Natural Phosphate Supported Titania as a Novel Solid Acid Catalyst

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Natural phosphate supported Titania as a novel solid acid catalyst for oleic acid esterification Younes Essamlali, Mohamed Larzek, Bilal Essaid, and Mohamed Zahouily Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 25 Apr 2017 Downloaded from http://pubs.acs.org on April 25, 2017

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Industrial & Engineering Chemistry Research

Natural phosphate supported Titania as a novel solid acid catalyst for oleic acid esterification Younes Essamlalia, Mohamed Larzekc, Bilal Essaid d and Mohamed Zahouily*a,b a

MAScIR Foundation, VARENA Center, Rabat Design, Rue Mohamed El Jazouli, Madinat El Irfane 10100Rabat, Morocco b Laboratoire de Matériaux, Catalyse et Valorisation des Ressources Naturelles, URAC 24, Faculté des Sciences et Techniques, Université Hassan II, B.P. 146, 20650, Morocco c Mohammed VI PolytechnicUniversity, Lot 660-Hay Moulay Rachid, 43150 Ben Guerir, Morocco d R&D OCP, OCP Group, Complexe industriel Jorf Lasfar, BP 118 El Jadida, Morocco 

ABSTRACT: In the present study, a novel solid acid catalyst based on titanium dioxide (2.5-10wt%) supported on natural phosphate (TiO2/NP) was prepared by sol-gel process, and 10wt% loaded catalyst was characterized by several physicochemical techniques. The performances of synthesized catalyst were investigated in esterification of oleic acid with methanol. The catalytic performance was screened under different reaction conditions, namely, loading amount of TiO2, calcination temperature, molar ratio of methanol to oleic acid, reaction temperature and amount of catalyst. Under optimal conditions, the catalyst loaded with 10wt% of TiO2 and calcined at 800°C showed the highest oleic acid conversion of 87%. The catalyst showed a good stability under high reaction temperature and can be reused without severe loss of activity. As a versatile application, the TiO2(10)/NP catalyst exhibits a good ability for the esterification of free fatty acids in a highly acidic feedstock.

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1. Introduction In the past decades renewable energies have received much attention as a viable solution to overcome the serious environmental and economical issues ascribed to the uses of fossil fuels such as carbon dioxide and greenhouse gas emissions, fossil fuel depletion and volatility of crude oil price1-2. Biodiesel, chemically defined as mixture of mono-alkyl esters of long-chain fatty acids, has been found to be one of the most promising renewable alternative fuels from environmental point of view. It is expected to be a good alternative fuel or an effective additive to the conventional diesel fuel. Biodiesel is typically produced by transesterification of a variety of feedstock such as vegetable oils, animal fats, and waste grease or through esterification of free fatty acids (FFAs).3-5 Recently, biodiesel production through esterification of long-chain fatty acids (FFAs) with short-chain alcohols has got much attention particularly when low-cost feedstock are used.6,7 Biodiesel is conventionally produced using traditional homogeneous alkali catalysts. However, biodiesel production from low cost feedstock such as waste cooking oils and animal fats containing high free fatty acids content required two step process involving combination between acid catalyzed esterification to reduce the acid content and to make these feedstock suitable for the conventional direct base-catalyzed transesterification.8,9 Oleic acid esterification has attracted much attention in the context of biodiesel production since oleic acid is present in many vegetable oils such as rapeseed, soybean and sunflower. Esterification reaction is conventionally carried out by homogeneous catalysis under acidic conditions. Sulfuric acid is the most widely used homogenous acid catalyst for fatty acids esterification.10 Homogeneous catalysts are highly efficient and provide high methyl esters yields in a short time and mild reaction condition but they are non reusable, highly corrosive and required several neutralization and washing steps making the process more complicated and environmental unfriendly.11 Therefore, many research attempts have been devoted to the 2 ACS Paragon Plus Environment

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development of new environmental friendly heterogeneous acid catalysts for biodiesel production by esterification of fatty acids. Among the reported heterogeneous acid catalysts, metal oxides as well as sulphated metal oxides12are the most common source of super acidity, but they were sensitive to moisture content and were unstable due to sulfur leaching. Ion-exchange resins such as Amberlyst-15 and Nafion-NR50 had been successfully demonstrated as excellent acid catalysts for FFA esterification. However, these catalysts are expensive and show bad stability.12,13 Several other solid acid catalysts were investigated in biodiesel production by esterification of fatty acids, some examples include clays,14 heteropolyacids,15,16 metal-containing molecular sieves,17,18 metal-organic frameworks-based solid acids,19,20 acid functionalized silica/mesoporous silica,21,22 mesoporeus cabon based solid acids23-25 and zeolites.26 Unfortunately, many of the aforementioned catalysts encounter several issues such as low surface area and small pore size and serious catalyst leaching.27,28 On the other hand, titanium based catalysts such titanium dioxide (TiO2)39 and titanium (II) oxide (TiO)30 were found to be effective catalysts for biodiesel production through transesterification of vegetable oils or esterification of free fatty acids in

supercritical

conditions. Moreover, Titanyl sulfate (TiOSO4) extracted from the mineral ilmenite (FeTiO3) and calcium titanate (CaTiO3) prepared by solid state reaction were also investigated in free fatty acids esterification and rapeseed oil transesterification, respectively.31,32 More recently, M.C. Manique et al.33 investigated the use of hydrothermally synthesized TiO2 nanotubes as photocatalysts in the esterification of oleic acid in photocatalytic reactor under UV light irradiation. Natural phosphate (NP), being the first mining sector in the Moroccan kingdom, is a natural and low-cost material that exhibits an interesting properties such as ionic substitution ability,



