Concurrent Desulfurization and Denitrogenation of Fuels Using Deep

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Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11341−11349

Concurrent Desulfurization and Denitrogenation of Fuels Using Deep Eutectic Solvents Filipa Lima,†,‡,§,∥ Maxime Dave,† Armando J. D. Silvestre,‡ Luis C. Branco,*,§,∥ and Isabel M. Marrucho*,†

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†

Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal ‡ CICECO-Aveiro Institute of Materials and Department of Chemistry, Universidade de Aveiro, 3810-193 Aveiro, Portugal § Solchemar, Lda, Rua 5 de Outubro no. 121C, 1°E, 7580-128 Alcácer do Sal, Portugal ∥ LAQV-REQUIMTE, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Campus da Caparica, 2829-516 Caparica, Portugal S Supporting Information *

ABSTRACT: The concurrent removal of both nitrogen- and sulfurcontaining compounds, from simulated fossil fuels, was studied in this work. Four different deep eutectic solvents (DES) were prepared and ranked according their performance to remove thiophene (Th), dibenzothiophene (DBT), pyridine (Py), and carbazole (Carb), from n-heptane in binary mixtures. The simultaneous extractive desulfurization (EDS) and extractive denitrogenation (EDN) was then performed with the TBPB:Sulf (1:4) DES, through the removal of a mixture of model compounds from n-heptane and also from a simulated gasoline and a simulated diesel. In the case of concurrent EDS and EDN from n-heptane, the capacity to extract the sulfur compounds only decreases slightly, but in the case of the nitrogen compounds, their removal is favored by the presence of the other compounds in the model mixture. Regarding the studied complex matrices, simulated gasoline and simulated diesel, the presence of diverse and more complex hydrocarbons has no practical effect on the extraction efficiencies of the nitrogen compounds, while a negative effect was registered for sulfur compounds in diesel. In addition, the reusability of the DES was carried over 5 cycles, where it lost 78% and 44% of its capacity to extract Th and DBT, respectively, but only 4% and 2% for Py and Carb, respectively. Moreover, after a regeneration step, the performance of the DES was fully recovered. KEYWORDS: Simultaneous extractive desulfurization and extractive denitrogenation, Desulfurization, Denitrogenation, Deep eutectic solvent, Simulated gasoline and diesel

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regulations have been implemented by many other countries.3,4 Moreover, in the last couple of years, jet fuels and more recently marine transportation fuels have also been under the target of regulations to reduce the SOx emissions of these transportation sectors. The International Maritime Organization (IMO) has decided, in 2016, to cut down global sulfur emissions for marine transportation fuels from 3.5% (in effect since 2012) to 0.5% by 2020.5 This limit refers to areas outside emission control areas (ECA), as inside the actual limit is already 0.1%, since 2015. All of these mandatory reductions of fuels impurities are quite challenging to oil refineries, which will need to produce ultralow sulfur fuels. In addition, the presence of such impurities is not desirable in the entire cycle of petroleum processing, fuel combustion engine, and also in fuel cells. Regarding refining

INTRODUCTION The continued development and industrialization of modern society have increased the energy demand. The depletion of conventional energy sources, together with the escalating environmental concerns, forced the change from nonrenewable to renewable energy sources. However, both conventional and unconventional fossil fuels will be simultaneously used, at least during the transition phase and especially in developing countries. Thus, there is a need not only to develop new clean sources of energy but also to clean up the conventional ones. Regarding the use of fossil fuels, besides carbon dioxide emission, sulfur and nitrogen oxides emissions are also main environmental concerns. Since 1990 and 1993, in United States and European Union, respectively, tighter regulations have been enforced on oil impurities with high environmental impact, particularly in highway fuels.1,2 For example, in Europe, the maximum sulfur content allowed in diesel and gasoline has been reduced from 2000 ppm (1994) to 10 ppm (2009), and similar © 2019 American Chemical Society

