Process for forming methyl methacrylate with fewer by-products from an oxidative esterification reactor

Abstract

A process for producing methyl methacrylate by oxidative esterification is disclosed. The process comprises reacting methanol and methacrolein in a reactor system containing an oxygen-containing gas and a heterogeneous precious metal-containing catalyst to produce methyl methacrylate. No Arrhenius base is added to the reactor system.

Description

[Technical field]

[0001] The present invention relates to a process for the production of methyl methacrylate by oxidative esterification of methacrolein and methanol using a heterogeneous catalyst. [Background technology]

[0002] The conversion of aldehydes and alcohols to carboxylic acid esters via oxidative esterification in the presence of oxygen, in particular the conversion of methacrolein and methanol to methyl methacrylate in the presence of oxygen, has been known for many years. For example, U.S. Patent No. 4,249,019 discloses the use of palladium (Pd)-lead (Pb) and other catalysts for this purpose.

[0003] Typical process configurations included slurry catalyst bubble column reactors and slurry catalyst continuous stirred tank reactors (CSTRs). Slurry reactors for this chemistry typically use catalysts less than 200 μm in size, and U.S. Patent No. 6,228,800 discloses the use of eggshell catalysts less than 200 μm in size for slurry reactions. Problems with the use of slurry catalysts result from catalyst attrition, which can limit catalyst life and make product stream filtration difficult. According to CN1931824, these problems can be addressed by using larger catalyst sizes packed into fixed bed reactors. However, as described in U.S. Patent Application Publication No. 2016 / 0251301, the use of larger catalyst particles results in reduced space-time yields and other potential disadvantages.

[0004] Fixed bed technology using larger catalyst particles has been demonstrated in U.S. Patent No. 4,520,125, which discloses the use of 4 mm particle size catalyst in a fixed bed system. The feed to the reactor in this case was relatively dilute, as fixed bed technology for this chemistry has been recently discussed, such as in U.S. Patent Application Publication No. 2016 / 0251301 and U.S. Patent Application Publication No. 2016 / 0280628.

[0005] In commercial production facilities, the oxidative esterification reactor is followed by a separation section consisting of a distillation column that purifies the product and recycles the unreacted reactants that have been dehydrated or otherwise purified (see, e.g., U.S. Pat. No. 5,969,178), where the product and recycle often constitute the majority of the product stream. In part, this is because an excess of methanol is typically fed to the oxidative esterification reactor to maximize the conversion of beneficial methacrolein (see, e.g., U.S. Pat. No. 7,326,806).

[0006] Feed concentrations of methacrolein into the oxidative esterification reactor vary in the literature from very low (see, e.g., U.S. Pat. No. 5,892,102) to about 35 wt. % (see, e.g., U.S. Pat. No. 8,461,373). Methanol is typically the major component of the feed and of the recycle stream returned to the oxidative esterification reactor from downstream separations.

[0007] Catalysts for this chemistry have included various precious metals such as palladium-based catalysts, including palladium-lead catalysts (see, e.g., U.S. Pat. No. 4,249,019), as well as gold-based or gold-containing catalysts (see, e.g., U.S. Pat. Nos. 7,326,806 and 8,461,373).

[0008] It is desirable to maximize selectivity and reduce the formation of all by-products, especially methyl isobutyrate (MIB), which is difficult to separate from the product MMA and is undesirable in the product. Summary of the Invention

[0009] The present invention is directed to a process for producing methyl methacrylate by oxidative esterification comprising reacting methanol and methacrolein in a reactor system comprising an oxygen-containing gas and a heterogeneous precious metal-containing catalyst to produce methyl methacrylate, wherein no Arrhenius base is added to the reactor system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] 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 methanol to methacrolein in a reactor system is calculated by dividing the average concentration of methanol entering and leaving the reactor system by the average concentration of methacrolein entering and leaving the reactor system.

[0011] 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.

[0012] "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.

