{'fraction_non_alphanumeric': 0.027696109761813456, 'fraction_numerical': 0.018151604243896204, 'mean_word_length': 5.016406049730838, 'pattern_counts': {'":': 0, '<': 1, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 1, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Kinetic Modeling of Jet Propellant-10 Pyrolysis

A detailed kinetic model for the thermal decomposition of the advanced fuel Jet-Propellant 10 (JP-10) was constructed using a combination of automated mechanism generation techniques and ab initio calculations. Rate coefficients for important unimolecular initiation routes of exo-TCD were calculated using the multireference method CAS-PT2, while rate coefficients for the various primary decompositions of the exo-TCD derived monoradicals were obtained using CBS-QB3. Rate-of-production analysis showed the importance of four dominating JP-10 decomposition channels. The model predictions agree well with five independent experimental data sets of JP-10 pyrolysis that cover a wide range of operating conditions (T = 300-1500K, P = 300 Pa – 1.7 10 5 Pa, dilution = 0.7 – 100 mol% JP-10, conversion = 0-100 %) without any adjustment of the model parameters. A significant part of the model comprises secondary conversion routes to aromatic and polyaromatic hydrocarbons (PAHs) and could thus be used to assess the tendency for deposit formation in fuel rich zones of endothermic fuel applications.


Introduction
Mono-and polycyclic hydrocarbons represent an interesting class of molecules that can be used as fuels in advanced aviation applications 1 . Many of them have superior physical properties such as a high energy content, a low viscosity at low temperatures, an elevated flash point and a higher density compared to the classically used n-alkanes 2,3 . Cyclic alkanes also have higher thermal stability limits and a larger heat sink capacity than their acyclic counterparts (cf. n-decane vs. decalin in ref. 4 ), which are important properties in the propulsion technology. Endothermic cracking of the fuel prior to the combustion chamber [5][6][7] not only amplifies the heat sink, it can also reduce ignition delays as illustrated by Colket et al. 8 . Hence, a thorough understanding of the decomposition chemistry of cyclic hydrocarbons is not only important for the design and optimization of the endothermic reaction technology itself, but also for the correct design of the combustor that converts the products of the endothermic decomposition.
This study focuses on the kinetic modeling of the thermal decomposition of the fuel Jet Propellant-10, "JP-10", used as the fuel for air-breathing missiles 9 . JP-10 consists of a tricyclic hydrocarbon, exo-tricyclo[5.2.1.0 2,6 ]decane (exo-TCD), cf. Scheme 1. Although a fair number of experimental JP-10 pyrolysis studies identified and quantified light products containing up to five carbons [10][11][12][13][14][15] , less is known about the quantities of heavier intermediates and products. Several studies noted the significant formation of aromatic components such as benzene and toluene under pyrolysis conditions 10,[12][13][14][15][16] . Even in the presence of an oxidizer, aromatic components represented an important share of the products as the comparative study between the flame structure of n-decane and JP-10 flames 17 indicated.
Vandewiele et al. 18 obtained a new data set of JP-10 pyrolysis experiments using GC×GC-FID/ToF-MS in which over seventy species with mass-to-charge ratio up to 178 (C 14 H 10 ) were identified and quantified. Validation of two available kinetic models of JP-10 thermal decomposition, the San Diego model 19 and the model by Magoon et al. 20 , revealed that they were largely inadequate in accurately predicting product distribution trends of the experiments of Vandewiele et al. 18 . This is not surprising since these kinetic models were designed for higher temperature combustion applications and not for the pyrolysis conditions investigated in that study. Herbinet 23 is an open-source software package that automatically constructs kinetic models based on the evaluation of reaction rates and species concentrations. The procedure used by RMG to expand the kinetic model by successive addition of kinetically significant species 24 is extensively discussed elsewhere (e.g. [25][26][27] ) and therefore only the elements specific to the modeling of the pyrolysis of polycyclic hydrocarbons are highlighted here. Temperature, pressure and initial exo-TCD dilution, which are used to evaluate rates of production in RMG, are set to 1100K, 1 10 5 Pa and 10 mol %, respectively. The main parameters determining the size of the generated reaction network, i.e. the time at which the integration of the ODEs is stopped and tolerance ε for the network generation, were set to 0.5s and 0.001, respectively. For these conditions the most important products observed in the experiments are included in the reaction network. The use of tighter tolerances significantly increases the size of the model, but does not affect the model predictions. The effect of both parameters on the size of the reaction network for steam cracking of n-hexane was illustrated by Van Geem et al. 27 .
RMG was previously employed to construct a model for combustion applications of JP-10 20 . The model described in this work is not an update of this combustion model but was built from scratch for endothermic fuel applications of JP-10 and aims at elucidating a complementary part of the thermal decomposition chemistry of JP-10. The pyrolysis of exo-TCD is described mainly by three elementary reaction families: intermolecular hydrogen abstraction reactions by radicals, intra-and intermolecular radical addition reactions and intermolecular radical recombination reactions. Details of the rate rules of these reaction families can be found elsewhere 22,23 . Note that the reactions belonging to the reverse reaction families, i.e. β-scission reactions and unimolecular scission reactions, are allowed too; rate coefficients of these reactions are calculated based on thermodynamic consistency.
In addition to built-in databases for reaction families, a number of reactions from literature were added that contain relevant exo-TCD pyrolysis chemistry. First of all, two sub-models were added that contain chemistry of species with two carbon atoms or less using the seed mechanism option in RMG. The Leeds methane oxidation model v1.5 28 , stripped of all oxygen chemistry, was used as a base model for high temperature pyrolysis reactions, resulting in 34 reactions between 22 species. An ethane steam cracking model by Sabbe et al. 26 , stripped of all reactions containing four or more carbons, was used as a base model for the low temperature pyrolysis and contains 277 reactions between 35 species. Next, a number of literature sub-models were added to RMG through the so-called "Reaction Library" option of RMG. The reactions in these submodels are considered during the iterative enlargement procedure and are added to the final model based on the kinetic significance of the species of these reactions. These literature submodels can be divided into two categories. First of all, sources of cyclic C 5 -C 6 -C 7 chemistry 29,30 were included, as it is believed that they are crucial in bridging the gap between the exo-TCD decomposition products and secondary products such as aromatics. Secondly, sources were included that contain pathways for the growth of polycyclic aromatic hydrocarbons such as indene and naphthalene 31,32 . Finally, thirty-three exo-TCD decomposition reactions were added, primarily originating from new transition state theory (TST) calculations performed in this study.
The automated calculation of pressure-dependent rate coefficients of reactions by RMG was not applied for this study. As a result, the assigned rate coefficients of the generated reactions in the model should be treated as high-pressure limits. Nevertheless, 20 reactions originating from ad hoc added reaction libraries have pressure-dependent rate expressions.

