Growth Of Zeolite Crystals Research Articles

Nanocrystalline mesoporous zeolite X with a considerable external surface area prepared via an ordered precursor: a potential adsorbent

A nanocrystal zeolite X with intracrystalline mesopores has been successfully prepared by using organofunctionalised mesoporous silica in hydrothermal system. The as-prepared zeolitic material persist the inherent framework structure of FAU zeolite, and present a crystallographic morphology of stacked three-dimensional intergrowths made up of rectangular nanocrystals. The intracrystalline mesopores are formed via a bond-blocking principle with hydrophobic carbon chains of organosilane inhibiting the growth of zeolite units and crystals in the channels of mesoporous silica during hydrothermal synthesis. The results showed that the zeolitic material has a considerable external surface area (247 m2g−1)and mesopore volume (0.44 cm3g−1), maintaining high Brunauer–Emmett–Teller surface area (991 m2g−1) and large total pore volume (0.71 cm3g−1), and clearly stating a larger adsorption capacity for p-xylene at relative higher pressure.

Mechanism of zeolites crystal growth: new findings and open questions

This paper will review the crystal growth mechanism of zeolites and discuss some of the latest findings in this field. A great effort has been dedicated in recent years to improving the fundamental understanding of zeolite crystal growth during the synthesis process, with the overall aim of increasing control over the zeolite properties and structure. The latest studies on zeolite crystallization in colloidal (clear) suspensions, prepared by addition of organic and inorganic structural directing agents, in comparison to traditional gel synthesis and seed-approach will be discussed. Further understanding of the imperative dependence of zeolite crystallization kinetics' on the nature and mode of mixing of the initial reactants, leading to the formation of amorphous particles, intermediates, and final crystalline zeolites will be presented.

Synthesis and evaluation of seed-directed hierarchical ZSM-5 catalytic supports: Inductive influence of various seeds and aluminosilicate gels on the physicochemical properties and catalytic dehydrogenative behavior

TPABr-silicalite-1 and TPAOH-silicalite-1 seeding suspensions were employed as structure directing agents through a seed-directed synthesis strategy to prepare a series of organotemplate-free hierarchical ZSM-5 zeolites with three different Si/Al molar ratios of 30, 60 and 120. No secondary meso-generating agent was implemented to construct wide supplementary pores. The physicochemical properties of the products were characterized by XRD, FESEM, FT-IR, EDX, AAS, TGA and N2 adsorption–desorption techniques. All zeolite samples with moderate hierarchy factors possessed a bi/tri-model porosity based on the governing nucleation mechanism and growth of zeolite crystals upon the surface of the silicalite-1 seeds. The prepared hybrid zeolites with intrinsic micropores and auxiliary meso/macro pores were served as the catalytic supports in propane dehydrogenation reaction to ascertain the influence of the seeding type, Si/Al ratio and consequent structural and particulate properties on the catalytic performance. The best catalytic result was acquired for Pt–Sn-based ZSM-5 catalyst, with Si/Al = 60, synthesized with less expensive TPABr-silicalite-1 seeds. Considerable activity (34.5% conversion) and high selectivity towards light olefins (90% for both ethylene and propylene); propylene in particular (84.5%) was observed after 4 h time on stream. This result is attributed to the high BET surface area and meso transport pores of the sample collaborated with high acidity to progressively impact on the reactivity and selectivity.

Effect of secondary and tertiary alkylamines for the synthesis of Zeolite L

Zeolite L was synthesized at 180°C/72h under hydrothermal condition in the presence of diethylamine and triethylamine. Effects of alkylamines on crystallization, textural and morphological behaviors of zeolite L were investigated. Alkylamines facilitated in the crystallization of phase pure zeolite L. The triethylamine modified samples exhibited highest surface area (367m2g−1) and microporosity compared to those prepared in the presence of diethylamine. Clam-shaped crystals with nonuniform thickness of the particles were obtained in the presence of diethylamine, while triethylamine yielded cylindrical shaped crystals with hexagonal faced crystal habit. The role of amines having different chemical behaviors toward crystal growth of zeolite L was illustrated with a tentative mechanism.