[0013] A reactor system refers to one or more reactors in which a specified reaction takes place. For example, oxidative esterification of methacrolein to produce methyl methacrylate may be the specified reaction taking place in the reactor system. A reactor system may include a single reactor or multiple reactors. In addition, a reactor system may be subdivided into multiple zones, i.e., a multi-zone reactor system. The zones may be defined by physical separation, such as by walls or barriers that define separate portions, or by differences in reaction conditions, such as, for example, pressure, temperature, composition or concentration of catalysts, reactants, or other reaction components (inerts, pH modifiers, etc.). For example, a reactor system may include 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 containing multiple zones, or multiple reactors each containing multiple zones. By definition, a reactor system containing multiple reactors is considered to be a multi-zone reactor system. An example of a multi-zone reactor can be a continuous tubular reactor that includes 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 can be a stirred bed reactor that includes an inner wall containing a catalyst that defines a catalytic zone through which liquid reactants are circulated, 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.

[0014] The reactor system may include 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 includes a packed bubble column reactor.

[0015] 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.

[0016] 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 particle size 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.

[0017] 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.

[0018] 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 particle size 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 following formula:

number

[0019] Preferably, the precious metal-containing catalyst further comprises titanium-containing particles.

[0020] Titanium-containing particles can be elemental titanium or titanium oxide (TiO). x Preferably, the titanium-containing particles comprise titanium oxide.

[0021] The titanium-containing particles preferably have an average particle size less than 5 times the average particle size of the precious metal-containing particles, more preferably less than 4 times the average particle size of the precious metal-containing particles, even more preferably less than 3 times the average particle size of the precious metal-containing particles, even more preferably less than 2 times the average particle size of the precious metal-containing particles, and still more preferably less than 1.5 times the average particle size of the precious metal-containing particles.

[0022] The amount by weight of the precious metal-containing particles relative 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.

[0023] 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 particle size 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 particle size 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 particle size of less than 15 nm.

[0024] 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 a particle of γ-, δ- or θ-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 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 particle size of the support and the average particle size of the final catalyst particles do not differ significantly.

[0025] 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.

[0026] 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% of the catalyst volume, preferably the outer 50%, preferably the outer 40%, preferably the outer 35%, preferably the outer 30%, preferably the outer 25%. Preferably, the outer volume of any particle shape is calculated for a volume that has 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% of the catalyst diameter, preferably no more than 25% by weight, preferably no more than 20% by weight, preferably no more than 15% by weight, preferably no more than 10% by weight, preferably no more than 8% by weight. The distance from the surface is measured along a line perpendicular to the surface.

[0027] 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.

[0028] Preferably, at least 0.1% by weight of the total weight of the gold particles is exposed on the surface of the catalyst, where the surface includes both the outer surface and the pores of the catalyst. As used herein, the term "exposed" means that at least some of the gold particles are not covered by other gold particles or titanium-containing particles, i.e., 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 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.

[0029] The catalyst is preferably produced by precipitating the precious metal from an aqueous solution of the metal salt in the presence of the support. Suitable precious metal salts can 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 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, 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 a C2-C aryl group containing at least one hydroxyl or carboxylic acid substituent. 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 4 or less, preferably 3 or less, preferably 2 or less, hydroxyl and carboxylic acid groups. 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.

[0030] 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 precious metal precursor salt and then adjusting the pH of the solution to allow 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 precious metal salt to the metal or metal oxide.

[0031] 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 particle size equal to or greater than the average particle size of the catalyst, preferably between 1 mm and 30 mm; preferably at least 2 mm; preferably 30 mm or less, preferably 10 mm or less, preferably 7 mm or less.

[0032] The present invention is useful in a process for producing methyl methacrylate (MMA) which involves reacting methacrolein with methanol in the presence of an oxygen-containing gas in an oxidative esterification reactor (OER) system containing a catalyst bed.

