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Article

Commercial hydrated lime as a cost-effectivesolid base for the transesterification of wasted

soybean oil with methanol for biodiesel productionManuel Sánchez-Cantú, Lydia María Pérez- Díaz, Rosalba Rosales, Elsie Ramírez, Alberto

Apreza-Sies, Israel Pala-Rosas, Efraín Rubio-Rosas, Manuel Aguilar-Franco, and Jaime S. ValenteEnergy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef200555r • Publication Date (Web): 09 June 2011

Downloaded from http://pubs.acs.org on June 22, 2011

Just Accepted

“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are postedonline prior to technical editing, formatting for publication and author proofing. The American ChemicalSociety provides “Just Accepted” as a free service to the research community to expedite thedissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscriptsappear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have beenfully peer reviewed, but should not be considered the official version of record. They are accessible to allreaders and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offeredto authors. Therefore, the “Just Accepted” Web site may not include all articles that will be publishedin the journal. After a manuscript is technically edited and formatted, it will be removed from the “JustAccepted” Web site and published as an ASAP article. Note that technical editing may introduce minorchanges to the manuscript text and/or graphics which could affect content, and all legal disclaimersand ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errorsor consequences arising from the use of information contained in these “Just Accepted” manuscripts.

1

Commercial hydrated lime as a cost-effective solid base

for the transesterification of wasted soybean oil with

methanol for biodiesel production

Manuel Sánchez-Cantú ‡*, Lydia M. Pérez- Díaz

‡, Rosalba Rosales

‡, E. Ramírez

‡, Alberto Apreza-Sies

‡,

Israel Pala-Rosas‡, Efraín Rubio-Rosas

‡, Manuel Aguilar-Franco

§, Jaime S. Valente

†

Benemérita Universidad Autónoma de Puebla, Facultad de Ingeniería Química, Avenida San Claudio y

18 Sur, C.P. 72570 Puebla, Puebla, México; Universidad Nacional Autónoma de México, Instituto de

Física, Circuito de la Inv. Científica S/N C.U., C.P 04510, D.F. México, Instituto Mexicano del

Petróleo, Eje Central #152, C.P. 07730 México, D. F., México.

*To whom correspondence should be addressed. Phone: + (5255) 2295500 Ext. 7254, E-mail:

manuel.sanchez@correo.buap.mx

‡ Benemérita Universidad Autónoma de Puebla

§ Universidad Nacional Autónoma de México

† Instituto Mexicano del Petróleo

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ABSTRACT.

The transesterification of used soybean oil with methanol was carried out over hydrated lime (HL),

Ca(OH)2, and its decomposition products in the 200-500ºC range. The catalysts were characterized by

X-ray powder diffraction (XRD), thermogravimetric analysis (TGA) and scanning electron microscopy

(SEM). The XRD powder patterns demonstrated that the pristine sample consisted of a mixture of

calcium hydroxide and calcite. It was noticed that the coexistence of CaO, Ca(OH)2 and CaCO3

remained up to 400ºC. At 500ºC Ca(OH)2 is transformed into CaO so that this and CaCO3 are the only

remaining phases. In the transesterification reaction the influence of calcination temperature, reaction

time, catalyst amount, methanol:oil ratio and reaction temperature were studied. Full conversion of the

raw materials into biodiesel (BD) was obtained with the fresh HL. In order to determine any change in

the solid it was recovered after 10, 30 and 60 minutes of reaction and analyzed by XRD analysis. Only

Ca(OH)2, CaCO3 and traces of monohydrocalcite were detected. From the results it was demonstrated

that the active phase for biodiesel production was calcium hydroxide. Furthermore, the catalyst was used

up to three times without deactivation. A simple, economic and environmental-friendly way for

biodiesel obtaining was developed considering: a) used soybean oil, considered as a waste, was

employed as raw material, b) hydrated lime is cheap and readily available and c) full conversion of the

raw materials into BD was achieved with the as-received HL.

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1. Introduction

The world’s oil dependence has reached historical levels and it’s imperative to find optimal

exploitation of the energetic resources in order to increase world’s energy security. Moreover, the

increment of the crude oil prices has forced the search for alternative fuels based on renewable energy

sources. In this sense, a potential diesel oil substitute is generally known as biodiesel (BD). BD from

vegetable oil resources can substitute diesel in considerable proportion and therefore, vegetable oil

resources, particularly, the non-edible one such as wasted oils, deserve our consideration.

