Investigation of Catalytic Activity of a Sulfonated Aluminum-Based Metal-Organic Framework: Design of Oleic Acid Esterication With Response Surface Methodology

In this study, the synthesis of MIL-53(Al) (Material Institute Lavoisier, MIL) material, which is an aluminum-containing metal-organic framework (MOF) and generally produced using the solvothermal method was carried out by microwave method at different synthesis times (30-180 min) and temperatures (120-180 °C). In order to improve the catalytic activity of the MIL-53(Al), it was functionalized by applying sulfonation process and SO 3 -MIL-53(Al) material was also provided. It has been observed that the changing in the synthesis time and temperature cause changes in the morphologies, surface areas and thermal resistance. The highest surface area value was obtained as 1256.3 m 2 /g at 180 °C-180 min synthesis condition. MIL-53(Al) and SO 3 -MIL-53(Al) were investigated as catalysts for the esterication of the oleic acid with methanol by designing response surface methodology (RSM). The SO 3 -MIL-53(Al) catalyst provided higher conversion of oleic acid to ester than MIL-53(Al) as 97.2% and 65.9%, respectively. the response with methanol. According to the results same experimental with CCD, the higher ester conversions were with the SO 3 compared to the This shows that in esterication reactions with MIL-53(Al) catalysts, surface acidity is more effective rather than surface areas of catalysts. For MIL-53(Al) and SO 3 -MIL-53(Al) materials, it has been observed that the molar ratio MetOH/OA (x 1 ), catalyst amount (x 2 ), temperature (x 3 ), reaction time (x 4 ) coded parameters positively affect the esterication according to the obtained RSM. It was determined that most effective variable among approximation coecients was temperature. The highest conversions were obtained as and for and SO 3 -MIL-53(Al), respectively, in studies performed at


Introduction
Due to the increasing environmental concerns and strict regulations in the world, environmentally friendly and e cient catalytic technologies attract great attention. Liquid catalysts which are very important in the chemical and re nery industry are highly dangerous, corrosive, polluting and non-reusable in catalysis processes. For this reason, the development of highly porous solid catalysts with high activity and stability is highly desirable for both scienti c and industrial aspects. Therefore, due to their high surface area, porosity and possibility to be modi ed and also effective usability in adsorption, separation and gas storage processes, in recent years, metal-organic frameworks (MOFs) are very popular especially in MOFs can be synthesized by many methods such as solvothermal, microwave, sonochemical, electrochemical, mechano-chemical [14,15]. Generally, it is the most used is solvothermal method, because it is very useful and simple, but requires long synthesis time. The long synthesis time of the solvothermal method has increased the tendency towards alternative methods [16]. Therefore, the microwave method has become an important alternative in MOF synthesis due to its simplicity as well as the fact that the synthesis takes place in a very short time [17]. For many MOF structures, there is still no comprehensive study investigating the synthesis parameters such as temperature and time using the microwave method.
Thanks to their unique properties, the use of MOFs as catalysts has been widely studied in recent years. However, there are limited studies on the use of MOFs as catalysts in esteri cation which are also used in biodiesel production. Esteri cation process plays an important role in numerous applications such as polymers for industrial use, paints, polishes, medicines, fragrances, softeners, solvents, and synthetic rubber production [18]. Esteri cation reactions are carried out with the help of catalysts such as H 2 SO 4 , HCl and H 3 PO 4 homogeneous acid catalysts. Recently, these homogeneous acid catalysts have been replaced by heterogeneous catalysts due to the disadvantages of separation, corrosion, environmental and reusability. Thanks to the reusability properties of heterogeneous catalysts, serious environmental and economic bene ts can be achieved. In recently, thanks to their unique properties in this eld, MOFs have been used extensively as a solid catalyst [3,[18][19][20][21].
One of the MOFs, Al 3 + , Fe 3+ and Cr 3+ based MILs (Materials of Institute Lavoisier) series can provide great advantage for esteri cation due to their high stability and available raw materials [22,23]. In this sense, MIL-53(Al), an aluminum-containing MOF, stands out with its high thermal stability and low toxicity. In this study, the synthesis of MIL-53(Al), was synthesized using microwave method at different synthesis times and temperatures. In addition, the MIL-53(Al) material was modi ed with post-synthetic modi cation (PSM) to obtain the sulfonated structure of MIL-53(Al), SO 3 -MIL-53(Al). The catalytic activities of the synthesized MIL-53 (Al) and SO 3 -MIL-53(Al) were investigated as catalysts for the esteri cation of oleic acid with methanol by designing with response surface methodology (RSM). Performing the design of the experiment with RSM helps to determine the effect of process variables and the interactions between all variables and provides a better understanding of the process. The effect of process variables (MetOH/OA, catalyst amount, temperature and time) on the esteri cation reaction and determination of the optimum reaction conditions by RSM is also studied.

