This study reports the synthesis of nanoporous Zn-MOF microspheres to develop an efficient and novel heterogeneous catalyst to synthesize polyhydroquinolines as an important dihydropyridine-containing scaffold via Hantzsch reaction. The resulting samples were characterized using various techniques.
3.1. Synthesis and characterization of the catalyst
The heterogeneous Zn-MOF microsphere catalyst was successfully prepared by hydrothermal reaction of Zn(NO3)2.6H2O and 4,6-Diamino-2-pyrimidinethiol, using the deprotonation of amine and thiol groups. Afterward, metal coordination bonding to the Zn ions was made to give a novel Zn-based microsphere catalyst. This newly synthesized nanoporous microsphere catalyst was fully characterized by various techniques; including, FT-IR, XRD, TGA, EDX, X-ray mapping, SEM, TEM and BET analysis. The results obtained from these techniques confirmed the successful preparation of this novel catalyst.
3.2. Structural and chemical composition analysis
The changes in the chemical features during the synthesis of the composite were measured by FT-IR, spectroscopy. Figure 1 shows the FT-IR spectra of a) 4,6-Diamino-2-pyrimidinethiol, b) Zn(NO3)2.6H2O and c) nanoporous Zn-MOF microspheres, respectively. In the FT-IR spectrum of the 4,6-Diamino-2-pyrimidinethiol (Figure. 1a), the bands at 3339 cm−1 and 3435 cm−1 are attributed to the stretching vibration of the N-H bonds of free NH2 groups while those at 1523 cm−1 are related to the C=C group of the pyrimidine ring. The Amine bands disappeared in the FT-IR spectra of the Zn-MOF microspheres (Figure. 1c), which show that the NH2 groups have been deprotonated and coordinated to the Zn atoms. Also, the bending vibration of NH2 at 1638 cm−1 in the Zn-MOF microspheres shows a shift towards higher frequencies as compared to the primary sample. This shift can be assigned to the presence of Zn ions in MOF structure and the reduction of the single bond nature of the amine and thiol groups, which confirms the successful complexion of Zn ions with the N and S atoms in the framework. The strong C=C stretching vibration band at 1468 cm−1 and C-N stretch band at 1102 cm−1 provide shreds of evidence confirming the successful synthesis of the final MOF (Figure. 1c).
The crystalline phases of nanoporous Zn-MOF microspheres were examined by X-ray diffraction (XRD) analysis as shown in Fig. 2. The characteristic diffraction peaks corresponding to the obtained framework are found at 2θ = 31.62 °, 34.49 °, 36.15 °, 47.42 °, 56.3 °, 62.57 °, 67.75 °, 68.95 °, 72.51 ° and 76.82 ° which effectively coincide with XRD patterns simulated from powder X-ray data of nanoporous Zn-MOF microspheres with a cubic phase [39–41]. Based on the XRD results, we can also observe that the Zn-MOF microspheres obtained from zinc nitrate show sharp characteristic peaks, suggesting the high crystalline nature of the obtained nanoporous Zn-MOF microspheres.
To estimate the mass ratios and the thermal stability of nanoporous Zn-MOF microspheres, the TGA-DSC technique was employed. Fig. 3. represents TGA (Maroon line) and DSC (Blue line) curves of Zn-MOF microspheres at the temperature range of 25-1500 ℃. In the TGA curve, the weight loss which was occurred below 200°C was attributed to the release of the physically adsorbed moisture and water and organic solvents from the sample [42]. The next weight loss (10.61% ) in the region of 200-800 ᵒC can be associated with the decomposition of 4,6-Diamino-2-pyrimidinethiol ligand [43, 44]. In addition, the weight loss at 450 ᵒC indicates the decomposition of the framework. A basic point about the nature of the Zn-MOF catalyst is the thermal stability of the catalyst up to 200 ᵒC; therefore, this catalyst can be used under various catalytic reaction conditions up to 200 ᵒC.
As shown in figure 4, energy dispersive X-ray (EDX) analysis was applied to determine the chemical composition of nanoporous Zn-MOF microspheres. The results indicate the presence of 53.25 W% of Zn species in the obtained nanoporous Zn-MOF microsphere catalyst. In addition, the presence of carbon (10.70 W%), nitrogen (9.11 W%) sulfur (13.97 W%) and oxygen (12.97 W%) elements in the prepared nanoporous material was also confirmed by these measurements. These observations support the high purity of the prepared Zn-MOF microspheres.
The uniform distribution of the index elements (C, S, N, O, and Zn) of the obtained nanoporous material is observed at the X-ray mapping analysis (Fig. 5). In addition, the uniform distribution of the Zn element shows that it has evenly coordinated to N and S elements, indicating the presence of ligand in the obtained frameworks, and the uniform incorporation of the activated Zn catalytic species with S and N groups showing that an excellent catalytic surface has been formed.
The surface morphology of the prepared nanoporous material was assessed by scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) analysis. The FE-SEM images for pure nanoporous Zn-MOF microspheres are presented in Fig. 6. A closer look at the SEM images of the obtained nanoporous material indicates that the Zn-MOF has been formed in uniform three-dimensional walnut-like hierarchically nanoporous microparticle morphology, which can be attributed to a proper bonding in-between the metal and linker. Moreover, the SEM images show that these Zn-MOF microparticles are composed of a collection of nanoparticles having a mean particle size of 23-27 nm. The results indicate that the Zn-MOF microspheres have been formed in a uniform manner. Besides, there is no indication of agglomeration of the particles and also this feature optimizes the catalytic properties of the samples.
By applying the transmission electron microscopy technique, high-quality images from the synthesized crystalline nanoporous Zn-MOF microspheres were obtained (Figure 7). As can be seen, the synthesized Zn-MOF microspheres exhibit characteristic yolk-shell structure and a clear gap existed between the shell and the core, confirming that the core is a solid sphere. Particle size distribution histogram of the as-prepared microspheres gave distribution dimensions across a range from 90 to 170 nm, with an average size of 130 nm (Figure 7).
The porosity and surface characteristics of nanoporous Zn-MOF microspheres were explored by the N2 adsorption/desorption analysis (Fig. 8). The obtained results are illustrated in Figure 8. Zn-MOF microspheres exhibited a Brunauer–Emmett–Teller (BET) surface area of 63.33 m2.g−1 and the pore volumes and pore size distribution of nanoporous Zn-MOF microspheres are calculated by BJH analysis, as the values are 0.088 cm3.g−1, and 1.22 nm, respectively.