1 Introduction

Renewable raw materials for the production of biodegradable materials have become a fundamental strategy for a global sustainable future alternative to replace the use of oil and its derivatives, which represent one of the main sources of environmental pollution [1,2,3]. Vegetable oils are one of the most promising, cheap, and available types of renewable sources, currently becoming the focus of interest for the chemical industry, and considered as environmentally friendly starting materials for the development of new processes and products [4,5,6].

Vegetable oils are products extracted from the seeds and/or fruits of oleaginous plants such as soybeans, palm and sunflower [2, 3], which at the molecular level are made up of triglycerides, glycerol esters, and straight-chain fatty acids [7]. They are currently of great importance for industry due to their use in food and in the production of biofuels [4, 8,9,10]. Because of the presence of unsaturation in their chemical structure, much research is focused on obtaining EVOs, which are used nowadays as precursors for the synthesis of polyurethane foams [11,12,13,14], plasticizers-stabilizers of high-use polymers such as PVC [15,16,17,18] and as essential components for obtaining biodegradable lubricants [19, 20].

In industry, EVOs are obtained through the Prileschajew reaction, which is a homogeneous classical epoxidation reaction catalyzed by percarboxylic acids (R–COO–OH) synthesized in situ from mineral acids, and used as oxidizing agents [21]. As highly toxic reagents are used in this reaction, the generated chemical waste is difficult to handle. On the other hand, the low selectivity, caused by the opening of the oxirane ring, leads to several by-products which decreases the process efficiency and increases the costs of the separation processes [3, 22, 23].

Various homogeneous catalysts have been proposed for this reaction, ranging from the use of enzymes to epoxidation in the presence of polyoxometalates or coordination complexes. [3, 24,25,26,27,28,29,30]. From these studies it was observed that catalysts based on active transition metal centers such as Ru, Co, Mo, Rh, and Ti showed a high reactivity using more environmental friendly oxidizing agents such as O2, H2O2, and TBHP [31,32,33,34]. Despite these advances, the problems associated with the recovery of the catalyst from the reaction medium, urged to examine heterogeneous catalysts such as ion exchange resins, clays, silicates, and inorganic oxides [3, 35,36,37,38]. Moreover, the incorporation of successful homogeneous catalysts on the surface of other inorganic solids have also been examined [39,40,41]. In the epoxidation of soybean oil, different heterogeneous catalysts have been evaluated, such as the Amberlite-16 resin, which was able to achieve a selectivity close to 80% or a natural zeolite with a selectivity of 82% and conversion of 96%, but using performic acid and H2O2 with formic acid, respectively [42, 43].

In the specific case of molybdenum, it represents one of the most studied structures in selective oxidation processes using enzymes known as oxotransferases, in which the active dioxomolybdenum (MoVIO2) unit is involved in the oxygen atoms transfer processes [44,45,46,47]. Adapting these natural systems into analogous bio-inspired solid materials, MoVIO2 active units have been incorporated into different supports such as TiO2, SiO2, and montmorillonite K10 through functional groups present on the ligands of their respective complexes. The obtained materials demonstrated to be highly selective in the oxidation of arylalkanes and epoxidation of both linear and cyclic alkenes. Moreover, their advantage of easy separation and reuse afterwards showed significant improvements in the conversion and selectivity compared to bulk catalysts doped or modified with transition metals [48,49,50,51,52,53].

Metal–organic frameworks better known as MOFs consist of metals linked systematically by organic ligands with suitable substituents [54, 55]. These alternating metal–ligand combinations result into highly crystalline solids with a defined topology, large internal surfaces that give rise to high specific surface areas (up to 7000 m2 g−1), low densities (up to 0.13 g-cm−3), and high metal content [56,57,58,59,60,61]. Additionally, the structural stability of the metal–organic framework and the functionality of the functional groups or heteroatoms present on the organic linker have been used for the post-synthetic functionalization of these materials with metals, or with catalytic centers [62, 63]. MOFs have been examined for a range of applications e.g. in the removal of pollutants [64,65,66], catalytic and photocatalytic coupling reactions [67,68,69] and CO2 cycloaddition with epoxides [70,71,72].

