Preparation method of paraformaldehyde

Abstract

The invention relates to a preparation method of paraformaldehyde, and belongs to the technical field of paraformaldehyde preparation. The preparation method of paraformaldehyde comprises the following steps: carrying out two-stage evaporation concentration on a 48 wt% formaldehyde solution until the concentration is 75-85 wt%, and then conveying the concentrated formaldehyde solution to a buffer tank; a polymerization inhibitor is added into a buffer tank, the temperature in the buffer tank is 90-110 DEG C, the concentrated formaldehyde solution and the polymerization inhibitor are mixed for 2 h, then a modified catalyst is added into the buffer tank, mixing continues to be conducted for 0.1-0.5 h, a reaction product obtained through mixing is filtered and then enters a fluidized bed drying device, cooling is conducted to 55-60 DEG C, and then a paraformaldehyde product is obtained. The paraformaldehyde preparation method has the advantages that the conversion rate is high, and the catalyst can be recycled.

Description

Technical Field

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

[0002] Paraformaldehyde is an important linear formaldehyde polymer in industry. It is a mixture of 8-100 units of polyoxymethylene glycol. It is usually a colorless crystalline solid with a strong formaldehyde odor. It is insoluble in ethanol, acetone and ether, but soluble in dilute acid and dilute alkali solutions. Paraformaldehyde has a higher effective component than industrial formaldehyde and is a solid particle. It is beneficial for chemical synthesis such as chemical industry, pharmaceutical industry and other industrial fields. It is widely used, especially in the synthesis that requires the use of anhydrous formaldehyde as a raw material. It can also be used as a disinfectant, bactericide, fumigant and herbicide, as well as in the manufacture of resins and artificial ivory.

[0003] At present, the more mature methods for preparing paraformaldehyde used at home and abroad mainly include vacuum rake dryer drying paraformaldehyde, metal conveyor belt drying paraformaldehyde, spray method, azeotropic distillation method, etc. The polymerization mechanism of paraformaldehyde solution shows that the equilibrium distribution of formaldehyde oligomers depends on the formaldehyde concentration and stability. If the formaldehyde concentration exceeds a certain limit, the long-chain oligomers formed will precipitate in the solution to form insoluble matter. Therefore, in order to control the degree of polymerization and stability of the paraformaldehyde product, it is necessary to add a suitable catalyst to the concentrated formaldehyde solution to control the growth of molecular weight.

[0004] Organic alkaline substances are currently the most ideal synthetic auxiliary agents in the production of polyformaldehyde. This auxiliary agent combines the dual functions of catalysis and polymerization degree regulation. In the formaldehyde polymerization and drying process, it has the advantages of low polymerization degree, good water solubility, no insoluble matter after long-term storage, no ash, and no residue after combustion.

[0005] Patent CN 101845130B discloses a method for preparing polyformaldehyde, which includes the following steps: concentrating a formaldehyde solution to a concentration of 75-85wt%, and then pumping the concentrated formaldehyde solution to a buffer tank; adding a polymerization inhibitor and a catalyst to the buffer tank in sequence, the reaction temperature in the buffer tank is 90-110°C, the average residence time of the above materials in the buffer tank is 0.1-5 hours, the concentrated formaldehyde solution undergoes a polymerization reaction, and the obtained reaction product is dried to obtain a polyformaldehyde product with a low degree of polymerization. However, the catalysts used in the preparation of polyformaldehyde in this patent and the prior art are difficult to recycle. Summary of the invention

[0006] In order to solve the problem that the catalysts used in the prior art for preparing paraformaldehyde are difficult to recycle, the present invention provides a method for preparing paraformaldehyde.

[0007] The purpose of the present invention can be achieved by the following technical solutions:

[0008] A method for preparing paraformaldehyde, the operating steps are:

[0009] Step A1: Concentrating a 48 wt % formaldehyde solution to a concentration of 75-85 wt % by two-stage evaporation, and then conveying the concentrated formaldehyde solution to a buffer tank;

[0010] Step A2: Add an inhibitor to a buffer tank with a temperature of 90-110°C, mix the concentrated formaldehyde solution with the inhibitor for 2 hours, then add the modified catalyst to the buffer tank, continue mixing for 0.1-0.5 hours, filter the reaction product, and then enter a fluidized bed drying device for drying and cooling to 55-60°C to obtain a polyformaldehyde product.

[0011] The dosage ratio of the polymerization inhibitor, the concentrated formaldehyde solution and the modified catalyst is 0.1-0.4 g: 1 L: 0.1-0.8 g.

