Ethanolysis and Methanolysis of Soybean and Macauba Oils

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Ethanolysis and methanolysis of soybean and macauba oils catalyzed by mixed oxide Ca-Al from hydrocalumite for biodiesel production Roberta Gomes Prado, Gilsélia Diniz Almeida, Ana Rita de Oliveira, Priscila Maria Teixeira Gonçalves Souza, Claudia C. Cardoso, Vera Regina-Leopoldo Constantino, Frederico Garcia Pinto, Jairo Tronto, and Vânya M.D. Pasa Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00005 • Publication Date (Web): 05 Jul 2016 Downloaded from http://pubs.acs.org on July 11, 2016

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Ethanolysis and methanolysis of soybean and macauba oils catalyzed by mixed oxide Ca-Al from hydrocalumite for biodiesel production Roberta G. Pradoa,b*; Gilsélia D. Almeidab; Ana Rita de Oliveirab; Priscila M. T. G. de Souzab, Claudia C. Cardosoc; Vera R. L. Constantinod; Frederico G. Pintob; Jairo Trontob; Vânya M. D. Pasaa

a

Laboratório de Ensaios de Combustíveis, Departamento de Química, Universidade Federal de Minas Gerais, Av. Antônio Carlos nº 6627, CEP: 31270-901, Belo Horizonte, Minas Gerais, Brazil. e-mail: [email protected], [email protected]

b

Universidade Federal de Viçosa, Campus de Rio Paranaíba, Rodovia BR 354, km 310, CEP: 38810-000, Rio Paranaíba, Minas Gerais, Brazil.

c

Universidade Federal Rural de Pernambuco, Departamento de Química, Av. D. Manuel de Medeiros s/n, CEP: 52171-900, Recife, Pernambuco, Brazil.

d

Universidade de São Paulo, Instituto de Química, Departamento de Química Fundamental, Av. Prof. Lineu Prestes 748, CEP: 05508-000, São Paulo, São Paulo, Brazil.

ABSTRACT In this study, Ca-Al mixed oxide produced from the thermal decomposition of a synthetic hydrocalumite was prepared and evaluated as catalyst in the transesterification reaction for biodiesel production, using the following reagents: refined soybean oil, crude macauba kernel oil, methanol, and ethanol. The synthetic hydrocalumite and the mixed oxide were characterized by powder X-ray diffraction, thermogravimetric1 ACS Paragon Plus Environment

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differential scanning calorimetry coupled with mass spectrometry, specific surface area measurement, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and temperature-programmed desorption of CO2. The catalytic tests indicated that the reactions using methanol exhibited more favorable activity than those employing ethanol regardless of the oil type used (soybean or macauba). Ethanolysis produced better results for the higher molar-mass oil (soybean) due to the effect of the ethanol cosolvent. The catalyst was efficient for transesterification, with conversions of 97% and 95% for soybean and macauba oil, respectively, after 1.5 h of reaction, under atmospheric pressure condition and reflux temperature.

KEYWORDS: layered double hydroxides, hydrocalumite, biodiesel, macauba, transesterification, heterogeneous catalyst.

1. Introduction Homogeneous catalysis is a regular industrial process for biodiesel production, in which alkali-metal methoxides are often used as catalysts. These catalysts allow for obtaining high conversion rates using reactions under low temperatures and times shorter than 1 h. However, these catalysts are unrecoverable, favour saponification reactions and generate large volumes of aqueous effluents with environmental impacts.1 The use of heterogeneous catalysts in transesterification reactions for biodiesel production has been extensively studied, and these catalysts have been found to be more adequate for processing acid oils.1–3 Typically, synthesis involving heterogeneous catalysts requires higher pressure, time, and temperature conditions than those performed with homogeneous catalysts. It is possible to overcome the less-favorable

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kinetics observed in heterogeneous catalysis focusing in its advantages, such as the potentially reusing of the catalysts1,2 and the easy biodiesel separation and purification. Taking into account the solid catalysts for transesterification, CaO has been recognized as the most promisor catalyst due to its high basicity, low cost, availability, and low toxicity.4 In recent years, the number of scientific studies involving the use of layered double hydroxides (LDHs) and their calcined products as heterogeneous catalysts for biodiesel production has grown.5 These materials of composition

