Combustion, Chemistry, and Carbon Neutrality

Combustion is a reactive oxidation process that releases energy bound in chemical compounds used as fuels─energy that is needed for power generation, transportation, heating, and industrial purposes. Because of greenhouse gas and local pollutant emissions associated with fossil fuels, combustion science and applications are challenged to abandon conventional pathways and to adapt toward the demand of future carbon neutrality. For the design of efficient, low-emission processes, understanding the details of the relevant chemical transformations is essential. Comprehensive knowledge gained from decades of fossil-fuel combustion research includes general principles for establishing and validating reaction mechanisms and process models, relying on both theory and experiments with a suite of analytic monitoring and sensing techniques. Such knowledge can be advantageously applied and extended to configure, analyze, and control new systems using different, nonfossil, potentially zero-carbon fuels. Understanding the impact of combustion and its links with chemistry needs some background. The introduction therefore combines information on exemplary cultural and technological achievements using combustion and on nature and effects of combustion emissions. Subsequently, the methodology of combustion chemistry research is described. A major part is devoted to fuels, followed by a discussion of selected combustion applications, illustrating the chemical information needed for the future.

Engineering the catalytic properties of CeO2 catalyst in HCl-assisted propane dehydrogenation by effective doping: A first-principles-based microkinetic simulation

HCl-assisted propane dehydrogenation (PDH) is an attractive route for propene production with good selectivity. In this study, the doping of CeO2 with different transition metals, including V, Mn, Fe, Co, Ni, Pd, Pt, and Cu, in the presence of HCl was investigated for PDH. The dopants have a pronounced effect on the electronic structure of pristine ceria that significantly alters the catalytic capabilities. The calculations indicate the spontaneous dissociation of HCl on all surfaces with a facile abstraction of the first hydrogen atom except on V- and Mn-doped surfaces. The lowest energy barrier of 0.50 and 0.51eV was found for Pd- and Ni-doped CeO2 surfaces. The surface oxygen is responsible for hydrogen abstraction, and its activity is described by the p-band center. Microkinetics simulation is performed on all doped surfaces. The increase in the turnover frequency (TOF) is directly linked with the partial pressure of propane. The adsorption energy of reactants aligned with the observed performance. The reaction follows first-order kinetics to C3H8. Furthermore, on all surfaces, the formation of C3H7 is found as the rate-determining step confirmed by the degree of rate control (DRC) analysis. This study provides a decisive description of catalyst modification for HCl-assisted PDH.

Ketenes in the Induction of the Methanol-to-Olefins Process

Ketene (CH2 =C=O) has been postulated as a key intermediate for the first olefin production in the zeolite-catalyzed chemistry of methanol-to-olefins (MTO) and syngas-to-olefins (STO) processes. The reaction mechanism remains elusive, because the short-lived ethenone ketene and its derivatives are difficult to detect, which is further complicated by the low expected ketene concentration. We report on the experimental detection of methylketene (CH3 -CH=C=O) formed by the methylation of ketene on HZSM-5 via operando synchrotron photoelectron photoion coincidence (PEPICO) spectroscopy. Ketene is produced in situ from methyl acetate. The observation of methylketene as the ethylene precursor evidences a computationally predicted ketene-to-ethylene route proceeding via a methylketene intermediate followed by decarbonylation.

Operando PEPICO unveils the catalytic fast pyrolysis mechanism of the three methoxyphenol isomers

The development of lignin valorization processes such as catalytic fast pyrolysis (CFP) to produce fine chemicals and fuels leads to a more sustainable future. The implementation of CFP is enabled by understanding the chemistry of lignin constituents, which, however, requires thorough mechanistic investigations by detecting reactive species. In this contribution, we investigate the CFP of the three methoxyphenol (MP) isomers over H-ZSM-5 utilizing vacuum ultraviolet synchrotron radiation and operando photoelectron photoion coincidence (PEPICO) spectroscopy. All isomers demethylate at first to yield benzenediols, from which dehydroxylation reactions proceed to produce phenol and benzene. Additional pathways to form benzene proceed over cyclopentadiene, methylcyclopentadiene, and fulvene intermediates. The detection of trace amounts of methanol in the product stream suggests a demethoxylation reaction to yield phenol. Guaiacol (2- or ortho-MP) exhibits slightly higher reactivity compared to 3-MP and 4-MP, due to the formation of the fulvenone ketene, which opens additional routes to benzene and phenol. When compared to benzenediol catalytic pyrolysis, the additional methyl group in MP leads to high conversion at lower reactor temperatures, which is mostly owed to the lower H₃C–O vs. H–O bond energy and the possibility to demethoxylate to produce phenol.

