Heterogeneous and efficient transesterification of Jatropha curcas L. seed oil to produce biodiesel catalysed by nano-sized SO42−/TiO2

Developing high-efficiency hetero-catalysts for transesterification reaction is of great importance in the production of biodiesel from Jatropha curcas L. seed oil (JO). Here, we synthesized a series of sulfated TiO2 by treating with varying H2SO4 concentration (SO42−/TiO2) and TiO2 catalysts and applied to the transesterification of JO. Furthermore, these heterostructures were characterized by many characterization methods including XRD, FT-IR, N2-adsorption, SEM, TEM, TG, py-IR and NH3-TPD, and their catalytic performance was investigated under various operating conditions. The results reveal that both the Brønsted and Lewis acid sites are presented in the SO42−/TiO2 catalysts, while only Lewis-type sites are observed in the TiO2 catalyst. And the acid intensity, surface area and mesoporous volume of catalysts are improved obviously after treating TiO2 with sulfuric acid. Then the SO42−/TiO2 catalysts exhibit much higher catalytic activity than TiO2 catalyst, which is attributed to the larger surface area and mesoporous volume and stronger acidity. Furthermore, the reusability behaviour of 1.5 SO42−/TiO2 catalyst in the transesterification of JO was also studied.


Introduction
The consumption of finite fossil fuels (e.g. petroleum, coal and natural gas) constantly and rapidly increases due to the course of industrialization and population growth. acid methyl ester standards were purchased from Nu-Chek-Prep, Inc., USA. Ammonia solution (28 wt% in H 2 O) was purchased from Chinasun Specialty Products Co. Ltd, China. Sulfuric acid (H 2 SO 4 ) and hexane were purchased from Tianjin Yongda Chemical Reagent Co. Ltd, China. Methanol and petroleum ether were purchased from Tianjin Damao Chemical Reagent Factory, China.

Synthesis of catalyst samples
The TiO 2 was prepared according to the literature with modification and carried out in a 500 ml threeneck round-bottom flask with mechanical stirring and the following procedure [40]. An amount of TiCl 4 (10 ml) was slowly added to deionized water (200 ml) and cooled by ice water. Ammonia solution was then added dropwise to the above mixture to a final pH around 8.0 9.0. The precipitate was filtered, washed with deionized water until no Cl 2 (tested by 1.0 mol l 21 AgNO 3 solution) and dried at 393 K for 12 h. The obtained solids (Ti(OH) 4 ) were powdered below 100 mesh, then calcined at 823 K for 3 h in air to obtain the TiO 2 sample.
The SO 2À 4 /TiO 2 was prepared by the impregnation of the above Ti(OH) 4

Catalyst characterization
X-ray powder patterns (XRD) were obtained with a RIGAKU Ultima IV diffractometer using Cu Ka radiation. Nitrogen physisorption (N 2 -adsorption) measurements were carried out on a Micromeritics ASAP 2020 M apparatus at the temperature of liquid nitrogen (77 K). The specific surface area and pore volume were calculated according to the Brunauer -Emmett-Teller (BET) and Barrett-Joyner -Halenda (BJH) methods, respectively. Fourier-transform infrared (FT-IR) spectra of the catalyst samples were recorded on a Thermo Nicolet iS5 FT-IR spectrometer (KBr pressed flake). The acidity of the catalysts was measured using temperature-programmed desorption of ammonia (NH 3 -TPD) on a Micromeritics ASAP 2920 instrument. The acid properties of the catalysts were also investigated by pyridine FT-IR (Py-IR), performed on a Bruker TENSOR 27 instrument equipped with an in situ reactor cell. The samples were pre-treated firstly, the system was then degassed and evacuated at designated temperature, and the IR spectra were recorded. Scanning electron microscopy (SEM) images were recorded on a SUPRA55 apparatus. Transmission electron microscopy (TEM) images were obtained on a JEM-2100F microscope. Thermo-gravimetric analyses (TG) of the samples were carried out on an SDT Q600 apparatus from 298 to 1073 K with a heating rate of 10 K min 21 in air (25 ml min 21 ).

