Process for preparation of alkyl methacrylate

Abstract

There is provided a process for preparing an alkyl methacrylate, the process comprising: (a) reacting propionaldehyde and 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 dimers and water in the methacrolein intermediate stream, the amount of methacrolein dimer leaving the at least one phase separation and the at least one distillation is less than 10% by weight, and the amount of water leaving 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 leaving the at least one phase separation and the at least one distillation; and (c) reacting the methacrolein with an alkyl alcohol in the presence of at least one inhibitor, an oxygen-containing gas, and a catalyst comprising gold particles and at least one metal oxide particle in an oxidative esterification reaction system to produce a product stream comprising alkyl methacrylate. The product stream comprises from 0.1 ppm to 5000 ppm of an alkyl isobutyrate and from 0.01% to 5% by weight 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 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 by 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 a commercial production facility, 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 into 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, the by-product methyl isobutyrate (MIB) is critical to reduce because 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:

[0010] (a) reacting propionaldehyde and formaldehyde to produce a methacrolein intermediate stream;

[0011] (b) subjecting the methacrolein intermediate stream produced in step (a) to at least one phase separation and at least one distillation to reduce the amounts of methacrolein dimer and water in the methacrolein intermediate stream, wherein based on the total weight of the methacrolein intermediate stream leaving the at least one phase separation and the at least one distillation, the amount of methacrolein dimer leaving the at least one phase separation and the at least one distillation is less than 10 wt%, and based on the total weight of the methacrolein intermediate stream leaving the at least one phase separation and the at least one distillation, the amount of water leaving the at least one phase separation and the at least one distillation is less than 10 wt%; and

[0012] (c) reacting the methacrolein with an alkyl alcohol in the presence of at least one inhibitor, an oxygen-containing gas, and a catalyst comprising gold particles and at least one metal oxide particle in an oxidative esterification reaction system 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;

[0013] wherein based on the total weight of the product stream, the product stream comprises 0.1 ppm to 5000 ppm of an alkyl isobutyrate and 0.01 wt% to 5 wt% of at least one Michael addition product. Detailed implementation mode

[0014] Unless otherwise specified, all percentage 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 value 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 a 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.

[0015] 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, and in this case, the limitation applies to the total amount of all noble metals.

[0016] "Catalyst center" is the centroid of the catalyst particle, that is, 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.

[0017] 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 an alkyl methacrylate can be the specified reaction that takes place in a reactor system. A reactor system can include a single reactor or multiple reactors. Additionally, a 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 walls or barriers that define the separation zones, or by differences in reaction conditions, such as pressure, temperature, catalyst, composition or concentration of reactants or other reaction components (such as inert materials, pH regulators, etc.). For example, a reactor system can include: a single reactor including a single zone, a single reactor including multiple zones, multiple reactors each including a single zone, 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. Examples of multi-zone reactors can be continuous tubular reactors including multiple zones, including one or more mixing zones, cooling zones, and one or more catalyst zones where the reaction takes place. 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 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 a reactor system, the average concentration or ratio is calculated based on the substances entering the reactor system and the substances leaving the reactor system.

[0018] One aspect of the present invention provides a method for preparing an alkyl methacrylate, the method comprising reacting propionaldehyde and formaldehyde to produce an intermediate stream of methacrolein, subjecting the intermediate stream of methacrolein 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 an alkyl methacrylate.

[0019] To prepare methacrolein from propionaldehyde and formaldehyde, a catalyst stream is provided by mixing water and an amine-acid catalyst. The water and the catalyst can be mixed in a catalyst tank before entering the reactor. The amine-acid catalyst is capable of catalyzing the Mannich condensation of propionaldehyde and formaldehyde to obtain methacrolein. The Mannich condensation method is known in the art, for example, as described in U.S. Patent No. 4,496,770 and U.S. Patent No. 7,141,702. Suitable amine-acid catalysts include, for example, those catalysts containing a secondary amine and an acid.

[0020] Examples of suitable acids for the amine-acid catalyst include inorganic acids and organic monocarboxylic acids, dicarboxylic acids, or polycarboxylic acids. Suitable carboxylic acids include, but are not limited to, aliphatic C1-C 10 monocarboxylic acids, C2-C 10Dicarboxylic acids, C2-C 10 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. Preferred inorganic acids include sulfuric acid and phosphoric acid.

