Recent Submissions

  • Autoignition of diethyl ether and a diethyl ether/ethanol blend

    Issayev, Gani; Sarathy, Mani; Farooq, Aamir (Fuel, Elsevier BV, 2020-07-04) [Article]
    Binary blends of fast-reacting diethyl ether (DEE) and slow-reacting ethanol (EtOH) are quite promising as renewable replacements for conventional fuels in modern compression ignition engines. In this work, pure diethyl ether and a 50/50 M binary blend of diethyl ether and ethanol (DEE/EtOH) were investigated in a shock tube and a rapid compression machine. Ignition delay times were measured over the temperature range of 550–1000 K, pressures of 20–40 bar, and equivalence ratios of 0.5–1. Literature reaction mechanisms of diethyl ether and ethanol were combined to simulate the reactivity trends of the blends. Species rate-of-production and sensitivity analyses were performed to analyze the interplay between radicals originating from the two fuels. Multistage ignition behavior was observed in both experiments and simulations, with peculiar 3-stage ignition visible at fuel-lean conditions. Kinetic analyses were used to identify the reactions controlling various stages of ignition. Reactivity comparison of DEE/EtOH and dimethyl ether/ethanol (DME/EtOH) blends showed that the oxidation of DEE blends is controlled by acetaldehyde whereas formaldehyde controls the oxidation of DME blends.
  • Cool flame chemistry of diesel surrogate compounds: n-Decane, 2-methylnonane, 2,7-dimethyloctane, and n-butylcyclohexane

    Wang, Zhandong; Hansen, Nils; Jasper, Ahren W.; Chen, Bingjie; Popolan-Vaida, Denisia M.; Yalamanchi, Kiran K.; Najjar, Ahmed; Dagaut, Philippe; Sarathy, Mani (Combustion and Flame, Elsevier BV, 2020-06-23) [Article]
    Elucidating the formation of combustion intermediates is crucial to validate reaction pathways, develop reaction mechanisms and examine kinetic modeling predictions. While high-temperature pyrolysis and oxidation intermediates of alkanes have been thoroughly studied, comprehensive analysis of cool flame intermediates from alkane autoxidation is lacking and challenging due to the complexity of intermediate species produced. In this work, jet-stirred reactor autoxidation of four C10 alkanes: n-decane, 2-methylnonane, 2,7-dimethyloctane, and n-butylcyclohexane, as model compounds of diesel fuel, was investigated from 500 to 630 K using synchrotron vacuum ultraviolet photoionization molecular beam mass spectrometry (SVUV-PIMS). Around 100 intermediates were detected for each fuel. The classes of molecular structures present during the autoxidation of the representative paraffinic functional groups in transport fuels, i.e., n-alkanes, branched alkanes, and cycloalkanes were established and were found to be similar from the oxidation of various alkanes. A theoretical approach was applied to estimate the photoionization cross sections of the intermediates with the same carbon skeleton as the reactants, e.g., alkene, alkenyl keto, cyclic ether, dione, keto-hydroperoxide, diketo-hydroperoxide, and keto-dihydroperoxide. These species are indicators of the first, second, and third O2 addition reactions for the four C10 hydrocarbons, as well as bimolecular reactions involving keto-hydroperoxides. Chemical kinetic models for the oxidation of these four fuels were examined by comparison against mole fraction of the reactants and final products obtained in additional experiments using gas chromatography analysis, as well as the detailed species pool and mole fractions of aforementioned seven types of intermediates measured by SVUV-PIMS. This works reveals that the models in the literature need to be improved, not only the prediction of the fuel reactivity and final products, but also the reaction network to predict the formation of many previous undetected intermediates.
  • A Systematic Theoretical Kinetics Analysis for the Waddington Mechanism in the Low-Temperature Oxidation of Butene and Butanol Isomers

