A synthesis process of methyl acetal

Abstract

This invention discloses a synthesis process for methylal, belonging to the field of methylal technology, comprising the following steps: In a reactor, formaldehyde and methanol are reacted at a molar ratio of 1:1.5-4.5 under the action of an organic-inorganic composite solid acid catalyst at 50-75°C for 40-240 min, with the reflux ratio controlled at 1-4:1, and the fraction collected at 40-46°C from the top of the column to obtain industrial-grade methylal product; the organic-inorganic composite solid acid catalyst is phosphotungstic acid supported on multi-component mesoporous microspheres grafted with sulfonic acid-based acrylamide. The methylal synthesis process provided by this invention uses an organic-inorganic composite solid acid catalyst to synthesize industrial-grade methylal. This organic-inorganic composite solid acid catalyst is easy to separate from the product, has low corrosiveness to equipment, high catalytic effect, wear resistance, and good recyclability, making it a novel, efficient, green, and environmentally friendly production method.

Description

Technical Field

[0001] This invention belongs to the field of methyl acetal technology, specifically relating to a synthesis process of methyl acetal. Background Technology

[0002] Methyl acetal, also known as dimethylformaldehyde or dimethoxymethane, can decompose into formaldehyde and methanol under acidic conditions. It is relatively stable under alkaline conditions and is a very important methanol derivative. It has strong solubility and a relatively low boiling point, and is widely used in industrial production.

[0003] In industrial production, the most mature process for preparing methylal is the aldol condensation method, which typically involves two consecutive steps. First, formaldehyde is formed through a gas-phase reaction with methanol. Then, in a second reaction vessel, formaldehyde undergoes a liquid-phase acetalization reaction with methanol. To meet the application requirements of this reaction, early methods used inorganic acids (such as H₂SO₄, HF, H₃PO₄, and p-toluenesulfonic acid) and Lewis acids (such as AlCl₃ and FeCl₃), which exhibited good catalytic activity. However, these methods suffered from drawbacks such as difficulties in separation and recovery, and severe corrosion of equipment. Therefore, solid acid catalysts were developed to replace liquid acids, achieving significant technical improvements.

[0004] For example, patent CN101857533B discloses a process for producing methyl acetal using a composite solid acid catalyst. The process uses a mixed solvent of ethyl acetate, acetone, tetrahydrofuran, and methanol as raw materials, adds formaldehyde, and obtains formaldehyde acetal through catalytic reactive distillation under the action of catalyst CSC-I. The composite solid acid catalyst is SO4. - The SnO-ZrO2-γ-Al2O3 composite solid acid catalyst process generates no waste gas and produces virtually no wastewater.

[0005] Patent CN102304030B discloses a method for preparing methylal using an activated carbon-supported catalyst. Methanol and formaldehyde react under normal pressure in the presence of a solid acid catalyst to obtain methylal. The solid acid catalyst is an activated carbon-supported acid catalyst, specifically obtained by impregnating pretreated activated carbon in concentrated sulfuric acid solution and refluxing it. This method has advantages such as economical catalyst preparation and easy separation from the product.

[0006] The solid acid catalysts in the aforementioned patents are obtained by loading transition metals or acidic groups onto the carrier using activated carbon or activated alumina through different loading processes. Although they are easy to separate, they have disadvantages such as weak strength and high wear rate. Furthermore, activated alumina has the problem of low catalytic efficiency due to its small specific surface area and small pore size. In addition, the use of concentrated sulfuric acid as a sulfonating agent poses risks such as high corrosivity. Summary of the Invention

[0007] The purpose of this invention is to provide a simple and efficient process for synthesizing methyl acetal, and to provide an organic-inorganic composite solid acid catalyst for the synthesis of methyl acetal, which has the characteristics of high strength and good catalytic performance.

[0008] The objective of this invention can be achieved through the following technical solutions:

[0009] A process for synthesizing methylal includes the following steps:

[0010] In the reactor, formaldehyde and methanol are reacted at a molar ratio of 1:1.5-4.5 under the action of an organic-inorganic composite solid acid catalyst at 50-75℃ for 40-240 min, with the reflux ratio controlled at 1-4:1. The fraction at the top of the column at 40-46℃ is collected to obtain industrial-grade methyl acetal.

