Maarten B. J. Roeffaers

Spatially resolved observation of crystal-face-dependent catalysis by single turnover counting

Catalytic processes on surfaces have long been studied by probing model reactions on single-crystal metal surfaces under high vacuum conditions. Yet the vast majority of industrial heterogeneous catalysis occurs at ambient or elevated pressures using complex materials with crystal faces, edges and defects differing in their catalytic activity. Clearly, if new or improved catalysts are to be rationally designed, we require quantitative correlations between surface features and catalytic activity—ideally obtained under realistic reaction conditions. Transmission electron microscopy and scanning tunnelling microscopy have allowed in situ characterization of catalyst surfaces with atomic resolution, but are limited by the need for low-pressure conditions and conductive surfaces, respectively. Sum frequency generation spectroscopy can identify vibrations of adsorbed reactants and products in both gaseous and condensed phases, but so far lacks sensitivity down to the single molecule level. Here we adapt real-time monitoring of the chemical transformation of individual organic molecules by fluorescence microscopy to monitor reactions catalysed by crystals of a layered double hydroxide immersed in reagent solution. By using a wide field microscope, we are able to map the spatial distribution of catalytic activity over the entire crystal by counting single turnover events. We find that ester hydrolysis proceeds on the lateral {1010} crystal faces, while transesterification occurs on the entire outer crystal surface. Because the method operates at ambient temperature and pressure and in a condensed phase, it can be applied to the growing number of liquid-phase industrial organic transformations to localize catalytic activity on and in inorganic solids. An exciting opportunity is the use of probe molecules with different size and functionality, which should provide insight into shape-selective or structure-sensitive catalysis and thus help with the rational design of new or more productive heterogeneous catalysts.

Interfacial synthesis of hollow metal–organic framework capsules demonstrating selective permeability

Metal–organic frameworks (MOFs) are a class of crystalline materials that consist of metal ions and organic ligands linked together by coordination bonds. Because of their porosity and the possibility of combining large surface areas with pore characteristics that can be tailored, these solids show great promise for a wide range of applications. Although most applications currently under investigation are based on powdered solids, developing synthetic methods to prepare defect-free MOF layers will also enable applications based on selective permeation. Here, we demonstrate how the intrinsically hybrid nature of MOFs enables the self-completing growth of thin MOF layers. Moreover, these layers can be shaped as hollow capsules that demonstrate selective permeability directly related to the micropore size of the MOF crystallites forming the capsule wall. Such capsules effectively entrap guest species, and, in the future, could be applied in the development of selective microreactors containing molecular catalysts. The intrinsically hybrid nature of metal–organic frameworks (MOFs) — microporous crystalline solids composed of metal ions and organic ligands — has been exploited to grow thin MOF films at the aqueous–organic interface of a biphasic reaction mixture. These materials exhibit selective permeability and can also be obtained as hollow capsules that have potential as microreactors.

Iron(III)-Based Metal–Organic Frameworks As Visible Light Photocatalysts

Herein, a new group of visible light photocatalysts is described. Iron(III) oxides could be promising visible light photocatalysts because of their small band gap enabling visible light excitation. However, the high electron-hole recombination rate limits the yield of highly oxidizing species. This can be overcome by reducing the particle dimensions. In this study, metal-organic frameworks (MOFs), containing Fe3-μ3-oxo clusters, are proposed as visible light photocatalysts. Their photocatalytic performance is tested and proven via the degradation of Rhodamine 6G in aqueous solution. For the first time, the remarkable photocatalytic efficiency of such Fe(III)-based MOFs under visible light illumination (350 up to 850 nm) is shown.

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

Fluorescence micro(spectro)scopy as a tool to study catalytic materials in action

Following its widespread use in biomedical research, fluorescence microscopy has recently been introduced in the catalysis field to study chemocatalytic processes with a high spatiotemporal resolution, a unique sensitivity down to the single molecule level and this under in situ conditions. This tutorial review is structured around the length scales that are currently accessible in fluorescence microscopy and discusses the different conceptual approaches that have been developed to study molecular concentration and dynamics like diffusion and catalytic conversion at these micron and sub-micron levels.

