Article pubs.acs.org/EF
Uncatalyzed Hydrogen Transfer during 100−250 °C Conversion of Asphaltenes Nazim Naghizada, Glaucia H. C. Prado, 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: The reactivity and reactions of asphaltenes were explored over the temperature range 100−250 °C following reports of reactivity and meaningful free radical content in asphaltenes. This study employed industrial pentane precipitated asphaltenes from Athabasca oilsands bitumen. The presence of free radicals in the asphaltenes feed was confirmed. On heating the asphaltenes to 150 °C, the aromatic hydrogen content increased relative to the feed by a factor 1.12. It was also found that on heating the asphaltenes to 150 °C the n-heptane insoluble fraction increased from 67% to 75%. Almost no gas phase products were produced. The observed changes were ascribed to hydrogen transfer reactions and addition products formed by combination reactions. Direct evidence of hydrogen transfer reactions taking place in the asphaltenes was obtained through the use of the model systems, α-methylstyrene and cumene, as well as anthracene and 9,10-dihydroanthracene. The extent of hydrogen transfer was of the order 1.8 mg H/g asphaltenes in 1 h at 250 °C. Asphaltenes also caused dimerization of model compounds, providing indirect evidence that free radical combination reactions took place in the asphaltenes. Interpretation relying on thermodynamic arguments combined with experimental results indicated that at 250 °C the reactive species in asphaltenes were incapable of abstracting hydrogen by hydrogen transfer that had bond strengths (based on homolytic bond dissociation energy) exceeding around 353 kJ mol−1. Using similar arguments, it was deduced that the ratio of reactive species in asphaltenes capable of abstracting hydrogen with bond strengths in the range around 315−353 kJ mol−1, compared to transferable hydrogen in asphaltenes with a bond strength less than 315 kJ mol−1 was about 2:1. which is equivalent to 0.3 × 10−3 mol kg−1 h−1 based on the reported gas composition. The major compounds identified in the gaseous product were CO2 > C1 ≈ C2. When industrially precipitated Athabasca asphaltenes were heated over the temperature range of 60−250 °C, changes in the n-pentane soluble yield were observed, as well as changes in the ratio of aliphatic to aromatic hydrogen of the n-pentane soluble fraction.10 All of these studies provided evidence of changes taking place when heating asphaltenes to temperature conditions found in solvent deasphalting processes. Analogous observations have been reported in the coal liquefaction literature, e.g., refs 11 and 12. It was postulated that the reactions taking place in the asphaltenes were mainly free radical type reactions. It is known that asphaltenes naturally have a high free radical content; in petroleum asphaltenes it is of the order 1018−1019 spins g−1.13−17 The persistence of free radicals in the asphaltenes was explained in terms of a “caging” effect, which was so effective that it precluded hydrogen transfer reactions at ambient conditions.16 Analogous explanations were forwarded in relation to the trapping of free radicals in coal to prevent further reaction.17 With an increase in temperature it was anticipated that the protection offered by “caging” of radicals in asphaltenes would decrease and that reactions involving hydrogen transfer would increase. The purpose of this investigation was to explore
1. INTRODUCTION Asphaltenes are industrially produced from heavy oils and bitumens by precipitation in a solvent deasphalting unit. The solvent used in the solvent deasphalting process determines the amount and the properties of the asphaltenes and deasphalted oil that are produced. As the carbon chain length of the paraffinic solvent is increased from C3 to C5, the yield of asphaltenes decreases and the yield of deasphalted oil increases.1 Solvent selection also affects the operating temperature of the solvent deasphalting process. When n-pentane solvent deasphalting is performed, the operating temperature range is typically in the range of 170−210 °C.2 Solvent deasphalting and supercritical solvent extraction, as is applied in the residuum oil supercritical extraction (ROSE) process for solvent deasphalting, are also used in oil characterization studies.3−6 The tacit assumption is that asphaltenes precipitation by solvent deasphalting is purely a physical separation process based on the insolubility of the asphaltenes in the paraffin-rich liquid phase. The notion that asphaltenes are unreactive at solvent deasphalting conditions is not supported by the literature.7−10 Traxler7 found that when asphalt was heated to temperatures in the range of 107−191 °C (225−375 °F) for a period of 4 h, while avoiding exposure to air and light, the viscosity increased. This was ascribed to polymerization. The rate of gas evolution from Athabasca asphaltenes was found to be 0.6 × 10−3 mol kg−1 h−1 at 210 °C.8 The major compounds identified in the gaseous product were COx > C5 ≫ C1. In a different study the gas evolution from Athabasca asphaltenes was measured at 200 °C over a period of 1.5 h, and it was found to be 1.7 g kg−1,9 © XXXX American Chemical Society
