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Efficient sulfated high silica ZSM-5 nanocatalyst for esterification of oleic acid with methanol
Journal Pre-proof Efficient sulfated high silica ZSM-5 nanocatalyst for esterification of oleic acid with methanol Saeed Mohebbi, Mohammad Rostamizadeh, Davood Kahforoushan PII:
S1387-1811(19)30702-4
DOI:
https://doi.org/10.1016/j.micromeso.2019.109845
Reference:
MICMAT 109845
To appear in:
Microporous and Mesoporous Materials
Received Date: 6 August 2019 Revised Date:
16 October 2019
Accepted Date: 28 October 2019
Please cite this article as: S. Mohebbi, M. Rostamizadeh, D. Kahforoushan, Efficient sulfated high silica ZSM-5 nanocatalyst for esterification of oleic acid with methanol, Microporous and Mesoporous Materials (2019), doi: https://doi.org/10.1016/j.micromeso.2019.109845. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Sulfated High silica ZSM-5
Biodiesel
Efficient sulfated high silica ZSM-5 nanocatalyst for esterification of oleic acid with methanol Saeed Mohebbia,b, Mohammad Rostamizadeha,b,1, Davood Kahforoushana,b a
Faculty of Chemical Engineering, Sahand University of Technology, Sahand New Town, Tabriz, Iran, P.O. Box: 51335-1996.
b
Research Center of Environmental Engineering, Sahand University of Technology, Sahand New Town, Tabriz, Iran, P.O. Box: 51335-1996.
Abstract The high silica ZSM-5 nanocatalyst was synthesized by hydrothermal technique and modified through the sulfation process. The nanocatalysts were applied in the esterification of free fatty acids (FFA) in the presence of methanol to produce biodiesel. The effect of different levels of sulfation and operating conditions were investigated. XRD, FT-IR, BET, FE-SEM, ICP-OES, TEM, and NH3-TPD techniques characterized the structure and acidity of the nanocatalysts. The loading of SO42- decreases the specific surface area and pore volume slightly. FE-SEM results represented spherical particles for both of the parent and modified nanocatalysts but the surface of the sulfated nanocatalyst was rough. The NH3-TPD pattern showed that the sulfation process slowly decreased the concentration of the acid sites. The sulfation with acid to catalyst ratio of 1 resulted in the best nanocatalyst. 1
Corresponding author. Tel: +98 4133459168, fax: +98 4133459152 Email address: [email protected] (M. Rostamizadeh)
1
The optimum reaction conditions were 5 wt.% of the nanocatalyst, methanol/oleic acid molar ratio of 20:1, T=190 °C, the stirring rate of 700 rpm, and reaction time of 8 h, leading to the highest FFA conversion (97%). The nanocatalyst had the high stability and reusability in which FFA conversion dropped only 9% after the five sequence cycles. The results confirmed the high potential of the developed nanocatalyst for biodiesel production. Keywords: Esterification; Biodiesel; Sulfation; ZSM-5; Nanocatalyst
1. Introduction
Industrialized world and population growth increase energy demand continuously. Fossil fuels are the biggest source of energy but have negative effects on the environment and human life. In this regard, the production of greenhouse gases is a worldwide issue, which results in global warming. Therefore, these challenges force the researchers to find alternative fuels including renewable, less pollution, and low-cost resources [1-3]. Among the fuels, biodiesel is one of the most considering fuels due to renewability, biodegradability, sustainable supply, non-toxicity, less pollutant emission to the environment, and lack of hazardous substances [1, 4]. According to the definition of ASTM, biodiesel is mono-alkyl esters of long-chain fatty acids, which can be produced by the process of esterification and/or transesterification [5, 6]. Renewable resources include long-chain fatty acids like oleic acid, palmitic acid, linoleic acid, etc. Esterification of fatty 2
acids in presence of short-chain alcohols (methanol or ethanol) has attracted much attention in biodiesel production [1, 7]. Homogeneous and heterogeneous catalysts can be used for biodiesel production. Homogeneous alkali catalysts lead to problems such as soap production, complex separation, and generation of the high amount of wastewater when feedstock contains a high amount of FFA and water. Homogeneous acid catalysts have serious problems such as corrosiveness, hazardous waste, non-reusability, and separation problems. Different heterogeneous acid catalysts have been investigated in the literature for the esterification reaction such as SO42-/ZrO2, SO42-/ZrO2-TiO2/La3+, SO42-/La2O3/HZSM-5, SO42-/La2O3,
Al2O3/SO42-/MCM-41,
SO42-/TiO2,
SO42-/ZrO2,
Fe2(SO4)3,
SO42-
/ZnAl2O4/kaolinite [5, 8-11]. Vieira et al. [9] reported that the sulfated catalysts had higher activity than the parent zeolites for the esterification reaction. Pan et al. [12] studied sulfated alumina supported on MCM-41 as a solid acid catalyst for the esterification reaction. They reported that sulfation increased catalyst activity and reusability of the catalyst. Hossain et al. [13] investigated sulfated zirconium oxide supported on SBA-15 for biodiesel production from hydrolyzed waste cooking oil. They reported the optimum conditions as reaction temperature of 140 °C, catalyst amount of 2%, methanol to oil molar ratio of 10:1, and reaction time of 10 min in which biodiesel yield was 96.38%. Ramli et al. [14] studied sulfated SiO2 for the production of ethyl levulinate. They reported that modification of SiO2 with sulfuric acid decreased the specific surface area and increased 3
the catalyst acidity. Wang et al. [15] found that S2O82-/ZnAl2O4 catalyst had a conversion of 91.7% in the esterification reaction including acetic acid to n-butyl molar ratio of 1:3, 1.55 wt.% of the catalyst, and reaction time of 3 h. In particular, ZSM-5 zeolite catalyst is an attractive and efficient choice for the esterification reaction because of the tunable surface area, acidity, and hydrophobicity properties [16]. The high hydrophobicity hinders the adsorption of the produced water in the esterification reaction on the surface of the catalyst, which increases the opportunity of methanol and FFA to access the catalyst and so provides the high conversion of FFA [17]. The hydrophobicity of the catalyst is an important parameter for the conversion of FFA, which can be adjusted by applying high silica ZSM5. The advantage of the high Si/Al molar ratio is the high hydrophobicity of the nanocatalyst, which suppresses the adsorption of the produced water on the surface of the nanocatalyst in the esterification process. This phenomenon simplifies the access of hydrophilic reactants like methanol and FFA to active sites and the exit of products, which shifts esterification (an equilibrium reaction) in forward. Furthermore, the physical, chemical, and textural properties of ZSM-5 are improved through post-treatments [18, 19]. As the best of our knowledge, there is no report on the development of the high silica sulfated ZSM-5 nanocatalyst for the esterification reaction. In this work, we synthesized the high silica ZSM-5 nanocatalyst by hydrothermal method and sulfated with different amounts of sulfuric acid aqueous. The nanocatalysts were applied in the esterification of
4
FFA with methanol. Also, the operating conditions were optimized to get the highest FFA conversion.
2. Experimental 2.1. Materials Methanol (99%), oleic acid (16% palmitic acid and 18% linoleic acid), silicic acid (SiO2.xH2O, > 99 wt.%), sodium aluminate (NaAlO2, Al2O3 55 wt.%), tetrapropyl ammonium bromide (TPABr, C12H28BrN, > 99 wt.%), ammonium nitrate (NH4NO3, 99 wt.%), sodium hydroxide (NaOH, 99.6 wt.%), and sulfuric acid (H2SO4, 98 wt.%) were purchased from Merck Company (Germany).
