Edited by: Saurabh Dhiman, South Dakota School of Mines and Technology, United States
Reviewed by: Jaime Puna, Instituto Superior de Engenharia de Lisboa, Portugal; Konstantinos Moustakas, National Technical University of Athens, Greece
This article was submitted to Bioenergy and Biofuels, a section of the journal Frontiers in Energy Research
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Application of acid-activated bentonite and SO3H-functionlized multiwall carbon nanotubes (SO3H-MWCNTs) for lowering free fatty acids (FFAs) content of low-quality residual olive oil, prior to alkali-catalyzed transesterification was investigated. The used bentonite was first characterized by Scanning Electron Microscopy (SEM), Inductively Coupled Plasma mass spectrometry (ICP-MS), and X-ray fluorescence (XRF), and was subsequently activated by different concentrations of H2SO4 (3, 5, and 10 N). Specific surface area of the original bentonite was measured by Brunauer, Emmett, and Teller (BET) method at 45 m2/g and was best improved after 5 N-acid activation (95–98°C, 2 h) reaching 68 m2/g. MWCNTs was synthesized through methane decomposition (Co-Mo/MgO catalyst, 900°C) during the chemical vapor deposition (CVD) process. After two acid-purification (HCl, HNO3) and two deionized-water-neutralization steps, SO3H was grafted on MWCNTs (concentrated H2SO4, 110°C for 3 h) and again neutralized with deionized water and then dried. The synthesized SO3H-MWCNTs were analyzed using Fourier-Transform Infrared Spectroscopy (FTIR) and Transmission Electron Microscopy (TEM). The activated bentonite and SO3H-MWCNTs were utilized (5 wt.% and 3 wt.%, respectively), as solid catalysts in esterification reaction (62°C, 450 rpm; 15:1 and 12:1 methanol-to-oil molar ratio, 27 h and 8 h, respectively), to convert FFAs to their corresponding methyl esters. The results obtained revealed an FFA to methyl ester conversion of about 67% for the activated bentonite and 65% for the SO3H-MWCNTs. More specifically, the acid value of the residual olive oil was decreased significantly from 2.5 to 0.85 and 0.89 mg KOH/g using activated bentonite and SO3H-MWCNTs, respectively. The total FFAs in the residual olive oil after esterification was below 0.5%, which was appropriate for efficient alkaline-transesterification reaction. Both catalysts can effectively pretreat low-quality oil feedstock for sustainable biodiesel production under a biorefinery scheme. Overall, the acid-activate bentonite was found more convenient, cost-effective, and environment-friendly than the SO3H-MWCNTs.
Graphical Abstract
Clay, particularly bentonites (clay mineral with different colors) are very popular in various industries (Christidis,
Bentonites, by virtue of montmorillonite, have high absorption capacity for exchanging some certain cations from solutions with their own molecules. Bentonite quality and properties (i.e., clay minerals loading, cation exchange capacity (CEC), porosity, selectivity, surface acidity, surface area) are very important in industrial applications and could be modified by activation methods (Önal and Sarıkaya,
Carbon nanotubes (CNTs), first synthesized in 1991, have many applications as fillers, chemical sensors, hydrogen storage, electronic devices, catalyst supports, and etc., due to their unique properties including chemical stability, electrical, and thermal properties, high surface area, mechanical characteristic, etc. (Ham et al.,
Oil feedstocks could be converted into biodiesel through the transesterification reaction (Aghbashlo et al.,
Alternatively, biodiesel may be produced from high FFA oil feedstock through acid-catalyzed (trans) esterification reaction, in which FFAs and triglyceride simultaneously react with alcohol to produce methyl ester (biodiesel) (Pan et al.,
Hydrophobic solid acid catalysts could be used to overcome the above-mentioned drawbacks of the homogeneous acid catalysts. Carbonized vegetable oil asphalt, ferric sulfate supported on silica, and tolune-4-sulfonic monohydrate acid are some solid acid catalysts investigated for pretreating low-quality oil feedstock for biodiesel production (Hayyan et al.,
Having considered the disadvantages of the acid pretreatment methods discussed above, searching for more efficient, less expensive, and easier to operate strategies is inevitable. Considering that, this study was set to investigate the catalytic performance of acid-activated bentonite in converting FFAs of low-quality, high FFA-containing residual olive oil into methyl esters. This could be regarded as a pretreatment step for FFA removal prior to basic-catalyzed transesterification reaction. To achieve that, bentonite was characterized, acid-activated, and its performance in decreasing FFA content of residual olive oil through conversion to methyl esters was evaluated. Moreover, MWCNTs were also synthesized, SO3H functionalized, and their performance in FFA removal from residual olive oil was evaluated and compared with that of the acid-activated bentonite.