Received: February 13, 2019 Revised: May 8, 2019 Published: May 28, 2019 11341

DOI: 10.1021/acssuschemeng.9b00877 ACS Sustainable Chem. Eng. 2019, 7, 11341−11349

Research Article

ACS Sustainable Chemistry & Engineering Table 1. List of Chemicals Used in This Work and Respective Abbreviations HBA (salt)

CA simulated fuels compounds

name

abbreviation

chemical formula

purity (%)

supplier

tetrabutylammonium chloride tetrabutylammonium bromide tetrabutylphosphonium bromide polyethylene glycol 400 sulfolane n-heptane n-hexane n-hexadecane isooctane n-dodecane toluene thiophene dibenzothiophene carbazole pyridine

TBAC TBAB TBPB PEG400 Sulf − − − − − − Th DBT Carb Py

N(C16H36)Cl N(C4H9)Br P(C16H36)Br C2nH4n+2On+1 (CH2)4SO2 C7H16 C6H14 C16H34 C8H18 C12H25 C7H8 C4H4S C12H8S C12H9N C5H5N

⩾97 ⩾97 98 − 99 >99 ⩾96 ⩾98 99.5 ⩾95 99.9 >99 98 96 99.5

Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Fisher Chemical Carlo Erba Fisher Chemical Carlo Erba Fisher Chemical Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Acros Organics Scharlau

greatly enhanced its efficiency. In relation to alternative denitrogenation processes, adsorptive denitrogenation (ADN) and extractive denitrogenation (EDN) have been widely studied. In the case of adsorption, materials such as silica gel, activated carbon, metal−organic frameworks, polymers, and zeolites have been tested in real fuel matrices, being the silica-gel that generally achieves the best results.24 Over the past few years, several organic solvents, from acidic solvents,25 passing through ILs26 and more recently with DES,27 have been tested for EDN. Only in 2016, the direct removal of both basic and nonbasic nitrogen compounds, from n-heptane, with several DES based on choline chloride was reported for the first time.27 Remarkable extraction efficiencies, higher than 98%, for both Py and Carb were achieved with DES composed of choline chloride and phenylacetic acid, in a molar ratio of 1:2. Hizaddin et al.28 reported ammonium- and phosphonium-based DES combined with ethylene glycol for the removal of model N-compounds from n-hexadecane, concluding that TBPB:EG (1:2) allowed higher partition coefficients and selectivity. An interesting option is the simultaneous implementation of EDS and EDN calling upon an adequate solvent which can efficiently extract both sulfur and nitrogen compounds from fuels. In 2018, Li et al.29 published the kick-off work for the simultaneous removal of nitrogen and sulfur compounds using DES. The authors concluded that the removal efficiency of sulfur and nitrogen compounds was closely related to the compatibility index. However, good extraction efficiencies were only obtained when an oxidation step was first performed, which presents some disadvantages compared to direct extraction, particularly the final quality of the fuel can be affected by the use of oxidants. Also, in 2018, Alli et al.30 studied the extraction of benzothiazole, which contains both sulfur and nitrogen, from n-heptane using two DES based on tetrahexylammonium bromide. In this work, the extraction ability of several DES for the simultaneous removal of both sulfur compounds (Th and DBT) and basic and nonbasic nitrogen compounds (Py and Carb), from different simulated fuel matrices, is investigated in order to achieve deep desulfurization and deep denitrogenation. Our main motivation is the integration of the two different fuel cleaning processes: desulfurization and denitrogenation. In our previous work,31 the reduction of the sulfur content to levels below the current regulation (10 ppm) was studied, according to a simple extractive approach, with polyethylene glycol-based DES. Thus, in the present study, on top of the previously

