EP4185405A1

EP4185405A1 - Method for producing a mixed oxide carrier and further finishing thereof into a catalyst for producing alkyl methacrylates

Info

Publication number: EP4185405A1
Application number: EP21739318.0A
Authority: EP
Inventors: Steffen Krill; Andreas RÜHLING; Andreas Tepperis
Original assignee: Roehm GmbH Darmstadt
Current assignee: Roehm GmbH Darmstadt
Priority date: 2020-07-24
Filing date: 2021-07-01
Publication date: 2023-05-31
Also published as: JP2023534751A; KR20230040372A; CN116157200A; US20230256416A1; WO2022017755A1; EP3943189A1

Abstract

The present invention relates to a new method for producing suitable improved carrier materials as a base material for catalysts for carrying out a direct oxidative esterification. In general, the catalyst is used to convert aldehydes with alcohols in the presence of oxygenic gases directly to the corresponding ester, by means of which, for example, (meth)acrolein can be converted to methyl(meth)acrylate. The catalysts used according to the invention are characterized in particular by high mechanical and chemical stability as well as by good catalytic performance even over very long periods of time. This applies in particular to an improvement of catalyst service life, activity and selectivity in comparison to catalysts from the prior art.

Description

Process for the production of a mixed oxide support and its further refinement to a catalyst for the production of alkyl methacrylates

field of invention

The present invention relates to a novel process for producing suitable, improved support materials as base material for catalysts for carrying out a direct oxidative esterification. In general, the catalyst serves to convert aldehydes with alcohols in the presence of oxygen-containing gases directly to the corresponding ester, by means of which (meth)acrolein, for example, can be converted to methyl (meth)acrylate.

The catalysts used for this purpose according to the invention are distinguished in particular by high mechanical and chemical stability and by good catalytic performance even over very long periods of time. This relates in particular to an improvement in catalyst service life, activity and selectivity compared with prior art catalysts.

What is special about this new catalyst according to the invention is that its catalytic performance can be significantly increased by the drying time and storage time between the process steps up to calcination. When using the catalysts according to the invention, the activity of the reaction is significantly more stable over a long period of operation.

The underlying mixed oxide carrier material and the resulting catalyst, based on silicon dioxide, aluminum oxide and magnesium oxide, also show a particle size distribution by means of classification, in which the fines content is greatly reduced, which is an important aspect of suppressing and significantly reducing the formation of by-products. which boil close to the desired ester, in particular methyl methacrylate, or form azeotropes with the product or the starting materials which are difficult to separate. The new type of catalyst makes it possible to produce MMA purities and qualities that are significantly higher than with the contacts previously described according to the prior art. State of the art

The catalytic, oxidative esterification of aldehydes for the preparation of carboxylic acid esters has been extensively described in the prior art.

For example, methyl methacrylate can be produced very efficiently from methacrolein (MAL) and methanol in this way. In particular, US Pat. No. 5,969,178 and US Pat. No. 7,012,039 describe a process for the continuous production of MMA from isobutene or tert-butanol. The process has the following steps: 1) oxidation of isobutene or tert-butanol to form methacrolein and 2) direct oxidative esterification of MAL with methanol to form MMA using a Pd-Pb catalyst which is on an oxidic support. Although conversion and selectivity are high in principle, a lead component has to be constantly fed in for continuous operation, since the catalyst has a continuous low loss of lead ions. The processing and removal of waste water containing lead requires a great deal of technical effort and ultimately also produces critical residues and waste water containing heavy metals, which must be treated separately, with high running costs.

EP 2210664 discloses a catalyst which, on the outside, in the form of a so-called egg-shell structure, has nickel oxide and gold nanoparticles on a support made of silicon dioxide, aluminum oxide and a basic element, in particular an alkali metal or alkaline earth metal. The nickel oxide is enriched on the surface, but is also present in lower concentrations in the deeper layers of the catalyst particle. Such a catalyst shows very good activities and selectivities. However, the catalyst produced according to the production instructions according to the invention from this application is relatively sensitive to attrition and unstable. In the experimental use of these catalysts described above, there is also a relatively high level of contamination with methyl isobutyrate, the formal hydrogenation product of MMA, as a result of which the effort involved in separating and the energy used in isolating the product are increased.

The special manufacturing method for creating the egg-shell structure and the use of not uncritical nickel salts in the manufacture of the catalyst make special demands on technical equipment and the handling of nickel-containing fine dust, as they inevitably occur in catalyst production, e.g. in the process step drying and calcination. Here, too, the doping component nickel is described as necessary in addition to the gold nanoparticles and the special anisotropic, inhomogeneous distribution of gold and doping in order to achieve high activity and selectivity over a long period of time. EP 3244996 discloses a similar catalyst system, with cobalt oxide being used as a component in addition to gold as a doping element instead of nickel oxide. A mixed oxide carrier is also used here, with overall better results than in EP 2210664 being achieved, with the hydrogenated MMA by-product methyl isobutyrate also being formed here in small traces. The influence of the grain size spectrum of the catalyst on the by-product production, as well as the problem of separating catalyst fines when the reaction is carried out continuously, is not discussed. Important manufacturing steps and manufacturing conditions that have a significant impact on the catalyst performance under reaction conditions are not described in this and other publications.

In the patent specifications US Pat. No. 7,326,806, US 2013/0172599 and US Pat. The parameters for spray drying, calcination and classification that are important on a technical and industrial scale and the resulting particle size distribution are not discussed. As is known to those skilled in the art, the grain size distribution, in particular the fines content of a catalyst, plays an important role in the choice of reactor and filtration in order to prevent the catalyst and thus the metallic active component from being lost during operation. Due to the contamination of process wastewater with metals, technical measures must be taken to protect people and the environment, which, together with the possible loss of precious metals, result in high costs.

The original composition of the mixed oxide carrier, which is included and adapted in the abovementioned documents, is described and discussed in US patent RE38,283, as a result of which good hydrolytic stability and stability to organic acids is achieved. However, the same aspects as in the documents mentioned above are missing here.