2.2. Nanocatalyst preparation The high silica ZSM-5 nanocatalyst (Si/Al=200) was prepared through the hydrothermal technique. At the first step, the solution A was formed by mixing of NaOH, sodium aluminate, deionized water, and TPABr for 90 min. Also, the solution of NaOH and silicic acid was prepared (solution B). Then, the solution A was added to the solution B drop by drop under continues agitation and stirred for 2 h. Sulfuric acid was used to adjust the pH of gel. The molar composition of the final gel was 209H2O: 20SiO2: 0.05Al2O3: 3TPABr: 1.51Na2O. Static stainless-steel and Teflon lined autoclave was applied for the crystallization at 180 °C under autogenous pressure for 48 h. The produced powder was 5
filtered, washed, and dried at 110 °C overnight. The calcination process was at 540 °C for 24 h (3 °C min-1) in air. The obtained Na-form ZSM-5 was converted to H-form ZSM-5 through the four times ion-exchange with NH4NO3 solution (1 M) for 10 h at 90 °C under continues agitation. The secondary calcination was at 540 °C for 12 h (3 °C min -1) in air. The sulfation process was in reflux condition at 70 °C for 2 h including 1 g of the parent nanocatalyst (ZSM-5) and different amounts of sulfuric acid solution (1 M). The sulfated ZSM-5 nanocatalyst was dried in the oven at 110 °C for 12 h and then calcined at 550 °C (3 °C min-1) for 3 h in air. The modified nanocatalysts were named XS/ZSM-5 where X showed the amount of sulfuric acid solution (ml). XRD, FE-SEM, TEM, FT-IR, ICP-OES, NH3-TPD, and N2 adsorption-desorption analyses characterized the synthesized nanocatalysts. The detail of the characterization techniques has been supplied in supporting information.
2.3. Esterification experiments The esterification of FFA in presence of methanol was studied for biodiesel production. The experiments were carried out in a batch system including a stainless steel reactor with a heating jacket at autogenous pressure. The reactants (FFA and methanol) and the nanocatalyst were placed directly into the reactor. After the reaction time, the mixture was centrifuged and the nanocatalyst was collected. The unreacted methanol was removed in the oven at 90 °C for 2 h. The initial and final acid values were determined by titrimetry method as described in the literature [6, 20, 21]. The titration was carried out by KOH and 6
phenolphthalein indicator in which the acid value was determined by Eq. (1). The conversion of FFA was calculated by Eq. (2). (%) =
= $% &$'
%$
× ×
(1)
× 100
(2)
where X is the conversion of FFA. N is the normality of the KOH solution (0.1 N). Si is the initial acid value of FFA and Sf is the final acid value of products at the end of the reaction. Titrimetry of each sample was performed twice and the results were averaged.
3. Results and discussion 3.1. Effect of acid to nanocatalyst ratio The sulfation process was carried out using a different amounts of sulfuric acid solution (1 M) and 1 g of the ZSM-5 nanocatalyst. The parent and modified nanocatalysts were tested in the esterification reaction including methanol/oleic acid molar ratio of 10:1, the nanocatalyst amount of 10 wt.%, reaction temperature of 160 °C, the stirring rate of 700 rpm, and reaction time of 4 h. The results in Fig. 1 shows that the sulfated nanocatalysts have higher conversion than the parent nanocatalyst (33%). It is reported that the addition of metal promoters to the zeolite catalyst improved the affinity of oleic acid to the metal surfaces and so the concentration of oleic acid near the active sites of the nanocatalyst was 7
increased [22]. This phenomenon can be explained by the direct adsorption/interaction between the metal phase and the oleic acid double bond. The high concentration of oleic acid near the nanocatalyst surface also enhances any ionic interaction of the zeolite surface and the carboxylic acid group of oleic acid, which leads to the high conversion of FFA. Furthermore, we studied the hydrophobicity/hydrophobicity properties of the nanocatalysts by water-droplet contact angle (CA) test. The parent and 1S/ZSM-5 nanocatalyst showed CA of 38.2° and 88.5°, respectively (Table 1). These results confirmed that the sulfation process increases the nanocatalyst surface hydrophobicity. Sun et al. [23] studied different kinetic models for the esterification over zeolites. They concluded that the adsorption of oleic acid molecules was more favorable than the adsorption of the polar molecules (like methanol) on the surface of hydrophobic ZSM-5 zeolite, which led to high coverage of oleic acid molecules on the surface of hydrophobic ZSM-5 catalysts and higher conversion. It is worth to note that the polar of water is stronger than methanol and so the adsorption capacity of water molecules is stronger than that of methanol molecules [24]. The water produced in the esterification reaction desorbs rapidly from the hydrophobic surface of zeolite nanocatalyst, increasing the surface coverage of the oleic acid. Therefore, the 1S/ZSM-5 nanocatalyst has a higher FFA conversion in compare with the parent nanocatalyst. The sulfation with the high amount of the solution leads to the low conversion of FFA. This can be explained by the high dealumination of the nanocatalyst through the sulfation with the high amount of acid solution. It is reported that the 8
dealumination of the ZSM-5 nanocatalyst decreased the acidity [19], which can influence the catalytic activity in the esterification reaction. As shown in Fig. 1, the acid to nanocatalyst ratio of 1 (ml g-1) results in the highest FFA conversion (95%). Therefore, the 1S/ZSM-5 nanocatalyst is the optimal nanocatalyst for the esterification of FFA.
