Metal-organic frameworks (MOFs) are porous materials composed of metal ions, clusters and organic ligands. Due to their outstanding chemical, thermal, and solvent stability, as well as numerous unsaturated metal sites, they have proven to be useful catalysts. In this study, MOFs were synthesized using hydrothermal methods with terephthalic acid and Ca, Mg, Al, and Cr nitrates. Subsequently, the MOFs were functionalized with diethylamine. The formation of MOF-Al and MOF-Cr structures was confirmed through characterization by XRD, FT-IR, and CHN analyses, while MOF-Ca and MOF-Mg could not be detected. SEM images revealed the particle size and morphology of the particles, which ranged between 0.2 and 1 mm. TGA/DTA curves revealed that the functionalized MOFs were the most thermally stable. Textural analysis by N2 adsorption/desorption showed that MOF-Cr and MOF-Cr-NH2 had high BET area values of 1,769.67 and 998.22 m2g− 1, respectively. The MOFs were employed as catalysts in Knoevenagel condensation reactions to synthesize (E)-ethyl 2-cyano-3-phenylacrylate and (E)-methyl 2-cyano-3-phenylacrylate, indicating their potential for reactions requiring acidic or basic sites.
Research Article
Synthesis, characterization and functionalization of MOFs and their use in Knoevenagel condensation reactions between ethyl cyanoacetate and 4-nitrobenzaldehyde
https://doi.org/10.21203/rs.3.rs-3793265/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 27 May, 2024
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Metal-organic framework
post-functionalization
MIL-101-Cr
terephthalic acid
Knoevenagel reaction
In recent years, researchers have focused on the synthesis and development of an emerging class of porous materials called porous coordination polymers (PCPs), also known as metal-organic frameworks (MOFs). These materials are formed by combining single metal ions, primary building units (PBUs), or clusters of metals; secondary building units (SBUs); and multidentate organic molecules to form one-, two-, or three-dimensional structures, respectively (Bhattacharjee et al. 2014; Banerjee et al. 2016; Yuan et al. 2018).
The primary goal when synthesizing MOFs is to establish conditions that yield inorganic building blocks of defined size and shape without decomposing the organic ligand. Additionally, the crystallization kinetics must facilitate successful nucleation and growth of the desired phase. Various methods are employed to synthesize these compounds, as different MOFs can be formed from the same reaction mixtures (Stock and Biswas 2012). The resulting MOFs possess unique characteristics, such as a high surface area and pore size, easy functionalization, high porosity and crystallinity, and strong organometallic interactions. These properties allow them to be used as adsorbents, in separation processes, in gas storage, as catalysts, in biomedical applications, and in proton conduction (He et al. 2021; Karimi Alavijeh et al. 2022).
One of the most prominent examples of MOFs is MIL-101, which possesses a high specific surface area and pore volume structure. Researchers have discovered that MOFs containing unsaturated metal-coordinated sites within their structure have potential as Lewis acid sites, exhibiting hydrothermal stability and high porosity conducive to catalytic activity (Wu et al. 2017; Zou et al. 2022). Due to the variety of unsaturated metal active sites present in their structures, as well as their Lewis or Brønsted acid properties, a wide range of organic reactions have been studied using these materials as catalysts, especially in condensation reactions (Noda-Pérez et al. 2009; Burgoyne and Meijboom 2013; Chughtai et al. 2015; Anılır et al. 2019; Wang and Astruc 2020; Panda et al.).
Knoevenagel condensation, used for the formation of new carbon‒carbon bonds, is widely known and applied in heterogeneous catalytic systems due to the ease of separation and reuse of the catalytic materials (Taher et al. 2020). A variety of different techniques for Knoevenagel condensation, such as Lewis’s acid/base sites, ultrasound irradiation, biotechnology, solid-phase reactions, ionic liquids, green solvents, and solvent-free methods, have been reported. However, long reaction times, high temperatures, expensive catalysts, and the disposal of toxic solvents and catalysts are among the drawbacks of these techniques. Therefore, there has been increasing interest in the development of simpler, more economical and more sustainable synthetic methods for Knoevenagel condensation (Taher et al. 2020).
