RETRACTED: Kinetic and theoretical study of the conversion reactions of methyl oleate with glycerol on MgO

Molecular Catalysis xxx (xxxx) xxx–xxx

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Kinetic and theoretical study of the conversion reactions of methyl oleate with glycerol on MgO C.A. Ferretti, J.I. Di Cosimo

⁎

Catalysis Science and Engineering Research Group (GICIC), INCAPE, UNL-CONICET, CCT CONICET Santa Fe, Colectora Ruta Nac. 168, km 0, Paraje “El Pozo”, 3000, Santa Fe, Argentina

A R T I C LE I N FO

A B S T R A C T

Keywords: Glycerolysis Transesterification Kinetic modeling DFT Monoglycerides

The reaction kinetics of the liquid-phase synthesis of glyceryl monooleates (monoglycerides, MG) and dioleates (diglycerides, DG) from methyl oleate (a fatty acid methyl ester, FAME) and glycerol (Gly) was studied on MgO. The reaction occurs in consecutive steps, with the MG formation by glycerolysis of FAME being the first step, followed by the transesterification of MG with FAME toward DG. The reaction system was investigated at different temperatures and reactant ratios. MG yields of up to 62% were obtained at 503 K using a Gly/FAME molar ratio of 4.5. A molecular modeling based on DFT calculations was used to study the Gly and FAME adsorptions and the reaction mechanism toward MG on a MgO (100) surface. Results showed that the adsorption of hydrophilic Gly on MgO is much stronger than that of hydrophobic FAME. Two transitions states and one tetrahedral intermediate participate in the reaction pathway toward MG. Based on the DFT results, a five-step heterogeneous Langmuir-Hinshelwood-Hougen-Watson mechanism and kinetic model were postulated considering that the reaction occurs between an adsorbed Gly molecule and a FAME molecule in the liquid phase. After preliminary discrimination based on initial rates, the surface reaction steps toward MG and DG were taken as rate-limiting. The model predicts the consecutive formation of DG after glycerolysis of FAME toward MG. In addition, the model anticipates that the MG/DG ratio will increase with temperature in agreement with the catalytic results and that the only relevant adsorption enthalpy is that of Gly. The kinetic parameters derived from regression of the experimental data at different temperatures satisfactorily describe also the catalytic results obtained at different Gly/FAME ratios.

1. Introduction Acylglycerols, the glycerol esters of fatty acids, such as monoglycerides (MG) and diglycerides (DG) are widely used as emulsifiers, surfactants, texturizing agents, flavor blooming ingredients and lubricants in many food, pharmaceutical, cosmetic, plasticizer and detergent formulations [1,2]. These compounds present amphiphilic character because they combine in the molecule a hydrophilic head (the ester function) with a lipophilic tail (usually a C11–C17 carbon chain). The base-catalyzed transesterification of fatty acid methyl esters (FAME) with glycerol (Gly), known as glycerolysis of FAME, Scheme 1, is the preferred pathway to synthesize these compounds for fine chemistry purposes [3]. During the glycerolysis reaction, Gly and FAME initially produce a mixture of monoester isomers (1- and 2-MG) and methanol; consecutive transesterification steps involving more FAME molecules lead to DG (1,2 and 1,3-DG isomers) or even triglycerides (TG). Recently, we reported our investigations on the glycerolysis of ⁎

methyl oleate (FAME) with Gly using solid catalysts with different acidbase properties; we demonstrated that the reaction proceeds faster on solid bases, with MgO being the most active catalyst [4]. Later, we investigated the chemical nature of the MgO sites responsible for the catalytic activity and concluded that the reaction is mainly promoted by strongly basic coordinatively unsaturated oxygen anions [5]. The reaction was further studied both, experimentally under different reaction conditions and by density functional theory (DFT); the molecular modeling elucidated the preferential formation of 1-MG regardless of the reaction temperature and Gly/FAME ratio [6]. Lately and due to the growth of the production of first generation biodiesel, many studies on the kinetics of oil transesterification have been published, either using homogeneous, heterogeneous or enzymatic catalysis. However, for substrates other than oil, the contributions dealing with reaction mechanism and kinetics under heterogeneous catalysis conditions are rare [7–12]. None of these previous contributions discusses a heterogeneous kinetic model for a system of complex reactions combining glycerolysis followed by other

Corresponding author. E-mail address: dicosimo@fiq.unl.edu.ar (J.I. Di Cosimo).

https://doi.org/10.1016/j.mcat.2018.02.019 Received 3 August 2017; Received in revised form 15 February 2018; Accepted 16 February 2018 2468-8231/ © 2018 Elsevier B.V. All rights reserved.

Please cite this article as: Ferretti, C.A., Molecular Catalysis (2018), https://doi.org/10.1016/j.mcat.2018.02.019