In addition, none of the systems, processes and catalysts described in the prior art describe, or only insufficiently describe, the formation of critical, in particular hydrogenated, by-products, which play an important role in the isolation of commercially available qualities of alkyl methacrylates, in particular MMA. Methyl isobutyrate, also referred to as methyl isobutyrate, is a critical by-product of this type, which can only be separated from MMA with considerable expenditure on apparatus and energy. This by-product is the corresponding, saturated hydrocarbon of an unsaturated compound produced in the process, such as the hydrogenated secondary product of alkyl methacrylate, and has a very similar boiling point, which makes separation by distillation very expensive and sometimes only possible with a loss of yield. This component occurs in many common technical methyl methacrylate processes and is ultimately also found in the commercial product MMA, such as is used for the production of PMMA, but in concentrations significantly less than 500 ppm, usually also less than 100 ppm.

All of the catalysts and mixed oxide support systems described in the prior art have in common that they undergo one or more drying steps and at least one calcination during the course of production, with the water used in the reaction and any salts, e.g. nitrates or acetates, being removed. While the literature is limited to examples in the laboratory, the parameters of the drying time and the associated residual moisture are consequently not examined and reported in a differentiated manner. However, Ma et. al. in "Heterogeneous gold catalysts and catalysis" that catalysts with gold as a noble metal component are often subject to the problem of sintering, especially when wet, which means that the catalyst activity drops sharply due to increasing nanoparticle diameters. Thus, the duration of the process steps of drying and calcination, as well as their time intervals for applying the (noble) metal active component to the support material, are of decisive importance for an active, long-term stable catalyst.

When reproducing the processes or catalysts described in the prior art, the water content of the intermediate stages, the residence time of the various individual process steps and, in particular, the storage times between the individual process steps have proven to be essential for the reproducible production of a catalyst produced in batches in sub-steps, or the underlying catalyst carrier material. In principle, these relationships are not described in the prior art.

Overall, various catalysts are therefore described in the prior art for direct oxidative esterification (DOE), for example for the reaction of unsaturated aldehydes such as acrolein and methacrolein with alcohols to give the respective carboxylic acid esters. However, the formation of by-products and the influence of an - industrially manufactured - support material on these by-products and the general handling of the catalyst during operation are only insufficiently addressed. task

The object of the present invention was primarily to develop a new process for producing a mixed oxide support and a catalyst based thereon covered with active metal-containing components on an industrial scale, the resulting catalyst being suitable for the direct oxidative esterification of aldehydes to carboxylic acid esters. The mixed oxide support itself and the catalyst produced on the basis of this mixed oxide support should have high mechanical and chemical stability, produce fewer by-products overall compared to the prior art and at the same time be easier to handle in filtration under reaction conditions.

In particular, the task was that this process should be suitable for the oxidative esterification of methacrolein to an alkyl methacrylate, in particular to MMA.

A particularly important aspect of the task on which the present invention is based is to realize the efficient and reduced use of (precious) metal components, which according to the invention are preferably deposited disproportionately on the catalyst fines. These catalyst fines increasingly lead to the formation of secondary components. Furthermore, when using the catalyst in long-term operation, a large part of the fines is lost as catalyst discharge, in the simplest case in a filtration.

In this context, a further object of the invention was to provide a catalyst material with a significantly reduced tendency towards sintering of the noble metal component or leaching. This applies not only to the use and application of the catalyst in a direct oxidative esterification of, for example, (meth)acrolein.

The suppression of sintering and leaching of the metal compounds is, in particular, an explicit task in the manufacture of the silicon oxide-based support material and the impregnated catalyst based thereon. If this is not adequately controlled and carried out using suitable measures according to the invention, the desired distribution structure of the active components in the catalyst material is partially lost or a less active catalyst is formed. In particular, compliance with storage times and production times of individual phases, and the service life of the intermediates between the individual phases of production according to a (i) to (iii) and b (i) to b (vi) is crucial for obtaining an improved catalyst and provides thus represents a further object of the present invention.

This defines the goal of stable catalyst activity over the entire life cycle of the catalyst in use.

A particularly important aspect of the task was to provide a new type of process which enables reduced formation of by-products and thus higher selectivity, particularly in the conversion of aldehydes to carboxylic acid esters. In the case of MMA synthesis, for example, such a by-product is methyl isobutyrate, the saturated or hydrogenated form of MMA.

Other objects that are not explicitly mentioned can result from the description, the examples, the claims or the overall context of the present invention.

solution

These problems are solved by providing a novel method for producing a support material and a catalyst based on this support material for an oxidative esterification. This new method has the two sub-processes a) production of a support and b) production of a catalyst. In particular, the new method is characterized by the following aspects of the two partial methods a) and b):

In partial process a), an oxidic support is produced. The resulting carrier has at least one or more oxides of at least one or more of the following elements: silicon, aluminum, one or more alkaline earth metals, titanium, zirconium, hafnium, vanadium, niobium, tantalum, yttrium and/or lanthanum.

Furthermore, sub-process a) includes the following process steps:

(i) The reaction of one or more compounds selected from silicon, aluminum, alkaline earth metal, titanium, zirconium, hafnium, vanadium, niobium, tantalum, yttrium and/or lanthanum compounds at a temperature Ti < 100°C. A suspension is obtained in this reaction. (ii) spray-drying the suspension from process step (i) at a temperature T2 which is >110° C. to obtain a solid. This solid has 0.1 to 20% by weight of water and 0.1 to 35% by weight of anions of one or more Bronsted acids.

(iii) calcination of the solid from (ii) at a temperature T ₃ comprised between 300 and 800°C. A second solid containing 0.01 to 5% by weight of water and 0.01 to 0.5% by weight of anions of a Bronsted acid is obtained.

(iv) Optionally, but preferably, the carrier powder resulting from one of the carrier steps (i) to (iii), preferably from carrier step (iii), is subjected to a classification step.

