J Am Oil Chem Soc (2017) 94:465–474 DOI 10.1007/s11746-016-2937-z
ORIGINAL PAPER
Biodiesel Synthesis from Palm Fatty Acid Distillate Using Tungstophosphoric Acid Supported on Cesium‑Containing Niobia Chanasuk Surasit1 · Boonyawan Yoosuk2 · Manat Pohmakotr3 · Jonggol Tantirungrotechai1
Received: 24 June 2016 / Revised: 8 December 2016 / Accepted: 12 December 2016 / Published online: 30 December 2016 © AOCS 2016
Abstract Tungstophosphoric acid supported on cesiumcontaining niobia (TPA/Csx/Nb2O5, x = 1.0–2.5) catalysts were prepared by a two-step impregnation method, and their physico-chemical properties were investigated. The initial studies on the esterification of oleic acid with methanol revealed that TPA/Cs ratio affected the acidity as well as the activity of the catalysts. Among the catalysts tested, TPA/Cs1.0/Nb2O5 exhibited the best performance. In addition, the efficiency of TPA/Cs1.0/Nb2O5 for biodiesel synthesis from palm fatty acid distillate (PFAD), a by-product from palm oil industry, was demonstrated, and the reaction parameters were also evaluated. Over 90% yield of FAME was achieved, and the properties of the biodiesel obtained from PFAD met the standard requirements for biodiesel fuel. However, deactivation of the catalysts was observed, possibly due to structural transformation or organic residues blocking the active sites. Electronic supplementary material The online version of this article (doi:10.1007/s11746-016-2937-z) contains supplementary material, which is available to authorized users. * Jonggol Tantirungrotechai
[email protected];
[email protected] 1
Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Center for Catalysis Science and Technology, Mahidol University, Rama 6 Road, Ratchathewi, Bangkok 10400, Thailand
2
National Metal and Materials Technology Center (MTEC), National Science and Technology Development Agency, Thailand Science Park, Phahonyothin Road, Klong Luang, Pathumthani 12120, Thailand
3
Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, Rama 6 Road, Ratchathewi, Bangkok 10400, Thailand
Keywords Palm fatty acid distillate · Biodiesel · Esterification · Solid acid catalyst · 12-Tungstophosphoric acid
Introduction Biodiesel has become a promising alternative fuel to substitute for petroleum diesel because it is renewable, nontoxic, and biocompatible, in addition to having fuel properties close to petroleum diesel [1]. Accordingly, the biodiesel production process has been extensively investigated in order to increase the efficiency and reduce the cost [2]. One attractive approach is to use industrial wastes or by-products as alternative low-cost feedstock. In the palm oil industry, crude palm oil is refined and separated into crude olein, crude stearin, and a lower value by-product, palm fatty acid distillate (PFAD). PFAD is composed mainly of free fatty acids and conventionally used in the manufacture of animal food and laundry detergent [3]. Because of the high free fatty acid content in PFAD, it potentially is an effective raw material for the production of fatty acid methyl ester (FAME) or biodiesel via esterification with alcohol. The commonly used acid catalysts in esterification are inorganic acids such as sulfuric (H2SO4) and hydrochloric (HCl) acids. Even though these homogeneous acid catalysts provide high activity, the complication of catalyst separation and no reusability are their disadvantages. Besides, corrosiveness and waste management are major drawbacks for the industry. Therefore, a number of heterogeneous acid catalysts have been developed for biodiesel production; for example, sulfonic-functionalized resins [4, 5], sulfonic acid-functionalized mesoporous silica [6], tungstated and sulfated metal oxides [7–9], carbohydrate-derived solid acids [10], and supported heteropoly acids [11, 12].
