Molecular sieve catalyst as well as preparation method and application thereof

Abstract

The invention discloses a molecular sieve catalyst as well as a preparation method and application thereof, and belongs to the technical field of petrochemical engineering catalysts. The preparation method adopts an etching-modification-confinement integrated strategy, and comprises the following steps: firstly, carrying out controllable etching on a pure silicon Silicalite-1 (S-1) molecular sieve by using a mixed alkali solution of alkali metal hydroxide and ethidene diamine, and in-situ constructing silicon hydroxyl nest defects in a skeleton and remaining atomic-scale dispersed alkali metal; and loading platinum and second metal salt on the etched molecular sieve, drying, calcining and reducing to obtain the final catalyst. The confinement microenvironment is pre-created through the etching step, precise positioning and strong interaction of platinum, second metal and alkali metal are achieved, and the problems that a traditional platinum-based catalyst is difficult in propane initial activation, low in propylene selectivity and poor in stability are effectively solved. The catalyst shows high conversion rate, high selectivity and excellent thermal stability in propane dehydrogenation reaction.

Description

Technical Field

[0001] This invention belongs to the field of petrochemical catalyst technology, specifically relating to a molecular sieve catalyst, its preparation method, and its application. Background Technology

[0002] Propane dehydrogenation is an important pathway to increase the production of high-value-added propylene. Currently, platinum-based catalysts are one of the mainstream catalytic systems for this reaction. However, existing platinum-based catalysts face two major bottlenecks in industrial applications: First, the high bond energy of the first carbon-hydrogen bond in the propane molecule makes initial activation difficult, limiting the intrinsic activity of the catalyst and propane conversion under mild conditions; second, the strong adsorption of the target product propylene at the active sites makes timely desorption difficult, easily leading to continuous deep dehydrogenation side reactions that generate low-carbon byproducts such as ethylene and methane or cause carbon deposition, severely reducing propylene selectivity and catalyst stability.

[0003] To address the aforementioned issues, precise control over the electronic and geometric structure of platinum active centers is crucial. Existing technologies primarily employ methods such as co-impregnation, stepwise loading, or one-pot hydrothermal synthesis to simultaneously introduce platinum along with a second metal (e.g., tin, indium) and an alkali metal (e.g., sodium, potassium) into the catalyst system. The alkali metal component can adjust the electron density of platinum, helping to lower the activation barrier of propane CH bonds; while the second metal component (e.g., tin, indium) can form alloys or interfacial sites with platinum, adjusting its electronic structure and effectively weakening its excessive adsorption of propylene, thereby synergistically enhancing the catalyst's activity and selectivity. However, the aforementioned conventional methods, especially the one-pot encapsulation of multiple components, have significant limitations: First, during the synthesis process, platinum, the second metal, and alkali metal ions may be randomly distributed in the molecular sieve channels or on the surface, making it difficult to precisely control the interaction sites and density among the three, resulting in a non-uniform active site structure; second, alkali metal ions are prone to migration, sintering, or loss during high-temperature treatment or reaction, making it difficult to maintain their stable modification effect for a long time; third, the direct encapsulation method has a weak "confining" effect on the active center, and under harsh reaction conditions, metal nanoparticles are prone to migration and aggregation, affecting catalytic stability.

[0004] Therefore, developing a new method to construct a stable, confined, and hydroxyl-defect-rich active microenvironment in situ, thereby achieving precise, stable, and strong interactions between platinum, second metal, and alkali metal components, is of great significance for overcoming the performance bottlenecks of existing catalysts. Summary of the Invention

[0005] In view of this, the present invention proposes a molecular sieve catalyst, its preparation method and application, to solve the problems of insufficient propane conversion and propylene selectivity in the prior art of platinum-based catalysts.

