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

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 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.

Kinetics and mechanisms of the catalytic thermal cracking of asphaltenes adsorbed on supported nanoparticles

“…As the temperature increases, radical species may be stabilized to form new lighter compounds, such as hydrocarbons with lower molecular weights, CO and CO 2 , among others (Savage et al 1985;Ali and Saleem 1991;Hosseinpour et al 2014). However, the recombination of free radial species leads to addition reactions that result in changes to the chemical structures of the n-C 7 asphaltenes and, therefore, result in heavier and more refractory compounds than the initial n-C 7 asphaltenes (Alshareef et al 2011;Habib et al 2013;Sanford 1994). …”

“…Reactions, such as addition of olefin free radicals (Gray and McCaffrey 2002), cross-linking between aromatic clusters (Habib et al 2013), and bridging of aromatics by alkyl chains (Alshareef et al 2011), were likely to be responsible for the formation of more refractory compounds. Figure 4 shows the results obtained for coke yield of the five evaluated samples.…”

Potential Raman parameters to assess the thermal evolution of kerogens from different pyrolysis experiments

“…In this case the main control on reaction rate, from a kinetic perspective, is pyrolysis temperature. And thus smaller aromatic rings are readily scatteredly developed and "archipelago" type architectures are produced in residual kerogens [27,28]. These smaller aromatic sub-units are connected by aliphatic chains.…”

“…However, chemical reactions in the semi-open or closed golden tube systems proceeded mainly under a thermodynamic equilibrium condition with the effect from extraneous pressure. So the larger conjugated aromatic clusters are developed and more "continental" type architectures produced in the residual kerogens, characterized by more bigger aromatic cores/clusters connected to aliphatic chains [27,28].…”

A study of the combustion chemistry of petroleum and bio-fuel oil asphaltenes

“…Thus, two markedly different structural types have been proposed [8,9]: one, the 'continental' or 'island' structure in which a single aromatic core containing approximately seven rings is substituted with pendant groups, especially long-chain alkyl groups, and a second, termed the 'archipelego' in which small naphthenic and aromatic groups ranging from monocycles upwards are joined together by alkyl and heteroatomic links [10,11]. A single asphaltene sample may contain mainly one structural type but may contain significant amounts of another.…”