Kinetics evaluation of the ethyl esterification of long chain fatty acids using commercial montmorillonite K10 as catalyst

Fuel 193 (2017) 265–274

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Full Length Article

Kinetics evaluation of the ethyl esterification of long chain fatty acids using commercial montmorillonite K10 as catalyst Luis Ricardo Shigueyuki Kanda a, Marcos Lucio Corazza a,⇑, Leandro Zatta b, Fernando Wypych c a

Department of Chemical Engineering, Universidade Federal do Paraná (UFPR), PO Box 19011, 81531-980 Curitiba, PR, Brazil Department of Chemistry, Universidade Tecnológica Federal do Paraná (UTFPR), Campus Pato Branco, Via do Conhecimento, 01 km, Pato Branco, PR, Brazil c Department of Chemistry, Universidade Federal do Paraná (UFPR), PO Box 19032, 81531-980 Curitiba, PR, Brazil b

h i g h l i g h t s
We used montmorillonite K10 as catalyst to alkyl esters production with ethanol.
Lauric, stearic and oleic acids were used as reactant.
Higher fatty acid conversions were observed independently of the fatty acid.

a r t i c l e

i n f o

Article history: Received 26 October 2016 Received in revised form 10 December 2016 Accepted 19 December 2016

Keywords: Montmorillonite K10 Esterification Free fatty acids Ethanol Reaction kinetics Biodiesel

a b s t r a c t This study reports an assessment of free fatty acid esterification reactions with ethanol using montmorillonite K10 as catalyst, a commercial solid catalyst known for having high performance in several catalytic processes. Therefore, the main objective of this work is to perform reactions of FFA (lauric, stearic and oleic acids) ethyl esterification, checking how different chain lengths and unsaturation presence affect the reaction kinetics. The catalyst was characterized by different techniques, in which the structure and acidic properties were evaluated. Esterification reactions were systematically evaluated by a factorial design (effects of temperature, molar ratio and catalyst content) to determine optimal conditions to be used in the reaction kinetics study. In addition, kinetics reactions without montmorillonite K10 were also carried out in order to evaluate the autocatalytic (or thermal) reaction contribution. The kinetic data obtained for all FFAs was used to adjust kinetic parameters and the model was capable to correlate the experimental data, providing values of the root mean square deviation (rmsd) around 3.5% in terms of fatty acid conversions. The low values obtained for the rmsd indicate the possibility to achieve a reactor design to process complex fatty matrices using data generated for a pure FFA without quality loss. Furthermore, due to the high activity presented in the free fatty acid esterification under the studied conditions, montmorillonite K10 proves to be a suitable catalyst for industrial-scale biodiesel obtention from raw materials with high content of free fatty acids. Ó 2016 Published by Elsevier Ltd.

1. Introduction The esterification of free fatty acids (FFA) into fatty acids alkyl esters (FAAE) takes place conventionally with the use of catalysts such as HCl or H2SO4 in a homogeneous medium. After the reaction, these catalysts are removed from the products throughout washing and purification steps. However, the catalyst cannot be reused in the reaction and a significant amount of wastewater is generated and must be neutralized before its discharge. Further-

⇑ Corresponding author. E-mail addresses: [email protected] (L.R.S. Kanda), [email protected] (M.L. Corazza), [email protected] (L. Zatta), [email protected] (F. Wypych). http://dx.doi.org/10.1016/j.fuel.2016.12.055 0016-2361/Ó 2016 Published by Elsevier Ltd.

more, the aforementioned homogenous catalysts can cause corrosion in piping, equipment and facilities and can also lead to the formation of undesirable co-products in the reaction. This background demonstrates the importance of developing processes that combine a satisfactory reaction yield with simple purification steps, sustaining the usage of solid catalysts as an alternative route [1,2]. In the biodiesel production context, solid acid catalysts can simultaneously catalyze both the esterification of free fatty acids and the transesterification of triacylglycerols (TAG). The standout feature for the use of this type of catalyst is the enhancement of both technical and economic practicability in biodiesel production,

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although it is beneficial in many ways, since it helps to avoid most of the drawbacks previously cited [3]. Among the several types of solids that have been studied, solid resins demonstrated to be suitable catalysts in esterification reactions and some works have explored the simultaneous esterification of FFAs and transesterification of vegetable oils. Given this background, Jasen and Marchetti [4] performed the ethyl esterification of a mixture of sunflower oil with oleic acid, and Tesser et al. [5] proceeded the methyl esterification of soybean oil with a high acid amount of oleic acid (acidity of nearly 50%). Supported heteropolyacids (HPA) are another type of solid catalyst that have been studied, e.g., Oliveira et al. [6] tested 12-tungstophosphoric acid supported in zirconia in ethyl esterification of oleic acid and Nandiwale and Bokade [7] reported studies regarding to the use of H3PW12O40 supported in montmorillonite K10 in methyl esterification of oleic acid. Recently, layered compounds, a very important class of solids whose crystals are built from the stacking of bi-dimensional units known as layers [8], have drawn especial attention as reaction catalysts, as layered metal laurates have demonstrated to be suitable catalysts for (m)ethyl esterification reactions of lauric acid [9,10]. Clay minerals are also classified as layered compounds and are considered to be very promising materials, not only due to the low cost and high availability, but also because they are activated by simple processes, are safe to handle, can be easily recovered to be reused for several reaction cycles, and after exhaustion can serve as raw material for other products such as ceramics [11]. Activated clay minerals can be applied in various organic reactions such as alkylation, condensation, dimerization, isomerization and others [12], but few studies have been published about their use in esterification reactions [11,13–17]. In these works, acidactivated Tunisian clay was studied by Neji et al. [16] in the ethyl esterification of stearic acid and montmorillonite STx1-b was applied by Zatta et al. [15] and Santos et al. [17] in the ethyl esterification of lauric acid. Another activated clay, whose trade name is montmorillonite K10, is a solid acid generated by acid and thermal activation of montmorillonite, resulting in a high surface area material that contains Brönsted and Lewis acids sites. Although K10 takes the name montmorillonite, it is a mixture of amorphous materials, contaminated with crystalline quartz, mica, crystobalite and feldspar and is widely used as a base for the production of various catalysts employed in organic processes because it enhances efficiency and is safer to handle, non-corrosive, inexpensive and easily separated from the reaction medium [18]. Available reports regarding the use of montmorillonite K10 are mainly related to chemical modifications such as intercalation/pillarization, to the use as support for other catalysts [7,19,20] and to its application in petroleum catalytic cracking [21]. As for esterification reactions, montmorillonite-based clays including K10 were studied by Neji et al. [12], in the ethyl esterification of stearic, oleic and palmitic acid, and by Pires et al. [22], as catalysts for the methyl esterification of stearic acid and the transesterification of waste oils. However, scarce studies aimed to investigate how the carbon chain length and unsaturation presence affect the kinetics of the esterification reaction. In this context, the present work intends to use montmorillonite K10 to accomplish the ethyl esterification of lauric acid, oleic acid and stearic acid and use this set of data to adjust the kinetic parameters, allowing the simulation of the reaction kinetics for all the aforementioned FFAs. Initially, the catalyst characterization was performed, presenting data on the acidic properties of montmorillonite K10 that contributes to the few works of literature, which focus mainly on the intrinsic properties of the material [23,24]. Therefore, the results obtained in this work are expected to be used as a manner to predict the esterification

