Distribution of Aluminum Species in Zeolite Catalysts: 27Al NMR of Framework, Partially-Coordinated Framework, and Non-Framework Moieties

Redox and Kinetic Properties of Composition-Dependent Active Sites in Copper-Exchanged Chabazite for Direct Methane-to-Methanol Oxidation

The CH4 oxidation performance of Cu-chabazite zeolites characterized by distinct Si/Al ratios and Cu loadings has been studied and the observed variations in reactivity have been correlated to the differences in the nature of the formed active centers. Plug flow reactor tests, in situ Fourier-transform infrared, and X-ray absorption spectroscopy demonstrate that a decrease in Cu loading shifts the reactivity/redox profile to higher temperatures and increases the CH3OH selectivity and Cu-efficiency. In situ electron paramagnetic resonance, Raman, ultraviolet-visible, Fourier-transform infrared, and photoluminescence spectroscopies reveal that this behavior is associated with the presence of monomeric Cu active sites, including bare Cu2+ and [CuOH]+ present at low Si/Al ratio and Cu loading. Formation of two distinct [Cu2(μ-O)]2+ moieties at higher Si/Al ratio or Cu loading forces these trends into the opposite direction. Operando electron paramagnetic resonance and ultraviolet-visible spectroscopy show that the apparent activation energy of monomeric Cu active species decreases with increasing Si/Al ratio, whereas the one of dimeric centers is unaffected.

Revealing the Bro̷nsted Acidic Nature of Penta-Coordinated Aluminum Species in Dealuminated Zeolite Y with Solid-State NMR Spectroscopy

The inevitable dealumination process of zeolite Y is closely related to ultrastabilization, enhanced Bro̷nsted acidity, and deactivation throughout its life cycle, producing complex aluminum and acidic hydroxyl species. Most investigations on dehydrated zeolites have focused on the Bro̷nsted acidity of tetra-coordinated Al (Al(IV)) and Lewis acidity associated with tricoordinated Al (Al(III)) sites, which has left the penta-coordinated Al (Al(V)) in dealuminated zeolites scarcely discussed. This is largely due to the oversimplified view of detectable Al(V) as an exclusively extra-framework species with Lewis acidity. Here we report the formation of Bro̷nsted acidic penta-coordinated Al species (Al(V)-BAS) in the dealumination process. Two-dimensional (2D) through-bond and multiquantum 1H-{27Al} correlation solid-state NMR experiments demonstrate the presence of a bridging Si-OH-Al(V) structure in dealuminated Y zeolites. Different from the conventional belief that water attack leads to the breaking of zeolite framework Al-O bonds in the initial stage of zeolite dealumination, the observed Al(V) as a dealumination intermediate is directly correlated with a BAS pair because of the direct dissociation of water on the framework tetrahedral aluminum (Al(IV)), thus bypassing the cleavage of Al-O bonds. 1H double-quantum solid-state NMR experiments and theoretical calculations provide further evidence for this mechanism, whereas pyridine adsorption experiments confirm stronger acidity of Al(V)-BASs than the traditional bridging hydroxyl groups associated with Al(IV). We were also able to detect the Al(V)-BAS site from dealuminated SSZ-13 zeolite with CHA topology, suggesting that its creation is not specific to the framework structure of zeolites.

Nature of Reactive Sites in TS-1 from 15N Solid-State NMR and Ti K-Edge X-Ray Absorption Spectroscopic Signatures Upon Pyridine Adsorption

Ti-containing zeotypes, notably titanosilicalite-1 (TS-1), are prominent examples of heterogeneous catalysts that have found applications in selective oxidation processes with hydrogen peroxide. Despite extensive characterization studies including using various probe molecules to interrogate the nature and the local environment of Ti sites, their detailed structure (as well as reactivity) remains elusive. Here, we demonstrate that using low temperature 15N magic angle spinning (MAS) ssNMR spectroscopy of adsorbed pyridine on TS-1 combined with Ti K-edge XANES on a range of samples (dehydrated, hydrated, contacted with H2O2 and pyridine) provides unique information regarding the Ti sites, highlighting their reactivity and dynamic nature. While dehydrated TS-1 shows only Lewis acid sites, the presence of H2O generates Brønsted acid sites, whose amount correlates with water loading. Moreover, the methodology─based on 15N ssNMR and Ti K-edge XANES─applied to a library of samples with various Ti-loadings and absence of extraframework TiO2 also enables quantification of the amount of Lewis acid sites and to establish a structure-activity descriptor (ratio of pyridine adsorbed on silanols vs titanium). Complementary analysis including computational modeling reveals that the reaction of Ti sites with H2O generates an acidic bridging silanol Ti-(OH)-Si, upon hydrolysis of one Ti-O-Si linkage, where Ti expands its coordination from four to pentacoordinated according to XAS.

Advances and challenges in designing active site environments in zeolites for Brønsted acid catalysis

Zeolites contain proton active sites in diverse void environments that stabilize the reactive intermediates and transition states formed in converting hydrocarbons and oxygenates to chemicals and energy carriers. The catalytic diversity that exists among active sites in voids of varying sizes and shapes, even within a given zeolite topology, has motivated research efforts to position and quantify active sites within distinct voids (synthesis-structure) and to link active site environment to catalytic behavior (structure-reactivity). This Feature Article describes advances and challenges in controlling the position of framework Al centers and associated protons within distinct voids during zeolite synthesis or post-synthetic modification, in identifying and quantifying distinct active site environments using characterization techniques, and in determining the influence of active site environments on catalysis. During zeolite synthesis, organic structure directing agents (SDAs) influence Al substitution at distinct lattice positions via intermolecular interactions (e.g., electrostatics, hydrogen bonding) that depend on the size, structure, and charge distribution of organic SDAs and their mobility when confined within zeolitic voids. Complementary post-synthetic strategies to alter intrapore active site distributions include the selective removal of protons by differently-sized titrants or unreactive organic residues and the selective exchange of framework heteroatoms of different reactivities, but remain limited to certain zeolite frameworks. The ability to identify and quantify active sites within distinct intrapore environments depends on the resolution with which a given characterization technique can distinguish Al T-site positions or proton environments in a given zeolite framework. For proton sites in external unconfined environments, various (post-)synthetic strategies exist to control their amounts, with quantitative methods to distinguish them from internal sites that largely depend on using stoichiometric or catalytic probes that only interact with external sites. Protons in different environments influence reactivity by preferentially stabilizing larger transition states over smaller precursor states and influence selectivity by preferentially stabilizing or destabilizing competing transition states of varying sizes that share a common precursor state. We highlight opportunities to address challenges encountered in the design of active site environments in zeolites by closely integrating precise (post-)synthetic methods, validated characterization techniques, well-defined kinetic probes, and properly calibrated theoretical models. Further advances in understanding the molecular details that underlie synthesis-structure-reactivity relationships for active site environments in zeolite catalysis can accelerate the predictive design of tailored zeolites for desired catalytic transformations.

