How is trioxane produced from formaldehyde?

Trioxane (TOX), which is usually produced from formaldehyde catalyzed by acid-catalysts, is an important polymerizing monomer to produce high-performance plastics polyoxymethylene polymers (POMs). In order to reduce the separation cost and increase the catalytic activity, hierarchical Beta zeolites with different pore size were synthesized by PDDA and used for TOX produce from formaldehyde in this work. Hierarchical Beta zeolites’ average pore size increased with the increasing of PDDAs’ molecular weight. Compare to conventional Beta zeolite, it exhibited a higher catalytic performance with 95.13% selectivity and 2118 g/kg/h space–time yield of TOX due to its excellent diffusion performance from extra mesopore. The HCHO conversion and space–time yield of TOX increase first and then decrease with the pore size increasing. Further research showed that the decrease of catalytic activity is mainly due to the increase of extra-framework Al percent. This work illustrates the influence of both pore size and extra-framework Al content in hierarchical Beta zeolites for TOX produce from formaldehyde reaction.

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Graphical Abstract

The composite catalyst composed of sulfuric acid and alkyl imidazole-based acidic ionic liquid was used to replace pure sulfuric acid as the catalyst for the synthesis of trioxane. 1-ethyl-3-methylimidazolium hydrosulfate ([EMIM][HSO4]), 1-butyl-3-methylimidazolium hydrosulfate ([BMIM][HSO4]), and 1-hexyl-3-methylimidazole hydrosulfate ([HMIM][HSO4]) were chosen to form composite catalysts with sulfuric acid. The effects of ionic liquid types, formaldehyde concentration (35 ~ 55 wt%), and temperature (353 ~ 368 K) on the reaction were studied. When H2SO4 / [EMIM][HSO4] was used as catalyst with a dosage of 0.6 mol/kg of formaldehyde solution, the optimal catalytic performance was obtained at 373 K with the 50 wt% formaldehyde solution as initial reactant. Compared to that with pure H2SO4 as the catalyst, the trioxane concentration in the reaction system was sharply increased with an increment of 17% by the synergy of ionic liquid [EMIM][HSO4]. The equilibrium constant was increased from 3.08 × 10−5 to 4.19 × 10−5 by the addition of [EMIM][HSO4]. The reaction was intensified by the inhibition of reverse reaction. It indicates that the addition of [EMIM][HSO4] has complicated effects on the reaction. The research results provide a deep understand for the catalytic performance of mineral acid/ionic liquid composite catalysts.

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