A quasi-homogeneous high-thermal-stable magnesio-alumina spinel aerogel, a preparation method and application thereof

Abstract

The application discloses pseudo-homogeneous high-thermal-stability magnesium-aluminum spinel aerogel as well as a preparation method and application thereof. The magnesium-aluminum spinel structure is controlled by changing experimental parameters such as a magnesium-aluminum molar ratio, an ethanol amount, a propylene oxide amount and the like, the magnesium-aluminum spinel structure is controllable, the control structure process is high in operability, the prepared high-thermal-stability magnesium-aluminum aerogel is high in porosity, large in specific surface area, small in size and uniform in particle, can still maintain the stable structure of the magnesium-aluminum spinel after high-temperature heat treatment, and the surface of the modified magnesium-aluminum spinel aerogel is successfully grafted with a hydrophobic group through modification by using dodecyl triethoxysilane, so that the magnesium-aluminum spinel aerogel is hydrophobic and oleophilic, and is stably dispersed in hydrocarbon fuel, thereby providing a possibility for realizing pseudo-homogeneous catalytic cracking of the hydrocarbon fuel.

Description

Technical Field

[0001] This invention belongs to the field of magnesium aluminum spinel aerogel technology, specifically relating to a method for regulating and modifying the structure of a pseudo-homogeneous, highly thermally stable magnesium aluminum spinel aerogel and its application. Background Technology

[0002] Aerogels are solid materials with a gaseous dispersion medium, consisting of a nanoporous network structure of colloidal particles or polymer molecules, and possessing a low density (0.002–0.35 g / cm³). 3 Aerogels possess properties such as low thermal conductivity (0.001–0.005 W / m·K) and high strength (capable of withstanding 1000 times their own weight). Due to their unique structure, aerogels exhibit many unique physicochemical properties, making them highly valuable in many fields. Their large specific surface area, high porosity, and interconnectedness make them excellent adsorbents and catalyst supports, applicable to catalysis, nuclear waste storage, and the containment of fusion fuels.

[0003] The unique environment of aerospace engines, characterized by high airspeeds and short dwell times, dictates that heterogeneous catalysts such as fixed-bed catalysts cannot be used; instead, homogeneous catalysis is required. Most homogeneous catalysts cannot withstand the high temperatures required for combustion or chemical reactions (dehydrogenation, cracking, reforming, etc.) in aerospace engines. Researchers have proposed a method for using "pseudo-homogeneous" catalysts for fuel chemical reactions (catalytic cracking): cracking catalysts are prepared as oil-soluble (lipophilic) nanoparticles and added as additives to the fuel for catalytic cracking reactions. Monodisperse nanoparticles protected by active groups are an ideal "pseudo-homogeneous" catalyst. First, nanoparticles can be smoothly transported and atomized with the fuel, and the large contact area provides a large number of highly active surface defect sites and contact sites. Second, the amphoteric groups of the protecting agent can "anchor" the solid particles in the fuel, and combined with the inherent dispersion stability of the monodisperse particles, the catalyst exhibits good oil solubility and can be stably dispersed in the fuel for a long period.

[0004] Aerogels, possessing both liquid and gaseous properties, are a relatively ideal "pseudo-homogeneous" catalyst. By controlling the structure and properties of aerogels, a relatively stable nanofluid can be formed by mixing oleophilic aerogels with hydrocarbon fuels. Suspended nano-aerogels can provide a higher specific surface area and sufficient active sites, offering a practical technical path for the pseudo-homogeneous catalytic cracking of fuels using aerogel catalysts. However, currently, there are still some technical challenges in using aerogels as "pseudo-homogeneous" catalysts for supercritical (high temperature and high pressure) hydrocarbon fuel catalytic cracking. For example, mature technologies such as silica aerogels and alumina aerogels still suffer from poor thermal stability at high temperatures (1000℃) and are prone to coking during the cracking process; magnesium aluminum spinel aerogels have bottlenecks such as poor structural controllability and difficulty in adjusting oleophilicity. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by providing a pseudo-homogeneous, highly thermally stable magnesium aluminum spinel (MgAl2O4) aerogel, its preparation method, and its application. By regulating the structure and modifying the oleophilicity of the magnesium aluminum spinel aerogel, pseudo-homogenization of the aerogel is achieved, and the magnesium aluminum aerogel is applied to the field of fuel catalytic cracking to further improve the catalytic effect.

