Catalyst for catalytically synthesizing C8+ higher alcohols from ethanol at low temperature, preparation method thereof, and method for synthesizing C8+ higher alcohols

Abstract

The present invention discloses a catalyst for catalytically synthesizing C8+ higher alcohols from ethanol at low temperature, a preparation method thereof, and a method for synthesizing C8+ higher alcohols; aiming to provide a catalyst Ni / MOFC prepared by combining impregnation and two-step annealing methods. In this catalyst, Ni forms a coordination with N, optimizing the electronic structure of the metal surface, having highly dispersed active sites and a unique electronic effect, reducing the ethanol dehydrogenation energy barrier, thereby enabling the catalytic generation of C8+ higher alcohols from an ethanol aqueous solution at an extremely low temperature, and having good catalytic efficiency and selectivity.

Description

Technical Field

[0001] The present invention discloses a catalyst, specifically, a catalyst for catalytically synthesizing C8+ higher alcohols from ethanol at low temperature. The present invention also discloses a preparation method of the catalyst and a synthesis method of C8+ higher alcohols catalyzed by the catalyst; it belongs to the technical field of catalysts. Background Art

[0002] With the increasing global attention to renewable energy and green chemistry, the research on developing high-value-added chemicals based on sustainable raw materials is becoming an important direction in the current chemical industry. Ethanol, as a basic chemical raw material with a wide source, low price and derived from renewable biomass, has important research value in green chemistry.

[0003] Higher alcohols, especially C8+ higher alcohols (such as n-octanol, 2-ethylhexanol, etc.) are important chemical intermediates, which are widely used in fields such as surfactants, lubricants, plasticizers, pharmaceuticals and cosmetics. Traditionally, the production of higher alcohols relies on petrochemical routes, but this process has problems such as non-renewable raw materials and high carbon emissions. In contrast, the preparation of higher alcohols from ethanol at low temperature has become a key research direction in recent years due to its lower energy consumption, higher product selectivity and longer catalyst life.

[0004] Although the preparation of higher alcohols from ethanol at high temperature is a feasible technology, this process has some obvious defects that restrict its wide application in industrialization. For example, the preparation of higher alcohols from ethanol usually requires a relatively high reaction temperature (such as above 200°C), and high-temperature operation directly leads to an increase in process energy consumption. Especially in large-scale production, the energy consumption of high-temperature heating is significant, reducing the economic efficiency of the process.

[0005] In addition, the traditional ethanol coupling reaction is carried out at high temperature using a Ni-based catalyst to catalyze the ethanol coupling reaction. However, at high temperature, the Ni-based catalyst catalyzes the excessive dehydrogenation of ethanol molecules, resulting in a large amount of small molecule gas by-products (CO2, CO, CH4), leading to low atom utilization rate.

[0006] However, at low temperature, side reactions are effectively suppressed, the chain growth of the target product is easier to control, and at the same time, catalyst sintering and carbon deposition phenomena are significantly reduced, significantly improving the environmental friendliness and economic efficiency of the process. In addition, the low-temperature process has lower requirements for equipment and is suitable for combination with green catalytic technologies (such as biocatalysis or homogeneous catalysis) to achieve a milder reaction process.

[0007] Therefore, the coupling of ethanol to prepare higher alcohols under low-temperature conditions has practical significance. However, the industrialization of current low-temperature processes still faces technical bottlenecks such as low reaction rates and insufficient catalyst development. In particular, there are still significant challenges in efficiently catalyzing the formation of C8+ higher alcohols under low-temperature conditions.

[0008] Therefore, the development of a catalyst that can achieve the formation of C8+ higher alcohols at low temperatures can not only further promote the efficient conversion of ethanol resources but also provide a new technical path to meet the growing market demand. Summary of the Invention

[0009] In view of the above deficiencies, the first object of the present invention is to provide a preparation method of a supported Ni-based catalyst Ni / MOFC. This method combines impregnation and two-step annealing methods to synthesize an atomic cluster catalyst highly dispersed on an N-doped porous carbon support.

