Local States in Microporous Silica and Aluminum Silicate Materials. 1. Modeling Structure, Formation, and Transformation of Common Hydrogen Containing Defects

Precise Structural and Dynamical Details in Zeolites Revealed by Coupling-Edited 1H-17O Double Resonance NMR Spectroscopy

Despite the extensive industrial and research interests in zeolites, their intrinsic catalytic nature is not fully understood due to the complexity of the hydroxyl-aluminum moieties. 17O NMR would provide irreplaceable opportunities for much-needed fine structural determination given the ubiquitous presence of oxygen atoms in nearly all species; however, the low sensitivity and quadrupolar nature of oxygen-17 make its NMR spectroscopic elucidation challenging. Here, we show that state-of-the-art double resonance solid-state NMR techniques have been combined with spectral editing methods based on scalar (through-bond) and dipolar (through-space) couplings, which allowed us to address the subtle protonic structures in zeolites. Notably, the often-neglected and undesired second-order quadrupolar-dipolar cross-term interaction ("2nd-QD interaction") can actually be exploited and can help gain invaluable information. Eventually, a comprehensive set of 1H-17O/1H-27Al double resonance NMR with J-/D-coupling spectral editing techniques have been designed in this work and enabled us to reveal atomic-scale precise structural and dynamical details in zeolites including: 1) The jump rate of the bridging acid site (BAS) proton is relatively low, i.e., far less than 100 s-1 at room temperature. 2) The Al-OH groups with 1H chemical shift at 2.6-2.8 ppm, at least for nonseverely dealuminated H-ZSM-5 catalysts, exhibit a rigid bridging environment similar to that of BAS. 3) The Si-OH groups at 2.0 ppm are not hydrogen bonded and undergo fast cone-rotational motion. The results in this study predict the 2nd-QD interaction to be universal for any rigid -17O-H environment, such as those in metal oxide surfaces or biomaterials.

Molecular Simulation of the Impact of Defects on Electrolyte Intrusion in Zeolites

We have investigated through molecular simulation the intrusion of electrolytes in two representative pure-silica zeolites, silicalite-1 and chabazite, in which point defects were introduced in varying amounts. We distinguish between two types of defects, considering either "weak" or "strong" silanol nest defects, resulting in different hydration behaviors. In the presence of weak defects, the hydration process occurs through a homogeneous nucleation process, while with strong defects, we observe an initial adsorption followed by a filling of the nanoporous volume at a higher pressure. However, we show that electrolytes do not penetrate the zeolites, and these defects appear to have only marginal influence on the thermodynamics of electrolyte intrusion. While replacing pure water by the electrolyte solution shifts the intrusion pressure toward higher values because of the drop of water saturation vapor pressure, an increase in hydrophilicity of the framework due to point defects has the opposite effect, showing that controlling the amount of defects in zeolites is crucial for storage energy applications.

Al distribution and structural stability of H-BEA zeolites at different Si/Al ratios and temperatures: a first-principles study

Beta zeolites have been widely used in acid-catalyzed reactions because of their excellent properties. An in-depth study of the position, quantity, and distribution of beta zeolites substituted by Al is significant to understand the catalytic performance of the active site of zeolite catalysts. The distribution of Al in H-BEA and the structure of silanol nests in dealuminated BEA at different Si/Al ratios and synthesis temperatures were studied by the DFT method. T1, T2, T7, and T9 sites were chosen to be simulated. The synthesis temperature can change the distribution of Al and the proportion of T sites at different Si/Al ratios. The proportion of T7 and T9 is more than 70% at different Si/Al ratios of H-BEA and decreases with the synthesis temperature. T1 and T2 sites begin to appear when Si/Al < 20 and the proportion of T1 and T2 sites is less than 20%. When Si/Al < 8, the substitution energy of the AlSiAl structure, which has Si(2Al, 2Si) species, is obviously lower than that of the normal structure, which indicates that the Al-O-Si-O-Al species will appear in H-BEA. The Al(T7)Si(T5)Al(T9)Si(T5)Al(T7) and Al(T1)Si(T1)Al(T9) groups can not only stabilize H-BEA but also play an essential role in the formation of Si(2Al, 2Si) species. For dealuminated BEA zeolites, the silanol nest forms four hydrogen bonds through four silanols. The orientation of silanol groups in the silanol nest formed after dealumination at different T sites is different. The T7 and T9 sites in H-BEA are more likely to undergo dealumination. By contrast, the dealumination of the T1 and T2 sites is a challenge.