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Scheme 1. Reaction scheme for the glycerolysis of methyl oleate (FAME) toward monoglycerides and consecutive transesterification reactions toward other glyceride products (acylglycerols). R: C17H33 (lipophilic tail). 0 FAME ratio (W / nFAME ) of 8 g/mol. More details are given elsewhere [14]. Prior to the catalytic test, MgO was pretreated ex situ at 773 K for 6 h in a N2 flow to remove adsorbed CO2 and water and then kept at 373 K under N2 atmosphere until use. The reactant mixture containing Gly and FAME was heated up under stirring (700 RPM). In some experiments FAME was diluted in pharmaceutical-grade liquid petrolatum (Vaseline). When the reaction temperature was reached, the catalyst was loaded to the reactor to start the reaction. During the 8 h reaction, samples were collected after specific time intervals, silylated and finally analyzed by gas chromatography (GC). Details of the analytical procedure are given elsewhere [15]. Quantification of samples was carried out in a SRI 8610C GC with a FID, on column injector port and HP-1 Agilent Technologies 15 m x 0.32 mm x 0.1 μm capillary column. Reaction products were 1- and 2-glyceryl monooleates (monoglycerides, MG) and 1,2 and 1,3-glyceryl dioleates (diglycerides, DG). Glyceryl trioleates (triglycerides, TG) were not observed under these reaction conditions. Conversion of FAME (XFAME) and Gly (XGly), selectivity (S) and yield (Y) of products were calculated using the following equations (nj = mol of product j; MG, total monoglycerides (both isomers); DG, 0 0 total diglycerides (both isomers); nFAME and nGly are the number of moles of FAME and Gly in the reactor at t = 0, respectively):

transesterification reactions. In this work, we continue our investigations on the kinetics of the MgO-promoted reactions between methyl oleate and glycerol toward MG and DG. We postulate a heterogeneous kinetic model that describes the experimental results at different reaction temperatures and reactant ratios; the reaction mechanism was based on theoretical results and previous experimental work [5,6]. The activation energies of MG and DG formation were estimated as well as the adsorption enthalpies of the different species. The kinetic model adequately describes the consecutive nature of the reactions involved, with MG formed by glycerolysis being the primary products and DG, formed consecutively by transesterification of FAME with MG, being the final products. 2. Experimental 2.1. Catalyst synthesis and characterization High surface area MgO catalyst was prepared by hydrating commercial MgO (Carlo Erba, 97%) with distilled water and subsequent decomposition of the resulting Mg(OH)2 precursor in a N2 flow at 773 K, as reported earlier [13]. The MgO obtained was sieved and for the catalytic experiments the particles with an average size of 177–250 μm were used. The prepared MgO catalyst was characterized by several techniques such as N2 physisorption at 77 K, X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR) and temperature programmed desorption (TPD) of CO2. More details are given elsewhere [4,14]. The density and the strength distribution of the MgO basic sites were quantified by TPD of CO2. In brief, after pretreatment at 773 K and cooling to 298 K, the sample was exposed to a 3% CO2/N2 flow; weakly adsorbed CO2 was removed then by flushing with N2. Later, CO2 desorption was carried out at a heating rate of 10 K/min in a N2 flow. CO2 was detected with a flame ionization detector (FID) after conversion into CH4 on methanation catalyst. The chemical nature of MgO basic sites was analyzed by FTIR of CO2 adsorbed at room temperature and evacuated at increasing temperatures. Data were collected in a Shimadzu FTIR Prestige-21spectrophotometer.

XFAME (%) =

nMG + 2nDG + 3nTG nMG + 2nDG + 3nTG + nFAME

XGly (%) = XFAME (%) × SMG (%) = SDG (%) =

0 nFAME 0 nGly

× ⎡SMG + ⎣

nMG nMG + 2nDG + 3nTG 2nDG nMG + 2nDG + 3nTG

Yj (%) =

1

2 SDG

× 100 +

1

3 STG ⎤

⎦

× 100 × 100

XFAME (%) × Sj (%) 100

2.3. Computational details Theoretical calculations reported in this work were performed in the framework of the density functional theory (DFT) using the Gaussian-03 program package [16] and the Vienna Ab Initio Simulations Package (VASP) [17–19]. Calculations carried out with Gaussian-03 were done using the gradient corrected Becke’s parameters hybrid exchange functional in combination with the correlation functional of Lee, Yang and Parr (B3LYP) [20]. The terrace site at MgO (100) surface was represented by the cluster indicated as Mg25O25(Mg-ECP)25 consisting of two layers. To take into account the Madelung field due to the rest of the extended oxide, the cluster was embedded with an array of ± 2 point charges and the positive point charges at the interface were replaced by effective core potentials (ECP) corresponding to Mg2+. For the defects at edges and corners similar clusters were used: a Mg22O22(Mg-ECP)19 cluster was used for modeling the edge defect of MgO; a Mg22O22(Mg-ECP)12

2.2. Catalytic tests The fatty acid methyl ester (methyl oleate, Fluka, > 60.0%, with 86% total C18 + C16 esters as determined by gas chromatography) is coded here as FAME. The glycerolysis between FAME and glycerol (Aldrich, 99.0%) was performed at 483–503 K in a glass batch reactor equipped with temperature and stirring control systems and a condenser to remove the methanol produced during the reaction. The experiments were carried out under a N2 flow (35 mL/min) at atmospheric pressure, using Gly/FAME molar ratios of 2–6 and a catalyst/ 2

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combining TPD and FTIR of CO2 adsorbed at room temperature. The total base site density, calculated by integration of the TPD profile (shown as Fig. 1S in the Supplemental Files section), was 3.47 μmol/m2. On the other hand, the different surface oxygen species responsible for the MgO basicity were identified by FTIR [22]. Several CO2 adsorbed species such as unidentate and bidentate carbonates as well as bicarbonate were detected [23,24]; the broad bands of the spectra (not shown here) indicated the presence oxygen species with different coordination number. Unidentate carbonates form on unsaturated oxygen anions (O3c or O4c) whereas bidentate carbonates form on metal-oxygen pairs (Mg5c–O5c) and bicarbonate species on surface hydroxyl groups. Based on the FTIR results, the surface oxygen species of MgO were classified in weak, medium and strong base sites and the following base strength order was assigned to them: low coordination O2− anions > oxygen in Mg2+-O2− pairs > OH groups [6]. Then, the contribution of each oxygen species was quantified by deconvolution of the TPD traces (Fig. 1S) in three peaks with maxima at 400, 450 and 550 K [5]; the density of weak, medium and strong base sites was 0.55, 1.26 and 1.66 μmol/m2, respectively. Thus, the MgO catalyst used here contains mainly strong and medium-strength basic sites.

