Process for preparation of alkyl methacrylate

Abstract

There is provided a process for preparing an alkyl methacrylate, the process comprising reacting methacrolein with an alkyl alcohol in an oxidative esterification reaction (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, the OER product stream comprises alkyl methacrylate, alkyl isobutyrate in an amount in the range of 0.1 ppm to 5000 ppm based on the total weight of the OER product stream, and at least one Michael addition product in an amount in the range of 0.01 wt% 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 alkyl alcohol and methacrolein and a bottoms stream comprising alkyl methacrylate, alkyl isobutyrate, acetal and / or hemiacetal of methacrolein. The bottoms stream of 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 method for preparing an alkyl methacrylate. Background Art

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

[0003] Typical process configurations include a slurry catalyst bubble column reactor and a slurry catalyst continuous stirred tank reactor (CSTR). Slurry-type reactors for this chemistry typically use catalysts smaller than 200 μm in size, and U.S. Patent No. 6,228,800 discloses the use of eggshell-type catalysts smaller than 200 μm in size for slurry reactions. The problems with using slurry catalysts stem from catalyst attrition, which can limit the catalyst life and make it difficult to filter the product stream. According to CN1931824, these problems can be solved by using larger-sized catalysts loaded in a fixed bed reactor. However, as described in U.S. Patent Application Publication No. 2016 / 0251301, using larger catalyst particles results in a lower space-time yield and other potential drawbacks.

[0004] Fixed bed technology with larger catalyst particles has been implemented in U.S. Patent No. 4,520,125, which discloses the use of 4 mm diameter catalysts in a fixed bed system. In this case, the reactor feed is relatively dilute, as is the case in recent discussions of fixed bed technology for this chemistry, such as 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 distillation columns for purifying the product and recycling the dehydrated and further purified unreacted reactants (see, for example, U.S. Patent No. 5,969,178), where the product and recycle typically make up the majority of the product stream. This is partly because an excess of methanol is typically provided to the oxidative esterification reactor to maximize the conversion of valuable methacrolein (see, for example, U.S. Patent No. 7,326,806).

[0006] The feed concentration of methacrolein entering the oxidative esterification reactor varies in the literature from very low (see, for example, U.S. Patent No. 5,892,102) to about 35 wt% (see, for example, U.S. Patent No. 8,461,373). Methanol is typically the main component of the feed and the recycle stream returned from the downstream separation section to the oxidative esterification reactor.

[0007] Catalysts for this chemistry include various noble metals, such as palladium-based catalysts, including palladium-lead catalysts (see, for example, U.S. Patent No. 4,249,019) and gold-based or gold-containing catalysts (see, for example, U.S. Patent No. 7,326,806 and U.S. Patent No. 8,461,373).

[0008] It is desirable to maximize selectivity and reduce the formation of all by-products. Specifically, methyl isobutyrate (MIB) as a by-product is critical to reduce, as it is difficult to separate from the product MMA and is undesirable in the product. SUMMARY OF THE INVENTION

[0009] One aspect of the present invention provides a method for preparing an alkyl methacrylate, the method comprising reacting methacrolein with an alkyl alcohol in an oxidative esterification reaction (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 that contains an alkyl methacrylate, an alkyl isobutyrate in an amount in the range of 0.1 ppm to 5000 ppm based on the total weight of the OER product stream, and at least one Michael addition product in an amount in the range of 0.01 wt% to 5 wt% based on the total weight of the OER product stream. Feeding the OER product stream to an alcohol recovery distillation column to provide an overhead stream that contains the alkyl alcohol and methacrolein and a bottoms stream that contains the alkyl methacrylate, the alkyl isobutyrate, an acetal and / or hemiacetal of methacrolein. Feeding the bottoms stream of the alcohol recovery distillation column to an acetal hydrolysis reactor and a phase separator. DETAILED DESCRIPTION

[0010] Unless otherwise specified, all percent compositions are by weight (wt%), all amounts provided in parts per million (ppm) are based on weight, and all temperatures are in °C. Unless otherwise specified, the average is the arithmetic mean. "Average concentration" is the arithmetic mean of the concentration entering a region and the concentration leaving that region, where the region is a single reactor, a reactor system, or a zone within a reactor or reactor system. "Average ratio" is the ratio of the average concentration of one component to the average concentration of another component. For example, the average ratio of alcohol to methacrolein in the reactor system is calculated by dividing the average concentration of alcohol entering and leaving the reactor system by the average concentration of methacrolein entering and leaving the reactor system.

