Acid catalyst and method for modifying acid catalyst and preparing methylal through aldol condensation

Abstract

The invention discloses an acid catalyst, modification and a method for preparing methylal through aldol condensation, and particularly relates to a method for preparing methylal through condensation of methanol and formaldehyde by modifying the acid catalyst with a metal oxide and then using the modified acid catalyst for preparing methylal through condensation of methanol and formaldehyde. Different types of acid catalysts are synthesized by adopting an impregnation loading-modification method and are used for the process of preparing methylal by acetalation. The method has the advantages of simplicity in synthesis, easiness in controlling the acid-base property of the catalyst, low reaction temperature, good activity, high selectivity and the like.

Description

Technical Field

[0001] The present invention discloses an acidic catalyst, its modification and a method for preparing dimethoxymethane by aldol condensation, specifically relating to an acidic catalyst modified with metal oxide and used for the condensation of methanol and formaldehyde to prepare dimethoxymethane. Background Art

[0002] Methanol is an important chemical raw material, which can be prepared from coal, natural gas, biomass, etc. through the syngas route. Due to the maturity of the syngas-to-methanol technology in recent years, the methanol production capacity has been seriously excessive, and the development of downstream products of methanol has received wide attention. Methanol can be used to produce organic chemical products such as olefins and alkanes, or can be oxidized to prepare oxygen-containing compounds such as aldehydes or esters. The oxidation path of methanol has good atom economy and low energy consumption.

[0003] Dimethoxymethane (DMM), commonly known as dimethoxymethane, is one of the important chemical materials. It has low toxicity, low boiling point, high oxygen content, and at the same time has hydrophilicity and lipophilicity, good solubility, small surface tension and low heat of vaporization. It is widely used in industries such as medicine, cosmetics, daily necessities, pesticides, automotive manufacturing supplies, cotton, rubber industry, detergents, paints, inks and diesel additives. Also, due to its excellent volatility and no damage to atmospheric ozone, it can be used as a refrigerant to replace Freon. Among the numerous processes for synthesizing dimethoxymethane (DMM), the process of reacting methanol and formaldehyde to produce DMM is the most mature and is also the main method for synthesizing DMM in industry at present. In this method, the reaction is carried out on an acidic catalyst, such as liquid acids (concentrated sulfuric acid, p-toluenesulfonic acid, ionic liquids), solid acids (strong acidic cation exchange resin, HZSM-5 molecular sieve, crystalline aluminosilicate), etc.

[0004] In recent years, researchers have conducted a lot of research on the catalytic condensation of methanol and formaldehyde to prepare dimethoxymethane using solid acid catalysts. For example, the invention patents EP 0327343, JP2000109443A, etc. disclosed by Asahi Kasei Corporation of Japan use solid acids such as cation exchange resin and crystalline aluminosilicate as catalysts and have been applied industrially; Jin Mingshan et al. (Journal of Fudan University (Natural Science), 2003, 42(3): 280-284.) prepared supported phosphotungstic heteropolyacid catalysts and Cs x H 3-x PW 12 O 40(X = 1, 1.5, 2, 2.5, 3) Cesium salt of phosphotungstic heteropolyacid catalyst; Acidic molecular sieve catalysts studied by Wang Shujuan et al. (Journal of Jilin University (Science Edition), 2002, 40(3): 320-323.); Compared with liquid acid catalysts and acidic ionic liquid catalysts, solid acids (strong acidic cation exchange resins, crystalline aluminosilicates, HZSM-5 molecular sieves, etc.) have the advantages of high activity, no corrosion, and easy separation, and are a type of green and environmentally friendly catalyst, which has attracted extensive attention in recent years. Summary of the Invention

[0005] The main technical problem to be solved by the present invention is to provide a method for modifying an acidic catalyst and using the modified catalyst for the synthesis of methylal by aldol condensation. By modifying and modifying the acidic catalyst with metal oxides, the acid strength and the number of acid centers of the catalyst are effectively changed, and by-products such as DMC are inhibited, thereby improving the selectivity of methylal.

