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Reformulation of Gasoline to Replace Aromatics by Biomass-Derived Alkyl Levulinates Gourav Shrivastav, Tuhin Suvra Khan, Manish Agarwal, and M. Ali Haider ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.7b01316 • Publication Date (Web): 08 Jul 2017 Downloaded from http://pubs.acs.org on July 13, 2017

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ACS Sustainable Chemistry & Engineering

Reformulation of Gasoline to Replace Aromatics by Biomass-Derived Alkyl Levulinates Gourav Shrivastav,†,‡,§ Tuhin S. Khan,∗,†,§ Manish Agarwal,∗,¶ and M. Ali Haider∗,† †Renewable Energy and Chemical Laboratory, Department of Chemical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India ‡Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi110016, India ¶Computer Services Centre, Indian Institute of Technology Delhi, Hauz Khas, New Delhi110016, India §Equal Contributing Authors *Corresponding Authors E-mail: [email protected]; [email protected]; [email protected] Phone: +91-11-2659-1016. Fax: +91-11-2658-2037 Abstract

In the search for a ‘green gasoline’, a new reformulation strategy, having no or reduced amount of aromatics is proposed. Biomass-derived alkyl levulinates (ALs) are prospected as oxygenated additives as well as blending components to circumvent the use of aromatics and methyl tertiary butyl ether (MTBE) in gasoline. By utilizing molecular dynamics (MD) simulations, the thermophysical and dynamical behavior of gasoline blends with four alkyl levulinates, viz. methyl levulinate (ML), ethyl levulinate (EL), propyl levulinate (PL), and butyl levulinate (BL), was scrutinized and compared with MTBE-gasoline mixtures. It is shown that at 300 K and 1 atm, ALs in conventional gasoline can be used for reformulation with

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amount upto 18 mol% while maintaining the density, viscosity, and compressibility within the recommended limits. However, this amount can be further increased to 35 mol% by modification of aromatic content. Among the studied oxygenates, BL was observed to have the lowest miscibility in water as compared to other ALs studied. The methodology may be applied to study similar biomass-derived oxygenates for their applicability as a fuel additive or blend.

Keywords Biomass, Oxygenates, Alkyl levulinates, Gasoline, Molecular dynamics

Introduction Gasoline is a complex hybrid of various hydrocarbon families viz. paraffins (16 wt%), isoparaffins (30 wt%), olefins (16 wt%), naphthenes (8 wt%), and aromatics (30 wt%).1 Octane rating of gasoline fuel is one of the most vital parameters for its use, in addition to other desired properties. Before the 1970s, toxic organo-lead compounds were used to improve the octane ratings. Since leaded fuel was discontinued due to its toxicity, expensive catalytic reforming units, converting paraffins (mostly C6 and C7) into aromatics were employed which helped in improving the octane ratings. The aromatics thus produced have benzene and its alkyl derivatives e.g. toluene, ethyl benzene and xylene as main constituents. All of these are well-known toxic and carcinogenic compounds, having adverse environmental impacts.2–7 Therefore, the United States with their amendments to the “Clean Air Act” in 1990 have called for regulations to reduce aromatics, leading to the reformulation of gasoline and thereby reducing the toxic content.8 Similar regulations and resolutions have been proposed in all major countries worldwide.9 In this regard, several ideas are proposed, foremost of which constitute the use of oxygenated additives, facilitating clean combustion by enhancing the local concentration of


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oxygen. It has been reported that higher oxygen content leads to a reduction of unburned hydrocarbon emission.10 While hydroxyl based oxygenates like ethanol, butanol, and bio-ethanol have been observed to show lower CO emission levels during engine testing,11–13 carbonyl based oxygenates such as acetone were reported to produce higher CO emission.10 Emission of NOx is dependent on the engine and temperature conditions and, in general, an increment of oxygen content at low loads maintains (or slightly reduces) the level of NOx emissions.10 For decades, MTBE, an oxygenated additive, has been widely used to improve octane ratings. However, now it is proven to be carcinogenic14,15 and a major contaminant of ground water16 owing to its high aqueous miscibility. Hence, an environmentally benign oxygenated additive is required for the reformulation of gasoline. Herein, biomass-derived ALs are suggested as an alternative, with a novel reformulation strategy, which may serve the desired dual purpose, to replace and reduce the aromatic content as well as complete elimination of MTBE in gasoline. The proposed reformulation strategy provides a generic framework for the study of biomass-derived oxygenates in the design of a relatively greener gasoline. An array of biomass-derived compounds i.e. levulinic acid (LA), dimethyltetrahydrofuran

