Monatomic solid base catalyst based on nitrogen and phosphorus co-doped graphene as well as preparation and application of monatomic solid base catalyst

Abstract

The invention discloses a monatomic solid base catalyst based on nitrogen and phosphorus co-doped graphene as well as a preparation method and application thereof. The monatomic solid base catalyst is suitable for reaction of methanol and ethylene carbonate to generate dimethyl carbonate. According to the catalyst, phosphorus-doped graphene serves as a carrier, alkali metal or alkaline earth metal nitrate serves as a precursor, the precursor is introduced into the phosphorus-doped graphene through an impregnation method, nitrogen in the precursor nitrate is doped to the carrier in situ in the high-temperature activation process, and finally the nitrogen-phosphorus co-coordinated monatomic solid base catalyst is formed. The monatomic solid base catalyst has highly dispersed active sites, and the utilization rate and catalytic activity of the metal sites are remarkably improved. Besides, heteroatom nitrogen and phosphorus are introduced to generate defect sites on the surface of the carrier, dispersion of metal sites is facilitated, and the heteroatom nitrogen and phosphorus are anchored on the carrier through coordination with the metal sites, so that loss of active sites is reduced, and the service life of the catalyst is prolonged.

Description

Technical Field

[0001] The present invention relates to the field of industrial catalysis technology, and in particular to a high-activity and high-stability single-atom solid base catalyst based on nitrogen and phosphorus co-doped graphene, and its preparation and application; in particular, to a single-atom solid base catalyst for preparing dimethyl carbonate by ester exchange reaction, and its preparation method. Background Art

[0002] The development and application of various environmentally friendly processes in the chemical industry are increasing, especially the process of replacing homogeneous liquid alkali catalysts with solid alkali catalysts has attracted widespread attention. Compared with traditional liquid alkali catalysts, solid alkali catalysts have the advantages of easy separation, mild reaction conditions, and environmental friendliness. From the perspective of actual production, solid alkali catalysts are less corrosive to equipment, extend the service life of equipment, reduce maintenance and replacement costs, and have the potential to become a new generation of environmentally friendly catalysts. However, traditional solid alkali catalysts currently have problems such as easy aggregation of alkaline sites, resulting in poor activity, and easy loss of alkaline sites, resulting in poor cyclic stability, which greatly limits their wide application. Therefore, the development of solid alkali catalysts with high activity and high stability is still a problem to be solved.

[0003] The rapid development of highly dispersed nano and sub-nano catalysts, especially the introduction of the concept of single-atom catalysts in recent years, has provided an opportunity to design more stable and efficient catalysts with their uniform active sites, extremely high atom utilization and excellent catalytic activity. For single-atom catalysts, the active sites are uniformly and uniformly dispersed on the carrier in the form of single atoms, and the catalytic reaction is carried out with a single atom as the catalytic active center. Single-atom catalysts have the characteristics of uniform and single active centers of homogeneous catalysts and stable and easy-to-separate structures of heterogeneous catalysts, and are expected to become a bridge between heterogeneous catalysis and homogeneous catalysis. Dispersing the basic sites of solid bases to the single-atom level and applying them to transesterification reactions is expected to solve the problems of traditional solid base catalysts in the process of transesterification reactions, such as the easy aggregation of basic sites leading to reduced activity and the easy loss leading to poor cyclic stability. Summary of the invention

[0004] The purpose of the present invention is to provide a single-atom solid base catalyst based on nitrogen-phosphorus co-doped graphene in order to improve the deficiencies of the prior art. Another purpose of the present invention is to provide a method for preparing the above catalyst. Another purpose of the present invention is to provide the use of the above catalyst. First, phosphorus atoms are introduced into the graphene structure to replace part of the carbon atoms to obtain phosphorus-doped graphene. Secondly, an alkali metal or alkaline earth metal nitrate precursor is introduced into the phosphorus-doped graphene by an impregnation method. During the high-temperature activation process, the nitrogen in the nitrate is in situ doped into the graphene, and finally a nitrogen-phosphorus co-coordinated single-atom solid base catalyst is obtained, which can catalyze the reaction of ethylene carbonate and methanol to produce dimethyl carbonate under mild conditions.

[0005] The technical scheme of the present invention is as follows: in view of the problem that the basic sites of the current conventional supported solid base catalysts are easy to aggregate and lose, resulting in poor activity and stability, the present invention designs a single-atom solid base catalyst with nitrogen and phosphorus co-doped graphene as a carrier and alkali metal or alkaline earth metal atoms as active sites. The introduction of heteroatoms nitrogen and phosphorus can significantly change the electronic structure and electron density of the single-atom solid base catalyst. Nitrogen and phosphorus atoms can change the electron density on the single-atom catalyst through electron supply, thereby affecting the electronic properties of the catalytic active center and further optimizing the catalyst's adsorption and activation ability for reactants. At the same time, the introduction of heteroatoms can form a strong interaction with the active site, stabilize the distribution of alkali metal / alkaline earth metal single atoms on the carrier, and prevent the aggregation of active sites and the loss during the reaction process.

