biodiesel production over lithium modified lime catalysts: activity and deactivation

Accepted Manuscript

Title: Biodiesel production over lithium modified limecatalysts: Activity and deactivation

Author: Jaime F. Puna Joao F. Gomes Joao C. Bordado M.Joana Neiva Correia Ana Paula Soares Dias

PII: S0926-860X(13)00712-6DOI: http://dx.doi.org/doi:10.1016/j.apcata.2013.11.022Reference: APCATA 14577

To appear in: Applied Catalysis A: General

Received date: 11-10-2013Revised date: 6-11-2013Accepted date: 18-11-2013

Please cite this article as: J.F. Puna, J.F. Gomes, J.C.Bordado, M.J.N. Correia, A.P.S.D. </sup>; <i><ce:inter-refid=intr0005xlink:href=mailto:[email protected]>[email protected]</ce:inter-ref> Biodiesel production over lithium modified lime catalysts:activity and deactivation, <i>Applied Catalysis A, General</i> (2013),http://dx.doi.org/10.1016/j.apcata.2013.11.022

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http://dx.doi.org/doi:10.1016/j.apcata.2013.11.022

http://dx.doi.org/10.1016/j.apcata.2013.11.022

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Biodiesel production over lithium modified lime catalysts: activity and deactivation

Jaime F. Puna1, João F. Gomes1, João C. Bordado2, M. Joana Neiva Correia2, Ana Paula Soares

Dias2*

1 ISEL – Instituto Superior de Engenharia de Lisboa, Chemical Engineering Department/CEEQ, R. Conselheiro Emídio Navarro 1, 1959‐007 Lisboa, Portugal

2 Instituto Superior Técnico, University of Lisbon, Av. Rovisco Pais, 1049‐001 Lisboa, Portugal

*Corresponding author: [email protected]; [email protected]

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Biodiesel production over lithium modified lime catalysts: activity and deactivation

Jaime F. Puna1, João F. Gomes1, João C. Bordado2, M. Joana Neiva Correia2, Ana Paula Soares

Dias2*

1 ISEL – Instituto Superior de Engenharia de Lisboa, Chemical Engineering Department/CEEQ, R. Conselheiro Emídio Navarro 1, 1959‐007 Lisboa, Portugal 2Instituto Superior Técnico, Chemical Engineering Department, University of Lisbon, Av. Rovisco Pais, 1049‐001 Lisboa, Portugal

*Corresponding author: [email protected]; [email protected]

Keywords: biodiesel, lime catalysts, basicity, lithium, methoxide, diglyceroxide, Ca leaching

ABSTRACT

Biodiesel production by methanolysis of semi‐refined rapeseed oil was studied over lime based

catalysts. In order to improve the catalysts basicity a commercial CaO material was

impregnated with aqueous solution of lithium nitrate (Li/Ca=0.3 atomic ratio). The catalysts

were calcined at 575 ºC and 800 ºC, for 5h, to remove nitrate ions before reaction.

The XRD patterns of the fresh catalysts, including the raw CaO, showed lines ascribable to CaO

and Ca(OH)2. The absence of XRD lines belonging to Li phases confirms the efficient dispersion

of Li over CaO.

In the tested condition (wcat/woil=5%; CH3OH/oil=12 molar ratio) all the fresh catalysts provided

similar biodiesel yields (FAME >93% after 4h) but the raw CaO catalyst was more stable. The

activity decay of the Li modified samples can be related to the enhanced, by the higher

basicity, calcium diglyceroxide formation during methanolysis which promotes calcium

leaching. The calcination temperature for Li modified catalysts plays an important role since

encourages the crystals sinterization which appears to improve the catalyst stability.

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1. INTRODUCTION

Today, the high cost of fossil fuels based energy and the environmental impact associated to

their consumption have been the major driving forces for the search of sustainable sources of

energy like biodiesel. Biodiesel, a mixture of long chain fatty acid methyl or ethyl esters, is an

environmental friendly alternative to fossil diesel. Actually, biodiesel does not contribute to

the increase of the net atmospheric CO2 level and allow a reduction of the emissions of several

pollutants such as particulate matter, carbon monoxide, sulfur and polycyclic aromatic

hydrocarbons [1].

