Desenvolvimento de métodos alternativos para a extração e determinação de compostos sulfurados e nitrogenados em óleo diesel

Abstract

This work aimed to developed methods to contribute to the understanding of the structure of sulfur and nitrogen compounds, as well as the fraction of basic nitrogen present in diesel oil. Sample preparation methods were evaluated for the extraction of sulfur and nitrogen compounds from diesel oil, as well as for the selective extraction of basic nitrogen compounds. For this purpose, liquid-liquid extraction, solid phase extraction (SPE) and reversed-phase liquid-liquid dispersive microextraction (RPDLLME, with and without the use of ultrasound) were evaluated, and the parameters influencing each procedure, such as sample mass, amount of solid phase, type of extract and dispersant solvent, proportion between solvents, among others, were optimized. The determination of total sulfur and nitrogen in the diesel oil before and after extraction, using an elemental analyzer was carried out to evaluate the extraction efficiency. The extraction efficiency for basic nitrogen was evaluated using the UOP 269 standard method, using potentiometric titration and with HClO4 as the titrant. About sulfur extraction, none of those methods provided extraction efficiency greater than 55%, which demonstrates the difficulty in extracting such compounds from diesel oil. Regarding nitrogen, it was possible to obtain an extraction efficiency of 91% when using the SPE method with 1 g of silica gel as a solid phase, 0.5 mL of diesel oil and acetic acid as the extract solvent. With the RP-DLLME method, 69% extraction efficiency was obtained when using 60 µL of formic acid as extract solvent, 200 µL of isopropanol as dispersant solvent and 0.5 g of diesel oil. Under specific conditions, it was possible to carry out the extraction of basic nitrogen fraction for RP-DLLME using 5 g of diesel oil, 400 µL of formic acid and 600 µL of isopropanol (extract and dispersant solvents, respectively), resulting in a concentration factor of 10.2. Extracts were analyzed by time-of-flight mass spectrometry with electrospray ionization (ESIToFMS). With emphasis on the basic nitrogen content, a miniaturized method was developed using a portable device with digital image acquisition and PhotoMetrix PRO, based on the colorimetric titration described in the UOP 269 standard method and making a multivariate calibration model with the partial least squares (PLS) method. Some conditions were optimized, such as the volume of indicator, volume and concentration of HClO4, as well as the accuracy with the variation of the volume of diesel oil. With the use of 60 µL of indicator, 20 µL of 20 mmol L-1 HClO4 and using from 50 to 100 µL of sample the best conditions for calibration were obtained (RMSEP of 0.38 mg kg-1 , RMSECV of 0.31 mg kg-1 with the use of 4 factors).