Polyaromatic Species Research Articles

Revealing Active Sites and Reaction Pathways in Methane Non-Oxidative Coupling over Iron-Containing Zeolites.

Non-oxidative coupling of methane is a promising route to obtain ethylene directly from natural gas. We synthesized siliceous [Fe]zeolites with MFI and CHA topologies and found that they display high selectivity (>90 % for MFI and >99 % for CHA) to ethylene and ethane among gas-phase products. Deactivated [Fe]zeolites can be regenerated by burning coke in air. In situ X-ray absorption spectroscopy demonstrates that the isolated Fe3+ centers in zeolite framework of fresh catalysts are reduced during the reaction to the active sites, including Fe2+ species and Fe (oxy)carbides dispersed in zeolite pores. Photoelectron photoion coincidence spectroscopy results show that methyl radicals are the reaction intermediates formed upon methane activation. Ethane is formed by methyl radical coupling, followed by its dehydrogenation to ethylene. Based on the observation of intermediates including allene, vinylacetylene, 1,3-butadiene, 2-butyne, and cyclopentadiene over [Fe]MFI, a reaction network is proposed leading to polyaromatic species. Such reaction intermediates are not observed over the small-pore [Fe]CHA, where ethylene and ethane are the only gas-phase products.

Revealing Active Sites and Reaction Pathways in Methane Non‐Oxidative Coupling over Iron‐Containing Zeolites

AbstractNon‐oxidative coupling of methane is a promising route to obtain ethylene directly from natural gas. We synthesized siliceous [Fe]zeolites with MFI and CHA topologies and found that they display high selectivity (>90 % for MFI and >99 % for CHA) to ethylene and ethane among gas‐phase products. Deactivated [Fe]zeolites can be regenerated by burning coke in air. In situ X‐ray absorption spectroscopy demonstrates that the isolated Fe3+ centers in zeolite framework of fresh catalysts are reduced during the reaction to the active sites, including Fe2+ species and Fe (oxy)carbides dispersed in zeolite pores. Photoelectron photoion coincidence spectroscopy results show that methyl radicals are the reaction intermediates formed upon methane activation. Ethane is formed by methyl radical coupling, followed by its dehydrogenation to ethylene. Based on the observation of intermediates including allene, vinylacetylene, 1,3‐butadiene, 2‐butyne, and cyclopentadiene over [Fe]MFI, a reaction network is proposed leading to polyaromatic species. Such reaction intermediates are not observed over the small‐pore [Fe]CHA, where ethylene and ethane are the only gas‐phase products.

Developing a parametric system model to describe the product distribution of steam pyrolysis in a Dual Fluidized bed

Steam pyrolysis is a thermochemical process that converts carbon-based materials into valuable gases. In general, the products of the reaction are syngas (H2,CO,CO2), low-molecular-weight hydrocarbon gases (methane, ethylene, and propylene), pyrolytic gasoline and oils, monoaromatic and polyaromatic species (tar), and carbonaceous residues (char) with ashes. However, the intricacy of the reactions comprising the process, the diversity of the product species, and the constraints linked to the sampling and measurement equipment, create a highly complex system. In this work, a method for data representation is presented based on a special Parametric System Model (PSM) that portrays product species measurements in a way that provides relevant information and valuable insights into the process. The method incorporates generic knowledge of the chemical nature of the reactions to create a constrained system in which the data can be expressed in parametric terms with meaningful statistical functions. The evaluated data were obtained from a high-temperature steam pyrolysis process performed in the 2–4-MW Dual Fluidized Bed reactor at Chalmers University using polyethylene as feedstock. The quantities of the hydrocarbon species detected in the gas product were taken for the PSM as a probabilistic system that can be described with a set of distribution functions. The carbon, hydrogen and oxygen balances were taken into account to build a constrained set of equations to find the parameters of the functions. The resulting model was proven to be useful as a prediction tool to quantify unmeasured carbon group species and to estimate process variables, such as the oxygen transport of the bed material. Also, it was demonstrated the potential of the model as a method to identify and estimate inconsistencies in the measurements, which improve the quality of the characterization data. The modeĺs outcomes find application in providing critical information for the control and evaluation of pyrolysis process and downstream operation of biorefineries.