Preparation of MIL materials
The synthesis of MIL-53(Al) was performed with the Microwave method by modifying the solvothermal method of Loiseau et al. (2004) [24]. 1 mmol Al(NO 3 ) 3 .9H 2 O and 1 mmol TPA were placed in 100 mL of te on reactor with 30 mL of distilled water. The reactors were held in the microwave oven for the desired test temperature and time (120, 150, 180 °C and 30, 60, 120 and 180 min). At the end of the studies carried out at the speci ed time and temperatures, the reactors were cooled to room temperature. After reaching room temperature, it was washed with 80 °C DMF to remove unreacted Al(NO 3 ) 3 and TPA reagents and washed 3 times with methanol to remove DMF from the cage structure and centrifuged.
Then, the mixture was ltered, the solid was washed 3 times with distilled water and once with acetone.
The obtained sulfonated MIL-53(Al) material was kept in ethanol at 70 °C for 24 hours and then in vacuum at 100 °C for 12 hours.

Characterization of MILs
The Scanning Electron Microscopy (SEM) images and Energy Dispersive X-ray (EDX) analyzes of MIL materials were performed in the ZEISS Supra 55 brand scanning electron microscope. Samples were coated with platinum-palladium plating under vacuum for 15 minutes before analysis to obtain a better image. X-Ray Diffraction (XRD) analyses were performed with a Rigaku Smartlab brand powder XRD device at 2-theta angle between 3 0 -30 0 with XRF mode open at 2°/min scanning speed. The FTIR spectra of the samples were recorded using with Perkin Elmer/MIR spectrophotometer and a Pike Technologies Gladi ATR accessory within the range of 450-4000 cm -1 , at a resolution of 1 cm -1 . Nitrogen adsorption isotherms were measured at -196 ℃ using a Micromeritics Tristar Orion II 3020 surface area and porosimetry analyzer. According to the resulting isotherms of the MIL-53(Al) samples, Brunauer-Emmet-Teller (BET) and Langmuir methods were used for the analysis of surface areas. Thermogravimetric analysis (TGA) were performed on a Mettler Toledo TGA/DSC 3+ thermogravimetric analyzer from room temperature to 600 ℃ at a ramp rate 10 ℃/min.

Surface acidity
Surface acidities of synthesized MIL-53(Al) and SO 3 -MIL-53(Al) materials was determined by potentiometric titration of n-butyl amine [26,27]. This measurement provides information on the acid regions of the catalyst relative to the Ei values of the initial electrode potential [26]. For measuring surface acidity, 200 mg of catalysts was shaken in 20 mL of acetonitrile (ACN) for 3 hours. The suspended material was then titrated with 0.02 N, 0.05 mL of n-butyl amine. Electrode potential change was measured with a digital pH meter in mV mode. The initial electrode potential indicates the acid strength of the catalyst.