Nevertheless, the major drawback of biodiesel production is its economic viability since its production

costs is higher than fossil derived diesel. The overall biodiesel cost consists of raw material (production

and processing), catalyst, biodiesel processing (energy, consumables and labour), transportation (raw

materials and final products) and local and national taxes.1 In this sense, the cost of refined vegetable

oils contribute between 60-80% of the overall biodiesel production cost.1, 2

Even more there is a severe controversy of using edible oils since it may cause an increase of their

costs and reduce their availability. Therefore, one way for reduction of BD production cost is employing

cheaper feedstocks such as used edible oils which are considered as waste. These are generally disposed

into the water drainage without any treatment, causing severe environmental problems. Nowadays, the

amount of these waste oils generated per year by any country is huge and will increase with the

population’s requirements.3

In general, biodiesel is produced by transesterification (alcoholysis) employing homogeneous bases

and/or acids.4 In the reported advantages of the homogeneous base catalyzed reaction are: very fast

reaction rate (4000 times faster than acid-catalyzed transesterification), catalysts such as NaOH and

KOH are relatively cheap and widely available, reaction can occur at mild reaction condition and less

energy intensive. In the same way, the advantages of the acid-catalyzed reaction are: is insensitive to free

fatty acids and water content in the oil, is the preferred-method if low-grade oil is used, esterification

and transesterification occur simultaneously, reaction can occur at mild reaction condition and less

energy intensive.

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Nevertheless, some of the problems associated with the homogeneous catalysts are, for instance, the

high consumption of energy, corrosion on reactor and pipelines by the use of high alkaline or acid

conditions, expensive separation of the homogeneous catalyst from the reaction mixture, the catalyst

cannot be reused or regenerated, generation of large amounts of wastewater during separation of the

products.1 In the base-catalyzed reaction, there is also the problem of formation of unwanted soap

byproduct by reaction of the free fatty acids.

Therefore, the use of solid catalysts represents an attractive solution, since the catalysts can be easily

separated from the products, the products do not contain impurities of the catalyst and the cost of final

separation could be reduced, they can be readily regenerated and reused and it is more environmentally

benign because there is no need for acid or water treatment in the separation step.5-7

The transesterification reaction in heterogeneous phase can be carried out by solids who disclose

Brönsted or Lewis basic sites. In this sense, a great variety of catalyst have been used for this purpose

which include: alumina, TiO2, ZrO2,8 zeolites,

9 anionic clays,

10-15 etc. In this context, the obtaining of a

highly active catalyst is not only required but also that the catalyst exhibit a poisoning resistance as well

as its non expensive production since this characteristics will not only impact in BD production but in

lowering its cost.

Among the most studied heterogeneous catalysts the Ca-derived bases are the most promising as they

are inexpensive, have long lifetimes, require only mild reaction conditions, they are the least toxic and

they exhibit low solubility in methanol.16

Kouzu et al.17

reported the obtaining of CaO from calcination of pulverized lime stone (CaCO3) at

900°C for 1.5 h in a helium gas flow reaching a 93 yield percentage at 1 h of reaction time for CaO.

Granados et al.18

evaluated the role of H2O and CO2 in the deterioration of the catalytic performance

of CaO in BD production by contacting it with room air and demonstrate that in order to recover the

catalyst’s activity a thermal treatment is required to revert CO2 poisoning.

Cho et al. obtained calcium oxide catalysts by calcining various precursors such as calcium acetate,

carbonate, hydroxide, nitrate and oxalate and their catalytic activities were examined in the

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transesterification of tributyrin with methanol.19

From their results, CaO obtained from calcium

hydroxide was the most active catalyst among the prepared catalysts, with 82% conversion in one hour.

From an industrial point of view, calcium oxide, also known as quicklime or burnt lime, is usually

produced by the thermal decomposition of a parent material such as its hydroxide or carbonate. If over

burning has not taken place, CaO reacts with water to form calcium hydroxide, by a process known as

slaking.

It is reported that calcium oxide produced from hydrated lime, Ca(OH)2, is more reactive in sulfur20

and CO221

capture than that derived from the respective limestone, CaCO3. The general reasons given

are:20, 22

a) the inherent particle size of hydroxide-CaO is much smaller than that of carbonate-CaO

giving rise to a greater reactivity and b) solid products of decomposing Ca(OH)2 are particles that have

approximately the same exterior dimensions as the parent Ca(OH)2 ones, showing slit-shaped geometry

and having higher internal surface areas than does calcium oxide obtained from CaCO3 calcination at

higher temperature. Taking this into account and in the knowledge that CaO disclose strong Lewis basic

sites whose nature and amount will depend on the treatment temperature,23

it is important to find the

optimal activation temperature for biodiesel production.

Moreover, as Ca(OH)2 structure resembles that of brucite, where M2+

(OH)6 octahedra share edges to

build infinite M(OH)2 sheets, and considering that the transesterification reaction can proceed via

surface OH- groups

15 it is

plausible to consider that hydrated lime can be used directly as a catalyst for

biodiesel production.

