Preparation of a highly active Ni-Co bimetallic catalyst and its application in catalytic hydrocracking of lignin

Abstract

The invention relates to the preparation of a high-activity Ni-Co bimetallic catalyst and its application in lignin hydrocracking. The catalyst comprises the following steps: adding aluminum nitrate, cobalt nitrate and nickel nitrate metal salt solutions dropwise into sodium carbonate and sodium hydroxide solutions, crystallizing the obtained suspension at 50-60 DEG C for 12-13 hours; centrifuging, washing the precipitate with deionized water until neutral, calcining the obtained solid at 400-800 DEG C for 2-4 hours after drying; cooling to room temperature, introducing hydrogen for reduction, and obtaining the high-activity Ni-Co bimetallic catalyst; the catalyst is prepared by a simple coprecipitation-calcination-reduction method, exhibits excellent lignin hydrocracking performance in the presence of the Ni-Co bimetallic catalyst, is low in price and high in activity, uses isopropanol as a solvent and a hydrogen donor in the catalytic reaction process to avoid the use of high-pressure hydrogen, and realizes the high-selective cleavage of C-O bonds under mild conditions.

Description

Technical Field

[0001] The invention relates to the technical field of catalyst preparation and biomass conversion, and in particular to the preparation of a high-activity Ni-Co bimetallic catalyst and its application in lignin catalytic hydrocracking. Background Art

[0002] Lignin is a three-dimensional polymer formed by random bonding of three phenylpropane structural units through CC bonds and CO bonds. It is also the only renewable resource in nature that can provide aromatic compounds. The unique chemical structure makes it possible for lignin to replace fossil resources to prepare fuels and chemicals to a certain extent. By hydrogenating lignin and breaking the covalent bonds between structural units, lignin can be depolymerized into small molecular compounds. The high-value utilization of lignin through hydrogenolysis can not only provide renewable aromatic hydrocarbon resources for downstream industries, but also greatly improve the utilization efficiency and economic benefits of lignin. However, in the process of lignin conversion, it is crucial to design a suitable new catalytic system and prepare and screen efficient catalysts by implementing the principles of green chemistry.

[0003] The traditional hydrocracking process usually uses high-pressure hydrogen as the hydrogen source. At present, hydrogen is mainly produced from fossil resources, which not only leads to a significant increase in carbon footprint, but also causes excessive hydrogenation of aromatic products to reduce the selectivity of aromatic compounds. Therefore, it is particularly important to reduce the use of high-pressure hydrogen and improve product selectivity and the overall efficiency of lignin depolymerization. A suitable catalytic system in the hydrocracking process of lignin can not only promote the depolymerization of lignin, but also inhibit the occurrence of side reactions, thereby improving the selectivity of hydrogenolysis products. Common precious metal catalysts such as ruthenium, palladium and rhodium have good catalytic performance in the process of lignin hydrogenolysis, but their cost for industrial production is too high, resulting in inability to use them on a large scale. Transition metal catalysts such as nickel, cobalt and copper have similar catalytic activity to precious metals, and have good economic performance, rich content and wide sources, and have received widespread attention. However, such catalysts reported so far still have problems such as complicated preparation process, difficulty in reuse, low yield and selectivity of lignin hydrogenolysis products, and easy coking. Therefore, it is an urgent technical problem to prepare new and efficient non-precious metal catalysts, improve the yield and selectivity of lignin hydrogenolysis products, achieve the breaking of characteristic chemical bonds of lignin under mild conditions, and strengthen the process of directional hydrogenolysis of lignin to prepare chemicals. Summary of the invention

[0004] In view of the above-mentioned technical deficiencies, the present invention provides a preparation method of a highly active Ni-Co bimetallic catalyst and its application in catalytic hydrocracking of lignin. The catalyst has the advantages of excellent hydrogenation capacity, high selectivity and low cost. For the hydrocracking reaction of lignin, isopropanol is used as the reaction solvent, and the depolymerization of lignin is achieved without an external hydrogen source.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] The first object of the present invention is to provide a method for preparing a highly active Ni-Co bimetallic catalyst, comprising the following steps:

[0007] S1: Weigh aluminum nitrate nonahydrate, cobalt nitrate hexahydrate and nickel nitrate hexahydrate and add them into deionized water. The concentration of the metal salt solution is 0.09-0.1 g / mL, and the molar ratio of the divalent salt to the trivalent salt is 2:1. The metal salt solutions are stirred at room temperature until they are uniformly dispersed.

