A catalyst for preparing melamine and a preparation method thereof

Abstract

The present invention discloses a method for preparing a catalyst for preparing melamine, comprising the following steps: S1: adding water glass to deionized water, then adding an aluminum salt solution, and adding an ammonium citrate solution while stirring until the pH is weakly alkaline to obtain a mixture I; S2: placing the mixture I on an extruder to form strips, followed by drying, roasting, grinding, and sieving to obtain a granular carrier; S3: dissolving a soluble salt containing platinum and magnesium in deionized water to obtain a mixed solution II, adding an aluminum chloride solution to the mixed solution II until the pH of the mixed solution II is 5-6, to prepare an impregnation solution; S4: placing the granular carrier in the impregnation solution, followed by filtering, drying, and roasting to obtain a catalyst. The catalyst prepared by this method has high melamine selectivity, a low content of the byproduct biuret, good wear resistance, and good stability.

Description

Technical Field

[0001] The present invention relates to the technical field of melamine production technology, and in particular to a catalyst for preparing melamine and a preparation method thereof. Background Art

[0002] Melamine is an important nitrogen-heterocyclic organic chemical intermediate. As a thermosetting resin, it exhibits heat resistance, aging resistance, acid and alkali resistance, and flame retardancy. It is widely used in industries such as wood processing, coatings, papermaking, textiles, leather, decorative panels, laminates, composite flooring, flame retardants, water reducers, adhesives, and melamine molding powder. Currently, melamine production mostly relies on the urea process, in which urea is added to a reactor and reacts with a catalyst (silica gel, alumina, or silica-alumina gel) in a fluidized contact reaction. This endothermic reaction is circulated and heated using a molten salt carrier, causing the urea to vaporize and decompose in the reactor to produce melamine, carbon dioxide, and ammonia.

[0003] The conventional and low-pressure melamine production processes, using urea as a raw material, place very stringent requirements on catalysts, requiring them to possess high strength, high activity, high loading capacity, and low price. The selection and performance of the catalyst directly impacts the production capacity, output quality, raw material consumption, and operating cycle of the entire melamine production plant. BASF's melamine technology abroad uses γ-Al2O3 as a catalyst, while the vast majority of melamine catalysts in China use coarse-porous silica gel. In the mid-to-late 1990s, some domestic melamine plants began using aluminum silicate catalysts.

[0004] Patent CN202111085647.5 discloses a catalyst for synthesizing melamine. The catalyst uses sodium silicate and sodium metaaluminate as raw materials. A silica-alumina melamine catalyst with an appropriate silica-alumina content is prepared by preparing silica-alumina, mixing and beating, spray granulation, roasting, and cooling and screening. The catalyst prepared by this method has a moderate ratio of SiO2 and Al2O3, and the catalyst surface has an appropriate acid strength and acid density. It has high activity and high selectivity for the vapor phase synthesis of melamine from urea, and the urea consumption is reduced to below 3.0t / t melamine. However, it was found that the content of the by-product biuret in the process of synthesizing melamine by this method needs to be further reduced, which will affect the production of melamine.

[0005] Patent CN201911041889.7 discloses a catalyst for synthesizing melamine and its preparation method. The catalyst contains 30-55 wt% Al2O3, 45-70 wt% SiO2, and 0-10 wt% P2O5. The Al2O3 consists of two components: one serving as the active component and the other as a binder. The SiO2 also consists of two components: one serving as a carrier for the active component Al2O3 and the other serving as a binder. The P2O5 also serves as a binder. The catalyst is prepared using the following method: the SiO2 carrier is slurried with water, the active component Al2O3 is added, an acid solution is added to adjust the pH, and finally a binder is added. The catalyst is spray granulated and calcined, washed, and dried to produce the melamine catalyst. The catalyst disclosed in this patent exhibits good initial activity, but this activity decreases significantly with age. Therefore, the catalyst's stability needs to be further improved.

[0006] Therefore, it is important to develop a catalyst with relatively stable activity and good performance in the process of preparing melamine in practical industrial applications. Summary of the Invention

[0007] In order to solve the problems in the prior art of high content of biuret as a byproduct and poor stability of the catalyst during the preparation of melamine, a catalyst for preparing melamine and a preparation method thereof are provided.

