Pyrolysis gas products from hexane were analyzed using Py-GC/MS coupling technology. Hexane served as a model compound to replace gaseous propane, indirectly validating the propane pyrolysis mechanism. A 5 μL aliquot of analytical-grade liquid hexane (purity ≥ 99.5%) was placed in a quartz pyrolysis tube. Inert quartz wool was used for fixation and sealing to prevent sample volatilization. After sealing, the tube was purged with nitrogen gas (flow rate 50 mL/min) for 3 minutes to remove oxygen. Heat at a rate of 10˚C/min to the target pyrolysis temperature of 800˚C, then hold at this temperature to complete pyrolysis. The gaseous products generated during pyrolysis were introduced into the gas chromatograph. Chromatographic separation and mass spectrometry enable qualitative and quantitative analysis of product composition, providing experimental data to support investigations into pyrolysis reaction pathways and selective control mechanisms.

With the support of GaussView software, Gaussian software can be applied in related fields such as chemistry, biochemistry, and physical chemistry [18]-[21]. Gaussian software calculates chemistry using two main methods: molecular mechanics methods and electronic structure theory, which in turn includes semi-empirical methods, ab initio calculations, and density-functional theory methods [22]. The M062X/6-31++G(d,p) level of calculation has been shown to give accurate thermodynamic properties and potential heights for hydrocarbons [23]-[26].

In this paper, the M06-2X/6-31++G(d,p) method was used to optimize the geometries of the reactants and products involved in the starting reaction, and all optimized structures were subjected to vibrational analysis by frequency calculations, which determined that there were no imaginary frequencies for either the reactants or the products. Transition states were identified using the Transition State (TS) method, ensuring that each transition state structure possesses only one imaginary frequency. Furthermore, each reaction underwent Intrinsic Reaction Coordinate (IRC) validation to ensure an accurate connection between reactants and products at every transition state.

All reaction path calculations were computationally realized using the G09W software with the implemented “strict” convergence criteria and “ultra-fine” integration grid. Figure 1 shows the optimized geometry of the species involved in the initial decomposition of propane.

Figure 1. The optimized geometry of the species involved in the initial decomposition of propane.

Since propane is a gas, making experimental control challenging, we supplemented the experiment with a pyrolysis study of hexane (another alkane with a representative structure). This indirectly validated the core conclusion that the primary products of alkane pyrolysis are olefins.

Tables 1-3 present the product distributions from three experimental pyrolysis studies of hexane. Based on the alkane radical chain pyrolysis mechanism, the validity of propane pyrolysis energy calculations is validated using hexane (an alkane homologue) pyrolysis products: The tabulated data indicate that the initial stage of hexane pyrolysis is a “primary cracking phase” dominated by radical chain reactions: The C-C bonds in the feedstock hexane undergo low-energy-barrier cleavage, yielding small-molecule alkanes and alkenes. At this stage, alkenes constitute a high proportion and are one of the primary products in the early pyrolysis phase. The later stage is the “product evolution phase” dominated by secondary reactions of the alkenes: The initially formed alkenes undergo secondary reactions such as hydrogenation and addition (e.g., alkenes reacting with radicals to form alkanes), leading to a significant decrease in the proportion of alkenes, while small-molecule alkanes become the dominant products. This temporal product evolution closely aligns with the reaction pathway derived from propane pyrolysis energy barriers, indirectly validating the reliability of propane pyrolysis energy calculations and providing experimental support for the rationality of the propane pyrolysis reaction mechanism.

Hexane was selected as the model compound to validate the propane pyrolysis mechanism, primarily based on the fact that homologous series of alkanes undergo pyrolysis via a unified radical chain reaction mechanism. Specifically, all undergo C-C/C-H bond cleavage to generate radicals, followed by chain propagation through β-scission and hydrogen abstraction reactions, ultimately yielding small-molecule alkanes and alkenes. This commonality provides a robust theoretical foundation for analogical validation. The core reaction pathways and radical evolution patterns of low-carbon alkanes (C3-C6) pyrolysis exhibit high consistency. Minor differences in energy barriers due to carbon chain length do not compromise the validity of the mechanism correlation.