From the above-mentioned, hydrated lime, obtained from the controlled hydration of CaO achieved

from the previous calcination of limestone, and the corresponding CaO obtained from its thermal

treatment represent not expensive catalysts for the obtaining of biodiesel since it is produced in large

amounts and it’s readily available.

Thus, in this work, hydrated lime and CaO produced by its thermal treatment were employed as

heterogeneous catalysts in the transesterification of used soybean oil and methanol.

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2. Experimental Section

2.1 Materials

The hydrated lime (HL), purchased from Cales Santa Emilia ubicated in Perote, Veracruz, México, is

sold in 25 kg paper sacks and was used as received without further purification. Virgin soybean oil was

purchased from a local store. The used soybean oil (USO) was obtained frying six batches of potatoes in

virgin soybean oil. Then, it was filtered under vacuum to eliminate the suspended solids followed by

heating to remove all the moisture. Methanol (98% purity) was purchased from Golden Bell. Methanol

was dried with metallic magnesium and iodine.

2.2 Catalyst characterization

The X-ray diffraction pattern of the sample was measured in a D8 Bruker Discover Series 2

diffractometer with CuKα radiation. Diffraction intensity was measured between 5° and 45°, with a 2θ

step of 0.02° and a counting time of 0.6s per point.

Thermogravimetric (TG) analysis was developed on a TGAi 1000 Series System which was operated

under nitrogen flow at a heating rate of 10 °C/min from 25 to 1000 °C. In the determination, ~100 mg of

finely powdered dried sample was used.

Scanning Electron Microscopy (SEM) analysis was carried out in a JEOL JSM-6610 LV with an

acceleration voltage of 20keV. Prior to analysis, the sample was covered with gold and mounted over a

carbon film.

2.3 Transesterification of used soybean oil with methanol

The experiments were conducted in a 100 mL three-neck glass flask coupled to a condenser and a

water cooling recirculation system. Stirring and heating were achieved using a magnetic stirrer/hot plate.

First, the USO was heated above 100°C for one hour to eliminate moisture; then the oil was cooled to

the required reaction temperature. By separate, methanol and the catalyst were added into the glass flask.

Then, USO was charged into the vessel and heated to the intended reaction temperature.

After the course of the reaction, the reaction mixture was centrifuged and the liquid phases were

separated from the catalyst and decanted in a separation funnel. Excess methanol was removed by

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evaporation, leaving biodiesel (BD) and glycerol as separate phases. The BD was analyzed by thin-layer

chromatography (TLC) and Proton Nuclear Magnetic Resonance (1H NMR) analysis.

TLC analysis was performed with ALUGRAM Sheets SIL G/UV 254 (Macherey-Nagel) and

developed in a solvent system of hexane/ethyl acetate (8:1). USO and methyl esters spots were revealed

by an iodine vapor stain. 1H NMR spectra were measured on a Varian Mercury spectrometer, using 400

MHz frequency, in deuterated chloroform (CDCl3) and tetramethylsilane (TMS) as reference. The

relevant signals chosen for integration were those of methoxy groups in the fatty acid methyl esters

(FAMES) (3.66 ppm, singlet) and the glyceridic protons of soybean oil (4.14-4.27 ppm, doublet of

doublets). The conversion was calculated directly from the integrated areas of the aforementioned

signals following the procedure described by Knothe.24

The studied operation variables were: catalyst activation temperature, catalyst concentration,

methanol/oil molar ratio, and reaction temperature. 12 mL of used soybean oil, 6mL of methanol, Fresh

HL/liquids mass ratio: 4%, reaction temperature of 60ºC and 120 min of reaction time were fixed as

initial reaction conditions. The catalyst mass was maintained the same in all experiments. The

experiments were performed three times and the average results from the 1H NMR spectra were

reported.

3. Results and discussion

3.1 Catalyst Characterization

Figure 1 shows the X-ray powder patterns of the pristine sample (PS) and the samples calcined at 200-

500°C. The PS exhibited the characteristic reflections of calcium hydroxide (JCPDS # 84-1263) and

calcium carbonate in its calcite form (JCPDS #88-1807), as the principal and secondary crystalline

phases, respectively. Since hydrated lime is generally obtained from the controlled rehydration of CaO,

the presence of the calcite phase may be caused by the reaction of atmospheric CO2 in the rehydration

process. It’s important to remark that both crystalline phases remained in the samples thermally treated

at 200 and 300ºC. These results can be corroborated by the analysis of the average crystal sizes shown in

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Table 1. Average crystal sizes were calculated from the most intense reflections employing the Scherrer

equation L = 0.9λ/Bcosθ, where L is the crystallite size, λ the X-ray wavelength, B the line broadening

and θ the Bragg angle.25

The crystal sizes of calcium hydroxide did not show any significant change in

the fresh and the calcined samples at 200 and 300°C (25, 22 and 24 nm, respectively) while calcite’s

crystallites presented a slight decrease from 56 to 41 nm.

