Fuel 93 (2012) 1–12

Contents lists available at SciVerse ScienceDirect

Fuel

journal homepage: www.elsevier .com/locate / fuel

Review article

Transesterification of vegetable oil into biodiesel catalyzed by CaO: A review

Masato Kouzu a,⇑, Jyu-suke Hidaka b

a Research Center for Fine Particle Science and Technology, Doshisha University, 4-1-1, Kizugawadai, Kizugawa, Kyoto 6190225, Japanb Department of Chemical Engineering and Material Science, Faculty of Science Engineering, Doshisha University, 1-3, Tataramiyakodani, Kyotanabe, Kyoto 6190321, Japan

a r t i c l e i n f o

Article history:Received 22 November 2010Received in revised form 1 September 2011Accepted 5 September 2011Available online 25 September 2011

Keywords:BiodieselTransesterificationSolid base catalystCalcium oxide

0016-2361/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.fuel.2011.09.015

⇑ Corresponding author. Tel.: +81 774 65 7654; faxE-mail address: mkozu@mail.doshisha.ac.jp (M. Ko

a b s t r a c t

Vegetable oil is one of the biomass resources generated from carbon dioxide and water with the aid ofphotosynthesis, and is converted into an alternative to fossil diesel fuel by transesterifying with metha-nol. The eco-friendly fuel, termed as ‘‘Biodiesel’’, is manufactured with the help of alkali hydroxide, but itshomogeneous catalysis gives rise to some technological problems: a massive amount of wastewater, soapformation and so on. Therefore, much interest has been taken in utilizing the heterogeneous catalysis ofsolid base for biodiesel production. Calcium oxide (CaO) is a candidate for the solid base catalyst from aneconomical point of view. In the present work, we reviewed CaO catalyst for the vegetable oil transeste-rification on the basis of a variety of the concerning research papers. After catalytic properties of the basicsites generated on CaO were described preliminarily, a mechanism on the vegetable oil transesterificationcatalyzed by CaO was explained. Then, procedure to prepare the active CaO catalyst, its deactivationoccurring under the reacting condition and modification of CaO catalyst were discussed. Finally, the prac-tical use of CaO catalyst for industrial biodiesel production was studied with pointing out the requiredfuture works.

� 2011 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12. Properties of basic sites on surface of CaO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23. Transesterification of soybean oil catalyzed by calcium oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44. Preparation of CaO catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55. Deactivation of CaO catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66. Modification of CaO catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77. Practical use of CaO catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88. Future study for practical use of CaO catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1. Introduction

The planet earth clearly tends toward warming clearly for thelast several decades. It is to be feared that the global warming leadsto the serious climatic change threatening human nature, so it isnecessary to make great efforts at reducing greenhouse gas emis-sions. This is a major reason that much interest has been takenin converting biomass resources, which are generated from carbon

ll rights reserved.

: +81 774 73 1902.uzu).

dioxide and water with the aid of photosynthesis, into the alterna-tive fuel and chemicals.

Vegetable oil is one of the biomass resources and is used as afeedstock of an alternative to fossil diesel fuel. The alternative fuel,which is termed as ‘‘Biodiesel’’, consists of fatty acid methyl estersproduced by transesterifying vegetable oil with methanol [1]. Inaddition to the renewable nature peculiar to the biomass re-sources, biodiesel has another advantage of the good fuel proper-ties: high flash point, good lubricity and so on [2,3]. Moreover, itwas reported that emissions of both carbon monoxide and partic-ulate was reduced by fueling the engine with biodiesel [4–6]. In2003, European Community has decided to replace at least 5.75%

2 M. Kouzu, J.-s. Hidaka / Fuel 93 (2012) 1–12

of the yearly consumed fossil fuels with biofuels, by the year 2010.This decision accelerated the use of biodiesel and its productionhas been constantly growing. The total of biodiesel yearlyproduced in the world was 7.75 million metric tons in 2008 [7].

For the existent biodiesel production process, vegetable oil istransesterified with the help of homogeneous base catalysis ofalkali hydroxide dissolved in methanol. The base-catalyzed transe-sterification is faster than the acid-catalyzed one for which sulfonicacid or p-toluenesulfonic acid is employed [8]. Furthermore, thereactants are accessible to the catalytic site in the homogeneousform. Thus, the existent biodiesel production process is character-ized by the very fast transesterification. For instance, Freedmanet al. reported that the yield of FAME produced by transesterifyingsunflower oil in the presence of 1% sodium hydroxide dissolvedinto methanol at the temperature of 333 K was above 90% for0.1 h [9]. Also, there is a research paper showing that 97% of palmoil was converted into its methyl esters by the base-catalyzedtransesterification performed at 306 K for 1 h on a continuous-flowreaction system [10]. Here, it should be noted that methanol is thealcohol appropriate to biodiesel production from an economicaland available point of view [11]. Additionally, methanol bringsabout the larger reaction rate than ethanol and propanol [12].

However, for the existent process, these are some technologicalproblems resulting in costly production of biodiesel. The typicalproblem is a massive amount of the waste water, which is due tothe purification to wash the homogeneous catalyst off the crudebiodiesel with water. And besides, emulsification of biodieseloccurs during the purifying operation, which causes not onlyobstruction of the process operation but also loss of biodiesel.

With a view of resolving the technological problems mentionedabove, Kusudiana and Saka studied a catalyst-free process in whichvegetable oil was transesterified with super-critical methanol[13,14]. It took only 4 min. to convert rapeseed oil into biodiesel,even though the high temperature (523–673 K) and high pressure(35–60 MPa) were required for making methanol reach the super-critical state. The very fast transesterification was due to the largesolubility of vegetable oil in super-critical methanol and intensifiednucleophilic nature of super-critical methanol. Apart from the cat-alyst-free process, the enzymatic process can be applied to biodie-sel production [15–17]. Although biocatalyst consisting of lipaseallows vegetable oil to be transesterified into its methyl esters atroom temperature, it takes more than 24 h to achieve the perfectconversion [18].

Solid base can leads to the heterogeneous catalytic process,which promises the cost reasonable biodiesel production. Sincethe solid base catalyst is easy to separate from the transesterifiedproduct, there is some expectation that the fixed-bed reactorsystem advantageous to process operation is applied to biodieselproduction. Also, the solid base catalyst is active in the transesteri-fication at the temperature around boiling point of methanol. Forthe purpose of studying the heterogeneous catalytic process, manyresearchers tested a variety of solid base for the catalytic activity.Xie et al. prepared potassium loaded on alumina as the solid basecatalyst and the prepared catalyst was employed for transesterifi-cation of soybean oil at reflux of methanol [19]. Under the sametransesterifying condition, they examined the solid base catalysisof the magnesium–aluminum (Mg–Al) mixed oxide that was pre-pared by calcining the corresponding hydrotalcite [20]. Shibasa-ki-Kitagawa et al. elucidated that anion-exchange resin catalyzeda transesterification of triolein which is a model of vegetable oil[21].

From the economical point of view, we have focused our atten-tions on calcium oxide (CaO) as a candidate for the solid basecatalyst. A major source of calcium oxide is limestone having theadvantages of good availability and cheap cost. Moreover, calciumoxide can be prepared from the waste matters consisting of

calcium carbonate, such as mollusk shells. The use of the wastematters is not only effective in enhancing the cost advantage ofCaO catalyst but also related to recycle of the naturally mineralresources. In addition to the economical advantage, the superiorcatalytic performance of CaO is described in a number of papersreviewing utilization of solid base for the heterogeneous catalyticreaction to produce biodiesel [22–25]. Also, there is a research pa-per in which CaO was compared with other solid bases in order toverify the superior catalytic performance [26]. CaO catalyst wasmore active in transesterification of soybean oil with methanolthan anion-exchange resin and the Mg–Al mixed oxide. Potassiumloaded on alumina resulted in the faster transesterification, butwas very inferior in the reusability to CaO catalyst.

In the present work, we reviewed CaO catalyst for the transeste-rification of vegetable oil into biodiesel in detail, on the basis of avariety of the concerning research papers. After catalytic propertiesof the basic sites generated on surface of CaO were described pre-liminarily, a mechanism on the vegetable oil transesterificationcatalyzed by CaO was explained with related to its peculiarities:variation in the catalytically active phase and the homogeneouscontribution. Then, procedure to prepare the active CaO catalyst,its deactivation occurring under the reacting condition and modi-fication of CaO catalyst were discussed. Finally, the practical useof CaO catalyst for the industrial biodiesel production was studiedwith pointing out the required future works.

2. Properties of basic sites on surface of CaO

According to a research paper reported by Iizuka et al., solidbase catalysis of CaO originates in its surface oxygen anion [27].CaO is one of alkaline earth metal oxides which are formed outof ionic crystal and Lewis acidity of the metal cation is very weakdue to its small electronegativity. Therefore, the conjugatedoxygen anion displays the strong basic property. In their researchpaper, the basic sites generated on surface of CaO were identifiedon IR spectra that were measured in the presence of acidic molec-ular probe such as benzaldehyde. The presence of the acidic molec-ular probe resulted in appearance of a new band of OH groups at3650 cm�1, in addition to a band assigned to the isolated OHgroups, which appeared at 3750 cm�1 in the absence of the acidicmolecular probe. They came into conclusion that the new band ofOH groups was due to abstraction of proton from the acidic molec-ular probe on the surface oxygen anion. Also, it seemed that theisolated OH groups functioned as the basic sites, since the corre-sponding IR band shifted toward the low wavenumber in the pres-ence of the acidic molecular probe. The experimental fact that theisolated OH groups were formed on surface of CaO calcined even invacuo was verified by XPS and LEED studies, carried out by Liu et al.[28].

