Pt nano catalyst, preparation method and application thereof

Abstract

The application provides a Pt nanometer catalyst which can be separated, recovered and recycled. The catalyst comprises a chiral ionic liquid and a Pt precursor or a reaction product thereof as a stabilizer, and the structure of the chiral ionic liquid is shown in formula (1). The application also provides a preparation method of the Pt nanometer catalyst and application of the Pt nanometer catalyst in asymmetric hydrogenation of alpha-keto ester. In addition, the application further provides a method for asymmetric hydrogenation of alpha-keto ester. The Pt nanometer catalyst provided by the application is used in asymmetric hydrogenation of alpha-keto ester, and has high enantiomeric selectivity. In addition, the catalyst can be recycled for at least 6 times, and has the advantages of long service life. In addition, the catalyst and the generated product are separated by extraction, and the recovered catalyst does not need to be treated and can be directly used for the next reaction.

Description

Technical Field

[0001] This invention belongs to the field of catalysis technology, and specifically relates to a recyclable Pt nanocatalyst, its preparation method, and its application. Background Technology

[0002] In recent years, optically active substances, such as α-hydroxy esters and their derivatives, and α-hydroxycarboxylic acids, have been widely used in the synthesis of fine chemicals such as pesticides and pharmaceuticals, as they are important components in the synthesis of bioactive natural products or analogues. Among different synthetic methods, the asymmetric hydrogenation reaction of α-keto esters, as one of the most effective synthetic methods, has gradually become a focus of research.

[0003] The asymmetric hydrogenation of α-keto esters catalyzed by cinchona alkaloid-modified nano-Pt has attracted widespread attention since its discovery by Orito et al. The most commonly used chiral modifiers of cinchona alkaloids are cinconidine and cinchonaine. In 1979, Orito et al. first used cinconidine as a chiral modifier with Pt / C as a catalyst to investigate the catalytic effect of asymmetric hydrogenation of methyl pyruvate in ethanol, achieving a maximum ee value of 78.7%. Cinchona alkaloids are currently the most widely used chiral modifiers in the asymmetric hydrogenation of α-keto esters, while other types of chiral modifiers have been studied less extensively.

[0004] Furthermore, in most reported literature, chiral modifiers are generally added directly to the catalytic system before the reaction, utilizing the large π bonds of their aromatic groups to adsorb onto the surface of the nanocatalyst. However, due to the relatively weak adsorption effect, and the competition for adsorption between the reaction substrate, products, and the chiral modifier on the nanocatalyst surface, some of the chiral modifier is lost into the product and cannot be recycled. Currently, the separation, recovery, and recycling of chiral modifiers remains a major challenge, and research on this is relatively limited (Catal. Sci. Technol., 2015, 5, 680). Therefore, researching novel (non-cinchona alkaloid) separable and recyclable chiral catalysts remains a significant challenge.

[0005] 1,1'-Bionaphthol (BINOL) possesses a chiral optical configuration with a C2 symmetry axis and achiral carbons. This unique stereostructure determines the molecule's combination of rigidity and high stereoselectivity. A series of novel compounds derived from the binaphthol backbone have attracted considerable attention due to their excellent chiral control and promising applications as chiral catalysts. Among these, phosphoric acid catalysts based on the chiral binaphthol backbone have exhibited highly efficient catalytic activity in various reactions, such as the Mannich reaction, the Aza-Diels-Alder reaction, and the Friedel-Crafts reaction. However, their application in asymmetric hydrogenation reactions has been rarely reported. Summary of the Invention

[0006] Based on the above, this invention introduces phosphoric acid, with chiral binaphthol as its backbone, into a chiral ionic liquid structure to construct a novel chiral ionic liquid, and uses it as a stabilizer to prepare Pt nanocatalysts. A recyclable Pt nanocatalyst is ultimately obtained, which is then used in the asymmetric hydrogenation of α-keto esters, achieving high enantioselectivity. Furthermore, the catalyst can be separated, recovered, and recycled multiple times.