[0006] An acidic catalyst and its modification and a method for using it in the synthesis of methylal by aldol condensation are specifically as follows:

[0007] Modification of the acidic catalyst for the condensation of methanol and formaldehyde to prepare methylal:

[0008] (1) Based on the dry acidic catalyst powder, dissolve the precursor of the modified metal oxide in a polar solvent, add the dried acidic catalyst powder to the metal oxide precursor solution, and stir at 25-80 °C for 1-12 h;

[0009] (2) Perform liquid-solid separation on the above mixed solution, wash the separated solid with the corresponding polar solvent 2-10 times, dry the solid after washing, and then calcine to obtain the modified acidic catalyst.

[0010] It is characterized in that the acidic catalyst includes one or more of HZSM-5, H-β zeolite molecular sieve, and HMCM-22 zeolite molecular sieve.

[0011] It is characterized in that the precursor of the modified metal oxide includes one or more of Fe(NO3)3·9H2O, Cu(NO3)2, CsNO3, Cs2CO3, Mg(NO3)2·6H2O, Zn(NO3)2·4H2O, KNO3, Ca(NO3)2, and K2CO3; in the modified acidic catalyst, the loading amount of the modified metal oxide is 10-60% (relative to the modified catalyst), preferably 35-55%.

[0012] It is characterized in that in the step (2), the drying temperature of the modified acidic catalyst is 100-120 °C, the time is 2-12 h, the calcination temperature is 400-700 °C (preferably 450-650 °C), and the time is 2-10 h (preferably 4-8 h).

[0013] It is characterized in that in the step (1), the polar solvent includes one or more of water, acetone, absolute ethanol, acetonitrile, isopropanol, and methanol; the mass ratio of the polar solution to the powder of the acidic catalyst before modification is 10:1-50:1, preferably 10:1-30:1.

[0014] The modified acidic catalyst is applied to the catalytic synthesis of methylal by the acetalization of methanol and formaldehyde.

[0015] It is characterized in that: using a fixed-bed reactor, the middle section of the bed is filled with the modified acidic catalyst (the formed catalyst is selected to be 20-60 mesh), and the upper and lower sections are filled with quartz sand; the raw material is a mixed solution of methanol and formaldehyde, and the molar ratio of methanol to formaldehyde is 2:1-10:1, preferably 2:1-4:1. The formaldehyde includes one or more of aqueous formaldehyde solution with a concentration of 10%-70%, trioxane, and paraformaldehyde; the reaction temperature is 40-150 °C (preferably 60-120 °C), and the reaction is carried out at atmospheric pressure. The WHSV during the reaction process = 0.5-10 h -1 (preferably 0.5-6 h -1 )

[0016] The reaction involved in the present invention can be represented by the following reaction equation:

[0017]

[0018] The advantages of the present invention are simple synthesis, easy control of the acidity and basicity of the catalyst, low reaction temperature, good activity, high selectivity, etc.

[0019] The beneficial technical effects of the present invention:

[0020] 1. After the catalyst is modified and decorated with metal, on the one hand, it can improve the acid strength of the catalyst. On the other hand, the loading of the weakly basic metal can have weakly basic sites that promote condensation without changing the acid strength of the catalyst, thereby improving the selectivity of the target product.

[0021] 2. The obtained catalyst is used in the process of synthesizing methylal by alcohol-aldehyde condensation, which is simple, easy to operate, and has no corrosion to equipment. It is a green synthesis route with good application prospects. Specific embodiments

[0022] The preferred embodiments of the present invention will be described in detail below, so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making a clearer and more definite definition of the protection scope of the present invention.

[0023] Using a fixed-bed reactor, the middle section of the bed is filled with the catalyst (the formed catalyst is selected with 20-100 mesh), and the upper and lower sections are filled with quartz sand respectively.

