JPS6337110B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a novel method for synthesizing trioxane, and more particularly to a method for converting formaldehyde in a gas phase to trioxane by contacting it with a heteropolyacid supported on a carrier. Trioxane is a cyclic trimer of formaldehyde and is mainly used as a starting material for polyoxymethylene. Many studies have been conducted on methods for synthesizing trioxane. The most conventional synthesis method is to heat formaldehyde in a liquid phase using an acidic catalyst such as sulfuric acid or phosphoric acid. For the synthesis of trioxane in liquid phase (1)
Problems include an increase in byproducts such as formic acid, methanol, and methyl formate due to side reactions, (2) precipitation of paraform, and (3) corrosion of materials due to acidic catalysts. On the other hand, Japanese Patent Publication No. 40-12898 discloses a method for producing trioxane using a metal oxide as a solid acid catalyst. Furthermore, Japanese Patent Publication No. 40-20552 discloses a method for synthesizing trioxane using a metal sulfate and a cation exchange resin as a solid acid catalyst. In these methods using solid acid catalysts, side reactions frequently occur, and the amount of byproducts contained in the obtained trioxane is significantly large, making it difficult to obtain trioxane with high purity. In order to obtain polyoxymethylene with sufficient molecular weight,
The trioxane to be used as starting material is
It needs to be extremely pure, and these methods cannot meet this requirement. Japanese Patent Publication No. 44-30735 describes bringing gaseous monomeric formaldehyde into contact with sulfuric acid or phosphoric acid held in silica gel. This method has the disadvantage that paraform precipitates when the water content in the gaseous formaldehyde exceeds 20%, which limits the formaldehyde raw material that can be used. That is, in this method, it is impossible to use formalin, which is normally used as a formaldehyde source, as a raw material, and expensive raw materials such as α-polyoxymethylene must be used. As a result of extensive studies on methods for synthesizing trioxane, the present inventors discovered that formaldehyde in the gas phase can be
The present inventors have discovered that trioxane of good purity can be synthesized in high yield by contacting it with a heteropolyacid supported on a carrier, leading to the completion of the present invention. That is, the present invention is a method for producing trioxane in which formaldehyde in a gas phase is brought into contact with a heteropolyacid supported on a carrier. In the present invention, a heteropolyacid supported on a carrier is used as a trioxane synthesis catalyst. Heteropolyacid is a heteronuclear condensed acid represented by the general formula Hn[MxM′ _y Oz]·mH ₂ O. Here, M represents the central element and M' represents the coordination element. Also, x is 0.1 to 10,
y is 6 to 30, z is 10 to 80, n is a number greater than 1,
m indicates a range of 0 to 40. The central elements represented by M are P, B, Si, Ge,
Sn, As, Sb, u, Mn, Re, Cu, Ni, Co, Fe,
Selected from the group consisting of Ce, Th, and Cr. As the central element, P, Si, B, Ge, Cu, and Sn are preferable, and P and Si are particularly preferable. On the other hand, the coordination elements represented by M′ are W, Mo,
It is selected from the group consisting of V and Nb, with W, Mo and V being particularly preferred. Among these heteropolyacids consisting of a combination of a central element and a coordination element, silicotungstic acid, silicomolybdic acid,
Particularly preferred are phosphotungstic acid and phosphomolybdic acid. One type of heteropolyacid can be used alone, or two or more types can be used in combination. Further, heteropolyacids whose central element is composed of two or more types of elements can also be effectively used in the present invention. In the present invention, the carrier supporting the heteropolyacid includes inorganic carriers such as silica, alumina, diatomaceous earth, activated carbon, zeolite, kaolin, titanium oxide, zirconium oxide, and activated clay, polyoxymethylene, polyamide, and polyphenylene sulfide. , polyphenylene oxide, polystyrene, ABS resin, and other organic carriers. The supporting rate, that is, the weight (parts) of the heteropolyacid excluding crystal water supported per 100 parts by weight of the carrier, is:
It is between 0.1 and 400. If the loading rate is too low, the conversion rate of formaldehyde to trioxane will be low, and if the loading rate is too high, side reaction products will increase. The loading rate should be determined experimentally in balance with the contact temperature and contact time. In the present invention, formaldehyde in a gas phase is used as a raw material for trioxane synthesis.
The concentration of formaldehyde to be contacted with the heteropolyacid ranges from 10 to 100%, with a more preferred concentration ranging from 30 to 100%. If the formaldehyde concentration is too low, the conversion to trioxane will be low. On the other hand, from the viewpoint of conversion rate, it is preferable to use a high concentration of formaldehyde;
