Evaluation of Adsorbent Materials for the Removal of Nitrogen Compounds in Vacuum Gas Oil by Positive and Negative Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

• Published in: Energy & fuels Vol. 31; no. 4; pp. 3454 - 3464
• Main Authors: Pinto, Fernanda E, Silva, Carlos F. P. M, Tose, Lilian V, Figueiredo, Marco A. G, Souza, Wallace C, Vaz, Boniek G, Romão, Wanderson
• Format: Journal Article
• Language: English
• Published: American Chemical Society 20-04-2017
• ISSN: 0887-0624; 1520-5029
• DOI: 10.1021/acs.energyfuels.6b02566

The purpose of this research is to evaluate, at a molecular level, the removal of nitrogen compounds from vacuum gas oil (VGO), which is used as feedstock for fluid catalytic cracking units. Here, a VGO sample was treated with two different adsorbents: an argillaceous material specifically developed for the removal of nitrogen compounds in middle distillate cuts (kerosene and diesel) and a commercial silica adsorbent. Breakthrough curves were built on two temperature levels (80 and 150 °C), containing different rupture times (from 60 to 420 min), to determine their influence on nitrogen compound removal. All samples, produced from each condition of adsorption, were analyzed by positive and negative electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry [ESI(±)­FT-ICR MS]. Besides FT-ICR MS characterization, the total nitrogen content was monitored. FT-ICR MS indicated that the removal of nitrogen compounds by the clay adsorbent was enhanced when the temperature was higher (150 °C). Conversely, silica has shown a rich adsorption capacity at moderate temperatures (80 °C). This result corroborates the existence of two different adsorption mechanisms. The clay adsorption mechanism is likely a chemisorption process, while the silica adsorption mechanism is related to physisorption. Both processes displayed better performance in short rupture times, for example, at 60 min. Longer rupture times require a saturation of the adsorption process through a packed bed. FT-ICR mass spectra detected a wide range of compounds from m/z 220 to 800, with average molecular weight distributions (M w) that increase as a function of decreasing the total nitrogen content (424 → 711 Da). Class distribution showed a removal preferential of N­[H] and N2[H] compounds with low carbon numbers (<C26) for ESI­(+) data. The ESI­(+)­FT-ICR MS data also revealed that clay is a more efficient adsorbent than silica for the removal of nitrogen compounds and should be used in the petroleum industry. However, the ESI(−)­FT-ICR MS data showed that, although adsorbents, such as clay, present acid sites and selectively adsorb basic nitrogen species, a great reduction of non-basic nitrogen species, such as carbazole and its analogues, is clearly observed mainly for VGO samples treated with clay at 150 °C and with silica at 80 and 150 °C, both with t = 60 min.