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Acidolysis of tripalmitin with capric acid using Nb2O5 and MgO as heterogeneous catalysis
Catalysis Communications 12 (2011) 362–367
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Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Short Communication
Acidolysis of tripalmitin with capric acid using Nb2O5 and MgO as heterogeneous catalysis Ana J. Avila, Gabriela M. Tonetto ⁎, Daniel E. Damiani Planta Piloto de Ingeniería Química PLAPIQUI (UNS-CONICET), Camino La Carrindanga Km 7, CC 717, CP 8000, Bahía Blanca, Argentina
a r t i c l e
i n f o
Article history: Received 21 July 2010 Received in revised form 5 October 2010 Accepted 7 October 2010 Available online 13 October 2010 Keywords: Acidolysis Structured lipids MgO Nb2O5
a b s t r a c t The present work reports the synthesis of structured lipids by acidolysis of tripalmitin (PPP) and capric acid (C) with heterogeneous catalysts. The inorganic oxides Nb2O5 and MgO were selected as catalysts for their acid and base properties, respectively. The solids were characterized by XRD, N2 adsorption isotherm, atomic absorption spectroscopy and SEM. The conversion of PPP at 20 h and 180 °C was 62% for MgO and 18% for Nb2O5. As regards the yield of PPC + PCP, the maximum value for MgO was 40% and 14% for Nb2O5. The yield of the structured lipid PCC + CPC was 14% and 2% for the base and acid solid, respectively. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Lipids are a diverse group of compounds. Their importance lies in that, along with proteins and carbohydrates, they constitute the main components of human nutrition. For a long time, however, they have been associated with some diseases, especially coronary diseases. That is why their composition and structure have been modified to obtain more appropriate components for different applications. Structured lipids (SLs) can be defined as triacylglycerols (TAG) that have been modified or restructured to change their fatty acid (FA) composition and/or their distribution in the glycerol molecule, designed for a nutritional or a specific technological use. Thus the physical and functional properties of the TAG can be determined by selecting the type of FA and its position in the SL. SLs with a high content of long-chain FA in the 2 position of the TAG and medium-chain FA in the 1, 3 positions are arousing great interest. These lipids have begun to be used in absorption studies and for clinical nutrition. Low calories SLs are also starting to be commercialized [1]. Low calories SLs are obtained by combining short- or mediumchain FAs, or both, and long-chain FA in the same molecule. Shortchain FAs (2 to 6 C atoms) are quickly absorbed in comparison with other FAs, and they have low heat of combustion, what makes them low in calorie. Medium-chain FAs (6 to 12 C atoms) are metabolized as fast as glucose because they are not reesterified within the TAG molecule, and they present low energy content. Long-chain saturated
⁎ Corresponding author. E-mail address: [email protected] (G.M. Tonetto). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.10.007
FAs (14 to 24 C atoms) have low gastric absorption, what makes them useful when producing reduced calorie food [2,3]. Some commercial low calorie SLs are available at present, such as Salatrim by Nabisco and Caprenin by Procter & Gamble [1,4]. Both present a caloric content of 5 kcal/g (vs. 9 kcal/g of a regular fat). The composition of the former is based on short-chain FAs (acetic acid, propionic or butyric) and long-chain saturated FAs (mainly estearic acid) randomly distributed in the glycerol molecule. The latter is an equimolar combination of caprylic, capric and behenic acids, all esterified randomly to the glycerol. Salatrim is industrially produced by chemical interesterification between short-chain TAG (triacetyl-, tripropionyl or tributyl-) and long-chain saturated TAG (hydrogenated vegetable oil). Caprenin is industrially produced by chemical acylation of behenic acid containing 1-monoacylglycerol with medium-chain FA. In general SLs are produced by base homogenous catalysis or enzymatic processes [1,5,6]. Even though enzymatic catalysis is characterized by a high regiospecificity and very high selectivity at low temperatures, difficulty in separating the catalyst from the reaction media and enzyme reusability issues make it more expensive than heterogeneous processes. Side reactions are inevitable, particularly acyl migration and hydrolysis, especially in batch or discontinuous reactors. The homogeneous process also presents technical disadvantages, because it requires a lot of control and consideration related to high toxicity, corrosion and disposal of spent base materials. In this case, several purification steps are required, such as neutralization, separation from the reaction mixture and washing, leading to a series of environmental problems related to the use of large amounts of solvents and energy. For these reasons, the use of heterogeneous catalysts that render a more environmentally friendly
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operation and more efficient processes in terms of catalyst reuse, lower corrosive character, elimination of separation stages, and lower catalyst costs (compared to enzymes) is an interesting aspect to study. Two attractive oxides to be used as catalysts are Nb2O5 and MgO. The niobium oxide is an acid solid with high acidity on the surface (Ho ≤ −5.6) even when calcined at relatively low temperatures (100– 300 °C) [7]. As regards magnesium oxide, it is known for its strong base properties [8]. Both oxides presented activity for another transesterification reaction, such as alcoholysis, which involves the exchange of an acyl radical between an ester and an alcohol [9–13], and in the esterification reaction to produce fatty acid esters [14,15]. Barrault et al. [9] studied the glycerol transesterification with methyl stearate over several heterogeneous base catalysts such as MgO, ZnO, La2O3, and CeO2. The transesterification was performed at 220 °C. The activity order was: La2O3 N MgO N ZnO N CeO2, and it showed that there was a relationship between the transesterification rate and the intrinsic basicity of the catalyst. Although La2O3 displayed the best activity for the reaction, it presented difficulty to control the surface area and the base properties