Esterification of free fatty acids in non- edible oils using partially sulfonated polystyrene for biodiesel feedstock

Industrial Crops and Products 95 (2017) 66–74

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Esterification of free fatty acids in non- edible oils using partially sulfonated polystyrene for biodiesel feedstock Remya Suresh a , Jolly Vakayil Antony a,b , Ragi Vengalil c , George Elias Kochimoolayil a , Rani Joseph a,∗ a

Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Cochin, Kerala, 682022 India Department of Chemistry, Maharajas College, Ernakulam, Kerala, 682011, India c Department of Chemistry, Sree Kerala Varma College, Thrissur, Kerala, 680011, India b

a r t i c l e

i n f o

Article history: Received 1 March 2016 Received in revised form 31 August 2016 Accepted 24 September 2016 Keywords: Sulfonated polystyrene Ion exchange resin Esterification Free fatty acid Acid value Biodiesel

a b s t r a c t Partially sulfonated polystyrene (PSS), synthesized from expanded polystyrene waste (EPS), was used as a catalyst for free fatty acid (FFA) conversion in non-edible oils. Acidic and water absorbing properties of the PSS facilitated the catalytic action for the FFA conversion by esterification reaction. The reaction was done on simulated acid oil (WCO) containing oleic acid and sunflower oil, and rubber seed oil (RSO). Effects of temperature, catalyst amount and alcohol to acid molar ratio were studied. FFA conversion increased with each of these factors. The advantage of this heterogeneous catalyst is that it is efficient as commercial ion exchange resin and easily removable from the reaction mixture. PSS is found to substantially reduce the acid value of WCO and RSO from 17 to 3.2 mg KOH/g and from 28.8 to 4.8 mg KOH/g respectively at 75◦ C. The WCO and RSO with reduced acid value may be used as a feedstock for biodiesel production. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Physical and chemical processes involved in industrial and scientific development are either pollution prone or energy intensive. Research and development are now redefined to be ‘green’ as the world is facing the twin perils of environmental pollution and energy crisis. The biggest contributions towards green environment initiatives include developing technologies for reducing waste and switching to renewable energy resources. Therefore, proper waste management and use of renewable energy resources are so indispensable for the survival of human society. Polymer waste management has emerged as a major concern these days. Modification of polymeric wastes into useful products is very attractive in this context. One of the high volume polymer waste is expanded polystyrene and the methods for utilization of this waste are reported by Sekharan et al. (2012) and Sulkowski et al. (2005). Our intention is to use this waste, after chemical modification, as a

Abbreviations: PSS, Partially sulfonated polystyrene; EPS, Expanded polystyrene waste; Tg , Glass transition temperature; x, Degree of sulfonation; FFA, Free fatty acid; WCO, Simulated waste cooking oil; RSO, Rubber seed oil; DCE, 1,2-Dichloroethane; IEC, Ion exchange capacity; A, Acid value (mg KOH/g oil ); STM, Standard titration method; ITM, Indirect titration method. ∗ Corresponding author. E-mail address: [email protected] (R. Joseph). http://dx.doi.org/10.1016/j.indcrop.2016.09.060 0926-6690/© 2016 Elsevier B.V. All rights reserved.

catalyst for the production of low cost biodiesel feed stock. Biodiesel defines by the ASTM as the mono alkyl ester of long chain fatty acids derived from a renewable lipid feedstock, such as vegetable oil or animal fat. It is an oxygenated, renewable, biodegradable and non toxic with similar flow and combustion properties as fossil diesel (Canakci, 2007; Sharma et al., 2008; Srivastava and Prasad, 2000). Depletion of fossil fuel is imminent as we largely depend upon this fuel. Biodiesel, an alternative for fossil diesel is widely encouraged as a need for promoting environment- friendly fuel to attain sustainable development. It can be produced from vegetable oils, fats etc. by transesterification (Marchetti et al., 2007a; Endalew et al., 2011; Zabeti et al., 2009). However, its high cost and food scarcity restricts the use of refined oil for diesel manufacturing. A study conducted in 2009–2012 by the European Union found that green house gas emissions were on the rise because of the conversion of agricultural land for planting first-generation biofuel crops. This calls for phasing out first-generation biofuels and replacing them with second-generation counterparts. However, the development of second generation biofuels from crop residues such as organic waste, algae and waste cooking oil in a cost-effective manner has been a challenge before the research community. Though biodiesel can be cost-effectively produced from non-edible oils such as waste cooking oil (WCO), animal fat, rubber seed oil (RSO) and waste grease through transesterification, high fatty acid (FFA) level in these oils leads to unnecessary reactions like saponification

