A process for the preparation of trimethylolpropane

Abstract

The application discloses a preparation method of trimethylolpropane. The trimethylolpropane is prepared by condensation hydrogenation reaction of 2-ethylpropenal (EA) and formaldehyde aqueous solution in the presence of a hydrotalcite supported palladium-based catalyst and an alcohol solvent. The condensation hydrogenation method TMP process is used, the by-product EA and equivalent formaldehyde are used as starting materials, and the total yield of the condensation hydrogenation method TMP can be significantly improved. The Pd / calcium / aluminum-hydrotalcite solid catalyst has condensation and hydrogenation activity, and has the advantages of high selectivity, easy separation and applicability.

Description

Technical Field

[0001] This invention belongs to the field of chemical technology, specifically relating to a method for preparing trimethylolpropane. Background Technology

[0002] Trimethylolpropane (TMP), a versatile polyol chemical raw material, plays an important role in many industries. It is favored for its excellent thermal stability, acid and alkali resistance, and superior weather resistance. These properties make TMP a key raw material in the manufacture of resins, plastics, coatings, adhesives, and many other chemical products.

[0003] Despite the wide range of applications of TMP, its industrial production process still faces some challenges. Currently, the main industrial method for producing TMP is the disproportionation process. While this method is mature, it has some inherent drawbacks. During the chemical reaction, it generates a large amount of formate as a byproduct, which not only increases the difficulty of product separation but also reduces atom economy, thus affecting production efficiency and economic benefits.

[0004] To overcome these limitations, the condensation hydrogenation method for producing TMP has begun to attract attention from researchers and industry. As described in patents such as GB1535826A, US4594461, CN1123558, and CN101395112, the condensation hydrogenation method offers some potential solutions that can avoid the formation of formate and improve the efficiency and sustainability of the production process.

[0005] However, in the process of preparing TMP by condensation hydrogenation, the reaction mechanism is as follows, and the condensation stage of n-butyraldehyde and formaldehyde still faces the following technical challenges. In this critical step, the intermediate MMB may not only continue to condense to form DMB, but also inevitably generate the byproduct 2-ethylpropenal (EA) through a dehydration reaction. This side reaction significantly reduces the overall reaction yield, which is the main reason why the TMP preparation process by condensation hydrogenation has failed to achieve industrial-scale production.

[0006]

[0007] Patent CN102304022A reported that the byproduct EA can be used to prepare TMP by organic base disproportionation. However, this method requires 2-4 equivalents of formaldehyde as raw material, and because the disproportionation reaction of formaldehyde produces TMP equivalent formate salt, it also faces the problems of difficult byproduct separation and low atom economy.

[0008] Therefore, the key to improving the economic efficiency of the TMP production process via condensation hydrogenation lies in how to efficiently utilize EA while avoiding the formation of formate as a byproduct. Solving this problem can not only improve the utilization rate of raw materials but also reduce production costs, which is of great significance for optimizing and improving the efficiency of the entire process. Summary of the Invention

[0009] To overcome the shortcomings of existing TMP synthesis techniques via condensation hydrogenation, this invention provides a novel TMP synthesis method. The method uses 2-ethylpropenal (EA) and an aqueous formaldehyde solution as starting materials, and in the presence of a hydrotalcite-supported palladium-based catalyst, undergoes a one-step condensation hydrogenation to obtain TMP (reaction formula below). EA can be a byproduct of the condensation hydrogenation method for TMP.

[0010]

[0011] To achieve the above objectives, the present invention adopts the following technical solution:

[0012] This invention provides a method for preparing trimethylolpropane (TMP), wherein the method involves preparing the trimethylolpropane by condensation hydrogenation reaction of 2-ethylpropenal (EA) and an aqueous formaldehyde solution in the presence of a hydrotalcite-supported palladium catalyst and an alcohol solvent.

[0013] In one embodiment, the 2-ethyl acrolein can be a commercially available fresh product or a byproduct of the preparation of trimethylolpropane by condensation hydrogenation. The present invention does not have any particular requirements on its source; however, from the perspective of improving the utilization rate of raw materials and reducing production costs, the byproduct 2-ethyl acrolein of the preparation of trimethylolpropane by condensation hydrogenation is preferred.

