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Ion-exchange resins as catalysts in transesterification of triolein
Catalysis Today 212 (2013) 157–163
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Ion-exchange resins as catalysts in transesterification of triolein Greg Paterson a , Titipong Issariyakul a , Chinmoy Baroi a , Amarjeet Bassi b , Ajay Dalai a,∗ a b
Department of Chemical and Biological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, Canada S7N 5A9 Department of Chemical and Biochemical Engineering, University of Western Ontario, 1151 Richmond Street, London, ON, Canada N6A 5B9
a r t i c l e
i n f o
Article history: Received 15 June 2012 Received in revised form 10 October 2012 Accepted 13 October 2012 Available online 21 November 2012 Keywords: Biodiesel Transesterification Ion-exchange resins Triolein Statistical optimization
a b s t r a c t The objective of the present study is to examine commercial ion exchange resins (IER) as heterogeneous catalysts at high temperatures for biodiesel production from triolein in a batch reaction system. Various commercial IER were screened as catalysts in transesterification of triolein and methanol. Of the catalysts tested, Amberlyst 15 was the most active catalyst in screening experiments. Reaction conditions were optimized within the experimental bounds using central composite design (CCD). At the optimized conditions, triolein conversion to products was 97 mol%. In longevity experiments Amberlyst 15 was reused once with no loss in catalytic activity, while successive experiments demonstrated decreases in catalytic activity. Product acid value, addition of free fatty acids as well as water addition to the reactants indicate that hydrolysis and esterification side reactions are taking place at the optimized conditions. Reaction kinetics were examined at 100–120 ◦ C. A mathematical model of transesterification was developed in MATLab and demonstrated a close approximation to experimental data. For the three forward reactions: triglyceride to diglyceride, diglyceride to monoglyceride, and monoglyceride to glycerol, reaction rate constants are reported to be 9.3 × 10−7 , 2.5 × 10−6 , and 7.5 × 10−5 L/mol/s. The first reversible reaction step converting triglycerides to diglycerides was found to be the rate limiting step. Activation energy for this step was 120 kJ/mol. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Biodiesel is a renewable and non-toxic alternative to petroleum diesel produced from biological sources such as oils and fats. Fossil fuels contain sequestered carbon, which release carbon as well as harmful pollutants into the atmosphere during the combustion process [1]. Biodiesel makes use of renewable organic feedstocks and does not add permanently sequestered carbon to the atmosphere [1]. Biodiesel is composed of fatty acid methyl esters (FAME) and is produced from triglycerides found in oils by transesterification. The majority of the costs associated with the production of biodiesel are primarily due to the high cost of feedstock and associated processing costs [2,3]. Commercial biodiesel is typically produced using non-reusable homogeneous catalysts resulting in corrosion of equipment and wastewater disposal problems [4]. The products of transesterification by homogeneous acids and bases require neutralization and washing with water to remove caustic substances. Homogeneous catalysts cannot be reused. Heterogeneous catalysts have the potential to replace homogeneous catalysts and circumvent some of the problems associated with current production. Various published works suggest that a solid
∗ Corresponding author. Tel.: +1 306 966 4768; fax: +1 306 966 4777. E-mail address: [email protected] (A. Dalai). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.10.013
acid catalyst (SAC) may play an important role for future commercial biodiesel production [4,5]. An ideal SAC for transesterification would consist of a hydrophobic surface to adsorb triglycerides, large pores to accommodate bulky molecules, and a moderate concentration of acidic functional groups [6,7]. The physical and chemical properties of ion exchange resins (IER) are highly customizable, as such; they may find a practical application as catalysts for biodiesel production [8]. IER are widely used in industrial water softening, food preparation, pharmaceuticals, and in medical applications [9]. As catalysts IER are commonly applied to organic chemical reactions such as: esterification, alcoholysis, hydrolysis and inversion of sugars [10]. Ion-exchange resins can be used in any type of reactor and in any solution. Ion-exchange resins offer a wide variety of support structures and functional groups. Several published works have examined IER in transesterification for biodiesel production [11–13]. Cation exchange resins are more commonly found in literature on biodiesel production [12,14–16]. One study found base-functionalized anion exchange resins to be highly active [17]. Bronsted acids and bases are proton donors and proton acceptors, respectively. Similarly, Lewis acids and bases are electron pair acceptors and electron pair donors, respectively. Each type of acid and base are capable to catalyzing the transesterification reaction. Bronsted acids are more active in the esterification reaction, while Lewis acids are more active in transesterification [5]. Basic
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catalysts are not suitable for use with low-quality feedstocks, since base-catalyzed transesterification reactions are intolerant to the presence of water as well as free fatty acids [7]. Laboratory-prepared cation exchange resins have been found to be highly active over long reaction times and with high alcohol to oil molar ratios [11,12]. One published work has been found examining the use of ion-exchange resins at high temperatures [18]. A Dowex 2-X8 anion exchange resin in the hydroxide form was tested at 200 ◦ C under 6.8 MPa of pressure. The resin was compared to other metal oxides doped with alkali functional groups. The tested resin displayed a higher activity than some of the alkali solid catalysts, but was less active than others [18]. Kinetics of the transesterification reaction using palm oil using sodium hydroxide as a catalyst have been previously examined [19]. The triglyceride to diglyceride step has been found to be the rate limiting step of the three reversible reactions. Reaction rate constants for the forward reactions ranged from 0.018 to 0.191 (wt% min)−1 temperatures ranging from 50 ◦ C to 65 ◦ C. Activation energy for the forward reactions were: 61.5, 59.4, and 26.8 kJ/mol for the tri-, di-, and monoglyceride reactions [19]. 2. Experimental 2.1. Materials Triolein (technical grade, 65%), Amberlite IR-120H, and Nafion SAC-13 were acquired from Sigma–Alrich Corporation (St. Louis, USA). Amberlyst 15 (hydrogen form, wet), Amberlite 200 (Na) and Amberlite 400 (Cl) ion-exchange resins were purchased from Alfa Aesar Co. (Ward Hill, USA). Methanol (American Chemical Society – ACS grade) was acquired from EMD Chemicals (Darmstadt, Germany). Ethanol was purchased from Commercial Alcohol Inc. (Brampton, Canada). Liquid nitrogen, nitrogen (N2 ), hydrogen (H2 ), and air (N2 , O2 ) gases were ordered from Praxair Inc. (Danbury, USA). Stearic acid (99%) was acquired from BDH Chemical Ltd. (Toronto, Canada). All materials were used as received. 2.2. Catalyst characterization Surface area, pore volume and pore diameter were measured using the Brunauer, Emmett and Teller (BET) method [20] using a Micrometrics ASAP 2000 adsorption apparatus at 78 K with liquid nitrogen. Prior to analysis, catalyst samples were evacuated under a vacuum of 50 Pa to remove moisture from the porous structure. Cation exchange capacity (CEC) is a measure of the acidity of cationic resins, comparable to measurements by temperature programmed desorption. To measure CEC, 5 g of ion-exchange resins was converted to the hydrogen form by washing with 1 L of nitric acid (HNO3 ) and rinsed with de-ionized water. One gram of the rinsed resin was accurately measured and placed in a 250 mL flask, the rest of the resin was placed in a 110 ◦ C oven for drying for 8 h. Standardized 0.1 N sodium hydroxide (NaOH) was added to the flask and salt was added to 5 wt% of the NaOH solution and left for 14 h. Back-titration was carried out with 50 mL aliquots of the resin solution with 0.05 N hydrochloric acid (HCl) and phenolphthalein as an indicator [9]. Fourier transform infrared (FTIR) spectroscopy with pyridine adsorption examines chemical structure and functional groups of the catalysts. FTIR was performed using a Perkin Elmer Spectrum GX-A system. The spectrum was recorded from 400 to 4000 cm−1 after 32 scans using a resolution of 8 cm−1 . A background scan was performed to subtract signal noise. Catalyst pellets were crushed using a mortar and pestle, a few milligrams were thoroughly mixed in to 200 mg of potassium bromide (99% A.C.S. reagent from Sigma–Aldrich). The mixture was loaded into a Pike Technologies