Monolithic metal-fiber@HZSM-5 core–shell catalysts for methanol-to-propylene

Monolithic metal-fiber@HZSM-5 core–shell catalysts have been developed by direct growth of zeolite crystals on a macroscopic 3D network of sinter-locked metal microfibers. This approach provides a combination of excellent thermal conductivity, hierarchical porous structure from micro- to macro-size, and unique form factor. The metal-fiber@HZSM-5 catalysts, with high HZSM-5 loadings (e.g., 27–30wt%) and excellent core–shell robustness, deliver dramatic selectivity and life-time improvement in the methanol-to-olefin process. Such unprecedented performance is due to propagation of the olefin methylation/cracking cycle over the aromatic-based cycle in the methanol-to-hydrocarbon catalysis. Using a feed of 30vol% methanol in N2, for example, at 480°C high propylene selectivity of ∼46% can be obtainable with a total C2–C4 olefin selectivity of ∼70%, being much higher than that (∼37%, C2–C4 olefin selectivity of ∼64%) for the corresponding zeolite powder. The core–shell catalyst is stable at least for 210h, almost 3-fold longer than the life-time of 60h for the powdered HZSM-5 catalysts, because the coking rate is obviously suppressed in association with the propagated olefin-based cycle.

In situ growth-etching approach to the preparation of hierarchically macroporous zeolites with high MTO catalytic activity and selectivity

Silicoaluminophosphate zeolite SAPO-34 with CHA topology is known as one of the best catalysts for methanol-to-olefin (MTO) conversion. In this work, we demonstrate a facile one-step hydrothermal synthesis of hierarchically macroporous SAPO-34 through the etching effect of hydrofluoric acid. The highly crystalline hierarchically macroporous SAPO-34 is prepared as central-holed rhombohedral crystals with particle size of ca. 5–10 μm that comprise intracrystalline parallel macrochannels of ca. 100 nm. The formation of macroporous structures via in situ growth-etching can be directly imaged by SEM. Strikingly, a particular crystal configuration consisting of eight pyramidal parts has been first observed during the zeolite crystal growth process, which eventually grows to form a perfect rhombohedral shape. The HF etching effect is further elucidated by the analysis of changes of pH values as well as of solid and liquid compositions following the evolution of crystallization. The texture properties, chemical environments of framework atoms, and acidity of the synthesized SAPO-34 are characterized by N2 adsorption/desorption, MAS NMR and NH3-TPD measurements. The hierarchically macroporous SAPO-34 shows larger micropore volume, slightly stronger acid strength, and lower external surface acidity than its conventional counterpart synthesized without using HF. Consequently, the hierarchically macroporous SAPO-34 exhibits excellent MTO catalytic performance, showing much higher selectivity to ethylene and propylene as well as longer lifetime than the conventional counterpart. In comparison with previously reported methods for the generation of hierarchical porosity, this one-step HF-assisted in situ growth-etching synthetic route is simple, straightforward and cost-effective, which offers a new approach to prepare hierarchically porous zeolites with improved catalytic activity.

Preparation of high silica chabazite with controllable particle size

In this work synthesis procedures for controlling particle size distributions of high silica chabazite (Si/Al=100) zeolites between 0.3 and 4.5μm is described. The chabzite zeolite crystallization and crystal growth have been monitored using XRD, SEM and DLS. Hydrothermal treatment parameters such as time, temperature and agitation and gel composition parameters such as amounts of water and structure directing agent (SDA) have been correlated with chabazite zeolite crystal size, crystal shape, synthesis yield and crystallinity. From this data a chabazite zeolite formation and crystal growth mechanism based on experimental studies and previous studies has been proposed. It was also found that large aggregates formed in the chabazite zeolite gel before any hydrothermal treatment (1μm large when using zeolite gel with composition 1Al2O3:100SiO2:60TMAdaOH:3100H2O). The size controlling parameter and size controlling synthesis step of the chabazite zeolite (below 1μm) has been identified as the primary fractal aggregation of the silicium/aluminium/SDA species.

Highly Crystallized Nanometer‐Sized Zeolite A with Large Cs Adsorption Capability for the Decontamination of Water

Nanometer-sized zeolite A with a large cesium (Cs) uptake capability is prepared through a simple post-milling recrystallization method. This method is suitable for producing nanometer-sized zeolite in large scale, as additional organic compounds are not needed to control zeolite nucleation and crystal growth. Herein, we perform a quartz crystal microbalance (QCM) study to evaluate the uptake ability of Cs ions by zeolite, to the best of our knowledge, for the first time. In comparison to micrometer-sized zeolite A, nanometer-sized zeolite A can rapidly accommodate a larger amount of Cs ions into the zeolite crystal structure, owing to its high external surface area. Nanometer-sized zeolite is a promising candidate for the removal of radioactive Cs ions from polluted water. Our QCM study on Cs adsorption uptake behavior provides the information of adsorption kinetics (e.g., adsorption amounts and rates). This technique is applicable to other zeolites, which will be highly valuable for further consideration of radioactive Cs removal in the future.

