Process for preparing alkyl methacrylates

Abstract

A process for preparing alkyl methacrylate is provided, comprising reacting methacrolein with an alkyl alcohol in an oxidative esterification (OER) system in the presence of at least one inhibitor, an oxygen-containing gas, and a noble metal-containing catalyst to produce an OER product stream, wherein the OER product stream comprises alkyl methacrylate, alkyl isobutyrate in an amount ranging from 0.1 to 5000 ppm, and at least one Michael addition product in an amount ranging from 0.01 to 5 wt %, based on the total weight of the OER product stream. The OER product stream is fed to an alcohol recovery distillation column to provide an overhead stream comprising the alkyl alcohol and methacrolein, and a bottoms stream comprising the alkyl methacrylate, alkyl isobutyrate, and acetals and / or hemiacetals of methacrolein. The bottoms stream from the alcohol recovery distillation column is fed to an acetal hydrolysis reactor and a phase separator.

Description

[Technical Field]

[0001] The present invention relates to a process for preparing alkyl methacrylates. [Background technology]

[0002] The conversion of aldehydes and alcohols to carboxylic acid esters via oxidative esterification in the presence of oxygen, particularly 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 with sizes less than 200 μm, and U.S. Patent No. 6,228,800 discloses the use of eggshell catalysts with sizes less than 200 μm for slurry reactions. Problems associated with the use of slurry catalysts result from catalyst attrition, which can limit catalyst life and make product stream filtration difficult. According to Chinese Patent No. 1931824, 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 a 4 mm particle size catalyst in a fixed bed system. The feed to the reactor in this case was relatively dilute, as have recent discussions of fixed bed technology for this chemistry, such as in U.S. Patent Application Publication Nos. 2016 / 0251301 and 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, dehydrates it, and recycles otherwise purified unreacted reactants (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 excess 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 oxidative esterification reactors 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 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), and 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), because it is difficult to separate from the product MMA and its presence in the product is undesirable. Summary of the Invention

[0009] One aspect of the present invention provides a process for preparing alkyl methacrylate, comprising reacting methacrolein with an alkyl alcohol in an oxidative esterification (OER) system in the presence of at least one inhibitor, an oxygen-containing gas, and a noble metal-containing catalyst to produce an OER product stream, wherein the OER product stream comprises alkyl methacrylate, alkyl isobutyrate in an amount ranging from 0.1 to 5000 ppm, and at least one Michael addition product in an amount ranging from 0.01 to 5 wt.%, based on the total weight of the OER product stream. The OER product stream is fed to an alcohol recovery distillation column to provide an overhead stream comprising the alkyl alcohol and methacrolein, and a bottoms stream comprising the alkyl methacrylate, alkyl isobutyrate, and acetals and / or hemiacetals of methacrolein. The bottoms stream from the alcohol recovery distillation column is fed to an acetal hydrolysis reactor and a phase separator. DETAILED DESCRIPTION OF THE INVENTION

[0010] Unless otherwise specified, all composition percentages are weight percentages (wt%), all amounts in parts per million (ppm) are by weight, and all temperatures are in °C. Averages are arithmetic means unless otherwise specified. An "average concentration" is the arithmetic mean of the concentration entering a region and the concentration 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 alcohol to methacrolein in a reactor system is calculated by dividing the average concentration of alcohol entering and exiting the reactor system by the average concentration of methacrolein entering and exiting the reactor system.

[0011] 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.

[0012] "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 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, the oxidative esterification of methacrolein to produce alkyl methacrylate may be a specified reaction taking place in a 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 defining separate sections, or by differences in reaction conditions, such as pressure, temperature, composition or concentration of catalysts, reactants, or other reaction components (inerts, pH adjusters, etc.). For example, a reactor system may 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 that includes 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 that includes an inner wall containing catalyst that defines 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 the reactor system and what exits the reactor system.

[0014] One aspect of the present invention relates to a process for producing alkyl methacrylate by oxidative esterification of methacrolein in the presence of an alkyl alcohol, at least one polymerization inhibitor, and an oxygen-containing gas, the process being carried out in an oxidative esterification reactor system ("OER system") containing a noble metal catalyst.

