Triacylglyceride Thermal Cracking: Pathways to Cyclic Hydrocarbons

Article pubs.acs.org/EF

Triacylglyceride Thermal Cracking: Pathways to Cyclic Hydrocarbons Alena Kubátová,† Jana Št’ávová,† Wayne S. Seames,‡ Yan Luo,‡ S. Mojtaba Sadrameli,‡,§ Michael J. Linnen,‡ Ganna V. Baglayeva,† Irina P. Smoliakova,† and Evguenii I. Kozliak*,† †

Department of Chemistry, and ‡Department of Chemical Engineering, University of North Dakota, Grand Forks, North Dakota 58202, United States S Supporting Information *

ABSTRACT: Thermal cracking of triacylglyceride (TG) oils results in complex mixtures, containing nearly 20% cyclic hydrocarbons, which can be further processed into middle-distillate transportation fuels and byproduct chemicals. The occurrence patterns of cyclic products obtained via the thermal cracking of several TG feedstocks, such as canola and soybean oils, as well as triolein and tristearin (conducted at 430−440 °C in the absence of catalysts under vacuum), were investigated to probe possible formation mechanisms. Detailed gas chromatographic characterization furnished full molar homology/molecular size and partial isomeric profiles for cyclopentanes, cyclopentenes, cyclohexanes, cyclohexenes, aromatics, and polycyclic aromatic hydrocarbons (PAHs). It was found that the data were inconsistent with previously proposed mechanisms involving the Diels− Alder reaction as a single pathway. An alternate mechanism was proposed and supported with experimental evidence based on the intramolecular cyclization of alkenyl and alkadienyl radicals formed as a result of TG cracking. The product homology profiles corroborate the proposed mechanism and show the depletion of medium-size alkenes coupled with the accumulation of corresponding monocyclic hydrocarbons (those with the matching number of carbon atoms). Similarly, the product mixtures were depleted of long-chain alkyl-substituted monocyclic hydrocarbons because of the formation of the corresponding PAHs as long as sufficient time is available. Entropy appears to determine the type and size of cyclic hydrocarbons formed.

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INTRODUCTION The thermal cleavage of triacylglyceride (TG) oils is a wellknown technique for transforming crop oils into chemicals that can be used as replacements for petroleum transportation fuels.1,2 For example, TG oil cracking is an important step in a recently developed process for the generation of renewable fuels and chemicals, e.g., fatty acids (FAs).3,4 A unique feature of cracking in comparison to alternative TG to transportation fuel development pathways, such as hydrotreating5 or transesterification,6 is the formation of cyclic hydrocarbons, including aromatics. Cyclic products are important as fuel components,7 as chemicals, and as precursors of polymers.8 Previous studies have shown that a significant fraction of the TG oil can be induced to form cyclic hydrocarbons, including aromatics in catalyst-facilitated reactions.9−14 In these studies, typically with zeolite catalysts, the TG oil feedstock is vaporized and then passed through the catalyst bed.9 However, TG molecules start decomposing at temperatures below their boiling points. Therefore, even when TG cracking is facilitated by the use of a catalyst,2 non-catalyzed free-radical reactions may still be important, depending upon the reactor setup used. This is reinforced by data that demonstrates that cyclic hydrocarbons are also formed, albeit in lower concentrations, in non-catalyzed thermal cracking reactions.4 Information on the chemical reaction mechanisms that can lead to cyclic hydrocarbon formation is limited and inconsistent, because of the inherent complexity of the diverse suite of reactions that can occur and the wide variety of experimental setups employed. Mechanistic considerations discussed in the literature are usually based on the major end-product distribution and common knowledge of pertinent high-temperature free-radical reactions.15−18 © 2011 American Chemical Society

When TG oils are thermally cracked at temperatures in the range of 400−440 °C, the resulting organic liquid product (OLP) typically contains linear, branched, and cyclic alkanes, alkenes, aromatics, and FAs.4,16,17,19 Initial thermal TG decomposition produces long-chain FAs. These intermediates may then undergo deoxygenation (e.g., decarboxylation) to form alkyl and alkenyl radicals, which may, upon stabilization, ultimately lead to alkanes and alkenes.20,21 Alternately, C−C bonds in the intermediate FAs may be cleaved to yield smaller sized FAs along with alkanes/alkenes.16−18 Our discovery of an efficient thermal cracking pathway for TG pyrolysis was recently reported.19 Thermal TG cracking exhibits two unique features: the generation of a mostly linear “diesel-like” fuel hydrocarbon structure and the avoidance of catalyst fouling issues experienced during catalytic cracking. The focus of the current work is on the mechanisms of cyclic product formation during this recently discovered path, using several feedstocks and providing detailed characterization of all quantifiable cyclic products and their precursors. To our best knowledge, the mechanisms that lead to cyclic product formation in free-radical-based petrogenic processes have not been thoroughly investigated. Alkenes (and/or their precursors in free-radical cracking reactions, alkenyl radicals) are a potential source of aromatic and other cyclic hydrocarbons. The common paradigm is that six-membered cyclic products, e.g., cycloalkanes and aromatics, are produced via the intermolecular Diels−Alder reaction (involving a diene and a Received: July 1, 2011 Revised: December 6, 2011 Published: December 7, 2011 672

dx.doi.org/10.1021/ef200953d | Energy Fuels 2012, 26, 672−685

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Scheme 1. Formation of Cyclic and Aromatic Hydrocarbons via the Diels−Alder Reaction

Scheme 2. Intramolecular Radical Cyclization to Produce Monocyclic Hydrocarbons

“dienophile”, e.g., alkene), followed by further processing of the cycloalkenes formed, as shown in Scheme 1.15,22 The Diels−Alder reaction is known to occur in similar systems at temperatures below 400 °C, e.g., the formation of dimeric polyunsaturated FAs.23 The formation of cyclic hydrocarbons via the intramolecular cyclization of alkenyl radicals is well-known in organic synthesis (Scheme 2).24−30 However, this path does not appear to have been considered in previous TG thermal cracking studies as a legitimate alternative to the Diels−Alder reaction, although intramolecular cyclization has been considered for alkene

cyclization on zeolites.31−33 However, surface reactions involve charged intermediate species. Thus, they are not pertinent to non-catalytic thermal cracking because the gas-phase reactions characteristic of non-catalyzed TG thermal cracking are based exclusively on free-radical intermediates because of the wellknown instability of charged species in the gas phase. Besides the Diels−Alder and intramolecular alkene cyclization pathways, cyclic products can also be formed by cyclization of alkyl radicals. However, those intermediates are amenable to cyclization only at temperatures above 570 °C, even if specific 673


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catalysts are present,34 which are much higher temperatures than considered here (430−440 °C). In this study, we performed a detailed identification/ quantification of individual species generated by the noncatalytic thermal cracking of soybean and canola oils at 430− 440 °C, the temperature range which was previously demonstrated to provide the highest yields of OLP with desirable properties.4,19 The ensuing investigation of full homology and isomer patterns within the pertinent classes of cyclic products of TG cracking allowed for a thorough evaluation of the hypothesis of intramolecular alkene cyclization during the non-catalytic thermal cracking of TGs. The role of the double-bond patterns within the structure of original TGs was confirmed by select experiments with individual TGs, triolein and tristearin, featuring one and zero double bonds per FA, respectively, as well as with crambe oil comprised of TGs with longer FAs.

