commercial hydrated lime as a cost-effective solid base for the transesterification of wasted...

Published: June 09, 2011

r 2011 American Chemical Society 3275 dx.doi.org/10.1021/ef200555r | Energy Fuels 2011, 25, 3275–3282

ARTICLE

pubs.acs.org/EF

Commercial Hydrated Lime as a Cost-Effective Solid Base for theTransesterification of Wasted Soybean Oil with Methanol forBiodiesel ProductionManuel S�anchez-Cant�u,*,† Lydia M. P�erez-Díaz,† Rosalba Rosales,† E. Ramírez,† Alberto Apreza-Sies,†

Israel Pala-Rosas,† Efraín Rubio-Rosas,† Manuel Aguilar-Franco,‡ and Jaime S. Valente§

†Facultad de Ingeniería Química, Benem�erita Universidad Aut�onoma de Puebla, Avenida San Claudio y 18 Sur, C.P. 72570 Puebla,Puebla, M�exico‡Instituto de Física, Universidad Nacional Aut�onoma de M�exico, Circuito de la Inv. Científica S/N C.U., C.P 04510, D.F. M�exico§Instituto Mexicano del Petr�oleo, Eje Central #152, C.P. 07730 M�exico, D. F., M�exico

ABSTRACT:The transesterification of used soybean oil with methanol was carried out over hydrated lime (HL), Ca(OH)2, and itsdecomposition products in the 200�500 �C range. The catalysts were characterized by X-ray powder diffraction (XRD),thermogravimetric analysis, and scanning electron microscopy. The XRD powder patterns demonstrated that the pristine sampleconsisted of a mixture of calcium hydroxide and calcite. It was noticed that the coexistence of CaO, Ca(OH)2, and CaCO3 remainedup to 400 �C. At 500 �C, Ca(OH)2 is transformed into CaO so that this and CaCO3 are the only remaining phases. In thetransesterification reaction, the influence of calcination temperature, reaction time, catalyst amount, methanol:oil ratio, and reactiontemperature was studied. Full conversion of the raw materials into biodiesel (BD) was obtained with the fresh HL. In order todetermine any change in the solid, it was recovered after 10, 30, and 60 min of reaction and analyzed by XRD analysis. OnlyCa(OH)2, CaCO3, and traces of monohydrocalcite were detected. From the results it was demonstrated that the active phase forbiodiesel production was calcium hydroxide. Furthermore, the catalyst was used up to three times without deactivation. A simple,economic, and environmentally friendly way to obtain biodiesel was developed considering (a) used soybean oil, considered waste,was employed as raw material, (b) hydrated lime is cheap and readily available, and (c) full conversion of the raw materials into BDwas achieved with the as-received HL.

1. INTRODUCTION

The world’s oil dependence has reached historical levels and itis imperative to find optimal exploitation of the energetic re-sources in order to increase the world’s energy security. More-over, the increment of the crude oil prices has forced the searchfor alternative fuels based on renewable energy sources. In thissense, a potential diesel oil substitute is generally known as bio-diesel (BD). BD from vegetable oil resources can substitute fordiesel in considerable proportion, and therefore, vegetable oilresources, particularly nonedible ones, such as wasted oils,deserve our consideration.

Nevertheless, themajor drawback of biodiesel production is itseconomic viability, since its production costs are higher thanfossil-derived diesel. The overall biodiesel cost consists of rawmaterial (production and processing), catalyst, biodiesel processing(energy, consumables, and labor), transportation (raw materialsand final products), and local and national taxes.1 In this sense,the cost of refined vegetable oils contribute between 60 and 80%of the overall biodiesel production cost.1,2

Even more, there is severe controversy in using edible oils,since it may cause an increase of their costs and reduce theiravailability. Therefore, one way to reduce BD production cost isemploying cheaper feedstocks, such as used edible oils that areconsidered as waste. These are generally disposed into the waterdrainage without any treatment, causing severe environmentalproblems. Nowadays, the amounts of these waste oils generated

per year by any country are huge and will increase with thepopulation’s requirements.3

In general, biodiesel is produced by transesterification(alcoholysis) employing homogeneous bases and/or acids.4

The reported advantages of the homogeneous base-catalyzedreaction are very fast reaction rate (4000 times faster than acid-catalyzed transesterification), catalysts such as NaOH and KOHare relatively cheap and widely available, the reaction can occurunder mild reaction conditions and are less energy intensive. Inthe same way, the advantages of the acid-catalyzed reaction are itsinsensitivity to free fatty acids andwater content in the oil, it is thepreferred-method if low-grade oil is used, esterification andtransesterification occur simultaneously, the reaction can occurat mild reaction conditions, and it is less energy intensive.

Nevertheless, some of the problems associated with thehomogeneous catalysts are, for instance, the high consumptionof energy, corrosion on reactor and pipelines by the use of highalkaline or acid conditions, expensive separation of the homo-geneous catalyst from the reactionmixture, the catalyst cannot bereused or regenerated, and generation of large amounts of waste-water during separation of the products.1 In the base-catalyzed

Received: April 12, 2011Revised: June 8, 2011

3276 dx.doi.org/10.1021/ef200555r |Energy Fuels 2011, 25, 3275–3282

Energy & Fuels ARTICLE

reaction, there is also the problem of formation of unwanted soapbyproduct by reaction of the free fatty acids.

