We report here an examination of the mass spectrometric fragmentation behavior of molecular ions generated (and excited) by electron ionization (EI) from several asphaltene model compounds simulating both the island and archipelago structural models. This behavior is compared to that of protonated molecules generated from the same compounds by atmospheric pressure chemical ionization (APCI) and excited by collision-activated dissociation (CAD). The fragmentation behavior of the protonated molecules and molecular ions is surprisingly similar. Both types of ions yielded distinct fragmentation patterns for both types of model compounds. Ions derived from the island-type model compounds fragment predominantly by losing their alkyl chains (with either all carbons or all but one), one after another, which allows for the identification of the chain lengths and counting the number of chains. Increasing the length of the alkyl chains reduces the extent of spontaneous fragmentation occurring upon EI, likely because of more efficient cooling of the fragmenting ions via emission of infrared (IR) light made possible by the reduced fragmentation rates of the longer chains. Ions derived from the archipelago model compounds with ethylene bridges connecting two or three aromatic cores (without alkyl side chains) readily undergo cleavages in these bridges. Increasing the length of the alkyl chain between the aromatic cores reduces the extent of fragmentation caused by EI. Similarly, the addition of long external alkyl chains to archipelago model compounds with an ethylene bridging two aromatic cores greatly hinders fragmentation upon EI. When these molecules are protonated and subjected to high-energy CAD, they appear to fragment almost randomly but, nevertheless, indicating some preference for cleavages of the bonds in the chain connecting the aromatic cores. A comparison of these findings to the fragmentation patterns observed for protonated asphaltenes indicates that the asphaltene molecules studied are likely composed of many isomeric and isobaric molecules. Each may contain several aromatic rings and a distribution of mostly aliphatic alkyl chains (and possibly naphthenic rings) ranging in size from 1 to at least 14 carbons, several containing methyl branching at the α carbons. The results do not allow for the unambiguous differentiation between island- and archipelago-type structures, although they are in a better agreement with the island model.

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 365À420 °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 cholestaneÀbenzoquinoline 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.

Cracking and coke formation of a series of pyrene-based model compounds were investigated by thermogravimetric analysis (TGA) and microreactor experiments. The structure of the model compounds is that of a three-island archipelago, consisting of two pyrenyl groups joined to a central aromatic or heteroaromatic group by ethano bridges. The molecular weights of these compounds range from 459–679 g/mol and have sufficiently high boiling points to remain liquid under the reaction conditions. TGA measured the cracking kinetics and the coke yield of each compound, where coke yield was defined as the solid residue remaining after a 10 °C/min ramp to 500 °C, followed by isothermal heating at 500 °C for 15 min. Microreactor experiments provided the yield and structure of both cracked and addition products. Analysis of the reaction products by gas chromatography, mass spectrometry (MS), high-pressure liquid chromatography, matrix-assisted laser desorption/ionization MS, and tandem MS/MS show that the initial cracked fragments combined to form larger structures through a process of alkyl–alkyl and, to a lesser extent, alkyl–aryl C–C bond-forming reactions. The most likely mechanism for these processes includes a sequence of free-radical addition reactions to an unsaturated bond, followed by rearrangement(s), dehydrogenation, and/or further cracking. Compounds with heteroatoms incorporated in the central ring typically gave higher yields of coke and different selectivity of the cracked products, compared to hydrocarbon compounds. The change in cracking selectivity is attributed to several possible factors including a neophyl–like rearrangement, while the coke yield is governed by the rate of addition reactions, as well as the nature and reactivity of both the starting compound and the initially formed products. To test the hypothesis that molecular alignment and aggregation play a role in the observed coke yield, six model compounds were examined for liquid crystalline behavior under cross-polarized light microscopy. In this series of compounds, the coke yield increased as the isotropic temperature decreased.

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A series of fully‐substituted 5,6‐benzoquinolines covalently fused to the chlolestane framework has been synthesized to serve as model compounds for the poorly characterized components found in the heaviest fractions of petroleum, namely the asphaltenes. Acid‐catalyzed cyclocondensation of an aromatic imine with 5‐α‐cholestan‐3‐one gives 5,6‐benzoquinoline cycloadducts incorporating the steroidal biomarker fused to the benzoquinoline system at the 3,4‐positions, with various pendent substituents in the 2‐position. X‐ray crystallography of three such derivatives shows that the solid‐state geometries of these molecules are quite similar, and packing in the solid state appears to be dominated to a great extent by the nature of the pendent substituent. X‐ray crystallography and solution phase circular dichroism spectroscopy together suggest that the conjugated benzoquinoline moiety adopts a helical conformation. Copyright © 2012 John Wiley & Sons, Ltd.