[0011] The noble metal is any one of gold, platinum, iridium, osmium, silver, palladium, rhodium, and ruthenium. More than one noble metal may be present in the catalyst, in which case the limitation applies to the total amount of all noble metals.

[0012] "Catalyst center" is the centroid of the catalyst particle, i.e., the average position of all points in all coordinate directions. The diameter is any linear dimension passing through the catalyst center, and the average diameter is the arithmetic mean of all possible diameters. The aspect ratio is the ratio of the longest diameter to the shortest diameter.

[0013] A reactor system refers to one or more reactors in which a specified reaction occurs. For example, the oxidative esterification of methacrolein to produce an alkyl methacrylate can be the specified reaction occurring in a reactor system. The reactor system can include a single reactor or multiple reactors. Additionally, the reactor system can be subdivided into multiple zones, i.e., a multi-zone reactor system. The zones can be defined by physical separation, such as by a wall or barrier defining a separation zone, or by differences in reaction conditions, such as pressure, temperature, catalyst, reactants, or other reaction components (such as inert materials, pH regulators, etc.) composition or concentration. For example, the reactor system can include: a single reactor including a single zone, a single reactor including multiple zones, multiple reactors each including a single zone in each reactor, multiple reactors in which one or more reactors have a single zone and one or more reactors include multiple zones, or multiple reactors each including multiple zones. By definition, a reactor system including multiple reactors will be considered a multi-zone reactor system. An example of a multi-zone reactor can be a continuous tubular reactor including multiple zones, including one or more mixing zones, cooling zones, and one or more catalyst zones where the reaction occurs. Another example of a multi-zone single reactor can be a stirred bed reactor including inner walls containing a catalyst, which define a catalyst zone and a feed / removal zone outside the catalyst zone, through which the liquid reactants circulate, and in which the reactants enter the reactor and the products leave the reactor. When referring to the average concentration or any ratio of the reactor system, the average concentration or ratio is calculated based on the substances entering the reactor system and the substances leaving the reactor system.

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

[0015] The OER reactor system may include a single reactor or multiple reactors. Additionally, the 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 zones, or by differences in reaction conditions, e.g., pressure, temperature, catalyst, composition or concentration of reactants or other reaction components (such as inert materials, pH regulators, etc.). For example, the reactor system may include: a single reactor including a single zone, a single reactor including multiple zones, multiple reactors each including a single zone in each reactor, multiple reactors where one or more reactors have a single zone and one or more reactors include multiple zones, or multiple reactors each including multiple zones. By definition, a reactor system including multiple reactors will be considered a multi-zone reactor system. Examples of multi-zone reactors may be continuous tubular reactors including multiple zones, including one or more mixing zones, cooling zones, and one or more catalyst zones where reactions occur. Another example of a multi-zone single reactor may be a stirred bed reactor including inner walls containing a catalyst, which define a catalyst zone and a feed / removal zone outside the catalyst zone, through which liquid reactants circulate, and in which the reactants enter the reactor and the products leave the reactor. When referring to the average concentration or any ratio of the reactor system, the average concentration or ratio is calculated based on the substances entering the reactor system and the substances leaving the reactor system.

[0016] 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.

[0017] Depending on the reactor in which the catalyst is present, the catalyst may be in the form of a slurry or a fixed bed. For example, a slurry catalyst may be used in a stirred bed reactor or a fluidized bed reactor, while a fixed bed catalyst may 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 may be selected based on the type of reactor. For example, a slurry catalyst may have an average particle diameter of less than 200 μm, e.g., 10 μm to 200 μm. A fixed bed catalyst may have an average particle diameter of 200 μm or greater, e.g., 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 still more preferably at least 800 μm; preferably not exceeding 30 mm, more preferably not exceeding 20 mm, and even more preferably not exceeding 10 mm.

[0019] The noble metal-containing catalyst preferably contains 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 contains gold particles having an average diameter of less than 12 nm, preferably less than 10 nm, and more preferably less than 8 nm. The standard deviation of the average diameter of the gold particles is + / - 4 nm, preferably + / - 2.5 nm, and more preferably + / - 2 nm. As used herein, the standard deviation is calculated by the following equation:

[0020]

[0021] where x is the size of each particle, is the average of n particles, and n is at least 500.

