Pyrolysis of Cyclopentadienone: Mechanistic Insights from a Direct Measurement of Product Branching Ratios

Gas-phase aluminium acetylacetonate decomposition: revision of the current mechanism by VUV synchrotron radiation

Although aluminium acetylacetonate, Al(C5H7O2)3, is a common precursor for chemical vapor deposition (CVD) of aluminium oxide, its gas-phase decomposition is not well-known. Here, we studied its thermal decomposition in a microreactor by double imaging photoelectron photoion coincidence spectroscopy (i2PEPICO) between 325 and 1273 K. The reactor flow field was characterized by CFD. Quantum chemical calculations were used for the assignment of certain species. The dissociative ionization of the room temperature precursor molecule starts at a photon energy of 8.5 eV by the rupture of the bond to an acetylacetonate ligand leading to the formation of the Al(C5H7O2)2+ ion. In pyrolysis experiments, up to 49 species were detected and identified in the gas-phase, including reactive intermediates and isomeric/isobaric hydrocarbons, oxygenated species as well as aluminium containing molecules. We detected aluminium bis(diketo)acetylacetonate-H, Al(C5H7O2)C5H6O2, at m/z 224 together with acetylacetone (C5H8O2) as the major initial products formed at temperatures above 600 K. A second decomposition channel affords Al(OH)2(C5H7O2) along with the formation of a substituted pentalene ring species (C10H12O2) as assigned by Franck-Condon simulations and quantum chemical calculations. Acetylallene (C5H6O), acetone (C3H6O) and ketene (C2H2O) were major secondary decomposition products, formed upon decomposition of the primary products. Three gas-phase aromatic hydrocarbons were also detected and partially assigned for the first time: m/z 210, m/z 186 (C14H18 or C12H10O2) and m/z 146 (C11H14 or C9H6O2) and their formation mechanism is discussed. Finally, Arrhenius parameters are presented on the gas-phase decomposition kinetics of Al(C5H7O2)3, aided by numerical simulation of the flow field.

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.

Comparing Positively and Negatively Charged Distonic Radical Ions in Phenylperoxyl Forming Reactions

In the gas phase, arylperoxyl forming reactions play a significant role in low-temperature combustion and atmospheric processing of volatile organic compounds. We have previously demonstrated the application of charge-tagged phenyl radicals to explore the outcomes of these reactions using ion trap mass spectrometry. Here, we present a side-by-side comparison of rates and product distributions from the reaction of positively and negatively charge tagged phenyl radicals with dioxygen. The negatively charged distonic radical ions are found to react with significantly greater efficiency than their positively charged analogues. The product distributions of the anion reactions favor products of phenylperoxyl radical decomposition (e.g., phenoxyl radicals and cyclopentadienone), while the comparable fixed-charge cations yield the stabilized phenylperoxyl radical. Electronic structure calculations rationalize these differences as arising from the influence of the charged moiety on the energetics of rate-determining transition states and reaction intermediates within the phenylperoxyl reaction manifold and predict that this influence could extend to intra-molecular charge-radical separations of up to 14.5 Å. Experimental observations of reactions of the novel 4-(1-carboxylatoadamantyl)phenyl radical anion confirm that the influence of the charge on both rate and product distribution can be modulated by increasing the rigidly imposed separation between charge and radical sites. These findings provide a generalizable framework for predicting the influence of charged groups on polarizable radicals in gas phase distonic radical ions. Graphical Abstract.

Thermal Decomposition of Potential Ester Biofuels. Part I: Methyl Acetate and Methyl Butanoate

Two methyl esters were examined as models for the pyrolysis of biofuels. Dilute samples (0.06-0.13%) of methyl acetate (CH₃COOCH₃) and methyl butanoate (CH₃CH₂CH₂COOCH₃) were entrained in (He, Ar) carrier gas and decomposed in a set of flash-pyrolysis microreactors. The pyrolysis products resulting from the methyl esters were detected and identified by vacuum ultraviolet photoionization mass spectrometry. Complementary product identification was provided by matrix infrared absorption spectroscopy. Pyrolysis pressures in the pulsed microreactor were about 20 Torr and residence times through the reactors were roughly 25-150 μs. Reactor temperatures of 300-1600 K were explored. Decomposition of CH₃COOCH₃ commences at 1000 K, and the initial products are (CH₂═C═O and CH₃OH). As the microreactor is heated to 1300 K, a mixture of CH₂═C═O and CH₃OH, CH₃, CH₂═O, H, CO, and CO₂ appears. The thermal cracking of CH₃CH₂CH₂COOCH₃ begins at 800 K with the formation of CH₃CH₂CH═C═O and CH₃OH. By 1300 K, the pyrolysis of methyl butanoate yields a complex mixture of CH₃CH₂CH═C═O, CH₃OH, CH₃, CH₂═O, CO, CO₂, CH₃CH═CH₂, CH₂CHCH₂, CH₂═C═CH₂, HCCCH₂, CH₂═C═C═O, CH₂═CH₂, HC≡CH, and CH₂═C═O. On the basis of the results from the thermal cracking of methyl acetate and methyl butanoate, we predict several important decomposition channels for the pyrolysis of fatty acid methyl esters, R-CH₂-COOCH₃. The lowest-energy fragmentation will be a 4-center elimination of methanol to form the ketene RCH═C═O. At higher temperatures, concerted fragmentation to radicals will ensue to produce a mixture of species: (RCH₂ + CO₂ + CH₃) and (RCH₂ + CO + CH₂═O + H). Thermal cracking of the β C-C bond of the methyl ester will generate the radicals (R and H) as well as CH₂═C═O + CH₂═O. The thermochemistry of methyl acetate and its fragmentation products were obtained via the Active Thermochemical Tables (ATcT) approach, resulting in Δ_fH₂₉₈(CH₃COOCH₃) = -98.7 ± 0.2 kcal mol^-1, Δ_fH₂₉₈(CH₃CO₂) = -45.7 ± 0.3 kcal mol^-1, and Δ_fH₂₉₈(COOCH₃) = -38.3 ± 0.4 kcal mol^-1.

