Catalytic Properties Of Zeolites Research Articles

Effect of zeolite structure and Si/Al ratio on cotton stalks hydropyrolysis

Zeolites, being acidic, act as the most promising catalytic materials for deoxygenating reactive oxygenated compounds produced during the pyrolysis of lignocellulosic biomass. Herein, two zeolites, HY and HZSM-5, with different Si/Al ratios, were employed to investigate the effect of zeolite structure on aromatic hydrocarbons (AHs) production during flash hydropyrolysis of cotton stalks (Temperature = 800 ˚C, H2 pressure = 10 bar). Both the zeolites enhanced AHs production. However, the pore structure and pore size of HZSM-5 marked a pronounced effect on the reduction of oxygenated compounds. With an increase in Si/Al ratio, AHs area% was found to decrease owing to the decrease in acidity. Ni/zeolite catalysts were also investigated to look into the effect of metal loading on the catalytic properties of zeolites. Ni/zeolite catalysts enhanced the aromatic and aliphatic hydrocarbon production by further conversion of phenolics and other oxygenated compounds due to the promotion of direct deoxygenation, decarbonylation and decarboxylation reactions.

Improvement of adsorption and catalytic properties of zeolites by precisely controlling their particle morphology.

An aluminosilicate zeolite has a porous structure with openings comparable to the molecular size, which endows it with unique adsorptive and catalytic properties that are highly dependent on its chemical composition and crystal morphology. Thus, the precise control or rational design of zeolite's particle morphology has attracted much attention as it can greatly improve the adsorptive separation and catalytic properties by effectively adjusting the diffusion path of adsorbates, reactants and products. This paper reviews the recent progress made in the synthesis and application of zeolites with a specific crystal/particle morphology with emphasis on the control of the crystal size and facet exposure degree, oriented assembly of crystals, creation of hierarchical porous structures and synthesis of core-shell structures. It is shown that an appropriate decrease of the crystal size and/or an increase of the exposure degree of certain facets by adding seeds and optimizing the synthesis conditions enhances the catalytic stability and product selectivity in some reactions. This can also be achieved by introducing plenty of mesopores and/or macropores in zeolites as a result of significant alleviation of diffusion limitation. Assembly of zeolite crystals into membranes on porous substrates improves the adsorptive separation performance of zeolites, for e.g. alcohol/water mixture and xylene and butane isomers. Core-shell-structured composites with metal nanoparticles or subnanoparticles as the core and the zeolite, including its modified counterpart, as the shell show excellent catalytic performance in some hydrogenation, dehydrogenation and oxidation reactions. In addition, attempts to illustrate the relationship between zeolite's particle morphology and its catalytic performance are discussed and strategies for the rational design of zeolite's particle size and behavior are envisioned.

Single-Atom Catalysts Supported by Crystalline Porous Materials: Views from the Inside.

Single-atom catalysts (SACs) have recently emerged as an exciting system in heterogeneous catalysis showing outstanding performance in many catalytic reactions. Single-atom catalytic sites alone are not stable and thus require stabilization from substrates. Crystalline porous materials such as zeolites and metal-organic frameworks (MOFs) are excellent substrates for SACs, offering high stability with the potential to further enhance their performance due to synergistic effects. This review features recent work on the structure, electronic, and catalytic properties of zeolite and MOF-protected SACs, offering atomic-scale views from the "inside" thanks to the subatomic resolution of synchrotron X-ray absorption spectroscopy (XAS). The extended X-ray absorption fine structure and associated methods will be shown to be powerful tools in identifying the single-atom site and can provide details into the coordination environment and bonding disorder of SACs. The X-ray absorption near-edge structure will be demonstrated as a valuable method in probing the electronic properties of SACs by analyzing the white line intensity, absorption edge shift, and pre-/postedge features. Emphasis is also placed on in situ/operando XAS using state-of-the-art equipment, which can unveil the changes in structure and properties of SACs during the dynamic catalytic processes in a highly sensitive and time-resolved manner.

Computational description of key spectroscopic features of zeolite SSZ-13.

