Organic-Inorganic Hybrid Materials Based on Oxyanion Linkers for Selective Adsorption of Polarizable Gases

Author

Mona Hanafy Mohamed, University of South FloridaFollow

Graduation Year

2015

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Chemistry

Degree Granting Department

Chemistry

Major Professor

Michael J. Zaworotko, Ph.D.

Co-Major Professor

Brian Space, Ph.D.

Committee Member

Julie Harmon, Ph.D.

Committee Member

Gregory McColm, Ph.D.

Committee Member

Shengqian Ma, Ph.D.

Keywords

Metal-Organic Frameworks, Porous Materials, Design, Topology, CO2 Capture, Xe/Kr Separation

Abstract

The separation of industrially important gases into pure supplies that can be used for many practical applications is based mainly on energy intensive methods such as the cryogenic distillation which is costly and energy intensive. Therefore other routes have been introduced to industrial separation of gases such as the selective adsorption using porous solid materials. Zeolites and activated carbon are the most widely used recyclable energy-efficient porous solid materials for industrial gas separations, however the low uptake and selectivity hurdles their commercialization in some separation applications. Metal organic frameworks (MOFs) have been extensively studied as solid porous materials in term of gas separations nevertheless the future of MOFs for practical gas separations is considered to be vague and stringent due to their low stability, low capacity and selectivity especially at low partial pressures of the adsorbed gas, the competitive adsorption of the contaminants such as H₂O, NO_X and SO_X, high cost of the organic ligands, besides the challenges of the formulation of MOFs which is very important in the MOFs marketing. In this context we present new porous materials based on inorganic linkers as well as the organic molecules, Organic-Inorganic Hybrid Materials, which were found to conquer the current challenges for the exploitation of MOFs in practical gas separation such as separation of trace and low CO₂ concentrations and Xe separation from Xe/Kr mixtures.

The work presented herein encompasses the development of novel 4⁸.6⁷ topology metal organic material (MOM) platform of formula [M(bp)₂(M'O₄)] (M= Co or Ni; bpe= bipyridine-type linkers; M'= W, Mo or Cr) that have been assigned RCSR code mmo based upon pillaring of [M(bp)₂] square grids by angular WO₄²-, MoO₄^2- or CrO₄^2- pillars. Such pillars are unexplored in MOMs. They represent ideal platforms to test the effect of pore size and chemistry upon gas sorption behavior since they are readily fine-tuned and can be varied at their 3-positions (metal, organic linker and the inorganic pillar) without changing the overall structure. Such an approach allows for systematic control of pore size to optimize interactions between the framework and the adsorbent in order to enhance selectivity and/or gas uptake. Interestingly, these nets showed a high chemical stability in air, water, boiling water and in a wide range of pH which is certainly a desirable property in industry and commercialization of MOMs.

[Ni(bpe)₂(MoO₄)] (bpe= 1,2-bis(4-pyridyl) ethane), MOOFOUR-1-Ni, and its chromate analog, CROFOUR-1-Ni, exhibit remarkable CO₂ affinity and selectivity, especially at low loading. This behavior can be attributed to exceptionally high isosteric heats of adsorption (Q_st) of CO₂ in MOOFOUR-1-Ni and CROFOUR-1-Ni of ~56 and ~50 kJ/mol, respectively, at zero loading. These results were validated by modeling which indicate that the electrostatics of such inorganic anions towards CO₂ affords favourable attractions to CO₂ that are comparable to the effect of unsaturated metal centres.

The use of WO₄^2- instead of CrO₄^2- or MoO₄^2- as an angular pillar in mmo topology nets has afforded two isostructural porous nets of formula [M(bpe)₂WO₄] (M = Co or Ni, bpe=1,2-(4-pyridyl)ethene). The Ni variant, WOFOUR-1-Ni, is highly selective towards CO₂ thanks to its exceptionally high isosteric heat of adsorption (Q_st) of -65.5 kJ/mol at zero loading.

The fine-tunability and the inherent modularity of this platform allow us exquisite design and control over the pore chemistry through the incorporation of different functionalities inside the channels of the networks which was then demonstrated as valuable strategy in terms of carbon dioxide capture at condition relevant to the direct CO₂ capture from air. The exploitation of 4,4'-azopyridine in the design and synthesis of CROFOUR-2-Ni, an isostructure of CROFOUR-1-Ni, affords a paradigm shift in the CO₂ adsorption properties as exemplified by the enhanced CO₂ isosteric heat of adsorption at moderate and high loading in CROFOUR-2-Ni and the superior CO₂ selectivity even for trace and low CO₂ concentration.

The two isostructures, CROFOUR-1-Ni and CROFOUR-2-Ni have been also investigated in term of Xe adsorption and separation from Xe/Kr mixtures. The two structures were found to exhibit the remarkable Xe affinity and selectivity which, together with high stability, good recyclability, low regeneration energy and low cost of the two materials could not only diminish the cost of the Xe and Kr production but also can potentially afford a high purity of the separated gases.

Scholar Commons Citation

Mohamed, Mona Hanafy, "Organic-Inorganic Hybrid Materials Based on Oxyanion Linkers for Selective Adsorption of Polarizable Gases" (2015). USF Tampa Graduate Theses and Dissertations.
https://digitalcommons.usf.edu/etd/5811