It’s worthwhile to stress the changes that took place at 400°C evidenced by the presence of CaO

(JCPDS #78-0649) which corresponds to calcium hydroxide decomposition26

followed by an increment

of the Ca(OH)2 and calcite crystal sizes. Finally, at 500°C the characteristic reflections of Ca(OH)2

completely disappeared remaining only calcite and CaO phases with no significant change in the average

CaO crystal size but with an increase of CaCO3 cystals. This behavior is attributed to the sintering

process27

explained by the Ostwald ripening effect which involves the growth of larger particles at the

expense of smaller ones.28

The thermogravimetric analysis is presented in Figure 2. From the first derivative of the thermogram

three thermal transitions were observed: the first one corresponds to the weight loss up to the point

where a relative stability is achieved, around 393°C; the second was set at 393-480°C and the third

interval was established at 480-743°C, respectively. In the first temperature range the sample lost 0.32 %

which was assigned to the elimination of physisorbed water. The second transition, where 17.81% was

lost, was attributed to the dehydroxylation of the Ca(OH)2 laminae. The last transition (7.56%)

corresponded to calcite’s decarbonation and the complete dehydroxylation of calcium hydroxide.26

From the high temperature X-ray powder patterns (see Figure 1) it can be seen that calcium hydroxide

and calcite crystalline phases remained almost unchanged up to 300°C which match with the TG results.

The second and third weight loss corroborated the decomposition of Ca(OH)2 and calcite to form the

CaO crystalline phase which was evidenced by the XRD analysis at 400°C and 500°C. Also, the well-

defined thermal transition in the 393-480ºC interval can be attributed to the OH’s nature, meaning that

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they are similar. This behavior has been observed in carbonate-hydrotalcite and hydrotalcite-like

compounds where the peak assigned to carbonate decomposition was related with their basic strength.29

Figure 3 exhibits the SEM images obtained with the JEOL JSM-6610 LV, employing the signals

generated by secondary electrons. SEM image of the pristine hydrated lime (Figure 3A) demonstrated

that the sample consisted of plate-like particles, a characteristic morphology of hydrated lime, with an

average particle size of 0.16 µm.30

The samples thermally treated at 200 and 300°C showed similar

results. However, when the sample was calcined at 400°C and 500°C (Figure 3B and 3C) an important

change in size was noticed since the average particle size increased up to 0.7 µm in both cases. Also,

from the micrographs the presence of necks that join the non-uniform grains collectively in a continuous

matrix was observed together with a transformation into sphere-like particles which is attributed to a

sintering process.31

These results agree with those obtained by the X-ray analysis where an increment in

the average crystal size was also noticed.

3.2. Transesterification reaction.

3.2.1 Effect of the catalyst activation.

According to the results reported in the literature, CaO acts as an efficient catalyst in the

transesterification reaction.32, 33

Therefore, we evaluated in first instance the calcined catalyst at 500°C

since CaO was detected as the main crystalline phase. From the thin-layer chromatography analysis we

observed the absence of the USO signal and only the biodiesel spot remained. This result was

corroborated by the 1H NMR analysis.

Figure 4 presents the 1H NMR spectra of the used soybean oil and the fatty acid methyl esters (FAME)

product resulting from its transesterification with methanol. Table 2 shows the chemical shifts and

characteristics signals of a typical 1H NMR spectrum of soybean oil (triglycerides) assigning each group

within it. The presence of olefinic hydrogens (-CH=CH-)-7 and methine group (-CH2-CH-CH2-)-2

triglyceride with a chemical shift at 5.34 and 5.26 ppm come out as multiplet signals, respectively.

Moreover, diallyl methylene hydrogens observed at 2.76 ppm, are presented as a triplet (=CH-CH2-

CH=)-8. The occurrence of methylene hydrogens of carboxyl groups at 2.31 ppm is seen as a triplet

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(-CH2-CH2-OCO-)-3 while hydrogens on neighboring methylene groups to unsaturated carbon chemical

shift of 2.03 ppm appeared as a multiplet signal (-(CH2)5-CH2-CH=)-6. Hydrogens neighboring

methylene groups of saturated carbon atoms are observed at 1.61 ppm (-(CH2)12-CH2-CH2-)-4. Likewise

the methylene groups’ hydrogens attached to two saturated carbon atoms arose as multiplet signals