due to the easy formation of carbonate or oxycarbonate species from the reaction of CO2 with the oxide. This makes MgO the major option. A similar result was reported by Di Cosimo et al. [10] in a recent work. The authors studied the glycerol transesterification with methyl oleate over solid base catalysts such as MgO, Y2O3 and CeO2 and acid catalysts such as Al2O3 and Nb2O5. The highest monoglyceride yields were found for MgO using reaction temperatures in the range of 220–250 °C. Aranda et al. [14,15] tested and compared different catalysts in the esterification of a mixture of fatty acids residue. They observed a better performance of Nb2O5 when compared to that of zeolite catalysts, such as HY, ZSM-5 and modernite. There are a number of publications on the catalytic application of MgO in the transesterification reaction to produce fatty acid alkyl esters for biofuel application [11,12], however little information is available on the use of Nb2O5 [13,16]. This work presents an unexplored alternative route to the current commercial technology (that uses liquid base catalysts) by heterogeneous catalysts that would simplify the process. This manuscript describes the acidolysis of a long-chain FA triglyceride (tripalmitin, C16:0) and a medium-chain FA (capric acid, C10:0) using Nb2O5 and MgO as catalyst. 2. Experimental
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CO2 adsorption over MgO: The sample was maintained at 30 °C under CO2 flow (Praxair 33% in Ar) for 30 min. Then the sample was flushed with N2 at the same temperature for 30 min, and at 80 °C for 15 min, and the spectra were recorded. Pyridine adsorption over Nb2O5: The sample was maintained at 100 °C and saturated with pyridine using N2 containing pyridine stream. Adsorption of pyridine was performed for 45 min. Then, the solid was flushed with N2 at the same temperature for 45 min to remove weakly adsorbed pyridine species. In order to establish the acid strength, the sample was also purged at 200 °C. In each stage, the spectra were recorded. The characterization of MgO was performed by atomic absorption spectroscopy (Instrumentation Laboratory 551) and fluorescence Xray spectrometry (PANalytical MagiX). 2.2. Catalytic tests The materials used were: tripalmitin (PPP, minimum purity of 95%) and capric acid (C, minimum purity of 98%) obtained from Fluka, n-heptane (Cicarelli), Nb2O5 HY-340 (CBMM), and MgO (Carlo Erba). Standards of triglycerides (tricaprin (CCC) and trimyristin (MMM)), acylglycerols from C10 and C16, and palmitic acid (P) were purchased from Sigma-Aldrich. The reaction was carried out in 7 ml vials with magnetic agitation. The operating conditions were: temperature = 180 °C, initial C/PPP molar ratio = 4 (with 500 mg of PPP), reaction time = 20 h, and catalyst mass percentage with respect to total substrate mass = 2%. The samples were pretreated in a vacuum stove for 30 min to remove the water, and for another 30 min in a nitrogen chamber to remove the oxygen. In order to study the reuse of the samples, three consecutive tests were performed (with no intermediate activation). Complementary analyses were also carried out using 1 and 3 wt% catalyst with respect to total substrate mass. The analysis of samples was performed in a PerkinElmer Clarus 500 gas chromatograph equipped with a flame ionization detector, a DB-17HT capillary column (30 m, 0.32 mm ID, 0.15 μm film thickness) and on-column injector. The injector and detector temperatures were maintained at 375 °C and 400 °C, respectively. The initial column temperature was maintained at 80 °C for 1 min, then increased to 250 °C at a rate of 40 °C/min, and maintained for 6 min, further increased to 350 at a rate of 10 °C/min, and maintained for 5 min. The carrier gas was H2, and the total gas flow rate was 15 mL/min. The identification of SL was performed with a GC–MS [17].
2.1. Catalyst characterization 3. Results and discussion The Nb2O5 sample was calcined at 300 °C. The MgO was hydrated and dried at 350 °C, and then calcined at 500 °C for 8 h. The purpose of the treatment was to expand its specific surface area and confer thermal stability. The inorganic oxides were characterized by X-ray diffraction (Philips PW1710, using Cu Kα radiation scan in the 2θ range of 5– 70° with a 0.0358° interval). The specific surface area and pore size distribution of the solids were measured by nitrogen adsorption isotherms at 77 K on a Nova 1200e Quantachrome system. The particle size distribution of the samples was determined with a JEOL 35 CP electron-scanning microscope (SEM), using the Analysis 3.1 software. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra were recorded in a Nicolet 6700 FT-IR spectrometer in the 4000–400 cm− 1 region. The spectra were recorded with a 4 cm− 1 resolution and 64 scans using a high-sensitivity mercury cadmium telluride (MCT-A) detector. The samples (ca. 12 mg) were finely ground and diluted in KBr (5%), and placed into a ceramic crucible in a DRIFTS chamber (Nexus Smart Collector, Nicolet). Before collecting the spectra, the catalysts were treated in situ by heating under N2 flow at 400 °C for 45 min to remove surface contaminants, and then cooled to 30 °C.
3.1. Catalyst characterization Fig. 1 presents the X-ray diffractograms for catalysts MgO and Nb2O5. In the case of MgO, the most intense peaks at 2θ 43.16 and 62.50 are characteristic of the periclase phase of MgO, and they indicated the high crystallinity obtained in calcinations [18]. The small peak at 29.48 revealed the presence of calcium carbonate as calcite. In the case of Nb2O5 calcined at 300 °C, the diffractogram exhibited an amorphous structure. The MgO sample was analyzed by atomic absorption spectroscopy to determine the concentration of Ca. This technique showed that the Ca content was 0.64%, a high value that cannot be overlooked when analyzing the results. For the Nb2O5 sample, the dehydration process was completed with a heat treatment at 300 °C. Under these conditions, the density of acid sites determined by thermal desorption of NH3 was 225 μmol NH3/g (data provided by the manufacturer, using NH3 diluted in He at 2 vol.%). Fig. 2A presents N2 adsorption–desorption isotherms. The isotherms were of type IV, typical for mesoporous solids. The shape of the
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Fig. 1. Nb2O5 and MgO DRX. References: ● Mg(OH)2,
MgO, ○ CaCO3.
