Using scCO2 in Preparation of Metal-Organic Frameworks: Investigating Effects on Crystallisation

In this report, we explore the use of supercritical CO2 (scCO2) in the synthesis of well-known metal-organic frameworks (MOFs) including Zn-MOF-74 and UiO-66, as well as on the preparation of [Cu24(OH-mBDC)24]n metal-organic polyhedra (MOPs) and two new MOF structures {[Zn2(L1)(DPE)]∙4H2O}n and {[Zn3(L1)3(4,4/-azopy)]∙7.5H2O}n, where BTC = benzene-1,3,5-tricarboxylate, BDC = benzene-1,4-dicarboxylate, L1 = 4-carboxy-phenylene-methyleneamino-4-benzoate, DPE = 1,2-di(4-pyridyl)ethylene, 4.4/-azopy = 4,4/- azopyridine, and compare the results versus traditional solvothermal preparations at low temperatures (i.e., 40 °Ϲ). The objective of the work was to see if the same or different products would result from the ssCO2 route versus the solvothermal method. We were interested to see which method produced the highest yield, the cleanest product and what types of morphology resulted. While there was no evidence of additional meso- or macroporosity in these MOFs/MOPs nor any significant improvements in product yields through the addition of scCO2 to these systems, it was shown that the use of scCO2 can have an effect on crystallinity, crystal size and morphology.

This work was published in the Special Issue Synthesis, Processing and Applications of Metal–Organic Frameworks using Compressed Fluids, Crystals 2020, 10(1), 17

Synthesis of zirconium-based MOFs for pheromones delivery: a study on NMR and molecular simulation

Controlled-release technologies are considered a key solution to deliver both pheromones and pesticides for better environmental and economical benefits. Metal-organic frameworks (MOFs) have gained a certain success in this area to deliver drug molecules. In this study, three different zirconium-based MOFs with different linkers (benzene-1,4-dicarboxylate, 2-aminobenzene-1,4-dicarboxylate and 2-(propylamino)benzene-1,4-dicarboxylate) were synthesised via a solvothermal method and examined for their possible use in pheromone delivery. The powder X-ray diffraction (PXRD) and nuclear magnetic resonance (NMR) results showed that these MOFs were produced successfully. 3-octanone, an archetypal pheromone used in this study, was loaded into the MOFs and the loading examined by NMR spectroscopy, showing that these MOFs are capable of accommodating pheromones in different quantities. Physical properties (e.g. surface area, pore volume and crystal density), nitrogen adsorption and pheromone diffusion of MOFs were studied in depth using simulation methods. The higher electrostatic contribution of UiO-66-NH2 leads to higher guest-host interaction energies than UiO-66. These findings suggest a promising porous material for use in sustainable agriculture.

Figure. a) Simulated nitrogen isotherm of MOFs at 77 K (logarithmic pressure axis) generated from Monte Carlo simulation. b) Mean square displacement of 3-octanone in MOFs generated from Molecular Dynamics simulation

Effect of mono- and divalent extra-framework cations on the structure and the internal surface area

Chabazite (CHA), one of the most common zeolite framework types, has been studied extensively in selective gas sorption. This is due to the remarkable capacity of this zeolite to accommodate cations within the unique CHA framework. Here, we report a systematic study on a series of chabazite zeolites exchanged by divergent extra-framework cations with valence and atomic radius difference. The results showed that chabazite (KNa-CHA) was synthesised successfully from zeolite Y, and six chabazite zeolites including K-CHA, Cs-CHA, Ca-CHA, Ba-CHA, Sr-CHA and Zn-CHA were prepared from the parent chabazite by ion exchange. These samples were examined by numerous techniques and it was found that the difference 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 the molecular sieve of the zeolite countercation which is a promising mechanism to selectively sequester and separate gases.

Figure. a) 27Al NMR spectrum of all chabazite zeolites. b) Nitrogen isotherms at 77 K of all chabazite zeolites

This work has been published in CrystEngComm