Utilization of metal-organic frameworks for gas separations and catalytic oxidation

Abstract

Metal-organic frameworks (MOFs) and their applications have been a rapidly growing area of research in recent years. The seemingly endless combinations of metal ions or clusters with various possible organic linkers, along with the methods of post-synthesis modification, has resulted in approximately 20,000 different MOFs to date. With such potential for variation, MOFs have shown to be useful in a whole host of applications. One such application of MOFs is the separation of natural gas. Currently, these separations require expensive methods, such as amine absorptions and cryogenic distillation. Polymers, such as polyimide, have been investigated, but the necessary high temperatures lead to plasticization and poor separation performance. Alternatively, ZIF-8 (zeolitic imidazolate framework 8), a member of the ZIF class of MOFs, is a viable option due to its inherent pore size and preferential adsorption of carbon dioxide. Microporous carbon membranes have shown increased chemical and thermal stability. By converting the microporous ZIF-8 to a carbon membrane, the resulting membrane can be expected to maintain the separation properties of the parent ZIF-8 while gaining additional stability provided by carbon. Unfortunately, synthesizing this material is a challenge to reproduce due to the wide variety of methods for ZIF-8 synthesis and the limited study of the effect of carbonization on the parent material A second application of MOFs is as heterogeneous catalysts. Cu-BTC, or HKUST-1, is used for the oxidation of benzyl alcohol to benzaldehyde, a chemical commonly used in perfumery and pharmaceuticals. However, the use of copper requires the presence of TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) as it is needed to deprotonate the alcohol. Unfortunately, TEMPO can interrupt the framework structure of Cu-BTC and poisons the catalysts. In order to help combat this problem, layer by layer synthesis of Cu-BTC was used to incorporate the MOF into the pores of a functionalized mesoporous silica nanoparticle (MSN). However, this incorporation did not protect the Cu-BTC, as the pores size of the protective MSN was insufficient to prevent TEMPO access. Additionally, only a small amount of Cu-BTC was synthesized within the pores, which magnified the effects of TEMPO poisoning.