Heterogenous PT-based catalyst

Abstract

The present document is directed to a heterogenous Pt-based catalyst for use in a continuous aerobic oxidation process. The heterogenous Pt-based catalyst comprises a solid support, platinum, and a promoter metal, wherein said platinum and promoter metal are dispersed in the outer part of said solid support, thereby forming a metal containing shell in the outer part of the solid support. The present document is also directed to a method for producing such a heterogeneous Pt-based catalyst and its use in continuous aerobic oxidation processes.

Description

[0001] HETEROGENOUS PT-BASED CATALYST

[0002] TECHNICAL FIELD

[0003] The present document is directed to the field of continuous aerobic oxidation processes. More particularly, the present document is directed to heterogeneous catalysts used in such processes and methods for their production.

[0004] BACKGROUND

[0005] Continuous aerobic oxidation processes are widely used for oxidizing substrates. In a continuous aerobic oxidation process, a feedstock with a substrate to be oxidized is continuously fed into a reactor where an oxidation occurs, and the resulting product continuously removed from the reactor.

[0006] One challenge with such oxidation processes is that in order to be efficient, they are dependent on catalysts with a long lifetime so that the catalyst does not have to be frequently replaced, thereby disturbing the oxidation process.

[0007] The synthesis of bio-based chemicals and polymers from renewable biomass has been developed as a sustainable alternative to reduce dependence on fossil-fuel resources. 2,5-furan dicarboxylic acid (FDCA) can e.g. be used for the production of polyethylene furanoate (PEF), which is a substitute for PET. The common route for synthesizing FDCA is by oxidation of 5-hydroxymethylfurfural (5-HMF). Oxidation of 5-HMF to produce FDCA is a complex reaction involving several intermediate molecules such as 5- hydroxy methyl- 2-furancarboxylic acid (HMFCA), 2,5-diformylfuran (DFF) and 5-formyl-2-furancarboxylic acid (FFCA).

[0008] WO 2017 / 123763 discloses a process for producing 2,5-furandicarboxylic acid pathway products, such as HMFCA, DFF, FFCA and FDCA. The process involves contacting an oxidation feedstock comprising a furanic oxidation substrate and an oxidation solvent with oxygen in the presence of a heterogenous oxidation catalyst comprising a solid support and a noble metal.

[0009] WO2019014382 discloses a process for producing a purified 2,5-furandicarboxylic acid pathway product. The process involves contacting an FDCA pathway product comprising FDCA and FFCA with hydrogen in the presence of a heterogenous reduction catalyst and a solvent. The heterogenous oxidation catalyst comprises a solid support and a noble metal. The solid support is e.g. carbon, zirconium dioxide, titanium dioxide, silicon carbide, silicon dioxide and / or AhOs.The catalyst may further comprise a promoter selected from Ti, Zr, Cr, Mo, W, Mn, Ru, Cu, Zn, Sb, Bi, Sn, Au, Ag, Pb, and / or Te.

[0010] In the process according to WO2017 / 123763 or WO2019 / 014382 for production of FDCA, 5-hydroxylmethyl furfural (HMF) is oxidized using a heterogenous oxidation catalyst in a continuous fixed-bed reactor together with oxygen in nitrogen as oxidant.

[0011] Pt-based catalysts used for the aerobic oxidation of aromatic alcohols and aldehydes, for example 5-hydroxymethyl furfural (HMF), furfural and furfuryl alcohol under non-alkaline conditions are thus known in the literature. However, when using these catalysts in continuous fixed-bed reactor over several hundred or thousand hours on stream in an industrial setting, catalyst stability and lifetime is of outmost importance. One issue is metal losses through leaching of the active metal species that overtime will lead to activity loss. The metal losses due to leaching can be due to poor interaction between the catalytically active metal and the catalyst support. Also, the surface area and material of the catalyst support is important for this interaction. The catalyst activity loss overtime can also be due to fouling of the catalyst surface due to build up of organic material that will block the pores of the catalyst preventing access of the oxidation substrate to the active site.

[0012] The catalyst deactivation will lead to more frequent catalyst changing, which is costly and will lead to lost production time for an industrial plant.

[0013] An object of the present invention is thus to overcome or at least mitigate one or more of the problems described herein.

[0014] SUMMARY

[0015] An object of the present document is therefore to provide an improved Pt-based catalyst for use in a continuous aerobic oxidation process. The heterogenous Pt-based catalyst provided in the present document comprises a solid support, platinum, and a promoter metal, wherein said platinum and promoter metal are dispersed in the outer part of the solid support, thereby forming a metal containing shell in the outer part of the solid support. The promoter metal may be bismuth and / or tellurium. The solid support may comprise or consist of a metal oxide, such as zirconium dioxide and / or titanium dioxide, preferably zirconium dioxide. The solid support may be in the form of particles having a diameter of 1 mm or more, such as 1 .2 mm or more, such as from about 1 .4 mm to 5 mm. The solid support may be in the form of extruded particles. Such extruded particles may have length of from about 1 .0 mm to about 15 mm, such as from about 1.5 mm to about 15 mm and / or a width of from about 1 .0 mm to about 5 mm, such as from about 1 .4 mm to about 5 mm. The Pt-based catalyst may be shaped as a sphere, cylinder, trilobe, or quadrilobe. The solid support may be treated by calcination before preparation of the heterogenous Pt-based catalyst. The heterogenous Pt-based catalyst may have a BET surface area of 100 m2 / g or less, such as 35 m2 / g or less, preferably from about 20 m2 / g to about 35 m2 / g. The pore volume of the Pt-based catalyst may be from about 130 mm3 / g to about 230 mm3 / g, wherein at least 50% of the pore volume is from pores having a pore diameter in the range from 10 nm to 100 nm. The solid support may have a radial crush strength of more than 14 N / mm. The thickness of said metal containing shell is typically from about 50 |j.m to about 300 |j.m. In the context of the present document, the terms “shell” and “layer” may be used interchangeably to denote this metal containing shell. The heterogenous Pt-based catalyst may comprise from about 0.4 wt% to about 3.5 wt% Pt, such as from about 0.4 wt% to about 3 wt% Pt, based on the total weight of the heterogenous Pt-based catalyst. The ratio of Pt and promoter metal may be from about 1 .5:1 to about 3:1 (wt:wt). An exemplary heterogenous Pt-based catalyst according to the present document has a solid support of zirconium dioxide shaped as pellets having a radial crush strength of more than 14 N / mm, wherein said metal containing shell has a thickness of from about 100 |j.m to about 150 |j.m and wherein the promoter metal is Bi.

