Preparation method of trimethylolpropane

Abstract

The invention discloses a preparation method of trimethylolpropane. According to the method, in the presence of a hydrotalcite-loaded palladium-based catalyst and an alcohol solvent, 2-ethyl acrolein (EA) and a formaldehyde aqueous solution are subjected to a condensation hydrogenation reaction to prepare the trimethylolpropane. The byproduct EA and equivalent formaldehyde in the condensation hydrogenation method TMP process are used as starting raw materials, and the total yield of the condensation hydrogenation method TMP can be remarkably increased. The Pd / calcium / aluminum-hydrotalcite solid catalyst has condensation and hydrogenation activity, and has the advantages of high selectivity, easy separation and mechanical application.

Description

Technical Field

[0001] The invention belongs to the technical field of chemical industry, and particularly relates to a method for preparing trimethylolpropane. Background Art

[0002] Trimethylolpropane (TMP), as 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 excellent weather resistance. These characteristics make TMP a key raw material for the manufacture of resins, plastics, coatings, adhesives and many other chemical products.

[0003] Although TMP is widely used, its industrial production process still faces some challenges. At present, the disproportionation method mainly used in industry to produce TMP is a mature process, but it has some inherent defects. During the chemical reaction process, this method will produce a large amount of formate as a by-product, which not only increases the difficulty of product separation, but also reduces the atom economy, thus affecting production efficiency and economic benefits.

[0004] In order to overcome these limitations, the condensation hydrogenation method for producing TMP has begun to attract the attention of researchers and the industry. As described in patents such as GB1535826A, US4594461, CN1123558, and CN101395112, the condensation hydrogenation method provides 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 process is as follows. The condensation stage of n-butyraldehyde and formaldehyde still has the following technical difficulties. In this key step, the intermediate MMB may not only continue to condense to form DMB, but also inevitably generate a by-product 2-ethylacrolein (EA) through a dehydration reaction. This side reaction significantly reduces the yield of the overall reaction, which is also the main reason why the condensation hydrogenation process for preparing TMP has not been successfully achieved in industrial production.

[0006]

[0007] Patent CN102304022A reports that the byproduct EA can be used to prepare TMP through an organic base disproportionation method, but this method requires the use of 2-4 equivalents of formaldehyde as a raw material, and because the disproportionation reaction of formaldehyde will produce TMP equivalent amine formate salt, it also faces the problems of difficult by-product separation and low atom economy.

[0008] Therefore, how to efficiently utilize EA while avoiding the generation of by-product formate has become the key to improving the economic benefits of the condensation hydrogenation process for producing TMP. Solving this problem can not only improve the utilization rate of raw materials, but also reduce production costs, which is of great significance for the optimization and efficiency improvement of the entire process. Summary of the invention

[0009] In order to overcome the shortcomings of the existing condensation hydrogenation method for preparing TMP synthesis technology, the present invention provides a new TMP synthesis method. The method uses 2-ethylacrolein (EA) and formaldehyde aqueous solution as starting materials, and in the presence of a hydrotalcite-supported palladium-based catalyst, one-step condensation hydrogenation to obtain TMP (reaction formula is as follows), wherein the EA raw material can be a by-product of TMP in the condensation hydrogenation method.

[0010]

[0011] To achieve the above object, the present invention adopts the following technical solutions:

[0012] The invention provides a method for preparing trimethylolpropane (TMP). The method comprises the following steps: in the presence of a hydrotalcite-supported palladium-based catalyst and an alcohol solvent, 2-ethylacrolein (EA) and a formaldehyde aqueous solution are subjected to a condensation hydrogenation reaction to prepare the trimethylolpropane.

[0013] In one embodiment, the 2-ethylacrolein can be a fresh product purchased from the market, or a byproduct of trimethylolpropane prepared by condensation hydrogenation. The present invention has no particular requirements for its source; however, from the perspective of improving the utilization rate of raw materials and reducing production costs, 2-ethylacrolein, a byproduct of trimethylolpropane prepared by condensation hydrogenation, is preferably used;

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

[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 loading amount of Pd is 0.1-0.7wt%, such as 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, etc.;

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

[0017] In one embodiment, the alcohol solvent can be selected conventionally in the field, and there is no special requirement 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 the alcohol solvent is 2-10 times the mass of 2-ethylacrolein, such as 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 commercial product, and there is no special requirement for its concentration, for example, a formaldehyde aqueous solution with a concentration of 37 wt %;

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

[0021] In one embodiment, the condensation hydrogenation reaction has a reaction temperature of 80-120°C, for example, 80°C, 90°C, 100°C, 110°C, 120°C, etc., a reaction time of 0.5-2h, for example, 0.5h, 0.7h, 1h, 1.2h, 1.5h, 1.8h, 2h, etc., and a pressure of 2-4Mpa, for example, 2Mpa, 2.5Mpa, 3Mpa, 3.5Mpa, 4Mpa, etc.

