CN117299168B - Method for preparing fructose by catalyzing glucose isomerization using nitrogen-doped carbon/magnesium oxide composite materials - Google Patents

Abstract

The invention discloses a method for preparing fructose by catalyzing glucose isomerization by utilizing a nitrogen-doped carbon/magnesium oxide composite material, which comprises the following steps: the nitrogen-doped carbon/magnesium oxide composite material is obtained by taking a metal organic framework ZIF-8 and magnesium chloride as raw materials through a two-step pyrolysis method. The nitrogen-doped carbon/magnesium oxide composite material is used as a catalyst to catalyze glucose to isomerise in a water phase to obtain fructose, so that the problems that high-concentration raw materials are difficult to directly convert in the water phase for preparing fructose by glucose isomerism, the reaction efficiency is low, the stability of the catalyst is poor and the recycling process is complex are solved, and the industrial application of preparing fructose by heterogeneous base-catalyzed glucose isomerism is promoted.

Description

Method for preparing fructose by catalyzing glucose isomerization using nitrogen-doped carbon/magnesium oxide composite materials

Technical Field

The invention belongs to the technical field of biomass catalytic conversion, and specifically relates to a nitrogen-doped carbon/magnesium oxide composite material and a preparation method and application thereof.

Background technique

Fossil fuels dominate the current energy structure in my country, but the environmental pollution caused by the large-scale use of fossil fuels and the depletion of fossil fuels are issues that cannot be ignored. Biomass refining technology is an emerging technology that is expected to replace petroleum refining. It can convert biomass into high-value chemicals or liquid fuels, providing a feasible strategy to solve the above problems. The use of fructose as a starting material to produce platform compounds and biofuels such as 5-hydroxymethylfurfural (HMF), lactic acid, and 5-ethoxymethylfurfural (EMF) has shown broad prospects. At the same time, fructose is also a widely used sweetener in the food industry and has high economic value. Therefore, glucose isomerization to produce fructose is one of the key reactions in biomass refining technology and has been a hot topic of research in the past few decades.

At present, the systems for glucose isomerization to produce fructose mainly include enzyme catalysis systems and homogeneous or heterogeneous acid-base catalysis systems. Among them, the enzyme catalysis system has been widely used in the food industry due to its high safety, which is one of the largest biocatalysis industries at present. The high efficiency of the food industry can make up for the high cost of the enzyme catalysis system to a certain extent, but it is impractical to apply the fructose produced by this process on a large scale in the field of biomass refining. Therefore, homogeneous and heterogeneous acid-base catalysts have attracted extensive attention from researchers. A few Lewis acid catalysts (Sn-Beta zeolite, Sn-MFI zeolite, H-USY zeolite, hafnium-containing zeolite, etc.) have shown good catalytic activity in specific solvents. However, these catalysts face the disadvantages of difficulty in large-scale preparation, high cost, easy deactivation and the need for a specific solvent environment, which limits their large-scale application in industry.

Compared with enzyme catalysis system and Lewis acid catalysis system, base catalysis system has the advantages of low cost and less restriction on reaction conditions. All kinds of Bronsted base catalysts, including amines, alkali metal and alkaline earth metal oxides, hydroxides, phosphates, carbonates, etc., have good catalytic performance. Whether homogeneous or heterogeneous base catalysts, the maximum fructose yield is below 30-40% when water is used as solvent. Recently, it has been reported that the use of magnesium aluminum hydrotalcite (HT) as an alkaline catalyst increased the fructose yield to 52% in butanol solvent and 56% in ethanol. However, although the use of base catalysts in organic solvent systems can obtain higher yields, even the use of the simplest and cheapest organic solvents requires a series of separation and purification processes, the construction or reconstruction of reaction equipment, and the adjustment of upstream and downstream processes, which is accompanied by an increase in solvent costs and safety risks, which limits the practical application of base catalysts. Similarly, using polystyrene-supported organic base as a catalyst, the addition of neutral salt can increase the fructose yield from 30% to 41%, but the subsequent treatment is complicated and it is difficult to avoid secondary pollution.