[0010] The second object of the present invention is to provide a supported Ni-based catalyst Ni / MOFC. In this catalyst, Ni forms coordination with N, optimizing the electronic structure of the metal surface. It has highly dispersed active sites and a unique electronic effect, reducing the ethanol dehydrogenation energy barrier, thereby realizing the catalytic upgrading of aqueous ethanol to C8+ higher alcohols at extremely low temperatures and having good catalytic efficiency and selectivity.

[0011] Another object of the present invention is to provide a method for synthesizing C8+ higher alcohols from ethanol at low temperatures. This method significantly improves the formation rate of C8+ higher alcohols, has high selectivity, and few by-products.

[0012] To this end, the first technical solution provided by the present invention is as follows: A preparation method of a catalyst for catalytically synthesizing C8+ higher alcohols from ethanol at low temperatures, successively including the following steps:

[0013] I) Synthesis of nitrogen-doped porous materials

[0014] 1) Slowly drop a zinc nitrate solution into a 2-methylimidazole solution, stir at room temperature for 3 - 9 h, centrifuge to collect the product, and dry to obtain a white powder ZIF-8;

[0015] The mass ratio of the zinc nitrate solution to 2-methylimidazole is 6.5:3.0 - 4.0;

[0016] Mix the ZIF-8 prepared in step 1) and KCl evenly in a desolvent, then evaporate the solvent and dry to obtain a powder. Place the powder in a corundum boat and put it in a tubular furnace, heat to 700 °C and hold for 3 - 5 h, and wash after natural cooling to obtain MOFC;

[0017] The mass ratio of ZIF-8 to KCl is 5:100;

[0018] II) Synthesis of Ni / MOFC

[0019] Disperse NiCl2·6H2O and the MOFC prepared in step 1) in ethanol, perform ultrasonic treatment, heat and stir to evaporate the solvent, and then dry; heat the powder in a nitrogen stream to 200 - 400 °C and keep it for 3 - 5 h; after low-temperature annealing, wash and dry the powder, and then heat it to 550 °C in a nitrogen stream and keep it for 3 - 5 h for annealing to obtain the catalyst;

[0020] The mass ratio of the described NiCl2·6H2O to MOFC is 52:30.

[0021] Furthermore, for the preparation method of the catalyst for low-temperature synthesis of C8+ higher alcohols from ethanol, it is characterized in that: in step i), the washing is sequentially carried out with hydrochloric acid, deionized water, and ethanol.

[0022] Furthermore, for the preparation method of the catalyst for low-temperature synthesis of C8+ higher alcohols from ethanol, it is characterized in that: the solvent of the 2-methylimidazole solution is methanol or water.

[0023] Furthermore, for the preparation method of the catalyst for low-temperature synthesis of C8+ higher alcohols from ethanol, it is characterized in that: in step ii), the cleaning is carried out by washing with an equal-volume ethanol-water mixed solution for 6 h.

[0024] Furthermore, for the preparation method of the catalyst for low-temperature synthesis of C8+ higher alcohols from ethanol, it is characterized in that: in step ii), the ultrasonic treatment time is 30 min; the drying is carried out at 80 °C for 12 h.

[0025] Furthermore, for the preparation method of the catalyst for low-temperature synthesis of C8+ higher alcohols from ethanol, the zinc nitrate solution is composed of Zn(NO3)2·6H2O and deionized water, and the mass ratio of Zn(NO3)2·6H2O to deionized water is 6.5:80.

[0026] The second technical solution of the present invention is to provide a catalyst for low-temperature synthesis of C8+ higher alcohols from ethanol, which is prepared by using the preparation method described in the first technical solution.

[0027] The present invention also has a technical solution to provide a method for low-temperature synthesis of C8+ higher alcohols from ethanol, and the C8+ higher alcohols are catalytically prepared by using the catalyst described in the second technical solution.