Although several studies of solid acid catalysts are available in the literature, research on the synthesis, characterization and catalytic properties of new basic solid catalysts is still underway. Therefore, the possible advantages of using MOF materials as heterogeneous catalysts, as well as their ability to be functionalized to increase the surface basicity and improve the catalytic activity of Knoevenagel condensation reactions, should be considered. The aim of this work was to synthesize and characterize MOFs containing the metals Ca, Mg, Al, and Cr with terephthalic acid as the organic ligand to functionalize these compounds with an amino group and use them as catalysts for Knoevenagel condensation reactions between ethyl cyanoacetate and 4-nitrobenzaldehyde.
Terephthalic acid (98% BDC, Merck), Mg(NO3)2.6H2O (P.A., Neon), Ca(NO3)2.4H2O (P.A., ACS) (P.A., Neon), Cr(NO3)3.9H2O (99%, Sigma‒Aldrich), Al(NO3)3.9H2O (P.A., Neon), tetramethylammonium hydroxide (TMAOH) solution (25 wt% in methanol), 4-nitrobenzaldehyde (98%, Sigma‒Aldrich), ethyl cyanoacetate (98%, Sigma‒Aldrich), diethylamine (99%, Baker Analyzed) and methanol (P.A., ACS) were used.
MOF synthesis
The MOF compounds were synthesized according to the methodology of Laredo et al. (2016). In a polytetrafluoroethylene (PTFE) reactor, 1 mmol of TPA and an alkali solution (TMAOH, 5 mL, 0.2% v/v) were added and stirred at room temperature for 10 min; then, 1 mmol of nitrate with the selected metal (Ca, Mg, Al, or Cr) was added to the mixture, and stirring was continued for 20 min. The reactor was placed in an autoclave and heated to 453 K in an oven for 24 hours. After cooling to room temperature, the product was washed with distilled water and methanol. The samples were dried at room temperature for 24 hours, after which the MOFs were dried at 373 K in an oven for 6 hours. These products were named MOF-Me, where "Me" corresponds to the metal present in the structure.
Functionalization
A quantity of 100 mg of each synthesized MOF-Me was suspended in 10 mL of toluene. Then, 0.3 mL of diethylamine was added. The mixture was then refluxed with toluene at 383 K for 8 h. At the end of the functionalization process, the product was filtered, washed with toluene, water and ethanol, and finally dried at room temperature for 24 h and at 373 K in an oven for 6 h. The amine-functionalized MOFs were named MOF-Me-NH2.
Physicochemical characterization
X-ray diffraction patterns were recorded using a DRX-6000 SHIMADZU diffractometer model using a Cu Kα1 1.54, operating at 40 kV and 20 mA and a Bragg‒Brentano configuration θ-2θ with a 0.01° step. Elemental analysis was performed on a Thermo Scientific FLASH 200 Organic Elemental Analyzer. The detection system used was a thermal conductivity detector (TCD), and a calibration curve was constructed using the sulfanilamide standard (P/N 338 25100). A JEOL JSM-6610 scanning electron microscope (Thermo Scientific NSS Spectral Imaging) equipped with EDS was used to determine the morphology of the synthesized samples.
The thermal decomposition of the MOFs, evaluated by thermogravimetric analysis (TGA) and derivative thermoanalysis (DTA), was carried out on a SHIMADZU DTG 60/60H, with a temperature range of 293 K to 1073 K, under a N2 atmosphere with a flow rate of 50 mL/min. The absorption spectra in the Fourier transform infrared (FT-IR) region were recorded on a Perkin Elmer Spectrum-400 instrument. The samples were inserted into KBr pellets with an acquisition range of 4000 − 400 cm− 1. The textural properties of the MOFs, such as BET specific area, external area (t-plot method) and pore volume (BJH method), were determined by N2 adsorption-desorption at 77 K on a Micromeritics ASAP 2020.
The 1H, 13C, 1H-13C HSQC, and HMBC NMR spectra were obtained at 298 K on a Bruker Avance III 500 spectrometer operating at 11.75 T (500 MHz for 1H) using deuterated chloroform (CDCl3) as the solvent. Approximately 20 mg of each sample was dissolved in 500 µL of CDCl3, transferred to 5 mm NMR tubes and immediately analyzed.
