Preparation Of Multifunctional Magnetic Metal-Organic Framework Catalysts And Their Catalytic Performance

Homogeneous catalysts are often used in traditional liquid organic reactions, such as the common homogeneous Lewis catalysts, organic catalysts and ionic liquid catalysts. However, the amount of the homogeneous catalyst in the liquid phase reaction is generally large, and the separation and recovery of homogeneous catalysts in the liquid phase after reaction is also a difficult problem. This leads to a huge waste of resources, and does not comply with the current advocated concept of green chemistry and energy efficient use. Moreover, some of the commonly used Lewis acid catalysts also cause the corrosion of the equipment, increase cost of operation and maintenance of equipment, and do not meet our requirements to maximize the economic benefits.For this reason, people have begun to focus on the development and preparation of various heterogeneous catalysts. Traditional γY-Al2O3, zeolite, molecular sieve, clay and other materials are widely used in the preparation of heterogeneous catalysts. Although when compared to homogeneous catalysts, the repeated usage of the homogeneous catalysts has been significantly improved, the recovery operation, during centrifugation, filtration or sedimentation and other separation methods, is also relatively complicated, and the recovery period is quiet long. Also in the solid-liquid separation process, it will inevitably result in the loss of the catalysts. Thus, the efficient heterogeneous catalysts of excellent catalytic performance and easy recycle for use would be people’s competitive research focus.MOFs materials because of its large specific surface area, controllable pore size and pore structure, easy functionalization in the metal center or ligand and many other advantages have received widespread attention. In particular, when MOFs as heterogeneous catalysts are applied to the solid-liquid phase organic reactions, the excellent catalytic activity and high selectivity for the desired product characteristics are fully reflected. Therefore, we assume that MOFs crystal structure was grown on the situ surface of the magnetic nanomaterials, so that we can take advantage of the catalytic activity of MOFs and superparamagnetic properties of the composite. This ensures the catalyst rapid separation with the presence of external magnetic field, and simplifies the operation time and the solid-liquid separation steps, and also minimizes the loss of the catalyst.In this work, we have prepared a series of different MOFs content of magnetic MOFs@SiO2@Fe3O4 materials, such as UiO-66-SO3H @SiO2@Fe3O4, MIL-101(Cr)@SiO2@Fe3O4, Y-MOF@SiO2@Fe3O4. A number of the catalysts also used functionalized means of rear modification or load to prepare the magnetic MOFs bifunctional catalyst. We used scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), in situ infrared FT-IR, N2 adsorption/desorption and Vibrating Sample Magnetometer (VSM) to characterize the structure of the magnetic MOFs bifunctional catalysts and Knoevenagel condensation reaction, acetal and Aza-Michael addition reaction was evaluated for catalytic activity of these magnetic bifunctional catalysts. We investigated the effects of different reaction conditions on the catalytic activity of the catalyst and repeating practicality.1. Different MIL-101(Cr) content of magnetic MIL-101(Cr)@SiO2@ Fe3O4 catalysts all have obvious XRD crystal structure and good superparamagnetic and exhibit good catalytic properties in furfural and ethyl cyanide Knoevenagel condensation reaction in liquid phase. Ethyl cyanide conversion rate increased with the increase content of MOFs. In case of MIL-101(Cr) with 30% content of magnetic MIL-101(Cr) @SiO2@Fe3O4 catalyst, at 40℃ for a reaction time of 1h, ethyl cyanide conversion is up to 99%; and when the benzene ring has electron-withdrawing group, the conversion rate will be significantly increased. After reaction, the catalysts are separated magnetically with the applied magnetic field, and after re-use for five times the rate of conversion of ethyl cyanoacetate has slightly decreased.2. Different amines grafted functionalized magnetic Amine-MIL-101 (Cr)@SiO2@Fe3O4 catalysts have good superparamagnetic and similar crystal structure, and exhibit good catalytic properties in the Knoevenagel condensation reaction of furfural and ethyl cyanoacetate. Wherein the grafted functionalized magnetic ethylenediamine 30% MIL-101(Cr)@ SiO2@Fe3O4 catalyst (referred to as NMM-2), the reaction at 40℃ for 1 hour, the conversion rate of ethyl cyanoacetate is up to 98.8%. Experiments confirmed that after repeated use, the catalyst has excellent recycle performance.3. The magnetic UiO-66-SO3H@SiO2@Fe3O4 catalyst with MOFs content of 10% has large specific surface area and good super paramagnetic properties. It has shown good catalytic performance in the acetalization of cyclohexanone with methanol. Among them, at 30℃ for reaction time of 10 h, MOFs content of 10% of the magnetic UiO-66-SO3H@SiO2@Fe3O4 catalyst has achieved the conversion rate of cyclohexanone which is closer to 80%. In addition, when the magnetic UiO-66-SO3H@SiO2@Fe3O4 catalyst had been reused for five times, the conversion rate of cyclohexanone remained at more than 70%, and it has excellent recycle performance.4. Magnetic Y-MOF@SiO2@Fe3O4 catalysts with different Y-MOF contents were prepared. The magnetic Y-MOF@SiO2@Fe3O4 catalyst showed a good catalytic performance in the Aza-Micheal addition reaction of aniline and methyl acrylate. Among them, the Y-MOF content of 43.3% of the magnetic Y-MOF@SiO2@Fe3O4 catalyst, at 80℃ and reaction time of 6 hours, the reaction conversion rate was up to 88%. The results of repeated experiments showed that the catalyst has excellent recycle performance.In conclusion, we can see that the four kinds of dual bifunctional magnetic MOFs@SiO2@Fe3O4 have two structures of MOFs and Fe3O4 magnetic core, and they are nanoparticles with core-shell structure in diameter 100-200 nm. It has been applied to heterogeneous organic reactions and has achieved good results. Dual function MOFs@ SiO2@Fe3O4 composites, inherited the characteristic of MOFs, large specific surface area, good catalytic performance, high selectivity of the target product; also as magnetic heterogeneous catalysts, they have the characteristics of convenient and quick recovery, which greatly enhance its repeated use performance, and has excellent industrial application potential.