High concentrations of formaldehyde include paraform, α-
Since it is obtained from an expensive substance such as polyoxymethylene, it is not necessarily industrially advantageous to use formaldehyde at a high concentration. The most industrially advantageous synthetic raw material is formalin (formaldehyde concentration 30-70%) heated and vaporized. In the present invention, this vaporized formalin can be efficiently used. Substances other than formaldehyde, such as formaldehyde in a gas phase containing a high concentration of compounds such as water and methanol, and inert gases such as nitrogen and carbon dioxide, can also be used in the present invention without any problems. It is. In the present invention, formaldehyde is brought into contact with a heteropolyacid supported on a carrier in a gaseous state. During contact, it is necessary to maintain the raw material formaldehyde and the reaction product in a gaseous state, and the lower limit of the contact temperature is determined from this point of view. Furthermore, as the contact temperature is increased, the conversion rate of formaldehyde to trioxane decreases, and at the same time, side reactions tend to increase. Therefore, too high a contact temperature should not be selected. From this point of view, the contact temperature is usually 50 to 200℃,
More preferably, the temperature is set between 70 and 150°C. Further, the contact time is set between 0.5 and 600 seconds. If the contact time is too short, the conversion of formaldehyde to trioxane will not proceed sufficiently. On the other hand, if the contact time is too long, side reaction products will increase. Optimal contact times can be easily determined by experiment. Next, an example of an embodiment for implementing the present invention industrially is as follows. A gas containing formaldehyde that has been heated and vaporized in advance is passed through a catalyst layer made of a heteropolyacid supported on a carrier, which is filled in a reaction tower as a fixed bed. The formaldehyde gas may flow either from top to bottom or from bottom to top. Further, the temperature of the catalyst layer is adjusted to a predetermined temperature by passing steam or cooling water through the jacket. The reaction gas containing trioxane that has exited the reaction tower is then led to a distillation tower where trioxane is concentrated. Alternatively, the reaction gas exiting the reaction tower is subjected to a partial condensation operation to partially condense a portion rich in trioxane. The trioxane thus concentrated is then extracted with an organic solvent and then purified. The features of the present invention described in detail above are as follows. (1) Few side reaction products and high purity trioxane When synthesizing trioxane using conventionally known acidic catalysts, compounds such as methanol, formic acid, methyl formate, and sugars are produced through side reactions. generate,
The purity of trioxane was reduced. In the method of the present invention, the amount of these compounds produced is small, and the purity of trioxane is high. The trioxane of the present invention can be easily purified into highly purified trioxane. (2) No precipitation of solid substances such as paraform (3) No corrosion of materials When the method of the present invention is used, there is virtually no corrosion of materials in the trioxane synthesis equipment. If acidic catalysts known prior to the present invention, such as sulfuric acid, are used in the reaction, the material is subject to significant corrosion.
However, in the method of the present invention, no corrosion of the material occurs and long-term stable operation of the trioxane synthesis apparatus is possible. The present invention will be explained below with reference to Examples, but the present invention is not limited to these Examples. Example 1 (1) Catalyst preparation Phosphortungstic acid H ₃ [PW ₁₂ O ₄₀ ]・29H ₂ O
50Wt% aqueous solution 135gr, particle size 20~80mesh
(Tyler mesh) dried silica gel
Add 100gr and stir gently for 15 minutes at 80℃.
Then, the stirring is stopped and the silica gel and the liquid are separated. The separated silica gel is dried at 120° C. for 5 hours under a vacuum of 10 mmHg. The supporting ratio of phosphotungstic acid in the catalyst thus prepared was 46.7 (phosphotungstic acid - parts by weight/silica - 100 parts by weight) according to fluorescent X-ray analysis. (2) Trioxane synthesis Fill a stainless steel tube (made of Sus 304) with an inner diameter of 2.5 cm and a jacket with 100 c.c. of the catalyst prepared in (1). Formalin vapor having the following composition was continuously supplied from above the stainless steel tube at a rate of 45 gr/H for 1500 hours. Formaldehyde 55.3 wt% Water 43.9 wt% Methanol 0.8 wt% Formic acid 490 ppm Methyl formate 83 ppm The temperature of the catalyst layer filled in the stainless steel tube is maintained at 106°C by passing a heat medium through the jacket. When the reaction gas coming out from the bottom of the stainless steel tube was sampled and analyzed at predetermined time intervals, it had the following composition.