The basic properties of the surface can be determined by CO2-TPD measurement, which is based on CO2 adsorption–desorptionprocess operated in vacuo. Prior to the measurement, CaO is heatedat the temperature required for the perfect evolution of CO2 andH2O, generally around 1000 K [29]. It is the scientifically commonsense that CaO is exposed to air as a result and the ambient CO2

and H2O are combined strongly with the basic sites. Therefore,the pre-treatment mentioned above is essential to collection ofthe exact CO2-TPD data. After the pre-treatment, CO2 is adsorbedon the fresh surface of CaO, and then the adsorbed CO2 is evolvedby heating up again. The total of the evolved CO2 and a tempera-ture at which the amount of the evolved CO2 is maximized corre-spond to the amount of the basic sites and the representativestrength of basic sites, respectively. Zhang et al. reported the basicproperties of alkaline-earth metal oxides from data collected byCO2-TPD measurement [30]. The strength of the basic sites was

Table 1Properties of CaO reported in a number of the research papers.

Entry Surface areaa (m2 g�1) Strength of basic siteb Crystalline sizec (nm) Source material Pre-treatment Ref.

1 6 26.5 < H_ N.A.d Commercially available CaO 1373 K [31]2 13 15.0 < H_ < 18.4 N.A.d Limestone 1173 K in helium [32]3 12 12.2 < H_ < 15.0 56 Limestone 873 K [33]4 2 9.8 < H_ < 12.2 >100 Commercially available CaO None [33]

a Calculated by BET method.b Determined by indicator method.c Determined from XRD data.d Not analyzed.

Fig. 1. Aldol addition of aldehydes catalyzed by CaO. (a) Abstraction of proton froma position of carbonyl carbon in the reactant, (b) formation of C–C bond betweenthe reactants after the nucleophilic attack of the carbonyl anion produced by theabstraction of proton and (c) proton capping with the anionic adduct.

Table 2Relations between basic properties and catalytic activity for a variety of the base-catalyzed reactions.

Entry Reaction Order of activitya Crucialproperty

Ref.

1 Hydrogenation of 1-butene MgO(1373) < CaO(1073) < SrO(1273)

Basicstrength

[34]

2 Tishchenko reaction betweenpivalaldehyde andcyclopropanecarbaldehyde

MgO (873) < CaO(873) < SrO(1273)

Basicstrength

[35]

3 Aldol addition of n-butyraldehyde

MgO (873) < CaO(873) < SrO(1073)

Basicstrength

[36]

4 Transesterification ofethylacetate with 2-propanol

MgO < CaO < SrOb Basicstrength

[37]

5 Self-Michael addition ofmethylcrotonate

CaO (673) < MgO(873)

Basicamount

[38]

6 Michael addition ofnitromethane with 2-cyclohexene-1-one

SrO (1073) < CaO(873) < MgO(873)

Basicamount

[39]

7 Nitroaldol reaction ofnitromethane withpropionaldehyde

SrO (1273) < CaO(873) < MgO(873)

Basicamount

[40]

a Value in parentheses is the calcination temperature to maximize catalyticactivity for each of the test reactions.

b Calcination temperature was not noted down in the reference.

M. Kouzu, J.-s. Hidaka / Fuel 93 (2012) 1–12 3

in the order of MgO < CaO < SrO < BaO, which agreed with a differ-ence in Lewis acidity among alkaline-earth metals. The order of theamount of the basic sites per unit weight approximately corre-sponded to that of the surface area: BaO < SrO < MgO < CaO.

Another technique to measure the basic properties is to observea change in color of acid–base indicators such as aniline and itsderivatives: the indicators are preliminarily dissolved in a hydro-phobic liquid [29]. This indicator method is characterized by theeasy discrimination, but the special device to protect the basic sitesfrom the ambient CO2 and H2O is required for collecting the exactdata. Table 1 lists the basic properties measured for CaO by theindicator method [31–33], and the listed data varies with the citedresearch paper. A maximum of 26.5 < H_ in the strength of thebasic sites generated on surface of CaO was reported by Zhu etal. [31]. For our experiment, the weakened basic sites were ob-served: 15.0 < H_ < 18.4 [32]. Probably, it was imperfect to preventsurface of CaO from coming into contact with air. In addition, thestrength of the basic sites was reduced to minimum for the com-mercially available CaO that did not undergo the activation [33].

Catalytic role of the basic sites generated on surface of CaO is toabstract proton from organic matter, which initiates the base-catalyzed reaction. One of the base-catalyzed reactions is aldoladdition, as illustrated with Fig. 1. For this reaction, the basic sitesabstract proton from a position of the reactant carbonylcompound, and then the resultant enolate serves as a nucleophileattacking carbonyl carbon in a molecule of the reactant. After the

nucleophilic attack, carbon–carbon bond is newly generated be-tween the reactant carbonyl compounds. In the case of Michaeladdition, a reaction of a,b-unsaturated carbonyl compound withnucleophile that is produced with the help of the basic sites causesformation of their 1,4-adduct. Double bond migration of olefin isinitiated by abstracting allylic proton of the reactant.

Table 2 shows relations between the basic properties and thecatalytic activity for a variety of the base-catalyzed reactions. Forappreciating the difference in the basic properties, MgO, CaO andSrO were employed for each of the base-catalyzed reactions. Asmentioned above, between these alkaline-earth metal oxides, thestrength of the basic sites is in the order of MgO < CaO < SrO. Theorder of the strength of the basic sites matched with that of thecatalytic activities for hydrogenation of 1-butene [34], Tishchenkoreaction between pivalaldehyde and cyclopropanecarbaldehyde[35], aldol addition of n-butyraldehyde [36], transesterification ofethylacetate with 2-propanol [37]. It was evident that thesebase-catalyzed reactions required the strong basic sites, so CaOcatalyst displayed the medium activity among the alkaline-earthmetal oxides. On the other hand, it seemed that the strength ofthe basic sites had no relation to catalytic activities for Michaeladdition [38,39] and nitroaldol reaction [40]. With respect to thesebase-catalyzed reactions, it was considered that the weak basicsites were active enough to abstract proton from the reactant.Probably, the reactants employed for Michael addition and nitroal-dol reaction displayed the rather high acidity [41]. Here, it should

Table 3Properties of calcium compounds active in the base-catalyzed reaction.

Surface areaa (m2 g�1) Strength of basic sitesb Ref.

Calcium oxide 13 15.0 < H_ < 18.4 [27]Calcium hydroxide 11 9.3 < H_ < 15.0 [27]Calcium

glyceroxide1 9.3 < H_ < 15.0 [39]

Calcium methoxide 44 15.0 < H_ < 18.4 [39]Ca-Xc 56 9.3 < H_ < 15.0 [40]

a Calculated by BET method.b Determined by indicator method.c Estimated chemical formula is CH3O–Ca–O(OH)2C3H5.

<Initial active phase>CaO (hydrated layer ?)

Ca(OCH3)2 producedon surface of CaO

or

<Final active phase>CH3O-Ca-[O(OH)2C3H5]produced on Ca-Gly

Reaction time

FAM

E y

ield

Fig. 2. Variation in active phase of CaO catalyst under transesterifying condition.

4 M. Kouzu, J.-s. Hidaka / Fuel 93 (2012) 1–12

be noted that each of alkaline-earth metal oxides used as the solidbase catalyst was calcined at the different temperature for maxi-mizing the catalytic activity for each of the test reactions.

3. Transesterification of soybean oil catalyzed by calcium oxide

For transesterification of vegetable oil with methanol, which is areaction to produce biodiesel, the catalytically basic sites allowmethanol to be transformed into nucleophile that attacks carbonylcarbon in a molecule of glycerides [37]. When CaO, MgO and SrOwere employed for the transesterification of soybean oil, the cata-lytic activity was in the order of MgO < CaO < SrO [32]. Therefore, itseemed that the strong basic sites were required for catalyzing thevegetable oil transesterification. However, SrO causing the fastesttransesterification was inferior in the reusability, because thegreater part of SrO was dissolved into the products mixture afterthe transesterification.

Concerning mechanism on the vegetable oil transesterificationcatalyzed by CaO, Gryglewicz pointed out that the catalytically ac-tive phase was calcium methoxide (Ca-Met) produced by a reac-tion of CaO with methanol under the transesterifying condition[42]: the chemical formula of Ca-Met is Ca(OCH3)2. Actually, CaOwas transformed into Ca-Met by only stirring into methanol atroom temperature [43]. On the other hand, the experimental datafrom our research work indicated that the conversion of CaO intoCa-Met was not appreciable [44]. Although no data to characterizethe surface of CaO was collected, there was a possibility that thevegetable oil transesterification was catalyzed by the originalsurface of CaO. Table 3 shows the properties of CaO and Ca-Met.