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Despite high fatty acid content, PFAD contains a small amount yet high number of other chemical components. Thus, not only must the catalyst used for the biodiesel production from PFAD be active, but it must also be chemically stable. There are only a small number of reports on biodiesel synthesis from PFAD using heterogeneous catalysts [13–18]. Furthermore, in those reports, leaching of catalytically active species was a problem, or high reaction temperature (>100 °C) was required. Therefore, the development of catalytic systems suitable for biodiesel production from PFAD is of interest. 12-Tungstophosphoric acid (TPA) is an efficient super acid catalyst that is soluble in polar media. There are two approaches to make TPA an insoluble solid catalyst. Cation exchange of proton(s) in TPA with Cs+ or NH4+ leads to lower solubility [19], and dispersion of TPA on a support such as ZrO2 [20], SiO2 [20], or Ta2O5 [11] results in a solid catalyst. In this study, TPA supported on cesiumcontaining niobia was investigated as an efficient catalyst for the synthesis of biodiesel from PFAD. Among metal oxides, niobia (Nb2O5) has moderate acidity, high stability, and good water tolerance, which are important properties for a heterogeneous solid acid catalyst [21]. Cesiumcontaining niobia was employed as a support to strengthen TPA-support interaction and improve the stability of the catalysts. The catalysts were successfully synthesized by a two-step impregnation method. The effect of the stoichiometric component (TPA/Cs ratio) of the catalysts on the esterification ability was examined in the reaction between oleic acid and methanol. The most active catalyst, among the ones tested, was further studied in the production of biodiesel from PFAD, and the effects of reaction parameters such as temperature, methanol/PFAD mole ratio, and catalyst loading were evaluated. In addition, the quality of the PFAD biodiesel was assessed according to the standard procedures.
Experimental Materials In the catalyst preparation, TPA (reagent grade), cesium chloride (CsCl, 99.9%), and niobium(V) oxide (Nb2O5, 99.99%) were purchased from Sigma-Aldrich (USA). Deionized water from Nanopure® Analytical Deionization Water System with an electronic resistance ≥18.2 MΩ-cm was used in all experiments. For the catalytic tests, analytical-grade methanol and n-hexane were obtained from J.T. Baker and RCI Labscan, respectively. Oleic acid (98%) was purchased from Alpha Chemical. PFAD was obtained from AI Energy Company Limited (Thailand), and it consists
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mainly of palmitic acid (C16:0, 36.2%), oleic acid (C18:1, 25.3%), linoleic acid (C18:2, 5.0%), stearic acid (C18:0, 3.0%), lauric acid (C12:0, 2.1%), and myristic acid (C14:0, 1.5%). Preparation of Catalyst Nb2O5 was calcined at 500 °C under air flow for 4 h prior to use. TPA supported on cesium-containing niobia (TPA/ Csx/Nb2O5) catalysts was prepared by a two-step impregnation method. TPA loading was kept constant at 20 wt% in all catalysts and the stoichiometric ratio of Cs/TPA was varied in TPA/Csx/Nb2O5 catalysts, where x was the Cs/ TPA mole ratio of 1.0, 1.5, 1.8, 2.0, 2.3, or 2.5. In the first impregnation step, cesium-containing niobia (Csx/Nb2O5) was prepared by refluxing 25 mL of aqueous solution of desired amount of CsCl with 1.0 g of Nb2O5 at 110 °C for 3 h. The solid product was isolated by slow evaporation of water in an 80-°C oil bath and further dried in an oven at 120 °C for 12 h. In the second impregnation step, a corresponding amount of TPA was added to the aqueous suspension of Csx/Nb2O5, and the impregnation was carried out under the same reaction condition as the first one. Finally, the obtained catalyst was calcined at 400 °C for 4 h to afford TPA/Csx/Nb2O5. Characterization of TPA/Csx/Nb2O5 Catalysts Powder XRD was performed on PANalytical (X’pert powder) diffractometer using a high-power CuKα source monochromator (λ = 1.54056 Å) operating at 45 kV and 45 mA. FT-IR spectra were collected on a Perkin-Elmer system 2000 spectrometer (KBr pellet). Surface area was measured by using Quantachrome Autosorb-1; samples were degassed at 250 °C for 12 h prior to the measurement. The acid strength of samples was estimated using the Hammett indicator method [22]. The catalysts were dried at 120 °C prior to the measurement. 0.5 mL of methanol solution of Hammett indicator was added to the catalyst sample and left to equilibrate for 2 h. The change of the indicator’s color was then noted. The indicators used were methyl orange (pKa = 3.47), benzeneazodiphenylamine (pKa = 1.5), 4-nitroaniline (pKa = 1.1), 2-nitroaniline (pKa = −0.2), 4-nitroazobenzene (pKa = −3.3), benzalacetophenone (pKa = −5.6), and 4-bezoylbiphenyl (pKa = −6.2). The acid site concentration was estimated using potentiometric titration. 75 mg of catalyst were suspended in 40 mL of acetonitrile and the mixture was stirred for 3 h. The suspension was then titrated with 0.0125 M n-butylamine by using Metrom auto-titrator equipped with a double-junction electrode with a flow rate of 0.01 mL/ min.
