Solvent-determined mechanistic pathways in zeolite-H-BEA-catalysed phenol alkylation

The Alkylation of Phenol with Methanol: The Influence of Acid-Base Properties of X Zeolite on the Selectivity of para- and meta-Cresol

The vapor-phase alkylation of phenol with methanol was investigated on X zeolites and modified X zeolites. First, the difference of product distribution was tested between acid zeolite (HZSM-5, HX, HMCM-22, and Hβ) and basic zeolite X (KX and CsX). Then, X zeolites were modified with Li, K, Cs, Ca, Mg, La, and Ce ion exchange to adjust the acid-base properties of the zeolites. Finally, the type of acid sites and strength of acid-base sites on zeolite catalysts were determined by characterization techniques such as Py-IR and TPD, and the alkylation of phenol with methanol was tested. The results showed that O-alkylation products convert to the C-alkylation products on the B acid sites, which enhances the isomerization reaction, thereby increasing the proportion of meta-cresol in cresol. The results of TPD and IR indicated that weak basic sites on the zeolite promote the vertical adsorption of the aromatic ring and strong basic sites promote side-chain activation, while acid sites determine whether ring substitution or side-chain substitution occurs. During the reaction of phenol with methanol, the phenolic hydroxyl group strongly interacts with the surface of zeolites, leading to differences in the adsorption mode of the aromatic ring (vertical or parallel), which, in turn, alters the position of alkyl substitution. It is found that the proper acid-base property of X zeolites can selectively determine the desired alkylated products.

Interplay Between Metal and Acid Sites Tunes the Catalytic Selectivity Over Pd/Nanodiamond Catalysts

Metal and acid sites are two of the most crucial catalytically active components in heterogeneous catalysis. While variations in the size, morphology, and heterogeneity of metal species, or the manipulation of the strength, location, and density of acid sites, could significantly impact the catalytic performance, the combination and interplay between these sites are even more critical and have been a recent research focus. To achieve highly efficient and selective synergistic catalysis, it is desired to design a catalyst capable of orchestrating the sequential transformation of all reactants and intermediates at different active sites. In this study, we demonstrate that both acid and metal (Pd) sites can be introduced onto a nanodiamond@graphene (NDG) support particle through simple air oxidation and metal salt deposition-precipitation methods, respectively. The presence and assembly of these two catalytically active sites significantly alter the reaction network for the cyclohexanol conversion reaction. Under this strategy, the selectivity toward designated products─cyclohexene, phenol, and benzene─can be precisely tuned by the presence and patterning of these two sites on the nanodiamond particles. Specifically, we show that the catalyst with both acid sites and Pd ensemble sites, i.e., Pd/NDG, can efficiently convert cyclohexanol through consecutive dehydration and dehydrogenation reactions to form benzene with high selectivity (>80%). These findings underscore the potential of integrating metal and acid sites to design advanced catalysts with tailored reactivity and selectivity, paving the way for more efficient and versatile catalytic processes in industrial applications.

Decarboxylation of Hydroxybenzoic Acids to Phenol Via Deep Eutectic Solvents

The development of greener and more sustainable synthesis processes for manufacturing commodity chemicals is of great importance. The majority of current phenol production methods involve harsh reaction conditions with high energy consumption, causing severe environmental pollution. In this study, we present a novel approach for the decarboxylation of hydroxybenzoic acids (HBAs) to phenol using a choline chloride-urea (ChCl-urea) deep eutectic solvent (DES). Our study reveals the remarkable dual performance of ChCl-urea both as a catalyst and solvent for the decarboxylation of HBA, resulting in a high phenol yield (94 mol %) under mild reaction conditions. The proposed reaction pathway, established through a combination of experiments and computational simulations, enhances our understanding of this process. The recyclability of the DES system during decarboxylation was also assessed. Our findings demonstrate that the integration of DES into conventional chemical processes can pave the way for sustainable manufacturing, exemplifying a novel approach for producing phenol from abundant natural resources using designer solvents.