3.2. Nanocatalyst characterization XRD pattern of the nanocatalysts in the range of 2θ = 5-65º is represented in Fig. 2. The pattern of the parent and sulfated nanocatalyst is well defined and matches with orthorhombic MFI-structure of ZSM-5 (JCPDS NO. 00-042-0023) [25]. The additional peak related to the sulfate groups is not detected, which confirms the uniform dispersion through the parent structure. Table 1 shows the relative crystallinity of the nanocatalysts that is measured by peak intensity at 2θ≈23.2º based on the parent nanocatalyst [26]. The sulfation process decreases the crystallinity owing to the dealumination through the sulfation and so the framework destruction. It is reported that the peak position moved to the high 2θ values due to dealumination [27], which supports the XRD results. FT-IR analysis for identification of functional groups was performed in the range of 400-4000 cm1
(Fig. 3). The band at 453 cm-1 is related to internal SiO4 and AlO4 tetrahedral. The band at
546 cm-1 is assigned to five-membered rings of the ZSM-5 with asymmetric stretching. The band at 793 cm-1 and 1097 cm-1 is attributed to the symmetric stretching at external and internal linkages, respectively. The band at 1228 cm-1 is related to the external asymmetric 9
stretching vibration because of the four chains of 5-rings in the structure [18]. The bands around 1385, 1463, and 1520 cm-1 are assigned to stretching vibration of S─O and S═O, which characterize the sulfate groups in the 1S/ZSM-5 nanocatalyst [15, 28]. FT-IR spectra in the range of 3500-3800 cm-1 characterize surface hydroxyl (OH) groups. The bands at 3615, 3675 and 3742 cm-1 are attributed to the presence of bridging structural hydroxyl groups in the form of Si-OH-Al, Al-OH and Si-OH groups, respectively [18, 29]. In general, dealumination initiates with the breakage of the Al-O-Si bond [30] because the AlO bond (1.77-1.89 Å) is weaker and longer than the Si-O bond (1.67-1.72 Å) [31]. The formed tri-coordinated Al reacts with H2O molecules and turns to the Al-OH group. The hydrolysis of water also produces a proton, which reacts with Si-O and forms Si-OH. Furthermore, the proton of the bridging hydroxyl group is moved to the basic extra framework aluminum oxide cluster [30]. Hence, the band at ca. 3740 cm-1 with high intensity for the 1S/ZSM-5 nanocatalyst reveals that the Si-OH groups are formed through dealumination, which is in agreement with the increasing Si/Al ratio according to ICP-OES results (Table 1). Fig. 4 shows the spherical morphology of the ZSM-5 and 1S/ZSM-5 nanocatalysts. It is clear that the morphology of the modified nanocatalyst almost is not changed through the sulfation process, which supports the XRD results. However, the surface of the 1S/ZSM-5 nanocatalyst is slightly rough due to acid washing in the sulfation process. Triantafillidis et al. [32] concluded that dealumination partially 10
broke some crystals and formed the extra-framework phase in HZSM-5 zeolite. Also, the modified zeolite had not sharp and well-defined edges compared with the parent zeolite, which supports the FE-SEM results of the 1S/ZSM-5 nanocatalyst. The TEM image of the nanocatalyst (Fig. 5) confirms that the microsphere surface is formed by aggregation of nanosized crystals. The nanocatalysts represent the combination of type I and IV Langmuir isotherms (Fig. 6) that relate to microporous and mesoporous structures [19, 33]. The BET isotherms of the parent and sulfated nanocatalysts are very similar because no significant destruction occurs in the microporous structure through the sulfation process. The results support the XRD and FE-SEM results. The rectangular type H4 hysteresis loop at the medium and high relative pressures (P/P0=0.45–0.95) reveals the mesoporosity, which is in agreement with BJH mean pore diameter (>2nm). The textural data of the nanocatalysts are shown in Table 1. The specific surface area and total pore volume of the ZSM-5 nanocatalyst are 356 m2 g-1 and 0.19 cm3 g-1, respectively. The attachment of the sulfate group decreases the specific surface area and total pore volume (0.17 cm3 g-1). These results can be explained by the partial poreblocking through the sulfation process in which the mean pore diameter decreases from 2.13 nm to 2.03 nm. The NH3-TPD pattern of the parent and sulfated nanocatalysts has two peaks at the low and high temperatures, which characterizes the weak and strong acid sites, respectively (Fig. 7). The peak temperature 11
indicates the strength and the peak area reveals the concentration of the acid sites. As can be seen, the sulfation process decreases the acidity strength and the peak temperature of the 1S/ZSM-5 nanocatalyst (Table 2). Also, the concentration of acid sites is reduced. This phenomenon can be explained by the partial pore blocking and inaccessibility of some of the internal acid sites in the 1S/ZSM-5 nanocatalyst, which is in agreement with the BET results. The parent nanocatalyst has a large number of weak (0.62 mmol NH3 g-1) and strong (0.31 mmol NH3 g-1) acid sites. Dealumination trough the sulfation process reduces the concentration of the weak and strong acid sites. The Al content in the HZSM-5 determines the acidity. Jin et al. [34] concluded that dealumination decreased the acidity of HZSM-5 zeolite due to the removal of Al species. The 1S/ZSM-5 nanocatalyst has a lower peak temperature for the weak and strong acid sites. It should be noted that Al and Si species in the ZSM-5 framework forms strong and weak acid sites, respectively [35, 36]. Thus, the extraction of the framework Al and the creation of the more Si-OH groups (based on FT-IR) results in the low strength of acidity for the 1S/ZSM-5 nanocatalyst.
3.3. Effect of reaction time The esterification reaction was carried out using the 1S/ZSM-5 nanocatalyst at different reaction times (Fig. 8). The long reaction time (<8 h) favors the conversion of FFA. The 12
experiment with 8 h time of reaction gets the highest conversion of FFA (95%). The more increasing of time does not change the conversion because the esterification reaction is reversible and the reaction reaches equilibrium state between 8 to 10 h. Chung et al. [37] applied the HZSM-5 (Si/Al=25) catalyst in the esterification of oleic acid at 60 °C, 0.5 g catalyst, and different reaction times. They found that conversion increased up to 60 min and then reached an equilibrium in which the highest conversion of oleic acid was 80%. Hence, the appropriate reaction time for the biodiesel production over the 1S/ZSM-5 nanocatalyst is 8 h.
3.4. Effect of nanocatalyst concentration Fig. 9 shows the effect of mass percent of the 1S/ZSM-5 nanocatalyst (based on the mass of oleic acid) on catalytic performance. The FFA conversion is increased by the high concentration of the nanocatalyst. The highest conversion of FFA (95%) is at the concentration of 5 wt.%. In the literature, a similar effect of the catalyst concentration has been reported for the esterification of FFA over different catalysts [12, 38, 39]. It is accepted that the high catalyst amount improves the number of active sites and the contact between methanol and catalyst [38]. Further increasing of the nanocatalyst concentration (10 wt.%) does not improve the conversion of FFA because of mass transfer problems. In these conditions, the mixture of methanol, FFA, and the nanocatalyst is very viscous and
13
requires a higher rate of stirring [40, 41]. Therefore, the 5 wt.% of the 1S/ZSM-5 nanocatalyst is the suitable nanocatalyst concentration for biodiesel production.
3.5. Effect of reaction temperature It is accepted that reaction temperature plays a key role in the esterification reaction [37]. The increasing of the reaction temperature from 100 °C to 190 °C enhances the FFA conversion from 88% to 97% (Fig. 10). The esterification is an endothermic reaction and so the high temperature favors the conversion of FFA. It is reported that the reaction rate increased with temperature in the esterification of sunflower oil with methanol [42]. Therefore, the reaction temperature of 190 °C is an optimal temperature for biodiesel production over the 1S/ZSM-5 nanocatalyst.
3.6. Effect of methanol/oleic acid molar ratio The esterification of FFA with alcohol is a reversible reaction. To raise the conversion of FFA, an excess amount of methanol is used to force the reaction towards the formation of methyl esters. Fig. 11 shows that the conversion of FFA is improved by the high methanol to oleic acid ratio. The highest FFA conversion (97%) is at the molar ratio of 20, which does not change by more increasing of the ratio. It is reported that the high molar ratio of alcohol resulted in the low concentration of the nanocatalyst in the reactor and so the
14
conversion did not improve [21]. Thus, the methanol to oleic acid ratio of 20 is appropriate for biodiesel production over the 1S/ZSM-5 nanocatalyst.
3.7. Catalyst reusability One of the most important factors for scale-up and commercialization of the heterogeneous catalysts is reusability. For this reason, the stability of the 1S/ZSM-5 nanocatalyst was investigated through the several sequence experiments under the optimal reaction conditions (methanol/oleic acid molar ratio of 20:1, 5 wt.% of the nanocatalyst, reaction temperature of 190 °C, and reaction time of 8 h). After each run, the nanocatalyst was twice washed with n-hexane and then dried in the oven at a temperature of 110 °C for 12 h. Fig. 12 shows the reusability results of the 1S/ZSM-5 nanocatalyst. It can be seen that the FFA conversion decreases only 9% after the five sequence cycles, which confirms the high stability and regeneration capacity of the 1S/ZSM-5 nanocatalyst. It is worth to note that the FFA conversion of the experiment without nanocatalyst is only 9.6% in the optimal operating conditions. This result can be attributed to the high crystallinity, high surface area, mesopore structure as well as the appropriate acidity of the developed nanocatalyst. Furthermore, the effective and suitable sulfation process leads to the uniform dispersion of the sulfate groups and hinders its aggregation in the nanocatalyst which results in the high stability of the nanocatalyst through the several regeneration cycles.
