The confinement effect in zeolites

Multilevel quantum mechanical calculations show the role of promoter molecules in the dehydration of methanol to dimethyl ether in H-ZSM-5

Methyl carboxylate esters promote the formation of dimethyl ether (DME) from the dehydration of methanol in H-ZSM-5 zeolite. We employ a multilevel quantum method to explore the possible associative and dissociative mechanisms in the presence, and absence, of six methyl ester promoters. This hybrid method combines density functional theory, with dispersion corrections (DFT-D3), for the full periodic system, with second-order Møller-Plesset perturbation theory (MP2) for small clusters representing the reaction site, and coupled cluster with single, double, and perturbative triple substitution (CCSD(T)) for the reacting molecules. The calculated adsorption enthalpy of methanol, and reaction enthalpies of the dehydration of methanol to DME within H-ZSM-5, agree with experiment to within chemical accuracy (∼4 kJ mol-1). For the promoters, a reaction pathway via an associative mechanism gives lower overall reaction enthalpies and barriers compared to the reaction with methanol only. Each stage of this mechanism is explored and related to experimental data. We provide evidence that suggests the promoter's adsorption to the Brønsted acid site is the most important factor dictating its efficiency.

Microporosity and Catalytic Activity for Hydrodesulfurization of Pharmacosiderite Mo4P3O16 Synthesized at a Moderate Temperature

Pharmacosiderite Mo4P3O16 (Pharma-MoPO) consists of [Mo4O4] cubane unit and [PO4] tetrahedral to form an open framework with a microporous structure similar to that of LTA-type zeolite. Although attractive applications are expected due to its microporous structure and redox-active components, its physicochemical properties have been poorly investigated due to the specificity of its synthesis, which requires a high hydrothermal synthesis temperature of 360 °C. In this study, we succeeded in synthesizing Pharma-MoPO by hydrothermal synthesis at 230 °C, which can be applied using a commercially available autoclave by changing the metal source. Through the study of the solids and liquids obtained after hydrothermal syntheses, the formation process of Pharma-MoPO under our studied synthesis conditions was proposed. Advanced characterizations provided detailed structural information on Pharma-MoPO, including the location site of a countercation NH4+. Pharma-MoPO could adsorb CO2 with the amount close to the number of cages without removing NH4+. Pharma-MoPO exhibited stable catalytic activity for the hydrodesulfurization of thiophene while maintaining its crystal structure, except for the introduction of sulfide by replacing lattice oxygens. Pharmacosiderite Mo4P3O16 was successfully obtained by hydrothermal synthesis at a moderate temperature, and its microporosity for CO2 adsorption and catalytic properties for hydrodesulfurization were discovered.

Unveiling the mechanisms of carboxylic acid esterification on acid zeolites for biomass-to-energy: A review of the catalytic process through experimental and computational studies

In recent years, there has been significant interest from industrial and academic areas in the esterification of carboxylic acids catalyzed by acidic zeolites, as it represents a sustainable and economically viable approach to producing a wide range of high-value-added products. However, there is a lack of comprehensive reviews that address the intricate reaction mechanisms occurring at the catalyst interface at both the experimental and atomistic levels. Therefore, in this review, we provide an overview of the esterification reaction on acidic zeolites based on experimental and theoretical studies. The combination of infrared spectroscopy with atomistic calculations and experimental strategies using modulation excitation spectroscopy techniques combined with phase-sensitive detection is presented as an approach to detecting short-lived intermediates at the interface of zeolitic frameworks under realistic reaction conditions. To achieve this goal, this review has been divided into four sections: The first is a brief introduction highlighting the distinctive features of this review. The second addresses questions about the topology and activity of different zeolitic systems, since these properties are closely correlated in the esterification process. The third section deals with the mechanisms proposed in the literature. The fourth section presents advances in IR techniques and theoretical calculations that can be applied to gain new insights into reaction mechanisms. Finally, this review concludes with a subtle approach, highlighting the main aspects and perspectives of combining experimental and theoretical techniques to elucidate different reaction mechanisms in zeolitic systems.