In order to achieve industrial application, heterogeneous catalysts should meet the requirements of low cost, less secondary pollution, recyclability and use pure water as reaction solvent. In past studies, fructose yield was usually used as an evaluation index of catalytic activity, but this cannot fully reflect the actual reaction efficiency of the catalytic system. Since the reaction in the aqueous phase is constrained by the reaction equilibrium and side reactions, it is inevitable that the selectivity of glucose isomerization to fructose decreases with the increase of glucose conversion rate. Therefore, there is very limited room for improving production efficiency by further improving the apparent fructose yield. In addition, the cost caused by the large-scale use of solvents and other related losses cannot be ignored. The substrate concentration of glucose isomerization to fructose reported in the public literature is generally less than 10wt%, and higher fructose yields are often achieved by reducing the substrate concentration to about 1wt%. For example, when an ordered mesoporous carbon and magnesium oxide composite material (OMC@MgO) is used as a catalyst, as the amount of glucose added increases from 1wt% to 5wt%, the fructose yield remains at 32%, and further increasing the amount of glucose added to 10wt% causes the fructose yield to drop to 27.1% (J.Fu, F.Shen, X.Liu, X.Qi, Synthesis of MgO-doped ordered mesoporouscarbons by Mg2+-tannin coordination for efficient isomerization of glucose tofructose, Green Energy & Environment, (2021)). Therefore, direct conversion of glucose with high substrate concentration under the action of a lower dose of catalyst is very meaningful for substantially improving the reaction efficiency. If the catalytic system can directly convert glucose into fructose under high substrate concentration conditions, the high-concentration product obtained is easier to separate, purify and subsequently convert, thereby greatly reducing the overall process cost. Taking into account the catalyst and solvent, the actual reaction efficiency should be evaluated by the average fructose yield per unit weight of the catalytic system per unit time.

In addition to catalytic activity and production efficiency, stability and recyclability should be considered in the catalyst design stage in order to speed up practical application. Acid, base catalysts and isomerases are all prone to deactivation. During the reaction, heterogeneous base catalysts will undergo multiple processes such as structural collapse, loss of active sites, and deposition of carbon species, resulting in catalyst deactivation. Removing deposited carbonaceous substances by calcination can restore the activity of the catalyst to a certain extent, but catalytic materials containing organic components are prone to irreversible deactivation processes. The deactivation process caused by the loss of active sites is also irreversible. So far, few heterogeneous base catalysts can maintain catalytic activity in cyclic experiments through a simple regeneration process. For example, magnesium aluminum hydrotalcites need to be regenerated by high temperature calcination, regenerated in ammonium carbonate solution and other complex processes to achieve regeneration (M. Yabushita, N. Shibayama, K. Nakajima, A. Fukuoka, Selective Glucose-to-Fructose Isomerization in Ethanol Catalyzed by Hydrotalcites, ACS Catalysis, 9 (2019) 2101-2109.). In the cycle experiment, the catalytic activity of magnesium oxide and carbon composite material (MgO/Carbon) decreased significantly. The fructose yield was 34.57% in the first reaction and 16.87% in the second cycle experiment. Even if the catalyst was calcined and regenerated, the fructose yield could only be restored to 29.5% (Y.Shao, D.-Y.Zhao, W.Lu, Y.Long, W.Zheng, J.Zhao, Z.-T.Hu, MgO/Carbon nanocomposites synthesized in moltensalts for catalytic isomerization of glucose to fructose in aqueous media, Green Chemical Engineering, 3 (2022) 359-366). Developing a simple and easy catalyst regeneration method is very meaningful for reducing costs and improving production efficiency.

Summary of the invention

In view of the deficiencies in the prior art, the purpose of the present invention is to provide a method for preparing a nitrogen-doped carbon/magnesium oxide composite material, which uses a metal organic framework ZIF-8 and magnesium oxide as raw materials to prepare the nitrogen-doped carbon/magnesium oxide composite material by a two-step pyrolysis method.

Another object of the present invention is to provide the use of the above-mentioned nitrogen-doped carbon/magnesium oxide composite material as a catalyst in catalyzing the isomerization of glucose to prepare fructose. The nitrogen-doped carbon/magnesium oxide composite material is used as a catalyst to catalyze the isomerization of glucose to obtain fructose in an aqueous phase, thereby solving the problems of difficulty in directly converting high-concentration raw materials, low reaction efficiency, poor catalyst stability and complex recycling process in the isomerization of glucose to prepare fructose in an aqueous phase, and promoting the industrial application of heterogeneous base-catalyzed glucose isomerization to prepare fructose.

The purpose of the present invention is achieved through the following technical solutions.

A method for preparing a nitrogen-doped carbon/magnesium oxide composite material comprises: using metal organic framework ZIF-8 and magnesium chloride as raw materials, and obtaining the nitrogen-doped carbon/magnesium oxide composite material through a two-step pyrolysis method.