13 mm die and compressed into a thin translucent circular disk under 55 MPa of compression in the die. Thermogravimetric analysis (TGA) was performed to test for thermal stability using a Perkin Elmer Pyris Diamond Thermogravimetric/Differential Thermal Analyzer (TG/DTA). Air flow rate was 10 mL/min, heating from 22 ◦ C to 500 ◦ C at 10 ◦ C/min holding for 15 min. 2.3. Analysis of feedstock Feedstock oil (triolein) was characterized by high performance liquid chromatography (HPLC) to determine the composition of triglycerides, diglycerides, monoglycerides, and ester content. An Agilent 1100 HPLC was used, equipped with a refractive index (RI) detector. Two 300 mm × 7.8 mm phenogel columns connected in series are used with tetrahydrofuran (THF, HPLC grade) as the mobile phase. Detector temperature was maintained at 35 ◦ C. Solvent pumping rate was 1 mL/min. Injection volume was 5 L. Samples were diluted to 5 vol% in THF. Content of free fatty acids in reactants and products is measured as the acid value using the American Oil Chemists’ Society (AOCS) standard method Ca 5a-40 [21]. Ethanol (95 vol%) was neutralized using 0.25 N standardized NaOH with phenolphthalein as an indicator. Five grams of sample was weighed into 75 mL of neutral ethanol, followed by titration to end point. Inductively coupled plasma mass spectrometry (ICP-MS) using a Perkin Elmer ELAN 5000 was used to determine sulfur content of the feedstock and products. A continuous flow system was employed with a channel electron multiplier as the detector. Water content was analyzed using a Mettler Toledo DL32 Karl Fischer Coulometer. The coulometer used Aquastar brand CombiCoulomat fritless Karl Fischer reagent for coulometric water determination, composed of methanol and bromoform. To calibrate the coulometer and determine error, a 1 wt% water standard from Aquastar supplied by EMD Chemicals was used. 2.4. Reaction procedure All experiments were performed in a Parr Instrumentation 300 mL stainless steel batch reactor. One hundred grams of oil was charged into the reactor, followed by catalyst beads at a specific loading relative to oil, then methanol at a specific molar ratio relative to oil. The reactor was sealed and pressurized with nitrogen. At set intervals, 1 mL samples were taken through a sampling valve. Nitrogen atmosphere pressure was maintained above 4 MPa to ensure methanol remained in a liquid state. Stirring rate was 600 rotations per minute (RPM) based on previous work [22]. Design Expert 6.0 statistical modeling software was used to design central composite design (CCD) optimization experiments. A statistical design with three numerical factors (temperature, alcohol molar excess, catalyst loading), one response factor (conversion to products) and six replicates was implemented. Variable ranges were selected based on initial experimentation with Amberlyst 15 and previous work using solid acid catalysts and ion-exchange resins [11,12,23]. Since the critical point of methanol is at 240 ◦ C, 220 ◦ C was selected as the temperature maximum to maintain methanol in a sub-critical state. Jacobson et al. [23] examined alcohol to oil ratios from 3:1 to 18:1. dos Reis et al. [11] and de Rezende et al. [12] examined ratios as high as 300:1. 24:1 was selected in CCD experiments to examine alcohol loadings above that of Jacobson et al. [23] while avoiding excessive quantities of methanol seen in other publications. Variable ranges are listed in Table 1. 2.5. Product analysis Reaction product composition, acid value, sulfur content, and water content were measured as in Section 2.3. In addition, fatty
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Table 1 Controlled variable ranges in CCD statistical design. Low test value
High test value
Optimized value
Temperature Catalyst loading Oil to alcohol ratio
160 ◦ C 1 wt% 1–9
220 ◦ C 20 wt% 1–24
200 ◦ C 13 wt% 1–24
acid methyl ester chain lengths and saturation extent were analyzed using an Agilent 7890A Gas Chromatograph System (GC) using a flame ionization detector (FID). The column was a J&W 1222362 DB23 60 m × 250 m × 0.25 m. Split mode of a split–splitless inlet injector was used with a ratio of 100:1 and injection volume was 1 L. The FID column was maintained at 260 ◦ C, hydrogen flow was 40 mL/min, and air flow was 400 mL/min. The temperature program was held at 140 ◦ C for 5 min and ramped at 4 ◦ C/min to 240 ◦ C. Final hold time was 10 min. 2.6. Kinetics study of transesterification reaction For kinetic studies a lower temperature was chosen to avoid the hydrolysis side reaction. Three temperatures were examined: 100 ◦ C, 110 ◦ C and 120 ◦ C. The oil quantity was 50 g and the molar ratio of oil to methanol was 1:77. Other reaction conditions are identical to the optimized conditions. Four samples were taken over the 72 h reaction time. Conversion to products was measured by HPLC. Results were iterated in MATLab and fit to the transesterification model developed from the rate law. Least squares non-linear regression was used with the Runge–Kutta method to solve for the reaction constants maximizing Pearson correlation coefficients. The activation energy was calculated from an Arrhenius plot and Eq. (1): k = Ae−E/RT
(1)
3. Results and discussion 3.1. Catalyst properties The five ion exchange resins under examination were characterized by the methods listed in Section 2.2. Results are summarized in Table 2. From the table, Amberlyst 15 has very similar physical surface properties compared to Amberlite 200. The physical properties such as surface area, pore diameter and pore volume of Amberlite 120 as well as Amberlite 400 are each smaller compared to Amberlyst 15 and Amberlite 200. Nafion SAC-13 was measured to have the highest surface area and pore volume of the IER tested, with a pore diameter lower than that of Amberlyst 15 and Amberlite 200. Cation exchange capacities follow similar trends comparable to physical properties. Amberlite 120 and Amberlite 400 have the lowest exchange capacities. Amberlyst 15 and Amberlite 200 have a larger value, and Nafion SAC-13 having the highest ion-exchange capacity. The FTIR spectra of these catalysts demonstrate similarities between Amberlyst 15 and Amberlite 120. The FTIR wavenumber Table 2 Catalyst characterization summary of surface properties and exchange capacities. Catalyst
AS (m2 /g)
˚ dP (A)
P (cm3 /g)
CEC (meq/g)
Amberlyst 15 Amberlite IR-120 Amberlite 200 Amberlite 400 Nafion SAC-13
43.1 4.2 45.4 5.0 233
231 12 204 22 143
0.25 1.3 × 10−3 0.23 2.7 × 10−3 0.83
4.4 1.0 4.5 1.94 [28] ≤11
˚ angstrom (10−10 m); P, pore volume; CEC, AS, surface area; dP, pore diameter; A, cation exchange capacity; meq, milliequivalents.
Amberlite 400 Amberlite 200
Amberlite 120H
Amberlyst 15 400
800
1200
1600
2000
2400
2800
3200
3600
4000
-1
Wavenumber (cm) Fig. 1. Pyridine adsorption FTIR analysis of various commercial ion-exchange resins.
spectra are illustrated in Fig. 1. Peaks observed at 580 cm−1 , 1600 cm−1 , and 2920 cm−1 are the aromatic ring stretching of the polystyrene supports of Amberlyst 15, Amberlite 120H, Amberlite 200 and Amberlite 400 [24]. The peaks at 1030 cm−1 for Amberlyst 15, Amberlite 120H and Amberlite 200 are from sulfur–oxygen double bonds. Weak Lewis acid functional groups of Amberlyst 15 and Amberlite 120 have peaks at wavenumbers 1440 cm−1 and 1600 cm−1 [25]. Amberlite 200 demonstrates sharp peaks at wavenumbers 1600 cm−1 and 1645 cm−1 associated with strong Lewis and Bronsted acid sites, respectively [25]. The Amberlite 400 functional group can be observed in a peak at 1480 cm−1 where nitrogen–carbon bonds stretch. The functional group of Nafion SAC-13 is sulfonic acid, is not observed at 1000 cm−1 since carbon–fluorine and sulfur–oxygen double bonds all absorb at wavenumbers between 1000 and 1250 cm−1 where there is no transmittance [24]. Thermogravimetric analysis (TGA) of each of the catalysts is produced in Fig. 2. The loss of mass up to 200 ◦ C is between 4 and 10% for each catalyst tested. It can be ascertained from this figure that the ion-exchange resins under study are stable up to 200 ◦ C. 3.2. Feedstock composition The feedstock triolein was found to contain (±0.7): 64 mol% triglycerides, 27 mol% diglycerides, and 9 mol% monoglycerides by HPLC analysis. The acid value of triolein was measured to be 2.5 ± 0.9 mg KOH/g sample. Water content of triolein was measured to be 0.01 wt%. ICP-MS analysis for sulfur content indicated a concentration of 21 (±1) parts per million (ppm) of sulfur in triolein. 100% Nafion SAC-13 Amberlite 200
Relative Mass of Sample
Property
Normalized Transmittance
Nafion SAC-13
Amberlite 120
80% Amberlyst 15
60%
Amberlite 400 40% 50
100
150
200
250
300
350
400
450
500
Temperature (˚C) Fig. 2. Thermogravimetric analysis of various ion-exchange resins. Conditions: 15 mL/min air, 22–500 ◦ C, 10 ◦ C/min, 1 sample/20 s, smoothed 20 points.