Raw bentonite with the following physicochemical properties was used: surface area of 45 m2.g−1, mean pore diameter of 7.7 nm, total pore volume of 0.0898 cm3.g−1, swelling index of 15 mL/2 g, and CEC of 58 meq/100 g. Residual olive oil was purchased from an olive oil refinery and its FFA profiles and physical properties were investigated. All the chemicals used in this study were purchased either from Merck (Germany) or Sigma-Aldrich (Germany).
A uniform sample of bentonite with the size of about 130 μm was obtained by passing it through different sieves with appropriate mesh sizes. Then, aliquots of 50 g were weighted for the acid activation step.
Three different concentrations of H2SO4 (i.e., 3, 5, and 10 N) were used for the activation of 50 g bentonite. The treatment was conducted in a water bath (95–98°C, atmospheric pressure) and 250 ml H2SO4 was slowly added while the mixture was agitated at 400 rpm using a magnetic stirrer. After 2 h, samples were washed (i.e., pH 7) with deionized water until reaching a neutral pH. The remaining slurry was subsequently dried in an oven at 130°C for 2 h. The dried bentonites were kept in a desiccator until further use. The acid-activated bentonite was characterized by Scanning Electron Microscopy (SEM) (Tescan Vega3, Czech Republic), Inductively Coupled Plasma Mass Spectrometry (ICP-MS) (Agilent, United States), and X-ray fluorescence (XRF) (Spectro, Germany). The specific area of the bentonite was measured by BET technique with N2 adsorption, at 77 K, using BELSORB Mini instrument (Bel Japan, Inc.) based on the ASTM 4567.
MWCNTs were synthesized through methane decomposition (900°C, atmospheric pressure, 20–50 min) over cobalt-molybdenum nanoparticles supported by nanoporous magnesium oxide during a chemical vapor deposition (CVD) process as described previously (Rashidi et al.,
The prepared MWCNTs were characterized by transmission electron microscopy (TEM, CM30, Philips, Netherland) for morphology determination. For FTIR analysis, the samples were milled with KBr to form a very fine powder, followed by their compression into pellets. FTIR spectra were recorded on a Thermo Nicolet Nexus 670 FT-IR ESP (Thermo Nicolet Corp., Madison, WI, United States). The specific area of the MWCNTs was measured by same BET technique used for bentonite, but was analyzed by an ASAP 2010 (Micromeritics, United States).
Catalytic activities of the acid-activated bentonite (5 wt.% of oil) and SO3H-MWCNTs (3 wt.% of oil) were calculated in respect to the esterification reaction with methanol-to-oil molar ratio of 15:1 and 12:1, respectively. The reaction was conducted in a magnetic stirred tank reactor (450 rpm) equipped with an alcohol reflux system at 62°C. The acid value was determined at certain intervals and the pretreatment step was continued until the value reached about 0.5 mg KOH/g. Samples were withdrawn and immediately placed in an ice bath to stop the reaction, followed by centrifugation (3,500 rpm, 5 min). Then, these samples were used for FFA reduction measurements.
The composition of the methyl esters produced by esterification of residual olive oil and methanol in the presence of acid-activated bentonite or SO3H-MWCNTs as solid acid catalyst was determined using gas chromatography (GC; Claus 580 GC model, Perkin Elmer Co., United States). The conversion of the waste oil into methyl esters was determined by using the following equation:
where ∑A is the total area of the peaks, AIS is the peak of the internal standard (C17:0), m is weight of internal standard, and M is the sample weight.
Acid value of the samples was also measured according to the method of Cd 3d-63 provided by American Oil Chemists' Society.
The morphology of the natural bentonite as shown by SEM are presented in Figure
Inductively coupled plasma mass spectrometry (ICP-MS) and X-ray fluorescence (XRF) results for the used bentonite as well as surface area data obtained using BET technique for bentonite samples activated by different concentration of H2SO4.