Received: March 6, 2017 Revised: May 23, 2017
A
DOI: 10.1021/acs.energyfuels.7b00661 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
diameter of 2.7 cm and length of 8.7 cm, is presented in the Supporting Information (Figure S2). It was speculated that the metal wall of the microbatch reactor could facilitate hydrogen transfer reactions in the asphaltenes. This was experimentally evaluated. A comparison of reactions performed in contact with the stainless steel vessel and that in contact with glass suggested that there was a difference, either due to the longer heating and cooling times of the reactor with the glass insert or due to the impact of the metal wall on the reactions that were taking place. The results of these tests can be found in Table S1 in the Supporting Information. If the metal wall had an impact, the results suggested that it suppressed hydrogen transfer compared to conversion that was performed in contact with glass. It was decided to perform the experiments in microbatch reactors with a glass vial insert, but in practice it was difficult for those experiments that involved industrial asphaltenes as only feed material (as is explained in the Supporting Information). The first set of experiments was performed to evaluate the impact of heating asphaltenes to 100−150 °C. In a typical experiment, 5.0 ± 0.1 g of industrial asphaltenes was loaded into a microbatch reactor (without glass vial) and the exact mass was recorded. The reactor was leak tested, before being flushed and then pressurized with nitrogen to a pressure of 4 MPa gauge. The pressurized reactor was then placed in a preheated fluidized sand bath heater (Omega fluidized bath FSB-3) to heat the reactors to reaction temperature. The temperature inside the reactor was measured using a thermocouple. The heating time was not taken into consideration for reaction time, which was 1 h measured from the time that reactor internal temperature reached the set point until the reactor was removed from the heater and cooled. The initial and final pressure in the reactor was similar. The reactors were connected to a multiport sampling valve of a gas chromatograph right after the reaction. The reactor was depressurized, and the gaseous product was injected through the sample loop of the multiport valve and analyzed by gas chromatography. The solid reaction product remaining in the reactor was manually removed by scraping out the reactor and without using any solvent. The reaction product was characterized in terms of its n-heptane soluble and insoluble subfractions (precipitation performed at 40:1 n-heptane/feed), as well as by nuclear magnetic resonance spectroscopy. All experiments were performed in triplicate. Following the same procedure as outlined above, one experiment in triplicate was performed at 150 °C for a reaction time of 5 h. The purpose of this experiment was to increase conversion in the hope of producing more gaseous products to facilitate analysis. 2.2.2. Asphaltenes with Model Compound Conversion. The remainder of the experiments was performed with a modified protocol. In these experiments the industrial asphaltenes feed was mixed with selected model compounds, and some model compound mixtures were studied on their own. Diphenylether was employed as insert solvent, subsequent to a report that it was not degraded during
asphaltenes conversion in the temperature range of 100−250 °C, with specific attention being paid to hydrogen transfer reactions.
2. EXPERIMENTAL SECTION 2.1. Materials. An industrially precipitated asphaltenes feed was used for this study. The asphaltenes feed was obtained from the solvent deasphalting unit at the Nexen Long Lake upgrader in Alberta, Canada. This unit employs C5 hydrocarbon deasphalting of Athabasca oilsands bitumen that is recovered subsurface by steam assisted gravity drainage. The industrial asphaltenes feed was characterized (Table 1).
