Characterization of Heteroatom-Containing Compounds in Thermally

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Characterization of Heteroatom-Containing Compounds in Thermally Cracked Naphtha from Oilsands Bitumen Yuan Rao and Arno de Klerk* Department of Chemical and Materials Engineering, University of Alberta, 9211 - 116th Street, Edmonton, Alberta T6G 1H9, Canada S Supporting Information *

ABSTRACT: Detailed characterization of feed materials can assist with development of new technology and troubleshooting existing units. In this work a methodology for the compound specific analysis of oxygen-, sulfur-, and nitrogen-containing compounds in naphtha is presented. It was applied to the analysis of industrial thermally cracked naphtha produced from Athabasca oilsands bitumen, with 0.25 wt % O, 0.90 wt % S, and 0.09 wt % N. The main oxygen-containing compound classes were acyclic carboxylic acids, ketones, and phenols. Interestingly, the carboxylic acids were almost exclusively linear and branched species in the C3−C11 range. These compounds could be explained by a thermal cracking pathway involving ring-opening of one or more adjacent naphthenic rings. The main sulfur-containing compound class was cyclic thioethers, with a minor amount of thiols. No thiophenes were identified and identification of the most abundant compound, 2-methyl tetrahydrothiophene, was confirmed with an authentic compound. This suggested that the hydrogen transfer during thermal cracking of oilsands derived material was high; the naphtha also had a correspondingly low aromatic content. The main nitrogen-containing compound class was pyridines, with a minor amount of pyrroles. for the partial upgrading of bitumen in a field upgrader.12,13 Our interest in the nature of the heteroatom-containing species was related to the downstream processing of the thermally cracked naphtha for olefins removal by olefin-aromatic alkylation.14 A methodology for the compound specific analysis of oxygen-, sulfur-, and nitrogen-containing compounds in naphtha is presented. Initial work on the identification and quantification of nitrogen-containing compounds was previously presented.15 The pre-separation and analytical methodology is applicable to naphtha-range materials in general, although this claim was not rigorously demonstrated.

1. INTRODUCTION Refining technologies have feed specifications to indicate whether a candidate feed material will be a suitable feed for the technology or not. When a feed material is not suitable, it is often related to compounds that would prematurely deactivate the catalyst and the feed has to be pretreated to remove such contaminants before it can be used. For example, it is standard practice in petroleum refineries to perform naphtha hydrotreating as a pretreatment step before catalytic naphtha reforming.1 Yet, irrespective of the refining technology and its limitations, it is necessary to characterize the feed material. In this work naphtha refining is of interest. Although naphtha is routinely characterized as part of refinery operations, detailed and compound specific characterization is seldom performed. For most applications it is not necessary to know the compound specific composition of naphtha, but when one wants to develop new technology or troubleshoot an existing unit, compound specific characterization is valuable. Studies that reported detailed naphtha characterization mainly used chromatographic separation in association with analysis using different detectors.2−9 Preseparation can be used to simplify the matrix by grouping specific compound classes together by extracting specific compound classes based on solubility and/or reaction.9,10 Once detailed characterization is available, spectroscopic methods can be used to replace chromatography for analysis, or to provide a more rapid way of tracking groups of compounds in the naphtha.11 Still, the detailed characterization of naphtha is onerous, because it is a complex mixture. In most refinery naphtha streams, not all oxygen-, sulfur-, and nitrogen-containing compound classes are considered, but in our work it was important due to the origin of the naphtha and the application being considered. The material studied was thermally cracked naphtha derived from oilsands bitumen. Thermal cracking is one of the conversion processes required © XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. The thermally cracked naphtha with boiling point distribution from 30 to 288 °C (Figure 1) was obtained from an industrial visbreaking unit that processed deasphalted oil. The deasphalted oil was obtained by pentane solvent deasphalting of bitumen that was produced by steam assisted gravity drainage from an oilsands deposit in the Athabasca region. A flow diagram of the Long Lake facility where the naphtha was produced can be found in the literature.16 Macroscopic characterization of the thermally cracked naphtha is presented in Table 1. These values are presented for reference and the characterization did not form part of the present study. Of particular relevance to this study is that the thermal cracked naphtha contained nitrogen 0.09 wt %, sulfur 0.90 wt %, and oxygen 0.25 wt % on a dry basis. Identification of the dienes in the naphtha was conducted and reported as a separate study.17 The chemicals used in this study (Table 2) were commercially obtained. Water used during the investigation was purified by a Millipore Milli-Q 5 Integral 5 system. The chemicals that were used as Received: June 8, 2017 Revised: August 11, 2017