Therefore, the use of solid catalysts represents an attractivesolution, since the catalysts can be easily separated from theproducts, the products do not contain impurities of the catalystand the cost of final separation could be reduced, they can bereadily regenerated and reused, and it is more environmentallybenign because there is no need for acid or water treatment in theseparation step.5�7

The transesterification reaction in heterogeneous phase can becarried out by solids that disclose Br€onsted or Lewis basic sites. Inthis sense, a great variety of catalysts have been used for thispurpose which include alumina, TiO2, ZrO2,

8 zeolites,9 anionicclays.10�15 In this context, obtaining a highly active catalyst is notonly required but also the catalyst must exhibit a poisoningresistance as well as possess an inexpensive production, sincethese characteristics will not only impact BD production butlower its cost.

Among the most studied heterogeneous catalysts, the Ca-derived bases are the most promising, as they are inexpensive,have long lifetimes, require only mild reaction conditions, are theleast toxic, and exhibit low solubility in methanol.16

Kouzu et al.17 reported the obtaining of CaO from calcinationof pulverized lime stone (CaCO3) at 900 �C for 1.5 h in a heliumgas flow reaching a 93% yield at 1 h of reaction time for CaO.

Granados et al.18 evaluated the role of H2O and CO2 in thedeterioration of the catalytic performance of CaO in BD produc-tion by contacting it with room air and demonstrate that in orderto recover the catalyst’s activity a thermal treatment is required torevert CO2 poisoning.

Cho et al. obtained calcium oxide catalysts by calcining variousprecursors such as calcium acetate, carbonate, hydroxide, nitrate,and oxalate, and their catalytic activities were examined in thetransesterification of tributyrin with methanol.19 From theirresults, CaO obtained from calcium hydroxide was the mostactive catalyst among the prepared catalysts, with 82% conver-sion in 1 h.

From an industrial point of view, calcium oxide, also known asquicklime or burnt lime, is usually produced by the thermaldecomposition of a parent material such as its hydroxide orcarbonate. If overburning has not taken place, CaO reacts withwater to form calcium hydroxide, by a process known as slaking.

It is reported that calcium oxide produced from hydrated lime,Ca(OH)2, is more reactive in sulfur20 and CO2

21 capture thanthat derived from the respective limestone, CaCO3. The generalreasons given are20,22 (a) the inherent particle size of hydroxide-CaO is much smaller than that of carbonate-CaO, giving rise to agreater reactivity, and (b) solid products of decomposing Ca-(OH)2 are particles that have approximately the same exteriordimensions as the parent Ca(OH)2 ones, showing slit-shapedgeometry and having higher internal surface areas than calciumoxide obtained from CaCO3 calcination at higher temperature.Taking this into account and in the knowledge that CaOdiscloses strong Lewis basic sites whose nature and amount willdepend on the treatment temperature,23 it is important to findthe optimal activation temperature for biodiesel production.

Moreover, as Ca(OH)2 structure resembles that of brucite,whereM2+(OH)6 octahedra share edges to build infiniteM(OH)2sheets, and considering that the transesterification reaction canproceed via surface OH� groups,15 it is plausible to consider thathydrated lime can be used directly as a catalyst for biodieselproduction.

From the above-mentioned, hydrated lime, obtained from thecontrolled hydration of CaO achieved from the previous calcina-tion of limestone, and the corresponding CaO obtained from itsthermal treatment represent inexpensive catalysts for obtainingbiodiesel, since they are produced in large amounts and arereadily available.

Thus, in this work, hydrated lime and CaO produced by itsthermal treatment were employed as heterogeneous catalysts inthe transesterification of used soybean oil and methanol.

2. EXPERIMENTAL SECTION

2.1. Materials. Hydrated lime (HL), purchased from Cales SantaEmilia ubicated in Perote, Veracruz, M�exico, is sold in 25 kg paper sacksand was used as received without further purification. Virgin soybean oilwas purchased from a local store. The used soybean oil (USO) wasobtained by frying six batches of potatoes in virgin soybean oil. Then, itwas filtered under vacuum to eliminate the suspended solids followed byheating to remove all the moisture. Methanol (98% purity) was pur-chased from Golden Bell. Methanol was dried with metallic magnesiumand iodine.2.2. Catalyst Characterization. The X-ray diffraction pattern of

the sample was measured in a D8 Bruker Discover Series 2 diffract-ometer with Cu KR radiation. Diffraction intensity was measuredbetween 5� and 45�, with a 2θ step of 0.02� and a counting time of0.6s per point.

Thermogravimetric (TG) analysis was developed on a TGAi 1000series system, which was operated under nitrogen flow at a heating rateof 10 �C/min from 25 to 1000 �C. In the determination, ∼100 mg offinely powdered dried sample was used.

Scanning electron microscopy (SEM) analysis was carried out in aJEOL JSM-6610 LV with an acceleration voltage of 20 keV. Prior toanalysis, the sample was covered with gold and mounted over acarbon film.2.3. Transesterification of Used Soybean Oil with Metha-

nol.The experiments were conducted in a 100mL three-neck glass flaskcoupled to a condenser and a water cooling recirculation system. Stirringand heating were achieved using a magnetic stirrer/hot plate.

First, the USOwas heated above 100 �C for 1 h to eliminate moistureand then the oil was cooled to the required reaction temperature.Methanol and the catalyst were separately added into the glass flask.Then, USO was charged into the vessel and heated to the intendedreaction temperature.

After the course of the reaction, the reaction mixture was centrifugedand the liquid phases were separated from the catalyst and decanted in aseparation funnel. Excess methanol was removed by evaporation, leavingBD and glycerol as separate phases. The BD was analyzed by thin-layer chromatography (TLC) and proton nuclear magnetic resonance(1H NMR) analysis.