hysteresis loop suggests that in Nb2O5 the pores presented an ink jar shape. In MgO, on the contrary, the pore morphology was compatible with a cylindrical geometry. The BET area of the samples was 97.9 m2/ g for Nb2O5 and 17.3 m2/g for MgO. The pore volume was 0.05 cm3/g and 0.11 cm3/g for MgO and Nb2O5, respectively. The pore size distribution is shown in Fig. 2B, where a thin peak is observed for Nb2O5 at 18.4 Å, and a wide distribution with a 52.4 Å radius for MgO. The particle size distribution was obtained by analysis of SEM micrographs (not shown here). For Nb2O5, 90% of the particles had a diameter of less than 80 nm, and for MgO, less than 50 nm. The study by IR of the changes in the nature of the species formed upon CO2 or pyridine adsorption provides information about the surface chemistry of the catalysts. The adsorption of CO2 seems to be a useful routine to evaluate the basicity of solids [19,20]. Fig. 3A shows FT-IR spectrum of MgO catalyst before and after CO2 adsorption. The sample was purged at different temperatures: 30, 80, 120, 160, 200 and 240 °C. Infrared bands were observed in the 800–1800 cm− 1 region. Unidentate and bidentate carbonate formation requires surface basic oxygen atoms. Unidentate carbonates exhibit bands at 1510, 1390, 1035, and 865 cm− 1 [21]. Bidentate carbonates show signals at 1665, 1325, 1005, and 850 cm− 1. Bicarbonate species formation involves surface hydroxyl groups. Bicarbonates show a C–OH bending mode at 1220 cm− 1 as well as symmetric and asymmetric O–C–O stretching modes at 1480 cm− 1 and 1650 cm− 1, respectively [10]. From these results, the bands at 1640 and 864 cm− 1 were assigned to bidentate carbonate. The bands at 1504, 1432, 1081 and 865 cm− 1 can be assigned to the unidentate carbonate complex. In any of the spectra, a signal for the bicarbonate species was found. It can be observed that the shoulder at 1640 cm− 1 decreased as temperature
increased (80–120 °C) and disappeared with purging at 160 °C. All the other bands remained constant with the purge temperature, indicating that only bidentate species present a weak interaction with the surface. It can also be noted that unidentate carbonate is predominantly formed when there is a large amount of CO2 adsorbed, as was reported by Fukuda et al. [21]. The FT-IR spectra after pyridine adsorption (a) and subsequent purging at 100 °C (b) and 200 °C (c) on Nb2O5 catalyst are reported in Fig. 3B. On spectra b and c it is possible to observe bands at around 1450, 1493 and 1614 cm− 1. These bands, all involving the C–C ring vibration of pyridine, are characteristic of the pyridine adsorption. Three modes of adsorption display IR-spectra peaks at different wavelengths and can be identified by their IR absorption bands [22]. Pyridine, coordinated with the Lewis center, yields a peak at 1450 cm− 1, and protonated pyridine on the Brönsted center a peak at 1540 cm− 1. Both complexes yield also a peak at 1490 cm− 1. The 1600 cm− 1 band is generally assigned to hydrogen-bonded pyridine. Rosenthal et al. [23] found that the extinction coefficients for these Lewis-pyridine and Brönsted-pyridine bands were equal. In view of that, and taking into account a work by Emeis, a Brönsted/Lewis ratio for the Nb2O5 was calculated. Brönsted/Lewis ratio was 4 for purging at 100 °C and 3 for purging at 200 °C. On this basis, it can be argued that there is a greater abundance of Brönsted sites. Furthermore, it can be argued that a fraction of Brönsted sites are weak given that their decrease with temperature is more noticeable than in the case of Lewis sites. 3.2. Catalytic tests The inorganic oxides Nb2O5 and MgO were studied in the synthesis of SLs, as detailed in the experimental section. Parallel to the acidolysis reaction (Fig. 4), the hydrolysis reaction takes place, producing mono-, diglycerides and glycerol. The capillary column used is specific to separate lipids. In the present work, PPP conversion values (limiting reactant) and SL yield are reported. The column does not allow separation of isomers, and thus their addition (i.e. PPC + PCP and PCC + CPC) is given. Yield is defined as: Yieldx =
moles of PPP that are transformed into the desired product x : initial moles of PPP in the reactor
ð1Þ The reaction time was 20 h, and a sample was extracted every 4 h. A blank reaction was performed, and a PPP conversion of 6% was found for the highest reaction time. Fig. 5A shows PPP conversion and yields of PPC + PCP and PCC + CPC for the catalyst MgO. Fig. 5B shows the same parameters for Nb2O5.
Fig. 2. A. N2 adsorption–desorption isotherms at 77 K. B. Pore size distribution (BJH).
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Fig. 3. A. FT-IR spectra of MgO catalyst before adsorbed CO2 (a), following purging at (g) 30 °C, (f) 80 °C, (e) 120 °C, (d) 160 °C, (c) 200 °C, (b) 240 °C. B. Pyridine adsorption on Nb2O5 at 100 °C following purging at (b) 100 °C, (c) 200 °C, (a) before pyridine adsorption.
PPP conversion at 20 h was of 62% for MgO and of 18% for Nb2O5. As regards the PPC + PCP yield, the maximum value for MgO was 40%, and 14% for Nb2O5. The yield of the structured lipid PCC + CPC was 14% and 2% for the base and acid solid, respectively. For MgO, it can be pointed out that after 12 h of reaction, the three parameters studied seemed to approximate a constant value. As regards Nb2O5, after 12 h the activity increased slowly, and a small increase in the SL yields was observed. In order to check the reusability of the catalyst, three consecutive tests were performed, and the results are shown in Fig. 6. After each reaction, the catalysts were washed with hexane, centrifugated, dried at room temperature, and used for reuse studies without any activation treatment. Nb2O5 was washed five times with hexane, and MgO only twice because, due to its small particle size and texture, the sample could be lost. When consecutive tests are compared, no important variation in activity for Nb2O5 was observed. In the case of MgO, activity fell to 34.3% in the second test (a 44% decrease), and it remained constant in
acidolysis P P + C
catalyst
P
P
P
C
P +
C +
C+ P
C
C
C
P
C
P
P +
C +
C +
P
Loss Fraction = 1−
+
hydrolysis Fig. 4. Scheme showing all the possible products for acidolysis and hydrolysis reactions (stereospecific isomers are not shown).
NCCC + NCCPþCPC + NPPCþ PCP NPPP;0 −NPPP;f
ð2Þ
where N is the mole number of the different species, and the “0” and “f” subscripts represent initial and final values. Fig. 7 presents “loss fraction” for the catalysts studied as a function of reaction time. In the case of MgO, it was observed that at 4 and 8 h of reaction half of the moles of PPP were lost in the hydrolysis reaction. This value decreased to 0.11 at 20 h of reaction. In the case of Nb2O5, the loss fraction reached a maximum value of 0.60 at 8 h, and decreased to 0.13 at the end of the reaction. The particle size of the catalysts is very small: 80 nm for Nb2O5 and 50 nm for MgO. Given that the speed expression is unknown, the Weisz–Prater module (Φ) was used in order to estimate if any problems of intraparticle mass transfer exist:
Φi =
C +
the third catalytic test. Since the decrease in activity was present in the second use only, it could be considered that the loss of activity is a consequence of the lack of treatment or intermediate activation. That is not minor if we take into account that the used sample could have residues from the previous reaction. Subsequent consecutive tests for Nb2O5 and MgO did not show any significant variations in selectivity (not shown). In order to quantify the moles of PPP lost in hydrolysis, originating mono- and diglycerides, palmitic acid and glycerol, the “loss fraction” was defined as:
2 −robs;i ρp Rp Def;i Ci
ð3Þ
where C is concentration (mol/m3), Def is the effective diffusion coefficient (m2/s x m3liq/m3cat), Rp is the mean particle size (m), robs is the observed rate (mol/s kgcat) and, ρp is the catalyst apparent density (g/cm3). For both catalysts the value obtained was Φ ≪ 1, indicating that it is possible to consider that there are no mass transfer limitations in the catalyst pores, even though they would differ in size for the
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Fig. 5. Tripalmitin conversion, and yields of PPC + PCP and PCC + CPC for: A, MgO; B, Nb2O5. Reference:
PPP conversion, □ PPC + PCP yield, ■ PCC + CPC yield.
