Preparation method and application of nickel magnesium oxide-melamine composite catalyst for preparing long-chain alcohol through ethanol Guerbet reaction

Abstract

The invention relates to a high-efficiency catalyst for converting ethanol into long-chain alcohol and a preparation method thereof. According to the catalyst, magnesium oxide serves as a main body, melamine is introduced to improve the water resistance of the catalyst and maintain a strong alkaline site, and then a nickel nitrate precursor and NaBH4 are utilized for in-situ reduction to generate Ni (OH) 2, so that the ethanol dehydrogenation and aldol condensation reaction performance is remarkably improved; the obtained catalyst shows a Janus-like particle structure, ethanol can be efficiently converted into C4 < + > long-chain alcohol in a microreactor, and excellent conversion rate and selectivity can be obtained within 12 hours. The invention not only expands the theoretical understanding of ethanol directional coupling conversion, but also provides a new design strategy for developing a Guerbet reaction catalyst with strong water resistance and high activity.

Description

Technical Field

[0001] This invention belongs to the fields of catalytic chemistry and energy chemical engineering, and relates to a composite catalyst for the Guerbet reaction of ethanol, specifically a nickel / magnesium oxide-melamine composite catalyst and its preparation method and application. Background Technology

[0002] With the continuous growth of global energy demand and the urgent need for renewable energy, long-chain alcohols are considered important candidates for future liquid fuels due to their excellent energy density and good combustion performance. Among these, the preparation of long-chain alcohols from ethanol via the Guerbet reaction has attracted considerable attention due to the wide availability of raw materials and the green and sustainable process route. This reaction typically involves the dehydrogenation of ethanol to acetaldehyde, the aldol condensation of acetaldehyde, the hydrogenation of the condensation product, and further chain growth. The properties of the catalyst directly determine the reaction efficiency and product distribution.

[0003] Currently, magnesium oxide, due to its abundant strong basic sites on its surface, has demonstrated outstanding performance in promoting aldol condensation and has been widely used in the Guerbet reaction (Appl Sci, 2019, 9, 1371). However, magnesium oxide has a significant drawback: it readily reacts with byproduct water during the reaction, leading to the deactivation of basic sites and thus significantly reducing the selectivity and yield of long-chain alcohols. To improve its performance, researchers have attempted to modify it by loading metals, introducing additives, or altering the crystal phase structure, but improvements in its water resistance remain limited (Ind Eng Chem Res, 2016, 55, 6579-6585).