processes, sulfur-containing compounds tend to deactivate some of the catalysts used and also lead to the formation of oxoacids, which brings corrosion problems in pipes and pumping and refining equipment.6−8 In the case of the nitrogen-containing compounds, they are responsible for disrupting the conventional desulfurization process through competitive adsorption and also by catalyst poisoning.9 In the same way, regarding automobile engines, sulfur and nitrogen also have an undesirable effect on the efficiency of catalytic converters, also leading to corrosion of engines.8,10 Finally, in the context of fuel cells, sulfur is very harmful to catalysts and electrodes.11,12 Industrially, catalytic hydrotreating processes, namely hydrodenitrogenation (HDN) and hydrodesulfurization (HDS), are the most common methods in the removal of nitrogen and sulfur from refined products. However, these processes are expensive, since they require expensive catalysts, large amounts of hydrogen, high temperatures, and high pressures.12,13 Moreover, HDS and HDN are less effective in the removal of heterocyclic sulfur and nitrogen compounds, which is quite problematic when it is necessary to reduce the amount of such compounds to extremely low levels. In addition, since nitrogen compounds tend to saturate the catalyst surface, HDN must be performed prior to HDS, in order to improve efficiency in the HDS unit.14 Although the extensive use of hydrotreatment in refineries around the world arose from the availability of hydrogen from catalytic reformers, today such hydrogen surplus is no longer enough to comply with legislation requirements.6,15 In this way, many efforts have been made to make these hydrotreating-based technologies cheaper and more efficient, either through the development of new catalysts formulations, as well as through new reactor’s design, or even different processing systems,7,16−19 however, without success until today. Therefore, it is crucial to find new alternative and sustainable solutions, in order to replace or complement the current petroleum processes for deep removal of sulfur and nitrogen. Different techniques that do not require hydrogen, such as adsorption, liquid−liquid extraction, oxidative extraction, or even biobased processes, have been widely investigated. Regarding desulfurization processes, extractive desulfurization (EDS) seems to be the one that has drawn the most attention due to its simple operation, low cost, benign effects on the final fuel quality, and high compatibility with other technologies.20,21 In addition, the possibility to use ionic liquids (ILs)22 and more recently deep eutectic solvents (DES),23 as task specific solvents, 11342

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(simulated diesel and simulated gasoline), as indicated in Table3. Higher sulfur compounds concentrations, rather than nitrogen

selected DES, DES based on sulfolane are also prepared and studied for the simultaneous removal of both sulfur and nitrogen compounds. Moreover, after the selection of the best system, the influence of the presence of each model compound is addressed as well as different mixtures of hydrocarbons. In the end, the reusability and regeneration of DES are also explored.

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Table 3. Specifications of the Different Multicomponent Simulated Fuel Matrices Studied in This Work basic model fuel n-heptane, 99.37 wt % thiophene, 0.15 wt %

EXPERIMENTAL SECTION

Materials. The chemicals used in this work are listed in Table 1 as well as their purities and suppliers. They are divided into hydrogenbond acceptors (HBA), complexing agents (CA), and compounds used in simulated fuel matrices. All compounds were acquired with high purity and used without any further purification. DES Preparation. The DES used in this study were prepared combining one ammonium or phosphonium-based salt (TBAC, TBAB and TBPB) with one complexing agent (CA) (PEG400 and Sulfolane). Each combination of organic salts and CA was selected by screening different molar ratios from 1:1 to 1:5. The organic salt:CA ratio chosen for this work was based on the lower proportion of CA to HBA which yields a homogeneous liquid (and not viscous) DES at room temperature. Since no thermally unstable compounds were selected, the DES preparation procedure followed in this work was the same as reported in our previous work.31,32 Briefly, the appropriate amount of each raw compound, according the chosen molar ratio (Table 2), was placed in a

dibenzothiophene, 0.3 wt % carbazole, 0.03 wt % pyridine, 0.15 wt %

simulated diesel

simulated gasoline

n-heptane, 29.81 wt % n-hexadecane, 29.81 wt % n-dodecane, 29.81 wt %

n-heptane, 29.81 wt % n-hexane, 29.81 wt %

toluene, 9.94 wt % thiophene, 0.15 wt % dibenzothiophene, 0.3 wt % carbazole, 0.03 wt % pyridine, 0.15 wt %