[0022] In an optional embodiment, 2-ethylacrolein and the formaldehyde aqueous solution are added continuously, in batches, or dropwise; preferably, the addition time is 0.5-2h, such as 0.5h, 0.7h, 1h, 1.2h, 1.5h, 1.8h, 2h, etc., and the addition time is not included in the aforementioned reaction time; a specific operation example of one of the addition methods is as follows: 2-ethylacrolein and the formaldehyde aqueous solution are mixed at room temperature and then added dropwise to a reactor containing a catalyst, the addition time is 0.5-2h, and the reaction is kept warm for 0.5-2h after the addition.

[0023] In an optional embodiment, the 2-ethylacrolein 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 of which are conventional adding methods in the field and the present invention has no specific requirements for this.

[0024] In an optional embodiment, during the above condensation hydrogenation reaction, the amount of hydrogen added satisfies the reaction pressure; that is, hydrogen is charged into the reactor and the hydrogen is pressurized to a preset reaction pressure.

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

[0026] Add methanol, ethanol or propanol and other alcohol solvents, hydrotalcite-supported palladium-based catalysts (such as calcium aluminum hydrotalcite-supported Pd catalysts) into a reactor, replace with hydrogen three times, heat to 80-120°C, and pressurize to 2-4Mpa with hydrogen. Dissolve EA in an alcohol solvent at room temperature to prepare a 20-30wt% solution, then add 37wt% formaldehyde aqueous solution and mix well. Then add it dropwise to the reactor containing the catalyst, control the temperature at 80-120°C during the addition process, and add for 0.5-2h. After the addition is completed, keep the temperature for 0.5-2h to obtain TMP.

[0027] After the reaction of the present invention is completed, the hydrophilic Pd-loaded solid catalyst can be recovered from the reaction solution by conventional separation methods such as filtration, and the recovered catalyst can be directly used.

[0028] The hydrotalcite-supported palladium-based catalyst of the present invention is a calcium-aluminum hydrotalcite-supported Pd catalyst. The hydrotalcite-supported palladium-based catalyst has been involved in the prior art and can be prepared by using the existing process, preparing the carrier through conventional reactions in the field and loading the active component. The operation and process conditions of the present invention, and the devices used therein, can all be carried out by the corresponding conventional selections in the field, and there is no particular limitation on this.

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

[0030] 1) mixing a calcium source and an aluminum source with water, adding an alkali solution to adjust the pH value, stirring for a certain period of time, and then filtering, washing, drying, grinding, and high-temperature calcining to obtain a calcium aluminum hydrotalcite carrier;

[0031] 2) The palladium source solution is brought into contact with the calcium aluminum hydrotalcite support by an equal volume impregnation method, and the solution is allowed to stand, dried, and calcined at high temperature to obtain the calcium aluminum hydrotalcite supported Pd catalyst.

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

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

[0034] In an optional embodiment, the calcium source and the aluminum source in step 1) are used in an amount of 3-4:1 in terms of the molar ratio of calcium to aluminum metal;

[0035] Mixing the calcium source, aluminum source and water into a solution is a common operation in the field, and there is no special requirement for the concentration. Preferably, the total concentration of the calcium source and aluminum source in water is 0.3-0.6 mol / L.

[0036] In an optional embodiment, in step 1), alkaline solution is added to adjust the pH value to 11-13, and the stirring time is 6-10 hours.

[0037] In an optional embodiment, the alkali solution in step 1) comprises one or more alkalis selected from LiOH, NaOH, KOH, etc.;

[0038] Optionally, the alkali solution is an aqueous solution of alkali, and using the alkali solution to adjust the pH of the system is a conventional operation method in the field, and its concentration and dosage are not specifically limited.