The catalytic properties of zeolites are intimately linked to the distribution and relative positions of Al atoms and defects in the pore network. However, characterizing this distribution is challenging, in particular when different local Al arrangements are considered. In this contribution we use a combination of first principles calculations and experimental measurements to develop a model for the Al-distribution in protonated SSZ-13. We furthermore apply this model to understand trends in OH-IR, 27Al-NMR and 29Si-NMR spectra. We use a Boltzmann distribution to predict the proton position for a given local Al configuration and show that for each configuration several H positions are occupied. Therefore a multi-peak spectrum in OH-IR vibrational spectroscopy is observed for all Al configurations, which is in line with experimentally measured spectra for zeolites at different Si/Al ratios. From NMR spectroscopy we find that the proton position leads to significant shifts in 27Al-NMR and 29Si-NMR spectra due to the modification of the local strain, which is lost when a uniform background charge is introduced. These findings are supported by experimental measurements. Finally we discuss the shortcomings of the presented model in terms of unit cell size and the impact of adjacent unit cells.

Structure and Thermal Stability of (H2O)4 Tetrahedron and (H2O)6 Hexagon Adsorbed on NaY Zeolite Studied by Synchrotron-Based Time-Resolved X-ray Diffraction

Water adsorption and desorption features play an important role in determining the adsorptive and catalytic properties of zeolites. In this study, the extraframework structure (water and cations) in NaY faujasite zeolite was studied by synchrotron-based in situ time-resolved X-ray diffraction (TR-XRD) in combination with the Rietveld refinement method. It was observed that Na cations locating at different sites migrated between each other during the dehydration process. Water molecules at three separate sites showed interesting geometries: (H2O)4 tetrahedron W1, (H2O)6 ice-like hexagonal W2, and disordered triangle W3. Interactions between each form of water and Na cations were deeply investigated. The stability of the waters followed the sequence of W1 > W3 > W2. This work provides insights into both framework structure and cation positions of model zeolite under actual working conditions, which helps fully understand the properties of zeolites.

Hierarchical zeolites Y obtained by desilication: Porosity, acidity and catalytic properties

The porosity, acidity and catalytic properties of zeolite Y desilicated in NaOH, tetrabutylammonium hydroxide (TBAOH) and NaOH/TBAOH mixture were investigated. The parent dealuminated zeolite Y of Si/Al = 31 contained very strongly acidic Si-OH-Al groups (more acidic than in other zeolites). These OH groups were also homogeneous. Desilication in NaOH solution extracted ca. 80% of Si but caused the amorphization of zeolite, loss of micropore system and of zeolitic acidity. On the other hand, TBAOH removed a small fraction of the Si, and preserved the crystallinity, porosity and acidity. Nevertheless, the NaOH/TBAOH treatment extracted about half of Si without a loss of crystallinity and zeolite microporosity. The mesopore volume and surface increased distinctly. Both concentration of Brønsted and Lewis acid sites also increased. Remarkably, the acid strength of the Si-OH-Al groups did not change upon desilication in the NaOH/TBAOH. The consequence of the enhanced mesoporosity, sites density, preservation of microporosity and very high acid strength was the catalytic activity in α-pinene isomerization that increased significantly.

Effect of acid leaching and catalytic properties of zeolite [Al,B]-MWW utilized as ethene methylation catalyst

Hydrothermally synthesized H-[Al,B]-MWW zeolite was dealuminated with oxalic, nitric and citric acids. The obtained samples were characterized with XRD, ICP-AES, FT-IR, NH3-TPD, pyridine-adsorption IR, 27Al MAS NMR techniques and nitrogen sorption experiment. Citric acid mainly removed the acid sites located on the external surface pockets with small amounts of extraframework Al species retained in the interlayer void space, while nitric acid leached both the internal (sinusoidal channels and supercages) and the external surface (side pockets) acid sites although significant amounts of extraframework Al species were generated in the sinusoidal channels. As for the oxalic acid, it primarily eliminated the acid sites in the supercages and side pockets without generation of considerable amounts of extraframework Al species in the internal channels, consequently greatly increasing the catalytic stability of the sample in methylation of ethene by methanol. The sample dealuminated with oxalic acid for 24 h showed a catalytic life of 300 h as a result of a reduction in coking rate.

