Defective hierarchical porous copper-based metal-organic frameworks synthesised via facile acid etching strategy

Introducing hierarchical pore structure to microporous materials such as metal-organic frameworks (MOFs) can be beneficial for reactions where the rate of reaction is limited by low rates of diffusion or high pressure drop. This advantageous pore structure can be obtained by defect formation, mostly via post-synthetic acid etching, which has been studied extensively on water-stable MOFs. Here we show that a water-unstable HKUST-1 MOF can also be modified in a corresponding manner by using phosphoric acid as a size-selective etching agent and a mixture of dimethyl sulfoxide and methanol as a dilute solvent. Interestingly, we demonstrate that the etching process which is time- and acidity- dependent, can result in formation of defective HKUST-1 with extra interconnected hexagonal macropores without compromising on the bulk crystallinity. These findings suggest an intelligent scalable synthetic method for formation of hierarchical porosity in MOFs that are prone to hydrolysis, for improved molecular accessibility and diffusion for catalysis.Comment: 14 pages, 8 figure

Acoustic Transmission Loss in Hilbert Fractal Metamaterials

Acoustic metamaterials are increasingly being considered as a viable technology for sound insulation. Fractal patterns constitute a potentially groundbreaking architecture for acoustic metamaterials. We describe in this work the behaviour of the transmission loss of Hilbert fractal metamaterials used for sound control purposes. The transmission loss of 3D printed metamaterials with Hilbert fractal patterns related to configurations from the zeroth to the fourth order is investigated here using impedance tube tests and Finite Element models. We evaluate, in particular, the impact of the equivalent porosity and the relative size of the cavity of the fractal pattern versus the overall dimensions of the metamaterial unit. We also provide an analytical formulation that relates the acoustic cavity resonances in the fractal patterns and the frequencies associated with the maxima of the transmission losses, providing opportunities to tune the sound insulation properties through control of the fractal architecture.Comment: Submitted to Scientific Report

Effect of mono- and divalent extra-framework cations on the structure and accessibility of porosity in chabazite zeolites

Chabazite (CHA), one of the most common zeolite framework types, has a remarkable capacity to accommodate a wide range of different cations within the unique CHA framework. This has led to CHA being applied extensively in ion exchange, and studied for highly selective gas sorption, most notably through a trapdoor mechanism. Here, we report the systematic study of a series of six chabazite zeolites (i.e.K-CHA, Cs-CHA, Ca-CHA, Ba-CHA, Sr-CHA and Zn-CHA) obtained by subjecting the parent chabazite (KNa-CHA) to exchange operations with cations of different valences and atomic radii. These samples were examined using numerous techniques and it was found that the differences in valence and size between extra-framework cations exert a significant effect on the abundance of these cations positioned in the framework, resulting in differing nitrogen sorption ability measured in the synthesised chabazite zeolites. These findings will help to understand how the zeolite counter-cation affects the ability of the CHA material to selectively sequester and separate gases through the use of the trapdoor mechanism.</p

Effect of mono- and divalent extra-framework cations on the structure and accessibility of porosity in chabazite zeolites

Chabazite (CHA), one of the most common zeolite framework types, has a remarkable capacity to accommodate a wide range of different cations within the unique CHA framework. This has led to CHA being applied extensively in ion exchange, and studied for highly selective gas sorption, most notably through a trapdoor mechanism. Here, we report the systematic study of a series of six chabazite zeolites (i.e.K-CHA, Cs-CHA, Ca-CHA, Ba-CHA, Sr-CHA and Zn-CHA) obtained by subjecting the parent chabazite (KNa-CHA) to exchange operations with cations of different valences and atomic radii. These samples were examined using numerous techniques and it was found that the differences in valence and size between extra-framework cations exert a significant effect on the abundance of these cations positioned in the framework, resulting in differing nitrogen sorption ability measured in the synthesised chabazite zeolites. These findings will help to understand how the zeolite counter-cation affects the ability of the CHA material to selectively sequester and separate gases through the use of the trapdoor mechanism.</p

