Article pubs.acs.org/EF
Adsorptive Removal of Nitrogen, Sulfur, and Aromatic Compounds from Gas Oil by Poly(glycidy methacrylate) Using Two Kinds of Graft Polymerization Methods Ali Abedi,† Ajay K. Dalai,*,† and John Adjaye‡ †
Department of Chemical and Biological Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5A9 Syncrude Edmonton Research Centre, Edmonton, Alberta, Canada T6N 1H4
‡
S Supporting Information *
ABSTRACT: Based on a polyglycidyl methacrylate-co-ethylene glycol dimethacrylate copolymer (PGMA-co-EGDMA), nitrogen, sulfur, and aromatic compounds were removed from light and heavy gas oil feeds. The method in which PGMA-coEGDMA is synthesized can influence the textural and chemical characteristics of the polymer and thus its adsorption capacity. Studies have shown that using cerium initiated graft polymerization in PGMA-co-EDGMA synthesis can improve the adsorption capacity of the polymer. In this work, nitrogen, sulfur, and aromatics removal capacity of (PGMA-co-EGDMA) polymer incorporated with tetranitrofluorenone (TENF) via 1,3 diaminopropane (PDA) using cerium initiated graft polymerization were compared with the same polymer without using cerium. A third polymer with different linker, ethylenediamine (EDA) instead of PDA, was synthesized using cerium initiated graft polymerization to inspect the impact of the linker on the removal efficiency. The synthesized polymers were characterized using different characterization methods. The synthesized polymers were tested at different nitrogen, sulfur, and aromatic content using light and heavy gas oil feeds. In addition, the removal capacity of the synthesized polymers toward nonbasic nitrogen were determined using automatic potentiometric titrator. Results have shown that using cerium graft polymerization on the synthesis of PGMA-co-EGDMA polymer reduced surface area, pore size and volume, and amount TENF grafted, thus decreasing the removal efficiency of nitrogen, sulfur, and aromatics. However, polymer selectivity toward nonbasic nitrogen was not affected by cerium graft polymerization. Furthermore, the adsorption capacity of the PGMA-co-EGDMA decreased with increasing linker length due to steric hindrance effect that influences the adsorption capacity of the polymer.
1. INTRODUCTION The increasing demand for crude oil has created interest in dealing with unconventional oil such as oil shales, oil sands, and heavy oil. However, the high nitrogen and sulfur contents of unconventional oil feedstocks make hydrotreating a challenging process for the petrochemical industries. In addition, the presence of high levels of nitrogen and sulfur species reduces fuel quality,1 impacts the environment by releasing pollutant upon combustion,2,3 and poisons the catalysts used in hydrotreating reactors downstream.4 In addition, Fu et al.,5 who evaluated the effect of various nitrogen compounds on the performance of hydrotreating catalysts, observed lower gasoline yields and higher coke formation as the nitrogen content increased in the feed. The inhibition effect of nitrogen compounds on nonconventional feedstocks is a major problem that affects the hydrodesulfurization (HDS) reactions by reducing the activity and lifetime of the catalysts.4,6−10 Based on reaction severity, nitrogen species can be irreversibly or reversibly adsorbed on the catalyst, thus preventing sulfur species from reaching active sites of the catalyst. 8,10 Furthermore, due to nitrogen compounds low reactivity in hydrotreating process as compared to sulfur species, removing nitrogen species is a challenging process.11−14 Nitrogen species present in oil consist of 2 groups: basic nitrogen and nonbasic nitrogen. According to Laredo et al.15 nonbasic nitrogen compounds compose 75% of the total © XXXX American Chemical Society
nitrogen content in atmospheric gas oil (AGO), whereas the remaining 25% are basic nitrogen compounds. Laredo16,17 found that the inhibition effect is similar for both types of nitrogen compounds. Furthermore, Li et al. found that basic nitrogen compounds hindered the conversion in fluid catalytic cracking by adsorption on acidic sites of the catalyst, while nonbasic nitrogen compounds are difficult to convert into smaller molecules, thus enhancing coke formation on the catalyst surface.18 In addition, more basic nitrogen compounds can be produced during HDS process by hydrogenation of nonbasic nitrogen compounds.19 Several studies showed that higher desulfurization can be achieved by removing nitrogen compounds prior to hydrotreating.20,21 Several methods have been reported in the literature on the removal of nitrogen compounds from gas oil including metal−organic framworks,22,23 ionic liquids,24 and functionalized polymers.25,26 Removing nitrogen compounds with functionalized polymers has shown significant impact on HDS process.25−29 Functionalized polymers adsorb nitrogen and sulfur species by π-acceptor molecules that are grafted on a polymer support. Due to its affinity to adsorb sulfur species, Milenkovic27 used tetranitrofluorenone (TENT) to selectively Received: September 19, 2016 Revised: February 3, 2017 Published: February 8, 2017 A
DOI: 10.1021/acs.energyfuels.6b02412 Energy Fuels XXXX, XXX, XXX−XXX