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At the end of each cracking experiment, the reaction products (present mostly as vapors at reaction pressure and temperature) were transferred from the reaction mixtures by slowly opening the highpressure vent valve. This gaseous reaction mixture passed through a water-cooled condenser (25 °C) where the OLP was condensed and then collected in two pre-weighed flash traps. For cracking of individual TGs at lower reaction times, a much smaller 5 mL reactor was used, because of the high costs of the individual TGs. This reactor had a similar design and allowed for the same operations as the larger reactors, except for the gas-phase product collection. Its small size allowed for fast cooling and heating, thus providing an opportunity to obtain product mixtures at lower reaction times. OLP Chemical Characterization. The detailed chemical analysis was performed using an Agilent 7890N gas chromatograph (GC) equipped with both a flame ionization detector (FID) and a mass spectrometric (MS) detector and with an Agilent 7683 series autosampler (Santa Clara, CA). The analyte separation was performed on a DB-Petro capillary column (100 m long, 0.25 mm inner diameter, with a 0.5 μm film thickness, J&W Scientific, Rancho Cordova, CA) at a constant helium flow rate of 1.5 mL min−1. Samples (0.2 μL) were injected in a split ratio of 1:30 into a programmed temperature vaporizing injector at 25 °C, which was heated at a rate of 720 °C min−1 to 350 °C. The column temperature program started at 5 °C, followed by a gradient of 2.5 °C min−1 up to 300 °C, with a final hold of 15 min. The FID and MS temperatures were set at 350 and 280 °C, respectively. The MS data were acquired using electron ionization in a full scan of 35−500 amu. For analysis, 1.0 mL of each OLP sample was weighed in a 2 mL autosampler vial and then 100 μL of the IS mixture was added. Identification was performed by matching the observed retention times of the products to those of known chemicals in standard mixtures (PIANO, etc., listed in the Materials) and confirmed using mass spectra. For chemicals whose standards were unavailable (e.g., long-chain substituted cyclic hydrocarbons, particularly, branched and polycyclic), the tentative identification was performed using the MS library with a minimum match of 80% (such compounds are labeled as “T” in the subsequent tables). The regularity of the retention time patterns within the full homology series and common ions for particular classes of compounds (listed in Table 2) enabled additional confirmation of the MS identification. Because calibration of all TG cracking products using individual chemicals as standards was not feasible, detailed quantitation was performed using GC−FID calibrated with a series of representative standards (provided under Materials) using the IS method. The response factors (RFs) of non-calibrated compounds were calculated as follows: For branched and cyclic alkanes, the RFs were assigned similar those of the corresponding linear alkanes found within the same retention time window. For alkenes and aromatics, the RF of the corresponding alkane eluting within a particular retention window was corrected by a factor representative for a particular class of compounds [e.g., the ratio of RFs of the nearest eluting alkene (aromatic) to that of the given alkane]. For a number of MS-identified products (using the extracted common ions), the FID signal was below the quantification limit, i.e., not significantly exceeding or similar to the background noise. The alternate quantification of a large number of compounds by MS was not feasible, because of the lack of individual standards, which are mandatory for accurate MS quantification as opposed to FID. Therefore, the mechanistic interpretations offered in this study were based on the more abundant species quantified using FID. The contribution of these peaks to mass balance closure was accounted for by the summation of unresolved peak areas, i.e., so-called “chromatographic hump”. This unresolved area was quantified, upon a blank signal deduction, using a representative (average) response factor, which was determined to be that of n-heptadecane. Further detail can be found in our publication on this analytical method development.35 The detailed identification included the analysis of each homology series within the specific classes of compounds while using the characteristic ions (Table 2) extracted from the original GC−MS

EXPERIMENTAL SECTION

Original Feedstocks. Refined food-grade soybean oil was obtained from Ag Processing, Inc. (AGP), a cooperative located in the state of Minnesota. Refined food-grade canola oil was obtained from ADM processing in Velva, ND. Composition data, reported as the percentages of the FAs incorporated into TGs, are shown in Table 1.

Table 1. FA Composition of Feedstock TGs44

a

FA

canola oil (wt %)

soybean oil (wt %)

palmitic C16:0 stearic C18:0 oleic C18:1 linoleic C18:2 linolenic C18:3 eicosenoic C20:1 erucic C22:1

4 2 60 20 10 1.6 2.4

12 3 23 56 6 NRa NRa

NR denotes not reported.

Crambe oil was obtained by direct crushing and pressing of the seeds with no further cleanup of the oil prior to experimentation. Materials. Defined mixtures of isoparaffins, aromatics, naphthenes, and olefins (Alphagaz PIANO), naphtha, reformate, and alkylate qualitative reference standards, as well as crude oil qualitative and quantitative standards (ASTM D5134) were purchased from Supelco (Bellefonte, PA). For analyte quantification, a mixture consisting of the complete series of n-alkanes (C5−C18), selected alkenes (C6, C9, C14, and C18), and aromatics/polycyclic hydrocarbons (benzene, toluene, ethylbenzene, p-xylene, 1,2,4-trimethylbenzene, indane, and naphthalene) was employed. A mixture of three internal standards (ISs) was employed to control for sample volume changes and aging, consisting of benzene-d6 (102.1 mg/mL), 2-chlorotoluene (100.1 mg/mL), and o-terphenyl (49.8 mg/ mL) in methylene chloride. FAs were quantified after derivatization, as described in detail previously.19 Thermal Non-catalytic Crop Oil Cracking Experiments. Experiments were performed using the experimental system and protocol described in our previous publication.19 The reactions were carried out in two batch autoclave reactor systems (0.5 and 3.75 L), as shown in Figure 1. Both reactors were rated for a pressure of up to 37 MPa (5400 psia) and were compatible with both hot-charge reactant injection and the removal of reaction-temperature volatile materials at a reaction pressure. Cracking experiments were carried out at 430−440 °C in a nitrogen-purged 3 kPa vacuum gaseous environment. After heating at a rate of 2 °C min−1 to the set temperature, the reaction mixtures were maintained at this temperature for 30 min, for a total heating time of 240 min. 674


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Figure 1. Experimental apparatus for batch TG thermal cracking experiments. that of a m/z of 92 (which, in turn, is formed by a hydrogen rearrangement followed by the α-cleavage) at a high ratio of the m/z 92/91 peaks (Table 2), n-alkylbenzenes can be readily identified (Figure 2C).36 The peaks featuring a m/z of 105 (Figure 2d) revealed nearly all possible isomers of C9 and heavier alkylbenzenes, with the exception of n-alkylbenzenes identified earlier. The m/z of 106 (as opposed to 105; Table 2) was particularly pronounced for the first eluting peak in each group of homologues shown in Figure 2d. This feature is a signature pattern for 1-methyl-3-alkylbenzenes, i.e., those with a methyl group at the meta position.36 This MS information was confirmed using pure standards that matched the chromatographic retention times for these peaks (Figure 2d). Thus, obtaining the ratios of these characteristic ions in the MS along with the characteristic elution order pattern in GC enabled us to provide a tentative identification of n-alkyl- and 1-methyl-n-alkylbenzenes for those products whose chromatographic standards were not available, as demonstrated in Figure 2e.

