Investigation of metal-organic frameworks as adsorbents for CO₂ capture from flue gas
Abstract
The post-synthetic modification of CPO-27(Mg) by introducing nickel cations into the
framework is described. A combination of surface sensitive XPS analysis, synchrotron powder X-ray diffraction and selected area and bulk EDX measurements revealed the distribution of Ni²⁺ throughout the crystal with the highest concentration towards the external surface forming a nickel-rich rim. By adding nickel acetate and chloride salts in combination with a weak acid, Ni²⁺ contents of up to 67% for the bulk material were achieved via a one-pot preparative procedure. A combined mechanism of overgrowth and isomorphous cation exchange is proposed. The study led to an improvement of porosity to N₂ (77 K) up to 17 mmol g⁻¹, close to values achieved elsewhere via complex activation procedures. High values for the adsorption of carbon dioxide of up to 6.7 mmol g⁻¹ at temperatures and partial pressure relevant for carbon capture from post-combustion power plants were accomplished (298 K, 0.1 bar) and were shown to be repeatable over cycles under dry conditions.
The synthesis, structure and adsorption properties of a series of zinc imidazolate
zeolitic imidazolate framework, ZIF, materials was also investigated. Structural
details of the zinc nitroimidazolate ZIF-65(Zn) were determined by Rietveld
refinement. Heating experiments of as prepared ZIF-65(Zn) revealed a partial
transformation from a cubic framework to an unknown structure, shown to be
reversible. The new phase possess high porosity to nitrogen and showed stepped,
hysteretic adsorption and desorption isotherms for at 77 K and CO₂ at 298 K. Using
methanol instead of DMF in synthesis yielded a novel dense non-porous zinc nitro
imidazolate structure.
A series of novel structures was prepared via synthesis including a mixture of 2-
nitroimidazole (NIm) and purine with different zinc metal sources. Two MOF
structures were found to consist of purine linkers only, but could not be rendered
porous. By changing the metal source or solvent an isoreticular structure of the ZIF-
68 (GME) family was obtained, composed of purine and NIm, as well as a novel form
of a mixed linker ZIF with the RHO topology with Im-3m symmetry that exhibits
large pores and exo-Zn and exo-NIm moieties decorating the cavity walls. The
nitrogen uptake (77 K) of 6.5 mmol g⁻¹could be increased to 12.5 mmol g⁻¹ by
removing exo-moieties through water washing.
The use of a diamino functionalised purine linker (DAP) together with NIm gave a
new ZIF material, STA-17, with a novel topology. The structure was found to exhibit
porosity to nitrogen (77 K) of 6.5 mmol g⁻¹ and carbon dioxide at (198 K) of 5 mmol
g⁻¹, but shows weak interaction with CO₂ at 298 K. Indexing from synchrotron
powder XRD data gave a hexagonal unit cell with a = b = 29.725 Å and c = 18.606 Å.
Subsequent analysis of the composition using NMR, TGA and IR techniques revealed
the presence of both linkers in the structure and a linker ratio of 2:1 (NIm : DAP).
Although crystals of suitable quality for single crystal X-ray diffraction were not
obtained, a partial model for the structure is proposed via analogy with a hypothetical
zeolite structure and analysis of powder X-ray diffraction data.
Type
Thesis, PhD Doctor of Philosophy
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