[0022] The noble metal-containing catalyst may further contain 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, more preferably less than 4 times, even more preferably less than 3 times, still more preferably less than 2 times, and even more preferably less than 1.5 times the average diameter of the noble metal particles. Preferably, the particles of the at least one metal oxide have an average diameter of at least half, and more preferably at least the same as the average diameter of the noble metal particles.

[0024] The amount by weight of the noble metal particles relative to the amount of the particles of the at least one metal oxide can be in the range of 1:1 to 1:20. Preferably, the weight ratio of the noble metal particles to the particles of the 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 still more preferably 1:5 to 1:8.

[0025] Preferably, the noble metal particles are evenly distributed among the particles of the at least one metal oxide. As used herein, the term "evenly distributed" means that the noble metal particles are randomly dispersed among the particles of the at least one metal oxide, with substantially no agglomeration of the noble metal particles. Preferably, at least 80% of the total number of noble metal particles are present in particles having an average diameter of less than 12 nm. More preferably, at least 90% of the total number of noble metal particles are present in particles having an average diameter of less than 12 nm. Even more preferably, at least 95% of the total number of noble metal particles are present in particles having an average diameter of less than 12 nm.

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

[0027] More preferably, at least 75% of the noble metal particles, counted by the number of noble metal particles, are within at least 20 nm of two metal oxide particles, i.e., the edge of the noble metal particle is within at least 20 nm of the edges of the two metal oxide particles closest to the noble metal particle. Preferably, at least 75% of the noble 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 noble metal particles, counted by the number of noble metal particles, are within at least 20 nm of at least three metal oxide particles, i.e., the edge of the noble metal particle is within at least 20 nm of the edges of the at least three metal oxide particles closest to the noble metal particle. Preferably, at least 75% of the noble metal particles are within at least 15 nm of at least three metal oxide particles, more preferably within at least 12 nm of at least three metal oxide nanoparticles, and even more preferably within at least 10 nm of at least three metal oxide particles.

[0029] The noble metal particles in the catalyst can be disposed on the surface of a support material. Preferably, the support material is particles of an oxide material; preferably γ-alumina, δ-alumina or θ-alumina, silica, magnesia, titania, zirconia, hafnia, vanadia, niobia, tantalum oxide, ceria, yttria, lanthanum oxide, or a combination thereof. Preferably, a part of the catalyst contains noble metals, and the support has a surface area greater than 10 m 2 / g, preferably greater than 30 m 2 / g, preferably greater than 50 m 2 / g, preferably greater than 100 m 2 / g, preferably greater than 120 m 2 / g. In the catalyst portion containing a small amount of noble metal or no noble metal, the carrier may have a surface area of less than 50 m 2 / g, preferably less than 20 m 2 / g. The average diameter of the carrier and the average diameter of the final catalyst particles are not significantly different.

[0030] Preferably, the aspect ratio of the catalyst particles is not greater than 10:1, preferably not greater than 5:1, preferably not greater than 3:1, preferably not greater than 2:1, preferably not greater than 1.5:1, preferably not greater than 1.1:1. Preferred shapes of the catalyst particles include spherical, cylindrical, rectangular solid, annular, multi-lobed (e.g., alfalfa leaf cross-section), shapes with multiple holes, and "wagon wheel"; preferably spherical. Irregular shapes can also be used.

[0031] The noble metal particles can be dispersed throughout the catalyst or have different concentration densities, such as gradient concentration or layered structure. Preferably, at least 90 wt% of the noble 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, for a volume having a constant distance measured along a line perpendicular to the outer surface from its inner surface to its outer surface (the surface of the particle), the outer volume of any particle shape is calculated. For example, for spherical particles, the outer x% of the volume is a spherical shell whose outer surface is the surface of the particle and whose volume is x% of the entire spherical volume. Preferably, at least 95 wt%, preferably at least 97 wt%, preferably at least 99 wt% of the noble metal is in the outer volume of the catalyst. Preferably, at least 90 wt% (preferably at least 95 wt%, preferably at least 97 wt%, preferably at least 99 wt%) of the noble metal is within a distance not exceeding 30%, preferably not exceeding 25%, preferably not exceeding 20%, preferably not exceeding 15%, preferably not exceeding 10%, preferably not exceeding 8% of the catalyst diameter from the surface. The distance from the surface is measured along a line perpendicular to the surface.