The kinetics of the two processes exhibit distinct differences in cage effects. The gas-phase pyrolysis of C3 propane shows no cage effect, with reaction rates primarily governed by molecular diffusion and collision frequency. In contrast, the cage effect during liquid-phase pyrolysis of C6 hexane slightly increases radical recombination probability, though this effect is significantly weakened at elevated temperatures. By precisely correlating the temporal distribution of hexane pyrolysis products with the radical reaction patterns of propane pyrolysis, this study effectively addresses the challenge of direct detection in gaseous propane and enhances the scientific rigor of mechanism validation.

Table 1. Product distribution of hexane pyrolysis experiment 1.

Table 2. Product distribution of hexane pyrolysis experiment 2.

Table 3. Product distribution of hexane pyrolysis experiment 3.

To determine the pyrolysis mechanism of propane, it is necessary to study the initial reactions of propane pyrolysis. The bond dissociation energy is an important reference for judging the ease of bond breaking; the larger the bond dissociation energy the less likely it is to break, and vice versa [27]. In terms of computational accuracy, the M06-2X functional demonstrates outstanding performance in calculating the thermochemical properties of alkanes, with its predicted bond dissociation energy errors typically ≤ 2% [28]. The 6-31++G(d,p) basis set, incorporating diffusion and polarization functions, effectively describes the electron distribution in radical systems.

Figure 2 shows the calculated bond dissociation energies for the three initial pyrolysis pathways of the propane molecule. The C-C bond dissociation energy of 95.587 kcal/mol obtained in this study is slightly higher than values reported in other literature [29][30]. However, it falls within the reasonable fluctuation range of calculations using different DFT methods, with the discrepancy primarily stemming from differences in how the functional describes electron correlation effects. The bond dissociation energy of secondary C-H bonds (104.672 kcal/mol) in this study is lower than that of primary C-H bonds (108.069 kcal/mol). This aligns with the general principle that “secondary carbon free radicals exhibit greater stability due to hyperconjugation, resulting in lower corresponding bond dissociation energies” [31][32]. The bond dissociation energy for the C-C bond is lower than that for the C-H bond. Therefore, Path 3 is the kinetically optimal path and is the most likely to occur in the initiation reaction.

Numerical discrepancies arising from different theoretical approaches are normal phenomena. The findings of this study do not contradict the core consensus in the literature: C-C bond dissociation energies are lower than C-H bonds, and secondary C-H bonds exhibit lower dissociation energies than primary C-H bonds. The methodological choices are based on clear scientific rationale, and the computational data provide a foundation for the future development of alkane pyrolysis technology.

Figure 2. Initial reaction path of propane pyrolysis.

From the experiments, the main products of propane pyrolysis are ethylene, propylene, hydrogen, methane and ethane [33][34]. The reaction pathways are significantly affected by radical stability and reaction conditions. For example, high temperatures may promote homocleavage (initiation phase) and β-breakage (transfer phase), increasing the proportion of ethylene [35]. Low temperatures may inhibit C-C bond breaking and promote C-H bond breaking to generate H- and propyl radicals. And low pressure reduces the probability of radical collision, decreases the termination reaction rate, extends the chain reaction lifetime, and enhances the main product selectivity.

Table 4 shows the various radical reactions involved in the pyrolysis of propane. Pyrolysis typically involves a free radical chain reaction consisting of three phases: initiation, transfer, and termination [36]. In the initiation phase, the propane molecule is homolyzed to generate the initial radicals. In the transfer stage, each radical reacts with the propane molecule to form products and regenerate radicals, maintaining the chain cycle. In the termination phase, the termination reaction usually requires a high concentration of radicals to occur, and its rate directly affects the duration and product selectivity of the chain reaction. The transition state structures involved in the free radical reactions 4, 5, 6...14 are named TS₄, TS₅, TS₆...TS₁₄, as shown in Figure 3.

Table 4. Free radical reactions of propane pyrolysis.

Figure 3. Transition state geometry involved in each free radical reaction.

Based on the free radical reaction mechanism of propane pyrolysis, four possible reaction paths were analyzed and organized for the formation of the products ethylene and propylene.

As shown in Figure 4 and Figure 5, Pathways 1 and 2 are reactions initiated by C-H bond cleavage. Experimental results from propane pyrolysis indicate that the primary products are propylene, ethylene, hydrogen, and methane. The initial step involves C-H bond cleavage: Path 1 forms n-C₃H₇•, while Path 2 formsi-C₃H₇•. In pathway 1, n-C₃H₇• generates methane via steps 7 and 9, and produces H₂ via steps 5 and 13. In pathway 2, i-C₃H₇• yields corresponding products via steps 8 and 10, and steps 6 and 14. The structural difference between n- and i-propyl radicals leads to distinct product ratios along the two pathways. This represents the proposed reaction pathways for propane’s primary olefin products. For other products, given their minor contributions, predicting their formation would be extremely complex and challenging; thus, this is not pursued here.

