Process for preparing alkyl methacrylates

Abstract

A process for preparing an alkyl methacrylate is provided, comprising: (a) reacting propionaldehyde with formaldehyde to produce a methacrolein intermediate stream; (b) subjecting the methacrolein intermediate stream produced in step (a) to at least one phase separation and at least one distillation to reduce the amount of methacrolein dimer and water in the methacrolein intermediate stream, wherein the amount of methacrolein dimer exiting the at least one phase separation and the at least one distillation is less than 10 wt. % based on the total weight of the methacrolein intermediate stream exiting the at least one phase separation and the at least one distillation, and the amount of water exiting the at least one phase separation and the at least one distillation is less than 10 wt. % based on the total weight of the methacrolein intermediate stream exiting the at least one phase separation and the at least one distillation; and (c) reacting methacrolein with an alkyl alcohol in an oxidative esterification reaction system in the presence of at least one inhibitor, an oxygen-containing gas, and a catalyst comprising gold particles and at least one metal oxide particles to produce a product stream comprising an alkyl methacrylate. The product stream comprises 0.1 to 5000 ppm alkyl isobutyrate and 0.01 to 5 wt % of at least one Michael addition product, based on the weight of the product stream.

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 is a process for preparing alkyl methacrylate, comprising: (a) reacting propionaldehyde with formaldehyde to produce a methacrolein intermediate stream; (b) subjecting the methacrolein intermediate stream produced in step (a) to at least one phase separation and at least one distillation to reduce the amount of methacrolein dimer and water in the methacrolein intermediate stream, wherein the amount of methacrolein dimer exiting the at least one phase separation and the at least one distillation is less than 10% by weight based on the total weight of the methacrolein intermediate stream exiting the at least one phase separation and the at least one distillation, and the amount of water exiting the at least one phase separation and the at least one distillation is less than 10% by weight based on the total weight of the methacrolein intermediate stream exiting the at least one phase separation and the at least one distillation; (c) reacting methacrolein with an alkyl alcohol in an oxidative esterification reaction system in the presence of at least one inhibitor, an oxygen-containing gas, and a catalyst comprising gold particles and particles of at least one metal oxide to produce a product stream comprising an alkyl methacrylate, wherein 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; The process provides a process wherein the product stream comprises 0.1 to 5000 ppm alkyl isobutyrate and 0.01 to 5 wt. % of at least one Michael addition product, based on the total weight of the product stream. 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 provides a process for preparing an alkyl methacrylate, the process comprising: reacting propionaldehyde and formaldehyde to produce a methacrolein intermediate stream; subjecting the methacrolein intermediate stream to at least one phase separation and at least one distillation; and reacting the methacrolein with an alkyl alcohol in an oxidative esterification reaction system to produce the alkyl methacrylate.

[0015] To produce methacrolein from propionaldehyde and formaldehyde, a catalyst stream is provided by mixing water with an amic acid catalyst. The water and catalyst may be mixed in a catalyst tank before entering the reactor. The amic acid catalyst can catalyze the Mannich condensation of propionaldehyde and formaldehyde to methacrolein. The Mannich condensation process is known in the art, for example, as described in U.S. Pat. Nos. 4,496,770 and 7,141,702. Suitable amic acid catalysts include, for example, those containing a secondary amine and an acid.

[0016] Suitable acids for the amino acid catalyst include, for example, inorganic acids and organic mono-, di-, or polycarboxylic acids. Suitable carboxylic acids include, for example, aliphatic C1-C 10 Monocarboxylic acids, C2-C 10 Dicarboxylic acids, C2-C 10 Examples of suitable carboxylic acids include polycarboxylic acids. Preferred carboxylic acids include at least one of acetic acid, propionic acid, methoxyacetic acid, n-butyric acid, isobutyric acid, oxalic acid, succinic acid, tartaric acid, glutaric acid, adipic acid, maleic acid, fumaric acid, and combinations thereof. Suitable inorganic acids include, for example, sulfuric acid and phosphoric acid.