A

DOI: 10.1021/acs.energyfuels.7b01646 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

reasons for selecting the oxygen- and sulfur-containing compounds are explained in Section 3. The selection and use of the nitrogencontaining compounds was described previously.15 2.2. Procedure. The procedure that was developed to characterize of the naphtha entailed two preseparation steps to produce four subfractions of the naphtha. a. Preseparation Step 1 (Base Pretreatment). The first compound class that had to be removed selectively was the acid fraction. This was performed using a base pretreatment using nickel(II) carbonate. During the development of the procedure other bases were tried,15 and nickel(II) carbonate was selected because it worked best. (A more aggressive alternative procedure employing 10 M NaOH was reported for analysis of biomass pyrolysis products.)10 The naphtha (200 mL) was mixed with nickel(II) carbonate (1.231 g, 10.378 mmol) and the reaction mixture was stirred overnight at room temperature, taking care that evaporative loss is prevented. The reaction was completed when the green nickel(II) carbonate completely turned into a brown color.18 Reaction at room temperature was not an arbitrary decision. As the reaction temperature is increased, an increasing amount of phenolic compounds are also removed with the carboxylic acids.15 The reaction mixture was then filtered. The acid free naphtha and solid residue containing the nickel−acid reaction product were collected separately. The base pretreatment procedure to remove acidic compounds from the naphtha is illustrated by Figure 2.

Figure 1. Distillation profile of the thermally cracked naphtha by simulated distillation following ASTM D 7169.

Table 1. Characterization of Bulk Properties of the Thermally Cracked Naphtha property

thermally cracked naphtha

Density at 20 °C (kg/m ) Elemental composition (wt %) Carbon Hydrogen Nitrogen Sulfur Oxygen Carbon distribution (mol %)a Aliphatic Aromatic Hydrogen distribution (mol %)b Aliphatic Saturated Aliphatic Olefinic Aromatic Olefin Content (as wt % 1-decene)b Acid Number (mg KOH/g) 3

762.7 83.34 13.76 0.09 0.90 0.25 91.49 8.51 96.42 1.26 2.32 4.7 0.12

a

Figure 2. Naphtha preseparation step 1, which is a base pretreatment to remove acids.

13

Analysis by carbon-13 nuclear magnetic resonance ( C NMR) spectroscopy. bAnalysis by proton magnetic resonance (1H NMR) spectroscopy.

The solid residue containing the nickel−acid reaction product was washed with n-heptane (50 mL). Then the residue was acidified with 1 N HCl (16 mL) and extracted with diethyl ether (3 × 20 mL), followed by washing with water (2 × 20 mL). The diethyl ether was dried with MgSO4. The dried diethyl ether contained the acid compounds from the naphtha. The diethyl ether was evaporated under reduced pressure and the remaining material was dissolved in acetone (1.5 mL) for analysis of acidic components. b. Preseparation Step 2 (Column Chromatography). The bulk of the naphtha consisted of hydrocarbons that did not contain heteroatoms. These compounds had to be removed to reduce the complexity of the mixture before analysis. A glass column containing silica gel (30 g) as the stationary phase was employed to perform column chromatography. The acid free naphtha (195−200 mL) was passed over the silica gel in the column. The solvents used for elution were selected based on polarity and to have low normal boiling point temperatures (Tb): n-pentane (Tb = 36 °C), dichloromethane (Tb = 40 °C), and acetone (Tb = 56 °C). In succession, the column was eluted with n-pentane (80 mL), dichloromethane (420 mL), and acetone (240 mL), each fraction being collected separately, as shown in Figure 3. The straight through naphtha fraction and n-pentane eluted naphtha fraction was collectively considered the hydrocarbon matrix. The dichloromethane and acetone fractions were concentrated by removing the solvents under reduced pressure, before the concentrated fractions were analyzed. There was a small risk that some volatile hetroatom-containing compounds were lost with the solvents.