Table 2. Characteristic Ions and Ranges for the Observed Homology Series of Cyclic TG Cracking Products class of compounds cycloalkanes n-akylcyclopentanes n-alkylcyclohexanes cycloalkenes alkylbenzenes metaalkylmethylbenzenes ortho- and paraalkylmethylbenzenes n-alkylbenzenes alkylphenols PAHs n-alkylnaphthalenes n-alkylindanes nalkylmethylindanes

characteristic fragment(s) m/z and their relative abundance ratios

last detected member of the homology series

[M+ •],a 69, 83 ratio of 68/69 > 0.7

C12bcyclopentane ratio of 82/83 >0.5 (for C2- C12-cyclohexane and higher substituted homologues) [M+ •], [M-15+], [M-29+] C3-cyclohexenes, C4 cyclopentenes [M+ •], 105, 106, 92, 91 C18-benzene ratio of 106/105 > 0.9

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105 ratio of 92/91 > 0.7 107 [M+ •], [M2+ •] ratio of 142/141 > 0.5 (for C4- and higher substituted homologues) 117, [M+ •] 131, [M+ •]

RESULTS AND DISCUSSION TG Cracking Product Distribution. TG cracking experiments yielded a small gas-phase fraction (4−7 wt % of the original feedstock), a larger solid fraction (coke and tar, 15− 20%), and nearly 5% of a polar phase, with the rest comprising the OLP. The bulk of the experiments were conducted with canola and soybean oils considered in this and subsequent sections. An overview of the overall TG cracking product distribution is shown in Table 3, while the gas-phase sample composition is shown in Table S1 of the Supporting Information. The detailed OLP GC analysis recovered only 54−60% as the total of all determined analytes. However, the rest of the OLP was accounted for by the sum of non-resolved peaks comprising chromatographic “humps”, a well-known feature of chromatograms of complete mixtures. The shapes of these humps were similar to the homology profiles of all of the products. An examination by mass spectrometry showed that the humps featured the characteristic ions of branched alkanes,

C12-phenol up to four rings C12-naphthalene C12-indane C11methylindane

a

Denotes the molecular ion. bThe subscript shows the number of carbon atoms in the single alkyl substituent. chromatograms. Figure 2 illustrates the application of this approach for alkylbenzenes. While the peaks of these products are difficult to discern in the total ion current (TIC) chromatograms (Figure 2a), they can be readily observed when extracting several characteristic ions. A m/z of 91 represents a well-known tropylium ion formed by the α-cleavage of benzylic bonds within the largest alkyl group of a substituted benzene (Figure 2b). When this ion is observed along with 675


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Figure 2. GC−MS (a) total ion current and (b−e) extracted ion chromatograms demonstrating the identification of alkylbenzenes. T denotes tentatively identified alkybenzenes based on the mass spectrum information, characteristic ion ratios, and chromatographic elution order. The identification of the rest of the products was confirmed by matching their chromatographic elution times with those of pure standards.

alkenes, and alicyclic and aromatic hydrocarbons present in small amounts. Qualitatively, the humps consisted of similar products as the identified portion of OLP, present in similar proportions. Thus, the quantified portion of OLP was deemed representative of the entire OLP composition. With the addition of unresolved and unidentified species, the mass balance closed satisfactorily (Table 3). Alicyclic, aromatic, and polycyclic aromatic hydrocarbons (PAHs) represented a significant fraction (∼20 mol %) of all determined GC-elutable OLP constituents. Thus, understanding

their formation mechanism is essential for optimization of TG cracking conditions. Each numerical value presented in Table 3 was obtained in triplicate, yielding reasonably small variances. The distribution of products was obtained in three different subsets, namely, cracking canola oil in two reactors of different sizes and also soybean oil in the smaller of the two reactors used for canola oil cracking. The values obtained for all cyclic products (Figure 3A) and their potential precursors, alkenes (Figure 3B), also turned out to be remarkably similar, as well as the distribution of each 676


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Table 3. Major Classes of Compounds (wt %) Recovered in OLPa feedstock/reactor scale

canola oil/small

canola oil/ large

soybean oil/small and large

alkanes linear branched cyclopentanes cyclohexanes total alkanes

20.4 0.61 2.29 2.58 25.9

± ± ± ± ±

1.4 0.07 0.26 0.18 1.6

20.2 0.94 2.85 3.02 27.1

± ± ± ± ±

1.6 0.18 0.38 0.36 2.5

16.3 0.46 1.75 1.86 20.4

± ± ± ± ±

2.9 0.16 0.43 0.45 3.9

alkenes linear branched linear nonterminal branched nonterminal cyclopentenes cyclohexenes total alkenes aromatics BTEX alkylbenzenes total aromatics polyaromatics indanes and indenes naphthalenes and fluorenes total polyaromatics ketones total FAs total total identified total unidentified total unresolved total a

2.29 ± 0.22 0.22 ± 0.04 0.054 ± 0.001

1.82 ± 0.18 0.23 ± 0.12 0.07 ± 0.02

2.24 ± 0.10 0.23 ± 0.05 0.05 ± 0.01

4.43 ± 0.35

4.77 ± 0.58

3.81 ± 0.92

0.88 ± 0.10 0.66 ± 0.12 8.54 ± 0.65

0.92 ± 0.03 0.66 ± 0.04 8.48 ± 0.81

0.94 ± 0.15 0.66 ± 0.11 7.93 ± 1.04

0.96 ± 0.07 1.68 ± 0.13 2.64 ± 0.20

1.41 ± 0.24 2.06 ± 0.33 3.47 ± 0.55

0.91 ± 0.35 1.63 ± 0.55 2.54 ± 0.90

1.15 ± 0.14

1.33 ± 0.19

0.98 ± 0.19

0.77 ± 0.04

0.86 ± 0.14

0.62 ± 0.15

1.92 ± 0.18

2.19 ± 0.33

1.60 ± 0.33

0.35 ± 0.12

0.23 ± 0.03

0.17 ± 0.05

12.9 52 17 25 94

± ± ± ± ±

2.9 4 1 3 2

15.9 57 15 28 101

± ± ± ± ±

1.1 3 2 5 1

18.9 51 10 37 99

± ± ± ± ±

Figure 3. Combined homology patterns of select groups of compounds quantified in the OLP obtained by TG thermal cracking: (A) alkenes and (B) cyclic products. The numerical values presented were obtained by summing the mole percentages of pertinent products having the same number of carbon atoms. The results are reported as mean values with one standard deviation.

1.1 7 1 4 3

This observation suggests that the traditional mechanism of their formation, via the Diels−Alder reaction (Scheme 1), cannot be the dominant path. Unlike the Diels−Alder reaction, products containing both five- and six-membered rings can be formed via the intramolecular path (Scheme 2). Thus, the abundance of cyclopentanes and cyclopentenes (Table 4) is evidence of the importance of intramolecular cyclization during the TG cracking process. The hypothesis of intramolecular cyclization is supported by the prevalent occurrence of monoalkylated cyclopentanes, which implies that cyclopentanes were formed primarily by folding (followed by cyclization) of alkenyl radicals with terminal double bonds (endocyclization in Scheme 2). Terminal alkenes were indeed recovered in significant concentrations among the TG cracking products (Table 3). An alkenyl radical formed upon cracking would have the double bond and unpaired electron located at opposite ends of the molecule, thus not immediately meeting the requirements shown in Scheme 2. However, the cyclization can be enabled by the movement of either the unpaired electron or a double bond along the carbon chain. The facile movement of the unpaired electron via hydrogen atom abstraction (radical translocation) is common at elevated temperatures.30 The observed broad size distribution of five-membered cyclic hydrocarbons (Figure 4A) corroborates this hypothesis as well as the observed similarity of the cyclic product size distribution for different feedstocks

Data are presented as the mean ± standard deviation (n = 3).

