ARTICLE pubs.acs.org/EF
Syntheses of Alkylated Aromatic Model Compounds To Facilitate Mass Spectral Characterization of Heavy Oils, Resids, and Bitumens M. A. Francisco,* R. Garcia, B. Chawla, C. Yung, K. Qian, K. E. Edwards, and L. A. Green Corporate Strategic Research, ExxonMobil Research and Engineering Company, Annandale, New Jersey 08801, United States ABSTRACT: This study summarizes the synthesis of model compounds. Long-chain alkyl multi-ring heteroaromatics are the synthetic targets. These compounds are considered analogues of similar compounds that are present at abundant levels in petroleum crude oils, resids, heavy oils, deasphalted oils, asphaltenes, and bitumens, and they are responsible for coke formation and other processing problems. Model compounds are not readily available. Some models have been synthesized to approximate the physical and chemical behavior of real asphaltenes, but the structures are not completely consistent with the current structural understanding. The composition of petroleum crudes, resids, heavy oils, and bitumens has been a subject of intense research for years. The structure of these coke-forming molecules is generally thought to be an aromatic core of critical molecular weight and number of fused aromatic rings. This critical ring number is around 4 5 fused aromatic rings, which bear alkyl side chains and heteroatoms. This general structural view is still being improved. During coking, these side chains thermally cleave off of the aromatic core and leave the coker as volatile liquids and gas. The aromatic cores are thermally stable, and they are not volatile. They remain in the coker until they condense and form low-value coke. The purpose of this research was to synthesize model compounds representing the molecular structure of the coke-forming molecules. These compounds will be used to model the behavior of resids, heavy oils, and bitumens to understand their processing chemistry and their physical and chemical properties. Model compounds that reasonably represent the structural data with boiling point ranges that map directly onto the boiling point ranges of real heavy oils, resids, and bitumens have been successfully synthesized. Important structural features are emphasized in these models, such as high molecular weight [∼1000 atomic mass units (amu)] with multi-ring aromatic cores, long alkyl side chains, and the inclusion of sulfur and nitrogen. These model compounds can be used to calibrate analytical methods to provide response factors for quantitative characterization of alkylated aromatics in real petroleum fractions and to better understand thermal chemistry kinetics.
’ INTRODUCTION This study summarizes the synthesis of model compounds consisting of long-chain alkyl multi-ring heteroaromatics. These compounds are considered analogues of similar compounds that are present at abundant levels in petroleum crude oils, resids, heavy oils, deasphalted oils, asphaltenes, and bitumens, and they are responsible for coke formation1,2 and other processing problems.3 5 Model compounds are not readily available for commercial purchase. Some models have been synthesized to approximate the physical and chemical behavior of real asphaltenes, but the structures are not completely consistent with the current structural understanding.6 10 The composition of petroleum crudes, resids, heavy oils, and bitumens has been a subject of intense research for years.11 15 The structure of these cokeforming molecules is generally thought to be an aromatic core of critical molecular weight or a number of fused aromatic rings. This critical ring number is around 4 5 fused aromatic rings, which bear alkyl side chains and heteroatoms. This general structural view is still being improved. During coking, these side chains thermally cleave off of the aromatic core and leave the coker as volatile liquids and gas. The aromatic cores are thermally stable, and they are not volatile. They remain in the coker until they coalesce and form low-value coke. The purpose of this research was to synthesize model compounds representing the molecular structure of the coke-forming molecules. These compounds will be used to model the behavior of resids, heavy oils, and bitumens to understand their processing chemistry and their physical and chemical properties. r 2011 American Chemical Society
Compounds with shorter alkyl chains and less than three rings in the aromatic/heteroaromatic portion of the molecule have been synthesized with difficulty. Synthesis of these compounds has been difficult because of aromatic tar formation and cracking of the longer alkyl chains. Tar formation and cracking becomes more of a problem with compounds that have more than three fused aromatic rings and longer alkyl side chains.16 21 Compounds with four fused aromatic rings and alkyl side chains of up to 22 carbon atoms were targeted because of the availability of pure starting materials and the attractiveness of using alkylation chemistry with zeolites and trifluoroacetic acid instead of Friedel Crafts catalysts. Friedel Crafts catalysts have been used in the past with alkyl halides to alkylate aromatics and produce alkyl aromatics, but this chemistry produced low yields of desired products, which had to be separated from intractable tars.22 33 Pyrene, phenanthrene, dibenzothiophene, and carbazole are commercially available multi-ring aromatics, as well as terminal olefins up to 22 carbon atoms. Zeolites34 and trifluoroacetic acid35 have been used to alkylate aromatics of three fused rings or less with short-chain olefins without the formation of tar, and the catalysts are easily removed from the products. All of these reactions (zeolites, trifluoroacetic acid, and Friedel Crafts catalysts) are referred to generally as alkylation reactions and proceed via an electrophilic aromatic substitution mechanism.36 Received: July 14, 2011 Revised: September 6, 2011 Published: September 28, 2011 4600
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Figure 1. Synthesis of alkyl aromatics.