Effect of the Topology on Wetting and Drying of Hydrophobic Porous Materials

Establishing molecular mechanisms of wetting and drying of hydrophobic porous materials is a general problem for science and technology within the subcategories of the theory of liquids, chromatography, nanofluidics, energy storage, recuperation, and dissipation. In this article, we demonstrate a new way to tackle this problem by exploring the effect of the topology of pure silica nanoparticles, nanotubes, and zeolites. Using molecular dynamics simulations, we show how secondary porosity promotes the intrusion of water into micropores and affects the hydrophobicity of materials. It is demonstrated herein that for nano-objects, the hydrophobicity can be controlled by changing the ratio of open to closed nanometer-sized lateral pores. This effect can be exploited to produce new materials for practical applications when the hydrophobicity needs to be regulated without significantly changing the chemistry or structure of the materials. Based on these simulations and theoretical considerations, for pure silica zeolites, we examined and then classified the experimental database of intrusion pressures, thus leading to the prediction of any zeolite's intrusion pressure. We show a correlation between the intrusion pressure and the ratio of the accessible pore surface area to total pore volume. The correlation is valid for some zeolites and mesoporous materials. It can facilitate choosing prospective candidates for further investigation and possible exploitation, especially for energy storage, recuperation, and dissipation.

Zeolite (In)Stability under Aqueous or Steaming Conditions

Zeolites are among the most environmentally friendly materials produced industrially at the Megaton scale. They find numerous commercial applications, particularly in catalysis, adsorption, and separation. Under ambient conditions aluminosilicate zeolites are stable when exposed to water or water vapor. However, at extreme conditions as high temperature, high water vapor pressure or increased acidity/basicity, their crystalline framework can be destroyed. The stability of the zeolite framework under aqueous conditions also depends on the concentration and character of heteroatoms (other than Al) and the topology of the zeolite. The factors critical for zeolite (in)stability in the presence of water under various conditions are reviewed from the experimental as well as computational sides. Nonreactive and reactive interactions of water with zeolites are addressed. The goal of this review is to provide a comparative overview of all-silica zeolites, aluminosilicates and zeolites with other heteroatoms (Ti, Sn, and Ge) when contacted with water. Due attention is also devoted to the situation when partial zeolite hydrolysis is used beneficially, such as the formation of hierarchical zeolites, synthesis of new zeolites or fine-tuning catalytic or adsorption characteristics of zeolites.

Consequences of secondary zeolite growth on catalytic performance in DMTO studied over DDR and CHA

Zeolites with DDR (Sigma-1 and ZSM-58) and CHA (SSZ-13) topology were synthesized by seed assisted and direct hydrothermal synthesis in order to investigate the effects of fast crystal growth on catalytic performance.

A molecular dynamics study of the interaction of water with the external surface of silicalite-1

The modeling study of the interaction of water with the external surface of silicalite-1 reveals retention of H₂O molecules at the interface because of the formation of a structured water layer.

