Chemical Kinetic Studies Research Articles

Theoretical study on the H-atom abstraction reactions of pentanol + HȮ2, part I: five branched pentanol isomers.

The chemical kinetic studies of hydrogen atom (H-atom) abstraction reactions by hydroperoxyl (HȮ2) radicals from five branched pentanol isomers, including 3-methyl-1-butanol, 2-methyl-1-butanol, 1,1-dimethyl-1-propanol, 1,2-dimethyl-1-propanol, and 2,2-dimethyl-1-propanol were investigated systematically through high-level ab initio calculations. Geometry optimization, frequency analysis, and zero-point energy (ZPE) corrections were performed for six reactants, twenty-three transition states (TSs), and twenty-four products at the M06-2X/6-311++G(d,p) level of theory. The intrinsic reaction coordinate calculation was performed at the same level of theory to confirm the transition state connection. The one-dimensional hindered rotor treatment for low-frequency torsional modes was also treated at the M06-2X/6-311++G(d,p) level of theory. The QCISD(T)/CBS level of theory was used to calculate the single-point energies for the species whose T1 diagnostic value was lower than 0.035. At the same time, the CASPT2/CBS level of theory was used to calculate the single-point energies for the channel in which the T1 diagnostic value of transition states was greater than 0.035. Rate constants for the H-atom abstraction reactions from the five branched pentanol isomers by HȮ2 radicals were calculated by using conventional transition state theory with asymmetric Eckart tunneling corrections in the temperature range of 500-2000 K. Rate constants and branching ratios for the title reactions and the rate rules for ten different H-atom abstraction types were investigated. Temperature-dependent thermochemistry properties for all reactants and products were calculated by the composite methods of G3/G4/CBS-QB3/CBS-APNO, which were in good agreement with the data available in the literature. Rate constants for the H-atom abstraction reactions by HȮ2 radical from branched pentanol isomers were investigated in this work as part I, and those for linear pentanol isomers will be analyzed in part II. All the calculated kinetics and thermochemistry data can be utilized in the model development for branched pentanol isomers oxidation.

The combustion chemistry of ammonia and ammonia/hydrogen mixtures: A comprehensive chemical kinetic modeling study

Ammonia (NH3) is relatively less reactive compared to hydrocarbon fuels. Therefore, ammonia mixtures blended with hydrogen (H2) have been shown to be a promising fuel for internal combustion engines. In this study, a detailed NH3/H2 chemical kinetic model is developed over a wide range of engine-relevant conditions and comprehensively validated to describe the combustion of NH3/H2 mixtures using available experimental literature data, including ignition delay times, laminar flame speeds and species concentration profiles. The new model captures very well the combustion properties of pure NH3 and NH3/H2 mixtures at most conditions. By performing sensitivity and reaction path flux analyses the key reactions controlling fuel reactivity at high-temperature (≥ 1500 K) and low-to-intermediate temperature (1000 ≤ T ≤ 1500 K) regimes are identified. Moreover, the formation and consumption pathways of nitrogen oxides (NOx) in NH3/H2 combustion at different conditions have also been investigated, which are found to be highly coupled to the underlying chemical reactions that dictate fuel reactivity. The kinetic data for the important reactions and species thermochemistry data used in our model are rigorously evaluated and are discussed in detail.

Two Tetra-Nuclear Ln-Substituted Prazine Dicarboxylic Acid-Functionalized Selenotungstates with Catalytic Oxidation of Thioether Properties.

Two two-dimensional Ln-substituted prazine dicarboxylic acid-functionalized selenotungstates Na3H9[(H2N(CH3)2]2{(Se4W27O100)[Ln4(H2O)8(Hpzdc)2(pzdc)]}·26H2O [Ln = Nd (1) and Ce (2)]; H2pzdc = 2,3-pyrazine dicarboxylic acid) have been synthesized by one-pot self-assembly strategy, in which the basic polyanion [Se4W27O100]22-was composed of two [SeW8O31]10- fragments, a [SeW9O33]8- segment and an intriguing {SeO} group, simultaneously tetra-nuclear Ln3+ ions with H2pzdc pendants were embedded. Compounds 1 and 2 showed excellent catalytic oxidation of thioether properties within a short time (20 min) with high 100% conversion and 98.9% selectivity. In addition, the pioneering Ln-substituted selenotungstates were used as catalysts to degrade sulfur mustard simulant 2-chloroethyl ethyl sulfide at room temperature with 99% conversion and 100% selectivity. The chemical kinetic experiment studies revealed that the catalytic reaction was in compliance with the first-order reaction, and the kinetic half-life (t1/2) values were 3.814 and 3.849 min, respectively.

