Christian Silvio POMELLI1, Andrea MEZZETTA1, Alberto DE ANGELIS2, Stefania GUIDETTI2, Lorenzo GUAZZELLI1
1Dept. of Pharmacy - University of Pisa , Pisa, Italy
2Eni S.p.A. Research and Technological Innovation Department, San Donato Milanese (MI), Italy
Some low value hydrocarbon stream from refinery processes are reservoirs of precious metals, which have been almost overlooked up to date. For instance, it has been shown that Molybdenum and Vanadium are present in relevant quantities (~10.000 ppm) in a carbonaceous heavy and danse product obtained from the purge stream of the proprietary EST (Eni Slurry Technology) plant, an hydroconversion process whose catalyst is constituted mainly of MoS2. The present study aims at exploring the capability of a panel of ionic liquids in extracting these metals, in conjunction with non-ionic additives, from such complex matrix.
A set of selected ILs characterized by a fixed cation (1-butyl-3-methylimidazolium) and a variety of anions (acetate, dicyanamide, salicylate and thiosalicylate) was tested. In the case of the acetate IL, thiourea and dithizone as non-ionic additives have been used.
The experimental results show that the acetate IL is highly selective for the extraction of Vanadium, while non-ionic additives do not significantly alter the extraction outcome.
To rationalize these results, a computational study of the stability of anion-metal complexes was performed. The intrinsic complexity of the matrix does not allow to infer the metal speciation. Therefore, a complete screening of the possible complexes, compatible with the chemistry of Vanadium and Molybdenum, with different oxidation and spin states needed to be performed.
To have a reasonable computational cost the semiempirical package XTB with the GFN2-xTB functional was used. This method has been recently assessed as highly efficient for the description of metal complexes. The most relevant structures have been validated at ab-initio DFT level.
This analysis showed the formation of the stable neutral Mo2Ac4, a well-known Molybdenum compound, a result which could explain the observed selectivity.