Table 1. Characterization of Industrial Asphaltenes Feed Material Derived from Athabasca Oilsands Bitumen
a
description
industrial asphaltenes feed
liquefying temperature (°C) n-pentane insolubles (wt %) n-heptane insolubles (wt %) elemental analysis carbon hydrogen sulfur nitrogen nature of hydrogen (%)a aliphatic hydrogen aromatic hydrogen
124−142 79.0 ± 0.9 66.9 ± 1.7 80.3 ± 0.1 8.0 ± 0.1 7.7 ± 0.1 1.1 ± 0.0 89.9 ± 0.1 10.1 ± 0.1
Spectrum shown as Figure S1 in the Supporting Information.
Some information was derived from a previous study.10 The temperature range over which the asphaltenes change from a solid material into liquid was labeled the liquefying temperature, because it is not associated with an energy change that is typical of enthalpy of melting. The proton nuclear magnetic resonance (1H NMR) spectrum of the asphaltenes feed is provided in the Supporting Information (Figure S1). In the literature the n-pentane insoluble and n-heptane insoluble fractions are often referred to as “C5-asphaltenes” and “C7asphaltenes” and may contain toluene insoluble material unless such material is explicitly removed. The use of the word “asphaltenes” should not be taken to mean the definition associated with the ASTM D 6560 standard test method.18 The chemicals employed during this investigation are listed in Table 2. These materials were commercially obtained and used without further purification. 2.2. Equipment and Procedure. 2.2.1. Asphaltenes Conversion. Conversion was performed in a stainless steel microbatch reactor. A detailed schematic of the microbatch reactor, which has an internal
Table 2. List of Chemicals and Cylinder Gases compound chemicals anthracene biphenyl cumene 9,10-dihydroanthracene diphenyl ether n-heptane methanol α-methylstyrene cylinder gases nitrogen
formula
CASRNa
purity (%)b
supplier
C14H10 C12H10 C9H12 C14H12 C12H10O C7H16 CH3OH C9H10
120-12-7 92-52-4 120-12-7 613-31-0 101-84-8 142-82-5 67-56-1 98-83-9
99 99.5 98 97 99 99 99.99 99
Across Organics Sigma-Aldrich Across Organics Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich
N2
7727-37-9
99.999
Praxair
a
CASRN = Chemical Abstracts Services registry number. bThis is the purity of the material guaranteed by the supplier; material was not further purified. B
DOI: 10.1021/acs.energyfuels.7b00661 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 3. List of Experiments Conducted in Microbatch Reactors with a Glass Vial Insert exp.
feed materials
mass ratio
total mass (g)
temperature (°C)
time (min)
A B C D Ea,b F G H I
asphaltenes/α-methylstyrene α-methylstyrene asphaltenes/cumene asphaltenes/α-methylstyrene asphaltenes: α-methylstyrene asphaltenes/anthracene asphaltenes/9,10-dihydroanthracene asphaltenes/α-methylstyrene/9,10-dihydroanthracene α-methylstyrene/9,10-dihydroanthracene
1:1 1 1:1 1:1 2−8:1 1:1 1:1 1:2: 1 1:1
1.14 0.57 1.14 1.14 0.75−2.28c 1.14 1.14 1.14 1.14
150−250 250 250 250 250 250 250 250 250
60 120 120 20−240 30−120 60 60 60 60
a These experiments were performed only in duplicate. bExtraction using only 50 mL of methanol for asphaltenes/α-methylstyrene at 4:1 and 8:1 ratios. cFeed mass of asphaltenes/α-methylstyrene: 2:1 ratio = 1.14 + 0.57 g; 3:1 ratio = 1.71 + 0.57 g; 4:1 ratio = 0.6 + 0.15 g; 8:1 ratio = 1.2 + 0.15 g.
