Biodiesel from sunflower oil by using activated calcium oxide

www.elsevier.com/locate/apcatb

Applied Catalysis B: Environmental 73 (2007) 317–326

Biodiesel from sunflower oil by using activated calcium oxide

M. Lopez Granados a,*, M.D. Zafra Poves a, D. Martın Alonso a, R. Mariscal a,F. Cabello Galisteo a, R. Moreno-Tost a, J. Santamarıa b, J.L.G. Fierro a

a Instituto de Catalisis y Petroleoquımica, CSIC, C/Marie Curie 2, Cantoblanco, 28049 Madrid, Spainb Departamento de Quımica Inorganica, Cristalografıa y Mineralogıa, Facultad de Ciencias, Universidad de Malaga,

Campus de Teatinos, 29071 Malaga, Spain

Received 9 October 2006; received in revised form 18 December 2006; accepted 23 December 2006

Available online 9 January 2007

Abstract

This work studies the activity of activated CaO as a catalyst in the production of biodiesel by transesterification of triglycerides with methanol.

Three basic aspects were investigated: the role of H2O and CO2 in the deterioration of the catalytic performance by contact with room air, the

stability of the catalyst by reutilization in successive runs and the heterogeneous character of the catalytic reaction. The characterization by X-ray

diffraction (XRD), evolved gas analysis by mass spectrometry (EGA-MS) during heating the sample under programmed temperature, X-ray

photoelectron (XPS) and Fourier transform-infrared (FT-IR) spectroscopies allowed to concluding that CaO is rapidly hydrated and carbonated by

contact with room air. Few minutes are enough to chemisorb significant amount of H2O and CO2. It is demonstrated that the CO2 is the main

deactivating agent whereas the negative effect water is less important. As a matter of fact the surface of the activated catalyst is better described as

an inner core of CaO particles covered by very few layers of Ca(OH)2. The activation by outgassing at temperatures�973 K are required to revert

the CO2 poisoning. The catalyst can be reused for several runs without significant deactivation. The catalytic reaction is the result of the

heterogeneous and homogeneous contributions. Part of the reaction takes place on basic sites at the surface of the catalyst, the rest is due to the

dissolution of the activated CaO in methanol that creates homogeneous leached active species.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Lime; CaO; Ca(OH)2; CaCO3; Transesterification; Fatty acid methyl esters (FAME); Heterogeneous basic catalyst

1. Introduction

Biodiesel is a non-toxic biodegradable fuel produced from

vegetable oils by the transesterification of triglycerides with

methanol. Therefore biodiesel can be considered an environ-

mental friendly and a renewable fuel arising from biomass

[1,2]. With regard to emissions it has been demonstrated that its

net CO2 emissions is rather low taking into account its

renewable origin. Other toxic emissions like CO, SOx,

unburned hydrocarbons and soot particles are also considerably

reduced when burnt in the Diesel engine (the results concerning

the NOx emission indicates slightly larger values than

conventional diesel) [3]. Other advantages of the biodiesel

are its good lubricant properties that extend the engine life, its

high cetane number, its high flash point and its acceptable cold

* Corresponding author. Tel.: +34 91 5854937; fax: +34 91 5854760.

E-mail address: [email protected] (M.L. Granados).

0926-3373/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcatb.2006.12.017

filter plugging point (CFPP) which makes it very attractive as a

alternative fuel [4].

However the high final cost of the production with respect to

that of petroleum-derived diesel fuel limits its widespread use.

An important contribution to the final cost arises from the

catalytic transesterification reaction. The current technology

utilizes homogeneous catalysts (NaOH or KOH dissolved in

methanol, a corrosive liquid) [1,5]. The produced biodiesel and

glycerine must be separated and purified to remove the basic

catalyst what requires time consuming and expensive separa-

tion steps [6,7]. Moreover the purification is a non-environ-

mental-friendly process because implies the consumption of

large amounts of water and the disposal a highly basic streams

[1,2].

The utilization of a successful heterogeneous catalyst will

cope with most of the economical and environmental draw-

backs of a homogeneous process. Thus, the heterogeneous

catalyst is not disposed but rapidly separated from the reaction

mixture by filtration avoiding the time consuming rinsing steps

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http://dx.doi.org/10.1016/j.apcatb.2006.12.017

M.L. Granados et al. / Applied Catalysis B: Environmental 73 (2007) 317–326318

to purify both the biodiesel and the glycerine and preventing the

consumption of large volume of water. And last but not least,

the filtered solid could be re-used. The catalyst is not consumed

in the production process as it is in the homogeneous case in

which a fresh batch of catalyst is loaded for each batch

production. Some solid catalysts, basic or acid, have been tested

in the transesterification reaction of triglycerides with

methanol. Basic catalysts showed higher reaction rate than

acid solids and they have been preferably studied. However,

although several basic catalysts have showed promising

activities like basic zeolites [8], alkali and alkali earth oxides

[9,10], alkali and alkali earth carbonates [11], supported

guanidines [12,13] and basic hydrotalcites [14,15] so far none

have replaced the homogeneous catalyst.

