Hydrogen Storage and Hydrocarbon Separations in Porous Framework Materials with Exposed Metal Sites

The introduction of coordinatively unsaturated metal centres in porous materials has significantly enhanced the performance of these systems for many adsorptive-based applications. The work discussed in this dissertation explores the synthesis and characterisation of several metal–organic frameworks and zeolites that contain these sites for hydrogen storage and olefin/paraffin separations. An emphasis is placed on understanding both the structural and electronic factors that influence adsorption as a means to design materials capable of storing and separating gases with the efficiency required for real-world implementation. Chapter 1 briefly introduces different synthetic approaches that can be taken to produce adsorbents with open metal centres and reviews the structural and spectroscopic techniques that are typically employed to characterise these sites. A number of reports are highlighted to compare the effect that these exposed metals have on gas sorption. Notably, the position of the open metal site within the framework (e.g., at the node vs in the pore) leads to vastly different adsorptive properties, which is a topic discussed in the following chapters. Chapter 2 describes the incorporation of various light metal cations (Li+, Na+, K+, Mg2+) into the metal–organic framework Fe2(bdp)3 (bdp2– = 1,4-benzenedipyrazolate) by reduction of the coordinatively saturated Fe(III) sites to Fe(II). Single-crystal X-ray diffraction is used to locate the inserted cations, and they are found to preferentially reside near the centre of the pores when solvated by THF. However, upon activation, the cations migrate and are positioned close to the corners of the pore to maximise stabilising interactions with the framework in the absence of solvation. The effect of this is to limit the accessibility of the cations to gas molecules and to diminish the charge density felt by gases. This is demonstrated through hydrogen adsorption isotherms, which show that inclusion of these exposed cations does not improve H2 uptake or binding strength as much as was predicted. The results obtained in Chapter 2 motivated the study of porous materials with more established structures in which the location of the extra-framework cations were already precisely defined; specifically, ion-exchanged zeolites. In Chapter 3, several variants of zeolite A (Mg-A, Ca-A, Sr-A, Ba-A, Zn-A, Cu-A) are examined as potential hydrogen storage materials. Of these frameworks, Mg-A and Ca-A are found bind H2 gas with enthalpies approaching the target set by the Department of Energy for on board hydrogen storage systems. Isotherms collected at 77 and 87 K show that, at low coverage, Mg-A interacts more strongly with H2 than Ca-A. Interestingly, infrared spectroscopy suggests that more than one hydrogen molecule can interact with a single metal site in Ca-A, which has important implications for improving overall hydrogen storage capacity. Finally, Chapter 4 details the design and synthesis of a series of metal–organic frameworks, M2(X-m-dobdc) (M = Mg, Mn, Fe, Co, Ni; X = F, Cl, Br, I; m-dobdc4¬– = 4,6-dioxido-1,3,-benzenedicarboxylate), that possess open metal sites at the inorganic node rather than the pore. These materials are derivatives of the well-known M2(m-dobdc) frameworks, which have shown exceptional ability to separate ethylene from ethane and propylene from propane. The addition of an electron withdrawing halogen atom onto the ligand is shown to drastically enhance the separative performance of these materials for propylene/propane, as verified by single-component gas adsorption isotherms and IAST selectivity calculations. Of particular note is the framework Co2(F-m-dobdc), which exhibits the highest propylene/propane selectivity known for materials that separate gases using a physisorptive mechanism.

Advisor: Long, Jeffrey R