[0021] Suitable amines for the amine-acid catalyst include, for example, amines of the formula NHR 2 R 3 wherein R 2 and R 3 are each independently a C1-C 10 alkyl group, which is optionally substituted with an ether, hydroxyl, secondary amino, or tertiary amino group, or R 2 and R 3 together with the adjacent nitrogen may form a C5-C7 heterocycle, optionally containing additional nitrogen atoms and / or oxygen atoms, and they are optionally substituted with a C1-C4 alkyl or C1-C4 hydroxyalkyl group. Preferred amines include dimethylamine, diethylamine, methylethylamine, methylpropylamine, dipropylamine, dibutylamine, diisopropylamine, diisobutylamine, methylisopropylamine, methylisobutylamine, methylsec-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 at least one of their combinations.

[0022] 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 have 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.

[0023] The Mannich condensation reaction can be carried out by feeding 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, such as 150 °C - 220 °C, and at superatmospheric pressure, i.e., greater than 1 bar. Preferably, the reaction is carried out in the liquid phase at a pressure in the range of greater than 1 bar to 150 bar, more preferably 10 bar to 120 bar.

[0024] The molar ratio of propionaldehyde to formaldehyde is not particularly limited. For example, the reaction stream may have a ratio of propionaldehyde to formaldehyde 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 because it contains a large amount of water, e.g., greater than 10 wt%, greater than 20 wt% or more, based on the total weight of the first intermediate stream.

[0025] The 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. Patent No. 4,427,486, U.S. Patent No. 5,087,763, U.S. Patent No. 4,716,250, U.S. Patent No. 4,731,486, and U.S. Patent No. 5,288,916. Hydroformylating ethylene to propionaldehyde involves contacting ethylene with carbon monoxide and hydrogen in the presence of a hydroformylation catalyst. Examples of hydroformylation catalysts include, for example, metal-organic phosphorus ligand complexes such as organophosphines, organophosphites, and organophosphinamides. The ratio of carbon monoxide to hydrogen can be in the range of 1:10 to 100:1, preferably 1:10 to 10:1. The hydroformylation process can be carried out at a temperature in the range of -25°C to 200°C, preferably 50°C to 120°C.

[0026] The ethylene used to prepare propionaldehyde can be prepared by dehydration of ethanol. For example, ethylene can be prepared by acid-catalyzed dehydration of ethanol. Ethanol dehydration is known in the art and is disclosed, for example, in U.S. Patent No. 9,249,066. Preferably, the ethanol is derived from renewable resources such as plant materials or biomass rather than ethanol prepared from petroleum-based sources. For example, the use of bio-derived ethanol alone in a process for producing MMA can result in up to 40% of the carbon atoms in MMA (i.e., 2 out of 5 carbon atoms in MMA) being from renewable resources.

[0027] To further increase the renewable carbon content in the alkyl methacrylate, additional starting materials can also be prepared from renewable resources. For example, formaldehyde can be prepared from syngas, where the syngas can be prepared from biomass. The carbon monoxide, which can also be used to prepare propionaldehyde, can also be prepared from renewable resources, as disclosed by Li et al., ACS Nano, 2020, 14, 4, 4905 - 4915. The use of these additional bio-sources can further increase the amount of renewable carbon.

[0028] 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.

[0029] Preferably, at least 40%, more preferably at least 60%, even more preferably at least 80% and still more preferably 100% of the carbon atoms in the alkyl methacrylate are derived from renewable or recycled content.

[0030] The methanol and formaldehyde present in the reaction stream can be provided in the form of formalin. The formalin used in the process of the present invention may comprise a saturated aqueous solution which, based on the total weight of the formalin, contains about 37% by weight of formaldehyde and 10% to 15% by weight of methanol. The methanol present in the formalin can be advantageously used in a subsequent oxidative esterification process which converts methacrolein to an alkyl methacrylate in the presence of an alkyl alcohol, for example converting methacrolein to methyl methacrylate in the presence of methanol. In certain embodiments, the methanol can be introduced at different locations in the process.

[0031] Subject the first intermediate stream to at least one distillation step and at least one phase separation step.