Single-molecule fluorescence spectroscopy in (bio)catalysis

The ever-improving time and space resolution and molecular detection sensitivity of fluorescence microscopy offer unique opportunities to deepen our insights into the function of chemical and biological catalysts. Because single-molecule microscopy allows for counting the turnover events one by one, one can map the distribution of the catalytic activities of different sites in solid heterogeneous catalysts, or one can study time-dependent activity fluctuations of individual sites in enzymes or chemical catalysts. By experimentally monitoring individuals rather than populations, the origin of complex behavior, e.g., in kinetics or in deactivation processes, can be successfully elucidated. Recent progress of temporal and spatial resolution in single-molecule fluorescence microscopy is discussed in light of its impact on catalytic assays. Key concepts are illustrated regarding the use of fluorescent reporters in catalytic reactions. Future challenges comprising the integration of other techniques, such as diffraction, scanning probe, or vibrational methods in single-molecule fluorescence spectroscopy are suggested.

High-Resolution Single-Turnover Mapping Reveals Intraparticle Diffusion Limitation in Ti-MCM-41-Catalyzed Epoxidation†

Microand mesoporous materials offer unique opportunities for catalysis thanks to their large surface area. By introducing active elements inside the pore walls of such materials, a wide range of acid–base or redox catalysts has been developed. For example, incorporation of Ti sites in silicalite resulted in the TS-1 catalyst, which is known for its high performance in the selective oxidation and epoxidation of hydrocarbons. However, the small (0.55 nm) micropores of this catalyst hinder the uptake of larger olefins as substrates for the epoxidation. To circumvent this limitation of TS-1, titanosilicates with larger pores, such as Ti-Beta and Ti-MWW zeolites, have been synthesized. Even mesoporous titanosilicates such as Ti-MCM-41 were developed with the aim of faster diffusion of more bulky substrates towards the inner active sites. 8] MCM-41 materials are characterized by a hexagonal array of pores with a uniform diameter that can be tuned between 1.5 and 10 nm. Despite the relatively large pore size, maximal utilization of the Ti sites in diffusion unlimited conditions remains a major challenge. Typically, TiMCM-41 is prepared in the form of particles with sizes of a few micrometers. It was recently demonstrated that a decrease in particle size to about 100 nm was accompanied with a relevant increase in selectivity and reaction rate for the epoxidation of cyclohexene and cholesterol. 11] It was reasoned that intraparticle diffusion limitations in the mesopores of the large particles hindered an optimal use of the active titanium sites, similarly as previously described for the microporous TS-1 catalyst. The kinetics of a catalytic process are often governed by the interplay between diffusion and reaction. Such insights are classically gathered by macroscopic kinetic experiments, for example, by comparing reaction rates using crystals with different sizes, by varying space velocities of the feed, or by measuring apparent activation energies. Pulsed-field gradient NMR spectroscopy has been used to determine intraparticle diffusion coefficients during catalysis, but this technique is restricted to extremely large particles (> 10 mm) and only yields ensemble-averaged results. Recent technological evolutions in optical microscopy now offer the opportunity to confront these insights with in situ observations for single catalyst particles. The high spatiotemporal resolution (submicrometer and milliseconds) of (single-molecule) fluorescence microscopy has proven to be extremely useful to study catalysis at the level of individual particles or even at the level of individual reaction events, as well as to investigate diffusion processes in mesoporous materials. However, so far these two phenomena, catalytic conversion and diffusion in porous materials, were treated separately in single-molecule studies; no direct information on the interplay between these two processes has been obtained. Moreover optical microscopy is subject to the laws of diffraction, limiting the spatial resolution to a few hundred nanometers, whereas the interesting processes related to catalysis within porous particles typically occur on smaller length scales. The present contribution circumvents the resolution discrepancy by applying a single-turnover-based strategy in fluorescence microscopy to provide diffractionunlimited resolution. This approach allows mapping the catalytic activity with nanometer-scale spatial resolution, that is, in the order of 10 to 30 nm, which is competitive with the most recent, but more complex nanoscopy tools such as PALM, STORM, STED, and related techniques. The high spatial resolution provides the direct visualization, and thus the immediate localization of active sites within individual particles, while recording the catalytic process under realistic conditions. By exploiting the milliseconds time resolution of the technique, the direct evaluation and quantification of the kinetics is within reach with a very limited number of experiments, as will be demonstrated below for epoxidation over Ti-MCM-41. Typical parameters such as the Thiele modulus and the related effectiveness [*] G. De Cremer, E. Bartholomeeusen, Dr. K. Lin, Prof. Dr. P. P. Pescarmona, Prof. Dr. P. A. Jacobs, Prof. Dr. D. E. De Vos, Prof. Dr. B. F. Sels Department of Microbial and Molecular Systems Katholieke Universiteit Leuven Kasteelpark Arenberg 23, 3001 Heverlee (Belgium) Fax: (+ 32)16-321-998 E-mail: bert.sels@biw.kuleuven.be