By the way, we found some interesting data indicating that an-other calcium compound acted as the catalytically active phaseafter the appreciable amount of glycerol was by-produced: CaOreacted with glycerol under the transesterifying condition. Theresultant Ca species was identified as calcium glyceroxide(Ca-Gly), whose chemical formula is Ca[O(OH)2C3H5]2 [44]. Fur-thermore, Ca-Gly was slightly less active in the vegetable oiltransesterification than CaO. Our last research paper made it clearthat Ca-Gly reacted with methanol under the transesterifying con-dition [45]. The resultants Ca species catalyzed the test reactionheterogeneously, and was termed as ‘‘Ca-X’’. For this catalyticallyactive phase, the chemical formula was estimated as CH3O–Ca–O(OH)2C3H5. As shown in Table 3, Ca-X was characterized bythe large surface area (56 m2 g�1), whereas the strength of thebasic site was less for Ca-X (9.3 < H_ < 15.0) than for CaO(15.0 < H_ < 18.4). The weakness of the basic property would prob-ably be made up for by chemical interaction between the reactantmethanol and the structural glyceroxide anion of Ca-X: glyceroxylOH groups on the surface would probably attract the reactantmethanol through hydrogen bond. It was small wonder that theattraction of methanol led to the easy access to the structuralmethoxide anion functioning as the basic site. However, transfor-mation of Ca-Gly into Ca-X was characteristics of reversibility

depending on the by-production of glycerol: Ca-X returned to Ca-Gly near the end of the transesterification. Unlike Ca-X, Ca-Glyhad none of the heterogeneous contribution toward the base-cata-lyzed reaction. Additionally, from the strength of the basic sites onCaO and the acidity of the reactant glycerides, it was inferred thatreaction kinetics of the transesterification catalyzed by Ca-X wasaccordant to the Eley–Rideal model.

From the experimental results mentioned above, it is deducedthat the active phase of CaO catalyst varies according with the con-version ratio, as illustrated with Fig. 2. When the vegetable oiltransesterification is operated in batchwise mode, at the low con-version ratio, either Ca-Met produced by a reaction of CaO withmethanol or the original surface of CaO catalyzes the reaction.Regrettably, it seems that no one collected the crucial data to iden-tify the initial active phase. After the conversion ratio becomeshigh enough to yield the appreciable amount of glycerol, CaO istransformed into Ca-Gly, and then Ca-X generated over Ca-Glyfunctions as the solid base catalyst.

Here, it should be noted that the vegetable oil transesterifica-tion is catalyzed by not only the basic sites generated on surfaceof CaO catalyst but also the soluble substance leached away fromCaO catalyst [46]. The leaching behavior and the following homo-geneous contribution toward the vegetable oil transesterificationappear independent of type of the active phase. In conclusion,due to a combination of the variable active phase and the homoge-neous contribution, the CaO-catalyzed transesterification is toocomplicated to be understood sufficiently without the furtherstudy.

Also, the kinetic study is important to understanding of mecha-nism on the CaO-catalyzed transesterification. A key point for thekinetic study is to express a shift in the rate determining step[47]. The reaction rate that is observed at the beginning of thetransesterification depends on a rate on mass transfer of the reac-tants, because vegetable oil and methanol do not merge togetherunder the transesterifying condition. Then, an increase in the con-version ratio causes the shift in the rate determining step towardthe chemical reaction controlled regime, since FAME produced bythe transesterification serves as a co-solvent for dissolving metha-nol into vegetable oil. Veljkovic et al. used a pair of the simplemodels on the basis of pseudo-first order kinetics for their investi-gation, and showed that the used simple models were consistentwell with a set of the conversion data collected around the timewhen the rate determining step was shifted [48]. Furthermore, itwas considered that the mass transfer became faster with anincrease in the available active surface of CaO catalyst. The internalmass transfer seemed to be too slow to affect the reaction rate.Dossin et al. simulated the triolein transesterification catalyzedby MgO on a kinetic model of the Eley–Rideal type, with the aidof computations [49].

M. Kouzu, J.-s. Hidaka / Fuel 93 (2012) 1–12 5

4. Preparation of CaO catalyst

From a variety of the research works to deepen understandingof the base-catalyzed reactions, it is considered that there are threekey points to prepare the active CaO catalyst: (1) calcination tem-perature, (2) calcination atmosphere and (3) type of the sourcematerial. The calcination temperature required for maximizingthe catalytic activity of CaO varies according to type of the base-catalyzed reaction. For example, the catalytic activity for Michaeldimerization of methyl crotonate was maximized by calcining at673 K [38], while the calcination temperature required for makingit fastest to catalyze hydrogenation of 1-butene was 1073 K [34].Therefore, it is very important to elucidate the calcination temper-ature appropriate for transesterification of vegetable oil withmethanol. The calcination atmosphere is controlled appropriatelyas a matter of course, because the basic sites are poisoned by theambient CO2 and H2O. Furthermore, as you know, CaO can beprepared from a variety of Ca salts by their calcination. But, amongthem, there is an appreciable difference in the calcination temper-ature required for the conversion into CaO.

Granados et al. examined an effect of calcination on the cata-lytic activity of the commercially available CaO [46]. Primarily,they appreciated by evolved gas analysis that surface of the com-mercially available CaO was covered with both the hydrate andthe carbonate. The catalytic activity was raised by calcining at973 K in vacuo, as shown in Table 4 (Entry 1). After the calcination,it was likely that several hydroxyl layers remained on surface ofCaO catalyst, even though the carbonate was perfectly removed.At the lower temperature, a small amount of the carbonate was leftwithout thermally decomposed, which retarded the calcinationeffect. Zhu et al. investigated the influence of the calcination tem-perature in the range of 1123–1373 K, and showed the catalyticactivity was maximized at the calcination temperature rangingfrom 1173 K to 1223 K [31]. At the higher temperature, surfacearea of CaO catalyst was reduced markedly. Accordingly, care toguard the produced rock-salt crystallites from the sintering is re-quired for the calcination to activate CaO catalyst.

The influence of the calcination atmosphere was described inour previous paper [50]. Calcium carbonate, which was preparedby a reaction of calcium hydroxide with CO2, was used as thesource material and its calcination was carried out at 1173 K in aflow of helium dosed with some contaminants: CO2 (500 ppm)and H2O (saturated at room temperature). Both CO2 and H2O added

Table 4Experimental data from a variety of research papers on preparation of active CaO catalyst

Entry Source material CaO catalyst Feed

Calcination Surface areaa

1 Commercially available CaO None N.A. Sunfl973 K N.A.

2 Precipitated calcium carbonate (He)d 13 m2 g�1 Soyb(He + CO2)d N.A.(He + H2O)d N.A.

3 Calcium nitrate 873 K N.A TribuCalcium acetate 1073 K 21 m2 g�1

Calcium oxalate 1073 K 15 m2 g�1

Calcium carbonate 1173 K 11 m2 g�1

Calcium hydroxide 973 K 25 m2 g�1

4 Limestone 1073 K 14 m2 g�1 PalmDolmite 20 m2 g�1 Palm

5 Oyster shell 1273 K N.A. Soyb6 Mud crab shell 1173 K 13 m2 g�1 Palm7 Egg shell 1073 K 1 m2 g�1 Palm

a Determined by BET method, and ‘‘N.A.’’ means not analyzed.b A methanol/oil molar ratio.c Value in parentheses is the reaction time.d Gaseous matter flowing in the calcination oven is noted in parentheses, ‘‘He’’ means

into helium brought about a decrease in the catalytic activity, asshown in Table 4 (Entry 2). In comparison to H2O, CO2 deactivatedCaO catalyst seriously. With a viewing of preparing of the activeCaO catalyst, the calcination atmosphere should be controlledeither by purging with highly pure gaseous matter or byevacuating.

Cho et al. prepared CaO catalyst from calcium carbonate, -hydroxide, -acetate, -nitrate and -oxalate, for the purpose ofscreening the source material [51]. Calcium carbonate is the majorsource of CaO used in a variety of industrial field and calciumhydroxide is proper for preparation of highly pure CaO. Calciumacetate and -nitrate are convenient for preparing the supportedCaO catalyst, due to their large solubility in water. Among thesecalcium salts, the calcination temperature required for maximizingthe catalytic activity of the prepared CaO was in the order of nitrate(873 K) < hydroxide (973 K) < acetate and oxalate (1073 K) < car-bonate (1173 K). However, the nitrate source was transformed intothe least active CaO, as shown in Table 4 (Entry 3). The least activeCaO catalyst was formed out of the largest particles. On the otherhand, the most active CaO catalyst was prepared from the hydrox-ide source, and was characterized by the large surface area. Amongcalcium carbonate, -acetate and -oxalate, there was no appreciabledifference in the catalytic potentiality.

From an economical point of view, it is considered that lime-stone and its derivatives are worthy of the source material ofCaO catalyst. Ngamcharussrivichai et al. claimed that a naturalCa–Mg mixed carbonate termed as dolomite was the source mate-rial appropriate to preparation of the active CaO catalyst [52,53]. Asshown in Table 4 (Entry 4), the dolomite-derived CaO was morecatalytically active than the limestone-derived CaO. The sourcematerial was calcined at 1073 K and the thermal decompositioninto its oxide form was perfect for dolomite. In the case of lime-stone source, a portion of calcite remained after the calcination.Probably, the remaining calcite resulted in the reduced catalyticactivity.

Recently, much interest has been taken in using waste matterssuch as mollusk shell and egg shell as the source material of CaOcatalyst [54–57]. These waste shells consist of calcium carbonateand the use of them is not only effective in enhancing the costadvantage of CaO catalyst but also related to recycle of thenaturally mineral resources. However, it seemed that the oystershell and egg shell were inferior in the catalytic potentiality, asshown in Table 4 (Entry 5–7). When they were used as the source

.

stock Catalyst (wt%) Transesterification FAMEc Ref.

M/Ob Temp.

ower 1 13 333 K ca. 60% (1.5 h) [46]>90% (1.5 h)

ean 0.8 12 Reflux 93% (1 h) [50]2% (1 h)53% (1 h)

tyrin 0.1 6 333 K ca. 50% (2 h) [51]ca. 80% (2 h)ca. 80% (2 h)ca. 80% (2 h)ca. 95% (2 h)

kernel 6 30 333 K 49% (3 h) [52]kernel 6 30 333 K 98% (3 h)

ean 20 6 338 K 96% (4 h) [54]olein 5 14 338 K 98% (2.5 h) [55]olein 10 18 333 K >90% (2 h) [56]

helium gas and the calcination temperature was 1173 K.