Table 2. List of Chemicals a

mass fraction purityb

compound

formula

CASRN

nickel(II) carbonate silica gel (63−200 μm) diethyl ether

NiCO3

3333−67−3

0.98

ACROS

supplier

SiO2

112926−00−8

-

C4H10O

60−29−7

0.999

hydrochloric acid n-heptane

HCl(aq) C7H16

7647−01−0 142−82−5

1.0 Nc 0.99

n-pentane

C5H12

109−66−0

0.997

dichloromethane

CH2Cl2

75−09−02

0.999

acetone

C3H6O

67−64−1

0.997

Fisher Scientific Fisher Scientific ACROS Fisher Scientific Fisher Scientific Fisher Scientific Fisher Scientific

a

CASRN = Chemical Abstracts Services Registry Number. bThis is the purity of the material guaranteed by the supplier; material was not further purified. cConcentration based on H+ activity. authentic compounds to confirm identification of products during the study are listed as Table S1 in the Supporting Information. The B


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Energy & Fuels Table 3. Identification and Relative Abundance of Compounds in the Acid Fraction, as Shown in the Chromatogram in Figure 4

Figure 3. Naphtha preseparation step 2, which entailed column chromatography to separate the heteroatom-containing compounds from the hydrocarbon matrix. 2.3. Analyses. The analysis of the different fractions in this study was performed using gas chromatography coupled with mass spectrometry (GC-MS). An Agilent 7820A gas chromatograph with Agilent 5977E mass selective detector was employed for compound identification. Sample separation was achieved with Agilent 19091S433 column (length: 30 m, diameter: 250 μm, film thickness: 0.25 μm, max temperature 325 °C). Helium was the carrier gas with flow of 1 mL/min. The split ratio was 1:100. The initial temperature was 80 °C, which was increased to 170 °C with a ramp rate of 6 °C/min and then increased to 300 °C with a ramp rate of 15 °C/min and held at 300 °C for 5 min. This study was primarily concerned with identifying the oxygen-, sulfur-, and nitrogen-containing compounds in the naphtha. The relative abundance of compounds based on their GC-MS peak area should be considered as semiquantitative only. An indication of the overall abundance of each heteroatom, irrespective of compound class, can be found in Table 1.

a

label

retention time (min)

compound

peak area (%)

a a to b b b to c c c to d d d to e e e to f f f to g g g to h h i j

1.83 2.01 2.13

propanoic acida 2-methyl propanoic acid butanoic acida C5 branched acids pentanoic acida C6 branched acids hexanoic acida C7 branched acids heptanoic acida C8 branched acids octanoic acida C9 branched acids nonanoic acida C10 branched acids decanoic acida undecanoic acida 2,6-di-tert-butyl phenol

0.8 1.3 3.8 1.7 14.0 5.8 18.4 6.9 16.7 5.1 7.8 3.6 6.4 2.6 2.2 0.5 2.4

b

2.77 b

3.80 b

5.19 b

6.89 b

8.78 b

10.75 12.73 13.87

Confirmed using authentic compounds. bMultiple peaks in this range.