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For comparison, recent studies on the esterification of oleic acid over the zeolite catalysts are summarized in Table 3. According to this table, the developed high silica 1S/ZSM-5 nanocatalyst in this study is more active than either of these catalysts. Vieira et al. [43] reported the conversion of 100% over La2O3/SO42-/HZSM-5 catalyst but the catalyst had not the high reusability in which the conversion was 45% and 44% for the second and third runs. The main advantages of the 1S/ZSM-5 nanocatalyst are the low catalyst loading, moderate methanol to oleic acid molar ratio, and the high capacity of reusability, which can be important at an industrial point of view, since it would allow using smaller size equipment and lower energy inputs. These results can be explained by the appropriate textural and acidity properties of the high silica nanocatalyst. Furthermore, it is reported that chemical and thermal stability, as well as the yield of synthesis of the high silica ZSM5, is more than the low silica ZSM-5 [44, 45] that can improve the economy of the catalyst preparation for biodiesel production.
4. Conclusion In this study, the high silica ZSM-5 nanocatalyst was synthesized by the hydrothermal method and sulfated with aqueous H2SO4 (1 M) at reflux conditions. The nanocatalyst was applied for biodiesel production in the esterification reaction. The acid to nanocatalyst ratio of 1 led to the best nanocatalyst. The characterization results showed the high crystallinity, 16
uniform dispersion of the sulfate group, and mesopore structure for the 1S/ZSM-5 nanocatalyst. The esterification was in a batch stainless steel reactor at different reaction conditions. The optimal conditions were 5 wt.% of the nanocatalyst, methanol/oleic acid molar ratio of 20:1, T=190 °C, the reaction time of 8 h, which resulted in the highest conversion of FFA (97%). The 1S/ZSM-5 nanocatalyst had a high capacity of reusability in the esterification reaction in which the FFA conversion dropped only 9% after the five sequence cycles. The results confirmed the potential of the developed nanocatalyst for biodiesel production through the esterification process.
Acknowledgments The authors also wish to acknowledge the Iran National Science Foundation (INSF) for their support of this study.
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[47] D.A.G. Aranda, J.d.A. Gonçalves, J.S. Peres, A.L.D. Ramos, C.A.R. de Melo, O.A.C. Antunes, N.C. Furtado, C.A. Taft, J. Phys. Org. Chem., 22 (2009) 709-716. [48] A.A. Costa, P.R.S. Braga, J.L. de Macedo, J.A. Dias, S.C.L. Dias, Microporous Mesoporous Mater., 147 (2012) 142-148.
19
Figure captions Fig. 1. Effect of acid to nanocatalyst ratio on the catalytic performance. Reaction conditions: 10wt.% of the nanocatalyst, methanol/oleic acid molar ratio 10:1, T=160 °C, and reaction time of 4 h. Fig. 2. XRD pattern of the nanocatalysts: a) ZSM-5 and b) 1S/ZSM-5. Fig. 3. FTIR spectra of the ZSM-5 and 1S/ZSM-5 nanocatalysts in range of 400-4000 cm-1. Fig. 4. FE-SEM images of the nanocatalysts: a) ZSM-5 and b) 1S/ZSM-5. Fig. 5. TEM image of the nanocatalyst. Fig. 6. Adsorption/Desorption isotherms of the ZSM-5 and 1S/ZSM-5 nanocatalysts. Fig. 7. NH3-TPD pattern of the ZSM-5 and 1S/ZSM-5 nanocatalysts. Fig. 8. Effect of reaction time on the conversion of FFA. Reaction conditions: 5wt.% of the nanocatalyst, methanol/oleic acid molar ratio of 10:1, and T=160 °C. Fig. 9. Effect of the nanocatalyst concentration on the conversion of FFA. Reaction conditions: methanol/oleic acid molar ratio of 10:1, T=160 °C, and reaction time of 8 h. Fig. 10. Effect of reaction temperature on the conversion of FFA. Reaction conditions: methanol/oleic acid molar ratio of 10:1, 5 wt.% of the nanocatalyst, and reaction time of 8 h. Fig. 11. Effect of methanol/oleic acid molar ratio on the conversion of FFA. Reaction conditions: T= 190 °C, 5 wt.% of the nanocatalyst, and reaction time of 8 h. Fig. 12. Reusability of the 1S/ZSM-5 nanocatalyst. Reaction conditions: 5 wt.% of the nanocatalyst, methanol/oleic acid molar ratio of 20:1, T=190 °C, and reaction time of 8 h.
20
Fig. 1
21
ZSM-5
Intensity (a.u.)
b) SO42-/ZSM-5
a) ZSM-5
Reference patterns
ZSM-5 (Orthorhombic , 00-042-0023)
10
20
30
40
2θ (degree)
Fig. 2
22
50
60
Fig. 3
23
Fig. 4
24
Fig. 5
25
Quantity adsorbed (cm3 g-1)
ZSM-5
1S/ZSM-5
Adsorption Desorption
0.0
0.2
0.4
0.6
Relative pressure (P/P0)
Fig. 6
26
0.8
1.0
TPD Signal (a.u.)
132 °C
112 °C 321 °C
ZSM-5 309 °C
1S/ZSM-5
60
100
140
180
220
260
Temperature (°C )
Fig. 7
27
300
340
380
Fig. 8
28
Fig. 9
29
Fig. 10
30
Fig. 11
31
Fig. 12
32
Table 1 Textural data of the nanocatalysts
ZSM-5
Relative crystallinity (%) 100
1S/ZSM-5
85
Nanocatalyst
a
SBET (m2 g-1)
Vtotal (cm3 g-1)
Vmicro (cm3 g-1)
Vmeso (cm3 g-1)
356
0.19
0.13
0.06
Mean pore diameter (nm) 2.13
334
0.17
0.12
0.05
2.08
Determined by ICP-OES
33
S (wt%)a
(Si/Al)bulk ratioa
-
193.70
1.50
196.50
Table 2 NH3-TPD data of the ZSM-5 and 1S/ZSM-5 nanocatalysts Distribution and concentration of acid sites (mmol NH3 g-1)
Peak temperature (°C)
Nanocatalyst Weak
Strong
Total
Strong/Weak
TP1
TP2
ZSM-5
0.63
0.31
0.94
0.49
132
321
1S/ZSM-5
0.54
0.26
0.81
0.48
112
309
34
Table 3 Comparison of catalytic activity of the zeolite catalysts in the esterification reaction. Catalyst Citric acid/HZSM-5 HZSM-5 La2O3/SO42-/HZSM-5 HBEA HMOR HZSM-5 HY SO42-/TiO2-SiO2 USY WO3/USY 1S/ZSM-5
Reaction conditions Temperature Catalysts Molar (%) ratio (°C) 100 10 1:45 100 10 1:45 100 10 1:45 130 2 1:3 130 2 1:3 130 2 1:3 130 2 1:3 120 10 1:20 200 10 1:6 200 10 1:6 190 5 1:20
35
Time (h) 4 7 7 1 1 1 1 3 2 2 8
Conversion (%)
References
83 80 100 35 50 40 35 93.4 74 80 97
[46] [43] [43] [47] [47] [47] [47] [28] [48] [48] Present study
- The sulfated high silica nanocatalyst has the high surface area. - The acid to catalyst ratio of 1 results in the best nanocatalyst for esterification. - The 1S/ZSM-5 nanocatalyst represents the highest FFA conversion of 97%. - We report the high reusability including only 9% conversion drop after 5 runs.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S1387-1811(19)30702-4
DOI:
https://doi.org/10.1016/j.micromeso.2019.109845
Reference:
MICMAT 109845
To appear in:
Microporous and Mesoporous Materials
Received Date: 6 August 2019 Revised Date:
16 October 2019
Accepted Date: 28 October 2019
Please cite this article as: S. Mohebbi, M. Rostamizadeh, D. Kahforoushan, Efficient sulfated high silica ZSM-5 nanocatalyst for esterification of oleic acid with methanol, Microporous and Mesoporous Materials (2019), doi: https://doi.org/10.1016/j.micromeso.2019.109845. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Sulfated High silica ZSM-5
Biodiesel
Efficient sulfated high silica ZSM-5 nanocatalyst for esterification of oleic acid with methanol Saeed Mohebbia,b, Mohammad Rostamizadeha,b,1, Davood Kahforoushana,b a
Faculty of Chemical Engineering, Sahand University of Technology, Sahand New Town, Tabriz, Iran, P.O. Box: 51335-1996.
