Article pubs.acs.org/EF
Transesterification of Low-Quality Triglycerides over a Zn/CaO Heterogeneous Catalyst: Kinetics and Reusability Studies Dinesh Kumar† and Amjad Ali* School of Chemistry and Biochemistry, Thapar University, Patiala 147 004, India S Supporting Information *
ABSTRACT: Zinc-doped (0.25−7 wt %) calcium oxide (Zn/CaO) has been prepared in nanocrystalline form by a simple wet chemical method followed by calcination up to 950 °C. The structural analysis has been investigated by powder X-ray diffraction (XRD), whereas the surface morphology and average particle size of Zn/CaO were determined by scanning electron and transmission electron microscopic studies, respectively. The catalytic activity of the prepared Zn/CaO toward the transesterification of cotton seed oil with methanol was found to be a function of its calcination temperature, crystallite size, and basic strength. A pseudo-first-order kinetic model was applied to evaluate the kinetic parameters for the transesterification of waste cotton seed oil with methanol, and a first-order rate constant (k) and activation energy (Ea) were found to be 0.10 min−1 and 43 kJ mol−1, respectively. The catalyst, Zn/CaO, was amenable to recovery and recycling for at least five consecutive reaction cycles. The Koros−Nowak criterion test has been employed to demonstrate that measured catalytic activity was independent of the influence of transport phenomenon. Further, Zn/CaO was also found as an efficient catalyst for the complete transesterification of a variety of triglycerides (having up to 8.4 wt % free fatty acids), such as virgin cotton seed oil, soybean oil, waste cotton seed oil, castor oil, karanja oil, jatropha oil, and mutton fat. Thus, the present work demonstrates the application of high free fatty acid containing waste or non-edible oils as feedstock, without any pre-treatment, for biodiesel production. production.7−10 Such feedstocks being less expensive than edible oils have the potential to reduce the overall biodiesel production cost. However, these oils usually contain high FFA (up to 12 wt %) and moisture (∼3 wt %) contents, and therefore, a homogeneous base catalyst could not be employed for their transesterification.11 To circumvent the above-mentioned problems associated with the application of low-quality feedstock for biodiesel production, research has been directed for the development of heterogeneous catalysts in the recent past.12,13 Heterogeneous catalysts have several advantages over a homogeneous catalyst, including easier catalyst operation and separation, recyclability, and high moisture and FFA content resistance.12,13 A commercial biodiesel plant based on Esterfip-H technology has been set up by the French Institute of Petroleum (IFP) to use the mixed oxides of Zn and Al as a heterogeneous catalyst for the transesterification of triglycerides. 14 The same technology does not require catalyst recovery and biodiesel washing with water and yielded >98% biodiesel yield.4,14 However, because of the relatively low activity under moderate reaction conditions, the same catalyst demanded a high reaction temperature (200−250 °C) and high pressure to obtain a specified FAME yield.14,15 Calcium oxide is one of the most frequently literaturereported heterogeneous catalysts for the transesterification reaction, owing to its non-toxicity, ease of availability, and lower cost.13,16−19 The main problem associated with commercially available CaO is its moisture sensitivity and lesser activity
1. INTRODUCTION A continuously growing energy demand, depleting fossil fuel resources, global warming, and environmental pollution are the diverse reasons for searching for alternate and renewable energy resources. Biodiesel, chemically known as fatty acid methyl esters (FAMEs), has been globally accepted as a renewable alternate to the fossil-based diesel fuel.1 Transesterification of triglycerides (vegetable oil or animal fat) with methanol in the presence of a catalyst (chemical or bio) leads to the formation of FAMEs and glycerol as a byproduct.2 Moreover, FAMEs not only provide an eco-friendly alternative to the conventional diesel fuel but could also be employed as a substrate for the preparation of diesel fuel additives,3 viz., lubricity and cetane number improvers. Homogenous base catalysts, such as hydroxides and methoxides of sodium or potassium, are frequently used for the transesterification reaction at an industrial scale.2 However, the same catalysts are non-recyclable and non-green and lead to the formation of sodium- or potassium-ion-contaminated biodiesel and glycerol.2,4 Additionally, same catalysts produce soap instead of biodiesel and become deactivated if moisture and/or free fatty acid (FFA) contents in feedstock are >0.3 and/or >0.5 wt %, respectively.5,6 Thus, the use of a homogeneous catalyst required refined and costly feedstocks for biodiesel production, which not only increase biodiesel production cost but also resulted in a fuel versus food situation. Despite several advantages over conventional diesel fuel, biodiesel could not obtain the desired commercial success in many countries, including India, because of the non-availability of adequate feedstock. The same problem could be pacified by employing non-edible oils (e.g., karanja and jatropha), animal fat, or waste cooking oils as feedstock for biodiesel © 2013 American Chemical Society
Received: January 15, 2013 Revised: June 16, 2013 Published: July 1, 2013 3758
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soybean oil (SO), mutton fat (MF), castor oil (CS), virgin cotton seed oil (CO), karanja oil (KO), and jatropha oil (JO) were purchased from local shops located in Patiala, India. The FFA, saponification, iodine value, and moisture content of the SO, CO, MF, CS, WO, KO, and JO were determined by following the literature-reported32 methods (Table 1).
