Analysis and Properties of the Decarboxylation Products of Oleic Acid

Sep 2, 2016 - Scribe , P.; Guezennec , J.; Dagaut , J.; Pepe , C.; Saliot , A. Anal. Chem. 1988, 60, 928– 931 DOI: 10.1021/ac00160a019. [ACS Full Te...
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Analysis and Properties of the Decarboxylation Products of Oleic Acid by Catalytic Triruthenium Dodecacarbonyl Bryan R. Moser,* Gerhard Knothe, Erin L. Walter, Rex E. Murray, Robert O. Dunn, and Kenneth M. Doll United States Department of Agriculture, Agricultural Research Service, National Center for Agricultural Utilization Research, Bio-Oils Research Unit, 1815 N. University Street, Peoria, Illinois 61604, United States ABSTRACT: Recently, ruthenium-catalyzed isomerization−decarboxylation of fatty acids to give alkene mixtures was reported. When the substrate was oleic acid, the reaction yielded a mixture consisting of heptadecene isomers. In this work, we report the compositional analysis of the mixture obtained by triruthenium dodecacarbonyl catalyzed decarboxylation of oleic acid. Surprisingly, the most prominent single compound identified was heptadecane at approximately 18 wt %. A mixture of heptadecene isomers constituted the greatest percentage of the product (>75%), where all positional isomers were confirmed by GC−MS analysis of dimethyl disulfide derivatives of the product. Besides these components, minor amounts of alkyl aromatics, each with a total of 17 carbons atoms, among them undecyl benzene, were observed. Minor amounts of other compounds, such as shorter-chain hydrocarbons and polyunsaturated C17 compounds, were also observed. A reaction pathway to explain the existence of these products is proposed. Heptadecenes first cyclize to the observed alkyl aromatics under liberation of hydrogen with the formed hydrogen, then in turn hydrogenating some heptadecenes to heptadecane. Thus, triruthenium dodecacarbonyl is suggested to also promote dehydrogenation, aromatization, and hydrogenation under the present conditions. Since the product mixture consisted mainly of long-chain hydrocarbons, its properties regarding diesel fuel application were studied and compared to biodiesel and petroleum diesel. The cetane number (86.9) was high, but oxidative stability (3.4 h; EN 15751) and cold flow (cloud point −1 °C) would require improvement to meet fuel specifications. All other fuel properties were within the limits prescribed in the petrodiesel standards.

1. INTRODUCTION Triacylglycerols, the major components of plant oils, animal fats, and algal oils, as well as fatty acids (FAs) and alkyl esters derived therefrom, are prominent sources of renewable products useful as alternatives to materials obtained from nonrenewable resources such as petroleum. Commercialized renewable materials produced from these feedstocks include lubricants, polymers, inks, paints, coatings, and solvents as well as fuels such as biodiesel.1−3 Another avenue for utilization of renewable feedstocks is preparation of compounds and chemical intermediates that are otherwise derived from nonrenewable sources, thus creating a route toward sustainable production and supply. Such materials may already possess commercial utilization potential without further chemical modification. A prominent example is the recently developed hydrotreatment process in which mixtures of hydrocarbons, mostly long-chain alkanes, are produced from triacylglycerols or fatty acid alkyl esters in the presence of hydrogen and which is summarized briefly elsewhere.4−8 Hydrotreatment involves reactions, such as hydrodeoxygenation, in which triacylglycerols are decarboxylated with concomitant formation of propane and hydrogenation of unsaturated FAs. The resulting mixture thus emulates the composition of conventional petrodiesel fuel. A typical catalytic system for this process is sulfided NiMo/γ-Al2O3 or CoMo/γAl2O3 with the addition of sulfur to prevent catalyst deactivation. The product mixture can be isomerized to give chain branching to improve cold flow properties, thus rendering this material of interest for use as jet fuel. Besides hydrotreatment, several procedures have been described for simple decarboxylation. Similar to hydrotreat© XXXX American Chemical Society