[0008] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0009] A method for preparing a catalyst for preparing melamine, comprising the following steps:

[0010] S1: adding water glass to deionized water, then adding aluminum salt solution, and adding ammonium citrate solution while stirring until the pH is weakly alkaline to obtain mixture I;

[0011] S2: placing the mixture I on an extruder to form extrusion strips, followed by drying, roasting, grinding, and sieving to obtain a granular carrier;

[0012] S3: dissolving a soluble salt containing platinum and magnesium in deionized water to obtain a mixed solution II, and adding an aluminum chloride solution to the mixed solution II until the pH of the mixed solution II is 5 to 6 to prepare an impregnation solution;

[0013] S4: placing the granular carrier in the impregnation solution, followed by filtering, drying, and calcining to obtain the catalyst.

[0014] Furthermore, in step S1, the concentration of water glass is 40 degrees Baume, and the volume ratio of water glass to deionized water is 1:1-3.

[0015] Furthermore, the aluminum salt solution in step S1 is an aluminum nitrate solution, the concentration of the aluminum nitrate solution is 20-30 g / L, and the volume ratio of the aluminum nitrate solution to water glass is 0.5:1.

[0016] Furthermore, the pH value after adding ammonium citrate in step S1 is 7.5-8.

[0017] Furthermore, in step S2, the mixture I is evenly mixed with the sesbania powder before being put into the extruder, and the mass ratio of the mixture I to the sesbania powder is 10:1.

[0018] Furthermore, in step S2, the drying condition is 50-65°C for 24-48 hours; and the roasting condition is a heating rate of 15°C / min to 550°C for 4-6 hours.

[0019] Furthermore, in step S2, the granular carrier is obtained by grinding and then passing through a 30-mesh sieve.

[0020] Furthermore, the soluble salt containing platinum and magnesium in step S3 is nitrate, the molar ratio of metal platinum to magnesium in the soluble salt containing platinum and magnesium is 0.01-0.05:1, and the mass of the soluble salt containing platinum and magnesium is 10% of the mass of the granular carrier.

[0021] Furthermore, in step S4, the volume ratio of the impregnation liquid to the granular carrier is 2.5 to 1:1, and the calcination condition is calcination at 550° C. for 3 to 5 hours in air.

[0022] A catalyst for preparing melamine obtained by any of the above preparation methods.

[0023] The present invention has the following beneficial effects:

[0024] 1. The present invention provides a catalyst for preparing melamine. Metal salts containing platinum and magnesium are added to the catalyst during the preparation process. While ensuring high catalytic activity of melamine, the content of the by-product biuret is reduced, resulting in a lower percentage of biuret in the product, which is beneficial to the subsequent purification of melamine.

[0025] 2. The present invention provides a catalyst for preparing melamine. By controlling the ratio of raw material components and strictly controlling the pH value during the preparation process, the prepared catalyst has a low wear index, can well adapt to the reaction conditions of preparing melamine from urea, improves the stability of the catalyst, and thus extends the service life of the catalyst.

[0026] 3. The present invention provides a catalyst for preparing melamine, which has good catalytic performance and wear resistance and is suitable for popularization and application. DETAILED DESCRIPTION

[0027] The technical solution of the present invention will be clearly and completely described below in conjunction with specific embodiments.

[0028] Example 1

[0029] S1: adding deionized water to water glass having a concentration of 40 degrees Baume, with the volume ratio of water glass to deionized water being 1:2, and then adding a prepared aluminum nitrate solution having a concentration of 30 g / L, with the volume ratio of the aluminum nitrate solution to the water glass being 0.5:1. During the stirring process, adding ammonium citrate solution until the pH is between 7.5 and 8, to obtain a mixture I;

[0030] S2: Mixture I and sesbania powder were mixed in a mass ratio of 10:1 and then placed on an extruder to form strips. The mixture was then dried in an oven at 50-65°C for 48 hours. The mixture was then placed in a muffle furnace and calcined at a heating rate of 15°C / min to 550°C for 4 hours. The calcined sample was taken out, ground, and passed through a 30-mesh sieve to obtain a granular carrier.

