Local Porosity Measurement From Scanning Electron Microscopy Images in the Backscattered Electrons Mode.

We propose a method to measure the local porosity of porous samples from scanning electron microscopy images in the backscattered electron mode. The porous samples are impregnated with a polymer resin and observed in polished cross sections. Image intensities are calibrated with intensities from pure resin and the bulk phase. The calibration model is justified with Monte Carlo simulations on perfectly homogeneous virtual samples. Uncertainties in measured porosity are given as a function of uncertainties on physical properties of the resin and the bulk phase and on measured signals. The methodology is applied to a series of heterogeneous alumina catalyst supports with varying porosities. A good agreement is found between the averaged local porosity by scanning electron microscopy and global porosity determined by mercury intrusion porosimetry. The use of local porosity statistics allowed the quantitative characterization of the porosity fluctuations of these supports that appeared to be linked with their preparation parameters.

High-power tunable low-noise coherent source at 1.06 µm based on a surface-emitting semiconductor laser.

Exploiting III-V semiconductor technologies, vertical external-cavity surface-emitting laser (VECSEL) technology has been identified for years as a good candidate to develop lasers with high power, large coherence, and broad tunability. Combined with fiber amplification technology, tunable single-frequency lasers can be flexibly boosted to a power level of several tens of watts. Here, we demonstrate a high-power, single-frequency, and broadly tunable laser based on VECSEL technology. This device emits in the near-infrared around 1.06 µm and exhibits high output power (>100 mW) with a low-divergence diffraction-limited TEM00 beam. It also features a narrow free-running linewidth of <400 kHz with high spectral purity (side mode suppression ratio >55 dB) and continuous broadband tunability greater than 250 GHz (<15 V piezo voltage, 6 kHz cutoff frequency) with a total tunable range up to 3 THz. In addition, a compact design without any movable intracavity elements offers a robust single-frequency regime. Through fiber amplification, a tunable single-frequency laser is achieved at an output power of 50 W covering the wavelength range from 1057 to 1066 nm. Excess intensity noise brought on by the amplification stage is in good agreement with a theoretical model. A low relative intensity noise value of -145 dBc/Hz is obtained at 1 MHz, and we reach the shot-noise limit above 200 MHz.

Tailoring metal-organic framework catalysts by click chemistry.

We successively introduce new catalytic centers through click reaction into MOFs and modify their environment by addition of lipophilic groups. The resulting bifunctionalized MOF provides an optimized balance between basicity and lipophilicity and shows outstanding performance for the transesterification reaction.

Catalysis of transesterification by a nonfunctionalized metal-organic framework: acido-basicity at the external surface of ZIF-8 probed by FTIR and ab initio calculations.

The zeolite imidazolate framework ZIF-8 is shown for the first time to be able to catalyze transesterification of vegetable oil with significant activity. Rationalization of this behavior at the atomic scale is provided by combining CO adsorption monitored by FTIR and DFT calculations (clusters and periodic models). We demonstrate that the acido-basic sites are located at the external surface of the material or at defects, but not in the microporosity of ZIF-8. A great variety of sites are found the surface: OH and NH groups, hydrogenocarbonates, low-coordinated Zn atoms, and free N(-) moieties belonging to linkers. Their proportions depend on the operating conditions (temperature and pressure). The acido-basicity of the surface is then probed by adsorption of CO at low temperature. In parallel, the species present are mapped by DFT calculations combined with a thermodynamic model. An assignment of the CO region of the FTIR spectra can thus be proposed. The complex infrared spectrum is attributed to the coexistence of classical C-adducts of CO with acid sites and other modes on basic sites (O-adducts and side-on adducts). Adsorption energies and CO frequency shifts show that some strong Lewis sites exist (in particular Zn(II) species), as well as strong Brønsted acid sites (NH groups), together with basic sites (OH groups and N(-) moieties). By calculating the co-adsorption of a model ester (methyl acetate) and methanol, we show the prevailing role of Zn(II) species as acid sites, combined with N(-) moieties and OH groups as basic ones, in determining the catalytic properties of ZIF-8. This work opens new perspectives on the use of MOFs in catalysis and, more generally, on the properties of their external surface.

Generic postfunctionalization route from amino-derived metal-organic frameworks.

This study deals with the development of a soft, generic, one-pot postfunctionalization method for metal-organic frameworks (MOFs) starting from compounds with an amino group on the linker. The first step consists of transforming the amino group into azide (N(3)) by an unconventional route using tBuONO and TMSN(3). In the same vessel, the desired functionalized MOF then is obtained by the Huisgen 1,3-dipolar cycloaddition of azides to alkynes, otherwise known as the "click" reaction. The method was applied to DMOF-NH(2) and MIL-68(In)-NH(2), which represent two distinct and important classes of MOF. For both, the functionalization was complete (>90% grafting) and the crystallinity was maintained. Thanks to the large diversity and availability of cyano- and acetylene-based chemicals, this method opens the door to tailor-made functionalized MOFs.