Currently, biodiesel is produced by transesterification of vegetable oils or animals fats using

basic homogeneous catalyzed batch processes. However, the use of heterogeneous catalysts is

advantageous as it allows to decrease operation costs due to the reduction of the catalysts

consumption and to simpler purification operations. Many heterogeneous solid basic catalysts,

such as alkaline earth metal oxides and hydroxides compounds supported on alumina, zeolites,

hydrotalcites and ion exchange resins have been used for the transesterification of vegetable

oils. From these, calcium oxide is among the best basic catalysts for the methanolysis of

vegetable oils [2,3] or waste frying oils [4].

According to Kouzu et al. [5], the transesterification reaction over CaO occurs through a

nucleophilic mechanism which involves, firstly, the activation of methanol by a basic site on

the catalyst surface. Therefore, the reaction may be accelerated by the enhancement of the

basic strength of the active sites using, for example, the doping with an alkaline metal salt, like

lithium nitrate. Actually, the replacement of Ca2+ by M+ in the crystal framework generates a O‐

basic site [6,7]. Hence, the wet impregnation of CaO catalysts with lithium nitrate was studied

by several authors. Watkins et al. [8] founded, for a CaO catalyst containing 1.23% (w/w) of

lithium, improved catalyst behavior ascribable to the formation of electron defect Li+ species.

These species were responsible for the enhanced availability of surface –OH. In fact, the

impregnation with Li increased the surface basicity from 8<pkBH+<10 for bare CaO up to

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15<pkBH+<17.2 for lithium doped material. Lately, Kumar and Ali [9] used a similar

impregnation technique to prepare nanostructured CaO catalysts modified with alkali

elements (Li, Na, K). These authors used nitrate and carbonate alkali salts and the catalysts

were only dried at 120 ºC after impregnation. They reported the best catalytic behavior for the

sample impregnated with lithium carbonate. Kaur and Ali [10] also underlined the beneficial

effect of Li on the catalytic behavior of CaO catalyst for biodiesel production. Endalew et al.

[11] tested lithium CaO doped catalysts for biodiesel production from high acidity jatropha oil

but, as expected, they found that both CaO and Li doped CaO catalysts led to the production of

large amounts of soaps.

From the published data, it can be inferred that lithium has positive effects on the catalytic

activity of CaO for biodiesel production by oil methanolysis. Since the catalyst stability is a

crucial issue, data on catalytic activity and stability of lithium doped catalysts will be discussed

in the following sections.

2. EXPERIMENTAL

2.1. Preparation of the catalysts

Catalysts containing 30% of lithium (Li/Ca molar ratio) were prepared by wet impregnation

technique. For each sample, an aqueous suspension (200 mL) containing 20g of commercial

CaO and the required amount of lithium nitrate was heated (80 ºC) under vigorous stirring

until dryness. The obtained powders were dried at 120 ºC overnight and then calcined in a

muffle during 5h. Samples were calcined at 575 ºC or 800 ºC using a slow heating rate (5

ºC/min). The as received CaO material, calcined in analogous conditions, was used as standard.

2.2 Characterization of fresh and post reaction catalysts

The fresh and post reaction samples were extensively characterized in order to find the

correlation between physical‐chemical properties and the catalytic activity and stability for the

methanolysis reaction.

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The surface basicity was assessed using basic Hammett indicators in methanolic solutions.

The crystallinity and the crystalline phases of both fresh and post reaction catalysts were

assessed by X‐Ray diffraction using a Rigaku Geigerflex diffractometer with Cu Kα radiation at

40 kV and 40 mA (2º/min).

Particles size distribution of the fresh catalysts was assessed by laser diffraction (blue

radiation, 455 nm) based on Lorenz Mie law. A Malvern Mastersizer 2000 equipment was

used. Water dispersions (with 10–20% of obscurity) of the materials were prepared using

ultrasound. The granulometric distributions were computed taking into account the refractive

index of CaO (1.838) and Li2O (1.644) (molar weighted). The external surface areas of the

catalysts were computed considering spherical particles.