[0028] Furthermore, in the above method for the low-temperature synthesis of C8+ higher alcohols from ethanol, the Ni / MOFC catalyst, water, ethanol, and sodium hydroxide are added to a reaction kettle, and the reaction is carried out at an initial pressure of 0 MPa and a temperature of 70-100 °C. After the reaction is completed, the reaction kettle is cooled to room temperature. After centrifugation and filtration, a liquid phase and a catalyst solid phase are obtained. The gas phase and liquid phase products are collected, and the main product in the organic phase of the liquid phase product is C4-C8+ higher alcohols.

[0029] Even further, in the above method for the low-temperature synthesis of C8+ higher alcohols from ethanol, the Ni / MOFC catalyst, water, ethanol, and sodium hydroxide are added to a reaction kettle, and the reaction is carried out at an initial pressure of 0 MPa and a temperature of 70-100 °C for 48 h. After the reaction is completed, the reaction kettle is cooled to room temperature. After centrifugation and filtration, a liquid phase and a catalyst solid phase are obtained. The gas phase and liquid phase products are collected, and the main product in the organic phase is C8+ higher alcohols;

[0030] The mass ratio of the Ni / MOFC catalyst, water, ethanol, and sodium hydroxide is 0.3:5:5:0.435.

[0031] Compared with the prior art, the technical solution provided by the present invention has the following technical advantages:

[0032] 1. The catalyst provided by the present invention combines impregnation and two-step annealing methods to synthesize an atom cluster catalyst highly dispersed on an N-doped porous carbon support. By forming a coordination between Ni and N, the electronic structure of the metal surface is optimized, and the geometric distribution and electronic structure of nickel atom clusters are precisely regulated. It has highly dispersed active sites and unique electronic effects; moreover, this catalyst has excellent thermal stability and anti-deactivation ability, and can maintain high reaction activity for a long time; it can selectively regulate the reaction path;

[0033] 2. KCl in the catalyst provided by the present invention is used as a salt template agent and pore-forming agent, which will sublime at high temperatures, causing ZIF-8 to form a pore structure during the carbonization process, greatly increasing the specific surface area of the carbon material;

[0034] 3. The catalyst provided by the present invention can effectively catalyze the coupling reaction of an ethanol aqueous solution at a low temperature of 70-100 °C to produce C8+ higher alcohols, avoiding common side reactions at high temperatures (such as CO2, CO, CH4), significantly improving the selectivity of the reaction and the stability of the catalyst, and increasing the yield of C8+ higher alcohols; low-temperature operation also significantly reduces energy consumption, providing an efficient, economical and environmentally friendly solution for industrial ethanol coupling reactions;

[0035] 4. The method for coupling ethanol aqueous solution to produce C8+ higher alcohols provided by the present invention can effectively inhibit carbon deposition under low-temperature conditions through the synergistic effect of water, and at the same time participate in the reaction activation, thereby further improving the stability and efficiency of the catalyst; NaOH is added during the reaction to provide additional basic sites to catalyze the reaction synergistically and promote the Aldol condensation of aldehydes and subsequent dehydration reactions. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 is the XRD pattern of the catalysts prepared in Examples 1-5;

[0037] Figure 2 is the TEM image of Ni / MOFC-300-550 prepared in Example 1;

[0038] Figure 3 is the XRD pattern of the catalyst prepared in Example 1 and the catalyst prepared in Comparative Example 1;

[0039] Figure 4 is the gas chromatogram of the final product in Application Example 5;

[0040] Figure 5 is the gas chromatogram of the final product in Comparative Application Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0041] The present invention is illustrated by the following examples, but the scope of protection of the present invention is not limited to the scope of the following examples.