Regarding epoxidation reactions catalyzed by metal–organic structures, new solid materials are reported every day, but their applications are restricted to low molecular weight unsaturated model molecules [55]. The novelty of this work was to evaluate the activity and stability of a metal–organic framework as a heterogenous catalyst in the epoxidation of unsaturated natural molecules with high molecular weight, which are of great industrial interest due to cheap and its availability as a renewable natural resource in Colombia and Latin-America. More specifically, it was examined the catalytic performance of the MoO2 catalytic sites incorporated in a metal–organic structure denoted as MoO2Cl2@COMOC-4, in the soybean oil epoxidation. Currently, different methods and catalysts are being studied to obtain epoxidized vegetable oils (EVO), seeking to replace highly harmful chemical agents and move towards more ecological and environmentally friendly processes [73,74,75,76]. To the best of our knowledge, this previously reported metal–organic framework functionalized with dioxo-molybdenum (VI) active center [77] represents the first example of this type of porous solids applied in the epoxidation of vegetable oils, resulting in an alternative oxidation catalyst with an enhanced selectivity. Moreover, compared to the conventional vegetable oil oxidation methods, the proposed method is also much greener than other processes.

2 Experimental

All the experiments were performed using standard Schlenk techniques under an inert atmosphere. Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectra (EDX) were obtained using a microscope FEI Quanta 200. Before analyses, the samples were metalized with a gold–palladium alloy using a Quorum Q150R ES metallizer. The surface area and porosity were determined from nitrogen (N2) gas adsorption isotherms, taken at 77 K with a Micromeretics ASAP 2010 adsorption analyzer in the P/P0 range of 1 × 10−5 to 0.99. Prior to the analyses, the solids were outgassed during 8 h at 110 °C and 1 µtorr. X-ray powder diffraction (XRPD) analyses were carried out using an X’Pert Pro MPD PANalytical equipment with Cu anode (Cu Kα radiation, λ = 1.54056 Å) and Bragg–Brentano configuration. The FT-IR ATR spectra were obtained using a Shimadzu IR prestige 21 spectrophotometer (Columbia, MD, USA). Diffuse reflectance UV–Vis spectra were recorded on a Hitachi U-3000 UV–Vis spectrophotometer. X-ray fluorescence (XRF) measurements were recorded with a NEX CG from Rigaku using a Mo-X-ray source. Differential Scanning Calorimetry (DSC) and Thermogravimetric analysis (TGA) was carried out using a Mettler Toledo Model TGA-1 thermal analyzer in a temperature range of 30–900 °C, under N2 at a heating rate of 10 °C min−1. A Bruker Avance 400 spectrometer was employed to measure the 1H NMR spectra of the collected samples (60 mg) during catalytic experiments. The samples were dissolved in deuterated chloroform (0.6 mL) containing TMS as internal standard.

2.1 Synthesis of COMOC-4

The synthesis of COMOC-4 was optimized at the gram scale based on a initially published procedure [61, 78]. Ga(NO3)3⋅H2O (1.2 g, 4.4 mmol) and 2,2’bipyridene-5,5’-dicarboxylic acid (H2bpydc, 1.2 g, 5 mmol) were added to 120 mL of DMF in a 250 mL Schlenk flask equipped with a magnetic stirrer. The mixture was heated to 110 °C and kept at this temperature for 0.5 h. Afterwards, the mixture was heated to 150 °C and held at this temperature for 48 h under gentle stirring. At the end of the reaction, an orange powder was separated by filtration and washed thoroughly with DMF, methanol, and acetone. In the following step, the as-synthesized MOF was suspended in DMF (0.5 g in 50 mL DMF) and heated at 80 °C for 2 h after which it was collected through filtration, washed with DMF and acetone, and dried under vacuum.

