Carlos Palomino Cabello

Thermodynamics of Hydrogen Adsorption on Metal‐Organic Frameworks

Interaction between adsorbed hydrogen and the coordinatively unsaturated Mg(2+) and Co(2+) cationic centres in Mg-MOF-74 and Co-MOF-74, respectively, was studied by means of variable-temperature infrared (VTIR) spectroscopy. Perturbation of the H(2) molecule by the cationic adsorbing centre renders the H--H stretching mode IR-active at 4088 and 4043 cm(-1) for Mg-MOF-74 and Co-MOF-74, respectively. Simultaneous measurement of integrated IR absorbance and hydrogen equilibrium pressure for spectra taken over the temperature range of 79-95 K allowed standard adsorption enthalpy and entropy to be determined. Mg-MOF-74 showed ΔH(0)=-9.4 kJ mol(-1) and ΔS(0)=-120 J mol(-1) K(-1), whereas for Co-MOF-74 the corresponding values of ΔH(0)=-11.2 kJ mol(-1) and ΔS(0)=-130 J mol(-1) K(-1) were obtained. The observed positive correlation between standard adsorption enthalpy and entropy is discussed in the broader context of corresponding data for hydrogen adsorption on cation-exchanged zeolites, with a focus on the resulting implications for hydrogen storage and delivering.

Automatic In-Syringe Dispersive Microsolid Phase Extraction Using Magnetic Metal–Organic Frameworks

A novel automatic strategy for the use of micro- and nanomaterials as sorbents for dispersive microsolid phase extraction (D-μ-SPE) based on the lab-in-syringe concept is reported. Using the developed technique, the implementation of magnetic metal-organic framework (MOF) materials for automatic solid-phase extraction has been achieved for the first time. A hybrid material based on submicrometric MOF crystals containing Fe3O4 nanoparticles was prepared and retained in the surface of a miniature magnetic bar. The magnetic bar was placed inside the syringe of an automatic bidirectional syringe pump, enabling dispersion and subsequent magnetic retrieval of the MOF hybrid material by automatic activation/deactivation of magnetic stirring. Using malachite green (MG) as a model adsorption analyte, a limit of detection of 0.012 mg/L and a linear working range of 0.04-2 mg/L were obtained for a sample volume equal to the syringe volume (5 mL). MG preconcentration was linear up to a volume of 40 mL, obtaining an enrichment factor of 120. The analysis throughput is 18 h(-1), and up to 3000 extractions/g of material can be performed. Recoveries ranging between 95 and 107% were obtained for the analysis of MG in different types of water and trout fish samples. The developed automatic D-μ-SPE technique is a safe alternative for the use of small-sized materials for sample preparation and is readily implementable to other magnetic materials independent of their size and shape and can be easily hyphenated to the majority of detectors and separation techniques.

Zeolitic imidazolate framework dispersions for the fast and highly efficient extraction of organic micropollutants

Development of advanced strategies for the extraction and preconcentration of trace levels of pollutants is essential for the quality control of water resources. A new procedure for the fast and highly efficient extraction of organic micropollutants from water using dispersions of zeolite imidazolate framework-8 (ZIF-8) crystals in a mixture of solvents is reported. The synergistic effect of using ZIF-8 dispersions in mixtures of water miscible and immiscible solvents enhances mass transfer and greatly improves extraction kinetics and capacity in comparison with the use of porous crystals or solvent microextraction separately. The effect of the ZIF-8 crystal size and surface composition has been evaluated using four different ZIF-8 samples spanning the micro- to nanometer range. The relevant parameters involved in the extraction such as the composition of the dispersion medium, the amount of ZIF-8 crystals, the extraction time, or the volume of dispersion required to ensure the maximum extraction efficiency, has also been studied using diethyl phthalate as a model compound. The use of 26 nm ZIF-8 crystals obtained using n-butylamine modulated synthesis has shown very fast extraction kinetics and excellent enrichment factors ranging from 150 to 380 for a mixture of six phthalate esters listed as priority pollutants by the United States EPA, allowing detection limits below the ng L−1 to be reached.