[0004] On the other hand, ethanol dehydrogenation is a crucial step in the entire reaction chain, and its activity and efficiency largely depend on the selection and dispersion of the metal component. Nickel-based materials are favored due to their moderate price and excellent dehydrogenation performance, but how to avoid their aggregation and deactivation under reaction conditions remains a problem to be solved. Therefore, developing novel composite catalysts with strong basicity sites, excellent water resistance, and efficient dehydrogenation capabilities has become an important direction for promoting the efficient conversion of ethanol into long-chain alcohols. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems of poor water resistance of magnesium oxide in the prior art, which easily leads to the deactivation of basic sites, and the easy agglomeration of nickel-based catalysts in the ethanol dehydrogenation process. A novel modified magnesium oxide composite catalyst is proposed for the efficient catalytic Guerbet reaction of ethanol to prepare long-chain alcohols.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: Step 1: Magnesium hydroxide is placed in a muffle furnace and calcined at approximately 400°C for 12 hours to remove structural water molecules and achieve a crystal phase transformation, yielding magnesium oxide (MgO) solid powder with a strongly alkaline surface. The resulting support provides the structural basis for subsequent modification. Step 2: Weigh a certain mass of melamine and mix it thoroughly with the MgO powder obtained in step (1). Under a nitrogen atmosphere (approximately 50 mL / min), heat the mixture to 600℃ at a rate of 3℃ / min and hold it at that temperature for 2 hours. After natural cooling, collect the resulting solid, which is the MgO-melamine composite carrier. The mass ratio of melamine to MgO (y:z) can be adjusted to 1:1 to 1:4 according to performance requirements. This step imparts excellent water resistance to MgO and maintains its strongly alkaline surface sites. Step 3: Dissolve nickel nitrate hexahydrate in deionized water, stir until homogeneous, then add the carrier (MgO or MgO-melamine) prepared in step (1) or (2), and continue stirring to form a suspension. Then slowly add sodium borohydride aqueous solution, maintaining vigorous stirring at room temperature for about 10 minutes, until the solution changes from transparent to turbid and a precipitate is formed. During this process, Ni 2+ The nickel nanoparticles are reduced to Ni(OH)2 or metallic nickel. The resulting solid is washed repeatedly with deionized water and ethanol, filtered and dried, and then passed through an 80-mesh sieve to obtain xNi / MgO or xNi / MgO-melamine composite catalyst, where x represents the mass fraction of nickel (preferably 1% to 5%). This in-situ reduction step effectively improves the dispersion of nickel and its interaction with the support. Step 4: By adjusting the mass ratio of melamine to magnesium oxide (1:1 to 1:4) and the nickel content (1% to 5%), the basicity, water stability, and distribution of metal active sites of the catalyst can be systematically optimized, thereby achieving targeted regulation of catalytic performance. Step 5: In a sealed microreactor, 20 mL of anhydrous ethanol, 0.5 g of potassium hydroxide, and 1.0 g of the catalyst prepared above were added sequentially. After the reactor was purged with nitrogen at 0.5 MPa five times, the nitrogen pressure was maintained at 0.5 MPa. The stirring rate was set to 600 rpm, and the reaction was carried out at the set temperature for 2 hours. After the reaction was completed, the mixture was immediately cooled with ice water and allowed to cool naturally to room temperature. The reaction mixture was collected, and gas chromatography analysis showed that C4 and above long-chain alcohols (including butanol, hexanol, octanol, etc.) were detected in the reaction products. Step Six: By comparing the experimental results of different catalytic systems (MgO, Ni / MgO, MgO-melamine, Ni / melamine, Ni / MgO-melamine), it was found that the Ni / MgO-melamine (1:2) catalyst exhibited the best catalytic performance, with the highest ethanol conversion and C4 concentration. +The selectivity of alcohols was significantly improved. Experiments showed that melamine modification effectively improved the water resistance and alkaline stability of MgO, while the in-situ generated Ni(OH)2 promoted the dehydrogenation and condensation process of ethanol, realizing the efficient synthesis of long-chain alcohols.

[0007] Compared with the prior art, the present invention has the following beneficial effects: 1. Melamine modification significantly enhances the water resistance of magnesium oxide, avoiding catalyst deactivation caused by water byproducts; 2. Maintaining abundant strong basic sites in magnesium oxide effectively promotes aldol condensation and ensures the continuity of chain growth reaction; 3. Treating the generated Ni(OH)2 with NaBH4 improves the dispersion and stability of nickel species, significantly enhancing the ethanol dehydrogenation efficiency; 4. Construct composite catalysts with Janus structural features to achieve synergistic effects between metal active sites and basic sites, thereby enabling efficient conversion of ethanol into C4 and above alcohols in a shorter reaction time.

[0008] Therefore, this invention not only deepens the understanding of the catalytic process for the preparation of long-chain alcohols from ethanol at the mechanistic level, but also provides a new design concept and application approach for developing high-performance, long-life Guerbet reaction catalysts. Attached Figure Description

[0009] Figure 1 Catalyst performance diagrams under different nickel loadings; Figure 2 Catalyst performance diagrams at different melamine and magnesium oxide mass ratios. Detailed Implementation

[0010] The present invention will now be described in detail through examples.

[0011] Example 1: Magnesium hydroxide was placed in a muffle furnace and calcined continuously at 400°C for 12 hours to obtain magnesium oxide (MgO) solid powder.