behavior, helping to evaluate the reactor design and accomplish the process prospecting. 2. Material and methods 2.1. Catalyst characterization The montmorillonite K10 used as catalyst was characterized prior to the studies. X-ray powder diffraction measurements (XRD) were obtained by placing the powdered material onto a neutral glass slide, using a Shimadzu XRD-6000 diffractometer with a Cu source (Ka = 1.5418 Å), between 3 and 90° (in 2h) at 30 mA and 40 kV, speed of 2° min1 and step of 0.02°. The infrared spectra (FTIR) were obtained in transmission mode with a Bio-Rad FTS 3500GX spectrometer, using KBr pellets with accumulation of 32 scans in the range of 4000–400 cm1 and resolution of 4 cm1. Nitrogen adsorption isotherms were obtained in a Quantachrome NOVA 2000e gas sorption analyzer. The samples were degassed at 250 °C under vacuum for three hours and the analysis was conducted at liquid nitrogen temperature (196 °C). The specific surface areas of the samples were calculated using the multi-point Brunauer-Emmet-Teller (BET) method and the volume and average pore size analyses were performed according to the Barrett-Joyner-Halenda (BJH) model. The identification of acid sites was accomplished by first heating a sample of 100 mg at 300 °C for 90 min and subsequently cooling it down to 120 °C. The sample was saturated with gaseous pyridine diluted in N2 for 60 min and then maintained under N2 flow for 60 min at 120 °C, in order to remove the physically adsorbed pyridine. The FTIR spectra were collected in the range of 1650–1350 cm1 [25]. The Lewis and Brönsted acid sites were identified by analysis of the FTIR spectra of the adsorbed pyridine, which generates species with vibrational frequency that are correlated to the sites. The catalyst was also evaluated for the presence of acid sites by thermal analysis measurement (TGA/DTA). TGA curves before (pure catalyst) and after the sorption of pyridine were generated and the number of acid sites was calculated from the difference between the masses of the samples. This value corresponds to the mass of sorbed pyridine, allowing the determination of the number of moles of pyridine per gram of sample [25]. The potentiometric titration technique was used to evaluate the solid’s acidic properties [26], in which approximately 50 mg of solid were suspended in 15 mL of acetonitrile (Vetec, 99.9%) and subjected to agitation for 3 h. Then the titration was conducted with a 0.05 molL1 solution of n-butylamine (Vetec, 98%) in acetonitrile. The change in electrode potential (Ag/AgCl) was measured with a digital Del-Lab microprocessed pH meter. With this technique, it was possible to estimate the strength and number of acidic sites present in the solid. The initial electrode potential (Ei) indicates the maximum strength of the acid sites, which can be classified according to the following scale: Ei > 100 mV (very strong sites); 0 < Ei < 100 mV (strong sites); Ei < 0 mV (weak sites), and Ei < 100 mV (very weak sites). The number of acidic sites, expressed as meq n-butylamine/g solid, was set as the value correspondent to the beginning of the constant potential region in the titration curve [27]. 2.2. Esterification reactions Esterification reactions were carried out using montmorillonite K10 (Sigma-Aldrich), lauric acid (Sigma-Aldrich, P98%), oleic acid (Sigma-Aldrich, P90%), stearic acid (Sigma-Aldrich, P95%) and anhydrous ethanol (Neon, P99.5%). All reagents were used as provided by the suppliers, without any additional treatment.

