Protic ionic liquids extract asphaltenes from direct coal liquefaction residue at room temperature

Abstract

Asphaltenes in direct coal liquefaction residue (DCLR) are highly aromatic and polydisperse mixtures, which are important precursors for producing high value-added carbon materials. In this research, a series of protic ionic liquids (PILs) with organic carboxylate anions were synthesized and applied to extract asphaltenes from DCLR at room temperature. The extracts were characterized by elemental analysis, FTIR, 13C-NMR, quinoline insolubles tests and so on. The results show that asphaltenes with the potential of precursors of carbon materials can be separated from DCLR at room temperature using PILs and the solid–liquid separation is carried out facilely due to the lower viscosities of PILs. The influences of PILs structures on extraction yields and atomic ratios of H/C were also investigated. As alkyl chain length of PILs anions increases, extraction yields of asphaltenes increase while H/C ratios decrease. Moreover, extraction yields vary with various cations of PILs and increase according to the order of N-methylimidazolium ([MIM]+)<triethylammonium ([TEtA]+)<3-methylpyridinium ([MPy]+), but H/C ratios decrease following the order of N-methylimidazolium ([MIM]+)>triethylammonium ([TEtA]+)>3-methylpyridinium ( [MPy]+). The asphaltenes extracted from DCLR with PILs have the excellent properties for preparing carbon materials, such as lower H/C ratios, higher aromaticities, lower sulfur contents and nearly no quinoline insolubles.