Quantitative 3D Fluorescence Imaging of Single Catalytic Turnovers Reveals Spatiotemporal Gradients in Reactivity of Zeolite H-ZSM-5 Crystals upon Steaming

Enabling the Methanol Economy: Opportunities and Challenges for Heterogeneous Catalysis in the Production of Liquid Fuels via Methanol

ConspectusThirty years ago, George A. Olah proposed the concept of the methanol economy, where methanol replaces fossil fuels as a means of energy storage, ground transportation fuel, and raw material for the manufacture of other carbon-based products. Over the years, with rising global warming concerns, the concept has evolved. A special interest is devoted to the development of catalytic processes that allow the transformation of carbon dioxide, via methanol, into CO2 neutral liquid hydrocarbons. These products could replace the oil-based fuels currently used by combustion engines. The rapid depletion of such fuels would avoid a considerable amount of CO2 emissions during the current energy transition.Over the past decade, we have focused on different key processes that should allow for maximal atom efficiency and, therefore, minimal energy consumption in a field, CO2 valorization, that can easily become a zero-sum game. In this Account, we highlight the importance of catalyst design to overcome the process challenges in the production of liquid fuels from methanol. Additionally, progress in multifunctional catalysts able to directly convert, in one single reactor, CO2 to liquid fuels is also discussed in detail. This integrated option is of particular interest since it allows an important decrease in operational units while increasing throughput by converting, in situ, a thermodynamically limited intermediate.

Droplet microreactor for high-throughput fluorescence-based measurements of single catalyst particle acidity

The particles of heterogeneous catalysts differ greatly in size, morphology, and most importantly, in activity. Studying these catalyst particles in batch typically results in ensemble averages, without any information at the level of individual catalyst particles. To date, the study of individual catalyst particles has been rewarding but is still rather slow and often cumbersome¹. Furthermore, these valuable in-depth studies at the single particle level lack statistical relevance. Here, we report the development of a droplet microreactor for high-throughput fluorescence-based measurements of the acidities of individual particles in fluid catalytic cracking (FCC) equilibrium catalysts (ECAT). This method combines systematic screening of single catalyst particles with statistical relevance. An oligomerization reaction of 4-methoxystyrene, catalyzed by the Brønsted acid sites inside the zeolite domains of the ECAT particles, was performed on-chip at 95 °C. The fluorescence signal generated by the reaction products inside the ECAT particles was detected near the outlet of the microreactor. The high-throughput acidity screening platform was capable of detecting ~1000 catalyst particles at a rate of 1 catalyst particle every 2.4 s. The number of detected catalyst particles was representative of the overall catalyst particle population with a confidence level of 95%. The measured fluorescence intensities showed a clear acidity distribution among the catalyst particles, with the majority (96.1%) showing acidity levels belonging to old, deactivated catalyst particles and a minority (3.9%) exhibiting high acidity levels. The latter are potentially of high interest, as they reveal interesting new physicochemical properties indicating why the particles were still highly acidic and reactive.

In situ quantitative single-molecule study of site-specific photocatalytic activity and dynamics on ultrathin g-C3N4 nanosheets

Graphitic carbon nitride (g-C₃N₄) has attracted extensive research attention in recent years due to its unique layered structure, facile synthetic route, visible-light-responsive nature, and excellent photocatalytic performance. However, an insightful investigation of site-specific catalytic activities and kinetics on g-C₃N₄ is still warranted. Here, we fabricated ultrathin g-C₃N₄ nanosheets through thermal exfoliation. The optimized sample exhibits a high specific surface area of 307.35 m² g^-1 and a remarkable H₂ generation activity of 2008 μmol h^-1 g^-1 with an apparent quantum efficiency of 4.62% at λ = 420 nm. Single-molecule fluorescence microscopy was applied for the first time to spatially resolve the reaction heterogeneities with nanometer precision (∼10 nm). The catalytic kinetics (i.e., reactant adsorption, conversion, and product dissociation) and temporal activity fluctuations were in situ quantified at individual structural features (i.e., wrinkles, edges, and basal planes) of g-C₃N₄. It was found that the wrinkle and edge exhibited superior photocatalytic activity due to the intrinsic band modulation, which are 20 times and 14.8 times that of the basal plane, respectively. Moreover, due to the steric effect, the basal plane showed the highest adsorption constant and the lowest direct dissociation constant. Density functional theory (DFT) simulations unveiled the adsorption energies of reactant and product molecules on each structure of g-C₃N₄, which support our experimental results. Such investigation would shed more light on the fundamental understanding of site-specific catalytic dynamics on g-C₃N₄, which benefits the rational design of 2D layered materials for efficient solar-to-chemical energy conversion.

