Pt nano-catalyst as well as preparation method and application thereof

Abstract

The invention provides a Pt nano-catalyst which can be separated, recovered and recycled. The catalyst comprises a chiral ionic liquid as a stabilizer and a Pt precursor or a reaction product thereof, wherein the structure of the chiral ionic liquid is shown as a formula (1). The invention also provides a preparation method of the Pt nano-catalyst and an application of the Pt nano-catalyst in asymmetric hydrogenation of alpha-keto ester. In addition, the invention also provides a method for asymmetric hydrogenation of the alpha-keto ester. The Pt nano-catalyst provided by the invention is used in asymmetric hydrogenation of alpha-keto ester, shows relatively high enantioselectivity, can be recycled for at least 6 times, and has the advantage of long service life. In addition, the catalyst and the generated product are separated through extraction, and the recovered catalyst does not need to be treated and can be directly used for next reaction. # imgabs0 #

Description

Technical Field

[0001] The invention belongs to the technical field of catalysis, and in particular relates to a recyclable Pt nanocatalyst and a preparation method and application thereof. Background Art

[0002] In recent years, optically active substances, such as α-hydroxy esters and their derivatives, α-hydroxy carboxylic acids, etc., are important components of synthetic biologically active natural products or analogs, and are therefore widely used in the synthesis of fine chemicals such as pesticides and pharmaceuticals. Among the different synthetic methods, the asymmetric hydrogenation reaction of α-keto esters is one of the most effective synthetic methods and has gradually become a hot topic for scientific researchers.

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

[0004] In addition, in most of the reported literature, the chiral modifier is generally added directly to the catalytic system before the reaction, and the large π bond of the aromatic group is used to adsorb on the surface of the nanocatalyst. However, since this adsorption effect is relatively weak, and there is also competitive adsorption between the reaction substrate, product, etc. and the chiral modifier on the surface of the nanocatalyst, the chiral modifier is partially lost to the product and cannot be recycled. At present, the separation, recovery and recycling of chiral modifiers is still a major problem, and there are relatively few studies (Catal. Sci. Technol., 2015, 5, 680). Therefore, it is still a great challenge to study new (non-cinchona alkaloids) and separable and recyclable chiral catalysts.

[0005] 1,1'-Binol (BINOL) has a chiral optical configuration of the C2 symmetry axis and achiral carbon. The unique stereostructure of this type of molecule determines that the molecule has both rigidity and high stereoselectivity. A series of novel compounds derived from binaphthol as a skeleton have attracted much attention for their excellent chiral control performance and excellent application as chiral catalysts. Among them, phosphoric acid catalysts with chiral binaphthol as a skeleton have shown efficient catalytic activity in different reactions, such as Mannich reaction, Aza-Diels-Alder reaction, Friedel-Crafts reaction, etc. However, its application in asymmetric hydrogenation reactions has rarely been reported. Summary of the invention

[0006] Based on the above, the present invention introduces phosphoric acid with chiral binaphthol as the skeleton into the chiral ionic liquid structure, constructs a new chiral ionic liquid, and uses it as a stabilizer to prepare a Pt nanocatalyst. Finally, a recyclable Pt nanocatalyst is obtained, and it is used in the asymmetric hydrogenation of α-keto esters to obtain a higher enantiomeric selectivity, and the catalyst can be separated, recovered and recycled for multiple times.

[0007] To this end, in a first aspect, the present invention provides a Pt nanocatalyst, comprising a chiral ionic liquid as a stabilizer and a Pt precursor or a reaction product thereof, 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 further 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 in the Pt nanocatalyst to the chiral ionic liquid 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] To this end, 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 a Pt nanocatalyst.

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

[0013] As a specific embodiment of the present invention, preferably, the solvent is deionized water; and / or, in terms of mass parts, based on 1 part of the Pt precursor, the amount of the solvent is 200-400 parts, and the amount of the chiral ionic liquid is 30-100 parts, and 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° C., and time 3-5 h.