[0012] Example 2: A certain mass of melamine was thoroughly mixed with the MgO powder prepared in Example 1, and placed in a tube furnace. Under a nitrogen flow of 50 mL / min, the temperature was raised from room temperature to 600 °C at a heating rate of 3 °C / min, and then kept at 600 °C for 2 h. After naturally cooling to room temperature, the resulting solid was collected, which is the MgO-melamine carrier (y:z), where y:z represents the mass ratio of melamine to MgO before mixing.

[0013] Example 3: Melamine was placed in a tube furnace and heated from room temperature to 600 °C at a rate of 3 °C / min under a nitrogen flow of 50 mL / min. The mixture was then kept at 600 °C for 2 h. After natural cooling to room temperature, the resulting solid was collected and designated as the melamine carrier.

[0014] Example 4: A certain amount of nickel nitrate hexahydrate was dissolved in deionized water and stirred until homogeneous. Then, MgO powder prepared in Example 1 was added, and stirring continued. Subsequently, sodium borohydride aqueous solution was slowly added dropwise to the above mixture. After vigorous stirring for 10 min, the resulting solid was washed repeatedly with deionized water and ethanol, excess liquid was removed by filtration, and the solid was dried and passed through an 80-mesh sieve to obtain xNi / MgO, where x represents the mass fraction of nickel.

[0015] Example 5: A certain amount of nickel nitrate hexahydrate was dissolved in deionized water and stirred until homogeneous. Then, the MgO-melamine (y:z) powder prepared in Example 2 was added, and stirring continued. A sodium borohydride aqueous solution was then slowly added dropwise, and after stirring for 10 min, the resulting solid was washed with deionized water and ethanol, filtered, dried, and finally passed through an 80-mesh sieve to obtain the xNi / MgO-melamine (y:z) catalyst, where x represents the mass fraction of nickel, and y:z represents the mass ratio of melamine to MgO before mixing.

[0016] Example 6: A certain amount of nickel nitrate hexahydrate was dissolved in deionized water and stirred until homogeneous. Then, the melamine support prepared in Example 3 was added, and stirring continued. A sodium borohydride aqueous solution was then slowly added dropwise, and after stirring for 10 minutes, the resulting solid was washed with deionized water and ethanol, filtered, dried, and finally passed through an 80-mesh sieve to obtain the xNi / melamine catalyst, where x represents the mass fraction of nickel.

[0017] Example 7: 20 mL of anhydrous ethanol and 0.5 g of potassium hydroxide were added to a microreactor. After sealing the reactor, nitrogen gas was purged at 0.5 MPa five times, maintaining a nitrogen pressure of 0.5 MPa. The stirrer was turned on (600 rpm), and the reaction was carried out at the set temperature for 2 h. After the reaction was completed, the mixture was immediately cooled with ice water and slowly lowered to room temperature to obtain a product mixture. Gas chromatography analysis detected butanol and higher carbon chain alcohols.

[0018] Example 8: 20 mL of anhydrous ethanol, 0.5 g of potassium hydroxide, and 1.0 g of the MgO catalyst prepared in Example 1 were added to a microreactor. After sealing the reactor, nitrogen gas was purged at 0.5 MPa five times and maintained at 0.5 MPa. Stirring was started (600 rpm), and the reaction was carried out at the set temperature for 2 h. After the reaction was completed, the mixture was immediately cooled with ice water and slowly lowered to room temperature to obtain a product mixture. Gas chromatography analysis detected butanol and higher carbon chain alcohols.

[0019] Example 9: 20 mL of anhydrous ethanol, 0.5 g of potassium hydroxide, and 1.0 g of the MgO-melamine catalyst (1:2) prepared in Example 2 were added to a microreactor. After sealing the reactor, nitrogen gas was purged at 0.5 MPa five times and maintained at 0.5 MPa. Stirring was started (600 rpm), and the reaction was carried out at the set temperature for 2 h. After the reaction was completed, the mixture was immediately cooled with ice water and slowly lowered to room temperature to obtain a product mixture. Gas chromatography analysis detected butanol and higher carbon chain alcohols.