[0014] The byproduct 2-ethylpropenal in the preparation of trimethylolpropane by the condensation hydrogenation method mainly refers to 2-ethylpropenal produced in the aldol condensation stage. The byproduct containing 2-ethylpropenal obtained from this process can be separated by conventional separation methods such as distillation to obtain 2-ethylpropenal byproduct with a purity of approximately 98 wt%.

[0015] In one embodiment, the hydrotalcite-supported palladium-based catalyst is a calcium aluminum hydrotalcite-supported Pd catalyst; preferably, based on the total mass of the hydrotalcite-supported palladium-based catalyst, the Pd loading is 0.1-0.7 wt%, for example, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, etc.

[0016] Optionally, the amount of the hydrotalcite-supported palladium-based catalyst is 1-5 wt% of the mass of 2-ethylpropenal, such as 15 wt%, 25 wt%, 35 wt%, 45 wt%, 55 wt%, etc., preferably 2-4 wt%.

[0017] In one embodiment, the alcohol solvent can be a conventionally selected type in the field, and there are no special requirements for its type and amount. For example, the alcohol solvent can be one or more of methanol, ethanol, and propanol.

[0018] Optionally, the amount of alcohol solvent used is 2-10 times the mass of 2-ethylpropenal, for example, 2 times, 4 times, 6 times, 8 times, 10 times, etc., preferably 2-4 times.

[0019] In one embodiment, the formaldehyde aqueous solution can be a conventional commercially available product, and its concentration is not particularly required, for example, a formaldehyde aqueous solution with a concentration of 37 wt%.

[0020] Optionally, the amount of formaldehyde aqueous solution used is calculated based on the formaldehyde therein, and the molar ratio of 2-ethylpropenal to formaldehyde is 1:1-1.2, such as 1:1, 1:1.05, 1:1.1, 1:1.15, 1:1.2, etc.

[0021] In one embodiment, the condensation hydrogenation reaction is carried out at a temperature of 80-120°C, such as 80°C, 90°C, 100°C, 110°C, 120°C, etc., for a reaction time of 0.5-2 hours, such as 0.5 hours, 0.7 hours, 1 hour, 1.2 hours, 1.5 hours, 1.8 hours, 2 hours, etc., and at a pressure of 2-4 MPa, such as 2 MPa, 2.5 MPa, 3 MPa, 3.5 MPa, 4 MPa, etc.

[0022] In one optional embodiment, 2-ethylpropenal and formaldehyde aqueous solution are fed continuously, in batches, or dropwise. Preferably, the feeding time is 0.5-2 hours, such as 0.5 hours, 0.7 hours, 1 hour, 1.2 hours, 1.5 hours, 1.8 hours, 2 hours, etc., and the feeding time is not included in the aforementioned reaction time. One specific example of the feeding method is as follows: 2-ethylpropenal and formaldehyde aqueous solution are mixed at room temperature and then added dropwise to a reactor containing a catalyst for 0.5-2 hours, followed by a reaction at a constant temperature for 0.5-2 hours.

[0023] In one alternative embodiment, the 2-ethyl acrolein raw material can be added in pure form, or it can be dissolved in the alcohol solvent to prepare a 20-30 wt% solution before use. Both are conventional addition methods in the field, and the present invention does not have specific requirements for them.

[0024] In one alternative embodiment, during the above-mentioned condensation hydrogenation reaction, the amount of hydrogen added meets the reaction pressure; that is, hydrogen is introduced into the reactor and pressurized to a preset reaction pressure.