The infrared spectra were collected using a spectrophotometer BOMEN FTLA200‐100 from

ABB. This equipment has a horizontal total attenuated reflection accessory (HATR), from PIKE

Technologies, with a ZnSe crystal. Sixty‐four scans were accumulated for each spectrum to

obtain an acceptable signal‐to‐noise ratio. To extract the maximum information from the FTIR

spectra of the different catalysts, principal components analysis (PCA) [12,13] was carried out

using Matlab version 7.9 (MathWorks, Natick, MA) and the PLS (partial least squares) Toolbox

version 3.0 (Eigenvector Research Inc ‐ USA) for Matlab, according to the description

presented elsewhere [14, 15]. Briefly, PCA is a method of data compression that transforms a

number of possibly correlated variables into a smaller number of non‐correlated ones called

principal components (PC). These components are linear combinations of original variables

whereas the first PC captures the greatest possible variance of the original data. A PCA model

decomposes the original data matrix (with m samples and n variables) into a product of two

smaller matrices, designated as scores and loadings. The scores matrix represent the

projection of each sample into the new coordinate system and gives information about the

similarity between samples, whereas the matrix of loadings represents the importance of each

original variable in the new space of principal components [12]. The criteria for choosing the

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appropriate number of principal components used in this work was to consider only the PCs

that lead to the capture of a cumulative variance of more than 90% [14,15].

2.3. Methanolysis tests

The methanolysis catalytic tests were carried out in a 500 mL round bottom pyrex reactor

equipped with a condenser and a mechanical stirrer (600 rpm). The temperature was kept at

the methanol reflux temperature using a temperature controlled water bath. Typically, 100 g

of soybean oil was transferred into the reactor and heated until the desired temperature. Then

the slurry of methanol and catalyst was added to the oil and the transesterification reaction

was run for 5 h. For each experiment, 5% (w/w, oil basis) of catalyst and a molar ratio of

methanol to oil of 12/1 was used. At the end of the reaction period, the catalyst was removed

by filtration and the reaction mixture was transferred into a decantation funnel to separate the

glycerin and biodiesel phases. The latter phase (crude biodiesel) was then washed with

distilled water followed by a 0.5 % HCl solution and again with water, and dried at 80‐90 ºC

under vacuum (0.05 bar).

The reproducibility of the catalytic tests was previously studied, for vegetable oil methanolysis,

using the same apparatus for SrO/MgO catalysts (variance of 0.1%) [16].

The FAME content of the samples was determined using near infrared spectroscopy (NIR) with

an ABB BOMEM MB160 spectrometer equipped with an InGaAs detector and a transflectance

probe from SOLVIAS, according to the procedure described elsewhere [12].

3. RESULTS AND DISCUSSION

3.1 Characterization of the catalysts

The surface basicity assessed using basic Hammett indicators in methanolic solutions showed

that the pKa of these catalysts is between 15 and 18.2.

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The XRD patterns of fresh catalysts showed diffraction lines ascribable to CaO, the major

phase, and calcium hydroxide (Fig.1). The absence of XRD lines belonging to Li phases seems to

indicate that this element was well dispersed over the CaO. It is worth noting that the

prepared weight content of Li2O (6% w/w) was higher than the detection limit of XRD (2%w/w

for mixed materials). Similar observations have been previously reported [7,8]. Alonso et al. [7]

reported the identification of the Ca(OH)2 pattern in the XRD of the 4.5LiCa catalyst calcined at

300 ºC but for activation temperatures higher than 700 ºC these authors refer the

identification of Li oxides that promote the activity of the catalyst.

As shown in Table 1, all samples were nanosized, with lime crystallites smaller than 125 nm

and portlandite (Ca(OH)2) crystallites smaller than 25 nm. Furthermore, the CaO material

seems to be sintering resistant since the samples calcined at 800 ºC, and dried at 120 ºC,

present the same size of the lime crystallites. On the other hand, the sintering effect of lithium

was only detectable for the samples calcined at 800 ºC. According to Borgwardt [17] Li and

other monovalent cations promotes solid state diffusion thus encouraging sintering.

