Persistent free radicals in asphaltenes and their reactions

• Author / Creator

  Alili, Aytan

• Asphaltenes, defined as a solubility class, are soluble in toluene and insoluble in paraffinic solvents such as 𝐶5 and 𝐶7. Asphaltenes have high heteroatom and a low H/C ratio compared to bitumen’s other fractions. Moreover, they have a complex structure, high molecular weight, and high melting point that give rise to its poor quality and therefore, asphaltenes have a low value as a product. Consequently, not much attention has been paid to the processing of asphaltenes. It is known that asphaltenes have high radical concentration that ranges between 1018 to 1020 spins/g, which leads to addition and combination reactions at high temperatures that in turn may result in undesired coke formation. In order to advance the asphaltenes processing, it is important to evaluate the nature of the radicals present in asphaltenes and causing the changes.
  This study was carried out to evaluate to what extent free radicals play a role in asphaltene chemistry. In addition, it was aimed to determine how much of the free radicals present in asphaltenes are persistent. In the current study, the change in free radical concentration of asphaltenes was tracked for 456 hours under air and nitrogen purges. It was observed that the concentration of the free radicals decreases in both cases. The reason for the persistency of the free radicals was investigated to identify whether it is due to caging or steric effects. According to the results of the current study, the reason for the free radical persistency in asphaltenes is mostly due to the steric effects. It was observed that with addition of 1,2-dihydronaphthalene the hydrogen transfer per gram of asphaltenes was 100 times larger than the amount of free radicals present.
  Different probe molecules were reacted with asphaltenes at 250 °C,4 MPa, to evaluate the steric requirements and behavior of the free radicals. It was found out that asphaltenes are capable of reacting with sterically hindered compound having three phenyl rings and complex geometry, such as 1,2-diphenylethenylbenzene in the presence of solvent to give 1,1,2-triphenylethane 0.03±0.01(mg/ml). Moreover, steric requirements of asphaltenes were dominant during their reactions with α-Methyl styrene, 2,4,6-trimethylstyrene, E-stilbene. It was shown that upon reaction of probe molecules with higher amount of asphaltenes, formation of addition products and combination products is inhibited. 1,1-diphenylethylene was found to be effective hydrogen acceptor for asphaltene chemistry evaluation that did not lead to formation of any addition or combination products.

• Subjects / Keywords

  • steric requirements
  • hydrogen abstraction
  • persistent free radicals
  • hydrogen acceptor
  • asphaltenes

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-3zvz-p769

• License

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