Submicrometric Magnetic Nanoporous Carbons Derived from Metal–Organic Frameworks Enabling Automated Electromagnet-Assisted Online Solid-Phase Extraction

We present the first application of submicrometric magnetic nanoporous carbons (μMNPCs) as sorbents for automated solid-phase extraction (SPE). Small zeolitic imidazolate framework-67 crystals are obtained at room temperature and directly carbonized under an inert atmosphere to obtain submicrometric nanoporous carbons containing magnetic cobalt nanoparticles. The μMNPCs have a high contact area, high stability, and their preparation is simple and cost-effective. The prepared μMNPCs are exploited as sorbents in a microcolumn format in a sequential injection analysis (SIA) system with online spectrophotometric detection, which includes a specially designed three-dimensional (3D)-printed holder containing an automatically actuated electromagnet. The combined action of permanent magnets and an automatically actuated electromagnet enabled the movement of the solid bed of particles inside the microcolumn, preventing their aggregation, increasing the versatility of the system, and increasing the preconcentration efficiency. The method was optimized using a full factorial design and Doehlert Matrix. The developed system was applied to the determination of anionic surfactants, exploiting the retention of the ion-pairs formed with Methylene Blue on the μMNPC. Using sodium dodecyl sulfate as a model analyte, quantification was linear from 50 to 1000 μg L(-1), and the detection limit was equal to 17.5 μg L(-1), the coefficient of variation (n = 8; 100 μg L(-1)) was 2.7%, and the analysis throughput was 13 h(-1). The developed approach was applied to the determination of anionic surfactants in water samples (natural water, groundwater, and wastewater), yielding recoveries of 93% to 110% (95% confidence level).

Incorporation of zeolitic imidazolate framework (ZIF-8)-derived nanoporous carbons in methacrylate polymeric monoliths for capillary electrochromatography

A series of metal organic frameworks-derived nanoporous carbons originating from zeolitic imidazolate framework-8 (ZIF-8) crystals as precursors have been prepared via varying the preparation conditions. The ZIF-8-derived carbons were subsequently admixed in the methacrylate monomers containing polymerization mixtures and polymerized to obtain monolithic columns for capillary electrochromatography (CEC). The effect of particle size and content of the ZIF-8-derived carbon materials in the polymerization mixture on the performance of the hybrid monolithic columns was investigated in detail. The resulting composites were characterized using scanning electron microscopy. Using short time UV-initiated polymerization, monolithic beds with homogenously dispersed ZIF-8-derived carbons were obtained. The chromatographic performance of these composites was demonstrated with separations of polycyclic aromatic hydrocarbons and non-steroidal anti-inflammatory drugs as test solutes. The incorporation of the ZIF-8-derived carbons into the organic polymer monoliths led to an increase in the retention of all the analytes compared to the parent monolith. Finally, the hybrid monolithic columns exhibited satisfactory run-to-run and batch-to-batch reproducibility.

Metal-organic framework mixed-matrix disks: Versatile supports for automated solid-phase extraction prior to chromatographic separation

We present for the first time the application of metal-organic framework (MOF) mixed-matrix disks (MMD) for the automated flow-through solid-phase extraction (SPE) of environmental pollutants. Zirconium terephthalate UiO-66 and UiO-66-NH2 MOFs with different size (90, 200 and 300nm) have been incorporated into mechanically stable polyvinylidene difluoride (PVDF) disks. The performance of the MOF-MMDs for automated SPE of seven substituted phenols prior to HPLC analysis has been evaluated using the sequential injection analysis technique. MOF-MMDs enabled the simultaneous extraction of phenols with the concomitant size exclusion of molecules of larger size. The best extraction performance was obtained using a MOF-MMD containing 90nm UiO-66-NH2 crystals. Using the selected MOF-MMD, detection limits ranging from 0.1 to 0.2μgL-1 were obtained. Relative standard deviations ranged from 3.9 to 5.3% intra-day, and 4.7-5.7% inter-day. Membrane batch-to-batch reproducibility was from 5.2 to 6.4%. Three different groundwater samples were analyzed with the proposed method using MOF-MMDs, obtaining recoveries ranging from 90 to 98% for all tested analytes.