[0024] Example 1

[0025] Weigh 3.636 g of Fe(NO3)3·9H2O and 30 ml of absolute ethanol respectively. Add Fe(NO3)3·9H2O to the absolute ethanol and stir; add 1 g of dried acidic support HZSM-5 to the above solution, mix and stir at room temperature of 25 °C for 12 h; then carry out solid-liquid separation on the above solution, wash the separated solid with absolute ethanol twice, put it in an oven at 100 °C and dry for 5 h, and finally calcine in a muffle furnace at 500 °C for 2 h to obtain the modified acidic catalyst 54.5% Fe2O3 / HZSM-5.

[0026] Press the obtained sample into tablets with 20-60 mesh, and then load it into a stainless steel tube fixed-bed reactor (inner diameter of the fixed bed is 8 mm) for evaluation. Using 37% aqueous formaldehyde solution by mass as the formaldehyde source, n 甲醇 :n 甲醛 = 7:3 (molar ratio, the same below), reaction temperature 120 °C, WHSV = 3 h -1 Finally, through analysis, the formaldehyde conversion rate is 87.5%, and the selectivity of methylal is 95.4%.

[0027] Example 2

[0028] Weigh 2.416 g of Cu(NO3)2 and 30 ml of absolute ethanol respectively. Add Cu(NO3)2 to the absolute ethanol and stir; add 1 g of dried acidic support HZSM-5 to the above solution, mix and stir at 70 °C in a water bath for 6 h; then carry out solid-liquid separation on the above solution, wash the separated solid with deionized water six times, put it in an oven at 100 °C and dry for 12 h, and finally calcine in a muffle furnace at 400 °C for 4 h to obtain the modified acidic catalyst 50.6% CuO / HZSM-5.

[0029] Press the obtained sample into tablets with 20-60 mesh, and then load it into a stainless steel tube fixed-bed reactor (inner diameter of the fixed bed is 8 mm) for evaluation. Using paraformaldehyde as the formaldehyde source, n 甲醇 :n 甲醛 = 10:1, reaction temperature 80 °C, WHSV = 10 h -1Finally, through analysis, the formaldehyde conversion rate was 32.6%, and the selectivity of methylal was 97.3%.

[0030] Example 3

[0031] Weigh 0.154 g of CsNO3 and 10 ml of deionized water respectively. Add CsNO3 to the deionized water and stir; add 1 g of dried acidic support H-β zeolite to the above solution. After mixing, heat it to 40 °C in a water bath and stir for 4 h; then carry out solid-liquid separation on the above solution. The separated solid is washed twice with deionized water, and after washing, it is placed in an oven at 120 °C and dried for 6 h. Finally, it is calcined in a muffle furnace at 650 °C for 3 h to obtain the modified acidic catalyst 10% Cs2O / H-β zeolite.

[0032] Press the obtained sample into tablets with a mesh size of 20 - 60, and then load it into a stainless steel tube fixed-bed reactor (inner diameter of the fixed bed is 8 mm) for evaluation. Using trioxane as the formaldehyde source, n 甲醇 :n 甲醛 = 9:1, reaction temperature 40 °C, WHSV = 8 h -1 Finally, through analysis, the formaldehyde conversion rate was 35.4%, and the selectivity of methylal was 96.5%.

[0033] Example 4

[0034] Weigh 0.289 g of Cs2CO3 and 20 ml of methanol respectively. Add Cs2CO3 to the methanol and stir; add 1 g of dried acidic support HZSM-5 to the above solution. After mixing, stir at room temperature of 25 °C for 6 h; then carry out solid-liquid separation on the above solution. The separated solid is washed twice with methanol, and after washing, the solid is placed in an oven at 120 °C and dried for 2 h. Finally, it is calcined in a muffle furnace at 550 °C for 4 h to obtain the modified acidic catalyst 20% Cs2O / HZSM-5.

[0035] Press the obtained sample into tablets with a mesh size of 20 - 60, and then load it into a stainless steel tube fixed-bed reactor (inner diameter of the fixed bed is 8 mm) for evaluation. Using paraformaldehyde as the formaldehyde source, n 甲醇 :n 甲醛 = 6:1, reaction temperature 150 °C, WHSV = 5 h -1 Finally, through analysis, the formaldehyde conversion rate was 46.8%, and the selectivity of methylal was 94.1%.