[0016] The present document also relates to a method for producing a heterogeneous Pt-based catalyst, such as a heterogeneous Pt-based catalyst as defined herein, said method comprising the steps of: i) providing a solid support as defined herein and an impregnation solution comprising a Pt salt and a promoter metal salt; ii) impregnating said solid support with said impregnation solution by applying said impregnation solution to said solid support in portions and letting said impregnation solution dry on said solid support; iii) repeating step ii) until the desired amount of Pt and promoter metal has been applied onto said solid support and the desired metal shell thickness have been obtained. The impregnation solution may be applied by spraying it onto said solid support. Step ii) of drying the impregnation solution on the solid support may be performed by application of heat and / or by air flow. Step ii) may be performed by applying the impregnation solution to the solid support in a tumble dryer using a spray nozzle and applying heat from below to effect drying of said impregnation solution on said solid support. The method may comprise a further step iv) of reducing the Pt and promoter metal, such as by using hydrogen in nitrogen.

[0017] The present document also relates to an impregnation solution for use in a method of preparing a heterogenous Pt-based catalyst, such as a heterogeneous Pt-based catalyst as defined herein, wherein said impregnation solution comprises or consists of a Pt salt, a promoter metal salt and a buffer in the form of citric acid. Preferably the impregnation solution does not comprise any organic solvent(s). The concentration of Pt in such an impregnation solution may be from about 1 wt% to about 10 wt%, of Bi from about 0.5 wt% to about 5 wt% and of citric acid from about 1 wt% to about 10 wt%.

[0018] The present document also relates to a heterogenous Pt-based catalyst, such as a heterogeneous Pt-based catalyst as defined herein, wherein the heterogenous Pt-based catalyst is obtained or obtainable by the method disclosed herein for producing a heterogenous Pt-based catalyst.

[0019] The present document is also directed to the use of an heterogenous Pt-based catalyst as defined herein in a continuous aerobic oxidation process.

[0020] Other features and advantages of the invention will be apparent from the following detailed description, drawings, examples, and from the claims.

[0021] BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 shows the amounts of different substances after oxidation of HMF using a catalyst in the form of 3 wt% Pt on small carbon particles (75-150 |j.m).

[0023] Figure 2 shows the amounts of different substances after oxidation of HMF using a catalyst in the form of 3 wt% Pt on small carbon particles (75-150 |j.m) with 0.3 wt% Bi.

[0024] Figure 3 shows the Pt and Bi leaching into the reactor effluent samples from catalysts in the form of 3 wt% Pt on small carbon particles (75-150 |j.m) and 3 wt% Pt on small carbon particles (75-150 |j.m) with 0.3 wt% Bi, respectively. Figure 4 shows the amounts of different substances after oxidation of HMF using a catalyst in the form of 3 wt% Pt on low surface carbon support (75-150 |j.m) with 0.3 wt% Bi.

[0025] Figure 5 shows the amounts of different substances after oxidation of HMF (A) and Pt leaching (B) using a catalyst in the form of 3 wt% Pt on small ZrO2particles (75-150 |j.m) (circles=Pt, squares=Zr).

[0026] Figure 6 shows the amounts of different substances after oxidation of HMF (A) and Pt and Bi leaching (B) using a catalyst in the form of 3 wt% Pt on small ZrO2particles (75-150 |j.m) with 0.3 wt% Bi.

[0027] Figure 7 shows the effect of increasing the concentration of Bi in the catalyst on yield (A) and metal loss (B).

[0028] Figure 8 shows a comparison of ZrO2and TiO2supported catalysts with different surface areas and different promoter metals.

[0029] Figure 9 shows a comparison between different Pt / Bi catalysts on ZrO2supports in pellet form.

[0030] Figure 10 shows a cross-section of a heterogenous Pt-based catalyst of the present document showing the shell-like black metal containing layer.

[0031] Figure 11 shows an illustrative set-up for preparing a heterogenous Pt-based catalyst according to the method of the present document.

[0032] Figure 12 shows the egg-shell impregnation pattern of a heterogenous Pt-based catalyst prepared with a method of the present document.

[0033] Figure 13 shows an EDX linescan measurement of the penetration depth and concentration profile of Pt and Bi of a heterogenous Pt-based catalyst prepared according to Example 5.

[0034] Figure 14 shows a photograph of the catalyst prepared according to Example 6 showing that catalytic metals (in black) are spread throughout the catalyst. Figure 15 shows an electron microscope photograph. The left photograph shows the Pt in the catalyst. As can be seen, the Pt is distributed throughout the scaffold. To the right the Zr in the solid support is labelled as a comparison.

[0035] DETAILED DESCRIPTION

[0036] As mentioned above, Pt-based catalysts suffer from metal losses through leaching of the active metal species. Overtime this leads to activity loss. The use of a second metal apart from Pt, a so-called promoter metal, on the catalyst surface can lead to a stabilization the active catalytic species.

[0037] In the oxidation of HMF to FDCA, a heterogenous Pt-based catalyst comprising a solid support, platinum, and a promoter metal in a continuous fixed-bed reactor, the presence of the promoter leads to enhanced FDCA yields relative to the non-promoted Pt-catalyst and to a significant reduction of Pt leaching from the catalyst support. A heterogenous catalyst is a catalyst that does not exist in the same phase as the reactants and are typically solid catalysts used for liquid reaction mixtures.

[0038] Industrial fixed bed reactors cannot operate using powdered catalysts since it would lead to excessive pressure drop in the reactor. Further, in contrast to batch processes, where the catalyst is soaked in the reaction medium over long time, a continuous fixed bed trickle reactor operates with much shorter contact time between the catalyst and a mixture of a liquid and a gas stream. For industrial production of e.g. FDCA using continuous fixed-bed oxidation reactors, the oxidation catalyst must thus be formulated on an extruded catalyst support material in a suitable shape and size. However, extruded catalyst supports have a much smaller surface area / volume ratio and a lot of the catalytically active components are therefore inaccessible to the reactants when the active components are dispersed throughout the support material. This leads to a waste of catalytically active components as they are never in contact with the reactants as they are situated too far inside the extruded catalyst support.

[0039] If the catalytically active components could be located in a shell in the outer part of the support material instead of being dispersed throughout the whole support material, the amount of catalytically active components could be reduced. It was surprisingly found that by carefully applying a stable Pt and promoter metal containing solution to the heterogenous support in a spray-dry operation, a catalyst with the catalytically active components located in a controllable shell in the outer part of the support material could be produced. Further, the heterogenous Pt-based catalyst showed increased stability and reduced leaching of Pt. Also, the yield of product when the catalyst was used in oxidation processes was shown to be increased.