Sucrose-derived porous carbon catalyzed lignin depolymerization to obtain a product with application in type 2 diabetes mellitus

As the most important phenolic biopolymer in nature, lignin shows promising application potentialities in various bioactivities in vivo and in vitro, mainly including antioxidant, anti-inflammatory, hypolipidemic, and antidiabetic control. In this work, several carbon-based solid acids were synthesized to catalyze the fragmentation of organosolv lignin (OL). The generated lignin fragments, with controllable molecular weight and functional groups, were further evaluated for their application in the prevention and treatment of type 2 diabetes mellitus (T2DM). The results suggested that the urea-doped catalyst (SUPC) showed a more excellent catalytic performance in producing diethyl ether insoluble lignin (DEIL) and diethyl ether soluble lignin (DESL). In addition, the lignin fragments have a good therapeutic effect on the cell model of T2DM. Compared with the insulin resistance model, DEIL obtained by catalytic depolymerization of OL with SUPC could improve the glucose consumption of insulin-resistant cells. Moreover, low-concentration samples (50 μg/mL) can promote glucose consumption (19.7 mM) more than the traditional drug rosiglitazone (17.5 mM). This work demonstrates the prospect of depolymerized lignin for the prevention and treatment of T2DM and provides a new application field for lignin degradation products.

Solvent effect in H-BEA catalyzed cyclohexanol dehydration reaction

The solvent effect on H-BEA catalyzed cyclohexanol dehydration was investigated in water, dioxane, and cyclohexanol. The dynamic evolution of the Brønsted acid site of zeolite and its interaction with reactant molecules in different solvents were explored with ab initio molecular dynamics simulations, providing reliable configuration sampling to obtain configurations at equilibrium. Solvent profoundly changes the adsorption as well as the dehydration reaction of cyclohexanol in H-BEA, where the reaction is determined to follow the E2 mechanism in water and dioxane but the E1 mechanism in cyclohexanol untill saturation uptake. Near saturation uptake, all three solvents significantly reduce the cyclohexanol dehydration rates in H-BEA. Cyclohexanol loading also dramatically affects the kinetics of the dehydration reaction, displaying an overall decreasing trend with a local minimum present at intermediate loading of 6 molecules per unit cell, which is a result of the entropic effect associated with greater freedom of motion of the transition state. Rigorous quantification of enthalpy and entropy contributions to cyclohexanol adsorption and activation shed light on the solvent effect of zeolite-catalyzed alcohol dehydration.

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.

Water-Induced Micro-Hydrophobic Effect Regulates Benzene Methylation in Zeolite

Water is a ubiquitous component in heterogeneous catalysis over zeolites and can significantly influence the catalyst performance. However, the detailed mechanism insights into zeolite-catalyzed reactions under microscale aqueous environment remain elusive. Here, using multiple dimensional solid-state NMR experiments coupled with ultrahigh magic angle spinning technique and theoretical simulations, we establish a fundamental understanding of the role of water in benzene methylation over ZSM-5 zeolite under water vapor conditions. We show that water competes with benzene for the active sites of zeolite and facilitates the bimolecular reaction mechanism. The growth of water clusters induces a micro-hydrophobic effect in zeolite pores, which reorients benzene molecules and drives their interactions with surface methoxy species (SMS) on zeolite. We identify the formation and evolution of active SMS-Benzene complexes in a microscale aqueous environment and demonstrate that their accumulation in zeolite pores boosts benzene conversion and methylation.

Decarboxylation and Tandem Reduction/Decarboxylation Pathways to Substituted Phenols from Aromatic Carboxylic Acids Using Bimetallic Nanoparticles on Supported Ionic Liquid Phases as Multifunctional Catalysts

Valuable substituted phenols are accessible via the selective decarboxylation of hydroxybenzoic acid derivatives using multifunctional catalysts composed of bimetallic iron-ruthenium nanoparticles immobilized on an amine-functionalized supported ionic liquid phase (Fe25Ru75@SILP+IL-NEt2). The individual components of the catalytic system are assembled using a molecular approach to bring metal and amine sites into close contact on the support material, providing high stability and high decarboxylation activity. Operating under a hydrogen atmosphere was found to be essential to achieve high selectivity and yields. As the catalyst materials enable also the selective hydrogenation and hydrodeoxygenation of various additional functional groups (i.e., formyl, acyl, and nitro substituents), direct access to the corresponding phenols can be achieved via integrated tandem reactions. The approach opens versatile synthetic pathways for the production of valuable phenols from a wide range of readily available substrates, including compounds derived from lignocellulosic biomass.