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structural stability and high adsorption capacity making it an attractive cost effective catalyst for several chemical transformations. Natural phosphate-based catalysts have been widely accepted as heterogeneous bifonctionnel catalyst, which can promote several organic reactions.34-38 Motivated by the abundance of phosphate mineral in Morocco, the main objective of this study was to investigate the catalytic activity of a new cost-effective heterogeneous catalyst based on titanium oxide supported on natural phosphate in biodiesel production through esterification of oleic acid as a model reaction. The catalyst has been prepared by a sol gel route using titanium isopropoxide as a precursor and characterized by several physicochemical techniques. The catalytic activity of the developed catalyst was investigated in oleic acid esterification with methanol in a batch reactor under autogenous pressure and correlated with the resulting physicochemical characteristics of TiO2/NP catalyst. Several impact factors were optimized for maximum methyl oleate yield. Moreover, the catalyst reusability was also investigated. Kinetic study was performed and showed that oleic acid esterification with methanol follows a first-order kinetic model. On the other hand, the catalytic behavior of the developed catalyst was also investigated in the esterification of free fatty acids in highly acidic feedstock using mixture oleic acid/rapeseed oil as a model. 2. Experimental section 2.1. Materials Titanium (IV) isopropoxideTi[OCH(CH3)2]4 was supplied by Sigma-Aldrich. Natural phosphate used in the present study was extracted from Khouribga region mines. Oleic acid (technical grade, 90%), methanol (HPLC grade), ethanol and diethyl ether (99.5%) were provided by Sigma-Aldrich. Potassium hydroxide, ammonia solution (28-30wt%) and phenolphthalein were of reagent grade.


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2.2. Preparation of the Catalyst Natural phosphate (NP) used in the present work was firstly sieved and the 100-400 μm fraction was subjected to repeated washing to eliminate soluble components. After drying at 80°C for 12h, the obtained dry powder was treated with 0.1 M HCl solution during 2h at pH = 5. The slurry was then filtered, washed with distilled water until a neutral filtrate. The taken powder was dried in an oven at 80°C to remove any water traces remained, calcined at 700°C for 2 hours and finally grind (63-125 μm) before being used. Natural phosphate-supported titania was prepared by following the same procedure described by Hidalgo-Carrillo et al.39 Briefly, 10 g of NP was introduced in 250 mL flask containing 60 mL of 2-propanol and an appropriate amount of titanium isopropoxide according to prepare a nominal content of 2.5wt% (0.925 mL), 5wt% (1.85 mL) and 10wt% (3.7 mL) (expressed as g of TiO2 per g catalyst). Afterward, a mixture of water/2-propanol (1/10, v/v) was added and resulting mixture was stirred for 30min. The solution was firstly adjusted to pH = 2.5 by adding nitric acid (65%) and then dropwise addition of ammonia solution was used to readjust the pH of the solution to pH = 9. The mixture was stirred at a room temperature for 3h, and then refluxed at 90°C steadily for another 5h. The solid precipitate was collected by centrifugation, washed with 2-propanol to remove the remaining titanium precursor, and dried at 80°C for 24 h. The resulting solid was calcined in air at 500, 700 or 800°C for 2 h. Hereafter, the TiO2/NP catalysts will be designated by TiO2(X)/NP where X stand for the loading amount of TiO2. 2.3. Characterization of the Catalyst The physicochemical characteristics of natural phosphate before and after the incorporation of TiO2 were investigated by several techniques. The structural properties of both NP and TiO2/NP samples were investigated by Powder X-ray diffraction (XRD) using Bruker AXS D8 advance diffractrometer with CuKα radiation (λ = 1.5418 Å) operating at 40 kV and 5 ACS Paragon Plus Environment


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40 mA. The diffraction patterns were scanned in the range of 10-90°, with a step size of 0.02. FT-IR spectra of samples were recorded over the range 4000-400 cm-1at a resolution of 16 cm-1 using ABB Bomem FTLA 2000 spectrometer. UV-vis spectra were recorded on a LAMBDA 1050 UV/Vis/NIR instrument in the range 200-800 nm. Thermogravimetric analysis (TGA) were recorded on a TA instrument Q500 apparatus under air atmosphere at a heating rate of 10°C/min. SEM, TEM and EDX analyses were carried out on a Tecnai G2 microscop at 120 KV. Textural characteristics of the prepared samples such as BET surface area and average pore diameter were determined by N2 adsorption-desorption technique using Micromeritics 3FLEX analyser. Prior to measurements, all samples were degassed at 250°C during 8h under vacuum. 2.4. Catalytic Experiments Esterification reactions were performed under autogenous pressure in a 300 mL stainless steel batch reactor fitted with a mechanical stirrer, thermal controller and sample outlet (Figure 1). In a typical experiment, an appropriate amount of oleic acid and methanol, with respect to methanol/acid molar ratio, and a predetermined amount of catalyst (based on the oleic acid weight) were introduced into the reactor, and then heated to the desired temperature under vigorous stirring during 8h of reaction. The reaction temperature was controlled by a temperature sensor. 0.5 mL samples were withdrawn from the reactor periodically and then analyzed. After reaction completion, the catalyst was separated by centrifugation and the excess of methanol and water formed during the reaction were eliminated by rotary evaporation under reduced pressure. The prepared methyl ester was characterized by 1H-NMR and GC-MS techniques. The percent conversion of oleic acid to the corresponding ester was estimated by the measurement of the acid value (AV)40 using the following equation: %𝐂 = [(𝐀𝐕𝐢 − 𝐀𝐕𝐭)/𝐀𝐕𝐢] × 𝟏𝟎𝟎


