Improving the sustainability of biodiesel by using imidazolium-based ionic liquid

Corrosion of biodiesel-filled fuel tanks has become a major problem in the use of biodiesel as a new green energy source. The ionic liquid 1-Hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C10H19N2]+[C2F6NO4S2]− was used to control corrosion of C-steel in non-edible biodiesel to resolve this problem. The anti-corrosion and antioxidant properties of the [C10H19N2]+[C2F6NO4S2]− were characterized using weight loss, electrochemical impedance spectroscopy, total acid number measurements beside SEM and EDX analysis. The findings show that [C10H19N2]+[C2F6NO4S2]− plays an important role in preventing C-steel corrosion in biodiesel with an efficiency close to 99 percent. The adsorption capability and antioxidant properties of [C10H19N2]+[C2F6NO4S2]− are the major contributors to the ionic liquid's anti-corrosion properties. We anticipate that this work will help to sustainable expand the use of biodiesel as a renewable energy source.

The synthesis of biodiesel was conducted in a conical flask containing 50 ml Neem oil, 200 ml methanol (Sigma-Aldrich) and 1.0 wt% KOH (Alfa Aesar). The experimental conditions were set at temperature of 333 K, experimental time of 3 h and stirring speed of 350 rpm. Finally, the resulting solution was allowed to settle for 24 h in order to separate the pure biodiesel and followed by washing with distilled water for several times. Water content in biodiesel was determined by coulometric Karl Fischer Titration (METTLER TOLEDO). Free and total glycerin in biodiesel was determined by gas chromatography (GC-2014, Shimadzu Corporation, Japan). Table 1 showed the physicochemical properties of the synthesized biodiesel 22,23 . The presence of water in the biodiesel was due to the synthesis process, which included washing the transesterification product.
Methods. The weight loss WL was calculated by weighing before and after the immersion of the electrode in biodiesel for 1440 h using METTLER analytical balance. All the steps of WL were conducted according to ASTM G31-72(2004) 24 . The initial mass and area of the substrate were 7.4763 g and 5.734 cm 2 , respectively. The volume of biodiesel used was 100 ml. Three independent repeated experiments at the same conditions were carried out to ensure results validity. The resulted data were presented by the means and the standard deviation.
The EIS experiments were conducted in the standard cell (three electrodes: C-steel, saturated calomel electrode (SCE) reference electrode, Pt counter electrode) connected with electrochemical work-station (Gamry-3000) 25 . EIS curves were recorded in the frequency range of 30 kHz-1.0 Hz at open circuit potential using 20 mV amplitude.
The antioxidant test and TAN calculation for biodiesel at different conditions were carried out according to ASTM D943-20 and ASTM D664-18e2, respectively 26,27 .
The surface morphology (SEM and EDX) were conducted for C-steel samples in pure biodiesel and biodiesel containing 80 mg/l of [C 10  on the rate of corrosion (ν) and anti-corrosion performance (η w %) of C-steel in biodiesel using the WL experiments is shown in Table 2. www.nature.com/scientificreports/ The ν and η w % were obtained using Eqs. (1) and (2) 28 : W = C-steel weight loss, S = surface area, t = time of experiment, ν 0 = corrosion rate in the blank solution.
[C 10 H 19 N 2 ] + [C 2 F 6 NO 4 S 2 ] − inhibitor elicited a decrease in ν at 20 mg/l (from 2.762 × 10 -4 to 1.692 × 10 -4 mg cm -2 h -1 ), and this effect was sustained until the highest inhibitor concentration (i.e. 0.049 × 10 -4 mg cm -2 h -1 at 120 mg/l) (see Table 2). Inhibition of corrosion activity of C-steel in biodiesel by [C 10 H 19 N 2 ] + [C 2 F 6 NO 4 S 2 ] − was observed, with η w % values ranging from 38.7% to 98.9%. We noted that [C 10 H 19 N 2 ] + [C 2 F 6 NO 4 S 2 ] − displayed the highest inhibition of 98.9% at 80 mg/l. Beyond concentration 80 mg/l, no significant change in the η w % values was observed. It appears that when 80 mg/l of ionic liquid was added, the ionic liquid molecules covered nearly all of the active centers on the C-steel, and that further addition had a limited impact on the inhibition efficiency. Similar observations were noted by Cao et al. 29 and Arellanes-Lozada, et al. 30 .
Further inspections on the performance of [C 10 H 19 N 2 ] + [C 2 F 6 NO 4 S 2 ] − were conducted by using EIS measurements for C-steel in biodiesel without and with 80 mg/l of inhibitor. Typical EIS plots (a = Nyquist, b = Bode-phase angle, c = Bode-module, d = equivalent circuit) are shown in Fig. 1. The Nyquist plots (Fig. 1a) show one slightly depressed semicircle. Such non-ideal in the Nyquist plots is due to heterogeneity at the C-steel surface [36][37][38] . Two plateaus were visible in Fig. 1c, one at high frequency and the other at low frequency. The Nyquist plots, with and without [C 10 H 19 N 2 ] + [C 2 F 6 NO 4 S 2 ] − , can be described by Randles equivalent circuit (EC) as presented in Fig. 1d. In Fig. 1d, R s is the solution resistance, C dl is the double layer capacitor and R ct is charge transfer resistance 39 . It is evident that for C-steel in biodiesel containing 80 mg/l of inhibitor, R ct increased from 10.5 Mohm.cm 2 (blank biodiesel) to 115.3 Mohm.cm 2 . Moreover, the addition of [C 10 H 19 N 2 ] + [C 2 F 6 NO 4 S 2 ] − in biodiesel led to the decrease in the C dl value from 1.51 nF cm −2 (blank biodiesel) to 0.13 nF cm −2 . Additionally, the width of the Bode-phase angle (Fig. 1b) increases by adding [C 10 H 19 N 2 ] + [C 2 F 6 NO 4 S 2 ] − , which indicates a lower corrosion rate 40,41 . This means that [C 10 H 19 N 2 ] + [C 2 F 6 NO 4 S 2 ] − is able to impede the corrosion of C-steel in biodiesel by forming a protective layer on the C-steel surface 42,43 .