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Prior to the kinetics study, the effect of factors such as temperature, molar ratio and catalyst load over the ethyl esterification of lauric acid were evaluated using an experimental factorial design. These reactions were conducted in a Büchiglasuster Cyclone miniclave drive with a 50 mL pressure vessel. Both the stirring speed and the reaction time were previously determined by Zatta et al. [13] and were kept constant at 500 rpm and 2 h, respectively. After this period, the reactor was cooled down and its content was removed and treated as a single sample. The reactions of the kinetics study were performed using a 300 mL Parr bench top stirred reactor, with the same stirring speed as that used for the factorial design (500 rpm). The stirring speed is considered to be sufficient for the concentration of each species in the samples to represent the concentration of the reaction media inside the reactor, avoiding that the amount withdrawn had a major influence over the experimental conversion values. For this set of experiments, the amount of each reactant was previous estimated in such a way that the final reaction media volume corresponded from 70% (due to the stirrer height) to 80% (for safety reasons) of the total volume of the reactor (300 mL). Six samples of 2 mL were withdrawn from the reactor vessel at the desired reaction times (0, 30, 60, 120, 240 and 360 min for the non-catalytic reactions and 0, 30, 60, 90, 120, and 240 min for the K10-catalyzed reactions). Therefore, a total of 12 mL was withdrawn from the reactor, representing around 6% of the initial volume. After the sampling, the catalyst was recovered from the reaction mixture by centrifugation and the excess alcohol was removed by evaporation at 50 °C using an air circulation oven at atmospheric pressure. After this treatment, the free fatty acid content in each sample was determined. All experiments for both the factorial design and the reaction kinetics were performed considering the initial time of reaction as the moment when the desired temperature was reached inside the reactor. Furthermore, reactions without K10 were also carried out in order to evaluate the conversion gain provided by the catalyst over the non-catalytic reaction performance. The pressure inside the reaction vessel was considered to be equivalent to the vapor pressure of the most volatile component of the mixture, i.e., the alcohol used (ethanol). 2.3. Acidity analysis The free fatty acid content was determined using the official method of the American Oil Chemist’s Society (AOCS methodCa5a-40) [28], with some modifications [29]. For this analysis, the chemicals used were hydrated ethanol (Neon, P95.0%) and sodium hydroxide (Vetec, P97%). In previous studies performed by our research group, the conversion of methyl laurate was determined using this method and good correlations were found with highperformance liquid chromatography (HPLC) [30] (rmsd of 1.33%) and nuclear magnetic resonance (MASNMR) [31] (rmsd of 0.59%). After the titration with a standardized 0.1 molL1 solution of NaOH, accomplished in duplicates, the acidity value was calculated according to Eq. (1), which is adapted from the literature [29].

Acidity value ¼

MV f C m

ð1Þ

where M is the molar mass (g/mol) of the free fatty acid, V is the volume (L) of the standardized NaOH solution used in the titration, f is the concentration factor of the standard NaOH solution, C is the molar concentration of the NaOH solution (mol/L), and m is the sample mass (g). Since the FFAs were used without previous purification steps, it was necessary to determine their acidity value before the reactions, to prevent that impurities (mostly other FFAs) affected the

267

conversion values. Therefore, conversion values, X (%), were calculated by comparison between the acidity value of the fatty material before the esterification reaction (CFFA0) and acidity values of the samples that were withdrawn from the reactor (CFFA), following the relationship expressed by Eq. (2).

X ð%Þ ¼ 100 

ðCFFA0  C FFA Þ C FFA0

ð2Þ

2.4. Kinetic modeling The non-isothermal approach was taken into account for the kinetic experiments carried out in this work. After the heating was turned on, temperature values were collected along the time for each reaction kinetic run, from the ambient temperature until the desired reaction temperature. The temperature profiles were correlated using a sigmoid function, T(t), as shown in Eq. (3), and were later considered in the kinetic modeling.

TðtÞ ¼

a b þ ect

ð3Þ

where T(t) is the sigmoid function, defined in the interval 0 6 T(t) 6 1, a, b, and c are adjustable parameters determined for each experimental heating ramp, and t is the time (min), counted from the moment when the heating was turned on. The volume of reaction media, required in the kinetic modeling, was calculated from the sum of the specific volumes of each pure component considering an ideal mixture, once the specific volume values were obtained from the relationship between mass and density of each pure compound. Densities of all compounds involved in the reaction were estimated at the temperatures of the reaction media, which were determined according to values predicted by the heating ramp, up to the temperature in which the reaction was held. Therefore, water density was calculated according to Eq. (4) (DIPPR 116), while the densities of the other components were calculated according to Eq. (5) (DIPPR 105).

y ¼ A þ B  s0:35 þ C  s2=3 þ D  s þ E  s4=3 y¼

A B

1þð1sÞD

ð4Þ ð5Þ

where s ¼ 1  T=Tc, Tc is the critical temperature (K), T is the reaction media temperature (K), y is the molar density (kmol/m3), and A, B, C, D, and E are adjustable parameters. The parameters of water and ethanol were obtained from DIPPR whereas those of free fatty acids and its esters were obtained through a fit of experimental data found in literature [32,33], although an extrapolation was necessary to infer the densities for the temperatures used in our work. Critical temperatures of the free fatty acids and its esters were also obtained from the available database (DDIPR) or literature [34]. All these values are shown in Table 1. The esterification reactions related with biodiesel systems occur between a free fatty acid (FFA) and a short chain alcohol, in our case ethanol (E), producing a fatty acid ethyl ester (FAEE) and water (W). This reaction can be performed in the absence of a catalyst with a significant reaction rate, if enough temperature is provided into the reaction media. Therefore, the global reaction rate, rFFA (G), considers the contributions of both the thermal reaction (also referred to as non-catalytic reaction or reaction without the solid catalyst), rFFA (NC), and the montmorillonite K10-catalyzed reaction, rFFA (C), according to Eq. (6).

rFFA ðGÞ ¼ rFFA ðNCÞ þ rFFA ðCÞ

ð6Þ

The mechanism for the non-catalytic reaction used in this work is the same earlier proposed in the literature [35,36], which considers the esterification reaction without any catalyst to be

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Table 1 Parameters used in this work for the density calculation of pure components. Component a

Water Ethanolb,c Lauric acidd,e Stearic acidd,e Oleic acidd,e Ethyl lauratef,g Ethyl stearatef,g Ethyl oleatef,g a b c d e f g

Tc (K)