[0006] The technical solution of this invention is implemented as follows: In a first aspect, the present invention provides a method for preparing a molecular sieve catalyst, characterized by comprising the following steps: S1. Dissolve alkali metal hydroxide and ethylenediamine in deionized water to obtain a mixed alkaline solution; S2. Dissolve the platinum salt and the precursor of the second metal in an aqueous solution to obtain a metal precursor solution; S3. Add the mixed alkaline solution obtained in step S1 to the S-1 molecular sieve under continuous stirring, perform etching treatment, wash, dry and then calcine. S4. Disperse the S-1 molecular sieve obtained in step S3 into the metal precursor solution of step S2, stir, dry, calcine, and roast under a reducing atmosphere to obtain the molecular sieve catalyst.

[0007] Based on the above technical solutions, preferably, in step S1, the molar ratio of alkali metal hydroxide to ethylenediamine is 1:10-10:1. In this invention, the ethylenediamine is preferably used in its monohydrate form. The alkali metal hydroxide provides an alkaline environment, while ethylenediamine acts as an organic base modifier; the combination of the two forms a mixed alkaline solution that can controllably etch pure silicon Silicalite-1 (S-1) molecular sieves. This etching process can construct abundant and uniform silanol nest defects in situ within the molecular sieve framework. These defects serve as anchoring points for subsequent metal loading, providing confined space for platinum and the second metal.

[0008] More preferably, in step S1, the molar ratio of alkali metal hydroxide to ethylenediamine is 2:1.

[0009] More preferably, in step S1, the alkali metal hydroxide is one or more of potassium hydroxide, sodium hydroxide, and lithium hydroxide; and the concentration of the aqueous solution of the alkali metal hydroxide is 50 g / L.

[0010] More preferably, in step S2, the platinum salt is one or more of chloroplatinic acid, platinum chloride, platinum nitrate, and dinitrosodiamineplatinum; and the second metal is one or more of indium, gallium, and tin.

[0011] More preferably, in step S2, the molar ratio of platinum to the second metal is 1:10-10:1; based on the final catalyst mass, the mass fraction of platinum is 0.05%~3%, and the mass fraction of the second metal is 0.05%~3%.

[0012] More preferably, in step S2, the molar ratio of platinum to the second metal is 3:1.

[0013] More preferably, in step S2, the second metal is indium, and its precursor is one or more of indium nitrate hydrate, indium chloride, or indium sulfate; or the second metal is gallium, and its precursor is one or more of gallium nitrate hydrate, gallium chloride, or gallium sulfate; or the second metal is tin, and its precursor is one or more of tin nitrate hydrate, tin chloride, or tin sulfate.

[0014] More preferably, in step S3, the concentration of the S-1 molecular sieve in the mixed alkaline solution is 1-100 g / L.

[0015] More preferably, in step S3, the etching time is 0.5-20 hours; the etching temperature is 20-100℃. During the etching process, alkali metal ions can stably remain on the surface of newly formed hydroxyl pits or channels in an atomically dispersed form. These residual alkali metals not only do not easily migrate or leak during high-temperature processing, but also generate strong electronic interactions with subsequently loaded platinum and the second metal, synergistically regulating the electron density of platinum, reducing the activation energy barrier of propane CH bonds, thereby improving catalyst activity and propylene selectivity.

[0016] More preferably, in step S3, the temperature for continuous stirring is 20-100℃, and the stirring time is 0.5-20 hours; the drying temperature is 40-200℃, and the drying time is 0.5-20 hours; the calcination temperature is 200-500℃, and the calcination time is 0.5-20 hours.

[0017] More preferably, in step S4, the reducing atmosphere is an H2 / Ar mixture with a hydrogen concentration of 1-100 vol%.

[0018] More preferably, in step S4, the stirring time is 0.5-100 hours; the drying temperature is 40-120℃ and the drying time is 0.5-20 hours; the calcination temperature is 400-600℃ and the calcination time is 0.5-20 hours; and the roasting temperature is 400-600℃ and the roasting time is 0.5-20 hours.

[0019] In a second aspect, the present invention provides a molecular sieve catalyst obtained by the preparation method described in the first aspect.

[0020] Thirdly, the present invention provides the application of the molecular sieve catalyst described in the second aspect in the propane dehydrogenation to propylene reaction.

