(21c) Reaction Kinetics of the Formation of Poly(oxymethylene) Dimethyl Ethers from Formaldehyde and Methanol in Aqueous Solutions

Poly(oxymethylene) dimethyl ethers (OMEs) are oxygenates of the chemical structure H₃C-O-(CH₂O)_n-CH₃ with n ≥ 2. OMEs are environmentally benign diesel fuel additives, which drastically reduce the soot formation during the combustion process. [1] In addition, OMEs can be beneficially employed as physical solvents for the absorption of carbon dioxide. [2] All the synthesis routes for the production of the OMEs start from methanol, which in turn is produced from synthesis gas. Hence, they open a route for the production of liquid fuel on the basis of alternative feedstocks, such as shale gas or biogas. The educts for the state-of-the-art process for the production of OMEs are trioxane and methylal, [3] which are produced in additional steps from methanol via the formaldehyde route. A direct synthesis of the OMEs from formaldehyde and methanol is a short-cut on the value added chain and is thus highly desirable. Liquid mixtures of formaldehyde and methanol/water are complex reacting systems in which formaldehyde is almost entirely bound in the oligomers poly(oxymethylene) hemiformals (structure: HO-(CH₂O)_n-CH₃) and poly(oxymethylene) glycols (structure: HO-(CH₂O)_n-H). The formation of OMEs in these mixtures occurs only under acidic conditions. [4] In the present work, the reaction kinetics of this formation is studied employing the ion exchange resin Amberlyst 46 as heterogeneous acidic catalyst. The experiments are conducted in a stirred batch reactor at different temperatures (303.15 K, 333.15 K, 363.15 K) and varying ratios of formaldehyde to methanol and varying water-contents. The measurements indicate that the OMEs of various chain lengths are formed rather by an etherification of poly(oxymethylene) hemiformals (n ≥ 2) and methanol than by the sequential addition of monomeric formaldehyde into OMEs of shorter length. Based on this assumption, a Langmuir-Hinshelwood-Hougen-Watson model is adjusted to the experimental data. The model is consistent to the model of the chemical equilibrium [5] and describes the experimental composition profiles with good accuracy. The results of this work enable a reliable reactor design for the OME synthesis from formaldehyde and methanol in the presence of water.

References

[1] Lumpp, B.; Rothe, D.; Pastötter, C.; Lämmermann, R.; Jacob, E. Oxymethylene ethers as diesel fuel additives of the future. MTZ 2011, 72, 34–38.                                  

[2] Burger, J.; Papaioannou, V.; Gopinath, S.; Jackson, G.; Galindo, A.; Adjiman, C.S. A hierarchical method to integrated solvent and process design of physical CO2 absorption using the SAFT-γ mie approach. AIChE J. In press.(2015) doi:10.1002/aic.14838

[3] Burger, J.; Ströfer, E.; Hasse, H. Production process for diesel fuel components poly(oxymethylene) dimethyl ethers from methane-based products by hierarchical optimization with varying model depth. Chem. Eng. Res. Des. 91(2013) 2648–2662.

[4] Drunsel, J.-O.; Renner, M.; Hasse, H. Experimental study and model of reaction kinetics of heterogeneously catalyzed methylal synthesis. Chem. Eng. Res. Des. 2012, 90, 696–703.

[5] Schmitz, N.; Homberg, F.; Berje, J.; Burger, J., Hasse, H.: Chemical Equilibrium of the Synthesis of Poly(oxymethylene) Dimethyl Ethers from Formaldehyde and Methanol in Aqueous Solutions. Ind. Eng. Chem. Res. submitted (2015).