CN117654467A

CN117654467A - Catalyst for melamine synthesis and preparation method thereof

Info

Publication number: CN117654467A
Application number: CN202311669863.3A
Authority: CN
Inventors: 项裕桥; 陈嘉梁; 何相君; 王永全; 赵柏
Original assignee: Ningbo Fareasttech Catalyst Engineering Co ltd
Current assignee: Ningbo Fareasttech Catalyst Engineering Co ltd
Priority date: 2023-12-04
Filing date: 2023-12-04
Publication date: 2024-03-08

Abstract

The invention relates to the technical field of melamine preparation, and discloses a preparation method of a catalyst for melamine synthesis, which comprises the following steps: 1) Dissolving 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) Dissolving fly ash with calcium oxide as primer in water, adding into molecular sieve-silica gel, and regulating pH with alkali solution while stirring to form molecular sieve-silica gel; 3) Washing molecular sieve-silicon-aluminum colloid with dilute acid solution, and then washing with deionized water; 4) Soaking the washed product in weak acid solution to obtain a soaked system; 5) And carrying out suction filtration, washing and preparing spray slurry on the immersed system, carrying out spray granulation and roasting on the spray slurry, and screening to remove fine particles with the granularity smaller than 8 mu m, thereby finally obtaining a catalyst finished product. The method realizes waste utilization, saves cost and saves procedures. The performance of the obtained catalyst reaches the equivalent level of the existing catalyst.

Description

Catalyst for melamine synthesis and preparation method thereof

Technical Field

The invention belongs to the technical field of melamine preparation, and particularly relates to a catalyst for melamine synthesis 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 production of melamine is mostly finished by a urea method, urea is added into a reactor, and is subjected to fluidization contact reaction with a catalyst (silica gel catalyst or alumina catalyst or silica-alumina catalyst), wherein the reaction is an endothermic reaction, and the melamine, carbon dioxide and ammonia are generated by circularly heating a fused salt carrier and gasifying and decomposing the urea in the reactor. The melamine production process using urea as raw material and adopting normal and low pressure method has very strict requirements on catalyst and high strength and high activityHigh load and low cost. 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. In the middle and late nineties of the twentieth century, aluminum silicate catalysts began to be used as part of the melamine plant catalysts in China.

In recent years, a melamine production device mostly uses a silica-alumina gel catalyst, and the silica-alumina gel catalyst is also studied intensively. CN102580711a discloses a method for producing melamine catalyst by gas phase urea synthesis, which adopts sodium silicate and aluminum sulfate as raw materials to produce melamine catalyst, but the method has the operations of soda boiling, reaming, acid soaking and the like in production, and has long process flow, complex operation and high production cost. CN110665521A discloses a catalyst for synthesizing melamine and a preparation method thereof, wherein the specific surface of a silica gel carrier is only 120m ² The catalyst prepared by the method has small specific surface area and small urea load, and the catalyst produced by the method has high aluminum content and high cost.

In recent years, I research shows that the doping of the molecular sieve can obviously improve the acid center number on the surface of the silica-alumina gel catalyst, so that the catalytic activity of the melamine catalyst is obviously improved, and meanwhile, the catalyst also has higher activity at low temperature by exploring the adding amount ratio of the molecular sieve, 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, and the carrying capacity and selectivity of the catalyst are improved. On the other hand, the fly ash is one of the solid wastes with the largest accumulation amount in China, and the accumulation and the discharge of the fly ash occupy a large amount of land resources and easily cause environmental pollution. The main components of the fly ash also contain silicon and aluminum, but the application of the fly ash in a melamine catalyst is not reported yet.

In view of this, the present invention has been proposed.

Disclosure of Invention

The invention aims to solve one of the problems and provides a novel catalyst for melamine synthesis and a preparation method thereof.

The invention aims at realizing the following technical scheme:

a method for preparing a catalyst for melamine synthesis, comprising the steps of:

1) Dissolving 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) Dissolving fly ash with calcium oxide as primer in water, adding into molecular sieve-silica gel, and regulating pH with alkali solution while stirring to form molecular sieve-silica gel;

3) Washing molecular sieve-silicon-aluminum colloid with dilute acid solution, and then washing with deionized water;

4) Soaking the washed product in a weak acid solution to obtain a soaked system;

5) And carrying out suction filtration, washing and preparing spray slurry on the immersed system, carrying out spray granulation and roasting on the spray slurry, and screening to remove fine particles with the granularity smaller than 8 mu m, thereby finally obtaining the finished product of the silica-alumina gel catalyst meeting the requirements.