Integrated Optimization of Crystal Facets and Nanoscale Spatial Confinement toward the Boosted Catalytic Performance of Pd Nanocrystals

Tuning the surface chemical property and the local environment of nanocrystals is crucial for realizing a high catalytic performance in various reactions. Herein, we aim to elucidate the structure sensitivity of Pd facets on the surface catalytic hydrogenation reaction and to identify what role the nanoconfinement effect plays in the catalytic properties of Pd nanocrystal catalysts. By controlling the coating structures of mesoporous silica (mSiO2) on Pd nanocrystals with different exposed facets that include {100}, {111}, and {hk0}, we present a series of Pd@mSiO2 nanoreactors in core-shell and yolk-shell structures and the discovery of a partial-coated structure, which can provide different types of nanoconfinement, and we propose a seed size-dominated growth mechanism. We demonstrate that a superior activity was exhibited in Pd nanocrystals enclosed by the {hk0} facet as compared to the Pd{100} and Pd{111} facets, and substantially enhanced efficiency and stability were achieved in Pd@mSiO2 particles with yolk-shell structures, indicating a crucial superiority of optimizing the configuration of crystal facets and nanoconfinement. Our study provides an efficient strategy to rationally design and optimize nanocatalysts for promoting catalytic performance.

Study of the diffusion properties of zeolite mixtures by combined gravimetric analysis, IR spectroscopy and inversion methods (IRIS)

We report the development of a new method of investigation of the mass transport properties of acidic zeolite-based materials aiming to overcome the limitations of classical approaches. It consists in hyphenating gravimetric analysis and infrared spectroscopy. The former allows assessing the diffusion from the gas phase to all the porosity, while IR allows for selective assessment of diffusion to the zeolite active sites located in the micropores. Furthermore, the data are processed by an original methodology allowing the recovery of the distribution of diffusion domains by inversion of the integral equations describing the uptake curves or the evolution of the infrared spectra. The combination of gravimetric analysis and IR spectroscopy makes it possible to monitor and distinguish diffusion within the various components of the material. The methodology has been applied to the isooctane uptake in the mechanical mixture of FAU and MFI zeolites. Analysis of both gravimetric uptake curves and evolving infrared spectra allows distinguishing and assigning diffusion domains to the H-FAU and H-MFI components of the mixture, with high and low effective diffusion rate constants, respectively. The advantages and limits of the methodology are discussed.

The role of 13X molecular sieves in photocatalytic nitrogen fixation

To overcome the weak adsorption and difficult activation of N2 on catalysts in the photocatalytic nitrogen reduction reaction (NRR), we put forward that the introduction of molecular sieve 13X may realize the enrichment and activation of N2. 13X and the photoactive substrate BiOBr were assembled electrostatically to construct composite catalysts. In the presence of 13X, they are rich in nitrogen adsorption and activation sites, and the highest ammonia yield can reach 360.5 μmol h-1 gcat-1. It is surprising to find that 13X is able to optimize the photoelectric properties. This work extends the function of molecular sieves in the NRR and offers guidance to design catalysts with high photocatalytic activity.

Zinc Single Atom Confinement Effects on Catalysis in 1T-Phase Molybdenum Disulfide

Active sites are atomic sites within catalysts that drive reactions and are essential for catalysis. Spatially confining guest metals within active site microenvironments has been predicted to improve catalytic activity by altering the electronic states of active sites. Using the hydrogen evolution reaction (HER) as a model reaction, we show that intercalating zinc single atoms between layers of 1T-MoS2 (Zn SAs/1T-MoS2) enhances HER performance by decreasing the overpotential, charge transfer resistance, and kinetic barrier. The confined Zn atoms tetrahedrally coordinate to basal sulfur (S) atoms and expand the interlayer spacing of 1T-MoS2 by ∼3.4%. Under confinement, the Zn SAs donate electrons to coordinated S atoms, which lowers the free energy barrier of H* adsorption-desorption and enhances HER kinetics. In this work, which is applicable to all types of catalytic reactions and layered materials, HER performance is enhanced by controlling the coordination geometry and electronic states of transition metals confined within active-site microenvironments.

