INTRODUCTION
Due to the ever-increasing energy demand and the problems associated with consuming petroleum fuels, there is a strong driving force worldwide to develop alternative fuels in order to reduce greenhouse gases [1]. Most petroleum fuels are consumed in the transportation system. Therefore, it is necessary to use a renewable fuel that can be utilized in the current engines. Among alternative fuels, biodiesel, which is usually made via the esterification/transesterification process from animal fat/vegetable oils and methanol in the presence of a catalyst [2], presents suitable properties for replacing the petroleum fuel due to its low toxicity, biodegradability, low emissions, and compatibility with current diesel engines. Therefore, the production of biodiesel fuels has increased over the last 20 years [3]. 
The industrial process of biodiesel production, which is usually performed on raw vegetable oils and homogeneous alkaline catalyst, is faced with major challenges and has to be modified due to the increase of deforestation for oil seed cultivation, conflict of demand between human nutrition and fuel production as a result of using vegetable oils, and the long and costly downstream process (separations, purification, and neutralization) which increases the biodiesel cost [4]. In this regard, utilizing the used and nonedible feedstock (waste cooking oil, Jatropha oil, etc.) and microalgae oil has been proposed in the second and third generations of biodiesel production. Although the production cost decreases considerably by reducing the feedstock price, the downstream cost increases significantly due to the reaction between the homogenous alkaline catalyst (NaOH or KOH) and free fatty acids (FFAs) of the feedstock (usually nonedible oil has FFA > 1 wt.%). 
In order to overcome such demerits, many researchers have studied heterogeneous catalysts for simultaneous esterification and transesterification processes of cheap feedstocks for biodiesel production [5]. According to the literature, most heterogeneous catalysts consist of two parts which are the catalyst support and the active phase. Catalyst supports have to be thermally stable and have a high surface area. This structure allows FFAs and triglyceride molecules access to the active sites of the catalyst and the internal surface area [6]. Various metal oxides , including silica [7], titania [8], alumina [9], and zirconia [10, 11] have been significantly highlighted as catalyst supports over recent years. Silica and titania are widely studied due to their favorable properties, including high surface area, low cost, porous structure, acidity, good mechanical and chemical stability, low toxicity, and easy availability [12, 13].
In order to achieve higher activity and stability as well as minimum structural weakness, bifunctional catalyst supports have been widely studied. Mahmoud et al. [14] synthesized ZrO2/SiO2 catalysts for stearic acid esterification and reported that the most active catalyst (ZrSiOC) exhibited good reusability until five runs. SiO2-Al2O3 supported bifunctional acid-base presented high activity and reusability [15, 16]. Kulyk et al. [17] reported that TiO2-SiO2 and Al2O3-SiO2 are proper catalysts. The ZnO-SiO2 nanocomposite has been used for biodiesel production and has obtained over 95% yield [18, 19]. Cobalt, chromium, calcium, etc. have been tested to obtain high active and stable catalyst support [20-22].
The active phase is usually chosen from acid or base components and is modified on the support with the impregnation method [23, 24]. Sulfated metal oxide is a good example of a solid super-acid catalyst and shows more tolerance to high amounts of FFAs, moisture, and other impurities [25, 26]. Manga et al. [27] investigated the activity of SO42-/TiO2-SiO2 catalysts and a 98.89% ethyl ester conversion was achieved with 5 wt.% catalyst, a molar ratio of 13, and a reaction time of 2.25 h. Shao et al. [28] fabricated a sulfated silica-titania catalyst via the sol-gel method followed by calcination at different temperatures. It was reported that the sulfated silica-titania catalyst exhibited less stability after the first run and the conversion also reduced by 60%. A similar conclusion was drawn by Sheikh et al. [29] for the esterification of oleic acid over sulfated silica-titania (50% reduction in the conversion after the first run). 
The instability of sulfated titania, silica, and titania-silica catalysts in the esterification and transesterification reactions are mainly related to the leaching of sulfate groups (weak bonds between the sulfate groups and the surface of the catalyst support) [28, 30]. Application of metal cations that make strong bonds with the sulfate group can overcome this weakness. As a result, the preparation of tri-functional catalysts has been suggested. Li et al. [31] modified SO4/TiO2–SiO2 with La3+ ions and found that the stability improved significantly, owing to the strong bonding of Si and La3+ ions with the sulfate groups. The La3+/ZnO-TiO2 tri-functional catalyst was utilized for biodiesel production from waste cooking oil by esterification of FFAs and showed high activity (over 96%) and stability (used for five cycles). Feyzi and Shahbazi [32] synthesized Cs–Ca/SiO2–TiO2 nanocatalysts for biodiesel production which obtained higher activity and stability compared with SiO2–TiO2 nanocatalysts. Based on previous research on bifunctional nanocatalysts including evaluations of the interaction between SiO2 or TiO2 and zirconia [33, 34], iron oxide [35], alumina [36, 37], and zinc oxide [38, 39], these cations can be suitable options to improve the weakness of sulfated silica-titania nanocatalysts.
To the best of the authors’ knowledge, there is a lack of studies on the modification of sulfated silica-titania catalysts in order to synthesize a tri-functional nanocatalyst with a proper catalytic activity and stability in the esterification reaction. Therefore, herein we report the preparation of a sulfated silica-titania catalyst modified by various metal cations with different valances (Zn2+, Co2+, Al3+, Cr3+, and Zr4+). The solid acid catalysts synthesized via a combined sol-gel and impregnation method were characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller (BET) surface area, field emission scanning electron microscopy (FESEM), and titrimetric analyses (the Hammett indicator method). The catalytic activity and stability of the synthesized catalysts were also examined in the esterification of oleic acid. Finally, the parameters of the esterification reaction were optimized.

