Enhanced Bimolecular Reaction in a Two-Component Fluid under

Feb 12, 2018 - Second, a time-to-temperature calibration is obtained with each data set so that the mass spectra, recorded with time using the Extrel ...
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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Enhanced Bimolecular Reaction in a Two-Component Fluid under Pyrolytic Conditions: In Situ Probing of the Pyrolysis of Jet Fuel Surrogates Using a Supersonic Expansion Molecular Beam Mass Spectrometer Christopher E. Bunker,* Andrew F. DeBlase, Taylor A. Youtsler,† Nathan L. Sanders,‡ and William K. Lewis Aerospace Systems Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio 45433, United States S Supporting Information *

ABSTRACT: In situ mass spectrometry is demonstrated as a technique to study the pyrolysis of neat fluids at supercritical conditions. These fluids included pure hexane, benzene, and binary mixtures of the two, which were sampled in a supersonic expansion to cool and trap reactants, intermediates, and products in a molecular beam. To identify the reacting species, the molecular beam was subjected to electron impact ionization prior to analysis in a quadrupole mass filter. In addition to the previously reported gas-phase pyrolysis products for hexane and benzene, we observe the enhanced production of biphenyl in the binary mixture, which can be attributed to an energetically favored pathway by which the initial production of alkyl radicals seeds the formation of phenyl radicals via H atom abstraction reactions. These phenyl radicals quickly react to form biphenyl because of the proximity of solvent reaction partners and high collision frequency in the fluid. Our results illustrate a simple model system that highlights contemporary difficulties associated with multicomponent fuels due to the new pathways that become available even in simple mixtures.

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INTRODUCTION

extreme complexity of the pyrolysis process for the compounds common to fuel, noting even simple hydrocarbons produce hundreds of reaction pathways, intermediates, and products when pyrolized.28,32,34 In an attempt to better understand the pyrolytic environment, we developed a specialized supersonic expansion molecular beam mass spectrometer to examine the in situ composition of neat fuel surrogates under pyrolytic conditions. The goal of this work is to make direct measurement of the high-temperature, high-pressure reaction environment and to observe the reactants, products, and intermediates that exist prior to any form of condensed-phase cooling. It is the action of the supersonic jet that allows for such study. The hydrodynamic forces in the supersonic expansion transfer momentum very efficiently into the downstream direction, resulting in a beam of molecules with very similar velocities.41 Because the spread in velocities is so small (typical effective translational temperatures42 in molecular beams are on the order of 1 K), the molecules in the beam rarely collide with one another after they have traveled more than a few nozzle diameters from the orifice. Therefore, chemical reactions are quenched, and fragile reaction intermediates are “frozen out.” The “supersonic expansion into vacuum” technique has even been demonstrated as capable of freezing out the very rapid chemistry occurring in steady-state detonations.43 Here we present data for two fuel surrogate homologues: hexane and benzene. The results for the neat fluids are found to be in general agreement with literature

Chemical and physical processes occurring in fuels at high temperatures and pressures are of considerable interest in the design and implementation of advanced fuel systems supporting high-speed flight. This is mostly driven by the necessity to utilize the fuel as the primary thermal management fluid. The exact conditions for any given application are highly system dependent, owing to the impact of flow rates and residence times. The high-temperature, high-pressure regime of interest is bounded by fuel temperatures of 200 and 1000 °C and pressures of 400 and 1000 psi. Such conditions capture a variety of fuel system configurations from advance thermal management for turbine engine technologies at the lower end to endothermic fuel systems supporting high-speed flight at the higher end.1−3 Of particular interest here are conditions that result in the pyrolytic decomposition of the fuel compounds. It is the controlled pyrolysis of fuels that will enable an endothermiccapable fuel system, i.e., a fuel or fuel system selective for endotherm-producing reactions and selective against the formation and deposition of coke. There is a tremendous amount of data in the literature for the pyrolysis of various fuelrelated compounds, with many gas-phase,4−17 shock tube,18−24 and reactor-based25−34 studies reported. Models describing the chemical pathways and reaction kinetics are available for many systems,35−40 yet it is currently not possible to design an endothermic-capable fuel or fuel system based on these data alone; experience has shown that significant trial-and-error testing is generally necessary to obtain acceptable performance results. Even then, the solution is likely to be applicable only to the designed-to requirements. This can be attributed to the © XXXX American Chemical Society