A

B

C

D

E

647.09 514.00 742.68 796.65 795.17 722.90 782.00 781.54

17.863 1.6288 0.0768 0.0775 0.0676 0.0846 0.0555 0.0728

58.606 0.2747 0.1200 0.1543 0.1337 0.1349 0.1286 0.1459

95.396 – – – – – – –

213.89 0.2317 0.1773 0.1011 0.2018 0.2119 0.2178 0.2325

141.26 – – – – – – –

DIPPR Equation 116. Tc available at NIST database. Parameters adapted from DIPPR Equation 105. Tc presented in the work by Sales-Cruz et al. [34]. Parameters fitted using experimental data obtained by Noureddini et al. [33]. Tc obtained using PC-SAFT (Perturbed Chain form of the Statistical Associating Fluid Theory). Parameters fitted using experimental data presented by Pratas et al. [32].

autocatalytic, since the FFA act as an acid catalyst. Thus, the FFA concentration is considered in both sides of the reaction equation, as presented in Eq. (7), which provides the reaction rate expression of the non-catalytic reaction for the limiting reactant (FFA). In Eq. (8), the kinetic model is written in terms of concentrations, since it is more suitable for reactor design, simulation and optimization. This approach takes into account a simplification hypothesis that the volume inside of the reactor remained approximately constant. k1

FFA þ E þ FFA ¢ FAEE þ W þ FFA k2

rFFA ðNCÞ ¼

dCFFA ¼ k1  C2FFA  CE  k2  CW  CFAEE  CFFA dt




rFFA ðCÞ ¼

OF ¼

dCFFA ¼ dt

M X N X j

CW CFAEE qcat  keff cat  CFFA  CE  KC

2

ð9Þ

ð10Þ

i

CFFA ¼ CFFA0  ð100  XFFA Þ

rmsdðx100Þ ¼ 100 

Moreover, the K10-catalyzed term in Eq. (6) followed the kinetic model proposed by Eley and Rideal for heterogeneous catalysis, with the hypothesis that the reaction takes place between a molecule of ethanol adsorbed in the catalyst surface and another of the free fatty acid from the bulk phase [37]. The reaction rate expression for the montmorillonite K10-catalyzed reaction is presented in Eq. (9), which contemplates adsorption/desorption coefficients (Ki) of the species on the catalyst surface. To proceed the kinetic modeling, the same methodology recently used in our research group by Santos et al. [17] was applied to the kinetic data obtained in this work, as well as the same software, algorithms and procedure for the kinetic parameters fitting. In summary, reaction kinetic constants were initially estimated for the non-catalytic reactions (using Eq. (8)) and then for the catalytic reactions (using Eq. (9)), considering the set of data obtained for all FFAs. The stochastic optimization technique of Particle Swarm Optimization (PSO) was used to estimate the initial guesses of the parameters and therefore the fminsearch subroutine of the software MatlabÒ was used to minimize the objective function (Eq. (10)). The experimental data of fatty acid conversion (%) were used as input in Eq. (11), which is an adapted form of Eq. (2), providing values of FFA concentration along the time in the reaction kinetics study, as well as concentrations of the other components. These values were used in Eq. (8) or (9) to calculate the differential equations using ODE23 s subroutine of MatlabÒ until the objective function (Eq. (10)) reached its minimum value. Arrhenius equation was adjusted concomitantly, in order to ensure the kinetic parameters (pre-exponential factors and activation energies) satisfy this equation as well. To evaluate the correlation between the kinetic modeling and the experimental data obtained the root mean square deviation, rmsd (x100), was calculated using Eq. (12).

  Pnc 1 þ i Ki  C i

Calc ðXExp FFA; ji  XFFA; ji Þ

ð7Þ

ð8Þ



ð11Þ

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uXM XN 2 Calc u ðXExp t j FFA; ji  XFFA; ji Þ i S

ð12Þ

where qcat = mcat/V, kcat ¼ kcat;1  ðcFFA  cE = C2t Þ and Ki = Kia  ci/Ct, eff

KC ¼

eff k1 =

eff k2 ,

XExp FFA;ji

and XCalc FFA;ji are the conversions (%) obtained

experimentally and predicted by the kinetic modeling, respectively, for each sample i of each kinetic j obtained, and S is the total number of experimental data obtained. 3. Results and discussion 3.1. Montmorillonite K10 characterization The X-ray diffraction pattern of the K10 catalyst (Fig. 1A) shows the peaks attributed to the impurities of quartz (Q), mica (M), crystobalite (C) and feldspar (F). Montmorillonite was identified with two main peaks attributed to the plane (0 2 0) and (2 0 0) [38,39], indicating that the layered structure of the montmorillonite was practically destroyed due to the montmorillonite K10 activation process. The absence of basal diffraction peaks attributed to layers packing is a result of the activation process and contributes to make the acid sites readily accessible for the adsorption of the reactants, allowing the reaction to take place [15]. Vibration bands are observed in the montmorillonite K10 FTIR spectrum (Fig. 1B) at 3628 and 3440 cm1 for OAH stretching of structural hydroxyl groups and hydration water molecules, respectively. SiAO vibrations are observed at 1045 cm1 (out of plane stretching); AlAAlAOH deformation at 915 cm1; free silica impurities at 793 cm1 and 690 cm1 (as verified by the XRD in Fig. 1A); AlAO and SiAO coupling out of the plane at 615 cm1; and AlAOASi and SiAOASi deformations at 526 and 468 cm1 respectively [40]. The presence of acidic sites in the montmorillonite K10 catalyst was qualitatively evaluated by analyzing the FTIR spectrum for the adsorbed pyridine and quantitatively by TGA/DTA analysis of the desorbed amount of pyridine (data not shown), in addition to the potentiometric titration with n-butylamine. The montmorillonite K10 catalyst has well defined vibration peaks in the FTIR

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Volume @ STP (cm3/g)

Intensity (a. u.)