[0020] Example 10: 20 mL of anhydrous ethanol, 0.5 g of potassium hydroxide, and 1.0 g of the 5Ni / melamine catalyst prepared in Example 3 were added to a microreactor. After sealing the reactor, nitrogen gas was purged at 0.5 MPa five times and maintained at 0.5 MPa. Stirring was started (600 rpm), and the reaction was carried out at the set temperature for 2 h. After the reaction was completed, the mixture was immediately cooled with ice water and slowly lowered to room temperature to obtain a product mixture. Gas chromatography analysis detected butanol and higher carbon chain alcohols.

[0021] Example 11: 20 mL of anhydrous ethanol, 0.5 g of potassium hydroxide, and 1.0 g of the 1Ni / MgO-melamine catalyst (1:2) prepared in Example 5 were added to a microreactor. After sealing the reactor, nitrogen gas was purged at 0.5 MPa five times and maintained at 0.5 MPa. Stirring was started (600 rpm), and the reaction was carried out at the set temperature for 2 h. After the reaction was completed, the mixture was immediately cooled with ice water and slowly lowered to room temperature to obtain a product mixture. Gas chromatography analysis detected butanol and higher carbon chain alcohols.

[0022] Example 12: 20 mL of anhydrous ethanol, 0.5 g of potassium hydroxide, and 1.0 g of the 2.5Ni / MgO-melamine catalyst (1:2) prepared in Example 5 were added to a microreactor. After sealing the reactor, nitrogen gas was purged at 0.5 MPa five times and maintained at 0.5 MPa. Stirring was started (600 rpm), and the reaction was carried out at the set temperature for 2 h. After the reaction was completed, the mixture was immediately cooled with ice water and slowly lowered to room temperature to obtain a product mixture. Gas chromatography analysis detected butanol and higher carbon chain alcohols.

[0023] Example 13: 20 mL of anhydrous ethanol, 0.5 g of potassium hydroxide, and 1.0 g of the 5Ni / MgO-melamine catalyst (1:2) prepared in Example 5 were added to a microreactor. After sealing the reactor, nitrogen gas was purged at 0.5 MPa five times and maintained at 0.5 MPa. Stirring was started (600 rpm), and the reaction was carried out at the set temperature for 2 h. After the reaction was completed, the mixture was immediately cooled with ice water and slowly lowered to room temperature to obtain a product mixture. Gas chromatography analysis detected butanol and higher carbon chain alcohols.

[0024] The yields of the long-chain alcohols obtained in Examples 7-13 are as follows: Figure 1 As shown.

[0025] Example 14: 20 mL of anhydrous ethanol, 0.5 g of potassium hydroxide, and 1.0 g of the 2.5Ni / MgO catalyst prepared in Example 4 were added to a microreactor. After sealing the reactor, nitrogen gas was purged at 0.5 MPa five times and maintained at 0.5 MPa. Stirring was started (600 rpm), and the reaction was carried out at the set temperature for 2 h. After the reaction was completed, the mixture was immediately cooled with ice water and slowly lowered to room temperature to obtain a product mixture. Gas chromatography analysis detected butanol and higher carbon chain alcohols.

[0026] Example 15: 20 mL of anhydrous ethanol, 0.5 g of potassium hydroxide, and 1.0 g of the 2.5Ni / MgO-melamine catalyst (1:4) prepared in Example 5 were added to a microreactor. After sealing the reactor, nitrogen gas was purged at 0.5 MPa five times and maintained at 0.5 MPa. Stirring was started (600 rpm), and the reaction was carried out at the set temperature for 2 h. After the reaction was completed, the mixture was immediately cooled with ice water and slowly lowered to room temperature to obtain a product mixture. Gas chromatography analysis detected butanol and higher carbon chain alcohols.

