A Mo@C3N4 catalyst, its preparation method and application

Abstract

This invention relates to the field of high-value utilization technology of biomass, specifically to a Mo@C3N4 catalyst, its preparation method, and its application. The preparation method of the Mo@C3N4 catalyst includes the following steps: dissolving ammonium molybdate and urea in water to obtain a mixed solution; freezing and then freeze-drying the mixed solution to obtain a catalyst precursor; and calcining the catalyst precursor at high temperature to obtain the final catalyst. This Mo@C3N4 catalyst uses urea as both a carbon and nitrogen source, and ammonium molybdate to provide catalytic activation sites for molybdenum. It is prepared by freeze-drying and high-temperature calcination, a simple and easy-to-operate method. Freeze-drying preserves the original chemical composition and physical properties of the dried material and maintains the dispersion of active sites, preventing agglomeration and affecting catalytic efficiency. High-temperature calcination forms a 2D nanosheet material from the C3N4 substrate and Mo metal active sites. This structure not only gives the catalyst high catalytic activity in water but also ensures high dispersion of the metal active sites.

Description

Technical Field

[0001] This invention belongs to the field of high-value utilization technology of biomass, specifically relating to a Mo@C3N4 catalyst, its preparation method, and its application. Background Technology

[0002] Biomass-based glucose isomerization can synthesize various value-added monosaccharides, such as fructose, mannose, and allulose. These high-value monosaccharides are precursors for many drugs and high-value-added chemicals. Mannose, for example, binds to human glycoproteins to regulate the human immune system and eliminate inflammation; it is also used as a preservative for fruits and vegetables; as a sweetener in health products and beverages; and adding mannose to livestock feed can inhibit Salmonella typhi infection in poultry and livestock. Enzymatic methods provide a reliable route for mannose production; however, the production efficiency is low due to the thermodynamic equilibrium control and enzyme activity limitations of enzymatic isomerization. Thermochemical isomerization remains one of the effective methods for industrial mannose production. However, chemical isomerization has environmental and energy consumption drawbacks, therefore, there is an urgent need to explore new and efficient catalysts that can efficiently catalyze the epimerization of glucose to mannose.

[0003] Graphite carbon-nitrogen materials (C3N4) have become a research hotspot in photocatalysis and thermochemical catalysts due to their low raw material cost, high thermal stability, high electron transport capacity, and non-toxicity. The nitrogen element on the C3N4 surface exhibits high plasticity, capable of generating functional groups such as -NH2, -NH-, and C=ON, which can synergistically catalyze active sites. The unique nanotube-like structure of C3N4 facilitates the dispersion of metal active sites. Currently, most research utilizes C3N4 supported catalysts for the photocatalytic degradation of organic pollutants. This invention successfully prepared a structured Mo@C3N4 catalyst and achieved highly efficient thermocatalytic glucose epimerization to mannose, providing a new synthetic approach for the efficient synthesis of value-added mannose from glucose.

[0004] Chinese patent application CN106032386A discloses a catalytic conversion method for aldoses and ketoses. This method involves adding oxidically modified carbon nanotubes or graphene with supported metal elements as a catalyst to an aldose solution, catalyzing the isomerization of aldoses to ketoses, and simultaneously causing C2 isomerization to obtain epimers. The metal element is one or more of iron, aluminum, calcium, magnesium, molybdenum, chromium, and manganese. However, this catalyst exhibits the highest glucose isomerization conversion rate at 140℃, but it still requires 3-4 hours to reach a conversion rate of over 80%. At 120℃, after 6 hours of reaction, the mannose yield is 40%, and the fructose yield is 51%. Using this catalyst to catalyze the isomerization of glucose to mannose and fructose requires a considerable amount of time, resulting in low catalytic efficiency. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing a Mo@C3N4 catalyst. The Mo@C3N4 catalyst prepared by this method has a 2D nanosheet structure and can efficiently catalyze the isomerization of glucose to produce mannose.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing a Mo@C3N4 catalyst, comprising the following steps: dissolving ammonium molybdate and urea in water to obtain a mixed solution; freezing the mixed solution and then freeze-drying it to obtain a catalyst precursor; and calcining the catalyst precursor at high temperature to obtain the catalyst.

