Assessment of Hydrophilicity/Hydrophobicity in Mesoporous Silica by Combining Adsorption, Liquid Intrusion, and Solid-State NMR Spectroscopy.

We have developed a comprehensive strategy for quantitatively assessing the hydrophilicity/hydrophobicity of nanoporous materials by combining advanced adsorption studies, novel liquid intrusion techniques, and solid-state NMR spectroscopy. For this, we have chosen a well-defined system of model materials, i.e., the highly ordered mesoporous silica molecular sieve SBA-15 in its pristine state and functionalized with different amounts of trimethylsilyl (TMS) groups, allowing one to accurately tailor the surface chemistry while maintaining the well-defined pore structure. For an absolute quantification of the trimethylsilyl group density, quantitative 1H solid-state NMR spectroscopy under magic angle spinning was employed. A full textural characterization of the materials was obtained by high-resolution argon 87 K adsorption, coupled with the application of dedicated methods based on nonlocal-density functional theory (NLDFT). Based on the known texture of the model materials, we developed a novel methodology allowing one to determine the effective contact angle of water adsorbed on the pore surfaces from complete wetting to nonwetting, constituting a powerful parameter for the characterization of the surface chemistry inside porous materials. The surface chemistry was found to vary from hydrophilic to hydrophobic as the TMS functionalization content was increased. For wetting and partially wetting surfaces, pore condensation of water is observed at pressures P smaller than the bulk saturation pressure p0 (i.e., at p/p0 < 1) and the effective contact angle of water on the pore walls could be derived from the water sorption isotherms. However, for nonwetting surfaces, pore condensation occurs at pressures above the saturation pressure (i.e., at p/p0 > 1). In this case, we investigated the pore filling of water (i.e., the vapor-liquid phase transition) by the application of a novel, liquid water intrusion/extrusion methodology, allowing one to derive the effective contact angle of water on the pore walls even in the case of nonwetting. Complementary molecular simulations provide density profiles of water on pristine and TMS-grafted silica surfaces (mimicking the tailored, functionalized experimental silica surfaces), which allow for a molecular view on the water adsorbate structure. Summarizing, we present a comprehensive and reliable methodology for quantitatively assessing the hydrophilicity/hydrophobicity of siliceous nanoporous materials, which has the potential to optimize applications in heterogeneous catalysis and separation (e.g., chromatography).

Mesoporous supraparticles with a tailored solid-liquid-gas interface for visual indication of H2 gas and NH3 vapours.

Dual-gasochromic supraparticles that undergo rapid eye-readable and gas-specific colour changes upon reaction with hydrogen or ammonia are reported. This functionality is achieved by tailoring the solid-liquid-gas interface within the mesoporous framework of supraparticles via spray-drying.

Catalyst Supraparticles: Tuning the Structure of Spray-Dried Pt/SiO2 Supraparticles via Salt-Based Colloidal Manipulation to Control their Catalytic Performance.

The structure of supraparticles (SPs) is a key parameter for achieving advanced functionalities arising from the combination of different nanoparticle (NP) types in one hierarchical entity. However, whenever a droplet-assisted forced assembly approach is used, e.g., spray-drying, the achievable structure is limited by the inherent drying phenomena of the method. In particular, mixed NP dispersions of differently sized colloids are heavily affected by segregation during the assembly. Herein, the influence of the colloidal arrangement of Pt and SiO2 NPs within a single supraparticulate entity is investigated. A salt-based electrostatic manipulation approach of the utilized NPs is proposed to customize the structure of spray-dried Pt/SiO2 SPs. By this, size-dependent separation phenomena of NPs during solvent evaporation, that limit the catalytic performance in the reduction of 4-nitrophenol, are overcome by achieving even Pt NP distribution. Additionally, the textural properties (pore size and distribution) of the SiO2 pore framework are altered to improve the mass transfer within the material leading to increased catalytic activity. The suggested strategy demonstrates a powerful, material-independent, and universally applicable approach to deliberately customize the structure and functionality of multi-component SP systems. This opens up new ways of colloidal material combinations and structural designs in droplet-assisted forced assembly approaches like spray-drying.

Chemically Reversible CO2 Uptake by Dendrimer-Impregnated Metal-Organic Frameworks.