A machine learning approach for dynamical modelling of Al distributions in zeolites via23Na/27Al solid-state NMR

One of the main limitations in supporting experimental characterization of Al siting/pairing via modelling is the high computational cost of ab initio calculations. For this reason, most works rely on static or very short dynamical simulations, considering limited Al pairing/siting combinations. As a result, comparison with experiment suffers from a large degree of uncertainty. To alleviate this limitation we have developed neural network potentials (NNPs) which can dynamically sample across broad configurational and chemical spaces of sodium-form aluminosilicate zeolites, preserving the level of accuracy of the ab initio (dispersion-corrected metaGGA) training set. By exploring a wide range of Al/Na arrangements and a combination of experimentally relevant Si/Al ratios, we found that the 23Na NMR spectra of dehydrated high-silica CHA zeolite offer an opportunity to assess the distribution and pairing of Al atoms. We observed that the 23Na chemical shift is sensitive not only to the location of sodium in 6- and 8MRs, but also to the Al-Sin-Al sequence length. Furthermore, neglect of thermal and dynamical contributions was found to lead to errors of several ppm, and has a profound influence on the shape of the spectra and the dipolar coupling constants, thus necessitating the long-term dynamical simulations made feasible by NNPs. Finally, we obtained a predictive regression model for the 23Na chemical shift in CHA (Si/Al = 35, 17, 11) that circumvents the need for expensive NMR density functional calculations and can be easily extended to other zeolite frameworks. By combining NNPs and regression methods, we can expedite the simulations of NMR properties and capture the effect of dynamics on the spectra, which is often overlooked in computational studies despite its clear manifestation in experimental setups.

Enhancing the Acidity Window of Zeolites by Low-Temperature Template Oxidation with Ozone

Revisiting the impact of the first and often deemed trivial postsynthetic step, i.e., a high-temperature oxidative calcination to remove organic templates, increases our understanding of thermal acid site evolution and Al distributions. An unprecedented degree of control over the acidity of high-silica zeolites (SSZ-13) was achieved by using a low-temperature ozonation approach. Fourier transform infrared spectroscopy of adsorbed probe molecules and solid-state NMR spectroscopy reveal the complexity of the thermal evolution of acid sites. Low-temperature activated (ozonated) zeolites maintain the original Brønsted acidity content and high defect content and have virtually no Lewis acidity. They also preserve the "as-made" Al distribution after crystallization and show a clear link between synthesis conditions and divalent cation capacity, as measured with aqueous cobalt ion uptake. The synthesis protocol is found to be the main contributor to Al proximity, yielding record high exchange capacity when ozonated. After conventional calcination at 500-600 °C, however, the presence of water leads to the gradual depletion of Brønsted acid sites, in particular, in small crystals. This work indicates that low-temperature ozonation followed by thermal activation at different temperatures can be used as a novel tool for tuning the amount and nature of acid sites, providing insights into the activity of zeolites in acid-catalyzed reactions, such as CO2 hydrogenation to dimethyl ether, and thereby expanding the possibilities of rational acidity tuning.

Constructing Hierarchical Zeolites with Highly Complete Framework via Controlled Desilication

Desilication in alkaline medium has been widely used in construction of hierarchical zeolites for industrially relevant catalytic processes. The built of hierarchy in zeolites, especially with low aluminum stability or high Si/Al ratio, often suffers from uncontrolled destruction of zeolitic framework, accompanied by a significant loss of microporous domains and intrinsic acidity after desilication. Here, we report a novel and simple methodology for preparation of hierarchical zeolites with highly complete framework and minimum sacrifice of microporosity and acidity. The pre-impregnated amines in zeolite micropores act as inner pore-directing agents (iPDAs), largely protecting the zeolitic framework and moderating the silicon extraction during the alkaline treatment. The resulting hierarchical zeolites exhibit high crystallinity, tunable hierarchy, stable framework, and well-preserved acidity, endowing them with significantly improved mass transport properties and enhanced activities in catalytic conversion of methanol or furfural.

Defects tune the acidic strength of amorphous aluminosilicates

Crystalline zeolites have high acidity but limited utility due to microporosity, whereas mesoporous amorphous aluminosilicates offer better porosity but lack sufficient acidity. In this work, we investigated defect engineering to fine-tune the acidity of amorphous acidic aluminosilicates (AAS). Here we introduced oxygen vacancies in AAS to synthesize defective acidic aluminosilicates (D-AAS). 1H, 27Al, and 17O solid-state nuclear magnetic resonance (NMR) studies indicated that defects induced localized structural changes around the acidic sites, thereby modifying their acidity. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy studies substantiated that oxygen vacancies alter the chemical environment of Brønsted acidic sites of AAS. The effect of defect creation in AAS on its acidity and catalytic behavior was demonstrated using four different acid-catalyzed reactions namely, styrene oxide ring opening, vesidryl synthesis, Friedel-Crafts alkylation, and jasminaldehyde synthesis. The defects played a role in activating reactants during AAS-catalyzed reactions, enhancing the overall catalytic process. This was supported by in-situ FTIR, which provided insights into the molecular-level reaction mechanism and the role of defects in reactant activation. This study demonstrates defect engineering as a promising approach to fine-tune acidity in amorphous aluminosilicates, bridging the porosity and acidity gaps between mesoporous amorphous aluminosilicates and crystalline zeolites.