Knoevenagel condensation reaction
The catalytic tests using the synthesized MOF-Me and MOF-Me-NH2 as catalysts in the Knoevenagel condensation reaction were performed between the reagents 4-nitrobenzaldehyde (1 mmol) and ethyl cyanoacetate (1 mmol) mixed in 25 mL of methanol and heated to a temperature of 333 K; then, 20 mg of catalyst was added to this mixture. After 6 hours of reaction, the catalyst was separated by filtration, and the filtrate was kept at room temperature overnight. The product was then obtained by filtration with cold methanol and drying at room temperature. All the assays were prepared in a round-bottomed glass flask immersed in a water bath equipped with a reflux condenser and a magnetic stirrer.
High-performance liquid chromatography (HPLC) analyses were performed on an Agilent 1220 chromatograph. A mixture of acetonitrile:water 60:40, v/v was used as the mobile phase. The flow rate of the mobile phase was 1 ml/min in isocratic mode. The column effluent was monitored at a wavelength of 254 nm for 10 minutes. An Agilent Zorbax Eclipse Plus C18 column (30 mm length, 4.6 mm OD and 3.5 µm particle size) was used as the stationary phase at room temperature. The retention times for 4-nitrobenzaldehyde and compounds 1 and 2 were 2.40, 2.9, and 3.3 min, respectively.
Physicochemical properties of the MOF
To confirm the successful synthesis and stability of the metal-organic materials, XRD analysis was performed. The XRD patterns of the MOFs synthesized with the metal ligands Mg(NO3)2˖6H2O and Ca(NO3)2˖4H2O showed well-defined crystalline patterns, with the two most intense peaks at 17.4°, 25.2°, and 27.9° (Figures S1 and S2 in Supplementary Information) corresponding to the crystalline planes (110), (011), and (200), respectively. Since these peaks were reported (Haque et al. 2009) for the pure ligand of 1,4-benzene dicarboxylic acid (H2BDC), the XRD showed that for both materials, we obtained Ca and Mg terephthalates and not MOF-type structures of both metals. The analyses of the amine-functionalized compounds showed peaks in the same crystal planes as those of the unfunctionalized compounds.
In contrast, the XRD patterns of MIL-53(Al) and NH2-MIL-53(Al) MOFs obtained through hydrothermal synthesis using H2O as a solvent, as reported in the literature (Gascon et al. 2009; Pera-Titus et al. 2012; Martinez Joaristi et al. 2012; Sánchez-Sánchez et al. 2015; Spekreijse et al. 2016; Martínez et al. 2017; Majchrzak-Kucęba and Ściubidło 2019), were found to be similar to those of MOF-Al and MOF-Al-NH2 synthesized in this study (Figure S3). The XRD diffraction patterns of the MOF-Cr and MOF-Cr-NH2 (Figure S4) compounds exhibited reflections with peaks below 2θ = 10°, which are similar to the patterns reported in the literature (Naghdi et al. 2015; Laredo et al. 2016; Zhao et al. 2018b, a; Chatterjee et al. 2018; Jouyandeh et al. 2020; Gholipour et al. 2022; Aljaddua et al. 2022; Ramsperger et al. 2022).
The absence of peaks at 2θ = 25.2° or 27.9° indicated the removal of unreacted H2BDC molecules. However, both MOFs exhibited crystallinity problems and amorphous byproducts, which can be attributed to the absence of fluorine anions in the structure since HF was not used in the synthesis of these compounds. Previous studies have shown that HF can act as a mineralizing agent, participating in the coordination of trimeric chromium species to enhance the crystallinity of microporous materials and promote the formation of highly crystalline phases in MOFs. High intensities indicated high crystallinity of the compound, and smaller angles indicated the abundant presence of pores in its structure (Jouyandeh et al. 2020; Gholipour et al. 2022).
The synthesized MOFs were analyzed using CHN elemental analysis to determine the amounts of carbon, hydrogen, and nitrogen present (Table S1). The analysis confirmed that the expected structures were achieved for MOF-Me and MOF-Me-NH2 with the cations Al and Cr, as their carbon and hydrogen were nearly identical to their theoretical values. However, the amount of nitrogen present in the amine-functionalized samples was lower than the theoretical value, which could be attributed to the incomplete binding of certain amine groups within the MOF-Me structure. The presence of residual TMAOH solvent molecules in the MOF-Me compounds may explain the occurrence of nitrogen, which was not entirely eliminated during the MOF washing process after synthesis.
