CO2 captured in zeolitic imidazolate frameworks: Raman spectroscopic analysis of uptake and host-guest interactions

Zeolitic imidazolate frameworks (ZIFs) exhibit enhanced selectivity and increased CO2 uptake due to the incorporation of functional imidazolate units in their structure as well as their extensive porosity and ring flexibility. In situ Raman investigation of a representative host compound, ZIF-69, in practical CO2 pressure and temperature regimes (0-10 bar and 0-64 °C) correlates well with corresponding macroscopic CO2 sorption data and shows clear clear spectroscopic evidence of CO2 uptake. Significant positive shift of the 159 cm-1 phenyl bending mode of the benzimidazole moiety indicates weak hydrogen bonding with CO 2 in the larger cavities of the ZIF matrix. Raman spectroscopy is shown to be an easy and sensitive tool for quantifying CO2 uptake, identifying weak host-guest interactions and elucidating CO2 sorption mechanism in ZIFs. Are you Raman enough? In situ Raman investigation of the interactions of zeolitic imidazolate frameworks (ZIFs) with CO2 in practical pressure and temperature regimes (0-10 bar and 0-64 °C) correlates well with corresponding macroscopic CO2 sorption data and shows clear spectroscopic evidence of CO2 uptake (see image). Raman is found to be an easy and sensitive tool for quantifying CO2 uptake, identifying weak host-guest interactions, and elucidating CO2 sorption mechanism in ZIFs. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Synthesis and characterisation of carbon nanotube modified anodised alumina membranes

Carbon nanotubes (CNTs) are considered as ideal model sorbent systems for studying the effect of pore size and surface characteristics on the sorption and transport properties of porous solids. In this study we synthesized CNTs on anodised alumina disks with pore sizes of 20, 100 and 200 nm. The disks were initially immersed into a nickel nitrate solution and purged with hydrogen at elevated temperatures in order to form metallic nickel nanoparticles. CNTs were thereinafter grown by CVD at 973 K using a mixture of acetylene and argon. When a nickel catalyst was involved, the CVD period adequate for CNT's growth was 15 min for all the templates. In absence of the catalyst the sufficient CVD duration was about 6 h for the 20 nm templates, while no CNT formation has been observed for the 100 and 200 nm templates during this period. Adsorption of n-hexane and relative permeability (RP) of the n-hexane/nitrogen system were applied to examine the resulting carbon nanotube membranes. Since the permeation of the non-condensable gas is governed by the Knudsen diffusion mechanism, a pore size distribution (PSD) was extracted directly from the derivative of the permeance curve against pore radii, after relating the latter with the equilibrium vapour pressure via the Kelvin equation and an accurate t-curve. These experiments were useful for the determination of the pore size distribution of the remaining open gaps of the developed membranes and gave an insight on the factors affecting the CNTs growth. © 2007 Elsevier Inc. All rights reserved

Correlating vibrational properties with temperature and pressure dependent CO2 adsorption in zeolitic imidazolate frameworks

Zeolitic imidazolate frameworks (ZIFs) feature a rigid porous structure where the interplay of pore merits and wall functionality, determined by the different imidazolate functional groups, results in superior CO2 capture ability. In this work, the vibrational properties of ZIF-68 and ZIF-69, two characteristic complex gmelinite (GME) type ZIFs comprising of benzimidazolate (bIm) and chloro-benzimidazolate (cbIm) linkers, respectively, were investigated by micro-Raman spectroscopy as a function of CO2 pressure and temperature, in combination with macroscopic adsorption experiments and extended molecular simulations, in order to explore the underlying host-guest interactions and particularly the variation of the framework lattice dynamics and flexibility to CO2 loading. The CO2 isosteric heat of adsorption (Qst) was quantitatively determined by the temperature dependence of the CO2 Fermi dyad intensity at constant pressure. ZIF-69 was consistently found to present higher Qst than ZIF-68 due to the cbIm polar functionality, in close agreement with macroscopic CO2 adsorption experiments and Monte Carlo analysis. More importantly, high CO2 uptake was found to cause significant blue shifts and enhancement of the frequency shift temperature gradients of several low-frequency Raman modes, which according to detailed polarization analysis of ZIF microcrystals, arise from free breathing vibrations of the functionalized ligands in the large ZIF pores. Low-frequency micro-Raman spectroscopy may accordingly constitute a sensitive spectroscopic tool for unveiling lattice dynamics upon CO2 sorption in ZIFs. © 2020 Elsevier B.V