[0021] The molecular sieve catalyst of the present invention has the following advantages over the prior art: (1) By using the integrated strategy of "etching-modification-confinement", abundant and uniform silanol nest defects are constructed in situ in the molecular sieve framework, providing excellent anchoring points for metal loading.

[0022] (2) Precise confinement: Hydroxyl nests provide size confinement and spatial binding for platinum and second metal nanoclusters, which can effectively suppress their high-temperature migration and sintering.

[0023] (3) Synergistic regulation: The pre-embedded and atomically dispersed residual alkali metal can generate sufficient and stable electronic interactions with the subsequently loaded platinum and second metal, thereby achieving synergistic optimization of the electronic structure.

[0024] (4) Structural stability: The metal active center is precisely anchored and confined in the hydroxyl nest defect sites of the molecular sieve framework. Combined with the electronic stabilizing effect of the residual alkali metal, the migration and sintering of the active components are inhibited in a synergistic manner from both spatial confinement and electronic regulation, thereby significantly improving the structural stability of the catalyst during high-temperature reduction and reaction processes. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 The propane conversion and propylene selectivity of the catalysts in Example 1 and Comparative Examples 1-3 of this invention at different temperature points are shown. Detailed Implementation

[0027] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0028] Table 1: Material Source Description Table

[0029] Example 1: A method for preparing a molecular sieve catalyst for propane dehydrogenation to propylene includes the following steps: S1. Dissolve 1 g of sodium hydroxide and 0.9764 g of ethylenediamine monohydrate in 20 mL of deionized water to prepare a mixed alkaline solution. The molar ratio of NaOH to ethylenediamine is 2:1.

[0030] S2. Dissolve 1 g of chloroplatinic acid hexahydrate and 0.1936 g of indium nitrate hydrate in 50 mL of deionized water to prepare a metal precursor solution with a platinum:indium molar ratio of 3:1.

[0031] S3. The mixed alkali obtained in step S2 is heated to 80°C, and 1.6 g of Silicalite-1 (S-1) molecular sieve is dispersed into it under continuous stirring. The concentration of S-1 molecular sieve in the mixed alkali solution is 80 g / L. Etching is performed for 10 hours; after washing, it is dried at 60°C for 10 hours, and then calcined in a muffle furnace at 300°C for 3 hours.

[0032] S4. Take 0.252 mL of the metal precursor solution obtained in step S2 and disperse it in 5 mL of deionized water. Then, place the molecular sieve powder obtained in step S3 into the solution and stir for 5 hours. After that, dry it under vacuum at 60°C for 5 hours, and finally calcine it in a muffle furnace at 550°C for 6 hours. The resulting powder is then calcined in a tube furnace under a 10 vol% H2 / Ar atmosphere, from room temperature to 500°C for 1 hour. The molecular sieve catalyst is thus obtained.

[0033] Examples 2-3: Unlike Example 1, the second metal salt in Example 2 was 0.1677 g of tin tetrachloride, with a platinum:tin molar ratio of 3:1; the second metal salt in Example 3 was 0.8063 g of gallium nitrate nonahydrate, with a platinum:gallium molar ratio of 1:1. The rest was the same as in Example 1, and will not be repeated here.

[0034] Example 4: A method for preparing a molecular sieve catalyst for propane dehydrogenation to propylene includes the following steps: S1. Dissolve 1 g of potassium hydroxide and 0.6960 g of ethylenediamine monohydrate in 20 mL of deionized water to prepare a mixed alkaline solution. The molar ratio of KOH to ethylenediamine monohydrate is 2:1.

[0035] S2. Dissolve 1 g of platinum chloride hexahydrate and 0.1424 g of indium chloride in 50 mL of deionized water to prepare a metal precursor solution with a platinum:indium molar ratio of 3:1.

[0036] S3. The mixed alkali obtained in step S2 is heated to 100°C, and 1.2 g of Silicalite-1 (S-1) molecular sieve is dispersed into it under continuous stirring. The concentration of S-1 molecular sieve in the mixed alkali solution is 60 g / L. Etching is performed for 20 hours; after washing, it is dried at 200°C for 0.5 hours, and then calcined in a muffle furnace at 500°C for 0.5 hours.