[0006] The method for preparing pseudo-homogeneous, highly thermally stable magnesium-aluminum aerogel provided by this invention includes the following steps:

[0007] (1) Dissolve a certain amount of aluminum nitrate nonahydrate and magnesium nitrate hexahydrate in anhydrous ethanol solution, and stir at 20-50℃ until completely dissolved to obtain a hydrolyzed binary magnesium-aluminum mixed salt solution system.

[0008] (2) Add propylene oxide to the binary mixed salt solution system obtained in step (1) and stir the mixture uniformly at 20-50°C until gel is formed.

[0009] (3) Place the gel obtained in step (2) in an environment of 20-40℃ and age it for 48 hours;

[0010] (4) The gel aged in step (3) is subjected to supercritical drying to obtain magnesium aluminum spinel aerogel precursor.

[0011] (5) The magnesium aluminum spinel aerogel precursor obtained in step (4) is heat-treated in a muffle furnace to obtain magnesium aluminum spinel aerogel.

[0012] (6) Grind the magnesium aluminum spinel aerogel into powder, mix it thoroughly with toluene, and then ultrasonically disperse it to obtain a suspension with an aerogel solid content of 3-8%.

[0013] (7) Add dodecyltriethoxysilane to the suspension described in step (1), disperse it by ultrasonication, and reflux it at 100°C for 24 to 72 hours; the amount of dodecyltriethoxysilane is 10% to 50% of the mass of magnesium aluminum spinel aerogel in the suspension.

[0014] (8) Centrifuge the refluxed mixed liquid to remove unreacted dodecyltriethoxysilane, and dry the separated solid vacuum tube to obtain modified magnesium aluminum spinel aerogel.

[0015] Furthermore, in step (1), the amounts of aluminum nitrate nonahydrate and magnesium nitrate hexahydrate used satisfy the molar ratio of magnesium to aluminum, Mg / Al = 0.5, 1.0, or 1.5.

[0016] Furthermore, in step (1), the molar ratio of ethanol to the total amount of magnesium and aluminum in the magnesium-aluminum spinel aerogel is (9.5~15.5):1.

[0017] Furthermore, in step (2), the molar ratio of propylene oxide to the total molar amounts of magnesium and aluminum in aluminum nitrate nonahydrate and magnesium nitrate hexahydrate is (1.0~3.0):1.

[0018] Further, the supercritical drying in step (4) is ethanol or carbon dioxide supercritical drying; ethanol supercritical drying uses a high-pressure reactor to control the supercritical pressure, the reaction temperature during drying is 250-270℃, the pressure inside the reactor is 7.0-9.0MPa, and the drying time is 1-4h; carbon dioxide supercritical drying uses a supercritical dryer, the displacement temperature during drying is 5℃, the displacement time is 5-8h, the reaction temperature for supercritical drying is 35-45℃, the pressure inside the supercritical dryer is 8.0-10.0MPa, the gas release rate is 2.5-5.0mL / min, and the drying time is 1-3h.

[0019] Further, the atmosphere for the heat treatment in step (5) is nitrogen or air, the temperature is 800-1000℃, the heating rate is 10℃ / min, and the heat treatment time is 1-5h.

[0020] Furthermore, the reflux operation in step (7) is carried out in an oil bath.

[0021] Furthermore, after centrifugation in step (8), the sample is repeatedly washed with toluene and then centrifuged again.

[0022] Furthermore, the vacuum drying described in step (8) is performed at 60–80°C for 24–72 hours.

[0023] The pseudo-homogeneous, highly thermally stable magnesium aluminum spinel aerogel prepared by this invention has a specific surface area of ​​10³ m² after aging at 1000℃. 2 It has a stable structure and complete spinel crystal form; and through modification, it has oleophilic properties, thus it can be stably dispersed in hydrocarbon fuels. After being dissolved in fuel oil and left to stand for 48 hours, no sedimentation occurs.

[0024] The present invention also provides a pseudo-homogeneous, highly thermally stable magnesium-aluminum aerogel prepared by the above method.

