Bronsted Acid Sites Of Zeolite Research Articles

Strong H-bonding from Zeolite Bro̷nsted Acid Site to Water: Origin of the Broad IR Doublet.

Hydrogen bonding between water molecules and zeolite Bro̷nsted acid sites (BAS) has received much attention due to the significant influence of water on the adsorption and catalytic properties of these widely used porous materials. When a single water molecule is adsorbed at the BAS, the zeolite O-H stretch vibration decreases in frequency and splits into two extraordinarily broad bands peaked near 2500 and 2900 cm-1 in the infrared (IR) spectrum. This broad doublet feature is the predominant IR signature used to identify and interpret water-BAS H-bonding at low hydration levels, but the origin of the band splitting is not well understood. In this study, we used broadband two-dimensional infrared (2D IR) spectroscopy to investigate zeolite HZSM-5 prepared with a single water molecule per BAS. We find that the 2D IR spectrum is not explained by the most common interpretation of Fermi resonance coupling between the stretch and the bend of the BAS OH group, which predicts intense excited-state transitions that are absent from the experimental results. We present an alternative model of a double-well proton stretch potential, where the band splitting is caused by excited-state tunneling through the proton-transfer barrier. This one-dimensional model reproduces the basic experimental pattern of transition frequencies and amplitudes, suggesting that the doublet bands may originate from a highly anharmonic potential in which the excited state proton wave functions are delocalized over the H-bond between zeolite BAS and adsorbed H2O. Additional details about molecular orientation and coordination of the adsorbed water molecule are also resolved in the 2D IR spectroscopy.

Mechanistic insights into the interaction between binders and Y-type zeolites in fluid catalytic cracking

Binders play essential roles in fluid catalytic cracking catalysts, and their interaction mechanism with zeolites remains to be explored. The effects of three binders on REUSY zeolite were investigated. The alumina sol most notably blocked the pore channels and destructed the framework of zeolites by removing the framework Al. The silica sol diffused into the silicon hydroxyl nests and form new Si-O tetrahedrons, which enhanced the structural stability and increased the number of Bronsted acid sites in zeolites, rendering the samples’ better hydrothermal stability and higher micro-reaction activity. The influence of the colloidal sol of pseudo boehmite on zeolites was between the two.

On the Nature of Extra-Framework Aluminum Species and Improved Catalytic Properties in Steamed Zeolites.

Steamed zeolites exhibit improved catalytic properties for hydrocarbon activation (alkane cracking and dehydrogenation). The nature of this practically important phenomenon has remained a mystery for the last six decades and was suggested to be related to the increased strength of zeolitic Bronsted acid sites after dealumination. We now utilize state-of-the-art infrared spectroscopy measurements and prove that during steaming, aluminum oxide clusters evolve (due to hydrolysis of Al out of framework positions with the following clustering) in the zeolitic micropores with properties very similar to (nano) facets of hydroxylated transition alumina surfaces. The Bronsted acidity of the zeolite does not increase and the total number of Bronsted acid sites decreases during steaming. O5Al(VI)-OH surface sites of alumina clusters dehydroxylate at elevated temperatures to form penta-coordinate Al1O5 sites that are capable of initiating alkane cracking by breaking the first C-H bond very effectively with much lower barriers (at lower temperatures) than for protolytic C-H bond activation, with the following reaction steps catalyzed by nearby zeolitic Bronsted acid sites. This explains the underlying mechanism behind the improved alkane cracking and alkane dehydrogenation activity of steamed zeolites: heterolytic C-H bond breaking occurs on Al-O sites of aluminum oxide clusters confined in zeolitic pores. Our findings explain the origin of enhanced activity of steamed zeolites at the molecular level and provide the missing understanding of the nature of extra-framework Al species formed in steamed/dealuminated zeolites.

Effect of Copper State in Cu/H-ZSM-5 on Methane Activation by Brønsted Acid Sites, Studied by 1H MAS NMR In Situ Monitoring the H/D Hydrogen Exchange of the Alkane with Brønsted Acid Sites

With regard to possible involvement of zeolite Bronsted acid sites (BAS) in the activation of methane molecules for methane transformation to methanol, the effect of different Cu(II) species loaded...

