Synthesis of Methylal and Poly(oxymethylene) Dimethyl Ethers from Dimethyl Ether and Trioxane

Abstract

Poly(oxymethylene) dimethyl ethers (OME) are interesting synthetic fuels that could replace fossil diesel fuel. Therefore, economic routes for OME production have to be developed. One particularly interesting route is the synthesis of OME from dimethyl ether (DME) and formaldehyde. The principal feasibility of this route is established, but the physicochemical information on essential steps is still lacking. In particular, there is no data on the first step in this synthesis, which is the formation of methylal (MAL) from DME and formaldehyde. Kinetic batch experiments were carried out in a stirred batch autoclave with a commercially available acidic ion-exchange resin as a catalyst in the temperature range of 353−373 K for up to 200 h. Trioxane was used as a water-free source of formaldehyde. Due to the volatility of DME, the experiments were carried out under pressure; high-pressure magnetic resonance (NMR) spectroscopy was applied for the analysis. During the reaction, not only MAL but also OME are formed, as well as a side product, methyl formate (MeFo). Therefore, the equilibrium constant of the MAL formation had to be determined based on a reaction kinetic model of the entire reaction network. The formation of MAL was found to have a larger equilibrium constant than the subsequent oligomerization reactions leading to OME, but it is also much slower than these reactions.

Continuous Anhydrous Synthesis of Oxymethylene Dimethyl Ethers by Reaction of Dimethyl Ether with Molecular Formaldehyde

“…Kinetic investigations also revealed that the oxymethylation from OME 1 to OME n proceeds much faster than the formation of OME 1 and MeFo. [ 7 ] However, the formation of OME n>1 was not observed to any greater extent even under kinetically more relevant reaction conditions (higher WHSV): Hence, the contact time of the feed gas with the catalyst was also varied by the addition of chemically inert and inactive silicon carbide (SiC) diluent. The zeolite H‐BEA‐25 (300 mg) was homogeneously mixed with SiC (1500 mg) resulting in a significantly longer catalyst bed.…”

“…[17] MeFo forms by disproportionation of two FA molecules (Tishchenko reaction; Table 1, Equation 3), [8,18,19] or through a recently reported irreversible OME 2 cleavage mechanism (Table 1, Equation 5). [7] From a thermodynamic perspective, both mechanisms are highly favored, presenting significant competitive reactions to the desired OME 3-5 formation. MeFo-formation is often described to be irreversible.…”

“…Whereas anhydrous synthesis routes starting from TRI and OME 1 are well-investigated, [6] the use of DME as a chain terminating agent is only very scarcely reported. [7][8][9][10] Hence, the reaction of DME with TRI was catalyzed by zeolites and ion exchange resins (IER). An initially investigated batch process [10] was transferred into a continuous, condensed-phase process.…”

A novel process towards the industrial realization of large-scale oxymethylene dimethyl ether production – COMET

“…Using DME as a feedstock for the OME synthesis, OME 1 is formed by the incorporation of FA into DME, as described by eqn (10).Furthermore, with DME and TRI as a feedstock for the OME synthesis, OME 3 is formed by direct incorporation of TRI into DME, as described by eqn (11). 29–31…”

“…Furthermore, with DME and TRI as a feedstock for the OME synthesis, OME 3 is formed by direct incorporation of TRI into DME, as described by eqn (11). [29][30][31] In addition to the main reaction network, side reactions lead to the formation of side products. Besides HF n , MG n , TRI and DME, the formation of tetroxane ((CH 2 O) 4 ), methyl formate (HCOOCH 3 , MEFO) and formic acid (HCOOH, FOAC) was observed, which strongly depends on the catalyst systems and increases with increasing temperatures.…”