Experimental and chemical-kinetic evaluation of a heavy-duty methanol PFI engine with direct water injection

Internal combustion engines are still widely used for propulsion in modern vehicles. Upcoming emission legislation imposes stricter limits on exhaust emissions. One method to achieve emission compliance is by using a three-way catalyst (TWC), which offers excellent emission reduction if the mixture is stoichiometric. However, stoichiometric mixtures in spark-ignited engines have the drawback of increased knock propensity. Knock can be mitigated by using water injection, which serves as both a diluent and utilizes latent heat to reduce the temperature, thereby reducing the reactivity of the unburned mixture. Methanol as a fuel has received more attention thanks to its high research octane number (RON) and its potential to contribute to decarbonization when produced as e- or bio-methanol. In the current study, Direct Water Injection (DWI) was evaluated on a Heavy-Duty (HD) single-cylinder research engine fueled by methanol. This work aims to fill a research gap on methanol-fueled engines with water injection. A direct injection system of water was chosen as it offers the freedom to inject during the closed cycle. Furthermore, a chemical kinetic study on the oxidation of stoichiometric methanol–water mixtures was conducted based on findings in the literature suggesting that, under certain conditions, water mixed with alcohol (in this case, ethanol) can reduce the ignition delay. The experimental results demonstrate that DWI effectively suppresses knock and reduced Nitrogen Oxides (NOx), albeit with deteriorated combustion efficiency. The chemical kinetic study suggested that at lower to intermediate temperatures, water acts as an efficient third-body collider, which lowers the ignition delay. However, this effect is not significant for the typical timescales encountered in HD engines.

Chemical kinetic and behavior study of the cracked gas of H2/N2 and DME addition on ammonia combustion in lean-burn condition

The comparisons between the cracked gas and DME addition with a certain proportion on combustion behaviors are studied. A mechanism for the prediction of ammonia/H2/DME is updated based on the previously developed one. Since it is important to use a simplified mechanism in 3D numerical simulations, mechanism simplification is conducted, resulting in one with 72 species and 459 reactions. The premixed laminar flame simulations indicate that both the fuel cracking with hydrogen production and DME addition with a small amount can enhance laminar burning velocity (LBV) and extend lean-burn limit. NO is increased with hydrogen or DME addition due to the chemical and thermal effects, and comparatively, adding cracked gas leads to lower NO generation for the same blending ratio. The 3D simulated results shows that a sudden increase of HNO for DME addition during the combustion induces a higher NO formation relatively to that for hydrogen addition.

(Invited) Hard Polymers: Irreversible Synthetic Routes to Ultra-Strong Two-Dimensional Polymers in Bulk

Polymers that extend covalently in two dimensions have attracted recent attention as a means of combining the mechanical strength and in-plane energy conduction of conventional two-dimensional (2D) materials with the low densities, synthetic processability, and organic composition of their one-dimensional counterparts. Efforts to date have proven successful in forms that do not allow full realization of these properties, such as polymerization at flat interfaces or fixation of monomers in immobilized lattices. A frequently employed synthetic approach is to introduce microscopic reversibility, at the cost of bond stability, to achieve 2D crystals after extensive error correction. Herein we demonstrate a synthetic route to 2D irreversible polycondensation directly in the solution phase, resulting in a covalently bonded 2D polymer that is chemically stable and highly processable. Further fabrication offers highly oriented, free-standing films which exhibit exceptional 2D elastic modulus and yield strength at 12.7 ± 3.8 GPa and 488 ± 57 MPa, respectively. The highly planar asymmetry of 2D molecules is evidenced by chemical atomic force microscopy, and molecular alignment in nanofilms is supported by a polarized photoluminescence centered at 580 and 680 nm from different dipole transitions. We have also made substantial progresses in the theory of 2D chain growth. We performed a chemical kinetic simulation study to understand 2D polymerization in homogeneous solution with irreversible chemical steps. We show that reaction sites for polymerization in 2D always scale unfavourably compared to 3D, growing as molecular weight to the 1/2 power versus 2/3 power for 3D. However, certain mechanisms can effectively suppress out-of-plane defect formation and subsequent 3D growth. We consider two such mechanisms, which we call bond-planarity and templated autocatalysis. In the first, although single bonds can easily rotate out-of-plane to render polymerization in 3D, some double-bond linkages prefer a planar configuration. In the second mechanism, stacked 2D plates may act as van der Waals templates for each other to enhance growth, which leads to an autocatalysis. When linkage reactions possess a 1000:1 selectivity (γ) for staying in plane versus rotating, solution-synthesized 2D polymers can have comparable size and yield with those synthesized from confined polymerization on a surface. Autocatalysis could achieve similar effects when self-templating accelerates 2D growth by a factor β of 106. A combined strategy relaxes the requirement of both mechanisms by over one order of magnitude. We map the dependence of molecular weight and yield for 2D polymer on the reaction parameters, allowing experimental results to be used to estimate β and γ. Our calculations show for the first time from theory, the feasibility of producing two-dimensional polymers from irreversible polymerization in solution. These new synthetic routes, now experimentally validated, provides exciting, new opportunities for 2D polymers in applications ranging from composite structures to barrier coating materials.

