Ionic Covalent Organic Framework as a Dual Functional Sensor for Temperature and Humidity.

Visual sensing of humidity and temperature by solids plays an important role in the everyday life and in industrial processes. Due to their hydrophobic nature, most covalent organic framework (COF) sensors often exhibit poor optical response when exposed to moisture. To overcome this challenge, the optical response is set out to improve, to moisture by incorporating H-bonding ionic functionalities into the COF network. A highly sensitive COF, consisting of guanidinium and diformylpyridine linkers (TG-DFP), capable of detecting changes in temperature and moisture content is fabricated. The hydrophilic nature of the framework enables enhanced water uptake, allowing the trapped water molecules to form a large number of hydrogen bonds. Despite the presence of non-emissive building blocks, the H-bonds restrict internal bond rotation within the COF, leading to reversible fluorescence and solid-state optical hydrochromism in response to relative humidity and temperature.

Study of the diffusion properties of zeolite mixtures by combined gravimetric analysis, IR spectroscopy and inversion methods (IRIS).

We report the development of a new method of investigation of the mass transport properties of acidic zeolite-based materials aiming to overcome the limitations of classical approaches. It consists in hyphenating gravimetric analysis and infrared spectroscopy. The former allows assessing the diffusion from the gas phase to all the porosity, while IR allows for selective assessment of diffusion to the zeolite active sites located in the micropores. Furthermore, the data are processed by an original methodology allowing the recovery of the distribution of diffusion domains by inversion of the integral equations describing the uptake curves or the evolution of the infrared spectra. The combination of gravimetric analysis and IR spectroscopy makes it possible to monitor and distinguish diffusion within the various components of the material. The methodology has been applied to the isooctane uptake in the mechanical mixture of FAU and MFI zeolites. Analysis of both gravimetric uptake curves and evolving infrared spectra allows distinguishing and assigning diffusion domains to the H-FAU and H-MFI components of the mixture, with high and low effective diffusion rate constants, respectively. The advantages and limits of the methodology are discussed.

Impact of the Si/Al ratio on the ethanol/water coadsorption on MFI zeolites revealed using original quantitative IR approaches.

Advanced IR vibrational spectroscopic techniques, e.g., using a coupled gravimetric-IR surface analyzer (AGIR) and a high-throughput in situ IR cell (Carroucell), have been used for the quantitative studies of the adsorption and coadsorption of ethanol and water on MFI zeolites with different Si/Al ratios. The AGIR coupling is a powerful tool for the accurate determination of the molar adsorption coefficients during coadsorption experiments since their evaluation is based on the measurement of the exact amount of adsorbed species. The use of the Carroucell set up allows characterizing all the samples simultaneously, strictly in the same gaseous and temperature environment. The molar absorption coefficients of pure adsorbed ethanol and water are determined: their values are constant whatever the Si/Al ratio of the MFI zeolites. Moreover, these coefficients are found to be identical in the case of the water-ethanol coadsorption experiments. Their use allows obtaining the exact quantity of each adsorbate specie in the binary system. At low partial pressures, the unary water adsorption experiments suggest that the amount of adsorbed water results mainly from the preferential adsorption on Brønsted acid sites in tetrameric clusters. In contrast, the adsorption of EtOH occurs on both silanol groups and Brønsted acid sites (BASs). The effect of the Si/Al ratio is only observed at relatively low partial pressures. The effect of the Si/Al ratio on the ethanol adsorption capacity is also investigated. This study directs the choice of an appropriate zeolite once it is used in membranes for drying ethanol.

Operando Reactor-Cell with Simultaneous Transmission FTIR and Raman Characterization (IRRaman) for the Study of Gas-Phase Reactions with Solid Catalysts.

Raman and transmission FTIR spectroscopic techniques have been coupled in a new homemade reactor-cell designed in a joint CSIC-LCS collaboration. The setup is easily adapted to any FTIR and fiber-coupled Raman spectrometers and gas analysis techniques. It allows for simultaneous operando FTIR and Raman spectroscopic measurement, which provide complementary characterization of adsorbed species, reaction intermediates, and structural properties of the catalyst. This system was validated with the study of vanadium-based catalysts during propane oxydehydrogenation (ODH). The combined use of both spectroscopies with gas analysis techniques to measure the activity contributes to the understanding of propane ODH and the identification of the role of different oxygen species bound to vanadium sites. For example, the simultaneous characterization of the catalyst under the same conditions by IR and Raman confirms that the V═O mode has the same frequency in both spectroscopies and that bridging oxygen sites (V-O-V, V-O-Zr) present higher activity than terminal V═O bonds. These results demonstrate the high potential of the new simultaneous transmission IR-Raman operando rig to correlate the activity and the structure of catalysts, thus assisting the rational design of catalytic processes.