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(isobutyronitrile). Alfa Aesar: 9-flourenone 98+%. Sigma-Aldrich: 1,3-diamnopropane (≥99%), ethylenediamine (reagent plus ≥99%) fuming nitric acid (>90%), glycidyl methacrylate (GMA), 1-dodecanol (≥98.0%) ethylene glycol dimethacrylate (EGDMA), poly[N-vinyl-2pyrrolidone] PVP (average Mw = 55 000 g/mol), and cyclohexanol (99.0%). Fisher Scientific: Cerium(III) nitrate hexahydrate (99%), concentrated 95−98 w/w % sulfuric acid, ethanol (95%). nitric acid (certified ACS plus 70%), toluene (ACS reagent (≥99.5%)), and acetic acid glacial (certified ACS), Prior to polymer synthesis, 0.5 M H2SO4 aqueous solution and 1.0 M nitric HNO3 solution were prepared by diluting concentrated H2SO4 acid and HNO3 acid, respectively. In addition, 0.1 M CAN solution was prepared by mixing appropriate amounts of cerium(III) nitrate hexahydrate powder with 1.0 M HNO3 acid solution. 2.2. Synthesis of Tetranitrofluorenone (TENF). Due to its high affinity to remove nitrogen and sulfur compounds, TENF was chosen as a π-acceptor for all polymers.25−28,36 Based on the method described by Newman,37 TENF was synthesized using three solutions. Please refer to Table S1 in the Supporting Information for the composition of the three solutions. Details of TENF synthesis can be found elsewhere.26 2.3. Synthesis of PGMA-co-EGDMA. PGMA-co-EGDMA polymer support was synthesized based on preparation procedures of Svec,38 and modified by Rizwan.25 The polymer was synthesized by preparing three solutions. The composition of the three solutions can be found in Table S2 in the Supporting Information. Details of PGMA-co-EGDMA synthesis can be found elsewhere.26 2.4. Synthesis of PGMA−PDA/EDA-TENF by Ce Initiated Graft Polymerization. Ce initiated graft polymerization was based on a method described by Li.34 In the first stage the epoxy ring of PGMA-co-EGDMA, which was prepared using Sevc et al.’s38 method and modified by Abedi et al.,26 was transformed to a hydroxyl group by adding 40 g of PGMA- co-EGDMA to 400 mL of 0.5 M H2SO4 aqueous solution at 30 °C for 48 h. Then the polymer was washed with deionized (DI) H2O, and vacuum filtered, to produce white powder. The powder was placed in an oven at 90 °C for 24 h. The product will be referred to as PGMA−OH. In the second stage, under nitrogen environment, 10 g of PGMA−OH was added to 700 mL of DI water in a 1000 mL three-neck round-bottom flask and mixed at 60 °C for 1 h. A total of 50 mL of 0.1 M CAN solution was added to the mixture, while stirring, then 30 min later, 25 mL of GMA was added. Then, 6 h later, the temperature was decreased to room temperature, and the product was washed with ethanol and deionized water and filtered. The product was allowed to dry at 90 °C for 24 h. The product of this stage will be referred to as PGMA-GMA. In the third stage, 10 g of the PGMA-GMA polymer, 200 mL of deionized water, and 200 mL of PDA linker were mixed. The reactor was heated to 80 °C and reaction proceeded for 24 h. Then the reaction was cooled and the product was filtered. Ethanol and deionized water were used to wash the filtrate. The product, PGMA− PDA, was allowed to dry for 24 h at 90 °C. Finally, TENF was grafted by dissolving 3.2 g of TENF powder in 150 mL of toluene and 15 mL of acetic acid. The temperature was increased to 90 °C while stirring to dissolve TENF. Once all TENF power was dissolved, 8 g of PGMA− PDA was added. After 24 h, the final product, PGMA−PDA-TENF powder, was washed with toluene, filtered and dried at 90 °C for 24 h. Similarly another polymer, PGMA-EDA-TENF was prepared following the same procedure except in the third stage EDA was used, instead of PDA. 2.5. Synthesis of PGMA-dPDA-TENF. To compare the performance of PGMA−PDA-TENF polymer prepared by Ce initiated graft polymerization, another polymer was synthesized without the formation of tentacle support attached to the polymer. The linker PDA was attached to the PGMA-co-EGDMA polymer directly; thus, the product was called PGMA-dPDA-TENF. A total of 5 g of PGMAco-EDGMA was mixed with 100 mL of PDA and 100 mL of DI water at 80 °C for 8 h. Then the polymer was washed with ethanol and DI water and filtered. The product, PGMA-dPDA, was allowed to dry at 90 °C for 24 h. Finally, TENF was attached to the polymer support
eliminate alkyldibenzothiophene from gas oil. Furthermore, several studies have used PGMA-co- EGDMA based copolymer with TENF π-acceptor to eliminate nitrogen species from light and heavy gas oil samples.25,26,28,29 Macaud et al. combined different polymer supports with TENF to enhance the removal of nonbasic nitrogen compounds.28,29 Hydrophilic polymer supports such as PGMA-co-EGDMA found to be selective toward nitrogen compounds in gas oil when combined with TENF π-acceptors.29 Rizwan et al.25 optimize the synthesis of PGMA-co-EGDMA incorporated with TENF to selectively eliminate nitrogen compounds from heavy gas oil. Abedi et al.26 have tested different polymer supports with TENF π-acceptor and found that PGMA-co-EGDMA had the highest nitrogen removal efficiency as compared to other polymers. Compared to nonpolymeric adsorbents, PGMA based copolymers can be easily separated from gas oil and regenerated using Soxhlet extractor. For example, Yang et al.30,31 used zeolite based adsorbents to achieve high sulfur removal from liquid fuel; however, the regeneration