Received: January 18, 2018 Revised: February 9, 2018 Published: February 12, 2018 A

DOI: 10.1021/acs.energyfuels.8b00232 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

The high-temperature reactor is a stainless steel tube equipped with a 10 μm diameter pinhole nozzle interfaced to a vacuum chamber. The reactor sits within a homemade furnace (Al2O3 tube inside a tungsten coil) that is resistively heated via a computer-controlled dc power supply. The temperature of the reactor is driven into and through the pyrolysis zone by stepping the voltage supplied to the furnace at a 0.1 V/min rate. The outer temperature (Touter) is monitored by a thermal couple embedded in the furnace. The reactor performance is recorded using two calibrations. First, a Touter to Tinner calibration is obtained for a particular furnace by placing a second thermocouple inside an empty stainless steel tube to measure the inner temperature of the tube (Tinner). This procedure is performed for each new furnace configuration. Second, a time-to-temperature calibration is obtained with each data set so that the mass spectra, recorded with time using the Extrel Merlin software, can be synched with the T inner measurement. Fluids within the reaction zone expand through the pinhole into the vacuum to form a supersonic jet, and the jet is then skimmed by a 0.5 mm skimmer to form a beam. Following supersonic expansion of the fluid to preserve the species involved in the nascent chemistry, the contents of the molecular beam are probed by a quadrupole mass spectrometer equipped with an electron ionizer (Extrel CMS). The incident electron energy can be adjusted from 3 to 150 eV with a resolution of ∼1 eV, allowing us to perform ionization at near-threshold energies (∼8−12 eV for most organic species) to minimize fragmentation, or to ionize at 70 eV to facilitate comparison with standard mass spectrum databases.44 The ions created by the electron impact source are then bent 90° into the quadrupole filter and detected by an electron multiplier. For these studies, the electron energy was set to 10 eV. The scan rate of the quadrupole was set to 0.53 amu/ms, and each spectrum saved by the Merlin software was averaged for 5 s (e.g., for an m/z range of 200, 13 scans are averaged). Using postprocessing software written in Python, the collected spectra were subjected to a moving window average (21 files) to perform high-pass filter and smoothing functions.

data, while the results obtained for mixtures of benzene with small amounts of hexane show a significant change in the yield of the benzene pyrolysis product biphenyl. The enhancement in the biphenyl yield is discussed within the context of the activation energies (Ea) of model reactions and solvation effects on bimolecular reactions.

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EXPERIMENTAL SECTION

Materials. Hexane and benzene (>99%) were obtained from Aldrich and used as received. High-pressure, high-temperature reactor tubes with a 10 μm laser-drilled hole were purchased from Lenox Laser. The tubes were 0.125 in. in o.d., 0.069 in. in i.d., and ∼10 in. in length. Visual analysis of the laser-drilled holes shows some inconsistency in hole location, shape, and diameter (Figure S1, Supporting Information). As a result, there is a tube-to-tube variation in properties associated with the expansion nozzle, i.e., the residence time and expansion to skimmer flux. In the Supporting Information, we estimate the residence time in the reactor to be on the order of 1s. The variability in residence times between nozzles is consistent with the observed change in chamber pressure between experiments (2 × 10−3 to 3 × 10−2 Torr) and can be attributed to the error in drilling the 10 μm apertures. Instrumentation. A schematic for our instrument is shown in Scheme 1. The instrument consists of a high-pressure sample feed, a