[0012] Furthermore, the inhibitor is any one of methanol and propanol, which has the function of blocking some end groups to prevent the growth of the molecular chain of the polymer already existing in the formaldehyde solution.

[0013] Furthermore, the specific steps of the two-stage evaporation and concentration are: sending a 48wt% formaldehyde solution to a first-stage falling film evaporator, where the formaldehyde solution concentration is 50-60wt%, and then performing a second-stage evaporation and concentration, inputting the 50-60wt% formaldehyde solution into a concentration kettle, and concentrating it to a concentration of 75-85wt%.

[0014] Furthermore, the temperature of the first stage of fluidized bed drying is 45-70°C, and after obtaining polyformaldehyde with an aldehyde content of 90-91%, it enters the second stage, and the temperature of the second stage is 70-100°C, until obtaining polyformaldehyde with an aldehyde content of 92.1-97.3%.

[0015] Furthermore, the preparation method of the modified catalyst is:

[0016] Step S1: dispersing Zn-MOF-74 in DMF and mixing by ultrasonication to obtain a Zn-MOF-74 solution;

[0017] Among them, the usage ratio of Zn-MOF-74 to DMF is 9-11 g:1L.

[0018] Step S2: dissolving ethanolamine in water, stirring and dissolving, adding Zn-MOF-74 solution, ultrasonically mixing to obtain a mixture, stirring the mixture, centrifuging, vacuum drying and taking out to obtain a modified catalyst.

[0019] Among them, the usage ratio of ethanolamine, water and Zn-MOF-74 solution is 16.5-25g:100mL:1L.

[0020] Furthermore, the preparation method of Zn-MOF-74 is:

[0021] Dissolve 2,5-dihydroxyterephthalic acid and zinc nitrate hexahydrate in a DMF aqueous solution, continue stirring for 1 hour, then transfer to a high-pressure reactor, heat at 100°C for 24 hours, place the reactor at room temperature to cool naturally after the reaction, collect the product and centrifuge once, immerse the obtained solid in DMF for washing for 24 hours, replace DMF every 12 hours, then immerse the solid in anhydrous methanol for solvent replacement for 48 hours, replace anhydrous methanol every 12 hours, and finally dry in a vacuum drying oven at 100°C for 12 hours to obtain Zn-MOF-74;

[0022] The mass ratio of 2,5-dihydroxyterephthalic acid to zinc nitrate hexahydrate is 1:4, and the volume ratio of DMF to deionized water in the DMF aqueous solution is 6:0.3.

[0023] Beneficial effects of the present invention:

[0024] 1. The present invention utilizes a modified catalyst to catalyze the preparation of paraformaldehyde, wherein ethanolamine is loaded in Zn-MOF-74. The rigid structure of Zn-MOF-74 can protect the ethanolamine catalyst loaded therein, prevent it from agglomerating or deactivating during the reaction, extend the service life of the catalyst, and improve the reusability of the catalyst. Since Zn-MOF-74 has good stability and processability, the Zn-MOF-74 loaded with ethanolamine can be separated from the reaction system by a simple physical method, reducing the residual and waste of the catalyst, and the prepared paraformaldehyde product has a high purity.

[0025] 2. Zn-MOF-74 exhibits high specific surface area and regular pore structure characteristics, which can effectively disperse and fix ethanolamine catalyst. This structure optimizes the contact area between the catalyst and the reactants, thereby improving the catalytic efficiency and significantly improving the conversion rate of formaldehyde to paraformaldehyde. Furthermore, Zn-MOF-74 contains metal ions. When the metal ions come into contact with organic amines, zinc ions and ethanolamine form stable coordination bonds through coordination bonds, which enhances the electron donating ability of ethanolamine, thereby promoting the process of chain initiation and chain growth, improving the catalytic efficiency, and thus improving the conversion rate of formaldehyde to paraformaldehyde. DETAILED DESCRIPTION

[0026] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0027] Example 1

[0028] The preparation method of Zn-MOF-74 of the present embodiment is:

[0029] 30 g of 2,5-dihydroxyterephthalic acid and 120 g of zinc nitrate hexahydrate were dissolved in 6 L of DMF and 0.3 L of deionized water, and the mixture was stirred for 1 h. The mixture was then transferred to a high-pressure reactor and heated at 100 °C for 24 h. After the reaction, the reactor was placed at room temperature for natural cooling. The product was collected and centrifuged once. The obtained solid was immersed in DMF for washing for 24 h, and the DMF was replaced every 12 h. The solid was then immersed in anhydrous methanol for solvent replacement for 48 h, and the anhydrous methanol was replaced every 12 h. Finally, the solid was dried in a vacuum drying oven at 100 °C for 12 h to obtain Zn-MOF-74.