Heterogeneous Photoredox Catalysis Based on Silica Mesoporous Material and Eosin Y: Impact of Material Support on Selectivity of Radical Cyclization

Heterogenization of the photocatalyst appears to be a valuable solution to reach sustainable processes. Rapid and efficient synthesis of supported photocatalyst is still a remaining challenge and the choice of the support material is crucial. The present study aims at preparing a new generation of hybrid inorganic/organic photocatalysts based on silica mesoporous material and Eosin Y. These results highlight the influence of non-covalent interactions between the material support and the reagent impacting the selectivity of the reaction.

Nitrogen Incorporation in Potassic and Micro- and Meso-Porous Minerals: Potential Biogeochemical Records and Targets for Mars Sampling

We measured the N concentrations and isotopic compositions of 44 samples of terrestrial potassic and micro- and meso-porous minerals and a small number of whole-rocks to determine the extent to which N is incorporated and stored during weathering and low-temperature hydrothermal alteration in Mars surface/near-surface environments. The selection of these minerals and other materials was partly guided by the study of altered volcanic glass from Antarctica and Iceland, in which the incorporation of N as NH4+ in phyllosilicates is indicated by correlated concentrations of N and the LILEs (i.e., K, Ba, Rb, Cs), with scatter likely related to the presence of exchanged, occluded/trapped, or encapsulated organic/inorganic N occurring within structural cavities (e.g., in zeolites). The phyllosilicates, zeolites, and sulfates analyzed in this study contain between 0 and 99,120 ppm N and have δ15Nair values of -34‰ to +65‰. Most of these minerals, and the few siliceous hydrothermal deposits that were analyzed, have δ15N consistent with the incorporation of biologically processed N during low-temperature hydrothermal or weathering processes. Secondary ion mass spectrometry on altered hyaloclastites demonstrates the residency of N in smectites and zeolites, and silica. We suggest that geological materials known on Earth to incorporate and store N and known to be abundant at, or near, the surface of Mars should be considered targets for upcoming Mars sample return with the intent to identify any signs of ancient or modern life.

Highly Active Ni-Ru Bimetallic Catalyst Integrated with MFI Zeolite-Loaded Cerium Zirconium Oxide for Dry Reforming of Methane

The dry reforming of methane (DRM) is a new potential technology that converts two major greenhouse gases into useful chemical feedstocks. The main challenge faced by this process is maintaining the catalyst with high catalytic activity and long-term stability. Here, a simple and effective preparation route for the synthesis of functional nanomolecular sieve catalysts (NiRu_xCZZ5) from kaolinite tailings was developed for dry reforming of methane with CO₂. The silica monoliths with flower-like spherical and micropore structures (ZSM-5) were prepared by crystal growth method, and the metal components were loaded by ultrasonic-assisted impregnation method. The NiRu_0.5CZZ5 catalyst exhibited excellent catalytic performance (maxmium CO₂ and CH₄ conversions up to 100 and 95.6%, respectively) and very good stability (up to 100h). The interfacial confinement and the strong support interaction are principally responsible for the excellent catalytic activity of the catalyst. The in situ DRIFTS was used to elucidate the possible carbon conversion steps, and stable surface intermediates were also identified.

Influence of ion mobility on the redox and catalytic properties of Cu ions in zeolites

This contribution aims at analysing the current understanding about the influence of Al distribution, zeolite topology, ligands/reagents and oxidation state on ions mobility in Cu-zeolites, and its relevance toward reactivity of the metal sites. The concept of Cu mobilization has been originally observed in the presence of ammonia, favouring the activation of oxygen by formation of NH3 oxo-bridged complexes in zeolites and opening a new perspective about the chemistry in single-site zeolite-based catalysts, in particular in the context of the NH3-mediated Selective Catalytic Reduction of NO x (NH3-SCR) processes. A different mobility of bare Cu+/Cu2+ ions has been documented too, showing for Cu+ a better mobilization than for Cu2+ also in absence of ligands. These concepts can have important consequences for the formation of Cu-oxo species, active and selective in other relevant reactions, such as the direct conversion of methane to methanol. Here, assessing the structure, the formation pathways and reactivity of Cu-oxo mono- or multimeric moieties still represents a challenging playground for chemical scientists. Translating the knowledge about Cu ions mobility and redox properties acquired in the context of NH3-SCR reaction into the field of direct conversion of methane to methanol can have important implications for a better understanding of transition metal ions redox properties in zeolites and for an improved design of catalysts and catalytic processes.