Demethylation, demethoxylation and fulvenone ketene formation determine the reactivity of methoxyphenols over H-ZSM-5 to yield phenols, benzene and toluene. Intermediates are isomer-selectively detected utilizing threshold photoelectron spectroscopy.

First-Generation Organic Reaction Intermediates in Zeolite Chemistry and Catalysis

Zeolite chemistry and catalysis are expected to play a decisive role in the next decade(s) to build a more decentralized renewable feedstock-dependent sustainable society owing to the increased scrutiny over carbon emissions. Therefore, the lack of fundamental and mechanistic understanding of these processes is a critical "technical bottleneck" that must be eliminated to maximize economic value and minimize waste. We have identified, considering this objective, that the chemistry related to the first-generation reaction intermediates (i.e., carbocations, radicals, carbenes, ketenes, and carbanions) in zeolite chemistry and catalysis is highly underdeveloped or undervalued compared to other catalysis streams (e.g., homogeneous catalysis). This limitation can often be attributed to the technological restrictions to detect such "short-lived and highly reactive" intermediates at the interface (gas-solid/solid-liquid); however, the recent rise of sophisticated spectroscopic/analytical techniques (including under in situ/operando conditions) and modern data analysis methods collectively compete to unravel the impact of these organic intermediates. This comprehensive review summarizes the state-of-the-art first-generation organic reaction intermediates in zeolite chemistry and catalysis and evaluates their existing challenges and future prospects, to contribute significantly to the "circular carbon economy" initiatives.

Continuous Pyrolysis Microreactors: Hot Sources with Little Cooling? New Insights Utilizing Cation Velocity Map Imaging and Threshold Photoelectron Spectroscopy

Resistively heated silicon carbide microreactors are widely applied as continuous sources to selectively prepare elusive and reactive intermediates with astrochemical, catalytic, or combustion relevance to measure their photoelectron spectrum. These reactors also provide deep mechanistic insights into uni- and bimolecular chemistry. However, the sampling conditions and effects have not been fully characterized. We use cation velocity map imaging to measure the velocity distribution of the molecular beam signal and to quantify the scattered, rethermalized background sample. Although translational cooling is efficient in the adiabatic expansion from the reactor, the breakdown diagrams of methane and chlorobenzene confirm that the molecular beam component exhibits a rovibrational temperature comparable with that of the reactor. Thus, rovibrational cooling is practically absent in the expansion from the microreactor. The high rovibrational temperature also affects the threshold photoelectron spectrum of both benzene and the allyl radical in the molecular beam, but to different degrees. While the extreme broadening of the benzene TPES suggests a complex ionization mechanism, the allyl TPES is in fact consistent with an internal temperature close to that of the reactor. The background, room-temperature spectra of both are superbly reproduced by Franck-Condon simulations at 300 K. On the one hand, this leads us to suggest that room-temperature reference spectra should be used in species identification. On the other hand, analysis of the allyl iodide pyrolysis data shows that iodine atoms often recombine to form molecular iodine on the chamber surfaces. Such sampling effects may distort the chemical composition of the scattered background with respect to the molecular beam signal emanating directly from the reactor. This must be considered in quantitative analyses and kinetic modeling.