[0007] Therefore, in a first aspect, the present invention provides a Pt nanocatalyst comprising a chiral ionic liquid as a stabilizer and a Pt precursor or its reaction product, wherein the structure of the chiral ionic liquid is shown in formula (1):

[0008]

[0009] As a specific embodiment of the present invention, preferably, the Pt precursor is a water-soluble platinum source, and more preferably, the water-soluble platinum source is selected from at least one of PtCl4 and H2PtCl6.

[0010] As a specific embodiment of the present invention, preferably, the mass ratio of the Pt precursor to the chiral ionic liquid in the Pt nanocatalyst is 1:30-100, more preferably 1:37-100; and / or, the average particle size of the Pt nanocatalyst is 2.0-3.0 nm.

[0011] Therefore, in a second aspect, the present invention also provides a method for preparing the above-mentioned Pt nanocatalyst, comprising the following steps: dissolving a Pt precursor in a solvent, adding a chiral ionic liquid, stirring evenly, reducing, and then removing the solvent to obtain the Pt nanocatalyst.

[0012] Specifically, since the chiral ionic liquid provided by the present invention as a stabilizer has a very good stabilizing effect, the catalyst will not agglomerate even when reduced in a solvent; and since the amount of chiral ionic liquid used in the present invention is small, it is more conducive to uniform stirring when reducing in the presence of a solvent.

[0013] As a specific embodiment of the present invention, preferably, the solvent is deionized water; and / or, based on the amount of the Pt precursor as 1 part by mass, the amount of the solvent is 200-400 parts, and the amount of the chiral ionic liquid is 30-100 parts, more preferably 37-100 parts.

[0014] As a specific embodiment of the present invention, preferably, the reduction conditions are: hydrogen pressure 2-4 MPa, temperature 70-80℃, and time 3-5 h.

[0015] Therefore, in a third aspect, the present invention also provides the application of the above-described Pt nanocatalyst or the Pt nanocatalyst prepared by the above-described preparation method in the asymmetric hydrogenation of α-keto esters, particularly in the asymmetric hydrogenation of methyl benzoylformate, ethyl benzoylformate, ethyl pyruvate and methyl pyruvate.

[0016] Therefore, in a fourth aspect, the present invention also provides a method for asymmetric hydrogenation of α-keto esters, comprising the following steps:

[0017] The above-mentioned nanocatalyst or the nanocatalyst prepared by the above-mentioned method is added to an organic solvent, followed by the addition of α-keto ester, and then hydrogen is introduced to carry out a hydrogenation reaction. Preferably, the amount of the organic solvent is 20-60 parts by mass, the amount of the nanocatalyst is 1-3 parts, and the amount of the α-keto ester is 1.5-3 parts.

[0018] As a specific embodiment of the present invention, preferably, the method further includes the following steps: adding an extraction solvent to the reaction mixture obtained after the hydrogenation reaction is completed, and separating the obtained hydrogenation product and the recovered nanocatalyst by extraction. More preferably, the extraction solvent is selected from at least one of n-heptane, n-decane, n-hexane, and cyclohexane, and even more preferably n-heptane or n-decane.

[0019] As a specific embodiment of the present invention, preferably, the α-keto ester is methyl benzoate, ethyl benzoate, ethyl pyruvate or methyl pyruvate; and / or, the organic solvent is selected from at least one of toluene, acetic acid, and propionic acid, more preferably toluene and acetic acid, and even more preferably the mass ratio of toluene and acetic acid is 5-10:1.

[0020] The beneficial effects of this invention are as follows:

[0021] The Pt nanocatalyst provided by this invention, used in the asymmetric hydrogenation of α-keto esters, achieves high enantioselectivity and can be recycled at least six times, exhibiting a long service life. Furthermore, the catalyst and the generated product are separated by extraction, and the recovered catalyst requires no further treatment and can be directly used in the next reaction. Detailed Implementation

[0022] The present invention will be further described below with reference to specific embodiments, but this does not constitute any limitation on the present invention.