Industrialization over the past two centuries has resulted in a continuous rise in global CO2 emissions. These emissions are changing ecosystems and livelihoods. Therefore, methods are needed to capture these emissions from point sources and possibly from our atmosphere. Though the amount of CO2 is rising, it is challenging to capture directly from air because its concentration in air is extremely low, 0.04%. In this study, amines installed inside metal-organic frameworks (MOFs) are investigated for the adsorption of CO2, including at low concentrations. The amines used are polyamidoamine dendrimers that contain many primary amines. Chemically reversible adsorption of CO2 via carbamate formation was observed, as was enhanced uptake of carbon dioxide, likely via dendrimer-amide-based physisorption. Limiting factors in this initial study are comparatively low dendrimer loadings and slow kinetics for carbon dioxide uptake and release, even at 80 °C.

Aspects of Gas Storage: Confined Geometry Effects on the High-Pressure Adsorption Behavior of Supercritical Fluids.

During the last decades, major progress was made concerning the understanding of subcritical low-pressure adsorption of fluids like nitrogen and argon at their boiling temperatures in nanoporous materials. It was possible to understand how structural properties affect the shape of the adsorption isotherms. However, within the context of gas storage applications, supercritical high-pressure gas adsorption is important. A key feature here is that the experimentally determined surface excess adsorption isotherm may exhibit a characteristic maximum at a certain pressure. For a given temperature and adsorptive/adsorbent system, the surface excess maximum (and the corresponding adsorbed amount) is related to the storage capacity of the adsorbent. However, there is still a lack of understanding of how key textural properties such as surface area and pore size affect details of the shape of supercritical high-pressure adsorption isotherms. To address these open questions, we have performed a systematic experimental study assessing the effect of pore size/structure on the supercritical adsorption isotherms of pure fluids such as C2H4, CO2, and SF6 over a wider range of temperatures and pressures on a series of model materials exhibiting well-defined pore sizes, i.e., ordered micro- and mesoporous materials (e.g., NaY zeolite, KIT-6 silica, and MCM-48 silica). A fundamental result of our experiments is a unique fluid-independent correlation between the pressure of the surface excess maximum pmax (at a given temperature) and the pore size (by taking into account the kinetic diameter of the fluid and the underlying effective attractive fluid-wall interaction). Summarizing, our results suggest important structure-property relationships, allowing one to determine, for given thermodynamic conditions, important information related to the optimal operating conditions for supercritical adsorption applications. The insights may also serve as a basis for optimizing and tailoring the properties of nanoporous adsorbent materials for gas storage applications.

The Nanoscale Wetting Parameter and Its Role in Interfacial Phenomena: Phase Transitions in Nanopores.

Through analysis of the statistical mechanical equations for a thin adsorbed film (gas, liquid, or solid) on a solid substrate or confined within a pore, it is possible to express the equilibrium thermodynamic properties of the film as a function of just two dimensionless parameters: a nanoscale wetting parameter, αw, and pore width, H*. The wetting parameter, αw, is defined in terms of molecular parameters for the adsorbed film and substrate and so is applicable at the nanoscale and for films of any phase. The main assumptions in the treatment are that (a) the substrate structure is not significantly affected by the adsorbed layer and (b) the diameter of the adsorbate molecules is not very small compared to the spacing of atoms in the solid substrate. We show that different surface geometries of the substrate (e.g., slit, cylindrical, and spherical pores) and various models of wall heterogeneity can be accounted for through a well-defined correction to the wetting parameter; no new dimensionless variables are introduced. Experimental measurements are reported for contact angles for various liquids on several planar substrates and are shown to be closely correlated with the nanoscale wetting parameter. We apply this approach to phase separation in nanopores of various geometries. Molecular simulation results for the phase diagram in confinement, obtained by the flat histogram Monte Carlo method, are reported and are shown to be closely similar to experimental results for capillary condensation, melting, and the triple point. The value of the wetting parameter, αw, is shown to determine the qualitative behavior (e.g., increase vs decrease in the melting temperature, capillary condensation vs evaporation), whereas the pore width determines the magnitude of the confinement effect. The triple point temperature and pressure for the confined phase are always lower than those for the bulk phase for all cases studied.

From Meso to Macro: Controlling Hierarchical Porosity in Supraparticle Powders.

