Factors that Determine Zeolite Stability in Hot Liquid Water

A reactive neural network framework for water-loaded acidic zeolites

Under operating conditions, the dynamics of water and ions confined within protonic aluminosilicate zeolite micropores are responsible for many of their properties, including hydrothermal stability, acidity and catalytic activity. However, due to high computational cost, operando studies of acidic zeolites are currently rare and limited to specific cases and simplified models. In this work, we have developed a reactive neural network potential (NNP) attempting to cover the entire class of acidic zeolites, including the full range of experimentally relevant water concentrations and Si/Al ratios. This NNP has the potential to dramatically improve sampling, retaining the (meta)GGA DFT level accuracy, with the capacity for discovery of new chemistry, such as collective defect formation mechanisms at the zeolite surface. Furthermore, we exemplify how the NNP can be used as a basis for further extensions/improvements which include data-efficient adoption of higher-level (hybrid) references via Δ-learning and the acceleration of rare event sampling via automatic construction of collective variables. These developments represent a significant step towards accurate simulations of realistic catalysts under operando conditions.

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.

Reactant-Induced Structural Evolution of Pt Catalysts Confined in Zeolite

Reactant-induced structural evolutions of heterogeneous metal catalysts are frequently observed in numerous catalytic systems, which can be associated with the formation or deactivation of active sites. In this work, we will show the structural transformation of subnanometer Pt clusters in pure-silica MFI zeolite structure in the presence of CO, O2, and/or H2O and the catalytic consequences of the Pt-zeolite materials derived from various treatment conditions. By applying the appropriate pretreatment under a reactant atmosphere, we can precisely modulate the size distribution of Pt species spanning from single Pt atoms to small Pt nanoparticles (1-5 nm) in the zeolite matrix, resulting in the desirably active and stable Pt species for CO oxidation. We also show the incorporation of Fe into the zeolite framework greatly promotes the stability of Pt species against undesired sintering under harsh conditions (up to 650 °C in the presence of CO, O2, and moisture).

Impact of Low-Temperature Water Exposure and Removal on Zeolite HY

Aqueous-phase postsynthetic modifications of the industrially important Y-type zeolite are commonly used to change overall acid site concentrations, introduce stabilizing rare-earth cations, impart bifunctional character through metal cation exchange, and tailor the distribution of Brønsted and Lewis acid sites. Zeolite Y is known to undergo framework degradation in the presence of both vapor- and liquid-phase water at temperatures exceeding 100 °C, and rare-earth exchanged and stabilized HY catalysts are commonly used for fluidized catalytic cracking due to their increased hydrothermal resilience. Here, using detailed spectroscopy, crystallography, and flow-reactor experiments, we reveal unexpected decreases in Brønsted acid site (BAS) density for zeolite HY following exposure even to room-temperature liquid water. These data indicate that aqueous-phase ion-exchange procedures commonly used to modify zeolite Y are impacted by the liquid water and its removal, even when fractional heating rates and inert conditions much less severe than standard practice are used for catalyst dehydration. X-ray diffraction, thermogravimetric, and spectroscopic analyses reveal that the majority of framework degradation occurs during the removal of a strongly bound water fraction in HY, which does not form when NH4Y is immersed in liquid water and which leads to reduced acidity in HY even when dehydration conditions much milder than those typically practiced are employed. Na+-exchanged HY prepared via room-temperature aqueous dissolution demonstrates that Brønsted acid sites are lost in excess of the theoretical maximum that is possible from sodium titration. The structural impact of low-temperature aqueous-phase ion-exchange methods complicates the interpretation of subsequent data and likely explains the wide variation in reported acid site concentrations and catalytic activity of HY zeolites with high-Al content.

ADOR zeolite with 12 × 8 × 8-ring pores derived from IWR germanosilicate

Zeolites have been well known for decades as catalytic materials and adsorbents and are traditionally prepared using the bottom-up synthesis method. Although it was productive for more than 250 zeolite frameworks, the conventional solvothermal synthesis approach provided limited control over the structural characteristics of the formed materials. In turn, the discovery and development of the Assembly-Disassembly-Organization-Reassembly (ADOR) strategy for the regioselective manipulation of germanosilicates enabled the synthesis of previously unattainable zeolites with predefined structures. To date, the family tree of ADOR materials has included the topological branches of UTL, UOV, IWW, *CTH, and IWV zeolites. Herein, we report on the expansion of ADOR zeolites with a new branch related to the IWR topology, which is yet unattainable experimentally but theoretically predicted as highly promising adsorbents for CO2 separation applications. The optimization of not only the chemical composition but also the dimensions of the crystalline domain in the parent IWR zeolite in the Assembly step was found to be the key to the success of its ADOR transformation into previously unknown IPC-17 zeolite with an intersecting 12 × 8 × 8-ring pore system. The structure of the as-prepared IPC-17 zeolite was verified by a combination of microscopic and diffraction techniques, while the results on the epichlorohydrin ring-opening with alcohols of variable sizes proved the molecular sieving ability of IPC-17 with potential application in heterogeneous catalysis. The proposed synthesis strategy may facilitate the discovery of zeolite materials that are difficult or yet impossible to achieve using a traditional bottom-up synthesis approach.