[0040] The present document is thus directed to a heterogenous Pt-based catalyst for use in a continuous aerobic oxidation process. The heterogenous Pt-based catalyst comprises or consists of a solid support, platinum, and a promoter metal, wherein the platinum and promoter metal are dispersed in the outer part of said solid support, thereby forming a metal containing shell in the outer part of the solid support. An illustrative photograph of the heterogenous Pt-based catalyst of the present document showing the metal containing shell (see the black layer) in the outer part, near the surface, of the solid support is shown in Fig. 10 and 12.

[0041] The heterogenous oxidation catalysts of the present document are particularly suitable for use in continuous aerobic oxidation processes. Such a continuous aerobic oxidation process may be any continuous oxidation process wherein a substrate to be oxidised is continuously fed into a reactor together with a solvent and an oxygen-containing gas. The term “heterogenous” in the context of the present document thus intends that the catalyst is in a different phase than the reactants and / or products, i.e. the catalyst is solid while the reactants and / or product are liquid and / or in gaseous form. The substrate for such a continuous aerobic oxidation process may be a substance comprising one or more of a primary hydroxy group, an aldehyde group or a carbonyl group. Examples of such substrates are a furanic oxidation substrates such as diformylfuran (DFF), hydroxymethylfurancarboxylic acid (HMFCA), formylfurancarboxylic acid (FFCA), furfural, and / or 5-(hydroxymethyl(furfural) (HMF). The oxidized product formed in the continuous aerobic oxidation process may e.g. be diformylfuran (DFF), hydroxymethylfurancarboxylic acid (HMFCA), formylfurancarboxylic acid (FFCA), 2-furoic acid, and / or 2,5- furandicarboxylic acid (FDCA). The heterogenous Pt-based catalysts of the present document are in particular advantageous to use in processes where HMF is converted to FDCA by contacting a solution of HMF with oxygen in the presence of such as catalyst in a continuous fixed-bed reactor and wherein the reaction is conducted below pH 7 in mixture containing a miscible solvent composition comprising water and an organic solvent. Such a process is for example disclosed in WO2017123763 and

[0042] WO2019014382. The present document is therefore also directed to the use of the heterogenous Pt-based catalyst according to the present document in a continuous aerobic oxidation process, such as such a process performed in a continuous fixed-bed reactor, such as a process according to WO2017123763 and WO2019014382.

[0043] The promoter metal is preferably Bi (bismuth) and / or Te (tellurium). It is possible to use a combination of different promoter metals. If a combination of promoter metals is used, in accordance with the present document the specified amounts, ratios etc. refer to the total amount of promoter metals. However, preferably only one promoter metal is used as this facilitates the production of the heterogenous Pt-based oxidation catalyst. Bi has been shown to be particular advantageous to use in the heterogenous Pt-based catalysts of the present document.

[0044] The solid support preferably comprises or consists of a metal oxide such as zirconium dioxide and / or titanium dioxide, preferably zirconium dioxide. Such a solid support is particularly preferred when the heterogenous Pt-based catalyst is to be used in a continuous aerobic oxidation process wherein the substrate(s) and / or product(s) are acidic, such as a continuous aerobic oxidation process wherein the substrate is a furanic oxidation substrate, such as diformylfuran (DFF), hydroxymethylfurancarboxylic acid (HMFCA), formylfurancarboxylic acid (FFCA), furfural, and / or 5-(hydroxymethyl(furfural) (HMF) and / or the oxidized product formed is diformylfuran (DFF), hydroxymethylfurancarboxylic acid (HMFCA), formylfurancarboxylic acid (FFCA), 2-furoic acid, and / or 2, 5-furandicarboxylic acid (FDCA). A zirconium dioxide and / or titanium dioxide solid support typically comprises 50 wt% or more of zirconium and / or titanium. Metal oxide catalyst supports, compared to carbon supports, when used for the preparation of heterogenous Pt-catalysts have the advantage of having surface properties, e. g. surface area and pore size distribution, that can be tuned by calcination (e.g. by different calcination temperatures and times) used in the preparation of the catalyst support. These properties impact the catalyst activity and stability and, as demonstrated herein, catalyst stability could be increased when using metal oxide based supports.

[0045] The solid support is porous. The BET surface area of the solid support is typically 100 m2 / g or less, such as less than 50 m2 / g, such as from about 5 m2 / g to about 50 m2 / g, such as from about 10 m2 / g to about 40 m2 / g, such as less than 35 m2 / g, such as from about 15 m2 / g to about 35 m2 / g, such as from about 20 m2 / g to about 35 m2 / g, such as from about 15 m2 / g to about 30 m2 / g, such as from about 17 m2 / g to about 30 m2 / g, such as from about 25 m2 / g to about 30 m2 / g. The specific surface can be determined using known methods, such as, for example, the method by Bruanauer, Emmett and Teller (J. Am. Chem. Soc. 1938, 60:309-311). See e.g. ASTM Test Method D3663. As disclosed elsewhere herein, a solid support in the form of zirconium dioxide and / or titanium dioxide having a BET surface area of 100 m2 / g or less may be prepared by calcination. However, such calcination results in a lack of (or reduced amount of) surface groups on the solid support. Therefore, when an impregnation solution comprising the catalytically active metals is applied to such a calcined solid support, the impregnation solution will easily flow into it. Therefore, as is demonstrated in the experimental section, in order to prepare the heterogeneous Pt-based catalysts of such low surface area solid supports, it is important to apply the impregnation solution to the solid support in small portions, so that the impregnation solution cannot fill the solid support, and allowing for sufficient drying before applying the next portion of impregnation solution as is further explained in the below.

[0046] The solid support should not have a too small size to avoid pressure drop in the continuous aerobic oxidation process. Typically, the solid support is thus in the form of particles having a mean diameter of 1 mm or more, such as 1 .2 mm or more, such as from about 1 .4 mm to 5 mm.

[0047] The solid support may be in the form of extruded particles. Such extruded particles may have a length of from about 1 .0 mm to about 15 mm, such as from about 1 .5 mm to about 15 mm, such as from about 1 .5 mm to about 10 mm, such as from about 1 .8 mm to about 5 mm, such as from about 2 mm to about 4 mm, such as from about 2.5 mm to about 3.5 mm, such as about 3 mm. The width may be width of from about 1 .0 mm to about 5 mm, such as from about 1 .4 mm to about 5 mm.

[0048] The solid support may have the shape of a sphere, cylinder, trilobe, or quadrilobe. The maximum ratio of the longest and shortest particles may be around 2 or lower.