samples studied. The Weisz–Prater module was calculated for PPP because it is the reactant of the biggest size. An approximate value of Def,PPP = 3.35 10− 11 m2/s was estimated [24]. Given that in the reaction system used it was not possible to change the agitation rate, different tests were performed varying the catalyst mass and maintaining the rest of the operating conditions constant. The results are shown in Table 1. It can be observed that for both catalysts the reaction rate remained constant (calculated as millimoles of PPP converted per mg of catalyst per min), indicating that there was no external mass transfer limitation. The MgO catalyst was more active than the Nb2O5 catalyst. The difference was attributed to the base properties of MgO, because the same tendency was observed in homogeneous catalysts. Both samples reached maximum PPP conversion values and yields at 12 h, and then catalytic activity ceased. An analysis on the active sites present in the catalysts and the possible reaction mechanisms is beyond the scope of this work. The mechanisms proposed by the literature and the characteristics of the catalytic surfaces will be reviewed only briefly. The acidolysis reaction is an interestification, an exchange of acyl radicals between an ester and an acid. Two reaction mechanisms have been proposed [25]: one considers the formation of an enolate ion as intermediate, by abstracting an α-proton of the glyceride by the base catalyst, and the other one proposes the formation of a complex by the addition of an initiator to the glycerol carboxyl. In the first case, the enolate ion reacts with an ester group in the triglyceride molecule to originate a β-keto-ester, which reacts with other esters to produce another β-keto-ester. The same applies to the exchange of esters between different molecules of triglycerides. In the second mecha-
nism, by addition to the carboxyl, the alkyl ion is incorporated to the ester carboxyl group producing a diglycerinate intermediate that reacts with another glyceride abstracting a FA, originating a new triglyceride and regenerating the diglyceride. This process would continue through a series of chain reactions until all the available FAs have exchanged position and the equilibrium composition is reached. When the reaction is catalyzed by acids, first the protonation of the ester group takes place (in the triglyceride), followed by the addition of the acid to provide an intermediate that can be dissociated via a transition state, to produce the new ester group. The surface of the solids used is complex. It will only be mentioned that in conditions similar to those of the reaction, the presence of Lewis and Brönsted sites on Nb2O5 was observed [26]. It should be borne in mind that rehydratation of Lewis acid sites by water sorption can occur during cooling, after calcination of the catalyst [27], or during storage and preparation of the reaction mixture. In the case of MgO, its surface basicity is the result of the electronic transfer between Mg and O. The O2− anions (electron rich) act as electrondonor strong base sites, whereas the Mg2+ cations (electron poor) act as electron-acceptor weak acid sites [8]. In the specific case of the MgO used in this work, it must be pointed out that the presence of 0.6% Ca increased the basicity of the sample. As mentioned above, the literature on the subject refers to the use of homogenous catalysis. Klinkesorn et al. [28] modified the composition of FAs in fish oil by chemical transesterification with eicosapentanoic (EPA) and docosahexanoic acid (DHA), using sodium methoxide as catalyst, but having previously transesterified the FAs to methilic esters. They obtained an incorporation of 60% of EPA and 50% of DHA into the triglyceride, with a 59% loss of oil by hydrolysis and soap production. In the present work, the use of a heterogeneous base catalyst allowed a FA incorporation of 23%, with 11% loss of triglyceride for the same reaction time than the work reported previously.
Fig. 6. Reuse studies: PPP conversion (%), reaction time = 20 h. Reference: ▲Nb2O5, ■ MgO.
Fig. 7. Loss fraction for the catalysts studied as a function of reaction time. Reference: Nb2O5, □ MgO.
A.J. Avila et al. / Catalysis Communications 12 (2011) 362–367 Table 1 Effect of the catalyst mass variation on conversion and activity for the catalysts studied. Reaction time: 4 h. Catalyst %
MgO X PPP (%)
1 2 3
21.8 30.9 44.5
367
tion, Argentina) and the Consejo Nacional de Investigaciones Científicas y Técnicas (National Council for Scientific and Technological Research (CONICET)) for the financial support.
Nb2O5 Activitya −5
5.83 10 3.79 10− 5 4.12 10− 5
X PPP (%)
Activitya
2.4 4.9 7.0
6.81 10− 6 6.72 10− 6 6.71 10− 6
a Integral activity calculated as millimoles of PPP converted per milligram of catalyst per minute.