[0032] For example, in one embodiment, the first intermediate stream can be subjected to phase separation to produce an organic phase comprising water, methanol and predominantly methacrolein and an aqueous phase comprising methacrolein, methanol, an amine-acid catalyst and predominantly water. In this embodiment, based on the total weight of the organic phase, methacrolein is present in the organic phase in an amount of at least 70% by weight, preferably at least 85% by weight and more preferably at least 90% by weight. Based on the total weight of the organic phase, methanol is present in the organic phase in an amount of less than 10% by weight, preferably less than 3% by weight and more preferably less than 2.5% by weight. Without wishing to be bound by theory, it is believed that operating the phase separator at a low temperature results in the organic phase containing a lower amount of methanol, which is beneficial for the 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 comprise at least 70% by weight, preferably at least 75% by weight and more preferably at least 80% by weight of water.

[0033] The organic phase can be distilled in a first distillation column to produce a second intermediate stream and an overhead stream. The first distillation column can operate as a stripping column, where the overhead vapor is condensed without any liquid reflux back to the column. The ratio of the second intermediate stream leaving 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 predominantly methacrolein. Based on the total weight of the second intermediate stream, water can be present in the second intermediate stream in an amount 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%. The overhead stream contains water, methanol, and predominantly methacrolein. Water can 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 part of the overhead stream is recycled to the phase separator.

[0034] The second intermediate stream can 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 predominantly methacrolein. Based on the total weight of the first product stream, methacrolein can 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%. In embodiments where the product of the subsequent oxidative esterification reaction is methyl methacrylate, based on the total weight of the first product stream, methanol can be present in the first product stream in an amount less than 30 wt%, preferably less than 10 wt%, and more preferably less than 2 wt%. Methacrolein and methanol can 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%. Based on the total weight of the first product stream, water can be present in the first product stream in an amount less than 2 wt%, preferably less than 1 wt%, and more preferably less than 0.5 wt%. In embodiments where methacrolein is used to form an alkyl methacrylate other than methyl methacrylate in the 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 dimers, 2-methyl-2-pentenal, inhibitors, and other heavy organic compounds from the process.

[0035] The aqueous phase can be distilled in a third distillation column to produce a second product stream, a bottoms stream, and a sidestream. The second product stream contains water, methanol, and methacrolein. Based on the total weight of the second product stream, water can 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 can 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%. In embodiments where methyl methacrylate is the product of a subsequent oxidative esterification reaction, based on the total weight of the second product stream, methanol can 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%. In embodiments where an alkyl methacrylate is the desired product of a subsequent oxidative esterification reaction, methanol is substantially removed from the second product stream. The bottoms stream contains the amine-acid catalyst of the recycled catalyst stream. Preferably, at least a portion of the bottoms stream is recycled to the catalyst stream, and in a preferred embodiment, the catalyst stream is mixed in a catalyst sump. The sidestream can mainly contain water and certain organic compounds from the process. The sidestream can contain methanol in an amount less than 2 wt%, preferably less than 1.5 wt%, and more preferably less than 1 wt%.

[0036] 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 predominantly methacrolein. Methacrolein is preferably present in the second intermediate stream in an amount of at least 70 wt%, preferably at least 80 wt%, and more preferably at least 85 wt% of the total weight of the second intermediate stream. The bottoms stream of the first distillation column contains methacrolein, the amine-acid catalyst, and predominantly water. Methacrolein can be present in the bottoms stream of the first distillation column in an amount less than 10 wt%, preferably less than 7.5 wt%, and 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 amine-acid catalyst in the bottoms stream can be recovered and recycled to the catalyst sump or discarded in a second distillation column.

[0037] The second intermediate stream can then be phase-separated to produce an organic phase containing water, methanol, and predominantly methacrolein, and an aqueous phase containing methacrolein, methanol, and predominantly water. Based on the total weight of the organic phase, methacrolein can be 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%. The aqueous phase contains methacrolein, methanol, and predominantly water. The organic phase can be further distilled in a third distillation column to further remove unwanted by-products, thereby producing a product stream containing predominantly methacrolein. In embodiments where 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.

[0038] The inhibitor can be introduced into the process through one or more locations such as a catalyst sump, reactor, phase separator, any distillation column, and in the intermediate stream or product stream. Suitable inhibitors include, for example, 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-hydroxy-TEMPO).

