Pyrene Derivatives of 2,2′-Bipyridine as Models for Asphaltenes: Synthesis, Characterization, and Supramolecular Organization

The behavior of 4,4′-bis-(2-pyren-1-yl-ethyl)-[2,2′]bipyridinyl (PBP) was studied as a model for petroleum asphaltenes with a bridged structure. PBP consists of two pyrene groups bridged by a bipyridyl spacer, and exhibits similar solubility and chromatographic properties to some fractions of asphaltenes. On the basis of nuclear magnetic resonance, steady state fluorescence, vapor pressure osmometry, solubility, and adsorption behavior studies, PBP gave self-association in solution. On the basis of these data and single crystal X-ray analysis, this behavior was attributed to π–π stacking interactions involving both pyrene rings and the bipyridine spacer. These results demonstrate that bridged aromatic species with up to four fused aromatic rings are capable of self-association in solution

Addition Reactions of Olefins to Asphaltene Model Compounds

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 <i>trans</i>-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 (i) the addition products are able to abstract hydrogen to give detectable products faster than they decompose, and (ii) the addition products can react even more readily than the parent compounds

Scalable, Chromatography-Free Synthesis of Alkyl-Tethered Pyrene-Based Materials. Application to First-Generation “Archipelago Model” Asphaltene Compounds

In this paper, we report a highly efficient, scalable approach to the total synthesis of conformationally unrestricted, electronically isolated arrays of alkyl-tethered polycyclic aromatic chromophores. This new class of modular molecules consists of polycyclic aromatic “islands” comprising significant structural fragments present in unrefined heavy petroleum, tethered together by short saturated alkyl chains, as represented in the “archipelago model” of asphaltene structure. The most highly branched archipelago compounds reported here share an architecture with first-generation dendrimeric constructs, making the convergent, chromatography-free synthesis described herein particularly attractive for further extensions in scope and applications to materials chemistry. The syntheses are efficient, selective, and readily adaptable to a multigram scale, requiring only inexpensive, “earth-abundant” transition-metal catalysts for cross-coupling reactions and extraction and fractional crystallization for purification. This approach avoids typical limitations in cost, scale, and operational practicality. All of the archipelago compounds and synthetic intermediates have been fully characterized spectroscopically and analytically. The solid-state structure of one archipelago model compound has been determined by X-ray crystallography

Characterization of Asphaltene Building Blocks by Cracking under Favorable Hydrogenation Conditions

The chemical building blocks that comprise petroleum asphaltene molecules were determined by thermal cracking of samples under conditions that minimized alterations to aromatic and cycloalkyl groups. Favorable hydrogenation conditions that used tetralin as a hydrogen-donor solvent and an iron-based catalyst allowed asphaltenes derived from different crude oils to yield approximately 50–60 wt % distillates (<538 °C fraction), with coke yields below 10 wt %, and reach conversions of the vacuum residue fraction between 65 and 75 wt %. Products in a wide range of boiling points, from naphtha to heavy material in the vacuum residue range, were observed by simulated distillation. Quantitative recovery of the cracked products, with mass balances above 96%, and characterization of the distillate fraction by gas chromatography–field ionization–time-of-flight high-resolution mass spectrometry (GC–FI–TOF HR MS) provided information on the abundance of building blocks, including saturates, 1–3-ring aromatics, 4+-ring aromatics, and nitrogen- and sulfide-containing molecules. Samples of asphaltenes from different geological basins exhibited a remarkable similarity in the yields of building blocks, with paraffins and 1–3-ring aromatics as the most abundant species. The diversity of molecules identified in the distillate products from the cracking of asphaltenes suggests a high degree of heterogeneity and complexity of asphaltene molecules, built up by smaller fragments attached to each other by bridges. The sum of material remaining in the vacuum residue fraction and the yield of coke were in the range of 35–45% and represent the maximum amount of large aromatic clusters present in asphaltenes that could not be converted to distillates or gases under the cracking conditions used in this study

Formation of Archipelago Structures during Thermal Cracking Implicates a Chemical Mechanism for the Formation of Petroleum Asphaltenes

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, alkyl-bridged 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 <i>n</i>-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

Catalytic Hydrodenitrogenation of Asphaltene Model Compounds

The catalytic hydrodenitrogenation of heavy petroleum fractions is important for the production of high-quality fuels, because the nitrogen-bearing compounds poison acidic catalysts and inhibit sulfur removal. Two families of synthetic nitrogen-containing model compounds representative of asphaltene molecular structures were catalytically hydrogenated over a commercial NiMo/γAl₂O₃ catalyst under industrial hydrotreating conditions, i.e., 370 °C and 18 MPa of hydrogen for 1 h, in a stainless steel batch reactor. The bridged compounds with pyridine as a center ring gave cracking, hydrogenation, and hydrodenitrogenation products with selectivities that depended on the position of substituents on the central pyridine ring. In contrast, a series of fused cholestane-benzoquinoline compounds gave only hydrogenation of all-carbon aromatic rings

Steroid-Derived Naphthoquinoline Asphaltene Model Compounds: Hydriodic Acid Is the Active Catalyst in I₂‑Promoted Multicomponent Cyclocondensation Reactions

A multicomponent cyclocondensation reaction between 2-aminoanthracene, aromatic aldehydes, and 5-α-cholestan-3-one has been used to synthesize model asphaltene compounds. The active catalyst for this reaction has been identified as hydriodic acid, which is formed <i>in situ</i> from the reaction of iodine with water, while iodine is not a catalyst under anhydrous conditions. The products, which contain a tetrahydro­[4]­helicene moiety, are optically active, and the stereochemical characteristics have been examined by VT-NMR and VT-CD spectroscopies, as well as X-ray crystallography