TLC analysis was performed with ALUGRAM Sheets SIL G/UV 254(Macherey-Nagel) and developed in a solvent system of hexane/ethylacetate (8:1). USO and methyl esters spots were revealed by an iodinevapor stain. 1H NMR spectra were measured on a Varian Mercuryspectrometer, using 400 MHz frequency, in deuterated chloroform(CDCl3) and tetramethylsilane (TMS) as reference. The relevant signalschosen for integration were those of methoxy groups in the fatty acidmethyl esters (FAMES) (3.66 ppm, singlet) and the glyceridic protonsof soybean oil (4.14�4.27 ppm, doublet of doublets). The conversionwas calculated directly from the integrated areas of the aforementionedsignals following the procedure described by Knothe.24

The studied operation variables were catalyst activation temperature,catalyst concentration, methanol/oil molar ratio, and reaction tempera-ture. Twelve milliliters of used soybean oil, 6 mL of methanol, fresh HL/liquids mass ratio of 4%, reaction temperature of 60 �C, and 120 min of



reaction time were fixed as initial reaction conditions. The catalyst masswas maintained the same in all experiments. The experiments wereperformed three times and the average results from the 1HNMR spectrawere reported.

3. RESULTS AND DISCUSSION

3.1. Catalyst Characterization. Figure 1 shows the X-raypowder patterns of the pristine sample (PS) and the samplescalcined at 200�500 �C. The PS exhibited the characteristicreflections of calcium hydroxide (JCPDS #84-1263) and calciumcarbonate in its calcite form (JCPDS #88-1807), as the principaland secondary crystalline phases, respectively. Since hydratedlime is generally obtained from the controlled rehydration ofCaO, the presence of the calcite phase may be caused by thereaction of atmospheric CO2 in the rehydration process. It isimportant to remark that both crystalline phases remained in thesamples thermally treated at 200 and 300 �C. These results canbe corroborated by the analysis of the average crystal sizes shownin Table 1. Average crystal sizes were calculated from the mostintense reflections employing the Scherrer equation L = 0.9λ/Bcos θ, where L is the crystallite size, λ the X-ray wavelength, B theline broadening, and θ the Bragg angle.25 The crystal sizes ofcalcium hydroxide did not show any significant change in thefresh and the calcined samples at 200 and 300 �C (25, 22, and24 nm, respectively), while calcite’s crystallites presented a slightdecrease from 56 to 41 nm.It is worthwhile to stress the changes that took place at 400 �C

are evidenced by the presence of CaO (JCPDS #78-0649), whichcorresponds to calcium hydroxide decomposition26 followed byan increment of the Ca(OH)2 and calcite crystal sizes. Finally, at

500 �C the characteristic reflections of Ca(OH)2 completelydisappeared, with only calcite and CaO phases remaining with nosignificant change in the average CaO crystal size but with anincrease of CaCO3 cystals. This behavior is attributed to thesintering process27 explained by the Ostwald ripening effectwhich involves the growth of larger particles at the expense ofsmaller ones.28

The thermogravimetric analysis is presented in Figure 2. Fromthe first derivative of the thermogram, three thermal transitionswere observed: the first one corresponds to the weight loss up tothe point where a relative stability is achieved, around 393 �C; thesecond was set at 393�480 �C, and the third interval wasestablished at 480�743 �C, respectively. In the first temperaturerange, the sample lost 0.32%, which was assigned to the elimina-tion of physisorbed water. The second transition, where 17.81%was lost, was attributed to the dehydroxylation of the Ca(OH)2laminae. The last transition (7.56%) corresponded to calcite’sdecarbonation and the complete dehydroxylation of calciumhydroxide.26

From the high-temperature X-ray powder patterns (seeFigure 1) it can be seen that calcium hydroxide and calcitecrystalline phases remained almost unchanged up to 300 �C,which match with the TG results. The second and third weightloss corroborated the decomposition of Ca(OH)2 and calcite toform theCaO crystalline phase, which was evidenced by the XRDanalysis at 400 and 500 �C. Also, the well-defined thermaltransition in the 393�480 �C interval can be attributed to theOH’s nature, meaning that they are similar. This behavior hasbeen observed in carbonate�hydrotalcite and hydrotalcite-likecompounds, where the peak assigned to carbonate decomposi-tion was related to their basic strength.29

Figure 3 exhibits the SEM images obtained with the JEOLJSM-6610 LV, employing the signals generated by secondaryelectrons. The SEM image of the pristine hydrated lime(Figure 3A) demonstrated that the sample consisted of platelikeparticles, a characteristic morphology of hydrated lime, with anaverage particle size of 0.16 μm.30 The samples thermally treatedat 200 and 300 �C showed similar results. However, when thesample was calcined at 400 and 500 �C (Figure 3B,C), animportant change in size was noticed, since the average particlesize increased up to 0.7 μm in both cases. Also, from themicrographs the presence of necks that join the nonuniform

Table 1. Average Crystal Sizes of Ca(OH)2, CaCO3, and CaO

sample L(011), nma L(104), nm

b L(200), nmc

fresh HL 26 58 �200 �C 23 48 �300 �C 25 42 �400 �C 37 52 36

500 �C � 65 31aCa(OH)2.

bCaCO3.cCaO.

Figure 1. X-ray powder patterns of fresh HL and the products obtainedafter its calcination in the 200�500 �C interval.

