Background
The activated alumina has good physicochemical properties such as large specific surface area, adjustable pore structure, different properties of acid centers on the surface, higher mechanical strength, thermal stability and the like, and has wide application in the fields of catalysis, adsorption separation and the like. The properties of the aluminum oxide material such as morphology and the like are one of the key factors influencing the performance of the catalytic material, and the controllable synthesis of the morphology and the size of the aluminum oxide is an important physical property regulation and control means. At present, aluminum oxide materials with different shapes such as flake shapes, rod shapes, spherical shapes and the like are prepared by adopting different methods.
Mater.Lett.2013,94,104-107 ] utilize ionic liquids to obtain fluorine-containing aluminum compounds with octahedral morphology with the size of about 5 μm, but the raw materials used in the method are expensive and seriously polluted.
Chem. mater.2009,21(24),5695-5697 takes m-trimellitic acid as a complexing agent to obtain the aluminum-containing octahedral MOF material, but the process conditions are harsh.
CN201610494090.3 provides a preparation method of octahedral alumina with micron size. The XRD spectrum of the product obtained by the method at 200 ℃ shows that the octahedral aluminum oxide does not have a boehmite structure or a pseudo-boehmite structure, so that the octahedral aluminum oxide cannot be converted into gamma-aluminum oxide under the conventional roasting condition of 500-700 ℃, and the application requirement in the field of catalysis cannot be well met. The repeatability tests show that the octahedral alumina provided by CN201610494090.3 has a single crystal structure, and the large grain size is not beneficial to improving the specific surface area of the material, so that the application of the octahedral alumina in various fields such as catalysis, adsorption separation and the like has great limitation.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides gamma-alumina octahedron particles with high specific surface area and a preparation method thereof.
The gamma-alumina octahedron particles with high specific surface area have a polycrystalline structure and approximate regular octahedron appearance, the edge length of the octahedron is 30-60 mu m, and the particle size is 45-85 mu m. The specific surface area of the octahedral polycrystalline gamma-alumina particles is 100-250m2The distribution range of the mesoporous aperture is 10-20 nm.
The preparation method of the gamma-alumina octahedral particle with high specific surface area comprises the following steps:
(1) adding a certain amount of inorganic sodium and/or inorganic potassium salt into alumina precursor powder, grinding the powder by using a ball mill, preparing the ground powder into suspension with a certain concentration, adding a certain amount of organic acid, and moderately evaporating excessive water under the stirring condition to form wet material;
(2) tabletting the wet material obtained in the step (1), forming and drying, wherein the dried material is divided into two parts, one part is subjected to mild roasting, then the other part is subjected to crushing and screening to obtain particles I with proper particle size, and the other part is subjected to severe roasting, then the particles II with proper particle size are crushed and screened;
(3) and (3) mixing the particles I and II obtained in the step (2), immersing the particles in a reaction medium, carrying out hydrothermal reaction under sealed autogenous pressure, and drying and roasting after the reaction is finished to obtain a product.
In the method of the present invention, the alumina precursor in step (1) refers to aluminum hydroxide and modified aluminum hydroxide powder including but not limited to aluminum hydroxide powder containing various elements such as silicon, boron, titanium, magnesium, lanthanum, etc.
In the method, the material in the step (1) is subjected to ball milling treatment to obtain powder particles above 2000 meshes, and the particle size of the powder is preferably 2000-10000 meshes.
In the method, the inorganic sodium salt in the step (1) is sodium nitrate and sodium chloride, and the inorganic potassium salt is potassium nitrate and potassium chloride. The amount of the inorganic sodium salt and/or the inorganic potassium salt accounts for 10-30% of the mass of the alumina precursor powder. The inorganic sodium salt and the inorganic potassium salt may be used in combination in any ratio.
In the method of the present invention, the concentration of the suspension in step (1) is 200-600g/L, preferably 300-500 g/L.
In the method of the present invention, the organic acid in step (1) is one or more of formic acid, acetic acid or citric acid, and the amount of the organic acid is 1 to 10%, preferably 4 to 8% of the mass of the alumina precursor powder.
In the method, the tabletting conditions in the step (2) are as follows: the applied pressure is 0.05-0.4MPa/mm2The pressing time is 5-100 seconds, and the preferable pressing pressure is 0.1-0.3 MPa/mm2The pressing time is 10-50 seconds.