Table 5Solubility of Ca-based catalysts into alcoholic liquid phase.

Entry Ca-compound Solubility (mg (ml-alcohol)�1) Ref.

In methanol Under reacting condition

1a CaO 0.028 0.4 [65]2b CaO 0.3 N.A. [42]

Ca-Met 0.2 N.A. [42]

6 M. Kouzu, J.-s. Hidaka / Fuel 93 (2012) 1–12

material, a very large amount of CaO catalyst was required forachieving the good reaction efficiency. On the other hand, it waslikely that the mud club shell-derived CaO was as catalyticallyactive as the natural mineral-derived CaO. The difference in thecatalytic potentiality among the waste shells corresponded to thatin the surface area of the prepared CaO: 1 m2 g�1 for the egg shell-derived CaO and 13 m2 g�1 for the mud club shell-derived CaO.

Ca(OH)2 0.1 N.A. [42]3c CaO N.A. 0.7 [61]

Ca-Gly N.A. 0.3 [61]

a Based on conductivity measured at 333 K.b The measurement procedure was not appreciable.c Filtrate containing the leached species was analyzed on an atomic absorption

spectrophotometer.

5. Deactivation of CaO catalyst

Causes for deactivation of CaO catalyst are classified generallyunder two routes: (1) the ambient CO2 and H2O are adsorbed onthe basic sites by carrying out the test reaction without the properprotection, and (2) the basic sites come into contact with H2O thatcontaminates the reactants. Also, it is considered that CaO catalystis deactivated by some causes peculiar to the transesterification ofvegetable oil for biodiesel production. One is neutralization of thebasic sites with the free fatty acids (FFA) contained in the low qual-ity oil such as waste cooking oil. Cost advantage of the low qualityoil is useful in achieving the economically reasonable biodieselproduction [58]. The other is the leaching of CaO catalyst, whichdisplays the homogeneous contribution toward the base-catalyzedtransesterification. The degree of the leaching affects the lifetimeexpectancy of CaO catalyst. And what is worse, biodiesel is con-taminated with the leached Ca species. For instance, the qualitystandard of European lays it down that calcium content of biodie-sel is limited to 5 ppm.

Our previous research paper elucidated that CaO catalyst wasrapidly deactivated by the ambient CO2 and H2O [44]: CaO catalystwas exposed to air for only 3 min after the thermal activation withthe result that the catalytic activity was reduced appreciably. Gra-nados et al. verified by some instrumental analysis that few minutewere enough to chemisorb the ambient CO2 and H2O on surface ofCaO catalyst [46]. The same deactivation was obvious for retro-al-dol reaction of diacetonealcohol, which is one of the representativebase-catalyzed reactions [59].

An influence of H2O that contaminates the reactants on the cat-alytic activity was examined by Liu et al. [60]: they employedmethanol dosed with some drop of H2O (2.03%) for the transeste-rification of soybean oil. Interestingly, the transesterifying effi-ciency was raised by the addition of H2O. From the raisedtransesterifying efficiency, it was deduced that the isolated OHgroups were dissociated from H2O with the help of the basic siteson the surface of CaO catalyst, and that the reaction was catalyzedby the isolated OH groups. In our opinion, it seems that an amountof H2O added into methanol was excessive for generating theisolated OH groups. For our research work in which the leachingof CaO catalyst was investigated, an amount of the leached Ca spe-cies increased by adding H2O into the reactant [61]. And besides, asmentioned above, the leached Ca species functions as the homoge-neous base catalyst for the vegetable oil transesterification. There-fore, it is considered that the enhancement of the transesterifyingefficiency by the addition of H2O originated in the remarkable in-crease in the leached Ca species. Actually, the similar experimentalresult was reported in our previous research paper [44]. Grade ofmethanol employed for the soybean oil transesterification changedfrom anhydrous level (H2O < 50 ppm) to regent one (H2O <1000 ppm) as a result and the catalytic activity of CaO was en-hanced appreciably.

The influence of FFA that contained in the low quality oil wasexamined by employing CaO catalyst for the transesterification ofwaste cooking oil, whose acid value was 5.1 mg-KOH g�1 [32]. Thisacid value corresponded to ca. 2.5 wt% FFA on the oil. The S-shapedcurve was plotted on a set of the FAME yields measured at an inter-val of the proper time, which indicated that the induction period

turned up for CaO catalyst. In the case of the refined high qualityoil containing a trace of FFA, the fast transesterification led to a lin-ear increase in the FAME yield. Since a large amount of the leachedCa species was contained in biodiesel deriving from waste cookingoil, it was considered that a neutralization of CaO with FFA yieldedCa–carboxylate soluble in the liquid phase consisting of the oilemulsified with methanol. Probably, the neutralization was com-pleted as a result and the fresh oxide surface came into contactwith the reactants. The similar result was described in a numberof research papers, from the experimental data collected byemploying the crude plant oils such as Jatrropha oil for theCaO-catalyzed transesterification [62–64].

The leaching of CaO catalyst is certainly related to highly polarproperty of methanol, from the scientific fact that CaO is slightlydissolved in water. It was small wonder that the leaching of CaOcatalyst do not occur in the initial oil phase. According to a researchpaper by Granados et al., an amount of the leached species was0.028 mg (ml-methanol)�1 at a temperature of 333 K based onelemental Ca, as shown in Table 5 (Entry 1) [65]. Also, there is a re-search paper showing that CaO was more soluble in methanol thanCa(OH)2 and Ca-Met (Entry 2) [42]. However, the leaching of CaOcatalyst is too complicated to be understood from only the solubil-ity of CaO in methanol, because a chemical property of the liquidphase mixed with CaO catalyst under the transesterifying condi-tion varies with an increase in the conversion ratio: FAME servesas a co-solvent for dissolving methanol into vegetable oil, whilepolarity of the alcoholic phase was raised with the by-productionof glycerol. Furthermore, as mentioned above, CaO catalyst reactswith the by-produced glycerol under the transesterifying condi-tion, and then is transformed into Ca-Gly.

For the purpose of understanding the leaching of CaO catalyst,the significant data were collected in some research works. Wemeasured the calcium content for both biodiesel and glycerol thatwere produced by the CaO-catalyzed transesterification [60]. Themeasured data were useful in evaluating the leaching of CaO cata-lyst quantitatively. As shown in Table 5 (Entry 3), the amount ofthe leached Ca-species was 0.7 mg (ml-methanol)�1 based onelemental Ca. Also, the leached Ca-species was quantified for Ca-Gly that served as a precursor of the final active phase for theCaO-catalyzed transesterification. It seemed that Ca-Gly was lesssoluble under the transesterifying condition than CaO. Althoughonly the static data were collected for our research work, it wasevident that the greater part of the leached Ca species was con-tained in the alcoholic phase containing the by-produced glycerol.

Granados et al. collected the dynamic data useful for deepunderstanding of the leaching of CaO catalyst, by their originaltechnique: the real leaching was evaluated by measuring conduc-tivity directly for a heterogeneous biodiesel–glycerol–methanolmixture into which CaO catalyst was stirred at 333 K [65]. Underthe measurement condition, the amount of the leached Ca species

M. Kouzu, J.-s. Hidaka / Fuel 93 (2012) 1–12 7

was 0.4 mg (ml-methanol)�1 based on elemental Ca, as shown inTable 5 (Entry 1). Their dynamic data elucidated that the leachingof CaO catalyst was promoted by an increase in the reactiontemperature, and that presence of biodiesel was effective in guard-ing CaO catalyst from the leaching. And what was more important,the homogeneous contribution toward the base-catalyzed transe-sterification was quantitatively evaluated: the homogeneous con-tribution deriving from the leached Ca species was much smallerthan the heterogeneous one arising from the basic sites, providedthat the catalyst loading was larger than 1 wt%. In addition, theamount of the leached Ca species was less for their research workthan for ours. The leached Ca species and its original solid formseparated out by filtration for our experiment, so there was possi-bility that a portion of solid form was fine enough to pass the filter.

6. Modification of CaO catalyst

CaO is superior to other solid bases in the catalytic performancefor transesterification of vegetable oil with methanol, as men-tioned above. But, the CaO-heterogeneous catalytic transesterifica-tion is much slower than the homogeneous catalytic reactionwhich is applied to the existent biodiesel production. Assumingthat the base-catalyzed transesterification is accordant to the pseu-do-first order kinetics, the reaction rate constant is much larger foralkali hydroxide (0.077 min�1) than for CaO catalyst (0.044 min�1)[32]. Therefore, many researchers who take interest in utilizingCaO catalyst for biodiesel production have been making great ef-fort to raise the catalytic activity. Also, it is important to enhancetolerance to the ambient CO2 and H2O, which poisons the catalyt-ically basic sites. Furthermore, the modification to guard CaO cat-alyst against the leaching should be studied.

Table 6 summarizes a variety of the modification studies forraising the catalytic activity of CaO. One of the modifying ap-proaches is to dope CaO catalyst with alkali-metal [66–69]. Forthe modification study by Watkins et al., the doped CaO catalystwas prepared by loading a proper amount of lithium nitrate withits aqueous solution onto surface of CaO, and then was employedfor the test reaction without calcining at the high temperature[66]. Data from the test reaction showed that the lithium dopingled to enhancement of the catalytic activity of CaO. The doping ef-fect was explained as follows: lithium cation loaded onto surface ofCaO was exchanged for the lattice calcium cation, which broughtabout generation of the oxygen anion vacancies. In addition tothe cation-exchange mentioned above, the defect sites were

Table 6Experimental data from a variety of research papers on modification of CaO catalyst.