A drying droplet containing colloidal particles can consolidate into a spherical assembly called a supraparticle. Such supraparticles are inherently porous due to the spaces between the constituent primary particles. Here, the emergent, hierarchical porosity in spray-dried supraparticles is tailored via three distinct strategies acting at different length scales. First, mesopores (<10 nm) are introduced via the primary particles. Second, the interstitial pores are tuned from the meso- (35 nm) to the macro scale (250 nm) by controlling the primary particle size. Third, defined macropores (>100 nm) are introduced via templating polymer particles, which can be selectively removed by calcination. Combining all three strategies creates hierarchical supraparticles with fully tailored pore size distributions. Moreover, another level of the hierarchy is added by fabricating supra-supraparticles, using the supraparticles themselves as building blocks, which provide additional pores with micrometer dimensions. The interconnectivity of the pore networks within all supraparticle types is investigated via detailed textural and tomographic analysis. This work provides a versatile toolbox for designing porous materials with precisely tunable, hierarchical porosity from the meso- (3 nm) to the macroscale (≈10 µm) that can be utilized for applications in catalysis, chromatography, or adsorption.

Reliable Surface Area Assessment of Wet and Dry Nonporous and Nanoporous Particles: Nuclear Magnetic Resonance Relaxometry and Gas Physisorption.

The reliable assessment of surface area is extremely important for many applications, e.g., catalysis, separation, and energy storage/conversion. Within this context, major progress has been made concerning the textural characterization of porous materials in the gas/dry state, e.g., gas physisorption and mercury porosimetry. However, these methods are not sufficient for the characterization of wet materials utilized in liquid-phase processes. For this, the application of nuclear magnetic resonance (NMR) relaxometry has been considered, but a systematic and rigorous assessment of the applicability of NMR relaxometry for reliable surface and pore size characterization of nanoporous materials is missing. Hence, we present a systematic study in which we assess the applicability of NMR relaxometry for reliable surface area assessment utilizing for the first time true surface area benchmark data based on argon 87 K adsorption on nonporous particles (silica and carbon black) coupled with the development of an advanced methodology including the investigation of the choice of the probe molecule and the effect of its accessibility to the pore network. Our results show that the method provides a fast (a few minutes per measurement) and reliable surface area of silica and carbon black model materials immersed in a liquid phase. In addition, our work clearly demonstrates the potential of NMR relaxometry for the targeted surface area assessment of defined pore classes (here ultramicropores) and suggests a new methodology for the characterization of pore entrances (pore window size). Furthermore, we investigate the effect of wettability and suggest that NMR relaxometry could be developed into a unique tool for assessing the wetting characteristics of adsorbate phases on pore surfaces. This fundamental study can be considered a first major step in enabling NMR relaxometry for reliable surface area assessment for wet materials, particularly relevant for materials used in processes occurring in a liquid phase.

Nucleation as a rate-determining step in catalytic gas generation reactions from liquid phase systems.

The observable reaction rate of heterogeneously catalyzed reactions is known to be limited either by the intrinsic kinetics of the catalytic transformation or by the rate of pore and/or film diffusion. Here, we show that in gas generation reactions from liquid reactants, the nucleation of gas bubbles in the catalyst pore structure represents an additional important rate-limiting step. This is highlighted for the example of catalytic hydrogen release from the liquid organic hydrogen carrier compound perhydro-dibenzyltoluene. A nucleation-inhibited catalytic system produces only dissolved hydrogen with fast saturation of the fluid phase around the active site, while bubble formation enhances mass transfer by more than a factor of 50 in an oscillating reaction regime. Nucleation can be efficiently triggered not only by temperature changes and catalyst surface modification but also by a mechanical stimulus. Our work sheds new light on performance-limiting factors in reactions that are of highest relevance for the future green hydrogen economy.

Classification and characterization of multimodal nanoparticle size distributions by size-exclusion chromatography.

Size-exclusion chromatography (SEC) is a well-known, versatile and scalable technique for the separation of molecules according to their hydrodynamic size in solution as well as for the determination of molecular weight distributions of polymers. In this paper we demonstrate and generalize the applicability of SEC to the classification and characterization of multimodal distributions of nanoparticles over a broad size range. After calibration with gold standards from 5 nm to 80 nm, the calibration curve is used to determine the particle size distributions (PSDs) of the standards which are in agreement with comprehensive nanoparticle size analysis by analytical ultracentrifugation. Universal calibration curves independent of the core material and surface functionality can be constructed if the pore diameter of the stationary phase exceeds the particle diameter by a factor of 2-3. Mixtures of gold standards are separated by SEC and evaluated in terms of peak resolution and size-dependent separation curves depending on how well the individual peaks are resolved. Baseline separation of a multimodal mixture is observed and its PSD is determined. Mixtures can be fractionated into coarse and fine fractions with nm precision at different switching times of the fraction collector. Our study demonstrates the strength of SEC to classify multimodal PSDs as well as to accurately determine size distributions of complex nanoparticle dispersions over a broad size range.