[0036] Example 5

[0037] Weigh 4.242 g of Mg(NO3)2·6H2O and 50 ml of acetone separately. Add Mg(NO3)2·6H2O to acetone and stir. Add 1 g of dried acidic support HMCM-22 to the above solution, mix, and stir at room temperature of 25 °C for 8 h. Then, perform solid-liquid separation on the above solution. Wash the separated solid 5 times with deionized water. After washing, place the solid in an oven at 100 °C and dry for 2 h. Finally, calcine in a muffle furnace at 600 °C for 5 h to obtain the modified acidic catalyst 50% MgO / HMCM-22.

[0038] Press the obtained sample into tablets of 20-60 mesh, and then load it into a stainless steel tube fixed-bed reactor (inner diameter of the fixed bed is 8 mm) for evaluation. Using an aqueous formaldehyde solution with a mass concentration of 37% as the formaldehyde source, n 甲醇 :n 甲醛 = 2:1, reaction temperature is 100 °C, WHSV = 1 h -1 Finally, through analysis, the formaldehyde conversion rate is 92.9%, and the selectivity of methylal is 91.8%.

[0039] Example 6

[0040] Weigh 4.817 g of Zn(NO3)2·4H2O and 30 ml of acetonitrile separately. Add Zn(NO3)2·4H2O to acetonitrile and stir. Add 1 g of dried acidic support H-β zeolite to the above solution, mix, and reflux and stir at 80 °C in a water bath for 1 h. Then, perform solid-liquid separation on the above solution. Wash the separated solid 5 times with deionized water. After washing, place the solid in an oven at 100 °C and dry for 3 h. Finally, calcine in a muffle furnace at 450 °C for 8 h to obtain the modified acidic catalyst 60% ZnO / H-β zeolite.

[0041] Press the obtained sample into tablets of 20-60 mesh, and then load it into a stainless steel tube fixed-bed reactor (inner diameter of the fixed bed is 8 mm) for evaluation. Using trioxane as the formaldehyde source, n 甲醇 :n 甲醛 = 2.5:1, reaction temperature is 90 °C, WHSV = 3 h -1 Finally, through analysis, the formaldehyde conversion rate is 91%, and the selectivity of methylal is 92.8%.

[0042] Example 7

[0043] Weigh 0.716 g of KNO3 and 30 ml of isopropanol respectively. Add KNO3 to isopropanol and stir; add 1 g of dried acidic support HMCM-22 to the above solution, mix, heat it to 60 °C in a water bath and stir for 6 h; then carry out solid-liquid separation on the above solution, wash the separated solid 10 times with absolute ethanol, put the washed solid into an oven at 120 °C and dry for 8 h, and finally calcine it in a muffle furnace at 600 °C for 2 h to obtain the modified acidic catalyst 25% K2O / HMCM-22.

[0044] Press the obtained sample into tablets of 20-60 mesh, and then load it into a stainless steel tube fixed-bed reactor (inner diameter of the fixed bed is 8 mm) for evaluation. Using an aqueous formaldehyde solution with a mass concentration of 20% as the formaldehyde source, n 甲醇 :n 甲醛 = 10:1, reaction temperature 120 °C, WHSV = 10 h -1 Finally, through analysis, the formaldehyde conversion rate is 25.6%, and the selectivity of methylal is 97.1%.

[0045] Example 8

[0046] Weigh 0.516 g of Ca(NO3)2 and 50 ml of deionized water respectively. Add Ca(NO3)2 to deionized water and stir; add 1 g of dried acidic support H-β zeolite to the above solution, mix, heat it to 50 °C in a water bath and stir for 3 h; then carry out solid-liquid separation on the above solution, wash the separated solid 4 times with absolute ethanol, put the washed solid into an oven at 120 °C and dry for 12 h, and finally calcine it in a muffle furnace at 650 °C for 8 h to obtain the modified acidic catalyst 15% CaO / H-β zeolite.