Sub-process b), in which a catalyst is produced from the oxidic support material from sub-process a), comprises in particular the following process steps:

(i) reacting the carrier material from a) with a water-soluble noble metal salt.

(ii) at the same time or after process step b (i), a further soluble metal salt is added.

(iii) The impregnated carrier is then separated from the mother liquor or the supernatant solution from process step b (ii) with subsequent washing. A washed, impregnated carrier is obtained which contains from 1.0 to 50% by weight of water.

(iv) drying of the impregnated support for 0.1 to 40 h at a temperature T4 comprised between 30 and 250°C. A dried, impregnated carrier is obtained which now contains only 0.1 to 10% by weight of water. Drying can take place, for example, in a paddle dryer or tray dryer. The drying can take place discontinuously or continuously.

(v) Calcination of the dried, impregnated support from (iv) at a temperature T ₅ between 250 and 700° C. and a residence time between 0.1 and 5 hours. A catalyst is obtained which has a BET surface area of 100 to 300 m ² /g with a pore volume of 0.2 to 2.0 ml/g and a pore diameter of 3 to 12 nm. Preferred embodiments for the individual process steps are specified below. Unless otherwise stated, these individual preferred features can be implemented separately from one another or together or synchronously, ie at the same time. It should be noted that not only do the process steps listed as essential up to here have synergetic effects in the production of the catalysts or the support materials, but that the preferred or optional embodiments can certainly bring about further such effects.

The following optional or preferred embodiments are mentioned in particular for the process steps of sub-process a):

(i) The reaction to obtain a suspension is preferably carried out in batch.

The oxidic support to be produced also preferably has silicon oxide, aluminum oxide and at least one alkaline earth metal oxide, particularly preferably magnesium oxide. Alternatively or additionally, the support to be produced contains at least 2% by weight of titanium dioxide, and the support can even consist entirely of titanium dioxide. In process step a (i), corresponding non-oxidic compounds are preferably used to obtain these oxides. In particular, compounds which are partially soluble or completely soluble under batch conditions and which form a precipitate after reacting with one another can be used. These soluble compounds are known to those skilled in the art, and preference is given to those salts which can be decomposed in the calcination step (iii), so that the desired oxides can be formed more or less without residues. Nitrate salts and acetates meet these requirements; many other anions of corresponding Bronsted acids also meet these criteria.

(ii) The spray drying of the suspension from process step (i) is preferably carried out continuously or semicontinuously. In spray drying, the temperature and volume flow of the drying gas are preferably adjusted in such a way that the laden drying gas at the outlet of the spray tower is 10 to 40° C., particularly preferably 20 to 30° C., above the condensation temperature of water. The suspension obtained in a) (i) can be sprayed using nozzles or

Atomizers take place, single-component nozzles and rotary atomizers being particularly preferred.

(iii) The calcination of the solid from (ii) is preferably carried out continuously or semicontinuously, particularly preferably in a rotary tube. Alternatively, the calcination can also take place in batches, in which case tray dryers or shaft furnaces can be used. The time between spray drying is particularly preferred and calcination according to steps a (ii) and a (iii) for no longer than 5 days. The calcination is particularly preferably carried out in the presence of an oxygen-containing gas. In this way, formed NO2 can be removed more efficiently.

(iv) Carrying out the optional classifying step is clearly preferred.

The solid from process step a (iii) is particularly preferably treated in such a way that the proportion of particles with a diameter of less than 20 μm is reduced.

(i) The reaction of the support material from a) with a water-soluble noble metal salt is preferably carried out in batches. It is particularly preferred, in process step b (i), first to use an aqueous suspension of the oxidic support from a. is prepared and this is mixed with the water-soluble precious metal salt. Optionally, the water-soluble noble metal salt can already be added while the suspension is being prepared, but preferably after the suspension is present.

(ii) In addition to or after the addition of the further soluble metal salt, a basic, aqueous solution is preferably additionally added to the mixture.

(iii) The mother liquor separated off in process step b (iii) is preferably worked up in such a way that residual noble metal and other metal salts are recovered.

(iv) The impregnated support is preferably dried at an absolute pressure of between 0.01 and 5 bar and/or in the presence of an inert drying gas. The time between washing and calcination according to steps b (iv) and b (v) is particularly preferably not longer than 4 days.

(v) The dried, impregnated support from (iv) is optionally calcined in batches. However, this preferably takes place continuously or semicontinuously in a rotary tube. The calcination is particularly preferably carried out in the presence of an oxygen-containing gas. In this way, formed NO2 can be removed more efficiently. A catalyst is preferably obtained which has a BET surface area of 180 to 250 m ² /g with a pore volume of 0.2 to 0.7 ml/g and a pore diameter of 3 to 9 nm. The supports and the catalysts resulting therefrom preferably have a diameter of between 10 and 200 μm. Such catalysts can be used well in a slurry reactor.

The particle size of the carrier comprising mixed oxides based on silicon dioxide according to features a (i) to a (iii) can be freely selected and maintained in various orders of magnitude depending on the manufacturing process selected and the apparatus used for this purpose. It is possible to adjust the order of magnitude as well as other physical characteristics such as BET surface area, pore volume and pore diameter by varying parameters in the spray drying and calcination steps.

Basically, there is a connection between particle size and spherical and geometric shape and the subsequent use of the catalysts under reaction conditions.

Powdered supports and catalysts can be prepared by the process according to the invention, which are then present in the form of a suspension in a stirred reactor system and are used as a slurry in the reaction medium. The support and the catalyst produced from it then reach a dimensional order of magnitude of 1 to 300 μm, with the fine fraction preferably being reduced and largely eliminated according to the invention in a classifying step a (iv) after production step a (iii).

