A method for synthesizing convallapyran

Abstract

The application discloses a synthesis method of lily pyran. The synthesis method uses 3-methyl-3-buten-1-ol (isopentenol) and isovaleraldehyde as raw materials, and performs a reaction in the presence of a catalyst to generate lily pyran. The catalyst is a supported catalyst, and comprises a carrier and a coating layer supported on the carrier. The coating layer comprises a Nafion polymer and an inorganic oxide, and the carrier is in a foamed state. Preferably, the carrier is selected from a combination of one or more of silicon carbide foam, aluminum foam, nickel foam and iron foam. A solution of the Nafion polymer, water and a precursor of the inorganic oxide are mixed to form a sol under stirring, and then the sol is coated on the carrier and dried to obtain the catalyst. The catalyst is used for the synthesis of lily pyran, and has the advantages of high raw material conversion rate, high product selectivity, less three-waste emissions and excellent catalyst stability.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of synthetic chemistry, and particularly relates to a synthesis method of lily pyran. BACKGROUND

[0002] Lily fragrance is one of the most popular fragrances in the world. Traditional lily aldehyde and new lily aldehyde are limited in use due to allergenicity. Lily pyran has a lily flower fragrance with a milk fragrance, and the flower fragrance is fresh, soft and natural, and well integrates and enhances the flower fragrance characteristics of rose and lily, and has a long-lasting fragrance, and is a most promising fragrance to replace lily aldehyde and new lily aldehyde, and the market volume is increasing.

[0003] Currently, in the industry, 3-methyl-3-buten-1-ol (isopentenol) and isovaleraldehyde are mainly used as raw materials, and an acid catalyst is used, and the method has the advantages of low raw material price, short synthesis route and low cost. For example, patent CN105175372A uses a solid super strong acid as a catalyst, and continuously synthesizes lily pyran, and the yield of lily pyran is 86% calculated based on isovaleraldehyde. Patent CN104529969A applied by BASF uses a macroporous acid resin AmberlystTM131 as a catalyst, and the yield is 79% calculated based on isopentenol.

[0004] The molecular structure of lily pyran is relatively large, and the reaction process is greatly affected by the pore size of the catalyst. After the reaction, lily pyran is not easy to diffuse out of the catalyst, and the tertiary carbon of lily pyran contains a hydroxyl group, which is easy to cause dehydroxylation to generate a double bond byproduct under the condition of using an acid catalyst, and the double bond byproduct is easy to polymerize on the acid catalyst. The combination of the above three factors leads to a decrease in product selectivity and catalyst deactivation.

[0005] Silicon carbide has a large pore structure, which is beneficial to the diffusion of macromolecular compounds. However, silicon carbide has a low specific surface area, and it is not easy to load an acid active component. In addition, the interaction between silicon carbide and the acid active component is weak, which leads to easy peeling of the active component and causes catalyst deactivation.

[0006] The molecular structure of Nafion is composed of polytetrafluoroethylene (PTFE) and a polymer containing fluorine ions. It is a copolymer of polytetrafluoroethylene and perfluoro-3,6-dioxepan-4-methyl-7-decene-sulfuric acid, and has a stable structure and is not easy to fall off. It is an excellent acid catalyst. However, pure Nafion has a very small specific surface area and is difficult to use as a catalyst. Therefore, Nafion is usually dissolved in isopropyl alcohol or ethanol to be used as a catalyst. At this time, it is a homogeneous catalyst, and the problem of catalyst recovery and reuse exists, and the reaction selectivity needs to be further improved. SUMMARY

[0007] In view of the defects and shortcomings of the prior art, the present application provides a synthesis method of lily pyran, which can realize high conversion rate of isopentanal and high reaction selectivity of target product lily pyran, and the catalyst has high stability and is easy to recover.