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that badly effects the biodiesel production. So prior to transesterification the FFA levels in the non-edible oils should be reduced by esterification using low molecular weight alcohols (Hayyan et al., 2014; Berrios et al., 2007; Marchetti and Errazu, 2008a). Homogeneous catalysts like sulfuric acid and heterogeneous catalysts like ion exchange resin, sulfonated zirconia and sulfonated carbon are used for the esterification of fatty acids in the biodiesel production (Cheng et al., 2012; Juan et al., 2007; Perez et al., 2012; Shu et al., 2010; Lilija et al., 2002; Tesser et al., 2005). Easy removal of catalyst from the reaction mixture, better efficiency and reusability are the advantages of heterogeneous catalysts over homogeneous catalysts. Various research papers have reported the use of high FFA oils for biodiesel production. Rubber seed oil and tobacco seed oil having FFA of 17% were esterified before transesterification and reduced the FFA level to 2%, which is equivalent to 4 mg KOH/g to avoid the difficulty of saponification (Ramadhas et al., 2005; Veljkovic et al., 2006). Though studies on the use of different ion exchange resins as heterogeneous catalyst for the FFA conversion has been undertaken earlier (Gan et al., 2012), ion exchange resin synthesized from the polymer waste and its application as catalyst in FFA conversion is reporting for the first time. Expanded polystyrene waste is utilized for the production of PSS, which is used as catalyst for the esterification of FFA in WCO and RSO. Effects of parameters like temperature, catalyst amount, alcohol to acid molar ratio and percentage composition of acid in the oil are proposed to be studied.

PSS solution with standard NaOH solution. About 0.1 g PSS dissolved in 10 ml methanol/toluene (6:4) mixture and titrate against standard methanolic NaOH solution (Bajdur et al., 2002). x = 0.104 ∗ CNaOH ∗ VNaOH ⁄[W − (0.08 ∗ CNaOH ∗ VNaOH )]

(2)

where CNaOH and VNaOH are the concentration (mol/L) and volume of standard methanolic NaOH solution (ml). W is the weight of PSS (g). ‘x’ was also calculated from the 1 H NMR spectrum of PSS and compared with that obtained by titration. IEC can also be calculated using ‘x’ (Smitha et al., 2003; Guan et al., 2005). IEC = 1000x⁄104 + 80x

(3)

2.3. Thermal analysis Differential Scanning Calorimetric (DSC) analysis of EPS and PSS were done in universal TA instruments, Q 100, under nitrogen atmosphere. Samples of about 6 mg taken in the pan were heated to 250 ◦ C at 20 ◦ C/min and cooled to 40 ◦ C at 20 ◦ C/min in nitrogen atmosphere. After keeping under isothermal condition for 5 min, the second run heating and cooling were done as in the first run. Thermal stability of EPS and PSS were compared using TGA/DTA of universal TA instruments Q50.The samples were heated from 25 ◦ C to 700 ◦ C at 10 ◦ C/min in nitrogen atmosphere. 2.4. Swelling capacity

2. Materials and methods 2.1. Materials Expanded polystyrene, a waste collected from packing cartons, was used for the synthesis of partially sulfonated polystyrene (PSS). PSS was synthesized from EPS using Sulfuric acid 98% (Merck) and Silver sulfate (Nice) in 1, 2-dichloroethane (DCE) (Merck). PSS was dissolved in toluene/methanol (Merck) mixture to cast film. Pure Oleic acid (Loba Chemie) and acid free commercial refined sunflower oil from local market were used to simulate the waste cooking oil. Crude rubber seed oil used in this study was obtained from VigneshTraders, Virudhanagar, Tamilnadu. The esterification of fatty acid was done using 1-butanol (Merck). Diethyl ether99.5%, ethanol-99.9%, triethanolamine and isopropanol (Merck) were used for FFA conversion analysis. Standard titration for the acid value of the oil was carried out using burette to an accuracy of 0.02 ml. 2.2. Sulfonation of expanded polystyrene waste Partial sulfonation of EPS to PSS was carried out earlier by our group and the PSS was characterized by NMR and FTIR (Antony et al., 2014). Ion exchange capacity (IEC) of the PSS was determined by measuring the amount of H+ ions that was exchanged with Na+ ions when PSS film was soaked with NaCl solution. It is the number of milli equivalence of H+ ions released by 1 g of dry polymer. A known weight of dry polymer (0.2 g) was soaked in 100 ml 0.1 M NaCl solution and shaken occasionally for 24 h. The amount of H+ released by the polymer was determined by titrating a definite volume of the NaCl solution against 0.01 M NaOH solution (Inagaki et al., 1999). IEC = (CNaOH ∗ VNaOH ∗ 100)⁄VNaCl ∗ 0.2