Binderless zeolite coatings on macroporous α-SiC foams

Several syntheses have been performed to allow the growth of zeolite crystals on α-silicon carbide supports (α-SiC). Silicon-carbide foams exhibit a duplex macroporous structure. MFI and ANA zeolites have been successfully coated on this relatively inert material. While the synthesis of MFI/SiC required the presence of an additional Si-containing source, in contrast, ANA/SiC composites have been unexpectedly obtained through the self-recrystallization of the silicon contained in the α-SiC substrate. In addition, the size of icositetrahedral ANA crystals was controlled to nearly 35μm, thanks to SiC open cells templating effect.The different composite materials were thoroughly characterized by SEM, complete textural analysis and XRD. The amount of zeolite coating as well as the coverage by ANA zeolite crystals on SiC surface were determined by both SEM observations and adsorption measurements.

First synthesis of SAPO molecular sieve with LTL-type structure by hydrothermal conversion of SAPO-37 with FAU-type structure

An interzeolite conversion method was applied to the synthesis of silicoaluminophosphate (SAPO) molecular sieves. In the interzeolite conversion process, a structural similarity between the starting zeolite and the final crystallized zeolite was the crucial factor for zeolite crystal growth. We succeeded in the first synthesis of a SAPO with LTL structure from SAPO-37 with FAU structure in the presence of KOH. The d6r unit was the common building unit between both SAPOs. The crystallization of the LTL-type SAPO was accomplished at 170°C and in only one day when tetramethylammonium hydroxide (TMAOH) was added to the starting materials. As the LTL-type SAPO could not be obtained from the amorphous hydrogel prepared from fumed silica, pseudoboehmite, and H3PO4, even with prolonged reaction time, it was found that the SAPO formation process via such interzeolite conversion was more efficient, with an enhanced crystallization rate. The hydrothermal conversion of SAPO-37 into LTL-type SAPO was also found to depend significantly upon the synthesis parameters of H2O/T (T=Si+Al+P), KOH/T, and TMAOH/T, as well as the synthesis temperature.

Controlled and rapid growth of MTT zeolite crystals with low-aspect-ratio in a microwave reactor

The nucleation and growth of MTT zeolite, an important 10-membered ring (10 MR) zeolite, was controlled by a microwave-assisted hydrothermal synthesis. The effect of irradiation time (12,18, and 24h), seed-assisted synthesis with 1wt.% seeds for 6h and desilication under microwave irradiation on crystallinity and porosity were investigated. Under microwave-assisted synthesis, uniform crystal size, with lower crystal aspect-ratio and suppressed phase-impurity has been obtained. The tendency of column-like crystal to form bundle of columns was reduced significantly. Seeded growth further reduced synthesis time to 6h. Post-synthetic treatment was used to obtain larger pore volume of MTT zeolites. Some of the protonated MTT samples were tested in n-hexane cracking as model reaction. Characterization techniques such as XRD, FE-SEM, N2 adsorption–desorption, 27Al MAS NMR and NH3-TPD have been carried out to obtain insight in the physico-chemical properties of zeolite catalysts.

Materials Discovery and Crystal Growth of Zeolite A Type Zeolitic–Imidazolate Frameworks Revealed by Atomic Force Microscopy

A new zeolitic-imidazolate framework (ZIF), [Zn(imidazolate)2-x(benzimidazolate)x], that has the zeolite A (LTA) framework topology and contains relatively inexpensive organic linkers has been revealed using in situ atomic force microscopy. The new material was grown on the structure-directing surface of [Zn(imidazolate)1.5(5-chlorobenzimidazolate)0.5] (ZIF-76) crystals, a metal-organic framework (MOF) that also possesses the LTA framework topology. The crystal growth processes for both [Zn(imidazolate)2-x(benzimidazolate)x] and ZIF-76 were observed using in situ atomic force microscopy; it is the first time the growth process of a nanoporous material with the complex zeolite A (LTA) framework topology has been monitored temporally at the nanoscale. The results reveal the crystal growth mechanisms and possible surface terminations on the {100} and {111} facets of the materials under low supersaturation conditions. Surface growth of these structurally complex materials was found to proceed through both "birth-and-spread" and spiral crystal-growth mechanisms, with the former occurring through the nucleation and spreading of metastable and stable sub-layers reliant on the presence of non-framework species to bridge the framework during formation. These results support the notion that the latter process may be a general mechanism of surface crystal growth applicable to numerous crystalline nanoporous materials of differing complexity and demonstrate that the methodology of seeded crystal growth can be used to discover previously unobtainable ZIFs and MOFs with desirable framework compositions.