toward the transesterification reaction. The activity of pure CaO could be enhanced by either preparing it in nanocrystalline form17 or impregnating it with lithium,12,19−21 potassium,22 or lanthanum ions.23 Although modified CaO was found to show better activity, it was not stable and suffered the leaching of Ca and/or active species into the reaction media. Mixed oxides of CaO/CeO2 have also been prepared via the coprecipitation method and employed as a reusable catalyst for the transesterification of palm oil to yield >90% FAMEs.24 Kim et al. reported25 the preparation of CaO-supported La2O3 or CeO2 and their application in a flow reactor for the transesterification of soybean oil. In both studies, the effect of moisture and FFA contents on catalytic activity has not been reported. It is crucial to study the effect of these factors on catalytic activity because low-quality feedstocks usually have high moisture and FFA contents. Among transition-metal oxides, ZnO is frequently studied26−28 as a heterogeneous catalyst for the transesterification reaction. Incomplete conversion (99% FAME yield in five successive catalytic runs. However, after the sixth cycle and onward, only partial conversion was achieved, even after 6 h of reaction period (see Figure S7 of the Supporting Information). The gradual loss of the catalytic activity could be due to (i) the blockage of active sites because of the adsorbed organic molecule, (ii) structural changes occurring in the catalyst during the regeneration process, and (iii) the partial leaching of the active species from the catalyst. The deposition of the adsorbed organic species on the catalyst support could partially deactivate the catalyst because of the blockage of catalyst active sites.14 The Fourier transform infrared (FTIR) spectrum (see Figure S8 of the Supporting Information) of the recovered and recalcined catalyst did not show vibrations corresponding to any adsorbed organic molecules to support that FAMEs or glycerol have not been accumulated on the recovered and regenerated Zn/CaO surface. To evaluate the structural changes that occurred in Zn/CaO, the XRD patterns of the fresh and regenerated Zn/CaO are compared (see Figure S9 of the Supporting Information). The XRD pattern of regenerated Zn/CaO (after 5 cycles) supports the formation of mixed phases, consisting of hexagonal Ca(OH)2 as the major phase (2θ = 18.1°, 28.7°, 29.4°, 34.2°, 47.2°, and 51.0°; JCPDS 841275) and cubic CaO (2θ = 32° and 54.4°; JCPDS 821691), hexagonal ZnO (2θ = 36.3°, 56.2°, and 62.8°; JCPDS 891397), and hexagonal CaCO3 (2θ = 43.04° and 64.3°; JCPDS 881811) as minor phases. Thus, upon repeated use and recalcination, CaO and ZnO become separated, and because of intrinsic moisture sensitivity, CaO reacts with atmospheric moisture easily. As a result of this, strong Lewis basic sites (−O−) become converted to weak Brønsted sites (−OH), and consequently, catalytic activity is lost. The concentration of metal ions (Ca2+ and Zn2+) in FAMEs and glycerol leached from the catalyst while catalyzing the 3765
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calculated from these plots and found to be 0.10, 0.04, 0.02, and 0.01 min−1 at 65, 55, 45, and 35 °C, respectively. The Arrhenius model49 was employed to estimate the activation energy (Ea) and pre-exponential factor (A) for the same reaction as given in eq 2
reaction has been analyzed by the atomic absorption spectroscopy (AAS) technique. The low zinc concentration in the reaction medium suggests that zinc has a strong interaction with CaO supports43 and, hence, resulted in either no or minor (8 ppm) lixiviation of Zn2+ in FAMEs and glycerol, respectively. On the other hand, a low calcium concentration was found in FAMEs (5 ppm) as well as in glycerol (120 ppm). Thus, gradual loss of the catalytic activity could be attributed to the (i) structural changes occurring in the catalyst and (ii) partial loss of Zn from the catalyst, upon successive reuse. As evident from the metal analysis, there is partial leaching of Ca2+ and Zn2+ in the reaction mixture and leached metal species could catalyze the reaction similar to a homogeneous catalyst.44 Hence, it is noteworthy to quantify the homogeneous contribution in the activity of a catalyst to be claimed as a heterogeneous catalyst. To investigate whether the leached metal ion has catalyzed the reaction, 1.5-Zn/CaO-550 (1 g) has been stirred vigorously with refluxing methanol (10 mL) for 45 min. After a stipulated time, the catalyst was removed by filtration and recovered methanol was mixed with WO (9:1 molar ratio) and heated again at 65 °C for 45 min. Under mentioned reaction conditions, not more than 5% FAME yield was obtained, and this was similar to the yield obtained in a blank experiment carried out in the absence of the catalyst. Hence, it is safe to assume that leached metal ions have not catalyzed the reaction to a significant extent and heterogeneous Zn/CaO is mainly responsible for the catalytic activity. 