ment, elevated temperatures are required, as decarboxylation is thermodynamically favorable at high temperatures.9 However, decarboxylation does not require hydrogen or elevated pressures, nor are the catalysts or reagents deactivated by the substrate. Classic examples, such as the Barton decarboxylation, Kolbe electrolysis, Kochi reaction, and Hunsdiecker reaction, proceed via free radical mechanisms and, therefore, require radical initiators and suitable hydrogen donors for completion. These reactions also require stoichiometric amounts of reagents, and in most cases, reductive decarboxylation is accompanied by additional chemical functionalization.10 For instance, end products of the Hunsdiecker and Kochi reactions are alkyl bromides and chlorides, respectively, whereas the Kolbe electrolysis yields dimerized alkanes. Lastly, terminal alkenes are produced from oxidative decarboxylation of FAs using, for example, lead(IV) tetraacetate in the presence of copper(II) and a base.11 Thermal decarboxylation without catalysts or reagents exposes FAs to elevated temperatures to yield hydrocarbons. However, complex liquid mixtures are produced, including cyclic and linear alkanes and alkenes, as well as gaseous alkanes and alkenes with carbon numbers up to C5. Thermal decarboxylation also promotes cracking of unsaturated constituents into smaller hydrocarbons and FAs.12−14 Therefore, it may be more appropriate for production of renewable chemicals as opposed to diesel-range fuels. More recently, hydrothermal decarboxylation of unsaturated fatty acids over Received: July 13, 2016

A

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5975C quadrupole mass detector, and an Agilent/J&W DB35-MS column. Reaction aliquots of 10 mg were diluted in acetone (1 mL) for analysis to give chromatographic signals that were identified by comparative retention time and by mass spectral analysis. The carrier gas was He with a 1:50 injector split. The injector and detector temperatures were 340 and 230 °C, respectively. The temperature program was maintained at 40 °C for 3 min, increased at 10 °C/min to 190 °C, held for 5 min, and then increased at 25 °C/min to 340 °C. The distilled product was reacted with dimethyl disulfide (DMDS) as described in previous literature,25,26 and this derivative was also analyzed by GC with an FID detector and GC−MS (MS in electron ionization mode; 70 eV) using for both a DB-88 [(88% cyanopropyl) methylarylpolysiloxane] (30 m × 0.25 mm ID × 0.20 μm film thickness) column. The GC-FID utilized for this analysis was a PerkinElmer (Norwalk, CT) gas chromatograph and for GC−MS, an Agilent 6890 GC equipped with an Agilent 5973 mass selective detector was used. The temperature program for GC−MS was 30 °C held for 10 min and then heating to 220 °C at 1 °C/min with H2 as carrier gas at 1.4 mL/min with an analogous procedure for GC. Structural composition was determined using GC−MS, and quantification of product distribution was accomplished using GCFID by calculation of relative response factors of the starting material and of 1-decene as a reference. 1 H and 13C NMR data were collected using a Bruker Avance-500 spectrometer (Billerica, MA) running Topspin 1.4 pl8 software operating at 500 MHz (125 MHz for 13C NMR) using a 5 mm BBO probe. Samples were dissolved in CDCl3 (Cambridge Isotope Laboratories, Andover, MA), and all spectra were acquired at 26.9 °C. Chemical shifts (δ) are reported as parts per million (ppm) from tetramethylsilane. FT-IR spectra were obtained on a Thermo-Nicolet Nexus 470 FTIR spectrometer (Madison, WI) with a Smart ARK accessory containing a 45 ZeSe trough in a scanning range of 650−4000 cm−1 for 64 scans at a spectral resolution of 4 cm−1. 2.3. Reaction of Oleic Acid with Ru3(CO)12. Catalyst activation was performed in a model IL-2GB Innovative Technology (Amesbury, MA) inert atmosphere drybox. Nitrogen was used as the inert gas in the chamber where water and oxygen levels were kept below 1 ppm. Into a 500 mL round-bottom flask, Ru3(CO)12 (0.501 g; 0.78 mmol) was mixed with oleic acid (60.1 g; 0.21 mol). The mixture was heated to 90 °C for 45 min, causing dissolution of solid material to form a light yellow solution. A Vigreux column capped with a septum was placed on the flask and removed from the drybox. The vessel was then connected to a Schlenk line with an argon flow and heated to a reaction temperature of 250 °C for 24 h using a hot plate. The flask was covered with glass wool and aluminum foil. After equilibration to room temperature, the crude product was a light yellow solid, which was purified three times by vacuum distillation (99%), triruthenium dodecacarbonyl (Ru3(CO)12; Strem Chemical, Newburyport, MA, 99%), acetone (Fisher Scientific, Fairlawn NJ, HPLC grade), and all other chemicals (Sigma-Aldrich Corp, St. Louis, MO) were used as received. 2.2. Chromatographic and Spectroscopic Characterization. Reaction progress was monitored on a GC−MS utilizing an Agilent (Santa Clara, CA) 7890A GC equipped with a 7683B series injector, a B

