Microporous Coordination Polymers: Exploring Heterogeneity with an Antimatter Probe.

Abstract

Microporous coordination polymers (MCPs) are a rapidly growing class of porous, crystalline materials derived from alternating organic and inorganic building blocks. While some MCPs exhibit exceptional sorption properties, many others do not show the performance expected based on their crystallographic models. Realizing the full potential of these materials requires a thorough understanding of why many fall short of such expectation. Obtaining such insight is hampered by a lack of methods to examine localized defects and heterogeneity within these materials. This dissertation focuses on the use of positron annihilation lifetime spectroscopy (PALS) to elucidate the reasons for low porosity in two well-known MCPs, Zn-HKUST-1 and IRMOF-8. PALS is used to show that while the Zn-HKUST-1 interior contains empty pores of diameter commensurate with the corresponding crystallographic model, the pores near the surface of Zn-HKUST-1 are inaccessible, thereby precluding access to the porous interior. The porosity of the material before solvent removal is confirmed by the facile diffusion of solution-phase guest species into the crystal interior. IRMOF-8, despite having been first reported more than a decade ago, has until now shown surface areas at best only half of that expected based on its crystallographic model. A combination of PXRD, gas sorption, and PALS are used to show that typical preparations of IRMOF-8 in fact produce an interpenetrated analogue. A route to synthesize and activate non-interpenetrated IRMOF-8 is developed. The material has high gravimetric adsorption capacities for gaseous fuels such as hydrogen and methane; however, in situ PALS reveals that even at high pressures, only monolayer sorption is achievable with light gases above their critical temperatures. Hence, to maximize volumetric adsorption, linker functionalization is necessary. The use of PALS to analyze MCPs also resulted in the serendipitous discovery that positronium assumes a quantum mechanical Bloch state in highly ordered and porous MCPs such as IRMOF-1 and non-interpenetrated IRMOF-8.