In the SEM images of MOF-Ca, MOF-Ca-NH2, MOF-Mg, and MOF-Mg-NH2, solids with layered lamellar morphologies of different sizes were observed. The functionalized MOF structures exhibited better defined shapes with smoother surfaces. According to the literature, plate or rod shapes, (Wang et al. 2015; Xiao et al. 2021; El-Shahat and Abdelhameed 2022; Dermanaki Farahani and Zolgharnein 2022; Gou et al. 2023) rhombic shapes, (Mazaj et al. 2013) and aggregated polyhedral (Dhawa et al. 2017) or shuttle-like shapes (Bao et al. 2011; Wu et al. 2013; Pu et al. 2018) have been reported. The particle sizes of MOF-Ca and MOF-Mg ranged from 0.4 to 1 mm, as illustrated in Fig. 1a-c. The MOF-Al compound consisted of solids with different polyhedral shapes, as presented in Fig. 1e. In the literature, we found structures with hexagonal morphologies, (Yang et al. 2015) plate types, (Pera-Titus et al. 2012; Peng et al. 2022) and MOF-Al-NH2 structures similar to those of MOF-Al but with agglomerates of small particles with irregular sizes and geometries (Martinez Joaristi et al. 2012; Chen et al. 2013; Sánchez-Sánchez et al. 2015; Autie-Castro and Jardim 2017). Finally, MOF-Cr and MOF-Cr-NH2 exhibited octahedral morphologies and agglomerated small particles with irregular sizes and geometries, as illustrated in Fig. 1g-h, in agreement with the literature (Ivanchikova et al. 2015; Belarbi et al. 2017; Niknam et al. 2018; Ho et al. 2021; Aljaddua et al. 2022). Notably, the morphology of MOF-Cr-NH2 remains unchanged even when NH2 is added to the structure, as confirmed by the XRD patterns. The particle sizes of the Al and Cr MOFs ranged from 0.3 to 0.6 µm and 0.2 to 0.5 µm, respectively.
The thermal stability characteristics of MOF-Ca and MOF-Ca-NH2, exhibited in Figure S5a-b, were analyzed through their DTA/TGA curves, and it was observed that they exhibited similar behavior. At 508 K, both MOFs experienced a mass loss of more than 90% in the TGA curve. MOF-Ca showed a minor mass loss of approximately 1.6% between 370 and 393 K, which was attributed to water molecules. The differential thermal curve of MOF-Ca displayed two endothermic peaks at 388 and 618 K, whereas MOF-Ca-NH2 had only one endothermic peak at 618 K. Figure S5c-d shows the thermal behavior of MOF-Mg and MOF-Mg-NH2. This behavior was similar to that of the Ca MOFs, with high mass loss and decomposition occurring in the range of 503 K until total decomposition.
The TGA curves of both the MOF-Al and MOF-Al-NH2 samples, illustrated in Fig. 2a-b, showed successive thermal degradation events. MOF-Al remained stable up to 583 K but decomposed until 933 K, with a mass loss of approximately 64.7%. MOF-Al-NH2 exhibited a loss of absorbed molecules in the temperature range of 371–473 K, followed by solvent degradation. The decomposition of the compound started at 633 K, resulting in a mass loss of approximately 61%. The presence of approximately 35% residue in both samples is likely associated with the metals present in their structure.
Figure 2c shows the TGA curve of MOF-Cr, which showed a loss of approximately 22% of its mass below 393 K due to the removal of solvents present on its surface. There was degradation of other molecules in the temperature range of 473–623 K, followed by structural decomposition in the range of 623–1073 K, leading to a mass loss of approximately 40%. MOF-Cr-NH2, Fig. 2d, exhibited two mass losses, the first being approximately 7% and the second being approximately 51% at 623K, which can be attributed to structural decomposition. Approximately 32% of the residues were observed in the structure of both samples.