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(1)

where CNaOH is the concentration (mol/L) of standard NaOH, VNaCl the volume of NaCl (ml) used in the titration and VNaOH the volume of NaOH (ml) required for the neutralization of released H+ ions. Degree of Sulfonation (x), is defined as the mole fraction of sulfonated monomers in PSS (0 ≤ x ≤ 1). It was determined by titrating

Water absorption capacity of the PSS film was determined using ‘Tea bag’ method (Kaiser et al., 2003). A known weight of dry PSS film was placed in a tea bag made of filter paper and soaked in distilled water at room temperature. A blank tea bag was also soaked in distilled water under same conditions. These bags were taken out at different time intervals and suspended for 5 min to allow the surface water to drop off. Weighed the bags to find the water retained by the film. Two more sets of bags were soaked and weighed to take the average. The amount of water (W) held by the film sample in the tea bag was determined. (Supplementary data) 2.5. Free fatty acid (FFA) esterification using PSS as catalyst Oleic acid in simulated waste cooking oil (WCO) and fatty acids in RSO were esterified in lab scale three- neck, round- bottom flask by heating in oil bath under magnetic stirring. Stirring speed was adjusted to 300 rpm. To check the evaporation loss, Liebig’s condenser was fitted to the centre neck; a thermometer was connected to the side neck and the reagents were fed through the third neck. WCO or RSO was stirred with 1-butanol. On attaining the desired temperature a definite percentage (w/w relative to oil) of the PSS catalyst was added. A known weight (1 g) of the sample drawn out from the reaction mixture at regular intervals and determined the FFA conversion by titration analysis (Ozbay et al., 2008). 2.5.1. Titration analysis A weighed amount of the withdrawn sample was dissolved in diethyl ether and ethanol mixture. It was titrated against aqueous solution of 0.02 M NaOH. Acid value of the sample (A), expressed in mg KOH/g oil was determined using the following equation (Aricetti and Tubino, 2012; Marchetti and Errazu, 2008b). A = 56.1 ∗ C ∗ V/M

(4)

where C and V are the concentration (mol/L) and volume of NaOH (ml) respectively at the equivalence point. Acid value of the sample was also determined using indirect titration method (ITM), in which the withdrawn sample was dissolved in isopropanol − water

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mixture (1:1) containing 0.05 mol/L triethanolamine (Kardash and Turyan, 2005). A − At FFA conversion (%) = i ∗ 100 Ai

(5)

Table 1 x and IEC of PSS.

1 H NMR Titration a

Where Ai and At are the initial acid value and acid value after certain time respectively.

2.5.2. Gas chromatography–mass spectrometry (GC–MS analysis) The fatty acid composition of RSO was determined by converting all fatty acids to the corresponding fatty acid methyl esters (FAME) followed by GC analysis. FAMEs were prepared by methylation of 10 ml oil with 0.5% methanolic NaOH at 60 ◦ C. The methyl esters were extracted with n-Hexane and analyzed for their composition with GC–MS (Agilent GC-7890A, Mass-5975C). The WCO and WCO after the FFA esterification were also analyzed for the presence of fatty acid and butyl ester of fatty acid (butyl oleate) using GC–MS.