    Li, Yang; Zhao, Qian; Zhang, Yingjia; Huang, Zuohua; Sarathy, Mani (The Journal of Physical Chemistry A, American Chemical Society (ACS), 2020-06-23) [Article]
    The Waddington mechanism, or the Waddington-type reaction pathway, is crucial for low-temperature oxidation of both alkenes and alcohols. In this study, the Waddington mechanism in the oxidation chemistry of butene and butanol isomers was systematically investigated. Fundamental quantum chemical calculations were conducted for the rate constants and thermodynamic properties of the reactions and species in this mechanism. Calculations were performed using two different ab initio solvers: Gaussian 09 and Orca 4.0.0, and two different kinetic solvers: PAPR and MultiWell, comprehensively. Temperature- and pressure-dependent rate constants were performed based on the transition state theory, associated with the Rice Ramsperger Kassel Marcus and master equation theories. Temperature-dependent thermochemistry (enthalpies of formation, entropy, and heat capacity) of all major species was also conducted, based on the statistical thermodynamics. Of the two types of reaction, dissociation reactions were significantly faster than isomerization reactions, while the rate constants of both reactions converged toward higher temperatures. In comparison, between two ab initio solvers, the barrier height difference among all isomerization and dissociation reactions was about 2 and 0.5 kcal/mol, respectively, resulting in less than 50%, and a factor of 2−10 differences for the predicted rate coefficients of the two reaction types, respectively. Comparing the two kinetic solvers, the rate constants of the isomerization reactions showed less than a 32% difference, while the rate of one dissociation reaction (P1 ↔ WDT12) exhibited 1−2 orders of magnitude discrepancy. Compared with results from the literature, both reaction rate coefficients (R4 and R5 reaction systems) and species’ thermochemistry (all closed shell molecules and open shell radicals R4 and R5) showed good agreement with the corresponding values obtained from the literature. All calculated results can be directly used for the chemical kinetic model development of butene and butanol isomer oxidation.
  • PAH formation from jet stirred reactor pyrolysis of gasoline surrogates

    Shao, Can; Kukkadapu, Goutham; Wagnon, Scott W.; Pitz, William J.; Sarathy, Mani (Combustion and Flame, Elsevier BV, 2020-06-20) [Article]
    Soot particles and their precursor polycyclic aromatic hydrocarbon (PAH) species, formed during combustion, are responsible for particulate emissions in gasoline direct injection (GDI) engines. To better understand the effects of fuel composition on formation of soot in GDI engines, the pyrolysis of several gasoline surrogates was studied in a jet-stirred reactor across a broad temperature range at atmospheric pressure and 1 s residence time. Fuel and intermediate species, including aromatics up to naphthalene, were measured using gas chromatography (GC). PAH concentrations from pyrolysis of surrogate fuels were compared to gain insight into the effects of fuel composition on PAH formation. In addition, synergistic effects were observed in pyrolysis experiments of binary blends. A detailed kinetic model, recently developed at Lawrence Livermore National Laboratory (LLNL), successfully captured the effects of blending and the concentration of major PAHs. Major reaction pathways are discussed, as well as the role of important intermediate species, such as acetylene, and resonantly stabilized radicals such as allyl, propargyl, cyclopentadienyl, and benzyl in the formation of PAH.
  • Multi-stage heat release in lean combustion: Insights from coupled tangential stretching rate (TSR) and computational singular perturbation (CSP) analysis

