Article pubs.acs.org/IECR
Biodiesel Production by Esterification of Oleic Acid and Transesterification of Soybean Oil Using a New Solid Acid Catalyst Comprising 12-Tungstosilicic Acid and Zeolite Hβ Nilesh Narkhede and Anjali Patel* Polyoxometalate and Catalysis Laboratory, Department of Chemistry, Faculty of Science, The M. S. University of Baroda, Vadodara, Gujarat 390002, India S Supporting Information *
ABSTRACT: A series of catalysts containing 10−40 wt % of 12-tungstosilicic acid anchored to zeolite Hβ were synthesized and characterized by different physicochemical techniques. Their catalytic activity was evaluated for esterification of free fatty acid and oleic acid as well as transesterification of soybean oil with methanol. The effects of reaction variables such as catalyst loading, methanol to acid ratio, reaction time, and temperature on the conversion were studied. The kinetic study was carried out for esterification of oleic acid, and its Arrhenius constant (A) and activation energy (Ea) were evaluated. The maximum conversion obtained for esterification of oleic acid and transesterification of soybean oil were 86% and 95%, respectively. Also the catalyst was recycled up to four cycles without any loss in the conversion.
1. INTRODUCTION Recently, fatty acid methyl esters, FAME (biodiesel), have attained great importance, and research has intensified for the use of plant/tree oil and animal fat as a source of fatty acids. In addition to known advantages, biodiesel offers the same performance and engine durability as petroleum diesel fuel and has almost the same flow properties (density and viscosity) as diesel fuel.1 The esterification reaction of long chain carboxylic acids such as oleic acid is interesting in the context of biodiesel production. Oleic acid is present in different extensions in vegetable oils like soybean, jatropha curcas, sunflower, rapeseed, pongamia, palm, and sea mango. Even though oleic acid is so important in the context of biodiesel production, not much work has been carried out for the same. Soybean oil is a vegetable oil extracted from the seeds of the soybean (Glycine max). It is one of the most widely consumed cooking oils. As a drying oil, processed soybean oil is also used as a base for printing inks (soy ink) and oil paints. The main free fatty acid (FFA) constituents of soybean oil are palmitic acid (11%), stearic acid (4%), oleic acid (23%), linoleic acid (54%), and linolenic acid (8%).2 This suggests that soybean oil possesses triglyceride esters of both saturated and unsaturated FFA and is perfect feedstock to study the catalytic behavior. Recently, supported heteropoly acids have gained tremendous interest in the synthesis of biodiesel.3−9 A literature survey shows that reports on biodiesel synthesis by esterification of oleic acid and transesterification of soybean oil over anchored heteropoly acids are few. Esterification of oleic acid has been successfully carried out over 12-tungstophosphoric acid and 12tungstosilicic acid supported on zirconia,10 Palygorskite,11 silica,12 and SBA-15.13 Guo et al.14 reported synthesis of mesoporous 12-tungstophosphoric acid-tantalum pentoxide (Ta2O5) composite catalyst, H3PW12O40/Ta2O5. The catalyst showed 76% soybean oil transesterification yield in 24 h with methanol to oil ratio of 90 at 65 °C. The same group reported © 2013 American Chemical Society
enhanced catalytic performance by introduction of hydrophobic character in the catalyst Ta2O5/SiO2-[H3PW12O40/R] (R = Me or Ph).15−17 In our previous work we have reported the synthesis of biodiesel by esterification of oleic acid over 12-tungstophosphoric acid anchored to Hβ zeolite.18 The obtained results encourage us to extend our work for 12-tungstosilicic acid, next acidic heteropoly acid in Keggin series. Here we are reporting biodiesel production by esterification of oleic acid; in addition, we are also reporting transesterification of soybean oil over 12tungstosilicic acid anchored to zeolite Hβ. A series of catalysts containing 10−40 wt % 12-tungstosilicic acid (SiW12) and Hβ were synthesized. The support and 30% loaded catalyst were characterized by various thermal and spectral techniques such as elemental analysis (EDX), thermogravimetric analysis (TGA), surface area measurement (BET method), Fourier transform infrared spectroscopy (FTIR), and powder X-ray diffraction (XRD). The surface morphology of the support and catalyst was studied by scanning electron microscopy (SEM). The acidity values were measured by NH3-TPD technique. The catalytic activity was evaluated for esterification of oleic acid with methanol. Also the catalyst was regenerated and reused up to four cycles. A detailed kinetic study was carried out, and it was found that the esterification reaction of oleic acid with methanol follows a firstorder dependency on the concentration of oleic acid and the catalyst. As an application of the above studies, detailed catalytic studies were carried out for biodiesel production from soybean oil without any pretreatment, with methanol over the present catalyst. Received: Revised: Accepted: Published: 13637