[0042] Example 1

[0043] A Ni / MOFC-300-550 provided in this example was prepared by the following method:

[0044] I. Synthesis of nitrogen-doped porous (MOFC) material

[0045] 1) At room temperature, Zn(NO3)2·6H2O (6.5 g) was dissolved in 80 ml of deionized water to obtain a zinc nitrate solution, and 2-methylimidazole (3.55 g) was dissolved in 40 ml of deionized water to obtain a 2-methylimidazole solution. Then the zinc nitrate solution was slowly dropped into the 2-methylimidazole solution and stirred for 6 h; the product was collected by centrifugation and placed in an oven at 70 °C overnight to dry, obtaining white powder ZIF-8;

[0046] 2) The ZIF-8 nanomaterial (5 g) prepared in step 1) and potassium chloride (KCl, 100 g) were uniformly dispersed in deionized water (300 mL), and continuously stirred on a magnetic stirrer until the solvent completely evaporated. Subsequently, the product was transferred to a vacuum drying oven and dried at 120 °C for 12 hours to obtain a ZIF-8 / KCl composite precursor. The precursor was placed in a corundum crucible and heated to 700 °C in a tubular furnace and held for 5 h. After the reaction system was naturally cooled, the product was washed successively with 2 mol / L hydrochloric acid, deionized water and ethanol, and finally a metal-organic framework composite carbon material (MOFC) was obtained;

[0047] II) Synthesis of Ni / MOFC-300-550

[0048] 1) NiCl2·6H2O (520 mg) and the MOFC prepared in step 2) above (300 mg) were dispersed in 40 ml of ethanol, sonicated at 20 W for 30 min, and the solvent was evaporated by magnetic stirring. Then it was dried in an oven at 80 °C to obtain Ni / MOFC powder;

[0049] 2) The Ni / MOFC powder after drying in step 1) was heated to 300 °C in a nitrogen stream and maintained for 5 h for low-temperature annealing; after low-temperature annealing, the powder was washed in an equal-volume ethanol-water mixed solution for 6 h and dried at 80 °C for 12 hours to obtain Ni / MOFC-300;

[0050] 3) The Ni / MOFC-300 after drying in step 2) was heated to 550 °C in a nitrogen stream and held for 5 h to obtain Ni / MOFC-300-550. The EDS elemental energy spectrum data are shown in Table 1; the TEM image is shown in Figure 2 ; where the Ni loading is as high as 14.78 wt%.

[0051] Table 1

[0052]

[0053] Example 2

[0054] The steps, raw materials, and process parameters provided in this example are basically the same as those in Example 1, except that the low-temperature annealing temperature is 200 °C to obtain Ni / MOFC-200-550.

[0055] Example 3

[0056] The steps, raw materials, and process parameters provided in this example are basically the same as those in Example 1, except that the low-temperature annealing temperature is 250 °C to obtain Ni / MOFC-250-550.

[0057] Example 4

[0058] The steps, raw materials, and process parameters provided in this example are basically the same as those in Example 1. The difference lies in that the temperature of low-temperature annealing is 350 °C, and Ni / MOFC-350-550 is obtained.

[0059] Example 5

[0060] The steps, raw materials, and process parameters provided in this example are basically the same as those in Example 1. The difference lies in that the temperature of low-temperature annealing is 400 °C, and Ni / MOFC-400-550 is obtained.

[0061] Comparative Example 1

[0062] A kind of Ni / MOFC-550 provided in this comparative example is prepared by the following method:

[0063] I. Synthesis of nitrogen-doped porous (MOFC) material

[0064] 1) At room temperature, dissolve Zn(NO3)2·6H2O (6.5 g) in 80 ml of deionized water to obtain a zinc nitrate solution, dissolve 2-methylimidazole (3.55 g) in 40 ml of deionized water to obtain a 2-methylimidazole solution, and then slowly drop the zinc nitrate solution into the 2-methylimidazole solution and stir for 6 h; centrifuge to collect the product, place it in an oven at 70 °C overnight to dry, and obtain white powder ZIF-8;