    AlRamadan, Abdullah; Galassi, Riccardo Malpica; Ciottoli, Pietro P.; Valorani, Mauro; Sarathy, Mani (Combustion and Flame, Elsevier BV, 2020-06-17) [Article]
    There is a growing interest in leaner burning internal combustion engines as an enabler for higher thermodynamic efficiency. The extension of knock-limited compression ratio and the increase in specific heat ratio with lean combustion are key factors for boosting efficiency. Under lean burning conditions, there is emerging evidence that certain fuels exhibit unusual heat release characteristics. It has been reported that fuel/air mixtures undergo three-stage heat release or delayed high temperature heat release: starting with an initial low temperature heat release, similar to the one observed in two stage ignition, followed by an intermediate stage where thermal runaway is inhibited, and then advances to a relatively slow third stage of combustion. The focus of this study is to examine the conditions under which various fuels exhibit three stage ignition or delayed high temperature heat release. The auto-ignition of hydrocarbons/air mixtures is simulated in a closed adiabatic homogenous batch reactor where the charge is allowed to auto-ignite at constant volume vessel under predefined initial temperature and pressure. The simulations cover pressures of 10–60 bar, temperatures of 600 K–900 K, and fuel to air ratio from stoichiometry (equivalence ratio) of 0.3–1.0. Tangential stretching rate (TSR) and the computational singular perturbation Slow Importance Indices for temperature are used to identify important reactions contributing to the temperature growth rate at critical time instants of the auto-ignition process. Overall, three-stage ignition or delayed high temperature heat release is found to be present for most fuels under lean fuel/air mixtures, high pressures, and low temperature conditions. The radical termination reactions of H, OH, and HO2 during the high temperature heat release are leading factors for the distinct separation of heat release stages.
  • Selective Electrocatalytic Oxidation of Biomass-Derived 5-Hydroxymethylfurfural to 2,5-Diformylfuran: from Mechanistic Investigations to Catalyst Recovery

    Kisszekelyi, Peter; Hardian, Rifan; Vovusha, Hakkim; Chen, Binglin; Zeng, Xianhai; Schwingenschlögl, Udo; Kupai, Jozsef; Szekely, Gyorgy (ChemSusChem, Wiley, 2020-06-15) [Article]
    The catalytic transformation of bio-derived compounds, specifically 5-hydroxymethylfurfural (HMF), into value-added chemicals may provide sustainable alternatives to crude oil and natural gas-based products. HMF can be obtained from fructose and successfully converted to 2,5-diformylfuran (DFF) by an environmentally friendly organic electrosynthesis performed in an ElectraSyn reactor, using cost-effective and sustainable graphite (anode) and stainless-steel (cathode) electrodes in an undivided cell, eliminating the need for conventional precious metal electrodes. In this work, the electrocatalysis of HMF is performed by using green solvents such as acetonitrile, γ-valerolactone, as well as PolarClean, which is used in electrocatalysis for the first time. The reaction parameters and the synergistic effects of the TEMPO catalyst and 2,6-lutidine base are explored both experimentally and through computation modeling. The molecular design and synthesis of a size-enlarged C 3-symmetric tris-TEMPO catalyst are also performed to facilitate a sustainable reaction work-up through nanofiltration. The obtained performance is then compared with those obtained by heterogeneous TEMPO alternatives recovered by using an external magnetic field and microfiltration. Results show that this new method of electrocatalytic oxidation of HMF to DFF can be achieved with excellent selectivity, good yield, and excellent catalyst recovery.
  • Illuminating Initial Carbon-Carbon Bond Formation during the Early Stages of Methane Dehydroaromatization.

    Caglayan, Mustafa; Paioni, Alessandra Lucini; Abou-Hamad, Edy; Shterk, Genrikh; Pustovarenko, Alexey; Baldus, Marc; Chowdhury, Abhishek Dutta; Gascon, Jorge (Angewandte Chemie (International ed. in English), Wiley, 2020-06-11) [Article]
    Still in 2020, methane dehydroaromatization (MDA) is among the most challenging processes in catalysis science due to the inherent harsh reaction conditions and fast catalyst deactivation. To improve it further, understanding the initial C-C bond formation mechanism is sine qua non. However, consensus about the actual reaction mechanism is still to be achieved. In this work, using advanced magic angle spinning (MAS) solid-state NMR spectroscopy, we study in detail the early stages of the reaction over a well-dispersed Mo/H-ZSM-5 catalyst. Simultaneous detection of acetylene (i.e., presumably the direct C-C bond forming product from methane), methylidene, allenes, acetal and surface-formate species along with the typical olefinic/aromatic species allow us to conclude the existence of two independent C-H activation pathways. Moreover, this study emphasizes the significance of mobility-dependent host-guest chemistry between inorganic zeolite and its organic trapped species during heterogeneous catalysis.
  • Triphenylphosphine-Based Covalent Organic Frameworks and Heterogeneous Rh-P-COFs Catalysts.

