Catalyst for methane oxidation

Abstract

This invention relates to a catalyst for the oxidation of methane. The catalyst comprises palladium supported on a zero-dimensional zeolite or Clasrasil. The palladium is present in an amount of 0.25 to 4% by weight based on the total weight of the catalyst. The catalyst contains less than 6% by weight of carbon based on the total weight of the catalyst. The zero-dimensional zeolite or Clasrasil is 100 m 2 It has a surface area of ​​less than / g.

Description

Technical Field

[0001] The present invention relates to a catalyst for the oxidation of methane, and more particularly to an oxidation catalyst for treating exhaust gas generated by a natural gas (NG) engine. A method for manufacturing the catalyst, as well as an exhaust system and apparatus including the catalyst, are also disclosed.

Background Art

[0002] Laws restricting the amount of pollutants that may be released into the atmosphere are becoming increasingly stringent. One category of pollutants regulated by intergovernmental agencies worldwide is unburned hydrocarbons (HC). Unburned HC of various compositions are typically present in the exhaust gases generated by various types of mobile or stationary engines such as spark ignition engines, compression ignition engines, and combustion turbines.

[0003] Natural gas (NG) typically contains a hydrocarbon (HC) gas mixture, a small amount of carbon dioxide (CO2), hydrogen sulfide (H2S), water vapor (H2O), and nitrogen (N2). The main component of NG is methane (CH4), but relatively small amounts of ethane (C2H6), propane (C3H8), and other hydrocarbons are also usually present. There is interest in using natural gas (NG) as a fuel for engines, particularly in the form of either compressed natural gas (CNG) or liquefied natural gas (LNG). In vehicle applications, the use of CNG as a fuel is typically preferred over LNG because CNG generally has both lower production and storage costs compared to LNG.

[0004] Engines specifically designed to use NG as fuel are being manufactured. Furthermore, it is possible to modify existing internal combustion engines to use NG. Engines are known to be able to use NG as fuel in various ways, such as alone (e.g., "dedicated" NG engines) or in combination with other fuels (e.g., bi-fuel engines), and engines may be operated with one fuel at a time, or both fuels may be used together. When NG is used as fuel, the exhaust gases produced by the engine contain a considerable amount of methane (so-called "methane slip"). Methane is a powerful greenhouse gas (GHG). Furthermore, compared to other HCs that are typically present in exhaust gases, methane and ethane are difficult to catalytically oxidize on catalytic converters, especially in the presence of excess oxygen, such as in the exhaust gases produced by lean-burn NG combustion engines.

[0005] Commercial oxidation catalysts for processing methane / ethane typically contain palladium (Pd) or platinum (Pt) and palladium (Pd) supported on alumina (Al2O3). These catalysts must be operated at high temperatures (e.g., >500°C) to achieve reasonable methane conversion efficiency. Other oxidation catalysts are being studied, but they often suffer from poor thermal stability. Furthermore, many methane oxidation catalysts are sensitive to sulfur poisoning.

[0006] U.S. Patent No. 2016236147 discloses a series of oxidation catalysts for treating exhaust gases produced by natural gas (NG) engines. The oxidation catalyst comprises a catalyst material and a substrate for treating the exhaust gases. The catalyst material comprises one of several specific molecular sieves and platinum group metals (PGMs) supported on the molecular sieve. These are said to be advantageous because they have good activity at low temperatures, both fresh and after aging, even in the presence of water vapor. The preferred zeolite has a SAR ≥ 1200, and the most preferred filling amount of total platinum group metals is 10–20% by weight.

[0007] U.S. Patent No. 2016236147 discloses a variety of suitable molecular sieves, including 137 different skeletal structure types. However, the examples focus only on BEA, MFI, and FAU. All of these are so-called medium-porous or large-porous zeolites. Furthermore, each example is a combination of Pt and Pd. [Overview of the project]

[0008] The object of the present invention is to provide an oxidation catalyst for treating methane-containing exhaust gases using platinum group metals more efficiently, or to address at least the problems associated therewith in the prior art, or to provide a commercially viable alternative thereto.

