Trioxane is widely used in many industrial fields and, for example, as a starting product in the production of polyoxymethylene (POM). POMs have very good physical characteristics, including their high tensile strength and impact resistance, excellent dimensional stability, good electrical insulation characteristics and a wide working temperature range. Currently, POMs are used in many industries, such as the motor vehicle industry, the sports industry and the electronics industry.
Since the POM market is in full expansion, there is therefore a need to produce trioxane, which is a POM precursor, with a high yield and selectivity, in large amounts, as economically as possible and without harming the environment.
Trioxane is usually produced by trimerization of formaldehyde, according to well-known techniques. Generally, trioxane is formed from a more or less concentrated aqueous solution of formaldehyde, in the presence of acid catalyst at high temperatures. The trioxane is then separated from the reaction medium by distillation. The synthesis vapor, which, in addition to the trioxane, still contains formaldehyde, water and reaction by-products, can be rectified in an enrichment column. The trioxane-rich fraction obtained is then subjected to extraction and/or any separation method known to those skilled in the art.
For example, it is possible to directly carry out the extraction of the trioxane after distillation thereof using a water-immiscible extraction agent. The extraction, which makes it possible to recover the trioxane in the extraction agent, is generally followed by distillation of the organic phase, which makes it possible to obtain trioxane which is pure, or at the very least of very high purity.
However, the methods known to those skilled in the art still suffer from numerous drawbacks and the improvements proposed have not actually made it possible to further improve or to optimize the trioxane preparation process.
Document GB949145 describes the preparation of trioxane from formaldehyde in aqueous solution in the presence of strong inorganic acids, for example sulfuric acid, perchloric acid and phosphoric acid, or of strong organic acids, such as aromatic sulfonic acids, particularly benzenesulfonic acid and homologs thereof, for example para-toluenesulfonic acid. Only sulfuric acid is exemplified in said document.
Document US2006/0058537 describes a process for preparing trioxane in the presence of catalysts. The catalysts are strong acids, for example sulfuric acid, trifluoromethanesulfonic acid or toluenesulfonic acid, or very acidic ion exchangers. It is also possible to use acidic zeolithes or heteropolyacids. Again in said document, only sulfuric acid is exemplified.
Document FR1549133 describes a process for preparing trioxane in the presence of acid catalysts. Among these acids, sulfuric acid is preferably used and, for example, phosphoric acid can be used. Other examples of catalysts are acidic salts, such as potassium hydrogen sulfate or zinc chloride, aliphatic and aromatic sulfonic acids, such as p-toluenesulfonic acid or 1,5-naphthalenedisulfonic acid or else acidic ion exchangers, for example commercial cation exchange resins bearing SO3H radicals. Only sulfuric acid is exemplified.
Document WO2013/076286 describes a process for preparing trioxane in the presence of various acid catalysts and in an aprotic solvent. However, a step of preparing formaldehyde in an aprotic solvent lengthens the duration of the process. Furthermore, this additional step requires modifying the already existing industrial equipment. Preferably, the catalysts used are Brønsted acids and Lewis acids. The catalyst is preferably chosen from the group consisting of trifluoromethanesulfonic acid, perchloric acid, methanesulfonic acid, toluenesulfonic acid and sulfuric acid or derivatives thereof, such as the anhydrides or esters, or any other derivative which generates the corresponding acid. Lewis acids such as boron trifluoride or arsenic pentafluoride can also be used. It is also possible to use a mixture of catalysts mentioned above. In the examples, sulfuric acid, trifluoromethanesulfonic acid, perchloric acid and a sulfolane ion exchange resin are used.
Document GB1064013 describes a process for preparing trioxane in the presence of an acidic emulsifier comprising one or more alkylsulfonic or arylalkylsulfonic or alkylarylsulfonic or arylsulfonic acids with one or more sulfonic acid functions. Preferably, the alkyl groups of the alkylsulfonic acids contain from 14 to 16 carbon atoms. Only said C14-C16 alkylsulfonic acids are exemplified.
Various catalysts, inorganic acids and organic acids have been mentioned, such as para-toluenesulfonic acid. However, it has the drawback of being in solid form at ambient temperature, thus requiring a step of dilution in a solvent. Moreover, only sulfuric acid, trifluoromethanesulfonic acid, perchloric acid and a sulfolane ion exchange resin are exemplified.
Although several acid catalysts have been mentioned in the prior art documents, and in particular the prior art documents mentioned above, sulfuric acid is at the current time the acid catalyst most widely used for the preparation of trioxane. Nevertheless, sulfuric acid has the drawback of being corrosive and of generating by-products which result in a significant loss of trioxane production yield. Indeed, the principal side reactions involved are:                the dismutation of formaldehyde (CH2O) in the presence of water (H2O) to give formic acid (HCOOH) and methanol (CH3OH), (Cannizzaro reaction), according to the following scheme:        
and                the following equilibrated reaction, resulting in the formation of methyl formate (HCOOCH3) from the formic acid and methanol previously formed:        

It is also possible to observe the following side reaction which generates methanol and carbon dioxide (CO2):
and also, in an acidic medium, the formation of glycolic acid (HOCH2COOH):