[0025] For example, in a specific application, the preparation method of trimethylolpropane (TMP) according to the present invention can be carried out according to the following steps:

[0026] Methanol, ethanol, or propanol (or other alcoholic solvents), along with a palladium-based catalyst supported on hydrotalcite (such as calcium aluminum hydrotalcite-supported Pd catalyst), are added to a reactor. After purging with hydrogen three times, the temperature is raised to 80-120°C, and the pressure is increased to 2-4 MPa using hydrogen. EA is dissolved in an alcoholic solvent at room temperature to prepare a 20-30 wt% solution, which is then mixed with a 37 wt% formaldehyde aqueous solution. This solution is then added dropwise to the reactor containing the catalyst, with the temperature controlled at 80-120°C and the addition time at 0.5-2 h. After the addition is complete, the reaction is maintained at this temperature for 0.5-2 h to obtain TMP.

[0027] After the reaction described in this invention is completed, the water-supported Pd solid catalyst can be recovered from the reaction solution by conventional separation methods such as filtration, and the recovered catalyst can be directly reused.

[0028] The hydrotalcite-supported palladium catalyst of the present invention is a calcium aluminum hydrotalcite-supported Pd catalyst. Hydrotalcite-supported palladium catalysts have been involved in the prior art and can be prepared by existing processes through conventional reactions in the field to prepare the support and load the active component. The operation and process conditions of the present invention, as well as the apparatus used, can be carried out by the corresponding conventional selections in the field, and there are no particular limitations.

[0029] For example, in one embodiment, the method for preparing the calcium aluminum hydrotalcite-supported Pd catalyst includes the following steps:

[0030] 1) Calcium source, aluminum source and water are mixed, alkali solution is added to adjust the pH value, stirred for a certain time, then filtered, washed, dried, ground and calcined at high temperature to obtain calcium aluminum hydrotalcite carrier.

[0031] 2) The palladium source solution was contacted with the calcium aluminum hydrotalcite support by the equal volume impregnation method, and then allowed to stand, dry and calcined at high temperature to obtain the calcium aluminum hydrotalcite supported Pd catalyst.

[0032] In one alternative embodiment, the calcium source in step 1) is a calcium-soluble compound, such as a calcium salt, preferably one or more of calcium nitrate and calcium chloride.

[0033] In one alternative embodiment, the aluminum source in step 1) is a soluble aluminum compound, such as an aluminum salt, preferably one or more of aluminum nitrate, aluminum sulfate, and aluminum chloride.

[0034] In one optional implementation, the amounts of calcium source and aluminum source used in step 1) are 3-4:1 in molar ratio of calcium to aluminum metal;

[0035] Mixing the calcium source and aluminum source with water to form a solution is a routine operation in the field, and there are no special requirements for the concentration. Preferably, the total concentration of the calcium source and aluminum source in the water is 0.3-0.6 mol / L.

[0036] In one alternative embodiment, step 1) involves adding alkali solution to adjust the pH value to 11-13, and the stirring time is 6-10 hours.

[0037] In one optional embodiment, the alkaline solution in step 1) is composed of one or more alkalis selected from LiOH, NaOH, KOH, etc.

[0038] Optionally, the alkaline solution is an aqueous solution of alkali. Adjusting the pH of the system with alkaline solution is a conventional operating method in the field, and its concentration and amount are not specifically limited.

[0039] In one alternative implementation, step 1) includes conventional operations in fields such as filtration, washing, drying, grinding, and high-temperature calcination. The present invention does not make specific requirements. For example, the high-temperature calcination temperature of the carrier is 460-600℃ and the time is 3-10h.

[0040] In one alternative embodiment, the palladium source in step 2) is a palladium-soluble compound, such as a palladium salt, preferably one or more of palladium nitrate and palladium sulfate;

[0041] Specifically, the palladium source solution is an aqueous solution of palladium source with a concentration of 0.02-0.1 mol / L.

[0042] In one alternative embodiment, the mass ratio of the palladium source to the calcium aluminum hydrotalcite carrier in step 2) is 0.005-0.025.

[0043] In one alternative implementation, step 2) involves loading the palladium source solution onto the calcium aluminum hydrotalcite carrier using an equal-volume impregnation method. Specifically, the palladium source solution is added dropwise to the calcium aluminum hydrotalcite carrier and then left to stand for 8-12 hours.