Engineering In-Co3O4/H-SSZ-39(OA) Catalyst for CH4-SCR of NOx: Mild Oxalic Acid (OA) Leaching and Co3O4 Modification

Zeolite-based catalysts efficiently catalyze the selective catalytic reduction of NOx with methane (CH4-SCR) for the environmentally friendly removal of nitrogen oxides, but suffer severe deactivation in high-temperature SO2- and H2O-containing flue gas. In this work, SSZ-39 zeolite (AEI topology) with high hydrothermal stability is reported for preparing CH4-SCR catalysts. Mild acid leaching with oxalic acid (OA) not only modulates the Si/Al ratio of commercial SSZ-39 to a suitable value, but also removes some extra-framework Al atoms, introducing a small number of mesopores into the zeolite that alleviate diffusion limitation. Additional Co3O4 modification during indium exchange further enhances the catalytic activity of the resulting In-Co3O4/H-SSZ-39(OA). The optimized sample exhibits remarkable performance in CH4-SCR under a gas hourly space velocity (GHSV) of 24,000 h-1 and in the presence of 5 vol% H2O. Even under harsh SO2- and H2O-containing high-temperature conditions, it shows satisfactory stability. Catalysts containing Co3O4 components demonstrate much higher CH4 conversion. The strong mutual interaction between Co3O4 and Brønsted acid sites, confirmed by the temperature-programmed desorption of NO (NO-TPD), enables more stable NxOy species to be retained in In-Co3O4/H-SSZ-39(OA) to supply further reactions at high temperatures.

Enhancing Stability of SAPO-37 Molecular Sieve through Aluminum Phosphate Utilization: Synthesis, Stability Mechanism, and Catalytic Performance

SAPO-37 molecular sieve, characterized by its three-dimensional 12-membered-ring FAU structure, has drawn wide attention due to its unique properties and catalytic potential. However, its susceptibility to framework collapse under low-temperature and humid conditions hinders practical applications, affecting both the reaction performance and sample storage. To tackle this, we utilized aluminum phosphate as a precursor for synthesizing SAPO-37, aiming to modify Si incorporation mechanisms and improve P and Al environments. Solid NMR spectroscopy combined with other techniques proves that the resulting SAPO-37-AP has enriched silicon islands, leading to reduced water adsorption, more reversible structural change, and significantly enhanced stability after low-temperature vapor treatment compared to conventional SAPO-37. Remarkably, SAPO-37-AP, after water vapor treatment, still exhibits superior performance in the liquid-phase Beckmann rearrangement reaction. This approach enhances stability, reduces templating agent amounts, and improves the solid product yield, offering promising practical applications.

Anchoring PdOx clusters on defective alumina for improved catalytic methane oxidation

Evolution of the Pd active centers in size and spatial distribution leads to an irreversible deactivation in many high-temperature catalytic processes. This research demonstrates the use of a defective alumina (Al2O3-x) as catalyst support to anchor Pd atoms and suppress the growth of Pd clusters in catalytic methane oxidation. A combination of operando spectroscopy and density functional theory (DFT) calculations provide insights into the evolution of Pd species and reveals distinct catalytic methane oxidation mechanisms on Pd single atoms, clusters, and nanoparticles (NPs). Among these Pd species, the cluster active centers are found to be the most favorable participants in methane oxidation due to their high dispersion, high content of Pd2+ oxidation state, and resistance to deactivation by carbonates, bicarbonates, and water. The Pd/Al2O3-x catalyst shows increased stability with respect to a Pd/Al2O3 counterpart during simulated aging in alternating reducing and oxidizing conditions due to stronger interactions with the support. This study demonstrates that defect engineering of non-reducible supports can constrain the evolution of active centers, which holds promising potential for widespread utilization across diverse industrial catalytic processes, including various hydrogenation and oxidation reactions.

Understanding the Structural and Catalytic Properties of Al(IV)-2 Acidic Sites of ZSM-5

It is crucial to identify the structures of active sites to understand how catalysts function and to use that understanding to develop better catalytic materials. ZSM-5 zeolites with dominant Al(IV)-2 sites have been developed in this work. 1H-27Al 2D HMQC and 2D 1H TQ(DQ)-SQ NMR experiments have been performed to investigate the structural properties of this acidic site. The Al(IV)-2 sites have Brønsted and Lewis acid characteristics. The catalytic performance of Al(IV)-2 sites has been tested by n-dodecane cracking reactions. The catalytic results show that the Brønsted acidic strength of the Al(IV)-2 sites is comparable to that of the Al(IV)-1 sites, but the Al(IV)-2 sites' Lewis acid characteristics provide extra catalytic activity. We have gained valuable insights into the characteristics of Al(IV)-2 acid sites within these materials.

Effective C4 Separation by Zeolite Metal-Organic Framework Composite Membranes

Inorganic zeolites have excellent molecular sieving properties, but they are difficult to process into macroscopic structures. In this work, we use metal-organic framework (MOF) glass as substrates to engineer the interface with inorganic zeolites, and then assemble the discrete crystalline zeolite powders into monolithic structures. The zeolites are well dispersed and stabilized within the MOF glass matrix, and the monolith has satisfactory mechanical stabilities for membrane applications. We demonstrate the effective separation performance of the membrane for 1,3-butadiene (C4H6) from other C4 hydrocarbons, which is a crucial and challenging separation in the chemical industry. The membrane achieves a high permeance of C4H6 (693.00±21.83 GPU) and a high selectivity over n-butene, n-butane, isobutene, and isobutane (9.72, 9.94, 10.31, and 11.94, respectively). This strategy opens up new possibilities for developing advanced membrane materials for difficult hydrocarbon separations.

Modulation of Al Distribution in High-Silica ZSM-5 Zeolites for Enhancing Catalytic Performance

The spatial distribution of framework Al (AlF) has been one of the important factors that affect the catalytic properties of zeolites in diverse chemical reactions; however, the synthesis of high-silica zeolites with special AlF distribution remains a challenge. In this study, we successfully synthesized high-silica ZSM-5 zeolites with a unique AlF distribution by employing pentaerythritol (PET) as an additive in the presence of a few tetrapropylammonium hydroxide (TPAOH). The results demonstrated that the introduction of PET led to a higher proportion of Al atoms located at the sinusoidal and/or straight channels. It was observed that the addition of PET prevented the interaction between TPA+ and tetrahedral [AlO4]- during the crystallization process, resulting in enhanced availability of TPA species in the form of ion-paired TPA+. This effect leads to AlF atoms dominantly distributed away from the intersection and located in narrow channels, where acidic sites more effectively inhibit hydrogen transfer and coke formation. In the reaction of dimethyl ether (DME) to olefins, the catalyst with a unique Al distribution exhibited a significant prolonged catalytic lifetime, surpassing traditional TPA-ZSM-5 by more than 2-fold and maintaining DME conversion above 90% for a maximum of 148 h. The results of multiple pulse experiments also showed that these PET-assisted ZSM-5 zeolites significantly enhanced the selectivity of propene and butene. This approach provides an effective strategy to regulate AlF distribution in high-silica ZSM-5 catalysts with the assistance of neutral alcohol. It holds great potential for application in the synthesis of other high-silica zeolites, thereby enriching the diversity of zeolite catalysis.

