Process for production of trioxane

#The present invention relates to a process for producing trioxane by cyclizing formaldehyde to convert it into a cyclic trimer. More particularly, the present invention relates to a process for producing trioxane by contacting an aqueous formaldehyde solution with an acidic catalyst, wherein the concentrations of methanol and formic acid present in the reaction system are each controlled and wherein the amounts of by-products formed are small.

#Trioxane has been produced by cyclizing formaldehyde in its aqueous solution in the presence of an acidic catalyst such as sulfuric acid, cation exchange resin having a sulfonic acid group, zeolite or the like.

#These conventional processes for trioxane production have not been satisfactory because of various problems such as corrosion of production equipment, precipitation of paraformaldehyde at high formaldehyde concentration, low trioxane selectivity and the like. For example, when a sulfuric acid catalyst is used, there is a problem of equipment corrosion by strong acid and it is necessary to use an expensive material(s) for equipment to prevent the problem. When a cation exchange resin having a sulfonic acid group is used as a catalyst, there are problems such as low trioxane selectivity, paraformaldehyde precipitation and consequent pressure loss of catalyst layer, and the like. Meanwhile, it is well known that increased methanol concentration in reaction system is effective for the prevention of paraformaldehyde precipitation in aqueous formaldehyde solution. Increased methanol concentration in reaction system, however, invites formation of larger amounts of by-products such as methyl formate, dioxymethylene dimethoxide, trioxymethylene dimethoxide and the like, which is undesirable. JP-A-59-134789 discloses that trioxane selectivity is higher when a macroreticular cation exchange resin having a sulfonic acid group is used and the concentration of formic acid is controlled at 0.025-15.0 wt. %. The concentration of formic acid in this literature, however, refers to a formic acid concentration in raw materials mixture fed into reactor, i.e. a formic acid concentration before start of reaction and is not a formic acid concentration in reaction system during reaction. Incidentally, too high a formic acid concentration causes equipment corrosion by formic acid.

#The present invention is hereinafter described in more detail by Examples and Comparative Examples.

#An aqueous formaldehyde solution 1 as raw material 1 consisting of 65 wt. % of formaldehyde, 3 wt. % of methanol and the remainder of water was fed into an intermediate tray (15th tray) of a distillation tower having 30 bubble cap trays. An aqueous formaldehyde solution 2 as raw material 2 having the same composition as the raw material 1 was fed into the bottom of the distillation tower. They were fed at a weight ratio of the raw material 1/the raw material 2 being 8/2. In a circulation line for tower bottom solution was provided a fixed catalyst bed, a reaction layer, filled with 10 l of a cation exchange resin (Amberlite 200C, a product of Organo Co.), and the bottom solution was circulated through the circulation line at a rate of 500 l/hr. While the total feed rate of the raw material formaldehyde solutions were being controlled to keep the bottom solution at a given level, the bottom solution was heated by a heater to conduct synthesis of trioxane; distillation was conducted at a reflux ratio of 2.0 to take out a distillate at a rate of 10 kg/hr, and trioxane was obtained. The bottom solution and the distillate were analyzed by gas chromatography to measure their compositions. In the circulation line of the bottom was provided a SUS 304 test piece, and its corrosion after continuous 10-day operation was examined visually.

#The results are shown in Table 1. There was neither precipitation of paraformaldehyde nor corrosion of test piece, and the total amount of dioxymethylene dimethoxide and trioxymethylene dimethoxide formed as by-products was 0.77 wt. % based on formed trioxane.