MMT

M

C

M

5

50

A

Q

10

15

20

F

25

MMT

30

35

40

2 Theta (Degrees)

40 30 20 10 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0) Fig. 2. Isotherm of adsorption/desorption of nitrogen for the K10 catalyst.

690

793

615

915

3440

3628

526

4000

3000

1000

468

1045

Transmittance (a. u.)

B

400

Wavenumber (cm-1) Fig. 1. X-ray diffraction pattern (A) and FTIR spectra (B) of the K10 catalyst. M, MMT, Q, C e F represent reflections of mica, montmorillonite, quartz, crystobalite and feldspar, respectively.

spectrum of adsorbed pyridine relating to Lewis (1490, 1590 and 1625 cm1) and Brönsted acid sites (1545–1640 cm1). The band at 1440 cm1 indicates physisorbed pyridine. The vibrations observed for this material are in agreement with literature reports [41]. By the potentiometric titration technique the number of acidic sites was 278 leqg1 (n-butylamine/mass of the catalyst) (data not shown). The initial potential of the montmorillonite K10 catalyst measured in the potentiometric titration is 309 mV, which indicates the presence of very strong acidic sites. The presence of a large number of Lewis and Brönsted acid sites indicate that montmorillonite K10 might be an effective catalyst for esterification reactions, following the results presented by Zatta et al. [15] regarding to acid leached clay-based catalyst. The textural properties determined for the montmorillonite K10 material used in this work showed the following values: surface area of 268.4 m2g1, pore volume of 0.35 cm3g1 and average pore radius of 17.4 Å. These data are in accordance with the specifications of the supplier [42]. Fig. 2 shows the isothermal adsorption/desorption of nitrogen by the K10 catalyst. The aspect of the curve is very similar to that reported in the literature, where the curve and hysteresis are of type IV and H3, respectively, according to IUPAC classifications and reports in the literature [43–45]. The opening of the curve at the relative pressure close to 0.4 indicates the presence of small mesopores in the adsorbate [43,44] and the hysteresis indicated particles in the form of plaques giving rise to pores in the form of slots [46], also indicating the presence of mesoporosity [47]. 3.2. Esterification reactions A factorial design for lauric acid ethyl esterification reactions using commercial montmorillonite K10 as a catalyst was performed.

Table 2 shows the conversions obtained while Table 3 summarizes the effects of the analyzed variables and also shows a cube-shaped diagram with the results obtained in the factorial design. In all conditions of the factorial design, esterification reactions in the presence of montmorillonite K10 had significantly higher conversions than those obtained in the blank reactions (without catalyst) under the same experimental conditions. Values of conversion for thermal reactions are also presented in Table 2. A remarkable conversion value was obtained in this study in Reaction 14 (95.06% of conversion in terms of fatty acid), whose conditions were 160 °C, RM = 12:1 and catalyst load of 12%, representing a conversion gain of 51.42 percentage points in relation to the non-catalytic reaction. Variable effects in the factorial design shown in Table 3 indicated that the first-order effects of temperature and catalyst load contributed significantly to reach higher conversions into ethyl laurate. The positive contribution provided by higher temperatures in the reaction conversions can be verified by comparing the experiments performed at 140 °C and 160 °C, while the same trend can be observed for the catalyst amount when comparing the conversions obtained for catalyst amounts of 8% and 12%. On the other hand, alcohol to fatty acid molar ratio (alcohol:FFA) presented a negative effect over the conversions, suggesting that a great excess of alcohol tends to be disadvantageous to the reaction. The second- and third-order effects were significant but with smaller magnitude compared to first-order effects. The analysis of variance (ANOVA) was performed for the previously mentioned factorial design, providing a coefficient of determination (R2) of 0.731, meaning the mathematical model explains 73.1% of the data. Regardless the difference between the values predicted by the statistical model and those obtained experimentally, the factorial design results were used to choose the reaction conditions of the kinetics study performed later. Concerning to the kinetics study, three different temperatures were evaluated to perform the kinetic parameters fitting (preexponential factor and activation energy in the Arrhenius equation), keeping constant both the catalyst amount (12%) and the alcohol to fatty acid molar ratio (6:1). The molar ratio of 6:1 was chosen in spite the slightly higher conversion presented when a MR of 12:1 was used, since a real industrial process whose reaction was carried out with a higher MR would require a larger reaction vessel. Other conditions were tested at the central temperature, 160 °C, whereas the molar ratio and the amount of catalyst were varied to 12:1 and 12%, respectively. Kinetic data for noncatalytic ethyl esterification reactions of oleic acid were measured later in this study, in order to evaluated the real contribution of montmorillonite K10 on the global reaction. All conditions of the kinetic reactions performed during this phase are presented in Table 4.

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Table 2 Experimental conditions and results obtained for lauric acid ethyl esterification using the montmorillonite K10 catalyst. Reaction

T (°C)

MR

Cat. (%)

Acidity (%)

Conversion (%)

Conversion gain (p.p.)