(DMTHF),

methylfuran

(MF),

dimethylfuran

(DMF),

ethoxymethylfurfural (EMF), nonanones, etc. have been investigated both theoretically and experimentally,17–23 with recent interest in exploring ALs as additives for diesel.24 Levulinates have been traditionally used in fragrances and as food flavors25,26 and, thus are expected to be environmentally and biologically benign. Non-compatibility of ALs with existing diesel engines causes a major hindrance in their commercial applications.27 However, recent experimental reports have suggested their suitability as an additive to gasoline.23,28 In addition to octane ratings, vapor pressure of the reformulated gasoline is an important property to be checked before its application. The volatility of gasoline should remain low for warm weather conditions and should stay in a higher range in cold weather conditions, for better driveability.28 In the study of McCormick and co-workers, three blend stocks of gasoline with various oxygenates which include ethanol, ML, EL, and BL were prepared volumetrically to achieve nominal weight 3 ACS Paragon Plus Environment


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percent of oxygen.28 Subsequently, effect of the presence of oxygenates on vapor pressure, distillation temperature, octane ratings, density, viscosity, and solubility was investigated.28 ML, EL, and BL blending were measured to introduce relatively mild lowering of vapor pressure. ML blending upto 20 vol% produced no significant effect on the lowering of gasoline vapor pressure.28 In addition, the relatively mild lowering in gasoline vapor pressure caused by ALs is expected to reduce the evaporative emission, eliminating expensive removal of light-end components. An improvement in the anti-knock index (average of research and motor octane number) with blending amount of alkyl levulinates in regular gasoline was reported.28 This is further confirmed by Tian et al. suggesting the superior anti-knock properties of ALs, compared to gasoline.29 This can be elucidated by focussing on the combustion properties of levulinates. ML is proposed to undergo H-abstraction reactions with hydroxyl and methyl radicals from fuels to give stable intermediates.29,30 The intermediates undergo subsequent beta-scission and H atomtransfer to yield methyl acrylate and methyl vinyl ketone as the final products.30 The formation of stable intermediates consequently slow down the overall reaction rate and thereby improve the anti-knocking properties of ALs as compared to the conventional gasoline.29 While engine efficiency of ALs blend with gasoline remained to be tested, ALs have shown similar engine efficiency with respect to diesel.31 Hence ALs may improve the octane ratings while maintaining other properties and can be used as gasoline blend stocks. In general, the reformulation strategies of gasoline reported so far in several reports were focused towards MTBE replacement while keeping the same aromatic content.32,33 The study presented here proposes a unique reformulation approach, in which ALs may be used as a conventional additives as well as a blending components, replacing aromatics. Moreover, ALs with higher oxygen content are expected to provide comparable octane ratings as that of MTBE.21 Using MD simulations, the study explores the extent to which AL’s can be added, without significantly altering the behavior of gasoline. Molecular simulations have been used to predict the thermophysical, dynamic, and fluid-phase equilibria properties of fluids including fuels.34–41 MD simulations with the generalized united atom forcefield TraPPE-UA (Transferable Potentials for Phase Equilibria-united atom) are 4 ACS Paragon Plus Environment

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known to produce accurate results for a number of systems including alkanes (acyclic and cyclic), alkenes, ethers, ketones, aldehydes, and esters.42–47 While in some studies deviations in transport properties are calculated to be higher than expected, the qualitative behavior is predicted to the desired accuracy.34 In the present study, the thermophysical and dynamical behavior of conventional and proposed gasoline blends with four alkyl levulinates (alkyl 4oxopentanoates), i.e. ML, EL, PL, and BL are investigated. Figure 1 shows the chemical structure of studied oxygenates. The obtained results are systematically compared with those obtained from fuel blends containing MTBE and aromatics.

Figure 1: Chemical structure of studied oxygenates.

Computational Details Two compositions of gasoline were used; gasoline with aromatic content (AG) and gasoline with no aromatic content (NAG), the later was the novel reformulation serving the dual purpose. AG was based on an “ideal” composition given in Ref. 35 and has aromatics, normal paraffins, isoparaffins, naphthenes, and olefins. In comparison, the composition of NAG was the same as of AG, except the aromatics were removed and other compounds were increased. In addition to 5 ACS Paragon Plus Environment


