Multifunctional Hydrogen-bonded Organic Frameworks (HOFs): Design, Synthesis and Applications

Microporous materials have been extensively studied in the past three decades for their high internal surface area where the pore/channel can be tuned to have multiple applications, gas separation, gas storage, heterogeneous catalysis, sensing, just name a few. As one of the most popular porous materials, metal−organic frameworks (MOFs) are crystalline hybrid solid materials constructed by organic building blocks and inorganic nodes which have emerged as a powerful platform to design and synthesize functional materials for specific applications.

As an alternative to the popular porous materials such as MOFs, hydrogen-bonded organic framework materials (HOFs) are an interesting type of crystalline porous materials just composed of organic linkers through multiple H-bounding interactions. HOFs have some unique advantages such as solution processability and characterization, easy purification, simple regeneration and reusage through routine recrystallization. A very outstanding feature is the metal-free nature of HOFs which can be good candidates in the biological applications. By pushing the edge of HOF materials, they can be potentially implemented in the industrial and/or pharmaceutical applications.

This dissertation projects have focused on developing multifunctional HOFs for their potential applications in small molecule recognition and gas separation through rational design of the organic linkers and fine tuning of the HOFs synthesis.

One project is about developing a series of nitrile-based HOFs (HOF-26, HOF-27, HOF-28) by using different solvents as to generate the well-defined pore structures of such HOFs. My investigations may lead to more explorations of HOF materials using this kind of method to tune the pore channels and structures. More potential applications, especially the biological ones, may be explored and studied to better the benefits inspired by the solvent dependent HOFs.

In the second project, HOF-29 was achieved using a nitrile-based organic linker. Further exploration of this HOF led to the discovery of HOF-29⊃p-xylene crystal structure, which was soaked in the commercial xylene mixtures containing m-xylene (mX), o-xylene (oX), p-xylene (pX) and ethylbenzene (EB). Detailed XRD studies coupled with the NMR studies revealed that the exclusive recognition of pX by HOF-29 is triggered by the encapsulation of pX molecules through 2D layers sliding and local distortion of the organic linkers. The realization of specific recognition of pX over other three isomers which is very challenging to separate each other even using other well-established porous materials. This showcased the potential of designing, synthesizing, and investigating HOFs for practical industrial applications.

In the third project, a Fe-containing porphyrin organic linker was achieved. The resulting crystal structure of HOF-30 was studied. Then our investigation extended to explore some gas separation applications. The results are fair enough to show that this HOF can have some separation of C2H2 over CH2H4, the pore channel and refined pore structure is interesting to study some more applications.