For acid catalysis, and qualitatively at least, the results obtained for Nb2O5 can be compared to those reported by Chakrabarty and Talapatra [29], who exchanged 14% of the FAs of cotton, peanut and palm oil with lauric acid using sulfuric acid as catalyst (working at 150° in 3 h). In the present work, for a PPP conversion of 18%, the incorporation of FAs was of 7%. As regards the deactivation observed (after 12 h) for MgO, a Claisen condensation cannot be ruled out, which would originate surface polymeric species. These polymers would block the reactive ionic pairs on the surface. In the case of Nb2O5, an exhaustion of the Brönsted acid sites can be considered. Nevertheless, both samples showed activity in two more subsequent catalytic tests. 4. Conclusions MgO and Nb2O5 samples were characterized and used in the synthesis of structured lipids by acidolysis of PPP with capric acid. The solids were active for reaction, showing a conversion of PPP of 62% for MgO and 18% for Nb2O5. The maximum value of PPC + PCP yield was 40% for the MgO catalyst, and 14% for Nb2O5. As regards the structured lipid PCC + CPC, its yield was 14% and 2% for the base and acid solid, respectively. Acknowledgements The authors thank the Agencia Nacional de Promoción Científica y Tecnológica (National Agency of Scientific and Technological Promo-
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Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Short Communication
Acidolysis of tripalmitin with capric acid using Nb2O5 and MgO as heterogeneous catalysis Ana J. Avila, Gabriela M. Tonetto ⁎, Daniel E. Damiani Planta Piloto de Ingeniería Química PLAPIQUI (UNS-CONICET), Camino La Carrindanga Km 7, CC 717, CP 8000, Bahía Blanca, Argentina
a r t i c l e
i n f o
Article history: Received 21 July 2010 Received in revised form 5 October 2010 Accepted 7 October 2010 Available online 13 October 2010 Keywords: Acidolysis Structured lipids MgO Nb2O5
a b s t r a c t The present work reports the synthesis of structured lipids by acidolysis of tripalmitin (PPP) and capric acid (C) with heterogeneous catalysts. The inorganic oxides Nb2O5 and MgO were selected as catalysts for their acid and base properties, respectively. The solids were characterized by XRD, N2 adsorption isotherm, atomic absorption spectroscopy and SEM. The conversion of PPP at 20 h and 180 °C was 62% for MgO and 18% for Nb2O5. As regards the yield of PPC + PCP, the maximum value for MgO was 40% and 14% for Nb2O5. The yield of the structured lipid PCC + CPC was 14% and 2% for the base and acid solid, respectively. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Lipids are a diverse group of compounds. Their importance lies in that, along with proteins and carbohydrates, they constitute the main components of human nutrition. For a long time, however, they have been associated with some diseases, especially coronary diseases. That is why their composition and structure have been modified to obtain more appropriate components for different applications. Structured lipids (SLs) can be defined as triacylglycerols (TAG) that have been modified or restructured to change their fatty acid (FA) composition and/or their distribution in the glycerol molecule, designed for a nutritional or a specific technological use. Thus the physical and functional properties of the TAG can be determined by selecting the type of FA and its position in the SL. SLs with a high content of long-chain FA in the 2 position of the TAG and medium-chain FA in the 1, 3 positions are arousing great interest. These lipids have begun to be used in absorption studies and for clinical nutrition. Low calories SLs are also starting to be commercialized [1]. Low calories SLs are obtained by combining short- or mediumchain FAs, or both, and long-chain FA in the same molecule. Shortchain FAs (2 to 6 C atoms) are quickly absorbed in comparison with other FAs, and they have low heat of combustion, what makes them low in calorie. Medium-chain FAs (6 to 12 C atoms) are metabolized as fast as glucose because they are not reesterified within the TAG molecule, and they present low energy content. Long-chain saturated
⁎ Corresponding author. E-mail address: [email protected] (G.M. Tonetto). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.10.007
FAs (14 to 24 C atoms) have low gastric absorption, what makes them useful when producing reduced calorie food [2,3]. Some commercial low calorie SLs are available at present, such as Salatrim by Nabisco and Caprenin by Procter & Gamble [1,4]. Both present a caloric content of 5 kcal/g (vs. 9 kcal/g of a regular fat). The composition of the former is based on short-chain FAs (acetic acid, propionic or butyric) and long-chain saturated FAs (mainly estearic acid) randomly distributed in the glycerol molecule. The latter is an equimolar combination of caprylic, capric and behenic acids, all esterified randomly to the glycerol. Salatrim is industrially produced by chemical interesterification between short-chain TAG (triacetyl-, tripropionyl or tributyl-) and long-chain saturated TAG (hydrogenated vegetable oil). Caprenin is industrially produced by chemical acylation of behenic acid containing 1-monoacylglycerol with medium-chain FA. In general SLs are produced by base homogenous catalysis or enzymatic processes [1,5,6]. Even though enzymatic catalysis is characterized by a high regiospecificity and very high selectivity at low temperatures, difficulty in separating the catalyst from the reaction media and enzyme reusability issues make it more expensive than heterogeneous processes. Side reactions are inevitable, particularly acyl migration and hydrolysis, especially in batch or discontinuous reactors. The homogeneous process also presents technical disadvantages, because it requires a lot of control and consideration related to high toxicity, corrosion and disposal of spent base materials. In this case, several purification steps are required, such as neutralization, separation from the reaction mixture and washing, leading to a series of environmental problems related to the use of large amounts of solvents and energy. For these reasons, the use of heterogeneous catalysts that render a more environmentally friendly