[0008] To achieve the above-mentioned purposes, the technical scheme adopted by the present application is as follows:

[0009] A synthesis method of lily pyran, which comprises the following steps: isopentenol (3-methyl-3-buten-1-ol) and isopentanal are used as raw materials, and a reaction is carried out in the presence of a catalyst to generate the lily pyran, wherein the catalyst is a supported catalyst, and comprises a carrier and a coating layer supported on the carrier, the coating layer comprises a Nafion polymer and an inorganic oxide, and the carrier is in a foam state.

[0010] In some embodiments, the specific surface area of the catalyst is 50-120 m 2 / g, and the average pore size is 10-30 nm. The catalyst of the present application has a large specific surface area and average pore size. When the specific surface area is too low, the average pore size is large, but the number of catalytic active sites is small, which reduces the conversion rate; when the specific surface area is too high, the coating layer destroys the three-dimensional pore structure of the foam, the average pore size is small, which affects diffusion, and thus the reaction selectivity and stability are reduced.

[0011] In some embodiments, the carrier is selected from a combination of one or more of silicon carbide foam, aluminum foam, nickel foam, and iron foam.

[0012] In some embodiments, the inorganic oxide is selected from a combination of one or more of silicon dioxide, aluminum oxide, titanium dioxide, and zinc oxide.

[0013] In some embodiments, the mass of the coating layer is 3%-50% of the mass of the catalyst.

[0014] In some embodiments, the catalyst is prepared by a preparation method comprising the following steps: 1) mixing a solution of the Nafion polymer, water, and a precursor of the inorganic oxide to form a sol under stirring, and then standing and aging; and 2) coating the sol after standing and aging on the carrier to obtain the catalyst after drying. The precursor of the inorganic oxide undergoes in-situ hydrolysis reaction in the Nafion system to form the inorganic oxide, and the sol contains the inorganic oxide. After drying, the Nafion is attached to the inorganic oxide, and the inorganic oxide provides a transitional carrier between the catalyst carrier and the Nafion.

[0015] In some embodiments, the solution of the Nafion polymer is an alcohol solution; preferably, the alcohol in the alcohol solution is selected from ethanol or isopropanol. The Nafion polymer is hydrophobic and is difficult to dissolve in water, while alcohol is miscible with water.

[0016] In some embodiments, the mass percentage concentration of the solution of the Nafion polymer is 10%-40%.

[0017] In some embodiments, the precursor of the inorganic oxide is selected from the group consisting of tetraethyl orthosilicate, tetrabutyl orthosilicate, tetraisopropyl orthosilicate, trimethylaluminum, triethylaluminum, triisobutylaluminum, diethylaluminum chloride, aluminum isopropoxide, aluminum n-butoxide, aluminum acetate, aluminum propionate, tetrabutyl titanate, isopropyl titanate, zinc acetylacetonate, or a combination of one or more thereof. These precursors can all undergo hydrolysis reaction.

[0018] In some embodiments, in the step 1), the mass ratio of the solution of the Nafion polymer to the precursor of the inorganic oxide is 0.05-10:1.

[0019] In some embodiments, the mass ratio of water to the precursor of the inorganic oxide is 1:0.05-20.

[0020] In some embodiments, the temperature of the standing aging is 20-60℃.

[0021] In some embodiments, the time of the standing aging is 0.5-24 hours.

[0022] In some embodiments, in the step 2), the coating and drying are repeated for multiple times; preferably, the coating and drying are repeated for 1-4 times. The coating and drying can also not be repeated, but only coated once. The more times of coating, the higher the content of the active component in the catalyst.

[0023] In some embodiments, the temperature of the drying is 80-140℃.

[0024] In some embodiments, the time of the drying is 3-48 hours.

[0025] In some embodiments, the reaction is carried out in a solvent selected from the group consisting of water, ethanol and water mixed solvent, isopropyl alcohol and water mixed solvent.

[0026] In some embodiments, the mass ratio of the solvent to the isopentenol is 1:10-300.

[0027] In some embodiments, the molar ratio of the isopentenol to the isovaleraldehyde is 0.5-5:1.

[0028] In some embodiments, the mass ratio of the catalyst to the isopentenol is 1:10-200.

