Solid-state nuclear magnetic resonance studies of acid sites in zeolites

Formation and local structure of framework Al Lewis sites in beta zeolites

Framework AlFR Lewis sites represent a substantial portion of active sites in H-BEA zeolite catalysts activated at low temperatures. We studied their nature by 27Al WURST-QCPMG nuclear magnetic resonance (NMR) and proposed a plausible mechanism of their formation based on periodic density functional theory calculations constrained by 1H MAS, 27Al WURST-QCPMG, and 29Si MAS NMR experiments and FTIR measurements. Our results show that the electron-pair acceptor of AlFR Lewis sites corresponds to an AlTRI atom tricoordinated to the zeolite framework, which adsorbs a water molecule. This AlTRI–OH2 complex is reflected in 27Al NMR resonance with δiso = 70 ± 5 ppm and CQ = 13 ± 2 MHz. In addition, the AlTRI atom with adsorbed acetonitrile- d3 (the probe of AlFR Lewis sites in FTIR spectroscopy) exhibits a similar 27Al NMR resonance. We suggest that these AlFR Lewis sites are formed from Al–OH–Si–O–Si–O–Si–OH–Al sequences located in 12-rings (i.e., close unpaired Al atoms).

Effect of aluminum and sodium on the sorption of water and methanol in microporous MFI-type zeolites and mesoporous SBA-15 materials

The interaction and nature of surface sites for water and methanol sorption on MFI-type zeolites and mesoporous SBA-15 were investigated by solid-state NMR spectroscopy and correlated with the desorption enthalpies determined via TGA/DSC. For siliceous Silicalite-1, 29Si CPMAS NMR studies support stronger methanol than water interactions with SiOH groups of Q3-type. On siliceous SBA-15, SiOH groups of Q2-type are accompanied by an enhanced hydrophilicity. In aluminum-containing Na-ZSM-5, Na+ cations are strong adsorption sites for water and methanol as evidenced by 23Na MAS NMR in agreement with high desorption enthalpies of ΔH = 66–74 kJ/mol. Solid-state NMR of aluminum-containing Na-[Al]SBA-15, in contrast, has shown negligible water and methanol interactions with sodium and aluminum. Desorption enthalpies of ΔH = 44–60 kJ/mol hint at adsorption sites consisting of SiOH groups influenced by distant framework aluminum. On H-ZSM-5, Brønsted acidic OH groups are strong adsorption sites as indicated by partial protonation of water and methanol causing low-field shifts of their 1H MAS NMR signals and enhanced desorption enthalpies. Due to the small number of Brønsted acid sites in aluminum-containing H-[Al]SBA-15, water and methanol adsorption on this material is suggested to mainly occur at SiOH groups with distant framework aluminum species, as in the case of Na-[Al]SBA-15.

Catalytic nanosponges of acidic aluminosilicates for plastic degradation and CO2 to fuel conversion

The synthesis of solid acids with strong zeolite-like acidity and textural properties like amorphous aluminosilicates (ASAs) is still a challenge. In this work, we report the synthesis of amorphous "acidic aluminosilicates (AAS)", which possesses Brønsted acidic sites like in zeolites and textural properties like ASAs. AAS catalyzes different reactions (styrene oxide ring-opening, vesidryl synthesis, Friedel-Crafts alkylation, jasminaldehyde synthesis, m-xylene isomerization, and cumene cracking) with better performance than state-of-the-art zeolites and amorphous aluminosilicates. Notably, AAS efficiently converts a range of waste plastics to hydrocarbons at significantly lower temperatures. A Cu-Zn-Al/AAS hybrid shows excellent performance for CO₂ to fuel conversion with 79% selectivity for dimethyl ether. Conventional and DNP-enhanced solid-state NMR provides a molecular-level understanding of the distinctive Brønsted acidic sites of these materials. Due to their unique combination of strong acidity and accessibility, AAS will be a potential alternative to zeolites.