The collision theory of unimolecular reactions: A second possibility to Lindemann’s proposal

Lindemann’s collision theory of unimolecular reactions was studied theoretically with comparative real-life assumptions. The result holds that there is a second possibility to the Lindemann’s proposal by applying quantitative/mathematical models, and assuming that in a bimolecular collision, both molecules are equally energized. This represents the second possibility of Lindemann’s proposal and is hereby presented to enhance the study of chemical kinetics.

New computational tools for chemical kinetics: the Cathedral Package

The advent of recent technological developments in software engineering has enabled the exploration of reaction mechanisms inside intricate reaction networks, thereby propelling the beginning of a new era in ab initio kinetics. While it is feasible to consider a substantial number of reactions, determining their rate constants with precision remains an arduous task, even for gas-phase processes. The difficulties are attributed not only to the inherent limitations in the calculation methodology but also to the manual labor and extensive chemical dynamics required, rendering these calculations inaccessible to the general public. As such, there is a pressing need for the development of automated codes and user-friendly interfaces to address this limitation. The present work focuses on the introduction of the Cathedral package, a unified computational code comprising the Q2DTor, TorsiFlex, and Pilgrim programs. This package serves to bridge the gap between theoretical studies in chemical kinetics and non-specialist users, making it more accessible and user-friendly.

Experimental and chemical kinetic modeling study of high-temperature oxidation of diisopropyl methylphosphonate (DIMP) - A sarin simulant

Chemical warfare (CW) agent simulants are used in laboratory experiments to study the combustion characteristics of CW agents due to their high toxicity. A crucial CW agent simulant with a chemical structure similar to the deadly nerve agent Sarin (GB) is diisopropyl methylphosphonate (DIMP), an organophosphate compound (OPC). In this study, the high-temperature oxidation of DIMP is investigated in a shock tube at temperatures between 1440 K and 1710 K and a pressure of 1–2 atm. The carbon monoxide mole fraction time histories near 4.9 µm were obtained using laser absorption spectroscopy. The rate parameters for DIMP's H-abstraction reactions with O, and OH radicals were determined using molecular simulations. The rates of reactions involving smaller phosphorous species were also calculated at the CBS-QB3 level. These reactions along with the isopropanol sub-mechanism from the literature were added to the LLNL model to obtain an improved chemical kinetic mechanism for DIMP. Since isopropanol was a major intermediate in DIMP decomposition, validations were conducted with CO time histories during the oxidation of isopropanol. The new model predicted CO during isopropanol oxidation reasonably well. Both the LLNL model and the model developed in this work could predict CO time histories during DIMP oxidation satisfactorily. To comprehend the CO formation pathways and sensitive reactions during DIMP oxidation, reaction path analysis and sensitivity analysis were also carried out. The reaction mechanism developed here will help in the design, development, and optimization of efficient, effective and secure CW destruction techniques.

Development and evaluation of a skeletal mechanism for EHN additized gasoline mixtures in large Eddy simulations of HCCI combustion

