Acid Sites In Zeolites Research Articles

Quantification of extracrystalline acid sites in MFI zeolites after post-synthetic passivation treatments using mesitylene benzylation kinetics

Mesitylene benzylation kinetics enable quantification of dilute amounts of external H+ sites in surface passivated MFI zeolites that are not assessable by conventional spectroscopic or titrimetric techniques.

Propane dehydrogenation on an extra-framework and framework-embedded metal site within ZSM-5 zeolite from first-principles microkinetic simulations

The Lewis acid sites in ZSM-5 zeolite involving Co, Ni, and Cu cations were computationally studied on the stability and the PDH catalytic activity.

Brønsted acid sites in the Beta zeolite induced highly efficient amidation of styrene in the aqueous phase

The acidic mesoporous Beta zeolite, which contains a substantial number of Brønsted acid sites, has been found to efficiently catalyze the amidation of styrene with benzonitrile in the aqueous phase.

Ce-modified USY zeolite catalysts for catalytic oxidation of chlorinated VOCs: Effect of the location of CeO2 and its synergistic effect with USY zeolite

The catalytic behavior of a catalyst for chlorine-containing volatile organic compounds (CVOCs) oxidation largely depends on the synergistic interaction between the oxidizing and acidic sites. In the present work, two catalysts with different distributions of CeO2 on the inner and outer surfaces of 4.0Ce-USY-ex and 4.0Ce-USY-dp (USY zeolite) were prepared respectively by ion exchange and deposition methods, with a purpose of finding out how the location of the oxidation sites (CeO2) influence its synergistic effect with the acidic sites of zeolite. The results show that 4.0Ce-USY-ex is much more active for catalytic degradation of 1,2-dichloroethane (DCE), while 4.0Ce-USY-dp catalyst exhibit higher catalytic degradation activity for other structured CVOCs (dichloromethane (DCM), trichloroethylene (TCE), chlorobenzene (CB)). CeO2 in 4.0Ce-USY-ex catalyst mainly disperses in the pore channels of USY zeolite, and there are many strong acid centers on the surface, which is conducive to the dechlorination conversion of CVOCs. However, CeO2 in 4.0Ce-USY-dp catalyst is mainly distributed on the outer surface of USY and has strong oxidation ability, which contributes to the deep oxidation of CVOCs. Moreover, the presence of a large number of strong acid centers on the catalyst surface of 4.0Ce-USY-ex catalysts leads to severe accumulation of surface carbon species and significantly decreases its stability towards DCE. However, a large number of active oxygen species on the surface of 4.0Ce-USY-dp and CeO2 catalysts are beneficial to the deep oxidation of DCE, reducing the formation of surface carbon and thus improving the stability of the catalyst. Thus, the influence of the location of the oxidation sites on its synergistic effect with the acidic sites was established in the present work, which could provide some new ideas for the rational design of CVOCs degradation catalyst with appropriate distribution of active sites.

Amphiphilic HZSM-5 for Cyclopentene Hydration at the Liquid-Liquid Interface in Pickering Emulsion.

Zeolite is considered an ideal catalyst for olefin hydration due to its high specific surface area and abundant acid sites. However, the immiscibility of the water-oil two phases in olefin hydration limits mass transfer, and the side reaction of etherification occurs acutely, resulting in a low yield of alcohol. Thus, water-oil amphiphilic HZSM-5 was prepared by sulfonating silanized zeolite. The successful introduction of organic and sulfonic acid groups is demonstrated by FT-IR, TG, and water contact angles. Amphiphilic HZSM-5 can stabilize the Pickering emulsion and catalyze cyclopentene hydration at the phase interface. In addition, NH3-TPD and Py-IR show that the amount of strong Bro̷nsted acid sites of zeolites increases significantly after sulfonation. This facilitates the rate-determining step of cyclopentene activation by H+ to form carbocation. Moreover, the nucleophilic side reactions are inhibited by a high concentration of H+. Finally, under the optimized reaction condition, the conversion of cyclopentene can achieve 5.066% with a selectivity of 85.37% to cyclopentanol, which almost reaches the reaction equilibrium.

