Mixed oxides of Ca, Mg and Zn as heterogeneous base catalysts for the synthesis of palm kernel oil methyl esters

Chemical Engineering Journal 225 (2013) 616–624

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Mixed oxides of Ca, Mg and Zn as heterogeneous base catalysts for the synthesis of palm kernel oil methyl esters Sasipim Limmanee a, Thikumporn Naree a, Kunchana Bunyakiat b, Chawalit Ngamcharussrivichai b,⇑ a b

Program in Petrochemistry and Polymer Science, Faculty of Science, Chulalongkorn University, Phayathai Road, Patumwan, Bangkok 10330, Thailand Fuels Research Center, Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Phayathai Road, Patumwan, Bangkok 10330, Thailand

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Effects of type of metal compounds in

a r t i c l e

i n f o

Article history: Received 14 November 2012 Received in revised form 15 March 2013 Accepted 24 March 2013 Available online 1 April 2013 Keywords: Mixed oxides Calcium oxide Base catalyst Transesterification Fatty acid methyl esters

14 CaMgZn-09

Calcination

Rate of FAME formation/ g gcat-1 h-1

the CaMgZn precipitates were studied. 2+ 2+ 2+  Ca , Mg and Zn were mainly precipitated in forms of CaMg(CO3)2 and CaZn(CO3)2.  The mixed metal carbonates yielded highly dispersed metal oxide crystallites.  The catalyst basicity was enhanced by the formation of mixed metal carbonates.  The rate of transesterification exponentially correlated to the total basicity.

12

CaMgZn-07

10 8 CaMgZn-03

6 CaMgZn-06

4 CaMgZn-05

CaMgZn-04

2 CaMgZn-01

0 0

200

400

600

800

Basicity/ μmol g-1

a b s t r a c t A series of mixed oxides of Ca, Mg and Zn were investigated as heterogeneous catalysts for the synthesis of palm kernel oil methyl esters (FAME) via the transesterification of palm kernel oil with methanol. The mixed oxides with different elemental compositions were prepared via the pH-controlled co-precipitation using Na2CO3 as a precipitant. The effects of the precipitation conditions on the physicochemical and catalytic properties of the resulting mixed metal precipitates and oxides were studied. The structural analyses indicated that the atomic ratio of Ca:Mg:Zn, the pH, the concentration of CO2 3 , and the molar ratio of CO2 3 /metal ions essentially determined the types of metal hydroxides and carbonates in the CaMgZn mixed precipitates. The substitution of Mg2+ and Zn2+ into the CaCO3 lattice as CaMg(CO3)2 and CaZn(CO3)2, respectively, resulted in highly dispersed metal oxide crystallites in the catalysts and enhanced the base properties of the mixed oxides. The initial rate of FAME formation was well correlated with the total basicity of the oxide catalysts. The FAME yield of 97.5 wt.% was achieved over the CaMgZn mixed oxide, prepared with the Ca:Mg:Zn ratio of 3:1:1 under the Na2CO3 concentration of 0.75 mol L1 and the CO2 3 /metal ions ratio of 1.0, when the reaction conditions were the methanol/oil molar ratio of 20, catalyst amount of 6 wt.% and temperature of 60 °C. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Transesterification, an interchange of alkoxy groups, is an important reaction in the oleochemical syntheses. Fatty acid alkyl ⇑ Corresponding author. Tel.: +66 2 218 7528/7523; fax: +66 2 255 5831. E-mail address: [email protected] (C. Ngamcharussrivichai). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.03.093

esters are primary oleochemicals produced from the transesterification of triglycerides, the major components in vegetable oils and animal fats, with alcohols. For biodiesel, they are principally produced as fatty acid methyl esters (FAME) using methanol. The reaction occurs spontaneously at temperatures above 200 °C but the desired esters are not selectively generated due to the presence of residual glyceride derivatives and thermally cracked products.

S. Limmanee et al. / Chemical Engineering Journal 225 (2013) 616–624

To overcome this, homogeneous acid or base catalysts are often applied to the reaction in order to accelerate the conversion of triglycerides at relatively low temperatures (60–120 °C) and to increase the selectivity for FAME production. FAMEs derived from palm kernel oil (PKO) and coconut oil are used as intermediates for the production of fatty alcohols and biodegradable lubes [1,2]. Co-production of FAME for oleochemical synthesis and fuel applications is performed with vegetable oils containing high-molecular weight fatty acids, such as found in palm oil [3]. The typical transesterification reaction is catalyzed by NaOH or KOH and can actively produce FAME with a high yield under mild conditions. However, in the homogeneous process, removal of the soluble catalysts from the reaction mixture is difficult and generates a large amount of alkali wastewater during the product washing stage. Utilization of solid catalysts can overcome these problems since they are insoluble and present heterogeneously in the reaction mixture. Several types of solid base catalysts for the transesterification of triglycerides have been reported in the literature, such as Al2O3-supported alkaline metal oxides [4–8], alkaline earth metal oxides [9–13], silica-supported CaO [14], Li2O-doped MgO and CaO [15], MgO modified with Sr [16], supported rare earth oxides [17,18], and layered double hydroxides [19–21]. Until now the only example of a commercialized process for FAME production based on the heterogeneous catalysis route is the EsterFip-H process developed by the Institut Français du Pétrole (IFP) [22]. The transesterification is continuously performed in two-consecutive fixed bed reactors filled with a spinel oxide of Zn and Al. Due to its relatively low basicity, compared to the catalysts containing alkaline and alkaline earth metal oxides, the reaction requires a high temperature and high pressure to achieve nearly complete conversion of vegetable oil to FAME and, as a by-product, glycerol. However, the resulting FAME and glycerol are very clean, and the catalyst has a long life time. The success of the process suggests an advantage of solid catalysts in the form of mixed oxides. Mixed metal oxide catalysts have attracted an increased amount of attention because of their tunable basicity via modification of their chemical composition and synthesis procedure. Among the mixed oxides reported previously, Ca-based mixed oxides have been extensively investigated in the transesterification since, in addition to their low cost and readily available status, CaO itself possesses a high basic strength (H_ = 26.5) [23], low solubility in methanol [9], and high activity [9–13]. Peterson and Scarrach observed an improvement in the obtained FAME yield over a mixed CaOMgO catalyst [24]. Further study on the physicochemical and catalytic properties of the CaMg mixed oxides was carried out by Taufiq-Yap et al. [25]. Evaluation of the catalytic performance of different MgAl or CaMg oxides in the transesterification of sunflower oil revealed that the CaMg oxide catalysts showed a superior activity [26]. The mixed oxide of Ca and La with a Ca:La molar ratio of 3:1 methanolyzed soybean oil completely within 2 h [27], whilst the textural properties and transesterification activity were enhanced when Ca was in the form of a CaO ZnO mixed oxide [28]. The mixed CaOCeO2 oxide, which has a high stability and water tolerance, was investigated comparatively with the Ca-based perovskites, including CaTiO3, CaMnO3, and CaZrO3 [29]. In order to yield a homogeneous distribution of metals in the catalysts, the synthesis of Ca-based mixed oxides was carried out by essentially following the co-precipitation method in which the white sediment was settled in an aqueous solution of mixed metals and in the presence of hydroxide and/or carbonate of Na+ or NHþ 4 as the precipitant. The precipitates obtained can be metal hydroxides and/or metal carbonates depending on the synthesis conditions, and these are then converted to the corresponding mixed oxides

