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Abstract: The synthesis and reactivity of single metal atoms in low–valence state bound to just water, rather than to organic ligands or surfaces, is conceptually relevant and a major experimental challenge. Here, we show a gram–scale wet synthesis of a Pt11+ complex stabilized in a confined space by a water cluster, formed by a well–defined crystallographic first water sphere and a second coordination sphere linked to a Metal–Organic Framework (MOF) through electrostatic and H–bonding interactions. The role of the water cluster is not only isolating and stabilizing the Pt atoms, but also regulating the charge of the metal and the adsorption of reactants. This is shown for the low–temperature water–gas shift reaction (WGSR: CO + H2O → CO2 + H2), where both metal coordinated and H–bonded water molecules trigger a double water attack mechanism to CO and give CO2 with both oxygen atoms coming from water. The stabilized Pt1+ single sites in confined water clusters allow performing the WGSR at temperatures as low as 50 ºC.

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Abstract: Understand/visualize the established interactions between gases and adsorbents is mandatory to implement better per-formant materials in adsorption/separation processes. Here we report the unique behavior of a rare example of a hemila-bile chiral three-dimensional metal-organic framework (MOF) with an unprecedented qtz-e-type topology, with formula CuII2(S,S)-hismox . 5H2O (1) (hismox = bis[(S)-histidine]oxalyl diamide). 1 exhibits a continuous and reversible breath-ing behavior, based on the hemilability of carboxylate groups from L-histidine. In-situ powder (PXRD) and single crystal X-ray diffraction (SCXRD) using synchrotron radiation allowed to unveil the crystal structures of 4 different host-guest adsorbates (Ar, N2, CO2 and C3H6@1), the rationalization of the breathing motion and unravel the mechanisms govern-ing the adsorption of these gases. Then, this information has been transferred to implement efficient separations of mix-tures of industrial and environmental relevance –CO2/N2, CO2/CH4 and C3H8/C3H6 using 1 in packed columns as the sta-tionary phase and dispersed in a mixed matrix membrane.

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Abstract: Since the advent of the first metal–organic frameworks (MOFs), we have witnessed an explosion of captivating architectures with exciting physicochemical properties and applications in a wide range of fields. This, in part, can be understood under the light of their rich host–guest chemistry and the possibility to use single-crystal X-ray diffraction (SC-XRD) as a basic characterization tool. Moreover, chemistry on preformed MOFs, applying recent developments in template-directed synthesis and postsynthetic methodologies (PSMs), has shown to be a powerful synthetic tool to (i) tailor MOFs channels of known topology via single-crystal to single-crystal (SC-SC) processes, (ii) impart higher degrees of complexity and heterogeneity within them, and most importantly, (iii) improve their capabilities toward applications with respect to the parent MOFs. However, the unique properties of MOFs have been, somehow, limited and underestimated. This is clearly reflected on the use of MOFs as chemical nanoreactors, which has been barely uncovered. In this Account, we bring together our recent advances on the construction of MOFs with appealing properties to act as chemical nanoreactors and be used to synthesize and stabilize, within their channels, catalytically active species that otherwise could be hardly accessible. First, through two relevant examples, we present the potential of the metalloligand approach to build highly robust and crystalline oxamato- and oxamidato-MOFs with tailored channels, in terms of size, charge and functionality. These are initial requisites to have a playground where we can develop and fully take advantage of singular properties of MOFs as well as visualize/understand the processes that take place within MOFs pores and somehow make structure–functionalities correlations and develop more performant MOFs nanoreactors. Then, we describe how to exploit the unique and singular features that offer each of these MOFs confined space for (i) the incorporation and stabilization of metals salts and complexes, (ii) the in situ stepwise synthesis of subnanometric metal clusters (SNMCs), and (iii) the confined-space self-assembly of supramolecular coordination complexes (SCCs), by means of PSMs and underpinned by SC-XRD. The strategy outlined here has led to unique rewards such as the highly challenging gram-scale preparation of stable and well-defined ligand-free SNMCs, exhibiting outstanding catalytic activities, and the preparation of unique SCCs, different to those assembled in solution, with enhanced stabilities, catalytic activities, recyclabilities, and selectivities. The results presented in this Accounts are just a few recent examples, but highly encouraging, of the large potential way of MOFs acting as chemical nanoreactors. More work is needed to found the boundaries and fully understand the chemistry in the confined space. In this sense, mastering the synthetic chemistry of discrete organic molecules and inorganic complexes has basically changed our way of live. Thus, achieving the same degree of control on extended hybrid networks will open new frontiers of knowledge with unforeseen possibilities. We aim to stimulate the interest of researchers working in broadly different fields to fully unleash the host–guest chemistry in MOFs as chemical nanoreactors with exclusive functional species.

Open Access

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Abstract: The most widely used method to measure the transport properties of dense polymeric membranes is the time lag method in a constant volume/pressure increase instrument. Although simple and quick, this method provides only relatively superficial, averaged data of the permeability, diffusivity, and solubility of gas or vapor species in the membrane. The present manuscript discusses a more sophisticated computational method to determine the transport properties on the basis of a fit of the entire permeation curve, including the transient period. The traditional tangent method and the fitting procedure were compared for the transport of six light gases (H2, He, O2, N2, CH4, and CO2) and ethane and ethylene in mixed matrix membranes (MMM) based on Pebax®1657 and the metal–organic framework (MOF) CuII2(S,S)-hismox·5H2O. Deviations of the experimental data from the theoretical curve could be attributed to the particular MOF structure, with cavities of different sizes. The fitting procedure revealed two different effective diffusion coefficients for the same gas in the case of methane and ethylene, due to the unusual void morphology in the MOFs. The method was furthermore applied to mixed gas permeation in an innovative constant-pressure/variable-volume setup with continuous analysis of the permeate composition by an on-line mass-spectrometric residual gas analyzer. This method can provide the diffusion coefficient of individual gas species in a mixture, during mixed gas permeation experiments. Such information was previously inaccessible, and it will greatly enhance insight into the mixed gas transport in polymeric or mixed matrix membranes.

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Abstract: The low‐temperature water‐gas shift reaction (WGSR, CO + H 2 O ⇔ H 2 + CO 2 ) is considered a very promising reaction –candidate for fuel cells– despite an efficient and robust catalyst is still desirable. One of the more prominent catalysts for this reaction is based on single Pt atoms (Pt 1 ) on different supports, which are supposed to manifold the reaction by the accepted mechanism for the general WGSR, i.e. by addition of one H 2 O molecule to CO, with generation of CO 2 and H 2 . Here we show, experimentally, that not one but two H 2 O molecules are added to CO on the Pt 1 catalyst, as assessed by a combination of reactivity experiments with soluble Pt catalysts, kinetic and spectroscopic measurements, and finally by in‐operando single crystal X‐ray diffraction on a Pt 1 ‐MOF, to visualize the formation of the hemiacetal intermediate on the solid catalytic site. These results confirm our previous DFT predictions and provide a paradigmatic shift in the assumed mechanism of the WGSR, which may open the debate if two H 2 O molecules are recurrently added during the WGSR, not only for Pt 1 catalysts but also for other metal catalysts.