[0015] An OER reactor system may include a single reactor or multiple reactors. Additionally, 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 defining separate sections, or by differences in reaction conditions, such as pressure, temperature, or the composition or concentration of catalysts, reactants, or other reaction components (e.g., inerts, pH adjusters). For example, a reactor system may include a single reactor containing a single zone, a single reactor containing multiple zones, multiple reactors containing a single zone in each reactor, multiple reactors in which 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 in which the reaction occurs. 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.

[0016] 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.

[0017] 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 reactor is in the form of a fixed bed reactor.

[0018] 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 particle diameter of 200 μm or more, such as 200 μm to 30 mm. Preferably, the average diameter of the catalyst particles is at least 200 μm, more preferably at least 400 μm, even more preferably at least 600 μm, and even more preferably at least 800 μm; preferably 30 mm or less, more preferably 20 mm or less, and even more preferably 10 mm or less.

[0019] The noble metal-containing catalyst preferably comprises a noble metal selected from gold and palladium, more preferably gold. Preferably, the noble metal is in the form of particles. In a preferred embodiment, the noble metal-containing catalyst comprises gold particles having an average diameter of less than 12 nm, preferably less than 10 nm, more preferably less than 8 nm. The standard deviation of the average diameter of the gold particles is + / - 4 nm, preferably + / - 2.5 nm, more preferably + / - 2 nm. As used herein, the standard deviation is calculated using the following formula:

[0020]

number

[0021]

number

[0022] The noble metal-containing catalyst may further comprise particles of at least one metal oxide. Preferably, the metal of the at least one metal oxide is selected from aluminum, titanium, lanthanides, zirconium, alkali metals, alkaline earth metals, nickel, cobalt, zinc, magnesium, tellurium, antimony, rhenium, tungsten, and bismuth. Preferably, the metal of the at least one metal oxide is selected from nickel and titanium.

[0023] The particles of the at least one metal oxide preferably have an average diameter less than 5 times the average diameter of the precious metal particles, more preferably less than 4 times the average diameter of the precious metal particles, even more preferably less than 3 times the average diameter of the precious metal particles, even more preferably less than 2 times the average diameter of the precious metal particles, and even more preferably less than 1.5 times the average diameter of the precious metal particles. Preferably, the particles of the at least one metal oxide have an average diameter at least half the average diameter of the precious metal particles, more preferably at least the same as the average diameter of the precious metal particles.

[0024] The weight ratio of the noble metal particles to the particles of at least one metal oxide may be in the range of 1:1 to 1:20. Preferably, the weight ratio of the noble metal particles to the particles of at least one metal oxide 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.

[0025] Preferably, the precious metal particles are uniformly distributed among the particles of at least one metal oxide. As used herein, the term "uniformly distributed" means that the precious metal particles are randomly dispersed among the particles of at least one metal oxide 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 12 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 12 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 12 nm.

[0026] Preferably, at least 75% of the precious metal particles by number are within at least 20 nm of the metal oxide particle. As used herein, the phrase "within at least X nm" means that the edge of a precious metal particle is within X nm of the edge of the metal oxide particle closest to the precious metal particle. Preferably, at least 75% of the precious metal particles are within at least 15 nm of the metal oxide particle, more preferably within at least 12 nm of the metal oxide particle, and even more preferably within at least 10 nm of the metal oxide particle.

[0027] More preferably, at least 75% of the precious metal particles by number are within at least 20 nm of two metal oxide particles, i.e., the edge of a precious metal particle is within at least 20 nm of the edges of the two metal oxide particles nearest it. Preferably, at least 75% of the precious metal particles are within at least 15 nm of two metal oxide particles, more preferably within at least 12 nm of two metal oxide particles, and even more preferably within at least 10 nm of two metal oxide particles.

[0028] Even more preferably, at least 75% of the precious metal particles by number are within at least 20 nm of at least three metal oxide particles, i.e., the edge of a precious metal particle is within at least 20 nm of the edge of the at least three metal oxide particles nearest it. Preferably, at least 75% of the precious metal particles are within at least 15 nm of at least three metal oxide-containing particles, more preferably within at least 12 nm of at least three metal oxide-containing particles, and even more preferably within at least 10 nm of at least three metal oxide-containing particles.

[0029] 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.

[0030] 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.

[0031] 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 particles are 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 external 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 its 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.

