Addition Reactions of Olefins to Asphaltene Model Compounds

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Addition Reactions of Olefins to Asphaltene Model Compounds Samuel D. Cardozo, Matthias Schulze, Rik R. Tykwinski, and Murray R Gray Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 01 Mar 2015 Downloaded from http://pubs.acs.org on March 3, 2015

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Addition Reactions of Olefins to Asphaltene Model Compounds Samuel D. Cardozo,† Matthias Schulze,‡ Rik R. Tykwinski,‡ Murray R. Gray*,†

†Department

of Chemical and Materials Engineering, University of Alberta,

Edmonton, Alberta T6G 2V4, Canada ‡Department

Molecular

of Chemistry and Pharmacy, & Interdisciplinary Center for

Materials

(ICMM),

Friedrich-Alexander-Universität

Erlangen-

Nürnberg (FAU), Erlangen 91054, Germany

ABSTRACT Addition reactions have been proposed as a significant pathway for coke formation in the liquid-phase cracking of heavy oils and bitumens. In order to study the kinetics of olefin addition in the liquid phase, two alkyl-bridged aromatic compounds, with molecular weights of 899.70 g/mol and 1127.99 g/mol, were thermally cracked with 1-hexadecene, 1-octadecene, or trans-stilbene, in a batch microreactor at 375–430 °C for 15 to 45 min. Reaction products were analyzed by gas chromatography, high-performance liquid chromatography, matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), and proton nuclear magnetic resonance (1H NMR) spectroscopy. Kinetic data indicate a first-order reaction in model compound concentration, with energetics consistent with a free-radical chain mechanism. Tandem MS/MS and 1H NMR spectra of the products are consistent with olefin addition through the alkyl bridge of the bridged aromatics. The results imply that the addition products are able to abstract hydrogen to give detectable products faster than they decompose, and that the addition products can react even more readily than the parent compounds. 1

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INTRODUCTION Asphaltenes, the heaviest fraction of heavy oils and bitumen, are an extremely complex mixture of polycyclic aromatic hydrocarbons (PAHs), sulfur, nitrogen, and organometallic compounds, defined as the oil fraction soluble in toluene but insoluble in paraffinic solvents, such as n-pentane or n-heptane.1 The solubility definition of asphaltenes generates a broad distribution of molecular structures with a high degree of heterogeneityi and complexity,2,

3

and despite

substantial research, no general consensus exists on a representative chemical structure for asphaltenes. Two structural models for asphaltenes have been proposed and are still under much debate: the archipelago model,4, 5 composed of polycyclic aromatic cores and cycloalkyl groups linked mainly via alkyl bridges, versus the island model, based on a dominant motif of an alkylated condensed polycyclic aromatic core.6 The most recent studies provide a strong support for significant concentrations of a multicore archipelago structure. For example, studies by Rueda et al.3 and Karimi et al.7 show evidence for a significant mass fraction of small, bridged fragments in the asphaltenes. Studies of model compounds demonstrate that such structures also arise due to addition reactions during thermal cracking in the liquid phase.8 The combination of phase separation and addition reactions is thought to lead to coke formation during cracking of heavy petroleum fractions.9 Commercial upgrading processes target the vacuum residue fractions of heavy oils and bitumen and are capable of dealing with the high heteroatom content and the high coke-forming tendency of these materials.10 The core step in any upgrading process for residues is cracking of the large molecules into smaller, more volatile fragments. Petroleum fractions begin to undergo rapid cracking at temperatures above ca. 400 °C. The extent of conversion increases with the time and temperature. At low conversion of vacuum residue, usually below 20%, the viscosity of the feedstock is reduced and the products are cracked vapor and a single liquid phase.11 At higher conversion, the asphaltenes become incompatible with the liquid phase, eventually giving rise to coke formation.9 At very high temperatures (>600°C), the cracking takes place mainly in the vapor phase, which minimizes addition reactions and leads to high gas yields.12 Regardless of the process conditions, the underlying chemistry is the

2


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same; thermal cracking of heavy oils and vacuum residues is controlled by a freeradical chain mechanism, illustrated in a highly simplified form in Scheme 1.

Initiation

R–R’

kin

R + R’

Propagation: kij

Hydrogen abstraction

βj + RjH

β-scission

R–C–C–βj

Radical addition

Ri + C=C–Rk

βjH + Rj ki

R–C=C + βj ka

kr

Radical rearrangement R–C–C–βj

Ri –C–C–Rk Ri

Termination: kc

Radical recombination Ri + Rj Disproportionation

Ri–Rj + Rk

Ri –Rj kd

Ri =Rj + RkH

Olefin removal Donor solvent Catalytic Aromatic hydrogenation

R–C=C + R’H2 R–C=C + H2 R’ + H2

ka

ks kh

R–C–C + R’ R–C–C

R’H2

Scheme 1. Chain reaction mechanism for liquid-phase cracking (adapted from Gray and McCaffrey13). Most hydrogen atoms are omitted for clarity.