Scheme 1. Schematic of the Supersonic Expansion Molecular Beam Instrument Coupled with the High-Temperature, High-Pressure Fluid Reactora

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RESULTS AND DISCUSSION Figure 1 shows the mass spectra collected for hexane at two temperatures and a constant pressure of 600 psi. The pressure of 600 psi was selected to ensure the fluid conditions were wellseparated from the complex behaviors experienced in the nearcritical region (hexane: Tc = 234.6 °C, Pc = 438 psi, and ρc = 2.71 mol/L). Such behaviors can result in density fluctuations

a

The instrument consists of three pressure zones: P1, which contains the fluid reactor, P2, which directly follows the skimmer and produces the molecular beam, and P3, which contains an electron impact ionizer and a quadrupole mass detector. The four boxes labeled “Feed”, “Reactor”, “Beam”, and “Detector” reflect regions within the instrument where the chemical composition is discussed (Table 1). high-temperature reaction zone, a supersonic expansion to quench pyrolysis and form a molecular beam, and a mass spectrometer to probe the contents of the beam. The high-pressure sample feed consists of a stainless steel vessel (∼400 mL volume) sealed with two flange caps and Teflon gaskets. The vessel is connected to a high-pressure nitrogen gas cylinder which is used to push the fuel through the system at a constant pressure. The in-line pressure of the fluid is monitored using a Heisse pressure gauge. All connections are made using a 1/16 in. stainless steel tubing and high-pressure valves and tees (HIP, Inc.). To prepare the precursor liquid samples, the high-pressure vessel is opened at one end, completely filled, and degassed by bubbling nitrogen gas for ∼1 h. Once sealed and attached to the system, all tubing is flushed with nitrogen gas to remove any residual oxygen from the lines.

Figure 1. Mass spectra collected for the fluid hexane obtained just prior to pyrolysis onset (top, T = 600 °C) and well into the pyrolysis reaction(s) (bottom, T = 640 °C). The pressure within the reactor was constant at 600 psi. B

DOI: 10.1021/acs.energyfuels.8b00232 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Table 1. Expected Composition at Various Experimental Conditions for a Select Location within the Instrument (Scheme 1)a fluid (T, °C)

feed

reactor

hexane (600) hexane (640)

hexane hexane

benzene (600) benzene (640)

benzene benzene

hex/ben (600)

hexane benzene hexane benzene

hexane Py int Py prod benzene benzene Py int Py prod hexane benzene Py int, hex Py prod, hex benzene Py int, ben Py prod, ben Py prod, hex + ben

hex/ben (640)

detectorb

beam hexane Py int Py prod benzene benzene

EI EI EI EI EI

hexane Py int Py prod benzene benzene

Py prod hexane benzene Py int, hex Py prod, hex benzene

EI EI EI EI EI EI

Py prod hexane benzene Py int, hex Py prod, hex benzene

Py prod, ben Py prod, hex + ben

EI Py prod, ben EI Py prod, hex + ben

a Py int = pyrolysis intermediates. Py prod = pyrolysis products. EI = electron impact fragmentation reaction. bExperimental observable. Contents of reactor and beam are hypothesized.

