HUSY zeolites: Influence of structural and acidity parameters

Applied Catalysis A: General 450 (2013) 114–119

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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Investigation of biodiesel production by HUSY and Ce/HUSY zeolites: Influence of structural and acidity parameters Luciana D. Borges a , Nayara N. Moura b , Andréia A. Costa b , Patrícia R.S. Braga b , José A. Dias a , Sílvia C.L. Dias a , Julio L. de Macedo a,∗ , Grace F. Ghesti a,∗ a b

Universidade de Brasília, Campus Darcy Ribeiro, Instituto de Química, Laboratório de Catálise, Caixa Postal 4478, Brasília, DF 70904-970, Brazil Universidade de Brasília, Faculdade UnB – Gama, Engenharia de Energia, Brasília, DF 72405-610, Brazil

a r t i c l e

i n f o

Article history: Received 3 August 2012 Received in revised form 25 September 2012 Accepted 6 October 2012 Available online 9 November 2012 Keywords: Biodiesel Transesterification USY zeolite Cerium Catalytic cycles

a b s t r a c t In this work, activities of HUSY and Ce/HUSY zeolites were studied in transesterification cycles of soybean oil and ethanol to produce biodiesel. The characterization of the materials was performed by FT-IR, XRD, BET method and pyridine adsorption followed by thermal analyses. TG/DTG results indicated a decrease of acid sites for both samples after each reaction cycle. However, Ce/HUSY zeolite showed a superior stabilization of acidic sites after three catalytic cycles and intermediary activation procedures. Biodiesel production exhibited high conversion levels (>96%) for both zeolites in all transesterification cycles. Surface area and pore volume measurements evidenced that cerium incorporation reduced the number of acid sites by interacting with OH groups in the micropore and external area of the zeolitic surface. This interaction resulted in an acid and structural stability, which provided a better activity (99%) than HUSY (96%). The higher conversion values obtained by zeolites showed a final product with a different distribution when compared with the traditional transesterification process. The identification of free fatty acids, diethyl and glycerol ethers in the final products and the reduction of unsaturated compounds indicated that parallel reactions also occurred in the studied systems. Nonetheless, the biofuel produced showed high ester content and did not present changes in its calorific power. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Environmental concerns and restrictive legislations have lead researches to develop clean technologies for energy production. Biodiesel, a biodegradable fuel and renewable form of energy, consists of mono alkyl esters of fatty acids that have been used as an alternative fuel in several countries [1–5]. The most widely used method to produced biodiesel is the transesterification of triacylglycerides using a homogeneous or heterogeneous catalyst [6]. Alkaline and acidic compounds are the two categories of catalysts used for the production of commercial available biodiesel [7,8]. Due to its low cost and high rate conversion at low temperature, homogeneous alkaline catalysts are commonly used in the industry for this purpose [9]. However, these catalysts cannot be recycled or regenerated and require a low content of free fatty acids (<0.5 wt.%) in the feedstock [10] to avoid soap formation. Besides, the glycerin obtained has a certain degree of impurity and the costs for its purification are very expensive [11–13].

∗ Corresponding authors. Tel.: +55 61 9277 2610; fax: +55 61 3368 6901. E-mail addresses: [email protected] (J.L. de Macedo), [email protected] (G.F. Ghesti). 0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2012.10.009

Brønsted acid catalysts can be applied in both transesterification and esterification reactions to obtain biodiesel [4–8] and have been used as an alternative to alkaline catalysts, especially in cheap feedstocks (e.g., unrefined oil, soapstock and waste cooking oil) [10]. Based on the above premises, the development of new heterogeneous acid catalysts is an appropriate solution to overcome the problems associated with homogeneous catalysis. As a consequence, the products do not contain impurities from the catalyst and the final cost of separation is reduced. These materials are easily regenerated, reused and environmental friendly [5,7]. Additionally, the degree of glycerin purity obtained is about 98%, which provides a higher market value for this byproduct [12–14]. Among several heterogeneous acid catalysts, zeolites have been used for the production of biofuels from renewable sources [13,15–20], e.g., the cracking of biomass to generate products in the diesel or gasoline fraction, using large (Y and ␤) and medium to small (ZSM-5) pore zeolites, respectively [20]. The main objective of this study was to investigate the activity of zeolitic catalysts (HUSY and Ce/HUSY) in transesterification cycles of soybean oil with ethanol and to evaluate their properties after each cycle. The materials were characterized by XRD, XRF/EDX, surface area by BET, and gas adsorption of pyridine followed by TG/DTG and FT-IR measurements.