Direct Oxidation of Methane to Methanol over Transition-Metal-Free Ferrierite Zeolite Catalysts

Direct oxidation of methane to methanol was reported to be highly dependent on the transition- or noble-metal-loading catalysts in the past decades. Here, we show that the transition-metal-free aluminosilicate ferrierite (FER) zeolite effectively catalyzed methane and N2O to methanol for the first time. The distorted tetracoordinated Al in the framework and pentacoordinated Al on the extra framework formed during calcination, activation, and reaction processes were confirmed as the potential active centers. The possible reaction pathway similar to the Fe-containing zeolites was advocated based on the reaction results using different oxidants, N2O adsorption FTIR spectra, and 27Al MAS NMR spectra. The stable and efficient methanol production capacity of FER zeolite was ascribed to the two-dimensional straight channels and its distinctive Al distribution of FER zeolite (CP914C) from Zeolyst. The transition-metal-free FER zeolite performed better than the record in the literature and our recent results using transition-metal-containing catalysts in terms of selectivity and formation rate of methanol and stability. This work has great significance and prospects for utilizing CH4 and N2O as resources and will open new avenues for methane oxidation.

Precise Structural and Dynamical Details in Zeolites Revealed by Coupling-Edited 1H-17O Double Resonance NMR Spectroscopy

Despite the extensive industrial and research interests in zeolites, their intrinsic catalytic nature is not fully understood due to the complexity of the hydroxyl-aluminum moieties. 17O NMR would provide irreplaceable opportunities for much-needed fine structural determination given the ubiquitous presence of oxygen atoms in nearly all species; however, the low sensitivity and quadrupolar nature of oxygen-17 make its NMR spectroscopic elucidation challenging. Here, we show that state-of-the-art double resonance solid-state NMR techniques have been combined with spectral editing methods based on scalar (through-bond) and dipolar (through-space) couplings, which allowed us to address the subtle protonic structures in zeolites. Notably, the often-neglected and undesired second-order quadrupolar-dipolar cross-term interaction ("2nd-QD interaction") can actually be exploited and can help gain invaluable information. Eventually, a comprehensive set of 1H-17O/1H-27Al double resonance NMR with J-/D-coupling spectral editing techniques have been designed in this work and enabled us to reveal atomic-scale precise structural and dynamical details in zeolites including: 1) The jump rate of the bridging acid site (BAS) proton is relatively low, i.e., far less than 100 s-1 at room temperature. 2) The Al-OH groups with 1H chemical shift at 2.6-2.8 ppm, at least for nonseverely dealuminated H-ZSM-5 catalysts, exhibit a rigid bridging environment similar to that of BAS. 3) The Si-OH groups at 2.0 ppm are not hydrogen bonded and undergo fast cone-rotational motion. The results in this study predict the 2nd-QD interaction to be universal for any rigid -17O-H environment, such as those in metal oxide surfaces or biomaterials.

Water structures on acidic zeolites and their roles in catalysis

The local reaction environment of catalytic active sites can be manipulated to modify the kinetics and thermodynamic properties of heterogeneous catalysis. Because of the unique physical-chemical nature of water, heterogeneously catalyzed reactions involving specific interactions between water molecules and active sites on catalysts exhibit distinct outcomes that are different from those performed in the absence of water. Zeolitic materials are being applied with the presence of water for heterogeneous catalytic reactions in the chemical industry and our transition to sustainable energy. Mechanistic investigation and in-depth understanding about the behaviors and the roles of water are essentially required for zeolite chemistry and catalysis. In this review, we focus on the discussions of the nature and structures of water adsorbed/stabilized on Brønsted and Lewis acidic zeolites based on experimental observations as well as theoretical calculation results. The unveiled functions of water structures in determining the catalytic efficacy of zeolite-catalyzed reactions have been overviewed and the strategies frequently developed for enhancing the stabilization of zeolite catalysts are highlighted. Recent advancement will contribute to the development of innovative catalytic reactions and the rationalization of catalytic performances in terms of activity, selectivity and stability with the presence of water vapor or in condensed aqueous phase.

Insights into the Structure of Sucralfate by Advanced Solid- and Liquid-State NMR

Sucralfate, which is a sucrose octasulfate aluminum complex, is an active pharmaceutical ingredient (API) falling in the category of cytoprotective agents which are very effective for gastric and duodenal ulcers. On interaction with stomach acid, it ionizes into aluminum and sucrose octasulfate ions to form a protective layer over the ulcerated region inhibiting further attack from acid. The mechanism of action of sucralfate in the context of its structure is not well understood. Considering that at least two forms of this API are available in the market, there are no reports on the various forms of sucralfate and differences in their pharmacological action. We characterized the two forms of sucralfate using multinuclear, multidimensional solid-state NMR, and the results show significant structural differences between them arising from variation in the aluminum environment and the level of hydration. The impact of structural differences on pharmacological action was examined by studying acid-induced Al release by 27Al liquid-state NMR. The sucralfate, European pharmaceutical standard, Form I, undergoes faster disruption in acid compared to Form II. The difference is explained on the basis of structural differences in the two forms which gives significant insights into the action of sucralfate in relation to its structure.

Hydrocarbon adsorption mechanism of modern automobile engines and methods of reducing hydrocarbon emissions during cold start process: A review

With the global emphasis on environmental protection and increasingly stringent emission regulations for internal combustion engines, there is an urgent need to overcome the problem of large hydrocarbon (HC) emissions caused by unstable engine cold starts. Synergistic engine pre-treatment (reducing hydrocarbon production) as well as after-treatment devices (adsorbing and oxidizing hydrocarbons) is the fundamental solution to emissions. In this paper, the improvement of hydrocarbon emissions is summarized from two aspects: pre-treatment and after-treatment. The pre-treatment for engine cold start mainly focuses on summarizing the intake control, fuel, and engine timing parameters. The after-treatment mainly focuses on summarizing different types of adsorbents and modifications (mainly including different molecular sieve structures and sizes, preparation conditions, silicon aluminum ratio, ion exchange modification, and heterogeneity, etc.), adsorptive catalysts (mainly including optimization of catalytic performance and structure), and catalytic devices (mainly including coupling with thermal management equipment and HC trap devices). In this paper, a SWOT (strength, weakness, opportunity, and threat) analysis of pre-treatment and after-treatment measures is conducted. Researchers can obtain relevant research results and seek new research directions and approaches for controlling cold start HC emissions.