617

by thermal treatment. Cho et al. demonstrated that the type of Ca precursor, such as Ca(OOCH3)2, CaCO3, Ca(OH)2, Ca(NO3)2 and CaC2O4, affected the catalytic properties of the resulting CaO in the transesterification [30]. Our previous study suggested that the CaCO3 polymorphs in different natural Ca-based materials (cuttlebone, dolomite and calcite) influenced the methanolysis activity of the CaO catalysts attained [31]. In case of the mixed oxide catalysts, the dependence of the metal phases in the precipitates on the physicochemical and the catalytic properties of the mixed oxides have never been reported. A number of reports associated with the influences of divalent ions on the nucleation and/or the growth rate of CaCO3 polymorphs are available [32–35]. For example, Zn2+ strongly suppressed the growth of calcite by adsorption onto the kink sites, resulting in the formation of small particles [32,33], whereas Mg2+ formed a Mg-calcite structure (MgxCa1x)CO3 with variable Mg contents [33–35]. The present work reported here is an attempt to improve the catalytic performance of the mixed oxides based on Ca, Mg and Zn for the methanolysis of PKO. The effects of precipitation conditions on the physicochemical and catalytic properties of the resulting mixed metal precipitates before and after the calcination were investigated. 2. Experimental 2.1. Catalyst preparation A mixed oxide of Ca, Mg and Zn was prepared by pH-controlled co-precipitation method. Typically, the required quantities of Ca(NO3)24H2O (Ajax Finechem), Mg(NO3)26H2O (Ajax Finechem) and Zn(NO3)26H2O (Ajax Finechem) were dissolved in deionized water. The mixed metal ions were precipitated by adding an aqueous solution of Na2CO3 under vigorous stirring in which the molar 2 ratio of CO2 3 /metal ions and the concentration of CO3 were controlled in the range of 0.75–1.5 and 0.5–1 M, respectively. The pH of the resulting slurry was maintained at 7, 8, 9 or 10 using HNO3 or NaOH as appropriate. The precipitate was further aged at either room temperature or 60 °C for 20 h. Finally, the white solid was filtered, washed thoroughly with deionized water, dried in an oven at 100 °C overnight, and then calcined in a muffle furnace at 800 °C for 2 h to obtain the mixed oxides of Ca, Mg and Zn. Hereafter, the mixed metal precipitates and oxides were designated as CaMgZn-XX, where XX referred to the synthesis conditions shown in Table 1. 2.2. Characterization The mixed metal oxide structure and crystallite size of the assynthesized and calcined CaMgZn were determined by means of powder X-ray diffraction (XRD) using a Bruker D8 ADVANCE diffractometer equipped with Cu Ka radiation with a 0.02° step size range at room temperature and recording the spectra over a 2h range of 10–80°. Assignments of diffraction peaks were performed after consulting the JCPDS powder diffraction files. Elemental composition was analyzed with a Philips PW-2400 ED-2000 energy dispersive X-ray fluorescence spectrometer (XRF). Morphological study was carried out with a JEOL JSM-5410 LV scanning electron microscope (SEM). Thermogravimetric analysis (TGA) of the mixed precipitates (10 mg) was performed on a Perkin Elmer Pyris Diamond thermogravimetric machine at a temperature ramp rate of 8 °C min1 under a dry air flow at a rate of 20 mL min1. Textural properties were measured by N2 physisorption using a Micromeritics ASAP 2020 surface area and porosity analyzer. The mixed oxides were degassed at 200 °C for 2 h prior to the measurement. Calculation of the specific surface area was based on the

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Table 1 Elemental and structural analyses of as-synthesized CaMgZn precipitates prepared under various conditions.a Precipitate