[0044] In one alternative implementation, step 2) includes conventional operations in the fields of drying and high-temperature calcination. The present invention does not make specific requirements. For example, the high-temperature calcination temperature of the catalyst is 450-550°C and the time is 3-10h.

[0045] For example, in a specific application, the preparation method of the catalyst of the present invention can be carried out according to the following steps:

[0046] This operation can be performed in air or under N2 protection. Specifically, a certain proportion of Ca(NO3)2·4H2O and Al(NO3)3·9H2O are added to deionized water. The pH value is adjusted to a certain value using an alkaline solution, preferably LiOH, NaOH, or KOH aqueous solution. After stirring at room temperature for a certain period of time, the mixture is filtered and washed, and then dried at 50-100℃ for 8-48 hours. The resulting solid is ground into powder, calcined at high temperature in air for a certain period of time, and then cooled to room temperature to obtain a support. A certain concentration of Pd(NO3)2 aqueous solution is added dropwise to the above support, stirred at room temperature until an equal volume impregnation state is achieved, and allowed to stand at room temperature for 8-24 hours. Then, it is dried in a vacuum drying oven at 50-100℃ for 8-48 hours. Subsequently, it is calcined at high temperature of 450-550℃ in air for 3-10 hours and then cooled to room temperature to obtain a Pd / calcium / aluminum-hydrotalcite solid catalyst.

[0047] Compared with the prior art, the positive effects of the present invention are as follows:

[0048] This invention prepares TMP via a one-step condensation-hydrogenation method, in which a water-slip supported Pd solid catalyst is used, which has both condensation and hydrogenation activities, and has the advantages of high selectivity, easy separation, and applicability.

[0049] The raw material EA used in the method of this invention can be a byproduct of the TMP condensation hydrogenation process, significantly improving the overall yield of TMP. Furthermore, the method provided by this invention can use near-equivalent amounts of formaldehyde, avoiding the formation of formate, a byproduct of the disproportionation reaction, and improving atom economy. Detailed Implementation

[0050] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention.

[0051] The main raw materials used in the embodiments of this invention are sourced as follows. Unless otherwise specified, other raw materials and reagents were obtained through commercially available channels:

[0052] 2-Ethyl acrolein (EA) virgin material, purchased from Aladdin Chemical Reagent Co., Ltd.

[0053] 2-Ethylpropenal (EA) byproduct, derived from the byproduct of the preparation of trimethylolpropane by condensation hydrogenation, is 2-ethylpropenal byproduct with a purity of approximately 98 wt% obtained by distillation.

[0054] Formaldehyde aqueous solution, purchased from Maclean's Reagent Co., Ltd., 37%;

[0055] Ca(NO3)2·4H2O, CaCl2, Al(NO3)3·9H2O, AlCl3, Al2(SO4)3·18H2O, Pd(NO3)2·2H2O, PdSO4, LiOH, NaOH, KOH, purchased from Sigma-Aldrich.

[0056] The main analytical methods used in the embodiments of this invention are as follows:

[0057] 1. Composition analysis of Pd solid catalyst supported on hydrotalcite was performed using inductively coupled plasma atomic emission spectrometry (ICP) with a Thermo iCAP 7000 spectroscopy system.

[0058] 2. The reaction products were identified using a high-resolution mass spectrometer, specifically the Thermo Q Exactive Focus.

[0059] 3. The selectivity of the EA reaction was determined by gas chromatography (Agilent 7820A) using an Rtx-5MS capillary column, FID detector, vaporization temperature 250℃, detector temperature 250℃, high-purity nitrogen as the carrier gas, and a flow rate of 30 ml / min. The temperature program was: 50℃ for 1 min; then increased to 250℃ at a rate of 10℃ / min and held for 10 min.