Advanced Low Temperature Combustion modes, such as the Sandia proposed Additive-Mixing Fuel Injection (AMFI), can unlock significant potential to boost fuel conversion efficiency and ultimately improve the energy conversion of internal combustion engines. This is a novel improved combustion process that is enabled by supplying small (<5%) variable amounts of autoignition improver to the fuel to enhance the engine operation and control. Common, diesel-fuel ignition-quality enhancing additive, 2-ethylexyl nitrate (EHN), is doped into gasoline to enable Sandia LTGC + AMFI combustion. This manuscript focuses on the development of a reduced sub-mechanism for EHN chemical kinetics at engine relevant conditions that is implemented into a skeletal mechanism for chemical kinetic studies of gasoline surrogate fuels. The mechanism validation utilized zero-dimensional numerical simulations and comparison to shock tube ignition-delay data of pure and EHN-doped n-heptane. Additional validation is presented with Homogeneous Charge Compression-Ignition (HCCI) engine data of pure and EHN-doped research-grade E10 gasoline. Then, the mechanism was deployed in a 3-D computational fluid dynamics (CFD) using Large Eddy Simulations (LES) to model the HCCI engine experiments of 0.4% vol EHN additized E10 gasoline at several equivalence ratios. Simulations showed a very good performance of the mechanism, and the model accurately reproduced (a) the ignition point, (b) combustion phasing, (c) combustion duration, and (d) the peak of the heat release rates of the engine experiments. The results show that EHN promotes Low-Temperature Heat Release, ultimately driving the gasoline to autoignite at thermodynamic conditions where the fuel would not otherwise ignite. Overall, this work demonstrates a viable reduced chemical-kinetic mechanism for EHN and shows that it can be combined with a skeletal gasoline mechanism for CFD-LES analysis of well-mixed LTGC that matches well with experimental results. The CFD-LES analysis also shows the spatial distribution of EHN-fuel interactions that control the autoignition throughout the combustion chamber.

Chemical kinetic study of the low temperature oxidation of Alkanes with a new scheme

Leading candidates of advanced internal combustion engine concepts are low temperature compression ignition engines. However, kinetic data for low-temperature oxidation reactions from experimental measurements was limited. Theoretical investigations on kinetic of low-temperature oxidation reactions with high-level quantum chemistry methods are too expensive especially for large fuels. Reliable relative energies can possibly be calculated with proper density functional approximations (DFAs) to obtain reasonable kinetic data for reactions of large fuels. In this work, we investigate performance of nineteen DFAs on reaction energies (REs), barrier heights (BHs) and relative energies of stationary points in seven reaction systems between C2-C4 alkyl radicals and oxygen, which are important low-temperature combustion reactions. Our results show that rDSD-PBEP86-D3(BJ) can provide reliable REs with a mean absolute deviation (MAD) of 0.62 kcal/mol, while different DFAs are suggested for different type of reactions so that MAD on BHs are within 1 kcal/mol. A scheme is proposed where relative energies of stationary points in these reaction systems are determined with rDSD-PBEP86-D3(BJ) for intermediates and final products and with corresponding optimal DFAs for BHs. Pressure-dependent rate constants for reactions in these reaction systems with this scheme are in good agreement with those where relative energies are determined with CCSD(T)-F12a. Moreover, error of relative energies with this scheme is not related to system size. Reasonable kinetic data for low-temperature combustion reaction systems of large hydrocarbon and oxygenated hydrocarbon fuels can be achieved based on relative energies determined with this scheme, which can be used for the construction of low-temperature combustion mechanisms.

Methane and n-hexane ignition in a newly developed diaphragmless shock tube

Shock tubes have been routinely used to generate reliable chemical kinetic data for gas-phase chemistry. The conventional diaphragm-rupture mode for shock tube operation presents many challenges that may ultimately affect the quality of chemical kinetics data. Numerous diaphragmless concepts have been developed to overcome the drawbacks of using diaphragms. Most of these diaphragmless designs require significant alterations in the driver section of the shock tube and, in some cases, fail to match the performance of the diaphragm-mode of operation. In the present work, an existing diaphragm-type shock tube is retrofitted with a fast-acting valve, and the performance of the diaphragmless shock tube is evaluated for investigating the ignition of methane and n-hexane. The diaphragmless shock tube reported here presents many advantages, such as eliminating the use of diaphragms, avoiding substantial manual effort during experiments, automating the shock tube facility, having good control over driver conditions, and obtaining good repeatability for reliable gas-phase chemical kinetic studies. Ignition delay time measurements have been performed in the diaphragmless shock tube for three methane mixtures and two n-hexane mixtures at P5 = 10–20 bar and T5 = 738–1537 K. The results obtained for fuel-rich, fuel-lean, and oxygen-rich (undiluted) mixtures show very good agreement with previously reported experimental data and literature kinetic models (AramcoMech 3.0 [1] for methane and Zhang et al. mechanism [2] for n-hexane). The study presents an easy and simple method to upgrade conventional shock tubes to a diaphragmless mode of operation and opens new possibilities for reliable chemical kinetics investigations.