reaction of a mixture of α-methylstyrene, 6H-benzo[cd]pyrene, and 9,10-dihydroanthracene in diphenylether at 250 °C.19 A list of the experiments that were performed is shown in Table 3. All experiments were performed in triplicate, unless otherwise indicated. In a typical experiment the feed mixture (around 1 g, the exact mass recorded) was prepared with 1 mL of diphenylether as solvent in a glass vial, which was placed inside the microbatch reactor. A small magnetic stirrer bar was added to the feed mixture in the glass vial. The reactor was leak tested and pressurized with nitrogen as described before. The reactor was then placed into a heated silicon oil bath on top of a heater−stirrer hot plate (Fischer Scientific Isotemp). Aluminum foil was used to provide additional insulation to limit heat loss. The temperature inside the reactor was monitored using a thermocouple. The content of the reactor was stirred using the magnetic stirrer bar at 250 rpm. The reaction time was taken from the time that the internal temperature reached set point. With the glass vial in the reactor heating time was 17 min and the cooling time was 8 min. At the conclusion of each experiment, the reactor was depressurized. Experiments that employed industrial asphaltenes as one of the feed components were extracted with 100 mL of methanol to recover the model compounds, unless otherwise noted. The solvent selection procedure that led to the selection of methanol is described in the Supporting Information. The methanol extraction was performed under constant stirring over a period of 30 min. The methanol extract was filtered, and 10 mg of biphenyl was added to a known amount of methanol extract to serve as internal standard for analysis. The methanol extract spiked with the internal standard was then analyzed by gas chromatography. The extraction efficiencies and error of quantification of α-methylstyrene and cumene were determined experimentally and was in the range 3−5% error over the concentration range of 0.4−5.0 mg mL−1, but higher errors were found for lower concentrations (Table S2 in the Supporting Information). Apart from these two model compounds, the quantification of all other feed materials and reaction products should be considered semiquantitative at best. 2.3. Analyses. 2.3.1. Gas Product Analysis. Composition of gaseous products formed from mild thermal treatment of asphaltenes was determined using an Agilent 7890A gas chromatograph (GC). The instrument is equipped with flame ionization and thermal conductivity detectors. Helium was used as carrier gas (25 mL/min), and a Hay Sep R column, 2.44 m × 3 mm, was used for separation. The injector temperature was kept at 200 °C, and the temperature program was isothermal at 70 °C for 7 min, increase from 70 to 250 °C at 10 °C/min, and isothermal at 250 °C for 2 min. 2.3.2. Nuclear Magnetic Resonance Spectroscopy of Asphaltenes. Proton nuclear magnetic resonance (1H NMR) spectra were measured using a 60 MHz benchtop NMR spectrometer from Nanalysis. The instrument is equipped with an electronic shimming system, pulse Fourier transform spectrometer, and temperature controlled permanent magnet. The samples were diluted in deuterated chloroform to a concentration equal to 0.1 g/mL; 0.7 mL of the
solution was taken for analysis. Each sample was scanned 32 times; the scan delay was 24.7 s. Carbon-13 nuclear magnetic resonance (13C NMR) spectra were measured by the Analytical Services of the Chemistry Department at University of Alberta. The instrument used was a 400 MHz Varian Unity Inova spectrometer. The same samples as used for 1H NMR analyses were submitted for 13C NMR analysis, but sample preparation for the analysis was different. For quantification of carbon, chromium(III) acetylacetonate was employed as a relaxation agent. The chromium(III) acetylacetonate was mixed with deuterated chloroform to make a 0.2 M solution. The asphaltenes and converted asphaltenes were dissolved in this solution for analysis. For analysis a NO NOE (no nuclear Overhauser enhancement) carbon experiment was set up where decoupling was on only during the acquisition. Additional details can be found in the Supporting Information. 2.3.3. Analysis of Methanol Extract. Reaction products extracted in methanol after reactions of mixtures containing asphaltenes and model compounds were analyzed by gas chromatography mass spectrometry (GC-MS). The instrument used was an Agilent 7820 with 5977E mass spectrometer. Separation was performed on an HP-5 column, 30 m × 0.25 mm × 0.25 μm, using helium as a carrier gas. The temperature program started at 90 °C and was increased by 5 °C/min up to 320 °C.