[0031] S3: dissolving a nitrate containing platinum and magnesium in deionized water to obtain a mixed solution II, wherein the molar ratio of metallic platinum to magnesium in the nitrate containing platinum and magnesium is 0.03:1, and the total mass of the nitrate containing platinum and magnesium is 10% of the mass of the granular support. Then, adding aluminum chloride solution to the mixed solution II until the pH of the mixed solution II is 5-6, thereby preparing an impregnation solution;

[0032] S4: Place the granular carrier in the impregnation liquid, and impregnate for 12-24 hours according to the volume ratio of the impregnation liquid to the granular carrier of 2:1. Then filter until there is no chloride ion in the filtrate, dry it in an oven at 65°C for 24 hours, and then place it in a muffle furnace and roast it at 550°C in an air atmosphere for 4 hours. After cooling to room temperature, continue to pass it through a 30-mesh sieve to remove samples with small particles to obtain a finished catalyst.

[0033] Example 2

[0034] S1: adding deionized water to water glass having a concentration of 40 degrees Baume, with the volume ratio of water glass to deionized water being 1:1, and then adding a prepared aluminum nitrate solution having a concentration of 30 g / L, with the volume ratio of the aluminum nitrate solution to the water glass being 0.5:1. During the stirring process, adding ammonium citrate solution until the pH is between 7.5 and 8, to obtain a mixture I;

[0035] S2: Mixture I and sesbania powder were mixed in a mass ratio of 10:1 and then placed on an extruder to form strips. The mixture was then dried in an oven at 50-65°C for 48 hours. The mixture was then placed in a muffle furnace and calcined at a heating rate of 15°C / min to 550°C for 4 hours. The calcined sample was taken out, ground, and passed through a 30-mesh sieve to obtain a granular carrier.

[0036] S3: dissolving a nitrate containing platinum and magnesium in deionized water to obtain a mixed solution II, wherein the molar ratio of metallic platinum to magnesium in the nitrate containing platinum and magnesium is 0.01:1, and the total mass of the nitrate containing platinum and magnesium is 10% of the mass of the granular support, and then adding an aluminum chloride solution to the mixed solution II until the pH of the mixed solution II is 5-6 to prepare an impregnation solution;

[0037] S4: Place the granular carrier in the impregnation liquid, and impregnate for 12-24 hours according to the volume ratio of the impregnation liquid to the granular carrier of 2:1. Then filter until there is no chloride ion in the filtrate, dry it in an oven at 65°C for 24 hours, and then place it in a muffle furnace and roast it at 550°C in an air atmosphere for 4 hours. After cooling to room temperature, continue to pass it through a 30-mesh sieve to remove samples with small particles to obtain a finished catalyst.

[0038] Example 3

[0039] S1: adding deionized water to water glass having a concentration of 40 degrees Baume, with the volume ratio of water glass to deionized water being 1:2, and then adding a prepared aluminum nitrate solution having a concentration of 30 g / L, with the volume ratio of the aluminum nitrate solution to the water glass being 0.5:3. During the stirring process, adding ammonium citrate solution until the pH is between 7.5 and 8, to obtain a mixture I;

[0040] S2: Mixture I and sesbania powder were mixed in a mass ratio of 10:1 and then placed on an extruder to form strips. The mixture was then dried in an oven at 50-65°C for 48 hours. The mixture was then placed in a muffle furnace and calcined at a heating rate of 15°C / min to 550°C for 4 hours. The calcined sample was taken out, ground, and passed through a 30-mesh sieve to obtain a granular carrier.

[0041] S3: dissolving a nitrate containing platinum and magnesium in deionized water to obtain a mixed solution II, wherein the molar ratio of metallic platinum to magnesium in the nitrate containing platinum and magnesium is 0.05:1, and the total mass of the nitrate containing platinum and magnesium is 10% of the mass of the granular support. Then, adding aluminum chloride solution to the mixed solution II until the pH of the mixed solution II is 5-6, thereby preparing an impregnation solution;

[0042] S4: Place the granular carrier in the impregnation liquid, and impregnate for 12-24 hours according to the volume ratio of the impregnation liquid to the granular carrier of 2:1. Then filter until there is no chloride ion in the filtrate, dry it in an oven at 65°C for 24 hours, and then place it in a muffle furnace and roast it at 550°C in an air atmosphere for 4 hours. After cooling to room temperature, continue to pass it through a 30-mesh sieve to remove samples with small particles to obtain a finished catalyst.

