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A comparative study on the catalytic performance of different types of zeolites for biodiesel production
Fuel 158 (2015) 848–854
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A comparative study on the catalytic performance of different types of zeolites for biodiesel production Kaian Sun a, Jie Lu b, Lingling Ma a, Ying Han a, Zhongjun Fu a,c,⇑, Jincheng Ding a,⇑ a
College of Chemical Engineering, Shandong University of Technology, 12 Zhangzhou Road, Zibo 255049, China Department of Resources and Environmental Engineering, Shandong University of Technology, 12 Zhangzhou Road, Zibo 255049, China c Shandong Engineering Research Center for Dyeing & Finishing of Textiles, 12 Zhangzhou Road, Zibo 255049, China b
h i g h l i g h t s Different kinetic models were used to describe the esterification over zeolites. Al was verified to detect the leaching of active component of zeolite. The Cassie–Baxter model was used to study the effect of hydrophobic property. The Thiele modulus was used to investigate the effect of zeolite pore size. Microporous and micro-mesoporous zeolites were compared in biodiesel production.
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Article history: Received 3 December 2014 Received in revised form 12 June 2015 Accepted 13 June 2015 Available online 17 June 2015 Keywords: Biodiesel Esterification Zeolites Thiele modulus Comparative kinetics Cassie–Baxter model
a b s t r a c t Microporous zeolites (BEA type Beta zeolite and MFI type ZSM-5 zeolite) and micro-mesoporous zeolites (MFI type ZRP-5 zeolite) with various Si/A1 ratios were employed in the esterification of oleic acid with ethanol. The effect of pore size on the internal mass transfer limitation was investigated by Thiele modulus calculation. The results showed that the zeolites with high Si/Al ratios had better catalytic performance, and of these three zeolites at the same Si/A1 ratios, the ZRP-5 zeolite exhibited the lowest internal mass transfer limitations but the worst catalytic performance. Through the comparison of the Eley–Rideal model and the Langmuir–Hinshelwood model, it was indicated that on the surface of hydrophilic ZRP-5 zeolites, the adsorption of the polar ethanol molecules were more favorable than the adsorption of oleic acid molecules, resulting in less coverage of oleic acid molecules on the surface of zeolites and lower conversion rate of esterification. Moreover, the Cassie–Baxter model and the water adsorption capacity test were used to further validate the assumption of kinetic model. The highest conversion rate of 73.6% was achieved when the reaction was catalyzed by high hydrophobic Beta (50) zeolites under optimized conditions of the molar ratio of oleic acid to ethanol of 1:20, catalyst loading of 0.167 meq/g (oleic acid), temperature of 78.0 °C, reaction time of 10.0 h and stirring speed of 600 rpm. The conversion rate of oleic acid remained above 70.0% after five runs and there was no apparent loss of the active component (Al) from the zeolite. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Biodiesel, a biodegradable fuel to replace traditional petroleum-derived diesels, has gaining much attention because of its renewability, non-toxic and low greenhouse gases emissions [1]. Generally, biodiesel is produced by the transesterification ⇑ Corresponding authors at: Shandong Engineering Research Center for Dyeing & Finishing of Textiles, and College of Chemical Engineering, Shandong University of Technology, 12 Zhangzhou Road, Zibo 255049, China. Tel.: +86 533 2781630; fax: +86 533 2781664 (Z. Fu). Tel.: +86 533 2783397; fax: +86 533 2781664 (J. Ding). E-mail addresses: [email protected] (Z. Fu), [email protected] (J. Ding). http://dx.doi.org/10.1016/j.fuel.2015.06.048 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
using alkaline catalysts [2]. However, the alkaline-catalyzed process suffers from serious limitations that the presences of free fatty acids (FFAs) and water in the feedstock may lead to the formation of soaps, increasing the product viscosity and separation difficulty of downstream products [3]. Compared with alkaline catalysts, the acid catalysts are known to react with the low-quality resources containing high FFAs and/or water under normal conditions. Moreover, the acid catalysts are effective for both esterification and transesterification reactions [4]. Some types of heterogeneous acid catalysts such as the zirconium sulfate [5], ion exchange resins [6], silica supported tin oxides [7] and tin oxide supported WO3 [8]
K. Sun et al. / Fuel 158 (2015) 848–854
have been used for biodiesel production recently. In contrast to the homogeneous catalysts, the heterogeneous catalysts have the advantages of non-corrosion, environmental friendliness, and easy removal from the products, showing the promising potentials for industrial biodiesel productions [9]. As one kind of the solid acids, zeolites are crystalline and porous materials with high specific surface area. Chung et al. [10] compared different microporous zeolites for reduction of free fatty acids in waste cooking oil in esterification reaction (ZSM-5, faujasite, beta and silicalite zeolites). Due to the limitation of the narrow channels, large molecules (like oleic acid molecule, length 2.4 nm and height 0.3 nm) cannot access the internal active sites of microporous zeolites. Moreover mesoporous zeolites can be potentially used in biodiesel production and attract many attentions. However, mesoporous zeolites show weaker catalytic activity, which seriously limit their extensive uses. In order to improve the catalytic activities of mesoporous zeolites, various approaches were used to modify mesoporous zeolites including phenylsulfonic acid functionalized SBA-15 zeolite [11], 12-tungstosilicic acid functionalized SBA-15 zeolite [12] and Al supported MCM-41 zeolite [13]. By contrast, micro-mesoporous zeolites have the advantages over either microporous zeolites or mesoporous zeolites [14]. And there were few reports about comparative studies on the catalytic performance of microporous and micro-mesoporous zeolites with various Si/Al ratios for biodiesel production. Besides, to meet the needs of theoretical researches and practical applications, multiple models were developed to describe the process of reaction. Jiang et al. [15] reported a pseudo-homogeneous model to describe the reaction progress of biodiesel production. Merchant et al. [16] compared the Langmuir–Hinshelwood model and the Eley–Rideal model on the biodiesel syntheses catalyzed by cation exchange resins. Konno et al. [17] used the Thiele modulus and the effectiveness factor to study the influence of mass transfer limitation. Han et al. [18] studied the wetting state on the surface of ZSM-5 zeolites using Cassie–Baxter model. However, to investigate the influence of pore size and hydrophobicity on catalysis performance of zeolites, there were few reports discussing the correlation between mass transfer model, wetting state model and kinetic model. In this study, oleic acid and ethanol were chosen as the model compounds for biodiesel production over zeolites, because both of them are renewable raw materials [19]. The zeolites included microporous zeolites (BEA type Beta zeolites and MFI type ZSM-5 zeolites) and micro-mesoporous zeolites (MFI type ZRP-5 zeolite) with various Si/Al ratio. Several models including the Cassie– Baxter model, the Thiele modulus, the Langmuir–Hinshelwood model and the Eley–Rideal model were used to discuss the correlation between physical prosperities of zeolite (the pore size and the pore surface hydrophilicity) and the performance of catalyst based on the experimental results. The influencing factors of esterification including reaction temperature, Si/Al ratios of zeolites, zeolite loading, the reusability of zeolites and molar ratio of oleic acid to ethanol were analyzed.
2. Experimental section 2.1. Materials The zeolites of ZRP-5 (Si/Al = 25), ZRP-5 (Si/Al = 50), ZSM-5 (Si/Al = 25) and ZSM-5 (Si/Al = 50) were kindly supplied by Sinopec Catalyst Co., Ltd., Zibo, China. The Beta (Si/Al = 25) zeolite and Beta (Si/Al = 50) zeolite were purchased from the Catalyst Plant of Nankai University, Tianjin, China. The oleic acid was obtained from Shuangshuang Chemical Reagent Co., Ltd., Yantai, China. Ethanol and NH4Cl were purchased from Tieta Chemical Reagent Co., Ltd., Laiyang, China. All the reagents were analytically pure.