Depending on the type of classification selected, the fine fraction and also the coarse fraction of the powdered carrier are influenced, which, after classification, are then used for catalyst production b(i) to b(vi). Preferred methods for influencing the particle size are air classifying and sieving, and a combination of these methods; the person skilled in the art is aware of other methods for achieving said task of limiting the particle spectrum. The classification adjusts the particle size spectrum of the material obtained after spray drying to 10 to 200 μm. This information relates to a result in which more than 95% by weight of the powdery material obtained is in this particle size range. A classification according to step a (iv) is particularly preferred, the result being that more than 95% by weight of the powdered material obtained has a particle size range between 20 and 150 μm. After spray drying and classification, the silicon dioxide-based material according to a (i) to (iv) is spherical or elliptical before. The sphericity here has an average value greater than 0.85, preferably greater than 0.90 and particularly preferably greater than 0.93.

Sphericity is the ratio of the circumference of the circle of equal area to the actual circumference. The result is a value between 0 and 1. The smaller the value, the more irregular the shape of the particle. This is the consequence of the fact that an irregular particle shape is exhibited to an increased extent. A comparison is always made with a circle of the same area, since this has the smallest of all possible circumferences for a projection surface.

In an alternative embodiment, however, catalysts for so-called fixed-bed reactors are produced by the process according to the invention. Such catalysts or the support materials on which they are based have a significantly larger diameter, particularly preferably between 0.1 and 100 mm. For the production of such a catalyst, a solid from one of the process steps a (ii), a (iii) or a (iv), preferably from process step a (iv), is therefore optionally subjected to a shaping step in a process step a (v) in such a way that a shaped body with a diameter between 0.1 and 100 mm is obtained. If a process step a (v) takes place after one of the process steps a (ii) or a (iii), the further process steps a (iii) and a (iv) or only a (iv) are carried out afterwards.

In a further embodiment, the mixed oxide material obtained according to a (iii) or the calcined and classified material optionally obtained according to a (iv) is subjected to a shaping step.

Specific examples of the shape of the resulting material are spherical, elliptical, tablet-shaped, cylindrical, annular, needle-shaped, hollow-cylindrical, honeycomb-shaped compacts ranging in size from 300 μm to several cm in dimension. The person skilled in the art knows how such shaping processes are technically implemented. In the simplest case, the material in powder form, with or without processing aids, is placed in an extruder as a paste and extruded under pressure at a nozzle that determines the shape.

When used as a catalyst or a catalyst carrier, the shape of the silica-based material of the present embodiment can be suitably changed according to a reaction system to be used. For example, when the silica-based material is used in a fixed-bed reaction, it is preferably in the form of a hollow cylinder or a honeycomb, which causes little pressure loss. In addition, a water-soluble Bronsted or Lewis acid can be added before, during or after the reaction of the oxidic support in process steps b) (i) and (ii). It is preferably an aqueous solution of a metal salt with the oxidation state +II or +III, for example aluminum nitrate or iron(III) nitrate. As a result, a thin (mantle) protective layer can be created around the shell with the active component, which further minimizes the loss of noble metal and thus catalyst activity. By adding a suitable metal salt, magnesium oxide present in the outer layer of the oxidic carrier is dissolved out in an acid-base reaction. The resulting defects can be filled with the added metal salt and also converted into an oxidic form in the calcination of the catalyst material, thereby maintaining the chemical and physical stability of the support or catalyst; The choice of metal salt preferably further increases the chemical and physical stability, for example against abrasion. No noble metal can be deposited on these magnesium oxide-free areas during the production of the catalyst, which results in a noble-metal-free outer layer that acts as a (mantle) protective layer on the catalyst. As described above, this minimizes the loss of noble metal and thus catalyst activity. To avoid any ambiguity, it should be noted that this further metal salt in the form of a Lewis acid is explicitly not the metal salt from process step b)(ii).

In a further preferred embodiment, it is possible to achieve the above-described shell structure, which is characterized by a very low proportion of noble metal, by adding a non-metal-containing, acidic compound. In the simplest case, this can be an aqueous solution of a Bronsted acid such as nitric acid. In this case, the basic alkali or alkaline earth oxide, such as magnesium oxide, is measurably dissolved from the shell, but not replaced by metal ions as in the case described above.

The resulting protective coat layer, caused both in the case of metal salts or Bronsted acids, preferably has a thickness of 0.01 to 10 μm, particularly preferably 0.1 to 5 μm, in order to prevent the catalyst activity from being impaired by mass transfer restrictions or by diffusion limitation of reactants and products is reduced.

As described under b) (iv), the drying time of the impregnated carrier material is less than 4 days, preferably less than 2 days and particularly preferably less than 1 day, the residual moisture content of the dried carrier material being less than 5% by weight, preferably less than 3% by weight and particularly preferably is below 2.5% by weight. as necessary The condition is that the remaining amount of water in the dried, impregnated carrier material cannot damage the calcination unit, such as a rotary kiln, during calcination, or that the water in the calcination unit or its exhaust system does not condense. A shortened drying time has the advantage that the gold, which is initially only weakly fixed, has less time for a sintering process, which is facilitated in the presence of water and salts, such as chloride or nitrate, especially at elevated temperatures, as is usual with drying. This sintering process increases the average particle diameter, which leads to a lower catalyst performance.

In addition to the process according to the invention for preparing the catalyst described, its use for the continuous preparation of carboxylic acid esters from aldehydes and alcohols in the presence of an oxygen-containing gas in the liquid phase is also part of the invention. The catalyst is heterogeneously suspended in the reaction matrix.

This reaction preferably takes place at a temperature between 20 and 120° C., a pH between 5.5 and 9 and a pressure between 1 and 20 bar. The reaction is particularly preferably carried out in such a way that the reaction solution contains between 2 and 10% by weight of water.

In an alternative embodiment of the present invention, the catalyst prepared according to the invention is used for the continuous preparation of carboxylic acid esters from aldehydes and alcohols in the presence of an oxygen-containing gas, using the catalyst in a fixed bed.

It should also be noted that the catalysts according to the invention can be used not only for direct, oxidative esterification but also for other oxidation reactions, such as the production of carboxylic acids from aldehydes in the presence of water and optionally a solvent.