[0023] This invention constructs a Pt nanocatalyst that can be used for the asymmetric hydrogenation reaction of α-keto esters to obtain high enantioselectivity, and can also be separated, recovered, and recycled. Specifically, this invention also provides a method for preparing the Pt nanocatalyst: 1.00 mg of PtCl4 is dissolved in 0.40 g of deionized water, and then 30-100 mg of chiral ionic liquid (1) is added. After stirring evenly, the mixture is then reduced in an autoclave at 70-80 °C for 3-5 h under 4 MPa of hydrogen gas to obtain the Pt nanocatalyst, which has an average particle size of 2.0-3.0 nm.

[0024] Then, the prepared Pt nanocatalyst was used in the asymmetric hydrogenation reaction of α-keto esters (methyl benzoylformate was selected as the template substrate). Specifically, the prepared Pt nanocatalyst was dissolved in 2.000 g of toluene and 0.2-0.4 g of acetic acid and placed in a high-pressure reactor. Methyl benzoylformate was then added, and hydrogen gas was introduced at 2 MPa. The reaction was carried out at 30 °C for 3-6 h. After the reaction was completed, n-heptane or n-decane was added for extraction to obtain the product, and the Pt nanocatalyst was simultaneously separated and recovered.

[0025] For clarity, the following examples will provide a detailed explanation:

[0026] Example 1

[0027] Synthesis of chiral ionic liquids (1)

[0028] Step 1: Following the synthesis steps in the literature "Appl. Organomet. Chem. 2008, 22, 620-623", prepare the ionic liquid [CH3(OCH2CH2)]. 16 NEt3] + [CH3SO3] -Step 2: Weigh a certain amount of (S)-(+)-binaphthol phosphoric acid and NaOH (molar ratio 1:1) and dissolve them in deionized water. After the reaction is complete, add the ionic liquid prepared in Step 1 at a molar ratio of (S)-(+)-binaphthol phosphoric acid to the ionic liquid of 1:1, and stir at 50℃ for 2 hours to obtain a mixed solution. Step 3: Transfer the mixed solution from Step 2 into a separatory funnel and add dichloromethane for extraction. After removing the solvent from the obtained dichloromethane solution, dry it to obtain the chiral ionic liquid (1). 1H NMR (400MHz, D2O): δ = 8.08 (d, J = 8.0Hz, 2H), 8.02 (d, J = 8.8Hz, 2H), 7.41-7.50 (m, 4H), 7.30-7.36 (m, 2H), 7.20 (d, J = 8.8Hz, 2H), 3.65 (m, 60H), 3.49 (q, J = 7.2Hz, 6H), 3.38 (s, 3H), 1.37 (t, J = 7.2Hz, 9H).

[0029] Example 2

[0030] Preparation of chiral ionic liquid (1) stabilized Pt nanocatalyst

[0031] 1.00 mg PtCl4 was dissolved in 400 mg deionized water, and then 70 mg of the chiral ionic liquid (1) prepared in Example 1 was added. After stirring evenly, the mixture was placed in a high-pressure reactor and charged with 4 MPa hydrogen gas at 70 °C for 5 h. After the reaction was completed, the water was removed to obtain Pt nanocatalyst. TEM test showed that the average particle size of the Pt nanocatalyst was 2.4 nm.

[0032] Example 3

[0033] Application of chiral Pt nanocatalysts in the asymmetric hydrogenation reaction of α-keto esters

[0034] The Pt nanocatalyst prepared in Example 2 was used in the asymmetric hydrogenation reaction of α-keto esters, with methyl benzoylformate selected as the template substrate. Specifically, 0.07 g of the prepared Pt nanocatalyst was dissolved in 2.000 g of toluene and 0.4 g of acetic acid and placed in an autoclave. Methyl benzoylformate was then added, and the mixture was purged with 2 MPa of hydrogen gas and reacted at 30 °C for 6 h. After the reaction was complete, n-heptane or n-decane was added for extraction to obtain the product, thus completing the separation and recovery of the Pt nanocatalyst.

