A one-pot synthesis is described to construct a composite of the amino-derivative Zr carboxylate metal–organic framework and silica gel (UiO-66-NH2@silica) as an efficient solid sorbent for hexavalent chromium.
The remarkable water stability of Zr-carboxylatebased metal−organic frameworks (MOFs) stimulated considerable interest toward their utilization in aqueous phase applications. The origin of such stability is probed here through pH titration and pK a modeling. A unique feature of the Zr 6 (μ 3 -OH) 4 (μ 3 -O) 4 (RCO 2 ) 12 cluster is the Zr-bridging oxo/hydroxyl groups, demonstrating several pK a values that appear to provide for the water stability at a wide range of pH. Accordingly, the tunability of the cage/surface charge of the MOF can feasibly be controlled through careful adjustment of solution pH. Such high stability, and facile control over cage/surface charge, can additionally be augmented through introducing chemical functionalities lining the cages of the MOF, specifically amine groups in the UiO-66-NH 2 presented herein. The variable protonation states of the Zr cluster and the pendant amino groups, their H-bond donor/acceptor characteristics, and their electrostatic interactions with guest molecules were effectively utilized in controlled experiments to demonstrate high uptake of model guest molecules (137 mg/g for Cr(VI), 1275 mg/g for methylene blue, and 909 mg/g for methyl orange). Additionally, a practical form of the silica-supported MOF, UiO-66-NH 2 @SiO 2 , constructed in under 2 h reaction time, is described, generating a true platform microporous sorbent for practical use in demanding applications.
Despite the large number of reports on the utilization of highly microporous solids, most relevant are metal−organic frameworks (MOFs), in different demanding applications, the successful hybridization of MOFs and moldable polymer matrices into flexible, water-permeable membranes exhibiting strong entanglement of the MOF and the polymer matrix properties is still lacking. We describe herein an efficient pathway to construct a mixed-matrix membrane (MMM) comprising a water-stable metal−organic framework (UiO-66-NH 2 ), as the active sorbent, and cellulose acetate (CA), as the polymer matrix, to construct a flexible membrane for water treatment applications. The MOF@CA MMM demonstrated superior performance in terms of exceptional removal of organic dyes (both cationic and anionic species) as well as hexavalent Cr ions, compared to the control CA membrane. The recorded high uptake of the MOF@CA MMM for this wide array of contaminants demonstrated the accessibility of the MOF nanocages immobilized within the MMM, in contrast to the common perception that the polymer matrix might act as a physical barrier to block the accessibility of the MOF cages. The negative surface charge of the matrix exerted a notable action to affect the diffusion of the negatively charged contaminants to reach the active sorbent filler. Moreover, the formed membrane demonstrated high durability and recyclability with no detected loss of performance over numerous cycles. This approach outlines the ability to formulate one of the most water-stable MOFs, as exceptional microporous sorbent, into a usable membrane form compatible with real-life applications.
A facile, postsynthetic treatment of a designed composite of pyrimidine-based porous-organic polymer and graphene (PyPOP@G) with ionic Pt, and the subsequent uniform electrodeposition of Pt metallic within the pores, led to the formation of a composite material (PyPOP-Pt@G). The pyrimidine porous-organic polymer (PyPOP) was selected because of the abundant Lewis-base binding sites within its backbone, to be combined with graphene to produce the PyPOP@G composite that was shown to uptake Pt ions simply upon brief incubation in H2PtCl6 solution in acetonitrile. The XPS analysis of PyPOP@G sample impregnated with Pt ions confirmed the presence of Pt(II/IV) species and did not show any signs of metallic nanoparticles, as further confirmed by transmission electron microscopy. Immediately upon electrochemical reduction of the Pt(II/IV), metallic Pt (most likely atomistic Pt) was observed. This approach stands out, as compared to Pt monolayer deposition techniques atop metal foams, or a recently reported atomic layer deposition (ALD), as a way of depositing submonolayer coverage of precious catalysts within the 1–10 nm pores found in microporous solids. The prepared catalyst platform demonstrated large current density (100 mA/cm2) at 122 mV applied overpotential for the hydrogen evolution reaction (HER), with measured Faradaic efficiency of 97(±1)%. Its mass activity (1.13 A/mgPt) surpasses that of commercial Pt/C (∼0.38 A/mgPt) at the overpotential of 100 mV. High durability has been assessed by cyclic and linear sweep voltammetry, as well as controlled potential electrolysis techniques. The Tafel plot for the catalyst demonstrated a slope of ∼37 mV/decade, indicating a Heyrovsky-type rate-limiting step in the observed HER.
A range of microporous, imide-based polymers were newly synthesized using two-step poly-condensation reactions of bis(carboxylic anhydride) and various aromatic diamines for CO 2 gas capture and storage applications. In this report, we attempted to assess the relative significance of molecular structural aspects through the manipulation of the conformational characteristics of the building blocks of the polymeric structures, the spiro-containing acid anhydride and the aromatic amines, to induce greater intrinsic microporosity and higher surface areas for the resulting solids. Results obtained from this study were thus used to outline a working relationship between the structural diversity of the constructed porous solids and their performance as CO 2 sorbents.
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