Promoting hydrogenation without side reactions over Pd nanoparticles uniformly dispersed in MFI zeolite

Zeolites have been widely applied as support materials for metal nanoparticles. However, when developing a metal/zeolite catalyst for hydrogenation reactions, the Brønsted acid sites of zeolites can promote side reactions and the zeolite structure can inhibit the access of reactants to the metal, resulting in low activity and selectivity. In this study, these challenges are addressed by preparing a metal/zeolite catalyst via a modified ion-exchange followed by reduction method, and subjecting the catalysts to two test reactions, hydrogenation of vanillin and fatty acid methyl esters (FAMEs). After forming Pd nanoparticles within ZSM-5, it was treated with a NaOH solution in a controlled manner to form mesopores around Pd and replace hydronium ions with sodium ions to remove Brønsted acid sites. Pd/ZSM-5 catalysts prepared with controlled NaOH treatment showed higher hydrogenation reactivity and improved selectivity toward the desired hydrogenation products than those prepared without NaOH treatment. This study presents a simple strategy to prepare metal/zeolite catalysts to promote hydrogenation without side reactions.

Front Cover: Coking Reduction of Cr‐loaded Beta Zeolite during Polymer Cracking: Hydrocracking of Aromatics by Synergistic Effect of Cr6+ and Zeolitic Acid Sites (ChemCatChem 21/2023)

Front Cover: Coking Reduction of Cr‐loaded Beta Zeolite during Polymer Cracking: Hydrocracking of Aromatics by Synergistic Effect of Cr⁶⁺ and Zeolitic Acid Sites (ChemCatChem 21/2023)

Tandem catalysts of different crystalline In2O3/sheet HZSM-5 zeolite for CO2 hydrogenation to aromatics

In tandem catalysts, not only good synergy between the two active components is required, but also the precise control of the spatial distribution between the two active components of metal oxides and zeolite is crucial for the migration and conversion of reaction intermediates in the direct conversion of CO2 to hydrocarbons. The correlation between the metal and the acidic site of zeolite has traditionally been simplified as “the closer, the better”. However, it should be noted that this principle only holds true for a portion of tandem catalysts. Therefore, this paper studied the effect of different crystalline In2O3 (cubic phase, hexagonal phase, and mixed cubic/hexagonal phase) and sheet HZSM-5 zeolite tandem catalysts on the activity of CO2 hydrogenation reaction under different spatial distribution. The generalized gradient approximation (GGA) of density functional theory (DFT) were used to simulate the adsorption energy of CO2 by oxygen vacancy on c-In2O3(111) and h-In2O3(104) planes, it was found that Ov1 on c-In2O3(111) and Ov4 on h-In2O3(104) had the strongest adsorption energy for CO2. In addition, it has been observed that the proximity of the two active components (e.g., during mortar mixing) results in decreased catalytic performance. This is due to the migration of metal In, which neutralizes the acid sites of zeolites and leads to inefficient conversion of methanol reaction intermediates to aromatics. As a result, CO2 conversion and aromatic selectivity are decreased.

Hollow STW‐Type Zeolite Single Crystals with Aluminum Gradient for Highly Selective Production of p‐Xylene from Methanol‐Toluene Alkylation

AbstractZeolites with uniform micropores are important shape‐selective catalysts. However, the external acid sites of zeolites have a negative impact on shape‐selective catalysis, and the microporosity may lead to serious diffusion limitation. Herein, we report on the direct synthesis of hierarchical hollow STW‐type zeolite single crystals with a siliceous exterior. In an alkalinous fluoride medium, the nucleation of highly siliceous STW zeolites takes place first, and the nanocrystals are preferentially aligned on the outer surface of the gel agglomerates to grow into single crystalline shells upon crystallization. The lagged crystallization of the internal Al‐rich amorphous gels onto the inner surface of nanocrystalline zeolite shells leads to the formation of hollow cavities in the core of the zeolite crystals. The hollow zeolite single crystals possess a low‐to‐high aluminum gradient from the surface to the core, resulting in an intrinsic inert external surface, and exhibit superior catalytic performance in toluene methylation reactions.

