Highly Selective Condensation of Biomass‐Derived Methyl Ketones as a Source of Aviation Fuel

Abstract

Aviation fuel (i.e., jet fuel) requires a mixture of C9 -C16 hydrocarbons having both a high energy density and a low freezing point. While jet fuel is currently produced from petroleum, increasing concern with the release of CO2 into the atmosphere from the combustion of petroleum-based fuels has led to policy changes mandating the inclusion of biomass-based fuels into the fuel pool. Here we report a novel way to produce a mixture of branched cyclohexane derivatives in very high yield (>94 %) that match or exceed many required properties of jet fuel. As starting materials, we use a mixture of n-alkyl methyl ketones and their derivatives obtained from biomass. These synthons are condensed into trimers via base-catalyzed aldol condensation and Michael addition. Hydrodeoxygenation of these products yields mixtures of C12 -C21 branched, cyclic alkanes. Using models for predicting the carbon number distribution obtained from a mixture of n-alkyl methyl ketones and for predicting the boiling point distribution of the final mixture of cyclic alkanes, we show that it is possible to define the mixture of synthons that will closely reproduce the distillation curve of traditional jet fuel.

Effects of Composition and Structure of Mg/Al Oxides on Their Activity and Selectivity for the Condensation of Methyl Ketones

“…Previous studies suggest that as-synthesized hydrotalcite possesses a single axially symmetric Mg(OMg) 6 environment similar to that observed for pure brucite, Mg(OH) 2 . 19 The one-dimensional 25 Mg MAS NMR spectrum of hydrotalcite pretreated at 623 and 823 K shows the presence of a single, relatively broad peak at 23 ppm, suggesting the formation of a poorly crystalline MgO phase ( Figure S4).…”

“…The MQMAS experiment for half integer quadrupolar nuclei is a powerful technique to separate anisotropic interactions from isotropic interactions. 31, 32 The MQMAS sheared spectrum of as-synthesized hydrotalcite (Mg/ Al = 3) exhibits a single peak at 10 ppm for an Al 3+ in an octahedral site (Al(OH) 6 ). The sharpness of the peak is reflective of the high symmetry of this site and suggests that Al cations in hydrotalcite are not randomly distributed in the layered structure but are arranged in a ordered fashion.…”

“…Condensation of these compounds produces molecules that serve as precursors to transportation fuels. 6,7,12,13 A classic example of this type of carbon−carbon bond formation is the base-catalyzed dimerization of propan-2-one (acetone) to produce mesityl oxide (MO) or its trimerization to produce cyclic trimers isophorone (IP) and mesitylene (MES) as shown in Scheme 2. 40 A comparison of activity of HT, (Mg/Al)O, MgO, and γ-Al 2 O 3 , calcined at 823 K, for the gas phase self-condensation of propan-2-one is shown in Figure 6.…”

“…These findings are similar to those reported for batch trimerization of C 3 −C 5 methyl ketones in the liquid phase. 6 The absence of oligomers higher than cyclic enone trimers is due to the steric hindrance around the reactive center of the molecule. It is notable that the conversion of pentan-3-one is lower and selectivity to the dimer product is >80%, suggesting that internal ketones are hard to condense further likely due to the lower acidity of the alpha hydrogen on the C 2 carbon of pentan-3-one and/or due to steric reasons.…”

“…14,15 We have recently shown that calcined hydrotalcite, HT, is an active and highly selective heterogeneous catalyst for the self-and crosscondensation of biomass-derived C 4 −C 11 methyl ketones to produce cyclic enones, which upon subsequent hydrodeoxygenation produce cyclic alkanes that have properties making them suitable for aviation fuel and lubricants. 6,13,16,17 Hydrotalcite-type layered double hydroxides (LDHs) are inorganic compounds with the chemical composition M 2+ 1−x M 3+ x (OH) 2 (A x/n ) n-·yH 2 O, where M 2+ and M 3+ are divalent and trivalent metal cations, x is the molar ratio of the trivalent cation [M 3+ /(M 2+ + M 3+ )] which can be varied between 17 and 33%, and A n− is an anion with charge n, most often a carbonate ion. 16 The structure of these materials is similar to that of brucite, Mg(OH) 2 , where magnesium is octahedrally surrounded by six oxygens in the form of hydroxides with the octahedral units forming infinite sheets through an edge sharing.…”