Efficient biodiesel production from oleic and palmitic acid using a novel molybdenum metal–organic framework as efficient and reusable catalyst

In this study, metal–organic framework based on molybdenum and piperidine-4-carboxylic acid, was synthesized through a simple solvothermal method and employed as an effective catalyst for biodiesel production from oleic acid and palmitic acid via esterification reaction. The prepared catalyst was characterized by XRD, FTIR, TGA, DSC, BET, SEM, TEM, ICP-OES, X-ray mapping and EDX analysis. The resulting Mo–MOF catalyst exhibit a rod-like morphology, specific surface area of 56 m2/g, and thermal stability up to 300 °C. The solid catalyst exhibited high activities for esterification of oleic acid and palmitic acid. Moreover, the catalyst could be simply recovered and efficiently reutilized for several times without significant loss in its activity, also obtained results revealed that metal–organic framework could be used for the appropriate and rapid biodiesel production.

www.nature.com/scientificreports/ palmitic acid via an esterification reaction. In recent years, metal-organic frameworks (MOFs) are attracting increasing attention due to their important properties such as controllable composition 25 , large surface area 26,27 , thermal stability 28 , flexibility, and easy preparation 29 . MOFs are constructed from SBU connected by organic linkers to form extended coordination networks. The linkers widely used in MOFs are rigid organic chelators such as amino acid, terephthalic acid, and polycarboxylate ligands. Many factors affect the activity of MOFs, such as the organic ligand, solvent type, particle size, and metal sort. Metal-organic frameworks (MOFs) known as coordination polymers, undergoing study by many researchers for advanced applications, including such as catalysis [30][31][32] , separation 33,34 , gas storage 35 , carbon dioxide capture 36 . MOFs mainly have their adjustable nanostructures and porous properties. However, as a good carrier, MOF also has its inherent catalytic performance. Moreover, the MOF-based functional catalytic materials show great potential in biodiesel production and other relevant biorefineries. In Table 1, typical heterogeneous bifunctional catalysts were chosen to compare with MOF-based bifunctional catalysts for biodiesel production. These studies revealed that the MOF with amino groups (Brønsted base) showed high catalytic activity and mild conditions for biodiesel production than another heterogeneous acid-base catalyst (Table 1). www.nature.com/scientificreports/ To the best of our knowledge, there have been few papers referred to preparation of Mo-MOFs [47][48][49][50][51][52] . In this study, the catalytic performance of metal-organic framework based on molybdenum and piperidine-4-carboxylic acid was employed in biodiesel production through the esterification of oleic acid and palmitic acid with methanol (Fig. 1).
The structure of a network is affected by several factors such as building blocks, solvent, temperature, pH, and so on. Also, its topology is mainly dependent on the connectivity and the symmetry of the metal ions (or metal clusters) and organic nodes. We find the rod-MOF topology [Rod MOFs are metal-organic frameworks in which the metal-containing secondary building units consist of infinite rods of linked metal-centered polyhedral], according to the present approach, the metal ions are connected to the framework by the organic SBUs (Scheme 1).

Experimental section
Solvothermal synthesis of Mo-MOF. Mo-MOF was prepared by mixing Na 2 MoO 4 with 4-piperidinecarboxylic acid (2:1 molar ratio M:L) in 20 mL of the DMF/H 2 O (18:2; v/v) and then magnetically stirred at 200 rpm for a 30 min at room temperature. Subsequently, the mixture was transferred to a Teflon-lined stainless steel autoclave and heated isothermally at 170 °C for 24 h. After, the autoclave was gradually cooled to room temperature. Finally, the resultant brown crystal was filtered by vacuum filtration followed by washing with ethyl acetate and dried at 60 °C in vacuum oven for 6 h (Scheme 1).
Biodiesel production. The catalytic activity of Mo-MOF was assessed for esterification reactions of oleic acid and palmitic acid by mixing oil (1 mol), methanol (13 mol), and MOF (300 mg) mixed in a round-bottom flask. The mixture was heated at 60 °C for 4 h. After the reaction is done, the catalyst was separated by centrifuges (5000 rmp), and the excess methanol was removed from the upper liquid phase using rotary evaporation completely. The extracted organic phase was further washed with distilled water to remove the residual impurities by decantation and dried with anhydrous Na 2 SO 4 .