Hollow STW-Type Zeolite Single Crystals with Aluminum Gradient for Highly Selective Production of p-Xylene from Methanol-Toluene Alkylation.

Zeolites with uniform micropores are important shape-selective catalysts. However, the external acid sites of zeolites have a negative impact on shape-selective catalysis, and the microporosity may lead to serious diffusion limitation. Herein, we report on the direct synthesis of hierarchical hollow STW-type zeolite single crystals with a siliceous exterior. In an alkalinous fluoride medium, the nucleation of highly siliceous STW zeolites takes place first, and the nanocrystals are preferentially aligned on the outer surface of the gel agglomerates to grow into single crystalline shells upon crystallization. The lagged crystallization of the internal Al-rich amorphous gels onto the inner surface of nanocrystalline zeolite shells leads to the formation of hollow cavities in the core of the zeolite crystals. The hollow zeolite single crystals possess a low-to-high aluminum gradient from the surface to the core, resulting in an intrinsic inert external surface, and exhibit superior catalytic performance in toluene methylation reactions.

Lewis acidity and substituent effects influence aldehyde enolization and C–C coupling in beta zeolites

The Claisen-Schmidt condensation between propanal and benzaldehyde was used a model aldol reaction to assess the reactivity of tetravalent framework Lewis acid sites (Ti, Zr, Sn, Hf) in hydrophobic beta zeolites (M-*BEA) and determine rates of enolization and carbon–carbon (C–C) bond formation along with selectivities among cross- and self-condensation products. Rates of enolate formation during liquid phase reactions (393 K) span nearly 100-fold, depending on the identity of the metal heteroatom, and correlate with enthalpies for adsorption of pyridine in liquid-filled pores of M-*BEA, as measured by isothermal titration calorimetry. Comparisons among adsorption enthalpies suggest that tetrahedrally substituted Zr and Hf are more effective Lewis acids than Sn and Ti for coordinating aldehydes within these environments. Ratios of initial rates of C–C coupling between propanal and benzaldehyde compared to propanal self-condensation were less than unity for all M-*BEA catalysts. These selectivity patterns reflect the combination of electronic and steric effects imposed by the larger aromatic substituent at the carbonyl carbon atom and its concerted interactions with the Lewis acid site, which causes rates for nucleophilic attack of enolates to benzaldehyde to be far less than those to propanal.

Shaping Y-Zeolites using in situ formation of alumina and AlPO binders: Coke-resistant catalysts for the cracking of methylcyclohexane

Endothermic catalytic cracking of hydrocarbon fuels over solid acid catalysts has been considered a potential heat sink to offset the thermal loads on high-speed flight vehicles, thus maintaining their catalyst stability and activity. Which is essential as an effective heat sink and produce fuels for sustained flight. The present work demonstrates a novel approach of dispersing zeolite in a catalyst matrix containing inorganic binders such as alumina and aluminum phosphate (ALPO) to improve the thermal, mechanical stability as well as the coke-resistance of Y-zeolites during catalytic cracking of endothermic fuels. Synthesis of such binders modified zeolite was achieved by adding Y-zeolite to pseudo boehmite, an alumina precursor followed by its hydrolysis in the presence of either nitric acid or phosphoric acid, which led to the formation of Alumina or ALPO modified zeolites. During this process, partial removal of alumina from the zeolite framework functions as seed for the growth of alumina and AlPO binders which together form a catalyst matrix consisting of zeolite and binder. Three different Y-Zeolites of varying Si/Al ratios were modified with alumina and ALPO binders using the afore-mentioned process. N2 sorption isotherms, Ammonia TPD, Solid state NMR and SEM imaging showed that these two binders modified the textural, structural, and acidic sites of the zeolites. Catalytic cracking of methylcyclohexane (model endothermic fuel) over these binder modified zeolites was carried out to study their performance and determine the conversion of methylcyclohexane, catalyst stability, product selectivity and coke deposition. Alumina and AlPO binders were observed to have differential binding with the zeolite acid sites, therefore the resultant catalyst activity, product activity and coking behavior were also found to be different. The alumina modified zeolites showed a slight decrease in the catalytic activity but promoted the formation of coke which can be easily oxidized and the selectivity towards cracked products than their isomers. In contrast, AlPO modified zeolites exhibited decreased catalytic activity but also decreased the coking considerably. Spent catalysts characterization clearly demonstrated that the binder modification of zeolites improved the catalyst stability, products selectivity, coke deposition, nature of coke and the amount of coke deposition.