MATERIALS AND METHODS
Synthesis of catalysts
Mixed metal oxide catalysts containing titania and silica as the main components were synthesized via the sol-gel method [40, 41] as follows: 
Titanium dioxide (98% purity, Merck Co.) was dispersed in a beaker containing the desired amount of nitric acid solution (4 M, Sigma-Aldrich). Then, tetraethyl orthosilicate (TEOS, 98% purity, ACROS Organics) with a constant Si/Ti molar ratio of 0.8 was added to the mixture. In order to enhance the activity of the Si-Ti complex metal oxide, other nitrate salts of metal cations, including Al(NO3)3.9H2O (Merck Co.), Co(NO3)2.6H2O (Merck Co.), ZrO(NO3)2.xH2O (Sigma-Aldrich), Cr(NO3)3.9H2O (Merck Co.), and Zn(NO3)2.6H2O (Merck Co.), at a metal cation/Ti molar ratio of 0.2 were separately added. 
In order to form the metal cation complex, citric acid as a complex agent with a ratio of 1.5 times of metal cations was added to the previous solutions and the mixtures were continuously stirred under reflux at 80°C. The resulting mixtures were then heated to evaporate the water and the obtained viscous gels were aged overnight. Finally, the gels were dried in an oven at 110°C and calcined in air at 550°C for 4 h. The same procedure was repeated for fabricating a silica-titania support without the addition of other nitrate salts.
The supports were activated by the modification of the sulfate group. In this regard, the sulfate groups were impregnated on the synthesized catalyst supports to enhance the catalytic activity towards the esterification reaction. For this purpose, 12.5 mL of H2SO4 solution (1 M, Shimi-Pjouhesh, 96%) was mixed with 1 g of the catalyst support and refluxed at 80°C for 2 h [42]. The mixtures were then oven-dried overnight at 110°C, prior to calcination under air at 550°C for 4 h. The SO4/SiO2–TiO2 promoted by Al3+, Co2+, Zr4+, Cr3+, and Zn2+ were labeled as S/Al-SiTi, S/Co-SiTi, S/Zr-SiTi, S/Cr-SiTi, and S/Zn-SiTi, respectively.