The initiation step consists of the homolytic cleavage of the chemical bonds of the molecules to produce radical species, which in turn can propagate through many scission and hydrogen abstraction steps before termination. The minimum energy required to break these covalent bonds in the initiation step is termed bond dissociation energy (BDE) and depends on the nature of the bond. At the typical temperatures for heavy oil/bitumen upgrading of ca. 400–540 °C,10 aromatic C–C and C-S bonds are not expected to break due to the high bond strength and aromatic stabilization, but aliphatic C–C and C-S bonds spontaneously cleave at these elevated temperatures without the use of a catalyst. This first initiation step is the most energy intensive, but the high energy requirement is compensated by the low activation energies of the subsequent steps and make the overall activation energies for cracking much lower.13 As first elucidated by Kossiakoff and Rice,14 both β-scission and hydrogen abstraction are 3


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fundamental steps for the propagation of free radicals. The cracking of radicals on alkyl chains via β-scission leads to the formation of an olefin product. These unsaturated compounds can then participate in radical addition reactions in the liquid phase and generate intermediate radicals that can further react or terminate via hydrogen abstraction, ultimately leading to the formation of higher molecular weight material and coke.13 At low temperatures, below circa 250 °C, these reactions are used in commercial polymerization processes and the addition reactions are thermodynamically favored.15 At the temperatures for residue conversion, over 400 °C, cracking becomes more thermodynamically favored, but radical addition remains kinetically accessible in the liquid phase.13, 16 For

non-volatile components in the vacuum residue fraction, thermal cracking is

a disproportionation process that at high conversions gives high yields of light gases and insoluble carbon-rich coke material. By carrying out the cracking at high partial pressures of hydrogen with a catalyst, the coke formation can be substantially suppressed without changing the underlying cracking reactions,16 likely by hydrogenating olefins to suppress addition.17 Studies by Khorasheh and Gray18-20 showed that α-olefins formed from cracking of n-hexadecane (n-C16) undergo addition reactions in the liquid phase, leading to the formation of larger compounds. Wu et al.21 demonstrated that the formation of branched alkanes with higher carbon number than the starting material was only significant at the high molecular density of liquid-phase cracking. Since the asphaltenes transition to a liquid melt prior to reaching reaction temperatures,22 even in the absence of the other components of vacuum residue, thermal cracking of this fraction must occur predominantly in the liquid-phase. Unfortunately, the extreme complexity of the vacuum residue components, especially the asphaltenes,23 makes direct study of reaction pathways extremely difficult. One method to investigate addition reactions in the liquid phase is by probing the behavior of defined synthetic compounds that incorporate substructures and functional groups that are known to be present in the asphaltenes, within the established range of molecular weights, and with sufficiently high boiling points to ensure liquid-phase cracking. Using this approach, Alshareef et al.8 showed that a range of alkyl aromatic compounds formed addition products with molecular weights higher than that of the starting compounds, fully consistent with a free-radical mechanism involving radical addition to cracked products containing unsaturated bonds. Their work indicated 4


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that substituent alkyl groups dominated this behavior, rather than direct aryl-aryl addition. The energetics of the types of reactions illustrated in Scheme 1 are well understood for vapor-phase reactions of small hydrocarbon species.14, 18, 24, 25 The formation of addition products during cracking would depend on the rate of radical addition to unsaturated intermediates, followed by hydrogen abstraction to stabilize the radical addition product. Assessing the relative importance of these competitive processes requires the Arrhenius parameters and the concentrations of the reacting species. Although little work has been done on large alkyl-aromatic compounds that characterize vacuum residues and asphaltenes, a general observation is that radical addition reactions have quite low activation energies (8–34 kJ/mol),19 and that rate constants for these bimolecular reactions tend to vary with radical and olefin substitution, decreasing with increasing radical size due to steric effects.26, 27 The objective of the present study was to investigate the kinetics of the addition reactions of olefins to model compounds for asphaltenes. Asphaltenes contain porphyrins, large aromatics, and bridging alkyl groups as discussed above. A model compound consisting of a nickel porphyrin with linked pyrene groups, which represents some chemical features of the asphaltenes, was thermally cracked to generate reactive intermediates. Simple alkyl olefins (1-hexadecene and 1-octadecene) and an aromatic olefin, trans-stilbene, were used to produce addition products under defined reaction conditions.

MATERIALS AND METHODS Materials. Olefins include two alpha-olefins, 1-hexadecene (99%, Sigma-Aldrich) and 1-octadecene (99.5%, Sigma-Aldrich), and one aromatic olefin, transstilbene (96%, Acros Organics), and were used as received. The asphaltene model compounds consisted of one or two pyrenyl groups linked together by ethano bridges to a central core consisting of a nickel(II) tetraphenylporphyrin (NiTPP).

Precursors

4-(2-(pyren-1-yl)ethyl)benzaldehyde28

and

5-

phenyldipyrromethane29 were prepared according to the literature protocols. Subsequent condensation of the aldehyde and the dipyrromethane provided a mixture of substituted free-base porphyrins,30 which were separated by column

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chromatography. Metallation with Ni(II) in toluene resulted in formation of the asphaltene model compounds [Et-Py]NiTPP in 3% and 2[Et-Py]NiTPP in 8% yield over the two steps. 2[Et-Py]NiTPP was obtained according to the reaction protocol as isomeric mixture of substituted A2B2-type cis- and trans-derivatives. Detailed data on the melting points and spectroscopic properties of the model compounds are given as Supporting Data. Table 1. Asphaltene model compounds investigated Compound

Structure

[Et-Py]NiTPP

2[Et-Py]NiTPP

a

a

MW, g/mol

Purity, %

899.70

98

1127.99

98

A mixture of cis and trans isomers was used; the trans isomer is illustrated.