(-(CH2)12-)-5 (1.27 ppm), also the hydrogen of the terminal methyl groups appeared as a multiple signal

(-CH3)-9 at 0.89 ppm. Moreover, it is important to mention that the 1H NMR spectrum of soybean oil

(Figure 4A) shows a splitting pattern for the protons glyceride which follows a ABX pattern due to the

asymmetric chemical shift of methylene group protons at 4.22 ppm (4 H), the value of the coupling

constants are: Jab = 12 Hz, Jax = 6 Hz, Jbx = 4 Hz for a system represented by RCO2-CHaHbCHx (CO2R’)

CHaHbCO2R´´. The inequivalence of the hydrogen a and b results from the fact that both are on opposite

sides of the plane that extends along the link to the methylene and methine, in this system the group has

O2CR and protons are on the same side of the plane, while Hb and Hx protons are on the other side of the

plane. Therefore, both hydrogens Ha and Hb appear as doublet of doublets (dd) at 4.27 and 4.14 ppm,

respectively. It is important to remark that both 1H NMR spectra of the fresh oil and the used soybean oil

spectra were identical. Although the soybean oil was used six times within a period of 12 hours, giving a

brownish product, the expected signals of the products formed during frying such as free fatty acid

and/or polymerized triglycerides were not appreciated in the used cooking oil which could be due to the

frying severity. Generally, in public restaurants, frying is conducted in the same oil for several days.3

Even though, their presence cannot be discarded since they could have exceeded the apparatus detection.

Figure 4B exhibit the 1H NMR spectrum of the produced FAMES (biodiesel). The major differences

between the spectrum of soybean oil and the resulting fatty acid methyl ester are the disappearance of

the glyceride protons and the appearance of a simple signal (s) corresponding to methyl ester protons at

3.66 ppm (9H). The chemicals shift of the hydrogens of aliphatic carbons corresponding to the 3

monoglycerides obtained are the same and can be verified in the spectrum.

As a complete conversion was achieved with the solid annealed at 500ºC, the calcination temperature

was decreased and the experiment was repeated employing the 400, 300 and 200°C calcined samples

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obtaining similar results. Finally, we used the as received hydrated lime in the transesterification

reaction achieving a 100% conversion.

These results differ from those reported by Kouzu et al.17

where they carried out the transesterification

of edible soybean oil with refluxing methanol in the presence of calcium oxide CaO, Ca(OH)2 and

CaCO3 achieving at 1 h of reaction time a BD yield of 93% for CaO, 12% for Ca(OH)2, and 0% for

CaCO3. Also they reported that in order to reach the same result with Ca(OH)2 it took up 3.5 h to gain

the same yield. Since CaCO3 is inactive in the transesterification reaction34

we assumed CaO, who

exhibit strong Lewis O2-

basic sites, was the responsible for the complete transformation of our raw

materials into BD in the sample calcined at 500°C. However, the results attained in the samples where

Ca(OH)2 was detected as the main crystalline phase must be explained in terms of their Brönsted, OH-,

basicity. This behavior has been also observed for other authors in the transesterification oleic acid

methyl ester with glycerol with rehydrated hydotalcite-like compounds.35

On the other hand, Kouzu et al.36

also evidenced that the active phase of calcium oxide in the

transesterification of edible soybean oil with methanol was calcium diglyceroxide (CD). When they took

a sample at 0.25h of the reaction only CaO and Ca(OH)2 crystalline phases were detected by means of

the XRD analysis. In this sense, taking into account that Ca(OH)2 was formed during the reaction it is

plausible to assume that CD is formed from Ca(OH)2 itself and not directly from CaO.

From the XRD powder pattern reported by Kouzu et al.36

the average crystal size of their calcium

hydroxide, calculated from the (011) reflection was ca. 10 nm. In this context, small crystal sizes favor

the activity, since it is well known that smaller crystals produce more reactive sites. According to our

results the formation of CD may be reached in a shorter period employing hydrated lime than when CaO

was used as catalyst precursor which explains the high conversions reached in this work.