[0008] Furthermore, the mass ratio of ammonium molybdate to urea is 0.5 to 1:5; the mass of ammonium molybdate added per mL of water is 0.01 to 0.02 g.

[0009] Furthermore, the calcination temperature is 450–600°C, the calcination heating rate is 4–6°C / min, and the calcination time is 3–5 hours.

[0010] Furthermore, the freezing temperature is -80 to -70°C, and the freezing time is 4 to 6 hours.

[0011] Furthermore, the ammonium molybdate and urea are dissolved in water and stirred at a speed of 550-650 r / min for 4-7 h to obtain the mixed solution.

[0012] A Mo@C3N4 catalyst was prepared using the method described above for preparing Mo@C3N4 catalysts.

[0013] An application of a Mo@C3N4 catalyst involves adding the Mo@C3N4 catalyst to a glucose solution and heating it to catalyze the isomerization of glucose to produce mannose and fructose.

[0014] Furthermore, the solvent of the glucose solution is water, and the concentration of the glucose solution is 8-10 g / L; the mass of the catalyst added in each mL of glucose solution is 0.004-0.005 g.

[0015] Furthermore, the heating catalysis is carried out at 100–170°C.

[0016] The beneficial effects of this invention are:

[0017] The Mo@C3N4 catalyst of this invention uses urea as both a carbon and nitrogen source, and ammonium molybdate to provide catalytic activation sites for molybdenum. It is prepared using a green and highly operable freeze-drying and high-temperature calcination method, which is simple and easy to operate. Freeze-drying allows the dried material to retain its original chemical composition and physical properties, and maintains the dispersion of active sites, preventing agglomeration and thus ensuring catalytic efficiency.

[0018] The Mo@C3N4 catalyst of this invention successfully prepared a 2D nanosheet material with C3N4 as the substrate and Mo as the metal active site by high-temperature calcination at 550℃. The 2D nanosheet structure not only gives the Mo@C3N4 catalyst high catalytic activity in water, but also allows for high dispersibility of the metal active site. The 2D nanosheet Mo@C3N4 catalyst prepared by this invention has certain advantages over powder in aqueous media. First, the C3N4 framework, as a non-metallic semiconductor, has a strong electron transfer effect. Compared with traditional catalysts, the nanosheet catalyst has excellent dispersibility in water. Due to the conjugated structure of the triazine units in the nanosheet catalyst, it also exhibits chemical and thermal stability under acid / alkali environments and high-temperature conditions. In summary, the excellent electron transfer effect of the C3N4 framework and the nanostructure of the nanosheet catalyst enable the 2D nanosheet Mo@C3N4 catalyst to have good catalytic activity in water, avoiding the use of toxic and harmful organic solvents.

[0019] This invention successfully prepared a 2D nanosheet-like Mo@C3N4 catalyst by freeze-drying combined with high-temperature calcination at 550℃, achieving efficient and uniform Mo loading on the nanosheets. This Mo@C3N4 catalyst was then applied to the high-value utilization of glucose, enabling efficient isomerization of glucose to mannose and fructose, with a high mannose yield. The mannose yield reached 31.47% within 5 minutes at 130℃. After three recycling cycles, the catalytic selectivity of the Mo@C3N4 catalyst still exceeded 90%.

[0020] In the preparation of the Mo@C3N4 catalyst and the catalytic process of glucose isomerization using the Mo@C3N4 catalyst, only water is used as a solvent, and no toxic byproducts are generated, providing an effective technical route for the green chemical synthesis of mannose. Attached Figure Description

[0021] Figure 1 This is a scanning electron microscope image of the Mo@C3N4 catalyst in Example 3;

[0022] Figure 2 The image shows a scanning electron microscope (SEM) image of the Al@C3N4 catalyst in Comparative Example 1.