Thermogravimetric analysis. A TA Instruments – Q500 Thermogravimetric analyzer was used to perform thermogravimetric analysis (TGA) of the model compounds in order to estimate and compare their initial cracking kinetics to series of pyrenebased bridged model compounds.31 For each experiment, a total of 4–5 mg of the sample was loaded on a platinum pan, heated at 10 °C/min from 25 °C to 500 °C, and then held at 500 °C for 15 min to obtain a cracking residue to approximate the microcarbon residue (MCR) content. All experiments were done using nitrogen as purge gas, at a flow rate of 60 mL/min. Thermal cracking experiments. To investigate the kinetics of the addition reactions between the olefins and asphaltene model compounds, thermal cracking experiments were carried out in ¼ in. stainless-steel batch microreactors under conditions relevant to bitumen upgrading. The batch reactors consisted of 5 cm long tubes (1/4 in. outer diameter) attached to Swagelok integral bonnet 6


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needle valves by a 9 cm section of 1/16 in. tubing, connected and capped with Swagelok fittings. Typically, a total of 1–2 mg of each the compound was loaded into a 3 x 45 mm one-end-sealed glass liner, which facilitated product recovery and helped avoid any catalytic activity with the reactor walls. The olefin was loaded into the body of the reactor to prevent carry-over of material when the olefin boils. Figure 1 illustrates the distribution of reactants inside the reactor. The amount of olefin loaded, approx. 10–60 mg, was based on calculation of vaporliquid phase equilibrium to ensure the presence of an olefin liquid phase. The loaded liner was then placed in the microreactor with the open side up, after which the reactor was sealed, purged with nitrogen at least 5 times, and heated by immersion into a fluidized sand bath (Tecam Fluidized Sand Bath Model No. SBS–4). Fluidization of the sand was achieved through a regulated airflow, which was kept constant at 40% throughout the experiments. Thermal cracking reactions were carried out at 375–430 °C for 15–45 min. At the end of each experiment, the reactor was air-cooled, and the products extracted with dichloromethane and concentrated using a rotary evaporator.

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Vapor Liquid MC

¼” SS microreactor (< 1 mL) Figure 1. Distribution of reactants inside the reactor: model compound (MC) is loaded in a glass liner and the olefin in the body of the reactor. At reaction conditions, the olefin is present in both vapor and liquid phases. Sample analysis. 1H

NMR spectroscopy, along with spectra calculated with MestReNova

software (Mestrelab Research, Santiago de Compostela, Spain) were used in elucidating the location of olefin addition to the model compounds. For spectra collection, approximately 2 mg of sample was dissolved in deuterated dichloromethane (CD2Cl2) and transferred into an NMR tube. The

1H

NMR

spectrum of samples was then collected on a Varian Inova four-channel 500 MHz Spectrometer at 27 °C. Analysis by gas chromatography-flame ionization detector (GC-FID) and gas chromatography-mass spectrometry (GC-MS) used a Thermo Scientific Trace GC Ultra gas chromatograph and attached Trace DSQII Mass Spectrometer. This allowed identification and quantification of small cracked products (mass-tocharge ratio, m/z < 300) in control experiments. For GC-FID runs, the column was 8


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a Thermo TR-5, 7 m x 0.32 mm x 0.25 µm. The GC oven was set at an initial temperature of 30 °C for 1 min, followed by continuous ramping at 10 °C /min to 300 °C, and held at 300 °C until elution was completed. Helium was used as the carrier gas at 2 mL/min in a split mode, with a split flow of 50 mL/min. The FID was operated at 250 °C, with the following gas flows: air at 350 mL/min, hydrogen at 35 mL/min, and make-up gas at 30 mL/min. GC-MS was equipped with a Thermo TR-5MS column, 15 m x 0.25 mm x 0.1 µm, and was run with the same temperature program in a split mode. Measurement of the model compound concentration after reaction was carried out using an Agilent Technologies 1200 Series High-Performance Liquid Chromatograph. The instrument was equipped with a Zorbax Eclipse PAH column of 4.6 x 150 mm with a C18 phase of 3.5 µm particles. The mobile phase was 65% methanol and 35% dichloromethane (isocratic flow), with a temperature of 23 °C, maximum pressure of 400 bar, and constant flow at 1 mL/min. The UV/Visible detector was set to record at a wavelength of 239 nm and 414 nm. An Applied Biosystems SCIEX 4800 Plus MALDI TOF/TOF Analyzer was used to carry out matrix-assisted laser desorption/ionization mass spectrometry (MALDIMS) analyses of the reaction mixtures, allowing the detection of cracked products and addition products that are too heavy to elute in a GC (m/z > 300). This instrument also had tandem MS/MS capabilities, which enabled further fragmentation of precursor ions of a selected mass-to-charge ratio previously identified in a MALDI-MS spectrum. Product samples to be analyzed by MALDI-MS and tandem MS/MS analysis were prepared as follows: trans-2-[3-(4-tertbutylphenyl)-2-methyl-2-propenylidene]malononitrile