process included heating the adsorbents to 350 °C. To enhance the removal efficiency of the polymer support, several synthesis procedures have been proposed, in which initiated graft polymerization using cerium(IV) was successfully used to enhance the adsorption capacity of different polymers.32−34 This method is based on the reaction between Ce (IV) ions and hydroxyl groups on the polymer to graft or obtain a copolymer.35 Cerium initiated graft polymerization was used to modify the surface of the polymer by the formation of tentacle support extensions, thus permitting the adsorbate to adsorb in multiple layer and to minimize undesirable interaction with the polymer base.32,33 Furthermore, high density tentacle polymer layers were synthesized by cerium initiated surface graft polymerization to increase the adsorption capacity of PGMA-NH2 microspheres toward Cr (VI).34 The purpose of this study was to examine the influence of polymerization method on the adsorption capacity PGMA-coEGDMA polymer toward basic and nonbasic nitrogen, sulfur, and aromatic compounds in light and heavy gas oil feeds. The performance of PGMA-co-EGDMA, with tentacle glycidyl methacrylate (GMA) polymer layers, synthesized by cerium initiated graft polymerization was compared to that without a tentacle polymer layer. TENF and PDA were used as a common π-acceptor and linker for both polymers. In addition, a third polymer with different linker, ethylenediamine (EDA), was synthesized using cerium initiated graft polymerization method to examine the effect of the linker on the removal efficiency of PGMA-co-EGDMA.
2. EXPERIMENTAL DETAILS AND METHODOLOGY Three polymers were synthesized using glycidyl methacrylate-coethylene glycol dimethacrylate (PGMA-co-EGDMA) and tetranitrofluorenone as a common polymer support and π-acceptor, respectively. For initial graft polymerization, glycol dimethacrylate (GMA) was attached to PGMA-co-EGDMA polymer using Ce before adding the linker. Two linkers were used, ethylenediamine (EDA) and 1,3 diaminopropane (PDA), to attach TENF to PGMA-co-EGDMA polymer. The produced functionalized polymers will be referred to as PGMA-EDA-TENF and PGMA−PDA-TENF. To compare the performance of the initiated graft polymerization method to that without grafting GMA, a third polymer was synthesized by attaching the PDA linker directly to the polymer support. This polymer will be referred to as PGMA-dPDA-TENF throughout the paper. 2.1. Materials. The chemicals used to synthesize the polymers were purchased from various sources: Molekula Ltd.: AzobisB
DOI: 10.1021/acs.energyfuels.6b02412 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels following the same procedures mentioned above. The final product was PGMA-dPDA-TENF.
Furthermore, the adsorption amount (q) was determined by the following equation: q = (C i − Cf )*a /1000
3. INSTRUMENTATION The synthesized polymers were characterized using different methods. Using PerkinElmer Spectrum GX instrument, Fourier transform infrared spectroscopy (FT-IR) was done to determine the characteristic functional groups of the polymers. A small amount of the sample was mixed with KBr and pressed to form a small pellet. The spectra were obtained based on an average of 16 scans between 400 and 4000 cm−1. The textural properties of the polymers were determined using a Brunauer− Emmett−Teller (BET) method. Based on N2 adsorption− desorption at 77 K, pore size and volume as well as BET surface area were determined using Micromeritics 2000 ASAP analyzer. A total of 0.1−0.2 g of polymer sample was prepared for BET analysis by degassing for 90 min at 150 °C under vacuum of 10 μm Hg to remove any moisture. The morphology of the polymers was analyzed using field emission scanning electron microscopy (FE-SEM), (Hitachi machine SU 6600 model) at 2 kV. The sample was prepared by gold coating the surface to prevent charge build-up during analysis. TGA Q500 model instrument was used to obtain thermo gravimetry (TGA) and differential thermal analysis (DTA) of the polymers. About 10 mg of the polymer sample was placed in the instrument at room temperature. Then, the temperature increased at a rate of 10 °C/min, under N2, to 800 °C. Vario EL III CHNS elemental analyzer was used to determine elemental carbon (C), hydrogen (H), nitrogen (N), and sulfur (S) contents of the polymers. The analysis was performed in triplicate, using 4−6 mg of the sample. A 500 MHz Bruker advance NMR spectrometer was used to perform liquid 13carbon nuclear magnetic resonance (13C NMR) in order to quantity the aromatic content of the oil. Details of the 13C NMR operating operating conditions can be found on Abedi et al.26 AntekModel 9000 N/S analyzer was used to determine nitrogen and sulfur contents of treated and untreated oil samples. Simulated distillation (SimDist) using Varian/AC Analytical instrument was used to determine the boiling point range of the gas oil feeds. The basic and nonbasic nitrogen contents of the gas oil were determined using Hanna HI 902C2−01 automatic potentiometric titrator. A small of the oil sample was dissolved in 100 mL of titrant solvent (toluene + isopropyl alcohol (IPA) + chloroform + a small amount of water), and then the sample was potentiometrically titrated with 0.1 M hydrochloric acid (HCl) in IPA solution.