insufficient chromatographic separation of the products made it difficult to make similar statements for the heavier acids. One of the minor peaks around the branched C8 acid isomers had a mass spectrum that indicated a molecular ion two less than the saturated acid, i.e., M+ = 142 m/z instead of 144 m/z. There were two possibilities, either it was an unsaturated acid, or it was a naphthenic acid. Spiking the mixture with trans-2octenoic acid revealed that it had a longer retention time than 2-octanoic acid and coeluted with the branched C9 acids. Although this is not conclusive proof that it was not an unsaturated acid, it appears more likely that it was a naphthenic acid. The acid fraction was spiked with authentic compounds of C8 naphthenic acids, 3-cyclopentyl propanoic acid and cyclohexane acetic acid, but neither of these compounds matched the unknown C8 acid isomer. Attempts to confirm the presence and/or absence of the C1 and C2 carboxylic acids failed, mainly due to interference of the solvent and residual water at short retention time in the chromatogram. It was found that when a stronger base than nickel(II) carbonate is used for the pretreatment, or if the pretreatment is performed at higher temperature, the amount of phenolic compound recovered as part of the acid fraction increases. The presence of 2,6-di-tert-butyl phenol specifically was cause for suspicion, because it is a well-known oxidation inhibitor.19 None was added when the samples were taken at the industrial facility. Although it was the only phenol recovered in the acid fraction using the procedure outlined, as mentioned before, other phenolic species could also be extracted when using higher temperatures and/or stronger bases. Based on the information available, it appears that 2,6-di-tert-butyl phenol was the most acidic phenolic compound in the naphtha fraction that was formed during thermal cracking of oilsands bitumen, or it is speculated that it was introduced at some point during the process as antioxidant in a process additive. 3.2. Dichloromethane Eluted Fraction. The chromatogram of the dichloromethane eluted fraction is shown in Figure 5. It can be seen that the separation of the compounds was not always complete, which complicated identification somewhat.

3. RESULTS 3.1. Acid Fraction. The chromatogram of the acid fraction is shown in Figure 4. Identification of the compounds corresponding to the peaks in the chromatogram is provided in Table 3.

Figure 4. Chromatogram of the acid fraction of naphtha, with the identification of the peaks indicated in Table 3.

The identity of the linear carboxylic acids (Table 3) was confirmed by spiking the acid fraction with authentic compounds, as explained in the Supporting Information. The branched carboxylic acids were only identified based on their electron impact mass spectra. The presence of only a single branched isomer of the C4 carboxylic acid, 2-methyl-propanoic acid, supports the interpretation. Likewise, there were two branched isomers of the C5 carboxylic acid, 2-methyl- and 3methyl-butanoic acid. The number of possible isomers and C


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Energy & Fuels Table 5. Phenolic Compounds Identified in the Dichloromethane Eluted Fraction, as Shown in the Chromatogram in Figure 5

Figure 5. Chromatogram of the dichloromethane eluted fraction of the naphtha, with peak identification provided for ketones (Table 4), phenolics (Table 5), thiols (Table 6), and cyclic thioethers (Table 7).


compound

1.71 1.82 1.91 2.05 2.25 2.59

2-butanone 2-pentanone 3-pentanone 4-methyl-2-pentanone (MIBK) 2-hexanone 2-methyl cyclopentanone

compound

3.78 4.83 5.13 5.79 6.24 6.46 6.76 7.02 7.20 7.59 7.98 8.15 8.20 8.59 8.81 9.07 9.38 10.02 10.76

phenol m-cresol p-cresol 2,5-dimethyl phenol 2-ethyl phenol 2,4-dimethyl phenol 4-ethyl phenol 2,3-dimethyl phenol 3-ethyl-5-methyl phenol 2,4,6-trimethyl phenol 3-ethyl-5-methyl phenol 2,3,6-trimethyl phenol 4-ethyl-3-methyl phenol 3-propyl phenol 2,4,5-trimethyl phenol 2-methyl-5-isopropyl phenol 4-methyl-2-isopropyl phenol 4-methyl-2-propyl phenol 2-butyl-5-methyl phenol

Table 6. Thiols Identified in the Dichloromethane Eluted Fraction, as Shown in the Chromatogram in Figure 5 label


compound

3 5

1.85 1.97

2-butanethiol 1-butanethiol

Cyclic thioethers were the most abundant sulfur-containing compounds in the dichloromethane eluted fraction (Table 7). The most abundant species in this fraction was 2-methyltetrahydrothiophene and this identification was confirmed using an authentic compound. The specific configuration of the other isomers of the cyclic thioethers was not confirmed with authentic compounds and identification was based on mass spectra only. Thus, the compound class identification was confirmed, but not the configuration of each individual isomer. Another interesting observation was that no thiophenes were identified. It is speculated that whatever thiophenes were present in the naphtha must have been present at low concentration. 3.3. Acetone Eluted Fraction. The chromatogram of the acetone eluted fraction is presented in Figure 6. Identification of the compounds corresponding to the peaks in the chromatogram is provided in Table 8. The total concentration of these nitrogen-containing compounds was estimated to be around 165 μg/g in the naphtha.15 Basic nitrogen-containing compounds were the dominant compound class in the acetone eluted fraction (Table 8). The

