Catalysis Communications 52 (2014) 1–4

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Catalysis Communications

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Short Communication

Basicity–FAME yield correlations in metal cation modified MgAl mixedoxides for biodiesel synthesis

Jian Sun, Jingyi Yang ⁎, Shaoping Li, Xinru XuState Key Laboratory of Chemical Engineering, East China University of Science and Technology, Meilong Road 130, Shanghai 200237, PR China

⁎ Corresponding author. Tel./fax: +86 021 64252160.E-mail address: jyyang@ecust.edu.cn (J. Yang).

http://dx.doi.org/10.1016/j.catcom.2014.03.0231566-7367/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 January 2014Received in revised form 18 March 2014Accepted 19 March 2014Available online 27 March 2014

Keywords:Metal oxideBasicityTransesterificationBiodiesel

In this paper Zn2+, Co2+, Fe3+ and La3+ were respectively introduced into Mg3Al1 mixed oxides to prepareMg1Zn2Al1, Mg1Co2Al1, Mg3Al0.6Fe0.4 and Mg3Al0.6La0.4 oxides by calcinations at 773 K. The XRD, XPS, BET,ICP-AES and CO2-TPD characterizations of samples exhibited that the effects of the substitution of trivalent cat-ions (Fe3+, La3+) for Al3+ on the basicity of samples were stronger than the substitution of divalent cations(Zn2+, Co2+) for Mg2+ because of high O2− concentration which were coincident with the FAME yield forthese catalysts. La3+ modified catalyst exhibited highest activity in transesterification.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Recently, biodiesel widely attracts increasing countries' atten-tions, because of its safety, clean burning, renewable property andenvironmental-friendly characteristic. Generally, natural oils arecomposed of straight-chain fatty acid triglycerides. Direct injectionof triglycerides will result in much engine problems. Therefore, triglyc-erides must be decomposed into small molecule compounds bytransesterification [1].

Transesterification catalyzed by heterogeneous catalysts has the ad-vantages of simple downstream separation and low pollution. Solidbase catalysts are mainly explored and reported in recent years suchas resins, clays, metal oxides and lipases [2–4]. Hydrotalcites (HTs)and hydrotalcite-like compounds, also called anionic clay are layereddouble hydroxides [5]. The conventional HT has the formula Mg6Al2(OH)16CO3·4H2O, in which parts of the Mg2+ cations are replaced byAl3+ cations forming positively charged layers. The charge balancinganions (CO2−) and water molecules are situated in the interlayers be-tween the stacked brucite-like cation layers [6,7]. The calcination ofMgAl HTs at a high temperature yields homogeneously dispersedMgAl mixed oxides which have surface basicity, much like the pureoxides, also potentially containing more surface defects due to theAl3+ incorporated in MgO lattice [8,9]. MgAl porous metal oxides ex-posed stronger Lewis basic sites which depend on the compositionand pretreatment parameters such as Mg/Al ratio and activation tem-perature [10,11]. In addition to ordinary MgAl catalyst [12–15], some

modified MgAl catalysts had been reported such as ZnAl complexoxide [16], KF/CaAl HT [17] and LiAl HT [18,19] to increase the catalyticactivities in transesterification. In this paper Zn2+, Co2+, Fe3+ and La3+

were introduced respectively into Mg3Al1 mixed oxides to obtain mod-ified catalysts by calcinations which aim to increase the basicity of cata-lysts. The characterization of catalysts was studied by XRD, XPS, BET,ICP-AES and CO2-TPD. The effects of the basicity of these catalysts onFAME yield were investigated.