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operation and more efficient processes in terms of catalyst reuse, lower corrosive character, elimination of separation stages, and lower catalyst costs (compared to enzymes) is an interesting aspect to study. Two attractive oxides to be used as catalysts are Nb2O5 and MgO. The niobium oxide is an acid solid with high acidity on the surface (Ho ≤ −5.6) even when calcined at relatively low temperatures (100– 300 °C) [7]. As regards magnesium oxide, it is known for its strong base properties [8]. Both oxides presented activity for another transesterification reaction, such as alcoholysis, which involves the exchange of an acyl radical between an ester and an alcohol [9–13], and in the esterification reaction to produce fatty acid esters [14,15]. Barrault et al. [9] studied the glycerol transesterification with methyl stearate over several heterogeneous base catalysts such as MgO, ZnO, La2O3, and CeO2. The transesterification was performed at 220 °C. The activity order was: La2O3 N MgO N ZnO N CeO2, and it showed that there was a relationship between the transesterification rate and the intrinsic basicity of the catalyst. Although La2O3 displayed the best activity for the reaction, it presented difficulty to control the surface area and the base properties due to the easy formation of carbonate or oxycarbonate species from the reaction of CO2 with the oxide. This makes MgO the major option. A similar result was reported by Di Cosimo et al. [10] in a recent work. The authors studied the glycerol transesterification with methyl oleate over solid base catalysts such as MgO, Y2O3 and CeO2 and acid catalysts such as Al2O3 and Nb2O5. The highest monoglyceride yields were found for MgO using reaction temperatures in the range of 220–250 °C. Aranda et al. [14,15] tested and compared different catalysts in the esterification of a mixture of fatty acids residue. They observed a better performance of Nb2O5 when compared to that of zeolite catalysts, such as HY, ZSM-5 and modernite. There are a number of publications on the catalytic application of MgO in the transesterification reaction to produce fatty acid alkyl esters for biofuel application [11,12], however little information is available on the use of Nb2O5 [13,16]. This work presents an unexplored alternative route to the current commercial technology (that uses liquid base catalysts) by heterogeneous catalysts that would simplify the process. This manuscript describes the acidolysis of a long-chain FA triglyceride (tripalmitin, C16:0) and a medium-chain FA (capric acid, C10:0) using Nb2O5 and MgO as catalyst. 2. Experimental
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CO2 adsorption over MgO: The sample was maintained at 30 °C under CO2 flow (Praxair 33% in Ar) for 30 min. Then the sample was flushed with N2 at the same temperature for 30 min, and at 80 °C for 15 min, and the spectra were recorded. Pyridine adsorption over Nb2O5: The sample was maintained at 100 °C and saturated with pyridine using N2 containing pyridine stream. Adsorption of pyridine was performed for 45 min. Then, the solid was flushed with N2 at the same temperature for 45 min to remove weakly adsorbed pyridine species. In order to establish the acid strength, the sample was also purged at 200 °C. In each stage, the spectra were recorded. The characterization of MgO was performed by atomic absorption spectroscopy (Instrumentation Laboratory 551) and fluorescence Xray spectrometry (PANalytical MagiX). 2.2. Catalytic tests The materials used were: tripalmitin (PPP, minimum purity of 95%) and capric acid (C, minimum purity of 98%) obtained from Fluka, n-heptane (Cicarelli), Nb2O5 HY-340 (CBMM), and MgO (Carlo Erba). Standards of triglycerides (tricaprin (CCC) and trimyristin (MMM)), acylglycerols from C10 and C16, and palmitic acid (P) were purchased from Sigma-Aldrich. The reaction was carried out in 7 ml vials with magnetic agitation. The operating conditions were: temperature = 180 °C, initial C/PPP molar ratio = 4 (with 500 mg of PPP), reaction time = 20 h, and catalyst mass percentage with respect to total substrate mass = 2%. The samples were pretreated in a vacuum stove for 30 min to remove the water, and for another 30 min in a nitrogen chamber to remove the oxygen. In order to study the reuse of the samples, three consecutive tests were performed (with no intermediate activation). Complementary analyses were also carried out using 1 and 3 wt% catalyst with respect to total substrate mass. The analysis of samples was performed in a PerkinElmer Clarus 500 gas chromatograph equipped with a flame ionization detector, a DB-17HT capillary column (30 m, 0.32 mm ID, 0.15 μm film thickness) and on-column injector. The injector and detector temperatures were maintained at 375 °C and 400 °C, respectively. The initial column temperature was maintained at 80 °C for 1 min, then increased to 250 °C at a rate of 40 °C/min, and maintained for 6 min, further increased to 350 at a rate of 10 °C/min, and maintained for 5 min. The carrier gas was H2, and the total gas flow rate was 15 mL/min. The identification of SL was performed with a GC–MS [17].
2.1. Catalyst characterization 3. Results and discussion The Nb2O5 sample was calcined at 300 °C. The MgO was hydrated and dried at 350 °C, and then calcined at 500 °C for 8 h. The purpose of the treatment was to expand its specific surface area and confer thermal stability. The inorganic oxides were characterized by X-ray diffraction (Philips PW1710, using Cu Kα radiation scan in the 2θ range of 5– 70° with a 0.0358° interval). The specific surface area and pore size distribution of the solids were measured by nitrogen adsorption isotherms at 77 K on a Nova 1200e Quantachrome system. The particle size distribution of the samples was determined with a JEOL 35 CP electron-scanning microscope (SEM), using the Analysis 3.1 software. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra were recorded in a Nicolet 6700 FT-IR spectrometer in the 4000–400 cm− 1 region. The spectra were recorded with a 4 cm− 1 resolution and 64 scans using a high-sensitivity mercury cadmium telluride (MCT-A) detector. The samples (ca. 12 mg) were finely ground and diluted in KBr (5%), and placed into a ceramic crucible in a DRIFTS chamber (Nexus Smart Collector, Nicolet). Before collecting the spectra, the catalysts were treated in situ by heating under N2 flow at 400 °C for 45 min to remove surface contaminants, and then cooled to 30 °C.
3.1. Catalyst characterization Fig. 1 presents the X-ray diffractograms for catalysts MgO and Nb2O5. In the case of MgO, the most intense peaks at 2θ 43.16 and 62.50 are characteristic of the periclase phase of MgO, and they indicated the high crystallinity obtained in calcinations [18]. The small peak at 29.48 revealed the presence of calcium carbonate as calcite. In the case of Nb2O5 calcined at 300 °C, the diffractogram exhibited an amorphous structure. The MgO sample was analyzed by atomic absorption spectroscopy to determine the concentration of Ca. This technique showed that the Ca content was 0.64%, a high value that cannot be overlooked when analyzing the results. For the Nb2O5 sample, the dehydration process was completed with a heat treatment at 300 °C. Under these conditions, the density of acid sites determined by thermal desorption of NH3 was 225 μmol NH3/g (data provided by the manufacturer, using NH3 diluted in He at 2 vol.%). Fig. 2A presents N2 adsorption–desorption isotherms. The isotherms were of type IV, typical for mesoporous solids. The shape of the
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Fig. 1. Nb2O5 and MgO DRX. References: ● Mg(OH)2,
MgO, ○ CaCO3.
