The hydrothermal synthesis of zeolites: precursors, intermediates and reaction mechanism

Composition and Microstructure of Alkali Activated Fly Ash Binder: Effect of the Activator

A hierarchical mesoporous network of zeolite beta with very high micropore as well as mesopore volume was synthesized without the need of a porogen at near 100% yield in the form of easily retrievable micrometer-sized particles. This was achieved by a dense-gel synthesis utilizing steam-assisted conversion (SAC) to induce a burst of nucleation. During the first phase of the synthesis, individual, evenly sized zeolite beta nanoparticles are formed that subsequently condense into a porous network displaying uniform mesopores. The final product consists of hierarchical self-sustaining macroscopic zeolite aggregates assembled from 20 nm crystalline domains of zeolite beta. The small size of the zeolite crystals in the resulting materials gives rise to mesopores with dominant pore sizes of about 13 nm. Large surface areas between 630 and 750 m2/g and total pore volumes up to 0.9 mL/g were obtained without sacrificing the microporosity (usually larger than 0.20 mL/g). Crystallization conditions were optimized f...

One-step synthesis of hierarchical zeolite beta via network formation of uniform nanocrystals.

Hierarchical zeolites combine the intrinsic catalytic properties of microporous zeolites and the enhanced access and transport of the additional meso- and/or macroporous system. These materials are the most desirable catalysts and sorbents for industry and become a highly evolving field of important current interests. In addition to the enhanced mass transfer leading to high activity, selectivity, and cycle time, another essential merit of the hierarchical structure in zeolite materials is that it can significantly improve the utilization effectiveness of zeolite materials resulting in the minimum energy, time, and raw materials consumption. Substantial progress has been made in the synthesis, characterization, and application of hierarchical zeolites. Herein, we provide an overview of recent achievements in the field, highlighting the significant progress in the past decade on the development of novel and remarkable strategies to create an additional pore system in zeolites. The most innovative synthesis approaches are reviewed according to the principle, versatility, effectiveness, and degree of reality while establishing a firm link between the preparation route and the resultant hierarchical pore quality in zeolites. Zeolites with different hierarchically porous structures, i.e., micro-mesoporous structure, micro-macroporous structure, and micro-meso-macroporous structure, are then analyzed in detail with concrete examples to illustrate their benefits and their fabrications. The significantly improved performances in catalytic, environmental, and biological applications resulting from enhanced mass transport properties are discussed through a series of representative cases. In the concluding part, we envision the emergence of "material-properties-by-quantitative and real rational design" based on the "generalized Murray's Law" that enables the predictable and controlled productions of bioinspired hierarchically structured zeolites. This Review is expected to attract important interests from catalysis, separation, environment, advanced materials, and chemical engineering fields as well as biomedicine for artificial organ and drug delivery systems.

Hierarchically Structured Zeolites: From Design to Application.

This review is a summary and critical analysis of recent advances in the understanding of (a) the nature and coordination state of Ti ions and other functional groups (such as OH) on dehydrated titanium silicate molecular sieves, (b) the type and structure of the oxo intermediates generated by the interaction of these active sites with oxidant/reactant molecules during catalytic reactions, and (c) the factors that influence the reactivity and selectivity of these active sites and reaction intermediates. In the dehydrated state, most of the Ti 4+ ions have the tetrapodal (Ti(OSi) 4 ) or the tripodal (Ti(OSi) 3 OH) structure. On contact with H 2 O 2 , titanium oxo species, Ti(O 2 H) and Ti(O 2 − ), respectively, are formed. On reaction with organic reactants, O–O bond cleavage in these titanium oxo species occurs in a hetero- or homolytic manner. Product selectivity is determined by the relative importance of these two modes of O–O cleavage. Factors such as the coordinative environment of titanium, substituents on the O–O bond (H or alkyl), temperature, solvent, nature of the organic reactant, etc. influence the mode of O–O cleavage. Correlations between the structure and catalytic activity of titanium sites and oxo-titanium intermediates are also described.