Oxygen vacancies-enriched Fe-Cu bimetallic minerals-based magnetic biochar activated peroxydisulfate for durable sulfonamides degradation: pH-dependence adsorption and singlet oxygen evolution mechanism

The construction of oxygen vacancies (Vo) is a promising strategy for designing an efficient catalyst toward peroxydisulfate (PDS) activation. However, the unstable interface electron transfer and deactivation of Vo have been the bottleneck, and Vo-mediated PDS activation mechanisms remain vaguely interpreted. Herein, we innovatively synthesized a low-cost and durable Fe-Cu bimetallic mineral-based magnetic biochar with abundant Vo (Fe-CuFe2O4@BC). Fe-CuFe2O4@BC exhibited 6.64, 22.91, and 79.22-fold PDS activation rates for sulfamethazine (SMT) degradation (100%) and mineralization (91.58%) compared with Fe@BC, Cu@BC, and pristine BC, respectively. The outstanding performance of the Fe-CuFe2O4@BC/PDS system was ascribed to the Vo-induced sustainable electron transfer and singlet oxygen (1O2) generation, determined by chemical probes and kinetics study. pH-dependence adsorption of SMT dominated by the strengthened H-bonding and π–π EDA interaction was the key rate-limited step to PDS activation. Based on the surface investigation, electronic measurement, and Density functional theory (DFT) calculations, we proposed new insights into the electron-rich Vo and electron-deficient Cu co-induced PDS activation mechanism. Different from conventional perspectives, Vo preferentially activated PDS to generate 1O2, subsequently delocalizing and accumulating residual electrons from the Cu site. Simultaneously, metastable copper intermediates activated PDS through thermodynamically feasible reactions, promoting nano-confinement Vo-triggered 1O2 pathway. This study provides a new perspective for synthesizing Vo-rich catalysts and essentially deepens insights into nonradical mechanisms of refractory organics.

Decompositional and chemical reaction kinetics study of eggshell powder waste for value-added materials

Decompositional and chemical reaction kinetics study of eggshell powder waste for value-added materials

New ε-N-thioglutaryl-lysine derivatives as SIRT5 inhibitors: Chemical synthesis, kinetic and crystallographic studies

SIRT5 has been implicated in various physiological processes and human diseases, including cancer. Development of new highly potent, selective SIRT5 inhibitors is still needed to investigate disease-related mechanisms and therapeutic potentials. We here report new ε-N-thioglutaryllysine derivatives, which were designed according to SIRT5-catalysed deacylation reactions. These ε-N-thioglutaryllysine derivatives displayed potent SIRT5 inhibition, of which the potential photo-crosslinking derivative 8 manifested most potent inhibition with an IC50 value of 120 nM to SIRT5, and low inhibition to SIRT1-3 and SIRT6. The enzyme kinetic assays revealed that the ε-N-thioglutaryllysine derivatives inhibit SIRT5 by lysine-substrate competitive manner. Co-crystallographic analyses demonstrated that 8 binds to occupy the lysine-substate binding site by making hydrogen-bonding and electrostatic interactions with SIRT5-specific residues, and is likely positioned to react with NAD+ and form stable thio-intermediates. Compound 8 was observed to have low photo-crosslinking probability to SIRT5, possibly due to inappropriate position of the diazirine group as observed in SIRT5:8 crystal structure. This study provides useful information for developing drug-like inhibitors and cross-linking chemical probes for SIRT5-related studies.

Chemical kinetic study of ammonia with propane on combustion control and NO formation

Chemical kinetic study of ammonia with propane on combustion control and NO formation

Jet-stirred reactor measurements and chemical kinetic study of ammonia with dimethyl ether

To investigate the chemical kinetic effect of dimethyl ether (DME) on ammonia combustion and NO formation, experiment of ammonia mixed with DME was conducted in a jet-stirred reactor. Mole fractions of reactants and intermediates were measured at the equivalence ratio of 0.5–2.0, at the temperature range of 800–1250 K and the DME ratio of 0.0–0.8. Meng model was selected as the base model of ammonia and DME. Zhang model was chosen to update the NH3 sub-mechanism through validated against large set of experimental data. Pathway and sensitivity analysis were conducted to figure out the key reactions on ammonia combustion and NO formation. The result showed that NH2 can react with carbon-containing species, such as DME and CH3, producing NH3 and methylamine, respectively. NH2 reacting with DME, HO2 generating NH3 + CH3OCH2 and NH3 + O2, plays dominant chain-termination role in ammonia oxidation. In addition, HO2 can react with NH2, CH3 and NO generating OH radicals, promoting ammonia oxidation. Sensitivity analysis indicated that reactions like CH3 + NO2 = CH3O + NO, HNO + O2 = NO + HO2, essential to NO formation have promoting effect on ammonia consumption. In contrast, NH2 + CH3 = CH3NH2 have high sensitivity coefficients, regarded as chain terminal reactions inhibiting the reactivity.