Thermodynamic activation and adsorption isotherms studies.
To estimate the performance of [C 10 H 19 N 2 ] + [C 2 F 6 NO 4 S 2 ] − at high temperatures circumstances, the ν and η w % values for C-steel in biodiesel without and with 80 mg/l of inhibitor were calculated in the range 298-328 K. It was noted that, under an elevated temperature of 298 K to 328 K, the η w % value slightly decreases from 98.9 to 91.9% and the corrosion rate increases from (0.030 ± 0.002) × 10 -4 to (0.396 ± 0.010) × 10 -4 mg cm −2 h −144 (see Fig. 2). This indicates that [C 10 H 19 N 2 ] + [C 2 F 6 NO 4 S 2 ] − retains its performance at high temperature, confirming its thermal stability 45 .
To assess the activation energy (E a ) for C-steel in biodiesel without and with 80 mg/l of [C 10 H 19 N 2 ] + [C 2 F 6 NO 4 S 2 ] − , the variation of log (ν) with (1/T) was plotted , as displayed in Fig. 2 48,49 . Where the ionic liquid molecules create a large energy barrier against the corrosion process of C-steel in biodiesel 50 .
The adsorption isotherm models that describe the adsorption of [C 10 H 19 N 2 ] + [C 2 F 6 NO 4 S 2 ] − on the C-steel surface based on the WL measurements were inspected. www.nature.com/scientificreports/ To choose the best isotherm for the current case, various adsorption isotherm models such as Langmuir, Freundlich, and Temkin were tested (Eqs. 4, 5 and 6).  www.nature.com/scientificreports/ where C inh is the ionic liquid concentration, K ads is the equilibrium constant, "a" is the molecules interaction parameter, and θ is the surface coverage = η w %/ 100. According to the data in Fig. 3, the Langmuir adsorption isotherm is the best isotherm for this case. This is dependent on the correlation coefficient (R 2 ) being close to unity 51 53 . Because the value of ∆G°a ds is less than-40 kJ mol −1 , the type of adsorption may be physisorption or mixed type (physisorption and chemisorption) 54 .
SEM and EDX analysis. The SEM and EDX analysis of C-steel in biodiesel without and with 80 mg/l of [C 10 H 19 N 2 ] + [C 2 F 6 NO 4 S 2 ] − are shown in Figs. 4 and 5. The C-steel surface, immersed in biodiesel for 1440 h, without [C 10 H 19 N 2 ] + [C 2 F 6 NO 4 S 2 ] − was extremely damaged due to the aggressive medium (Fig. 4a). EDX analysis for this case (Fig. 4b), reveals the signals for C-steel composition (i.e. C, Si, Mn, Fe) and corrosion products (i.e. iron oxide).
The impact of adding 80 mg/l of [C 10 H 19 N 2 ] + [C 2 F 6 NO 4 S 2 ] − to the biodiesel on the C-steel surface is shown in Fig. 5a. It is clear that the surface of C-steel is smooth and no corrosion products were observed on the metal (5) θ = K ads (C inh ) 1/n Freundlich  www.nature.com/scientificreports/ surface. EDX analysis for this case (Fig. 5b) 55,56 . This layer can isolate the C-steel surface from the biodiesel 57 . The presence of hetero-atoms (O, S, and N atoms) in the ionic liquid molecule affects the efficiency of this inhibitor. These atoms are commonly regarded as the reaction centre for initiating the adsorption process [58][59][60] . The nonbonding electrons present on hetero-atoms, as well as π-electrons, will be transferred into the d-orbitals of the Fe atoms on the steel surface, leading to the formation of coordinate bonds between C-steel and the adsorbed ionic liquid, as observed for many organic inhibitors 61,62 . SEM and EDX analysis verified the ionic liquid's adsorption on the C-steel surface, as shown in Figs. 4 and 5.
The second factor is the antioxidant properties of ionic liquid 63 . This leads to the decrease in the oxidation of biodiesel and consequently, prevents the formation corrosion compounds such as free acids and aldehydes 16

Conclusions
The necessity to control corrosion in fuel tanks containing biodiesel motivated us to explore the anti-corrosion properties of ionic liquid [C 10 H 19 N 2 ] + [C 2 F 6 NO 4 S 2 ] − , that could serve as informative to control the corrosion of C-steel in biodiesel. [C 10 H 19 N 2 ] + [C 2 F 6 NO 4 S 2 ] − , reveals an effective new C-steel corrosion inhibitor in biodiesel. The inhibition mechanism is based on the ionic liquid's mixed physisorption and chemisorption.
[C 10 H 19 N 2 ] + [C 2 F 6 NO 4 S 2 ] − molecules cover the surface of C-steel sheets, preventing biodiesel corrosive attack on steel sites. The inhibition effect is explained by this protective layer and the adsorption of an ionic liquid compound. It was clear that the [C 10 H 19 N 2 ] + [C 2 F 6 NO 4 S 2 ] − displayed the highest inhibition 98.9% at 80 mg/l. The   www.nature.com/scientificreports/