[0043] Example 4

[0044] S1: adding deionized water to water glass having a concentration of 40 degrees Baume, with the volume ratio of water glass to deionized water being 1:2, and then adding a prepared aluminum nitrate solution having a concentration of 30 g / L, with the volume ratio of the aluminum nitrate solution to the water glass being 0.5:1. During the stirring process, adding ammonium citrate solution until the pH is between 7.5 and 8, to obtain a mixture I;

[0045] S2: Mixture I and sesbania powder were mixed in a mass ratio of 10:1 and then placed on an extruder to form strips. The mixture was then dried in an oven at 50-65°C for 48 hours. The mixture was then placed in a muffle furnace and calcined at a heating rate of 15°C / min to 550°C for 4 hours. The calcined sample was taken out, ground, and passed through a 30-mesh sieve to obtain a granular carrier.

[0046] S3: dissolving a nitrate containing platinum and magnesium in deionized water to obtain a mixed solution II, wherein the molar ratio of metallic platinum to magnesium in the nitrate containing platinum and magnesium is 0.03:1, and the total mass of the nitrate containing platinum and magnesium is 10% of the mass of the granular support. Then, adding aluminum chloride solution to the mixed solution II until the pH of the mixed solution II is 5-6, thereby preparing an impregnation solution;

[0047] S4: Place the granular carrier in the impregnation liquid, and impregnate for 12-24 hours according to the volume ratio of the impregnation liquid to the granular carrier of 1:1. Then filter until there is no chloride ion in the filtrate, dry it in an oven at 65°C for 24 hours, and then place it in a muffle furnace and roast it at 550°C in an air atmosphere for 4 hours. After cooling to room temperature, continue to pass it through a 30-mesh sieve to remove samples with small particles to obtain a finished catalyst.

[0048] Example 5

[0049] S1: adding deionized water to water glass having a concentration of 40 degrees Baume, with the volume ratio of water glass to deionized water being 1:2, and then adding a prepared aluminum nitrate solution having a concentration of 30 g / L, with the volume ratio of the aluminum nitrate solution to the water glass being 0.5:1. During the stirring process, adding ammonium citrate solution until the pH is between 7.5 and 8, to obtain a mixture I;

[0050] S2: Mixture I and sesbania powder were mixed in a mass ratio of 10:1 and then placed on an extruder to form strips. The mixture was then dried in an oven at 50-65°C for 48 hours. The mixture was then placed in a muffle furnace and calcined at a heating rate of 15°C / min to 550°C for 4 hours. The calcined sample was taken out, ground, and passed through a 30-mesh sieve to obtain a granular carrier.

[0051] S3: dissolving a nitrate containing platinum and magnesium in deionized water to obtain a mixed solution II, wherein the molar ratio of metallic platinum to magnesium in the nitrate containing platinum and magnesium is 0.03:1, and the total mass of the nitrate containing platinum and magnesium is 10% of the mass of the granular support. Then, adding aluminum chloride solution to the mixed solution II until the pH of the mixed solution II is 5-6, thereby preparing an impregnation solution;

[0052] S4: Place the granular carrier in the impregnation liquid, and impregnate for 12-24 hours according to the volume ratio of the impregnation liquid to the granular carrier of 2:1. Then filter until there is no chloride ion in the filtrate, dry it in an oven at 65°C for 24 hours, and then place it in a muffle furnace and roast it at 550°C in an air atmosphere for 4 hours. After cooling to room temperature, continue to pass it through a 30-mesh sieve to remove samples with small particles to obtain a finished catalyst.

[0053] Comparative Example 1

[0054] S1: adding deionized water to water glass having a concentration of 40 degrees Baume, with the volume ratio of water glass to deionized water being 1:2, and then adding a prepared aluminum nitrate solution having a concentration of 30 g / L, with the volume ratio of the aluminum nitrate solution to the water glass being 1:1. During the stirring process, ammonium citrate solution was added until the pH was between 7.5 and 8, to obtain a mixture I;

[0055] S2: Mixture I and sesbania powder were mixed in a mass ratio of 10:1 and then placed on an extruder to form strips. The mixture was then dried in an oven at 50-65°C for 48 hours. The mixture was then placed in a muffle furnace and calcined at a heating rate of 15°C / min to 550°C for 4 hours. The calcined sample was taken out, ground, and passed through a 30-mesh sieve to obtain a granular carrier.