[0032] Preferably, the catalyst comprises gold particles and at least one metal oxide particle on a support material comprising silica, the metal of the at least one metal oxide being selected from titanium and nickel. Preferably, the gold particles and the at least one metal oxide particle 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.

[0033] Preferably, at least 0.1 wt.% of the total weight of the precious metal particles is exposed on the surface of the catalyst, where the surface includes both the outer surface and the pores of the catalyst. As used herein, the term "exposed" means that at least a portion of the precious metal particles is not covered by other precious metal particles or by particles of at least one metal oxide, i.e., reactants can directly contact the gold particles. Thus, the precious metal particles may be disposed within the pores of the support material and still be exposed to reactants that can directly contact the precious metal particles within the pores. More preferably, at least 0.25 wt.% of the total weight of the precious metal particles is exposed on the surface of the catalyst, even more preferably, at least 0.5 wt.% of the total weight of the precious metal particles is exposed on the surface of the catalyst, and even more preferably, at least 1 wt.% of the total weight of the precious metal particles is exposed on the surface of the catalyst.

[0034] 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.

[0035] 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 can be at least 10 bar, more preferably at least 20 bar, even more preferably at least 30 bar, preferably at least less than 150 bar, and preferably at least less than 120 bar. When the OER system includes multiple reactors and / or zones, the pressure in each reactor and / or zone can be the same or different.

[0036] The heterogeneous noble 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 alkyl methacrylate exiting the reactor system over one hour. Preferably, the heterogeneous noble metal-containing catalyst in the OER system is present in an amount of at least 0.02 kg to 0.5 kg of catalyst per gram-mole of alkyl methacrylate exiting the reactor system over one hour. Preferably, the heterogeneous noble metal-containing catalyst in the OER system is present in an amount of less than 0.4 kg of catalyst, more preferably less than 0.3 kg of catalyst, 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 alkyl methacrylate exiting the reactor system over one hour.

[0037] 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 alkyl methacrylate 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 alkyl methacrylate 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 alkyl methacrylate exiting the reactor system over one hour.

[0038] 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 rate 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 one hour. At a 25% conversion rate 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 one hour. At a 75% conversion rate 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 0.075 kg of catalyst per gram-mole of methacrolein entering the reactor system over one hour.

[0039] In the OER reactor system, alkyl methacrylates are produced by reacting methacrolein with alkyl alcohols in the presence of an oxygen-containing gas. The alkyl groups of the alkyl methacrylates can be linear or branched C1-C 12 The alkyl alcohol is an alkyl group. Alkyl alcohols include linear or branched alcohols containing 1 to 12 carbon atoms. Preferably, the alkyl alcohol is selected from the group consisting of methanol, ethanol, propanol, butanol, hexanol, 2-ethylhexanol, and octanol, in all isomeric forms thereof. More preferably, the alkyl alcohol is selected from the group consisting of methanol, ethanol, butanol, and 2-ethylhexanol. Even more preferably, the alkyl alcohol is methanol.

[0040] Preferably, the concentration of alkyl alcohol entering the OER system is greater than 32 wt.%, based on the total weight of alkyl alcohol and methacrolein entering the reactor system. More preferably, the concentration of alkyl alcohol entering the OER system is greater than 35 wt.%, and even more preferably greater than 40 wt.%, based on the total weight of alkyl alcohol and methacrolein entering the reactor system. Preferably, the concentration of alkyl alcohol entering the OER system is less than 75 wt.%, based on the total weight of alkyl alcohol and methacrolein entering the reactor system. More preferably, the concentration of alkyl alcohol entering the OER system is less than 60 wt.%, and even more preferably less than 50 wt.%, based on the total weight of alkyl alcohol and methacrolein entering the reactor system.

[0041] Preferably, the average concentration of alkyl alcohol in the OER system (i.e., the arithmetic average of the concentrations of alkyl alcohol entering and leaving the OER system) is greater than 70 wt. % based on the average total weight of alkyl alcohol and methacrolein entering the reactor system (i.e., the arithmetic average of the total weight of methanol and methacrolein entering the OER system and the total weight of methanol and methacrolein leaving the OER system). More preferably, the average concentration of alkyl alcohol in the OER system is greater than 75 wt. % based on the average total weight of alkyl alcohol and methacrolein entering and leaving the reactor system.