Design Strategies for Hydroxyapatite-Based Materials to Enhance Their Catalytic Performance and Applicability

Hydroxyapatite (HAP) is a green catalyst that has a wide range of applications in catalysis due to its high flexibility and multifunctionality. These properties allow HAP to accommodate a large number of catalyst modifications that can selectively improve the catalytic performance in target reactions. To date, many studies have been conducted to elucidate the effect of HAP modification on the catalytic activities for various reactions. However, systematic design strategies for HAP catalysts are not established yet due to an incomplete understanding of underlying structure-activity relationships. In this review, tuning methods of HAP for improving the catalytic performance are discussed: 1) ionic composition change, 2) morphology control, 3) incorporation of other metal species, and 4) catalytic support engineering. Detailed mechanisms and effects of structural modulations on the catalytic performances for attaining the design insights of HAP catalysts are investigated. In addition, computational studies to understand catalytic reactions on HAP materials are also introduced. Finally, important areas for future research are highlighted.

Aldehyde Hydrogenation by Pt/TiO2 Catalyst in Aqueous Phase: Synergistic Effect of Oxygen Vacancy and Solvent Water

The aldehyde hydrogenation for stabilizing and upgrading biomass is typically performed in aqueous phase with supported metal catalysts. By combining density functional theory calculations and ab initio molecular dynamics simulations, the model reaction of formaldehyde hydrogenation with a Pt/TiO₂ catalyst is investigated with explicit solvent water molecules. In aqueous phase, both the O vacancy (Ov) on support and solvent molecules could donate charges to a Pt cluster, where the Ov could dominantly reduce the Pt cluster from positive to negative. During the formaldehyde hydrogenation, the water molecules could spontaneously protonate the O in the aldehyde group by acid/base exchange, generating the OH* at the metal-support interface by long-range proton transfer. By comparing the stoichiometric and reduced TiO₂ support, it is found that the further hydrogenation of OH* is hard on the positively charged Pt cluster over stoichiometric TiO₂. However, with the presence of Ov on reduced support, the OH* hydrogenation could become not only exergonic but also kinetically more facile, which prohibits the catalyst from poisoning. This mechanism suggests that both the proton transfer from solvent water molecules and the easier OH* hydrogenation from Ov could synergistically promote aldehyde hydrogenation. That means, even for such simple hydrogenation in water, the catalytic mechanism could explicitly relate to all of the metal cluster, oxide support, and solvent waters. Considering the ubiquitous Ov defects in reducible oxide supports and the common aqueous environment, this synergistic effect may not be exclusive to Pt/TiO₂, which can be crucial for supported metal catalysts in biomass conversion.

Maximum Impact of Ionic Strength on Acid-Catalyzed Reaction Rates Induced by a Zeolite Microporous Environment

The intracrystalline ionic environment in microporous zeolite can remarkably modify the excess chemical potential of adsorbed reactants and transition states, thereby influencing the catalytic turnover rates. However, a limit of the rate enhancement for aqueous-phase dehydration of alcohols appears to exist for zeolites with high ionic strength. The origin of such limitation has been hypothesized to be caused by the spatial constraints in the pores via, e.g., size exclusion effects. It is demonstrated here that the increase in turnover rate as well as the formation of a maximum and the rate drop are intrinsic consequences of the increasingly dense ionic environment in zeolite. The molecularly sized confines of zeolite create a unique ionic environment that monotonically favors the formation of alcohol-hydronium ion complexes in the micropores. The zeolite microporous environment determines the kinetics of catalytic steps and tailors the impact of ionic strength on catalytic rates.