Table 4. Ketones Identified in the Dichloromethane Eluted Fraction, as Shown in the Chromatogram in Figure 5 1 2 4 6 7 9


19 23 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

in the dichloromethane eluted fraction was possible by performing a reaction with a base-extraction. Such a baseextraction was not performed, because aldol condensation and thiol to disulfide conversion would be likely side-reactions when employing a strong base. Thiols (mercaptans) were present only in minor amounts (Table 6). Heavier thiols were likely also present at low concentration, but could not be identified due to overlap with compounds present at higher concentration.

The species in the chromatogram was not exhaustively assigned, with only 41 species being identified. Dichloromethane (DCM) eluted mainly oxygen- and sulfurcontaining compounds. A minor amount of neutral nitrogencontaining compounds was present, but none of these compounds could be separated and identified. The presence of the neutral nitrogen-containing compounds (pyrroles) could only be established by making use of a nitrogen−phosphorusselective detector and the total concentration was estimated to be around 20 μg/g.15 The compound class with the shortest retention time was the aliphatic ketones and a number of ketones could be identified (Table 4). The ketones were present at low concentration

label

label

compared to the bulk and it is likely that heavier ketones were present, but could not be identified against the background of the rest of the matrix. All of the other oxygenates that could be identified were phenolic compounds (Table 5). The specific isomers indicated should be viewed as a guideline only, since the specific isomers were not confirmed using authentic samples. At retention times longer than 6 min, the chromatogram of the dichloromethane eluted fraction was dominated by phenolic compounds. Further separation of the phenolic compounds from the other material D


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Energy & Fuels Table 7. Cyclic Thioethers Identified in the Dichloromethane Eluted Fraction, as Shown in the Chromatogram in Figure 5

a

label


compound

8 10 11 12 13 14 15 16 17 18 20 21 22 24

2.42 2.69 2.84 2.89 2.97 3.11 3.20 3.38 3.48 3.61 3.96 4.24 4.69 4.92

tetrahydrothiophenea 2-methyl-tetrahydrothiophenea 3-methyl-tetrahydrothiophene dimethyl-tetrahydrothiophene isomer dimethyl-tetrahydrothiophene isomer dimethyl-tetrahydrothiophene isomer dimethyl-tetrahydrothiophene isomer methylthiane dimethylthiane ethyl-tetrahydrothiophene ethyl-methyltetrahydrothiophene n-butyl-tetrahydrothiophene ethylthiane propyl-tetrahydrothiophene

Table 8. Identification and Relative Abundance of Compounds in the Acetone Eluted Fraction Shown by the Chromatogram in Figure 6 label


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 23 24

2.43 2.75 2.89 3.07 3.32 3.48 3.56 3.68 3.86 4.00 4.08 4.21 4.46 4.52 4.63 4.84 4.95 5.13 5.24 5.44 5.75 6.40 9.47

25

9.77

26 27 28 -

10.31 12.50 13.83 -

Confirmed using authentic compounds.

a

compound 2-methyl pyridinea 3-methyl pyridinea 2,6-lutidinea 2-ethyl pyridinea 2,5-lutidinea 2,3-lutidinea 2-isopropyl pyridine 2-ethyl-6methyl pyridine 3-ethyl pyridine 2,4,6-trimethyl pyridinea 2-propyl pyridine 3-ethyl-5methyl pyridine 5,6,7,8-tetrahydroindolizine 3-ethyl-4-methyl pyridine 2-ethyl-benzenamine 2-methyl-6-propylpyridine 2,4-dimethyl-benzenamine 2-ethyl-4,6-dimethyl pyridine 2-ethyl-3,5-dimethylpyridine 2,6-diethyl-pyridine 2-ethyl-6-isopropyl pyridine N,N-3,5-tetramethyl-benzenamine 6-methyl-1,2,3,4,tetrahydroquinoline 3-ethyl-5,6,7,8tetrahydroquinoline 4-amino-1,5-pentandioic acid 2,4-dimethyl quinoline 2,4,6-trimethyl quinoline not identified