[0060] Example 1

[0061] (1) Preparation of Pd catalyst supported on calcium aluminum hydrotalcite

[0062] Add 0.04 mol of Ca(NO3)2·4H2O and 0.01 mol of Al(NO3)3·9H2O to 100 mL of deionized water to prepare an aqueous solution. Then, add 100 mL of LiOH aqueous solution (1.1 M) to adjust the pH value, and stir at room temperature for 6 hours at a pH of 11. Then filter, wash with water, and dry at 60 °C for 12 hours. Grind the obtained solid into powder, heat in air at 460 °C for 3 hours, and calcine to obtain a calcium-aluminum hydrotalcite carrier, which is then cooled to room temperature for later use.

[0063] 2 ml of a 0.02 mol / L Pd(NO3)2·2H2O aqueous solution (containing 0.009 g of Pd(NO3)2·2H2O) was added dropwise to 2 g of catalyst support. The mixture was stirred at room temperature for 10 min and allowed to stand for 8 hours. It was then dried in a vacuum drying oven at 60 °C for 12 hours. Subsequently, it was calcined at 450 °C for 3 hours in air and then cooled to room temperature to obtain a calcium-aluminum hydrotalcite-supported Pd catalyst with a Pd loading of 0.15 wt%.

[0064] (2) Synthesis of TMP

[0065] 5g of methanol and 0.2g of calcium aluminum layered double hydroxide (CALD) supported Pd catalyst were added to a reactor. After purging with hydrogen three times, the temperature was raised to 80℃, and the pressure was increased to 2MPa with hydrogen. At room temperature, 10g (0.12mol) of EA (virgin material) was dissolved in 40g of methanol to prepare a 20wt% solution, followed by the addition of 9.6g (0.12mol) of 37% formaldehyde aqueous solution, which was mixed thoroughly. This solution was then added dropwise to the reactor containing the CALD supported Pd catalyst. The temperature was controlled at 80℃ during the dropwise addition process, and the addition time was 0.5h. After the dropwise addition was completed, the reaction was maintained at this temperature for 0.5h.

[0066] After being restored to room temperature and pressure, samples were taken for analysis. The EA conversion rate was 96% and the selectivity was 95%.

[0067] Example 2

[0068] (1) Preparation of Pd catalyst supported on calcium aluminum hydrotalcite

[0069] Add 0.035 mol of anhydrous CaCl2 and 0.01 mol of AlCl3 to 100 mL of deionized water to prepare an aqueous solution. Then, add 100 mL of NaOH aqueous solution (1.1 M) to adjust the pH value, and stir at room temperature for 8 hours at a pH of 12. Then filter, wash with water, and dry at 80°C for 24 hours. Grind the obtained solid into powder, heat in air at 500°C for 8 hours, and calcine to obtain a calcium-aluminum hydrotalcite carrier, which is then cooled to room temperature for later use.

[0070] 2 ml of a 0.06 mol / L PdSO4 aqueous solution (containing 0.024 g of PdSO4) was added dropwise to 2 g of catalyst support, stirred at room temperature for 15 min, and allowed to stand for 10 h. Then, it was dried in a vacuum drying oven at 80 °C for 24 h. Subsequently, it was calcined at 500 °C for 8 h in air, and then cooled to room temperature to obtain a calcium-aluminum-hydrotalcite-supported Pd catalyst with a Pd loading of 0.4 wt%.

[0071] (2) Synthesis of TMP

[0072] 5g of ethanol and 0.3g of calcium aluminum layered double hydroxide (CALD) supported Pd catalyst were added to a reactor. After purging with hydrogen three times, the temperature was raised to 100℃ and the pressure was increased to 3MPa with hydrogen. At room temperature, 10g (0.12mol) of EA (byproduct) was dissolved in 30g of ethanol to prepare a 25wt% solution, followed by the addition of 10.6g (0.13mol) of 37% formaldehyde aqueous solution and mixing. This solution was then added dropwise to the reactor containing the CALD supported Pd catalyst. The temperature was controlled at 100℃ during the dropwise addition process, and the addition time was 1h. After the dropwise addition was completed, the reaction was maintained at this temperature for 1h.

[0073] After being restored to room temperature and pressure, samples were taken for analysis. The EA conversion rate was 97% and the selectivity was 92%.