The reactions consist of an aromatic compound, an alkylating agent, and an acid catalyst. Figure 1 illustrates a general example. Alkylating agents can be alkyl halides,37 alkenes,38 or alcohols and their derivatives.39 Carbonyl compounds have also been used as alkylating agents, but the products are hydroxyl alkylated aromatics and not the desired alkyl aromatics.40 42 The soluble protic43 or Lewis acid44 or the solid acid catalysts45,46 (used in these reactions) coordinate to the chlorine atom of an alkyl halide or the hydroxyl group of alcohols and polarize the C Cl bond or the C OH bond, making the carbon electrophilic. They also coordinate to the double bond of an alkene, making one of the carbon atoms electrophilic. The electrophilic carbon atom can attack the nucleophilic aromatic compound, and the positively charged intermediate loses a proton to generate an alkyl aromatic and a proton. These reactions result in complex product mixtures consisting of multiple alkylations, regioisomers, and insoluble tars. The desired products are isolated by distillation and/or chromatography. The objective of this study was to synthesize high-molecular-weight [∼1000 atomic mass units (amu)] model compounds with multi-ring aromatic cores, long alkyl side chains, and the inclusion of sulfur and nitrogen heteroatoms. We believe these molecules reasonably emphasize the key features of real petroleum heavy oils, resids, bitumens, and asphaltenes and could be used to better understand their thermal and analytical chemistry. The synthetic work described in this report is based on the alkylation of pyrene, chrysene, and perylene with long-chain terminal olefins and alcohols using zeolites and trifluoroacetic acid as catalysts.
’ EXPERIMENTAL SECTION Each of these model compounds required somewhat different conditions to synthesize depending upon the starting aromatic compound. Alkylation of Toluene with Hexadecene. Hexadecene (8.36 g, 0.0373 mol, 10.6 mL) was put into a 50 mL round-bottom flask equipped with a magnetic stir bar. Toluene (17.30 g, 0.188 mol, 20.0 mL) was added along with finely ground MCM-22 powder (0.503 g). The mixture was stirred, refluxed under nitrogen at 90 °C for 2 h, and allowed to cool to room temperature. The mixture was suction-filtered to remove the solid catalyst. The solid catalyst was washed several times with small portions of toluene or methylene chloride. The filtrates were evaporated to dryness on a rotary evaporator and then in a vacuum oven at 50 °C overnight. The product yield was 9.48 g (80.3%). The products were analyzed by gas chromatography mass spectrometry (GCMS), field-desorption mass spectrometry (FDMS), and Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS). The workup procedure used in this experiment with toluene is repeated for the workup and product isolation of all of the other experiments, unless otherwise stated. Alkylation of Naphthalene with Hexadecene. Hexadecene (11.25 g, 0.050 mol, 14.3 mL) was put into a 50 mL round-bottom flask equipped with a magnetic stir bar. Naphthalene (4.79 g, 0.0451 mol) was added along with finely ground MCM-22 powder (0.534 g). The
