Synthesis and Characterization of New Metal-Organic Frameworks for Gas Adsorption Studies and as Solid Superacids
- Author(s): Jiang, Juncong
- Advisor(s): Yaghi, Omar M
- et al.
The development of metal-organic frameworks (MOFs) has experienced three main periods: (1) discovery of new structures; (2) structure design using the principles of reticular chemistry; and (3) study of these new materials in various applications. We are now in the third period as MOFs are extensively tested in a large number of potential applications, such as gas-storage and separation, catalysis, design of energy storage devices, drug delivery, and molecule sensing and recognition. The increasing applications of MOFs is raising the demand for the materials, therefore in this dissertation, I will focus on design and synthesis of new MOFs with high storage capacity for various gases including water, ammonia, and methane; and design and synthesis of MOF-based solid acids and the study of their liquid and vapor phase catalytic properties.
The strategy of material design targets practical applications. Thus, with different gas storage purposes in mind, different materials are proposed. In the first example, the need to capture water vapor at low humidity requires MOFs to exhibit pore filling or condensation of water into the pores at P/P0 < 0.1. Thus we have designed a small pore zirconium MOF, MOF-801, for this purpose. MOF-801 is shown to have high uptake of water, recyclability, and water stability. In the second example, the need to capture harmful ammonia gas requires MOFs with strong binding sites for the basic gas at low concentration. Thus we have designed and installed strongly acidic sites in a zirconium MOF, sulfated MOF-808, for this purpose. This sulfated MOF-808 is shown to take up 5.3 mmol ammonia per gram at P < 1.5 Torr, and 16.7 mmol ammonia at P = 760 Torr, one of the highest numbers for a MOF-based ammonia capture material. In the third example, high methane gas storage capacity at 80 bar was attempted using MOFs. This increase on the working capacity (between 5 and 80 bar) requires MOFs to have both high surface area and suitable pore sizes. For this purpose, we have designed zinc MOFs using organic linkers having delocalized π-electrons and with suitable pore metrics. The compound, MOF-905, shows 200 cm3 cm-3 working capacity between 5 and 80 bar, the highest of all zinc MOFs and is equivalent to the benchmark HKUST-1 compound.
With regard to the design and synthesis of MOF-based solid acids, three approaches have been proposed: (1) acidic functional groups bound to the organic linker; (2) acidic ligands bound to the inorganic cluster; and (3) acidic molecules encapsulated in MOF pores. While in the first approach, the acidity of the MOF is mostly determined by the acidity of the free acidic functional group, the latter two strategies provide for more interaction between the framework and the acidic groups, which is of great importance to understand the chemistry within the pores. A new type of MOF-based solid acid is demonstrated by the controllable sulfation of a zirconium MOF, MOF-808, on the inorganic cluster. The substitution of terminal formate groups in MOF-808 with sulfate groups has imparted strong acidity onto the sulfated MOF-808. The material has shown activity for acid catalyzed Friedel-Crafts acylation, esterification, isomerization, as well as the conversion of methylcyclopentane (MCP) into various hydrocarbons at 150-200 oC. Another MOF-based acid is synthesized by including phosphotungstic acid (PTA) into the large cages of MIL-101. Interestingly, we have found that the Brønsted acidity (originating from PTA) of the composite material is not directly proportional to the loading of PTA, but instead, it is not exhibited until certain loading of PTA is reached. This is explained by the level of protonation of PTA incorporated into the material, where the material with the highest loading, Pt/60PTA/MIL-101, is shown to effectively catalyze hydroisomerization of n-hexane at 250 oC.