[0037] S4. Take 0.252 mL of the metal precursor solution obtained in step S2 and disperse it in 5 mL of deionized water. Then, place the molecular sieve powder obtained in step S3 into the solution and stir for 100 hours. After that, vacuum dry at 120°C for 0.5 hours, and finally calcine at 600°C for 0.5 hours in a muffle furnace. The resulting powder is then calcined in a tube furnace under a 20 vol% H2 / Ar atmosphere, from room temperature to 600°C for 0.5 hours. The molecular sieve catalyst is thus obtained.

[0038] Example 5: A method for preparing a molecular sieve catalyst for propane dehydrogenation to propylene includes the following steps: S1. Prepare a mixed alkaline solution by dissolving 1 g of lithium hydroxide and 1.6307 g of ethylenediamine monohydrate in 20 mL of deionized water, with a molar ratio of LiOH to ethylenediamine of 2:1.

[0039] S2. Dissolve 1 g of dinitrosodiammineplatinum and 0.3333 g of indium sulfate in 50 mL of deionized water to prepare a metal precursor solution with a platinum:indium molar ratio of 3:1.

[0040] S3. The mixed alkali obtained in step S2 is heated to 60°C, and 2.0 g of Silicalite-1 (S-1) molecular sieve is dispersed into it under continuous stirring. The concentration of S-1 molecular sieve in the mixed alkali solution is 100 g / L. Etching is performed for 0.5 hours; after washing, it is dried at 40°C for 20 hours, and then calcined in a muffle furnace at 200°C for 20 hours.

[0041] S4. Take 0.252 mL of the metal precursor solution obtained in step S2 and disperse it in 5 mL of deionized water. Then, add the molecular sieve powder obtained in step S3 and stir for 0.5 hours. Dry it under vacuum at 40°C for 20 hours, and finally calcine it in a muffle furnace at 400°C for 20 hours. The resulting powder is then calcined in a tube furnace under a 30 vol% H2 / Ar atmosphere, from room temperature to 400°C for 20 hours. The molecular sieve catalyst is thus obtained.

[0042] Examples 6-9: Unlike Example 1, the molar ratios of NaOH to ethylenediamine monohydrate were 1:11, 1:10, 10:1, and 11:1, respectively, and the amount of NaOH added was fixed at 1 g in all examples. The rest were the same as in Example 1 and will not be repeated here.

[0043] Comparative Example 1: Unlike Example 1, Comparative Example 1 does not use a second metal (only platinum is used), but otherwise it is the same as Example 1, and will not be repeated here.

[0044] Comparative Example 2: Unlike Example 1, Comparative Example 2 did not have an alkali metal etching step (platinum and indium were directly loaded onto the S-1 molecular sieve), and the rest was the same as in Example 1, so it will not be described again here.

[0045] Comparative Example 3: Unlike Example 1, Comparative Example 3 did not have a second metal or an alkali metal etching step (it only directly loaded platinum onto the S-1 molecular sieve), and the rest was the same as Example 1, so it will not be repeated here.

[0046] The catalysts obtained in Examples 1-9 and Comparative Examples 1-3 were subjected to propylene dehydrogenation to ethylene activity tests. The catalytic activity experiments were conducted in a fixed-bed reactor with an initial gas concentration of 100 vol% propane and a mass hourly space velocity (MSV) of 21.55 / h based on the propane mass (main test conditions: catalyst mass 100 mg, propane flow rate 20 mL / min). The activity test temperatures were 500 / 550 / 600 °C, and the stability test temperature was 550 °C.

[0047] The test results are shown in Table 2 and Figure 1 As shown.