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Energy & Fuels (S, ppm), ASTM D5453; surface tension (mN/m), ASTM D3825. Oxidation onset temperature (OOT, °C) was determined by pressurized differential scanning calorimetry (PDSC) following a procedure described previously.31 Crystallization onset temperature (COT, °C) was measured by subambient DSC using equipment and a modified procedure outlined in an earlier study.32 The cooling scans were conducted from 40 to −85 °C at 10 °C/min. Derived cetane number (DCN) was determined (n = 32) at Southwest Research Institute (San Antonio, TX) following ASTM D6890. For greater precision, PP was measured with a resolution of 1 °C instead of the specified 3 °C increment. 2.5. Preparation of Blend. A precisely measured volume of product mixture was blended with ULSD at 5 vol % with continuous stirring to ensure homogeneity and will be termed H5. The sample was stored in an amber jar under argon at 4 °C prior to use. Before analysis, the blend was equilibrated to room temperature and agitated to ensure homogeneity, and aliquots were removed from the middle of the solution using a glass pipet.

3. RESULTS AND DISCUSSION 3.1. Reaction of Oleic Acid with Ru3(CO)12 and Product Composition. Decarboxylation of oleic acid into a mixture of heptadecene isomers along with other materials was accomplished in 80% conversion employing Ru3(CO)12 at 250 °C for 24 h. The crude product was a solid at room temperature due to the presence of trans isomers of octadecenoic acid, which have melting points > 44 °C. These isomeric acids resulted from incomplete decarboxylation and collectively represented 7% of the crude product. Therefore, the crude product was distilled under vacuum to yield a translucent mixture of liquid product (42% yield), the composition of which is described below (section 3.2). The precatalyst, Ru3(CO)12, reacts with dicarboxylic acids to form isolatable ruthenium−carboxylate complexes.33 Complexes of this type catalyze the isomerization of alkenes,34 often in the presence of phosphine ligands.35−37 Under our conditions, isomerization of oleic acid afforded a number of intermediates, most of which underwent subsequent decarboxylation24 and secondary reactions as described in the present work. Examples of these intermediates included positional isomers of oleic acid in which the double bond was shifted to other carbons. Conversion of oleic acid was considerably higher (80%) when Ru3(CO)12 was activated at 90 °C prior to reaching the reaction temperature (250 °C). In contrast, a conversion of only 39% was obtained when the mixture was heated directly to 250 °C. Such a stark contrast in conversion indicated the importance of forming the active catalyst species, which appeared to be more likely at a lower temperature. We assume that the active catalyst species was [Ru(CO)2RCO2]n, where R represents the hydrocarbon tail of oleic acid or a positional isomer.24 3.2. Composition of the Distilled Product Mixture. Spectroscopic and chromatographic characterization of the product was consistent with a mixture of linear C17 hydrocarbons (95.6% collectively) along with a small contingent of aromatics (4.4%). GC and GC−MS analysis showed that the distilled product mixture consisted of more than 22 components, all containing 17 carbons. The most prominent component in this mixture was heptadecane which was identified at a level of about 18%. Figure 1 presents a possible reaction scheme accounting for the various products observed as discussed below.

Figure 1. Proposed reaction scheme accounting for the observed products of the reaction of oleic acid with Ru3(CO)12.