[0056] S3: dissolving a nitrate containing platinum and magnesium in deionized water to obtain a mixed solution II, wherein the molar ratio of metallic platinum to magnesium in the nitrate containing platinum and magnesium is 0.03:1, and the total mass of the nitrate containing platinum and magnesium is 10% of the mass of the granular support. Then, adding aluminum chloride solution to the mixed solution II until the pH of the mixed solution II is 5-6, thereby preparing an impregnation solution;

[0057] S4: Place the granular carrier in the impregnation liquid, and impregnate for 12-24 hours according to the volume ratio of the impregnation liquid to the granular carrier of 2:1. Then filter until there is no chloride ion in the filtrate, dry it in an oven at 65°C for 24 hours, and then place it in a muffle furnace and roast it at 550°C in an air atmosphere for 4 hours. After cooling to room temperature, continue to pass it through a 30-mesh sieve to remove samples with small particles to obtain a finished catalyst.

[0058] Comparative Example 2

[0059] S1: adding deionized water to water glass having a concentration of 40 degrees Baume, with the volume ratio of water glass to deionized water being 1:2, and then adding a prepared aluminum nitrate solution having a concentration of 30 g / L, with the volume ratio of the aluminum nitrate solution to the water glass being 0.5:1. During stirring, adding ammonium citrate solution until the pH reaches 9.5, to obtain a mixture I;

[0060] S2: Mixture I and sesbania powder were mixed in a mass ratio of 10:1 and then placed on an extruder to form strips. The mixture was then dried in an oven at 50-65°C for 48 hours. The mixture was then placed in a muffle furnace and calcined at a heating rate of 15°C / min to 550°C for 4 hours. The calcined sample was taken out, ground, and passed through a 30-mesh sieve to obtain a granular carrier.

[0061] S3: dissolving a nitrate containing platinum and magnesium in deionized water to obtain a mixed solution II, wherein the molar ratio of metallic platinum to magnesium in the nitrate containing platinum and magnesium is 0.03:1, and the total mass of the nitrate containing platinum and magnesium is 10% of the mass of the granular support. Then, adding aluminum chloride solution to the mixed solution II until the pH of the mixed solution II is 5-6, thereby preparing an impregnation solution;

[0062] S4: Place the granular carrier in the impregnation liquid, and impregnate for 12-24 hours according to the volume ratio of the impregnation liquid to the granular carrier of 2:1. Then filter until there is no chloride ion in the filtrate, dry it in an oven at 65°C for 24 hours, and then place it in a muffle furnace and roast it at 550°C in an air atmosphere for 4 hours. After cooling to room temperature, continue to pass it through a 30-mesh sieve to remove samples with small particles to obtain a finished catalyst.

[0063] Comparative Example 3

[0064] S1: adding deionized water to water glass having a concentration of 40 degrees Baume, with the volume ratio of water glass to deionized water being 1:4, and then adding a prepared aluminum nitrate solution having a concentration of 30 g / L, with the volume ratio of the aluminum nitrate solution to the water glass being 0.5:1. During the stirring process, adding ammonium citrate solution until the pH is between 7.5 and 8, to obtain a mixture I;

[0065] S2: Mixture I and sesbania powder were mixed in a mass ratio of 10:1 and then placed on an extruder to form strips. The mixture was then dried in an oven at 50-65°C for 48 hours. The mixture was then placed in a muffle furnace and calcined at a heating rate of 15°C / min to 550°C for 4 hours. The calcined sample was taken out, ground, and passed through a 30-mesh sieve to obtain a granular carrier.