Lewis Acidic Aluminosilicates: Synthesis, 27Al MQ/MAS NMR, and DFT-Calculated 27Al NMR Parameters

Porous aluminosilicates are functional materials of paramount importance as Lewis acid catalysts in the synthetic industry, yet the participating aluminum species remain poorly studied. Herein, a series of model aluminosilicate networks containing [L-AlO3] (L = THF, Et3N, pyridine, triethylphosphine oxide (TEPO)) and [AlO4]- centers were prepared through nonhydrolytic sol-gel condensation reactions of the spherosilicate building block (Me3Sn)8Si8O20 with L-AlX3 (X = Cl, Me, Et) and [Me4N] [AlCl4] compounds in THF or toluene. The substoichiometric dosage of the Al precursors ensured complete condensation and uniform incorporation, with the bulky spherosilicate forcing a separation between neighboring aluminum centers. The materials were characterized by 1H, 13C, 27Al, 29Si, and 31P MAS NMR and FTIR spectroscopies, ICP-OES, gravimetry, and N2 adsorption porosimetry. The resulting aluminum centers were resolved by 27Al TQ/MAS NMR techniques and assigned based on their spectroscopic parameters obtained by peak fitting (δiso, CQ, η) and their correspondence to the values calculated on model structures by DFT methods. A clear correlation between the decrease in the symmetry of the Al centers and the increase of the observed CQ was established with values spanning from 4.4 MHz for distorted [AlO4]- to 15.1 MHz for [THF-AlO3]. Products containing exclusively [TEPO-AlO3] or [AlO4]- centers could be obtained (single-site materials). For L = THF, Et3N, and pyridine, the [AlO4]- centers were formed together with the expected [L-AlO3] species, and a viable mechanism for the unexpected emergence of [AlO4]- was proposed.

Post-Synthetic Ensembling Design of Hierarchically Ordered FAU-type Zeolite Frameworks for Vacuum Gas Oil Hydrocracking

Zeolites hold importance as catalysts and membranes across numerous industrial processes that produce most of the world's fuels and chemicals. In zeolite catalysis, the rate of molecular diffusion inside the micropore channels defines the catalyst's longevity and selectivity, thereby influencing the catalytic efficiency. Decreasing the diffusion pathlengths of zeolites to the nanoscopic level by fabricating well-organized hierarchically porous architecture can efficiently overcome their intrinsic mass-transfer limitations without losing hydrothermal stability. We report a rational post-synthetic design for synthesizing hierarchically ordered FAU-type zeolites exhibiting 2D-hexagonal (P6mm) and 3D-cubic (Ia3 ‾ ${\bar{3}}$ d) mesopore channels. The synthesis involves methodical incision of the parent zeolite into unit-cell level zeolitic fragments by in situ generated base and bulky surfactants. The micellar ensembles formed by these surfactant-zeolite interactions are subsequently reorganized into various ordered mesophases by tuning the micellar curvature with ion-specific interactions (Hofmeister effect). Unlike conventional crystallization, which offers poor control over mesophase formation due to kinetic constraints, crystalline mesostructures can be developed under dilute, mild alkaline conditions by controlled reassembly. The prepared zeolites with nanometric diffusion pathlengths have demonstrated excellent yields of naphtha and middle-distillates in vacuum gas oil hydrocracking with decreased coke deposition.

Revealing the evolution of local structures in the formation process of alkaline earth metal cation-containing zeolites from glasses

Alkaline earth metal cations are ubiquitously present in natural zeolites but less exploited in synthetic zeolites due to their low solubility in water, and hence it remains elusive how they contribute to zeolite formation. Herein, harmotome, a PHI-type zeolite with Ba2+, is readily synthesized from a Ba-containing aluminosilicate glass. This glass-to-zeolite transformation process, in particular the structure-regulating role of Ba2+, is investigated by anomalous X-ray scattering and high-energy X-ray total scattering techniques. The results demonstrate that the steady Ba2+-aluminosilicate interactions not only help prevent the precipitation of barium species under alkaline synthetic conditions, but also dictate the local structures with distinct interatomic distances between the Ba2+ and the surrounding aluminosilicate species throughout the transformation process, which lead to the successful formation of harmotome without detectable impurities. This study highlights the usefulness of the comprehensive X-ray scattering techniques in revealing the formation scheme of the zeolites containing specific metal species. In addition, a promising alternative approach to design and synthesize zeolites with unique compositions and topologies by using well-crafted glasses with suitable metal cation dopants is demonstrated.

Imaging of Single Molecular Behaviors Under Bifurcated Three-Centered Hydrogen Bonding

The mechanism for interaction and bonding of single guest molecules with active sites fundamentally determines the sorption and subsequent catalytic processes occurring in host zeolitic frameworks. However, no real-space studies on these significant issues have been reported thus far, since atomically visualizing guest molecules and recognizing single Al T-sites in zeolites remain challenging. Here, we atomically resolved single thiophene probes interacting with acid T-sites in the ZSM-5 framework to study the bonding behaviors between them. The synergy of bifurcated three-centered hydrogen bonds and van der Waals interactions can "freeze" the near-horizontal thiophene and make it stable enough to be imaged. By combining the imaging results with simulations, direct atomic observations enabled us to precisely locate the single Al T-sites in individual straight channels. Then, we statistically found that the thiophene bonding probability of the T11 site is 15 times higher than that of the T6 site. For different acid T-sites, the variation in the interaction synergy changes the inner angle of the host-guest O-H⋅⋅⋅S hydrogen bond, thereby affecting the stability of the near-horizontal thiophene and leading to considerable bonding inhomogeneities.