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where C is the percent conversion of oleic acid, AVi and AVt represent the initial and final acid values of the reaction mixture, respectively. All the experiments were performed in triplicate and the reported values have the same averages of three experimental results with a relative standard deviation of ±2%. The conversion values determined by titrimetric method were further confirmed by 1H-NMR according to the method developed by Satyarthi and co-workers.41

Figure 1. The overall oleic acid esterification with methanol

2.5. Methyl oleate characterization Gas chromatography-Mass spectroscopy (GC-MS) of methyl oleate was performed on Agilent 7890A equipped with multimode injector and DB-17HT column with dimension of 30 m x 250 m x 0,15 m using methyl 10-undecenoate as an internal standard. Two µL of sample in chloroform was injected into column by splitless mode using helium as carrier gas at 1.5 mL/min. The column temperature program was started at 50°C for 1 min and then increased at 15°C/min until 180°C before to being ramped to 230°C at a heating rate of 7°C/min, followed by a further increase to 340°C at 10°C/min and finally kept constant for 7 ACS Paragon Plus Environment


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10 min. The ion source and quadruple temperatures were 230 and 150°C respectively. MS detection was done in the m/z range of 35-600 with gain factor 5. Fatty acid esters identification was performed using the National Institute of Standard and Test (NIST) 2011 MS Library. Nuclear magnetic resonance (1H-NMR) spectra of the produced methyl ester were recorded on a BRUKER AVANCE 600 MHz spectrometer. Deuterated chloroform (CDCl3) was used as solvent and TMS as an internal standard. 3. Results and discussion 3.1. Catalyst Characterization. XRD patterns of pure NP, TiO2 and TiO2(10)/NP catalyst calcined at different temperatures are plotted in Figure2. The XRD pattern of pure NP (Figure 2A, spectrum a) exhibits clear diffraction peaks of carbonate fluorapatite according to JCPDS: 01-073-9697. No characteristic peaks of other secondary phases such as calcium carbonate (CaCO3) or quartz (SiO2) were observed in the pattern, indicating pure carbonate fluorapatite structure. The diffraction peaks are very intense, suggesting a high crystallinity the apatite phase. The chemical analysis of the support indicates that the measured Ca/P atomic ratio is 1.71which is higher than the stoichiometric ratio of 1.67, suggesting partial substitution of phosphate groups by carbonate ones. Furthermore, XRD pattern of TiO2 prepared by sol gel process (Figure 2A, spectrum b) under the same reaction condition showed a broad characteristic anatase peaks at 2θ ≈ 25.2, 37.7, 53.8, and 55° assigned to the diffraction of (101), (004), (105), and (211) planes of TiO2 anatase phase, respectively. In the XRD patterns of TiO2(10)/NP dried at 80°C, only the diffraction peaks of NP were observed. The absence of characteristic peaks of TiO2 could be explained by retardation of TiO2 crystallisation when it is supported on natural phosphate40 or by the fact that the TiO2 particles could be very small in size and highly dispersed on NP surface there for not be detected by XRD. When the catalyst was calcined at 500°C and 700°C for 2h, new diffraction peaks were observed. The 8 ACS Paragon Plus Environment

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most characteristic peak at 2θ ≈ 25.2°, due to the anatase (TiO2) phase, indicating the successfully incorporation of titania into the NP support surface. Subsequent calcination at 800°C resulted in the disappearance of anatase peaks and the appearance of a new diffraction peaks attributed to the phase of rutile. We also observed the presence of a new phase that was related to calcium titanate (CaTiO3) of perovskite-type structure. The formation of rutile and calcium titanate was confirmed by the appearance of new intensive peaks at 2θ ≈ 27.4° and 33° according to the JCPDS files N°: 01-089-0552 and 00-042-0423, respectively. According to the XRD results, a partial breaking of the crystalline structure of the natural apatite was probably the main cause of the formation of calcium titanate phase.