In the method, the wet material in the step (1) has the dry and wet degree based on that no adhesion occurs after the step (2) is formed and the demoulding is easy.
In the method of the invention, the drying temperature in the step (2) is 20-250 ℃, preferably 60-150 ℃, and the drying time is 1-12 hours, preferably 3-6 hours.
In the method, the mild roasting conditions in the step (2) are as follows: roasting at 250-450 deg.C for 2-48 hr, preferably at 300-400 deg.C for 6-12 hr.
In the method, the severe roasting condition in the step (2) is as follows: roasting at 850 ℃ for 2-48 hours at 550 ℃, preferably at 750 ℃ for 6-12 hours at 600 ℃.
In the method of the invention, the particle size range of the particle I in the step (2) is 2000-5000 meshes, and the particle size range of the particle II is 100-1000 meshes.
In the method, the mass ratio of the particles I to the particles II in the step (3) is 2: 1-6: 1, preferably 3: 1-5: 1.
in the method, the closed hydrothermal reaction in the step (3) is carried out, wherein a reaction medium is a mixture of water, alcohol amine and ethanol, the mass percentage of the mixture is taken as a reference, the alcohol amine accounts for 5-35%, the low-carbon alcohol accounts for 10-45%, and the balance is water; the alcohol amine is one or more of ethanolamine, diethanolamine or triethanolamine; the lower alcohol is one or more of methanol, ethanol or propanol, preferably ethanol.
In the method, the closed hydrothermal treatment conditions in the step (3) are as follows: the temperature is 100-300 ℃, preferably 150-200 ℃, and the time is 2-72 hours, preferably 12-48 hours.
In the method of the invention, the drying temperature in the step (3) is not more than 250 ℃, preferably not more than 120 ℃, and the drying degree is the constant weight of the material at the drying temperature.
In the method, the roasting conditions in the step (3) are as follows: the calcination at 750 ℃ for 1-24 hours in 500-750 ℃, preferably at 650 ℃ for 3-12 hours in 500-650 ℃.
The gamma-alumina octahedral particles with high specific surface area can be used as catalyst carrier materials for reactions such as selective hydrogenation and olefin reduction of catalytic gasoline, deep hydrodesulfurization of distillate oil, propylene preparation by propane dehydrogenation, selective hydrogenation of acetylene and the like, or adsorption separation materials in other fields.
Detailed Description
The process of the present invention is illustrated in detail by the following examples. The shape and size of the alumina particles were observed and measured according to a scanning electron microscope. The crystal form is characterized by X-ray diffraction, and the sample is judged to be single crystal or polycrystal through electron diffraction. According to the scanning electron microscope image, the distance between two top ends of the octahedral crystal grains is taken as the crystal grain size.
Example 1
A proper amount of aluminum hydroxide powder is uniformly mixed with sodium chloride with the mass content of 20%, particles with the particle size not less than 3000 meshes are screened after ball milling, and then 200g/L of water suspension is prepared. And adding acetic acid into the suspension, wherein the dosage of the acetic acid is 6 percent of the mass of the calcined powder. Evaporating water to moderate humidity under stirring, and molding with tablet press (tabletting condition: 0.30 MPa/mm)2And 20 seconds. ) The formed block is dried for 12 hours at 120 DEG CThen, the mixture was lightly and heavily calcined at 400 ℃ and 650 ℃ for 8 hours and 6 hours, respectively. After roasting, crushing the lightly roasted and heavily roasted substances, respectively screening particles of 2000 meshes and 500 meshes, and simultaneously immersing the particles and the particles in a hydrothermal medium according to the mass ratio of 4/1. The hydrothermal medium comprises the following components in percentage by mass: 15% of amine alcohol, 40% of ethanol and the balance of water. The temperature and time of the hydrothermal reaction were 200 ℃ and 12 hours, respectively. After the hydrothermal reaction, the sample was washed. After drying at 120 ℃, roasting two samples at 200 ℃ and 550 ℃ for 5 hours respectively to obtain the product. The observation of a scanning electron microscope shows that the appearance of the product is approximately regular octahedron, the edge length is about 35 mu m, and the particle size is about 50 mu m. The BET specific surface area of the material is 187m2The mesoporous distribution interval is 11-15 nm.