[0023] Figure 3 Scanning electron microscope image of the C3N4 catalyst prepared after removing ammonium molybdate;

[0024] Figure 4 (a) XPS analysis (elemental peaks of Mo 3d) of the Mo@C3N4 catalyst before and after catalysis at 130°C for 5 min in Example 3;

[0025] Figure 4 (b) XPS analysis (C 1s elemental peak) of the Mo@C3N4 catalyst before and after catalysis at 130°C for 5 min in Example 3;

[0026] Figure 4 (c) XPS analysis (N 1s elemental peak) of Mo@C3N4 catalyst before and after catalysis at 130℃ for 5 min in Example 3;

[0027] Figure 4 (d) is a survey diagram of the four elements C, N, O and Mo before and after the reaction of the Mo@C3N4 catalyst in Example 3 at 130°C for 5 min.

[0028] Figure 5 The FTIR spectra of the Mo@C3N4 catalyst in Example 3 before and after catalysis at 130°C for 5 min are shown.

[0029] Figure 6 The catalytic efficiency graph of the Mo@C3N4 catalyst in Example 3 at different catalytic temperatures over 30 minutes is shown.

[0030] Figure 7 The graph shows the catalytic efficiency of the Mo@C3N4 catalyst in Example 3 at a catalytic temperature of 130°C and different catalytic times.

[0031] Figure 8 The catalytic efficiency graph of the Mo@C3N4 catalyst in Example 3 within 5 minutes at a catalytic temperature of 130°C (repeated three times). Detailed Implementation

[0032] The freeze dryer used in this invention is from Virtis BTP-8ZLEOX SP Scientific, the muffle furnace is from Hefei Kejing Materials Technology Co., Ltd. (KSL-1200X), the oil bath is from Zhengzhou Huatai Instrument Equipment Co., Ltd. (ZNCL-GS type), the X-ray photoelectron spectrometer is from Thermo Fisher Nexsa (USA), and the scanning electron microscope is from Hitachi Regulus 8230 (Japan).

[0033] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0034] Example 1

[0035] The preparation method of the Mo@C3N4 catalyst in this embodiment is as follows:

[0036] 1. Dissolve 0.75g of ammonium molybdate and 5g of urea in 40mL of deionized water, stir magnetically at 600r / min for 6h to obtain a mixed solution;

[0037] 2. Freeze the mixed solution at -80℃ for 4 hours, then freeze-dry it in a freeze dryer to obtain the catalyst precursor;

[0038] 3. The precursor was activated at high temperature in a muffle furnace at 450℃ for 4 hours, and then naturally cooled to room temperature to obtain the Mo@C3N4 catalyst. The heating rate during calcination was 4℃ / min.

[0039] Example 2

[0040] The preparation method of the Mo@C3N4 catalyst in this embodiment is as follows:

[0041] 1. Dissolve 0.5g of ammonium molybdate and 5g of urea in 50mL of deionized water, stir magnetically at 600r / min for 6h to obtain a mixed solution;

[0042] 2. Freeze the mixed solution at -75℃ for 5 hours, then freeze-dry it in a freeze dryer to obtain the catalyst precursor;

[0043] 3. The precursor was activated at high temperature in a muffle furnace at 500℃ for 4 hours, and then naturally cooled to room temperature to obtain the Mo@C3N4 catalyst. The heating rate during calcination was 5℃ / min.

[0044] The obtained 4 mL of 9 g / L glucose aqueous solution was placed in a 10 mL microreactor with a polytetrafluoroethylene liner. 0.02 g of Mo@C3N4 catalyst was placed in the liner. The microreactor was then placed in an oil bath at 130 °C and heated to catalyze the isomerization of glucose to obtain mannose and fructose.

[0045] Example 3

[0046] The preparation method of the Mo@C3N4 catalyst in this embodiment is as follows:

[0047] 1. Dissolve 0.75g of ammonium molybdate and 5g of urea in 50mL of deionized water, stir magnetically at 600r / min for 6h to obtain a mixed solution;

[0048] 2. Freeze the mixed solution at -75℃ for 5 hours, then freeze-dry it in a freeze dryer to obtain the catalyst precursor;

[0049] 3. The precursor was activated at high temperature in a muffle furnace at 550℃ for 4 hours, and then naturally cooled to room temperature to obtain the Mo@C3N4 catalyst. The heating rate during calcination was 5℃ / min.