Among the alkali and alkali earth oxides, CaO is one of the

solids that have displayed higher transesterification activity

[16–18]. As any other basic catalyst, its surface sites can be

poisoned by contact with room air due to the adsorption of CO2

and H2O at the surface of the solid as carbonates and hydroxyls

groups [19]. To our knowledge, the effect that this poisoning

mechanism may have on the transesterification rate has not

been investigated yet. The second goal of the study is to test the

stability of the CaO. Thus the CaO was reutilised for several

runs. Finally other alkali earth oxides or hydroxides, like for

instance SrO or Ba(OH)2 present high activity buy they are

fully dissolved in the reaction media that means that in such

cases the catalysis is homogeneous. CaO is only little dissolved

in the reaction media [16] and it is not clear yet whether the

reaction takes place on the basic surface sites of the CaO or

involved the few homogeneous basic species leached to the

reaction media (for instance it seems plausible that soluble Ca

methoxide may be formed in the reaction between methanol

and the surface of CaO). The third goal of this study is to

investigate how important the homogeneous contribution is

versus that occurring at the surface of the solid (heterogenoeus).

The three goals are essential for its industrial application.

2. Experimental

2.1. Sample preparation and storing

The CaO was supplied as a fine powder by Aldrich (99.9% of

the metallic atoms are Ca). The supplier indicates that ca. 10%

of the weight is lost by calcination at 1000 8C that indicates that

CaO is partially carbonated and hydrated. The as received CaO

was stored under vacuum in a desiccator that contains silica gel

and KOH pellets to remove the H2O and CO2 of the residual

desiccator atmosphere. This ‘‘fresh CaO’’ sample was labelled

as CaO.

The CaO, incipiently carbonated and hydrated, was exposed

to room air in a flat plate for different periods of time to extend

the carbonation and hydration processes. After the contacting

with room air for a given period of time the solid was stored

under vacuum in the desiccator. The samples exposed to air

were labelled a-CaO followed by the days in contact with air.

Thus a-CaO-20 means that the fresh CaO was exposed to room

air for 20 days.

2.2. Catalyst characterisation

2.2.1. Evolved gas analysis by mass spectrometry (EGA-

MS)

EGA-MS profiles were obtained from a 50 mg powder

sample placed in a U-shaped quartz reactor, the exit of which

was connected to a Balzer PrismaTM quadrupole mass

spectrometer (QMS 200) for on line gas analysis. The sample

consisted of solid particles in the 0.42–0.5 mm range. The gas

flow rate was 50 mL min�1 (20% O2 in Ar). The analysis was

conducted while heating the sample at 10 K min�1 (298–

1100 K). The signals from CO2 (m/z = 44), CO (m/z = 28) and

H2O (m/z = 18) were continuously monitored by the mass

spectrometer in order to follow the processes of decarbonation

and dehydration of the solid. Gas lines from the reactor to the

inlet of the mass spectrometer were heated to 393 K to avoid

water condensation.

2.2.2. X-ray diffraction and X-ray thermodiffractometry

studies

Powder X-ray diffraction (XRD) patterns were recorded in

the 5–858 2u range in the scan mode (step size 0.028, counting

1 s per step) using a Seifert 3000 XRD diffractometer equipped

with a PW goniometer with Bragg–Brentano u/2u geometry, an

automatic slit, and a bent graphite monochromator. The

samples were stored in an eppendorf filled with N2 until the

recording of the XRD to prevent the contact with room air.

Thermodiffactometric data were recorded on a Siemens

D5000 automated diffractometer with Bragg–Brentano geo-

metry using a graphite monochromator equipped with HTK10

heating chamber that allows heating treatments of the sample

under the flow of an inert gas (ca. 25 mL min�1 of He). Samples

were heated up to a given temperature (5 K min�1). The

patterns were recorded (over the angular range 15–458,counting 1 s per step and a step size of 0.048) after a delay

time of 10 min to ensure sample thermal stabilisation.

2.2.3. X-Ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) studies were

performed with a VG Escalab 200 R spectrometer equipped

with a hemispherical electron analyzer and a Mg Ka

(1253.6 eV) X-ray source. The sample was first placed in a

copper holder mounted on a sample-rod in the pretreatment

chamber of the spectrometer. It was then outgassed at room

temperature at 473 and at 773 K for 1 h before transferred to the

analysis chamber. In order to study the treatment at 973 K the

following procedure was carried out. Sample was ex situ

outgassed in a quartz cell at 973 K and once at room

temperature, isooctane was added in excess to cover the solid. A

portion of the soaked solid was then mounted in the copper

holder and outgassed at room temperature in the pretreatment

chamber for removing the weakly physisorbed isooctane.

O 1s, Ca 2p and C 1s regions of the XPS spectrum was

scanned a number of times in order to obtain a good signal-to-

noise ratio. For removing the shifts caused by charging effects

the binding energies (BE) were referenced by C 1s level of

carbonates (290.0 eV) as it was presumed that carbonates are

M.L. Granados et al. / Applied Catalysis B: Environmental 73 (2007) 317–326 319

present in the samples. This selection was found to be

consistent as the BE of C 1s level from adventitious

hydrocarbon and the position of O 1s core level from O2�,

OH� and CO32� were in agreement with the values reported in

literature [23–27]. When the carbonate peak was not observed,

then the C 1s peak of saturated hydrocarbon (285.0 eV) was

then used. The areas of the peaks were computed by fitting the

experimental spectra to Gaussian/Lorenztian curves after

removal of the background (use of Shirley function).