[0025] This invention also provides the application of the above-mentioned pseudo-homogeneous, highly thermally stable magnesium-aluminum aerogel in the field of fuel catalytic cracking. The application is as a catalyst for pseudo-homogeneous catalytic cracking of fuels.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] 1. This invention regulates the structure of magnesium-aluminum aerogels by changing experimental parameters such as the magnesium-aluminum molar ratio, the amount of ethanol as solvent, and the amount of propylene oxide as gelling agent. This achieves controllable magnesium-aluminum spinel structure, and the structure regulation process is highly operable. The prepared high-thermal-stability magnesium-aluminum aerogels exhibit high porosity and large specific surface area (Table 2, Appendix). Figure 1-2 The small size and uniform particle size of the magnesium aluminum spinel allow it to maintain a stable structure even after high-temperature heat treatment, which provides a guarantee for the subsequent preparation of catalysts supported on metallic Ni. Catalysts prepared using it as a support exhibit superior catalytic performance when used in hydrocarbon fuel cracking processes.

[0028] 2. The method of this invention utilizes dodecyltriethoxysilane modification. Under certain conditions, the ethoxy group at the end of the silane coupling agent reacts with the hydroxyl group on the surface of the prepared aerogel, successfully grafting hydrophobic groups onto the surface of the modified magnesium aluminum spinel aerogel. This results in a hydrophobic and oleophilic composition, thus stably dispersing it in hydrocarbon fuels (Table 1, Appendix). Figure 3 , 4 This provides the possibility of achieving "pseudo-homogeneous" catalytic cracking of hydrocarbon fuels. Attached Figure Description

[0029] Figure 1 This is a flowchart illustrating the preparation method and structural property control of the present invention;

[0030] Figure 2 The structural properties of magnesium aluminum spinel aerogel catalyst after high-temperature aging;

[0031] Figure 3 The contact angle with fuel oil and water in the oleophilicity test before and after modification of magnesium aluminum spinel aerogel;

[0032] Figure 4 The dispersion of magnesium aluminum spinel aerogel in n-decane is shown in the following diagrams: (a) coprecipitation method; (b) oleic acid modification; (c) Example 1; (d) Example 1 after standing for 48 hours. Detailed Implementation

[0033] The present invention will be further illustrated below through embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the above-described invention, and these improvements and adjustments still fall within the scope of protection of the invention.

[0034] Example 1

[0035] (1) The amount of aluminum nitrate nonahydrate and magnesium nitrate hexahydrate precursors was calculated according to the magnesium / aluminum molar ratio Mg / Al=0.5. They were dissolved in anhydrous ethanol solution and stirred at a temperature of about 30℃ until completely dissolved to obtain a hydrolyzed binary mixed solution system; wherein the molar ratio of ethanol to magnesium and aluminum was 10.5:1.

[0036] (2) Add propylene oxide to the above binary mixed solution; stir evenly and react until gel forms. The molar ratio of propylene oxide to magnesium and aluminum is 2.5:1.

[0037] (3) Place the obtained gel in an environment of 40°C for 48 hours to age.

[0038] (4) The aged gel was subjected to supercritical carbon dioxide drying to obtain magnesium aluminum spinel aerogel precursor.

[0039] (5) The magnesium aluminum spinel aerogel precursor was placed in a muffle furnace and heat-treated at 800°C under a nitrogen atmosphere to finally obtain magnesium aluminum spinel aerogel.

[0040] (6) Grind the spinel aerogel into powder, then add 2g of powder to 30mL of toluene and mix evenly. After ensuring thorough mixing, ultrasonically disperse for 1h.

[0041] (7) Take 200 mg of dodecyltriethoxysilane and add it to the above mixed suspension. Disperse it by ultrasonication for 1 h; then reflux it at 100 °C for 24 h.

[0042] (8) The mixed liquid after reflux reaction was centrifuged on a high-speed (10000rpm) centrifuge for 10min, and the unreacted silane coupling agent was removed by repeated washing with toluene; finally, it was vacuum dried at 60℃ for 48h to obtain the modified magnesium aluminum spinel aerogel.