[0032] Preferably, the catalyst comprises gold particles and particles of at least one metal oxide on a support material comprising silica, wherein the metal of the at least one metal oxide is selected from titanium and nickel. Preferably, the gold particles and the particles of at least one metal oxide 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 noble 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 noble metal particles is not covered by another noble metal particle or a particle of at least one metal oxide, i.e., the reactants can directly contact the gold particles. Thus, the noble metal particles can be disposed within the pores of the support material and are still exposed since the reactants can directly contact the noble metal particles within the pores. More preferably, at least 0.25 wt% of the total weight of the noble metal particles is exposed on the surface of the catalyst, even more preferably, at least 0.5 wt% of the total weight of the noble metal particles is exposed on the surface of the catalyst, and still more preferably, at least 1 wt% of the total weight of the noble metal particles is exposed on the surface of the catalyst.

[0034] The catalyst bed may further comprise an inert material or an acidic material. Preferred inert materials or acidic materials include, for example, alumina, clay, glass, silicon carbide, and quartz. Preferably, the inert material or acidic material located before and / or after the catalyst bed has an average diameter equal to or greater than the average diameter of the catalyst, preferably 1 mm to 30 mm; preferably at least 2 mm; preferably not greater than 30 mm, preferably not greater than 10 mm, preferably not greater than 7 mm.

[0035] Preferably, the catalyst bed in the OER system is under a pressure of 1 bar to 150 bar (100 kPa to 15000 kPa). Without wishing to be bound by theory, it is believed that operating the OER system at increased pressure will reduce the amount of MIB present in the product stream by increasing the amount of oxygen present in the liquid phase. Thus, the pressure in the catalyst bed of the OER system can be at least 10 bar, more preferably at least 20 bar, even more preferably at least 30 bar, and preferably less than 150 bar, and more preferably less than 120 bar. When the OER system includes more than one reactor and / or zone, the pressure in each reactor and / or zone can be the same or different.

[0036] For each gram mole of alkyl methacrylate leaving the reactor system within 1 hour, the heterogeneous noble metal-containing catalyst in the OER system can be present in an amount in the range of 0.02 kg to 2 kg of catalyst. Preferably, for each gram mole of alkyl methacrylate leaving the reactor system within 1 hour, the heterogeneous noble metal-containing catalyst in the OER system is present in an amount in the range of at least 0.02 kg to 0.5 kg of catalyst. Preferably, for each gram mole of alkyl methacrylate leaving the reactor system within 1 hour, 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, still more preferably less than 0.25 kg of catalyst, and even more preferably less than 0.2 kg of catalyst.

[0037] When the noble metal-containing catalyst contains gold, for each gram mole of alkyl methacrylate leaving the reactor system within 1 hour, gold can be present in an amount in the range of 0.0001 kg to 0.1 kg. Preferably, for each gram mole of alkyl methacrylate leaving the reactor system within 1 hour, gold is present in an amount of at least 0.0001 kg to 0.005 kg. Preferably, for each gram mole of alkyl methacrylate leaving the reactor system within 1 hour, gold is present in an amount of less than 0.004 kg.

[0038] Regarding the amount of the heterogeneous noble metal-containing catalyst in the OER system relative to the amount of methacrolein entering the reactor system, when the conversion of methacrolein entering the OER system is 50%, for each gram mole of methacrolein entering the reactor system within 1 hour, gold in the heterogeneous noble metal-containing catalyst in the OER system can be present in an amount in the range of 0.00005 kg to 0.05 kg of gold. When the conversion of methacrolein entering the OER system is 25%, for each gram mole of methacrolein entering the reactor system within 1 hour, gold in the heterogeneous noble metal-containing catalyst in the OER system can be present in an amount in the range of 0.000025 kg to 0.025 kg of catalyst. When the conversion of methacrolein entering the OER system is 75%, for each gram mole of methacrolein entering the reactor system within 1 hour, gold in the heterogeneous noble metal-containing catalyst in the OER system can be present in an amount in the range of 0.000075 kg to 0.075 kg of catalyst.