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Fig. 1 XRD patterns of TPA/ Csx/Nb2O5 catalysts. Filled square and filled triangle denote the crystalline phases of Nb2O5 and CsxH3-xPW12O40, respectively
Intensity (A.U.)
TPA/Cs2.5/Nb2O5 TPA/Cs2.3/Nb2O5 TPA/Cs2.0/Nb2O5 TPA/Cs1.8/Nb2O5 TPA/Cs1.5/Nb2O5 TPA/Cs1.0/Nb2O5 10
20
30
40
50
2-Theta (°)
Biodiesel Synthesis The biodiesel synthesis via the esterification reaction was carried out in a 100-mL two-neck round bottom flask equipped with a condenser. A mixture of methanol and catalyst was added into the reaction flask containing oleic acid or melted PFAD under stirring. The reaction was then set to the desired temperature. A small aliquot was collected from the reaction mixture at different reaction times and then extracted with hexane and water. The hexane layer was collected and dried with anhydrous sodium sulfate. Then, the hexane solvent was evaporated by purging with nitrogen gas. The sample was then submitted to 1H NMR analysis (Bruker Advance DPX 300 MHz) in CDCl3 and the FAME yield was calculated based on the molar ratio of FAME obtained relative to the total fatty acids in the starting material [23]. An example of 1H NMR spectrum of biodiesel (FAME) synthesized from PFAD is included in the supplementary material. No 1H NMR signals of mono-, di-, and triglycerides were observed in any samples.
Results and Discussion Characterization of TPA/Csx/Nb2O5 Catalysts XRD patterns of TPA supported on cesium-containing niobia (TPA/Csx/Nb2O5) are presented in Fig. 1. The impregnation of cesium salt did not alter the crystalline structure of niobia as the diffraction peaks of orthorhombic Nb2O5 were clearly observed at 2θ = 22.6°, 28.4°, 29.0°, 36.7°, 37.1°, and 46.2° [24]. Although cesium species was already impregnated in the niobia support, the cation exchange between the cesium ion
and proton in TPA still proceeded. The characteristic XRD pattern of cesium salt of TPA was observed for all samples at 2θ = 10.6°, 18.5°, 26.2°, 30.3°, 35.6°, and 38.9° [19]. It was previously reported that CsxH3-xPW12O40 with a composition of 0 ≤ x < 2 was a mixture of triclinic H3PW12O40 and cubic Cs2HPW12O40 phases, whereas the one with 2 < x < 3 exhibited a mixture of Cs2HPW12O40 and Cs3PW12O40 [25]. Here, even though the XRD peaks attributed to CsxH3-xPW12O40 were quite broad, possibly due to the low crystallinity and small crystallite sizes, all samples displayed the same pattern. As a result, they must have the same crystalline structure. The calcination in the final synthetic step may ease ion diffusion and homogenize the crystalline phase [26]. FT-IR spectroscopic study (Fig. 2) also confirmed the successful incorporation of TPA into the cesium-containing niobia. All TPA/Csx/ Nb2O5 samples displayed similar FT-IR spectra. The characteristic peaks of the Keggin structure of TPA were found at 1080 and 983 cm−1 which were assigned to P–O and W=O vibrations, respectively [26]. It was reported that the surface area of CsxH3-xPW12O40 increased when the amount of Cs was raised from x = 0 to x = 3 due to the morphological changes [27]. In this study, the surface area of TPA/Csx/Nb2O5 catalysts tend to increase with the increasing content of cesium (Table 1). However, overall, the catalysts possessed low specific surface area (<20 m2/g). All TPA/Csx/Nb2O5 catalysts also exhibited comparable acid strength in the range of −5.6 ≤ H0 ≤ −3.3, suggesting a similar type of acid sites. Nevertheless, the acid site concentration depended on the Cs/TPA ratio (Table 1). As expected, the acid site concentration decreased when the amount of cesium increased. This results from the lower number of acidic protons present at a higher cesium content [27, 28].