Entry Modification CaO catalyst

Specification

1 Alkali metal doping No Li-doping, no calcination1.23 wt%-Li, no calcination

2 Alkali metal doping No calcination (4.5 wt%-Li)Calcined at 773 K (4.5 wt%-Li)

3 Nano-sizing Calcined limestone, 12 m2 g�1

Nano-sized CaO from hydrate4 Nano-sizing 6 wt%-CaO on SBA-15 supporti

14 wt%-CaO on SBA-15 suppor5 Combination with metal oxide No combination

Ca–La mixed oxide, Ca/La = 3/16 Combination with metal oxide No combination

Ca–Zn mixed oxide, Ca/Zn = 1/17 Pasting biodiesel on surface No pasting

Pasting 3%-biodiesel8 Surface regeneration Commercially available CaO

Stirring into methanol for 1 h

a Value in parentheses is the reaction time.

yielded by a reaction of lithium cation with surface of CaO as a re-sult and the isolated OH groups were formed at the defect sites.Due to the oxygen anion vacancy and the isolated OH groups, thebasic properties were intensified for the doped CaO catalyst. Theproper amount of the loaded lithium cation was 1.23 wt% onCaO, at which the catalytic activity was maximized. Meher et al. re-ported that the doping effect on CaO catalyst was largest for lith-ium among alkali metals [67]. Alonso et al. appreciated that thecatalytic activity of the doped CaO was raised by calcining at thehigh temperature [68]. The proper calcination temperature, whichled to the maximized catalytic activity, was around 773 K. Thedoped CaO catalyst was calcined at 973 K with the result that thehomogeneous contribution originating in the leached lithium spe-cies was crucial for the transesterifying efficiency.

A number of the research groups studied preparation of nano-sized CaO, with a view of raising the catalytic activity [33,70,71].It is small wonder that the large surface area, which is characteris-tic of nano-sized material, results in an increase in the amount ofthe catalytically basic sites. Yoosuk et al. described their own pro-cedure to prepare the nano-sized CaO in detail [33]. Limestone wasselected as the source material of the nano-sized CaO, and was pre-liminarily turned into the hydroxide precursor by calcination fol-lowed by hydration. As mentioned above, for CaO catalyst, theuse of the hydroxide precursor is effective in increasing the surfacearea. Finally, the hydroxide precursor was transformed into thenano-sized CaO by calcining at 873 K. As compared to the denseCaO (12 m2 g�1,) which was prepared by calcining as-receivedlimestone, the nano-sized CaO (25 m2 g�1) led to the fast transeste-rification. Additionally, their CO2-TPD measurement verified thatthe larger amount of the basic sites was generated on the nano-sized CaO than the dense CaO. The much finer particles of CaOwere tested for the catalytic activity in the research work per-formed by Reddy et al. (90 m2 g�1) [70] and Verziu et al.(74 m2 g�1) [71].

Also, the nano-sized CaO can be prepared with the aid of theimpregnation technique: the proper Ca salt is loaded in its solutionform onto the porous supporting material, and then the resultantprecursor is calcined at the temperature appropriate for conversioninto CaO. There is some expectation that the nano-sized pores ofthe supporting material functioned as a mold to limit the particlesize of CaO. Albuquerque et al. used SBA-15, which is one of mes-oporous silica-based materials, as the supporting material for pre-paring nano-sized CaO [72]. SBA-15 was so thermally stable that itsmesoporous structure was not collapsed at the temperature re-quired for transforming the loaded Ca salts into CaO. On surface

Transesterification Ref.

Feedstock FAMEa

Tributyrin <5% (10 min) [66]ca. 100% (10 min)

Sunflower <5% (1 h) [68]ca. 90% (1 h)

Palm olein ca. 80% (1 h) [33]source 25 m2 g�1 ca. 95% (1 h)ng material Ethyl butyrate ca. 20%(1 h) [72]ting material ca. 40% (1 h)

Soybean ca. 10% (1 h) [64](mol/mol) >90% (1 h)

Palm kernel >90% (3 h) [80](mol/mol) >90% (3 h)

Sunflower ca. 10% (2 h) [84]ca. 80% (2 h)

Rapeseed <5% (3 h) [86]ca. 60% (3 h)

8 M. Kouzu, J.-s. Hidaka / Fuel 93 (2012) 1–12

of SBA-15, however, CaO particles were formed into aggregateswhose size distribution was within the range of 0.5–3.0 lm.Although there was none of the nano-sized CaO dispersed on thesurface with the help of the porosity of the supporting material,it seemed that SBA-15 provided the stability against the leachingfor CaO catalyst. In their manner to test for the leaching, the solidbase catalyst was preliminarily stirred into methanol, and then themethanolic solution was collected by filtration. Finally, the testreaction was carried out in the presence of the methanolic solution,instead of its original solid form. Since the methanolic solution didnot catalyze the test reaction, they concluded that none of the lea-ched Ca species was contained in the methanolic solution. Addi-tionally, silica-gel, alumina and zeolite, which are the industriallyavailable material, were used as the support to mold the nano-sized CaO [73–77]. For these supporting materials, it was not re-ported that CaO particles formed on the surface was guarded fromthe leaching.

There are some researchers who tested the mixed oxide pre-pared by combining CaO with other metal oxide, with a view ofenhancing the catalytic activity [64,78–83]. Yan et al. combinedCaO with lanthanum oxides (La2O3), with the precipitation tech-nique: lanthanum nitrate and calcium acetate merging togetherin the aqueous solution was carbonated for yielding their precipi-tate. Then, the precipitated carbonate was turned into the Ca–Lamixed oxide by calcination [64,78]. The Ca–La mixed oxide wassuperior to both CaO and La2O3 in the catalytic activity. The com-bination of CaO with La2O3 intensified the basic properties: a largeramount of the stronger base sites were generated for the Ca–Lamixed oxide. Their unique technique termed as ammonia–etha-nol–CO2 precipitation allowed La-species to be highly dispersedin CaO solid phase. On the other hand, it was reported that the bothCa–Zn and Ca–Mg mixed oxides had the advantage of good stabil-ity against the leaching that occurs under the transesterifying con-dition, but any cause for the appearance of the good stability wasnot reasoned for each of the mixed oxide catalysts [80–82]. Hsinet al. prepared calcium containing silicate by mixing CaO with tet-raethylorthosilicate in the aqueous phase [83]. Although the pre-pared Ca-silicate catalyzed the transesterification without anycalcination, very large amounts of the catalyst (20 wt% on the oil)and methanol (530 of the methanol/oil molar ratio) were em-ployed for the test reaction.

Next, significant attempts to protect the basic sites on the sur-face of CaO catalyst from ambient CO2 and H2O are introduced[84,85]. Granados et al. was successful in protecting the basic sitesby pasting up a small amount of biodiesel on CaO catalyst [84].According to their successive technique, the catalytically basic siteswas not deteriorated even after the biodiesel–CaO paste was ex-posed to air for 24 h. Moreover, the catalytic activity of CaO wasremarkably enhanced by pasting biodiesel. Methanol and vegeta-ble oil was inferior to biodiesel in the pasting effect, so they inves-tigated origin of the superior pasting effect of biodiesel in detail[85]. From the detail investigation, it seemed that minute amountsof mono-glycerides, di-glycerides and glycerol, which were presentin biodiesel, caused the superior pasting effect. Mono-glyceridesand di-glycerides were converted into glycerol on surface of CaOwith the aid of methanol, and then the produced glycerol reactedwith CaO. The glycerolysis of CaO resulted in formation of Ca-Gly, which not only was easy to change into the real active phaseby coming into contact with methanol but also was difficult tobe contaminated with CO2 and H2O, as shown in our previous re-search papers [44,45].

In the case that CaO is unfortunately deactivated by exposing toair, it is possible to regenerate the catalytic sites by a reaction ofthe fouled surface with methanol [86–89]. Kawashima et al. foundthat a commercially available CaO, whose surface was fouled withthe ambient CO2 and H2O, could be activated by only stirring it into

methanol at 298 K for 1 h [86]. By the regenerating operation, sur-face of the catalyst consisted of calcium methoxide that is catalyti-cally active in the transesterification of vegetable oil. For thecatalytic behavior of calcium methoxide, it was reported that thehomogeneous contribution toward the base-catalyzed reactionwas negligible [87]. The same regenerating effect would possibly ap-pear by stirring the deactivated CaO catalyst into ethanol [88,89].

7. Practical use of CaO catalyst

The heterogeneous catalytic process applied to the industrialbiodiesel production is only the Esterfif™M process, whose outlinewas described in a research paper by Bournay et al. [90]. Funda-mentals on the heterogeneous catalytic process were studied bythe French Institute of Petroleum (IFP) and technologies for thepractical use were developed by Axens. The Zn–Al mixed oxide isused as the heterogeneous catalyst, which is packed into twofixed-bed reactor located in a stream of the continuous-flowtransesterifying system. According to a review paper by Yan et al.[25], the heterogeneous catalytic transesterification is operated atthe temperature ranging from 483 K to 523 K under pressure be-tween 3 and 5 MPa with the liquid hourly space velocity of 0.3–3 h�1. Excess of methanol is removed after each reactor by partialevaporation, and then esters and glycerol separate in settlers. Forthe produced biodiesel, FAME content is above 98%. The Esterfif™process was first used in an industrial context in 2006, by Sofipro-teol in Sete.