[0009] Accordingly, the present invention provides a catalyst for the oxidation of methane, wherein the catalyst comprises palladium supported on a zero-dimensional zeolite or clathracil, the palladium is present in an amount of 0.25 to 4% by weight, and the catalyst contains less than 6% by weight of carbon based on the total weight of the catalyst.

[0010] Herein lies a further explanation of this disclosure. Different aspects / embodiments of this disclosure are defined in more detail in the following sections. Each of the aspects / embodiments defined in this way may be combined with any other aspects / embodiments or more aspects / embodiments unless otherwise expressly indicated. In particular, any feature shown as preferred or advantageous may be combined with any other or more features shown as preferred or advantageous. Features disclosed in relation to a product may be combined with features disclosed in relation to a method, and vice versa.

[0011] The inventors have found that the catalyst described herein can provide complete methane conversion at 550°C and >95% conversion at 430°C in the presence of a gas feed representing exhaust gas from a CNG combustion engine. The catalyst exhibits high water and sulfur resistance, as well as good desulfurization properties under low temperature and lean conditions.

[0012] Zeolites are microporous crystalline aluminosilicate materials commonly used as catalysts. They primarily contain silicon, aluminum, and oxygen. Zeolite structures are identified by their three-letter code skeletal type.

[0013] There are many parameters that can be used to characterize the framework. Internal voids are generally classified as cages and channels, and their size is given as the number of tetrahedral coordinating atoms (T atoms) that form the ring, or as a unit of length suitable for the atomic scale, such as angstroms.

[0014] Channels present in a given skeleton are generally classified according to the size of the largest channel: (i) small if the largest channel is assembled from 8 T atoms (and the same number of oxygen atoms), (ii) medium if the largest channel is assembled from 10 T atoms, (iii) large if the largest channel is assembled from 12 T atoms, and (iv) extra-large if the number of T atoms is greater than 12.

[0015] Channel system dimensionality is an alternative approach to classifying zeolites. It determines the number of dimensions (D) through which a guest molecule can freely move through the framework once it enters the pore. Frameworks having only cages or channels composed of six or fewer T atoms are considered inaccessible, and therefore their dimension is 0 (D=0, or zero-dimensionality). Frameworks composed of more than six T atoms and having channels running in only one direction have D=1. For frameworks with multiple channels running in more than one direction, the pore dimension depends on how the channels are interconnected. If a molecule can move between two orthogonal channel systems, D=2; if it can move between three orthogonal channel systems, D=3.

[0016] Common zeolites used in catalytic applications include BEA (D=3), MFI (D=3), and FAU (D=3). These are typically chosen for their high internal bonding properties, which allow for easy diffusion of the gas being treated.

[0017] Clasrasil are crystalline silica inclusion compounds consisting of a three-dimensional silica host skeleton. Similar to zeolites, they are synthesized using organic structure-directing agents that can be decomposed and removed to provide a cage-like internal structure. Therefore, clasrasil have an extended, regular skeleton similar to that of zeolites. They typically have smaller pore diameters than zeolites. To avoid doubt, the clasrasil contemplated herein can also be considered zero-dimensional.

[0018] Therefore, zero-dimensional zeolites, or clathracil, are characterized by the absence of channels or voids having openings larger than six T atoms, and are characterized by the lack of large channels or voids within them. Another term for zero-dimensional zeolites, sometimes used in the art, is "non-porous" zeolite. Nevertheless, zero-dimensional zeolites, or clathracil, still possess a highly ordered repeating structure defined by a skeletal structure type that distinguishes them from amorphous materials such as silica.

[0019] Focusing on high-SAR zeolites described in U.S. Patent No. 2016236147, the inventors investigated the possibility of applying palladium to silica. Silica, being entirely silicon-based, inexpensive, and abundant, represents a logical extension to the use of high-SAR zeolites. However, the inventors found that performance was significantly degraded, at least partially, by palladium sintering.

[0020] The inventors have found that the use of zero-dimensional zeolites or clathracil as carriers for palladium provides an alternative to the known high-SAR zeolites disclosed in U.S. Patent No. 2016236147. While not bound by theory, it is believed that because there are no open pores in the zeolite, the applied palladium cannot access the exchange sites and becomes inactive. This means that since no palladium is unusable, less palladium needs to be added for a given activity benefit.