[0049] The solid support may be treated by calcination. Such calcination can be used to reduce the surface area of the solid support. The BET surface area of the heterogenous Pt-based catalyst is typically 100 m2 / g or less, such as 50 m2 / g or less, such as from about 5 m2 / g to about 50 m2 / g, such as from about 10 m2 / g to about 40 m2 / g, such as less than 35 m2 / g, such as from about 15 m2 / g to about 35 m2 / g, such as from about 20 m2 / g to about 35 m2 / g, such as from about 15 m2 / g to about 30 m2 / g, such as from about 17 m2 / g to about 30 m2 / g, such as from about 25 m2 / g to about 30 m2 / g. The specific surface can be determined using known methods, such as, for example, the method by Bruanauer, Emmett and Teller (J. Am. Chem. Soc. 1938, 60:309-311). See e.g. ASTM Test Method

[0050] D3663.

[0051] The pore volume of the solid support and / or the heterogenous Pt-based catalyst is typically from about 130 mm3 / g to about 230 mm3 / g, wherein at least 50% of the pore volume is from pores having a pore diameter in the range from 10 nm to 100 nm. The pore volume and pore volume distribution can be determined either by nitrogen desorption isotherms in conjunction with BET surface area determinations and / or by mercury intrusion porosimetry. See e.g. ASTM test methods D4641 , and D4284.

[0052] The solid support typically has a radial crush strength of more than 14 N / mm. The crush strength may e.g. be determined according to e.g. ASTM test method D6175. It is important that the crush strength is not too low, in order to avoid the catalyst being crushed in the oxidation reactor.

[0053] As mentioned above, the active catalytic components, i.e. Pt and the promoter, are dispersed in the outer part of the solid support, thereby forming a shell-like structure (see Fig. 10 and 12). Some of the active catalytic components are also dispersed in the inner parts of the solid support, but the absolute majority is located in the shell. The metal containing shell typically has a thickness of from about 50 |j.m to about 300 |j.m, such as from about 75 |j.m to about 250 |j.m, such as from about 90 |j.m to about 160 |j.m, such as from about 100 |j.m to about 150 |j.m, such as from about 110 |j.m to about 200 |j.m, such as from about 130 |j.m to about 170 |j.m, such as from about 140 |j.m to about 160 |j.m, such as about 150 |j.m. The shell thickness can be determined by measuring the thickness of the black metal containing layer using a light microscope seen in a cross section of a catalyst particle, see e.g. Fig. 10 and 12. The Pt and promoter metal distribution can be determined using a SEM-EDX linescan analysis of the cross section of the catalyst particle, see e.g. Fig. 13. Typically, at least about 50 wt% of the catalytically active metals of the catalyst are located in this shell, such as at least 60 wt%, at least 70 wt%, at least 80 wt%, at least 90 wt%, at least 95 wt% or at least 98 wt%.

[0054] The amount of Pt in the heterogenous Pt-based catalyst of the present document is typically from about 0.4 wt% to about 3.5 wt% Pt, such as from about 0.4 wt% to about 3 wt% Pt, based on the total weight of the heterogenous Pt-based catalyst. The ratio of Pt and promoter metal is typically from about 1 .5:1 to about 3:1 , such as about 2:1 (wt:wt). The total weight of the Pt and promoter metal, respectively, in the heterogenous Pt-based catalyst can be measured by digesting the catalyst particle(s) in a strong acid and then analyzing the metal content in the resulting solution using ICP-AES (Inductive Coupled Plasma Atomic Emission Spectroscopy).

[0055] As demonstrated in the experimental section, a catalyst with Pt-Bi on ZrO2with 100-150 pm shell thickness was to very efficient in the oxidation of HMF to FDCA. A preferred heterogenous Pt-based catalyst for use in a continuous aerobic oxidation process according to the present document therefore comprises or consists of a solid support in the form of pellets of ZrO2and / or TiO2, Pt and a promoter metal in the form of Bi, and has a metal containing shell thickness of from about 100 pm to about 150 pm. The size of such pellets is preferably as described above. The amount of Pt and Bi in such a heterogenous Pt-based catalyst may be about 1 .5 wt% Pt and about 0.5 wt% Bi, based on the total weight of the heterogenous Pt-based catalyst. Preferably, such a catalyst is prepared by using a citric acid buffered impregnation solution (see below). The BET surface area of such a catalyst may be 100 m2 / g or less, preferably 50 m2 / g or less.

[0056] The heterogenous Pt-based oxidation catalysts of the present document have a low surface area and a suitable shell thickness with the right amounts of metals. These features make the heterogenous Pt-based oxidation catalysts of the present document very efficient when used in the oxidation reactions referred to herein.

[0057] Method for producing the heterogenous Pt-based catalyst

[0058] For low surface area metal oxide supports, when traditional wet impregnation methods are used for catalyst production, this will result in that the Pt / promoter metal becomes distributed throughout the catalyst extrudate. This is due to that the catalyst support lacks sufficient surface groups due to the high-temperature calcination such metal oxide supports are exposed to during their production. As mentioned above, this means that a lot of the catalytically active components will never come in contact with the reactants as they are positioned too deep inside the solid support. For larger solid support particles that consequently have a low surface area, it would thus be preferred to have the catalytically active components located mainly in the outer part of the solids support, where they could come in contact with the reactants. The present inventor was able to solve this problem by a method where an impregnation solution comprising a Pt salt and a promoter metal salt repeatedly is applied in small portions onto the solid support with a drying step between each application. The volume of each portion of the impregnation solution that is applied should be lower than the total pore volume of the catalyst support to prevent the solution to spread too far into the solid support. The impregnation solution is thus repeatedly applied and dried on the solid support’s surface. In this way, a highly controlled shell thickness of the active metal components on the solid support’s outer part could be obtained. Due to the sequential application and drying steps, the present method allows for a very accurate control of the shell thickness of the heterogenous Pt-based catalyst produced as the shell is built up slowly and it is possible to stop the application of more impregnation solution once the desired shell thickness has been achieved.

[0059] The present document is therefore also directed to a method for producing a heterogeneous Pt-based catalyst, such as a heterogeneous Pt-based catalyst as defined herein, wherein the method comprises the steps of: i) providing a solid support as defined elsewhere herein and an impregnation solution comprising a Pt salt and a promoter metal salt; ii) impregnating said solid support with said impregnation solution by applying said impregnation solution to said solid support in portions and letting said impregnation solution dry on said solid support; iii) repeating step ii) until the desired amount of Pt and promoter metal has been applied onto said solid support and the desired metal shell thickness have been obtained.

[0060] Details regarding the solid support, promoter metal salts and impregnation solution are disclosed elsewhere herein.

[0061] The application of the impregnation solution is preferably performed by spraying onto the solid support.

[0062] Typically, the application and drying step (i.e. step ii) is repeated several hundred times. For example, the application and drying steps may be repeated at least about 50 times, at least about 100 times, at least about 150 times, at least about 200 times at least about 250 times, at least about 300 times, at least about 400 times, at least about 500 times, such as from about 50 times to about 1000 times, from about 100 times to about 1000 times, from about 200 times to about 1000 times, from about 200 times to about 800 times, from about 200 times to about 700 times or from about 200 t times o about 500, from about 400 times to aboutl 000 times, or from about 400 to about 800 times.