Influence of Levulinic Acid Hydrogenation on Aluminum Coordination in Zeolite-Supported Ruthenium Catalysts: A 27 Al 3QMAS Nuclear Magnetic Resonance Study

The influence of a highly oxygenated, polar protic reaction medium, that is, levulinic acid in 2-ethylhexanoic acid, on the dealumination of two zeolite-supported ruthenium catalysts, namely Ru/H-β and Ru/H-ZSM-5, has been investigated by 27 Al triple-quantum magic-angle spinning nuclear magnetic resonance spectroscopy (3QMAS NMR). Upon use of these catalysts in the hydrogenation of levulinic acid, the heterogeneity in aluminum speciation is found to increase for both Ru/H-ZSM-5 and Ru/H-β. For Ru/H-ZSM-5, the symmetric, tetrahedral framework aluminum species (FAL) were found to be mainly converted into distorted tetrahedral FAL species, with limited loss of aluminum to the solution by leaching. A severe loss of both FAL and extra-framework aluminum (EFAL) species into the liquid phase was observed for Ru/H-β instead. The large decrease in tetrahedral FAL species, in particular, results in a significant decrease in strong acid sites, as corroborated by Fourier transform infrared spectroscopy (FT-IR). This decrease in acidity, evidence of the inferior stability of the strongly acidic sites in Ru/H-β relative to Ru/H-ZSM-5 under the applied conditions, is considered as the main reason for differences seen in catalyst performance.

Probing atomic positions of adsorbed ammonia molecules in zeolite

The atomic positions and interactions between adsorbed guest molecules, such as ammonia in H-ZSM-5 microporous solids, are for the first time revealed by using in situ synchrotron powder X-ray diffraction combined with refinement within experimental errors.

Investigating adsorption of organic compounds in metal-organic framework MIL-53

Metal-organic frameworks (MOFs) are versatile materials that incorporate metal centers along with organic linkers in highly ordered, intricate structures. MIL-53 is a MOF that exhibits a “breathing effect,” where the pore size and MOF topology are profoundly influenced by the identity and binding mechanism of guest molecules. This phenomenon renders MIL-53 a promising candidate for sensing applications. In this report, the adsorption of various organic compounds within MIL-53 is investigated using a combination of complementary techniques. Thermal gravimetric analysis experiments confirm loading of the guest molecules and yield insight into adsorption interactions and strengths. Significant guest-induced changes in the crystal structure of MIL-53 are revealed by powder X-ray diffraction experiments; specific unique phases of MIL-53 are related to the identity of the guest molecule and its binding mechanism to the framework. 27Al and 13C solid-state NMR experiments probe the interaction between guest molecules and MIL-53. The relationship between the nature of the guest, the structure of MIL-53, and 27Al NMR parameters is explored. 27Al NMR parameters are sensitive to the host-guest binding mechanism (i.e., hydrogen-bonding or π–π stacking interactions) and yield valuable information regarding the influence of the adsorbates on the local aluminum environment. This combination of physical characterization techniques is a useful probe of guest adsorption and the breathing effect within MIL-53 and should prove useful for investigation of related MOFs.

An efficient approach to explore the adsorption of benzene and phenol on nanostructured catalysts: a DFT analysis

The adsorption behavior of benzene and phenol on the zeolite is attributed to the differences in the strength of their interactions.

Solid state NMR characterization of zeolite beta based drug formulations containing Ag and sulfadiazine

The paper presents an investigation of structural changes of zeolite carrier, drug–matrix interactions and localization of drug molecules within the zeolite framework in dual drug formulations with antibacterial properties.

Unique properties of silver cations in solid-acid catalysis by zeolites and heteropolyacids

Reversible interconversion of Ag⁺ and Ag⁰ occurs in zeolite. Heterolytic dissociation of alkanes including methane and H₂ proceeds over Ag⁺-zeolite.