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FTIR spectra of NP and TiO2(10)/NP catalyst (see Figure S1 in the SI) showed many characteristic bands of apatite structure. The typical bands at 1095 and 1034 (υ3), 962 (υ1), and 602 and 565 cm-1 (υ4) could be assigned to the vibration modes of PO43- groups. Compared to the NP support, the infrared spectra of the TiO2(10)/NP catalyst showed broad peak in the region of 600-800 cm-1 which become more intensive and evident with subsequent calcination temperatures. This broad peak may be attributed to the Ti-O-Ti band of TiO2 as detected by XRD, suggesting the successfully incorporation of TiO2 into the NP support. Thermal stability of the TiO2(10)/NP was evaluated from its TGA thermogram (see Figure S2 in the SI). The TGA results show two characteristic in interval from 270 and 432°C. The first one, up to 270°C, is mainly attributed to the loss of low volatile components such as water and isopropanol and physisorbed water. The second mass-loss at 432°C can be attributed to the decomposition of residual organic matter mainly the non-reactive titanium precursor and the elimination of the OH groups attached to the surface of TiO2 nanoparticles.39 The UV-visible spectra of TiO2 and TiO2(10)/NP samples calcined at different temperatures are depicted in Figure S2 (see Figure S2 in the SI). The calcined samples show similar and very strong absorption bands in the 200-350 nm wavelength regions, centered at around 215, 270 and 307 nm. The first band was attributed to Ti(IV) species in tetrahedral coordination39,42 while the other adsorption bands could be assigned to Ti atoms with octahedral symmetry or to partially polymerized Ti species, which contain Ti-O-Ti bonds.43-45 We note that the obtained results are in good agreement with those reported by HidalgoCarrillo et al.39 The N2 adsorption-desorption isotherm of TiO2(10)/NP catalyst corresponds to type-IV isotherm according to the IUPAC classification and exhibited an H3 hysteresis loopappearing at high P/P0 region of 0.5-0.96, suggesting the presence of very homogeneous slit-shaped


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mesopores (Figure 3). As expected, incorporation of TiO2 on NP results in a significant increase in surface area of the prepared catalyst (35.55 m2/g) compared to the NP alone (2 m2/g) or pure TiO2 prepared in the same conditions as TiO2(10)/NP (30 m2/g). The average pores size estimated according to the BJH method were about 18.93 nm, which reflect the existence of textural mesopores. Similar results were reported by J. Hidalgo-Carrillo et al.39, in the investigation of the photocatalytic activity of different titanium-based systems supported on natural phosphate. The increase in surface area may be attributed to the existence of a small TiO2 nanoparticles highly dispersed on the NP surface.

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Figure 3. Nitrogen adsorption-desoption isotherm and pore size distribution of TiO2(10)/NP catalyst calcined at 800°C The scanning electron microscope images of natural phosphate and TiO2(10)/NP samples calcined at 700 and 800°C show that the surface morphology of the catalyst is not identical to that of NP support. A significant change in the surface morphology of the support was observed (Figure 4). The SEM image of the NP shows the presence of micro-particles of



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irregular geometries in the agglomerated form. Also the SEM images show that the incorporation of TiO2 resulted in the deposition of small sphere like TiO2 nanoparticles with uniform shape and size.

Figure 4. SEM images of: traited NP and TiO2(10)/NP catalyst calcined at 700 and 800°C/2h; Semi quantitative EDX analysis of TiO2(10)/NP catalyst calcined at 800°C/2h Elemental analysis by energy-dispersive spectroscopy (EDS) confirmed the successfully incorporation of TiO2 into the NP surface. As it was observed, the EDS spectra of the NP showed the characteristic peaks of Ca, P, O, and C elements, whereas, the EDS spectra of TiO2(10)/NP clearly demonstrates the existence of Ca, P, O, Ti, and C elements in the synthesized catalyst (Figure 4). The Ca/P atomic ratio of NP was found to be 1.7, which is quite close to the Ca/P ratio measured by chemical analysis. Additionally, no significant difference was detected for TiO2(10)/NP which exhibits a Ca/P atomic ratio of 1.79. Semi quantitative analysis of this sample showed the following elemental composition in % weight: C: 9.52; O: 45.77; F: 6.78; P: 10.06; Ca: 18.03; Ti: 9.84.


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TEM images of TiO2(10)/NP (Figure 5) shows that the catalyst consist of very small TiO2 nanoparticles, which have sizes between 3 and 6 nm, highly dispersed on NP support. The analysis of different zones of TiO2(10)/NP catalyst indicates more or less a homogeneous distribution of TiO2 nanoparticles on the surface of the NP. The use of a large ratio of water to alkoxide promotes the anisotropic crystal growth of TiO2, producing particles with morphology nearly spherical.

Figure 5. TEM images of TiO2(10)/NP catalyst calcined at 800°C/2h

3.2. Catalytic Activity in oleic acid esterification The esterification reaction is influenced by several parameters. In an attempt to find out the suitable reaction condition providing a maximum methyl ester yield, effects of different reaction conditions such as the TiO2 loading, the calcination temperature, the reaction temperature, the molar ratio of methanol to oleic acid, the amount and type of catalyst and the reaction time were studied and discussed. It is well known that the catalyst preparation condition strongly modifies its physicochemical properties and the catalytic activity of a given catalyst. To improve the acidic properties and therefore the catalytic activity of the prepared catalysts, TiO2 loading amount and calcination 13 ACS Paragon Plus Environment


temperature were varied during the catalyst preparation. The activity of the prepared catalyst was studied in the esterification of oleic acid with methanol in a period of 8h using a molar ration of 8:1 at 150°C. As shown in Figure 6, the catalytic activity of TiO2(2.5)/NP, TiO2(5)/NP, TiO2(10)/NP and TiO2(15)/NP catalysts was far superior to that corresponding to NP alone indicating that the incorporation of titanium species generates a considerable amount of acidic sites and clearly improve the catalytic activity of the prepared catalysts. When the loading amount of TiO2 was increased from 2.5 to 10wt%, there was an obviously increase in oleic acid conversion from 77 to 87%. However, no improvement was detected after using increasing loading amount of TiO2 up to 15wt%. Thus, the TiO2(10)/NP catalyst could be selected as the best catalyst and was used for further studies.

