Small Oxygenates Research Articles

Response of protonated, adduct, and fragmented ions in Vocus proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS)

Abstract. Volatile organic compounds (VOCs) affect secondary pollutant formation via active chemistry. Proton-transfer-reaction mass spectrometry (PTR-MS) is one of the most important techniques to study the highly variable spatial and temporal characteristics of VOCs. The response of protonated, adduct, and fragmented ions in PTR-MS in changing instrument settings and varying relative humidity (RH) requires rigorous characterization. Herein, we present dedicatedly designed laboratory experiments conducted to investigate the response of these ions for 21 VOCs, including 12 oxygenated VOCs and 2 nitriles, using the recently developed Vocus PTR-MS. Our results show that the focusing ion–molecule reactor (FIMR) axial voltage increases sensitivity by 3 to 4 orders of magnitude but does not significantly change the fractions of protonated ions. Reducing the FIMR pressure, however, substantially increases fragmentation. Applying a high radio frequency (RF) amplitude radially to the FIMR can enhance sensitivity by 1 to 2 orders of magnitude without affecting the protonated ion fractions. The change in big segmented quadrupole (BSQ) amplitude mainly affects sensitivity and protonated ion fraction by modifying ion transmission. The relationship between sensitivity and proton-transfer reaction rate constant is complicated by the influences from both ion transmission and protonated ion fraction. The protonated ions of most VOCs studied (19 out of 21) show less than 15 % variations in sensitivity as RH increases from ∼ 5% to ∼ 85 %, except for some long-chain aldehydes which show a positive RH variation of up to 30 %. Our results suggest that the Vocus PTR-MS can reliably quantify the majority of VOCs under ambient conditions with varying RH. However, caution is advised for small oxygenates such as formaldehyde and methanol due to their low sensitivity and for long-chain aldehydes due to their slight RH dependence and fragmentation.

Simulation of the VUV Absorption Spectra of Oxygenates and Hydrocarbons: A Joint Theoretical–Experimental Study

Vacuum UV absorption spectroscopy is regularly used to provide unambiguous identification of a target species, insight into the electronic structure of molecules, and quantitative species concentrations. As molecules of interest have become more complex, theoretical spectra have been used in tandem with laboratory spectroscopic analysis or as a replacement when experimental data is unavailable. However, it is difficult to determine which theoretical methodologies can best simulate experiment. This study examined the performance of EOM-CCSD and 10 TD-DFT functionals (B3LYP, BH&HLYP, BMK, CAM-B3LYP, HSE, M06-2X, M11, PBE0, ωB97X-D, and X3LYP) to produce reliable vacuum UV absorption spectra for 19 small oxygenates and hydrocarbons using vertical excitation energies. The simulated spectra were analyzed against experiment using both a qualitative analysis and quantitative metrics, including cosine similarity, relative integral change, mean signed error, and mean absolute error. Based on our ranking system, it was determined that M06-2X was consistently the top performing TD-DFT method with BMK, CAM-B3LYP, and ωB97X-D also producing reliable spectra for these small combustion species.

Direct conversion of almond waste into value-added liquids using carbon-neutral catalysts: Hydrothermal hydrogenation of almond hulls over a Ru/CNF catalyst

The almond industry leaves behind substantial amounts of by-products, with almond hulls being the primary residue generated. Given that one way to improve food security is by decreasing waste to reduce environmental impacts, developing sustainable processes to manage this by-product is necessary. Herein, we report on the hydrothermal hydrogenation of almond hulls over a carbon-neutral Ru supported on carbon nanofibres (Ru/CNF) catalyst, addressing the temperature, H2 pressure, time and catalyst loading. These variables controlled the distribution of the reaction products: gas (0–5%), liquid (49–82%) and solid (13–51%), and ruled the composition of the liquid effluent. This aqueous fraction comprised oligomers (46–81 wt%), saccharides (2–7 wt%), sugar alcohols (2–15 wt%), polyhydric alcohols (1–8 wt%) and carboxylic acids (7–31 wt%). The temperature and reaction time influenced the extension of hydrolysis, depolymerisation, deamination, hydrolysis, hydrogenation and dehydration reactions. Additionally, the initial H2 pressure and catalyst loading kinetically promoted these transformations, whose extensions were ruled by the amount of H2 effectively dissolved in the reaction medium and the prevalence of hydrogenations over dehydration/decarboxylation reactions or vice versa depending on the catalyst loading.Process optimisation revealed that it is feasible to convert up to 67% of almond hulls into merchantable oligomers at 230 °C, 35 bar initial H2, using 1 g catalyst/g biomass (0.4 g Ru/g biomass) for 360 min. Additionally, decreasing the temperature to 187 °C without modifying the other parameters could convert this material into oligomers (31 wt%) and small oxygenates (17 wt% carboxylic acids, 11 wt% sugar alcohols and 6 wt% polyhydric alcohols) concurrently. The theoretical energy assessment revealed that the total and partial combustion of the spent solid material could provide the required energy for the process and allow catalyst recovery and reutilisation. This environmental friendliness and holistic features exemplify a landmark step-change to valorising unavoidable food waste.