Effects of the Acidic and Textural Properties of Y-Type Zeolites on the Synthesis of Pyridine and 3-Picoline from Acrolein and Ammonia

A set of Y-type zeolites with Si/Al atomic ratios between 7–45 were studied as catalysts in the aminocyclization reaction between acrolein and ammonia to produce pyridine and 3-picoline. The catalytic activity tests at 360 °C revealed that the acrolein conversion increased in the order Z45 < ZY34 < ZY7 < ZY17, in agreement with the increase of the total acidity per gram of catalyst. In all cases, pyridine bases and cracking products (acetaldehyde and formaldehyde) were detected in the outflow from the reactor. The total yield of pyridines was inversely proportional to the total acidity for the catalysts, which presented large surface areas and micro- and mesoporosity. The selectivity towards 3-picoline was favored when using catalysts with a Brønsted/Lewis acid sites ratio close to 1. The formation of pyridine occurred more selectively over Lewis acid sites than Brønsted acid sites. The deactivation tests showed that the time on stream of the catalysts depended on the textural properties of zeolites, i.e., large pore volume and large BET area, as evidenced by the deactivation rate constants and the characterization of the spent catalysts. The physicochemical properties of the catalysts were determined by XRD, UV-vis, and Raman spectroscopies, infrared spectroscopy with adsorbed pyridine, N2 physisorption, and SEM-EDXS. After the reaction, the spent catalysts were characterized by XRD, Raman spectroscopy, TGA, and SEM-EDXS, indicating that the uniform deposition of polyaromatic species on the catalyst surface and within the porous system resulted in the loss of activity.

Atmospheric aging increases the cytotoxicity of bare soot particles in BEAS-2B lung cells

Soot particles (SP) are ubiquitous components of atmospheric particulate matter and have been shown to cause various adverse health effects. In the atmosphere, freshly emitted SP can be coated by condensed low-volatility secondary organic and inorganic species. In addition, gas-phase oxidants may react with the surface of SP. Due to the chemical and physical resemblance of SP carbon backbone with polyaromatic hydrocarbon species and their potent oxidation products, we investigated the biological responses of BEAS-2B lung epithelial cells following exposure to fresh- and photochemically aged-SP at the air–liquid interface. A comprehensive physical and chemical aerosol characterization was performed to depict the atmospheric transformations of SP, showing that photochemical aging increased the organic carbon fraction and the oxidation state of the SP. RNA-sequencing and qPCR analysis showed varying gene expression profiles for fresh- and aged-SP. Exposure to aged-SP increased DNA damage, oxidative damage, and upregulation of NRF2-mediated oxidative stress response genes compared to fresh-SP. Furthermore, aged-SP augmented inflammatory cytokine secretion and activated AhR-response, as evidenced by increased expression of AhR-responsive genes. These results indicate that oxidative stress, inflammation, and DNA damage play a key role in the cytotoxicity of SP in BEAS-2B cells, where aging leads to higher toxic responses. Collectively, our results suggest that photochemical aging may increase SP toxicity through surface modifications that lead to an increased toxic response by activating different molecular pathways.

Solid-State Synthesis of Zeolite-Templated Carbons

The first solid-state synthesis of zeolite-templated carbons (ZTCs) is reported using anthracene as the carbon precursor in FAU-structured zeolite. The evolution of the anthracene molecule upon sublimation inside of the zeolite micropore system and its further condensation into larger polyaromatic species could be followed by DRUV and Raman spectroscopy. The development of the anthracene radical cation through spontaneous charge separation was found to play a fundamental role in the formation of the carbon skeleton. A cyclic radical formation and recombination behavior could be revealed by in situ EPR spectroscopy. Textural and chemical characterization of the hybrid (zeolite/ZTC) and ZTC upon zeolite removal allowed us to conclude that anthracene allows for achieving ZTC materials with transcribed morphological and textural properties of the template zeolite. The impact of the thermal treatment on the ZTC properties was further studied. ZTCs with emerging structural order could be achieved.