Carbocation chemistry confined in zeolites: spectroscopic and theoretical characterizations

Acid-catalyzed reactions inside zeolites are one type of broadly applied industrial reactions, where carbocations are the most common intermediates of these reaction processes, including methanol to olefins, alkene/aromatic alkylation, and hydrocarbon cracking/isomerization. The fundamental research on these acid-catalyzed reactions is focused on the stability, evolution, and lifetime of carbocations under the zeolite confinement effect, which greatly affects the efficiency, selectivity and deactivation of zeolite catalysts. Therefore, a profound understanding of the carbocations confined in zeolites is not only beneficial to explain the reaction mechanism but also drive the design of new zeolite catalysts with ideal acidity and cages/channels. In this review, we provide both an in-depth understanding of the stabilization of carbocations by the pore confinement effect and summary of the advanced characterization methods to capture carbocations in zeolites, including UV-vis spectroscopy, solid-state NMR, fluorescence microscopy, IR spectroscopy and Raman spectroscopy. Also, we clarify the relationship between the activity and stability of carbocations in zeolite-catalyzed reactions, and further highlight the role of carbocations in various hydrocarbon conversion reactions inside zeolites with diverse frameworks and varying acidic properties.

HZSM-5 zeolite modification and catalytic reaction mechanism in the reaction of cyclohexene hydration

This study investigated a three-phase (liquid–liquid–solid) reaction system of cyclohexene hydration where the catalyst was hydrophilic at the bottom of the water phase.

Ab initio molecular dynamics with enhanced sampling in heterogeneous catalysis

Enhanced sampling ab initio simulations enable to study chemical phenomena in catalytic systems including thermal effects & anharmonicity, & collective dynamics describing enthalpic & entropic contributions, which can significantly impact on reaction free energy landscapes.

Insight into Carbocation-Induced Noncovalent Interactions in the Methanol-to-Olefins Reaction over ZSM-5 Zeolite by Solid-State NMR Spectroscopy

Carbocations such as cyclic carbenium ions are important intermediates in the zeolite-catalyzed methanol-to-olefins (MTO) reaction. The MTO reaction propagates through a complex hydrocarbon pool process. Understanding the carbocation-involved hydrocarbon pool reaction on a molecular level still remains challenging. Here we show that electron-deficient cyclopentenyl cations stabilized in ZSM-5 zeolite are able to capture the alkanes, methanol, and olefins produced during MTO reaction via noncovalent interactions. Intermolecular spatial proximities/interactions are identified by using two-dimensional ¹³ C-¹³ C correlation solid-state NMR spectroscopy. Combined NMR experiments and theoretical analysis suggests that in addition to the dispersion and CH/π interactions, the multiple functional groups in the cyclopentenyl cations produce strong attractive force via cation-induced dipole, cation-dipole and cation-π interactions. These carbocation-induced noncovalent interactions modulate the product selectivity of hydrocarbon pool reaction.

Influence of solvent structure and hydrogen bonding on catalysis at solid-liquid interfaces

Solvent molecules interact with reactive species and alter the rates and selectivities of catalytic reactions by orders of magnitude. Specifically, solvent molecules can modify the free energies of liquid phase and surface species via solvation, participating directly as a reactant or co-catalyst, or competitively binding to active sites. These effects carry consequences for reactions relevant for the conversion of renewable or recyclable feedstocks, the development of distributed chemical manufacturing, and the utilization of renewable energy to drive chemical reactions. First, we describe the quantitative impact of these effects on steady-state catalytic turnover rates through a rate expression derived for a generic catalytic reaction (A → B), which illustrates the functional dependence of rates on each category of solvent interaction. Second, we connect these concepts to recent investigations of the effects of solvents on catalysis to show how interactions between solvent and reactant molecules at solid-liquid interfaces influence catalytic reactions. This discussion demonstrates that the design of effective liquid phase catalytic processes benefits from a clear understanding of these intermolecular interactions and their implications for rates and selectivities.

Theoretical characterization of zeolite encapsulated platinum clusters in the presence of water molecules

Zeolite encapsulated metal clusters have shown high catalytic activity and superior stability due to confinement effects, the synergy between acidic and metal active sites, and strong metal-zeolite interactions. In the present work, density functional theory calculations were employed to study the stability of encapsulated Pt_n (n = 1-6) clusters in the zeolitic frameworks including Silicalite-1 and H-MFI. It has been found that the metal-zeolite interaction becomes stronger with the increasing Pt_n cluster size for both zeolitic frameworks. The encapsulated Pt_n clusters in the vicinity of the Brønsted acid site (BAS) of H-MFI form more stable Pt_nH_x (x = 1, 2) clusters. The presence of water molecules around the encapsulated Pt₆ cluster further enhances its stability, while the oxidation states of the encapsulated Pt_n cluster are largely affected by the BAS site and the surrounding water molecules. As the water concentration increases, water dissociation becomes more facile on the Pt₆@Silicalite-1 cluster while an opposite trend is found over the Pt₆H₂@H-MFI cluster. The proton of the BAS site can be transferred to the encapsulated Pt₆ cluster via a hydronium cluster H⁺(H₂O)_n, leading to the formation of the Pt₆H₂@H-MFI cluster.