3 Synthesis of MoO2Cl2@COMOC-4

MoO2Cl2@COMOC-4 was synthesized according to the previously published procedure [77]. Typically, 1.8 g MoO2Cl2 was added to 75.0 mL of THF and stirred for 10 min at room temperature. The yellowish solution was filtrated to remove the solid impurities and evaporated up to dryness to obtain the MoO2Cl2 (THF)2 complex. The obtained complex was dissolved in 100 mL of THF, after which 2.5 g COMOC-4 was added to this solution and vigorously stirred at room temperature for 2 h. The solid product was filtered, washed with acetone, and activated before use.

3.1 Soybean Oil Epoxidation

The catalytic tests were carried out in a 25.0 mL round bottom flask equipped with a reflux condenser containing 1.0 g of soybean oil (1.0 mmol, equivalent to 4.0 mmol of double bonds), 10.0 mL of toluene, 66.0 mg of MoO2Cl2@COMOC-4 (0.04 mmol of Mo, equivalent to 1% of the double bonds present in oil), and a 70% TBHP in aqueous solution used as the oxidizing agent. Several reaction parameters were examined including the substrate/TBHP ratio and the temperature. At the end of the required reaction time, the mixture was filtered, and the catalyst was separated and washed (initially with toluene and then with acetone to be dried under vacuum). Before reuse and respective characterization of the catalyst, the solid was left under stirring in toluene at room temperature for 12 h after which it was filtered and dried under vacuum at 110 °C for 4 h. Consequently, the reaction mixture was subjected to a liquid–liquid extraction with 4.0% saline solution (15.0 mL × 2) and 15.0% w/v sodium bisulfite solution (0.5 mL), in order to decompose the remaining oxidizing agent and remove its by-products (tert-butyl alcohol), from the reaction mixture. After that the organic phase was separated, dried over anhydrous Na2SO4, and the solvent was removed by rotaevaporation to obtain an off-white viscous liquid.

3.2 Analytical Quantification Methods

The monitoring of the epoxidation reaction was carried out by determination of the oxirane oxygen content (% O.O, expressed as grams of oxirane oxygen per 100 g of oil) and the iodine number (IY) (defined as g I2 per 100 g of oil) before and after each catalytic test. The I.Y was determined by the classical method of Wijs [79], while the % O.O was calculated by applying the AOCS Official Method CD 9–57 [80]. The catalytic activity was evaluated by determination of the conversion, selectivity, and yield of the epoxidation reaction, using the following equations, where i and f represent initial and final values respectively:

$$\mathrm{\% }Conversion= \frac{moles \, C=C consumed}{{moles \left(C=C\right)}_{i}}\mathrm{x}100=\frac{{(I.Y)}_{i}-{(I.Y)}_{f}}{{(I.Y)}_{i}} x 100$$
$$\mathrm{\% }Selectivity= \frac{moles \, C=C \, converted \, to \, epoxide}{moles \, de \, C=C \, consumed} x 100$$
$$\mathrm{\% }Epoxidation= \frac{moles \, C=C \, converted \, to \, epoxide}{{(I.Y)}_{i}} x 100$$

The previously described catalytic activity parameters were validated by applying a second quantification method previously published using 1H NMR in CDCl3 [81]. The molecular weight (M) of the original soybean oil (equal to 871.5 g-mol−1) was calculated from its 1H NMR spectrum (Fig. 1), using the following equation:

Fig. 1
figure 1

1H NMR spectrum of the soybean oil sample represented by a triglyceride of linoleic acid (R1 and R2 are alkyl groups)

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$$M=\frac{15.034{\varvec{G}}}{3NF}+ \frac{14.026(C+D+E+F+H}{2NF}+\frac{26.016(A-NF)}{2NF}+173.1,$$

where NF is the normalization factor (the relative peak area of one hydrogen) calculated from the signal area associated with the four hydrogens of the methylene groups of the glycerol moiety (signal B in Fig. 1).