[0017] Suitable amines of the amic acid catalyst include, for example, amines of the formula NHR 2 R 3 wherein R 2 and R 3 are each independently C1 to C 10 alkyl, optionally substituted with an ether, hydroxyl, secondary amino, or tertiary amino group, or R 2 and R 3can be taken together with the adjacent nitrogen to form a C5-C7 heterocycle, optionally containing additional nitrogen and / or oxygen atoms and optionally substituted by C1-C4 alkyl or C1-C4 hydroxyalkyl. Preferred amines include at least one of dimethylamine, diethylamine, methylethylamine, methylpropylamine, dipropylamine, dibutylamine, diisopropylamine, diisobutylamine, methylisopropylamine, methylisobutylamine, methyl-sec-butylamine, methyl-(2-methylpentyl)-amine, methyl-(2-ethylhexyl)-amine, pyrrolidine, piperidine, morpholine, N-methylpiperazine, N-hydroxyethylpiperazine, piperazine, hexamethyleneimine, diethanolamine, methylethanolamine, methylcyclohexylamine, methylcyclopentylamine, and dicyclohexylamine, and combinations thereof.

[0018] Preferably, the amine acid catalyst comprises dimethylamine and acetic acid. The molar ratio of amine to acid can be such that the resulting pH is 2.5 to 7. For example, the amine acid catalyst can contain a molar ratio of dimethylamine to acetic acid in an amount of 10:1 to 1:10, preferably 5:1 to 1:5, and more preferably 1:1 to 1.2:1.

[0019] The Mannich condensation reaction can be carried out by sending a catalyst stream and a reaction stream containing propionaldehyde, formaldehyde, and methanol to a reactor to produce a first intermediate stream containing methacrolein, methanol, and water via the Mannich condensation reaction. The reaction can be carried out under any suitable conditions under which the reaction proceeds. For example, the reaction can be carried out at a temperature of at least 20°C and at least atmospheric pressure. Preferably, the reaction is carried out in the liquid phase at a temperature above 150°C, e.g., 150 to 220°C, and at superatmospheric pressure, i.e., above 1 bar. Preferably, the reaction is carried out in the liquid phase at a pressure in the range of from above 1 bar to 150 bar, more preferably from 10 bar to 120 bar.

[0020] The molar ratio of propionaldehyde to formaldehyde is not particularly limited. For example, the reaction stream may contain a propionaldehyde to formaldehyde ratio in an amount of 1.1:1 to 1:2, preferably 1.1:1 to 1:1.5, and more preferably 1.05:1 to 1:1.05. The first intermediate stream is considered a "wet" methacrolein stream in that it contains a significant amount of water, for example, more than 10 wt. %, more than 20 wt. %, or more, based on the total weight of the first intermediate stream.

[0021] Propionaldehyde, used to prepare methacrolein, can be prepared by hydroformylation of ethylene. Hydroformylation processes are known in the art and are disclosed, for example, in U.S. Pat. Nos. 4,427,486, 5,087,763, 4,716,250, 4,731,486, and 5,288,916. The hydroformylation of ethylene to propionaldehyde involves contacting ethylene with carbon monoxide and hydrogen in the presence of a hydroformylation catalyst. Examples of hydroformylation catalysts include metal-organophosphorus ligand complexes, such as organophosphines, organophosphites, and organophosphoramidites. The ratio of carbon monoxide to hydrogen may range from 1:10 to 100:1, preferably 1:10 to 10:1. The hydroformylation process may be carried out at a temperature ranging from -25°C to 200°C, preferably from 50°C to 120°C.

[0022] The ethylene used to prepare propionaldehyde can be prepared from the dehydration of ethanol. For example, ethylene can be prepared by acid-catalyzed dehydration of ethanol. Ethanol dehydration is known in the art and is disclosed, for example, in U.S. Pat. No. 9,249,066. Preferably, the ethanol is sourced from a renewable resource, such as plant material or biomass, as opposed to ethanol prepared from petroleum-based sources. For example, using bio-sourced ethanol alone in a process for producing MMA can result in up to 40% of the carbon atoms of MMA being derived from renewable resources (i.e., 2 out of 5 carbon atoms in MMA).

[0023] To further increase the renewable carbon content in alkyl methacrylate, additional starting materials can be prepared from renewable resources. For example, formaldehyde can be prepared from syngas, which in turn can be prepared from biomass. Carbon monoxide, which can also be used to prepare propionaldehyde, can also be prepared from renewable resources, as disclosed in Li et al., ACS Nano, April 14, 2020, pp. 4905-4915. The use of these additional biological resources can further increase the amount of renewable carbon.