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2. Experimental 2.1. Materials USY zeolite (ultrastable Y, CBV500) in the ammonium form (NH4 USY) was obtained from Zeolyst International with the following characteristics: SiO2 /Al2 O3 mole ratio = 2.6, surface area = 750 m2 g−1 and 0.2 wt.% of Na2 O. Commercial refined soybean oil (SoyaTM ), NaOH (Vetec, 99%), NaCl (Vetec, 99.5%) and Ce(NO3 )3 ·6H2 O (BDH Chemicals) were used as received. Ethanol (Vetec, 99.8%) was dried over molecular sieves 3A (Aldrich) for at least 24 h before the experiments. MgSO4 ·7H2 O (Vetec, 98.0%) was dried at 300 ◦ C for 4 h. Pyridine (Vetec, 99%) was distilled over CaH2 (Merck).

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(1000 rpm) and autogeneous pressure (∼20 bar). After the reaction, the system was cooled to room temperature, centrifuged to remove the catalyst, washed three times with a 5 wt.% NaCl solution, dried over anhydrous magnesium sulfate and the residual alcohol was removed in a rotary evaporator at 70 ◦ C. Each run was made under identical conditions in order to reproduce a continuous 24 h reaction. The catalysts were recycled by calcination at 550 ◦ C (10 ◦ C min−1 ) for 8 h under air just before the transesterification reactions. The biodiesel produced was analyzed by 1 H NMR [23,24], HPLC and FT-Raman.

2.5. Reaction analysis 2.2. Catalysts preparation Ce/HUSY and HUSY were prepared according to Ghesti and co-authors [21] in USY zeolite in the protonic form (HUSY) was obtained by calcination of the NH4 USY material in a muffle furnace using ambient air at 550 ◦ C (10 ◦ C min−1 ) for 8 h. Ce/HUSY zeolite with 5 wt.% of cerium was prepared by aqueous impregnation. NH4 USY was stirred in a cerium nitrate solution at 80 ◦ C until dryness and calcined under the same conditions used to obtain the HUSY zeolite. 2.3. Catalysts characterization X-ray powder diffraction (XRD) analysis was performed at 2◦ min−1 on a Bruker D8 FOCUS X-ray diffractometer with CuK␣ ( = 0.15418 nm) radiation at 40 kV and 30 mA. X-ray fluorescence (XRF) measurements were obtained under vacuum conditions on a model 720 Energy Dispersive X-Ray (EDX) spectrometer (Shimadzu) equipped with a rhodium X-ray tube at 50 (Ti-U) and 15 (Na-Sc) kV. Thermogravimetry (TG), derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC) curves were obtained on a model 2960 Simultaneous DSC-TGA (TA Instruments) from ambient temperature (∼25 ◦ C) to 1000 ◦ C at a rate of 10◦ min−1 using N2 (99.999%) as the purge gas (100 mL min−1 ). FT-IR spectra were recorded on a Nicolet 6700 spectrometer (Thermo Scientific) equipped with a DTGS detector using 128 scans and 4 cm−1 resolution. The samples were prepared as standard KBr pellets. Framework Si/Al ratio was calculated by Eq. (1) [22]: x = 3.857 − 0.00621wDR (cm−1 )

(1)

where 0.1 < x < 0.3, wDR is the zeolite specific double ring vibration mode between 570 and 600 cm−1 and x = [1 + (Si/Al)]−1 . Gas phase pyridine (Py) adsorption was made simultaneously for both zeolites. Platinum crucibles loaded with samples (∼20 mg) were placed in a shallow porcelain plate and inserted in a glass reactor adapted to a tubular furnace (Model F21135, Thermolyne). The catalysts were dehydrated in dried N2 (100 mL min−1 ) at 300 ◦ C for 1 h and cooled to 100 ◦ C. Thereafter, gaseous pyridine diluted in N2 was allowed to pass through the samples for 1 h. The temperature was held at 100 ◦ C under N2 for 2 h to remove physically adsorbed pyridine. After that, the samples were analyzed by TG/DTG and FT-IR. 2.4. Transesterification reactions The reactions were carried out in a high pressure autoclave from Berghof (HR-200) with Teflon® walls. The catalysts were activated in a muffle furnace (EDG, model 3P-S) at 300 ◦ C for 4 h. The alcohol to oil molar ratio and catalyst amount were 30:1 and 0.001 mol, respectively. The system was kept at 200 ◦ C under constant stirring