hysteresis loop suggests that in Nb2O5 the pores presented an ink jar shape. In MgO, on the contrary, the pore morphology was compatible with a cylindrical geometry. The BET area of the samples was 97.9 m2/ g for Nb2O5 and 17.3 m2/g for MgO. The pore volume was 0.05 cm3/g and 0.11 cm3/g for MgO and Nb2O5, respectively. The pore size distribution is shown in Fig. 2B, where a thin peak is observed for Nb2O5 at 18.4 Å, and a wide distribution with a 52.4 Å radius for MgO. The particle size distribution was obtained by analysis of SEM micrographs (not shown here). For Nb2O5, 90% of the particles had a diameter of less than 80 nm, and for MgO, less than 50 nm. The study by IR of the changes in the nature of the species formed upon CO2 or pyridine adsorption provides information about the surface chemistry of the catalysts. The adsorption of CO2 seems to be a useful routine to evaluate the basicity of solids [19,20]. Fig. 3A shows FT-IR spectrum of MgO catalyst before and after CO2 adsorption. The sample was purged at different temperatures: 30, 80, 120, 160, 200 and 240 °C. Infrared bands were observed in the 800–1800 cm− 1 region. Unidentate and bidentate carbonate formation requires surface basic oxygen atoms. Unidentate carbonates exhibit bands at 1510, 1390, 1035, and 865 cm− 1 [21]. Bidentate carbonates show signals at 1665, 1325, 1005, and 850 cm− 1. Bicarbonate species formation involves surface hydroxyl groups. Bicarbonates show a C–OH bending mode at 1220 cm− 1 as well as symmetric and asymmetric O–C–O stretching modes at 1480 cm− 1 and 1650 cm− 1, respectively [10]. From these results, the bands at 1640 and 864 cm− 1 were assigned to bidentate carbonate. The bands at 1504, 1432, 1081 and 865 cm− 1 can be assigned to the unidentate carbonate complex. In any of the spectra, a signal for the bicarbonate species was found. It can be observed that the shoulder at 1640 cm− 1 decreased as temperature
increased (80–120 °C) and disappeared with purging at 160 °C. All the other bands remained constant with the purge temperature, indicating that only bidentate species present a weak interaction with the surface. It can also be noted that unidentate carbonate is predominantly formed when there is a large amount of CO2 adsorbed, as was reported by Fukuda et al. [21]. The FT-IR spectra after pyridine adsorption (a) and subsequent purging at 100 °C (b) and 200 °C (c) on Nb2O5 catalyst are reported in Fig. 3B. On spectra b and c it is possible to observe bands at around 1450, 1493 and 1614 cm− 1. These bands, all involving the C–C ring vibration of pyridine, are characteristic of the pyridine adsorption. Three modes of adsorption display IR-spectra peaks at different wavelengths and can be identified by their IR absorption bands [22]. Pyridine, coordinated with the Lewis center, yields a peak at 1450 cm− 1, and protonated pyridine on the Brönsted center a peak at 1540 cm− 1. Both complexes yield also a peak at 1490 cm− 1. The 1600 cm− 1 band is generally assigned to hydrogen-bonded pyridine. Rosenthal et al. [23] found that the extinction coefficients for these Lewis-pyridine and Brönsted-pyridine bands were equal. In view of that, and taking into account a work by Emeis, a Brönsted/Lewis ratio for the Nb2O5 was calculated. Brönsted/Lewis ratio was 4 for purging at 100 °C and 3 for purging at 200 °C. On this basis, it can be argued that there is a greater abundance of Brönsted sites. Furthermore, it can be argued that a fraction of Brönsted sites are weak given that their decrease with temperature is more noticeable than in the case of Lewis sites. 3.2. Catalytic tests The inorganic oxides Nb2O5 and MgO were studied in the synthesis of SLs, as detailed in the experimental section. Parallel to the acidolysis reaction (Fig. 4), the hydrolysis reaction takes place, producing mono-, diglycerides and glycerol. The capillary column used is specific to separate lipids. In the present work, PPP conversion values (limiting reactant) and SL yield are reported. The column does not allow separation of isomers, and thus their addition (i.e. PPC + PCP and PCC + CPC) is given. Yield is defined as: Yieldx =
moles of PPP that are transformed into the desired product x : initial moles of PPP in the reactor
ð1Þ The reaction time was 20 h, and a sample was extracted every 4 h. A blank reaction was performed, and a PPP conversion of 6% was found for the highest reaction time. Fig. 5A shows PPP conversion and yields of PPC + PCP and PCC + CPC for the catalyst MgO. Fig. 5B shows the same parameters for Nb2O5.
Fig. 2. A. N2 adsorption–desorption isotherms at 77 K. B. Pore size distribution (BJH).
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Fig. 3. A. FT-IR spectra of MgO catalyst before adsorbed CO2 (a), following purging at (g) 30 °C, (f) 80 °C, (e) 120 °C, (d) 160 °C, (c) 200 °C, (b) 240 °C. B. Pyridine adsorption on Nb2O5 at 100 °C following purging at (b) 100 °C, (c) 200 °C, (a) before pyridine adsorption.
PPP conversion at 20 h was of 62% for MgO and of 18% for Nb2O5. As regards the PPC + PCP yield, the maximum value for MgO was 40%, and 14% for Nb2O5. The yield of the structured lipid PCC + CPC was 14% and 2% for the base and acid solid, respectively. For MgO, it can be pointed out that after 12 h of reaction, the three parameters studied seemed to approximate a constant value. As regards Nb2O5, after 12 h the activity increased slowly, and a small increase in the SL yields was observed. In order to check the reusability of the catalyst, three consecutive tests were performed, and the results are shown in Fig. 6. After each reaction, the catalysts were washed with hexane, centrifugated, dried at room temperature, and used for reuse studies without any activation treatment. Nb2O5 was washed five times with hexane, and MgO only twice because, due to its small particle size and texture, the sample could be lost. When consecutive tests are compared, no important variation in activity for Nb2O5 was observed. In the case of MgO, activity fell to 34.3% in the second test (a 44% decrease), and it remained constant in
acidolysis P P + C
catalyst
P
P
P
C
P +
C +
C+ P
C
C
C
P
C
P
P +
C +
C +
P
Loss Fraction = 1−
+
hydrolysis Fig. 4. Scheme showing all the possible products for acidolysis and hydrolysis reactions (stereospecific isomers are not shown).
NCCC + NCCPþCPC + NPPCþ PCP NPPP;0 −NPPP;f
ð2Þ
where N is the mole number of the different species, and the “0” and “f” subscripts represent initial and final values. Fig. 7 presents “loss fraction” for the catalysts studied as a function of reaction time. In the case of MgO, it was observed that at 4 and 8 h of reaction half of the moles of PPP were lost in the hydrolysis reaction. This value decreased to 0.11 at 20 h of reaction. In the case of Nb2O5, the loss fraction reached a maximum value of 0.60 at 8 h, and decreased to 0.13 at the end of the reaction. The particle size of the catalysts is very small: 80 nm for Nb2O5 and 50 nm for MgO. Given that the speed expression is unknown, the Weisz–Prater module (Φ) was used in order to estimate if any problems of intraparticle mass transfer exist:
Φi =
C +
the third catalytic test. Since the decrease in activity was present in the second use only, it could be considered that the loss of activity is a consequence of the lack of treatment or intermediate activation. That is not minor if we take into account that the used sample could have residues from the previous reaction. Subsequent consecutive tests for Nb2O5 and MgO did not show any significant variations in selectivity (not shown). In order to quantify the moles of PPP lost in hydrolysis, originating mono- and diglycerides, palmitic acid and glycerol, the “loss fraction” was defined as:
2 −robs;i ρp Rp Def;i Ci
ð3Þ
where C is concentration (mol/m3), Def is the effective diffusion coefficient (m2/s x m3liq/m3cat), Rp is the mean particle size (m), robs is the observed rate (mol/s kgcat) and, ρp is the catalyst apparent density (g/cm3). For both catalysts the value obtained was Φ ≪ 1, indicating that it is possible to consider that there are no mass transfer limitations in the catalyst pores, even though they would differ in size for the
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Fig. 5. Tripalmitin conversion, and yields of PPC + PCP and PCC + CPC for: A, MgO; B, Nb2O5. Reference:
PPP conversion, □ PPC + PCP yield, ■ PCC + CPC yield.