R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16

140 160 150 140 160 140 160 140 160 140 160 140 160 150 150 150

6:1 6:1 9:1 12:1 12:1 6:1 () 6:1 () 12:1 (+) 12:1 (+) 6:1 () 6:1 () 12:1 (+) 12:1 (+) 9:1 (0) 9:1 (0) 9:1 (0)

0 0 0 0 0 8 () 8 () 8 () 8 () 12 (+) 12 (+) 12 (+) 12 (+) 10 (0) 10 (0) 10 (0)

50.15 ± 1.19 28.30 ± 0.47 47.88 ± 0.92 65.88 ± 0.94 53.36 ± 1.37 25.99 ± 0.49 14.13 ± 0.18 36.79 ± 0.18 21.97 ± 0.09 15.66 ± 0.39 6.09 ± 0.24 29.84 ± 0.08 4.94 ± 0.46 7.24 ± 0.07 6.84 ± 0.10 7.44 ± 0.14

49.85 71.70 52.12 34.12 43.64 74.01 85.87 63.21 78.03 84.34 93.91 70.16 95.06 92.76 93.15 92.56

– – – – – 24.16 14.17 29.09 34.39 34.49 21.61 36.04 51.42 40.64 41.03 40.44

() (+) () (+) () (+) () (+) (0) (0) (0)

MR = molar ratio alcohol:FFA; Cat. = catalyst load as percentage of the mass of FFA; the symbols +,  and 0 represent the major, minor and focal point of the factorial planning; p.p. = percentage points. The acidity is the percentage of fatty acids present in the sample, determined according to the standard AOCS Ca-5a-40.

Table 3 Results of factorial design 23 for the ethyl esterification of lauric acid using the K10 catalyst. Graphical representation

Factorial design effects Factor

Effect

S.D.

t8  S.D.

R2 (ANOVA)

Global average T MR Cat TxMR TxCat MRxCat TxMRxCat

83.91 15.29 7.92 10.59 4.57 1.95 1.40 3.09

3.15 7.39 7.39 7.39 7.39 7.39 7.39 7.39

7.26 17.04 17.04 17.04 17.04 17.04 17.04 17.04

0.7308 (73.08%)

T = temperature in °C; MR = molar ratio alcohol:FFA; Cat = catalyst load as percentage of the mass of FFA; SD = standard deviation; R2 = coefficient of determination of the model; t8 - likely point of the t distribution with t = 8 degrees of freedom and 95% confidence = 2.306.

Table 4 Esterification reaction conditions performed during the kinetics study. Reaction

FFA

T (°C)

MR

Cat (%)

C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 NC1 NC2 NC3 NC4 NC5 NC6

Lauric acid Lauric acid Lauric acid Stearic acid Stearic acid Stearic acid Oleic acid Oleic acid Oleic acid Lauric acid Lauric acid Stearic acid Stearic acid Oleic acid Oleic acid Lauric acid Stearic acid Oleic acid Oleic acid Oleic acid Oleic acid

140 160 180 140 160 180 140 160 180 160 160 160 160 160 160 160 160 160 140 180 160

6:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1 12:1 6:1 12:1 6:1 12:1 6:1 6:1 6:1 6:1 6:1 12:1

12 12 12 12 12 12 12 12 12 6 12 6 12 6 12 0 0 0 0 0 0

In order to assess the complete profile of the temperature during the reaction, a two-step equation approach was considered to take into account the heating of the reactant system, using

Eq. (3). Step one was considered from the initial temperature of the reaction media until 120 °C and step two from this temperature until the desired reaction temperature. Heating ramps for each reaction kinetics were adjusted separately, generating different set of parameters for each run. All ramps are presented in Supplementary Material (Fig. A-C), showing the experimental data and values predicted by the model. This modeling was necessary in order to predict the reactants consumption from the moment when the batch reactor was closed and the heating was turned on, until the temperature reached the desired value for the reaction. Finally, the experimental kinetic data obtained for montmorillonite K10-catalyzed and for the non-catalytic esterification reactions considering all the free fatty acids tested in this study (lauric, stearic and oleic acid) were used to estimate a single set of both kinetic and equilibrium constants of the kinetic model proposed. Experimental and calculated values of FFA conversions are presented in Figs. 3–5 and the kinetic parameters fitted, as well as the rmsd, are presented in Table 5. The first conclusion that can be drawn by the low rmsd (x100) value, around 3.5%, is that the FFAs tested in this work are indeed very similar in what concerns to the reaction kinetics. Experimental data showed that an alcohol to fatty acid molar ratio of 12:1 led to values of conversion slightly lower than those obtained for a MR of 6:1. This observation is in accordance with the results reported by several authors, such as Zatta et al.

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100

100

A

90

60 50 40

Kinetics: C1 C2 C3

30 20 10 0

50

100

150

200

250

Conversion (%)

Conversion (%)

80

70

0

A

90

80

70 60 50

Kinetics: C2 NC1 C10 C11

40 30 20 10 0

300

0

50

100

150

t (min)

B

90

60 50 40

Kinetics: C4 C5 C6

30

Conversion (%)

Conversion (%)

70

10

400

450

70 60 40 30 20 10 0

0

50

100

150

200

250

Kinetics: C5 NC2 C12 C13

50

300

0

50

100

150

200

250

300

350

400

450

t (min)

t (min) 100

100

C

90

C

90

80

Conversion (%)

80

Conversion (%)

350

80

20

70 60 50

Kinetics: C7 C8 C9

40 30 20

70 60 Kinetics: C8 NC3 C14 C15 NC6

50 40 30 20 10

10

0 0

50

100

150

200

250

300

t (min)

D

90

0

50

100

150

200

250

300

350

400

450

t (min) Fig. 4. Effect of MR and catalyst load at 160 °C for (A) lauric acid, (B) stearic acid, and (C) oleic acid. The symbols stand for experimental data and lines represent values calculated using the kinetic model. A MR = 6:1 was used to perform the following kinetics: C2, C5 and C8 (Cat = 12%), C10, C12 and C14 (Cat = 6%), and noncatalytic kinetics NC1, NC2 and NC3 (Cat = 0%). A MR = 12:1 was used to perform the following kinetics: C11, C13 and C15 (Cat = 12%) and non-catalytic kinetic NC6 (Cat = 0%).

