DE102013106637A1 - Catalytically active carbon materials, processes for their preparation and their use as catalysts - Google Patents

Abstract

The present invention relates to catalytically active, carbonized nitrogen-containing carbon materials modified with transition metal, processes for their production and their use as catalysts.

Description

The present invention relates to catalytically active transition metal-modified carbonated nitrogen-containing carbonaceous materials, processes for their preparation and their use as catalyst.

Methane is diverse and is constantly being regenerated on Earth, e.g. in biological and geological processes. Today, methane is an integral part of our energy industry and is often produced, for example, as fuel in biogas plants. Furthermore, the methanation after previous electrolysis of water is the basic principle for the production of wind or solar gas, to which in the field of renewable energies is attributed an increasing importance. In addition, an estimated 600 million tons of methane are emitted or flared annually, which is the inefficient use of this raw material.

However, like all alkanes, methane exhibits relatively low reactivity because the C-H bonds are relatively stable and can not be easily broken and no functional groups are present.

Thus, the direct conversion of methane by means of C-H activation presents a challenge, since the development of efficient catalysts with sufficient activities and selectivities is hampered by the high C-H bond energy. Thus, a competitive process for the direct conversion of methane into liquid products due to inefficient catalysts is currently not available.

The currently most active catalytic system has been developed by Periana and his co-workers ( RA Periana, DJ Taube, S. Gamble, H. Taube, T. Satoh and H. Fujii, Science, 1998, 280, 560-564 ). The dichloro-bipyrimidyl-platinum (II) complex developed by this group (in the more specific Pt (bpym) Cl ₂ ) allows the catalytic oxidation of methane in concentrated or fuming sulfuric acid with high yields and methanol selectivities of over 90%. The product formed first methyl bisulfate, which can then be hydrolyzed to methanol. Thus, within the process, the acid serves as both an oxidant and a protective reagent. However, the difficult catalyst and product separation, as well as the limited catalytic activity, still present considerable difficulties for industrial application. Currently, neither homogeneous nor heterogeneous or enzymatic catalyst systems offer a ready-for-use alternative solution.

A novel catalyst system similar to the Periana catalyst through coordinate binding of Pt ²⁺ to a covalent triazine framework (CTF) was developed by the inventors some time ago and showed comparable activities to the Periana catalyst. However, the synthesis of the catalyst is complicated and the reaction rates achieved are still to be improved for possible industrial application.

Another, known in the prior art catalyst material is based on precursors from biomass ( Green, Vol. 2 (2012), pp. 25-40 ). The hydrothermal carbonization followed by thermal treatment of biomaterials such as chitin-based starting compounds (in particular the exoskeleton of a lobster) resulted in porous carbon material (ExLOB) with large specific surface areas (> 400 m ² / g) and variable chemical properties. Further modification of these ExLOB-NDC materials with a Pt ²⁺ species yielded coordinatively modified Pt ²⁺ -ex-LOB-NDC materials. Although some of these materials had high catalytic activity, it has been shown in recycling experiments that the high activity could not be retained and deactivated the materials. Moreover, in such biomass-based catalyst materials, complete reproducibility due to natural diversity is rarely realizable.

There is therefore a need for a catalyst system which is improved, in particular with respect to reproducibility, which on the one hand exhibits high catalytic activities, in particular for the direct oxidation of methane to methanol, on the other hand is stable and in particular does not tend to leach the catalytically active metal.

Investigations on nitrogenous carbon materials (hereinafter referred to as NDC materials) have been made by the inventors, which proved to be particularly suitable catalyst materials because of their thermal and oxidative stability. Nitrogen-containing carbon material according to the invention means a carbon skeleton or network in which individual sites within the framework are occupied by nitrogen atoms and which can coordinate a transition metal.

Thus, the inventors studied simple and inexpensive synthetic routes to produce nitrogen-enriched carbon networks.

The investigations of the inventors revealed that catalyst systems based on nitrogen-containing carbon materials are well suited for the oxidation process according to the invention. Such catalyst systems can be prepared by subjecting nitrogen and carbon based polymer materials under inert atmosphere or under vacuum to thermal carbonation. Optionally, for these materials, either by templating during the polymer synthesis or by known from the prior art steam activation the material targeted a pore structure can be imparted.

The present invention is therefore directed to processes for producing nitrogen-containing carbon materials, which are characterized in that a nitrogen and carbonaceous starting material is subjected to a Karbonisierungsbehandlung and the material obtained a loading step with a catalytically active metal salt, in particular Pt, Hg or Pd, or a be subjected to another metal compound.