[0029] In some embodiments, the temperature of the reaction is 40-100℃.

[0030] In some embodiments, the reaction time is 3-24 hours.

[0031] In some embodiments, the reaction is carried out under normal pressure.

[0032] In some embodiments, the synthesis method comprises the following steps: 1) adding the isopentenol, isovaleraldehyde, solvent and catalyst into a reaction kettle; 2) heating the reaction kettle to a reaction temperature so that the isopentenol and isovaleraldehyde react; 3) after the reaction is completed, separating and recovering the catalyst to obtain the lily pyran.

[0033] The application also provides the aforementioned catalyst. The catalyst is used for the synthesis of lily pyran, and has the advantages of high raw material conversion rate, high product selectivity, less three-waste discharge and excellent catalyst stability.

[0034] Compared with the prior art, the application has the following advantages:

[0035] The synthesis method of the application can achieve high conversion rate of isovaleraldehyde and high reaction selectivity of the target product lily pyran, and the catalyst has high stability and is easy to recover.

[0036] In the application, the supported catalyst used for the synthesis of lily pyran has the advantages of high catalytic efficiency, high product selectivity, many batches of reuse and high catalytic stability. DETAILED DESCRIPTION

[0037] In the prior art, the molecular structure of lily pyran is large, and the reaction process is greatly affected by the pore size of the catalyst. After the reaction, the lily pyran is not easy to diffuse out of the catalyst, and the tertiary carbon of lily pyran contains a hydroxyl group, which is easy to dehydroxylate to generate a double bond byproduct under the condition of using an acidic catalyst, and the double bond byproduct is easy to polymerize on the acidic catalyst. The combination of the above factors leads to a decrease in product selectivity and catalyst deactivation. Although the foam carrier such as silicon carbide has a large pore structure, the specific surface area is low, and it is not easy to load an acidic active component. In addition, the interaction between silicon carbide and the acidic active component is weak, which leads to easy peeling of the active component and causes catalyst deactivation. Although the Nafion polymer is an excellent acid catalyst, its specific surface area is very small, and it is difficult to directly use it in a solid state form as a catalyst. In addition, the homogeneous catalyst in the form of a solution is difficult to recover and reuse.

[0038] Based on this, the inventors of the present application found that by mixing a solution of Nafion polymer, water and a precursor of inorganic oxide to form a sol, the precursor of inorganic oxide is hydrolyzed in situ in the Nafion system to form inorganic oxide, so that the Nafion is attached to the formed inorganic oxide (compared with silicon carbide, the specific surface area is increased), the specific surface area of the Nafion is increased, the Nafion, the inorganic oxide and the carrier are increased, the loading capacity of the carrier such as silicon carbide for the Nafion is also obviously improved, and the Nafion is not easy to fall off. If the inorganic oxide is not added, the Nafion is directly coated on the carrier such as silicon carbide foam, the coating amount is small, and the catalyst stability is poor. The present application can improve the catalytic activity and stability of the catalyst by the interaction of the Nafion, the inorganic oxide and the carrier, and the coating layer containing the Nafion and the inorganic oxide on the carrier.

[0039] The Nafion polymer in the catalyst coating layer is a catalytically active component, and the inorganic oxide therein does not act as an active component.

[0040] The catalyst of the present application is a solid acid catalyst, which has suitable acidity, and has the following characteristics:

[0041] 1. The foam carrier with developed pores such as silicon carbide foam is used as the carrier, which is beneficial to the diffusion of the substrate and the product, improves the substrate conversion rate, the product selectivity and the catalyst stability;

[0042] 2. In the preparation process of the catalyst, the inorganic oxide is introduced into the Nafion polymer by in situ hydrolysis of the precursor, which increases the specific surface area of the catalyst and improves the number of active sites, so that the catalyst is used in the synthesis method of the present application, which is beneficial to improve the substrate conversion rate;

[0043] 3. The Nafion-inorganic oxide coating layer on the carrier has strong interaction with the foam carrier, which avoids the falling off of the active component Nafion, improves the catalyst stability, and makes the catalyst can be recycled and used for more than 40 times, and the catalytic activity remains basically unchanged.