Coupling a Rapid-Scan FT-IR Spectrometer with Quantum Cascade Lasers within a Single Setup: An Easy Way to Reach Microsecond Time Resolution without Losing Spectral Information.

For the first time, a standard rapid-scan Fourier-transform infrared (FT-IR) spectrometer was coupled with quantum cascade lasers (QCLs) tunable within the 1876-905 cm-1 spectral range, within one single setup, by keeping one single sample compartment. The aim was to extend the time resolution of absorption measurements by several orders of magnitude thanks to the fast pulsed QCL technology without losing the spectral information provided by standard FT-IR spectroscopy, both probing the same sample. By slightly modifying the optical bench arrangement, the spectrometer now enables a fast and easy switch between the standard FT-IR mode, used for classical broadband scans from 6000 to 650 cm-1, and the new QCL-irradiation mode, used for ultrafast recording at specific wavenumbers (the two diagnostics have superimposed beam paths). So, one can study a sample (in condensed or gaseous state) during a physical or chemical transformation first as a whole in a broadband configuration and then immediately switch to the QCL mode to monitor a selected absorption feature (associated with an intermediate, a structural change, a diffusing substance, etc., for example) versus time. The QCL mode then drastically boosts the time resolution from tens of milliseconds (in rapid-scan FT-IR) to a few microseconds, as demonstrated here in the case of ammonia diffusion into a commercial zeolite ZSM-5.

In Situ FTIR Reactor for Monitoring Gas-Phase Products during a (Photo)catalytic Reaction in the Liquid Phase.

Various catalytic and photocatalytic reactions in the liquid phase give rise to gas products. Therefore, the identification and quantification of these products are of high importance and are even essential for some reactions. In this paper, a new in situ FTIR reactor is designed and used for analyzing the gas headspace of a (photo)catalytic reaction in solution. It allows the identification and quantification of the gas-phase products of a liquid reaction under operating conditions and in real time. The new reactor has been tested in three representative photocatalytic reactions widely studied as model reactions in the liquid phase: i.e., (i) decomposition of formic acid, (ii) oxidation of methylene blue, and (iii) reduction of CO2. The validity of the results has been confirmed by analyzing the headspace at the end of the reaction using gas chromatography technique. The new reactor opens the possibility to follow online the (photo)catalyst activity. This is useful for ensuring the stability of the catalyst and studying the evolution of the selectivity during the reaction. The nondestructive behavior of the FTIR technique allows its coupling with other techniques for obtaining complementary results. The new reactor setup is easy to handle and to ship and is very efficient, which makes it very suitable for performing complementary, fast and/or preliminary studies.

Combining Multiscale Approaches for the Structure Determination of an Iron Layered Oxysulfate: Sr4Fe2.5O7.25(SO4)0.5.

The new iron layered oxysulfate Sr4Fe2.5O7.25(SO4)0.5 has been prepared by a solid-state reaction in closed ampules into the form of ceramics and single crystals. Its atomic structure has been solved by means of spectroscopy, diffraction techniques, and high-resolution electron microscopy. Sr4Fe2.5O7.25(SO4)0.5 is a layered structure that derives from the Ruddelsden-Popper (RP) phases with the layer stacking sequence SrO/SrFeO2.5/SrFe0.5(SO4)0.5O1.25/SrFeO2.5. Within the mixed Fe3+/SO42- layer, the sulfur atoms are slightly shifted from the B site of the perovskite and each sulfate group shares two corners with iron pyramids in the basal plan without any order phenomenon. The electronic conductivity is thermally activated, while no ionic conductivity is detected.

Nanosized Na-EMT and Li-EMT zeolites: selective sorption of water and methanol studied by a combined IR and TG approach.

Nanosized EMT-type zeolite crystals in sodium (Na-EMT) and ion-exchanged lithium (Li-EMT) forms were prepared. The sorption behavior of Li(Na)-EMT samples towards water, methanol and a mixture of both (50 : 50) was studied by combined thermogravimetric and infrared spectroscopic methods. The stability of the samples prior to and after the sorption measurements in two subsequent cycles was confirmed by X-ray diffraction, N2 sorption and NMR spectroscopy. The high sorption capacity of the Li-EMT sample towards water was demonstrated. It was found that the methanol is replaced by water faster in the Li-EMT sample in comparison to the Na-EMT sample. At low temperature, the methanol shows weak adsorption on each cationic site and no side products during desorption for both samples were obtained.