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All H-type zeolites were prepared from Na-type zeolites by following the ion exchange procedure reported by Patel et al. [20]. The Na-type zeolite was added to a 2 M NH4Cl solution at the mass ratio of 1.0:1.0, and the mixture was stirred at 80.0 °C for 2.0 h. Then, the samples were washed twice with the ultrapure water (resistance 18.2 MX, deionized and distilled from the Ulupure UPT-2-401 pure water system), subsequently filtered and dried at 100.0 °C for 12.0 h, and finally calcined at 550.0 °C for 6.0 h. The prepared zeolites were denoted by their Si/Al ratio in the parenthesis after each zeolite name. 2.2. Characterization Prior to testing, all samples were calcined at 500.0 °C for 6.0 h to remove the templates and moisture. The infrared spectra were recorded by using a Nicolet 5700 FTIR (Thermo Electron, USA) in the scanning range of 4000–400 cm1 with the KBr pellet method. XRD spectra were obtained by using a D8 Advance XRD instrument (Brucker, Germany) with a Cu Ka (k = 1.5418 Å) radiation and collected in the range of 2h = 3–60°. The pore size, pore volume and surface area of the samples were measured at 196.0 °C by N2 sorption (ASAP 2020 system, Micrometitics, USA). Atomic absorption spectroscopy (AA-6601F Shimadzu, Japan) was used to study the leaching of Al from zeolites. The rigorous leaching test was conducted by studying the filtrate at the reaction temperature before the completion of reaction [21]. The reaction mixture was separated by simple filtration. The particle size distribution analysis was performed using a JL 9200 laser particle size analyzer (Jinan Winner Particle Instruments Stock Co., Ltd., China). The samples were introduced into the dispersion module with ultrapure water as the solvent and sonicated for 1 min at 70 W and 40 kHz. The apparent contact angle of the zeolites was characterized by JY-82 contact angle measuring device (Chengde Dingsheng Testing Machine Equipment Co., Ltd., China) at room temperature. Prior to the measurement, the powder samples (0.2 g) were pressed into tablet under 30.0 MPa for 20 min. Every sample was tested several times and three results which the difference between each other was less than 0.5° were taken into average. The semi-quantitative analysis of hydrogen ion contents in the zeolite samples was performed by measuring the ion exchange capacity (IEC), namely the number of milli-equivalents of ions (hydrogen ion) in 1.0 g of zeolites. The values of IEC was determined following the method described by Zhu et al. [22]. All samples were measured in duplicate within the error of 3.0%. 2.3. Evaluation of catalytic performance of zeolites The reaction was carried out in a three-necked 250 ml round-bottomed flask, fitted with a water refluxing condenser. The temperature was controlled using a heating jacket which was connected to a thermocouple. The oleic acid, ethanol and zeolites were placed directly into the reactor, and the mixture was stirred at a constant rate by the magnetic agitator in the course of the reaction. When the reaction was completed, the zeolites were separated by filtration, and the excess ethanol was removed by using a rotary evaporator. After that, the zeolites were washed twice with ethanol and dried at 70.0 °C overnight. The recovered zeolites were charged for the next run. The initial acid value of oleic acid (198.73 mg KOH/g) and the acid value of the reacted mixture were determined by titrimetry following the procedure described by Ding et al. [23]. The conversion rates of oleic acid were calculated by Eq. (1).
X¼
S0 Sn 100% S0
ð1Þ
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where X (%) is the conversion rate of the oleic acid, S0 is the initial acid value of the oleic acid, and Sn is the acid value of the reaction mixture at a specific time point. Using the same method, within a short interval of time, each sample was tested twice and averaged the results. The absolute difference between the two independent single test results should not exceed 3.0% of mean value.
3. Results and discussion 3.1. Physical properties of different zeolites Fig. 1 showed the FTIR spectra of ZSM-5 (50) and ZRP-5 (50) samples. The characteristic bands of the silicon–oxygen tetrahedron such as the asymmetric stretching of external vibration bands, asymmetric stretching of internal vibration bands, symmetric stretching, double ring vibration and bending were present in all samples, corresponding to the characteristic peak at 1224, 1098, 799, 545 and 450 cm1, respectively [24]. Therefore, the ZRP-5 zeolite was assigned to the MFI structure. Meanwhile, the peak observed at 877 cm1 was due to the P–O–H stretching, verifying that the phosphorus was introduced into the zeolite frameworks [25]. Moreover, the absorption peak near 3610 cm1 in the spectrum of ZRP-5 (50) was stronger than that of ZSM-5 (50), which suggested that the amount of terminal hydroxyl groups of ZRP-5 (50) was more than ZSM-5 (50) [18], making the ZRP-5 (50) more hydrophilic than ZSM-5 (50), as verified by the contact angle measurement (see below). Fig. 2 illustrated the XRD patterns of Beta (50), ZRP-5 (50) and ZSM-5 (50) samples. As illustrated in Fig. 2, the diffraction peaks in the range of 2h = 6.0–9.0° and 23.0–25.0° indicated that the framework of ZRP-5 corresponded well with the MFI structure [26]. The Beta (50) sample showed the typical XRD patterns of the BEA framework structure. Fig. 3a demonstrated the N2 adsorption–desorption isotherms and the pore size distribution of ZRP-5 zeolites. The ZRP-5 sample exhibited a type IV isotherm with an obvious hysteresis loop, indicating that the presence of mesopores in the ZRP-5 samples. Furthermore, ZRP-5 zeolites showed an abundant mesopores centralized distribution near 4 nm. The isotherms of ZSM-5 and Beta zeolites were shown in Fig. 3b, indicating ZSM-5 zeolites displayed a type I isotherm, which were the predominant characteristic of micropores. Since poor degree of order of Beta zeolites, the adsorption curve had evidently increased at high P/P0 range (P/P0 = 0.9–1). But the micropores were still dominantly in Beta zeolites (shown in Table 1). Thus, the ZRP-5 zeolites were micro-mesoporous structure while the ZSM-5 and Beta zeolites were microporous
Fig. 1. The FTIR spectra of ZRP-5 (50) and ZSM-5 (50) samples.
Fig. 2. The XRD patterns of Beta (50), ZRP-5 (50) and ZSM-5 (50) samples.
structure. The physical and chemical properties of different zeolites were shown in Table 1. ZRP-5 zeolite exhibited the highest values of IEC, probably caused by the phosphorus introduced (ZRP-5 (50) > Beta (50) > ZSM-5 (50), ZRP-5 (25) > Beta (25) > ZSM-5 (25)). The values of IEC of the same zeolites significantly decreased as the Si/Al ratios of zeolites increased (ZSM-5 (25) > ZSM-5 (50), ZRP-5 (25) > ZRP-5 (50), Beta (25) > Beta (50)). It could be inferred that the values of IEC of zeolites depending on the Al content in zeolites. The difference of D50 (the diameter of 50% cumulative distribution of zeolite particle size or average particle sizes) of different types of zeolites were less than 5%, indicating the average particle sizes of different types of zeolites were similar. The values of intrinsic water contact angle of different zeolites which was computed using apparent contact angle coupling Cassie–Baxter equation [27] were shown in Table 1. It was indicated that the zeolite with high Si/Al ratios was hydrophobic and the orderliness of surface hydrophobicity was as follows: Beta > ZSM-5 > ZRP-5. Moreover, these results were further testified by the water adsorption capacity test which was conducted following the literature (see Supporting Information Table S1) [28]. 3.2. The comparison of catalytic performance of different zeolites The effect of pore sizes on the esterification needed to be checked whether the reaction was influenced by pore size on internal mass transfer limitations – the limitations of reactant diffusion inside the pores of the zeolites [29]. In order to investigate the effects of the pore size and surface hydrophobicity on the catalyst performance, the total hydrogen ion contents of different zeolite were kept at the same level, from 0.042 to 0.251 meq/g (oleic acid). The optimized conditions were the reaction temperature of 78.0 °C, molar ratio of oleic acid to ethanol of 1:20, stirring rate of 600 rpm and reaction time of 10.0 h (All the samples reached the equilibrium at 10.0 h). As shown in Fig. 4, the conversion rate increased with increasing the amount of the acid sites before the equilibrium. The optimum loading of Beta (50) zeolites was 0.167 meq/g and the catalysis performance of the zeolites decreased in the following order: Beta (50) > ZSM-5 (50)>ZRP-5 (50) > Beta (25) > ZSM (25) > ZRP-5 (25). Therefore, the effects of reaction conditions and reusability study were conducted in the presence of optimum Beta (50) zeolite for the esterification of oleic acid with ethanol. The pore size on internal mass transfer limitations were estimated by Thiele modulus and effectiveness factor calculation [30]. As shown in Fig. 5, the calculated values of Thiele modulus of micro-mesoporous ZRP-5 (50) and ZRP-5 (25) zeolites were
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Fig. 3. (a) The N2 adsorption–desorption isotherms and the pore size distribution of the zeolites of ZRP-5 zeolite, (b) the N2 adsorption–desorption isotherms of ZSM-5 and Beta zeolites.
Table 1 The physical and chemical properties of different zeolites. Zeolites
ZSM-5 (25)
ZSM-5 (50)
ZRP-5 (25)
ZRP-5 (50)
Beta (25)
Beta (50)
IEC (meq/g) BET specific surface area (m2/g) Total pore volume (cm3/g) Mesopore volume (cm3/g) Micropore volume (cm3/g) Intrinsic Water contact angle (°) D50 (lm)
0.427 325 0.183 0.021 0.163 7.7 31.3
0.236 333 0.182 0.027 0.155 12.1 31.3
0.643 312 0.240 0.148 0.092 3.2 30.8
0.466 336 0.241 0.164 0.077 5.8 30.7
0.553 509 0.196 0.035 0.161 10.2 31.9
0.306 546 0.194 0.041 0.153 14.3 32.2
much less than the other microporous zeolites, indicating that the internal mass transfer limitations of esterification of the long-chain oleic acid with ethanol could be reduced by designing zeolites with large pore size. However, the esterification reactions catalyzed by each samples were controlled under the surface reaction mechanism because the effectiveness factor was near 1.0 and the Thiele
Fig. 4. The comparison of different types of zeolites on the esterification of oleic acid with ethanol at the molar ratio of oleic acid to ethanol of 1:20, reaction temperature of 78.0 °C, stirring rate of 600 rpm, reaction time 10.0 h. (j) Beta (50), (d) ZSM-5 (50), (N) ZRP-5 (50), (.) Beta (25), (J) ZSM-5 (25) and (I) ZRP-5 (25).
modulus was less than 0.4 [31]. The effect of pore sizes on the internal mass transfer limitations could be neglected in this study. 3.3. The optimized reaction conditions of esterification 3.3.1. Effect of stirring rates Experiments were conducted to investigate the effect of external mass transfer limitations during the esterification at the
Fig. 5. The relationship between the Thiele modulus and the effectiveness factor of different zeolites. (j) Beta (50), (d) ZSM-5 (50), (.) Beta (25), (J) ZSM-5 (25), (N) ZRP-5 (50) and (I) ZRP-5 (25).
