That composition and structure profoundly impact the properties of crystalline solids has provided impetus for exponential growth in the field of crystal engineeringover the past 25 years. Crystal engineering has evolved from structure design (form) to control over bulk properties (function). Today, when coupled with molecular modeling, crystal engineering can offer a paradigm shift from the more random, high-throughput methods that have traditionally been utilized in materials discovery and development. Custom-design of the right crystalline material for the right application could therefore be at hand.

Porous crystalline materials exemplify this situation. Whereas purely inorganic materials (e.g. zeolites) and those based upon coordination chemistry (e.g. Metal-Organic Frameworks, MOFs, and Porous Coordination Polymers, PCPs) are well studied and offer great promise for separations and catalysis, they are often handicapped by cost or performance (e.g. poor chemical stability, interference from water vapour, low selectivity) limitations. Ultramicroporous Materials, UMs, are built from metal or metal cluster “nodes” and combinations of organic and inorganic “linkers” and their pore chemistry and size (< 0.7 nm) can overcome some of the weaknesses of existing classes of porous material (Figure 1). Three families (platforms) of UMs will be detailed and their performance with respect to important gas separation (e.g. CO₂ capture [1], C₂H₂ capture [2]) and water purification applications will be discussed. Most recently, we have shown that UMs can function synergistically to address complex gas mixtures [3] or perform effectively for CO₂ capture even in the presence of humidity [4].

[1] Nugent, P. Nugent, P.; Belmabkhout, Y.; Burd, S.D.; Cairns, A.J.; Luebke, R.; Forrest, K.; Pham, T.; Ma, S.; Space, B.; Wojtas, L.; Eddaoudi, M.; Zaworotko, M.J. (2013) Nature 495, 80-84.

[2] Cui, X.; Chen, K.J.; Xing, H.; Yang, Q.; Krishna, R.; Bao, Z.; Wu, H.; Zhou, W.; Dong, X.; Li, B.; Ren, Q.; Zaworotko, M.J.; Chen, B. (2016). Science 353, 141-144.

[3] Chen, K.J.; Madden, D.G.; Mukherjee, S.; Pham, T.; Forrest, K.A.; Kumar, A.; Space, B.; Kong, J.; Zhang, Q.Y. Zaworotko, M.J. (2019). Science, 366, 241-246.

[4] Mukherjee, S.; Sikdar, N.; O’Nolan, D.; Franz, D.M.; Gascon, V.; Kumar, A.; Kumar, N.; Scott, H.S.; Madden, D.G.; Kruger, P.E.; Space, B.; Zaworotko, M.J. (2019). Science Adv. 5, eaax9171.

Noncovalent interactions play a fundamentally important role in the properties of solid materials. For instance, guests are taken up into the host framework of porous materials as a result of the interactions between these species, while the manner in which they interact has an influence on the sorption ability of the porous material. In this work calculations on a range of porous frameworks allow us to explain the role that noncovalent interactions play in the sorption properties of these compounds. For instance, the origin of anomalous sorption isotherms are shown to be the result of interactions between acetylene^[1] or carbon dioxide^[1,2] and the host frameworks, as well as interactions between guests. Similarly, noncovalent interactions are responsible for the change in colour along an hourglass pattern of a crystalline porous compound during sorption of particular solvents. Calculations show that the origin of this effect is that the channels in the porous framework are anisotropic, allowing sorption only from particular faces.^[3]

References

[1] Jacobs, T.; Lloyd, G. O.; Gertenbach, J. A.; Esterhuysen, C.; Müller-Nedebock, K. K.; Barbour, L. J., Angew. Chem. Int. Ed., 2012, 51, 4913-4916.

[2] Bezuidenhout, C. X.; Smith, V. J.; Bhatt, P. M.; Esterhuysen, C.; Barbour, L. J., Angew. Chem. Int. Ed. 2015, 54, 2079–2083.

[3] Bezuidenhout, C. X.; Esterhuysen, C.; Barbour, L. J., Chem. Commun., 2017, 53, 5618–5621.

Mesoporous materials are excellent materials to be used in energy and environmental related applications. Methods to characterize the pore structures and the filling and emptying processes are physisorption and small-angle scattering. Gas physisorption in mesoporous materials and the associated capillary hysteresis intrigue the scientific community since decades. These phenomena are largely exploited for the characterization of porous solids, which justify the strong need for their complete understanding. To date, the major hurdle lies in a reliable description of the state of the confined fluid, which is usually given by measuring macroscopic observable, i.e. the amount of adsorbed gas.