CaMgZn-01 CaMgZn-02 CaMgZn-03 CaMgZn-04 CaMgZn-05 CaMgZn-06 CaMgZn-07 CaMgZn-08k CaMgZn-09 MgZn

c d e f g h i j k

0.5 0.5 0.5 0.75 0.75 0.75 0.75 1.0 1.0 0.75

c CO2 3 /metal ions

0.75 1.0 1.5 1.0 1.0 1.0 1.0 1.5 1.5 1.0

As-synthesized phasee

Ca:Mg:Zn atomic ratio Theoretical

Precipitate

1:1:1 1:1:1 1:1:1 1:1:1 1:1:3 1:3:1 3:1:1 1:1:1 1:1:1 0:1:1

8:1:9 9:1:9 3:1:4 6:1:7 1:1:4 1:1:1 19:1:7 1:1:1 1:1:1 0:1:4

d

Carbonate

f

Hydroxide

C, M, CM, CZ C, M, CM, CZ C, CM, CZ C, M, CM, CZ C, M, CM, CZ M, CM, CZ C, M, CM, CZ C, CM, CZ CM, CZ M

C, M, C, M, C, M, C, M, C, M, M, Z C, Z Z Z M, Z

d104h (Å)

a0i (Å)

2.9388 n.d.j 2.9128 2.9298 2.9360 2.8581 2.9717 2.8752 2.8890 n.d.

13.6076 n.d. 13.4872 13.5660 13.5947 13.2340 13.7600 13.3127 13.3770 n.d.

g

Z Z Z Z Z

The synthesis pH was maintained at 7. Initial concentration of Na2CO3 used in the synthesis. Molar ratio of CO2 3 /metal ions used in the synthesis. Ca:Mg:Zn atomic ratio in the final precipitates determined by XRF spectroscopy. Phase presenting in the as-synthesized precipitates determined by XRD. Metal carbonate species: C = CaCO3, M = MgCO3, CM = CaMg(CO3)2 and CZ = CaZn(CO3)2. Metal hydroxide species: C = Ca(OH)2, M = Mg(OH)2 and Z = basic Zn carbonates, typically Zn2CO3(OH)2 and Zn3CO3(OH)4. d104 represents d spacing of (1 0 4) plane. a0 represents unit cell parameter, calculated according to Eq. (1). n.d. means not determined. Synthesis mixture was aged at 60 °C for 8 h.

Braunauer–Emmett–Teller (BET) equation using the linear-relationship data attained in the P/P0 range of 0.02–0.2. The basic properties of the catalysts were investigated by the temperatureprogrammed desorption (TPD) of CO2 (CO2-TPD) measurement using a Micromeritics AutoChem II 2920 chemisorption analyzer. The mixed oxides were pretreated in situ at 500 °C for 1 h under an Ar flow (50 mL min1), after which the adsorption of CO2 (10 vol.% in Ar) was performed at 100 °C and the temperature was increased to 900 °C at 10 °C min1 for desorbing CO2. 2.3. Transesterification procedure A 100-mL three-neck round bottom flask equipped with a reflux condenser and a magnetic stirrer was used as the reactor for the transesterification of refined PKO with methanol. In a typical reaction, 0.6 g of the calcined catalyst was suspended in methanol (99.5%, commercial grade) and temperature of the mixture was controlled at 65 °C using a water bath. Then, PKO was added into the mixture under vigorous stirring. After the course of reaction, the catalyst was separated from the reaction mixture by centrifugation and the excess methanol was removed by using rotary evaporator. The composition of the FAME produced was determined by gas chromatography (GC) using a Shimadzu 14A gas chromatograph equipped with a 30-m DB-Wax capillary column and a flame ionization detector (FID). The FAME yield (wt.%) was calculated based on the external standardization method (EN14103) using methyl undecanoate (Aldrich) as the reference standard.

both parameters, the atomic ratio of the CaMgZn mixed precipitates became close to the theoretical ratios. XRD analysis revealed that, without Ca2+ in the solution, Mg2+ and Zn2+ were precipitated in the forms of MgCO3 and Mg(OH)2, and basic Zn carbonates, such as Zn2CO3(OH)2 and Zn3CO3(OH)4, respectively (Supplementary Information Fig. S1). However, in the presence of Ca2+, the main phases in the precipitates were in the forms of mixed calcium carbonates, i.e. CaMg(CO3)2 and CaZn(CO3)2 (Fig. 1). CaCO3 has three kinds of crystal polymorphs, vaterite, aragonite, and calcite, with hexagonal, orthorhombic and rhombohedral structures, respectively, of which calcite is the most stable and vaterite is the least stable phase. In this study, the carbonates of Ca were all in the stable calcite form. The broad characteristics of the XRD peaks, indicating a low crystallinity, are attributed to the incorporation of Mg2+ and Zn2+ into the CaCO3 lattice, which then impedes the crystallization into the rhombohedral

(d)

Intensity/ a.u.

a b

b 1 [CO2 3 ] (mol L )

(c)

3. Results and discussion (b)

3.1. Characterization of the CaMgZn mixed precipitates The elemental compositions of the CaMgZn mixed precipitates synthesized under different conditions are summarized in Table 1. The Ca:Mg:Zn atomic ratios found in the precipitated solids in most cases deviated from the theoretical ratios, which is attributed to the fact that Ca2+ and Mg2+ are favorably precipitated at a relatively high pH, while Zn2+ is readily solidified at a pH of 4 [28]. Under the studied conditions, the elemental composition was determined by the concentration of Na2CO3 ([CO2 3 ]) and the molar ratio of CO2 3 /metal ions used in the precipitation. With increasing

(a)

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

2θ/ ° Fig. 1. XRD patterns of the as-synthesized CaMgZn-09 precipitates prepared at pH: (a) 7, (b) 8, (c) 9 and (d) 10. (Symbols: = CaCO3, . = CaZn(CO3)2, = CaMg(CO3)2, N = Ca(OH)2, = Mg(OH)2 and  = basic Zn carbonates).