Influence of Aromatic Structure on the Thermal Behaviour of Lignin

Lignin, a natural biopolymer and abundant by-product, is a particularly promising feedstock for carbon-based materials and a potentially sustainable alternative to phenolic resins, which are typically derived from crude oil. The source and method used to isolate lignin have a large impact on the thermal properties of the polymer, and can affect resultant materials prepared from lignin. Previous investigations into lignin characterisation often utilise a variety of feedstocks and isolation methods, which can make robust comparisons challenging. We present a systematic investigation into the chemical composition of lignins extracted using an identical Organosolv isolation method but from different biomass feedstocks: hemp hurds, eucalyptus chips, flax straw, rice husk and pine. We show how the aromatic structure of lignin can affect the thermal behaviour of the polymer, which correlates to the structure of resulting carbons. Carbons from lignins with a high syringyl unit content display a pronounced foaming behaviour which, on activation, results in a high-surface area material with hierarchical porosity

Structure-property relationships in metal-organic frameworks for hydrogen storage

Experimental hydrogen isotherms on several metal-organic frameworks (IRMOF-1, IRMOF-3, IRMOF-9, ZIF-7, ZIF-8, ZIF-9, ZIF-11, ZIF-12, ZIF-CoNIm, MIL-101 (Cr), NH2-MIL-101 (Cr), NH2-MIL-101 (Al), UiO-66, UiO-67 and HKUST-1) synthesized in-house and measured at 77 K and pressures up to 18 MPa are presented, along with N2 adsorption characterization. The experimental isotherms together with literature high pressure hydrogen data were analyzed in order to search for relationships between structural properties of the materials and their hydrogen uptakes. The total hydrogen capacity of the materials was calculated from the excess adsorption assuming a constant density for the adsorbed hydrogen. The surface area, pore volumes and pore sizes of the materials were related to their maximum hydrogen excess and total hydrogen capacities. Results also show that ZIF-7 and ZIF-9 (SOD topology) have unusual hydrogen isotherm shapes at relatively low pressures, which is indicative of "breathing", a phase transition in which the pore space increases due to adsorption. This work presents novel correlations using the modelled total hydrogen capacities of several MOFs. These capacities are more practically relevant for energy storage applications than the measured excess hydrogen capacities. Thus, these structural correlations will be advantageous for the prediction of the properties a MOF will need in order to meet the US Department of Energy targets for the mass and volume capacities of on-board storage systems. Such design tools will allow hydrogen to be used as an energy vector for sustainable mobile applications such as transport, or for providing supplementary power to the grid in times of high demand.</p

Understanding the AC conductivity and permittivity of trapdoor chabazites for future development of next-generation gas sensors

Synthetic K+ chabazite (KCHA), Cs+ chabazite (CsCHA) and Zn2+ chabazite (ZnCHA) have been synthesized and investigated in order to relate the differences in their crystalline structures to their thermal stability, moisture content and frequency dependent alternating current (AC) conductivity, permittivity and phase angle at a range of temperatures. The materials are shown to exhibit the universal dielectric response, which is typical of materials consisting of both conductive and insulating regions. Due to the presence of porosity, the three chabazites were hydrated significantly at room temperature and so the dehydrated state was achieved by heating the chabazites to high temperatures to ensure that all different energetic types of water were removed. Cation migration activation energies for KCHA (0.66 ± 0.10) eV, CsCHA (0.88 ± 0.01) eV and ZnCHA (0.90 ± 0.01) eV were determined during the cooling cycle from the fully dehydrated state to provide an accurate measurement of the activation energies. Good thermal stability of the materials was observed up to 710 °C and below 200 °C the electrical properties can be strongly influenced by hydration level. Overall, it was determined that when either hydrated or dehydrated, KCHA had the highest conductivity and lowest cation migration activation energy of the three studied chabazites and thus has the most promising electrical properties for potential use as a gas sensing material in next-generation electrical-based gas sensors.</p