Growth Of Zeolite Crystals Research Articles

Controlling Crystal Morphology of Anisotropic Zeolites with Elemental Composition.

The morphology of zeolite crystals strongly affects their textural, catalytic, and mechanical attributes. However, controlling zeolite crystal morphology without using modifiers or structure-directing agents remains a challenging task because of our limited understanding of the relationships between zeolite crystal shape, crystallization mechanism, and composition of the starting synthesis mixture. In this study, we aimed at developing a general method for controlling the morphology of zeolites by assessing the impact of the Si/T molar ratio of the synthesis gel on the growth rate of zeolite crystals in various crystallographic directions and on the final crystal morphology of the UTL germanosilicate with a 2D system of intersecting 14- and 12-ring pores. Our results showed that flat UTL crystals progressively thicken with the Si/Ge molar ratio, demonstrating that Ge concentration controls the relative rate of crystal growth in the perpendicular direction to the pore system. The morphology of other zeolites and zeotypes with an anisotropic structure, including AFI (12R), IFR (12R), MWW (10-10R), and IWW (12-10-8R), can also be predicted based on their Si/T ratio, suggesting a systematic pattern across zeolite structures and in a wide range of zeolite framework elements. Combined, these findings introduce a facile and cost-efficient method for directly controlling crystal morphology of zeolites with anisotropic structures with a high potential for scale-up while providing further insights into the role of elemental composition in zeolite crystal growth.

Double Four Ring Units-Containing Zeolites: Synthesis, Structural Modification and Catalytic Applications.

In zeolite frameworks, double four-ring (d4r) configurations are among the most frequent composite building units. The composition variations in d4r units greatly influence the energy and structural modifiability of the zeolitic framework. The introduction of germanium, with a larger ionic radius than silicon or aluminum, not only reduces the energy constraints of d4r in the nucleation and crystal growth of zeolites, but also opens a new window for constructing novel crystalline structures, especially with large or extra-large pores and channels. Ge-enriched d4r units endow germanosilicates with structure diversity readily for post treatments. Promising catalytic materials have been gradually developed and increasingly studied by direct synthesis or post-synthetic isomorphous substitution for Ge. This review focuses on the recent progress in the synthesis, modification, and catalytic application of d4r-containing zeolites, including germanosilicates, aluminosilicates, and silicates.

A Nanostrips-Assemble Morphology of ZSM-5 Zeolite for Efficient Propylene Production from Methanol Conversion

Turning the morphology and orientated growth of zeolite crystals is essential to alter their catalytic performances. MFI-type ZSM-5 nanostripes (named as D-Z5) with a sea-urchin-like overall morphology were directly synthesized with a commercially available and small-molecular organic structure-directing agent (OSDA) of 1-butyl-1-methylpyrrolidinium hydroxide. Their nanostrip crystals exhibited a unique dioriented morphology with thin thicknesses of ∼8 nm and ∼70 nm along the a-axis and b-axis, respectively. The dealuminated D-Z5 displayed higher propylene selectivity (∼57%), longer lifetime (60 h) and much higher propylene-to-ethylene ratio (>12) in methanol-to-propylene (MTP) reaction than those conventional ZSM-5 with known crystal morphologies. Computational simulation and experimental measurements provided solid proof that the diorientated crystals decreased the bulky molecular diffusion resistance, postponed the secondary reaction and then promoted propylene to diffuse rapidly out of two sets of micropores in zeolites. This unusual diorientation phenomenon is not only beneficial for improving the MTP catalytic performance of ZSM-5, but also expected to be versatile to develop other useful zeolite catalysts.

Synthesis of a Coal Fly Ash‐Based NaP Zeolite Using the Microwave‐Ultrasonic Assisted Method: Preparation, Growth Mechanism, and Kinetics

AbstractFly ash was used as the raw material to prepare coal fly ash‐based NaP zeolite using the microwave‐ultrasonic‐hydrothermal method. The obtained NaP zeolite has a completed crystal structure, regular surface morphology, uniform particle size (about 1.6 um), large specific surface area (48.85 m2/g) and good thermal stability. Under microwave and ultrasonic external fields, the coal fly ash‐based NaP zeolite crystals grow in an S‐shape, achieving single crystal growth and uniform crystal growth size. The activation energy of NaP zeolite crystal growth Ea is 2.08 kJ/mol, the pre‐exponential factor A is 85.63.

Reprint of : Recent strategies for synthesis of metallosilicate zeolites

Incorporation of catalytically active metallic sites into the framework of silica-based zeolites is highly efficient for design of active, selective, and stable heterogeneous catalysts, but there are still many challenges for this kind of preparation such as the presence of aggregated metallic sites due to the alkaline media in the synthesis, relatively low concentration of these metallic sites in the zeolite frameworks, and difficulty for morphological control of metallosilicate zeolites. To overcome these issues, it has been developed new strategies for synthesis of metallosilicate zeolites such as introduction of additives to adjust crystallization rate, addition of chelating agents to form heteroatom complexes, growth of zeolite crystals along with targeted directions, zeolite transformation, and heteroatom replacements. In this review, the recent progresses for syntheses of metallosilicate zeolites are concisely summarized. In addition, the fundamental principles are briefly discussed for the formation of the metallosilicate zeolites. This review should be helpful for design and preparation of the heteroatom-substituted silica-based zeolites as highly efficient heterogeneous catalysts.