The number of double bonds (ND) present in the initial soybean oil sample was determined by the following equation:

$${ND}_{i}=\frac{A-NF}{2NF}$$

which was used to determine the percentage of conversion, epoxidation, and selectivity according to [82]:

$$\mathrm{\% }Conversion= \frac{{ND}_{i}-{ND}_{f}}{{ND}_{i}} x 100,$$
$$\mathrm{\% }Epoxidation = \frac{(K+L)/2}{NF\times {ND}_{i}} x 100,$$
$$\mathrm{\% }Selectivity= \frac{\mathrm{\% }Epoxidation }{\mathrm{\% }Conversi\mathrm{o}n} x 100,$$

where NDf is the number of double bonds that remain unreacted and K and L are the peak areas associated with the hydrogens of the epoxide groups that are identified by new signals at chemical shifts of 2.9 (monoepoxide) and 3.1 (diepoxide) ppm, respectively.

4 Results and Discussion

4.1 Synthesis, Characterization, and Structural Information

The X-ray diffraction analysis of COMOC-4 confirmed its crystallinity which corresponds to an open architecture typically observed for MIL-47 and MIL-53 series but constructed by infinite chains of octahedral GaO4(OH)2 units, in which each Ga3+ ion is bound to four dicarboxy-bipyridine ligands and two µ2-trans hydroxide anions (Fig. 2).

Fig. 2
figure 2

XRPD patterns of COMOC-4 and MoO2Cl2@COMOC-4

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After incorporation of MoO2Cl2, the main Bragg diffraction angles of COMOC-4 are preserved, but a slight decrease in the intensities of some peaks in the MoO2Cl2@COMOC-4 diffractogram was observed, suggesting that the crystallinity of the COMOC structure might be partially impaired in the post-functionalization process [83]. Additionally, new Bragg reflections (or splitting) have been identified close to the original ones at of lower intensity, originated by slight distortions in the shape and angle of the linkers because of the interaction between the Mo(VI) center and the chelating nitrogen atoms (about 2θ = 14°). The results obtained from nitrogen adsorption, infrared and UV–Vis diffuse reflectance spectroscopy for COMOC-4 and MoO2Cl2@COMOC-4 are in agreement with the previously reported data (see Supporting information, Figure S1-S4 and Table S1). BET surface areas and pore volumes are 742 m2/g and 1.66 cm3/g for COMOC-4 and 214 m2/g and 0.78 cm3/g for MoO2Cl2@COMOC-4, reveling a partial deterioration of the textural properties as consequence of the incorporation of MoO2Cl2 units. A quantity of 5.9% Mo in the MoO2Cl2@COMOC-4 sample was determined by means of XRF. In other words, 22% of bipyridine sites was loaded with the active dioxo-molybdenum (VI) complex (the Mo/Ga molar ratio is 0.22) based on the empirical formula of the catalysts, namely C12H7N2Cl0.44GaMo0.22O4.44.

4.2 Soybean Oil Epoxidation Reaction

The catalytic performance of the MoO2Cl2@COMOC-4 catalyst was evaluated in the epoxidation of soybean oil using TBHP as oxygen donor agent. In the experimental design, the molar concentration of the oil (≈0.1 M) and the molar relation between the number of unsaturations and Mo active centers were kept constant (100:1), using toluene as solvent. To demonstrate the participation of the MoO2 active center and the role of the oxidizing agent in the catalytic epoxidation process, different control experiments were carried out. An initial control reaction was performed at 80 °C in the presence of MoO2Cl2@COMOC-4 and in the absence of the oxygen donor agent (TBHP). A conversion of 3.3% and a selectivity of 17.7% (Table 1, Entry 1) were obtained, due to the oxygen atom transfer from the dioxo-molybdenum (VI) unit towards the double bond [48, 84,85,86]. A scheme of the stoichiometric epoxidation process of soybean oil from MoO2Cl2@COMOC-4 catalyst without oxygen donor agent is presented in Fig. 3.