[0030] Example 2

[0031] The preparation method of the modified catalyst of this embodiment is:

[0032] Step S1: 9 g of Zn-MOF-74 was dispersed in 1 L of DMF and ultrasonically mixed (frequency: 40 KHz, power: 200 W, time: 25 min) to obtain a Zn-MOF-74 solution;

[0033] Step S2: Dissolve 16.5 g of ethanolamine in 100 mL of water, stir to dissolve, then add 1 L of Zn-MOF-74 solution, and perform ultrasonic mixing (frequency of 40 kHz, power of 200 W, time of 25 min) to obtain a mixture. Stir the mixture at 300 r / min for 5 h, centrifuge, and vacuum dry at 100 °C for 5 h before taking out to obtain a modified catalyst.

[0034] The preparation method of Zn-MOF-74 is the same as that in Example 1.

[0035] Example 3

[0036] The preparation method of the modified catalyst of this embodiment is:

[0037] Step S1: Disperse 10 g of Zn-MOF-74 in 1 L of DMF and mix by ultrasonication (frequency: 40 KHz, power: 200 W, time: 25 min) to obtain a Zn-MOF-74 solution;

[0038] Step S2: Dissolve 20.5 g of ethanolamine in 100 mL of water, stir to dissolve, then add 1 L of Zn-MOF-74 solution, and perform ultrasonic mixing (frequency of 40 kHz, power of 200 W, time of 25 min) to obtain a mixture. Stir the mixture at 300 r / min for 5 h, centrifuge, and vacuum dry at 100 ° C for 5 h before taking out to obtain a modified catalyst.

[0039] The preparation method of Zn-MOF-74 is the same as that in Example 1.

[0040] Example 4

[0041] The preparation method of the modified catalyst of this embodiment is:

[0042] Step S1: Disperse 11 g of Zn-MOF-74 in 1 L of DMF and mix by ultrasonication (frequency: 40 KHz, power: 200 W, time: 25 min) to obtain a Zn-MOF-74 solution;

[0043] Step S2: dissolve 25 g of ethanolamine in 100 mL of water, stir to dissolve, then add 1 L of Zn-MOF-74 solution, and perform ultrasonic mixing (frequency of 40 kHz, power of 200 W, time of 25 min) to obtain a mixture, stir the mixture at 300 r / min for 5 h, centrifuge, and vacuum dry at 100 ° C for 5 h before taking out to obtain a modified catalyst.

[0044] The preparation method of Zn-MOF-74 is the same as that in Example 1.

[0045] Example 5

[0046] This embodiment provides a method for preparing paraformaldehyde, and the operation steps are as follows:

[0047] Step A1: sending a 48wt% formaldehyde solution to a primary falling film evaporator, where the concentration of the formaldehyde solution is 50wt%, and then performing secondary evaporation and concentration, inputting the 50wt% formaldehyde solution into a concentration kettle, and concentrating it to a concentration of 75wt%, and then conveying the concentrated formaldehyde solution to a buffer tank equipped with a stirring and heating device;

[0048] Step A2: Add 0.1g of propanol to the buffer tank, the temperature in the buffer tank is 90°C, mix 1L of concentrated formaldehyde solution with the inhibitor for 2h, then add 0.1g of modified catalyst to the buffer tank, continue mixing for 0.1h, filter the reaction product obtained by mixing to remove the modified catalyst, and dry the filtrate in a fluidized bed and cool it to 60°C to obtain a polyformaldehyde product. The temperature of the first stage of fluidized bed drying is 45°C, and after obtaining polyformaldehyde with an aldehyde content of 90%, enter the second stage, the temperature of the second stage is 70°C, until obtaining polyformaldehyde with an aldehyde content of 92.1%.

[0049] The preparation method of the modified catalyst is the same as that in Example 2.