[0027] Example 16: 20 mL of anhydrous ethanol, 0.5 g of potassium hydroxide, and 1.0 g of the 2.5Ni / MgO-melamine catalyst (1:1) prepared in Example 5 were added to a microreactor. After sealing the reactor, nitrogen gas was purged at 0.5 MPa five times and maintained at 0.5 MPa. Stirring was started (600 rpm), and the reaction was carried out at the set temperature for 2 h. After the reaction was completed, the mixture was immediately cooled with ice water and slowly lowered to room temperature to obtain a product mixture. Gas chromatography analysis detected butanol and higher carbon chain alcohols.

[0028] The yields of the long-chain alcohols obtained in Examples 14-16 are as follows: Figure 2 As shown.

Claims

1. A method for preparing a nickel-based composite magnesium oxide-melamine catalyst, characterized by, The method comprises the following steps: (1) calcining magnesium hydroxide in a muffle furnace at 400℃ for 12 hours to obtain magnesium oxide; (2) mixing melamine and magnesium oxide in a certain mass ratio, and then introducing 50 mL / min of nitrogen into a tube furnace, and raising the temperature to 600℃ at a rate of 3℃ / min and maintaining for 2 hours, and then cooling to obtain a magnesium oxide-melamine composite carrier; (3) dissolving a certain amount of nickel nitrate hexahydrate in deionized water, adding the magnesium oxide-melamine composite carrier and stirring uniformly to obtain a mixed solution; (4) adding a sodium borohydride solution dropwise into the mixed solution to perform a reduction reaction, continuing to stir for 10 minutes, and then separating to obtain a solid; (5) washing the obtained solid with deionized water and ethanol in sequence, filtering, drying and then passing through an 80-mesh sieve to obtain a nickel-loaded composite catalyst Ni / MgO-melamine.

2. A method for preparing a nickel supported on magnesium oxide catalyst Ni / MgO, characterized by, The method comprises the following steps: (1) dissolving nickel nitrate hexahydrate in water and stirring intensively to obtain a solution; (2) adding a magnesium oxide carrier into the solution and continuing to stir to obtain a mixed system; (3) adding a sodium borohydride solution dropwise into the mixed system to perform a reduction reaction, continuing to stir for 10 minutes, and then separating to obtain a solid; (4) washing, filtering, drying and sieving the solid to obtain a Ni / MgO catalyst.

3. The preparation method according to claim 1, characterized in that, The mass ratio of melamine to magnesium oxide is 1:5-5:

1.

4. The production method according to claims 1 and 2, characterized by, The concentration of the sodium borohydride solution is 0.1-0.5 mol / L, and the dropwise adding rate is 1-3 mL / min.

5. The production method according to claims 1 and 2, characterized by, The mass fraction x of the obtained nickel is 1-10 wt%.

6. The catalyst according to any one of claims 1 or 2, characterized in that, The catalyst is used in a reaction of catalytic coupling of ethanol to prepare long-chain alcohols.

7. The reaction method according to claim 6, characterized by, In an SLM25 micro-reactor, 20 mL of ethanol, 0.5 g of potassium hydroxide and 1 g of the catalyst are added, 0.5 MPa of nitrogen is introduced and repeatedly replaced for 5 times, and the reaction is performed under stirring at 600 rpm, and after the reaction is completed, the product is obtained by cooling to room temperature with ice water.

8. The reaction method according to claim 6 or 7, characterized by, The product is mainly C4-C8 long-chain alcohols.

9. A catalyst for the coupling of ethanol to produce long chain alcohols based on a nickel-magnesia-melamine composite system, characterized in that, The catalyst comprises a magnesium oxide-melamine composite carrier and a nickel active component loaded on the surface of the carrier, the nickel is distributed in the form of metal or hydroxide on the surface of the carrier to form island-shaped active centers, and the magnesium oxide provides alkaline sites.

10. The catalyst of claim 9, wherein The catalyst can significantly improve the selectivity and yield of C4+ alcohols in the ethanol coupling reaction.

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