In Situ Diagnostic of Supercritical Fuel Surrogates: Probing Heterogeneous Catalysis by Collision-Induced Dissociation in a Molecular Beam Tandem Mass Spectrometer

Abstract

High-speed flight is limited by the amount of heat absorbed by the aviation fuel in a thermal management system. One approach to increase the thermal capacity of the fuel is via endothermic heterogeneous catalysis within the heat exchanger (HEX). To optimize such chemistry, better tools are needed to probe the chemical composition inside the HEX in situ. Toward this aim, we demonstrate the first on-line analysis of supercritical fuel surrogates (n-hexane and n-dodecane) over a zeolite catalyst (H-ZSM-5) using tandem mass spectrometry (MS). In our approach, a supersonic expansion is generated directly from the supercritical state (200–1000 °C and 400–1000 psi) of the neat fluid to capture a “snapshot” of the reactive intermediates and products inside the reactor by isolating these species in the gas phases. A molecular beam is generated, which is ionized by 10 eV electron-impact ionization (EI), and the mass spectrum is acquired using a triple quadrupole mass spectrometer. Based on precursor scans, we distinguish between EI fragments and cracking products from the furnace. Product scans from the triple quadrupole reveal a structural similarity between the EI fragments and pyrolysis products. By directly identifying the light C2 and C3 radicals from pyrolysis, our results corroborate those in our previous study [DeBlase, Energy Fuels 2018, 32, 12289], which suggested that C2 and/or C3 intermediates are the key building blocks for aromatic synthesis by supercritical alkane pyrolysis. We anticipate that the new tandem MS capability will be widely used for catalyst development and to study the endothermic chemistry of fuels.