3. RESULTS 3.1. Asphaltenes Conversion at 100−150 °C. A set of experiments was performed with industrially precipitated asphaltenes that were heated to 100−150 °C for 1 h. The material balances for these reactions are presented in Table 4. This set of experiments evaluated three different aspects of low temperature asphaltenes reactivity: gaseous product release, solubility class distribution, and hydrogen disproportionation. The first objective was to determine whether there was a release of volatile products during the reaction and the nature Table 4. Material Balance of Microbatch Reactor Experiments Conducted with Industrial Asphaltenes Feed at 100−150 °C for 1 h under 4 MPa N2 Pressure material balance (%)a
a
C
gas product (mg g−1)a
temperature (°C)
x
s
x
s
100 110 120 130 140 150
101.8 102.8 99.5 101.2 100.8 101.1
0.7 0.5 0.3 1.1 0.4 0.5
6 4 6 8 8 8
2 4 4 2 0 2
Average (x) and sample standard deviation (s) of triplicates. DOI: 10.1021/acs.energyfuels.7b00661 Energy Fuels XXXX, XXX, XXX−XXX
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The third objective was to determine the extent of hydrogen disproportionation that took place. Hydrogen disproportionation was evaluated by 1H NMR spectroscopy. The industrial asphaltenes feed material, as well as the reaction products, were analyzed after separation into C7-asphaltenes and C7-maltenes subfractions. The ratio of aromatic hydrogen of the reaction products relative to that of the feed material is shown in Figure 1. Compared to the C7-asphaltenes subfraction of the feed
of the gaseous products. Gaseous products were released (Table 4), but in all experiments the total amount of gaseous products were C6 > CO2 > C1−C4. No hydrogen sulfide (H2S) or carbon monoxide (CO) were detected in any of the gaseous products. The second objective was to determine whether heating the material affected the solubility class distribution. The solubility class distribution was determined by measuring the n-heptane insoluble (C7-asphaltenes) content and n-heptane soluble (C7maltenes) content and is reported in Table 6. The C7-
material, the aromatic hydrogen content of the C7-asphaltenes of the reaction products is meaningfully higher. At the same time the aromatic hydrogen content of the C7-maltenes of the reaction products is meaningfully lower than that of the C7maltenes subfraction of the feed material. In both instances a Student’s t test at 95% confidence level was performed on the 1 H NMR data to evaluate whether the difference was meaningful or not. The aromatic hydrogen content of the total product was calculated (Figure 1) from the material balance and the 1H NMR analyses. It indicated that there was an increase in the aromatic hydrogen content compared to that of the feed, reaching a ratio of 1.12 at 150 °C, i.e., the aromatic hydrogen content increased by 12% over that in the feed after heating to 150 °C for 1 h. The aromatic hydrogen content is not a reliable indicator of the aromatic carbon content, because aromatic carbons are not necessarily bonded to hydrogen. To obtain some indication of the extent to which the change in aromatic hydrogen content paralleled a change in aromatic carbon content, the C7asphaltenes subfraction of the feed and the 150 °C reaction product were analyzed by 13C NMR (Table 7). There was an
Table 6. Solubility Class Distribution of Products after Conversion Conducted with Industrial Asphaltenes Feed at 100−150 °C for 1 h under 4 MPa N2 Pressure C7-asphaltenes (wt %)a
C7-maltenes (wt %)a
temperature (°C)
x
s
x
s
material balance (%)
feedb 100 110 120 130 140 150
66.9 66.5 64.3 67.1 67.5 71.1 74.8
1.7 2.1 1.3 0.8 1.4 1.2 3.1
32.5 33.2 35.1 32.6 31.9 28.9 25.3
1.5 0.3 0.9 0.5 0.4 1.7 1.0
99.4 99.7 99.4 99.7 99.4 100.0 100.1
Table 7. Aliphatic and Aromatic Carbon Content of the C7Asphaltenes Subfraction of the Feed and Product after Reaction at 150 °C for 1 h
a
Average (x) and sample standard deviation (s) of triplicates. bC7asphaltenes (n-heptane insoluble) content repeated from Table 1.