a in the equation is the mass ratio of liquid fuel to the polymer. The adsorption performance of the synthesized polymers was tested using light and heavy gas oil samples. Table 1 shows the Table 1. Characteristic Properties of LGO and HGO parameters
LGO
HGO
density [g/mL] boiling range [°C] total N content [ppm] basic N content [ppm] nonbasic N content [ppm] total S content [ppm] aromatic content [%]
0.91 180−430 1800 470 1330 32 000 38
0.96 230−520 3600 1060 2540 42 000 45
characteristic properties of LGO and HGO feeds. It is noticed that the nitrogen content of HGO is twice as much as that of LGO. In addition sulfur content in both LGO and HGO was much higher than LGO feed. A total of 26% of the total nitrogen content in LGO feed was basic nitrogen, while the basic nitrogen was 29% of the total nitrogen content of HGO feed. For the adsorption isotherms, different quantity of nitrogen compounds were dissolved in LGO to obtain the equilibrium adsorption capacity over PGMA-dPDA-TENF and PGMA−PDA-TENF at 23 and 55 °C. A batch system was used to mix the synthesized polymers with gas oil feeds. 0.5 g of the polymer was mixed with the gas oil feed at 1:5 polymer to oil (P/O) ratio. The mixture was stirred at 200 rpm for 24 h at room temperature, about 21 °C. This temperature was chosen based on a study of the adsorption temperature using similar polymer. Figure S2, in the Supporting Information, shows the removal efficiency of nitrogen and sulfur from LGO at different adsorption temperature. Thus, adsorption performance was examined at 21 °C. For isotherms, the ratio of liquid fuel to polymer was kept constant at 5, and the mixing time was 24 h. After mixing, the polymer particles were separated from the gas oil using vacuum filtration.
5. RESULTS AND DISCUSSION Three functionalized polymers were synthesized to study the effects of Ce initiated graft polymerization method on the removal efficiency of nitrogen, sulfur, and aromatics using PGMA-co-EGDMA polymer support. The performance of the PGMA−PDA-TENF polymer, which was synthesized by formation of tentacle polymer on the surface using Ce initiated graft polymerization method, was compared to the PGMAdPDA-TENF polymer that was synthesized without modifying the surface of the polymer. LGO and HGO feeds were used to determine the adsorption capacity of the polymers. 5.1. Polymer Characterizations. The characterizations of the prepared polymers, PGMA−PDA-TENF, PGMA-EDATENF, and PGMA-dPDA-TENF, are presented in this section. In the first stage of this work, PGMA-co-EGDMA and TENF were synthesized. Then, TENF was grafted on the polymer support via PDA linker using two different methods: (A) attaching the linker to the polymer support, PGMA-coEGDMA, without modifying the surface to form PGMAdPDA-TENF, and (B) using Ce initiated graft polymerization
4. ADSORPTION STUDY The adsorption capacities of the synthesized polymers were determined by analyzing different gas oil samples with different nitrogen and sulfur concentrations. A total of 0.5 g of the polymer was used for each adsorption experiment, and 0.03− 0.04 of the treated oil was used for nitrogen and sulfur analysis. In addition, 0.1 g of the treated oil samples was used for potentiometric titration. The removal efficiency was determined by measuring the nitrogen and sulfur contents of the gas oil before and after polymer adsorption (treatment). Nitrogen, sulfur, and aromatics removal percentages were calculated based on the following equation: R = (C i − Cf )/C i × 100
where R (wt %) is removal percentage, C i is initial concentration [ppmw], and Cf is final concentration [ppmw]. C
DOI: 10.1021/acs.energyfuels.6b02412 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 1. Synthesis of (A) PGAM-dPDA-TENF and (B) PGMA−PDA-TENF polymers.