Table 1 Results of the preliminary experiments of epoxidation of soybean oil at 80 °C
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Fig. 3
figure 3

Stoichiometric epoxidation of soybean oil from MoO2Cl2@COMOC-4 catalyst without oxygen donor agent. No reaction products were obtained when COMOC-4 solid was evaluated under the same conditions

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A second control reaction using the same temperature was carried out with TBHP as oxygen donor agent using a molar ratio TBHP: double bonds: catalyst of 100:100:0 (without Mo catalyst). In this test, a conversion of 37.7% of the respective oil was observed but with a very low selectivity (13.3%) towards the epoxide (Table 1, Entry 2). This low selectivity is caused by side reactions that result in different products such as alcohols, ketones, or carboxylic derivates (Fig. 4), which is the main problem associated with conventional epoxidation methods [3].

Fig. 4
figure 4

Scheme of some side reactions under different conditions in soybean oil epoxidation process

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A first test in the presence of MoO2Cl2@COMOC-4 and the oxygen donor agent (TBHP) was carried out at 80 °C during 4 h employing a molar ratio TBHP: double bonds: catalyst of 100:100:1. As shown in Table 1, entry 3, a conversion of 17.0% and a selectivity of 56.4% were obtained. The respective monitoring of the oil sample, before and at the end of the reaction, using 1H NMR spectroscopy corroborated the appearance of the oxirane ring (signals at 3.1 and 2.9 ppm) and the respective decrease in the signal corresponding to the proton A, as seen in Fig. 5. A preference towards the formation of monoepoxide was deduced because of the higher intensity of the signal at 2.9 ppm compared to the characteristic diepoxide signal at 3.1 ppm [87].

Fig. 5
figure 5

1H NMR spectrum of soybean oil after epoxidation reaction

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It is well known that transition metal ions in their highest oxidation state such as Mo(VI) can simultaneously support one or several oxygen ligands such as oxo, peroxide, hydroxide or hydroperoxide in their coordination sphere, and the type of oxygen ligand that is incorporated into the active molybdenum center depends on the oxidizing agent used in the respective catalytic process [85, 86]. On the other hand, although the mechanism of epoxidation of alkenes is still controversially discussed [84, 88, 89], the epoxidation mechanism in vegetable oils using THBP as an oxidizing agent has been previously reported [90], and the results are in agreement with the ideas proposed by Sobczak [29], that begins with the formation of the species Mo(VI)–O–O–(CH3)3 as an intermediary through coordination via the peroxo oxygen bonded to the molybdenum atom, and formation of a hydrogen bonding with one of the terminal oxygens. The metal center acts as a Lewis acid by removing charge from the O–O bond, facilitating its dissociation, combined it with the olefin nucleophilic attack to the electrophilic oxygen atom of the coordinated peroxide (Fig. 6) [91, 92].

Fig. 6
figure 6

Illustration of the soybean epoxidation reaction catalyzed by MoO2Cl2@COMOC-4 with hydroperoxides

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To evaluate the stability of the MoO2Cl2@COMOC-4 catalyst during the soybean oil epoxidation reaction, the used catalyst was characterized by means of XRPD, IR spectroscopy, thermogravimetric analyses, and SEM microscopy (Figs. 7 and 8).