July 12, 2013 August 24, 2013 August 27, 2013 August 27, 2013 dx.doi.org/10.1021/ie402230v | Ind. Eng. Chem. Res. 2013, 52, 13637−13644
Industrial & Engineering Chemistry Research
Article
2. EXPERIMENTAL SECTION 2.1. Materials. The chemicals used were of A.R. grade. 12Tungstosilicic acid was purchased from Merck. Sodium forms of zeolite beta (β) with an Si/Al ratio of 10 was procured from zeolites and Allied Products, Bombay, India, and used as received. Oleic acid, methanol, and ammonium chloride were purchased from Merck. Soybean oil was purchased from local market. 2.2. Treatment of the Support (Hβ). The zeolite support (Naβ) was converted into NH4+ form by a conventional ion exchange method19 using a 10 wt %, 1 M NH4Cl aqueous solution. The resulting NH4+ type zeolite was further converted to H+ type by calcination in air at 550 °C for 6 h. 2.3. Synthesis of the Catalyst (SiW12 Anchored to Hβ). A series of catalysts containing 10−40% of SiW12 anchored to Hβ were synthesized by an impregnation method. One gram of Hβ was impregnated with an aqueous solution of SiW12 (0.1/ 10−0.4/40 g/mL of double distilled water) and dried at 100 °C for 10 h. The obtained materials were labeled as 10% SiW12/ Hβ, 20% SiW12/Hβ, 30% SiW12/Hβ, and 40% SiW12/Hβ, respectively. 2.4. Catalyst Characterization. Energy Dispersive X-ray Analysis (EDX) analysis was carried out using JSM 5910 LV mutual with INCA instrument for EDX-SEM. Thermogravimetric analysis (TGA) was executed on METTLER TOLEDO STAR SW 7.01 instrument. BET surface area of the samples was computed on a Micromeritics ASAP 2010 surface area analyzer. The FT-IR data were noted on Perkin-Elmer. The Raman spectra were recorded on a FT-Raman spectrophotometer Model Bruker FRA 106. The XRD patterns of the samples were recorded on PHILIPS PW-1830 at Cu Kα: 1.5417 Å. Scanning electron microscope depictions were recorded on a JEOL-SEM instrument (model-JSM-5610LV). NH3 chemisorption studies were carried out using Micromeritics Pulse Chemisorb-2705. 2.5. Catalytic Reaction. The esterification of (0.01 mol) oleic acid with (0.2 mol) methanol was executed in a 50 mL batch reactor provided with Dean−Stark apparatus, double walled air condenser, a guard tube, and magnetic stirrer. The apparatus was attached with the round-bottom flask to separate the water from the reaction mixture. The reaction mixture was refluxed for 10 h at 60 °C. The products were estimated on a gas chromatograph (Nucon-5700) by using a BP1 capillary column. All the products were recognized by comparison with authentic samples. The typical transesterification of soybean oil (5 g) with a corresponding amount of methanol was carried out in a 100 mL round reactor, provided with thermometer, mechanical stirring, and condenser. The reaction mixture was held at 65 °C for 20 h with stirring at 800 rpm in order to keep the system uniform in temperature and suspension. After the reaction is completed, the mixture was rotary evaporated at 50 °C to separate the methyl esters. The conversion of FFA to biodiesel was calculated by means of the acid value (AV) of the oil layer with the following equation
necessary to study the stability as well as leaching of SiW12 from the support. Heteropoly acids can be quantitatively characterized by the heteropoly blue color, which is observed when it reacted with a mild reducing agent such as ascorbic acid. In the present study, this method was used for determining the leaching of SiW12 from the support. One gram of catalyst with 10 mL of conductivity water was refluxed for 24 h. Then, 1 mL of the supernatant solution was treated with 10% ascorbic acid. Development of blue color was not observed, indicating that there was no leaching. The same procedure was repeated with alcohols and the filtrate of the reaction mixture after completion of reaction in order to check the presence of any leached SiW12. The absence of blue color indicates no leaching of SiW12.