XRD results of the products after roasting at 200 ℃ and 550 ℃ show that the phase states of the products are pseudo-boehmite and gamma-alumina respectively. The electron diffraction of the particles is annular, indicating a polycrystalline structure.
Example 2
A proper amount of aluminum hydroxide powder is uniformly mixed with 30% by mass of sodium chloride, particles with the particle size not less than 4000 meshes are screened after ball milling, and then 300g/L of water suspension is prepared. And adding citric acid into the suspension, wherein the dosage of the citric acid is 7 percent of the mass of the roasted powder. Evaporating water to moderate humidity under stirring, and molding with tablet press (tabletting condition: 0.25 MPa/mm)2And 30 seconds. ) The shaped block is dried at 120 deg.C for 12 hours and then lightly and heavily calcined at 400 deg.C and 750 deg.C for 8 hours and 8 hours, respectively. After roasting, crushing the lightly roasted and heavily roasted substances, respectively screening particles of 2000 meshes and 1000 meshes, and simultaneously immersing the particles and the particles in a hydrothermal medium according to the mass ratio of 5/1. The hydrothermal medium comprises the following components in percentage by mass: 20% of amine alcohol, 35% of ethanol and the balance of water. The temperature and time of the hydrothermal reaction were 180 ℃ and 18 hours, respectively. After the hydrothermal reaction, the sample was washed. After drying at 120 ℃, roasting two samples at 200 ℃ and 550 ℃ for 5 hours respectively to obtain the product. The observation of a scanning electron microscope shows that the product appearance is approximately regular octahedron, the edge length is about 39 mu m, and the particle size is about 56 mu m. The BET specific surface area of the material is161m2The mesoporous distribution interval is 14-18 nm.
XRD results of the products after roasting at 200 ℃ and 550 ℃ show that the phase states of the products are pseudo-boehmite and gamma-alumina respectively. The electron diffraction of the particles is annular, indicating a polycrystalline structure.
Example 3
Uniformly doping a proper amount of aluminum hydroxide powder into 25 mass percent of potassium chloride, screening particles with the particle size not less than 4000 meshes after ball milling, and preparing 350g/L aqueous suspension. And adding citric acid into the suspension, wherein the dosage of the citric acid is 8 percent of the mass of the roasted powder. Evaporating water to moderate humidity under stirring, and molding with tablet press (tabletting condition: 0.20 MPa/mm)2And 50 seconds. ) The formed block is dried at 120 deg.C for 12 hr, and then lightly and heavily calcined at 350 deg.C and 750 deg.C for 8 hr and 4 hr, respectively. After roasting, crushing the lightly roasted and heavily roasted substances, respectively screening particles of 2000 meshes and 500 meshes, and simultaneously immersing the particles and the particles in a hydrothermal medium according to the mass ratio of 5/1. The hydrothermal medium comprises the following components in percentage by mass: 30% of amine alcohol, 30% of ethanol and the balance of water. The temperature and time of the hydrothermal reaction were 250 ℃ and 24 hours, respectively. After the hydrothermal reaction, the sample was washed. After drying at 120 ℃, roasting two samples at 200 ℃ and 550 ℃ for 5 hours respectively to obtain the product. The observation of a scanning electron microscope shows that the appearance of the product is approximately regular octahedron, the edge length is about 55 mu m, and the particle size is about 75 mu m. The BET specific surface area of the material is 138m2The mesoporous distribution interval is 15-20 nm.
XRD results of the products after roasting at 200 ℃ and 550 ℃ show that the phase states of the products are pseudo-boehmite and gamma-alumina respectively. The electron diffraction of the particles is annular, indicating a polycrystalline structure.
Comparative example 1
Octahedral alumina particles were prepared according to the method of CN201610494090.3 example 1. The obtained product is roasted for 6 hours at 200 ℃ and 550 ℃ respectively. The 200 ℃ roast did not have a pseudo-boehmite or boehmite structure, the 550 ℃ roast failed to form gamma-alumina, and the particles at this time were analyzed by electron diffraction to have a single crystal structure. The BET specific surface area of the calcined product at 550 ℃ is only 84m2/g。