Dissolution Behavior and Varied Mesoporosity of Zeolites by NH4 F Etching.

The mesopores formation in zeolite crystals has long been considered to occur through the stochastic hydrolysis and removal of framework atoms. Here, we investigate the NH4 F etching of representative small, medium, and large pore zeolites and show that the zeolite dissolution behavior, therefore the mesopore formation probability, is dominated by zeolite architecture at both nano- and sub-nano scales. At the nano-scale, the hidden mosaics of zeolite structure predetermine the spatio-temporal dissolution of the framework, hence the size, shape, location, and orientation of the mesopores. At the sub-nano scale, the intrinsic micropore size and connectivity jointly determine the diffusivity of reactant and dissolved products. As a result, the dissolution propensity varies from removing small framework fragments to consuming nanodomains and up to full digestion of the outmost part of zeolite crystals. The new knowledge will lead to new understanding of zeolite dissolution behavior and new adapted strategies for tailoring hierarchical zeolites.

Unraveling the Effect of Silanol Defects on the Insertion of Single-Site Mo in the MFI Zeolite Framework.

The preparation of defect-free MFI crystals containing single-site framework Mo through a hydrothermal postsynthesis treatment is reported. The insertion of single Mo sites in the MFI zeolite samples with different crystal sizes of 100, 200, and 2000 nm presenting a diverse concentration of silanol groups is revealed. The nature of the silanols and their role in the incorporation of Mo into the zeolite structure are elucidated through an extensive spectroscopic characterization (29Si NMR, 1H NMR, 31P NMR, and IR) combined with X-ray diffraction and HRTEM. In addition, a DFT-based theoretical modeling of a large Si154O354H92 nanoparticle containing 600 atoms is carried out to understand the expansion of the unit cell volume measured by X-ray diffraction. An accurate quantification of the silanols in the MFI crystals with different particle sizes and the insertion of Mo in the zeolitic framework is reported for the first time. The results confirmed that the non-H-bonded silanols seem to be the gateway for the insertion of single Mo atoms in the zeolite structure. Such materials with single metal sites present high crystallinity and perfect structure, thus providing great stability in catalytic applications.

Silanol defect engineering and healing in zeolites: opportunities to fine-tune their properties and performances.

Zeolites have been game-changing materials in oil refining and petrochemistry over the last 60 years and have the potential to play the same role in the emerging processes of the energy and environmental transition. Although zeolites are crystalline inorganic solids, their structures are not perfect and the presence of defect sites - mainly Brønsted acid sites and silanols - influences their thermal and chemical resistance as well as their performances in key areas such as catalysis, gas and liquid separations and ion-exchange. In this paper, we review the type of defects in zeolites and the characterization techniques used for their identification and quantification with the focus on diffraction, spectroscopic and modeling approaches. More specifically, throughout the review, we will focus on silanol (Si-OH) defects located within the micropore structure and/or on the external surface of zeolites. The main approaches applied to engineer and heal defects and their consequences on the properties and applications of zeolites in catalysis and separation processes are highlighted. Finally, the challenges and opportunities of silanol defect engineering in tuning the properties of zeolites to meet the requirements for specific applications are presented.

Unlocking the Potential of Hidden Sites in Faujasite: New Insights in a Proton Transfer Mechanism.

Zeolite Y and its ultra-stabilized hierarchical derivative (USY) are the most widely used zeolite-based heterogeneous catalysts in oil refining, petrochemisty, and other chemicals manufacturing. After almost 60 years of academic and industrial research, their resilience is unique as no other catalyst displaced them from key processes such as FCC and hydrocracking. The present study highlights the key difference leading to the exceptional catalytic performance of USY versus the parent zeolite Y in a multi-technique study combining advanced spectroscopies (IR and solid-state NMR) and molecular modeling. The results highlight a hitherto unreported proton transfer involving inaccessible active sites in sodalite cages that contributes to the exceptional catalytic performance of USY.

Probing the Brønsted Acidity of the External Surface of Faujasite-Type Zeolites.