cluster was used for modeling the oxygen corner defect of MgO; and a similar Mg23O23(Mg-ECP)14 cluster was used for modeling a magnesium apical corner. The atoms belonging to Gly and FAME molecules were described by the 6–31G(d,p) basis set. The adsorption energy Eads of Gly or FAME was evaluated according to the following equation: Eads = Emolecule-MgOcluster − EMgOcluster − Emolecule. Atomic net charges were calculated following the natural bond orbital (NBO) scheme. Details are given elsewhere [5]. For the calculations made with VASP, a slab representing a monoatomic step on the MgO (100) surface that contains 90 atoms placed in three layers was constructed. Taking into account the periodic calculation, only the atoms corresponding to the first layer were allowed to relax while all the adsorbed molecules were completely optimized. In the perpendicular direction, the slabs are separated with a vacuum region of ∼15 Å, corresponding to seven ideal bulk layers. For isolated Gly and FAME molecules, the gas-phase geometries were completely optimized using a 20 × 20 × 20 supercell. More details are given in a previous work [6]. To reduce calculation time, instead of methyl oleate (C18:1), the unsaturated FAME molecule used in the catalytic experiments, a shorter saturated methyl ester (C5:0) was used in all the calculations.

3.2. Glycerolysis of methyl oleate (FAME) on MgO 2.4. Kinetic modeling, data fitting and statistical analysis Detailed studies of the glycerolysis reaction were carried out previously. Under heterogeneous catalysis conditions, the reaction medium in a batch reactor consists of four phases: the lower liquid layer formed by Gly, the upper liquid layer (the “oily” phase) containing FAME and the glyceride products, the solid catalyst and the gas phase methanol that is continuously removed from the reactor. Since FAME is not soluble in Gly, the reaction occurs in the top layer and therefore Gly, which is slightly soluble in the oily phase, must be transferred from the bottom layer. Our preliminary works included the optimization of the reaction conditions [14] and the identification of the catalyst acid-base site requirements [4]. But most of the effort was devoted to elucidate the main reaction features when MgO is used as catalyst; we demonstrated experimentally that the active sites participating in kinetically relevant reaction steps are coordinatively unsaturated oxygen anions such as those on corners and edges (O3c or O4c) of the MgO surface [5] and discussed the effect of adding basic promoters (Li+) on the monoglyceride selectivity [25]. The glycerolysis of methyl oleate on MgO was studied at different reaction temperatures and Gly/FAME molar ratios. Fig. 1 shows the reactant conversions (XFAME and XGly) and product yields (YMG and YDG) as a function of time for the experiments carried out at 483, 493 and 503 K and with a Gly/FAME molar ratio of 4.5. The FAME conversion at the end of the 8 h run increases with increasing the reaction temperature from 80% at 483 K to reach values close to 100% at 503 K. Due to the excess of glycerol respect to the stoichiometric condition (Gly/ FAME = 1) for these experiments, the effect of temperature on the Gly conversion is less noticeable, but XGly clearly increases from 14% up to 18% in the temperature range under study. Regardless of the reaction temperature, monoglycerides are the main products throughout the entire run. High YMG can be obtained (≈62%) at the end of the experiment at 503 K. In spite of the positive effect of increasing the reaction temperature, this parameter should be kept below 533 K to avoid the undesirable formation polyglycerols and glyceride product degradation [26,27]. Fig. 2 summarizes the results at 8 h of reaction showing an enhancement of the YMG/YDG ratio at higher reaction temperatures that attains a value of ≈2 at 503 K. Also, in Fig. 2, the yields to MG and DG are split to show the contribution of each isomer. Glycerides with the acyl group at terminal positions of the glycerol backbone (Scheme 1, 1MG and 1,3-DG) prevail over the isomers having acyl chains positioned at the middle carbon of the Gly molecule (2-MG and 1,2-DG). In a previous work, we explained by theoretical calculations the preferential formation 1-MG [6].

The kinetic study of the glycerolysis of methyl oleate on MgO was carried out using a model based on a single site Langmuir-HinshelwoodHougen-Watson (LHHW) surface reaction mechanism for complex reactions. The system of differential equations was solved with the Runge0 Kutta-Merson algorithm. The relative concentrations (C j* = nj / nFAME )of the compounds (j) involved in the reaction were calculated from the * ) and compared with the experimental values model parameters (C jcalc * ). The model parameter estimation was performed by nonlinear (C jobs regression, using a Levenberg-Marquardt algorithm [21], which minimizes the sum of squared errors (SSE) between the data estimated by the model and the experimental data, according to Eq. (1):

SSE =

∑

* − C jobs * )2 (C jcalc

all data

(1)

The fitting quality was evaluated through the coefficient of determination (R2), calculated according to Eq. (2): n

R2 =

∑i =1 n ∑i =1

* )2 * − C jobs (C jcalc * )2 * − C jobs (C jobs

(2)

* is the mean of measured values. where C jobs The global significance of the regression was evaluated using the Ftest according to Eq. (3) [11]: 2

∑ alldata ⎛⎜C *jcalc ⎞⎟ samples ⎝

⎠

p

Fcalc =

2

> Ftab (p , m − p , 95%)