In addition to the process according to the invention for the production of a catalyst and the use of precisely this catalyst, a catalyst which can be produced according to the invention itself is also part of the present invention. Examples:

Example 1a - Support Preparation & Spray Drying

434 kg of silica sol (Köstrosol 1530, 15 nm primary particles, 30% by weight S1O2 in H2O) were placed in a reactor lined with enamel and cooled to 10° C. with vigorous stirring. The silica sol dispersion was adjusted to pH 2 with 60% nitric acid to break the basic (sodium oxide) stabilization.

A mixture of 81.2 kg of aluminum nitrate nonahydrate, 55.6 kg of magnesium nitrate hexahydrate and 108.9 kg of deionized water was prepared in a second, enamelled receiver. The mixture cooled as it dissolved with stirring and had a pH of just below 2. After complete dissolution, 3.2 kg of 60% nitric acid was added.

The metal salt solution was then added to the silica sol dispersion in a controlled manner over a period of 30 minutes. After the addition was complete, the mixture was heated to 50° C. and the resulting dispersion gelled for 4 hours, the pH being 1 at the end. The viscosity established was below 10 mPas.

The suspension (approx. 30% by weight solids) was pumped at a temperature of 50 °C at a feed rate of 20 kg/h into a pilot spray tower with a diameter of approx. 1.8 m and in this at 10000 sprayed revolutions per minute; whereby a spherical material was obtained. The drying gas supplied at 180°C was adjusted in such a way that the exiting, cold drying gas had a temperature of 120°C. The white, spherical material obtained had a residual moisture content of 10% by weight. The residual moisture was determined by drying at 105° C. to constant weight.

The use of nitrates per kg of material was just below 0.5 kg, which corresponded to a proportion of about 30% by weight in the spray-dried material. Example 1b - Calcination

The material spray-dried in Example 1a was calcined in air at 650° C. in a rotary tube-type, continuous unit. The residence time was adjusted by fittings and optimization of the angle of inclination in such a way that the material obtained had a nitrate content of less than 1000 ppm after calcination. The residual amount of nitrate was determined using ion chromatography with a conductivity detector and relates to the soluble amount of nitrate in deionized water.

At an angle of inclination of 0.5 degrees, a residence time of 45 minutes was established, with the nitrate content being quantified at 936 ppm by means of double determination by ion chromatography. The water content of the material directly after the calcination was determined analogously to Example 1a and was 1.3% by weight.

The nitrogen oxides released - calculated as NO2 - amounted to 0.3 kg / kg material and were collected in a DeNO _x scrubber.

Example 1c - Classification

The material obtained from Example 1b was first freed from agglomerates larger than 150 μm, which are formed by adhesions in the rotary tube or spray tower, by means of a coarse screening. The desired particle size range was then adjusted by means of air classification, with fine particles being separated off.

The final, white, spherical support material had a D10 of 36 μm, a D50 of 70 μm, a D90 of 113 μm, a fines content below 25 μm of less than 2.5 vol%, a coarse content greater than 150 μm of less than 0.1 vol %, an average sphericity greater than 0.8, and an average symmetry greater than 0.85. Sphericity and symmetry were determined using dynamic image analysis (Retsch HORIBA CamsizerX2) as the deviation of the 2D projected particle surface from an ideal circle, with a value of 1 corresponding to a perfect sphere or circle in 2D projection. The BET was 140 m ² /g, the pore volume 0.34 mL/g and the pore diameter 8.1 nm. The carrier material was amorphous and the individual components were randomly distributed; 86.8% by weight S1O2, 5.8% by weight MgO and 7.4% by weight Al2O3 were present.

The yield in steps 1a to 1c was over 80%. Comparative Example 1a Support Production with Shortened Residence Time and Catalyst Synthesis and Batch Testing

The support was produced analogously to Examples 1a to 1c on a 10 kg scale, but the residence time during calcination was reduced to 15 minutes, resulting in a nitrate content of 10,000 ppm in the support.

200 mg of the carrier material obtained were suspended in 20 g of deionized water in the pressure vessel and heated to 180° C. for 1 hour. After cooling, the loss of magnesium and silicon was measured as a measure of hydrolytic stability. Compared to the support material from Examples 1a to 1c, the support showed four times the loss of magnesium and silicon, as a result of which the mechanical stability of the support material turns out to be lower.

The catalyst was prepared analogously to Example 2a on a 1 kg scale. Due to the increased nitrate content in the carrier, the amount of wash water increased, resulting in slightly less gold being impregnated on the final catalyst. The final gold content was 0.78% by weight

384 mg of catalyst were placed in a steel autoclave with a magnetic stirrer and suspended with a mixture of methacrolein (1.20 g) and methanol (9.48 g). The methanolic solution contained 50 ppm Tempol as a stabilizer. The steel autoclave was sealed, pressurized to 30 bar with 7% by volume air and stirred at 60 degrees for 2 hours. The mixture was cooled to −10° C., the autoclave was carefully degassed, and the suspension was filtered and analyzed by GC. The conversion of methacrolein was about 61%, the selectivity to MMA was 89%.

The example shows that shortening the residence time during support calcination and the resulting increased nitrate content leads to problems in catalyst production and in the synthesis of MMA from methacrolein and methanol. In addition to the more pronounced hydrolytic instability, which is critical for long-term use, only a lower catalyst performance can be found in short-term production.

Example 2a - Catalyst production

167 kg of deionized water were placed in an enamel kettle with a propeller stirrer, and 50 kg of the support material from Example 1c were added. The following steps were carried out using the steam heating of the reactor under isothermal conditions. right in A solution of 611 g of aluminum nitrate nonahydrate in 10 kg of deionized water was then added. The suspension was heated to 90°C and then aged for 15 minutes. 2845 g of cobalt nitrate hexahydrate were dissolved in 20 kg of deionized water and metered in over 10 minutes after the end of aging and reacted with the carrier material for 30 minutes.

