CN114887657B

CN114887657B - Catalyst for synthesizing melamine and preparation method thereof

Info

Publication number: CN114887657B
Application number: CN202210660516.3A
Authority: CN
Inventors: 臧未一; 刘琛; 陈观堃; 雷婧; 王硕
Original assignee: Ningbo Fareasttech Catalyst Engineering Co ltd
Current assignee: Ningbo Fareasttech Catalyst Engineering Co ltd
Priority date: 2022-06-10
Filing date: 2022-06-10
Publication date: 2024-01-26
Anticipated expiration: 2042-06-10
Also published as: CN114887657A

Abstract

The invention discloses a preparation method of a catalyst for synthesizing melamine, which comprises the following steps: (1) Dissolving solid sodium silicate in water, adding molecular sieve raw powder when the pH value is 8 by dripping dilute acid, and continuing to add the dilute acid until molecular sieve-silica gel is formed; (2) aging the molecular sieve-silica gel by placing it; (3) Addition of Al to aged molecular sieve-silica gel ₂ (SO ₄ ) ₃ Adjusting the pH value of the solution by using alkali solution to form molecular sieve-silica-alumina gel; (4) Soaking molecular sieve-silica-alumina gel in salt solution to obtain a soaked system; (5) And (3) carrying out suction filtration and washing on the immersed system to prepare spray slurry, carrying out spray granulation, and roasting to obtain the molecular sieve type silica-alumina gel catalyst finished product. The catalyst prepared by the method has high activity, slow deactivation and good wear resistance, and can be suitable for preparing melamine by fixed bed catalytic reaction.

Description

Catalyst for synthesizing melamine and preparation method thereof

Technical Field

The invention relates to the technical field of catalysts, in particular to a catalyst for synthesizing melamine and a preparation method thereof.

Background

Melamine is an important intermediate product of nitrogen heterocyclic organic chemical industry and is mainly used for producing triamine formaldehyde resin. As a thermosetting resin, the resin has the characteristics of heat resistance, aging resistance, acid and alkali resistance, flame retardance and the like. The modified polyurethane resin is widely applied to industries such as wood processing, coating, papermaking, textile, leather, decorative board, laminated board, composite floor, flame retardant material, water reducing agent, adhesive, melamine molding powder and the like. In recent years, along with the deep research on melamine and the continuous improvement of the living standard of people, the application field and the range of the melamine are also continuously expanded, and after twenty-first century, the annual growth rate of melamine exceeds 15 percent due to the market demand.

At present, the melamine production is mostly finished by a urea method, and the method is characterized in thatAdding urea into a reactor, carrying out fluidization contact reaction with a catalyst (silica gel catalyst or alumina catalyst or silica-alumina catalyst), wherein the reaction is endothermic, and carrying out cyclic heating by using a fused salt carrier, so that the urea is gasified and decomposed in the reactor to generate melamine, carbon dioxide and ammonia. The melamine production process using urea as raw material and adopting normal and low pressure method has very strict requirements on catalyst, and has the advantages of high strength, high activity, large load, low price and the like. Therefore, the selection of the catalyst and the exertion of the service performance thereof are directly related to various indexes such as the production capacity, the output quality, the raw material consumption, the operation period and the like of the whole melamine production device. Foreign BASF melamine technology, and gamma-Al is adopted as a catalyst ₂ O ₃ Most of the melamine catalysts in China adopt coarse pore silica gel, and aluminum silicate catalysts are used for the melamine device catalysts in the middle and later stages of nineties of the twentieth century.

CN1846848A discloses a method for producing an alumina catalyst for melamine production, which is prepared by mixing alumina and aluminum hydroxide, spraying water, rolling into pellets, drying, sieving, and roasting. However, the catalyst only contains one component of alumina, so that the catalyst has low strength, serious abrasion, low specific surface area, poor carbon precipitation resistance and quick inactivation.

CN101024171a discloses a catalyst for synthesizing melamine and a preparation method, wherein a commercially available aluminum silicon oxide and a commercially available aluminum oxide are put into a kneader, water or an acid solution is added, and after granulation, drying, sieving and activation treatment, the melamine catalyst is obtained. Because the preparation process belongs to mechanical mixing, the two raw materials cannot be fully combined, and the problems of more fine powder, quick activity reduction and the like can occur in the use process.