[0039] In the OER reactor system, alkyl methacrylate is produced by reacting methacrolein with an alkyl alcohol in the presence of an oxygen-containing gas. The alkyl group of the alkyl methacrylate is a straight-chain or branched C1 to C 12Alkyl groups. The alkyl alcohol includes a straight-chain or branched-chain alcohol 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 their isomeric forms. 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, based on the total weight of the alkyl alcohol and methacrolein entering the reactor system, the concentration of the alkyl alcohol entering the OER system is greater than 32% by weight. More preferably, based on the total weight of the alkyl alcohol and methacrolein entering the reactor system, the concentration of the alkyl alcohol entering the OER system is greater than 35% by weight, and even more preferably greater than 40% by weight. Preferably, based on the total weight of the alkyl alcohol and methacrolein entering the reactor system, the concentration of the alkyl alcohol entering the OER system is less than 75% by weight. More preferably, based on the total weight of the alkyl alcohol and methacrolein entering the reactor system, the concentration of the alkyl alcohol entering the OER system is less than 60% by weight, and even more preferably less than 50% by weight.

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

[0042] Preferably, the average weight ratio of the alkyl alcohol to methacrolein in the OER system is from 20:1 to 2:1, where the average weight ratio is based on the average concentrations of the alkyl alcohol entering and leaving the OER system and the average concentrations of the 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 and preferably at least 55°C. The temperature of the liquid phase in the OER system is preferably not greater than 110°C and preferably not greater than 100°C. When the OER system includes more than one reactor and / or more than one zone, the temperature in each reactor and / or zone can be the same or different. For example, the reaction mixture leaving the reactor or zone can be cooled before entering the next reactor or zone.

[0044] The pH in the catalyst bed can be in the range of 2 to 10. Some catalysts can be deactivated under acidic conditions. Thus, when the catalyst is acid-intolerant, the pH in the catalyst bed is from 4 to 10; preferably at least 5, preferably at least 5.5; preferably not greater than 9, preferably not greater than 8, preferably not greater than 7.5.

[0045] To increase the pH in the reactor system, an alkaline material can be added. The alkaline material can include an Arrhenius base (i.e., a compound that dissociates in water to form hydroxide ions), a Lewis base (i.e., a compound capable of donating a pair of electrons), or a Bronsted-Lowry base (i.e., a compound capable of accepting a proton). Examples of Arrhenius bases include, but are not limited to, hydroxides of alkali metals and alkaline earth metals. Examples of Lewis bases include, but are not limited to, amines, sulfates, and phosphines. Examples of Bronsted-Lowry bases include, but are not limited to, halides, nitrates, nitrites, chlorites, chlorates, etc. Ammonia can be a Lewis base or a Bronsted-Lowry base.

[0046] The inventors have found that a high local concentration of the alkaline material in the reactor system can lead to the formation of unwanted Michael addition products as by-products. Thus, to help minimize the formation of Michael addition products, preferably the alkaline material is mixed with at least one other material before entering the reactor system. Preferably, the alkaline material is introduced at a location external to the reactor system and mixed with one or more reactants or diluents to form an alkali-containing feed stream. For example, the alkaline material can be mixed with an alkyl alcohol, water, or a non-reactive solvent (i.e., a solvent that does not negatively impact the formation of alkyl methacrylate in the reactor system). The location external to the reactor system can be a mixing vessel. Alternatively, the location external to the reactor can be a pipeline through which the components travel to the reactor system, such as a feed pipeline or a recycle pipeline, where sufficient mixing occurs, such as by turbulence, baffles, jet mixers, or other mixing methods.

[0047] Preferably, based on the total weight of the alkali-containing feed stream, the amount of the alkaline material in the alkali-containing feed stream is 50 wt% or less, preferably 25 wt% or less, preferably 20 wt% or less, preferably 15 wt% or less, preferably 10 wt% or less, preferably 5 wt% or less, or preferably 1 wt% or less. Before entering the reactor system, the alkaline material is preferably diluted by a factor of less than 1:2, such as less than 1:3, less than 1:4, less than 1:5, less than 1:10, less than 1:20, or less than 1:100 relative to the total weight of the alkali-containing feed stream. Preferably, based on the total weight of the reactants in the OER reactor system, the amount of the alkaline 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%.

[0048] Preferably, the alkali-containing feed stream is sufficiently mixed to avoid localized peaks in the concentration of the alkaline material within the alkali-containing feed stream before adding the alkali-containing feed stream to the reactor system. For example, it is preferred that the alkali-containing feed stream reaches a uniformity of at least 95%, i.e., before entering the reactor system, the variation in the concentration of the alkaline material deviates within + / -5% of the average concentration of the alkaline material in the alkali-containing feed stream. Preferably, the alkali-containing feed stream reaches 95% uniformity within 4 minutes of introducing the alkaline material, more preferably within 2 minutes of introducing the alkaline material, and even more preferably within 1 minute of introducing the alkaline material.

