Photoinduced transposed Paternò–Büchi reaction for effective synthesis of high-performance jet fuel

Abstract

High-energy-density fuels are important for volume-limited aerospace vehicles, but the increase in fuel energy density always leads to poor cryogenic performance. Herein, we investigated the transposed Paternò–Büchi reaction of biomass cyclic ketone and cyclic alkene to synthesize a new kind of alkyl-substituted polycyclic hydrocarbon fuel with high energy density and good cryogenic performance. The triplet-energy-quenching results and phosphorescent emission spectra reveal the sensitization mechanism of the reaction, including photosensitizer excitation, triplet-triplet energy transfer, cyclization, and relaxation, and the possible reaction path was revealed by the density functional theory (DFT) calculations. The reaction conditions of photosensitizer type and addition, molar ratio of substrates, reaction temperature, and incident light intensity were optimized, with the target product yield achieving 65.5%. Moreover, the reaction dynamics of the reaction rate versus the light intensity are established. After the hydrogenation-deoxygenation reaction, three fuels with a high density of 0.864–0.938 g·ml⁻¹ and a low freezing point of < −55 °C are obtained. This work provides a benign and effective approach to synthesize high-performance fuels.

1. Introduction

High-energy-density (HED) fuels, with their high density and high volumetric net heat of combustion (NHOC), are very important for volume-limited aerospace vehicles to extend their flight range [1]. Cycloaddition of cyclic alkene is the most commonly used method to synthesize HED fuels, but higher carbon content leads to poor cryogenic performance (high freezing point and low-temperature viscosity) [2]. Actually, the construction of bicyclic and alkyl-branched structures can realize high density and well-cryogenic properties simultaneously [3–5]. This kind of molecule structure could be obtained by thermal catalytic alkylation, but it seems difficult to synthesize the target products with high yields for the abundant byproducts [6–9].

Alternatively, photoinduced reactions have attracted much attention for their advantages of mild reaction conditions, high product selectivity, and low process energy consumption [10]. Specifically, an efficient C–C coupling process between the ketone and cyclic alkene can produce oxetane products using the [2+2] cycloaddition reaction (Paternò–Büchi reaction) triggered by UV light [11–13], whose hydrodeoxygenation can result in the HED fuels with bicyclic and alkyl-branched structures. As for the substrates of HED fuels, the biomass resources attract more attention than the traditional petroleum-derived compounds for their green and sustainable properties [14]. It is worth noting that the cyclic ketones, like cyclohexanone, cyclopentanone, isophorone, etc., can be easily obtained from the degradation and isomerization of lignocellulose [15], which is very promising to be used as the substrate to produce HED fuels.