    Liu, Yubing; Dikhtiarenko, Alla; Xu, Naizhang; Sun, Jiawei; Tang, Jie; Wang, Kaiqiang; Xu, Bolian; Tong, Qing; Heeres, Hero Jan; He, Songbo; Gascon, Jorge; Fan, Yining (Chemistry (Weinheim an der Bergstrasse, Germany), Wiley, 2020-06-04) [Article]
    The synthesis of phosphine-based functional covalent organic frameworks (COFs) has attracted great attention recently. Here, we present two examples of triphenylphosphine-based COFs (termed as P-COFs) with well-defined crystalline structures, high specific surface areas and good thermal stability. Furthermore, the rhodium catalysts using the obtained P-COFs as support materials show high turnover frequency (TOF) for the hydroformylation of olefins, as well as the excellent recycling performance. This work not only extended phosphine-based COFs family, but also demonstrated their application in immobilizing homogeneous metal-based (e.g., Rh-phosphine) catalysts for heterogeneous catalysis application.
  • Oxidation kinetics of n-pentanol: A theoretical study of the reactivity of the 1‑hydroxy‑1-peroxypentyl radical

    Duan, Yaozong; Monge Palacios, Manuel; Grajales Gonzalez, Edwing; Han, Dong; Møller, Kristian H.; Kjaergaard, Henrik G.; Sarathy, Mani (Combustion and Flame, Elsevier BV, 2020-06-04) [Article]
    n-Pentanol has been considered as a promising alternative fuel for compression-ignition engines due to its potential to reduce greenhouse gases and pollutant emissions. Engine performance is strongly dominated by fuel oxidation chemistry, and thus a more accurate determination of the coefficients of the reactions ruling its oxidation is essential for the utilization of n-pentanol in combustion engines. The reactions involving 1‑hydroxy‑1-pentyl and molecular oxygen were found to play an important role in controlling the low temperature oxidation chemistry, but have not been investigated experimentally or theoretically; this is also the case for the reactions of the 1‑hydroxy‑1-peroxypentyl radical, which is formed by the addition of oxygen to the radical center of 1‑hydroxy‑1-pentyl. This work presents a theoretical study with high level ab initio calculations at the CCSD(T)/aug-cc-pVTZ//M06-2X/cc-pVTZ level of theory to shed light on the fate of the 1‑hydroxy‑1-peroxypentyl radical. The rate coefficients of all the possible intra-molecular hydrogen shift reactions of that radical were computed using variational transition state theory with small curvature tunneling corrections. For certain reactions, tunneling and variational effects are very pronounced, proving the need for robust methodologies to account for these effects. The hydrogen shift reaction leading to a concerted HO2 elimination and formation of n-pentanal is the dominant pathway and governs the reactivity of 1‑hydroxy‑1-peroxypentyl radical at any temperature. The reverse of this reaction was thereby investigated as well. For this prominent pathway, the effects of multistructural (multiple conformers) torsional anharmonicity of the stationary points were taken into account in order to refine the forward and reverse rate coefficients. The rate coefficients calculated at room temperature are compared to those calculated using a previously developed cost-effective multi-conformer transition state theory approach. The system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory was used to compute the pressure-dependent rate coefficients, which indicate significant pressure dependence at intermediate and high temperatures. Implementation of the calculated reaction rate coefficients in chemical kinetics models of n-pentanol revealed that our computed rate coefficients enable better insights into the chemistry of n-pentanol, and help to understand how n-pentanal is formed.
  • Enhancement of critical current density in a superconducting NbSe2 step junction.

    He, Xin; Wen, Yan; Zhang, Chenhui; Lai, Zhiping; Chudnovsky, Eugene M; Zhang, Xixiang (Nanoscale, Royal Society of Chemistry (RSC), 2020-06-02) [Article]
    We investigate the transport properties of a NbSe2 nanodevice consisting of a thin region, a thick region and a step junction. The superconducting critical current density of each region of the nanodevice has been studied as a function of temperature and magnetic field. We find that the critical current density has similar values for both the thin and thick regions away from the junction, while the critical current density of the thin region of the junction increases to approximately 1.8 times as compared with the values obtained for the other regions. We attribute such an enhancement of critical current density to the vortex pinning at the surface step. Our study verifies the enhancement of the critical current density by the geometrical-type pinning and sheds light on the application of 2D superconductors.
  • Identification of volatile constituents released from IQOS heat-not-burn tobacco HeatSticks using a direct sampling method.