[MIIR MIII (OH)2 ]An- nH2O (abbreviated MIIRMIII-A, where R is the metals molar ratio and An- is an anion) are precursors of oxy-hydroxides and/or mixed oxides. The specific architecture of these 2D materials produces a synergistic effect between the layered inorganic matrix and the intercalated anions, generating materials with physical and/or chemical properties distinct from those of their individual components. These compounds represent innovative alternatives to produce and explore new oxide catalysts. Cantrell et al.6 have reported that in the transesterification of glycerol tributyrate with methanol for biodiesel production, the metal oxides MgO and Al2O3 were less active as catalysts when compared to the mixed oxides derived from calcined carbonate intercalated LDHs with different Mg / Al molar ratios. Liu et al.7 have investigated LDH of composition ZnRAl-CO3 calcined at different temperatures (140, 200, 300, 400, and 500°C) as catalysts for biodiesel synthesis in a fixed-bed reactor, employing soybean oil as reagent. The usage of the LDH treated at 200°C allowed to obtain approximately 76% yield at 140°C and 1.7 MPa after 1 h of reaction. This catalyst exhibited activity for up to approximately 150 h. Prado et al.8 synthesized LDHs with composition MgRAl-CO3 with a Mg/Al molar ratio ranging from 2 to 6. The LDHs were calcined and used as catalysts in the transesterification reaction of soybean oil for biodiesel production. A multivariate 3 ACS Paragon Plus Environment

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method was employed in the transesterification reactions to optimize the synthesis. Using the determined optimal conditions, it was possible to obtain 95.4% of soybean oil conversion into methyl esters after 2 h of reaction at 115°C, employing 14:1 molar ratio between alcohol and soybean oil, 4% of catalyst, and an initial N2 pressure of 1380 kPa. The performance of the selected catalyst against acidity and moisture added to the soybean oil was evaluated. Considering the usage of CaO and calcined LDHs as solid catalysts for transesterification, in this study a LDH with Ca2+ in its structure was synthetized and explored as the catalyst precursor for transesterification. For this purpose, Al3+ is the unique trivalent metal cation suitable to be used with calcium to prepare hydrocalumite, the LDH of our interest. The hydrocalumite group belongs to the huge class of LDHs, in which Ca2+ and Al3+ ions in a molar ratio equal 2 are ordered distributed in the hydroxide layers; the interlayer region can accommodate the hydrated anions chloride, carbonate, hydroxyl or sulfate and water.9 The Al3+ cations are hexa-coordinated, whereas Ca2+ cations exhibit a [6 + 1] coordination, where the seventh oxygen atom is from

the

intercalated

H2 O

molecule.10

The

hydrocalumite

of

composition

[Ca2Al(OH)6]Cl·2H2O is also known as Friedel’s salt.11 After thermal treatment of suitable temperature this material can be converted into mixed oxides with basic character to be used as catalysts1.Non-edible oils with high productivity have been preferentially used to ensure more sustainable biodiesel production, replacing edible oils such as soybean oil. Palms produce 400 - 5,000 kg oil/ha, whereas soybean produces between 400 - 600 kg oil/ha.12–14 Among the various palms, macauba (Acrocomia aculeata) stands out as an important alternative to biodiesel production because of its robustness and low edafoclimatic (relative to the soil and climate) and rainfall requirements. This palm is native of South America, commonly occurring in Brazil.


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Macauba has two oil types: one from the pulp, comprising predominantly oleic, linoleic, and linolenic acids, and the other one from the kernel, which is rich in linoleic and lauric acids. Oil from the macauba kernel is more saturated, has shorter carbon chains and lower acidity than the oil from the pulp15; in the present study, the oil from the kernel was investigated for biodiesel production. Because of its high reactivity and low cost, methanol is the most commonly alcohol used in biodiesel synthesis via esterification and transesterification. Although Brazil is the world’s largest producer of ethanol from sugarcane and can produce biodiesel from renewable sources, Brazilian industries still use predominantly methanol for biodiesel synthesis. The employment of ethanol leads to a reduced reaction rate and occasionally hinders phase separation, since ethanol can act as a co-solvent in the glycerin and biodiesel phases.16 This study has the following objectives: (i) to synthesize and characterize a mixed oxide of Ca and Al using the synthetic hydrocalumite precursor; (ii) to evaluate the calcined material as a catalyst in transesterification reactions (ethyl and methyl routes) employing refined soybean oil or crude macauba kernel oil; and (iii) to compare the calcined hydrocalumite catalytic performance with the commercial CaO. To the best of our knowledge, studies on macauba biodiesel production are rare17–19, especially those using the ethyl route catalyzed by LDH derivatives materials.