[0074] Example 3

[0075] (1) Preparation of Pd catalyst supported on calcium aluminum hydrotalcite

[0076] Add 0.03 mol of Ca(NO3)2·4H2O and 0.005 mol of Al2(SO4)3·18H2O to 100 mL of deionized water to prepare an aqueous solution. Then, add 100 mL of KOH aqueous solution (1.1 M) to adjust the pH value, and stir at room temperature for 10 hours at a pH of 13. Then filter, wash with water, and dry at 100 °C for 48 hours. Grind the obtained solid into powder, heat in air at 560 °C for 10 hours, and calcine to obtain a calcium-aluminum hydrotalcite carrier, which is then cooled to room temperature for later use.

[0077] 2 ml of a 0.1 mol / L Pd(NO3)2·2H2O aqueous solution (containing 0.046 g of Pd(NO3)2·2H2O) was added dropwise to 2 g of catalyst support. The mixture was stirred at room temperature for 20 min and allowed to stand for 12 h. Subsequently, it was dried in a vacuum drying oven at 100 °C for 48 h. Following this, it was calcined at 550 °C for 10 h in air and then cooled to room temperature to obtain a calcium-aluminum hydrotalcite-supported Pd catalyst with a Pd loading of 0.7 wt%.

[0078] (2) Synthesis of TMP

[0079] 5g of propanol and 0.4g of calcium aluminum layered double hydroxide (CALD) supported Pd catalyst were added to the reactor. After purging with hydrogen three times, the temperature was raised to 120℃ and the pressure was increased to 4MPa with hydrogen. At room temperature, 10g (0.12mol) of EA (byproduct) was dissolved in 20g of propanol to prepare a 33wt% solution, followed by the addition of 11.6g (0.14mol) of 37% formaldehyde aqueous solution and mixing. This solution was then added dropwise to the reactor containing the CALD supported Pd catalyst. The temperature was controlled at 120℃ during the dropwise addition process, and the addition time was 2h. After the dropwise addition was completed, the reaction was maintained at this temperature for 2h.

[0080] After being restored to room temperature and pressure, samples were taken for analysis. The EA conversion rate was 98% and the selectivity was 90%.

[0081] Example 4 (Catalyst Recycling)

[0082] Application of calcium aluminum hydrotalcite-supported Pd catalyst:

[0083] The reaction solution in Example 1 was filtered under a nitrogen atmosphere, and the filter cake was the recovered calcium aluminum hydrotalcite supported Pd catalyst.

[0084] TMP was synthesized according to step (2) in Example 1, except that the catalyst was replaced with the above-mentioned recycled catalyst, while other operations and conditions remained unchanged. The sampling and analysis results are shown in Table 1.

[0085] The activity did not decrease significantly after 5 applications.

[0086] Table 1

[0087] Apply batch EA conversion rate Selective 1 95％ 96％ 2 94％ 97％ 3 96％ 94％ 4 94％ 96％ 5 95％ 97％

[0088] Comparative Example 1

[0089] The method in step (2) of Example 1 is followed, except that the catalyst is replaced with Pd / C (5% loading) (palladium-based hydrogenation catalyst), and other operations and conditions remain unchanged. Sampling and analysis are performed, and the products are 2-methylbutanol and methanol.

[0090] This indicates that catalysts with only hydrogenation effects cannot achieve the condensation hydrogenation objective of this invention, and no TMP is generated.

[0091] It is readily understood that the above embodiments are merely illustrative examples for clear explanation and do not imply that the invention is limited thereto. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations. However, obvious variations or modifications derived therefrom remain within the scope of protection of this invention.

Claims

1. A process for the preparation of trimethylolpropane, characterized in that, The method is to prepare the trimethylolpropane by condensation hydrogenation reaction of 2-ethylpropenal and aqueous formaldehyde solution in the presence of hydrotalcite supported palladium-based catalyst and alcohol solvent. The hydrotalcite supported palladium-based catalyst is calcium-aluminum hydrotalcite supported Pd catalyst.