[0050] The obtained 4 mL of 10 g / L glucose aqueous solution was placed into a 10 mL microreactor with a polytetrafluoroethylene liner. 0.02 g of Mo@C3N4 catalyst was placed into the liner. The microreactor was then placed in an oil bath at 130 °C and heated to catalyze the isomerization of glucose to obtain mannose and fructose.

[0051] The Mo@C3N4 catalyst prepared in this embodiment has a 2D nanosheet structure. A scanning electron microscope image of the 2D nanosheet Mo@C3N4 catalyst prepared in this embodiment is shown below. Figure 1 As shown in the figure, after Mo is loaded, the Mo@C3N4 catalyst exhibits a relatively regular lamellar cuboid aggregate block structure, with each cuboid structure having a length of approximately 2 μm. Figure 3 Scanning electron microscope image of g-C3N4 prepared by calcination of urea, from Figure 3 As can be seen, g-C3N4 exhibits an irregular, unstable, plate-like structure. From... Figure 1 and Figure 3 The comparison shows that Mo salt has a significant impact on the surface morphology and structure of C3N4, resulting in the formation of a regular 2D nanosheet stable structure in the prepared Mo@C3N4 catalyst. The presence of Mo not only enables the active sites to be loaded on the support, but also provides a stable structure for the support through calcination.

[0052] 4 mL of the obtained 10 g / L glucose aqueous solution was placed in a 10 mL microreactor with a polytetrafluoroethylene liner. 0.02 g of Mo@C3N4 catalyst was placed inside the liner. The microreactor was then placed in a 130°C oil bath for heating to catalyze the isomerization of glucose to obtain mannose and fructose. Table 1 shows the results of glucose catalysis using the Mo@C3N4 catalyst in this example at a catalytic temperature of 130°C and different catalytic times.

[0053] Table 1

[0054]

[0055] As can be seen from Table 1, the catalyst in this embodiment catalyzes the isomerization of glucose to produce fructose and mannose. At 130°C and within 5 minutes, the selectivity of mannose is the highest, reaching 85%, and the yield of mannose reaches 31.47%. However, as the catalytic time increases, the produced mannose decomposes or isomerizes into fructose, resulting in a decrease in the selectivity of mannose.

[0056] XPS analysis images of the 2D nanosheet Mo@C3N4 catalyst prepared in this embodiment before and after catalysis are shown below. Figure 4 As shown, Figure 4 'a' represents the Mo 3d element peak. The Mo 3d spectrum can be divided into two pairs of peaks, with the peaks at 231.2 eV and 234.2 eV corresponding to Mo 3d elements. 4+ The peaks at 232.2 eV and 235.8 eV correspond to Mo. 6+ The peak, of which Mo 6+ Pre-reaction Mo accounts for the majority 6+ The proportion was 85.61%, Mo 4+ The proportion was 14.39%, and after the reaction, Mo... 6+ The proportion was 90.21%, Mo 4+ The percentage was 9.79%, Mo 6+ The ratio of Mo before and after the reaction did not change significantly. 6+ Dominant; Figure 4 b is the elemental peak of C1s, with a peak value of 284.8 eV corresponding to uncertain C-C hydrocarbon hybridization, and a peak value of 286.5 eV corresponding to sp. 3 CN coordination hybridization, peak value of 288.6 eV corresponds to sp 2 C=N hybridization is a fundamental structure of carbon-based materials, indicating the presence of C3N4 in the 2D nanosheet Mo@C3N4 catalyst prepared in this embodiment. The peak at 284.2 eV is generated by C-Mo hybridization, indicating that Mo was successfully loaded onto C3N4. Before the reaction, the C=N hydrocarbon hybridization ratio was 56.06%, sp 3 The CN hybridization ratio was 26.29%, and the CC hydrocarbon hybridization ratio after the reaction was 81.32%. 3 The CN hybridization ratio was 7.06%, and the C-C hydrocarbon hybridization and sp hybridization before and after the reaction were... 3 The proportion of CN hybridization varies considerably; Figure 4 c is the elemental peak of N1s. The peak values ​​at 398.4 eV and 400.8 eV are the hybridization peaks of N=C and NH, respectively. A new peak belonging to N-Mo is formed at 396.3 eV, which is the result of strong electronic coupling between the Mo atom and the g-C3N4 support. Figure 4 d is a survey plot of the four elements C, N, O, and Mo before and after the reaction. Figure 4As can be seen from d, the elemental peaks of C, N, O, and Mo did not change significantly before and after the reaction, indicating that the 2D nanosheet Mo@C3N4 catalyst prepared in this embodiment is structurally stable and can be reused under high temperature, high pressure, and aqueous phase catalytic conditions. The method for recovering the Mo@C3N4 catalyst is to wash it with water and ethanol after the catalytic reaction, and then dry it.