Design of Esteri cation Experiments with RSM
Esteri cation reactions are affected by temperature change, reaction time, catalyst amount and alcohol concentration. These variables can either increase or decrease the conversion of fatty acids to esters. Therefore, it is important to determine the most favorable reaction conditions for the conversion of fatty acids to their esters and also to evaluate the in uence of process variables to maximize yield and reduce costs in the ester production. The catalytic activity of the prepared catalysts was tested for the esteri cation of oleic acid with methanol. To optimize the esteri cation process, experiments were designed according to the response surface methodology (RSM) by using the Design Expert-12 statistical program. The most basic experimental design model is central composite design (CCD) which is used in the RSM. Central composite design consists of 2k (k is the number of independent parameters) factorial points consisting of two axial points on each axis and some central points at a distance alpha (α) from the center of the design. Different α, i.e., different CCD can be obtained by taking the distance of the axial point from the center of the design. In this design, depending on the factor of k, the number of experiments (N) was determined by the following formula: N = 2 k + 2k + 6 (1) The experiments were carried out according to a central composite design with six center points for each catalyst resulting in 30 experiments. The level selection was determined according to the results obtained in pre-tests. The independent parameters of molar ratio (x 1 ), catalyst concentration (x 2 ), temperature (x 3 ) and time (x 4 ) were operated in the range of 20-100 mmol/mmol, 1-5 %, 70-150 ℃, 30-180 min, respectively. The codes and levels of the independent variables are given in Table 1.
Esteri cation studies were also carried out by the microwave process. Before the esteri cation experiments, the MOF materials were stored in the oven at 105 °C for 12 hours. In each study, 10 mL of alcohol was kept constant and the molar ratio and catalyst concentration were determined. At the end of each study, the MOF was separated from the liquid phase with 0.2 micron lters and ester conversions were determined by Gas Chromatography-Flame Ionization Detector (GC-FID, Agilent 7890A, HP Innowax column 30m*250µm*0.25µm). The percent of conversion of oleic acid to ester were determined by using equation 2 from the areas of the obtained chromatograms.

Reusability
Reusability studies were carried out for each catalyst at the central points of the experimental program prepared with CCD (molar ratio:60 mmol alcohol/mmol FA, catalyst amount: 3 %, temperature: 110 °C and time: 120 min). After each run, the catalysts were recovered by centrifugation and washed 3 times with methanol. The recovered catalysts were dried under vacuum at 70 °C and their reusability was investigated in 3 repetitions.

Results And Discussion
First of all, MIL-53(Al), one of the MOF structures, was synthesized at different temperatures and times with the microwave method instead of the common solvothermal method in order to examine the change in synthesis yield of the materials and their textural properties. Then, the sulfonation process was performed to optimum MIL-53(Al) structure and catalytic activities of both MIL-53(Al) structures were investigated for the esteri cation process.

Characterization of MIL-53(Al) Materials
Microwave method production parameters of the synthesized MIL-53(Al) catalyst at different temperatures and times are given in Table 2. Pressure and power values naturally increased with the increasing temperature applied in the production of 100 mL closed Te on reactors. The general reaction of the MIL-53(Al) formation and the obtained MOF structure is shown in Fig 1. As can be seen in the formation reaction of the MIL-53(Al) structure, H + concentration and therefore the acidity of the reaction solution will increase with the increase of conversion rate to MIL-53(Al). The nal pH values measured after synthesis and also the calculated synthesis yield values con rm this result (Table 2). When the synthesis time and temperature increased, the synthesis yield of MIL-53(Al) is increased, but the more increase at the time and temperature, MIL-53(Al) cannot be obtained. White crystals which is speci c to MIL-53(Al) could not be obtained at temperatures above 180 ° C. It was not suitable for the microwave method over 180 min.