peak area (%) 0.6 1.2 1.8 3.3 2.8 1.2 1.8 1.8 0.3 2.5 1.6 2.9 1.2 0.3 0.2 0.7 0.6 6.0 1.2 5.5 5.5 6.4 1.5 1.6 1.7 1.2 1.4 43.0

Confirmed using authentic compounds.

Information. Quinoline derivatives were found in the retention time range from 8 to 13 min (Figure 6). Identification was difficult, because these compounds overlapped with compounds containing alcohol groups. A cluster of compounds that could not be chromatographically separated was observed in the retention time range from 16.5 to 21.5 min (Figure 6). These molecules contained both ketone and alcohol functional groups. The mass spectra of the more abundant compounds resembled that of steroids. Steroid-type compounds are potential biomarkers in crude oils,20 which can be used to estimate properties such as the extent of biodegradation. It was surprising to see such material in cracked naphtha. The samples were checked for evidence of bacterial growth, but none was found. It suggested that these compounds survived thermal cracking and were derived from the bitumen.

4. DISCUSSION The results of the study can be used in two ways. They can directly be used as information on the properties of the naphtha and how it will influence downstream processing, or, they can indirectly be used to reconstruct information about the properties of oilsands bitumen and the pathway by which such compounds are formed. Both will be discussed. 4.1. Implications for Hydroprocessing. Sulfur in the naphtha fraction was present mainly as thioethers. Although the

Figure 6. Chromatogram of the acetone eluted fraction of the naphtha, with peak identification provided in Table 8.

identity of some of the compounds was confirmed with authentic compounds, as explained in the Supporting E


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Figure 7. Possible thermal cracking reaction pathway to explain the formation of acyclic acids from naphthenic acids.

For example, no olefinic acids were detected (Table 3) and the sulfur-containing compounds were dominated by cyclic thioethers, rather than thiophenes (Table 7). Further thermal cracking of (3) could lead to further ring-opening. It is worthwhile pointing out that none of the thermal cracking reactions shown in Figure 7 required the formation of a primary carbon centered radical. The disadvantage of the illustration in Figure 7 is that it is two-dimensional and it does not reflect the nonplanar nature of (2). Depending on the direction in which the alkyl chain folds, intramolecular 1,5- or 1,6-transfer is possible (Figure 8). Both