3.2.7. Koros−Nowak (KN) Criterion Test. The KN criterion test modified by Madon−Boudart has been performed to establish that the measured catalytic activity is independent of the influence of transport phenomena.45,46 The reaction rates in the kinetic regime should be proportional to the concentration of the active material (Zn2+ in the present case). Hence, a series of the Zn/CaO catalyst was prepared by varying the Zn2+ doping in CaO from 0.25 to 1.5 wt % (for example, 0.25-Zn/ CaO-550, 0.50-Zn/CaO-550, 1.0-Zn/CaO-550, and 1.5-Zn/ CaO-550) but following the identical experimental conditions. The prepared catalysts were employed (5 wt % of oil) for the transesterification of WO with methanol (1:9 molar ratio) at 35 and 65 °C reaction temperature. The KN criterion has been explained by plotting the reaction rate (r) in mol h−l g−1 of catalyst versus the weight of Zn2+ (f w) in g−1 of CaO, as shown in Figure 10. The value of the slope was calculated from the graph shown in Figure 10 and found to be 0.94 and 0.96 at 35 and 65 °C, respectively. Because the calculated values of the slope are close to unity, hence, it could be concluded that transesterification of cotton seed oil in the presence of the Zn/CaO catalyst obeyed the KN criterion and the rate of transport has not influenced the reaction rates. 3.3. Kinetic Study. The transesterification of triglycerides in the presence of excess methanol has been reported47,48 to follow a pseudo-first-order kinetic model as given in eq 1 −ln(1 − X me) = kt
ln k = −Ea /RT + ln A
(2) −1
−1
where R is the gas constant (8.31 J K mol ) and T is the reaction temperature in Kelvin. The values of Ea and A, calculated from 1/T versus ln k plot (see Figure S10 of the Supporting Information), were found to be 43 kJ mol−1 and 1.65 × 107 min−1, respectively. The calculated activation energy (43 kJ mol−1) for the WO transesterification was found within the range (26−84 kJ mol−1) reported for the transesterification of various vegetable oils (see Table S2 of the Supporting Information).23,47,49−55 3.4. Comparison of the Zn/CaO Activity with Few Literature-Reported Similar Catalysts. To show the efficacy and advantages of 1.5-Zn/CaO-550 over literaturereported similar catalysts, a comparison between two has been made and shown in Table 3. As evident from the comparison, the use of 1.5-Zn/CaO-550 as a solid catalyst is more advantageous than its literature-reported counterparts because the former (i) yielded a higher FAME yield (99 ± 2%), (ii) required less methanol/oil molar ratio (9:1), (iii) needed shorter reaction duration (0.75 h for WO) at 65 °C, (iv) was reusable (up to 5 catalytic runs), (v) leached lesser metal in FAMEs and glycerol, (vi) demonstrated high FFA and moisture tolerance, and (vii) was effective for the transesterification of a variety of triglycerides with alcohols having up to a 12 carbon atom chain. 3.5. Physicochemical Properties of Prepared FAMEs. FAMEs obtained from the transesterification of triglycerides should satisfy EN 14214 or ASTM standards before they could be employed in diesel engines as biodiesel. To study the physicochemical properties, transesterification of WO, JO, and KO with methanol were carried out in the presence of 1.5-Zn/ CaO-550 under optimal reaction conditions. Upon completion of the reaction, the catalyst was filtered out and the liquid phase was kept in a separating funnel for 12 h to separate the upper FAME layer from the lower glycerol layer. FAMEs thus obtained were rotary-evaporated to remove the excess methanol, washed, and finally studied for a few physicochemical properties. The values of studied properties of prepared FAMEs were found within the acceptable limits of EN 14214, as given in Table 4.
