Anton-Jan Bons

High-flux MFI membranes

The synthesis and evaluation of high performance MFI-type membranes is described. These systems exhibit fluxes that are one to two orders of magnitude higher than previous literature reports, with ...

Super‐Resolution Reactivity Mapping of Nanostructured Catalyst Particles

For almost a century, heterogeneous catalysts have been at the heart of countless industrial chemical processes, but their operation at the molecular level is generally much less understood than that of homogeneous catalysts or enzymes. The principal reason is that despite the macroscopic dimensions of solid catalyst particles, their activity seems to be governed by compositional heterogeneities and structural features at the nanoscale. Progress in understanding heterogeneous catalysis thus requires that the nanoscale compositional and structural data be linked with local catalytic activity data, recorded in the same small spatial domains and under in situ reaction conditions. Light microscopy is a recent addition to the toolbox for in situ study of solid catalytic materials. It combines high temporal resolution and sensitivity with considerable specificity in distinguishing reaction products from reagents. However, lens-based microscopes are subjected to light diffraction which limits the optical resolution to 250 nm in the image plane. This resolution is far too limited to resolve the nanosized domains on solid catalysts. Nanometer-accurate localization of single emitters can be achieved by fitting a Gaussian distribution function to the intensity of the observed fluorescence spot (point-spread function, PSF). This method has been used to map out diffusion pathways in mesoporous or clay materials under highly dilute conditions. However, for more concentrated systems, several molecules simultaneously located within a diffraction-limited area cannot be distinguished. Separating the emission of the different fluorescent labels in time, for example by selective photoactivation, solves the problem for imaging of static systems, 13–18] but not when looking at the dynamics of a working catalyst. Herein, we used single catalytic conversions of small fluorogenic reactants, which occurred stochastically on the densely packed active sites of the catalyst, to reconstruct diffraction-unlimited reactivity maps of catalyst particles. As successive catalytic reactions do not overlap in time, one can precisely determine the location of reaction sites that show turnovers at different moments in time, even if the distance between them is only 10 nm (or less, depending on the signal-to-noise ratio), and reconstruct images of catalytically active zones with super-resolution. Although fluorogenic substrates are widely used in singlemolecule enzymology, so far only a few studies have reported single-turnover counting using fluorescence microscopy on solid chemocatalysts. 24, 25] Such studies typically use large polycyclic substrates, which cannot enter the micropores of many heterogeneous catalysts. Hence, similar experiments on microporous materials critically depend on identifying a small reagent that is converted to a product detectable at the single-molecule level. Surprisingly, furfuryl alcohol is such a reagent, and it appears that after acid-catalyzed reaction (see the Supporting Information), the pore-entrapped products are sufficiently fluorescent to be individually observed using a standard microscope equipped with a single excitation source (532 nm diode laser) and sensitive CCD camera (for experimental details, see the Supporting Information). We refer to this novel high-resolution reconstruction method based on catalytic conversion of fluorogenic substrates as NASCA microscopy, or nanometer accuracy by stochastic catalytic reactions microscopy. Figure 1a and b show the concept of NASCA microscopy and a 2D fluorescence intensity image of individual product molecules formed by an acid zeolite crystal, respectively. The fluorescence intensity plot of Figure 1c proves how well the intensity of the individual product molecules allows them to be distinguished from background signals, caused by scatter[*] Dr. M. B. J. Roeffaers, Dr. P. Dedecker, Prof. Dr. J. Hofkens Department of Chemistry, Katholieke Universiteit Leuven Celestijnenlaan 200F, 3001 Heverlee (Belgium) Fax: (+ 32)163-2799 E-mail: johan.hofkens@chem.kuleuven.be

The development of oblique preferred orientations in zeolite films and membranes

We propose a model that can explain the development of 101 and other oblique crystallographic preferred orientations in thin MFI-type zeolite films and membranes. The model is a refinement of the competitive growth model. In the classic competitive growth model the preferred orientation is determined by the fastest growth direction of the crystals. We show that the lateral growth component also plays a role. By the combination of outward growth and lateral growth, those crystals that have their fastest growth direction at an angle to the normal to the membrane will dominate in the early stages of growth, and an oblique orientation develops. With prolonged growth the crystals with the fastest growth direction perpendicular to the membrane will dominate and the classic competitive growth model applies again. This concept has been verified using two-dimensional growth simulations. The new model also applies to other types of thin films.