from the electron impact ionization of the intermediates and products (Table 1, hexane at 640 °C). The peaks in the 640 °C mass spectrum can be separated into four general groups with slightly offset progressions. The first group contains the low-mass peaks in the C5, C4, C3, and C2 regions. These peaks show maximum contribution at the aliphatic or olephenic masses and follow the CH2 elimination progression. The relative contributions differ significantly from those of hexane at 600 °C and therefore suggest pyrolysis products such as butene, propene, and ethylene, among others. The second group begins in the C6 region with the peak at m/z = 78 and progresses through the C7, C8, and C9 regions with a CH2 mass increment. These peaks are not observed for hexane at 600 °C and are therefore directly attributable to pyrolysis products. The peak at m/z = 78 is assigned to benzene, with the CH2 additions resulting in toluene, xylene(s), and trimethylbenzene(s). A third group then appears at m/z = 128 (C10) and progresses through the C11 and C12 regions. These peaks are likely due to naphthalene and methyl-substituted naphthalenes. The last group, located at m/z > 160, shows no clear correlation or progression and may have many possible assignments. These peaks might represent the types of compounds or fragments that make up or lead to the coke formation that ultimately plugs the nozzle. In general, the observed results are in good agreement with literature studies for hexane pyrolysis.34 Figure 2 shows the mass spectra collected for benzene at five temperatures (600, 610, 620, 630, and 640 °C from top to bottom, respectively) and at a constant pressure of 800 psi. This pressure was also selected to avoid proximity to the nearcritical region (benzene: Tc = 289 °C, Pc = 709 psi, and ρc = 3.9 mol/L). The top spectrum was obtained at the reactor temperature of 600 °C and is representative of all benzene spectra recorded from room temperature to 600 °C. The spectrum is characterized by the strong molecular ion at m/z = 78 and two very weak peaks at m/z = 41 and 55. The weak peaks are due to fragmentation caused by the electron ionization reaction, but because of the stability of the aromatic ring, yields for fragmentation are very low. The composition of the reactor is shown in Table 1 (benzene at 600 °C). At higher temperatures, the spectrum for benzene remains relatively unchanged, with the minor exception of a small peak formed at

and instabilities within the reactor and supersonic expansion. The top spectrum was obtained at a reactor temperature of 600 °C and is representative of hexane at all temperatures from room temperature to 600 °C. The only differences observed (not shown) over that temperature range are small changes in the relative intensities of the observed ions. This is due to the fact that during ionization the nascent molecular ion inherits the thermal energy contained in the hexane molecule prior to its ionization. Although one might expect residual internal energy to be diminished by the supersonic expansion, it is possible that cooling is incomplete in these studies because the expansion starts from a neat supercritical fluid at high temperatures (>600 °C) and no buffer gases are used. However, the overall change to the fragmentation pattern is quite small, and its impact is assumed negligible for this analysis. The hexane spectrum is characterized by a molecular ion at m/z = 86, followed by four groups of peaks attributable to the known ion fragmentation pathways of the molecular ion. The groups lie in the C5, C4, C3, and C2 mass regions with a CH2 elimination step increment. The spectrum agrees well with published hexane ionization fragmentation spectra, noting the difference in incident electron energy (10 eV vs 70 eV). To relate the mass spectrum to the contents of the reactor, we should consider the fluid composition at each stage of the instrument (Scheme 1; feed, reactor, beam, and detector). Table 1 provides the expected composition of the fluid for each region at a number of experimental conditions. For hexane at 600 °C, the scheme is simple because the reactor temperature is below the pyrolysis onset temperature. The bottom spectrum in Figure 1 was obtained at a reactor temperature of 640 °C and is qualitatively representative of the end-state spectrum, i.e., the final spectrum obtained prior to definitive nozzle plugging. Within the end-state zone, significant changes in the relative intensities of the peaks are possible with slight changes in temperature; however, the ions shown are consistently observed within the end state. Also, characteristic of the end-state spectrum is the absence of the hexane molecular ion; no peak at m/z = 86 is observed above the background noise signals. Thus, the peaks that are observed must originate from the pyrolysis of hexane and are intermediates (Py int), products (Py prod), or fragments C

DOI: 10.1021/acs.energyfuels.8b00232 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 2. Mass spectra for the fluid benzene recorded at temperatures of 600 °C (top) to 640 °C (bottom) with a 10° increment. The pressure within the reactor was constant at 800 psi. All plots share the same y-axis scale for fractional contribution. The inset plots are ×10 magnification to highlight the only product observed from benzene pyrolysis.