[0015] To this end, in a third aspect, the present invention also provides an application of the above-mentioned Pt nanocatalyst or the Pt nanocatalyst prepared by the above-mentioned preparation method in the asymmetric hydrogenation of α-ketoesters, especially in the asymmetric hydrogenation of methyl benzoylformate, ethyl benzoylformate, ethyl pyruvate and methyl pyruvate.

[0016] To this end, in a fourth aspect, the present invention also provides a method for asymmetric hydrogenation of an α-keto ester, comprising the following steps:

[0017] The above-mentioned nanocatalyst or the nanocatalyst prepared by the above-mentioned preparation method is added to an organic solvent, and then α-keto ester is added, and then hydrogen is introduced to carry out a hydrogenation reaction. Preferably, in terms of mass parts, the amount of the organic solvent is 20-60 parts, 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 following steps are also included: adding an extraction solvent to the reaction mixture obtained after the hydrogenation reaction is completed, separating the obtained hydrogenation product and recovering the nanocatalyst by extraction, and further preferably, the extraction solvent is selected from at least one of n-heptane, n-decane, n-hexane, and cyclohexane, and further preferably n-heptane or n-decane.

[0019] As a specific embodiment of the present invention, preferably, the α-ketoester is methyl benzoylformate, ethyl benzoylformate, 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 further preferably, the mass ratio of toluene to acetic acid is 5-10:1.

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

[0021] The Pt nanocatalyst provided by the present invention is used in the asymmetric hydrogenation of α-keto esters, and can obtain high enantiomeric selectivity. Moreover, the Pt nanocatalyst can be recycled at least 6 times, and has the advantage 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 processed and can be directly used for the next reaction. DETAILED DESCRIPTION

[0022] The present invention will be further described below in conjunction with specific embodiments, but they do not constitute any limitation to the present invention.

[0023] The present invention constructs a Pt nanocatalyst that can be used for asymmetric hydrogenation of α-keto esters to obtain high enantiomeric selectivity and can also be separated, recovered and recycled. Specifically, the present invention also provides a method for preparing the Pt nanocatalyst: 1.00 mg of PtCl4 is dissolved in 0.40 g of deionized water, 30-100 mg of chiral ionic liquid (1) is added, and after stirring evenly, 4 MPa of hydrogen is filled into an autoclave and reduced at 70-80° C. for 3-5 hours to obtain a Pt nanocatalyst having an average particle size of 2.0-3.0 nm.

[0024] Then, the prepared Pt nanocatalyst is used in the asymmetric hydrogenation reaction of α-ketoester (methyl benzoylformate is selected as the template substrate). The specific operation is: the prepared Pt nanocatalyst is dissolved in 2.000g toluene and 0.2-0.4g acetic acid and placed in an autoclave, and then methyl benzoylformate is added, and 2MPa hydrogen is charged, and the reaction is carried out at 30°C for 3-6h. After the reaction is completed, n-heptane or n-decane is added to extract the product, and at the same time, the separation and recovery of the Pt nanocatalyst is completed.

[0025] For the sake of clarity, the following examples are used to explain in detail:

[0026] Example 1

[0027] Synthesis of Chiral Ionic Liquids (1)

[0028] Step 1: Prepare the ionic liquid [CH3(OCH2CH2)] by referring to the synthesis steps in the literature "Appl.Organomet.Chem.2008,22,620-623". 16 NEt3] + [CH3SO3] -. Step 2: Weigh a certain amount of (S)-(+)-binaphthol phosphate and NaOH (molar ratio of 1:1) and dissolve them in deionized water. After the reaction is completed, add the ionic liquid prepared in step 1 at a molar ratio of (S)-(+)-binaphthol phosphate to the ionic liquid of 1:1, and stir at 50°C for 2 hours to obtain a mixed solution. Step 3: Transfer the mixed solution in step 2 to a separatory funnel and add dichloromethane for extraction. Remove the solvent from the obtained dichloromethane solution and dry it to obtain a 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 Pt Nanocatalysts Stabilized by Chiral Ionic Liquids (1)