Synthesis and characterization of zoned MFI films by seeded growth

Supported zoned and sandwiched MFI films were prepared by a two-step crystallization procedure, using seeds. In this work, a zoned MFI film is defined as one assembled by crystals propagating from the support to the film top surface with varying Al content along the length of the crystal. A sandwiched MFI film is referred to as one assembled by at least two layers of crystals. Six types of films were prepared, both zoned and sandwiched, with a high or a low Al-content in the ZSM-5 part and with varying order of the layers, i.e. ZSM-5 coated with silicalite-1 or vice versa. The films were characterized by SEM and TEM. The Al-distribution was measured by cross-sectional EDS, and the preferred orientation of the crystals could be determined by XRD. Truly zoned films are obtained when the compositional difference between the layers is relatively small, and the synthesis conditions are similar or when the first layer is silicalite-1. If the first layer is ZSM-5 and the synthesis conditions and/or the composition vary too much, a discontinuity occurs at the interface between the layers, and sandwiched film results, where nucleation of the second layer is initiated by secondary nucleation or by applying seeds.

Binder Effects in SiO2- and Al2O3-Bound Zeolite ZSM-5-Based Extrudates as Studied by Microspectroscopy

Microspectroscopic methods were explored to investigate binder effects occurring in ZSM‐5‐containing SiO2‐ and Al2O3‐bound millimetre‐sized extrudates. Using thiophene as a selective probe for Brønsted acidity, coupled with time‐resolved in situ UV/Vis and confocal fluorescence microspectroscopy, variations in reactivity and selectivity between the two distinct binder types were established. It was found that aluminium migration occurs in ZSM‐5‐containing Al2O3‐bound extrudates, forming additional Brønsted acid sites. These sites strongly influence the oligomer selectivity, favouring the formation of thiol‐like species (i.e., ring‐opened species) in contrast to higher oligomers, predominantly formed on SiO2‐bound ZSM‐5‐containing extrudates. Not only were the location and distribution of these oligomers visualised by 3 D analysis, it was also observed that more conjugated species appeared to grow off the surface of the zeolite ZSM‐5 crystals (containing less conjugated species) into the surrounding binder material. Furthermore, a higher binder content resulted in an increasing overall reactivity owing to the greater number of stored thiophene monomers available per Brønsted acid site.

Automated discrete electron tomography – Towards routine high-fidelity reconstruction of nanomaterials

Electron tomography is an essential imaging technique for the investigation of morphology and 3D structure of nanomaterials. This method, however, suffers from well-known missing wedge artifacts due to a restricted tilt range, which limits the objectiveness, repeatability and efficiency of quantitative structural analysis. Discrete tomography represents one of the promising reconstruction techniques for materials science, potentially capable of delivering higher fidelity reconstructions by exploiting the prior knowledge of the limited number of material compositions in a specimen. However, the application of discrete tomography to practical datasets remains a difficult task due to the underlying challenging mathematical problem. In practice, it is often hard to obtain consistent reconstructions from experimental datasets. In addition, numerous parameters need to be tuned manually, which can lead to bias and non-repeatability. In this paper, we present the application of a new iterative reconstruction technique, named TVR-DART, for discrete electron tomography. The technique is capable of consistently delivering reconstructions with significantly reduced missing wedge artifacts for a variety of challenging data and imaging conditions, and can automatically estimate its key parameters. We describe the principles of the technique and apply it to datasets from three different types of samples acquired under diverse imaging modes. By further reducing the available tilt range and number of projections, we show that the proposed technique can still produce consistent reconstructions with minimized missing wedge artifacts. This new development promises to provide the electron microscopy community with an easy-to-use and robust tool for high-fidelity 3D characterization of nanomaterials.

Operando micro-spectroscopy on ZSM-5 containing extrudates during the oligomerization of 1-hexene

The influence of the binder material in an industrial-type catalyst material is often neglected, although the addition of a binder can cause a significant change in the performance of the catalyst. It is difficult to visualize the effects of the different components in these multi-complex materials, and therefore, high spatiotemporal resolution techniques need to be employed. In this work, two complementary micro-spectroscopic techniques; operando UV-vis diffuse reflectance micro-spectroscopy (coupled to on-line mass spectrometry), and in situ confocal fluorescence microscopy were used to investigate the 1-hexene oligomerization reaction. The reaction was performed on both Al2O3- and SiO2-bound ZSM-5-containing extrudates at 250 °C and 300 °C. By employing operando UV-vis micro-spectroscopy, coupled with on-line mass spectrometry, Al2O3-bound catalysts were found to form larger reaction products, as well as more and larger hydrocarbon deposits, compared to the SiO2-bound catalysts. Furthermore, the extrudate containing Al2O3 deactivated slower than the extrudate containing SiO2 binder. Time-resolved chemical maps of the location of the reaction products were visualized using in situ confocal fluorescence microscopy. The maps show that, after reaction, the zeolite crystals contain different coke species than the Al2O3 binder.