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pure systems, four variants of each oxygenate in AG and NAG with mole fractions (x) = 0.1, 0.2, 0.3, and 0.4 were studied with a total number of 10,000 molecules. These mole fractions for MTBE corresponds to range of ≈ 1.5-7.25 wt% while for ALs, this range corresponds to ≈ 4.7518.25 wt%. Protocols discussed by Christensen et al.28 advise a 3.7 wt% oxygen to be the maximum limit fuel for the conventional gasoline combustion engine. However, it should be noted that this limit is set for the application of ethanol and other similar oxygenates. For structurally different ALs, this limit may change significantly. Furthermore, flexible fuel vehicles with internal combustion engine have been designed to take higher oxygen content, such as 85% ethanol, and are likely to be applicable for the reformulated gasoline with higher content of ALs proposed in this study. The representative compositions of the conventional gasoline AG, its formulation, and proposed gasoline NAG are tabulated in Table 1. Similar procedure was used to prepare the other studied formulations.

Potential Energy Surface Gasoline and its additives were modeled using a united-atom approach under the TraPPE-UA force field.42 Although the original TraPPE-UA force field considers the bonds to be fixed; here, the harmonic potential was used to maintain the bond stretchings as well as bond bendings of these pseudoatoms.48,49 Torsional movements were governed by triple cosine series potential. For aromatic components, a rigid model of atomic sites was used. Non-bonded interactions between united-atoms were modeled through pairwise additive standard 12-6 Lennard-Jones potential. These nonbonding interactions included the interactions of united-atoms separated by three or more bonds in same molecule and the interaction between united-atoms of different molecules.42 Non-bonded parameters for unlike pair of united atoms were obtained by Lorentz-Berthelots mixing rules and electrostatic interactions were parametrized using Coulombic potential as per the TraPPE-UA model used here.47


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Table 1: Composition of AG gasoline, NAG gasoline, and AG formulation containing 0.1 mole fraction of oxygenate. Major categories are shown in bold face. Values denote number of molecules, totalling to 10,000 in the simulation box. Components

AG

NAG

AG with AL/MTBE

Paraffins Pentane Hexane Heptane Octane Isoparaffins Isopentane Isohexane Isoheptane Isooctane Olefins Pent-2-ene Hex-2-ene Naphthenes Cyclohexane Aromatics Benzene Toluene o-Xylene Oxygenate

1600 400 400 400 400 3000 700 700 800 800 1600 800 800 200 200 3600 100 1800 1700 0

2500 650 650 600 600 4500 1100 1100 1150 1150 2500 1250 1250 500 500 0 0 0 0 0

1440 360 360 360 360 2700 630 630 720 720 1440 720 720 180 180 3240 90 1620 1530 1000

For simulating water environment, the flexible model was used.50 In this approach, water molecules had charges situated on each of the three atoms, with flexible bonds and angle. Hydrogen atoms were used as a point charges interacting only with Coulombic potential. In addition, molecules can also interact using a Lennard-Jones potential on the oxygen sites.



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Simulation Details Simulations for all the systems were performed in the NPT ensemble using GPU accelerated LAMMPS package.51–53 Equations of motions were integrated using the velocity-Verlet scheme with a time step of 1 fs. Cubic periodic boundary condition was used to ensure the bulk limit. A global cut-off of 14 Å was used in the interaction potential and long-range corrections to the energy and pressure were also included. Temperature and pressure were maintained using Nosé Hoover thermostat and barostat with the damping constant of 1 ps and 5 ps, respectively. Particle-particle-particle-mesh was used for the electrostatic calculations for compounds containing charged atoms, as implemented in LAMMPS. For all the systems, initial configuration with randomly packed molecules was obtained using PACKMOL software package.54 Thereafter, the system was allowed to equilibrate for 5 ns with rescaling of velocities every 10 steps followed by the production run of 11 ns. For all the simulations, temperature and pressure were maintained at 300 K and 1 atmospheric pressure, respectively. The data from last 5 ns was used for further analysis. We confirmed that this protocol ensured equilibration of all properties. The trajectory was stored every 1000 time steps. For viscosity calculations, the system was further simulated for 1 ns each in NVT and NVE ensembles. Subsequently, reverse non-equilibrium molecular dynamics (rNEMD) simulations in NVE ensemble were performed for 0.5 ns to generate initial conditions. This was subsequently followed by 10 ns of rNEMD simulations in order to estimate the viscosity at 300 K.55 The box was divided into 20 slabs and velocities were exchanged for every 100 time steps using 10 swaps as implemented in LAMMPS. Rigid aromatic molecules were not used for the velocity exchange. Data was collected for every 1000 steps to compute the viscosity. For the computation of diffusivity, data from the NPT simulation was used. Self-diffusion constants were estimated using the Einstein relation, mathematically given as39


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= lim→ |( () − (0)|

(1)

where ri(t) corresponds to the position of carbon atom i at time t, while the angular bracket denotes the ensemble average of mean square displacement (MSD). Mean-square displacements were calculated by parallel GPU in-house codes. The trajectory was split into 8 parts to estimate the error which was within 0.5%-3% for all the systems. Isothermal compressibility (κT) of studied formulations of gasoline was estimated by utilizing the volumetric fluctuations obtained from NPT simulations. It is calculated using56

= − =

〈 ! 〉#〈〉! 〈〉$% !

