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Sodiumzirconate(Na2ZrO3)asacatalystinasoybeanoiltransesterificationreactionforbiodieselproduction

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IssisRomero

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HeribertoPfeiffer

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Fuel Processing Technology 120 (2014) 34–39

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Fuel Processing Technology

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Sodium zirconate (Na2ZrO3) as a catalyst in a soybean oiltransesterification reaction for biodiesel production

Nicolás Santiago-Torres, Issis C. Romero-Ibarra, Heriberto Pfeiffer ⁎Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Circuito Exterior s/n Cd. Universitaria, Del. Coyoacán, CP 04510 Mexico DF, Mexico

⁎ Corresponding author. Tel.: +52 55 5622 4627; fax:E-mail address: pfeiffer@iim.unam.mx (H. Pfeiffer).

0378-3820/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.fuproc.2013.11.018

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 February 2013Received in revised form 24 November 2013Accepted 24 November 2013Available online xxxx

Keywords:Basic catalystBiodieselSodium zirconateSoybean oil

Sodium zirconate (Na2ZrO3) was tested as a basic catalyst for the production of biodiesel using a soybeanoil transesterification reaction. Initially, Na2ZrO3 was synthesized via a solid-state reaction. The structure andmicrostructure of the catalyst were characterized using X-ray diffraction, scanning electron microscopy and N2

adsorption. Various transesterification reactions were then conducted using soybean oil and methanol underdiffering reaction conditions. The influence of some parameters, such as the reactant concentrations (molarratios), catalyst percentage, reaction time, temperature and re-use of the catalyst, on the transesterificationprocess in the presence of Na2ZrO3 was investigated. The maximum FAME conversion efficiency was 98.3% at3 h of reaction time and 3% of catalyst. Additionally, the produced biodiesel was characterized using infraredspectroscopy, gas chromatography coupled to mass spectrometry and proton nuclear magnetic resonance. Theresulting biodiesel showed good purity, composition and degree of unsaturation in comparison to previousreports. According to these results, Na2ZrO3 could become an alternative solid base catalyst for scalable andcost-effective biodiesel production.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

One promising approach to mitigate global energy problems isthe use of biomass bio-fuels, such as bioethanol and biodiesel, as abun-dant and environmentally friendly energy sources. Biodiesel is a non-petroleum based, alternative diesel fuel that is relatively clean burning,non-toxic, biodegradable and renewable [1–6]. It consists of a mixtureof esters composed ofmono-methyl esters of long chain fatty acid deriv-atives [7–9]. The combustion of biodiesel can decrease carbon monox-ide (CO) emissions by 46.7%, particulate matter emissions by 66.7%and unburned hydrocarbons by 45.2% [9].

Themost commonmethod for producing biodiesel is via triglyceride(TG) transesterification from vegetable oils and animal fats in thepresence of short-chain alcohols and homogeneous acid or base cata-lysts; these yield fatty acid methyl esters (FAME) or fatty acid ethylesters (FAEE) and glycerol (also called glycerin) as products [10].Every triglyceride molecule reacts with 3 equivalents of alcohol toproduce glycerol and three fatty acid (methyl) ester molecules[11–13]. The alcohols (R-OH) used in the transesterification are normal-lymethanol or ethanol,wheremethanol is themost commonbecause ofits low cost, low reaction temperatures, fast reaction times and higherquality methyl ester products [9–11].

Themost commonly usedmethod for triglyceride transesterificationrelies on the use of batch plants, where homogeneous base-catalyzedreactions are used [10,14]. NaOH, KOH or methoxides are typically

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used as the homogenous catalyst due to their high activity, amongother advantages that make them economically superior over mineralacids and lipases [12]. In this catalytic method, the base catalysts areunrecyclable after the reaction and suffer from separation problems.Additionally, the current aqueous quenching practice, which isperformed by acid quenching, results in saponification and emulsionformation. This process also results in an alkaline wastewater stream,which leads to the corrosion of equipment and high operating costs.Moreover, the recovered glycerol is normally impure due to thepresence of salts, soaps, monoglycerides and diglycerides, and the glyc-erol purification process consequently adds an additional cost. There-fore, for both environmental and economic reasons, there is increasinginterest in the possibility of replacing homogeneous bases by heteroge-neous solid catalysts for triglyceride transesterification [2,5,9,15–19].