Thermochemistry
A detailed decomposition model necessitates the inclusion of thermochemical parameters for all intermediate species. This can be achieved on the basis of group and bond additivity methods 33 but the accuracy of these estimates may be doubtful for species with structures that fall beyond the scope of the regressed group additive parameters. More specifically, the influence of ring strain on the thermochemistry estimates of polycyclic intermediates remains a point of concern.
Magoon et al. 34,35 addressed this issue by incorporating on-the-fly thermochemical calculations for polycyclic intermediates using three-dimensional atomic coordinates in molecules 20 . In the present work, several important modifications in the calculation of thermochemical parameters for polycyclic intermediates were incorporated. First of all, a database of polycyclic ring strain corrections (RSC) was added containing ca. one hundred polycyclic structures [36][37][38][39][40] . If an appropriate polycyclic RSC is found for a component, it is used in the estimation of the thermochemical parameters. If not, the molecule is passed-on to the framework that generates 3D atomic coordinates on-the-fly using a distance-geometry algorithm implemented in the chemoinformatics toolkit RDKit 41 to convert the chemical graph representation into a 3D geometry. Secondly, the database of hydrogen bond increments (HBIs), a method developed by Lay et al. 42 to estimate the thermochemistry of radicals, was expanded with groups for cyclic and polycyclic radicals found in literature [43][44][45][46] . The radical atom in strained species cannot adopt the energetically favored planar geometry because of the constriction of the rigid carbon frame and therefore significantly alters the value of the corresponding HBI in comparison with the acyclic analog. Table 1 shows a comparison of the thermochemistry estimates for a selected set of polycyclic strained species relevant for exo-TCD decomposition. Three values are reported: the Benson group additivity method including polycyclic ring strain corrections (RMG-GA), the semi-empirical PM3 method using 3D atomic coordinates that are generated on-the-fly (RMG-PM3) and finally the best available benchmark value. It was observed that neither RMG-GA nor RMG-PM3 reproduces all benchmark data well. In general, RMG-GA scores better for enthalpies of formation than for entropies or heat capacities because more data is available in literature. In the worst case, S and C p contributions of polycyclic RSCs were estimated as a sum of contributions of smaller rings. The example of norbornene illustrates this principle. The value of the standard enthalpy of formation at 298 K, estimated through RMG-GA corresponds well with the benchmark value, whereas significantly differences are observed for the standard entropy at 298 K. The values for the enthalpy contribution to the norbornene RSC originate from Wiberg 37 .
Due to the lack of values for the entropy contribution of the norbornene RSC, the value is taken equal to the cyclopentane RSC. Note that the thermochemistry of species in the model is always based on the best available estimates, e.g. CBS-QB3 values for the six tricyclodecyl C 10 H 15 radicals from 47 .