∑ alldata ⎛⎜C *jcalc − C *jobs⎞⎟ samples ⎝

⎠

m−p

(3)

where Ftab is obtained from tables [21]. 3. Results and discussion 3.1. Catalyst characterization A detailed characterization of MgO catalysts prepared by similar procedures is given elsewhere [4–6]. Briefly, the BET surface area of the MgO calcined at 773 K was 189 m2/g, whereas the pore volume and the mean pore size were 0.38 cm3/g and 80 Å, respectively. By XRD analysis the presence of only a periclase phase (ASTM 4-0829) was confirmed. The basic properties of the MgO catalyst were investigated 3

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Fig. 1. Reactant conversions and product yields as a function of time at different reaction temperatures. (A) 483 K; (B) 493 K; (C) 503 K. Symbols: experimental data; Lines: kinetic modeling results [MgO, Gly/FAME = 4.5].

Φ=

Before any kinetic analysis, the results of Fig. 1 were checked to verify the absence of external and internal mass and heat transfer limitations. The reaction enthalpy of the reactions toward MG and DG, Scheme 1, is negligible (ΔHR0 ≈ 0 kcal/mol), as in other similar transesterification reactions [28]. Thus, heat transfer limitations were ruled out [29]. The extent of the external mass transfer resistance was evaluated by calculating the Mears number (ω) [30]: 0 rFAME ρp Rp 0 kc CFAME

<

0.15 n

(4)

0 rFAME

where is the initial FAME conversion rate (44.8 mol/h kg) for the experiment at 493 K, ρp is the catalyst particle density (1550 kg/m3), Rp 0 is the average particle radius (1.25 × 10−4 m), CFAME is the FAME 3 concentration in the reactor at t = 0 (2720 mol/m ) and n is the reaction order taken as 1. The value of kc (0.46 m/h), the mass transfer coefficient, was calculated with Eq. (5):

kc =

Sh Dm 2Rp

0 Deff CFAME

<1

(6)

where Φ is the dimensionless Weisz-Prater modulus, Deff is the effective diffusion coefficient (2.8 × 10−6 m2/h) calculated after multiplying the Dm value by the particle porosity (0.57). The absence of internal diffusional limitations was confirmed from the value of Φ = 0.14, which corresponds to a Thiele modulus of 0.12 and an effectiveness factor of 0.99. Thus, at typical reaction conditions the reaction takes place under kinetic control. Nevertheless, at Gly/FAME ratios close to one, the depletion of the Gly concentration in the reaction zone might increase the influence of the Gly mass transfer limitations and the consequent shift of the reaction pathways toward unwanted reactions. In a kinetically controlled reaction system, the positive effect of increasing the reaction temperature on XFAME and YMG (Fig. 1) must be entirely ascribed to an increase of the overall kinetic rate constant, as predicted by the Arrhenius equation, and to the enhanced Gly solubility in the oily phase [6,38,39]. The glycerolysis reaction was further studied on MgO at 493 K and at different reactant ratios (Gly/FAME = 2; 3; 4,5; 6). Results are presented in Figs. 1B and 3 . According to the stoichiometry of the MG synthesis (Scheme 1), a Gly/FAME = 1 molar ratio would be enough for the reaction to proceed; however, an excess of Gly is usually employed in glycerolysis reactions to obtain high MG yields by avoiding thermodynamic limitations and the shift of the reaction pathway toward competitive reactions [28]. In particular, we have reported that under stoichiometric conditions, MG formed at low reaction times convert to DG as the reaction continues thereby decreasing the final MG yield [4]. This is the consequence of the low Gly concentration in the reaction zone that favors the competitive transesterification of MG with a second molecule of FAME giving DG. As expected, Figs. 1B and 3 indicate that larger final FAME conversion values are obtained at high Gly/FAME ratios, in detriment of the XGly values. Fig. 4 shows that at 8 h of reaction the XFAME increased from 75% for a Gly/FAME = 2 to 93% for the experiment with the largest Gly excess. On the other hand, MG were the main products in the four catalytic runs. As the final XFAME increases under excess of Gly conditions, the glycerolysis reaction toward MG predominates over the consecutive conversion of MG into DG. Thus, the YMG/YDG ratio increases with Gly/FAME because of the enhanced YMG whereas YDG remains almost constant, Fig. 4. From the experiments of Figs. 1 and 3 we can conclude that high YMG (≈58-62%) are obtained on MgO at temperatures in the range of 493–503 K and Gly/FAME molar ratios of 3–6. Furthermore, the monoester isomer substituted at terminal positions of the glycerol carbon chain (1-MG) prevails at all the reactant conditions of this study.

Fig. 2. Effect of the reaction temperature on FAME conversion and product yields at 8 h of reaction [MgO, Gly/FAME = 4.5].

