Lewis Acid Sites In Zeolites Research Articles

Qualitative and quantitative analysis of Brønsted and Lewis acid sites in zeolites: A combined probe-assisted 1H MAS NMR and NH3-TPD investigation

Zeolite is one of the most important heterogeneous catalysts in acid catalytic reactions. Considering that the catalytic behaviors of zeolites are mostly related to their acidic characteristics, extensive attention has been attracted to the measurements of acid type, strength and concentration in zeolites. Numerous techniques including Fourier-transform infrared spectroscopy (FTIR), probe-assisted 1H, 13C and 31P magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) as well as temperature programmed desorption of ammonia (NH3-TPD) have been developed for determining the acid sites. Nevertheless, a single approach is defective to characterize the acid sites comprehensively. Herein, combining the probe-assisted (e.g., NH3 and CD3CN) 1H MAS NMR and NH3-TPD, the acid sites in different zeolites including the acid type, density and strength were determined. The commonly utilized NH3-TPD to determine the acid strength of zeolite samples with different topologies should be rigorously considered owing to zeolite confinement effect. Controlling the desorption temperature of NH3 probe molecules, the acid type (i.e., Brønsted acid sites (BAS) and Lewis acid sites (LAS)) and the corresponding density could be determined by NH3 probe-assisted 1H MAS NMR spectroscopy, while the acid strength could be investigated via CD3CN probe-assisted 1H MAS NMR spectroscopy.

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.

Promotional effect of Co cations on the catalytic activity of Co/ZSM-5 in 1,2-dichloroethane dehydrochlorination for the production of vinyl chloride monomer

In the catalytic dehydrochlorination of 1, 2-dichloroethane (DCE) for Vinyl chloride monomer (VCM) production, it is important to design catalysts with appropriate acid properties which can activate DCE, while inhibiting the further reaction of VCM. In this study, Co species were introduced into ZSM-5 through an ion-exchange method to change acid site distribution of ZSM-5. The introduction of Co cations significantly improved DCE conversion and VCM selectivity, achieving 6 and 2-fold greater than bare HZSM-5, respectively. Postmortem analyses allowed us to delineate the role of acid sites: Lewis acid sites are the major active sites, whereas Brønsted acid sites give rise to undesired secondary reactions. Thus, our results demonstrate that the introduction of Co cations increases the number of relevant active sites, and simultaneously eliminates undesirable sites. These findings help understand the effect of Brønsted and Lewis acid sites in zeolites on the DCE dehydrochlorination, providing useful information for designing rational catalysts.

IR-spectroscopic study of complex formation of nitrogen oxides (NO, N2O) with strong Lewis acid sites and the reactivity of adsorbed species in CO and CH4 oxidation

The formation of complexes and transformations of nitrogen oxides (NO, N2O) on strong Lewis acid sites of HZSM-5, H[Ga]ZSM-5, ZnO/HZSM-5 zeolites and dealuminated mordenites was investigated by diffuse-reflectance IR spectroscopy. Adsorbed N2O that is formed by disproportionation of NO is capable of oxidizing CO and CH4 molecules to CO2. The behaviour of strong Lewis acid sites in zeolites and cationic forms of zeolites in the NO disproportionation and CO or CH4 oxidation was comparatively studied.

Towards a better understanding of Lewis acidic aluminium in zeolites.

Zeolites are a class of materials that are of great relevance for industrial catalysis. Several fundamental questions relating to the structure and role of the Lewis acid sites in these materials remain unanswered. Proposals for the origin of such species can broadly be classified into three categories, which have distinct structures: extra-framework, framework-associated and framework aluminium. In this Perspective, we review each of these proposals and proceed to analyse their suitability to understand experimental results. Contrary to traditional belief, the number of Lewis acid sites does not always correlate to extra-framework aluminium content. As a result, we highlight that the terms 'extra-framework' and 'framework-associated' aluminium should be used with caution. We propose how the usage of different characterization techniques can enable the closure of knowledge gaps concerning the strength, multiplicity, localization and structure of catalytically active Lewis acid sites in zeolites.

A Supramolecular View on the Cooperative Role of Brønsted and Lewis Acid Sites in Zeolites for Methanol Conversion.