Different heterogeneous base catalysts have been investigated forthe production of biodiesel [10]. Some of these solid catalysts includealkali-metal and alkaline earth metal carbonates and oxides (MgO,CaO, BaO and SrO [19–23]), lithium base ceramics (Li4SiO4 and Li2SiO3

[24,25]), Na2SiO3 [26], transition metal oxides (ZnO, PbOx, ZrO2 etc.[27,28]), basic ion-exchange resins [29], layered double hydroxides[30–33], supported bases (basic alumina, [34]), salts impregnated onAl2O3 and ZnO [35–37] and zeolites [38]. CaO has been one of themost widely used alkaline earth metal oxides as a catalyst for thetransesterification reaction, as the transesterification reaction yields a98% FAME efficiency during the first cycle [39]. Modifications of ZnOor CaO with lanthanum [40] and lithium [41] have attracted consider-able attention for the production of biodiesel due to their high basicity,low solubility and low preparation cost [1,10].

35N. Santiago-Torres et al. / Fuel Processing Technology 120 (2014) 34–39

Sodium zirconate (Na2ZrO3) has recently been examined for hightemperature CO2 capture and has shown some interesting behavior[42–44]. It has been found that Na2ZrO3 is able to trap CO2 due to itslamellar crystal structure and because of its high surface basicity [42].Therefore, the aim of the present work is to determine the influence ofsodium zirconate as a catalyst in the transesterification reaction ofsoybean oil, where the parameters of temperature, time and reagent:catalyst molar ratios were examined.

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Fig. 1. XRD pattern of the Na2ZrO3 catalyst synthesized by solid-state reaction.

2. Experimental section

Sodium zirconate (Na2ZrO3) was prepared via a solid state reaction.Stoichiometric amounts of zirconium oxide (ZrO2, Sigma-Aldrich) andsodium carbonate (Na2CO3, Aldrich) were mixed and heat-treatedat 900 °C for 4 h. The sample was then characterized using X-raydiffraction (XRD), scanning electron microscopy (SEM) and nitrogenadsorption. X-ray diffraction patterns were obtained on a Bruker AXSD8 Advance diffractometer coupled to a copper anode X-ray tube. Theresulting compounds were identified according to their correspondingJCPDS files (Joint Committee Powder Diffraction Standards). Themicrostructural characteristics of Na2ZrO3 were determined via N2

adsorption–desorption and scanning electron microscopy (SEM) mea-surements. For the N2 adsorption–desorption experiments, isothermswere acquired on a Bel-Japan Minisorp II at 77 K using the multi-pointtechnique. The samples were degassed at room temperature for 24 hunder vacuum before analysis. SEM experiments were performed on aJEOL JMS-7600F SEM.

Transesterification reactions catalyzed by Na2ZrO3 were performedin a 25 mL three-neck round-bottomed glass flask equipped with amechanical stirrer, a reflux condenser and a thermometer. The batchreactions were performed with various stoichiometric methanol/soybeanoil molar ratios. Analytical grade methanol (Sigma-Aldrich, 99.9% purity)was used along with pure Soybean oil, Nutrioli brand NOM-051-SCFI-SSA12010, that was purchased at a local food store (Mexico, D.F.).The fatty acid composition of the soybean oil was determined by gaschromatography (GC) analysis and is reported in the Results anddiscussion section. Various catalyst concentrations between 1 and 10%sodium zirconate under vigorousmagnetic stirring at 65 °Cwere consid-ered. Additional experiments were performed at different temperatures(45, 65 and 85 °C). A reflux condenser was employed to prevent theevaporation of alcohol. The progress of the transesterification reactionwas monitored at various reaction times between 0.5 and 8 h. After thecorresponding reaction times, the samples were immediately quenchedin an ice bath (~4 °C) to control the conversion rate, and the differentphases were separated for analysis. The top liquid layer was the FAME(fatty acid methyl ester) phase, whereas the bottom liquid phasewas glycerol. Both liquid layers were separated, and the catalyst wasrecovered and washed several times with methanol.