CN1493565a discloses a co-production method for producing melamine by a one-step method, which adopts silica gel/silica-alumina gel as a catalyst, overcomes the defects of the existing co-production process by a one-step method, a two-step method and a two-step method in the normal pressure method, and provides a co-production process with low investment and low cost.

CN1723203a discloses a melamine two-step reactor, a process for the preparation of melamine by decomposing urea on a solid catalyst using a main reactor and a post reactor. The main reactor is a fluidized bed, a catalyst with low Lewis acidity is adopted, the post reactor is a fixed bed, a catalyst with high Lewis acidity is adopted, the two-step reactor can avoid the premature deactivation of the catalyst caused by excessive deposition of condensation products on the surface of the catalyst in the reaction, and the post reactor is provided with the catalyst with high Lewis acidity, so that high conversion rate can be realized. However, the problems of high investment cost, overlong reaction flow and more severe reaction conditions are caused.

Disclosure of Invention

The invention mainly aims to provide a catalyst for synthesizing melamine and a preparation method thereof, which are used for solving the problems of low catalyst activity, serious abrasion and quick deactivation in the catalyst preparation process in the prior art.

In order to achieve the above purpose, the present invention is realized by the following technical scheme:

the preparation method of the catalyst for synthesizing melamine specifically comprises the following steps:

(1) Dissolving solid sodium silicate in water, adding molecular sieve raw powder when the pH value is 8 by dripping dilute acid, and continuing to add the dilute acid until molecular sieve-silica gel is formed;

(2) Placing the molecular sieve-silica gel to age;

(3) Addition of Al to aged molecular sieve-silica gel ₂ (SO ₄ ) ₃ Adjusting the pH value of the solution by using alkali solution to form molecular sieve-silica-alumina gel;

(4) Soaking molecular sieve-silica-alumina gel in salt solution to obtain a soaked system;

(5) And carrying out suction filtration and washing on the immersed system to prepare spray slurry, and carrying out spray granulation, roasting and screening on the spray slurry to obtain the molecular sieve type silica-alumina gel catalyst finished product.

Further, the molecular sieve is a mesoporous molecular sieve, and the mass ratio of the molecular sieve raw powder to the solid sodium silicate is (0.2-0.5) to 1;

preferably, the molecular sieve is selected from one or more of MCM-22, ZSM-48 and MCM-56;

preferably, the molecular sieve has a silica to alumina ratio of 20 to 150.

Further, the dilute acid in step (1) is dilute sulfuric acid.

Further, the aging time of the molecular sieve-silica gel in the step (2) is 1 to 3 hours.

Further, al in the step (3) ₂ (SO ₄ ) ₃ The mass fraction of the solution is 40% -50%, the alkali liquor is added dropwise, and the pH is adjusted to 7.5-8 by the alkali liquor;

preferably, the alkali liquor is ammonia water or sodium hydroxide solution.

Further, the weak acid solution in the step (4) is a citric acid solution, and the volume ratio of the citric acid solution to the molecular sieve-silica alumina gel is (4-6) to 1.

Further, in the step (5), the spray slurry is prepared by adding dilute sulfuric acid after suction filtration and washing of a immersed system, the solid content of the spray slurry is 25wt%, the inlet and outlet temperature of a sprayer for spray granulation is 450 ℃, the outlet temperature is 115 ℃, the feeding pressure is 1.2MPa, spray granulation is carried out, and the granularity of catalyst particles obtained after spray granulation is 90-115 mu m.

Further, the roasting temperature in the step (5) is 300-350 ℃, and the roasting time is 0.5-1 h.

Further, the attrition parameter of the finished molecular sieve-type silica alumina gel catalyst product in the step (5) is less than 3.5%.

A catalyst for the synthesis of melamine obtained according to the process for the preparation of the catalyst described above.

