Andrea Santiago-Portillo

This work highlights the benefit of designing mixed-metal (Cr/Fe) MOFs for enhanced chemical stability and catalytic activity. A robust and stable mixed-metal MIL-101(Cr/Fe) was prepared through a HF-free direct hydrothermal route with Fe³⁺ content up to 21 wt%. The incorporation of Fe³⁺ cations in the crystal structure was confirmed by ⁵⁷Fe Mössbauer spectrometry. The catalytic performance of the mixed metal MIL-101(Cr/Fe) was evaluated in the Prins reaction. MIL-101(Cr/Fe) exhibited a higher catalytic activity compared to MIL-101(Cr), improved chemical stability compared to MIL-101(Fe) and a higher catalytic activity for bulky substrates compared to MIL-100(Fe). In situ infra-red spectroscopy study suggests that the incorporation of Fe³⁺ ions in MIL-101 structure leads to an increase in Lewis acid sites. It was thus concluded that the predominant role of Cr³⁺ ions was to maintain the crystal structure, while Fe³⁺ ions enhanced the catalytic activity.

Subphthalocyanine has been incorporated into a robust metal–organic framework having amino groups as binding sites. The resulting SubPc@MIL-101(Cr)-NH₂ composite has a loading of 2 wt%. Adsorption of subphthalocyanine does not deteriorate host crystallinity, but decreases the surface area and porosity of MIL-101(Cr)-NH₂. The resulting SubPc@MIL-101(Cr)-NH₂ composite exhibits a 575 nm absorption band responsible for the observed photoredox catalytic activity under simulated sunlight irradiation for hydrogenative dehalogenation of α-haloacetophenones and for the coupling of α-bromoacetophenone and styrene. The material undergoes a slight deactivation upon reuse. In comparison to the case of phthalocyanines the present study is one of the few cases showing the use of subphthalocyanine as a photoredox catalyst, with its activity derived from site isolation within the MOF cavities.

Prolonged (weeks) UV/Vis irradiation under Ar of UiO‐66(Zr), UiO66 Zr‐NO₂, MIL101 Fe, MIL125 Ti‐NH₂, MIL101 Cr and MIL101 Cr(Pt) shows that these MOFs undergo photodecarboxylation of benzenedicarboxylate (BDC) linker in a significant percentage depending on the structure and composition of the material. Routine characterization techniques such as XRD, UV/Vis spectroscopy and TGA fail to detect changes in the material, although porosity and surface area change upon irradiation of powders. In contrast to BCD‐containing MOFs, zeolitic imidazolate ZIF‐8 does not evolve CO₂ or any other gas upon irradiation.

Stable mixed-metal Fe^III/Ni^II MIL-100 MOFs have been synthesized de novo and have been explored as superior heterogeneous catalysts in acid catalyzed reactions, presenting superior performances.

This manuscript reports a comparative study of the catalytic performance of gold nanoparticles (NPs) encapsulated within MIL‐101(Cr) with or without amino groups in the terephthalate linker. The purpose is to show how the amino groups can influence the microenvironment and catalytic stability of incorporated gold nanoparticles. The first influence of the presence of this substituent is the smaller particle size of Au NPs hosted in MIL‐101(Cr)‐NH₂ (2.45±0.19 nm) compared with the parent MIL‐101(Cr)‐H (3.02±0.39 nm). Both materials are highly active to promote the aerobic alcohol oxidation and exhibit a wide substrate scope. Although both catalysts can achieve turnover numbers as high as 10⁶ for the solvent‐free aerobic oxidation of benzyl alcohol, Au@MIL‐101(Cr)‐NH₂ exhibits higher turnover frequency values (12 000 h⁻¹) than Au@MIL‐101(Cr)‐H (6800 h⁻¹). Au@MIL‐101(Cr)‐NH₂ also exhibits higher catalytic stability, being recyclable for 20 times with coincident temporal conversion profiles, in comparison with some decay observed in the parent Au@MIL‐101(Cr)‐H. Characterization by transmission electron microscopy of the 20‐times used samples shows a very minor particle size increase in the case of Au@MIL‐101(Cr)‐NH₂ (2.97±0.27 nm) in comparison with the Au@MIL‐101(Cr)‐H analog (5.32±0.72 nm). The data presented show the potential of better control of the microenvironment to improve the performance of encapsulated Au nanoparticles.

Despite the higher efficiency, larger color range and faster stimulus response of polymeric electrochromic materials, their poor cyclability strongly hampers their application in optoelectronics. As an original strategy to stabilize and further nanostructure these polymers, herein an efficient encapsulation and in situ polymerization inside highly porous metal–organic frameworks (MOFs) is reported. In particular, the successful accommodation of poly(3,4-ethylendioxythiophene) (PEDOT) and its partially oxidized polarons inside the mesopores of the nontoxic iron trimesate MIL-100(Fe) is convincingly proved by a large panel of experimental techniques. Remarkably, the polymer–MOF interaction occurring for entrapped PEDOT within the pores (deeply assessed by experimental and simulation methods) might be responsible for the enhanced electrical conductivity of the resulting PEDOT@MIL-100(Fe) composite when compared to the insulating MIL-100(Fe) and the conductive free PEDOT. Furthermore, it was possible to observe the electrochromic properties of the PEDOT@MIL-100(Fe) composite, achieving an improved stability and good cyclability as a consequence of the effective protection by the MOF matrix.

Metal organic frameworks (MOFs) are currently attracting considerable attention as heterogeneous catalysts at moderate temperatures, mainly for liquid‐phase reactions. Since structural stability is one of the major concerns for the use of MOFs in catalysis, particularly considering that frequently some of the reported MOF materials are very unstable, the interest in this area has been focused on those MOFs exhibiting the highest structural robustness, UiO‐66 being among the most widely used. Two introductory sections deal with the synthesis, structure and main properties of UiO‐66, including its remarkable thermal (up to 350 °C) and chemical (aqueous solution in a wide pH range) stability. The main body of the review summarizes those examples of using UiO‐66 in catalysis grouped according to the nature of the active sites, starting with the use of UiO‐66 as Lewis acids. In this section, emphasis has been made in the recent strategies to create structural defects in a controlled way that generate Lewis acidity. Other sections cover examples illustrating substituted terephthalate as active sites and the evidence that there is a synergy between acid and basic sites in close proximity that make some of these UiO‐66 with substituents at the linker to act as dual acid‐base catalysts. Other sections are focused on the use of UiO‐66 as hosts of metal nanoparticles, metal oxides and other host, remarking the influence that the nature of UiO‐66 and the possible presence of substituents in the framework play on the activity of the incorporated guest. The last section summarizes the current state of the art in the use of UiO‐66 in catalysis and provides our views on future developments regarding the application of UiO‐66 in industrial processes.