[0049] For a mixing vessel, the time required for the additive to reach 95% uniformity is defined as Θ 95 , which can be calculated by the method disclosed by Grenville and Nienow, "The Handbook of Industrial Mixing", pages 507 - 509, which gives the following expression for a stirred tank in turbulent flow:

[0050]

[0051] where T is the tank diameter, H is the liquid level, D is the impeller diameter, N p is the characteristic power number of the impeller, and N is the impeller speed. Similar expressions exist for static mixers, jet mixing vessels, etc.

[0052] Preferably, no basic material is added to the reactor system, whether inside or outside the reactor system. Preferably, when no basic material is added to the reactor system, the noble metal-containing catalyst comprises an acid-resistant catalyst, such as a catalyst comprising gold and titanium-containing particles. Operating the OER system in the absence of a basic material offers several advantages. One advantage is an increase in selectivity and space-time yield (STY) due to a lower production of Michael addition products. Another advantage is a cost reduction due to a reduced cost of treating aqueous waste. The aqueous waste leaving the oxidative esterification process using a basic material can produce a large amount of inorganic salts, which may be difficult or impossible to treat with biological water treatment processes. This may in turn require the use of other waste treatment methods, such as incineration.

[0053] An example of an OER system includes a multi-zone or multi-reactor system. In the first zone or reactor, based on the average total amount of alkyl alcohol and methacrolein entering and leaving the first zone or reactor, the average concentration of alkyl alcohol in the first zone or reactor is from 50 wt% to 80 wt%. Based on the average total amount of alkyl alcohol and methacrolein entering and leaving the final zone or reactor, the final zone or reactor has an average alkyl alcohol concentration in the range of 80 wt% to 100 wt%. Between the first zone or reactor and the final zone or reactor, the reactor mixture can be cooled and / or additional oxygen can be added, such as, for example, by adding air to the gas phase entering the final zone or reactor.

[0054] A polymerization inhibitor is introduced into the OER system. The inhibitor can also be introduced into the process at additional locations to control unwanted polymerization. For example, the inhibitor 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-tetramethylpiperidin-1-oxyl (4-hydroxy-TEMPO).

[0055] The OER system produces a product stream containing a liquid phase and a gas phase. The liquid phase contains methacrolein, an alkyl alcohol, and an alkyl methacrylate. The gas phase contains oxygen. The liquid phase may also contain by-products such as Michael addition products, methacrolein acetals, and methacrolein hemiacetals (such as, for example, methacrolein dimethyl acetal (MDA) or methacrolein dibutyl acetal) and isobutyrates of alkyl alcohols (such as, for example, methyl isobutyrate (MIB) or butyl isobutyrate (BIB)). In the absence of steps to control its formation, the alkyl isobutyrate can be present in the alkyl methacrylate product stream in an amount exceeding 1 wt% (10,000 ppm) relative to the total weight of the alkyl methacrylate, methacrolein, and alkyl alcohol in the product stream leaving the OER system. The alkyl isobutyrate may be difficult to separate from the alkyl methacrylate. Thus, the present invention seeks to limit the amount of alkyl isobutyrate formed such that the amount of alkyl isobutyrate 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, still more preferably 0.1 ppm to 2000 ppm, and yet more preferably 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 is in the range of 0.01 wt% to 5 wt%, more preferably 0.01 wt% to 3 wt%, still more preferably 0.01 wt% to 2 wt%, and even more preferably 0.01 wt% to 1 wt%, based on the total weight of the product stream. Preferably, the amount of methacrolein acetals and hemiacetals in the product stream is in the range of 0.01 wt% to 10 wt%, more preferably 0.01 wt% to 5 wt%, and even more preferably 0.01 wt% to 3 wt%, based on the total weight of the alkyl methacrylate and the acetals and hemiacetals of methacrolein in the product stream leaving the OER system.

