Ionic liquid-catalyzed isomerization of tetrahydrotricyclopentadiene using various chloroaluminate complexes
Graphical abstract
Introduction
High-energy–density-fuels (HEDFs) for volume-limited aircrafts have received a great deal of attention due to the recent rapid progress of aeronautical technologies. HEDFs provide high flight range and carrying capacity owing to their high energy content and density [1], [2]. In the case of JP-10 (primarily composed of exo-tetrahydrodicyclopentadiene), the density and volumetric energy content are 0.94 g mL−1 and 39.6 MJ L−1, respectively, and JP-10 also has a low freezing point (∼−79 °C). RJ-5 (primarily composed of endo, endo-dihydrodinorbornadiene) has been developed to increase energy density (density: 1.08 g mL−1, volumetric energy content: 44.9 MJ L−1). However, RJ-5 cannot be used as a HEDF because of its poor low-temperature performance (freezing point: >0 °C). In general, the low-temperature performance is very important because HEDFs commonly experience harsh cold weather during their operation at high altitude [1].
Many researchers have attempted to develop novel HEDFs containing higher volumetric energy contents and low freezing points to meet the requirements of next-generation advanced aircraft fuel [3], [4], [5], [6], [7]. For example, exo-tetrahydrotricyclopentadiene (exo-THTCPD) has great potential as a new generation HEDF having higher energy density with an adequately low freezing point (density: 1.04 g mL−1, volumetric energy content: 44.1 MJ L−1, freezing point: <−40 °C) compared with JP-10 [3]. Aluminum chloride (AlCl3) catalyst, which acts as a Lewis acid, can effectively transform tetrahydrotricyclopentadiene (THTCPD) into a HEDF [3]. However, conventional AlCl3 has serious disadvantages such as difficulty of recycling, the need for a laborious post-treatment process, and the generation of a corrosive waste product. The chloroaluminate IL catalyst is a potential substitute for AlCl3 because it has many advantages in terms of its adjustable acidity, non-combustibility, and ease of recycling [8]. To date, ILs have been presented as “green” solvents that allow catalyst recovery and have lower environmental impact and production costs [8], [9], [10], [11]. Therefore, IL catalysts have been applied in many kinds of reactions such as dimerization [12], [13], [14], alkylation [15], Diels–Alder addition [16] and Friedel–Crafts reaction [17]. Recently, Wang et al. reported that an acidic IL could effectively induce isomerization of THTCPD [5]. Moreover; the product of the reaction that used IL was considerably different from that of the previously reported AlCl3-based isomerization of THTCPD. A compact polycyclic hydrocarbon, so called, “diamondoid,” is a promising source of HEDF, which can be artificially synthesized via catalytic rearrangement of THTCPD [6], [7]. This diamondoid-based product is expected to perform better in propulsion applications due to its high density (∼1.04 g cm−3), low freezing point (−41 °C to −70 °C) and high hydrogen content, resulting in reduced soot formation [5]. However, there are no systematic studies regarding the comparison of catalytic behaviors of THTCPD isomerization between IL catalysts with different cations such as ammonium, alkyl imidazolium, and pyridinium.
In this study, we examined three kinds of typical chloroaluminate IL catalysts for isomerization of THTCPD to HEDF. The reaction parameters such as the catalyst/reactant ratio, the acidity of the chloroaluminate IL catalyst, the reaction time, and the polar solvent content were investigated to optimize the isomerization reaction conditions. In addition, FT-IR analysis using pyridine as a probe molecule was performed to characterize the acidity of the IL catalysts.
Section snippets
Materials
Triethylamine hydrochloride (TEAC, Sigma Aldrich, USA), 1-butyl-3-methylimidazolun chloride (BMIC, Sigma Aldrich, USA), and pyridine hydrochloride (PHC, Sigma Aldrich, USA) were used as cation precursors of IL catalysts. THTCPD was prepared according to a published procedure [3]. Three kinds of isomers, namely: an endo, exo, endo norbornyl (NB) adduct (compound 1: 82.1%); an endo, exo, exo NB adduct (compound 2: 7.2%); and an endo, exo, exo cyclopentyl (CP) adduct (compound 3: 10.7%) were
Results and discussion
Dicyclopentadiene isomer has two carbon–carbon double bonds, one in the NB ring and the other in the CP ring [1]. During the Diels–Alder addition, cyclopentadiene (CPD) may attack the NB or CP ring to form NB or CP adducts. Moreover, the newly formed bond can be 3-dimesionally orientated as an exo or endo position. Therefore various stereoisomers should be appeared during THTCPD isomerization reaction as shown in Fig. 1 [18]. Major stereoisomers from the reaction of THTCPD isomerization by
Conclusion
In conclusion, the combination of three kinds of cation precursors and AlCl3 were compared in terms of the catalytic performance of THTCPD isomerization. The most effective cation for use with the IL catalyst in the THTCPD isomerization was a PHC species. It is attributed to the highest Lewis-acidic character of the PHC-based catalysts compared with other IL catalysts. The catalytic behavior was confirmed by FT-IR analysis of the Lewis acid character of the used IL catalysts. The Lewis acidity
Acknowledgements
This research was supported by Agency for Defense Development and BK21plus program through the National Research Foundation (NRF) of South Korea funded by the Ministry of Education (2013).
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