Ag | <0.01 | Li | 0.04 |
Al | 270 | Mg | 22.8 |
As | 0 | Mn | 1.4 |
B | 99 | Mo | 0.1 |
Ba | 9 | Na | 94 |
Be | <0.01 | Ni | 0.3 |
Ca | 68 | Pb | 0.3 |
Cd | <0.01 | Sb | 0.0 |
Co | 0.03 | Sc | 0.0 |
Cr | 0.17 | Se | <0.02 |
Cu | 0.06 | Sr | 0.7 |
Fe | 125 | Ti | 11.0 |
K | 32 | V | 0.2 |
La | 0.04 | Zn | 0.3 |
L.O.I. |
8.37 | K2O | 0.94 |
Na2O | 3.17 | MgO | 0.95 |
CaO | 2.38 | TiO2 | 0.46 |
Al2O3 | 12.75 | Fe2O3 | 4.46 |
SiO2 | 64.13 | ||
Raw bentonite | 45 | ||
3N acid-activated bentonite | 62 | ||
5N acid-activated bentonite | 68 | ||
10N acid-activated bentonite | 66 |
The BET surface area of the synthesized SO3H-MWCNTs was measured at 230 m2/g, with the pore volume and diameter recorded at 0.76 cm3/g and 11 nm, respectively. This high BET surface area was because of inducing the repulsion force between SO3H and COOH groups on the MWCNTs surface (Shuit et al.,
FTIR spectroscopy of the SO3H-MWCNTs.
The characteristics of the residual olive oil used in this study, i.e., FFA profiles and physicochemical properties are presented in Table
Free fatty acid profile and physical properties of residual olive oil used in the present study.
Palmitic (C16:0) | 12.29 |
Palmitoleic (C16:1) | 0.61 |
Stearic (C18:0) | 2.44 |
Oleic (C18:1) | 73.67 |
Linoleic (C18:2) | 9.42 |
Linolenic (C18:3) | 0.40 |
Arachidic (C20:0) | 0.36 |
Others | 0.82 |
Free Fatty Acid (wt. %) | 1.3 |
Mean Molecular weight (g/mol) | 885.4 |
Viscosity (mPa.s) | 34.52 |
Density (g/cm3) | 0.89 |
Acid value (mg KOH/g) | 2.5 |
Iodine No. | 94 |
The impact of 5 N acid-activated bentonite and SO3H-functionalized MWCNT as catalyst in the esterification reaction (methanol-to-oil molar ratio of 15:1 and 12:1, respectively), on FFA removal through its conversion into methyl esters and acid value over time.
20 min | 1.27 | – | 2.3 | – | 2.45 | – |
40 min | 1.21 | – | 6.9 | – | 2.39 | – |
1 h | 1.18 | 1.2 | 9.2 | 7.7 | 2.30 | 2.4 |
2 h | 1.14 | 1.14 | 12.3 | 12.3 | 2.27 | 2.27 |
3 h | 1.03 | 0.98 | 20.8 | 24.6 | 2.04 | 1.95 |
4 h | 0.99 | 0.81 | 23.8 | 37.7 | 1.97 | 1.6 |
5 h | 0.97 | 0.71 | 25.4 | 45.4 | 1.91 | 1.41 |
6 h | 0.95 | 0.68 | 26.9 | 47.7 | 1.95 | 1.35 |
7 h | – | 0.60 | – | 53.8 | – | 1.20 |
8 h | – | 0.47 | – | 64.4 | – | 0.89 |
23 h | 0.51 | – | 60 | – | 1.00 | – |
25 h | 0.53 | – | 59.2 | – | 1.08 | – |
27 h | 0.43 | – | 66.9 | – | 0.85 | – |
GC chromatograms used for calculating methyl ester yield through the esterification reaction, with 5 N acid-activated bentonite and the SO3H-MWCNTs as catalyst.
Acid value and FFA content reduction through the esterification reaction
The acid-activated bentonite acted as a solid acid catalyst due to the hydrogen ions already anchored among its smectite's layers during the activation process. In another word, acid-activated bentonite acted as a proton donor catalyst expediting the esterification reaction. As seen in Figure
Moreover, SO3H-MWCNTs was shown to possess favorable catalytic activity as 65% of the contained FFAs were converted, reaching the final concentration of 0.47%, in just 8 h of esterification reaction (Figure
Table
Comparison of some studies in which acid-activated bentonites and SO3H-functionlized multiwall carbon nanotubes (MWCNT) were used as catalysts in FFA esterification process.