possible presence of thiophenes could not be ruled out, no thiophenes were identified. From a hydrodesulfurization (HDS) perspective this does not appear to matter, because it was found that the rate of HDS of thiophene and tetrahydrothiophene was near-similar over a number of hydrotreating catalysts.21 The presence of both phenols and pyridines in the naphtha indicate that hydrodeoxygenation (HDO) and hydrodenitrogenation (HDN) might be incomplete and affect the overall conversion that can be achieved during hydroprocessing. Generally speaking high nitrogen removal by HDN of naphtha that contains pyridinic compounds is very difficult.22 Prior removal of phenols is also beneficial for sulfur and nitrogen removal, as well as olefin saturation during hydroprocessing.23 Regardless, the influence of oxygen-, sulfur-, and nitrogencontaining compounds on hydroprocessing is complex,24 and the presence of phenolic and pyridinic compounds in the thermally cracked naphtha will have an influence on the hydroprocessing conditions for high heteroatom removal. The concentration of carboxylic acids in the naphtha is low; the acid number is 0.12 mg KOH/g (Table 1), which is equivalent to 70 μg O/g present as carboxylic acids. The present study could not determine whether the more aggressive C1 and C2 carboxylic acids were present, but C3 and heavier aliphatic carboxylic acids were definitely present (Table 3) in the thermally cracked naphtha. Aliphatic carboxylic acids can cause problems with hydroprocessing due to their ability to preferentially adsorb on metals, leach metals, and deposit metals during decomposition reactions,25 and these issues are not limited to the short chain carboxylic acids. 4.2. Carboxylic Acids in Cracked Naphtha. Carboxylic acids naturally present in oilsands bitumen are mainly naphthenic acids. The relative abundance of the acids is in the order: tricyclic acids > pentacyclic acids > acyclic acids > other acids.26 It was further noted that linear carboxylic acids that naturally occur in bitumen are in the C8−C28 range, and that even-numbered linear acids are more abundant that the odd-numbered linear acids.26 The carboxylic acids in the naphtha fraction displayed no odd−even bias and are shorter in chain length (Table 3). The carboxylic acids in the naphtha fraction are products resulting from thermal cracking reactions. One plausible reaction pathway that could explain the formation of acyclic carboxylic acids is shown in Figure 7. The tricyclic backbone of naphthenic acid (1) used to illustrate the reaction sequence was not arbitrary. The presence of the structural motif in (1) in bitumen is supported by analysis using field ionization mass spectrometry in combination with selective reduction.26 The first cracking event shown in Figure 7 causes ring-opening to produce a diradical intermediate (2). The fate of the diradical determines what subsequent reactions will take place. One possibility is that intermolecular hydrogen transfer converts (2) to (3). The nature of the products identified in the thermally cracked naphtha suggested that hydrogen transfer activity during thermal cracking was high.

Figure 8. Intramolecular hydrogen transfer during thermal cracking of naphthenic acids. Only the hydrogen that will be involved in the transfer is explicitly shown.

the 1,5- and the 1,6-transfer shown in Figure 8 abstract hydrogen from a tertiary carbon, which makes these reactions not only geometrically favored, but also energetically advantageous.27 For the configuration shown, 1,5-transfer would have the added advantage of forming an internal olefin as product, rather than a free radical product. The purpose of Figures 7 and 8 is not to claim that this represents the free radical chemistry leading to the formation of the acyclic carboxylic acids identified in the naphtha, but to show that there is a plausible reaction pathway that could explain these products. Carboxylic acids could also have been produced from esters in an analogous fashion, but where the first step is the thermal cracking of the ester to produce the carboxylic acid and olefin (eq 1).28 However, due to the nature of the reaction, ester groups in five- and six-membered rings are thermally stable,28 which implies that carboxylic acid formation from esters by thermal cracking more likely involves acyclic ester groups and not lactones. R1−CH 2−CH 2−O(CO)−R 2 → R1−CHCH 2 + R 2−COOH F

(1)


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Energy & Fuels 4.3. Ketones in Cracked Naphtha. In the native bitumen ketones occur mainly as fluorenone-type derivatives.26 Thermal decomposition of fluorenones leads to deoxygenation,29 which makes it unlikely that the acyclic naphtha range ketones (Table 4) are cracking products from ketones already present in the bitumen. It is more likely that the ketones were formed by ketonization of acyclic carboxylic acids (eq 2).30

the naphtha fraction is provided by the results from the naphtha characterization, and is particularly evident from the product spectrum of the sulfur-containing compounds. 4.6. Nitrogen-Containing Compounds in Cracked Naphtha. Heterocyclic pyridine-type compounds dominate the nitrogen-containing compounds that are present in thermally cracked naphtha (Table 8). It is likely that most of these products occurred in the bitumen as part of larger molecules. Whether the nitrogen-containing heterocycles were attached as pendant groups, or through naphthenic structures, cannot be ascertained from the cracked products in the naphtha, but the number of compounds with two or more alkyl groups suggests that at least some of the pyridines were attached to naphthenic structures.