A systematic molecular level and spectroscopic investigation is presented to show the cooperative role of Brønsted acid and Lewis acid sites in zeolites for the conversion of methanol. Extra-framework alkaline-earth metal containing species and aluminum species decrease the number of Brønsted acid sites, as protonated metal clusters are formed. A combined experimental and theoretical effort shows that postsynthetically modified ZSM-5 zeolites, by incorporation of extra-framework alkaline-earth metals or by demetalation with dealuminating agents, contain both mononuclear [MOH]+ and double protonated binuclear metal clusters [M(μ-OH)2M]2+ (M = Mg, Ca, Sr, Ba, and HOAl). The metal in the extra-framework clusters has a Lewis acid character, which is confirmed experimentally and theoretically by IR spectra of adsorbed pyridine. The strength of the Lewis acid sites (Mg > Ca > Sr > Ba) was characterized by a blue shift of characteristic IR peaks, thus offering a tool to sample Lewis acidity experimentally. The incorporation of extra-framework Lewis acid sites has a substantial influence on the reactivity of propene and benzene methylations. Alkaline-earth Lewis acid sites yield increased benzene methylation barriers and destabilization of typical aromatic intermediates, whereas propene methylation routes are less affected. The effect on the catalytic function is especially induced by the double protonated binuclear species. Overall, the extra-framework metal clusters have a dual effect on the catalytic function. By reducing the number of Brønsted acid sites and suppressing typical catalytic reactions in which aromatics are involved, an optimal propene selectivity and increased lifetime for methanol conversion over zeolites is obtained. The combined experimental and theoretical approach gives a unique insight into the nature of the supramolecular zeolite catalyst for methanol conversion which can be meticulously tuned by subtle interplay of Brønsted and Lewis acid sites.

Dominant Role of Entropy in Stabilizing Sugar Isomerization Transition States within Hydrophobic Zeolite Pores

Lewis acid sites in zeolites catalyze aqueous-phase sugar isomerization at higher turnover rates when confined within hydrophobic rather than within hydrophilic micropores; however, relative contributions of competitive water adsorption at active sites and preferential stabilization of isomerization transition states have remained unclear. Here, we employ a suite of experimental and theoretical techniques to elucidate the effects of coadsorbed water on glucose isomerization reaction coordinate free energy landscapes. Transmission IR spectra provide evidence that water forms extended hydrogen-bonding networks within hydrophilic but not hydrophobic micropores of Beta zeolites. Aqueous-phase glucose isomerization turnover rates measured on Ti-Beta zeolites transition from first-order to zero-order dependence on glucose thermodynamic activity, as Lewis acidic Ti sites transition from water-covered to glucose-covered, consistent with intermediates identified from modulation excitation spectroscopy during in situ attenuated total reflectance IR experiments. First-order and zero-order isomerization rate constants are systematically higher (by 3-12×, 368-383 K) when Ti sites are confined within hydrophobic micropores. Apparent activation enthalpies and entropies reveal that glucose and water competitive adsorption at Ti sites depend weakly on confining environment polarity, while Gibbs free energies of hydride-shift isomerization transition states are lower when confined within hydrophobic micropores. DFT calculations suggest that interactions between intraporous water and isomerization transition states increase effective transition state sizes through second-shell solvation spheres, reducing primary solvation sphere flexibility. These findings clarify the effects of hydrophobic pockets on the stability of coadsorbed water and isomerization transition states and suggest design strategies that modify micropore polarity to influence turnover rates in liquid water.

Motions of Trimethylphosphine Oxide in Carbon Nanotubes as Revealed by Solid‐state NMR

Trimethylphosphine (TMP) has been adsorbed in carbon nanotubes and oxidized to trimethylphosphine oxide (TMPO). 31P spin–lattice relaxation T1 measurements at temperatures from 203 to 333 K have been employed to investigate the motions of TMPO. The 31P T1 is found to be monoexponential and isotropic. The activation energy is measured as 4.4 ± 0.3 kcal•mol−1, which is comparable to that of TMP adsorbed on Lewis acid sites in zeolites.