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solids (Table 1). Mixed metal carbonate phases with a high purity were attained when the CO2 concentration and the CO2 3 3 /metal ions molar ratio were 1.0 mol L1 and 1.5, respectively (CaMgZn08 and CaMgZn-09). A reduction of either the initial concentration of Na2CO3 or the CO2 3 /metal ions ratio enhanced the precipitation of various hydroxide compounds and decreased the crystallinity of the mixed metal carbonates (Supplementary Information Fig. S2). Fig. 2 shows a representative TGA profile of the as-synthesized CaMgZn precipitate in comparison with the thermal decomposition profiles of authentic CaCO3 and binary metal precipitates (CaZn and MgZn) prepared by the co-precipitation method under the similar conditions. The pure CaCO3 released CO2 via decarbonation at 785 °C giving CaO (Fig. 2A). The basic Zn carbonates in the CaZn precipitate were decomposed at 242 °C, whilst the formation of the CaZn mixed carbonates decreased the temperature required for the decarbonation to 734 °C (Fig. 2B). We postulate that the liberation of CO2 from CaCO3 was facilitated by a number of voids produced via the earlier decomposition of basic Zn carbonate species in the CaZn precipitates [28]. The mixed precipitate of Mg and Zn exhibited the weight loss in three merged steps (Fig. 2B), in which the temperatures of 235 and 318 °C were related to the basic Zn carbonates with low and high contents of carbonate group, respectively, while the weight change at 400 °C is attributed to the transformation of precipitated Mg compounds into MgO. In accordance, Sawada et al. reported that Zn2+ was precipitated as basic Zn carbonates with variable formulas of Znx(CO3)y(OH)2(xy) depending on the synthesis conditions [38], whilst the precipitation of Mg2+ in the different forms of hydrates, such as MgCO32H2O, and hydroxy carbonates, e.g. MgCO3Mg(OH)23H2O, was also found. In the case of the CaMgZn precipitate, the weight loss relating to the Mg and Zn compounds were shifted to the higher temperatures of 320 and 491 °C, respectively, but the decomposition temperature of CaCO3 was remarkably decreased to 710 °C (Fig. 2D). This observation reflected the formation of the mixed metal carbonates as CaMg(CO3)2 and CaZn(CO3)2. The XRD patterns of the calcined CaMgZn precipitates prepared under different conditions are compared in Figs. 3 and 4. The calcination at 800 °C decomposed all the precipitated carbonate and hydroxide phases entirely to the corresponding metal oxides. Note that the Ca(OH)2 that appeared in all the samples, was derived subsequently from the hydrolysis of CaO by ambient moisture (rh 70%) in the environment during the analysis. Increasing the concentration of Na2CO3 and the molar ratio of CO2 3 /metal ions resulted in an increase in the peaks related to the ZnO phase, which is the opposite of that seen for CaO and MgO (Fig. 3). Since the elemental analysis suggested the improvement nearer to the theoretical ratio of the metal composition in the mixed precipitates attained under a higher concentration of CO2 (Table 1), it was 3 likely that Ca2+ and Mg2+ were highly dispersed in the mixed oxide. This result correlates with the XRD patterns of these precipitates

structure. Indeed, Loste et al. demonstrated that the transformation of amorphous CaCO3, initially formed in the precipitates, into the crystalline phases was retarded at a high concentration of Mg2+ [35]. By using the Sherrer’s equation [36], the crystallite sizes of CaMg(CO3)2 and CaZn(CO3)2 were estimated to be 11.2 and 9.3 nm. Other possible phases observed in this study were CaCO3, MgCO3 and Ca(OH)2 (see also Table 1). At a pH of 7, where the hydroxide ion concentration is relatively low, the majority of the metal ions crystallized with carbonate ions (Fig. 1a). When the pH was increased from 7 up to 10, the formation of the metal hydroxides was enhanced concordantly with a decrease in the intensity of the diffraction peaks corresponding to the mixed calcium carbonates, which is due to the increasing concentration of hydroxide ions (Fig. 1b–d). Moreover, synthesis from a higher pH resulted in a divergence of the Ca:Mg:Zn atomic ratio of the precipitates from the theoretical ratio, and facilitated the generation of CaCO3 as a separate phase (Table 2). A preliminary evaluation of the transesterification activity of these mixed precipitates after the calcination at 800 °C indicated that the CaMgZn mixed oxide catalyst derived from precipitation at pH 7 gave the highest yield of FAME, and increasing the precipitation pH from 7 to 10 reduced the amount of FAME formed. Consequently, pH 7 was selected as the most suitable for the synthesis of the CaMgZn mixed precipitates. Table 1 summarizes the structural analysis results of the assynthesized CaMgZn precipitates prepared under various conditions, whilst the relevant representative XRD patterns are shown in the Supplementary Information (Figs. S2 and S3). The degree of Mg2+ and Zn2+ substitution into the lattice of CaCO3 to form the mixed metal carbonates was partly reflected by a shift in the XRD peak at 2h  30.2°, corresponding to the (1 0 4) plane of the CaCO3 crystal with a rhombohedral polymorph, to higher diffraction angles, since Mg2+ (0.89 Å) and Zn2+ (0.90 Å) are smaller than Ca2+ (1.23 Å). Furthermore, a shrinkage of the d spacing (d104) and the unit cell parameter (a0) of the rhombohedral lattice was inevitable. The unit cell parameter can be calculated according to Eq. (1) in which the rhombohedral angle (a) was assumed to be 102° [37].

a20 2

d104

2

¼

2

2

2

ðh þ k þ l Þsin a þ 2ðhk þ kl þ hlÞðcos2 a  cosaÞ ð1  3cos2 a þ 2cos3 aÞ

ð1Þ

Pure CaCO3 possessed a unit cell size of 14.053 Å and the formation of CaMg(CO3)2 and CaZn(CO3)2 phases in the precipitates reduced the d-spacing and the unit cell parameter (Table 1). Broadly speaking, the more Mg2+ and Zn2+ that were incorporated into the structure of CaCO3, the more the unit cell size was decreased. The initial concentration of Na2CO3 and the molar ratio of CO2 3 / metal ions influenced the types of metal species in the precipitated