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Fig. 2 FT-IR spectra of bulk TPA, TPA/Cs1.0/Nb2O5, and Nb2O5
1080
985
890
820
Transmittance (%)
TPA
TPA/Cs1.0/Nb2O5
Nb2O5
1200
1100
1000
900
800
700
600
Wavenumber (cm-1)
Table 1 Specific surface area and acid site concentration of TPA/Csx/ Nb2O5 catalysts Catalyst
Surface areaa (m2/g)
Acid site concentration (mmol/g)
TPA/Cs1.0/Nb2O5 TPA/Cs1.5/Nb2O5 TPA/Cs1.8/Nb2O5 TPA/Cs2.0/Nb2O5 TPA/Cs2.3/Nb2O5
8.1 –b 13.0 –b –b
0.220 0.190 0.121 0.111 0.081
TPA/Cs2.5/Nb2O5
19.1
0.027
a
BET surface area
b
Not determined
Esterification of Oleic Acid with Methanol Using TPA/ Csx/Nb2O5 Catalysts The performance of the catalysts with different Cs/TPA stoichiometric ratio was evaluated in the esterification of oleic acid with methanol; the results are shown in Fig. 3. Under the same reaction condition, TPA/Cs1.0/Nb2O5 was the most active catalyst, giving 95% yield of oleic acid methyl ester (OAME) at 8 h. With a higher amount of cesium in the catalyst, the catalytic performance was lessened. The surface areas of all catalysts were similar. Hence, these catalytic results are correlated well with the acid site concentration of the catalysts (Table 1). The higher cesium content in the TPA structure led to a lower number of acidic protons which is the active species for the esterification reaction [27–29]. As a result, the more cesium content, the lower acid site concentration, and the less active the catalyst was.
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TPA/Cs1.0/Nb2O5 was, therefore, used for further studies on the biodiesel synthesis from PFAD. Biodiesel Synthesis from PFAD Using TPA/Cs1.0/Nb2O5 Catalysts Reaction conditions can influence both rate and product yield. Therefore, the reaction parameters such as temperature, methanol/PFAD mole ratio, and catalyst loading were evaluated in order to find the suitable condition for biodiesel synthesis from PFAD. Biodiesel or FAMEs can be synthesized from fatty acids and methanol via esterification which is a reversible reaction. In order to drive the reaction forward, an excess amount of methanol is usually required. Thus, the effect of the methanol/PFAD mole ratio was investigated. Figure 4 illustrates the reaction profiles of TPA/Cs1.0/Nb2O5 catalyst for the esterification of PFAD with methanol at 65 °C using 15 wt% catalyst loading, and the methanol/PFAD ratio was varied from 5:1 to 20:1. The result showed that the yield of FAMEs at 8 h was increased from 59 to 90% when the methanol/PFAD ratio was raised from 5:1 to 15:1. In contrast, when the ratio was 20:1, the yield was dropped to 83%, possibly due to the lower concentration of the catalyst and PFAD in the reaction mixture. Hence, the 15:1 ratio was selected for the study of other reaction parameters. Mass transfer is an important issue in heterogeneous catalysis [5, 30]. Even though higher catalyst loading normally yields higher reaction rate, excessive loading may encounter mass transfer limitations due to the aggregation and/or the poor dispersion of catalyst particles. As a consequence, the effect of the catalyst loading on FAME