Enhanced CO2 adsorption capacity of amine-functionalized MIL-100(Cr) metal–organic frameworks

Amine-functionalized metal–organic frameworks EN-MIL-100(Cr) and MMEN-MIL-100(Cr) were prepared by grafting ethylenediamine and N,N′-dimethylethylenediamine molecules, respectively, on coordinatively unsaturated chromium(III) cations in MIL-100 metal–organic frameworks. Although amine grafting results in a decrease in the surface area and pore volume of the functionalized samples compared to those of the bare MIL-100(Cr), results showed that the incorporation of amine groups into the MIL-100 framework improved both carbon dioxide adsorption capacity and kinetics, especially in the case of ethylenediamine. Heats of carbon dioxide adsorption in EN-MIL-100(Cr) and MMEN-MIL-100(Cr) were about 80 kJ mol−1 at zero coverage, a value higher than that shown by MIL-100(Cr), which suggests chemisorption between CO2 and pendant amine groups. Measurement of CO2 adsorption–desorption cycles proved that functionalized materials show good regenerability and stability.

Metal Oxide Assisted Preparation of Core–Shell Beads with Dense Metal–Organic Framework Coatings for the Enhanced Extraction of Organic Pollutants

Dense and homogeneous metal-organic framework (MOF) coatings on functional bead surfaces are easily prepared by using intermediate sacrificial metal oxide coatings containing the metal precursor of the MOF. Polystyrene (PS) beads are coated with a ZnO layer to give ZnO@PS core-shell beads. The ZnO@PS beads are reactive in the presence of 2-methylimidazole to transform part of the ZnO coating into a porous zeolitic imidazolate framework-8 (ZIF-8) external shell positioned above the internal ZnO precursor shell. The obtained ZIF-8@ZnO@PS beads can be easily packed in column format for flow-through applications, such as the solid-phase extraction of trace priority-listed environmental pollutants. The prepared material shows an excellent permeance to flow when packed as a column to give high enrichment factors, facile regeneration, and excellent reusability for the extraction of the pollutant bisphenol A. It also shows an outstanding performance for the simultaneous enrichment of mixtures of endocrine disrupting chemicals (bisphenol A, 4-tert-octylphenol and 4-n-nonylphenol), facilitating their analysis when present at very low levels (<1 μg L(-1) ) in drinking waters. For the extraction of the pollutant bisphenol A, the prepared ZIF-8@ZnO@PS beads also show a superior extraction and preconcentration capacity to that of the PS beads used as precursors and the composite materials obtained by the direct growth of ZIF-8 on the surface of the PS beads in the absence of metal oxide intermediate coatings.

Emerging materials for sample preparation

This review provides an update on the implementation of emerging materials as sorbents for sample preparation in combination with chromatographic separation. We have focused on recent applications of metal-organic frameworks, layered double hydroxides, porous carbons obtained from polymers or biomass precursors, and silicates (clays and zeolites). The review is directed toward the strategies followed by the authors to engineer suitable supports enabling the application of materials with unconventional size and shape as high-performance sorbents to explore new boundaries in sample pretreatment in manual or automated modes.

Immobilization of Metal–Organic Frameworks on Supports for Sample Preparation and Chromatographic Separation

Metal–organic frameworks (MOFs) are porous crystalline materials with large surface areas, uniform pore size, and tunable selectivity. In the last few years, the number of analytical applications of MOFs has been growing constantly. However, the direct use of as-synthesized MOFs in packed column format is rather limited for analytical separations because of the small size and non-spherical shape of MOF crystals. In this review, we outline and critically discuss the advantages and limitations of the different methods described to immobilize MOFs into functional supports for analytical separations, including beads, monoliths, and fibers. These methods are based on embedding MOF crystals into functional supports, in situ MOF growth, controlled layer-by-layer MOF growth, or the in situ conversion of immobilized MOF metal oxide precursors. Representative examples of immobilized MOFs for sample preparation and chromatographic separation are overviewed. We also overview recent progress on the use of MOFs as precursors to obtain other functional materials such as layered double hydroxides or porous carbons.