[0043] Comparative Example 1

[0044] Preparation of nanostructured magnesium aluminum spinel by co-precipitation method:

[0045] Magnesium nitrate hexahydrate and aluminum nitrate nonahydrate, as magnesium-aluminum precursors, were mixed and dissolved in deionized water at a mass ratio of 3:7. The mixture was then co-titrated with an ammonia buffer solution (pH = 10) prepared from concentrated ammonia and ammonium carbonate at a volume ratio of 1:1.25 to induce precipitation. After aging and filtration, a white precipitate was obtained, which was then dried at 90°C for 24 h and pre-decomposed at 300°C. Finally, it was calcined in air at 800°C for 4 h to obtain magnesium-aluminum spinel nanomaterials.

[0046] Compared with the magnesium aluminum spinel aerogel prepared in Example 1, the magnesium aluminum spinel support prepared in Comparative Example 1 has a larger particle size, higher density, and very poor oleophilicity, making it unable to be stably dispersed in fuel oil (see attached). Figure 4 a).

[0047] Example 2

[0048] (1) The amount of aluminum nitrate nonahydrate and magnesium nitrate hexahydrate precursors was calculated according to the magnesium / aluminum molar ratio Mg / Al=1.0. They were dissolved in anhydrous ethanol solution and stirred at a temperature of about 30℃ until completely dissolved to obtain a hydrolyzed binary sol system. The molar ratio of ethanol to magnesium and aluminum was 10.5:1.

[0049] (2) Add propylene oxide to the above solution; stir evenly and react until gel forms. The molar ratio of propylene oxide to magnesium and aluminum is 2.5:1.

[0050] (3) The obtained gel was aged in an environment of 40°C for 48 hours.

[0051] (4) The aged gel was subjected to supercritical carbon dioxide drying to obtain magnesium aluminum spinel aerogel precursor.

[0052] (5) The magnesium aluminum spinel aerogel precursor was placed in a muffle furnace and heat-treated at 800°C under a nitrogen atmosphere to finally obtain magnesium aluminum spinel aerogel.

[0053] (6) Grind the spinel aerogel into powder, then add 2g of powder to 30mL of toluene and mix evenly. After ensuring thorough mixing, ultrasonically disperse for 1h.

[0054] (7) Take 200 mg of dodecyltriethoxysilane and add it to the above mixed suspension. Disperse it by ultrasonication for 1 h; then reflux it at 100 °C for 24 h.

[0055] (8) The mixed liquid after reflux reaction was centrifuged on a high-speed (10000rpm) centrifuge for 10min, and the unreacted silane coupling agent was removed by repeated washing with toluene; finally, it was vacuum dried at 60℃ for 48h to obtain the modified magnesium aluminum spinel aerogel.

[0056] Example 3

[0057] (1) The amount of aluminum nitrate nonahydrate and magnesium nitrate hexahydrate was calculated according to the magnesium / aluminum molar ratio Mg / Al = 1.5. They were dissolved in anhydrous ethanol solution and stirred at a temperature of about 30°C until completely dissolved to obtain a hydrolyzed binary sol system. The molar ratio of ethanol to magnesium and aluminum, n(Mg+Al), was 10.5.

[0058] (2) Add propylene oxide to the above solution; stir evenly and react until gel forms. The molar ratio of propylene oxide to magnesium and aluminum, n(Mg+Al), is 2.5.

[0059] (3) The obtained gel was aged in an environment of 40°C for 48 hours.

[0060] (4) The aged gel was subjected to supercritical carbon dioxide drying to obtain magnesium aluminum spinel aerogel precursor.

[0061] (5) The magnesium aluminum spinel aerogel precursor was placed in a muffle furnace and heat-treated at 800°C under a nitrogen atmosphere to finally obtain magnesium aluminum spinel aerogel.

[0062] (6) Grind the spinel aerogel into powder, then add 2g of powder to 30mL of toluene and mix evenly. After ensuring thorough mixing, ultrasonically disperse for 1h.

[0063] (7) Take 200 mg of dodecyltriethoxysilane and add it to the above mixed suspension. Disperse it by ultrasonication for 1 h; then reflux it at 100 °C for 24 h.

[0064] (8) The mixed liquid after reflux reaction was centrifuged on a high-speed (10000rpm) centrifuge for 10min, and the unreacted silane coupling agent was removed by repeated washing with toluene; finally, it was vacuum dried at 60℃ for 48h to obtain the modified magnesium aluminum spinel aerogel.

