Extractive denitrogenation of fuel oils with dicyanamide-based ionic liquids

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The removal of nitrogen-compounds (N-compounds), e.g. basic and neutral species, from fuel oils is necessary because of their inhibiting effect on the hydrodesulfurization process. In this work, the extractive denitrogenation performance of four dicyanamide-based ionic liquids (ILs) with different cationic characteristics, i.e., aromatic 1-butyl-3-methylimdazolium dicyanamide ([BMI][N(CN)2]) and 1-ethyl-3-methylimdazolium dicyanamide ([EMI][N(CN)2]), cyclic ethylated tetrahydrothiophenium dicyanamide ([S2][N(CN)2]), and tetrahedral ethyldimethylsulfonium dicyanamide ([EtMe2S][N(CN)2]), is investigated using basic pyridine and neutral carbazole as representative N-compounds. These ILs are capable of effectively extracting the N-compounds from the fuel oils with carbazole being more efficiently extracted than pyridine; also, aromatic imidazolium ILs exhibit better performance than cyclic thiophenium and tetrahedral trialkylsulfonium ILs in the order [BMI][N(CN)2] > [EMI][N(CN)2] > [S2][N(CN)2] > [EtMe2S][N(CN)2]. Under ambient conditions, 1 : 1 (w/w) IL : oil, the N-content in the raffinate phase of the carbazole-containing fuel oil is undetected after <5 min of contact with [BMI][N(CN)2] and [EMI][N(CN)2], while 96.8% and 84.3% N-extraction efficiency is obtained after contact with [S2][N(CN)2] and EtMe2S][N(CN)2] respectively; for pyridine-containing fuel oil, the N-extraction efficiency in the aforementioned ILs is 72.7%, 69.1%, 63.5% and 59.8%, respectively. Compared with other ILs reported, the extractive performance of these ILs is competitive. [BMI][N(CN)2] is selected as a representative IL to undergo a series of parallel experiments to determine the influence of IL : oil mass ratio, temperature, initial N-content, and multiple extractions; a recyclability test is also performed. This work may present a new approach to fuel denitrogenation.