[0042] It is preferred that the average weight ratio of alkyl alcohol 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 alkyl alcohol entering and leaving the OER system and the average concentrations of methacrolein entering and leaving the OER system.

[0043] 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.

[0044] 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 9 or less, preferably 8 or less, preferably 7.5 or less.

[0045] 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.

[0046] The inventors have discovered that high local concentrations of base material in a reactor system can cause the formation of unwanted Michael addition products. Therefore, to help minimize the formation of Michael addition products, 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 an alkyl alcohol, water, or a non-reactive solvent, i.e., a solvent that does not adversely affect the formation of alkyl 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.

[0047] 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 before entering the reactor system. Preferably, the amount of base material added to the OER reactor system is less than 10 wt%, more preferably less than 5 wt%, and even more preferably less than 2 wt%, based on the total weight of the reactants in the OER reactor system.

[0048] 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.

[0049] 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:

[0050]

number

[0051] Preferably, no basic material is added to the 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 materials can provide several advantages. One advantage is increased selectivity and space-time yield (STY) due to reduced production of Michael addition products. Another advantage is cost savings due to reduced costs for treating aqueous waste. Aqueous waste from an oxidative esterification process in which basic materials are 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.

[0052] An example of an OER system includes a multi-zone or multi-reactor system. In the first zone or reactor, the average alkyl alcohol concentration in the first zone or reactor ranges from 50% to 80% by weight, based on the average total amount of alkyl alcohol and methacrolein entering and leaving the first zone or reactor. The final zone or reactor has an average alkyl alcohol concentration in the range of 80% to 100% by weight, based on the average total amount of alkyl alcohol and methacrolein entering and leaving 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.

[0053] A polymerization inhibitor is introduced into the OER system. Inhibitors can also be introduced into the process at additional locations to control undesired polymerization. For example, inhibitors can be added to any intermediate or product stream, any phase separator, and any distillation column present in subsequent purification operations. Suitable inhibitors include, for example, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (4-hydroxy-TEMPO).

[0054] The OER system produces a product stream comprising a liquid phase containing methacrolein, alkyl alcohol, and alkyl methacrylate, and a gas phase containing oxygen. The liquid phase may further comprise by-products, such as Michael addition products, methacrolein acetals and hemiacetals, such as methacrolein dimethyl acetal (MDA) or methacrolein dibutyl acetal, and isobutyric acid of the alkyl alcohol, such as methyl isobutyrate (MIB) or butyl isobutyrate (BIB). If steps are not taken to control its formation, alkyl isobutyrate may be present in the alkyl methacrylate product stream in an amount greater than 1 wt. % (10,000 ppm), based on the total weight of alkyl methacrylate, methacrolein, and alkyl alcohol in the product stream exiting the OER system. Alkyl isobutyrate may be difficult to separate from the alkyl methacrylate. Thus, the present invention seeks to limit the amount of alkyl isobutyrate formed so that the amount of alkyl isobutyrate in the product stream ranges from 0.1 ppm to 5000 ppm, preferably from 0.1 ppm to 4000 ppm, more preferably from 0.1 ppm to 3000 ppm, even more preferably from 0.1 ppm to 2500 ppm, even more preferably from 0.1 ppm to 2000 ppm, and even more preferably from 0.1 ppm to 1000 ppm, based on the total weight of the product stream. Preferably, the amount of Michael product in the product stream ranges from 0.01 to 5 wt.%, more preferably from 0.01 to 3 wt.%, even more preferably from 0.01 to 2 wt.%, and even more preferably from 0.01 to 1 wt.%, based on the total weight of the product stream. Preferably, the amount of acetals and hemiacetals of methacrolein in the product stream ranges from 0.01 to 10 wt. %, more preferably from 0.01 to 5 wt. %, and even more preferably from 0.01 to 3 wt. %, based on the total weight of alkyl methacrylates and acetals and hemiacetals of methacrolein in the product stream exiting the OER system.