[0050] Example 6

[0051] This embodiment provides a method for preparing paraformaldehyde, and the operation steps are as follows:

[0052] Step A1: sending a 48wt% formaldehyde solution to a primary falling film evaporator, where the concentration of the formaldehyde solution is 55wt%, and then performing secondary evaporation and concentration, inputting the 55wt% formaldehyde solution into a concentration kettle, and concentrating it to a concentration of 85wt%, and then conveying the concentrated formaldehyde solution to a buffer tank equipped with a stirring and heating device;

[0053] Step A2: 0.3g of methanol is added to the buffer tank, the temperature in the buffer tank is 90°C, 1L of concentrated formaldehyde solution is mixed with the inhibitor for 2h, and then 0.4g of modified catalyst is added to the buffer tank, and the mixing is continued for 0.3h. The reaction product obtained by mixing is filtered to remove the modified catalyst, and the filtrate is dried in a fluidized bed and cooled to 55°C to obtain a polyformaldehyde product. Among them, the temperature of the first stage of fluidized bed drying is 45°C, and after obtaining polyformaldehyde with an aldehyde content of 91%, it enters the second stage, and the temperature of the second stage is 80°C until a polyformaldehyde with an aldehyde content of 95.4% is obtained.

[0054] The preparation method of the modified catalyst is the same as that in Example 3.

[0055] Example 7

[0056] This embodiment provides a method for preparing paraformaldehyde, and the operation steps are as follows:

[0057] Step A1: sending a 48wt% formaldehyde solution to a primary falling film evaporator, where the concentration of the formaldehyde solution is 60wt%, and then performing two-stage evaporation and concentration, inputting the 60wt% formaldehyde solution into a concentration kettle, and concentrating it to a concentration of 80wt%, and then conveying the concentrated formaldehyde solution to a buffer tank equipped with a stirring and heating device;

[0058] Step A2: 0.4g methanol is added to the buffer tank, the temperature in the buffer tank is 90°C, 1L of concentrated formaldehyde solution is mixed with the inhibitor for 2h, and then 0.8g of modified catalyst is added to the buffer tank, and the mixing is continued for 0.5h. The reaction product obtained by mixing is filtered to remove the modified catalyst, and the filtrate is dried in a fluidized bed and cooled to 58°C to obtain a polyformaldehyde product. Among them, the temperature of the first stage of fluidized bed drying is 45°C, and after obtaining polyformaldehyde with an aldehyde content of 90.5%, it enters the second stage, and the temperature of the second stage is 100°C until a polyformaldehyde with an aldehyde content of 97.3% is obtained.

[0059] The preparation method of the modified catalyst is the same as that of Example 4.

[0060] Comparative Example 1

[0061] This comparative example provides a method for preparing paraformaldehyde, and its operating steps are:

[0062] Step A1: sending a 48wt% formaldehyde solution to a primary falling film evaporator, where the concentration of the formaldehyde solution is 50wt%, and then performing secondary evaporation and concentration, inputting the 50wt% formaldehyde solution into a concentration kettle, and concentrating it to a concentration of 75wt%, and then conveying the concentrated formaldehyde solution to a buffer tank equipped with a stirring and heating device;

[0063] Step A2: 0.1 g of propanol is added to a buffer tank, the temperature in the buffer tank is 90°C, 1 L of concentrated formaldehyde solution is mixed with the inhibitor for 2 hours, and then 0.1 g of ethanolamine is added to the buffer tank, and the mixing is continued for 0.1 hour. The reaction product obtained by mixing is dried in a fluidized bed and cooled to 60°C to obtain a polyformaldehyde product. The temperature of the first stage of fluidized bed drying is 45°C, and after obtaining polyformaldehyde with an aldehyde content of 90%, the second stage is entered, and the temperature of the second stage is 70°C until a polyformaldehyde with an aldehyde content of 92.1% is obtained.

[0064] Performance tests were performed on Examples 5 to 7 and Comparative Example 1, and the test items were as follows:

[0065] 1. Conversion rate: The conversion rate of formaldehyde to paraformaldehyde in the preparation process of paraformaldehyde in Examples 5 to 7 and Comparative Example 1 was detected by gas chromatography, conversion rate (%) = (initial formaldehyde amount - residual formaldehyde amount) / initial formaldehyde amount × 100%. The results are shown in Table 1.

[0066] 2. Purity: The purity of the paraformaldehyde prepared in Examples 5 to 7 and Comparative Example 1 was tested by gas chromatography. The results are shown in Table 2.

[0067] 3. Experiment on repeated use of modified catalyst: The modified catalyst used in Examples 5 to 7 was washed 4 times with methanol, centrifuged, washed, and dried, and then recycled for the preparation process of paraformaldehyde in Example 5. The catalyst was recycled five times to calculate the yield of the product prepared by the catalyst at different numbers of cycles. The yield calculation formula is: Yield (%) = actual mass of target product obtained / theoretical mass of target product obtained × 100%. The results are shown in Table 3.