Evaluating the efficacy of zeolites synthesized from natural clay for the methanol-to-hydrocarbon process

Introducing sustainability into advanced catalytic material design is essential to address growing environmental concerns. Among them, synthesizing inorganic zeolite materials from non-traditional sources (like natural clay) offers several advantages, contributing to sustainability and environmental stewardship. With this objective, we used kaolin to synthesize zeolites with different topologies: SSZ-13 (8-MR with CHA topology), ZSM-5 (10-MR with MFI topology), and Beta (12-MR with BEA topology) (MR: member ring), where a simple and flexible synthetic protocol was adopted without any significant changes. All these zeolites were subjected to catalytic performance evaluation concerning the industrially relevant methanol-to-hydrocarbon (MTH) process. Herein, the kaolin-derived zeolites, especially ZSM-5, led to superior performance and demonstrated enhanced catalyst deactivation-resistant behavior compared to their zeolite counterparts prepared from traditional synthetic routes. Various characterization tools (including under operando conditions) were employed to understand their reactions and deactivation mechanisms. Overall, making zeolites from non-traditional sources presents a pathway for sustainable and environmentally friendly material production, offering benefits such as reduced resource dependence, lower energy consumption, and tailored physicochemical properties beneficial to catalysis. In a broader context, such a research approach contributes to the transition toward a more sustainable and circular economy.

Xylene Synthesis Through Tandem CO2 Hydrogenation and Toluene Methylation Over a Composite ZnZrO Zeolite Catalyst

Selective synthesis of specific value-added aromatics from CO2 hydrogenation is of paramount interest for mitigating energy and climate problems caused by CO2 emission. Herein, we report a highly active composite catalyst of ZnZrO and HZSM-5 (ZZO/Z5-SG) for xylene synthesis from CO2 hydrogenation via a coupling reaction in the presence of toluene, achieving a xylene selectivity of 86.5 % with CO2 conversion of 10.5 %. A remarkably high space time yield of xylene could reach 215 mg gcat -1  h-1 , surpassing most reported catalysts for CO2 hydrogenation. The enhanced performance of ZZO/Z5-SG could be due to high dispersion and abundant oxygen vacancies of the ZZO component for CO2 adsorption, more feasible hydrogen activation and transfer due to the close interaction between the two components, and enhanced stability of the formate intermediate. The consumption of methoxy and methanol from the deep hydrogenation of formate by introduced toluene also propels an oriented conversion of CO2 .

Operando Modeling of Zeolite-Catalyzed Reactions Using First-Principles Molecular Dynamics Simulations

Within this Perspective, we critically reflect on the role of first-principles molecular dynamics (MD) simulations in unraveling the catalytic function within zeolites under operating conditions. First-principles MD simulations refer to methods where the dynamics of the nuclei is followed in time by integrating the Newtonian equations of motion on a potential energy surface that is determined by solving the quantum-mechanical many-body problem for the electrons. Catalytic solids used in industrial applications show an intriguing high degree of complexity, with phenomena taking place at a broad range of length and time scales. Additionally, the state and function of a catalyst critically depend on the operating conditions, such as temperature, moisture, presence of water, etc. Herein we show by means of a series of exemplary cases how first-principles MD simulations are instrumental to unravel the catalyst complexity at the molecular scale. Examples show how the nature of reactive species at higher catalytic temperatures may drastically change compared to species at lower temperatures and how the nature of active sites may dynamically change upon exposure to water. To simulate rare events, first-principles MD simulations need to be used in combination with enhanced sampling techniques to efficiently sample low-probability regions of phase space. Using these techniques, it is shown how competitive pathways at operating conditions can be discovered and how broad transition state regions can be explored. Interestingly, such simulations can also be used to study hindered diffusion under operating conditions. The cases shown clearly illustrate how first-principles MD simulations reveal insights into the catalytic function at operating conditions, which could not be discovered using static or local approaches where only a few points are considered on the potential energy surface (PES). Despite these advantages, some major hurdles still exist to fully integrate first-principles MD methods in a standard computational catalytic workflow or to use the output of MD simulations as input for multiple length/time scale methods that aim to bridge to the reactor scale. First of all, methods are needed that allow us to evaluate the interatomic forces with quantum-mechanical accuracy, albeit at a much lower computational cost compared to currently used density functional theory (DFT) methods. The use of DFT limits the currently attainable length/time scales to hundreds of picoseconds and a few nanometers, which are much smaller than realistic catalyst particle dimensions and time scales encountered in the catalysis process. One solution could be to construct machine learning potentials (MLPs), where a numerical potential is derived from underlying quantum-mechanical data, which could be used in subsequent MD simulations. As such, much longer length and time scales could be reached; however, quite some research is still necessary to construct MLPs for the complex systems encountered in industrially used catalysts. Second, most currently used enhanced sampling techniques in catalysis make use of collective variables (CVs), which are mostly determined based on chemical intuition. To explore complex reactive networks with MD simulations, methods are needed that allow the automatic discovery of CVs or methods that do not rely on a priori definition of CVs. Recently, various data-driven methods have been proposed, which could be explored for complex catalytic systems. Lastly, first-principles MD methods are currently mostly used to investigate local reactive events. We hope that with the rise of data-driven methods and more efficient methods to describe the PES, first-principles MD methods will in the future also be able to describe longer length/time scale processes in catalysis. This might lead to a consistent dynamic description of all steps-diffusion, adsorption, and reaction-as they take place at the catalyst particle level.

The need for operando modelling of 27Al NMR in zeolites: the effect of temperature, topology and water

Solid state (ss-) 27Al NMR is one of the most valuable tools for the experimental characterization of zeolites, owing to its high sensitivity and the detailed structural information which can be extracted from the spectra. Unfortunately, the interpretation of ss-NMR is complex and the determination of aluminum distributions remains generally unfeasible. As a result, computational modelling of 27Al ss-NMR spectra has grown increasingly popular as a means to support experimental characterization. However, a number of simplifying assumptions are commonly made in NMR modelling, several of which are not fully justified. In this work, we systematically evaluate the effects of various common models on the prediction of 27Al NMR chemical shifts in zeolites CHA and MOR. We demonstrate the necessity of operando modelling; in particular, taking into account the effects of water loading, temperature and the character of the charge-compensating cation. We observe that conclusions drawn from simple, high symmetry model systems such as CHA do not transfer well to more complex zeolites and can lead to qualitatively wrong interpretations of peak positions, Al assignment and even the number of signals. We use machine learning regression to develop a simple yet robust relationship between chemical shift and local structural parameters in Al-zeolites. This work highlights the need for sophisticated models and high-quality sampling in the field of NMR modelling and provides correlations which allow for the accurate prediction of chemical shifts from dynamical simulations.