toluene, 9.94 wt % thiophene, 0.15 wt % dibenzothiophene, 0.3 wt % carbazole, 0.03 wt % pyridine, 0.15 wt %

isooctane, 29.81 wt %

compounds, were used, in order to mimic their concentrations in real fuels. The initial concentration of each model compound was 1500 ppmw for Th, 3000 ppmw for DBT, 1500 ppmw for Py and 300 ppmw for Carb. Extraction Procedure. Extractive desulfurization and denitrogenation processes, simultaneous or not, were carried out at 25 °C, using an oil bath. The only exception was for sulfolane (when tested alone), which needs 27.5 °C to be liquid. In each experiment, the DES was mixed with simulated fuel, in a volume ratio of 1:1. In general, the stirring time was 15 min, and the settling time was 12 h, to replicate the same conditions for all samples. After that, the upper layer (fuel phase) was collected and analyzed by UV−vis spectroscopy and/or by GCFID, using previously established calibration curves, in order to quantify the amount of sulfur and/or nitrogen compounds. The values of extraction efficiency (EE) were obtained by relating the amount of sulfur or nitrogen compound in the simulated fuel phase before (Ci) and after extraction (Cf), as shown in the eq 1. The Nernst partition coefficients were also determined by relating the concentration of each compound (here denoted by x) in the two phases after the extraction, as presented in the eq 2. All experiments in this study were performed in triplicate.

Table 2. HBA and CA Structures and HBA:CA Molar Ratio for the DES Used in This Work

EE (%) =

Ci − Cf × 100 Ci

(1)

mx ,DES

KN =

mDES mx ,fuel m fuel

(2)

In order to achieve deep desulfurization and deep denitrogenation, a sequence of cleaning cycles was performed. In this way, after a first extraction cycle, the treated simulated fuel was collected and exposed to fresh DES. This process was repeated until an extremely low concentration of sulfur and nitrogen was obtained. The reuse of the DES was also evaluated for the DES with the maximum EE. After a first extraction cycle, the DES was separated from the treated model fuel oil, and used DES was exposed to fresh simulated fuel oil. This process was repeated for five cycles or until the extraction capacity of the DES was greatly reduced. After that, the regeneration of the extractant (DES) was performed, by the addition of water, which allowed the precipitation of the DBT and Carb and in the case of Th the formation of a second liquid phase. After the separation of the different phases, the DES was regenerated by vacuum drying, where the Py was evaporated, and a new cycle of extraction was carried out in order to understand whether the extractant capacity was compromised or not. Analytical Methods/Instrumentation. In this work, for simple matrices of simulated fuel (binary mixtures of one model compound in n-heptane), the amount of sulfur and nitrogen was quantified by UV− vis spectroscopy (UV-1800 Simadzu), using quartz cells with 10 mm of path length. The detection wavelengths used for Th, DBT, Py, and Carb

sealed flask and heated to 80 °C, with continuous and vigorous mechanic stirring (≈800 rpm), for 1 h or until a clear and homogeneous liquid was obtained. After that, the liquid was slowly cooled to room temperature and kept in the sealed flasks. Model Fuels Preparation. Th and DBT were chosen as model sulfur compounds as well as nonbasic Carb and basic Py as model nitrogen compounds. Binary mixtures were prepared by dissolving each model compound in n-heptane. In order to understand the influence of the presence of other compounds in the extraction efficiencies, different multicomponent mixtures were also prepared by dissolving all model compounds in n-heptane (basic model fuel). Afterward, several hydrocarbons were also introduced to modulate diesel and gasoline 11343