All possible 15 isomers of heptadecene, resulting from positional and geometric isomerization of the original double bond, were present with 1-heptadecene being the least prominent compound. The presence of all possible heptadecene isomers was shown by DMDS addition to the double bond in the isomers, yielding vicinal di(thiomethyl) compounds.25,26 In mass spectrometric analysis, these derivatives cleaved at the former double bond position, with each characteristic fragment carrying a thiomethyl moiety and were thus easily identified. The mass spectra of the DMDS derivative of 1-heptadecene and either isomer of 8-heptadecene are depicted in Figure 2 and may serve as representative examples. For example, the DMDS derivative of 1-heptadecene gave a strong peak at m/z 271 corresponding to a loss of the terminal −CH2SCH3 moiety. The DMDS derivatives of the other heptadecene isomers provided strong peaks at successively 14 amu less. A second strong peak corresponding to the fragment of the other side of the former double bond became easily visible when its m/z value was over m/z 100. Thus, the DMDS derivative of the two 8-heptadecene isomers displayed strong peaks at m/z 173 and m/z 159. The present results are in close agreement with the mass spectra of the DMDS derivatives of 1and 3-heptadecene reported previously.38 At least six alkylbenzene compounds with a molecular weight of 232 and formula of C17H28 were identified by their GC−MS patterns (base peaks at m/z 91, 105, 119). The mass spectra of these compounds are shown in Figure 6 in order of elution. The least prominent of the six alkylbenzenes was identified as undecylbenzene by the fragmentation pattern corresponding to that of an authentic sample eluting at the same retention time (spectrum e in Figure 6). One alkylbenzene compound (spectrum f in Figure 6) had a longer retention time than undecylbenzene while the other four had shorter retention times. For identifying these compounds, a comparison with literature data on the mass spectra of some C 17 H 26 alkylbenzenes as obtained in the present work as well as C12H28 alkylbenzenes with 12 carbon atoms in the side chains was conducted.39,40 This comparison revealed that spectrum a in Figure 6 was likely that of pentylhexyl benzene, spectrum b is assignable to butylheptylbenzene, spectrum c to propyloctyl C

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hydrocarbons of 1:24 (4% aromatics). Aromatic signals in the FT-IR and 13C NMR spectra were too weak to be visible. More accurate quantification of aromatic content (4.4%) was achieved via GC−MS (Figure 5). The identification of

Figure 3. 1H NMR spectrum of the reaction product of oleic acid with Ru3(CO)12.

Figure 2. Mass spectra (EI) of the (a) DMDS derivative of 1heptadecene (1,2-dithiomethyl heptadecane) and (b) the DMDS derivative of 8-heptadecene (8-9-dithiomethyl heptadecane). The fragment at m/z 271 in (a) “migrates” with the double bond position to m/z 173 in (b) with increments of 14 amu observed for the “inbetween” double positions. Similarly, the fragment at m/z 61 (CH2SCH3+) “migrates” to m/z 159, although m/z 61 is observed in the spectra of all heptadecene DMDS derivatives. The fragments at m/z 285 and m/z 238 observed in all heptadecene DMDS derivatives result from loss of one and two SCH3 moieties.

benzene, spectrum d to ethylnonyl benzene (m/z 119 as base peak as reported previously39 and similar to ethyldecyl benzene40), and spectrum f to methyldecyl benzene. The base peaks at m/z 105 (except the base peak of m/z 119 for ethylnonyl benzene) demonstrated that the compounds were dialkylbenzenes with minor peaks at m/z 203, 189, 175, etc., serving to identify the nature of the side chains. The substitution pattern (ortho, meta, or para) on the benzene ring could not be unambiguously established, although para substitution appears most likely based on previously published data,39 as the base peaks reported there correspond to the present results. Alkylbenzenes with branched alkyl chains appear unlikely as these compounds would exhibit base peaks of higher m/z values. Besides these C17H28 alkylbenzenes, minor amounts of compounds with a molecular weight of 230 were also identified. Their mass spectra indicated that they were alkylbenzenes with one unsaturated side chain, thus corresponding to C17H26. Interestingly, a very minor amount of dodecylbenzene was also observed, indicating that dehydrogenation and aromatization were not necessarily subsequent to isomerization and decarboxylation as well as showing that minor deoxygenation occurred. The presence of the alkylbenzenes was also indicated by weak signals in the 1H NMR spectrum at around δ 7.1−7.2 ppm, which, after integration, revealed a molar ratio of aromatics to other

Figure 4. 13C NMR spectrum of the reaction product of oleic acid with Ru3(CO)12. The signals at 77 ppm are from CDCl3.