[0066] S3: dissolving a nitrate containing platinum and magnesium in deionized water to obtain a mixed solution II, wherein the molar ratio of metallic platinum to magnesium in the nitrate containing platinum and magnesium is 0.03:1, and the total mass of the nitrate containing platinum and magnesium is 10% of the mass of the granular support. Then, adding aluminum chloride solution to the mixed solution II until the pH of the mixed solution II is 5-6, thereby preparing an impregnation solution;

[0067] S4: Place the granular carrier in the impregnation liquid, and impregnate for 12-24 hours according to the volume ratio of the impregnation liquid to the granular carrier of 2:1. Then filter until there is no chloride ion in the filtrate, dry it in an oven at 65°C for 24 hours, and then place it in a muffle furnace and roast it at 550°C in an air atmosphere for 4 hours. After cooling to room temperature, continue to pass it through a 30-mesh sieve to remove samples with small particles to obtain a finished catalyst.

[0068] Comparative Example 4

[0069] S1: adding deionized water to water glass having a concentration of 40 degrees Baume, with the volume ratio of water glass to deionized water being 1:2, and then adding a prepared aluminum nitrate solution having a concentration of 30 g / L, with the volume ratio of the aluminum nitrate solution to the water glass being 0.5:1, and stirring to obtain a mixture I;

[0070] S2: Mixture I and sesbania powder were mixed in a mass ratio of 10:1 and then placed on an extruder to form strips. The mixture was then dried in an oven at 50-65°C for 48 hours. The mixture was then placed in a muffle furnace and calcined at a heating rate of 15°C / min to 550°C for 4 hours. The calcined sample was taken out, ground, and passed through a 30-mesh sieve to obtain a granular carrier.

[0071] S3: dissolving a nitrate containing platinum and magnesium in deionized water to obtain a mixed solution II, wherein the molar ratio of metallic platinum to magnesium in the nitrate containing platinum and magnesium is 0.03:1, and the total mass of the nitrate containing platinum and magnesium is 10% of the mass of the granular support. Then, adding aluminum chloride solution to the mixed solution II until the pH of the mixed solution II is 5-6, thereby preparing an impregnation solution;

[0072] S4: Place the granular carrier in the impregnation liquid, and impregnate for 12-24 hours according to the volume ratio of the impregnation liquid to the granular carrier of 2:1. Then filter until there is no chloride ion in the filtrate, dry it in an oven at 65°C for 24 hours, and then place it in a muffle furnace and roast it at 550°C in an air atmosphere for 4 hours. After cooling to room temperature, continue to pass it through a 30-mesh sieve to remove samples with small particles to obtain a finished catalyst.

[0073] Comparative Example 5

[0074] S1: adding deionized water to water glass having a concentration of 40 degrees Baume, with the volume ratio of water glass to deionized water being 1:2, and then adding a prepared aluminum nitrate solution having a concentration of 30 g / L, with the volume ratio of the aluminum nitrate solution to the water glass being 0.5:1. During the stirring process, adding ammonium citrate solution until the pH is between 7.5 and 8, to obtain a mixture I;

[0075] S2: Mixture I and sesbania powder were mixed in a mass ratio of 10:1 and then placed on an extruder to form strips. The mixture was then dried in an oven at 50-65°C for 48 hours. The mixture was then placed in a muffle furnace and calcined at a heating rate of 15°C / min to 550°C for 4 hours. The calcined sample was taken out, ground, and passed through a 30-mesh sieve to obtain a granular carrier.

[0076] S3: dissolving a magnesium-containing nitrate in deionized water to obtain a mixed solution II, wherein the mass of the magnesium-containing nitrate is 10% of the mass of the granular support, and then adding an aluminum chloride solution to the mixed solution II until the pH of the mixed solution II is 5-6 to prepare an impregnation solution;

[0077] S4: Place the granular carrier in the impregnation liquid, and impregnate for 12-24 hours according to the volume ratio of the impregnation liquid to the granular carrier of 2:1. Then filter until there is no chloride ion in the filtrate, dry it in an oven at 65°C for 24 hours, and then place it in a muffle furnace and roast it at 550°C in an air atmosphere for 4 hours. After cooling to room temperature, continue to pass it through a 30-mesh sieve to remove samples with small particles to obtain a finished catalyst.