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ACS Sustainable Chemistry & Engineering were at 231, 286, 250, and 254 nm, respectively. In the case of the complex matrices, all quantification was performed using a Thermo Scientific gas chromatograph (GC-Trace 1300) equipped with a flame ionization detector (FID) and a SPB-5 capillary column (30 m × 0.25 mm × 0.23 μm of film thickness). Helium was used as a carrier gas at a flow rate of 1.5 mL min−1 and split ratio of 30:1. The injector and detector temperatures were set at 265 and 250 °C, respectively. The oven temperature program was started at 50 °C for 3 min, then increased to 100 °C at 15 °C·min−1 (held for 2 min), and finally increased to 300 °C at 45 °C min−1. The retention time of the Th, Py, DBT, and Carb peaks were 2.78, 3.75, 12.58, and 12.74 min, respectively. Differential scanning calorimetry (NETZSCH DSC 200 F3) was used to determine the melting points of the prepared DES. Measurements were carried out over two heating/cooling cycles between −70 and 80 °C, both with a heating rate of 2 °C·min−1 and a cooling rate of 3 °C·min−1, under nitrogen flow. Between heating and cooling cycles, samples were held isothermally for 5 min. DES viscosities and densities were measured between 20 and 80 °C, using an Anton Paar (model SVM 3000) automated rotational Stabinger viscometer-densimeter, and the obtained data were validated with simple correlations.

heptane, separately. The results, in terms of EE and Nernst partition coefficients (KN), are displayed in Figure 1. In general, sulfur compounds are the hardest to be removed, with Th exhibiting the lowest extraction efficiencies for all extraction systems tested. This trend was also observed by several authors, who extracted both sulfur and nitrogen compounds with ILs36−38 and more recently also with different DES.29 In this context, Li et al.29 pointed out the fact that the low polarity of the sulfur compounds is the most likely explanation for this difficulty in extracting sulfur compounds, since all of the DES studied in this work are composed of salts and thus charged. Regarding the nature and the type of HBA or CA of each DES, the removal of the Th and DBT is favored by the presence of sulfolane in detriment of PEG400. However, in the case of Py and Carb, their extraction efficiencies decrease slightly when sulfolane is the CA. The same trend was confirmed when sulfolane and PEG400 were tested alone as extractant solvents. Concerning DES containing salts, it is possible to conclude that (a) the ef fect of the anion: chloride seems to yield slightly better results than bromide; and (b) the ef fect of the cation: regarding the sulfur compounds, the presence of phosphonium or ammonium cations does not have a great effect on the extraction efficiencies; however, in the case of the nitrogen compounds, phosphonium cation-based DES provide better extraction efficiencies, especially for Py, increasing its removal from around 86% to 99%, and also the KN, increasing this parameter from about 4 to 37. Finally, an analysis of all partition coefficients allows to conclude that they are in agreement with the determined extraction efficiencies, ranging from about 1 for Th to 37 for Py. Viscosity and density are important thermophysical properties of any solvent, since they influence mass transport phenomena and also the settling time between the 2 immiscible phases, affecting their suitability for specific applications. The viscosity and density of the four different DES, PEG400, and sulfolane were measured, between 20 and 80 °C, and are depicted in Figure 2. The fitted parameters of the models used to correlate and validate data temperature dependence are depicted in Tables S1 and S2. As expected, temperature has a higher influence on viscosity than on density. All the DES present higher viscosities than its corresponding CA, which was also expected due to the interaction between the HBA and the CA. However, sulfolane is only liquid around 30 °C, which makes comparisons at room temperature (25 °C) impossible. Even so, if a temperature of 30 °C is considered, TBPB:Sulf DES showed the lowest increase on viscosity when comparing with sulfolane, from 7.95 to 36.391 mPa·s. The TBAC:PEG400 DES is the most viscous, presenting a viscosity of 154.685 mPa·s at 30 °C. In the case of density, TBAC:PEG400 is the less dense DES (1.1843 g·cm−3), and the TBPB:Sulf DES the most dense (1.2732 g·cm−3). Thus, with respect to the physicochemical properties, the TBPB:Sulf DES presents the most attractive properties to carry out a liquid−liquid extraction. At this point, the TBPB:Sulf (1:4) was selected as the most promising system to perform simultaneously the removal of the four model compounds, since it presents good capacities of extraction and the lowest viscosity as well as the highest density from all tested DES. Simultaneous EDS and EDN using the TBPB:Sulf DES. Effect of the Fuel Matrix. In order to step forward in the application of EDS and EDN to fuels and taking into account that most works on desulfurization and denitrogenation with DES focus only on simple fuel matrices (such as one model compound dissolved in one model hydrocarbon), this work