Acid Catalysis with Alkane/Water Microdroplets in Ionic Liquids

Ionic liquids are composed of an organic cation and a highly delocalized perfluorinated anion, which remain tight to each other and neutral across the extended liquid framework. Here we show that n-alkanes in millimolar amounts enable a sufficient ion charge separation to release the innate acidity of the ionic liquid and catalyze the industrially relevant alkylation of phenol, after generating homogeneous, self-stabilized, and surfactant-free microdroplets (1-5 μm). This extremely mild and simple protocol circumvents any external additive or potential ionic liquid degradation and can be extended to water, which spontaneously generates microdroplets (ca. 3 μm) and catalyzes Brönsted rather than Lewis acid reactions. These results open new avenues not only in the use of ionic liquids as acid catalysts/solvents but also in the preparation of surfactant-free, well-defined ionic liquid microemulsions.

Confinement effects and acid strength in zeolites

Chemical reactivity and sorption in zeolites are coupled to confinement and-to a lesser extent-to the acid strength of Brønsted acid sites (BAS). In presence of water the zeolite Brønsted acid sites eventually convert into hydronium ions. The gradual transition from zeolite Brønsted acid sites to hydronium ions in zeolites of varying pore size is examined by ab initio molecular dynamics combined with enhanced sampling based on Well-Tempered Metadynamics and a recently developed set of collective variables. While at low water content (1-2 water/BAS) the acidic protons prefer to be shared between zeolites and water, higher water contents (n > 2) invariably lead to solvation of the protons within a localized water cluster adjacent to the BAS. At low water loadings the standard free energy of the formed complexes is dominated by enthalpy and is associated with the acid strength of the BAS and the space around the site. Conversely, the entropy increases linearly with the concentration of waters in the pores, favors proton solvation and is independent of the pore size/shape.

Insights into protonation for cyclohexanol/water mixtures at the zeolitic Brønsted acid site

Proton transfer from Brønsted acid sites (BASs) to alcohol molecules ignites the acid-catalyzed alcohol dehydration reactions. For aqueous phase dehydration reactions in zeolites, the coexisting water molecules around BASs in the zeolite pores significantly affect the alcohol dehydration activity. In the present work, proton transfer processes among the BASs of H-BEA zeolites, the adsorbed cyclohexanol and surrounding water clusters with different sizes up to 8 water molecules were investigated using ab initio molecular dynamics (AIMD) simulations combined with the multiple-walker well-tempered metadynamics algorithm. The plausible proton locations and proton transfer processes were characterized using two/three-dimensional free energy landscapes. The strong proton affinity makes the protonated cyclohexanol stable species until a water trimer is formed. The proton either is shared between protonated cyclohexanol and the water trimer or remains with the water trimer (H7O3+). With a further increase in water concentrations, the proton prefers to remain with the water clusters.

Directing the Rate-Enhancement for Hydronium Ion Catalyzed Dehydration via Organization of Alkanols in Nanoscopic Confinements

Alkanol dehydration rates catalyzed by hydronium ions are enhanced by the dimensions of steric confinements of zeolite pores as well as by intraporous intermolecular interactions with other alkanols. The higher rates with zeolite MFI having pores smaller than those of zeolite BEA for dehydration of secondary alkanols, 3-heptanol and 2-methyl-3-hexanol, is caused by the lower activation enthalpy in the tighter confinements of MFI that offsets a less positive activation entropy. The higher activity in BEA than in MFI for dehydration of a tertiary alkanol, 2-methyl-2-hexanol, is primarily attributed to the reduction of the activation enthalpy by stabilizing intraporous interactions of the Cβ -H transition state with surrounding alcohol molecules. Overall, we show that the positive impact of zeolite confinements results from the stabilization of transition state provided by the confinement and intermolecular interaction of alkanols with the transition state, which is impacted by both the size of confinements and the structure of alkanols in the E1 pathway of dehydration.

