Stability, Transport and Modification of Zeolitic Imidazolate Framework-8 Membranes for Light Hydrocarbon Separations

abstract: Membrane technology is a viable option to debottleneck distillation processes and minimize the energy burden associated with light hydrocarbon mixture separations. Zeolitic imidazolate frameworks (ZIFs) are a new class of microporous metal-organic frameworks with highly tailorable zeolitic pores and unprecedented separation characteristics. ZIF-8 membranes demonstrate superior separation performance for propylene/propane (C3) and hydrogen/hydrocarbon mixtures at room temperature. However, to date, little is known about the static thermal stability and ethylene/ethane (C2) separation characteristics of ZIF-8. This dissertation presents a set of fundamental studies to investigate the thermal stability, transport and modification of ZIF-8 membranes for light hydrocarbon separations. Static TGA decomposition kinetics studies show that ZIF-8 nanocrystals maintain their crystallinity up to 200○C in inert, oxidizing and reducing atmospheres. At temperatures of 250○C and higher, the findings herein support the postulation that ZIF-8 nanocrystals undergo temperature induced decomposition via thermolytic bond cleaving reactions to form an imidazole-Zn-azirine structure. The crystallinity/bond integrity of ZIF-8 membrane thin films is maintained at temperatures below 150○C. Ethane and ethylene transport was studied in single and binary gas mixtures. Thermodynamic parameters derived from membrane permeation and crystal adsorption experiments show that the C2 transport mechanism is controlled by adsorption rather than diffusion. Low activation energy of diffusion values for both C2 molecules and limited energetic/entropic diffusive selectivity are observed for C2 molecules despite being larger than the nominal ZIF-8 pore aperture and is due to pore flexibility. Finally, ZIF-8 membranes were modified with 5,6 dimethylbenzimidazole through solvent assisted membrane surface ligand exchange to narrow the pore aperture for enhanced molecular sieving. Results show that relatively fast exchange kinetics occur at the mainly at the outer ZIF-8 membrane surface between 0-30 minutes of exchange. Short-time exchange enables C3 selectivity increases with minimal olefin permeance losses. As the reaction proceeds, the ligand exchange rate slows as the 5,6 DMBIm linker proceeds into the ZIF-8 inner surface, exchanges with the original linker and first disrupts the original framework’s crystallinity, then increases order as the reaction proceeds. The ligand exchange rate increases with temperature and the H2/C2 separation factor increases with increases in ligand exchange time and temperature.Dissertation/ThesisDoctoral Dissertation Chemical Engineering 201

Unveiling the Temperature Influence on the Sorptive Behaviour of ZIF-8 Composite Materials Impregnated with [Cn MIM][B(CN)4 ] Ionic Liquids

LA/P/0008/2020 PTDC/CTM-CTM/ 30326/2017Composite sorbent materials (IL@MOF) with a metal-organic framework (MOF) ZIF-8 and [B(CN)4 ]−-based ionic liquids (ILs) were produced for the first time. Characterization results indicate the successful IL impregnation and conservation of the ZIF-8 crystalline structure and morphology. The data collected from the nitrogen (N2 ) physisorption at 77 K suggest that these IL@ZIF-8 materials are nonporous as their textural properties, such as BET specific surface area and total pore volume, are negligible. However, CO2, CH4, and N2 adsorption/desorption measurements in the IL@ZIF-8 composites at 303 and 273 K contradict the N2 data at 77 K, given that the obtained isotherms are Type I, typical of (micro)porous materials. Their gas adsorption capacity and ultramicroporous volume are in the same order of magnitude as the pristine microporous ZIF-8. The case study [C6 MIM][B(CN)4 ] IL revealed a high affinity to both CO2 and CH4 . This compromised the selectivity performance of its respective composite when compared with pristine ZIF-8. This work highlights the importance of accurate experimental gas adsorption/desorption equilibrium measurements to characterize the adsorption uptake and the porous nature of adsorbent materials.publishersversionpublishe

Computational discovery of metal-organic frameworks for separations of organic molecules