2. Experimental 2.1 Materials The reagents used in this study have analytical degrees of purity. The following reagents were used for hydrocalumite synthesis: CaCl2·2H2O (99%), AlCl3·6H2O (99.5%), CaO (90%), NaOH (min. 99%), and ethanol (min. 96%) acquired from the


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Vetec Company. For the transesterification reaction, 99.5% methanol and 99.8% absolute ethanol (Vetec) were used in addition to refined commercial soybean oil (Liza®) and crude macauba kernel oil; the crude macauba oil was acquired from the Agro-extractive

Cooperative

-

Cooperativa

dos

Agricultores

Familiares

e

Agroextrativista Grande Sertão - Minas Gerais, Brazil.

2.2 Syntheses Hydrocalumite was synthesized by the slow addition of 50 mL of an aqueous solution containing 0.66 mol·L-1 of CaCl2·2H2O and 0.33 mol·L-1 of AlCl3·6H2O to a solution containing 500 mL of H2O and 750 mL of ethyl alcohol. During the addition, 2.0 mol·L-1 of NaOH solution was added to keep the pH value constant at 11.5. The synthesis was carried under N2 atmosphere in a closed system, as described in the literature10. The obtained solid material was washed with deionized water using reduced pressure filtration. Then the synthesized hydrocalumite, denoted Ca2Al, was calcined under a flow of 150 cm3·min-1 of O2 gas at 750°C for 4 h; the heating rate used was 10 °C·min-1, yielding the catalyst denoted Ca2Al-c. For comparison purposes, a commercial calcium oxide, abbreviated CaO-c, was calcined under the same conditions. However, the temperature was set at 900°C since most works in the literature employs this condition for the thermal treatment of CaO catalyst.

2.3 Catalytic tests The transesterification reactions were performed using an alcohol/oil molar ratio of 14:1, under alcohol reflux temperature, atmospheric pressure condition and mechanical stirring at 600 rpm. The proportion of Ca2Al-c catalyst used was 3% (w/w) relative to the weight of the oil. Aliquots were removed at certain time intervals for


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determining fatty acid methyl esters (FAME) and fatty acid ethyl esters (FAEE). The catalyst and glycerin were removed by centrifugation. The residual alcohol in the biodiesel was evaporated using a water bath.

2.4 Kinetic study The kinetic model was obtained according to Dang et al.20, Kaur et al.21, and Balat et al.22, where the overall transesterification reaction is represented as follows (Equation 1):

(1) The general equation of the reaction rate can be described as follows (Equation 2):

∙

(2)

where

−

d [TG] = triglyceride (TG) consumption per time unit; dt

k = reaction rate constant; [TG] = TG concentration; [ALCOHOL] = methanol or ethanol concentration; a = TG reaction order; b = alcohol reaction order. 7 ACS Paragon Plus Environment

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Some considerations for this kinetic study involving heterogeneous catalysis are as follows: a) In this reaction, only alcohol adsorbs on the catalyst surface because it is considered to follow the Eley-Rideal mechanism, according to Dang et al.20; b) The variation in the catalyst concentration during the reaction is negligible; c) The concentrations of the intermediates, mono- and diglycerides, are constant, and very low values are assumed23; d) The reaction occurs in three steps, and the reverse reaction is not considered; e) Because there is a large excess of alcohol in the reaction, it is considered as a pseudofirst order reaction as follows:

∙ ℎ

(3)

where alcohol = degree of alcohol recovery after adsorption at the catalytic sites;

∙ ℎ ≈ , when alcohol is used in excess. Rearranging and integrating Equation (3), we have:

ln ln " ′ ∙

(4)

To find k´, graphs of ln[TG] versus time were plotted, where the slope is the determined k´ value.

2.5 Characterization Powder X-ray diffraction (PXRD) patterns were recorded in a Shimadzu XRD6000 instrument using a graphite crystal as monochromator to select Cu-Kα radiation (λ 8 ACS Paragon Plus Environment