(99%,

Santa

Cruz

Biotechnology) matrix solution was made to a concentration of 10 mg/mL in dichloromethane, and vortex-mixed with sub-samples of the reaction solution in a 4:1 (v/v) ratio. To minimize variability due to sub-sampling of the reaction solution, a minimum of three sub-samples were prepared this way. Next, 1.0 µL portions of the final mixtures were spotted onto a 384 Opti-TOF 123 x 81 mm stainless steel MALDI plate from Applied Biosystems, a minimum of 3 spots per sample, and allowed to dry, leaving an opaque crystalline layer. The plate was then magnetically affixed to a stainless steel plate holder and placed into the loading chamber of the instrument. MS data acquisition was carried out in the reflector positive mode. MALDI-MS and MS/MS spectra shown in this study are

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representative of the typical spectrum collected for a particular sample. Acquisition and averaging of multiple spectra allowed minimizing shot-to-shot, region-to-region, and sample-to-sample variability due to heterogeneous cocrystallization of the matrix and analyte.32

RESULTS AND DISCUSSION Thermogravimetric analysis (TGA) of model compounds. The data of Figure 2 show the weight loss and temperature curves of the model compounds from TGA experiments. Both compounds have comparable weight-loss curves, and show little change before the onset of cracking (2.3 wt% and 2.5±0.2 wt% for [EtPy]NiTPP and 2[Et-Py]NiTPP, respectively). These small losses were likely due to the elimination of small amounts of impurities, such as adsorbed solvent molecules from synthesis,33, 34 since cracking is not expected at such low temperatures. The TGA data confirm that these compounds do not evaporate prior to reaching the onset of cracking at ca. 350 °C and so are suitable for investigating thermal cracking reactions in the liquid phase. The weight loss observed after the onset of decomposition corresponds to cracking of the molecules and subsequent evaporation of the lighter fragments. For these MCs, the first bond that is likely to cleave is the aliphatic C–C bond in the ethyl bridge (lowest BDE); therefore, cleavage of this bond should initiate radical chain reactions. Similar observations are reported by Lebedeva et al.,35 whose experimental results show that cracking of the substituent alkyl groups was the first stage of decomposition of zinc(II) tetratert-butyltetraphenylporphyrin in TGA experiments. Coke residue was observed at the end of TGA experiments at 500 °C, and the yield of coke is reported in Table 2. These yields were adjusted for mass loss due to the evaporation of solvents and impurities at low temperatures. The high coke yields, compared to those observed by Alshareef et al.31 for a series of three-island archipelago compounds, can be attributed to the large size of the NiTPP aromatic core. Under thermal cracking conditions in the absence of hydrogen, Ni and V are stably held in the porphyrin ring even at very high reaction severity.36 The metal-bearing porphyrinic fragment would give a low rate of evaporation even after cleavage of the side groups, leading to a more conserved liquid phase, and hence, an increased coke yield in comparison to

10


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the compounds with smaller building blocks. The coke yields are not, however, completely consistent with this interpretation. In the [Et-Py]NiTPP, the porphyrin and its attached phenyl groups is 75% of the MW, while for 2[Et-Py]NiTPP it is 59%. This difference was not reflected in the coke yield, which was the same for both compounds (Table 2). Despite the large size of the NiTPP group, some of this material must have evaporated.

100

500

400

2[Et-Py]NiTPP 80

Temperature 300

70 200 60

Temperature (°C)

[Et-Py]NiTPP

90

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100

50 40

0 0

10

20

30 40 Time (min)

50

60

Figure 2. Typical weight loss and temperature curves of model compound pyrolysis in TGA experiments.

The apparent Arrhenius activation energy of cracking (Ea) and preexponential factor (A), via first-order kinetics of mass loss from the compounds, were calculated using the differential method of Alshareef et al.,37 based on the normalized weight loss versus temperature, and can also be found in Table 2. Initial cracking kinetics for the porphyrin compounds fell within the range reported by Alshareef et al.31 Ea values of 140.6–253.1 kJ/mol for these mass-loss kinetics are consistent with cleavage of the ethano bridge and with reported values from Poutsma38 for liquid-phase pyrolysis of 1,2-diphenylethane.

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Table 2. Thermogravimetric analysis (TGA) results for the model compounds. Error bounds for 2[Et-Py]NiTPP are standard deviations from three repeat experiments. Results for [Et-Py]NiTPP are means of two repeats. Compound

MW, g/mol

Loss before cracking, wt%

Coke yield, wt%

Ea, kJ/mol

log(A, min-1)

[Et-Py]NiTPP

899.7

2.3

53.4

212.9

13.86

2[Et-Py]NiTPP

1128.0

2.5±0.2

54.0±0.2

190±2

12.27±0.09

3–33

140.6– 253.1

9.6–18.3

Py–[Φ]–Py

a

540.7– 678.9

2.9

b

a

Three-island archipelago compounds studied by Alshareef et al.31 consist of two pyrenyl groups tethered by ethano bridges to a central aromatic group, [Φ] b

Average weight loss before 200 °C for most of the model compounds

Phase equilibrium calculations. The initial reaction mixtures consisted of two phases; a liquid phase of the model asphaltene compound with dissolved olefin, and an olefin-rich vapor phase. The phase compositions would change as the cracking and addition reactions proceed. We approximated the initial conditions as equilibrium between the two phases, which requires that the chemical potential of each component in the liquid phase µiL must equal the chemical potential of each component in the vapor phase µiV.

