Vanadium and nickel distributions in selective-separated n-heptane asphaltenes of heavy crude oils

Abstract

Understanding vanadium and nickel distributions in asphaltene fractions have significant commercial importance throughout the petroleum value chain and the potential use of heavy feedstocks as a precursor of carbon-based materials, such as carbon fibers. This work extends our previous studies and aims to characterize volatile and non-volatile vanadium and nickel distributions by selective separation of n-heptane asphaltenes obtained from two Venezuelan heavy crude oils and the NIST Standard Reference Material (SRM) 8505. Asphaltenes were separated by extrography, i.e., adsorption on SiO2 and subsequent extraction with acetone, heptol (n-heptane/toluene 1:1 vol), and a mixture of toluene, THF, and methanol (TTM). The results suggest that their solubility and aggregation strongly correlate to a higher hydrogen deficiency and increased heteroatom levels. The qualitative analysis of the spent silica gel, containing irreversibly-adsorbed asphaltenes, by Light-Induced Breakdown Spectroscopy, suggests that regardless of the solvent power used during the extraction, strong chemisorbed Ni and V species remain on the SiO2, presumable associated with porphyrin molecules present on such feeds. High-Temperature Gas Chromatography coupled with Inductively Coupled Plasma Mass Spectrometry (HTGC-ICP-MS) showed that vanadium and nickel compounds have boiling points starting at 1050 °F. Quantification of the V-content below 1300 °F for the Venezuelan crude 1 indicated that the acetone fraction contains a large amount of distillable vanadium (∼62 %wt.). Interestingly, whole/unfractionated asphaltenes revealed only 47% of distillable vanadium, which suggests that the extrography method can obtain asphaltene fractions with “improved” properties (weaker aggregation, increased solubility, and lower boiling points). The characterization by atmospheric pressure photoionization Fourier Transform ion cyclotron resonance mass spectrometry (APPI FT-ICR MS) and HTGC-ICP-MS revealed that the acetone asphaltene fractions have a higher relative abundance of vanadyl porphyrins (with and without sulfur) than heptol and TTM.