[0044] The present application will be further described in conjunction with the following examples. However, the present application is not limited to the following examples. The implementation conditions used in the examples can be further adjusted according to different requirements of specific use, and the implementation conditions not specified are the conventional conditions in the industry. The technical features involved in each embodiment of the present application can be combined with each other as long as there is no conflict between them.

[0045] Preparation Example 1

[0046] The present preparation example provides a supported catalyst, which is prepared as follows:

[0047] 1) A 2L glass reactor was charged with Nafion solution 100 g (Nafion was commercially available, the mass percentage concentration of Nafion was 30%, and the solvent was isopropyl alcohol), water 50 g, and tetraethyl orthosilicate 1000 g. After stirring for 2 hours, a uniform sol was formed (in which ethyl silicate was hydrolyzed to silicon dioxide), and the mixture was aged at 40 °C for 24 hours;

[0048] 2) 300 g of foamed silicon carbide was weighed, and then the sol prepared in step 1) was uniformly coated on the foamed silicon carbide. The mixture was dried at 80 °C for 24 hours;

[0049] 3) Step 2) was repeated 3 times to obtain a supported catalyst, which was denoted as Nafion-SiO2 / SiC-foam.

[0050] By weighing method, it was found that the Nafion-SiO2 coating on the Nafion-SiO2 / SiC-foam catalyst accounted for 32% of the total mass of the catalyst. By BET specific surface area test method, it was found that the specific surface area of the Nafion-SiO2 / SiC-foam catalyst was 68 m 2 / g, and the average pore size was 17 nm.

[0051] In the present preparation example, adjusting the coating times in step 3) can obtain catalysts with different mass percentages of the above Nafion-SiO2 coating.

[0052] In the present preparation example, adjusting the organic oxide precursor in step 1) can obtain different Nafion coatings. For example, replacing tetraethyl orthosilicate with aluminum isopropoxide, tetrabutyl titanate, and zinc acetylacetonate can obtain Nafion-Al2O3 / SiC-foam, Nafion-TiO2 / SiC-foam, and Nafion-ZnO / SiC-foam, respectively.

[0053] Comparative Preparation Example 1

[0054] The present comparative preparation example provides a comparative supported catalyst, which is prepared as follows:

[0055] 1) A 2L glass reactor was charged with Nafion solution 100 g (Nafion was commercially available, the mass percentage concentration of Nafion was 30%, and the solvent was isopropyl alcohol), water 50 g, and tetraethyl orthosilicate 1000 g. After stirring for 2 hours, a uniform sol was formed (in which ethyl silicate was hydrolyzed to silicon dioxide), and the mixture was aged at 40 °C for 24 hours;

[0056] 2) 300 g of foamed silicon carbide was weighed, and then the sol prepared in step 1) was uniformly coated on the foamed silicon carbide. The mixture was dried at 80 °C for 24 hours;

[0057] 3) Repeat step 2) for 3 times, i.e. to obtain the supported catalyst, denoted as Nafion / SiC-foam.

[0058] The specific surface area of the Nafion / SiC-foam catalyst is 5.4 m 2 / g, and the average pore size is 38 nm.

[0059] Comparative Preparation Example 2

[0060] This comparative preparation example provides a comparative supported catalyst, and the preparation method is as follows:

[0061] 1) In a 2L glass kettle, 50g of water and 1000g of tetraethyl orthosilicate were added, stirred for 2 hours to form a uniform sol, and then aged at 40°C for 24 hours;

[0062] 2) 300g of foamed silicon carbide was weighed, and then the sol prepared in step 1) was uniformly coated on the foamed silicon carbide, and dried at 80°C for 24 hours;

[0063] 3) Repeat step 2) for 3 times, i.e. to obtain the supported catalyst, denoted as SiO2 / SiC-foam.