[0031] 1.00 mg of PtCl4 was dissolved in 400 mg of deionized water, and 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 filled with 4 MPa of hydrogen gas to reduce the mixture at 70°C for 5 h. After the reaction, the water was removed to obtain a 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 Asymmetric Hydrogenation of α-Ketoesters

[0034] The Pt nanocatalyst prepared in Example 2 was used in the asymmetric hydrogenation reaction of α-keto esters, and methyl benzoylformate was selected as the template substrate. The specific operation was as follows: 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, and then methyl benzoylformate was added, and 2 MPa of hydrogen was charged, and the reaction was carried out at 30° C. for 6 hours. After the reaction was completed, n-heptane or n-decane was added to extract the product, and at the same time, the separation and recovery of the Pt nanocatalyst was completed.

[0035] Example 4

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

[0037] The chiral Pt nanocatalyst was prepared by the method shown in Example 2. According to the mass fraction, the amount of Pt precursor was 1 part, the amount of acetic acid was 400 parts, and the amount of toluene was 2000 parts. The amount of the chiral ionic liquid was 37 parts, 74 parts, and 100 parts, respectively. The average particle size of the prepared Pt nanocatalyst was 3.0 nm, 2.4 nm, and 2.0 nm, respectively. And it was used in the asymmetric hydrogenation reaction of methyl benzoylformate (MBF) with reference to Example 3. The reaction results are shown in Table 1.

[0038] Table 1

[0039] Chiral ionic liquid dosage (parts) MBF conversion rate (%) Enantiomeric excess (%) 37 ＞99 54 74 ＞99 61 100 ＞99 56

[0040] As can be seen from Table 1, when the amount of chiral ionic liquid is 37 parts, the enantiomeric excess of the product is 54%. As the amount of chiral ionic liquid increases, the enantiomeric excess of the product significantly increases to 61%. When the amount of chiral ionic liquid continues to increase, the enantiomeric excess of the product decreases.

[0041] Example 5

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

[0043] The chiral Pt nanocatalyst was prepared by referring to the method of Example 2. Then, the chiral Pt nanocatalyst was used in the asymmetric hydrogenation reaction of methyl benzoylformate by referring to the method of Example 3. The reaction temperature and hydrogen pressure in Example 3 were adjusted according to Table 2 to investigate the effects of the reaction temperature and hydrogen pressure on the reaction. The reaction results are shown in Table 2.

[0044] Table 2

[0045]

[0046]

[0047] As can be seen from Table 2, when the temperature is 25°C, the MBF conversion rate is 89% and the enantiomeric excess value is 62%. When the temperature is raised to 30°C, the substrate conversion rate is >99% and the product enantiomeric excess value is 61%, which is not much changed. When the temperature is further increased, the MBF conversion rate is still >99%, but the enantiomeric excess value decreases. The effect of hydrogen pressure was then investigated. When the hydrogen pressure was 1MPa, the MBF conversion rate was 92% and the enantiomeric excess value was 60%. When the hydrogen pressure was increased to 2MPa, the MBF conversion rate was >99% and the enantiomeric excess value was 61%. However, when the hydrogen pressure was further increased, the enantiomeric excess value decreased slightly.

[0048] Example 6

[0049] Investigation on the service life of Pt nanocatalyst

[0050] The Pt nanocatalyst was prepared by referring to the method of Example 2. Then, referring to the method of Example 3, the chiral Pt nanocatalyst was used in the MBF asymmetric hydrogenation reaction. After the reaction was completed, n-heptane or n-decane was added for extraction, and the system was separated into two phases. The upper organic phase was taken out and analyzed by gas chromatography. After the solvent in the organic phase was evaporated, the hydrogenation product was obtained, and the substance at the bottom was the catalyst, which was continuously recycled. The reaction results are shown in Table 3.

