UC Berkeley

The work herein describes progress toward designing, synthesizing, and characterizing new metal–organic frameworks for gas separation applications. Metal–organic frameworks (MOFs) are a class of porous materials synthesized from molecular inorganic nodes and organic bridging linkers that can be designed to have very precise gas binding sites comprising of coordinatively unsaturated transition metal centers. These binding sites can be modified using standard coordination chemistry concepts to tune the metal site to selectively bind one gas from a given mixture, allowing for effective separations through selective adsorption. The research addresses improvements in selectivity for gas binding in several different examples and in reducing the energy input required for a separation cycle by engineering new functionalities at these transition metal binding sites in new metal–organic frameworks.

Chapter 1 details the current literature on using adsorbents for gas separation applications, with an emphasis on utilizing tunable metal–organic frameworks as high-capacity, highly selective adsorbents. Separations utilizing coordinatively unsaturated transition metals within these frameworks as selective binding sites for small molecules are highlighted, and the efficacy and mechanisms for a variety of gas separations are described. However, as shown, these metal sites generally operate through one type of mechanism in which the metal centers act as exposed, Lewis acidic cations. While effective for some separations, the types of separations that can be performed are limited. Additionally, this adsorption mechanism limits the energy efficiencies of the separation processes. As such, the performance of these materials can be improved with new design principles. The chapter concludes with describing proposed new functionalities at these open metal sites as basic design principles to either make frameworks that can perform new gas separations or design more energy efficient adsorbents.

Chapter 2 describes the design and synthesis of a new metal–organic framework containing exposed vanadium(II) sites that have been designed to contain metal centers that perform electron back-donation to π-acidic gases. This is the first example of such an electron-donating metal center with the proper electronic structure to effectively donate electrons to π* orbitals of gas molecules. This is demonstrated through the strong adsorption of N2 and characterization of the adsorbed species. In assessing it for potential gas separation performance, particularly for N2-based separations such as the removal of N2 from CH4, the material displays excellent capacity and record-setting selectivity for N2. This mechanism for adsorption is also applied to ethylene/ethane separations, where this material shows excellent selectivity at high temperatures.

Chapter 3 discusses the rational design of highly ethylene-specific binding sites in a manganese(II)-based metal–organic framework for ethylene capture in the presence of ethane, CO2, CO, CH4, and H2. In particular, this emphasizes creating a metal site with the proper ionic radius, electronegativity, and π-basicity to selectively adsorb ethylene from this complex mixture, which is produced through the oxidative coupling of methane as an alternative process to making ethylene from petroleum sources. By adsorbing ethylene selectively, several different separation steps required for this process can be eliminated, reducing the energy and capital cost significantly. In assessing a series of different materials, the choice of a high-spin Mn(II) ion to act as an effective ethylene adsorbent was demonstrated, and this material was evaluated under realistic process conditions to show very selective ethylene adsorption.

Chapter 4 focuses on a new metal–organic framework that performs selective and reversible adsorbate-induced spin state transitions at the framework metal sites. This allows for highly selective gas separations for strong field ligand gases and the reversible desorption of these gases for facile regeneration of the adsorbent. The compound features open, high-spin Fe(II) sites ligated by triazolate ligands that undergo a spin transition to low-spin Fe(II) when exposed to CO. Applying vacuum then allows the CO to be efficiently removed as the iron sites convert back to high-spin Fe(II). This allows for reversible capture of CO at very low pressures, enabling a reversible CO scrubber for purification of other gases, and also allows for the capture of highly pure CO for potential use as a reagent with some of the highest selectivity values ever reported for metal–organic frameworks.

Chapter 5 reports the realization of the first cooperative adsorbent featuring open metal sites by using a spin transition mechanism in two Fe(II)-based frameworks. Cooperative adsorption for selective adsorbents has recently been demonstrated to have several desirable energy-efficient properties in gas separation applications, but general guidelines currently do not exist for the design of such materials. This new mechanism uses communicating metal centers to perform cooperative adsorption using a concerted spin transition spin transition mechanism for highly efficient CO separations. The iron centers in these frameworks are situated in highly interconnected Fe–triazolate chains with open, high-spin Fe(II) sites, such that CO binding at one iron site prompts a spin transition that subsequently promotes adsorption at neighboring iron sites. While being selective, this adsorption process allows for high working capacities with small temperature swings, resulting in very low regeneration energies for use in gas separations. Ultimately, due to the highly tunable nature of these metal sites, we envision that this can be a broadly applicable design principle toward making next-generation, highly efficient adsorbents.