[0055] Preferably, the concentration of alkyl alcohol in the liquid-phase product stream exiting the OER system ranges from 15% to 95% by weight, based on the total weight of the liquid-phase product stream exiting the OER system. For example, the concentration of alkyl alcohol in the liquid-phase product stream exiting the OER system may be at least 20%, at least 25%, or at least 30% by weight, based on the total weight of the liquid-phase product stream exiting the OER system. Preferably, the concentration of alkyl alcohol in the liquid-phase product stream exiting the OER system is less than 90% by weight, more preferably less than 80% by weight, even more preferably less than 70% by weight, even more preferably less than 60% by weight, and even more preferably less than 50% by weight, based on the total weight of the liquid-phase product stream exiting the OER system.

[0056] 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.

[0057] The amount of alkyl 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 alkyl 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 one hour. At a 25% conversion rate of methacrolein entering the OER system, 4 moles of methacrolein would be required for every mole of alkyl methacrylate produced, and the 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 1 gram-mole of methacrolein entering the reactor system over one hour. At a 75% conversion rate of methacrolein entering the OER system, 1.33 moles of methacrolein would be required for every mole of alkyl 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 per gram-mole of methacrolein entering the reactor system over one hour. Disregarding any external recycle stream, the OER system preferably exhibits a conversion rate of methacrolein to alkyl 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.

[0058] The product stream from the OER system is preferably subjected to at least one distillation and at least one phase separation to purify and recover components within the product stream. For example, the product stream contains unreacted methacrolein and alkyl alcohol, which can be separated and returned to the OER system. The acetals and hemiacetals of methacrolein are preferably subjected to hydrolysis reactions to recover additional methacrolein and alkyl alcohol. Michael addition products and alkyl isobutyrate present in the product stream are preferably removed.

[0059] Preferably, the product stream is fed to an alcohol recovery distillation column which provides an overhead stream rich in alkyl alcohol and methacrolein, which is preferably recycled to the OER system. In the alcohol recovery distillation column, some hydrolysis of the acetals and hemiacetals of methacrolein occurs, allowing for the recovery of additional alkyl alcohol and methacrolein in the alcohol recovery distillation column.

[0060] The bottoms stream from the alkyl alcohol recovery distillation column contains alkyl methacrylate, isobutyric acid, methacrylic acid, salts of alkyl alcohol, and water. The bottoms stream further contains acetals and hemiacetals of methacrolein that were not hydrolyzed in the alcohol recovery distillation column. In one embodiment, the bottoms stream from the alkyl alcohol recovery distillation column is sent to an acetal hydrolysis reactor for further hydrolysis of the acetals and hemiacetals of methacrolein, followed by phase separation to separate the organic phase from the aqueous phase. In an alternative embodiment, the acetals and hemiacetals of methacrolein can be hydrolyzed in a separate acetal hydrolysis reactor after phase separation of the alkyl alcohol recovery bottoms stream. It may be necessary to add water to the organic phase to ensure sufficient water for hydrolysis of the methacrolein dialkyl acetal, and the amount 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 methacrolein dialkyl acetal removal. Preferably, the amount of acetals and hemiacetals of methacrolein exiting the acetal hydrolysis reactor and the phase separator is in the range of 0.01 to 100 ppm, more preferably 0.01 to 25 ppm, and even more preferably 0.01 to 5 ppm, based on the total weight of the streams exiting the acetal hydrolysis reactor and the phase separator.

[0061] In either embodiment, the stream subjected to hydrolysis in the acetal reactor and phase separator is then sent to a heavies removal column to remove the Michael addition products. Preferably, the overhead stream of the heavies removal column contains 0.01 to 1 wt. %, more preferably 0.01 to 0.5 wt. %, and even more preferably 0.01 to 0.25 wt. % of the Michael addition products, based on the total weight of the overhead stream of the heavies removal column.

[0062] The overhead stream from the heavy components removal column is then sent to an alkyl isobutyrate removal column to further reduce the amount of alkyl isobutyrate in the product stream. Preferably, the amount of alkyl isobutyrate in the bottoms stream exiting the alkyl isobutyrate column ranges from 0.01 to 800 ppm, more preferably from 0.01 to 600 ppm, and even more preferably from 0.01 to 400 ppm, based on the total weight of the bottoms stream exiting the alkyl isobutyrate column.

[0063] The bottoms stream of the alkyl isobutyrate column may be sent to an alkyl methacrylate product column for further purification of the alkyl methacrylate, for example, to remove and recycle process inhibitors that may have been added during either the distillation or phase separation process.