[0024] Alternatively, the starting materials for producing alkyl methacrylates can be prepared from recycled materials, for example, recycled carbon dioxide can be used to produce methanol, and methanol can be used to produce formaldehyde.

[0025] Preferably, at least 40%, more preferably at least 60%, even more preferably at least 80%, and even more preferably 100% of the carbon atoms in the alkyl methacrylate are derived from renewable or recycled content.

[0026] The methanol and formaldehyde present in the reaction stream may be provided in the form of formalin. The formalin utilized in the process of the present invention may comprise a saturated aqueous solution containing formaldehyde in an amount of about 37% by weight and methanol in an amount of 10-15% by weight, based on the total weight of the formalin. The methanol present in the formalin can be advantageously used in a subsequent oxidative esterification process to convert methacrolein to alkyl methacrylate in the presence of an alkyl alcohol, e.g., to methyl methacrylate in the presence of methanol. In certain embodiments, methanol can be introduced at various locations during the process.

[0027] The first intermediate stream is subjected to at least one distillation step and at least one phase separation step.

[0028] For example, in one embodiment, the first intermediate stream can be subjected to phase separation to produce an organic phase containing water, methanol, and primarily methacrolein, and an aqueous phase containing methacrolein, methanol, an amine-acid catalyst, and primarily water. In this embodiment, methacrolein can be present in the organic phase in an amount of at least 70 wt %, preferably at least 85 wt %, and more preferably at least 90 wt %, based on the total weight of the organic phase. Methanol can be present in the organic phase in an amount of less than 10 wt %, preferably less than 3 wt %, and more preferably less than 2.5 wt %, based on the total weight of the organic phase. Without wishing to be bound by theory, it is believed that operating the phase separator at a lower temperature results in the organic phase containing less methanol, which is beneficial for downstream distillation of the organic phase. The phase separator can be operated at a temperature below 15°C, preferably below 10°C, and more preferably below 5°C. The aqueous phase can contain water in an amount of at least 70 wt %, preferably at least 75 wt %, and more preferably at least 80 wt %.

[0029] The organic phase can then be distilled in a first distillation column to produce a second intermediate stream and a top stream. The first distillation column can be operated as a stripping column, and the overhead vapor can be condensed without refluxing any liquid to the column. The ratio of the second intermediate stream exiting the first distillation column to the organic phase entering the first distillation column can be in the range of 1:10 to 8:10, preferably 3:10 to 7:10, and more preferably 5:10 to 6:10. The second intermediate stream contains water, methanol, and primarily methacrolein. Water can be present in the second intermediate stream in an amount of less than 2 wt.%, preferably less than 1 wt.%, and more preferably less than 0.5 wt.%, based on the total weight of the second intermediate stream. Methacrolein can be present in the second intermediate stream in an amount of at least 70 wt.%, preferably 85 wt.%, and more preferably 95 wt.%, based on the total weight of the second intermediate stream. The top stream contains water, methanol, and primarily methacrolein. Water may be present in the overhead stream in an amount greater than 2 wt%, preferably greater than 3 wt%, and more preferably greater than 4 wt%. Preferably, at least a portion of the overhead stream is recycled to the phase separator.

[0030] The second intermediate stream may then be distilled in a second distillation column to produce a first product stream and a waste stream. The first product stream contains water, methanol, and primarily methacrolein. Methacrolein may be present in the first product stream in an amount of at least 70 wt%, preferably at least 85 wt%, and more preferably at least 95 wt%, based on the total weight of the first product stream. In embodiments in which the product of the subsequent oxidative esterification reaction is methyl methacrylate, methanol may be present in the first product stream in an amount of less than 30 wt%, preferably less than 10 wt%, and more preferably less than 2 wt%, based on the total weight of the first product stream. Methacrolein and methanol may be present in the first product stream in a combined amount of at least 97 wt%, preferably at least 98 wt%, and more preferably at least 99 wt%. Water may be present in the first product stream in an amount of less than 2 wt%, preferably less than 1 wt%, and more preferably less than 0.5 wt%, based on the total weight of the first product stream. In embodiments in which methacrolein is used to form alkyl methacrylates other than methyl methacrylate in a subsequent oxidative esterification reaction, methanol is preferably substantially removed from the product stream to avoid the formation of methyl methacrylate. The waste stream contains undesired organic compounds from the process, such as methacrolein dimer, 2-methyl-2-pentenal, inhibitors, and other heavy organic compounds from the process.