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100
TPA/Cs1.0/Nb2O5
90 80
TPA/Cs1.5/Nb2O5
70 OAME yield (%)
Fig. 3 Effect of cesium content in TPA/Csx/Nb2O5 catalysts on the esterification of oleic acid with methanol. Reaction condition: methanol/oleic acid mole ratio 20:1, catalyst amount 15 wt%, reaction temperature 65 °C
469
60
TPA/Cs1.8/Nb2O5 TPA/Cs2.0/Nb2O5
50 40 30 20
TPA/Cs2.3/Nb2O5
10 0
Fig. 4 Effect of methanol/ PFAD mole ratio on the esterification of PFAD with methanol using 15 wt% loading of TPA/ Cs1.0/Nb2O5 catalyst and 65 °C reaction temperature
TPA/Cs2.5/Nb2O5 0
1
2
3
4 Time (h)
5
6
7
8
100 90 80
FAME yield (%)
70 60 50 MeOH : PFAD
40
20 : 1
30
15 : 1
20
10 : 1
10 0
5:1 0
1
yield was examined. The loading amount of TPA/Cs1.0/ Nb2O5 catalyst was evaluated in the range of 5–20 wt% (with respect to the weight of PFAD) for the esterification at 65 °C with a 15:1 methanol/PFAD ratio and a constant agitation speed of 700 rpm. The yields as a function of time are presented in Fig. 5. The increase of the catalyst loading raised the catalytic activity as higher catalyst loading means higher number of acidic sites. The maximum yield of 90% was achieved at 8 h of reaction with 15 wt% loading of TPA/Cs1.0/Nb2O5. A comparable yield was observed for 20 wt% catalyst loading because of the aggregation and
2
3
4 Time (h)
5
6
7
8
poorer dispersion of the catalyst particles at this high loading [15]. Reaction temperature always has great influence on the rate of reactions. Therefore, the effect of the reaction temperature on the esterification of PFAD was also studied by varying the temperature from 50 to 80 °C. The reaction profiles for this study are shown in Fig. 6. As the reaction temperature was raised, the yield of FAME was increased. Although the reaction temperature of 80 °C offered the highest initial rate, the yield at 8 h (>90%) was comparable to that of the reaction operated at 65 °C. At 80 °C,
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Fig. 5 Effect of catalyst loading on the esterification of PFAD with methanol using TPA/Cs1.0/Nb2O5 catalyst with a 15:1 methanol/PFAD mole ratio and a 65-°C reaction temperature
100 90 80
FAME yield (%)
70 60 50 40
20 wt.%
30
15 wt.%
20
10 wt.%
10 0
Fig. 6 Effect of reaction temperature on the esterification of PFAD with methanol using 15 wt% loading of TPA/ Cs1.0/Nb2O5 catalyst and a 15:1 methanol/PFAD mole ratio
5 wt.% 0
1
2
3
4 Time (h)
5
6
7
8
100 90 80
FAME yield (%)
70 60 50 40 80 °C
30
65 °C
20
60 °C
10 0
50 °C 0
1
methanol was quickly evaporated and a lot of methanol bubbles were formed in the reaction mixture. These created a gas–liquid system and the mass transfer on the phase’s interface was obstructed. The catalyst was dispersed in the liquid phase. As a result, the performance of the catalyst in the liquid–liquid system should be better than that in the gas–liquid system. Consequently, the reaction operated at 80 °C did not produce better results than the one at 65 °C. Thus, the temperature of 65 °C was preferred to carry out the reaction.