We outline two methodologies to selectively characterize the Brønsted acidity of the external surface of FAU-type zeolites by IR and NMR spectroscopy of adsorbed basic probe molecules. The challenge and goal are to develop reliable and quantitative IR and NMR methodologies to investigate the accessibility of acidic sites in the large pore FAU-type zeolite Y and its mesoporous derivatives often referred to as ultra-stable Y (USY). The accessibility of their Brønsted acid sites to probe molecules (n-alkylamines, n-alkylpyridines, n-alkylphosphine- and phenylphosphine-oxides) of different molecular sizes is quantitatively monitored either by IR or 31 P NMR spectroscopy. It is now possible, for the first time to quantitatively discriminate between the Brønsted acidity located in the microporosity and on the external surface of large pore zeolites. For instance, the number of external acid sites on a Y (LZY-64) zeolite represents 2 % of its total acid sites while that of a USY (CBV760) represents 4 % while the latter has a much lower framework Si/Al ratio.

Emphasis on the Properties of Metal-Containing Zeolites Operating Outside the Comfort Zone of Current Heterogeneous Catalytic Reactions.

The development of catalysts that can operate under exceptionally harsh and unconventional conditions is of critical importance for the transition of the energy and chemicals industries to low-emission and renewable chemical feedstocks. In this review we will highlight materials and more specifically metal-containing zeolite catalysts that have been tested under harsh reaction conditions such as high temperature light alkane conversion and biomass valorization. Particular attention will be given to studies that explore the stability and recyclability of metal-containing zeolite catalysts operating in continuous modes. Metal-containing zeolites are considered as an important class of catalysts operating outside the comfort zone of current heterogeneous catalytic reactions in both gas and liquid phase reactions. The relationship between the properties of the metal-containing zeolite and catalytic performance will be explored.

Novel Strategy for the Synthesis of Ultra-Stable Single-Site Mo-ZSM-5 Zeolite Nanocrystals.

The current energy transition presents many technological challenges, such as the development of highly stable catalysts. Herein, we report a novel "top-down" synthesis approach for preparation of a single-site Mo-containing nanosized ZSM-5 zeolite which has atomically dispersed framework-molybdenum homogenously distributed through the zeolite crystals. The introduction of Mo heals most of the native point defects in the zeolite structure resulting in an extremely stable material. The important features of this single-site Mo-containing ZSM-5 zeolite are provided by an in-depth spectroscopic and microscopic analysis. The material demonstrates superior thermal (up to 1000 °C), hydrothermal (steaming), and catalytic (converting methane to hydrogen and higher hydrocarbons) stability, maintaining the atomically disperse Mo, structural integrity of the zeolite, and preventing the formation of silanols.

Direct Evidence for Single Molybdenum Atoms Incorporated in the Framework of MFI Zeolite Nanocrystals.

Direct evidence of the successful incorporation of atomically dispersed molybdenum (Mo) atoms into the framework of nanosized MFI zeolite is demonstrated for the first time. Homogeneous distribution of Mo with a size of 0.05 nm is observed by scanning transmission electron microscopy high-angle annular dark-field imaging (STEM-HAADF). 31P magic-angle spinning nuclear magnetic resonance (MAS NMR) and Fourier-transform infrared (FT-IR) spectroscopy, using trimethylphosphine oxide (TMPO) and deuterated acetonitrile as probe molecules, reveal a homogeneous distribution of Mo in the framework of MFI nanozeolite, and the presence of Lewis acidity. 31P MAS NMR using TMPO shows probe molecules interacting with isolated Mo atoms in the framework, and physisorbed probe molecules in the zeolite channels. Moreover, 2D 31P-31P MAS radio frequency-driven recoupling NMR indicates the presence of one type of Mo species in different crystallographic positions in the MFI framework. The substitution of framework Si by Mo significantly reduces the silanol defect content, making the resulting zeolite highly hydrophobic. In addition, the insertion of Mo into the MFI structure induces a symmetry lowering, from orthorhombic ( Pnma), typical of high silica MFI, to monoclinic ( P21/ n), as well as an expansion of unit cell volume. The novel material opens many opportunities of catalysts design for application in mature and emerging fields.

One-pot synthesis of silanol-free nanosized MFI zeolite.