Molecular transport in zeolite catalysts: depicting an integrated picture from macroscopic to microscopic scales

Increasing social sustainability triggers the persistent progress of industrial catalysis in energy transformation and chemical production. Zeolites have been demonstrated to be pivotal catalysts in chemical industries due to their moderate acidity and versatile well-defined pore structures. However, in the context of enhancing the performances of zeolite catalysts, the perspectives on the diffusion regulations within the pores and channels in the bulk phases or external surfaces of the zeolites are often overlooked. Establishing the structure-transport-reactivity relationships in heterogeneous catalysis can provide rational guidelines to design high-performance catalysts. Herein, this tutorial review attempts to systematically depict an integrated picture of molecular transport behaviors in zeolite catalysts from macroscopic to microscopic perspectives. The advances in the accurate diffusion measurements employing both macroscopic and microscopic techniques are briefly introduced. The diffusion characteristics in zeolite catalysts under working conditions (e.g., high temperature, multi-components, and reaction coupling) are then addressed. The macroscopic internal diffusion and the microscopic diffusion occurring in the micro-zones of zeolite crystals (e.g., surface diffusion, diffusion anisotropy, and confined diffusion) are reviewed and discussed in more detail. These diffusion behaviors highly impact the underlying reaction mechanism, catalytic performances, and catalyst optimization strategies. Finally, the multi-type pore systems of practical zeolite catalysts in industrial reactors and their transport behaviors are analyzed. The fully-crystalline monolithic zeolites in the absence of binders are highlighted as rising-star catalytic materials for industrial applications. The research challenges in this field and the potential future development directions are summarized.

In Situ Imaging of Catalytic Reactions on Tungsten Oxide Nanowires Connects Surface-Ligand Redox Chemistry with Photocatalytic Activity

Semiconductor nanocrystals are promising candidates for generating chemical feedstocks through photocatalysis. Understanding the role of ligands used to prepare colloidal nanocrystals in catalysis is challenging due to the complexity and heterogeneity of nanocrystal surfaces. We use in situ single-molecule fluorescence imaging to map the spatial distribution of active regions along individual tungsten oxide nanowires before and after functionalizing them with ascorbic acid. Rather than blocking active sites, we observed a significant enhancement in activity for photocatalytic water oxidation after treatment with ascorbic acid. While the initial nanowires contain inactive regions dispersed along their length, the functionalized nanowires show high uniformity in their photocatalytic activity. Spatial colocalization of the active regions with their surface chemical properties shows that oxidation of ascorbic acid during photocatalysis generates new oxygen vacancies along the nanowire surface. We demonstrate that controlling surface-ligand redox chemistry during photocatalysis can enhance the active site concentration on nanocrystal catalysts.

Progress and perspectives in single-molecule optical spectroscopy

We review some of the progress of single-molecule optical experiments in the past 20 years and propose some perspectives for the coming years. We particularly focus on methodological advances in fluorescence, super-resolution, photothermal contrast, and interferometric scattering and briefly discuss a few of the applications. These advances have enabled the exploration of new emitters and quantum optics; the chemistry and biology of complex heterogeneous systems, nanoparticles, and plasmonics; and the detection and study of non-fluorescing and non-absorbing nano-objects. We conclude by proposing some ideas for future experiments. The field will move toward more and better signals of a broader variety of objects and toward a sharper view of the surprising complexity of the nanoscale world of single (bio-)molecules, nanoparticles, and their nano-environments.