Structure of framework aluminum Lewis sites and perturbed aluminum atoms in zeolites as determined by 27Al{1H} REDOR (3Q) MAS NMR spectroscopy and DFT/molecular mechanics

Zeolites are highly important heterogeneous catalysts. Besides Brønsted SiOHAl acid sites, also framework AlFR Lewis acid sites are often found in their H-forms. The formation of AlFR Lewis sites in zeolites is a key issue regarding their selectivity in acid-catalyzed reactions. The local structures of AlFR Lewis sites in dehydrated zeolites and their precursors--"perturbed" AlFR atoms in hydrated zeolites--were studied by high-resolution MAS NMR and FTIR spectroscopy and DFT/MM calculations. Perturbed framework Al atoms correspond to (SiO)3AlOH groups and are characterized by a broad (27)Al NMR resonance (δi = 59-62 ppm, CQ = 5 MHz, and η = 0.3-0.4) with a shoulder at 40 ppm in the (27)Al MAS NMR spectrum. Dehydroxylation of (SiO)3AlOH occurs at mild temperatures and leads to the formation of AlFR Lewis sites tricoordinated to the zeolite framework. Al atoms of these (SiO)3Al Lewis sites exhibit an extremely broad (27)Al NMR resonance (δi ≈ 67 ppm, CQ ≈ 20 MHz, and η ≈ 0.1).

Translational and rotational mobility of methanol-d4 molecules in NaX and NaY zeolite cages: a deuteron NMR investigation

Nuclear magnetic resonance (NMR) provides means to investigate molecular dynamics at every state of matter. Features characteristic for the gas phase, liquid-like layers and immobilized methanol-d(4) molecules in NaX and NaY zeolites were observed in the temperature range from 300 K down to 20K. The NMR spectra at low temperature are consistent with the model in which molecules are bonded at two positions: horizontal (methanol oxygen bonded to sodium cation) and vertical (hydrogen bonding of hydroxyl deuteron to zeolite framework oxygen). Narrow lines were observed at high temperature indicating an isotropic reorientation of a fraction of molecules. Deuteron spin-lattice relaxation gives evidence for the formation of trimers, based on observation of different relaxation rates for methyl and hydroxyl deuterons undergoing isotropic reorientation. Internal rotation of methyl groups and fixed positions of hydrogen bonded hydroxyl deuterons in methyl trimers provide relaxation rates observed experimentally. A change in the slope of the temperature dependence of both relaxation rates indicates a transition from the relaxation dominated by translational motion to prevailing contribution of reorientation. Trimers undergoing isotropic reorientation disintegrate and separate molecules become localized on adsorption centers at 166.7 K and 153.8K for NaX and NaY, respectively, as indicated by extreme broadening of deuteron NMR spectra. Molecules at vertical position remain localized up to high temperatures. That indicates the dominating role of the hydrogen bonding. Mobility of single molecules was observed for lower loading (86 molecules/uc) in NaX. A direct transition from translation to localization was observed at 190 K.

Solid-state nuclear magnetic resonance investigations of the nature, property, and activity of acid sites on solid catalysts

Further progress in the field of heterogeneous catalysis depends on our knowledge of the nature and behavior of surface sites on solid catalysts and of the mechanisms of chemical reactions catalyzed by these materials. In the past decades, solid-state NMR spectroscopy has been developed to an important tool for routine characterization of solid catalysts. The present work gives a review on experimental approaches and applications of solid-state NMR spectroscopy for investigating Brønsted and Lewis sites on solid acids. Studies focusing on the generation of surface sites via post-synthesis modification routes of microporous and mesoporous materials support the development of new and the improvement of existing catalyst systems. High-temperature and flow techniques of in situ solid-state NMR spectroscopy allow a deeper insight into the mechanisms of heterogeneously catalyzed reactions and open the way for studying the activity of acidic surface sites. They help to clarify the activation of reactants on Brønsted and Lewis acid sites and improve our understanding of mechanisms affecting the selectivity of acid-catalyzed reactions.