The separation of para-xylene from a stream of mixed xylenes and ethylbenzene is critical for the large-scale production of plastics in the petrochemical industry. Several groups have identified metal-organic frameworks (MOFs) as having desirable characteristics for this separation. In this thesis, we demonstrate that molecular simulations can be used to efficiently screen large databases of MOFs to identify promising materials for this separation. We validated our approach in conjunction with our experimental collaborators and discovered that two of the top-performing materials from our screening procedure have similar performance to the zeolites used in industrial practice for xylene separations. We also developed a classical force field parameterization approach for refining the interactions between C8 alkyl aromatic hydrocarbons and MOFs using Density Functional Theory (DFT) calculations. We demonstrate that our DFT-based force field gives better predictions of some adsorption properties than generic force fields. A major technological hurdle to using small alcohols as biofuels is in their separation from aqueous fermentation broths. To address this issue, we developed classical models to identify hydrophobic MOFs capable of efficiently performing this separation. We were then able to use our models in a different context to understand the factors governing the thermodynamic stability and structural flexibility of MOFs. The methods developed in this thesis provide unique insight into chemical separations and material properties that would be challenging to obtain from experiments and promote the development of MOFs for industrial applications.Ph.D

Computational study of adsorption and diffusion in metal- organic frameworks

Ph.DDOCTOR OF PHILOSOPH

AN EXPERIMENTAL STUDY OF THE EFFECTS OF SUBSTRATE POROSITY, MORPHOLOGY, AND FLEXIBILITY ON THE EQUILIBRIUM THERMODYNAMICS AND KINETICS OF ADSORPTION FOR ATOMIC AND MOLECULAR ADSORBATES

Five systems consisting of different sorbate-sorbent combinations were studied using experimental volumetric adsorption techniques. Multiple adsorption isotherms were measured at low temperatures and low pressures for all of the systems studied which included CO2 adsorption on single walled carbon nanotubes (CO2 – SWCNT), Ethane adsorption on closed carbon nanohorns (Ethane-cNH), Ar adsorption on open carbon nanohorns (Ar – oNH), CO2 adsorption on zeolitic imidazolate framework-8 (CO2 – ZIF-8), and O2 adsorption on ZIF-8 (O2 – ZIF-8). Each of these systems offers a unique study of the relationship between the physical properties of the adsorbate and substrate and the effects of these properties on the thermodynamics and kinetics of adsorption. In addition to being of fundamental interest, the thermodynamics and kinetics of adsorption are important to understand for practical considerations in research fields such as gas storage and gas separation via adsorption processes, among other applications. CO2 – SWCNT is a system with a small linear molecular adsorbate with a permanent quadrupole moment adsorbing on a substrate with quasi-1D grooves and convex outer adsorption sites. Ethane-cNH is a system with a linear alkane adsorbing on a substrate with conical pores and convex outer adsorption sites. Ar – oNH is a system with a spherical atom sorbing in a substrate with two different groups of conical adsorption sites and both convex and concave surface sites. CO2 – ZIF-8 and O2 – ZIF-8 are both systems with small linear molecules sorbing in a flexible microporous scaffold-like substrate. Adsorption isotherms were analyzed to identify features corresponding to adsorbate-adsorbate and adsorbate-substrate interactions. Namely, the presence of substeps in the semi-logarithmic data were identified and interpreted to correspond to groups of adsorption sites of similar binding energy which likely depend on the morphology and/or structural flexibility of the substrates. All of the systems, with the exception of CO2 - SWCNTs, yielded at least some isotherms with substeps at pressures below that corresponding to saturation. Effective specific surface areas for all adsorbent-substrate combinations were calculated using the BET and Point-B methods for the sake of comparison. These surface area measurements are very dependent on the porosity and morphology of the substrate as well as the size and shape of the adsorbate atoms/molecules and therefore the values vary greatly between the different systems. The isosteric heat of adsorption was calculated using isotherms over the full range of temperatures for each system. A variant of the Clausius-Clapeyron equation was used for this purpose and the results were analyzed based on adsorbate-adsorbate and adsorbate-substrate interactions. Plateaus in the isosteric heat data for Ethane – cNH and Ar – oNH were related to the morphology of the substrates and properties of the adsorbate species. For CO2 – SWCNTs, the isosteric heat at all but the lowest coverages is below the latent heat of deposition. This is due to the quadrupole moment of CO2. For both of the studies of adsorption on ZIF-8, the isosteric heat contains peaks in the data which likely are the result of the flexibility of the ZIF-8 structure. The kinetics of adsorption (or, the rates at which the adsorption systems approach equilibrium) were analyzed as functions of isotherm temperature and coverage, vapor pressure, and fractional uptake point by point along individual isotherms using the linear driving force model. Certain trends in the kinetics of adsorption are consistent for all the systems studied and others vary depending on the specific adsorbate-substrate combination. As with the thermodynamic results, trends in the kinetics of adsorption are discussed in terms of the effects of adsorbate-adsorbate and adsorbate-substrate interactions

