High‐Performance Jet Fuels Derived from Bio‐Based Alkenes by Iron‐Catalyzed [2+2] Cycloaddition

Abstract

A series of high‐performance cycloparaffinic fuels have been generated by [2+2] cycloaddition of the bio‐derived alkenes 1‐hexene, isoprene, and 1‐pentene, catalyzed by a low‐valent iron pyridine(diimine) complex [(MePDI)Fe(N2)2(μ‐N2)] [MePDI=N,N′‐(2,6‐pyridinediyldiethylidyne)bis(2,6‐dimethylbenzenamine)]. Reactions with 1‐pentene and 1‐hexene resulted in 85 % selectivity to 1,2‐cyclobutanes, and 12 % selectivity to acyclic alkenes generated by β‐hydride elimination. Self‐dimerization of isoprene was sluggish and generated heavier oligomer products, but cross‐dimerization of isoprene with 1‐hexene afforded primarily a 1,3‐cyclobutane product, along with isomers of acyclic C11 mixed dimers. Hydrocarbon mixtures were hydrogenated and fractionally distilled to yield finished fuel mixtures in overall yields of 83–93 % at the multigram scale. The fuels exhibited densities ranging between 0.767 and 0.783 g mL−1, and net heats of combustion (NHOC) of up to 120.6 kBtu gal−1 (43.8 MJ kg−1). These values are higher than conventional synthetic paraffinic kerosenes owing to the higher density and ring strain afforded by the cyclobutane rings. The fuel mixtures also exhibited extremely low viscosities ranging from 2.38 to 4.78 mm2 s−1 at −20 °C, due in part to the presence of the acyclic dimers. The excellent fuel properties of the product mixtures, selectivity for dimer products, high yields, and the ability to use simple bio‐derived alkenes as substrates, make the [Fe]‐catalyzed [2+2] cycloaddition of unactivated alkenes a compelling route to the synthesis of sustainable high‐performance fuels.

Bio‐Based Cycloalkanes: The Missing Link to High‐Performance Sustainable Jet Fuels

“…Hydrocarbons containing cyclobutanes have been proposed for high energy density fuel applications due to the ring strain afforded by the 4-membered ring, which can impart higher NHOCs and densities. [17,288] Compared to cyclopentane with a strain energy of 5.4 kJ mol À 1 , cyclobutane offers 27.2 kJ mol À 1 of ring strain energy, while not suffering from the relatively low thermal stability of cyclopropane rings. [17] Ryan et al [289] generated a cyclobutane containing fuel blend via [2 + 2]-cycloaddition [290][291][292][293][294] of isophorone.…”

“…[17,288] Compared to cyclopentane with a strain energy of 5.4 kJ mol À 1 , cyclobutane offers 27.2 kJ mol À 1 of ring strain energy, while not suffering from the relatively low thermal stability of cyclopropane rings. [17] Ryan et al [289] generated a cyclobutane containing fuel blend via [2 + 2]-cycloaddition [290][291][292][293][294] of isophorone. Previous work on photochemical [2 + 2]-cycloaddition of isophorone with bio-derived olefins [295] demonstrated the benefit of isophorone's enone functionality, which allows it to absorb long wavelengths (> 310 nm), circumventing the need for a photosensitizer.…”

“…[331] To explore ring-strained jet fuel range hydrocarbons derived from C 5 and C 6 olefins, Harvey and co-workers synthesized alkyl-substituted cyclobutane fuels via [2 + 2]cycloaddition of 1-hexene, isoprene, and 1-pentene. [17] The cycloaddition was promoted by a low-valent iron compound containing a redox-active pyridine(diimine) ligand. [332][333][334] Dimerization reactions were accomplished with 1 mol % of catalyst at ambient temperature in a solvent-free process (Scheme 39).…”

“…The TON was 3680, 42 times higher than the [2 + 2]-cycloaddition reactions described above. [17] Hydrogenation of 32 was conducted with PtO 2 under mild conditions (RT, 50 psi H 2 ) using acetic acid as a heterogeneous cosolvent. This resulted in the isolation of DMCO (33) as a 50 : 50 mixture of the cis-and trans-isomers (Scheme 40).…”

“…[9][10][11] Other approaches include alcohol-to-jet (ATJ), which is based on the conversion of bio-based alcohols (e. g., ethanol, butanol) to alkenes, followed by oligomerization and hydrogenation. [12][13][14][15][16][17][18][19] Direct sugar to hydrocarbon (DSH) approaches focus on the biosynthetic conversion of biomass sugars to hydrocarbons (e. g., terpenes) via fermentation with metabolically engineered microorganisms. [20][21][22][23] The acyclic HEFA, FT, ATJ, and DSH jet fuels meet most of the requirements of conventional jet fuel, but exhibit lower densities and volumetric heats of combustion.…”