Quantification of Malondialdehyde in Atmospheric Aerosols: Application of the Thiobarbituric Acid Method

Based on available toxicity data, malondialdehyde (MDA; O=CHCH2CH=O) has been designated as a potential human carcinogen. A handful of studies suggest that MDA forms in the gas and aerosol phase in the troposphere, potentially contributing to inhalation toxicity, yet it has never been quantified in ambient air. The thiobarbituric acid (TBA) acid assay for (MDA) has been used as a marker for reactive oxygen species (ROS), oxidative stress, and lipid peroxidation in biological samples for decades. Here we apply the TBA assay to estimate the amount of MDA in ambient fine particulate matter (PM2.5) for the first time, in samples containing biomass burning/urban aerosol from Fresno, CA, and urban aerosol from Los Angeles. We found 0.31–0.75 ng m-3 MDA in the particle phase, similar to the low end, but about to three orders of magnitude lower than the upper end of reported concentrations of the common C3 oxygenates methylglyoxal and malonic acid. Additionally, we investigated the response in the assay to seven common small oxygenates, and found interference only from acrolein, and that only when the acrolein was at millimolar concentrations, well above expected levels in aerosol extracts. In sum, this work suggests that MDA is present at moderate levels in biomass burning and urban aerosols; more may be in the gas phase.

Enhanced Nitrite Production from the Aqueous Photolysis of Nitrate in the Presence of Vanillic Acid and Implications for the Roles of Light-Absorbing Organics.

A prominent source of hydroxyl radicals (•OH), nitrous acid (HONO) plays a key role in tropospheric chemistry. Apart from direct emission, HONO (or its conjugate base nitrite, NO2-) can be formed secondarily in the atmosphere. Yet, how secondary HONO forms requires elucidation, especially for heterogeneous processes involving numerous organic compounds in atmospheric aerosols. We investigated nitrite production from aqueous photolysis of nitrate for a range of conditions (pH, organic compound, nitrate concentration, and cation). Upon adding small oxygenates such as ethanol, n-butanol, or formate as •OH scavengers, the average intrinsic quantum yield of nitrite [Φ(NO2-)] was 0.75 ± 0.15%. With near-UV-light-absorbing vanillic acid (VA), however, the effective Φ(NO2-) was strongly pH-dependent, reaching 8.0 ± 2.1% at a pH of 8 and 1.5 ± 0.39% at a more atmospherically relevant pH of 5. Our results suggest that brown carbon (BrC) may greatly enhance the nitrite production from the aqueous nitrate photolysis through photosensitizing reactions, where the triplet excited state of BrC may generate solvated electrons, which reduce nitrate to NO2 for further conversion to nitrite. This photosensitization process by BrC chromophores during nitrate photolysis under mildly acidic conditions may partly explain the missing HONO in urban environments.

Mechanism investigation and product selectivity control on CO-assisted direct conversion of methane into C1 and C2 oxygenates catalyzed by zeolite-supported Rh

Methane has emerged as an important energy source and chemical feedstock, and thus breakthrough strategies for the direct partial oxidation of CH4 into small oxygenates have been desired. Here, CO-assisted CH4 conversion towards C1 and C2 oxygenates over zeolite-supported single-atom Rh catalysts was demonstrated, and the key role of CO, as an indispensable additive, and the reaction mechanism were experimentally investigated. Step-by-step introduction of the reactants resulted in the formation of stoichiometric amounts of product. Replacement of O2 by H2O2 revealed that gaseous O2 acts as a true oxidant, and CO enhanced the reaction as a ligand. The critical effect of acid sites in the formation of the C2 product was also confirmed. Based on these results, a plausible reaction mechanism was proposed. Finally, a small-pore zeolite, SSZ-13, was found to be superior for the selective production of methanol.