39

For convenience, we use

the fugacity, which must also be the same for each component.40 Due to their relatively low boiling point, the olefins (1-C16, 1-C18, and transstilbene) are expected to vaporize at reaction conditions. Thus, the amount of olefin loaded for microreactor experiments was selected based on calculation of vapor-liquid phase equilibrium to ensure the presence of a liquid phase. Intrinsic kinetics of elementary reactions depend on the chemical potential of the reacting species, because at equilibrium microscopic reversibility is achieved.40 Although kinetic analyses often use concentration or partial pressure as an empirical substitution for more precise measures of chemical potential, for multiphase reactions the gas phase fugacity of a component is more appropriate Thus, fugacity was chosen to represent the chemical potential of the olefin reactants. Products obtained in thermal cracking of 1-C16 at 410 °C for 20 min were detected by GC-FID and GC-MS, and fall into two general categories; those with molecular weight lower than 1-C16 (complete series of alkanes/alkenes 12


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in the C7 to C15 range, including isomeric products) and those with higher molecular weights (series of alkanes/alkenes in the C17 to C32 range). Due to the low extent of cracking, addition, and homopolymerization of the olefin, the changes in 1-C16 fugacity during reaction were insignificant and fall within the range of experimental error. The fugacity of the olefin-rich vapor phase was estimated with VMGSim (Virtual Materials Group, Calgary, Canada) using the Peng–Robinson cubic equation of state. To validate the calculation of the fugacities by this method, vapor-liquid equilibrium data for boiling point and vapor pressure for trans-stilbene were compared to VMGSim estimates. The simulations were accurate to within 5%, indicating that the calculations were accurate for the simple components (trans-stilbene, 1-C16, and 1-C18). The asphaltene model compounds were in the liquid phase since [Et-Py]NiTPP and 2[Et-Py]NiTPP have melting points in the range of 203–283 °C, and TGA confirmed the low volatility of the samples prior to cracking; therefore, mass concentration was used for these compounds in the kinetic expressions. Cracking of 2[Et-Py]NiTPP with 1-hexadecene and 1-octadecene. To identify the main addition products of the olefin in the liquid-phase cracking of the model compounds, pure 2[Et-Py]NiTPP was thermally cracked at 375 °C for 15 min, and then cracked with added 1-C16 and 1-C18 in subsequent experiments. The reaction products were analyzed by MALDI-MS. The mass spectrum of the products obtained from cracking of 2[Et-Py]NiTPP with and without 1-C16 (Figure 3) shows cracked products with molecular weights lower than the parent compound and heavier species arising from addition reactions (up to ca. m/z 2500 Da). The MALDI-MS of 2[Et-Py]NiTPP does not contain these mass peaks with m/z greater than the parent compound, except for some impurities; therefore, these heavy ions are not instrumental artifacts. Cracking with added 1-C16 shows the formation of an addition product of m/z value consistent with that of the olefin plus 2[Et-Py]NiTPP, and the MS/MS spectrum of the ion from the addition product at m/z = 1350.1 supports addition through the alkyl bridge. This pathway for addition has been observed by Alshareef et al.8 who emphasized that addition reactions occur predominantly on the alkyl chains and bridges attached to aromatic ring systems, rather than to the aryl carbons. Furthermore, in reactions with 1-C18 the olefin addition peaks shifted by 28 mass units, consistent with the 2 additional CH2 in the alkyl chain. Likewise, in 13


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reactions with 1-C18, the fragmentation pattern of the precursor ion by MALDI MS/MS spectrometry is also in accordance with addition to the bridge. Detection by MALDI-MS spectrometry confirms addition of the olefin to 2[Et-Py]NiTPP; hence, the olefin-2[Et-Py]NiTPP coupling product must form at a rate higher than it is consumed by reaction, giving the strong signal of the ion due to the addition product illustrated in the mass spectrum of Figure 3.

100000 Parent compound

80000 Cracking products

Addition products

60000

40000

intensity (arbitrary units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20000

0 100000 1350.1

80000

60000

40000

1345

1350

1355

1360

1365

20000

0 400

700

1000

1300

1600

1900

2200

2500

mass/charge ratio (m/z)

Figure 3. MALDI mass spectrum of products from cracking of (a) 2[Et-Py]NiTPP at 375 °C for 15 min and (b) 2[Et-Py]NiTPP with 1-C16 under equivalent conditions. Reaction with 1-C16 shows the formation of the coupling product at m/z 1350.1 and corresponding isotopic envelope.

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1H

NMR spectroscopy of reaction products. 1H NMR spectrum of the soluble

products obtained from thermal cracking of 2[Et-Py]NiTPP with 1-C16 at 375 °C for 15 min show new resonances arising at δ 5.45 ppm that are not present in the initial 1-C16. These signals are consistent with substituted internal olefins from homo-addition of 1-C16 under thermal conditions. On the other hand, no significant change is observed for signals in the aromatic region (Figure 4). The spectral observations exclude the possibility of significant substitution on the aromatic rings, but do not, unfortunately, offer evidence of the where the substitution does occur.

ppm

ppm

Figure 4. 1H NMR (500 MHz) spectrum of products from pyrolysis of 2[Et-Py]NiTPP with 1-C16 at 375 °C for 15 min. The aromatic region is enlarged.