(2)

Here, V, P, T, and kB represents the volume, applied pressure, temperature and the Boltzmann’s constant, respectively. Angular brackets denote ensemble averages. Data was divided into 8 parts to estimate the error. The estimated error was ≈ 14%.

Results and Discussions The proposed formulations of AG and NAG were tested considering the fact that when gasoline reaches the engine, it is subjected to a multitude of flow, compression, and temperature gradients. Therefore, the thermophysical and dynamic behavior of a fuel would play a crucial and decisive role. Furthermore, significant deviations from the desired limits affect engine’s performance and emission characteristics. In order to avoid considerable modifications in the current engine styles, these properties should be maintained within standard specifications after addition of new additives.



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Density Table 2: Data for experimental and simulation densities (g cm−3) for pure alkyl levulinates, MTBE, and gasoline at 300 K and 1 atm. Superscripts represent the appropriate references.

System

Experimental Density

Simulation Density

ML EL PL BL MTBE AG NAG

1.0492528 1.0228 0.989857 0.9728 0.7458 0.694-0.79423 -

1.004 0.982 0.965 0.956 0.747 0.720 0.650

Density is one such physical property and has been known to be accurate from the calculation based on MD simulations.42 The complete details of experimental and computed densities for pure ALs and MTBE are tabulated in Table 2. The computed densities for ML, BL, and MTBE were 1.004 g cm−3, 0.956 g cm−3, and 0.747 g cm−3, respectively. These values were in fair agreement with their corresponding experimental densities of 1.05 g cm−3, 0.97 g cm−3, and 0.74 g cm−3, respectively.42 With the increase of alkyl group’s chain length in ALs, a monotonic reduction in the density was observed. It was apparent that simulation densities were only within a deviation of ≈ 4% of experimental densities. Considering the fact that the forcefield parameters were not specifically trained for ALs, the observed deviations were considered acceptable. These results complement the transferable behavior of TraPPE-UA forcefield and support its employment for ALs in the present study. The density of fuel blends is a useful parameter for assessing the power of fuel and its spray characteristics during the combustion in an engine. Moreover, it plays a central role in the


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correlations with the other properties of the fuel, viz. octane index, kinetics, and thermodynamic properties. As per ASTM international standards, the recommended range of density for gasoline is 0.694-0.794 g cm−3 at 293 K and ambient pressure.23,35

Figure 2: Multiplot shows the variation in density with the mole fraction of oxygenated additive in gasoline samples for (a) AG formulations, (b) NAG formulations, and (c) RAG formulation, at 300 K and 1 atm. The shaded region denotes recommended density range.23

The same range is used here to screen the studied gasoline blends, as shown in a blue shaded region in Figure 2. As the first test of these oxygenates as additives in gasoline, densities of existing and proposed gasoline blends were computed at 300 K and 1 atm. The density for the chosen composition of neat gasoline sample AG was computed to be 0.72 g cm−3 (Table 2), which is within the range of international standards. The density variations for the AG formulations as a function of added mole fraction of oxygenates are shown in Figure 2(a). In case of blends containing ALs, density linearly increased from 0.72 g cm−3 at x = 0 to 0.839 g cm−3, 0.835 g cm−3, 0.837 g cm−3, and 0.837 g cm−3 for ML, EL, PL, and BL, respectively, at x = 0.4. All the studied ALs demonstrated increment in the density and, at a given mole fraction, the variation of density among the ALs was only about 1-2%. Compared to ALs-AG mixtures, an insignificant density variation, from 0.72 g cm−3 at x = 0 to 0.725 g cm−3 at x = 0.4, was seen for MTBE-AG mixtures. This difference in the density variation for AG formulations containing ALs and MTBE can be attributed to the difference in alkyl chain length. Longer the chain length,



