(717d) Low Sooting Tendencies and Structure-Property Relationships of Individual Polyoxymethylene Ethers As Alternative Diesel Fuels

Soot has been identified as the second largest source of global warming and it contributes to ambient fine particulates that cause millions of deaths worldwide each year. Diesel fuels have large sooting tendencies and diesel engines produce about two-fifths of soot emissions in the US. Polyoxymethylene ethers (POMEs) are alternative diesel fuels that can be produced from biomass or that can be synthesized from CO₂. Some POMEs have been reported to have low soot emissions from engines and they consisted of methyl-terminated oxymethylene backbones containing alternating oxygen and carbon atoms. Unfortunately, these methyl-POMEs suffer from high water solubility and poor lower heating value (LHV). Replacing the methyl end groups with larger alkyl groups can overcome these disadvantages. However, larger alkyl groups will lead to higher sooting tendencies. To provide a basis for optimizing the trade-off between soot production and other properties, we measured the sooting tendencies of 13 individual POMEs.

The sooting tendencies were characterized by Yield Sooting Index (YSI), which is based on the maximum soot concentration measured in coflow diffusion flames whose fuel is methane doped with a low concentration of each test compound. The figure shows the results in terms of YSI (~soot/mole) and YSI/LHV (~soot/energy). The sooting tendencies of all the POMEs are much lower than a certification diesel fuel (“CF diesel”), which demonstrates that all the POMEs have soot emission benefits. The YSIs of the POMEs decrease with more oxymethylene units (e.g., from M1M to M5M) and increase with more carbon atoms in the end groups (e.g., from M1M to B1B). Reaction pathway analysis indicates that POMEs with more oxymethylene units dissociate to larger amounts of formaldehyde, which dilutes soot formation from the other products, while POMEs with longer end groups dissociate to larger amounts of ethylene, which increases soot production.