[0006] The specific technical solution of the present invention is: a single-atom solid base catalyst based on nitrogen-phosphorus co-doped graphene, characterized in that it is composed of a nitrogen-phosphorus co-doped graphene carrier and an active component, and the active component is a single alkali metal or alkaline earth metal atom.

[0007] Preferably, the nitrogen-phosphorus co-doped graphene carrier is a graphene skeleton in which nitrogen and phosphorus atoms are doped to replace part of the carbon atoms, wherein the mass content of phosphorus is 0.2% to 1%, and the mass content of nitrogen is 1% to 3%.

[0008] Preferably, the active component is a single potassium, sodium or calcium atom; the mass content of the active component is 0.5% to 2.5%.

[0009] The present invention also provides a method for preparing the above-mentioned single-atom solid base catalyst, and the specific steps are as follows:

[0010] (1) weighing a phosphorus source precursor and dispersing it in an organic solvent to obtain a phosphorus-containing dispersion, adding graphene to the phosphorus-containing dispersion, stirring evenly and continuously stirring for a period of time, then evaporating to dryness in a water bath, and calcining the evaporated sample in an inert atmosphere to obtain a phosphorus-doped graphene carrier;

[0011] (2) The active component precursor is dissolved in water to obtain a solution, the phosphorus-doped graphene carrier is added to the solution, and after stirring for a period of time, the solution is evaporated in a water bath to dryness, and the evaporated sample is calcined at a high temperature in an inert atmosphere to obtain a single-atom solid base catalyst.

[0012] Preferably, the phosphorus source precursor described in step (1) is triphenylphosphine or phosphorus oxychloride, and the organic solvent is methanol or ethanol; the calcination temperature is 450-550° C., and the calcination time is 2-2.5 h; the mass ratio of the phosphorus source precursor to graphene is 1.2-3:1.

[0013] Preferably, the active component precursor in step (2) is potassium nitrate, sodium nitrate or calcium nitrate; the mass ratio of the active component precursor to phosphorus-doped graphene is 0.05-0.2:1; the stirring time is 12-24 hours; the calcination temperature is 850-950° C., and the calcination time is 2-2.5 hours. The inert gas is preferably argon or nitrogen.

[0014] The present invention also provides an application of the above-mentioned single-atom solid base catalyst in the ester exchange reaction to produce dimethyl carbonate. The specific steps are as follows: methanol, ethylene carbonate and the catalyst are placed in a reaction bottle, then heated to a specific temperature, and the product is obtained after a period of reaction.

[0015] The present invention catalyzes methanol and ethylene carbonate to undergo ester exchange reaction to generate dimethyl carbonate; preferably, the mass ratio of methanol to ethylene carbonate is 2.1-3.1:1; the mass ratio of the catalyst dosage to ethylene carbonate is 0.02-0.05:1; the reaction temperature is 65-75°C, and the reaction is carried out for 4-5 hours under normal pressure condensation reflux conditions.

[0016] Beneficial effects:

[0017] The single-atom solid base catalyst of the present invention is a new type of solid base catalyst with highly dispersed metal active sites and resistance to loss. Since the carrier graphene is modified by the introduction of heteroatom nitrogen and phosphorus, it has the following advantages:

[0018] (1) Highly dispersed active sites: The doping of heteroatoms such as nitrogen and phosphorus can significantly increase the density of active sites of the catalyst. The introduction of nitrogen and phosphorus atoms provides more anchoring sites for active components. Every atom in the single-atom catalyst can serve as an active site. The doped heteroatoms can further optimize the electronic structure of these sites, thereby improving the catalytic performance.

[0019] (2) Electronic structure regulation: Nitrogen and phosphorus atom doping can regulate the electronic structure of single-atom catalysts and improve their electron density. This regulation can enhance the catalyst's adsorption capacity and activation energy for reactants, thereby improving catalytic efficiency.

[0020] (3) Enhanced stability: Nitrogen and phosphorus atom doping can enhance the stability of single-atom catalysts, reduce the loss of active sites during recycling, and inhibit the aggregation of single atoms during the reaction. Heteroatoms can provide additional stabilization to maintain the dispersion of single-atom catalysts. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The transmission electron microscope and spherical aberration corrected high-angle annular dark field scanning electron microscope images of the sample in Example 1; (A) is the transmission electron microscope image, and (B) is the scanning electron microscope image. DETAILED DESCRIPTION

[0022] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with specific embodiments.