Concerning CaO catalyst, the practical use for the industrial bio-diesel production was studied from data collected by operating thelaboratory scale pilot plant [91]. The employed CaO catalyst wasprepared from limestone whose particle size ranged from 1.0 mmto 1.7 mm, and was packed into a fixed-bed reactor located inthe circulating stream of the batch transesterifying system. Thelaboratory scale pilot plant was operated at 333 K under atmo-spheric pressure as a result and the yield of FAME from rapeseedoil reached around 97% with the catalytic contact time of 2 h.The reason for the circulating flow was applied to the laboratoryscale pilot plant was to make it possible that the emulsified reac-tants came in contact with CaO catalyst for the sufficient time atthe large feeding rate. The fast feeding is required for keepingthe emulsification enough to reduce the mass transfer resistancearound CaO catalyst. The reactants do not merge together underthe transesterifying condition of our process: vegetable oil wastransesterified at 333 K under atmospheric pressure. In the caseof continuous-flow transesterifying system, there is a need of thevolumetrically huge reactor for achieving both the sufficient con-tact time and the fast feeding. Thus, our process mentioned aboveis appropriate to utilization of CaO catalyst for biodiesel produc-tion. Table 7 summarizes feature of our CaO-heterogeneous cata-lytic process, as compared to the industrial Esterfif™ process.

For our process to utilize CaO catalyst, after the transesterifyingoperation, the product mixture is moved to the purifying vessel forremoving excess of methanol and the by-produced glycerol. Inaddition to the mild transesterifying condition, the simplified sys-tem is characteristics of our process. But the batch transesterifyingsystem is disadvantageous to the mass production. In the case thatthe large numbers of the oil gathering points with the small capac-ity are distributed over the extensive area, our CaO-heterogeneouscatalytic process would probably allow the reasonable biodieselproduction to be achieved on sites. On the other hand, it was evi-dent that the industrial Esterfif™ process is proper for mass pro-duction of biodiesel, due to the good productivity originating inthe continuous-flow transesterifying system.

Here, it should be noted that the leaching of CaO catalyst is amajor problem obstructing the practical use for the industrial

Table 7Industrial Esterfif™ heterogeneous catalytic process vs. laboratory CaO-heterogeneous catalytic process.

Industrial Esterfif™ Laboratory CaO

Catalyst Zn–Al mixed oxide Calcined lime stone (size distribution: 1.0–1.7 mm)Reactor Fixed-bed reactor (continuous-flow reaction system) Fixed-bed reactor (batch reaction system with circulating stream)Reacting

conditionTemperature: 483–503 K Temperature: 333 K

Pressure: 3–5 MPa Pressure: atmospheric pressureLiquid space hourly velocity: 0.3–3 h�1 Catalyst contact time: 2 h

Feature Good productivity proper for mass production Simplified system proper for on-site productionProcess

diagram

Methanol

&

Vegetable oil

RX.aRX.a

Methanol

Glycerol

Biodiesel

Biodiesel

Head

tank

Methanol

&

Vegetable oil

Purifying

vessel

Methanol

Glycerol

RX.a PL. b

a ‘‘RX’’ means fixed-bed reactor.b ‘‘PL’’ means polisher.

Pre-treatment

Biodiesel synthesis

Purification

FFA Esterification1) Optimization of reacting condition2) Screening of solid acid catalyst

Transesterification

1) Intrinsic solution for the leaching of CaO catalyst2) Enhancement of the catalytic activity

Polishing

1) Evaluation of lifetime expectancy 2) Optimization of regenerating operation

Fig. 3. Major technological elements on CaO-heterogeneous catalytic process to produce biodiesel and the assignment for the practical use in future research work.

M. Kouzu, J.-s. Hidaka / Fuel 93 (2012) 1–12 9

biodiesel production. For the major problem, one of the solutionswas verified in our previous papers: [61,91]. The verified solution

is to polish biodiesel with the absorbent for elimination of theleached Ca species. Since the leaching of CaO catalyst is not

10 M. Kouzu, J.-s. Hidaka / Fuel 93 (2012) 1–12

intrinsically resolved yet, it is certain that the transesterifying effi-ciency is reduced by repeating the transesterifying batch succes-sively for many times with reusing CaO catalyst. Actually, thegradual reduction of the transesterifying efficiency was obviousfor the laboratory scale pilot plant test, in which the transesterify-ing batch was successively repeated 17 times [91]. On the otherhand, even for the Esterfif™ process, it seems that the Zn–Al heter-ogeneous catalyst is deactivated as the operation time goes by [25].Accordingly, we come into conclusion that the CaO-heterogeneouscatalytic process is technologically feasible for the industrial bio-diesel production.

Additionally, there is an interesting attempt to apply CaO cat-alyst to the continuous-flow transesterifying system [75]. Forthis attempt, more than 92% of soybean oil was transesterifiedinto FAME at 331 K with the resident time of 2.0 h. However,it seemed that the external mass transfer around CaO catalystpacked into the fixed-bed reactor was not reasoned.

8. Future study for practical use of CaO catalyst

Due to the great efforts made by many researchers, technolog-ical elements to utilize CaO catalyst for the industrial biodiesel pro-duction seem to gradually near the practical level. In order toachieve the practical use, the assignments that are illustrated withFig. 3 must be studied for the future research work.

The urgent assignment is to complete the polishing technologyto remove the leached Ca species that contaminates biodiesel. Theperfect removal of the leached Ca species is achieved with cation-exchange resin [61,91], but the lifetime expectancy of the resinabsorbent is not appreciable yet. And besides, it is important tostudy the economical operation to regenerate the cation-exchang-ing ability crucial for the removal of the leached Ca species. Takingboth the selling price of biodiesel and the cost for procuring cation-exchange resin into consideration, the resin absorbent should bereused as long as possible with regenerated in the proper manner.

Since the removal of the leached Ca species is only the provi-sional solution, the future study on CaO catalyst must be advancedtoward the intrinsic solution of the leaching problem. Fortunately,there are a lot of the research papers showing the potential modi-fication of CaO catalyst for appearance of the good stability againstthe leaching, so it is necessary to verify the potential modificationon the basis of the experimental data collected under the real oper-ating condition. Also, scientific fundamentals on the leachingshould be examined for the future study, with a view of findingthe novel modifying approach. As a matter of course, the furtherenhancement of the catalytic activity is essential to the economi-cally reasonable biodiesel production.

Finally, it should be noted that much interest is recently takenin using the low quality plant oil, containing a large amount ofFFA that makes it serious to leach CaO catalyst, as the feedstockof biodiesel. 85% of the biodiesel production costs originate in pro-curement of the refined plant oil with the high quality, so the eco-nomical advantage of the low quality plant oil is reflectedmarkedly by the production cost of biodiesel. A technologicallyreasonable way to convert the low quality plant oil into biodieselwith the help of CaO catalyst is to combine the heterogeneous cat-alytic transesterification with the preliminarily elimination of FFA.The acid-catalyzed esterification is proper for the preliminaryelimination of FFA, so there are many researchers investigatingthe FFA esterification catalyzed by solid acid such as sulfonatedcation-exchange resin [92–94], sulfated zirconia (SO4/ZrO2), [95],tungstated zirconia (WO3/ZrO2) [96] and so on. For the purposeof raising the esterifying efficiency to the reasonable level, optimi-zation of the reacting conditionand screening of the solid acid cat-alyst should be studied for the future research works.

9. Conclusions

This review paper indicated a number of issues to utilize CaOcatalyst for biodiesel production, from a variety of the concerningresearch papers. The primary issue is to protect CaO catalyst fromthe ambient CO2 and H2O, which deteriorates the catalytically ba-sic sites. For this study assignment, some practical solutions havebeen found: pasting biodiesel on surface of CaO catalyst, regenera-tion of the active surface with methanol and so on. The secondaryissue is the catalytic activity inferior to homogeneous base such asalkali hydroxide, which is applied to the existent process for bio-diesel production. Although great efforts to enhance the catalyticactivity have been made, the further enhancement should be stud-ied for the future research work. Concerning the secondary issue,some fundamentals such as the catalytically active phase and thereaction kinetics still remain to be investigated. The latter issueis the leaching that occurs under the transesterifying condition.The potential modification to resolve the leaching problem shouldbe verified on the basis of data collected under the real transeste-rifying condition. Also, it is necessary to study fundamentals re-quired for deep understanding of the leaching of CaO catalyst.Although there is the provisional solution making it technologi-cally feasible that CaO catalyst is applied to biodiesel production,the intrinsic solution is essential to the economical reasonable bio-diesel production. Finally, it should be noted that FFA esterificationcatalyzed by solid acid catalyst is a significant technology toachieve the utilization of CaO catalyst for the industrial biodieselproduction, because much interest is recently taken in using thelow quality plant oil as the feedstock of biodiesel.

Acknowledgment

The authors gratefully acknowledge support for this research byKyoto Environmental Nanotechnology Cluster from Ministry ofEducation, Culture, Sports, Science and Technology.

References

[1] Ma F, Hanna HA. Biodiesel production: a review. Bioresour Technol1999;70:1–15.

[2] Dorado MP, Ballesteros E, Arnal JM, Gomez J, Lopez FJ. Exhaust emissions froma diesel engine fueled with transesterified waste olive oil. Fuel2003;82:1311–5.

[3] Lin CY, Lin HA, Hung LB. Fuel structure and properties of biodiesel produced byperoxidation process. Fuel 2006;86:1743–9.

[4] Karmee SK, Chadha A. Preparation of biodiesel from crude oil of Pongamiapinaanta. Bioresour Technol 2005;96:1425–9.