The invention has the beneficial effects that:

1. according to the catalyst for synthesizing the melamine, the doping of the molecular sieve can obviously improve the acid center number on the surface of the catalyst, so that the catalytic activity of the catalyst is obviously improved, and meanwhile, the catalyst also has higher activity at low temperature by exploring the optimal mass ratio of the molecular sieve to solid sodium silicate, so that the occurrence probability of side reaction is reduced, the deposition of byproducts on the surface of the catalyst is reduced, the deactivation rate of the catalyst is slowed down, the bearing capacity and the selectivity of the catalyst are improved, the reaction conversion rate is finally improved, and the yield of the melamine is improved.

2. According to the catalyst for synthesizing melamine, disclosed by the invention, through the impregnation step of adding the weak acid solution in the preparation process, the number of acid centers of the active center of the catalyst is increased, the catalytic activity is also improved, and the problem of premature deactivation of the catalyst is avoided.

3. The catalyst for synthesizing the melamine provided by the invention has the advantages that the silicon-aluminum ratio of the added molecular sieve is 20-150, and the finished product of the finally prepared formed molecular sieve type silicon-aluminum gel catalyst has high wear resistance and small wear index, and can be more suitable for the actual preparation process of the melamine on a fixed bed.

Detailed Description

The technical scheme of the present invention will be clearly and completely described in the following in connection with specific embodiments.

Example 1

The embodiment provides a catalyst for synthesizing melamine and a preparation method thereof, and the specific steps are as follows:

(1) Weighing 100g of solid sodium silicate, dissolving in 500mL of water, stirring and dissolving to form uniform liquid, slowly dropwise adding 15% by mass of dilute sulfuric acid solution, adjusting the pH of the material until the pH is 8.0, adding 20g of MCN-22 molecular sieve raw powder, continuously stirring, and dropwise adding 15% by mass of dilute sulfuric acid until the pH is 8.7 to obtain molecular sieve-silica gel;

(2) Placing the molecular sieve-silica gel for 3 hours at room temperature to age;

(3) Adding 40% by mass of Al to the aged molecular sieve-silica gel ₂ (SO ₄ ) ₃ Dropwise adding ammonia water into the solution to adjust the pH value to 7.5 to form molecular sieve-silica-alumina gel;

(4) Putting the molecular sieve-silica-alumina gel into a citric acid solution with the volume of 4 times to be immersed for 2 hours to obtain an immersed system;

(5) Suction filtering the immersed system to obtain filter cake, and washing the filter cake with water to remove NH ⁺ And SO ₄ ^2- Adding dilute sulfuric acid after washing to prepare spray slurry with solid content of 25wt%, and spraying to granulate, wherein a sprayer is arranged at the inlet of the sprayerThe temperature is 450 ℃, the outlet temperature is 115 ℃, the feeding pressure is 1.2Mpa, and the granularity of the catalyst particles obtained after spray granulation is 90-115 mu m; roasting the dried catalyst particles in a fluidized bed furnace at the temperature of 300 ℃ for 30min to obtain a molecular sieve type silica-alumina gel catalyst finished product.

Example 2

(1) Weighing 100g of solid sodium silicate, dissolving in 500mL of water, stirring and dissolving to form uniform liquid, slowly dropwise adding a dilute sulfuric acid solution, adding 50g of ZSM-48 molecular sieve raw powder when the pH value of the material is regulated to 8.0, continuously stirring, and dropwise adding dilute sulfuric acid until the pH value is 9.0 to obtain molecular sieve-silica gel;

(2) Placing the molecular sieve-silica gel for 1h at room temperature to age;

(3) Adding 50% by mass of Al to the aged molecular sieve-silica gel ₂ (SO ₄ ) ₃ Dropwise adding ammonia water into the solution to adjust the pH value to 8.0, so as to form molecular sieve-silica-alumina gel;

(4) Putting the molecular sieve-silica-alumina gel into a citric acid solution with the volume of 6 times to be immersed for 2 hours to obtain an immersed system;

(5) Suction filtering the immersed system to obtain filter cake, and washing the filter cake with water to remove NH ⁺ And SO ₄ ^2- Adding dilute sulfuric acid after washing to prepare spray slurry with solid content of 25wt%, and during spray granulation, the inlet temperature of a sprayer is 450 ℃, the outlet temperature is 115 ℃, the feeding pressure is 1.2Mpa, and the granularity of catalyst particles obtained after spray granulation is 90-115 mu m; roasting the dried catalyst particles in a fluidized bed furnace at the temperature of 300 ℃ for 1h to obtain a molecular sieve type silica-alumina gel catalyst finished product.