Catalyst characterization
Powder X-ray diffraction (XRD, UNISANTIS XMD 300) was utilized to determine the crystal structure of the synthesized catalysts. The test was performed using Cu Kα radiation and scanned in the range of 10-80° (2θ) at rating of 10°/min. The surface functional groups of catalysts were analyzed by FT-IR spectroscopy in the range of 4000-400 cm-1 using a Thermo Nicolet AVATAR 370 spectrometer. The textural properties of the synthesized catalysts were determined by multipoint nitrogen adsorption-desorption method at -196 °C using AUTOSORB 1 analyzer (Quantachrome Co.). All samples were degassed at 120 °C for 8 h. Particle shape, morphology and elemental composition of the synthesized catalysts were detected using a field emission scanning electron microscope (FESEM, TESCAN Mira 3-XMU, Czech Republic) coupled with energy dispersive X-ray spectrometer (EDS). The acid strength and amount of acid sites on the surface of the synthesized catalysts were measured by Hammett indicator method in which methyl red (Hammett acidity function H0 ≤+4.8), p-dimethylaminoazobenzene (H0 ≤ +3.3), crystal violet (H0 ≤+0.8) and dicinnamalacetone (H0 ≤ -3.0) were used as indicators. The acidity was determined using titration the discolored solution by 0.01 N n-butylamine solution [11].

Esterification reaction
Esterification reaction was performed in an autoclave stainless steel reactor equipped with a thermocouple (Type-K) and a manometer. 20 g of oleic acid (Fluka Analytical, ≥99.0%), 0.6 g of catalyst and 25.8 mL methanol were added to the reactor at room temperature [11]. The mixture was then stirred and heated at 90 °C and 120 °C for 4 h. Afterward, the ester (methyl oleate) layer was separated from the catalyst and by-product (water) by centrifugation at 4000 rpm for 20 min. The obtained methyl oleate was slowly heated to remove excess methanol and water and then titrated with 0.1 M alcoholic potassium hydroxide solution to obtain the acid value. The conversion was calculated according to the follow equations (Eqns. 1 & 2) [38]:
 (1)
                               

                                                           (2)
where AV is the acid value, V is the volume of 0.1 M alcoholic solution of KOH used for titration, and W is the sample weight.

RESULTS AND DISCUSSION
Characterization of the synthesized solid acid catalysts
XRD analysis
The XRD patterns of the synthesized catalysts are shown in Fig. 1(I). As can be seen, the diffraction peaks of the SO4/SiO2–TiO2 catalyst confirm the anatase phase of titania (JCPDS No. 71-1164) linked with the sulfate groups to form titanium oxysulfate (TiO(SO4), JCPDS No. 85-2452) and Ti2(SO4)3 (JCPDS No. 22-0947). No characteristic peak was observed for the SiO2 particles, which may suggest that SiO2 diffused in the titania lattice to form a Ti‒O‒Si structure or an amorphous structure that would be found in the FTIR analysis [28, 43]. The SO4/SiO2–TiO2 catalyst supported by metal cations (Al, Co, Zr, Cr, and Zn) leads to the formation of narrower peaks with higher intensity (and probably higher crystallinity degree; e.g. 2θ observed at 25.3°). 
 The results also revealed that no characteristic peaks related to the crystalline reinforced metal cations were observed, which indicated that the metal cations were probably doped into the titania lattice [44]. The overall diffraction peaks may confirm the homogenous distribution of the reinforcing metal cations within the structure of the silica-titania catalyst support [45]. 
The loading of sulfate groups on the support also changed the chemical and crystal structure of titania into TiO(SO4), and the Ti2(SO4)3 phase mostly disappeared in the case of modification with the Al3+ cation. It was reported that TiO(SO4) was much more active than Ti2(SO4)3 and could transform into other sulfate phases at high concentrations of sulfate groups [46]. Based on this result, the loading of metal cations on the catalyst supports predominated the formation of the TiO(SO4) phase by replacing one oxygen atom from the titania crystals.  
The summary of the calculation of the crystallite size based on the strongest peak (2θ at 25.3°) of the catalysts using Scherer’s equation [47] is presented in Table 1. The results revealed that the crystallite size was significantly reduced due to the diffusion of the metal cations into titania lattice and the change of crystal sheet bonds [48]. The difference in the crystallite size can be due to the difference of the atomic radius and the valence electron of metal cations. 