[0023] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0024] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive with other embodiments.

[0025] Embodiment 1:

[0026] (1) Weigh 0.25 g of triphenylphosphine into a beaker, add 40 mL of ethanol and stir evenly, then add 0.2 g of graphene and stir for 12 h, then evaporate in a water bath, and calcine the obtained sample at 450 °C in an argon atmosphere for 2 h. The obtained sample is phosphorus-doped graphene.

[0027] (2) Weigh 0.03g of calcium nitrate and dissolve it in 20mL of deionized water to fully dissolve it. Continue to add 0.55g of carrier phosphorus-doped graphene under stirring, continue stirring for 14h to fully mix it, then evaporate it in a water bath, and calcine the obtained sample at 850℃ for 2.5h under argon atmosphere. After naturally cooling down to room temperature, a single-atom calcium solid base catalyst (sample 1-1) is obtained, in which the mass content of phosphorus is 0.23%, the mass content of nitrogen is 1.05%, and the mass content of active components is 0.54%. Figure 1 (A) is a transmission electron microscopy image of the sample, in which no aggregation of nanoparticles is observed. Figure 1 (B) is a spherical aberration electron microscope image of the sample, in which the bright spots marked with red circles are single calcium atoms, which intuitively proves the successful synthesis of the single-atom catalyst. For the convenience of comparison and explanation, the above single-component metal precursors were added respectively and the comparison samples were prepared by the same preparation method: Sample 1-2 used graphene as a carrier and calcium nitrate as a precursor to obtain a single-atom sample coordinated by nitrogen, and Sample 1-3 used phosphorus-doped graphene as a carrier and calcium acetate as a precursor to obtain a single-atom sample coordinated by phosphorus.

[0028] (3) The obtained catalyst was tested for catalytic transesterification reaction. 4.4g of ethylene carbonate, 9.6g of anhydrous methanol and 0.1g of catalyst were added to a double-necked flask. The reaction was stirred in a water bath at 75℃ for 4h. The supernatant was filtered and analyzed for yield by gas chromatography. After the reaction, the catalyst was recovered and activated at high temperature and a cycle experiment was carried out. The results showed that the yield of dimethyl carbonate of sample 1-1 was 41.5%, while the yields of dimethyl carbonate of samples 1-2 and 1-3 were 34.2% and 33.3%, respectively, which indicated that the single-atom solid base with nitrogen and phosphorus co-doped graphene as the carrier had high catalytic activity. At the same time, after five cycles of use, the yield of sample 1-1 did not show a significant decrease, indicating that the introduction of heteroatom nitrogen and phosphorus anchored the calcium atom on the carrier well, giving it good cycle stability.

[0029] Embodiment 2:

[0030] (1) Weigh 0.4 g of phosphorus oxychloride into a beaker, add 40 mL of ethanol and stir evenly, then add 0.2 g of graphene and stir at room temperature for 18 h, then evaporate in a water bath, and calcine the obtained sample at 500 °C in an argon atmosphere for 2.2 h to obtain phosphorus-doped graphene.

[0031] (2) Weigh 0.03g of sodium nitrate and dissolve it in 20mL of deionized water to fully dissolve it, then add 0.2g of carrier phosphorus-doped graphene, continue stirring for 20h to fully mix it, then evaporate it in a water bath, and calcine the obtained sample at 900℃ in an argon atmosphere for 2h. When it naturally cools to room temperature, an atomic sodium solid base catalyst (sample 2-1) is obtained, in which the mass content of phosphorus is 0.56%, the mass content of nitrogen is 2.2%, and the mass content of active components is 1.42%. For the convenience of comparison, the above single component metal precursors are added respectively and the same preparation method is used to prepare comparative samples: Sample 2-2 is a single atomic sodium sample coordinated with nitrogen using graphene as the carrier and sodium nitrate as the precursor; Sample 2-3 is a single atomic sodium sample coordinated with phosphorus using phosphorus-doped graphene as the carrier and sodium acetate as the precursor.

[0032] (3) The obtained catalyst was tested for catalytic transesterification reaction. 4.4 g of ethylene carbonate, 12.6 g of anhydrous methanol and 0.2 g of catalyst were added to a double-necked flask. The reaction was stirred for 4.5 h in a 70°C water bath. The supernatant was filtered and the yield was analyzed by gas chromatography. After the reaction, the catalyst was recovered and activated at high temperature and a circulation experiment was carried out. The results showed that the yield of dimethyl carbonate of sample 2-1 was 49.3%, while the yields of dimethyl carbonate of sample 2-2 and sample 2-3 were 41.5% and 38.2%, respectively.