R1−COOH + R 2−COOH → R1−(CO)−R 2 + CO2 +H 2O

(2)

Ketonization of metal carboxylates is also a well-known reaction,18 and ketonization could be the product of metal or mineral assisted reactions. Bitumen typically contains metal ions and mineral matter and metal or mineral assisted ketonization of carboxylic acids is a more likely pathway than thermal ketonization. Should this be the case, the ketones with the carbonyl on the second carbon (Table 4) provide indirect evidence of the formation of ethanoic acid, even though no ethanoic acid was reported in Table 3. 4.4. Phenols in Cracked Naphtha. Phenols were reported to be an important oxygen-containing functional group in the asphaltene fraction of bitumen.31,32 By extension, phenols likely existed as phenols in bitumen, although it can be speculated that additional phenols may have formed by thermal cracking of aryl−alkyl ethers. It is known from the coal literature that phenol formation during pyrolysis is affected by many parameters.33 4.5. Sulfur-Containing Compounds in Cracked Naphtha. Aliphatic C−S bonds are around 40−60 kJ/mol weaker than aliphatic C−C bonds.34 During thermal cracking, aliphatic C−S linkages are readily broken and hydrogen sulfide is a major gas phase product.35 The survival of some sulfur as aliphatic sulfur-containing compounds was anticipated, but the extent to which sulfur was present in aliphatic heterocycles, rather than aromatic heterocycles, was surprising (Table 7). Thiophenes are commonly reported in cracked naphtha.3 Considering that the naphtha contained 0.9 wt % sulfur (Table 1), it appeared unlikely that so much aliphatic sulfur would survive the thermal cracking process. The sulfur-containing compounds that were identified included five-membered cyclic thioethers (tetrahydrothiophenes) and six-membered cyclic thioethers (thianes). The mass spectra of branched tetrahydrothiophenes and thianes with the same molecular formula are very similar.36 It is possible that some compounds identified as thianes were indeed tetrahydrothiophenes with one more carbon present in an alkyl group. Regardless, the most prominent compound was 2-methyl tetrahydrothiophene and sulfur was present mainly as cyclic thioethers. It is likely that products identified as tetrahydrothiophenes were not originally present in the feed material to the thermal cracker as tetrahydrothiophenes, but as thiophenes.26 It is speculated that cracked monocyclic thiophenic products were converted to tetrahydrothiophenes by hydrogen transfer (eq 3). (R)n (C4 H(4 − n)S) + 4H• → (R)n (C4 H(8 − n)S)

5. CONCLUSIONS Oxygen-, sulfur-, and nitrogen-containing compounds in industrial thermally cracked naphtha produced from oilsands bitumen was characterized. The compound classes identified in the naphtha were acyclic carboxylic acids, ketones, phenols, cyclic thioethers, and pyridines. Thiols and pyrroles were also present. The following specific conclusions are highlighted: Linear and branched carboxylic acids in the C3−C11 range were identified in the naphtha, but little naphthenic acids. These compounds could be explained by a thermal cracking pathway involving ring-opening of one or more adjacent naphthenic rings. Indirect evidence of the formation ethanoic acid was also found in the ketonization products. The sulfur-containing compounds were predominantly cyclic thioethers and not thiophenes, which suggested that the hydrogen transfer during thermal cracking of the oilsands derived material was high. The naphtha also had a correspondingly low aromatic content. The main nitrogen-containing compound class was the pyridines. Based on quantification reported previously,15 the ratio of nonbasic (pyrrole-type) to basic (pyridine-type) compounds were 20 versus 165 μg/g in the naphtha.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b01646. List of authentic compounds used to confirm identification of species in the thermally cracked naphtha (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +1 780-248-1903. Fax: +1 780-492-2881. ORCID

Arno de Klerk: 0000-0002-8146-9024 Notes

The authors declare no competing financial interest.

(3)

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The inherent hydrogen-donor capacity of heavy petroleum fractions has been remarked upon before.37 Evidence of hydrogen transfer was found even at low temperatures.38 It can further be pointed out that the thermally cracked naphtha is not very aromatic, with only 8.5 mol % of the carbon being aromatic (Table 1). Indirect evidence of hydrogen transfer to

ACKNOWLEDGMENTS This work was funded through the NSERC/Nexen-CNOOC Ltd. Industrial Research Chair program in Field Upgrading and Asphaltenes Processing. Nexen Energy ULC funded the initial work on nitrogen-containing compound identification, and G


Article

Energy & Fuels

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permission to disclose general characterization data is appreciated.

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