[0065] 2) Uniformly disperse the ZIF-8 nanomaterial (5 g) prepared in step 1) and potassium chloride (KCl, 100 g) in deionized water (300 mL), place it on a magnetic stirrer and continuously stir until the solvent completely evaporates. Subsequently, transfer the product to a vacuum drying oven and dry it at 120 °C for 12 hours to obtain a ZIF-8 / KCl composite precursor. Place this precursor in a corundum crucible, heat it to 700 °C in a tubular furnace and hold for 5 h. After the reaction system naturally cools, wash the product with 2 mol / L hydrochloric acid, deionized water, and ethanol in sequence, and finally obtain a metal-organic framework composite carbon material (MOFC).

[0066] II. Synthesis of Ni / MOFC-550

[0067] 1) Disperse NiCl2·6H2O (520 mg) and the MOFC prepared in step 2) above (300 mg) in 40 ml of ethanol, perform ultrasonic treatment at 20 W for 30 min, magnetically stir to evaporate the solvent, and then dry it in an oven at 80 °C to obtain Ni / MOFC powder;

[0068] 2) Heat the Ni / MOFC powder dried in step 1) to 550 °C in a nitrogen stream and hold for 5 h to obtain Ni / MOFC-550.

[0069] The XRD patterns of the catalysts prepared in Examples 1-5 are shown in Figure 1 , and through Table 1, Figure 1 , it can be seen that the diffraction peaks corresponding to the three crystal planes of Ni do not appear in Ni / MOFC-300-550, indicating good dispersion of Ni particles. And when the first annealing temperature is less than 300 °C, the Ni loading does not reach the maximum, and the loading increases with the increase of the first annealing temperature. When the first annealing temperature exceeds 300 °C, Figure 1 narrow, thin and sharp diffraction peaks corresponding to the three crystal planes of Ni appear, indicating that NiCl2 decomposes to form nanoparticles when the temperature exceeds 300 °C.

[0070] Through Figure 2 , no obvious Ni agglomeration phenomenon appears in the TEM image of Ni / MOFC-300-550, which is consistent with the XRD pattern. The mapping spectrum shows that Ni particles are evenly distributed on the carbon support and are similar to the distribution of N, indicating that it is very likely that Ni forms a coordination with N, changing the electronic environment, thereby improving the ethanol dehydrogenation activity.

[0071] The catalysts Ni / MOFC-550 prepared in Comparative Example 1 and Ni / NC-300-550 prepared in Example 1 were analyzed by XRD, and the results are shown in Figure 3 ; From Figure 3 , it can be seen that narrow, thin and sharp diffraction peaks corresponding to the three crystal planes of Ni appear in the XRD spectrum of Ni / MOFC-550. Specifically, the diffraction peak at 44.1° corresponds to the (111) crystal plane of Ni, the diffraction peak at 51.5° corresponds to the (200) crystal plane of Ni, and the diffraction peak at 76.08° corresponds to the (220) crystal plane of Ni. The presence and sharpness of these characteristic peaks indicate that Ni particles aggregate to form a nanoscale cluster structure in the Ni / MOFC-550 catalyst.

[0072] Application Example 1

[0073] 0.3 g of Ni / MOFC-300-550 catalyst, 5 g of water, 5 g of ethanol, and 0.435 g of NaOH were added to a 70 ml steel magnetic stirring reactor, and the low-temperature ethanol coupling experiment was carried out at 70 °C respectively. The temperature was kept constant during the reaction, and the reaction duration was 48 h. After the reaction, the reactor was cooled to room temperature, centrifuged and filtered to obtain the liquid phase and the catalyst solid phase. The gas phase and liquid phase products were collected and analyzed by gas chromatography respectively. The main product in the organic phase is C4-C8+ higher alcohols. The ethanol conversion rate and the selectivity of each substance are shown in Table 2.