    Ilies, Dragos-Bogdan; Moosakutty, Shamjad; Kharbatia, Najeh M.; Sarathy, Mani (Tobacco control, BMJ, 2020-05-28) [Article]
    OBJECTIVES:To identify the chemicals released in I Quit Ordinary Smoking (IQOS) heat-not-burn tobacco aerosol and to assess their potential human health toxicity. METHODS:The heating temperature window of the IQOS heat-not-burn device was determined using a thermographic camera over a period of 100 s. Qualitative studies were performed using a novel real-time gas chromatograph-mass spectrometer set-up. Aerosols from six tobacco-flavoured IQOS HeatSticks (Amber, Blue, Bronze, Sienna, Turquoise and Yellow) were collected in a 1 mL loop via a manual syringe attached to the sample-out port of the valve. The gas transport line was heated to 200°C in order to prevent the condensation of volatile species. Compound identification was performed using the NIST11 mass spectrometry database library (US National Institute of Standards and Technology), where only chemicals with a match of 70% and above were listed as identifiable. RESULTS:The temperature profile of the IQOS device revealed a non-combustive process employed in generating the tobacco aerosol. Real-time qualitative analysis revealed 62 compounds encompassing a broad spectrum of chemicals such as carbonyls, furans and phthalates, which are highly toxic. DISCUSSION:Our findings complement the qualitative studies previously performed by Philip Morris International and others via indirect sampling methods. By analysing the aerosols in real time, we have identified a total of 62 compounds, from which only 10 were in common with previous studies. Several identified species such as diacetyl, 2,3-pentanedione, hydroxymethylfurfural and diethylhexyl phthalate are classified as highly toxic, with the latter considered carcinogenic.
  • Investigating the effects of C3 and C4 alcohol blending on ignition quality of gasoline fuels

    Angikath Shamsudheen, Fabiyan; Naser, Nimal; Sarathy, Mani (Energy & Fuels, American Chemical Society (ACS), 2020-05-28) [Article]
    The study of the ignition quality of alcohol blends with petroleum fuels is a subject of practical interest. It is well known that the ignition delay time (IDT), as well as octane number (ON), increases when gasoline fuels are blended with ethanol. This study focuses on the impact on inverse ignition delay time (IDT-1) when alcohols, such as n-propanol and n-butanol, are blended with gasoline fuels. A non-linear decrease in the IDT-1 of the blends was observed. Predicting the extent of non-linearity in blends is complicated because it involves unknown inter-molecular interactions between base fuel components and the blended components. The purpose of this study is to establish the dependence of base fuel composition (in terms of functional groups) on observed non-linearity. Gasoline fuel contains hundreds of compounds (predominantly hydrocarbons), making it a challenge to understand observed non-linearity when they blend with other components. In this study, the IDT of primary reference fuels (PRF, a binary mixture of iso-octane and n-heptane) and FACE gasolines (fuels for advanced combustion engines) blended with two alcohols (n-propanol and n-butanol) were obtained with an ignition quality tester (IQT) following ASTM D6890 standards. A mole-based Gaussian fit was used to model the blending effects of alcohol with gasoline. The synergistic effect of the different mixtures tested in this study was investigated by analyzing the Gaussian parameters. A multiple linear regression model was formulated to provide information about the impact of the structural composition (functional group) on the synergistic blending effects of gasoline-alcohol mixtures. Constant volume homogenous batch reactor simulations were also conducted, using Chemkin-Pro for alcohols blended with a FACE J surrogate mixture to provide kinetic information about the blending effects observed in the IQT measurements.
  • Bimetallic metal-organic framework mediated synthesis of Ni-Co catalysts for the dry reforming of methane