Active Sites and Reactive Intermediates in Titanium Silicate Molecular Sieves

ZEOLITES and other microporous molecular sieves1–4 exhibit useful catalytic, absorption and ion-exchange properties. Most synthetic zeolites are made by hydrothermal methods, which generally suffer from the drawback of long crystallization times5. Here we show that the crystallization rate of zeolites and related molecular sieves can be accelerated by a factor of as much as about five by the addition of various 'promoter' ions, such as perchlorate, phosphate, nitrate, sulphate and carbonate. By using enhanced-sensitivity 29Si NMR studies to monitor soluble silicate species during crystallization, we show that the promoter ions enhance the rate of formation of the oligomeric (mainly tetrameric) silicate ions responsible for nucleation and growth of the crystals.

Promoter-induced enhancement of the crystallization rate of zeolites and related molecular sieves

Synthesis and Characterization of Ferrierite-Type Zeolite in the Presence of Nonionic Surfactants

Silicalite-1 has been hydrothermally synthesized in a bicontinuous microemulsion of tetraethylenepentamine–sodium bis(2-ethylhexly) sulfosuccinate (AOT)–water system containing fluoride ions. The presence of AOT (an anionic surfactant) has been found to be essential for the formation of Silicalite-1 in bulk tetraethylene pentamine solvent. The tetrahedral binding sites of silicon atom have been confirmed by29Si magic angle spinning NMR and FT-IR spectra. Thermogravimetric and differential thermal analysis studies on as-synthesized Silicalite-1 and X-ray diffraction pattern for calcined product established their structural stability. The scanning electron micrographs show the twinned and uniform morphology of crystals. The presence of additive (AOT) lengthens the nucleation period and helps to produce a large size crystal of Silicalite-1.

Synthesis of Silicalite-1 in Bicontinuous Microemulsion Containing AOT.

Synthesis of ferrierite (FER) type zeolite with varying Si to Al ratios in the presence of a catalytic amount of pyrrolidine and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in aqueous media is reported. The surfactant moieties direct pyrrolidine molecules in a particular fashion, resulting in a sixfold decrease in the required amount of template as compared to conventional procedure, for FER crystallization. The product obtained was highly crystalline and pure. In the presence of AOT alone, ferrierite co-crystallized with the ZSM-5 phase, indicating AOT is not acting as a structure-directing agent and a small concentration of pyrrolidine ( approximately 2 wt%) template is essential for the ferrierite crystallization. A scanning electron micrograph showed uniformity in crystals (average 2-3 µm) consisting of broad plate type morphology. The crystal structure of FER (AOT/Py) maintains structural integrity until about 1000 degrees C. FER (AOT/Py) has been further characterized by employing XRD, XRF, IR, and TG/DTA techniques. Copyright 2001 Academic Press.

Synthesis of Ferrierite-Type Zeolite in the Presence of a Catalytic Amount of Pyrrolidine and Sodium Bis(2-ethyhlhexyl) Sulfosuccinate

The zeolite Ferrierite (FER) was synthesized from the hydrogel containing template and promoting medium. The kinetic features of the ferrierite crystallization with varying SiO2/Al2O3 ratio are reported. The apparent activation energies for nucleation (En) and crystallization (Ec) were evaluated applying the Arrhenius equation. The rates of both nucleation and crystallization have been found to increase significantly in the presence of promoters like ClO4
-, PO4
-3 and SO4
-2 oxyanions during hydrothermal synthesis. The significant enhancement of the FER crystal growth rate due to promoting medium has been attributed to its greater polarizing and rapid condensation ability of tetrasiloxy silane species [Si-(O-Si)4]. The crystalline FER products obtained from promoting medium were characterized by XRD, SEM, TG/DTA, IR, NMR and sorption techniques.

Ranjeet Kaur Ahedi

Papers

Promoter-induced enhancement of the crystallization rate of zeolites and related molecular sieves

Synthesis and Characterization of Ferrierite-Type Zeolite in the Presence of Nonionic Surfactants

Synthesis of Silicalite-1 in Bicontinuous Microemulsion Containing AOT.

Synthesis of Ferrierite-Type Zeolite in the Presence of a Catalytic Amount of Pyrrolidine and Sodium Bis(2-ethyhlhexyl) Sulfosuccinate

Studies in the Crystallization of Ferrierite (FER) Type Zeolites in Presence of Promoting Medium