Amino-induced cadmium metal–organic framework based on thiazole ligand as a heterogeneous catalyst for the epoxidation of alkenes

Selective epoxidation of olefins is of high interest in the chemical industry due to the many applications of epoxides. This study reports on the synthesis of Cd-MOF, [Cd(DPTTZ)(5-AIP)] (IUST-1) (where DPTTZ = 2, 5-di (pyridine-4-yl) thiazolo [5, 4-d] thiazole, 5-AIP = 5-Aminoisophthalic acid), by a reflux method, which can be considered as a fast and simple process. The morphology and structure of the synthesized IUST-1 were determined by using FE-SEM (Field Emission Scanning Electron Microscopy), EDX (Energy Dispersive Analysis of X-ray), Mapping (Elemental Mapping), CHNS (Elemental analysis), XRD (X-Ray Diffraction), FT-IR (Fourier Transform Infrared), and TGA (Thermo Gravimetric Analysis). The epoxidation of cyclooctene was investigated using the activity of catalytic IUST-1. The results showed that in the presence of tert-butyl hydroperoxide and CCl4 in a 1:2 alkene/oxidant ratio, a high epoxide yield (99.8%) was obtained. In addition, IUST-1 can be easily separated by simple filtration and recycled five times successfully with a slight decrease in activity. This compound has some advantages such as high yield, short reaction time, and ease of reuse, which make it a suitable heterogeneous catalyst for the epoxidation of cyclooctene.

of olefins.This is because of having high-potential active sites such as modified ligands with active sites, metal centers with exchangeable coordination positions, and empty cavities with active species 26 .
In spite of the numerous reported catalytic uses of MOFs, the development of truly efficient and selective catalytic processes using MOFs remains a challenge.Some MOFs have low stability and are susceptible to self-decompose in organic solvents and water, which limits their use as photocatalysis, biocatalysis, and electrocatalysis 27 .Moreover, the ability to obtain monodisperse nano-sized MOFs is of major importance for prospective applications in heterogeneous catalysis 28 .
In this work, the reflux method was used to prepare an amino-functionalized Cd-MOF (IUST-1) based on the thiazole ligand in a fast, simple, and promising manner for a large-scale.IUST-1 displays an excellent ability in the epoxidation of olefins.The catalytic properties of IUST-1 for the epoxidation of cyclooctene were investigated using different reaction parameters, such as temperature, catalyst amount, oxidant, reaction time, and solvent effect.The IUST-1 demonstrated a high yield (99.8%) in the epoxidation of cyclooctene in CCl 4 in the presence of TBHP in a 1:2 alkene/oxidant ratio.In addition, the IUST-1 catalyst can be used for the oxidation of various olefins with high efficiency.

Experimental
Materials and measurements.All chemicals and solvents used in the syntheses were of analytical reagent grade and were used without further purification.Elemental analysis (C, H, N, and S) was carried out on a Thermo Finniga Flash 1112 series elemental analyzer.FT-IR spectra were recorded using the KBr pellet method on a Shimaduz FT-IR-8400 spectrometer.Power X-ray diffraction (PXRD) was conducted by a Philips X'pert X-ray powder diffractometer (Cu-Kα, λ = 1.5418Å).TGA analysis was carried out using Perkin Elmer Pyris 1 thermo gravimeter under an argon (Ar) atmosphere in the range from 50 to 800 °C with a ramp rate of 10 °C.min -1 .Field emission scanning electron microscope (FE-SEM) measurement was performed using the FE-SEM TESCAN MIRA3 microscope.GC-MS samples were registered on a Shimadzu QP-5050 GC-MS device.
The leaching metals of catalyst was analyzed using ICP-MS (Inductively coupled plasma mass spectrometry) on a Vavian 715-ES.
Catalyst recycling.Testing of catalyst recycling involved filtering, washing with ethanol, and drying at 80°C.It can then be reused for up to five runs under similar conditions.
Leaching test.After the catalytic reaction, ICP analysis was performed to measure the cadmium concentration in the solution and to determine whether cadmium metal might leach under the reaction conditions.