Property-activity relations of multifunctional reactive ensembles in cation-exchanged zeolites: a case study of methane activation on Zn2+-modified zeolite BEA

The reactivity theories and characterization studies for metal-containing zeolites are often focused on probing the metal sites. We present a detailed computational study of the reactivity of Zn-modified BEA zeolite towards C-H bond activation of the methane molecule as a model system that highlights the importance of representing the active site as the whole reactive ensemble integrating the extra-framework Zn_EF²⁺ cations, framework oxygens (O_F^2-), and the confined space of the zeolite pores. We demonstrate that for our model system the relationship between the Lewis acidity, defined by the probe molecule adsorption energy, and the activation energy for methane C-H bond cleavage performs with a determination coefficient R² = 0.55. This suggests that the acid properties of the localized extra-framework cations can be used only for a rough assessment of the reactivity of the cations in the metal-containing zeolites. In turn, studying the relationship between the activation energy and pyrrole adsorption energy revealed a correlation, with R² = 0.80. This observation was accounted for by the similarity between the local geometries of the pyrrole adsorption complexes and the transition states for methane C-H bond cleavage. The inclusion of a simple descriptor for zeolite local confinement allows transferability of the obtained property-activity relations to other zeolite topologies. Our results demonstrate that the representation of the metal cationic species as a synergistically cooperating active site ensembles allows reliable detection of the relationship between the acid properties and reactivity of the metal cation in zeolite materials.

Direct assessment of confinement effect in zeolite-encapsulated subnanometric metal species

Subnanometric metal species confined inside the microporous channels/cavities of zeolites have been demonstrated as stable and efficient catalysts. The confinement interaction between the metal species and zeolite framework has been proposed to play the key role for stabilization, though the confinement interaction is elusive to be identified and measured. By combining theoretical calculations, imaging simulation and experimental measurements based on the scanning transmission electron microscopy-integrated differential phase contrast imaging technique, we have studied the location and coordination environment of isolated iridium atoms and clusters confined in zeolite. The image analysis results indicate that the local strain is intimately related to the strength of metal-zeolite interaction and a good correlation is found between the zeolite deformation energy, the charge state of the iridium species and the local absolute strain. The direct observation of confinement with subnanometric metal species encapsulated in zeolites provides insights to understand their structural features and catalytic consequences.

Zeolite-encapsulated metal nanoparticles have important catalytic properties, but their effect on the zeolite local structure has been difficult to characterize. Here the authors, using DFT calculations and scanning transmission electron microscopy, characterize the local strain due to confinement effects in metal-zeolite catalysts.

Driving Forces of Cationic Dye Adsorption, Confinement, and Long-Range Correlation in Zeolitic Materials

Zeolitic materials are commonly used to capture emergent contaminants in water or complex aqueous effluents. The efficiency of this adsorption depends strongly on the guest-host interactions and on the surrounding environment with possible coadsorption of the solvent. Only a few experimental techniques are available to probe in situ the sequestration processes at the solid/liquid interface. We propose in the present work to combine the second harmonic scattering technique with isothermal titration calorimetry in order to investigate the adsorption and the confinement of a hemicyanine dye adsorbed inside faujasite materials. The methodology described here permits the quantification of the correlations between the confined dyes in the material and thus gives local information about the organization at the nanometer scale. Various impacts, such as the effect of the solvent type and the silicon to aluminum ratio of the zeolitic adsorbent, are quantitatively estimated and discussed. This work highlights that the most correlated system matches the higher adsorption capacity associated with the lower entropic contribution.