SEM-EDX analysis
SEM images of the materials obtained in the synthesis of MIL-53(Al) at different temperatures and times are given in Fig. 2 (a-m). Synthesized MIL-53 (Al) appears as a trapezoidal crystal, similar to those previously reported in solvothermal method [28,29]. In Fig. 2, while the lowest crystal sizes (300-500 nm) are observed in the synthesis at 180 ° C for 30-180 minutes, it is seen that the crystal sizes are between 500-800 nm in the synthesis at 120 °C for 180 minutes. It has been reported in the literature that overmicron crystal sizes are obtained in the synthesis of MIL-53(Al) by the solvothermal method [30,31]. SEM image of SO 3 -MIL-53(Al) material obtained by functionalization after synthesis is given in Figure 2m.
There is no signi cant change in the morphology and particle size of the MIL-53 (Al) material after the sulfonation process.
The EDX results of the synthesized MIL-53(Al) materials are given according to atomic percentages in Table 3. Elemental composition and distribution analysis by EDX measurement shows the presence of Al, C and O elements in synthesized MIL-53 (Al). Considering MIL-53 (Al) with the molecular formula Al(OH)-[O 2 C-C 6 H 4 -CO 2 ], the % atomic C/Al and C/O ratios are compared to the theoretical ratios (theoretical C/Al and C/O ratios of 8.0 and 1.6, respectively). It can be said to be closer for materials synthesized at temperatures of 180 °C. The presence of element S in the EDX results of SO 3 -MIL-53 (Al) con rms the sulfonation process. The C/S ratio shows that 54% of the aromatic terephthalate units are successfully sulfated, assuming that there is mono-sulfonation to the MIL-53(Al) structure.

XRD analysis
The XRD spectra of the MIL-53 (Al) structure and the sulfonated SO 3 -MIL-53(Al) structure are shown in  [2,32,33]. Peak intensities con rm the high crystallinity. Also, the XRD spectrum of the sulfonated material shows that the MIL-53(Al) lattice integrity is preserved. Sulfonated MIL-53(Al) material appears to have characteristic peaks at 2ϴ =9.3, 12.5 and 17.8 degrees in the XRD spectrum [25,34] reported in the literature.

N 2 Adsorption Isotherms
It is important to determine textural parameters such as surface areas, pore volumes, average pore size and pore size distributions in MOF characterization. The BET surface area, Langmuir surface area, total pore volume and average pore diameter of MIL-53(Al) and SO 3 -MIL-53(Al) materials calculated from N 2 adsorption isotherm data are given in Table 4. The highest BET and Langmuir surface areas of MIL-53(Al) were obtained at 180 °C -180 min synthesis as 1256.3 and 1404.3 m 2 /g, respectively. The BET surface area for MIL-53(Al) ranges from 1000 to 1600 m 2 /g in the literature [24,[35][36][37]. Theoretically, MIL-53(Al) has a surface area of 1632.8 m 2 /g [33]. The pore diameters of MIL-53(Al) materials vary between 14-16 Â and their total pore volume between 41-57 cm 3 /g [32,36,37]. This shows that the synthesized MIL-53(Al) materials has been produced in large pore form. It is seen that the increase in   Fig. 4, it is seen that the nitrogen adsorption capacity has signi cantly reduced after sulfonation. The isotherms obtained by nitrogen adsorption of MIL-53(Al) crystals are similar to the Type I isotherm. The similarity of the isotherms to the Type I isotherm model con rms that the synthesized crystals have a microporous structure. isotherms [36,38]. As given in Fig. 4 show that the materials have a microporous.