aromatics (Figure 6) indicated that the present combination of catalyst and reaction conditions promoted not only aromatization to a minor extent but also dehydrogenation. Such a result is not unprecedented, as ruthenium-catalyzed aromatization of alkenes is reported. 41,42 Besides the components discussed above, trace amounts of heptadecadienes were identified by their molecular ions of m/z 236 in GC−MS, but their exact nature and number of components was not established. From the compositional data, it is clear that subjection of oleic acid to catalytic Ru3(CO)12 under the present reaction conditions yields reactions beyond just isomerization and decarboxylation. These included not only aromatization as discussed above but also dehydrogenation and hydrogenation as proposed in Figure 1. According to that scheme, a combination of isomerization and decarboxylation initially led to the various heptadecenes. Dehydrogenation of the heptadecenes caused the formation of heptadecadienes (not D

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present in sufficient concentration to be detected by either FTIR or 1H NMR. In the 13C NMR spectrum (Figure 4), a signal at 124 ppm corresponding to β-unsaturation was identified. 3.3. Fuel Properties of the Product Mixture. Presented in Table 1 are the fuel properties of the hydrocarbon mixture (HM) resulting from reaction of oleic acid with Ru3(CO)12, a 5% blend thereof with ULSD (H5), and unblended ULSD. Also presented for comparison are the petrodiesel standards ASTM D975 and EN 590. Fuel properties of interest included cold flow, oxidative stability, AV, energy content, cetane number, KV, density, moisture content, specific gravity, lubricity, and sulfur content. Cold flow properties were assessed through measurement of CP, PP, and COT. The petrodiesel standards do not specify limits for CP and PP, although both offer guidance. The CP and PP values for HM were −1 and −4 °C, respectively. These results were comparable to FA methyl esters (biodiesel) prepared from canola and soybean oils. The similar cold flow properties can be traced to the melting points of the hydrocarbon components in the present mixture being in a range similar to that of FA methyl esters. The melting points of heptadecane (22 °C) and 1-heptadecene (11 °C) are close to those of the saturated fatty esters methyl palmitate (28.5 °C) and methyl myristate (18.5 °C).46−48 However, both ULSD and the H5 blend yielded CP (−15 °C) and PP (−24 °C) values that were considerably lower than that of the neat product mixture. Although the CP and PP values for HM were significantly higher than the corresponding values for ULSD, HM did not negatively affect CP and PP when blended with ULSD at 5 vol % (H5). Subambient DSC analysis of HM yielded a COT of −7.69 °C. This value was lower than CP and PP due to supercooling of the sample during analysis.32 The COT was low compared to the melting points of heptadecane (22 °C) and 1-heptadecene (11 °C).47 The product sample was a diverse mixture of C17 hydrocarbon and aryl compounds. The crystallization temperatures of such mixtures depend on the concentration and melting properties of the individual molecular species present. Thus, the relatively low COT measured for HM was not unexpected. It is of interest to compare the oxidative stability of HM to that of biodiesel because both materials contain unsaturated long-chain compounds. Furthermore, the standard method (EN 15751) used for biodiesel employing the so-called Rancimat instrument, yielding a minimum induction period (IP, 110 °C) prescribed in the standards, is now also included in the European petrodiesel standard EN 590, thus enabling comparability between fuels of various kinds. The IP obtained for HM was 3.4 h, which was considerably below the minimum limit of 20 h specified in EN 590. Such an IP was comparable to values reported previously for biodiesel prepared from canola and soybean oils along with pure methyl oleate.46,49 For comparison, the IP of ULSD was greater than 24 h. The IP of HM was sufficiently low to cause the H5 blend (17.7 h) to not meet the 20 h threshold prescribed in EN 590. Oxidative stability (OOT) was also determined via PDSC, which has proved to be an effective tool for comparison of samples.31 Higher OOT values indicate greater resistance to oxidation and are hence more desirable. The OOT results corroborated those reported for IP in which HM (175.8 °C) was less stable to oxidation than ULSD (198.0 °C) and addition to ULSD (H5 blend) negatively affected OOT (195.8 °C). The presence of unsaturation is presumed to be the cause of reduced oxidative