Unusual Acid Sites in LSX Zeolite: Formation Features and Physico-Chemical Properties

The advanced approach for the preparation of the NH₄ form of highly crystalline LSX zeolite under gentle drying conditions (40 °C, membrane pump dynamic vacuum) is discussed. Decationization of this form at moderate temperatures led to the formation of Brønsted acid sites (BASs), whose concentration and strength were characterized by IR spectroscopy. It was found that a maximum concentration of three BASs per unit cell can be achieved at 200 °C prior to the initiation of zeolite structure degradation. The proton affinity of BASs is unusual, and aspires 1240 kJ/mol, which is significantly higher compared to faujasites with higher moduli. The increase in temperature of the heat treatment (up to 300 °C) resulted in thermal decomposition of BASs and the manifestation of amorphous phase with corresponding Lewis acid sites (LASs) as well as terminal Si-OH groups. Both the destruction of BASs and formation of the LAS-containing amorphous phase are the key reasons for the significant decrease in the adsorption capacity in the micropore region revealed for the sample decationized at 300 °C.

Direct TEM Observation of Vacancy-Mediated Heteroatom Incorporation into a Zeolite Framework: Towards Microscopic Design of Zeolite Catalysts

Incorporating hetero-metal-atom, e.g., titanium, into zeolite frameworks can enhance the catalytic activity and selectivity in oxidation reactions. However, the rational design of zeolites containing titanium at specific sites is difficult because the precise atomic structure during synthesis process remained unclear. Here, a titanosilicate with predictable titanium distribution was synthesized by mediating vacancies in a defective MSE-type zeolite precursor, based on a pre-designed synthetic route including modification of vacancies followed by titanium insertion, where electron microscopy (EM) plays a key role at each step resolving the atomic structure. Point defects including vacancies in the precursor and titanium incorporated into the vacancy-related positions have been directly observed. The results provide insights into the role of point defects in zeolites towards the rational synthesis of zeolites with desired microscopic arrangement of catalytically active sites.

A review of catalytic hydrogenation of carbon dioxide: From waste to hydrocarbons

With the rapid development of industrial society and humankind’s prosperity, the growing demands of global energy, mainly based on the combustion of hydrocarbon fossil fuels, has become one of the most severe challenges all over the world. It is estimated that fossil fuel consumption continues to grow with an annual increase rate of 1.3%, which has seriously affected the natural environment through the emission of greenhouse gases, most notably carbon dioxide (CO₂). Given these recognized environmental concerns, it is imperative to develop clean technologies for converting captured CO₂ to high-valued chemicals, one of which is value-added hydrocarbons. In this article, environmental effects due to CO₂ emission are discussed and various routes for CO₂ hydrogenation to hydrocarbons including light olefins, fuel oils (gasoline and jet fuel), and aromatics are comprehensively elaborated. Our emphasis is on catalyst development. In addition, we present an outlook that summarizes the research challenges and opportunities associated with the hydrogenation of CO₂ to hydrocarbon products.

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.

The challenge of silanol species characterization in zeolites

The chemistry of silica-based materials, including zeolites, is strongly influenced by the nature and amount of their silanols. In zeolites, they are either isolated (non H-bonded) silanol species, or silanol...

Recent Advances in the Mitigation of the Catalyst Deactivation of CO2 Hydrogenation to Light Olefins

The catalytic conversion of CO2 to value-added chemicals and fuels has been long regarded as a promising approach to the mitigation of CO2 emissions if green hydrogen is used. Light olefins, particularly ethylene and propylene, as building blocks for polymers and plastics, are currently produced primarily from CO2-generating fossil resources. The identification of highly efficient catalysts with selective pathways for light olefin production from CO2 is a high-reward goal, but it has serious technical challenges, such as low selectivity and catalyst deactivation. In this review, we first provide a brief summary of the two dominant reaction pathways (CO2-Fischer-Tropsch and MeOH-mediated pathways), mechanistic insights, and catalytic materials for CO2 hydrogenation to light olefins. Then, we list the main deactivation mechanisms caused by carbon deposition, water formation, phase transformation and metal sintering/agglomeration. Finally, we detail the recent progress on catalyst development for enhanced olefin yields and catalyst stability by the following catalyst functionalities: (1) the promoter effect, (2) the support effect, (3) the bifunctional composite catalyst effect, and (4) the structure effect. The main focus of this review is to provide a useful resource for researchers to correlate catalyst deactivation and the recent research effort on catalyst development for enhanced olefin yields and catalyst stability.

