Article pubs.acs.org/EF
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 Fernanda E. Pinto,† Carlos F. P. M. Silva,‡ Lilian V. Tose,† Marco A. G. Figueiredo,§ Wallace C. Souza,§ Boniek G. Vaz,∥ and Wanderson Romaõ *,†,⊥ †
Laboratório de Petroleômica e Química Forense, Departamento de Química, Universidade Federal do Espírito Santo (UFES), Avenida Fernando Ferrari, 514, Goiabeiras, Vitória, Espírito Santo 29075-910, Brazil ‡ Refinaria Duque de Caxias (REDUC), Petrobras, Rodovia Washington Luiz, km 113.7, BR 040, Duque de Caxias, Rio de Janeiro 25225-970, Brazil § Laboratório de Engenharia e Tecnologia de Petróleo e Petroquímica, Universidade Estadual do Rio de Janeiro, Rua São Francisco Xavier 524, Rio de Janeiro, Rio de Janeiro 20550-900, Brazil ∥ Instituto de Química, Universidade Federal de Goiás, Goiânia, Goiás 74001-970, Brazil ⊥ Instituto Federal do Espírito Santo (IFES), Avenida Ministro Salgado Filho, Soteco, Vila Velha, Espírito Santo 29106-010, Brazil S Supporting Information *
ABSTRACT: 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 (Mw) 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 ( O[H] > O3S[H] > other (Figure S7 of the Supporting Information). Although the adsorbents, such as clay, present acid sites and selectively adsorb basic nitrogen species, a greater reduction of acid nitrogen species, such as carbazole and its analogues, is clearly observed when visualizing the plots of DBE versus CN for the N[H] class (Figure 9). Note that, for sample 1 of the VGO treated with clay at 150 °C for 60 min (Figure 9f) and the VGO treated with silica at 80 and 150 °C for 60 min (panels j and n of Figure 9), practically all acid nitrogen compounds were removed, leaving only some with a CN of C26−C40 for the VGO samples treated with silica. However, when the rupture time increases to t = 180 min, there is clearly a saturation process of the bed, where the abundance of the acid nitrogen species increases, reducing
only when t ≥ 240 min. This behavior is similar to that observed in Figure 6.
4. CONCLUSION The use of clay and silica as adsorbents has been proposed as a method of pretreatment for VGO. The analysis by ESI(+)FTICR-MS showed the efficiency of the proposed processes for the removal of nitrogenous compounds. A good correlation was shown between ESI(+)FT-ICR MS data and total nitrogen contents, providing chemical information on a molecular level (molecular formula, CN, and DBE distributions). FT-ICR MS indicated that the adsorption mechanism that occurred for the clay was different from the mechanism that occurred for the silica. The clay had a high density of acid sites, which likely triggered the chemisorption of the nitrogen compounds from the VGO. Higher temperatures are favorable for chemisorption. The acidity of the silica was low. The adsorption mechanism enabled by the silica was a weak van der Waals interaction or physisorption, which was limited at the higher temperature. The results of the breakthrough curves (Table S1 of the Supporting Information) show that silica is more efficient at 80 °C while the clay is more efficient at 150 °C in the removal of nitrogenous compounds. This behavior can be explained by 3462
DOI: 10.1021/acs.energyfuels.6b02566 Energy Fuels 2017, 31, 3454−3464
Article
Energy & Fuels
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the fact that the physical adsorption process (that occurs with the silica adsorbent) is disadvantaged with the increase in the temperature, whereas the chemical adsorption process (that occurs with the clay) is favored by the increase in the temperature. 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 was observed at the lower rupture time (t = 60 min) on the VGO samples treated with clay at 150 °C and with silica at 80 and 150 °C. Both materials yielded promising results in the removal of nitrogenous compounds in heavy hydrocrbon fractions and could be suggested for application in the industrial process. FT-ICR MS can also be used as an auxiliary method for the determination of different reaction mechanisms.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b02566. Schematic of the adsorption column, showing the temperature control system for the packed bed (Figure S1), total nitrogen content versus Mw plot for VGO samples treated with (a) clay and (b) silica adsorbents (Figure S2), DBE distribution of (a) N[H] and (b) N2[H] classes of pure VGO and VGO treated with silica (Figure S3), diagram of DBE versus CN for N[H] class compounds of the (a) pure VGO and (b−i) VGO treated with silica at different times and temperatures of adsorption (Figure S4), van Krevelen diagrams of Nx species generated from ESI(+)FT-ICR MS data of the (a) pure VGO and (b−i) VGO treated samples with silica at different times of adsorption and temperatures (80 and 150 °C) (Figure S5), ESI(−)FT-ICR mass spectra of the pure VGO and its samples treated with clay with different times of rupture and temperatures (80 and 150 °C) (Figure S6), class distribution from ESI(−)FT-ICR MS data for pure VGO and its fractions treated with (a) clay and (b) silica (Figure S7), total nitrogen contents in the samples collected to obtain the breakthrough curves at 80 and 150 °C using the clay and silica as adsorbent materials (Table S1), and operating conditions of rupture curves (Table S2) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Wanderson Romaõ : 0000-0002-2254-6683 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank CNPq, CAPES (23038.007083/2014-40), and FAPES (73309516/16) for the grant and financial support.
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DOI: 10.1021/acs.energyfuels.6b02566 Energy Fuels 2017, 31, 3454−3464
