Critical Factors Driving the High Volumetric Uptake of Methane in Cu-3(btc)(2)

A thorough experimental and computational study has been carried out to elucidate the mechanistic reasons for the high volumetric uptake of methane in the metal-organic framework Cu-3(btc)(2) (btc(3-) = 1,3,5-benzenetricarboxylate; HKUST-1). Methane adsorption data measured at several temperatures for Cu-3(btc)(2), and its isostructural analogue Cr-3(btc)(2), show that there is little difference in volumetric adsorption capacity when the metal center is changed. In situ neutron powder diffraction data obtained for both materials were used to locate four CD4 adsorption sites that fill sequentially. This data unequivocally shows that primary adsorption sites around, and within, the small octahedral cage in the structure are favored over the exposed Cu2+ or Cr2+ cations. These results are supported by an exhaustive parallel computational study, and contradict results recently reported using a time-resolved diffraction structure envelope (TRDSE) method. Moreover, the computational study reveals that strong methane binding at the open metal sites is largely due to methane-methane interactions with adjacent molecules adsorbed at the primary sites instead of an electronic interaction with the metal center. Simulated methane adsorption isotherms for Cu-3(btc)(2) are shown to exhibit excellent agreement with experimental isotherms, allowing for additional simulations that show that modifications to the metal center, ligand, or even tuning the overall binding enthalpy would not improve the working capacity for methane storage over that measured for Cu-3(btc)(2) itself.

Characterization of metal organic frameworks of interest for gas adsorption/separation applications

Mehrdad Asgari

Metal-Organic Frameworks (MOFs) are a class of porous materials that are applicable in many energy and environmentally relevant areas, due to their unique features including unprecedented internal surface areas and easy chemical tunability. Gas separation and storage are among the most important applications for which MOFs have been extensively studied. Considering the large number of potential MOFs that can be accessed, through a combination of numerous metal clusters and organic linkers, determining the relationship between their structural features (such as pore size and shape and chemical functionalities) and corresponding gas adsorption properties is of utmost importance. In this regard, in-situ characterization, especially diffraction, can be advantageous. MOFs can offer high crystallinity, and so their structure can readily be characterized via diffraction. In addition, due to the presence of metal ions, and predefined molecular building blocks, which exhibit multiple chemical moieties, the potential energy surface for a guest molecule within a MOF cavity possesses multiple minima that correspond to well-defined adsorption sites with varying binding energies. This makes diffraction the most direct way to probe static site-specific binding properties. Given this, the focus of this work is on understanding the structure-derived function of MOFs of interest in gas separation/storage applications. To do this, standard adsorption experiments are coupled with in-situ diffraction techniques. The latter is able to reveal the location and orientation of adsorbed guest species inside the framework, provide insights into the framework response to varying pressure, temperature, and atmosphere, and allow one to determine the relative differences in binding energy between neighboring adsorption sites. The aforementioned information can then be used to rationalize the relationship between different physiochemical features of a given framework and their performance in an adsorption process. In addition to establishing structure-property relations for the usage of MOFs, the obtained experimental results are used to corroborate those calculated by DFT methods, providing a stress test for computational methods aimed at predicting the structures and properties of hypothetical MOFs. The first chapter of this thesis, the introduction, offers a brief review of different characterization methods for studying gas adsorption/separation applications in MOFs. Although the focus of this chapter is on carbon dioxide capture as the application, these characterization methods are also used to study other adsorption processes. The next three chapters, chapters 2-4, present several prominent MOF families that are of interest in hydrogen storage and carbon dioxide capture applications. These case studies demonstrate how using in-situ diffraction techniques coupled with adsorption measurements and DFT calculations can be used to gain molecular level insight into how small molecules bind inside the selected MOF structures. Further, these structure-property correlation studies are able to help unveil how altering MOF building blocks, such as metals and ligands, gives rise to enhanced or diminished adsorption properties. The last chapter, chapter 5, is a study of the response of an activated MOF to varying temperature; in this chapter various structural motions, which are responsible for a large negative thermal expansion (NTE), are elucidated.

EPFL2020

Etude de l'effet du brassage électromagnétique sur les microstructures d'alliages cuivreux