[0011] Furthermore, the amount of organic-inorganic composite solid acid catalyst used is 2-8% of the combined mass of formaldehyde and methanol.

[0012] The organic-inorganic composite solid acid catalyst is phosphotungstic acid supported on multi-component mesoporous microspheres grafted with sulfonic acid-based acrylamide.

[0013] Furthermore, the organic-inorganic composite solid acid catalyst is prepared by the following steps:

[0014] Phosphotungstic acid loaded on multi-component mesoporous microspheres was added to a mixture containing KH-570 and DMF. After reacting at 110℃ for 4-6 hours, the temperature was lowered to 70℃ and azobisisobutyronitrile and 2-acrylamido-2-methylpropanesulfonic acid were added. The mixture was kept warm and stirred for 8-10 hours. After the reaction was completed, the mixture was filtered, and the filter cake was washed 3-5 times with anhydrous ethanol and then dried.

[0015] Using phosphotungstic acid supported on multi-component mesoporous microspheres as the base material, KH-570 as the bridging component, and azobisisobutyronitrile as the initiator, 2-acrylamido-2-methylpropanesulfonic acid was chemically bonded to the surface of phosphotungstic acid supported on multi-component mesoporous microspheres to obtain an organic-inorganic composite solid acid catalyst.

[0016] Furthermore, the ratio of phosphotungstic acid, KH-570, DMF, and 2-acrylamido-2-methylpropanesulfonic acid loaded on the multi-component mesoporous microspheres is 0.5-1g:0.1-0.2g:40-60mL:0.2-0.4g, and the amount of azobisisobutyronitrile is 0.1-0.3% of the total mass of KH-570 and 2-acrylamido-2-methylpropanesulfonic acid.

[0017] Furthermore, the phosphotungstic acid-loaded multi-component mesoporous microspheres are prepared by the following steps:

[0018] Mix the multi-component mesoporous microspheres and deionized water ultrasonically for 10-20 min, add phosphotungstic acid aqueous solution, stir at 60℃ and 400 r / min for 20-40 min, then centrifuge, wash the precipitate with deionized water 3-5 times, and dry.

[0019] Furthermore, the ratio of the amount of multi-component mesoporous microspheres, deionized water, and phosphotungstic acid aqueous solution is 1g:20mL:5-10mL. The phosphotungstic acid aqueous solution is composed of phosphotungstic acid and deionized water in a ratio of 0.5-1g:5-10mL. Under stirring conditions, the phosphotungstic acid is made to reach adsorption saturation in the channels of the multi-component mesoporous microspheres, thereby obtaining multi-component mesoporous microspheres with phosphotungstic acid loaded on the surface.

[0020] Furthermore, the multi-component mesoporous microspheres are prepared by the following steps:

[0021] Mesoporous silica microspheres and hexadecyltrimethylammonium bromide were ultrasonically dispersed in deionized water for 10-20 min. A mixture of resorcinol, ammonia, and anhydrous ethanol (a) was added, and the mixture was ultrasonically dispersed for 30 min. The temperature was controlled at 35℃, and formaldehyde solution was added. The mixture was stirred for 6 h and aged for 12 h. After centrifugation, washing, and drying, an intermediate product was obtained. The intermediate product was heated to 150℃ under argon protection and held for 1 h. Then it was heated to 800℃ and held for 2 h to obtain a semi-finished product. The semi-finished product and magnesium powder were ground and mixed evenly at a mass ratio of 2:1. Under argon protection, the mixture was heated to 800℃ and held for 5 h. After cooling in the furnace, the product was washed with 2 mol / L hydrochloric acid for 5 h and then washed with deionized water until the washing solution was neutral. After drying, multi-component mesoporous microspheres were obtained.