Different Efficiency of Zn2+ and ZnO Species for Methane Activation on Zn-Modified Zeolite

Understanding methane activation pathways on Zn-modified high-silica zeolites (ZSM-5, BEA) is of particular importance because of the possibility of methane involvement in coaromatization with higher alkanes on this type of zeolites. Herein, two samples of Zn-modified zeolite BEA containing exclusively either small zinc oxide clusters or isolated Zn2+ cations have been synthesized and thoroughly characterized by a range of spectroscopic methods (1H MAS NMR, DRIFTS, XPS, EXAFS, HRTEM) to show that only one of the Zn-species, either Zn2+ cations or ZnO small clusters, exists in the void of zeolite pores. The ability of zinc sites of different nature to promote the activation of methane C–H bond with the zeolite Bronsted acid sites (BAS) has been examined in the reactions of methane H/D hydrogen exchange with BAS and the alkylation of benzene with methane. It has been found that both ZnO and Zn2+ species promote the reaction of H/D exchange of methane with BAS. The rate of H/D exchange is higher by 2 and 3 o...

Inelastic Neutron Scattering Study on the Location of Brønsted Acid Sites in High Silica LTA Zeolite

The presence of Bronsted acid sites (SiOHAl) is crucial in many of the applications of zeolites in petroleum refining and related industries. This acidity relies on the number and location of acid sites, but the latter is difficult to know accurately in spite of a large number of experimental techniques available. Zeolites usually contain many types of acid sites depending on the crystallographic different O-sites and their chemical environment. In the present work we show how inelastic neutron scattering (INS) can be employed for an accurate determination of the location of Bronsted acid sites in zeolites. This is achieved using a sample of LTA zeolite with few defects of connectivity, few (O1H, O2H, O3H) crystallographically different oxygens, and high Si/Al ratio. The narrow and intense peaks corresponding to SiOH bending modes ca. 1100 cm–1 have been shown, using periodic DFT calculations, to strongly depend on the type of oxygen to which the proton is bonded. A comparison between calculations and INS...

DFT-D2 Study of the Adsorption of Bio-Oil Model Compounds in HZSM-5: C1–C4 Carboxylic Acids

C1–C4 primary carboxylic acids were selected as model compounds to observe the nature and stability of bio-oil adsorption in zeolite HZSM-5 via DFT-D2 theoretical calculation. Adsorptions with and without solvents of water, dimethyl sulfoxide, methanol, and ethanol were studied. The most stable adsorption configuration is hydrogen bonding (HB) through the carbonyl oxygen of acid over the Bronsted acid site of zeolite with the strong energies range from −133.2 to −154.8 kJ/mol. Partial transfer of the Bronsted proton to the acid was observed. Strength of intramolecular-zeolite acid site interaction increases in the order: carbonyl oxygen HB > hydroxyl oxygen HB > CH3 physisorption. The vdW interactions increase notably with the size of the acids. The adsorption energies are significantly reduced by solvation effect.

Dehydration of 1-Octadecanol over H-BEA: A Combined Experimental and Computational Study

Liquid-phase dehydration of 1-octadecanol, which is intermediately formed during the hydrodeoxygenation of microalgae oil on zeolite H-BEA, has been studied, combining experiment and theory. Both the OH group and the alkyl chain of 1-octadecanol interact with zeolite Bronsted acid sites, inducing inefficient utilization in the presence of high acid-site concentrations. The parallel intramolecular and intermolecular dehydration pathways, leading to octadecene and dioctadecyl ether, have different activation energies and pass through different reaction intermediates. The formation of surface alkoxides is the rate-limiting step in the intramolecular dehydration, whereas the intermolecular dehydration proceeds via a bulky dimer intermediate, occurring preferentially at the pore mouth or outer surface of zeolite crystallites. Despite the main contribution of Bronsted acid sites toward both dehydration pathways, Lewis acid sites are also active to form dioctadecyl ether.