[5] Szybist JP, Song J, Alam M, Boehman AL. Biodiesel combustion, emissions andemission control. Fuel Process Technol 2007;88:679–91.

[6] Micheal SG, McCormick RL. Combustion of fat and vegetable oil derived fuels indiesel engines. Prog Energy Combust Sci 1998;24:125–64.

[7] Knothe G. Biodiesel: current trends and properties. Top Catal 2010;53:714–20.[8] Nimcevic D, Puntigam R, Worgetter M, Gapes JR. Preparation of rapeseed oil

esters of lower aliphatic alcohols. J Am Oil Chem Soc 2000;77:275–80.[9] Freedman B, Pryde EH, Mounts TL. Variables affecting the yield of fatty esters

from transesterified vegetable oils. J Am Oil Chem Soc 1984;61:1638–43.[10] Mao V, Konor SK, Boocoke DGB. The pseudo-single-phase, transmethylation of

soybean oil. J Am Oil Chem Soc 2004;81:803–8.[11] Formo MW. Ester reactions of fatty materials. J Am Oil Chem Soc

1954;31:548–59.[12] Xie W, Yang Z. Ba–ZnO catalysts for soybean oil transesterification. Catal Lett

2007;117:159–65.[13] Kusudiana D, Saka S. Kinetics of transesterification in rapeseed oil to biodiesel

fuel as treated in supercritical methanol. Fuel 2001;80:693–8.[14] Kusudiana D, Saka S. Effect of water on biodiesel fuel production by

supercritical methanol treatment. Bioresour Technol 2004;91:289–95.[15] Nelson LA, Foglia TA, Marmer WN. Lipase-catalyzed production of biodiesel. J

Am Oil Chem Soc 1996;73:1191–5.[16] Shimada Y, Watanabe W, Samukawa T, Sugihara TA, Noda H, Fukuda H, et al.

Conversion of vegetable oil to biodiesel using immobilized Candia antarcticalipase. J Am Oil Chem Soc 1999;76:789–93.

[17] Iso M, Chen B, Eguchi M, Kubo T, Shrestha S. Production of biodiesel fuel fromtriglycerides and alcohol using immobilized lipase. J Mol Catal B Enzym2001;16:53–8.

M. Kouzu, J.-s. Hidaka / Fuel 93 (2012) 1–12 11

[18] Oda M, Kaieda M, Hama S, Yamaji H, Kondo A, Izumoto E, et al. Facilitatoryeffect of immobilized lipase-producing Rhizopus oryzae cells on acyl migrationin biodiesel-fuel production. Biochem Eng J 2005;23:45–51.

[19] Xie W, Peng H, Chen L. Transesterification of soybean oil catalyzed bypotassium loaded on alumina as a solid-base catalyst. Appl Catal A Gen2006;300:67–74.

[20] Xie W, Peng H, Chen L. Calcined Mg–Al hydrotalcites as solid base catalysts formethanolysis of soybean oil. J Mol Catal A Chem 2006;246:24–32.

[21] Shibasaki-Kitagawa N, Honda H, Kuribayashi H, Toda T, Fukumura T,Yonemoto T. Biodiesel production using anionic ion-exchange resin asheterogeneous catalyst. Bioresour Technol 2007;98:416–21.

[22] DiSerio M, Tesser R, Pengmei L, Santacesaria E. Heterogeneous catalysts forbiodiesel production. Energy Fuels 2008;22:201–17.

[23] Zabeti M, Daud WMAW, Aroua MK. Activity of solid catalysts for biodieselproduction: a review. Fuel Process Technol 2009;90:770–7.

[24] Helwani Z, Othman MR, Aziz N, Kim J, Fernando WJN. Solid heterogeneouscatalysts for transesterification of triglycerides with methanol: a review. ApplCatal A Gen 2009;363:1–10.

[25] Yan S, DiMaggio C, Mohan S, Kim M, Salley SO, Ng KYS. Advancements inheterogeneous catalysis for biodiesel synthesis. Top Catal 2010;53:721–36.

[26] Kouzu M. Calcium oxide as a catalyst for a reaction to produce biodiesel fuelhaving an advantage of carbon neutral property. J Soc Inorg Mater Jpn2010;17:365–70.

[27] Iizuka T, Hattori H, Ohno Y, Sohma J, Tanabe K. Basic sites and reducing sites ofcalcium oxide and their catalytic activities. J Catal 1971;22:130–9.

[28] Liu P, Kendelewicz T, Brown Jr GE, Parks GA, Pianetta P. Reaction water of withvacuum-cleaved CaO (100) surfaces: an X-ray photoemission spectroscopystudy. Surf Stud 1998;416:326–40.

[29] Hattori H. Solid base catalysts: generation, characterization, and catalyticbehavior of basic sites. J Jpn Petrol Inst 2004;47:67–81.

[30] Zhang G, Hattori H, Tanabe K. Aldol addition of acetone, catalyzed by solid basecatalysts: magnesium oxide, calcium oxide, strontium oxide, barium oxide,lanthanum oxide and zirconium oxide. Appl Catal 1988;36:189–97.

[31] Zhu H, Wu Z, Chen Y, Zhang P, Duan S, Liu X, et al. Preparation of biodieselcatalyzed by solid super base of calcium oxide and its refining process. Chin JCatal 2006;27:391–6.

[32] Kouzu M, Kasuno T, Tajika M, Sugimoto Y, Yamanaka S, Hidaka J. Calcium oxideas a solid base catalyst for transesterification of soybean oil and its applicationto biodiesel. Fuel 2008;87:2798–806.

[33] Yoosuk B, Udomsap P, Puttasawat B, Krasae P. Modification of calcite byhydration–dehydration method for heterogeneous biodiesel process: theeffects of water on properties and activity. Chem Eng J 2010;162:135–41.

[34] Hattori H, Tanaka Y, Tanabe K. Hydrogenation of olefins catalyzed by alkalineearth metal oxides. Chem Lett 1975;7:659–60.

[35] Seki T, Kabashima H, Akutsu K, Tachikawa H, Hattori H. Mixed Tishchenkoreaction over solid base catalysts. J Catal 2001;204:393–401.

[36] Zhang G, Hattori H, Tanabe K. Aldol addition of butyraldehyde over solid basecatalysts. Bull Chem Soc Jpn 1989;62:2070–2.

[37] Hattori H, Shima M, Kabashima H. Alcoholysis of ester and epoxide catalyzedby solid bases. Stud Surf Sci Catal 2000;130:3507–10.

[38] Kabashima H, Tsuji H, Hattori H. Michael addition of methyl crotonate oversolid base catalysts. Appl Catal A Gen 1997;165:319–25.

[39] Kabashima H, Tsuji H, Shibuya T, Hattori H. Michael addition of nitromethaneto a,b-unsaturated carbonyl compounds over solid base catalysts. J Mol Catal AChem 2000;155:23–9.

[40] Akutsu K, Kabashima H, Seki T, Hattori H. Nitroaldol reaction over solid basecatalysts. Appl Catal A Gen 2003;247:65–74.

[41] Hattori H. Solid base catalysts: generation of basic sites and application toorganic synthesis. Appl Catal A Gen 2001;222:247–59.

[42] Gryglewicz S. Rapeseed oil methyl esters preparation using heterogeneouscatalyst. Bioresour Technol 1999;70:249–53.

[43] Yasue T, Wakui Y, Arai Y. Methoxidation of calcium hydroxide andcharacteristics of the product. Chem Soc Jpn 1981;9:1402–8.

[44] Kouzu M, Kasuno T, Tajika M, Yamanaka S, Hidaka J. Active phase of calciumoxide used as solid base catalyst for transesterification of soybean oil withrefluxing methanol. Appl Catal A Gen 2008;334:357–65.

[45] Kouzu M, Hidaka J, Wakabayashi K, Tsunomori M. Solid base catalysis ofcalcium glyceroxide for a reaction to convert vegetable oil into its methylesters. Appl Catal A Gen 2010;390:11–8.

[46] Granados ML, Poves MDZ, Alonso DM, Mariscal R, Galisteo FC, Moreno-Tost R,et al. Biodiesel from sunflower oil by using activated calcium oxide. Appl CatalB Environ 2007;73:317–26.

[47] Vicente G, Martinez M, Aracil J, Esteban A. Kinetics of sunflower oilmethanolysis. Ind Eng Chem Res 2005;44:5447–54.

[48] Veljkovic VB, Stamenkovic OS, Todorovic ZB, Lazic ML, Skela DU. Kinetics ofsunflower oil methanolysis catalyzed by calcium oxide. Fuel2009;88:1554–62.

[49] Dossin TF, Reyniers MF, Berger RJ, Marin GB. Kinetics of heterogeneously MgO-catalyzed transesterification. Appl Catal B Environ 2006;61:35–45.

[50] Kouzu M, Umemoto M, Kasuno T, Tajika M, Aihara Y, Sugimoto Y, et al.Biodiesel production from soybean oil using calcium oxide as a heterogeneouscatalyst. J Jpn Inst Energy 2006;85:135–41.

[51] Cho YB, Seo G, Chang DR. Transesterification of tributyrin with methanol overcalcium oxide catalysts prepared from various precursors. Fuel ProcessTechnol 2009;90:1252–8.

[52] Ngamcharussrivichai C, Wiwatnimit W, Wangnoi S. Modified dolomites ascatalysts for palm kernel oil transesterification. J Mol Catal A Chem2007;276:24–33.

[53] Ngamcharussrivichai C, Nunthasanti P, Tanachai S, Bunyakiat K. Biodieselproduction through transesterification over natural calciums. Fuel ProcessTechnol 2010;91:1409–15.