Dynamics of hydroxyl deuterons and bonded water molecules in NaDY(0.8) zeolite as studied by means of deuteron NMR spectroscopy and relaxation

Deuteron spin-lattice relaxation and spectra were measured for NaDY (0.8) zeolite containing some heavy water. Two subsystems of deuterons with different mobility were disclosed at low temperatures with their respective relaxation rates differing by two orders of magnitude. Spectra exhibit different shapes related directly to a specific motional model. Hydroxyl deuterons perform incoherent tunneling along the hydrogen bond, then on increasing temperature jumps to excited states and over the barrier appear. Hydrogen bonded water molecules perform 180 degrees rotational jumps about the twofold symmetry axis. Spectral amplitudes are consistent with the water content of 13 D(2)O molecules per unit cell. Above about 240K translational mobility becomes significant and finally water molecules diffuse across the free space of cages. Diversity in temperature dependence of hydroxyl deuteron dynamics may indicate location of adsorbed molecules.

17O Magic Angle Spinning NMR Studies of Brønsted Acid Sites in Zeolites HY and HZSM-5

High-resolution 17O/1H double resonance NMR spectra were obtained for two zeolites, one with a low Si/Al ratio (zeolite HY) and one with a high Si/Al ratio (HZSM-5), to investigate their local structure and Brønsted acidity. Two different oxygen signals, corresponding to Brønsted acid sites in supercages and sodalite cages of zeolite HY were readily resolved in the two-dimensional (2-D) 1H-17O heteronuclear correlation (HETCOR) NMR spectra allowing the 17O isotropic chemical shift (deltaCS) and quadrupolar coupling parameters (quadrupolar coupling constant, QCC, and asymmetry parameter, eta) for the two oxygen atoms to be extracted. Similar experiments for HZSM-5 showed that the sites in this system are associated with a much larger distribution in NMR parameters than found in HY. 17O-1H rotational echo double resonance (REDOR) NMR was applied to probe the O-H distances in zeolites HY and HZSM-5. Weaker 17O-1H dephasing was observed for zeolite HZSM-5 in comparison to that of HY, consistent with longer O-H bonds and/or increased proton mobility.

Effects of Adsorbate Molecules on the Quadrupolar Interaction of Framework Aluminum Atoms in Dehydrated Zeolite H,Na-Y

The effect of adsorbate molecules on the quadrupolar interaction of framework aluminum atoms with the electric field gradient in dehydrated zeolite H,Na-Y has been studied by (27)Al MAS NMR and (27)Al MQMAS NMR spectroscopy at magnetic fields of 9.4 and 17.6 T. Upon adsorption of molecules interacting with bridging OH groups by hydrogen bonds (acetonitrile and acetone), the quadrupole coupling constant of framework aluminum atoms was found to decrease from 16.0 MHz (unloaded zeolite) to 9.4 MHz. Adsorption of molecules, which cause a proton transfer from the zeolite framework to the adsorbates (ammonia and pyridine), reduces the quadrupole coupling constant to 3.8 MHz for coverages of 0.5-2 molecules per bridging OH group. The experiments indicate that the quadrupole coupling constant of framework aluminum atoms in dehydrated zeolite H,Na-Y reflects the chemical state of adsorbate complexes formed at bridging OH groups. In agreement with earlier investigations it was found that a proton affinity of the adsorbate molecules of PA = 812-854 kJ/mol is necessary to induce a proton transfer from the zeolite framework to the adsorbed compounds. This proton transfer is accompanied by a strong improvement of the tetrahedral symmetry of zeolitic framework AlO(4) tetrahedra and a decrease of the electric field gradient.