[0031] The aqueous phase may be distilled in a third distillation column to produce a second product stream, a bottoms stream, and a side-draw stream. The second product stream contains water, methanol, and methacrolein. Water may be present in the second product stream in an amount less than 5 wt. %, preferably less than 2 wt. %, and even more preferably less than 1 wt. %, based on the total weight of the second product stream. Methacrolein may be present in the second product stream in an amount greater than 25 wt. %, preferably greater than 35 wt. %, and more preferably greater than 45 wt. %, based on the total weight of the second product stream. In embodiments in which methyl methacrylate is the product of the subsequent oxidative esterification reaction, methanol may be present in the second product stream in an amount greater than 25 wt. %, preferably greater than 40 wt. %, and more preferably greater than 55 wt. %, based on the total weight of the second product stream. In embodiments in which alkyl methacrylate is the desired product of the subsequent oxidative esterification reaction, methanol is substantially removed from the second product stream. The bottoms stream contains the amino acid catalyst of the recovered catalyst stream. In certain embodiments, at least a portion of the bottoms stream is recycled to the catalyst stream, which in preferred embodiments is mixed in the catalyst tank. The side-draw stream may contain primarily water and certain organic compounds from the process. In certain embodiments, the side-draw stream may contain less than 2 wt. %, preferably less than 1.5 wt. %, and more preferably less than 1 wt. % methanol.

[0032] In an alternative embodiment, the first intermediate stream can be subjected to at least one distillation step prior to at least one phase separation step. In this embodiment, the first intermediate stream is distilled in a first distillation column to produce a second intermediate stream containing water, methanol, residual formaldehyde and propionaldehyde, and primarily methacrolein. Methacrolein is preferably present in the second intermediate stream in an amount of at least 70 wt.%, preferably at least 80 wt.%, more preferably at least 85 wt.%, based on the total weight of the second intermediate stream. The bottoms stream of the first distillation column contains methacrolein, an amino acid catalyst, and primarily water. Methacrolein may be present in the bottoms stream of the first distillation column in an amount of less than 10 wt.%, preferably less than 7.5 wt.%, more preferably less than 5 wt.%, of the total weight of the bottoms stream. The bottoms stream of the first distillation column can be recycled to the first distillation column to recover additional methacrolein. The amino acid catalyst in the bottoms stream can be recovered in a second distillation column and recycled to the catalyst tank or discarded.

[0033] The second intermediate stream can then be subjected to phase separation to produce an organic phase containing water, methanol, and primarily methacrolein, and an aqueous phase containing methacrolein, methanol, and primarily water. Methacrolein is present in the organic phase in an amount of at least 75 wt%, preferably at least 85 wt%, and more preferably at least 95 wt%, based on the total weight of the organic phase. Water can be present in the organic phase in an amount of less than 8 wt%, preferably less than 5 wt%, more preferably less than 2 wt%, and even more preferably less than 1 wt%, based on the total weight of the organic phase. The aqueous phase contains methacrolein, methanol, an amino acid catalyst, and primarily water. The organic phase can be subjected to further distillation in a third distillation column to further remove undesired by-products and produce a product stream containing primarily methacrolein. In embodiments in which an alkyl methacrylate other than methyl methacrylate is the desired product of the subsequent oxidative esterification reaction, methanol can be removed from the product stream prior to entering the oxidative esterification reactor.

[0034] The inhibitor can be introduced into the process at one or more locations, such as through the catalyst tank, reactor, phase separator, distillation column, and in the intermediate or product stream. Suitable inhibitors include, for example, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (4-hydroxy-TEMPO).