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stirring rate of 600 rpm. The effect of external mass transfer limitations could be examined basing on the Mears criterion.
CM ¼
r0 qdn 2kc C o0
ð2Þ
where CM is the dimensionless Mears parameter, r0 is the initial reaction rate which equals to the reaction rate at 60 min of oleic acid, q is the framework density of zeolite, d is the mean diameter of sample particles, Co0 is the initial concentration of oleic acid, n is the reaction order (the reaction was second-order and was proofed by Pseudo-homogeneous second order reaction kinetic model in Supporting Information), and kc is the mass transfer coefficient, estimated by the Dwivedi–Upadhyay equation [32]. The calculated value of dimensionless Mears parameter was much smaller than 0.15, indicating that the external mass transfer limitations in the solid–liquid interface were overcome and the boosting effect of agitation on the conversion rate was negligible when the stirring speed reached 600 rpm. 3.3.2. Effect of the molar ratio of oleic acid to ethanol on esterification The esterification reaction between oleic acid and ethanol is a reversible process. To achieve a higher conversion rate, the backward reaction should be prevented by the introduction of excess ethanol into the esterification reaction system. The molar ratio of oleic acid to ethanol varied from 1:10 to 1:30 under otherwise similar conditions (reaction temperature 78.0 °C, stirrer speed 600 rpm and Beta (50) zeolite loading 0.167 meq/g oleic acid). The influence of ratios on the conversion rate of oleic acid was depicted in Fig. 6. The conversion rate of oleic acid increased from 62.5% to 73.6% as the molar ratio of oleic acid to ethanol did from 1:10 to 1:20. This is in agreement with the established principle that the equilibrium can be shifted toward the direction of biodiesel formation when an excess of ethanol is used in the reaction [33]. However, when the molar ratio of oleic acid to ethanol was above 1:20, the increase in the conversion rate of oleic acid was marginal (only about 2.0% as the molar ratio increased from 1:20 to 1:30). Besides, the excess ethanol would cause more difficulties in the separation of catalyst and production of ethyl ester. Hence, the molar ratio of 1:20 (oleic acid to ethanol) was applied as the optimum molar ratio for the esterification reaction.
Fig. 6. Effect of the molar ratio of oleic acid to ethanol on the esterification of oleic acid with ethanol at the stirring rate of 600 rpm, reaction temperature of 78.0 °C, Beta (50) catalyst loading of 0.167 meq/g (oleic acid). (j) 1:10; (J) 1:15; (N) 1:20; (.) 1:25 and (d) 1:30.
Fig. 7. Effect of the reaction temperature on the esterification of oleic acid with ethanol at the molar ratio of oleic acid to ethanol of 1:20, stirring rate of 600 rpm, Beta (50) catalyst loading of 0.167 meq/g (oleic acid). (j) 69.0 °C; (d) 73.0 °C; (N) 78.0 °C and (.) 83.0 °C.
3.3.3. Effect of reaction temperature on esterification Fig. 7 showed the effect of temperature on the conversion rate of oleic acid. The reaction temperature for Beta (50) zeolite increased from 68.0 °C to 83.0 °C at optimum reaction conditions of the molar ratio of oleic acid to ethanol of 1:20, catalyst loading of 0.167 meq/g (oleic acid) and stirring rate of 600 rpm. The conversion rate of oleic acid increased gradually with increasing the temperature, but declined as the temperature approached 83.0 °C. The highest conversion rate of oleic acid (73.6%) was obtained at 78.0 °C (the boiling point of ethanol at atmospheric pressure) within 10.0 h. The possible reason for the decline was that the ethanol vapor filling in the headspace of reactor was not available for the reaction when the temperature exceeded the boiling point of ethanol. Liu et al. [34] also reported that the alcohol vaporized rapidly and large amount of bubbles formed when the reaction temperature exceeded the boiling point, resulting in a decrease in the conversion rate of FFAs. In conclusion, the optimum reaction temperature was 78.0 °C. 3.3.4. The reusability study and leaching test The reusability of the zeolite is one of the most important factors to evaluate whether the catalyst is appropriate for practical applications. The operational stability of the catalyst was investigated by recycling the Beta (50) zeolite five times under the following conditions: molar ratio of oleic acid to ethanol of 1:20, catalyst loading of 0.167 meq/g (oleic acid), temperature of 78 °C, reaction time of 10.0 h and stirring rate of 600 rpm. The conversion rates of oleic acid remained above 70.0% after five runs (see Supporting Information Fig. S1). However, the zeolite was easy to run off during washing and separation processes because fine particles suspended readily in water and the maximal dosage of zeolite was limited by the volume of the reactor. After five runs, the zeolites collected were not enough to complete the next regeneration experiment. In general, the acid property was determined by the Si/Al ratios of the zeolites [35], and the results agreed well with the IEC test. The acid sites were mainly introduced by the Al in the framework and extra-framework from the zeolite. The Al in the framework of zeolites introduced negative charges, which were equilibrated by extra-framework cations. If these cations were hydrogen ions (belonged to H-type zeolites), the zeolites would exhibit an acidic property. Therefore, it was necessary to investigate the leaching of Al from the zeolite. Further study verified that no obvious leaching
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of Al from Beta (50) zeolite was found in the product mixture detected by AAS (the leaching of Al in the mixture was less than 1 ppm). It was strongly evidenced that the zeolites had better reproducibility and stability for the esterification. 3.4. Kinetic model The Eley–Rideal model and Langmuir–Hinshelwood model were verified to represent the biodiesel production with oleic acid and ethanol over zeolites. On the basis of the experiment results above, there were no external mass transfer limitations in the esterification over zeolites under 600 rpm stirring, and the surface reaction step was the rate determining step. All kinetic data were obtained at the optimized conditions (reaction temperature 78.0 °C, stirrer speed 300 rpm, molar ratio of oleic acid to ethanol 1:20, Beta (50) zeolite loading 0.167 meq/g oleic acid, ZSM-5 (50) zeolite loading 0.125 meq/g oleic acid, and ZRP-5 (50) zeolite loading 0.251 meq/g oleic acid). The k, Ko, Kw, Ke, Koe and Keq were the rate constant, adsorption equilibrium constant of oleic acid, adsorption equilibrium constant of water, adsorption equilibrium constant of ethyl oleate, adsorption equilibrium constant of ethanol and reaction equilibrium constant, respectively. The k1 was the forward reaction constant, and k2 was reverse reaction constant. Co, Ce, Coe and Cw were the concentrations of oleic acid, ethanol, ethyl oleate and water, respectively. Co0 and Ce0 were the initial concentrations of oleic acid and ethanol. The objective model was solved using the lsqcurvefit subroutine in the First Optimization (7D-Soft High Technology Inc., China), where the Quasi-Newton method was adopted. If the reaction took place between adsorbed oleic acid and ethanol in the liquid phase, and there existed the competing adsorption phenomena between all the components, the kinetic rate equation corresponding to the Eley–Rideal mechanism could be described as below. (The detailed deducing of Eley–Rideal model was illuminated in Supporting Information.)
r¼
k1 ðK o C o C e K oeKCeqoe C w Þ 1 þ K o C o þ K e C e þ K oe C oe þ K w C w
ð3Þ
Assuming Eq. (3) at an initial state, neglecting the initial concentration of water and ethyl oleate, the initial reaction rate could be obtained.
r0 ¼
kK o C o0 C e0 1 þ K o C o0 þ K e C e0
ð4Þ
The conversion rate at 60 min of oleic acid was calculated by the value of r0. Fig. 8 showed the fitting curve of the experimental data of the conversion rate at 60 min assuming the Eley–Rideal kinetic model. The estimated parameters and the correlation coefficients were listed in Table 2. The correlation coefficients of fitting curve showed good fits to Eley–Rideal kinetic model, indicating that the Eley–Rideal kinetic model was valid. Besides, it should be noted that the order of the Ke/Ko of different zeolites was according to ZRP-5 (50) > ZSM-5 (50) > Beta (50) and the initial conversion rate of different zeolites increased with the Ke/Ko values decreasing. It was indicated that the stronger absorbent ability of oleic acid on the surface of zeolites could aid the esterification. In association with the results mentioned in Cassie–Baxter model and water adsorption capacity test above, on the surface of high hydrophobic Beta (50) zeolites, the adsorption of oleic acid molecules was more favorable than the adsorption of the polar ethanol molecules, resulting in higher coverage of oleic acid molecules on the surface of zeolites and higher conversion rate of esterification. In contrast, due to the pore surface hydrophobicity, the oleic acid molecules were less adsorbed on the surface of ZRP-5 zeolites than other zeolites. Moreover, according to Eq. (4), the existent of the water term
Fig. 8. The fitting curve of the experimental data assuming the Eley–Rideal kinetic model (the change in initial conversion rate of oleic acid over Ce0/Co0). (j) Experimental data of Beta (50) samples, (d) Experimental data of ZSM-5 (50) samples and (N) Experimental data of ZRP-5 (50) samples; Line (1) fitting curve of Beta (50) samples, Line (2) fitting curve of ZSM-5 (50) samples and Line (3) fitting curve of ZRP-5 (50) samples.
