Effect of Chemical Structure on the Cracking and Coking of Archipelago Model Compounds Representative of Asphaltenes

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 degrees C/min ramp to 500 degrees C, followed by isothermal heating at 500 degrees 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.