As reported before, the oil molecules cannot diffuse into the narrow pores and the reaction

occurs over the external surface of the catalyst [16]. Therefore the particles sizes distribution

of catalysts plays an important role in the catalytic process. The granulometric distribution (Fig.

2) of fresh and post reaction catalysts were assessed by laser light scattering. For the raw CaO

material the calcination promotes the disappearance of the larger particles which can be

ascribable to the thermal decomposition of carbonate species which are larger than the

corresponding lime particles [18]. The samples modified with lithium showed larger particles

than the bare lime material. CaO calcined at 800 ºC presented a surface weighted mean

diameter of 3.61 µm versus 17.41 µm of the Li‐CaO sample. Raising the calcination

temperature from 575 to 800 ºC led to an increase of the particle size of the catalyst powders

(1.2 times for the CaO catalysts and 1.7 times for the Li‐CaO samples), which confirms that high

calcination temperatures promotes sinterization. All the post reaction samples showed

formation of larger particles during the first reaction batch which are vanished for the later

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reaction batches. This occurrence can be related with calcium leaching. Eventually the larger

particles are formed by calcium methoxide and calcium diglyceroxide. The raw CaO sample

(unmodified with Li), only dried, showed dissimilar behavior probably due to the lesser

formation of calcium diglyceroxide.

The diffractograms (Fig.3) for the post reaction Li modified catalysts showed an important

effect of the calcination temperature on the crystalline phases, formed during reaction. As

observed for raw CaO catalyst, during the methanolysis reaction, lime is converted into

calcium hydroxide with crystallites smaller than 50 nm. The main crystalline phase in of the

post reaction Li_CaO catalysts was always calcium hydroxide, thus indicating that this Ca phase

has an important role on the catalytic process. Additionally, as shown in Fig. 3, the catalyst

annealed at 800 oC showed the formation of calcium diglyceroxide (C6H14CaO6, JCPDS 00‐21‐

1544). Nevertheless, according to the standard XRD file of calcium diglyceroxide, the line at

8.2o should be more intense than the line at 10.1o. However, the XRD patterns of the post

reaction samples calcined at 800 oC showed the reverse relative intensities for these two lines.

Therefore, the presence of calcium methoxide cannot be discarded. In fact, the higher

intensity of the line at around 10o can be due to the overlapping with the intense line

ascribable to calcium methoxide [19]. It is worth noting that the diffractogram of the catalyst,

after the fourth reaction batch presented in Fig. 2, does not show the lines correspondent to

diglyceroxide or methoxide phases and only the diffraction lines ascribable to Ca(OH)2 are

visible. Nevertheless, the infrared spectrum showed that calcium methoxide remains on the

surface of the catalyst.

The formation of calcium diglyceroxide [19‐21] and methoxide [11, 22] during oil methanolysis

over lime catalyst is reported by other researchers. Actually, the diglyceroxide phase is

reported to be highly active. Nevertheless, Kouzu et al. [19] mentioned that the process is

catalyzed by a soluble compound and recent results also showed that the high activity of

diglyceroxide can be due, at least partially, to a homogeneous catalyzed process [19, 23]. This

dissolution may also explain the disappearance of the calcium diglyceroxide phase with the

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increase of the number of reaction batches. Recently, Urasaki et al. [24] also reported the

instability of calcium diglyceroxide during alcoholysis of triolein and concluded that the

formation of calcium diglyceroxide leads to calcium leaching.

The FTIR analysis of the catalysts was also carried out in an attempt to identify the different

species on the catalysts surface. Thus, the infrared spectra of fresh and post reaction catalysts

were collected using a horizontal attenuated total reflectance device (HATR) and some of the

spectra collected are presented in Fig. 4. To help this identification, the main IR bands of the

main species involved in the transesterification reaction of vegetable oils using CaO derived

catalysts are presented in Table 2.

The spectra of the fresh catalyst presented in Fig. 4 shows a low intensity band at around 3630

cm‐1 that may be ascribed to the stretching of the O‐H groups of Ca(OH)2, which was also

identified in the XRD analysis (Fig.1) . The peak at around 2390 cm‐1,that appears in the Li

doped catalysts, may be ascribed to the adsorbed atmospheric CO2, whereas the band at 1610‐

1150 cm‐1 may be attributed to the adsorbed water and/or to the CaO carbonate species

derived from the carbonation of the surface [21].