[0021] As a result of the absence of pores, palladium remains on the surface of the zeolite or clathracil crystal. This ensures that the palladium is accessible and active. Again, without wanting to be bound by theory, it is understood that the regular surface structure of zeolites or clathracil helps to keep the palladium well distributed on the crystal surface, resulting in reduced sintering (as observed in amorphous silica) and retained high activity after aging.

[0022] The catalyst comprises palladium supported on a zero-dimensional zeolite or clathracil. This is typically achieved by conventional application methods that involve contacting the support with a palladium-containing solution, such as palladium nitrate.

[0023] Palladium is present in an amount of 0.25 to 4% by weight, based on the total weight of the catalyst (i.e., further components including zeolite or clathracil and palladium). Preferably, palladium is present in an amount of 0.5 to 3% by weight. Preferably, palladium is the only PGM present.

[0024] The catalyst contains less than 6% by weight of carbon, based on the total weight of the catalyst. The as-synthesized zeolite or clathracil contains carbon from the structure-directing agent. This needs to be removed to ensure that the catalyst remains stable over its useful life. However, the inventors have found that this must be removed carefully to avoid damaging the structure (including loss of crystallinity). This is discussed in more detail below and cannot be achieved using conventional calcination without considering the structure of the material. Preferably, the zero-dimensional zeolite or clathracil contains less than 5% by weight of carbon, preferably less than 4% by weight, and preferably less than 2% by weight, based on the total weight of the catalyst. This can be measured by CHN analysis or other known techniques.

[0025] According to alternative embodiments, the carbon present in the catalyst is evaluated relative to the carbon present in the as-synthesized, uncalcined zeolite or clathracil form. Preferably, the zero-dimensional zeolite or clathracil in the catalyst contains less than 50% by weight, preferably less than 40% by weight, preferably less than 20% by weight, and most preferably less than 10% by weight of the carbon present in the as-synthesized, uncalcined zeolite form. Reduction of carbon in this manner while maintaining the zeolite or clathracil structure can be achieved using the method described herein. This ensures that the advantages of the present invention are achieved.

[0026] The surface area can be measured using the commonly used Brunauer-Emmett-Teller (BET) method. The BET method uses the measurement of physicoadsorption of a gas (adsorbate) to obtain the surface area of ​​the sample. Preferably, the BET method is used to determine the surface area using argon as the adsorbate. Preferably, the zero-dimensional zeolite is 100 m 2 Less than / g, preferably 30m 2 Less than 20mg / g, preferably 20mg 2 It has a BET surface area of ​​less than 1g. These low surface areas are unusual for catalyst materials and help ensure good stability over the catalyst's lifetime.

[0027] The zero-dimensional zeolite can have any suitable framework type that does not have any pore openings of 8 atoms or more. That is, the framework is not of a small (8-membered ring), medium (10-membered ring), or large (12-membered ring) structural type (or larger). Suitable framework types include AFG, AST, DOH, FAR, FRA, GIU, LIO, LOS, LTN, MAR, MEP, MSO, MTN, NON, RUT, SGT, SOD, SVV, TOL, and UOZ. Preferably, the zero-dimensional zeolite has a NON framework structure.

[0028] The zeolite may consist of an aluminosilicate framework structure. Alternatively, one or more heteroatoms may be substituted into the framework. The zeolite may be provided in different forms, such as the H or Na form, for example.

[0029] Preferably, the zero-dimensional zeolite has a SAR of 5 to 200, preferably 40 to 80. These lower SAR values have been found to be related to good stability and ease of synthesis.

[0030] Preferably, the zero-dimensional zeolite or classisil has a particle size of less than 500 nm, preferably 1 to 100 nm. The particle size (or crystal size) determines the surface area of the bulk powder. The surface area varies depending on the form and use of the final catalyst article. These small particle sizes are optimal for good filling of palladium considering the absence of an accessible internal pore surface. The particle size of the zero-dimensional zeolite or classisil is well-known and can be measured by several suitable analytical techniques commonly used, such as scanning electron microscopy (SEM), laser diffraction particle size analysis using, for example, a Malvern Mastersizer 2000, or X-ray diffraction (XRD). Preferably, SEM is used to determine the particle size.