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Figure 6. Effect of TiO2 loading on oleic acid conversion, reaction conditions: oleic acid/methanol molar ratio 1:8, amount of catalyst 10wt%, reaction temperature 150°C and reaction time 8h. The influence of the calcination temperature of the TiO2(10)/NP catalyst on the percentage of the conversion was also investigated (Figure 7). Indeed, a series of experiments were carried out using TiO2(10)/NP catalyst calcined at 550, 700 and 800°C. From the obtained result, it 14 ACS Paragon Plus Environment

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can be observed that the oleic acid conversion increased simultaneously with the calcinations temperature increased. The calcination of TiO2(10)/NP at 500°C gives 76% conversion. When the catalyst is calcined at 700°C, a slight increase in the conversion ratio is observed (80%). Subsequent calcination at 800°C led to the maximum conversion of 87%. Higher calcinations temperature leads to the breaking of the crystalline structure of the natural apatite and the catalyst become no more stable above 800°C as shown by TGA curve (see Figure S2 B in the SI). In addition, this increase in calcination temperature would lead to the collapse of the mesoporosity and increase the size of the crystallites. On the other hand, the increase in calcination time of the catalyst will ensure its crystallization and the elimination of organic matter but increase the cost of the process. The optimum calcination temperature was to be 800°C during 2h.

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The aforementioned explanations are consistent with XRD results shown in Figure 2A, where the anatase phase is detected for samples calcined at temperatures below 700°C. The conversion reaction increase in this range might be justified by the improvement of the crystallization of the anatase phase of TiO2(10)/NP catalyst. According to the XRD patterns (Figure 2) the intensity of the anatase peaks increase significantly when the temperature was increased from 550 to 700°C. For sample calcined at 800°C, the transformation of anatase into rutile and the formation of calcium titanate (CaTiO3) phase might be the reason for the conversion improvement. According to the results of catalyst preparation condition, the TiO2(10)/NP catalyst calcined at 800°C/2h was chosen as the most active one and was used for the investigation of other reaction parameters. It is well known that the stoichiometric methanol to oleic acid molar ratio is 1, however an excess amount of methanol is required to drive the reaction equilibrium towards methyl oleate production. Dependence of the oleic acid conversion on the methanol: oleic acid molar ratio is shown in Figure 8. The esterification reaction was performed by varying the molar ratio from 4:1 to 20:1, while keeping the other parameters constant. At molar ratio of 4:1, the oleic acid conversion was close to 40%, further increase in molar ratio to 6:1 and 8:1 resulted in a significant increase in the conversion up to 85 and 87%, respectively. With further increase in molar ratio above 8:1, a drop of about 20% in the conversion was observed (67% at 20:1 molar ratio). The decrease conversion of oleic acid at higher methanol ratio is already described in the literature.46 It is attributed to the formation of a greater amount of water, moving the equilibrium in the direction of the inverse reaction. These findings were further supported by Manique al. when using hydrothermally produced TiO2 nanotubes in photocatalytic esterification of oleic acid.33 Furthermore, Zhang et al. also reported that further excess of methanol might also introduce more water, which may react


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with the FAME to convert back to oleic acid.47 Hence, the molar ratio of 8:1 was selected to be the optimal oleic acid/methanol molar ratio.

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Reaction temperature plays an important role in a heterogeneous catalytic process. Figure 9 depicts the influence of reaction temperature on the oleic acid esterification catalyzed by TiO2(10)/NP. Obviously, the oleic acid conversion increases from 16 to 87% as the reaction temperature increases from 80 to 150°C, indicating that the oleic acid conversion has a strong dependency on the reaction temperature. At low reaction temperatures (80°C), the reaction did not fully complete and showed low conversion value (16%). When the reaction temperature was increased from 80 to 100°C, the oleic acid conversion increased from 16 to 50%, respectively. Figure 9 shows a little difference in conversion between the reaction performed at 100°C (50%) and 120°C (53%). At higher reaction temperature (150°C), the oleic acid conversion reached a maximum value of 87%. Such behavior might not be only



explained by the increase of the reaction rate at a higher reaction temperature but can also be due to some improvement of the mass transfer limitation between the reactant and the catalyst.46 As a result, 150°C was chosen as the optimized reaction temperature. 100 90 80


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Reaction temperature (°C)

Figure 9. Dependence of oleic acid conversion on reaction temperature

To investigate the effect of the amount of catalyst on oleic acid conversion, the esterification reaction has been studied using different amounts of the catalyst ranging from 5 to 15wt% while keeping the other experimental conditions constant. As depicted in figure 10, when the catalyst amount is increased from 5 to 10wt%, the conversion slightly increases from 80 to 87%. However, further increase in the amount of catalyst up to 15wt%, do not affect the oleic acid conversion which does not exceed 87%. Hence, the optimal catalyst amount was found to be 10wt%.