100 80

Conversion (%)

300

B

90

80

0

250

t (min) 100

100

0

200

70 60 50 Kinetics: NC3 NC4 NC5

40 30 20 10 0

0

50

100

150

200

250

300

350

400

450

t (min) Fig. 3. Effect of temperature over K10-catalyzed reactions with MR = 6:1 and Cat = 12%: (A) Lauric acid, (B) Stearic Acid, and (C) Oleic acid, and (D) Non-catalytic reactions of oleic acid. The symbols stand for experimental data and lines represent values calculated using the kinetic model. Kinetics C1, C4, C7 and NC4 were obtained at T = 140 °C, kinetics C2, C5, C8 and NC3 were obtained at T = 160 °C, and kinetics C3, C6, C9 and NC5 were obtained at T = 180 °C.

[14,15] and Minami and Saka [35], suggesting the reaction has an autocatalytic effect in the absence of montmorillonite K10. Since the FFA itself is responsible to catalyze these type of reactions, higher amounts of alcohol lead to the dilution of the catalyst (FFA) causing a conversion loss, which was also observed in the factorial design evaluation. However, when montmorillonite K10 is present in the reaction media, the heterogeneous catalyst seems to supplant the autocatalytic effect. The low rmsd value also allows confirming these hypotheses, since the proposed kinetic model is capable to adequately correlate the experimental data. Fig. 3 depicts a comparison between experimental and calculated values of conversion in the ethyl esterification of lauric acid

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100 80

80

70 60 50

Kinetics: C1 C4 C7

40 30 20

70 60 50 30 10

0

50

100

150

200

250

0

300

t (min)

80

70 60 50 40

Kinetics: C2 C5 C8

30 20 10

200

250

300

60 50

Kinetics: C10 C12 C14

40 30 20 10

0

50

100

150

200

250

0

300

80

50

100

150

90

250

300

F

80

Conversion (%)

70 60 50

Kinetics: C3 C6 C9

40 30 20

70 60 50 40

Kinetics: NC1 NC2 NC3

30 20

10

10 0

200

100

C

90

0

t (min)

100

Conversion (%)

150

70

t (min)

0

100

E

90

Conversion (%)

Conversion (%)

80

50

100

B

90

0

t (min)

100

0

Kinetics: C11 C13 C15

40 20

10 0

D

90

Conversion (%)

Conversion (%)

100

A

90

50

100

150

200

250

300

t (min)

0

0

50

100 150 200 250 300 350 400 450 500

t (min)

Fig. 5. Comparison between FFAs at MR = 6:1, Cat = 12% and temperatures of (A) 140 °C, (B) 160 °C, and (C) 180 °C, (D) MR = 12:1, Cat = 12% and T = 160 °C, (E) MR = 6:1, Cat = 6% and T = 160 °C, and (F) MR = 6:1, Cat = 0 and T = 160 °C. The symbols stand for experimental data and lines represent values calculated using the kinetic model. Square markers represent lauric acid data, triangle markers are related to stearic acid and circle symbols to oleic acid.

Table 5 Kinetic parameters estimated for K10-catalyzed and non-catalytic esterification of fatty acids with ethanol. Non-catalytic k0 (1) (L mol1 min1) k0 (2) (L mol1 min1) Ea (1)/R (K) Ea (2)/R (K) rmsd (%)

Catalyzed 9649.66 2829.64 6993.42 6813.02 3.48

kcat (L mol1 min1) Ea/R (K) KE (L2 mol1 min1)

4.82E+03 7.21E+03 7.00E01

rmsd (%)

3.34

(Fig. 3A), stearic acid (Fig. 3B), and oleic acid (Fig. 3C), in which the reactions were performed at different temperatures and with catalyst load of 12%, as well as the results for non-catalytic ethyl esterification of oleic acid (Fig. 3.D). Experimental conversion data are presented in all Figures as the mean value and whiskers values correspond to mean value ± standard deviation, obtained after titration analyses performed in duplicates.

Considering only the reactions with montmorillonite K10, all FFAs have presented the same behavior. At higher temperature conditions, higher conversions of FFAs were achieved. Experimental data acquired for oleic acid does not enable to evaluate if the equilibrium conversion at 140 °C and catalyst load of 12% would reach the same value of those obtained at 160 °C and 180 °C. However, once the kinetic model and the adjusted set of parameters were capable to properly describe the behavior of all FFAs tested, it was possible to simulate results which allowed the observation of a tendency that after around 3000 min, the same equilibrium conversion is reached for all temperatures (Supplementary Material – Fig. D). Thus, the equilibrium conversion is not shifted with the temperature, but rather the initial reaction rate of consumption of the FFA. These results also confirm the endothermic nature of this reaction for all FFAs tested, what is in good agreement with other works [10,17]. Concerning to the effect of the catalyst load, presented in Fig. 4, non-catalytic reactions (without montmorillonite K10) have