Ab initio calculations
High level thermochemical data for species involved in the JP-10 thermal decomposition is very scarce. Rate coefficients are even less available, in particular regarding the initial decomposition of exo-TCD. Therefore in this work new high-level ab initio calculations were carried out on a set of reactions that are believed to be important for the decomposition of exo-TCD. Three types of reactions were considered.
Unimolecular C-C scission reactions of exo-TCD leading to biradicals and subsequent intramolecular H-abstraction reactions of the biradicals were explored using density functional theory (DFT) and multireference methods. In the DFT methodology, geometries were optimized using UB3LYP (unrestricted B3LYP) and energies were calculated using CBS-QB3 to explore the PES and obtain initial refinements of kinetic parameters. Using the DFT geometries as initial

Reactor modeling and experimental data
A wide variety of experimental data on the thermal decomposition of exo-TCD published in open literature were used for validating the model. The kinetic model was compared to the product distribution data obtained by Herbinet et al. 10 (hereafter referred to as the "Nancy data"), Nakra et al. 13 ("Nakra data"), Van Devener and Anderson 14 ("Van Devener data"), Rao and Kunzru ("Rao data"). 12 , Xing et al. 15 ("Xing data") and Vandewiele et al. 18 , referred to as "Ghent data" in this work. Table 2 shows an overview of the operating conditions of each experimental data set.  of these impurities typically does not exceed 3.5% of JP-10 feedstock 60 , the presence of endo-TCD may be of importance for the reactivity of exo-TCD. Hudzik et al. 47 reported that the standard enthalpy of formation of endo-TCD is 17 kJ mol -1 higher than the value for exo-TCD, suggesting that the endo-isomer may be less stable than the exo-isomer. Information on the initial decomposition of endo-TCD is non-existing in literature; therefore, only one reaction was added that converts endo-TCD into BR1, similar to the initiation of exo-TCD.
The activation energy of this reaction is lowered by 17 kJ mol -1 , relative to the value of the reaction with exo-TCD, to reflect the higher enthalpy of formation of the endo-isomer. The JP-10 feedstock of the modeled experiments was treated as a mixture of exo-and endo-TCD whenever information on the content of endo-TCD was available, cf. Table 2. If not, JP-10 was considered as 100% exo isomer.