Computational Characterization of Defects in Metal-Organic Frameworks: Spontaneous and Water-Induced Point Defects in ZIF-8

Zeolitic imidazolate frameworks (ZIFs) are an important class of porous crystalline metal-organic framework (MOF) materials that have attracted widespread attention for applications ranging from gas adsorption and separation to catalysis. Although the bulk crystal structures of MOFs are typically well-characterized, comparatively little is known regarding MOF defect structures. Drawing on analogies with conventional silicon-based zeolites, we utilize computational methods to examine the structure and stability of putative point-defect structures (including vacancies, substitutions, and "dangling" linkers) within the prototypical ZIF-8 structure. Considering both postsynthetic (gas-phase) and synthetic (solution-phase) conditions, we find that several of the defect structures lie low in energy relative to the defect-free parent crystal, with barriers to defect formation that are large but surmountable under relevant temperatures. These results are consistent with prior experimental observations of ZIF stability and reactivity and suggest that defects may play an important role in influencing the long-term stability of MOFs under conditions that include exposure to water vapor and trace contaminants such as acid gases.

Disruptive catalysis by zeolites

Emerging concepts and novel possibilities in catalysis by zeolites for a new scenario in chemical and energy vector production.

Optical investigation of the intergrowth structure and accessibility of Brønsted acid sites in etched SSZ-13 zeolite crystals by confocal fluorescence microscopy

Template decomposition followed by confocal fluorescence microscopy reveals a tetragonal-pyramidal intergrowth of subunits in micrometer-sized nearly cubic SSZ-13 zeolite crystals. In order to accentuate intergrowth boundaries and defect-rich areas within the individual large zeolite crystals, a treatment with an etching NaOH solution is applied. The defective areas are visualized by monitoring the spatial distribution of fluorescent tracer molecules within the individual SSZ-13 crystals by confocal fluorescence microscopy. These fluorescent tracer molecules are formed at the inner and outer crystal surfaces by utilizing the catalytic activity of the zeolite in the oligomerization reaction of styrene derivatives. This approach reveals various types of etching patterns that are an indication for the defectiveness of the studied crystals. We can show that specially one type of crystals, denoted as core-shell type, is highly accessible to the styrene molecules after etching. Despite the large crystal dimensions, the whole core-shell type SSZ-13 crystal is utilized for catalytic reaction. Furthermore, the confocal fluorescence microscopy measurements indicate a nonuniform distribution of the catalytically important Brønsted acid sites underlining the importance of space-resolved measurements.

Aluminum Siting in the ZSM-22 and Theta-1 Zeolites Revisited: A QM/MM Study

The Al siting in the silicon rich ZSM-22 and Theta-1 zeolites of the TON structure was investigated analyzing already published 27Al 3Q MAS NMR experimental data using QM/MM calculations. The results of our computations show that Al atoms can be located in 6 framework T positions because the two eightfold sites (T1 and T2) split into four fourfold T sites after an Al/Si substitution. The observed resonance at 55.5 ppm corresponds to the T4 site which is predominantly occupied by Al. This site is not located on the surface of the TON ten-membered ring channel and thus the protonic sites related with the majority of Al atoms in the TON structure exhibit a significantly limited reaction space. The 27Al NMR signals centered at 57.6 and 58.7 ppm correspond to either the T2 and T3 sites, respectively, or only to T2. The T2 and T3 sites accommodate some 40% and up to 10%, respectively, of Al while the T1 site is unoccupied by Al. Isotropic shifts of 61.1 and 61.6 ppm were calculated for Al atoms located in the T1-1 and T1-2 sites, respectively. The effect of a silanol "nest" as a next-next-nearest neighbor on the 27Al isotropic chemical shift of Al located in the T4 site is calculated to be less than 1 ppm.