[0078] Comparative Example 6

[0079] S1: adding deionized water to water glass having a concentration of 40 degrees Baume, with the volume ratio of water glass to deionized water being 1:2, and then adding a prepared aluminum nitrate solution having a concentration of 30 g / L, with the volume ratio of the aluminum nitrate solution to the water glass being 0.5:1. During the stirring process, adding ammonium citrate solution until the pH is between 7.5 and 8, to obtain a mixture I;

[0080] S2: Mixture I and sesbania powder were mixed in a mass ratio of 10:1 and then placed on an extruder to form strips. The mixture was then dried in an oven at 50-65°C for 48 hours. The mixture was then placed in a muffle furnace and calcined at a heating rate of 15°C / min to 550°C for 4 hours. The calcined sample was taken out, ground, and passed through a 30-mesh sieve to obtain a granular carrier.

[0081] S3: dissolving a nitrate containing platinum and magnesium in deionized water to obtain a mixed solution II, wherein the molar ratio of metallic platinum to magnesium in the nitrate containing platinum and magnesium is 0.05:10, and the total mass of the nitrate containing platinum and magnesium is 10% of the mass of the granular support. Then, adding aluminum chloride solution to the mixed solution II until the pH of the mixed solution II is 5-6, thereby preparing an impregnation solution;

[0082] S4: Place the granular carrier in the impregnation liquid, and impregnate for 12-24 hours according to the volume ratio of the impregnation liquid to the granular carrier of 2:1. Then filter until there is no chloride ion in the filtrate, dry it in an oven at 65°C for 24 hours, and then place it in a muffle furnace and roast it at 550°C in an air atmosphere for 4 hours. After cooling to room temperature, continue to pass it through a 30-mesh sieve to remove samples with small particles to obtain a finished catalyst.

[0083] Comparative Example 7

[0084] S1: adding deionized water to water glass having a concentration of 40 degrees Baume, with the volume ratio of water glass to deionized water being 1:2, and then adding a prepared aluminum nitrate solution having a concentration of 30 g / L, with the volume ratio of the aluminum nitrate solution to the water glass being 0.5:1. During the stirring process, adding ammonium citrate solution until the pH is between 7.5 and 8, to obtain a mixture I;

[0085] S2: Mixture I and sesbania powder were mixed in a mass ratio of 10:1 and then placed on an extruder to form strips. The mixture was then dried in an oven at 50-65°C for 48 hours. The mixture was then placed in a muffle furnace and calcined at a heating rate of 15°C / min to 550°C for 4 hours. The calcined sample was taken out, ground, and passed through a 30-mesh sieve to obtain a granular carrier.

[0086] S3: dissolving a nitrate containing platinum and magnesium in deionized water to obtain a mixed solution II, wherein the molar ratio of metallic platinum to magnesium in the nitrate containing platinum and magnesium is 0.03:1, and the total mass of the nitrate containing platinum and magnesium is 10% of the mass of the granular support, to prepare an impregnation solution;

[0087] S4: Place the granular carrier in the impregnation liquid, and impregnate for 12-24 hours according to the volume ratio of the impregnation liquid to the granular carrier of 2:1. Then filter until there is no chloride ion in the filtrate, dry it in an oven at 65°C for 24 hours, and then place it in a muffle furnace and roast it at 550°C in an air atmosphere for 4 hours. After cooling to room temperature, continue to pass it through a 30-mesh sieve to remove samples with small particles to obtain a finished catalyst.

[0088] The catalytic performance data is based on the following: using urea as the raw material, the raw material was added to a Mini-DPL mini experimental multifunctional fluidized bed purchased from Chongqing Dongsheng Precision Co., Ltd., ammonia was introduced, heated to 200°C, and ammonia was introduced to maintain the ammonia partial pressure at 1.12 MPa. The catalysts prepared in Examples 1-5 and Comparative Examples 5-7 were added at a rate of 5 wt% of the raw material mass. After the reaction stabilized, the yield of melamine and the percentage of the by-product biuret were measured after 24 hours. The yield of melamine was based on the ratio of the melamine content to the theoretical melamine content of urea, and the percentage of biuret was based on the percentage of biuret content in the product. The detection method was a liquid phase method.

[0089] The high-speed air jet wear index method is used to analyze the catalyst wear index. A certain amount of catalyst sample is placed in a straight glass tube. Humidified air is blown through the catalyst at high speed to fluidize it. The resulting fine powder is carried by the airflow into a filter paper cylinder, which then passes through the filter paper cylinder into the atmosphere. The mass of fine powder (less than 15 μm) generated within 4 hours is measured and its percentage of the total catalyst mass is calculated to provide the catalyst wear index.