[0031] In a further embodiment, a method for producing catalysts described herein is provided. The inventors have found that in the synthesis process, it is important to remove carbon from the structure to ensure the necessary activation, but at the same time, it is difficult to remove carbon without damaging the structure (due to the absence of larger pores). Therefore, gentle heat treatment is required to remove carbon without damaging the zeolite or clathracil structure. Preferably, the method is for the production of zeolites having a non-non-linear framework structure.

[0032] This method, a) Synthesizing a zeolite or clathracil containing an organic structure-directing agent, b) Heat-treating a zeolite or clathracil containing an organic structure-directing agent at a temperature exceeding 500°C under an inert gas, and then calcining the heat-treated intermediate in air at a temperature exceeding 450°C, thereby substantially removing the organic structure-directing agent from the zeolite or clathracil, and then c) Preferably comprising depositing palladium on a zeolite or clathracil by an initial wet impregnation method.

[0033] The heat treatment under an inert gas is preferably carried out at temperatures up to 700°C. Preferably, the treatment is carried out at temperatures in the range of 550°C to 650°C, most preferably at about 600°C. This temperature is for the holding temperature, and it will be understood that the heat treatment is associated with the heating temperature (slowly at a rate of 1 to 5°C / min to avoid damage). The holding temperature is preferably maintained for at least 30 minutes, more preferably for 1 to 3 hours. The inert gas is preferably oxygen and moisture-free, and preferably substantially nitrogen.

[0034] Firing in air is preferably carried out at temperatures up to 600°C. Preferably, the firing is carried out at temperatures in the range of 400°C to 550°C, most preferably at about 500°C. This temperature is the holding temperature, and it will be understood that the firing is related to the heating temperature (e.g., at a rate of 1 to 15°C / min, e.g., 10°C / min, slowly to avoid damage). The holding temperature is preferably maintained for at least 30 minutes, more preferably 1 to 3 hours. The gas is preferably air, but it should be understood that it is important that some oxygen is present.

[0035] An alternative to firing in air is processing in the presence of an ozone source. This can be carried out at lower temperatures and can reduce the risk of structural damage.

[0036] After each heat treatment / sintering step, the zeolite may be cooled. The temperature gradient for cooling is much faster than the heating gradient, for example, at least 20°C / min or at least 30°C / min.

[0037] The inventors have found that this two-stage heat treatment is necessary for complete zero-D zeolite synthesis and activation (for methane conversion), because calcination appears to make it more susceptible to decomposition and structural loss. While not bound by theory, this may be due to the difficulty in removing SDA from the micropore system, and oxygen-free treatment may promote less severe decomposition of SDA.

[0038] In a further embodiment, a catalyst article is provided comprising a substrate and one or more wash coat layers, wherein at least one of the wash coat layers comprises the catalyst described herein.

[0039] In a further embodiment, an exhaust system including the catalyst article described herein is provided.

[0040] In a further embodiment, a device is provided comprising an engine and an exhaust system as described herein.

[0041] Preferably, the engine is a natural gas combustion engine. Most preferably, the engine is a stationary natural gas combustion engine.

[0042] In a further embodiment, a method for the oxidation of methane is provided, the method comprising contacting a gas containing methane with a catalyst described herein. Preferably, the methane is present in the exhaust gas of a stationary methane combustion engine.

[0043] In all embodiments of this specification, it is preferable to use the catalyst under conditions in which sulfur may be present, as this is the most advantageous for catalyst consistency. Sulfur may be present at a level of at least 0.5 ppm, preferably 1 to 5 ppm. [Brief explanation of the drawing]

