MgAlLi Mixed Oxides Derived from Hydrotalcite for CatalyticTransesterification

Cınthia S. Castro • Dilson Cardoso •

Pedro A. P. Nascente • Jose Mansur Assaf

Received: 27 April 2011 / Accepted: 13 June 2011 / Published online: 29 June 2011

� Springer Science+Business Media, LLC 2011

Abstract MgAl hydrotalcite was synthesized and used as

support for Li impregnation. MgAlLi oxides were obtained

from heat treatment of the Li/MgAl hydrotalcite. These

materials were characterized and evaluated as catalysts for

model transesterification reactions. MgAl showed negligi-

ble activity under mild reaction conditions (50 �C and

0.5 h) whereas Li incorporation greatly increased the

activity. The activities were correlated to their basicity

determined by TPD of CO2. Reuse tests showed catalyst

deactivation after the third cycle, probably due to lithium

leaching. However, the contribution to a homogeneous

reaction has been dismissed. MgAlLi revealed to be a

promising catalysts for transesterification reaction and thus

for biodiesel production.

Keywords Mixed oxide � Hydrotalcite �Transesterification reaction � Biodiesel

1 Introduction

Hydrotalcite compounds are layered double hydroxides with

the general formula [M1-x2? Mx

3?(OH)2]x?[Ax/n]-�mH2O,

where M2? is a divalent cation, M3? is a trivalent cation,

x (molar ratio): M3?/(M2? ? M3?) and A is a compensation

anion with charge n. The structure of these compounds is

constituted by brucite-type octahedral layers, in which a part

of M2? cations (e.g. Mg, Cu, Co, Ni, Mn, Zn, Fe) are

substituted by those of M3? (e.g. Al, Cr, Fe, Co, Mn, V, Ga).

The resulting excess of positive charge is compensated by

hydrated anions in the interlayer space, the CO32- ions being

the most common in the hydrotalcite-like clays [1].

Hydrotalcite calcination at high temperature promotes

collapse of the crystalline structure leading to a basic solid

with interesting properties for application in catalysis

composed of homogeneous mixtures of oxides. These

oxides are characterized as having very small crystal size

(*10 nm), surface area of 100–300 m2 g-1, stable against

thermal treatments and surface basic properties, presum-

ably Lewis base sites [1].

Recently, these compounds have been applied to transe-

sterification reactions in heterogeneous phase revealing to

be promising heterogeneous catalysts for biodiesel produc-

tion [2–6]. Moreover, lithium incorporation onto solid cat-

alysts is reported in literature to be capable of increasing the

catalysts basicity and therefore the activity in basic cata-

lyzed reactions such as transesterification [7–10]. Compared

to other alkali (Na, K and Cs) or alkaline earth (Ca, Sr and

Ba) metal ions, lithium incorporation onto MgO is described

to create the strongest basic sites [11, 12].

Considering this, the present work deals with the

investigation of the catalytic properties for transesterifica-

tion reactions of mixed oxides obtained from thermal

treatment of MgAl hydrotalcite compounds modified by

lithium impregnation. The structure, textural, and basicity

of the catalysts were determined by different techniques.

Furthermore, the correlation between the basic properties

of the catalysts and their catalytic activities was discussed.

Stability studies covering catalyst reuse experiments and

metal leaching were also performed.

C. S. Castro (&) � D. Cardoso � J. M. Assaf

Chemical Engineering Department, Federal University of Sao

Carlos, Rodovia Washington Luıs, Km 235, Sao Carlos,

SP CEP 13565-905, Brazil

e-mail: cicastro14@yahoo.com.br

P. A. P. Nascente

Materials Engineering Department, Federal University of Sao

Carlos, Rodovia Washington Luıs, Km 235, Sao Carlos,

SP CEP 13565-905, Brazil

123

Catal Lett (2011) 141:1316–1323

DOI 10.1007/s10562-011-0650-y

2 Experimental

2.1 Synthesis of the MgAl Hydrotalcite

Mg–Al hydrotalcite (HT Mg–Al) was synthesized under

high supersaturation conditions from aqueous solutions at

room temperature using an alkali-free coprecipitation route

[4]. 100 cm3 of a solution containing a mixture of the nitrate

salts Mg(NO3)2�6H2O (Mallinckrodt Chemicals) and

Al(NO3)3�9H2O (Mallinckrodt Chemicals) with Al/(Al ?