[0008] S2: Weigh sodium carbonate and sodium hydroxide and add them into deionized water. The concentration of sodium carbonate in the prepared solution is 0.02-0.03 g / mL, and the concentration of sodium hydroxide is 0.08-0.1 g / mL. The prepared solutions are stirred at room temperature until they are evenly dispersed.

[0009] S3: adding the metal salt solution in step S1 dropwise to the solution prepared in step S2 under room temperature while stirring, and crystallizing the resulting suspension at 50-60° C. for 12-13 hours;

[0010] S4: The suspension obtained in step S3 is centrifuged, and the precipitate is washed with deionized water until it is neutral. The solid obtained after drying is ground into powder, and calcined at 400-800° C. for 2-4 hours; after cooling to room temperature, hydrogen is introduced for reduction to obtain a highly active Ni-Co bimetallic catalyst.

[0011] Preferably, in step S1, the molar ratio of nickel, cobalt and aluminum in the catalyst is 1:3:2, and the catalyst is recorded as Ni0.5Co1.5Al.

[0012] Preferably, in step S2, the molar ratio of sodium carbonate to sodium hydroxide is 1:10.

[0013] Preferably, the calcination in step S4 is carried out in a muffle furnace, and the calcination condition is a heating rate of 2 to 5° C. / min.

[0014] Preferably, the flow rate of hydrogen introduced in step S4 is 60-80 mL / min, and the reduction temperature is set to 500-600°C.

[0015] Another object of the present invention is the use of a highly active Ni-Co bimetallic catalyst in lignin hydrocracking, comprising the following steps:

[0016] S5: Weigh the reaction substrate and the catalyst according to the proportion and add them to the reaction system, add the reaction solvent, replace the air in the reaction kettle with nitrogen, repeat the replacement three times and then fill with 0.5-1.0 MPa nitrogen, in the reaction system, the concentration of the reactants is 2 mg / mL-20 mg / mL, and the concentration of the catalyst is 0.4 mg / mL-4 mg / mL;

[0017] S6: In step S5, the temperature of the reactor is set at 180-300° C., and after the reaction is completed, the reactor is cooled to room temperature, the gas in the reactor is collected, the catalyst is filtered and separated, and the product distribution is analyzed by characterization.

[0018] Preferably, the reaction substrate includes any one of 2-phenoxy-1-phenylethanol and lignin.

[0019] Preferably, the reaction solvent is one of methanol, ethanol and isopropanol.

[0020] Preferably, in step S5, the heating rate is set to 8-10°C / min, the rotation speed is set to 220-300r / min, and the reaction time is set to 1-10h.

[0021] Preferably, the products obtained after conversion are detected and analyzed using GC-MS, and the lignin model compound is completely converted into aromatic monomers and cyclohexanol.

[0022] Preferably, the product of the catalytic hydrocracking reaction is mainly aromatic monomers, the lignin oil yield is 14.56-43.30 wt%, and the guaiacol selectivity is 39.98-47.96 wt%.

[0023] Preferably, the catalyst obtained after separation can be reused after drying in a vacuum drying oven, and the reaction steps are the same as the above steps S5 to S6.