Fig. 7
figure 7

XRPD, IR and thermogravimetric analysis of MoO2Cl2@COMOC-4 catalyst fresh and used in the epoxidation reaction with TBHP

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Fig. 8
figure 8

SEM images and EDX of a COMOC-4, b MoO2Cl2@COMOC-4 catalyst, and c used MoO2Cl2@COMOC-4 catalyst

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XRPD analyses of the used catalyst corroborated the structural integrity of the support in the soybean epoxidation reaction (see Fig. 7a), and the IR spectrum of the catalyst after reaction (Fig. 7b) has the same signals in the range 600–1800 cm−1 assigned to the stretching vibrations of the bipyridine ligand in the fresh MoO2Cl2@COMOC-4 catalyst. Furthermore, no changes in the intensity of the symmetric and asymmetric vibrations of dioxo-molybdenum (MoO2) were observed in the region between 890 and 950 cm−1, evidencing the preservation of the oxidation state in the metal center during the catalytic test. The thermogravimetric analyses, presented in Fig. 7c, demonstrate the high stability of the metal–organic structure since no significant changes in the loss of mass of the catalyst were observed between the fresh and spend catalyst. The temperature decomposition of the catalysts was determined by DSC [93] (Figure S5 -Supporting information) obtaining a value close to 500 °C for the fresh solid, which presented a slight decrease (around 8 °C) when compared to the used MoO2Cl2@COMOC-4 catalyst.

Finally, SEM images of the MoO2Cl2@COMOC-4 catalyst revealed heterogeneous morphology and particle sizes, and EDX analyses confirmed the presence of molybdenum in the metal–organic framework catalyst before and after the epoxidation process (Fig. 8). Evidently, irregular morphologies were observed for MoO2Cl2@COMOC-4 catalyst before and after used, but the particle size was decreased for the used solid. This fractionation is a consequence of the mechanical attrition of the catalyst throughout the catalytic process. From these initial results, it was proposed to examine the influence of concentration of the oxidizing agent, the reaction time, and the reaction temperature in the heterogenous soybean oil epoxidation catalyzed by the MoO2Cl2@COMOC-4 solid.

4.3 Influence of the Concentration of Oxidizing Agent

To determine the optimal reaction conditions in the presence of the oxidizing agent (TBHP), three additional experiments were carried out in which the molar ratio of the oxidizing agent was varied with respect to the number of unsaturation contained in the oil (Fig. 9 and Table 2), while keeping the reaction time (4 h), the temperature (80 °C) and the moles of double bonds constant (Entries 2–4, Table 2). Initially, a 2:1 molar ratio of oxidizing agent: unsaturations was evaluated, which resulted in an increase in the conversion, selectivity, and percentage of epoxidation (Entry 2, Table 2).

Fig. 9
figure 9

Analysis of the molar ratio variation oxidizing agent: double bonds in soybean oil epoxidation at 80 °C catalyzed by MoO2Cl2@COMOC-4 with TBHP as an oxidizing agent

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Table 2 Results obtained to determine the optimal reaction conditions in soybean oil epoxidation catalyzed by MoO2Cl2@COMOC-4 with TBHP as oxidizing agent
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Using an oxidizing agent:doble bond ratio to 4:1 resulted only in a slightly further increase in the selectivity percentage (Fig. 9, and Entry 3 of Table 2) while the conversion and percentage of epoxidation decreased. This is because the generated by-product tert-butanol interferes in the catalytic cycle, which becomes more evident in our case when an oxidizing agent:double bond ratio of 8:1 was used (Entry 4, Table 2). In other words, increasing the concentration of the oxidizing agent leads to its decomposition and formation of larger amounts of by-product (tert-butanol), which was subsequently detected through the purification processes.

4.4 Influence of Reaction Time

Once the best oxidizing agent:double bond molar ratio was determined at 80 °C, the influence of the reaction time was studied. Initially, two reaction times were evaluated, 4 and 24 h (Entry 2 and 5, Table 2). After 4 h of reaction, the conversion reached a value of 18.6%, while after 24 h a conversion of 75.8% was obtained (Fig. 10). Although a higher conversion was noted, the selectivity decreased from 66.6% to 14.8% after 24 h of reaction. This result shows that the epoxidation product is formed quite fast and that long reaction times favors the formation of by-products due to collateral reactions [94]. Additionally, a prolonged reaction time leads to a significant darkening (browning) of the oil after prolonged contact with atmospheric oxygen, which indicates the occurrence of undesired oxidation processes [95].