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larger is the molecular mass and, thus, stronger will be the intermolecular forces, resulting in a higher density. Hence, the overall change observed in density for mixtures of ALs and MTBE were ≈ 16% and 1%, respectively. This can be further depicted by higher density of neat ALs than that of neat AG and MTBE, as shown in Table 2. As a result, an increase in the fraction of such additives would increase the density of the blend considerably. Since the density of MTBE is 0.746 g cm−3, close to that of AG, only small modifications in density for MTBE-AG blends were observed. Like MTBE, alcohols are of lesser density as compared to ALs. Experiments have revealed that the change in densities of the gasoline with ALs is greater than that of alcohols.28 Furthermore, for ALs, per gram of oxygen addition in experiments increments density by ≈ 0.0065 g cm−3. In this study, a similar amount of variation in density (≈ 0.0066 g cm−3 per gram of oxygen) was observed for AG formulations. These observations indicate that only 20 mol% of ALs can be capitalized without causing the density to exceed the recommended specifications, however, MTBE can be used to a large extent in AG. Complete removal of aromatics reduced the density significantly from 0.72 g cm−3 to 0.65 g cm−3 for neat gasoline NAG, which was clearly well below the acceptable range of density (Table 2). In Figure 2(b), the formulations of NAG feature similar uptrends in the density but lead to different consequences. Here, higher mol% of ALs can be tolerated, in contrast to those in ALs-AG mixtures. It was apparent that initially the densities were below the recommended standards and entered into the acceptable limits only when x > 0.2. In comparison, addition of 0.4 mole fraction of MTBE exhibited an increase in density from 0.65 g cm−3 to 0.69 g cm−3. This unequivocally highlighted that MTBE was unable to compensate the loss of aromatic content; however, a higher mole fraction of ALs could be used to retain the density in specified allowed limits. Since current engine design and emission specification may not allow complete removal of aromatics, progressive replacement of aromatics by MTBE and ALs was investigated next. Note that while replacing aromatics with oxygenates, the proportion of other components remains unchanged. Therefore, the role of added oxygenates can be considered as a blending component 12 ACS Paragon Plus Environment

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rather than an additive. For example, 10 mol% of aromatics were exactly replaced by BL by replacing 360 molecules of aromatic compounds out of 3600 (Table 1) in proportion by 360 molecules of BL. Subsequently, other blends were also prepared by replacing the 20 mol%, 30 mol%, and 36 mol% of aromatics. The highest amount of oxygenates was 36 mol% corresponded to the exact amount of aromatics in AG. The newly proposed resultant formulation is termed as replaced aromatic gasoline (RAG) and the alterations in density are shown in Figure 2(c). Akin to the mixtures of ALs in AG and NAG, accumulation of ALs mol% in RAG increased the density from 0.72 g cm−3 at x = 0 to 0.778 g cm−3 for ML and EL whereas 0.783 g cm−3 and 0.785 g cm−3 for PL and BL, respectively, at x = 0.36. Contrary to this, the density of MTBE-RAG blends continuously decreased from 0.72 g cm−3 to 0.67 g cm−3 at the corresponding mole fractions. This was understandable as ALs are denser than the aromatics which in turn are denser than MTBE. As noted before, MTBE was deficient in compensating the loss occurred in density by removal of aromatics. At lower concentration of ALs, higher content of aromatics maintained the density and at higher concentration, ALs themselves took over and maintained the density. As a result, it was interesting that, unlike AG and NAG blends, RAG formulations allowed access to the full concentration range of levulinates-gasoline mixtures studied here, since the density of all four samples were within the recommended range. It is also apparent from all the formulations that variation at a given density across the four levulinates were within 1-2%, Figure 2(a)- 2(c). Therefore, in order to determine a good choice out of the four ALs studied, effects on other key fuel properties were considered.

Solubility in water Miscibility in water plays a decisive role to judge the environmental impact of such oxygenated fuel additives and blends. To inspect the aggregation behavior, which is inversely proportional to solubility, of each oxygenate in water, MD simulation for a periodic cubic box consisting o20-60 molecules of oxygenates in 10,000 water molecules were performed at 300 K and 1 atm. Subsequently, the aggregation tendency was monitored visually. Figure 3 shows representative



snapshots from the simulations containing 20-40 molecules of oxygenates in water. These snapshots clearly show a greater clustering of BL in water than MTBE and other ALs. While ML and EL formed multiple small agglomerates, PL and BL quickly formed a single mass. It was

MTBE

ML

EL

PL

BL

40 Molecules

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Figure 3: Representative snapshots from 20 ns simulation of oxygenate molecules in water at 300 K and 1 atm. (a, f, k) MTBE, (b, g, l) ML, (c, h, m) EL, (d, i, n) PL and (e, j, o) BL observed that BL phase separates within ≈ 10 nanoseconds of simulation time, while MTBE and other ALs remained dispersed even for 20 nanoseconds. The preference to form stable dispersion followed the order; ML > MTBE ≈ EL > PL > BL. This observed qualitative trend of miscibility of studied oxygenates corroborates with the experimental trends based on solubility. Experimental investigations28 have suggested ML to be miscible in water while MTBE and BL 14 ACS Paragon Plus Environment

