Accelerated degradation of MOFs under flue gas conditions

The zero length column (ZLC) technique is used to investigate the stability of Mg- and Ni-CPO-27 metal organic framework (MOF) crystals to the presence of water and humid flue gas. The design of the ZLC enables the stability test to be conducted over a considerably shorter time period and with lower gas consumption than other conventional techniques. A key advantage over other experimental methods to test the stability of adsorbents is the fact that the ZLC allows to quantify the amount adsorbed of every component present in the gas mixture. The developed protocol is based on a two-stage stability test. The samples were first exposed to a humid carbon dioxide and helium mixture in order to study the effect of water on the carbon dioxide adsorption capacity of the samples. In the second stage the samples were exposed to a flue gas mixture containing water. From the preliminary water stability test, the Ni-sample exhibited the highest tolerance to the presence of water, retaining approximately 85% of its pristine CO2 capacity. The Mg-MOFs deactivated rapidly in the presence of water. The Ni-CPO-27 was then selected for the second stage of the protocol in which the material was exposed to the wet flue gas. The sample showed an initial drop in the CO2 capacity after the first exposure to the wet flue gas followed by a stabilisation of the performance over several cycles.PostprintPeer reviewe

Investigation of zeolitic imidazolate frameworks using 13C and 15N solid-state NMR spectroscopy

The authors are grateful to EPSRC computational support through the Collaborative Computational Project on NMR Crystallography (CCP-NC), via EP/M022501/1, and for other support through EP/G062129/1 (JK) and EP/M506631/1 (SS). AFO acknowledges funding from the European Community Seventh Framework Program (FP7/2007-2013 [grant agreement number 608490], Project M4CO2). SEA would also like to thank the Royal Society and Wolfson Foundation for a merit award. AFO would also like to acknowledge the SCI for a scholarship for her PhD studies. Some of the calculations were performed on the ARCHER UK National Supercomputing Service, and were supported by CCP-NC. The research data (and/or materials) supporting this publication can be accessed at http://dx.doi.org/10.17630/7959a81e-161d-4ada-9914-08d3d235ce88Zeolitic imidazolate frameworks (ZIFs) are a subclass of metal-organic frameworks (MOFs) with extended three-dimensional networks of transition metal nodes (bridged by rigid imidazolate linkers), with potential applications in gas storage and separation, sensing and controlled delivery of drug molecules. Here, we investigate the use of 13C and 15N solid-state NMR spectroscopy to characterise the local structure and disorder in a variety of single- and dual-linker ZIFs. In most cases, a combination of a basic knowledge of chemical shifts typically observed in solution-state NMR spectroscopy and the use of dipolar dephasing NMR experiments to reveal information about quaternary carbon species are combined to enable spectral assignment. Accurate measurement of the anisotropic components of the chemical shift provided additional information to characterise the local environment and the possibility of trying to understand the relationships between NMR parameters and both local and long-range structure. First-principles calculations on some of the simpler, ordered ZIFs were possible, and provided support for the spectral assignments, while comparison of these model systems to more disordered ZIFs aided interpretation of the more complex spectra obtained. It is shown that 13C and 15N NMR are sufficiently sensitive to detect small changes in the local environment, e.g., functionalisation of the linker, crystallographic inequivalence and changes to the framework topology, while the relative proportion of each linker present can be obtained by comparing relative intensities of resonances corresponding to chemically-similar species in cross polarisation experiments with short contact times. Therefore, multinuclear NMR spectroscopy, and in particular the measurement of both isotropic and anisotropic parameters, offers a useful tool for the structural study of ordered and, in particular, disordered ZIFs.Publisher PDFPeer reviewe

Porous zinc and cobalt 2-nitroimidazolate frameworks with six-membered ring windows and a layered cobalt 2-nitroimidazolate polymorph

This work was supported by the European Community Seventh Framework Program (FP7/2007-2013) [grant agreement number 608490] (project M4CO2). A.O. would like to acknowledge the SCI for awarding a scholarship for her PhD research. G.M. thanks Institut Universitaire de France for its support. S.S. would like to thank the EPSRC for support EP/M506631/1. S.E.A would like to thank the Royal Society and Wolfson Foundation for a merit award.Polymorphs of Zn(2-nIm)2 (compound 1 ) and Co(2-nIm)2 (compounds 2 and 3 ) (2-nIm = 2-nitroimidazole) have been prepared by two routes: solvothermal synthesis and recrystallisation of ZIF-65(Zn/Co). Compounds 1 and 2 are isostructural, with a tetrahedrally-connected framework topology related to, but different from, that of tridymite (lonsdaleite). Single crystal X-ray diffraction analysis showed that in compound 1 (Pccn, Z = 8; a = 8.462(8) Å, b = 14.549(15) Å, c = 18.799(18) Å, V = 2314(4) Å3) there is rotational disorder for two of the three crystallographically-distinct linker types, which has been investigated computationally and by solid-state NMR spectroscopy. Detailed adsorption studies on a sample of 1 prepared by recrystallisation show 1.1 mmol g-1 uptake of CO2 at 0.1 bar (25 °C) with high affinity for CO2 over CH4 and N2 (adsorption enthalpies of 39.5, 26.0 and 18.5 kJ mol-1, respectively). A cobalt analogue (compound 2 ) with improved water stability (but lower porosity) has also been prepared. Changing the conditions of synthesis and recrystallisation gives rise to a cobalt 2-nitroimidazolate (Co(2-nIm)2, compound 3 ), which has a layered structure (I41/amd, a = 6.025(18) Å, c = 26.95(8) Å, V = 978.3(5) Å3) containing sheets of tetrahedrally-connected Co2+ cations composed of four membered rings, without porosity.PostprintPostprintPeer reviewe

Investigation of metal-organic frameworks as adsorbents for CO₂ capture from flue gas

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

Post-synthetic incorporation of nickel into CPO-27(Mg) to give materials with enhanced permanent porosity

Nickel cations have been introduced into framework sites of the magnesium dioxiterephthalate MOF, CPO-27(Mg), via a procedure involving post-synthetic treatment with an aqueous solution of Ni2+ cations and a weak acid. The final solids have Ni concentrations up to 70 mol% of the total cation content. Selected area EDX analysis, synchrotron X-ray powder diffraction and XPS surface analysis indicate that Ni2+ is distributed throughout the crystals with highest concentration at the external surface. A mechanism involving both crystallisation and predominant isomorphous cation replacement is proposed. The modified solids have much enhanced adsorption capacities following simple thermal evacuation under vacuum than unmodified CPO-27(Mg), particularly for N-2 at 77 K, for which uptakes of 17 mmol g(-1) are achieved. This is attributed to surface modification that makes the surface more stable to heating under evacuation.</p

Synthetic control of framework zinc purinate crystallisation and properties of a large pore, decorated, mixed-linker RHO-type ZIF

A novel form of mixed-linker ZIF with the RHO topology is one of four zinc-imidazolate frameworks prepared with purine and 2-nitroimidazole. In this structure the linkers order to give a large pore solid with a high pore volume and an unusual symmetry and linker orientation. It possesses extra-framework zinc imidazolate units decorating the internal surface which can be removed to give high porosity.</p

Synthetic control of framework zinc purinate crystallisation and properties of a large pore, decorated, mixed-linker RHO-type ZIF

A novel form of mixed-linker ZIF with the RHO topology is one of four zinc-imidazolate frameworks prepared with purine and 2-nitroimidazole. In this structure the linkers order to give a large pore solid with a high pore volume and an unusual symmetry and linker orientation. It possesses extra-framework zinc imidazolate units decorating the internal surface which can be removed to give high porosity.</p