Development of a cell for in-situ/operando Mössbauer spectroscopy studies of Catalysts: Application to the characterization of iron nanoparticles used in Fischer-Tropsch reaction

In this paper, we report the design of a stainless-steel cell with polyimide film windows for carrying out in situ Mossbauer spectroscopy studies with a horizontal x-ray beam. It allows recording spectra at pressures up to 0.2-0.3 MPa and temperatures up to 500 °C under a gas flow rate of up to 100 ml min-1. The catalyst is either directly deposited on the heating element or pressed into the form of a pellet for larger quantities. A wide range of heating or cooling rates can be used, and a very accurate sample temperature can be monitored for several days. An example of application to the study of a catalyst based on iron nanoparticles entrapped in silicalite-1 used for the Fischer-Tropsch reaction is presented to illustrate the use of the cell.

Evaluation of HKUST-1 as Volatile Organic Compound Adsorbents for Respiratory Filters

Cyclohexane is a representative of volatile organic compounds (VOCs). VOCs can cause serious health problems in case of continuous exposure; therefore, it is essential to develop efficient personal protective equipment. Historically, activated carbons are used as VOC adsorbents. However, the emergence of promising novel adsorbents, such as metal-organic frameworks, has pushed the research to study their behavior under the same conditions. In this work, the use of the well-known HKUST-1 MOF of different particle sizes (20 μm, 300-600 μm, and 1-1.18 mm) for the adsorption of low-grade (5000 ppm) cyclohexane combined with different water concentrations (dry, 27 and 80% RH) in a fixed bed is proposed. The results were compared under the same conditions for a typically used activated carbon, PICACTIF TA 60. HKUST-1 has higher affinity to cyclohexane than PICACTIF for the whole pressure range studied, especially at low partial pressures. It begins to adsorb much earlier (0.0025 kPa) than the activated carbon (0.01 kPa). However, a different adsorption behavior is evidenced for both materials in the presence of water vapor since HKUST-1 is very hydrophilic in the zone near to the copper open metal sites, whereas PICACTIF is hydrophobic. After three consecutive cycles, good stability results were obtained for the MOF, comparable to activated carbon, even in the presence of water. As the main finding, although the unstability of HKUST-1 is well established under high humid conditions, the kinetic of degradation has not been established so far. Here, it is shown that the time usage of HKUST-1 as the adsorbent for respiratory mask (single pass) is not affected by the degradation of the structure, which may occur on a longer time scale. Finally, shaping by tableting provides good results since it is possible to increase the MOF density by around 69% with minor loss of adsorption capacity. The best fraction is 300-600 μm, reaching cyclohexane breakthrough times around 85 min/cm³ at 80% RH, comparable with PICACTIF-activated carbon and promising for practical applications.

Controlled grafting of dialkylphosphonate-based ionic liquids on γ-alumina: design of hybrid materials with high potential for CO2 separation applications

In this work we provide a detailed study on grafting reactions of various dialkylphosphonate-based ILs. Special attention has been devoted to a comprehensive investigation on how the nature of the anion and the organic spacer composition (hydrophilic or hydrophobic groups) could impact the grafting densities and bonding modes of phosphonate-based ILs anchored to γ-alumina (γ-Al₂O₃) powders. For the first time, the bonding of phosphonate-based ILs with only surface hexacoordinated aluminum nuclei was established using both solid-state ³¹P–²⁷Al D-HMQC and ³¹P NMR experiments. It has been demonstrated that the grafting of dialkylphosphonate-based ILs is competing with a hydrolysis and/or precipitation process which could be attractively hindered by changing the anion nature: bis(trifluoromethane)sulfonylimide anion instead of bromide. In additon, independently of the chosen spacer, similar reaction conditions led to equivalent grafting densities with different bonding mode configurations. The CO₂ physisorption analysis on both pure ILs and grafted ILs on alumina powders confirmed that the initial sorption properties of ILs do not change upon grafting, thus confirming the attractive potential of as-grafted ILs for the preparation of hybrid materials in a form of selective adsorbers or membranes for CO₂ separation applications.

Grafting of diethylphophonate-based ILs onto γ-Al₂O₃ powder in solvothermal condition was achieved on mesoporous γ-alumina powder and membrane (A = organic spacer).

Quantitative structure-property relationship approach to predicting xylene separation with diverse exchanged faujasites

Streamlining the xylene separation process on faujasites is a promising way to design innovative adsorbents for this application. For this purpose, we present herein an original quantitative structure-property relationship (QSPR) approach. It deals with the development of a multi-linear predictive model correlating the separation properties with a set of structural descriptors for the adsorbents. The implementation of such an approach makes it necessary to (i) set an appropriate design of experiment (DOE), (ii) prepare an adsorbent database, (iii) test the adsorbent database for xylene separation and (iv) compute a set of relevant descriptors. The selected descriptors essentially characterize the nature of the confinement in the faujasite supercage, i.e., the size of the cations localized in adsorption sites II, as well as the occupancy ratio of both adsorption sites II and III. Two different statistical methods were applied to develop a structure-property relationship model linking experimental selectivity and the set of descriptors. A multiple linear regression model enables the prediction of para/meta-xylene selectivity with a correlation coefficient R2 of 0.78, while a linear discriminant analysis predicts the assignment of the adsorbents to four identified classes with a total prediction percentage of 76%.

Monovalent and bivalent cations exchange isotherms for faujasites X and Y

This study addresses the modeling of exchange isotherms for faujasite-type zeolites X and Y with K⁺, Cs⁺, Ca²⁺ and Ba²⁺ cations based on a large experimental dataset obtained under operating conditions of 0.5 N total normality and an exchange temperature of 80 °C.

Systematic study of the impact of MOF densification into tablets on textural and mechanical properties

Densification process of MOF powders (HKUST-1, UiO-66, UiO-66-NH₂, and UiO-67) into mechanically resistant pellets with maintained microporosity and enhanced volumetric uptake.

Origin of highly active metal–organic framework catalysts: defects? Defects!

This article provides a comprehensive review of the nature of catalytic sites in MOFs.

Determination of oxygen adsorption–desorption rates and diffusion rate coefficients in perovskites at different oxygen partial pressures by a microkinetic approach

We show a generic method for the measurement of oxygen surface exchange and diffusion parameters from powders allowing the prediction of oxygen flux in membrane configuration.