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are slightly soluble with solubility of 5.1 wt% and 1.3 wt% at 293K.28,59 Furthermore, Christensen et al. have showed relatively lesser extraction of BL from gasoline to water as compared to EL and ML.28 This can be elucidate by the fact that at a given temperature and pressure, solubility in water depends on the solute-solvent interactions. The oxygenates exhibit some polarity due to the presence of oxygen which becomes subordinate with an increase in carbon chain length. Therefore, BL is expected to show a relatively lesser miscibility in water as compared to MTBE due to the longer n-alkyl chain. Considering the density behavior and miscibility, it was concluded that BL was a better additive blend candidate among the studied ALs. Furthermore, the effective

Table 3: Effective C/H ratio for aromatics and additives used in gasoline blends. Compounds

C

H

O

C/H

Benzene Toluene o-Xylene MTBE ML EL PL BL

6 7 8 5 6 7 8 9

6 8 10 12 10 12 14 16

0 0 0 1 3 3 3 3

1.00 0.88 0.80 0.50 1.50 1.17 1.00 0.90

C/H ratio, which is proportional to the energy density of a given component, of BL (0.90) was similar to that of the aromatic fragments (0.8-1.0), as given in Table 3. Thus, in the following discussion, only MTBE and BL are compared in the proposed NAG and RAG formulations.



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Viscosity Viscosity is another property of fuels which is inter-related to the density. It affects the lubrication and atomization phenomenon during the spray, controlling the carbon deposition on the fuel

Figure 4: Plot for the variations in shear viscosity for (a) AG formulations, (b) NAG formulations, and (c) RAG formulations at 300 K and 1 atm. The blue shaded region denotes recommended viscosity range as described in the text. For RAG, the highest mole fraction of oxygenates is 0.36 which corresponds to the exact amount of aromatics in AG.

filter.60 In molecular simulations, viscosities have been known to be underestimated,41,61 especially with the TraPPE-UA potential.34,62 As a result, the acceptable experimental viscosity range of 0.37 to 0.44 mPa s, at 293 K and ambient pressure,35 cannot be used as a reference range here. Hence, a modified viscosity range is defined as follows. The lower bound was the computed viscosity for neat AG (0.26 mPa s) and the upper bound was the corresponding value for 0.23 mol% AG-BL (0.38 mPa s) blend. The choice of these limits was based on Figure 4(a), corresponding to the viscosities of the systems which have the densities in the acceptable range. Viscosities for all systems were estimated by reverse non-equilibrium MD (rNEMD) simulations to avoid convergence issues with traditional Green-Kubo formalism.55 It should be noted that although simulations do not give good quantitative results, the qualitative behavior obtained for viscosity among ALs in AG was similar to those obtained from experiments.28 For a given 16 ACS Paragon Plus Environment

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amount of oxygen content, BL was observed to increase viscosity the most while ML the least. Furthermore, the ratio of increment in viscosity for BL to ML was calculated ≈ 2.2 from experiments28 and ≈ 2.3 from the simulations performed. Increasing the amount of BL in NAG showed an increase in computed viscosity from 0.175 mPa s at x = 0 to 0.449 at the x = 0.4, as shown in Figure 4(b). The relatively smaller increment was also apparent in the case of MTBE, which displayed an increase in viscosity from 0.175 mPa s at x = 0 to 0.190 at x = 0.4, as shown in Figure 4(c). It was evident that BL blending was able to bring the viscosity within the recommended limit in the range of x = 0.18 and x = 0.35; Post this range, viscosity exceeded the allowed limit (Figure 4(b)). The range of mole fraction corresponding to suitable viscosity correlated with the acceptable range of mole fraction obtained from the density profiles in corresponding blends which may be explained by the direct proportionality between the two. Similarly, the viscosity for MTBE-NAG mixtures negligibly varied from 0.175 mPa s to 0.190 mPa s (Figure 4(b)), which was inferior to the specified limit, suggesting MTBE unlikely to compensate the decrease in viscosity caused by the removal of aromatics. This observed behavior can be attributed to the higher viscosity of BL than MTBE, likely due to its higher density. From Figure 4(c), it is apparent that progressive replacement of aromatics with BL, in RAG formulation, increased the viscosity from 0.26 at x=0, to 0.40 mPa s at x = 0.36, and were well within the allowed limit. However, the viscosity for MTBE-RAG blends continuously decreased from 0.26 to 0.183 mPa s (Figure 4(c)), indicating that the rise in viscosity due to MTBE addition was lower than the fall of viscosity due to the removal of aromatics. Viscosities for all the formulations are shown in Figure S1 in the SI.