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= 1.5406 Å) and a 0.02º s-1 step. Thermogravimetric analysis with differential scanning calorimetry coupled to mass spectrometry (TG-DSC-MS) was performed in a Netzsch STA 409 CP Luxx instrument coupled to a Netzsch QMS 403 C-Aeölos from 20°C to 1200°C with a heating rate of 10°C·min-1 and a synthetic air atmosphere (20% of O2 and 80% of N2). Specific area analyses were performed by N2 adsorption-desorption (BET) in a BelSorp Japan instrument. The samples were previously degasified for 2 h at 120°C. Scanning electron microscopy (SEM) analyses were performed with a Quanta 3D FEG scanning microscope. A gold coating was applied to the samples before analysis using a Sputter BAL-TEC, MED 0.20. Energy dispersive X-ray spectroscopy (EDS) was carried out using a Jeol JXA-8900RL instrument. Temperature-programmed desorption of CO2 (CO2-TPD) was performed using a reactor. The gas mixture was administered by mass flow controllers, and the gas composition at the end of the reactor was determined using a Multigas 2000 FTIR analyzer (MKS). Approximately 250 ± 1 mg of the sample was placed in the reactor, and CO2 adsorption was performed using a 0.1% CO2/N2 mixture with a flow rate of 500 mL·min-1 with N2 as a carrier gas. Next, the samples were heated up to 800°C, with a heating rate of 10°C·min-1, using N2 as carrier gas at a flow rate of 500 mL·min-1. The fatty acid profile of the oils was determined by gas chromatography with a flame ionization detector (GC-FID), according to EN14103:2011, involving a hydrolysis step followed by a methylation step under the same conditions described for Bejan et al24. The moisture content present in the oils was determined in triplicate according to the ASTM D6304 standard using the Karl Fischer coulometric titration method, with a Metrohm 831 KF coulometer. The acidity index was determined by colorimetric titration using a standardized aqueous 0.1 mol·L-1 NaOH solution.25 1H nuclear magnetic resonance (1H NMR) was used to determine the oil conversion into


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esters using a 200 MHz Bruker Avance DPX 200 spectrometer. The samples were diluted in CDCl3, and tetramethylsilane (TMS) was used as reference.26,27

3. Results and Discussion 3.1 Catalyst characterization The Ca2Al and Ca2Al-c X-ray diffraction patterns are shown in Figure 1. The diffractogram in Figure 1(a) has the characteristic profile of hydrocalumite: hexagonal lattice and R-3 rhombohedral symmetry.11 The high intensity of the peaks, indicated by the small width at the half-height, shows the formation of a layered compound with excellent structural organization and phase purity. PXRD profile of Ca2Al sample does not show peaks related to other phases such as CaCO3 (most intense signal at (2θ) 29.9o for calcite).10 The Ca2Al basal spacing value was calculated using the Bragg equation; the mean of the basal peaks was 7.96 Å (co equal to 23.9 Å). This value is characteristic of the intercalation of chloride anions between the inorganic hydrocalumite layers.28 The calcined material, Ca2Al-c, exhibits new phases as noticed in Figure 1(b) that are mainly attributed to the presence of mayenite (Ca12Al14O33) and calcium oxide (CaO)10; these phases are identified with the symbols * and # in the diffractogram, respectively. The mayenite and CaO phases present metallic cations and negative oxygen ions. These metallic cations are considered Lewis acid sites whereas the negative oxygen ions are considered Brønsted basic sites.1 The heterogeneous catalytic mechanism currently accepted is based on homogeneous catalysis.29 In this mechanism, the Brønsted basic sites adsorb the proton from methanol during the first phase of transesterification reaction. In this work, basic sites are responsible for the catalytic behaviour of the studied system. It is worth to remember that other phases, non-crystalline, can be


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present in the sample. Then the calcination process caused the collapse of hydrocalumite layered structure producing a mixed oxides material. The CaO-c PXRD pattern Figure 1(c), is identical to that one reported in the JCDPS 77-2376 powder diffraction file databank, which confirm the high purity and crystallinity of CaO after the heat treatment.

Figure 1

Figure 2 shows the TG-DSC-MS data of Ca2Al performed under synthetic air atmosphere. The first thermal event occurs from room temperature (RT) to 190°C, with about 13.6% of mass loss (DTG peaks at 125 and 150oC). The DSC curve in this temperature range indicates an endothermic process, and MS curve shows a fragment with m/z equal 18 (Figure 2), assigned to the Ca2Al dehydration (event 1), i.e., release of intercalated and/or adsorbed H2O molecules in the sample structure. In the 190 570°C range (DTG peak at 320oC), an endothermic process related to 16.3% of mass loss and water release (MS detects a fragment with m/z equal 18) can be attributed to the hydrocalumite layers dehydroxylation (event 2). About 6.3% of sample weight is lost from 570°C to 750°C (DTG peak at about 690oC; exothermic process), and MS curve indicates the release of a very low intensity fragment with m/z equal 44 (amplified 10 times in Figure 2), attributed to the sample decarbonation (event 3). In spite of precaution to avoid the contact of the alkaline suspension with atmospheric CO2, carbonate is present in Ca2Al sample. Above 750°C, it is observed mainly an exothermic event and a gentle weight decrease that intensifies near 1200°C, suggesting that a forth event should occur above the operation limit of the equipment used is this study. Unfortunately, the mass variation between 800°C and 1200oC is not high (∆m ca. 11 ACS Paragon Plus Environment