DFT study of phenol alkylation with propylene on H-BEA in the absence and presence of water

Water reduces the activation barrier of the rate-limiting step of phenol alkylation with propylene in H-BEA. This, in turn, increases the transition-state theory rate coefficient by two orders-of-magnitude, suggesting much faster alkylation.

Rigid Arrangements of Ionic Charge in Zeolite Frameworks Conferred by Specific Aluminum Distributions Preferentially Stabilize Alkanol Dehydration Transition States

Zeolite reactivity depends on the solvating environments of their micropores and the proximity of their Brønsted acid sites. Turnover rates (per H⁺ ) for methanol and ethanol dehydration increase with the fraction of H⁺ sites sharing six-membered rings of chabazite (CHA) zeolites. Density functional theory (DFT) shows that activation barriers vary widely with the number and arrangement of Al (1-5 per 36 T-site unit cell), but cannot be described solely by Al-Al distance or density. Certain Al distributions yield rigid arrangements of anionic charge that stabilize cationic intermediates and transition states via H-bonding to decrease barriers. This is a key feature of acid catalysis in zeolite solvents, which lack the isotropy of liquid solvents. The sensitivity of polar transition states to specific arrangements of charge in their solvating environments and the ability to position such charges in zeolite lattices with increasing precision herald rich catalytic diversity among zeolites of varying Al arrangement.

Structure and solvation of confined water and water-ethanol clusters within microporous Brønsted acids and their effects on ethanol dehydration catalysis

Water networks confined within zeolites solvate clustered reactive intermediates and must rearrange to accommodate transition states that differ in size and polarity, with thermodynamic penalties that depend on the shape of the confining environment.

Aqueous-phase reactions within microporous Brønsted acids occur at active centers comprised of water-reactant-clustered hydronium ions, solvated within extended hydrogen-bonded water networks that tend to stabilize reactive intermediates and transition states differently. The effects of these diverse clustered and networked structures were disentangled here by measuring turnover rates of gas-phase ethanol dehydration to diethyl ether (DEE) on H-form zeolites as water pressure was increased to the point of intrapore condensation, causing protons to become solvated in larger clusters that subsequently become solvated by extended hydrogen-bonded water networks, according to in situ IR spectra. Measured first-order rate constants in ethanol quantify the stability of S_N2 transition states that eliminate DEE relative to (C₂H₅OH)(H⁺)(H₂O)_n clusters of increasing molecularity, whose structures were respectively determined using metadynamics and ab initio molecular dynamics simulations. At low water pressures (2–10 kPa H₂O), rate inhibition by water (–1 reaction order) reflects the need to displace one water by ethanol in the cluster en route to the DEE-formation transition state, which resides at the periphery of water–ethanol clusters. At higher water pressures (10–75 kPa H₂O), water–ethanol clusters reach their maximum stable size ((C₂H₅OH)(H⁺)(H₂O)_4–5), and water begins to form extended hydrogen-bonded networks; concomitantly, rate inhibition by water (up to –3 reaction order) becomes stronger than expected from the molecularity of the reaction, reflecting the more extensive disruption of hydrogen bonds at DEE-formation transition states that contain an additional solvated non-polar ethyl group compared to the relevant reactant cluster, as described by non-ideal thermodynamic formalisms of reaction rates. Microporous voids of different hydrophilic binding site density (Beta; varying H⁺ and Si–OH density) and different size and shape (Beta, MFI, TON, CHA, AEI, FAU), influence the relative extents to which intermediates and transition states disrupt their confined water networks, which manifest as different kinetic orders of inhibition at high water pressures. The confinement of water within sub-nanometer spaces influences the structures and dynamics of the complexes and extended networks formed, and in turn their ability to accommodate the evolution in polarity and hydrogen-bonding capacity as reactive intermediates become transition states in Brønsted acid-catalyzed reactions.