[Table] * Elapsed time from the start of formalin vapor supply As is clear from the above table, the conversion rate of formaldehyde to trioxane is high. Also methanol,
The increase rate of formic acid and methyl formate was very low, and trioxane of high purity was obtained. The reaction stopped after 1500 hours,
The catalyst layer was observed, but no precipitation of solid substances such as paraform was observed. Further, as a result of microscopic structure observation of the inner surface of the stainless steel tube, no abnormality was found. Examples 2 to 10 (1) Preparation of catalyst To 200 gr of a 45 wt% aqueous solution of the heteropolyacid shown in Table 1, 150 gr of the carrier shown in Table 1 whose particle size was adjusted to 20 to 80 mesh was added, and the mixture was heated at 95°C for 30 min. Stir for a minute. After separating the carrier and the liquid, the carrier was vacuum dried. For each of the catalysts prepared in this way, the supported ratio of heteropolyacid was determined using fluorescent X-rays, and the results are shown in Table 1. (2) Trioxane synthesis 120 c.c. of the catalyst prepared in (1) was transferred to a stainless steel tube with an inner diameter of 1 inch (Sus316
(manufactured by). From the bottom of this stainless steel tube, 65 gr/g of formalin vapor having the following composition is poured into
H was continuously supplied for 500 hours. During this time, the temperature of the catalyst layer was maintained at 108°C. The reaction gas coming out from the top of the stainless steel tube was analyzed, and the results are also shown in Table 1. Formalin vapor composition Formaldehyde (wt%) 62.5 wt% Water (wt%) 36.8 wt% Methanol (wt%) 0.7 wt% Formic acid (ppm) 390 ppm Methyl formate (ppm) 90 ppm Side reactions Trioxane with low product content has been efficiently synthesized. Further, in all of the examples, no precipitation of solid precipitates or corrosion was observed. Comparative Example 1 (1) Preparation of solid acid catalyst (method disclosed in Japanese Patent Publication No. 44-30735). Silica gel 100gr with particle size 20~80mesh
200 gr of a 5% aqueous sulfuric acid solution is added thereto, then evaporated with water and finally dried at 150° C. for 5 hours under a pressure of 5 mmHg. The sulfuric acid loading ratio of the catalyst obtained by this method was 10 (sulfuric acid - parts by weight/silica gel - 100 parts by weight). (2) Trioxane synthesis Instead of the catalyst used in Example 1, the above
The same operations as in Example 1 were repeated using all the same reagents and equipment as in Example 1, except that the catalyst prepared in (1) was used. The results are also shown in Table 1. It is clear from Table 1 that when a sulfuric acid catalyst is used, a large amount of methanol, formic acid, and methyl formate are produced. Furthermore, precipitation of paraform and sugary solids was observed in the catalyst layer after 1500 hours. In addition, corrosion of the stainless steel tube caused the catalyst layer to become Fe,
It was contaminated with Cr and Ni. Comparative Examples 2 to 4 All the same as Example 1 except that the solid acid catalyst shown in Table 1 was used instead of the catalyst used in Example 1.
The same operations as in Example 1 were repeated using the same reagents and equipment. The results are also shown in Table 1. In all comparative examples, a large amount of byproducts were generated, and solid precipitates were observed in the catalyst layer. Furthermore, activated clay and silica gel-alumina mixtures have low catalytic activity and cannot sufficiently convert formaldehyde to trioxane. Examples 11-15 (1) Preparation of catalyst Tungstic silicoic acid H ₄ [Siw ₁₂ O ₄₀ ]・30H ₂ O
60wt% aqueous solution 1250gr, particle size 20~80mesh
Acid-resistant zeolite adjusted to
After stirring at 90°C for 30 minutes, the carrier and the liquid were separated. The carrier was heated at 180°C under reduced pressure for 10 hours to evaporate water. The loading rate of silicotungstic acid determined using fluorescent X-rays was 40%. (2) Trioxane synthesis 150 c.c. of the catalyst prepared in (1) was transferred to a stainless steel tube (Sus304) with an inner diameter of 1 inch and a jacket.
(manufactured by). Formaldehyde vapor having the composition shown in Table 2 was supplied from above the stainless steel tube at a rate of 105 gr/H. During this time, the temperature of the catalyst layer was maintained at 118°C. The reaction gas coming out from the top of the stainless steel tube was analyzed and the results are shown in Table 2. In all Examples, trioxane with a low content of side reaction products was obtained.