[0074] Application Examples 2-4

[0075] The preparation steps and the amounts of each substance used in Application Example 2-4 are the same as those in Application Example 1. The difference lies in that the specific reaction temperature is different. For the specific reaction temperature, the ethanol conversion rate, and the selectivity of each substance, refer to Table 2.

[0076] Comparative Application Example 1

[0077] The preparation steps, the amounts of each substance used, and its process parameters in Comparative Application Example 1 are the same as those in Application Example 2. The difference lies in that Ni@MOFC-550 prepared in an equal amount in Comparative Example 1 is used to replace the catalyst Ni / MOFC-300-550 in Application Example 2. The results are shown in Table 2.

[0078] Comparative Application Example 2

[0079] The preparation steps, the amounts of each substance used, and its process parameters in Comparative Application Example 2 are the same as those in Application Example 2. The difference lies in that Ni@MOFC-550 prepared in an equal amount in Comparative Example 1 is used to replace the catalyst Ni / MOFC-300-550 in Application Example 3. The results are shown in Table 2.

[0080] Table 2

[0081]

[0082] As can be seen from Table 2, in Application Examples 1 to 5, as the reaction temperature increases, the conversion rate of ethanol gradually increases. In Application Example 1, using Ni / MOFC-300-550 as the catalyst, the ethanol conversion rate is 4% at 70 °C, and the selectivities of n-butanol, C6 alcohol, and C8+ alcohol are 48.09%, 41.04%, and 10.87% respectively, and no gas is generated. As the reaction temperature further increases to 80 °C (Application Example 2), the ethanol conversion rate significantly increases to 5.8%, the selectivity of n-butanol is 56.93%, the selectivity of C6 alcohol is 34.73%, and at this time the selectivity of C8+ alcohol is 8.34%. When the temperature is increased to 90 °C (Application Example 3), the conversion rate continues to increase to 6.4%, the selectivity of C8+ alcohol increases to 18.07%, and at the same time C6 alcohol is 36.00%. Under the high temperature condition of 100 °C (Application Example 4), the ethanol conversion rate is increased to 12.2%, the selectivity of C6 alcohol is 30.96%, and the selectivity of C8+ alcohol is 12.80%. Comparing Application Example 1 (the conversion rate is only 2.5% at 80 °C) and Comparative Application Example 2 (the selectivity of C8+ alcohol is zero at 90 °C), it can be seen that under low temperature conditions, compared with Ni / MOFC-550, the Ni / MOFC-300-550 provided in this application has a lower ethanol conversion rate on Ni / MOFC-550 at 80-90 °C, and no C8+ alcohol products appear at 90 °C. When the temperature is further reduced to 80 °C, the ethanol conversion rate is further reduced, and only the n-butanol product of C4 appears, while the Ni / MOFC-300-550 during the same period can still produce C8+ alcohol products. This shows that the highly dispersed Ni atomic cluster catalyst Ni / MOFC-300-550 prepared by the two-step annealing method has stronger ethanol coupling performance than the nanocluster catalyst Ni / MOFC-550 at low temperature. Therefore, the catalyst provided in this application significantly improves the selectivity of the reaction and the stability of the catalyst, and improves the yield of C8+ higher alcohols.

[0083] Application Examples 1-4 also show that the Ni / MOFC-300-550 catalyst can effectively catalyze the synthesis of C8+ higher alcohols from ethanol within a wide temperature range, and the product selectivity is stable.

[0084] Application Example 5

[0085] The preparation steps and the amounts of each substance used in Application Example 5 are the same as those in Application Example 1. The difference is that the reaction temperature is 150 °C, and the results are shown in Table 3 and Figure 4 。

[0086] Comparative Application Example 3

[0087] The preparation steps and the amounts of each substance used in Comparative Application Example 3 are the same as those in Application Example 5. The difference is that the Ni@MOFC-550 prepared in Comparative Example 1 in an equal amount is used to replace the catalyst Ni / MOFC-300-550 in Application Example 5. The results are shown in Table 3 and Figure 5 .