2. Experimental

2.1. Catalyst preparation

Hydrotalcite samples were prepared with a 3 mol ratio of M2+ toM3+ (M for metal cation) by co-precipitation. Solution (A) withMg(NO3)2·6H2O (56.40 g), Al(NO3)3·9H2O (16.43 g) and La(NO3)3·6H2O (13.51 g) in 167mLdeionizedwaterwas uniformlymixed. Anoth-er solution (B)withNaOH (26.70 g) andNa2CO3 (20.90 g) in 167mL de-ionized water was prepared in the same way. Then solution A andsolution B were slowly mixed together with constant stirring. The pHof the mixture was maintained at 9.8–10.2 by controlling the titrationspeed. The precipitate was left to age in the reaction mixture at 348 Kovernight and subsequently washed with deionized water until thewashings attained a pH of 7. The solid was dried at 333 K in vacuumoven for 20 h. Other corresponding nitric acid compounds Zn(NO3)2·6-H2O (43.63 g), Co(NO3)2·6H2O (42.69 g) and Fe(NO3)3·9H2O (11.85 g)were used to prepare Mg1Zn2Al1, Mg1Co2Al1 and Mg3Al0.6Fe0.4 HT sam-ples by the same co-precipitation. The HTs were calcined in air at 773 Kfor 8 h to produce corresponding metal oxides.

Fig. 1. XRD patterns of catalysts. Mg3Al1 (a), Mg1Zn2Al1 (b), Mg1Co2Al1 (c) Mg3Al0.6Fe0.4(d), Mg3Al0.6La0.4 (e). *: MgO, #: ZnO, : Co3O4, ♦: Fe2O3, △: La2O3.

2 J. Sun et al. / Catalysis Communications 52 (2014) 1–4

2.2. Characterization method of catalyst

The chemical composition of catalystswas determined by inductive-ly coupled plasma atomic emission spectrometry (Varian 710ES). Solidmorphologies and crystallite sizes of the catalysts were determined bypowder X-ray diffraction (Rigaku D/max 2550VB/PC) operating withCuKα radiation (λ = 1.5406 Å), at 100 mA, 45 kV, 2θ scanning rangeof 10–80° and a step size of 0.02° (2θ). N2 adsorption anddesorption iso-therms were recorded by Micromeritics 2010 and were calculated byusing BET method and BJH method. X-ray photoelectron spectroscopyspectra were performed with AlKα radiation (Thermo Fisher ESCALAB250Xi). The charging effect was corrected by the C1s peak at 284.6 eV.The basicity of catalysts was determined by the temperatureprogrammed desorption with CO2 (Micromeritics Autochem 2920).

2.3. Transesterification

Catalysts were tested in transesterification of soybean oil andmethanol at 338 K. Transesterification reacted in a glass reactor withcondenser andmagnetic stirring. After a certain reaction time, the prod-uct was filtered and stratified. The upper layer product was extracted,weighed and analyzed by a gas chromatograph (Jinghe GC-7860),equipped with phenyl-methyl-polysiloxane capillary column and aflame ionization detector. A known amount of methyl heptadecanoatewas added as internal standard (EN 14103).

3. Results and discussion

3.1. Catalyst characterization

The incorporation ofMg2+, Al3+ and other divalent/trivalent cationswithin Mg3Al1, Mg1Zn2Al1, Mg1Co2Al1, Mg3Al0.6Fe0.4 and Mg3Al0.6La0.4oxides was verified by elemental analysis. All samples were in goodagreement with small deviation between theoretical and experimentalvalues (Table 1). The modified oxides exhibited advanced specificsurface area (162.7–198.2 m2/g) and mesoporous structure (15.0–19.7 nm). The advanced specific surface area could account for defectswhich were caused by divalent/trivalent cation modification [20].

The XRD patterns of oxides are shown in Fig. 1. After dehydrationand decarburization process in 773 K, two sharp and symmetrical re-flections of Mg3Al1 mixed oxide (a) at 42.9° and 62.3° correspond toMgO crystalline phase. When some Mg2+ cations were replaced by di-valent cations Zn2+ and Co2+ in Mg3Al1 framework, several new crys-talline phases could be observed in addition to quondam MgO andAl2O3 crystalline phases. Two main diffraction peaks (b) at 31.8° and36.3° were corresponding to ZnO. Three main diffraction peaks (c) at36.9°, 44.8° and 65.2° were assigned to Co3O4. The substitution of Al3+

cations by Fe3+ or La3+ cations in the framework resulted in corre-sponding iron oxide and lanthanum oxide. In the XRD patterns of theMg3Al0.6Fe0.4 oxide (d) the diffraction peaks at 35.4°, 43.1° and 62.6°(d) were characteristic of Fe2O3. Mg3Al0.6La0.4 oxide (e) showed diffrac-tion peaks at 22.0°, 29.1° and 30.0°, which were characteristic of La2O3

phases.Fig. 2 presents the XPS spectra of catalysts in O1s region. All the sam-

ples could be deconvoluted to two peaks at binding energy of ~531 and

Table 1Chemical composition and textural characterization of catalysts.