[0064] The specific surface area of the SiO2 / SiC-foam catalyst is 72 m 2 / g, and the average pore size is 15 nm.

[0065] Comparative Preparation Example 3

[0066] This comparative preparation example provides a comparative supported catalyst, and the preparation method is as follows:

[0067] 1) In a 2L glass kettle, 100g of Nafion solution (Nafion is commercially available, the mass percentage concentration of Nafion is 30%, and the solvent is isopropyl alcohol), 50g of water and 1000g of tetraethyl orthosilicate were added, stirred for 2 hours to form a uniform sol, and then aged at 40°C for 24 hours;

[0068] 2) After aging, the sample was dried at 80°C for 24 hours, i.e. to obtain the supported catalyst, denoted as Nafion-SiO2;

[0069] The specific surface area of the Nafion-SiO2 catalyst is 197 m 2 / g, and the average pore size is 4 nm.

[0070] Example 1

[0071] This example provides a synthesis method of lirinapyrane, which is specifically as follows:

[0072] 1) Add 3.5 kg of isoamylene alcohol, 3 kg of isoamyl aldehyde, 0.2 kg of water and 0.3 kg of each catalyst prepared in the foregoing in a 10 L reaction kettle at room temperature;

[0073] 2) After starting stirring, heat the reaction kettle to 60°C, and react for 12 hours under normal pressure;

[0074] 3) After the reaction is completed, cool to room temperature, then separate the catalyst by filtration, and the separated catalyst is used for the next batch of experiments.

[0075] Quantitative analysis of the reaction liquid is performed on an Agilent 7890 gas chromatograph, using a chromatographic column HP-INNOWax and an FID detector for detection.

[0076] Table 1 is the reaction results of the Nafion-SiO2 / SiC-foam prepared in Preparation Example 1, the Nafion / SiC-foam, SiO2 / SiC-foam, Nafion-SiO2 catalyst prepared in Comparative Preparation Examples 1, 2 and 3, and the commercial macroporous acidic resin Amberlyst™ 131 catalyst (used in patent CN104529969A), solid Nafion, and the isopropyl alcohol solution of Nafion (the mass percentage concentration of Nafion is 30%, the solvent is isopropyl alcohol, the reaction system is a homogeneous catalyst, the catalyst cannot be separated from the reaction system and cannot be recycled).

[0077] As shown in Table 1, the Nafion-SiO2 / SiC-foam catalyst of Preparation Example 1 has the best reaction performance, with a conversion rate of 98% for isopentanal and a selectivity of 97.6% for lily of the valley pyran. The activity and selectivity of the Nafion / SiC-foam catalyst of Comparative Preparation Example 1 are greatly reduced, with a conversion rate of 28.4% for isopentanal and a selectivity of 85.6% for lily of the valley pyran, and the byproduct of dehydration is obviously increased. The comparison between the two shows that the addition of the oxide effectively improves the conversion rate of isopentanal. The SiO2 / SiC-foam catalyst has no catalytic activity, which shows that Nafion is the reaction active site and the oxide has no catalytic activity and is not an active component. However, the conversion of the solid Nafion as a catalyst is only 2.3%, which is mainly because the specific surface area of the solid Nafion is too low and the catalytic sites are too few. The isopropyl alcohol solution of Nafion as a catalyst has a conversion rate of 99% for isopentanal, but the selectivity for lily of the valley pyran is 86%, which is because the reaction system is homogeneous when the isopropyl alcohol solution of Nafion is used as a catalyst, and the generated lily of the valley pyran will continue to undergo dehydration reaction. The comparison between the Nafion-SiO2 catalyst of Preparation Example 3 and the Nafion-SiO2 / SiC-foam catalyst of Preparation Example 1 shows that the conversion rate of isopentanal is 96.8% and the selectivity for lily of the valley pyran is 87.4%, which is obviously lower than that of the Nafion-SiO2 / SiC-foam catalyst of Preparation Example 1, which is mainly because the pore of the Nafion-SiO2 catalyst is smaller than that of the Nafion-SiO2 / SiC-foam catalyst, resulting in more dehydration side reactions and reducing the reaction selectivity.

