Alkylation (the transfer of an alkyl group—a piece of a molecule with the general formula C_nH_2n+1, where n is an integer—from one molecule to another) has been investigated as a potential method for the production of longer chain components more suitable for the current fuel market. (In petroleum refining, alkylation is used to convert isobutane and low-molecular-weight alkenes into a high-octane gasoline component.)

Other work has shown that the alkylation of alcohols results in the formation of long-chain ketones in the C₅ to C₁₁ range, which may be deoxygenated to paraffins suitable as components in gasoline.

However, the straight-chain ketones produced from the upgrading of these ABE mixtures have too high freezing points to be considered for aviation, while the derived hydrocarbons are only suitable for gasoline applications due to the low flash point. In this investigation the suitability of producing ketones with superior low temperature properties was examined by introducing branching into the ketone chain. This was achieved through the alkylation of isobutanol and isoamyl alcohol with acetone. The key fuel parameters of both the unpurified alcohol/ketone product mixtures and the mixtures after removal of remaining alcohol were assessed.

—Donnelly et al.

For the Bath study, the researchers synthesized the fuels under solvent-free conditions using a Pd/C catalyst with K₃PO₄, which has been used elsewhere for the alkylation of acetone, butanol, and ethanol (ABE) fermentation mixtures. The Bath team acvhieved reasonable yields and selectivity for branched alkylation products with up to 61% produced from isoamyl alcohol and 64% from isobutanol.

They then tested the key aviation fuel properties of the mixtures unblended and in 50% and 20% blends with Jet A-1 aviation kerosene. Among their findings:

• The freezing points of the fuels were all found to be below the required −47 °C irrespective of blend or the temperature of the reaction.

• The energy density of the unblended fuels ranged between 30.4 and 41.36 MJ/kg depending on the temperature of the reaction and whether remaining alcohols were removed. While this is below the higher heating value (HHV) of the Jet A-1 used (45.69 MJ/kg), the energy densities of the 50% and 20% blends were more suitable with the isoamyl alcohol derived fuels having a maximum HHV of 44.31 MJ/kg at 50% blending and 44.99 MJ/kg at 20% blend with Jet A-1.

• The fuels derived from isoamyl alcohol produced above 140 °C were found to satisfy the flash point criterion (>38 °C) of the Jet A-1 specification, though the isobutanol derived fuels did not, producing fuels with flash points between 33 and 35 °C.

• Unblended, only a few of the fuels analyzed met the maximum kinematic viscosity requirement at −20 °C of 8 mm² s^–1; this fuel property was improved substantially on blending with jet fuel.

They attributed the improvement in the low temperature properties to the presence of branching in the constituent molecules. Branching acts to disrupt intermolecular interactions and thus suppresses the melting point temperature improving the cold flow characteristics.

Resources

• Joseph Donnelly, Richard Horton, Kesavan Gopalan, Chris D. Bannister and Christopher J. Chuck (2015) “Branched Ketone Biofuels as Blending Agents for Jet-A1 Aviation Kerosene” Energy & Fuels doi: 10.1021/acs.energyfuels.5b01629

• Gy. Onyestyák, Gy. Novodárszki, R. Barthos, Sz. Klébert, Á. Farkas Wellischa and A. Pilbátha (2015) “Acetone alkylation with ethanol over multifunctional catalysts by a borrowing hydrogen strategy,” RSC Adv., 5, 99502-99509 doi: 10.1039/C5RA17889D