[0039] 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. Patent No. 4,427,486, U.S. Patent No. 5,087,763, U.S. Patent No. 4,716,250, U.S. Patent No. 4,731,486, and U.S. Patent No. 5,288,916. Hydroformylation of ethylene 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 organophosphonamides. In certain embodiments, the ratio of CO to hydrogen 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 -25°C to 200°C, preferably 50°C to 120°C.

[0040] Any product stream containing methacrolein can be used in a downstream oxidative esterification ("OER") process to form an alkyl methacrylate from methacrolein and an alkyl alcohol in an OER reactor system. Preferably, based on the total weight of the stream entering the OER reactor system, 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, and still more preferably less than 1 wt% methacrolein dimer, and less than 10 wt% water, more preferably less than 8 wt% water, still more preferably less than 6 wt% water, and even more preferably less than 5 wt% water.

[0041] The OER 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, 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 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 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. 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.

[0042] The reactor system can 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.

[0043] Depending on the reactor in which the catalyst is present, the catalyst can be in the form of a slurry or a fixed bed. 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.

[0044] The size of the catalyst can be selected based on the type of reactor. For example, the slurry catalyst can have an average particle size of less than 200 μm, for example, 10 μm to 200 μm. The fixed bed catalyst can have an average particle size of 200 μm or greater, for example, 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.

[0045] 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.

[0046] The gold particles preferably have 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:

[0047]

[0048] where x is the size of each particle, is the average of n particles, and n is at least 500.

[0049] The average diameter of the at least one metal oxide particle is preferably less than 5 times the average diameter of the gold particles, more preferably the average diameter is less than 4 times the average diameter of the gold particles, even more preferably the average particle size is less than 3 times the average diameter of the gold particles, still more preferably the average particle size is less than 2 times the average diameter of the gold particles, and even more preferably the average particle size is less than 1.5 times the average diameter of the gold particles. Preferably, the average diameter of the at least one metal oxide particle is at least half of the average diameter of the gold particles, and more preferably at least the same as the average diameter of the gold particles.

[0050] By weight, the amount of gold particles relative to the amount of the at least one metal oxide particle can be in the range of 1:1 to 1:20. Preferably, the weight ratio of gold particles to the at least one metal oxide particle 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.

[0051] Preferably, the gold particles are uniformly distributed in 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, with substantially no agglomeration of the gold particles. 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.

[0052] Preferably, at least 75% of the gold particles, by number of gold 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 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 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.

[0053] More preferably, at least 75% of the gold particles, by number of gold particles, are within at least 20 nm of two metal oxide particles, i.e., the edge of the gold particle is within at least 20 nm of the edges of the two metal oxide particles closest to the gold particle. 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.

[0054] Even more preferably, at least 75% of the gold particles, by number of gold particles, are within at least 20 nm of at least three metal oxide particles, i.e., the edge of the gold particle is within at least 20 nm of the edges of the at least three metal oxide particles closest to the gold particle. Preferably, at least 75% of the gold 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.

[0055] The gold 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 portion of the catalyst contains a noble metal, 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 m2 / g, preferably greater than 120 m 2 / g of specific surface area. In the catalyst portion containing a small amount of noble metal or no noble metal, the support may have a specific surface area of less than 50 m 2 / g, preferably less than 20 m 2 / g of specific surface area. The average diameter of the support and the average diameter of the final catalyst particles are not significantly different.

[0056] 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.

[0057] The gold 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 gold particles are in the outer 70% of the catalyst volume (i.e., the volume of the average catalyst particles), preferably in the outer 60% of the catalyst volume, preferably in the outer 50%, preferably in the outer 40%, preferably in the outer 35%, preferably in the outer 30%, preferably in the outer 25%. Preferably, for a volume having a constant distance from its inner surface to its outer surface (the surface of the particle) measured along a line perpendicular to the outer surface, 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 located 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.

[0058] Preferably, the catalyst comprises gold particles and at least one metal oxide particle on a support material containing silica. Preferably, the gold particles and at least one metal oxide particle form an eggshell structure on the support particles. The eggshell layer can have a thickness of 500 microns or less, preferably 250 microns or less, more preferably 100 microns or less.

[0059] 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 another gold particle or at least one metal oxide particle, i.e., the reactants can directly contact the gold particles. Thus, the gold particles can be disposed within the pores of the support material and still be exposed since the reactants 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 still more preferably, at least 1 wt% of the total weight of the gold particles is exposed on the surface of the catalyst.