Cracking of [Et-Py]NiTPP with 1-hexadecene. Thermal cracking reactions at different times and temperatures of [Et-Py]NiTPP with 1-C16 allowed a more detailed kinetic analysis. The amount of [Et-Py]NiTPP remaining at each reaction time was determined by HPLC. Thermal cracking of [Et-Py]NiTPP with 1-C16 at 375 °C gave conversions ranging from 25% at 20 min to 40% at 45 min. The data indicated a first-order reaction in [Et-Py]NiTPP concentration, with a rate constant for [Et-Py]NiTPP conversion of (1.9 ± 0.1) x 10-4 s-1. Conversion of [Et-Py]NiTPP was lower without the presence of olefin; only 13.3% after 20 min. Since [Et-Py]NiTPP is 15


Energy & Fuels

the main radical generator during cracking, it is likely that these unsaturated species act as radical scavengers, reacting with the parent radicals that could otherwise reform the parent compound by hydrogen abstraction. Thus, the olefin addition reaction would contribute to an increase in the rate of disappearance of the parent compound, as more pathways for reaction are added, causing an increase in conversion. Two series of experiments were performed, one at 390 °C and the other at 410 °C, and the corresponding rate constants were obtained. Figure 5 shows the Arrhenius plot of first-order rate constants for overall conversion of [Et-Py]NiTPP in reactions with 1-C16. The reaction is relatively slow, but is accelerated considerably with temperature with an estimated activation energy of 180 ± 40 kJ/mol. This value is consistent with a mechanism dominated by free-radical chain reactions, where the propagation steps are significant, and is comparable with available literature data. For example, Smith and Savage41 report an apparent activation energy of 167 ± 25 kJ/mol for cracking of 1,3-bis(1pyrene)propane, while Savage et al.42 report a value of 170.7 kJ/mol for cracking of 1-dodecylpyrene. An estimate of log(A) equal to 10.9 ± 3.0 s-1 was obtained from the Arrhenius plot (Figure 5).

R² = 0.9561 1

k (s-1), x 103

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Page 16 of 27

0.1 1.44

1.46

1.48

1.50

1.52

1.54

1.56

1/T (K-1), x 103

Figure 5. Arrhenius plot of first-order rate constants for conversion of [Et-Py]NiTPP in thermal cracking reactions with 1-C16.

16


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The molar yield for the olefin-[Et-Py]NiTPP addition product, defined as the number of moles of product formed from the decomposition of 100 moles of the parent model compound in the microreactor experiments, is shown in Figure 6 as a function of temperature. A combination of the data from HPLC and MALDI-MS were used to calculate the yield of addition product. Only peaks above the noise level of the spectrum were included. The data suggest that the coupling product is an intermediate that is subject to formation and destruction, consistent with observations by Alshareef et al. for other three island MCs,31 who found that addition products formed at low conversions remain reactive, and would thus continue to react to give smaller cracked fragments and still larger structures as conversion increases. In this sense, the resulting coupling product from the olefin[Et-Py]NiTPP addition will participate in further reactions, such as hydrogen abstraction to produce a radical, which might subsequently rearrange to a more stable radical, combine with another radical, undergo cracking via β-scission, or add to another olefin, amongst other possibilities.

Yield (mol/100 mol of model compound converted)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

40

30

20

10

0 370

380

390

400

410

420

430

440

Temperature (°C)

Figure 6. Yield of the coupling product in reactions of [Et-Py]NiTPP with 1-C16 as a function of temperature (reaction time: 20 min) Given formation and cracking, the following rate expression was derived for the olefin-[Et-Py]NiTPP coupling product, C: dሾCሿ = k1 fo ൣPሶ ൧ − k2 ሾCሿ dt

(1)

17


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where [C] (mg/cm3) is the addition product mass concentration, k1 (s-1 kPa-1) is the rate constant for the parent radical-olefin addition step, fo (kPa) is the olefin fugacity, ൣPሶ ൧ (mg/cm3) is the parent radical concentration, and k2 (s-1 kPa-1) is the rate constant for consumption of the addition product. At low conversion, the concentration of the parent radical will be constant and proportional to the concentration of the parent compound: ൣPሶ ൧=βሺTሻ ∙ ሾPሿ. In this expression, the temperature dependence of the constant of proportionality, β, will depend on the underlying elementary reactions and their activation energies. This result gives: dሾCሿ = k1 βfo ሾPሿ − k2 ሾCሿ dt

(2)

where k1, β, and k2 are all functions of temperature. Additionally, from the kinetics derived for model compound conversion in reaction with 1-C16:

dሾPሿ = − kሾPሿ dt

(3)

where k (s-1) is defined by the following Arrhenius parameters: Ea of 180.6 ± 38.7 kJ/mol and log(A) of 10.9 ± 3.0 s-1 (as previously estimated). Solving these two differential equations gives (for a given temperature so that k1, β, and k2 are constant):

[C]=

k1 βfo [P]o −kt ൣe − e−k2t ൧ k2 − k

(4)

where [P]o is the concentration of the parent compound at t = 0. Fitting this model to the experimental yield of addition product at 375 °C by minimizing the total sum of squares gives the following estimates for the rate constants: k = 2 x 104

s-1, (k1 β) = 5.1 x 10-7 s-1 kPa-1, and k2 = 2 x 10-3 s-1. These rate estimates suggest that

the addition product is consumed at a rate faster than the parent compound, indicating that these alkylated adduct structures react even more readily than the parent compounds to form volatile fragments. For example, cleavage of the H2C–CH2 bridge between the aromatic groups would generate a more stable secondary benzylic radical, which in turn is a more favorable reaction intermediate than in the case of the parent compound.