[0022] Using mesoporous silica microspheres as a matrix, phenolic resin is coated onto their surface through chemical means, and a semi-finished product is obtained through high-temperature carbonization. Finally, a multi-component mesoporous microsphere is obtained through a magnesothermic reaction (magnesium and silica) at 800℃. It contains SiC, amorphous SiO2, and amorphous C phases, and has a large number of mesoporous structures on its surface. The fine pore structure can ensure uniform support of active components and provide a site for catalytic reaction. Based on the tetrahedral structural unit of silicon carbide, which is similar to diamond, the multi-component mesoporous microspheres are endowed with high mechanical strength and hardness, reducing breakage during transportation and wear during use. In addition, silicon carbide has high thermal conductivity, giving the multi-component mesoporous microspheres good heat dissipation performance, which can quickly transfer the heat of the reactants away and reduce the damage of thermal shock.

[0023] Furthermore, in the preparation of the intermediate product, the ratio of mesoporous silica microspheres, hexadecyltrimethylammonium bromide, deionized water, resorcinol, ammonia, anhydrous ethanol, and formaldehyde solution is 0.3-0.4 g: 0.9 g: 30 mL: 0.6 g: 0.1 mL: 15 mL: 0.8 mL, the mass fraction of ammonia is 25-28%, and the mass fraction of formaldehyde solution is 37%.

[0024] Furthermore, the heating process has a heating rate of 5°C / min.

[0025] Furthermore, the mesoporous silica microspheres are prepared by the following steps:

[0026] Hexadecyl bromopyridine, urea, deionized water, cyclohexane, and isopropanol were added to a flask and ultrasonically dispersed for 10-15 min. Then, tetraethyl orthosilicate was added dropwise to the flask at a speed of 700-900 r / min. After the addition was complete, the mixture was stirred at 70 °C for 20 h. After the reaction was completed, the reaction product was washed, vacuum dried at 60 °C for 12 h, and then calcined at 550 °C for 5 h to obtain hollow mesoporous silica microspheres. The ratio of hexadecyl bromopyridine, urea, deionized water, cyclohexane, isopropanol, and tetraethyl orthosilicate was 0.75-1.1 g: 0.6 g: 30 mL: 30 mL: 0.9 mL: 2.7 mL. The mass fraction of SiO2 in the tetraethyl orthosilicate was 38-43%.

[0027] Furthermore, the reactor is equipped with a stirrer, a distillation column, and a packed column.

[0028] Furthermore, the formaldehyde is a 37 wt% formaldehyde solution.

[0029] Furthermore, the methanol is refined methanol or industrial-grade methanol.

[0030] The beneficial effects of this invention are:

[0031] This invention provides a synthesis process for methylal, which uses an organic-inorganic composite solid acid catalyst to synthesize industrial-grade methylal. This organic-inorganic composite solid acid catalyst is easy to separate from the product, has low corrosiveness to equipment, high catalytic effect, wear resistance, and good recycling effect. It is a new, efficient, green and environmentally friendly production method.

[0032] In this invention, the organic-inorganic composite solid acid catalyst is a multi-component mesoporous microsphere supported on phosphotungstic acid, with sulfonic acid-based acrylamide grafted onto its surface. By first impregnating the multi-component mesoporous microspheres with an inorganic acid and then chemically grafting an organic acid, the acidity of the catalyst is increased, thereby enhancing the catalytic effect. Furthermore, the catalyst uses multi-component mesoporous microspheres as a support, which has a high specific surface area, allowing for sufficient contact with the reactants. It also possesses good mechanical strength and thermal conductivity, reducing catalyst wear and thermal shock damage, and giving the catalyst good recyclability. Moreover, the catalyst surface is composed of sulfonic acid-based polyacrylamide, which provides acidic sulfonic acid groups to enhance the catalyst's catalytic performance. Additionally, the amide bonds in acrylamide can form strong hydrogen bonds with water, absorbing the water generated in the acetal reaction, promoting the forward reaction, and increasing the yield of methyl acetal. The coupling agent KH-570 enhances the interaction between the sulfonic acid-based polyacrylamide and phosphotungstic acid, enabling them to synergistically exert their catalytic effect. Detailed Implementation

