Solid catalyst, preparation method thereof and method for preparing ammonia through urea hydrolysis

Abstract

The invention relates to the field of catalysts, and discloses a solid catalyst, a preparation method of the solid catalyst and a method for preparing ammonia through urea hydrolysis, and the catalyst comprises the following components in parts by weight: 70-95 parts of SiO2; 2 to 30 parts by weight of Al2O3; and 0.1 to 3 parts by weight of Dy2O3. According to the solid catalyst, SiO2, Al2O3 and Dy2O3 in specific parts by weight are matched, the dispersity of active components of the catalyst is good, the catalyst does not contain phosphorus, secondary pollution cannot be caused, and the urea hydrolysis rate is high and the ammonia production rate is high in the reaction of preparing ammonia through urea hydrolysis.

Description

Technical Field

[0001] The present invention relates to the field of catalysts, and particularly to a solid catalyst, a preparation method thereof, and a method for producing ammonia by urea hydrolysis. Background Art

[0002] The reducing agent ammonia in the SCR flue gas denitrification system can be prepared from liquid ammonia, urea, and ammonia water. Among them, the production of ammonia by urea hydrolysis is safe and economical, and the hydrolysis rate can be increased by adding a specific catalyst. The commonly used catalysts for producing ammonia by urea hydrolysis in engineering include ammonium dihydrogen phosphate and diammonium hydrogen phosphate, which will introduce phosphorus-containing compounds into the wastewater, and direct discharge will cause secondary pollution. Therefore, it is necessary to develop a pollution-free and efficient solid catalyst.

[0003] CN203513297U discloses a system for producing ammonia by hydrolyzing urea solution, which solves the problem of complex equipment in the urea hydrolysis ammonia production system, but does not use a catalyst, and the efficiency needs to be further improved.

[0004] CN102658159B discloses a catalyst coating for producing ammonia by hydrolyzing solid urea and a preparation method thereof. When the carrier temperature is 150 °C, the conversion rate of isocyanic acid reaches more than 95%. However, the main function of this catalyst is to convert isocyanic acid, not a direct catalyst for urea hydrolysis to produce ammonia, and it belongs to the field of diesel vehicle exhaust purification and is not suitable for large-scale industrial flue gas denitrification.

[0005] Therefore, there is a need for a catalyst that can improve the efficiency of urea hydrolysis to produce ammonia while reducing the generation of secondary pollution. Summary of the Invention

[0006] The purpose of the present invention is to overcome the problems of low hydrolysis rate of urea hydrolysis to produce ammonia and secondary pollution caused by phosphorus in homogeneous catalysts in the prior art, and to provide a solid catalyst, a preparation method thereof, and a method for producing ammonia by urea hydrolysis. The solid catalyst is combined with SiO 2 , Al 2 O 3 and Dy 2 O 3 in specific weight parts, with good dispersion of the catalyst active components, no phosphorus in the catalyst, no secondary pollution, high urea hydrolysis rate in the reaction of urea hydrolysis to produce ammonia, and fast ammonia production rate.

[0007] To achieve the above object, in a first aspect of the present invention, a solid catalyst is provided. Based on 100 parts by weight of the total amount of the catalyst, the catalyst comprises: 70 - 95 parts by weight of SiO 2 ; 2 - 30 parts by weight of Al 2 O 3 ; 0.1 - 3 parts by weight of Dy 2 O 3 .

[0008] Preferably, in the catalyst, the weight ratio of SiO 2 to Al 2 O 3 is 2 - 45:1, preferably 5 - 20:1.

[0009] Preferably, in the catalyst, the weight ratio of Al 2 O 3 to Dy 2 O 3 is 1 - 50:1, preferably 5 - 30:1.

[0010] The second aspect of the present invention provides a method for preparing a solid catalyst, wherein the method comprises the following steps:

[0011] (1) Prepare a mixed solution I containing an aluminum source and a dysprosium source;

[0012] (2) Mix a silicon source and a dispersion aid with the mixed solution I to obtain a mixed solution II, adjust the pH value of the mixed solution II to 2 - 6, and dry and calcine.

[0013] Preferably, the dispersion aid is tetraethylammonium hydroxide and / or ammonium bicarbonate.

[0014] Preferably, the dispersant is tetraethylammonium hydroxide and ammonium bicarbonate, and the mass ratio of tetraethylammonium hydroxide to ammonium bicarbonate is 0.1 - 1.