[0060] The catalyst can be prepared by a method comprising the steps of: providing a support and providing at least one metal oxide particle on the surface of the support, contacting the support with a gold salt, and heating the support at a temperature in the range of 50 °C to 600 °C for a time in the range of at least 0.1 h to 48 h 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 particles.

[0061] The at least one metal oxide particle can be formed on the support by first contacting the support with a metal salt and then oxidizing the metal in an environment containing an oxygen-containing gas (such as air or oxygen).

[0062] 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. The gold salt is then converted into metallic gold particles by heating the support at a temperature in the range of 50 °C to 600 °C for a time in the range of 0.1 h to 48 h.

[0063] In one embodiment, the step of heating the support to convert the gold salt into metallic gold can be carried out in the presence of an oxygen-containing gas (such as air or oxygen) to calcine the catalyst. The calcination can be carried out at a temperature in the range of 150 °C to 500 °C, preferably 200 °C to 450 °C. Calcination is the preferred method for converting the gold salt into metallic gold because it can simultaneously form metal oxide particles from the metal salt.

[0064] In an alternative embodiment, the step of heating the support to convert the gold salt into metallic gold can be carried out in the presence of a reducing gas containing at least 0.1 vol% of a reducing agent.

[0065] In another alternative embodiment, the step of heating the support to convert the gold salt into metallic gold can be carried out in the presence of an inert atmosphere, in which case the inert gas causes the gold salt to self-reduce to metallic gold.

[0066] In yet another alternative embodiment, the step of heating the support to convert the gold salt to metallic gold can be carried out in the liquid phase in the presence of a solvent and a reducing agent, where the ratio of the reducing agent to the solvent is at least 0.01. In this embodiment, the support is heated at a temperature from 50 °C to less than the boiling point of the solvent.

[0067] 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, spraying the support with a solution containing the gold salt, or by sequentially impregnating the support by first 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 the gold salt, the support can be dried first before heating the support to convert the gold salt to metallic gold particles.

[0068] Preferably, the catalyst is produced by precipitating gold and a metal (i.e., the metal of the metal oxide) from an aqueous solution of a metal salt in the presence of a support. In a preferred embodiment, the catalyst is produced by contacting an aqueous solution of a suitable gold precursor salt and a nickel salt with a porous inorganic oxide such that the pores are filled with the solution, and then removing the water by drying. The resulting material is then converted to the finished catalyst by calcination or reduction to decompose the gold salt and the metal salt into gold and the metal oxide. Preferably, a C2-C 18 thiol containing at least one hydroxyl or carboxylic acid substituent is present in the solution. Preferably, the C2-C 18 thiol has from 2 to 12, preferably from 2 to 8, preferably from 3 to 6 carbon atoms. Preferably, the thiol compound contains no more than 4, preferably no more than 3, preferably no more than 2 total hydroxyl and carboxylic acid groups. Preferably, the thiol compound has no more than 2, preferably no more than one thiol group. If the thiol compound contains a carboxylic acid substituent, they can be present in the acid form, the conjugate base form, or a mixture thereof. Particularly preferred thiol compounds include thiomalic acid, 3-mercaptopropionic acid, mercaptoacetic acid, 2-mercaptoethanol, and 1-thioglycerol, including their conjugate bases.

[0069] The catalyst bed can also contain 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 from 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.

[0070] In an OER reactor system, an alkyl methacrylate is prepared 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 12 alkyl group. The alkyl alcohol includes straight-chain 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 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.

[0071] The catalyst bed, which may include a slurry bed or a fixed bed, contains catalyst particles. The OER reactor produces a product stream comprising a liquid phase and a gas phase, the liquid phase containing methacrolein, an alkyl alcohol, and an alkyl methacrylate, and the gas phase containing oxygen. The liquid phase may also contain by-products such as Michael addition products, dialkyl acetals of methacrolein (such as dimethyl acetal of methacrolein (MDA) or dibutyl acetal of methacrolein), and isobutyrates of alkyl alcohols (such as methyl isobutyrate (MIB) or butyl isobutyrate (BIB)). In the absence of steps to control its formation, 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 present in the alkyl methacrylate product stream in an amount exceeding 1 wt% (10,000 ppm). The alkyl isobutyrate may be difficult to separate from the alkyl methacrylate. Therefore, the present invention attempts to limit the amount of alkyl isobutyrate formed such that, based on the total weight of the product stream, 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. Preferably, based on the total weight of the product stream, 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%. Preferably, based on the total weight of the alkyl methacrylate and the acetals and hemiacetals of methacrolein in the product stream leaving the OER system, the amount of acetals and hemiacetals of methacrolein 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%.