MOFs vs. zeolites: carbonyl activation with M(iv) catalytic sites

Comparative review of the catalytic performance of isolated tetravalent metals as Lewis acid sites in zeolite and MOF materials.

Hydrodeoxygenation of lignin-derived phenolic compounds over bi-functional Ru/H-Beta under mild conditions

Bifunctional ruthenium catalysts supported on H-Beta zeolite were designed and applied in the hydrodeoxygenation of lignin-derived phenolic compounds for bio-oil upgrading. The oxygen-containing groups in lignin-derived phenolic monomers and dimers could be efficiently removed through aqueous phase hydrodeoxygenation catalyzed by Ru/H-Beta under very mild conditions, i.e. at 413K and 4MPa H2. Characterization results revealed that ruthenium species existed in the form of highly dispersed nanoparticles on H-Beta zeolite, while the presence of both Brønsted and Lewis acid sites in zeolite was crucial for the remarkable hydrodeoxygenation activity. The effects of reaction conditions and catalyst constitution on the catalytic performance of Ru/H-Beta in hydrodeoxygenation were systematically investigated. The hydrodeoxygenation pathway of diphenyl ether as a model compound catalyzed by optimized 0.5%Ru/H-Beta was discussed and the structure-activity relationship in hydrodeoxygenation was proposed.

Periodic density functional investigation of Lewis acid sites in zeolites: relative strength order as revealed from NH 3 adsorption

Different types of cationic species acting as Lewis acid sites as well as Na-counter ion in dealuminated zeolites are investigated, for the first time, using periodic density functional method. The energy of ammonia adsorption is taken as a measure for the acidity. The results reveal that the relative strength of the investigated active sites increases in the order: Na + < Al(OH) 2 + < AlO + < Si +. Moreover, the interaction of NH 3 with tri-coordinated aluminum dislodged in the framework is less favorable than that with the neighboring silicon ion.

Thermal effects of the interactions of carbon monoxide with zeolites

The interactions of carbon monoxide with Lewis acid sites of zeolites were investigated using microcalorimetric and temperature programmed desorption (TPD) techniques. Carbon monoxide was adsorbed at 25°C on transition-metal ion-exchanged FAU (Y and X) and MOR type zeolites. The differential heats of adsorption ( Q dif) indicate the heterogeneity of the acid sites, in all the investigated systems. The specific interactions of CO with the strongest acid sites were recognized. The results of TPD confirm that CO molecules were adsorbed on the sites of different strength.

Spectroscopic and Photophysical Behavior of 2-Quinolone as a Probe for Brønsted and Lewis Acid Sites in Zeolites

Abstract Brønsted and Lewis acid sites in zeolites were characterized by using 2-quinolone as a probe, by means of ultraviolet and infrared absorption spectra, fluorescence decay lifetime, and ESR. The results showed clearer differences in the acidity of zeolites compared to those determined by using Hammett indicators. The species observed in the zeolites depended on the acidity, the cavity size, and the amount of 2-quinolone loaded.

Molecular models of catalytically active sites in zeolites. Quantum chemical approach

A brief review of quantum chemical studies of different active sites in zeolites is presented. Various factors that significantly affect the strength of Brønsted acid sites in zeolites are discussed. An interaction of zeolite protons with entrapped metal particles is considered as a reason of electron-deficiency of metal clusters in zeolite cavities on the Pd and Pt species as an example. Probable precursors of Lewis acid sites (LAS) and reliable molecular models of the LAS in zeolites are discussed on the basis of quantum chemical analysis. Transition metal ions can be catalytically active in the lattice or extra-lattice zeolite positions and the two possibilities are considered for selective oxidation site in titanium silicalite and FeZSM-5 zeolite catalysts, respectively.

