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.