[0033] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0034] Example 1

[0035] Mesoporous silica microspheres are prepared by the following steps:

[0036] 0.75 g of hexadecyl pyridine bromide, 0.6 g of urea, 30 mL of deionized water, 30 mL of cyclohexane, and 0.9 mL of isopropanol were added to a flask and ultrasonically dispersed for 15 min. Then, 2.7 mL of tetraethyl orthosilicate was added dropwise to the flask at 700 r / min. After the addition was complete, the mixture was stirred at 70 °C for 20 h. After the reaction was completed, the reaction product was repeatedly washed with deionized water and anhydrous ethanol, dried under vacuum at 60 °C for 12 h, and then calcined at 550 °C for 5 h to obtain hollow mesoporous silica microspheres. The mass fraction of SiO2 in the tetraethyl orthosilicate was 38%.

[0037] Example 2

[0038] Mesoporous silica microspheres are prepared by the following steps:

[0039] 1.1 g of hexadecyl pyridine bromide, 0.6 g of urea, 30 mL of deionized water, 30 mL of cyclohexane, and 0.9 mL of isopropanol were added to a flask and ultrasonically dispersed for 10 min. Then, 2.7 mL of tetraethyl orthosilicate was added dropwise to the flask at 900 r / min. After the addition was complete, the mixture was stirred at 70 °C for 20 h. After the reaction was completed, the reaction product was repeatedly washed with deionized water and anhydrous ethanol, dried under vacuum at 60 °C for 12 h, and then calcined at 550 °C for 5 h to obtain hollow mesoporous silica microspheres. The mass fraction of SiO2 in the tetraethyl orthosilicate was 43%.

[0040] Example 3

[0041] Multi-component mesoporous microspheres are manufactured by the following steps:

[0042] 0.3g of mesoporous silica microspheres obtained in Example 1 and 0.9g of hexadecyltrimethylammonium bromide were ultrasonically dispersed in 30mL of deionized water for 10min. A mixture of 0.6g of resorcinol, 0.1mL of ammonia, and 15mL of anhydrous ethanol (solution a) was added and ultrasonically dispersed for 30min. The temperature was controlled at 35℃, and 0.8mL of formaldehyde solution was added. After stirring for 6h, the mixture was aged for 12h, centrifuged, washed, and dried to obtain an intermediate product. The intermediate product was heated to 150℃ under argon protection and held for 1h, then heated to 800℃ and held for 2h to obtain a semi-finished product. The semi-finished product and magnesium powder were ground and mixed evenly at a mass ratio of 2:1. Under argon protection, the mixture was heated to 800℃ and held for 5h. After cooling in the furnace, the product was washed with 2mol / L hydrochloric acid for 5h, then washed with deionized water until the washing solution was neutral, and dried to obtain multi-component mesoporous microspheres. The formaldehyde solution had a mass fraction of 37%. The heating rate during the heating process was 5℃ / min.

[0043] Example 4

[0044] Multi-component mesoporous microspheres are manufactured by the following steps:

[0045] 0.4 g of mesoporous silica microspheres obtained in Example 2 and 0.9 g of hexadecyltrimethylammonium bromide were ultrasonically dispersed in 30 mL of deionized water for 20 min. A mixture of 0.6 g of resorcinol, 0.1 mL of ammonia, and 15 mL of anhydrous ethanol (solution a) was added and ultrasonically dispersed for 30 min. The temperature was controlled at 35 °C, and 0.8 mL of formaldehyde solution was added. The mixture was stirred for 6 h and aged for 12 h. After centrifugation, washing, and drying, an intermediate product was obtained. The intermediate product was heated to 150 °C under argon protection and held for 1 h. Then it was heated to 800 °C and held for 2 h to obtain a semi-finished product. The semi-finished product and magnesium powder were ground and mixed evenly at a mass ratio of 2:1. Under argon protection, the mixture was heated to 800 °C and held for 5 h. After cooling in the furnace, the product was washed with 2 mol / L hydrochloric acid for 5 h and then washed with deionized water until the washing solution was neutral. After drying, a multi-component mesoporous microsphere formaldehyde solution with a mass fraction of 37% was obtained. The heating rate during the heating process was 5 °C / min.