In parallel, 12.4 L of a NaOH solution were prepared in such a way that the ratio of hydroxide ions to auric acid was 4.75. The NaOH solution was added over 10 minutes with the suspension darkening.

After addition of the NaOH solution, 1250 g of an auric acid solution (41% gold content) were diluted in 20 kg of deionized water and added to the reaction suspension over 10 minutes and stirred for a further 30 minutes.

After the reaction, the reaction suspension was cooled to 40° C. and pumped into a centrifuge with a filter cloth, the filtrate being recycled until a sufficient filter cake had built up. It was washed with deionized water until the filtrate had a conductivity below 100 pS/cm and then drained for 30 minutes. The filter cake then had a residual moisture content of almost 30% by weight. The filtrates were first pumped through a selective ion exchanger to remove residual cobalt and then residual gold was absorbed on activated carbon. The recovery rate of both metals after the reaction was greater than 99.5% as determined by ICP analysis.

Immediately after the dewatering was complete, the filter cake was dried in a paddle dryer at 105° C. to a residual moisture content of 2%. The drying process in the paddle dryer was carried out discontinuously with the addition of a drying gas - in this case nitrogen - within 8 hours.

Directly after drying, the dried material was continuously fed into the rotary tube described in Example 1b, which was operated at 450° C. under air. The residence time was set at 30 minutes.

The final catalyst had a loading of 0.91 wt% gold, 1.10 wt% cobalt, 2.7 wt% magnesium, a BET of 236 m ² /g, a pore volume of 0.38 mL/g and a pore diameter of 4.1nm Example 2b - Testing Catalyst in a Continuous Direct Oxidative Esterification

1 kg of the catalyst from Example 1c was dispersed in methanol/water (95/5) in a stainless steel pressure vessel equipped with an EKATO Phasejet and an EKATO Combijet stirrer, resulting in a solids concentration in the suspension of 9%.

The suspension was pressurized to 5 bar absolute at a temperature of 80° C. under nitrogen. Methacrolein and methanol were metered in continuously in a molar ratio of 1 to 4 in the feed, and the TEMPOL stabilizer content was 100 ppm. The reactor was gassed with oxygen at the same time, so that the oxygen concentration in the off-gas from the reactor was adjusted to 4% by volume (explosion limit is 7.8% by volume oxygen). The feed rate and thus residence time were adjusted so that the space velocity over the catalyst was 10 mol methacrolein/kg catalyst×hr. The pH of the reaction was kept constant at 7 by adding a solution of 4% NaOH, 5.5% H2O and 90.5% methanol. The reaction was continuously operated for 2000 hours with this configuration. The average conversion of methacrolein was about 80%, the selectivity for MMA was 94.5%. Conversion and selectivity were determined by GC-FID. The MMA selectivity showed no change during the 2000 hours of operation within the measurement accuracy (+/- 0.5%); the conversion changed in the first almost 500 hours from the initial values of 82% to 79% and remained stable at this level for the remaining operating time.

Example 2c - variation drying time catalyst and testing

The catalyst was produced on a 1 kg scale analogously to example 2a on the support from example 1c, only this time the drying time was extended from 8 to 20 hours. The residual moisture after drying was 1%.

The catalyst was tested analogously to example 2b in a smaller test apparatus suitable for the use of 100 g of catalyst. After 1000 hours of operation, the average conversion of methacrolein was 75% and the selectivity for MMA was 94.5%.

Example 2c shows that, compared to example 2b, a longer drying time has a significant effect on the conversion and thus on the activity in continuous operation. Comparative example 2a thus shows a greater loss of initial activity, with no further deactivation being observed after the initial drop in conversion. Example 2d - variation drying time catalyst and testing

The catalyst was produced on a 1 kg scale analogously to example 2a on the support from example 1c, only this time the drying time was extended from 8 to 40 hours. The residual moisture after drying was 0.8%.

The catalyst was tested analogously to example 2b in a smaller test apparatus suitable for the use of 100 g of catalyst. After 1000 hours of operation, the average conversion of methacrolein was 70% and the selectivity for MMA was 94.5%.

Example 2d shows that, compared to example 2b, a longer drying time has a significant effect on the conversion and thus on the activity in continuous operation. Comparative example 2a thus shows a greater loss of initial activity, with only minimal deactivation being observed after the initial drop in conversion.

Comparative example 2a - variation drying time catalyst and testing

The catalyst was produced on a 1 kg scale analogously to example 2a on the support from example 1c, only this time the drying time was extended from 8 to 70 hours. The residual moisture after drying was 0.8%.

The catalyst was tested analogously to example 2b in a smaller test apparatus suitable for the use of 100 g of catalyst. After 1000 hours of operation, the average conversion of methacrolein was 64% and the selectivity for MMA was 92.5%. Sales were not stable, falling more than 5% over time.

Comparative example 2a shows that, compared to example 2b, a longer drying time has a significant influence on the conversion and thus on the activity in continuous operation. In addition to the initial loss of activity, the catalyst with a drying time of 70 hours also showed a continuous drop in activity and conversion during further operation. Comparative Example 2b - support without classification, catalyst synthesis and testing

The carrier was produced on a 10 kg scale according to Examples 1a and 1b, but the classification step was omitted. The proportion of fines below 25 μm was larger this time at 10% by volume. The catalyst was produced on this—unclassified—support material on a 1 kg scale according to Example 2a. The testing was carried out according to Example 2c, but this had to be stopped less than 24 hours after the start because the fines blocked the sintered metal filter of the reactor and only 60% of the original output could be achieved. Up until then, an average methacrolein conversion of 85% with an MMA selectivity of 94.5% had been achieved.

Flushing with the reaction mixture and nitrogen could not adequately clear the blockage.

Comparative example 2b shows that although the reaction without the removal of fines proceeds chemically without losses in terms of conversion and selectivity, the effect of the fines on the reaction technology does not permit continuous operation.