[0063] The drying of the impregnation solution may be performed in any way that allows the impregnation solution to dry on the solid support surface but is preferably performed by the application of heat and / or by air flow, such as by blowing warm air or by using an infrared heater. The drying temperature is typically from about 50 °C to about 90 °C, such as from about 70 °C to about 80 °C.

[0064] The impregnation may be performed by applying the impregnation solution to the solid support in a tumble dryer using a spray nozzle and applying heat from below to effect drying of said impregnation solution on said solid support. An exemplary set-up for the application and drying steps is illustrated in Fig. 11 , where a rotary vessel heated electrically from bottom is used. The vessel opening is equipped with a computer- controlled spraying nozzle. When using such a set-up, the solution may be impregnated by ca 1 s spraying and ca 10 s delay on a moving layer (approx. 40 rpm) of hot carrier (70 - 80 °C from IR-thermometer) during 160 min.

[0065] As is evidenced in the experimental section, it is important to apply the impregnation solution in small amounts to ensure that it only spreads into the outer part of the solid support. Further, it is important to ensure that an applied portion of impregnation solution is allowed to dry before applying the next portion. Otherwise, the impregnation solution will spread too far into the solid support and the catalytic metals will not be present mainly in the outer part of the solid support. This is particularly important when applying an impregnation solution to a solid support having a low surface area, such as a zirconium dioxide and / or titanium dioxide supports having surface area of 100 |j.m or less, due to a lack of (or low amount of) surface groups of such solid supports.

[0066] Typically, in the method for producing a heterogeneous Pt-based catalyst of the present document, from about 0.5 vol% to about 3 vol% of impregnation solution is applied in each application step, based on the total pore volume of the solid support. Typically, an amount of impregnation solution corresponding to about 1 vol % of the total pore volume is applied with each application (i.e. each portion). The total volume of impregnation solution to be applied will depend upon e.g. the desired thickness of the metal containing layer but is typically around two times the total pore volume of the solid support. Typically, in this case, the number of times the impregnation solution is applied is from about 400 to about 500.

[0067] When the desired thickness of the metal containing shell has been obtained, the impregnated solid support may be further dried, such as at an elevated temperature (such as about 120 °C for about 2 hours). The impregnated solid support may further be calcined, such as at a temperature of about 350 °C for about 4 hours. The method for producing a heterogenous Pt- based catalyst may further comprise a step iv) of reducing the Pt and promoter metal, such as by using hydrogen in nitrogen, e.g. 5% H2 in N2. Such a reduction step may be performed at from about 250 °C to about 500 °C. For example, such reduction may be performed at about 350 °C for about 2 - 4 h starting at 0.5 vol% H2 in N2and finishing at 5 vol% H2in N2, keeping the inner temperature under control by slowly increasing the H2 concentration. After cooling down to 120 °C, passivation of the catalyst surface was carried out using 4 - 100 vol. % air in N2 for approximate 3 h.

[0068] The impregnation solution used for impregnating a solid support comprises a Pt salt and a promoter metal salt in an acidic solution, such as a hydrochloric acid, nitric acid or a mildly acidic solution, such as citric acid buffer, or in a mildly alkaline solution, such as NH4OH. Typically, such an impregnation solution only comprises water as the solvent, i.e. the impregnation solution is aqueous. Preferably, no organic solvents are present in the impregnation solution. In particular, the impregnation solution may comprise or consist of Pt salt, promoter metal salt, citric acid and water. The concentration of Pt in the impregnation solution may be from about 1 wt% to about 10 wt%. The concentration of Bi in the impregnation solution may be from about 0.5 wt% to about 5 wt%. The concentration of citric acid in the impregnation solution may be from about 1 wt% to about 10 wt%. The Pt salt may suitably be provided as [Pt(NH3)4](HCOs)2. The Bi salt may be provided as ammonium bismuth citrate.

[0069] The impregnation solution may be prepared by suspending the [Pt(NH3)4](HCOs)2 in water, adding citric acid and then heating until a colourless solution is obtained. Ammonium bismuth citrate may then be added before dilution with water to obtain the desired concentrations of the respective ingredient.

[0070] The impregnation method is not limited to using [Pt(NH3)4](HCOs)2 and ammonium bismuth citrate in a citric acid solution. K2PtCl4 or FhPtCle and BiCH dissolved in hydrochloric acid (1 M) or Pt(NOs)2 and Bi(NOs)s dissolved in nitric acid (1 M) can also successfully be used as impregnation solutions. However, the highly corrosive nature of these solutions may be prohibitive when applied on large scale and the use of less corrosive solutions, such as citric acid, may be preferred.

[0071] When the solid support is in the form of a metal oxide, such as ZrO2or TiO2or mixed ZrO2-TiO2, the solid support may be calcined before the application of the impregnation solution to reduce its surface area (for exemplary suitable BET surface areas to achieve, see above). Such calcination may be performed at a temperature of from about 500 °C to about 950 °C with about 4-6 h dwell time at target temperature.

[0072] The present document is also directed to a heterogenous Pt-based catalyst as defined herein, wherein said heterogenous Pt-based catalyst is obtained or obtainable by the method for producing a heterogeneous Pt-based catalyst as described herein. The present document is thus also directed to a heterogenous Pt-based catalyst for use in a continuous aerobic oxidation process, said heterogenous Pt-based catalyst comprising a solid support, platinum, and a promoter metal, wherein said platinum and promoter metal are dispersed in the outer part of said solid support, thereby forming a metal containing shell in the outer part of the solid support wherein said heterogenous Pt-based catalyst is obtained or obtainable by a method comprising the steps of: i) providing a solid support as defined elsewhere herein and an impregnation solution comprising a Pt salt and a promoter metal salt; ii) impregnating said solid support with said impregnation solution by applying said impregnation solution to said solid support in portions and letting said impregnation solution dry on said solid support; iii) repeating step ii) until the desired amount of Pt and promoter metal has been applied onto said solid support and the desired metal shell thickness have been obtained.

[0073] Additional steps and details regarding the different steps may be found above.

[0074] One advantage with the method for producing such heterogenous Pt-based oxidation catalysts disclosed herein is that it is scalable.

[0075] The invention will be further described in the following examples, which do not limit the scope of the invention described in the claims. EXPERIMENTAL SECTION

[0076] In this experimental section, all references to metal content of catalysts are in wt% of the total weight of the catalysts unless otherwise specified. Examples 1-3 demonstrate the differences between different supports, choice of promoter metal and choice of surface area and are prepared with prior art methods, such as incipient wetness impregnation methods.