[0065] Comparative Example 2

[0066] Oleic acid modification of magnesium-aluminum aerogel in Example 1:

[0067] The magnesium-aluminum aerogel from Example 1 was placed in a flask with excess oleic acid, and the mixture was continuously stirred with a magnetic stirrer to ensure uniform mixing of the catalyst and oleic acid. Nitrogen gas was purged into the flask for 30 minutes at room temperature to purge air, and nitrogen gas was continuously purged for protection. The mixture was heated in an oil bath to 120°C and reacted for 2 hours. The mixture was then cooled to room temperature, washed repeatedly with ethanol, centrifuged, dried at 80°C, ground, and sieved.

[0068] Compared with the magnesium aluminum spinel aerogel modified with dodecyltriethoxysilane in Example 1, the oleic acid-modified magnesium aluminum spinel aerogel of Comparative Example 2 has poorer oleophilicity and is partially dispersed in hydrocarbon fuels (see attached). Figure 4 b).

[0069] Performance test comparison between the examples and the comparative examples

[0070] Ni / MgAl2O4 catalysts were prepared using the magnesium aluminum spinel materials prepared in Examples 1-3 and Comparative Examples 1-2 as supports via a wet impregnation method, with a Ni loading of 7 wt.% in each case. n-Decane was used as a hydrocarbon fuel, and its supercritical cracking was used as a probe reaction to evaluate the catalyst performance. The experimental pressure was 3.5 MPa, and the reaction temperature range was 550–750 °C. The activity measured at 750 °C is shown in Table 1.

[0071] Table 1 Catalytic activity of catalyst

[0072]

[0073] Among them, the conversion rate (α), gas production rate (γ), and heat sink (Q) m The definition of ) is:

[0074]

[0075]

[0076]

[0077] In the formula, m in and m out These are the mass of n-decane flowing into the reactor and the mass of n-decane in the effluent, respectively; m flu is the mass of the effluent; G is the mass flow rate; W is the heating power; U is the heating voltage; I is the heating current. The amount of carbon deposited is the total amount of carbon deposited on the inner wall of the reaction tube 5 cm from the outlet after the n-decane catalytic cracking activity test.

[0078] Table 1 shows that the Ni-based catalyst using the pseudo-homogeneous high thermal stability magnesium-aluminum spinel prepared in Example 1 as a support exhibits better conversion rate, gas production rate, and heat sink performance in the cracking of n-decane than Examples 2, 3, and Comparative Example 2. Comparative Example 4 is the worst. Furthermore, Example 1 also shows less carbon deposition. This indicates that the pseudo-homogeneous magnesium-aluminum aerogel preparation method and the structure and property control method of this invention can effectively improve catalytic activity and inhibit carbon deposition.

[0079] Table 2 shows the analysis of the catalyst texture properties. It can be seen that the catalyst using magnesium aluminum spinel prepared in Example 1 as the support exhibits the largest specific surface area, which remains >100 μm² even after aging at 1000℃. 2 / g. Figure 2 The XRD pattern of the Ni-supported catalyst in the pseudo-homogeneous magnesium aluminum spinel aerogel of Example 1, as well as the N2 adsorption-desorption and pore size distribution, show that the spinel structure is stable and the crystal form is complete after high temperature; and no Ni particles were detected, indicating high Ni dispersion.

[0080] Table 2 Analysis of catalyst texture properties

[0081]

[0082] In addition, the contact angles between the modified aerogel powder and water and n-decane were determined using a contact angle meter, and the results are as follows: Figure 3As shown. The static contact angles of the obtained material with water were 146.9° and 146.6°, confirming the superhydrophobicity of the material prepared in Example 1. The contact angle between n-decane and the modified aerogel powder was also measured. The n-decane droplets dissolved and disappeared immediately after contacting the prepared catalyst material tablet, and no droplet rolling could be observed on the surface of the prepared sample, indicating that these materials have excellent oleophilicity. Figure 4 In the oil solubility experiment of the modified magnesium-aluminum aerogel in Example 1, the magnesium-aluminum spinel prepared in Comparative Examples 1 and 2 could not be well dispersed in n-decane, while the modified magnesium-aluminum aerogel prepared in Example 1 could be well dissolved in n-decane and did not settle after 48 hours. Hydrophobicity is a key factor in achieving pseudo-homogeneous catalysis, because the hydrophobicity of the aerogel allows the aerogel catalyst to be uniformly dispersed in the fuel (such as n-decane), thus achieving pseudo-homogeneous catalysis.