Carbon Nanotube Selective Membranes with Subnanometer, Vertically Aligned Pores, and Enhanced Gas Transport Properties

Membranes consisting of ultrathin, oriented, single-wall carbon nanotube (SWCNT) micropores with a diameter of ∼4 Å were developed. c-Oriented AFI-type aluminophosphate (AlPO) films (AlPO4-5 and CoAPO-5), consisting of parallel channels 7.3 Å in diameter, were first fabricated by seeded growth on macroporous alumina supports, and used as templates for synthesis of CNTs inside the zeolitic channels by thermal treatment, utilizing the structure directing agent (amine) occluded in the channels as carbon source. Incorporation of CNTs inside the AFI channels altered the transport mechanism of all permeating gases tested, and imposed a substantial increase in their permeation rates, in comparison to the AlPO4-5 membrane, despite the pore size reduction due to nanotube growth. The enhancement of the permeation rates is attributed to repulsive potentials between gas molecules and occluded nanotubes, which limit adsorption strength and enhance diffusivity, coupled to the smooth SWCNT surface that enables fast diffusion through the nanotube interior. Separation ability, evaluated with respect to H2 and CO2 gases, was enhanced by using polysterene as defect-blocking medium on both AlPO and CNT/AlPO membranes and was preserved after CNT growth. © 2015 American Chemical Society

Zeolite imidazolate framework-Ionic liquid hybrid membranes for highly selective CO2 separation

Zeolitic imidazolate framework ZIF-69 membranes were grown on porous a-alumina substrates via seeded secondary growth and further functionalized by a CO2-selective tricyanomethanide anion/alkylmethylimidazolium cation-based ionic liquid (IL) to plug the gaps between the ZIF crystals yet leave the framework pores open for gas diffusion. In this configuration, ZIF intergrain boundaries and defects were repaired by a medium that exhibits high selectivity for CO2. As a result, the selectivity of the hybrid membrane was significantly higher than that of as-grown ZIF membranes and, because of the existence of the ZIF channels, the permeability was higher than that corresponding to bulk IL. Specifically, CO2 permeated 20 times faster than N2 through the intact ZIF pores and 65 times faster than through the bulk IL phase. The developed membranes at room temperature and under a 2 bar transmembrane pressure exhibited CO2 permeance of 5.6 × 10–11 and 3.7 × 10–11 mol m–2 s–1 Pa–1 and real CO2/N2 selectivities of 44 and 64 for CO2/N2 mixtures consisting of 44% and 75% (v/v) CO2, respectively. In addition, on the basis of the experimental evidence from the hybrid membranes, predictions were made on the expected performance of an ideal, crack-free, and homogeneous ZIF-69 membrane. This work provides a promising solution to the challenges associated with defect formation experienced during growth not only of ZIFs but also of other zeolite and inorganic membranes used for CO2 separation

Zeolite imidazolate framework-Ionic liquid hybrid membranes for highly selective CO2 separation

Zeolitic imidazolate framework ZIF-69 membranes were grown on porous a-alumina substrates via seeded secondary growth and further functionalized by a CO2-selective tricyanomethanide anion/alkylmethylimidazolium cation-based ionic liquid (IL) to plug the gaps between the ZIF crystals yet leave the framework pores open for gas diffusion. In this configuration, ZIF intergrain boundaries and defects were repaired by a medium that exhibits high selectivity for CO2. As a result, the selectivity of the hybrid membrane was significantly higher than that of as-grown ZIF membranes and, because of the existence of the ZIF channels, the permeability was higher than that corresponding to bulk IL. Specifically, CO2 permeated 20 times faster than N2 through the intact ZIF pores and 65 times faster than through the bulk IL phase. The developed membranes at room temperature and under a 2 bar transmembrane pressure exhibited CO2 permeance of 5.6 × 10–11 and 3.7 × 10–11 mol m–2 s–1 Pa–1 and real CO2/N2 selectivities of 44 and 64 for CO2/N2 mixtures consisting of 44% and 75% (v/v) CO2, respectively. In addition, on the basis of the experimental evidence from the hybrid membranes, predictions were made on the expected performance of an ideal, crack-free, and homogeneous ZIF-69 membrane. This work provides a promising solution to the challenges associated with defect formation experienced during growth not only of ZIFs but also of other zeolite and inorganic membranes used for CO2 separation