Emerging Analytical Methods to Characterize Zeolite-Based Materials

Zeolites and zeolitic materials are, through their use in numerous conventional and sustainable applications, very important to our daily lives, including to foster the necessary transition to a more circular society. The characterization of zeolite-based materials has a tremendous history and a great number of applications and properties of these materials have been discovered in the past decades. This review focuses on recently developed novel as well as more conventional techniques applied with the aim of better understanding zeolite-based materials. Recently explored analytical methods, e.g. atom probe tomography, scanning transmission X-ray microscopy, confocal fluorescence microscopy and photo-induced force microscopy, are discussed on their important contributions to the better understanding of zeolites as they mainly focus on the micro- to nanoscale chemical imaging and the revelation of structure–composition–performance relationships. Some other techniques have a long and established history, e.g. nuclear magnetic resonance, infrared, neutron scattering, electron microscopy and X-ray diffraction techniques, and have gone through increasing developments allowing the techniques to discover new and important features in zeolite-based materials. Additional to the increasing application of these methods, multiple techniques are nowadays used to study zeolites under working conditions (i.e. the in situ/operando mode of analysis) providing new insights in reaction and deactivation mechanisms.

A microwell array-based approach for studying single nanoparticle catalysis with high turnover frequency

Measuring the catalytical activities of single catalysts in the case of high turnover frequency (TOF, realistic conditions) is highly desirable to accurately evaluate the functional heterogeneities among individuals and to understand the catalytic mechanism. Herein, we report a microwell array-based method to in operando measure the photocatalytic kinetics of single CdS nanoparticles (NPs) with high TOF. This was realized by sealing individual CdS NPs into separated micrometer-sized polydimethylsiloxane wells, thus eliminating the diffusion of products among individuals in the case of high concentration of reactants. This method allowed us to monitor the activities of single catalysts with an average TOF up to 2.1 × 10⁵ s^-1. Interestingly, two types of catalytical behaviors were revealed during single CdS photocatalysis: a rapid decline in activity for most CdS NPs and an initial increase in activity followed by a decrease for a minor population of individuals. The developed method will facilitate the investigation of catalytic activities of single particles under realistic conditions and hold great potential in the fields of photo/electro-catalysts, enzymes, functional bacteria, and so on.

Single-Molecule Colocalization of Redox Reactions on Semiconductor Photocatalysts Connects Surface Heterogeneity and Charge-Carrier Separation in Bismuth Oxybromide

The surface structure of semiconductor photocatalysts controls the efficiency of charge-carrier extraction during photocatalytic reactions. However, understanding the connection between surface heterogeneity and the locations where photogenerated charge carriers are preferentially extracted is challenging. Herein we use single-molecule fluorescence imaging to map the spatial distribution of active regions and quantify the activity for both photocatalytic oxidation and reduction reactions on individual bismuth oxybromide (BiOBr) nanoplates. Through a coordinate-based colocalization analysis, we quantify the spatial correlation between the locations where fluorogenic probe molecules are oxidized and reduced on the surface of individual nanoplates. Surprisingly, we observed two distinct photochemical behaviors for BiOBr particles prepared within the same batch, which exhibit either predominantly uncorrelated activity where electrons and holes are extracted from different sites or colocalized activity in which oxidation and reduction take place within the same nanoscale regions. By analyzing the emissive properties of the fluorogenic probes, we propose that electrons and holes colocalize at defect-deficient regions, while defects promote the selective extraction of one carrier type by trapping either electrons or holes. Although previous work has used defect engineering to enhance the activity of bismuth oxyhalides and other semiconductor photocatalysts for useful reductive half-reactions (e.g., CO₂ or N₂ reduction), our results show that defect-free regions are needed to promote both oxidation and reduction in fuel-generating photocatalysts that do not rely on sacrificial reagents.