Role of Zinc in Zn-Loaded ZSM-5 Zeolites in the Aromatization of n-Hexane

Aromatization of n-hexane over zinc-modified ZSM-5 zeolites was investigated. It was shown that incorporation of zinc by ion exchange into cationic positions of NH4-ZSM-5 zeolite causes acid-site strength redistribution and generation of new relatively strong Lewis acid sites in zeolite increasing the selectivity of n-hexane aromatization in comparison with the parent NH4-ZSM-5 zeolite. Simultaneous presence of Lewis and Broensted acid sites in ZSM-5 zeolite does not affect the strength of Broensted acid sites in zeolite. For the activity/selectivity of aromatization of n-hexane on Zn-modified ZSM-5 zeolites, the amount of Zn and its localization in the cationic positions are decisive. The reaction of n-hexane can be also initiated by the Zn species alone in the cationic positions. ZnO species alone as an extraframework phase was found inactive in the catalyst for aromatization of n-hexane. The influence of ZnO addition on the performance of pure ammonium forms of ZSM-5 zeolites in n-hexane conversion is a result of partial migration of zinc into cationic positions of zeolite by solid-state ion exchange.

Probing cationic and ‘true’ Lewis acid sites in zeolite Y with NO and CO. An adsorption and EPR study

Adsorption isotherms of CO and NO in HY and ZnY zeolites at −25 and 25°C as well as EPR spectra of NO at −196°C with and without coadsorbed CO were measured. From a quantitative analysis of the adsorption isotherms, the concentration of Lewis acid sites in HY and cation sites in ZnY were determined. The spin concentrations evaluated from the calibrated intensities of the EPR signals originating from NO, were found to be about 10–20% of the NO concentrations from the adsorption experiments. Coadsorbed CO in ZnY drastically reduces the EPR signal intensity permitting the identification of the adsorption sites of EPR active NO.

Characterization of Lewis-acid sites in zeolites by EPR using the NO molecule as a probe

EPR spectra of NO adsorbed on both Na-ZSM-5 and H-ZSM-5 zeolites have been recorded at 10 and 78 K. Using the experimental parameters, it was possible to simulate the spectra on the assumption that in the studied zeolites, an NO molecule is tightly bound to an Na+ ion, already at 78 K. In the case of a ‘true’ Lewis site in these zeolites, a tight binding of NO at 10 K, but a rotation of the molecule around its z axis at 78 K, was assumed. It was possible to estimate the Al–N distance and to propose a structure of the ‘true’ Lewis site.

On the generation mechanism of aromatic cation-radicals under adsorption on zeolites

Nonempirical quantum-chemical calculations suggest a molecular model for oxygen adsorption on Lewis acid sites of zeolites. This adsorption complex is assumed to be capable of acting as an electron acceptor in the generation of benzene cation-radicals.

Magic-angle spinning nuclear magnetic resonance studies of water molecules adsorbed on Brønsted- and Lewis-acid sites in zeolites and amorphous silica–aluminas

On weakly rehydrated dealuminated zeolites and amorphous silica–aluminas the presence of different types of acidic centres can be ascertained by proton magic-angle spinning nuclear magnetic resonance (1H MAS NMR) measurements. A 1H NMR line at ca. 6.5 ppm is caused by water adsorption on Lewis-acid sites. The shift of the 1H NMR line of Bronsted-acid sites (bridging OH groups) to lower field for hydrated samples can be interpreted quantitatively by a fast proton exchange between water molecules, bridging OH groups and hydroxonium ions.

Characterization of acidity in ZSM-5 zeolites: an x-ray photoelectron and IR spectroscopy study

An x-ray photoelectron spectroscopic (XPS) method is proposed for the identification and quantitation of Broensted and Lewis acid sites in ZSM-5 zeolites. The method consists of deconvoluting the N{sub 1s}XPS level of chemisorbed pyridine and measuring the relative intensities of the peak components. It was found that pyridine is chemisorbed in three different states on ZSM-5 zeolites corresponding to N{sub 1s} binding energy of 398.7, 400.0, and 401.8 eV, respectively. The first peak at 398.7 eV was assigned to N{sub 1s} level of pyridine adsorbed on Lewis sites, while the second and third were assigned to N{sub 1s} levels of pyridine adsorbed on relatively weak and strong Broensted acid sites, respectively. Comparison of the concentrations of the various acid sites as determined from the relative intensities of the N{sub 1s} components with IR spectroscopic data showed that XPS has potential applications in the identification and the quantitative determination of Broensted and Lewis acid sites in zeolites.