Molecular Views on Mechanisms of Brønsted Acid-Catalyzed Reactions in Zeolites

The Brønsted acidity of proton-exchanged zeolites has historically led to the most impactful applications of these materials in heterogeneous catalysis, mainly in the fields of transformations of hydrocarbons and oxygenates. Unravelling the mechanisms at the atomic scale of these transformations has been the object of tremendous efforts in the last decades. Such investigations have extended our fundamental knowledge about the respective roles of acidity and confinement in the catalytic properties of proton exchanged zeolites. The emerging concepts are of general relevance at the crossroad of heterogeneous catalysis and molecular chemistry. In the present review, emphasis is given to molecular views on the mechanism of generic transformations catalyzed by Brønsted acid sites of zeolites, combining the information gained from advanced kinetic analysis, in situ, and operando spectroscopies, and quantum chemistry calculations. After reviewing the current knowledge on the nature of the Brønsted acid sites themselves, and the key parameters in catalysis by zeolites, a focus is made on reactions undergone by alkenes, alkanes, aromatic molecules, alcohols, and polyhydroxy molecules. Elementary events of C-C, C-H, and C-O bond breaking and formation are at the core of these reactions. Outlooks are given to take up the future challenges in the field, aiming at getting ever more accurate views on these mechanisms, and as the ultimate goal, to provide rational tools for the design of improved zeolite-based Brønsted acid catalysts.

Hydrothermal Aging Alleviates the Phosphorus Poisoning of Cu-SSZ-39 Catalysts for NH3-SCR Reaction

As a new type of catalyst with the potential for commercial application in NO_x removal from diesel engine exhausts, Cu-SSZ-39 catalysts must have excellent resistance to complex and harsh conditions. In this paper, the effects of phosphorus on Cu-SSZ-39 catalysts before and after hydrothermal aging treatment were investigated. Compared with fresh Cu-SSZ-39 catalysts, phosphorus poisoning significantly decreased the low-temperature NH₃-SCR catalytic activity. However, such activity loss was alleviated by further hydrothermal aging treatment. To reveal the reason for this interesting result, a variety of characterization techniques including NMR, H₂-TPR, X-ray photoelectron spectroscopy, NH₃-TPD, and in situ DRIFTS measurements were employed. It was found that Cu-P species produced by phosphorus poisoning decreased the redox ability of active copper species, resulting in the observed low-temperature deactivation. After hydrothermal aging treatment, however, Cu-P species partly decomposed with the formation of active CuO_x species and a release of active copper species. As a result, the low-temperature NH₃-SCR catalytic activity of Cu-SSZ-39 catalysts was recovered.

Active Sites in H-Mordenite Catalysts Probed by NMR and FTIR

Mordenites are widely used in catalysis and environmental protection. The catalytic properties of mordenite are largely determined by the composition of its crystal framework, i.e., the SiO2/Al2O3 molar ratio (MR), and the cationic form. In H-mordenites, the most important characteristic becomes the structure and distribution of acid sites, which depends on the number and distribution of Al tetrahedra in the framework. In the present work, the local structure of these centers in H-mordenite catalysts with a nominal MR varied from 9.9 to 19.8 was studied in detail using a combination of magic angle spinning nuclear magnetic resonance (MAS NMR) and Fourier transform infrared spectroscopy (FTIR). 27Al MAS NMR indicates the presence of extra-framework Al in most of the studied samples that results in a higher real MR of the zeolitic framework compared to the nominal value. Concentrations of Lewis and Brønsted acid sites, as well as of silanol groups were estimated by elemental analysis, NMR, and FTIR spectroscopy. The values of site concentrations obtained from band intensities of adsorbed CO and those of OH groups are compared with the amount of framework and extra-framework aluminum. The advantages and restrictions of different methods of active site characterization are discussed.

How Water and Ion Mobility Affect the NMR Fingerprints of the Hydrated JBW Zeolite: A Combined Computational-Experimental Investigation

An important aspect within zeolite synthesis is to make fully tunable framework materials with controlled aluminium distribution. A major challenge in characterising these zeolites at operating conditions is the presence of water. In this work, we investigate the effect of hydration on the 27 Al NMR parameters of the ultracrystalline K,Na-compensated aluminosilicate JBW zeolite using experimental and computational techniques. The JBW framework, with Si/Al ratio of 1, is an ideal benchmark system as a stepping stone towards more complicated zeolites. The presence and mobility of water and extraframework species directly affect NMR fingerprints. Excellent agreement between theoretical and experimental spectra is obtained provided dynamic methods are employed with hydrated structural models. This work shows how NMR is instrumental in characterising aluminium distributions in zeolites at operating conditions.

Solid-State NMR Spectra of Protons and Quadrupolar Nuclei at 28.2 T: Resolving Signatures of Surface Sites with Fast Magic Angle Spinning

Advances in solid-state nuclear magnetic resonance (NMR) methods and hardware offer expanding opportunities for analysis of materials, interfaces, and surfaces. Here, we demonstrate the application of a very high magnetic field strength of 28.2 T and fast magic-angle-spinning rates (MAS, >40 kHz) to surface species relevant to catalysis. Specifically, we present as case studies the 1D and 2D solid-state NMR spectra of important catalyst and support materials, ranging from a well-defined silica-supported organometallic catalyst to dehydroxylated γ-alumina and zeolite solid acids. The high field and fast-MAS measurement conditions substantially improve spectral resolution and narrow NMR signals, which is particularly beneficial for solid-state 1D and 2D NMR analysis of 1H and quadrupolar nuclei such as 27Al at surfaces.