Formation of a super-dense hydrogen monolayer on mesoporous silica.

Adsorption on various adsorbents of hydrogen and helium at temperatures close to their boiling points shows, in some cases, unusually high monolayer capacities. The microscopic nature of these adsorbate phases at low temperatures has, however, remained challenging to characterize. Here, using high-resolution cryo-adsorption studies together with characterization by inelastic neutron scattering vibration spectroscopy, we show that, near its boiling point (~20 K), H2 adsorbed on a well-ordered mesoporous silica forms a two-dimensional monolayer with a density more than twice that of bulk-solid H2, rather than a bilayer. Theoretical studies, based on thorough first-principles calculations, rationalize the formation of such a super-dense phase. The strong compression of the hydrogen surface layer is due to the excess of surface-hydrogen attraction over intermolecular hydrogen repulsion. Use of this super-dense hydrogen monolayer on an adsorbent might be a feasible option for the storage of hydrogen near its boiling point, compared with adsorption at 77 K.

Spray-Drying and Atomic Layer Deposition: Complementary Tools toward Fully Orthogonal Control of Bulk Composition and Surface Identity of Multifunctional Supraparticles.

Spray-drying is a scalable process enabling one to assemble freely chosen nanoparticles into supraparticles. Atomic layer deposition (ALD) allows for controlled thin film deposition of a vast variety of materials including exotic ones that can hardly be synthesized by wet chemical methods. The properties of coated supraparticles are defined not only by the nanoparticle material chosen and the nanostructure adjusted during spray-drying but also by surface functionalities modified by ALD, if ALD is capable of modifying not only the outer surfaces but also surfaces buried inside the porous supraparticle. Simultaneously, surface accessibility in the porous supraparticles must be ensured to make use of all functionalized surfaces. In this work, iron oxide supraparticles are utilized as a model substrate as their magnetic properties enable the use of advanced magnetic characterization methods. Detailed information about the structural evolution upon individual ALD cycles of aluminium oxide, zinc oxide and titanium dioxide are thereby revealed and confirmed by gas sorption analyses. This demonstrates a powerful and versatile approach to freely designing the functionality of future materials by combination of spray-drying and ALD.

Fingerprinting diverse nanoporous materials for optimal hydrogen storage conditions using meta-learning.

Adsorptive hydrogen storage is a desirable technology for fuel cell vehicles, and efficiently identifying the optimal storage temperature requires modeling hydrogen loading as a continuous function of pressure and temperature. Using data obtained from high-throughput Monte Carlo simulations for zeolites, metal-organic frameworks, and hyper-cross-linked polymers, we develop a meta-learning model that jointly predicts the adsorption loading for multiple materials over wide ranges of pressure and temperature. Meta-learning gives higher accuracy and improved generalization compared to fitting a model separately to each material and allows us to identify the optimal hydrogen storage temperature with the highest working capacity for a given pressure difference. Materials with high optimal temperatures are found in close proximity in the fingerprint space and exhibit high isosteric heats of adsorption. Our method and results provide new guidelines toward the design of hydrogen storage materials and a new route to incorporate machine learning into high-throughput materials discovery.

Porosimetry for Thin Films of Metal-Organic Frameworks: A Comparison of Positron Annihilation Lifetime Spectroscopy and Adsorption-Based Methods.

Thin films of crystalline and porous metal-organic frameworks (MOFs) have great potential in membranes, sensors, and microelectronic chips. While the morphology and crystallinity of MOF films can be evaluated using widely available techniques, characterizing their pore size, pore volume, and specific surface area is challenging due to the low amount of material and substrate effects. Positron annihilation lifetime spectroscopy (PALS) is introduced as a powerful method to obtain pore size information and depth profiling in MOF films. The complementarity of this approach to established physisorption-based methods such as quartz crystal microbalance (QCM) gravimetry, ellipsometric porosimetry (EP), and Kr physisorption (KrP) is illustrated. This comprehensive discussion on MOF thin film porosimetry is supported by experimental data for thin films of ZIF-8.

