Comprehensive study of carbon dioxide adsorption in the metal-organic frameworks M-2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn)

Analysis of the CO2 adsorption properties of a well-known series of metal-organic frameworks M-2(dobdc) (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate; M = Mg, Mn, Fe, Co, Ni, Cu, and Zn) is carried out in tandem with in situ structural studies to identify the host-guest interactions that lead to significant differences in isosteric heats of CO2 adsorption. Neutron and X-ray powder diffraction and single crystal X-ray diffraction experiments are used to unveil the site-specific binding properties of CO2 within many of these materials while systematically varying both the amount of CO2 and the temperature. Unlike previous studies, we show that CO2 adsorbed at the metal cations exhibits intramolecular angles with minimal deviations from 180 degrees, a finding that indicates a strongly electrostatic and physisorptive interaction with the framework surface and sheds more light on the ongoing discussion regarding whether CO2 adsorbs in a linear or nonlinear geometry. This has important implications for proposals that have been made to utilize these materials for the activation and chemical conversion of CO2. For the weaker CO2 adsorbents, significant elongation of the metal-O(CO2) distances are observed and diffraction experiments additionally reveal that secondary CO2 adsorption sites, while likely stabilized by the population of the primary adsorption sites, significantly contribute to adsorption behavior at ambient temperature. Density functional theory calculations including van der Waals dispersion quantitatively corroborate and rationalize observations regarding intramolecular CO2 angles and trends in relative geometric properties and heats of adsorption in the M-2(dobdc)-CO2 adducts.

Systematic Tuning and Multifunctionalization of Covalent Organic Polymers for Enhanced Carbon Capture

Johanna Maria Huck, Rocio Mercado, Berend Smit, Hui Wang

Porous covalent polymers are attracting increasing interest in the fields of gas adsorption, gas separation, and catalysis due to their fertile synthetic polymer chemistry, large internal surface areas, and ultrahigh hydrothermal stabilities. While precisely manipulating the porosities of porous organic materials for targeted applications remains challenging, we show how a large degree of diversity can be achieved in covalent organic polymers by incorporating multiple functionalities into a single framework, as is done for crystalline porous materials. Here, we synthesized 17 novel porous covalent organic polymers (COPs) with finely tuned porosities, a wide range of Brunauer Emmett Teller (BET) specific surface areas of 430-3624 m(2) g(-1), and a broad range of pore volumes of 0.24-3.50 cm(3) g(-1), all achieved by tailoring the length and geometry of building blocks. Furthermore, we are the first to successfully incorporate more than three distinct functional groups into one phase for porous organic materials, which has been previously demonstrated in crystalline metal organic frameworks (MOFs). COPs decorated with multiple functional groups in one phase can lead to enhanced properties that are not simply linear combinations of the pure component properties. For instance, in the dibromobenzene-lined frameworks, the bi- and multifunctionalized COPs exhibit selectivities for carbon dioxide over nitrogen twice as large as any of the singly functionalized COPs. These multifunctionalized frameworks also exhibit a lower parasitic energy cost for carbon capture at typical flue gas conditions than any of the singly functionalized frameworks. Despite the significant improvement, these frameworks do not yet outperform the current state-of-art technology for carbon capture. Nonetheless, the tuning strategy presented here opens up avenues for the design of novel catalysts, the synthesis of functional sensors from these materials, and the improvement in the performance of existing covalent organic polymers by multifunctionalization.

American Chemical Society2015

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

Systematic Tuning and Multifunctionalization of Covalent Organic Polymers for Enhanced Carbon Capture

Johanna Maria Huck, Rocio Mercado, Berend Smit, Hui Wang

American Chemical Society2015

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

Mehrdad Asgari

EPFL2020