Single molecule fluorescence imaging of nanoconfinement in porous materials

This review covers recent progress in using single molecule fluorescence microscopy imaging to understand the nanoconfinement in porous materials. The single molecule approach unveils the static and dynamic heterogeneities from seemingly equal molecules by removing the ensemble averaging effect. Physicochemical processes including mass transport, surface adsorption/desorption, and chemical conversions within the confined space inside porous materials have been studied at nanometer spatial resolution, at the single nanopore level, with millisecond temporal resolution, and under real chemical reaction conditions. Understanding these physicochemical processes provides the ability to quantitatively measure the inhomogeneities of nanoconfinement effects from the confining properties, including morphologies, spatial arrangement, and trapping domains. Prospects and limitations of current single molecule imaging studies on nanoconfinement are also discussed.

Recent progress in the improvement of hydrothermal stability of zeolites

Zeolites have been successfully employed in many catalytic reactions of industrial relevance. The severe conditions required in some processes, where high temperatures are frequently combined with the presence of steam, highlight the need of considering the evolution of the catalyst structure during the reaction. This review attempts to summarize the recently developed strategies to improve the hydrothermal framework stability of zeolites.

Multimodal Imaging of Autofluorescent Sites Reveals Varied Chemical Speciation in SSZ-13 Crystals

A multimodal imaging study of chabazite is used to show the distribution of and discriminate between different emissive deposits arising as a result of the detemplation process. Confocal imaging, 3D fluorescence lifetime imaging, 3D multispectral fluorescence imaging, and Raman mapping are used to show three different types of emissive behaviours each characterised by different spatial distributions, trends in lifetime, spectral signals, and Raman signatures. A notable difference is seen in the morphology of agglomerated surface deposits and larger subsurface deposits, which experience lifetime augmentation due to spatial confinement. The distribution of organic residue throughout the crystal volume is comparable to XRF mapping that shows Si enrichment on the outer edges and higher Al content through the centre, demonstrating that a fluorescence-based technique can also be used to indirectly comment on the compositional chemistry of the inorganic framework.

Single-molecule observation of diffusion and catalysis in nanoporous solids

Nanoporous solids, including microporous, mesoporous and hierarchically structured porous materials, are of scientific and technological interest because of their high surface-to-volume ratio and ability to impose shape- and size-selectivity on molecules diffusing through them. Enormous efforts have been put in the mechanistic understanding of diffusion–reaction relationships of nanoporous solids, with the ultimate goal of developing materials with improved catalytic performance. Single-molecule localization microscopy can be used to explore the pore space via the trajectories of individual molecules. This ensemble-free perspective directly reveals heterogeneities in diffusion and diffusion-related reactivity of individual molecules, which would have been obscured in bulk measurements. In this article, we review developments in the spatial and temporal characterization of nanoporous solids using single-molecule localization microscopy. We illustrate various aspects of this approach, and showcase how it can be used to follow molecular diffusion and reaction behaviors in nanoporous solids.

Zeolite (In)Stability under Aqueous or Steaming Conditions

Zeolites are among the most environmentally friendly materials produced industrially at the Megaton scale. They find numerous commercial applications, particularly in catalysis, adsorption, and separation. Under ambient conditions aluminosilicate zeolites are stable when exposed to water or water vapor. However, at extreme conditions as high temperature, high water vapor pressure or increased acidity/basicity, their crystalline framework can be destroyed. The stability of the zeolite framework under aqueous conditions also depends on the concentration and character of heteroatoms (other than Al) and the topology of the zeolite. The factors critical for zeolite (in)stability in the presence of water under various conditions are reviewed from the experimental as well as computational sides. Nonreactive and reactive interactions of water with zeolites are addressed. The goal of this review is to provide a comparative overview of all-silica zeolites, aluminosilicates and zeolites with other heteroatoms (Ti, Sn, and Ge) when contacted with water. Due attention is also devoted to the situation when partial zeolite hydrolysis is used beneficially, such as the formation of hierarchical zeolites, synthesis of new zeolites or fine-tuning catalytic or adsorption characteristics of zeolites.