Recent Advances in Catalysis Based on Transition Metals Supported on Zeolites

This article reviews the current state and development of thermal catalytic processes using transition metals (TM) supported on zeolites (TM/Z), as well as the contribution of theoretical studies to understand the details of the catalytic processes. Structural features inherent to zeolites, and their corresponding properties such as ion exchange capacity, stable and very regular microporosity, the ability to create additional mesoporosity, as well as the potential chemical modification of their properties by isomorphic substitution of tetrahedral atoms in the crystal framework, make them unique catalyst carriers. New methods that modify zeolites, including sequential ion exchange, multiple isomorphic substitution, and the creation of hierarchically porous structures both during synthesis and in subsequent stages of post-synthetic processing, continue to be discovered. TM/Z catalysts can be applied to new processes such as CO₂ capture/conversion, methane activation/conversion, selective catalytic NO_x reduction (SCR-deNO_x), catalytic depolymerization, biomass conversion and H₂ production/storage.

Zeolite (In)Stability under Aqueous or Steaming Conditions

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

Investigation of Suitable Templates for One-Pot-Synthesized Cu-SAPO-34 in NOx Abatement from Diesel Vehicle Exhaust

The control of NO_x emission from diesel vehicles is of great importance to the environment, and Cu-SAPO-34 is considered to be an effective catalyst for the abatement of NO_x from diesel vehicles. Along with catalytic activity, hydrothermal stability is a key property for NO_x abatement catalysts. The attack of Cu species and framework atoms by H₂O may result in activity loss under both low/high temperature humid conditions, which are inevitable in practical application. Therefore, apart from good catalytic activity, hydrothermal stability under both low/high temperatures for Cu-SAPO-34 is also critical for NO_x control in diesel vehicles. Three Cu-SAPO-34 samples were prepared by a one-pot hydrothermal method using propylamine, triethylamine, and morpholine, with Cu-TEPA (tetraethylenepentamine) as the cotemplate. The NH₃-SCR activity and the effects of hydrothermal aging at 70 and 800 °C on these Cu-SAPO-34 samples were investigated. The type of cotemplate can affect the Si and Cu species in one-pot-synthesized Cu-SAPO-34 catalysts, so that the catalytic activity as well as the low/high temperature hydrothermal stability is affected by the choice of template. Generally speaking, Cu-SAPO-34 prepared using PA as cotemplate showed superior catalytic activity and hydrothermal stability under low/high temperatures compared with the other two catalysts, which makes PA a more suitable template for one-pot-synthesized Cu-SAPO-34 for use in NO_x abatement from diesel vehicle exhaust.

One-step synthesis of hydrophobic clinoptilolite modified by silanization for the degradation of crystal violet dye in aqueous solution

One-step synthesis of hydrophobic CPs was demonstrated, in which the kinetically-controlled induction period and thermodynamically-based rapid growth process were elucidated.

The effect of crystallite size on low-temperature hydrothermal stability of Cu-SAPO-34

Two Cu-SAPO-34 catalysts with different crystallite sizes were obtained, and catalyst with smaller crystallite size had better low-temperature hydrothermal stability because of less Cu²⁺ species on the surface.

Discussing the performance of beta zeolites in aqueous-phase valorization of xylose

Beta zeolites are potential catalysts for xylose upgrade to bioproducts and selectivity is determined by the balance between water-tolerant Lewis and Brønsted acid sites.

Fast room temperature lability of aluminosilicate zeolites

Aluminosilicate zeolites are traditionally used in high-temperature applications at low water vapour pressures where the zeolite framework is generally considered to be stable and static. Increasingly, zeolites are being considered for applications under milder aqueous conditions. However, it has not yet been established how neutral liquid water at mild conditions affects the stability of the zeolite framework. Here, we show that covalent bonds in the zeolite chabazite (CHA) are labile when in contact with neutral liquid water, which leads to partial but fully reversible hydrolysis without framework degradation. We present ab initio calculations that predict novel, energetically viable reaction mechanisms by which Al-O and Si-O bonds rapidly and reversibly break at 300 K. By means of solid-state NMR, we confirm this prediction, demonstrating that isotopic substitution of ¹⁷O in the zeolitic framework occurs at room temperature in less than one hour of contact with enriched water.

While aluminosilicate zeolites are of interest for many applications, the affect of water on zeolite stability in mild aqueous conditions has yet to be established. Here, using ab initio calculations and NMR spectroscopy, the authors show that covalent bonds in the zeolite chabazite are labile when in contact with neutral liquid water.