Thermogravimetric analysis
The proportion of organic or inorganic impurities attached to the lattice structure can be determined by thermogravimetric analysis depending on the loss of weight [39]. Fig. 5 shows the TGA curves of the MIL-53(Al) and SO 3 -MIL-53(Al) materials. As seen in the TGA curves of MIL-53(Al) materials performed at 120, 150 and 180 °C for 180 min, the weight loss of structures occurred in 3 steps with a similar trend. Weight loss in the rst step in the temperature range 60-120 °C; 5.1% for 120 °C synthesis, 3.4% for 150 °C synthesis, 4.3% for 180 °C synthesis. This result is due to the transition of water or solvent molecules in the pore or on the surface of the lattice structure to the gas phase [40]. The second step weight loss was determined at temperatures between 120-540 °C, around 2.1% and 1.0% for synthesis at 150 ° C and 180°C , respectively, and a little more 5.4% for a temperature of 120 °C. This loss of weight is due to the removal of organic and inorganic impurities due to the cage structure. The lower values obtained in 120°C production in BET surface area measurements con rm the TGA result of the lattice structure produced at 120 ° C. It shows that the channels and pores in 120 °C production cannot be completely free of impurities. The weight loss in the third step was between 540-580 ° C, similarly between 34-37% of the total weight for the three substances. This step is the stage where TPA begins to decompose and the cage structure begins to degrade. The TGA curve of sulfonated MIL-53(Al) material (Fig. 5) shows that the structure is thermally stable up to a temperature slightly above 300 °C. After this temperature, it is seen that the cage structure has completely deteriorated. The rst step weight loss was 45.8% at a temperature of about 300 °C. Then, weight losses in 4 different steps up to 568.7 °C, respectively, 9.4%, 364.5 ° C, 9.2%, 419.5 ° C, 8.2%, 474.4 ° C and 21.6%, 568.7 ° C has been realized. The rst step is thought to be due to the degradation of the TPA binder while the other steps are due to various impurities and Al compounds. Thermal resistance of MIL-53(Al) material has decreased signi cantly after sulfonation.

FTIR analysis
The FTIR spectra of the MIL-53 (Al) obtained at optimum condition (180 °C-180 min) and SO 3 -MIL-53(Al) structures is shown in Fig 6. In the MIL-53(Al) spectrum, the coordination of TPA organic ligand and Al metal centers is observed with asymmetric -COO stress in the 1601 cm -1 band and symmetric -COO stress in the 1415 cm -1 band. The stress in the region of 3672 cm -1 is due to the µ2-OH stress of the AlO 6 octahedra structure [24,[41][42][43]. The FTIR spectrum of SO 3 -MIL-53(Al) shows similarities to non-sulfated MIL-53(Al). In the SO 3 -MIL-53(Al) spectrum, the asymmetric and symmetrical -COO stresses observed in the MIL-53 (Al) material are observed in the band 1601 and 1415 cm -1 , respectively. Unlike pure MIL-53 (Al) material, the O=S=O tensile vibration in sulfated material was detected in the band of 1006 cm -1 . Again, it can only be explained by the settlement of the peak sulfoxy groups in the benzene rings, which appear in the 929 cm -1 band in the sulfated sample [25,34].

Surface acidity
Based on the initial electrode potential value of the catalysts, the amount of acid sites and their relative acid strength can be estimated. N-butylamine is a strong base and can be attached to acid sites of different strengths and types, so it will titrate both the Lewis and Brønsted sites [27,44]. Potentiometric titration curves of MIL-53(Al) materials obtained with n-butylamine are given in Fig 7. Acid strength of these sites can be classi ed as: Ei> 100 mV (very strong sites), 0 <Ei <100 mV (strong sites), 100 <Ei <0 mV (weak sites) and Ei <100 mV (very weak sites) [26]. According to this classi cation, it can be said that MIL-53 (Al) and SO 3 -MIL-53 (Al) materials have very strong acid sites. The initial electrode potential (Ei) of SO 3 -MIL-53(Al) (313 mV) material is quite high compared to MIL-53(Al) (116 mV). In processes such as esteri cation, the effectiveness of catalysts depends more on the active acid sites they have. This indicates that SO 3 -MIL-53(Al) will have higher catalytic e ciency than MIL-53(Al).  It is desired that the signal-to-noise ratio showing the model sensitivity is greater than 4.
In Fig. 8 a,  shows that the response is more sensitive to temperature. For MetOH/OA molar ratio, it is seen in Fig. 8c that the increase in the MetOH/OA molar ratio does not cause a signi cant change in the response, especially for MIL-53(Al).
The interactive effect of temperature and reaction time of methyl oleate conversion at a constant molar ratio (60 mmol MetOH/mmol OA) and catalyst amount (3% for MIL-53(Al) and SO 3 -MIL-53(Al)) is shown in contour and 3-D plots in Fig. 9(a-d). In Figures 9 a and b, it can be seen that the conversion e ciency of MIL-53(Al) increases with increasing reaction time and temperature. It can also be seen in Fig 9 c and d that the conversion of methyl oleate does not change signi cantly with increasing reaction time at a constant temperature of 150 °C. On the contrary, it is observed that the conversion to methyl oleate signi cantly increases with the increase in reaction time for 70 °C temperature. This shows that the e ciency of the reaction time decreases as the temperature increases in the esteri cation studies with the SO 3 -MIL-53(Al) catalyst.

