| One-pot tandem reactions refer to those reactions in which two or more reactions proceed consecutively in a same reaction system. Since this type of reactions has no need to isolate or purify the intermediate products, it has the characteristics of simple operation, high yield and less by-products. So it is a kind of atom economic and environment friendly chemical reaction. Many heterogeneous catalysts containing various catalytic sites have been used in the study of cascade reactions, metal-organic framework (MOF) is one hot spot among them. As a new type of solid porous material, MOFs have been studied widely in the field of catalysis due to their high specific surface area, highly ordered channels and adjustable pore size. Furthermore, MOF can be modified by pre-functionalization or post-synthetic modification to obtain various catalytic sites for further organic catalytic use. In this paper, we prepared a kind of Cr-MOF, MIL-101(Cr)-NH2, and obtained a series of acid-base bifunctionalized MOF, MIL-101-AB-x, with different modification proportions by varying the reaction conditions via post-synthetic modification. Deacetalization-Knoevenagel condensation reaction was chosen as a model reaction to study the catalytic properties of these acid-base bifunctionalized MOF. The best catalytic performance was achieved at a moderate modification ratio(x=0.32). We have demonstrated that proton transfer exists in these sulfonic and amino bifunctional materials. Because of the acid-base effect, different modification proportions of MOF showed different catalytic properties.This thesis is mainly divided into two parts:1. Synthesis and characterization of acid-base bifunctional MIL-101(Cr)In this part, we introduced the synthetic procedures of MIL-101(Cr)-NO2 and MIL-101(Cr)-NH2. We have prepared a series of post-synthetic modification products MIL-101-AB-x via ring-opening reaction between amino and 1,3-propanesultone, and obtained products with different modification ratios through adjusting the reaction time and the concentration of 1,3-propanesultone. We characterized these MOF through a variety of methods such as XRD, IR, TG, NMR, etc. It turned out that the MOF maintained integrity after the post-synthetic modification and there was no significantly decrease in the specific surface area, pore volume and pore diameter. Thermogravimetric analysis showed that the catalysts have good thermal stability. The modification proportion was calculated by 1H NMR data.2. Catalytic properties of acid-base bifunctional MIL-101(Cr) for the one-pot tandem deacetalization-Knoevenagel condensation reactionIn this part, we studied the catalytic properties of MIL-101-AB-x for the one-pot tandem deacetalization-Knoevenagel condensation reaction, including the effects of solvent, temperature and modification proportion. We found that the optimal solvent and temperature is DMF and 90℃, respectively. When x is 0.32, the catalyst has the best catalytic property, and higher proportions have adverse effect on the Knoevenagel condensation reaction. As for the best modification proportion, we proposed two possible assumptions:(i) steric effect; (ii) acid-base effect. By comparing the catalytic properties and the XPS data of MIL-101-AB-0.52 before and after alkali treatment, we eventually proved the existence of proton transfer in the catalyst. It is the ammonium rather than sulfonic group that acts as the catalytic acid site. The sulfopropylamino group attached to the MOF takes the zwitterionic form. Due to the acid-base effect, different modification proportions of MOF showed different catalytic properties. Alkali content reduces along with the increase of the modification ratio, thus higher modification ratio has adverse effect on the second step reaction. So we perfectly explain the existence of best modification ratio finally. Filtration and cycle experiments demonstrated that the catalytic process was heterogeneous. The loss of the active sites did not take place in the reaction process. |
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