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Table 3 Fatty acid methyl ester chain length and saturation extent by gas chromatograph. Amount (mg/mL)
Percent (mass%)
Name
Group
0.76 0.20 0.07 1.60 1.68 0.08 0.56 28.8 1.23 3.34 0.84 0.02 0.22 2.11
1.8% 0.5% 0.2% 3.8% 4.1% 0.2% 1.4% 69.3% 3.0% 8.1% 2.0% 0.0% 0.5% 5.1%
Myristic Myristoleic Pentadecanoic Palmitic Palmitoleic Heptadecanoic Stearic Oleic Elaidic Linoleic Linolenic Arachidic cis-11-Eicosenoic Unknowns
14:0 14:1 15:0 16:0 16:1 17:0 18:0 18:1 18:1 18:2 18:3 20:0 20:1 –
Table 3 summarizes the results from gas chromatography examining the chain length and saturation composition of methyl esters. The methyl ester form of oleic acid is the primary component of the FAME mixture in the products. The fatty acid composition of the esters consists of 85 mass% unsaturated and monounsaturated fatty acids. 3.3. Catalyst screening Each of the catalysts was tested using reaction conditions used in previous work [23]. Fig. 3 illustrates results of the catalyst screening. Amberlyst 15 was the most active catalyst and was selected for further investigation. The weak Lewis acid functional groups observed in Amberlyst 15 and Amberlite 120 are responsible for the high activity observed in screening experiments. Our observation of the physical and chemical properties of the various IER and contrasting them suggests that weakly Lewis acid sites are responsible for their performance as catalysts. This is confirmed in literature, where Lewis acids are more active in transesterification and strong acids groups hinder the release of product species [5]. The strong acid functional groups of Amberlite 200 are detrimental to transesterification due to the restriction of desorption of reacted species [5]. 3.4. Reaction optimization A central composite design was used to optimize the reactor conditions using Amberlyst 15 within the variable ranges listed in Table 1. The number of experiments in the statistical design was based on Eq. (2), where N is the number of experiments (20), k is
Conversion to Products (mol%)
100%
Amberlyst 15
80%
Amberlite 120H
60%
Nafion SAC-13 Amberlite 200
40%
100
95
Conversion to Products (mol%)
90
6
9
Alcohol:Oil Ratio 12 18 24 2
5
10
15
95-100 90-95 85-90
85 20
Catalyst Loading (wt%)
Fig. 4. Numerical model of transesterification yield of triolein over Amberlyst 15. Conditions: 200 ◦ C, 4 MPa, 600 RPM, and 2 h.
the number of controlled variables (temperature, catalyst loading and alcohol molar excess) and NC is the number of repeated experiments (six replicates). Conversion to products was measured as the response factor in the design, measured by HPLC. From the collected data, confidence intervals were calculated and a numerical model was developed. N = 2k + 2(k) + NC
(2)
The F-value of the polynomial model was found to be 0.01 meaning the model is significant (<0.05). The polynomial equation using coded factors is listed in Eq. (3), limited to the statistically significant variables. The variable X is conversion to products, T is temperature, A is alcohol molar excess and C is catalyst loading. The numerical model as illustrated in Fig. 4, is found to be a hyperbolic paraboloid. This shape is due to local maxima in the alcohol to oil axis and local minima on the loading axis. The local maximum on the catalyst loading axis may be due to loading effects of excess catalyst. The minima may be due to the formation of methyl sulfonates by esterification on the sulfonic acid groups of Amberlyst 15 [26]. From the numerical optimization, the optimized reactor conditions were selected to be 200 ◦ C, 13 wt% catalyst loading, 1–24 oil to alcohol molar ratio, 4 MPa nitrogen atmosphere, stirring speed 600 RPM using 100 g of oil indicating that the IER performance was very good. However, further work can be undertaken to examine the effects of higher alcohol loadings for the increase in conversion beyond 97%. It was the intent of this work to examine wide temperature range up to 220 ◦ C in conjunction with IER for biodiesel production in order to find temperature for high oil conversion. The upper temperature of 220 ◦ C was fixed as the material was unstable thermally beyond this temperature (see Fig. 2). Also, since the critical point of methanol is at 240 ◦ C, 220 ◦ C was selected as the temperature maximum to maintain methanol in a sub-critical state. The experimental variable values were selected from the model, and are valid within the variable ranges from Table 1. The local maxima for each of the controlled variables were at the selected values. These maximum values are visible in Fig. 4. X = 89 + 21.5T + 3.6C + 7.1A − 8.7T 2 −11.5C 2 + 5.1A2 − 10.7T · A
Amberlite 400 20%
(3)
0% 0
60
120
180
240
Time (minutes) Fig. 3. Transesterification of triolein to methyl esters using various resins as catalysts. Conditions: 200 ◦ C, 1:9 oil:methanol, 3 wt% loading, 600 RPM, 4 MPa, and 100 g oil.
3.5. Product analysis The predicted conversion from the model at optimized conditions was 98.1 mol%, while the experimental value at the same conditions was 97.0 (±0.7) mol%. Fig. 5 demonstrates the high
G. Paterson et al. / Catalysis Today 212 (2013) 157–163
100%
Original Experiment
Conversion to Products (mol%)
Conversion to Products (mol%)
100%
80% Repeat Experiment 60%
40%
20%
0%
0
161
60
120
180
240
80%
60%
40%
20%
0% Fresh Catalyst
Time (minutes)
Used Twice
Used Three Times
Used Four Times
Successive Catalyst Reusage
Fig. 5. Transesterification of triolein using optimized reactor conditions. Conditions: 200 ◦ C, 1:24 oil:methanol, 13 wt% loading, 600 RPM, 4 MPa,and 100 g oil.
Fig. 6. Amberlyst 15 longevity in successive transesterification of triolein. Conditions: 200 ◦ C, 1:24 oil:methanol, 13 wt% loading, 600 RPM, 4 MPa, and 100 g oil.
conversion of the optimized experiment. The experimental results were reliably repeated. The acid value of the products was typically titrated to be in the range of 50 mg KOH/g sample. To test for catalyst leaching, ICP-MS was employed to measure sulfur content in triolein and the reaction products. From Section 3.2, triolein was found to contain 21 (±1) ppm, while the products contain 12 (±1) ppm. The decrease in sulfur content can be attributed to the glycerol layer containing a portion of the sulfur present in the feedstock. It can be concluded that sulfur leaching did not take place, given the sulfonic functional group of Amberlyst 15 and therefore the high acid value is transesterification products is caused by the presence of free fatty acids. In this case, the formation of free fatty acids is due to hydrolysis of FAME.
that water is being produced and consumed in side reactions and approaching equilibrium of 0.7 wt% at the optimized conditions. Therefore, Amberlyst 15 is catalyzing the hydrolysis, esterification, and transesterification reactions.
3.6. Effect of water and free fatty acid additions Water was added to the reactant mixture to determine if hydrolysis of methyl esters was taking place in competition with transesterification. Results are illustrated in Table 4. Adding water to feedstock mixtures (up to 1 wt% relative to oil) did not have an effect on the reaction products as measured by HPLC or the acid value. At 2 wt% loading of water, a decrease in conversion to products is apparent. The water content of the products for each of the water addition reactions (0.1, 0.5, 1 and 2 wt%) were 0.67, 0.72, 0.71 and 0.72 (±0.01) wt%, respectively. Comparatively, the water content of the optimized reaction conditions without water addition had a water content of 0.69 wt%. See Table 4. The addition of water did not have a significant impact on the conversion, water content or the acid value of the transesterification products. Ester content and the acid value were not affected when stearic acid was added to the feedstock up to 15 wt% see Table 4. It is concluded Table 4 Conversion, acid value and water content of feedstock and various experiments. Feedstock or experiment
Conversion to products (±0.7 mol%)
Water content Acid value (±0.9 mg KOH/g) (±0.01 wt%)
Triolein Canola oil Greenseed oil Optimized conditions 0.1 wt% water products 0.5 wt% water products 1 wt% water products 2 wt% water products 5 wt% FFA products 10 wt% FFA products 15 wt% FFA products
0 0 0 96.8 95.8 95.2 96.3 92.6 96.7 97.1 97.5
2.5 0 8.2 55.0 57.8 55.5 58.5 56.1 55.9 48.1 53.3
0.01 0.02 0.05 0.69 0.67 0.72 0.71 0.72 0.66 0.62 0.60
3.7. Catalyst longevity Successive reactions reused Amberlyst 15 to test catalyst longevity. Following each trial, the catalyst was collected by gravity filtration allowed to dry and charged into the subsequent reaction. As shown in Fig. 6, there is a negligible decrease in the conversion of triolein to products during the second use. Subsequent reuses of the catalyst show significant decrease in conversion to products after 2 h. The third use of Amberlyst 15 achieved a final conversion of 81 mol%, while the fourth successive use demonstrated 54 mol% conversion to products. Amberlyst 15 can be reused once under the current reaction conditions, while maintaining high conversion to products. 3.8. The acid value and temperature A lower temperature, a higher alcohol molar ratio and a longer reaction duration were selected for the kinetics study of transesterification to avoid the hydrolysis side reaction, while maintaining the conversion to products. Temperature was varied from 100 ◦ C to 120 ◦ C. Oil quantity was decreased to 50 g per reaction. Oil to alcohol molar ratio was raised to 1:77. The reaction duration was increased to 72 h. 3.9. Reaction kinetics Kinetics experiments at 100 ◦ C, 110 ◦ C, and 120 ◦ C are illustrated in Fig. 7. The acid values of the products increase with temperature increase. The acid value for 100 ◦ C, 110 ◦ C, and 120 ◦ C are 0.35, 0.46, and 2.1 (±0.9) mg KOH/g sample, respectively. Previous work [27] has successfully been modeled similar reactions using the power rate law, while others [28] have found that the pseudo-homogeneous and Langmuir–Hinshelwood kinetics also yield similar results. Langmuir–Hinshelwood (or pseudohomogeneous) kinetics could have been examined in the reaction kinetics modeling of this work. The rate law kinetics were selected for simplicity and based on previous work [28]. Results for the kinetic parameters from the model are tabulated in Table 5. An increase in reaction rate constants is seen in each reaction as temperature increases. Reaction rate constants are higher in the forward reactions compared to the reverse reactions. The majority
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Table 5 Reaction rate constants for reversible transesterification reactions (conditions: 100 ◦ C, 110 ◦ C and 120 ◦ C, 1:77 oil:methanol, 13 wt% loading, 600 RPM, and 50 g oil). Temperature (◦ C)
Rate constant, k (L/mol/s)
Pearson correlation coeff.
