Identification of organic nitrogen compounds in methanol-soluble portion from sweet sorghum stalk methanolysis

• Numerous ONCs in methanol-soluble portion (MSP) from SSS methanolysis were detected.
• N1On and N2On class species are dominant NNCs and BNCs in MSP, respectively.
• Pyrroles and indoles should be the characteristic structures of NNCs.
• Benzimidazoles, indazoles, and isoquinolinamines are the characteristic structures for BNCs.

Understanding the chemical composition of organic nitrogen compounds (ONCs) in biomass and its derived liquids is important because of their negative impacts on the environment during biomass utilization as fuel. In this study, X-ray photoelectron spectrometer (XPS) was used to identify the nitrogen forms on sweet sorghum stalk (SSS) surface and gas chromatograph/mass spectrometer (GC/MS) together with electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometer (FTICRMS) were employed to analyze ONCs in methanol-soluble portion (MSP) from SSS methanolysis. Analysis with XPS shows that the nitrogen forms on SSS surface mainly include pyrrolic, amino, and quaternary nitrogen species. GC/MS identified 14 ONCs in MSP, including 3 pyrroles, 3 indoles, 2 imidazoles, 3-methoxypyridine, 1-(hex-5-enyl)-1-methylhydrazine, cyclohexanecarbohydrazide, N-pyridin-3-ylacetamide, 1,3,4-trimethyl-1H-pyrazol-5(4H)-one, and pyridin-3-amine. According to analysis with negative-ion ESI FTICRMS, N1On class species are the main neutral nitrogen compounds (NNCs) in MSP. N1O3 class species are the most abundant NNCs, followed by N1O4 and N1O5 class species. Pyrroles and indoles should be dominant NNCs based on double bond equivalent (DBE) distributions. In positive-ion mode, most of the detected basic nitrogen compounds in MSP are N2On class species, in which N2O2–N2O4 class species with DBE = 8–12 and carbon atom numbers of 19–28 are the most abundant. N2On class species with DBE < 4 are likely to be diamines, hydrazines, hydrazides, imidazoles, and/or pyrazoles, while those with DBE ≥ 4 could be ascribed to pyridinamine, pyrazines, pyrimidines, benzoimidazoles, indazoles, and/or isoquinolinamines.

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