Synthesis of high-density aviation fuels with methyl benzaldehyde and cyclohexanone†
A new two-step process was developed for the synthesis of high-density jet fuel range tricyclic alkanes with methyl benzaldehydes and cyclohexanone which can be derived from lignocellulose. In the first step, C14 oxygenates (i.e. 2-(2-methylbenzylidene)cyclohexanone or 2-(4-methylbenzylidene)cyclohexanone) were obtained by the solvent-free aldol condensation of 2-methyl benzaldehyde (or 4-methyl benzaldehyde) and cyclohexanone. Among the investigated catalysts, the EAOAc ionic liquid (a renewable catalyst which was prepared with ethanolamine and acetic acid) exhibited the highest activity and good stability for this reaction. Over it, high carbon yields (∼85%) of C14 oxygenates were achieved under mild reaction conditions (353 K, 6 h). In the second step, the C14 oxygenates were selectively converted into 1-methyldodecahydro-1H-fluorene and 3-methyldodecahydro-1H-fluorene by the aqueous phase hydrodeoxygenation (APHDO) over a commercial Pd/C catalyst. According to our measurement, the 1-methyldodecahydro-1H-fluorene and 3-methyldodecahydro-1H-fluorene as obtained have high densities (0.99 g mL−1 and 0.96 g mL−1, respectively, at 298 K). As a potential application, they can be used as additives to improve the volumetric heat values of the current bio-jet fuels.