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2
3
4 Time (h)
5
6
7
8
Hence, in this study, the most efficient catalytic system realized for the biodiesel synthesis from PFAD and methanol was to employ TPA/Cs1.0/Nb2O5 catalyst with 15 wt% loading and a 15:1 methanol/PFAD ratio for the reaction at 65 °C. Recyclability of Catalyst In this work, TPA/Cs1.0/Nb2O5 catalyst was recovered and reused in the esterification of PFAD with methanol. After
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100
Fig. 7 Reusability of TPA/ Cs1.0/Nb2O5 catalyst in the esterification of PFAD with methanol at 65 °C using 15 wt% catalyst loading and a 15:1 methanol/PFAD mole ratio
90 80
FAME yield (%)
70 60 50 40 30 20 10 0
1
the reaction run, the catalyst was separated from the reaction mixture by simple decantation of the liquid layer, washed with hexane and water followed by drying in vacuo. Then, prior to the next catalytic run, the catalyst was dried in an oven at 120 °C for 15 h. The result of the catalyst’s reusability is shown in Fig. 7. The yield was gradually decreased from 90% in the first run to 84% in the fourth. A marked loss was observed in the fifth run where the yield was down to 75%. Furthermore, the catalyst appeared as sludge after the second use. The minor components such as carotene and vitamin E present in PFAD might be absorbed onto the catalyst. Hence, the reasons for the deactivation could be due to the organic residues on the catalyst surface blocking the active sites and the loss of the catalyst mass during the recovery process. In addition, the structure of the catalyst might be altered during the course of the reaction. To test the leaching of the catalyst, the amounts of cesium and tungsten in the reaction products from the first catalytic run were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES, Spectro CIROSCCD). Only a small amounts of cesium (<1 ppm) and tungsten (6 ppm) were detected. These results suggested that the prior incorporation of cesium ions onto the niobia support and the low solubility of cesium-exchanged TPA concurrently enhance the stability of the catalyst against dissolution and the leaching may not be the cause for the drop of FAME yields in the subsequent runs. Nevertheless, the blockage by adsorbed species and the alteration of the structure could be responsible for the decline in the catalytic activity. Attempts to reactivate the catalyst by calcination proved unsuccessful.
2
3 Number of reaction runs
4
5
Comparison of Catalytic Activity To further evaluate the catalytic activity of TPA/Cs1.0/ Nb2O5 catalyst, the performance of catalytic systems reported for the synthesis of biodiesel from PFAD are summarized and compared in Table 2. Because of the different reaction conditions, direct comparison of the catalytic activity is difficult. TPA/Cs1.0/Nb2O5 catalyst could catalyze the reaction between PFAD and methanol to produce FAME yield similar to other reported catalysts [13–18]. Our catalyst afforded 90% yield after 8 h of reaction at 65 °C with 15 wt% loading and a 15:1 methanol/PFAD ratio. Higher reaction temperatures (80–170 °C) were required to produce a comparable yield (80–97%) using catalysts such as sugar cane bagasse [14], ZeFeTiO [15], WO3–ZrO2 [16], SO4–ZrO2 [16], and sulfonated beet pulp [18], even though shorter reaction times (0.5–5 h) were needed for these catalytic systems. In addition, no reusability was reported for sulfonated beet pulp catalyst, and leaching of catalytically active species was determined as the problem for the others. In the reusability studies for sugar cane bagasse, ZeFeTiO, WO3–ZrO2, and SO4–ZrO2 catalysts, the yields were dropped by 8–35% after 4 or 5 reaction cycles. For our TPA/Cs1.0/Nb2O5 catalyst, the yield was decreased from 90% in the first run to 84 and 75% in the fourth and fifth runs, respectively. Therefore, in term of the reusability, our catalyst is superior to ZrFeTiO and SO4–ZrO2, but not as good as WO3–ZrO2 and only comparable to sugar cane bagasse. Amberlyst 15 [13] and carbonized sulfonated microcrystalline cellulose [17] could catalyze the reaction to produce 97% yield and 89% conversion, respectively, at
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Table 2 Comparison of catalytic systems reported for the synthesis of biodiesel from PFAD Catalyst
Catalyst loading (wt%)
MeOH/PFAD mole ratio
Reaction temp. Reaction time (°C) (h)
FAME (%) at 1st reaction cycle
No. of reaction FAME (%) at cycles last reaction cycle
Refs.