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2.7%) and not rapid enough to permit characterize by MS the species leaving the sample. The Ca2Al residue is 61 wt.% at 1200oC (Figure 2). Mesbah et al. have reported the existence of a solid solution of general formulae [Ca2Al(OH)6](Cl1-x(CO3)x/2)~2.25H2O], in which chloride and carbonate ions are cointercalated in the interlayer domain30,31. In view of these studies and the thermal analyses

data

mentioned

above,

the

chemical

composition

[Ca2Al(OH)6]Cl0.2(CO3)0.4·2.1H2O (277.9 g mol-1) is proposed to hydrocalumite isolated in this work. Taking into account this proposal, the decomposition steps can be interpreted as follow. Dehydration of hydrocalumites (step 1) occurs up to 200oC, as was noticed in other works:10,32

[Ca4Al2(OH)12]Cl0.4(CO3)0.8∙4.2H2O

1 Dehydration (RT – 190oC) - 4.2 H2O

wt.% weight loss exp. = 13.6%

[Ca4Al2(OH)12]Cl0.4(CO3)0.8 wt.% weight loss calc. = 13.6%

Such as observed in other studies10,33, the second event of weight loss corresponds to a partial dehydroxylation process:

[Ca4Al2(OH)12]Cl0.4(CO3)0.8

2 Dehydroxilation (190oC – 570oC) - 5 H2O

{Ca4Al2(OH)2O5Cl0.4(CO3)0.8} wt.% weight loss exp. = 16.3% wt.% weight loss calc. = 16.2%

It was expected a mass loss of 19.4 % if the layers hydroxylation is complete. The parentheses indicate the species present in the heated sample and the molar relation among them. In situ X-ray diffraction data have shown that the product isolated after step 2 is amorphous but crystalline CaCO3 is formed at 500°C and converted to CaO above 700oC.10 MS curve indicates that CO2 (and not HCl or H2O) is lost in the third decomposition event (Figure 2), as was also observed previously in the literature.32


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Decarbonation (step 3) can encompass mainly carbonate intercalated between the layers, as is suggested in this work:

{Ca4Al2(OH)2O5Cl0.4O0.8}

3 Decarbonation (570oC – 750oC)

{Ca4Al2(OH)2O 5Cl0.4O0.8} wt.% weight loss exp. = 6.0% wt.% weight loss calc. = 6.3%

- 0.8 CO2

Again, the parentheses indicate the species present in the heated sample and the molar relation among them. However, considering the XRD profile of the product obtained at 750oC (Figure 1), the calcium and aluminum cations, and the oxide anion are organized in crystalline Ca12Al14O33 and CaO; non-crystalline phases should be present. Some works have detected Ca(OH)Cl phase in the XRD of the residue obtained around 9001000oC and, at temperatures higher than 830oC - 1000oC, the release of chloride as HCl fragment was observed32,34. Hence, the mass loss observed at about 1000oC in Figure 2 is related to the decomposition of phases containing Cl- and OH- anions. In this study, the hydrocalumite calcination temperature (750°C) was selected based on the TG-DSC-MS analyses in order to isolate a catalyst with strong basic sites, i.e., a mixed metal oxide. According to literature35, in this temperature strong basic sites are formed from the calcined hydrocalumite.

Figure 2

The BET area values obtained for Ca2Al and Ca2Al-c were 10.7 and 6.0 m2·g-1, respectively. The materials exhibited practically the same total pore volume values, 4.86x10-2 and 4.85x10-2 cm3·g-1 for Ca2Al and Ca2Al-c, respectively. Generally, after the LDHs calcination processes, their specific areas and pore volumes increase because layer decomposition contributes to the formation of micro and/or mesopores. However, 13 ACS Paragon Plus Environment

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this pattern is not observed for Ca2Al and Ca2Al-c samples. The presence of phases with high crystallinity in the calcined materials such as mayenite and CaO (Figures 1b and 1c) is the main reason for the low values of surface area. The specific surface area of the phases is directly related to the state of division of the solid. Thus, an increased particle size causes a lower specific area. The specific area for CaO-c was also measured and presented 1.0 m2·g-1. This area value is also low, similar to others catalysts studied in this work. The SEM micrograph of Ca2Al sample shows particles with a hexagonal morphology, which is characteristic of compounds of the LDH family (Figure 3a). For this material, the particles shape and size are uniform and homogeneously dispersed. As expected, Ca2Al-c has a very different morphology from its precursor, with the presence of compact aggregates without regularity or defined shape (Figure 3b). The EDS results are presented in Figure 3. EDS spectra show qualitatively the composition of Ca2Al and Ca2Al-c. The presence of chlorine in Ca2Al-c validates the results of TG-DSC-MS in which the temperature of 750oC is not enough for the chlorine removal.