Mg) = 0.2 and a total metal concentration of 1.0 mol L-1

was slowly dropped into 200 cm3 of another solution con-

taining 2.4 mol L-1 of (NH4)2CO3 (Mallinckrodt Chemi-

cals) under vigorous stirring. The pH during the synthesis

was monitored by a pH meter and held at 10.0 ± 0.5 by the

addition of NH4OH 30% (Mallinckrodt Chemicals). The

precipitate was aged at 65 ± 5 �C for 18 h and then washed

several times with warm distilled water. The solid obtained

was oven dried at 110 �C for 24 h.

2.2 Synthesis of the MgAl and MgAlLi Mixed Oxides

The as-synthesized magnesium and aluminum hydrotalcite

(HT Mg–Al) was used as support for LiNO3 (Acros

Organics 99?%) impregnation using the lithium loadings

of 1, 5, 8 and 10 wt% (wt of Li/wt of Li ? calcined

hydrotalcite support). The determination of the weight of

the calcined hydrotalcite support at the desired calcination

temperature was established by TGA analyses. The wet

impregnation method was used to synthesize the Li/HT

MgAl. For that, LiNO3 was dissolved in 20 cm3 of distilled

water, placed in contact with an appropriated amount of the

support and heated at 80 �C under magnetic stirring until

complete drying. The resulting powder mixture was dried

further in an oven at 110 �C for 24 h.

The MgAl and MgAlLi mixed oxides were obtained by

calcination of the Mg–Al hydrotalcite and hydrotalcite

impregnated Li, respectively. The thermal treatment was

performed in a tubular furnace under air flow of

80 mL min-1, using a heating rate of 10 �C min-1 from

room temperature until 600 �C and remaining at this

temperature for a variable period time from 1 min to 10 h.

The resulting mixed oxides were stored in a vacuum desic-

cator. The catalysts were denoted as MgAl and MgAlLi-x,

where x is the lithium loading expressed in wt%.

2.3 Support and Catalysts Characterization

The catalysts were characterized by powder X-ray dif-

fractometry (XRD) in a Rigaku Multiflex spectrometer

using Cu Ka radiation (k = 1.5406 A) in an angular range

from 5� to 80�, a step width of 0.02� and an acquisition

time of 2 s for each time. The phases were identified using

the Powder Diffraction File (PDF) database (JCPDS,

International Centre for Diffraction Data). Field emission

gun (FEG) scanning electron microscopy (SEM) was used

to obtain information about the hydrotalcite and oxides

morphology. The images were obtained at 10.0 kV on a

Philips XL-30 FEG microscope. X-ray photoelectron

spectroscopy (XPS) analysis was performed using a Kratos

XSAM HS spectrometer. The measurements were obtained

in ultrahigh vacuum (low 10-7 Pa range) with non-mono-

chromatic Mg Ka (hm = 1253.6 eV) X-ray source, with an

emission current of 5 mA and a voltage of 12 kV. An

electron flood gun was used to reduce charge effects.

Gaussian curves were used to fit the peaks. The Shirley

background subtraction methods and a least-square routine

were used for fitting. The binding energies were referenced

to the adventitious hydrocarbon C 1s level set at 284.8 eV.

The sensitivity factors for quantitative analysis were ref-

erenced to SF1s = 1.0. Due to the overlap of the Al 2p and

Mg KLL peaks, the Al 2s peak was used in the analysis.

