Tailoring the Pore Environment of Metal-Organic and Molecular Materials Decorated with Inorganic Anions: Platforms for Highly Selective Carbon Capture

Author

Patrick Stephen Nugent, University of South FloridaFollow

Graduation Year

2015

Document Type

Thesis

Degree

M.S.

Degree Name

Master of Science (M.S.)

Degree Granting Department

Chemistry

Major Professor

Michael J. Zaworotko, Ph.D.

Co-Major Professor

Shengqian Ma, Ph.D.

Committee Member

Julie Harmon, Ph.D.

Keywords

metal-organic frameworks, coordination polymers, carbon capture, CO2 capture, gas separation

Abstract

Due to their high surface areas and structural tunability, porous metal-organic materials, MOMs, have attracted wide research interest in areas such as carbon capture, as the judicious choice of molecular building block (MBB) and linker facilitates the design of MOMs with myriad topologies and allows for a systematic variation of the pore environment. Families of MOMs with modular components, i.e. MOM platforms, are eminently suitable for targeting the selective adsorption of guest molecules such as CO₂ because their pore size and pore functionality can each be tailored independently. MOMs with saturated metal centers (SMCs) that promote strong yet reversible CO₂ binding in conjunction with favorable adsorption kinetics are an attractive alternative to MOMs containing unstaurated metal centers (UMCs) or amines. Whereas MOMs with SMCs and exclusively organic linkers typically have poor CO₂ selectivity, it has been shown that a versatile, long known platform with SMCs, pillared square grids with inorganic anion pillars and pcu topology, exhibits high and selective CO₂ uptake, a moderate CO₂ binding affinity, and good stability under practical conditions. As detailed herein, the tuning of pore size and pore functionality in this platform has modulated the CO₂ adsorption properties and revealed variants with unprecedented selectivity towards CO₂ under industrially relevant conditions, even in the presence of moisture.

With the aim of tuning pore chemistry while preserving pore size, we initially explored the effect of pillar substitution upon the carbon capture properties of a pillared square grid, [Cu(bipy)₂(SiF₆)] (SIFSIX-1-Cu). Room temperature CO₂, CH₄, and N₂ adsorption isotherms revealed that substitution of the SiF₆^2- (“SIFSIX”) inorganic pillar with TiF₆^2- (“TIFSIX”) or SnF₆^2- (“SNIFSIX”) modulated CO₂ uptake, CO₂ affinity (heat of adsorption, Q_st), and selectivity vs. CH₄ and N₂. TIFSIX-1-Cu and SNIFSIX-1-Cu were calculated to exhibit the highest CO₂/N₂ and CO₂/CH₄ adsorption selectivites of the series, respectively. Modeling studies of TIFSIX-1-Cu and SIFSIX-1-Cu suggested that the enhancements in low pressure CO₂ uptake and CO₂ selectivity in the former arose from the stronger polarization of CO₂ molecules by TIFSIX-1-Cu. The stronger framework-CO₂ interaction at the primary binding site in TIFSIX-1-Cu correlates with the greater electronegativity of the pillar fluorine atoms relative to those in SIFSIX-1-Cu, and in turn to the higher polarizability of Ti⁴⁺ vs. Si⁴⁺.

The effect of tuning pore size upon the carbon capture performance of pillared square grid nets was next investigated. Linker substitution afforded three variants, SIFSIX-2-Cu, SIFSIX-2-Cu-i, and SIFSIX-3-Zn, with pore sizes ranging from nanoporous (13.05 Å in SIFSIX-2-Cu) to ultramicroporous (3.84 Å in SIFSIX-3-Zn). Single-gas adsorption isotherms showed that SIFSIX-2-Cu-i, a doubly interpenetrated polymorph of SIFSIX-2-Cu with contracted pores (5.15 Å), exhibited far higher CO₂ uptake, Q_st towards CO₂, and selectivity towards CO₂ vs. CH₄ and N₂ than its non-interpenetrated counterpart. Further contraction of the pores afforded SIFSIX-3-Zn, a MOM with enhanced CO₂ binding affinity and selectivity vs. SIFSIX-2-Cu-i. Remarkably, the selectivity of SIFSIX-3-Zn towards CO₂ was found to be unprecedented among porous materials. Equilibrium and column breakthrough adsorption tests involving gas mixtures meant to mimic post-combustion carbon capture (CO₂/N₂), natural gas/biogas purification (CO₂/CH₄), and syngas purification (CO₂/H₂) confirmed the high selectivities of SIFSIX-2-Cu-i and SIFSIX-3-Zn. Gas mixture experiments also revealed that SIFSIX-3-Zn exhibited optimal CO₂ adsorption kinetics. Most importantly, the CO₂ selectivity of SIFSIX-2-Cu-i and SIFSIX-3-Zn was minimally affected in the presence of moisture. Modeling studies of CO₂ adsorption in SIFSIX-3-Zn (experimental Q_st ~ 45 kJ/mol at all loadings) revealed strong yet reversible electrostatic interactions between CO₂ molecules and the SIFSIX pillars lining the confined channels of the material.

Porous materials based upon the non-covalent assembly of discrete MBBs can also exhibit high surface areas and systematically tunable pore environments. Molecular porous material (MPM) platforms have begun to emerge despite the greater challenge of designing such materials in comparison to MOMs. Herein we report the tuning of pore functionality in an MPM platform based upon an extensive hydrogen-bonded network of paddlewheel-shaped [Cu(ade)₄L₂] complexes (ade = adenine; L = axial ligand). The substitution of Cl axial ligands with inorganic TIFSIX moieties has produced [Cu₂(ade)₄(TiF₆)₂], MPM-1-TIFSIX, a variant with enhanced CO₂ separation performance and stability. Single-gas adsorption isotherms reveal that MPM-1-TIFSIX exhibits the highest CO₂ uptake and CO₂ Q_st yet reported for an MPM as well as high selectivity towards CO₂ vs. CH₄ and N₂. Modeling studies indicated strong electrostatic interactions between CO₂ and the TIFSIX ligands lining the pores of MPM-1-TIFSIX. In addition to dramatically surpassing MPM-1-Cl with regard to CO₂ separation performance, MPM-1-TIFSIX exhibits thermal stability up to 568 K and retains its performance even after immersion in water for 24 hrs.

Comprehensively, the results presented herein affirm that porous materials featuring inorganic anions and SMCs can exhibit high and selective CO₂ uptake, sufficient stability, and facile activation conditions without the drawbacks associated with UMCs and amines, i.e. competitive water adsorption and high regeneration energy, respectively.

Scholar Commons Citation

Nugent, Patrick Stephen, "Tailoring the Pore Environment of Metal-Organic and Molecular Materials Decorated with Inorganic Anions: Platforms for Highly Selective Carbon Capture" (2015). USF Tampa Graduate Theses and Dissertations.
https://digitalcommons.usf.edu/etd/5889