Table 2 Preliminary results of the structure analysis and reaction test obtained from the CaMgZn precipitates synthesized at different pH.a Precipitate

CaMgZn-09

Synthesis pH

7 8 9 10

As-synthesized phasec

Ca:Mg:Zn molar ratio b

FAME yieldd (wt.%)

Theoretical

Precipitate

Carbonate

Hydroxide

1:1:1 1:1:1 1:1:1 1:1:1

1:1:1 2:1:2 2:1:3 2:1:2

CM, CZ C, CM, CZ C, CM, CZ C, CM, CZ

Z C, M, Z C, M, Z C, M, Z

91.4 80.2 72.0 5.1

1 HNO3 or NaOH was used for the pH adjustment. Initial concentration of Na2CO3 and molar ratio of CO2 and 1.5, respectively. 3 / metal ions were maintained at 1.0 mol L Ca:Mg:Zn molar ratio in the final precipitate, as determined by XRF spectroscopy. c Phases present in the as-synthesized catalysts, as determined by XRD. See Table 1 for details. d Yield of fatty acid methyl esters. Prior to use in the reaction, the precipitates were calcined at 800 °C for 2 h. Transesterification conditions: catalyst amount, 6 wt.%; methanol/oil molar ratio, 20; temperature, 65 °C; time, 3 h. a

b

S. Limmanee et al. / Chemical Engineering Journal 225 (2013) 616–624

950

785° C

(A)

850

0

450

-10

550 -20

450 350 150

734° C

0 -10

350 242° C

250

-20

150

-30

-40

50

-40

-50

-50

-30

250

10

(B)

Weight loss/ %

650

550

Weight loss/ %

DTG/ μg min-1

750

10

DTG/ μg min-1

620

50 -50

300

300 600 Temperature/ °C

10

(C)

318° C

-20

400° C

-30

50 0 -50 0

300 600 Temperature/ °C

900

10

(D) 491° C 710° C

0

200

-10

150

320° C

-20 100 -30

50

-40

0

-50

-50

900

Weight loss/ %

-10 235° C

100

300 600 Temperature/ °C

250

0

200 150

300

Weight loss/ %

DTG/ μg min-1

250

-50 0

900

DTG/ μg min-1

0

-40 -50 0

300 600 Temperature/ °C

900

Intensity/ a.u.

Intensity/ a.u.

Fig. 2. Weight loss (TG) and DTG curves of (A) CaCO3, and the as-synthesized mixed metal precipitates of: (B) CaZn, (C) MgZn and (D) CaMgZn.

(c)

(d)

(c)

(b) (b)

(a) 10

15

(a)

20

25

30

35

40

45

50

55

60

65

70

75

80

2θ/ ° Fig. 3. XRD patterns of the calcined CaMgZn precipitates prepared with a Ca:Mg:Zn ratio of 1:1:1 under different Na2CO3 concentrations and CO2 3 /metal ions molar ratios of: (a) CaMgZn-01 (0.5 mol L1; 0.75), (b) CaMgZn-03 (0.5 mol L1; 1.5) and (c) CaMgZn-09 (1.0 mol L1; 1.5). (Symbols: s = CaO, N = Ca(OH)2, = MgO and } = ZnO).

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

2 /° Fig. 4. XRD patterns of the calcined CaMgZn precipitates prepared from a Na2CO3 concentration of 0.75 mol L1 and a CO2 3 /metal ions ratio of 1.0 with Ca:MgZn molar ratios of: (a) CaMgZn-04 (1:1:1), (b) CaMgZn-07 (3:1:1), (c) CaMgZn-06 (1:3:1) and (d) CaMgZn-05 (1:1:3). (Symbols: s = CaO, N = Ca(OH)2, = MgO and } = ZnO).

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S. Limmanee et al. / Chemical Engineering Journal 225 (2013) 616–624 Table 3 Physicochemical and catalytic properties of various CaMgZn precipitates after the calcination at 800 °C for 2 h.

a b c d e f g

Catalyst

Ca fractiona

Particle sizeb (lm)

CaMgZn-01 CaMgZn-02 CaMgZn-03 CaMgZn-04 CaMgZn-05 CaMgZn-06 CaMgZn-07 CaMgZn-08 CaMgZn-09 MgZn

0.44 0.46 0.36 0.46 0.22 0.32 0.70 0.34 0.36 –

0.2–4.5 0.4–4.2 0.7–4.0 0.4–5.0 0.2–0.3 0.2–1.6 0.8–3.4 0.2–1.2 1.1–3.8 0.2–0.5

Crystallite sizec (nm) CaO

MgO

ZnO

46.1 44.0 43.6 45.0 52.1 51.5 56.8 40.4 38.1 –

44.6 45.4 37.9 44.6 44.7 46.9 39.4 36.8 – 39.3

42.2 43.9 44.3 45.4 50.8 40.6 40.7 53.5 54.2 45.7

Basicityd (lmol g1)

Initial rate of FAME formatione (gFAME gcat1 h1)

FAME yieldf (wt.%)

139 n.d.g 469 449 198 382 563 n.d. 712 –

1.17 n.d. 6.86 3.45 2.76 3.80 10.93 11.02 12.83 n.d.

16.0 29.5 88.6 47.8 87.1 55.2 97.5 89.5 91.4 3.1

Atomic fraction of Ca presenting in the final precipitate. Determined by SEM technique. Calculated from XRD pattern using Sherrer’s equation. Total basicity determined by temperature-programmed desorption of CO2. Calculated from reaction data obtained at 30 min. Transesterification conditions: see Table 2. Attained at 3 h of reaction course. n.d. means not determined.