4. CONCLUSION The catalyst, 1.5-Zn/CaO-550, has been prepared by a wet chemical method in nanocrystalline form as revealed by TEM and powder XRD studies. The same catalyst was found to have the highest surface area and basic strength, among the prepared catalysts, as supported by the BET surface area and Hammett indicator analysis, respectively. The Zn/CaO nanocatalyst was found to be efficient even at room temperature (35 °C) for complete transesterification reactions of triglycerides with methanol. Under optimized reaction conditions (methanol/oil molar ratio, 9:1; catalyst concentration, 5 wt %; reaction temperature, 65 °C; and reaction duration, 45 min), Zn/CaOcatalyzed transesterification of WO was found to follow pseudo-first-order kinetics, and the apparent first-order rate constant and activation energy for the same reaction were found to be 0.10 min−1 (at 65 °C) and 43 kJ mol−1,
(1)
where Xme is the fraction of FAME content at time t (min). The kinetics of the Zn/CaO-catalyzed transesterification of WO has been studied at a 9:1 methanol/oil molar ratio in the temperature range of 35−65 °C. Figure 11 shows the linear nature of −ln(1 − Xme) versus t (time) plots to maintain that the Zn/CaO-catalyzed reaction follows pseudo-first-order kinetics. The rate constants were 3766
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respectively. Further, the catalyst could be employed for 5 catalytic cycles without significant loss in activity. The catalyst was found to be effective even for the transesterification of lowquality feedstock having up to 4.3 wt % moisture and 8.4 wt % FFA contents. The lixiviation study supported negligible homogeneous contribution in catalytic activity, and the KN test demonstrates that activity is independent of the influence of transport phenomenon. Few physicochemical properties of the FAMEs derived from the WO, JO, and KO have also been studied, and the same was found within the limits of EN 14214 specifications.
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ASSOCIATED CONTENT
S Supporting Information *
Comparative proton NMR spectrum of triglycerides and their respective FAMEs, histogram showing particle size distribution, effect of the catalyst concentration on the FAME yield, effect of the methanol/oil molar ratio on the FAME yield, effect of the reaction temperature on the FAME yield, comparative proton NMR spectrum of fatty acid alkyl esters derived from CO with various alcohols, reusability studies of 1.5-Zn/CaO-550, FTIR spectra of fresh and recycled Zn/CaO, XRD patterns of fresh and recycled Zn/CaO, and Arrhenius plot for Zn/CaOcatalyzed transesterification of WO (Figures S1−S10, respectively) and comparison of the basic strength of M/CaO (M = Mn, Fe, Co, Cu, Ni, Zn, and Cd) and comparison of catalytic activities of a few literature-reported catalysts with the present report (Tables S1 and S2, respectively). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +91-175-2393832. Fax: +91-175-2393005. E-mail:
[email protected] and/or
[email protected]. Present Address †
Dinesh Kumar: Department of Chemistry, Sri Guru Granth Sahib World University, Fatehgarh Sahib 140 406, India. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the Council of Scientific and Industrial Research (CSIR) [Grant 01(2503)/11/EMR-II] and the Defence Research and Development Organisation (DRDO) [Grant ERIP/ER/1103933/M/01/1453] for the financial support. We are also thankful to the Sophisticated Analytical Instrumentation Facility (SAIF, Panjab University, Chandigarh, India) for powder XRD, NMR, and TEM, Matter Lab (Thapar University, Patiala, India) for SEM, and Kunash Instruments, Thane, India, for surface area analysis. We are also thankful to the Associate Editor, both the reviewers, and Dr. B. K. Chudasama for their valuable comments and suggestions.
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