individual cyclic product (Tables 4−8), despite the complex homology and isomer patterns obtained. Minor differences were observed in the percentages of FAs (Table 3) and homology profiles of n-alkanes (not shown) but not for the pertinent products, cyclic hydrocarbons and alkenes (Figure 3). Because of the similarity of pertinent TG cracking product patterns among the different feedstocks and reactor sizes, a further discussion is based on the mean values obtained for these three above-mentioned subsets. For a correct comparison of the amounts of chemicals with different molecular weights (MWs), the molar percentages (rather than weight percentages) are discussed henceforth. More detailed information on the mechanism of formation of cyclic hydrocarbons was obtained through an analysis of distribution patterns for individual classes of pertinent products, i.e., their homology profiles, as in Figure 3, plus isomer profiles (Tables 4−8). We shall start from the five-membered alicyclic hydrocarbons because their abundance is critical for determining the process mechanism. Cycloalkanes and Cycloalkenes. Five-membered cyclic products represent ca. 30 mol % of the total concentration of cyclic hydrocarbon products (including aromatics) obtained, including both saturated and unsaturated species (Tables 3 and 4). 677


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Table 4. Cyclopentanes and Cyclopentenes in OLP (mol % of GC Elutable)a feedstock/reactor scale cyclopentane n-alkylcyclopentanes methylcyclopentane ethylcyclopentane Tb n-propylcyclopentane n-butylcyclopentane n-pentylcyclopentane T n-hexylcyclopentane T n-heptylcyclopentane T n-nonylcyclopentane T n-decylcyclopentane T n-undecylcyclopentane other cyclopentanes cis-1,2-dimethylcyclopentane cis-1,3-dimethylcyclopentane trans-1,3-dimethylcyclopentane cis-1-ethyl-2-methylcyclopentane trans-1-ethyl-2-methylcyclopentane trans-1-ethyl-3-methylcyclopentane ethylmethylcyclopentane T 1,2,3-trimethylcyclopentane 1,2,4-trimethylcyclopentane 1,3-diethylcyclopentane T methylpropylcyclopentane T methylpropylcyclopentane T total cyclopentanes cyclopentenes cyclopentene T 1-methylcyclopentene methylcyclopentene T dimethylcyclopentene T dimethylcyclopentene T ethylcyclopentene T dimethylcyclopentene T trimethylcyclopentene T propylcyclopentene T propylcyclopentene T ethylmethylcyclopentene T pentylcyclopentene T octylcyclopentene T total cyclopentenes

carbon number

canola oil/small

canola oil/large


5

0.47 ± 0.05

0.41 ± 0.05

0.38 ± 0.04

6 7 8 9 10 11 12 14 15 16

0.65 ± 0.02 0.47 ± 0.01 0.274 ± 0.004 0.27 ± 0.01 0.21 ± 0.01 0.21 ± 0.01 0.15 ± 0.03 0.33 ± 0.03 0.13 ± 0.02 0.056 ± 0.002

0.75 ± 0.11 0.58 ± 0.06 0.31 ± 0.03 0.31 ± 0.03 0.23 ± 0.02 0.22 ± 0.02 0.16 ± 0.01 0.36 ± 0.04 0.11 ± 0.01 0.051 ± 0.005

0.52 ± 0.10 0.39 ± 0.08 0.20 ± 0.04 0.18 ± 0.03 0.19 ± 0.02 0.23 ± 0.01 0.07 ± 0.01 0.17 ± 0.03 0.081 ± 0.005 0.027 ± 0.003

7 7 7 8 8 8 8 8 8 9 9 9

0.09 ± 0.01 0.049 ± 0.005 0.07 ± 0.01 0.10 ± 0.01 0.20 ± 0.01 0.030 ± 0.002 0.17 ± 0.02 0.040 ± 0.003 0.012 ± 0.004 0.05 ± 0.01 0.069 ± 0.002 0.144 ± 0.004 4.3 ± 0.2

0.09 ± 0.01 0.06 ± 0.02 0.10 ± 0.01 0.13 ± 0.01 0.27 ± 0.03 0.05 ± 0.01 0.20 ± 0.02 0.049 ± 0.004 0.02 ± 0.01 0.09 ± 0.01 0.077 ± 0.004 0.20 ± 0.03 4.8 ± 0.4

0.07 ± 0.01 0.03 ± 0.01 0.06 ± 0.01 0.12 ± 0.02 0.20 ± 0.05 0.02 ± 0.01 0.15 ± 0.04 0.04 ± 0.01 0.009 ± 0.002 0.04 ± 0.01 0.06 ± 0.01 0.12 ± 0.03 3.4 ± 0.5

5 6 6 7 7 7 7 8 8 8 8 10 13

0.5 ± 0.1 0.41 ± 0.06 0.091 ± 0.004 0.19 ± 0.02 0.10 ± 0.01 0.041 ± 0.004 0.16 ± 0.01 0.14 ± 0.01 0.148 ± 0.004 0.04 ± 0.01 0.13 ± 0.01 0.10 ± 0.01 0.07 ± 0.01 2.1 ± 0.3

0.34 ± 0.07 0.46 ± 0.12 0.095 ± 0.001 0.20 ± 0.01 0.102 ± 0.003 0.04 ± 0.01 0.17 ± 0.01 0.17 ± 0.01 0.137 ± 0.003 0.042 ± 0.002 0.14 ± 0.01 0.099 ± 0.001 0.09 ± 0.04 2.1 ± 0.2

0.7 ± 0.2 0.41 ± 0.01 0.11 ± 0.01 0.182 ± 0.003 0.098 ± 0.003 0.044 ± 0.004 0.15 ± 0.01 0.14 ± 0.01 0.133 ± 0.001 0.044 ± 0.002 0.13 ± 0.01 0.11 ± 0.01 0.08 ± 0.02 2.3 ± 0.2

a Data are presented as the mean ± standard deviation (n = 3). bT denotes tentative MS identification. All other species were confirmed with standards.

(Figure 3A), despite the difference in the double-bond patterns for the most abundant FAs in the canola and soybean oil reactants (Table 1). The isomer distribution pattern for cyclohexanes (Table 5) is similar to that of cyclopentanes (Table 4), i.e., with the prevalence of monoalkyl-substituted products among higher MW homologues. This observation indicates that high-MW cycloalkanes (both five- and six-membered) appear to be formed almost exclusively as the cyclization products of linear terminal alkenes, as shown in Scheme 2. As for the alternate, intermolecular Diels−Alder path, the formation of monoalkyl-substituted cyclic hydrocarbons (e.g., cyclohexenes, cyclohexanes, or monocyclic aromatic hydrocarbons), according to Scheme 1, would be possible only in two cases: (1) when the diene reactant was 1,3-butadiene or (2) when the alkene reactant was ethylene (non-substituted),

reacting with a second Diels−Alder reactant that, in both cases, had a terminal double bond.37 However, 1,3-butadiene was not detected, and ethylene was observed only in minor amounts among the TG cracking gaseous products (see Table S1 of the Supporting Information). Because other alkenes were formed in much greater amounts (Table 3), the Diels−Alder reaction should be ruled out as the major pathway for the formation of monoalkyl-substituted six-membered cyclic hydrocarbons. Another confirmation of the intramolecular path can be obtained by analyzing the cyclohexane homology pattern, which is shown in Figure 4B for the products recovered in quantifiable amounts. Additional information on detectable products comprising the chromatographic humps, i.e., those below the quantification limit, is provided in Table 2. Cyclohexanes were detected only up to 18 carbon atoms (quantified up to C17), i.e., with the alkyl substituents, up to C12, attached to a six-membered 678