Fig. 5. SEM images of the (A) pure Ca precipitate, and the as-synthesized (B) MgZn, (C) CaMgZn and (D) the CaMgZn precipitate after calcination at 800 °C.

before their calcination (Supplementary Information Fig. S2), revealing the large extent of the mixed carbonate phases, as CaMg(CO3)2 and CaZn(CO3)2, that are formed at high concentrations of Na2CO3 and high molar ratios of CO2 3 /metal ions. At a fixed CO2 3 concentration (Fig. 4), the intensities of the diffraction peaks corresponding to CaO, MgO and ZnO were consistent with the elemental composition (Table 1). The generation of CaO and MgO with nanosizes as a result of the thermal treatment of dolomite, a natural CaMg(CO3)2, at 900 °C has been reported previously by Wilson et al. [39], whilst we calculated the crystallite sizes of CaO and MgO derived from a dolomitic rock to be 103.2 and 127.6 nm, respectively (data not shown). As

summarized in Table 3, the sizes of CaO, MgO and ZnO crystallites formed in the CaMgZn mixed oxides were significantly smaller than those produced from the natural mixed metal carbonates. The shrinkage of the CaO crystallite size with increasing Zn content of CaOZnO mixed precipitates was reported by our group [28]. In the present case, the crystallite size of CaO decreased with increasing CO2 3 /metal ions ratio in the synthesis mixture (compare CaMgZn-01, CaMgZn-03, CaMgZn-04, and CaMgZn-09 in Table 3). Fig. 5 shows representative SEM images of the as-synthesized and the calcined CaMgZn precipitates in comparison with those of the as-synthesized pure Ca and MgZn precipitates. The high purity Ca precipitate with a rhombohedral structure exhibited

S. Limmanee et al. / Chemical Engineering Journal 225 (2013) 616–624

812 °C

14

Rate of FAME formation/ g gcat-1 h-1

CaMgZn-09

621 °C

482 °C

TCD Signal/ a.u.

726 °C

763 °C

622

Calcined Ca precipitate

12

CaMgZn-07

10 8 CaMgZn-03

6 CaMgZn-06

4 CaMgZn-05

CaMgZn-04

2 CaMgZn-01

(c) CaMgZn-09

0 0

(b) CaMgZn-03

200

400 600 Basicity/ μmol g-1

800

(a) CaMgZn-04

100

200

300

400

500

600

700

800

900

Fig. 8. Relationship between the total basicity of the CaMgZn mixed oxides and the initial formation rate of FAME from PKO and methanol. Reaction conditions: see Table 3.

Temperature/ °C

90

FAME yield/ wt.%

80

857 °C

812 °C

763 °C

100

70 60 50 40 30 20

621 °C

TCD Signal/ a.u.

726 °C

Fig. 6. Representative profiles of the temperature-programmed desorption of CO2 attained from the CaMgZn mixed oxides prepared with a Ca:Mg:Zn ratio of 1:1:1 under Na2CO3 concentrations and CO2 3 /metal ions molar ratios of: (a) CaMgZn-04 (0.75 mol L1; 1.0), (b) CaMgZn-03 (0.5 mol L1; 1.5) and (c) CaMgZn-09 (1.0 mol L1; 1.5).

10 0 0

Calcined Ca precipitate (×0.4) (d) CaMgZn-07

30

(b) CaMgZn-06

0

(a) CaMgZn-05

300

400

500

600

700

800

900

180

Fig. 7. Profiles of the temperature-programmed desorption of CO2 attained from the CaMgZn mixed oxides prepared from a Na2CO3 concentration of 0.75 mol L1 and a CO2 3 /metal ions ratio of 1.0 with metal compositions of: (a) CaMgZn-05 (1:1:3), (b) CaMgZn-06 (1:3:1), (c) CaMgZn-04 (1:1:1) and (d) CaMgZn-07 (3:1:1).

essentially square-shaped particles (Fig. 5A). The mixed precipitate of Mg and Zn was in the form of thin flakes, similarly to brucite, with the particle sizes of 0.3–0.5 lm (Fig. 5B). The observed morphology supported the XRD and TGA results that indicated the favorable precipitation of Mg2+ as Mg(OH)2. With the co-existence of Ca2+, the small flakes were aggregated to form large spheres in the CaMgZn precipitate (Fig. 5C). The agglomerate nature is analogous to that observed for the framboidal vaterite, synthesized under mild conditions [40]. The calcination at 800 °C transformed the aggregate flakes into smaller spheres, some of which were broken down due to the extensive loss of hydroxide and carbonate groups and the shrinkage of the unit cell (Fig. 5D).

2

Catalyst amount/ wt.% 4 6 8 10

12

100

100

95

95

90

90

85

85

80

80

75

75

70

FAME yield/ wt.%

Temperature/ °C

FAME yield/ wt.%

200

150

Fig. 9. Dependence of the FAME formation on reaction time in the transesterification of PKO over: () CaMgZn-01, (d) CaMgZn-03, (N) CaMgZn-04, (h) CaMgZn07 and (j) CaMgZn-09. Reaction conditions: See Table 3.

(c) CaMgZn-04

100

60 90 120 Reaction time/ min

70 3

6

9 12 15 18 21 24 Methanol/oil molar ratio

27

Fig. 10. Effect of catalyst amount and methanol/oil molar ratio on FAME formation over CaMgZn-07. Other reaction conditions: See Table 3.