Figure 3

The CO2-TPD analyses are show in Figure 4. The Ca2Al-c and CaO-c samples present elevated desorption temperatures: 675oC and 610oC, respectively. The results obtained for Ca2Al-c indicated the presence of stronger basic sites compared to CaO-c. The Ca2Al-c has higher amount of CO2 desorbed (1616 mmol·g-1) than CaO-c (143 mmol·g1

), indicating the presence of large amount of basic sites in the first material. Moreover,


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Ca2Al-c sample presents higher amount and stronger basic sites compared to other calcined LDH compositions as for example ZnAl-LDH and MgAl-LDH.36–38

Figure 4

3.2 Characterization of the soybean and macauba kernel oils The soybean oil used in the transesterification reaction exhibited acidity index lower than 0.99% (1.97 mg KOH/g) and moisture content of 713 mg·kg-1. Its fatty acid composition is predominantly (79.70%) unsaturated chains (C16:1, 0.09%; C18:1, 24.04%; C18:2, 50.50%; C18:3, 5.07%) vs. 18.36% saturated chains (C10:0 0.11%; C12:0 0.78%; C14:0 0.23%; C16:0 13.51%; C17:0 0.07%; C18:0 3.45%; C20:0 0.14%; C21:0 0.07%). The molar mass of soybean oil, 1288 g·mol-1, was calculated based on its composition. The crude macauba kernel oil used in the transesterification reactions showed 4.9% of acidity (9.7 mg KOH/g) and moisture content of 763.9 mg·kg-1. In contrast to soybean oil, the fatty acid composition was predominantly (64.72%) saturated chains (C6:0 0.07%; C8:0 2.83%; C10:0 3.41%; C11:0 0.15%; C12:0 35.43%; C14:0 9.83%; C16:0 9.41%; C18:0 3.59%) with 35.27% of unsaturated chains (C18:1 30.15% and C18:2 5.12%). The crude macauba kernel oil exhibited a molar mass of 751 g·mol-1. The kinematic viscosity of the studied oils was measured; the soybean oil displayed viscosity of 32.6 mm2·s-1 while crude macauba kernel oil exhibited viscosity of 35.0 mm2·s-1. These results are explained by the fact that soybean oil is more unsaturated, although it has a higher mean molar mass than macauba kernel oil.39

3.3 Biodiesel


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A method using gas chromatography, proposed by the EN 14214 specification, is used to determine the FAME content in biodiesel samples. However, this technique is not adequate for measuring higher alcohol ester levels and samples with high contents of intermediates because these compounds can cause clogging problems in the chromatographic column. For this reason, 1H NMR spectroscopy was used to determine the ester content throughout the reaction, both in the transesterification reactions with methanol and with ethanol.24 The FAME content was analyzed according to the method proposed by Gelbard et al26, whereas the FAEE content was determined according to the method proposed by Ghesti et al.27 Kinetic curves for the transesterification reactions were constructed from the ester content values obtained, using soybean oil or macauba kernel oil, 3% (w/w) of catalyst and alcohol:oil ratio of 14:1. The conversion vs. time curves for the transesterification reactions using the methanol and ethanol alcohols are shown in Figures 5a and 5b, respectively.

Figure 5

The reaction with macauba oil exhibited a higher initial rate than that observed for soybean oil, both in the methyl route (Figure 5a) and the ethyl route (Figure 5b). After some time, conversion with the macauba oil occurred at a slightly lower rate compare to that observed for soybean oil. Using soybean oil, it was possible to obtain 88% of FAME conversion after 1 h of reaction; after 1.5 h of reaction, the conversion was approximately 97% (Figure 5a). Under the same conditions, the FAEE conversion was slower; with a conversion of 11%