Reaction conversion values were analyzed with a HPLC system from Shimadzu (model CTO-20A) equipped with a UV–vis adsorption detector (205 nm). Separation was carried out with a Shim-Pack VP-ODS column (C-18, 250 mm length and 4.6 mm diameter). All sample injections were held constant at 10 ␮L and 1 mL min−1 of flow rate. The settings and conditions of analysis included a linear gradient of 100% of methanol to 50% of methanol and 50% of a 2-propanol/hexane mixture (4:4, v/v) for 15 min. The temperature of the column was held at 40 ◦ C and the samples were dissolved with a 2-propanol/hexane mixture (5:4, v/v). The quantitative analysis of the chromatograms was performed by integrating the area of the normalized peaks of fatty acids ethyl esters (FAEE) and triacylglycerides (TAG) [25]. The results obtained were double-checked by 1 H NMR measurements. NMR experiments were performed at 7.05 T in a Varian Mercury Plus NMR spectrometer equipped with 5 mm Varian probe using CDCl3 as solvent. 1 H (300 MHz) spectra were recorded with pulse duration of 45◦ , a recycle delay of 1.36 s, and 16 scans. The spectra were referenced to TMS (ı = 0.0 ppm). Triacylglyceride conversion into FAEE (%C) was determined by the equation proposed by Ghesti and co-authors [23] (Eq. (2)):

%C = 100



4(ITAG+FAEE − ITAG ) 4(ITAG+FAEE − ITAG ) + 6(2ITAG )

 (2)

where ITAG = integration of glyceryl methylenic protons at 4.25–4.35 ppm and ITAG+FAEE = integration of glyceryl methylenic protons and OCH2 of ethoxy protons superimposed at 4.10–4.20 ppm. FT-Raman spectra were recorded on a Bruker FRA 106/S module attached to a Bruker Equinox 55 spectrometer using a 1 cm quartz cuvette with a mirrored surface toward the scattering direction (128 scans and 4 cm−1 resolution). The laser excitation (Nd:YAG) and laser power were 1064 nm and 250 mW, respectively. The signal was detected by a Ge detector cooled with liquid N2 and the spectra recorded at room temperature. GC analyses were obtained on a GC-FID (Shimadzu, model GC2010) with a RTX® -WAX column (carbowax, polyethylene glycol) from Restek (30 m × 0.32 mm × 0.5 ␮m). FT-IR spectra were recorded on the same equipment and conditions described in Section 2.3, except that samples were analyzed in NaCl windows. The biodiesel assay was performed according to ABNT (Brazilian Technical Standards Association) method for total ester content (NBR 15764) and ASTM (American Society for Testing and Materials) method for free and total glycerin, mono-, di- and triacylglycerides content (D 6584).

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Table 1 Textural properties of parent HUSY and Ce/HUSY zeolites and after the catalytic cycles. Parameters

Parent samples

Surface area (m2 g−1 )a Micropore area (m2 g−1 )b External area (m2 g−1 )b Pore volume (cm3 g−1 )c Micropore volume (cm3 g−1 )b Average pore size (nm)a a b c

After catalytic cycles

HUSY

Ce/HUSY

HUSY

Ce/HUSY

635 571 64 0.36 0.26 2.28

565 530 35 0.31 0.25 2.23

442 440 2 0.26 0.20 2.39

452 400 53 0.31 0.19 2.72

Determined by BET method. Determined by t-plot method. Determined from single point desorption isotherm at P/P0 = 0.97.

