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

Pillar modularity in fsc topology hybrid ultramicroporous materials based upon tetra(4-pyridyl)benzene

.Hybrid ultramicroporous materials (HUMs) are porous coordination networks composed of combinations of organic and inorganic linker ligands with a pore diameter of 10 for 1:99 C2H2/C2H4 and >5 for 1:1 C2H2/CO2. The approach taken, systematic variation of pillars with retention of structure, enables differences in selectivity to be attributed directly to the choice of the inorganic pillar. This study introduces fsc topology HUMs as a modular platform that is amenable to fine-tuning of structure and properties</p

Metal-organic frameworks as regeneration optimized sorbents for atmospheric water harvesting

As the freshwater crisis looms, metal-organic frameworks (MOFs) with stepped isotherms lie at the forefront of desiccant development for atmospheric water harvesting (AWH). Despite numerous studies on water sorption kinetics in MOF desiccants, the kinetics of AWH sorbents  are a challenge to quantify. Here, we report that the AWH kinetics of  seven known MOFs and the industry-standard desiccant Syloid are limited  by diffusion  to the sorbent bed surface. A quantitative model that exploits isotherm  shape enables simulation of sorption cycling to evaluate sorbent  performance through productivity contour plots  (“heatmaps”). These heatmaps reveal two key findings: steady-state  oscillation around partial loading optimizes productivity, and dense  ultramicroporous MOFs with a step at low relative humidity afford superior volumetric  performance under practically relevant temperature swing conditions  (27°C, 30% relative humidity [RH] − 60°C, 5.4% RH). Cellulose-desiccant  composites of two such regeneration optimized sorbents retain the  kinetics of powders, producing up to 7.3 L/kg/day of water under these  conditions.</p

CO₂ capture by hybrid ultramicroporous TIFSIX-3-Ni under humid conditions using non-equilibrium cycling

Although pyrazine-linked hybrid ultramicroporous materials (HUMs, pore size <7 Å) are benchmark physisorbents for trace carbon dioxide (CO2) capture under dry conditions, their affinity for water (H2O) mitigates their carbon capture performance in humid conditions. Herein, we report on the co-adsorption of H2O and CO2 by TIFSIX-3- Ni—a high CO2 affinity HUM—and find that slow H2O sorption kinetics can enable CO2 uptake and release using shortened adsorption cycles with retention of ca. 90% of dry CO2 uptake. Insight into co-adsorption is provided by in situ infrared spectroscopy and ab initio calculations. The binding sites and sorption mechanisms reveal that both CO2 and H2O molecules occupy the same ultramicropore through favorable interactions between CO2 and H2O at low water loading. An energetically favored water network displaces CO2 molecules at higher loading. Our results offer bottom-up design principles and insight into co-adsorption of CO2 and H2O that is likely to be relevant across the full spectrum of carbon capture sorbents to better understand and address the challenge posed by humidity to gas capture.</p

Structural phase transformations induced by guest molecules in a nickel-based 2D Square lattice coordination network

Herein, we report the crystal structure and guest binding properties of a new two-dimensional (2D) square lattice (sql) topology coordination network, sql-(azpy)(pdia)-Ni, which is comprised of two linker ligands with diazene (azo) moieties, (E)-1,2-di(pyridin-4-yl)diazene(azpy) and (E)-5-(phenyldiazenyl)isophthallate(pdia). sql-(azpy)(pdia)-Ni underwent guest-induced switching between a closed (nonporous) β phase and several open (porous) α phases, but unlike the clay-like layer expansion to distinct phases previously reported in switching sql networks, a continuum of phases was formed. In effect, sql-(azpy)(pdia)-Ni exhibited elastic-like properties induced by adaptive guest binding. Single-crystal X-ray diffraction (SCXRD) studies of the α phases revealed that the structural transformations were enabled by the pendant phenyldiazenyl moiety on the pdia2– ligand. This moiety functioned as a type of hinge to enable parallel slippage of layers and interlayer expansion for the following guests: N,N-dimethylformamide, water, dichloromethane, para-xylene, and ethylbenzene. The slippage angle (interplanar distances) ranged from 54.133° (4.442 Å) in the β phase to 69.497° (5.492 Å) in the ethylbenzene-included phase. Insight into the accompanying phase transformations was also gained from variable temperature powder XRD studies. Dynamic water vapor sorption studies revealed a stepped isotherm with little hysteresis that was reversible for at least 100 cycles. The isotherm step occurred at ca. 50% relative humidity (RH), the optimal RH value for humidity control.</p

Rapid determination of experimental sorption isotherms from non-equilibrium sorption kinetic data

Herein, we report a new method for rapid determination of experimental sorption isotherms that exploits gravimetric non-equilibrium sorption kinetics data in a thin sorbent bed. In comparison with equilibrium-based isotherm determination methods, this sorption kinetics isotherm determination (SKID) method needs only two equilibrium points, P/P0min and P/P0max. SKID requires up to an order of magnitude less data collection time than conventional methods and is suitable for high-throughput sorbent discovery and evaluation. SKID was validated by testing a library comprising 30 sorbents, including rigid and flexible metal-organic frameworks (MOFs), inorganics (zeolites), and organics (microcrystalline cellulose). Average data collection time for water vapor was 3.1 vs. 23 h for dynamic vapor sorption experiments. SKID was also demonstrated for other vapors (C8 aromatics) and a gas (CO2), making it a promising tool for rapid screening of new or existing sorbents for applications such as water harvesting and dehumidification and carbon capture.</p

