Most current bio-jet fuels consist primarily of linear or branched chain alkanes; they suffer from low densities (~0.76 g/mL) and low volumetric heating values compared with those of petro-jet fuels. As a result, most alternative fuels have to blend with petro-jet fuels to meet the energy density requirements.
Cyclic hydrocarbons (i.e. cycloalkanes) can be used to make dense jet fuels with high thermal stability. However, industrial synthesis is costly, and the precursor from hydrocracking of petroleum has low selectivity. Now, researchers from the University of Nevada and Washington State University have developed a novel efficient biphasic tandem catalytic process (biTCP) for synthesizing cycloalkanes from renewable terpenoid biomass (such as 1,8-cineole). A paper on their work is published in the RSC journal Green Chemistry.
Emerging synthetic routes are being developed to produce biomass-derived cycloalkanes via alkylation of biomass intermediates such as furfural, phenols, and cyclopentanone. For instance, Huber and coworkers synthesized high-density (0.82 g mL−1) cycloalkanes in the jet-fuel range with a high overall yield (∼ 80%) by aldol-condensation of cyclopentanone and butanal followed by hydrodeoxygenation (HDO). However, this process requires purified and thus relatively expensive fuel precursors that are not abundantly available. Chen’s group developed the process to produce alkanes and aromatics in the jet fuel range by microwave-induced pyrolysis of intact biomass (Douglas fir) integrated with hydrotreating upgrading of pyrolysis oil. However, high hydrogen gas consumption and severe reaction conditions were needed in this process. Therefore, the current technologies of producing bio-derived cycloalkanes are still far from cost-competitive in large scales.
Terpenoids (e.g., isoprenoids), containing one or more cyclic hydrocarbon rings, are commonly found in many biomass genera such as mint, pine, eucalyptus, etc. Dated back to 1980s, Calvin has suggested terpenoids as a potential feedstock for synthesizing advanced biofuels. The ring structures in their hydrocarbon chains are advantageous toward synthesizing cycloalkanes. Unfortunately, terpenoids are usually produced in limited quantities in natural plants that can be used as biofuel feedstock.
However, recent developments in metabolic engineering shed light on the mass production of terpenoids. In particular, the potential global production capacity of eucalyptus oil (more than 90% of the components is 1,8-cineole) has been estimated to be in millions of metric tonnes per year. However, recent developments in metabolic engineering shed light on the mass production of terpenoids. In particular, the potential global production capacity of eucalyptus oil (more than 90% of the components is 1,8-cineole) has been estimated to be in millions of metric tonnes per year. 1,8-cineole is readily converted to p-menthane, which is a C10 cycloalkane in the jet fuel range.—Yang et al.
Biphasic catalysis (i.e., two separate and immiscible catalysis phases) is used in many processes with homogenous catalysts in aqueous phase such as olefin oligomerization, hydroformylation, hydrogenation, carbon-carbon cross coupling, etc. The conventional biphasic approach facilitate the separation of hydrophobic products from aqueous homogeneous catalysts and thus enables the easy recycling of homogenous catalysts.
In the proposed biTCP for cylcoalkane production, both hydrophilic and hydrophobic catalysts are added into two immiscible solven—an organic phase and an aqueous phase—creating a biphasic environment for a “cascade” of chemical reactions in both phases.
This approach of biphasic tandem catalysis not only reserves the feature of convenient catalyst/product separation but also accomplishes multiple reaction steps in a “one-pot” process.—Yang et al.
Using 1,8-cineole as a model terpenoid compound, the researchers attained a ~100% conversion of 1,8-cineole and a ~100% selectivity to p-menthane at 120°C within one hour.
The concept of biTCP has the potential to be generalized for the conversion of various biomass feedstocks to renewable hydrocarbon fuels with high carbon efficiencies.—Yang et al.
Xiaokun Yang, Teng Li, Kan Tang, Xinpei Zhou, Mi Lu, Whalmany L Ounkham, Stephen M Spain, Brian J Frost and Hongfei Lin (2017) “Highly Efficient Conversion of Terpenoid Biomass to Jet-fuel Range Cycloalkanes in a Biphasic Tandem Catalytic Process” Green Chemistry doi: 10.1039/C7GC00710H