Table 2 Conversion results of triacylglyceride transesterification to produce FAEE and the number of acid sites for the three catalytic cycles with HUSY and Ce/HUSY zeolites. Cycle

n (mmol g−1 )a HUSY

Ce/HUSY

0 1 2 3

0.88 0.55 0.50 0.48

a

Determined by TG/DTG analyses. Determined by HPLC after 24 h of reaction. Initial activity determined from the data of Fig. 2.

b c

TOF (mmol mmolzeo −1 min−1 )c

Conversion (%)b

0.78 0.69 0.62 0.54

HUSY – 99.7 97.9 96.4

3. Results and discussion 3.1. Structural and textural analysis of the catalysts The structural preservation of HUSY and Ce/HUSY zeolites after activation and/or impregnation procedures were verified by XRD (Fig. 1d and c, respectively). No significant changes were observed in the intensities of the peaks for both samples, except for an increased background in the Ce/HUSY pattern between 5 and 15◦ . The conditions used to activate the materials, i.e., 550 ◦ C for 8 h, were sufficient to completely convert the cerium nitrate precursor to cerium oxide (Fig. 1a). The absence of cerium oxide peaks in the Ce/HUSY pattern (Fig. 1c) can be related to a high dispersion of CeO2 particles on the zeolite surface [21,26]. For comparison, a mechanical mixture between CeO2 and NH4 USY at the same weight proportion (5 wt.%) was carefully grounded and activated under the same conditions used Ce/HUSY. The XRD pattern of the CeO2 /HUSY mixture showed peaks related to CeO2 at 28.7, 33.3, and 47.7◦ (peaks marked with a * in Fig. 1a).

Ce/HUSY – 99.8 99.6 99.5

HUSY

Ce/HUSY −2

6.8 × 10 – – –

3.5 × 10−2 – – –

The textural properties of the materials measure by N2 adsorption/desorption revealed that all parameters were reduced after cerium impregnation (Table 1). The values obtained for micropore and external areas evidenced that cerium species were deposited on the inner micropores and on the external surface of the zeolite. In a previous work [21], we have shown by DRIFTS measurements that cerium impregnation clearly changes the hydroxyl group region of HUSY spectrum. In addition, micropore volume values did not change after impregnation, suggesting that cerium particles were very well dispersed on the zeolite surface, as observed by XRD. These analyses also showed that HUSY and Ce/HUSY zeolites have an average pore diameter of 2.28 and 2.23 nm, which can be attributed to the formation of mesopores during the ultrastabilization process of zeolite Y. This mesoporosity can reduce mass transfer resistances and accessibility to the active sites. However, the external surface area and sites can also play an important role on the materials activities.

3.2. Activity and acidity studies of the catalysts

Fig. 1. XRD patterns of the samples after impregnation and/or thermal activation procedure: (a) CeO2 , (b) mechanical mixture between CeO2 (5 wt.%) and NH4 USY (*indicates CeO2 peaks), (c) Ce/HUSY and (d) HUSY.

A full acidity characterization of HUSY and Ce/HUSY materials has been reported previously [21]. It was shown that HUSY zeolite exhibits two strong Brønsted sites and that cerium species interacted with these sites and generated Lewis sites. TG/DTG curves after pyridine adsorption were used to analyze the total number of acid sites of both catalysts. The acidity analysis of HUSY and Ce/HUSY showed the number of sites of 0.88 and 0.78 mmol g−1 , respectively (Table 2). DRIFTS study of the hydroxyl region [21] also suggested that cerium species could be located at ion exchange positions. The impregnation of cerium introduced a new band at 3663 cm−1 and a shoulder at about 3540 cm−1 (Figure S1) that can be assigned to OH groups associated with Ce(III) species [27–31]. Moreira and co-workers [31] and Garcia and co-workers [29] evidenced that both CeO2 particles and Ce(III) exchanged species were present in Y zeolites impregnated with cerium. The stability achieved by cerium impregnation caused an acidity reduction due to the interaction of CeO2 particles and/or cerium ions with Brønsted sites.

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Table 3 Si/Al framework ratio determined by FT-IR for the parent HUSY and Ce/HUSY zeolites and after the catalytic cycles. Sample

HUSY Ce/HUSY a b c

Ce content (%)a

0.0 4.2

(Si/Al)total b

2.2 ± 0.1 2.2 ± 0.1

(Si/Al)framework c Parent samples

After catalytic cycles

5.1 4.0

5.8 7.1

Determined by XRF/EDX. Determined by ICP-AES [21]. Determined by FT-IR.