Vibrational Analysis of Pyridine Adsorption on the Brønsted Acid Sites of Zeolites Based on Density Functional Cluster Calculations

Adsorption of pyridine on Bronsted acid sites of protonic zeolites has been studied using density functional calculations within the cluster approach. Two types of cluster models consisting of 3 (3T: 2Si, Al) and 12 ring (12T: 8Si, 4Al) tetrahedral atoms (with Si/Al ratio of 2) with dangling bonds saturated by either hydrogen atoms or hydroxyl groups have been considered to represent the zeolite acid sites. The B3LYP functional with the 6-31+G* basis set are used to calculate the geometrical structures, adsorption energies, and vibrational frequencies of the complexes formed upon interaction of pyridine on the zeolite clusters. The minimum energy pathways for proton transfer from the Bronsted acid site of zeolite (ZOH) to pyridine molecule are characterized. For all clusters considered, the ion-pair complexes (PYH+···OZ−) are the most stable, while the stability of the hydrogen-bonded complexes (PY···HOZ) depends on the size and nature of the terminating groups of the clusters. Since the proton-transfer b...

High-Temperature Dehydrogenation of Brønsted Acid Sites in Zeolites

We present evidence for the dehydrogenation of zeolite Bronsted acid sites at temperatures above 500 °C forming hydrogen gas and a [AlO4]0 site that could participate in one-electron oxidations or react further to form extraframework aluminum.

Highly active and reusable heterogeneous catalysts for acylation of anisole

In the present study, non-conventional solid acid catalysts such as NaY, metal ion exchanged zeolite NaY (Zn2+, Fe3+, Ce3+, La3+ and Nd3+), H-mordenite, H-β and HZSM-5 were used in order to overcome the disadvantages of conventional Friedel-Crafts catalysts for the acylation of anisole with acetic anhydride. Among the various zeolites studied, the HY zeolite shows an intermediate activity. Zeolite containing transition metal ions (Zn2+ and Fe3+) are less active and zeolite NaY is nearly inactive. The catalysts exhibit the activity in the order H-β>transition metal ions (Zn2+ and Fe3+)>HY>NaY zeolite. The highest catalytic activity of H-β could be due to its larger pore size. The type of acidity and the acid strength in zeolite Y were determined by FTIR and differential scanning calorimetric (DSC) studies on the pyridine adsorbed catalysts. The correlation of catalytic activity with acidity reveals that Bronsted acid sites in zeolite promote the acylation of anisole.

Promotion effects of Pt and Rh on catalytic performances of Mo/HZSM-5 and Mo/HMCM-22 in selective methane-to-benzene reaction

The promotion effects of Pt and Rh on catalytic performances of Mo/HZSM-5 and Mo/HMCM-22 in selective methane-to-benzene reaction were studied in the presence of additive H2. The selectivity to naphthalene was effectively suppressed and highly selective and stable benzene formation was obtained by the addition of noble metal to the Mo/HZSM-5 and Mo/HMCM-22 catalysts, due to the suppression of carbon deposition on the Bronsted acid sites of zeolite.

N2 and CO as probe molecules for determining the properties of acid sites on the surface of zeolites

The applicability of molecular nitrogen as a probe for the Bronsted and Lewis acid sites of HNaY and HZSM-5 zeolites was studied by Fourier transform IR spectroscopy. The integrated absorption coefficients of bands due to N-N vibrations in complexes with Bronsted and Lewis acid sites were determined. The correlation between the integrated absorption coefficients and the positions of bands due to N-N vibrations in nitrogen interacting with the acid sites of test samples is discussed. We propose using the low-temperature adsorption of nitrogen for express determination of the concentrations of strong Lewis and Bronsted acid sites in zeolites.

Quantitative Determination of Brönsted Acid Sites on Zeolites: A New Approach Towards the Chemical Composition of Zeolites

An original quantitative method based on H/D exchange between H2O/D2O molecules and the OH groups of different zeolites has been developed for the titration of the Bronsted acid sites present on the solid surface. The measured Bronsted acid sites density appears to be in good agreement with the theoretical amount estimated by the Si/Al ratio. In contrary to classical methods, this non-destructive anhydride method titrates the whole quantity of Bronsted acid sites of zeolites.

Quantitative Investigations of Acidity, and Transient Acidity, in Zeolites and Molecular Sieves

To demonstrate that transient acidity exists in certain solid acid catalysts, a convenient and robust 1H solid-state NMR spin-counting method is introduced for the accurate quantification of Bronsted acid sites in zeolites and molecular sieves. Poly(dimethylsiloxane) is used as an inert and easily handled spin-counting standard, allowing internal calibration of MAS NMR peaks arising from both acidic and nonacidic hydrogens in the catalyst. Results from example systems including H-ZSM5, H-ferrierite (H-FER), and SAPO-34 are presented, and the relevance of these measurements to zeolite synthesis conditions, postsynthetic treatments, and reaction mechanisms are discussed. Using this technique, we show the first direct spectroscopic proof that framework Bronsted acidity in SAPO-34 molecular sieves decreases with time following removal of the synthesis template molecules. This effect is similar to hydrothermally catalyzed dealumination in traditional zeolites, except that it occurs at ambient temperature and m...