Impact of the Zeolite Cage Structure on Product Selectivity in CO-assisted Direct Partial Oxidation of Methane over Rh Supported AEI-, CHA-, and AFX-type Zeolites

In this manuscript, we have demonstrated CO-assisted methane conversion into small oxygenates using a Rh catalyst supported on zeolites with AEI-, CHA-, and AFX-type structures (specifically, SSZ-39, SSZ-13, and SSZ-16 zeolites, respectively). Methanol, formic acid, and acetic acid were obtained as the oxygenate products and their selectivities were unique to each of the zeolite framework structures. Smaller cage structures were preferable to produce smaller oxygenates and vice versa. Among them, the AEI-type structure was highly selective (>80%) in the formation of methanol.

Pretreatment Effects on the Surface Chemistry of Small Oxygenates on Molybdenum Trioxide

Understanding surface reactions of biomass-derived oxygenates on metal oxides is important for designing catalysts for valorization of biomass. This work elucidated the effect of different pretreat...

Density Functional Theory Study of Decarboxylation and Decarbonylation of Acetic Acid on Pd(111)

Acetic acid (CH3COOH) decomposition over Pd (111) surface is a good model system for decomposition of acids and small oxygenates common in biomass conversion. Here we present Density Functional The...

Thermogravimetric-mass spectrometric characterization of thermal decomposition of lignite with attention to the evolutions of small molecular weight oxygenates

Thermogravimetry coupled with electron impact mass spectrometry (TG-EI MS) was used to investigate the evolutions of gases and small molecular weight oxygenates during the pyrolysis of a raw lignite and a potassium (K)-impregnated lignite. By clarifying the effects of ionization on the fragmentations of various compounds, it was found that a low ionizing energy of 20 eV was better for detecting oxygenates. The study further proposed a unique approach of the installment of a cold trap between the TG-MS interfaces. This vapor trapping approach is useful to distinguish between some compounds of equal mass in terms of their different boiling points. The real-time release of various oxygenates during the pyrolysis of the raw lignite and the K-impregnated lignite were detected. It was found that some highly volatile oxygenates such as formic acid (46 m/z), acetone (58 m/z) and butanone (72 m/z) started to release at a temperature of 200 °C or lower. Many oxygenates such as furan (68 m/z), methylfuran (82 m/z), cyclopentenone (82 m/z), cyclopentanone (84 m/z), furfural (96 m/z), dimethylfuran (96 m/z), cyclohexenone (96 m/z) and dimethoxyphenol (154 m/z) could be explained as the products degraded from the preserved woody components in this low-maturity lignite. The K impregnation overall facilitated the formations of oxygenates, while it had an inhibitory effect on the formation of CO and an insignificant effect on the formations of some oxygenates such as acetic acid and phenol. The curves of differential thermogravimetry (DTG) were deconvoluted to correspond to the staged release characteristics of main gases and various oxygenates.

Thermal Cracking of Sugars for the Production of Glycolaldehyde and Other Small Oxygenates.

Thermal cracking of sugars for production of glycolaldehyde, a potential renewable platform molecule, in yields up to 74 % with up to 95 % carbon recovered in the condensed product is demonstrated using glucose as the feed. The process involves spraying an aqueous sugar solution into a fluidized bed of glass beads. Continuous operation is carried out for more than 90 h with complete conversion and stable product selectivity. Besides glycolaldehyde, the other identified condensed products are pyruvaldehyde (9 %), formaldehyde (7 %), glyoxal (2 %), acetol (2 %), and acetic acid (1 %). The effects of temperature, glucose feed concentration, and type of sugar feedstock are investigated. Cracking the monosaccharides fructose and xylose leads to very different product distributions from glucose, but similar carbon recovery. A reaction network in agreement with the main observed products from cracking of monosaccharide sugars is proposed.

Consequence of replacing nitrogen with carbon dioxide as atmosphere on suppressing the formation of polycyclic aromatic hydrocarbons in catalytic pyrolysis of sawdust

This study evaluates the effect of replacement of N2 with CO2 as atmosphere in catalytic pyrolysis of waste lignocellulosics with acidic and metal-modified zeolites, respectively, on the 16 EPA priority pollutant polycyclic aromatic hydrocarbons (PAHs) in bio-oils. By coupling solid phase extraction pretreatment with single ion monitoring detection, it is found that the replacement alleviates PAHs in bio-oil concerning synchronously abating the 16 PAHs with low, medium and high molecular weights, and the benzo[a]pyrene equivalent toxicity of bio-oil decreases. Meanwhile, CO2 decreases the content of small oxygenates, e.g. furans, ketones, acids, and increases phenolics and aromatics affording more stable and valuable bio-oils. Moreover, CO2 enhances carbon conversion efficiency, especially in combination with Fe-modified zeolite, which presents a synergistic effect. This study indicates the practical application of CO2 as an atmosphere in catalytic pyrolysis to improve the bio-oil quality by suppressing PAHs formation and adjusting compound constituent.