Table 2 shows the conversion results for HUSY and Ce/HUSY zeolites after the 24 h transesterification reaction cycles. Both zeolites exhibited high conversion levels of triacylglycerides into FAEE (>96%) in all transesterification cycles, the blank reaction was 49.7%. HUSY zeolite showed a small decrease of conversion from the first (99.7%) to the third (96.4%) cycle, but the values obtained for Ce/HUSY zeolite were practically kept constant throughout the three cycles (99.8, 99.6 and 99.5% for the first, second and third cycle, respectively). To further investigate the effect of the transesterification reaction on the materials acidity and vice versa, TG/DTG analyses of pyridine adsorption were conduct before and after the reaction cycles. The results showed a reduction of acid sites after each reaction cycle for both samples (Table 2), but Ce/HUSY zeolite showed a greater stability of the sites after catalytic cycles and intermediate heat treatments, when compared to HUSY. This stability can be attributed to the homogeneous distribution of cerium species on the zeolitic surface [21,27,29]. In fact, the framework Si/Al ratio of Ce/HUSY zeolite showed a small increase from 3.6 to 4.0 (NH4 USY and Ce/HUSY, respectively) after the calcination procedure (Table 3), while for HUSY zeolite an increase from 3.6 to 5.2 was observed. This result indicates that cerium impregnation prevents the removal of Al atoms from the zeolite matrix. The effect of time on the activity of HUSY and Ce/HUSY was also studied in this work. Fig. 2 shows the conversion of triacylglyceride into FAEE during 24 h for both zeolites. The initial activity of each zeolite was determined as turnover frequency (TOF) by fitting the data from Fig. 2 (mmol versus min) to a third-order polynomial function, differentiating and evaluating at t = 0 (Table 2). The values obtained are in agreement with the first 4 h of reaction, in which HUSY zeolite showed higher conversion values than Ce/HUSY, which can be attributed to the higher amount of acid sites on its structure, see Table 2. Canakci and Van Gerpen [32] found that the reaction rate of transesterification reactions was dependent of the catalyst concentration, i.e., an increase in the number of active

Fig. 2. Kinetic curve of triacylglyceride conversion into FAEE for HUSY and Ce/HUSY zeolites.

sites. In the interval of 6–8 h of reaction, both catalysts presented similar values, but after 12 h it was observed that Ce/HUSY zeolite showed a conversion rate that exceed HUSY. These later results can be explained by the stabilization of the zeolite acidity found by pyridine adsorption after cerium impregnation. The stabilization of the acid sites is related to the maintenance of the zeolite structure by cerium species well distributed on the surface. After the final catalytic cycle, a loss of crystallinity of 18.2% for Ce/HUSY and 35.9% for HUSY was observed by XRD [27,29]. Moreover, textural analyses after the final cycle (Table 1) showed that Ce/HUSY zeolite exhibited a smaller loss of specific surface area and pore volume (reduction of 20.0% and 2.4%, respectively) than HUSY zeolite (reduction of 30.4% and 22.7%, respectively). Nevertheless, the analysis of the other parameters evidenced that the reduction of micropore area and micropore volume and the increase of pore size were greater for Ce/HUSY (24.7, 24.0 and 21.9%, respectively) than HUSY (22.9, 23.1 and 4.5%, respectively). These later results might be related to the leaching of cerium from the external surface and micropores of the zeolite. Indeed, XRF/EDX analyses verified that the initial cerium content after the impregnation procedure (4.2 wt.%) was no longer observed in the Ce/HUSY zeolite after the third catalytic cycle, indicating a leaching process along the reactions. Sajith and co-workers [33] showed that cerium oxide nanoparticles improve the physicochemical properties of the biodiesel, but XRF/EDX analyses of the biodiesel produced by the Ce/HUSY zeolite were made and no metal content was observed. Rare earth elements are widely used to modify zeolite properties by enhancing its catalytic activity and thermal stability [27]. Our research group is currently investigating the activity of homogeneous cerium(III) chloride hydrated into esterification and transesterification reactions. It was observed that even at low concentration, homogeneous cerium(III) species darkened the fatty acid ethyl esters (FAEE) at high temperatures and longer reaction times. Since the FAEE obtained by the Ce/HUSY zeolite was clear, cerium leaching might occur as CeOx species. The transesterification with CeO2 as catalyst was not active in the reaction, but some polymerization was observed. Thitsartarn and Kawi [34] evidenced that CeO2 has low basicity using the Hammett indicator method. The effect of cerium leaching from the zeolitic surface after the catalytic cycles could also be studied by analyzing the infrared region of the double ring vibration mode (570–600 cm−1 ). This band is very sensitive to any change in Y zeolite structure and can be used to calculate framework Si/Al ratios [22]. The framework Si/Al ratio of both zeolites increased after the third reaction cycle (Table 3). The value obtained for Ce/HUSY (7.1) indicates an additional dealumination of the zeolite structure after cerium leaching, which explains the loss of crystallinity, textural properties and the number of sites. Cerium species are expected to be located inside the sodalite cages and the leaching process breaks the bonds between cerium species and the hexagonal prism tetrahedra leading to the partial destruction of the crystalline structure. Ce/HUSY conversion values were compared to results obtained in an earlier work with HUSY zeolite modified with barium and strontium (15 wt.%) [35]. Ce/HUSY showed higher conversions in