[0068] Table 1

[0069] project Conversion rate (%) Example 5 90.1 Example 6 92.3 Example 7 95.1 Comparative Example 1 87.6

[0070] It can be seen from Table 1 that the conversion rate of formaldehyde to paraformaldehyde in the paraformaldehyde preparation process of Examples 5 to 7 is higher than that of Comparative Example 1, indicating that the modified catalyst can improve the conversion rate in the paraformaldehyde preparation process.

[0071] Table 2

[0072]

[0073]

[0074] It can be seen from Table 2 that the purity of the paraformaldehyde prepared in Examples 5 to 7 is higher than the purity of the paraformaldehyde prepared in Comparative Example 1, indicating that the modified catalyst is easy to remove.

[0075] Table 3 Yield

[0076]

[0077] It can be seen from Table 3 that after the modified catalysts used in Examples 5 to 7 are recycled 5 times, the modified catalysts still have excellent catalytic performance.

[0078] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0079] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing paraformaldehyde, characterized in that: The following steps are involved: Step A1: Concentrating a 48 wt % formaldehyde solution to a concentration of 75-85 wt % by two-stage evaporation, and then conveying the concentrated formaldehyde solution to a buffer tank; Step A2: Add an inhibitor to a buffer tank with a temperature of 90-110°C, mix the concentrated formaldehyde solution with the inhibitor for 2 hours, then add the modified catalyst to the buffer tank, continue mixing for 0.1-0.5 hours, filter the reaction product, and then enter a fluidized bed drying device for drying and cooling to 55-60°C to obtain a polyformaldehyde product.

2. The method for preparing paraformaldehyde according to claim 1, characterized in that: The polymerization inhibitor is either methanol or propanol.

3. The method for preparing paraformaldehyde according to claim 1, characterized in that: The dosage ratio of the polymerization inhibitor, the concentrated formaldehyde solution and the modified catalyst is 0.1-0.4g:1L:0.1-0.8g.

4. The method for preparing paraformaldehyde according to claim 1, characterized in that: The specific steps of the two-stage evaporation and concentration are: sending a 48wt% formaldehyde solution to a first-stage falling film evaporator, where the formaldehyde solution concentration is 50-60wt%, and then performing a second-stage evaporation and concentration, inputting a 50-60wt% formaldehyde solution into a concentration kettle, and concentrating it to a concentration of 75-85wt%.

5. The method for preparing paraformaldehyde according to claim 1, characterized in that: The temperature of the first stage of fluidized bed drying is 45-70°C, and after obtaining polyformaldehyde with an aldehyde content of 90-91%, it enters the second stage, and the temperature of the second stage is 70-100°C, until obtaining polyformaldehyde with an aldehyde content of 92.1-97.3%.

6. The method for preparing paraformaldehyde according to claim 1, characterized in that: The preparation method of the modified catalyst is: Step S1: dispersing Zn-MOF-74 in DMF and mixing by ultrasonication to obtain a Zn-MOF-74 solution; Step S2: dissolving ethanolamine in water, stirring and dissolving, adding Zn-MOF-74 solution, ultrasonically mixing to obtain a mixture, stirring the mixture, centrifuging, vacuum drying and taking out to obtain a modified catalyst.

7. The method for preparing paraformaldehyde according to claim 6, characterized in that: In step S1, the usage ratio of Zn-MOF-74 to DMF is 9-11 g:1 L.

8. The method for preparing paraformaldehyde according to claim 6, characterized in that: In step S2, the usage ratio of ethanolamine, water, and Zn-MOF-74 solution is 16.5-25 g: 100 mL: 1 L.

9. The method for preparing paraformaldehyde according to claim 6, characterized in that: The preparation method of Zn-MOF-74 is: 2,5-Dihydroxyterephthalic acid and zinc nitrate hexahydrate were dissolved in a DMF aqueous solution and stirred for 1 hour. Then the mixture was transferred to a high-pressure reactor and heated at 100°C for 24 hours. After the reaction, the reactor was placed at room temperature for natural cooling. The product was collected and centrifuged. The obtained solid was immersed in DMF for washing for 24 hours. The DMF was replaced every 12 hours. The solid was then immersed in anhydrous methanol for solvent replacement for 48 hours. The anhydrous methanol was replaced every 12 hours. Finally, the solid was dried for 12 hours to obtain Zn-MOF-74.

10. The method for preparing paraformaldehyde according to claim 9, characterized in that: The mass ratio of 2,5-dihydroxyterephthalic acid to zinc nitrate hexahydrate is 1:4, and the volume ratio of DMF to deionized water in the DMF aqueous solution is 6:0.3.

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