TG ⇒ DG
100 110 120
1.273 × 10−7 2.083 × 10−7 9.259 × 10−7
0.957 0.990 0.991
DG ⇒ TG
100 110 120
1.157 × 10−8 1.273 × 10−8 4.630 × 10−7
0.823 0.991 0.972
DG ⇒ MG
100 110 120
1.042 × 10−6 1.505 × 10−6 2.546 × 10−6
0.987 0.905 0.998
MG ⇒ DG
100 110 120
3.472 × 10−7 5.787 × 10−7 2.315 × 10−6
0.938 0.992 0.991
MG ⇒ GL
100 110 120
1.134 × 10−5 2.662 × 10−5 7.523 × 10−5
0.928 0.992 0.991
GL ⇒ MG
100 110 120
2.894 × 10−6 4.630 × 10−5 5.787 × 10−5
0.939 0.992 0.991
Reaction
4. Conclusions
of the Pearson correlation coefficients are above 0.98, demonstrating a close approximation to the experimental data. An Arrhenius plot in Fig. 8 shows the forward reaction rate constants for the three temperatures examined. Linear best fit slopes for each reaction rate constant are natural logarithmic
Conversion to Products (mol%)
100%
80% 120˚C 60%
40%
110˚C
20% 100˚C 1
2
3
Time (days) Fig. 7. Transesterification of canola oil at various temperatures over Amberlyst 15. Conditions: 1:77 oil:methanol, 13 wt% loading, 600 RPM, and 50 g oil.
Natural Logarithm of Reaction Constants (L/mol/s)
-11 -12 -13
MG to GL y = -13863x + 25.727 R2 = 0.9949 DG to MG y = -6544.7x + 3.735 R2= 0.9864
-14 -15 -16 2.5E-03
The authors acknowledge the funding from Natural Sciences and Engineering Council of Canada (NSERC) and the Canada Research Chair (CRC) program. References
-9 -10
Amberlyst 15 was found to be the most active ion-exchange resin tested as a transesterification catalyst and reaction conditions were optimized in a 300 mL batch reactor. Within the experimental bounds, the optimized conditions were 200 ◦ C, 1:24 oil to methanol molar ratio, 13 wt% catalyst loading, 600 RPM stirring speed, 4 MPa nitrogen atmosphere using 100 g of triolein with conversion to products of 97 wt% as measured by HPLC. The acid value of the products is higher than that of the feedstock. No leaching of functional groups was detected in the products. The addition of water to the reactants did not have a significant effect of the reaction products up to a 1 wt% loading. The addition of up to 15 wt% stearic acid as a FFA did not impact the reaction products. Hydrolysis and esterification are taking place as side reactions at the optimized reaction conditions. Amberlyst 15 can be reused once at the stated conditions with no loss in conversion to products. Successive uses cause a reduction in conversion to products from 97 mol% down to 81 mol% and then to 54 mol%. Reaction rate constants were determined for forward and reverse transesterification between 100 ◦ C and 120 ◦ C. The rate determining step is the TG to DG reaction. Activation energies for the TG to DG, DG to MG, and MG to GL reactions are 120, 54.4 and 115 kJ/mol, respectively. Future work should be undertaken to examine lower temperatures, and pressures to improve commercial viability. The local maxima observed in optimization experiments can be further examined using a wider variety of reaction conditions. In addition, other IER should be examined in a variety of reactor types using a variety of feedstocks and an economic feasibility study based on the energy requirement should be carried out for this system. Acknowledgements
0% 0
derivations of Eq. (1). From the linear equations the activation energy for the triglyceride (TG) to diglyceride (DG) reaction was 120 kJ/mol. The activation energy for the diglyceride to monoglyceride (MG) reaction was 54 kJ/mol and 115 kJ/mol for the monoglyceride to glycerol (GL) reaction. Literature values for the three reactions using homogeneous catalysts are 61.5, 59.4, and 26.8 kJ/mol, respectively [19]. Similar reactions using heterogeneous catalysts report activation energies ranging from 49 kJ/mol to 110 kJ/mol [27,29].
TG to DG y = -14487x + 22.776 R2= 0.9138 2.6E-03
2.7E-03
Inverse Temperature (K-1 ) Fig. 8. Arrhenius plot tri-, di- and monoglyceride consumption during transesterification. Conditions: 100 ◦ C, 110 ◦ C and 120 ◦ C, 1:77 oil:methanol, 13 wt% loading, 600 RPM, and 50 g oil.
[1] V. Patil, K. Tran, H. Giselrød, International Journal of Molecular Sciences 9 (2008) 1188–1195. [2] M. Haas, A. McAloon, W. Yee, T. Foglia, Bioresource Technology 97 (2006) 671–678. [3] A.A. Apostolakos, I.K. Kookos, C. Marazioti, K.C. Angelopoulos, Fuel Processing Technology 90 (2009) 1023–1031. [4] J. Melero, J. Iglesias, G. Morales, Green Chemistry 11 (2009) 1285–1308. [5] M. Di Serio, R. Tesser, L. Pengmei, E. Santacesaria, Energy & Fuels 22 (2008) 207–217. [6] R. Jothiramalingam, M. Wang, Industrial & Engineering Chemistry Research 48 (2009) 6162–6172. [7] E. Lotero, Y. Liu, D. Lopez, K. Suwannakarn, D. Bruce, J. Goodwin Jr., Industrial & Engineering Chemistry Research 44 (2005) 5353–5363. [8] M. Sharma, Reactive & Functional Polymers 26 (1995) 3–23. [9] K. Dorfner, Ion Exchangers Properties and Applications, Ann Arbor Science Publishers Inc., Ann Arbor, 1972. [10] G. Osborn, Synthetic Ion-Exchangers, Chapman & Hall Ltd., London, 1961. [11] S. dos Reis, E. Lachter, R. Nascimento, J. Rodrigues Jr., M. Reid, Journal of the American Oil Chemists’ Society 82 (2005) 661–665. [12] S. de Rezende, M. de Castro Reis, M. Reid, P. Silva Jr., F. Coutinho, R. da Silva San Gil, E. Lachter, Applied Catalysis A: General 349 (2008) 198–203.
G. Paterson et al. / Catalysis Today 212 (2013) 157–163 [13] M. Talukder, J. Wu, S. Lau, L. cui, G. Shimin, A. Lim, Energy & Fuels 23 (2008) 1–4. [14] A. Kiss, A. Dimian, G. Rothenberg, Advanced Synthesis & Catalysis 348 (2006) 75–81. [15] Z. Helwani, M. Othman, N. Aziz, J. Kim, W. Fernando, Applied Catalysis A: General 363 (2009) 1–10. [16] M. Harmer, Q. Sun, Applied Catalysis A: General 221 (2001) 45–62. [17] N. Shibasaki-Kitakawa, H. Honda, H. Kuribayashi, T. Toda, T. Fukumura, T. Yonemoto, Bioresource Technology 98 (2007) 416–421. [18] G. Peterson, W. Scarrah, Journal of the American Oil Chemists’ Society 61 (1984) 1593–1597. [19] D. Darnoko, M. Cheryan, Journal of the American Oil Chemists’ Society 77 (2000) 1263–1267. [20] S. Brunauer, P. Emmett, E. Teller, Journal of the American Oil Chemists’ Society 60 (1938) 309–319. [21] Official Methods and Recommended Practices of the AOCS, American Oil Chemists’ Society, Champaign, 1998.
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[22] M. Kulkarni, R. Gopinath, L. Meher, A. Dalai, Green Chemistry 8 (2006) 1056–1062. [23] K. Jacobson, R. Gopinath, L. Meher, A. Dalai, Applied Catalysis B: Environmental 85 (2008) 86–91. [24] R. Silverstein, F. Webster, Spectrometric Identification of Organic Compounds, sixth ed., John Wiley & Sons, Inc., New York, 1998. [25] D. Yazici, C. Bilgic¸, Surface and Interface Analysis 42 (2010) 959–962. [26] J. Melero, L. Bautista, G. Morales, J. Iglesias, D. Briones, Energy & Fuels 22 (2009) 539–547. [27] H. Hamid, R. Yunus, T. Choong, Chemical Product and Process Modeling 5 (2010) 1–17. [28] E. Bozek-Winkler, J. Gmehling, Industrial & Engineering Chemistry Research 45 (2006) 6648–6654. [29] D. López, J. Goodwin Jr., D. Bruce, Journal of Catalysis 245 (2007) 381–391.