[0024] The beneficial effects of the present invention are:

[0025] 1. The Ni0.5Co1.5Al catalyst of the present invention is prepared by a coprecipitation-calcination-reduction method, which has a simple preparation process and low cost, has high activity and selectivity, can realize the hydrocracking conversion of lignin model compounds and lignin under mild conditions, and the catalyst has magnetism for easy recycling, which provides great convenience for post-reaction processing;

[0026] 2. The catalyst prepared by the present invention can use low-carbon alcohol as solvent and hydrogen source, and can achieve the cracking of CC and CO bonds and lignin catalytic hydrogenation cracking under mild conditions without external hydrogen source. Compared with hydrogenation reactions involving hydrogen, on the one hand, competitive reactions and excessive hydrogenation of aromatic rings are reduced, and on the other hand, energy input and equipment requirements are reduced.

[0027] 3. The catalyst particles of the present invention have small particle size, and the active components are evenly dispersed. In the process of catalyzing lignin conversion, it has the advantages of strong catalytic reaction activity and high selectivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0029] Figure 1 is the X-ray diffraction pattern of the NixCoyAl catalyst prepared in Example 1 of the present invention;

[0030] Figure 2 This is a scanning electron microscope energy spectrum of the Ni0.5Co1.5Al catalyst prepared in Example 1 of the present invention;

[0031] Figure 3 This is a hydrogen temperature-programmed reduction diagram of the NixCoyAl catalyst prepared in Example 1 of the present invention;

[0032] Figure 4 This is a diagram showing the conversion of lignin using the Ni0.5Co1.5Al catalyst prepared in Example 3 of the present invention;

[0033] Figure 5 This is a cyclic experimental test diagram of the Ni0.5Co1.5Al catalyst prepared in Example 4 of the present invention. DETAILED DESCRIPTION

[0034] The following will be combined with the accompanying drawings in the examples of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. The following examples are provided for a better understanding of the present invention, and are not limited to the best implementation mode, and do not limit the content and protection scope of the present invention. Any product that is the same or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with other prior arts shall fall within the protection scope of the present invention.

[0035] If no specific experimental steps or conditions are specified in the examples, the conventional experimental operations or conditions described in the literature in the field can be followed. The reagents or instruments used are marked with the manufacturer and are all conventional reagent products that can be purchased commercially.

[0036] Example 1: Preparation of Ni0.5Co1.5Al catalyst

[0037] 2.5mmol nickel nitrate hexahydrate, 7.5mmol cobalt nitrate hexahydrate and 5mmol aluminum nitrate nonahydrate were dissolved in 50mL deionized water to obtain solution A. 6.25mmol sodium carbonate and 62.5mmol sodium hydroxide were dissolved in 25mL deionized water to obtain solution B. Solution B was preheated to 55°C in a 250ml beaker and stirred for 15min. Under stirring at 450r / min, solutions A and B were dripped into the beaker with two separate injection pumps. The pH value and temperature of the mixture in the beaker were strictly controlled at 10 and 55°C. After the dropwise addition, it was stirred at 55°C for 12 hours. The precipitate was separated from the liquid by centrifugation and washed with deionized water until neutral. The washed precipitate was dried in an oven at 60°C for 12h and then ground into powder. The ground sample was placed in a muffle furnace and calcined at 700°C for 2h; after cooling to room temperature, hydrogen was introduced for reduction to obtain the Ni0.5Co1.5Al catalyst.

[0038] Comparative Example 1: Preparation of Ni1Co1Al bimetallic catalyst

[0039] The preparation method is similar to that of Example 1, except that the added amounts of nickel nitrate hexahydrate, cobalt nitrate hexahydrate and aluminum nitrate nonahydrate in solution A are 5 mmol, 5 mmol and 5 mmol respectively.

[0040] Comparative Example 2: Preparation of Ni1.5Co0.5Al bimetallic catalyst

[0041] The preparation method is similar to that of Example 1, except that the added amounts of nickel nitrate hexahydrate, cobalt nitrate hexahydrate and aluminum nitrate nonahydrate in solution A are 7.5 mmol, 2.5 mmol and 5 mmol, respectively.