[0033] The catalyst bed, which may include a slurry bed or a fixed bed, contains catalyst particles. The OER system further includes a liquid phase containing methacrolein, methanol, and MMA, and a gas phase containing oxygen. The liquid phase may further include 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 still more preferably 0.1 ppm to 2000 ppm.

[0034] 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 reactor 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 reactor system.

[0035] 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 average 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 average 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.

[0036] It is preferred that the average weight ratio of methanol to methacrolein in the OER system ranges from 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.

[0037] An example of an OER system includes a multi-zone or multi-reactor system. In the 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 leaving the first zone or reactor. The final zone or reactor has an average methanol concentration in the range of 80% to 100% by weight based on the average total amount of methanol and methacrolein entering and leaving the final zone or reactor. Between the first zone or reactor and the final zone or reactor, 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.

[0038] 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.

[0039] Preferably, the liquid phase in the OER system is at a temperature of from 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 no greater than 110° C., preferably no greater than 100° C. When the OER system comprises multiple reactors and / or multiple 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.

[0040] 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, it is believed that operating the OER system under elevated pressure reduces 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.

[0041] The heterogeneous precious metal-containing catalyst in the OER system may be present in an amount ranging from 0.02 kg to 2 kg of catalyst per gram-mole of methyl methacrylate that exits the reactor system over one hour. Preferably, the 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 that exits the reactor system over one hour. Preferably, the 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 that exits the reactor system over one hour.

[0042] 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 precious 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 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 heterogeneous precious 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 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 for every gram-mole of methacrolein entering the reactor system over an hour. Ignoring 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 that recycles unreacted methacrolein to the OER system can also be used to improve the overall conversion efficiency of the process.

[0043] 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 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.

[0044] With respect to the amount of heterogeneous precious metal-containing catalyst in the OER system relative to the amount of methacrolein entering the reactor system, at a 50% conversion of methacrolein entering the OER system, the gold in the 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 over an hour. At a 25% conversion of methacrolein entering the OER system, the gold in the 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 over an hour. At a 75% conversion of methacrolein entering the OER system, the gold in the heterogeneous precious metal-containing catalyst in the OER 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.

[0045] 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 in 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.

[0046] A base material may be added to increase the pH in the reactor system. The inventors have discovered that Arrhenius bases (i.e., compounds that dissociate in water to form hydroxide ions, such as hydroxides of alkali and alkaline earth metals) can result in a detrimental build-up of salts in the catalyst bed. Therefore, the OER system does not include Arrhenius bases. When a base is used to adjust the pH, the base material is preferably 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) that is not also classified as an Arrhenius base. In other words, the base material can be selected from bases that do not include hydroxides of alkali or 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.

[0047] The inventors have discovered that high local concentrations of base material in the reactor system can cause the formation of unwanted by-product 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 to the reactor system, such as a feed line or a recycle line, in which sufficient mixing is achieved, such as by turbulence, baffles, jet mixers, or other mixing methods.

[0048] 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.

[0049] 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 remains within + / - 5% deviation from 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.

[0050] 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 gives the following equation for a stirred vessel in turbulent flow:

number

[0051] Preferably, no basic material is added to the reactor system, either inside or outside the reactor system. Preferably, when 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 OER system in the absence of basic materials can provide several advantages. One advantage is increased selectivity and space-time yield (STY) due to reduced production of Michael adducts. Another advantage is reduced costs due to reduced costs of treating aqueous waste. Aqueous waste from an oxidative esterification process in which basic materials are used can generate large amounts of inorganic salts that may be difficult or impossible to treat by biological water treatment processes. This, in turn, may require the use of other waste treatment methods, such as incineration.