[0078] Table 1 results of synthesis of lily of the valley pyran by different catalysts

[0079]

[0080]

[0081] Table 2 is the catalysts prepared by different coating times in Preparation Example 1 and their catalytic results. As shown in Table 2, the conversion rate of isopentanal increases with the increase of the coating times, and then slightly decreases, which is mainly because the number of active sites gradually increases with the increase of the coating times, so the conversion rate increases, but the conversion rate slightly decreases when the coating times are too much, which is mainly because the pore is too small. The selectivity for lily of the valley pyran gradually decreases with the increase of the coating times, which is mainly because the pore is too small when the coating times are too much.

[0082] Table 2 performance of catalysts with different coating times for synthesis of lily of the valley pyran

[0083] Coating times Isovaleraldehyde conversion (%) Lyratene selectivity (%) Average pore size (nm) 1 90.2 98.3 28 2 93.4 97.9 24 3 98.0 97.6 17 4 98.8 97.4 16 5 97.2 91.4 12

[0084] Table 3 is the catalysts prepared by different oxides in Preparation Example 1 and their catalytic results. As shown in Table 3, the catalysts prepared by different oxides all have excellent yield of lily of the valley pyran.

[0085] Table 3 Performance of different oxide catalysts for synthesizing lily pyran

[0086] Catalyst Isovaleraldehyde conversion (%) Lyratene selectivity (%) Nafion-SiO2 / SiC-foam 98.0 97.6 Nafion-Al203 / SiC-foam 97.4 96.7 Nafion-TiO2 / SiC-foam 94.5 98.1 Nafion-ZnO / SiC-foam 94.2 96.9

[0087] Table 4 is the reuse activity of the Nafion-SiO2 / SiC-foam catalyst prepared in Preparation Example 1. As shown in Table 4, the activity of the Nafion-SiO2 / SiC-foam catalyst remains basically unchanged after 40 batches of reuse. Table 5 is the reuse activity of the Nafion-SiO2 catalyst prepared in Comparative Preparation Example 3. As shown in Table 5, the conversion rate and selectivity of the Nafion-SiO2 catalyst decrease obviously during 14 batches of reuse. The comparison of the two shows that the developed pore structure of the Nafion-SiO2 / SiC-foam catalyst helps to increase the stability of the catalyst, and the catalyst can still maintain high reaction activity and selectivity after multiple reuse.

[0088] Table 4. Reuse activity of Nafion-SiO2 / SiC-foam catalyst

[0089]

[0090] Table 5. Reuse activity of Nafion-SiO2 catalyst

[0091] Coating times Isovaleraldehyde conversion (%) Lyratene selectivity (%) 1 96.8 87.4 2 96.4 87.2 3 96.4 86.9 4 95.1 86.9 5 95.3 86.4 6 93.3 85.2 7 92.2 85.4 8 91.9 83.2 9 91.1 83.0 10 88.4 81.2 11 84.2 80.8 12 78.6 80.4 13 72.4 78.6 14 60.6 77.1

[0092] It can be seen that the catalyst of the present application can achieve high conversion rate of raw material isovaleraldehyde and high reaction selectivity of target product lily pyran in the synthesis of lily pyran, and the catalyst has high stability, the catalytic activity remains basically unchanged after more than 40 batches of reuse, and the catalyst is a heterogeneous catalyst, which is easy to recover and use in the reaction system.

[0093] The above examples are only for illustrating the technical concept and characteristics of the present application, and the purpose is to enable those skilled in the art to understand the content of the present application and implement it, and cannot limit the protection scope of the present application. Any equivalent changes or modifications made according to the spirit and principle of the present application shall be covered within the protection scope of the present application.

[0094] The endpoints of the ranges and any values disclosed herein are not limited to the precise values stated. The endpoints of the ranges and any values are understood to be approximate values. The endpoints of the ranges of values and the values of individual points are not to be construed as limiting. It is intended to cover any and all variations of addition to the example values disclosed herein.

