Characterization Techniques Coupled with Mathematical Tools for Deepening Asphaltene Structure

Abstract

Asphaltenes are the heavy fraction of fossil fuels, whose characterization remains a very difficult and challenging issue due to the still-persisting uncertainties about their structure and/or composition and molecular weight. Asphaltene components are highly condensed aromatic molecules having some heteroatoms and aliphatic functionalities. Their molecular weights distribution spans in a wide range, from hundreds to millions of mass units, depending on the diagnostic used, which is mainly due to the occurrence of self-aggregation. In the present work, mass spectrometry along with size exclusion chromatography and X-ray diffraction analysis have been applied to asphaltenes for giving some further insights into their molecular weight distribution and structural characteristics. Relatively small polycyclic aromatic hydrocarbons (PAHs) with a significant degree of aliphaticity were individuated by applying fast Fourier transform (FFT) and double bond equivalent (DBE) number analysis to the mass spectra. X-ray diffraction (XRD) confirmed some aliphaticity, showing peaks specific of aliphatic functionalities. Size exclusion chromatography indicated higher molecular weight, probably due to the aliphatic substituents. Mass spectrometry at high laser power enabled observing a downward shift of molecular masses corresponding to the loss of about 10 carbon atoms, suggesting the occurrence of aryl-linked core structures assumed to feature asphaltenes along with island and archipelago structures.

Hydrogen, sp2 Carbon Hybridization, and sp2 Clustering as Pieces of the Puzzling Nanostructure of Soot: A Closer Look

“…In the same figure we report the m/I(G) evaluated on Raman spectra of soot and different carbonaceous materials listed in Table 1 that are coal tar pitch (CTP) 74 and its toluene insoluble fraction (CTP-TI), 75 a naphthalene pitch (NP) 74 and its toluene soluble (NP-TS) and insoluble (NP-TI) fractions, 75 and heavy fuel oil asphaltenes, obtained by heptane insolubilization. 80 The m/I(G) has been estimated also for hydrogen-rich soot samples like those derived from premixed fuel-rich ethylene and methane flames at low heights above the burner (4.5 mm for ethylene and 5 and 7 mm for the methane flame). 29 Only some samples could be analyzed by combustion analysis due to the low sample amounts, but hydrogen data obtained from other techniques, namely Raman (m/I(G), H mol % (using the Casiraghi formula)) and FTIR spectroscopy (H mol %, aromatic hydrogen (H ar ) mol % and aliphatic hydrogen (H ali ) mol %), could be obtained as listed in Table 1.…”

“…In the same figure we report the m / I (G) evaluated on Raman spectra of soot and different carbonaceous materials listed in Table that are coal tar pitch (CTP) and its toluene insoluble fraction (CTP-TI), a naphthalene pitch (NP) and its toluene soluble (NP-TS) and insoluble (NP-TI) fractions, and heavy fuel oil asphaltenes, obtained by heptane insolubilization . The m / I (G) has been estimated also for hydrogen-rich soot samples like those derived from premixed fuel-rich ethylene and methane flames at low heights above the burner (4.5 mm for ethylene and 5 and 7 mm for the methane flame) …”

Analytics for Recovery and Reuse of Solid Wastes from Refineries

“…Indeed, different groups have reported mean MW spanning two orders of magnitude for similar LDI setups and asphaltene samples [69][70][71][72]. However, the variation of some experimental parameters, such as laser pulse energy [73] and sample concentration, can heavily affect the measured mass spectrum. In general, heavier measured masses are observed if a higher pulse energy or concentration are employed, even if a too high energy can even cause fragmentation and it is necessary to choose a good compromise [69].…”

“…These results suggest that asphaltenes may be composed primarily of small molecules (less than ~2 kDa) with highly condensed aromatic ring systems [77]. In order to investigate deeply the effects of ionization source and mass analyzer, some authors [73,78,79] compared Laser Desorption/Ionization-Time of Flight mass spectrometry (LDI-TOFMS) with APPI-MS for asphaltenes analysis. Both the MS spectra (reported in Figures 5 and 6) of asphaltenes, obtained with the two techniques, present similar MW range with a broad structure extending in the range m/z 300-1100 (the peaks around m/z 200, in the case of APPI-MS spectrum, are due to toluene) with peaks periodicities of 1 (peaks at every nominal mass), 2 and 14 units, indicating the presence of homologues with different unsaturation degrees [65].…”

“…The mass range found, between m/z 300-1100 with a maximum at m/z 500-600, is in very good agreement with previous works [65,80,81], whereas the eventual presence of higher masses (>100,000 u), as found by other authors [82,83], cannot be investigated by APPI-MS because the mass limit of the ion trap MS is about 3000-4000 u for single-ionized analytes. Adding mathematical tools for mass spectra interpretation, such as Fourier transform (FFT) and double bond equivalent (DBE) number analysis, also with the support of other techniques such as X-ray diffraction (XRD) and Size exclusion chromatography, the occurrence of aryl-linked core structures [84] was hypothesized to feature asphaltenes, along with island and archipelago structures [73].…”