Oxymethylene ethers (OMEs) have been studied as possible
additives
or replacements for diesel fuels. Typically, studies have considered
only methyl-terminated OMEs. Recent structure–property relationship
models suggest that extended-alkyl OMEs may provide improvements to
many of the properties of methyl-terminated OMEs that make them less
suitable as diesel fuel blendstocks. In this work, we describe the
synthesis and characterization of 16 different OMEs with methyl-,
ethyl-, propyl-, butyl-, isopropyl-, and isobutyl-terminating alkyl
groups with varying oxymethylene chain lengths. Indicated cetane number,
lower heating value, flash point, density, viscosity, vapor pressure,
and oxidative stability are tested via ASTM standard methods. Additionally,
water solubility, boiling point, seal material compatibility, and
sooting propensity (via the yield sooting index) are measured for
these fuels. For diesel compatibility, all tested OMEs except smaller
methyl and ethyl OMEs and the branched isopropyl OME meet cetane number
requirements. Extending the alkyl end group increases the heating
value, but all OMEs, due to their oxygen content, have heating values
less than diesel. Despite this, all OMEs show significant reductions
in soot production per unit heating value. Only the heaviest OMEs
meet diesel viscosity requirements, and most are higher density than
diesel. OMEs with larger alkyl groups show the highest stability under
accelerated auto-oxidation conditions. Increases in alkyl group length
cause order of magnitude reduction in water solubility, from hundreds
of g/L for methyl terminated OMEs to hundreds of mg/L for butyl terminated
OMEs. Limited seal material testing indicates that PEEK polymers are
unaffected by OMEs, and while extended alkyl groups may improve compatibility
with FKM (Viton), other common elastomers (NBR, silicone) remain incompatible
with all tested OMEs. Overall, it is found that methyl-terminated
OMEs exhibit the most potential for soot reduction, but OMEs with
larger propyl- and butyl-terminating alkyl groups show improved compatibility
with existing diesel systems.