Sample Mole ratios SBET (m2/g) D (nm) V (cm3/g)

Theoretical Experimental

Mg3Al1 3:1 3:1 82.2 21.9 0.85Mg1Zn2Al1 1:2:1 1.1:2.1:1 162.7 16.8 1.26Mg1Co2Al1 1:2:1 1.1:1.9:1 164.9 19.7 0.92Mg3Al0.6Fe0.4 3:0.6:0.4 3:0.6:0.3 198.2 18.1 0.98Mg3Al0.6La0.4 3:0.6:0.4 3:0.6:0.3 189.9 15.0 0.98

~529 eV. The lower binding energy at ~529 eV was consistent with thecharacteristic of O2− [21,22]. The higher binding energy at ~531 eVcould correspond to the oxygen of M–O groups. It was significant thatthe peaks of samples turned to broad after the incorporation of Zn2+,Co2+, Fe3+ and La3+. These changes indicated that the oxygen chemicalenvironment inMg3Al1 mixed oxide precursor changed by substitution.

The basicity of catalysts characterized by CO2-TPD is presented inFig. 3. There were three desorption peaks at around 373 K, 473 K and673 K in Mg3Al1 mixed oxide precursor. The three active sites could beattributed to OH− groups (373 K), Mg–O pairs (473 K) and O2− anions(673 K) [7]. When Zn2+ and Co2+ were respectively exchanged intoMg3Al1 framework, the desorption at around 473 K peaks translatedto broad and intense which was due to the formation of ZnO andCo3O4. When Fe3+ and La3+ were respectively exchanged into Mg3Al1framework, the broad desorption peaks appeared at around 673 K inthe CO2-TPD profiles of Mg3Al0.6Fe0.4 and Mg3Al0.6La0.4 oxides, whichcould be due to the increase of O2− concentration.

3.2. Transesterification

The effects of the incorporation of Zn2+, Co2+, Fe3+ and La3+ onFAME yield were investigated. The transesterification of methanol andsoybean oil reacted with 2 wt.% catalyst amount and 12 methanol tooil molar ratio at 338 K.

Compared withMg3Al1 mixed oxide, the transesterification efficien-cies of Zn2+, Co2+, Fe3+ and La3+ modified oxides raised (Fig. 4). TheFAME yields catalyzed by Mg1Zn2Al1, Mg1Co2Al1, Mg3Al0.6Fe0.4 andMg3Al0.6La0.4 oxides were respectively 64.7%, 66.4%, 95.7% and 98.3%in 5 h.

Fig. 2. O1s XPS patterns of catalysts. Mg3Al1 (a), Mg1Zn2Al1 (b), Mg1Co2Al1 (c) Mg3Al0.6-Fe0.4 (d), Mg3Al0.6La0.4 (e).

Fig. 3. CO2-TPD profiles of catalysts. Mg3Al1 (a), Mg1Zn2Al1 (b), Mg1Co2Al1 (c)Mg3Al0.6Fe0.4(d), Mg3Al0.6La0.4 (e).

Fig. 5. Correlative effect between total basicity and catalysis activity of samples. Reactioncondition: 338 K, methanol/oil = 12, 2 wt.% catalyst amount, 5 h reaction time.

3J. Sun et al. / Catalysis Communications 52 (2014) 1–4

The basicity of Mg3Al1 mixed oxide was related to O2− groups(strong), M–O pairs (medium) and OH− groups (weak) [7,23]. The sub-stitution of two Al3+ for three Mg2+ could create the cation vacancy,and then the surface acted as a sink for the vacancy to optimize thelattice energy. Therefore, the Mg2+ was removed from the surfaceresulting in O2− [24]. The substitution caused effects of Schottky de-fects, which resulted in the unsaturation of adjacent oxygen anions(O2−) [25], so that the solid solution showed strong basicity.