Characterization of Mo-based metal-organic frameworks. The catalytic system has been studied by FTIR, XRD, BET, SEM, TEM, EDX, ICP-OES, TGA, DSC, and EDS elemental mapping. Moreover, Fourier transforms infrared (FT-IR) spectroscopy has been introduced for the quantitative analysis of Mo-based metal-organic frameworks. Figure 2 provides the FT-IR spectra of Na 2 MoO 4 . 2H 2 O (a), 4-piperidinecarboxylic acid (b), Mo-based metal-organic framework (c). FT-IR spectrum of 4-piperidine carboxylic acid exhibited two peaks at 3525 and 3450 cm −1 corresponding to stretching vibrations of -OH and -NH groups, respectively. The peak at 1645 cm −1 was attributed to the carboxylic (-COO) group of piperidine. The peaks at 2854 and 2967 cm −1 are corresponded to -C-H stretching of the methylene group. The strong peak of C-N stretching was observed at 1406 cm -1 . Moreover, -OH bending and -N-H wagging of secondary amine were observed at 970 and 686 cm −1 , respectively (  (Fig. 2c). It is worth noting that the C-N stretch band, which was present at 1406 cm −1 in the spectrum of 4-piperidine carboxylic acid, which is absent in the spectrum of the Mo-based metal-organic framework (Fig. 2c).
Powder X-ray diffraction was used to determine the chemical composition and crystal structure of the typical synthesized products. Obviously, Mo-MOF exhibits a series of sharp peaks, indicating its good crystallization. The characteristic peaks are at 28.17, 33.12, 49.37, 52.7, 57.2, 65.17, and 68.2, which are identical to representative reference 47,48 , (Fig. 3). All of these characteristics indicate that Mo-MOF was successfully synthesized. The morphologies of Mo-MOF are monitored by scanning electron microscopy. The SEM images (Fig. 5) revealed that morphology of the product featured a rod-like structure with a well-defined plane facing along the transversal direction at approximately 500 nm-2 µm. The inset in Fig. 5a displays the edge of a bar with the typical cleavage for monoclinic structures (inset Fig. 5a).  www.nature.com/scientificreports/ The structure and morphology of the Mo-metal-organic framework (MOF) have been studied by TEM. Figure 6 shows that the particles have a nanorod-like structure.
TGA (Thermo-gravimetric analysis) was performed to investigate the thermal stability of the synthesized materials and was carried out from room temperature to 1000 K by using a TGA analyzer at a heating rate of 10 K/min under air atmosphere (Fig. 7). The result demonstrated thermodynamic stability of the materials up to 300 °C. The weight loss was observed in two steps: the first weight loss (3%) occurred in the range of 100 °C related to the loss of physically adsorbed H 2 O molecules and organic solvents. The second weight loss (19%) between 330 and 600 °C is corresponding to the decomposition of the organic species. The DSC shows two endotherms at 270 and 330 °C which corresponds to the successive release of water and DMF molecules from the host channels. The DSC analysis also displays an exothermic after complete loss of functional groups of guest molecules and coordination modes at approximately 420 °C which show that the compound undergoes phase transformation.
The texture and porosity of the Mo-MOF was quantified by measuring the nitrogen adsorption isotherm (Fig. 8). The results show that according to the IUPAC classification of adsorption isotherms, the N 2 isotherm resembles the type III having sharp adsorption capacity while indicating the presence of broader pore size distributions, and narrower mesopores, and wider micropores. According to The Brunauer-Emmett-Teller (BET) method, the specific surface area and pore volume were estimated to be 56 m 2 g −1 and 12. 98 cm 3 g −1 . The BJH pore size calculations using the adsorption branch of the nitrogen isotherm indicate a micropore peak at about 1.66 nm of diameter for Mo-MOF (Fig. 9).