2.2.4. Fourier transform-infrared spectra

Infrared spectra of self supported wafers (ca. 12 mg cm�2)

were recorded with a Nicolet 5700 Fourier transform spectro-

photometer equipped with an Hg–Cd–Te cryodetector, working

in the range of wavenumbers 4000–650 cm�1 at a resolution of

4 cm�1. Each spectra was averaged over 256 scans. The

recording of the spectra was carried out in an infrared vacuum

cell equipped with greaseless stopcocks and KBr windows.

This cell allows outgassing of the samples at different

temperatures and the dosing of pulses of gases to submit the

samples to different controlled gas environment.

2.2.5. Catalytic activity measurements and re-utilization of

the catalysts

The catalytic reaction was carried out in a three-necked

jacketed batch reactor. A reflux condenser was connected to one

of the necks, another neck was for introducing the (helix) stirrer

and a dropping funnel was connected to the third one. The

finely grounded sample (ca. 0.5 g) was previously in situ

pretreated at a given temperature for 2 h by outgassing (heating

rate = 5 K min�1). This treatment of the CaO sample results in

a weight loss (ca. 5% at 773 K and ca. 10% at 973 K). Once the

sample was at room temperature, 50 g of oil (refined sunflower

oil, food grade) was added. The solid–oil mixture was heated

under strong agitation (1000 rpm) up to 333 K. Then methanol

previously heated at 333 K was added to the oil–catalyst

mixture by using the dropping funnel (t = 0 min of the catalytic

reaction). The reaction mixture was maintained under static N2

and under vigorous agitation (1000 rpm) while performing the

catalytic reaction. The methanol/oil molar ratio was ca. 13

(more than four times the stoichiometric value) in an attempt to

increase the FAME yield by displacing the equilibrium towards

the FAME formation and to minimize any effect of the

methanol losses. Methanol losses can occur either during the

methanol addition or during sampling of the reaction mixture.

Aliquots (ca. 2 mL) were taken from the agitating reaction

mixtures at different reaction times. The reaction was quenched

by addition to an aqueous HCl solution containing twice the

stoichiometric amount required to neutralize the CaO. After

agitation for few minutes, dichloromethane was added, this

mixture was again agitated and set aside to develop two layers:

the ester layer containing dichloromethane, mono, di and

triglycerides and FAME (and traces of methanol and glycerine)

and the polar layer containing glycerine, methanol, water

dissolved HCl and CaCl2 (and traces of esters). The ester layer

was separated and subjected once more to the treatment with

HCl solution and dichloromethane. The dichloromethane was

then removed from the organic layer by evaporation at 333 K.

In this study the yields to mono and diglycerides were not

determined and it is assumed that the FAME yield was close to

the triglyceride conversion. This can be considered a good

approximation only for conversion larger than 30–40% since

the selectivity to these products are important for lower

triglyceride conversion [5–15]. Their selectivities rapidly

decrease as the reaction proceeds to larger conversion, the

yield to these products become very small for conversion larger

than 30–40% and minor in comparison with the FAME yield.

Consequently it is widely accepted that the yield to FAME is a

reasonable measurement to estimate the catalytic activity of the

solids studied in this work. This approximation is only true for

large conversion but it was valid for the conclusions derived in

this article. The content in FAME of the organic layer was

determined by following the European regulated procedure EN

14103. Basically, 250 mg of the organic layer was added to

5 mL of a heptane solution of the internal standard

methylheptadecanoate (10 g/L of C17 ester in heptane). The

FAME content (wt%) was calculated from the formula:

wt% ¼�ðP

Ai � AMHÞAMH

�CMHVMH � 100

W

whereP

Ai is the total peak area from methyl ester, from

methyl miristate (C14) to methyl nervonate (C24:1); AMH the

area of methyl heptadecanoate, which response factor is equal

to those of FAME; CMH the concentration in mg/mL of the

methyl heptadecanoate (10 mg/mL); VMH the volume in mL of

the methyl heptadecanoate solution (5 mL); W is the weigh in

mg of the sample (250 mg).

This solution was analysed in an Agilent 6890 GC with a HP

INNOwax capillary column. The weight content in FAME of

the organic layer was considered to represent the wt% yield in

FAME of the catalytic reaction assuming that, during the

neutralisation and the rinsing process of the ester phase, only

traces of esters was transferred to the polar phase and that only

the extraction of methanol and glycerine takes place.

The experimental procedure to test the reutilisation of the

catalyst was as follows: the first run was carried out as described

above (1 wt% of catalyst in 50 g of oil, methanol/oil molar

ratio = 13, 333 K, 1000 rpm, reaction time 100 min). Before

stopping the agitation an aliquot was sampled and analysed

following the method described above. Then the agitation was

stopped and the mixture was rapidly centrifuged at 6000 rpm

for 30 min. The solid was recovered and subjected to the

reaction-sampling-analysis-centrifugation sequence for several

successive runs without being subjected to any previous

activation-outgassing step.

