Characterization of Oxygen-Containing Species in Methanolysis Products of the Extraction Residue from Xianfeng Lignite with Negative-Ion Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Abstract

Methanolysis of an extraction residue (ER) from Xianfeng lignite was carried out to obtain extracts 1–4 (E1–E4). The molecular compositions (MCs) of oxygen-containing species (OCSs) in E1–E4 were characterized using negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS). In addition, solid-state 13C nuclear magnetic resonance and X-ray photoelectron spectrometry were used to analyze the carbon types and oxygen functional groups (OFGs) in the ER. The results show that the carbon skeleton structures in the ER are dominated by aliphatic (50.2%) and aromatic (44.9%) carbons. Methylene and methoxy carbons are the most abundant among the aliphatic carbons. Each aromatic cluster contains three rings on average with two substituent groups on each ring. The OFGs in the ER include hydroxy, ether, carbonyl, and carboxyl groups, among which the hydroxy group is the most abundant. The ESI FT-ICR MS analysis shows that the molecular mass distributions of E1–E4 range from 150 to 500 u. The On (n = 1–6) class species are the predominant OCSs in E1–E4, with 0–14 double bond equivalent (DBE) values and 9–34 carbon numbers (CNs). The most abundant On class species in E1–E4 are O2, O2, O2–O3, and O3, respectively. The OCSs in E4 contain low abundances of O1 and O2 class species but relatively high abundances of O4–O6 class species. In addition, the On class species in E4 have narrower ranges of DBE values and CNs than those in E1–E3. A series of acidic species with different DBE values and CNs are assigned to alkanoic acids, alkanedioic acids, alkanetricarboxylic acids, alkylarenols, alkylarenediols, alkylarenetriols, and alkylarenoic acids. With high resolving power and mass accuracy, ESI FT-ICR MS is an effective technique for characterizing MC of the soluble portion from lignites, which will facilitate producing important chemicals from lignites.