Catalytic Activation of Polyethylene Model Compounds Over Metal-Exchanged Beta Zeolites.

Decomposition of polymers by heterogeneous catalysts presents a promising approach for reuse of waste plastics. We demonstrated non-hydrogenative decomposition of model polyolefins over proton-form and metal (Cu, Ni) ion-exchanged beta (BEA) zeolites at moderate temperatures (around 300 °C). Near complete polyolefin decomposition was observed in batch reactions monitored by thermogravimetric analysis, while decomposition at partial conversion was studied in flow reactions. Ni-exchanged zeolites produced H2 at substantially higher rates (>10x) than other catalysts while also uniquely resisting deactivation over time. Application of the delplot formalism offered insights into the reaction network for polyolefin decomposition over Ni/BEA most notably that H2 is solely a primary product. We deduce that H2 production is catalyzed by activation of C-H bonds at ionic Ni sites, and H2 prevents buildup of polyaromatic coke species in Ni-exchanged zeolites that deactivate Cu-exchanged and protonic zeolites.

Elucidation of active species and reaction mechanism of sulfide V-K/Al2O3 catalyst for isobutane dehydrogenation

Supported sulfide vanadium-based catalyst in dehydrogenation of light alkanes is becoming a promise to replace the commercial used Pt- and Cr-based catalysts, due to the lower cost and toxicity. Nonetheless, the nature of active sites and reaction mechanism for dehydrogenation of isobutane over sulfide vanadium-based catalysts are still ambiguous. In this study, a series of sulfide V-K/γ-Al2O3 catalysts were prepared, presenting diverse distribution of isolated, oligomeric and polymeric vanadium species on catalyst surface. The oligomeric species were demonstrated to be the active dehydrogenation sites, and the TOFoligo showed a closely correlation with the isobutane conversion. Besides, the dehydrogenation reaction mechanism was studied by in-situ DRIFTS. The results showed that isobutane would first dissociative adsorbed on catalyst surface, and the stable intermediates of vanadium-isobutenyl were detected. Furthermore, the deposited coke rather than sulfur loss was found to play the crucial role in deactivation. The vast majority of deposited coke existed as polyaromatic species and the other slight ones presented as aliphatic form. This study enriches the fundamental explorations in active sites and reaction mechanism of sulfide V-K/γ-Al2O3 catalysts during isobutane dehydrogenation, and provides an in-depth strategy to improve the activity and stability of vanadium-based dehydrogenation catalysts.

Pore Blocking by Phenolates as Deactivation Path during the Cracking of 4-Propylphenol over ZSM-5

Cracking of propyl side chains from 4-propylphenol, a model compound for lignin monomers, is studied for a commercial ZSM-5 zeolite catalyst. The decline of 4-propylphenol conversion with time on stream can be delayed by co-feeding water. FTIR spectroscopy shows the formation of chemisorbed phenolates during reactions and significant amounts of phenolics are detected by GC-MS of the extract from the spent catalysts. Thus, chemisorbed phenolates are identified as the main reason for deactivation in the absence of water. Regardless of the amount of co-fed water, substituted monoaromatics and polyaromatic species are formed. Comprehensive characterization of the spent catalysts including Raman and solid-state 27Al NMR spectroscopy, and thermogravimetric analysis points to a combination of deactivation processes. First, phenolates bind to Lewis acid sites within the zeolite framework and hinder diffusion unless they are hydrolyzed by water. In addition, light olefins created during the cracking process react to form a polyaromatic coke that deactivates the catalyst more permanently.