[0039] In an optional embodiment, step 1) includes conventional operations in the fields of filtration, washing, drying, grinding, and high-temperature calcination, and the present invention does not make specific requirements. For example, the high-temperature calcination temperature of the carrier is 460-600°C and the time is 3-10h.

[0040] In an optional embodiment, the palladium source in step 2) is a soluble compound of palladium 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 a palladium source with a concentration of 0.02-0.1 mol / L.

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

[0043] In an optional embodiment, step 2) uses an equal volume impregnation method to load the palladium source solution onto the calcium aluminum hydrotalcite support, specifically, the palladium source solution is dropped onto the calcium aluminum hydrotalcite support and then allowed to stand for 8-12 hours.

[0044] In an optional embodiment, step 2) includes conventional operations in the fields of drying and high-temperature calcination, and 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 3-10 h.

[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 carried out in an air environment or under N2 protection. Specifically, a certain proportion of Ca(NO3)2·4H2O and Al(NO3)3·9H2O are added to deionized water, and an alkali solution preferably LiOH, NaOH or KOH aqueous solution is used to adjust the pH value, and then the solution is filtered and washed after stirring at room temperature for a certain period of time, and then dried at 50-100°C for 8-48 hours. The obtained solid is ground into powder, calcined at high temperature for a certain period of time in an air environment, and then cooled to room temperature to obtain a carrier. A certain concentration of Pd(NO3)2 aqueous solution is added dropwise to the above-mentioned carrier, stirred at room temperature until an equal volume impregnation state is reached, and allowed to stand at room temperature for 8-24 hours, and then dried at 50-100°C in a vacuum drying oven for 8-48 hours. Subsequently, the solution is calcined at a high temperature of 450-550°C for 3-10 hours in an air environment and then cooled to room temperature to obtain a Pd / calcium / aluminum-hydrotalcite solid catalyst.

[0047] Compared with the prior art, the present invention has the following positive effects:

[0048] The invention prepares TMP by a one-step condensation hydrogenation method, wherein a hydrophilic 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 in the method of the present invention can use the byproduct of the condensation hydrogenation TMP process, which significantly improves the total yield of the condensation hydrogenation TMP. In addition, the method provided by the present invention can use nearly equivalent formaldehyde, which can avoid the formation of formate, a byproduct of the disproportionation reaction, and improve the atom economy. DETAILED DESCRIPTION

[0050] The technical solution of the present invention will be clearly and completely described below in conjunction with specific implementation methods. However, those skilled in the art will understand that the embodiments described below are only part of the embodiments of the present invention, rather than all of the 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 sources of the main raw materials used in the examples of the present invention are as follows. Other raw materials and reagents, unless otherwise specified, were purchased from commercial sources:

[0052] Fresh 2-ethylacrolein (EA) was purchased from Aladdin Chemical Reagent Co., Ltd.;

[0053] 2-Ethyl acrolein (EA) by-product, which is derived from the by-product of preparing trimethylolpropane by condensation hydrogenation method, and the 2-Ethyl acrolein by-product with a purity of about 98wt% is separated by distillation method;

[0054] Formaldehyde aqueous solution, purchased from MacLean 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 analysis methods used in the embodiments of the present invention are as follows:

[0057] 1. The composition of the hydrotalcite-supported Pd solid catalyst was analyzed by inductively coupled plasma emission spectroscopy (ICP), model: Thermo iCAP 7000;

[0058] 2. The reaction products were characterized by a high-resolution mass spectrometer, model Thermo Q Exactive Focus;

[0059] 3. The selectivity of EA reaction was determined by gas chromatography (Agilent 7820A), Rtx-5MS capillary column, FID detector, vaporization temperature 250°C, detector temperature 250°C, carrier gas was high-purity nitrogen, flow rate 30 ml / min. Heating program: 50°C, 1 min; 10°C / min to 250°C, hold for 10 min.

[0060] Example 1

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

[0062] To 100 mL of deionized water, add 0.04 mol of Ca(NO3)2·4H2O and 0.01 mol of Al(NO3)3·9H2O to prepare an aqueous solution. Then add LiOH aqueous solution (100 mL, 1.1 M) to adjust the pH value, and stir at room temperature for 6 hours at a pH value 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, calcine to obtain a calcium aluminum hydrotalcite carrier, and cool to room temperature for use.