[0072] 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 wt%. 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 wt%, and even more preferably greater than 40 wt%. 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 wt%. 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 wt%, and even more preferably less than 50 wt%.

[0073] 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%.

[0074] 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 the methanol and methacrolein entering the OER system and the total weight of the 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 wt%. 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 wt%.

[0075] 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.

[0076] 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 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 by adding air to the gas phase entering the final zone or reactor.

[0077] 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%.

[0078] 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.

[0079] 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. Therefore, 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 more than one zone, the pressure in each reactor and / or zone can be the same or different.

[0080] 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 within 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 within 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.

[0081] The amount of alkyl methacrylate leaving the reactor depends on the conversion of methacrolein in the OER system. For example, at a conversion of methacrolein entering the OER system of 50%, 2 moles of methacrolein are required to produce each mole of alkyl methacrylate. In this example, for each gram mole of methacrolein entering the reactor system over the course of 1 hour, 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. At a conversion of methacrolein entering the OER system of 25%, 4 moles of methacrolein are required to produce each 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. At a conversion of methacrolein entering the OER system of 75%, 1.33 moles of methacrolein are required to produce each 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 streams, 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.

[0082] When the noble metal-containing catalyst contains gold, gold can be present in an amount in the range of 0.0001 kg to 0.1 kg per gram mole of alkyl methacrylate leaving the reactor system within 1 hour. Preferably, gold is present in an amount of at least 0.0001 kg to 0.005 kg per gram mole of alkyl methacrylate leaving the reactor system within 1 hour. Preferably, gold is present in an amount less than 0.004 kg per gram mole of alkyl methacrylate leaving the reactor system within 1 hour.

[0083] 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 rate of methacrolein entering the OER system is 50%, 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 per gram mole of methacrolein entering the reactor system within 1 hour. When the conversion rate of methacrolein entering the OER system is 25%, 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 per gram mole of methacrolein entering the reactor system within 1 hour. When the conversion rate of methacrolein entering the OER system is 75%, 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 per gram mole of methacrolein entering the reactor system within 1 hour.

[0084] The pH in the catalyst bed can be in the range of 2 to 10. Some catalysts can be deactivated under acidic conditions. Therefore, when the catalyst is not acid-resistant, the pH in the catalyst bed is 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.

[0085] 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 that can donate a pair of electrons), 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, etc. Ammonia can be a Lewis base or a Bronsted-Lowry base.

[0086] The present inventors have found that high local concentrations of basic materials in a reactor system can lead to the formation of unwanted Michael adducts as by-products. Thus, to help minimize the formation of Michael adducts, the basic material is preferably mixed with at least one other material prior to entering the reactor system. Preferably, the basic material is introduced at a location external to the reactor system and mixed with one or more reactants or diluents to form a basic-containing feed stream. For example, the basic 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 methacrylates 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 line through which the components travel to the reactor system, such as a feed line or a recycle line, where sufficient mixing occurs, such as by turbulence, baffles, jet mixers, or other mixing methods.

[0087] Preferably, based on the total weight of the basic-containing feed stream, the amount of basic material in the basic-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. Prior to entering the reactor system, the basic 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 basic-containing feed stream. Preferably, based on the total weight of the reactants in the OER reactor system, the amount of basic 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%.

[0088] Preferably, the basic-containing feed stream is thoroughly mixed to avoid localized peaks in the concentration of the basic material within the basic-containing feed stream prior to adding the basic-containing feed stream to the reactor system. For example, it is preferred that the basic-containing feed stream reaches a level of uniformity of at least 95%, i.e., prior to entering the reactor system, the variation in the concentration of the basic material deviates within + / -5% of the average concentration of the basic material in the basic-containing feed stream. Preferably, the basic-containing feed stream reaches a level of uniformity of 95% within 4 minutes of introducing the basic material, more preferably within 2 minutes of introducing the basic material, and even more preferably within 1 minute of introducing the basic material.

[0089] For a mixing vessel, the time required for the additive to reach a level of uniformity of 95% 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 turbulence:

[0090]

[0091] 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.