The textural properties analysis of the calcined CaMgZn precipitates, as evaluated by the physisorption of N2, indicated a low BET surface area (1.8–4.8 m2 g1) and a small pore volume (3.8– 8.6 mm3 g1). These results suggested the absence of intraparticle

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pores in the mixed metal oxides. Nevertheless, the aggregation of very small oxide spheres generated interparticle voids with a diameter of 63.2–84.8 Å. Pore sizes of a meso-scale should not hamper the diffusion of triglyceride molecules to the basic sites. From the ChemDraw Ultra program, the kinetic molecular size of the triglyceride with three molecules of lauric acid (C12), since this is the major fatty acid component in PKO (60%), was estimated to be 29.04 Å. Fig. 6 compares the CO2-TPD profiles of the CaMgZn oxides prepared with a Ca:Mg:Zn ratio of 1:1:1 under different concentrations of CO2 3 . The total basicities of the mixed oxides are reported in Table 3. The majority of the mixed oxides basicity was classified as high strength, relating to the desorption of CO2 at temperatures of >500 °C. CaO, precipitated from pure Ca(NO3)2 solution followed by calcination at 800 °C, exhibited a broad desorption peak with three maxima, suggesting that there should be at least three types of CaO derived from different precipitated phases, such as Ca(OH)2 and CaCO3. The mixed oxide synthesized under a low CO2 3 /metal ions ratio showed one desorption peak at ca. 726 °C (CaMgZn-04), corresponding to CaO as the major basic site. When the amount of CO2 3 increased, a new type of basic site was found at 621 °C simultaneously with a slight shift of the CaO peak to a higher temperature (CaMgZn-03). This behavior was 1 more pronounced when using a CO2 3 concentration of 1.0 mol L 2 and a CO3 /metal ions ratio of 1.5 (CaMgZn-09). The peaks appearing at 482 and 621 °C should be ascribed to the CO2 desorbed from ZnO and MgO, respectively, which were formerly derived from CaZn(CO3)2 and CaMg(CO3)2, as evidenced by the structural analysis (Supplementary Information Fig. S2). These results suggest that the extensive formation of the mixed carbonate phases in the CaMgZn precipitates improved the basicity of the mixed oxides by an enhanced dispersion of CaO, MgO and ZnO crystallites. The effect of metal composition on the basicity of CaMgZn mixed oxides is shown in Fig. 7. The amount of basic sites (Table 3), associated with the area of the desorption peaks, varied with the Ca content (Table 1). A relationship between the TPD profiles (Fig. 7) and the metal phases presenting in the CaMgZn precipitates before (Supplementary Information Fig. S3) and after (Fig. 4) the calcination was seen. CaMgZn-05, which possessed a low crystallinity of the mixed metal carbonates and a relatively large amount of hydroxide phases (Supplementary Information Fig. S3), exhibited a low basicity. The relatively intense peak at 621 °C attained from CaMgZn-06 with a high Mg content (Table 1) confirmed the basicity contributed by MgO. In the case of CaMgZn-07, with the largest Ca content (Table 1), the desorption of CO2 appeared at temperatures of >700 °C. The broad and asymmetric peak indicated the existence of CaO with different chemical natures similarly to the case of the calcined Ca precipitate. Interestingly, there was a new basic site with very high strength as reflected by the presence of the peak at 857 °C. Yu and coworkers reported that the substitution of Ce ions into the CaO lattice enhanced the basicity of CaO– CeO2 catalysts [41], whilst a synergistic effect between CaO and MgO in CaMg mixed oxides was suggested by Taufiq-Yap et al. [25]. Therefore, the shift of the desorption peak relating to CaO to higher temperatures than those found for the calcined Ca precipitate may well be an enhanced effect of the formation of mixed oxides in the CaO matrix by which the basic strength of the resulting mixed oxide was improved.

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gZn-07 possessed the highest FAME yield at 98% due to the largest Ca content and the highest basicity (Table 3). However, by comparing the mixed oxides synthesized with the same concen2 tration of CO2 3 and molar ratio of CO3 /metal ions (from CaMgZn04 to CaMgZn-07 in Table 3), the amount of FAME produced did not increase with increasing Ca content. As discussed in the effects of the solution pH in the precipitation of mixed metal ions, the increase in the formation of hydroxide phases, preferentially formed at high pH, retarded the FAME synthesis (Table 2). By considering the results from catalysts with a similar Ca content (CaMgZn-03, CaMgZn-06 and CaMgZn-09 in Table 3), CaMgZn-09 with a relatively high purity of the mixed metal carbonate phases provided the highest yield of FAME. Therefore, the transesterification activity of the CaMgZn mixed oxides was essentially determined by the types of metal phases presenting in the mixed precipitates before their calcination. An attempt to correlate the reaction results with the amount of the metal compounds contained in the mixed precipitates and oxides revealed no simple relationship between the FAME yield and the quantity of the metal compounds. It could rather be accounted for by the presence of the metal oxides (CaO, MgO and ZnO) with different basicities that promoted the reaction at different rates. Wilson et al. reported that the nanocrystallites of MgO dispersed over CaO particles in calcined dolomite are the catalytically active sites for the transesterification of model triglycerides [39]. In contrast, the effects of Mg compounds on the physicochemical properties and the transesterification activity of the industrial catalysts prepared by using dolomite as the raw material revealed that CaO was the major active site in the dolomite calcined at 800 °C [42]. The controversy over the active site identity can be ascribed to a strongly crystal size-dependent activity of MgO and to the reaction conditions applied in the methanolysis of triglycerides [19]. Fig. 8 provides the plot of the total basicity of the CaMgZn mixed oxides with the initial rate of FAME formation, where the catalytic activity broadly correlated with the total basicity of the CaMgZn mixed oxides. This is consistent with Ebiura et al. [5], who demonstrated that the transesterification activity of the basic oxide catalysts was strongly dependent on the total basicity but not on the basic strength. Dependence of the FAME yield on time in the transesterification over different CaMgZn mixed oxides prepared with the Ca:Mg:Zn molar ratio of 1:1:1 is illustrated in Fig. 9. An absence of induction period at the early stage of the reaction should be ascribed to the fact that the mixed oxides possessed the large pore sizes as the interparticle voids. The catalysts with higher basicity promoted the transesterification faster. The FAME yield reached the maximum after 120 min. As shown in Fig. 10, reducing the methanol/ oil molar ratio decreased the FAME formation. The yield of more than 80% was still obtained at the methanol/oil molar ratio of 9. The results indicated a high activity of the catalyst. However, whilst a low FAME yield was obtained when the catalyst level was reduced from 6 wt.% to 2 wt.%, as expected, it also declined when the amount of catalyst was increased to 8 wt.% (Fig. 10), presumably due to mass transfer limitations resulted from agglomeration of the oxide particles.