Shuit et al., |
SO3H-MWCNTs | 10 wt.% (NH4)2SO4, ultrasonication (10 min), heating (235°C, 30 min) | 92.37 | 12.3 | 0.25 | Palm fatty acid distillate + methanol (1:20) | 2 wt.% catalyst, 170°C, 1 MPa, 3 h | 84.9 |
Current study | SO3H-MWCNTs | Concentrated H2SO4, 110°C, 3 h | 230 | 11 nm | 0.76 | Residual olive oil + methanol (1:12) | 3 wt.% catalyst, 450 rpm, 62°C, 8 h | 64.4 |
Rezende and Pinto, |
Acid-activated bentonite | 10% w/v suspension of clay, H2SO4 (4 mol.L−1), 90°C, 2 h | 137 | 56.6 | 0.21 | Fatty acids from residual palm oil + methanol (1:3) methanol | 50 g catalyst/mol fatty acid, 100°C, 4 h | 89 |
Jeenpadiphat and Tungasmita, |
H2SO4 or HNO3 (bentonite-to acid ratio of 1 g/30 mL), 120°C, 1 h | 42 | 21 | 0.16 | Oleic acid in palm oil + methanol (1:23) | 10 wt.% catalyst, 60°C, 1 h | 99 | |
Current study | Acid-activated bentonite | 5 N H2SO4 (??? wt.%), 95–98°C, 400 rpm, 2 h | 68 | – | – | Residual olive oil + methanol (1:15) | 5 wt.% catalyst, 450 rpm, 62°C, 27 h | 66.9 |
This study showed that bentonite could be applied as an efficient raw material for synthesizing solid acid catalyst, through a simple chemical (i.e., acid) activation method, for using in solid acid catalyzed-esterification of high FFA oils. The application of this natural clay may encourage the production of sustainable fuels, as there is no requirement for sophisticated methods, and environmental hazardous chemicals as well. Acid activation of benonite using 5 N H2SO4 was found to considerably improve its acidity as well as its specific surface area (more than half-time), and in turn, its catalytic activity for FFA content reduction of residual olive oil to <0.5 wt.%. It should also be noted that bentonite is very abundant and cheap (<20 USD/ton) and therefore, 5 N acid-activated bentonite could serve as a promising pre-treatment process for esterification of low-quality oil feedstock prior to alkali-catalyzed transesterification. Moreover, SO3H-MWCNTs were also synthesized and their catalytic performance was characterized. It showed excellent specific surface area (230 m2/g), and good pore diameter (11 nm) and pore volume (0.76 cm3/g), enhancing mass transfer of reaction. Compared with the acid-activated bentonite, SO3H-MWCNTs provided better catalytic performance. Despite more favorable economic preparation of the 5 N acid-activated bentonite and its relatively similar performance in terms of FFA reduction with the SO3H-MWCNTs, the latter catalyst proved more feasible for industrial application. This feasibility can be attributed to its considerably shorter reaction time due to more than 3.3 times higher specific surface area, significantly lowering mass transfer limitation. Further optimization of the esterification reaction conditions would be necessary for full exploitation of these two catalysts for FFA conversion into biodiesel. Overall, both of the prepared catalysts could be used for effective pretreatment of low-quality residual olive oil for sustainable biodiesel production under a biorefinery scheme.
HR performed the bentonite catalyst production, SG and MM assisted with GC analyses, MeA assisted with some characterizations, MT and AR developed the idea and led the research. MT, MoA, and A-SN wrote the manuscript. HP as of substantial help during the revision process.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
We are thankful to Biofuel Research Team (BRTeam) for supporting this study.
Emmett and Teller
Cation exchange capacity
Carbon nanotube
Chemical vapor deposition
Free fatty acid
Fourier-Transform Infrared Spectroscopy
Inductively Coupled Plasma mass spectrometry
Scanning Electron Microscopy
SO3H-functionlized multiwall carbon nanotube
Transmission Electron Microscopy
X-ray fluorescence.