Figure 2. Thermogravimetric analysis of the HL.



grains collectively in a continuous matrix was observed togetherwith a transformation into spherelike particles, which is attributedto a sintering process.31 These results agree with those obtainedby the X-ray analysis, where an increment in the average crystalsize was also noticed.3.2. Transesterification Reaction. 3.2.1. Effect of the Catalyst

Activation. According to the results reported in the literature,CaO acts as an efficient catalyst in the transesterificationreaction.32,33 Therefore, we evaluated in the first instance thecalcined catalyst at 500 �C, since CaO was detected as the maincrystalline phase. From the thin-layer chromatography analysiswe observed the absence of the USO signal, and only thebiodiesel spot remained. This result was corroborated by the1H NMR analysis.Figure 4 presents the 1H NMR spectra of the used soybean oil

and the fatty acid methyl esters (FAME) product resulting fromits transesterification with methanol. Table 2 shows the chemicalshifts and characteristics signals of a typical 1HNMR spectrum ofsoybean oil (triglycerides) assigning each group within it. Thepresence of olefinic hydrogens, (�CHdCH�)-7, and methinegroup, (�CH2�CH�CH2�)-2, triglyceride with a chemicalshift at 5.34 and 5.26 ppm is observed as multiplet signals,respectively. Moreover, diallyl methylene hydrogens observed at2.76 ppm are presented as a triplet, (dCH�CH2�CHd)-8.The occurrence of methylene hydrogens of carboxyl groups at2.31 ppm is seen as a triplet, (�CH2�CH2�OCO�)-3, whilehydrogens onmethylene groups neighboring unsaturated carbonof chemical shift 2.03 ppm appeared as a multiplet signal,(�(CH2)5�CH2�CHd)-6. Hydrogens neighboringmethylenegroups of saturated carbon atoms are observed at 1.61 ppm,(�(CH2)12�CH2�CH2�)-4. Likewise, the methylene groups’

hydrogens attached to two saturated carbon atoms arose asmultiplet signals, (�(CH2)12�)-5 (1.27 ppm); also the hydro-gen of the terminal methyl groups appeared as a multiple signal,(�CH3)-9, at 0.89 ppm. Moreover, it is important to mentionthat the 1H NMR spectrum of soybean oil (Figure 4A) shows asplitting pattern for the glyceride protons, which follows a ABXpattern due to the asymmetric chemical shift of methylene groupprotons at 4.22 ppm (4H); the values of the coupling constantsare Jab = 12 Hz, Jax = 6 Hz, Jbx = 4 Hz for a system represented byRCO2�CHaHbCHx(CO2R0)CHaHbCO2R00. The inequivalenceof the hydrogens a and b results from the fact that both are onopposite sides of the plane that extends along the link to themethylene and methine; in this system, the group has O2CR andprotons are on the same side of the plane, while Hb and Hx

Figure 4. 1H NMR spectra of (a) used soybean oil and (b) fatty acidmethyl esters product from soybean oil transesterification with methanol.

Figure 3. SEM images of the samples covered with gold: fresh HL (A)and calcined at 400 (B) and 500 �C (C). Table 2. Soybean Oil Protons’ Chemical Shift Assignments

and Its Splitting

assignments

δ

(ppm) signal integration group

9 0.89 multiplet 9H �CH3

5 1.27 multiplet 51H �(CH2)5�4 1.61 multiplet 6H �(CH2)4�CH2�CH2�6 2.03 multiplet 10H �CH2�CH2�(CH2)4�3 2.31 triplet 6H �CH2�COO�8 2.76 triplet 4H �CH2�CHdCH

1 4.22 doublet

of doublets

4H RCO�O�CH2�CH�CH2

2 5.26 multiplet 1H RCO�O�CH2�CH�CH2RCO

7 5.34 multiplet 8H �CHdCH�



protons are on the other side of the plane. Therefore, bothhydrogens Ha and Hb appear as doublet of doublets (dd) at 4.27and 4.14 ppm, respectively. It is important to remark that both 1HNMR spectra of the fresh oil and the used soybean oil spectrawere identical. Although the soybean oil was used six times withina period of 12 h, giving a brownish product, the expected signalsof the products formed during frying such as free fatty acid and/or polymerized triglycerides were not appreciated in the usedcooking oil, which could be due to the frying severity. Generally,in public restaurants, frying is conducted in the same oil forseveral days.3 However, their presence cannot be disregardedsince they could have exceeded the apparatus detection.Figure 4B exhibit the 1H NMR spectrum of the produced

FAMES (biodiesel). The major differences between the spec-trum of soybean oil and the resulting fatty acid methyl ester arethe disappearance of the glyceride protons and the appearanceof a simple signal(s) corresponding to methyl ester protons at3.66 ppm (9H). The chemicals shift of the hydrogens of aliphaticcarbons corresponding to the three monoglycerides obtained arethe same and can be verified in the spectrum.As a complete conversion was achieved with the solid annealed

at 500 �C, the calcination temperature was decreased and theexperiment was repeated employing the 400, 300, and 200 �Ccalcined samples, with similar results being obtained. Finally, weused the as received hydrated lime in the transesterificationreaction, achieving a 100% conversion.These results differ from those reported by Kouzu et al.,17 who

carried out the transesterification of edible soybean oil withrefluxing methanol in the presence of calcium oxide CaO,Ca(OH)2, and CaCO3, achieving at 1 h of reaction time a BDyield of 93% for CaO, 12% for Ca(OH)2, and 0% for CaCO3.Also they reported that in order to reach the same result withCa(OH)2 it took up 3.5 h to gain the same yield. Since CaCO3 isinactive in the transesterification reaction,34 we assumed CaO,which exhibits strong Lewis O2- basic sites, was responsible forthe complete transformation of our raw materials into BD in thesample calcined at 500 �C. However, the results attained in thesamples where Ca(OH)2 was detected as the main crystallinephase must be explained in terms of their Br€onsted, OH�,basicity. This behavior has been also observed for other authors

in the transesterification of oleic acid methyl ester with glycerolwith rehydrated hydotalcite-like compounds.35