Results and Discussion
The final RMG-generated, kinetic model for the pyrolysis of exo-TCD contains 5261 reactions between 384 species, i.e. 234 radicals and 150 molecular species and can be found in Section S1 of the Supplementary

Exo-TCD initiation
It is generally assumed that the pyrolysis of hydrocarbons such as exo-TCD proceeds through a free radical mechanism. The initiation of exo-TCD involves the creation of radicals through unimolecular bond scission reactions of exo-TCD. Although the rupture of C-H bonds is possible as an exo-TCD initiation route, these bonds are stronger than C-C bonds and were consequently neglected. The unimolecular initiation of exo-TCD can proceed through unimolecular C-C bond scission reactions resulting in the creation of seven distinct biradicals named BRx, x=1..7, cf.
The resulting highly unstable biradicals can further isomerize to stable molecules via intramolecular H-abstraction reactions (disproportionation reactions), proposed by Herbinet et al. 10 upon detecting 3-cyclopentylcyclopentene, further referred to as "MA110", as a major primary decomposition product.
Intramolecular H-abstraction reactions are thought to proceed via multiple distinct transition states that differ by the polycyclic entity that is formed through the course of the reaction, cf. Scheme 3.

Scheme 3: The unimolecular C-C scission of exo-TCD yielding biradicals and subsequent intramolecular Habstraction reactions yielding molecular intermediates taken into account in this work.
Herbinet et al. 10 20 suggested that initiation of exo-TCD towards MA110 may proceed as a concerted step rather than via a distinct biradical intermediate 20 .
In the present work, rate coefficients for 10 ring-opening and intramolecular H-abstraction reactions were calculated. The latter are particularly useful in the re-evaluation of the relative importance of the different exo-TCD initiation routes. Table 3 shows values for the Arrhenius parameters for these reactions. Activation energies for the ring-opening reactions are similar to those reported by Herbinet et al. 10 .

Tricyclodecyl decomposition reactions
It is expected that a large part of exo-TCD conversion is attributed to H-abstraction reactions by radicals. The abstraction of a hydrogen atom from exo-TCD yields six distinct C 10  Nomenclature for the tricyclodecyl monoradicals is adopted from Hudzik et al. 47 .
The tricyclodecyl radicals subsequently react via various C-H and C-C β-scission reactions leading to either C 10 H 14 molecules or to new C 10 H 15 radicals. A single monoradical can react through multiple parallel decomposition channels. The kinetics for these elementary reactions were not available and therefore they were calculated.
Literature providing kinetic data for cycloalkyl decomposition reactions is very scarce, which makes a comparison of the calculated rate coefficients with previously reported data for these  Figure 1a. In some cases, the joining of the cyclopentane and norbornane ring in tricyclodecyl radicals leads to reactions whose TS structure is not comparable to TS's of the smaller units.
Therefore, Figure 1 only compares tricyclodecyl reactions whose TS's are located in one of the two rings.

Kinetic model validation
The generated kinetic model was validated against five data sets of exo-TCD pyrolysis experiments. None of the rate coefficients or thermochemical parameters were adjusted by regression of the experimental data, i.e. these comparisons are a test of the predictive ability of the model.    Figure 3b shows mass fractions of the major products as a function of the equivalent space time for the Rao data. In this case the agreement between model predictions and experimental data is better than for the Ghent data. The main difference is that in this case model predictions of methane agree with experiments, while methane was slightly underpredicted for the Ghent experiments. Figure 4 shows the mole fractions of the major products as a function of the mean reactor temperature for the Nancy data. Again, good agreement is found for the majority of the major products and confirms the trends seen in Figure 3.   as an important primary decomposition product and which also appears in the current model. As can be seen on Figure 7, the temperature of peak concentration is well predicted, but concentrations are slightly mispredicted.

Major decomposition pathways of exo-TCD
While the thermal decomposition of exo-TCD is initiated by the scission of a C-C bond yielding BR1, at the conditions corresponding to Scheme 5, 85% of the decomposition rate of exo-TCD is accounted for by H-abstraction reactions by H. The main decomposition channels involve TCDR5, TCDR8 and TCDR4 with contributions of 33%, 26% and 14% respectively.
Decomposition pathways through TCDR7 and TCDR10 are negligible due to the significantly higher bond dissociation energies of the corresponding C-H bonds 47 . The important C 10 radicals lead to different primary products: TCDR8 leads primarily to cyclopentene and 1,3-