A radiolytic origin of organic matter in primitive chondrites and trans-neptunian objects? New clues from ion irradiation experiments

We question here the radiolytic origin of (i) polyaromatic insoluble organic matter (IOM) recovered from primitive chondrites, and (ii) organics at the surface of reddish Trans-Neptunian Objects (TNOs), some minor planets and icy satellites. Organic synthesis by ion irradiation was investigated through experiments on a variety of targets: Polyethylene glycol 1450, lignin, cellulose and sucrose, exposed to low (C 40 keV and Ne 170 keV) and high energy (C 12 MeV, Ni 17 MeV, 78Kr 59 MeV) ions. These experiments show that all carbonaceous precursors evolve towards a sp2-rich amorphous carbon (a-C) above a critical nuclear dose of 10−7+10 eV.atom−1. A thorough review of the literature shows that this value applies for a large range of carbonaceous materials, including C-rich simple ices. Below this critical dose, irradiated targets are carbonized and transformed into cross-linked polymeric disordered solids, with abundant olefinic and acetylenic bonds, but devoid of aromatic or polyaromatic species. Ion irradiation of simple compounds, e.g. ices, is thereby not a viable process to synthesize IOM. However, in the case of aromatic-rich precursors, swift heavy ions irradiation leads to polyaromatic materials, by bridging existing aromatic or polyaromatic units. In the context of Early Solar System, i.e. Galactic Cosmic Rays (GCR) irradiation during 10–20 Myr, the formation of chondritic IOM from simple ices mixed with interstellar Polycyclic Aromatic Hydrocarbons (PAHs) appears as a plausible mechanism. This scenario, based on the recycling of existing carbonaceous interstellar grains under low-temperature conditions, would account for the heterogeneity of the D, 15N and 13C isotopic fractionations at the molecular scale, and the preservation of deuterium hot spots that are highly sensitive to high-temperature conditions (> 300 °C). At the surface of TNOs, sp2-rich amorphous carbons are formed by the implantation of GCRs and Solar wind ions. The electronic dose is also very high for an irradiation time of several Gyr (> 100 eV.atom−1), leading to the formation of reddish disordered solids, provided that the surface contains a minimum abundance of carbonaceous species. Finally, sp2-rich amorphous carbons produced in the laboratory (e.g. the ACAR compound from Zubko et al., 1996) are fair analogues of the darkening agent produced by radiolysis.

Benzofuran as deactivation precursor molecule: Improving the stability of acid zeolites in biomass pyrolysis by co-feeding propylene

Benzofuran (BZF) is produced during the biomass and waste pyrolysis, playing a key role in the catalytic deactivation in the processing of the resulting fractions, as well as in catalytic pyrolysis reactions. The transformation of this chemical over a typical pyrolysis catalyst (ZSM-5) is studied in this article. When this compound is processed alone, it presents a very high reactivity. At low temperature, the formation of large amount of coke precursors leads to a fast catalyst deactivation. As temperature increases, the weight of cracking reactions (yielding benzenes or naphtalenes) becomes more important, increasing the catalyst’s stability, although new polyaromatic carbon species are detected.A new strategy to minimize the deactivation caused by this compound, consisting of co-feeding propylene, is proposed. The presence of an alkene leads to the formation of alkylated derivatives of Diels-Alder condensation products, hindering their oligomerization capacity and minimizing the formation of polyaromatic coke deposits. Different temperatures and alkene/BZF ratios were tested, concluding that increasing the alkene concentration and the reaction temperature monotonously increase the catalyst’s stability in presence of BZF.