FTIR analysis
The FTIR spectra of the synthesized catalysts are presented in Fig. 1(II). The broad band at 3400 cm-1 and the weak band at 1640 cm-1 are related to the O-H stretching and bending vibrations, respectively [49]. The band in the region of 1000-1500 cm-1 can be attributed to the vibration of S=O symmetric and asymmetric vibrations [33, 50]. 
The strong band at 960 cm-1 is attributed to the Ti‒O‒Si vibrations in the nano-structured frameworks [51]. Moreover, the peak for M=O‒S (M=metal cation) can also be detected in this region and overlaps with the vibrational peak for Ti‒O‒Si. The band is more visible for the silica-titania catalyst modified by Al, Co, and Zr cations. A strong interaction between the sulfate groups and metal cations leads to an increase in the stability of the catalyst [37]. Moreover, the formation of the tetrahedral framework through the Si‒O‒Ti bond increases the acid sites in the catalyst which can enhance the activity of the catalyst in the esterification reaction [52]. The broad peaks that appeared in the range of 550-900 cm-1 are attributed to the M‒O‒M bridging stretches such as Ti‒O‒Ti and Si‒O‒Si for the S/SiTi catalyst [53]. 

BET-BJH analysis
In general, a larger surface area and a bigger pore size in the catalysts facilitate the accessibility of the reactant molecules (triglyceride/FFA) to the external and internal acid sites. As can be seen from Table 1, the loading of metal cations (Al, Co, Zr, Cr, and Zn) on the sulfated silica-titania catalyst resulted in a relatively lower surface area compared with the sulfated silica-titania catalyst. This is probably due to the particle surface coverage by the modified metal cations [36]. However, an insignificant change was observed in the mean pore diameter and pore volume.
The N2 adsorption-desorption curves and insets of the pore size distribution for the sulfated silica-titania and sulfated silica-titania catalyst modified with various metal cations are illustrated in Fig. 2. It can be seen that the adsorption-desorption isotherm for all the synthesized catalysts is type IV corresponding to the mesoporous materials [54]. Moreover, the hysteresis loop of the samples is assigned to type H1, which is witnessed for the cylindrical and spherical pores [3, 55]. 

Acidity analysis
The catalytic activity of a catalyst has a direct relationship with its acid sites (see Table 1). According to the Hammett indicator measurement, all the synthesized catalysts present an acid strength between 48%–90% sulfuric acid (-3.0 < H0 < -8.2). The number of acid sites on a solid surface, which was measured by amine titration, indicated that the sulfated silica-titania catalyst reinforced by alumina had the highest acidity. This was also consistent with the FTIR results in which the S/Al-SiTi catalyst exhibited sulfate bands with higher intensity.  
FESEM analysis
The FESEM images of S/SiTi and S/Al-SiTi catalysts are illustrated in Fig. 3. The sulfated silica-titania catalyst showed agglomerated clusters with large particles. In the case of the catalyst support modified by Al3+ cations, however, a smaller particle size with lower agglomeration and more obvious external pores was observed. This morphology leads to the dispersion of sulfate groups on the surface of individual particles and inside the pores to make stronger bonds with the support surfaces (Al-SiTi). 
The surface particle size of S/SiTi and S/Al-SiTi nanocatalysts was measured and the distribution histograms are depicted in Fig. 4. The size of surface particles was reduced by the loading of Al3+ ions such that S/SiTi had a particle size in the range of 5-25 nm and the particle size of S/Al-SiTi was lower than 25 nm. These findings were in good agreement with the crystallite size of catalysts listed in Table 1. Moreover, the frequency of particles with a size below 10 nm increased by the doping of Al3+ ions. In addition, the average particle size reduced from 9.7 nm to 8.2 nm by incorporating Al3+ ions into the SiO2-TiO2 structure. Reduction of particle size can enhance the interaction of sulfated groups with the surface support and consequently, increase the catalytic activity of the Al-promoted catalyst. 
The surface roughness of the samples was also evaluated by analyzing the FESEM images as illustrated in Fig. 5. The 3D surface overview reveals that the surface roughness was also reduced by the doping of Al3+ ions. Reduction of the surface roughness by incorporating the Al3+ cation had a positive influence on the interaction of the reactants and active phases on the surface of the catalyst [56]. Better interaction between the reactants and the surface of the catalyst leads to the formation of higher amounts of the product. Moreover, simple egression of the product from the catalyst surface with lower roughness was reported [57]. Therefore, it seems that S/Al-SiTi can have better interaction with the reactants in order to produce ester (biodiesel).

