Student Ph.D. Defense - Ayalew H. Assen

Sep 10 2018 01:00 PM - Sep 10 2018 02:00 PM

Polynuclear Rare-earth (RE) based Metal-Organic Frameworks (MOFs): From Topological Exploration to Preparation of Tailor-made MOF's

Monday, September 10, 2018
01:00 PM - 02:00 PM
Auditorium Between Builidng 4 & 5

Abstract:  Metal-organic frameworks (MOFs) have emerged as a unique class of solid-state materials, exemplifying the power of combining organic and inorganic chemistries to address the enduring challenge pertaining to designing solid state materials with desired attributes. Notably, a myriad of MOFs were constructed in the last two decades. In particular, the use of well-defined polyatomic clusters as molecular building blocks (MBBs) permitted access to the looked-for geometrical features, incorporated in preselected building units prior to the assembly process, guiding the assembly of a targeted network. Nevertheless, the diverse coordination modes and geometries of rare-earth (RE) elements requires the introduction of a sophisticated controlled approach for their use as polynuclear cluster MBBs. Subsequently, our group has introduced the use of 2-fluorobenzoic acid (2-FBA) modulator that consistently allows the in situ control and formation of multi-nuclear RE MBBs. The presented work in this thesis demonstrates the use of elaborate RE MBBs and their successful deployment in reticular chemistry for the construction of particular MOF platforms expressing unique properties in term of gas separations. Accordingly, the RE hexanuclear clusters were used to construct fcu- and flu- MOF platforms with controlled pore-aperture sizes. Markedly, the isolated RE-MOFs, RE-fum-fcu-MOF and RE-bqdc-flu-MOF, showed unprecedented paraffin/isoparaffin molecular sieving. Further tuning of the windows of RE-fcu-MOFs afforded the assembly of a MOF suitable for propylene/propane separation. The exceptional thermal and chemical stability and high adsorption selectivity of some of these MOFs prompted us to explore the fcu-MOF platform for selective removal of H2S/CO2 from CH4 and for sensing of toxic gases, namely H2S and NH3. 

Additionally, the research presented in this dissertation highlights the topological exploration for the formation of new MOFs:  i) highly-connected polyatomic RE-MOFs in combination with tetrahedrally oriented tetracarboxylate ligands afforded the formation MOFs with new underlying topologies, namely kna-, kel- and kem-MOFs; ii) mixed-metal approach (RE plus other elements) was employed to fabricate MOFs containing in situ formed metalo-linker MBBs that are difficult to be pre-assembled by organic synthesis; iii) supermolecular building layer (SBL) approach was extended from the prevalent sql to the less explored double sql layer for the  rational design of pillared MOFs.