Influence of ZSM-5 porosity and binder introduction on the coke formation in the cracking of 1,3,5-triisopropylbenzene

Hierarchical ZSM-5 catalysts with different pore sizes from micropore to macropore and their corresponding ZSM-5-based extrudates using SiO2 binders have been successfully prepared. The series of catalysts were used for exploring the influence of pore size as well as the addition of non-acidic SiO2 binders on the catalytic performance and coke formation in the cracking of 1,3,5-triisopropylbenzene (TIPB). It is demonstrated that the TIPB conversion and stability of catalysts improve with increasing the pore sizes of ZSM-5 catalysts. The hierarchical ZSM-5 catalyst including a macropore diameter of about 60 nm exhibited the highest TIPB conversion (98.6 %) and selectivities to propylene (34.2 %) and benzene (60.8 %) with the least amount of carbonaceous compounds, compared to the microporous and mesoporous ZSM-5 catalysts. Also, the addition of non-acidic SiO2 binders has a positive effect on limiting the deactivation of ZSM-5-based extrudates during the cracking of TIPB. Less poly-aromatic species are formed for the catalyst extrudates with SiO2 binders.

Laboratory formation and photochemistry of covalently bonded polycyclic aromatic nitrogen heterocycle (PANH) clusters in the gas phase

ABSTRACT To examine the evolution processes of the nitrogen-containing polycyclic aromatic hydrocarbon (PAH) molecules occurring in interstellar environments, in this work we focus on the formation of large covalently bonded N-substituted polyaromatic species and their photochemistry behaviour in the gas phase. The experimental results show large PANH (e.g. DC/acridine and DC/phenazine) cluster cations formed in a chemical reaction between large PAH (e.g. dicoronylene, DC, C48H20) cations and small PANHs (e.g. acridine, C13H9N, or phenazine, C12H8N2) by gas-phase condensation through ion–molecule reactions. With laser irradiation, PANH cluster cations are involved in a complex photofragmentation process (e.g. dehydrogenation, HCN/CN, C2 or N2 units lost) and then form large PANH/PAH or multiple dehydrogenated molecules; in particular, the dehydrogenation of PANH clusters provides a possible way to synthesize large nitrogen-containing graphene species (e.g. C59N+ and C61N+). Also, we perform quantum-theoretical calculations on the formation and photochemistry of DC/acridine and DC/phenazine cluster cations: two types of molecular cluster are considered (C–C and C–N bond type) and the formation pathway and dissociation energy for each isomer are determined. The experimental and theoretical findings obtained give a general molecular growth pathway toward all-benzenoid aromatic species with size (> 60 C atoms) in the astrophysically relevant range, during a ground-up formation process, and offer understanding of the nitrogen element effect on their chemical-evolutionary behaviour. Also, studies of DC/acridine and DC/phenazine clusters (89–112 atoms, ∼2 nm in size) offer a feasible means of explanation for the formation of nanoscale dust grains (nitrogen element included) in space.

Effect of biphenyl biimide structure on the thermal stability, flame retardancy and pyrolysis behavior of PET

At present, developing a flame-retardant unit to solve the flammability and melt-dripping of poly(ethylene terephthalate) (PET) still is a key issue. To endow PET with good flame retardancy and low smoke releasing, a third monomer, N, N′-bis(2-hydroxyethyl)-biphenyl-3,4,3′,4′-tetracarboxylic diimide (BPDI) containing biphenyl biimide and without any traditional flame-retardant elements, is synthesized and incorporated to the main chain of PET to obtain the P(ET-co-BP)n copolyester via melt polymerization. The thermal stability, combustion and pyrolysis behaviors of the obtained copolyesters have been well investigated. Thermogravimetric analysis (TGA) results demonstrate that introducing BPDI into PET obviously improves its thermal stability and the forming ability of char residue. LOI, vertical UL-94 and cone calorimeter measurements have been applied to investigate the combustion behaviors of P(ET-co-BP)n. The results prove that P(ET-co-BP)n copolyesters containing biphenyl biimide show better fire safety, reflected by lower fire growth rate (FIGRA) and low smoke production. SEM and Raman results suggest that the char layers of P(ET-co-BP)n become denser and mainly consist of polyaromatic species with small and uniform microstructures. The pyrolysis behaviors of the copolyesters are investigated by Py-GC/MS, and the results showed that the biphenyl biimide structure units can lead to rearrangement reactions at high temperature, ultimately forming the phenanthrene ring structures during combustion. Without traditional flame-retardant elements like halogen and phosphorus, this novel PET copolyester is environmentally friendly and more safety.