In the above technical solution, the preparation method of the nitrogen-doped carbon/magnesium oxide composite material comprises the following steps:

Step 1, uniformly dispersing the metal organic framework ZIF-8 and magnesium chloride in ethanol to obtain a suspension, and drying the suspension to obtain a white powder, wherein the ratio of the metal organic framework ZIF-8 to the magnesium chloride is 1:(1-10) based on the amount of the substance;

In step 1, the ratio of the mass fraction of the metal organic framework ZIF-8 to the volume fraction of the ethanol is (3-10):1, the unit of the mass fraction is mg, and the unit of the volume fraction is mL;

In step 1, the metal organic framework ZIF-8 and magnesium chloride are uniformly dispersed in ethanol by ultrasound, and the ultrasound time is 5 to 60 minutes.

In step 1, the drying method is to first stir at 20-100° C. at a speed of 100-3000 r/min for 0.5-5 h, and then stand at 60-100° C. for 5-12 h.

Step 2: calcining the white powder for the first time in a nitrogen or inert gas environment, washing, drying, and then calcining for the second time in a nitrogen or inert gas environment to obtain a nitrogen-doped carbon/magnesium oxide composite material.

In step 2, the first calcination is performed at 300-500° C. for 3-8 hours.

In the above technical solution, the first calcination is to increase the temperature to 300-500°C at a rate of 3-10°C/min and calcine at 300-500°C for 3-8h.

In step 2, the second calcination is performed at 500-900° C. for 3-8 hours.

In the above technical solution, the second calcination is to increase the temperature to 500-900°C at a rate of 1-5°C/min and calcine at 500-900°C for 3-8h.

In step 2, the washing method is to use ethanol and water for alternating washing.

In step 2, the drying temperature is 60-100° C., and the drying time is 8-12 hours.

The method for preparing a metal organic framework ZIF-8 comprises: dissolving 2-methylimidazole in water to obtain a first solution; dissolving a zinc salt in water to obtain a second solution; mixing the first solution and the second solution, stirring until uniform, standing, centrifuging, obtaining a white precipitate, washing the white precipitate, and drying to obtain a metal organic framework ZIF-8, wherein, in terms of the amount of substance, the ratio of zinc in 2-methylimidazole and zinc salt is (3 to 10):1.

In the method for preparing the metal organic framework ZIF-8, the concentration of 2-methylimidazole in the first solution is 0.2 to 1 mol/L, and the concentration of zinc in the zinc salt in the second solution is 0.02 to 0.08 mol/L.

In the method for preparing the metal organic framework ZIF-8, the stirring speed is 100 to 3000 r/min, and the stirring time is 0.5 to 5 h.

In the method for preparing the metal organic framework ZIF-8, the standing time is 5 to 12 hours.

In the method for preparing the metal organic framework ZIF-8, the centrifugal rotation speed is 5000-12000 r/min.

In the method for preparing the metal organic framework ZIF-8, the washing is performed by alternately washing with water and ethanol.

In the method for preparing the metal organic framework ZIF-8, the drying temperature is 40 to 100°C.

The nitrogen-doped carbon/magnesium oxide composite material is used as a catalyst in catalyzing glucose isomerization to prepare fructose.

In the above technical scheme, the nitrogen-doped carbon/magnesium oxide composite material is used as a catalyst to catalyze the isomerization of glucose to prepare fructose, which includes the following steps: mixing the nitrogen-doped carbon/magnesium oxide composite material, glucose and water, and stirring the reaction at 80-120°C.

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

The nitrogen-doped carbon/magnesium oxide composite material prepared by the present invention has higher reaction efficiency of glucose isomerization to prepare fructose as a catalyst in an aqueous phase, better catalyst stability, and can realize catalyst recycling, which is helpful to promote the industrial application of glucose isomerization to prepare fructose.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is (a-c) SEM and (d-h) EDS images of the nitrogen-doped carbon/magnesium oxide composite material prepared in Example 2;

FIG2 is an XPS spectrum of the nitrogen-doped carbon/magnesium oxide composite material prepared in Examples 1 to 3 and the magnesium oxide in Comparative Example 3;

FIG3 is an XPS spectrum of relevant elements in the nitrogen-doped carbon/magnesium oxide composite material prepared in Examples 1 to 3;

FIG4 is an XRD spectrum of the nitrogen-doped carbon/magnesium oxide composite material prepared in Examples 1 to 3 and the magnesium oxide in Comparative Example 3;

FIG5 is a Raman spectrum of the nitrogen-doped carbon/magnesium oxide composite material prepared in Examples 1 to 3;

FIG6 is a Fourier transform infrared spectrum of the nitrogen-doped carbon/magnesium oxide composite material prepared in Examples 1 to 3;