Fig. 5 illustrates the correlative effect between basicity and FAMEyield. The evolved CO2 was calculated by integrating the TPD curves.The basicity values of Mg1Zn2Al1 and Mg1Co2Al1 oxides were respec-tively 250.1 μmol/g and 251.4 μmol/g which were more than the basic-ity of Mg3Al1 oxide (219.8 μmol/g). After trivalent cation modification,the basicity values of Mg3Al0.6Fe0.4 and Mg3Al0.6La0.4 oxides increasedwhich were respectively 261.4 μmol/g and 276.2 μmol/g. For trivalentcation modified oxides, the migration of Fe3+ (0.65 Å) and La3+

(1.03 Å) into the framework enhanced the Schottky defects whichwere caused by the incorporation of Al3+ (0.54 Å) into the framework.

The trivalent cation modified Mg3Al0.6Fe0.4 and Mg3Al0.6La0.4 oxideshad much higher activities in transesterification. The high activity ofmodified catalysts in transesterification could be attributed to high ba-sicity. The Lewis basic sites were modified by the substitution of Mg2+

by Zn2+ and Co2+ or Al3+ by Fe3+ and La3+. And trivalent cation(Fe3+, La3+) modified oxides exhibited higher activity than divalentcation (Zn2+, Co2+) modified oxides because of the increase of O2−

concentration. Therefore, the FAME yields catalyzed by Fe3+ and La3+

modified oxides in transesterification were higher than Zn2+ and

Fig. 4. Effect of reaction time on FAME yield. Mg3Al1 (a), Mg1Zn2Al1 (b), Mg1Co2Al1 (c)Mg3Al0.6Fe0.4 (d), Mg3Al0.6La0.4 (e). Reaction condition: 338 K, methanol/oil = 12,2 wt.% catalyst amount.

Co2+ modified oxides in Fig. 5. The FAME yield was found to followthe same trend as that of the basicity of the samples, i.e. Mg3Al0.6La0.4 -Mg3Al0.6Fe0.4 N Mg1Co2Al1 N Mg1Zn2Al1 N Mg3Al1 oxides. La3+ cationmodified oxide showed highest activity in transesterification. TheFAME yield catalyzed by Mg3Al0.6La0.4 oxide in transesterificationreached 95.0% in 4 h and 98.3% in 5 h when the reaction was carriedout with the methanol to oil molar of 12:1, 338 K, and 2 wt.% catalystamount.

4. Conclusions

Zn2+, Co2+, Fe3+ and La3+ modified Mg3Al1 mixed oxides exhibitedhigher activities in the transesterification of soybean and methanol at338 K than Mg3Al1 mixed oxide precursor. The higher activity could beattributed to higher basicity of oxides. After divalent and trivalent cationmodifications, the basic sites in oxideswere improved accounting for thedefects, which were caused by the substitution of the metal cations. Theeffects of the substitutionof trivalent cations (Fe3+, La3+) for Al3+ on thebasicity of samples were stronger than that of divalent cations (Zn2+,Co2+) for Mg2+ because of the higher O2− concentration. As the result,activities of the Fe3+ and La3+ modified oxides in transesterificationwere higher than Zn2+ andCo2+modified oxides. The order of the basic-ity of the samples was Mg3Al0.6La0.4 N Mg3Al0.6Fe0.4 N Mg1Co2Al1 N

Mg1Zn2Al1 N Mg3Al1 oxides, which were coincident with the FAMEyields for these catalysts in transesterification. The FAME yield forMg3Al0.6La0.4 oxide in transesterification was 98.3% when thetransesterification was carried out at 338 K, with a methanol to oilmolar ratio of 12:1, 5 h reaction time and 2 wt.% catalyst amount. La3+

modified oxides exhibited highest activity in transesterification.

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