Table 2 The estimated values of Eley–Rideal model parameters. Zeolites
Beta (50) ZSM-5 (50) ZRP-5 (50)
Eley–Rideal model k
Ke
Ko
Ke/Ko
R2
8.11e05 9.01e05 6.52e05
2.26e02 6.25e02 7.76e02
0.998 0.996 0.999
2.26e02 6.27e02 7.76e02
0.997 0.998 0.995
in the denominator meant that its existence continuously inhibited the reaction kinetics. Because the polar of water was stronger than ethanol, the adsorption capacity of water molecules was stronger than that of ethanol molecules. The water produced in the esterification desorbed rapidly from the hydrophobic surface of zeolites, increasing the surface coverage of oleic acid. Thus, the ZRP-5 zeolite exhibited the lowest internal mass transfer limitations but the worst catalyst performance, and the zeolites with high Si/Al ratios had better catalyst performance [36]. Compared with Langmuir–Hinshelwood model, the experimental data were highly fitting the Eley–Rideal model that the esterification took place between adsorption oleic acid molecules and ethanol molecules in the liquid [37]. There existed the competing adsorption phenomena between all the components on the surface of zeolites. Thus, the conversion rate of oleic acid could be enhanced by designing zeolites with hydrophobicity. (The comparison between the Langmuir–Hinshelwood model were listed in Supporting Information.) 4. Conclusions Compared with the Langmuir–Hinshelwood model, the Eley– Rideal model could better describe the esterification of oleic acid with ethanol over zeolites. There existed the competing adsorption phenomena between all the components on the surface of zeolites and the reaction took place between adsorbed oleic acid and ethanol in solution. By discussing the correlation between experimental results and various modes including Eley–Rideal model, Thiele modulus and Cassie–Baxter model, it was proved that the conversion rate of oleic acid with ethanol could be enhanced by designing zeolites with hydrophobicity and increasing the amount of secondary mesoporous of zeolites. The high hydrophobic Beta (50) zeolites exhibited better catalytic performance than any other
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zeolites and the maximum conversion rate of oleic acid was 73.6% under the optimum reaction conditions. Although the maximum rate of ethyl esters did not meet the standard to be considered biodiesel, the zeolites showed the excellent stability and reproducibility. The zeolites could be reused for a minimum of five times with negligible decreases in the conversion rate and no obvious leaching of actives components after regeneration. Thus, the final product could be adjusted by the multi-stage reaction. Acknowledgments The authors thank the support from the Natural Science Foundation of Shandong Province (Grant No. ZR2013BL010) and Research Excellence Award of Shandong University of Technology and Zibo Technology Research and Development Program of China (Grant No. 2013GG04110). The authors also wish to express their thanks to Zibo Jinxuan Resources and Environmental Technology Development Co., Ltd. for their sincere help during this work. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2015.06.048. References [1] Kouzu M, Hidaka J. Transesterification of vegetable oil into biodiesel catalyzed by CaO: a review. Fuel 2012;93:1–12. [2] Tubino M, Junior JGR, Bauerfeldt GF. Biodiesel synthesis with alkaline catalysts: a new refractometric monitoring and kinetic study. Fuel 2014;125:163–72. [3] Atadashi IM, Aroua MK, Abdul AAR, Sulaiman NMN. The effects of catalysts in biodiesel production: a review. J Ind Eng Chem 2013;19:14–26. [4] Xie W, Yang D. Silica-bonded N-propyl sulfamic acid used as a heterogeneous catalyst for transesterification of soybean oil with methanol. Bioresour Technol 2011;102:9818–22. [5] Ding JC, He BQ, Tu JG, Li JX. Heat-activated zirconium sulfate as acid heterogeneous catalyst for biodiesel production. Biobased Mater Bio 2012;6:142–7. [6] Ding JC, He BQ, Li JX. Cation ion-exchange resin/polyethersulfone hybrid catalytic membrane for biodiesel production. Biobased Mater Bio 2011;5:85–91. [7] Xie W, Wang H, Li H. Silica-supported tin oxides as heterogeneous acid catalysts for transesterification of soybean oil with methanol. Ind Eng Chem Res 2012;51:225–31. [8] Xie W, Wang T. Biodiesel production from soybean oil transesterification using tin oxide-supported WO3 catalysts. Fuel Process Technol 2013;109:150–5. [9] Lee JS, Saka S. Biodiesel production by heterogeneous catalysts and supercritical technologies. Bioresour Technol 2010;470:7191–200. [10] Chung KH, Chang DR, Park BG. Removal of free fatty acid in waste frying oil by esterification with methanol on zeolite catalysts. Bioresour Technol 2008;99:7438–43. [11] Xie W, Qi C, Wang H, Liu Y. Phenylsulfonic acid functionalized mesoporous SBA-15 silica: a heterogeneous catalyst for removal of free fatty acids in vegetable oil. Fuel Process Technol 2014;119:98–104. [12] Narkhede N, Brahmkhatri V, Patel A. Efficient synthesis of biodiesel from waste cooking oil using solid acid catalyst comprising 12-tungstosilicic acid and SBA15. Fuel 2014;135:253–61. [13] Carmo Jr AC, Souza LKC, Costa CEF, Longo E, Zamian JR, Filho GNR. Production of biodiesel by esterification of palmitic acid over mesoporous aluminosilicate Al-MCM-41. Fuel 2009;88:461–8.