Figure 5. GC trace of the reaction product of oleic acid with Ru3(CO)12 after 24 h of reaction. Peaks after 18.1 min are aromatics, whereas those before 18.1 min are heptadecenes and heptadecane.

shown in Figure 1) and eventually the formation of the observed alkyl aromatics. It is, therefore, likely that dehydrogenation occurs stepwise but with dehydrogenation of heptadecadienes causing nearly immediate formation of the alkyl aromatics, as no heptadecatrienes were observed. In turn, hydrogen liberated by dehydrogenation and aromatization saturated a portion of the heptadecenes to heptadecane, thus explaining the existence of the latter in the product mixture. However, it cannot be fully excluded that some hydrogenation of oleic acid to stearic acid occurred before decarboxylation and that, accordingly, stearic acid was decarboxylated to afford heptadecane (this sequence is not shown in Figure 1). A further step in this case could be dehydrogenation of heptadecane to the heptadecene isomers. Similar to aromatization by ruthenium-based catalysis, reactions by ruthenium-based catalysts promoting hydrogenation43,44 and dehydrogenation45 are reported. Previously, it was speculated that the active catalyst species in isomerization and decarboxylation was [Ru(CO)2RCO2]n, with R representing the hydrocarbon tail of oleic acid or a positional isomer.24 Considering the complexity of the product mixture as analyzed presently and the various reactions apparently occurring stepwise and/or simultaneously, it appears possible that several catalyst species were active. The nature of any such species was not determined and may be the subject of future work. Another issue is to investigate if reaction conditions can be modified to favor the formation of certain desired products, for example, suppressing the formation of aromatics and increasing the relative amount of heptadecenes or, conversely, the formation of certain aromatics should they be of interest. The FT-IR spectrum confirmed that the product consisted primarily of internal isomers of heptadecene. From 1H NMR spectroscopy (Figure 3), integration values close to theoretical (in parentheses) for an internal alkene in a position of γ or greater were observed: 2.0 (2) alkene protons to 4.0 (4) protons α to an alkene. However, the higher relative ratios of other methylene protons, 29.3 (22), and terminal methyl protons, 7.1 (6), indicated a significant contribution from βunsaturation. The terminal alkene, 1-heptadecene, was not E

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Figure 6. Mass spectra of alkylbenzene compounds with a formula of C17H28 arising in the reaction of oleic acid with Ru3(CO)12 in order of gas chromatographic elution. Spectrum e is assigned to undecyl benzene. For a comparative evaluation of the other spectra with literature data, see the text.

length but decrease with branching and unsaturation with number and position of double bonds playing a role, an example being the low cetane standard (2,2,4,4,6,8,8-heptamethylnonane; HMN) with an assigned value of 15.51 Thus, heptadecane was found to have a higher DCN than hexadecane, 101.6 versus 96 (note experimental deviation of the CN of hexadecane).52 1-Hexadecene (cetene) as an example of an unsaturated long-chain hydrocarbon had a CN of 87.5,52 thus the CN or DCN of HM can be assumed to be slightly higher than that of 1-hexadecene because of the slightly increased chain length. On the other hand, aromatic compounds such as benzene and toluene have very low CN and DCN, but CN increases with increasing chain length of an alkyl side chain.

stability relative to ULSD. As a consequence, antioxidant additives are recommended to improve oxidative stability. The minimum limits specified for cetane number in ASTM D975 and EN 590 are 40 and 51, respectively. Reported herein is derived cetane number (DCN). The ASTM D6890 method (DCN), which requires a much smaller sample size, is approved as an alternative to the more traditional CN method (ASTM D613) specified in ASTM D975.50 The DCN of HM was 86.9, which was considerably higher than the limits specified in the petrodiesel standards. For comparison, the value obtained for ULSD was 42.5. Such a result is not surprising, as the high cetane standard is n-hexadecane (trivial name cetane) with an assigned value of 100. The CN (and DCN) increase with chain F