The effect of local defects on water adsorption in silicalite-1 zeolite: a joint experimental and molecular simulation study

We report a joint experimental and molecular simulation study of water condensation in silicalite-1 zeolite. A sample was synthesized using the fluoride route and was found to contain essentially no defects. A second sample synthesized using the hydroxide route was found to contain a small amount of silanol groups. The thermodynamics of water condensation was studied in these two samples, as well as in a commercial sample, in order to understand the effect of local defects on water adsorption. The molecular simulation study enabled us to qualitatively reproduce the experimentally observed condensation thermodynamics features. A shift and a rounding of the condensation transition was observed with an increasing hydrophilicity of the local defect, but the condensation transition was still observed above the water saturation vapor pressure P0. Both experiments and simulations agree on the fact that a small water uptake can be observed at very low pressure, but that the bulk liquid does not form from the gas phase below P0. The picture that emerges from the observed water condensation mechanism is the existence of a heterogeneous internal surface that is overall hydrophobic, despite the existence of hydrophilic "patches". This heterogeneous surface configuration is thermodynamically stable in a wide range of reduced pressures (from P/P0 = 0.2 to a few thousands), until the condensation transition takes place.

Oxidation State of Vanadium Introduced in Dealuminated β Zeolite by Impregnation with VIVOSO4 Solution: Influence of Preparation Parameters

Vanadium was introduced in dealuminated beta zeolite by impregnation with a VIVOSO4 aqueous solution at 353 K in air or argon (to prevent oxidation of VIV), leading to VSibeta and VSibeta-Ar zeolites, respectively. The samples were characterized by spectroscopy, XRD, and N2-physisorption. The oxidation state and environment of V in Sibeta zeolite depend on the preparation parameters (i.e., on the way the solid is recovered after impregnation and on the drying temperature). In solids recovered by centrifugation, washed with distilled water, and then dried overnight at 298 K in argon, vanadium is found as extra-lattice octahedral VIV ions as evidenced by EPR. In contrast, in solids not washed but directly dried overnight at 353 K in air or argon, vanadium is found in both cases as lattice tetrahedral VV ions. These ions are incorporated into vacant T sites associated with SiOH, SiO-, oxygen vacancies (OVs) or nonbridging oxygen (NBOs) defects as shown by diffuse reflectance UV-visible, 51V MAS NMR, FT-IR, and photoluminescence. The oxidation to VV ions is suggested to be due to an electron transfer from VO2+ to trigonal identical with Si+ defect sites followed by reaction of the resulting VO2+ ions with particular defects of vacant T sites. These processes occur already upon drying of V-impregnated Sibeta at 353 K. 51V MAS NMR allows detection of one kind of lattice tetrahedral V ions in VSibeta and two kinds in VSibeta-Ar. The formation of different kinds of tetrahedral V species is related to the presence in vacant T sites of Sibeta zeolite of different types of defect sites such as trigonal identical with Si+ defect or SiOH and SiO- groups.

Spectroscopy and Computations of Supported Metal Adducts. 1. DFT Study of CO and NO Adsorption and Coadsorption on Cu/SiO2

Interactions of the CO and NO molecules with the Cu(II) and Cu(I) isolated sites on the amorphous silica surface are investigated by means of density functional theory (DFT) methods within the finite cluster model approach. The clusters of silica of increasing nT size (T = Si) are used, with n from 2 to 6. The Cu(II) sites are characterized by calculated g-tensors and hyperfine coupling constants (HFCCs) and compared with experiment. On this basis, the three-coordinated complexes are the most plausible. Due to the charge transfer from the silica "ligand", the metal charge shrinks and the spin density is distributed over silanol and siloxy groups up to 50%. The reduced sites are exclusively two-coordinated. Strong interaction of CO with Cu(I)-nT sites (31-39 kcal/mol) gives rise to the formation of carbonyl adducts with planar coordination around copper. The population of the ligand pi system shifts downward the stretching frequency in agreement with experiment. Reaction with a second CO molecule gives a geminal dicarbonyl of very uniform structure independent of the site. Carbonyl complexes with Cu(II) are less stable and of tetrahedral coordination of the metal. Accumulation of the positive charge on the complex along with sigma overlap with d orbitals locates the calculated CO stretching frequency above free molecule value. NO molecule is preferably bound to the Cu(II)-nT sites, forming a tetrahedral complex with tilted adsorbate and NO stretching frequency blue-shifted with respect to the free molecule value. The full set of electron paramagnetic resonance (EPR) parameters and vibrational frequencies for the copper(I) mononitrosyl, {CuNO}(11), though not observed experimentally, are predicted and compared to the same magnetophore inside the ZSM-5 zeolite. The interaction energies show that in the CO/NO reaction mixture adsorption is selective and allows discrimination between Cu(I) and Cu(II) sites. However, for the Cu(I) complex, formation of mixed-ligand structures of the {Cu(CO)(NO)}(11) type is possible.