[0035] Example 4

[0036] Effect of chiral ionic liquid dosage on hydrogenation effect

[0037] Chiral Pt nanocatalysts were prepared according to the method shown in Example 2. By weight, the amounts of Pt precursor were 1 part, acetic acid 400 parts, toluene 2000 parts, and the chiral ionic liquid 37 parts, 74 parts, and 100 parts, respectively. The average particle sizes of the prepared Pt nanocatalysts were 3.0 nm, 2.4 nm, and 2.0 nm, respectively. They were then used in the asymmetric hydrogenation reaction of methyl benzoylformate (MBF) according to Example 3. The reaction results are shown in Table 1.

[0038] Table 1

[0039] Dosage (parts) of chiral ionic liquid MBF conversion rate (%) Enantiomer excess (%) 37 ＞99 54 74 ＞99 61 100 ＞99 56

[0040] As shown in Table 1, when the amount of chiral ionic liquid was 37 parts, the enantiomeric excess of the product was 54%. With the increase of the amount of chiral ionic liquid, the enantiomeric excess of the product increased significantly to 61%. When the amount of chiral ionic liquid was further increased, the enantiomeric excess of the product decreased.

[0041] Example 5

[0042] Effects of reaction temperature and hydrogen pressure on the reaction

[0043] Chiral Pt nanocatalysts were prepared according to the method in Example 2. Then, following the method in Example 3, the chiral Pt nanocatalysts were used in the asymmetric hydrogenation reaction of methyl benzoylformate. The reaction temperature and hydrogen pressure in Example 3 were adjusted according to Table 2, and the effects of these factors on the reaction were investigated. The reaction results are shown in Table 2.

[0044] Table 2

[0045]

[0046]

[0047] Table 2 shows that at 25℃, the MBF conversion was 89% and the enantiomeric excess was 62%. When the temperature increased to 30℃, the substrate conversion was >99% and the product enantiomeric excess was 61%, showing little change. With further increases in temperature, the MBF conversion remained >99%, but the enantiomeric excess decreased slightly. The effect of hydrogen pressure was then investigated. At a hydrogen pressure of 1 MPa, the MBF conversion was 92% and the enantiomeric excess was 60%. When the hydrogen pressure increased to 2 MPa, the MBF conversion was >99% and the enantiomeric excess was 61%. However, with further increases in hydrogen pressure, the enantiomeric excess decreased slightly.

[0048] Example 6

[0049] Investigation of the lifespan of Pt nanocatalysts

[0050] Pt nanocatalysts were prepared according to the method in Example 2. Then, following the method in Example 3, the chiral Pt nanocatalysts were used in the asymmetric hydrogenation reaction of MBF. After the reaction, n-heptane or n-decane was added for extraction, and the system separated into two phases. The upper organic phase was removed and analyzed by gas chromatography. After removing the solvent from the organic phase, the hydrogenation product was obtained. The bottom layer was the catalyst, which was recycled. The reaction results are shown in Table 3.

[0051] Table 3

[0052] Number of times used MBF conversion rate (%) Enantiomer excess (%) 1 ＞99 61 2 ＞99 60 3 ＞99 60 4 ＞99 59 5 ＞99 60 6 ＞99 57

[0053] As shown in Table 3, the Pt nanocatalyst can be recycled at least 6 times, and the MBF conversion rate and the enantiomeric excess value of the product remain basically unchanged.

[0054] Any numerical value mentioned in this invention, if there is only a two-unit interval between any minimum and any maximum value, includes all values ​​that increase by one unit each time from the minimum to the maximum value. For example, if the amount of a component, or the value of a process variable such as temperature, pressure, or time, is stated as 50-90, in this specification it means specifically listing values ​​such as 51-89, 52-88… and 69-71 and 70-71, etc. For non-integer values, it may be appropriately considered that a unit is 0.1, 0.01, 0.001, or 0.0001. These are merely some specifically specified examples. In this application, in a similar manner, all possible combinations of numerical values ​​between the listed minimum and maximum values ​​are considered to have been disclosed.