[0052] 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 providing an overhead stream rich in methanol and methacrolein, 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 may be hydrolyzed in the bottoms stream from the methanol recovery distillation column. This hydrolysis may be carried out in the methanol recovery column. The bottoms stream from the methanol recovery distillation column can be sent to a separate acetal hydrolysis reactor for additional MDA hydrolysis. Alternatively, MDA may be hydrolyzed in a separate acetal hydrolysis reactor after the organic phase is separated from the methanol recovery bottoms stream. To ensure sufficient water for hydrolysis of MDA, it may be necessary to add water to the organic phase, the amount of which can be easily 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 passed through one or more distillation columns to produce the MMA product and light and / or heavy by-products.

[0053] Methacrolein used in the oxidative esterification 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.

[0054] 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, an amine of the formula NHR 1 R 2 (In the formula, R 1 and R 2 each independently represents a C1-C substituted or unsubstituted 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 C5-C7 heterocycle, optionally containing additional nitrogen atoms and / or oxygen atoms, and optionally substituted with C1-C4 alkyl or C1-C4 hydroxyalkyl.

[0055] 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 150 bar.

[0056] 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.

[0057] Propionaldehyde used to prepare methacrolein can be prepared by hydroformylation of ethylene. The process of hydroformylation is known in the art and is 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 from 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.

[0058] 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 a petroleum-based source. Using biosourced ethanol alone in the MMA production process can result in up to 40% of the carbon atoms of MMA being derived from renewable resources (i.e., 2 out of 5 carbon atoms in MMA).

[0059] To further increase the renewable carbon content in MMA, additional starting materials can also be prepared from renewable resources. For example, formaldehyde can be prepared from syngas, which in turn 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.

[0060] Alternatively, the starting materials for producing MMA 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.

[0061] Preferably, at least 40%, more preferably at least 60%, even more preferably at least 80%, and even more preferably 100% of the carbon atoms in the MMA are derived from renewable or recycled content.

Claims

1. 1. A method for producing methyl methacrylate by oxidative esterification, comprising: reacting methanol and methacrolein in a reactor system containing an oxygen-containing gas and a heterogeneous noble metal-containing catalyst to produce methyl methacrylate; The method wherein no Arrhenius base is added to the reactor system.

2. 10. The method of claim 1, wherein a base material comprising a Lewis base is added to the reactor system to maintain a pH of 4 to 10.

3. 3. The method of claim 2, wherein the base material is added directly or indirectly to the reactor system.

4. 4. The method of claim 3, wherein the base material is added to the reactor system indirectly by mixing with one or more reactants to form a base-containing stream and subsequently adding the base-containing stream to the reactor system.

5. 10. The method of claim 1, wherein no base is added to the reactor system.

6. 6. The method of claim 5, wherein the pH of the reactor system ranges from 2 to 7.

7. 10. The method of claim 1, wherein the reactor system comprises a fluidized bed, a fixed bed, a trickle bed, a packed bubble column, or a stirred bed reactor.

8. 10. The method of claim 1, wherein the heterogeneous precious metal-containing catalyst is in the form of a slurry or a fixed bed.

9. 10. The method of claim 1, wherein a portion of the product stream exiting the reactor system is recycled back to the reactor system without the addition of base material.

10. 10. The method of claim 9, wherein the portion of the product stream recycled back to the reactor system is distilled to remove methyl methacrylate and / or heavy and / or light materials prior to entering the reactor system.

11. The method of claim 1 , wherein the heterogeneous noble metal-containing catalyst comprises gold.

12. 10. The method of claim 1, wherein the gas phase stream exiting the reactor system comprises oxygen in an amount ranging from 1 mol % to 7.5 mol %, based on the total amount of the gas phase.

13. 10. The method of claim 1, wherein the reactor system is maintained at a temperature in the range of 40°C to 120°C.

14. 10. The method of claim 1, wherein the reactor system is maintained at a pressure ranging from 0 psig to 300 psig.

15. 15. The process of any one of claims 1 to 14, wherein the molar ratio of methanol to methacrolein entering the reactor system ranges from 1:100 to 1000:

1.

16. 15. The process of claim 14, wherein the molar ratio of methanol to methacrolein entering the reactor system ranges from 0.1:1 to 10:1.