In order to clarify the differences between the spectra, a PCA (principal component analysis)

was developed [12, 13]. Before model development, the spectra were pre‐processed in order

to reduce variations not directly related to the analysis, such as random noise, baseline drift,

light scattering, etc. In this case, the standard normal variate scaling plus mean centering were

used as pre‐processing. As presented in Fig. 5, the model developed for the fresh catalyst

captured 90.1% of the overall variance of the original data with only two PCs (PC1‐75.6%; PC2‐

14.5%). This Figure presents the scores plot of PC2 versus PC1 for this model, as well as the

loadings plots of the variables (in cm‐1) in PC1, which captured the higher variance. Thus, as

shown in the scores plots, the first PC allows to distinguish the CaO from the Li_CaO catalysts.

Additionally, the loading plots indicates that the variables that are more important to describe

this PC are at around 870 cm‐1 and 1300‐1400 cm‐1, which may be attributed to the CaO

carbonate species (Table 2); and at around 3400‐3600 cm‐1, which are assigned to the

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stretching vibrations of the OH groups of Ca(OH)2 present at the surface of the Ca oxide [18].

It is worth noting that these ‐OH surface groups play an important role on the methanol

activation and thus on the methanolysis catalyzed process [26].

The spectra of the post reaction samples presented in Fig. 4 show bands at around 3600 cm‐1,

2800‐3000 cm‐1 and 1500‐900 cm‐1 that ascribable to calcium methoxide and glyceroxide

species (Table 2). Fig. 4 also shows that the intensity of these bands increase with the raise of

the number of reaction batches. The quantity of the methoxide species, on the catalyst

surface, increases with the number of reaction cycles.

Fig. 6 presents the spectra of the several catalysts samples after the third reaction stage

(PR#3). These spectra show the main IR bands in the range 1200‐1350 cm‐1, attributed by

Reina et al. [21] to calcium glyceroxide species. These bands are more intense for the

Li_CaO_800 ºC possibly due to its higher basicity. Furthermore, Fig. 3 shows that the samples

of this catalyst, after the first and second reaction batches, present the higher amount of the

calcium diglyceroxide phase. However, the diffractogram of the PR#4 sample did not show this

crystalline phase, which is an indication that, as found elsewhere [11,23,24], this species is

soluble in the reaction medium.

The differences between the spectra presented in Fig. 6 are visible in the scores plot of the

PCA model presented in Fig. 7. In fact, the Li_CaO catalysts obtained after the third reaction

batch are grouped in the negative side of the PC1 axis, whereas the CaO samples are in the

positive side. Furthermore, from the loadings plots of PC1, it is possible to conclude that the

variables that are more important to describe this PC, which captured 82.5% of the variance of

the spectra, are at around 3600 cm‐1, 3000 cm‐1 and 1400 cm‐1. Therefore, these differences

between the Li modified and the raw CaO catalysts identified by this PCA model seem to be

due to the presence of a higher amount of methoxide and glyceroxide species on the catalyst

surface. As mentioned above, the XRD analysis allowed to identify calcium diglyceroxide in the

sample calcined at 800 0C (Fig. 3).

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3.2 Catalytic tests

The main data on the catalytic behavior during rapeseed oil methanolysis, for the prepared

catalysts, are presented in Table 3. For fresh samples no significant differences were observed

in the catalytic activity. These results are in agreement with those of Alonso et al. [7]. These

researchers reported that a minimum of 4.5% (w/w) of Li is necessary to promote the activity

of CaO catalyst and the tested catalyst only had 3.72% (w/w) of lithium. Endalew et al. [11]

also reported an improved activity for Li‐CaO catalyst, containing 7% (w/w) of Li, when acid

jatropha curcas oil was used to produces biodiesel whereas an apparent null effect was

observed when rapeseed oil was used. The fair results obtained using acid oil seems to

emphasize that Li is crucial to obtain strong basic active sites.