carbon in C7-asphaltenes (%)a
asphaltenes and C7-maltenes obtained from each reaction were weighed individually after solvent removal, and the material balance closure was always within the range 99−101%. The results indicated that, after reaction at 140 and 150 °C for 1 h, the C7-asphaltenes content of the reaction product was meaningfully higher than the C7-asphaltenes content of the industrial asphaltenes feed; Student’s t test at 95% confidence level.
temperature (°C)
aliphatic
aromatic
aromatic H/aromatic C molar ratiob
feedc 150
49.7 ± 0.3 48.0 ± 0.6
50.3 ± 0.3 52.0 ± 0.6
0.17 0.20
a
Average of duplicate analyses. bAromatic hydrogen content used in calculation taken from 1H NMR analysis. cThe n-heptane insoluble subfraction of industrial asphaltenes feed prepared without heating. D
DOI: 10.1021/acs.energyfuels.7b00661 Energy Fuels XXXX, XXX, XXX−XXX
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extract, the uncertainty in the selectivity is high. After reaction at 180 and 200 °C, no byproducts were recovered by methanol extraction, but after reaction at 220 and 250 °C, byproducts were observed in the methanol extract. Two control experiments were performed (Table 3, exp. B and C). In the first control experiment it was shown that, when α-methylstyrene was heated to 250 °C in the absence of asphaltenes, no reaction took place. In the second control experiment, a mixture of cumene and asphaltenes was heated to 250 °C, and no α-methylstyrene, or any other products apart from cumene, was detected in the reaction product. The absence of α-methylstyrene in the reaction product did not exclude the possibility that cumene could have reacted with the asphaltenes, but if it did, none of the products were extracted by methanol. Likewise, if some α-methylstyrene was formed as an intermediate product it could have been converted again to either cumene or a different product that was not extracted with methanol. The control experiment could therefore not conclusively show that cumene did not participate in any reactions. The reaction of α-methylstyrene with asphaltenes was performed at 250 °C varying reaction time to obtain an indication of reaction rate (Table 3, exp. D). The conversion of α-methylstyrene and selectivity to cumene as a function of reaction time at 250 °C is shown in Figure 4.
increase in the aromatic carbon content after reaction at 150 °C, which was accompanied by an increase in the ratio of aromatic hydrogen to aromatic carbon content from 0.17 to 0.20. Differently put, the aromatic carbon content increased after heating, but the aromatic carbon was associated with more hydrogen than in the feed. 3.2. Asphaltenes and α-Methylstyrene Conversion. The challenge with studying hydrogen transfer reactions in asphaltenes is that the asphaltenes are difficult to analyze. Spectroscopy can be used to monitor and quantify macroscopic changes, but it is not practical to explore hydrogen transfer at a species-level. It is for this reason that two probe molecules were selected to evaluate hydrogen transfer on a species-level, namely, α-methylstyrene and cumene. The reported ceiling temperature α-methylstyrene was 61 °C,20 which is the temperature above which α-methylstyrene will not polymerize, making it a suitable probe molecule over the temperature range studied. When α-methylstyrene is added to asphaltenes, hydrogen transfer from the asphaltenes to α-methylstyrene will form cumene (Figure 2). The reverse reaction, with
Figure 2. Hydrogen transfer between α-methylstyrene and cumene.
cumene transferring hydrogen to asphaltenes, is in principle also possible, although it will be shown that the reverse reaction was not observed. The impact of three variables was investigated in relation to the conversion of asphaltenes and α-methylstyrene mixtures: reaction temperature, reaction time, and asphaltenes to α-methylstyrene feed ratio. The conversion of α-methylstyrene with asphaltenes was studied over the temperature range of 150−250 °C (Table 3, exp. A). The conversion of α-methylstyrene and selectivity to cumene at each reaction temperature is shown in Figure 3. Over a 1 h reaction period at 150 °C, no conversion of αmethylstyrene was observed and the methanol extract of the reaction product contained no other reaction products beyond that in the feed. The conversion of α-methylstyrene increased with an increase in temperature, whereas selectivity to cumene decreased. Due to the low concentration of cumene in the
Figure 4. Conversion of α-methylstyrene (●) and selectivity to cumene (■) during the reaction of a 1:1 mixture of α-methylstyrene and asphaltenes at 250 °C under 4 MPa N2.