Figure 4. Design of propane pyrolysis path 1.

Figure 5. Design of propane pyrolysis path 2.

After inferring the pyrolysis pathway of propane based on its product distribution, it is necessary to calculate the reaction energy barrier pathway to validate its feasibility. The energy barrier diagrams for propane pyrolysis Path 1 and Path 2 are shown in Figure 6 and Figure 7, respectively, clearly illustrating the energy level differences among the reactions.

Figure 6. Schematic representation of the reaction energy barriers during the formation of ethylene and propylene in path 1.

Figure 7. Schematic of the reaction energy barriers during the formation of ethylene and propylene in path 2.

The energy barrier difference between propane pyrolysis Path 1 and Path 2 fundamentally stems from the distinct properties of radical structures and reaction bonds. During the initial C-H bond cleavage stage, Path 2 exhibits a lower energy barrier (104.67 kcal/mol) than Path 1 (108.07 kcal/mol). This arises because the isopropyl radical possesses a stronger hyperconjugation effect and a more dispersed electron cloud, which weakens the bond energy of the initial C-H bond, making it easier to break.

The difference in product energy barriers is directly related to the bond types involved in the reaction. The energy barrier for ethylene formation (approximately 142.59 kcal/mol) is nearly identical for both pathways because the corresponding β-cleavage involves a terminal C-C bond. This bond exhibits uniform electron density distribution and no significant side-chain disturbance affecting its bond energy. In contrast, the energy barrier for propylene formation (Path 1: 149.59 kcal/mol; Path 2: 148.22 kcal/mol) is higher. This is because the corresponding C-C bond is adjacent to a side chain, where the hyperconjugation effect stabilizes the bond, increasing the energy required for cleavage. Comparatively, ethylene has a slightly lower energy barrier, making its formation more favorable during the initial stages of pyrolysis. Among other products, the energy barriers for CH₄ (Path 1: 151.21 kcal/mol; Path 2: 148.99 kcal/mol) are lower than those for H₂ (Path 1: 158.80 kcal/mol; Path 2: 156.12 kcal/mol). This stems from CH₄ formation involving radical hydrogen abstraction (single bond transfer, requiring lower energy barriers), whereas H₂ formation involves dehydrogenation (involving double electron pair bond breaking, consuming more bond energy). Thus, the alkane product CH₄ forms earlier than H₂.

From the pyrolysis mechanism perspective, the structural differences of initial radicals determine pathway initiation ease, while the height of product energy barriers directly corresponds to formation timing. Ethylene (low energy barrier) accumulates initially, while propylene and CH₄ (medium energy barrier) increase as the reaction progresses. H₂ (high energy barrier) gains a larger proportion in the later stages. These energy barrier differences clearly illustrate pyrolysis patterns: the energy level differences in the energy barrier diagram directly reflect reaction priority, fundamentally embodying the pyrolysis principle that “the lower the energy barrier, the earlier the product forms”.

As shown in Figure 8 and Figure 9, pathways 3 and 4 are reactions triggered by C-C bond cleavage. Path 3 initiates with the initial C-C bond cleavage of propane (distinct from the C-H cleavage in Paths 1 and 2), generating two types of radicals: CH₃• and C₂H₅•. After conversion to n-C₃H₇• via hydrogen abstraction reactions (Steps 9, 11, 13), β-cleavage (Step 7) and dehydrogenation (Step 5) concurrently yield CH₄, C₂H₄, and C₃H₆. This pathway centers on multi-radical product evolution driven by carbon-carbon bond cleavage. Pathway 4 features i-C₃H₇• as its core radical. Due to the insufficient terminal carbon chain length of this branched radical, direct ethylene formation via β-cleavage is unfeasible. Therefore, only the energy barrier for propylene generation was calculated: i-C₃H₇• undergoes dehydrogenation in Step 6 to form C₃H₆. Pathways 4-1 to 4-3 all focus on propylene production, demonstrating how the structural properties of branched radicals limit product formation. This contrasts with the multi-product pathways of Pathway 3, reflecting structurally driven evolutionary differences.

Figure 8. Design of propane pyrolysis path 3.