FIG7 is a carbon dioxide temperature programmed desorption experiment (CO ₂ -TPD) spectrum of the nitrogen-doped carbon/magnesium oxide composite material prepared in Example 2;

Figure 8 (a-c) shows the changes in the fructose yield, glucose conversion rate and fructose selectivity over time when the nitrogen-doped carbon/magnesium oxide composite material prepared in Examples 1 to 3 catalyzes the isomerization of glucose to fructose within 240 min; Figure 8 (d) shows the fructose yield, glucose conversion rate and fructose selectivity of the methods in Examples 35 to 38;

FIG9 is a test result of the catalytic effect of the nitrogen-doped carbon/magnesium oxide composite material prepared in Example 2 after cyclic use;

FIG10 shows the changes in fructose yield, glucose conversion rate and fructose selectivity with catalyst dosage in Examples 7 to 14;

Figure 11 shows the changes in fructose yield, glucose conversion rate and fructose selectivity over time at different substrate concentrations (the abscissa in Figure 11 represents T in Table 2), where (a) is Examples 15 to 19, (b) is Examples 20 to 24, (c) is Examples 25 to 29, and (d) is Examples 30 to 34.

Detailed ways

The technical solution of the present invention is further described below in conjunction with specific embodiments.

The related instruments and equipment used in the following examples are as follows:

High performance liquid chromatograph: Thermo Fisher U3000

The determination method of fructose yield, glucose conversion rate and fructose selectivity in the embodiment of the present invention is: fructose or glucose are used to configure multiple aqueous solutions of different concentrations as standard solutions, and the peak area of fructose or glucose in the standard solution is detected by high performance liquid chromatography, and the relationship curve between the peak area and the concentration of the corresponding substance (the corresponding substance is fructose or glucose) is established as a standard curve, and the reaction sample solution is filtered through a 0.25 μm filter membrane, diluted 5-100 times with water, and the reaction sample solution is detected by high performance liquid chromatography to obtain the peak area of the corresponding substance, which is substituted into the equation of the standard curve, and then the concentration of the corresponding substance in the reaction sample solution is calculated. The concentration of the corresponding substance before and after the reaction is determined respectively. Fructose yield, glucose conversion rate and fructose selectivity are calculated according to the following formula.

Calculation method for the average production efficiency of the catalytic system

The catalytic system consists of catalyst, glucose and water.

Comparative Example 1

An ordered mesoporous carbon and magnesium oxide composite material (OMC@MgO), and its preparation method reference: J.Fu, F.Shen, X.Liu, X.Qi, Synthesis of MgO-doped ordered mesoporous carbons by Mg2+-tannin coordination for efficient isomerization of glucose to fructose, Green Energy & Environment, (2021)).

Ordered mesoporous carbon and magnesium oxide composite material (OMC@MgO) was used as a catalyst. A certain amount of catalyst was added to a 10mL polytetrafluoroethylene-lined reactor, and a certain concentration of glucose solution was added. The thermal catalytic reaction was carried out at a specific temperature under magnetic stirring at 1000r/min. After the reaction, the reaction vessel was rinsed with cold water to achieve the purpose of rapid quenching of the reaction. After filtering the reaction product, the concentrations of glucose and fructose before and after the reaction were determined by Shodex SH1011 chromatographic column and differential detector high performance liquid chromatography (Waters Acquity UPLC H-Clasa). The mobile phase was 5mmol/L sulfuric acid solution, the flow rate was 0.5mL/min, and the column temperature and detector temperature were 50℃ and 35℃, respectively. Then the glucose conversion rate, fructose yield and selectivity were calculated.

As the amount of glucose added increased from 1wt% to 5wt%, the fructose yield remained at 32%, at which point the highest average production efficiency of the catalytic system was 0.088mmol g ^- 1h ^-1 . When the amount of glucose added was further increased to 10wt%, the fructose yield dropped to 27.1%.

When the amount of glucose added was fixed at 5 wt %, as the amount of catalyst added decreased from 30 mg to 5 mg (the mass ratio relative to glucose decreased from 0.12 to 0.02), the fructose yield decreased from 23% to 11.7%.