The FT-IR spectra indicated the presence of carboxylate groups (COO-) with characteristic bands between 1300 and 1700 cm− 1. These bands showed asymmetric (1490–1600 cm− 1) and symmetric (1350–1450 cm− 1) stretching vibrations. Additionally, the absorption band between 3200 and 3600 cm− 1 suggested the presence of water molecules (OH-) in the sample. The bands between 690 and 900 cm− 1 were associated with aromatic ring vibrations, and those between 540 and 670 cm− 1 were related to (Metal-O) stretching vibrations (Gumilar et al. 2020, 2022). The absorption peak observed at 1690 cm− 1, as observed in the spectra of the MOFs containing Ca and Mg (Figure S6), suggested the presence of free molecules of terephthalic acid. This finding provides another explanation for the production of Ca and Mg terephthalates rather than the expected MOF-like structures (Chen et al. 2013).
The MOF-Al and MOF-Al-NH2 infrared spectra, Fig. 3a, exhibit a narrow band at 3680 cm− 1, which is attributed to the bridging hydroxyl group (Martinez Joaristi et al. 2012). The FT-IR spectra of the MOFs containing chromium, illustrated in Fig. 3b, display peaks in the range of 1300–1700 cm− 1, indicating the presence of dicarboxylate ligands in their structures. Moreover, weak bands at approximately 1000 and 750 cm− 1 were also observed, which could be associated with the C-H bending vibrations of the aromatic rings. The peak at 585 cm− 1 is due to Cr-O stretching, while the band at 1620 cm− 1 can be attributed to C = N bonds (Gholipour et al. 2022).
The specific surface area characteristics of the metalorganic materials were analyzed in relation to pore size and distribution, and the results are listed in Table 1. Based on the IUPAC classification, the samples containing Ca and Mg cations exhibited type III isotherms (Fig. 7 in the SI). This indicates the formation of nonporous or macroporous solids with weak interactions between the adsorbent and adsorbate, with the adsorbed molecules concentrated at the most favorable adsorption sites (Thommes et al. 2015).
The adsorption-desorption isotherms of MOF-Al showed Type III isotherms (Figure S8). The material's BET surface area was low at 35.1 m2g− 1, while other studies reported BET areas of approximately 1200 m2g− 1 and pore volumes of 0.5–0.8 cm3g− 1 (Martinez Joaristi et al. 2012; Sánchez-Sánchez et al. 2015; Peng et al. 2022). The functionalized MOF-Al-NH2 displayed an increase in surface area, but the obtained value of 310.19 m2g− 1 was lower than the reported values from other studies (Serra-Crespo et al. 2011; Sánchez-Sánchez et al. 2015; Martínez et al. 2017). This discrepancy may be attributed to the presence of chemical species occupying the pores of the MOF structures.
Among the synthesized MOF-Me materials, MOF-Cr demonstrated the highest BET surface area and external area. It also exhibited a high pore volume. MOF-Cr-NH2 displayed the largest pore volume and the highest BET and external surface areas among all the functionalized MOFs. However, both MOF-Cr and MOF-Cr-NH2 had the smallest pore diameters. The BET surface area ranged from 1,769.67 to 998.2 m2g− 1, and the pore volume decreased from 0.82 to 0.35 cm3g− 1. These results suggest that while the amino functional group does not alter the morphology of MOF-Cr, it affects its surface area and pore volume. The MOF-Cr and MOF-Cr-NH2 samples both exhibited Type I isotherms (Figure S8), which are attributed to microporous solids. The presence of hysteresis in both MOFs over the higher relative pressure region indicates the development of mesopores in the structure. These results are consistent with those obtained in previous studies (Chen et al. 2017; Fallah et al. 2019; Aljaddua et al. 2022).
| Sample | BET surface area (m2 g− 1) | External surface area (m2 g− 1) | BJH* pore volume (cm3 g− 1) | Pore diameter (nm) |
|---|---|---|---|---|
| MOF-Ca | 31,56 | 28,96 | 0,0468 | 5,96 |
| MOF-Mg | 7,99 | 11,29 | 0,0240 | 12,06 |
| MOF-Al | 35,09 | 54,09 | 0,1033 | 11,72 |
| MOF-Cr | 1.769,67 | 1.695,62 | 0,8177 | 2,84 |
| MOF-Ca-NH2 | 21,83 | 24,30 | 0,0408 | 7,51 |
| MOF-Mg-NH2 | 9,59 | 16,29 | 0,0324 | 13,53 |
| MOF-Al-NH2 | 310,19 | 116,42 | 0,1057 | 3,26 |
| MOF-Cr-NH2 | 998,22 | 610,97 | 0,3481 | 2,83 |
*BJH pore volume of the adsorbent.