[0077] EXAMPLE 1 : Carbon-supported catalysts: comparison between carbon-supported Ptbased oxidation catalysts with or without Bi-promoter addition.

[0078] Reaction conditions: Liquid feed: 2 wt% HMF in 1 ,4-dioxane:water (60:40 w:w). Gas feed: 5 vol% O2in 95% N2. Catalyst amount: 3.7 g with an estimated bed length of 46 cm.

[0079] Reactor pressure: 68.9 - 103.4 barg (68.9 barg for Example 1 D). Reactor temperature: 90 - 140 °C (125 °C for Example 1 D).

[0080] A. 3 wt% Pt on small carbon particles (75-150 tm)

[0081] When 3 wt% of Pt on carbon particles having a size of 75-150 |j.m was used as the heterogenous Pt- based catalyst, the yield of FDCA was around 20-25% depending on process conditions, see Fig. 1.

[0082] B. 3 wt% Pt on small carbon particles (75-150 tm) with 0.3 wt% Bi

[0083] When 10 wt% Bi (based on Pt-loading) was added to a 3 wt% Pt on small carbon particle catalyst, there was a significant increase in FDCA yield up to 70-75%, see Fig. 2. However, this catalyst showed deactivation and lower FDCA yield over 400 h time on stream.

[0084] C. Pt and Bi leaching from catalysts A and B above

[0085] ICP-analysis of the reactor effluent showed higher levels of Pt leaching when using a nonpromoted catalyst (1-3 ppm of Pt in the effluent) compared to the Bi-promoted catalyst (<0.5 ppm), see Fig. 3. However, there are signs of high Bi-leaching during the first 200 h which could explain the activity losses for the Bi-promoted catalyst (catalyst B above).

[0086] Conclusion Addition of Bi as a promoter to the carbon-supported Pt catalyst gave a significant increase in FDCA yield using the same process conditions and also a stabilization of the Pt on the support seen as decreased Pt-leaching. D. 3 wt% Pt on low surface carbon support (75-150 tm) with 10 wt% Bi (based on Pt- loadinq)

[0087] Pt-Bi on a low surface carbon supports show initial high activity with around 75% FDCA yield, see Fig. 4. However, the carbon-supported catalyst shows significant deactivation over only 200 h of continuous time on stream.

[0088] Conclusion Decreasing the surface area of the catalyst support material increases the

[0089] FDCA yield slightly but the carbon-supported oxidation catalyst still suffers from rapid deactivation over a few hundred hours’ time on stream.

[0090] EXAMPLE 2: Zirconium oxide-supported catalysts: comparison between Zr02-supported Pt-based oxidation catalysts with or without Bi-promoter addition.

[0091] Reaction conditions: Liquid feed: 2 wt% HMF in 1 ,4-dioxane:water (60:40 w:w) for Example 2A and 2B and 5 wt% for Example 2C. Gas feed: 5 vol% O2in 95% N2. Catalyst amount: 4.2 g with an estimated bed length of 23 cm. Reactor pressure: 68.9 barg.

[0092] Reactor temperature: 125 °C.

[0093] A. 3 wt% Pt on small ZrO2particles (75-150 urn)

[0094] When a catalyst with 3wt% Pt on small ZrO2particles (75-150 |j.m) was used, the FDCA yield was very low and a rapid Pt-loss due to severe leaching of Pt into reactor effluent was observed. Almost all Pt was lost over 100 h continuous processing. See Fig. 5A and

[0095] B.

[0096] B. 3 wt% Pt on small ZrO2particles (75-150 tm) with 0.3 wt% Bi

[0097] Addition of 10 wt% Bi (based on Pt-loading) resulted in a significant increase in FDCA yield up to 25-30%. ICP-analysis if the reactor effluent showed very low Pt and Bi content, <0.1 ppm, i.e. there was no significant leaching of Pt and Bi from the catalyst. See Fig. 6A and B.

[0098] C. Effect of increased Bi-loading

[0099] Increasing the Bi-loading to 1 / 3 of the Pt-loading (1 wt% Bi and 3 wt% Pt) increased the activity of the ZrC2-supported catalyst to give 55% FDCA at 5 wt% HMF feed.

[0100] The metal losses to the reactor effluent were low, <0.1 ppm for both Pt and Bi. See Fig. 7A and B.

[0101] Conclusion: Addition of Bi as a promoter to the ZrC>2-supported Pt catalyst gave a significant increase in FDCA yield using the same process conditions and also a greatly enhanced stabilization of the Pt on the support seen as decreased Pt-leaching. The ZrC>2- supported Bi-promoted Pt catalysts have lower initial activity than the carbon-based catalysts but have much higher stability and less leaching of the active metals.

[0102] EXAMPLE 3: Comparison of ZrC>2 and TiC>2 supported catalysts with different surface areas and different promoter metals

[0103] Reactor setup: The catalysts were loaded into a stainless-steel reactor (4.6 mm ID) connected with a liquid feed and gas feed supply. The reactor was filled with 4 mL of catalyst.

[0104] Commercially available catalyst supports where either dried (350 °C) or calcined to reduce the surface area. ZrC>2 support SZ-31247 (350 °C, 43 m2 / g) was calcined to 800 °C and the surface area was reduced to 20 m2 / g. TiO2support ST-31119 (350 °C, 35 m2 / g) was calcined to 750 °C and the surface area was reduced to 19 m2 / g. The catalyst supports were crushed and sieved and a particle size fraction of 100-150 pm was used. The dried and calcined catalyst support particles (100-150 pm particle size) were impregnated with a Pt and Bi containing solution or a Pt and Te containing solution in HCI using an incipient wetness method followed by drying and reduction using 5% H2in N2at 350 °C.

[0105] Liquid feed: 5 wt% of HMF in 1 ,4-dioxane:water (60:40 wt:wt). Gas feed: 5 vol% O2in 95 vol% N2. Table 1 : Reaction conditions

[0106] From the catalyst screen it was found that the calcined supports with reduced surface area performed better than the non-calcined supports (unfilled vs filled circles). Also, it was found that Bi performed better as a promoter metal than Te using both ZrC>2 and TiC>2 supports (unfilled circles vs unfilled triangles and filled triangles vs filled squares). Finally, ZrO2supported catalyst with Pt and Bi with reduced surface area performed better than the corresponding catalyst supported on TiC>2 (unfilled circles vs filled triangles). See Fig. 8.