Cu-containing MFI zeolites as catalysts for wet peroxide oxidation of formic acid as model organic contaminant

The catalytic behavior of different Cu(Fe)/zeolite catalysts with the MFI morphology in wet peroxide oxidation of formic acid as a model organic substrate was thoroughly examined. The influence of the method for Cu2+ ions introduction into the zeolite matrix, Cu loading and the electron state of Cu on catalytic properties of zeolites, including their stability to leaching of Cu was studied. Cu-substituted ZSM-5 zeolites with the atomic ratio Si/Al=30 and Cu content of 0.5–1.5, which were prepared by ion exchange, were shown most promising for wet peroxide oxidation. The impact of temperature, initial pH and concentrations of reagents on the catalytic performance of the catalyst with the optimal composition also was studied. The characterization of fresh and spent catalysts using UV–vis DR and ESR spectroscopic techniques led to suppose that the high efficiency of Cu-ZSM-5 to redox reactions in aqueous media is provided by nanostructured square-planar copper oxide clusters localized in the zeolite channels.

ChemInform Abstract: Delineating Similarities and Dissimilarities in the Use of Metal Organic Frameworks and Zeolites as Heterogeneous Catalysts for Organic Reactions

AbstractReview: 160 refs.

Delineating similarities and dissimilarities in the use of metal organic frameworks and zeolites as heterogeneous catalysts for organic reactions

This perspective article is aimed at providing a comparison of similarities and dissimilarities between the catalytic properties of zeolites and metal organic frameworks (MOFs). In the first part of the paper, we comment the general characteristics of MOFs with relevance to catalysis, making emphasis of how the properties of MOFs can serve to compliment those of zeolites as catalysts. The lower chemical and thermal stability of MOFs compared to zeolites is commented and correlated to the requirements for some liquid-phase reactions conducted under mild conditions. In the second part, we discuss the behaviour of zeolites and MOFs for four types of general organic reactions (acid catalysed, base catalysed, oxidation and hydrogenations). Particular attention is paid to provide critical comments on how MOFs could be adapted by design or can be modified by post-synthetic treatments to give well performing catalysts.

Interaction of [rad]CH2OH with silver cation in Ag–A/CH3OH zeolite: A DFT study

Density functional theory (DFT) has been applied to model the structure of (AgCH2OH/A)+ complexes previously experimentally characterized by electron paramagnetic resonance (EPR) in zeolite matrices. The magnetic parameters of (AgCH2OH/A)+ were found to depend on the local structure of the zeolite represented by clusters referred to as 3T and 6T, and also on the applied computational method. A spin distribution analysis confirms the one-electron silver–carbon bonding, showing delocalization of an unpaired electron density distributed between the sliver and carbon atoms along the one-electron silver–carbon bond. The results are of relevance for a deeper understanding of the electronic and catalytic properties of zeolites containing silver atoms and clusters.

In situ IR and catalytic studies of the regeneration of acid sites in coked zeolite Y

The process of regeneration of zeolite catalyst by coke oxidation was followed by “in situ” experiments in which the coke deposition, and subsequently coke oxidation were carried out inside flow IR cell. The characterization of acid sites by ammonia sorption and desorption, as well as by CO sorption, was done also in the same IR cell without exposing the sample to the air. The information on the nature of OH groups was obtained by recording the spectra at low temperature (170 K) which narrows IR bands and improves resolution. The effect of coke deposition and regeneration on catalytic properties of zeolite was studied in the experiments in which coke deposition and zeolite regeneration were carried out inside IR cell (the properties of such zeolite were characterized by IR spectroscopy) and afterwards zeolite was transmitted to a pulse microreactor and catalytic test was realized. Coke deposition eliminated some hydroxyls (most acidic ones were eliminated the first): IR bands diminished, concentration and acid strength of OH groups decreased. As a consequence, catalytic activity in m-xylene reaction decreased too. Oxygen treatment at 720 K decreased IR bands of coke by ca. 40% but all protonic sites became able to protonate ammonia and became active in m-xylene reaction. However, OH bands which decreased by coke deposition restored their intensity only a little, suggesting that some protonic sites (of relatively low acidity) were “invisible” in IR spectra as OH groups, they were still able to protonate ammonia and to be active sites in m-xylene reaction.

Effects of the Method of Introducing Iron and Iron Concentration on the Acid and Catalytic Properties of Zeolite

The effects of the method of introducing iron and iron concentration on the acid properties, catalytic activity, and stability of a zeolite in methanol conversion to hydrocarbons were studied. The state of active sites in the zeolite depends on the distribution of the metal in its structure, which depends on the method of zeolite modification. The addition of iron to zeolite increased the catalyst run time between regenerations, and the samples in which iron was introduced at the stage of hydrothermal synthesis exhibited the maximum catalytic stability.

