Interpenetrated hybrid ultramicroporous materials: insight into structure-property relationships

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

A coordination network that reversibly switches between two nonporous polymorphs and a high surface area porous phase

We report a 2-fold interpenetrated primitive cubic (pcu) network X-pcu-5-Zn, [Zn2(DMTDC)2(dpe)] (H2DMTDC = 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylic acid, dpe = 1,2-di(4-pyridyl)ethylene), that exhibits reversible switching between an as-synthesized “open” phase, X-pcu-5-Zn-α, and two nonporous or “closed” polymorphs, X-pcu-5-Zn-β and X-pcu-5-Zn-γ. There are two unusual features of X-pcu-5-Zn. The first relates to its sorption properties, which reveal that the α form exhibits high CO2 uptake (ca. 255 cm3/g at 195 K) via reversible closed-to-open switching (type F-IV isotherm) of the type desirable for gas and vapor storage; there are only three other reports of porous materials that combine these two features. Second, we could only isolate the β form by activation of the CO2 loaded α form and it persists through multiple CO2 adsorption/desorption cycles. We are unaware of a new polymorph having been isolated in such a manner. That the observed phase changes of X-pcu-5-Zn-α occur in single-crystal-to-single-crystal fashion enabled structural characterization of the three forms; γ is a coordination isomer of α and β, both of which are based upon “paddlewheel” clusters

The effect of centred versus offset interpenetration on C2H2 sorption in hybrid ultramicroporous materials

Fine-tuning of hybrid ultramicroporous materials (HUMs) can significantly impact their gas sorption performance. This study reveals that offset interpenetration can be antagonistic with respect to C2H2 separation from C2H2/C2H4 gas mixtures

Recyclable switching between nonporous and porous phases of a square lattice (sql) topology coordination network

A nonporous square lattice (sql) coordination network [Co(bipy)2(NCS)2]n (sql-1-Co-NCS) exhibits recyclable switching induced by CO2. The sorption isotherms are stepped with moderate hysteresis, temperature controlled and saturation uptake is fixed. Such switching, which has rarely been observed, offers the promise of exceptional working capacity for gas storage

Controlling the uptake and regulating the release of nitric oxide in microporous solids

Representative compounds from three classes of microporous solids, namely metal-organic frameworks (MOFs), hybrid ultramicroporous materials (HUMs) and porous-organic polymers (POPs), were investigated for their nitric oxide gas uptake and release behavior. Low pressure sorption studies indicated strong chemisorption of NO on the free amine groups decorating the MOF UiO-66-NH2 when compared to its non-amine functionalized parent. The HUMs demonstrated reversible physisorption within the low pressure regime but interestingly in one case there was evidence for chemisorption following pressurization with NO at 10 bar. Significant release of chemisorbed NO from the UiO-66-NH2 and one of the HUMs was triggered by addition of acid to the medium, a pH change from 7.4 to 5.4 being sufficient to trigger NO release. An imidazole-based POP exhibited chemisorption of NO at high pressure wherein the ring basicity facilitated both NO uptake and spontaneous release upon contact with the aqueous release medium

Metal–organic material polymer coatings for enhanced gas sorption performance and hydrolytic stability under humid conditions

Physisorbent metal–organic materials (MOMs) have shown benchmark performance for highly selective CO2 capture from bulk and trace gas mixtures. However, gas stream moisture can be detrimental to both adsorbent performance and hydrolytic stability. One of the most effective methods to solve this issue is to transform the adsorbent surface from hydrophilic to hydrophobic. Herein, we present a facile approach for coating MOMs with organic polymers to afford improved hydrophobicity and hydrolytic stability under humid conditions. The impact of gas stream moisture on CO2 capture for the composite materials was found to be negligible under both bulk and trace CO2 capture conditions with significant improvements in regeneration times and energy requirements

Crystal engineering of a rectangular sql coordination network to enable xylenes selectivity over ethylbenzene†

Separation of the C8 aromatic isomers, p-xylene (PX), m-xylene (MX), o-xylene (OX) and ethylbenzene (EB), is relevant thanks to their widespread application as chemical feedstocks but challenging because of their similar boiling points and close molecular dimensions. Physisorptive separation could offer an energy-efficient solution to this challenge but sorbents which exhibit strong selectivity for one of the isomers remain a largely unmet challenge despite recent reports of OX or PX selective sorbents with high uptake capacity. For example, the square lattice, sql, topology coordination network [Co(bipy)2(NCS)2]n (sql-1-Co-NCS) exhibits the rare combination of high OX selectivity and high uptake capacity. Herein we report that a crystal engineering approach enabled isolation of the mixed-linker sql coordination network [Co(bipy)(bptz) (NCS)2]n (sql-1,3-Co-NCS, bipy = 4,4′-bipyridine, bptz = 4,4′-bis(4-pyridyl)tetrazine) and study of its C8 vapour and liquid sorption properties. sql-1,3-Co-NCS was found to exhibit high adsorption capacity from liquid xylenes (∼37 wt%) and is to our knowledge the first sorbent to exhibit high selectivity for each of xylene isomer over EB (SOX/EB, SMX/EB, SPX/EB > 5). Insights into the performance of sql-1,3-Co-NCS are gained from structural studies which reveal stacking interactions between electron-deficient bptz linkers and the respective xylenes. sql-1,3-Co-NCS is the first N-donor mixed-linker sql coordination network studied for its gas/vapour sorption properties and represents a large and diverse class of understudied coordination networks

Impact of partial interpenetration in a hybrid ultramicroporous material on C2H2/C2H4 separation performance

Phases of a 2‐fold pcu Hybrid Ultramicroporous Material (HUM), SIFSIX‐14‐Cu‐i, exhibiting 99%, 93%, 89%, and 70% partial interpenetration have been obtained. 1:99 C2H2/C2H4 gas separation studies reveal that as the proportion of interpenetrated component decreases, so does the separation performance