3. Results and discussion

3.1. Bulk and surface characterisation of the samples

Fig. 1 shows the XRD patterns of the CaO sample and of the

a-CaO samples after exposure to room air for 10, 20, 40 and 120

days. The strongest reflections of the CaO sample are assigned

Fig. 1. XRD patterns of the CaO sample and of CaO aged under air for 10, 20,

40 and 120 days; (1) CaO, JCPDS file 77-2376; (2) Ca(OH)2, JCPDS file 84-

1264; (3) calcite–CaCO3, JCPDS file 86-174; (4) aragonite–CaCO3, JCPDS file

76-606.

Fig. 2. EGA-MS of CaO (dashed trace) and of a-CaO-40 (solid line) samples.


to CaO (lime). Weak peaks of Ca(OH)2 (portlandite) were also

visible that indicates that fresh CaO is partially hydrated. After

exposure to room air for 10 days, the sample is fully

transformed to Ca(OH)2 and no CaO reflections could be

detected anymore. Incipient peaks from calcite–CaCO3 could

be alleged. The clear observation of calcite required exposure to

room air for 20 days. After contact for 40 days reflections from

aragonite–CaCO3 phase were also visible and the intensity of

the Ca(OH)2 peaks seems to be slightly weaker than those

detected in a-CaO-20. This effect is much clearer in the a-CaO-

120 sample that indicates that calcium carbonates develop at

the expense of Ca(OH)2. The XRD results here presented are in

agreement with the data reported by Lanas and Alvarez [20].

The carbonation process of the CaO is slow when compared

with the rapid hydration. This is because the own calcium

carbonate layer acts as a protective layer that prevent the rapid

carbonation of the Ca(OH)2 [21].

Fig. 2 represents the analysis by mass spectrometry of the gas

evolved from a given sample while heating. The analysis of the

figure corresponded to the CaO and a-CaO-40 samples. Only

H2O and CO2 were detected (monitored by following the

fragments m/z = 18 and 44, respectively), representing the

decomposition of the Ca(OH)2 and Ca carbonates present in the

samples. The m/z = 28 trace was also followed but it

corresponded to the less intense fragmentation of the CO2

molecule. Results showed that decomposition of Ca(OH)2 occurs

at lower temperatures than those required to decompose

carbonates. Carbonates were not detected by XRD in the

CaO. Therefore the CO2 detected in the EGA-MS pattern of the

CaO solid must be assigned to the decomposition of amorphous

CaCO3 present in the fresh sample. Specifications of the CaO by

supplier indicate that it is already partially hydrated and

carbonated (the amount of carbonate can be slightly over-

estimated because CaO is exposed to air for few minutes during

loading of the reactor). Lanas and Alvarez have already reported

the existence of XRD-silent (amorphous) Ca carbonates during

the initial stages of the exposure of CaO to ambient air [20]. Our

XRD data demonstrated that several days are required to detect

calcite crystals. In the a-CaO-40 sample the H2O and CO2 peaks

are more intense than those from CaO as expected considering

the deeper hydration and carbonation of this sample showed by

XRD. Finally it must be stressed that decomposition at high

temperatures, around 1000 K, is required to remove the

carbonates even for the CaO sample. The other samples exposed

to air for longer time displayed similar EGA-MS pattern to that of

a-CaO-40 sample, the CO2 peak was more intense.

Fig. 3 represents the thermodiffractometry results obtained

on an extensively carbonated and hydrated CaO (exposed to air

for 200 days) while heated in an in situ cell under a flow of

helium (only the reflections between 2Q = 15–408 were

recorded for reducing the dwelling time at a given temperature).

Before heating, the a-CaO-200 sample displayed essentially the

same crystalline phases detected for the a-CaO-120 (Ca(OH)2,

calcite and aragonite). Vaterite, that was only incipiently

detected in a-CaO-120 sample, is visible in the a-CaO-200

sample. No effect was observed by heating at 473 K in

agreement with the EGA-MS pattern that did not report any gas

evolution during the heating up to 473 K. However after heating

Fig. 3. Thermodiffractometry of a-CaO-200 sample. (1) CaO, JCPDS file 77-

2376; (2) Ca(OH)2, JCPDS file 84-1264; (3) calcite–CaCO3, JCPDS file 86-

174; (4) aragonite–CaCO3, JCPDS file 76-606; (5) vaterite–CaCO3, JCPDS file

74-1867.


at 773 K the following changes can be mentioned. First the

portlandite, Ca(OH)2, peaks disappear whereas reflections from

lime, CaO, start to be detected that indicates that the H2O peak

detected in the EGA-MS is due to the formation of CaO by

dehydration of Ca(OH)2. Furthermore the only polymorph of

calcium carbonate detected at 773 K is calcite. It is clear that

Fig. 4. O 1s, Ca 2p and C 1s core levels of CaO sample after in situ

during heating vaterite and aragonite are transformed to the

more stable calcite [22]. The reflections from calcite are much

more intense and narrower than those from any other calcium

carbonate phase detected when calcining at lower temperature

suggesting that the calcite formed by heating at 773 K is more

crystalline than previously detected calcium carbonates and

that amorphous carbonates were present in the initial sample.