Examples 1 to 3

A method for preparing a nitrogen-doped carbon/magnesium oxide composite material comprises: using metal organic framework ZIF-8 and magnesium chloride hexahydrate as raw materials, and obtaining the nitrogen-doped carbon/magnesium oxide composite material by a two-step pyrolysis method. The two-step pyrolysis method comprises the following steps:

Step 1, mixing the metal organic framework ZIF-8, magnesium chloride hexahydrate and ethanol, ultrasonicating for 10 minutes, so that the metal organic framework ZIF-8 and magnesium chloride hexahydrate are uniformly dispersed in the ethanol to obtain a suspension, stirring the suspension at 26°C at a speed of 2500r/min for 60 minutes, and then standing in an oven at 80°C for 600 minutes to obtain a white powder, wherein, in terms of the amount of substance, the ratio of the metal organic framework ZIF-8 to magnesium chloride hexahydrate is 1:4, the ratio of the mass fraction of the metal organic framework ZIF-8 to the volume fraction of ethanol is 100:20, the unit of mass fraction is mg, and the unit of volume fraction is mL;

Step 2, the white powder is placed in a tube furnace, heated to 300°C at a rate of 5°C/min in a nitrogen environment, and calcined at 300°C for the first time for 5 hours, washed alternately with ethanol and water for 5 times each, dried in an oven at 80°C for 600 minutes, and then placed in a tube furnace, heated to T°C at a rate of 1°C/min in a nitrogen environment, and calcined at T°C for the second time for 5 hours to obtain a nitrogen-doped carbon/magnesium oxide composite material. The value of T is shown in Table 1.

The method for preparing the above-mentioned metal organic framework ZIF-8 includes: dissolving 2-methylimidazole in deionized water to obtain a first solution; dissolving zinc nitrate hexahydrate in deionized water to obtain a second solution; mixing the first solution and the second solution, stirring at a speed of 2500r/min for 2h until uniform, standing for 12h, centrifuging at a speed of 10000r/min to obtain a white precipitate, washing the white precipitate alternately with water and ethanol for 3 times, and drying in an oven at 80°C to obtain a metal organic framework ZIF-8, wherein, in terms of the amount of substance, the ratio of zinc in 2-methylimidazole and zinc nitrate hexahydrate is 8.3:1, the concentration of 2-methylimidazole in the first solution is 0.355mol/L, and the concentration of zinc in the zinc salt in the second solution is 0.043mol/L.

Table 1

Example T(℃) Numbering of nitrogen-doped carbon/magnesium oxide composites Example 1 550 MgO-NC-550 Example 2 700 MgO-NC-700 Example 3 800 MgO-NC-800

Comparative Example 2

A method for preparing a metal organic framework ZIF-8 comprises the following steps:

2-Methylimidazole was dissolved in deionized water to obtain a first solution; zinc nitrate hexahydrate was dissolved in deionized water to obtain a second solution; the first solution and the second solution were mixed, stirred at a speed of 2500 r/min for 2 hours until uniform, allowed to stand for 8 hours, and centrifuged at a speed of 10000 r/min to obtain a white precipitate, the white precipitate was washed alternately with water and ethanol for 3 times each, and dried in an oven at 80°C to obtain a metal organic framework ZIF-8, wherein, based on the amount of substance, the ratio of zinc in 2-methylimidazole and zinc nitrate hexahydrate was 8.3:1, the concentration of 2-methylimidazole in the first solution was 0.355 mol/L, and the concentration of zinc in the zinc salt in the second solution was 0.043 mol/L.

Comparative Example 3

A magnesium oxide purchased from Macklin, with a purity of 99.9% metals basis.

Comparative Example 4

A method for preparing NC-700 is provided, wherein the metal organic framework ZIF-8 in comparative example 2 is calcined at 700° C. for 5 hours in an air atmosphere to obtain NC-700.

Comparative Example 5

A method for preparing MgO-700 is provided. The magnesium oxide in Comparative Example 3 is calcined at 700° C. for 5 hours in an air atmosphere to obtain MgO-700.

Embodiments 4 to 38

A method for preparing fructose by catalyzing glucose isomerization using a catalyst, comprising the following steps: adding a catalyst, glucose and water into a pressure-resistant bottle, placing the pressure-resistant bottle in an oil bath pot and stirring and heating at 90° C. for 4 h (stirring speed is 2500 r/min), wherein the fructose yield, glucose conversion rate and fructose selectivity are detected at the Tth min of stirring, wherein Tmin is 30 min, 60 min, 120 min, 180 min and 240 min, the ratio of the catalyst, glucose and water is X in parts by mass, the fructose yield obtained at the Tth min of stirring is Y, and the catalyst is one of MgO-NC-550, MgO-NC-700, MgO-NC-800 prepared in Examples 1 to 3, the metal organic framework ZIF-8 prepared in Comparative Example 2, the magnesium oxide of Comparative Example 3, the NC-700 prepared in Comparative Example 4 and the MgO-700 prepared in Comparative Example 5. When X is 0.2:0.5:5, the effect curves of the nitrogen-doped carbon/magnesium oxide composite materials prepared in Examples 1 to 3 catalyzing glucose isomerization to produce fructose within 240 min are shown in a to c of Figure 8. When X is 0.1:0.25:5, the metal organic framework ZIF-8 prepared in Comparative Example 2, the magnesium oxide in Comparative Example 3, the NC-700 prepared in Comparative Example 4, and the MgO-700 prepared in Comparative Example 5 catalyzing glucose isomerization to produce fructose at 120 min, the fructose yield, glucose conversion rate, and fructose selectivity are shown in d of Figure 8.