Fig. 10
figure 10

Evaluation of the reaction time in the soybean oil epoxidation at 80 °C catalyzed by MoO2Cl2@COMOC-4 with TBHP

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4.5 Influence of Temperature

To evaluate the influence of the reaction temperature, two additional experiments were carried out (at room temperature and at 110 °C) during 4 h of reaction, using the optimal concentration of the oxidizing agent. The catalytic activity of MoO2Cl2@COMOC-4 with TBHP at room temperature showed no formation of reaction products. On the contrary, an increase of the temperature to 110 °C (Entry 6, Table 2) led to a significant increase in the reaction yield (26.6% epoxidation, 29.4% conversion, and 90.6% selectivity) compared to the results obtained at 80 °C (Entry 2), as evidenced in Fig. 11.

Fig. 11
figure 11

Evaluation of the reaction temperature in the soybean oil epoxidation at 4 h of reaction catalyzed by MoO2Cl2@COMOC-4 with TBHP

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Such an increase in activity for the vegetable oil oxidation was already observed for different catalysts in literature. However, it is important to note that the conversion is relatively low compared to the oxidation of molecules such as cyclohexene, cyclooctene or small linear alkenes with this type of catalyst [57, 77]. This low conversion is probably the result of the larger size of the constituent molecules of soybean oil, restricting the entering of the molecules into the pores of MOF.

Finally, two more reactions were carried out to establish the optimal reaction time at 110 °C. The reactions carried out for 8 h and 2 h (Entries 7 and 8, Table 2) did not show better results than the reaction performed during 4 h of catalysis at the same temperature. In this way, a prolonged reaction time largely affects the selectivity of the reaction. Also, from the TON and TOF (26.1 and 6.5 h−1) values, it is clear the catalyst exhibits its highest catalytic activity and selectivity after 4 h of reaction at 110 °C (Fig. 12).

Fig. 12
figure 12

Evaluation of the reaction time for the soybean oil epoxidation at 110 °C catalyzed by MoO2Cl2@COMOC-4 with TBHP

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Compared to published Mo-based heterogeneous and homogeneous catalyst systems (Entry 1–3), presented in Table 3, all these catalysts show more conversion than MoO2Cl2@COMOC-4 in epoxidation of soybean oil, reaching transformations of the aliphatic double bonds above 30%. The selectivity values observed, on the contrary, do show that the MoO2 unit is more selective being directly functionalized onto the solid metal–organic structure. This phenomenon has been previously observed in epoxidation reactions of natural products such as Limonene and alpha-pinene, using different supports, and has been associated with the stability generated at the active center, when it is incorporated covalently on a solid structure, originating a more selective process towards the epoxide [102,103,104]. Additionally, when MoO2Cl2@COMOC-4 was compared with heterogeneous catalysts based on oxo-tungsten active centers (Entry 4–5, Table 3), SiO2, Amberlite IR-120, or AlO3 solid supports that use inorganic acids as co-catalysts (Entry 7–9, Table 3), the same trend was observed in the conversion and selectivity values. Finally, it is important to highlight that in our case, both the mass of catalyst used and the optimal reaction time are significantly lower than the heterogeneous processes that have shown good results in conversion and selectivity (Entry 6–10, Table 3), which may be favorable in economic terms in the future [105].

Table 3 Comparison of catalytic activity of MoO2Cl2@COMOC-4 with other heterogeneous catalysts in the soybeam oil epoxidation
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4.6 Catalytic Reuse Evaluation

To evaluate the reusability of the MoO2Cl2@COMOC-4 catalyst in the soybean oil epoxidation at 110 °C, the solid catalyst was separated from the reaction medium by means of filtration after 4 h of reaction, washed with toluene and acetone, and dried in vacuum. Interestingly, as shown in Table 4, the catalytic activity is preserved during a new successive catalytic test with a slight decrease in the % conversion and % epoxidation (22.0% and 20.3% respectively), but an increase in the selectivity values reaching 92.3%.