With respect to the sample after treatment at 973 K, only very

intense peaks from CaO are visible. According to EGA-MS

results, decarbonation rate is important at these temperatures

and thus the decomposition of calcite to CaO has taken place at

973 K.

In order to gain information of the state of the surface of the

samples the X-ray photoelectron spectra of O 1s, C 1s and Ca

2p core levels of the CaO sample were recorded after

outgassing at 473 and 773 K in the in situ cell (higher

temperatures can not be reached by the cell). The sample was

also subjected to outgassing at 973 K ex situ in a quartz reactor

and contact of the treated sample with room air was prevented

by soaking in isooctane. The Fig. 4 shows the X-ray

photoelectron spectra for the C 1s, O1s and Ca 2p core levels

of the sample after outgassing at 473 and 773 K in the in situ

XPS chamber. The Ca 2p level shows the spin-orbit doublet Ca

2p3/2 and Ca 2p1/2. Between the two Ca 2p3/2 and Ca 2p1/2 levels

there should be a well-defined valley, absent in our samples.

Taking into account that the samples were initially hydrated and

carbonated, in principle the Ca 2p features were assumed to

arise from two components, one from Ca(OH)2 and another

from CaCO3. It is well known that these two types of Ca species

present different binding energies (BE) [23,24]. The BE of the

two Ca 2p3/2 components were at 347.6 � 0.2 eV for CaCO3

and 346.5 � 0.2 eV for Ca(OH)2 species, respectively (the

corresponding Ca 2p1/2 peaks are at 3.5 eV higher BE). The

difference in BE between the contributions is in agreement with

that published [23–25]. The presence of CaO can be discarded

since the BE of Ca in CaO is below those of Ca(OH)2 [23,24].

outgassing at 473 K and 773 K and ex situ outgassing at 973 K.


Therefore it can be concluded that the outermost surface layers

of CaO solid is extensively carbonated and hydrated.

Evacuation at 773 K should have resulted in the dehydroxyla-

tion of the bulk Ca(OH)2 to yield CaO whereas decarbonation

cannot be achieved at these temperatures. However it is well

known that only under very low residual H2O pressure in the

XPS chamber CaO surface can be fully dehydroxylated. It has

been estimated that partial pressure PH2O � 5� 10�9 Torr is

required to fully prevent hydroxylation of the CaO surface

[23,24]. This estimation was done on CaO single crystal but is

still valid for polycrystalline samples. Therefore although the

Ca(OH)2 is transformed to CaO during the treatment at 773 K,

it is expected that, once the sample is cooled down to room

temperature in the in situ XPS chamber, the surface of the

resulting CaO is covered by OH groups in a large extension.

This justifies why if samples were evacuated at temperatures

high enough to decompose the Ca(OH)2, the Ca 2p core level

still shows a contribution from Ca-OH species. The compo-

nents at higher BE (CaCO3) represent in both samples ca. 60%

of the total weighted area whereas that at lower BE (Ca(OH)2)

corresponds to 35–40%.

The BE of the O1s core level of the samples after evacuation

at 473 and 773 K were at 531.7 � 0.2 eV. In principle the O 1s

peak can arise from OH or CO3 groups which peaks are

separated by less than 0.5 eV [23,24] and the width of the peak

clearly indicates that is the result of these contributions. The

deconvolution into two peaks was attempted but none of the

found solutions could be considered statistically definitive.

The C 1s core level depicted in Fig. 4 shows that once the

samples are outgassed at 473 and 773 K two components can

be clearly observed. A very intense contribution at

290.1 � 0.1 eV was assigned to surface carbonate group in

agreement with the Ca 2p results (it was used as a reference for

the rest of the core levels). Another intense peak at

285.1 � 0.1 eV was also clearly visible that is assigned to

carbon from adventitious hydrocarbons (C–H species) [26]. A

third peak (much weaker) was also visible after deconvolution

of the C level and located at 287–288 eV that is due to C present

in carbonyl, alcohols or ether groups (C O or C–O species)

[26]. Therefore the sample is carbonated and hydrocarbon

species are also present at the surface of the solids. Actually,

after evacuation at 773 K the C 1s contribution from

hydrocarbon represents more than 40%. The origin of this

hydrocarbon species is very likely the contamination from the

adventituous hydrocarbon, which presence in the ambient or in

the in situ cell of the XPS spectrophotometer cannot be

avoided.

