Interpenetrated hybrid ultramicroporous materials: insight into structure-property relationships

Abstract

Crystal Engineering is the field of chemistry that studies the design, properties, and application of crystalline materials. An aspect of crystal engineering is the design of coordination networks using linker ligands that cross-link transition metal nodes. Coordination networks that can exhibit permanent porosity have attracted attention for their potential application in gas storage, separation, and catalysis. In the context of separations, 15% of global energy costs are associated with separation of chemical commodities. That some coordination networks are inherently modular through node/linker substitution enables crystal engineering studies that can provide insight into structure-function relationships. Square lattice (sql) coordination networks were perhaps the first class of coordination networks to undergo systematic study; thanks mainly to their propensity to form from many nodes and linkers. Further, some sql coordination networks can be pillared to afford primitive cubic (pcu) coordination networks, offering modularity that, in principle, has at least four variables: node, linker, pillar, interpenetration. A class of pillared sql coordination networks known as Hybrid Ultramicroporous Materials (HUMs) has recently set new benchmarks for several important gas separations thanks to their ultramicropores (≤0.7 nm) which are lined by inorganic pillars that can act as molecular traps for small gas molecules. For example, ethylene (C2H4) is the highest volume chemical feedstock and contains ca. 1% acetylene (C2H2) impurities that must be removed. The goal herein is to conduct crystal engineering studies of interpenetrated HUMs in the context of C2H2/C2H4 gas separations and hydrolytic stability. The insight found herein may afford better design principles for future porous coordination networks in terms of performance and stability. Chapter 1 introduces crystal engineering, coordination networks, and HUMs. Chapter 2 addresses the C2H2/C2H4 separation performance of the two-fold interpenetrated pcu (pcu-c) HUM, SIFSIX-14-Cu-i ([Cu(1,2-bis(4- pyridyl)diazene)2(SiF6)]n). Sorption-based gas separation/purification is hindered by a general inverse relationship between selectivity and uptake capacity in porous materials. Ideal molecular sieves could be a compromise with pores that block larger gas molecules and adsorb high quantities of smaller gas molecules. SIFSIX-14-Cu-i has ultramicropores (3.4 Å) that effectively exclude C2H4 molecules but is constructed from SiF6 2- pillars yielding benchmark C2H2 uptake (58 cm3 cm-3 at 0.01 bar) and selectivity at 298 K (>6000 vs 44 for the previous benchmark, SIFSIX-2-Cu-i ([Cu(1,2-bis(4- pyridyl)acetylene)2(SiF6)]n)). Dynamic gas breakthrough studies further confirm separation performance with an effluent C2H4 production of 87.5 mmol/g (99.9999% pure) and capturing 1.18 mmol/g C2H2 per cycle. Chapter 3 reports on the rare and poorly understood phenomenon of partial interpenetration and its potential relevance to gas separations as it could, in principle, enable an increase in uptake capacity without reducing selectivity. Systematic synthesis afforded solid solutions of SIFSIX-14-Cu-i and its non-interpenetrated pcu polymorph SIFSIX-14-Cu. Solid solutions exhibited proportions of two-fold interpenetration ranging from 70-99%. C2H2/C2H4 gas separation studies reveal that partial interpenetration negatively affects separation performance and is attributed to a reduction in the bulk density of C2H2 molecular traps. Chapter 4 details the study of linker and pillar substitution, enabling greater understanding of how subtle differences in structure may affect properties. The pcu-c HUMs TIFSIX-2-Cu-i ([Cu(1,2-bis(4-pyridyl)acetylene)2(TiF6)]n) and TIFSIX-4-Cu-i ([Cu(1,4-bis(4-pyridyl)benzene)2(TiF6)]n) demonstrate that variations in linkers and pillars can affect C2H2/C2H4 separation performances. Whereas TiF6 2- pillars impart stronger electrostatics and improved performance in TIFSIX-2-Cu-i (compared with SIFSIX-2-Cu-i), the longer ligand in TIFSIX-4-Cu-i leads to larger pores and weaker sorbent-sorbate interactions. Indeed, TIFSIX-4-Cu-i exhibits offset interpenetration resulting in two types of pores. Gas sorption studies of TIFSIX-4-Cu-i exhibited a stepped isotherm as a result of sequential pore filling. Chapter 5 continues the study of linker/pillar substitution, with TIFSIX-14-Cu-i ([Cu(1,2-bis(4-pyridyl)diazene)2(TiF6)]n) and NbOFFIVE-2-Cu-i ([Cu(1,2-bis(4- pyridyl)acetylene)2(NbOF5)]n), and its effect on C2H2/C2H4 gas separations. Although these pillars would be expected to afford the strongest electrostatics, an evaluation of bond lengths reveals that subtle pore size effects can be more influential. This observation leads to the conclusion that there is an optimal balance between pore size and pore chemistry that yields benchmark performances. Chapter 6 reports water vapour sorption in four hybrid materials; benchmarks for C2H2 capture (SIFSIX-14-Cu-i, SIFSIX-2-Cu-i, and SIFSIX-1-Cu) and CO2 capture (SIFSIX-3-Ni). The effects of water vapour on performance and stability remain understudied, despite practical relevance. Three materials exhibit a negative-water vapour-sorption phenomenon wherein adsorbed vapour uptake decreases as pressure increases and is attributed to a water-vapour-induced phase transformation, where initial structures convert to sql or interpenetrated square lattices (sql-c*). Although studied, the mechanisms by which coordination networks change degrees and modes of interpenetration are not understood. SIFSIX-2-Cu-i retained its structure leading to an understanding of the interactions controlling hydrolytic stability. Chapter 7 extends the study of water vapour sorption with SIFSIX-7-Cu, TIFSIX-7-Cu, and GEFSIX-7-Cu ([Cu(1,2-bis(4-pyridyl)ethylene)2(MF6)]n; M = Si, Ti, Ge). Water vapour adsorption is observed to lead each compound to undergo the pcu to sql-c* phase transformation at different relative humidity levels, underlining the different interaction strengths imparted by each pillar. Further, a structural analysis suggests that the close packing of the sql-c* phase may inhibit structures with longer ligands from undergoing this irreversible phase transformation. Chapter 8 offers a conclusion to the crystal engineering of interpenetrated HUMs reported herein and looks towards possible future directions. The synthesis of solid solutions and substitution of linkers and pillars provide an understanding of structure-property relationships in C2H2/C2H4 gas separations and water vapour sorption with a view to designing future porous coordination networks with improved performance and stability