Pore selectivity and electron transfers in HZSM-5 single crystals: a Raman microspectroscopy mapping and confocal fluorescence imaging combined study

Electron transfers at the single particle level in HZSM-5 zeolite are followed by combining Raman microspectroscopy mapping and confocal fluorescence imaging. The effects of pore accessibility and guest diffusion on reactivity are investigated.

Fluorescence spectroscopy studies of crossed aldol reactions: a reactive Nile red dye reveals catalyst-dependent product formation

A highly fluorescent, aldol-reactive derivative of the dye Nile red is synthesized and evaluated as an in situ probe of crossed aldol reactions.

Deciphering nanoconfinement effects on molecular orientation and reaction intermediate by single molecule imaging

Nanoconfinement could dramatically change molecular transport and reaction kinetics in heterogeneous catalysis. Here we specifically design a core-shell nanocatalyst with aligned linear nanopores for single-molecule studies of the nanoconfinement effects. The quantitative single-molecule measurements reveal unusual lower adsorption strength and higher catalytic activity on the confined metal reaction centres within the nanoporous structure. More surprisingly, the nanoconfinement effects on enhanced catalytic activity are larger for catalysts with longer and narrower nanopores. Experimental evidences, including molecular orientation, activation energy, and intermediate reactive species, have been gathered to provide a molecular level explanation on how the nanoconfinement effects enhance the catalyst activity, which is essential for the rational design of highly-efficient catalysts.

Rational design of functional materials guided by single particle chemiluminescence imaging

Chemiluminescence (CL) functionalized materials have found tremendous value in developing CL assays for clinical assays and point-of-care tests. To date, the design and optimization of these materials have mainly relied on conventional trial-and-error procedures in which the ensemble performance is evaluated using conditional experiments. Here we have built an optical microscope to acquire the CL emission from single magnetic-polymer hybrid microbeads functionalized with luminol analogues, and to access the CL kinetics of each individual particle. It was incidentally found that a minor subpopulation of microbeads exhibited intense and delayed CL emission while the majority showed transient and weak emission. Structural characterization of the very same individual particles uncovered that the amorphous multi-core microstructures were responsible for the enhanced encapsulation efficiency and optimized CL reaction kinetics. Guided by this knowledge stemming from single particle CL imaging, the synthesis procedure was rationally optimized to enrich the portion of microbeads with better CL performance, which was validated by both single particle imaging and the significantly improved analytical performance at the ensemble level. The present work not only demonstrates the CL imaging and CL kinetics curve of single microbeads for the first time, but also sets a clear example showing the capability of single particle studies to investigate the structure-activity relationship in a bottom-up manner and to help the rational design of ensemble materials with improved performance.

3D Raman Spectroscopy of Large Zeolite ZSM-5 Crystals

Hydrothermal treatment is a common method used to modify the physicochemical properties of zeolite‐based catalyst materials. It alters the number and type of acid sites through dealumination and increases molecular diffusion by mesopore formation. Steaming also reduces the structural integrity of zeolite frameworks. In this study, Raman microscopy has been used to map large zeolite ZSM‐5 crystals before and after steaming. 3D elemental maps of T−O (T: Al or Si) sites of the zeolite were obtained. The Raman active vibrational bands were determined, which are indicative of (non‐) framework Al, as well as of structural integrity. Zeolite steaming caused the introduction of additional heterogeneities within the zeolite framework. Al migration and the formation of extra‐framework Al species were observed. The described experiments demonstrate the capability of 3D Raman spectroscopy as a valuable tool to obtain information on the spatial distributions of framework elements as well as defects within a zeolite‐based material.