On the mechanism of methanol photooxidation to methylformate and carbon dioxide on TiO2: an operando-FTIR study.

This work is a mechanistic study of total and partial methanol photooxidation using operando FTIR coupled to gas phase analysis techniques (gas-IR and MS). Methoxy and formate/formyl species play a key role in the reaction. Methoxy species are formed by thermal and photochemical dissociation of methanol. The formation of methylformate is favored by a high surface coverage by methoxy species. Surface and/or bridged oxygen atoms are also important actors. Steady State Isotopic Transient Kinetic Analysis (SSITKA) experiments showed that the limiting step is the conversion of chemisorbed formyl/formate and that methylformate is a secondary product from a reaction between methoxy and neighboring formyl species. Methanol concentration, among other reaction parameters, influences greatly the selectivity of photooxidation.

Room Temperature Reduction of Nitrogen Oxide on Iron Metal-Organic Frameworks.

Nitrogen oxides represent one of the main threats for the environment. Despite decades of intensive research efforts, a sustainable solution for NOx removal under environmental conditions is still undefined. Using theoretical modelling, material design, state-of-the-art investigation methods and mimicking enzymes, it is found that selected porous hybrid iron(II/III) based MOF material are able to decompose NOx, at room temperature, in the presence of water and oxygen, into N2 and O2 and without reducing agents. This paves the way to the development of new highly sustainable heterogeneous catalysts to improve air quality.

Speciation of adsorbed CO2 on metal oxides by a new 2-dimensional approach: 2D infrared inversion spectroscopy (2D IRIS).

A new methodology based on the inversion of adsorption isotherms obtained using infrared spectroscopy has been developed. It provides a description of coexisting surface species in terms of their individual IR spectra and surface affinities in a new two dimensional, 2D IR spectroscopic technique. When implemented with simultaneous gravimetric analysis, it further provides the quantification of adsorbed species. The adsorption of CO2 on monoclinic ZrO2 was investigated using this technique with temperature and pressure ranges of 353-673 K and 10(-4)-0.4 bar, respectively. The sets of spectra obtained at constant temperature and variable pressures (spectroscopic isotherms) were inverted assuming they obey a generalized Langmuir isotherm. This procedure yields a 2D map in which the IR spectra of the prominent surface species formed upon CO2 adsorption are resolved in one dimension - hydrogen carbonates, bidentate carbonates and polydentate carbonates - while these species are resolved according to their surface adsorption affinities (logarithm of adsorption equilibrium constants, ln K) on the other dimension. This technique also allows for the unambiguous determination of the thermodynamic stabilities of the various adsorbed species. The inversion of the gravimetric isotherms recorded simultaneously with the infrared spectra leads to a quantitative distribution function of CO2 adsorption sites whose components match those of the 2D infrared map and allows for a straightforward quantification of the corresponding sites, namely (i) weakly basic sites leading to bridged carbonates, hydrogen carbonates and bidentate carbonates (~0.7 µmol m(-2), Δ(ads)H = -70 to 90 kJ mol(-1)), (ii) mild basic sites leading to a second type of bidentate carbonates (~0.8 µmol m(-2), Δ(ads)H = -110 to 120 kJ mol(-1)) and (iii) strong basic sites leading to polydentate carbonate species (~0.1 µmol m(-2), Δ(ads)H < -120 kJ mol(-1)). Finally, the advantages and limitations of the present methodology are discussed. Because this technique is not limited to a particular spectroscopy or physical process, it should find other applications in the field of spectroscopic characterization of surfaces.

Interplay between alkali-metal cations and silanol sites in nanosized CHA zeolite and implications for CO2 adsorption.

Silanols are key players in the application performance of zeolites, yet, their localization and hydrogen bonding strength need more studies. The effects of post-synthetic ion exchange on nanosized chabazite (CHA), focusing on the formation of silanols, were studied. The significant alteration of the silanols of the chabazite nanozeolite upon ion exchange and their effect on the CO2 adsorption capacity was revealed by solid-state nuclear magnetic resonance (NMR), Fourier-transform infrared (FTIR) spectroscopy, and periodic density functional theory (DFT) calculations. Both theoretical and experimental results revealed changing the ratio of extra-framework cations in CHA zeolites changes the population of silanols; decreasing the Cs+/K+ ratio creates more silanols. Upon adsorption of CO2, the distribution and strength of the silanols also changed with increased hydrogen bonding, thus revealing an interaction of silanols with CO2 molecules. To the best of our knowledge, this is the first evidence of the interplay between alkali-metal cations and silanols in nanosized CHA.