Figure 3. Mass spectra recorded for the fluid mixture of benzene with hexane (88:12 mass fraction) at temperatures of 600 °C (top) to 640 °C (bottom) with a 10° increment. The pressure within the reactor was constant at 650 psi. All plots share the same y-axis scale for fractional contribution.

m/z = 154 (highlighted by the inset plots in Figure 2). Studies of benzene pyrolysis have shown multiple intermediates and products for the decomposition of benzene; however, it is welldemonstrated that the first observable product is the formation of biphenyl: 2C6H6 → PhPh + H 2

(1)

The biphenyl product can form via two pathways:

12,45,46

Ph• + Ph• → PhPh •

Ph + C6H6 → PhPh + H

constant pressure of 650 psi. The critical parameters for this mixture were estimated using a mole fraction average of the pure components: Tc = 283 °C, Pc = 679 psi, and ρc = 3.76 mol/L. At 650 psi, we are away from the near-critical region but below the critical pressure for this fluid mixture. The top spectrum is representative of the fluid mixture from room temperature to 600 °C and is equivalent to the superposition of the unreacted hexane spectrum (Figure 1, top) with the unreacted benzene spectrum (Figure 2, top), scaled to the mole fraction ratio. The similarity in the spectra indicates nothing unusual with respect to the chemistry of benzene and hexane below the pyrolysis onset temperature. The bottom spectrum in Figure 3 represents the end state for the benzene/hexane fluid mixture. The spectrum is characterized by a strong signal for benzene, and smaller contributions that directly correlate with either the hexane or benzene endstate spectrum shown in Figure 1. None of the peaks observed are unaccounted for by the pure fluid spectra; however, there is one significant discrepancy between the individual end-state spectra and the benzene/hexane mixture end-state spectrum, namely, the large contribution of the biphenyl product (m/z = 154). The fractional contribution for m/z = 154 is approximately an order of magnitude greater than that for neat benzene. The intermediate spectra collected at temperatures of 610, 620, and 630 °C show that the peak at m/z = 154 undergoes a smooth, continuous growth with increasing temperature. To examine in detail the temperature-dependent behavior of biphenyl, we compare the temperature dependence of the total ion count (TIC) of pure benzene to that of the benzene/ hexane mixture (Figure 4, top). The steeper decrease of the TIC in the case of pure benzene is indicative of the more rapid nozzle plugging in the pyrolysis regime. This result, taken alone, implies two possibilities: (1) addition of hexane interferes with the coking process in a way that allows further reaction progress to be monitored before the nozzle plugs, or (2) a chemical pathway favoring biphenyl becomes preferred when hexane is added to benzene. If the system obeyed case 1,

(2) •

(3) •

Both reactions require the initial pyrolysis radical Ph . For our experiments, the end-state spectrum for benzene shows only benzene and a very small contribution of biphenyl. We do not detect any contribution from Ph• (m/z = 77) beyond experimental uncertainty, indicating that the intermediate Ph• is transient between the pyrolysis and ionization regions of the instrument and is not detected in abundance. For our system, any further increase in temperature results in complete plugging of the nozzle. The contents of the reactor and its relation to the mass spectrum obtained at the detector can be viewed as follows (Table 1, benzene at 640 °C): feed [benzene] → reactor [benzene, Py int, Py prod] → beam [benzene, Py prod] → detector [benzene, EI benzene, Py prod, EI prod] (4)

The fact that no pyrolysis intermediates for benzene (EI Py int) are observed suggests that the bimolecular reactions shown in eqs 2 and 3 are fast relative to production of new phenyl radicals. With a data record for both hexane and benzene pyrolysis under our experimental conditions and with knowledge that both compounds behave in general as indicated by the literature, we sought to explore the effects of a two-component fluid and the possibility of mixed pyrolysis products. Figure 3 shows the mass spectra obtained for a benzene/hexane solution (89% benzene, 11% hexane). The data were collected with a D