Pore Size Gradient Effect in Monolithic Silica Mesopore Networks Revealed by In-Situ SAXS Physisorption.

In disordered mesopore networks, the size distribution and connection between adjacent pores control desorption. How network characteristics can be extracted from corresponding physisorption isotherms is still a matter of research. To elucidate this, we study krypton physisorption (117.8 K) in the mesopore networks of "Nakanishi"-type monolithic silica. Combining physisorption in scanning acquisition mode with synchrotron-based in-situ SAXS provides complementary information on pore-filling states. These data reveal a mean pore size gradient in which pores grow smaller towards the material's network center. This structural motif cannot be derived through conventional isotherm analysis, but it is clearly exposed through scanning desorption curves which do not quite converge but merge individually with the main desorption isotherm before the lower hysteresis closing point. Hence, our findings provide the basis to build advanced models for analyzing scanning isotherms and extracting network characteristics through new descriptors, such as pore size and connectivity distributions as a function of the distance from the network center.

Exploring the confinement of polymer nanolayers into ordered mesoporous silica using advanced gas physisorption.

Over the last two decades, in parallel to the rise of ordered mesoporous silica, porous nanostructured polymer-silica composites have attracted the interest of material scientists due to their promising perspectives of application as sorbents, ion-exchangers, supports, and catalysts. While knowledge is available regarding their synthesis and applications, understanding and controlling their pore properties in order to rationalize their performances remain challenging tasks. Greater knowledge is therefore needed regarding their precise characterization, especially using gas adsorption. To this aim, mesoporous polymer-silica nanocomposites were synthesized from two ordered mesoporous silica materials using a pore-surface restricted polymerization technique. Hydrophobic polystyrene, PS, and hydrophilic poly(2-hydroxyethyl methacrylate), PHEMA, were specifically confined and polymerized in the pores of high-quality SBA-15 and KIT-6 silicas of different pore sizes. The physico-chemical characteristics of the resulting hybrid materials were probed in detail using gas physisorption at cryogenic temperatures (Ar at 87 K and N2 at 77 K). The polymer loadings and the interactions between the silica host and the polymer were investigated using thermogravimetric analysis coupled with differential thermal analysis (TGA-DTA) and attenuated total reflection infrared spectroscopy (ATR-FTIR). The effects of the pore structure, mode pore size and presence or absence of intra-wall pores in the silica hosts on the final composite characteristics were assessed as a function of the polymer type and loading. Two different polymer filling mechanisms were identified as a function of the polymer-silica interactions, resulting in important changes on the pore topology of the composites. The results of this study allow a better understanding of the nature of the confined interactions between hydrophilic and hydrophobic polymers and large pore mesoporous silicas and shed some light on fundamental aspects regarding the design of silica-based composites.

Phase Behavior and Capillary Condensation Hysteresis of Carbon Dioxide in Mesopores.

Carbon dioxide adsorption on micro- and mesoporous carbonaceous materials in a wide range of temperatures and pressures is of great importance for the problems of gas separations, greenhouse gas capture and sequestration, enhanced hydrocarbon recovery from shales and coals, as well as for the characterization of nanoporous materials using CO2 as a molecular probe. We investigate the influence of temperature on CO2 adsorption focusing on the capillary condensation and hysteresis phenomena. We present experimental data on the adsorption of CO2 on CMK-3, ordered carbon with mesopores of ∼5-6 nm, at various temperatures (185-273 K) and pressures (up to 35 bars). Using Monte Carlo (MC) simulations in the grand canonical and mesocanonical ensembles, we attempt to predict the transition from reversible capillary condensation to hysteretic adsorption-desorption cycles that is experimentally observed with the decrease of temperature. We show that although the desorption at all temperatures occurs at the conditions of pore vapor-liquid equilibrium, the capillary condensation is a nucleation-driven process associated with an effective energy barrier of ∼43 kT, specific to the sample used in this work. This barrier can be overcome at the equilibrium conditions in the region of reversible condensation at temperatures higher than 240 K. At lower temperatures, the regime of developing hysteresis is observed with progressively widening hysteresis loops. The position of capillary condensation transition is estimated using the pressure dependence of the energy barrier calculated by the thermodynamic integration of the van der Waals-type continuous canonical isotherm simulated with the gauge cell MC method. These findings lay the foundation for developing kernels of CO2 adsorption and desorption isotherm for calculating the pore size distribution in the entire range of micropore and mesopore sizes from one high-pressure experimental isotherm.