[14] Parsafard N, Peyrovi MH, Rashidzadeh M. N-Heptane isomerization on a new kind of micro/mesoporous catalyst: Pt supported on HZSM-5/HMS. Micropor Mesopor Mater 2014;200:190–8. [15] Jiang YW, Lu J, Sun KA, Ma LL, Ding JC. Esterification of oleic acid with ethanol catalyzed by sulfonated cation exchange resin: experimental and kinetic studies. Energy Convers Manage 2013;76:980–5. [16] Merchant SQ, Almohammad KA, Bassam AAMA, Ali SH. Biofuels and additives: comparative kinetic study of Amberlite IR 120-catalyzed esterification of ethanol with acetic, propanoic and pentanoic acids to produce eco-ethylesters. Fuel 2013;111:140–7. [17] Ðokic´ M, Kesic´ Zˇ, Krstic´ J, Jovanovic´ D, Skala D. Decrease of free fatty acid content in vegetable oil using silica supported ferric sulfate catalyst. Fuel 2012;97:595–602. [18] Han X, Wang L, Li J, Zhan X, Chen J, Yang J. Tuning the hydrophobicity of ZSM-5 zeolites by surface silanization using alkyltrichlorosilane. Appl Surf Sci 2011;257:9525–31. [19] Fauzi AHM, Amin NAS. Optimization of oleic acid esterification catalyzed by ionic liquid for green biodiesel synthesis. Energy Convers Manage 2013;76:818–27. [20] Patel A, Narkhede N. 12-Tungstophosphoric acid anchored to zeolite Hb: synthesis characterization, and biodiesel production by esterification of oleic acid with methanol. Energy Fuels 2012;26:6025–32. [21] Brahmkhatri V, Patel A. Esterification of lauric acid with butanol-1 over H3PW12O40 supported on MCM-41. Fuel 2012;102:72–7. [22] Zhu ML, He BQ, Shi WY, Feng YH, Ding JC, Li JX, et al. Preparation and characterization of PSSA/PVA catalytic membrane for biodiesel production. Fuel 2010;89:2299–304. [23] Ding JC, He BQ, Li JX. Biodiesel production from acidified oils via supercritical methanol. Energies 2011;4:2212–23. [24] Liu GP, Xiangli FJ, Wei W, Liu SN, Jin WQ. Improved performance of PDMS/ ceramic composite pervaporation membranes by ZSM-5 homogeneously dispersed in PDMS via a surface graft/coating approach. Chem Eng J 2011;174:495–503. [25] Chen FQ, Ma LY, Cheng DG, Zhan XL. Synthesis of hierarchical porous zeolite and its performance in n-heptane cracking. Catal Commun 2012;18:110–4. [26] Zhang K, Liu Y, Tian S, Zhao E, Zhang J, Liu C. Preparation of bifunctional NiPb/ ZnO-diatomite-ZSM-5 catalyst and its reactive adsorption desulfurization coupling aromatization performance in FCC gasoline upgrading process. Fuel 2013;103:201–7. [27] Gambaryan-Roisman T. Liquids on porous layers: wetting, imbibition and transport processes. Curr Opin Colloid Interface Sci 2014;29:320–35. [28] Nur H, Kee GL, Hamdan H, Mahlia TMI, Efendi J, Metselaar HSC. Organosulfonic acid functionalized zeolite ZSM-5 as temperature tolerant proton conducting material. Int J Hydrogen Energy 2012;37:12513–21. [29] Abidin SZ, Haigh KF, Saha B. Esterification of free fatty acids in used cooking oil using ion-exchange resins as catalysts: an efficient pretreatment method for biodiesel feedstock. Ind Eng Chem Res 2012;51:14653–64. [30] Altıokka MR, Ödes E. Reaction kinetics of the catalytic esterification of acrylic acid with propylene glycol. Appl Catal A – Gen 2009;362:115–20. [31] Konno H, Okamura T, Kawahara T, Nakasaka Y, Tago T. Kinetics of n-hexane cracking over ZSM-5 zeolites – Effect of crystal size on effectiveness factor and catalyst lifetime. Chem Eng J 2012;207–8:490–6. [32] Sert E, Buluklu AD, Karakus S, Atalay FS. Kinetic study of catalytic esterification of acrylic acid with butanol catalyzed by different ion exchange resins. Chem Eng Process 2013;73:23–8. [33] Xie W, Zhao L. Heterogeneous CaO–MoO3–SBA-15 catalysts for biodiesel production from soybean oil. Energy Convers Manage 2014;79:34–42. [34] Liu Y, Wang L, Yan Y. Biodiesel synthesis combining pre-esterification with alkali catalyzed process from rapeseed oil deodorizer distillate. Fuel Process Technol 2009;90:857–62. [35] Tian FP, Wu YH, Shen QC, Li X, Chen YY, Meng CG. Effect of Si/Al ratio on mesopore formation for zeolite beta via NaOH treatment and the catalytic performance in a-pinene isomerization and benzoylation of naphthalene. Micropor Mesopor Mater 2013;173:129–38. [36] Khalid M, Joly G, Renaud A, Magnoux P. Removal of phenol from water by adsorption using zeolites. Ind Eng Chem Res 2004;43:5275–80. [37] Ilgen O. Investigation of reaction parameters, kinetics and mechanism of oleic acid esterification with methanol by using Amberlyst 46 as a catalyst. Fuel Process Technol 2014;124:134–9.
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Fuel journal homepage: www.elsevier.com/locate/fuel
A comparative study on the catalytic performance of different types of zeolites for biodiesel production Kaian Sun a, Jie Lu b, Lingling Ma a, Ying Han a, Zhongjun Fu a,c,⇑, Jincheng Ding a,⇑ a
College of Chemical Engineering, Shandong University of Technology, 12 Zhangzhou Road, Zibo 255049, China Department of Resources and Environmental Engineering, Shandong University of Technology, 12 Zhangzhou Road, Zibo 255049, China c Shandong Engineering Research Center for Dyeing & Finishing of Textiles, 12 Zhangzhou Road, Zibo 255049, China b
h i g h l i g h t s Different kinetic models were used to describe the esterification over zeolites. Al was verified to detect the leaching of active component of zeolite. The Cassie–Baxter model was used to study the effect of hydrophobic property. The Thiele modulus was used to investigate the effect of zeolite pore size. Microporous and micro-mesoporous zeolites were compared in biodiesel production.
a r t i c l e
i n f o
Article history: Received 3 December 2014 Received in revised form 12 June 2015 Accepted 13 June 2015 Available online 17 June 2015 Keywords: Biodiesel Esterification Zeolites Thiele modulus Comparative kinetics Cassie–Baxter model
a b s t r a c t Microporous zeolites (BEA type Beta zeolite and MFI type ZSM-5 zeolite) and micro-mesoporous zeolites (MFI type ZRP-5 zeolite) with various Si/A1 ratios were employed in the esterification of oleic acid with ethanol. The effect of pore size on the internal mass transfer limitation was investigated by Thiele modulus calculation. The results showed that the zeolites with high Si/Al ratios had better catalytic performance, and of these three zeolites at the same Si/A1 ratios, the ZRP-5 zeolite exhibited the lowest internal mass transfer limitations but the worst catalytic performance. Through the comparison of the Eley–Rideal model and the Langmuir–Hinshelwood model, it was indicated that on the surface of hydrophilic ZRP-5 zeolites, the adsorption of the polar ethanol molecules were more favorable than the adsorption of oleic acid molecules, resulting in less coverage of oleic acid molecules on the surface of zeolites and lower conversion rate of esterification. Moreover, the Cassie–Baxter model and the water adsorption capacity test were used to further validate the assumption of kinetic model. The highest conversion rate of 73.6% was achieved when the reaction was catalyzed by high hydrophobic Beta (50) zeolites under optimized conditions of the molar ratio of oleic acid to ethanol of 1:20, catalyst loading of 0.167 meq/g (oleic acid), temperature of 78.0 °C, reaction time of 10.0 h and stirring speed of 600 rpm. The conversion rate of oleic acid remained above 70.0% after five runs and there was no apparent loss of the active component (Al) from the zeolite. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Biodiesel, a biodegradable fuel to replace traditional petroleum-derived diesels, has gaining much attention because of its renewability, non-toxic and low greenhouse gases emissions [1]. Generally, biodiesel is produced by the transesterification ⇑ Corresponding authors at: Shandong Engineering Research Center for Dyeing & Finishing of Textiles, and College of Chemical Engineering, Shandong University of Technology, 12 Zhangzhou Road, Zibo 255049, China. Tel.: +86 533 2781630; fax: +86 533 2781664 (Z. Fu). Tel.: +86 533 2783397; fax: +86 533 2781664 (J. Ding). E-mail addresses: [email protected] (Z. Fu), [email protected] (J. Ding). http://dx.doi.org/10.1016/j.fuel.2015.06.048 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
using alkaline catalysts [2]. However, the alkaline-catalyzed process suffers from serious limitations that the presences of free fatty acids (FFAs) and water in the feedstock may lead to the formation of soaps, increasing the product viscosity and separation difficulty of downstream products [3]. Compared with alkaline catalysts, the acid catalysts are known to react with the low-quality resources containing high FFAs and/or water under normal conditions. Moreover, the acid catalysts are effective for both esterification and transesterification reactions [4]. Some types of heterogeneous acid catalysts such as the zirconium sulfate [5], ion exchange resins [6], silica supported tin oxides [7] and tin oxide supported WO3 [8]
K. Sun et al. / Fuel 158 (2015) 848–854
have been used for biodiesel production recently. In contrast to the homogeneous catalysts, the heterogeneous catalysts have the advantages of non-corrosion, environmental friendliness, and easy removal from the products, showing the promising potentials for industrial biodiesel productions [9]. As one kind of the solid acids, zeolites are crystalline and porous materials with high specific surface area. Chung et al. [10] compared different microporous zeolites for reduction of free fatty acids in waste cooking oil in esterification reaction (ZSM-5, faujasite, beta and silicalite zeolites). Due to the limitation of the narrow channels, large molecules (like oleic acid molecule, length 2.4 nm and height 0.3 nm) cannot access the internal active sites of microporous zeolites. Moreover mesoporous zeolites can be potentially used in biodiesel production and attract many attentions. However, mesoporous zeolites show weaker catalytic activity, which seriously limit their extensive uses. In order to improve the catalytic activities of mesoporous zeolites, various approaches were used to modify mesoporous zeolites including phenylsulfonic acid functionalized SBA-15 zeolite [11], 12-tungstosilicic acid functionalized SBA-15 zeolite [12] and Al supported MCM-41 zeolite [13]. By contrast, micro-mesoporous zeolites have the advantages over either microporous zeolites or mesoporous zeolites [14]. And there were few reports about comparative studies on the catalytic performance of microporous and micro-mesoporous zeolites with various Si/Al ratios for biodiesel production. Besides, to meet the needs of theoretical researches and practical applications, multiple models were developed to describe the process of reaction. Jiang et al. [15] reported a pseudo-homogeneous model to describe the reaction progress of biodiesel production. Merchant et al. [16] compared the Langmuir–Hinshelwood model and the Eley–Rideal model on the biodiesel syntheses catalyzed by cation exchange resins. Konno et al. [17] used the Thiele modulus and the effectiveness factor to study the influence of mass transfer limitation. Han et al. [18] studied the wetting state on the surface of ZSM-5 zeolites using Cassie–Baxter model. However, to investigate the influence of pore size and hydrophobicity on catalysis performance of zeolites, there were few reports discussing the correlation between mass transfer model, wetting state model and kinetic model. In this study, oleic acid and ethanol were chosen as the model compounds for biodiesel production over zeolites, because both of them are renewable raw materials [19]. The zeolites included microporous zeolites (BEA type Beta zeolites and MFI type ZSM-5 zeolites) and micro-mesoporous zeolites (MFI type ZRP-5 zeolite) with various Si/Al ratio. Several models including the Cassie– Baxter model, the Thiele modulus, the Langmuir–Hinshelwood model and the Eley–Rideal model were used to discuss the correlation between physical prosperities of zeolite (the pore size and the pore surface hydrophilicity) and the performance of catalyst based on the experimental results. The influencing factors of esterification including reaction temperature, Si/Al ratios of zeolites, zeolite loading, the reusability of zeolites and molar ratio of oleic acid to ethanol were analyzed.