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RESULTS AND DISCUSSION DES Preparation. Based on our previous work on EDS,31 TBAC:PEG400 (1:2) DES was chosen since it allowed to reach the best results in terms of Th and DBT extraction efficiency (65% and 85%, respectively). In this work, TBAC:PEG400 (1:2) and other similar DES were selected to extract four different model fuel impurities: Th, DBT, Py, and Carb. The different combinations of HBA:CA (Table 3) were prepared using the heating method. Regarding the HBAs, in addition to TBAC, two other salts were tested in order to study the influence of different anions (chloride and bromide) and also different cations (ammonium and phosponium). In the case of the CAs, besides PEG400, sulfolane was also used as the second component. Sulfolane is the most commercially used solvent for the separation of aromatics,33 and in recent years, it has been combined with TBPB34 and TBAB33 for the same purpose. Recently, TBAB:Sulf (1:7) was also used to remove Th from nheptane,35 with a maximum EE of 35% for one cycle. To the best of our knowledge, TBAC has never been combined with sulfolane, and the removal of nitrogen compounds has never been studied with sulfolane-based DES. In the case of TBAC:PEG400, the same molar ratio (1:2) used in our previous work was also used here. For the other DES, a molar ratio study was performed, and 1:4 was selected in all cases, as this choice is based on the final physical state of the eutectic solvent, liquid and not liquid with solid in suspension and particularly on its fluidity. Table 4 illustrates the melting temperature of DES used during this work, determined by DSC analysis (available in the Supporting Information). DES Selection for Simultaneous Desulfurization and Denitrogenation. All DES were first screened as extractant solvents for the removal of Th, DBT, Py, and Carb from nTable 4. List of the Prepared DES and Their Respective Physical State and Melting Point Determined by DSC DES

physical state at room temperature

melting point (°C)

TBAC:PEG400 (1:2) TBAC:Sulf (1:4) TBAB:Sulf (1:4) TBPB:Sulf (1:4)

fluid clear liquid fluid clear liquid fluid clear liquid fluid clear liquid

−0.8 −5.7 −8.8 −6.6 11344

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Figure 1. Extraction efficiencies of Th, DBT, Py, and Carb from n-heptane using four different DES, PEG400, and sulfolane. Each bar represents the average of three different experiments and the error bars the standard deviations. Nernst partition coefficient is also depicted through darker bars.

Figure 2. Experimental viscosities and densities of the four different studied DES as well as the two pure complexing agents.

explores the influence of the presence of various model compounds in several hydrocarbons. This study was done in two steps: First, all four model compounds (Th, DBT, Py, and Carb) were dissolved in n-heptane in order to understand if any competition in removal occurs; second, different hydrocarbons

were introduced to simulate a gasoline and a diesel (Table 3). The extraction efficiencies of each model compound from different fuel matrices are compared with the single extractions from n-heptane in the Figure 3. Comparing the single and the simultaneous extractions of both sulfur compounds from n11345

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Figure 3. Comparison between single and simultaneous removal of nitrogen and sulfur compounds from n-heptane as well as the effect of the fuel matrix complexity on the removal of Th, DBT, Py, and Carb by the TBPB:Sulf DES.