17O Labeling Reveals Paired Active Sites in Zeolite Catalysts

Current needs for extending zeolite catalysts beyond traditional gas-phase hydrocarbon chemistry demand detailed characterization of active site structures, distributions, and hydrothermal impacts. A broad suite of homonuclear and heteronuclear NMR correlation experiments on dehydrated H-ZSM-5 catalysts with isotopically enriched ¹⁷O frameworks reveals that at least two types of paired active sites exist, the amount of which depends on the population of fully framework-coordinated tetrahedral Al (Al(IV)-1) and partially framework-coordinated tetrahedral Al (Al(IV)-2) sites, both of which can be denoted as (SiO)_4-n-Al(OH)_n. The relative amounts of Al(IV)-1 and Al(IV)-2 sites, and subsequent pairing, cannot be inferred from the catalyst Si/Al ratio, but depend on synthetic and postsynthetic modifications. Correlation experiments demonstrate that, on average, acidic hydroxyl groups from Al(IV)-1/Al(IV)-2 pairs are closer to one another than those from Al(IV)-1/Al(IV)-1 pairs, as supported by computational DFT calculations. Through-bond and through-space polarization transfer experiments exploiting ¹⁷O nuclei reveal a number of different acidic hydroxyl groups in varying Si/Al catalysts, the relative amounts of which change following postsynthetic modifications. Using room-temperature isotopic exchange methods, it was determined that ¹⁷O was homogeneously incorporated into the zeolite framework, while ¹⁷O → ²⁷Al polarization transfer experiments demonstrated that ¹⁷O incorporation does not occur for extra-framework Al_nO_m species. Data from samples exposed to controlled hydrolysis indicates that nearest neighbor Al pairs in the framework are more susceptible to hydrolytic attack. The data reported here suggest that Al(IV)-1/Al(IV)-2 paired sites are synergistic sites leading to increased reactivity in both low- and high-temperature reactions. No evidence was found for paired framework/nonframework sites.

Catalytic Enhancement of Cyclohexene Hydration by Ga-Doped ZSM-5 Zeolites

Ga-doped ZSM-5 zeolites were directly synthesized by a facile one-step hydrothermal method without organic templates and calcination and then investigated in the cyclohexene hydration reaction. The structure, component, textural properties, and acidity of the as-prepared samples were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray fluorescence (XRF), Brunauer-Emmett-Teller (BET), ammonia temperature-programmed desorption (NH₃-TPD), pyridine-chemisorbed IR (Py-IR), and ⁷¹Ga, ²⁷Al, ²⁹Si, and ¹H magic-angle spinning (MAS) NMR techniques. The characterization results showed that the introduction of Ga atoms into the ZSM-5 zeolite framework is much easier than Al atoms and beneficial to promote the formation of small-sized crystals. The number of Brønsted acid sites of Ga-doped ZSM-5 samples obviously increased compared with Ga0-ZSM-5. Additionally, the highest cyclohexanol yield (10.1%) was achieved over the Ga3-ZSM-5 sample, while the cyclohexanol yield of the Ga0-ZSM-5 sample was 8.6%. This result indicated that the improved catalytic performance is related to its larger external surface area, smaller particle size, and more Brønsted acid sites derived from Si-OH-Al and Si-OH-Ga of Ga3-ZSM-5. Notably, the green route reduces harmful gas emission and provides a basis for doping other heteroatoms to regulate the catalytic performance of zeolites, especially in industrial production.

Kinetic Model of Acid Gas Induced Defect Propagation in Zeolitic Imidazolate Frameworks

Understanding the degradation of nanoporous materials under exposure to common acid gas contaminants (e.g., SO₂, CO₂, NO₂, and H₂S) is essential to elongate their lifetime and thus enable their practical applications in separations and catalysis. Previous theoretical investigations have focused on the formation of isolated point defects, which are insufficient to provide direct insights into the long-term evolution of the bulk properties of materials such as zeolitic imidazolate frameworks (ZIFs) under sustained acid gas exposure. To bridge this divide in both length and time scales, we developed a first-principles lattice-based kinetic model to simulate the defect propagation and bulk material breakdown in ZIFs. This model closely reproduces the experimentally measured macroscopic evolution of the time-dependent bulk materials proprieties and also yields important new insights regarding the autocatalytic nature of ZIF degradation and the spatial distribution of defects. Our results suggest new experimental directions to identify nascent defect clusters in degraded ZIFs and avenues to mitigate degradation under challenging conditions of acid gas exposure.

Selective catalytic reduction of NOx with ammonia (NH3-SCR) over copper loaded LEV type zeolites synthesized with different templates

LEV type zeolites were synthesized with four different structure-directing agents and converted to copper loaded NH₃-SCR catalysts. The synthesis recipe was found to impact the respective Al population in the two topologically different framework sites in double and single 6-rings, resolvable by ²⁷Al MAS NMR spectroscopy. Hydrothermal stability was found to be related to the silanol concentration, Si/Al ratio, particle size, crystal morphology, crystal defects, external surface area, and microporosity. Catalytic activity in NH₃-SCR was dependent on preferential Al siting in the double 6-rings. Levinite synthesized using adamantylamine showed the strongest preference for Al atoms sitting in double 6-ring sites, and showed the highest catalytic turnover frequency. Unfortunately, because of the large crystal size, copper loading of this sample was limited to 0.6 wt% while other samples could be loaded with copper up to 3.3 wt%. An optimum combination of hydrothermal stability and catalytic activity was obtained with N,N'-bis-dimethylpentanediyldiammonium dibromide as structure-directing agent.

Clarification of acid site location in MSE-type zeolites by spectroscopic approaches combined with catalytic activity: comparison between UZM-35 and MCM-68

MSE-type zeolites synthesized by different organic structure-directing agents (OSDAs), UZM-35 and MCM-68, were prepared. The location of Brønsted acid sites derived from the framework Al atoms and acidic properties were investigated based on ²⁷Al MQMAS NMR and in situ IR techniques combined with the evaluation of the catalytic activity. We have successfully found a significant difference in the location of Brønsted acid sites in the MSE-type framework; 61 and 33% of acid sites were located at the 12-ring channel for MCM-68 and UZM-35, respectively. The differences in the location of the acid sites yielded their unique catalytic activities for the hydrocarbon cracking reactions, indicating that a well-chosen type of OSDAs for the synthesis is one of the possibilities for controlling the distribution of the framework Al atoms in the MSE-type framework.

Long-Range Spatial Distribution of Single Aluminum Sites in Zeolites

How aluminum distributes during synthesis and rearranges after processing within the zeolite framework is a central question in heterogeneous catalysis, as it determines the structure and location of the catalytically active sites of the one of the most important classes of industrial catalysts. Here, exploiting the dipolar interaction between paramagnetic metal ions, we derive the spatial distribution of single aluminum sites within the ZSM-5 zeolite framework in the nanometer range, in polycrystalline samples lacking long-range order. We use a Monte Carlo approach to validate the findings on a pristine ZSM-5 sample and demonstrate that the method is sensitive enough to monitor aluminum redistribution induced in the framework by chemical stress.

Dynamic evolution of Al species in the hydrothermal dealumination process of CHA zeolite

The hydrothermal stability of zeolites is an important factor being considered which could restrict their scope of industrial application. Revealing the water-induced dealumination mechanism is crucial for improving the hydrothermal...