Nitrogen physisorption at 77 K was employed for surface

area determination on Quantachrome equipment model

NOVA-1200. The samples were previously outgassed at

200 �C for several hours and the specific surface area

was obtained using the Brunauer–Emmett–Teller (BET)

method. Thermogravimetric analyses were carried out on

TA Instrument (SDT 2960) equipment. The samples were

heated at a rate of 10 �C min-1 from 25 to 1,000 �C under

O2 flow of 40 mL min-1.

The temperature programmed desorption technique (TPD

of CO2) was employed in the determination of basicity and

basic strength of the catalysts. The 0.1 g sample was heated

at a rate of 20 �C min-1 until the corresponding calcination

temperature of 600 �C in a helium atmosphere (flow of

50 mL min-1) to remove the adsorbed impurities species.

CO2 adsorption was performed afterwards at 200 �C for 1 h

using a CO2 flow of 50 mL min-1. This was followed by

helium purge to remove physically adsorbed CO2. The CO2

desorption amount was then measured by TCD detector

heating the samples at a rate of 10 �C min-1 under helium

atmosphere (flow of 30 mL min-1) until a catalyst calci-

nation temperature of 600 �C.

2.4 Catalytic Tests

Mechanistic studies of biodiesel production from crude

feedstock are made difficult by the range of C14–C20 fatty

acid components; hence methyl acetate has been chosen as

a simple model reagent for screening the active catalysts.

The catalytic activity was evaluated in the transesterifica-

tion of methyl acetate (Vetec) and ethanol (Synth) used as

renewable alcohol source. The experiments were per-

formed under mild conditions in a 2 mL volumetric

capacity batch reactor using a mol ratio of ethanol/methyl

MgAlLi Mixed Oxides Derived from Hydrotalcite 1317

123

acetate = 6/1, 4% of catalyst (wt catalyst/wt of the reac-

tion mixture) at a temperature of 50 �C controlled by

immersion of the reactor in warm water for 30 min. The

reaction was then interrupted by submerging the reactor in

an ice bath. The catalyst was separated by centrifugation

and the reaction products were analyzed in a gas chro-

matograph (Varian Star GC model 3400) equipped with

FID and an SPB1 fused silica capillary column.

2.5 Stability Tests

The reuse of the catalysts was investigated. After the first

reaction cycle, the catalyst was filtered and placed in an oven

until complete drying. Then, an appropriated amount of a

new reaction mixture was placed in contact with the used

catalyst and the following reaction was performed under

the conditions described for the catalytic tests. Moreover,

chemical analysis of the reaction solution was performed in

order to verify the possible lithium leaching from the cata-

lysts to solution. The lithium content in the solution was

analyzed by SpectrAA Varian flame emission spectrometer.

For this purpose, the supernatant solution was collected after

reaction and the catalyst was filtered off using syringe filters

(pore size of 0.45 lm). Thus, the volatile compounds were

evaporated and residual metals resuspended with HNO3 1%

solution for the chemical analysis.

3 Results and Discussion

3.1 Characterization of the MgAl Hydrotalcite

The XRD of the MgAl hydrotalcite (Fig. 1a) showed

characteristic diffractions of hydrotalcite structure at

2h = 11.4�; 22.7�; 34.4�; 38.5�; 45.4�; 60.3� and 61.6�[JCPDS 22-0700]. The TGA and DTA profiles (Fig. 1b)

presented typical weight losses of hydrotalcite compounds

associated with endothermic transformations. The first

weight loss below 100 �C is related to weakly held surface

water. The second weight loss, with a maximum at around

215 �C, corresponds to the elimination of interlayer water

and the third, at near 390 �C, is associated with the dehy-

droxylation and decarboxylation processes [2, 13].

