Synthesis And Catalytic Properties Of Functionalized-Mil-101Solid-Acid Catalysts

Hydrolysis and esterification are important reactions in chemical industry, usually carried out in the present of liquid acid catalysts such as sulfuric and hydrofluoric acid. Since these liquid acids are corrosive and produce a large amount of waste, replacement of the liquid catalysts with water-tolerant solid-acid catalysts is desirable from an environmental protection point of view. Heteropolyacids (HPAs) possess high acidity, but with low surface area; and for supported HPAs, the leaching of HPAs into the polar media cannot be negligible. On the other hand, the leaching problem of sulfonic acid functionalized solid catalysts also ofter occurred in case they were used as acidic catalysts in the above mentioned reactions. Apparently, development on highly active and stable solid acidic catalysts is desirably required.Among more than10000known MOFs, there are only several transition-metal MOFs that have been proven to be stable under liquid-phase reaction conditions. These include mesoporous MIL-101formed by rigid terephthalate ligands and Cr3+octahedral clusters. Due to its high thermal and chemical stability, MIL-101exhibits no detectable leaching of chromium into aqueous solutions, allowing its safe use in different applications. In this thesis, several different solid acidic catalysts were developed by functionalization of MIL-101. The main results are summarized as follows:1) A heterogeneous catalyst, chromium(III) terephthalate metal organic framework (MIL-101) encapsulated12-tungstophosphoric heteropolyacid (HPW), HPW@MIL-101, was prepared by a simple one-step hydrothermal reaction of aqueous mixtures of Cr(NO3)3, terephthalic acid, and HPW. The obtained catalyst was systemically characterized by XRD, N2adsorption, FT-IR, acid-base titration, and31P MAS NMR techniques as well as elemental analysis. The catalytic activity and reusability of HPW@MIL-101were evaluated in liquid-phase hydrolysis of ethyl acetate and in esterification of acetic acid with n-hexanol. In both of the reactions, water exists as a reactant/solvent and as a product. HPW@MIL-101shows a high activity and excellent reusability in both of the reactions, behaving as a water-tolerant heterogeneous acid catalyst without agglomeration, leaching and deactivation of HPW during its repeated use. The unique characteristics of MIL-101and the well-dispersed level of HPW confined as noncoordinating guests in the mesoporous cages of the MIL-101matrix with the integrated protonic acidity of HPW may account for the high catalytic properties of HPW@MIL-101.2) The sulfoxy or sulfonic acid-functionalized MIL-101catalysts were prepared by either post-functionalization or pre-modification of the organic linkers with the functionalized ligands, the latter is so-called the one-pot synthesis method. These catalysts were systemically characterized by XRD, N2adsorption, acid-base titration, and FT-IR techniques and applied in the liquid-phase esterification of monocarboxylic acids with monohydric alcohols. The results show that the sulfonic acid-functionalized MIL-101(S-MIL-101) prepared by the one-pot synthesis method possesses a higher catalytic activity in the esterification than its counterpart the sulfoxy acid-functionalized MIL-10(S/MIL-101) prepared by a post-functionalized method, due to the fact that S-MIL101has a higher acid loading and a high utilization efficiency of the functionalized acid sites. Moreover, S-MIL-101is stable to leaching, behaves as a true heterogeneous catalyst, can be easily recovered by filtration, and reused for five times in succession without any loss of its catalytic activity.