The components of the FAME and liquid samples as well as the puri-ty of the biodiesel product were determined using infrared spectrosco-py (ATR-FTIR), proton nuclear magnetic resonance (1H NMR) and gaschromatography coupled with mass spectrometry. For ATR-FTIR spec-troscopy measurements, the samples were analyzed using a Nicolet6700-FTIR spectrometer. Several 1H NMR analyses were performedusing a Bruker Advance II spectrometer (200 MHz) with CDCl3 (99.8%,Aldrich) as the solvent. A Hewlett Packard-5973 GC–MS was usedto evaluate the conversion and selectivity of the product over thetransesterification reactions. The mobile phase was hexane (HPLCgrade, Sigma-Aldrich). The final flow ratewas 0.9 mL/min, and the sam-ple injection volume was 1 μL. The temperature of the detector was250 °C, and the temperature analyses were initially performed from150 to 300 °C at a rate of 20 °C/min. All analyses were performed atroom temperature. Finally, the catalyst was re-characterized by XRDafter several transesterification cycles to evaluate the structural stabilityof the catalyst.

3. Results and discussion

The purity of Na2ZrO3 was analyzed by XRD. Fig. 1 shows the XRDpattern of the sample after thermal treatment at 900 °C for 4 h. The dif-fraction pattern was fitted to the JCPDS file 35-0770, which correspondsto Na2ZrO3 with amonoclinic structure. Therefore, Na2ZrO3 was obtain-ed without the presence of any impurities at the XRD detection level(N3%). The Na2ZrO3 microstructural characteristics were determinedusing SEM and N2 adsorption. The morphology and particle size of thesample were analyzed by SEM (Fig. 2). The Na2ZrO3 particles exhibiteda dense polyhedral morphology, with an average particle size of ~1 μm.These particles produced large agglomerates with sizes greater than15–20 μm. The dense morphology was corroborated by the texturalanalysis performed using nitrogen adsorption–desorption experiments,in which a surface area of 1.3 m2/g was determined using the BETmodel [45]. All microstructural results are in good agreement with thesolid-state reaction synthesis method.

Once the Na2ZrO3 was characterized, the material was used as acatalyst in the transesterification reaction of soybean oil and methanol.Initially, the objective was to investigate the influence of the amountof catalyst used on the conversion. The amount of catalyst wasvaried between 1 and 10 wt.% (catalyst/oil weight ratio), and thetransesterification reactions were performed at 65 °C for 3 h.These experimental conditions were established based on differentreferences reporting that some alkaline ceramics show the bestconversion under similar conditions [24,25]. As shown in Fig. 3,the conversion increased as the amount of catalyst increased from1 to 3 wt.%. In this range, the conversion varied from 84.5 to98.3%, with the best conversion ratio at 3 wt.% catalyst. The conver-sion tended to decrease for higher catalyst concentrations. With10 wt.% catalyst, the conversion ratio decreased to 93.3%. Notethat in heterogeneous catalysis, mass transfer and reactant adsorp-tion on the catalyst are extremely crucial. Therefore, a molar ratiothat is greater than the stoichiometric molar ratio of methanol isrequired to shift the equilibrium of the reaction [46]. Because theoil/methanol ratio was not varied, but the catalyst content wasincreased, it may have partially decreased the final conversionobserved at catalyst concentrations greater than 3 wt.%.

After the optimal catalyst amount was determined to be 3 wt.%, thetemperature conditions were optimized for this amount of catalyst. Asshown in Fig. 3, varying the temperature to greater than or less than65 °C resulted in decreased reaction conversion efficiencies. At 45 °C,the conversion ratio decreased to 78.4%, indicating a decrease in thekinetic properties of the reaction. Increasing the temperature to 85 °Cstrongly favored the conversion to glycerol, and the final conversionratio was a modest 83.3%.

Fig. 2. SEM images of the Na2ZrO3 catalyst synthesized by solid-state reaction.

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Fig. 4. Kinetic curves of the FAME conversion efficiency of the soybean oil and methanoltransesterification reaction, varying the Na2ZrO3 amount as catalyst.

36 N. Santiago-Torres et al. / Fuel Processing Technology 120 (2014) 34–39

Fig. 4 shows three different kinetic curves for the FAME conversionas a function of time and with varying amounts of catalyst (3, 5 and10 wt.%). This figure shows that the experiments performed with3 wt.% catalyst resulted in the highest FAME conversion for each timeexamined relative to the other kinetic series. This result is in good agree-ment with the previous results presented in Fig. 3. Additionally, thethree kinetic curves show important increases in efficiency during thefirst three hours, which correspond to the highest FAME conversion.

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Fig. 3. FAME conversion efficiency of the soybean oil and methanol transesterificationreaction, using Na2ZrO3 as catalyst and varying temperature and the catalyst amount.