Further, the adding amount of the sodium silicate, the molecular sieve and the fly ash is 1: (0.1-0.5): (2-3) by weight ratio.

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 the step 1) and the step 3) is dilute nitric acid or dilute sulfuric acid or dilute hydrochloric acid.

Further, the addition amount of the calcium oxide in the step 2) is 0.5-3% of the weight of the fly ash; the alkali solution is sodium hydroxide solution or potassium hydroxide solution, the alkali solution is added in a dropwise manner, and the pH is adjusted to 7-8 by adding the alkali solution.

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

Further, in the step 5), the solid content of the spray slurry is 25wt%, and the spray granulation is performed by spraying the slurry at an inlet and outlet temperature of 450 ℃, an outlet temperature of 115 ℃ and a feed pressure of 1.2 MPa.

Further, the roasting in the step 5) adopts a temperature programming mode, the temperature is increased to 500-600 ℃ at a temperature increasing speed of 5-10 ℃/min, and then the roasting is continued for 2-20h at the temperature.

A catalyst for producing melamine, which is prepared by the method.

Further, the catalyst product contains 65 to 85 percent of SiO ₂ 15 to 35 percent of Al ₂ O ₃ 0 to 5% of P ₂ O ₅ 。

Compared with the prior art, the invention has the following main advantages:

1. the invention uses the fly ash to replace the aluminum source used by the commercial silica-alumina gel to prepare the novel silica-alumina gel catalyst, realizes the reutilization of solid waste and saves the cost.

2. In the presence of primer calcium oxide, in an alkaline system, silicate compound anions in the fly ash can react with hydroxyl groups in the solution in a bonding way, so that the volume of the fly ash is increased, the probability of bonding the fly ash and silica gel in a recombination way is also given, the bonding strength between formed silica-alumina colloid atoms is high, and the abrasion index of the final catalyst is smaller than that of a molecular sieve doped catalyst prepared before I.

3. The dropwise addition of the alkali solution causes volume increase and bonding recombination, and the subsequent acid washing can basically dissolve out metal elements Fe, ga, ti, K, na and the like in the fly ash, so that the system is reamed, a separate aging reaming step is not needed, and the preparation process of the catalyst is saved.

4. Doping of the molecular sieve and weak acid impregnation can obviously improve the acid center number of the surface of the catalyst; meanwhile, under the impregnation of weak acid, the pore volume and specific surface area of the catalyst are increased, and the catalytic activity is improved; meanwhile, some fly ash contains P element, namely P ₂ O ₅ In the form of (2) can further improve the catalytic activity of the catalyst.

5. The catalyst has higher activity at low temperature, so that the occurrence rate 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 carrying capacity and selectivity of the catalyst are improved, the conversion rate of the reaction is finally improved, and the yield of melamine is improved.

Detailed Description

Further, the adding amount of the sodium silicate, the molecular sieve and the fly ash is 1: (0.1-0.5): (2-3). 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 the step 1) and the step 3) is dilute nitric acid or dilute sulfuric acid or dilute hydrochloric acid. The addition amount of the calcium oxide in the step 2) is 0.5-3% of the weight of the fly ash; the alkali solution is sodium hydroxide solution or potassium hydroxide solution, the alkali solution is added in a dropwise manner, and the pH is adjusted to 7-8 by adding the alkali solution. The weak acid solution in the step 4) is citric acid solution, and the volume ratio of the citric acid solution to the molecular sieve-silica-alumina colloid is (5-6) to 1. The solid content of the spray slurry in the step 5) is 25wt%, and the inlet and outlet temperatures of the spray granulating sprayer are 450 ℃, the outlet temperature is 115 ℃ and the feeding pressure is 1.2MPa for spray granulation. And 5) roasting in a temperature programming mode, increasing the temperature to 500-600 ℃ at a temperature increasing speed of 5-10 ℃/min, and then continuously roasting at the temperature for 2-20h.