Despite computational methods, in situ techniques combining gas physisorption with X-ray scattering methods showed in the last years to be valuable tools to get deeper insights into gas adsorption phenomena [1, 2]. Combining the different contrasts of SAXS and SANS and applying contrast matching [1], a more detailed, locally resolved description of the process could be given by the analysis of the scattering signals of the material pore structure. However, clear and comprehensive assessment of the adsorption process is still missing since the adsorbate evolution in the mesoporous host could be only indirectly investigated.

This presentation deals with the development of a novel in situ method based on the combination of anomalous small-angle X-ray scattering (ASAXS) and X-ray absorption near edge structure (XANES) spectroscopy to directly probe the evolution of the xenon adsorbate phase in mesoporous silicon during gas adsorption at its boiling point of 165 K [3]. The interface area and size evolution of the confined xenon phase alone were determined from ASAXS demonstrating that filling and emptying the pores follows two distinct mechanisms. The mass density of the confined xenon was found to decrease prior pore emptying. XANES analyses showed that Xe exists in two different species when confined in mesopores. This combination of methods provides a smart new tool for the study of nanoconfined matter for catalysis, battery electrodes, and for gas and energy storage applications.

The instrumental setup used allowed us to reach the Xenon L₃ X-ray absorption edge at 4.781 keV. The combination of that three experiments, ASAXS, XANES and physisorption were done in situ on different points of the adsorption and desorption branch of the isotherm. Thus, from the resonant scattering curves of xenon the mesoscopic evolution of the adsorbate (multilayer formation, capillary condensation and desorption) could be directly investigated.

[1] Mascotto, S., Wallacher, D., Brandt, A., Hauss, T., Thommes, M., Zickler, G. A., Funari, S. S., Timmann, A. & Smarsly, B. M. (2009). Langmuir 25, 12670−12681.

[2] Jähnert, S., Müter, D., Prass, J., Zickler, G. A., Paris, O. & Findenegg, G. H. (2009). J. Phys. Chem. C 113, 15201−15210.

[3] Gericke, E., Wallacher, D., Wendt, R., Greco, G., Krumrey, M., Raoux, S., Hoell, A. & Mascotto, S. (2021). J. Phys. Chem. Lett. 12, 4018−4023.

Mankind is now in the “age of gas”[1] and there are urgent needs in gas purification that will likely only be solved by a new generation of physisorbent porous materials that offer reduced cost and superior performance. Engaging the principles of crystal engineering, hybrid ultramicroporous materials, HUMs (pore size < 0.7 nm) [2], by means of combining small pores (< 0.7 nm) with strong electrostatics offer an ideal sorbent platform suited for tight-fit of the target sorbate, resulting in performance benchmarks over the recent years [3, 4]. However, due to narrow pore networks imposing steric restrictions, crystal engineering of modular HUMs on account of organic ligand functionalisation has remained largely elusive.

Moving one step ahead of the synergistic sorbent separation technology[5], herein we address single-step purification of ethylene (C₂H₄), the highest volume product of the chemical industry, by crystal engineering of two HUMs of formula [Ni(pyz-NH₂)₂(MF₆)]_n (pyz-NH₂ = aminopyrazine, 17; M = Si, Ti), MFSIX-17-Ni [6]. Isostructural pyrazine analogues (SIFSIX-3-Zn [7], SIFSIX-3-Ni [8]) are the benchmark physisorbents for trace carbon capture but are unsuited for acetylene capture. No single physisorbent has the requisite selectivity to purify C₂H₄ from ternary C₂-CO₂ mixtures (C₂H₄/C₂H₂/CO₂) under ambient conditions until now. Indeed, both MFSIX-17-Ni sorbents produce polymer grade ethylene (> 99.95% purity) from a 1:1:1 ternary mixture (Figure 1). Regarding insights for the future, we attribute the observed properties to the unusual binding sites in MFSIX-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 molecular modelling provide insight into these binding sites and why they differ from those of the pyrazine-linked materials.

Metal-Organic frameworks (MOFs) and porous molecular materials represent a new platform for achieving and exploring high-performance sorptive properties and gas transport. The key lies in the modular nature of these materials, which allows for tuning and functionalization towards improved gas capture.