On the other hand, Kouzu et al.36 also evidenced that theactive phase of calcium oxide in the transesterification of ediblesoybean oil with methanol was calcium diglyceroxide (CD).When they took a sample at 0.25 h of the reaction only CaO andCa(OH)2 crystalline phases were detected by means of the XRDanalysis. In this sense, taking into account that Ca(OH)2 wasformed during the reaction, it is plausible to assume that CD isformed from Ca(OH)2 itself and not directly from CaO.From the XRD powder pattern reported by Kouzu et al.36 the

average crystal size of their calcium hydroxide, calculated from the(011) reflection, was ca. 10 nm. In this context, small crystal sizesfavor the activity, since it is well-known that smaller crystalsproducemore reactive sites. According to our results, the formationof CD may be reached in a shorter period when employinghydrated lime than when CaO was used as catalyst precursor,which explains the high conversions reached in this work.To verify this assumption, the reaction was performed by

employing hydrated lime as catalyst, and the solid was collectedafter 10, 30, and 60 min of reaction. The sample was onlyfiltrated, and the paste consisting of a mixture of the catalyst, oil,glycerin, BD, andmethanol was analyzed by XRD. The results arepresented in Figure 5. The XRD results revealed only thepresence of calcium hydroxide, calcite, and traces of mono-hydrocalcite (JCPDS #83-1923). These results disagree with thoseof Kouzu, since we expected to obtain CD. Kouzu et al. reportedthat after 2 h of reaction, the catalyst was collected by filtrationand washed with methanol to remove methyl esters and glycerol,followed by vacuum drying at 80 �C. According to Ngamchar-ussrivichai et al.,34 methanol washing is not enough to removethe sorbed organic compounds, and as a consequence weattributed the formation of CD in Kouzu’s samples to thepresence of organic molecules that could not be removed bythe methanol washing and further reacted with CaO and Ca-(OH)2 favored by the 80 �C drying. Again, we took a sample at60 min of reaction and repeated Kouzu’s activation. After thereaction, we washed the collected catalyst with 50 mL ofanhydrous methanol and dried it at 80 �C. From the XRDpowder pattern we did not detect calcium diglyceroxide (seeFigure 5D). It appears that during the reaction the CaOemployed by Kouzu et al.36 was covered by a Ca(OH)2 film thatis also responsible for biodiesel production, and calcium diglycer-oxide is produced by the reaction of CaO with the organiccompounds sorbed on its surface. This also explains the reportedconversion decrease when CD itself was employed in BDproduction. In this sense, at least for our results, the active phasein the transesterification of soybean oil with methanol is con-sidered to be calcium hydroxide. This suggests that during thetransesterification reaction surface-bound OH groups of hy-drated lime are converted to surface-bound water groups uponreaction with methanol, thus liberating methoxide, which isrecognized as a very active catalyst for BD production.17,32,37

This behavior is in good agreement with the results obtained byReddy et al.,38 which also confirmed the importance of theBr€onsted basic sites in obtaining biodiesel.These results are attractive in the sense that, besides its high

activity, the catalyst itself is economic, commercially available, andwill not require an activation process. Also, it can be used as receivedwithout any special care to avoid atmospheric contamination.The TGA results (see Figure 2) revealed that the sample

contained physisorbed water (0.32 wt % corresponding to 0.21%

Figure 5. XRD powder patterns of the samples collected at distinctreaction times: (A) 10min, (B) 30min, (C) 60min, (D) 60min, washedand dried at 80 �C, and (E) after the third reuse.



by weight of USO), which according to some reports could favorthe saponification reaction and in consequence diminish bio-diesel yield.6,7,39 Nonetheless, Liu et al.,32 who carried out thetransesterification of soybean oil with methanol, reported that ifwater content is less than 2.8% by weight of soybean oil, thetransesterification reaction rate can be accelerated and thebiodiesel yield is improved within a short reaction time, whichmatches our results.In order to evaluate the optimal reaction conditions of the

transesterification process with the as-received hydrated lime, weexamined the effect of the following reaction parameters: reac-tion time, catalyst amount, and methanol:oil ratio, and reactiontemperature. The initial reaction conditions were 12 mL of USO,6 mL of methanol, 1 g of catalyst, and a reaction temperature of60 �C.3.2.2. Reaction Time Effect. Figure 6 exhibits the influence of

reaction time at 10, 30, 60, and 120 min. From the resultsobtained, it is evident that reaction time affected significantly thecatalytic activity, since the conversion percentage graduallyincreased with the reaction time, achieving in 10, 30, 60, and120 min conversions of 12, 58, 86, and 100%, respectively. By theanalysis of the catalyst’s average crystal size (calculated from thereflection at 34.1� of 2θ) it can be seen that the catalyst crystalsize was inversely proportional to the reaction time; for instance,the pristine hydrated lime exhibited a Ca(OH)2 crystal size of26 nm, while at 10, 30, and 60 min the crystal sizes were 24, 22,and 19 nm, respectively. This decrease can be ascribed to thereaction conditions and the vigorous stirring. However, it is well-known that smaller crystals are more active than the bigger ones.Eventually, although 86% is an acceptable conversion percentage,we selected the 120 min period for the following experiments.3.2.3. Effect of Catalyst Loading. The effect of the catalyst

loading on BD production is shown in Figure 7. The studiedrange was 0.3�0.60 g (2.1�4.2% in relation to the mass of theUSO and methanol). As can be observed, the conversionpercentage increase was proportional to the catalysts loading.The best result was obtained with a catalyst amount of 0.57 g,which corresponds to a weight ratio of 4%. With 4% of catalyst,2 h is enough to reach the complete conversion of the rawmaterials.3.2.4. Effect of the Methanol:Oil Ratio. Theoretically, to carry

out the transesterification reaction, 3 mol of alcohol is requiredfor 1 mol of triglyceride to produce 3 mol of fatty acid ester and