[0015] The third aspect of the present invention provides a solid catalyst prepared by the preparation method described in the second aspect.

[0016] The fourth aspect of the present invention provides a method for hydrolyzing urea to ammonia. In the presence of the solid catalyst described in the first aspect or the third aspect, an aqueous urea solution is subjected to a hydrolysis reaction.

[0017] Through the above technical solutions, the following beneficial effects are obtained:

[0018] (1) The solid catalyst provided by the present invention is combined with specific parts by weight of SiO 2 , Al 2 O 3 and Dy 2 O 3 , the dispersibility of the catalyst active components is good, the catalyst does not contain phosphorus, and no secondary pollution will be caused;

[0019] (2) The preparation method of the solid catalyst of the present invention is simple, and the catalyst has a high urea hydrolysis rate and a fast ammonia production rate in the reaction of hydrolyzing urea to ammonia. Specific Embodiments

[0020] The endpoints and any values in the ranges disclosed herein are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be considered to be specifically disclosed herein.

[0021] The first aspect of the present invention provides a solid catalyst, wherein, based on 100 parts by weight of the total amount of the catalyst, the catalyst comprises: 70 - 95 parts by weight of SiO 2 ; 2 - 30 parts by weight of Al 2 O 3 ; 0.1 - 3 parts by weight of Dy 2 O 3 .

[0022] The solid catalyst provided by the present invention is formulated with specific parts by weight of SiO 2 , Al 2 O 3 and Dy 2 O 3 . The active components of the catalyst are well-dispersed, the catalyst does not contain phosphorus, and it will not cause secondary pollution. In the reaction of urea hydrolysis to ammonia, the urea hydrolysis rate is high and the ammonia production rate is fast.

[0023] According to the present invention, preferably, based on 100 parts by weight of the total amount of the catalyst, the catalyst comprises: 78 - 93 parts by weight of SiO 2 ; 5 - 20 parts by weight of Al 2 O 3 ; 0.5 - 2 parts by weight of Dy 2 O 3 .

[0024] According to the present invention, preferably, the pore diameter of the catalyst is 1 - 5 nm, preferably 1 - 3 nm.

[0025] According to the present invention, preferably, the specific surface area of the catalyst is 300 - 400 m 2 / g, such as 300 m 2 / g, 310 m 2 / g, 320 m 2 / g, 330 m 2 / g, 340 m 2 / g, 350 m 2 / g, 360 m 2 / g, 370 m 2 / g, 380 m 2 / g, 390 m 2 / g, 400 m 2 / g, or in the range between any two of them, preferably 340 - 380 m 2 / g. In the present invention, the catalyst has a small pore size and a large specific surface area, which helps to increase the active sites in the catalyst, has good dispersion of the active components, and has a high hydrolysis rate and a fast ammonia production rate in the reaction for urea hydrolysis to produce ammonia.

[0026] In the present invention, the pore size and specific surface area of the catalyst are detected by a TRISTAR Ⅱ 302 specific surface area and porosity analyzer of Micromeritics Corporation of the United States.

[0027] According to the present invention, preferably, in the catalyst, the weight ratio of SiO 2 and Al 2 O 3 is 2 - 45:1, such as 2:1, 5:1, 10:1, 15:1, 20:1, 25:1, 30:1, 35:1, 40:1, 45:1, or in the range between any two of them, preferably 5 - 20:1.

[0028] According to the present invention, preferably, in the catalyst, the weight ratio of Al 2 O 3 and Dy 2 O 3 is 1 - 50:1, such as 1:1, 2:1, 5:1, 10:1, 15:1, 20:1, 25:1, 30:1, 35:1, 40:1, 45:1, 50:1, or in the range between any two of them, preferably 5 - 30:1.

[0029] In the present invention, for the solid catalyst satisfying the above mass ratio limit, Dy 2 O 3 is more uniformly dispersed, and can significantly improve the hydrolysis catalytic activity.

[0030] The second aspect of the present invention provides a method for preparing a solid catalyst, wherein the method includes the following steps:

[0031] (1) Prepare a mixed solution I containing an aluminum source and a dysprosium source;

[0032] (2) Mix a silicon source and a dispersion aid with the mixed solution I to obtain a mixed solution II, adjust the pH value of the mixed solution II to 2 - 6, and dry and calcine.