samples studied. The Weisz–Prater module was calculated for PPP because it is the reactant of the biggest size. An approximate value of Def,PPP = 3.35 10− 11 m2/s was estimated [24]. Given that in the reaction system used it was not possible to change the agitation rate, different tests were performed varying the catalyst mass and maintaining the rest of the operating conditions constant. The results are shown in Table 1. It can be observed that for both catalysts the reaction rate remained constant (calculated as millimoles of PPP converted per mg of catalyst per min), indicating that there was no external mass transfer limitation. The MgO catalyst was more active than the Nb2O5 catalyst. The difference was attributed to the base properties of MgO, because the same tendency was observed in homogeneous catalysts. Both samples reached maximum PPP conversion values and yields at 12 h, and then catalytic activity ceased. An analysis on the active sites present in the catalysts and the possible reaction mechanisms is beyond the scope of this work. The mechanisms proposed by the literature and the characteristics of the catalytic surfaces will be reviewed only briefly. The acidolysis reaction is an interestification, an exchange of acyl radicals between an ester and an acid. Two reaction mechanisms have been proposed [25]: one considers the formation of an enolate ion as intermediate, by abstracting an α-proton of the glyceride by the base catalyst, and the other one proposes the formation of a complex by the addition of an initiator to the glycerol carboxyl. In the first case, the enolate ion reacts with an ester group in the triglyceride molecule to originate a β-keto-ester, which reacts with other esters to produce another β-keto-ester. The same applies to the exchange of esters between different molecules of triglycerides. In the second mecha-
nism, by addition to the carboxyl, the alkyl ion is incorporated to the ester carboxyl group producing a diglycerinate intermediate that reacts with another glyceride abstracting a FA, originating a new triglyceride and regenerating the diglyceride. This process would continue through a series of chain reactions until all the available FAs have exchanged position and the equilibrium composition is reached. When the reaction is catalyzed by acids, first the protonation of the ester group takes place (in the triglyceride), followed by the addition of the acid to provide an intermediate that can be dissociated via a transition state, to produce the new ester group. The surface of the solids used is complex. It will only be mentioned that in conditions similar to those of the reaction, the presence of Lewis and Brönsted sites on Nb2O5 was observed [26]. It should be borne in mind that rehydratation of Lewis acid sites by water sorption can occur during cooling, after calcination of the catalyst [27], or during storage and preparation of the reaction mixture. In the case of MgO, its surface basicity is the result of the electronic transfer between Mg and O. The O2− anions (electron rich) act as electrondonor strong base sites, whereas the Mg2+ cations (electron poor) act as electron-acceptor weak acid sites [8]. In the specific case of the MgO used in this work, it must be pointed out that the presence of 0.6% Ca increased the basicity of the sample. As mentioned above, the literature on the subject refers to the use of homogenous catalysis. Klinkesorn et al. [28] modified the composition of FAs in fish oil by chemical transesterification with eicosapentanoic (EPA) and docosahexanoic acid (DHA), using sodium methoxide as catalyst, but having previously transesterified the FAs to methilic esters. They obtained an incorporation of 60% of EPA and 50% of DHA into the triglyceride, with a 59% loss of oil by hydrolysis and soap production. In the present work, the use of a heterogeneous base catalyst allowed a FA incorporation of 23%, with 11% loss of triglyceride for the same reaction time than the work reported previously.
Fig. 6. Reuse studies: PPP conversion (%), reaction time = 20 h. Reference: ▲Nb2O5, ■ MgO.
Fig. 7. Loss fraction for the catalysts studied as a function of reaction time. Reference: Nb2O5, □ MgO.
A.J. Avila et al. / Catalysis Communications 12 (2011) 362–367 Table 1 Effect of the catalyst mass variation on conversion and activity for the catalysts studied. Reaction time: 4 h. Catalyst %
MgO X PPP (%)
1 2 3
21.8 30.9 44.5
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tion, Argentina) and the Consejo Nacional de Investigaciones Científicas y Técnicas (National Council for Scientific and Technological Research (CONICET)) for the financial support.
Nb2O5 Activitya −5
5.83 10 3.79 10− 5 4.12 10− 5
X PPP (%)
Activitya
2.4 4.9 7.0
6.81 10− 6 6.72 10− 6 6.71 10− 6
a Integral activity calculated as millimoles of PPP converted per milligram of catalyst per minute.
For acid catalysis, and qualitatively at least, the results obtained for Nb2O5 can be compared to those reported by Chakrabarty and Talapatra [29], who exchanged 14% of the FAs of cotton, peanut and palm oil with lauric acid using sulfuric acid as catalyst (working at 150° in 3 h). In the present work, for a PPP conversion of 18%, the incorporation of FAs was of 7%. As regards the deactivation observed (after 12 h) for MgO, a Claisen condensation cannot be ruled out, which would originate surface polymeric species. These polymers would block the reactive ionic pairs on the surface. In the case of Nb2O5, an exhaustion of the Brönsted acid sites can be considered. Nevertheless, both samples showed activity in two more subsequent catalytic tests. 4. Conclusions MgO and Nb2O5 samples were characterized and used in the synthesis of structured lipids by acidolysis of PPP with capric acid. The solids were active for reaction, showing a conversion of PPP of 62% for MgO and 18% for Nb2O5. The maximum value of PPC + PCP yield was 40% for the MgO catalyst, and 14% for Nb2O5. As regards the structured lipid PCC + CPC, its yield was 14% and 2% for the base and acid solid, respectively. Acknowledgements The authors thank the Agencia Nacional de Promoción Científica y Tecnológica (National Agency of Scientific and Technological Promo-
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