1 mol of glycerol. It is well-known that the transesterificationreaction is a reversible one, and with the intention to shifting theequilibrium toward biodiesel production, an excess of methanolis generally employed. Thus, the yield of biodiesel is increasedwhen the alcohol:triglyceride ratio is raised above 3 and reaches amaximum. It is important to consider that increasing the alcoholamount beyond the optimal ratio will not increase the yield butwill increase the cost for alcohol recovery. The stoichiometry ofthis reaction requires 0.134 volume of methanol per volume oftriglyceride (the molar ratio is 3:1).32 Taking this into account,the methanol:oil ratio (vol) was varied from 0.08 to 0.5.The experimental results, illustrated in Figure 8, demonstrate

the significant impact on biodiesel obtained since the conversionincreased with the increment of the methanol:oil ratio from 0.08to 0.17, achieving conversions of 73 and 100%, respectively.Although conversions higher than 97% were obtained withhigher methanol:oil ratios, the methanol excess could raise thebiodiesel cost due to the additional process needed for alcoholrecovery, as stated before.3.2.5. Effect of Reaction Temperature. To elucidate the effect

of reaction temperature, the transesterification of USO wasconducted with the fresh HL, Ca(OH)2, and with the samplecalcined at 500 �C (CaO), maintaining constant the catalystamount (4%), 0.17 methanol:oil ratio (vol), and 2 h reaction

Figure 6. Effect of reaction time on biodiesel yield. FreshHL:liquidsmassratio, 3.8%; methanol:oil ratio (vol), 0.5; reaction temperature, 60 �C.

Figure 7. Effect of the catalyst loading on biodiesel production. Reactiontime, 2 h; methanol:oil ratio (vol), 0.5; reaction temperature, 60 �C.

Figure 8. Effect of the methanol:oil ratio on biodiesel manufacture.Reaction time, 2 h; catalyst loading, 3.6%; reaction temperature, 60 �C.



time. The results are presented in Figure 9. The results revealedthat conversion of the raw materials increased with the reactiontemperature. No conversion was achieved with the fresh HL at25 �C, while at 35, 45, 55, and 60 �C conversions of 2, 19, 57 and100% were accomplished, correspondingly. On the other hand,by using the calcined sample (CaO), conversions of 45, 89,91, 93, and 100% were achieved at 25, 35, 45, 55, and 60 �C,respectively.The transesterification is generally assumed to be a pseudo-

first-order reaction in excess of methanol.40,41 Thus, the apparentrate constant is

k ¼ � lnð1� yÞ

tð1Þ

where k is the apparent rate constant, y is the FAME conversion,and t is the reaction time. Moreover, according with Arrheniusequation the overall reaction rate constant has a relationship withtemperature as follows

ln k ¼ � EaRT

+ C ð2Þ

where Ea is the activation energy, R is the gas constant(J mol�1 K�1),T is the absolute temperature, andC is a constant.The average rate constant at different temperatures can beobtained from Figure 9. Hence, activation energies of 53.8 and279.2 kJ/mol were calculated for the reaction based on the k valuesat distinct temperatures for CaO and Ca(OH)2, correspondingly.3.2.6. Catalyst Reutilization.Recovery and reuse of the catalyst

is one of the most important characteristics of heterogeneouscatalysis which also would impact the economics for biodieselproduction. Reutilization of the catalyst was carried out byrecovering the liquids after a catalytic run, maintaining thecatalyst inside the reactor, and adding new fresh portions ofmethanol and used soybean oil. From the above-studied reactionparameters the operating conditions for the reutilization studywere 2 h of reaction time, methanol:oil ratio (vol) of 0.17, catalystconcentration of 3.6 wt %, and a reaction temperature of 60 �C.The catalyst was highly active up to the second reutilization,

which means, after using the catalyst three times, a 100%conversion was achieved. However, an important decrease wasidentified in the third reuse, reaching a conversion of 62%. From

the X-ray analysis of the paste (see Figure 5E), we observed thatwhile calcite was identified as a secondary crystalline phase in thefirst use, which we consider to remain up to the second reuse,after the third one it was detected as the main crystalline phase,thus decreasing the amount of the catalytic active phase, and as aconsequence, the biodiesel conversion decreased. The deactiva-tion of catalyst was also attributed to the blockage of the activesites by adsorbed intermediates or product species, such asdiglyceride, monoglyceride, and/or glycerin. However, consider-ing that hydrated lime is slightly soluble in water (0.05 mol ofOH�/L)42 we cannot discard that the deactivation was caused bythe catalyst components’ leaching. Further study should be done.

4. CONCLUSIONS

In this work an environmental friendly process for biodieselproduction was developed. As-received hydrated lime and theproducts of its thermal decomposition were evaluated in thetransesterification of used soybean oil and methanol. It wasdemonstrated that hydrated lime did not required an activationprocess for achieving a complete conversion of the raw materialsinto biodiesel. From the XRD analysis it was confirmed that theactive phase was calcium hydroxide, so it was established thatsurface Br€onsted basic sites of Ca(OH)2 were highly active in thetransesterification reaction. Moreover, 100% conversion wasobtained up to the second reuse of the catalyst without anyspecial reactivation procedure. Due to its low cost, commercialavailability, resistance to atmospheric poisoning, and reusability,hydrated lime represents a new alternative for lowering the costof biodiesel production.