Revelation on the Complex Nature of Mesoporous Hierarchical FAU-Y Zeolites.

The texture of mesoporous FAU-Y (FAUmes) prepared by surfactant-templating in basic media is a subject of debate. It is proposed that mesoporous FAU-Y consists of: (1) ordered mesoporous zeolite networks formed by a surfactant-assisted zeolite rearrangement process involving local dissolution and reconstruction of the crystalline framework, and (2) ordered mesoporous amorphous phases as Al-MCM-41, which coexist with zeolite nanodomains obtained by a dissolution-reassembly process. By the present systematic study, performed with FAU-Y (Si/Al = 15) in the presence of octadecyltrimethylammonium bromide and 0 < NaOH/Si ratio < 0.25 at 115 °C for 20 h, we demonstrate that mesoporous FAU zeolites consist, in fact, of a complex family of materials with textural features strongly impacted by the experimental conditions. Two main families have been disclosed: (1) for 0.0625 < NaOH/Si < 0.10, FAUmes are ordered mesoporous materials with zeolite walls, which coexist with zeolite nanodomains (100-200 nm) and (2) for 0.125 < NaOH/Si < 0.25, FAUmes are ordered mesoporous materials with amorphous walls as Al-MCM-41, which coexist with zeolite nanodomains (5-100 nm). The zeolite nanodomains decrease in size with the increase of NaOH/Si ratio. Increasing NaOH/Si ratio leads to an increase of mesopore volume, while the total surface area remains constant, and to a decrease of strong acidity in line with the decrease of micropore volume. The ordered mesoporous materials with zeolite walls feature the highest acidity strength. The ordered mesoporous materials with amorphous walls present additional large pores (50-200 nm), which increase in size and amount with the increase of NaOH/Si ratio. This alkaline treatment of FAU-Y represents a way to obtain ordered mesoporous materials with zeolite walls with high mesopore volume for NaOH/Si = 0.10 and a new way to synthesize mesoporous Al-MCM-41 materials containing extralarge pores (50-200 nm) ideal for optimal diffusion (NaOH/Si = 0.25).

Identification of the Basic Sites on Nitrogen-Substituted Microporous and Mesoporous Silicate Frameworks Using CO2 as a Probe Molecule.

Carbon dioxide was shown to identify surface basic properties of nitrogen-substituted microporous and mesoporous silicas, in addition to conventional basic oxides, by a detailed study using isotherm and heat of adsorption measurements as well as by infrared spectroscopy. A hydrogen-bonded weak interaction was primarily observed between CO2 and silanol (Si-OH) and silamine (Si-NH-Si) groups. The heat of adsorption of CO2 demonstrated that the latter adspecies were formed preferentially over the former, although a much higher amount of linear CO2 adspecies were found on SBA-15 mesoporous silica because of the presence of a large quantity of silanol groups on its surface. Carbamate-type chemisorbed adspecies were not detected on silamino sites, whereas carbonate-type adspecies were formed on alkali ion-exchanged zeolites and also residual sodium ions on the surface of silicalite-1. CO2 was shown to be a successful probe molecule for identifying weakly interactive hydrogen-bonding sites, and it has potential as a surface probe for strongly interactive nucleophilic sites derived from alkaline ions or a methylated silamino group, Si-N(CH3)-Si.

Opening the Cages of Faujasite-Type Zeolite.

Zeolites are widely used in industrial processes, mostly as catalysts or adsorbents. Increasing their micropore volume could further improve their already exceptional catalytic and separation performances. We report a tunable extraction of zeolite framework cations (Si, Al) on a faujasite-type zeolite, the archetype of molecular sieves with cages and the most widely used as a catalyst and sorbent; this results in ca. 10% higher micropore volume with limited impact on its thermal stability. This increased micropore volume results from the opening of some of the small (sodalite) cages, otherwise inaccessible to most molecules. As more active sites become accessible, the catalytic performances for these modified zeolites are substantially improved. The method, based on etching with NH4F, is also applicable to other cage-containing microporous molecular sieves, where some of the most industrially relevant zeolites are found.