Claims

1. 1. A process for preparing alkyl methacrylate, comprising: reacting methacrolein with an alkyl alcohol in an oxidative esterification (OER) system in the presence of at least one inhibitor, an oxygen-containing gas, and a noble metal-containing catalyst to produce an OER product stream, wherein the OER product stream comprises alkyl methacrylate, alkyl isobutyrate in an amount ranging from 0.1 to 5000 ppm, and at least one Michael addition product in an amount ranging from 0.01 to 5 wt %, based on a total weight of the OER product stream; feeding the OER product stream to an alcohol recovery distillation column to provide an overhead stream comprising alkyl alcohols and methacrolein and a bottoms stream comprising alkyl methacrylates, alkyl isobutyrates, and acetals and / or hemiacetals of methacrolein; and feeding the bottoms stream of the alcohol recovery distillation column to an acetal hydrolysis reactor and a phase separator.

2. The alkyl group of the alkyl methacrylate is a linear or branched C １ ~C １２ 2. The process of claim 1, wherein the alkyl group of the alkyl alcohol is a straight or branched chain alcohol containing 1 to 12 carbon atoms.

3. 3. The process of claim 2, wherein the alkyl alcohol is selected from the group consisting of methanol, ethanol, propanol, butanol, hexanol, 2-ethylhexanol, and octanol in all isomeric forms thereof.

4. 4. The process of claim 1, wherein feeding the bottoms stream of the alcohol recovery distillation column to an acetal hydrolysis reactor and a phase separator comprises feeding the bottoms stream of the alcohol recovery distillation column to the acetal hydrolysis reactor and then to a phase separator to produce an organic phase and an aqueous phase, wherein the organic phase comprises acetals and / or hemiacetals of methacrolein in an amount ranging from 0.01 ppm to 100 ppm, based on the combined weight of the streams exiting the acetal hydrolysis reactor and the phase separator.

5. 4. The process of claim 1, wherein feeding the bottoms stream of the alcohol recovery distillation column to an acetal hydrolysis reactor and a phase separator comprises feeding the bottoms stream of the alcohol recovery distillation column to a phase separator to produce an organic phase and an aqueous phase, and feeding the organic phase to the acetal hydrolysis reactor to produce a stream comprising acetals and / or hemiacetals of methacrolein in an amount ranging from 0.01 ppm to 100 ppm, based on the combined weight of the streams exiting the acetal hydrolysis reactor and the phase separator.

6. 6. The process of claim 4 or 5, further comprising feeding the streams exiting the acetal hydrolysis reactor and the phase separator to a heavy components removal column to produce an overhead stream, wherein the overhead stream comprises 0.01 to 1 wt. % of the at least one Michael addition product, based on the total weight of the overhead stream.

7. 7. The process of claim 6, further comprising feeding the overhead stream of the heavy components removal column to an alkyl isobutyrate removal column, wherein the amount of alkyl isobutyrate in the bottoms stream exiting the alkyl isobutyrate column ranges from 0.01 to 800 ppm, based on the total weight of the bottoms stream exiting the alkyl isobutyrate column.

8. 8. The process of claim 7, further comprising feeding the bottoms stream of the alkyl isobutyrate removal column to an alkyl methacrylate product column to reduce the amount of polymerization inhibitor present in the bottoms stream.

9. 9. The process of any one of claims 1 to 8, wherein the alkyl alcohol comprises methanol and the alkyl methacrylate comprises methyl methacrylate.

10. The process of any one of claims 1 to 9, wherein the noble metal-containing catalyst comprises gold.

11. 11. The process of claim 10, wherein the gold is in the form of gold particles having an average diameter of less than 12 nm.

12. 12. The process of claim 11, wherein the precious metal-containing catalyst further comprises particles of at least one metal oxide, the metal of the at least one metal oxide being nickel or titanium.

13. 13. The process of claim 12, wherein the particles of the at least one metal oxide preferably have an average diameter less than 5 times the average diameter of the gold particles.

14. 14. The process of claim 12 or 13, wherein at least 75% of the gold particles are within at least 20 nm of particles of the at least one metal oxide.

15. The process of any one of claims 1 to 14, wherein the noble metal-containing catalyst has an average particle diameter of from 200 μm to 30 mm.

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