The synthesis of nanostructured zeolites enables modification of catalytically relevant properties such as effective surface area and diffusion path length. Nanostructured zeolites may be synthesized either in alkaline media, and so contain significant numbers of hydrophilic silanol groups, or in expensive and harmful fluoride-containing media. Here, we report and characterize, using a combination of experimental and theoretical techniques, the one-pot synthesis of silanol-free nanosized MFI-type zeolites by introducing atomically dispersed tungsten; this prevents silanol group occurrence by forming flexible W-O-Si bridges. These W-O-Si bonds are more stable than Si-O-Si in the all-silica MFI zeolite. Tungsten incorporation in nanosized MFI crystals also modifies other properties such as structural features, hydrophobicity and Lewis acidity. The effect of these is illustrated on the catalytic epoxidation of styrene and separation of CO2 and NO2. Silanol-free nanosized W-MFI zeolites open new perspectives for catalytic and separation applications.

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.

Template-free nanosized faujasite-type zeolites.

Nanosized faujasite (FAU) crystals have great potential as catalysts or adsorbents to more efficiently process present and forthcoming synthetic and renewable feedstocks in oil refining, petrochemistry and fine chemistry. Here, we report the rational design of template-free nanosized FAU zeolites with exceptional properties, including extremely small crystallites (10-15 nm) with a narrow particle size distribution, high crystalline yields (above 80%), micropore volumes (0.30 cm(3) g(-1)) comparable to their conventional counterparts (micrometre-sized crystals), Si/Al ratios adjustable between 1.1 and 2.1 (zeolites X or Y) and excellent thermal stability leading to superior catalytic performance in the dealkylation of a bulky molecule, 1,3,5-triisopropylbenzene, probing sites mostly located on the external surface of the nanosized crystals. Another important feature is their excellent colloidal stability, which facilitates a uniform dispersion on supports for applications in catalysis, sorption and thin-to-thick coatings.

The Mosaic Structure of Zeolite Crystals.

Zeolites are widely used in many commercial processes, mostly as catalysts or adsorbents. Understanding their intimate structure at the nanoscale is the key to control their properties and design the best materials for their ever increasing uses. Herein, we report a new and controllable fluoride treatment for the non-discriminate extraction of zeolite framework cations. This sheds new light on the sub-structure of commercially relevant zeolite crystals: they are segmented along defect zones exposing numerous nanometer-sized crystalline domains, separated by low-angle boundaries, in what were apparent single-crystals. The concentration, morphology, and distribution of such domains analyzed by electron tomography indicate that this is a common phenomenon in zeolites, independent of their structure and chemical composition. This is a milestone to better understand their growth mechanism and rationally design superior catalysts and adsorbents.

Photochemical preparation of silver nanoparticles supported on zeolite crystals.

A facile and rapid photochemical method for preparing supported silver nanoparticles (Ag-NPs) in a suspension of faujasite type (FAU) zeolite nanocrystals is described. Silver cations are introduced by ion exchange into the zeolite and subsequently irradiated with a Xe-Hg lamp (200 W) in the presence of a photoactive reducing agent (2-hydroxy-2-methylpropiophenone). UV-vis characterization indicates that irradiation time and intensity (I0) influence significantly the amount of silver cations reduced. The full reduction of silver cations takes place after 60 s of a polychromatic irradiation, and a plasmon band of Ag-NPs appears at 380 nm. Transmission electron microscopy combined with theoretical calculation of the plasmon absorbance band using Mie theory shows that the Ag-NPs, stabilized in the micropores and on the external surface of the FAU zeolite nanocrystals, have an almost spheroidal shape with diameters of 0.75 and 1.12 nm, respectively. Ag-NPs, with a homogeneous distribution of size and morphology, embedded in a suspension of FAU zeolites are very stable (∼8 months), even without stabilizers or capping agents.

Methane-to-chemicals: a pathway to decarbonization.

The utilization of methane for chemical production, often considered as the future of petrochemistry, historically could not compete economically with conventional processes due to higher investment costs. Achieving sustainability and decarbonization of the downstream industry by integration with a methane-to-chemicals process may provide an opportunity to unlock the future for these technologies. Gas-to-chemicals is an efficient tool to boost the decarbonization potential of renewable energy. While the current implementation of carbon capture utilization and storage (CCUS) technologies is of great importance for industrial decarbonization, a shift to greener CO2-free processes and CO2 utilization from external sources for manufacturing valuable goods is highly preferred. This review outlines potential options for how a methane-to-chemicals process could support decarbonization of the downstream industry.

Exploration, explanation and exploitation of hydroxyls in zeolites.

The precise location and role of all types of hydroxyls in zeolites are still enigmatic, and their control permits tailoring of novel properties increasing the efficiency of catalysts and adsorbents in industrial processes for cleaner energy.