Performance of mesoporous HZSM-5 and Silicalite-1 coated mesoporous HZSM-5 catalysts for deoxygenation of straw fast pyrolysis vapors

•Catalytic treating of biomass fast pyrolysis vapors offers a promising route to convert many different types of lignocellulosic biomass into biofuels. Considerable research efforts have focused on using zeolite catalysts, especially the microporous HZSM-5 zeolite. Unfortunately, the zeolite’s initial high activity in forming coke and light gases reduces the overall oil yield. In this contribution, we synthesized conventional HZSM-5 zeolite, mesoporous HZSM-5 zeolite, and a core-shell catalyst consisting of mesoporous HZSM-5 zeolite coated with a thin layer (<20 nm) of silicalite-1 zeolite. The catalysts were characterized by inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 and Ar physisorption, temperature programmed desorption of ammonia (NH3-TPD), diffuse reflectance infrared fourier transform spectroscopy (DRIFT) spectra, and 27Al nuclear magnetic resonance (NMR). The catalysts were tested in a tandem micro-pyrolyzer system for deoxygenation of wheat straw derived fast pyrolysis vapors in a downstream catalytic fixed bed reactor and the quantitative product analysis was performed with GC-MS/FID/TCD and TGA-MS. The change in yields of different product groups was monitored during catalyst deactivation for an increasing number of pyrolysis vapor pulses and the cumulative carbon recoveries of all product streams (char, vapors, gas, and coke) were compared at different biomass-to-catalyst ratios (∼4 and ∼1) for catalyst loadings of 2 and 8 mg, respectively. In addition, the yields were compared for the same number of zeolitic acid sites. Even though the coking propensity was highest for the mesoporous HZSM-5 (5.1% carbon recovery of fed biomass at B:C = 1), it showed improved conversion of oxygenates (wt% O of vapors = 15) and a higher tolerance towards deactivation by coking compared to the conventional HZSM-5 (wt% O of vapors = 20). Coating of mesoporous HZSM-5 with silicalite-1 reduced the number of acid sites per mass of catalyst by ∼20%, which is attributed to the addition of the inert silicalite-1 layer and passivation of the external acid sites of the mesoporous HZSM-5. Compared to mesoporous HZSM-5, the added silicalite-1 shell reduced the coke yield by 43% (from 5.1–2.9 wt% of fed biomass carbon) for the same catalyst mass and by 8% (4.7 wt% of fed biomass carbon) for the same total number of acid sites. Furthermore, the tolerance towards deactivation observed for the mesoporous HZSM-5 core was largely preserved—especially for conversion of small oxygenates like acids, alcohols and aldehydes, resulting in vapors with 14 wt% O.

Catalytic pyrolysis of soda lignin over zeolites using pyrolysis gas chromatography-mass spectrometry

Catalytic fast pyrolysis of soda lignin was examined at different temperatures (500,600,700,800 and 900 °C) in the presence of three zeolites with different Si/Al ratio using the Py–GC/MS in order to investigate best catalytic system. The three zeolites are y-zeolite (8–9), mordenite (15–17), ZSM-5 (30–40), which have static pore sizes 0.74, 0.65, and 0.59 nm respectively. The shape and acidity of zeolites, as well as pyrolysis temperature, have a significant effect on product distribution in catalytic fast pyrolysis. Y-zeolite was the most effective catalytic system among all catalysts for deomethoxylation and dehydroxylation of small oxygenates as well as bulky oxygenates to produce aromatics. However, mordenite and ZSM-5 could not convert the large oxygenates due to size exclusion and pore blockage. Highest yield of aromatics with significant amount of aromatic dimers was obtained over y-zeolite and then yield of aromatics followed in order by mordenite and ZSM-5 at 800 °C.