Suitable nitrogen-containing materials according to the invention are, in particular, those materials which have polymeric structures or which form them under the conditions of a carbonization. Particularly suitable are nitrogen-containing hydrocarbon polymers which have amino, imino, amido, nitrile units and are composed of monomeric hydrocarbons or combinations thereof. Examples are given in the following table:

The above compounds are meant to be exemplary and those skilled in the art can identify other C-N containing polymers that are readily carbonizable and therefore useful as precursor compounds.

Although it is generally possible for nitrogen-containing hydrocarbons to be formed from hydrocarbons and nitrogen-containing compounds such as amines or ammonia under the carbonation conditions and / or hydrothermal treatment conditions, those compounds which are already preferred as starting materials are amino, imino, imido, and or nitrile groups in the framework as in a polymer as indicated in the table above.

Thus, according to the invention, preference is given to CN-containing polymers based on C-containing monomers such as polyacrylonitrile, acrylonitrile-butadiene-styrene copolymers (ABS), SAN, ASA, polyamides such as polycaprolactam, (PA 6), nylon, (PA 6.6), poly - (hexamethylene sebacamide), (PA 6.10), poly (hexamethylene dodecane diamide), (PA 6.12), polyundecanolactam, (PA 11), polylauryllactam, (PA 12), polybenzimidazole, polypyrrole, polyamines, polyimides and polyurethanes, which also may be substituted with C ₁ - to C ₁₂ -alkyl groups, and mixtures thereof. Particular preference is given to the use of polyacrylonitrile, polyurethanes, polyamides, etc., which are subsequently subjected to a carbonization treatment.

The carbonization of the nitrogen-containing polymer of the invention is carried out in the absence of a reactive atmosphere, e.g. under an inert protective gas, usually in a temperature range of 600 ° to 1200 ° C over a period of 0.5 to 12 hours. The carbonization is preferably carried out under inert atmosphere at temperatures of 900 ° to 1200 ° C over a period of 2 to 6, especially 3 to 5 hours, for example over 4 hours.

The carbon material thus obtained is treated with a solution of a catalytically active metal salt or a mixture of several metal salts. The catalytically active metal is usually selected from metals of subgroups I-VII, especially from subgroups I, II and VIII and more particularly from subgroup VIII of the PSE, or mixtures thereof. The amount of catalytically active metal is 0.01 to 5 wt .-% based on the amount of carbonized nitrogen-containing carbon material used.

The use of Pt, Pd, Hg, Co, Rh, Ir, Ni, Ru, Fe, Au, Ag and Cu is thus possible according to the invention as the catalytically active metal, the use of Pt, in particular in the form of K ₂ PtCl, being preferred ₄ , PtCl ₂ (NH ₃ ) ₂ , Pt (acetylacetonate) ₂ , Pt (1,5-cyclooctadiene) Cl ₂ , K ₂ PtCl ₆ , H ₂ PtCl ₆ or PtCl ₂ .

The resulting catalytically active material based on nitrogen-containing carbon materials is very well suited for use as a catalyst in oxidation processes. Thus, it can be used in particular for the conversion of methane to methanol in the Periana system. In addition, uses for oxidation, hydrogenation, hydroamination, isomerization, rearrangement and hydroformylation reactions are possible.

If desired, the material may also be subjected to a forming step in a molding apparatus such as tabletting press, extruder, pelletizer.

The present invention will be further illustrated by the following examples and figures. Showing:

1 the procedure for carrying out the reaction of Example 8 with the reaction autoclave (autoclave 1) and preheating autoclave for methane (autoclave 2);

2 the course of the reaction for various catalysts according to Example 8 ("PAN-C-1100": polyacrylonitrile-based catalyst carbonated at 1100 ° C. (corresponding to Example 1); "PAN-C-1100-WA": polyacrylonitrile-based carbonated at 1100 ° C. Catalyst (according to Example 2); "ExLOB-900": biomass-based catalyst material; "CTF": covalent triazine framework).

Preparation Examples

example 1

Nitrogen-containing carbon material was detected according to the method Lu et al. (Chem. Mater., Vol. 16, No. 1, 2004) produced. 9.88 g of silica (Davisil, 60 Å pore size, 100-200 sieve) were dried for 12 hours at 150 ° C. in vacuo (10 ^-3 bar). The white powder was then impregnated with a solution of 7.5 mL of acrylonitrile and 40 mg of AMBN (2'-azobis (2-methylbutyronitrile)) using the incipient wetness method. The polymerization reaction to the polyacrylonitrile was carried out at 50 ° C for 15 hours and then at 60 ° C for 8 hours in an autoclave. The resulting white powder was then treated for 15 hours at 200 ° C in a muffle furnace. The carbonization was carried out in a tube furnace in a nitrogen stream, at 5 ° C / minute heated to 1100 ° C and this temperature was held for 4 hours. After cooling to room temperature, the black powder was transferred into a 500 ml glass flask and treated with 400 ml of 1M sodium hydroxide solution at 60 ° C. for at least 3 days. Finally, the solid was filtered off, washed thoroughly with water and acetone and dried at 90 ° C.