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Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Ion-exchange resins as catalysts in transesterification of triolein Greg Paterson a , Titipong Issariyakul a , Chinmoy Baroi a , Amarjeet Bassi b , Ajay Dalai a,∗ a b
Department of Chemical and Biological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, Canada S7N 5A9 Department of Chemical and Biochemical Engineering, University of Western Ontario, 1151 Richmond Street, London, ON, Canada N6A 5B9
a r t i c l e
i n f o
Article history: Received 15 June 2012 Received in revised form 10 October 2012 Accepted 13 October 2012 Available online 21 November 2012 Keywords: Biodiesel Transesterification Ion-exchange resins Triolein Statistical optimization
a b s t r a c t The objective of the present study is to examine commercial ion exchange resins (IER) as heterogeneous catalysts at high temperatures for biodiesel production from triolein in a batch reaction system. Various commercial IER were screened as catalysts in transesterification of triolein and methanol. Of the catalysts tested, Amberlyst 15 was the most active catalyst in screening experiments. Reaction conditions were optimized within the experimental bounds using central composite design (CCD). At the optimized conditions, triolein conversion to products was 97 mol%. In longevity experiments Amberlyst 15 was reused once with no loss in catalytic activity, while successive experiments demonstrated decreases in catalytic activity. Product acid value, addition of free fatty acids as well as water addition to the reactants indicate that hydrolysis and esterification side reactions are taking place at the optimized conditions. Reaction kinetics were examined at 100–120 ◦ C. A mathematical model of transesterification was developed in MATLab and demonstrated a close approximation to experimental data. For the three forward reactions: triglyceride to diglyceride, diglyceride to monoglyceride, and monoglyceride to glycerol, reaction rate constants are reported to be 9.3 × 10−7 , 2.5 × 10−6 , and 7.5 × 10−5 L/mol/s. The first reversible reaction step converting triglycerides to diglycerides was found to be the rate limiting step. Activation energy for this step was 120 kJ/mol. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Biodiesel is a renewable and non-toxic alternative to petroleum diesel produced from biological sources such as oils and fats. Fossil fuels contain sequestered carbon, which release carbon as well as harmful pollutants into the atmosphere during the combustion process [1]. Biodiesel makes use of renewable organic feedstocks and does not add permanently sequestered carbon to the atmosphere [1]. Biodiesel is composed of fatty acid methyl esters (FAME) and is produced from triglycerides found in oils by transesterification. The majority of the costs associated with the production of biodiesel are primarily due to the high cost of feedstock and associated processing costs [2,3]. Commercial biodiesel is typically produced using non-reusable homogeneous catalysts resulting in corrosion of equipment and wastewater disposal problems [4]. The products of transesterification by homogeneous acids and bases require neutralization and washing with water to remove caustic substances. Homogeneous catalysts cannot be reused. Heterogeneous catalysts have the potential to replace homogeneous catalysts and circumvent some of the problems associated with current production. Various published works suggest that a solid
∗ Corresponding author. Tel.: +1 306 966 4768; fax: +1 306 966 4777. E-mail address: [email protected] (A. Dalai). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.10.013
acid catalyst (SAC) may play an important role for future commercial biodiesel production [4,5]. An ideal SAC for transesterification would consist of a hydrophobic surface to adsorb triglycerides, large pores to accommodate bulky molecules, and a moderate concentration of acidic functional groups [6,7]. The physical and chemical properties of ion exchange resins (IER) are highly customizable, as such; they may find a practical application as catalysts for biodiesel production [8]. IER are widely used in industrial water softening, food preparation, pharmaceuticals, and in medical applications [9]. As catalysts IER are commonly applied to organic chemical reactions such as: esterification, alcoholysis, hydrolysis and inversion of sugars [10]. Ion-exchange resins can be used in any type of reactor and in any solution. Ion-exchange resins offer a wide variety of support structures and functional groups. Several published works have examined IER in transesterification for biodiesel production [11–13]. Cation exchange resins are more commonly found in literature on biodiesel production [12,14–16]. One study found base-functionalized anion exchange resins to be highly active [17]. Bronsted acids and bases are proton donors and proton acceptors, respectively. Similarly, Lewis acids and bases are electron pair acceptors and electron pair donors, respectively. Each type of acid and base are capable to catalyzing the transesterification reaction. Bronsted acids are more active in the esterification reaction, while Lewis acids are more active in transesterification [5]. Basic
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catalysts are not suitable for use with low-quality feedstocks, since base-catalyzed transesterification reactions are intolerant to the presence of water as well as free fatty acids [7]. Laboratory-prepared cation exchange resins have been found to be highly active over long reaction times and with high alcohol to oil molar ratios [11,12]. One published work has been found examining the use of ion-exchange resins at high temperatures [18]. A Dowex 2-X8 anion exchange resin in the hydroxide form was tested at 200 ◦ C under 6.8 MPa of pressure. The resin was compared to other metal oxides doped with alkali functional groups. The tested resin displayed a higher activity than some of the alkali solid catalysts, but was less active than others [18]. Kinetics of the transesterification reaction using palm oil using sodium hydroxide as a catalyst have been previously examined [19]. The triglyceride to diglyceride step has been found to be the rate limiting step of the three reversible reactions. Reaction rate constants for the forward reactions ranged from 0.018 to 0.191 (wt% min)−1 temperatures ranging from 50 ◦ C to 65 ◦ C. Activation energy for the forward reactions were: 61.5, 59.4, and 26.8 kJ/mol for the tri-, di-, and monoglyceride reactions [19]. 2. Experimental 2.1. Materials Triolein (technical grade, 65%), Amberlite IR-120H, and Nafion SAC-13 were acquired from Sigma–Alrich Corporation (St. Louis, USA). Amberlyst 15 (hydrogen form, wet), Amberlite 200 (Na) and Amberlite 400 (Cl) ion-exchange resins were purchased from Alfa Aesar Co. (Ward Hill, USA). Methanol (American Chemical Society – ACS grade) was acquired from EMD Chemicals (Darmstadt, Germany). Ethanol was purchased from Commercial Alcohol Inc. (Brampton, Canada). Liquid nitrogen, nitrogen (N2 ), hydrogen (H2 ), and air (N2 , O2 ) gases were ordered from Praxair Inc. (Danbury, USA). Stearic acid (99%) was acquired from BDH Chemical Ltd. (Toronto, Canada). All materials were used as received. 2.2. Catalyst characterization Surface area, pore volume and pore diameter were measured using the Brunauer, Emmett and Teller (BET) method [20] using a Micrometrics ASAP 2000 adsorption apparatus at 78 K with liquid nitrogen. Prior to analysis, catalyst samples were evacuated under a vacuum of 50 Pa to remove moisture from the porous structure. Cation exchange capacity (CEC) is a measure of the acidity of cationic resins, comparable to measurements by temperature programmed desorption. To measure CEC, 5 g of ion-exchange resins was converted to the hydrogen form by washing with 1 L of nitric acid (HNO3 ) and rinsed with de-ionized water. One gram of the rinsed resin was accurately measured and placed in a 250 mL flask, the rest of the resin was placed in a 110 ◦ C oven for drying for 8 h. Standardized 0.1 N sodium hydroxide (NaOH) was added to the flask and salt was added to 5 wt% of the NaOH solution and left for 14 h. Back-titration was carried out with 50 mL aliquots of the resin solution with 0.05 N hydrochloric acid (HCl) and phenolphthalein as an indicator [9]. Fourier transform infrared (FTIR) spectroscopy with pyridine adsorption examines chemical structure and functional groups of the catalysts. FTIR was performed using a Perkin Elmer Spectrum GX-A system. The spectrum was recorded from 400 to 4000 cm−1 after 32 scans using a resolution of 8 cm−1 . A background scan was performed to subtract signal noise. Catalyst pellets were crushed using a mortar and pestle, a few milligrams were thoroughly mixed in to 200 mg of potassium bromide (99% A.C.S. reagent from Sigma–Aldrich). The mixture was loaded into a Pike Technologies