Comparative Example 2c Catalyst Synthesis Without Building Up a Protective Coating Layer

The catalyst was produced on a 1 kg scale analogously to Example 2a on the support from Example 1c, except this time the addition of aluminum nitrate was omitted.

The catalyst was tested analogously to example 2b in a smaller test apparatus suitable for the use of 100 g of catalyst. After 1000 hours of operation, the average conversion of methacrolein was 75% and the selectivity for MMA was 94.3%. Sales were initially unstable and fell by almost 5%, but then remained stable. MMA selectivity was not affected.

Comparative example 2c shows that, compared to example 2b, if the addition of aluminum salt and the consequent lack of (mantle) protective shell are omitted, a functioning catalyst is obtained, but a drop in catalyst activity is observed as a result of attrition of gold and cobalt lying on the outside. After abrasion of the active components on the outside, the activity remains constant, but at a lower level. The gold loss was quantified at 0.10% absolute, the cobalt loss over the same period was 0.15% absolute. Example 3a - Catalyst screening and batch testing

1 kg of the catalyst produced in Comparative Example 2b was sieved out using various sieves and a shaking tower. To avoid clogging of the screens, the individual screens were periodically cleaned with compressed air. The screening took place in 6 fractions:

The individual fractions were analyzed using laser diffraction and ICP with regard to their grain size distribution and gold and cobalt content:

In the context of this invention, the particle size distribution was determined using ISO 13320:2020 Particle size analysis—laser diffraction methods.

It is easy to see that the content of gold and cobalt increases with decreasing particle diameter, with a disproportionately strong increase in gold. It is therefore of particularly great interest that the fines are already removed in the carrier, because on the one hand the catalyst based on fines has a negative effect on the filtration technology and thus the reaction technology and on the other hand this - unwanted - catalyst fines absorbs a disproportionately large amount of gold and cobalt. As a result, the precious metal costs can be reduced. Furthermore, the smallest particles must be removed to prevent the cobalt it contains from ending up in the sewage, where these particles are toxic to humans and the environment due to the cobalt. 384 mg of catalyst from the respective fraction were placed in a steel autoclave with a magnetic stirrer and suspended with a mixture of methacrolein (1.20 g) and methanol (9.48 g). The methanolic solution contained 50 ppm Tempol as a stabilizer. The steel autoclave was sealed, pressurized to 30 bar with 7% by volume air and stirred at 60 degrees for 2 hours. The mixture was cooled to −10° C., the autoclave was carefully degassed, and the suspension was filtered and analyzed by GC. To determine the selectivity towards methyl isobutyrate, 1% by weight of sodium formate, which serves as a reducing equivalent, was also added to the mixture.

Consistent with the ICP results, there is a higher activity with smaller particles and also a higher selectivity towards methyl isobutyrate; this must be kept as low as possible in the final, specification-compliant product in order to be able to use the MMA for optical applications, among other things. Ultimately, the maximum content of methyl isobutyrate must be significantly below 1000 ppm. A process-related depletion by e.g. rectification, distillation, extraction, hydrolysis is very difficult and requires such large investments and running costs that a process becomes economically and/or technically uninteresting. This shows once again that a support that is not classified accordingly is not suitable for the manufacture of the catalyst and the conduct of the reaction.

Example 4a - Alternative Catalyst Preparation with PVP & Sodium Citrate (Pre-Colloid Formation)

16.7 kg of deionized water were placed in an enamel kettle with a propeller stirrer, and 5 kg of the support material from Example 1c were added. The following steps were carried out using the steam heating of the reactor under isothermal conditions. Immediately afterwards, a solution of 61.1 g of aluminum nitrate nonahydrate in 1 kg of deionized water was added. The suspension was heated to 90°C and then aged for 15 minutes. In parallel, 284.5 g of cobalt nitrate hexahydrate were dissolved in 2 kg of deionized water and dosed over 10 minutes after completion of the aging and reacted with the carrier material for 30 minutes.

In parallel, 1.24 L of a NaOH solution were prepared in such a way that the ratio of hydroxide ions to auric acid was 4.75. The NaOH solution was added over 10 minutes with the suspension darkening.

In a second enamel kettle with a propeller stirrer, 62.5 g of auric acid were diluted in 2 kg of deionized water and 65 g of polyvinylpyrrolidone (8000 to 10000 g/mol average molecular weight) were added. After brief stirring, 62.5 g of sodium citrate were added and the mixture was heated to 70° C., as a result of which a purple to black colored colloid solution was formed within 0.5 hour.

The colloid solution was pumped to the support suspension and the resulting mixture was passively cooled to room temperature followed by a 10 hour post-stirring period.

The suspension was then washed, centrifuged, dried and calcined analogously to Example 2a. During the calcination, the polyvinylpyrrolidone was also removed from the gold nanoparticles by oxidation.

Due to the polyvinylpyrrolidone stabilization, the complete adsorption of the gold nanoparticles in the filtrate & washing solution of the catalyst synthesis on activated carbon to recover the noble metal took about 24 hours, which is significantly longer than the synthesis method without polyvinylpyrrolidone.

The final catalyst had a loading of 0.48 wt% gold and 1.09 wt% cobalt.

Example 4b - Alternative Catalyst Preparation with PVP & Sodium Citrate (Pre-Colloid Formation)

The synthesis was carried out analogously to Example 4a, but instead of cobalt nitrate, 195 g of copper nitrate and 156 g of lanthanum nitrate were used and the addition of NaOH solution was dispensed with. Furthermore, after the calcination, the catalyst was converted into a reductive form by adding hydrogen at 100° C. for 1 hour.

The final catalyst had a loading of 0.48 wt% gold, 1.01 wt% copper and 0.96 wt% lanthanum. This shows that the deposition of lanthanum is complete at 99% of theory, but in the case of copper only 58% of the theoretical deposition is possible was. Since copper nitrate is very toxic to aquatic organisms, the copper has to be recovered in a complex manner analogous to the removal of cobalt.