Nevertheless, at prolonged reaction times, the FAME conversiondecreases, with final FAME efficiencies of approximately 87–90%.Similar effects have been previously observed [47]. Amin and co-workers have reported that high quantities of methanol (30:1) intransesterification reaction systems, as in the present case, tend todecrease the FAME conversion over long periods of time [47].Alkaline-modified zirconias were used as the catalyst materials inthat report; therefore, similar effects may be produced in thisstudy. In our case, the decrease in FAME conversion was associatedwith further glycerol production.

The biodiesel was characterized using ATR-FTIR, 1H NMR and GC–MS. Fig. 5 shows the ATR-FTIR spectra of the biodiesel produced overdifferent reaction times at 65 °C with 3 wt.% Na2ZrO3 as a catalyst.Although the spectra do not exhibit any major changes, there aresome specific vibration bands that must be analyzed. At approximately3330 cm−1, there is a wide vibration band associated with O\H bondsthat corresponds to the presence of methanol [48]. As expected, thisO\H vibration band can be clearly observed in the biodiesel sample

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Fig. 5. ATR-FTIR spectra of the biodiesel produced at different reaction times at 65 °Cwith3 wt.% of Na2ZrO3 as catalyst.

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Fig. 7. Typical gas chromatogram obtained from the different biodiesel products. In thiscase the chromatogram corresponds to the biodiesel produced after 3 h of reaction at65 °C, using 3 wt.% of Na2ZrO3 as catalyst.

37N. Santiago-Torres et al. / Fuel Processing Technology 120 (2014) 34–39

prepared over 0.5 h, indicating an incomplete reaction. In this case,the FAME conversion was 86.7%. This band disappeared over longerreaction times, showing good progress of the transesterification reac-tion. Aside from this band, the ATR-FTIR spectra were in very goodagreementwith previous reports [49–51]. The threewell-defined vibra-tion bands located at 3004, 2926 and 2853 cm−1 correspond to carbox-ylic acid compounds. The presence of a strong C_O double bond bandat ~1748 cm−1 corresponds to the ester group, which confirms thetransesterification of the oil [50]. In addition to the previous vibrationbands, other vibration bands were observed between 1480 and1100 cm−1, which correspond to the\CH2 scissor and\CH3 antisym-metric vibrations. These spectra also exhibited a specific vibration bandat approximately 720 cm−1, corresponding to the cis isomers of \CH.Finally, the first (0.5 h) and last (8 h) ATR-FTIR spectra exhibited a vi-bration band located at 1015 cm−1, which can be associated withC\(OH) vibrations. In the first case, this band can be attributed to thepresence of methanol, whereas in the second case, this vibration bandmay be associated to the partial dissolution of glycerol because theformation of glycerol is favored for long reaction times. A closer analysisof the 8 h spectrum shows an O\H vibration band between 3100 and3400 cm−1. This result is in good agreement with the decreasein FAME conversion observed over long reaction times, which waspreviously associated with a higher glycerol conversion.

The soybean oil and aliquots of different biodiesel products werecharacterized by NMR. Fig. 6 shows the 1H NMR spectra of the initialsoybean oil and biodiesel producedwith 3, 5 and 10 wt.% of theNa2ZrO3

catalyst during 3 h at 65 °C. Initially, the pure soybean oil spectrum fitsvery well with previously reported 1H NMR spectra [11]. The transfor-mation of soybean oil into biodiesel (FAME) could then be determinedby the appearance of a signal at ~3.5 ppm (9H), corresponding to themetylenic \CH3 protons. Additionally, some specific differences wereobserved among the spectra. The 3 wt.% spectrum shows three verysmall peaks located at 0.9, 4.1 and 4.2 ppm. These peaks were observedin the spectrum for the soybean oil and correspond to the \CH2 and\CH3 protons of the palmitic and linoleic chains. The presence ofthese peaks indicates incomplete FAME conversion, which corroboratesthe gravimetric quantification. The presence of these peaks was notobserved in the other 1H NMR spectra. Residual methanol was alsoobserved in all transesterification reaction samples at 3.4 ppm. Integra-tion of the signals showed that themethanol concentration increased asa function of the catalyst content, from 0.16 to 1.14. The presence andincrease in methanol concentration can be correlated to the incompleteFAME conversion observed when the amount of catalyst was increasedfrom 3 to 10 wt.%. It must be mentioned that glycerol is insoluble inCDCl3; therefore, it was not detected.