TPA/Cs1.0/ Nb2O5 Amberlyst 15
15
15:1
65
8
90
5
75
This work
30
20 wt%a
11.5
20 wt%a
60 170
7 0.5
97 80
15 5
97 68
[13] [14]
3 0.5 1 7
3:1 6:1 6:1 3:1
170 80 80 60
5 2 3 3
96.5 84.9 93.7 89b
4 5 5 N/Ac
80 78 61 N/Ac
[15] [16] [16] [17]
14.4
5:1
85
5
92%
N/Ac
N/Ac
[18]
Sugar cane bagasse ZrFeTiO WO3–ZrO2 SO4–ZrO2 Carbonized sulfonated microcrystalline cellulose Sulfonated beet pulp a
wt% of methanol with respect to the weight of PFAD
b
Conversion
c
Information is not available
Table 3 Comparison of the properties of the biodiesel produced from PFAD in this work with the standard specifications
Property
ASTM D6751
EN 14214
Biodiesel produced in this work
Density (g/cm3), 15 °C Kinematic viscosity (mm2/s), 40 °C Flash point (°C) Pour point (°C) Cloud point (°C) Oxidation stability (h), 110 °C
– 1.9–6.0 >130 – – >3
0.860–0.900 3.5–5.0 >101 – – >6
0.899 6.2 135 12 16 11
Copper strip corrosion
3
1
1a
slightly lower temperature (60 °C). However, no result of reusability was available for carbonized sulfonated microcrystalline cellulose catalyst, while Amberlyst 15 could be recycled more than 15 times without losing its activity. Considering the benefit in terms of reactivity and reusability, TPA/Cs1.0/Nb2O5 catalyst would be one of the suitable candidates for the synthesis of biodiesel from PFAD. Although its performance may not be as excellent as Amberlyst 15, it offers very similar, if not better, results when compared to the other reported catalysts. TPA/Cs1.0/Nb2O5 catalyst could provide good yields at reasonable temperature and time, albeit the persisting deactivation problem.
ratio was evaluated according the standard ASTM methods, and the results are shown in Table 3. The viscosity of the biodiesel product was slightly higher than the standard values because of the high content of saturated fatty acids in PFAD. However, this viscosity can be easily adjusted by blending the biodiesel with petroleum diesel. Cloud point and pour point were the same as the reported values for biodiesel produced from palm oil [31, 32]. Furthermore, all other properties tested including density, flash point, oxidation stability, and corrosiveness complied with the biodiesel standards specified in ASTM D6751 and/or EN 14214. Therefore, the biodiesel produced from PFAD in this study is a promising alternative to diesel fuel.
Quality Assessment of Biodiesel Synthesized from PFAD
Conclusions The quality of the biodiesel obtained from the reaction between PFAD and methanol at 65 °C with 15 wt% loading of TPA/Cs1.0/Nb2O5 catalyst and a 15:1 methanol/PFAD
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Tungstophosphoric acid (TPA) supported on cesium-containing niobia prepared by the two-step impregnation method
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exhibited efficient catalytic activity for the biodiesel synthesis. The catalytic performance depended on the amount of cesium. Among the catalysts tested, TPA/Cs1.0/Nb2O5 offered the best activity. The suitable condition for the biodiesel production from PFAD was found to be at 65 °C with 15 wt% loading of TPA/Cs1.0/Nb2O5 catalyst and a 15:1 methanol/PFAD mole ratio, for which a yield of 90% was achieved at 8 h. The catalyst can also be isolated and reused. However, the activity dropped by 15% at the fifth run, perhaps due to the organic residues blocking the active sites and the change of the structure during the reaction course. Nonetheless, most properties of the PFAD biodiesel obtained in this study were acceptable according to the American and European standards. Acknowledgements Financial supports from the Thailand Research Fund (MRG5280098) and Center of Excellence for Innovation in Chemistry (PERCH-CIC), Office of the Higher Education Commission, Ministry of Education are gratefully acknowledged. The authors also acknowledge the support of National Metal and Materials Technology through the P-10-10382 project.
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