Fig. 4 also shows the O 1s, C 1s and Ca 2p core levels for the

CaO sample after outgassing (in an ex situ cell) at 973 K. The C

1s level is dominated by peak at 285.0 � 0.2 eV that is due to

the isooctane that cannot be removed by the very low pressures

required for the spectra recording. The very wide and weak

peak at 288.0 eV intends to represent the sum of contributions

from partially oxygenated hydrocarbons (C–O and C O

functionalities). Any attempt of deconvoluting such weak peak

into several contributions was considered questionable and

irrelevant for the conclusions. It must be stressed that the

treatment at 973 K has completely removed the carbonate

groups as there is a lack of contribution from these species at

290 � 0.2 eV. The O 1s core level is clearly composed of two

contributions, both with similar wt%, one at 531.3 � 0.2 eV

that is assigned to Ca-OH species (CO3 groups are absent) and

another peak at 528.7 � 0.1 eV. The latter arises from the CaO

framework O2�, its very low BE is due to the basic character of

such species [25,27]. The Ca 2p core level also confirms the

absence of CO3 species: the peaks of the Ca 2p spin-orbit

doublet at 349.5 and 346.2 eV arise from Ca-OH species

whereas the new peaks at 348.7 and 345.3 eV (ca. 1 eV below

the Ca-OH peaks) clearly arise from CaO species [23–25]. The

weight (%) of the Ca 2p contributions are in agreement with

those found in the O 1s level, both indicate that 50% of the

species detected by XPS are CaO and the other 50% are

Ca(OH)2. Considering that the hydroxylation of CaO cannot be

avoided due to the extremely high reactivity of the CaO surface

to the tiny residual traces of H2O that are always present in any

atmosphere, the surface of the sample after outgassing at very

high temperatures and cooling down to room temperature can

be described as CaO covered by Ca(OH)2. Considering the

inelastic mean free path of the O 1s photoelectrons, it is very

reasonable to assume that several layers of Ca(OH)2 are

covering the fully decarbonated CaO formed by the treatment at

973 K [23,25].

The FWMH of all the peaks were between 2 and 2.5 eV.

These values are in some cases slightly larger than those

published for the bulk pure compounds. This difference can be

due to slight variations in the chemical nature of the surface

species present at the surface of the polycrystalline solids

studied in this work and/or to the amorphous character of some

of the phases.

The surface atomic CO32�/Ca ratio was estimated from the

area of their XPS core levels and by using the sensitivity factors

reported by Sosulnikov and Teterian [25] and in the case of the

CaO evacuated at 473 and 773 K was 0.37 as expected from a

surface partially hydrated and carbonated (fully carbonation

should have been resulted in ratio closer to 1).

Summarising the XPS results it can be said that the surface

of solid after outgassing at 473 and 773 K is dominated by Ca-

OH and CaCO3 groups. The outgassing at very high

temperatures (�973 K) results in the fully decarbonation of

the solid. Then the surface of the resulting activated CaO can be

described as CaO covered by a layer (or several layers) of

Ca(OH)2. Similar situation was found for a CaO sample

exposed to room air for more than 120 days.

In an attempt of tracing how fast a ‘‘clean’’ CaO is

hydrated and carbonated, the a-CaO-120 sample previously

outgassed at 973 K in the IR cell was contacted with room air

at atmospheric pressure for different periods. The Fig. 5

shows the different FTIR spectra obtained. The bottom

spectrum represents the initial situation, the CaO evacuated

at 973 K. Only a band at 3736 cm�1 is detected that can be

assigned to isolated OH groups present at the surface of the

CaO solid. No IR feature is visible in the region of carbonate

stretching vibrations. The spectrum immediately above is

that obtained after the sample was first contacted the room air

Fig. 5. FT-IR of the a-CaO-120 sample after outgassing at 973 K for 2 h and contacted with room air for different periods of time: (a) 4000–3000 cm�1 region and (b)

2000–800 cm�1 region.


in the IR cell (the recording of the spectrum took ca. four

minutes, so this spectrum is the average spectrum during the

recording for the first 4 min). Two new hydroxyls bands

appear, those at 3647 and 3572 cm�1 that, in principle can be

assigned to OH groups of Ca(OH)2 [28]. Apparently, their

intensity do not growth with longer contact time with room

air, suggesting that the solid is hydrated very fast during the

first minutes of contacting and that further hydration is a

much slower process. The bands at 1479 and 1419 cm�1 can

be assigned to the symmetric and asymmetric stretching

vibrations of O–C–O bonds of unidentate carbonate at the

surface of the CaO [29,30]. The bands at 1074 and 864 cm�1

arises also from these carbonates groups. The band at

1130 cm�1 has been detected in calcite or aragonite solids.

Therefore the spectrum clearly shows that after 5 min the

surface sample is also already carbonated and the formation

of incipient nuclei of calcite or aragonite are formed. Longer

contact time with room air just results in the increase of the

intensity of all the carbonate related bands and the detection

of a band at 1647 cm�1 that is assigned to the bending OH

mode of water molecules physisorbed on the surface of the

solid [28,31]. The intensity of any the bands from carbonate

groups increase during the first minutes of contacting and

actually a lack of resolution in the bands between 1600 and

1200 cm�1 region is observed after contact with room air for

more than 15 min. This is due to the increasingly higher

carbonation of the sample. However the intensity of these

bands after 3 days of contact is similar to those after the first

3 h. Summarising the FT-IR results it can be concluded that

the carbonation and the hydration of CaO is very rapid and

just few minutes are required to extensively carbonate and

hydrate the sample. However after the initial hours of contact

these processes are not as fast.

3.2. Catalytic activity

All the previous characterisation results indicate that the

fresh solid (CaO) is partially hydrated and carbonated. The

storing of the sample under room air results in a progressive

hydration and carbonation of an activated CaO, such

phenomena are very rapid during the first minutes and only

few minutes are enough to carbonate the surface of this solid.