Thomas Campanella

In order to eliminate internal and surface defects, which appear in the ingots during continuous casting of metallic alloys, homogeneous and fine structures are often required. Therefore, when the adjunction of inoculant substances is not possible, the electromagnetic stirring (EMS) process is used during solidification. It consists to create convective mouvements in the liquid part of the casting, by the action of an electromagnetic field. A fine equiaxed grain structure can then be observed. The main goal of this research is to study the effect of the EMS process on grain structures for two industrial copper-base alloys : C97 (Cu-1%Ni-1%Pb-0.2%P) and BZ4 (Cu-4%Zn- 4%Sn-4%Pb). In particular, it included the determination of the optimal experimental parameters to obtain a grain refinement, the characterization of their effect on the solidification and the understanding of the physical mechanisms driving grain refinement. For that purpose, a Bridgman type furnace was modified in order to add an electromagnetic stirring device. Indeed, as the industrial continuous casting process is rigid and complex, the investigations were easier to carry out on the Bridgman installation, enabling us to define the influence of each experimental parameter : power of the induction, casting speed, coil position, temperature of the liquid and composition of the alloy. This experimental study was completed by the numerical simulation of the experimental conditions. The analysis of the structures, refined or not, showed that the dominant factors concerning the capacity of EMS to refine the structure, are the position of the inductor with respect to the liquidus, and the permeability of the mushy zone of the alloy. The more permeable the alloy is near the liquidus the more effective is the grain refinement. Concerning the position of the inductor, in-situ temperature measurements clearly revealed the local reheating of the liquid induced by convection. To correctly refine the low concentration alloy (C97), this local reheating must be brought on the position close to the dendrite position. During EMS-induced grain refinement, fragmentation of the dendrite arms by remelting and their survival in the melt are prevalent. Remelting can occur only if the "hot" liquid penetrates the mushy zone. Thus, the concept of permeability according to the model of Karman-Cozeny was used in simulation in order to characterize the penetration of the liquid in the mushy zone. From these results, a criterion of remelting, based on Flemings model of macrosegregation, was adapted and applied to our case. This criterion shows that remelting takes place if the velocity of the interdendritic liquid is lager than the speed of the isotherms. The velocity of the interdendritic liquid was calculated by numerical simulation for both alloys, under various experimental conditions. These results confirmed the importance to bring the inductor closer to the position of the liquidus to remelt dendrites, in particular in the case of the C97 alloy, which proves to be much more difficult to refine than the BZ4 alloy. Lastly, survival of the fragments was also studied. It depends on the casting conditions (casting speed and thermal gradient) and on the composition of the alloy. From the differences in composition of both alloys, it could be shown that the survival of dendrites fragments is improved during the solidification of the BZ4 alloy, compared with the C97 alloy.

EPFL2003

Computational Studies of the Proton-Coupled Metal Ion Transport in the SLC11/NRAMP Family of Transporters

Maria Letizia Merlini

In the last years, it has been demonstrated a link between the overload of metal ions inside nervous system cells and the onset of severe neurodegenerative diseases. This prompted the investigation of the structural and functional properties of transporters responsible for the uptake and regulation of metal ions inside cells. In particular, members of the Nramp/SLC11 ("natural resistance-associated macrophage proteins"/"solute carrier 11") family of transporters tightly regulate the influx of divalent transition-metal ions across cellular membranes contributing to cellular homeostasis. In these secondary active transporters, the translocation of divalent transition metal ions from the extracellular matrix to the cellular cytoplasm is coupled to proton transport (symport). The co-transport is realized using an alternate access mechanism in which the protein is alternately open towards the extracellular matrix, ("outward facing conformation", OFC) or towards the cellular cytoplasm ("inward facing conformation", IFC).Although a wide range of functional and structural studies provided fundamental insights on the proton-coupled transport, most mechanistic details are still unknown. As an example, the coordination sphere of the metal ion in the inward-facing conformation of the protein has only been partially resolved by crystallographic experiments and later attempts to provide comprehensive descriptions led to contradictory models. Furthermore, several experimental evidence proved that proton uniport in the absence of the metal ion, is also possible, suggesting an unusual symporter behaviour for Nramps, in which the two substrates are not tightly coupled. Despite the recent advances in the comprehension of Nramps proton transport, the underlying mechanistic details remain still ambiguous, as demonstrated by inconsistent mechanistic models proposed during the last years.In this thesis, computational methods have been used in order to provide new molecular level insights in the proton-coupled divalent transition metal ions transport performed by Nramps. In particular, a combination of classical molecular dynamics (MD) and QM/MM ab initio MD methods have been applied in order to refine the partially resolved coordination sphere of Mn2+ in the active site of Staphylococcus Capitis Divalent Metal-ion Transporter (ScaDMT), representing the only available crystal structure of a substrate-bound member of the family in an IFC. This QM/MM multiscale approach, using an accurate quantum treatment of the active site and a classical treatment of the surroundings, allowed for state-of-the-art calculations of the entire system that led to the unequivocal identification of the metal's coordination sphere. Furthermore, MD simulations have been performed for both the IFC ScaDMT and Eremococcus Coleocola DMT (EcoDMT), whose crystal structure has been obtained in OFC. Since experimental evidences proved that proton transport can occur even in the absence of the metal ion substrate, we performed simulations of both proteins in their apo forms. These simulations allowed us to characterize four amino acidic residues identified as likely key players in the proton transport process and to determine which role they assume in the process. Finally, we were able to identify the likely primary proton acceptor and proposed a potential route followed by protons in their influx toward the intracellular cytoplasm.

EPFL2022