Quantification of Redox Sites during Catalytic Propane Oxychlorination by Operando EPR Spectroscopy

Identification and quantification of redox-active centers at relevant conditions for catalysis is pivotal to understand reaction mechanisms and requires development of advanced operando methods. Herein, we demonstrate operando EPR spectroscopy as an important technique to quantify the oxidation state of representative CrPO₄ and EuOCl catalysts during propane oxychlorination, an attractive route for propylene production. In particular, we show that the space-time-yield of C₃ H₆ correlates with the amount of Cr²⁺ and Eu²⁺ ions generated over the catalysts during reaction. These results provide a powerful strategy to gather quantitative understanding of selective alkane oxidation, which could potentially be extrapolated to other functionalization approaches and operating conditions.

Status and prospects of the decentralised valorisation of natural gas into energy and energy carriers

We critically review the recent advances in process, reactor, and catalyst design that enable process miniaturisation for decentralised natural gas upgrading into electricity, liquefied natural gas, fuels and chemicals.

Combustion in the future: The importance of chemistry

Combustion involves chemical reactions that are often highly exothermic. Combustion systems utilize the energy of chemical compounds released during this reactive process for transportation, to generate electric power, or to provide heat for various applications. Chemistry and combustion are interlinked in several ways. The outcome of a combustion process in terms of its energy and material balance, regarding the delivery of useful work as well as the generation of harmful emissions, depends sensitively on the molecular nature of the respective fuel. The design of efficient, low-emission combustion processes in compliance with air quality and climate goals suggests a closer inspection of the molecular properties and reactions of conventional, bio-derived, and synthetic fuels. Information about flammability, reaction intensity, and potentially hazardous combustion by-products is important also for safety considerations. Moreover, some of the compounds that serve as fuels can assume important roles in chemical energy storage and conversion. Combustion processes can furthermore be used to synthesize materials with attractive properties. A systematic understanding of the combustion behavior thus demands chemical knowledge. Desirable information includes properties of the thermodynamic states before and after the combustion reactions and relevant details about the dynamic processes that occur during the reactive transformations from the fuel and oxidizer to the products under the given boundary conditions. Combustion systems can be described, tailored, and improved by taking chemical knowledge into account. Combining theory, experiment, model development, simulation, and a systematic analysis of uncertainties enables qualitative or even quantitative predictions for many combustion situations of practical relevance. This article can highlight only a few of the numerous investigations on chemical processes for combustion and combustion-related science and applications, with a main focus on gas-phase reaction systems. It attempts to provide a snapshot of recent progress and a guide to exciting opportunities that drive such research beyond fossil combustion.

Operando Photoelectron Photoion Coincidence Spectroscopy Unravels Mechanistic Fingerprints of Propane Activation by Catalytic Oxyhalogenation

Herein, we demonstrate operando photoelectron photoion coincidence (PEPICO) spectroscopy as a pivotal technique for evidencing unprecedented mechanistic insights by isomer-selective radical detection within complex hydrocarbon-functionalization reaction networks, such as those of catalyzed propane oxychlorination and oxybromination. In particular, while the oxychlorination is surface-confined, we show that in oxybromination alkane activation follows a gas-phase reaction mechanism with evolved bromine and bromine radicals, favoring 2-propyl over 1-propyl radical formation, as evidenced by isomer-selective threshold photoelectron analysis. Furthermore, we provide new mechanistic insights into the cracking and coking pathways that are observed in oxybromination. The first entails propargyl radical formation from consecutive hydrogen abstraction of propyl radicals, ultimately yielding benzene. The second originates from C-C bond cleavage in propane to ethyl and methyl radicals, which produce CH₄ and C₂H₄, or undergo chain-growth reactions, forming C₄-C₆ species.

New analytical tools for advanced mechanistic studies in catalysis: photoionization and photoelectron photoion coincidence spectroscopy

How can we detect reactive and elusive intermediates in catalysis to unveil reaction mechanisms? In this mini review, we discuss novel photoionization tools to support this quest.

Metamorphic meta isomer: carbon dioxide and ketenes are formed via retro-Diels–Alder reactions in the decomposition of meta-benzenediol

Deoxygenation of the lignin model compound resorcinol was investigated using VUV synchrotron radiation: Formation of two reactive ketenes and decarboxylation are the dominating pathways, much different from the other two benzenediol isomers.