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Fig. 3. HPLC chromatograms for HUSY zeolite reaction cycles.

the three catalytic cycles than Ba/HUSY (97, 97 and 98%, respectively) and Sr/HUSY (69, 71 and 71%, respectively). Nevertheless, the activity differences observed between the basic zeolites was attributed to a higher number of basic sites found for Ba/HUSY and a reduced interaction of barium species with HUSY Brønsted sites. 3.3. Biodiesel characterization The biodiesel produced by the transesterification cycles was evaluated by HPLC [25], GC-FID, 1 H NMR [23], FT-IR and FT-Raman [36]. The HPLC profile (Figs. 3 and 4) of the biodiesel obtained from both catalysts in the three reaction cycles exhibited a high ester production (Table 2) and low content of triacylglycerides (TAG), diacylglycerides (DAG) and monoacylglycerides (MAG). It also indicated that the use of acidic zeolites produces a biofuel with different ester distribution, when compared to acidic and alkaline homogeneous catalysts. The HPLC profile of the biodiesel obtained by the zeolites showed an additional peak at ∼7.5 min and the intensity of the peak at ∼6.5 min in the 1st catalytic cycle is significantly inferior when compared to the biodiesel produced by NaOH. FT-Raman spectroscopy is a powerful technique for the nondestructive analysis of biodiesel samples [36]. The differences observed in the Raman spectra of ethyl esters obtained by HUSY zeolite and via conventional alkaline transesterification (Fig. 5) evidenced the intensity decrease of bands associated to the C C bonds (arrows in Fig. 5). This reduction can be attributed to the interaction between the oil phase and the acidic sites of the catalysts at high temperatures. It is well known that molecular oxygen reacts under mild conditions with the double bonds of vegetable oil [19]. Therefore, the

Fig. 4. HPLC chromatograms for Ce/HUSY zeolite reaction cycles.

Fig. 5. FT-Raman spectra of biodiesel produced by conventional method (dotted line) and catalyzed by HUSY zeolite after 1st cycle (solid line).