ω=

0 rFAME ρp Rp2

(5)

where Dm (5.0 × 10−6 m2/h) is the molecular diffusion coefficient calculated from the Wilke-Chang correlation [31], Sh is the Sherwood number calculated with the Frösslinǵs equation (Sh = 2 + 0.55Re 0.5Sc 0.33) , Re is the Reynolds number and Sc is the Schmidt number [32–36]. Eq. (4) predicted a ω value of 0.007, what confirms the absence of interparticle diffusion limitations. The magnitude of the intraparticle mass transfer limitations was evaluated with the Weisz-Prater criterion [37] for a first order reaction and spherical catalyst particles, Eq. (6): 4

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Fig. 3. Reactant conversions and product yields as a function of time at different reactant ratios. (A) Gly/FAME = 2; (B) Gly/FAME = 3; (C) Gly/FAME = 6. Symbols: experimental data; Lines: kinetic modeling results [MgO, T = 493 K].

with an acyl chain shorter than that of methyl oleate was used as FAME. In all the geometries investigated the FAME-surface interaction took place through the oxygen of the C]O group, step 1, Scheme 2. The Eads for the adsorption geometries on the different cluster sites was much smaller than that of Gly, suggesting a weaker adsorption; Eads values varied between −1.15 kcal/mol on the terrace site to −15.4 kcal/mol on the Mg-corner site. The FAME molecule adsorbs non-dissociatively on a Mgs atom, preserving its integrity. The charge difference of the carbonyl oxygen(Δq (OC=O) ), which is an indication of the polarization of the C]O bond, depended on the type of cluster site. However, the values of Δq(OC=O) were always small, varying between −0.02 for the terrace site and −0.07 for the Mg-corner site, suggesting negligible polarization of the C]O bond for all the geometries. After comparing the modeling of Gly and FAME adsorptions on the MgO (100) surface with typical terrace, edge and corner sites, we can conclude that glycerol binds to the surface with higher adsorption energies than those of FAME. The stronger Gly-surface interaction is in line with the hydrophilic properties of Gly. Using the shorter saturated methyl ester (C5:0), the calculations predict a weak FAME-surface interaction even on low coordination Mg3c and O3c sites and therefore, we anticipate that more hydrophobic methyl oleate (C18:1) would behave in a similar fashion with its adsorption step being probably kinetically irrelevant.

Fig. 4. Effect of the reactant ratio on FAME conversion and product yields at 8 h of reaction [MgO, T = 493 K].

3.3. Theoretical studies of the Gly and FAME adsorptions on MgO With the purpose to gain insight into the magnitude and nature of the reactant-MgO surface interaction to postulate then a suitable kinetic model for the glycerolysis reaction, the molecular modeling of the adsorption of both reactants on MgO was carried out within the density functional theory (DFT) formalism. Initially, the adsorption calculations were performed on a MgO (100) surface using the cluster model approximation with four different clusters (terrace, edge, O-corner and Mg-corner sites) and the Gaussian-03 package [5]. Results were later corroborated using periodic calculations, the VASP package and a slab representing a stepped MgO surface [6]. The main findings of these studies were that the Gly adsorption on a MgO surface occurs dissociatively with breaking of the secondary OeH bond on strongly basic coordinatively unsaturated oxygen anions such as those on edges (O4c) and to lesser extent, on corners (O3c). Upon dissociation, a surface 2-glyceroxide species (2-Glyox) is formed as well as a new surface OH group, step 1, Scheme 2. The adsorption energies (Eads) for the geometries that dissociate on an edge site were between −37.6 and −42.7 kcal/mol and the charge difference (Δq (OO−Mgs) )of the oxygen attached to a surface Mg cation (Mgs) was ≈-0.19, indicating that the 2-glyceroxide species gained negative charge. These results agreed with the experimental results showing that the reaction is promoted on strong base sites. On the other hand, non-dissociative adsorption takes place on medium-strength base sites such as those on terrace sites (O5c). Similar calculations were carried out to model the FAME adsorption. As described in section 2.3, for calculation purposes a molecule

3.4. Theoretical studies of the FAME glycerolysis reaction mechanism on MgO After the theoretical studies of the adsorption of Gly and FAME on the MgO surface, a kinetic modeling of the FAME glycerolysis reaction was carried out. Again, a shorter FAME molecule was used in the calculations, as explained above. The aim of these studies was to gain insight into the reaction mechanism leading to MG. A stepped MgO surface and the VASP package were used in the calculations [6]. The reaction toward formation of both MG isomers (1-MG and 2MG) proceeds through two transitions states (TS) and one tetrahedral intermediate (TI), Fig. 2S. Scheme 2 summarizes the main findings for the 1-MG synthesis. The reaction starts by formation of the surface 2glyceroxide (2-Glyox) species after dissociative adsorption of Gly on a MgO edge site (step 1). The reaction continues by dissociation of one of the terminal OH groups of 2-Glyox and formation of a new CeO bond between 2-Glyox and FAME (tetrahedral intermediate), after crossing the energetic barrier of the first transition state, step 2. The following is step 3 in which both, the surface 1-monoglyceroxide (1-MGox) and CH3O species form. The transition state associated with this step requires a higher energy (13.6 kcal/mol) than the first one (9.9 kcal/mol). Fig. 2SA presents the tetrahedral intermediate and both transition states predicted by the calculations for the pathway toward 1-MG. In step 4, adsorbed species (1-MG and methanol) form by reaction of 1-MGox and 5

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Scheme 2. Reaction pathway of FAME glycerolysis toward 1-MG on MgO predicted by the molecular modeling.

reaction steps participating in the synthesis of the most abundant diglyceride isomer, 1,3-DG, (Scheme 3) would involve the formation of a new CeO bond between the FAME molecule and the terminal oxygen of the 1-MGox surface species, followed by steps equivalent to those described above for 1-MG formation.