Coking Reduction of Cr‐loaded Beta Zeolite during Polymer Cracking: Hydrocracking of Aromatics by Synergistic Effect of Cr6+ and Zeolitic Acid Sites

AbstractZeolite deactivation induced by coke deposition is a critical problem in the industrialization of the chemical recycling (CR) of plastics. We previously demonstrated that Cr6+ species bonded to silanol groups in the zeolite framework inhibit coke deposition in low‐density polyethylene (LDPE) cracking. In this study, the role of Cr6+ species and the mechanism of coke reduction were determined experimentally. During LDPE cracking, hydrogen was generated from the alkane products at the Cr6+ sites. Furthermore, the hydrogenation and decomposition of aromatics, which are coke precursors, occurred at the zeolitic acid sites, resulting in the reduction of coke deposition. Owing to the synergistic effect of the Cr6+ and zeolitic acid sites, coke reduction was achieved without reducing cracking activity.

Acidity: A Key Parameter in Zeolite-Templated Carbon Formation.

This work reveals the crucial role of zeolite acidity in the synthesis of zeolite-templated carbons (ZTCs). While textural and chemical properties appear to be independent from acidity at a given synthesis temperature, the spin concentration in hybrid materials appears to be strongly impacted by the zeolite acid site concentration. The electrical conductivity of the hybrids and resulting ZTCs are closely related to the spin concentration in the hybrid materials. The amount of zeolite acid sites hence fundamentally impacts the electrical conductivity of the samples that spans over a range of four magnitudes. Electrical conductivity reveals as key parameter to describe the quality of ZTCs.

Dynamical Equilibrium between Brønsted and Lewis Sites in Zeolites: Framework-Associated Octahedral Aluminum.

While the structures of Brønsted acid sites (BAS) in zeolites are well understood, those of Lewis acid sites (LAS) remain an active area of investigation. Under hydrated conditions, the reversible formation of framework-associated octahedral aluminum has been observed in zeolites in the acidic form. However, the structure and formation mechanisms are currently unknown. In this work, combined experimental 27 Al NMR spectroscopy and computational data reveal for the first time the details of the zeolite framework-associated octahedral aluminium. The octahedral LAS site becomes kinetically allowed and thermodynamically stable under wet conditions in the presence of multiple nearby BAS sites. The critical condition for the existence of such octahedral LAS appears to be the availability of three protons: at lower proton concentration, either by increasing the Si/Al or by ion-exchange to non-acidic form, the tetrahedral BAS becomes thermodynamically more stable. This work resolves the question about the nature and reversibility of framework-associated octahedral aluminium in zeolites.

Dynamical Equilibrium between Brønsted and Lewis Sites in Zeolites: Framework‐Associated Octahedral Aluminum

AbstractWhile the structures of Brønsted acid sites (BAS) in zeolites are well understood, those of Lewis acid sites (LAS) remain an active area of investigation. Under hydrated conditions, the reversible formation of framework‐associated octahedral aluminum has been observed in zeolites in the acidic form. However, the structure and formation mechanisms are currently unknown. In this work, combined experimental 27Al NMR spectroscopy and computational data reveal for the first time the details of the zeolite framework‐associated octahedral aluminium. The octahedral LAS site becomes kinetically allowed and thermodynamically stable under wet conditions in the presence of multiple nearby BAS sites. The critical condition for the existence of such octahedral LAS appears to be the availability of three protons: at lower proton concentration, either by increasing the Si/Al or by ion‐exchange to non‐acidic form, the tetrahedral BAS becomes thermodynamically more stable. This work resolves the question about the nature and reversibility of framework‐associated octahedral aluminium in zeolites.