To verify this assumption the reaction was performed employing hydrated lime as catalyst and the

solid was collected after 10, 30 and 60 minutes of reaction. The sample was only filtrated and the paste

consisting of a mixture of the catalyst, oil, glycerin, BD and methanol was analyzed by XRD. The

results are presented in Figure 5. The XRD results revealed only the presence of calcium hydroxide,

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calcite and traces of monohydrocalcite (JCPDS #83-1923). These results disagree with those of Kouzu’s

since we expected CD obtaining. Kouzu et al. reported that after 2 hours of reaction, the catalyst was

collected by filtration, washed with methanol to remove the stains such as methyl esters and glycerol

followed by vacuum drying at 80ºC. According with Ngamcharussrivichai et al.34

methanol washing is

not enough for the sorbed organic compounds removal, and by consequence we attributed the formation

of CD in Kouzu’s samples to the presence of organic molecules which could not be removed by the

methanol washing and further reacted with CaO and Ca(OH)2 favored by the 80ºC drying. Again, we

took a sample at 60 minutes of reaction and repeated Kouzu’s activation. After the reaction we washed

the collected catalyst with 50 mL of anhydrous methanol and dried at 80°C. From the XRD powder

pattern we did not detect calcium diglyceroxide (see Figure 5D). It appears that during reaction the CaO

employed by Kouzu et al.36

was covered by a Ca(OH)2 film which is also resposible for biodiesel

production and calcium diglyceroxide is produced by the reaction of CaO with the organic compounds

sorbed on its surface. This also explains the reported conversion decrease when CD itself was employed

in BD production. In this sense, at least for our results, the active phase in the transesterification of

soybean oil with methanol is considered to be calcium hydroxide. This suggests that during the

transesterification reaction surface-bound OH groups of hydrated lime are converted to surface bound

water groups upon reaction with methanol, thus liberating methoxide which is recognized as a very

active catalyst for BD production.17, 32, 37

This behavior is in good agreement with the results obtained by

Reddy et al.38

which also confirmed the importance of the Brönsted basic sites in biodiesel obtaining.

These results are attractive in the sense that besides its high activity the catalyst itself is economic,

commercially available and will not require an activation process. Also, it can be used as received

without any special care to avoid the atmospheric contamination.

The TGA results (see Figure 2) revealed that the sample contained physisorbed water (0.32 weight %

corresponding to 0.21% by weight of USO) which according to some reports could favour the

saponification reaction and in consequence diminish biodiesel yield.6, 7, 39

Nonetheless, Liu et al.32

who

carried out the transesterification of soybean oil with methanol reported that if water content is less than

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2.8% by weight of soybean oil, the transesterification reaction rate can be accelerated and the biodiesel

yield is improved within a short reaction time which matches our results.

In order to evaluate the optimal reaction conditions of the transesterification process with the as-

received hydrated lime we examined the effect of the following reaction parameters: reaction time,

catalyst amount, and methanol:oil ratio and reaction temperature. The initial reaction conditions were 12

mL of USO, 6 mL of methanol, 1 gram of catalyst and a reaction temperature of 60ºC.

3.2.2 Reaction time effect.

Figure 6 exhibits the influence of reaction time at 10, 30, 60 and 120 minutes. From the results

obtained, it is evident that reaction time affected significantly the catalytic activity since the conversion

percentage gradually increased with the reaction time achieving in 10, 30, 60 and 120 minutes

conversions of 12, 58, 86 and 100%, respectively. By the analysis of the catalyst’s average crystal size

(calculated from the reflection at 34.1º of 2θ) it can be seen that the catalyst crystal size was inversely

proportional to the reaction time, for instance, the pristine hydrated lime exhibited a Ca(OH)2 crystal

size of 26 nm while at 10, 30 and 60 minutes the crystal sizes were 24, 22 and 19 nm, respectively. This

decrease can be ascribed to the reaction conditions and the vigorous stirring. However, it is well known

that smaller crystals are more active than the bigger ones. Eventually, although 86% is an acceptable

conversion percentage we selected the 120 minutes period for the following experiments.

3.2.3 Effect of catalyst loading.

The effect of the catalyst loading in BD production is shown in Figure 7. The studied range was in the

0.3 to 0.60 grams (2.1-4.2% in relation to the mass of the USO and methanol). As can be observed, the

conversion percentage increase was proportional to the catalysts loading. The best result was obtained

with a catalyst amount of 0.57 grams which corresponds to a weight ratio of 4%. With 4% of catalyst, 2

hours is enough to reach the complete conversion of the raw materials.

3.2.3 Effect of the methanol:oil ratio.

Theoretically, to carry out the transesterification reaction it’s required 3 mol of alcohol for 1 mol of

triglyceride to produce 3 mol of fatty acid ester and 1 mol of glycerol. It’s well known that the

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transesterification reaction is a reversible one and with the intention to shifting the equilibrium towards

biodiesel production an excess of methanol is generally employed. Thus, the yield of biodiesel is

increased when the alcohol/triglyceride ratio is raised above 3 and reaches a maximum. It is important to

consider that increasing the alcohol amount beyond the optimal ratio will not increase the yield but will

increase the cost for alcohol recovery. The stoichiometry of this reaction requires 0.134 volume

methanol per volume triglyceride (the molar ratio is 3:1).32

Taking this into account, the methanol:oil

ratio (vol.) was varied from 0.08 to 0.5.

