ARTICLE pubs.acs.org/EF
Formation of Archipelago Structures during Thermal Cracking Implicates a Chemical Mechanism for the Formation of Petroleum Asphaltenes Ali H. Alshareef,† Alexander Scherer,‡ Xiaoli Tan,† Khalid Azyat,§ Jeffrey M. Stryker,§ Rik R. Tykwinski,‡ and Murray R. Gray*,† †
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 0G3, Canada Department of Chemistry, Friedrich Alexander University, 91054 Erlangen, Germany § Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada ‡
bS Supporting Information ABSTRACT: A series of model compounds for the large components in petroleum, with molecular weights from 534 to 763 g/mol, was thermally cracked in the liquid phase at 365 420 °C to simulate catagenesis over a very short time scale and reveals the selectivity and nature of the addition products. The pyrolysis of three types of compounds was investigated: alkyl pyrene, alkylbridged pyrene with phenyl or pyridine as a central ring group, and a substituted cholestane benzoquinoline compound. Analysis of the products of reaction of each compound by mass spectrometry, high-pressure liquid chromatography, and gas chromatography demonstrated that a significant fraction of the products, ranging from 26 to 62 wt %, was addition products with molecular weights higher than that of the starting compounds. Nuclear magnetic resonance (NMR) spectroscopic analysis showed that the pyrene compounds undergo addition through the attached alkyl groups, giving rise to bridged archipelago products. These results imply that the same geochemical processes that generate the light components of petroleum, such as n-alkanes, simultaneously produce some of the most complex heavy components in the asphaltenes. Similarly, thermal cracking reactions during refinery processes, such as visbreaking and coking, will drive addition reactions involving the alkyl groups on large aromatic compounds.
’ INTRODUCTION The molecular structure of petroleum asphaltenes is critical to understanding the origin and migration of these components.1 3 These asphaltenes comprise a significant portion of heavy petroleum and bitumen and determine the properties4 and commercial processing5 of these resources, which are consumed at an increasing rate worldwide. Petroleum is an extremely complex mixture of compounds, comprising tens to hundreds of thousands of components,6 which hampers efforts for identification and quantitative analysis of the largest components present. The main two structural motifs suggested for the asphaltene fraction are the “archipelago” compounds, composed of alkyl-bridged aromatic and cycloalkyl groups linked together mainly with alkyl carbon bridges,1,7 and the “continental” compounds that are based on highly alkylated condensed polycyclic aromatic compounds.8 The prevalent view of the origin of petroleum is that organic material is accumulated in sedimentary deposits and transformed into a polymeric material called kerogen. Geothermal heating after burial of these deposits gives catagenesis or thermal cracking, releasing petroleum, which migrates and accumulates in traps to form commercial deposits.1 Modeling of these chemical processes is used to identify promising locations for exploration within a given sedimentary basin.2,3,9 11 The least soluble fraction of petroleum is designated “asphaltenes” and is defined as the components in petroleum that are insoluble in n-heptane. This fraction contains any compounds in the crude oil mixture r 2011 American Chemical Society
with low solubility because of high molecular weight, high polarity, highly aromatic character, or any combination of these features. In comparison to the remainder of petroleum, the asphaltenes are rich in sulfur, nitrogen, oxygen, vanadium, and nickel compounds. Heavy oils and bitumens are rich in asphaltenes, because of either a low degree of thermal cracking of the original kerogen or extensive biodegradation, which removes the lighter fractions.12 Both kerogen and asphaltenes release steroid-like biomarker compounds during thermal cracking under hydrogen atmosphere in the laboratory, and these compounds can be used to relate asphaltene character to the original source kerogen.1,13 Such pendent cycloalkanes are also released from the heavy fractions of petroleum during refining.14 The importance of pendent aromatic, alkyl, and cycloalkyl groups to refinery processing,15,16 the characterization of alkyl bridges between aromatic groups,7 and the detection of sulfide and ether bridges1,17 all support a structural paradigm for asphaltenes constructed of polycyclic aromatic and aliphatic groups connected by short alkyl bridges, similar to an archipelago of islands. An alternate, continental, structural motif has also been posited, consisting of highly condensed polyalkylated aromatic compounds, some with fused saturated rings.8,18,19 The most recent Received: January 31, 2011 Revised: April 8, 2011 Published: April 11, 2011 2130
dx.doi.org/10.1021/ef200170a | Energy Fuels 2011, 25, 2130–2136
Energy & Fuels
ARTICLE
Table 1. Estimated Yield of Addition Products from Thermal Cracking of Model Compounds
Conversion is defined as the difference between initial and final weights of the starting compound divided by the initial weight. The final weight is determined by HPLC. 1
arguments promulgated in favor of the condensed alkyl aromatic motif rely heavily on either fluorescence spectroscopy,8 which cannot be used to determine “average” or most probable structures in a complex mixture of components,20 or mass spectrometry without proper calibration by suitable reference compounds. In this work, we subjected a series of well-characterized synthetic model compounds to condensed-phase thermal cracking at 365 420 °C. These compounds were designed to incorporate substructures known to be present in the asphaltenes and within the established range of molecular weights, with sufficiently high boiling points to ensure that they remain in the liquid phase during cracking reactions. Cracking reactions of simple hydrocarbons in the vapor phase favor fragmentation to lighter products, but cracking in the liquid phase simultaneously produces compounds heavier than the starting material.21 24 The purpose of this study was to reveal the importance of addition reactions in the liquid phase by quantitatively determining yields and to identify the structure of such products.