[0056] Preferably, based on the total weight of the liquid phase product stream leaving the OER system, the concentration of the alkyl alcohol in the liquid phase product stream leaving the OER system ranges from 15 wt% to 95 wt%. For example, based on the total weight of the liquid phase product stream leaving the OER system, the concentration of the alkyl alcohol in the liquid phase product stream leaving the OER system can be at least 20 wt%, at least 25 wt%, or at least 30 wt%. Preferably, based on the total weight of the liquid phase product stream leaving the OER system, the concentration of the alkyl alcohol in the liquid phase product stream leaving the OER system is less than 90 wt%, more preferably less than 80 wt%, even more preferably less than 70 wt%, still more preferably less than 60 wt%, and yet more preferably less than 50 wt%.

[0057] Preferably, based on the total volume of the gas stream leaving the OER system, the oxygen concentration in the gas stream leaving the OER system is at least 1 mol%, more preferably at least 2 mol%, even more preferably at least 2.5 mol%, still more preferably at least 3 mol%, yet more preferably at least 3.5 mol%, even yet more preferably at least 4 mol%, and most preferably at least 4.5 mol%. Preferably, based on the total amount of the gas stream leaving the OER system, the oxygen concentration in the gas stream leaving the OER system is not greater than 7.5 mol%, preferably not greater than 7.25 mol%, preferably not greater than 7 mol%.

[0058] The amount of alkyl methacrylate leaving the reactor depends on the conversion of methacrolein in the OER system. For example, when the conversion of methacrolein entering the OER system is 50%, 2 moles of methacrolein are required to produce 1 mole of alkyl methacrylate. In this example, for each gram-mole of methacrolein entering the reactor system over a 1-hour period, the heterogeneous noble metal-containing catalyst in the OER system can be present in an amount within the range of 0.01 kg to 1 kg of catalyst. When the conversion of methacrolein entering the OER system is 25%, 4 moles of methacrolein are required to produce 1 mole of alkyl methacrylate, and for each gram-mole of methacrolein entering the reactor system within 1 hour, the heterogeneous noble metal-containing catalyst in the OER system can be present in an amount within the range of 0.005 kg to 0.5 kg of catalyst. When the conversion of methacrolein entering the OER system is 75%, 1.33 moles of methacrolein are required to produce 1 mole of alkyl methacrylate, and for each gram-mole of methacrolein entering the reactor system within 1 hour, the heterogeneous noble metal-containing catalyst in the OER system can be present in an amount within the range of 0.015 kg to 1.5 kg of catalyst. Without considering any external recycle stream, the OER system preferably exhibits a conversion of methacrolein to alkyl methacrylate of at least 25%, more preferably at least 35%, and even more preferably at least 40% of methacrolein to alkyl methacrylate in the OER system. 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.

[0059] 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 the 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. Preferably, the acetals and hemiacetals of methacrolein are subjected to a hydrolysis reaction to recover additional methacrolein and alkyl alcohol. Preferably, the Michael addition products and alkyl isobutyrate present in the product stream are removed.

[0060] Preferably, the product stream is fed to an alcohol recovery distillation column which provides an overhead stream rich in alkyl alcohol and methacrolein; preferably, this stream is recycled back to the OER system. Some hydrolysis of the acetals and hemiacetals of methacrolein may occur in the alcohol recovery distillation column, thus allowing additional alkyl alcohol and methacrolein to be recovered in the alcohol recovery distillation column.

[0061] The bottoms stream from the alkyl alcohol recovery distillation column contains alkyl methacrylate, isobutyrate of alkyl alcohol, methacrylic acid, salts and water. The bottoms stream also contains the 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, followed by phase separation of the alkyl alcohol recovery column bottoms stream. It may be necessary to add water to the organic phase to ensure that there is sufficient water for the hydrolysis of dialkyl acetals of methacrolein; these amounts can be determined by the composition of the organic phase. An acid stream can also be added to the hydrolysis reactor to ensure complete removal of the dialkyl acetals of methacrolein. Preferably, based on the total weight of the stream leaving the acetal hydrolysis reactor and the phase separator, the amount of the acetals and hemiacetals of methacrolein in the stream leaving the acetal hydrolysis reactor and the phase separator is in the range of 0.01 ppm to 100 ppm, more preferably 0.01 ppm to 25 ppm, and even more preferably 0.01 ppm to 5 ppm.