Knoevenagel condensation reaction
The synthesized MOFs were tested as catalysts in the Knoevenagel condensation reaction. The tests were carried out in triplicate and analyzed by HPLC. Table 2 presents the percentage yield and selectivity of reaction products. During the experiments, MOF catalysts containing Ca and Mg could not be recovered through filtration because they dissolved in the reaction mixture. Essentially, these MOFs functioned as homogeneous catalysts, remaining in the same phase as the reactants and products. However, due to their methanol solubility, they could be isolated from the products, which were insoluble at room temperature, leaving behind only the reaction compounds.
Table 2. Knoevenagel reaction yield and selectivity of reaction products.
The MOF-Cr catalyst achieved the highest Knoevenagel condensation yield of 83.37%, whereas MOF-Cr-NH2 yielded only 55.49%. Although the yield was lower, MOF-Cr-NH2 was the only MOF tested that produced additional compounds, as confirmed by the NaOH test. These compounds included (E)-methyl-2-cyano-3-(4-nitrophenyl) acrylate, (Z)-methyl-2-cyano-3-(4-nitrophenyl) acrylate, and (E)-ethyl-2-cyano-3-(4-nitrophenyl)-acrylate. This outcome was unexpected since the use of methanol as a solvent in the presence of catalysts with basic sites resulted in a transesterification reaction of ethyl cyanoacetate with methanol, leading to the formation of methyl cyanoacetate (Figs. S9 and S10).
During the catalytic tests utilizing MOFs, two types of Knoevenagel condensation compounds, namely, (E) and (Z) ethyl 2-cyano-3-(4-nitrophenyl) acrylate, were produced. The product was confirmed by 1H, 13C, 1H-13C HSQC, and HMBC NMR spectroscopy. The probe was tuned, matched and shimmed before the images were acquired. NMR data were acquired and processed using Bruker TopSpin software under standard conditions, and the parameters are shown in the corresponding figures (S11 to S15). All NMR chemical shifts are given in δ (ppm) relative to the TMS signal at δ 0.00 as an internal reference shown in Table 3.
Table 3. 1H and 13C NMR spectral data for ethyl 2-cyano-3-(4-nitrophenyl)acrylate (1) and methyl-2-cyano-3-(4-nitrophenyl)acrylate (2) in CDCl3 (500 MHz)
The use of certain MOFs resulted in the formation of Z/E isomers of ethyl-2-cyano-3(4-nitrophenyl) acrylate, with a selectivity ratio of 15/85. However, the behavior of MOF-Cr-NH2 was distinct from that of other MOFs, as it produced both Z/E methyl-2-cyano-3(4-nitrophenyl) acrylate (selectivity ratio of 7/50) and (E)-ethyl-2-cyano-3(4-nitrophenyl) acrylate (selectivity ratio of 42). This can be attributed to the greater number of basic sites generated by the nitrogen present in the MOF-Cr-NH2 structure, its high BET specific surface area and external area, and its smaller particle size. As a result, MOF-Cr-NH2 did not exhibit selectivity for the expected compound, and both methyl and ethyl compounds were observed with a selectivity ratio of 58/42.
Elazarifi et al. (2004) conducted a study on the Knoevenagel condensation reaction involving benzaldehyde and ethyl cyanoacetate. Their findings revealed that a transesterification reaction occurred in the presence of methanol as the solvent and MgO catalyst. They observed the formation of trans-α-methyl-2-cyanocinnamate at a faster rate than trans-α-ethyl-2-cyanocinnamate. This was due to the relatively slow transesterification of trans-α-ethyl-2-cyanocinnamate in a large amount of methanol. The transesterification of ethyl cyanoacetate to methyl cyanoacetate occurs at the initial stage of the reaction, while methyl cyanoacetate is rapidly consumed to produce trans-α-methyl-2-cinnamate. However, when Mg and Al were used as mixed oxide catalysts, the formation of trans-ethyl-α-cyanocinnamate occurred at a faster rate than did the formation of trans-methyl-2-cyanocinnamate at a ratio of 22/78. This was due to the formation of pairs of Mg2+-O2− and Al3+-O2− acid‒base sites in the mixed oxide of Mg and Al, which were more active than the O2− species isolated as basic sites in MgO. The mixed Mg-Al oxide catalyst had a greater number of basic sites and was more active than the MgO and apatite catalysts, despite having a similar or greater total basicity.