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after 2 h of reaction, and 91% after 12 h (Figure 5b). The results indicate much slower kinetics for ethanolysis compared to methanolysis for the oils studied. The transesterification data of crude macauba kernel oil, which has been seldom studied for biofuel production, presented a high conversion: 71% of FAME was obtained after 1 h of reaction, and 95% after 1.5 h. Unlike most heterogeneous catalytic processes, this reaction was performance under atmospheric pressure and low temperature (65oC).40,41 This aspect contributes for the economic feasibility of this specific process. Wang et al.42 achieved a 90% of FAME yield from rapeseed oil after 3 h of reaction, using a methanol:oil ratio of 15:1 and 6% of basic Ca/Al oxides. The catalyst content used by these authors was twice that one used in this study, and the alcohol:oil ratio was similar. Crude macauba kernel oil exhibited a slower conversion in the ethyl route: 12 h of reaction was necessary to obtain 69% of FAEE. These results indicate that for ethanolysis, the conversion rate of macauba oil is much slower compared to refined soybean oil. Table 1 shows the values observed for k´ (the apparent rate constant), calculated for each reaction studied. The transesterification reactions using ethanol showed lower k´ values regardless of the oil type used. This finding may be explained by methanol having higher acidity (pKa = 15.5) than ethanol (pKa = 15.9), which is a slightly stronger acid; thus, methanol favors the transesterification reaction.43

Table 1

The difference in kinetics observed in the reactions was measured by the ratio between the rate constants. In the ethyl route, the k´ (soybean) / k´ (macauba kernel)


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ratio is 1.7, whereas in the methyl route, the k´ (soybean) / k´ (macauba kernel) ratio is 1.1. These results may be associated with the effect of the ethanol co-solvent for soybean oil, whose chains are longer; thus, there is a greater contribution from nonpolar groups. The co-solvent effect is explained by ethanol's ability to partially mix with the soybean oil, increasing the contact and miscibility of the reagents and favoring the reaction, although this alcohol should be less effective than methanol. The lowest k´ values obtained for the transesterification reaction using macauba oil compared to soybean oil may be due to the different compositions of these oils: macauba kernel oil has more saturated chains than soybean oil (as shown in section 3.2). A higher number of saturated chains present in the oils causes a higher viscosity (35.0 mm2·s-1 for crude macauba kernel oil, and 32.6 mm2·s-1 for refined soybean oil), which hinders the diffusion step. Additionally, macauba kernel oil has 4% of acidity, whereas soybean oil has a value lower than 1%; these free fatty acids are prone to undergo undesirable reactions with basic catalysts, such as saponification. These parallel reactions can consume the catalyst, reducing the transesterification reaction speed. Moreover, the crude oil contains impurities that can adsorb onto the catalyst, leading to parallel reactions or competing with the catalytic sites used in transesterification. Under the same reaction conditions, Ca2Al sample was used as a catalyst in the transesterification of the soybean oil with methanol for 8 h, resulting in 12% of FAME conversion, and indicating that calcination is an essential step; the active phase of the catalyst is generated during the calcination process. Tests performed with soybean oil and methanol without catalyst addition, denoted as “blank”, showed that only 0.4% of FAME was obtained after 8 h of reaction. Hence, it is impossible to undergo transesterification without the catalyst.


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The biodiesel produced with the macauba kernel oil exhibited lower than 1% of acidity, and this parameter decreased compared to the starting oil, which had 4% of acidity. Silva et al.41 used calcined hydrotalcite-type materials in the transesterification of an acid oil (9.5% of acidity) and found that the acidity of the products decreased by 1%. The authors concluded that the calcined hydrotalcite exhibits a catalytic activity for triglyceride transesterification and fatty acid esterification and that this activity is related to the Lewis acid sites generated by the hydrotalcite calcination41. Because hydrocalumite has characteristics similar to these materials, the same effect likely occurs with Ca2Al-c. The results of the transesterification reaction using CaO-c soybean and macauba oils with methanol, under the same conditions in which Ca2Al-c was evaluated, are shown in Figure 6. It can be seen that the Ca2Al-c showed a more rapid kinetics than the CaO-c and, consequently, a higher k´ value. The calculated k´value for reaction with CaO-c was 1.13 h-1 with soybean oil and 1.17 h-1 with macauba, while Ca2Al-c presented a k´constant of 2.88 h-1 with soybean oil and 2.57 h-1 with macauba (as shown in Table 1). This result shows that the mixed oxide derivative of hydrocalumite, Ca2Alc, was more active than the simple oxide, CaO-c, in the transesterification reaction. In addition, the results presented in Figure 6 show that the reaction using CaO as a catalyst and soybean oil as a reagent, presented a lower value of k' (1.13 h-1) compared to the macauba oil (1.17 h-1). A different result was obtained when the Ca2Al-c was used as catalyst, in which k' value was lower to the macauba oil. Probably this result is due to the fact that these catalysts have different particularity, for example, different compositions, different amounts of force and basic sites, etc that make them act differently in the transesterification reaction of used oils.