Variable Temperature and Pressure Operando MAS NMR for Catalysis Science and Related Materials

The characterization of catalytic materials under working conditions is of paramount importance for a realistic depiction and comprehensive understanding of the system. Under such relevant environments, catalysts often exhibit properties or reactivity not observed under standard spectroscopic conditions. Fulfilling such harsh environments as high temperature and pressure is a particular challenge for solid-state NMR where samples spin several thousand times a second within a strong magnetic field. To address concerns about the disparities between spectroscopic environments and operando conditions, novel MAS NMR technology has been developed that enables the probing of catalytic systems over a wide range of pressures, temperatures, and chemical environments. In this Account, new efforts to overcome the technical challenges in the development of operando and in situ MAS NMR will be briefly outlined. Emphasis will be placed on exploring the unique chemical regimes that take advantage of the new developments. With the progress achieved, it is possible to interrogate both structure and dynamics of the environments surrounding various nuclear constituents (1H, 13C, 23Na, 27Al, etc.), as well as assess time-resolved interactions and transformations.Operando and in situ NMR enables the direct observation of chemical components and their interactions with active sites (such as Brønsted acid sites on zeolites) to reveal the nature of the active center under catalytic conditions. Further, mixtures of such constituents can also be assessed to reveal the transformation of the active site when side products, such as water, are generated. These interactions are observed across a range of temperatures (-10 to 230 °C) and pressures (vacuum to 100 bar) for both vapor and condensed phase analysis. When coupled with 2D NMR, computational modeling, or both, specific binding modes are identified where the adsorbed state provides distinct signatures. In addition to vapor phase chemical environments, gaseous environments can be introduced and controlled over a wide range of pressures to support catalytic studies that require H2, CO, CO2, etc. Mixtures of three phases may also be employed. Such reactions can be monitored in situ to reveal the transformation of the substrates, active sites, intermediates, and products over the course of the study. Further, coupling of operando NMR with isotopic labeling schemes reveals specific mechanistic insights otherwise unavailable. Examples of these strategies will be outlined to reveal important fundamental insights on working catalyst systems possible only under operando conditions. Extension of operando MAS NMR to study the solid-electrolyte interface and solvation structures associated with energy storage systems and biomedical systems will also be presented to highlight the versatility of this powerful technique.

The Steric Effect in Green Benzylation of Arenes with Benzyl Alcohol Catalyzed by Hierarchical H-beta Zeolite

For decades the steric effect was still ambiguously understood in catalytic benzylation reactions of arenes with benzyl alcohol, which limited the green synthesis of phenylmethane derivates in industrial scale. This research applies a series of silica–alumina beta zeolites to systematically evaluate factors like catalyst porosity, reactants molecule size, and reaction temperature on catalytic benzylation. First, a suitable hierarchical beta zeolite catalyst was screened out by X-ray powder diffraction, N2 adsorption−desorption, and probe benzylation with p-xylene. In the following substrates expanding study, for a typical benzylation of benzene, it showed extraordinary performance among literature reported ones that the conversion was 98% while selectivity was 90% at 353 K only after 10 min. The steric effect of aromatics with different molecular sizes on benzylation was observed. The reaction activities of four different aromatics followed the order: benzene > toluene > p-xylene > mesitylene. Combined with macroscopic kinetic analysis, this comprehensive study points out for the first time that the nature of this steric effect was dominated by the relative adsorption efficiency of different guest aromatic molecules on the host zeolite surface.

A Method of Calculating the Kamlet-Abboud-Taft Solvatochromic Parameters Using COSMO-RS

There is demand for safer and bio-based solvents, brought on by legislation and sustainability objectives. The prediction of physical properties is highly desirable to help design new molecules. Here we present an in silico approach to obtain calculated Kamlet-Abboud-Taft solvatochromic parameters using virtual experiments. The tautomerisation equilibrium of methyl acetoacetate and dimedone was calculated in different solvents with COSMO-RS theory and converted into estimates of solvent dipolarity and hydrogen bond accepting ability, respectively. Hydrogen bond donating ability was calculated as a function of the electron deficient surface area on protic solvents. These polarity descriptors correlate with rate constants and equilibria, and so ability of calculated Kamlet-Abboud-Taft solvatochromic parameters to recreate experimental free energy relationships was tested with sixteen case studies taken from the literature. The accuracy of the calculated parameters was also satisfactory for solvent selection, as demonstrated with a 1,4-addition reaction and a multicomponent heterocycle synthesis.