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Table 5. Cyclohexanes and Cyclohexenes in OLP (mol % of GC Elutable)a feedstock/reactor scale cyclohexanes cyclohexane n-alkylcyclohexanes methylcyclohexane n-ethylcyclohexane n-propylcyclohexane Tb n-butylcyclohexane T n-pentylcyclohexane T n-hexylcyclohexane T n-heptylcyclohexane T n-octylcyclohexane T n-nonylcyclohexane T n-decylcyclohexane T other cyclohexanes methylenecyclohexane T cis-1,2-dimethylcyclohexane trans-1,2-dimethylcyclohexane cis-1,3-dimethylcyclohexane dimethylcyclohexane T trans-1,4-dimethylcyclohexane 1,1,2-trimethylcyclohexane ethylmethylcyclohexane T ethylmethylcyclohexane T dimethylethylcyclohexane T ethylpropylcyclohexane T ethylpropylcyclohexane T n-undecylcyclohexane T total cyclohexanes cyclohexenes cyclohexene T methylcyclohexene T methylcyclohexene T methylcyclohexene T dimethylcyclohexene T dimethylcyclohexene T dimethylcyclohexene T dimethylcyclohexene T ethylcyclohexene T ethylcyclohexene T propylcyclohexene T propylcyclohexene T total cyclohexenes

carbon number

canola oil/large

canola oil/small


6

0.302 ± 0.004

0.31 ± 0.05

0.22 ± 0.02

7 8 9 10 11 12 13 14 15 16

0.58 ± 0.02 0.42 ± 0.02 0.29 ± 0.01 0.20 ± 0.01 0.253 ± 0.002 0.146 ± 0.004 0.34 ± 0.02 0.14 ± 0.01 0.29 ± 0.02 0.12 ± 0.01

0.68 ± 0.08 0.52 ± 0.04 0.35 ± 0.01 0.23 ± 0.02 0.26 ± 0.02 0.14 ± 0.01 0.33 ± 0.01 0.12 ± 0.03 0.29 ± 0.01 0.09 ± 0.01

0.54 ± 0.07 0.34 ± 0.07 0.20 ± 0.04 0.13 ± 0.02 0.23 ± 0.03 0.18 ± 0.01 0.20 ± 0.03 0.08 ± 0.01 0.16 ± 0.01 0.10 ± 0.01

7 8 8 8 8 8 9 9 9 10 11 11 17

0.024 ± 0.002 0.089 ± 0.003 0.150 ± 0.004 0.05 ± 0.01 0.031 ± 0.003 0.008 ± 0.001 0.025 ± 0.002 0.16 ± 0.01 0.07 ± 0.01 0.043 ± 0.003 0.10 ± 0.02 0.104 ± 0.004 0.11 ± 0.02 4.04 ± 0.01

0.022 ± 0.004 0.10 ± 0.01 0.13 ± 0.11 0.05 ± 0.01 0.04 ± 0.01 0.01 ± 0.01 0.032 ± 0.003 0.23 ± 0.02 0.10 ± 0.01 0.042 ± 0.001 0.10 ± 0.01 0.11 ± 0.00 0.08 ± 0.02 4.36 ± 0.31

0.023 ± 0.004 0.09 ± 0.01 0.16 ± 0.03 0.04 ± 0.00 0.03 ± 0.01 0.002 ± 0.003 0.024 ± 0.001 0.15 ± 0.03 0.07 ± 0.02 0.040 ± 0.002 0.07 ± 0.02 0.10 ± 0.01 0.07 ± 0.02 3.23 ± 0.37

6 7 7 7 8 8 8 8 8 8 9 9

0.32 ± 0.03 0.09 ± 0.01 0.061 ± 0.002 0.37 ± 0.04 0.12 ± 0.01 0.02 ± 0.01 0.08 ± 0.01 0.02 ± 0.01 0.072 ± 0.003 0.08 ± 0.08 0.13 ± 0.03 0.05 ± 0.01 1.4 ± 0.2

0.27 ± 0.02 0.10 ± 0.01 0.08 ± 0.01 0.41 ± 0.02 0.133 ± 0.005 0.032 ± 0.002 0.082 ± 0.003 0.030 ± 0.002 0.07 ± 0.01 0.14 ± 0.01 0.13 ± 0.01 0.03 ± 0.002 1.52 ± 0.07

0.31 ± 0.03 0.095 ± 0.004 0.06 ± 0.01 0.38 ± 0.01 0.130 ± 0.004 0.030 ± 0.002 0.089 ± 0.004 0.031 ± 0.001 0.07 ± 0.01 0.10 ± 0.06 0.10 ± 0.01 0.05 ± 0.01 1.45 ± 0.08

Data are presented as the mean ± standard deviation (n = 3). bT denotes tentative MS identification. All other species were confirmed with standards. a

intermediates that transformed into either cyclohexanes or aromatics. Thus, only some cyclohexenes, those present in large abundance, survived. Hydrogen essential for cycloalkane formation was observed in the gas phase. The low hydrogen mass concentrations listed in Table S1 of the Supporting Information, 0.3%, increase near 20-fold when molar concentrations are considered. In addition, the high concentration of CO in the gas-phase products (37−43 mass % compared to that of CO2, 27−30%; see Table S1 of the Supporting Information) is evidence that more hydrogen is actually formed than is measured because the most likely source of CO is CO2 produced as a result of FA decarboxylation. This conversion would require equimolar amounts of H2. Hydrogen appears to be formed as a result of the aromatization of the

ring, yielding the maximum total of C18. This observation is consistent with the intramolecular formation pathway because this number of carbon atoms matches that in the original FAs present in TGs upon decarboxylation (Table 1). The observed relatively high abundance of nonylcyclohexane (C15; Figure 4B) was expected because its parent FA, upon decarboxylation, could be C16, i.e., palmitic acid, a common saturated component of TGs. Because of the lack of double bonds, palmitic acid is not as prone to cracking as other parent FAs. The concentration of cyclohexenes was smaller than that of cyclohexanes (Figure 4B). This feature indicated that cyclohexenes were not particularly stable within the cracking mixtures. Through their subsequent dehydrogenation, most of the cyclohexene molecules formed most likely served as 679


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Table 6. Aromatic Hydrocarbons in OLP (mol % of GC Elutable)a feedstock/reactor scale benzene toluene trimethylbenzene Tb methylpropylbenzene T total unclassifiedc ortho-substituted benzenes o-xylene 1-methyl-2-ethylbenzene 1,2-diethylbenzene 1-methyl-2-propylbenzene 1-hexyl-2-methylbenzene 1-methyl-2-isopropylbenzene total meta- and para-substituted benzenes m-xylene p-xylene 1,2,4-trimethylbenzene 1,3,5-trimethylbenzene 1-ethyl-3-methylbenzene 1-ethyl-4-methylbenzene 4-ethyl-1,2-dimethylbenzene 2-ethyl-1,4-dimethylbenzene 1-methyl-3-propylbenzene 1-isopropyl-4-methylbenzene total monoalkylbenzenes isopropylbenzene n-propylbenzene n-butylbenzene n-pentylbenzene n-hexylbenzene n-heptylbenzene n-octylbenzene n-nonylbenzene total total aromatics

carbon number

canola oil/small

canola oil/large


6 7 9 10

0.28 ± 0.02 0.83 ± 0.01 0.118 ± 0.001 0.22 ± 0.03 1.45 ± 0.03

0.36 ± 0.02 1.13 ± 0.13 0.14 ± 0.01 0.15 ± 0.12 1.8 ± 0.2

0.28 ± 0.08 0.83 ± 0.23 0.11 ± 0.02 0.18 ± 0.02 1.4 ± 0.3

8 9 10 10 13 10

0.41 ± 0.01 0.21 ± 0.01 0.050 ± 0.003 0.195 ± 0.002 0.074 ± 0.004 0.014 ± 0.001 0.95 ± 0.02

0.52 ± 0.08 0.24 ± 0.04 0.05 ± 0.01 0.19 ± 0.02 0.08 ± 0.01 0.02 ± 0.00 1.09 ± 0.13

0.39 ± 0.10 0.19 ± 0.05 0.09 ± 0.10 0.15 ± 0.06 0.10 ± 0.01 0.02 ± 0.00 0.93 ± 0.31