The experimental results, illustrated in Figure 8, demonstrate the significant impact on biodiesel

obtaining since the conversion increased with the increment of the methanol:oil ratio from 0.08 to 0.17

achieving conversions of 73 and 100%, respectively. Although conversions higher than 97% were

obtained with higher methanol:oil ratios the methanol excess could raise the biodiesel cost due to the

additional process needed for alcohol recovery as stated before.

3.2.3 Effect of reaction temperature.

To elucidate the effect of reaction temperature the transesterification of USO was conducted with the

fresh HL, Ca(OH)2, and with the sample calcined at 500ºC (CaO), maintaining constant the catalyst

amount (4 %), 0.17 methanol:oil ratio (vol.) and 2 hours of reaction time. The results are presented in

Figure 9. The results revealed that conversion of the raw materials increased with the reaction

temperature. No conversion was achieved with the fresh HL at 25°C while at 35, 45, 55 and 60°C

conversions of 2, 19, 57 and 100% were accomplished, correspondingly. On the other hand, by using

the calcined sample (CaO), conversions of 45, 89, 91, 93 and 100% were achieved at 25, 35, 45, 55 and

60ºC, respectively.

The transesterification is generally assumed to be a pseudofirst-order reaction in excess of

methanol.40, 41

Thus, the apparent rate constant is:

(1)

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where k is the apparent rate constant; y is the FAME conversion and t is the reaction time. Moreover,

according with Arrhenius equation the overall reaction rate constant has a relationship with temperature

as follows:

(2)

where Ea is the activation energy, R is the gas constant (J mol-1

K-1

), T is the absolute temperature and C

is a constant. The average rate constant at different temperatures can be obtained from Figure 9. Hence,

activation energies of 53.8 and 279.2 KJ/mol were calculated for the reaction based on the k values at

distinct temperatures for CaO and Ca(OH)2, correspondingly.

3.2.4 Catalyst reutilization

Recovery and reuse of the catalyst is one of the most important characteristic in heterogeneous

catalysis which also would impact the economics for biodiesel production. Reutilization of the catalyst

was carried out by recovering the liquids after a catalytic run, maintaining the catalyst inside the reactor

and adding new fresh portions of methanol and used soybean oil. From the above-studied reaction

parameters the operating conditions for the reutilization study were: 2 hours of reaction time

methanol/oil ratio (vol.) 0.17, catalyst concentration 3.6 weight percent and a reaction temperature of

60°C.

The catalyst was highly active up to the second reutilization, that means, after using the catalyst three

times, achieving a 100% of conversion. However, an important decrease was identified in the third reuse

reaching a conversion of 62%. From the X-ray analysis of the paste (see Figure 5E) we observed that

whilst calcite was identified as a secondary crystalline phase in the first use which we consider remained

up to the second reuse, after the third one it was detected as the main crystalline phase, thus decreasing

the amount of the catalytic active phase and, by consequence, the biodiesel conversion decreased. The

deactivation of catalyst was also attributed to the blockage of the active sites by adsorbed intermediates

or product species, such as diglyceride, monoglyceride, and/or glycerin. However, considering that

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hydrated lime is slightly soluble in water (0.05 mol of OH-/L)

42 we cannot discard that the deactivation

was caused by the catalyst components’ leaching. Further study should be done.

4. Conclusions.

In this work an environmental friendly process for biodiesel production was developed. As-received

hydrated lime and the products of its thermal decomposition were evaluated in the transesterification of

used soybean oil and methanol. It was demonstrated that hydrated lime did not required an activation

process for achieving a complete conversion of the raw materials into biodiesel. From the XRD analysis

it was confirmed that the active phase was calcium hydroxide so it was established that surface Brönsted

basic sites of Ca(OH)2 were highly active in the transesterification reaction. Moreover, 100% conversion

was obtained up to the second reuse of the catalyst without any special reactivation procedure. Due to its

low cost, commercial availability, resistance to atmospheric poisoning and reusability, hydrated lime

represents a new alternative for lowering the cost of biodiesel production.

ACKNOWLEDGMENT

We thank Consejo Nacional de Ciencia y Tecnología, PROMEP and Fondos Mixtos-Puebla’s

Government for the financial support (projects CB 2008/100355, 103.5/10/2124, and 2009-01/128348,

respectively) and BUAP’s Centro Universitario de Vinculación for the help given in catalyst’s

characterizations.

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FIGURE CAPTIONS.

Figure 1. X-ray powder patterns of fresh HL and the products obtained after its calcination in the 200-

500°C interval.

Figure 2. Thermogravimetric analysis of the HL.

Figure 3. SEM images of the samples covered with gold: fresh HL (A), calcined at 400 (B) and 500°C

(C).