The concept of active site in heterogeneous catalysis

Catalysis is at the core of chemistry and has been essential to make all the goods surrounding us, including fuels, coatings, plastics and other functional materials. In the near future, catalysis will also be an essential tool in making the shift from a fossil-fuel-based to a more renewable and circular society. To make this reality, we have to better understand the fundamental concept of the active site in catalysis. Here, we discuss the physical meaning - and deduce the validity and, therefore, usefulness - of some common approaches in heterogeneous catalysis, such as linking catalyst activity to a 'turnover frequency' and explaining catalytic performance in terms of 'structure sensitivity' or 'structure insensitivity'. Catalytic concepts from the fields of enzymatic and homogeneous catalysis are compared, ultimately realizing that the struggle that one encounters in defining the active site in most solid catalysts is likely the one we must overcome to reach our end goal: tailoring the precise functioning of the active sites with respect to many different parameters to satisfy our ever-growing needs. This article ends with an outlook of what may become feasible within the not-too-distant future with modern experimental and theoretical tools at hand.

The confinement effects of ordered mesoporous carbon on copper nanoparticles for methanol oxidative carbonylation

The confinement of Cu nanoparticles in narrow mesopores strengthens the Cu–carbon interactions. This can promote the auto-reduction of CuO to form active Cu2O species, resulting in high activity and stability during DMC synthesis.

A Ferrocene Metal-Organic Framework Solid for Fe-Loaded Carbon Matrices and Nanotubes: High-Yield Synthesis and Oxygen Reduction Electrocatalysis

Using a carbon-rich designer metal-organic framework (MOF), we open a high-yield synthetic strategy for iron-nitrogen-doped carbon (Fe-N-C) nanotube materials that emulate the electrocatalysis performance of commercial Pt/C. The Zr(IV)-based MOF solid boasts multiple key functions: (1) a dense array of alkyne units over the backbone and the side arms, which are primed for extensive graphitization; (2) the open, branched structure helps maintain porosity for absorbing nitrogen dopants; and (3) ferrocene units on the side arms as atomically dispersed precursor catalyst for targeting micropores and for effective iron encapsulation in the carbonized product. As a result, upon pyrolysis, over 89% of the carbon component in the MOF scaffold is successfully converted into carbonized products, thereby contrasting the easily volatilized carbon of most MOFs. Moreover, over 97% of the iron ends up being encased as acid-resistant Fe/Fe₃C nanoparticles in carbon nanotubes/carbon matrices.

Activation and conversion of alkanes in the confined space of zeolite-type materials

Microporous zeolite-type materials, with crystalline porous structures formed by well-defined channels and cages of molecular dimensions, have been widely employed as heterogeneous catalysts since the early 1960s, due to their wide variety of framework topologies, compositional flexibility and hydrothermal stability. The possible selection of the microporous structure and of the elements located in framework and extraframework positions enables the design of highly selective catalysts with well-defined active sites of acidic, basic or redox character, opening the path to their application in a wide range of catalytic processes. This versatility and high catalytic efficiency is the key factor enabling their use in the activation and conversion of different alkanes, ranging from methane to long chain n-paraffins. Alkanes are highly stable molecules, but their abundance and low cost have been two main driving forces for the development of processes directed to their upgrading over the last 50 years. However, the availability of advanced characterization tools combined with molecular modelling has enabled a more fundamental approach to the activation and conversion of alkanes, with most of the recent research being focused on the functionalization of methane and light alkanes, where their selective transformation at reasonable conversions remains, even nowadays, an important challenge. In this review, we will cover the use of microporous zeolite-type materials as components of mono- and bifunctional catalysts in the catalytic activation and conversion of C₁⁺ alkanes under non-oxidative or oxidative conditions. In each case, the alkane activation will be approached from a fundamental perspective, with the aim of understanding, at the molecular level, the role of the active sites involved in the activation and transformation of the different molecules and the contribution of shape-selective or confinement effects imposed by the microporous structure.