3.2 Characterization of the MgAl and MgAlLi

Catalysts

The MgAl mixed oxide was obtained from heat treatment

of MgAl hydrotalcite precursor at 600 �C for 0.5 h. The

XRD of MgAl (Fig. 2) exhibits diffractions of an Mg(Al)O

mixed oxide, poorly crystallized, with MgO periclase-type

structure [JCPDS 75-1525]. The representation Mg(Al)O is

used to indicate that the Al3? ions are highly dispersed in

the MgO lattice without segregation of Al3? species. The

incorporation of Al3? in the MgO network generates cat-

ionic defects producing a low degree of crystallinity [14].

For the MgAlLi oxides, new diffractions, probably related

to LiAlO2 [JCPDS 44-0224], were also observed. The

expected Li2O was not detected for the MgAlLi which can

be due to low scattering factor for the light atom of Li

[15, 16]. Li2O can also be in small particle size or highly

dispersed over the oxide matrix.

Moreover, it is clearly noticed that the Li addition

increased the crystallinity of the mixed oxides, showing

more intense and narrower peaks than MgAl (Fig. 2). The

crystallite size of the oxides was calculated by the Scherrer

equation (Table 1) using the main reflection at 2h = 43.0�from (200) MgO plane [17]. The lithium addition increased

the crystallite size of the mixed oxides. For example, the

crystallite size of MgAl is 2.6 and 25.5 nm for the sample

with the highest Li content, MgAlLi-10. It is known that

some alkali metals can distort the structure of oxides upon

heating, causing sintering [18, 19]. Accordingly, the surface

area of the oxides is reduced with the Li addition (Table 1).

This detrimental effect on the surface area has already been

reported for the Li supported on MgO [18–20].

In order to investigate the morphology of the Mg–Al

hydrotalcite and mixed oxides we have selected

0

200

400

600

800

1000

1200

1400

1600

1800

(113)(110)

(018)(015)

(009)(006)

Inte

nsity

2θ

(003)(a)

10 20 30 40 50 60 70 80 0 100 200 300 400 500 600 700 800 900 1000-50

-40

-30

-20

-10

0

DT

A/a.u.

Temperature/ °C

Wei

ght l

oss/

%

endo

(b)Fig. 1 XR diffractogram

(a) and TGA/DTA profiles

(b) of the MgAl hydrotalcite

1318 C. S. Castro et al.

123

representative samples which were studied by FEG-SEM

(Fig. 3). Mg–Al hydrotalcite (Fig. 3a) presents flat struc-

ture with ‘‘platelet’’ morphology, representative of layered

materials [21, 22]. The hydrotalcite calcination did not

cause any change in the crystal morphology of the MgAl

oxide and the flat structure is still retained (Fig. 3b). This is

in agreement with the results reported previously [1, 2]

which show that the decomposition of the Mg–Al hydro-

talcite produces a Mg–Al–O periclase-like structure pre-

serving the original morphology. On the other hand, Li

impregnation caused considerable morphologic modifica-

tions (Fig. 3c, d). By increasing the Li loading on the

catalysts, the typical layered morphology of hydrotalcite

gradually disappears. For the MgAlLi-8, is still possible to

observe crystals with platelet structure along with round

particles (Fig. 3c). However, for the sample containing the

highest lithium amount, MgAlLi-10, only large aggregates

composed by round particles are observed (Fig. 3d). This

could be explained by the dissolution of the hydrotalcite

structure due to the strong basic character of the impreg-

nating solution. Besides, the morphologic changes due to

the lithium incorporation onto the oxides are in line with

the sintering effect verified by XRD and surface area

reduction.

XPS analyses of the catalysts were performed and Fig. 4

displays the location of the Mg 2p and Li 1s levels as a

function of the Li loadings.