mixture was stirred under nitrogen at 150 °C for 4 h and allowed to cool to room temperature. The product yield was 13.79 g (86.7%). The products were analyzed by GCMS, FDMS, and FTICR MS. Alkylation of Pyrene with Hexadecene. Hexadecene (5.731 g, 0.0255 mol, 7.2 mL) was put into a 50 mL round-bottom flask equipped with a magnetic stir bar. Pyrene (1.707 g, 0.008 44 mol) was added along with finely ground MCM-22 powder (0.5 g). The mixture was stirred under nitrogen at 150 °C for 6 h and allowed to cool to room temperature. The product yield was 5.31 g (71.4%). The products were analyzed by GCMS, FDMS, and FTICR MS. The reaction mixture was separated by high-performance liquid chromatography (HPLC) into fractions to remove starting materials and to isolate the different alkylated products. The fractions were then analyzed by gas chromatography (GC) and nuclear magnetic resonance (NMR). Alkylation of Phenanthrene with Hexadecene. Hexadecene (7.85 g, 0.035 mol, 10.0 mL) was put into a 50 mL round-bottom flask equipped with a magnetic stir bar. Phenanthrene (2.0785 g, 0.0117 mol) was added along with finely ground MCM-22 powder (0.573 g). The mixture was stirred under nitrogen at 150 °C for 6 h and allowed to cool to room temperature. The product yield was 5.96 g (59.6%). The products were analyzed by GCMS, FDMS, and FTICR MS. The reaction mixture was separated by HPLC into fractions to remove starting materials and to isolate the different alkylated products. The fractions were then analyzed by GC and NMR. Alkylation of Dibenzothiophene with Hexadecene. Hexadecene (7.85 g, 0.035 mol, 10.0 mL) was put into a 50 mL roundbottom flask equipped with a magnetic stir bar. Dibenzothiophene (2.15 g, 0.0117 mol) was added along with finely ground MCM-22 powder (0.534 g). The mixture was stirred under nitrogen at 150 °C for 6 h and allowed to cool to room temperature. The mixture was suction-filtered to remove the solid catalyst. The product yield was 8.42 g (84.2%). The products were analyzed by GCMS, FDMS, and FTICR. The reaction mixture was separated by HPLC into fractions to remove starting materials and to isolate the different alkylated products. The fractions were then analyzed by GC and NMR. Alkylation of N-Phenylcarbazole with Hexadecene. Hexadecene (7.85 g, 0.035 mol, 10.0 mL) was put into a 50 mL roundbottom flask equipped with a magnetic stir bar. N-Phenylcarbazole (2.80 g, 0.0115 mol) was added along with finely ground MCM-22 powder (0.573 g). The mixture was stirred under nitrogen at 150 °C for 6 h and allowed to cool to room temperature. The product yield was 8.40 g (78.9%). The products were analyzed by GCMS, FDMS, and FTICR MS. The reaction mixture was separated by HPLC into fractions to remove starting materials and to isolate the different alkylated products. The fractions were then analyzed by GC and NMR.
Preparative Separation/Purification/Isolation of Alkylated Reaction Mixtures. Initially, one of the reaction mixtures was separated using a semi-preparative HPLC, and the resulting fractions were analyzed by GC and NMR. Because the GC and NMR analyses indicated that there were significant overlaps between the fractions, we decided to use a preparative HPLC technique for the separation. The HPLC system consisted of a Waters quaternary solvent delivery system, a 2 mL sample loop injector, a 10 position electrically actuated Valco valve, a Valco backflush valve, and a Waters photodiode-array 4601
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Table 1. Alkylation Reactions GC peak area (%) starting aromatics
unreacted aromatic unreacted olefin dimer of olefin
pyrene
1.8
2.7
3.4
phenanthrene
4.6
24.8
21.7
dibenzothiophene
2.3
12.5
24.8
9-phenylcarbazole
0.0
8.0
10.1
Figure 2. GCMS analysis of alkylated toluene.