The final catalyst product contains 65-85% SiO ₂ 15 to 35 percent of Al ₂ O ₃ 0 to 5% of P ₂ O ₅ The sum of the total mass percent is 100 percent. The apparent bulk density of the catalyst is 0.58-0.95 g/cm ³ The abrasion index is less than 2.0 percent h ^-1 BET specific surface area of more than 200m ² /g, pore diameter greater than 4nm, pore volume greater than 0.3m ³ And/g. The catalyst product has a median particle size of 80-115 μm, a particle size of not more than 3% below 40 μm and a particle size of not more than 5% above 150 μm. The catalyst is used for synthesizing melamine by taking urea as a raw material, and the synthesis reaction is carried out in a fluidized bed reactor.

The present invention will be further illustrated with reference to the following specific examples, but is not limited thereto.

Example 1

The embodiment provides a preparation method of a catalyst for melamine synthesis, which comprises the following specific steps:

(1) Weighing 100g of solid sodium silicate, dissolving in 500mL of water, stirring and dissolving to form uniform liquid, slowly dripping 15% by mass of dilute nitric acid solution, adjusting the pH of the material until the pH is 8.0, adding 10g of MCN-22 molecular sieve raw powder, continuously stirring, and dripping 15% by mass of dilute nitric acid until the pH is 8.7 to obtain molecular sieve-silica gel;

(2) Uniformly mixing 200g of fly ash with 1g of calcium oxide, adding 1000ml of water for dissolution, then adding a vertical horse into the molecular sieve-silica gel, stirring the system, and dropwise adding sodium hydroxide solution while stirring to adjust the pH to 7.5 to form a molecular sieve-silica-alumina colloid;

(3) Stirring and washing molecular sieve-silicon-aluminum colloid with 15% dilute nitric acid solution to remove Fe ³⁺ 、Ca ² ⁺ 、Ti ²⁺ 、K ⁺ 、Na ⁺ Waiting for the stirring with deionized waterWashing to neutrality;

(4) Immersing the washed product in a citric acid solution with the volume of 4 times for 2 hours to obtain an immersed system;

(5) Filtering the immersed system, washing with water to obtain a filter cake, adding dilute nitric acid after washing to prepare spray slurry with the solid content of 25wt%, and carrying out spray granulation, wherein 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, adopting a temperature programming mode to perform roasting, heating to 600 ℃ at a temperature rising speed of 5-10 ℃/min, continuously roasting at the temperature for 2 hours, naturally cooling roasting materials, screening, removing fine particles smaller than 8 mu m, and finally obtaining the catalyst product meeting the requirements.

Example 2

(1) Weighing 100g of solid sodium silicate, dissolving in 500mL of water, stirring and dissolving to form uniform liquid, slowly dripping 15% by mass of dilute sulfuric acid solution, adjusting the pH of the material until the pH is 8, adding 50g of ZSM-48 molecular sieve raw powder, continuously stirring, and dripping 15% by mass of dilute sulfuric acid until the pH is 9.0 to obtain molecular sieve-silica gel;

(2) Uniformly mixing 300g of fly ash and 9g of calcium oxide, adding 1000ml of water for dissolution, then adding a vertical horse into the molecular sieve-silica gel, stirring the system, and dropwise adding potassium hydroxide solution while stirring to adjust the pH to 8.0 to form a molecular sieve-silica-alumina colloid;

(3) Stirring and washing molecular sieve-silicon-aluminum colloid with 15% dilute sulfuric acid solution to remove Fe ³⁺ 、Ca ² ⁺ 、Ti ²⁺ 、K ⁺ 、Na ⁺ Waiting, and then stirring and washing with deionized water to neutrality;

(4) Immersing the washed product in a citric acid solution with the volume of 6 times for 2 hours to obtain an immersed system;

(5) Filtering the immersed system, washing with water to obtain a filter cake, adding dilute sulfuric acid to prepare spray slurry with the solid content of 25wt%, and carrying out spray granulation, wherein the inlet temperature of a sprayer is 450 ℃, the outlet temperature of the sprayer 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, adopting a temperature programming mode to perform roasting, heating to 550 ℃ at a temperature rising speed of 5-10 ℃/min, continuously roasting for 2 hours at the temperature, naturally cooling roasting materials, screening, removing fine particles smaller than 8 mu m, and finally obtaining the catalyst product meeting the requirements.