Then the hydration and carbonation processes slow down but

the longer the contact time the deeper these processes are. The

catalytic activity of the CaO and a-CaO-120 samples before and

after dehydration and decarbonation by outgassing at different

temperatures was then measured in an attempt of gaining

information of how hydration and carbonation deteriorate the

catalytic activity. The Fig. 6 displays the yield to fatty acid

methyl esters obtained. If the a-CaO-120 sample is mildly

pretreated (evacuation at 473 K) presents a very low activity.

An evacuation of the sample at 773 K results in an improvement

of the catalytic activity that indicates that dehydration of the

Ca(OH)2 present in the a-CaO-120 sample to CaO results in a

partial activation. The transformation of the CaCO3 to CaO by

outgassing at 973 K causes a larger activation of the catalytic

activity. The CaO sample without activation presents a higher

catalytic activity than the a-CaO-120 untreated sample as it is

expected considering the lower hydration and carbonation

degree of the former. A further improvement of the activity of

CaO sample was achieved by outgassing at 973 K. It must be

stressed that the activity of both CaO and a-CaO-120 samples

after the treatment at 973 K are quite similar. Thus the yield to

FAME of both samples after outgassing at 973 K is slightly

larger than 90 % after 90 min of reaction.

In order to discard external mass transfer effects on the

FAME yield, particles with size <105 mm (mesh size 140) was

Fig. 6. Yield to FAME of the CaO sample previously outgassed at 973 K after

successive cycles of reaction.

Table 1

Yield to FAME of the CaO sample previously outgassed at 973 K after

successive cycles of reaction

Run number FAME yield (wt%)

First 94

Second 79

Fifth 75

Sixth 73

Seventh 78

Eighth 81


sieved and used for the catalytic tests. The problems related

with the access of the voluminous triglyceride molecules to the

active sites within the nanopores of the solid can be evaluated

when comparing the mean pore size diameter of CaO after

activation at 973 K (�25–30 nm as determined from the

desorption branch of the N2 adsorption isotherm, specific

surface 32 m2/g) with the dimensions of the triglycerides. The

latter can be estimated from the largest dimension of for

instance methyl oleate (that is around 2.5 nm) and of glycerine

(around 0.6 nm). Even by assuming that the relevant dimension

of triglyceride molecule is the dimension along the fatty acid

chain (which is not true as triglyceride can go through pores by

accommodating the smallest dimension, that for the glycerine

part, which is around 0.6 nm) the pore size of the CaO solid is

still close to one order of magnitude larger that this triglyceride

dimension and restrictions to their access inside the nanopores

are expected to be not very relevant (the specific surface and the

mean pore size of the samples activated at lower temperatures

were similar).

All the above results demonstrate that both the hydration and

the carbonation of CaO oxide can deteriorate its catalytic

performance. Since the decomposition of the Ca(OH)2 (to CaO)

that is present in a-CaO-120 sample takes place with treatment

at 773 K and this results in a very limited improvement in the

FAME yield, it is very reasonable to assume that the

carbonation of the CaO is more detrimental for the catalytic

activity than the hydroxylation. Furthermore, the total removal

of surface OH groups cannot be achieved after outgassing at

high temperatures (973 K) because when cooling down to room

temperature tiny traces are enough to cover the surface by

several layers of Ca(OH)2. Then it is proposed that water

adsorption on CaO surface (i) is not the main deactivating

process and (ii) in fact the surface state of the working catalyst

must be thought as fully hydroxylated. Finally, it should be

recalled that the activated CaO was rapidly carbonated and

hydrated, and hence in order to prevent deterioration of the

catalytic reaction extreme care must be taken to avoid any

contact of the CaO with room air.

Table 1 summarizes the reutilization of the CaO (the sample

was previously outgassed at 973 K) for several runs. Although

the yield to FAME decreases from more than 90% in the first

run to ca. 80% in the second run, the latter performance is kept

even after eight cycles of reutilization. Verkade and coworkers

have also recently demonstrated that the CaO can be reutilised

for several runs [18]. The former results stress the possibility of

using CaO as a catalyst when compared with the conventional

homogeneous catalyst, KOH dissolved in methanol. In spite of

the transesterification activity of CaO is not as large as that of

KOH or NaOH catalysts, this can be compensated with a larger

concentration of CaO as long as CaO can be reutilised for a

greater number of runs.

CaO is slightly dissolved in the reaction media as it is

soluble to some extent in methanol (0.035 wt%) [16]. The

reactions (I) and (II) account for the most likely dissolution

reactions:

CaðOHÞ2 ? Ca2þ þ 2OH� (I)

CaO ? Ca2þ þO2� (II)

And reactions (III) and (IV) explains the formation of

methoxide anion from the O2� and OH� soluble species:

O2� þCH3OH ? OH� þCH3O� (III)

OH� þCH3OH ? H2O þ CH3O� (IV)

Then one of the questions that can be raised is to estimate the

contribution of the heterogeneous reaction by surface basic

sites of the solid and the participation of homogeneous reaction

involving the CaO that can be dissolved in the reaction media.

Fig. 7. Yield to FAME obtained by catalytic reaction with different initial

weights of a-CaO-120 solid (the final weight of the loaded solid after the

outgassing at 973 K was 0.148, 0.275 and 0.382 g).