To coordinate platinum, 46 mg of K ₂ PtCl _{4 was dissolved} in 1 L of deionized water, after which 400 mg of nitrogen-containing carbon material were added and the mixture was stirred for at least 48 hours. The black powder was filtered off, washed extensively with 3 L deionized water and dried at 90 ° C.

Example 2

Nitrogen-containing carbon material was added according to a modified method Lu et al. (Chem. Mater., Vol. 16, No. 1, 2004) produced. 15 mL of acrylonitrile and 80 mg of AMBN (2, 2'-azobis (2-methylbutyronitrile) were mixed and heated in an autoclave at 50 ° C for 15 hours and subsequently at 60 ° C for 8 hours The white polyacrylonitrile was then added to The black-brown material obtained was then powdered in a ball mill (400 revolutions / minute, silicon nitride grinding jar, 6 balls / 3g material) for 15 hours at 200 ° C. The carbonization was carried out in a tube furnace under nitrogen flow heating at 5 ° C./minute to 1100 ° C. and maintaining this temperature for 4 hours The nonporous material was then heated to 850 ° C. at 5 ° C./minute in a tube furnace under an inert gas stream, after which the inert gas stream was heated at the same temperature Finally, the sample was cooled to room temperature and dried in vacuo at 50 ° C. The coordination of Pt within the nitrogen-containing Carbon material is as described in Example 1.

Example 3

Nitrogen-containing carbon material was prepared according to the method described in Example 2, with the difference that the polymerization to polyacrylonitrile in a molding container was performed and was dispensed with a ball mill.

Example 4

Nitrogen-containing carbon material was prepared according to the method described in Example 2, except that a polyacrylonitrile granulate was pressed into tablets with a pelleting press before the carbonization step and ball mill treatment was omitted.

Example 5

Nitrogen-containing carbon material was prepared according to the method described in Example 2, except that the carbonation step was carried out at 650 ° C instead of 1100 ° C.

Example 6

Nitrogen-containing carbon material was prepared according to the method described in Example 2, except that the carbonation step was carried out at 850 ° C instead of 1100 ° C.

application examples

Catalyst assays were performed in a batch setup with similar reaction conditions as periana and his co-workers. The pressure loss, as a sign of the conversion of methane to methyl bisulfate, was recorded by means of pressure sensors.

Example 7

For the low-temperature direct oxidation of methane, 60 mg of the platinum-modified nitrogen-containing carbon material together with 15 mL oleum (20 wt% SO ₃ ) were placed in a 30 mL Hastelloy autoclave with glass insert. This was then purged with argon, sealed and charged with 45 bar of methane. The autoclave was then heated to 215 ° C with intensive stirring of the reaction mixture contained (1000 revolutions / minute), held at this temperature for 2.5 hours and finally cooled to room temperature by means of a water bath. The gas phase in the autoclave was analyzed by FTIR spectroscopy. The solid catalyst was separated by filtration from the reaction solution, which was then mixed with 2 volumes of deionized water while cooling, hydrolyzed for 3 hours at 90 ° C with stirring and finally cooled to room temperature. The methanol content of the hydrolyzed reaction solution was determined by HPLC.

Example 8

For the low-temperature direct oxidation of methane, 40 mg of the platinum-modified nitrogen-containing carbon material together with 15 mL oleum (20% by weight SO ₃ ) were placed in a 30 mL Hastelloy autoclave with glass insert and sealed under argon atmosphere. The reactor was then heated with stirring (1000 revolutions / minute) to 215 ° C and then pressurized with 70 bar to 215 ° C preheated methane. After 30 minutes, the reactor was quenched in a water bath. The gas phase in the autoclave was analyzed by FTIR spectroscopy. The solid catalyst was separated by filtration from the reaction solution, which was then mixed with 2 volumes of deionized water while cooling, hydrolyzed for 3 hours at 90 ° C with stirring and finally cooled to room temperature. The methanol content of the hydrolyzed reaction solution was determined by HPLC.

As the evaluation of Example 8 showed, the turnover frequency could be calculated from the pressure drop between 69-67.5 bar or from the produced amount of methanol within half an hour.