reaction conditions used in this work (200 ◦ C and ∼20 bar) and in the presence of the zeolite catalysts, the above reaction may have been favored. These changes could be attributed to the most acidic sites of the catalysts at elevated temperatures, since the intensity of the peak at ∼7.5 min in the HPLC profile decreased in the 2nd and 3rd cycles and the peak at ∼6.5 min increases. To further investigate the transesterification process catalyzed by HUSY and Ce/HUSY, the biodiesel produced was additionally studied by FT-IR, GC-FID and 1 H NMR, which showed the presence of free fatty acids, diethyl and glycerol ethers. The identification of these products suggests that the partial cracking of the vegetable oil, the condensation of ethanol and the etherification of glycerol are also parallel reactions of the process. The biodiesel FTIR spectra revealed a shoulder at 1715 cm−1 in the C O region of ethyl esters (1739 cm−1 ) that is characteristic of free fatty acids (Figure S2) [37]. The diethyl and glycerol ethers 1 H NMR spectra were calculated (SpecManager, version 10.08 from ACD/Labs) and compared with the biodiesel obtained from Ce/HUSY zeolite. The signals of the two methylene groups of diethyl ether were identified at ı = 3.41 ppm (quartet, 4H). Signals at ı = 3.9 (multiplet), 3.8 (duplet), 3.7 (quartet), and 3.6 ppm (multiplet) can be associate with mono- and di-ethers of glycerol. However, an accurate identification of these products is complicated due to superimposed signals. This result suggests that the glycerol produced during the transesterification also interacts with the zeolites. GC-FID analyses were used to verify if the zeolite was transforming glycerol into acrolein and water [38]. The profile of the chromatograms of the biofuel produced before the purification steps was compared against a standard acrolein solution and the peak of acrolein was not observed. The formation of the abovementioned products can be attributed to the reaction conditions used [39]. The heat capacity of the biodiesel was evaluated by differential scanning calorimetry (DSC), Fig. 6. The biodiesel produced by the conventional alkaline method presented a value of 48.69 kJ mol−1 (159.1 J g−1 ) and boiling point of 255 ◦ C, while the biodiesel obtained by heterogeneous acid catalysis showed 48.67 kJ mol−1 (159.0 J g−1 ) and the same temperature of boiling point. Thus, it is possible to conclude that the parallel reactions in catalytic process did not change its calorific power. Finally, the biodiesel produced by Ce/HUSY (8 and 12 h of reaction) and analyzed by ABNT and ASTM methods (Table 4) showed high ester content (86.6 and 90.5%, respectively) and the reduction of free and total glycerol from 8 to 12 h. Both samples presented values for ester and total glycerol above the establish limits for standard biodiesel (96.5 and 0.25%, respectively), which can be associated with the parallel reactions observed during the transesterification reaction with the zeolites. The reaction for longer

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(LAPREN/UnB) for HPLC analyses, Mr. Eder Márcio Silva de Oliveira (CPT/ANP) for biodiesel assays, and financial support of DEX/UnB, DPP/PPGQ/IQ/UnB, MCT/CNPq, FINEP-CTPetro, FINEP-CTInfra, FAPDF, FINATEC and PETROBRAS. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apcata. 2012.10.009. References

Fig. 6. DSC curves of biodiesel produced by conventional method (a) and catalyzed by HUSY zeolite after 1st cycle (b). Table 4 Biodiesel assay for total ester, free and total glycerin, mono-, di- and triacylglycerides. Parameter

Biodiesel samples Ce/HUSY – 8 h

Total ester (wt.%)a Free glycerin (wt.%)b Total glycerin (wt.%)b Monoacylglycerides (wt.%)b Diacylglycerides (wt.%)b Triacylglycerides (wt.%)b a b

86.6 0.030 2.70 4.83 7.91 2.350

± ± ± ± ± ±

2.5 0.006 0.03 0.32 0.17 0.23

Ce/HUSY – 12 h 90.5 0.018 2.14 5.53 4.63 0.032

± ± ± ± ± ±

2.5 0.006 0.03 0.32 0.17 0.23

Determined by ABNT method NBR 15764. Determined by ASTM method D 6584.

periods of time increases the ester content and reduces mono-, di-, and triacylglycerides, as observed for the 24 h reaction, but to avoid parallel products a different reaction condition has to be used (e.g., lower temperature, inert atmosphere, smaller oil to ethanol ratios, etc.). 4. Conclusion The results showed that the incorporation of cerium reduced the number of acid sites by interacting with the OH groups of the zeolite, both in the micropore and external surface areas. This interaction resulted in an enhanced structural and acidic stability than HUSY, which kept its activity above 99% during the reaction cycles. The biodiesel produced using these zeolites presented a distinct profile of the products when compared to conventional transesterification. These products were attributed to side reactions occurring during the transesterification process and were identified as free fatty acids, diethyl ether, mono- and/or di-ethers of glycerol and products derived from the partial oxidation of unsaturated compounds. Nevertheless, the biofuel produced showed high ester content and no change was observed in its calorific power. These catalysts showed not only a high ester production, but also that can be reused, which makes this process environmentally desirable. Acknowledgements The authors are grateful to Prof. Inês Sabioni Resck for NMR measurements at LRMN/UnB, Dr. Melquizedeque Bento Alves

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