Understanding the Fundamentals of Microporosity Upgrading in Zeolites: Increasing Diffusion and Catalytic Performances.

Hierarchical zeolites are regarded as promising catalysts due to their well-developed porosity, increased accessible surface area, and minimal diffusion constraints. Thus far, the focus has been on the creation of mesopores in zeolites, however, little is known about a microporosity upgrading and its effect on the diffusion and catalytic performance. Here the authors show that the "birth" of mesopore formation in faujasite (FAU) type zeolite starts by removing framework T atoms from the sodalite (SOD) cages followed by propagation throughout the crystals. This is evidenced by following the diffusion of xenon (Xe) in the mesoporous FAU zeolite prepared by unbiased leaching with NH4 F in comparison to the pristine FAU zeolite. A new diffusion pathway for the Xe in the mesoporous zeolite is proposed. Xenon first penetrates through the opened SOD cages and then diffuses to supercages of the mesoporous zeolite. Density functional theory (DFT) calculations indicate that Xe diffusion between SOD cage and supercage occurs only in hierarchical FAU structure with defect-contained six-member-ring separating these two types of cages. The catalytic performance of the mesoporous FAU zeolite further indicates that the upgraded microporosity facilitates the intracrystalline molecular traffic and increases the catalytic performance.

Hierarchical ZSM-5 zeolites in shape-selective xylene isomerization: role of mesoporosity and acid site speciation.

The isomerization of o-xylene, a prototypical example of shape-selective catalysis by zeolites, was investigated on hierarchical porous ZSM-5. Extensive intracrystalline mesoporosity in ZSM-5 was introduced by controlled silicon leaching with NaOH. In addition to the development of secondary porosity, the treatment also induced substantial aluminum redistribution, increasing the density of Lewis acid sites located at the external surface of the crystals. However, the strength of the remaining Brønsted sites was not changed. The mesoporous zeolite displayed a higher o-xylene conversion than its parent, owing to the reduced diffusion limitations. However, the selectivity to p-xylene decreased, and fast deactivation due to coking occurred. This is mainly due to the deleterious effect of acidity at the substantially increased external surface and near the pore mouths. A consecutive mild HCl washing of the hierarchical zeolite proved effective to increase the p-xylene selectivity and reduce the deactivation rate. The HCl-washed hierarchical ZSM-5 displayed an approximately twofold increase in p-xylene yield compared to the purely microporous zeolite. The reaction was followed by operando infrared spectroscopy to simultaneously monitor the catalytic performance and the buildup of carbonaceous deposits on the surface. Our results show that the interplay between activity, selectivity, and stability in modified zeolites can be optimized by relatively simple post-synthesis treatments, such as base leaching (introduction of mesoporosity) and acid washing (surface acidity modification).

The preparation of hierarchical SAPO-34 crystals via post-synthesis fluoride etching.

SAPO-34 crystals are etched in fluoride medium. The interface between crystalline domains is dissolved and yields a hierarchical material with a system of straight intersecting mesopores that improve the access to micropore space.

Advances in nanosized zeolites.

This review highlights recent developments in the synthesis of nanosized zeolites. The strategies available for their preparation (organic-template assisted, organic-template free, and alternative procedures) are discussed. Major breakthroughs achieved by the so-called zeolite crystal engineering and encompass items such as mastering and using the physicochemical properties of the precursor synthesis gel/suspension, optimizing the use of silicon and aluminium precursor sources, the rational use of organic templates and structure-directing inorganic cations, and careful adjustment of synthesis conditions (temperature, pressure, time, heating processes from conventional to microwave and sonication) are addressed. An on-going broad and deep fundamental understanding of the crystallization process, explaining the influence of all variables of this complex set of reactions, underpins an even more rational design of nanosized zeolites with exceptional properties. Finally, the advantages and limitations of these methods are addressed with particular attention to their industrial prospects and utilization in existing and advanced applications.

Hydroisomerization of emerging renewable hydrocarbons using hierarchical Pt/H-ZSM-22 catalyst.

Last site standing: A new generation of hierarchical Pt/H-ZSM-22 zeolites is designed for the efficient processing of upcoming renewable feedstocks. The enhanced accessibility of the active sites is vital for the superior activity and exceptional selectivity in the hydroisomerization of model molecules such as nonadecane and pristane.