Separation of carvone by batch distillation from the mixture obtained from limonene oxidation

Limonene is the main constituent of citrus oils whose oxidation produces a set of fine chemical compounds such as carvone, carveol, and limonene 1,2-epoxide. This contribution reports the results of the experimental evaluation and computational simulation of carvone separation by fractional distillation from the reaction mixture. Carvone was obtained from limonene oxidation over a perchlorinated iron phthalocyanine supported on modified silica catalyst (FePcCl16-NH2-SiO2) and t-butyl hydroperoxide (TBHP) as oxidant. Both experimental and simulation results support that fractional distillation (in batch and continuous) is a suitable technique for concentrating carvone. However, in the presence of water, the formation of immiscible L-L phases makes the experimental separation of carvone more difficult. Simulation results of the batch distillation incorporating the NRTL-RK thermodynamic model indicate that if water, acetone, and t-butanol are previously removed from the reaction mixture, carvone composition can be enriched in the reboiler from 4% up to 50%, or around 86.5% if the removal is in a third distillate cut under vacuum conditions.

On the similarities and differences between the products of oxidation of hydrocarbons under simulated atmospheric conditions and cool flames

Atmospheric oxidation chemistry and, more specifically, photooxidation show that the long-term oxidation of organic aerosol (OA) progressively erases the initial signature of the chemical compounds and can lead to a relatively uniform character of oxygenated organic aerosol (OOA). This uniformity character observed after a long reaction time seems to contrast with the great diversity of reaction mechanisms observed in the early stages of oxidation. The numerous studies carried out on the oxidation of terpenes, and more particularly on limonene for its diversity of reaction sites (endo- and oxocyclic), allow this evolution to be studied. We have selected, for their diversity of experimental conditions, nine studies of limonene oxidation at room temperature over long reaction times to be compared to the present data set obtained at elevated temperature and short reaction time in order to investigate the similarities in terms of reaction mechanisms and chemical species formed. Here, the oxidation of limonene–oxygen–nitrogen mixtures was studied using a jet-stirred reactor at elevated temperature and atmospheric pressure. Samples of the reacting mixtures were collected and analyzed by high-resolution mass spectrometry (Orbitrap) after direct injection or after separation by reverse-phase ultra-high-pressure liquid chromatography and soft ionization, i.e., (+/-) HESI and (+/-) APCI. Unexpectedly, because of the diversity of experimental conditions in terms of continuous-flow tank reactor, concentration of reactants, temperature, reaction time, mass spectrometry techniques, and analysis conditions, the results indicate that among the 1138 presently detected molecular formulae, many oxygenates found in earlier studies of limonene oxidation by OH and/or ozone are also produced under the present conditions. Among these molecular formulae, highly oxygenated molecules and oligomers were detected in the present work. The results are discussed in terms of reaction pathways involving the initial formation of peroxy radicals (RO2), isomerization reactions yielding keto-hydroperoxides, and other oxygenated intermediates and products up to C25H32O17, products which could derive from RO2 autoxidation via sequential H shift and O2 addition (C10H14O3,5,7,9,11) and products deriving from the oxidation of alkoxy radicals (produced by RO2 self-reaction or reaction with HO2) through multiple H shifts and O2 additions (C10H14O2,4,6,8,10). The oxidation of RO2, with possible occurrence of the Waddington mechanism and of the Korcek mechanism, involving H shifts is also discussed. The present work demonstrates similitude between the oxidation products and oxidation pathways of limonene under simulated atmospheric conditions and in those encountered during the self-ignition of hydrocarbons at elevated temperatures. These results complement those recently reported by Vereecken and Nozière and confirm for limonene the existence of an oxidative chemistry of the alkylperoxy radical beyond 450 K based on the H shift (Nozière and Vereecken, 2019; Vereecken and Nozière, 2020).

Carboxylic acids from limonene oxidation by ozone and hydroxyl radicals: insights into mechanisms derived using a FIGAERO-CIMS

This work presents the results from a flow reactor study on the formation of carboxylic acids from limonene oxidation in the presence of ozone under NOx-free conditions in the dark. A High-Resolution Time-of-Flight acetate Chemical Ionisation Mass Spectrometer (HR-ToF-CIMS) was used in combination with a Filter Inlet for Gases and AEROsols (FIGAERO) to measure the carboxylic acids in the gas and particle phases. The results revealed that limonene oxidation produced large amounts of carboxylic acids which are important contributors to secondary organic aerosol (SOA) formation. The highest 10 acids contributed 56 %–91 % to the total gas-phase signal, and the dominant gas-phase species in most experiments were C8H12O4, C9H14O4, C7H10O4 and C10H16O3. The particle-phase composition was generally more complex than the gas-phase composition, and the highest 10 acids contributed 47 %–92 % to the total signal. The dominant species in the particle phase were C8H12O5, C9H14O5, C9H12O5 and C10H16O4. The measured concentration of dimers bearing at least one carboxylic acid function in the particle phase was very low, indicating that acidic dimers play a minor role in SOA formation via ozone (O3)/hydroxyl (OH) oxidation of limonene. Based on the various experimental conditions, the acidic compositions for all experiments were modelled using descriptions from the Master Chemical Mechanism (MCM). The experiment and model provided a yield of large (C7–C10) carboxylic acid of the order of 10 % (2 %–23 % and 10 %–15 %, respectively). Significant concentrations of 11 acids, from a total of 16 acids, included in the MCM were measured with the CIMS. However, the model predictions were, in some cases, inconsistent with the measurement results, especially regarding the OH dependence. Reaction mechanisms are suggested to fill-in the knowledge gaps. Using the additional mechanisms proposed in this work, nearly 75 % of the observed gas-phase signal in our lowest concentration experiment (8.4 ppb converted, ca. 23 % acid yield) carried out under humid conditions can be understood.

Carboxylic acids from limonene oxidation by ozone and OH radicals: Insights into mechanisms derived using a FIGAERO-CIMS

This work presents the results from flow reactor studies on the formation of carboxylic acids from limonene oxidation under various conditions. A High Resolution Time Of Flight acetate Chemical Ionisation Mass Spectrometer (HR – TOF – CIMS) was used in combination with the Filter Inlet for Gases and AEROsols (FIGAERO) to measure the carboxylic acid profile in the gas and particle phases. The results revealed that limonene oxidation produced large amounts of carboxylic acids which are important contributors to secondary organic aerosol (SOA) formation. The highest 10 acids contributed 56–91 % to the total gas-phase signal and the dominant gas-phase species in most experiments were C8H12O4, C9H14O4, C7H10O4 and C10H16O3. The particle-phase composition was generally more complex than the gas-phase composition and the highest 10 acids contributed 47–92 % to the total signal. The dominant species in the particle phase were C8H12O5, C9H14O5, C9H12O5 and C10H16O4. The measured concentrations of dimers in the particle phase were very low, indicating that acidic dimers play a minor role in SOA formation via ozone/OH oxidation of limonene. Spearman correlation analysis of the produced carboxylic acid species and experimental parameters were helpful in interpreting the results. Based on the various experimental conditions, the acidic composition for all experiments were modelled using the Master Chemical Mechanisms (MCM). Significant concentrations of 11 acids, from a total of 16 acids, included in MCM were measured with the CIMS. However, the model predictions were, in some cases, inconsistent with the measurement results, especially in the case of the OH dependence. Reaction mechanisms are suggested to fill-in the knowledge gaps. Based on the mechanisms proposed in this work, nearly 75 % of the qualitative gas-phase signal of the low concentration (ppb converted), humid, mixed oxidant experiment can be explained.