[0044] The present invention will be further explained in the following diagram. [Figure 1] The plots of methane conversion with increasing temperature are shown for a commercially available MFI with a SAR of 2120 and a catalyst containing the Zero-D zeolite disclosed herein. Plots are provided for an artificial gas mixture and a gas mixture representing engine exhaust (engine simulated gas mixture). As the temperature increases, a 50% conversion is achieved first for the high-SAR MFI catalyst (artificial), then for the Zero-D catalyst (artificial), then for the MFI catalyst (engine simulated), and then for the Zero-D catalyst (engine simulated). [Figure 2] The graph shows plots of methane conversion with increasing temperature for a range of materials. The samples tested were a commercially available MFI catalyst, the Zero-D catalyst disclosed herein, and a silica comparative catalyst. All contained 1 wt% Pd and were tested under an engine simulated gas mixture. As the temperature increased, a 50% conversion was achieved first for the MFI catalyst (fresh), then the Zero-D catalyst (fresh), then the silica catalyst (fresh), then the MFI catalyst (sulfated), then the Zero-D catalyst (sulfated), and then the silica catalyst (sulfated). As can be seen from the graph, the Zero-D catalyst is less affected by sulfation, and the silica catalyst does not function well. [Figure 3] This graph shows methane conversion over a simulated engine operating cycle. Temperatures are shown over a 3-hour cycle, with sulfur present in the first half of the cycle. The Zero-D catalyst shows good performance across the operating cycle duration and temperature. At 60 minutes, the high-SAR MFI catalyst shows the highest conversion, followed by the Zero-D catalyst, and then the silica catalyst. At 140 minutes, the lines for the MFI and Zero-D catalysts are indistinguishable. The temperature line indicates the inlet temperature. [Modes for carrying out the invention]

[0045] The data demonstrate that the Zero-D catalyst exhibits performance comparable to known high-SAR MFI catalysts, making them a reliable alternative. Further advantages of the Zero-D catalyst were observed, such as more efficient use of palladium and improved sulfur tolerance. In particular, the degradation of performance after sulfation was reduced, resulting in a more consistent catalyst. [Examples]

[0046] The present invention will now be further described with respect to the following non-limiting embodiments.

[0047] Zero-D zeolite, nonacil (NON), was prepared using a reaction mixture consisting of 60% SiO2, 1% Al2O3, and 6-7% Na2O.

[0048] In a typical preparation, 15.65 g of colloidal silica (Ludox PX30), 0.96 g of NaOH, and 51 g of water were mixed. 0.80 g of aluminum sulfate hydrate and 5.10 g of 97% by weight of SDA trimethylcyclohexylammonium iodide (TMCHI) were successively added to obtain the following gel components. 60 SiO2:1 Al2O3:6 Na2O:14 TMCHI:2700 H2O

[0049] After stirring at room temperature for 1 hour, the gel was placed in a 125 mL cylinder and heated at 170°C for 5 days while rotating at 30 rpm under self-pressure. Finally, it was cooled, the product was filtered, washed, and dried at 110°C.

[0050] The bulk SAR of the material was measured as 46 by ICP. The weight loss determined by TGA between 200 and 900°C was approximately 10 wt%, which was in good agreement with CHN analysis (total organic content of 9.8 wt%).

[0051] Direct firing of this material in air at 550°C resulted in the detrimental effect of disintegrating the zeolite structure (amorphous, black) while retaining the template, as revealed by TGA and XRD. Alternatively, a heat treatment involving first exposing the material to N2 at 600°C / 2 hours (graded at 2°C / min), followed by air firing at 500°C / 6 hours, resulted in a greater degree of organic matter removal, leaving only 3% carbon with a crystalline structure, as revealed by CHN analysis (XRD shows 86% crystallinity).

[0052] The BET surface area of ​​the zeolite was 13 m² before heat treatment. 2 / g is 19m after heat treatment 2 It was / g. This is 323m for the firing MFI of SAR 2120. 2 It is comparable to / g.

[0053] 1 wt% Pd / zeolite was prepared by initial wet impregnation using a nitrate Pd precursor. It was dried in a drying oven at 100°C / 1 hour, followed by calcination in still air at 500°C for 2 hours (gradient at 2°C / min). The catalyst powder was pelletized, and 0.2 g was tested for CH4 oxidation activity using SCAT.

[0054] SCAT conditions: Simple (artificial) gas mixture - 4000 ppm CH4 and 10% O2 Engine simulation gas mixture - 4000 ppm CH4, 100 ppm C2H6, 35 ppm C3H8, 1000 ppm CO, 500 ppm NO, 7% CO2, 10% O2, 10% H2O+ (2 ppm SO2)

[0055] Impregnation of the above material with 1% Pd using a nitrate Pd precursor showed high activity comparable to the standard reference 1 Pd / MFI (SAR 2120). In particular, this example demonstrated a highly active lean-burn CNG catalyst with high on-stream stability in the presence of water and sulfur and good desulfurization properties at low temperatures.