Only the sample with Li calcined at 575 ºC showed a slightly lower FAME yield, 93.1% instead

96‐97%, eventually due to morphologic differences (larger particles). The same effect for the

sample calcined at 800 ºC is compensated by the stronger surface basicity. As reported by

Alonso et al. [7] the sample calcined at higher temperatures will processes stronger basicity

even if the Hammett indicators technique showed analogous results for both samples.

The catalysts deactivation was studied using the same catalyst sample in consecutive reaction

batches without any intermediate regeneration procedure. Data in Table 3 show that Li had a

negative effect on the catalyst stability which can be due to the enhanced formation of calcium

diglyceroxide for the Li modified samples. Previous results appears to point out glyceroxide

species as responsible for Ca leaching [23] and thus for the activity decay.

Finally, it must be emphasized that based on a model developed by West, Posarac and Ellis

[27], it is estimated that the use of heterogeneous catalysts for biodiesel production with the

above presented performances could reduce the equipment costs in about 25%, when

compared to the traditional production process using homogeneous catalysts.

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CONCLUSIONS

The methanolysis of vegetable oils over CaO catalysts involves methanol activation by basic

active sites on the catalyst surface (‐OH or other). Thus, it seems likely that an improvement of

the surface basicity will be beneficial for the catalytic performances.

In order to improve surface basicity, a commercial CaO material was modified with lithium

(Li/Ca=0.3 molar) by wet impregnation (nitrate salt) followed by calcination at 575 ºC and 800

ºC. The highest calcination temperature promotes sinterization and enhances the formation of

glyceroxide and methoxide species on the catalyst surface.

In the tested conditions, the catalysts showed analogous initial catalytic activities and allowed

to produce a biodiesel containing 93‐98 % of FAME after 4 h of reaction. However the Li doped

catalysts presented a faster deactivation than the raw CaO. The surface infrared spectra

showed that Li modified catalysts have more glyceroxide surface species than the raw CaO

catalyst. Bulk calcium diglyceroxide and methoxide species were also founded by XRD for the

sample calcined at 800 ºC. The formation of the Ca‐diglycerol phase promotes the Ca leaching

and thus contributes to catalysts deactivation.

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26. I. Choedkiatsakul, K. Ngaosuwan, S. Assabumrungrat, Application of heterogeneous

catalysts for transesterification of refined palm oil in ultrasound‐assisted reactor, Fuel

Proces Technol, 111 (2013), pp. 22‐28.

27. A. West, D. Posarac, N. Ellis, Simulation, case studies and optimization of a biodiesel

process with a solid acid catalyst, Int J Chem React Eng, 5 (2007), A37.

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Figures Captions

Fig. 1– XRD patterns of fresh catalysts.

Fig. 2 – Granulometric distributions of fresh and post reaction catalysts (laser diffraction).

Fig. 3 – XRD patterns of the fresh and post reaction of Li modified catalysts.

Fig. 4 – Fresh and post reaction HATR‐FTIR spectra of CaO and Li modified catalysts.

Fig. 5 – Scores and loadings plots of the PCA model derived the FTIR spectra of the fresh

catalysts.

Fig. 6 – HATR‐FTIR spectra of post batch #3 CaO and Li‐CaO catalysts (FAME yields: CaO‐120 ºC‐

96%; CaO‐575 ºC ‐98%; LiCaO‐575 ºC‐76%; LiCaO‐800 ºC‐96%)

Fig.7 – PCA analysis of HTAR‐FTIR spectra of the PR#3 catalysts.

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5 15 25 35 45 55 65 75

Inte

nsity

(a.u

.)

2θ (º)

CaO, 120ºC

CaO, 575ºC

CaO, 800ºC

30Li ‐ CaO, 575ºC

30Li ‐ CaO, 800ºC

CaO 00‐001‐1160

Ca(OH)2 00‐001‐1079

Figure 1

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0

1

2

3

4

5

6

0.1 1 10 100

Surface area

(%)

dp (μm)

120ºC

575ºC

800ºC

120ºC; PR#1

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0.1 1 10 100

Surface area

(%)

dp (μm)

fresh

PR#1

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0.1 1 10 100

Surface area

(%)

dp (μm)

fresh

PR#1

PR#2

Figure 2

30Li_CaO_800ºC

30Li_CaO_575ºC

CaO

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10 20 30 40 50 60 70

Intensity

(a.u.)