Preparation of JO
Jatropha curcas seeds were put in a desiccation oven and dried at 393 K for 24 h. They were then chopped using a grinder to a size of 80 meshes. Extracting the Jatropha curcas L. seed oil (JO) with solvents was carried out in a 500 ml three-necked round-bottom flask fitted with a reflux condenser under vigorous stirring. In a case, 40 g Jatropha curcas seeds powder and 250 ml of hexane (solvent) were used. The extraction process was heated at 343 K for 5 h. A rotavapor was used at 348 K for 25 min in order to remove the solvent. Finally, the JO was dried at 333 K until its weight was consistent. And the acid value and FFA content of the JO are 15.30 mg KOH g 21 oil and 7.65% w/w, respectively. Methyl heptadecanoate was applied as an internal standard [41]. After transesterification of Jatropha curcas L. seed oil, the composition of FAMEs was determined by GC analysis, which was as follows (wt%): methyl palmitate ¼ 14.6%, methyl stearate ¼ 7.6%, methyl oleate ¼ 44.5%, methyl linoleate ¼ 32.5%, other products ¼ 0.8%, which is very close to that result reported in the reference [42]. And the detail GC graph of biodiesel from JO is shown in figure 1. Biodiesel (FAMEs) was the main target product. X JO , S MP , S MS , S MO , S ML and S Biodiesel denote the conversion of JO (JO conv.) and the selectivity of methyl palmitate, methyl stearate, methyl oleate, methyl linoleate and biodiesel (biodiesel sel.), respectively. These key parameters were calculated as follows:

Transesterification of JO
where n 0 JO and n JO denote the initial and final molar content of JO, respectively. n MP , n MS , n MO , n ML and n Other represent the mole content of methyl palmitate, methyl stearate, methyl oleate, methyl linoleate and other products, respectively. In all the experiments, the amounts of other products are too low to be measured, so they were not shown in tables and figures.    [43]. Additionally, the similar characteristic peaks at 2u of 25.38, 37.98, 48.08, 54.08, 55.18 and 62.78 are observed in the xSO 2À 4 /TiO 2 samples, indicating that the structures of these samples have no differences [40,44]. The N 2 adsorption -desorption isotherm and NH 3 -TPD measurements of all samples are shown in figure 3 and table 1, respectively. The adsorption and desorption isotherms of TiO 2 and xSO 2À 4 /TiO 2 samples show type IV behaviour with the typical hysteresis loop, suggesting narrow slit-shaped pores that are generally associated with plate-like particles. And these existing pores are ascribed to the aggregation of single crystals [43]. In addition, the results of the specific surface area and pore volume calculated with the BET and BJH method, respectively, indicated that the TiO 2 and xSO 2À 4 /TiO 2 samples are mesoporous structures without any micropore. However, the xSO 2À 4 /TiO 2 samples exhibit much larger surface area and mesopore volume than TiO 2 sample, and the surface area increases with the impregnation sulfuric acid content, which is in agreement with the results obtained in the previous reference [45]. This may be because the TiO 2 particles are partially dissolved by H 2 SO 4 , which would reduce the particle size and lead to the increase in BET area [46]. Similarly, the mesopore volume, which is the hole between the crystal particles, will increase when the catalyst particle size decreases in these xSO 2À 4 /TiO 2 samples, in good agreement with the results obtained by nitrogen sorption (figure 3).
The FT-IR spectra of all samples are given in figure 4. The broad peak at about 500 cm 21 belongs to Ti-O stretching vibration. As for the xSO 2À 4 /TiO 2 samples, the absorbance bands at 1177 cm 21 demonstrate that the SO 2À 4 -dopant might coordinate with TiO 2 in the network [47,48]. And these samples exhibit obvious absorption at 1640 cm 21 , corresponding to the stretching vibration of the hydroxyl group on the surface [49]. As we know, although TiO 2 itself possesses the acidity, the acidity of TiO 2 can be further enhanced by modifying its surface with sulfate groups [50]. The NH 3 -TPD spectra of TiO 2 and SO 2À 4 /TiO 2 with different impregnation sulfuric acid concentration samples are shown in figure 5 and table 1. The peaks shown in these profiles are assigned to the desorption of NH 3 from the acid sites of the sample surface. Desorption temperature is related to the acid strength of xSO 2À 4 /TiO 2 , and the higher desorption temperature, the stronger acid strength [51]. There are three principal desorption peaks in the range of 390-450, 530-590 and 720-790 K. And these peaks correspond to the weak acid sites, middle acid sites and strong acid sites, respectively [52]. It can be obviously seen that TiO 2 contains some weak acids and middle acids. The acidity of xSO 2À 4 /TiO 2 is stronger than that of TiO 2 after treatment of sulfuric acid. And the laws remain static that more middle acids and strong acids between SO 2À 4 and TiO 2 form with the increase in sulfuric acid concentration. And these stronger acids can promote the transesterification reactions [3,4]. The acid  type of the solid samples is further determined by using pyridine-IR technology ( figure 6). The peaks around 1542 and 1445 cm 21 can be assigned to the adsorption of coordinated pyridine in Brønsted (B) acid sites and Lewis (L) acid sites, respectively, and the peak at 1490 cm 21 is ascribed to a combination of B and L acid sites [53]. The results show all the samples contain an amount of L acid sites, which may result from the coordination between SO 2À 4 and TiO 2 in the network [48]. However, the Brønsted acidic sites are formed in the xSO 2À 4 /TiO 2 but are not presented in TiO 2 sample, attributing to the surface adsorption between SO 2À 4 species and TiO 2 . As shown in figure 7, in one SO 2À 4 /TiO 2 unit, two oxygen atoms from S -O bonds are bonded to Ti atoms in addition to the coordination of an S ¼ O group with a Ti atom, and the acidic proton is derived from the surface hydroxyl group of TiO 2 induced by the sulfate group. This acidic proton can be released easily owing to three S -O -Ti bonds linking one SO 2À 4 group with the TiO 2 matrix, which results in the Brønsted acidic strength of the SO 2À 4 /TiO 2 [4]. That is to say, the sulfated TiO 2 samples exhibit stronger acidity than pure TiO 2 , which is in good agreement to the experimental results of NH 3 -TPD. And the stronger acidity of the solid acid catalysts could promote the transesterification reactions [4]. The texture and morphology of catalysts are very important parameters and may influence the catalytic activity. The SEM images of pure TiO 2 and xSO 2À 4 /TiO 2 are shown in figure 8. As shown in figure 8a, the SEM image of the TiO 2 depicts that the particles are in the form of aggregates and the surface is irregular. The xSO 2À 4 /TiO 2 samples show the particles are not only massively agglomerated, but also the particle size of samples is smaller than that of TiO 2 , which corresponds with the report of reference [54]. These results are further confirmed by TEM images (figure 9). Figure 9 shows that all samples have similar particle morphology. And the nanoparticle size of the TiO 2 and xSO 2À 4 /TiO 2 is around 20 nm ( figure 9a) and 15 nm (figure 9b-e), respectively, which is consistent with previous report [54].     4 shows the highest catalytic activity and the JO conversion is 81.4%, which is attributed to a strong acidity of the homogeneous catalyst that has a full contact with the reactants and promotes the conversion of JO. However, this process shows some drawbacks of environment unfriendly, limitations of separating and difficult recycling use of catalysts. Compared with the liquid catalyst, TiO 2 catalyst exhibits the lowest catalytic activity and the JO conversion is only 24.8%. However, the catalytic activity of the xSO 2À 4 /TiO 2 catalysts is enhanced remarkably and the conversion of JO reaches up to above 70.9%, which is three times higher than pure TiO 2 . The reason is that the acidity and surface area of xSO 2À 4 /TiO 2 catalysts are much higher than that of TiO 2 (figure 5 and table 1), which can promote the transesterification of JO. Furthermore, the 1.5SO 2À 4 /TiO 2 catalyst reveals the highest JO conversion (73.1%) and others perform a little lower than it, because these catalysts have similar structure, surface, texture and acidic properties. Additionally, the high biodiesel selectivity (above 98%) is obtained and FAMEs composition is almost unchanged in the reaction over all catalysts. According to Scheme 1, it will see the amount and composition proportion of the various FAME products produced are as the same as the ones of JO, although the transesterification reaction is catalysed over different catalysts.