[0035] The propionaldehyde in the reaction stream can be prepared by hydroformylation of ethylene. Hydroformylation processes are known in the art, for example, as described in U.S. Pat. Nos. 4,427,486, 5,087,763, 4,716,250, 4,731,486, and 5,288,916. The hydroformylation of ethylene to propionaldehyde involves contacting ethylene with CO and hydrogen in the presence of a hydroformylation catalyst. Suitable hydroformylation catalysts include, for example, metal-organophosphorus ligand complexes. Suitable organophosphorus ligands include, for example, organophosphines, organophosphites, and organophosphoramidites. In certain embodiments, the CO to hydrogen ratio is in the range of 1:10 to 100:1, preferably 1:10 to 10:1. In certain embodiments, the hydroformylation reaction is carried out at a reaction temperature of from -25°C to 200°C, preferably from 50°C to 120°C.

[0036] Any of the methacrolein-containing product streams may be utilized in a downstream oxidative esterification ("OER") process to form alkyl methacrylate from methacrolein and an alkyl alcohol in an OER reactor system. Preferably, the product stream entering the OER reactor system contains less than 10 wt.% methacrolein dimer, more preferably less than 6 wt.% methacrolein dimer, even more preferably less than 2 wt.% methacrolein dimer, even more preferably less than 1 wt.% methacrolein dimer, and less than 10 wt.% water, more preferably less than 8 wt.% water, even more preferably less than 6 wt.% water, and even more preferably less than 5 wt.% water, based on the total weight of the stream entering the OER reactor system.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] The catalyst preferably comprises gold particles and 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.

[0042] The gold particles preferably have 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:

[0043]

number

[0044]

number

[0045] 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, more preferably less than 4 times the average diameter of the gold particles, even more preferably less than 3 times the average diameter of the gold particles, even more preferably less than 2 times the average diameter of the gold particles, and even more preferably less than 1.5 times the average diameter of the gold particles. Preferably, the particles of the at least one metal oxide have an average diameter at least half the average diameter of the gold particles, more preferably at least the same as the average diameter of the gold particles.

[0046] The weight ratio of gold particles to particles of at least one metal oxide may be in the range of 1:1 to 1:20. Preferably, the weight ratio of gold particles to 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.

[0047] Preferably, the gold particles are uniformly distributed among the particles of at least one metal oxide. As used herein, the term "uniformly distributed" means that the gold 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 gold particles are present in particles having an average diameter of less than 12 nm. More preferably, at least 90% of the total number of gold 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 gold particles are present in particles having an average diameter of less than 12 nm.

[0048] Preferably, at least 75% of the gold 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 gold particle is within X nm of the edge of the metal oxide particle closest to the gold particle. Preferably, at least 75% of the gold 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.

[0049] More preferably, at least 75% of the gold particles by number are within at least 20 nm of two metal oxide particles, i.e., the edge of a gold particle is within at least 20 nm of the edges of the two metal oxide particles nearest it. Preferably, at least 75% of the gold 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.

[0050] Even more preferably, at least 75% of the gold particles by number are within at least 20 nm of at least three metal oxide particles, i.e., the edge of a gold 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 gold 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.

[0051] The gold 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 precious 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.

[0052] 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.

[0053] The gold 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 gold particles are in the outer 70% of the catalyst volume (i.e., the volume of the average catalyst particle), preferably the outer 60%, preferably the outer 50%, preferably the outer 40%, preferably the outer 35%, preferably the outer 30%, preferably the outer 25% of the catalyst volume. Preferably, the 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%, preferably at least 97%, 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.

[0054] Preferably, the catalyst comprises gold particles and at least one metal oxide particles on a support material comprising silica. Preferably, the gold particles and at least one metal oxide particles form an eggshell structure on the support particles. The eggshell layer may have a thickness of 500 microns or less, preferably 250 microns or less, more preferably 100 microns or less.

[0055] Preferably, at least 0.1 wt.% of the total weight of the gold particles is exposed on the surface of the catalyst, where the surface includes both the outer surface and the pores of the catalyst. As used herein, the term "exposed" means that at least a portion of the gold particles is not covered by other gold particles or by particles of at least one metal oxide, i.e., the gold particles can be directly contacted by reactants. Thus, the gold particles may be disposed within the pores of the support material and still be exposed to reactants that can directly contact the gold particles within the pores. More preferably, at least 0.25 wt.% of the total weight of the gold particles is exposed on the surface of the catalyst, even more preferably, at least 0.5 wt.% of the total weight of the gold particles is exposed on the surface of the catalyst, and even more preferably, at least 1 wt.% of the total weight of the gold particles is exposed on the surface of the catalyst.