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

A square lattice topology coordination network that exhibits highly selective C2H2/CO2 separation performance

C2H2/CO2 separation is an industrially important process that remains challenging because of the similar physicochemical properties of C2H2 and CO2. We herein report that the new square lattice (sql) coordination network [Cu (bipy‐xylene)2(NO3)2]n, sql‐16‐Cu‐NO3, 16 = bipy‐xylene = 4,4′‐(2,5‐dimethyl‐ 1,4‐phenylene)dipyridine, exists in at least three forms, as‐synthesised (α), activated (α′) and hydrated (β). The activated phase, sql‐16‐Cu‐NO3‐α′, is an ultra microporous material that exhibits high selectivity towards C2H2 over CO2 as revealed by dynamic gas breakthrough experiments (1:1, C2H2/CO2) that afforded 99.87% pure CO2 in the effluent stream. The separation selectivity at 298 K and 1 bar, 78, is the third best value yet reported for C2H2 selective physisorbents whereas the mid‐loading performance sets a new benchmark. The performance of sql‐16‐Cu‐NO3‐α′ is attributed to a new type of C2H2 binding site in which CH···ONO2 interactions enable moderately strong sorbent‐sorbate binding (Qst (C2H2) = 38.6 kJ/mol) at low loading. Conversely, weak CO2 binding (Qst (CO2) = 25.6 kJ/mol) at low loading means that (ΔQst)AC [Qst (C2H2)–Qst (CO2)] is 13 kJ/mol at low coverage and 11.4 kJ/mol at mid‐loading. Analysis of in situ powder X‐ray diffraction and modelling experiments provide insight into the sorption properties and high C2H2/CO2 separation performance of sql‐16‐Cu‐NO3‐α′

Water vapour and gas induced phase transformations in an 8-fold interpenetrated diamondoid metal–organic framework

In this work, we report the synthesis, structural characterisation and sorption properties of an 8-fold interpenetrated diamondoid (dia) metal–organic framework (MOF) that is sustained by a new extended linker ligand, [Cd(Imibz)2], X-dia-2-Cd, HImibz or 2 = 4-((4-(1H-imidazol-1-yl)phenylimino)methyl)benzoic acid. X-dia-2-Cd was found to exhibit reversible single-crystal-to-single-crystal (SC–SC) transformations between four distinct phases: an as-synthesised (from N,N-dimethylformamide) wide-pore phase, X-dia-2-Cd-α; a narrow-pore phase, X-dia-2-Cd-β, formed upon exposure to water; a narrow-pore phase obtained by activation, X-dia-2-Cd-γ; a medium-pore CO2-loaded phase X-dia-2-Cd-δ. While the space group remained constant in the four phases, the cell volumes and calculated void space ranged from 4988.7 Å3 and 47% (X-dia-2-Cd-α), respectively, to 3200.8 Å3 and 9.1% (X-dia-2-Cd-γ), respectively. X-dia-2-Cd-γ also exhibited a water vapour-induced structural transformation to the water-loaded X-dia-2-Cd-β phase, resulting in an S-shaped sorption isotherm. The inflection point occurred at 18% RH with negligible hysteresis on the desorption profile. Water vapour temperature-humidity swing cycling (60% RH, 300 K to 0% RH, 333 K) indicated hydrolytic stability of X-dia-2-Cd and working capacity was retained after 128 cycles of sorbent regeneration. CO2 (at 195 K) was also observed to induce a structural transformation in X-dia-2-Cd-γ and in situ PXRD studies at 1 bar of CO2, 195 K revealed the formation of X-dia-2-Cd-δ, which exhibited 31% larger unit cell volume than X-dia-2-Cd-γ.</p

One atom can make all the difference: Gas-induced phase transformations in bisimidazole-linked diamondoid coordination networks

Coordination networks (CNs) that undergo gas-induced transformation from closed (nonporous) to open (porous) structures are of potential utility in gas storage applications, but their development is hindered by limited control over their switching mechanisms and pressures. In this work, we report two CNs, [Co(bimpy)(bdc)]n (X-dia-4-Co) and [Co(bimbz)(bdc)]n (X-dia-5-Co) (H2bdc = 1,4-benzendicarboxylic acid; bimpy = 2,5-bis(1H-imidazole-1-yl)pyridine; bimbz = 1,4-bis(1H-imidazole-1-yl)benzene), that both undergo transformation from closed to isostructural open phases involving at least a 27% increase in cell volume. Although X-dia-4-Co and X-dia-5-Co only differ from one another by one atom in their N-donor linkers (bimpy = pyridine, and bimbz = benzene), this results in different pore chemistry and switching mechanisms. Specifically, X-dia-4-Co exhibited a gradual phase transformation with a steady increase in the uptake when exposed to CO2, whereas X-dia-5-Co exhibited a sharp step (type F-IV isotherm) at P/P0 0.008 or P 3 bar (195 or 298 K, respectively). Single-crystal X-ray diffraction, in situ powder XRD, in situ IR, and modeling (density functional theory calculations, and canonical Monte Carlo simulations) studies provide insights into the nature of the switching mechanisms and enable attribution of pronounced differences in sorption properties to the changed pore chemistry.</p