b
Research Center of Environmental Engineering, Sahand University of Technology, Sahand New Town, Tabriz, Iran, P.O. Box: 51335-1996.
Abstract The high silica ZSM-5 nanocatalyst was synthesized by hydrothermal technique and modified through the sulfation process. The nanocatalysts were applied in the esterification of free fatty acids (FFA) in the presence of methanol to produce biodiesel. The effect of different levels of sulfation and operating conditions were investigated. XRD, FT-IR, BET, FE-SEM, ICP-OES, TEM, and NH3-TPD techniques characterized the structure and acidity of the nanocatalysts. The loading of SO42- decreases the specific surface area and pore volume slightly. FE-SEM results represented spherical particles for both of the parent and modified nanocatalysts but the surface of the sulfated nanocatalyst was rough. The NH3-TPD pattern showed that the sulfation process slowly decreased the concentration of the acid sites. The sulfation with acid to catalyst ratio of 1 resulted in the best nanocatalyst. 1
Corresponding author. Tel: +98 4133459168, fax: +98 4133459152 Email address: [email protected] (M. Rostamizadeh)
1
The optimum reaction conditions were 5 wt.% of the nanocatalyst, methanol/oleic acid molar ratio of 20:1, T=190 °C, the stirring rate of 700 rpm, and reaction time of 8 h, leading to the highest FFA conversion (97%). The nanocatalyst had the high stability and reusability in which FFA conversion dropped only 9% after the five sequence cycles. The results confirmed the high potential of the developed nanocatalyst for biodiesel production. Keywords: Esterification; Biodiesel; Sulfation; ZSM-5; Nanocatalyst
1. Introduction
Industrialized world and population growth increase energy demand continuously. Fossil fuels are the biggest source of energy but have negative effects on the environment and human life. In this regard, the production of greenhouse gases is a worldwide issue, which results in global warming. Therefore, these challenges force the researchers to find alternative fuels including renewable, less pollution, and low-cost resources [1-3]. Among the fuels, biodiesel is one of the most considering fuels due to renewability, biodegradability, sustainable supply, non-toxicity, less pollutant emission to the environment, and lack of hazardous substances [1, 4]. According to the definition of ASTM, biodiesel is mono-alkyl esters of long-chain fatty acids, which can be produced by the process of esterification and/or transesterification [5, 6]. Renewable resources include long-chain fatty acids like oleic acid, palmitic acid, linoleic acid, etc. Esterification of fatty 2
acids in presence of short-chain alcohols (methanol or ethanol) has attracted much attention in biodiesel production [1, 7]. Homogeneous and heterogeneous catalysts can be used for biodiesel production. Homogeneous alkali catalysts lead to problems such as soap production, complex separation, and generation of the high amount of wastewater when feedstock contains a high amount of FFA and water. Homogeneous acid catalysts have serious problems such as corrosiveness, hazardous waste, non-reusability, and separation problems. Different heterogeneous acid catalysts have been investigated in the literature for the esterification reaction such as SO42-/ZrO2, SO42-/ZrO2-TiO2/La3+, SO42-/La2O3/HZSM-5, SO42-/La2O3,
Al2O3/SO42-/MCM-41,
SO42-/TiO2,
SO42-/ZrO2,
Fe2(SO4)3,
SO42-
/ZnAl2O4/kaolinite [5, 8-11]. Vieira et al. [9] reported that the sulfated catalysts had higher activity than the parent zeolites for the esterification reaction. Pan et al. [12] studied sulfated alumina supported on MCM-41 as a solid acid catalyst for the esterification reaction. They reported that sulfation increased catalyst activity and reusability of the catalyst. Hossain et al. [13] investigated sulfated zirconium oxide supported on SBA-15 for biodiesel production from hydrolyzed waste cooking oil. They reported the optimum conditions as reaction temperature of 140 °C, catalyst amount of 2%, methanol to oil molar ratio of 10:1, and reaction time of 10 min in which biodiesel yield was 96.38%. Ramli et al. [14] studied sulfated SiO2 for the production of ethyl levulinate. They reported that modification of SiO2 with sulfuric acid decreased the specific surface area and increased 3
the catalyst acidity. Wang et al. [15] found that S2O82-/ZnAl2O4 catalyst had a conversion of 91.7% in the esterification reaction including acetic acid to n-butyl molar ratio of 1:3, 1.55 wt.% of the catalyst, and reaction time of 3 h. In particular, ZSM-5 zeolite catalyst is an attractive and efficient choice for the esterification reaction because of the tunable surface area, acidity, and hydrophobicity properties [16]. The high hydrophobicity hinders the adsorption of the produced water in the esterification reaction on the surface of the catalyst, which increases the opportunity of methanol and FFA to access the catalyst and so provides the high conversion of FFA [17]. The hydrophobicity of the catalyst is an important parameter for the conversion of FFA, which can be adjusted by applying high silica ZSM5. The advantage of the high Si/Al molar ratio is the high hydrophobicity of the nanocatalyst, which suppresses the adsorption of the produced water on the surface of the nanocatalyst in the esterification process. This phenomenon simplifies the access of hydrophilic reactants like methanol and FFA to active sites and the exit of products, which shifts esterification (an equilibrium reaction) in forward. Furthermore, the physical, chemical, and textural properties of ZSM-5 are improved through post-treatments [18, 19]. As the best of our knowledge, there is no report on the development of the high silica sulfated ZSM-5 nanocatalyst for the esterification reaction. In this work, we synthesized the high silica ZSM-5 nanocatalyst by hydrothermal method and sulfated with different amounts of sulfuric acid aqueous. The nanocatalysts were applied in the esterification of
4
FFA with methanol. Also, the operating conditions were optimized to get the highest FFA conversion.
2. Experimental 2.1. Materials Methanol (99%), oleic acid (16% palmitic acid and 18% linoleic acid), silicic acid (SiO2.xH2O, > 99 wt.%), sodium aluminate (NaAlO2, Al2O3 55 wt.%), tetrapropyl ammonium bromide (TPABr, C12H28BrN, > 99 wt.%), ammonium nitrate (NH4NO3, 99 wt.%), sodium hydroxide (NaOH, 99.6 wt.%), and sulfuric acid (H2SO4, 98 wt.%) were purchased from Merck Company (Germany).