Molecular Modelling of Stimuli Responsive Gate Effects in Flexible Metal Organic Frameworks

Metal-Organic Frameworks (MOFs) are new remarkable nano-porous materials that exhibit exceptional thermal and chemical stabilities. They have a broad range\ud of applications ranging from, not limited to, gas storage/separation, molecular separations, sensor, catalysis, drug delivery and removal of toxic chemicals and detoxification of warfare agents. The properties of the materials are highly dependent on the nature of the atomic interactions and structural arrangement of the crystallise material. This thesis focused on the MOFs that respond to stimuli where the structure will undergo reversible transformation in which it can lead to remarkable sorption properties that enhance the material performance. The selected MOFs, ZIF-8 and Mg-MOF-74 were studied in this thesis. The materials were evaluated at atomistic and quantum levels. Here, we proposed a new theoretical concept regarding controlling molecular movements by implanting molecular machines in Mg-MOF-74. We designed the molecular machines to respond to external electric field and the machines are anchored within the onedimensional pore channel of Mg-MOF-74. The pore opening and closing was simultaneously controlled by the induced electric field; thus, the flow directions of methane can be controlled at molecular level. Moreover, this thesis moved onto the study regarding mechanical ‘gate’ opening movements in ZIF-8 which are stimulated by introduction of water molecules to the system. First, we examined with water models response in ZIF-8. Five types of different water models and six different ZIF-8 force fields were chosen and simulated with Grand Canonical Monte Carlo method. We found that the simulated adsorption isotherms are significant diverse in relation to water models. Afterward, we mimicked experimental water adsorption set up through the use of a graphene piston in molecular dynamics simulation. Here, we demonstrated through the use of water that we witnessed experimental ‘gate’ effect. However, we also found that current atomics force fields in literature were unable to replicate the adsorption isotherm at experimental conditions, resulting in the development and modifications for new force field that tune to water responses in ZIF-8

Quantitative Prediction of Adsorption and Diffusion in Pure Silica and Cationic Zeolites

Zeolites are a class of nanoporous aluminosilicate materials. They are often used industrially for separations and catalysis because of their low cost and high thermal stability. The variety of exchangeable cations, Si/Al ratio and aluminum distribution can affect the adsorption and diffusion properties of these materials. Molecular simulations provide an inexpensive, well-defined way to study the effects of these properties on measurable quantities, such as adsorption and diffusion. In this work, we developed methods to examine the effects of aluminum distribution in zeolites and more accurate force fields for predicting adsorption and diffusion. We first examined the effect of aluminum distribution on CO2 adsorption in cationic zeolites. We observed a significant dependence of extra-framework cation distributions and CO2 adsorption properties on aluminum distribution. This indicated that aluminum ordering should be considered when screening cationic zeolites for CO2 adsorption and that CO2 adsorption isotherms can be used to probe aluminum distribution. Next, we developed accurate, transferable force field methods that are used to examine adsorption and diffusion in both pure-silica and cationic zeolites. In both cases, the force fields were fit to reproduce DFT/CC energies of both transition state configurations and energy minimum configurations to enable accurate predictions for both adsorption and diffusion data for a wide array of adsorbates in both pure-silica zeolites and cationic zeolites. Overall, in this work we developed more transferable tools for predicting both adsorption and diffusion in both pure-silica and cationic zeolites, which previous classical simulation methods were limited to predicting adsorption for pure-silica, Na-exchanged and K-exchanged zeolites.Ph.D

Gas adsorption in the MIL-53(AI) metal organic framework. Experiments and molecular simulation

Dissertação para obtenção do Grau de Doutor em Engenharia QuímicaFCT - PhD Fellowship at Universidade Nova de Lisboa, Department of Chemistry (bolsa N SFRH/BD/45477/2008); FCT Program, project PTDC/AAC-AMB/108849/2008; NANO_GUARD, Project N°269138; Programme “PEOPLE” – Call ID “FP7-PEOPLE-2010-IRSES