[0047] Press the obtained sample into tablets of 20-60 mesh, and then load it into a stainless steel tube fixed-bed reactor (inner diameter of the fixed bed is 8 mm) for evaluation. Using paraformaldehyde as the formaldehyde source, n 甲醇 :n 甲醛 = 4:1, reaction temperature 100 °C, WHSV = 2 h -1 Finally, through analysis, the formaldehyde conversion rate is 89.1%, and the selectivity of methylal is 95.5%.

[0048] Example 9

[0049] Weigh 1.467 g of K2CO3 and 10 ml of absolute ethanol separately. Add K2CO3 to the absolute ethanol and stir. Add 1 g of dried acidic support HMCM-22 to the above solution, mix, and stir at room temperature of 25 °C for 8 h. Then perform solid-liquid separation on the above solution. Wash the separated solid with deionized water 5 times. After washing, place the solid in an oven at 100 °C and dry for 3 h. Finally, calcine in a muffle furnace at 400 °C for 8 h to obtain the modified acidic catalyst 50% K2O / HMCM-22.

[0050] Press the obtained sample into tablets with a particle size of 20 - 60 mesh, and then load it into a stainless steel fixed-bed reactor (inner diameter of the fixed bed is 8 mm) for evaluation. Using an aqueous formaldehyde solution with a mass concentration of 55% as the formaldehyde source, n 甲醇 :n 甲醛 = 8:1, reaction temperature is 90 °C, WHSV = 9 h -1 Finally, through analysis, the formaldehyde conversion rate is 30.2%, and the selectivity of methylal is 87.9%.

[0051] Example 10

[0052] Weigh 1.24 g of Cu(NO3)2 and 30 ml of acetone separately. Add Cu(NO3)2 to the acetone and stir. Add 1 g of dried acidic support H-β zeolite to the above solution, mix, and heat to 70 °C in a water bath and stir for 6 h. Then perform solid-liquid separation on the above solution. Wash the separated solid with deionized water 6 times. After washing, place the solid in an oven at 120 °C and dry for 6 h. Finally, calcine in a muffle furnace at 400 °C for 8 h to obtain the modified acidic catalyst 34.5% CuO / H-β zeolite.

[0053] Press the obtained sample into tablets with a particle size of 20 - 60 mesh, and then load it into a stainless steel fixed-bed reactor (inner diameter of the fixed bed is 8 mm) for evaluation. Using paraformaldehyde as the formaldehyde source, n 甲醇 :n 甲醛 = 10:1, reaction temperature is 80 °C, WHSV = 10 h -1 Finally, through analysis, the formaldehyde conversion rate is 43.4%, and the selectivity of methylal is 92.0%.

[0054] Example 11

[0055] Weigh 1.036 g of Mg(NO3)2·6H2O and 50 ml of absolute ethanol respectively. Add Mg(NO3)2·6H2O to the absolute ethanol and stir. Add 1 g of dried acidic support H-β zeolite to the above solution, mix and stir at room temperature of 25 °C for 8 h. Then carry out solid-liquid separation on the above solution. Wash the separated solid with deionized water 5 times. After washing the solid, put it into an oven at 100 °C and dry for 2 h. Finally, calcine it in a muffle furnace at 650 °C for 4 h to obtain the modified acidic catalyst 14% MgO / H-β zeolite.

[0056] Press the obtained sample into tablets of 20-60 mesh, and then load it into a stainless steel tube fixed-bed reactor (inner diameter of the fixed bed is 8 mm) for evaluation. Using 37% formaldehyde aqueous solution by mass concentration as the formaldehyde source, n 甲醇 :n 甲醛 =2:1, reaction temperature is 100 °C, WHSV = 1 h -1 . Finally, through analysis, the formaldehyde conversion rate is 56.3%, and the selectivity of methylal is 94.6%.