【table】

[Table] Examples 16 to 29 (1) To 200g of a 50wt% aqueous solution of the heteropolyacid shown in Table 3, 150g of the carrier shown in Table 3 whose particle size was adjusted to 40 to 80mesh was added, and the mixture was heated at 95°C for 30 minutes. Stir for a minute. After separating the carrier and the liquid, the carrier was vacuum dried. Using fluorescent X-rays for each of the catalysts prepared in this manner, the supported ratio of heteropolyacid was determined, and the results are shown in Table 3. (2) Trioxane synthesis 120 c.c. of the catalyst prepared in (1) above was transferred to a stainless steel tube with an inner diameter of 1 inch (DP
-3: Made of two-layer alloy). Formalin vapor having the following composition was continuously supplied from above the stainless steel tube at a rate of 108 gr/H for 800 hours. During this time, the temperature of the catalyst layer was maintained at 118°C. The reaction gas coming out from the bottom of the stainless steel tube was analyzed, and the results are also shown in Table 3. Formalin vapor composition Formaldehyde (wt%) 98.7 Water (wt%) 1.2 Methanol (wt%) 0.05 Formic acid (ppm) 75 Methyl formate (ppm) 30 In all examples, trioxane with a low content of side reaction products was used. Synthesized efficiently. Furthermore, no solid precipitates or corrosion were found.

【table】

【table】

Claims (1)

[Scope of Claims] 1. A method for vapor phase synthesis of trioxane, which comprises bringing formaldehyde in a vapor phase into contact with a heteropolyacid supported on a carrier. 2. The synthesis method according to claim 1, wherein the heteropolyacid is a heteropolyacid containing P and/or Si as a central element. 3. The synthesis method according to claim 1, wherein the heteropolyacid has a metal selected from the group consisting of W, Mo, and V as a coordination element. 4. The synthesis method according to claim 1, wherein the heteropolyacid is silicotungstic acid, silicomolybdic acid, phosphotungstic acid, or phosphomolybdic acid alone or in a mixture thereof. 5. The synthesis method according to claim 1, wherein the carrier on which the heteropolyacid is to be supported is an inorganic carrier. 6. The synthesis method according to claim 1, wherein the carrier on which the heteropolyacid is to be supported is an organic carrier.