CH3O species with surface hydrogen fragments. Finally, both, 1-MG and methanol desorb to the fluid phase. Contrarily, the calculations of the reaction pathway toward 2-MG showed that formation of this monoglyceride isomer (not shown in Scheme 2) is not favored, because the oxygen in position 2 of the 2Glyox species is sterically hindered to form a new CeO bond with FAME (Fig. 2SB). Therefore, formation of the first transition state requires crossing a barrier of 22.1 kcal/mol, a much higher energy than those involved in the pathway toward 1-MG. Similarly, and on the basis of the calculations presented above, the

3.5. Kinetic modeling of the MG and DG formation from FAME and Gly on MgO From the experimental and molecular modeling results, a

Scheme 3. Postulated reaction pathway for the transesterification of FAME with 1-MG toward 1,3-DG on MgO. 6

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0 rI0 = kI •CGly =

HGly (t 0) =

0G kI •CGly

HGly (t 0)

(12)

0G CGly 0 CGly

(13) 0 CGly

where kI• is the kinetic constant of step I and is the initial Gly 0 is linked to the Gly concentration in concentration in the oily phase. CGly 0G the glycerol phase (CGly ) by the partition coefficient of Gly between both phases at t = 0, HGly (t 0) , given by Eq. (13). If the surface reaction step toward MG formation (step II) is ratelimiting:

rII0 =

0 0G kII •KI CFAME CGly / HGly (t 0) 0G [1 + KI CGly / HGly (t 0) ]

(14) 0 CFAME

where kII• is the kinetic constant of step II and concentration in the oily phase. If the MG desorption (step III) is rate limiting:

0 Fig. 5. Initial FAME conversion rate as a function of CFAME . Symbols: experimental data; Line: kinetic modeling results [MgO, T = 493 K, Gly/ FAME = 4.5].

0 rIII = kIII •

– Reactions products were only MG, DG and methanol (C1OL). – Gly adsorbs on the unsaturated oxygen anions of the MgO surface whereas the FAME-MgO surface interaction is weak and can be disregarded. Therefore, the general LHHW approach eventually turns into an Eley-Rideal-type mechanism. – Gly adsorption and product desorption steps are equilibrated. – The C1OL concentrations in the liquid phase and on the surface during the course of reaction are negligible since this product is continuously removed from the reactor. Based on these assumptions, the following mechanism comprising five elementary steps (steps I to V) was postulated:

=

I

CGly • CGly C•

(7)

Gly•+FAME ↔ MG•+C1 OL (II ) K

MG•↔MG + • (III) K

desIII

=

II

CMG C• CMG •

MG•+FAME ↔ DG•+C1 OL (IV) K

DG•↔DG + • (V) K

desV

=

CDG C• CDG •

=

=

=

CMG •CC1OL CFAME CGly •

(8)

1

(9)

K III

IV

1 KV

=

CDG •CC1OL CFAME CMG •

(15)

where kIII• is the kinetic constant of step III. From inspection of Eq. (12), (14) and (15) it can be deducted that 0 only Eq. (14) depends on CFAME whereas Eq. (15) is independent of the 0G . Thus, two addireactant concentrations and Eq. (12) depends on CGly 0 tional experiments were carried out at 493 K in which CFAME was varied in the oily phase by diluting it with liquid Vaseline; this solvent is not soluble in glycerol so that the glycerol phase remained pure throughout 0 the catalytic run. From those experiments, the initial rate values (rFAME ) 0 were calculated and plotted as a function of CFAME in Fig. 5 together 0 with the data point corresponding to pure FAME (CFAME = 2.72 mol/L). 0 Clearly, the experimental results agree with rII , Eq. (14), that antici0 .Therefore, the pates an increasing initial rate with increasing CFAME surface reaction steps toward MG and DG formation were considered rate-limiting for the modeling of the complete data set. The mass balances for the different glyceride components of the reaction system in the oily phase are given below as the differential Eqs. (16)–(18), whereas the mass balance of Gly in the glycerol phase is given in Eq. (19):

heterogeneous kinetic model was postulated for the process sketched in Scheme 1. To interpret the catalytic results, a LHHW kinetic model was used with the following assumptions:

Gly + •↔Gly• (I ) K

is the initial FAME

(10)

1 dnFAME = −(rII + rIV ) W dt

(16)

1 dnMG = rII − rIV W dt

(17)

1 dnDG = rIV W dt

(18)

G 1 dnGly = −rII W dt

(19)

where rII and rIV are the reaction rates of steps II and IV in mol/h kg, nj G is number of moles of species j in the oily phase and nGly is the number of moles of Gly in the glycerol phase. Then, the rate expressions for the two kinetically relevant steps are given by Eqs. (20) and (21):

(11)

where Gly, FAME, MG, DG and C1OL are the compounds in the liquid phase, • is the surface site, and Gly• , MG• and DG• are surface species. Cj is the molar concentration of species j in the oily phase and Cj• is the surface concentration of species j. KI, KII, KdesIII, KIV and KdesV are the thermodynamic constants for steps I, II, III, IV and V, respectively; KIII and KV are the adsorption equilibrium constants for MG and DG, respectively. Preliminary discrimination between the possible rate-limiting steps was performed based on initial reaction rates (r0), i.e., assuming a negligible concentration of products at t = 0. Thus, the rate expressions were derived for the three relevant steps under initial conditions (steps I, II and III), considered as rate limiting. Thus, if Gly adsorption (step I) is rate limiting:

rII =

rIV =

kII •KI CFAME CGly [1 + KI CGly + KIII CMG + KV CDG]

(20)

kIV •KIII CFAME CMG [1 + KI CGly + KIII CMG + KV CDG]

(21)

Cj is the molar concentration of species j in the oily phase and kII• and kIV• (the kinetic constants for steps II and IV) include the total active site concentration (C•T ), calculated with Eq. (22): C•T = C• + CGly • + CMG • + CDG • In a kinetically controlled regime, the concentration of Gly in both 7