Hole localization in [AlO4]0 defects in silica materials

First-principles calculations based on cluster models have been performed to investigate the ground state and the optically excited states of the [AlO(4)](0) hole in alpha-quartz and in the siliceous zeolite ZSM-5. The structure and spectroscopic properties of this defect have been studied using the recently developed Becke88-Becke95 one-parameter model for kinetics (BB1K) functional of Zhao et al., [J. Phys. Chem. A 108, 2715 (2004)]. Our results show that the BB1K method is significantly more reliable and more accurate than the standard density-functional theory (DFT) functionals at reproducing the localized spin density on one oxygen atom and the hyperfine coupling constants associated with the hole. Furthermore, we find that the BB1K results are in close agreement with experiments, and with the self-interaction-free unrestricted Hartree-Fock (UHF) and unrestricted second-order Møller-Plesset perturbation theory (UMP2) calculations. For the first time, we present results of the ground-state paramagnetic properties of the Al defect in ZSM-5. Similar to the theoretical work for defective alpha-quartz, we find that the BB1K, UHF, UHFLee-Yang-Parr, and UMP2 calculations show a localized hole on one oxygen neighboring the Al, while even the best to date thermochemically derived hybrid generalized gradient approximation density-functional, B97-2, predicts a different model where the hole is distributed over two oxygen. We have further considered the optical transitions of the [AlO(4)](0) center in alpha-quartz and ZSM-5. In both systems, our BB1K time-dependent density-functional theory (TDDFT) and configuration interaction singles (CIS) calculations predict that the most likely transition involves electron transfer from the hole-bearing oxygen to other neighboring oxygen ions. This reinforces the experimental conclusions obtained for defective alpha-quartz. Notably, the two lowest, most dominant excitation energies calculated by BB1K-TDDFT (1.99 and 3.03 eV) show excellent agreement with experiment (1.96 and 2.85 eV [B. K. Meyer, J.M. Spaeth, and J.A. Weil, J. Phys. C: Solid State Phys. 17, L31 (1987)]) clearly outperforming the CIS method and other DFT calculations available in the literature.

Wear of mica under aqueous environments:direct observation of defect nucleation by AFM

The generation of defects at surfaces in sliding contacts is the catalyst for the eventual wear of the materials. Here, the wear of muscovite mica has been investigated under aqueous environments using atomic force microscopy (AFM). Through concomitant acquisition of topography, friction, and adhesion data under controlled pH conditions, defect nucleation on the atomic scale prior to gross wear may be directly observed. Nucleation is found to present itself initially as charging of the surface due to stress-induced tribochemical bond scission as OH- breaks open the surface terminating Si-O-Si or Si-O-Al bonds. As the surface bonds are continually cleaved, an ensemble of defects (e.g., Si-OH/Al-OH and Si-O-) contribute to a crystal lattice reconstruction (from approximately 5.2 to approximately 3 A), as observed in AFM topographic and frictional force micrographs. Following lattice restructuring, displacement/abstraction of mica surface materials ensues, yielding readily discernible wear scars ranging from approximately 2 to 10 A in depth. The environmental OH- concentration profoundly affects the efficacy of this sequence of events leading to wear and is illustrated by the acceleration or inhibition of wear with adjustment of pH under identical load and scan conditions.