Active Ensembles in Methane Dehydroaromatization over Molybdenum/ZSM‐5 Zeolite Identified by 2D 1H−95Mo Magic Angle Spinning Nuclear Magnetic Resonance Correlation Spectroscopy

AbstractMethane dehydroaromatization (MDA) over Mo‐modified zeolite is a potential catalytic route for converting natural gas into valuable aromatics. However, the active species in this reaction are highly complex, involving diverse Mo species, acidic sites of zeolite, and organic molecules. Herein, we apply 1D 95Mo NMR and 2D 1H‐95Mo heteronuclear correlation solid‐state NMR spectroscopy to directly observe the active ensembles in the confined channels of Mo/ZSM‐5 zeolite during the MDA reaction. We monitor the evolution of the spatial correlations of Mo species with the Brønsted acid sites and organic products (olefins and aromatics) in the zeolite channels. We identified two kinds of MoOxCy species, with the more carbidic one (MoOxCy‐II) exhibiting higher activity for methane activation and benzene formation. The strong spatial interactions between the active Mo species and the organic species in the Mo/ZSM‐5 pores are related to the MDA activity.

Active Ensembles in Methane Dehydroaromatization over Molybdenum/ZSM-5 Zeolite Identified by 2D 1 H-95 Mo Magic Angle Spinning Nuclear Magnetic Resonance Correlation Spectroscopy.

Methane dehydroaromatization (MDA) over Mo-modified zeolite is a potential catalytic route for converting natural gas into valuable aromatics. However, the active species in this reaction are highly complex, involving diverse Mo species, acidic sites of zeolite, and organic molecules. Herein, we apply 1D 95 Mo NMR and 2D 1 H-95 Mo heteronuclear correlation solid-state NMR spectroscopy to directly observe the active ensembles in the confined channels of Mo/ZSM-5 zeolite during the MDA reaction. We monitor the evolution of the spatial correlations of Mo species with the Brønsted acid sites and organic products (olefins and aromatics) in the zeolite channels. We identified two kinds of MoOx Cy species, with the more carbidic one (MoOx Cy -II) exhibiting higher activity for methane activation and benzene formation. The strong spatial interactions between the active Mo species and the organic species in the Mo/ZSM-5 pores are related to the MDA activity.

Reaction mechanism of co-coupling conversion of propane and methanol over H-ZSM-5 zeolite

Co-coupling conversion of propane and methanol is an effective method to promote the activation of propane; however, the reaction mechanism has not been fully understood due to the complex reaction network. In this study, the reaction kinetics and thermodynamics of various elementary steps for propane activation under the assistance of methanol over H-ZSM-5 zeolite were systemically investigated by density functional theory (DFT) calculations combining with model experiments. The results show that direct dehydrogenation of propane on the acid sites of zeolite is very difficult, while introduction of methanol considerably decreases the free energy barrier for propane activation. This is because large amounts of active intermediates, such as surface methoxy and alkene-based species, are generated in methanol conversion that promote the propane dehydrogenation through hydrogen transfer reactions. Among these active intermediates, alkene-based species (including adsorbed alkene and tertiary carbocations) show higher activity towards C–H bond activation of propane.

Investigating trimethylphosphine oxide interactions with Brønsted and Lewis acid sites in zeolites: A comparative solid-state NMR study of wet-phase and gas-phase adsorption techniques

This study examines the differences in using two primary methods for adsorbing trimethylphosphine oxide (TMPO) onto acid catalysts and their influence on the NMR spectra of TMPO-loaded zeolites. The findings show that TMPO molecules interact with both Brønsted and Lewis acid sites, and that the amount of (TMPO)2H+ dimers formed increases with the amount of TMPO added. The use of dichloromethane as a solvent inhibits the formation of dimers and crystalline TMPO but may lead to inaccurate estimations of acid site strength and quantification due to interactions with residual solvent molecules. The gas-phase method is preferred for adsorbing TMPO onto zeolites, but caution is needed to avoid forming a high number of dimers that ca pose challenges in the interpretation of the NMR spectra.