The comparison of the pressure curves shows a clear difference in activity between the catalysts. ( 2 ). Reaction rates determined in accordance with the pressure drop confirm this observation and show that the materials according to the invention Pt @ PAN-C-1100-WA or Pt @ PAN-C-1100 in comparison to prior art materials such as Pt @ CTF, Pt @ ExLOB -900 or Pt (2,2'-bipyrimidine) Cl ₂ (Pt (bpym) Cl ₂ ) can achieve significantly higher turnover frequencies (TOF). Table 1 gives the calculated reaction rates based on the pressure curves in the range 69-67.5 bar ("PAN-C-1100": at 1100 ° C carbonated polyacrylonitrile-based catalyst (corresponding to Example 1); "PAN-C-1100-WA": carbonated polyacrylonitrile-based catalyst (corresponding to Example 2) at 1100 ° C; "ExLOB-900": biomass-based catalyst material; "CTF": covalent triazine framework). Table 1 material Load% Reaction rate g / (hg (Pt)) Pt @ CTF 5.41 28.6 Pt @ ExLOB-900 5.55 340.6 Pt @ PAN-C-1100 4.66 595.4 Pt @ PAN-C-1100-WA 4.43 1371.6 Pt (2,2'-bipyrimidine) Cl ₂ 45,99 149.7

Table 2 gives the calculated conversions, yields and selectivities based on the partial pressure of methane at the start and end of the reaction and the amount of methanol after the hydrolysis step on the experiments at. Table 2 material Sales % Yield of methanol after hydrolysis% Selectivity to methanol after hydrolysis% Pt @ CTF 7.0 6.0 85.4 Pt @ ExLOB-900 33.8 31.7 94.0 Pt @ PAN-C-1100 36.2 34.6 95.9 Pt @ PAN-C-1100-WA 50.2 48.5 96.7 Pt (2,2'-bipyrimidine) Cl ₂ 17.9 17.2 96.3

The results demonstrate that the catalytic activity of the materials of the present invention is significantly higher than that of the known Pt @ ExLOB-900, Pt @ CTF or Pt (2,2'-bipyrimidine) Cl ₂ , suggesting that the carbon support is a large one Has an influence on the catalytic activity.

Surprisingly, the materials of the present invention allow selective methane direct oxidation even at low temperature in the temperature range of 150 ° to 250 ° C over unselective materials (e.g., metal oxides) in the temperature range> 300 ° C of the prior art.

In summary, it can be said that the thermal carbonation of carbon networks obtained by nitrogen-containing carbon polymers according to the invention, followed by metal coordination such as platinum coordination, leads to novel, highly active solid catalysts, inter alia for direct methane oxidation in the Periana system. The catalytic activity of these materials is higher than that of the homogeneous reference catalyst and significantly better than that of the previously developed solid catalysts.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• RA Periana, DJ Taube, S. Gamble, H. Taube, T. Satoh and H. Fujii, Science, 1998, 280, 560-564 [0005]
• Green, Vol. 2 (2012), pp. 25-40 [0007]
• Lu et al. (Chem. Mater., Vol. 16, No. 1, 2004) [0025]
• Lu et al. (Chem. Mater., Vol. 16, No. 1, 2004). [0027]

Claims (6)

A carbonated nitrogen-containing carbonaceous material containing catalytically active metal selected from transition group I-VII metals, especially from subgroups I, II and VIII and more particularly from subgroup VIII of the PSE, or mixtures thereof in an amount of from 0.01 to 5 wt .-%, based on the amount of carbonized nitrogen-containing carbon material used by carbonizing CN-containing polymer selected from polyacrylonitrile, acrylonitrile-butadiene-styrene copolymers, polyamides, polybenzimidazole, polypyrrole, polyamines, polyimides, polyurethanes, also be substituted with C ₁ - to C ₁₂ alkyl groups, or mixtures thereof is obtainable. A carbonated nitrogen-containing carbon material according to claim 1, which is obtainable via a process for thermally carbonizing a nitrogen-containing hydrocarbon polymer, followed by loading the thus-obtained carbon material with the catalytically active metal. A method for producing the catalytically active nitrogen-containing carbon material according to claim 1 or 2, comprising the steps of: thermal carbonization of a nitrogen-containing hydrocarbon polymer selected from polyacrylonitrile, acrylonitrile-butadiene-styrene copolymers, polyamides, polybenzimidazole, polypyrrole, polyamines, polyurethanes, also be substituted with C ₁ - to C ₁₂ -alkyl groups, and mixtures thereof, • loading of the nitrogen-containing carbon skeleton thus obtained with a compound of the catalytically active metal with coordination of the active metal within the carbonated nitrogen-containing carbon material. A process according to claim 3, wherein the thermal carbonization is carried out under an atmosphere inert to the nitrogen-containing hydrocarbon polymer or under vacuum in a temperature range of 500 ° to 1200 ° C. Use of the catalytically active carbonated nitrogenous carbon material of any one of claims 1 to 2 as a catalyst in oxidation, hydrogenation, hydroamination, isomerization, rearrangement and hydroformylation processes, especially in C-H bond activation processes. Use according to claim 5, wherein the C-H bond activation process is a process for the direct oxidation of methane in the Periana system.

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