2.5.3. Esterification of FFA in WCO FFA conversion in WCO containing 10% (w/w) oleic acid was carried out at 75 ◦ C. Effect of catalyst amount (1% and 2% by weight relative to the weight of sunflower oil and oleic acid) and alcohol to acid molar ratio (5:1, 10:1, and 20:1) for the FFA conversion in WCO were studied. By changing the initial amount of FFA in WCO, three acid oils were prepared and its effect on fatty acid esterification was also noticed. Composition of oleic acid in the three oils was 10%, 20% and 40% (w/w). Butanol was mixed with these oils in the molar proportion, 10:1(butanol: oleic acid). Esterification was carried out in these three reaction mixtures using PSS catalyst (2% w/w relative to oil) at 75 ◦ C.

2.5.4. Esterification of FFA in RSO FFA conversion in RSO was carried out at 50 ◦ C, 60 ◦ C, and 75 ◦ C by varying the amount of catalyst (1%, 2%, and 4% by weight relative to RSO). Esterification of FFA in RSO was also done in the absence of catalyst. Also the effect of alcohol (1-butanol) amount in the esterification of FFA was studied. Three different mixtures of RSO and 1-butanol were prepared with different weight percentage of 1-butanol. The weight percentage of 1-butanol with respect to RSO was 25%, 50% and 75% (w/w). In these FFA conversion studies, 1 h intervals were maintained to take the sample from RSO for the determination of acid value. FFA of the reaction mixture at the successive time was determined using the reaction mixture remains after each sampling. STM and ITM for acid value determination in vegetable oils were used for the determination of acid value of the reaction mixture (Marchetti and Errazu, 2008b; Kardash and Turyan, 2005; Pasias et al., 2006).

b c

xa

IECb (meq/g)

IECc (meq/g)

0.273 0.258

2.16 2.07

2.06

Degree of sulfonation. Calculated using x. Obtained by soaking in NaCl.

3. Results and discussion 3.1. Sulfonated polystyrene (PSS) The EPS has been modified by a simple sulfonation reaction for its productive use as a heterogeneous catalyst. The degree of sulfonation (x = 0.27) of PSS was determined previously (Antony et al., 2014). IEC of the PSS is calculated as 2.07 meq/g using the equation (3), which agrees with the value obtained by finding the equivalence of H+ ions generated by PSS in NaCl solution using the titrant 0.01 M NaOH (Guan et al., 2005). Ozbay et al. (2008) have reported the IEC of the conventional ion exchange resins as 5 meq/g. The degree of sulfonation (x) of PSS has a significant role when it is used as heterogeneous esterification catalyst. High sulfonation level increases the efficiency of the catalyst as there is increase in the acid site but dissolves in water and loses its heterogeneity. So in the present work the degree of sulfonation is optimized to obtain a water insoluble but water absorbing ion exchange resin. The degree of sulfonation of PSS used in the present study is 0.27. ‘x’ obtained from 1 H NMR spectrum and titration is comparable. ‘x’ and IEC of PSS are given in Table 1. The glass transition temperature (Tg ) of a material indicates its crosslink density and the temperature below which it can be used without any structural loss. The DSC curves have shown in Fig. 1, which compares the Tg of PSS with EPS. Tg of 102 ◦ C was obtained for EPS in the first (a) and second run (b). For PSS a broad irreversible endothermic peak in the first run may be due to relaxation of polymeric molecule. Tg of the PSS was higher than that of EPS and it was calculated from the second run as 121 ◦ C. The decreased Cp and increased Tg accounted for the more rigidity and strong ionic interaction in the ionomer, PSS. DSC curves obtained were same as that obtained earlier by Martins et al. (2007) where sulfonation was carried out by acetyl sulfate. Fig. 2 compares the thermogram of EPS and PSS. Low thermal stability and hygroscopic nature of PSS relative to EPS was confirmed from the TGA. In PSS 10% weight loss below 200 ◦ C was attributed to the absorbed water. Thermal degradation of EPS and PSS occurred in the temperature range 360 ◦ C − 450 ◦ C and 300 ◦ C–450 ◦ C respectively (Piboonsatsanasakul et al., 2008; Smitha et al., 2003). Thermal degradation temperature from DTG peaks was obtained at 418 ◦ C for EPS and 401 ◦ C for PSS. 3.2. Swelling capacity

2.6. Reusability of the catalyst Recycling of catalyst is an attractive option in any industrial process. The reuse of the catalyst in the successive reaction was carried out. After the first esterification reaction in WCO at 75 ◦ C, the catalyst was filtered out and washed several times with water and ◦ toluene. It was dried at 70 C for 24 h and used in second run for the FFA conversion in WCO under similar conditions. The catalyst was recovered again and used for a third run by adopting the same procedure. In order to compensate for the loss of some quantity of catalyst during the recovery process, and for maintaining a constant catalyst (2% w/w relative to acid oil), the amount of WCO used in the successive reactions was decreased.