[0033] Embodiment 3:

[0034] (1) Weigh 0.6 g of triphenylphosphine into a beaker, add 40 mL of ethanol and stir evenly, then add 0.2 g of graphene and stir at room temperature for 20 h, then evaporate in a water bath, and calcine the obtained sample at 550 °C in a nitrogen atmosphere for 2.2 h to obtain phosphorus-doped graphene.

[0035] (2) Weigh 0.03g potassium nitrate and dissolve it in 20mL deionized water to fully dissolve it. Continue to add 0.17g of carrier phosphorus-doped graphene while stirring, continue stirring for 20h to fully mix it, then evaporate it in a water bath, and finally keep the obtained sample at 950℃ in an argon atmosphere for 2h. When it naturally drops to room temperature, the atomic potassium solid base catalyst (sample 3-1) is obtained, in which the mass content of phosphorus is 0.98%, the mass content of nitrogen is 2.93%, and the mass content of active components is 2.3%. For the convenience of comparison, the above single component metal precursors are added respectively and the same preparation method is used to prepare comparative samples: Sample 3-2 is a single atomic potassium sample coordinated with nitrogen using graphene as the carrier and potassium nitrate as the precursor; Sample 3-3 is a single atomic potassium sample coordinated with phosphorus using phosphorus-doped graphene as the carrier and potassium acetate as the precursor.

[0036] (3) The obtained catalyst was tested for catalytic transesterification reaction. 4.4 g of ethylene carbonate, 13.6 g of anhydrous methanol and 0.15 g of catalyst were added to a double-necked flask. The reaction was stirred for 5 h in a water bath at 65 ° C. The supernatant was filtered and the yield was analyzed by gas chromatography. After the reaction was completed, the catalyst was recovered and activated at high temperature and a circulation experiment was carried out. The results showed that the yield of dimethyl carbonate of sample 3-1 was 52.4%, while the yields of dimethyl carbonate of sample 3-2 and sample 3-3 were 49.8% and 41.2%, respectively.

Claims

1. A single-atom solid base catalyst based on nitrogen and phosphorus co-doped graphene, characterized in that: It is composed of a nitrogen-phosphorus co-doped graphene carrier and an active component, wherein the active component is a single alkali metal or alkaline earth metal atom.

2. The single-atom solid base catalyst according to claim 1, characterized in that The mass content of phosphorus in the nitrogen-phosphorus co-doped graphene carrier is 0.2% to 1%, and the mass content of nitrogen is 1% to 3%.

3. The single-atom solid base catalyst according to claim 1, characterized in that The active component is a single potassium, sodium or calcium atom; the mass content of the active component is 0.5% to 2.5%.

4. A method for preparing the single-atom solid base catalyst as claimed in claim 1, wherein the specific steps are as follows: (1) weighing a phosphorus source precursor and dispersing it in an organic solvent to obtain a phosphorus-containing dispersion, adding graphene to the phosphorus-containing dispersion, stirring evenly and continuously stirring for a period of time, then evaporating to dryness in a water bath, and calcining the evaporated sample in an inert atmosphere to obtain a phosphorus-doped graphene carrier; (2) The active component precursor is dissolved in water to obtain a solution, the phosphorus-doped graphene carrier is added to the solution, and after stirring for a period of time, the solution is evaporated in a water bath to dryness, and the evaporated sample is calcined at a high temperature in an inert atmosphere to obtain a single-atom solid base catalyst.

5. The method according to claim 4, characterized in that The phosphorus source precursor described in step (1) is triphenylphosphine or phosphorus oxychloride, and the organic solvent is methanol or ethanol; the calcination temperature is 450-550° C., and the calcination time is 2-2.5 hours; and the mass ratio of the phosphorus source precursor to graphene is 1.2-3:

1.

6. The method according to claim 4, characterized in that The active component precursor described in step (2) is potassium nitrate, sodium nitrate or calcium nitrate; the mass ratio of the active component precursor to the phosphorus-doped graphene is 0.05-0.2:1; the stirring time is 12-24 hours; the calcination temperature is 850-950° C., and the calcination time is 2-2.5 hours.

7. Use of the single-atom solid base catalyst as claimed in claim 1 in producing dimethyl carbonate by ester exchange reaction.

8. The use according to claim 7, wherein the specific steps are as follows: placing methanol, ethylene carbonate and a catalyst in a reaction bottle, then heating to a specific temperature, and reacting for a period of time to obtain a product.

9. The use according to claim 8, characterized in that The mass ratio of methanol to ethylene carbonate is 2.1-3.1:1; the mass ratio of the catalyst dosage to ethylene carbonate is 0.02-0.05:1; the reaction temperature is 65-75°C, and the reaction is carried out for 4-5 hours under normal pressure condensation reflux conditions.

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