Results and discussion
Characterization of the IUST-1.Using a reflux method, 5-amino isophthalic acid, 2, 5-di-pyridine-4-ylthiazolo [5, 4-d] thiazole, and cadmium nitrate tetrahydrate were combined to produce IUST-1 (Fig. 1).The X-ray diffraction analysis exhibits that IUST-1 has a space group of Pbam and crystallizes in the orthorhombic crystal system.The central Cd(II) ion is seven-coordinated with five equatorial oxygen donors of carboxylate groups from three different 5-AIP ligands and two axial nitrogen donors of DPTTZ ligands, indicating a distorted [CdN 2 O 5 ] pentagonal bipyramidal geometry.The IUST-1 is based on binuclear Cd(II) units, Cd 2 (μ-OCO) 2 , that is 6-coordinated and linked through 5-AIP and DPTTZ ligands.By coordinating the 5-AIP and linear DPTTZ ligands, a 3D framework is created.
Figure S1 shows the simulated powder X-ray diffraction (PXRD) pattern from single crystal X-ray data of IUST-1 in comparison with the PXRD pattern of a nano-particles typical sample of IUST-1 prepared by the reflux process.The prepared IUST-1 by the reflux method exhibits a good agreement with the simulated model on its PXRD pattern, demonstrating its phase purity.On FT-IR spectra of IUST-1 (see supplementary Fig. S2 online), the vibrational bands for the primary amine (NH 2 ) of 5-Aminoisophthalic acid as a ligand can be seen at 3355 and 3258 cm −1 .The asymmetric and symmetric vibrations of the dicarboxylate groups of the 5-Aminoisophthalic acid ligand appear at 1606 and 1381 cm −1 , respectively.In the reaction with Cd ions, all carboxyl groups of 5-Aminoisophthalic acid ligands have been deprotonated, as suggested by the absence of the expected absorption bands at 1700 cm −1 .As part of the thermal gravimetric analysis (TGA) of the nanostructured IUST-1 synthesized by the reflux process, the thermal stability is tested between 50 and 800 °C underflows of argon (see supplementary Fig. S3 online).Thermal analysis results determined that the overall thermal stability of the IUST-1 prepared by the reflux process is similar to those presented in the literature 29 .The IUST-1 prepared by the reflux process shows stability up to approximately 370 °C.The adsorption-desorption isotherm of the IUST and BET results have been given in Fig. S4.According to this figure, the IUST is a mesoporous compound and the average size of pores is 32 nm.
The FE-SEM images of the reflux-produced IUST-1 have be given in Fig. 2. In the FE-SEM photographs, the structure of IUST-1 can be seen as a series of nanoplates formed by juxtaposing nanoparticles.

Catalytic activity of the IUST-1 for epoxidation of cyclooctene.
In this part of the study, the catalytic activity of IUST-1 in the epoxidation of cyclooctene was investigated.This reaction is one of the most commonly used procedures in biomedical and pharmaceutical synthesis.To begin with, the impact of temperature on the epoxidation of cyclooctene was investigated to identify the optimal conditions (Table 1, Entries 1-4).
In this part, it is found that higher temperatures give higher yields.As the temperature increases, the kinetic energy increases, and the number of interactions between the initial materials increases.Thus, 76°C is optimal temperature for cyclooctene epoxidation in the presence of the IUST-1.
By comparing all three oxidants, it can be seen that TBHP obtains a higher product efficiency.The reason may be that there is peroxide oxygen in TBHP, which has a greater electrophilic character 30 .
Also, TBHP has a benefit compared to H 2 O 2 due to the absence of water formation resulting from the reduction of this oxidant.Water is indeed considered to be at the origin of the IUST-1 catalyst deactivation (Table 1, Entries 4-6).In many cyclooctene oxidation reactions, Using TBHP as an oxidant, the reaction leads to a mixture of compounds but with the use of the IUST-1 catalyst, no side product was created and the creation of a single product in the catalytic reaction is a very important advantage 31,32 .
In the following step, the catalyst value and time were optimized (see supplementary Tables S1 and S2 online) and 20 mg of the IUST-1 and time 2 h were obtained as the optimal conditions of catalyst for the epoxidation of cyclooctene.
In the epoxidation of cyclooctene, solvents play a critical role in determining the rate and yield of products.In order to obtain the best solvent for epoxidation cyclooctene with the IUST-1 and NH 4 VO 3 as co-catalysts, some reactions were performed (Table 2).Based on the results, carbon tetrachloride (Table 2, Entry 2) is the best solvent with a 99.8% yield.In comparison with carbon tetrachloride, other solvents like DMF, methanol, ethanol, acetonitrile, and acetone (Table 2, Entries 4-9) show lower yields and almost 0%.Furthermore, 1,2-dichloroethane (CH 2 Cl 2 ) had relatively good yields in the product (Table 2, Entry 3), but carbon tetrachloride was selected as the solvent in the end.In the absence of the catalyst, no cyclooctene conversion was observed (Table 2, Entries 1).
The reported solvents in Table 2 due to their stability in oxidation processes were selected.Moreover, the results indicate that cyclooctene exhibits more reactivity in CH 2 Cl 2 and CCl 4 rather than CH 3 CN and other polar solvents.This can be explained by the coordination ability of CH 3 CN that inhibits the reaction by competing with the oxidant to occupy the coordination sites of the catalyst.These results are consistent with the previous reports 30 .
The IUST-1 was investigated as a catalyst in the epoxidation of alkenes using different olefins (Table 3).This catalyst displays a good catalytic performance for the epoxidation of various olefins.NH 4 VO 3 was used as a co-catalysts in order to obtain the best yield for epoxidation cyclooctene with the IUST-1.After performing GC/Mass analysis, the results showed the necessity of NH 4 VO 3 use (see supplementary  www.nature.com/scientificreports/Table S3 online).In the presence of only the IUST-1, the product yield was 53%, but with co-catalyst (NH 4 VO 3 ), the product yield was 99.8%.