2. The production method according to claim 1, characterized by, The 2-ethylpropenal is a by-product in the preparation of trimethylolpropane by condensation hydrogenation method.

3. The production method according to claim 1, characterized by, The loading amount of Pd is 0.1-0.7wt% based on the total mass of the hydrotalcite supported palladium-based catalyst; and / or The amount of the hydrotalcite supported palladium-based catalyst is 1-5wt% of the mass of 2-ethylpropenal.

4. The production method according to claim 3, characterized by, The amount of the hydrotalcite supported palladium-based catalyst is 2-4wt% of the mass of 2-ethylpropenal.

5. The preparation method according to claim 1, characterized in that, The alcohol solvent is one or more of methanol, ethanol and propanol; and / or The amount of the alcohol solvent is 2-10 times of the mass of 2-ethylpropenal.

6. The production method according to claim 5, wherein The amount of the alcohol solvent is 2-4 times of the mass of 2-ethylpropenal.

7. The preparation method according to claim 1, characterized in that, The amount of the aqueous formaldehyde solution is 2-10 times of the mass of 2-ethylpropenal.

8. The method of claim 1, wherein, The molar ratio of 2-ethylpropenal to formaldehyde is 1:1-1.2 based on the formaldehyde in the aqueous formaldehyde solution.

9. The preparation method according to claim 6, characterized in that, The condensation hydrogenation reaction is carried out at a temperature of 80-120℃, a reaction time of 0.5-2h and a pressure of 2-4Mpa.

10. The method of claim 9, wherein, 2-ethylpropenal and aqueous formaldehyde solution are added continuously, or in batches, or dropwise.

11. A method for preparing the Pd catalyst supported on the calcium-aluminum hydrotalcite of claim 1, characterized in that, The feeding time is 0.5-2h, which is not included in the reaction time. The method comprises the following steps: 1) mixing a calcium source, an aluminum source and water, adjusting the pH value by adding alkali solution, stirring for a certain time, then filtering, washing, drying, grinding and high-temperature calcination to obtain a calcium-aluminum hydrotalcite carrier; 12. The method of claim 11, wherein, 2) contacting a palladium source solution with the calcium-aluminum hydrotalcite carrier by equal-volume impregnation, standing, drying and high-temperature calcination to obtain the calcium-aluminum hydrotalcite supported Pd catalyst. In step 1), the calcium source is one or more of soluble compounds of calcium; and / or In step 1), the aluminum source is one or more of soluble compounds of aluminum; and / or In step 1), the molar ratio of the calcium source to the aluminum source is 3-4:1; and / or In step 1), the pH value is adjusted to 11-13 by adding alkali solution, and the stirring time is 6-10h; and / or In step 1), the alkali solution contains one or more of LiOH, NaOH and KOH; and / or 13. The method of claim 12, wherein, In step 1), the high-temperature calcination is carried out at a temperature of 460-600℃ for 3-10h.

14. The method of claim 12, wherein, The calcium source is one or more of calcium nitrate and calcium chloride.

15. The method of claim 11, wherein, The aluminum source is one or more of aluminum nitrate, aluminum sulfate and aluminum chloride. In step 2), the palladium source is one or more of soluble compounds of palladium; and / or In step 2), the palladium source solution is an aqueous solution of the palladium source with a concentration of 0.02-0.1mol / L; and / or In step 2), the mass ratio of the palladium source to the calcium-aluminum hydrotalcite carrier is 0.005-0.025; and / or In step 2), the equal-volume impregnation is used to load the palladium source solution onto the calcium-aluminum hydrotalcite carrier; and / or 16. The method of claim 15, wherein, In step 2), the high-temperature calcination is carried out at a temperature of 450-550℃ for 3-10h. The palladium source is one or more of palladium nitrate and palladium sulfate.

17. The method of claim 15, wherein, The equal volume impregnation method is to drop the palladium source solution onto the calcium-aluminum hydrotalcite carrier, and then stand for 8-12 hours. The equal volume impregnation method is to drop the palladium source solution onto the calcium-aluminum hydrotalcite carrier, and then stand for 8-12 hours.

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