2. Experimental section 2.1. Materials The zeolites of ZRP-5 (Si/Al = 25), ZRP-5 (Si/Al = 50), ZSM-5 (Si/Al = 25) and ZSM-5 (Si/Al = 50) were kindly supplied by Sinopec Catalyst Co., Ltd., Zibo, China. The Beta (Si/Al = 25) zeolite and Beta (Si/Al = 50) zeolite were purchased from the Catalyst Plant of Nankai University, Tianjin, China. The oleic acid was obtained from Shuangshuang Chemical Reagent Co., Ltd., Yantai, China. Ethanol and NH4Cl were purchased from Tieta Chemical Reagent Co., Ltd., Laiyang, China. All the reagents were analytically pure.
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All H-type zeolites were prepared from Na-type zeolites by following the ion exchange procedure reported by Patel et al. [20]. The Na-type zeolite was added to a 2 M NH4Cl solution at the mass ratio of 1.0:1.0, and the mixture was stirred at 80.0 °C for 2.0 h. Then, the samples were washed twice with the ultrapure water (resistance 18.2 MX, deionized and distilled from the Ulupure UPT-2-401 pure water system), subsequently filtered and dried at 100.0 °C for 12.0 h, and finally calcined at 550.0 °C for 6.0 h. The prepared zeolites were denoted by their Si/Al ratio in the parenthesis after each zeolite name. 2.2. Characterization Prior to testing, all samples were calcined at 500.0 °C for 6.0 h to remove the templates and moisture. The infrared spectra were recorded by using a Nicolet 5700 FTIR (Thermo Electron, USA) in the scanning range of 4000–400 cm1 with the KBr pellet method. XRD spectra were obtained by using a D8 Advance XRD instrument (Brucker, Germany) with a Cu Ka (k = 1.5418 Å) radiation and collected in the range of 2h = 3–60°. The pore size, pore volume and surface area of the samples were measured at 196.0 °C by N2 sorption (ASAP 2020 system, Micrometitics, USA). Atomic absorption spectroscopy (AA-6601F Shimadzu, Japan) was used to study the leaching of Al from zeolites. The rigorous leaching test was conducted by studying the filtrate at the reaction temperature before the completion of reaction [21]. The reaction mixture was separated by simple filtration. The particle size distribution analysis was performed using a JL 9200 laser particle size analyzer (Jinan Winner Particle Instruments Stock Co., Ltd., China). The samples were introduced into the dispersion module with ultrapure water as the solvent and sonicated for 1 min at 70 W and 40 kHz. The apparent contact angle of the zeolites was characterized by JY-82 contact angle measuring device (Chengde Dingsheng Testing Machine Equipment Co., Ltd., China) at room temperature. Prior to the measurement, the powder samples (0.2 g) were pressed into tablet under 30.0 MPa for 20 min. Every sample was tested several times and three results which the difference between each other was less than 0.5° were taken into average. The semi-quantitative analysis of hydrogen ion contents in the zeolite samples was performed by measuring the ion exchange capacity (IEC), namely the number of milli-equivalents of ions (hydrogen ion) in 1.0 g of zeolites. The values of IEC was determined following the method described by Zhu et al. [22]. All samples were measured in duplicate within the error of 3.0%. 2.3. Evaluation of catalytic performance of zeolites The reaction was carried out in a three-necked 250 ml round-bottomed flask, fitted with a water refluxing condenser. The temperature was controlled using a heating jacket which was connected to a thermocouple. The oleic acid, ethanol and zeolites were placed directly into the reactor, and the mixture was stirred at a constant rate by the magnetic agitator in the course of the reaction. When the reaction was completed, the zeolites were separated by filtration, and the excess ethanol was removed by using a rotary evaporator. After that, the zeolites were washed twice with ethanol and dried at 70.0 °C overnight. The recovered zeolites were charged for the next run. The initial acid value of oleic acid (198.73 mg KOH/g) and the acid value of the reacted mixture were determined by titrimetry following the procedure described by Ding et al. [23]. The conversion rates of oleic acid were calculated by Eq. (1).
X¼
S0 Sn 100% S0
ð1Þ
850
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where X (%) is the conversion rate of the oleic acid, S0 is the initial acid value of the oleic acid, and Sn is the acid value of the reaction mixture at a specific time point. Using the same method, within a short interval of time, each sample was tested twice and averaged the results. The absolute difference between the two independent single test results should not exceed 3.0% of mean value.
3. Results and discussion 3.1. Physical properties of different zeolites Fig. 1 showed the FTIR spectra of ZSM-5 (50) and ZRP-5 (50) samples. The characteristic bands of the silicon–oxygen tetrahedron such as the asymmetric stretching of external vibration bands, asymmetric stretching of internal vibration bands, symmetric stretching, double ring vibration and bending were present in all samples, corresponding to the characteristic peak at 1224, 1098, 799, 545 and 450 cm1, respectively [24]. Therefore, the ZRP-5 zeolite was assigned to the MFI structure. Meanwhile, the peak observed at 877 cm1 was due to the P–O–H stretching, verifying that the phosphorus was introduced into the zeolite frameworks [25]. Moreover, the absorption peak near 3610 cm1 in the spectrum of ZRP-5 (50) was stronger than that of ZSM-5 (50), which suggested that the amount of terminal hydroxyl groups of ZRP-5 (50) was more than ZSM-5 (50) [18], making the ZRP-5 (50) more hydrophilic than ZSM-5 (50), as verified by the contact angle measurement (see below). Fig. 2 illustrated the XRD patterns of Beta (50), ZRP-5 (50) and ZSM-5 (50) samples. As illustrated in Fig. 2, the diffraction peaks in the range of 2h = 6.0–9.0° and 23.0–25.0° indicated that the framework of ZRP-5 corresponded well with the MFI structure [26]. The Beta (50) sample showed the typical XRD patterns of the BEA framework structure. Fig. 3a demonstrated the N2 adsorption–desorption isotherms and the pore size distribution of ZRP-5 zeolites. The ZRP-5 sample exhibited a type IV isotherm with an obvious hysteresis loop, indicating that the presence of mesopores in the ZRP-5 samples. Furthermore, ZRP-5 zeolites showed an abundant mesopores centralized distribution near 4 nm. The isotherms of ZSM-5 and Beta zeolites were shown in Fig. 3b, indicating ZSM-5 zeolites displayed a type I isotherm, which were the predominant characteristic of micropores. Since poor degree of order of Beta zeolites, the adsorption curve had evidently increased at high P/P0 range (P/P0 = 0.9–1). But the micropores were still dominantly in Beta zeolites (shown in Table 1). Thus, the ZRP-5 zeolites were micro-mesoporous structure while the ZSM-5 and Beta zeolites were microporous
Fig. 1. The FTIR spectra of ZRP-5 (50) and ZSM-5 (50) samples.
Fig. 2. The XRD patterns of Beta (50), ZRP-5 (50) and ZSM-5 (50) samples.