[0042] Comparative Example 3: Preparation of Ni2Al Monometallic Catalyst

[0043] The preparation method is similar to that of Example 1, except that only nickel nitrate hexahydrate and aluminum nitrate nonahydrate are added to solution A, wherein the amount of nickel nitrate hexahydrate added is 10 mmol, and the amount of aluminum nitrate nonahydrate added is 5 mmol.

[0044] Comparative Example 4: Preparation of Co2Al monometallic catalyst

[0045] The preparation method is similar to that of Example 1, except that only cobalt nitrate hexahydrate and aluminum nitrate nonahydrate are added to solution A, wherein the amount of cobalt nitrate hexahydrate added is 10 mmol, and the amount of aluminum nitrate nonahydrate added is 5 mmol.

[0046] Figure 1 The X-ray diffraction patterns of the NixCoyAl catalysts prepared in Example 1 and Comparative Examples 1-4 are shown in FIG. Figure 1As shown in a, the characteristic diffraction peaks of Co3O4 and Co2AlO4 were detected in the Co2Al catalyst before reduction, and the diffraction peaks corresponding to NiO and NiAl2O4 were detected for the Ni2Al catalyst. With the increase of Ni content, the diffraction peaks gradually shifted to the peaks of 37.44° (003), 43.52° (012), and 62.14° (104) corresponding to the high-angle NiO. Figure 1 b is the X-ray diffraction pattern of the reduced NixCoyAl catalyst. Characteristic diffraction peaks of metal Ni, Co and part of the spinel structure were detected in the reduced Co2Al and Ni2Al catalysts, respectively. When the Ni content increases, the diffraction peaks at 51.83° and 76.24° shift significantly to higher angles. The changes in the intensity and corresponding positions of the catalyst diffraction peaks indicate to some extent the local interaction between Ni and Co.

[0047] Figure 2 This is a scanning electron microscope energy spectrum of the catalyst Ni0.5Co1.5Al obtained in Example 1. Figure 2 It can be seen that the surface morphology of the Ni0.5Co1.5Al catalyst is rough, and the Ni, Co and Al elements are evenly dispersed. The SEM-EDS atomic percentage results show that the atomic percentages of Ni, Co and Al are 13.26%, 38.22% and 48.42% respectively. The calculated atomic ratio of Ni and Co is about 1:3, which is consistent with the stoichiometric ratio of Ni0.5Co1.5Al. No peaks of other elements are detected in the EDS surface spectrum, indicating that the prepared catalyst is relatively pure.

[0048] Figure 3 The hydrogen temperature-programmed reduction diagram of the NixCoyAl catalyst prepared in Example 1 and Comparative Examples 1-4 is shown in FIG. Figure 3 As shown in the figure, the Ni2Al catalyst has three obvious reduction peaks, among which the reduction peak at 440℃ corresponds to the reduction of NiO to elemental Ni, which is consistent with the XRD results, while the peaks at 624℃ and 712℃ belong to the reduction of octahedral coordinated nickel ions and the reduction of tetrahedral coordinated nickel ions with strong interactions with the support, respectively. Co2Al shows two broad hydrogen consumption peaks at 420℃ and 710℃, the peak at 420℃ is the reduction of Co3O4 to CoO, and the peak between 710℃ is the reduction of CoO to Co. Compared with the Co2Al catalyst, after the introduction of metallic Ni, the reduction temperature of Co shifts to the low temperature zone, indicating that the addition of Ni can reduce the reduction temperature of Co, and there is an interaction between Ni and Co.

[0049] Example 2: Hydrocracking of 2-phenoxy-1-phenylethanol

[0050] (1) 50 mg of 2-phenoxy-1-phenylethanol, 10 mg of catalyst and 25 ml of isopropanol solvent were added to a high-pressure reactor, which was sealed and nitrogen was introduced to replace the air in the reactor. The replacement was repeated three times and then filled with 1 MPa nitrogen;

[0051] (2) the high pressure reactor in step (1) is heated from room temperature to 180° C. at a heating rate of 8 to 10° C. / min, and reacted at a rotation speed of 220 to 300 r / min for 60 to 120 min;

[0052] (3) After the reaction is completed, the high-pressure reactor is cooled to room temperature, the gas in the reactor is collected, the catalyst is filtered and separated, and the product distribution is analyzed by characterization.