The binding energy (B.E.) of the Mg 2p peak for the

MgAl was 50.0 eV, in agreement with previous reports

[23, 24]. All the other samples exhibited a single feature for

Mg 2p, indicative of a unique chemical environment at

49.5–50.0 eV expected for MgO. The Li 1s peak is near the

Mg 2p peak, making it difficult to accurately assign the

B.E. values for the lithium species. Li signal was not

detected for MgAlLi-1 due to the low Li content in this

sample and very weak XPS sensitivity for this element. Only

one chemical environment was identified for MgAlLi-5

(54.7 eV), possibly related to LiOH [25], whereas two Li

1s components were observed for MgAlLi-8 and MgAlLi-

10: one at 55.4 eV which is attributed to Li2CO3 [25], and

another at 52.0 eV that might be associated to Li0 [20]. Li0

may be formed by long X-ray exposure during XPS mea-

surements [26]. Possibly, the Li2O formed from the calci-

nation of LiNO3 reacted with atmospheric water and CO2

producing the LiOH and Li2CO3 detected on the catalysts

surface. The Al 2s level was used in the analyses because

the most commonly used Al 2p peak overlaps the Auger

Mg KLL peak. The B.E. of Al signal for MgAl was at

119.2 eV and at 118.6–118.8 eV for MgAlLi oxides. This

indicates that the aluminum component corresponds to

Al3? in the Al2O3 structure [4].

The temperature-programmed desorption of CO2 was

used to obtain information about the basic properties of the

catalysts. The amount of CO2 irreversibly adsorbed is

associated with the total amount of basic sites and the CO2

desorption temperature is a function of the basic site

strength. The stronger the basic site, the higher the CO2

desorption temperature. The TPD of CO2 profiles for the

catalysts are presented in Fig. 5.

Broad desorption bands centered at ca. 150 �C are

observed for all samples (Fig. 5). These bands are assigned

to the interaction of CO2 with sites of weak basic strength

(associated with OH- surface groups) and medium basic

strength (Mg2?–O2- or Al3?–O2- pairs) [3, 24]. Interest-

ingly, this is the only band detected for the MgAl. On the

other hand, the lithium incorporation created strong basic

sites which are observed with the appearance of new bands

centered at 350 �C for MgAlLi-5 and 480 �C for MgAlLi-

10. Because the higher the CO2 desorption temperature, the

higher the basic strength, the MgAlLi-10 presents the

strongest basic sites. It is reported in the literature that

addition Li onto MgO for example, causes a great increase

in its surface basicity, especially strong basic sites [9, 27].

These differences in basic properties of the MgAlLi deeply

10 20 30 40 50 60 70 80

MgAlLi-10

MgAlLi-8

MgAlLi-5

MgAlLi-1

Inte

nsity

/a.u

.

2θ

Mg(Al)O

MgAl LiAlO2

Fig. 2 XRD of the mixed oxides of MgAl and MgAlLi obtained from

heat treatment at 600 �C/0.5 h

Table 1 Crystallite size obtained from XRD and BET surface area of

the catalysts

Catalysts L200 (nm) Surface area (m2 g-1)

MgAl 2.6 201

MgAlLi-1 9.4 147

MgAlLi-5 19.4 38

MgAlLi-8 22.0 8

MgAlLi-10 25.5 5

MgAlLi Mixed Oxides Derived from Hydrotalcite 1319

123

affected the catalytic performances as will be seen in the

forthcoming section.

3.3 Catalytic Tests

The catalytic tests were monitored through the transesterifi-

cation reaction using methyl acetate and ethanol as model

reactants. Figure 6 displays the catalyst activity as a function of

its Li content. The samples were calcined at 600 �C for 0.5 h.

MgAl showed insignificant transesterification activity

(Fig. 6). Fraile et al. [23] also found very low activity for

MgAl in the reaction of methyl palmitate with methanol.