Secondary chemistry and aromatics formation
Many studies of JP-10 pyrolysis indicated the presence of significant quantities of aromatic components, with benzene, toluene, styrene, indene and naphthalene as the most prominent species 10,12  Secondary conversion of cyclopentene leads to 1,3-cyclopentadiene, with the direct dehydrogenation pathway accounting for ca. 80% of the total cyclopentene decomposition rate.
The conversion of propene produces methyl radical and is the second major source for methyl radicals. The latter recombine with CPDyl yielding 5-methylcyclopentadiene. This C 6 H 8 molecule can rearrange into the other methylcyclopentadiene isomers or can undergo Habstraction resulting in methyl-1,3-cyclopentadien-5-yl and isomeric H-fulvenyl radicals. The latter rearrange via various exit channels to fulvene, 1,3-cyclohexadienyl and eventually benzene.
The kinetics and thermochemistry for these reactions and species were taken from Sharma and Green 30 .
Another important aspect of the kinetic model at high exo-TCD conversion, is the secondary conversion of ethenylcyclopentenes. The latter produce fulvene and represents an important pathway to α-H-fulvenyl, cf. Scheme 6. Next to the primary decomposition channels from TCDR6 and TCDR8, the hydrogen assisted fulvene isomerization and the decomposition of 5methyl-1,3-cyclopentadien-5-yl 30 become importance sources of benzene too. A sensitivity analysis of benzene predictions at 1100K is shown in Figure 9 and illustrates again the importance of the unimolecular ring opening reaction of exo-TCD yielding BR1, initiating the chemistry. The majority of the reactions shown in Figure 9 were based on estimations by analogy from RMG's databases. More accurate rate coefficients for these reactions based on ab initio techniques may therefore be valuable.

Conclusions
A detailed kinetic model was developed to simulate JP-10 pyrolysis using the RMG. New elements of this model compared to previously developed JP-10 models are the inclusion of more accurate kinetics for initial exo-TCD decomposition chemistry, improved fused ring thermochemistry and the incorporation of many secondary conversion routes to aromatics.
The good agreement between model predictions and five independent sets of experimental data

Supporting Information
Section S1: Kinetic model in Chemkin format Section S2: Ab initio reaction barriers at 0K Section S3: Modified Arrhenius parameter fit

References
Section S1 Kinetic model in Chemkin format Cf. separate text files.

Section S2 Ab initio reaction barriers at 0K
Tricyclodecyl beta-scission reactions The structure and nomenclature of the tricyclodecyl radicals is shown in Scheme 1. Structures were optimized with "verytight" convergence criteria and, correspondingly, a very fine integration grid was used ("int=ultrafine"). In the case of TCDR10, there are two minima corresponding to inversion of the radical center; the lower energy conformation was considered here. Saddle points were confirmed to have a single imaginary frequency and the geometries were visually inspected to confirm that the structure was intermediate between the intended reactant and product(s) and intrinsic reaction coordinate calculations were performed to confirm that the saddle point connected the intended reactant and product. Transition state theory calculations were performed using results for the saddle point and corresponding tricyclodecyl reactant.
Reaction barriers at 0K (including zero-point energy effects) are tabulated in Table 1.
Decomposition channels for which saddle point could not be successfully located are   The modified Arrhenius parameters are obtained through a least-squares fit using calculated rate coefficients k training , at ten equidistant temperatures between 298 -3000K. The value of k training is obtained by multiplying the ab initio contribution k AI at temperature T and the Eckart tunneling coefficient κ at temperature T:

Exo-TCD initiation reactions and intramolecular H-abstraction reactions
In general, the uncertainties associated with the parameter fitting are significantly smaller than the uncertainties associated with the level-of-theory of the employed ab initio methods. For example, uncertainties of CBS-QB3 derived reaction barriers typically amount to 4-8 kJ mol -1 .
Statistical analysis has shown that the 95% confidence interval of the E a parameter in the modified Arrhenius expression typically amounts to ± 0.05 kJ mol -1 , for the A parameter a multiplicative uncertainty of 10% and for the n parameter an uncertainty of ±0.01. Nevertheless, it should also be noted that for reactions with substantial tunneling coefficients at lower temperatures (< 500K) a less optimal correspondence between the value of the fitted rate coefficient k fit and the rate coefficient k training from the training set can be expected.