Cysteamine-decorated gold nanoparticles for plasmon-based colorimetric on-site sensors for detecting cyanide ions using the smart-phone color ratio and for catalytic reduction of 4-nitrophenol

In this paper, we have reported the cyanide ions (CN−) sensing in environmental water samples using cysteamine-capped gold nanoparticles (Cyst-AuNPs) by spectrophotometric, colorimetric, and smartphone-based RGB color detection. The surface plasmon resonance shift at around 525 nm for the Cyst-AuNPs could be used to detect quantitatively the amounts of CN− with concomitant alteration of their color from wine red to purple visualized by the naked eye. For the first time, the Cyst-AuNPs-based visual sensing of CN− was performed using smartphone-based detection with its detection limit of 159 × 10−9 M, ten times lower than that of the highest tolerance level (2 × 10−6 M) permitted by the world health organization. The Cyst-AuNPs displayed excellent specificity for detecting the concentration of 30 × 10−6 M even amid the presence of other interfering inorganic anions with their concentrations about five times higher than it. Environmental real water samples were used to arrange the three different CN− concentrations for plasmon-based colorimetric detection and smartphone-based method. Additionally, the catalytic performance of Cyst-AuNPs was demonstrated for the fast catalytic conversion of hazardous 4-nitrophenol (selected environmental contaminant) to the analogous amino aromatic compounds. A chemical kinetic study showed the conversion rate to be estimated as 1.65 × 10−2 s−1. Cyst-AuNPs can find an application in colorimetric sensing of CN− while being able to be utilized as a catalytic nanomaterial for ecological remedies associated with health care.

Room-Temperature ZnBr2 -Catalyzed Regioselective 1,6-Hydroarylation of Electron-Rich Arenes to para-Quinone Methides: Synthesis of Unsymmetrical Triarylmethanes.

A mild and efficient Zn(II)-catalyzed regioselective 1,6-hydroarylation of para-quinone methides (p-QMs) with electron-rich arenes protocol is reported. A variety of electron-rich arenes and para-quinone methides are well tolerated under mild conditions, delivering a broad range of triarylmethanes in good to excellent yields. The present method also works well for the hydroarylation of p-QMs with other nucleophiles, such as aniline, indole and phenol derivatives, offering the corresponding triarylmethanes with good yields under the standard conditions. The possible mechanism for the formation of C(sp3 )-C(sp2 ) bonds in hydroarylation reactions has been explored by step-by-step control experiments, and the reaction may follow a second-order manner in a chemical kinetic study.

Dual-camera high-speed imaging of n-hexane oxidation in a high-pressure shock tube

Shock tubes are widely used in the study of chemical kinetics. Its benefits rely on the almost ideal shock-heating process that provides high temperatures and pressures to a chemical system for a limited test time. Just like any reactor, shock tubes are not immune to non-ideal effects. The study of conditions that might deviate experiments from ideal conditions is thus of the utmost importance. High-speed imaging has been proven to be a powerful tool to analyze non-ideal / non-homogenous combustion in shock tubes. In this work, dual-camera high-speed imaging experiments were performed at 10, 15 and 20 bar in a high-pressure shock tube (HPST). An optical section was designed as an extension of the HPST which enabled simultaneous visualization from the endwall and the sidewall of the driven section of the shock tube. n-Hexane, a fuel with a negative temperature coefficient (NTC) behavior that has been identified as prone to non-homogenous ignition, is used as a test fuel. Reactive mixtures and thermodynamic conditions were selected to visually analyze ignition processes at the high-temperature, NTC and low-temperature regimes. Non-homogeneous ignition was observed mostly at the local maximum of the IDT, which is comprised by the high-temperature and NTC regions. Stoichiometric n-hexane mixture with high fuel loading (5% n-hexane) presented the highest deviation from constant volume chemical kinetic simulations. The inclusion of helium as a bath gas to mitigate preignition was tested and it showed to improve the susceptibility of the mixtures to develop reaction fronts. The modified Sankaran criterion for the identification of ignition regimes in shock tubes was tested and it showed an overall good agreement against the experimental observations.