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13.6 kcal/mol for 1-MG, which are close to 23.6 kcal/mol predicted by the model. In a kinetically controlled system of complex reactions such as that of Scheme 1, it can be easily demonstrated that a higher Ea of the first kinetic step compared to the consecutive one, will enhance the YMG/YDG ratio at higher reaction temperatures; that is in fact the experimental result of Fig. 2. The adsorption equilibrium coefficients for Gly, MG and DG (KI, KIII and KV, respectively) were also estimated with the model. The values follow the order KI > > KIII > KV, in agreement with previous results by Mazzieri et al. [41], who showed that the MG adsorption coefficient is much smaller than that of Gly on silica at 298 K. There is a wide span of Gly adsorption coefficient values reported in the literature for measurements at 298–448 K; KI varies from 1.3 to 2200 L mol−1 on solids such as silica, activated carbon, sulfonated acid resins or sulfated iron oxide [41–44]. The values predicted by our model (∼20 L mol−1) suggest a strong Gly adsorption on a solid with basic properties such as MgO. Moreover, from the values of KI, KIII and KV, the adsorption enthalpies were calculated using the vańt Hoff equation. Only the enthalpy of the Gly adsorption (ΔHGly) was significant (-8.6 kcal/mol) in the temperature range of 483–503 K; the negative value confirmed the exothermic nature of the Gly adsorption process on MgO. On the other hand, KIII and KV resulted rather independent of the temperature (Table 1), thereby predicting small adsorption enthalpies of the products on MgO. These results are in line with the hydrophilic properties of Gly and the amphiphilic features of the reaction products. From the theoretical calculation described in section 2.3 for the Eads of the most stable geometry of the dissociative Gly adsorption on an edge of MgO (100) (-42.7 kcal/mol), the ΔHGly was calculated using Eq. (23). For this purpose, frequency calculations were performed for that optimized geometry and energy corrections were obtained from the thermochemical data at 298 K and 1.0 atm. The zero-point energy (ΔE0) and the thermal (ΔET) and enthalpic (ΔEH) corrections were added [45].

Table 1 Kinetic and statistical parameters estimated with the model for the reaction between Gly and FAME at different temperatures. Parameters

Estimated valuesa

483

kII ● (L kg−1 h−1)

18.09 ± 0.92 5.96 ± 1.1

493

kIV ● (L kg−1 h−1) KI (L mol−1) KIII (L mol−1) KV (L mol−1) SSE R2 kII ● (L kg−1 h−1)

503

kIV ● (L kg−1 h−1) KI (L mol−1) KIII (L mol−1) KV (L mol−1) SSE R2 kII ● (L kg−1 h−1)

Temperature (K)

kIV ● (L kg−1 h−1) KI (L mol−1) KIII (L mol−1) KV (L mol−1) SSE R2 a

22.14 ± 1.85 1.23 ± 0.14 0.042 ± 0.004 0.030 0.999 24.15 ± 1.35 7.23 ± 1.28 21.37 ± 2.99 1.21 ± 0.26 0.041 ± 0.005 0.055 0.999 48.11 ± 4.31 8.65 ± 1.91 15.49 ± 2.60 1.18 ± 0.36 0.037 ± 0.015 0.076 0.999

With 95% confidence interval.

phases at any time during reaction is related by HGly =

G CGly

CGly

.

Measurements of the Gly solubility in the oily phase under no reaction conditions (t = 0, pure FAME) were carried out at different temperatures and the corresponding HGly (t 0) values were calculated with Eq. (13), Table 1S. Results of Table 1S agree well with what predicted by Kimmel [38] for the solubility of Gly in methyl palmitate at 493 K (1.2 wt.%) and with the value calculated by Negi [40] at 408 K for methyl oleate (0.55 wt.%). We found that the Gly solubility in both, pure FAME and oily phase (glyceride product mixture) increases with temperature [38,40]. Furthermore, the Gly solubility in the oily phase increases with time because of formation of products with surfactant properties. Thus, a function relating HGly with time and temperature was determined (as explained in the Supplemental Files section) and used for modeling purposes to link the concentration of Gly in both phases. Firstly, the model was applied to the experiments carried out at different reaction temperatures, Fig. 1. With these experimental data, the set of equations (16) to (21) was solved as described in section 2.4. The values of the five kinetic parameters (kII•, kIV•, KI, KIII and KV) estimated with the model are given in Table 1. The lines in Fig. 1, representing the modeling results, indicate that the experimental data obtained at different reaction temperatures are well described by the postulated model. The initial slopes (slopes at t = 0) of the MG and DG curves in Fig. 1 suggest that in agreement with Scheme 1, the modeling results predict that MG are primary products (non-zero initial slope) formed directly from FAME and Gly, whereas DG are secondary products (zero initial slope). Moreover, the SSE parameter calculated with Eq. (1) was in all the regressions below to 0.08 whereas the value of the correlation coefficient (R2) was always ≈0.999. From the kII• and kIV• values predicted by the model, the corresponding activation energies (Ea) were calculated using the Arrhenius equation. The highest Ea value (23.6 kcal/mol) was obtained for step II leading to MG, whereas the consecutive formation of DG (step IV) has an Ea value of 9.0 kcal/mol. Time ago we reported 26 and 19 kcal/mol for the apparent Ea of MG and DG formation on similar MgO catalysts but obtained in a wider temperature range of 483–523 K [14]. Thus, the values predicted here by the kinetic model for the elementary reaction steps agree well with the apparent values. Furthermore, as explained in section 3.3, the molecular modeling of the reaction pathway toward MG predicts energy barriers between 22.1 kcal/mol for 2-MG and

