Physico-chemical properties and fuel characteristics of oxymethylene dialkyl ethers
Graphical abstract
Introduction
Even for modern diesel engines, soot formation during the combustion process is still a problem. New emission standards (e.g. Euro VI) define, especially for commercial vehicles, a significant reduction of hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxide (NOx) emissions as well as a decrease of particulate matter (PM) and threshold for the particle number (PN). To meet these high emission standards, sophisticated exhaust gas aftertreatment is needed on a high level of complexity. Due to technical restrictions this comes along with high costs.
To lower the formation of pollutants during combustion, fuel additives can be blended into diesel. Several studies have shown that additives with high oxygen content and without carbon–carbon bonds lead to a reduction of soot formation and lower other harmful emissions (CO, NOx, HC, PM and PN) [1], [2], [3], [4]. Oligomeric oxymethylene dimethyl ethers (OMDME, CH3–(OCH2)n–OCH3) are particularly suitable as soot reducing diesel fuel additives due to their favorable physico-chemical properties [5], [6]. Because of the predicted high flash points and cetane numbers (CN) [7] as well as the matching boiling points [8], OMDMEs with n = 3, 4 and 5 (OMDME 3, OMDME 4 and OMDME 5) are especially preferred as diesel additives. These can be employed with slight engine modifications and nearly no changes in the fuel supply infrastructure. Further studies have shown that OMDMEs also enhance the thermal efficiency and decrease the brake specific fuel consumption by supporting the combustion process with oxygen [2]. Besides the benefit of lower pollutant emissions, OMDMEs can be synthesized on large scale from methanol, which can be obtained from biomass via syngas or CO2 [9].
Oligomeric OMDMEs and the corresponding polymers (polyoxymethylene, POM) are stable acetals, which were first synthesized by Descude [10] from methanol and formaldehyde in 1904 and studied in detail by Staudinger et al. in the 1920s [11], [12]. Due to the fact that formaldehyde is a highly reactive aldehyde, which is prone to polymerize spontaneously, various acidic catalysts can be used for the etherification of the end groups under mild reaction conditions [13], [14]. Unwanted hemiformals are always formed as byproducts, decreasing the yields of OMDMEs significantly. In 1948, Gresham and Brooks [15] reported a new synthesis method for OMDMEs using dimethoxymethane (DMM), instead of methanol, and formaldehyde sources such as monomeric formaldehyde, para-formaldehyde or trioxane. Similar to the previous synthesis from methanol, the oligomerization reaction of DMM can be performed under mild conditions using acidic catalysts (Scheme 1). Due to the absence of methanol and water, the formation of unwanted hemiformals is inhibited and OMDMEs can be obtained with high yields and selectivities in a continuous process [16], [17].
Although there is an extensive interest in OMDMEs with n = 3, 4 and 5 as diesel additives, fundamental and comprehensive studies on the pure components have not been carried out so far because of their poor availability. To fulfill diesel standards, fuel characteristics such as cold filter plugging point (CFPP), autoignition point, flash point, CN as well as lubricity, kinematic viscosity and surface tension must be met. In this work, OMDMEs were synthesized, separated, purified by fractional distillation and examined with respect to their fuel properties. Additionally, the analogous oxymethylene diethyl ethers (OMDEE) were also synthesized (Scheme 1), separated, fractional distilled and characterized. Due to the fact that OMDME and OMDEE are linear compounds similar to n-alkanes, a comprehensive comparison between these three substance classes has been carried out.
Section snippets
Materials and synthesis
The reagents DMM (ReagentPlus, 99.9% GC-Area), diethoxymethane (DEM, 99.9% GC-Area) and trioxane (99.98% GC-Area) were purchased from Sigma Aldrich and dried with CaH2 (Sigma Aldrich) to minimize the formation of hemiformals, formaldehyde and methanol as byproducts. As acidic catalyst, the ion exchanger Amberlyst36 supplied by Rohm&Haas was used. The catalyst was dried at 100 °C under reduced pressure before use. To avoid the formation of waxes consisting of higher oligomers the molar
Characterization of OMDMEs and OMDEEs
OMDME 3–5 and OMDEE 2–4 were synthesized, purified and distilled until the purity of each compound was above 99.9% with respect to GC-FID area. The pure compounds of OMDME and OMDEE were characterized by 1H NMR – as well as FTIR-spectroscopy. Exemplarily, 1H NMR and FTIR spectra of OMDME 4 and OMDEE 4 are shown in Fig. 1, Fig. 2. Spectra of the other OMDMEs and OMDEEs are very similar and therefore not displayed. Additionally, refractive indices were measured (Table 1). Due to similar boiling
Conclusion
Within this work the oxygenates OMDME 3, 4 and 5 as well as OMDEE 2, 3 and 4 were synthesized and purified by multiple fractional distillation. The pure components (GC-Area 99.9+%) were characterized via 1H NMR and FTIR spectroscopy. Furthermore, melting points of OMDEE 2, 3 and 4 were complemented and physico-chemical properties of the oxygenates were compared to the corresponding n-alkanes. In general, boiling points of the different compound classes are very similar, whereas the melting
Acknowledgement
Financial support from the Helmholtz Association is gratefully acknowledged.
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