Table 2

The elemental analysis results of the nitrogen-doped carbon/magnesium oxide composite materials prepared in Examples 1 to 3 of the present invention are shown in Table 3, wherein the element content is determined by elemental content analysis, the metal content is determined using ICP-OES, and the surface element composition is determined by XPS analysis.

table 3

It can be seen from Table 3 that the carbon and nitrogen contents in the nitrogen-doped carbon/magnesium oxide composite materials prepared in Examples 1 and 2 are similar, while the carbon and nitrogen contents in the nitrogen-doped carbon/magnesium oxide composite materials prepared in Example 3 are significantly lower. These results show that suitable temperature is the key to maintaining the nitrogen-doped carbon structure. The metal content was analyzed by ICP-OES, which verified that Mg was successfully loaded on the metal organic framework ZIF-8. The zinc content in the nitrogen-doped carbon/magnesium oxide composite materials prepared in Examples 2 and 3 is less than that in Example 1, and most of the zinc is removed during the calcination process. In general, the elemental composition of the nitrogen-doped carbon/magnesium oxide composite material prepared in Example 2 is relatively balanced.

The specific surface area and internal pore analysis of the nitrogen-doped carbon/magnesium oxide composite materials prepared in Examples 1 to 3 of the present invention are shown in Table 4, and the results are calculated from the nitrogen adsorption curve using the BET formula.

Table 4

As shown in Table 4, the specific surface areas of the nitrogen-doped carbon/magnesium oxide composite material prepared in Example 2 and the nitrogen-doped carbon/magnesium oxide composite material prepared in Example 3 are 5.8 times and 4.9 times that of magnesium oxide, respectively (Table 4). The significant increase in specific surface area can enhance the mass transfer capacity between the catalyst and the reactants, thereby facilitating the improvement of catalytic performance.

The average production efficiency (average fructose production per unit weight of the catalytic system per unit time) of the catalytic system of the nitrogen-doped carbon/magnesium oxide composite material prepared in Example 2 of the present invention is shown in Table 5.

table 5

Note: Glucose concentration = mass of glucose/mass of water; Catalyst dosage = mass of catalyst/mass of water.

Compared with previously reported catalysts, the nitrogen-doped carbon/magnesium oxide composite material prepared in Example 2 has outstanding advantages in reducing the amount of catalyst used and tolerating high-concentration glucose raw materials. If limited to the aqueous phase catalytic system, the fructose yield of the nitrogen-doped carbon/magnesium oxide composite material system prepared in Example 2 is the highest. The average production efficiency of the nitrogen-doped carbon/magnesium oxide composite catalytic system prepared in Example 2 is as high as 0.2267 mmol g-1h-1, which is 2.29 times that of the most efficient catalytic system reported previously (A.A.Marianou, C.M.Michailof, A.Pineda, E.F.Iliopoulou, K.S.Triantafyllidis, A.A.Lappas, Glucose to Fructose Isomerization in Aqueous Media over Homogeneous and Heterogeneous Catalysts, ChemCatChem, 8 (2016) 1100-1110) (A.A.Marianou, C.M.Michailof, D.K.Ipsakis, S.A.Karakoulia, K.G.Kalogiannis, H.Yiannoulakis, K.S.Triantafyllidis, A.A.Lappas, Isomerization of Glucose into Fructose over Natural and Synthetic MgO Catalysts, Acs Sustainable Chemistry & Engineering, 6 (2018) 16459-16470).

Figure 1 shows that the nitrogen-doped carbon/magnesium oxide composite material is composed of irregular particles of micrometer size. EDS spectrum further illustrates that carbon, nitrogen, oxygen and magnesium elements are distributed on the nitrogen-doped carbon/magnesium oxide composite material.