Fig. 8. Yield to FAME obtained (A) by using the homogeneous species created

by contacting the methanol and the activated CaO for 2 h at 333 K and (B) by

using the activated CaO solid.


Fig. 7 presents the catalytic activity obtained when different

amounts of a-CaO-120 catalyst were used. The sample was

previously outgassed at 973 K. The amounts that were utilized

are indicated in the figure and they are the final weight after

the outgassing treatment. It is assumed that the dissolution of

CaO is a fast process and takes only few minutes to proceed.

The extension of the dissolution, and therefore the presence of

homogeneous species, is in principle controlled by the

solubility product of the solid in the reaction media and

therefore a constant value of dissolved CaO should be

obtained irrespective the amount of CaO used in the catalytic

reaction. If the reaction is caused by active homogeneous

species related with the dissolution of CaO, similar FAME

yield should have been obtained by using different loadings of

catalyst. Fig. 7 clearly shows that the yield to FAME increases

with the amount of used catalyst and suggests that the

contribution from the surface mediated heterogeneous

catalytic reaction is relevant. Species created by the

dissolution of the CaO dissolved can not be fully discarded.

It is also worthy to stress that the leaching of active species

occurs only on activated CaO as Fig. 6 indicates.

In an attempt to clarify this point the following experiment

was carried out (this experiment has been already conducted by

the authors in alumina-supported K carbonate catalyst used in

the oil transesterification and demonstrated that in the latter

case the homogeneous species were responsible of the catalytic

reaction [32]. First 0.5 g of CaO was activated by outgassing at

973 K and once the sample is at room temperature the required

amount of methanol was added preventing the contact with

room air. Then the methanol–solid mixture was at 333 K for 2 h

under strong agitation. Subsequently the agitation was stopped

and the solid left to settle down maintaining the temperature at

333 K. Once the solid was decanted the methanol was

transferred to another reactor set also at 333 K in order to

prevent the cooling of the methanol. The transfer was carried

out rapidly under N2 atmosphere to prevent the contact with

room air. Although care was taken to minimize the agitation of

the decanted solid and to reduce as much as possible the

presence of solid, a weak turbidity was evident in the

transferred methanol due to the presence of traces of suspended

solid. Then, the oil was added at 333 K and the transesterifica-

tion reaction started. After 2 h of reaction, 0.2 mL of mixture

was sampled and the FAME content analyzed as explained in

the experimental section. Fig. 8 compares the FAME yield

obtained with this biphasic mixture, that does not contain solid

CaO but the hypothetical soluble species, with the yield

obtained by using the conventional triphasic mixture containing

the activated CaO solid. The figure clearly shows that the

contribution from the hypothetical homogeneous species is

important but it also shows that there is a contribution from the

heterogeneous sites on the activated CaO. The contribution

assigned to the homogeneous species cannot arise from the

traces of solid causing the weak turbidity since reaction carried

out with ca. 100 mg of activated CaO, an amount much larger

than that causing the turbidity, yielded less than 20% of FAME

(results are not shown in the figure) and therefore the

contribution from the turbidity is residual.

The dissolution of the CaO in the reaction media or the

leaching of active species from the surface of the solid is not

as intense as to result in the decline of the catalytic

performance while using the solid for several runs (eight in

this article) as long as enough amount of CaO is used. It is

important to stress that the presence of leached species in the

reaction media may be also taken into consideration in other

basic reactions in which basic solids and polar reactants

participate [19,33]. A research effort is required and currently

performed in our laboratory devoted to the quantification of

leaching and to find the favorable conditions (activation

temperature, reaction temperature, contact time, etc.) to

minimize the leaching.


4. Conclusions

The active surface sites of CaO are unavoidably poisoned by

the atmospheric H2O and CO2. The catalytic activity of CaO

can be improved if before the reaction the CaO is subjected to

an activation treatment to remove and to clean the surface of the

main poisoning species (the carbonate groups) and if after this

treatment the contact with room air is prevented. The poisoning

effect of H2O adsorption is less relevant and as matter of fact the

surface the activated CaO can be described as few layers of

Ca(OH)2 covering an inner core of CaO.

CaO is an attractive catalyst for the transesterification reaction

of the sunflower oil with methanol that can be re-used for several

runs without significant deactivation. The basic surface sites

present at the surface of the solid are partly responsible of the

catalytic reaction, the contribution of the homogeneous reaction

due to soluble species created by the dissolution of the solid in

methanol is also important. Thermal activation is required for

both homogeneous and heterogeneous reactions to take place.

The dissolution of the CaO in the reaction media is not as intense

as to result in the decline of the catalytic performance while using

the solid for eight runs providing that enough amount of CaO is

added to the reaction media.

Acknowledgements

Financial support from European Commission Sixth Frame-

work Programme (under project BIOCARD, contract no.

019829), Spanish Ministry of Education and Science

(MAT2003-08348-C04-01 project) and Regional Government

of Madrid (GR/AMB/0686/2004 project) is gratefully acknowl-

edged. D.M. A. and M.D. Z. P. thank to the Regional

Government of Madrid and the Spanish Ministry of Education

and Science, respectively, for their fellowships.

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Biodiesel from sunflower oil by using activated calcium oxide

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