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Figure 6

Ca2Al-c sample presented higher catalytic activity than simple calcium oxide in the methyl route. These results point to a synergy between mayenita and CaO phases, obtained by heat treatment of hydrocalumita, resulting in higher catalytic activity than pure CaO calcined.44 Lee et al.45 also obtained similar results: the physicochemical properties of the binary system MgO-ZnO were superior than those of the individual oxides of MgO and ZnO in the transesterification. This result was attributed to the synergistic effect between Mg and Zn promoting an increase in the an catalyst basicity.

3.4 Catalyst stability The catalytic stability test was conducted with the recovered catalyst after the transesterification reaction under conditions which provided the highest k´value: soybean oil and methylic route. The results showed that without further heat treatment, the catalyst did not exhibit catalytic activity. This is probably due to the reactants and/or products impregnation on the catalyst surface during the reaction, requiring a heat treatment for its recovery. Thus, after the catalyst recuperation from the reaction medium, it was heattreated (750°C for 2 h) before each new use cycle. The Ca2Al-c sample provided high levels of soybean oil conversion into methyl esters in the first, second and third reaction cycles (95, 97 and 97%, respectively), after 2 h of reaction and 3% m/m catalyst. Deactivation took place in the fourth reaction cycle, with only 50% of methyl ester conversion. Sankaranarayanan

et al.46

have

employed calcined

hydrocalumite

in

transesterification reactions using different oils. It was possible to use the catalyst for 4 20 ACS Paragon Plus Environment

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consecutive cycles, obtaining 96, 87, 85 and 77% of FAME, respectively. Wang et al.42 also used a catalyst consisting of Ca12Al14O33 and CaO for biodiesel production. These authors reported that it was possible to use the catalyst in at least 7 cycles, showing a good stability of the synthesized material.

4. Conclusions In this study, a mixed oxide of Ca and Al derived from synthetic hydrocalumite (Ca2Al-c) was prepared and characterized. The calcination temperature used (750°C) produced the collapse of Ca2Al layered structure, yielding Ca12Al14O33 and CaO phases with high crystallinity. The produced oxide was used as catalyst in transesterification reactions using refined soybean oil and crude macauba kernel oil, which exhibited high FAME and FAEE conversions. The results indicate that methanolysis catalyzed by Ca2Al-c has faster kinetics than ethanolysis regardless of the oil type used. In the ethyl route, oils with longer chains led to faster alcoholysis due to effect of the ethanol co-solvent. The Ca2Al-c presented more favorable kinetics than the CaO-c using soybean oil or methanol. The present study showed that Ca2Al-c is a catalyst easily synthesized and very efficient for transesterification reactions under mild conditions (atmospheric pressure, reflux temperature, and short reaction time); therefore, it is adequate for industrial use in processes using the methyl and ethyl routes. The study also showed that crude macauba kernel oil has potential for use as a non-edible source for biodiesel production.

Acknowledgment


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The authors gratefully acknowledge the financial support from the ANP – Agência Nacional de Petróleo, Gás Natural e Biocombustíveis (Human Resources Program – PRH-46), FINEP – Financiadora de Estudos e Projetos, MCTI – Ministery of Science Technology, and Inovation. This work is a collaboration research project of members of the Rede Mineira de Química (RQ-MG) supported by FAPEMIG (Project: CEX - RED00010-14), and CNPq – Conselho Nacional de Desenvolvimento Científico e Tecnológico. The authors are grateful the Professor Luiz Carlos Alves de Oliveira (UFMG) for TPD analysis.


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Figures:

Figure 1: PXRD patterns of (a) Ca2Al, (b) Ca2Al-c and (c) CaO-c samples. * Mayenite (Ca12Al14O33); # CaO.


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Figure 2: TG-DSC-MS curves of Ca2Al sample in air atmosphere.


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Figure 3: SEM images (left) and EDS spectra (right) for (a) Ca2Al and (b) Ca2Al-c.


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Figure 4: Temperature-programmed CO2 desorption profile of Ca2Al-c and CaO-c.


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Figure 5: Variation in conversion over time for the transesterification reaction, using Ca2Al-c, soybean oil, and macauba kernel oil exploiting the (a) methyl route and (b) ethyl route.


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Figure 6: Variation in conversion over time for the transesterification reaction, using CaO-c, methyl route, soybean oil and macauba kernel oil.


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Table 1: Apparent rate constant values using refined soybean oil and macauba kernel oil with methanol and ethanol. Raw material Soybean Macauba kernel

Methanol k´ (h-1) R2 2.88 0.98 2.57

0.95

Ethanol k´ (h-1) R2 0.22 0.98 0.13

0.95

k´ methanol/ k´ ethanol 13.1 19.8


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Ethanolysis and Methanolysis of Soybean and Macauba Oils

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