Rate-determining step. Scheme 2 illustrates the two elementary steps required to form the olefin-[Et-Py]NiTPP addition product: olefin addition to the parent radical and subsequent hydrogen abstraction that forms the stable addition product. 18


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Energy & Fuels

Activation energies and pre-exponential factors are averaged from values given by Freund and Olmstead,43 Smith and Savage,41, 44, 45 Khorasheh and Gray,18 and Fischer and Radom.26 Olefin addition to the parent radical is significantly slower than the subsequent hydrogen abstraction step, based on these literature sources. This observation is consistent with the high concentration of available abstractable hydrogen in the liquid phase, both benzylic and aliphatic. In turn, the addition reaction between the olefin and the parent radical requires that these two species collide with sufficient energy and in the correct orientation. Taking these factors into account, addition of the olefin to the parent radical is likely rate limiting, so that the intermediate radical formed stabilizes rapidly via hydrogen abstraction.

Scheme 2. Representative elementary steps involved in the formation of the coupling product, where R1 = C14H29: (a) radical addition to the unsaturated bond (Ea of 33.5 kJ/mol, log(A) of 7.36 L mol-1 s-1), and (b) hydrogen abstraction (Ea of 29.3 kJ/mol, log(A) of 8.00 L mol-1 s-1). Reaction rate estimates at 400 °C: 5.8 x 104 L mol-1 s-1and 5.3 x 105 L mol-1 s-1for kadd and kabs, respectively.

Olefin reactivity. The data of Table 3 show the impact of the added olefin on the yield of the coupling product in thermal cracking reactions with [Et-Py]NiTPP at 375 and 400 °C for 20 min. At least two experiments were performed with each 19


Energy & Fuels

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Page 20 of 27

olefin to confirm repeatability, and average values are shown. According to the data in Table 3, formation of the addition product is less favored in reactions with

trans-stilbene. The difference in the yield between trans-stilbene and the α-olefins is statistically significant, and outside the range of experimental error and uncertainty in the calculated fugacity. Either the rate constant for addition of

trans-stilbene is much lower, or its subsequent reaction is much faster than in the case of the larger species. In either case, the rate constant for addition to transstilbene suggests that some other factor, aside from fugacity of the olefin reactant, must affect the main reaction pathway for radical addition to the unsaturated bond. Normally, the preferred pathway for free-radical reactions is dominated by the stability of the parent radical, the stability of the intermediate radical addition product, and/or the reactivity of the reactants. In this case, the most favorable parent radical is adjacent to the pyrene due to delocalization of the unpaired electron over the π orbital system.46 Consequently, unsaturated species probably add next to the pyrene, forming a family of addition products all in the same location. As discussed above, preference for the two carbons in the bridge is difficult to establish from MALDI MS/MS and 1H NMR spectra data.

Table 3. Impact of the added olefin on the yield of the coupling product in thermal cracking reactions with [Et-Py]NiTPP (reaction time: 20 min)

Olefin

Conversion, wt%

Yield, mol/100 mol of converted model compound

Olefin fugacity, kPa

375 °C

400 °C

375 °C

400 °C

375 °C

400 °C

1-C16

25±6

50.5

29±8

15.8

432

583

1-C18

21.9

44.9

29.0

14.5

277

387

trans-stilbene

23.9

40.7

18.2

3.4

308

434

The stability of the intermediate radical addition product must take into account the fact that addition of the radical to the π-bond of the olefin is regioselective,47 giving the more stable carbon radical as an intermediate. Thus, the addition reaction causes the radical center to move away from the pyrene, resulting in either a secondary alkyl radical in the case of reaction with α-olefin, or a more stable benzylic radical if reaction were with trans-stilbene. The added 20