Role of adsorption in zeolite-catalyzed hydrocarbon cracking reaction

The activity for hydrocarbon cracking of zeolite catalysts depends on the type of zeolite and steam treatment. By comparing the activities of different zeolites under the condition where the catalyst did not undergo deactivation and the product distributions were identical, and presumably for the monomolecular mechanism, it was found that there were no differences in the chemical properties of the active sites in the zeolites. The apparent differences in catalytic activities among zeolites could be attributed to different concentrations of adsorbed alkane reactants, which is a consequence of the different strength of interaction of the alkyl groups with the zeolite walls. The latter may also be a significant contribute to the observed different strengths of interaction of alkyl amines with the Bronsted acid sites in zeolites.

The dynamics of hydrogen bonds and proton transfer in zeolites - joint vistas from solid-state NMR and quantum chemistry

The different aspects of zeolite Bronsted acid sites are reviewed from the perspective of solid-state NMR spectroscopy and quantum-chemical calculations. The strength of the combined use of these two methods is demonstrated. Special emphasis is dedicated to the structure and dynamics of hydrogen-bonded complexes of zeolites with H2O and CH3OH.

Vibrational Spectroscopy of NH4+ Ions in Zeolitic Materials: An IR Study

It is now well accepted that the interaction of NH3 with the Bronsted acid sites of zeolites leads to the formation of NH4+ ions that are strongly H-bonded to the basic oxygens of the lattice; nevertheless, unambiguous evidence of their local siting is still lacking. In this contribution we report a detailed assignment of the IR vibrational features of NH4+ ions which reveals that mainly bidentate and tridentate species with various local symmetry are stabilized inside the cages and channels of ZSM-5, MORD, β, SAPO-34, and Y zeolites. Tetradentate species, mainly in MORD, are also found. In addition, it is shown that Fermi-type resonance interactions lead to the appearance of several bands in the ν(NH) stretching region of the vibrational spectrum that do not correspond to different NH oscillators. Finally, the solvation of NH4+ in the presence of excess ammonia is also investigated, and the IR manifestations, observed when reversibly adsorbed NH3 is present in the channels and cavities, are explained in ...

Quantum-Chemical Justification of the Zeolite Acid Strength Measurement by Infrared Spectroscopy

The quantitative measurement of the strength of Bronsted acid sites in zeolites would be very useful in catalyst development. One possible method is the measurement of the shift in the stretching frequency of Bronsted hydroxyls on adsorption of a weak base. An empirical correlation for deprotonation energy versus infrared shift has been suggested in the literature, but it cannot be directly experimentally tested. We have performed quantum-chemical calculations of the variation of the infrared shift with Bronsted acid strength. The three probe bases considered are as follows: ethene, carbon monoxide, and nitrogen. The calculations were carried out at the MP2 level with a basis set close to 6-31G** and with explicit consideration of anharmonicity of the OH stretching potential. The results show that the experimental correlation for carbon monoxide gives both absolute acid site deprotonation energies and differences between them to within 10%, supporting the quantitative use of this correlation.

Measurement of vibrational energy relaxation of a surface hydroxyl group by ultrashort tunable infrared pulses

Measurements were made in real time of the vibrational energy relaxation time, T 1, of the O-H stretching mode of surface hydroxyl groups at the so-called Bronsted acid sites of zeolites, by using tunable ultrashort infrared pulses (30 ps) generated by parametric amplification in two successive LiNbO 3 crystals. T 1 ( ν = 1 → ν = 0) of the O-H stretching mode was 161 ± 8 ps for mordenite zeolite and 200 ± 17 ps for Y-type zeolite both at 300 K. The dependence of T 1 on the sample temperature was measured between 145 and 700 K, and it was suggested that the multiphonon (four phonons) decay of the vibrational energy to substrate phonons was operative.