The Way You Make It

The way that OMEs and related fuel substitutes are made is very important as regards cost as well as atom economy.

In what follows, I have drawn from this research article :-

“Production of oxymethylene dimethyl ether (OME)-hydrocarbon fuel blends in a one-step synthesis/extraction procedure”, by Dorian Oestreich, Ludger Lautenschütz, Ulrich Arnold and Jörg Sauer, in Fuel, 214, 2018, p 39-44, DOI : http://dx.doi.org/10.1016/j.fuel.2017.10.116

In general, the authors comment that, “[…] an enormous interest in oligomeric oxymethylene dimethyl ethers (OMEs, CH3O-(CH2O)n-CH3, n=1–5) awakened and activities in this research field extremely increased in recent years. OMEs are related to DME dimethyl ethern-CH3, n=0) and exhibit an enormous potential for the reduction of soot and NOx emissions. Due to their high oxygen content and the absence of carbon-carbon bonds in the molecular structure, formation of pollutants is suppressed during combustion. Thus, strict exhaust emission standards can be met and exhaust gas treatment can be simplified. Properties of OMEs strongly depend on the chain length and OMEs with n=3–5 exhibit physicochemical as well as fuel properties similar to conventional diesel. Therefore, no serious changes of the fuel supply infrastructure and engines are necessary. Further advantages are their good miscibility with established fuels, low corrosivity as well as favorable health- and safety-related properties.”

These authors point out the predominance of methanol as a chemical feedstock, as written extensively about by George Olah – “Forget about the hydrogen economy. Methanol is the key to weaning the world off oil. George Olah tells us how to do it.”

They also point out that OMEs produced via renewable resources addresses climate change in addition to air pollution : “Regarding the production of OMEs and other fuel-related ethers, many strategies are based on methanol. Methanol is produced from synthesis gas, which is usually stemming from fossil resources, especially natural gas. Synthesis gas can also be obtained from renewable resources via different pretreatment technologies, depending on the feedstock type, and subsequent gasification. If renewable feedstocks are employed for OME synthesis, not only soot and NOx emissions can be reduced but also total CO2 emissions considering the entire system from feedstocks to combustion.”

But these wonderfuels don’t necessarily come easy – especially where routes include reagents that have a complicated synthesis of their own, such as trioxane (1,3,5 trioxane, a cyclic trimer of formadehyde) : “[…] a highly optimized and efficient production of OMEs is still a major challenge. Thus, availability is restricted at present and sufficient quantities of OMEs for intense testing can only be purchased from a few Chinese suppliers. However, capacities of Chinese plants are currently exceeding 40,000 tons per year and activities in the field of OMEs are rapidly developing there. Production of OMEs can be carried out employing different educts like methanol, DME, dimethoxymethane (OME 1) and formaldehyde sources like formalin, p-formaldehyde, or trioxane. Different homogeneous and heterogeneous acidic catalysts such as sulfuric acid, zeolites, ion exchange resins, metal-oxides or heteropoly acids are typically used.” (What do you know ? China is ahead of the curve again.)

There is a general divergence of choice in processing : “[…]A distinction can be drawn between OME synthesis in aqueous reaction systems (e.g. reaction of methanol with formalin or p-formaldehyde) and synthesis in anhydrous systems (e.g. reaction of dimethoxymethane with trioxane). In aqueous systems significant amounts of water, hemiformals and glycols are formed as by-products. In contrast, formation of such byproducts is largely suppressed in anhydrous systems. However, the use of aqueous systems, especially the reaction of methanol with formaldehyde, is highly desired since low-cost educts can be employed.”

The writers of this paper elucidate clearly the problems posed by dealing with controlling the chemical equilibrium in the production of OMEs and similar molecules; and then they introduce their contribution, “[…] a convenient one-step procedure for the production of OME-hydrocarbon blends is proposed. Selective extraction of OMEs from aqueous reaction solutions is described employing hydrocarbons such as n-dodecane, diesel and hydrogenated vegetable oil (HVO) as extraction agents. The corresponding oxymethylene diethyl ethers (OMDEEs) have also been synthesized and investigated.”

This use of straight chain hydrocarbons to “wash” or extract the OMEs is not completely described, neither the recycling of by-products – especially as one of the by-products of the aqueous process is trioxane, which could be used in a later anhydrous OME production process. However, I think I grasp enough of this to see that there could be a fairly strong atom economy – so high selectivity for the desired products, and not large percentages of rejected “waste” molecules that need to be ultimately disposed of. This is important because it shows that chemical synthesis of liquid fuels from base, simple molecules can be more efficient in terms of making atoms useful, compared to chemical processes using whole biological complexes – for example, the use of lignocellulose (lignin, cellulose and hemicellulose) in wood.

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