Example 3

(1) Weighing 100g of solid sodium silicate, dissolving in 500mL of water, stirring and dissolving to form uniform liquid, then slowly dropwise adding a dilute sulfuric acid solution, adding 40g of MCM-56 molecular sieve raw powder when the pH value of the material is regulated to 8.0, continuously stirring, and dropwise adding dilute sulfuric acid until the pH value is 9.2 to obtain molecular sieve-silica gel;

(2) Placing the molecular sieve-silica gel for 2 hours at room temperature to age;

(4) Putting the molecular sieve-silica-alumina gel into a citric acid solution with the volume of 5 times to be immersed for 2 hours to obtain an immersed system;

(5) Suction filtering the immersed system to obtain filter cake, and washing the filter cake with water to remove NH ⁺ And 5O ₄ ^2- Adding dilute sulfuric acid after washing to prepare spray slurry with solid content of 25wt%, and during spray granulation, the inlet temperature of a sprayer is 450 ℃, the outlet temperature is 115 ℃, the feeding pressure is 1.2Mpa, and the granularity of catalyst particles obtained after spray granulation is 90-115 mu m; roasting the dried catalyst particles in a fluidized bed furnace at the temperature of 350 ℃ for 40min to obtain a molecular sieve type silica-alumina gel catalyst finished product.

Comparative example 1

The embodiment provides a catalyst for synthesizing melamine and a preparation method thereof, wherein 10g of MCN-22 molecular sieve raw powder is added when the pH value of a material in the step (1) is regulated to 8.0, and other operation steps are the same as those in the embodiment 1.

Comparative example 2

The embodiment provides a catalyst for synthesizing melamine and a preparation method thereof, 60g of MCN-22 molecular sieve raw powder is added when the pH value of a material in the step (1) is regulated to 8.0, and other operation steps are the same as in the embodiment 1.

Comparative example 3

The embodiment provides a catalyst for synthesizing melamine and a preparation method thereof, which do not comprise the steps of adding 20g of MCN-22 molecular sieve raw powder in the step (1), directly dripping dilute sulfuric acid until the pH value is 8.7 to obtain silica gel, and other operation steps are the same as those in the embodiment 1.

Comparative example 4

The embodiment provides a catalyst for synthesizing melamine and a preparation method thereof, which do not include the citric acid solution impregnation operation in the step (4), directly suction-filtering molecular sieve-silica-alumina gel and subsequent operation, and other operation steps are the same as those in the embodiment 1.

Comparative example 5

The embodiment provides a catalyst for synthesizing melamine and a preparation method thereof, wherein in the step (4), a molecular sieve-silica-alumina gel is placed in a citric acid solution with a volume of 3 times to be immersed for 3 hours, so as to obtain an immersed system, and other operation steps are the same as those of the embodiment 1.

Catalyst performance and attrition determination:

the catalytic performance data are that urea, biuret or cyanuric acid is taken as a raw material, the raw material is added into a reactor, ammonia gas is introduced, the temperature is heated to 200 ℃, the ammonia gas is introduced under the condition that the partial pressure of the ammonia gas is kept at 10MPa, the catalysts prepared in examples 1-3 and comparative examples 1-5 accounting for 5 weight percent of the mass of the raw material are added, and the reaction is stopped after two time points of 15min and 24h respectively.

The melamine as the reaction product is solid, centrifugal separation is carried out, thus obtaining a solid melamine crude product, and white solid melamine pure product is obtained by water purification and the content is calculated.

Catalyst attrition was determined using a catalyst of specified particle size, continuously ball milled for 2 hours, and then re-sieved, with the fraction of the catalyst mass reduction of specified particle size as attrition parameter.

For a clearer description of the catalyst preparation process, specific process parameters are summarized in table 1 below:

TABLE 1

The activity of the prepared catalyst for catalyzing melamine was measured by the catalyst activity measurement, and the specific results are shown in the following table 2.