[0056] The figure shows the on-stream performance of the catalyst in the presence of water, compared to high-silicicity zeolite and amorphous silica.

[0057] As used herein, the singular forms "a," "an," and "the" refer to multiple objects unless the context clearly indicates otherwise.

[0058] The use of the term “comprising” is intended to be interpreted as including such features but not excluding other features, and also intended to include a selection of features that are necessarily limited to those described. In other words, the term also includes the limitations of “essentially consisting of” (intended to mean that certain further components may exist on the condition that they do not substantially affect the essential nature of the described feature) and “consisting of” (intended to mean that if the components are expressed as percentages by their proportions, these together make up 100%, while explaining any unavoidable impurities, but not including any other features).

[0059] As used herein, the term “above” is intended to mean “directly above,” such that there is no intervening layer between one material and another. Spatially relative terms such as “below,” “beneath,” “lower,” “above,” and “upper” may be used herein to facilitate descriptions of the relationship between one element or feature and another. It will be understood that spatially relative terms are intended to encompass different orientations of the catalyst in use or operation, in addition to the orientation shown in the figures.

[0060] All percentages are by weight unless otherwise specified.

[0061] The detailed description above is provided for illustrative and illustrative purposes only and is not intended to limit the scope of the appended claims. Many modifications of the currently preferred embodiments shown herein will be obvious to those skilled in the art and remain within the scope of the appended claims and their equivalents.

[0062] To avoid any ambiguity, the entire contents of all documents found herein are incorporated herein by reference.

Claims

1. A catalyst for the oxidation of methane, wherein the catalyst comprises palladium supported on a zero-dimensional zeolite or Clasrasil, the palladium present in an amount of 0.25 to 4% by weight based on the total weight of the catalyst, the catalyst contains less than 6% by weight of carbon based on the total weight of the catalyst, and the zero-dimensional zeolite or Clasrasil is 100 m ２ A catalyst having a surface area of ​​less than 1g.

2. The catalyst according to claim 1, wherein the palladium is present in an amount of 0.5 to 3% by weight.

3. The catalyst according to claim 1 or claim 2, wherein the palladium is the only platinum group metal present in the catalyst.

4. The zero-dimensional zeolite or the Clasrasil is 30 m ２ Less than 20 m / g, preferably 20 m ２ The catalyst according to any one of claims 1 to 3, having a surface area of ​​less than / g.

5. The catalyst according to any one of claims 1 to 4, wherein the zero-dimensional zeolite or the clathracil has a particle size of less than 500 nm, preferably 1 to 100 nm.

6. The catalyst according to any one of claims 1 to 5, wherein the zero-dimensional zeolite has a NON skeleton.

7. The catalyst according to any one of claims 1 to 6, wherein the zero-dimensional zeolite has a SAR of 5 to 800, preferably 10 to 200, more preferably 40 to 80.

8. The catalyst according to any one of claims 1 to 7, wherein the zero-dimensional zeolite or the clathracil contains less than 3% by weight of carbon, preferably less than 1% by weight of carbon, based on the total weight of the catalyst.

9. A method for producing the catalyst, wherein the zeolite has the NON skeleton structure, and the method is a) Synthesizing a zeolite or clathracil containing an organic structure-directing agent, b) The zeolite or the clasrasil containing the organic structure directing agent is subjected to heat treatment at a temperature exceeding 500°C under an inert gas, and the heat-treated intermediate is then calcined in air at a temperature exceeding 450°C, thereby substantially removing the organic structure directing agent from the zeolite or the clasrasil, and then c) The method according to any one of claims 1 to 8, which preferably comprises depositing palladium on the zeolite or the clathracil by an initial wet impregnation method.

10. A catalyst article comprising a substrate and one or more wash coat layers, wherein at least one of the wash coat layers comprises the catalyst described in any one of claims 1 to 8.

11. An exhaust system comprising the catalyst article described in claim 10.

12. An apparatus comprising an engine and the exhaust system described in claim 11.

13. The apparatus according to claim 12, wherein the engine is a natural gas combustion engine.

14. A method for the oxidation of methane, the method comprising contacting a gas containing methane with a catalyst according to any one of claims 1 to 8.

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