Material performance. In this study, the catalytic properties of the investigated Mo-MOF systems were performed by the esterification of oleic acid and palmitic acid with methanol. The biodiesel production was optimized using Mo-MOF as catalyst (100-300 mg), and various oleic acid/alcohol molar ratios at different temperatures. According to the obtained results, the yield of the reaction decreased with the catalyst amount decreasing. This may be due to the decrease in the total sorption surface area available to adsorbates resulting www.nature.com/scientificreports/ from the overlapping of active sites. According to these results, the maximum biodiesel production was achieved by300 mg of catalyst. A maximum conversion of 95% (oil to ester) was achieved for the temperature of 60 °C. The molar ratio between methanol and oil was considered to be 13:1 in this study for the completion of the esterification process (Table 2).   Table 3, the flashpoint of biodiesels made from palmitic acid and oleic acid are measured to be 140, and 160 °C, respectively which is in agreement with ASTM D92. The pour point represents the lowest temperature at which a liquid will begin to flow. The pour point of biodiesels made from palmitic acid and oleic acid is calculated to be 12 and − 3 °C, respectively, which agrees with ASTM D97. The cloud point is the temperature at which wax crystals begin to form in a liquid as it is cooled. The cloud point of the biodiesel from palmitic acid and oleic acid were 17 and − 1 °C, respectively. Cloud points of biodiesels were almost within the ASTM D2500 standard range. Ash content: when organic compounds are decomposed at high temperatures  www.nature.com/scientificreports/ (500-600 °C), the leftover residue is called ash. The approved limits of the carbon and ash residue after biofuel ignition are 0.01 wt% according to ASTM D-482, respectively. The obtained ash residues of the biodiesel from palmitic acid and oleic acid were between 0.004 and 0.002 wt% using Mo-MOF. These values are comparatively lower than those obtained in the case of petroleum fuel.
Comparison. In order to show efficiency of the catalytic activity of Mo-MOF, we compared our results for the esterification of reaction of oleic acid has been shown in Table 4. It is evident that our protocol shows excellent catalytic activity in terms of yield of product.
Catalyst reusability and catalytic performance comparison of Mo-MOF. Reusability emerged as a very important factor to determine the robustness of a heterogeneous solid catalyst and its commercial potential, particularly from economic and practical viewpoints. Filtration and centrifugation are two of the methods traditionally employed on a laboratory scale to allow handling, separation, recovery, and recycling of heterogeneous catalysts. Furthermore, the MOFs are often suitable as highly recyclable catalytic systems with easy and efficient recovery of catalysts through simple filtration or centrifugation. In this regard, the catalytic stability of Mo-MOF was investigated by the esterification of oleic acid. After the reaction was completed, the catalyst was   www.nature.com/scientificreports/ separated using centrifugation. Then, it was washed with methanol and dried entirely at 80 °C. The dried catalyst was reused in the second reaction under optimum reaction conditions (Fig. 10). The catalyst was recycled efficiently for four cycles and the catalyst showed almost constant activity in the esterification of oleic acid (92% after the 4th recycle was obtained). The retrieved catalyst after four recycles maintained its original framework structure, as evidenced by FT-IR technique. The FT-IR spectrum of the recovered Mo-MOF indicates that this catalyst can be recycled without any change in its structure (Fig. 2d). Moreover, the nature of the catalyst was also determined by investigating the leaching of the molybdenum species by using ICP-AES analysis. ICP-OES analysis exhibited very low leaching of molybdenum species upon reuse. Based on the results from the ICP-OES analysis, the amount of molybdenum in the fresh catalyst and the recovered catalyst after four runs was 0.077 mol g −1 and 0.072 mol g −1 respectively, which this observation was attributed to the strong interactions of heteroatoms in the piperidine-4-carboxylic acid with molybdenum. Moreover, the results confirmed the heterogeneous nature of the catalysis.

Conclusion
In summary, a novel Mo-MOF was prepared by solvothermal method. Structural features of the material were ascertained by several analytical techniques like XRD, FTIR, TGA, DSC, BET, SEM, TEM, ICP-OES, X-ray mapping, and EDX analysis. Interestingly, obtained solid catalyst exhibits more highly catalytic activity in the esterification of oleic acid and palmitic acid with methanol. The influential reaction parameters, including the methanol/oil molar ratio, temperature, and catalyst amount, are optimized. The physicochemical properties of the biodiesel, such as density, viscosity, flash point, pour points, and ash was determined according to the standard ASTM methods. Control experiments showed that the fuel properties of prepared biodiesel are found to meet the international biodiesel standards. Moreover, the catalyst could be simply recovered and efficiently reutilized several times without significant loss in its activity, also obtained results revealed that this metal-organic framework could be used for the appropriate and rapid biodiesel production. This strategy can produce a solid catalyst with catalytic site accessibility, which has a good application future in the field of green chemistry.