An efficient, biphasic route towards oxymethylene dimethyl ethers (OMEs) allowing for catalyst recycling and reuse is presented (OMEs=CH 3 (À OCH 2 À ) n OÀ CH 3 , n = 3-5). OMEs are an interesting novel class of non-toxic, diesel-like synthetic fuels with soot-free combustion properties. A solution of commercial OMe 3 + BF 4 À in the ionic liquid (IL) 1-ethyl-3-methylimadazolium tetrafluoroborate acts as the immobilized catalyst. Upon addition of dimethoxymethane (OME 1 ) and anhydrous formaldehyde (FA) very pure OMEs form in the upper phase of the biphasic mixture. In the lower IL-phase, the catalyst remains immobilized. After phase separation and removal of the top OME layer, the catalytically active IL-phase is reusable for at least ten times without loss of activity and selectivity.Climate change and air pollution are two of the most critical health and sustainability challenges facing society today. They are also closely related: the major sources of CO 2 emissions are the most significant sources of air pollution. So it would seem only logical to seek joint solutions to these two problems. [1] Amongst others, the mobility sector makes a huge contribution to CO 2 emission (ca. 23 %). [2] This sector could be reduced drastically by substituting conventional fossil-based fuels by sustainable synthetic fuels, which are produced on the basis of CO 2 and H 2 . In this context oxymethylene dimethyl ethers (OMEs) with the general formula CH 3 (À OCH 2 À ) n OÀ CH 3 (n = 3-5) have diesel-like properties, are suited for the substitution of diesel and are classified as nontoxic. [3][4][5] Furthermore, based on the lack of covalent CÀ C-bonds and the high oxygen content, OMEs have soot-free combustion properties and can reduce the emission of nitrogen oxides (NO x ) significantly. [4,6] In addition, Hank et al. elaborated in a well-to-wheel analysis a potential reduction of CO 2 emissions of up to 86 % by using OMEs instead of diesel fuel. [7] However, the diesel consumption is enormous (e. g. 38.7 Mt in Germany in 2017), [8] whereas the largest OMEproduction plants produce 400 kt a À 1 and an optimized sustainable and technically viable OME production route is still lacking. [9] Conventional OME-syntheses are differentiated into aqueous and anhydrous syntheses. The aqueous route starts from methanol (MeOH) and aqueous formaldehyde (FA) solution, whereas anhydrous synthesis routes typically start from a FA-source (e. g. 1,3,5-trioxane, TRI; para-FA) and dimethoxymethane (OME 1 ) or dimethyl ether (DME). [10,11,12] The easily available educts MeOH and aqueous FA seemingly favor the aqueous route. However, the presence of water produces a large amount of side products (ca. 65 %), the separation of which requires about a third of the OME 3-5 energy content. [9,13] Instead, the avoidance of water in the anhydrous synthesis route provides relatively pure OME-mixtures. Yet, conventional anhydrous syntheses require the use of very energy-intensively synthesized anhydrous TRI, [11,14,15] largely increasing the CO 2footprint o...