8 8 9 9 9 9 10 10 10 10

0.24 ± 0.02 0.11 ± 0.01 0.13 ± 0.01 0.02 ± 0.01 0.174 ± 0.001 0.09 ± 0.01 0.07 ± 0.01 0.036 ± 0.002 0.085 ± 0.004 0.067 ± 0.003 1.0 ± 0.1

0.34 ± 0.05 0.15 ± 0.02 0.15 ± 0.02 0.03 ± 0.01 0.27 ± 0.05 0.12 ± 0.02 0.08 ± 0.01 0.04 ± 0.01 0.11 ± 0.02 0.08 ± 0.01 1.4 ± 0.2

0.25 ± 0.05 0.12 ± 0.03 0.12 ± 0.02 0.01 ± 0.01 0.16 ± 0.04 0.09 ± 0.03 0.07 ± 0.01 0.03 ± 0.01 0.10 ± 0.02 0.05 ± 0.01 1.0 ± 0.2

9 9 10 11 12 13 14 15

0.08 ± 0.01 0.34 ± 0.01 0.12 ± 0.00 0.16 ± 0.04 0.13 ± 0.01 0.13 ± 0.01 0.13 ± 0.01 0.08 ± 0.01 1.54 ± 0.02 3.5 ± 1.3

0.10 ± 0.01 0.41 ± 0.04 0.16 ± 0.02 0.21 ± 0.03 0.13 ± 0.04 0.12 ± 0.04 0.11 ± 0.02 0.08 ± 0.00 1.80 ± 0.24 3.1 ± 0.5

0.08 ± 0.01 0.34 ± 0.05 0.15 ± 0.06 0.26 ± 0.03 0.17 ± 0.05 0.13 ± 0.02 0.08 ± 0.01 0.06 ± 0.01 1.61 ± 0.34 2.7 ± 0.6

Various types of isomers are grouped according to their substitution pattern in the benzene ring. Data are presented as the mean ± standard deviation (n = 3). bT denotes tentative MS identification. All other species were confirmed with standards. cTentatively identified analytes (with unknown positions of functional groups) as well as benzene and toluene that cannot be classified.

a

OLP components (see the next two sections) and formation of alkenes. As shown in Figure 4 and Table 3, the cycloalkenes recovered among the cracking products were smaller in size (i.e., average number of carbon atoms in their molecules) than the recovered cycloalkanes. This trend, particularly pronounced for six-membered cyclic hydrocarbons (Figure 4B), indicates that alkylcycloalkenes with long substituents may participate in subsequent “downstream” reactions (e.g., PAH formation; see below), altering their molecular size distribution. Monocyclic Aromatics (Alkylbenzenes). In contrast to the previously considered cyclic products, a much broader range of commercial standards was available for aromatic hydrocarbons, enabling their accurate identification (especially that of low-MW aromatics). The way the identification was performed was explained in the Experimental Section (cf. Figure 2). Thus, we were able to differentiate with certainty the positional isomers among the aromatic products (Table 6). The differentiation of positional aromatic isomers was used to match the resulting isomer distribution with the projected

substitution pattern characteristic for intermolecular (Diels− Alder; Scheme 1) and intramolecular (Scheme 2) pathways. The Diels−Alder path (Scheme 1) would mostly yield polysubstituted products, particularly, 1,2,3,4-tetraalkyl or 1,2,3-trialkyl and 1,2,4-trialkyl derivatives if one of the reactants possessed a terminal double bond.37 If the dienophile was ethylene, the presence of para-substituted products was expected. If both the reacting diene and dienophile featured terminal double bonds, a near-random mixture of ortho and meta isomers was expected. In contrast, the intramolecular path (Scheme 2) was expected to yield monoalkyl- and orthodialkylbenzenes (particularly, ortho-methylalkyl derivatives if the endocyclization of terminal alkenes made a sizable contribution). Similar to cycloalkanes as discussed in the previous section, the experimentally observed isomeric distribution of the aromatic hydrocarbon products in the OLP from TG cracking was significantly biased toward Scheme 2, thus presenting clear evidence for the intramolecular path of cyclization as the major route. All three groups of aromatic isomers (monoalkyl-, ortho-, and then para-, meta-, and polysubstituted) were present 680


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Table 7. PAHs in OLP (mol % of GC Elutable)a feedstock/reactor scale total indanes/indenes indane methylindane Tb methylindane T methylindane T dimethylindane T dimethylindane T dimethylindane T dimethylindane T dimethylindane T ethylindane T ethylindane T indene trimethylindene T octahydroindene T total naphthalenes/fluorenes fluorene T methylfluorene T naphthalene tetrahydronaphthalene T methylnaphthalene T methylnaphthalene T dimethylnaphthalene T dimethylnaphthalene T ethylnaphthalene T ethylnaphthalene T isopropylnaphthalene T total total PAHs

carbon number

canola oil/small

canola oil/large


9 10 10 10 11 11 11 11 11 11 11 9 12 9

0.32 ± 0.01 0.31 ± 0.02 0.19 ± 0.01 0.19 ± 0.02 0.027 ± 0.001 0.12 ± 0.02 0.064 ± 0.001 0.23 ± 0.01 0.074 ± 0.004 0.126 ± 0.002 0.08 ± 0.01 0.036 ± 0.001 0.07 ± 0.01 0.08 ± 0.02 1.9 ± 0.1

0.33 ± 0.04 0.32 ± 0.04 0.20 ± 0.02 0.21 ± 0.05 0.03 ± 0.00 0.12 ± 0.01 0.07 ± 0.01 0.23 ± 0.01 0.07 ± 0.01 0.14 ± 0.01 0.07 ± 0.02 0.04 ± 0.01 0.10 ± 0.01 0.10 ± 0.01 2.0 ± 0.2

0.29 ± 0.03 0.26 ± 0.02 0.18 ± 0.01 0.17 ± 0.01 0.027 ± 0.005 0.115 ± 0.002 0.06 ± 0.02 0.22 ± 0.01 0.07 ± 0.00 0.14 ± 0.01 0.06 ± 0.01 0.03 ± 0.01 0.04 ± 0.02 0.07 ± 0.02 1.7 ± 0.1

13 14 10 11 11 11 12 12 12 12 13

0.10 ± 0.01 0.06 ± 0.01 0.18 ± 0.00 0.20 ± 0.01 0.13 ± 0.04 0.12 ± 0.04 0.04 ± 0.01 0.10 ± 0.03 0.13 ± 0.01 0.075 ± 0.004 0.044 ± 0.002 1.2 ± 0.1 3.1 ± 0.1

0.085 ± 0.003 0.07 ± 0.01 0.21 ± 0.03 0.18 ± 0.01 0.06 ± 0.01 0.18 ± 0.03 0.06 ± 0.02 0.09 ± 0.03 0.12 ± 0.05 0.09 ± 0.01 0.048 ± 0.004 1.2 ± 0.2 3.2 ± 0.4

0.07 ± 0.01 0.03 ± 0.01 0.14 ± 0.04 0.19 ± 0.03 0.14 ± 0.03 0.138 ± 0.003 0.030 ± 0.003 0.06 ± 0.02 0.11 ± 0.02 0.06 ± 0.01 0.028 ± 0.003 1.0 ± 0.1 2.7 ± 0.2

a Data are presented as the mean ± standard deviation (n = 3). bT denotes tentative MS identification. All other species were confirmed with standards.