Claims

1. A method for synthesizing lily of the valley pyran, comprising reacting isopentenol and isovaleraldehyde as raw materials in the presence of a catalyst to produce the lily of the valley pyran, characterized in that: The reaction is carried out in a solvent. The catalyst is a supported catalyst and includes a support and a coating supported on the support. The coating includes a Nafion polymer and an inorganic oxide. The support is in a foamed state. The specific surface area of ​​the catalyst is 50-120 m 2 / g, and an average pore size of 10-30 nm; the catalyst is prepared by a preparation method comprising the following steps: 1) mixing the Nafion polymer solution, water and the inorganic oxide precursor to form a sol under stirring, and standing and aging; 2) coating the sol after standing and aging on the support, and obtaining the catalyst after drying.

2. The synthetic method of lily of the valley pyran according to claim 1, wherein: The carrier is selected from one or more combinations of silicon carbide foam, aluminum foam, nickel foam, and iron foam.

3. The synthetic method of lily of the valley pyran according to claim 1, wherein: The inorganic oxide is selected from one or more combinations of silicon dioxide, aluminum oxide, titanium dioxide and zinc oxide.

4. The method for synthesizing lily of the valley pyran according to claim 1, wherein: The mass of the coating layer is 3%-50% of the mass of the catalyst.

5. The method for synthesizing lily of the valley pyran according to claim 1, wherein: The solution of the Nafion polymer is an alcohol solution; and / or the mass percentage concentration of the Nafion polymer solution is 10%-40%; and / or the precursor of the inorganic oxide is selected from a combination of one or more of tetraethyl silicate, tetrabutyl silicate, tetraisopropyl silicate, trimethylaluminum, triethylaluminum, triisobutylaluminum, diethylaluminum chloride, aluminum isopropoxide, aluminum n-butoxide, aluminum acetate, aluminum propionate, tetrabutyl titanate, isopropyl titanate, and zinc acetylacetonate.

6. The method for synthesizing lily of the valley pyran according to claim 5, wherein: The alcohol in the alcohol solution is selected from ethanol or isopropanol.

7. The method for synthesizing lily of the valley pyran according to claim 1, wherein: In the step 1), the mass ratio of the Nafion polymer solution to the inorganic oxide precursor is 0.05-10:1; and / or the mass ratio of water to the inorganic oxide precursor is 1:0.05-20.

8. The method for synthesizing lily of the valley pyran according to claim 1, wherein: The temperature of the static aging is 20-60°C; and / or the time of the static aging is 0.5-24 hours; and / or, in the step 2), the coating and drying are repeated multiple times; and / or the temperature of the drying is 80-140°C, and / or the time of the drying is 3-48 hours.

9. The method for synthesizing lily of the valley pyran according to claim 1, wherein: The solvent is selected from one of water, a mixed solvent of ethanol and water, and a mixed solvent of isopropyl alcohol and water.

10. The method for synthesizing lily of the valley pyran according to claim 9, wherein: The mass ratio of the solvent to prenol is 1: 10-300.

11. The method for synthesizing lily of the valley pyran according to claim 1, wherein: The molar ratio of isopentenol to isovaleraldehyde is 0.5-5:1; and / or the mass ratio of the catalyst to isopentenol is 1:10-200.

12. The method for synthesizing lily of the valley pyran according to claim 1, wherein: The reaction temperature is 40-100° C., and / or the reaction time is 3-24 hours, and / or the reaction is carried out under normal pressure.

13. The method for synthesizing lily of the valley pyran according to claim 1, wherein: The synthesis method comprises the following steps: a) adding the isopentenol, isovaleraldehyde, a solvent and the catalyst into a reaction kettle; b) heating the reaction kettle to a reaction temperature to allow the isopentenol and isovaleraldehyde to react; and c) separating and recovering the catalyst after the reaction to obtain the lily of the valley pyran.

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