Morphology of large ZSM-5 crystals unraveled by fluorescence microscopy

Understanding the internal structure of ZSM-5 crystallites is essential for improving catalyst performance. In this work, a combination of fluorescence microscopy, AFM, SEM, and optical observations is employed to study intergrowth phenomena and pore accessibility in a set of five ZSM-5 samples with different crystal morphologies. An amine-functionalized perylene dye is used to probe acid sites on the external crystal surface, while DAMPI (4-(4-diethylaminostyryl)- N-methylpyridinium iodide) is used to map access to the straight channels in MFI from the outer surface. The use of these dyes is validated by studying the well-understood rounded-boat type ZSM-5 crystals. Next coffin-shaped ZSM-5 crystals are considered; we critically evaluate the seemingly conflicting 2-component and 3-component models that have been proposed to account for the hourglass structure in these crystals. The data prove that observation of an hourglass structure is essentially unrelated to a 90 degree rotation of the pyramidal crystal components under the (010) face. Hence, in perfectly formed coffin-shaped crystals, the straight channels can be accessed from (010). However, in other crystal batches, sections with a 90 degrees rotation can be found; they are indeed located inside the crystal sections under (010) but often only partially occupy these pyramidal components. In such a case, both straight and sinusoidal pores surface at the hexagonal face. The results largely support the 3-component model, but with the added notion that 90 degree rotated sections (as proposed in the 2-component model) are most likely to be formed inside the defect-rich, pyramidal crystal sections under the (010) faces.

Characterization of fluorescence in heat-treated silver-exchanged zeolites.

Thermal treatment of Ag(+)-exchanged zeolites yields discrete highly photostable luminescent clusters without formation of metallic nanoparticles. Different types of emitters with characteristic luminescence colors are observed, depending on the nature of the cocation, the amount of exchanged silver, and the host topology. The dominant emission bands in LTA samples are situated around 550 and 690 nm for the samples with, respectively, low and high silver content, while in FAU-type materials only a broad band around 550 nm is observed, regardless of the degree of exchange. Analysis of the fluorescent properties in combination with ESR spectroscopy suggests that a Ag(6)(+) cluster with doublet electronic ground state is associated with the appearance of the 690-nm emitter, having a decay of a few hundred microseconds. Tentatively, the nanosecond-decaying 550-nm emitter is assigned to the Ag(3)(+) cluster. This new class of photostable luminescent particles with tunable emission colors offers interesting perspectives for various applications such as biocompatible labels for intracellular imaging.

p-Xylene-Selective Metal-Organic Frameworks: A Case of Topology-Directed Selectivity

Para-disubstituted alkylaromatics such as p-xylene are preferentially adsorbed from an isomer mixture on three isostructural metal-organic frameworks: MIL-125(Ti) ([Ti(8)O(8)(OH)(4)(BDC)(6)]), MIL-125(Ti)-NH(2) ([Ti(8)O(8)(OH)(4)(BDC-NH(2))(6)]), and CAU-1(Al)-NH(2) ([Al(8)(OH)(4)(OCH(3))(8)(BDC-NH(2))(6)]) (BDC = 1,4-benzenedicarboxylate). Their unique structure contains octahedral cages, which can separate molecules on the basis of differences in packing and interaction with the pore walls, as well as smaller tetrahedral cages, which are capable of separating molecules by molecular sieving. These experimental data are in line with predictions by molecular simulations. Additional adsorption and microcalorimetric experiments provide insight in the complementary role of the two cage types in providing the para selectivity.