The PSS film has very high water absorption property. 1 g of the film can absorb 12.8 g of distilled water in 20 min and absorption level remains almost same even after 40 min (percentage water absorption is > 1200%). In 48 h 1 g of the film has absorbed 15.4 g of water. Most of the water absorption has taken place during the initial hours of soaking. High water absorption capacity of PSS is due to the hydrophilic −SO3 H functional groups, porous structure and low cross linking (Ozbay et al., 2008). The photograph of hydrogelated PSS is shown in Fig. 3. On comparing PSS with conventional ion exchange resins like Amberlyst, Dowex, Relite CFS etc. IEC and ‘x’ of the PSS is less while its water absorption capacity is very high (Ozbay et al., 2008). The

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Fig. 1. DSC curves of EPS and PSS in the a) first run and b) second run.

is able to shift the reactants more to products and can favor FFA conversion. 3.3. Composition of RSO The RSO is highly acidic with acid value of 41.5 mg KOH/g oil. The fatty acid composition as obtained from GC–MS analysis (gas chromatogram is shown in Fig. 4) follows: palmitic acid 7.7%, linoleic acid 32.1%, stearic acid 10.7%, and oleic acid 49.5%. The molecular weight of the oil is calculated as 876.7 g/mol (Gan et al., 2012). 3.4. FFA conversion in WCO

Fig. 2. TGA of a) EPS and b) PSS.

Fig. 3. Hydrogelated PSS.

combination of swelling in water and low sulfonation level, benefits in heterogeneous catalyzed esterification reaction. The cross linking of PSS is less than that of commercial resins because of the polymer structure disruption by sulfonation reaction. The low cross-linked polymer possesses porosity, which favors the reagent transportation. The mass transfer in the polymer catalyst results acid sites more accessible to the reagents and facilitate the esterification reaction (Barbaro and Liguori, 2009; Kiss et al., 2006). As esterification is a water removal reaction, high water absorbing PSS

Standard titration procedure gives acidity or acid value of the reaction mixture. The procedure is considered as a green method for the acid number determination in biodiesel (Aricetti and Tubino, 2012).The acid value determined using STM is same as that determined using ITM. FFA conversion in the reaction mixture at different time intervals calculated using STM and ITM are comparable as reported earlier (Ozbay et al., 2008). 3.4.1. Effect of catalyst amount for the FFA conversion in WCO Effect of catalyst amount in the esterification of oleic acid in WCO at 75 ◦ C is shown in Fig. 5. FFA conversion in presence of triglycerides was an appreciable 80.8% compared to 53% in pure oleic acid using 2% w/w PSS catalyst at 75 ◦ C (Fig. 1S Supplementary data for pure oleic acid esterification). In the presence of triglycerides the amount of water produced by the esterification reaction is small compared with reaction volume, facilitate higher FFA conversion (Marchetti et al., 2007b; Tesser et al., 2005). The acid value (A) of the reaction mixture reduced from 16.5 mg KOH/g to 3.2 mg KOH/g in 5 h reaction. Acid value was that of the mixture containing WCO and alcohol. FFA conversion achieved in this work is similar to the results published earlier (Ozbay et al., 2008; Aricetti and Tubino, 2012; Marchetti and Errazu, 2008b; Kardash and Turyan, 2005; Pasias et al., 2006; Marchetti et al., 2007b). The gas chromatogram with its peaks specified by m/z value and percentage are shown in Figs. 6 and 7, which confirms the ability of PSS in the FFA conversion to reduce the acid value below 4 mg KOH/g. 3.4.2. Effect of alcohol to acid mole ratio for the FFA conversion in WCO With the alcohol to acid molar ratio, FFA conversion increases in 10% w/w acid oil using PSS catalyst (1 wt% relative to oil) at 75 ◦ C

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Fig. 4. Gas chromatogram of FAME.