Example 3

f1 100g of solid sodium silicate is weighed and dissolved in 500mL of water, stirred and dissolved into uniform liquid, then dilute hydrochloric acid solution with the mass fraction of 15% is slowly dripped, the pH of the material is regulated until the pH is 8, 25g of MCM-56 molecular sieve raw powder is added, stirring is continued, and dilute hydrochloric acid with the mass fraction of 15% is dripped until the pH is 9.2, so as to obtain molecular sieve-silica gel;

(2) Uniformly mixing 250g of fly ash and 4g of calcium oxide, adding 1000ml of water for dissolution, then adding a vertical horse into the molecular sieve-silica gel, stirring the system, and dropwise adding potassium hydroxide solution while stirring to adjust the pH to 7.5 to form a molecular sieve-silica-alumina colloid;

(3) Stirring and washing molecular sieve-silicon-aluminum colloid with 15% diluted hydrochloric acid solution to remove Fe ³⁺ 、Ca ² ⁺ 、Ti ²⁺ 、K ⁺ 、Na ⁺ Waiting, and then stirring and washing with deionized water to neutrality;

(4) Immersing the washed product in 5 times volume of citric acid solution for 2 hours to obtain an immersed system;

(5) Filtering the immersed system, washing with water to obtain a filter cake, adding dilute hydrochloric acid to prepare spray slurry with the solid content of 25wt%, and carrying out spray granulation, wherein the inlet temperature of a sprayer is 450 ℃, the outlet temperature of the sprayer 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, adopting a temperature programming mode to perform roasting, heating to 500 ℃ at a temperature rising speed of 5-10 ℃/min, continuously roasting at the temperature for 2 hours, naturally cooling roasting materials, screening, removing fine particles smaller than 8 mu m, and finally obtaining the catalyst product meeting the requirements.

Comparative example 1

This example provides a method for preparing a catalyst for melamine synthesis, which differs from example 1 only in that no molecular sieve is added in step (1), and silica gel is directly obtained after adjusting the pH of the material to 8.0, and other operation steps are the same as in example 1.

Comparative example 2

This example provides a method for preparing a catalyst for melamine synthesis, which is different from example 1 only in that in step (2), no alkali solution is added, fly ash and calcium oxide are directly mixed and uniformly dissolved in water, and then added into molecular sieve-silica gel, and other operation steps are the same as in example 1.

Comparative example 3

This example provides a process for the preparation of a catalyst for melamine synthesis, which differs from example 1 only in that there is no impregnation operation of step (4) with citric acid solution, and other operating steps are the same as in example 1.

Comparative example 4

This example provides a method for preparing a catalyst for melamine synthesis, which differs from example 1 only in that the amount of MCN-22 molecular sieve raw powder added in step (1) is 5g, and other operation steps are the same as in example 1.

Comparative example 5

This example provides a method for preparing a catalyst for melamine synthesis, which differs from example 1 only in that the amount of MCN-22 molecular sieve raw powder added in step (1) is 55g, and other operation steps are the same as in example 1.

Comparative example 6

The present example provides a method for preparing a catalyst for melamine synthesis, which is different from comparative example 1 only in that in step (2), no alkali solution is added, fly ash and calcium oxide are directly mixed and uniformly dissolved in water, and then added into silica gel, and other operation steps are the same as comparative example 1.

Comparative example 7

This example provides a method for preparing a catalyst for melamine synthesis, which differs from comparative example 1 only in that there is no operation of impregnating step (4) with citric acid solution, and other operation steps are the same as comparative example 1.

Comparative example 8

This example provides a process for the preparation of a catalyst for melamine synthesis which differs from comparative example 1 only in that there is no washing operation of step (3) with a dilute acid solution, and the other operating steps are the same as in comparative example 1.