The Knoevenagel reaction, involving 4-nitrobenzaldehyde and ethyl cyanoacetate, has been proposed to occur through a mechanism in which the metal site in the MOF acts as a Lewis acid and the organic ligand acts as a Lewis base (Fig. 4). The first step involves the activation of the carbonyl group by the metal center, which coordinates with the carbonyl oxygen atom of benzaldehyde. The organic ligand then attracts ethyl cyanoacetate (A), forming a nucleophilic species that attacks the carbonyl group (B), resulting in the formation of a C‒C bond (aldol condensation) while eliminating water molecules (C) (Karmakar and Pombeiro 2019; Mannarsamy and Prabusankar 2022). Analysis of this reaction mechanism confirmed that the strength of the metal ion, the basicity of the oxygen ion, and the length of the M-O bond are crucial for the activation of reactants and the rearrangement of intermediates on the metal-carboxylate surface (Panchenko et al. 2014). The coordination environment around the metal center is expected to prevent the formation of sterically congested Z-type coordination in intermediates (B) and (C), leading to less crowded orientations and the formation of only E isomers (Mannarsamy and Prabusankar 2022).
Physicochemical evaluations, such as XRD, FT-IR, and CHN analyses, confirmed the successful hydrothermal synthesis of MOF-Cr and MOF-Al. However, MOF-Ca and MOF-Mg did not yield the expected structures and instead formed materials similar to terephthalate. SEM images revealed different morphologies in the metal-organic frameworks, with layered or agglomerated characteristics being the most prominent.
The TGA/DTA results indicated that MOFs derived from Ca and Mg ions had a greater mass loss, while functionalized MOFs exhibited greater thermal stability. This suggests that the presence of an amino group affects the material’s stability. The MOF-Cr and MOF-Cr-NH2 structures had the highest BET surface areas and external areas. The BET surface area and pore volume decreased from MOF-Cr to MOF-Cr-NH2, indicating that the amino functional groups impacted these textural parameters.
The catalysts based on H2BDC MOF showed higher yields in the Knoevenagel condensation reaction than did the basic NaOH catalyst, making them promising materials for catalyzing reactions that require basic sites. All the MOF-Me and MOF-Me-NH2 catalysts, except for MOF-Cr-NH2, exhibited selectivity for the formation of the compound Z/E-ethyl-2-cyano-3(4-nitrophenyl) acrylate at a ratio of 15/85. However, using methanol as a solvent in the reaction led to the transesterification of ethyl cyanoacetate to methyl cyanoacetate using NaOH and MOF-Cr-NH2 catalysts, resulting in the formation of two other products with a methyl radical. MOF-Cr-NH2 was the only MOF that did not show high selectivity for the expected compound.
Further analysis of the acidity and basicity of the synthesized MOFs is recommended, along with additional investigations into the effects of variations in catalyst amount and reaction temperature on MOF-catalyzed reactions. Additionally, testing these catalysts in other catalytic reactions and examining opportunities for catalyst reuse are also worth considering.
Acknowledgments: The authors thank to Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES-DS) for the master´s scholarship, to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) number 310286/2021-2 and to Fundação de Amparo à Pesquisa do Estado de Goiás (FAPEG) for the financial support, number CAP2022061000091.
Funding: Conselho Nacional de Desenvolvimento Científico e Tecnológico, (CNPs -project 310286/2021-20), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES-DS) for the master´s scholarship and Fundação de Amparo à Pesquisa do Estado de Goiás (FAPEG -project CAP2022061000091).
Supplementary information: Powder X-ray diffractogram, CHN elemental analysis, thermogravimetric analysis, FT-IR spectra, N2 sorption isotherms, and compound structures are described in the supplementary material and are freely available at -- the Brazilian Journal of Chemical Engineering.
Conflict of interest: The authors declare no conflicts of interest.
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