[0057] Example 12

[0058] Weigh 0.919 g of KNO3 and 30 ml of acetonitrile respectively. Add KNO3 to the acetonitrile and stir. Add 1 g of dried acidic support HZSM-5 to the above solution, mix and heat it to 60 °C in a water bath and stir for 6 h. Then carry out solid-liquid separation on the above solution. Wash the separated solid with absolute ethanol 10 times. After washing the solid, put it into an oven at 100 °C and dry for 12 h. Finally, calcine it in a muffle furnace at 550 °C for 6 h to obtain the modified acidic catalyst 30% K2O / HZSM-5.

[0059] Press the obtained sample into tablets of 20-60 mesh, and then load it into a stainless steel tube fixed-bed reactor (inner diameter of the fixed bed is 8 mm) for evaluation. Using 62.5% formaldehyde aqueous solution by mass concentration as the formaldehyde source, n 甲醇 :n 甲醛 =10:1, reaction temperature is 120 °C, WHSV = 1 h -1 . Finally, through analysis, the formaldehyde conversion rate is 64.8%, and the selectivity of methylal is 92.7%.

[0060] Example 13

[0061] Weigh 0.163 g of K2CO3 and 10 ml of isopropanol separately. Add K2CO3 to isopropanol and stir. Add 1 g of dried acidic support HMCM-22 to the above solution, mix, and stir at room temperature of 25 °C for 8 h. Then, perform solid-liquid separation on the above solution. Wash the separated solid 5 times with deionized water. After washing, place the solid in an oven at 100 °C and dry for 3 h. Finally, calcine in a muffle furnace at 500 °C for 4 h to obtain the modified acidic catalyst 10% K2O / HMCM-22.

[0062] Press the obtained sample into tablets of 20 - 60 mesh, and then load it into a stainless steel tube fixed-bed reactor (inner diameter of the fixed bed is 8 mm) for evaluation. Using 70% aqueous formaldehyde solution by mass as the formaldehyde source, n 甲醇 :n 甲醛 = 8:1, reaction temperature is 90 °C, WHSV = 0.5 h -1 Finally, after analysis, the formaldehyde conversion rate is 73.1%, and the selectivity of methylal is 90.5%.

[0063] Comparative Example 1

[0064] Using dried HZSM-5 as the catalyst, press it into tablets of 20 - 60 mesh, and then load it into a stainless steel tube fixed-bed reactor (inner diameter of the fixed bed is 8 mm) for evaluation. Using 37% aqueous formaldehyde solution as the formaldehyde source, n 甲醇 :n 甲醛 = 7:3, reaction temperature is 120 °C, WHSV = 3 h -1 Finally, after analysis, the formaldehyde conversion rate is 83.5%, and the selectivity of methylal is 85.0%.

[0065] Comparative Example 2

[0066] Using dried H-β zeolite as the catalyst, press it into tablets of 20 - 60 mesh, and then load it into a stainless steel tube fixed-bed reactor (inner diameter of the fixed bed is 8 mm) for evaluation. Using paraformaldehyde as the formaldehyde source, n 甲醇 :n 甲醛 = 4:1, reaction temperature is 100 °C, WHSV = 2 h -1 Finally, after analysis, the formaldehyde conversion rate is 87.2%, and the selectivity of methylal is 90.5%.

[0067] Comparative Example 3

[0068] Using dried HMCM-22 as the catalyst, press it into tablets of 20 - 60 mesh, and then load it into a stainless steel tube fixed-bed reactor (inner diameter of the fixed bed is 8 mm) for evaluation. Using 37% aqueous formaldehyde solution as the formaldehyde source, n 甲醇 :n 甲醛 = 2:1, reaction temperature is 100 °C, WHSV = 1 h -1Finally, the formaldehyde conversion rate was analyzed to be 89.1%, and the selectivity of methylal was 81.5%.