    Khan, Il Son; Galilea, Adrian; Shterk, Genrikh; Garzon Tovar, Luis Carlos; Gascon, Jorge (Catalysts, MDPI AG, 2020-05-25) [Article]
    Dry reforming of methane (DRM) involves the conversion of CO2 and CH4, the most important greenhouse gases, into syngas, a stoichiometric mixture of H2 and CO that can be further processed via Fischer–Tropsch chemistry into a wide variety of products. However, the devolvement of the coke resistant catalyst, especially at high pressures, is still hampering commercial applications. One of the relatively new approaches for the synthesis of metal nanoparticle based catalysts comprises the use of metal-organic frameworks (MOFs) as catalyst precursors. In this work we have explored MOF-74/CPO-27 MOFs as precursors for the synthesis of Ni, Co and bimetallic Ni-Co metal nanoparticles. Our results show that the bimetallic system produced through pyrolysis of a Ni-Co@CMOF-74 precursor displays the best activity at moderate pressures, with stable performance during at least 10 h at 700◦ C, 5 bar and 33 L·h−1·g−1.
  • Facile synthesis and gas transport properties of Hünlich's base-derived intrinsically microporous polyimides

    Wang, Yingge; Ghanem, Bader; Han, Yu; Pinnau, Ingo (Polymer, Elsevier BV, 2020-05-23) [Article]
    Tröger's base (TB) has been utilized as an important building block in designing ladder polymers of intrinsic microporosity (PIMs) and microporous polyimides (PIM-PIs) for membrane-based gas separations due to its unique V-shaped bicyclic structure and versatile molecular chemistry. Nearly a century after its discovery, Hünlich's base (HB) was recently reintroduced as a valuable diamine derivative of TB made by a single-step reaction of 2,4-diaminotoluene and formaldehyde, spurring use in molecular devices such as molecular tweezers and photo-switches. Unlike TB, HB has not been explored as a building block of PIMs and PIM-PIs for membrane-based gas separations. In this study, we synthesized two soluble PIM-PIs for the first time by reaction of HB as diamine and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) or 9,10-diisopropyltriptycene tetracarboxylic dianhydride (TDAi3), respectively. 6FDA-HB exhibited high Brunauer-Teller-Emmett (BET) surface area of 415 m2 g−1 and fractional free volume (FFV) of 0.26. The gas separation performance of the 6FDA-HB polymer was similar to other 6FDA-based dimethyl-substituted Tröger's base polyimide analogues, exhibiting O2, CO2 and H2 permeability of 62, 286, and 391 Barrer with O2/N2, CO2/CH4 and H2/CH4 selectivity of 4.4, 26 and 36, respectively. Compared to 6FDA-HB, the triptycene-containing Hünlich's base polyimide (TDAi3-HB) displayed a higher BET surface area (501 m2 g−1) owing to the presence of the rigid bridged tricyclic triptycene contortion site, resulting in about two-fold increase in O2 permeability to 188 Barrer coupled with slightly compromised O2/N2 selectivity of 4.1. Beside the merit of facile synthesis, the Hünlich's base-derived polyimides experienced relatively modest effects of physical aging on gas permeation properties.
  • Quantification of sorption, diffusion, and plasticization properties of cellulose triacetate films under mixed-gas CO2/CH4 environment

    Genduso, Giuseppe; Pinnau, Ingo (Journal of Membrane Science, Elsevier BV, 2020-05-23) [Article]
    Membrane technology is employed in large-scale removal of acid gases from natural gas, and cellulose acetate is by far the most adopted material for this application. Because of its utmost industrial relevance, we analyzed the gas sorption behavior of CO2–CH4 mixtures in cellulose triacetate (CTA) at 35 °C. CO2 solubility in CTA was only slightly affected by the presence of methane, whereas competition effects sharply reduced CH4 uptake. Regardless of mixture concentration, CO2 vs. CH4 solubility coefficients regressed linearly, which translated in solubility selectivities that increased as equilibrium pressures increased. Specifically, compared to other relevant glassy polymer membrane materials, CTA positioned very close to the solubility selectivity upper bound at infinite dilution and demonstrated the highest affinity to CO2 at all investigated pressures. The experimental solubility and permeability data were used in the framework of the solution-diffusion theory to determine pure- and mixed-gas concentration averaged diffusion coefficients of CTA. CO2 diffusion was essentially unaffected by mixture effects, whereas methane diffusivity was boosted by the CO2-induced plasticization of CTA. The ratio between the pure- and mixed-gas concentration averaged diffusion coefficients of methane was used to quantify the effect of plasticization on the mixed-gas performance of CTA and other relevant membrane materials previously analyzed in similar experimental studies. When we further extended this comparison in a mixed-gas diffusion analysis (at 10 atm partial pressure), we observed that CTA had lower diffusion selectivity due to an inferior size-sieving capability than a reference material, 6FDA-mPDA polyimide, but displayed superior solubility selectivity.
  • Impact of small promoter amounts on coke structure in dry reforming of methane over Ni/ZrO2