Optimization of esteri cation reaction conditions
Optimum conditions for the esteri cation of oleic acid with MIL-53(Al) and SO 3 -MIL-53 catalysts were determined by Design Expert 12 software numerical optimization. In order to obtain maximum ester conversions (response), all independent parameters (x1, x2, x3 and x4) were set to in the test range and are shown in Table 7. In the test range, the maximum ester conversions with MIL-53 and SO 3 -MIL-53 were determined from the model results 63.5% and 91.6%, respectively. It can be seen in Table 5 that the experimental results are quite consistent with the model results. This con rms that model results can predict experimental results. In Table 5, the model results obtained by adjusting the independent variables as minimum and the ester conversion as maximum and the experimental results also con rm the model t.

Comparision of catalytic effects of MIL-53(Al) and SO 3 -MIL-53(Al) catalysts
According to the results obtained, SO 3 -MIL-53(Al) material showed higher catalytic e ciency in the esteri cation of oleic acid with methanol than MIL-53(Al). This shows that in esteri cation reactions with MIL-53(Al) catalysts, surface acidity is effective rather than surface areas of catalysts. With MIL-53(Al) and SO 3 -MIL-53(Al), the highest ester conversions were obtained as 65.9% and 97.2%, respectively, in studies conducted at temperatures of 150 °C. Table 8 presented examples of the use of some heterogeneous catalysts at different reaction conditions in the esteri cation of fatty acids. Catalyst amount, reaction temperature, alcohol/fatty acid ratio and reaction time are known as important parameters in esteri cation process. In the esteri cation process, the suitability of these parameters to the process should be evaluated in the selection of the appropriate catalyst. According to the heterogeneous solid catalysts used in the literature, it can be said that MIL-53(Al) catalysts are effective in ester conversions. Especially the conversions over 90% obtained with SO 3 -MIL-53(Al) material are well above the average. Although the high conversion to methyl oleate with MIL-53(Al) catalysts at high temperatures, the use of low catalyst amount is an important advantage.

Reusability of catalysts
One of the most important desired properties of heterogeneous catalysts is their stability in the reaction medium; the catalyst must be recyclable and reusable. Therefore

Conclusions
The synthesis of MIL-53(Al) has been generally carried out by the solvothermal/hydrothermal methods at 220 °C for 72 hours. In this study, MIL-53(Al) synthesis was performed using a microwave method at lower temperatures and duration. It was determined by SEM-EDX, XRD, surface area, TGA and FTIR analysis that the optimum MIL-53(Al) structure can be obtained at the 180 °C-180 min synthesis conditions. Especially, TGA and surface area results showed that the structure reached its purest form in studies performed at 180 degrees. As a detail, it was determined in the synthesis studies performed with microwave in water that the crystal sizes (300-500 nm) were decreased compared to the solvothermal method.
The catalytic activity of the obtained MIL-53(Al) and SO 3 -MIL-53(Al) structures was evaluated by using the response surface method in the esteri cation experiments of oleic acid with methanol. According to the results obtained in the same experimental program created with CCD, the higher ester conversions were realized with the SO 3 -MIL-53(Al) compared to the MIL-53. This shows that in esteri cation reactions with MIL-53(Al) catalysts, surface acidity is more effective rather than surface areas of catalysts. For MIL-53(Al) and SO 3 -MIL-53(Al) materials, it has been observed that the molar ratio MetOH/OA (x 1 ), catalyst amount (x 2 ), temperature (x 3 ), reaction time (x 4 ) coded parameters positively affect the esteri cation process according to the approximation functions obtained from RSM. It was determined that the most effective independent variable among the approximation function coe cients was temperature.