It can be seen from Figures 2 and 3 that the surface Mg content of MgO/NC-700 (nitrogen-doped carbon/magnesium oxide composite material prepared in Example 2) is comparable to that of MgO/NC-550 (nitrogen-doped carbon/magnesium oxide composite material prepared in Example 1), while the surface N content of MgO/NC-700 is more than twice that of MgO/NC-550. The N/Mg atomic ratios on the surface and inside of MgO/NC-550 (nitrogen-doped carbon/magnesium oxide composite material prepared in Example 1) are roughly equal (0.282 and 0.257, respectively), indicating that the element distribution of the nitrogen-doped carbon/magnesium oxide composite material prepared in Example 1 is relatively uniform. In contrast, the surface N/Mg atomic ratio (0.671) of MgO/NC-700 (nitrogen-doped carbon/magnesium oxide composite material prepared in Example 2) is higher than the bulk N/Mg atomic ratio (0.124). Corroborated by the results of the carbon element peak, it can be inferred that the structure of nitrogen-doped carbon is more inclined to gather on the surface of the composite material and can be used as a protective layer.

As can be seen from Figure 4, MgO/NC-550 (nitrogen-doped carbon/magnesium oxide composite material prepared in Example 1), MgO/NC-700 (nitrogen-doped carbon/magnesium oxide composite material prepared in Example 2) and MgO/NC-800 (nitrogen-doped carbon/magnesium oxide composite material prepared in Example 3) have characteristic peaks of crystalline magnesium oxide at 37.0°, 43.0°, 62.4°, 74.7° and 78.6°. The peak intensity of the nitrogen-doped carbon/magnesium oxide composite material prepared in Example 2 is greater than the peak intensity of the nitrogen-doped carbon/magnesium oxide composite materials prepared in Example 1 and Example 3, indicating that the MgO/NC-700 of Example 2 has a higher magnesium oxide crystallinity.

In Figure 5, graphitized carbon (G band) and disordered carbon (D band) appear at 1586 cm ^-1 and 1323 cm ^-1 , respectively. As shown in Figure 5, the peak area ratios of the D band and the G band of the nitrogen-doped carbon/magnesium oxide composites prepared in Examples 1 to 3 are 1.34, 1.14 and 1.06, respectively, indicating that the increase in the secondary calcination temperature is conducive to the graphitization of the material.

As shown in FIG6 , the OH peak in MgO/NC-700 (the nitrogen-doped carbon/magnesium oxide composite material prepared in Example 2) is significantly lower than that of the other two materials, and may have better thermal stability.

Figure 7 is a carbon dioxide temperature programmed desorption (CO ₂ -TPD) spectrum of the nitrogen-doped carbon/magnesium oxide composite material prepared in Example 2, and the basicity of the material is detected. As can be seen from the figure, the strong peak is between 400 and 750°C, and the weak peak is centered at 150°C, which confirms the existence of strong basicity and weak basicity, respectively, indicating that the obtained nitrogen-doped carbon/magnesium oxide composite material is a typical solid base catalyst.

Figure 8 (a), (b), (c) are the effect curves of the nitrogen-doped carbon/magnesium oxide composite materials prepared in Examples 1, 2, and 3 catalyzing the isomerization of glucose to produce fructose within 240 min. The maximum fructose yields of the nitrogen-doped carbon/magnesium oxide composite materials prepared in Examples 1, 2, and 3 catalyzing the isomerization of glucose to produce fructose were 36.2%, 39.9%, and 37.2%, respectively, and the fructose selectivities were 80.8%, 67.4%, and 56.4%, respectively.

Figure 8(d) shows the fructose yield, glucose conversion rate and fructose selectivity of Examples 35 to 38 in Table 2 at the 120th minute of stirring, among which the metal organic frameworks ZIF-8 and NC-700 have poor activity, and the glucose conversion rate does not exceed 10%. The fructose yield of magnesium oxide is 31.1%, but the selectivity is relatively low. The glucose conversion rate of magnesium oxide after calcination is slightly increased, while the fructose yield and selectivity are low. The fructose yield of nitrogen-doped carbon/magnesium oxide composite material is significantly higher than that of magnesium oxide, metal organic framework ZIF-8 and NC-700 materials, indicating that its unique composite structure is the key to producing superior catalytic performance.

FIG9 is a test result of the catalytic effect of the nitrogen-doped carbon/magnesium oxide composite material (MgO/NC-700) prepared in Example 2 after being recycled according to the method for preparing fructose by catalyzing glucose isomerization using a catalyst in Example 12 in Table 2 (after each reaction, the solid catalyst is recovered by centrifugation or filtration, washed alternately with water and ethanol three times each, dried at 80°C, and then the next cycle experiment is carried out). As shown in FIG9, after 4 cycles of use (recycling and drying), the catalytic performance of the nitrogen-doped carbon/magnesium oxide composite material is relatively stable.