A Facile Strategy To Design Zeolite L Crystals with Tunable Morphology and Surface Architecture

Tailoring the anisotropic growth rates of materials to achieve desired structural outcomes is a pervasive challenge in synthetic crystallization. Here we discuss a method to selectively control the growth of zeolite crystals, which are used extensively in a wide range of industrial applications. This facile method cooperatively tunes crystal properties, such as morphology and surface architecture, through the use of inexpensive, commercially available chemicals with specificity for binding to crystallographic surfaces and mediating anisotropic growth. We examined over 30 molecules as potential zeolite growth modifiers (ZGMs) of zeolite L (LTL type) crystallization. ZGM efficacy was quantified through a combination of macroscopic (bulk) and microscopic (surface) investigations that identified modifiers capable of dramatically altering the cylindrical morphology of LTL crystals. We demonstrate an ability to tailor properties critical to zeolite performance, such as external porous surface area, crystal shape, and pore length, which can enhance sorbate accessibility to LTL pores, tune the supramolecular organization of guest-host composites, and minimize the diffusion path length, respectively. We report that a synergistic combination of ZGMs and the judicious adjustment of synthesis parameters produce LTL crystals with unique surface features, and a range of length-to-diameter aspect ratios spanning 3 orders of magnitude. A systematic examination of different ZGM structures and molecular compositions (i.e., hydrophobicity and binding moieties) reveal interesting physicochemical properties governing their efficacy and specificity. Results of this study suggest this versatile strategy may prove applicable for a host of framework types to produce unrivaled materials that have eluded more conventional techniques.

In situ crystallization of b-oriented MFI films on plane and curved substrates coated with a mesoporous silica layer

A simple and reproducible method is presented for preparing b-oriented MFI films on plane (disc) and curved (hollow fiber) supports by in situ hydrothermal synthesis. A mesoporous silica (sub-)layer was pre-coated on the supports by dip coating followed by a rapid thermal calcination step (973K during 1min) to reduce the number of grain boundaries while keeping the hydrophilic behavior of silica. The role of the silica sub-layer is not only to smoothen the substrate surface, but also to provide a silica source to promote the nucleation and growth of zeolite crystals via a heterogeneous nucleation mechanism (zeolitization), and adsorb zeolite moieties generated in the synthesis solution via a homogeneous nucleation mechanism. A monolayer of b-oriented MFI crystals was obtained on both supports after 3h synthesis time with a moderate degree of twinning on the surface.

Nucleation and crystal growth of zeolite A synthesised from hydrogels of different density

Various factors affect the process of zeolite formation and thus the characteristics of the ultimate material. The present study is devoted to the influence of the density of the starting aluminosilicate hydrogels, i.e. the amount of the solid phase in the system, on the characteristics of the final product. Different density (viscosity) of the five alumosiliate gels was obtained by changing the ratio SiO2/H2O, while the ratios Na2O/H2O and SiO2/Al2O3 remained constant. The freshly prepared hydrogels were hydrothermally treated at 80 °C under static conditions. The reaction course was followed by a set of complementary analyses including XRD, chemical analysis, microscopy and thermogravimetry. The linear rate of crystal growth was calculated according to Zhdanov and Samulevich's method. The crystal size distribution curves of the end products were measured and the corresponding curves of nuclei distribution within gel matrix and specific number of crystals were calculated. The observed effect of “local ordered structure” (potential nuclei) formation was commented on from the viewpoint of the autocatalytic nucleation theory.