How water fosters a remarkable 5-fold increase in low-pressure CO2 uptake within mesoporous MIL-100(Fe).

The uptake and adsorption enthalpy of carbon dioxide at 0.2 bar have been studied in three different topical porous MOF samples, HKUST-1, UiO-66(Zr), and MIL-100(Fe), after having been pre-equilibrated under different relative humidities (3, 10, 20, 40%) of water vapor. If in the case of microporous UiO-66, CO(2) uptake remained similar whatever the relative humidity, and correlations were difficult for microporous HKUST-1 due to its relative instability toward water vapor. In the case of MIL-100(Fe), a remarkable 5-fold increase in CO(2) uptake was observed with increasing RH, up to 105 mg g(-1) CO(2) at 40% RH, in parallel with a large decrease in enthalpy measured. Cycling measurements show slight differences for the initial three cycles and complete reversibility with further cycles. These results suggest an enhanced solubility of CO(2) in the water-filled mesopores of MIL-100(Fe).

Discovering the active sites for C3 separation in MIL-100(Fe) by using operando IR spectroscopy.

A reducible MIL-100(Fe) metal-organic framework (MOF) was investigated for the separation of a propane/propene mixture. An operando methodology was applied (for the first time in the case of a MOF) in order to shed light on the separation mechanism. Breakthrough curves were obtained as in traditional separation column experiments, but monitoring the material surface online, thus providing evidences on the adsorption sites. The qualitative and quantitative analyses of Fe(II) and, to some extent, Fe(III) sites were possible, upon different activation protocols. Moreover, it was possible to identify the nature and the role of the active sites in the separation process by selective poisoning of one family of sites: it was clearly evidenced that the unsaturated Fe(II) sites are mainly responsible for the separation effect of the propane/propene mixture, thanks to their affinity for the unsaturated bonds, such as the C=C entities in propene. The activity of the highly concentrated Fe(III) sites was also highlighted.

Monitoring catalysts at work in their final form: spectroscopic investigations on a monolithic catalyst.

A monolithic vanadia-titania based catalyst has been subjected to studies with in situ FTIR spectroscopy coupled with mass spectrometry, during the SCR (Selective Catalytic Reduction) reaction. A device based on a transmission reactor cell for monolithic samples was constructed, dedicated to the study of surface species during reaction. After analysing the steady state SCR activity under industrially relevant conditions, NH(3) chemisorption phenomena as a function of temperature and the subsequent SCR reaction of NO + O(2) with chemisorbed ammonia and ammonium ion species were also investigated. The observations reported here serve as a demonstration of the great potential for the application of operando spectroscopy on monolithic systems. This cross disciplinary approach aims to identify reaction pathways, active sites, intermediate- and spectator-species for catalytic reactions under truly industrial conditions in a shaped monolithic catalyst based on monitoring chemical profiles along its channels. In particular, by demonstrating the feasibility of the approach using the technically challenging operando transmission FTIR spectroscopy methodology, we foresee easy future adaption of this approach with other surface or bulk sensitive techniques, e.g. Raman and UV-vis spectroscopy.

The porosity, acidity, and reactivity of dealuminated zeolite ZSM-5 at the single particle level: the influence of the zeolite architecture.

A combination of atomic force microscopy (AFM), high-resolution scanning electron microscopy (HR-SEM), focused-ion-beam scanning electron microscopy (FIB-SEM), X-ray photoelectron spectroscopy (XPS), confocal fluorescence microscopy (CFM), and UV/Vis and synchrotron-based IR microspectroscopy was used to investigate the dealumination processes of zeolite ZSM-5 at the individual crystal level. It was shown that steaming has a significant impact on the porosity, acidity, and reactivity of the zeolite materials. The catalytic performance, tested by the styrene oligomerization and methanol-to-olefin reactions, led to the conclusion that mild steaming conditions resulted in greatly enhanced acidity and reactivity of dealuminated zeolite ZSM-5. Interestingly, only residual surface mesoporosity was generated in the mildly steamed ZSM-5 zeolite, leading to rapid crystal coloration and coking upon catalytic testing and indicating an enhanced deactivation of the zeolites. In contrast, harsh steaming conditions generated 5-50 nm mesopores, extensively improving the accessibility of the zeolites. However, severe dealumination decreased the strength of the Brønsted acid sites, causing a depletion of the overall acidity, which resulted in a major drop in catalytic activity.