4. Conclusions 3.2. Transesterification over CaMgZn mixed oxide catalysts The initial rates and the yields of FAME produced via the transesterification of PKO with methanol over several CaMgZn mixed oxides are summarized in Table 3. Under the conditions studied, the catalyst composed of only Mg and Zn was not active in the reaction, presumably due to the low basicity of MgO and ZnO. CaM-

Nanocrystallite CaMgZn mixed oxides were successfully prepared by the pH-controlled co-precipitation method. The as-synthesized precipitates were present in the form of CaMg(CO3)2 and CaZn(CO3)2 as the major phases and metal hydroxides as the minor phases. By increasing the concentration of CO2 3 , the molar ratio of CO2 3 /metal ions and the aging time, the formation of the

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metal carbonate species in the precipitate before calcining was enhanced. The synthesis at pH 7, and not more alkaline, was important to attain precipitates with pure mixed carbonate phases. The incorporation of Mg2+ and Zn2+ into the rhombohedral lattice of CaCO3 reduced the unit cell parameter (a0), as evidenced by the XRD analysis, while the SEM analysis indicated a very unique morphology of the precipitate and oxide particles. The basicity of the mixed oxides was determined by the elemental composition and the types of metal compounds in the CaMgZn mixed precipitates. The initial rate of FAME formation was well correlated with the total basicity of the CaMgZn mixed oxides. The transesterification of PKO was catalyzed actively even at a low catalyst loading and low methanol/oil molar ratio. Acknowledgements The authors are grateful to the Chumporn Palm Oil Industry Public Company, Limited for the PKO sample. This work was financially supported by ‘‘The 90th Anniversary of Chulalongkorn University Fund (Ratchadaphiseksomphot Endowment Fund)’’ from the Graduate School, Chulalongkorn University; and the Thai Government Stimulus Package 2 (TKK 2555) under the Project for Establishment of Comprehensive Center for Innovative Food, Health Products and Agriculture (PERFECTA). In addition, the research grant (MRG52113) from the Thailand Research Fund (TRF) and the Commission on Higher Education (CHE) is acknowledged. The authors also wish to express their thanks to Dr. Robert Douglas John Butcher for English language editing. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2013.03.093. References [1] H. Wagner, R. Luther, T. Mang, Lubricant base fluids based on renewable raw materials: their catalytic manufacture and modification, Appl. Catal. A: General 221 (2001) 429–442. [2] J. Salimon, N. Salih, E. Yousif, Industrial development and applications of plant oils and their biobased oleochemicals, Arab. J. Chem. 5 (2012) 135–145. [3] Annual report 2011, PTT Global Chemical Co., Ltd. . [4] H.J. Kim, B.S. Kang, M.J. Kim, Y.M. Park, D.K. Kim, J.S. Lee, K.Y. Lee, Transesterification of vegetable oil to biodiesel using heterogeneous base catalyst, Catal. Today 93–95 (2004) 315–320. [5] T. Ebiura, T. Echizen, A. Ishikawa, K. Murai, T. Baba, Selective transesterification of triolein with methanol to methyl oleate and glycerol using alumina loaded with alkali metal salt as a solid-base catalyst, Appl. Catal. A 283 (2005) 111– 116. [6] W. Xie, H. Peng, L. Chen, Transesterification of soybean oil catalyzed by potassium loaded on alumina as a solid-base catalyst, Appl. Catal. A 300 (2006) 67–74. [7] W. Xie, H. Li, Alumina-supported potassium iodide as a heterogeneous catalyst for biodiesel production from soybean oil, J. Mol. Catal. A: Chem. 255 (2006) 1– 9. [8] S. Benjapornkulaphong, C. Ngamcharussrivichai, K. Bunyakiat, Al2O3supported alkali and alkali earth metal oxides for transesterification of palm kernel oil and coconut oil, Chem. Eng. J. 145 (2009) 468–474. [9] S. Gryglewicz, Rapeseed oil methyl esters preparation using heterogeneous catalysts, Bioresour. Technol. 70 (1999) 249–253. [10] C. Reddy, V. Reddy, R. Oshel, J.G. Verkade, Room-temperature conversion of soybean oil and poultry fat to biodiesel catalyzed by nanocrystalline calcium oxides, Energy Fuels 20 (2006) 1310–1314. [11] M. López Granados, M.D. Zafra Poves, D. Martín Aloson, R. Mariscal, F. Cabello Galisteo, R. Moreno-Tost, J. Santamaría, J.L.G. Fierro, Biodiesel from sunflower oil by using activated calcium oxide, Appl. Catal. B 73 (2007) 317–326. [12] P. Patil, V.G. Gude, S. Pinappu, S. Deng, Transesterification kinetics of Camelina sativa oil on metal oxide catalysts under conventional and microwave heating conditions, Chem. Eng. J. 168 (2011) 1296–1300. [13] M. Kouzu, T. Kasuno, M. Tajika, Y. Sugimoto, S. Yamanaka, J. Hidaka, Calcium oxide as a solid base catalyst for transesterification of soybean oil and its application to biodiesel production, Fuel 87 (2008) 2798–2806.

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