[0069] By comparing Comparative Example 1 with Example 1, Comparative Example 2 with Example 8, and Comparative Example 3 with Example 5, it was found that for the catalysts without modification by metal oxides, the formaldehyde conversion rate and the selectivity of methylal both decreased to varying degrees; since the strength of the acidity on the catalyst surface is the key to the aldol condensation reaction, compared with Example 1, the acidity of the catalyst could be slightly enhanced after loading iron in Comparative Example 1; the comparison of the catalytic performances of Comparative Example 2 with Example 8 and Comparative Example 3 with Example 5 were both for the catalysts modified with weak basic metals, and it had basically no effect on the strength of the acidity of the catalyst, but the weak basic sites could promote the condensation reaction to a certain extent.

[0070] The above are only the embodiments of the present invention, and do not limit the patent scope of the present invention accordingly. Any equivalent structure or equivalent process transformation made by using the content of the specification of the present invention, or directly or indirectly applied in other related technical fields, shall be similarly included in the patent protection scope of the present invention.

Claims

1. A method for modifying an acidic catalyst, characterized in that: (1) Based on the dry acidic catalyst powder, dissolve the precursor of the modified metal oxide in a polar solvent, add the dried acidic catalyst powder to the metal oxide precursor solution, and stir at 25 - 80 °C for 1 - 12 h; (2) Perform liquid-solid separation on the above mixed solution, wash the separated solid with the corresponding polar solvent 2 - 10 times, dry the solid after washing, and then calcine to obtain the modified acidic catalyst.

2. The method for modifying an acidic catalyst according to claim 1, characterized in that, The acidic catalyst includes one or more of HZSM-5, H-β zeolite molecular sieve, and HMCM-22 zeolite molecular sieve.

3. The method for modifying an acidic catalyst according to claim 1, characterized in that, The precursor of the modified metal oxide includes one or more of Fe(NO3)3·9H2O, Cu(NO3)2, CsNO3, Cs2CO3, Mg(NO3)2·6H2O, Zn(NO3)2·4H2O, KNO3, Ca(NO3)2, and K2CO3; in the modified acidic catalyst, the loading of the modified metal oxide is 10 - 60% (relative to the modified catalyst), preferably 35 - 55%.

4. The method for modifying an acidic catalyst according to claim 1 or 2 or 3, characterized in that, In step (2), the drying temperature of the modified acidic catalyst is 100 - 120 °C, the time is 2 - 12 h, the calcination temperature is 400 - 700 °C (preferably 450 - 650 °C), and the time is 2 - 10 h (preferably 4 - 8 h).

5. The method for modifying an acidic catalyst according to claim 1, characterized in that, In step (1), the polar solvent includes one or more of water, acetone, absolute ethanol, acetonitrile, isopropanol, and methanol; the mass ratio of the polar solution to the acidic catalyst powder before modification is 10:1 - 50:1, preferably 10:1 - 30:

1.

6. A modified acidic catalyst obtained by the preparation method according to any one of claims 1 - 5.

7. An application of the catalyst according to claim 6, characterized in that: This catalyst can be applied to the catalytic synthesis of methylal by the acetalization of methanol and formaldehyde.

8. The application of the catalyst according to claim 7, characterized in that: The formaldehyde includes one or more of aqueous formaldehyde solution with a mass concentration of 10% - 37%, trioxymethylene, and paraformaldehyde.

9. The application of the catalyst according to claim 7 or 8, characterized in that: Using a fixed-bed reactor, the raw material is a mixed solution of methanol and formaldehyde, and the molar ratio of methanol to formaldehyde is 2:1 - 10:1, preferably 2:1 - 4:1; the reaction temperature is 40 - 150 °C (preferably 60 - 120 °C), the reaction is carried out at atmospheric pressure, and the WHSV during the reaction process is 0.5 - 10 h -1 (preferably 0.5 - 6 h -1 ).

10. The application of the catalyst according to claim 9, characterized in that: Using a fixed-bed reactor, fill the middle section of the bed with the catalyst (the formed catalyst is selected from 20 - 100 mesh), and fill the upper and lower sections with quartz sand respectively.

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