In situ infrared molecular detection using palladium-containing zeolite films.

In situ IR detection of carbon monoxide in the presence of hydrocarbons (methanol and pentane) using Pd-containing zeolite thin films is reported. The thin films are prepared by spin coating deposition of nanosized LTL and BEA type zeolites suspensions; the palladium clusters are introduced in the nanosized zeolites by ion exchange followed by γ radiolysis of the coating suspensions. The Pd-containing zeolite films with a thickness of 200 nm are exposed to a single gas (either CO or hydrocarbons) or gas mixtures in the presence of water (100 ppm), and the IR spectra are collected continuously at 25, 75, and 100 °C. The fast recognition of very low concentrations of CO (2-100 ppm) in the presence of highly concentrated vapors of methanol or pentane (400-4000 ppm) with the Pd-containing zeolite films is demonstrated. The detection of CO and hydrocarbons is instant, which is a function of the low thickness of the films, small size of the individual zeolite crystals, and regular size and high stability of the Pd clusters in the zeolite films. The heat of adsorption for all experiments is similar (15 kJ.mol(-1)), which is explained with weak interactions between the carbon monoxide and palladium clusters in the zeolite films at temperatures below 100 °C. The nanosized zeolites with homogeneously distributed Pd clusters deposited in thin films demonstrate high molecular recognition capacity toward low concentrations of carbon monoxide under real environmental conditions, i.e., in the presence of water and hydrocarbons.

How to determine IR molar absorption coefficients of co-adsorbed species? Application to methanol adsorption for quantification of MgO basic sites.

Methanol adsorption on MgO samples with different morphologies is studied by infrared (IR) spectroscopy coupled to volumetry or thermogravimetry measurements to probe qualitatively and quantitatively the acid-base paired sites. The molar absorption coefficients of ν(OC) vibration of non-dissociated methanol, type I and type II methoxy species are determined by analyzing data obtained under specific adsorption conditions (ε(ND)=2.5 cm µmol(-1); ε(I)=ε(II)=6.1 cm µmol(-1)). Thanks to these results, the amounts of different adsorbed methanol species are evaluated. These various species are formed on surface sites presenting different topologies. Hence, the IR method is the only one allowing us to both discriminate and quantify the defects on the MgO surfaces, in terms of concentrations of convex and concave defective zones. This study reveals that sol-gel preparation leads to a MgO surface presenting a greater amount of concave defective zones than precipitated MgO.

Investigation of methanol oxidation over Au/catalysts using operando IR spectroscopy: determination of the active sites, intermediate/spectator species, and reaction mechanism.

FTIR spectroscopy coupled with mass spectrometry has been used to study the mechanism of methanol oxidation at low temperatures on nanostructured Au/CeO(2) and Au/TiO(2) catalysts. Activity and selectivity toward CO(2) have been investigated through simultaneous analysis of adsorbed surface species and gaseous species, and some key steps in the oxidation pathway, active sites, and intermediate species are proposed. Among the detected species, some kinds of methoxy species formed on the support were identified as intermediates, which further transform into formates whose oxidation was found to be the rate-determining step for the reaction. The role of the support and the noble metal in the mechanism are revealed using operando spectroscopy.

Selective elimination of phenol from hydrocarbons by zeolites and silica-based adsorbents-Impact of the textural and acidic properties.

This paper investigates the parameters that influence the selective adsorption of phenol, toxic molecule, from a semi-model biofuel mixture containing alkanes and different proportions of aromatic compounds. The adsorption capacity, selectivity and regeneration ability of different adsorbents, i.e. zeolites, silica-based solids, alumina and activated carbon, were related to their textural properties and the nature, strength or location of their acidic sites. This work demonstrates that phenol differently adsorbs in the micropores and mesopores. In the micropores of faujasites, phenol is condensed into the supercages. Otherwise, in the mesopores of the zeolite, phenol interacts with the silanol groups. On purely siliceous adsorbents, a ratio of one phenol adsorbed on one silanol group could be established. As for selectivity, the strong acidic sites of the faujasites are necessary to favor phenol adsorption compared to toluene. By contrast, the amount of strong Brønsted and Lewis acid sites limits regeneration. Hence, a compromise has to be found and the best performances were obtained using a slightly dealuminated zeolitic adsorbent presenting both micro and mesopores.