[0046] The specific surface area and porosity of the samples obtained in Examples 1, 2, 3, and 4 were tested using a BET 3Flex 5.02 (Micromeritics, USA). The test results are shown in Table 1.

[0047] Table 1

[0048] project Example 1 Example 2 Example 3 Example 4 <![CDATA[Specific surface area (m 2 / g)]]> 407.1247 408.3596 501.1891 500.4563 Average pore size (nm) 9.2762 9.2850 8.8903 8.8671

[0049] As can be seen from Table 1, compared with Examples 1 and 2, the average pore size of the materials obtained in Examples 3 and 4 is reduced, but the specific surface area is larger.

[0050] Example 5

[0051] Organic-inorganic composite solid acid catalysts are prepared by the following steps:

[0052] 0.5 g of phosphotungstic acid supported on multi-component mesoporous microspheres was added to a mixture containing 0.1 g KH-570 and 40 mL LDM. After reacting at 110 °C for 4 h, the temperature was lowered to 70 °C and 0.9 mg azobisisobutyronitrile and 0.2 g 2-acrylamido-2-methylpropanesulfonic acid were added. The mixture was kept at this temperature and stirred for 8 h. After the reaction was completed, the mixture was filtered, and the filter cake was washed three times with anhydrous ethanol and dried.

[0053] Phosptonic acid supported on multi-component mesoporous microspheres is prepared by the following steps:

[0054] 1g of the multi-component mesoporous microspheres from Example 3 and 20mL of deionized water were ultrasonically mixed for 10min. Then, 5mL of phosphotungstic acid aqueous solution was added and stirred for 20min at 60℃ and 400r / min. After centrifugation, the precipitate was washed three times with deionized water and dried. The phosphotungstic acid aqueous solution was composed of phosphotungstic acid and deionized water in a ratio of 0.5g:5mL.

[0055] Example 6

[0056] Organic-inorganic composite solid acid catalysts are prepared by the following steps:

[0057] 1 g of phosphotungstic acid loaded on multi-component mesoporous microspheres was added to a mixture containing 0.2 g KH-570 and 60 mL DMF. After reacting at 110 °C for 6 h, the temperature was lowered to 70 °C and 0.6 mg azobisisobutyronitrile and 0.4 g 2-acrylamido-2-methylpropanesulfonic acid were added. The mixture was kept at this temperature and stirred for 10 h. After the reaction was completed, the mixture was filtered, and the filter cake was washed 5 times with anhydrous ethanol and dried.

[0058] Phosptonic acid supported on multi-component mesoporous microspheres is prepared by the following steps:

[0059] 1g of the multi-component mesoporous microspheres from Example 4 and 20mL of deionized water were ultrasonically mixed for 20min. Then, 10mL of phosphotungstic acid aqueous solution was added and stirred at 60℃ and 400r / min for 40min. After centrifugation, the precipitate was washed 5 times with deionized water and dried. The phosphotungstic acid aqueous solution was composed of phosphotungstic acid and deionized water in a ratio of 1g:10mL.

[0060] Comparative Example 1

[0061] Organic-inorganic composite solid acid catalysts are prepared by the following steps:

[0062] Compared with Example 5, the multi-element mesoporous microspheres in Example 5 were replaced with mesoporous silica microspheres of the same mass as in Example 1, while the other raw materials and preparation process were the same as in Example 5.

[0063] Comparative Example 2

[0064] This comparative example uses multi-component mesoporous microspheres loaded with phosphotungstic acid, and the specific preparation method is the same as in Example 5.

[0065] Comparative Example 3

[0066] Organic-inorganic composite solid acid catalysts are prepared by the following steps:

[0067] Add 0.5g of multi-component mesoporous microspheres to a mixture containing 0.2g of KH-570 and 40mL of DMF. After reacting at 110℃ for 4h, cool to 70℃ and add 0.6mg of azobisisobutyronitrile and 0.4g of 2-acrylamido-2-methylpropanesulfonic acid. Keep the mixture warm and stir for 8h. After the reaction is complete, filter the mixture, wash the filter cake three times with anhydrous ethanol, and dry it.