[0033] The method for preparing the solid catalyst provided by the present invention is simple in preparation method, easy to obtain raw materials, environmentally friendly and will not cause secondary pollution through the cooperation of a dispersion aid with a silicon source, a dysprosium source and an aluminum source. The prepared solid catalyst is used for urea hydrolysis to produce ammonia and has high hydrolysis catalytic activity.

[0034] According to the present invention, preferably, in the mixed solution I, based on oxides, the weight ratio of the aluminum source to the dysprosium source is 1 - 50:1, preferably 5 - 30:1.

[0035] In the present invention, in the mixed solution I, the mass concentrations of the aluminum source and the dysprosium source are not particularly limited, and those skilled in the art can adaptively adjust the addition amount of the solvent according to the dissolution conditions of the aluminum source and the dysprosium source in the mixed solution I. The solvent of the mixed solution I is water.

[0036] In the present invention, preferably, the mixed solution I is prepared under ultrasonic conditions, and the ultrasonic conditions are not particularly limited. Preferably, the ultrasonic conditions include: at room temperature, the ultrasonic time is 5 - 30 min.

[0037] According to the present invention, the type of the aluminum source is not particularly limited, and those skilled in the art can select from conventional soluble aluminum-containing compounds. Preferably, the aluminum source is AlCl 3 and / or Al(NO 3 ) 3 .

[0038] According to the present invention, the type of the dysprosium source is not particularly limited, and those skilled in the art can select from conventional soluble dysprosium-containing compounds. Preferably, the dysprosium source is Dy(NO 3 ) 3 and / or DyCl 3 .

[0039] According to the present invention, preferably, based on silicon dioxide for the addition amount of the silica sol and based on aluminum oxide for the addition amount of the aluminum source, the weight ratio of the silica sol to the aluminum source is 2 - 45:1, preferably 5 - 20:1.

[0040] According to the present invention, preferably, the weight ratio of the silica sol to the dispersion aid is 100:1 - 5, preferably 100:2 - 4.

[0041] According to the present invention, the dispersion aid is an aid that can make the silicon source, aluminum source, and dysprosium source disperse uniformly. Preferably, the dispersion aid is tetraethylammonium hydroxide and / or ammonium bicarbonate. More preferably, the dispersant is tetraethylammonium hydroxide and ammonium bicarbonate, and the mass ratio of tetraethylammonium hydroxide to ammonium bicarbonate is 0.1 - 1. The above combination of dispersion aids is more helpful for making the silicon source, aluminum source, and dysprosium source disperse uniformly.

[0042] According to a preferred embodiment of the present invention, the dispersion aid is formulated into an aqueous solution of tetraethylammonium hydroxide and ammonium bicarbonate, and then mixed with the mixed solution I.

[0043] According to the present invention, preferably, the mixing in step (2) is carried out under ultrasonic conditions.

[0044] According to the present invention, the conditions of the ultrasound are not particularly limited, and those skilled in the art can select conventional ultrasound mixing conditions to ensure that the substances in the mixed solution II are evenly mixed. Preferably, the ultrasound conditions include: the ultrasound temperature is 20 - 100 °C, preferably 40 - 80 °C; the ultrasound time is 10 - 30 min, preferably 15 - 25 min. In the present invention, under the above conditions, ultrasound can make the silicon source, aluminum source, and dysprosium source more evenly dispersed, improving the catalyst activity.

[0045] According to the present invention, preferably, in step (2), the pH value of the mixed solution II is adjusted to 3 - 5. In the present invention, adjusting the pH value of the mixed solution II to acidic can increase the specific surface area of the catalyst.

[0046] According to the present invention, the method of adjusting the pH value of the mixed solution II is not particularly limited, as long as the pH value of the mixed solution II meets the above - defined range. Preferably, an acid is added to adjust the pH value of the mixed solution II.

[0047] More preferably, the type of the acid is not particularly limited, and it is preferably selected from at least one of nitric acid, hydrochloric acid, and sulfuric acid. The mass concentration and the addition amount of the acid are not particularly limited, and those skilled in the art can select the mass concentration and the addition amount of the acid according to the pH value of the mixed solution II.

[0048] In the present invention, before drying, it further includes the steps of filtering and washing the solid product obtained by ultrasound. The filtering and washing methods are conventional filtering and washing methods in the art.

[0049] According to the present invention, the drying method and drying conditions are not particularly limited, and those skilled in the art can select conventional drying methods and drying conditions. Preferably, the drying conditions include: the drying temperature is 80 - 160 °C, and the drying time is 1 - 12 h.