Table 4 Results of the catalyst reuse evaluation in soybean oil epoxidation catalyzed by MoO2Cl2@COMOC-4 with TBHP as oxidizing agent a 110 °C
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Initially, to analyze the stability of the active center on the surface, an elemental analysis of Mo using Plasma Mass Spectrometry (ICP-MS) was carried out, evidencing a slow reduction of the concentration of active center (decrease 0.7% wt Mo), but confirming the presence of molybdenum in the metal–organic framework catalyst after the epoxidation process. The oxidation state stability of MoO2 (VI) entity in MoO2Cl2@COMOC-4 catalyst was confirmed by XPS measurements before and after catalysis (Fig. 13). In both samples Mo is present in the oxidation state + 6, confirmed by the molybdenum 3d peak signals (Mo3d5/2 and Mo 3d3/2 Peaks) localized at an average value 232.4 and 235.6 eV, characteristics of this oxidation state [106].

Fig. 13
figure 13

The Mo 3d3/2 and Mo 3d5/2 peak of MoO2Cl2@COMOC-4 catalyst before and after catalysis

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Further, the surface properties of the metal–organic structure obtained by N2 adsorption–desorption measurements of the new and used catalyst were compared. Evidently, the N2 adsorption–desorption isotherms of MoO2Cl2@COMOC-4 fresh and used are different (Fig. 14), revealing a decrease in the BET surface area and porosity. Adsorption–desorption isotherms type IVa (IUPAC classification), typical of mesoporous materials, was maintained. The BJH pore size distributions (Fig. 14-inset) show monomodal functions with pore diameters centered about 14 nm and 12 nm for the catalysts fresh and used, respectively. However, the population of pores (cumulative pore volume) was clearly reduced for the used catalyst. Furthermore, values of BET area and pore volume of 214 m2/g and 0.78 cm3/g for fresh MoO2Cl2@COMOC-4, and 73  m2/g and 0.31 cm3/g for used MoO2Cl2@COMOC-4, were obtained (Table 5). These reductions of porosity and surface areas likely are the result of the partial blockage of the pores by the large molecules of vegetable oil, which is understandable considering the oleic nature and bulky size of oleic and linoleic acid (main constituents of the vegetable oil under study). A similar effect has been previously observed by other authors [83]. These reductions of porosity and surface area and this low conversion are probably the result of the size of the constituent molecules of soybean oil, restricting the entering of the molecules into the pores of MOF, suggesting that the reaction takes place mainly on the active sites available on the surface of the catalyst. The results suggest that MoO2Cl2@COMOC-4 has good potential to be reusable catalysis for the epoxidation processes of vegetable oils; nevertheless, more research is necessary to diminish or reduce the effect of pores clogging.

Fig. 14
figure 14

Nitrogen adsorption/desorption isotherm and pore size distribution analysis (inset) of MoO2Cl2@COMOC-4 catalyst before and after catalysis

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Table 5 Results of the Mo loading, surface area and pore size distribution of MoO2Cl2@COMOC-4 catalyst before and after catalysis
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5 Conclusions

The catalytic activity of MoO2Cl2@COMOC-4 was demonstrated in the selective epoxidation of commercial soybean oil in the presence of tert-butyl-hydroperoxide as an oxidizing agent. The analysis of the temperature, the reaction time, and the TBHP: double bonds molar ratio in the oil revealed that the best conversion, selectivity, and epoxidation results were obtained at 110 °C for 4 h and a 200: 100: 1 molar ratio (TBHP: double bonds: catalyst). The results obtained from the stability of the catalyst during two cycles confirmed the catalyst’s ability to be reused in catalytic processes for the recovery of vegetable oils.