Curiously, the MgAl mixed oxide is reported in many

papers as an active catalyst for biodiesel production [2, 4–

6]. Possibly, this MgAl which only possess weak basic sites

revealed by TPD of CO2 (Fig. 5), is solely active under

more drastic reaction conditions like those reported in the

literature. Under the mild reaction conditions studied in

this work, the product formation is negligible. On the other

hand, the Li incorporation onto MgAl greatly increased the

activity (Fig. 6). It is well established that Li addition onto

MgO induces defect generations that are important for

heterogeneous catalysis. Li is capable of altering surface

reactivity by increasing the number of surface defects that

are chemically active, like oxygen vacancies and low

coordination sites, onto MgO. Li? can replace lattice Mg2?

due to their similar ionic radii, leading to the formation of

isolated low coordination O2- anions with high basic

strength [9, 19]. These isolated O2- anions may be

responsible for the extraordinary properties of MgAlLi in

transesterification reaction compared to MgAl. The presence

of highly basic Li2O in small amorphous domains not

detected by XRD can as well contribute for the activity [9].

Moreover, the ester conversion was dependent on the Li

content, the most active being the catalyst with the highest

Li loading (MgAlLi-10) (Fig. 6). The transesterification

activity became enhanced with the Li content despite surface

area reduction (see Table 1). The catalytic tests are in a good

agreement with the TPD of CO2 that revealed that the higher

the Li content, the higher the basic strength (see Fig. 5).

Therefore, the results suggest that, under mild reaction

conditions, only the strongest basic sites are involved.

The calcination temperature of the catalysts has also been

investigated. Temperatures C600 �C were studied (decom-

position temperature of LiNO3 used for impregnation, veri-

fied by thermogravimetric analysis). No difference was

observed in the activity of samples containing Li 10 wt%

treated at 600, 700 and 800 �C for 0.5 h. It is worth men-

tioning that, in the absence of thermal activation, the hydro-

talcite compounds were inactive, as also verified in other

works [2, 28, 29]. Nevertheless, the calcination time at a

temperature of 600 �C strongly affected the catalytic per-

formances. Thus, samples containing 10 wt% of Li (MgAlLi-

10) were calcined using a heating rate of 10 �C min-1 from

25 until 600 �C and kept at 600 �C for different time periods.

The catalytic tests are displayed in Fig. 6b.

The maximum ester conversion (ca. 94%) was obtained

for the sample calcined for 10 h at 600 �C. The reduction

Fig. 3 Field-emission scanning electron micrographies (FEG-SEM) of the MgAl hydrotalcite (a) and of the mixed oxides: MgAl (b), MgAlLi-8

(c) and MgAlLi-10 (d)

1320 C. S. Castro et al.

123

of the calcination time slightly decreased the activity until

0.5 h (ca. 86% of conversion). At 0 h at 600 �C though, the

conversion is considerably reduced (Fig. 6b). The presence

of residual LiNO3 not completely decomposed at 0 h of

calcination was detected by TGA (not shown). This can

explain the reduction in activity with the reduction of

calcination time. Indeed, LiNO3 salt was not active in the

transesterification reaction (conversion of \0.1% after

0.5 h and 4 wt% of LiNO3).

In summary, the optimization of the activating condi-

tions of the MgAlLi oxides revealed that a temperature of

600 �C and calcination time of 0.5 h produced active cat-

alysts (ca. 86% of conversion) in expense of long periods

of calcination requiring energy.

3.4 Catalysts Stability

The investigation of catalyst stability in the reaction

medium is of extreme importance for heterogeneous cata-

lysts considering industrial applications. Chemical analysis

of the supernatant solution after reaction with MgAlLi-10

revealed the presence of about 490 mg L-1 of lithium.

This represents &15% of the total lithium content for

MgAlLi-10. In fact, the cation leaching from hydrotalcite

compounds (e.g. potassium and sodium cations) has been

pointed out in the literature as the main drawback of this

type of catalyst applied for biodiesel production [23, 30].

The cation leaching can lead to catalyst deactivation,

product contamination and also limits the reutilization of

Fig. 4 XPS spectra of the Mg

2p and Li 1s levels

MgAlLi Mixed Oxides Derived from Hydrotalcite 1321

123

the catalyst in a subsequent batch process. In addition, the

cation leaching can form active species in the solution

capable of reacting in homogeneous phase. Considering

this, it is important to investigate if the Li leached from

MgAlLi oxide was able to form active homogeneous spe-

cies in the solution.