to form tentacle polymers, GMA, on the surface of the polymer support then adding the linker and TENF to form PGMA− PDA-TENF. Figure 1 illustrates the synthesis steps of both methods. The third polymer PGMA-EDA-TENF was synthesized following the preparation procedures of method (B), except that the linker, PDA, was replaced with shorter linker, EDA, to determine the influence of linker length on the removal efficiency of the polymer. In method (B), the epoxy group of PGMA-co-EGDMA was opened to form two hydroxyl groups, PGMA−OH, by using diluted H2SO4 acid. Then, GMA was grafted on PGMA−OH polymer using Ce ions to initiate the reaction between GMA and hydroxyl group.34,35 Then the linker was attached to the polymer by reacting with the epoxy ring of GMA tentacles on the polymer surface. Finally, TENF was attached to the polymer support via PDA linker. The formation of PGMA−PDA-TENF polymer was evident by the FT-IR analysis after each stage of the synthesis. Figure 2 shows the FT-IR spectra of (a) PGMA-co-EGDMA, (b) PGMA−OH, and (c) PGMA−PDA-TENF. In Figure 2a, the absorbance bands corresponding to the epoxy ring and carbonyl group (CO) were observed at 800−1000 and 1730 cm−1 respectively. After dissolving the PGMA-co-EGDMA polymer in acidic solution, the epoxy ring was converted to hydroxyl groups as shown in Figure 2b by the disappearance of characteristic peaks of the epoxy ring, and the existence of the hydroxyl group absorbance peaks at 3200−3600 cm−1. In Figure 2c, the attachment of the TENF, via PDA linker, to the polymer support was observed by absorbance bands of (N−O),
Figure 2. FT-IR spectra of (a) PGMA-co-EGDMA, (b) PGMA−OH, and (c) PGAM- PDA-TENF.
symmetric and asymmetric, at 1350 and 1540 cm −1 , respectively. The results of FT-IR analysis of PGMA-EDATENF and PGMA-dPDA-TENF polymers are found in Figure S1, available in the Supporting Information. The characteristic peaks of these polymers were similar to PGMA−PDA-TENF polymer and evidenced the grafting of TENF. The thermal stability and decomposition temperatures of the synthesized polymers before and after attaching TENF were determined using TGA/DTG analysis. In addition, grafted TENF was determined using TGA thermograms. Figures 3 and 4 illustrate TGA and DTG of (a) PGMA-co-EGDMA, (b) PGMA-EDA-TENF, (c) PGMA−PDA-TENF, and (d) PGMAdPDA-TENF. All polymers were degraded in two stages between 200 and 500 °C as shown by the two peaks in Figures D
DOI: 10.1021/acs.energyfuels.6b02412 Energy Fuels XXXX, XXX, XXX−XXX
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CHNO elemental analysis of the synthesized polymers at different stages and TENF are shown in Table 3. Elemental Table 3. Elemental CHNO Analysis of the Synthesized Materials at Different Stages element
Figure 3. TGA thermograms of (a) PGMA-co-EGDMA, (b) PGMAEDA-TENF, (c) PGAM−PDA-TENF, and (d) PGMA-dPDA-TENF.
polymer
carbon [wt %]
hydrogen [wt %]
nitrogen [wt %]
oxygen [wt %]
TENF (theoretical) TENF (experimental) PGMA-co-EGDMA PGMA-dPDA PGMA-dPDA-TENF PGMA-co-EGDMA PGMA−OH PGMA-GMA PGMA-EDA PGMA−PDA PGMA-EDA-TENF PGMA−PDA-TENF
43.4 42.5 59.6 56.9 56.1 59.6 54.8 51.5 54.5 52.9 53.5 56.5
1.1 1.9 7.1 8.0 7.8 7.1 7.4 7.4 8.1 7.9 7.6 8.1
15.6 15.2 0.0 4.8 5.1 0.0 0.0 0.0 0.9 0.9 1.3 1.4
40.0 40.0 32.8 30.4 30.8 32.8 37.5 40.4 36.4 38.2 37.5 33.7
CHNO of TENF agreed with the theoretical as well as experimental values determined by Abedi et al.26 CHNO elemental analysis of PGMA-dPDA-TENF polymer confirmed polymer synthesis as the elemental nitrogen composition increased to 4.8 and 5.1% after grafting PDA and TENF, respectively. Similarly, PGMA-EDA-TENF and PGMA−PDATENF syntheses were also confirmed by CHNO analysis of the polymer at different stages. For example, elemental oxygen increased after opening the epoxy ring, PGMA- OH, and elemental nitrogen increased after attaching EDA/PDA linkers and TENF. The textural properties of the polymer, such as surface area, pore size, and pore volume, play a significant role in the adsorption capacity of the polymer.25,26 For this reason, pore size and volume as well as BET surface area of the synthesized polymers at each stage were determined using nitrogen physisorption isotherms. Table 4 shows BET the textural
Figure 4. DTG thermograms of (a) PGMA, (b) PGMA-EDA-TENF, (c) PGAM−PDA-TENF, and (d) PGMA-dPDA-TENF.
