Roxanne L. Quintana

High-Density Renewable Fuels Based on the Selective Dimerization of Pinenes

High-density fuel candidates have been synthesized in up to 90%yield from β-pinene, a renewable strained bicyclic compound derived fromwood and plant sources. The process is based on the heterogeneous acidic catalysts Montmorillonite-K10 and Nafion, which promote selective isomerization and dimerization of pinenes under moderate conditions (100 C and atmospheric pressure). Montmorillonite-K10 was shown to be a highly active catalyst for dimerization but was also active in the ring opening of β-pinene followed by dehydrogenation to produce p-cymene, which limited the yield of the dimer to ca. 75%. Nafion was capable of producing dimers in up to 90%yield primarily through isomerization followed by dimerization. Amberlyst-15, a common industrial catalyst had very poor activity and conversion even at 150 C. The dimer mixtures were upgraded through hydrogenation over PtO2 and fractional distillation. The synthesized fuels have a density of 0.94 g/cm and a net volumetric heating value of 39.5 MJ/L (141 745 BTU/gallon). These values are nearly identical to those for the widely used tactical fuel JP-10 (primarily composed of exotetrahydrodicyclopentadiene), suggesting that these renewable fuels may have applications for rocket propulsion.

Solvent‐Free Conversion of Linalool to Methylcyclopentadiene Dimers: A Route To Renewable High‐Density Fuels

The development of techniques for the efficient synthesis of custom fuels and chemicals from sustainable natural feedstocks is of fundamental importance to society as the direct and indirect costs of petroleum use continue to increase. For general transportation fuels, complex mixtures of molecules that have somewhat lower utility than petroleum-based analogs may be sufficient; however, for specific applications, such as jet and missile propulsion, a more selective model that produces molecules with defined and specialized properties is required. Well-characterized, single-site catalysis is the basis of elegant synthetic strategies for the production of pure compounds. In particular, ruthenium-based olefin metathesis catalysts are known to catalyze a number of reactions, including self-metathesis, cross-metathesis, ring-closing metathesis (RCM), and ring-opening metathesis polymerization (ROMP). This family of catalysts is ubiquitous in the literature and has been applied in many fields of chemistry, ranging from natural product synthesis to polymer chemistry. The transition of these catalysts to large-scale industrial processes has in the past been hindered by their modest turnover numbers and high cost. To overcome these difficulties, catalytic systems that can efficiently yield pure products while maintaining low catalyst concentrations need to be developed. In this report, we detail a ruthenium-catalyzed method for the synthesis of dimethyldicyclopentadiene from linalool, a linear terpene alcohol. Recent work in our laboratory has focused on the conversion of terpenes into high-density fuel surrogates. Although terpenes are naturally produced by pine trees and a variety of plants, a truly sustainable method may require the utilization of bioengineered microbes to produce specific molecules or families of molecules from waste cellulose. Within the terpene family, linalool is a particularly intriguing feedstock for fuels because of its molecular structure. Although RCM of linalool must proceed through a sterically hindered transition state, the reaction is facilitated by coordination of the allylic alcohol. This results in an efficient method for the synthesis of 1-methylcyclopent-2-enol (1) and isobutylene (Scheme 1). Both products are of significant interest as they can be converted to renewable fuel and polymer products. Isobutylene is a valuable side-product that can be selectively trimerized to produce jet fuel, dimerized or alkylated with C4 raffinate to produce high-octane gasoline, or polymerized to polyisobutylene. Meanwhile, 1 is a promising precursor for the synthesis of methylcyclopentadiene dimer, which can be hydrogenated and isomerized to produce the high-density missile fuel RJ-4 (Scheme 1). NMR-scale conversions of linalool to 1 under dilute conditions and at elevated temperatures have been reported in the literature. Catalysts used for this reaction (Figure 1) have included the first-generation Grubbs catalyst (2), both a second-generation Grubbs (5) and Grubbs–Hoveyda catalyst

Synthesis of renewable jet and diesel fuels from 2-ethyl-1-hexene

An efficient method for the selective dimerization of the renewable feedstock, 2-ethyl-1-hexene, to a complex mixture of C16H32 hydrocarbons has been developed. To optimize the process, the activity of a variety of strongly acidic heterogeneous catalysts was investigated. Montmorillonite K-10 and sulfated zirconia readily isomerized 2-ethyl-1-hexene to a mixture of the cis- and trans isomers of 3-methyl-2-heptene and 3-methyl-3-heptene, but were inactive for the dimerization of 2-ethyl-1-hexene at temperatures up to 116 °C. In contrast, the cation exchange resins Amberlyst-15 and Nafion, readily dimerized 2-ethyl-1-hexene at elevated temperatures. For both sets of catalysts, the degree of hydration strongly affected the rate of isomerization/dimerization. After hydrogenation over PtO2 and fractional distillation, saturated dimer mixtures could be isolated in up to 90% yield. The dimers have a density of 0.78 g mL−1 and a freezing point <−60 °C, suggesting that they can be blended with renewable or conventional jet fuels, without adversely affecting the overall density and low temperature viscosity of the mixtures.

Renewable Gasoline, Solvents, and Fuel Additives from 2,3-Butanediol.

2,3-Butanediol (2,3-BD) is a renewable alcohol that can be prepared in high yield from biomass sugars. 2,3-BD was selectively dehydrated in a solvent-free process to a complex mixture of 2-ethyl-2,4,5-trimethyl-1,3-dioxolanes and 4,5-dimethyl-2isopropyl dioxolanes with the heterogeneous acid catalyst Amberlyst-15. The purified dioxolane mixture exhibited an anti-knock index of 90.5, comparable to high octane gasoline, and a volumetric net heat of combustion 34 % higher than ethanol. The solubility of the dioxolane mixture in water was only 0.8 g per 100 mL, nearly an order of magnitude lower than the common gasoline oxygenate methyl tert-butyl ether. The dioxolane mixture has potential applications as a sustainable gasoline blending component, diesel oxygenate, and industrial solvent.

Synthesis of renewable plasticizer alcohols by formal anti-Markovnikov hydration of terminal branched chain alkenes via a borane-free oxidation/reduction sequence

An efficient method for the formal anti-Markovnikov hydration of 1,1-disubstituted alkenes has been developed. The utility of the process has been demonstrated by conversion of bio-derived butene oligomers into primary alcohols through initial oxidation to vicinal acetoxy-alcohols, diols, or diacetates, followed by selective dehydration/tautomerization of the diols, and hydrogenation of the intermediary aldehydes. This approach allows for the isolation of important industrial plasticizer alcohols from a renewable source. In a broader context, this pathway, which can be conducted with sustainable, conventional reagents under mild conditions, represents a unique alternative to hydroboration for a challenging subset of hindered olefins.