2.2. Nanocatalyst preparation The high silica ZSM-5 nanocatalyst (Si/Al=200) was prepared through the hydrothermal technique. At the first step, the solution A was formed by mixing of NaOH, sodium aluminate, deionized water, and TPABr for 90 min. Also, the solution of NaOH and silicic acid was prepared (solution B). Then, the solution A was added to the solution B drop by drop under continues agitation and stirred for 2 h. Sulfuric acid was used to adjust the pH of gel. The molar composition of the final gel was 209H2O: 20SiO2: 0.05Al2O3: 3TPABr: 1.51Na2O. Static stainless-steel and Teflon lined autoclave was applied for the crystallization at 180 °C under autogenous pressure for 48 h. The produced powder was 5
filtered, washed, and dried at 110 °C overnight. The calcination process was at 540 °C for 24 h (3 °C min-1) in air. The obtained Na-form ZSM-5 was converted to H-form ZSM-5 through the four times ion-exchange with NH4NO3 solution (1 M) for 10 h at 90 °C under continues agitation. The secondary calcination was at 540 °C for 12 h (3 °C min -1) in air. The sulfation process was in reflux condition at 70 °C for 2 h including 1 g of the parent nanocatalyst (ZSM-5) and different amounts of sulfuric acid solution (1 M). The sulfated ZSM-5 nanocatalyst was dried in the oven at 110 °C for 12 h and then calcined at 550 °C (3 °C min-1) for 3 h in air. The modified nanocatalysts were named XS/ZSM-5 where X showed the amount of sulfuric acid solution (ml). XRD, FE-SEM, TEM, FT-IR, ICP-OES, NH3-TPD, and N2 adsorption-desorption analyses characterized the synthesized nanocatalysts. The detail of the characterization techniques has been supplied in supporting information.
2.3. Esterification experiments The esterification of FFA in presence of methanol was studied for biodiesel production. The experiments were carried out in a batch system including a stainless steel reactor with a heating jacket at autogenous pressure. The reactants (FFA and methanol) and the nanocatalyst were placed directly into the reactor. After the reaction time, the mixture was centrifuged and the nanocatalyst was collected. The unreacted methanol was removed in the oven at 90 °C for 2 h. The initial and final acid values were determined by titrimetry method as described in the literature [6, 20, 21]. The titration was carried out by KOH and 6
phenolphthalein indicator in which the acid value was determined by Eq. (1). The conversion of FFA was calculated by Eq. (2). (%) =
= $% &$'
%$
× ×
(1)
× 100
(2)
where X is the conversion of FFA. N is the normality of the KOH solution (0.1 N). Si is the initial acid value of FFA and Sf is the final acid value of products at the end of the reaction. Titrimetry of each sample was performed twice and the results were averaged.
3. Results and discussion 3.1. Effect of acid to nanocatalyst ratio The sulfation process was carried out using a different amounts of sulfuric acid solution (1 M) and 1 g of the ZSM-5 nanocatalyst. The parent and modified nanocatalysts were tested in the esterification reaction including methanol/oleic acid molar ratio of 10:1, the nanocatalyst amount of 10 wt.%, reaction temperature of 160 °C, the stirring rate of 700 rpm, and reaction time of 4 h. The results in Fig. 1 shows that the sulfated nanocatalysts have higher conversion than the parent nanocatalyst (33%). It is reported that the addition of metal promoters to the zeolite catalyst improved the affinity of oleic acid to the metal surfaces and so the concentration of oleic acid near the active sites of the nanocatalyst was 7
increased [22]. This phenomenon can be explained by the direct adsorption/interaction between the metal phase and the oleic acid double bond. The high concentration of oleic acid near the nanocatalyst surface also enhances any ionic interaction of the zeolite surface and the carboxylic acid group of oleic acid, which leads to the high conversion of FFA. Furthermore, we studied the hydrophobicity/hydrophobicity properties of the nanocatalysts by water-droplet contact angle (CA) test. The parent and 1S/ZSM-5 nanocatalyst showed CA of 38.2° and 88.5°, respectively (Table 1). These results confirmed that the sulfation process increases the nanocatalyst surface hydrophobicity. Sun et al. [23] studied different kinetic models for the esterification over zeolites. They concluded that the adsorption of oleic acid molecules was more favorable than the adsorption of the polar molecules (like methanol) on the surface of hydrophobic ZSM-5 zeolite, which led to high coverage of oleic acid molecules on the surface of hydrophobic ZSM-5 catalysts and higher conversion. It is worth to note that the polar of water is stronger than methanol and so the adsorption capacity of water molecules is stronger than that of methanol molecules [24]. The water produced in the esterification reaction desorbs rapidly from the hydrophobic surface of zeolite nanocatalyst, increasing the surface coverage of the oleic acid. Therefore, the 1S/ZSM-5 nanocatalyst has a higher FFA conversion in compare with the parent nanocatalyst. The sulfation with the high amount of the solution leads to the low conversion of FFA. This can be explained by the high dealumination of the nanocatalyst through the sulfation with the high amount of acid solution. It is reported that the 8
dealumination of the ZSM-5 nanocatalyst decreased the acidity [19], which can influence the catalytic activity in the esterification reaction. As shown in Fig. 1, the acid to nanocatalyst ratio of 1 (ml g-1) results in the highest FFA conversion (95%). Therefore, the 1S/ZSM-5 nanocatalyst is the optimal nanocatalyst for the esterification of FFA.
