Low-temperature oxidative asphaltenes liquefaction for petrochemicals: fact or fiction?

Abstract

Asphaltene is the heavy and heteroatom-rich fraction of petroleum that is rejected during a solvent deasphalting process. In patent literature there are claims that state that this material can be converted into an aromatic petrochemical feedstock by oxidative liquefaction at low temperature. To evaluate the validity of these claims, asphaltenes from an industrial solvent deasphalting process were oxidized with dry and water-saturated air at temperatures in the range 45-100°C. Infrared spectroscopy of the oxidized product confirmed that oxygen was incorporated as C=O and C-O. Under all experimental conditions studied little oxidative degradation was observed that would lead to the production of a petrochemical feedstock. Nevertheless, some observations of scientific value were made about the low-temperature conversion of asphaltenes. During autoxidation with dry air, the n-pentane-insoluble fraction increased. On the contrary, when oxidation was conducted with water-saturated air, the formation of additional n-pentane-insoluble material was suppressed. Mild heating of asphaltenes under nitrogen atmosphere also caused the n-pentane-insoluble content to increase. Spectroscopic evidence showed that esters are formed during oxidation at *100°C. The temperature dependence of this reaction was explained and a possible reaction pathway for cycloalkane to ester conversion was presented. Ester selectivity was determined by the competition between hydrogen abstraction and b-scission of the alkoxy radical.

Thermo-Oxidative Decomposition Behaviors of Different Sources of n-C7 Asphaltenes under High-Pressure Conditions

“…Depending on the origin, asphaltenes can have around 10 18 spins•g −1 free radicals, which can promote reactions during oxygen chemisorption, favoring the interactions over different points in the asphaltene molecular structure. 62 However, for all cases, the pressure increase favors the amount of chemisorbed oxygen in the n-C 7 asphaltene structure due to the thermal expansion of the aggregates, increasing the number of active sites available for the interaction with oxygen atoms. 27 Although the oxygen content follows another order, a direct relationship with oxygen in the form of carboxyl groups in the molecular structure of the system was found.…”

“…62 A possible pathway for ester formation is explained in Scheme 1, where oxidation of cycloalkanes as proposed by Klerk et al is shown. 62 Because asphaltenes are rich in multicycle molecules where cycloalkane and aromatic rings are adjoining, indan hydroperoxide functions as a good representative of a substructure obtained in the complex molecules. 62 The suggested pathway begins with the formation of an oxygen-centered alkoxy radical from indan, which can produce alcohol by taking out hydrogen from another hydrocarbon.…”

“…62 Because asphaltenes are rich in multicycle molecules where cycloalkane and aromatic rings are adjoining, indan hydroperoxide functions as a good representative of a substructure obtained in the complex molecules. 62 The suggested pathway begins with the formation of an oxygen-centered alkoxy radical from indan, which can produce alcohol by taking out hydrogen from another hydrocarbon. The alkoxy radicals also can undergo β-scission, leading to ring-opening to produce a carbon-centered radical.…”

“…During this reaction, an ester is formed (e) during the elimination of a water molecule. 62 The diameter of the cluster also influences the oxygen chemisorption yield. 52 With the increase in L c , the OC zone is favored because there is a greater active area available for oxygen absorption inside the intermolecular spaces of aggregates.…”

“…From the results obtained, it was observed that oxygen chemisorption percentage increases in the order D < F < E < B < A < C for 3.0 and 6.0 MPa. Depending on the origin, asphaltenes can have around 10 18 spins·g –1 free radicals, which can promote reactions during oxygen chemisorption, favoring the interactions over different points in the asphaltene molecular structure . However, for all cases, the pressure increase favors the amount of chemisorbed oxygen in the n -C 7 asphaltene structure due to the thermal expansion of the aggregates, increasing the number of active sites available for the interaction with oxygen atoms .…”