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Energy & Fuels

stability of the benzylic radical is primarily due to the delocalization resonance effect. As noted by Fischer and Radom,26 benzyl radicals reacted 100–400 times more slowly than methyl in addition reactions to alkenes, and that much of this difference could be the result of the radical-stabilizing effect of the phenyl group. Therefore, if the stability of the intermediate radical controlled the rate of formation of the addition product, higher yields would be expected in reactions with stilbene. Given the contrary observation that stilbene is less reactive under the same conditions, the reactivity of the reactants must have a greater impact on the outcome. Two important factors influence the reactivity of olefins in addition reactions with radicals: the electron density on the double bond and steric hindrance. A lower electron density in the double bond of stilbene due to delocalization of the electrons from conjugation, would lower its reactivity towards radical addition,48, 49 compared to α-olefins with localized π-electrons. The nature of the substituents on the olefinic bond affects the rate of addition of radicals, as these influence the interactions between the singly occupied orbital of the radical (SOMO) and the lowest unoccupied orbital (LUMO) and/or the highest occupied orbital (HOMO) of the π-C–C bond.50, 51 Likewise, substituents on the alkene could sterically hinder addition to radicals and reduce the rates of addition,52, 53 as in the present case, where stilbene has bulkier phenyl substituents on the double bond, compared to the linear alkyl α-olefins. The difference in reactivity of stilbene is consistent with a combination of these factors, both electron density and steric hindrance effects. For example, Cameron and Grassie54 reported that in polymerization reactions with styrene and trans-stilbene, the reactivity of styrene towards radical addition was much higher than that of

trans-stilbene, and showed that addition of the second benzene ring onto the double bond lowered the reactivity of the olefin. Contrary to reactions with the α-olefins, the major addition product in reactions of [Et-Py]NiTPP with trans-stilbene was observed with m/z 1594.4. This main peak in the product spectrum cannot be reconciled with addition to transstilbene. The formation of the addition product m/z = 1594.4 was also significant in reactions of [Et-Py]NiTPP without trans-stilbene and would involve the addition of fragments of the original molecule to [Et-Py]NiTPP (Supporting Information). These

21


Energy & Fuels

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Page 22 of 27

observations imply that the stilbene is not highly reactive after the rapid formation of an initial adduct.

CONCLUSIONS (1) Reaction of olefins with [Et-Py]NiTPP and 2[Et-Py]NiTPP during cracking at 375–430 °C gives addition products. MALDI-MS shows the formation of an addition product of molecular weight consistent with that of the olefin plus the model compound, and MS/MS and

1H

NMR spectra are

consistent with addition to the alkyl bridge. (2) Kinetic

data

indicate

first-order

reaction

in

parent

compound

concentration, with energetics consistent with a free-radical chain mechanism. (3) Addition of the olefin to the parent radical is likely rate limiting, so that the intermediate radical formed terminates rapidly via hydrogen abstraction. (4) Lower reactivity of trans-stilbene in free-radical addition is consistent with a combination of electron density and steric hindrance effects.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We are grateful for the financial support from the Institute for Oil Sands Innovation at the University of Alberta and to Lisa Brandt, Xiaoli Tan, and Keith Draganiuk for their technical support and assistance.

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37. Alshareef, A. H.; Azyat, K.; Tykwinski, R. R.; Gray, M. R., Measurement of Cracking Kinetics of Pure Model Compounds by Thermogravimetric Analysis. Energy Fuels 2010, 24, 3998-4004. 38. Poutsma, M. L., A review of thermolysis studies of model compounds relevant to processing of coal. Energy Fuels 1990, 4, 113-131. 39. Whitson, C. H.; Brule, M. R., Phase behavior. Society of Petroleum Engineers: Richardson, TX, 2000; Vol. 20. 40. Prausnitz, J. M.; Lichtenthaler, R. N.; Gomes de Azevedo, E., Molecular Thermodynamics of Fluid-Phase Equilibria. Prentice Hall: Upper Saddle River, NJ, 1999. 41. Smith, C. M.; Savage, P. E., Reactions of polycyclic alkylaromatics. 2. Pyrolysis of 1,3-diarylpropanes. Energy Fuels 1991, 5, 145-155. 42. Savage, P. E.; Jacobs, G. E.; Javanmardian, M., Autocatalysis and aryl alkyl bond cleavage in 1-dodecylpyrene pyrolysis. Ind. Eng. Chem. Res. 1989, 28, 645-653. 43. Freund, H.; Olmstead, W. N., Detailed chemical kinetic modeling of butylbenzene pyrolysis. Int. J. Chem. Kinet. 1989, 21, 561-574. 44. Smith, C. M.; Savage, P. E., Reactions of polycyclic alkylaromatics. 4. Hydrogenolysis mechanisms in 1-alkylpyrene pyrolysis. Energy Fuels 1992, 6, 195-202. 45. Smith, C. M.; Savage, P. E., Reactions of polycyclic alkylaromatics—VI. Detailed chemical kinetic modeling. Chem. Eng. Sci. 1994, 49, 259-270. 46. Ahluwalia, V. K.; Parashar, R. K., Organic Reaction Mechanisms. Alpha Science International: Harrow, Middlesex, 2005. 47. De Vleeschouwer, F.; Jaque, P.; Geerlings, P.; Toro-Labbe, A.; De Proft, F., Regioselectivity of radical additions to substituted alkenes: Insight from conceptual density functional theory. J. Org. Chem. 2010, 75, 4964-4974. 48. De Mico, A.; Margarita, R.; Mariani, A.; Piancatelli, G., Radical additions to olefins in the presence of iodobenzenediacetate: An easy route to alkyl dithiocyanates. Tetrahedron Lett. 1996, 37, 1889-1889. 49. Moore, L. O., Radical reactions of highly polar molecules. Relative reactivity of halogenated olefins in haloalkyl radical additions. J. Phys. Chem. 1970, 74, 3603-3607.

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Addition Reactions of Olefins to Asphaltene Model Compounds

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