Figure 4. 1H NMR spectra of: a) used soybean oil and b) fatty acid methyl esters product from soybean

oil transesterification with methanol.

Figure 5. XRD powder patterns of the samples collected at distinct reaction times: A) 10 minutes, B) 30

minutes, C) 60 minutes, D) 60 minutes, washed and dried at 80°C and E) After the third reuse.

Figure 6. Effect of reaction time on biodiesel yield. Fresh HL/liquids mass ratio: 3.8%; methanol:oil

ratio (vol): 0.5; reaction temperature: 60°C.

Figure 7. Effect of the catalyst loading on biodiesel production. Reaction time: 2h; methanol:oil ratio

(vol): 0.5; reaction temperature: 60°C.

Figure 8. Effect of the methanol:oil ratio on biodiesel manufacture. Reaction time: 2h; catalyst loading

3.6%; reaction temperature: 60°C.

Figure 9. Effect of reaction temperature on biodiesel yield. Reaction time: 2h; catalyst loading 3.6%;

methanol:oil ratio (vol): 0.17.

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TABLES.

Table 1. Average crystal sizes of Ca(OH)2, CaCO3 and CaO

Sample L(011), (nm)+ L(104),

(nm)ǂ

L(200), (nm)ǂ

Fresh HL 26 58 -

200°C 23 48 -

300°C 25 42 -

400°C 37 52 36

500°C - 65 31

+ Ca(OH)2, ǂ CaCO3, ǂ CaO.

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Table 2. Soybean oil protons’ chemical shift assignments and its splitting.

Assignments δ δ δ δ (ppm) signal Integration Group

9 0.89 multiplet 9 H -CH3

5 1.27 multiplet 51H -(CH2)5-

4 1.61 multiplet 6 H -(CH2)4-CH2-CH2-

6 2.03 multiplet 10 H -CH2-CH2-(CH2)4-

3 2.31 triplet 6H -CH2-COO-

8 2.76 triplet 4H -CH2-CH=CH

1 4.22 doublet of doublets 4H RCO-O-CH2-CH-CH2

2 5.26 multiplet 1H RCO-O-CH2-CH-CH2 RCO

7 5.34 multiplet 8H -CH=CH-

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Figure 1.

15 20 25 30 35 40 45

(110)

(011)

(202)

(113)

(200)

(111)

(100) (1

04)

(001)

xxx

∆

***

∆

* CaCO3

X Ca(OH)2

∆ CaO

*500°C

400°C

300°C

200°C

As received

Rel

ativ

e In

tensi

ty (

a.u

)

2-Theta

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Figure 2.

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

70

75

80

85

90

95

100

1s

t. de

riva

tive

(dP

/dt)

We

igh

t L

os

s %

Temperature (°C)

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Figure 3.

0.5 µm0.5 µm

0.5 µm

0.5 µm0.5 µm

A

B

C

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Figure 4.

CHx-OCO-CH2-CH2-(CH2)4-CH2-CH=CH-CH2

-(CH2)5-CH2-CH3

C

C

OCO-CH2-CH2-(CH2)12-CH3

OCO-CH2-CH2-(CH2)5-CH2-CH=CH-CH2-CH=CH-CH2

-(CH2)2-CH2-CH3

1

Hb

Ha1

Ha

Hb

3 4 5 9

8

2

6

7'7

CH3-O-CO-CH2-CH2-(CH2)4-CH2-CH=CH-CH2

-(CH2)5-CH2-CH3

CH3-O-CO-CH2-CH2-(CH2)12-CH3

CH3-O-CO-CH2-CH2-(CH2)5-CH2-CH=CH-CH2-CH=CH-CH2

-(CH2)2-CH2-CH3

5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 ppm

5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0ppm

a)

b)

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Figure 5.

10 15 20 25 30 35 40

x Ca(OH)2

* CaCO3

****

*x

x

x

E)

D)

C)

B)

A)

Rel

ativ

e In

tensi

ty (

a.u.)

2-Theta

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Figure 6.

20 40 60 80 100 120

10

20

30

40

50

60

70

80

90

100

Conver

sion (

%)

Reaction time (min)

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Figure 7.

0.30 0.35 0.40 0.45 0.50 0.55 0.6070

75

80

85

90

95

100

Conver

sion (

%)

Catalyst amount (g)

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Figure 8.

0.1 0.2 0.3 0.4 0.570

75

80

85

90

95

100

Conver

sion (

%)

Methanol:oil ratio (vol.)

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Figure 9.

20 25 30 35 40 45 50 55 60 65

0

20

40

60

80

100

Ca(OH)2

CaO

Conver

sion %

Temperature (°C)

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