Terpenes such as α-pinene, β-pinene, camphene and limonene are components of pine resin, and currently produced industrially either by tapping trees (gum turpentine) or as a byproduct of paper pulping.

Turpentine and ethanol were both used as renewable fuels as far back as the early 19^th century, the researchers note in their paper. Mixtures were used for lighting and for early internal combustion engines. Although ethanol has since gone on to become the most abundant global biofuel, with production estimated to reach 73 billion gallons per year by 2030, turpentine is not not used as a transportation fuel—despite several advantages over ethanol, including an exceptional volumetric net heat of combustion and low volatility.

This disparity is not difficult to reconcile in light of the limited scale of turpentine production based on the availability of pine resin. However, given the natural advantages of these hydrocarbons, the development of methods to both produce and utilize terpenes as fuels, on a relevant scale, is quite compelling.

An elegant route toward the development of saturated hydrocarbon fuels based on terpene feedstocks may lie in a biosynthetic approach in which engineered organisms convert cellulose and/or hemicellulose derived sugar solutions into pure terpene products. In this manner, full performance and even ultra-performance renewable fuels can be synthesized from low-value and abundant waste materials.

—Meylemans et al.

In an earlier study, the team reported the synthesis of high density fuel mixtures by the heterogeneous, acid-catalyzed dimerization of β-pinene; they chose β-pinene as a starting point due to its ease of dimerization. However, they also found that the terpene was subject to isomerization—resulting in the main isomeric products α-pinene, limonene and camphene—prior to dimerization. This variety of structures raised a number of fuel property issues as well as process issues, such as the need for strong heterogeneous acids to produce a complex distribution of C₂₀ molecules.

To provide insight into how different monomers impact key fuel properties, the current work explores the dimerization of pure camphene, α-pinene, limonene, and mixtures of these terpenes with heterogeneous acid catalysts. In addition, the use of “crude” turpentine was investigated to evaluate the ability of these catalysts to produce terpene dimers in high yield while using cheap, unpurified feedstocks.

—Meylemans et al.

Among their findings were:

• Fuels derived from both α-pinene and camphene have similar viscosities, densities and net heats of combustion due to the isomerization of camphene prior to dimerization.

• Limonene dimers exhibited a much lower density and viscosity (due to their ring opened structures).

• Dimers derived from camphene, α-pinene, and limonene had freezing points of -54, -52 and -78 °C, respectively.

• Nafion SAC-13, the preferred catalyst, could be recovered by simple filtration and reused up to 8 times.

Although the high viscosity of these fuels may limit their applications as standalone fuels, their low freezing points suggests that they can be major components of high density fuel mixtures or additives to conventional fuels.

—Meylemans et al.

Resources

• Heather A. Meylemans, Roxanne L. Quintana, Benjamin G. Harvey (2012) Efficient conversion of pure and mixed terpene feedstocks to high density fuels, Fuel, Volume 97 Pages 560-568 doi: 10.1016/j.fuel.2012.01.062