13 mm die and compressed into a thin translucent circular disk under 55 MPa of compression in the die. Thermogravimetric analysis (TGA) was performed to test for thermal stability using a Perkin Elmer Pyris Diamond Thermogravimetric/Differential Thermal Analyzer (TG/DTA). Air flow rate was 10 mL/min, heating from 22 ◦ C to 500 ◦ C at 10 ◦ C/min holding for 15 min. 2.3. Analysis of feedstock Feedstock oil (triolein) was characterized by high performance liquid chromatography (HPLC) to determine the composition of triglycerides, diglycerides, monoglycerides, and ester content. An Agilent 1100 HPLC was used, equipped with a refractive index (RI) detector. Two 300 mm × 7.8 mm phenogel columns connected in series are used with tetrahydrofuran (THF, HPLC grade) as the mobile phase. Detector temperature was maintained at 35 ◦ C. Solvent pumping rate was 1 mL/min. Injection volume was 5 L. Samples were diluted to 5 vol% in THF. Content of free fatty acids in reactants and products is measured as the acid value using the American Oil Chemists’ Society (AOCS) standard method Ca 5a-40 [21]. Ethanol (95 vol%) was neutralized using 0.25 N standardized NaOH with phenolphthalein as an indicator. Five grams of sample was weighed into 75 mL of neutral ethanol, followed by titration to end point. Inductively coupled plasma mass spectrometry (ICP-MS) using a Perkin Elmer ELAN 5000 was used to determine sulfur content of the feedstock and products. A continuous flow system was employed with a channel electron multiplier as the detector. Water content was analyzed using a Mettler Toledo DL32 Karl Fischer Coulometer. The coulometer used Aquastar brand CombiCoulomat fritless Karl Fischer reagent for coulometric water determination, composed of methanol and bromoform. To calibrate the coulometer and determine error, a 1 wt% water standard from Aquastar supplied by EMD Chemicals was used. 2.4. Reaction procedure All experiments were performed in a Parr Instrumentation 300 mL stainless steel batch reactor. One hundred grams of oil was charged into the reactor, followed by catalyst beads at a specific loading relative to oil, then methanol at a specific molar ratio relative to oil. The reactor was sealed and pressurized with nitrogen. At set intervals, 1 mL samples were taken through a sampling valve. Nitrogen atmosphere pressure was maintained above 4 MPa to ensure methanol remained in a liquid state. Stirring rate was 600 rotations per minute (RPM) based on previous work [22]. Design Expert 6.0 statistical modeling software was used to design central composite design (CCD) optimization experiments. A statistical design with three numerical factors (temperature, alcohol molar excess, catalyst loading), one response factor (conversion to products) and six replicates was implemented. Variable ranges were selected based on initial experimentation with Amberlyst 15 and previous work using solid acid catalysts and ion-exchange resins [11,12,23]. Since the critical point of methanol is at 240 ◦ C, 220 ◦ C was selected as the temperature maximum to maintain methanol in a sub-critical state. Jacobson et al. [23] examined alcohol to oil ratios from 3:1 to 18:1. dos Reis et al. [11] and de Rezende et al. [12] examined ratios as high as 300:1. 24:1 was selected in CCD experiments to examine alcohol loadings above that of Jacobson et al. [23] while avoiding excessive quantities of methanol seen in other publications. Variable ranges are listed in Table 1. 2.5. Product analysis Reaction product composition, acid value, sulfur content, and water content were measured as in Section 2.3. In addition, fatty
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Table 1 Controlled variable ranges in CCD statistical design. Low test value
High test value
Optimized value
Temperature Catalyst loading Oil to alcohol ratio
160 ◦ C 1 wt% 1–9
220 ◦ C 20 wt% 1–24
200 ◦ C 13 wt% 1–24
acid methyl ester chain lengths and saturation extent were analyzed using an Agilent 7890A Gas Chromatograph System (GC) using a flame ionization detector (FID). The column was a J&W 1222362 DB23 60 m × 250 m × 0.25 m. Split mode of a split–splitless inlet injector was used with a ratio of 100:1 and injection volume was 1 L. The FID column was maintained at 260 ◦ C, hydrogen flow was 40 mL/min, and air flow was 400 mL/min. The temperature program was held at 140 ◦ C for 5 min and ramped at 4 ◦ C/min to 240 ◦ C. Final hold time was 10 min. 2.6. Kinetics study of transesterification reaction For kinetic studies a lower temperature was chosen to avoid the hydrolysis side reaction. Three temperatures were examined: 100 ◦ C, 110 ◦ C and 120 ◦ C. The oil quantity was 50 g and the molar ratio of oil to methanol was 1:77. Other reaction conditions are identical to the optimized conditions. Four samples were taken over the 72 h reaction time. Conversion to products was measured by HPLC. Results were iterated in MATLab and fit to the transesterification model developed from the rate law. Least squares non-linear regression was used with the Runge–Kutta method to solve for the reaction constants maximizing Pearson correlation coefficients. The activation energy was calculated from an Arrhenius plot and Eq. (1): k = Ae−E/RT
(1)
3. Results and discussion 3.1. Catalyst properties The five ion exchange resins under examination were characterized by the methods listed in Section 2.2. Results are summarized in Table 2. From the table, Amberlyst 15 has very similar physical surface properties compared to Amberlite 200. The physical properties such as surface area, pore diameter and pore volume of Amberlite 120 as well as Amberlite 400 are each smaller compared to Amberlyst 15 and Amberlite 200. Nafion SAC-13 was measured to have the highest surface area and pore volume of the IER tested, with a pore diameter lower than that of Amberlyst 15 and Amberlite 200. Cation exchange capacities follow similar trends comparable to physical properties. Amberlite 120 and Amberlite 400 have the lowest exchange capacities. Amberlyst 15 and Amberlite 200 have a larger value, and Nafion SAC-13 having the highest ion-exchange capacity. The FTIR spectra of these catalysts demonstrate similarities between Amberlyst 15 and Amberlite 120. The FTIR wavenumber Table 2 Catalyst characterization summary of surface properties and exchange capacities. Catalyst
AS (m2 /g)
˚ dP (A)
P (cm3 /g)
CEC (meq/g)
Amberlyst 15 Amberlite IR-120 Amberlite 200 Amberlite 400 Nafion SAC-13
43.1 4.2 45.4 5.0 233
231 12 204 22 143
0.25 1.3 × 10−3 0.23 2.7 × 10−3 0.83
4.4 1.0 4.5 1.94 [28] ≤11
˚ angstrom (10−10 m); P, pore volume; CEC, AS, surface area; dP, pore diameter; A, cation exchange capacity; meq, milliequivalents.
Amberlite 400 Amberlite 200
Amberlite 120H
Amberlyst 15 400
800
1200
1600
2000
2400
2800
3200
3600
4000
-1
Wavenumber (cm) Fig. 1. Pyridine adsorption FTIR analysis of various commercial ion-exchange resins.
spectra are illustrated in Fig. 1. Peaks observed at 580 cm−1 , 1600 cm−1 , and 2920 cm−1 are the aromatic ring stretching of the polystyrene supports of Amberlyst 15, Amberlite 120H, Amberlite 200 and Amberlite 400 [24]. The peaks at 1030 cm−1 for Amberlyst 15, Amberlite 120H and Amberlite 200 are from sulfur–oxygen double bonds. Weak Lewis acid functional groups of Amberlyst 15 and Amberlite 120 have peaks at wavenumbers 1440 cm−1 and 1600 cm−1 [25]. Amberlite 200 demonstrates sharp peaks at wavenumbers 1600 cm−1 and 1645 cm−1 associated with strong Lewis and Bronsted acid sites, respectively [25]. The Amberlite 400 functional group can be observed in a peak at 1480 cm−1 where nitrogen–carbon bonds stretch. The functional group of Nafion SAC-13 is sulfonic acid, is not observed at 1000 cm−1 since carbon–fluorine and sulfur–oxygen double bonds all absorb at wavenumbers between 1000 and 1250 cm−1 where there is no transmittance [24]. Thermogravimetric analysis (TGA) of each of the catalysts is produced in Fig. 2. The loss of mass up to 200 ◦ C is between 4 and 10% for each catalyst tested. It can be ascertained from this figure that the ion-exchange resins under study are stable up to 200 ◦ C. 3.2. Feedstock composition The feedstock triolein was found to contain (±0.7): 64 mol% triglycerides, 27 mol% diglycerides, and 9 mol% monoglycerides by HPLC analysis. The acid value of triolein was measured to be 2.5 ± 0.9 mg KOH/g sample. Water content of triolein was measured to be 0.01 wt%. ICP-MS analysis for sulfur content indicated a concentration of 21 (±1) parts per million (ppm) of sulfur in triolein. 100% Nafion SAC-13 Amberlite 200
Relative Mass of Sample
Property
Normalized Transmittance
Nafion SAC-13
Amberlite 120
80% Amberlyst 15
60%
Amberlite 400 40% 50
100
150
200
250
300
350
400
450
500
Temperature (˚C) Fig. 2. Thermogravimetric analysis of various ion-exchange resins. Conditions: 15 mL/min air, 22–500 ◦ C, 10 ◦ C/min, 1 sample/20 s, smoothed 20 points.