ΔHGly = Eads + ΔE0 + ΔET + ΔEH

(23)

After corrections, the ΔHGly value predicted by the molecular modeling was −34.6 kcal/mol at 298 K. The value predicted by the kinetic model in the range of 483–503 K is much smaller reflecting a decrease of the Gly adsorption exothermicity at increasing temperatures. The kinetic parameters of Table 1 do not depend on the reactant ratio and therefore they were used to evaluate the model predictions for the experiments of Fig. 3 at different Gly/FAME ratios and 493 K. The concentrations of all the chemical species were calculated with the parameters of Table 1 and the results are presented as lines in Fig. 3, showing a good agreement between experimental and model results. 0 Furthermore, with the parameters of Table 1, the rFAME values 0 predicted by the model at 493 K and at different CFAME were calculated and contrasted with the experimental values. Previously, the new value of HGly (to) corresponding to the FAME-Vaseline solution was determined (Table 1S). The model prediction is shown as a line in Fig. 5. The broken line is due to the fact that the solubility of Gly in FAME-Vaseline is smaller (higher HGly (t 0) ) than that in pure FAME and therefore, the slope of the curve according to Eq. (14) is also smaller. * ) of all the Finally, with the experimental relative concentration (C jobs species for all the catalytic experiments and the relative concentrations * ) predicted by the model, a parity graph was plotted, Fig. 6. Con(C jcalc sidering that the kinetic modeling involved 288 experimental data obtained at different operational conditions, the statistical parameters of Fig. 6 indicate that the model is suitable to represent the reactions between FAME and glycerol on MgO. That is confirmed by the value of Fcalc = 821138 obtained from Eq. (3), in contrast to a Ftab = 1.3. 4. Conclusions The conversion of methyl oleate (a fatty acid methyl ester, FAME) with glycerol (Gly) was studied on MgO at different reaction conditions. The FAME conversion at the end of the 8 h run increased with the 8

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del Litoral, Santa Fe, Argentina (grant CAID PI 64-103/11) for financial support of this work. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mcat.2018.02.019. References [1] Y. Zheng, X. Chen, Y. Shen, Commodity chemicals derived from glycerol, an important biorefinery feedstock, Chem. Rev. 108 (2008) 5253–5277. [2] A. Corma, S. Iborra, A. Velty, Chemical routes for the transformation of biomass into chemicals, Chem. Rev. 107 (2007) 2411–2502. [3] A. Corma, S.B.A. Hamid, S. Iborra, A. Velty, Lewis and Brønsted basic active sites on solid catalysts and their role in the synthesis of monoglycerides, J. Catal. 234 (2005) 340–347. [4] C.A. Ferretti, A. Soldano, C.R. Apesteguia, J.I. Di Cosimo, Monoglyceride synthesis by glycerolysis of methyl oleate on solid acid-base, Chem. Eng. J. 161 (2010) 346–354. [5] C.A. Ferretti, S. Fuente, R. Ferullo, N. Castellani, C.R. Apesteguía, J.I. 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* ) relative * ) and predicted (C jcalc Fig. 6. Parity plot of the experimental (C jobs concentrations of reactants and products.

reaction temperature reaching ≈100% at 503 K. The effect of temperature on the Gly conversion was less marked due to the excess of glycerol used in the experiments. Monoglycerides were the main products regardless of the operational conditions and a ≈62% monoglyceride yield was obtained at 503 K. The monoglyceride isomer having the acyl group in the middle of the glycerol backbone prevailed in all the experiments. Molecular modeling of the Gly and FAME adsorptions on a MgO (100) surface using four different clusters (terrace, edge, O-corner and Mg-corner sites) showed that the adsorption energy of FAME was much smaller than that of Gly, in line with the more hydrophobic character of methyl oleate. Furthermore, Gly dissociated on an edge site whereas FAME adsorption was weak and non-dissociative on all the cluster sites. Theoretical calculations of the mechanism of monoglyceride formation from FAME and Gly indicated that the reaction proceeds through two transitions states and one tetrahedral intermediate, with activation energies between 13.6–22.1 kcal/mol, depending on the monoglyceride isomer. A Langmuir-Hinshelwood-Hougen-Watson kinetic model with five elementary steps was postulated to interpret the catalytic data for the synthesis of MG and DG from FAME and Gly on MgO. The surface reaction steps toward MG and DG formation were considered rate-limiting and the adsorption/desorption of the different compounds were assumed to be quasi-equilibrated steps. Based on the theoretical adsorption studies, the adsorption of FAME was considered kinetically irrelevant. The kinetic parameters associated to MG and DG formation were calculated as well as the adsorption equilibrium coefficients of glycerol and products. The temperature dependence of these parameters was discussed by calculating the activation energies and heat of adsorptions of the different chemical species. The activation energy of the MG formation (23.6 kcal/mol) was close to the value predicted for this species by the theoretical calculations. Moreover, the calculated activation energies for MG and DG predict that the MG/DG ratio will increase with temperature in agreement with the catalytic results. Gly adsorption was exothermic and its adsorption enthalpy was much higher than those of MG and DG, in parallel with the more hydrophilic nature of Gly compared with the glyceride products. The kinetic model was found to give a suitable description of the experimental data resulting from testing MgO in a wide range of experimental conditions. Acknowledgements Authors thank the Agencia Nacional de Promoción Científica y Tecnológica (ANPCyT), Argentina (grant PICT 1888/10), CONICET, Argentina (grant PIP 11220090100203/10) and Universidad Nacional 9

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