[0050] According to the present invention, preferably, the calcination conditions include: the calcination temperature is 400 - 600 °C, preferably 440 - 560 °C; the calcination time is 2 - 8 hours, preferably 4 - 7 hours.

[0051] The third aspect of the present invention provides a solid catalyst prepared by the preparation method described in the second aspect.

[0052] The fourth aspect of the present invention provides a method for hydrolyzing urea to produce ammonia. In the presence of the solid catalyst described in the first aspect or the third aspect, an aqueous urea solution is subjected to a hydrolysis reaction.

[0053] In the present invention, when using the above - mentioned solid catalyst for the reaction of hydrolyzing urea to produce ammonia, the urea hydrolysis rate is high and the ammonia production rate is fast.

[0054] According to the present invention, preferably, the conditions for the hydrolysis reaction include: the reaction temperature is 100 - 300 °C, preferably 120 - 180 °C; the reaction pressure is 0.2 - 5 MPa, preferably 0.5 - 2 MPa; based on 1 L of the volume of the urea aqueous solution, the catalyst dosage is 0.5 - 5 g.

[0055] According to the present invention, preferably, the mass concentration of the urea aqueous solution is 10 - 70 wt%, preferably 30 - 50 wt%.

[0056] In the present invention, the reaction equipment for urea hydrolysis to ammonia is not particularly limited, and those skilled in the art can select conventional reaction equipment. Preferably, the reaction of urea hydrolysis to ammonia is carried out in a hydrolysis reaction kettle.

[0057] According to a particularly preferred embodiment of the present invention, a method for preparing a solid catalyst, the method includes the following steps:

[0058] (1) Prepare a mixed solution I containing an aluminum source and a dysprosium source;

[0059] (2) Mix a silica source and a dispersion aid with the mixed solution I to obtain a mixed solution II, adjust the pH value of the mixed solution II to 3 - 5, and dry and calcine.

[0060] In the mixed solution I, based on oxides, the weight ratio of the aluminum source to the dysprosium source is 5 - 30:1;

[0061] The addition amount of the silica sol is based on silicon dioxide, and the addition amount of the aluminum source is based on aluminum oxide. The weight ratio of the silica sol to the aluminum source is 5 - 20:1;

[0062] The weight ratio of the silica sol to the dispersion aid is 100:2 - 4;

[0063] The dispersion aid is tetraethylammonium hydroxide and ammonium bicarbonate, and the mass ratio of tetraethylammonium hydroxide to ammonium bicarbonate is 0.1 - 1.

[0064] The present invention will be described in detail below through examples and comparative examples. In the following examples and comparative examples, unless otherwise specified, the reagents used are commercially available;

[0065] The test methods for the pore size and specific surface area of the catalyst are described in the foregoing specification and will not be elaborated here.

[0066] Example 1

[0067] Take 10 g of Al(NO 3 ) 3 and 1 g of Dy(NO 3 ) 3 ·6H 2O was dissolved in 20 mL of water to prepare a mixed solution I. 100 g of silica sol (silicon dioxide content 29 - 31 wt%) and 20 mL of an aqueous solution containing 0.5 g of tetraethylammonium hydroxide and 2 g of ammonium bicarbonate were added to the prepared mixed solution I, and the mixture was ultrasonically treated for 10 min. Then 5 mL of dilute nitric acid (mass concentration 1%) was added, the pH value of the solution was adjusted to 5, and it was ultrasonically treated at 80 °C for 20 min, dried at 120 °C for 6 h, and calcined at 450 °C for 4 h to obtain the catalyst.

[0068] The conditions for urea hydrolysis to ammonia were as follows: feed urea aqueous solution concentration: 50 wt%;

[0069] Solid catalyst addition amount: 1 g / L urea aqueous solution;

[0070] Hydrolysis reactor volume: 500 mL;

[0071] Reaction temperature: 150 °C;

[0072] Reaction pressure: 0.6 MPa.