3. RESULTS AND DISCUSSION The analytically computed percentages of elements for 30% SiW12/Hβ are as follows, (%): W, 17.66; Si, 26; Al, 2.5. Values acquired from EDX are as follows (%): W, 18.5; Si, 26.6; Al, 2.2. The EDX results were comparable with the expected ones. TGA-DTA curves of Hβ and 30% SiW12/Hβ are displayed in Supporting Information, Figure S1. A unique weight loss of 13−15% was observed up to 250 °C for zeolite support, which is attributed to desorption of physically adsorbed water. No further weight loss was observed beyond 250 °C which indicates zeolite Hβ retains its framework structure up to 600 °C. For the catalyst 30% SiW12/Hβ, a similar weight loss of 10−12% up to 200 °C assigned to adsorbed water was detected. A second weight loss of 1.2−1.5% was observed between 200 and 300 °C due to the loss of water of crystallization of the Keggin anion. After 480 °C gradual weight loss was observed for 30% SiW12/Hβ . TGA analysis suggests that catalyst is stable up to 480 °C. Textural properties of Hβ and 30% SiW12/Hβ like surface area, pore diameter, and pore volumes are presented in Supporting Information, Table S3. Specific surface area, pore diameter, and pore volume all strongly decreased for the catalyst relative to the support. This indicates that SiW12 occupies sites inside the zeolite framework. Also the decrease in surface area is the first evidence of chemical interaction between SiW12 and Hβ. The N2 adsorption isotherms of support and catalyst are displayed in Supporting Information, Figures S2a and S2b. Both isotherms showed a Type (IV) pattern with three stages: monolayer adsorption of nitrogen on the walls of pores at P/Po < 0.4, the part characterized by a sharp increase in adsorption due to capillary condensation in mesopores with hysteresis at P/Po = 0.4−0.8, and multilayer adsorption on the outer surface of the particles. It is seen from the Figure S2b that the pore diameter of the catalyst is decreased after anchoring SiW12 inside the zeolite framework, as anticipated. FT-IR of SiW12 (Figure 1) has four characteristic bands at 1020, 926, 878, and 779 cm−1 corresponding to W=Od asymmetrical, Si−Oa asymmetrical, W−Ob−W asymmetrical, and W−Oc−W asymmetrical, respectively, where Oa, Ob, Oc, and Od attributed to the oxygen atoms connected to silicon, to oxygen atoms bridging to two tungsten (from two different triads for Ob and from the same triad for Oc), and to the terminal oxygen W=O, respectively.20 FT-IR spectra for Hβ and 30% SiW12/Hβ (Figure 1) shows a large and broad peak appearing in the range of 1020−1090 cm−1 is due to asymmetric stretching vibration O−T−O (νasym), which is sensitive to the silicon and aluminum contents in the zeolite Hβ
⎛ AVOL ⎞ Conversion (%) = ⎜1 − ⎟ × 100 AVSO ⎠ ⎝
where AVOL is acid value of biodiesel (oil layer), and AVSO refers to acid value of soybean oil. 2.6. Leaching Test. Any leaching of the active species from the support makes the catalyst unattractive, and hence, it is 13638
dx.doi.org/10.1021/ie402230v | Ind. Eng. Chem. Res. 2013, 52, 13637−13644
Industrial & Engineering Chemistry Research
Article
Crystalline peaks for 30% SiW12/Hβ (Figure 3) suggests that zeolite framework has been retained after incorporation of
Figure 1. FT-IR spectra of (a) Hβ, (b) 30% SiW12/Hβ, and c) SiW12.
framework. A broad band between 3700 and 3200 cm−1 is assigned as hydrogen bonds of silanol groups. The typical band for SiW12, at 920 cm−1 corresponding to Si−Oa (νasym), is clearly observed in 30% SiW12/Hβ. FT-IR spectra indicate that the SiW12 anions preserve the Keggin unit even after anchoring to the support. FT-Raman spectra of SiW12 and 30% SiW12/Hβ are displayed in Figure 2. FT-Raman of SiW12 shows bands at
Figure 3. X-ray diffraction patterns of (a) Hβ and (b) 30% SiW12/Hβ.
SiW12. The reflections at 2θ = 26 and 29° indicate the presence of the crystalline phase of SiW12 onto the support.21 The catalyst shows reflections of SiW12 with very low intensity indicating a well dispersion of SiW12 inside the channels of support.22 The SEM images show that the surface morphology of 30% SiW12/Hβ (Figure 4b) is identical to that of Hβ (Figure 4a). This indicates that the framework structure of Hβ is retained after incorporation of SiW12. Thus, XRD and SEM confirm the uniform distribution of SiW12 inside the framework of zeolite Hβ. NH3-TPD is a useful tool for determining the surface acid sites of the catalyst. The concentration of acid sites was estimated by quantity of NH3 adsorbed, and the strength of the acid strength was evaluated by the desorption temperature. The NH3-TPD profile of both support (Figure 5a) and catalyst (Figure 5b) exhibits two types of acid sites, weak (