4. However, the PGMA-co-EGDMA polymer was more thermally stable after functionalization as the degradation temperatures and remaining amount of polymer was higher after attaching TENF. For example, at 300 °C, the remaining weight of PGMA-EDA- TENF, PGMA−PDA-TENF, and PGMA-dPDA-TENF polymers were more than twice as much as that of the PGMA-co-EGDMA, as shown in Figures 3 and 4. Table 2 summarizes the characteristic properties of TGA/DTG analysis. Table 2. TGA/DTG Characteristic Properties of the Synthesized Polymers
Table 4. Textural Properties of the Synthesized Polymers
polymer
Tmax1 [°C]
Tmax2 [°C]
residue @ 700 °C [wt %]
TENF grafted [mmol/g]
PGMA-co-EGDMA PGMA-dPDA-TENF PGMA−PDA-TENF PGMA-EDA-TENF
230 296 348 323
302 403 404 405
0 12 9 9
0 0.37 0.32 0.22
At 700 °C, the PGMA-co-EGDMA polymer was totally decomposed, while 9% of the PGMA−PDA-TENF and PGMA-EDA-TENF and 12% of the PGMA-dPDA-TENF were remaining. In addition, the maximum decomposition temperatures of functionalized polymers were higher than that of PGMA-co-EGDMA, which indicates that the functionality of the polymer has changed after attaching the π-acceptor. Based on TGA/DTA curves, the total grafted TENF was determined. It was observed that PGMA-dPDA-TENF had the highest TENF loading, then PGMA−PDA-TENF and PGMA-EDATENF. These results confirmed the attachment of TENF to the polymer support and suggested that PGMA-dPDA- TENF adsorption is expected to be higher than other polymers, due to the higher amount of TENF grafted.
sample
BET surface area [m2/g]
pore diameter [nm]
pore volume [cm3/g]
PGMA-co-EGDMA PGMA−OH PGMA-GMA PGMA-EDA PGMA-EDA-TENF PGMA−PDA PGMA−PDA-TENF PGMA-dPDA PGMA-dPDA-TENF
26 24 3 4 3 3 4 27 27
14 16 3 4 9 5 9 14 19
0.094 0.092 0.002 0.004 0.008 0.004 0.009 0.104 0.130
properties of the synthesized polymers at various stages. It was observed that the textural properties of the PGMA-co-EGDMA polymer decrease significantly after the formation of tentacle GMA on the surface of the polymer. This was evident by the decrease of BET surface area from 24 to 3 m2/g, pore diameter from 16 to 3 nm, and pore volume from 0.092 to 0.002 cm3/g. Therefore, the surface area, pore diameter, and pore volume of PGMA−PDA-TENF and PGMA-EDA-TENF polymers were lower than that of PGMA-dPDA-TENF. The textural properE
DOI: 10.1021/acs.energyfuels.6b02412 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 5. SEM images of (A) PGMA-co-EGDMA, (B) PGMA-GMA, (C) PGMA−PDA-TENF, and (D) PGMA-dPDA-TENF at 10 000× magnification.
ties of PGMA-dPDA-TENF with higher surface area, pore diameter, and pore volume will affect its affinity to adsorb larger molecules compared to other polymers. For example, the surface area of PGMA-dPDA-TENF was 6.8 times higher than that of PGMA−PDA-TENF, while the pore diameter of PGMA-dPDA-TENF was about twice as much as that of PGMA−PDA-TENF. The morphology of the synthesized polymers was analyzed using SEM Figure 5 shows SEM micrographs of PGMA-coEGDMA, PGMA- GMA, PGMA−PDA-TENF, and PGMAdPDA-TENF at 10 000× magnification. It was observed that PGMA-co-EGDMA spherical beads, shown in Figure 5A, were agglomerated into clusters of beads with porous structure. Previous studies25,26,39,40 observed similar structure of PGMAco-EGDMA polymer, which was affected by synthesis conditions including stirring rate and temperature. After adding GMA polymer into PGMA-co-EGDMA, the spherical particles was covered with extra layer that changed the spherical bead clusters into irregular structure as shown in Figure 5B. The morphology of PGMA-co-EGDMA polymer with TENF attached using graft polymerization method, see Figure 5C, was similar to that without using graft polymerization method, see Figure 5D. 5.2. Adsorption Performance of the Polymers. To compare the adsorption capacity of the initial polymer, PGMAco-EGDMA, before and after attaching the π-acceptor, the adsorption capacity of the initial polymer with no linker or πacceptor was tested with LGO feed. No nitrogen or sulfur adsorption was observed with the initial polymer. The removal efficiencies of the grafted polymers using light gas oil feed (LGO) are shown in Figure 6. PGMA-dPDA-TENF showed better adsorption performance than PGMA−PDA- TENF. Nitrogen removal of PGMA-dPDA-TENF was about twice as much as PGMA−PDA-TENF. Similarly sulfur and aromatics removal were higher for PGMA-dPDA-TENF. The superior performance of PGMA-dPDA-TENF was because of its high surface area and pore diameter, 27 m2/g and 19 nm, compared
Figure 6. Nitrogen (N), sulfur (S), and aromatics (A) removal percentage using LGO feed.
to, 4 m2/g and 9 nm, for PGMA−PDA-TENF. These results were inconsistent with our previous studies that concluded that polymer surface area is an important factor that effects the adsorption capacity of the polymer.26 Moreover, higher TENF loading of PGMA-dPDA-TENF as compared to PGMA−PDATENF and PGAM-EDA-TENF contributed to the superior adsorption capacity of PGMA-dPDA-TENF polymer. PGMAEDA-TENF polymer showed the lowest adsorption capacity toward nitrogen, sulfur, and aromatics as it showed the lowest surface area, 3 m2/g, and TENF loading, 0.22 mmol/g. In addition, steric hindrance around the attached TENF could have affected the adsorption capacity of shorter linker length polymer PGMA-EDA-TENF by limiting the interaction with the feed, as suggested by Chitanda et al.40 For all polymers, the removal of aromatics was higher than that of nitrogen and sulfur. It was confirmed in our study that during adsorption process there is aromatics removal associated with denitrogenation and desulfurization of real industrial feedstocks. The polymers were regenerated using a soxhelt extractor at 120 °C, with toluene as a solvent, for 24 h. Then the regenerated polymers were allowed to dry at 110 °C for 12 h. The results of the removal efficiencies of the regenerated polymers in LGO feed are shown in Table 5. Results have shown that all polymers F
DOI: 10.1021/acs.energyfuels.6b02412 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 5. Fresh and Regenerated Polymers Removal Percentages Using in LGO feed PGMA-dPDA-TENF PGMA−PDA-TENF PGMA-EDA-TENF a
nitrogen [wt %] a
8.3 (8.9) 3.3 (4.8)a 2.6 (2.5)a
Table 6. Isotherm Parameters and Regression Data for PGMA-dPDA-TENF and PGMA−PDA-TENF Polymers at 23 and 55 °C
sulfur [wt %] 5.5 (5.7)a 4.3 (3.7)a 0.6 (1.7)a
The removal efficiency of fresh polymer.
were regenerated as the removal efficiencies of the regenerated polymers were similar to that of fresh polymers, except the nitrogen removal of regenerated PGMA−PDA-TENF and the sulfur removal of regenerated PGMA-EDA-TENF, which were lower than that of the fresh polymer. The difference in removal efficiency between the fresh and regenerated polymer could be attributed to insufficient regeneration time. Figures 7 and 8 show the adsorption isotherms of nitrogen and sulfur compounds using PGMA-dPDA-TENF and
polymer
Qm [mg/g]
b
RL
R2
PGMA-dPDA-TENF @ 23 °C PGMA-dPDA-TENF @ 55 °C PGMA−PDA-TENF @ 23 °C PGMA−PDA-TENF @ 55 °C
2.44 2.33 2.20 1.86
0.00028 0.00016 0.00016 0.00013
0.67 0.78 0.78 0.82
0.98 0.93 0.97 0.94
R2 values showed a good fit with Langmuir isotherm equation. The maximum adsorption capacity (qm), and adsorption equilibrium constant (b), were higher for the PGMA-dPDATENF polymer compared to that for PGMA−PDA-TENF polymer. To predict the adsorption affinity of the polymers, Langmuir parameters were used to determine the dimensionless separation factor (RL). RL is determined as the following:
RL = 1/(1 + bCo) where Co is the initial concentration of nitrogen compounds and b is the adsorption equilibrium constant.43 Based on the value of RL, the type of adsorption is identified as unfavorable (RL > 1), favorable if (0 < RL < 1), irreversible (RL = 0), or linear (RL = 1). The adsorptions of PGMA-dPDA-TENF and PGMA−PDA-TENF polymers were favorable since the RL values were between 0 and 1. Figure 9 shows the results for basic and nonbasic nitrogen removal efficiency using LGO feed. It was observed that all
Figure 7. Adsorption isotherms for nitrogen compounds using PGMA-dPDA-TENF and PGMA−PDA-TENF polymers at 23 and 55 °C.
Figure 9. Basic nitrogen (basic N) and nonbasic nitrogen (nonbasic N) removal efficiency of the synthesized polymers using light gas oil feed (LGO).
polymers removed more nonbasic nitrogen with PGMA-dPDATENF being the most efficient polymer. Because of the higher content of nonbasic nitrogen in the feed, its removal was much greater than that of the basic nitrogen. For example, PGMAdPDA-TENF removed 9.6% of the nonbasic nitrogen compounds, corresponding to 128 ppm, and 7.5% of basic nitrogen, corresponding to 35 ppm only. Comparing the basic and nonbasic nitrogen removal, all polymers removed more nonbasic nitrogen. Thus, using the Ce graft polymerization method reduced the adsorption capacity of the polymers toward nitrogen compounds, both basic and nonbasic, sulfur compounds, and aromatics. The order of removal efficiency, in term of %, of all polymers was as follow: sulfur species < basic N species < nonbasic N species < aromatics. Aromatics removal was the highest because it includes both nitrogen and sulfur species as well as other species that the polymers can adsorb. In addition, the polymers were more selective toward nitrogen
Figure 8. Adsorption isotherms for sulfur compounds using PGMAdPDA-TENF and PGMA−PDA-TENF polymers at 23 and 55 °C.