[0051] Table 3

[0052] Use times MBF conversion rate (%) Enantiomeric 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 product enantiomeric excess remain basically unchanged.

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

[0055] It should be noted that the embodiments described above are only used to explain the present invention and do not constitute any limitation to 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 words, rather than restrictive words. The present invention may be modified as specified within the scope of the claims of the present invention, and the present invention may be revised without departing from the scope and spirit of the present invention. Although the present invention described therein relates to specific methods, materials and embodiments, it does not mean that the present invention is limited to the specific examples disclosed therein, on the contrary, the present invention can be extended to all other methods and applications with the same functions.

Claims

1. A Pt nanocatalyst, characterized in that: It includes a chiral ionic liquid as a stabilizer and a Pt precursor or a reaction product thereof, wherein the structure of the chiral ionic liquid is shown in formula (1):

2. The Pt nanocatalyst according to claim 1, characterized in that The Pt precursor is a water-soluble platinum source. Preferably, the water-soluble platinum source is selected from at least one of PtCl4 and H2PtCl6.

3. The Pt nanocatalyst according to claim 1 or 2, characterized in that: In parts by mass, the mass ratio of the Pt precursor in the Pt nanocatalyst to the chiral ionic liquid is 1:30-100, preferably 1:37-100; and / or the average particle size of the Pt nanocatalyst is 2.0-3.0 nm.

4. A method for preparing a Pt nanocatalyst according to any one of claims 1 to 3, characterized in that: The method comprises the following steps: dissolving a Pt precursor in a solvent, adding a chiral ionic liquid, stirring evenly, reducing, and then removing the solvent to prepare a Pt nano catalyst.

5. The preparation method according to claim 4, characterized in that: The solvent is deionized water; and / or, based on mass parts, based on 1 part of the Pt precursor, the amount of the solvent is 200-400 parts, and the amount of the chiral ionic liquid is 30-100 parts, preferably 37-100 parts.

6. The preparation method according to claim 4 or 5, characterized in that: The reduction conditions are: hydrogen pressure 2-4 MPa, temperature 70-80° C., and time 3-5 h.

7. Use of the Pt nanocatalyst described in any one of claims 1 to 3 or the Pt nanocatalyst prepared by the preparation method described in any one of claims 4 to 6 in the asymmetric hydrogenation of α-ketoesters, especially in the asymmetric hydrogenation of methyl benzoylformate, ethyl benzoylformate, ethyl pyruvate and methyl pyruvate.

8. A method for asymmetric hydrogenation of α-keto esters, characterized in that: The following steps are involved: Add the Pt nanocatalyst according to any one of claims 1 to 3 or the Pt nanocatalyst prepared by the preparation method according to any one of claims 4 to 6 to an organic solvent, then add α-keto ester, and then fill with hydrogen to carry out a hydrogenation reaction. Preferably, in parts by mass, the amount of the organic solvent is 20-60 parts, the amount of the nanocatalyst is 1-3 parts, and the amount of the α-keto ester is 1.5-3 parts.

9. The method according to claim 8, characterized in that The following steps are also included: An extraction solvent is added to the reaction mixture obtained after the hydrogenation reaction, and the hydrogenation product is separated and the Pt nanocatalyst is recovered by extraction. Preferably, the extraction solvent is selected from at least one of n-heptane, n-decane, n-hexane, and cyclohexane, and more preferably n-heptane or n-decane.

10. The method according to claim 8 or 9, characterized in that: The α-keto acid ester is methyl benzoylformate, ethyl benzoylformate, ethyl pyruvate or methyl pyruvate; and / or, the organic solvent is selected from at least one of toluene, acetic acid and propionic acid, preferably toluene and acetic acid, more preferably the mass ratio of toluene to acetic acid is 5-10:1.

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