Claims

1. A method for preparing a pseudo-homogeneous, highly thermally stable magnesium-aluminum aerogel, characterized in that, Includes the following steps: (1) Dissolve aluminum nitrate nonahydrate and magnesium nitrate hexahydrate in anhydrous ethanol solution and stir at 20-50 °C until completely dissolved to obtain a hydrolyzed binary magnesium-aluminum mixed salt solution system; the amount of aluminum nitrate hydrate and magnesium nitrate hexahydrate used satisfies the molar ratio of magnesium to aluminum Mg / Al = 0.5 or 1.0 or 1.

5. (2) Add propylene oxide to the binary mixed salt solution system obtained in step (1) and stir uniformly at 20-50 °C until gel is formed; the molar ratio of propylene oxide to the total molar amounts of magnesium and aluminum in aluminum nitrate nonahydrate and magnesium nitrate hexahydrate is (1.0-3.0):1; (3) Place the gel obtained in step (2) in an environment of 20-40 °C for 48 h; (4) The gel aged in step (3) is subjected to supercritical drying to obtain magnesium aluminum spinel aerogel precursor; the supercritical drying is ethanol or carbon dioxide supercritical drying method; the ethanol supercritical drying adopts a high-pressure reactor to control the supercritical pressure, the reaction temperature during drying is 250~270 ℃, the pressure inside the reactor is 7.0~9.0 MPa, and the drying time is 1~4 h; the carbon dioxide supercritical drying adopts a supercritical dryer, the displacement temperature during drying is 5 ℃, the displacement time is 5~8 h, the reaction temperature of supercritical drying is 35~45 ℃, the pressure inside the supercritical dryer is 8.0~10.0 MPa, the gas release rate is 2.5~5.0 mL / min, and the drying time is 1~3 h; (5) The magnesium aluminum spinel aerogel precursor obtained in step (4) is heat-treated in a muffle furnace to obtain magnesium aluminum spinel aerogel; the atmosphere of the heat treatment is nitrogen or air, the temperature is 800~1000 ℃, the heating rate is 10 ℃ / min, and the heat treatment time is 1~5 h. (6) Grind the magnesium aluminum spinel aerogel into powder, mix it thoroughly with toluene, and then ultrasonically disperse it to obtain a suspension with an aerogel solid content of 3-8%. (7) Add dodecyltriethoxysilane to the suspension described in step (1), disperse it by ultrasonication, and reflux it at 100 °C for 24 to 72 h; the amount of dodecyltriethoxysilane used is 10% to 50% of the mass of the aerogel powder; (8) Centrifuge the refluxed mixed liquid to remove unreacted dodecyltriethoxysilane, and dry the separated solid vacuum tube to obtain modified magnesium aluminum spinel aerogel.

2. The method according to claim 1, characterized in that, The amounts of aluminum nitrate nonahydrate and magnesium nitrate hexahydrate used in step (1) satisfy the molar ratio of magnesium to aluminum, Mg / Al = 0.5, 1.0 or 1.

5.

3. The method according to claim 1, characterized in that, In step (1), the molar ratio of ethanol to the total amount of magnesium and aluminum in the magnesium-aluminum spinel aerogel is (9.5~15.5):

1.

4. The method according to claim 1, characterized in that, The reflux operation in step (7) is carried out in an oil bath.

5. The method according to claim 1, characterized in that, Furthermore, after centrifugation in step (8), the product is repeatedly washed with toluene and centrifuged again; the vacuum drying in step (8) is performed at 60~80 ℃ for 24~72 h.

6. The pseudo-homogeneous, thermally stable magnesium-aluminum aerogel prepared by the method of any one of claims 1 to 5.

7. The application of the pseudo-homogeneous high thermal stability magnesium-aluminum aerogel as described in claim 6 in the field of fuel catalytic cracking.

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