EDS analysis
The EDS analysis of S/SiTi and S/Al-SiTi catalysts is shown in Fig. 6. The samples only showed the cations used in the parent solution (the mixture of precursors) and no other impurities were detected. The EDS analysis confirmed the better bonding of the sulfate group and the support when Al3+ cation was modified into the SiO2–TiO2 structure, whereas half of the sulfate concentration was detected on the surface of the final powder (S/SiTi) compared with its parent solution. It can be concluded that the sulfate groups were not purchased on the surface of SiTi as support or agglomerated on the surface, which was not detected in this region of the catalyst surface [49, 58].

Catalyst performance
Catalytic activity in the esterification reaction
According to the literature [59, 60], the esterification reaction over various catalysts is carried out at around 120°C, and all the synthesized catalysts exhibited good activities under a methanol/oleic acid molar ratio of 9:1, 3 wt.% catalyst, and the reaction run time of 4 h. The results of the catalytic activity of the samples in the esterification reaction are illustrated in Fig. 7. The synthesized samples exhibited high activity (over 94%,  except for the S/SiTi catalyst modified by Zr4+ and Zn2+) in the esterification reaction at 120°C. This can probably be due to their smaller surface area, mean pore size, and acidity. Since the reaction temperature has a direct relationship with the reaction rate, the reaction temperature was decreased to 90°C for better assessment of the activity of the catalysts. Among the samples, the S/Al-SiTi catalyst exhibited less reduction in the conversion of oleic acid. It seems likely that increasing the interaction between the metal cations and sulfate groups, surface area, mean pore size and reducing the particle size of the support led the S/Al-SiTi catalyst to exhibit the highest activity in the esterification reaction. Komintarachat et al. [61] reported that a solid acid catalyst containing Al2O3 as a porous support had higher surface area and greater pore volume compared with those including SiO2 and ZnO. Large pores can facilitate the easy diffusion of fatty acid molecules in the internal surface area and active sites, resulting in the highest activities in the esterification reaction. Rahmani Vahid et al. [55] also showed that alumina-supported sulfated zirconia had the best catalytic activity in the esterification reaction due to its low crystalline size, well-bonded sulfate groups with zirconium and aluminum ions, and high acidity. They also reported that adding alumina enhanced the reusability of the nanocatalysts. Yee et al. [62] prepared a sulfated zirconia catalyst loaded on alumina for the transesterification of Jatropha Curcas L. oil with methanol to biodiesel and the catalyst had the basic characteristic of an acidic catalyst.
In this study, the nanocatalysts modified with Al3+ cations exhibited the highest acidity (1.98 mmol/g). Although S/Al-SiTi (184 m2/g) had the highest surface area after S/Cr-SiTi (206 m2/g) nanocatalysts, S/Al-SiTi nanocatalysts exhibited higher pore volume (0.63 cc/g) and mean pore size (8.9 nm). The proper textural properties of S/Al-SiTi nanocatalysts helped the reactant molecules to be able to truly make a connection with all the available surface area and active sites (especially in porosities), which resulted in higher activity in the esterification reaction [63].

Optimization of the esterification reaction conditions
After selecting S/Al-SiTi nanocatalyst as the most active catalyst, several reaction parameters, including the molar ratio of methanol to oleic acid, reaction time, reaction temperature, and amount of catalyst to oleic acid were examined to obtain an appropriate conversion in the optimum reaction conditions and the obtained results are present in Table 2.
The reaction temperature is an important factor that leads to accelerating the reaction rate as shown in Fig. 7. The results revealed that the conversion using the S/Al-SiTi catalyst drastically reduced to 73.8% by reducing the reaction temperature to 65°C. Therefore, the temperature of 90°C was selected as the optimum temperature. 
Maximum conversion of oleic acid into methyl oleate can be ensured by using the excess methanol due to the fact that esterification is a reversible reaction [55]. Increasing the molar ratio of methanol/oleic acid from 3:1 to 9:1 considerably enhanced the conversion from 23.2% to 91.1%. However, further increase in the amount of methanol leads to the reduction of the conversion. This is probably due to the dilution of the reaction medium, filling the catalyst pores, and covering the active acid sites on the surface. Additionally, this offers better miscibility of methyl oleate and water which could raise the cost of production [10]. 
The amount of catalyst is also an essential factor in the esterification reaction to provide the required surfaces for the interaction of the reactants and active sites. Therefore, the effect of the catalyst was also studied between 2-4 wt.%. It was found that 3 wt.% catalyst provided sufficient catalyst surfaces to push the esterification reaction towards the maximum conversion, and a higher catalyst loading had an insignificant effect [54]. It is well established that the esterification reaction is a reversible reaction. At low catalyst amounts, there were not enough active sites for the reaction while the reverse reaction might take place when an excessive amount of catalyst was employed [61].
The reaction time was also examined and varied from 2 to 5 h while the other process conditions were kept constant at 90°C, 9:1 molar ratio of methanol/oleic acid, and 3 wt.% catalyst. By raising the reaction time from 2 h to 3 h, a slight increase in the conversion was observed. 

