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.

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%.

Fast 'Operando' electron nanotomography.

Electron tomography in transmission electron microscopy provides valuable three-dimensional structural, morphological and chemical information of condensed matter at nanoscale. Current image acquisitions require at least tens of minutes, which prohibits the analysis of nano-objects evolving rapidly such as under dynamic environmental conditions. Reducing the acquisition duration to tens of seconds or less permits to follow in 3D the same object during its evolution under varying temperatures and pressures. We report Operando Electron nanotomography using image series acquired in less than 230 seconds instead of typically 15 min in the best cases so far. The in situ calcination of silica zeolites encaging silver nanoparticles, a catalytic nanosystem of potential interest for, e.g., nuclear waste treatments or selective heterogeneous catalysis, was successfully studied. Kinetic environmental Operando 3D electron microscopy becomes possible, as well as real time observation of beam sensitive samples (polymers, biological objects) without prior preparation, which reduces their contrast and reactivity.

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. The isotherm models are based on the mass action law. Ideal solution phase is assumed. Heterogeneity of the solid phase is taken into account by using Barrer and Klinowski's approach to multi-site exchange. Three types of exchange sites are identified on these zeolites. To each exchange site j corresponds a fitted selectivity coefficient Kj. These parameters, estimated by least square method, evaluate the affinity of the studied cations for the identified exchange site. Globally, these fitted coefficients show that the cations considered present better affinity than Na+, especially for type III sites in faujasite X and type II sites in faujasite Y. For bivalent cations, an exchange with Ba2+ is always more favorable than with Ca2+. On faujasite X, type II sites are more strongly preferred by monovalent cations (with the exception of Cs+) than by bivalent ones. The opposite trend is observed on faujasite Y, even for Cs+. These conclusions have been confirmed and are supported by bibliographic data.

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

This article provides a comprehensive review of the nature of catalytic sites in MOFs. In the last decade, a number of striking studies have reported outstanding catalytic activities of MOFs. In all cases, the authors were intrigued as it was unexpected from the ideal structure. We demonstrate here that (surface) defects are at the origin of the catalytic activities for the reported examples. The vacancy of ligands or linkers systematically generates (surface) terminations which can possibly show Lewis and/or Brønsted acido-basic features. The engineering of catalytic sites at the nodes by the creation of defects (on purpose) appears today as a rational approach for the design of active MOFs. Similarly to zeolite post-treatments, post-modifications of MOFs by linker or metal cation exchange appear to be methods of choice. Despite the mild acidity of defective MOFs, we can account for very active MOFs in a number of catalytic applications which show higher performances than zeolites or benchmark catalysts.

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

A novel, powerful method based on a microkinetic approach is described for the estimation of the oxygen transport parameters of mixed electronic conducting materials (MIECs). This method is validated on the perovskite La0.6Sr0.4Co0.2Fe0.8O3-δ and has been applied on Ba0.5Sr0.5Co0.8Fe0.2O3-δ. This approach is original and relevant in that the surface kinetic rate constants are measured using a sample in powder form. In contrast to methods previously used, such as isotope exchange depth profiling (IEDP) and electrical conductivity relaxation (ECR), which determine the global exchange kinetic parameter, our microkinetic modelling approach allows the estimation of the forward and reverse kinetic rates accounting for the oxygen vacancy concentration. Also, the self-diffusion rate coefficient has been estimated at different oxygen partial pressures. This microkinetic approach, which combines SSITKA (steady-state isotopic transient kinetic analysis) and thermogravimetric measurements at controlled oxygen partial pressure, has the potential to significantly accelerate the characterization of oxygen transport in perovskites and related materials in the future. In this study, the kinetic parameters were measured in a temperature window between 873 K and 1173 K, and at two oxygen pressure conditions (21 kPa and 1 kPa) that are appropriate for simulating the semi-permeability of oxygen in a membrane in a process of oxygen separation from air.