’AUTHOR INFORMATION

Corresponding Author*Phone: + (5255) 2295500, Ext. 7254. E-mail: [email protected].

’ACKNOWLEDGMENT

We thankConsejoNacional deCiencia y Tecnología, PROMEP,and Fondos Mixtos-Puebla’s Government for the financial sup-port (projects CB 2008/100355, 103.5/10/2124, and 2009-01/128348, respectively) and BUAP’s Centro Universitario deVinculaci�on for the help given in catalyst’s characterizations.

’REFERENCES

(1) Lam, M. K.; Lee, K. T.; Mohamed, A. R. Homogeneous, hetero-geneous and enzymatic catalysis for transesterification of high free fattyacid oil (waste cooking oil) to biodiesel: A review. Biotechnol. Adv. 2010,28, 500–518.

(2) C) etinkaya, M.; Karaosmano�glu, F. Optimization of base-catalyzedtransesterification reaction of used cooking oil. Energy Fuels 2004,18, 1888–1895.

(3) Kulkarni, M. G.; Dalai, A. K. Waste cooking oils—An economicalsource for biodiesel: A review. Ind. Eng. Chem. Res. 2006, 45, 2901–2913.

(4) Helwani, Z.; Othman,M. R.; Aziz, N.; Kim, J.; Fernando,W. J. N.Solid heterogeneous catalysts for transesterification of triglycerides withmethanol: A review. Appl. Catal., A 2009, 363, 1–10.

(5) Helwani, Z.; Othman,M. R.; Aziz, N.; Fernando,W. J. N.; Kim, J.Technologies for production of biodiesel focusing on green catalytictechniques: A review. Fuel Process. Technol. 2009, 90, 1502–1514.

(6) Vyas, A. P.; Verma, J. L.; Subrahmanyam, N. A review on FAMEproduction processes. Fuel 2010, 89, 1–9.

Figure 9. Effect of reaction temperature on biodiesel yield. Reactiontime, 2 h; catalyst loading, 3.6%; methanol:oil ratio (vol), 0.17.



(7) Zabeti, M.; Daud, W. M. A. W.; Aroua, M. K. Activity of solidcatalysts for biodiesel production: A review. Fuel Process. Technol. 2009,90, 770–777.(8) McNeff, C. V.; McNeff, L. C.; Yan, B.; Nowlan, D. T.; Rasmussen,

M.; Gyberg, A. E.; Krohn, B. J.; Fedie, R. L.; Hoye, T. R. A continuouscatalytic system for biodiesel production.Appl. Catal., A 2008, 343, 39–48.(9) Suppes, G. J.; Dasari, M. A.; Doskocil, E. J.; Mankidy, P. J.; Goff,

M. J. Transesterification of soybean oil with zeolite and metal catalysts.Appl. Catal., A 2004, 257, 213–223.(10) Xie, W.; Peng, H.; Chen, L. Calcined Mg�Al hydrotalcites as

solid base catalysts for methanolysis of soybean oil. J. Mol. Catal. A:Chem. 2006, 246, 24–32.(11) Campos-Molina, M. J.; Santamaría-Gonz�alez, J.; M�erida-Robles,

J.; Moreno-Tost, R.; Albuquerque, M. C. G.; Bruque-G�amez, S.; Rodrí-guez-Castell�on, E.; Jim�enez-L�opez, A.; Maireles-Torres, P. Base catalystsderived from hydrocalumite for the transesterification of sunflower oil.Energy Fuels 2010, 24, 979–984.(12) Xu, C.; Sun, J.; Zhao, B.; Liu, Q. On the study of KF/Zn(Al)O

catalyst for biodiesel production from vegetable oil. Appl. Catal., A 2010,99, 111–117.(13) Cantrell, D. G.; Gillie, L. J.; Lee, A. F.; Wilson, K. Structure

�reactivity correlations in MgAl hydrotalcite catalysts for biodieselsynthesis. Appl. Catal., A 2005, 287, 183–190.(14) Di Serio, M.; Ledda, M.; Cozzolino, M.; Minutillo, G.; Tesser,

R.; Santacesaria, E. Transesterification of soybean oil to biodiesel byusing heterogeneous basic catalysts. Ind. Eng. Chem. Res. 2006, 45,3009–3014.(15) Xi, Y.; Davis, R. J. Influence of water on the activity and stability

of activated Mg�Al hydrotalcites for the transesterification of tributyrinwith methanol. J. Catal. 2008, 254, 190–197.(16) Zabeti, M.; Daud, W. M. A. W.; Aroua, M. K. Activity of solid

catalysts for biodiesel production: A review. Fuel Process. Technol. 2009,90, 770–777.(17) Kouzu, M.; Kasuno, T.; Tajika, M.; Sugimoto, Y.; Yamanaka, S.;

Hidaka, J. Calcium oxide as a solid base catalyst for transesterification ofsoybean oil and its application to biodiesel production. Fuel 2008,87, 2798–2806.(18) Lopez, M.; Zafra, M. D.; Martín, D.; Mariscal, R.; Cabello,