Valorization of Biomass‐derived Small Oxygenates: Kinetics, Mechanisms and Site Requirements of H2‐involved Hydrogenation and Deoxygenation Pathways over Heterogeneous Catalysts

AbstractHydrodeoxygenation (HDO) is a key step in upgrading of biomass‐derived feedstocks to fuels and valuable chemicals. In this review, I select to discuss the current understanding of heterogeneous HDO catalysis of representative small oxygenates that are present in the bio‐crude obtained from thermochemical conversions of biomass raw materials, as the efficient valorization of these presently underutilized carbon sources would significantly improve carbon recovery and the overall process techno‐economics, in addition to facilitating downstream processing in some cases. A primary focus is laid on the kinetics, mechanisms and active site requirements of molecular H2‐involved hydrogenation and deoxygenation reactions of small aliphatic oxygenates. More often than not, surprising contrasts are found between gas‐solid and polar liquid‐solid interfaces and hint toward substantial restructuring and modifications of the catalytic surfaces and chemistries in polar liquids, particularly induced by protic molecules (the most abundant being water). Knowledge gaps, uncharted territories and associated challenges for the interrogations of these fundamental aspects are also discussed in this review.

Online photoionization mass spectrometric evaluation of catalytic co-pyrolysis of cellulose and polyethylene over HZSM-5

The hydrogen-deficient and oxygen-rich nature of lignocellulosic biomass prohibits effective conversions of biomass to fuels and chemicals via catalytic pyrolysis due to significant coking of the catalysts. Co-feeding of biomass feedstock with hydrogen-rich and oxygen-deficient thermoplastics could improve the process. Herein, thermal and catalytic co-pyrolysis of cellulose and polyethylene (PE) was studied via thermogravimetry combined with an online photoionization time-of-flight mass spectrometry (PI-TOF-MS). No notable synergetic effect was found in the thermal co-pyrolysis process while a considerable synergetic effect was observed during the catalytic co-pyrolysis. In the case of catalytic pyrolysis, co-feeding of cellulose with PE significantly improved the aromatic formation. Detailed reaction intermediates and products were detected by PI-TOF-MS and the process of aromatization could be ascribed to aromatization of small oxygenates and olefins, as well as Diels-Alder reaction and dehydration by HZSM-5. Moreover, this study provides a reliable tool for screening and optimizing of catalytic co-pyrolysis.

Synergistic Bronsted‐Lewis Acidity Effect on Upgrading Biomass‐Derived Phenolic Compounds: Statistical Optimization of Process Parameters, Kinetic Investigations and DFT Study

AbstractBiomass‐derived phenolic compounds and small oxygenates obtained from pyrolysis of lignocellulosic biomass needs to be upgraded to obtain the C10‐C13 range fuel precursors. In this context, iso‐propylation of m‐cresol was performed in batch reactors using multifunctional ionic liquids (ILs) as catalysts. Environmentally benign Bronsted‐Lewis acidic ILs were synthesized and characterized using 1H NMR, 13C NMR and Raman spectroscopic data. Relative Bronsted and Lewis acidity of these ILs were compared using UV‐Visible and FTIR spectroscopic data and the effect of acidity on the catalytic activity and product selectivity in alkylation of m‐cresol with iso‐propyl alcohol was investigated. Correlation of acidity and catalytic activity was confirmed by theoretical DFT study on IL structures. High conversion of m‐cresol (95.6%) was obtained using ZnCl2 containing IL, at 393 K temperature and 2:1:1 alcohol: m‐cresol: IL molar ratio, within 5 h of reaction time. The activity and recoverability of these ILs were retained even after 8 recycles. The effect of process parameters like reaction time, temperature, reactant mole ratio and catalyst to reactant mole ratio was investigated and optimized from design of experiments results. A plausible reaction mechanism was formulated based on the product distribution and a kinetic model was developed to estimate the activation energy (45.70 kJ mol−1) and pre‐exponential factor (247.78 lit. mol−1 min−1).

Methanol Production – a Key Intermediate in the Electrochemical Interconversion of Organics to Chemicals and Fuels

