Two methods are commonly used to produce biofuels from biomass. The first method is fermenting sugars with yeasts or bacteria to produce ethanol, butanol, acetic acid, and other products. Sugars used in this process can be derived from corn, sugar cane, or wood (lignocellulosic biomass). The second method requires a thermochemical conversion process that converts biomass to a synthesis gas (or syngas) composed primarily of hydrogen (H₂) and carbon monoxide (CO). Using the Fischer−Tropsch (F−T) process followed by an isomerization process, syngas can be synthesized to a straight-chain hydrocarbon with a distribution of chain lengths.

Competing reactions in the F−T process can be used to produce a mixture of straight-chain alcohols (R−OH), which generally have high octane numbers. These high-octane mixed alcohols can be used directly as a transportation fuel in spark-ignited combustion engines or as an antiknock blending component in gasoline, as is currently done with ethanol.

—Rapp et al.

In the reported study, the team from UC Berkeley, UC San Diego, Graz University of Technology, and Vienna University of Technology, investigated the antiknock properties of the AlcoMix alcohol mixture produced using a MoS₂-based catalyst described in a foundational Dow Chemical Company patent with methanol recycling. They selected AlcoMix because the US EPA has already approved it for blending with gasoline, and because it can be separated by distillation into individual alcohols and sold in the marketplace.

The team said that the benefits of using AlcoMix in comparison to pure ethanol include:

• elimination of the need for additional distillation and decreased production costs in terms of both energy consumption and capital costs;

• a higher energy content on a volume basis and reduced hydroscopic properties; and

• reduction of the effect on the Reid vapor pressure of gasoline, enabling higher alcohol concentrations when blending with gasoline without increasing volatile organic compounds.

The team estimated the research octane numbers (RONs) of AlcoMix and ethanol using a modified, validated method in a Waukesha Cooperative Fuel Research (CFR) F-4 research engine. The CFR F-4 engine is a spark-ignited, single-cylinder, variable- compression-ratio engine. They then estimated the blending research octane number (BRON); the BRON represents a fuel’s ability to increase the octane number at low blend compositions.

They determined that AlcoMix has a BRON of 130.4 and a RON of 110.8; by comparison, ethanol has a measured BRON of 134.4 and a measured RON of 111.2. The BRON of 130.4 indicates that AlcoMix could be a good candidate for use as an antiknock blending component, despite containing 25% alcohols with lower octane numbers than ethanol.

Resources

• Vi H. Rapp, J. Hunter Mack, Philipp Tschann, Wolfgang Hable, Robert J. Cattolica, and Robert W. Dibble (2014) “Research Octane Numbers of Primary and Mixed Alcohols from Biomass-Based Syngas,” Energy & Fuels doi: 10.1021/ef5001453