6 5 4 3 2 1Chemical shift (ppm)

(A)

(B)

(C)

(D)

Fig. 6. 1H NMR spectra of the initial soybean oil (A) and biodiesels produced with 3 (B),5 (C) and 10 wt.% (D) of Na2ZrO3 as catalyst during 3 h at 65 °C.

The average degree of FAME unsaturation (DU) was calculated bycomparing the 1H NMR integration values for the methyl group(3.67 ppm) and the olefin protons (5.35 ppm) in the methyl esterproducts [11,38]. The DU values from soybean oil obtained in thismanner were 0.89, 1.09 and 1.07 for the 3, 5 and 10 wt.% of catalyst,respectively. These values are similar to previous reports, where valuesbetween 0.96 and 1.06 were obtained [11,38,52,53].

The biodiesels were finally characterized by GC–MS to moreprecisely elucidate their chemical compositions. Fig. 7 shows a typicalchromatogram obtained from the different biodiesel products. At lowretention times, the GC curves showed an incomplete peak, whichmay be associated with the methanol originally present in the samples(see RMN results). The first well-defined peak, which was observedafter 1.7 min, corresponds to the hexane used as a solvent in theseexperiments. After the hexane peak, three peaks appeared at 5.4, 6.1and 6.4 min, and the chemical compositions of these peaks were identi-fied by mass spectrometry, which showed that these were a typicalmixture of biodiesel esters. The first peak (5.4 min) was associated

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Fig. 8. Na2ZrO3 reutilization analysis of five transesterification reactions performed with3 wt.% of catalyst, at 65 °C for 3 h.

38 N. Santiago-Torres et al. / Fuel Processing Technology 120 (2014) 34–39

with hexadecanoic acid methyl ester (CH3-O-CO-(CH2)14-CH3), thesecond peak (6.1 min) corresponded to 9-, 12-, octadecadienoic acid,methyl ester (CH3-O-CO-(CH2)7-CH = CH-CH2-CH = CH-(CH2)4-CH3),and the third peak (6.4 min) was determined to be octadecanoic acidmethyl ester (CH3-O-CO-(CH2)16-CH3).

Finally, to evaluate the cyclability performance of the Na2ZrO3, thetransesterification process was repeated five times using the samecatalyst. Fig. 8 shows that the FAME conversion was diminished as afunction of the cycle number, from 98.3% in the first cycle to 81.3% inthe fourth cycle. The catalyst was then washed after the fourth cycle,and it exhibited some recovery in the FAME conversion (84.0%) in thefollowing cycle. The reduction in FAME conversion observed duringthe cycles may be attributed to several factors, as follows: 1) Aftereach cycle, the catalyst was recovered by decantation, and part of thecatalyst was lost (~5% after five cycles). 2) Although the crystallinestructure of Na2ZrO3 did not change (XRD analysis not shown), theparticle surface must suffer inactivation, as it was evidenced duringthe washing process.

4. Conclusions

Na2ZrO3 was synthesized, characterized and evaluated as a basiccatalyst for the production of biodiesel. The biodiesel produced througha transesterification reaction was characterized. Qualitative and quanti-tative results showed that Na2ZrO3 has excellent catalytic activity andgood stability due to its basic strength and because it is stable duringtransesterification reactions. The FAME conversion reached an excellentefficiency of 98.3% after 3 h at 65 °C. Additionally, the biodiesel charac-terization corroborated the FAME conversion and other parameters,such as its high purity, composition (typically 9, 12, octadecadienoic,hexadecanoic and octadecanoic acid methyl esters) and degree ofunsaturation (between 0.89 and 1.09). The biodiesel produced withNa2ZrO3 showed similar conversions relative to other basic heteroge-neous and homogeneous systems, and the obtained biodiesel hadgood properties. Finally, the cyclic behavior revealed that the catalysthad a relatively stability. Nevertheless, Na2ZrO3 should be further ana-lyzed and improved upon to maintain its high initial FAME conversion.All of these results suggest that Na2ZrO3 could become an alternativesolid base catalyst for scalable and cost-effective production of biodiesel.

Acknowledgments

The present work was financially supported by the projectIN102313-PAPIIT-UNAM. The authors thank A. Tejeda and O. Novelofor technical help.

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