Reusability assessment
The reusability of S/Al-SiTi catalyst was also assessed in order to evaluate the catalyst stability and to understand the effect of the loading of Al3+ cation on the stability of the sulfated silica-titania catalyst. After each run, the catalyst particles were separated and washed several times with a mixture of methanol/n-hexane (1:1 vol.%) to remove any remaining methyl oleate and oleic acid from the surface and pores of the recovered catalyst. The collected particles were finally oven-dried overnight and then calcined at 300°C for 2 h. The reusability of the catalyst was plotted based on the activity reduction measured using Equation 3.
                                                                               (3)

As observed in Fig. 8, the S/SiTi catalyst drastically lost its catalytic activity after the first run. This might be due to the leaching of the sulfate groups from the surface of the catalyst in the reaction medium. This result was also consistent with that of the studies on the sulfated silica-titania catalyst [28]. In the case of catalyst modified by Al3+ cation, the sulfate groups make stronger bonds with the catalyst surface, and the catalyst shows better stability in the esterification reaction with a conversion greater than 85% through five consecutive runs.

Comparison of the results
The catalytic activity and stability of SO4/SiO2–TiO2 modified by the aluminum cation compared with those of some solid acid catalysts reported in the literature are summarized in Table 3. The catalyst synthesized in the current study converted an appropriate amount of oleic acid feedstock into methyl oleate at a lower reaction temperature and shorter time. Moreover, the sample exhibited higher stability for several uses. These desirable properties can be due to the presence of the Al3+ cation, which may consequently increase the metal cation-SO4 bands as well as the acidity [37]. Therefore, the leaching of the sulfate group may reduce. 

CONCLUSIONS
Sulfated silica and titania as catalysts were widely studied in many catalytic reactions including esterification and transesterification reactions. However, these solid acid catalysts suffer from a low number of acid sites and weak bonding of sulfate groups with the catalyst surface. Therefore, the metal cations of Al, Co, Zr, Cr, and Zn were reinforced on the sulfated silica-titania catalyst to form a ternary metal oxide system to overcome these problems. The sol-gel method was utilized for the fabrication of the catalysts, and different techniques were used to analyze the physical-chemical properties of the catalysts. It was found that the incorporation of metal cations in the sulfated silica-titania catalyst inhibited the formation of Ti2(SO4)3 phase. The FTIR analysis confirmed that among the metal cations the catalyst modified by the Al3+ cation had an appropriate interaction between the sulfate groups and the metal cation and catalyst support. Therefore, the catalyst showed higher acidity which is an important parameter for the catalytic esterification reaction. The results showed that the S/Al-SiTi catalyst exhibited high catalytic activity in the esterification reaction at a low temperature (90°C). The reaction conditions using this catalyst (S/Al-SiTi) were also optimized, and the reusability of S/SiTi and S/Al-SiTi catalysts was examined under optimized conditions. High stability of S/Al-SiTi along with the high reduction in the activity of S/SiTi after the first reaction could confirm the positive effect of Al3+ cation on improving the catalytic activity and stability. The S/Al-SiTi catalyst can be a good candidate for biodiesel industrial application after conducting further kinetic studies on different chain lengths of the fatty acid composition and/or cheap feedstocks, e.g. waste cooking oil, microalgae oil, and Jatropha oil.

ACKNOWLEDGMENT
The authors gratefully acknowledge Esfarayen University of Technology (93/9317).

CONFLICT OF INTEREST
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.