Table 2 Initial and final acidity of WCO and FFA conversion using 2% PSS catalyst. Acid in WCOa

Initial acidityb

Final acidity

FFA Conversion

10% 20% 40%

8.34 10.91 13.7

1.6 2.29 3.78

6.74 8.82 9.92

a b

Oleic acid weight percentage in WCO. Initial acidity of the mixture containing WCO and 1-butanol.

also enhances the FFA conversion (Gan et al., 2012). At high butanol/acid ratio, the FFA conversion is not so high because of the release of more water, which slows down the forward reaction (Thiruvengadaravi et al., 2012).

Fig. 5. FFA conversion in WCO (10% acid) using different catalyst amount.

(Feng et al., 2010). A considerable increase in the FFA conversion as the alcohol/acid mole ratio increases from 5 to 10, but thereafter not much increase in FFA conversion (Fig. 8). Increasing trend in FFA conversion is in accordance to the LeChatelier principle; with the increase of reactants more esterification reaction takes place to shift the equilibrium of reversible reaction in favor of ester formation. By increasing the butanol/acid molar ratio, the viscosity of the reaction mixture decreases. This

3.4.3. Effect of initial amount of free fatty acid for the FFA conversion in WCO Initial acidity of the three reaction mixtures containing WCO and butanol are 8.34%, 10.91% and 13.7% w/w. Butanol was mixed in molar ratio relative to oleic acid in WCO; there is not much difference in initial FFA in the three reaction mixtures. With the increase in initial acidity of WCO, the FFA conversion (initial acidity-final acidity) increases using 2% PSS catalyst at 75◦ C as shown in Table 2. (Supplementary data for acidity calculation). Since FFA conversion is not in proportion to the initial acidity of the mixture, % FFA conversion decreases (Fig. 8 inset). This is due to the increase in viscosity of the reaction medium which limits the mobility of molecules.

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Fig. 6. Gas chromatogram of WCO (10% by weight oleic acid relative to oil).

3.5. FFA conversion in RSO Acid value of RSO is determined to be 42 mg KOH/g using STM described earlier. For esterification reaction the RSO is mixed with 1- butanol of different composition. The acid value of the resultant mixture is decreased due to the dilution. A further decrease in acid value in presence of PSS is attributed to the FFA esterification.

3.5.1. Effect of alcohol weight percentage for the conversion of FFA in RSO In practice the molar ratio of one reagent should be higher than that of stoichiometric ratio in order to drive the reaction towards completion. Therefore to study the effect of butanol we have increased the amount of butanol. Canakci and Gerpen (2001) advocate the use of large quantities of methanol while using the H2 SO4 as catalyst. The FFA conversion in relation with butanol weight percentage is shown in Fig. 9. The maximum conversion of FFA is achieved when the butanol is 50% by mass. The acid value of RSO containing 50% by mass of butanol has been reduced from 31.37 mg KOH/g to 9.18 mg KOH/g. (FFA conversion is 70.7%).FFA conversion is not appreciable in the absence of PSS catalyst. In the absence of catalyst there is only 3.7% FFA conversion in RSO containing 50% (w/w) butanol. We have also repeated the experiment with 75% (w/w) butanol without catalyst at 75 ◦ C and it shows only 14.8% conversion, while there is 53.7% FFA conversion with catalyst. (Fig. 2S included in Supplementary data). Increasing trend in FFA conversion is in accordance with Le Chatelier principle. But, further increase in the butanol to 75% by weight decreases the FFA conversion because of the release of more water that slows down the forward reaction.

3.5.2. Effect of temperature for the conversion of FFA in RSO In RSO, FFA conversion of 70.7%, 50.4% and 11.1% has been achieved at 75 ◦ C, 60 ◦ C and 50 ◦ C respectively using 2% by weight of PSS (Fig. 10). At room temperature the conversion efficiency is very low. With increase in temperature the conversion takes place at a faster rate. The optimum temperature for this reaction is found to be at 75 ◦ C. A positive effect on FFA conversion with temperature is due to the increase in kinetic energy of the reactant molecule. Furthermore esterification is an endothermic process. At high temperature removal of the PSS catalyst from the reaction mixture become difficult due to the dispersion of the catalyst in the reaction mixture.