    Franz, Robert; Kühlewind, Tobias; Shterk, Genrikh; Abou-Hamad, Edy; Parastaev, Alexander; Uslamin, Evgeny; Hensen, Emiel J.M.; Kapteijn, Freek; Gascon, Jorge; Pidko, Evgeny A. (Catalysis Science & Technology, Royal Society of Chemistry (RSC), 2020-05-22) [Article]
    Coke deposition is one of the main challenges in the commercialisation of dry reforming of methane over supported Ni catalysts. Besides the coke quantity, the structure of the deposits is also essential for the catalyst lifetime. Accordingly, in this study, we analysed the effect of Na, K, and Cs promoters on both these variables over Ni/ZrO2 catalysts. Besides blocking the most active coke-forming sites already at low loading, the promoting effect of the alkali metals is also contributed to by their coke gasification activity. To evaluate the additional impact of the latter, the behaviour of alkali-doped catalysts was compared to that for Mn-doped catalysts, exclusively featuring the site-blocking promotion mechanism. While the conversion is barely affected by the type of promoter, it has a profound effect on the amount and the composition of carbon deposits formed during the reaction. Promoting with K or Mn reduces the coke content to a similar degree but with less carbon fibres observed in the case of K. The promotion by Cs and Na results in the lowest coke content. The superior performance of Cs and Na-doped Ni/ZrO2 catalysts is attributed to the enhanced coke gasification via carbonate species on top of the site blocking effects.
  • Screening gas-phase chemical kinetic models: Collision limit compliance and ultrafast timescales

    Yalamanchi, Kiran K.; Tingas, Alexandros; Im, Hong G.; Sarathy, Mani (International Journal of Chemical Kinetics, Wiley, 2020-05-22) [Article]
    Detailed gas-phase chemical kinetic models are widely used in combustion research, and many new mechanisms for different fuels and reacting conditions are developed each year. Recent works have highlighted the need for error checking when preparing such models, but a useful community tool to perform such analysis is missing. In this work, we present a simple online tool to screen chemical kinetic mechanisms for bimolecular reactions exceeding collision limits. The tool is implemented on a user-friendly website, cloudflame.kaust.edu.sa, and checks three different classes of bimolecular reactions; (ie, pressure independent, pressure-dependent falloff, and pressure-dependent PLOG). In addition, two other online modules are provided to check thermodynamic properties and transport parameters to help kinetic model developers determine the sources of errors for reactions that are not collision limit compliant. Furthermore, issues related to unphysically fast timescales can remain an issue even if all bimolecular reactions are within collision limits. Therefore, we also present a procedure to screen ultrafast reaction timescales using computational singular perturbation. For demonstration purposes only, three versions of the rigorously developed AramcoMech are screened for collision limit compliance and ultrafast timescales, and recommendations are made for improving the models. Larger models for biodiesel surrogates, tetrahydropyran, and gasoline surrogates are also analyzed for exemplary purposes. Numerical simulations with updated kinetic parameters are presented to show improvements in wall-clock time when resolving ultrafast timescales.
  • Laminar Burning Velocities and Kinetic Modeling of a Renewable E-Fuel: Formic Acid and Its Mixtures with H2 and CO2