[0073] Example 2

[0074] 3.3 g of AlCl 3 and 0.6 g of DyCl 3 ·6H 2 O was dissolved in 15 mL of water to prepare a mixed solution I. 100 g of silica sol (silicon dioxide content 29 - 31 wt%) and 20 mL of an aqueous solution containing 0.6 g of tetraethylammonium hydroxide and 2.6 g of ammonium bicarbonate were added to the prepared mixed solution I, and the mixture was ultrasonically treated for 10 min. Then 8 mL of dilute nitric acid (mass concentration 1%) was added, the pH value of the solution was adjusted to 4, and it was ultrasonically treated at 70 °C for 20 min, dried at 120 °C for 6 h, and calcined at 450 °C for 4 h to obtain the catalyst.

[0075] The conditions for urea hydrolysis to ammonia are shown in Table 2.

[0076] Example 3

[0077] The catalyst was prepared according to the method of Example 1, except that the dispersing aid was an aqueous ammonium bicarbonate solution with a mass concentration of 5% (20 mL was added), and other conditions were the same as those in Example 1.

[0078] The conditions for urea hydrolysis to ammonia are shown in Table 2.

[0079] Example 4

[0080] The reaction of urea hydrolysis to ammonia was carried out using the catalyst of Example 1, except that the urea hydrolysis reaction temperature was 110 °C, the reaction pressure was 0.3 MPa, and the feed urea solution concentration was 10%.

[0081] Example 5

[0082] The catalyst was prepared according to the method of Example 1, except that the addition amount of Dy(NO 3 ) 3 ·6H 2 O was reduced, so that the weight ratio of Al 2 O 3 and Dy 2 O 3 in the catalyst was 40:1, and other conditions were the same as those in Example 1.

[0083] The conditions for urea hydrolysis to ammonia are shown in Table 2.

[0084] Example 6

[0085] The catalyst was prepared according to the method of Example 1, except that the addition amount of dilute nitric acid (mass concentration 1%) was changed and the pH value of the solution was adjusted to 6, and other conditions were the same as those in Example 1.

[0086] The conditions for urea hydrolysis to ammonia are shown in Table 2.

[0087] Example 7

[0088] The catalyst was prepared according to the method of Example 1, except that 10 g of Al(NO 3 ) 3 and 1 g of Dy(NO 3 ) 3 ·6H 2 O, 100 g of silica sol (silica content 29 - 31 wt%), and 20 mL of an aqueous solution containing 0.5 g of tetraethylammonium hydroxide and 2 g of ammonium bicarbonate were dissolved in 20 mL of water, and dilute nitric acid (mass concentration 1%) was added to adjust the pH value of the solution to 5, and other conditions were the same as those in Example 1.

[0089] The conditions for urea hydrolysis to ammonia are shown in Table 2.

[0090] Example 8

[0091] The catalyst was prepared according to the method of Example 1, except that the dispersion aid was 0.5 g of tetraethylammonium hydroxide and ammonium bicarbonate was not contained, and other conditions were the same as those in Example 1.

[0092] The conditions for urea hydrolysis to ammonia are shown in Table 2.

[0093] Comparative Example 1

[0094] The reaction of urea hydrolysis to ammonia was carried out under the conditions of Example 1, except that no catalyst was added.

[0095] Comparative Example 2

[0096] The catalyst was prepared according to the method of Example 1, except that Dy(NO3 ) 3 ·6H 2 O, with other conditions the same as in Example 1, a catalyst was prepared.

[0097] The conditions for urea hydrolysis to ammonia are shown in Table 2.

[0098] Comparative Example 3

[0099] A catalyst was prepared according to the method of Example 1, except that Dy(NO 3 ) 3 ·6H 2 O was replaced by La(NO 3 ) 3 ·6H 2 O, such that the mass of La 2 O 3 in the catalyst was equal to that of Dy 2 O 3 . With other conditions the same as in Example 1, a catalyst was prepared.

[0100] Test Example

[0101] The test results of the pore size and specific surface area of the catalyst are shown in Table 1.

[0102] Table 1

[0103] Aperture (nm) <![CDATA[Specific surface area (m 2 / g)]]> Example 1 1.9 366 Example 2 2.4 358 Example 3 3.3 330 Example 4 1.9 366 Example 5 3.7 327 Example 6 3.8 319 Example 7 4.7 322 Example 8 4.4 303 Comparative Example 2 5.7 288 Comparative Example 3 5.2 291

[0104] It can be seen from Table 1 that the catalyst prepared by the preparation method of the embodiment of the present invention has a smaller pore size and a larger specific surface area.

[0105] The conditions and results of the urea hydrolysis to ammonia reaction are shown in Table 2.