Relationship between viscosity and diffusivity The Stokes-Einstein (SE) relation was examined next by computing self-diffusivity of the gasoline blends. In general, the SE relationship can be used to determine the hydrodynamic radius of a solute in the viscous medium and is given as = &' (/6+, where kB, T, η, and r



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represent the Boltzmann’s constant, temperature, viscosity, and hydrodynamic radius, respectively.63 Furthermore, the existence of SE relationship in these systems can be fruitful to determine the variation of transport properties by considering the self-diffusivity rather than the viscosity. This aids in alleviating an excess computation overhead of reverse non-equilibrium MD simulation for

Figure 5: Correlation plot of the viscosity and inverse of diffusivity for (a) AG formulations (red star), NAG formulation (green square), and RAG formulations (blue triangle) at 300 K and 1 atm.

viscosity calculations. In comparison, self-diffusivity is relatively rapidly converged, and easily obtained from equilibrium MD simulations. Here, it was computed using the Einstein relation and the estimated error was within 0.5-3%. For all the studied formulations of AG, NAG, and RAG, a linear fit (R2 = 0.981-0.995) was observed between inverse diffusivity, 1/D, and viscosity, η, shown in Figure 5. Equations obtained from the linear fits are given in Table 4. This indicated that there was a direct correlation between the viscosity and inverse diffusivity in SE relationship for the entire range of oxygenates in the conventional and proposed formulations of gasoline. As far as the authors’ knowledge, this is the first report of SE relationship in gasoline blends using molecular simulations.


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Table 4: Linear polynomial fitting data of inverse diffusivity as a function of viscosity for AG formulations, NAG formulations, and RAG formulations. The equation used is y = ax + b and the corresponding R-square values are also given. System AG NAG RAG

a

b

R2

0.954 -0.016 0.995 0.984 -0.031 0.982 0.961 -0.035 0.991

Compressibility

Figure 6: The figure shows the variation in isothermal compressibility for (a) AG formulations, (b) NAG formulations, and (c) RAG formulations at 300 K and 1 atm. The shaded region denotes recommended compressibility range as described in the text. For RAG, the highest mole fraction of oxygenates is 0.36 which corresponds to the exact amount of aromatics in AG.

Among the thermodynamic properties of gasoline, isothermal compressibility, (κT), is known to affect the fuel injection timing in a diesel engine.64 Here, the compressibility of studied formulations of gasoline was estimated as the function of mole fraction of oxygenates by utilizing volumetric fluctuations.56 The compressibility of AG, given in Figure 6(a), (κT = 1.4 GPa−1) was used as a reference to define a range of 1.2 GPa−1 to 1.8 GPa−1 as the acceptable limits, following the same strategy used for defining the viscosity limits. From Figure 6(b), it was



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observed that neat NAG had a relatively higher compressibility (at x = 0) due to the absence of aromatic compounds. On increasing the concentration of BL the compressibility dropped down from 2.6 to 1.3 GPa−1, likely due to the relatively lesser compressibility of BL component. Since MTBE is relatively more compressible, due to less density, only small drop (from 2.6 to 2.4 GPa−1) was observed on adding MTBE up to a mole fraction of x = 0.4 (Figure 6(b)). Since compressibility is inversely proportional to density, the observed lower compressiblities with increasing BL content is expected. Interestingly, for RAG in Figure 6(c), increase in compressibility due to the removal of aromatics was counter balanced by BL with a compressibility value of 1.4 ± 0.2 GPa−1. These compressibility values were well within the recommended limits (in the blue region of Figure 6(c)). In comparison, the compressibility of RAG-MTBE mixtures increased with increment in mol% of MTBE and exceeded the upper limit of the specified range (Figure 6(c)). Once again, these observations and including those from density and viscosity calculations suggested BL to be a promising additive and blending component for gasoline. Complete data for the compressibility is shown in Figure S2 of SI.