DOI: 10.1021/acs.energyfuels.8b00232 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

One aspect of our results that is particularly remarkable is that the addition of a small amount of hexane into a solution with benzene results in a dramatic enhancement in the rate of biphenyl formation at lower temperatures. This phenomenon can be understood in terms of the reaction pathways accessible in neat benzene and in a benzene/hexane mixture. In neat benzene, initiation occurs via C−H bond breaking to form a phenyl radical and a hydrogen atom: C6H6 → Ph• + H•

(5)

The reported Eact for the process is 4.9 eV.48 While this process must occur as the first step in the pyrolysis of benzene, we do not detect phenyl radical in our experimental data. That is likely due to the subsequent reactions that consume phenyl radicals and produce the biphenyl molecule (eqs 2 and 3). Both biphenyl formation routes must begin with initiation via eq 5 with a corresponding activation energy of 4.9 eV. We note that the hydrogen atom can then undergo an abstraction reaction with another benzene to produce an additional phenyl radical:45

Figure 4. Top: Total ion count vs reactor temperature for two representative data sets (red, hexane/benzene, 725 psi, 12% hexane; black, benzene, 800 psi). Bottom: Reactor temperature vs natural logarithm of the ratio of mass 154 to mass 78 for the same data sets.

C6H6 + H• → Ph• + H 2

(6)

If an alkane is added to the fluid, additional reaction pathways become available in the mixture. To keep the number of possible pathways manageable, it is helpful to consider a simpler analogue, such as a benzene/ethane mixture. In such a system, initiation can occur either via benzene as described in eq 5 or via C−C or C−H bond cleavage in ethane (eqs 7 and 8,

then the mass spectra for benzene/hexane should show a biphenyl abundance increase with the same slope as was obtained with the pure benzene because the chemical mechanism of biphenyl formation is unaltered. However, if the system followed case 2, then a change in the ratedetermining step would cause the reaction progress to depend differently on the temperature. To further evaluate these possibilities, we plot the natural logarithm of the ratio of the biphenyl (m/z = 154) to benzene (m/z = 78) abundances as a function of the temperature in the bottom panel of Figure 4. For the binary mixture, biphenyl forms at a lower temperature relative to the TIC decay (nozzle plugging) and increases in abundance with a reduced slope as the temperature is increased in contrast to pure benzene. It is important to note that the temperature axes cannot be directly compared between different experiments because the variability in residence times (Figure S3, Supporting Information) resulting from imprecise drilling of the 10 μm apertures (Figure S1, Supporting Information) influences the temperature at which pyrolysis is observed. Therefore, we compare relative rather than absolute temperatures in our analysis. Although the biphenyl abundance increases at a faster rate in the case of pure benzene, the onset of its formation corresponds to the onset of rapid nozzle plugging, beyond which the TIC is very small. Replicate studies of neat benzene and hexane/benzene mixtures (Figure S2, Supporting Information) confirm the impact of hexane on the rate of biphenyl formation. Because pressure is known to play a role in the kinetics of small alkane pyrolysis in the gas phase,47 data were also collected at 800 psi and are included in Figure S2. The data taken at different pressures did not differ within the experimental uncertainty. Therefore, it is likely that the change in fluid density due to pressure does not significantly alter the chemistry in this study. We are interested in unraveling the chemical consequences of such a rationally tuned density change and are currently investigating systems with lower critical pressures so that we can make greater changes to the fluid density.

CH3CH3 → CH3• + CH3•

(7)

CH3CH3 → CH3CH 2• + H•

(8)

respectively). Given the much lower activation energies of the reactions in eqs 7 and 8 (3.7 and 4.3 eV, respectively)49 compared to that of eq 5 (4.9 eV), we would expect both eqs 7 and 8 to be several orders of magnitude faster than eq 5 at 600 °C. Because the variations in CH and CC bond dissociation energies are small for n-alkanes (