[0106] Urea hydrolysis rate = (total urea weight - remaining urea weight at 1 h of reaction) / total urea weight;

[0107] Ammonia production rate = weight of ammonia produced in the reaction / reaction time;

[0108] Table 2

[0109]

[0110]

[0111] It can be seen from the results in Table 2 that when the catalyst of the embodiment of the present invention is used for urea hydrolysis to ammonia, the urea hydrolysis rate is high and the ammonia production rate is fast.

[0112] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A solid catalyst, characterized in that, Based on 100 parts by weight of the total catalyst, the catalyst comprises: 70 - 95 parts by weight of SiO 2 ; 2 - 30 parts by weight of Al 2 O 3 ; 0.1 - 3 parts by weight of Dy 2 O 3 .

2. The catalyst according to claim 1, wherein, Based on 100 parts by weight of the total catalyst, the catalyst comprises: 78 - 93 parts by weight of SiO 2 ; 5 - 20 parts by weight of Al 2 O 3 ; 0.5 - 2 parts by weight of Dy 2 O 3 ; preferably, the pore diameter of the catalyst is 1-5 nm, preferably 1-3 nm; Preferably, the specific surface area of the catalyst is 300-400 m 2 / g, preferably 340-380 m 2 / g.

3. The catalyst according to claim 1 or 2, wherein, In the catalyst, the weight ratio of SiO 2 and Al 2 O 3 is 2 - 45:1, preferably 5 - 20:1; Preferably, Al 2 O 3 and Dy 2 O 3 are in a weight ratio of 1 - 50:1, preferably 5 - 30:

1.

4. A method for preparing a solid catalyst, characterized in that, the method comprises the following steps: (1) Prepare a mixed solution I containing an aluminum source and a dysprosium source; (2) Mix a silica source and a dispersion aid with the mixed solution I to obtain a mixed solution II, adjust the pH value of the mixed solution II to 2-6, and dry and calcine.

5. The preparation method according to claim 4, wherein, in the mixed solution I, calculated as oxides, the weight ratio of the aluminum source to the dysprosium source is 1-50:1, preferably 5-30:1; Preferably, the aluminum source is AlCl 3 and / or Al(NO 3 ) 3 ; Preferably, the dysprosium source is Dy(NO 3 ) 3 and / or DyCl 3 .

6. The preparation method according to claim 4 or 5, wherein, the addition amount of the silica sol is calculated as silicon dioxide, the addition amount of the aluminum source is calculated as aluminum oxide, and the weight ratio of the silica sol to the aluminum source is 2-45:1, preferably 5-20:1; preferably, the weight ratio of the silica sol to the dispersion aid is 100:1-5, preferably 100:2-4; preferably, the dispersion aid is tetraethylammonium hydroxide and / or ammonium bicarbonate; preferably, the dispersant is tetraethylammonium hydroxide and ammonium bicarbonate, and the mass ratio of tetraethylammonium hydroxide to ammonium bicarbonate is 0.1-1.

7. The preparation method according to any one of claims 4-6, wherein, the mixing in step (2) is carried out under ultrasonic waves; preferably, the ultrasonic conditions include: the ultrasonic temperature is 20-100 °C, preferably 40-80 °C; the ultrasonic time is 10-30 min, preferably 15-25 min; preferably, in step (2), the pH value of the mixed solution II is adjusted to 3-5; preferably, an acid is added to adjust the pH value of the mixed solution II.

8. The preparation method according to any one of claims 4-7, wherein, the drying conditions include: the drying temperature is 80-160 °C, and the drying time is 1-12 h; preferably, the calcination conditions include: the calcination temperature is 400-600 °C, preferably 440-560 °C; the calcination time is 2-8 hours, preferably 4-7 hours.

9. A solid catalyst prepared by the preparation method according to any one of claims 4-8.

10. A method for hydrolyzing urea to produce ammonia, characterized in that, in the presence of the solid catalyst according to any one of claims 1-3, 9, the urea aqueous solution is subjected to a hydrolysis reaction; preferably, the conditions of the hydrolysis reaction include: the reaction temperature is 100-300 °C, preferably 120-180 °C; the reaction pressure is 0.2-5 MPa, preferably 0.5-2 MPa; based on 1 L of the volume of the urea aqueous solution, the catalyst dosage is 0.5-5 g; preferably, the mass concentration of the urea aqueous solution is 10-70 wt%, preferably 30-50 wt%.

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