[0092] 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 includes 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 adducts. Another advantage is a cost reduction due to a decrease in the 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.

[0093] Preferably, the product stream from the OER system is 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.

[0094] 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 can occur in the alcohol recovery distillation column, allowing for the recovery of additional alkyl alcohol and methacrolein in the alcohol recovery distillation column.

[0095] 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 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 is present for the hydrolysis of dialkyl acetals of methacrolein; these amounts can be determined from the composition of the organic phase. An acid stream can also be added to the hydrolysis reactor to ensure complete removal of 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 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.

[0096] In either embodiment, the organic phase that has been hydrolyzed in the acetal reactor is then sent to a heavy removal column to remove Michael addition products. Preferably, based on the total weight of the overhead stream from the heavy removal column, the overhead stream from 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 Michael addition products.

[0097] The overhead stream from 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.

[0098] The bottoms stream from the alkyl isobutyrate column can be sent to an alkyl methacrylate product column to further purify the alkyl methacrylate. For example, process inhibitors that may have been added during any distillation or phase separation process can be removed and recycled.

Claims

1. A method for preparing an alkyl methacrylate, the method comprising: (a) reacting propionaldehyde and 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 amounts of methacrolein dimer and water in the methacrolein intermediate stream, wherein based on the total weight of the methacrolein intermediate stream leaving the at least one phase separation and the at least one distillation, the amount of methacrolein dimer leaving the at least one phase separation and the at least one distillation is less than 10% by weight, and based on the total weight of the methacrolein intermediate stream leaving the at least one phase separation and the at least one distillation, the amount of water leaving the at least one phase separation and the at least one distillation is less than 10% by weight; (c) reacting the methacrolein with an alkyl alcohol in the presence of at least one inhibitor, an oxygen-containing gas, and a catalyst comprising gold particles and at least one metal oxide particle in an oxidative esterification reaction system to produce a product stream comprising an alkyl methacrylate, wherein the metal of the at least one metal oxide is selected from aluminum, titanium, the lanthanide elements, zirconium, alkali metals, alkaline earth metals, nickel, cobalt, zinc, magnesium, tellurium, antimony, rhenium, tungsten, and bismuth; wherein based on the total weight of the product stream, the product stream comprises 0.1 ppm to 5000 ppm of an alkyl isobutyrate and 0.01% to 5% by weight of at least one Michael addition product.

2. The method according to claim 1, wherein the propionaldehyde is produced by contacting ethylene with CO and H2 in the presence of a hydroformylation catalyst.

3. The method according to any one of the preceding claims, wherein the alkyl alcohol is a straight-chain or branched alcohol having 1 to 8 carbon atoms.

4. The method according to claim 3, 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.

5. The method according to any one of the preceding claims, based on the total weight of the methacrolein intermediate stream leaving the at least one phase separation and the at least one distillation, the methacrolein dimer leaving the at least one phase separation and the at least one distillation is less than 5% by weight, and based on the total weight of the methacrolein intermediate stream leaving the at least one phase separation and the at least one distillation, the amount of water leaving the at least one phase separation and the at least one distillation is less than 5% by weight.

6. The method according to any one of the preceding claims, the method further comprising adding a basic material to the oxidative esterification reactor system in an amount less than 10% by weight based on the total weight of the reactants in the oxidative esterification reaction system.

7. The method according to any one of the preceding claims, wherein based on the total weight of the product stream, the product stream comprises 0.1 ppm to 1000 ppm of an alkyl isobutyrate.

8. The method according to any one of the preceding claims, wherein based on the total weight of the product stream, the product stream comprises from 0.01% to 1% by weight of at least one Michael addition product.

9. The method according to any one of the preceding claims, wherein the gold particles have an average diameter of less than 12 nm.

10. The method according to any one of the preceding claims, wherein the metal of the at least one metal oxide is nickel or titanium.

11. The method according to any one of the preceding claims, wherein the catalyst has an average particle size in the range of 200 μm to 30 mm.

12. The method according to any one of the preceding claims, wherein the average diameter of the at least one metal oxide particle is preferably 5 times lower than the average diameter of the gold particles.

13. The method according to any one of the preceding claims, wherein at least 75% of the gold particles are within at least 20 nm of at least one metal oxide particle.

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