TABLE 2

As can be seen from table 2, in example 1, compared with comparative examples 3 and 4, the initial activity of the catalyst prepared finally without adding the molecular sieve is lower, the catalyst prepared finally without adding the molecular sieve is lower in initial activity but lower in final activity for 24 hours although the catalyst prepared finally without citric acid impregnation, which indicates that the molecular sieve can improve the activity of the catalyst of the present invention, and the catalyst prepared finally by citric acid impregnation does not experience the problem of premature deactivation, which indirectly indicates that the catalyst activity corresponds to the number of acid centers on the catalyst surface, the doping of the molecular sieve can obviously improve the number of acid centers on the catalyst surface, thereby obviously improving the catalytic activity, and the impregnation operation of the weak acid solution further increases the number of acid centers on the catalyst, so that the problem of premature deactivation of the catalyst is avoided.

In example 1, the amount of the molecular sieve raw powder added should be within a certain range to exhibit a good catalytic effect as compared with comparative examples 1 and 2, beyond which the catalytic effect of the finally prepared catalyst is reduced.

The attrition parameters of the final prepared catalyst were measured and the specific results are shown in table 3 below.

TABLE 3 Table 3

Numbering device	Catalytic active attrition parameter
		Example 1	2.5％
Example 2	2.9％
		Example 3	3.2％
Comparative example 3	8.5％

From the attrition resistance parameters of the catalyst measured in table 2 above, it can be seen that the attrition resistance parameters of comparative example 3 are significantly higher, indicating that the addition of a molecular sieve having a silica-alumina ratio of 20 to 150 can indeed improve the attrition resistance of the final catalyst, more adapt to the severe reaction conditions of the fixed bed, reduce the catalyst loss, and indirectly reduce the catalyst cost.

Claims

1. The preparation method of the catalyst for synthesizing melamine is characterized by comprising the following steps:

(2) Placing the molecular sieve-silica gel to age;

(4) Soaking molecular sieve-silica-alumina gel in weak acid solution to obtain soaked system;

(5) Carrying out suction filtration and washing on the immersed system to prepare spray slurry, and carrying out spray granulation, roasting and screening on the spray slurry to obtain a molecular sieve type silica-alumina gel catalyst finished product;

the molecular sieve is selected from one or more of MCM-22, ZSM-48 and MCM-56.

2. The method for preparing the catalyst according to claim 1, wherein the mass ratio of the molecular sieve raw powder to the solid sodium silicate is (0.2-0.5): 1.

3. The method for preparing a catalyst according to claim 1 or 2, wherein the molecular sieve has a silica-alumina ratio of 20 to 150.

4. The method for preparing a catalyst according to claim 1, wherein the dilute acid in step (1) is dilute sulfuric acid.

5. The method for preparing a catalyst according to claim 1, wherein the aging time of the molecular sieve-silica gel in the step (2) is 1 to 3 hours.

6. The method for producing a catalyst according to claim 1, wherein Al in the step (3) ₂ (SO ₄ ) ₃ The mass fraction of the solution is 40% -50%, the alkali liquor is added dropwise, and the pH is adjusted to 7.5-8 by the alkali liquor.

7. The method for preparing a catalyst according to claim 6, wherein the alkali solution is ammonia water or sodium hydroxide solution.

8. The method for preparing a catalyst according to claim 1, wherein the weak acid solution in the step (4) is a citric acid solution, and the volume ratio of the citric acid solution to the molecular sieve-silica alumina gel is (4-6): 1.

9. the method for preparing a catalyst according to claim 1, wherein in the step (5), the spray slurry is prepared by adding dilute sulfuric acid after suction filtration and washing of a immersed system, the solid content of the spray slurry is 25wt%, the inlet and outlet temperatures of a sprayer for spray granulation are 450 ℃, the outlet temperature is 115 ℃, the feeding pressure is 1.2MPa, spray granulation is performed, and the particle size of catalyst particles obtained after spray granulation is 90-115 μm.

10. The method for preparing a catalyst according to claim 1, wherein the calcination temperature in step (5) is 300 to 350 ℃ and the calcination time is 0.5 to 1h.

11. The method for preparing a catalyst according to claim 1, wherein the attrition parameter of the finished molecular sieve type aluminosilicate catalyst in step (5) is less than 3.5%.

12. Catalyst for the synthesis of melamine obtained by the process for the preparation of the catalyst according to any one of claims 1 to 11.