For all of the experiments conducted at 250 °C, irrespective of the reaction length, the selectivity to form cumene remained in the range of 30−45%. There was no relationship between cumene selectivity and overall α-methylstyrene conversion at 250 °C. Conversion increased with time. Over the first hour the rate of conversion was high and near linear with respect to reaction time (r2 = 0.999 for linear regression), where after the rate of conversion decreased. The last series of experiments performed with asphaltenes and α-methylstyrene as probe molecule investigated the effect of the asphaltenes to α-methylstyrene ratio on the reaction at 250 °C at different reaction times (Table 3, exp. E). It was suspected that the rate of α-methylstyrene conversion depended on the availability of transferable hydrogen and hence the amount of asphaltenes in the reaction mixture. The change in conversion of α-methylstyrene as a function of asphaltenes to α-methylstyrene ratio, as well as reaction time, is shown in Figure 5.
Figure 3. Conversion of α-methylstyrene (●) and selectivity to cumene (■) during the reaction of a 1:1 mixture of α-methylstyrene and asphaltenes for 1 h under 4 MPa N2. E
DOI: 10.1021/acs.energyfuels.7b00661 Energy Fuels XXXX, XXX, XXX−XXX
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Table 8. Product Selectivity from Conversion of αMethylstyrene and Asphaltenes at 250 °C under 4 MPa N2 selectivity (%) asphaltene/αmethylstyrene ratio
reaction time (min)
cumene
recovered byproducts
unrecovered byproducts
1:1a
20 40 60 80 100 120 240 30 60 120 60 30 60 120 30 60 120
36 34 30 37 39 44 35 37 52 42 43 47 62 52 54 66 61
54 55 62 52 51 42 53 60 44 54 12 51 33 47 43 33 36
10 11 8 10 10 13 13 3 4 4 45 2 5 1 3 1 3
2:1b
Figure 5. Conversion of α-methylstyrene in the reaction of asphaltenes and α-methylstyrene at 250 °C under 4 MPa N2.
3:1b 4:1b
There was a monotonic increase in α-methylstyrene conversion with an increase in asphaltenes to α-methylstyrene ratio, as well as reaction time. Over the reaction time period studied, the relationship between α-methylstyrene conversion and reaction time appeared near linear. This was particularly apparent for the conversion of the 8:1 asphaltenes to αmethylstyrene ratio, where the relationship between αmethylstyrene conversion and reaction time was linear (r2 = 0.999 for linear regression), despite the conversion of αmethylstyrene reaching 56% after 2 h. 3.3. Asphaltenes and α-Methylstyrene Reaction Products. The conversion of α-methylstyrene by asphaltenes resulted in the formation of cumene as a major product from hydrogen transfer, as illustrated by the reaction in Figure 2. However, cumene was not the only product. The products of α-methylstyrene conversion were grouped into three selectivity categories: cumene, recovered byproduct, and unrecovered byproducts. Focusing on the conversion of asphaltenes and αmethylstyrene at 250 °C (Table 3, exp. D and E), the selectivity toward each of these categories was determined by analysis of the methanol extract in combination with material balance (Table 8). These results are only semiquantitative, due to uncertainty in the quantification of cumene, and by extension the recovered byproducts. The unrecovered byproducts could only be determined indirectly by material balance closure with respect to the amount of α-methylstyrene that was converted. Since it appeared that there is no systematic change in selectivity with respect to reaction time (Figure 4), the average selectivity values were calculated and are shown in Figure 6. There was too much variability in the data to conclude trends with certainty, but some general observations can be made. Cumene selectivity appeared to increase slightly with an increase in the asphaltenes content, whereas the recovered byproducts decreased slightly with an increase in the asphaltenes content. The unrecovered byproducts that remained in the asphaltenes were
