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.
The simple definition of asphaltenesthe material that dissolves in aromatic solvents and precipitates in normal alkanesproved to be not very defining at all. The nature of this material, how it formed, what its key properties are, and how it behaves under thermal or catalytic reaction conditions has extensively evolved over decades of continuous research and debate. One of the main sources of the debate was the apparent conflicting findings between numerous analytical studies and thermal reaction studies of asphaltenes and their model compounds. Similar to the blind men describing different parts of an elephant, many studies were describing some aspect of the material as it is observed but not realizing the existence of a “larger” object, and even the observations were very subjective to how asphaltenes were precipitated and analyzed and how data was generated and interpreted. In this paper, a brief summary is given on how asphaltenes are defined, how they form, and the current consensus on their properties, and finally a focus on thermal behavior, as observed from thermal studies on asphaltenes and their model compounds. The collective knowledge from analytical and thermal studies on the asphaltenes and various model compounds helped settle most of the debate on the notoriously complex material named asphaltenes.
Six asphaltene model compounds incorporating the biomarker structure of 5α-cholestane, covalently fused to a range of differentially substituted benzoquinoline groups, were subjected to thermal cracking. Thermogravimetric analysis of the six compounds showed similar cracking kinetics and yields of solid residue (coke), with the heaviest compound, bearing a pyrenyl substituent, forming the largest amount of residue. Analysis of the products formed from thermal cracking of three such model compounds in a stainless steel microreactor showed mainly dehydrogenation of the saturated hydrocarbon rings, along with some peripheral demethylation, and steroid side-chain fragmentation, with no significant ring opening or release of cyclic substructures from the steroid moiety. Some loss of the aromatic substituents appended to the benzoquinoline moiety was also detected. Methylated products and dimers of the parent compounds were formed by addition reactions, which participated in further cracking and addition reactions as the conversion increased.
Determination of cracking kinetics by thermogravimetric analysis (TGA) is attractive but requires careful experimental design to ensure consistent results. Here, we examine three methods for determining kinetic parameters by TGA: differential analysis at a constant heating rate (differential), temperature for constant conversion at multiple heating rates (isoconversional), and maximum rate at multiple heating rates (peak temperature). The test compounds used in this study are based on pyrene, with boiling points high enough that mass loss was dominated by the reaction rather than evaporation. The methods using multiple heating rates gave comparable results to each other but were very different from the results from the differential method. The results of the differential method were insensitive to the heating rate and consistent with kinetics of cracking reported in the literature for similar structures. The differential analysis method for cracking of model compounds was the best approach, because the methods of multiple heating rates consumed more sample and were reported in the literature to be very sensitive to minor experimental errors. The conversion in a microreactor experiment was accurately predicted using the kinetics from the differential TGA method, suggesting a consistent reaction rate in these two different reactions. Hence, the differential approach was concluded to be much more accurate when compared to isoconversional and peak temperature methods.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.