Self-assembly of polyfunctional molecules containing multiple charges, namely, tetrahedral tetra-sulfonate anions and bi-functional linear cations, resulted in a permanently porous crystalline material in which the channels are decorated by double helices of electrostatic charges that governed the association and transport of CO₂ molecules (Fig. 1). These channels electrostatically compliment the CO₂ molecules and forms strong interactions of 35 kJ mol⁻¹, ideal for CO₂ capture/release cycles.[1]

The CO₂ adsorption properties were modulated for an isoreticular series of Fe-MOFs by varying the decoration of fluorine atoms within their channel (Fig. 2). A host of complementary experimental and computational techniques gives a holistic view of the host-CO₂ properties towards the potential selective removal of CO₂ from other gases. GCMC and DFT were employed for a detailed description of the CO₂ diffusion and interactions in the porous materials. CO₂–matrix adsorption enthalpies of 33 kJ mol−1 was accurately measured in-situ by simultaneous acquisition of micro-calorimetric and volumetric-isotherm data. Direct measurements of adsorption heats are not common and such data helps to validate mathematical models and protocols for sorption-derived adsorption enthalpies. [2]

[1] Xing, G.; Bassanetti, I.; Bracco, S.; Negroni, M.; Bezuidenhout, C.; Ben, T.; Sozzani, P.; Comotti, A., Chemical Science 2019, 10 (3), 730-736.

[2] Perego, J.; Bezuidenhout, C. X.; Pedrini, A.; Bracco, S.; Negroni, M.; Comotti, A.; Sozzani, P., Journal of Materials Chemistry A 2020, 8 (22), 11406-11413.

Metal-Organic Frameworks (MOFs) are a class of synthetic porous crystalline materials based on metal ions connected through spacing ligands. They possess interesting properties such as high porosity [1], high concentration of metal centres and flexibility [2]. Additionally, MOFs can maintain their crystal structure upon removal, inclusion, exchange or reaction of a wide selection of guests, making them useful for multiple applications, e.g. in selective gas adsorption/separation. The synthesis of chemically and thermally stable MOFs, the comprehension of their properties and knowledge of their crystallographic features, are indispensable for the design and development of well performing materials. As MOFs’ properties are intrinsically related to their crystal structure, a deep understanding of the host-guest interactions during adsorption processes is a fundamental aspect [3].

Here, a high-resolution powder X-ray diffraction (HR-PXRD) crystallographic study of the host-guest interactions in Fe₂(BDP)₃ [H₂BDP = 1,4-bis(pyrazol-4-yl)benzene] upon CO₂ adsorption is presented. This MOF is characterised by a 3D network with 1D triangular channels. The peculiar shape of its channels and its good Brunauer-Emmett-Teller specific surface area (1230 m²/g) [4] prompted its investigation as CO₂ storage material, revealing an uptake capacity of 298.0 cm³/g at P_CO2 = 0.99 bar and T = 195 K.

At the ESRF ID22 beamline, HR-PXRD data were collected in situ and operando at T = 273 and 298 K while varying the CO₂ loading in the pressure range 0-8 bar. The obtained results will be presented after an in-depth data analysis, ranging from assessment of unit cell parameters variation to location of the primary adsorption sites and quantification of the adsorbed guest (Fig. 1). These results provide key information to better understand the CO₂-host interactions during the whole adsorption process, thus disclosing the chemical and structural features a MOF should possess to favour CO₂ uptake at mild conditions.

[1] I. M. Hönicke, I. Senkovska, V. Bon, I. A. Baburin, B. S. Raschke, J. D. Evans, S. Kaskel, Angew. Chem. 2018, 57, 42, 13780-13783 [2] A. Schneemann, V. Bon, I. Schwedler, I. Senkovska, S. Kaskel, R. A. Fischer, Chem. Soc. Rev. 2014, 43, 6062-6096; J. H. Lee, S. Jeoung, Y. G. Chung, H. R. Moon, Coord. Chem. Rev. 2019, 389, 15, 161-188 [3] C. Giacobbe, E. Lavigna, A. Maspero, S. Galli, J. Mater. Chem. A 2017, 5, 16964 [4] Z. R. Herm, B. M. Wiers, J. A. Mason, J. M. van Baten, M. R. Hudson, P. Zajdel, C. M. Brown, N. Masciocchi, R. Krishna, J. R. Long, Science 2013, 340, 960-964