Effect of various parameters over 1.5SO 2À 4 /TiO 2 catalyst
In order to further study systematically, the catalytic performance of the 1.5SO 2À 4 /TiO 2 catalyst on the transesterification of JO, the various reaction parameters (reaction time, n CH 3 OH : n JO , catalyst content and reaction temperature) were investigated carefully. And these researches are also very useful in the production of biodiesel in industry.

Effects of reaction time
The transesterification of JO is carried out for 8, 12, 16 and 24 h, and the results are given in figure 10. The conversion of JO increases remarkably with the increase in reaction time within 16 h. And the JO conversion increases slowly from 70.3% to 73.1% with further increasing the reaction time to 24 h. This result may be ascribed to the JO molecules being adsorbed and reacted gradually on the active sites of the solid acid catalyst along with the transesterification reaction progressed. In addition, as the reaction time increases, the selectivity to biodiesel is relatively stable and remains above 99.0%, and the composition of FAMEs is also almost unchanged (methyl palmitate ¼ 15%, methyl stearate ¼ 8%, methyl oleate ¼ 44%, methyl linoleate ¼ 31%). Thus, the time of 24 h is selected as the reaction time in the following transesterification process.

Effects of n CH 3 OH : n JO
The appropriate methanol content is a crucial parameter in JO transesterification that could affect the reaction rate and conversion significantly. The effect of the different molar ratio of methanol and JO from 4.5 to 27 on biodiesel production was investigated and the results are shown in figure 11. The JO conversion increases with the increase in the n CH 3 OH : n JO and reaches the maximum (84.6%) at the n CH 3 OH : n JO of 18 then begins to decrease. It is well known that the addition of methanol into the reaction mixture during the stage of transesterification could enhance the JO conversion and biodiesel selectivity [21]. However, excessive methanol results in the JO conversion decrease, which may be due to the decline of the contact probability between catalyst and reactants. This is also in good agreement with the literature reported earlier [46]. Clearly, the selectivity to biodiesel (greater than 98.8%) and FAMEs composition is almost unchanged with the rising of n CH 3 OH : n JO .

Effects of catalyst content
The results that the transesterification of JO is carried out in several different catalyst amounts (0.06, 0.09, 0.12 and 0.15 g) are shown in figure 12. As catalyst amount is increased from 0.06 to 0.12 g, the conversion of JO increases remarkably from 45.5% to 73.1%, while the X JO almost remains constant even though increasing the loading amount of 1.5SO 2À 4 /TiO 2 . This can be because the higher amounts of available catalyst allow more JO molecules to be absorbed to the catalytic active centre. Additionally, both the S biodiesel and fatty acids composition have no obvious change with the increasing catalyst content. Thus, 0.12 g is the best 1.5SO 2À 4 /TiO 2 catalyst content for biodiesel synthesis from JO considering the low production cost.

Effects of reaction temperature
Reaction temperature is a key factor that influences biodiesel production. The effect of reaction temperature on JO conversion and selectivity to biodiesel is shown in figure 13. The conversion of JO increases remarkably from 23.3% to 85.8% with the increase in reaction temperature from 353 to 413 K. It is generally known that the catalytic activity is improved when the reaction temperature is risen, attributing to the formation of more active species in the catalyst at a higher temperature. Meanwhile, the selectivity of biodiesel remains unchanged (nearly 99.0%) as the temperature increases, and the composition and content of all fatty acids are also not found obviously different.