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Table 3 Fatty acid methyl ester chain length and saturation extent by gas chromatograph. Amount (mg/mL)
Percent (mass%)
Name
Group
0.76 0.20 0.07 1.60 1.68 0.08 0.56 28.8 1.23 3.34 0.84 0.02 0.22 2.11
1.8% 0.5% 0.2% 3.8% 4.1% 0.2% 1.4% 69.3% 3.0% 8.1% 2.0% 0.0% 0.5% 5.1%
Myristic Myristoleic Pentadecanoic Palmitic Palmitoleic Heptadecanoic Stearic Oleic Elaidic Linoleic Linolenic Arachidic cis-11-Eicosenoic Unknowns
14:0 14:1 15:0 16:0 16:1 17:0 18:0 18:1 18:1 18:2 18:3 20:0 20:1 –
Table 3 summarizes the results from gas chromatography examining the chain length and saturation composition of methyl esters. The methyl ester form of oleic acid is the primary component of the FAME mixture in the products. The fatty acid composition of the esters consists of 85 mass% unsaturated and monounsaturated fatty acids. 3.3. Catalyst screening Each of the catalysts was tested using reaction conditions used in previous work [23]. Fig. 3 illustrates results of the catalyst screening. Amberlyst 15 was the most active catalyst and was selected for further investigation. The weak Lewis acid functional groups observed in Amberlyst 15 and Amberlite 120 are responsible for the high activity observed in screening experiments. Our observation of the physical and chemical properties of the various IER and contrasting them suggests that weakly Lewis acid sites are responsible for their performance as catalysts. This is confirmed in literature, where Lewis acids are more active in transesterification and strong acids groups hinder the release of product species [5]. The strong acid functional groups of Amberlite 200 are detrimental to transesterification due to the restriction of desorption of reacted species [5]. 3.4. Reaction optimization A central composite design was used to optimize the reactor conditions using Amberlyst 15 within the variable ranges listed in Table 1. The number of experiments in the statistical design was based on Eq. (2), where N is the number of experiments (20), k is
Conversion to Products (mol%)
100%
Amberlyst 15
80%
Amberlite 120H
60%
Nafion SAC-13 Amberlite 200
40%
100
95
Conversion to Products (mol%)
90
6
9
Alcohol:Oil Ratio 12 18 24 2
5
10
15
95-100 90-95 85-90
85 20
Catalyst Loading (wt%)
Fig. 4. Numerical model of transesterification yield of triolein over Amberlyst 15. Conditions: 200 ◦ C, 4 MPa, 600 RPM, and 2 h.
the number of controlled variables (temperature, catalyst loading and alcohol molar excess) and NC is the number of repeated experiments (six replicates). Conversion to products was measured as the response factor in the design, measured by HPLC. From the collected data, confidence intervals were calculated and a numerical model was developed. N = 2k + 2(k) + NC
(2)
The F-value of the polynomial model was found to be 0.01 meaning the model is significant (<0.05). The polynomial equation using coded factors is listed in Eq. (3), limited to the statistically significant variables. The variable X is conversion to products, T is temperature, A is alcohol molar excess and C is catalyst loading. The numerical model as illustrated in Fig. 4, is found to be a hyperbolic paraboloid. This shape is due to local maxima in the alcohol to oil axis and local minima on the loading axis. The local maximum on the catalyst loading axis may be due to loading effects of excess catalyst. The minima may be due to the formation of methyl sulfonates by esterification on the sulfonic acid groups of Amberlyst 15 [26]. From the numerical optimization, the optimized reactor conditions were selected to be 200 ◦ C, 13 wt% catalyst loading, 1–24 oil to alcohol molar ratio, 4 MPa nitrogen atmosphere, stirring speed 600 RPM using 100 g of oil indicating that the IER performance was very good. However, further work can be undertaken to examine the effects of higher alcohol loadings for the increase in conversion beyond 97%. It was the intent of this work to examine wide temperature range up to 220 ◦ C in conjunction with IER for biodiesel production in order to find temperature for high oil conversion. The upper temperature of 220 ◦ C was fixed as the material was unstable thermally beyond this temperature (see Fig. 2). Also, since the critical point of methanol is at 240 ◦ C, 220 ◦ C was selected as the temperature maximum to maintain methanol in a sub-critical state. The experimental variable values were selected from the model, and are valid within the variable ranges from Table 1. The local maxima for each of the controlled variables were at the selected values. These maximum values are visible in Fig. 4. X = 89 + 21.5T + 3.6C + 7.1A − 8.7T 2 −11.5C 2 + 5.1A2 − 10.7T · A
Amberlite 400 20%
(3)
0% 0
60
120
180
240
Time (minutes) Fig. 3. Transesterification of triolein to methyl esters using various resins as catalysts. Conditions: 200 ◦ C, 1:9 oil:methanol, 3 wt% loading, 600 RPM, 4 MPa, and 100 g oil.
3.5. Product analysis The predicted conversion from the model at optimized conditions was 98.1 mol%, while the experimental value at the same conditions was 97.0 (±0.7) mol%. Fig. 5 demonstrates the high
G. Paterson et al. / Catalysis Today 212 (2013) 157–163
100%
Original Experiment
Conversion to Products (mol%)
Conversion to Products (mol%)
100%
80% Repeat Experiment 60%
40%
20%
0%
0
161
60
120
180
240
80%
60%
40%
20%
0% Fresh Catalyst
Time (minutes)
Used Twice
Used Three Times
Used Four Times
Successive Catalyst Reusage
Fig. 5. Transesterification of triolein using optimized reactor conditions. Conditions: 200 ◦ C, 1:24 oil:methanol, 13 wt% loading, 600 RPM, 4 MPa,and 100 g oil.
Fig. 6. Amberlyst 15 longevity in successive transesterification of triolein. Conditions: 200 ◦ C, 1:24 oil:methanol, 13 wt% loading, 600 RPM, 4 MPa, and 100 g oil.
conversion of the optimized experiment. The experimental results were reliably repeated. The acid value of the products was typically titrated to be in the range of 50 mg KOH/g sample. To test for catalyst leaching, ICP-MS was employed to measure sulfur content in triolein and the reaction products. From Section 3.2, triolein was found to contain 21 (±1) ppm, while the products contain 12 (±1) ppm. The decrease in sulfur content can be attributed to the glycerol layer containing a portion of the sulfur present in the feedstock. It can be concluded that sulfur leaching did not take place, given the sulfonic functional group of Amberlyst 15 and therefore the high acid value is transesterification products is caused by the presence of free fatty acids. In this case, the formation of free fatty acids is due to hydrolysis of FAME.
that water is being produced and consumed in side reactions and approaching equilibrium of 0.7 wt% at the optimized conditions. Therefore, Amberlyst 15 is catalyzing the hydrolysis, esterification, and transesterification reactions.
3.6. Effect of water and free fatty acid additions Water was added to the reactant mixture to determine if hydrolysis of methyl esters was taking place in competition with transesterification. Results are illustrated in Table 4. Adding water to feedstock mixtures (up to 1 wt% relative to oil) did not have an effect on the reaction products as measured by HPLC or the acid value. At 2 wt% loading of water, a decrease in conversion to products is apparent. The water content of the products for each of the water addition reactions (0.1, 0.5, 1 and 2 wt%) were 0.67, 0.72, 0.71 and 0.72 (±0.01) wt%, respectively. Comparatively, the water content of the optimized reaction conditions without water addition had a water content of 0.69 wt%. See Table 4. The addition of water did not have a significant impact on the conversion, water content or the acid value of the transesterification products. Ester content and the acid value were not affected when stearic acid was added to the feedstock up to 15 wt% see Table 4. It is concluded Table 4 Conversion, acid value and water content of feedstock and various experiments. Feedstock or experiment
Conversion to products (±0.7 mol%)
Water content Acid value (±0.9 mg KOH/g) (±0.01 wt%)
Triolein Canola oil Greenseed oil Optimized conditions 0.1 wt% water products 0.5 wt% water products 1 wt% water products 2 wt% water products 5 wt% FFA products 10 wt% FFA products 15 wt% FFA products
0 0 0 96.8 95.8 95.2 96.3 92.6 96.7 97.1 97.5
2.5 0 8.2 55.0 57.8 55.5 58.5 56.1 55.9 48.1 53.3
0.01 0.02 0.05 0.69 0.67 0.72 0.71 0.72 0.66 0.62 0.60
3.7. Catalyst longevity Successive reactions reused Amberlyst 15 to test catalyst longevity. Following each trial, the catalyst was collected by gravity filtration allowed to dry and charged into the subsequent reaction. As shown in Fig. 6, there is a negligible decrease in the conversion of triolein to products during the second use. Subsequent reuses of the catalyst show significant decrease in conversion to products after 2 h. The third use of Amberlyst 15 achieved a final conversion of 81 mol%, while the fourth successive use demonstrated 54 mol% conversion to products. Amberlyst 15 can be reused once under the current reaction conditions, while maintaining high conversion to products. 3.8. The acid value and temperature A lower temperature, a higher alcohol molar ratio and a longer reaction duration were selected for the kinetics study of transesterification to avoid the hydrolysis side reaction, while maintaining the conversion to products. Temperature was varied from 100 ◦ C to 120 ◦ C. Oil quantity was decreased to 50 g per reaction. Oil to alcohol molar ratio was raised to 1:77. The reaction duration was increased to 72 h. 3.9. Reaction kinetics Kinetics experiments at 100 ◦ C, 110 ◦ C, and 120 ◦ C are illustrated in Fig. 7. The acid values of the products increase with temperature increase. The acid value for 100 ◦ C, 110 ◦ C, and 120 ◦ C are 0.35, 0.46, and 2.1 (±0.9) mg KOH/g sample, respectively. Previous work [27] has successfully been modeled similar reactions using the power rate law, while others [28] have found that the pseudo-homogeneous and Langmuir–Hinshelwood kinetics also yield similar results. Langmuir–Hinshelwood (or pseudohomogeneous) kinetics could have been examined in the reaction kinetics modeling of this work. The rate law kinetics were selected for simplicity and based on previous work [28]. Results for the kinetic parameters from the model are tabulated in Table 5. An increase in reaction rate constants is seen in each reaction as temperature increases. Reaction rate constants are higher in the forward reactions compared to the reverse reactions. The majority
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Table 5 Reaction rate constants for reversible transesterification reactions (conditions: 100 ◦ C, 110 ◦ C and 120 ◦ C, 1:77 oil:methanol, 13 wt% loading, 600 RPM, and 50 g oil). Temperature (◦ C)
Rate constant, k (L/mol/s)
Pearson correlation coeff.