3.5.3. Effect of amount of catalyst for FFA conversion in RSO The effect of the catalyst amount on the conversion efficiency at 75 ◦ C is shown in Fig. 11. The catalytic process attains the maximum conversion efficiency at 4% by weight of PSS. 82.9% of FFA is converted in 5 h to bring down the acid value from 28 mg KOH/g oil to 4.8 mg KOH/g oil. More water absorption and large surface contact of the catalyst with reactant molecules are the reasons for increasing FFA conversion with catalyst amount. Only 7% FFA conversion has been attained for 1% by weight of PSS. Although the sulfonation level is low, the water absorption and minimum acid functional leaching compensate for the esterification reaction. Thus the efficiency of PSS in esterification of FFA in WCO and RSO is comparable to that of commercial resins. Too many hydrophilic acid sites in the heterogeneous catalysts inhibit catalyst activity by water absorption. The water formed in the reaction site is easily transferred to nearby sulfonic acid groups. The water inter-

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Fig. 7. Gas chromatogram of WCO (10% by weight oleic acid relative to oil) after FFA conversion using 2% by weight PSS at 75 ◦ C.

Fig. 8. Effect of alcohol to acid molar ratio for the FFA conversion in WCO, Inset: Effect of acid percentage for the FFA conversion in WCO. Fig. 9. FFA conversion in RSO with respect to the butanol amount.

acts with sulfonic acid leads to its leaching (Barbaro and Liguori, 2009; Kiss et al., 2006). The possibility of acid group leaching is less in PSS because of the low level of sulfonation as compared to conventional ion exchange resin. The porosity and hydrophobic surface of the polymer facilitate the transfer of water to dissolve in alcohol and reduce the possibility of sulfonic acid leaching. The polar water-alcohol mixture permeates to the porous structure and gelate the polymer.

3.6. Reusability of the catalyst Economic gain of an industrial process depends upon the recovery and reusability of the catalyst. Easy recovery of the PSS catalyst from the reaction mixture and its reuse makes this catalysis reaction more advantageous over homogeneous catalysis. However the efficiency of the PSS catalyst decreases with reuse. In the second use the FFA conversion decreased from 80% to 50% and then there

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Fig. 12. Reusability of PSS (䊏 first run second run 䊉 third run). Fig. 10. Effect of temperature on FFA conversion in RSO.

and biodiesel production from non-edible oils, the present work can be considered as an environment friendly approach in meeting the needs of sustainable development. The activity of the PSS catalyst in transesterification and improving activity for its reuse is included in our future work. 4. Conclusion The PSS, 2% by weight relative to oil, obtained from the waste has achieved an appreciable FFA conversion of 53.4% in pure oleic acid and 80.8% in WCO at 75 ◦ C. The acid value of WCO has been reduced to 3.2 mg KOH/g. The catalyst is also proved its efficiency in lowering the FFA of RSO by 83% to bring down the residual acid value of the reaction mixture to 4.8 mg KOH/g. The significance of the FFA esterification in WCO and RSO lies in utilizing a waste polymer as a catalyst and in the generation of oil feed stock for biodiesel. Acknowledgments

Fig. 11. FFA conversion in RSO with different amount of PSS catalyst.

is not much change in the efficiency of catalyst in the third use (Fig. 12). Reused catalyst’s less efficiency in FFA conversion is due to the fall in water swelling capacity. The recovered catalyst, after the first run, contains absorbed water and oil. Heating of the recovered PSS catalyst at 70 ◦ C is only sufficient enough to remove water completely but oil remains in the porous structure of the PSS. High temperature is required to remove oil and is difficult in the case of PSS as its Tg is only 121◦ C. Thermal stability of PSS is also less due to low cross-linking (Barbaro and Liguori, 2009). The Fig. 2 gives a slanting thermogram around 300 ◦ C, which is attributed to the structural degradation by the thermal leaching of sulfonic acid functional groups. The remaining oil residue in the catalyst restricts further absorption of water which will affect the% FFA conversion. The absorbed oil in the active site of the PSS decreases its efficiency in its second use. The absorption of oil in the catalyst attains saturation even in the first use itself and the recovered catalyst retains same amount of oil after the second use. So the FFA conversion in the second and third run is comparable. The sulfonated polystyrene has proved its catalytic activity in converting FFA by esterification, which is an important step in biodiesel production. In terms of reaction simplicity, EPS utilization

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