Conclusion ALs possess good octane ratings, similar C/H ratio to that of aromatics and better local oxygen concentration than that of MTBE. Considering these facts, ALs were studied to reduce the amount of aromatics as well as MTBE in gasoline simultaneously. In order to avoid the modifications in current engine style, it was crucial to study the effect of new components on the thermophysical and dynamical behavior of gasoline. MD simulations were performed on the selected blends of gasoline with four alkyl levulinates (ML, EL, PL, and BL) with three mixing strategies, (i) increasing the amount of additives while reducing the mol% of other components of gasoline in proportion in sample AG, (ii) increasing the amount of additives while reducing the mol% of other components of gasoline in proportion in sample NAG, and (iii) increasing the amount of additives while reducing the mol% of only aromatic content of gasoline in sample AG. It was clearly demonstrated that for all the three formulations, the density of fuel increased


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with an increase in mole fraction of ALs. The amounts of ALs which maintained the density of blends in the specified limit were < 20 mol% for AG, 20 - 40 mol% for NAG, and < 36 mol% for RAG. For a given formulation and mol%, the variation in density amongst the ALs studied was 1-2%. Since MTBE showed calculated density similar to that of gasoline, negligible increments in the density were observed for the blends of MTBE with AG and NAG. Contrary to this, in the case of RAG, the density was decreasing with the progressive replacement of aromatics with MTBE. Interestingly, for MTBE-NAG and MTBE-RAG, the calculated densities were below the acceptable standards, indicating MTBE unlikely to compensate the loss of aromatics. Out of all the ALs, BL was preferred for further comparison with MTBE, due to its superior aggregation behavior in water and similar C/H ratio to that of aromatics. In addition to density, viscosity and compressibility were also investigated for the effect of BL and MTBE addition in proposed gasoline. In both formulations (NAG and RAG), MTBE was observed to be inadequate to compensate the loss in viscosity and gain in compressibility, due to the removal of aromatics. However, 20-35 mol% (NAG) and 0-35 mol% (RAG) of BL blends were able to keep these properties in the recommended limit. The theoretical results direct towards the need for measuring these properties experimentally with the novel reformulation idea proposed in this work. In addition, a more detailed study needs to verify some other desired properties like vapor pressure, octane ratings, and drive-ability index. Nevertheless, the proposed reformulated gasoline is likely to propel ideas on using environment friendly biofuels as the blend and aromatic replacement. All previous studies on ALs were conducted with a motivation to use the biofuels as an additive to the aromatic enriched gasoline. With the new reformulation strategy, aromatics of high value and versatile chemical application may be extracted (up to 15-45 wt% from the gasoline), adding substantial economic benefits. This study shows biomass-derived alkyl levulinates have the potential to revolutionize the conventional refinery and petrochemical industry, where the energy and cost intensive reforming and isomerization units currently used to convert the hydro-treated naphtha to high octane rating gasoline can be effectively replaced by a 21 ACS Paragon Plus Environment


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green, cheaper cost and reduced energy liquid phase process to obtain alkyl levulinate, which can be easily blended with hydro-treated naphtha, resembling sample NAG used in this study to obtain motor fuel grade gasoline. However, at present, high volume synthesis of ALs is not achieved. Few companies have announced commercial scale synthesis of levulinic acid (LA).65– 68

Esterification reactions of LA will produce ALs in high yields27 and therefore high volume

synthesis of ALs is likely to follow from LA commercialization. In order to access competitiveness with respect to gasoline, a techno-economic analysis of ALs synthesis from biomass substrate is desirable. Acknowledgments G.S. thanks the CSIR, India for granting the Senior Research Fellowship ((F. No. - 09/086(1141)/2012-EMR-I). Authors thank the IIT Delhi HPC facility for computational resources. Supporting Information The Supporting Information for more details on the variation of viscosity and compressibility for the blends of MTBE, ML, EL, PL, and BL for AG, NAG, and RAG formulations is available on the ACS Publications website at DOI:

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Reformulation of Gasoline to Replace Aromatics by Biomass-Derived Alkyl Levulinates Gourav Shrivastav, Tuhin S. Khan, Manish Agarwal, and M. Ali Haider Synopsis In search of a green gasoline a reformulation strategy is proposed wherein biomass-derived alkyl levulinates are evaluated to replace aromatics.


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Reformulation of Gasoline to Replace Aromatics by Biomass-Derived Alkyl Page 33 of 33 ACS Sustainable Chemistry & Engineering Levulinates

Gourav Shrivastav, Tuhin S. Khan, Manish Agarwal, and M. Ali Haider

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Synopsis In19search of 20

ACS Paragon Plus Environment

a green gasoline a reformulation strategy is proposed wherein biomass-derived alkyl levulinates are evaluated to replace aromatics.

Reformulation of Gasoline To Replace Aromatics ... - ACS Publications

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