Table 2. Product Yields (as a Weight Percentage of the Reaction Mixture) starting aromatics
total products
mono
di
tri
tetra
pyrene
92.1
18.3
41.3
28.7
11.7
phenanthrene dibenzothiophene
48.9 60.4
45.2 43.0
40.9 47.4
14.0 5.8
none none
9-phenylcarbazole
81.8
4.1
60.2
26.2
9.6
Figure 3. GCMS analysis of alkylated naphthalene. detector. A column (50 cm 25 mm inner diameter) pre-packed with 10 μm particles of [3-(2,4-dinitroanilino)propyl]silica (DNAP-silica or DNAP) was used. HPLC has been optimized to separate about 200 mg of a reaction mixture. GC Analysis. GC analyses were performed on the reaction mixture, as well as the separated fractions. The sample concentration was about 20 mg/2 mL of methylene chloride. GC chromatograms were collected on a HP6890 instrument equipped with a Sievers chemiluminescence sulfur detector. The signals from the sulfur detector confirmed the presence of a sulfur-containing compound, for example, in the alkylation reaction of dibenzothiophene. The sample was injected with an oncolumn injection and onto a Restek MXT-1 (15 m 0.53 mm inner diameter). The column temperature was held at 40 °C for 2 min, ramped to 360 °C at 6 °C/min, and then held at the final temperature for 20 min. The GC peak area percent yields (uncorrected) were obtained from the integrated area under the peaks. NMR Analysis. NMR analyses were performed on the separated fractions to confirm the degree of alkylation based on GC analysis. 1 H NMR spectra were collected on Bruker AMX360 at 360 MHz. The samples were dissolved in CDCl3 spiked with Si(CH3)4, and the spectra were calibrated to the tetramethylsilane (TMS) peak (0 ppm) internal standard. FDMS. About 0.1 μL of the liquid was applied onto a FD emitter (purchased from Linden ChroMasSpec) using a syringe. The FD emitter was then inserted into the ion source of a mass spectrometer near the extraction electrodes. During the operation, the FD emitter was set to 8 kV. The extraction electrodes were set at 3 kV. The distance and orientation of the emitter to the extraction electrodes were optimized by monitoring the acetone molecular ion signal at the mass-to-charge ratio (m/z) of 58. This voltage difference created a field strength of roughly 107 108 V/cm near the carbon dendrimers of the FD emitter. The emitter was heated with a current manually varied from 0 to 65 mA to assist desorption of the molecules. The extraction electrodes were heated with 1.2 A current (corresponding to about 225 °C) to avoid condensation of the analytes. The ions generated by the process were analyzed by a magnetic sector mass spectrometer capable of detecting ions in the range of m/z 100 5000. Data were collected in either centroid or multichannel analyzer (MCA) mode. GCMS. A total of 1.0 μL of the neat sample was injected with a split injection onto a DB-1 column (30 m 0.25 mm inner diameter) on a HP6890 gas chromatograph. The column temperature was ramped from
Figure 4. GC chromatogram of alkylated pyrene. 30 to 325 °C at 10 °C/min and then held at the final temperature for 15 min. GC eluents were detected by a HP5989B mass spectrometer engine with 70 eV electron impact (EI) ionization. Both the starting material and reactant product were run under the same conditions. The percent conversion was estimated from the integrated areas under the peak because of the aromatic reactant in the starting material and product.
’ RESULTS AND DISCUSSION All of the aromatics illustrated in this section are shown as n-alkyl side chains. This is just a representation of the number of alkyl groups added to each aromatic. The representations do not show the isomerized versions of the side chains, which are likely given in the chemistry nor do they represent the correct regioisomer. We sought to control the alkylation chemistry by trying different types of solid and soluble catalysts. Initial scoping experiments were with solid zeolite-type catalysts. These experiments focused, initially, on the alkylation of toluene and naphthalene with hexadecene to develop the right conditions. The conditions were optimized with these base case reactions and then adapted to the other aromatics by adjusting the time, temperature, and amount of olefin. The focus moved to pyrene, phenanthrene, dibenzothiophene, and 9-phenylcarbazole alkylations with hexadecane. The reaction with toluene was successful. GCMS analysis (Figure 2) shows that the product contains mostly regioisomers 4602
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Figure 8. Pyrene alkylation products.
Figure 5. GC chromatogram of alkylated phenanthrene.
Figure 6. GC chromatogram of alkylated dibenzothiophene.
Figure 9. FDMS spectrum of pyrene alkylation products.
Figure 7. GC chromatogram of alkylated 9-phenylcarbazole.
of alkylated toluene and traces of unreacted hexadecene, with the double bond isomerized. The alkylation of naphthalene was also successful after raising the temperature of the reaction to 150 °C and increasing the reaction time to 4 h (Figure 3). The reaction conditions were established, and attention was turned to alkylating the four aromatic target compounds. Table 1 lists the four compounds that were alkylated with hexadecene. Unreacted aromatic compounds and olefin were determined by GC and are given as a percentage of the final reaction mixture. Hexadecene oligomerized under reaction conditions, the trimer of hexadecene coeluted with some alkylated products in the GC chromatogram. Although the yield of the trimer is small (