[0107] EXAMPLE 4: Shaped metal oxide-supported catalysts containing Pt and Bi

[0108] To evaluate industrially relevant oxidation catalysts, the catalyst support was changed from small particle to extruded pellets (1.5 x 3 mm). The support pellets were calcined using the same calcination conditions as in Example 3 to reduce the surface area to around 20-25 m2 / g. The impregnation solutions, containing both Pt and Bi, were applied to the support pellets using a spray nozzle where the solution was sprayed onto the pellets placed in a rotating tumbler / beaker. The solution was sprayed onto the support pellets in small portions with a drying step between where hot air (ca 150-180 °C) was blown onto the wet pellets to allow the water to evaporate. This spray-dry sequence was repeated until the desired amount of the impregnation solution had been applied to reach the target metal loading. By alternating the length of the spraying and / or drying step, different shell thicknesses could be targeted. After the impregnation was completed, the catalysts were further dried and reduced using forming gas at 350 °C.

[0109] The composition of the impregnation solution was also investigated using either highly acidic solutions based on hydrochloric and nitric acid or mildly acidic solutions based on citric acid.

[0110] Table 2: Reactor set-up

[0111] Reactor setup: The catalysts were loaded into two stainless steel reactors (4.6 mm ID) connected in series with a liquid feed and gas feed to the first reactor and additional gas feed between the first and second reactor. Each reactor was filled with 2 mL of catalyst, 4 mL of catalyst per trial. The reactor setup was operated at 70 barg pressure.

[0112] Liquid feed: 5 wt% of HMF in 1 ,4-dioxane:water (d:w). Gas feed: 5 vol% O2in 95 vol% N2.

[0113] Reaction conditions: Table 3: Reaction conditions From screening a large number of variables, it was found that Pt-Bi on ZrO2with 100-150 pm shell thickness were most active in the oxidation of HMF to FDCA (Reactor 1 , 3 and 5). The catalyst with higher shell thickness (Reactor 7) and the catalyst with higher surface area (Reactor 8) were less productive. TiC>2-supported catalysts were also less productive even though they had the same metal loading and shell thickness (Reactor 2, 4 and 6). The ZrO2-based catalyst prepared using citric acid buffered impregnation solution gave the highest FDCA yield over almost 800 h time on stream using different process conditions (Reactor 6). See Fig. 9.

[0114] Catalyst characterization data

[0115] Metal loading:

[0116] The Pt and Bi content were determined by ICP-AES analysis of the freshly prepared catalyst.

[0117] Shell thickness:

[0118] The thickness of the applied metal containing shell was determined by analyzing a cross section of a catalyst pellet with an optical microscope. The analysis confirmed the 100- 150 pm shell thickness. See Fig. 10.

[0119] Conclusions Examples 1-4:

[0120] -ZrO2or TiC2performed better as support compared to carbon-based supports. -Increasing the size of the support particles improved the performance of the catalysts. -Increasing the amount of promoter metal in view of the amount of Pt increased the performance (up to 1 / 3 Bi to Pt).

[0121] -Decreasing the surface area had a positive effect on the performance of the catalysts. -Catalyst performance was also improved by applying the metal containing solutions to the support by repeatedly spraying and drying them on the support material

[0122] -A mildly acidic impregnation solution instead of a highly acidic impregnation solution when preparing the catalysts provided better performing catalysts.

[0123] EXAMPLE 5: Preparation of Pt / Bi metal catalysts on ZrO2supports Support material

[0124] Zirconia pellets with a particle size of 1 .4 mm or more, pre-calcined at approx. 800 °C in order to reduce the specific area (BET) to 25 m2 / g, was used as the support material (carrier). The radial crush strength of these pellets was 14.4 N / mm and the total pore volume was 0.16 ml / g. The crush strength was calculated by crushing a pellet and measuring the force (in N) necessary to obtain this. To obtain the crush value in N / mm, the force necessary for crushing was divided by the length of the pellet. The crush value was obtained by calculating the mean from the crushing of 20 pellets.

[0125] 540 g portions of material were impregnated with 162 g impregnation solution. Composition of the impregnation solution varied in each impregnation as target concentrations of platinum and bismuth were different as shown in Table 4.

[0126] Table 4: Target metal concentrations and compositions of impregnation solutions

[0127] Impregnation

[0128] Each impregnation solution was freshly prepared by suspending [Pt(NH3)4](HCO3)2 (50.96 % Pt, Heraeus Deutschland GmbH, Hanau, Germany) in water, 0.5 M citric acid was carefully added and the mixture was gently heated to obtain colourless solution. Then, solid ammonium bismuth citrate (46.13 % Bi, Glentham Life Sciences Ltd, Corsham, United Kingdom) was added to give a colourless solution. Finally, water was added to give 162.0 g of the solution.

[0129] The impregnation apparatus consisted of a stainless-steel rotary vessel heated electrically from bottom. See Fig. 11 , wherein 1 is a rotating spherical stainless-steel vessel (diameter 25 cm), 2 is a computer-controlled spraying nozzle, 3 is a carrier to be impregnated and 4 is an IR heating (power ~2000 W). The vessel opening was equipped with a computer- controlled spraying nozzle. The solution was impregnated by 1 s spraying and 10 s delay on a moving layer (approx. 40 rpm) of hot carrier (70 - 80 °C from IR-thermometer) during 160 min. Calcination and Reduction

[0130] Each of the impregnated materials was dried at 120 °C for 2 h and calcined at 350 °C for 4 h. The samples were then reduced in a larger reduction unit (diameter 16 cm, length 170 cm) in four separated zones at 350 °C for 2 h starting at 0.2 L / min H2+ 10 L / min N2and finishing at 0.2 L / min H2+ 2 L / min N2. The unit was cooled down using air-fan while flushed with N2and as soon inner temperature reached 100 - 120 °C, passivation was carried out using 4 - 100 vol. % air in N2in such manner to control inner temperature increase. The yields are given in Table 5. Table 5: Yields of calcined and reduced products

[0131] Characterization

[0132] The prepared samples (four batches) were analysed by inductively coupled plasma atomic emission spectroscopy (ICP-AES), mercury intrusion porosimetry (MIP), adsorption / desorption of nitrogen, chemisorption of H2, optical microscopy, and scanning electron microscopy (SEM-EDX).

[0133] Table 6: Analysis of different batches of catalyst

[0134] Optical microscope investigations of all four samples showed similar egg-shell impregnation patterns - shell approx. 0.1 - 0.2 mm thick. See Fig. 12.

[0135] SEM

[0136] Four samples were investigated - platinum- and bismuth-containing shell impregnated on ZrO2extrudates. The sample surface was investigated from a morphological and elemental composition point of view. The focus was on radial profiles of characteristic elements on extrudate intersection. Prior to the analysis, the samples were stuck down an SEM stub using copper tape and then coated by the 5 nm carbon layer to avoid a sample charging. The sample surface morphology was investigated using scanning electron microscopy (SEM) with a FEG electron gun (FIB-SEM TESCAN LYRA3GMU). Elemental analysis and radial profiles were measured using an energy dispersive spectroscopy (EDS) analyzer with an 80 mm2SDD detector (X-MaxN, Oxford instruments) and AZtecEnergy software. The radial concentration profiles were measured for at least 3 independent particles. The SEM-analysis with elemental mapping confirmed the colocation of Pt and Bi within the shell. For an example, see Fig. 13.