[0056] The catalyst may be prepared by a process comprising providing a support and providing at least one metal oxide particle on a surface of the support, contacting the support with a gold salt, and heating the support at a temperature ranging from 50°C to 600°C for a time ranging from at least 0.1 hours to 48 hours to convert the gold salt into gold nanoparticles having an average diameter of less than 12 nm and a standard deviation of + / - 4 nm, wherein at least 75% by number of the gold nanoparticles are located within 20 nm of the metal oxide particle.

[0057] Particles of at least one metal oxide may be formed on the support by first contacting the support with a salt of the metal and then oxidizing the metal in an environment comprising an oxygen-containing gas, such as air or oxygen.

[0058] To form gold nanoparticles on the support, the support is contacted with a gold salt, preferably in the form of an aqueous solution containing the gold salt, which is then converted to metallic gold particles by heating the support at a temperature ranging from 50°C to 600°C for a time ranging from 0.1 to 48 hours.

[0059] In one embodiment, the step of heating the support to convert the gold salt to metallic gold may be carried out in the presence of an oxygen-containing gas, such as air or oxygen, to calcinate the catalyst. Calcination may be carried out at a temperature ranging from 150°C to 500°C, preferably from 200°C to 450°C. Calcination is a preferred process for converting gold salt to metallic gold because it can simultaneously form metal oxide particles from the metal salt.

[0060] In an alternative embodiment, the step of heating the support to convert the gold salt to metallic gold may be carried out in the presence of a reducing gas comprising at least 0.1% by volume of a reducing agent.

[0061] In another alternative embodiment, the step of heating the support to convert the gold salt to metallic gold may be carried out in the presence of an inert atmosphere, where the inert gas allows the gold salt to self-reduce to metallic gold.

[0062] In yet another alternative embodiment, the step of heating the support to convert the gold salt to metallic gold may be carried out in the liquid phase in the presence of a solvent and a reducing agent, the ratio of reducing agent to solvent being at least 0.01, In this embodiment, the support is heated at a temperature ranging from 50°C to below the boiling point of the solvent.

[0063] Contacting the support with the gold salt can be carried out by several different methods. For example, contacting the support with the gold salt can be carried out by impregnating the support with an aqueous solution containing the gold salt, dip-coating the support with a solution containing the gold salt, spray-coating the support with a solution containing the gold salt, or sequentially impregnating the support by impregnating the support with a first solution to fill at least 80% by volume of any pores that may be present in the support, and then impregnating the support with a second solution containing the gold salt. When contacting the support with a solution or aqueous solution of a gold salt, the support may first be dried before heating the support to convert the gold salt to metallic gold particles.

[0064] Preferably, the catalyst is produced by precipitating gold and metal (i.e., metal of the metal oxide) from an aqueous solution of metal salts in the presence of a support. In one preferred embodiment, the catalyst is produced by contacting a porous inorganic oxide with an aqueous solution of suitable gold precursor salts and nickel salts to fill the pores with the solution, followed by removal of the solvent and water by drying. The resulting material is then converted to the final catalyst by calcination or reduction to decompose the gold and metal salts to gold and metal oxide. Preferably, the catalyst is a C2-C cation salt containing at least one hydroxyl or carboxylic acid substituent. 18 Thiols are present in solution. Preferably, C2-C thiols containing at least one hydroxyl or carboxylic acid substituent. 18 The thiol has 2 to 12, preferably 2 to 8, and preferably 3 to 6 carbon atoms. Preferably, the thiol compound contains a total of 4 or less, preferably 3 or less, and preferably 2 or less, hydroxyl and carboxylic acid groups. Preferably, the thiol compound has 2 or less, preferably 1 or less, thiol groups. When the thiol compound contains a carboxylic acid substituent, it can exist in the acid form, the conjugate base form, or a mixture thereof. Particularly preferred thiol compounds include thiomalic acid, 3-mercaptopropionic acid, thioglycolic acid, 2-mercaptoethanol, and 1-thioglycerol, including their conjugate bases.

[0065] 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.

[0066] 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-C12 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.