Enhanced Catalysis under 2D Silica: A CO Oxidation Study

Interfacially confined microenvironments have recently gained attention in catalysis, as they can be used to modulate reaction chemistry. The emergence of a 2D nanospace at the interface between a 2D material and its support can promote varying kinetic and energetic schemes based on molecular level confinement effects imposed in this reduced volume. We report on the use of a 2D oxide cover, bilayer silica, on catalytically active Pd(111) undergoing the CO oxidation reaction. We "uncover" mechanistic insights about the structure-activity relationship with and without a 2D silica overlayer using in situ IR and X-ray spectroscopy and mass spectrometry methods. We find that the CO oxidation reaction on Pd(111) benefits from confinement effects imposed on surface adsorbates under 2D silica. This interaction results in a lower and more dispersed coverage of CO adsorbates with restricted CO adsorption geometries, which promote oxygen adsorption and lay the foundation for the formation of a reactive surface oxide that produces higher CO₂ formation rates than Pd alone.

In situandoperandoelectron microscopy in heterogeneous catalysis-insights into multi-scale chemical dynamics

This review features state-of-the-artin situandoperandoelectron microscopy (EM) studies of heterogeneous catalysts in gas and liquid environments during reaction. Heterogeneous catalysts are important materials for the efficient production of chemicals/fuels on an industrial scale and for energy conversion applications. They also play a central role in various emerging technologies that are needed to ensure a sustainable future for our society. Currently, the rational design of catalysts has largely been hampered by our lack of insight into the working structures that exist during reaction and their associated properties. However, elucidating the working state of catalysts is not trivial, because catalysts are metastable functional materials that adapt dynamically to a specific reaction condition. The structural or morphological alterations induced by chemical reactions can also vary locally. A complete description of their morphologies requires that the microscopic studies undertaken span several length scales. EMs, especially transmission electron microscopes, are powerful tools for studying the structure of catalysts at the nanoscale because of their high spatial resolution, relatively high temporal resolution, and complementary capabilities for chemical analysis. Furthermore, recent advances have enabled the direct observation of catalysts under realistic environmental conditions using specialized reaction cells. Here, we will critically discuss the importance of spatially-resolvedoperandomeasurements and the available experimental setups that enable (1) correlated studies where EM observations are complemented by separate measurements of reaction kinetics or spectroscopic analysis of chemical species during reaction or (2) real-time studies where the dynamics of catalysts are followed with EM and the catalytic performance is extracted directly from the reaction cell that is within the EM column or chamber. Examples of current research in this field will be presented. Challenges in the experimental application of these techniques and our perspectives on the field's future directions will also be discussed.

Reactivity descriptors in acid catalysis: acid strength, proton affinity and host–guest interactions

Acid strength and proton affinity, the independent properties of catalysts and molecules, are incomplete descriptors of because cations and conjugate anions reorganize their charges as they interact as bound intermediates and transition states.

Tuning product selectivity in the catalytic oxidation of glycerol by employing metal-ZSM-11 materials

Selective oxidation of glycerol towards dihidoxyacetone and lactic acid, employing micro/mesoporous zeolites with copper(ii) and chromium(iii).

Interplay of the adsorption of light and heavy paraffins in hydroisomerization over H-beta zeolite

Hydroisomerization: controlling selectivity by tuning the Pt/zeolite properties.

A Universal Descriptor for the Entropy of Adsorbed Molecules in Confined Spaces

Confinement of hydrocarbons in nanoscale pockets and pores provides tunable capability for controlling molecules in catalysts, sorbents, and membranes for reaction and separation applications. While computation of the enthalpic interactions of hydrocarbons in confined spaces has improved, understanding and predicting the entropy of confined molecules remains a challenge. Here we show, using a set of nine aluminosilicate zeolite frameworks with broad variation in pore and cavity structure, that the entropy of adsorption can be predicted as a linear combination of rotational and translational entropy. The extent of entropy lost upon adsorption is predicted using only a single material descriptor, the occupiable volume (V _occ). Predictive capability of confined molecular entropy permits an understanding of the relation with adsorption enthalpy, the ability to computationally screen microporous materials, and an understanding of the role of confinement on the kinetics of molecules in confined spaces.