Methanol is the world’s 5th largest commodity chemical by volume. It can be used to synthesize numerous products, and has been touted as an important energy carrier of the future. It has a high energy density and is a liquid at atmospheric conditions, which make it ideal for stable transportation and storage that is compatible with existing infrastructure, which is quite the opposite of methane or even hydrogen gas.1 The production of methanol and its interconversion is typically achieved through the steam reforming of methane to syngas – this is a very energy intensive pathway, along with many intermediate reaction steps that result in an inelegant and energetically circuitous process. Therefore, a low temperature, low energy process is desired to produce methanol from methane. Electrochemical processes are ideal because they allow for control of the catalyst surface free energy directly, thus methanol production may be realized in a single reactor.2 Continuing to use methane for this conversion makes sense from an economic perspective because of the very low cost of natural gas today. However, the low cost of natural gas also ensures that carbon sources will be used for electricity generation for the foreseeable future – releasing carbon dioxide. CO2 is believed to be the largest contributor to global climate change, with more than 35 gigatons per year (and climbing) being emitted into the atmosphere. However, the global demand to reduce carbon dioxide emissions means that soon we will need to capture this power plant derived CO2, which could flood the market with a new chemical feedstock. Another interesting source for emerging chemical feedstocks, such as acetate (acetic acid) is biomass fermentation. As we move forward in the 21st century, it is becoming clear that in the very near term, we will have access to a diversity of feedstocks and it is very important to find low energy pathways convert between several small hydrocarbon oxygenates that have market value including: methanol, ethanol, formate, acetate, and carbon monoxide have industry and commercial value. Since electrochemistry can play a critical role in this paradigm, a deeper understanding of an electrochemical pathway from methane to methanol and beyond is desired, from a deeper fundamental understanding to a larger commercial application. In this poster, we will show recent experimental work in our group regarding the transformation of methane, CO2 and acetate to methanol. We will show the utility of catalysts such as copper, silver, gold, and palladium/copper, and others, to develop a link and fundamental pathway between oxygenates. We will also discuss possibilities other than methanol, and to start to build an electrochemical map from methane to carbon dioxide and back, stopping at all the compounds in between.

Zn-loaded HY zeolite as active catalyst for iso-propylation of biomass-derived phenolic compounds: A comparative study on the effect of acidity and porosity of zeolites

Liquid bio-oil formed from pyrolysis of lignocellulosic biomass needs further upgradation by CC bond formation reaction between the phenolic compounds and small oxygenates to retain the maximum amount of carbon in the fuel range (C10C13), prior to hydrodeoxygenation step. In this contribution, we have explored the gas-phase alkylation of m-cresol with iso-propanol over zeolites like HZSM5, HBEA, HMCM22, HY, and ZnY solid acid catalysts. The fresh and spent catalysts were characterized by XRD, N2 sorption, ICP-MS, Py-FTIR, NH3-TPD, SEM, and TEM techniques. The time-on-stream and temperature studies were carried out over these catalysts at a WHSV value of 2.8h−1 to analyze the role of different active acidic sites as well as their pore dimensions on the catalytic activity and product distribution. Among all the catalysts, formation of thymol by C-alkylation was highly dominant over ZnY which has strong Lewis acid sites. Coke analysis revealed that HBEA was found to rapidly deactivate by coking (16.4% coke) and pore blocking. Time-on-stream study at 250°C showed that maximum conversion of m-cresol (92%) was obtained on fresh ZnY catalyst having strong Lewis acidity and moderate Bronsted acidity, confirming the fact that strong Lewis acidity in combination with Bronsted acidity improved the stability of iso-propyl cation and consequently, enhanced the conversion of m-cresol. Moreover, the presence of Zn2+ content in FAU zeolite framework was found to alter its acid site distribution and pore volume as compared to HY zeolite and resulted in predominant mono-alkylated product formation, especially thymol.

Experimental and kinetic modeling study of laminar coflow diffusion methane flames doped with iso-butanol

In this work, laminar coflow diffusion flames fueled with methane/iso-butanol were investigated with synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Mole fractions of C2–C16 flame species, including small hydrocarbons, oxygenates and aromatics were measured along the flame centerline. The enhancement in the formation of aromatics was observed through the increment of the iso-butanol doping ratio (1.95% and 3.90% in two flames) in the inlet fuel gas mixture. A detailed kinetic model was developed from previous models to help understand the experimental findings. According to rate of production analysis in laminar coflow diffusion flames, unimolecular decomposition and H abstraction reactions provide equivalent contributions to the consumption of iso-butanol. Compared to the coflow flames doped with other butanol isomers with the identical condition, the water elimination reaction of iso-butanol has the lowest contribution to fuel consumption. Two reaction pathways provide major contribution to the formation of propargyl radical, i.e. iC4H9OH→iC3H7→C3H6→aC3H5→aC3H4→C3H3 and iC4H9OH→βC4H8OH→iC4H8→iC4H7→aC3H4→C3H3. Propargyl is concluded as the key precursor for aromatics, while phenyl and benzyl radicals are the main PAH precursors. Finally, based on the studies of laminar coflow flames fueled with butanol isomers, the sooting tendency of butanol isomers relates strictly to the efficiency of the propargyl formation pathways in fuel decomposition.