[0067] The catalyst bed, which may comprise a slurry bed or a fixed bed, contains catalyst particles. The OER reactor 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 dialkyl acetals, such as methacrolein dimethyl acetal (MDA) or methacrolein dibutyl acetal, and isobutyric acid of alkyl alcohols, 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 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] It is preferred that the average weight ratio of alkyl alcohol to methacrolein in the OER system be in the range of 20:1 to 2:1, where this average weight ratio is based on the average concentrations of alkyl alcohol entering and leaving the OER system and the average concentrations of methacrolein entering and leaving the OER system.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] The inventors have discovered that high local concentrations of base material in a reactor system can cause the formation of unwanted Michael adducts. Therefore, to help minimize the formation of Michael adducts, the base material is preferably mixed with at least one other material before entering the reactor system. Preferably, the base material is introduced at a location external to the reactor system and mixed with one or more reactants or diluents to form a base-containing stream. For example, the base material may be mixed with 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.

[0083] 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.

[0084] 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.

[0085] 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 by Grenville and Nienow, The Handbook of Industrial Mixing, pp. 507-509, which gives the following equation for a stirred vessel in turbulent flow:

[0086]

number

[0087] 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 Michael adduct formation. Another advantage is cost savings due to reduced costs for treating aqueous waste. Aqueous waste from an oxidative esterification process in which basic 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] In either embodiment, the organic phase subjected to hydrolysis in the acetal reactor 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.

[0092] 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.

[0093] 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: (a) reacting propionaldehyde with formaldehyde to produce a methacrolein intermediate stream; (b) subjecting the methacrolein intermediate stream produced in step (a) to at least one phase separation and at least one distillation to reduce the amount of methacrolein dimer and water in the methacrolein intermediate stream, wherein the amount of methacrolein dimer exiting the at least one phase separation and the at least one distillation is less than 10 wt % based on the total weight of the methacrolein intermediate stream exiting the at least one phase separation and the at least one distillation, and the amount of water exiting the at least one phase separation and the at least one distillation is less than 10 wt % based on the total weight of the methacrolein intermediate stream exiting the at least one phase separation and the at least one distillation; (c) reacting the methacrolein with an alkyl alcohol in an oxidative esterification reaction system in the presence of at least one inhibitor, an oxygen-containing gas, and a catalyst comprising gold particles and particles of at least one metal oxide to produce a product stream comprising an alkyl methacrylate, wherein 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; wherein the product stream comprises 0.1 to 5000 ppm alkyl isobutyrate and 0.01 to 5 wt. % of at least one Michael addition product, based on the total weight of the product stream.

2. The propionaldehyde is reacted with ethylene to form CO and H in the presence of a hydroformylation catalyst. ２ The process of claim 1 , wherein the compound is produced by contacting the compound with

3. 3. The process of claim 1 or 2, wherein the alkyl alcohol is a straight-chain or branched alcohol containing 1 to 8 carbon atoms.

4. 4. The process of claim 3, 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.

5. 5. The process according to claim 1, wherein the amount of methacrolein dimers exiting the at least one phase separation and the at least one distillation is less than 5% by weight, based on the total weight of the methacrolein intermediate stream exiting the at least one phase separation and the at least one distillation, and the amount of water exiting the at least one phase separation and the at least one distillation is less than 5% by weight, based on the total weight of the methacrolein intermediate stream exiting the at least one phase separation and the at least one distillation.

6. 6. The process of any one of claims 1 to 5, further comprising adding a base material to the oxidative esterification reactor system in an amount less than 10 wt%, based on the total weight of reactants in the oxidative esterification reaction system.

7. 7. The process of any one of claims 1 to 6, wherein the product stream comprises from 0.1 to 1000 ppm alkyl isobutyrate, based on the total weight of the product stream.

8. 8. The process of any one of claims 1 to 7, wherein the product stream comprises 0.01 to 1 wt. % of at least one Michael addition product, based on the total weight of the product stream.

9. 9. The process of any one of claims 1 to 8, wherein the gold particles have an average diameter of less than 12 nm.

10. The process of any one of claims 1 to 9, wherein the metal of the at least one metal oxide is nickel or titanium.

11. The process of any one of claims 1 to 10, wherein the catalyst has an average particle diameter of from 200 μm to 30 mm.

12. A process according to any one of claims 1 to 11, 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.

13. 13. The process of any one of claims 1 to 12, wherein at least 75% of the gold particles are within at least 20 nm of particles of the at least one metal oxide.

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