F.; Moreno-Tost, R.; Santamaría, J.; Fierro, J. L.G. Biodiesel from sun-flower oil by using activated calcium oxide. Appl. Catal., A 2007, 73,317–326.(19) Cho, Y. B.; Seo, G.; Chang, D. R. Transesterification of

tributyrin with methanol over calcium oxide catalysts prepared fromvarious precursors. Fuel Process. Technol. 2009, 90, 1252–1258.(20) Bruce, K. R.; Gullet, B. K.; Beach, L. O. Comparative SO2

reactivity of CaO derived from CaCO3 and Ca(OH)2. AIChE J. 1989,35, 37–41.(21) Wu, S. F.; Beum, T. H.; Yang, J. I.; Kim, J. N. Properties of

Ca-base CO2 sorbent using Ca(OH)2 as precursor. Ind. Eng. Chem. Res.2007, 46, 7896–7899.(22) Irabien, A.; Viguri, J. R.; Ortiz, I. Thermal dehydration of

calcium hydroxide. 1. Kinetic model and parameters. Ind. Eng. Chem.Res. 1990, 29, 1599–1606.(23) Hattori, H. Heterogeneous basic catalysis. Chem. Rev. 1995,

95, 537–550.(24) Knothe, G. Monitoring a progressing transesterification reac-

tion by fiber-optic near infrared spectroscopy with correlation to 1Hnuclear magnetic resonance spectroscopy. J. Am. Oil Chem. Soc. 2000, 77(5), 489–493.(25) Azaroff, L. V.; Buerger, M. J. The Powder Method in X-ray

Crystallography; McGraw-Hill: New York, 1958.(26) Richardson, F. Analysis of hydrated lime by a thermo-chemical

method. Ind. Eng. Chem. 1927, 19, 625–629.(27) Mikhail, R. SH. Activation and sintering behavior of calcium

oxide—The effect of hydration of the surface area of the oxide producedby thermal decomposition. J. Phys. Chem. 1963, 67, 2050–2054.(28) Rouquerol, F.; Rouquerol, J.; Sing, K.Adsorption by Powders and

Porous Solids; Academic Press: San Diego, CA, 1999.

(29) S�anchez, J.; Figueras, F.; Gravelle, M.; Kumbhar, P.; Lopez, J.;Besse, J.-P. Basic properties of the mixed oxides obtained by thermaldecomposition of hydrotalcites containing different metallic composi-tions. J. Catal. 2000, 189, 370–381.

(30) Lin, R.-B.; Shih, S.-M.; Liu, C.-F. Structural properties andreactivities of Ca(OH)2/fly ash sorbents for flue gas desulfurization. Ind.Eng. Chem. Res. 2003, 42, 1350–1356.

(31) Milne, C. R.; Silcox, G. D.; Pershing, D. W.; Kirchgessner, D. A.Calcination and sintering models for application to high-temperature,short-time sulfation of calcium-based sorbents. Ind. Eng. Chem. Res.1990, 29, 139–149.

(32) Liu, X.; He, H.;Wang, Y.; Zhu, S.; Piao, X. Transesterification ofsoybean oil to biodiesel using CaO as a solid base catalyst. Fuel 2008,87, 216–221.

(33) Martín, D.; Vila, F.; Mariscal, R.; Ojeda, M.; L�opez, M.;Santamaría-Gonz�alez, J. Relevance of the physicochemical propertiesof CaO catalysts for the methanolysis of triglycerides to obtain biodiesel.Catal. Today 2010, 158, 114–120.

(34) Ngamcharussrivichai, C.; Totarat, P.; Bunyakiat, K. Ca and Znmixed oxide as a heterogeneous base catalyst for transesterification ofpalm kernel oil. Appl. Catal., A 2008, 341, 77–85.

(35) Corma, A.; Hamid, S. B. A.; Iborra, S.; Velty, A. Lewis andBr€onsted basic active sites on solid catalysts and their role in thesynthesis of monoglycerides. J. Catal. 2005, 234, 340–347.

(36) Kouzu, M.; Kasuno, T.; Tajika, M.; Yamanaka, S.; Hidaka, J.Active phase of calcium oxide used as solid base catalyst for transester-ification of soybean oil with refluxing methanol. Appl. Catal., A 2008,334, 357–365.

(37) Liu, X.; Piao, X.; Wang, Y.; Zhu, S.; He, H. Calcium methoxideas a solid base catalyst for the transesterification of soybean oil tobiodiesel with methanol. Fuel 2008, 87, 1076–1082.

(38) Reddy, C. R. V.; Oshel, R.; Verkade, J. G. Room-temperatureconversion of soybean oil and poultry fat to biodiesel catalyzed bynanocrystalline calcium oxides. Energy Fuels 2006, 20, 1310–1314.

(39) Encinar, J. M.; Gonzalez, J. F.; Rodríguez-Reinares, A. Biodieselfrom used frying oil. Variables affecting the yields and characteristics ofthe biodiesel. Ind. Eng. Chem. Res. 2005, 44, 5491–5499.

(40) Yan, S.; Kim,M.; Salley, S. O.; Ng, K.Y. S. Oil transesterificationover calcium oxides modified with lanthanum. Appl. Catal., A 2009,360, 163–170.

(41) Liu, X.; Piao, X.; Wang, Y.; Zhu, S. Calcium ethoxide as a solidbase catalyst for the transesterification of soybean oil to biodiesel. EnergyFuels 2008, 22, 1313–1317.

(42) Busca, G. Bases and basic materials in industrial and environ-mental chemistry: A review of commercial processes. Ind. Eng. Chem.Res. 2009, 48, 6486–6511.

commercial hydrated lime as a cost-effective solid base for the transesterification of wasted...

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