[54] Nakatani N, Takamori H, Takeda K, Sakugawa H. Transesterification of soybeanoil using combusted oyster shell waste as a catalyst. Bioresour Technol2009;100:1510–3.

[55] Boey PL, Maniam GP, Hamid SA. Biodiesel production via transesterification ofpalm olein using waste mud crab (Scylla serrara) shell as a heterogeneouscatalyst. Bioresour Technol 2009;100:6362–8.

[56] Viriya-empikul N, Krasae P, Puttasawat B, Yoosuk B, Chollacoop N,Faungnawakij K. Waste shells of mollusk and egg as biodiesel productioncatalysts. Bioresour Technol 2010;101:3765–7.

[57] Wei Z, Xu C, Li B. Application of waste eggshell as low-cost solid base catalystfor biodiesel production. Bioresour Technol 2009;100:2883–5.

[58] Kulkarni MG, Dalai AK. Waste cooking oil – an economical source for biodiesel:a review. Ind Eng Chem Res 2006;45:2901–13.

[59] Tanabe T, Fuluda Y. Basic properties of alkaline earth metal oxides and theircatalytic activity in the decomposition of diacetone alcohol. React Kinet CatalLett 1974;1:21–4.

[60] Liu X, He H, Wang Y, Zhu S, Piao X. Transesterification of soybean oil tobiodiesel using CaO as a solid base catalyst. Fuel 2008;87:216–21.

[61] Kouzu M, Yamanaka S, Hidaka J, Tsunomori M. Heterogeneous catalysis ofcalcium oxide used for transesterification of soybean oil with refluxingmethanol. Appl Catal A Gen 2009;355:94–9.

[62] Wen L, Wang Y, Lu D, Hu S, Han H. Preparation of KF/CaO nanocatalyst and itsapplication in biodiesel production from Chinese tallow seed oil. Fuel2010;89:2267–71.

[63] Taufiq-Yap YH, Lee HV, Hussein MZ, Yunus R. Calcium-based mixed oxidecatalysts for methanolysis of Jatropha curcas oil to biodiesel. BiomassBioenergy 2011;35:827–34.

[64] Yan S, Kim M, Salley SO, Ng KYS. Oil transesterification over calcium oxidesmodified with lanthanum. Appl Catal A Gen 2009;360:163–70.

[65] Granados ML, Alomso DM, Sadaba I, Mariscal R, Ocon P. Leaching andhomogeneous contribution in liquid phase reaction catalyzed by solids: thecase of triglycerides methanolysis using CaO. Appl Catal B Environ2009;89:265–72.

[66] Watkins RS, Lee AF, Wilson K. Li–CaO catalyzed tri-glyceridetransesterification for biodiesel applications. Green Chem 2004;6:335–40.

[67] Meher LC, Kulkarni MG, Dalai AK, Naik SN. Transesterification of karanja(Pongamia pinnata) oil by solid base catalysts. Eur J Lipid Sci Technol2006;108:389–97.

[68] Alonso DM, Mariscal R, Granados ML, Maireles-Torres P. Biodiesel preparationusing Li/CaO catalysts: activation process and homogeneous contribution.Catal Today 2009;143:167–71.

[69] Kumar D, Ali A. Nanocrystalline lithium ion impregnated calcium oxide asheterogeneous catalyst for transesterification of high moisture containingcotton seed oil. Energy Fuels 2010;23:2091–7.

[70] Reddy CRV, Oshel R, Verkade JG. Room-temperature conversion of soybean oiland poultry fat to biodiesel catalyzed by nanocrystalline calcium oxides.Energy Fuels 2006;20:1310–4.

[71] Verziu M, Coman SM, Richards R, Parvulescu VI. Transesterification ofvegetable oils over CaO catalysts. Catal Today 2011;167:64–70.

[72] Albuquerque MCG, Jimenez-Urbistondo I, Santamaria-Gonzalez J, Merida-Robles JM, Moreno-Tost R, Rodriguez-Castellon E, et al. CaO supportedmesoporous silicas as basic catalysts for transesterification reactions. ApplCatal A Gen 2008;334:35–43.

[73] Samart C, Chaiya C, Reubroycharoen P. Biodiesel production by methanolysisof soybean oil using calcium supported on mesoporous silica catalyst. EnergyConvers Manage 2010;51:1428–31.

[74] Yamanaka S, Kouzu M, Kadota K, Shimosaka A, Shirakawa Y, Hidaka J. Catalysisby CaO/SiO2 composite particle for biodiesel production. Kagaku KougakuRonbunsyu 2007;33:483–9.

[75] Yan S, Lu H, Liang B. Supported CaO Catalysts used in the transesterification ofrapeseed oil for the purpose of biodiesel production. Energy Fuels2008;22:646–51.

[76] Umdu ES, Tuncer M, Seker E. Transesterification of Nannochloropsis oculatamicroalga’s lipid to biodiesel on Al2O3 supported CaO and MgO catalysts.Bioresour Technol 2009;100:2828–31.

[77] Zabeti M, Daud WMAW, Aroua MK. Biodiesel production using alumina-supported calcium oxide: an optimization study. Fuel Process Technol2010;91:243–8.

[78] Yan S, Kim M, Mohan S, Salley SO, Ng KYS. Effect of preparative parameters onthe structure and performance of Ca–La metal oxide catalysts for oiltransesterification. Appl Catal A Gen 2010;373:104–11.

[79] Kawashima A, Matsubara K, Honda K. Development of heterogeneous basecatalysts for biodiesel production. Bioresour Technol 2008;99:3439–43.

[80] Ngaamcharussrivichai C, Totarat P, Bunyakiat K. Ca and Zn mixed oxide as aheterogeneous base catalyst for transesterification of palm kernel oil. ApplCatal A Gen 2008;341:77–85.

[81] Rubio-Caballero JM, Santamaria-Gonzalez J, Merida-Robles J, Moreno-Tost R,Jimenez-Lopez A, Maireles-Torres P. Calcium zincate as precursor of activecatalysts for biodiesel production under mild condition. Appl Catal B Environ2009;91:339–46.

12 M. Kouzu, J.-s. Hidaka / Fuel 93 (2012) 1–12

[82] Albuquerque MCG, Azevedo DCS, Cavalcante Jr CL, Santamaria-Gonzalez J,Merida-Robles JM, Moreno-Tost R, et al. Transesterification of ethylbutyratewith methanol using MgO/CaO catalysts. J Mol Catal A Chem 2009;300:19–24.

[83] Hsin TM, Chen S, Guo E, Tsai CH, Pruski M, Lin VSY. Calcium containing silicatemixed oxide-based heterogeneous catalysts for biodiesel. Top Catal2010;53:746–54.

[84] Granados ML, Alonso DM, Aluba-rubio AC, Mariscal R, Ojeda M, Brettes P.Transesterification of triglycerides by CaO: increase of the reaction rate bybiodiesel addition. Energy Fuels 2009;23:2259–63.

[85] Granados ML, Alba-Rubio AC, Vila F, Alonso DM, Mariscal R. Surface chemicalpromotion of Ca oxide catalysts in biodiesel production reaction by theaddition of monoglycerides, diglycerides and glycerol. J Catal2010;276:229–36.

[86] Kawashima A, Matsubara K, Honda K. Acceleration of catalytic activity ofcalcium oxide for biodiesel production. Bioresour Technol 2009;100:696–700.

[87] Martyanov IN, Sayari A. Comparative study of triglyceride transesterificationin the presence of catalytic amounts of sodium, magnesium, and calciummethoxide. Appl Catal A Gen 2008;339:45–52.

[88] Liu X, Piao X, Wang Y, Zhu S, He H. Calcium methoxide as a solid base catalystfor the transesterification of soybean oil to biodiesel with methanol. Fuel2008;87:1076–82.

[89] Liu X, Piao X, Wang Y, Zhu S. Calcium ethoxide as a solid base catalyst for thetransesterification of soybean oil to biodiesel. Energy Fuels 2008;22:1313–7.

[90] Bournay L, Casanave D, Delfort B, Hillion G, Chodorge JA. New heterogeneousprocess for biodiesel production: a way to improve the quality and the value ofthe crude glycerin produced by biodiesel plants. Catal Today 2005;106:190–2.

[91] Kouzu M, Hidaka J, Komichi Y, Nakano H, Yamamoto M. A process totransesterify vegetable oil with methanol in the presence of quick lime bitfunctioning as solid base catalyst. Fuel 2009;88:1983–90.

[92] Tesser R, DiSerio M, Guida M, Nastai M, Santacesaria E. Kinetics of oleic acidesterification with methanol in the presence of triglycerides. Ind Eng Chem Res2005;44:7978–82.

[93] Russbueldt BME, Hoelderich WF. New sulfuric acid ion-exchange resins for thepreesterification of different oils and fats with high content of free fatty acids.Appl Catal A Gen 2009;365:47–57.

[94] Shibasaki-Kitagawa N, Tsuji T, Chida K, Kubo M, Yonemoto T. Simplecontinuous production process of biodiesel fuel from oil with high contentof free fatty acid using ion-exchange resin catalysts. Energy Fuels2010;24:3634–8.

[95] Ni J, Meunier FC. Esterification of free fatty acids in sunflower oil over solidacid catalysts using batch and fixed bed-reactor. Appl Catal A Gen2007;333:122–30.

[96] Park YM, Chung SH, Eom HJ, Lee JS, Lee KY. Tungsten oxide zirconia as solidsuperacid catalyst for esterification of waste acid oil (dark oil). BioresourTechnol 2010;101:6589–93.