Rigorous proof of heterogeneity can be obtained only by

filtering the catalysts at the reaction temperature before

completion of the reaction and testing the filtrate for

activity [31]. For that, the MgAlLi-10 was filtered off after

20 min of reaction (ca. 30% of conversion) and the com-

position of the remaining solution was monitored. No

further reaction was observed afterwards (Fig. 7a). Thus,

the catalytic process showed to be strictly dependent on the

solid presence and the reaction occurs through heteroge-

neous process in the case of MgAlLi.

Another important characteristic of a heterogeneous

catalyst is the possibility of its recycling. After each

reaction, the solid was recovered by filtration, oven dried

and a new reaction mixture was placed in contact with the

used catalyst. Tests performed with MgAlLi-10 showed

significant deactivation after the third reaction cycle

(Fig. 7b). Strong adsorption of reactants or products during

transesterification reaction could be the responsible for the

catalyst deactivation. To investigate this, after each cycle

of reaction, the catalyst was recovered by filtration, oven

dried and recalcined (600 �C for 0.5 h) before being reused

in a new cycle of reaction. However, a similar deactivation

profile was obtained. Therefore, the cause of the deacti-

vation seems not to be ascribed to the strong adsorption of

0 100 200 300 400 500 600

0

100

200

300

400

500

600

MgAlLi - 10MgAlLi - 5

Des

orbe

d C

O2/

mV

g-1

Temperature/ºC

MgAl

Fig. 5 TPD-CO2 of the catalysts

0

10

20

30

40

50

60

70

80

90

100

Con

vers

ion/

%

Li content/ wt %

(a)

0 1 32 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 110

10

20

30

40

50

60

70

80

90

100C

onve

rsio

n/%

Calcination time/h

(b)Fig. 6 Effect of the lithium

content on the catalytic activity

of samples calcined at 600 �C

for 0.5 h (a) and effect of the

calcination permanence time at

600 �C of samples containing

10 wt% Li on the catalytic

transesterification (b)

0 20 40 60 80 100 120 140 160

0

10

20

30

40

50

with catalyst

Con

vers

ion/

%

Time/min

without catalyst

(a)

catalyst removal

3210

10

20

30

40

50

60

70

80

90

100

Con

vers

ion/

%

Reuse cycles

(b)

Fig. 7 Transesterification reaction in the presence of the MgAlLi-10 as function of the time and after catalyst separation from the reaction

medium (a) and the reuse cycles for the same catalyst (b)

1322 C. S. Castro et al.

123

reactants or products. The catalyst deactivation may be a

result of the gradual loss of lithium ions as observed from

the supernatant chemical analysis by AA.

4 Conclusions

MgAl hydrotalcite was synthesized by coprecipitation and

used as support for Li impregnation. After heat treatment,

MgAl mixed oxide presented negligible catalytic activity in

the transesterification reaction of the model reactants

methyl acetate and ethanol. However, MgAlLi mixed

oxides revealed to be very active under mild reaction

conditions. The catalyst performance was associated with

the Li loadings, the MgAlLi-10 being the most active

despite low surface area. The catalysts activities were

closely related to their basicity determined by TPD of CO2.

Li increased the basic strength possibly as function of the

defect generation onto solid surfaces. The temperature of

600 �C and time of 0.5 h were the result of the activation

step optimization for samples containing 10 wt% Li. Sta-

bility tests showed that the reaction is strictly dependent on

the solid presence in the solution and confirmed the process

heterogeneity in the case of MgAlLi. Li leaching from the

catalyst to the solution was detected. Indeed, alternative

synthesis methods have to be investigated aiming at the

increase of the catalyst stability in the reaction medium.

However, the high activity presented by the MgAlLi mixed

oxides showed that these materials are promising for

transesterification reactions and consequently for biodiesel

production.

Acknowledgments The authors are grateful to the CNPq (Brazil)

and to PRH-ANP for financial support.

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