Possible transesterification reaction mechanism over SO 2À 4 /TiO 2 catalyst
The results of NH 3 -TPD and Py-IR spectra show that the SO 2À 4 /TiO 2 catalyst possesses many acid sites (figures 5 and 6), and it is well known that both Brønsted and Lewis acid sites are presented. Furthermore, the Brønsted acidity of the catalyst can be further enhanced by modifying its surface with sulfate groups. Furthermore, based on previous research reports on the principle of transesterification reaction catalysed by sulfated metal oxides [55,56], the possible mechanism for the transesterification of triglyceride using SO 2À 4 /TiO 2 catalyst is provided in figure 14. In order to further explore the causes of the descending activity of the used catalyst in JO transesterification reaction. The fresh and used 1.5SO 2À 4 /TiO 2 catalysts are characterized by the methods of N 2 -adsorption, NH 3 -TPD, XRD and TG. The XRD diffraction patterns have confirmed that the structure destruction of used 1.5SO 2À 4 /TiO 2 did not occur ( figure 15a). However, as shown in table 1, compared with the fresh catalyst, the S BET and V Mesopore of used catalyst decrease obviously from 76.4 m 2 g 21 and 0.26 cm 3 g 21 to 62.1 m 2 g 21 and 0.18 cm 3 g 21 , respectively. And the TG analyses also show that the weight loss of the used catalyst (3.0%) is much greater than that of the fresh catalyst (0.6%), indicating that some carbonaceous residues (cokes) are formed on the used catalyst (figure 15b). It is well known that deposits of cokes on the catalyst surface can cover the active centre and block the mesopore channels [57,58]. So the catalytic activity of the used catalyst decreases significantly. Additionally, the regenerated 1.5SO 2À 4 /TiO 2 is obtained by calcining at high temperature, which successfully removes these cokes on the surface of the used 1.5SO 2À 4 /TiO 2 , being verified sufficiently by the results of TG analysis. However, compared with the used catalyst (27.5%), the JO conversion merely rises slightly over a regenerated catalyst (31.8%). In other words, the deposition of coke is unlikely to be a major reason for the deactivation of SO 2À 4 /TiO 2 catalyst in this reaction. On the other hand, the result of NH 3 -TPD analysis (table 1) shows that the total NH 3 desorbed of the used 1.5SO 2À 4 /TiO 2 (0.24 mmol g 21 ) is much less than that of the fresh 1.5SO 2À 4 /TiO 2 (0.39 mmol g 21 ), which means the acidity and activity of the catalyst reduces significantly after being used in transesterification reaction. The reason is that conventional preparation of SO 2À 4 /TiO 2 is via a post-synthesis grafting method, which leads to relatively weak interactions between SO 2À 4 and TiO 2 framework and thereby poor catalytic stability, and the leaching of sulfate group also easily happens from SO 2À 4 /TiO 2 catalyst in a solvent system for a longrunning process [4,59,60]. Consequently, the 1.5SO 2À 4 /TiO 2 catalyst shows poor catalytic stability and deactivates easily in the transesterification of JO, attributing to leaching of SO 2À 4 and blocking of the active sites by carbonaceous residues deposition. Therefore, the approaches of improving the reusability of sulfated TiO 2 need to be developed by the researchers in the future. Table 4 compares the catalytic performance of 1.5SO 2À 4 /TiO 2 with the solid acidic catalysts reported by other researchers for transesterification of JO. The results show that the high biodiesel yield of 85.3% obtained in this study was obtained under much milder reaction condition and less CH 3 OH dosage and catalyst loading. Although slightly higher biodiesel yields (greater than 92%) were reported by other literature, these transesterification processes generally require much higher reaction temperature and pressure. Therefore, the process puts forward a higher requirement for reaction equipment and has an