[0063] 2 ml of 0.02 mol / L Pd(NO3)2·2H2O aqueous solution (including 0.009 g of Pd(NO3)2·2H2O) was added dropwise onto 2 g of the catalyst carrier, stirred at room temperature for 10 min, and allowed to stand for 8 hours. Thereafter, it was dried at 60°C in a vacuum drying oven for 12 hours. Subsequently, it was calcined at 450°C for 3 hours in an air environment, 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 methanol and 0.2g calcium aluminum hydrotalcite supported Pd catalyst were added to the reactor, replaced with hydrogen for 3 times, and then heated to 80°C, and the pressure was increased to 2Mpa with hydrogen. 10g (0.12mol) EA (new material) was dissolved in 40g methanol at room temperature to prepare a 20wt% solution, and then 9.6g (0.12mol formaldehyde) 37% formaldehyde aqueous solution was added and mixed. Then it was added dropwise to the reactor containing the calcium aluminum hydrotalcite supported Pd catalyst, and the temperature was controlled at 80°C during the addition process, and the addition time was 0.5h. After the addition was completed, the reaction was kept warm for 0.5h.

[0066] After returning to normal temperature and pressure, sampling and analysis were performed, and the EA conversion rate was 96% and the selectivity was 95%.

[0067] Example 2

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

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

[0070] 2 ml of 0.06 mol / L PdSO4 aqueous solution (including 0.024 g of PdSO4) was added dropwise onto 2 g of the catalyst support, stirred at room temperature for 15 min, and allowed to stand for 10 hours. Thereafter, it was dried at 80°C in a vacuum drying oven for 24 hours. Subsequently, it was calcined at 500°C for 8 hours in an air environment, 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 hydrotalcite-supported Pd catalyst were added to the reactor, replaced with hydrogen three times, and then heated to 100°C and pressurized to 3Mpa with hydrogen. 10g (0.12mol) of EA (byproduct) was dissolved in 30g of ethanol at room temperature to prepare a 25wt% solution, and then 10.6g (0.13mol of formaldehyde) of 37% formaldehyde aqueous solution was added and mixed. Then, it was added dropwise to the reactor containing the calcium aluminum hydrotalcite-supported Pd catalyst, and the temperature was controlled at 100°C during the addition process, and the addition time was 1h. After the addition was completed, the reaction was kept warm for 1h.

[0073] After returning to normal temperature and pressure, sampling and analysis were performed, and the EA conversion rate was 97% and the selectivity was 92%.

[0074] Example 3

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

[0076] To 100 mL of deionized water, add 0.03 mol of Ca(NO3)2·4H2O and 0.005 mol of Al2(SO4)3·18H2O to prepare an aqueous solution. Then add KOH aqueous solution (100 mL, 1.1 M) to adjust the pH value, and stir at room temperature for 10 hours at a pH value 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, calcine to obtain a calcium aluminum hydrotalcite carrier, and cool to room temperature for use.

[0077] 2 ml of 0.1 mol / L Pd(NO3)2·2H2O aqueous solution (including 0.046 g of Pd(NO3)2·2H2O) was added dropwise onto 2 g of the catalyst carrier, stirred at room temperature for 20 min, and allowed to stand for 12 hours. Thereafter, it was dried at 100°C in a vacuum drying oven for 48 hours. Subsequently, it was calcined at 550°C for 10 hours in an air environment, 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 propanol and 0.4g calcium aluminum hydrotalcite supported Pd catalyst were added to the reactor, replaced with hydrogen for 3 times, and then heated to 120°C, and the pressure was increased to 4Mpa with hydrogen. 10g (0.12mol) EA (byproduct) was dissolved in 20g propanol at room temperature to prepare a 33wt% solution, and then 11.6g (0.14mol formaldehyde) 37% formaldehyde aqueous solution was added and mixed. Then it was added dropwise to the reactor containing the calcium aluminum hydrotalcite supported Pd catalyst, and the temperature was controlled at 120°C during the addition process, and the addition time was 2h. After the addition was completed, the reaction was kept warm for 2h.

[0080] After returning to normal temperature and pressure, sampling and analysis were performed, and the EA conversion rate was 98% and the selectivity was 90%.

[0081] Example 4 (Cycling Catalyst)

[0082] Calcium aluminum hydrotalcite supported Pd catalyst application:

[0083] The reaction solution in Example 1 was filtered under a nitrogen atmosphere to obtain a filter cake, i.e., the recovered calcium aluminum hydrotalcite-loaded Pd catalyst.