Table 8. Relative Occurrence of Non-oxygenated Products (Hydrocarbons) in the OLPs Obtained upon Thermal Cracking tristearina

feedstock

trioleina

canola oil

soybean oil

mean temperature (°C)

377

400

458

430

time of cracking (min)

10

10

10

10

90−150

90−150

63 NDc ND 35 2.45 ND ND ND

65 ND ND 35 ND ND ND ND

21 1.6 0.07 65 17 3.4 0.54 0.06

5.5 2.5 0.11 57 13 4.7 1.34 traces

53.1 ± 1.1 12.7 ± 0.9

51.1 ± 1.3 11.2 ± 0.8

18.2 ± 0.3

19.8 ± 0.9

4.0 ± 0.6 6.9 ± 0.5 5.0 ± 0.5

5.1 ± 1.1 7.8 ± 1.4 5.0 ± 0.1

productsb linear alkanes cycloalkanes bicycloalkanes alkenes dienes cycloalkenes aromatics PAHs

a The reported data were obtained in preliminary experiments. bThe data are normalized to exclude the oxygenated products, which are different at different reaction times, i.e., primarily tri-, di-, and monoglycerides and high-MW FAs for triolein and tristearin (10 min cracking time), as opposed to short-chain FAs for canola and soybean oils (>1 h cracking time). cND denotes not detected.

carbons. In contrast, the concentration of polysubstituted aromatics, the main products expected from the Diels−Alder reaction, was rather small (ca. 3%), thus confirming that the Diels−Alder reaction cannot be viewed as a major cyclization path. The Diels−Alder path cannot be ruled out completely. It may be important in the formation of smaller molecules when lining up the reactants is more likely (because of steric considerations), whereas longer, more flexible molecules are

among the TG cracking products (Table 5). However, with the exception of two low-MW homologues (benzene and toluene) and two tentatively identified products (Table 5), the combination of n-alkylbenzenes and ortho-substituted methyl alkylbenzenes prevailed in all aromatic hydrocarbons recovered as TG cracking products. The remaining aromatics were metaand para-methylalkylbenzenes (e.g., alkyl toluenes) present in smaller amounts that could be produced as a result of isomerization of the corresponding ortho-substituted hydro681


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Low concentrations (below the FID quantification limit) of monoalkylphenols (forming a full homology series) were detected (Table 2). This was expected because water is known to be formed during TG thermal cracking.18 Water may react with double bonds of cyclic intermediates followed by aromatization. PAHs. The distribution pattern of quantifiable PAH products in the OLP generated during TG cracking is shown in Table 7. Scheme 3 illustrates how these bi- and tricyclic compounds could be formed via intramolecular cyclization (shown for six-membered bi- and tricyclic PAHs; a similar path for indanes is not shown). The essential intermediates, monoalkyl- and ortho-methyl-substituted six-membered monocyclic radicals shown in Scheme 3, were assumed to be abundant on the basis of the occurrence of the corresponding hydrocarbons (Tables 4−6). According to the literature, intramolecular cyclizations into polycyclic products are possible either when an intermediate featuring a monocyclic ring contains non-conjugated double bonds or when it is fully aromatic.25,27,38 Recovery of the full homology series of bicyclic PAHs (i.e., naphthalenes and indanes) identified by MS (Table 2) corroborates the intramolecular mechanism of cyclization. This conclusion is based on the predominant number of carbon atoms, less than 18−20, thus matching their count in the original TG FAs. Unlike the corresponding monocyclic hydrocarbons, long-chain alkyl-substituted PAHs were less abundant (at least 10-fold), possibly because the number of carbon atoms in the cyclic product approached that in the original FAs of triglycerides. The observed PAH profile was rather selective, being skewed toward compounds with lower numbers of condensed rings (predominantly bi- and tricyclic). This information may be of interest for envisioning the initial (low temperature) steps of PAH formation upon fat/oil and/or hydrocarbon incomplete

Figure 4. Molecular size distributions of (A) five-membered and (B) six-membered cycloalkanes and cycloalkenes. The numerical values presented were obtained by summing the mole percentages of pertinent isomers having the same number of carbon atoms. The data for canola oil, large reactor are shown as representative for all reactors.

subject to intermolecular cyclizations. A significant fraction of benzene and toluene among the products (Table 6) indicates that the intermolecular reaction may be a minor path for the formation of low-MW cyclic compounds.

Scheme 3. Six-Membered PAH Formation via Intramolecular Radical Cyclizations

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combustion. For this purpose, a comparison can be made with other PAH sources differing in their formation temperatures. Three-ring PAHs, rather than two-ring PAHs, are dominant in diesel fuel and are believed to be produced during hightemperature petroleum cracking.39 Furthermore, four- and fivering PAHs are predominant in diesel exhaust PM, i.e., even higher temperature combustion.39 Thus, the number of PAH rings increases with the temperature. Note that the number of carbon atoms, in up to five rings, is below 18−20, i.e., sufficient for intramolecular cyclization of alkenyl radicals formed upon thermal cracking. Apparently, at the relatively low temperatures used in our TG cracking experiments, entropy plays a significant role in defining the size of the cyclic products, whereas at higher combustion temperatures, entropy effects are less pronounced. Recently, the formation of naphthalene and methyl naphthalenes was reported for zeolite-catalyzed TG cracking via an intramolecular cyclization.32 This observation is consistent with the results obtained in the current study in terms of the apparent dominance of intramolecular cyclization mechanisms in thermal methods of biofuel production. However, one has to keep in mind that the mechanisms of thermal (free radical) and acid-catalyzed (electrophilic) TG cracking reactions are fundamentally different. Fully and partially hydrogenated analogues of the abovediscussed PAHs were also identified in the current study, e.g., tetra- and pentahydronaphthalene. Thus, both hydrogenation and further dehydrogenation of unsaturated intermediates were possible, as shown in Scheme 3. On the basis of much lower amounts of hydrogenated PAHs, e.g., tetrahydronaphthalene (Table 7), compared to regular PAHs, dehydrogenation of radical intermediates to form highly conjugated PAHs was more favorable, as expected. Comparison of the Molecular Size Distribution Profiles for Cyclic Products and Alkenes. The experimental information discussed in the previous sections suggests an explanation for the formation of cyclic TG cracking products via the intramolecular cyclization of alkenyl radicals. However, the quantitative information discussed thus far has not included the precursors of cyclic species, i.e., alkenes. Additional mechanistic insights can be obtained by focusing on the size of cyclic molecules formed with respect to the size of their precursor alkenes. Figure 5 presents the molecular size distribution profiles of the cyclic products grouped according to the specific size and number of their ring(s) matched with the homology profile of alkenes. The comparison of these two size distribution patterns shows that low- and high-MW homologues were more abundant among the alkenes than among the cyclic hydrocarbons. This trend was similar for all three experimental subsets, cf. Figure 2, which suggests that those alkenyl radicals that were either too small or too large were more likely to stabilize as alkenes than to form cyclic products. This partial regioselectivity of cyclization may be due to entropy/probability reasons, as suggested in the previous section while discussing PAH size. Apparently, “longer” alkenyl radicals, which exhibit multiple conformations, are too flexible. Therefore, only a few of them are eligible for cyclization. At the opposite end of the size distribution range, “shorter” alkenyl radicals possessed too few combinatorial options to form fiveand six-membered cyclic products [zero for C4, one for C5, two (exo- and endo-) for C6, three for C7 (exo- and endo- for both 1- and 2-alkenes), etc.].

Figure 5. Molecular size distributions of cycles (combined PAHs and monocycles) and their apparent parent species (alkenes). The numerical values presented were obtained by summing the mole percentages of all pertinent products having the same number of carbon atoms. The data are shown for the mean values of all nine runs, with both feedstocks and both reactors.