[0137] EXAMPLE 6: Comparative example using a wet impregnation method for preparing a shaped metal oxide-supported catalyst containing Pt and Bi

[0138] The catalyst was prepared with 1 ,5%Pt and 0.75% Bi on a ZrO2support with a pore volume of 0.15 mL / g.

[0139] Citric acid was dissolved in 3.2 L water under stirring, warmed to 30°C and

[0140] Pt(NH3)4](HCO3)2was added to give a colorless solution. Ammonium bismuth citrate was dissolved in 3.2 L water under stirring and combined with the former solution to give a colorless solution. Finally, water was added to get 11 .2 kg impregnation solution, which was homogenized.

[0141] Impregnation

[0142] The support was pre-heated in a drum to 66°C (maximum temperature with this heating capacity). The impregnation solution was applied using a spray equipment and was added in doses of 150 g solution over 6.5 h.

[0143] The time interval between two sprayings was 5 minutes. Although a minimum temperature of 80 °C was targeted inside the drum, the temperature decreased to 45°C due to a low heating capacity. The water content of impregnated ZrO2increased during the impregnation of max. 8.6% H2O, corresponding to 60 % of the pore volume.

[0144] Finishing

[0145] The impregnated support was dried at 140°C, calcinated at 350°C for 10 h and finally reduced using forming gas at 350°C for 16 h.

[0146] Analytics

[0147] Optical analysis

[0148] As can be seen in Figure 14, the resulting catalyst was homogeneous and fully impregnated with the catalytic metals. The catalyst on the lower right corner shows the cross section of a divided catalyst. Figure 14 can be compared to Figure 12, which shows a catalyst of the present document where the catalytic metals (in black) are distributed only in the outer part of the solid support.

[0149] Electron microscope analysis

[0150] Further, as is also evidenced by electron microscopy the catalytic metals are distributed throughout the solid support (Figure 15). The left photograph shows the Pt in the catalyst. As can be seen, the Pt is distributed throughout the scaffold. To the right the Zr in the solid support is labelled as a comparison. Conclusions Example 6

[0151] As is demonstrated in this example, if not sufficient time is given or if too low temperature is applied for the impregnation solution to dry between each application step, the impregnation solution will spread into the solid support and the catalytic metals will be spread throughout the solid support and not be present mainly in the outer part of the solid support.

[0152] It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.

[0153] Unless expressly described to the contrary, each of the preferred features described herein can be used in combination with any and all of the other herein described preferred features.

Claims

CLAIMS1 . A heterogenous Pt-based catalyst for use in a continuous aerobic oxidation process, said heterogenous Pt-based catalyst comprising a solid support, platinum, and a promoter metal, wherein said platinum and promoter metal are dispersed in the outer part of said solid support, thereby forming a metal containing shell in the outer part of the solid support.

2. The heterogenous Pt-based catalyst according to claim 1 , wherein said promoter metal is bismuth and / or tellurium.

3. The heterogenous Pt-based catalyst according to claim 1 or 2, wherein said solid support comprises or consists of a metal oxide, such as zirconium dioxide and / or titanium dioxide, preferably zirconium dioxide.

4. The heterogenous Pt-based catalyst according to any one of the preceding claims, wherein said solid support is in the form of extruded particles, such as extruded particles having a length of from about 1.0 mm to about 15 mm, such as from about 1 .5 mm to about 15 mm and / or a width of from about 1 .0 mm to about 5 mm, such as from about 1 .4 mm to about 5 mm.

5. The heterogenous Pt-based catalyst according to any one of the preceding claims, wherein said heterogenous Pt-based catalyst has a BET surface area of 100 m2 / g or less, such as 35 m2 / g or less, preferably from about 20 m2 / g to about 35 m2 / g and / or wherein the pore volume of said heterogenous Pt-based catalyst is from about 130 mm3 / g to about 230 mm3 / g, wherein at least 50% of the pore volume is from pores having a pore diameter in the range from 10 nm to 100 nm.

6. The heterogenous Pt-based catalyst according to any one of the preceding claims, wherein said solid support has a radial crush strength of more than 14 N / mm.

7. The heterogenous Pt-based catalyst according to any one of the preceding claims, wherein the thickness of said metal containing shell is from about 50 |j.m to about 300 |j.m.

8. The heterogenous Pt-based catalyst according to any one of the preceding claims, wherein said heterogenous Pt-based catalyst comprises from about 0.4 wt% to about 3.5 wt% Pt, such as from about 0.4 wt% to about 3 wt% Pt, based on the total weight of the heterogenous Pt-based catalyst.

9. The heterogenous Pt-based catalyst according to any one of the preceding claims wherein the solid support is zirconium dioxide shaped as pellets having a radial crush strength of more than 14 N / mm, wherein said metal containing shell has a thickness of from about 100 |j.m to about 150 |j.m.

10. A method for producing a heterogeneous Pt-based catalyst as defined in any one of claims 1-9, said method comprising the steps of: i) providing a solid support as defined in any one of claims 1-9 and an impregnation solution comprising a Pt salt and a promoter metal salt; ii) impregnating said solid support with said impregnation solution by applying, such as by spraying, said impregnation solution to said solid support in portions and letting said impregnation solution dry on said solid support; iii) repeating step ii) until the desired amount of Pt and promoter metal has been applied onto said solid support and the desired metal shell thickness have been obtained.11 . The method according to claim 10, wherein said method comprises a step iv) of reducing the Pt and promoter metal, such as by using hydrogen in nitrogen.

12. An impregnation solution for use in a method of preparing a heterogenous Ptbased catalyst according to any one of claims 10-11 , said solution comprising or consisting of a Pt salt, a promoter metal salt and a buffer in the form of citric acid.

13. The impregnation solution according to claim 12, wherein the concentration of said Pt is from about 1 wt% to about 10 wt%, of said Bi from about 0.5 wt% to about 5 wt% and of said citric acid from about 1 wt% to about 10 wt%.

14. A heterogenous Pt-based catalyst, such as a heterogenous Pt-based catalyst as defined in any one of claims 1-9, wherein said heterogenous Pt-based catalyst is obtained or obtainable by a method according to any one of claims 10-11.

15. Use of the heterogenous Pt-based catalyst as defined in any one of claims 1-9 and 14 in a continuous aerobic oxidation process.

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