[0090] The adsorbents prepared in Examples 1-5 and Comparative Examples 5-7 were used to test the urea conversion rate and the percentage of the by-product biuret. The specific test results are shown in Table 1 below.

[0091] Table 1

[0092]

[0093] According to the comparison of the test results in Table 1 above, it can be seen from the results of Comparative Examples 5 and 6 that changing the molar ratio of platinum to magnesium, or not adding platinum metal salt during catalyst preparation, the catalytic effect of the final catalyst will be significantly reduced. In addition, the content of the by-product biuret will also increase.

[0094] In addition, according to the test results of Comparative Example 7, it can be seen that the pH of the impregnation solution also has a certain influence on the catalytic effect of the subsequent catalyst, and not controlling the pH of the impregnation solution by adding aluminum chloride also has a certain influence on the formation of biuret by-products.

[0095] At the same time, during the catalyst preparation process, zinc chloride was tried to replace aluminum chloride to adjust the pH of the impregnation solution. The test results showed that the melamine yield was still not high at 75.8%, and the biuret content was 11.9%.

[0096] The catalysts prepared in Examples 1-5 and Comparative Examples 1-4 were placed in an intelligent ball mill to test the wear index of the catalysts. The test results are summarized in Table 2 below.

[0097] Table 2

[0098]

[0099] Combined with the wear index detected in Table 2 above, changing the addition ratio of each reagent during the catalyst preparation process and controlling the pH of the mixture I will affect the wear index of the subsequently prepared catalyst, and the wear index will affect the strength of the catalyst to a certain extent. Catalysts with lower wear indexes can adapt to more stringent reaction conditions, have more stable catalytic performance, and extend their service life.

Claims

1. A method for preparing a catalyst for preparing melamine, characterized in that: The following steps are involved: S1: Add water glass to deionized water, then add aluminum salt solution, and add ammonium citrate solution while stirring until the pH becomes weakly alkaline to obtain mixture I; S2: Mixture I is placed on an extruder to form extrusion strips, which are then dried, roasted, ground, and sieved to obtain a granular carrier; S3: dissolving a soluble salt containing platinum and magnesium in deionized water to obtain a mixed solution II, and adding an aluminum chloride solution to the mixed solution II until the pH of the mixed solution II is 5 to 6 to prepare an impregnation solution; S4: placing the granular support in the impregnation solution, followed by filtering, drying, and calcining to obtain the catalyst; The aluminum salt solution in step S1 is an aluminum nitrate solution, the concentration of the aluminum nitrate solution is 20-30 g / L, the volume ratio of the aluminum nitrate solution to water glass is 0.5:1; the volume ratio of the water glass to deionized water is 1:1-3, and the pH is 7.5-8 after adding ammonium citrate; The soluble salt containing platinum and magnesium in step S3 is nitrate, and the molar ratio of metal platinum to magnesium in the soluble salt containing platinum and magnesium is 0.01-0.05:

1.

2. The method for preparing the catalyst according to claim 1, wherein The concentration of water glass in step S1 is 40 degrees Baume.

3. The method for preparing the catalyst according to claim 1, wherein In the step S2, the mixture I is evenly mixed with the sesbania powder before being put into the extruder, and the mass ratio of the mixture I to the sesbania powder is 10:

1.

4. The method for preparing the catalyst according to claim 1, wherein In the step S2, the drying condition is 50-65° C. for 24-48 hours; and the roasting condition is a heating rate of 15° C. / min to 550° C. for 4-6 hours.

5. The method for preparing the catalyst according to claim 1, wherein In the step S2, the granular carrier is obtained by grinding and passing through a 30-mesh sieve.

6. The method for preparing the catalyst according to claim 1, wherein The mass of the soluble salt containing platinum and magnesium is 10% of the mass of the granular carrier.

7. The method for preparing the catalyst according to claim 1, wherein In step S4, the volume ratio of the impregnation liquid to the granular carrier is 2.5 to 1:1, and the calcination condition is calcination at 550° C. for 3 to 5 hours under air conditions.

8. A catalyst for preparing melamine obtained by the method for preparing a catalyst according to any one of claims 1 to 7.

Images