Crystal engineering of layered and pillared square lattice networks for adsorptive separation of hydrocarbons

Separation of hydrocarbons (HCs) is industrially relevant thanks to their widespread utility in the petrochemical industry but remains a challenge because of the similar physicochemical properties of the components of important gas mixtures such as those produced during manufacture of C2 and C3 HCs. Technologies to separate such HCs currently rely upon energy-intensive separations such as cryogenic distillation, chemisorption, or solvent extraction. Physisorbents offer the potential to enable energy-efficient adsorptive separation technologies for purification of HCs and there is a growing activity in this area. In this context, metal-organic materials (MOMs), including metal-organic frameworks (MOFs) and porous coordination polymers (PCPs), have emerged as leading candidates for addressing energy-efficient gas/vapour/liquid separations such as C2H2/CO2, C2H2/C2H4, C3H4/C3H6, C8 aromatic isomers etc. Crystal engineering, the field of chemistry that studies the design, properties, and applications of crystals, has evolved from a focus upon the design of new crystalline materials and their properties to an emphasis upon creating the right materials for the right applications. MOMs that are amenable to crystal engineering are important in this context as they offer a means of precise control over pore size/chemistry and they have recently emerged as benchmark physisorbents for separating HCs. Herein, we address structure-property relationships with respect to HC adsorption in two subclasses of MOMs, layered square lattice (sql) coordination networks and hybrid ultramicroporous materials (HUMs, also known as inorganic linker pillared sql networks). Chapter 1 reviews the importance of separating small molecules of industrial relevance, especially C1-C8 HCs. Herein, present, and emerging technologies for HCs’ separation and purification, contextualizing their energy efficiency and regenerability are reviewed comprehensively. Adsorptive separation based on physisorbents, an alternative technology that is more energy efficient to separate HCs, has been discussed. Guided by crystal engineering blueprints, the development of two generations of MOM based HC adsorbents have been discussed, in terms of fine-tuning their pore size/chemistry. Further we discuss about layered sql coordination networks, an underexplored class of MOMs for their emerging role in separation and purification of HCs and its switching behaviour that exhibit high adsorption capacity and selectivity in the pressure region where one component can open the framework while others cannot. Hybrid ultramicroporous (pore size < 0.7 nm) materials (HUMs), can also be called as pillared square grid networks, also a subclass of MOMs, are outstanding candidates for physisorptive separation of HCs, as they offer benchmark selectivities for several C1-C3 separations. This chapter also reviews HUMs and other sorbents and their performance for separation and purification of light hydrocarbons (LHs). A brief discussion on how to control the pore environments of MOMs in ways to elicit optimal binding sites specific for target HC molecules concludes the chapter. Chapter 2 highlights the role of crystal engineering to rationally design mixed linker/rectangular square grid networks and elucidates our studies upon their sorption properties. We demonstrate that a new rectangular sql coordination network [Co(bipy)(bptz)(NCS)2]n, abbreviated as sql-1,3-Co-NCS, exhibits strong selectivity towards all three xylene isomers over EB with high uptake capacities. In fact, a comparative analysis of the key performance parameters viz. selectivity of xylene isomers over EB and gravimetric uptake reveals that sql-1,3-Co-NCS outperforms physisorbents reported thus far. To the best of our knowledge, sql-1,3-Co-NCS is the first adsorbent to exhibit a combination of high xylene adsorption capacity (~ 37 wt%) and high xylene selectivity over EB (SOX/EB, SMX/EB, SPX/EB > 5). Thanks to the use of mixed-linker guided fine-tuning of pore size and chemistry, this work establishes the importance of crystal engineering the modular class of rectangular sql coordination networks to design top-performing C8 sorbents. Chapter 3 reports an ultramicroporous sql coordination network. We report efficient C2H2/CO2 separation using an ultramicroporous coordination network, [Cu(4,4-(2,5-dimethyl 1,4-phenylene)dipyridine)2(NO3)2]n (sql-16-Cu-NO3-), a new member of the understudied class/family of sorbents of sql topology. sql-16-Cu-NO3- exhibits both flexible and rigid behaviour with C2H2 at cryogenic and ambient temperatures respectively. A new type of C2H2 binding site CH∙∙∙ONO2, in sql-16-Cu-NO3- offers highly selective C2H2/CO2 separation performance. sql-16-Cu-NO3- exhibits highly selective C2H2/CO2 separation performance offering the combination of a) only the third best experimentally derived equimolar separation selectivity for C2H2/CO2 among physisorbents; b) benchmark difference between the C2H2 and CO2 adsorption enthalpies at half loading. In situ powder X-ray diffraction, molecular modelling studies and their analysis provide insights into the sorption properties and high C2H2/CO2 separation performances revealed by sql-16-Cu-NO3-. Chapter 4 breaks the existing trade-off between adsorption capacity and selectivity with porous materials, which is major roadblock to reducing the energy footprint of gas separation technologies. In this regard, we report a family of six new hybrid ultramicroporous materials (HUMs) based upon a ligand that enables higher surface area than existing HUMs; strong binding sites for C2H2; weak binding for CO2. Only minor structural differences across this isostructural family of six HUMs enabled fine-tuning of pore size and pore chemistry. We demonstrate that four of the new HUMs, [Ni(pypz)2SiF6]n, SIFSIX-21-Ni; [Ni(pypz)2NbOF5]n, NbOFFIVE-3-Ni; [Cu(pypz)2TiF6]n, TIFSIX-4-Cu; and [Cu(pypz)2NbOF5]n, NbOFFIVE-3- Cu, (pypz: 4-(3,5-dimethyl-1H-pyrazol-4-yl)pyridine) break the aforementioned selectivity/capacity trade-off with adsorption capacities ≥ 3.5 mmol∙g -1 and high separation selectivities ≥ 5. SIFSIX-21-Ni is the new benchmark among C2H2/CO2 selective sorbents since it combines exceptional separation selectivity (27.7) with high adsorption capacity (4 mmol∙g-1 ). In situ infrared (IR) spectroscopy and molecular modelling studies provide insights into the acetylene binding sites in this family of HUMs and critically interrogates why they differ from those of structurally related HUMs. Chapter 5 addresses single-step purification of ethylene (C2H4), by crystal engineering of two HUMs, [Ni(aminopyrazine)2(SiF6)]n (SIFSIX-17-Ni) and [Ni(aminopyrazine)2(TiF6)]n (TIFSIX-17-Ni). No single physisorbent meets the requisite selectivity required to purify pure (> 99.9%) C2H4 from ternary C2-CO2 mixtures (C2H4/C2H2/CO2) under ambient conditions. Indeed, both SIFSIX-17-Ni and TIFSIX-17-Ni produce polymer-grade ethylene (> 99.9% purity) from a 1:1:1 ternary C2-CO2 mixture. We attribute the observed properties to the unusual binding sites in SIFSIX-17-Ni and TIFSIX-17-Ni that offer comparable affinity to both CO2 and C2H2, thereby enabling coadsorption of C2H2 and CO2. In situ synchrotron x-ray diffraction, in situ IR spectroscopy and computational simulations provide in-depth understanding of these binding sites and explains how the amino substitution profoundly impacts the prototypal HUM pore environment in the isostructural pyrazine-linked SIFSIX-3-Zn. Chapter 6 presents a conclusion and explores future potential applications of layered sql coordination networks and HUMs to purify commodity chemicals, including HCs. We explain how modularity and amenability to crystal engineer sql coordination networks and HUMs, make them potential sorbents to enable adsorptive separation and purification of industrially and environmentally relevant pure chemicals from the industrial feedstocks during downstream processing of mixtures. Our findings provide improved structure-property relationships, key to explain how fine-tuning of pore size and pore chemistry will enhance HC separation performances. Our studies lead to new design principles which can be further developed in future to generate bespoke sorbents for myriad properties and applications. Relevant lead sorbents can be applied to “synergistic sorbent separation technology”, SSST, to enable one step purification from ternary and quaternary gas mixtures. This chapter concludes by highlighting the yet unaccomplished objectives of sql coordination networks and HUMs, such as the formulation of several benchmark sorbents into regular shaped/ sized pellet based fixed bed development to translate into higher technological readiness level research and to examine the effects of particle size, defects, hierarchical MOMs derived porous solids, composites and membranes to build upon the status quo.