[0057] Figure 5 The images show the FTIR spectra of the 2D nanosheet Mo@C3N4 catalyst prepared in this embodiment before and after catalysis. The FTIR spectra of the 2D nanosheet Mo@C3N4 catalyst before and after catalysis are approximately 750–1000, 1500–2000, and 3000–3400 cm⁻¹, respectively. -1 There are distinct peaks at all locations. These are at 1500–2000 cm⁻¹. -1 The stretching peaks corresponding to CN in carbonyl and amino groups are 3000–3400 cm⁻¹. -1 The absorption peaks between these peaks correspond to the vibrational peaks of CH and NH in carbonyl and amino groups, respectively, ranging from 750 to 1000 cm⁻¹. -1 Middle, 1000cm -1 The presence of an N-Mo vibrational peak at 810 cm⁻¹ confirms successful Mo loading on C₃N₄. -1 The peak at that point represents the out-of-plane bending of the heptaazine ring in carbon-based materials, and is a fundamental characteristic peak of carbon-based materials.

[0058] Figure 6 The catalytic efficiency of the 2D nanosheet Mo@C3N4 catalyst prepared in this embodiment is shown in the graphs under different temperatures and 30-minute conditions. Figure 6 It can be seen that when the reaction temperature is 100℃, the combined selectivity of mannose and fructose is 84.57%, and the total conversion rate of mannose and fructose produced by isomerization is 19.445%. As the temperature increases, the combined selectivity of mannose and fructose and the total value of mannose and fructose produced by isomerization reach their highest values ​​at 130℃. When the temperature continues to rise, glucose, mannose, and fructose all show a significant decrease because monosaccharides are easily degraded in high-temperature aqueous solutions. At 170℃, the total amount of glucose, mannose, and fructose is 8.79%, but some monosaccharides produce humic substances at high temperatures.

[0059] Figure 7 This is a graph showing the catalytic efficiency of the 2D nanosheet Mo@C3N4 catalyst prepared in this embodiment under different reaction times at 130°C. From... Figure 7As can be seen, the yield of mannose was highest at 5 min (31.47%), while the yield of fructose was 0.49%, with a comprehensive selectivity of 86%. At 130℃ and 25 min, the total yield of mannose and fructose reached its highest value (53.02%), with a fructose yield of 29.97% and a mannose yield of 23.05%, resulting in a comprehensive selectivity of 97.16%. This indicates that the 2D nanosheet Mo@C3N4 catalyst prepared in this embodiment first promotes the conversion of glucose to mannose. With increasing time, glucose and mannose are converted to fructose to varying degrees. After 30 min, the three sugars begin to decompose to varying degrees with further time. The main objective of this application is to obtain mannose, therefore the catalytic reaction time can be controlled to 5 min.

[0060] Figure 8 The prepared 2D nanosheet Mo@C3N4 catalyst was subjected to three repeated catalytic experiments at 130℃ for 5 min, as shown in the figure. Figure 8 As can be seen, after three recycling cycles, the catalytic selectivity of the Mo@C3N4 catalyst in this embodiment still reaches over 90%, indicating that the Mo@C3N4 catalyst in this embodiment can be reused after repeated washing and drying. The Mo@C3N4 catalyst in this embodiment is prepared by washing with water and ethanol and then drying.

[0061] Example 4

[0062] The preparation method of the Mo@C3N4 catalyst in this embodiment is as follows:

[0063] 1. Dissolve 1g of ammonium molybdate and 5g of urea in 50mL of deionized water, stir magnetically at 600r / min for 6h to obtain a mixed solution;

[0064] 2. Freeze the mixed solution at -70℃ for 6 hours, then freeze-dry it in a freeze dryer to obtain the catalyst precursor;

[0065] 3. The precursor was activated at high temperature in a muffle furnace at 600℃ for 4 hours, and then naturally cooled to room temperature to obtain the Mo@C3N4 catalyst. The heating rate during calcination was 6℃ / min.