Effect of Spectrum of Acidity of Zeolites of Various Structural Types on Their Catalytic Characteristics in the Synthesis of Ethyl tert -Butyl Ether

The catalytic properties of zeolites of various structural types in the liquid phase synthesis of ethyl tert-butyl ether from isobutylene and ethanol have been studied. The activity and selectivity of the catalysts depend on the concentration and strength of the acid centers. A possible mechanism for the synthesis of ethyl tert-butyl ether is proposed, suggesting that isobutylene and ethanol are activated on the weak and strong acid centers respectively.

ESRF/ILL hold workshop on environmental studies using neutrons and synchrotron radiation

ESRF and ILL Grenoble organized a workshop on Environmental Studies from February 20 to 21,2001, attracting over 100 participants from Europe, theUSA and Australasia to hear 19 invited lectures and discussa large selection of posters. Many of the lecturers had an opportunity to describe the practical importance of their work at a press conference attended by a half dozen journalists. John Parise (Stony Brook, NY) gave the plenary lecture, on new materials used for heavy metal recovery from industrial and nuclear waste. Instead of minimizing the Si4+/A13+ ratio inalumino-silicates to increase their ion exchange capacity, it has been possibleto synthesize new Si4+/Li1+ zeolites, and to study their crystal chemistry using a combination of synchrotron and neutron diffraction techques, including in-situ chemical synthesis. Herve Jobic (CNRS Lyon) also spoke about the catalytic properties of zeolites, and how neutron diffractioncan be used to understand the structure of adsorbed hydrocarbons, and neutroninelastic scattering to understand diffusion in the zeolite channels, andthe strength of bonding to the framework.

Crafting Small Siloxanes

CHEMISTRY One route that chemists have taken to modify the catalytic properties of zeolites (aluminosilicates) is to anchor small transition molecule catalysts within their channels. Yoshizawa et al. have now turned the tables on this kind of “ship in a bottle” chemistry. They have enlisted the

Monte Carlo simulations of extraframework cation positions in dehydrated zeolite LiX (Si:Al = 1.25)

Abstract A knowledge of extraframework cation positions is crucial for understanding the adsorption and catalytic properties of zeolites. However, diffraction methods are not always able to provide this information with sufficient reliability. Li-containing zeolites (LiLSX, LiX), for example, are particularly difficult to characterize experimentally because of the small X-ray and neutron scattering amplitudes of lithium. The situation is aggravated by the location of Li ions in general positions with low partial occupancies. We show how simulation tools can deal with the partial statistical disorder of the supercage Li cations in zeolite LiX (Si:Al = 1.25) and especially demonstrate the efficacy of canonical ( N, V, T ) Monte Carlo simulations for understanding the cation placements in such systems.

Sorption, diffusion and catalytic properties of zeolites containing 10- and 12-member ring pores in the same structure

The structure, adsorption, diffusion and catalytic properties of zeolites containing 10- and 12-member ring pores in the same structure, i.e. SSZ-33, SSZ-26, CIT-1, MCM-22, NU-86, NU-87, are presented. The adsorption and diffusion properties are well explained on the basis of the pore topologies. In those structures in which the 10- and 12-MR channels cross, the catalytic behaviour is strongly influenced by the void space left in the crossing points. This is not the case in the structures in which the pores are not connected. In this case, the catalytic performance is the average of the primary processes occurring in each one of the channels.

Alcohol Reactivity on Zeolites and Molecular Sieves

Abstract Zeolites and molecular sieves have been extensively studied and applied in different fields. Their specific crystalline structure and composition make it possible to obtain high-activity stable catalyst systems for a wide range of chemical reactions. There are many investigations on the structural characteristics and adsorption and catalytic properties of zeolites and molecular sieves as well as on the relation between the different properties. The nature of active sites in zeolite catalysts and the mechanism of catalytic processes on their surface belong to the most important problems of the investigations.

Oxidation of benzene to phenol by nitrous oxide over Fe-ZSM-5 zeolites

One step oxidation of benzene to phenol by nitrous oxide was studied over Fe-ZSM-5 zeolites. In addition to the case of nitrous oxide decomposition studied previously, catalytic properties of zeolites were determined by the presence of iron. New effective catalysts were synthesized. Some of them provided 20–25% benzene conversion with 100% phenol selectivity. In addition to the chemical composition of the catalyst the nature of the oxidant (N 2O, O 2) was shown to be of exceptional importance for the rate and the direction of benzene oxidation.