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

A novel gaseous synthesis route to oxymethylene dimethyl ethers (OMEn, n = 3-5) starting from CO2 and green H2 by using molecular formaldehyde (FA) and dimethyl ether (DME) is presented. The anhydrous reaction runs in a pressure free, gaseous, and continuous reaction setup. Hetero-geneous cata-lysts including zeolites and ion exchange resins (IER) are investigated, if they catalyze this reaction. While IER is almost inactive, zeolites with a 3D pore structure and an acidity exceeding ρm,H+ (NH3,ads ) = 250 µmol·gcat.-1 proved to be catalytically active. DME conversions of up to 2.76 mol-% are observed. The observed product gas stream compositions confirm thermo-dynamic considerations with back reactions / OMEn decomposition occurring as part of the equilibria under the investigated reaction conditions (90…180 °C). However, feed gas ratio variations (FA:DME = 1:2 to 1:9.5) highlighted the possibility to shift the product selectivity in favor of OMEn and suppress FA disproportionation to methyl formate. FA trimerization to trioxane is almost completely suppressed by running the reaction at 120 °C. The results presented here provide an important and unprecedented contribution to understand the complex reaction network in the OMEn synthesis reaction necessary to establish an energy efficient sustainable OMEn production process.