Confined Carbon Mediating Dehydroaromatization of Methane over Mo/ZSM-5.

Non‐oxidative dehydroaromatization of methane (MDA) is a promising catalytic process for direct valorization of natural gas to liquid hydrocarbons. The application of this reaction in practical technology is hindered by a lack of understanding about the mechanism and nature of the active sites in benchmark zeolite‐based Mo/ZSM‐5 catalysts, which precludes the solution of problems such as rapid catalyst deactivation. By applying spectroscopy and microscopy, it is shown that the active centers in Mo/ZSM‐5 are partially reduced single‐atom Mo sites stabilized by the zeolite framework. By combining a pulse reaction technique with isotope labeling of methane, MDA is shown to be governed by a hydrocarbon pool mechanism in which benzene is derived from secondary reactions of confined polyaromatic carbon species with the initial products of methane activation.

Confined Carbon Mediating Dehydroaromatization of Methane over Mo/ZSM‐5

AbstractNon‐oxidative dehydroaromatization of methane (MDA) is a promising catalytic process for direct valorization of natural gas to liquid hydrocarbons. The application of this reaction in practical technology is hindered by a lack of understanding about the mechanism and nature of the active sites in benchmark zeolite‐based Mo/ZSM‐5 catalysts, which precludes the solution of problems such as rapid catalyst deactivation. By applying spectroscopy and microscopy, it is shown that the active centers in Mo/ZSM‐5 are partially reduced single‐atom Mo sites stabilized by the zeolite framework. By combining a pulse reaction technique with isotope labeling of methane, MDA is shown to be governed by a hydrocarbon pool mechanism in which benzene is derived from secondary reactions of confined polyaromatic carbon species with the initial products of methane activation.

Study of coke deposited on a VOx-K2O/γ-Al2O3 catalyst in the non-oxidative dehydrogenation of isobutane

Coke deposition is commonly identified as the main cause of deactivation for dehydrogenation (DH) catalysts. The VOx-K2O/γ-Al2O3 catalysts for non-oxidative DH of isobutane were studied to better understand the formation and nature of coke. Coke deposition influenced VOx species and acid sites, leading to a deterioration of DH performances. The coke was identified as aliphatic and polyaromatic species. As the DH reaction proceeding, the deposited carbonaceous species progressively transformed from aliphatic into aromatic species, with aromaticity increasing. They continuously shift between metallic VOx species and acid sites. Of note is that the aromatic species eventually condense onto the support resulting in polyaromatic and graphitic carbon mixtures. A possible coke generation process was proposed, which contained consecutive reactions such as cracking, oligomerization, and aromatization. The establishment of the coke formation mechanisms and study of the nature of the deposited coke provide guidance for future DH catalyst optimization while reducing coke formation.

Sandwich-structured CeO2@ZSM-5 hybrid composites for catalytic oxidation of 1, 2-dichloroethane: An integrated solution to coking and chlorine poisoning deactivation