    Sarathy, Mani; Brequigny, Pierre; Katoch, Amit; Elbaz, Ayman M.; Roberts, William L.; Dibble, Robert W.; Foucher, Fabrice (Energy & Fuels, American Chemical Society (ACS), 2020-05-22) [Article]
    Formic acid is a promising fuel candidate that can be generated by reacting renewable hydrogen with carbon dioxide. However, the burning characteristics of formic acid/air mixtures have not been extensively studied. Furthermore, due to its low reactivity, the addition of hydrogen to formic acid/air mixtures may help with improving burning characteristics. This paper presents the first extensive study of formic acid/air premixed laminar burning velocities, as well as mixtures with hydrogen and carbon dioxide. Unstretched laminar burning velocities and Markstein lengths of formic acid in air for two different unburnt gas temperatures and equivalence ratios are presented. Measurements of formic acid mixed with various proportions of hydrogen and carbon dioxide in air are also studied as a potential renewable fuel for the future. Experimental results demonstrate the low burning velocities of formic acid and the ability to significantly enhance flame speeds by hydrogen addition. A modified detailed kinetic model for combustion of formic acid and its mixtures with hydrogen is proposed by merging well-validated literature models. The proposed model reproduces the experimental observations and provides the basis for understanding the combustion kinetics of formic acid laminar premixed flames, as well as mixtures with hydrogen. It is shown that the HOCO radical is the principal intermediate in formic acid combustion, and hydrogen addition accelerates the decomposition of HOCO radical thereby accelerating burning velocities.
  • Understanding the blending octane behaviour of unsaturated hydrocarbons: A case study of C4 molecules and comparison with toluene

    Li, Yang; Shankar, Vijai; Yalamanchi, Kiran K.; Badra, Jihad; Nicolle, André; Sarathy, Mani (Fuel, Elsevier BV, 2020-05-11) [Article]
    Octane number (ON) is an important empirical parameter for developing and optimizing internal combustion engine (ICE) for knock resistance. Primary reference fuels (PRF) comprising iso-octane and n-heptane are the simplest gasoline surrogates. C4 hydrocarbons: butane isomers (n-butane and isobutane), butene isomers (1-butene, 2-butene and isobutene) and 1,3-butadiene are the smallest hydrocarbons with isomeric, saturated, unsaturated and conjugated bond structures, which makes them good candidates for understanding the blending octane behavior of saturated and unsaturated hydrocarbons. In this study, the blending octane behaviors of six PRF60 & C4 hydrocarbon mixtures were systematically investigated. A state-of-the-art kinetic models were used by merging the latest KAUST gasoline surrogate model with the AramcoMech 3.0 model. IDTs of stoichiometric fuel/air mixtures were simulated at wide range of pressure (20–50 atm) and temperature (600–1400 K). Three correlation equations were employed from the literature to predict the research octane number (RON) and motor octane number (MON) of all blends based on these calculated IDTs. Compared with the experimentally measured ON, the best correlation conditions and errors were identified. With the highest degree of unsaturation, 1,3-butadiene was found to be the strongest ON enhancer. Moreover, based on a polynomial correlation, a TPRF (PRF + toluene) blend was formulated by matching the RON and MON of PRF plus 1,3-butadiene blend, for a comparative analysis. Finally, the reactants’ consumption profile, flux and sensitivity analysis were simultaneously performed for explaining the chemistry behind the blending octane behavior of the PRF blends with 1,3-butadiene and toluene.
  • Pore engineering of ultrathin covalent organic framework membranes for organic solvent nanofiltration and molecular sieving

    Shinde, Digambar; Cao, Li; Wonanke, A. D. Dinga; Li, Xiang; Kumar, Sushil; Liu, Xiaowei; Hedhili, Mohamed N.; Emwas, Abdul-Hamid M.; Addicoat, Matthew; Huang, Kuo-Wei; Lai, Zhiping (Chemical Science, Royal Society of Chemistry (RSC), 2020-04-30) [Article]
    <p>Pore surface engineering of ultrathin COF membranes by introducing different lengths of alkyl chains into the skeleton, which allows us to precisely control the pore size of COF membranes for OSN applications and molecular sieving.</p>

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