structure. The physical and chemical properties of different zeolites were shown in Table 1. ZRP-5 zeolite exhibited the highest values of IEC, probably caused by the phosphorus introduced (ZRP-5 (50) > Beta (50) > ZSM-5 (50), ZRP-5 (25) > Beta (25) > ZSM-5 (25)). The values of IEC of the same zeolites significantly decreased as the Si/Al ratios of zeolites increased (ZSM-5 (25) > ZSM-5 (50), ZRP-5 (25) > ZRP-5 (50), Beta (25) > Beta (50)). It could be inferred that the values of IEC of zeolites depending on the Al content in zeolites. The difference of D50 (the diameter of 50% cumulative distribution of zeolite particle size or average particle sizes) of different types of zeolites were less than 5%, indicating the average particle sizes of different types of zeolites were similar. The values of intrinsic water contact angle of different zeolites which was computed using apparent contact angle coupling Cassie–Baxter equation [27] were shown in Table 1. It was indicated that the zeolite with high Si/Al ratios was hydrophobic and the orderliness of surface hydrophobicity was as follows: Beta > ZSM-5 > ZRP-5. Moreover, these results were further testified by the water adsorption capacity test which was conducted following the literature (see Supporting Information Table S1) [28]. 3.2. The comparison of catalytic performance of different zeolites The effect of pore sizes on the esterification needed to be checked whether the reaction was influenced by pore size on internal mass transfer limitations – the limitations of reactant diffusion inside the pores of the zeolites [29]. In order to investigate the effects of the pore size and surface hydrophobicity on the catalyst performance, the total hydrogen ion contents of different zeolite were kept at the same level, from 0.042 to 0.251 meq/g (oleic acid). The optimized conditions were the reaction temperature of 78.0 °C, molar ratio of oleic acid to ethanol of 1:20, stirring rate of 600 rpm and reaction time of 10.0 h (All the samples reached the equilibrium at 10.0 h). As shown in Fig. 4, the conversion rate increased with increasing the amount of the acid sites before the equilibrium. The optimum loading of Beta (50) zeolites was 0.167 meq/g and the catalysis performance of the zeolites decreased in the following order: Beta (50) > ZSM-5 (50)>ZRP-5 (50) > Beta (25) > ZSM (25) > ZRP-5 (25). Therefore, the effects of reaction conditions and reusability study were conducted in the presence of optimum Beta (50) zeolite for the esterification of oleic acid with ethanol. The pore size on internal mass transfer limitations were estimated by Thiele modulus and effectiveness factor calculation [30]. As shown in Fig. 5, the calculated values of Thiele modulus of micro-mesoporous ZRP-5 (50) and ZRP-5 (25) zeolites were
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Fig. 3. (a) The N2 adsorption–desorption isotherms and the pore size distribution of the zeolites of ZRP-5 zeolite, (b) the N2 adsorption–desorption isotherms of ZSM-5 and Beta zeolites.
Table 1 The physical and chemical properties of different zeolites. Zeolites
ZSM-5 (25)
ZSM-5 (50)
ZRP-5 (25)
ZRP-5 (50)
Beta (25)
Beta (50)
IEC (meq/g) BET specific surface area (m2/g) Total pore volume (cm3/g) Mesopore volume (cm3/g) Micropore volume (cm3/g) Intrinsic Water contact angle (°) D50 (lm)
0.427 325 0.183 0.021 0.163 7.7 31.3
0.236 333 0.182 0.027 0.155 12.1 31.3
0.643 312 0.240 0.148 0.092 3.2 30.8
0.466 336 0.241 0.164 0.077 5.8 30.7
0.553 509 0.196 0.035 0.161 10.2 31.9
0.306 546 0.194 0.041 0.153 14.3 32.2
much less than the other microporous zeolites, indicating that the internal mass transfer limitations of esterification of the long-chain oleic acid with ethanol could be reduced by designing zeolites with large pore size. However, the esterification reactions catalyzed by each samples were controlled under the surface reaction mechanism because the effectiveness factor was near 1.0 and the Thiele
Fig. 4. The comparison of different types of zeolites on the esterification of oleic acid with ethanol at the molar ratio of oleic acid to ethanol of 1:20, reaction temperature of 78.0 °C, stirring rate of 600 rpm, reaction time 10.0 h. (j) Beta (50), (d) ZSM-5 (50), (N) ZRP-5 (50), (.) Beta (25), (J) ZSM-5 (25) and (I) ZRP-5 (25).
modulus was less than 0.4 [31]. The effect of pore sizes on the internal mass transfer limitations could be neglected in this study. 3.3. The optimized reaction conditions of esterification 3.3.1. Effect of stirring rates Experiments were conducted to investigate the effect of external mass transfer limitations during the esterification at the
Fig. 5. The relationship between the Thiele modulus and the effectiveness factor of different zeolites. (j) Beta (50), (d) ZSM-5 (50), (.) Beta (25), (J) ZSM-5 (25), (N) ZRP-5 (50) and (I) ZRP-5 (25).
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stirring rate of 600 rpm. The effect of external mass transfer limitations could be examined basing on the Mears criterion.
CM ¼
r0 qdn 2kc C o0
ð2Þ
where CM is the dimensionless Mears parameter, r0 is the initial reaction rate which equals to the reaction rate at 60 min of oleic acid, q is the framework density of zeolite, d is the mean diameter of sample particles, Co0 is the initial concentration of oleic acid, n is the reaction order (the reaction was second-order and was proofed by Pseudo-homogeneous second order reaction kinetic model in Supporting Information), and kc is the mass transfer coefficient, estimated by the Dwivedi–Upadhyay equation [32]. The calculated value of dimensionless Mears parameter was much smaller than 0.15, indicating that the external mass transfer limitations in the solid–liquid interface were overcome and the boosting effect of agitation on the conversion rate was negligible when the stirring speed reached 600 rpm. 3.3.2. Effect of the molar ratio of oleic acid to ethanol on esterification The esterification reaction between oleic acid and ethanol is a reversible process. To achieve a higher conversion rate, the backward reaction should be prevented by the introduction of excess ethanol into the esterification reaction system. The molar ratio of oleic acid to ethanol varied from 1:10 to 1:30 under otherwise similar conditions (reaction temperature 78.0 °C, stirrer speed 600 rpm and Beta (50) zeolite loading 0.167 meq/g oleic acid). The influence of ratios on the conversion rate of oleic acid was depicted in Fig. 6. The conversion rate of oleic acid increased from 62.5% to 73.6% as the molar ratio of oleic acid to ethanol did from 1:10 to 1:20. This is in agreement with the established principle that the equilibrium can be shifted toward the direction of biodiesel formation when an excess of ethanol is used in the reaction [33]. However, when the molar ratio of oleic acid to ethanol was above 1:20, the increase in the conversion rate of oleic acid was marginal (only about 2.0% as the molar ratio increased from 1:20 to 1:30). Besides, the excess ethanol would cause more difficulties in the separation of catalyst and production of ethyl ester. Hence, the molar ratio of 1:20 (oleic acid to ethanol) was applied as the optimum molar ratio for the esterification reaction.
Fig. 6. Effect of the molar ratio of oleic acid to ethanol on the esterification of oleic acid with ethanol at the stirring rate of 600 rpm, reaction temperature of 78.0 °C, Beta (50) catalyst loading of 0.167 meq/g (oleic acid). (j) 1:10; (J) 1:15; (N) 1:20; (.) 1:25 and (d) 1:30.
Fig. 7. Effect of the reaction temperature on the esterification of oleic acid with ethanol at the molar ratio of oleic acid to ethanol of 1:20, stirring rate of 600 rpm, Beta (50) catalyst loading of 0.167 meq/g (oleic acid). (j) 69.0 °C; (d) 73.0 °C; (N) 78.0 °C and (.) 83.0 °C.
3.3.3. Effect of reaction temperature on esterification Fig. 7 showed the effect of temperature on the conversion rate of oleic acid. The reaction temperature for Beta (50) zeolite increased from 68.0 °C to 83.0 °C at optimum reaction conditions of the molar ratio of oleic acid to ethanol of 1:20, catalyst loading of 0.167 meq/g (oleic acid) and stirring rate of 600 rpm. The conversion rate of oleic acid increased gradually with increasing the temperature, but declined as the temperature approached 83.0 °C. The highest conversion rate of oleic acid (73.6%) was obtained at 78.0 °C (the boiling point of ethanol at atmospheric pressure) within 10.0 h. The possible reason for the decline was that the ethanol vapor filling in the headspace of reactor was not available for the reaction when the temperature exceeded the boiling point of ethanol. Liu et al. [34] also reported that the alcohol vaporized rapidly and large amount of bubbles formed when the reaction temperature exceeded the boiling point, resulting in a decrease in the conversion rate of FFAs. In conclusion, the optimum reaction temperature was 78.0 °C. 3.3.4. The reusability study and leaching test The reusability of the zeolite is one of the most important factors to evaluate whether the catalyst is appropriate for practical applications. The operational stability of the catalyst was investigated by recycling the Beta (50) zeolite five times under the following conditions: molar ratio of oleic acid to ethanol of 1:20, catalyst loading of 0.167 meq/g (oleic acid), temperature of 78 °C, reaction time of 10.0 h and stirring rate of 600 rpm. The conversion rates of oleic acid remained above 70.0% after five runs (see Supporting Information Fig. S1). However, the zeolite was easy to run off during washing and separation processes because fine particles suspended readily in water and the maximal dosage of zeolite was limited by the volume of the reactor. After five runs, the zeolites collected were not enough to complete the next regeneration experiment. In general, the acid property was determined by the Si/Al ratios of the zeolites [35], and the results agreed well with the IEC test. The acid sites were mainly introduced by the Al in the framework and extra-framework from the zeolite. The Al in the framework of zeolites introduced negative charges, which were equilibrated by extra-framework cations. If these cations were hydrogen ions (belonged to H-type zeolites), the zeolites would exhibit an acidic property. Therefore, it was necessary to investigate the leaching of Al from the zeolite. Further study verified that no obvious leaching
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of Al from Beta (50) zeolite was found in the product mixture detected by AAS (the leaching of Al in the mixture was less than 1 ppm). It was strongly evidenced that the zeolites had better reproducibility and stability for the esterification. 3.4. Kinetic model The Eley–Rideal model and Langmuir–Hinshelwood model were verified to represent the biodiesel production with oleic acid and ethanol over zeolites. On the basis of the experiment results above, there were no external mass transfer limitations in the esterification over zeolites under 600 rpm stirring, and the surface reaction step was the rate determining step. All kinetic data were obtained at the optimized conditions (reaction temperature 78.0 °C, stirrer speed 300 rpm, molar ratio of oleic acid to ethanol 1:20, Beta (50) zeolite loading 0.167 meq/g oleic acid, ZSM-5 (50) zeolite loading 0.125 meq/g oleic acid, and ZRP-5 (50) zeolite loading 0.251 meq/g oleic acid). The k, Ko, Kw, Ke, Koe and Keq were the rate constant, adsorption equilibrium constant of oleic acid, adsorption equilibrium constant of water, adsorption equilibrium constant of ethyl oleate, adsorption equilibrium constant of ethanol and reaction equilibrium constant, respectively. The k1 was the forward reaction constant, and k2 was reverse reaction constant. Co, Ce, Coe and Cw were the concentrations of oleic acid, ethanol, ethyl oleate and water, respectively. Co0 and Ce0 were the initial concentrations of oleic acid and ethanol. The objective model was solved using the lsqcurvefit subroutine in the First Optimization (7D-Soft High Technology Inc., China), where the Quasi-Newton method was adopted. If the reaction took place between adsorbed oleic acid and ethanol in the liquid phase, and there existed the competing adsorption phenomena between all the components, the kinetic rate equation corresponding to the Eley–Rideal mechanism could be described as below. (The detailed deducing of Eley–Rideal model was illuminated in Supporting Information.)