[0088] Table 3

[0089]

[0090] As can be seen from Table 3, the ethanol conversion rate of the Ni / MOFC-300-550 catalyst reaches 25.6% under the reaction conditions of 150 °C. Among them, the yield of straight-chain higher alcohols (16.2%) is 67% higher than that of the comparative catalyst Ni / MOFC-550 (9.7%). However, we also found that aldehyde substances are generated at higher temperatures, and the side reactions of the generated products are also increasing; that is, as the carbon chain grows, the selectivity of higher alcohols gradually decreases, which is because the coupling of ethanol to form higher alcohols requires multiple consecutive reactions. For example, the butanol formation path: ethanol → acetaldehyde (dehydrogenation) → crotonaldehyde (aldol condensation) → butanol (hydrogenation); this process involves two dehydrogenation-condensation-hydrogenation cycles; the formation of octanol requires an additional two condensation steps (such as crotonaldehyde further condensing with acetaldehyde to form a C6 intermediate, and then repeating the condensation to C8). As the number of steps increases, the cumulative activation energy increases significantly, resulting in a decrease in the thermodynamic probability of higher carbon products and an increase in by-products.

Claims

1. A method for low-temperature synthesis of C8+ higher alcohols from ethanol, characterized in that, Add the Ni / MOFC catalyst, water, ethanol, and sodium hydroxide into a reaction kettle, and carry out the reaction at an initial pressure of 0 MPa and a temperature of 70 - 100 °C. After the reaction is completed, cool the reaction kettle to room temperature. After centrifugation and filtration, a liquid phase and a catalyst solid phase are obtained. The main product of the organic phase in the liquid phase product is C4 - C8+ higher alcohols; The Ni / MOFC catalyst described above is prepared successively through the following steps: I) Synthesis of nitrogen-doped porous materials 1) Slowly drip the zinc nitrate solution into the 2-methylimidazole solution, stir at room temperature for 3 - 9 h, centrifuge to collect the product, and dry to obtain white powder ZIF-8; The mass ratio of the zinc nitrate solution to 2-methylimidazole is 6.5:(3.0 - 4.0); 2) Mix the ZIF-8 prepared in step 1) and KCl evenly in a solvent, then evaporate the solvent and dry to obtain a powder. Place the powder in a corundum boat and put it in a tubular furnace, heat to 700 °C and keep for 3 - 5 h, and wash after natural cooling to obtain MOFC; The mass ratio of ZIF-8 to KCl is 5:100; II) Synthesis of Ni / MOFC Disperse NiCl2·6H2O and the MOFC prepared in step 1) in ethanol, carry out ultrasonic treatment, heat and stir to evaporate the solvent, and then dry; Heat the powder in a nitrogen stream to 200 - 400 °C and keep for 3 - 5 h; After low-temperature annealing, wash and dry the powder, and then heat it to 550 °C in a nitrogen stream and keep it for 3 - 5 h for annealing to obtain the Ni / MOFC catalyst; The mass ratio of NiCl2·6H2O to MOFC is 52:

30.

2. The method for synthesizing C8+ higher alcohols from ethanol at low temperature according to claim 1, wherein: In step I), the washing is successively carried out with hydrochloric acid, deionized water, and ethanol.

3. The method for synthesizing C8+ higher alcohols from ethanol at low temperature according to claim 1, characterized in that: The solvent of the 2-methylimidazole solution is methanol or water.

4. The method for low-temperature synthesis of C8+ higher alcohols from ethanol according to claim 1, characterized in that: In step II), the cleaning is carried out by washing with a mixed solution of ethanol and water with equal volume for 6 h.

5. The method for synthesizing C8+ higher alcohols from ethanol at low temperature according to claim 1, characterized in that: In step II), the ultrasonic treatment time is 30 min; the drying is carried out at 80 °C for 12 h.

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