Example 4c - testing catalyst in a continuous direct oxidative esterification

The catalyst from example 4a was carried out analogously to example 2b in a smaller test apparatus, suitable for the use of 100 g of catalyst. After 1000 hours of operation, the average conversion of methacrolein was 45.8% and the selectivity for MMA was 91.0%.

Example 4d - Testing Catalyst in a Continuous Direct Oxidative Esterification

The catalyst from example 4a was carried out analogously to example 2b in a smaller test apparatus, suitable for the use of 100 g of catalyst. After 1000 hours of operation, the average conversion of methacrolein was 47.3% and the selectivity for MMA was 91.5%.

Claims

Expectations

1. A process for preparing a catalyst for an oxidative esterification, comprising the two sub-processes a) preparing a support and b) preparing a catalyst, characterized in that in sub-process a) an oxidic carrier is prepared, this carrier having at least one or contains several oxides of silicon, aluminum, one or more alkaline earth metals, titanium, zirconium, hafnium, vanadium, niobium, tantalum, yttrium and/or lanthanum, comprising the process steps

(i) Reaction of one or more compounds selected from silicon, aluminium, alkaline earth metal, titanium, zirconium, hafnium, vanadium, niobium, tantalum, yttrium and/or lanthanum compounds at a temperature Ti < 100 °C to obtain a suspension

(ii) spray drying of the suspension at a temperature T2 > 110 °C

(i) to obtain a solid containing 0.1 to 20% by weight of water and 0.1 to 35% by weight of anions of one or more Brönsted acids,

(iii) Calcination of the solid from (ii) at a temperature T3 between 300 and 800° C., a second solid comprising 0.01 to 5% by weight of water and 0.01 to 0.5% by weight of anions of a Bronsted acid being obtained become, and

(iv) the support powder resulting from (i) to (iii) is optionally subjected to a classification step, and that in sub-process b) the oxidic support from sub-process a. is converted to a catalyst, comprising the process steps

(i) reacting the carrier material from a) with a water-soluble precious metal salt,

(ii) simultaneous or subsequent addition of a further soluble metal salt,

(iii) separating the impregnated support from the mother liquor from (ii) and then washing it, the washed impregnated support containing 1.0 to 50% by weight of water,

(iv) drying of the impregnated support for 0.1 to 40 h at a temperature T4 between 30 and 250 °C, a dried impregnated carrier containing 0.1 to 10% by weight of water is obtained,

(v) calcination of the dried impregnated support from (iv) at a temperature T ₅ between 250 and 700° C. and a residence time between 0.1 and 5 h, a catalyst having a BET surface area of 100 to 300 m ² /g at a pore volume of 0.2 to 2.0 m/g and a pore diameter of 3 to 12 nm.

2. The method according to claim 1, characterized in that the process steps a (i), b (i) and b (ii) in batch and the process steps a (ii) and a (iii) are carried out continuously or semi-continuously.

3. The method according to claim 1 or 2, characterized in that in process step a (iv) the second solid from process step a (iii) is treated in such a way that the proportion of particles with a diameter of less than 20 μm is reduced.

4. The method according to at least one of claims 1 to 3, characterized in that in or after process step b (ii) a basic, aqueous solution is additionally added to the mixture.

5. The method according to at least one of claims 1 to 4, characterized in that the mother liquor separated off in process step b (iii) is worked up in such a way that remaining noble metal and other metal salts are recovered.

6. Process according to at least one of Claims 1 to 5, characterized in that the drying in process step b (iv) takes place at an absolute pressure of between 0.01 and 5 bar and/or in the presence of an inert drying gas.

7. The method according to at least one of claims 1 to 6, characterized in that the calcination of process step a (iii) and optionally the calcination of process step b (v) is carried out batchwise.

8. The method according to at least one of claims 1 to 6, characterized in that the calcination of process step a (iii) and optionally the calcination of process step b (v) are carried out continuously or semi-continuously in a rotary kiln.

9. The method according to at least one of claims 1 to 8, characterized in that in process step b (i) an aqueous suspension of the oxidic support from a. is prepared and this is mixed with the water-soluble precious metal salt.

10. The method according to at least one of claims 1 to 10, characterized in that the oxidic support has silicon oxide, aluminum oxide and at least one alkaline earth metal oxide.

11. The method according to at least one of claims 1 to 11, characterized in that in a method step a (v) a solid from one of the method steps a (ii), a (iii) or a (iv) is subjected to a shaping step such that that a shaped body with a diameter between 0.1 and 100 mm is obtained.

12. The method according to at least one of claims 1 to 11, characterized in that the time between spray drying and calcination according to steps a (ii) and a (iii) is no longer than 5 days.

13. The method according to at least one of claims 1 to 12, characterized in that the time between washing and calcination according to steps b (iv) and b (v) is no longer than 4 days.

14. The method according to at least one of claims 1 to 12, characterized in that before, during, or after the reaction of the oxidic support in process steps b) (i) and (ii) a water-soluble Bronsted or Lewis acid, preferably an aqueous solution of a Metal salt is added with the oxidation state +II or +III.

15. Use of a catalyst produced by a process according to at least one of claims 1 to 14 for the continuous production of carboxylic acid esters from aldehydes and alcohols in the presence of an oxygen-containing gas in the liquid phase, the catalyst being suspended heterogeneously in the reaction matrix.

16. Use according to claim 15, characterized in that the reaction takes place at a temperature between 20 and 120 °C, a pH between 5.5 and 9 and a pressure between 1 and 20 bar, the reaction being carried out in such a way that the reaction solution contains between 2 and 10% by weight of water.

17. Use of a catalyst produced by a process according to claim 11 for the continuous production of carboxylic acid esters from aldehydes and alcohols in the presence of an oxygen-containing gas in the liquid phase, the catalyst being used as a fixed bed.

18. Catalyst, characterized in that it can be produced by means of a method according to at least one of claims 1 to 14.