r¼
k1 ðK o C o C e K oeKCeqoe C w Þ 1 þ K o C o þ K e C e þ K oe C oe þ K w C w
ð3Þ
Assuming Eq. (3) at an initial state, neglecting the initial concentration of water and ethyl oleate, the initial reaction rate could be obtained.
r0 ¼
kK o C o0 C e0 1 þ K o C o0 þ K e C e0
ð4Þ
The conversion rate at 60 min of oleic acid was calculated by the value of r0. Fig. 8 showed the fitting curve of the experimental data of the conversion rate at 60 min assuming the Eley–Rideal kinetic model. The estimated parameters and the correlation coefficients were listed in Table 2. The correlation coefficients of fitting curve showed good fits to Eley–Rideal kinetic model, indicating that the Eley–Rideal kinetic model was valid. Besides, it should be noted that the order of the Ke/Ko of different zeolites was according to ZRP-5 (50) > ZSM-5 (50) > Beta (50) and the initial conversion rate of different zeolites increased with the Ke/Ko values decreasing. It was indicated that the stronger absorbent ability of oleic acid on the surface of zeolites could aid the esterification. In association with the results mentioned in Cassie–Baxter model and water adsorption capacity test above, on the surface of high hydrophobic Beta (50) zeolites, the adsorption of oleic acid molecules was more favorable than the adsorption of the polar ethanol molecules, resulting in higher coverage of oleic acid molecules on the surface of zeolites and higher conversion rate of esterification. In contrast, due to the pore surface hydrophobicity, the oleic acid molecules were less adsorbed on the surface of ZRP-5 zeolites than other zeolites. Moreover, according to Eq. (4), the existent of the water term
Fig. 8. The fitting curve of the experimental data assuming the Eley–Rideal kinetic model (the change in initial conversion rate of oleic acid over Ce0/Co0). (j) Experimental data of Beta (50) samples, (d) Experimental data of ZSM-5 (50) samples and (N) Experimental data of ZRP-5 (50) samples; Line (1) fitting curve of Beta (50) samples, Line (2) fitting curve of ZSM-5 (50) samples and Line (3) fitting curve of ZRP-5 (50) samples.
Table 2 The estimated values of Eley–Rideal model parameters. Zeolites
Beta (50) ZSM-5 (50) ZRP-5 (50)
Eley–Rideal model k
Ke
Ko
Ke/Ko
R2
8.11e05 9.01e05 6.52e05
2.26e02 6.25e02 7.76e02
0.998 0.996 0.999
2.26e02 6.27e02 7.76e02
0.997 0.998 0.995
in the denominator meant that its existence continuously inhibited the reaction kinetics. Because the polar of water was stronger than ethanol, the adsorption capacity of water molecules was stronger than that of ethanol molecules. The water produced in the esterification desorbed rapidly from the hydrophobic surface of zeolites, increasing the surface coverage of oleic acid. Thus, the ZRP-5 zeolite exhibited the lowest internal mass transfer limitations but the worst catalyst performance, and the zeolites with high Si/Al ratios had better catalyst performance [36]. Compared with Langmuir–Hinshelwood model, the experimental data were highly fitting the Eley–Rideal model that the esterification took place between adsorption oleic acid molecules and ethanol molecules in the liquid [37]. There existed the competing adsorption phenomena between all the components on the surface of zeolites. Thus, the conversion rate of oleic acid could be enhanced by designing zeolites with hydrophobicity. (The comparison between the Langmuir–Hinshelwood model were listed in Supporting Information.) 4. Conclusions Compared with the Langmuir–Hinshelwood model, the Eley– Rideal model could better describe the esterification of oleic acid with ethanol over zeolites. There existed the competing adsorption phenomena between all the components on the surface of zeolites and the reaction took place between adsorbed oleic acid and ethanol in solution. By discussing the correlation between experimental results and various modes including Eley–Rideal model, Thiele modulus and Cassie–Baxter model, it was proved that the conversion rate of oleic acid with ethanol could be enhanced by designing zeolites with hydrophobicity and increasing the amount of secondary mesoporous of zeolites. The high hydrophobic Beta (50) zeolites exhibited better catalytic performance than any other
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zeolites and the maximum conversion rate of oleic acid was 73.6% under the optimum reaction conditions. Although the maximum rate of ethyl esters did not meet the standard to be considered biodiesel, the zeolites showed the excellent stability and reproducibility. The zeolites could be reused for a minimum of five times with negligible decreases in the conversion rate and no obvious leaching of actives components after regeneration. Thus, the final product could be adjusted by the multi-stage reaction. Acknowledgments The authors thank the support from the Natural Science Foundation of Shandong Province (Grant No. ZR2013BL010) and Research Excellence Award of Shandong University of Technology and Zibo Technology Research and Development Program of China (Grant No. 2013GG04110). The authors also wish to express their thanks to Zibo Jinxuan Resources and Environmental Technology Development Co., Ltd. for their sincere help during this work. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2015.06.048. References [1] Kouzu M, Hidaka J. Transesterification of vegetable oil into biodiesel catalyzed by CaO: a review. Fuel 2012;93:1–12. [2] Tubino M, Junior JGR, Bauerfeldt GF. Biodiesel synthesis with alkaline catalysts: a new refractometric monitoring and kinetic study. Fuel 2014;125:163–72. [3] Atadashi IM, Aroua MK, Abdul AAR, Sulaiman NMN. The effects of catalysts in biodiesel production: a review. J Ind Eng Chem 2013;19:14–26. [4] Xie W, Yang D. Silica-bonded N-propyl sulfamic acid used as a heterogeneous catalyst for transesterification of soybean oil with methanol. Bioresour Technol 2011;102:9818–22. [5] Ding JC, He BQ, Tu JG, Li JX. Heat-activated zirconium sulfate as acid heterogeneous catalyst for biodiesel production. Biobased Mater Bio 2012;6:142–7. [6] Ding JC, He BQ, Li JX. Cation ion-exchange resin/polyethersulfone hybrid catalytic membrane for biodiesel production. Biobased Mater Bio 2011;5:85–91. [7] Xie W, Wang H, Li H. Silica-supported tin oxides as heterogeneous acid catalysts for transesterification of soybean oil with methanol. Ind Eng Chem Res 2012;51:225–31. [8] Xie W, Wang T. Biodiesel production from soybean oil transesterification using tin oxide-supported WO3 catalysts. Fuel Process Technol 2013;109:150–5. [9] Lee JS, Saka S. Biodiesel production by heterogeneous catalysts and supercritical technologies. Bioresour Technol 2010;470:7191–200. [10] Chung KH, Chang DR, Park BG. Removal of free fatty acid in waste frying oil by esterification with methanol on zeolite catalysts. Bioresour Technol 2008;99:7438–43. [11] Xie W, Qi C, Wang H, Liu Y. Phenylsulfonic acid functionalized mesoporous SBA-15 silica: a heterogeneous catalyst for removal of free fatty acids in vegetable oil. Fuel Process Technol 2014;119:98–104. [12] Narkhede N, Brahmkhatri V, Patel A. Efficient synthesis of biodiesel from waste cooking oil using solid acid catalyst comprising 12-tungstosilicic acid and SBA15. Fuel 2014;135:253–61. [13] Carmo Jr AC, Souza LKC, Costa CEF, Longo E, Zamian JR, Filho GNR. Production of biodiesel by esterification of palmitic acid over mesoporous aluminosilicate Al-MCM-41. Fuel 2009;88:461–8.
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