Leaching and recycling tests.
To confirm the catalytic nature of IUST-1, the leaching and recycling tests were performed.The Cadmium-based catalyst was tested for its reusability for cyclooctene epoxidations.At the end of the reaction, the catalyst was centrifuged, washed many times with ethanol, dried, and used for another reaction with a similar method for 5 more runs.As a result, this catalyst can be recycled five times successfully with a slight decrease in activity (Fig. 3).
Table 3. Substrate scope for epoxidation reaction a of olefins using the IUST-1 as a catalyst.a Reaction conditions: Substrate (1 mmol), 20mg IUST-1 as a catalyst, CCl 4 (0.5 mL), NH 4 VO 3 (4mg), TBHP (2 mmol), 76 °C.b TON = (mol of substrate /mol of catalyst)*yield of product 33 .c TOF = TON/Time (h) 33 .After the reaction, the IUST-1 catalyst was separated from the liquid and the supernatant was stirred for another 10 h at room temperature.GC/MS was used to analyze the products (see chromatogram graph as Fig. S8 in supplementary).Epoxide yield was preserved at 99.8% after the filtration stage, indicating no leakage of the cadmium catalyst.Moreover, a Cd metal leaching test was performed by ICP after the reaction, and the Cd concentration was below the detection limit, ruling out the possibility of Cd metal leaching.In addition, the structure of the recovered IUST-1 after the epoxidation of cyclooctene has been well maintained after five cycles, as shown by the PXRD patterns, FTIR spectra, and EDX analysis (see supplementary Figs.S5-S7 online).

Proposed mechanism for the epoxidation of olefin with TBHP by the IUST-1
A probable mechanism for the epoxidation of cyclooctene with TBHP has been proposed by the IUST-1 based on Sobczak's ideas and other theoretical and experimental reports [43][44][45][46] .On the basis of this proposed mechanism, the first step in the epoxidation of an alkene is the coordination of the TBHP to the metal center by the terminal oxygen, thereby activating the peroxide for oxygen transfer (the observed trend for solvent effect also agrees with this mechanism.As the coordination ability of the solvent is increased, the solvent binding instead of TBHP binding to the metal center is also increased and the formation of species II is prevented and retarded the progress of the epoxidation reaction).The peroxidic oxygen in this species has an electrophilic character.
Then, the olefin substrate coordinates to the metal center and, as a nucleophile, inserts into the metal-oxygen bond of coordinated peroxide electrophile anion (the higher electron density of the double bond is expected to show more epoxidation reactivity).Then the epoxide was produced, and at the same time, the tert-butylperoxide anion converted to the tert-butoxide anion.After that, the peroxide product will be released and the cycle of catalytic reactions continues with the substitution of a new tert-butylperoxide instead of a tert-butoxide anion (Fig. 4).

Conclusion
In conclusion, amino-functionalized Cd-MOF (IUST-1) based on thiazole ligand was prepared by reflux method as a fast, simple, and promising method for large scale and ultimately used for the epoxidation of olefins.The IUST-1 behaves as a very effective and selective heterogeneous catalysts for the epoxidation of olefins.The IUST-1 demonstrated a high yield (99.8%) using NH 4 VO 3 as a co-catalyst.Meanwhile, FT-IR, EDX, and PXRD of the fresh and fifth recycled catalyst reuse demonstrated that the IUST-1 can efficiently catalyze the epoxidation of cyclooctene as a recyclable and stable heterogeneous catalyst.Moreover, in the presence of IUST-1, diverse types of olefins can be converted selectively into the corresponding epoxides with excellent selectivity and conversion.

Figure 1 .Figure 2 .
Figure 1.The schematic diagram for the preparation of the IUST-1 and the epoxidation of cyclooctene by it.

Figure 4 .
Figure 4.A schematic of the proposed mechanism for the reaction of cyclooctene oxidation in the presence of the IUST-1.

Table 2 .
The impact of diverse solvents on the oxidation reaction cyclooctene a .