Reversible and Site-Dependent Proton-Transfer in Zeolites Uncovered at the Single-Molecule Level

Zeolite activity and selectivity is often determined by the underlying proton and hydrogen-transfer reaction pathways. For the first time, we use single-molecule fluorescence microscopy to directly follow the real-time behavior of individual styrene-derived carbocationic species formed within zeolite ZSM-5. We find that intermittent fluorescence and remarkable photostability of carbocationic intermediates strongly depend on the local chemical environment imposed by zeolite framework and guest solvent molecules. The carbocationic stability can be additionally altered by changing para-substituent on the styrene moiety, as suggested by DFT calculations. Thermodynamically unstable carbocations are more likely to switch between fluorescent (carbocationic) and dark (neutral) states. However, the rate constants of this reversible change can significantly differ among individual carbocations, depending on their exact location in the zeolite framework. The lifetimes of fluorescent states and reversibility of the process can be additionally altered by changing the interaction between dimeric carbocations and solvated Brønsted acid sites in the MFI framework. Advanced multidimensional magic angle spinning solid-state NMR spectroscopy has been employed for the accurate structural elucidation of the reaction products during the zeolite-catalyzed dimerization of styrene in order to corroborate the single-molecule fluorescence microscopy data. This complementary approach of single-molecule fluorescence microscopy, NMR, and DFT collectively indicates that the relative stability of the carbocationic and the neutral states largely depends on the substituent and the local position of the Brønsted acid site within the zeolite framework. As a consequence, new insights into the host–guest chemistry between the zeolite and aromatics, in terms of their surface mobility and reactivity, have been obtained.

Imaging the chemical activity of single nanoparticles with optical microscopy

Chemical activity of single nanoparticles can be imaged and determined by monitoring the optical signal of each individual during chemical reactions with advanced optical microscopes. It allows for clarifying the functional heterogeneity among individuals, and for uncovering the microscopic reaction mechanisms and kinetics that could otherwise be averaged out in ensemble measurements.

Photochemistry and Photophysics in Silica-Based Materials: Ultrafast and Single Molecule Spectroscopy Observation

Silica-based materials (SBMs) are widely used in catalysis, photonics, and drug delivery. Their pores and cavities act as hosts of diverse guests ranging from classical dyes to drugs and quantum dots, allowing changes in the photochemical behavior of the confined guests. The heterogeneity of the guest populations as well as the confinement provided by these hosts affect the behavior of the formed hybrid materials. As a consequence, the observed reaction dynamics becomes significantly different and complex. Studying their photobehavior requires advanced laser-based spectroscopy and microscopy techniques as well as computational methods. Thanks to the development of ultrafast (spectroscopy and imaging) tools, we are witnessing an increasing interest of the scientific community to explore the intimate photobehavior of these composites. Here, we review the recent theoretical and ultrafast experimental studies of their photodynamics and discuss the results in comparison to those in homogeneous media. The discussion of the confined dynamics includes solvation and intra- and intermolecular proton-, electron-, and energy transfer events of the guest within the SBMs. Several examples of applications in photocatalysis, (photo)sensors, photonics, photovoltaics, and drug delivery demonstrate the vast potential of the SBMs in modern science and technology.

Solvent Polarity-Induced Pore Selectivity in H-ZSM-5 Catalysis

Molecular-sized micropores of ZSM-5 zeolite catalysts provide spatial restrictions around catalytic sites that allow for shape-selective catalysis. However, the fact that ZSM-5 has two main pore systems with different geometries is relatively unexploited as a potential source of additional shape selectivity. Here, we use confocal laser-scanning microscopy to show that by changing the polarity of the solvent, the acid-catalyzed furfuryl alcohol oligomerization can be directed to selectively occur within either of two locations in the microporous network. This finding is confirmed for H-ZSM-5 particles with different Si/Al ratios and indicates a general trend for shape-selective catalytic reactions.