Page 19 of 35

88 87 86

Oleic acide conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


85 84 83 82 81 80 79 5

7,5 Catalyst amount (wt%)

10

Figure 10. The effect of catalyst loading on oleic acid conversion

The catalytic behavior of our catalyst could be attributed to both the acid species present in the support and the well-dispersed TiO2 nanoparticles containing Lewis acid sites. Moreover, the incorporation of TiO2 nanoparticule on NP resulted in a significant increase in surface area and pore diameter and consequently the surface contact between the catalyst and reactants has been increased. Moreover, since TiO2(10)/NP exhibits a mesoporous structure with broad pore size distribution, effective diffusion and accessibility of reactant molecules to the catalytic active sites occur. The effect of the reaction time on the oleic acid esterification was studied over the TiO2(10)/NP catalyst (10wt%) using a molar ratio of 8:1 at 150°C. The conversion increases proportionally as a function of the reaction time (Figure 11). Indeed, when the reaction time was increased from 1 to 6h, the conversion value also increased and reached 19 and 73%, respectively. A near-equilibrium oleic acid conversion was found 87% at 8h reaction time. Prolonged reaction time does not affect the conversion rate since a value of 87% was obtained after 9h of reaction. Therefore, the optimum reaction time for high oleic acid conversion is 8h. 19 ACS Paragon Plus Environment


100

80


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

40

20

0 0

1

2

3

4

5

6

7

8

9

Time (hours)

Figure 11. The effect of reaction time on the oleic acid esterification reaction

During the esterification of oleic acid with methanol, aliquots of reaction mixture were withdrawn at different interval of time and then analyzed by 1H-NMR to monitor the esterification progression (see Figure S3 in the SI). The withdrawn samples were firstly evaporated and then diluted with CDCl3 before being analyzed. At the beginning of the reaction, the oleic acid exhibited one triplet at 2.34-2.37 ppm belonged to the methylene group (α-CH2) adjacent to the carbonyl group. Due to the deshielding effect of the carboxylic group, α-CH2 signals of oleic acid (2.34-2.37 ppm) appear at chemical shift higher than those of the methyl oleate (2.30-2.33 ppm).45 As the esterification progressed, the intensity of the α-CH2 signals due to oleic acid decreased and those corresponding to methyl oleate increased (see Figure S3 in the SI). In addition, the appearance of new signal at 3.6 ppm attributed to the protons of methoxy group of methyl oleate confirms the successfull esterification of oleic acid.


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Quantification of the FFA content was performed by 1H-NMR through the measurement of the areas of the methylene group of both oleic acid and methyl oleate.45 The FFA content can be estimated using the following equation:

% oleic acid= [

Area of α-CH2 of oleic acid ] ×100 Total area of α-CH2 of both oleic acid and ester % Conversion = 100 - % Oleic acid

The results showed that the oleic acid content in sample withdrawn at t = 3h (see Table S1 in the SI) determined by titration (47%) is slightly higher than that determined by the 1H-NMR (41%) technique. Moreover, when the contact time was extended to 7h, the oleic acid content was found to be decreased (see Table S1 in the SI) towards methyl oleate production. The values estimated by the titrimetric method (19%) were similar to those determined by 1

H-NMR (18%). After 8h of reaction, the oleic acid content measured by both titration and

1

H-NMR was about 13 and 11%, respectively. These results suggest that the conversion rate

estimated by titrimetric method correlated well with that determined by

1

H-NMR

spectroscopy. In the present study the esterification of oleic acid with methanol was performed in the presence of a large excess of methanol corresponding to the molar ratio of 8:1. Thus, the concentration of methanol remains constant throughout the esterification reaction which is expected to follow a pseudo first order kinetic model44 as expressed by the following equation (1). − ln(1 − 𝐶) = 𝑘𝑡 where k = rate constant of the pseudo first order model, C = conversion of oleic acid and t = time. The relationship between ln(1-C) and time should be represented by a linear plot, the slope of this plot represents the pseudo first order rate constant (k). The plot of ln(1-conversion) versus time shows a linear relationship (see S4 in the SI) with high linear 21 ACS Paragon Plus Environment


Page 22 of 35

correlation coefficients (R2), therefore, the kinetic behavior of the oleic acid esterification catalyzed by TiO2(10)/NP followed the pseudo-first-order kinetic model. The plot was used to measure the rate constant (k) of this chemical transformation using the pseudo-first-order rate law. The first order rate constant of TiO2(10)/NP-catalyzed oleic acid esterification, evaluated from the slope of the plot, was found to be 0.23 h-1. The obtained value is very close to that found by Hasan and co-workers (0.28 h-1) in the kinetic study of oleic acid esterification catalyzed by mesoporous sulfonated silica.44 It has been reported that the support plays an important role in a catalytic system. It can act as a simple dispersant or by developing a strong interaction with the supported active specie. In this context, the catalytic activity of our catalyst was compared to other catalyst prepared in the same way as TiO2(10)/NP and containing the same amount of TiO2 such as TiO2(10)/FAP and TiO2(10)/HAP. The esterification reaction was performed at 150°C during 8h using a molar ratio of 8:1 and an amount of catalyst of 10wt%. As illustrated in Figure 12, all the prepared catalysts displayed a good catalytic activity in the oleic acid esterification. The TiO2(10)/FAP and TiO2(10)/NP catalysts exhibited similar catalytic activity. The oleic acid conversion obtained after 8h of reaction at 150°C was 74 and 87% using TiO2(10)/HAP and TiO2(10)/FAP, respectively. Hence, the catalytic behavior of these catalysts could be attributed to the different nature of the support employed.


Page 23 of 35

90 80


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70 60 50 40 30 20 TiO2/HAP

TiO2/PN

TiO2/FAP

Catalysts

Figure 12. Catalytic performance of the catalysts with TiO2 loaded on HAP or FAP.