Sandwich-structured CeO2@HZSM-5 core-shell hybrid composites were designed and prepared by an in situ two-step hydrothermal growth method under dynamic condition, and SEM indicated that CeO2 nanosheets were completely encapsulated by HZSM-5 particles which formed continuous and dense zeolite membranes (exposed HZSM-5). The catalytic total oxidation of 1, 2-dichloroethane (DCE) over CeO2@HZSM-5 was investigated, compared with CeO2/HZSM-5 (CeO2 dispersed on HZSM-5) and CeO2+HZSM-5 (CeO2 mixed with HZSM-5) reference catalysts. Although the latter two showed a better catalytic activity due to the synergies of ceria and HZSM-5 zeolite, the formation of poly-chlorinated hydrocarbons (PCHs) by-products over CeO2@HZSM-5 was significantly inhibited since CeO2 with high activity for Deacon reaction was not directly exposed to DCE or HCl molecules. Moreover, CeO2@HZSM-5 also exhibited a better resistance to the coking and chlorine poisoning, because the formed non-activated coke species or polyaromatic species could be easier to be removed via in situ oxidation by the active oxygen species from CeO2, and the exposed HZSM-5 was tolerant to the chlorination of acid sites and avoided the direct adsorption of HCl on CeO2. Additionally, the presence of water and ethyl acetate (EA) dramatically inhibited the activity of CeO2@HZSM-5 for DCE oxidation, but water also completely suppressed the formation of PCHs by-products. The high Si/Al ratio could improve the water-resistance due to the increase of hydrophobicity, and the induction of trace Pd (0.1wt%) further increased the CO2 selectivity.

Measurements of the concentrations of positive and negative ions along premixed fuel-rich flames of methane and oxygen

The positive and negative ions in fuel-rich, pre-mixed flames of CH4+O2+Ar (0.4 ≤ C/O (atomic basis) ≤0.75) burning at 1 atm have been studied mass spectrometrically. The ions in the most oxygen-rich of these non-turbulent flames were found to be similar to those in more fuel-lean flames, previously studied by Goodings et al. In particular, in this flame, ions are produced in only one chemi-ionisation step in the reaction zone, i.e. by CH+O→CHO++e−. Subsequently a pool of positive ions is then formed rapidly, mainly by protons being transferred from CHO+ to intermediates and final products of combustion. Some negative ions were detected in the reaction zone, but none were found in this flame more than 1mm downstream of the reaction zone, i.e. in the burnt gas. These negatively charged species, like the positive ions, reflect the presence of intermediates in and near the reaction zone. In the burnt gas, the dominant positive ion, H3O+, recombines with free electrons, the principal negatively charged species, in H3O++e−→products. The recombination coefficient was measured to be 2.9 ± 1.5×10−7ml ion−1 s−1 at ∼2250K. However, in the richer oxy-methane flames with C/O ≥ 0.50, another ion-producing step was found to operate from 1 to at least ∼5mm downstream of the reaction zone. This new process is deduced to be CH+C2H2→C3H3++e−, and maybe involves electronically excited CH radicals. Certainly, C3H3+ becomes the major positive ion in the more fuel-rich flames. The richest flame studied, with C/O=0.75, is weakly sooting; in its burnt gas, both positive and negative ions with masses exceeding 170 a.m.u. were formed throughout the first 8 or so mm of the burnt gas. It is likely that the heavy positive ions are produced by proton transfer from a pool of positive ions (with the primary ion being C3H3+) to precursors of soot, including polyaromatic species. Likewise, the pool of massive negative ions reflects these precursors of soot.

Effects of Coke Deposits on the Catalytic Performance of Large Zeolite H-ZSM-5 Crystals during Alcohol-to-Hydrocarbon Reactions as Investigated by a Combination of Optical Spectroscopy and Microscopy.

The catalytic activity of large zeolite H-ZSM-5 crystals in methanol (MTO) and ethanol-to-olefins (ETO) conversions was investigated and, using operando UV/Vis measurements, the catalytic activity and deactivation was correlated with the formation of coke. These findings were related to in situ single crystal UV/Vis and confocal fluorescence micro-spectroscopy, allowing the observation of the spatiotemporal formation of intermediates and coke species during the MTO and ETO conversions. It was observed that rapid deactivation at elevated temperatures was due to the fast formation of aromatics at the periphery of the H-ZSM-5 crystals, which are transformed into more poly-aromatic coke species at the external surface, preventing the diffusion of reactants and products into and out of the H-ZSM-5 crystal. Furthermore, we were able to correlate the operando UV/Vis spectroscopy results observed during catalytic testing with the single crystal in situ results.