3.2. Nanocatalyst characterization XRD pattern of the nanocatalysts in the range of 2θ = 5-65º is represented in Fig. 2. The pattern of the parent and sulfated nanocatalyst is well defined and matches with orthorhombic MFI-structure of ZSM-5 (JCPDS NO. 00-042-0023) [25]. The additional peak related to the sulfate groups is not detected, which confirms the uniform dispersion through the parent structure. Table 1 shows the relative crystallinity of the nanocatalysts that is measured by peak intensity at 2θ≈23.2º based on the parent nanocatalyst [26]. The sulfation process decreases the crystallinity owing to the dealumination through the sulfation and so the framework destruction. It is reported that the peak position moved to the high 2θ values due to dealumination [27], which supports the XRD results. FT-IR analysis for identification of functional groups was performed in the range of 400-4000 cm1
(Fig. 3). The band at 453 cm-1 is related to internal SiO4 and AlO4 tetrahedral. The band at
546 cm-1 is assigned to five-membered rings of the ZSM-5 with asymmetric stretching. The band at 793 cm-1 and 1097 cm-1 is attributed to the symmetric stretching at external and internal linkages, respectively. The band at 1228 cm-1 is related to the external asymmetric 9
stretching vibration because of the four chains of 5-rings in the structure [18]. The bands around 1385, 1463, and 1520 cm-1 are assigned to stretching vibration of S─O and S═O, which characterize the sulfate groups in the 1S/ZSM-5 nanocatalyst [15, 28]. FT-IR spectra in the range of 3500-3800 cm-1 characterize surface hydroxyl (OH) groups. The bands at 3615, 3675 and 3742 cm-1 are attributed to the presence of bridging structural hydroxyl groups in the form of Si-OH-Al, Al-OH and Si-OH groups, respectively [18, 29]. In general, dealumination initiates with the breakage of the Al-O-Si bond [30] because the AlO bond (1.77-1.89 Å) is weaker and longer than the Si-O bond (1.67-1.72 Å) [31]. The formed tri-coordinated Al reacts with H2O molecules and turns to the Al-OH group. The hydrolysis of water also produces a proton, which reacts with Si-O and forms Si-OH. Furthermore, the proton of the bridging hydroxyl group is moved to the basic extra framework aluminum oxide cluster [30]. Hence, the band at ca. 3740 cm-1 with high intensity for the 1S/ZSM-5 nanocatalyst reveals that the Si-OH groups are formed through dealumination, which is in agreement with the increasing Si/Al ratio according to ICP-OES results (Table 1). Fig. 4 shows the spherical morphology of the ZSM-5 and 1S/ZSM-5 nanocatalysts. It is clear that the morphology of the modified nanocatalyst almost is not changed through the sulfation process, which supports the XRD results. However, the surface of the 1S/ZSM-5 nanocatalyst is slightly rough due to acid washing in the sulfation process. Triantafillidis et al. [32] concluded that dealumination partially 10
broke some crystals and formed the extra-framework phase in HZSM-5 zeolite. Also, the modified zeolite had not sharp and well-defined edges compared with the parent zeolite, which supports the FE-SEM results of the 1S/ZSM-5 nanocatalyst. The TEM image of the nanocatalyst (Fig. 5) confirms that the microsphere surface is formed by aggregation of nanosized crystals. The nanocatalysts represent the combination of type I and IV Langmuir isotherms (Fig. 6) that relate to microporous and mesoporous structures [19, 33]. The BET isotherms of the parent and sulfated nanocatalysts are very similar because no significant destruction occurs in the microporous structure through the sulfation process. The results support the XRD and FE-SEM results. The rectangular type H4 hysteresis loop at the medium and high relative pressures (P/P0=0.45–0.95) reveals the mesoporosity, which is in agreement with BJH mean pore diameter (>2nm). The textural data of the nanocatalysts are shown in Table 1. The specific surface area and total pore volume of the ZSM-5 nanocatalyst are 356 m2 g-1 and 0.19 cm3 g-1, respectively. The attachment of the sulfate group decreases the specific surface area and total pore volume (0.17 cm3 g-1). These results can be explained by the partial poreblocking through the sulfation process in which the mean pore diameter decreases from 2.13 nm to 2.03 nm. The NH3-TPD pattern of the parent and sulfated nanocatalysts has two peaks at the low and high temperatures, which characterizes the weak and strong acid sites, respectively (Fig. 7). The peak temperature 11
indicates the strength and the peak area reveals the concentration of the acid sites. As can be seen, the sulfation process decreases the acidity strength and the peak temperature of the 1S/ZSM-5 nanocatalyst (Table 2). Also, the concentration of acid sites is reduced. This phenomenon can be explained by the partial pore blocking and inaccessibility of some of the internal acid sites in the 1S/ZSM-5 nanocatalyst, which is in agreement with the BET results. The parent nanocatalyst has a large number of weak (0.62 mmol NH3 g-1) and strong (0.31 mmol NH3 g-1) acid sites. Dealumination trough the sulfation process reduces the concentration of the weak and strong acid sites. The Al content in the HZSM-5 determines the acidity. Jin et al. [34] concluded that dealumination decreased the acidity of HZSM-5 zeolite due to the removal of Al species. The 1S/ZSM-5 nanocatalyst has a lower peak temperature for the weak and strong acid sites. It should be noted that Al and Si species in the ZSM-5 framework forms strong and weak acid sites, respectively [35, 36]. Thus, the extraction of the framework Al and the creation of the more Si-OH groups (based on FT-IR) results in the low strength of acidity for the 1S/ZSM-5 nanocatalyst.
3.3. Effect of reaction time The esterification reaction was carried out using the 1S/ZSM-5 nanocatalyst at different reaction times (Fig. 8). The long reaction time (<8 h) favors the conversion of FFA. The 12
experiment with 8 h time of reaction gets the highest conversion of FFA (95%). The more increasing of time does not change the conversion because the esterification reaction is reversible and the reaction reaches equilibrium state between 8 to 10 h. Chung et al. [37] applied the HZSM-5 (Si/Al=25) catalyst in the esterification of oleic acid at 60 °C, 0.5 g catalyst, and different reaction times. They found that conversion increased up to 60 min and then reached an equilibrium in which the highest conversion of oleic acid was 80%. Hence, the appropriate reaction time for the biodiesel production over the 1S/ZSM-5 nanocatalyst is 8 h.
3.4. Effect of nanocatalyst concentration Fig. 9 shows the effect of mass percent of the 1S/ZSM-5 nanocatalyst (based on the mass of oleic acid) on catalytic performance. The FFA conversion is increased by the high concentration of the nanocatalyst. The highest conversion of FFA (95%) is at the concentration of 5 wt.%. In the literature, a similar effect of the catalyst concentration has been reported for the esterification of FFA over different catalysts [12, 38, 39]. It is accepted that the high catalyst amount improves the number of active sites and the contact between methanol and catalyst [38]. Further increasing of the nanocatalyst concentration (10 wt.%) does not improve the conversion of FFA because of mass transfer problems. In these conditions, the mixture of methanol, FFA, and the nanocatalyst is very viscous and
13
requires a higher rate of stirring [40, 41]. Therefore, the 5 wt.% of the 1S/ZSM-5 nanocatalyst is the suitable nanocatalyst concentration for biodiesel production.
3.5. Effect of reaction temperature It is accepted that reaction temperature plays a key role in the esterification reaction [37]. The increasing of the reaction temperature from 100 °C to 190 °C enhances the FFA conversion from 88% to 97% (Fig. 10). The esterification is an endothermic reaction and so the high temperature favors the conversion of FFA. It is reported that the reaction rate increased with temperature in the esterification of sunflower oil with methanol [42]. Therefore, the reaction temperature of 190 °C is an optimal temperature for biodiesel production over the 1S/ZSM-5 nanocatalyst.
3.6. Effect of methanol/oleic acid molar ratio The esterification of FFA with alcohol is a reversible reaction. To raise the conversion of FFA, an excess amount of methanol is used to force the reaction towards the formation of methyl esters. Fig. 11 shows that the conversion of FFA is improved by the high methanol to oleic acid ratio. The highest FFA conversion (97%) is at the molar ratio of 20, which does not change by more increasing of the ratio. It is reported that the high molar ratio of alcohol resulted in the low concentration of the nanocatalyst in the reactor and so the
14
conversion did not improve [21]. Thus, the methanol to oleic acid ratio of 20 is appropriate for biodiesel production over the 1S/ZSM-5 nanocatalyst.
3.7. Catalyst reusability One of the most important factors for scale-up and commercialization of the heterogeneous catalysts is reusability. For this reason, the stability of the 1S/ZSM-5 nanocatalyst was investigated through the several sequence experiments under the optimal reaction conditions (methanol/oleic acid molar ratio of 20:1, 5 wt.% of the nanocatalyst, reaction temperature of 190 °C, and reaction time of 8 h). After each run, the nanocatalyst was twice washed with n-hexane and then dried in the oven at a temperature of 110 °C for 12 h. Fig. 12 shows the reusability results of the 1S/ZSM-5 nanocatalyst. It can be seen that the FFA conversion decreases only 9% after the five sequence cycles, which confirms the high stability and regeneration capacity of the 1S/ZSM-5 nanocatalyst. It is worth to note that the FFA conversion of the experiment without nanocatalyst is only 9.6% in the optimal operating conditions. This result can be attributed to the high crystallinity, high surface area, mesopore structure as well as the appropriate acidity of the developed nanocatalyst. Furthermore, the effective and suitable sulfation process leads to the uniform dispersion of the sulfate groups and hinders its aggregation in the nanocatalyst which results in the high stability of the nanocatalyst through the several regeneration cycles.
