 75% conversion rate of methacrolein entering the oxidation reactor system, the gold in the heterogeneous precious metal-containing catalyst in the oxidation reactor system may be present in an amount ranging from 0.000075 kg to .075 kg of catalyst per gram-mole of methacrolein entering the reactor system over an hour.

[0038] The pH in the 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 not more than 9, preferably not more than 8, preferably not more than 7.5.

[0039] 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 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 may be either a Lewis base or a Bronsted-Lowry base.

[0040] The inventors have discovered that high local concentrations of base material in the reactor system can cause the formation of undesirable Michael adducts as 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 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 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 travel to the reactor system, such as a feed line or a recycle line, where sufficient mixing occurs, such as by turbulence, baffles, jet mixers, or other mixing methods.

[0041] 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, based on the total weight of the base-containing stream prior to entering the reactor system.

[0042] Preferably, the base-containing stream is mixed thoroughly 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 reaches 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 it enters 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.

[0043] For a mixing vessel, the time required for the additive to reach 95% homogeneity is Θ 95 and can be calculated by the method disclosed by Grenville and Nienow, The Handbook of Industrial Mixing, Pages 507-509, which for a stirred tank in turbulent flow can be calculated using the formula:

number

[0044] Preferably, no base material is added to the reactor system, either inside or outside the reactor system. Preferably, when no base material is added to the reactor system, the precious metal-containing catalyst comprises an acid-resistant catalyst, such as a catalyst made of gold and titanium-containing particles. Operating the oxidation reactor system in the absence of a base material can provide several advantages. One advantage is increased selectivity and space-time yield (STY) due to less Michael adduct production. Another advantage is reduced costs due to reduced costs of treating aqueous waste. Aqueous waste leaving the oxidation process in which a base material was used can generate large amounts of inorganic salts, which may be difficult or impossible to treat with biological water treatment methods. This, in turn, may require the use of other waste treatment methods, such as incineration.

[0045] Methacrolein used in the oxidation reaction is preferably produced by either aldol condensation or Mannich condensation. Preferably, methacrolein is formed by 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, even more preferably 1:1.1 to 1.1:1.

[0046] 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 Suitable amines for the amine acid catalyst include, but are not limited to, amines of the formula NHR 1 R 2 (In the formula, R 1 and R 2 are each independently C1 to C 10 alkyl, optionally substituted with an ether, hydroxyl, secondary amino or tertiary amino group, or R 1 and R 2 may be taken together with the adjacent nitrogen to form a C5-C7 heterocycle, optionally containing additional nitrogen and / or oxygen atoms, and optionally substituted by C1-C4 alkyl or C1-C4 hydroxyalkyl).

[0047] 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 of more 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 more than 1 bar to 120 bar.

[0048] Inhibitors can be added to the reactor to prevent the formation of undesired products, for example, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (4-hydroxy-TEMPO) can be added to the reactor.

[0049] Propionaldehyde used to prepare methacrolein can be prepared by hydroformylation of ethylene. Hydroformylation methods are known in the art and are disclosed, for example, in U.S. Pat. No. 4,427,486, U.S. Pat. No. 5,087,763, U.S. Pat. No. 4,716,250, U.S. Pat. No. 4,731,486, and U.S. Pat. No. 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 ratio of carbon monoxide to hydrogen may range from 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.

[0050] The ethylene used to prepare propionaldehyde can be prepared from the 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 a plant material or biomass, as opposed to ethanol prepared from a petroleum-based source. Using only biosourced ethanol in the process for producing methacrylic acid can result in up to 50% of the carbon atoms of the methacrylic acid being derived from renewable resources (i.e., 2 of the 4 carbon atoms in methacrylic acid).

[0051] 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, 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. By using these additional biological resources, the amount of renewable carbon can be further increased.

[0052] Alternatively, the starting material for producing methacrylic acid can be prepared from recycled materials, for example recycled carbon dioxide can be used to produce methanol, which can be used to produce formaldehyde.

[0053] 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.

Claims

1. 1. A method for producing methacrylic acid, comprising: a) producing methacrolein from propionaldehyde and formaldehyde; b) producing methacrylic acid from the methacrolein produced in step a) and water in an oxidation reaction; step b) is carried out at a pressure above 1 bar, step b) is carried out in a liquid phase reaction in the presence of a heterogeneous noble metal-containing catalyst in a reactor system, said reactor system containing an oxygen-containing gas; the average concentration of methacrolein in step b) is less than 40 wt. % based on the total weight of water and methacrolein; The process of claim 1, wherein the reactor system of step b) 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.

2. 10. The process of claim 1, wherein the oxygen in the gas phase exiting the reactor system of step b) is present in an amount ranging from 1 mol % to 7.5 mol % oxygen based on the total amount of the gas phase.

3. 3. The method of claim 2, wherein the oxygen in the gas phase exiting the reactor system of step b) is present in an amount ranging from 2 mol % to 7.25 mol %, based on the total amount of the gas phase.

4. 4. The method of claim 3, wherein oxygen in the gas phase exiting the reactor system of step b) is present in an amount ranging from 4 mol % to less than 7 mol %, based on the total amount of the gas phase.

5. The process of any one of claims 1 to 4, wherein the heterogeneous noble metal-containing catalyst is in the form of a slurry or a fixed bed.

6. The method of any one of claims 1 to 4, wherein the heterogeneous noble metal-containing catalyst comprises gold.

7. 5. The method of any one of claims 1 to 4, wherein the heterogeneous noble 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 reactor system over one hour.

8. 5. The method of any one of claims 1 to 4, wherein the heterogeneous precious metal-containing catalyst is present in an amount ranging from 0.0001 kg to 0.1 kg of gold per gram-mole of methacrylic acid exiting the reactor system over one hour.

9. The process of any one of claims 1 to 4, wherein the reactor of step b) comprises a multi-zone reactor.

10. 5. The method according to any one of claims 1 to 4, wherein the reactor system of step b) comprises a single reactor.

11. 5. The method according to any one of claims 1 to 4, wherein the reactor system of step b) comprises two or more reactors.

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