Figure 4. Evolution of the sulfur and nitrogen model compounds concentration in the simulated gasoline phase, with multiple extraction cycles, using TBPB:Sulf DES.

heptane (first two bars in the graph), a slight decrease can be observed for the latter case, from 67% and 88% to 63% and 86%, for Th and DBT, respectively. However, in the case of the nitrogen compounds, their extraction efficiencies are favored by the presence of sulfur compounds, increasing from 99% and 97% to 100% and 100%, for Py and Carb, respectively. Concerning simulated gasoline and simulated diesel (third and fourth bars in the Figure 3), in general, the presence of more complex hydrocarbons seems to have no effect on the extraction efficiencies. At least, in the case of nitrogen compounds, their removal rates remain unchanged and at a maximum value. In the case of the simulated gasoline, the presence of additional hydrocarbons, n-hexane, isooctane, and toluene, seems to even slightly improve the extraction of Th, since it increases from 63% to 68%. However, when the fuel matrix is the simulated diesel, the Th extraction efficiency decreases again to 63%, and in the case of DBT, a slight decrease, from 86% to 83%, can also be observed. These different trends obtained for simulated gasoline

and diesel may be explained by the fact that, in simulated diesel, hydrocarbons with much longer alkyl chains are present. Multistage Extraction. In the last section, it was shown that both nitrogen compounds are deeply removed in a single cycle of extraction. However, the same does not happen with the sulfur compounds. Thus, it is worthwhile considering a multistage extraction approach, using the TBPB:Sulf DES, so that deep desulfurization can be also achieved. The evolution of the amount of nitrogen and sulfur along the extraction cycles is depicted in Figure 4. As expected, after two successive extraction steps, the amount of nitrogen compounds is already zero. In the case of the sulfur compounds, after three extraction cycles, their concentration is almost zero regarding DBT. However, in the case of Th, a fourth cycle is required to bring its concentration below the targeted environmental sulfur regulation (10 ppm). Therefore, a set of four cycles is necessary to achieve both deep desulfurization and denitrogenation of a simulated gasoline. These results clearly show the high potential of DES as extractants in simultaneous EDS and EDN in mild conditions. 11346

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Figure 5. Reuse capacity of the TBPB:Sulf DES to extract the four model compounds at the same time from the simulated gasoline, without regeneration for five cycles and after regeneration.

Reusability and Regeneration. The capacity to be reused and regenerated are two extremely important features of any solvent. From both economic and environmental points of view, minimizing the volume of solvent used is mandatory. In this way, the capacity of TBPB:Sulf DES to be reused, first without regeneration, was investigated (Figure 5). This study was carried out with one of the complex matrices: simulated gasoline. Right from the first reuse, which corresponds to the second extraction cycle, the DES extraction capacity decreased for the sulfur compounds, but remained unchanged in the case of nitrogen compounds. After five cycles of reuse, the DES lost 78% (from 68 to 15%) and 44% (from 87 to 49%) of its capacity to extract Th and DBT, respectively. The same trend was already found in our previous work when a PEG-based DES was used.31 In the case of the nitrogen compounds, after the five cycles of reutilization, the DES maintains almost the same extraction capacity, losing about 4% and 2% for Py and Carb removal, respectively. Finally, after the five cycles of reuse, the regeneration of TBPB:Sulf DES was performed so that the recycling of DES could be implemented. The regeneration was carried out by adding water to the DES phase (containing Th, DBT, Py, and Carb), which allowed the precipitation of the DBT and Carb and in the case of Th the formation of a second liquid phase. After the separation of the different phases, the DES was regenerated by vacuum drying, where the Py was also separated. As shown in Figure 5, after the regeneration step, the performance of the DES to extract the four model compounds was fully recovered, once the extraction efficiencies remain the same as for the fresh DES in cycle 1. Regarding the regeneration method, considering the energy costs for water separation, this method is suitable only for laboratory scale, since the purpose was simply to show that DES capacity could be fully recovered. Thus, further research is still needed to make its industrial application economically viable.