L.R.S. Kanda et al. / Fuel 193 (2017) 265–274

presented lower conversion values for all FFAs, confirming the positive catalytic effect of montmorillonite K10 over the esterification of FFAs. Furthermore, in the conditions tested in this work, it is observed that the use of higher catalyst load (12%) is advantageous for the reaction rates, rather than the use of 6% of catalyst. This is due to the availability of more acid sites with the more catalyst load, allowing the reaction to take place readily. Industrially, the use of higher catalyst load does not necessarily constitute a disadvantage, since clay-based catalysts, so as other solid catalysts, tend to be easily recovered and reused in the reaction [13–15,17]. Simulations concerning to the effect of the catalyst load over the reaction rate and the equilibrium conversion were also possible to be performed. For this simulation, the heating ramp used was chosen as the same of reaction C8. From the calculated results it can be seen that the catalyst amount does not affect the equilibrium conversion, but rather increases the reaction rate, as it is widely known, confirming the consistency of the model. Furthermore, the model simulation have indicated that the time required for the equilibrium to be achieved shortens with higher catalyst loads (Supplementary Material – Fig. E). Since the model input for the catalyst load is in terms of concentration (g/L), every molar ratio change has an important impact over the catalyst concentration. Thus, the observation of raw experimental data presents a consequence of the effect of the dilution of the catalyst. This affirmation is confirmed by the fact that the kinetics obtained at MR of 12:1 and catalyst load of 12% presented higher conversions, for all FFA tested. Therefore, simulations for changes in the MR with the acquired data were also carried out, considering both the K10-catalyzed and the noncatalytic (without K10) reactions (Supplementary Material – Fig. F and G). The simulation of the catalytic esterification of oleic acid was performed using a catalyst load of 12% (nearly 42.69 g/L, in case of the oleic acid reactions). Several values of MR were tested and the simulation showed that an increase in this variable led to higher initial reaction rates until a MR of 6:1 and then starts to slightly decrease beyond this value. Furthermore, the reaction presents a tendency to reach the equilibrium after longer times when the MR is increased. The non-catalytic reaction simulation presented the same behavior, showing that higher MR requires more time to reach the equilibrium and also that the equilibrium conversion increases with an increase in the MR. The equilibrium is reached after several hours of reaction, also confirming the catalytic effect of K10 in the reactions of esterification of free fatty acids with ethanol. Since all the previous assessments imply that the behavior of the different FFAs tested are very similar, a graphical comparison between the kinetics of all the FFAs was compiled in Fig. 5. Differences in the heating ramps accounted for the higher differences between the results (as it can be noticed for the kinetics obtained at 140 °C), whereas similar heating ramps provided nearly identical results, with a minor difference for the lauric acid, which achieved slightly higher conversions. In a general way, kinetics data of FFA with different degrees of unsaturation and chain lengths presented very similar results, as the kinetic modeling have shown by the low value of the rmsd achieved. It is worth mentioning that the model parameters were fitted using the whole set of experimental data obtained for these different FFAs and it turns out that the model was capable to correlate very well all the experimental data set. The modeling results obtained in this work demonstrate that kinetic information of few representative FFAs is enough to provide a robust and reliable kinetic model. Moreover, it ascertain the possibility that the kinetic model obtained could be used in the design of industrial equipment to operate with much more complex raw material composed by FFAs mixtures without a significant lack of quality in the reactor design.

273

4. Conclusion Additional characterization analysis to those already available in the literature for the commercial montmorillonite K10 catalyst is presented in this article. Among the results, we highlight the textural and acidic properties. This material has high surface area and high density of acid sites (Lewis and Brönsted), awarding good catalytic activity to the material towards a large number of organic reactions, including fatty acid ethyl esterification reactions. Regarding to its application as a catalyst in fatty acid esterification processes, the material is catalytically active and provided high conversions of free fatty acids to fatty acid ethyl esters. Considering the temperature range investigated in this work, the endothermic nature of this type of reaction was also confirmed by both the experimental results and simulated data. Moreover, the catalyst dilution was proven to have a major effect over the conversion of FFA into FAEE, since it affects both the reaction rate and the equilibrium conversion. Results obtained for ethyl esterification of lauric, stearic and oleic acids have shown good consistency and also have indicated the possibility that a process to operate with a complex fatty material could be designed based on information gathered for only a few FFAs. This work also contributes to shed light on the fact that the esterification reaction is autocatalytic (the FFA act as a catalyst), what implies the necessity to evaluate the optimum molar ratio of this kind of reaction, since larger amounts of the alcohol would shift the equilibrium towards the product formation only until a certain value. The possibility found of using the montmorillonite K10 catalyst in the processing of fatty materials with low quality, i.e., with high acidity, eliminating the need of pretreatment steps of the fatty precursor material, can significantly reduce production costs and brings the possibility for the use of such catalyst in industrial scale. Acknowledgements The authors gratefully acknowledge the Brazilian funding agencies CNPq (Proc. Num. 406737/2013-4 and 303846/2014-3), CAPES and Fundação Araucária for the financial support of this work. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2016.12.055. References [1] Liu W, Yin P, Liu X, Chen W, Chen H, Liu C, et al. Microwave assisted esterification of free fatty acid over a heterogeneous catalyst for biodiesel production. Energy Convers Manage 2013;76:1009–14. [2] Han X, He Y, Hung C, Liu L. Efficient and reusable polyoxometalate-based sulfonated ionic liquid catalysts for palmitic acid esteri fi cation to biodiesel. Chem Eng Sci 2013;104:64–72. [3] Melero JA, Bautista LF, Iglesias J, Morales G, Sánchez-vazquez R. Production of biodiesel from waste cooking oil in a continuous packed bed reactor with an agglomerated Zr-SBA-15/bentonite catalyst. Appl Catal B Environ 2014;145:197–204. [4] Jasen P, Marchetti JM. Kinetic study of the esterification of free fatty acid and ethanol in the presence of triglycerides using solid resins as catalyst. Int J LowCarbon Technol 2012;7:325–30. [5] Tesser R, Di Serio M, Guida M, Nastasi M, Santacesaria E. Kinetics of oleic acid esterification with methanol in the presence of triglycerides. Ind Eng Chem Res 2005;44:7978–82. [6] Oliveira CF, Dezaneti LM, Garcia FAC, de Macedo JL, Dias JA, Dias SCL, et al. Esterification of oleic acid with ethanol by 12-tungstophosphoric acid supported on zirconia. Appl Catal A Gen 2010;372:153–61. [7] Nandiwale KY, Bokade VV. Process optimization by response surface methodology and kinetic modeling for synthesis of methyl oleate biodiesel over H 3 PW 12 O 40 anchored montmorillonite K10. Ind Eng Chem Res 2014;53:18690–8.

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