3.3. Effect of alcohol chain length In the present study the esterification of oleic acid with different alcohols such as ethanol and 2-propanol was also studied. The obtained results indicate that the effectiveness of the esterification process was much higher when short chain alcohol such as methanol was used. The oleic acid conversion was found to be 87, 35 and 30% using methanol, ethanol and 2-propanol, respectively. Similar results were observed for the esterification of palmitic and oleic acid over Al-MCM-41 and activated metakaolin, respectively.45,46 The observed behavior was probably due to the lower alcohol nucleophilicity and the steric hindrance effect when long chain alcohols were used.48 3.4. Catalyst stability and reusability Life cycle and reusability are one of the most important features of a heterogeneous catalyst. To investigate the reusability of the TiO2(10)/NP catalyst, the catalytic activity of this catalyst was evaluated in six consecutive cycles. After the first batch reaction, the used catalyst was



separated from the reaction mixture by centrifugation, and then it was washed with ethanol and water for complete removal of residual reactants, and finally dried at 100°C for 4h before being reused. As shown in Figure 13, TiO2(10)/NP catalyst still exhibits a good catalytic activity in the first three reuse cycles, above 82% conversion. However, noticeable drop in oleic acid conversion (17%) was observed after six consecutive runs. This drop in catalytic activity may be due to adsorption of the reactants, which poison the catalytic surface of the catalyst. The TiO2(10)/NP catalyst exhibits a good reusability and stability in the oleic acid esterification due to the high stability of NP support and the strong interaction between NP and titanium species.

100 87%

84%

82%

80


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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79%

76% 70%

60

40

20

0 1

2

3

4

5

6

Number of cycle

Figure 13. Reusability of TiO2(10)/NP-800 in esterification of oleic acid. (Reaction conditions: Catalyst/Oleic acid 10wt%, Methanol/Oleic acid molar ratio 8:1, 150°C, 8 h)

Additionally, in order to check the evolution of the TiO2(10)/NP catalyst regenerated after the sixth cycle, the characterization of the reused catalyst compared with fresh one especially in SEM and elemental analysis was performed. SEM images (Figure 14) showed that the


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recycled catalyst still maintains a good morphology, but the particles tend to agglomerate and formed a small block.

Figure 14. SEM images of recycled TiO2(10)/NP catalyst after the sixth cycle

EDS analysis of the surface of the recycled TiO2(10)/NP catalyst (6th use) was carried out and the result was summarized in Table 1. Three zones on the catalyst surface were analyzed and the values in Table 1 were the averages of the three measurements. Table 1. EDS analysis of the surface of the fresh and recycled TiO2(10)/NP Catalyst

Elements content (wt%) Ca

P

O

Ti

C

TiO2(10)/NP (fresh)

18.03

10.06

45.77

9.84

9.52

TiO2(10)/NP (6th use)

17.8

9.32

45.22

9.2

12.3

From table 1, it is clear that the recycled TiO2(10)/NP catalyst showed the same characteristic peaks of Ca, P, O, Ti, and C elements compared to the fresh one. Indeed, the Ti contents was slightly decreased from 9.84 to 9.2% by weight and the carbone content was found to be increased from 9.52 to 12,3% by weight. The drop in oleic acid conversion could be mainly due to the gradual poisoning of the catalyst by adsorbed organic matter and the blocking of the pores of the catalyst.



Page 26 of 35

In the light of the ofermentioned results, the good stability of the TiO2(10)/NP catalyst might be due to the high calcination temperature of catalyst (800°C) and the good distribution of the TiO2 species on the NP support. 3.5. Possible reaction mechanism A possible oleic acid esterification mechanism involves several reaction steps (Figure 15). The presence of Ti species on the surface of the NP support can act as Lewis acid in catalyzing oleic acid esterification. Initially, oleic acid molecule was adsorbed on catalyst surface, the interaction between carbonyl oxygen of oleic acid and acidic site of the catalyst generate an electro-deficient carbon. Nucleophilic attack by the methanol’s oxygen atom present in the reaction mixture on the generated carbocation gives a tetrahedral intermediate. Internal rearrangement, followed by elimination of water molecule from the tetrahedral intermediate resulted in the production of methyl oleate.

Figure 15. Plausible reaction mechanism of oleic acid esterification over TiO2(10)/NP-800 catalyst 26 ACS Paragon Plus Environment

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3.6. Qualitative analysis of the methyl oleate The profile of the produced methyl oleate was investigated by gas chromatography (see Figure S5 in the SI). The reaction product was mainly consisting of: methyl oleate (C18:1), methyl linoleate (C18:2), methyl linolenate (C18:3) and other Fatty acid methyl esters such as methyl myristate (C14:0), methyl palmitate (C16:0) and palmitoleate (C16:1) and trace of methyl heptadecanoate (C17:0) and heptadecenoate (C17:1). 3.7. Esterification of acidic feedstock As a versatile catalytic application of TiO2(10)/NP, the catalytic performance of the developed catalyst was also investigated for the esterification of acidic feedstock using a mixture of 5wt% oleic acid in rapeseed oil as a model of high free fatty acid oil. The initial acid value of the mixed oleic acid/rapeseed was found to be 29.43 ± 1.0 mg KOH/g. The esterification experiment was performed in a batch type stainless steel batch reactor under the optimal reaction condition obtained in this work. After reaction completion, the catalyst was collected by centrifugation and the reaction mixture was subjected to rotary evaporation for excess metal recovery. The acid value of the resulting mixture was determined by titrimetric method according to the previous report.40 Based on Figure 16, it was found that our catalyst exhibits a good catalytic activity in the esterification of free fatty acids in a fatty acid/triglyceride mixture. At a temperature of 150°C for a time period of 8 h, a maximum conversion of 74.21% was reached. It can be concluded that the TiO2(10)/NP catalyst has the ability to be used as a heterogeneous acid catalyst for the removal of FFA in acidic feedstock.