Figure 9. Design of propane pyrolysis path 4.

Hydrogen atom abstraction is the core step in radical chain propagation during gas-phase alkane pyrolysis. In pathway 3, radicals such as CH₃• and C₂H₅• generated from the initial C-C bond cleavage of propane abstract hydrogen atoms from the secondary carbon (-CH₂) and primary carbon (-CH₃) sites of propane molecules within the system, converting them into n-C₃H₇• to sustain radical chain propagation. The secondary carbon hydrogen bond, possessing lower bond energy and higher electron density, becomes the preferred site for hydrogen abstraction. This mechanism directly determines the reaction priority for subsequent β-cleavage to produce products like CH₄ and C₂H₄. The primary sources of hydrogen atom abstraction are: 1) Unreacted propane molecules: The main hydrogen donor, from which radicals can abstract hydrogen from both secondary and primary carbon sites. Secondary carbon hydrogen, possessing lower bond energy and higher electron density, is the preferred abstraction site. 2) Reaction intermediates: Generated alkyl radicals such as n-C₃H₇• can also act as hydrogen donors in abstraction reactions, sustaining radical chain propagation. In the gas phase environment, molecular diffusion and high-frequency collisions provide ample contact opportunities for these abstraction reactions, ensuring the continuous advancement of the chain reaction.

The energy barrier diagrams for propane cracking pathways 3 and 4 are depicted in Figure 10 and Figure 11, respectively, clearly illustrating the energy level differences between the respective reactions. Path 3 initiates with propane C-C bond cleavage, whose activation energy (95.59 kcal/mol) is lower than the C-H bond cleavages in Paths 1 and 2. This stems from the inherently weaker C-C bond energy compared to C-H bonds. In Path 3, the energy barrier for CH₄ (109.96 kcal/mol) is the lowest, as its formation relies on hydrogen abstraction and single-bond electron transfer, resulting in minimal energy consumption. In contrast, the barriers for ethylene (133.63, 137.55, 178.41 kcal/mol) and propylene (126.63, 130.55, 170.4 kcal/mol) are higher. This is because the β-cleavage of ethylene involves a C-C bond whose bond strength is enhanced by the electron density concentration effect from adjacent radicals, requiring higher energy input for cleavage. In contrast, the C-C bond corresponding to propylene experiences weaker electron density disturbance from radicals. Path 4’s core radical is i-C₃H₇•. Its branched structure cannot undergo β-cleavage to form ethylene, thus yielding only propylene and alkane products. Its propylene formation energy barriers (132.26, 136.18, 177.03 kcal/mol) are higher than those in Path 3. This is because the hyperconjugation effect of the side chain disperses the electron cloud of the C-C bond, enhancing bond stability and increasing the fracture energy barrier. Conversely, the energy barrier for CH₄ in pathway 4 (107.74 kcal/mol) is lower than in pathway 3, as the side chain structure of i-C₃H₇• increases the electron cloud density at the hydrogen abstraction site, facilitating the abstraction reaction.

Regarding pyrolysis mechanisms, the C-C initiation fracture in Path 3 broadens the product spectrum. Low-energy hydrogen abstraction first drives alkane formation, followed by β-cleavage yielding alkenes. In contrast, the branched radical structure in Path 4 restricts the product range, clearly demonstrating the dual regulatory role of reactant bond properties and radical structure on pyrolysis energy barriers.

Figure 10. Schematic of the reaction energy barriers during the formation of ethylene and propylene in path 3.

Figure 11. Schematic of reaction energy barriers during the formation of ethylene and propylene in Path 4.

In summary, based on the previously calculated bond dissociation energies, the initial activation energy barrier for C-C bond cleavage in propane molecules is lower than that for the C-H cleavage pathway. Pathways 3 and 4 emerge as critical pathways, primarily due to the synergistic effect between bond energy differences and reaction energy barriers. According to the Arrhenius equation, the lower activation energy corresponds to a higher reaction rate constant, enabling C-C initiation to dominate the kinetics during the early pyrolysis stage. Concurrently, the subsequent disproportionation/β-cleavage reactions of the C-C initiated propyl radical exhibit product selectivity: the activation energy for ethylene formation is significantly lower than that for propylene or methane pathways. This dual characteristic of “low initiation energy barrier + high product selectivity” directly determines ethylene’s dominant position in the overall product composition, making it a core target for optimizing pyrolysis products.