[0053]

[0054] It can be seen from Table 1 that Ni0.5Co1.5Al exhibits the best catalytic performance. Under the conditions of a reaction temperature of 180°C and isopropanol solvent as a hydrogen donor, it can catalyze the complete conversion of 2-phenoxy-1-phenylethanol, and the yields of ethylbenzene and cyclohexanol in the products are relatively high, indicating that the prepared Ni0.5Co1.5Al catalyst can achieve the breaking of the CO bond under relatively mild conditions.

[0055] Example 3: Ni0.5Co1.5Al catalyst for hydrocracking of lignin

[0056] (1) 500 mg of lignin (de-alkalized), 100 mg of catalyst and 25 mL of isopropanol solvent were added to a high-pressure reactor, which was sealed and nitrogen was introduced to replace the air in the reactor. The replacement was repeated three times and then filled with 1 MPa nitrogen;

[0057] (2) the high pressure reactor in step (1) is heated from room temperature to 240-300° C. at a heating rate of 8-10° C. / min, and reacted at a rotation speed of 220-300 r / min for 4-10 h;

[0058] (3) After the reaction is completed, the autoclave is cooled to room temperature, the gas in the autoclave is collected, the catalyst is filtered and separated, and the product distribution is characterized and analyzed by GC-MS.

[0059] The dealkalized lignin was reacted at 300°C for 8 hours to obtain a lignin oil yield of 43.3 wt%. Figure 4The changes in the yields of lignin oil and guaiacol and the selectivity of the products with the reaction temperature and reaction time show that the yields of lignin oil and guaiacol are significantly increased with the increase of reaction temperature and the extension of reaction time. The yields of the two reach the maximum value when the reaction is carried out at 300°C for 8 hours. Under this condition, the content of phenolic compounds in the product is the highest. When the reaction time is further extended to 10 hours at a reaction temperature of 300°C, the yield of lignin oil and the selectivity of phenolic compounds are significantly reduced. It may be that with the extension of reaction time, the products after depolymerization undergo re-condensation reaction. The highly selective phenolic compounds and high-yield guaiacol compounds in the lignin hydrogenolysis products indicate that the Ni0.5Co1.5Al catalyst prepared by the present invention has good catalytic activity and selectivity, as well as the applicability of the catalyst in lignin.

[0060] Example 4: Recycling of Ni0.5Co1.5Al catalyst

[0061] (1) 50 mg of 2-phenoxy-1-phenylethanol, 10 mg of catalyst and 25 mL of isopropanol solvent were added to a high-pressure reactor, which was sealed and nitrogen was introduced to replace the air in the reactor. The replacement was repeated three times and then filled with 1 MPa nitrogen;

[0062] (2) the high pressure reactor in step (1) is heated from room temperature to 180° C. at a heating rate of 8 to 10° C. / min, and reacted at a rotation speed of 220 to 300 r / min for 60 to 120 min;

[0063] (3) After the reaction is completed, the high-pressure reactor is cooled to room temperature, the gas in the reactor is collected, the catalyst is filtered and separated, and the product distribution is analyzed by characterization.

[0064] (4) The catalyst is separated by centrifugation, and the filtered catalyst is washed three times with isopropanol, dried in a vacuum drying oven at 60-80° C., and the catalytic reaction of 2-phenoxy-1-phenylethanol is repeated. The reaction operation is the same as the above steps (1) to (3).