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Table 1. Fuel Properties of HM, a 5 vol % (H5) Blend with ULSD, and Unblended ULSD. Also Shown Are the Petrodiesel Standards ASTM D975 and EN 590 for Comparison ASTM D975 AV, mg KOH/g cold flow CP, °C PP, °C COT, °C oxidative stability IP, 110 °C, h OOT, °C cetane number KV, 40 °C, mm2/s moisture, ppm density, 15 °C, kg/m3 specific gravity, 15 °C HFRR wear scar, μm, 60 °C sulfur, ppm higher heating value, MJ/kg surface tension 25 °C, mN/m 40 °C, mN/m

-

a

EN 590

HM

H5

a

b

b

-

N/D

N/D

ULSD N/Db

-a -a -a

-a -a -a

−1 −4 −7.69 (0.04)

−15 −24 (1)c -e

−15 −24 (1) -e

-a -a 40.0 min 1.9−4.1 -a -a -a 520 max 15 max -a

20 min -a 51.0 min 2.0−4.5 200 max 820−845 -a 460 max 10 max -a

3.4 (0.3) 175.8 (0.3) 86.9 (3.5)d 3.14 5 (2) 791 0.792 299 (1) N/Db 47.08 (0.12)

17.7 (0.1) 195.8 (0.3) -e 2.35 9 839 0.839 545 (11) 7.3 45.58 (0.23)

>24 h 198.0 (0.4) 42.5 (0.8)d 2.23 10 842 (1) 0.842 567 (17) 7.5 44.23 (0.10)

-a -a

-a -a

24.3 23.7 (0.1)

26.6 25.3 (0.3)

27.3 (0.2) 25.9 (0.1)

a

Not specified. bNone detected. cValue in parentheses represents standard deviation (SD) from the reported means (n = 3). Where none is indicated, SD = 0. dDerived cetane number (n = 32). eNot determined.

The density of HM at 15 °C was 791 kg/m3, which was below the minimum limit specified in EN 590 (820−845 kg/ m3). ASTM D975 does not contain a density specification. The density of HM was also lower than that of ULSD (842 kg/m3) and the H5 blend (839 kg/m3), both of which yielded values within the range specified in EN 590. Analogously, the specific gravity (15 °C) of HM (0.792) was lower than those of ULSD (0.842) and the H5 blend (0.839). Neither of the petrodiesel standards contains specifications for specific gravity. The lower density of HM versus ULSD was most likely a result of aromatics and cyclic hydrocarbons found in ULSD, which have higher densities than linear hydrocarbons largely comprising HM. For instance, the densities reported for naphthalene, hexyl benzene, cyclodecane, 1-heptadecene, and heptadecane are 1025, 858, 854, 785, and 778 kg/m3, respectively.47 Saturated hydrocarbons, including cyclic, usually comprise approximately 75−80% of ULSD with the remaining 20−25% consisting of various aromatics.55 The gross heat of combustion of HM was 47.08 MJ/kg, which was higher than the energy content of ULSD (44.23 MJ/ kg). Correspondingly, the value obtained for the H5 blend (45.58 MJ/kg) was greater than that of ULSD. Energy content is largely dependent on the number of energetic C−H bonds. Aromatics contain less of these bonds due to their polyunsaturation. HM yielded a higher gross heat of combustion because it contained fewer aromatics than ULSD while possessing more alkane components. Neither ASTM D975 nor EN 590 includes specifications for energy content. Although surface tension is not specified in either ASTM D975 or EN 590, it is nevertheless an important fuel property that affects fuel atomization in combustion chambers in diesel engines.56 HM afforded lower surface tension at both 25 °C (24.3 mN/m) and 40 °C (23.7 mN/m) than ULSD (27.3 and 25.9 mN/m, respectively). As expected, surface tension decreased at the higher temperature. The values obtained for the H5 blend were also lower than that of ULSD. Such a result was not surprising, because it is known that a relationship