[0068] The catalysts obtained in Examples 5, 6, 1, 2, and 3 were tested. Strength was tested according to standard HG / T 2782-2011; annual wear rate was tested according to standard HG / T 2976-1999. The test results are shown in Table 2.

[0069] Table 2

[0070] project Example 5 Example 6 Comparative Example 1 Comparative Example 2 Comparative Example 3 <![CDATA[Strength (N / m 2 )]]> 312 318 275 308 310 Annual wear rate (%) 1.2 0.8 2.5 1.5 1.3

[0071] As can be seen from Table 2, compared with Comparative Examples 1, 2 and 3, the catalysts obtained in Examples 5 and 6 have greater mechanical strength and lower annual wear rate.

[0072] Example 7

[0073] A process for synthesizing methylal includes the following steps:

[0074] Formaldehyde, methanol, and the organic-inorganic composite solid acid catalyst of Example 5 were added to a reactor equipped with a stirrer, a distillation column, and a packed column. The molar ratio of formaldehyde to methanol was controlled at 1:1.5, and the amount of organic-inorganic composite solid acid catalyst was 2% of the total mass of formaldehyde and methanol. The reaction was carried out at 50°C for 240 min, and the reflux ratio was controlled at 1:1. The fraction at the top of the column at 40-46°C was collected to obtain industrial-grade methylal product.

[0075] The formaldehyde is a 37wt% formaldehyde solution, and the methanol is refined methanol.

[0076] Example 8

[0077] A process for synthesizing methylal includes the following steps:

[0078] Formaldehyde, methanol, and the organic-inorganic composite solid acid catalyst of Example 6 were added to a reactor equipped with a stirrer, a distillation column, and a packed column. The molar ratio of formaldehyde to methanol was controlled at 1:3.0, and the amount of organic-inorganic composite solid acid catalyst was 6% of the total mass of formaldehyde and methanol. The reaction was carried out at 65°C for 140 min, and the reflux ratio was controlled at 3:1. The fraction at the top of the column at 40-46°C was collected to obtain industrial-grade methylal product.

[0079] The formaldehyde is a 37wt% formaldehyde solution, and the methanol is industrial-grade methanol.

[0080] Example 9

[0081] A process for synthesizing methylal includes the following steps:

[0082] Formaldehyde, methanol, and the organic-inorganic composite solid acid catalyst of Example 6 were added to a reactor equipped with a stirrer, a distillation column, and a packed column. The molar ratio of formaldehyde to methanol was controlled at 1:4.5, and the amount of organic-inorganic composite solid acid catalyst was 8% of the total mass of formaldehyde and methanol. The reaction was carried out at 75°C for 40 min, and the reflux ratio was controlled at 4:1. The fraction at the top of the column at 40-46°C was collected to obtain industrial-grade methylal product.

[0083] The formaldehyde is a 37wt% formaldehyde solution, and the methanol is refined methanol.

[0084] Comparative Example 4

[0085] A process for synthesizing methylal, compared with Example 7, is described in which the organic-inorganic composite solid acid catalyst in Example 7 is replaced with the product prepared in Comparative Example 1, and the remaining raw materials and steps are the same as in Example 7.

[0086] Comparative Example 5

[0087] A process for synthesizing methylal, compared with Example 7, is described in which the organic-inorganic composite solid acid catalyst in Example 7 is replaced with the substance in Comparative Example 2, and the remaining raw materials and steps are the same as in Example 7.

[0088] Comparative Example 6

[0089] A process for synthesizing methylal, compared with Example 7, involves replacing the organic-inorganic composite solid acid catalyst in Example 7 with the product prepared in Comparative Example 3, while the remaining raw materials and steps are the same as in Example 7.

[0090] The yields of methylal obtained in Examples 7-9 and Comparative Examples 4-6 were tested and calculated, and a reusability test was performed. Specifically, the catalysts in each group were reused 6 times without changing them, and then the yield of the product obtained from the seventh synthesis process was measured. The test results are shown in Table 3.