applications, such as aromatic extraction, extractive distillation, reaction solvent, printing ink, among others.39 Comparing sulfolane with other common organic solvents, although sulfolane presents a low vapor pressure, 0.0091 kPa at 30 °C, it has the disadvantage of being a water-soluble compound, which can easily cause water contamination. Nevertheless, sulfolane presents a low acute oral and skin toxicity and a low skin penetration in comparison to other commonly used solvents.40 In the present work, DES based on sulfolane are tested in the integration of EDS and EDN of fossil fuels in one single process and the simultaneous development of a circular process, which is achieved through regeneration of the DES used as extractant. The use of sulfolane in oil and gas refining is not new, since there are several sulfolane-containing solutions commercially available, mainly for sour gas treatment, but also for BTX extraction.41 Regarding the development and application of sulfolane-based DES, they have also been studied for the separation of aromatic hydrocarbons33,34 and in sulfur removal,35 but have never been studied for denitrogenation. In our work, with the addition of TBAC/TBAB/TBPB salts to the sulfolane, in order to produce the DES, a new liquid window of sulfolane is opened, by decreasing its freezing point to almost −9 °C (Figure S1). Taking into account the work of Shahbaz et al.,42 who pointed out that the final vapor pressures of DES formed by glycerol and several salts are much lower than vapor pressure of glycerol, we believe that these sulfolane-based DES have lower vapor pressure than sulfolane (work in progress). The volatility of DES as extractant solvents is thus a crucial property for their proper selection and application. In addition, our final solvent (TBPB:Sulf (1:4)) has a higher extraction capacity for aromatic sulfur compounds as well as nitrogen compounds, in comparison to sulfolane. Furthermore, it can be reused and recycled, making it possible to fully recover its extraction capacity.

SUSTAINABILITY AND NOVELTY Sulfolane is a polar aprotic solvent, widely used in the industry. It presents a high boiling point and good chemical stability which makes it a versatile solvent for a wide spectrum of organic

CONCLUSIONS In this work, four different DES were prepared, and their performances in the removal of both nitrogen and sulfur compounds from fuels were evaluated. All four DES could be

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Research Article

ACS Sustainable Chemistry & Engineering

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successfully used for Th, DBT, Py, and Carb removal, with EE ranging from 65% to 99%, regarding binary mixtures. The sulfur compounds were the hardest to be removed, especially Th. The TBPB:Sulf DES was chosen to perform the simultaneous EDS and EDN, allowing the best results in terms of EE in the binary mixtures, while being the most dense and less viscous liquid. In general, the concurrent removal of the all four model compounds leads to a slight decrease on the EE of both sulfur compounds but, on the other hand, to an increase on the nitrogen-containing compounds removal. In addition, when other types of hydrocarbons were introduced, to simulate diesel and gasoline, practically no changes were observed with the EEs, except in the case of the simulated diesel, which led to a decrease on the EE of DBT. Taking sustainability into account, further research was undertaken to evaluate the reuse of the TBPB:Sulf DES. After five cycles of reusing, the capacity of the DES to extract Th and DBT was decayed considerably, and a regeneration step is enough to fully recover its performance.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b00877. Density- and viscosity-temperature model parameters and DSC thermograms (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *[email protected]. Phone: +351 218413385. ORCID

Armando J. D. Silvestre: 0000-0001-5403-8416 Luis C. Branco: 0000-0003-2520-1151 Isabel M. Marrucho: 0000-0002-8733-1958 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS F.L., I.M.M., and L.C.B. gratefully acknowledge the financial support of FCT/MCTES (Portugal) for the Ph.D. fellowship PD/BDE/114355/2016 and for the contract under Programa Investigador FCT 2012 and 2013 (IF/363/2012 and IF/0041/ 2013), respectively. This work was financed by CQE project (UID/QUI/00100/2013), the Associate Laboratory CICECO, Aveiro Institute of materials (UID/CTM/50011/2013) and Associated Laboratory for Green Chemistry, LAQV-REQUIMTE (UID/QUI/50006/2013), and Solchemar company.

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REFERENCES

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DOI: 10.1021/acssuschemeng.9b00877 ACS Sustainable Chem. Eng. 2019, 7, 11341−11349

Research Article

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DOI: 10.1021/acssuschemeng.9b00877 ACS Sustainable Chem. Eng. 2019, 7, 11341−11349