[0065] Figure 5The stability test diagram of the Ni0.5Co1.5Al catalyst synthesized in Example 1 of the present invention. Compared with the unused catalyst, the conversion rate of 2-phenoxy-1-phenylethanol is significantly reduced after the third cycle experiment, and the conversion rate of 2-phenoxy-1-phenylethanol is only 63.84% in the sixth cycle experiment, which may be related to the valence change of the active component in the catalyst. After six cycles of recycling, the catalyst was characterized by XPS. The presence of metal elements was not detected in the XPS spectrum of the used catalyst, which may be due to the oxidation of the Ni0.5Co1.5Al catalyst during the recycling process, resulting in a decrease in the catalytic performance of the catalyst. The catalyst after the activity reduction is reduced again, and the conversion rate of 2-phenoxy-1-phenylethanol is increased to 98.00%, and the yields of ethylbenzene and cyclohexanol are also significantly improved. This result shows that the catalyst is easy to regenerate, and the recovered catalyst can still be put into use again after reduction.

[0066] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection of the invention.

Claims

1. Application of a highly active Ni-Co bimetallic catalyst in lignin hydrocracking, characterized in that: The preparation method of the catalyst comprises the following steps: S1: Weigh aluminum nitrate nonahydrate, cobalt nitrate hexahydrate and nickel nitrate hexahydrate and add them into deionized water. The concentration of the metal salt solution is 0.09-0.1 g / mL, and the molar ratio of the divalent salt to the trivalent salt is 2:

1. The metal salt solutions are stirred at room temperature until they are evenly dispersed. S2: Weigh sodium carbonate and sodium hydroxide and add them into deionized water. The concentration of sodium carbonate in the prepared solution is 0.02-0.03 g / mL, and the concentration of sodium hydroxide is 0.08-0.1 g / mL. The prepared solutions are stirred at room temperature until they are evenly dispersed. S3: adding the metal salt solution of step S1 dropwise to the solution prepared in step S2 under room temperature while stirring, and crystallizing the obtained suspension at 50-60°C for 12-13 h; S4: centrifuge the suspension obtained in step S3, wash the precipitate with deionized water until it is neutral, grind the solid obtained after drying into powder, and calcine at 400-800 °C for 2-4 h; cool to room temperature, introduce hydrogen for reduction, and obtain a highly active Ni-Co bimetallic catalyst, wherein the molar ratio of nickel, cobalt and aluminum in the catalyst is 1:3:2, and the catalyst is recorded as Ni0.5Co1.5Al; The hydrocracking method of the highly active Ni-Co bimetallic catalyst comprises the following steps: S5: Weigh the reaction substrate and catalyst according to the proportion and add them to the reaction system, add the reaction solvent, replace the air in the reaction kettle with nitrogen, repeat the replacement three times and then fill with 0.5-1.0 MPa nitrogen. In the reaction system, the concentration of the reactants is 2 mg / mL-20 mg / mL, and the concentration of the catalyst is 0.4 mg / mL-4 mg / mL; S6: In step S5, the temperature of the reactor is set at 180-300° C., and after the reaction is completed, the reactor is cooled to room temperature, the gas in the reactor is collected, the catalyst is filtered and separated, and the product distribution is analyzed by characterization; The reaction substrate includes any one of 2-phenoxy-1-phenylethanol and lignin; The reaction solvent is isopropanol.

2. The use of a highly active Ni-Co bimetallic catalyst in lignin hydrocracking as claimed in claim 1, characterized in that: In step S2, the molar ratio of sodium carbonate to sodium hydroxide is 1:

10.

3. The use of a highly active Ni-Co bimetallic catalyst in lignin hydrocracking as claimed in claim 1, characterized in that: The calcination in step S4 is carried out in a muffle furnace, and the calcination heating rate is 2-5 °C / min.

4. The use of a highly active Ni-Co bimetallic catalyst in lignin hydrocracking as claimed in claim 1, characterized in that: In step S4, the flow rate of hydrogen is 60-80 mL / min, and the reduction temperature is set to 500-600 °C.

5. The use of a highly active Ni-Co bimetallic catalyst in lignin hydrocracking as claimed in claim 1, characterized in that: The catalyst obtained after separation in step S6 can be reused after drying, and the reaction is the same as the above steps S5-S6.

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