PGMA−PDA-TENF polymers at 23 and 55 °C. The adsorption capacities of both polymers toward nitrogen and sulfur compounds decreased as higher adsorption temperature was used. These results confirmed the adsorption temperature study in Figure S2, in the Supporting Information, that at higher temperatures nitrogen and sulfur adsorption capacity of the polymers decreased. This decrease of adsorption capacity is due to the exothermic nature of adsorption process. Several sources in the literature have reported similar results.23,41 The adsorption isotherms for nitrogen, in Figure 7, showed typical curves of Langmuir type (L-type), whereas that of sulfur, in Figure 8, showed S-type isotherms.42 Table 6 lists the isotherm parameters and regression data for nitrogen compounds. The G
DOI: 10.1021/acs.energyfuels.6b02412 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
TENF and PGMA−PDA-TENF removed more nonbasic nitrogen compounds than basic nitrogen; however, the removal percentage of nonbasic nitrogen compounds was lower for both polymers in HGO feed. PGMA-dPDA-TENF showed higher removal percentage, 5.8%, of nonbasic nitrogen compared to PGMA−PDA-TENF that removed 4.2%. Therefore, the superior performance of PGMA-dPDA-TENF over PGMA− PDA-TENF was confirmed using the HGO feed. In addition, the performance of the polymer with longer linker length, PGMA−PDA-TENF, was better than that with shorter linker, PGMA-EDA-TENF.
species as they removed higher percentage of nitrogen species as compared to that of sulfur. The removal efficiency of the polymers for higher nitrogen, sulfur, and aromatics content environment was tested using HGO feed. Figure 10 illustrates the removal efficiency of the
6. CONCLUSIONS In this work compared the performance of PGMA-co-EGDMA polymer, with tentacle glycidyl methacrylate (GMA) polymer layers, synthesized by cerium initiated graft polymerization, with that without tentacle GMA for determining the removal efficiency of nitrogen, sulfur, and aromatics content in light and heavy gas oil samples. TENF and PDA were used as common π-acceptor and linker for both polymers. In addition, EDA linker was also used to examine the influence of the linker on the performance of PGMA-co-EGDMA polymer. Different characterization methods were used to confirm the formation of PGMA-dPDA-TENF, PGMA−PDA-TENF, and PGAMEDA-TENF polymers. Adsorption results in LGO feed showed that PGMA-co-EGDMA polymer, without tentacle glycidyl methacrylate (GMA) polymer layers, was better than that synthesized by cerium initiated graft polymerization. The higher adsorption capacity of PGMA-dPDA-TENF compared to other polymers was attributed to its textural properties and higher amount of π-acceptor grafted. These results were confirmed in a higher nitrogen, sulfur, and aromatics content using HGO feed. In addition, the analysis of basic and nonbasic nitrogen showed that all polymers removed more nonbasic nitrogen species than basic. Furthermore, the adsorption capacity of PGMA-EDA-TENF polymer was lower than that of PGMA−PDA-TENF polymer due to lower surface area, amount TENF grafted, and steric hindrance effect of the shorter linker length polymer.
Figure 10. Nitrogen (N), sulfur (S), and aromatics (A) removal percentage using HGO feed.
polymers in HGO. Like LGO feed, in HGO feed PGMAdPDA-TENF achieved the highest nitrogen, sulfur, and aromatics removal percentage, while PGMA-EDA-TENF had the lowest. For all polymer the adsorption capacities of toward nitrogen and sulfur were in the same order of magnitude in LGO and HGO, except the nitrogen adsorption capacity for PGMA-EDA-TENF, as shown in Table 7. This indicates that Table 7. Nitrogen and Sulfur Adsorption Capacities in LGO and HGO Feeds adsorption capacity feed PGMA-dPDA-TENF PGMA−PDA-TENF PGMA-EDA-TENF
N adsorption capacity [mgN/g] LGO 0.72 0.45 0.23
HGO 0.90 0.72 0.00
S adsorption capacity [mgS/g] LGO 9.60 6.40 3.20
HGO 11.55 8.40 3.15
the performance of the polymers were similar in higher nitrogen, sulfur, and aromatic contents. Furthermore, the adsorption capacity of sulfur compounds was higher than that of nitrogen compounds for all polymers. For example, in HGO the nitrogen adsorption capacity of PGMA-dPDA-TENF was 0.9 mg/g whereas the sulfur adsorption capacity was 11.5 mg/g. Figure 11 shows the removal efficiencies toward basic and nonbasic nitrogen compounds using HGO feed. PGMA-dPDA-
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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/acs.energyfuels.6b02412. Composition of TENF solutions; composition of PGMA-co-EDGMA solutions; FT-IR spectra of (a) PGMA-co-EGDMA, (b) PGMA-EDA-TENF, and (c) PGAM-dPDA; nitrogen and sulfur removal efficiency as a function of adsorption temperature using light gas oil feed (LGO) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel: +1 (306) 966-4771. Fax: +1 (306) 966-4777. E-mail:
[email protected]. ORCID
Ajay K. Dalai: 0000-0002-3083-2217
Figure 11. Basic nitrogen (basic N) and nonbasic nitrogen (nonbasic N) removal efficiency of the synthesized polymers using heavy gas oil feed (HGO).
Notes
The authors declare no competing financial interest. H
DOI: 10.1021/acs.energyfuels.6b02412 Energy Fuels XXXX, XXX, XXX−XXX
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ACKNOWLEDGMENTS The authors wish to acknowledge the financial support from Mitacs and Syncrude Canada Ltd.
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DOI: 10.1021/acs.energyfuels.6b02412 Energy Fuels XXXX, XXX, XXX−XXX