TG ⇒ DG
100 110 120
1.273 × 10−7 2.083 × 10−7 9.259 × 10−7
0.957 0.990 0.991
DG ⇒ TG
100 110 120
1.157 × 10−8 1.273 × 10−8 4.630 × 10−7
0.823 0.991 0.972
DG ⇒ MG
100 110 120
1.042 × 10−6 1.505 × 10−6 2.546 × 10−6
0.987 0.905 0.998
MG ⇒ DG
100 110 120
3.472 × 10−7 5.787 × 10−7 2.315 × 10−6
0.938 0.992 0.991
MG ⇒ GL
100 110 120
1.134 × 10−5 2.662 × 10−5 7.523 × 10−5
0.928 0.992 0.991
GL ⇒ MG
100 110 120
2.894 × 10−6 4.630 × 10−5 5.787 × 10−5
0.939 0.992 0.991
Reaction
4. Conclusions
of the Pearson correlation coefficients are above 0.98, demonstrating a close approximation to the experimental data. An Arrhenius plot in Fig. 8 shows the forward reaction rate constants for the three temperatures examined. Linear best fit slopes for each reaction rate constant are natural logarithmic
Conversion to Products (mol%)
100%
80% 120˚C 60%
40%
110˚C
20% 100˚C 1
2
3
Time (days) Fig. 7. Transesterification of canola oil at various temperatures over Amberlyst 15. Conditions: 1:77 oil:methanol, 13 wt% loading, 600 RPM, and 50 g oil.
Natural Logarithm of Reaction Constants (L/mol/s)
-11 -12 -13
MG to GL y = -13863x + 25.727 R2 = 0.9949 DG to MG y = -6544.7x + 3.735 R2= 0.9864
-14 -15 -16 2.5E-03
The authors acknowledge the funding from Natural Sciences and Engineering Council of Canada (NSERC) and the Canada Research Chair (CRC) program. References
-9 -10
Amberlyst 15 was found to be the most active ion-exchange resin tested as a transesterification catalyst and reaction conditions were optimized in a 300 mL batch reactor. Within the experimental bounds, the optimized conditions were 200 ◦ C, 1:24 oil to methanol molar ratio, 13 wt% catalyst loading, 600 RPM stirring speed, 4 MPa nitrogen atmosphere using 100 g of triolein with conversion to products of 97 wt% as measured by HPLC. The acid value of the products is higher than that of the feedstock. No leaching of functional groups was detected in the products. The addition of water to the reactants did not have a significant effect of the reaction products up to a 1 wt% loading. The addition of up to 15 wt% stearic acid as a FFA did not impact the reaction products. Hydrolysis and esterification are taking place as side reactions at the optimized reaction conditions. Amberlyst 15 can be reused once at the stated conditions with no loss in conversion to products. Successive uses cause a reduction in conversion to products from 97 mol% down to 81 mol% and then to 54 mol%. Reaction rate constants were determined for forward and reverse transesterification between 100 ◦ C and 120 ◦ C. The rate determining step is the TG to DG reaction. Activation energies for the TG to DG, DG to MG, and MG to GL reactions are 120, 54.4 and 115 kJ/mol, respectively. Future work should be undertaken to examine lower temperatures, and pressures to improve commercial viability. The local maxima observed in optimization experiments can be further examined using a wider variety of reaction conditions. In addition, other IER should be examined in a variety of reactor types using a variety of feedstocks and an economic feasibility study based on the energy requirement should be carried out for this system. Acknowledgements
0% 0
derivations of Eq. (1). From the linear equations the activation energy for the triglyceride (TG) to diglyceride (DG) reaction was 120 kJ/mol. The activation energy for the diglyceride to monoglyceride (MG) reaction was 54 kJ/mol and 115 kJ/mol for the monoglyceride to glycerol (GL) reaction. Literature values for the three reactions using homogeneous catalysts are 61.5, 59.4, and 26.8 kJ/mol, respectively [19]. Similar reactions using heterogeneous catalysts report activation energies ranging from 49 kJ/mol to 110 kJ/mol [27,29].
TG to DG y = -14487x + 22.776 R2= 0.9138 2.6E-03
2.7E-03
Inverse Temperature (K-1 ) Fig. 8. Arrhenius plot tri-, di- and monoglyceride consumption during transesterification. Conditions: 100 ◦ C, 110 ◦ C and 120 ◦ C, 1:77 oil:methanol, 13 wt% loading, 600 RPM, and 50 g oil.
[1] V. Patil, K. Tran, H. Giselrød, International Journal of Molecular Sciences 9 (2008) 1188–1195. [2] M. Haas, A. McAloon, W. Yee, T. Foglia, Bioresource Technology 97 (2006) 671–678. [3] A.A. Apostolakos, I.K. Kookos, C. Marazioti, K.C. Angelopoulos, Fuel Processing Technology 90 (2009) 1023–1031. [4] J. Melero, J. Iglesias, G. Morales, Green Chemistry 11 (2009) 1285–1308. [5] M. Di Serio, R. Tesser, L. Pengmei, E. Santacesaria, Energy & Fuels 22 (2008) 207–217. [6] R. Jothiramalingam, M. Wang, Industrial & Engineering Chemistry Research 48 (2009) 6162–6172. [7] E. Lotero, Y. Liu, D. Lopez, K. Suwannakarn, D. Bruce, J. Goodwin Jr., Industrial & Engineering Chemistry Research 44 (2005) 5353–5363. [8] M. Sharma, Reactive & Functional Polymers 26 (1995) 3–23. [9] K. Dorfner, Ion Exchangers Properties and Applications, Ann Arbor Science Publishers Inc., Ann Arbor, 1972. [10] G. Osborn, Synthetic Ion-Exchangers, Chapman & Hall Ltd., London, 1961. [11] S. dos Reis, E. Lachter, R. Nascimento, J. Rodrigues Jr., M. Reid, Journal of the American Oil Chemists’ Society 82 (2005) 661–665. [12] S. de Rezende, M. de Castro Reis, M. Reid, P. Silva Jr., F. Coutinho, R. da Silva San Gil, E. Lachter, Applied Catalysis A: General 349 (2008) 198–203.
G. Paterson et al. / Catalysis Today 212 (2013) 157–163 [13] M. Talukder, J. Wu, S. Lau, L. cui, G. Shimin, A. Lim, Energy & Fuels 23 (2008) 1–4. [14] A. Kiss, A. Dimian, G. Rothenberg, Advanced Synthesis & Catalysis 348 (2006) 75–81. [15] Z. Helwani, M. Othman, N. Aziz, J. Kim, W. Fernando, Applied Catalysis A: General 363 (2009) 1–10. [16] M. Harmer, Q. Sun, Applied Catalysis A: General 221 (2001) 45–62. [17] N. Shibasaki-Kitakawa, H. Honda, H. Kuribayashi, T. Toda, T. Fukumura, T. Yonemoto, Bioresource Technology 98 (2007) 416–421. [18] G. Peterson, W. Scarrah, Journal of the American Oil Chemists’ Society 61 (1984) 1593–1597. [19] D. Darnoko, M. Cheryan, Journal of the American Oil Chemists’ Society 77 (2000) 1263–1267. [20] S. Brunauer, P. Emmett, E. Teller, Journal of the American Oil Chemists’ Society 60 (1938) 309–319. [21] Official Methods and Recommended Practices of the AOCS, American Oil Chemists’ Society, Champaign, 1998.
163
[22] M. Kulkarni, R. Gopinath, L. Meher, A. Dalai, Green Chemistry 8 (2006) 1056–1062. [23] K. Jacobson, R. Gopinath, L. Meher, A. Dalai, Applied Catalysis B: Environmental 85 (2008) 86–91. [24] R. Silverstein, F. Webster, Spectrometric Identification of Organic Compounds, sixth ed., John Wiley & Sons, Inc., New York, 1998. [25] D. Yazici, C. Bilgic¸, Surface and Interface Analysis 42 (2010) 959–962. [26] J. Melero, L. Bautista, G. Morales, J. Iglesias, D. Briones, Energy & Fuels 22 (2009) 539–547. [27] H. Hamid, R. Yunus, T. Choong, Chemical Product and Process Modeling 5 (2010) 1–17. [28] E. Bozek-Winkler, J. Gmehling, Industrial & Engineering Chemistry Research 45 (2006) 6648–6654. [29] D. López, J. Goodwin Jr., D. Bruce, Journal of Catalysis 245 (2007) 381–391.