As shown in Figure 10, the excellent catalytic activity of the nitrogen-doped carbon/magnesium oxide composite material prepared in Example 2 is also reflected in its ability to play a catalytic role under conditions of very low catalyst dosage (the test results in Figure 10 are the corresponding test results at 120 minutes in the method of preparing fructose by catalyzing glucose isomerization using different amounts of catalyst, and Figure 10 is Examples 7 to 14 in order from small to large from left to right). For example, as in Example 7 in Table 2, when the ratio of catalyst, glucose and water is 0.005:0.25:5, the fructose yield is still as high as 28.23%. As the ratio of catalyst, glucose and water increases to 0.05:0.25:5 (Example 10), the fructose yield is increased to 42.6%.

As shown in Figure 11, keeping the dosage of the nitrogen-doped carbon/magnesium oxide composite material prepared in Example 2 unchanged, when the ratio of catalyst, glucose and water is 0.1:0.5:5 (Example 22), the fructose yield is further increased to 42.9%, reaching the limit of the reaction equilibrium in the water phase. When the ratio of catalyst, glucose and water is 0.1:1:5 (Example 32), the fructose yield is still as high as 40.8%.

The present invention is described above by way of example. It should be noted that, without departing from the core of the present invention, any simple deformation, modification or other equivalent replacement that can be made by those skilled in the art without inventive labor falls within the protection scope of the present invention.

Claims (9)

1. A method for preparing a nitrogen-doped carbon/magnesium oxide composite material for catalyzing glucose isomerization to prepare fructose, characterized in that it comprises: using metal organic framework ZIF-8 and magnesium chloride as raw materials, and obtaining the nitrogen-doped carbon/magnesium oxide composite material by a two-step pyrolysis method; The preparation method of the nitrogen-doped carbon/magnesium oxide composite material specifically comprises the following steps: Step 1, uniformly dispersing the metal organic framework ZIF-8 and magnesium chloride in ethanol to obtain a suspension, and drying the suspension to obtain a white powder, wherein the ratio of the metal organic framework ZIF-8 to the magnesium chloride is 1:(1-10) based on the amount of the substance; Step 2: calcining the white powder for the first time in a nitrogen or inert gas environment, washing, drying, and then calcining for the second time in a nitrogen or inert gas environment to obtain a nitrogen-doped carbon/magnesium oxide composite material. 2. The preparation method according to claim 1, characterized in that in step 1, the ratio of the mass fraction of the metal organic framework ZIF-8 to the volume fraction of the ethanol is (3 to 10): 1, the unit of mass fraction is mg, and the unit of volume fraction is mL. 3. The preparation method according to claim 2 or 1, characterized in that in step 1, the metal organic framework ZIF-8 and magnesium chloride are uniformly dispersed in ethanol by ultrasound, and the ultrasound time is 5 to 60 minutes. 4. The preparation method according to claim 1, characterized in that in step 1, the drying method is firstly stirring at 20-100°C at a speed of 100-3000 r/min for 0.5-5h, and then standing at 60-100°C for 5-12h; In step 2, the washing method is to use ethanol and water for alternating washing; In step 2, the drying temperature is 60-100° C., and the drying time is 8-12 hours. 5. The preparation method according to claim 1, characterized in that, in step 2, the first calcination is calcined at 300-500°C for 3-8h. 6. The preparation method according to claim 5 or 1, characterized in that the second calcination is calcined at 500-900°C for 3-8h. 7. The preparation method according to claim 6 is characterized in that the first calcination is to increase the temperature to 300-500°C at a rate of 3-10°C/min and calcine at 300-500°C for 3-8h; the second calcination is to increase the temperature to 500-900°C at a rate of 1-5°C/min and calcine at 500-900°C for 3-8h. 8. Use of the nitrogen-doped carbon/magnesium oxide composite material obtained by the preparation method according to any one of claims 1 to 7 as a catalyst in catalyzing glucose isomerization to produce fructose. 9. A method for preparing fructose by catalyzing glucose isomerization using the nitrogen-doped carbon/magnesium oxide composite material obtained by the preparation method according to any one of claims 1 to 7, characterized in that it comprises the following steps: mixing the nitrogen-doped carbon/magnesium oxide composite material, glucose and water, and stirring the mixture at 80 to 120°C for reaction.