15
For comparison, recent studies on the esterification of oleic acid over the zeolite catalysts are summarized in Table 3. According to this table, the developed high silica 1S/ZSM-5 nanocatalyst in this study is more active than either of these catalysts. Vieira et al. [43] reported the conversion of 100% over La2O3/SO42-/HZSM-5 catalyst but the catalyst had not the high reusability in which the conversion was 45% and 44% for the second and third runs. The main advantages of the 1S/ZSM-5 nanocatalyst are the low catalyst loading, moderate methanol to oleic acid molar ratio, and the high capacity of reusability, which can be important at an industrial point of view, since it would allow using smaller size equipment and lower energy inputs. These results can be explained by the appropriate textural and acidity properties of the high silica nanocatalyst. Furthermore, it is reported that chemical and thermal stability, as well as the yield of synthesis of the high silica ZSM5, is more than the low silica ZSM-5 [44, 45] that can improve the economy of the catalyst preparation for biodiesel production.
4. Conclusion In this study, the high silica ZSM-5 nanocatalyst was synthesized by the hydrothermal method and sulfated with aqueous H2SO4 (1 M) at reflux conditions. The nanocatalyst was applied for biodiesel production in the esterification reaction. The acid to nanocatalyst ratio of 1 led to the best nanocatalyst. The characterization results showed the high crystallinity, 16
uniform dispersion of the sulfate group, and mesopore structure for the 1S/ZSM-5 nanocatalyst. The esterification was in a batch stainless steel reactor at different reaction conditions. The optimal conditions were 5 wt.% of the nanocatalyst, methanol/oleic acid molar ratio of 20:1, T=190 °C, the reaction time of 8 h, which resulted in the highest conversion of FFA (97%). The 1S/ZSM-5 nanocatalyst had a high capacity of reusability in the esterification reaction in which the FFA conversion dropped only 9% after the five sequence cycles. The results confirmed the potential of the developed nanocatalyst for biodiesel production through the esterification process.
Acknowledgments The authors also wish to acknowledge the Iran National Science Foundation (INSF) for their support of this study.
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Figure captions Fig. 1. Effect of acid to nanocatalyst ratio on the catalytic performance. Reaction conditions: 10wt.% of the nanocatalyst, methanol/oleic acid molar ratio 10:1, T=160 °C, and reaction time of 4 h. Fig. 2. XRD pattern of the nanocatalysts: a) ZSM-5 and b) 1S/ZSM-5. Fig. 3. FTIR spectra of the ZSM-5 and 1S/ZSM-5 nanocatalysts in range of 400-4000 cm-1. Fig. 4. FE-SEM images of the nanocatalysts: a) ZSM-5 and b) 1S/ZSM-5. Fig. 5. TEM image of the nanocatalyst. Fig. 6. Adsorption/Desorption isotherms of the ZSM-5 and 1S/ZSM-5 nanocatalysts. Fig. 7. NH3-TPD pattern of the ZSM-5 and 1S/ZSM-5 nanocatalysts. Fig. 8. Effect of reaction time on the conversion of FFA. Reaction conditions: 5wt.% of the nanocatalyst, methanol/oleic acid molar ratio of 10:1, and T=160 °C. Fig. 9. Effect of the nanocatalyst concentration on the conversion of FFA. Reaction conditions: methanol/oleic acid molar ratio of 10:1, T=160 °C, and reaction time of 8 h. Fig. 10. Effect of reaction temperature on the conversion of FFA. Reaction conditions: methanol/oleic acid molar ratio of 10:1, 5 wt.% of the nanocatalyst, and reaction time of 8 h. Fig. 11. Effect of methanol/oleic acid molar ratio on the conversion of FFA. Reaction conditions: T= 190 °C, 5 wt.% of the nanocatalyst, and reaction time of 8 h. Fig. 12. Reusability of the 1S/ZSM-5 nanocatalyst. Reaction conditions: 5 wt.% of the nanocatalyst, methanol/oleic acid molar ratio of 20:1, T=190 °C, and reaction time of 8 h.
20
Fig. 1
21
ZSM-5
Intensity (a.u.)
b) SO42-/ZSM-5
a) ZSM-5
Reference patterns
ZSM-5 (Orthorhombic , 00-042-0023)
10
20
30
40
2θ (degree)
Fig. 2
22
50
60
Fig. 3
23
Fig. 4
24
Fig. 5
25
Quantity adsorbed (cm3 g-1)
ZSM-5
1S/ZSM-5
Adsorption Desorption
0.0
0.2
0.4
0.6
Relative pressure (P/P0)
Fig. 6
26
0.8
1.0
TPD Signal (a.u.)
132 °C
112 °C 321 °C
ZSM-5 309 °C
1S/ZSM-5
60
100
140
180
220
260
Temperature (°C )
Fig. 7
27
300
340
380
Fig. 8
28
Fig. 9
29
Fig. 10
30
Fig. 11
31
Fig. 12
32
Table 1 Textural data of the nanocatalysts
ZSM-5
Relative crystallinity (%) 100
1S/ZSM-5
85
Nanocatalyst
a
SBET (m2 g-1)
Vtotal (cm3 g-1)
Vmicro (cm3 g-1)
Vmeso (cm3 g-1)
356
0.19
0.13
0.06
Mean pore diameter (nm) 2.13
334
0.17
0.12
0.05
2.08
Determined by ICP-OES
33
S (wt%)a
(Si/Al)bulk ratioa
-
193.70
1.50
196.50
Table 2 NH3-TPD data of the ZSM-5 and 1S/ZSM-5 nanocatalysts Distribution and concentration of acid sites (mmol NH3 g-1)
Peak temperature (°C)
Nanocatalyst Weak
Strong
Total
Strong/Weak
TP1
TP2
ZSM-5
0.63
0.31
0.94
0.49
132
321
1S/ZSM-5
0.54
0.26
0.81
0.48
112
309
34
Table 3 Comparison of catalytic activity of the zeolite catalysts in the esterification reaction. Catalyst Citric acid/HZSM-5 HZSM-5 La2O3/SO42-/HZSM-5 HBEA HMOR HZSM-5 HY SO42-/TiO2-SiO2 USY WO3/USY 1S/ZSM-5
Reaction conditions Temperature Catalysts Molar (%) ratio (°C) 100 10 1:45 100 10 1:45 100 10 1:45 130 2 1:3 130 2 1:3 130 2 1:3 130 2 1:3 120 10 1:20 200 10 1:6 200 10 1:6 190 5 1:20
35
Time (h) 4 7 7 1 1 1 1 3 2 2 8
Conversion (%)
References
83 80 100 35 50 40 35 93.4 74 80 97
[46] [43] [43] [47] [47] [47] [47] [28] [48] [48] Present study
- The sulfated high silica nanocatalyst has the high surface area. - The acid to catalyst ratio of 1 results in the best nanocatalyst for esterification. - The 1S/ZSM-5 nanocatalyst represents the highest FFA conversion of 97%. - We report the high reusability including only 9% conversion drop after 5 runs.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: