Ht

Ca

b

c

a

ARRAA

KBTHC

1

aobaccfwsct

owcvra[g

t

0d

Applied Catalysis A: General 382 (2010) 158–166

Contents lists available at ScienceDirect

Applied Catalysis A: General

journa l homepage: www.e lsev ier .com/ locate /apcata

ighly effective MnCeOx catalysts for biodiesel production byransesterification of vegetable oils with methanol

. Cannillab, G. Bonuraa, E. Rombic, F. Arenaa,b, F. Frusteri a,b,∗

CNR-ITAE “Nicola Giordano”, Salita S. Lucia sopra Contesse, 5, I-98126, Messina, ItalyDip. di Chimica Industriale ed Ing. Materiali, Università di Messina, Salita Sperone 31, I-98166, Messina, ItalyDip. di Scienze Chimiche, Università di Cagliari-CSGI, Cittadella Universitaria, S.S. 554 bivio Sestu, 09042 Monserrato, Cagliari, Italy

r t i c l e i n f o

rticle history:eceived 25 February 2010eceived in revised form 14 April 2010

a b s t r a c t

This paper reports the results obtained using a novel MnCeOx system in the transesterification reactionof refined sunflower oil with methanol. The performance of such catalysts has been compared with thatof common acid supported catalysts. Results obtained revealed that MnCeOx system possesses a superior

ccepted 15 April 2010vailable online 4 May 2010

eywords:iodieselransesterification reaction

activity especially by operating at low temperatures (≤120 ◦C). Independently of Mn loading, the redox-precipitation method for the preparation of Mn-based systems allowed to obtain always high dispersedcatalysts and, as a consequence, a linear relationship between reaction rate and Mn loading was obtained.NH3-TPD and CO2-TPD measurements indicate that MnCeOx systems are characterized by a prevalentnature of basic sites. However, the catalyst performance is the result of a synergic role played by both

racte

eterogeneous (liquid–solid) catalysiseria–manganese catalysts

the surface acid/base cha

. Introduction

Biodiesel is an environmentally friendly, non-toxic, biodegrad-ble mixture, which can be used either as alternative “pure” fuelr for blending with conventional oil-derived fractions [1]. Sinceiodiesel is produced from renewable vegetable sources, over-ll “well-to-wheel” CO2 emissions are lower, thus reducing theontribution to the “greenhouse” effect [2]. The physico-chemicalharacteristics of biodiesel are similar to those of the fossil dieseluel in terms of energy density, cetane number and phase change,hile it is characterized by a higher flash point value. Moreover, a

uperior oxygen content (10–11%) improves the combustion effi-iency and the lubricant properties, the last contributing to reducehe engine stress and maintenance [1,3].

Biodiesel consists of long-chain fatty acid alkyl esters (FAAEs),btained by transesterification reaction of triacylglycerides (TGs)ith alcohols. Depending upon the climate and environmental

onditions, several countries are looking for different sources ofegetable oils [1]; soybean oil in the United States and Brazil [4–8],

apeseed [9,10] and sunflower [11,12] oil in Europe, palm oil [13]nd coconut oil [14] in the Asian countries. Cotton oil [15], safflower16], canola [17], jatropha [18], brassica carinata oil [19] and wasterease [3,20] are also considered as potential biodiesel feedstock.

∗ Corresponding author at: CNR-ITAE “Nicola Giordano”, Salita S. Lucia sopra Con-esse, 5, I-9816, Messina, Italy.

E-mail address: Francesco.Frusteri@itae.cnr.it (F. Frusteri).

926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apcata.2010.04.031

r and textural porosity.© 2010 Elsevier B.V. All rights reserved.

The structural composition, such as chain length and branching ordegree of unsaturation, affects the physico-chemical properties ofa fatty ester molecule, thus determining the overall properties ofthe biodiesel. Saturated compounds (myristic acid, C14:0; palmiticacid, C16:0; stearic acid, C18:0) exhibit high heat of combustionvalues, high cetane numbers and are less ready to oxidation thanunsaturated compounds, however they crystallize at high temper-ature [3,18]. Biodiesel from sunflower oil is a highly unsaturatedmixture, being more prone to oxidation, but its cold flow propertiesare acceptable. Although simple linear alcohols, such as methanol,ethanol, propanol and butanol, are generally used for the oil trans-esterification, methanol is widely used for its lower cost and itsphysico-chemical characteristics (polarity and the shortest chainalcohol) [21].

Biodiesel production involves three consecutive reactions, withdiglycerides (DGs) and monoglycerides (MGs) as intermediates andglycerol as a valuable by-product [22]:

TG + CH3OH � diglyceride + R1COOCH3 (1)

DG + CH3OH � monoglyceride + R2COOCH3 (2)

MG + CH3OH � glycerol + R3COOCH3 (3)

All the above reactions are reversible and then an excess of alcohol

is used to obtain high yields (>99%) of methyl esters (MEs) that alsoallow an easier separation of ME from glycerol [2]. Reaction temper-ature, pressure, alcohol–oil molar ratio, water and free fatty acids(FFAs) content are the main parameters affecting the efficiency ofthe process. However, a limited solubility of TGs in the alcoholic

C. Cannilla et al. / Applied Catalysis A

Table 1Composition of the sunflower oil.

Acidity indexa Composition (wt.%)

Palmitic (C16:0) 6.8Stearic (18:0) 3.8Oleic (C18:1) 27.1Linoleic (C18:2) 61.1Linolenic (C18:3) 0.2

pm

pagtrbucutdpplFdahmfa[o[hz[gt5

oer

2

2

t

tion of a 0.3 mol L−1 KOH solution. After titration, the solid wasdigested for 6 h at 60 ◦C and then filtered and repeatedly washedwith hot distilled water. Then, the solid was dried at 110 ◦C for 16 h

◦

TP

Arachidate (C20:2) 0.3

a Numbers in parenthesis show the number of carbon atoms andthe unsaturated centres.

hase and the presence of the solid catalyst require an efficientixing to improve the interphase mass transfer rate [21].Kinetics of the transesterification reactions can be efficiently

romoted by either basic or acid catalysts, while high temper-ture (350–400 ◦C) and pressure (100–250 bar) are required toet reasonable yields in supercritical methanol [23]. However,he nature of the catalyst depends upon feedstock composition,eaction conditions and post-separation steps. Although low-costasic catalysts, such as potassium or sodium hydroxide, can besed, more than half of the current production processes areatalyzed by sodium methoxide [5,19]. Basic catalysts are activender mild conditions (80 ◦C), though their suitability is limitedo refined oils with low FFAs concentration (<0.5 wt.%) and anhy-rous (H2O content < 0.06 wt.%) alcohol [9,24], as side reactions canroduce soaps and gels requiring additional separation steps toroduce commercial-grade biodiesel. On the contrary, acid cata-

ysts can simultaneously promote the TGs transesterification andFAs esterification of low-cost feedstock [20,25] even if severalrawbacks exist: longer reaction time, higher operating temper-ture and the requirement of acid-resistant reactors. Then, manyeterogeneous basic [7] and acidic systems [26], as well as enzy-atic catalysts [23,27], have been proposed in the last decade

or the transesterification process. In particular, alkali [9], suchs Na/NaOH/�-Al2O3 [28], alkaline-earth compounds, such as CaO13], Li/CaO [29] and SrO [4], rare-earth metal-loaded inorganicxides [5,10], Amberlyst-15 [30], Fe–Zn double-metal cyanide31], vanadyl phosphate [32], zirconia-supported isopoly- andeteropoly-tungstate [12], Cs-doped heteropolyacid [33], modifiedeolites [8,34], sulphated metal oxides [14,25,35] and hydrotalcites8,36] ensure the best catalytic performances. Recently, a hetero-eneous system, based on a mixture of ZnO, Al2O3 and ZnAl2O4 forransesterification of vegetable oils with methanol at 230 ◦C and0 atm, has been also proposed [37].

This work is aimed at investigating the catalytic performancef a new class of manganese–ceria mixed oxides in the trans-sterification of sunflower oil with methanol, in comparison toepresentative acid and basic solid catalysts.

. Experimental

.1. Chemicals

Commercial-grade sunflower oil (density, 0.85 g mL−1), con-aining saturated (10%), mono-unsaturated (27%) and poly-

able 2hysico-chemical properties of the solid acid systems.

Catalyst Active phase (loading wt.%) SABET (m

HPW-17 Phosphotungstic acid (15) 200HPMo-17 Phosphomolybdic acid (15) 264Cs-HPW Phosphotungstic acid exchanged with cesium (0.5) 207NS-20 Nafion® (20) 187

a Average pore diameter (4 PV/SABET).b ZPC method.

: General 382 (2010) 158–166 159

unsaturated (63%) esters, and methanol (MeOH, HPLC-grade, 99.8%)were used. Pure GC standard chemicals, including fatty acidmethyl esters mix and methyl eptadecanoate, were supplied bySigma–Aldrich. The typical composition of a commercial sun-flower oil with a low content of FFAs (0.07 wt.%) is reported inTable 1.

2.2. Catalysts preparation

2.2.1. Silica-supported tungstophosphoric (HPW-17) andmolybdophosphoric (HPMo-17) heteropolyacids

Catalysts were prepared by the incipient wetness method usingSiO2 (Cab-O-Sil “LM50”; SABET, 250 m2 g−1) as support and twoaqueous solutions containing 17 wt.% of tungstophosphoric het-eropolyacid and 17 wt.% of molybdophosphoric heteropolyacid,respectively. At the end of the impregnation, the samples weredried at 120 ◦C for 2 h and activated in a stream of dry nitrogenfor 2 h at 300 ◦C.

2.2.2. Cs-exchanged phosphotungstic acid (Cs-HPW)The sample was prepared by partial neutralization of the

phosphotungstic acid (H3PW12O40) using an aqueous solution ofCs2CO3, added drop-wise according to the procedure elsewheredescribed [38,39]. The Cs0.5H2.5PW12O40 salt obtained was filtered,dried and activated in a stream of dry nitrogen for 2 h at 300 ◦C.

2.2.3. Nafion®/SiO2 (NS-20)A commercial sample of silica (Cab-O-Sil® “M5”, SABET,

207 m2 g−1) was dispersed under stirring in an ethanolic solutioncontaining a known amount of Nafion®. After filtration, the solidsample was dried at 80 ◦C overnight. The catalyst so obtained waspressed, crushed and sieved. The 40–70 mesh fraction was usedboth for characterization and catalytic measurements.

2.2.4. MnCeOx systemsA series of manganese–ceria catalysts, with Mn/Ce atomic

ratio ranging between 0.4 and 3.4, was prepared via theredox-precipitation route, according to the procedure elsewheredescribed [40]. The catalysts were dried at 100 ◦C and calcined inair at 400 ◦C for 6 h.

2.2.5. “Bulk” CeO2, MnO2, ZnOA known amount of (NH4)2Ce(NO3)6 was dissolved in hot deion-

ized water and slowly added to a solution of KOH maintained at60 ◦C, under vigorous stirring, at constant pH (8.0 ± 0.2) by addi-

and calcined for 6 h in air at 400 C.A commercial “precipitated-activated” MnO2 sample (Fluka

product) was selected as a “standard” sample.A commercial ZnO sample (Aldrich product) was also used as a

reference system.

2 g−1) PV (cm3 g−1) APDa (Å) Acidityb (mmolH+ g−1)

0.62 124 0.740.46 70 1.130.80 155 0.810.52 111 0.92

1 lysis A: General 382 (2010) 158–166

2

2

mAw(

2d

dagaatmt

2

ppAs“

2

r2oud(slqC

2

fawsmOstmftt

2

uCr

alan

dac

id–b

asic

pro

per

ties

ofM

nC

eOx

syst

ems

alon

gw

ith

“bu

lk”

Mn

O2

and

CeO

2sa

mp

les.

Mn

at/C

e at

Mn

(wt.

%)

K(w

t.%

)SA

BET

(m2

g−1)

PV(c

m3

g−1)

APD

(Å)

Aci

dit

yB

asic

ity

B/A

ara

tio

Spec

ific

(�m

olN

H3

g cat

−1)

Surf

ace

(�m

olN

H3

m−2

)Sp

ecifi

c(�

mol

CO

2g c

at−1

)Su

rfac

e(�

mol

CO

2m

−2)

0.41

10.9

–17

30.

4268

187

1.1

460

2.7

2.4

0.87

19.3

0.54

150

0.59

133

N.D

.–

365

2.4

–1.

2925

.41.

7014

30.

7319

136

.90.

336

02.

58.

33.

4041

.05.

2012

40.

9031

464

.00.

524

01.

93.

8–

––

920.

274

N.D

.–

271

2.9

––

––

890.

125

N.D

.–

237

2.6

–

twee

nsu

rfac

eba

sici

tyan

dac

idit

y.

60 C. Cannilla et al. / Applied Cata

.3. Catalysts characterization

.3.1. X-ray fluorescence (XRF)The analytical composition of the Mn-based catalysts was deter-

ined by X-rays fluorescence (XRF) measurements, using a BrukerXS-S4 Explorer Spectrometer. The concentration of elementsas determined by the emission value of K�1 transitions of Mn

E = 5.9 keV), Ce (E = 4.8 keV) and K (E = 3.3 keV).

.3.2. Surface area (SABET), pore volume (PV) and pore sizeistribution (PSD)

Surface area, pore volume and pore size distribution wereetermined from the nitrogen adsorption/desorption isothermst −196 ◦C, using a Carlo Erba (Sorptomatic Instruments CE Series)as adsorption device. Before analysis, all samples were outgassedt 120 ◦C under vacuum for 2 h. The isotherms were elaboratedccording to the BET method for surface area calculation, withhe Horwarth–Kavazoe (HK) and Barrett–Joyner–Halenda (BJH)

ethods used for the evaluation of micropore and mesopore dis-ributions, respectively.

.3.3. Acid capacityAcid capacity was determined by potentiometric titrations (zero

oint charge method, ZPC). About 0.1 g of each sample was dis-ersed under stirring in an aqueous solution of NaNO3 0.5 mol L−1.cidity, measured by an electrode Orion ROSS, was evaluated con-idering the pH value at which particles suspended in solution havezero charge”.

.3.4. CO2–temperature programmed desorptionCO2-TPD measurements in the range of 20–350 ◦C were car-

ied out using a linear quartz micro-reactor (dint, 4 mm; l,00 mm) loaded with ca. 50 mg of catalyst, using a heating ratef 20 ◦C min−1. After an in situ pre-treatment at 400 ◦C (30 min)nder a 10% O2/He flow (25 STP mL min−1), the catalyst was cooledown to room temperature, switched in the He carrier flow30 STP mL min−1) and saturated with CO2 pulses (0.2 mL). Afteraturation, the sample was purged in the carrier flow until stabi-ization of the baseline. The desorption process was monitored anduantified by a TCD, preliminarily calibrated by the injection of pureO2 pulses.

.3.5. NH3–temperature programmed desorptionNH3-TPD measurements in the range 100–500 ◦C were per-

ormed in a flow apparatus, using a heating rate of 12 ◦C min−1 andHe carrier flow rate of 25 STP mL min−1. The desorption processas monitored by a quadrupole mass spectrometer, acquiring the

ignal relative to the mass-to-charge (m/z) ratio 17 (NH3). Beforeeasurements, the samples were treated in situ for 30 min in a 10%

2/He flow (25 STP mL min−1) at 400 ◦C, except for the HPMo-17ample that was pre-treated at 300 ◦C in helium flow to avoid anyhermal decomposition phenomenon. After the activation treat-

ent, the samples were cooled down to 150 ◦C and then saturatedor 30 min in a 5% NH3/He stream (25 STP mL min−1). Thereafter,he samples were purged in He carrier flow until stabilization ofhe signal.

.3.6. X-ray diffractionX-ray diffraction analysis of powdered samples was performed

sing a Philips X-Pert diffractometer equipped with a Ni �-filteredu K� radiation at 40 kV and 30 mA. Data were collected over a 2�ange of 20–85◦, with a step size of 0.05◦ at a speed of 0.05◦ s−1. Ta

ble

3Ph

ysic

o-ch

emic

Cat

alys

t

Mn

0.3C

e 0.7

Mn

0.4C

e 0.6

Mn

0.5C

e 0.5

Mn

0.7C

e 0.3

Mn

O2

CeO

2

aB/

Ara

tio

be

C. Cannilla et al. / Applied Catalysis A: General 382 (2010) 158–166 161

2

cIr

F1t

Fig. 1. PSD of the MnCeOx systems: influence of the Mn/Ce atomic ratio.

.4. Catalyst testing

Transesterification reaction of sunflower oil with methanol wasarried out in a 300 mL stainless steel AISI 316 autoclave (Parrnstruments) equipped with a magnetic stirrer, in the temperatureange of 65–200 ◦C, according to the following procedure: a known

ig. 2. NH3 desorption profiles as a function of temperature in the range of00–500 ◦C: silica-supported HPMo-17 heteropolyacid and structured MnCeOx sys-ems.

Fig. 3. Deconvolution plot of the CO2-TPD profiles in the range of 0–350 ◦C.

amount of sunflower oil (80 g) was charged into the reactor, adding44 mL of methanol, which corresponds to a MeOH/oil molar ratio(Rmet/oil) of 12. Methanol was previously mixed with the catalyst, inorder to operate with a concentration of 1 wt.% of catalyst referredto the oil. The reactor was preliminarily flushed under a nitrogenflow to remove the air and heated up to the reaction temperature(ca. 15 min). At the end of the run, the reactor was cooled down toroom temperature by putting it into an ice-bath, thus allowing allthe gas-phase components to be condensed. Then, the reactor wasopened and the catalyst separated by centrifugation and filtration.The reaction mixture, after the evaporation of unreacted methanol,resulted to be composed by a top layer containing oil and methylesters products and a bottom layer of glycerol that were furtherseparated by centrifugation.

According to the EN 14103 method suitable for analysis ofmethyl esters C14–C24, the reaction mixture was analyzed off-line,by a gas chromatograph HP 7890N, provided with a capillary col-umn HP Innowax (l, 30 m; i.d., 0.32 �m; film thickness, 0.25 �m)and a flame ionization detector (FID), using He as carrier gas. Thefollowing temperature program was adopted for the separationof the various compounds: 9 min at 210 ◦C; from 210 to 230 ◦C(with a heating rate of 20 ◦C min−1) and 230 ◦C for 10 min. Thesamples (1 �l) were injected in split mode (split ratio of 80:1)into the GC using an automatic sampler Agilent 7683B Series, eachconversion-selectivity data set being calculated from the average ofthree independent measurements with an accuracy of ± 1%. Methylpalmitate, methyl stearate, methyl oleate, methyl linoleate, methyllinolenate and methyl arachidate were used as chemical GC stan-dards for analysis calibration. The biodiesel analysis was carriedout dissolving 50 mg of sample into 1 mL of methyl heptadecanoatesolution (internal standard).

The yield to biodiesel was calculated from the content of methylesters analyzed by the following equation:

Yield (%) = 100 × molME

3 × moloil

162 C. Cannilla et al. / Applied Catalysis A: General 382 (2010) 158–166

Table 4Basic sites distribution evaluated on the basis of CO2-TPD profiles.

Catalyst Basic strength (�molCO2 m−2) Contribution (%) of medium and strong basic sites

Weak (T1, 93 ◦C) Medium (T2, 165 ◦C) Strong (T3, 289 ◦C)

0010

Tybo

3

3

c

uapyt

mwgrdtpaoi

ampctB

1mtic(Mat1loTamb

Me(

trend with the Mn/Ce ratio.The highest basicity of Mn0.3Ce0.7 (either specific or surface)

could be justified considering that the contribution of weak basicsites of ceria in such sample is significant [42]. On the contrary,

Mn0.3Ce0.7 0.85 0.90Mn0.4Ce0.6 0.83 0.93Mn0.5Ce0.5 0.38 0.96Mn0.7Ce0.3 0.35 0.78

he factor “3” takes into account that each triglyceride moleculeields three methyl esters molecules, while the molecular weight ofiodiesel was calculated from the composition of a representativef sunflower oil (see Table 1).

. Results and discussion

.1. Structural and morphological properties

The list of the studied catalysts along with their main physico-hemical properties are reported in Tables 2 and 3.

In spite of different surface area (187–264 m2 g−1) and pore vol-me (0.5–0.8 cm3 g−1), the solid acid catalysts are characterized bysimilar proton-exchange capacity (≈1.0 mmolH+ gcat

−1), while theartial neutralization of the HPW-17 sample with Cs+ ions does notield any significant changes neither in terms of surface area nor inhe proton-exchange capacity (Table 2) [33].

The MnCeOx catalysts are characterized by SA values decreasingonotonically with the Mn/Ce atomic ratio from 173 to 124 m2 g−1,hile a PV growing from 0.42 to 0.90 cm3 g−1 accounts for a pro-

ressive broadening of the APD from 68 to 314 Å (Table 3). Suchegular trends points out as the textural properties are tightlyependent on the MnOx loading [40]. Indeed, the pore size dis-ribution (see Fig. 1) shows for all the systems a Gaussian-shapedrofile, whose maximum shifts from ca. 8 to 40–50 nm as the Mn/Cetomic ratio rises from 0.41 to 3.40. This is related to the formationf large clusters at higher MnOx loading, giving rise to a comparablenter-particle porosity [43].

As the transesterification reaction can be catalyzed by eithercidic or basic systems, NH3 and CO2 chemisorption/desorptioneasurements were carried out to determine the surface acid–base

roperties; namely, NH3 desorption data probe strength and con-entration of acidic sites irrespective of their nature [39,41], whilehe chemisorption of the acidic CO2 molecule monitors bothrønsted and Lewis basicity.

The ammonia desorption profiles in Fig. 2 shown that the HPMo-7 sample has a broad peak spanned in the range 150–400 ◦C, with aaximum at ca. 200 ◦C and markedly tailed at higher temperatures;

hat is diagnostic of a rather large strength distribution, account-ng for a concentration of surface acid sites of 1.16 mmol g−1 wellomparing with data obtained by titration method (1.13 mmol g−1)see Table 2). Still in agreement with potentiometric data, the

nCeOx systems feature a much poorer surface affinity towardsmmonia, with some slight influence of the Mn loading. In par-icular, Mn0.3Ce0.7 shows a higher acid character since absorbs86 �mol NH3 gcat

−1, while Mn0.5Ce0.5 and Mn0.7Ce0.3 have a muchower acidity, as they adsorb an amount of NH3 which is close to onerder of magnitude lower (37 and 64 �mol NH3 gcat

−1 respectively).he maximum of desorption peak of Mn0.3Ce0.7 and Mn0.5Ce0.5 isround 150–200 ◦C, whereas the broad signal of Mn0.7Ce0.3 has aaximum at around 320 ◦C. The highest acidity of Mn0.3Ce0.7 could

e related to a higher availability of weak acid sites of ceria [42].As regards the bulk oxides (see Table 3), the commercial

nO2 results to be characterized by significant basic properties,ven more pronounced than CeO2 under the same surface area2.9 �mol m−2). Moreover, differently from ceria, MnO2 exhibits

.90 67

.93 69

.17 85

.81 82

two desorption peaks centred at around 107 and 270 ◦C, accountingfor weak and strong basic strength sites (see Fig. 3).

The CO2-TPD profiles of Mn-based catalysts show for all thesamples a desorption profile spanned in the range 20–350 ◦C, mon-itoring the presence of surface basic sites with a broad strengthdistribution, the specific (�molCO2 gcat

−1) and intrinsic basicity(�molCO2 m−2) values being summarized in Table 3. Interestingly,in all the MnCeOx systems a dominant basic character is observable(B/A > 1). Really, CO2 strongly interacts with surface basic centers ofMn-based systems, but, from a quantitative point of view, it is noteasy to state a well defined trend with the Mn loading. Indeed, byincreasing the Mn/Ce ratio, several reactive surface oxygen speciescan be involved in CO2 adsorption, leading to different distribu-tion profiles [40,48]. Then, in order to discriminate the contributionof basic sites of different strength, a deconvolution analysis of theMnCeOx profiles obtained by CO2 chemisorption has been done. Byan iterative data treatment, it was found that all TPD profiles canbe described by a linear combination of three Gaussian peaks withmaxima at 93, 165 and 289 ◦C respectively (see Table 4). For all thesystems, this deconvolution analysis provides a very accurate fit-ting (r2 > 0.99) of the experimental curves, as shown in Fig. 3. On thebasis of literature evidences [7], the desorption peak at low tem-perature (T1) was attributed to the interaction of CO2 with sitesof weak basic strength, mostly corresponding to OH− groups onthe catalyst surface. Instead, the peaks at higher temperature (T2and T3) were attributed to both medium and strong sites, relatedrespectively to the oxygen of Men+–O2− ion pairs and isolated O2−

anions (250–350 ◦C), that can be expected to possess Lewis basecharacter.

Taking into account such considerations, it is possible to statethat by increasing the Mn loading, the concentration of weaksites progressively decreases from 0.85 until 0.35 �mol m−2 (seeTable 4), while medium and strong sites do not follow a progressive

Fig. 4. XRD patterns of MnCeOx systems.

C. Cannilla et al. / Applied Catalysis A: General 382 (2010) 158–166 163

Fwt

a(w

emswrTrtiha

3

ssMvattt

FR

ig. 5. ME yield (%) with differently functionalized solid catalysts: sunflower oileight, 80 g; Rmet/oil , 12 mol/mol; Rcat/oil , 1 wt.% (3 wt.% for ZnO); TR, 200 ◦C; reaction

ime, 5 h.

lthough Mn0.7Ce0.3 exhibits the lowest value of both specific240 �mol gcat

−1) and surface basicity (1.9 �mol m−2), it shows aider population of medium and strong basic sites (82%).

In order to attain further information on the structural prop-rties of the MnCeOx systems, samples were analyzed by XRDeasurements. As shown in Fig. 4, irrespective of the compo-

ition, the redox precipitated samples show diffraction patternsith a main broad and “smoothed” signal spanned in the 2�

ange 20–40◦ and a less intense component between 40◦ and 60◦.hese diffraction patterns are diagnostic of the lack of a “long-ange” crystalline order which, according to the characteristics ofhe redox-precipitation process, can be attributed to an intimatenteraction of MnOx and CeOx species at a (quasi)molecular level,indering the growth of “large” crystalline domains [43–45] andllowing the formation of nanosized MnCeOx oxide particles.

.2. Catalytic activity

The results of a preliminary screening of different catalysts in theunflower oil transesterification with methanol at 200 ◦C, includingilica-supported heteropolyacids, the commercial bulk ZnO and onenCeOx catalyst, are shown in Fig. 5. Such data refer to the oil con-

ersion values taken after 5 h of reaction using a Rmet/oil of 12 andcatalyst/oil mass ratio (Rcat/oil) of 1 wt.%. In addition, considering

hat at 200 ◦C the uncatalyzed reaction could occur [32], a blankest was carried out. The result of the blank test reveals that theransesterification reaction proceeds, but with a rate much lower

ig. 7. Influence of reaction time on oil conversion and methyl esters yield [A]; reactioncat/oil , 1 wt.%; TR, 140 ◦C.

Fig. 6. Oil conversion and ME yield as a function of reaction temperature: Mn0.5Ce0.5

catalyst; sunflower oil weight, 80 g; Rmet/oil , 12 mol/mol; Rcat/oil , 1 wt.%; reaction time,5 h.

(ME yield of about 10%) than that observed in presence of cata-lyst. The methyl esters yield of the commercial ZnO sample (Rcat/oil,3 wt.%) is comparable with literature data (75%) under analogousreaction conditions [44], while the activity scale of the supportedand precipitated systems, in terms of ME yield, resulted to be thefollowing:

Mn0.5Ce0.5 > HPMo-17 > HPW-17 > NS-20 > Cs-HPW

indicating that the Mn0.5Ce0.5 sample ensured the best catalyticperformance. It is important also to underline that with such a cat-alyst, an easier separation of the reaction products at the end of run(clean solution) was allowed. However, the results confirmed thatthe typical solid acidic systems (HPW-17 and HPMo-17 samples)also exhibit a considerable transesterification activity due to thepresence of surface Brønsted acid sites [46,47].

The preliminary screening of the different catalytic systemsprompted us to shed light into the behaviour pattern of the novelMnCeOx systems and factors affecting a potential process develop-ment based on their use.

The effect of the reaction temperature on biodiesel yield in therange of 120–200 ◦C, using Mn0.5Ce0.5, catalyst is shown in Fig. 6.As expected, the ME yield increases with reaction temperature. Inaddition the higher temperature mitigated the interphase diffusion

phenomena favouring a better mixing of the two phases, alcoholand oil [17]. It is important to observe that, the reaction proceedswith a significant rate even at 120 ◦C obtaining an oil conversion ofca. 40% and a ME yield of 37%. Considering the low Rcat/oil (1 wt.%)used, in comparison to higher ratios normally used in similar exper-

kinetics for Mn0.5Ce0.5 catalyst [B]. Sunflower oil weight, 80 g; Rmet/oil , 12 mol/mol;

164 C. Cannilla et al. / Applied Catalysis A: General 382 (2010) 158–166

F1

iii

tuiMsooito

liweiccu5M

rFtNsiwteMrp

pvb(

reaction, like TGs, could be limited by the catalyst pore diameter.Considering that the redox-precipitation method employed to

prepare the MnCeOx catalysts foresees the use of potassium (KOH)that could remain in the structure (mostly in the form of carbonatespecies) and that K also could play a catalytic role in the transesteri-

ig. 8. ME yield as a function of Mn loading: sunflower oil weight, 80 g; Rmet/oil ,2 mol/mol; Rcat/oil , 1 wt.%; TR, 140 ◦C; reaction time, 1 h.

ments (i.e., 3–5 wt.%) [5,12,31] and the low process temperaturenvestigated, it is evident that the result obtained appears verynteresting in view of potential practical exploitation.

Indeed, the data in Fig. 6 show that the catalytic transesterifica-ion of sunflower oil with methanol can be effectively run at 140 ◦Csing the MnCeOx system. Therefore the attention was focused to

nvestigate the influence of reaction time on oil conversion andE yield. Results obtained at 140 ◦C (see Fig. 7A), using Mn0.5Ce0.5

ample show that oil conversion trend satisfactorily obeys to first-rder reaction kinetics (see Fig. 7 B). Such result, contrarily to whatbserved by other authors [17,49], points out the absence of anynduction time due to interphase diffusional resistances. In addi-ion, results reveal that, by operating with Rcat/oil 1 wt.%, to obtainil conversion close to 100%, long reaction time is requested.

As the physico-chemical properties can affect the functiona-ity of the MnCeOx system, the influence of the manganese load-ng on the catalytic activity was investigated at 140 ◦C. Experiments

ere stopped after 1 h of reaction time to obtain data far away fromquilibrium conditions. A blank test was also carried out, indicat-ng that the contribution of the thermal conversion is very low (oilonversion < 1%) whereas a marked influence of MnOx loading onatalyst performance is evident. As shown in Fig. 8, ME yield reg-larly increases with Mn loading up to a maximum value of ca.0% obtained with Mn0.7Ce0.3 catalyst, characterized by the highestn/Ce atomic ratio (3.4).Taking into account the results obtained in kinetic regime, the

eaction rate was plotted as a function of the Mn loading (seeig. 9). Referring the rate to both catalyst and manganese mass,wo straight-line relationships with a different slope were found.otably, this finding would signal a constant intrinsic activity of the

urface active sites due to a constant dispersion of the active phasen the whole range of the Mn loading (9–41 wt.%). These evidences

ell match with previous characterization results of similar sys-ems [43], since it was shown that the redox-precipitation methodffectively enables a uniform “molecular-like” dispersion of thenOx species across the ceria matrix, irrespective of the Mn/Ce

atio, due to the selective “co-generation” of the MnOx and CeOx

hases [43–45].

Then, to disclose the influence of the main physico-chemical

roperties on the catalytic activity of the MnCeOx system, thealues of the reaction rate have been plotted as a function ofoth average pore diameter (APD) and intrinsic basic capacitysee Fig. 10). It is evident that the surface chemical proper-

Fig. 9. Rate of ME formation under kinetic regime: sunflower oil weight, 80 g;Rmet/oil , 12 mol/mol; TR, 140 ◦C; reaction time, 1 h.

ties of the systems do not allow a proper rationalization of thecatalyst performance, as the reaction rate decreases with the con-centration of surface basic sites, resulting the highest for theMn0.7Ce0.3 sample characterized by the lower intrinsic basic capac-ity (1.9 �molCO2 m−2). In spite of this, the largest population ofmedium and strong basic sites per surface unit on the Mn0.7Ce0.3and Mn0.5Ce0.5 (see Table 4) reflects a lower concentration of weaksites. That evidence could explain their superior activity in respectto the Mn0.4Ce0.6 and Mn0.3Ce0.7 catalysts. However, the differentreactivity of Mn0.7Ce0.3 and Mn0.5Ce0.5 points out that the basicstrength controls the reaction but, evidently, other factors couldplay an important role, like the porosity. Really, the main differ-ences between the two systems are represented by PV and APDvalues, being respectively 0.73 cm3 g−1 and 191 Å for Mn0.5Ce0.5and 0.90 cm3 g−1 and 314 Å for Mn0.7Ce0.3. The reaction rate sig-nificantly increased with APD (see Fig. 10), thus highlighting thatcatalyst porosity strictly influences the active sites accessibilitythat, in case of big molecules involved in the transesterification

Fig. 10. Correlation among textural or surface properties with catalytic data (TR,140 ◦C; reaction time, 1 h) of the Mn-based systems.

C. Cannilla et al. / Applied Catalysis A: General 382 (2010) 158–166 165

Table 5Influence of K content on the catalyst performance.

Catalyst K (wt.%) Mn (wt.%) Reaction rate (molME gcat−1 min−1) ME yield (%)

Mn0.5Ce0.5 ma – 25 5.5 × 10−4 10Mn0.5Ce0.5 1.7 25 7.9 × 10−4 15Mn0.7Ce0.3 eb 1.7 41 1.1 × 10−3 20Mn0.7Ce0.3 wc 4.3 41 2.3 × 10−3 40Mn Ce 5.2 41 2.9 × 10−3 50

fipppafnKr

IdMMKyMviopwapstTsisrtsscscmtn

4

Mfif

-

[[

[

[

[

[

[[[[

[[[

[[[[

[

[

0.7 0.3

a Precipitation with NH4OH .b Washing with a 10−3 M solution of HNO3.c Washing with deionized water repeatedly.

cation reaction, some experiments were performed with samplesreviously treated to reduce or eliminate K from the catalyst com-osition. On this account, a MnCeOx sample (Mn0.7Ce0.3 w) wasrepared by using the redox-precipitation method, washing it forlong time with deionized water. An amount of such sample was

urther washed in an acid solution (10−3 mol L−1, HNO3) to elimi-ate K (Mn0.7Ce0.3 e). In addition, a new MnCeOx sample without(Mn0.5Ce0.5 m) was prepared by using NH4OH as the precipitant

eactant.The results obtained using such catalysts are reported in Table 5.

t can be seen that, in terms of reaction rate and ME yield,ata obtained are compared with that of two representativenCeOx samples prepared by redox-precipitation route with KOH,n0.5Ce0.5 and Mn0.7Ce0.3. Using the Mn0.5Ce0.5 m catalyst without

, it is possible to attain a ME yield of 10% in comparison to a MEield of 15% obtained with Mn0.5Ce0.5 containing 1.7% of K. At highn/Ce ratio the ME yield significantly increases, with a maximum

alue of 50% on the Mn0.7Ce0.3 sample. Such a catalyst, character-zed by Mn loading of 41 wt.%, exhibits the best performance in termf reaction rate as well. Indeed, on similar but timely washed sam-les, like Mn0.7Ce0.3 w and Mn0.7Ce0.3 e, a lower conversion rateas obtained in correspondence of the sample containing a lower

mount of K. This result has forced us to investigate the leachinghenomena that could occur during reaction. Then, the Mn0.7Ce0.3ample was soaked with pure methanol for 1 h at 140 ◦C and thenhe methanol solution was analyzed by X-ray fluorescence (XRF).he XRF analysis revealed that only trace of both Mn and K goes inolution. However, to exclude that such low amount of Mn and Kn liquid phase could affect the reaction, the recovered methanololution was used as medium reaction for a new experiment. Sameesult of the uncatalyzed run was obtained, demonstrating so thathe low amount of Mn and K dissolved in liquid phase does notignificantly contribute to alter the catalytic activity of Mn-basedystems and therefore the homogeneous catalytic route could beonsidered not very significant. As a first hypothesis, it could be con-idered that potassium act as a “simple” structural promoter (notompletely removable by washing) or rather as an electronic pro-oter able to improve the functionality of manganese. At moment,

his is only a mere speculation and additional investigations areecessary to confirm the assumption.

. Conclusions

The results here reported clearly demonstrated that the novelnCeOx systems are very active and selective in the transesteri-

cation reaction of vegetable oils with alcohols. In particular theollowing conclusions can be drawn:

MnCeOx catalytic systems, characterized by high surface area,

high chemical and thermal stability and high Mn dispersion,result to be very active in the biodiesel production by transester-ification reaction of sunflower oil with methanol, at temperaturelower than that employed for acid catalytic reactions. High MEyield could be achieved using MnCeOx systems at TR not higher

[[

[

than 140 ◦C in 5 h of reaction, by operating at low catalyst/oil ratio(1 wt.%), without evident catalyst deactivation phenomena;

- The obtainment of a linear relationship between reaction rate andMn loading confirms that the redox-precipitation method allowsto obtain highly dispersed catalysts even at high Mn loading;

- Correlations between catalytic activity of MnCeOx catalysts andphysico-chemical properties highlight that the basic strengthcontrols the reaction but, owing to the large dimension of themolecules involved in the transesterification reaction, the acces-sibility of active sites has also to be taken into account. The activitydoes not depend only on the acid–base capacity, but it is the resultof a synergic role played by both the surface properties and thetextural porosity;

- K, incorporated in the catalyst structure during the preparationprocedure, could contribute to enhance the catalytic performanceof MnCeOx system. Further investigations are necessary to clarifyits role.

References

[1] R. Jothiramalingam, M.K. Wang, Ind. Eng. Chem. Res. 48 (2009) 6162–6172.[2] A. Demirbas, Energ. Convers. Manage. 50 (2009) 14–34.[3] M. Canakci, Bioresour. Technol. 98 (2007) 183–190.[4] X. Liu, H. He, Y. Wang, S. Zhu, Catal. Commun. 8 (2007) 1107–1111.[5] Z. Yang, W. Xie, Fuel Process. Technol. 88 (2007) 631–638.[6] W. Xie, H. Peng, L. Chen, J. Mol. Catal. A: Chem. 246 (2006) 24–32.[7] M.D. Serio, M. Ledda, M. Cozzolino, G. Minutillo, R. Tesser, E. Santacesaria, Ind.

Eng. Chem. Res. 45 (2006) 3009–3014.[8] G. Suppes, M. Dasari, E. Doskocil, P. Mankidy, M. Goff, Appl. Catal. A 257 (2004)

213–223.[9] S. Gryglewich, Bioresour. Technol. 70 (1999) 249–253.10] H. Sun, K. Hu, H. Lou, X. Zheng, Energy Fuels 22 (2008) 2756–2760.11] G. Arzamendi, I. Campo, E. Arguinarena, M. Sánchez, M. Montes, L.M. Gandía,

Chem. Eng. J. 134 (2007) 123–130.12] G. Sunita, B.M. Devassy, A. Vinu, D.P. Sawant, V.V. Balasubramanian, S.B. Hal-

ligudi, Catal. Commun. 9 (2008) 696–702.13] K. Noiroj, P. Intarapong, A. Luengnaruemitchai, S.A. Jai-In, Renew. Energy 34 (4)

(2009) 1145–1150.14] J. Jitputti, B. Kitiyanan, P. Rangsunvigit, K. Bunyakiat, L. Attanatho, P. Jenvanit-

panjakul, Chem. Eng. J. 116 (2006) 61–66.15] A. Keskin, M. Gürü, D. Altiparmak, K. Aydin, Renew. Energy 33 (4) (2008)

553–557.16] U. Rashid, F. Anwar, Energy Fuels 22 (2008) 1306–1312.17] F. Ataya, M.A. Dubé, M. Ternan, Energy Fuels 21 (2007) 2450–2459.18] A.K. Tiwari, A. Kumar, H. Raheman, Biomass Bioenergy 31 (2007) 569–575.19] M.P. Dorado, E. Ballesteros, F.J. Lopez, M. Mittelbach, Energy Fuels 18 (2004)

77–83.20] S. Yan, S.O. Salley, K.Y. Simon Ng, Appl. Catal. A 353 (2009) 203–212.21] G.W. Huber, S. Iborra, A. Corma, Chem. Rev. 106 (2006) 4044–4098.22] F. Frusteri, F. Arena, G. Bonura, C. Cannilla, L. Spadaro, O. Di Blasi, Appl. Catal. A

367 (2009) 77–83.23] A. Demirbas, Prog. Energy Combust. Sci. 33 (2007) 1–18.24] F. Ma, M.A. Hanna, Bioresour. Technol. 70 (1999) 1–15.25] S. Futura, H. Matsuhashi, K. Arata, Appl. Catal. A 269 (2004) 187–191.26] E. Lotero, Y. Liu, D.E. Lopez, K. Suwannakarn, D.A. Bruce, J.G. Goodwin Jr., Ind.

Eng. Chem. Res. 44 (2005) 5353–5363.27] A. Salis, M. Pinna, M. Monduzzi, V. Solinas, J. Mol. Catal. B: Enzym. 54 (2008)

19–26.28] H.J. Kim, B.S. Kang, M.J. Kim, Y.M. Park, D.K. Kim, J.S. Lee, K.Y. Lee, Catal. Today

315 (2004) 93–95.29] R.S. Watkins, A.F. Lee, K. Wilson, Green Chem. 6 (2004) 335–340.30] S.P. Chavan, Y.T. Subbarao, S.W. Dantale, R. Sivappa, Synth. Commun. 31 (2001)

289–294.31] P.S. Sreeprasanth, R. Srivasta, D. Srinivas, P. Ratnasamy, Appl. Catal. A 314 (2006)

148–159.

1 lysis A

[

[

[

[[[

[[[

[

[

[[[

2769–2773.

66 C. Cannilla et al. / Applied Cata

32] M.D. Serio, M. Cozzolino, R. Tesser, P. Patrono, F. Pinzari, B. Bonelli, E. Santace-saria, Appl. Catal. A 320 (2007) 1–7.

33] K. Narasimharao, D.R. Brown, A.F. Lee, A.D. Newman, P.F. Siril, S.J. Tavener, K.Wilson, J. Catal. 248 (2007) 226–234.

34] S.M. De Rezende, M.D. Castro Reis, M.C. Reid, P.L. Silva Jr., F.M.B. Coutinho, R.A.da Silva San Gil, E.R. Lachter, Appl. Catal. A 349 (2008) 198–203.

35] X.R. Chen, Y.H. Ju, C.Y. Mou, J. Phys. Chem. C 111 (2007) 18731–18737.36] D.G. Cantrell, L.J. Gillie, A.F. Lee, K. Wilson, Appl.Catal. A 287 (2005) 183–190.

37] R. Stern, G. Hillion, J.J. Rouoxel, S. Leport, Institute Francais de Petrole , US Patent

5,908,946 (1999).38] T. Nishimura, T.M. Okuhara, M. Mison, Appl. Catal. 73 (1991) L7.39] F. Arena, R. Dario, A. Parmaliana, Appl. Catal. A 170 (1998) 127–137.40] F. Arena, G. Trunfio, J. Negro, B. Fazio, L. Spadaro, Chem. Mater. 19 (9) (2007)

2270–2276.

[

[[[

: General 382 (2010) 158–166

41] K. Tanabe, in: B. Imelik, C. Naccashe, G. Goudurie, Y. Ben Taarit, J.C. Vedrine(Eds.), Catalysis by Acids and Bases, Elsevier, Amsterdam, 1985, pp. 1–14.

42] S. Bancquart, C. Vanhove, Y. Pouilloux, J. Barrault, Appl. Catal. A 218 (2001)1–11.

43] F. Arena, G. Trunfio, J. Negro, L. Spadaro, Appl. Catal. B 85 (2008) 40–47.44] R. Craciun, Solid State Ionics 110 (1998) 83–93.45] X. Zheng, S. Wang, X. Wang, S. Wang, X. Wang, A. Wu, Mater. Lett. 59 (2005)

46] A. Alsalme, E.F. Kozhevnikova, I.V. Kozhevnikov, Appl. Catal. A 349 (2008)170–176.

47] J. Ni, F.C. Meunier, Appl. Catal. A 333 (2007) 122–130.48] J. Shen, M. Tu, C. Hu, J. Solid State Chem. 137 (1998) 295–301.49] D. Darnoko, M. Cheryan, J. Am. Oil Chem. Soc. 77 (2000) 1263–1267.