The melamine catalysts prepared in examples 1-3 and the catalysts obtained in comparative examples 1-8 were analyzed and the catalytic performance was determined, wherein each of the prepared catalysts and the comparative catalysts were tested according to the following criteria: determination of solid substance specific surface area by GB/T19587-2004 gas adsorption BET method GB/T21650.3-2011 mercury intrusion method and determination of solid material pore size distribution and porosity part 3 by gas adsorption method: determination of the content of elements of the micropore GB/T30905-2014 inorganic chemical product by a gas adsorption method, and determination of apparent bulk density of the catalytic cracking catalyst by an X-ray fluorescence spectrometry NB/SH/T0954-2017, HG/T4861-2015, chapter 4, section 2 of the industrial standard of low-carbon olefin catalysts prepared from methanol: the measured catalytic performance data of the attrition index are obtained by testing when the operation is stable on a fluidized bed reactor for producing melamine by a gas phase method, a gas phase distributor is arranged above an air inlet of the fluidized bed reactor, carrier gas is mixed gas of ammonia and carbon dioxide in a weight ratio of 1:1, the carrier gas flow is 25-40 kg/h, the reaction temperature is 390-420 ℃, the catalyst loading amount is 50-60 kg, the urea feeding amount is 6-10 kg/h, the primary urea conversion rate is the ratio of the feeding amount of a urea washing tower for 72h of stable operation of the device to the feeding amount of urea of the reactor, and the melamine yield is the ratio of theoretical urea amount required by melamine produced by the device for 72h of stable operation to the urea feeding amount of the reactor.

The specific results are as follows:

physical property index of melamine catalyst prepared in Table 1

The catalysts of the above examples and comparative examples were carried on a catalyst h of 160g urea/kg ^-1 Melamine production tests were carried out under the conditions of (2) to obtain the following catalytic performance data.

Table 2 comparison of the catalytic performance data of the melamine catalysts prepared according to the invention with the comparative catalysts

Numbering device	Urea primary conversion%	Melamine yield%
			Example 1	99.5	97.6
Example 2	99.5	97.6
			Example 3	99.6	98.0
Comparative example 1	94.3	93.9
			Comparative example 2	92.2	92.2
Comparative example 3	95.5	94.4
			Comparative example 4	96.1	95.1
Comparative example 5	96.1	95.2
			Comparative example 6	89.1	88.1
Comparative example 7	91.8	91.1
			Comparative example 8	91.5	90.6
Commercially available silica-alumina gel	99.5	97.2

The above description is intended to be illustrative and not limiting, and variations, modifications, alternatives, and variations of the above embodiments may be made by those of ordinary skill in the art within the scope of the present disclosure. Also, the above examples (or one or more aspects thereof) may be used in combination with each other, and it is contemplated that the embodiments may be combined with each other in various combinations or permutations.

Claims

1. A process for the preparation of a catalyst for melamine synthesis, characterized in that it comprises the following steps: 1) Dissolving 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;

4) Soaking the washed product in weak acid solution to obtain a soaked system;

2. The method for preparing the catalyst for producing melamine according to claim 1, wherein the addition amount of sodium silicate, molecular sieve and fly ash is 1: (0.1 to 0.5): (2-3);

3. The method for preparing a catalyst for the production of melamine according to claim 1, characterized in that the dilute acid in step 1) and step 3) is dilute nitric acid or dilute hydrochloric acid.

4. The method for preparing a catalyst for producing melamine according to claim 1, characterized in that the addition amount of calcium oxide in step 2) is 0.5% -3% by weight of fly ash; the alkali solution is sodium hydroxide solution or potassium hydroxide solution, and the pH is adjusted to 7-8 by adding the alkali solution.

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

6. the method for preparing a catalyst for the production of melamine according to claim 1, characterized in that in step 5) the solid content of the spray slurry is 25wt%, and the spray granulation is carried out with a sprayer inlet-outlet temperature of 450 ℃, an outlet temperature of 115 ℃, and a feed pressure of 1.2 MPa.

7. The method for preparing a catalyst for melamine production according to claim 1, characterized in that the calcination in step 5) is performed by a temperature programming method, the temperature is raised to 500-600 ℃ at a temperature raising rate of 5-10 ℃/min, and then the calcination is continued for 2-20 hours at the temperature.

8. A catalyst for the production of melamine, characterized in that it is prepared by a process according to any one of claims 1 to 7.

9. The catalyst according to claim 8, wherein the catalyst product contains 65-85% of SiO ₂ 15-35% of Al ₂ O ₃ 0-5% of P ₂ O ₅ 。