[0055] It should be noted that the embodiments described above are only for explaining the present invention and do not constitute any limitation on the present invention. The present invention has been described with reference to typical embodiments, but it should be understood that the words used therein are descriptive and explanatory terms, not limiting terms. Modifications can be made to the present invention within the scope of the claims, and revisions can be made to the present invention without departing from the scope and spirit of the present invention. Although the present invention described herein relates to specific methods, materials, and embodiments, it does not mean that the present invention is limited to the specific examples disclosed herein; on the contrary, the present invention can be extended to all other methods and applications with the same function.

Claims

1. A Pt nanocatalyst, characterized in that, Including chiral ionic liquids as stabilizers and Pt precursors or their reaction products, the structure of the chiral ionic liquid is shown in formula (1): ； The preparation method of the Pt nanocatalyst includes the following steps: dissolving the Pt precursor in a solvent, adding a chiral ionic liquid, stirring evenly, reducing, and then removing the solvent to obtain the Pt nanocatalyst; the reduction conditions are: hydrogen pressure 2-4 MPa, temperature 70-80℃, and time 3-5 h. Based on mass fractions, the mass ratio of the Pt precursor (calculated as Pt) to the chiral ionic liquid in the Pt nanocatalyst is 1:30-100.

2. The Pt nanocatalyst according to claim 1, characterized in that, The Pt precursor is a water-soluble platinum source.

3. The Pt nanocatalyst according to claim 2, characterized in that, The water-soluble platinum source is selected from at least one of PtCl4 and H2PtCl6.

4. The Pt nanocatalyst according to any one of claims 1-3, characterized in that, The average particle size of the Pt nanocatalyst is 2.0-3.0 nm.

5. The Pt nanocatalyst according to any one of claims 1-3, characterized in that, Based on mass fractions, the mass ratio of the Pt precursor (calculated as Pt) to the chiral ionic liquid in the Pt nanocatalyst is 1:37-100.

6. The Pt nanocatalyst according to any one of claims 1-3, characterized in that, The solvent is deionized water; and / or, based on the amount of the Pt precursor as 1 part by mass, the amount of the solvent is 200-400 parts and the amount of the chiral ionic liquid is 30-100 parts.

7. The Pt nanocatalyst according to claim 6, characterized in that, Based on the mass fraction, with 1 part of the Pt precursor, the amount of the chiral ionic liquid is 37-100 parts.

8. The application of a Pt nanocatalyst according to any one of claims 1-7 in the asymmetric hydrogenation of α-keto esters.

9. The application according to claim 8, characterized in that, The application is in the asymmetric hydrogenation of methyl benzoate, ethyl benzoate, ethyl pyruvate and methyl pyruvate.

10. A method for asymmetric hydrogenation of α-keto esters, characterized in that, The process includes the following steps: adding the Pt nanocatalyst according to any one of claims 1-7 to an organic solvent, then adding α-keto ester, and then introducing hydrogen gas to carry out a hydrogenation reaction.

11. The method according to claim 10, characterized in that, The amount of the organic solvent used is 20-60 parts by weight, the amount of the nanocatalyst is 1-3 parts, and the amount of the α-keto ester is 1.5-3 parts.

12. The method according to claim 10 or 11, characterized in that, It also includes the following steps: An extraction solvent was added to the reaction mixture obtained after the hydrogenation reaction was completed, and the hydrogenation product and the recovered Pt nanocatalyst were separated by extraction.

13. The method according to claim 12, characterized in that, The extraction solvent is selected from at least one of n-heptane, n-decane, n-hexane, and cyclohexane.

14. The method according to claim 13, characterized in that, The extraction solvent is n-heptane or n-decane.

15. The method according to claim 10 or 11, characterized in that, The α-keto ester is methyl benzoate, ethyl benzoate, ethyl pyruvate, or methyl pyruvate; and / or, the organic solvent is selected from at least one of toluene, acetic acid, and propionic acid.

16. The method according to claim 15, characterized in that, The organic solvent is toluene and acetic acid.

17. The method according to claim 16, characterized in that, In the organic solvent, the mass ratio of toluene to acetic acid is 5-10:1.

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