’ MATERIALS AND METHODS A series of four model compounds was selected to investigate the reactions of polyaromatic compounds under conditions relevant to thermal conversion and to mimic catagenesis over a much shorter time scale.9 These model compounds, shown in Table 1, were based on either pyrene as the main aromatic group (compounds 1 3) or a condensed cholestane benzoquinoline structure (compound 4) based on aromatic biomarkers identified in crude oil25 (see the Supporting Information for synthetic details). Thermogravimetric analysis confirmed that compounds 1 4 do not evaporate prior to the onset of cracking at ca. 350 °C; therefore, they were suitable for investigating cracking reactions in the liquid phase.
Thermal cracking experiments were carried out in a tubular stainlesssteel microreactor, 5 mm in diameter and 5 cm in length, attached to a hightemperature valve, with a 1 mm (1/16 in.) diameter and 9 cm tube, connected and capped with Swagelok fittings. A total of 2 3 mg of each compound was loaded into a 3 45 mm one-end-sealed glass tube using a microspatula or micropipet. The loaded glass linear was placed in the microreactor with the open side up to collect easily the coke and heavy products that formed. The reactor was purged with nitrogen at least 3 times, closed, and then heated by immersion into a fluidized sand bath. At the end of each experiment, the reaction was terminated instantly by immersing the sealed reactor in cold water. Products were extracted with methylene chloride and concentrated using a rotary evaporator. Thermogravimetric measurements were performed on a Thermo Cahn TherMax 400 thermogravimetric analyser (TGA) (Thermo Electron Corporation, Waltham, MA), heating 4 5 mg of sample at 10 K/min. Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) and tandem MS/MS analyses (Bruker Ultraflextreme MALDI TOF/TOF, Bremen, Germany, or Applied BioSystems Voyager Elite MALDI TOF, Foster City, CA) were used to reveal the masses of the products and the fragmentation of selected products. In all cases, 2-[(2E)-3-(4-tertbutylphenyl)-2-methylprop-2-enylidene]malononitrile (DCTB) was used as the matrix for the MALDI experiments. Dimers of the parent compounds were not observed, and only the nitrogen-containing compounds (3 and 4) and associated pyrolysis products gave adducts incorporating the DCTB matrix. All peaks present before the reaction or attributable to adducts with the matrix (mass 250 Da) were subtracted in the ratio calculations. 1H nuclear magnetic resonance (NMR) spectroscopy (500 MHz Varian Inova, Santa Clara, CA) was conducted in CDCl3 solution. NMR spectra of addition products were calculated using MestReNova software (Mestrelab Research, Santiago de Compostela, Spain). High-pressure liquid chromatography (HPLC) analysis (Agilent Technologies, Santa Clara, CA) was performed using a Zorbax Eclipse PAH column of 4.6 150 mm with a C18 phase of 3.5 μm particles. The mobile phase was 70 75% methanol and 30 25% methylene chloride, 2131
dx.doi.org/10.1021/ef200170a |Energy Fuels 2011, 25, 2130–2136
Energy & Fuels
ARTICLE
Figure 1. MALDI mass spectrum of products from cracking of compound 1. The cracked products resemble the parent compound but with fewer or shorter side chains, while the addition products are bridged structures comprising the different cracking fragments.
Figure 3. MALDI mass spectrum of products from cracking of compound 3. Cracked products are fragments of the parent, while addition products are mainly alkyl alkyl bridged structures composed of adducts of the cracked fragments and the original material.
Figure 2. MALDI mass spectrum of products from cracking of compound 2. Cracked products are fragments of the parent, while addition products are mainly alkyl alkyl bridged structures composed of adducts of the cracked fragments and the original material.
Figure 4. MALDI mass spectrum of products from cracking of compound 4. Cracked products resemble the parent with shorter side chains or methyl groups and dehydrogenated products. The amount of addition products was much less compared to compounds 1 3; nevertheless, a dimer is observed at m/z 1402.
with a temperature of 23 °C and maximum pressure of 400 bar. The ultraviolet (UV) detector was set at either 239 or 270 nm. The yield of cracked products, such as pyrene and methylpyrene, which are too small to appear in the MALDI spectrum (m/z