Probing Zeolite Crystal Architecture and Structural Imperfections using Differently Sized Fluorescent Organic Probe Molecules

A micro‐spectroscopic method has been developed to probe the accessibility of zeolite crystals using a series of fluorescent 4‐(4‐diethylaminostyryl)‐1‐methylpyridinium iodide (DAMPI) probes of increasing molecular size. Staining large zeolite crystals with MFI (ZSM‐5) topology and subsequent mapping of the resulting fluorescence using confocal fluorescence microscopy reveal differences in structural integrity: the 90° intergrowth sections of MFI crystals are prone to develop structural imperfections, which act as entrance routes for the probes into the zeolite crystal. Polarization‐dependent measurements provide evidence for the probe molecule's alignment within the MFI zeolite pore system. The developed method was extended to BEA (Beta) crystals, showing that the previously observed hourglass pattern is a general feature of BEA crystals with this morphology. Furthermore, the probes can accurately identify at which crystal faces of BEA straight or sinusoidal pores open to the surface. The results show this method can spatially resolve the architecture‐dependent internal pore structure of microporous materials, which is difficult to assess using other characterization techniques such as X‐ray diffraction.

Surface acid–base catalytic activity of ZIF-8 revealed by super-resolution fluorescence microscopy

Fluorescence microscopy uncovers the surface-only catalytic activity of ZIF-8, but its accessibility is improved via extra porosity introduction by oleic acid treatment.

Hierarchically porous materials: synthesis strategies and structure design

This review addresses recent advances in synthesis strategies of hierarchically porous materials and their structural design from micro-, meso- to macro-length scale.

Single Molecule Nanospectroscopy Visualizes Proton-Transfer Processes within a Zeolite Crystal

Visualizing proton-transfer processes at the nanoscale is essential for understanding the reactivity of zeolite-based catalyst materials. In this work, the Brønsted-acid-catalyzed oligomerization of styrene derivatives was used for the first time as a single molecule probe reaction to study the reactivity of individual zeolite H-ZSM-5 crystals in different zeolite framework, reactant and solvent environments. This was accomplished via the formation of distinct dimeric and trimeric fluorescent carbocations, characterized by their different photostability, as detected by single molecule fluorescence microscopy. The oligomerization kinetics turned out to be very sensitive to the reaction conditions and the presence of the local structural defects in zeolite H-ZSM-5 crystals. The remarkably photostable trimeric carbocations were found to be formed predominantly near defect-rich crystalline regions. This spectroscopic marker offers clear prospects for nanoscale quality control of zeolite-based materials. Interestingly, replacing n-heptane with 1-butanol as a solvent led to a reactivity decrease of several orders and shorter survival times of fluorescent products due to the strong chemisorption of 1-butanol onto the Brønsted acid sites. A similar effect was achieved by changing the electrophilic character of the para-substituent of the styrene moiety. Based on the measured turnover rates we have established a quantitative, single turnover approach to evaluate substituent and solvent effects on the reactivity of individual zeolite H-ZSM-5 crystals.

X-ray Excited Optical Fluorescence and Diffraction Imaging of Reactivity and Crystallinity in a Zeolite Crystal: Crystallography and Molecular Spectroscopy in One

Structure-activity relationships in heterogeneous catalysis are challenging to be measured on a single-particle level. For the first time, one X-ray beam is used to determine the crystallographic structure and reactivity of a single zeolite crystal. The method generates μm-resolved X-ray diffraction (μ-XRD) and X-ray excited optical fluorescence (μ-XEOF) maps of the crystallinity and Brønsted reactivity of a zeolite crystal previously reacted with a styrene probe molecule. The local gradients in chemical reactivity (derived from μ-XEOF) were correlated with local crystallinity and framework Al content, determined by μ-XRD. Two distinctly different types of fluorescent species formed selectively, depending on the local zeolite crystallinity. The results illustrate the potential of this approach to resolve the crystallographic structure of a porous material and its reactivity in one experiment via X-ray induced fluorescence of organic molecules formed at the reactive centers.