80

70


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 35

60

50

40

30

20 1

2

3

4

5

6

7

8

Reaction time (h)

Figure 16. Free fatty acids esterification in an oleic acid/rapeseed oil mixture

3.8. Comparison with the Reported Catalysts Several solid acid catalysts have been investigated for the oleic acid esterification. The competitiveness of our catalyst was examined against the other reported catalysts as illustrated in Table 2. As shown in Table 2, the synthesized catalyst showed competitive catalytic activity with respect to the other catalytic systems, such as organophosphonic acidfunctionalized silica,49 Zr/Al2O3,50 WO3/ZrO2,51 sulfonated OMCs (SOMCs),52 Nb2O5,53 aminophosphonic acid resin D418,54 which not only required high reaction temperature, but also high alcohol to substrate molar ratio and longer reaction period to achieve complete conversion of fatty acid into respective ester.


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Table 2. A comparison of catalytic activity of TiO2(10)/NP-800 with other catalysts for the esterification of oleic acid.

Entry

Catalyst

1 2

TiO2(10)/NP-800 Organophosphonic acid-functionalized silica Zr/Al2O3 WO3/ZrO2 The sulfonated OMCs (S-OMCs) Nb2O5 Aminophosphonic acid resin D418

3 4 5 6 7

Type of alcohol

Time (h)

Temp. (°C)

Methanol Ethanol

8 10

150 120

Methyl oleate yield (%) 87C 78Y

Methanol Methanol Methanol

7 10 8

200 200 160

90Y 67 97

[50] [51] [52]

Ethanol Ethanol

80 10

200 120

81,92 92

[53] [54]

Reference This work [49]

4. Conclusions In this study, mesoporous TiO2/NP with TiO2 loadings up to 10wt% was prepared by a sol gel technique and its catalytic activity was investigated in the oleic acid esterification with methanol at 150°C under autogeneous pressure. Among all the catalysts examined, TiO2(10)/NP calcined at 800°C exhibited the best catalytic activity and provided a 87% conversion under the following reaction conditions: reaction temperature 150°C, methanol to oleic acid molar ratio of 8:1, catalyst loading of 10wt% and reaction period of 8h. Furthermore, the TiO2(10)/NP-800 catalyst exhibited good stability and reusability since it maintains its activity up to six runs without significant decrease in activity. Additionally, the kinetic of the esterification of oleic acid was also studied and was found to follow pseudo first order kinetic model with an apparent first order rate constant of about 0.233 h-1. As a conclusion, TiO2(10)/NP catalyst is a potential candidate for the esterification of free fatty acids in highly acidic feed stocks since it can significantly reduce the FFA content in low cost feedstock.



Page 30 of 35

Supporting Information Additional experimental data are available is supporting information: Figure S1. FTIR spectrum of a) pure NP and calcined TiO2(10)/NP at (b) 550°C, (c) 700°C and (d) 800°C, Figure S2. A) UV-visible spectra of a) calcined TiO2, calcined TiO2(10)/NP at b) 550°C, c) 700 and d) 800°C. B) TGA curve of TiO2(10)/NP catalyst, Table S1. Comparison between oleic acid content and conversion determined by titration and 1H-NMR, Figure S3. Evolution of the 1H-NMR spectra of the reaction mixture of oleic acid esterification over TiO2(10)/NP as a function of time, Figure S4. Plot of -Ln (100-C) versus time (h) for oleic acid esterification catalyzed by TiO2(10)/NP, Figure S5. Typical GC-MS chromatogram of methyl oleate. Corresponding Author * Telephone: +212- 661 416359. E-mail: [email protected]. Acknowledgments: The financial assistance of the MAScIR Foundation and the Office Chérifien des Phosphates in the Moroccan Kingdom (OCP Group) is hereby acknowledged. We equally thank the administrative and technical support team of the MAScIR Foundation. References (1) Banković-Ilić, I. B.; Stojković, I. J.; Stamenković, O. S.; Veljkovic, V. B.; Hung, Y-T. Waste animal fats as feedstocks for biodiesel production. Renew. Sustain.Energy Rev. 2014, 32, 238-254. (2) Balat, M.; Balat, H. Progress in biodiesel processing. Appl. Energy 2010, 87, 18151835. (3) Soldi, R. A.; Oliveira, A. R. S.; Ramos, L. P.; César-Oliveira, M. A. F. Soybean oil and beef tallow alcoholysis by acid heterogeneous catalysis. Appl. Catal. A 2009, 361, 42-48. (4) Verziu, M.; Florea, M.; Simon, S.; Simon, V.; Filip, P.; Parvulescu, V. I.; Hardacre, C. Transesterification of vegetable oils on basic large mesoporous alumina supported alkaline


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For Table of Contents Only

Natural phosphate supported titania (TiO2/NP) was investigated in the esterification of oleic acid with methanol, in a stainless steel batch reactor for biodiesel production in good to excellent yield.


Natural Phosphate Supported Titania as a Novel Solid Acid Catalyst

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