[0091] Table 3

[0092]

[0093] As can be seen from Table 3, compared with the synthesis processes of methylal described in Comparative Examples 4, 5, and 6, the synthesis processes of methylal described in Examples 7, 8, and 9 have better product yields and better catalyst recycling effects.

[0094] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0095] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A process for synthesizing methylal, characterized in that, Includes the following steps: In the reactor, formaldehyde and methanol are reacted at a molar ratio of 1:1.5-4.5 under the action of an organic-inorganic composite solid acid catalyst at 50-75℃ for 40-240 min, with the reflux ratio controlled at 1-4:

1. The fraction at 40-46℃ at the top of the column is collected to obtain industrial-grade methyl acetal product. The organic-inorganic composite solid acid catalyst is phosphotungstic acid supported on multi-component mesoporous microspheres grafted with sulfonic acid-based acrylamide on the surface. Organic-inorganic composite solid acid catalysts are prepared by the following steps: Add phosphotungstic acid loaded on multi-component mesoporous microspheres to a mixture containing KH-570 and DMF. After reacting at 110℃ for 4-6 hours, cool to 70℃ and add azobisisobutyronitrile and 2-acrylamido-2-methylpropanesulfonic acid. Keep warm and stir for 8-10 hours. Filter, wash the filter cake, and dry. Phosptonic acid supported on multi-component mesoporous microspheres is prepared by the following steps: Mix the multi-component mesoporous microspheres and deionized water ultrasonically, add phosphotungstic acid aqueous solution, stir at 60℃ for 20-40 min, centrifuge, wash the precipitate, and dry. Multi-component mesoporous microspheres are manufactured by the following steps: Mesoporous silica microspheres and hexadecyltrimethylammonium bromide were ultrasonically dispersed in deionized water. A mixture of resorcinol, ammonia, and anhydrous ethanol (a) was added. After ultrasonic dispersion, the temperature was controlled at 35°C. Formaldehyde solution was added, stirred for 6 hours, and aged for 12 hours. The mixture was centrifuged, washed, and dried to obtain an intermediate product. The intermediate product was heated to 150°C under argon protection and held for 1 hour. Then it was heated to 800°C and held for 2 hours to obtain a semi-finished product. The semi-finished product and magnesium powder were ground and mixed evenly at a mass ratio of 2:

1. Under argon protection, the mixture was heated to 800°C and held for 5 hours. After cooling in the furnace, the product was washed with hydrochloric acid solution and deionized water and dried to obtain multi-component mesoporous microspheres.

2. The synthesis process of methylal according to claim 1, characterized in that, The amount of organic-inorganic composite solid acid catalyst used is 2-8% of the total mass of formaldehyde and methanol.

3. The synthesis process of methylal according to claim 1, characterized in that, The ratio of phosphotungstic acid, KH-570, DMF and 2-acrylamido-2-methylpropanesulfonic acid loaded on the multi-component mesoporous microspheres is 0.5-1g: 0.1-0.2g: 40-60mL: 0.2-0.4g, and the amount of azobisisobutyronitrile is 0.1-0.3% of the total mass of KH-570 and 2-acrylamido-2-methylpropanesulfonic acid.

4. The synthesis process of methylal according to claim 1, characterized in that, The ratio of the amount of multi-component mesoporous microspheres, deionized water and phosphotungstic acid aqueous solution is 1g:20mL:5-10mL. The phosphotungstic acid aqueous solution is composed of phosphotungstic acid and deionized water in a ratio of 0.5-1g:5-10mL.

5. The synthesis process of methylal according to claim 1, characterized in that, In the preparation of the intermediate product, the ratio of mesoporous silica microspheres, hexadecyltrimethylammonium bromide, deionized water, resorcinol, ammonia, anhydrous ethanol and formaldehyde solution is 0.3-0.4g:0.9g:30mL:0.6g:0.1mL:15mL:0.8mL, the mass fraction of ammonia is 25-28%, and the mass fraction of formaldehyde solution is 37%.

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