[0066] The obtained 4 mL of 8 g / L glucose aqueous solution was placed into a 10 mL microreactor with a polytetrafluoroethylene liner. 0.02 g of Mo@C3N4 catalyst was placed into the liner. The microreactor was then placed in an oil bath at 130 °C and heated to catalyze the isomerization of glucose to obtain mannose and fructose.

[0067] Comparative Example 1

[0068] 1. Dissolve 0.75g aluminum chloride and 5g urea in 50mL of deionized water, stir magnetically at 600r / min for 6h to obtain a mixed solution;

[0069] 2. Freeze the mixed solution at -65℃ for 12 hours, then freeze-dry it in a freeze dryer to obtain the catalyst precursor;

[0070] 3. The precursor was activated at high temperature in a muffle furnace at 650℃ for 5 hours, and then naturally cooled to room temperature to obtain the Al@C3N4 catalyst. The heating rate during calcination was 10℃ / min. Figure 2 This is a scanning electron microscope (SEM) image of the Al@C3N4 catalyst.

[0071] The obtained 4 mL of a 12 g / L glucose aqueous solution was placed in a 10 mL microreactor with a polytetrafluoroethylene liner. 0.02 g of Al@C3N4 catalyst was added to the liner. The microreactor was then placed in an oil bath and heated to 110 °C to catalyze the isomerization of glucose to obtain mannose and fructose. The results of catalyzing the isomerization of glucose to obtain fructose and mannose using the catalysts prepared in this comparative example and Example 1 are shown in Table 2.

[0072] Table 2

[0073]

[0074] As shown in Table 2, at a temperature of 110℃ and a catalytic time of 0.5 h, the catalyst prepared in Example 1 exhibited a higher glucose conversion rate, with a fructose selectivity of approximately 67% and a mannose selectivity of approximately 29%. At a catalytic time of 1 h, the catalyst prepared in Example 1 achieved a glucose conversion rate of 74.11%, but the fructose and mannose selectivities decreased. This is because, with prolonged catalytic time, the isomerized fructose and mannose, due to their unstable structures, undergo dehydration, leading to a decrease in actual yield and selectivity. Since excessively long catalytic times cause dehydration of the isomerized fructose and mannose, the catalytic time is best controlled within 30 min.

Claims

1. An application of a Mo@C3N4 catalyst, characterized in that, The Mo@C3N4 catalyst is added to a glucose solution, and the solution is heated to catalyze the isomerization of glucose to produce mannose and fructose; the preparation method of the Mo@C3N4 catalyst includes the following steps: Ammonium molybdate and urea were dissolved in water to obtain a mixed solution; the mixed solution was frozen and then freeze-dried to obtain a catalyst precursor; the catalyst precursor was calcined at high temperature to obtain the final product.

2. The application of the Mo@C3N4 catalyst according to claim 1, characterized in that, The mass ratio of ammonium molybdate to urea is 0.5 to 1:5; the mass of ammonium molybdate added per mL of water is 0.01 to 0.02 g.

3. The application of the Mo@C3N4 catalyst according to claim 1, characterized in that, The calcination temperature is 450–600℃, the calcination heating rate is 4–6℃ / min, and the calcination time is 3–5h.

4. The application of the Mo@C3N4 catalyst according to claim 1, characterized in that, The freezing temperature is -80 to -70°C, and the freezing time is 4 to 6 hours.

5. The application of the Mo@C3N4 catalyst according to claim 1, characterized in that, The ammonium molybdate and urea are dissolved in water and stirred at a speed of 550-650 r / min for 4-7 h to obtain the mixed solution.

6. The application of the Mo@C3N4 catalyst according to claim 1, characterized in that, The glucose solution is in water as a solvent and has a concentration of 8–10 g / L; the catalyst is added in 0.004–0.005 g per mL of glucose solution.

7. The application of the Mo@C3N4 catalyst according to claim 1, characterized in that, The heating catalysis is carried out at 100–170°C.

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