Thus, the CN of undecylbenzene was determined as 59.1 in the present work and is likely the alkylaromatic compound with the highest CN in the product mixture. Therefore, considering the contribution and amounts of heptadecane, the heptadecene isomers, and the various alkylaromatics, the CN/DCN of the product mixture is well-explained. The DCN of ULSD was low because, as mentioned above, the ULSD used here contained significant amounts of compounds with low cetane numbers such as the alkylaromatics and polyhydroaromatics along with lesser amounts of branched hydrocarbons. The KV (40 °C) of HM was 3.14 mm2/s, which was higher than that of ULSD (2.23 mm2/s) but within the ranges specified in the petrodiesel standards. This value can be explained by the KV of the individual components of HM. The KV of heptadecane is 3.40 mm2/s at 40 °C.53 The KV of the various heptadecene isomers can be assumed to be in the range of 3.00−3.10 mm2/s at 40 °C, but the difference between cis and trans isomers should be considered because trans isomers display higher KV, closer to that of the saturated analogues. Overall, the KV of HM is, therefore, within the expected range. The H5 blend yielded a KV of 2.35 mm2/s at 40 °C, which was similar to the value obtained for ULSD. Lubricity specifications are included in ASTM D975 and EN 590 with maximum wear scars (60 °C) of, respectively, 520 and 460 μm prescribed using the high-frequency reciprocating rig (HFRR) lubricity test (ASTM D6079). The wear scar of 299 μm generated by HM was below the prescribed limits and significantly shorter than that measured for ULSD (567 μm). Accordingly, the H5 blend provided a smaller wear scar (545 μm) than that of ULSD. However, both ULSD and the H5 blend exhibited wear scars in excess of the maximum limits specified in the petrodiesel standards. While a previous study elucidated that unsaturation results in smaller wear scars, thus contributing to the present result, it is also likely that trace amounts of FAs remaining are responsible for this result. Even minor amounts of free FAs improve lubricity considerably more than unsaturation.54 G

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NMR spectra and Dr. Michael Wibbens of Southwest Research Institute, San Antonio, TX, for cetane testing.

(termed the MacLeod−Sugen parachor) exists between the molecular volume of a liquid, its density, and surface tension.57 As noted previously, HM provided a lower density than ULSD. The moisture content of HM was 5 ppm, which was below the maximum limit of 200 ppm prescribed in EN 590. ASTM D975 does not contain a dissolved water specification. Both ULSD (10 ppm) and the H5 blend (9 ppm) also afforded low moisture contents. In addition, no sulfur was detected in HM. Both ULSD (7.5 ppm) and the H5 blend (7.3 ppm) had sulfur levels below the maximum limits of 15 and 10 ppm specified in ASTM D975 and EN 590, respectively. Lastly, HM did not contain a detectable amount of carboxylic acids by AV titration. AV is not specified in the petrodiesel standards, but was of interest because the starting material was oleic acid. On the other hand, the results regarding lubricity may imply that trace amounts of free FAs not detected by AV titration remain in HM.

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4. CONCLUSIONS The mixture of hydrocarbons prepared from oleic acid by Ru3(CO)12-based catalysis consisted of all linear isomers of heptadecene (77.6%), several alkylaromatics (4.4%), and heptadecane (18.0%) as the most prominent single component. The reaction was more effective when the medium was preheated to 90 °C (80% conversion) prior to reaching the reaction temperature of 250 °C versus heating directly to 250 °C (39%). The method is catalytic and requires no coreagents, and the only coproduct is CO2, so that further refinement and development may render it useful for the production of unsaturated hydrocarbons from natural oils. Diesel fuel properties of HM such as AV, cold flow, density, DCN, HHV, KV, lubricity, moisture content, oxidative stability, specific gravity, sulfur content, and surface tension were measured and compared against the petrodiesel standards ASTM D975 and EN 590. Also of interest was a comparison with ULSD along with the properties of an H5 blend of the product mixture in ULSD. HM yielded higher energy content (47.08 MJ/kg) and DCN (86.9) but lower sulfur content (not detected) and surface tension (24.3 mN/m at 25 °C) than ULSD. Cold flow (CP −1 °C) and oxidative stability (3.4 h; EN 15751), however, were problematic, which is reminiscent of the problems besetting biodiesel. All other fuel properties, including sulfur content, were within the ranges specified in the petrodiesel standards.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +1-309-681-6511. Fax: +1-309-681-6524. Notes

Disclosure: Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Kim L. Ascherl, Benetria N. Banks, and Kevin R. Steidley for technical assistance. Dr. Karl Vermillion is acknowledged for acquisition and analysis of H

DOI: 10.1021/acs.energyfuels.6b01728 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.6b01728 Energy Fuels XXXX, XXX, XXX−XXX