2θ(0)

30_LiCaO, 575ºC

PR#4

PR#2

fresh

5 15 25 35 45 55 65

Intensity

(a.u.)

2θ(0)

30_LiCaO, 800ºC

PR#2

PR#1

fresh

PR#4

C6H14CaO6 00‐021‐1544

Figure 3

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500750100012501500175020002250250027503000325035003750

Absorban

ce (a.u.)

Wave number (cm‐1)

CaO

fresh

PR#1PR#5

PR#4

500750100012501500175020002250250027503000325035003750

Absorban

ce (a.u.)


Li CaO_575ºC

PR#2

fresh

PR#3

Figure 4

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Figure 5

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500750100012501500175020002250250027503000325035003750

Absorban

ce (a.u.)


after reaction batch #3

30Li_CaO, 800ºC

30Li_CaO, 575ºC

CaO, 575ºC

Figure 6

CaO, 120ºC

-methoxide

-hydroxyl

-glyceroxide

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Figure 7

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Table 1– XRD phases of fresh catalysts and crystallite sizes (Scherrer law).

Crystalline phases* Crystallites size (nm)

CaO, 120 ºC CaO 75.6

CaO, 575 ºC CaO Ca(OH)2

73.6 11.7

CaO, 800 ºC CaO 75.6

Ca(OH)2 15.2 30_LiCaO, 575 ºC CaO 79.6

Ca(OH)2 48.2 30_LiCaO, 800 ºC CaO 123.4

Ca(OH)2 25.6 *CaO JCPDS 001‐1160; Ca(OH)2 JCPDS 01‐072‐0156

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Table 2 – Main Infrared bands of several species eventually present on the catalyst surface (CaO catalytic system for FAME production). Absorption range (cm‐1) Reference

Biodiesel

C‐H stretching

C=O stretching

C‐O‐C stretching

≈3000‐2850

1750

≈1170

[20]

Methoxide species

O‐H stretching

CH3 stretching vibrations

C‐H bending

C‐O stretching

3650

2800‐3000

1465

1084, 1063

[25]

[25]

[25]

[20]

Ca glyceroxide species

O‐H stretching

C‐H stretching

C‐H, C‐O‐H bending

C‐O Stretching

CH2 vibration

3641

2927,2883,2841

1469‐919

1138, 1080

700‐1000 and 1200‐1350

[21]

[25]

Ca(OH)2

O‐H

3400‐3600

3645‐3658

[18]

[20]

CaO carbonate species

1694,1651,1612,1354

1490‐1419

870

[21]

[20]

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Table 3 – FAME content (weight basis), by NIR, of the purified biodiesel phase (Wcatal/Woil=5%; methanol/oil=12 molar ratio; 4h at 62 ºC).

FAME content ±1.5% (w/w) Catalyst Thermal treatment (ºC) Fresh PR#1 PR#2 PR#3 PR#4 PR#5

120 97.2 96.9 96.1 96.2 96.4 62.4

575 94.9 97.6 97.6 96.7 28.3

CaO 800 98.1 97.7 97.4

575 93.1 96.2 76.0 16.5 30%Li‐CaO

800 96.8 97.9 96.0 67.2

maximum error for FAME content previsions in the range 78.4‐99.3% [14].

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Highlights Biodiesel from rapeseed oil was produced over lithium modified CaO catalysts Lithium improve the surface basicity by formation of electron defects The calcination temperature plays an important role on the catalyst performances Calcium diglyceroxide formation is enhanced by catalyst basicity Leaching of Ca seems be due to calcium glyceroxide formed during reaction

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Anionic vacancy

created by Li+

Improved

catalytic

activity due

higher basicity

FAME + Glycerin Vegetable oil + Methanol

Improved formation of Ca-diglyceroxide

Faster catalyst deactivation due

to Ca leaching

*Graphical Abstract (for review)

biodiesel production over lithium modified lime catalysts: activity and deactivation

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