Detection of Brønsted acid sites in zeolite HY with high-field 17O-MAS-NMR techniques

The acidity and unique porous structures of zeolites play an important role in controlling the activity and selectivity of many zeolite-based catalysts. Although (27)Al, (29)Si and (1)H NMR spectroscopy represent standard analytical tools with which to study these materials, (17)O-NMR investigations are much less routine, owing to the very low natural abundance of (17)O (0.037%), its relatively low resonant frequency and its large quadrupole moment. (17)O-NMR resonances from framework oxygen sites in a variety of zeolites have been detected, but the (17)O-NMR resonance from oxygen directly bound to the Brønsted acid site (Si-O(H)-Al) has remained elusive. Here we report the direct observation of this resonance in dehydrated zeolite HY, by using high magnetic-field strengths. (17)O-(1)H double-resonance NMR experiments are used to prove unambiguously that the (17)O signal arises from O nearby H atoms. A large quadrupolar coupling constant, the measure of the local distortion of this site, of 6.6 MHz is determined, which is similar to that obtained in ab initio calculations of zeolite HY-like clusters; this value drops to 5 MHz on acetone binding. The results presented in this paper open up methods for characterizing zeolite acidity and investigating H(+)-sorbent interactions.

Characterization of framework and extra-framework aluminum species in non-hydrated zeolites Y by 27Al spin-echo, high-speed MAS, and MQMAS NMR spectroscopy at B0= 9.4 to 17.6 T

27Al spin-echo, high-speed MAS (nu(rot) = 30 kHz), and MQMAS NMR spectroscopy in magnetic fields of B0 = 9.4, 14.1, and 17.6 T were applied for the study of aluminum species at framework and extra-framework positions in non-hydrated zeolites Y. Non-hydrated gamma-Al2O3 and non-hydrated aluminum-exchanged zeolite Y (Al,Na-Y) and zeolite H,Na-Y were utilized as reference materials. The solid-state 27Al NMR spectra of steamed zeolite deH,Na-Y/81.5 were found to consist of four signals. The broad low-field signal is caused by a superposition of the signals of framework aluminum atoms in the vicinity of bridging hydroxyl protons and framework aluminum atoms compensated in their negative charge by aluminum cations (delta(iso) = 70 +/- 10 ppm, C(QCC) = 15.0 +/- 1.0 MHz). The second signal is due to a superposition of the signals of framework aluminum atoms compensated by sodium cations and tetrahedrally coordinated aluminum atoms in neutral extra-framework aluminum oxide clusters (delta(iso) = 65 +/- 5 ppm, C(QCC) = 8.0 +/- 0.5 MHz). The residual two signals were attributed to aluminum cations (delta(iso) = 35 +/- 5 ppm, C(QCC) = 7.5 +/- 0.5 MHz) and octahedrally coordinated aluminum atoms in neutral extra-framework aluminum oxide clusters (delta(iso) = 10 +/- 5 ppm, C(QCC) = 5.0 +/- 0.5 MHz). By chemical analysis and evaluating the relative solid-state 27Al NMR intensities of the different signals of aluminum species occurring in zeolite deH,Na-Y/81.5 in the non-hydrated state, the aluminum distribution in this material was determined.

Three-coordinate aluminum in zeolites observed with in situ x-ray absorption near-edge spectroscopy at the Al K-edge: flexibility of aluminum coordinations in zeolites

Application of in situ X-ray absorption near-edge spectroscopy (XANES) at the Al K-edge provides unique insight into the flexibilty of the aluminum coordinations in zeolites as a function of treatment or during true reaction conditions. A unique, previously not observed, pre-edge feature is detected in zeolites H-Mordenite and steamed and unsteamed H-Beta at temperatures above 675 K. Spectra simulations using the full multiple scattering code Feff8 identify the unique pre-edge feature as three-coordinate aluminum. The amount of three-fold coordinated aluminum is a function of temperature and pretreatment of a zeolite: a steamed zeolite Beta contains more three-coordinate aluminum than an unsteamed sample. No clear differences between zeolites H-Mordenite and H-Beta were observed. Octahedrally coordinated aluminum forms in zeolites H-Mordenite and H-Beta at room temperature in a stream of wet helium. This octahedrally coordinated aluminum is unstable at temperatures higher than 395 K, where it quantitatively reverts to the tetrahedral coordination.