Using the same entropy/probability arguments, if the primary process of cyclic product formation was the Diels− Alder or any other cycloaddition reaction, the pool of alkenes would be depleted rather than enriched with shorter alkenes because smaller molecules are more likely to line up properly in intermolecular reactions. While the combined pool of cyclopentanes and cyclopentenes peaked at C5−C8 and their homology profiles were similar to that of the parent linear alkenes, Figures 4A and 5, both six-membered monocyclic hydrocarbons and PAHs exhibited maxima at greater numbers of carbon atoms in their structures (Figure 5). This feature corroborates the hypothesis that alkenyl radicals smaller than C6−C7 tend to stabilize as alkenes rather than forming cyclic hydrocarbons. For instance, the exocyclization to form six-membered cyclic structures would require a minimum of seven carbon atoms in the parent terminal alkene (Scheme 2). If the parent alkene was not terminal, the minimum size of the precursor of the cyclic product would further increase. The observed homology pattern of five-membered cyclic products is similar to that of alkenes but dissimilar to that of sixmembered cyclic products (Figures 4 and 5). The formation of smaller size five-membered hydrocarbons compared to sixmembered products can be explained by the difference between endo- and exocyclizations, respectively. Endocyclizations, essential for the formation of five-membered cyclic products (Scheme 2), are known to be favored kinetically,30 thus in part offsetting entropy limitations and yielding a greater fraction of smaller molecules. Also, the formation of five-membered cyclic hydrocarbons via the endocyclization of alkenyl radicals is known to be a fast and reversible process.39 Thus, while fivemembered monocyclic products were near equilibrium with their parent alkenes having the matching number of carbon atoms, some of their six-membered isomers could be converted into polycyclic species, thus skewing the resulting molecular size distributions. This observation was corroborated by the absence of five-membered bicyclic hydrocarbons among the 683


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products (unlike six-membered bicyclic hydrocarbons, e.g., naphthalenes). Finally, the observed homology patterns corroborate the hypothesis that alkenes are the origin of monocyclic hydrocarbons, whereas six-membered cyclic products are the source of PAHs. Figure 5 shows that the molecular size distributions of alkenes and six-membered cyclic products were complementary within the C6−C9 range. Similarly, those of six-membered monocyclic products and PAHs were complementary within the C9−C11 range. Just as the pool of alkenes was enriched with those of lower MW compared to six-membered cyclic hydrocarbons, the pool of six-membered cyclic products, the presumed source of PAHs, was enriched with those that are too short to form PAHs, i.e., below C9 (Figure 5). Cracking of Individual TGs and Crambe Oil. To confirm the mechanistic conclusions obtained upon the analysis of crop oil (i.e., TG mixtures) cracking products, select experiments were conducted, in a smaller 5 mL reactor (because of high costs of individual TGs), with tristearin (no double bonds) and triolein (one double bond per each of the three oleic acid residues). The results are presented in Table 8. Note that, as specified in the footnotes of Table 8, these are preliminary results, which did not allow for accurate mass balance closure, thus providing a semi-quantitative product profile. The first goal of these experiments was to confirm our hypothesis that alkenyl radicals, as opposed to just alkenes, are essential for the formation of cyclic hydrocarbons. Indeed, tristearin cracking yielded a sizable concentration of alkenes (mostly, 1heptadecene), presumably obtained via dehydrogenation of the corresponding terminal C17 alkyl radical formed as a result of stearic acid decarboxylation. However, no cyclic products were observed. The abundant presence of dienes in this OLP did not make any difference, as consistent with the mechanism suggested in this study but inconsistent with the Diels−Alder path. In contrast, the entire spectrum of cyclic hydrocarbons, similar to that obtained with crop oils, was observed upon triolein cracking. The extracted ion chromatograms shown in Figure 6 further underscore the similarity of aromatic hydrocarbon profiles in cracking products of crop oils and triolein and their dissimilarity to those of tristearin. The spikes that could be perceived as “peaks” in the tristearin profile (Figure 6c) are due to an irreproducible noise, which becomes significant at a high sensitivity selected to prove the lack of aromatic hydrocarbons in these samples (note the different scales used in panels a−c of Figure 6). Both the homology and isomer profiles of triolein and canola oil OLPs were similar (panels a and b of Figure 6). Similar profiles were observed for crambe oil cracking products peaking at C8−C9 for aromatics (not shown). The only difference observed was that, for this feedstock, dominated by longer, predominantly C20 FAs, the homology profiles were broader, yielding a slightly greater fraction of higher MW aromatics and other cyclic hydrocarbons, as expected. The observation of a key role of alkenyl radicals in hydrocarbon product processing after TG cracking corroborates the findings by Al-Amrousi et al.,40,41 who attempted to accelerate TG thermal cracking by adding phenols as a source of free radicals. However, these radicals may also serve as scavengers of more reactive radicals, e.g., alkenyl radicals essential for the formation of cyclic products. As a result, the most abundant products obtained in the work by Al-Amrousi et al. were the uncracked C16 and C18 FAs comprising the initial TGs, with little aromatics, i.e., a product composition in

Figure 6. GC−MS extracted ion chromatogram with a m/z of 105 representative for alkylbenzenes for the cracking products of (a) canola oil, (b) triolein, and (c) tristearin.

between the canola/soybean cracking products and those of tristearin. The second goal of experiments on triolein cracking was to lower the total reaction time by taking advantage of a fast heating and cooling regime available for the small reactor used. As a result of this arrangement, a significant fraction of uncracked tri-, di-, and monoglycerides was observed (not shown). The rest of the products were dominated by alkenes, with cyclic products present in smaller amounts than in the experiments with crop oils; the PAHs were barely detectable (Table 8). The only exception was cycloalkenes present in nearly equal percentages in crop oil and triolein cracking products. This observation corroborates the process mechanism proposed in the previous section, which could be formulated in its final form as follows. Alkenyl radicals and alkenes are formed first, as a result of FA decarboxylation and cracking. Then, as long as the products do not undergo significant aromatization and hydrogenation (which are coupled via hydrogen formation), high-temperature cyclization processes progress in time to yield, first, monocyclic and, then, polycyclic hydrocarbons (along with polymerization, leading to coke and tar). Cycloalkenes are key intermediates of this process; the ensuing aromatization of six-membered cyclic hydrocarbons and hydrogenation of alkenes (including both five- and sixmembered cycloalkenes) stabilizes the final product mixture. Importance for Commercial Biofuels. In general, the presence of 5−20 wt % cyclic hydrocarbons, including aromatics, in transportation fuels improves operational performance, although this varies by engine type and the specific compression ratio and ignition characteristics of the engine. When cyclic compound concentrations become too high, fuel burning may not be complete. As a result, PAH generation may be observed.42,43 In this respect, the findings of this study, that five-membered cyclic hydrocarbons, which are abundant in the OLP of crop oil thermal cracking, are not as prone to forming PAHs as their six-membered analogues, may 684


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be of interest. However, only future engine testing can lead to verification that cycle size significantly affects the operational and environmental performance of a fuel.

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ASSOCIATED CONTENT

S Supporting Information *

Table S1 on gas-phase analysis and related details of the analytical protocol used. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 701-777-2145. Fax: 701-777-2331. E-mail: jkozliak@ chem.und.edu. Present Address §

Tarbiat Modares University, Jalal Ale Ahmad Highway, Post Office Box 14115-111, Tehran, Iran.

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ACKNOWLEDGMENTS Funding was provided by the North Dakota Agricultural Production Utilization Commission, the North Dakota State Board of Agricultural Research, the North Dakota Soybean Council, and the National Science Foundation (Grants EPS081442 and EPS-0447679).

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REFERENCES

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