Formation Pathway for LTA Zeolite Crystals Synthesized via a Charge Density Mismatch Approach

A solid understanding of the molecular-level mechanisms responsible for zeolite crystallization remains one of the most challenging issues in modern zeolite science. Here we investigated the formation pathway for high-silica LTA zeolite crystals in the simultaneous presence of tetraethylammonium (TEA(+)), tetramethylammonium (TMA(+)), and Na(+) ions as structure-directing agents (SDAs) with the goal of better understanding the charge density mismatch synthesis approach, which was designed to foster cooperation between two or more different SDAs. Nucleation was found to begin with the formation of lta-cages rather than the notably smaller sod and d4r-cages, with concomitant incorporation of TMA(+) and Na(+) into a very small amount of the solid phase with a low Si/Al ratio (ca. 2.5). The overall characterization results of our work demonstrate that sod-cages are first built around the preorganized lta-cages and that d4r-cages are in turn constructed by the progressive addition of low-molecular-weight (alumino)silicate species, which promotes the formation and growth of embryonic LTA zeolite crystals. We also show that the crystal growth may take place by a similar process in which TEA(+) is also incorporated, forming a single LTA zeolite phase with a higher Si/Al ratio (ca. 3.3).

Growth of zeolite crystals with graphene oxide nanosheets

Growth of silicalite with graphene oxide (GO) nanosheets occurred via attachment of GO onto the silicalite surface, and entrapment of GO nanosheets inside single crystals. Electrically conductive composites were produced by calcination in nitrogen whereas silicalite crystals with slit-like mesopores of sizes 20-25 Å were obtained after GO burn-off.

NaA zeolite cubic crystal formation and deformation: cubes with crystalline core, simultaneous growth of surface and core crystals, and layer-by-layer destruction

The entire sequence of NaA zeolite crystal growth mechanism, involving the formation of initial inorganic precursor gel, NaA zeolite nucleation, primary intermediate crystalline building block formation followed by their successive oriented aggregation and then finishing with cubic NaA zeolite crystal formation, has been monitored by light scattering analysis (LSA), X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM) and N2 adsorption–desorption. The presented crystal growth mechanism in this work not only provides a prominent feature in NaA zeolite cubic crystal formation but also in the deformation of cubes. The deformation of micron-size NaA zeolite cubes first involves layer-by-layer destruction followed by complete breakage of cubes due to core–shell crystal expansion, resulting in nanosize cubic crystals having the same characteristics as the original NaA zeolite before destruction. Therefore, the present study provides a first demonstration about an occurrence of multiple phenomena during the NaA zeolite crystal growth process such as formation of intermediate crystalline building blocks which are structurally similar to NaA nuclei, multiple nucleation (crystalline island formation), heat facilitated fast oriented aggregation, formation of chamfered edged cubic crystals, successive layer-by-layer destruction and finally breakage of cubic crystals. Furthermore, LSA, SEM and TEM results reveals the formation of nanocrystals with size ranging from 50 to 500 nm during the crystal growth process.

Synthesis of highly b-oriented ZSM-5 membrane on a rough surface modified simply with TiO2 by in situ crystallization

A simple, reliable and reproducible approach for the fabrication of highly b-oriented ZSM-5 zeolite membranes on rough surface is presented. The approach consists of depositing a TiO2 intermediate layer on the rough surface of substrate, followed by UV irradiation, and subsequently in situ crystallization on the TiO2 modified rough surface. The TiO2 interlayer combined UV irradiation provides a smooth and high Ti–OH density surface for the growth of b-oriented ZSM-5 crystals. By comparing the morphology of zeolite membranes on Al2O3, SiO2, ZrO2, and TiO2, the impacts of smoothness, dispersion force between the zeolite crystal phase and substrate, and density of surface hydroxy on the formation of a highly b-oriented and continuous ZSM-5 membrane have been investigated. A smooth surface rich in Ti-OH was helpful for the continuous and uniform spread of the synthesis solution on the support, resulting in the oriented nucleation and accelerated growth of zeolite crystals, which formed the highly b-oriented and covered ZSM-5 membrane.

Bead-Milling and Postmilling Recrystallization: An Organic Template-free Methodology for the Production of Nano-zeolites

Organic template-free synthesis of nano-zeolite has been an important subject of both scientific and industrial applications. Most research has focused on the fabrication of nano-zeolite by a bottom up approach, that is, control of zeolite nucleation and crystal growth during the hydrothermal synthesis. This communication reports a new method for the production of nano-zeolite powder by a top-down approach. In this study, the zeolite was first milled to produce a nanopowder. This technique can cause destruction of the outer portion of the zeolite framework and hence cause pore blocking, which deactivates various properties of the zeolite. To remedy this, the damaged part was recrystallized using a dilute aluminosilicate solution after bead milling. As a result of the combination of bead milling and postmilling recrystallization, nano-zeolite A (LTA type zeolite) about 50 nm in size with high crystallinity was successfully obtained.