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

[0085] After 5 applications, there was no significant decrease in activity.

[0086] Table 1

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

[0088] Comparative Example 1

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

[0090] This indicates that a catalyst with only hydrogenation function cannot achieve the purpose of condensation hydrogenation of the present invention, and no TMP is generated.

[0091] It is easy to understand that the above embodiments are only examples for clear explanation and do not mean that the present invention is limited thereto. For ordinary technicians in the relevant field, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from this are still within the protection scope of the present invention.

Claims

1. A method for preparing trimethylolpropane, characterized in that: The method comprises the following steps: in the presence of a hydrotalcite-supported palladium-based catalyst and an alcohol solvent, 2-ethylacrolein and a formaldehyde aqueous solution are subjected to a condensation hydrogenation reaction to prepare the trimethylolpropane.

2. The preparation method according to claim 1, characterized in that: The 2-ethylacrolein is a by-product in the preparation of trimethylolpropane by a condensation hydrogenation process.

3. The preparation method according to claim 1, characterized in that: The hydrotalcite-supported palladium-based catalyst is a calcium-aluminum hydrotalcite-supported Pd catalyst; and / or Based on the total mass of the hydrotalcite-supported palladium-based catalyst, the Pd loading is 0.1-0.7 wt %; and / or The amount of the hydrotalcite-supported palladium-based catalyst is 1-5 wt %, preferably 2-4 wt %, based on the mass of 2-ethylacrolein.

4. 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 alcohol solvent is used in an amount of 2-10 times, preferably 2-4 times, the mass of 2-ethylacrolein.

5. The preparation method according to claim 1, characterized in that: The dosage of the formaldehyde aqueous solution is calculated based on the formaldehyde therein, and the molar ratio of 2-ethylacrolein to formaldehyde is 1:1-1.

2.

6. The preparation method according to claim 1, characterized in that: The condensation hydrogenation reaction has a reaction temperature of 80-120° C., a reaction time of 0.5-2 h, and a pressure of 2-4 MPa.

7. The preparation method according to claim 1, characterized in that: 2-Ethyl acrolein and formaldehyde aqueous solution are added continuously, in batches, or dropwise; Preferably, the feeding time is 0.5-2h, and the feeding time is not included in the aforementioned reaction time.

8. A method for preparing the calcium aluminum hydrotalcite-supported Pd catalyst according to claim 3, characterized in that: The following steps are involved: 1) mixing a calcium source and an aluminum source with water, adding an alkali solution to adjust the pH value, stirring for a certain period of time, and then filtering, washing, drying, grinding, and high-temperature calcining to obtain a calcium aluminum hydrotalcite carrier; 2) The palladium source solution is brought into contact with the calcium aluminum hydrotalcite support by an equal volume impregnation method, and the solution is allowed to stand, dried, and calcined at a high temperature to obtain the calcium aluminum hydrotalcite supported Pd catalyst.

9. The preparation method according to claim 8, characterized in that: Step 1) The calcium source is a soluble compound of calcium, preferably one or more of calcium nitrate and calcium chloride; and / or Step 1) The aluminum source is a soluble compound of aluminum, preferably one or more of aluminum nitrate, aluminum sulfate, and aluminum chloride; and / or Step 1) The calcium source and the aluminum source are used in an amount of 3-4:1 in terms of the molar ratio of calcium to aluminum metal; and / or Step 1) adding alkali solution to adjust the pH value to 11-13, and the stirring time is 6-10h; and / or In step 1), the alkali solution comprises one or more alkalis selected from LiOH, NaOH and KOH; and / or Step 1) The high temperature calcination temperature is 460-600°C and the time is 3-10h.

10. The preparation method according to claim 8, characterized in that: Step 2) the palladium source is a soluble compound of palladium, preferably one or more of palladium nitrate and palladium sulfate; and / or Step 2) the palladium source solution is an aqueous solution of a palladium source with a concentration of 0.02-0.1 mol / L; and / or Step 2) the mass ratio of the palladium source to the calcium aluminum hydrotalcite carrier is 0.005-0.025; and / or Step 2) loading the palladium source solution onto the calcium aluminum hydrotalcite support by an equal volume impregnation method, specifically, adding the palladium source solution dropwise onto the calcium aluminum hydrotalcite support and leaving it to stand for 8-12 hours; and / or Step 2) The high temperature calcination temperature is 450-550°C and the time is 3-10h.

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