[0062] In any embodiment, the stream that has undergone hydrolysis in the acetal reactor and the phase separator is then sent to a heavy removal column to remove the Michael addition product. Preferably, the overhead stream of the heavy removal column contains 0.01 wt% to 1 wt%, more preferably 0.01 wt% to 0.5 wt%, and even more preferably 0.01 wt% to 0.25 wt% of the Michael addition product based on the total weight of the overhead stream of the heavy removal column.

[0063] The overhead stream of the heavy removal column is then sent to an alkyl isobutyrate removal column to further reduce the amount of alkyl isobutyrate in the product stream. Preferably, based on the total weight of the bottoms stream leaving the alkyl isobutyrate column, the amount of alkyl isobutyrate in the bottoms stream leaving the alkyl isobutyrate column is in the range of 0.01 ppm to 800 ppm, more preferably 0.01 ppm to 600 ppm, and even more preferably 0.01 ppm to 400 ppm.

[0064] The bottoms stream of the isobutyric acid alkyl ester column can be sent to the alkyl methacrylate product column for further purification of the alkyl methacrylate. For example, process inhibitors that may have been added during any distillation or phase separation process during the distillation or phase separation process can be removed and recycled.

Claims

1. A method for preparing an alkyl methacrylate, the method comprising: reacting methacrolein with an alkyl alcohol in an oxidative esterification reaction (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 comprising an alkyl methacrylate, an alkyl isobutyrate in an amount in the range of 0.1 ppm to 5000 ppm based on the total weight of the OER product stream, and at least one Michael addition product in an amount in the range of 0.01 wt% to 5 wt% based on the total weight of the OER product stream; feeding the OER product stream to an alcohol recovery distillation column to provide a top stream comprising an alkyl alcohol and methacrolein and a bottom stream comprising an alkyl methacrylate, an alkyl isobutyrate, an acetal and / or hemiacetal of methacrolein; and feeding the bottom stream of the alcohol recovery distillation column to an acetal hydrolysis reactor and a phase separator.

2. The method according to claim 1, wherein the alkyl group of the alkyl methacrylate is a straight-chain or branched C1 to C 12 alkyl group, and the alkyl group of the alkyl alcohol is a straight-chain or branched alcohol containing 1 to 12 carbon atoms.

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

4. The method according to any one of the preceding claims, wherein feeding the bottom stream of the alcohol recovery distillation column to an acetal hydrolysis reactor and a phase separator comprises feeding the bottom stream of the alcohol recovery distillation column to the acetal hydrolysis reactor and then to the phase separator to produce an organic phase and an aqueous phase, wherein the organic phase comprises an acetal and / or hemiacetal of methacrolein in an amount in the range of 0.01 ppm to 100 ppm based on the total weight of the stream leaving the acetal hydrolysis reactor and the phase separator.

5. The method according to any one of claims 1 to 3, wherein feeding the bottom stream of the alcohol recovery distillation column to an acetal hydrolysis reactor and a phase separator comprises feeding the bottom stream of the alcohol recovery distillation column to the 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 an acetal and / or hemiacetal of methacrolein in an amount in the range of 0.01 ppm to 100 ppm based on the total weight of the stream leaving the acetal hydrolysis reactor and the phase separator.

6. The method according to claim 4 or 5, the method further comprising feeding the stream leaving the acetal hydrolysis reactor and the phase separator to a heavy matter removal column to produce a top stream comprising the at least one Michael addition product in an amount of 0.01 wt% to 1 wt% based on the total weight of the top stream.

7. The method according to claim 6, the method further comprising feeding the top stream of the heavy matter removal column to an alkyl isobutyrate removal column, wherein the amount of alkyl isobutyrate in the bottom stream leaving the alkyl isobutyrate column is in the range of 0.01 ppm to 800 ppm based on the total weight of the bottom stream leaving the alkyl isobutyrate column.

8. The method according to claim 7, wherein the method further comprises feeding the bottoms stream of the alkyl isobutyrate removal column to a methyl acrylate product column to reduce the amount of polymerization inhibitor present in the bottoms stream.

9. The method according to any one of the preceding claims, wherein the alkyl alcohol comprises methanol and the methyl acrylate comprises methyl methacrylate.

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

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

12. The method according to claim 11, wherein the noble metal-containing catalyst further comprises particles of at least one metal oxide, wherein the metal of the at least one metal oxide is nickel or titanium.

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

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

15. The method according to any one of the preceding claims, wherein the noble metal-containing catalyst has an average particle diameter in the range of 200 μm to 30 mm.

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