The effect of water on the validity of Löwenstein's rule

Löwenstein's rule is explained in terms of the level of solvating water inside zeolite pores, along with the formation of Brønsted acidic water clusters derived from framework sites.

The common understanding of zeolite acidity is based on Löwenstein's rule, which states that Al–O–Al aluminium pairs are forbidden in zeolites. This rule is generally accepted to be inviolate in zeolites. However, recent computational research using a 0 K DFT model has suggested that the rule is violated for the acid form of several zeolites under anhydrous conditions [Fletcher et al., Chem. Sci., 8, (2017), 7483]. The effect of water loading on the preferred aluminium distribution in zeolites, however, has so far not been taken into account. In this article, we show by way of ab initio molecular dynamics simulations that Löwenstein's rule is obeyed under high water solvation for acid chabazite (H-CHA) but disobeyed under anhydrous conditions. We find that varying the water loading in the pores leads to dramatic effects on the structure of the active sites and the dynamics of solvation. The solvation of Brønsted protons in the surrounding water was found to be the energetic driving force for the preferred Löwenstein Al distribution and this driving force is absent in non-Löwenstein (Al–O(H)–Al) moieties. The preference for solvated protons further implies that the catalytically active species in zeolites is a protonated water cluster, rather than a framework Brønsted site. Hence, an accurate treatment of the solvation conditions is crucial to capture the behaviour of zeolites and to properly connect simulations to experiments. This work should lead to a change in modelling paradigm for zeolites, from single molecules towards high solvation models where appropriate.

High-Temperature Grafting Silylation for Minimizing Leaching of Acid Functionality from Hydrophobic Mesoporous Silicas Used as Catalysts in the Liquid Phase

Ordered-hexagonal silica materials, such as Mobil crystalline material-41 and Santa Barbara amorphous-15, have important applications in heterogeneous catalysis and biomass conversion due to their chemical stability and mesoporous structure. Low-temperature grafting (LG) is one of the most common functionalization methods used to modify the acidity/basicity or hydrophobicity/hydrophilicity of the surface. However, the materials prepared by this method are prone to leaching of functional groups into the reaction medium. The exact nature of the leaching phenomenon has not been fully addressed in the literature. In this contribution, we have investigated this process at the molecular level by combining well-controlled reaction experiments and several characterization techniques (Fourier transform infrared, ¹H-²⁹Si cross-polarization magic-angle spinning NMR, X-ray diffraction, thermogravimetric analysis, and N₂ adsorption-desorption). We have found that leaching is originated by the presence of terminal surface silanols, which render the catalysts susceptible to the attack of water and polar compounds. Hence, instead of simple detaching of functional groups, leaching can be better described as a partial dissolution of the surface layers of the silica, which of course also removes the functional groups during this process. Therefore, an effective strategy to minimize leaching is to reduce the density of free silanols via full functionalization of the surface. We propose a novel silylation method, high-temperature grafting, which allows the grafting process to be conducted at high temperatures (180 °C) under solvent-free conditions. By this method, a more complete silylation of surface silanols can be obtained. Consequently, the samples prepared by this high-temperature grafting method show to be highly stable during acid-catalyzed alkylation reaction, conducted under severe conditions (high temperature and in the presence of polar solvents).

Deactivation of Sn-Beta zeolites caused by structural transformation of hydrophobic to hydrophilic micropores during aqueous-phase glucose isomerization

Spectroscopic, titration and kinetic methods were used to probe the deactivation of Sn-Beta in water.

ZSM-5 decrystallization and dealumination in hot liquid water

ZSM-5 zeolite degrades the crystal surface framework and internal acid sites, dependent on the unique thermophysical nature of water solvent.

Interplay of Support Chemistry and Reaction Conditions on Copper Catalyzed Methanol Steam Reforming

A series of Cu catalysts supported on SiO₂, Al₂O₃-SiO₂, TiO₂ rutile, and Cu/TiO₂ anatase metal oxides has been studied for methanol reforming in the vapor phase. The highest activity was obtained on Cu/SiO₂ catalysts (5493 μmol H₂ min^-1·g_cat ^-1) followed by Cu/TiO₂ rutile, Cu/Al₂O₃-SiO₂, and anatase. XRD and HRTEM characterization after reaction revealed that on Cu/SiO₂ significant sintering occurred during reaction. In contrast, the particle size growth on Cu/TiO₂ rutile and anatase was less pronounced, which could be associated with the interaction between Cu clusters and TiO₂. Characterization by TGA showed that on Cu/Al₂O₃-SiO₂ the main cause of deactivation was coke deposition.