[0048] As shown in Table 2, the catalyst prepared in Example 1 of this invention exhibits the highest propylene yield at 500 / 550 / 600 degrees Celsius. Compared to the catalysts of Examples 2 and 3, it is evident that indium optimizes the regulation of the platinum electronic structure compared to tin and gallium; compared to the catalysts of Examples 4 and 5, it is evident that sodium hydroxide optimizes the regulation of the platinum electronic structure compared to lithium hydroxide and sodium hydroxide; comparative analysis of the catalysts of Example 1 and Comparative Examples 1-3 shows that the introduction of an alkali metal alone can promote the improvement of propane conversion, the introduction of a second metal alone can promote the improvement of propylene selectivity, while the simultaneous introduction of an alkali metal and a second metal can synergistically optimize the carbon-hydrogen bond activation and propylene adsorption capacity, promoting different improvements in propane conversion and propylene selectivity.

[0049] Examples 6 and 9 are examples where the molar ratio of NaOH to ethylenediamine monohydrate is greater than and less than the scope of this invention, respectively. Comparison of Examples 6-9 with Comparative Example 2 shows that when the molar ratio of NaOH to ethylenediamine monohydrate reaches 1:10, the improvement in propane conversion is relatively limited compared to the treatment without mixed alkali. When the molar ratio reaches 1:11, the propane conversion is essentially not improved, because the low NaOH concentration makes it difficult to construct defects. When the molar ratio of NaOH to ethylenediamine monohydrate reaches 10:1, the improvement in propane conversion is also relatively limited compared to the treatment without mixed alkali. When the molar ratio reaches 11:1, the propane conversion even decreases. This is because an excessively high NaOH concentration leads to over-etching of the S-1 molecular sieve, resulting in a significant decrease in the hydroxyl group content.

[0050] Depend on Figure 1 (Test conditions: catalyst mass is 100 mg, propane flow rate is 20 mL / min) It can be seen that the catalyst prepared in Example 1 of the present invention has good catalytic stability for propane dehydrogenation at 550 °C.

[0051] Table 2: Propane conversion and propylene selectivity at different temperatures

[0052] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a molecular sieve catalyst, characterized in that, Includes the following steps: S1. Dissolve alkali metal hydroxide and ethylenediamine in deionized water to obtain a mixed alkaline solution; S2. Dissolve the platinum salt and the precursor of the second metal in an aqueous solution to obtain a metal precursor solution; S3. Add the mixed alkaline solution obtained in step S1 to the S-1 molecular sieve under continuous stirring, perform etching treatment, wash, dry and then calcine. S4. Disperse the S-1 molecular sieve obtained in step S3 into the metal precursor solution of step S2, stir, dry, calcine, and roast under a reducing atmosphere to obtain the molecular sieve catalyst.

2. The preparation method according to claim 1, characterized in that, In step S1, the molar ratio of alkali metal hydroxide to ethylenediamine is 1:10-10:

1.

3. The preparation method according to claim 1, characterized in that, In step S2, the platinum salt is one or more of chloroplatinic acid, platinum chloride, platinum nitrate, and dinitrosodiammineplatinum; the second metal is one or more of indium, gallium, and tin.

4. The preparation method according to claim 3, characterized in that, In step S2, the molar ratio of platinum to the second metal is 1:10-10:

1.

5. The preparation method according to claim 3, characterized in that, In step S2, the second metal is indium, and its precursor is one or more of indium nitrate hydrate, indium chloride, or indium sulfate; or the second metal is gallium, and its precursor is one or more of gallium nitrate hydrate, gallium chloride, or gallium sulfate; or the second metal is tin, and its precursor is one or more of tin nitrate hydrate, tin chloride, or tin sulfate.

6. The preparation method according to claim 1, characterized in that, In step S3, the concentration of the S-1 molecular sieve in the mixed alkaline solution is 1-100 g / L.

7. The preparation method according to claim 1, characterized in that, In step S3, the etching time is 0.5-20 hours; the etching temperature is 20-100℃.

8. The preparation method according to claim 1, characterized in that, In step S4, the reducing atmosphere is an H2 / Ar mixture with a hydrogen concentration of 1-100 vol%.

9. A molecular sieve catalyst prepared by any one of the preparation methods described in claims 1-8.

10. The application of the molecular sieve catalyst as described in claim 9 in the propane dehydrogenation to propylene reaction.

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