Cyclopentadienone Oxidation Reaction Kinetics and Thermochemistry for the Alcohols, Hydroperoxides, and Vinylic, Alkoxy, and Alkylperoxy Radicals

Cyclopentadienone has one carbonyl and two olefin groups resulting in 4n + 2 π-electrons in a cyclic five-membered ring structure. Thermochemical and kinetic parameters for the initial reactions of cyclopentadienone radicals with O2 and the thermochemical properties for cyclopentadienone-hydroperoxides, alcohols, and alkenyl, alkoxy, and peroxy radicals were determined by use of computational chemistry. The CBS-QB3 composite and B3LYP density functional theory methods were used to determine the enthalpies of formation (ΔfH°298) using the isodesmic reaction schemes with several work reactions for each species. Entropy and heat capacity, S°(T) and Cp°(T) (50 K ≤ T ≤ 5000 K) are determined using geometric parameters, internal rotor potentials, and frequencies from B3LYP/6-31G(d,p) calculations. Standard enthalpies of formation are reported for parent molecules as cyclopentadienone, cyclopentadienone with alcohol, hydroperoxide substituents, and the cyclopentadienone-yl vinylic, alkoxy, and peroxy radicals corresponding to loss of a hydrogen atom from the carbon and oxygen sites. Entropy and heat capacity vs temperature also are reported for the parent molecules and for radicals. The thermochemical analysis shows The R(•) + O2 well depths are deep, on the order of 50 kcal mol(-1), and the R(•) + O2 reactions to RO + O (chain branching products) for cyclopentadienone-2-yl and cyclopentadienone-3-yl have unusually low reaction (ΔHrxn) enthalpies, some 20 or so kcal/mol below the entrance channels. Chemical activation kinetics using quantum RRK analysis for k(E) and master equation for falloff are used to show that significant chain branching as a function of temperature and pressure can occur when these vinylic radicals are formed.

An optically accessible pyrolysis microreactor

We report an optically accessible pyrolysis micro-reactor suitable for in situ laser spectroscopic measurements. A radiative heating design allows for completely unobstructed views of the micro-reactor along two axes. The maximum temperature demonstrated here is only 1300 K (as opposed to 1700 K for the usual SiC micro-reactor) because of the melting point of fused silica, but alternative transparent materials will allow for higher temperatures. Laser induced fluorescence measurements on nitric oxide are presented as a proof of principle for spectroscopic characterization of pyrolysis conditions.

Formation and stability of gas-phase o-benzoquinone from oxidation of ortho-hydroxyphenyl: a combined neutral and distonic radical study

The o-hydroxyphenyl radical reacts with O₂ to form o-benzoquinone + OH and cyclopentadienone is assigned as a secondary product.

Pyrolysis Pathways of the Furanic Ether 2-Methoxyfuran

Substituted furans, including furanic ethers, derived from nonedible biomass have been proposed as second-generation biofuels. In order to use these molecules as fuels, it is important to understand how they break apart thermally. In this work, a series of experiments were conducted to study the unimolecular and low-pressure bimolecular decomposition mechanisms of the smallest furanic ether, 2-methoxyfuran. Electronic structure (CBS-QB3) calculations indicate this substituted furan has an unusually weak O-CH3 bond, approximately 190 kJ mol(-1) (45 kcal mol(-1)); thus, the primary decomposition pathway is through bond scission resulting in CH3 and 2-furanyloxy (O-C4H3O) radicals. Final products from the ring opening of the furanyloxy radical include 2 CO, HC≡CH, and H. The decomposition of methoxyfuran is studied over a range of concentrations (0.0025-0.1%) in helium or argon in a heated silicon carbide (SiC) microtubular flow reactor (0.66-1 mm i.d., 2.5-3.5 cm long) with reactor wall temperatures from 300 to 1300 K. Inlet pressures to the reactor are 150-1500 Torr, and the gas mixture emerges as a skimmed molecular beam at a pressure of approximately 10 μTorr. Products formed at early pyrolysis times (100 μs) are detected by 118.2 nm (10.487 eV) photoionization mass spectrometry (PIMS), tunable synchrotron VUV PIMS, and matrix infrared absorption spectroscopy. Secondary products resulting from H or CH3 addition to the parent and reaction with 2-furanyloxy were also observed and include CH2═CH-CHO, CH3-CH═CH-CHO, CH3-CO-CH═CH2, and furanones; under the conditions in the reactor, we estimate these reactions contribute to at most 1-3% of total methoxyfuran decomposition. This work also includes observation and characterization of an allylic lactone radical, 2-furanyloxy (O-C4H3O), with the assignment of several intense vibrational bands in an Ar matrix, an estimate of the ionization threshold, and photoionization efficiency. A pressure-dependent kinetic mechanism is also developed to model the decomposition behavior of methoxyfuran and provide pathways for the minor bimolecular reaction channels that are observed experimentally.