Recent strategies for synthesis of metallosilicate zeolites

Incorporation of catalytically active metallic sites into the framework of silica-based zeolites is highly efficient for design of active, selective, and stable heterogeneous catalysts, but there are still many challenges for this kind of preparation such as the presence of aggregated metallic sites due to the alkaline media in the synthesis, relatively low concentration of these metallic sites in the zeolite frameworks, and difficulty for morphological control of metallosilicate zeolites. To overcome these issues, it has been developed new strategies for synthesis of metallosilicate zeolites such as introduction of additives to adjust crystallization rate, addition of chelating agents to form heteroatom complexes, growth of zeolite crystals along with targeted directions, zeolite transformation, and heteroatom replacements. In this review, the recent progresses for syntheses of metallosilicate zeolites are concisely summarized. In addition, the fundamental principles are briefly discussed for the formation of the metallosilicate zeolites. This review should be helpful for design and preparation of the heteroatom-substituted silica-based zeolites as highly efficient heterogeneous catalysts.

Time-resolved dissolution elucidates the mechanism of zeolite MFI crystallization.

Zeolite crystal growth mechanisms are not fully elucidated owing to their complexity wherein the formation of a particular zeolite can occur by more than one crystallization pathway. Here, we have conducted time-resolved dissolution experiments of MFI-type zeolite crystals in ammonium fluoride medium where detailed structural analysis allowed us to extrapolate and elucidate the possible mechanism of nucleation and crystal growth. A combination of electron and scanning probe microscopy shows that dissolution initiates preferentially at lattice defects and progressively removes defect zones to reveal a mosaic structure of crystalline domains within each zeolite crystal. This mosaic architecture evolves during the growth process, reflecting the changing conditions of zeolite formation that can be retroactively assessed during zeolite crystal dissolution. Moreover, a more general implication of this study is the establishment that dissolution can be used successfully as an ex situ technique to uncover details about crystal growth features inaccessible by other methods.

Calcination-Free Fabrication of Highly b-Oriented Silicalite-1 Zeolite Films by Secondary Growth in the Absence of Organic Structure-Directing Agents

Calcination-free fabrication of highly b-oriented silicalite-1 (S-1) zeolite films in the absence of organic structure-directing agents (OSDAs) is attractive but still challenging. Herein, for the first time, we show a novel route for calcination-free synthesis of a b-oriented S-1 zeolite film in the absence of OSDAs. The key to this success is a rational secondary growth of zeolite crystals on the surface of supports by avoidance of the formation of zeolite nuclei in the system due to the absence of OSDAs. Because of OSDA-free synthesis of S-1, it not only reduces the cost of zeolite films but also avoids the calcination of OSDAs in the synthesis, thus avoiding the formation of cracks in the zeolite films. In addition, the calcination-free fabrication is favorable for reduction of zeolite film defects such as a terminal OH group. As a result, the zeolite films obtained from this work exhibit higher hydrophobicity than the conventional one. Furthermore, the current polarization curve of this S-1 zeolite film shows excellent corrosion resistance. The aforementioned features would be potentially important for industrial application of zeolite films in the near future.

Single- and multi-component adsorption of selected contaminants of emerging concern from water and some of their metabolites onto hierarchical porous copper(II)-zeolite -activated carbon composite

A copper(II) functionalized hierarchical faujasite zeolite/activated carbon composite adsorbent (Cu-CFAU) was prepared in an attempt to develop a platform for the effective removal of contaminants of emerging concern (CECs: caffeine, carbamazepine, naproxen, and metabolites clofibric acid, 10,11-epoxy-carbamazepine, o-desmethyl-naproxen, paraxanthine, salicylic acid) from water at ambient conditions. CFAU was prepared hydrothermally via seeded in situ growth of zeolite crystals within the pores of an activated carbon (AC, Darco-KB-G) and subsequently exchanged with copper(II). The materials were characterized using XRD, TEM, TGA, porosimetry, and elemental analysis. TEM, TGA and textural properties showed direct evidence of the in situ growth of the zeolite phase. Performance of the CFAU composite for CECs equilibrium adsorption was addressed in both single- and multi-component fashion and for CECs aqueous phase concentrations covering the μg L−1 – mg L−1 range. Cu-CFAU displayed the best overall adsorption working capacities, probably due to greater affinity for all CECs by relying on complexation interaction with the transition metal sites along with an enhancement of electrostatic and hydrophobic-hydrophilic interactions. Compared to AC, the composite excelled in adsorbing ionic CECs and was able to offer at least equal capacity toward neutral CECs. Compared to other adsorbents reported so far in the literature, the Cu-CFAU offers adsorption capacities that are larger by an order of magnitude in many cases. Preliminary tests for the reusability of the composite after various adsorption/desorption cycles suggest that Cu-CFAU might be regenerated via thermal treatment under inert atmosphere when the adsorbed CEC possesses a relative low melting point.

A facile route to synthesize mesoporous ZSM-5 with hexagonal arrays using P123 triblock copolymer

Mesoporous ZSM-5 samples were synthesized using Pluronic P123 triblock copolymer (EO20PO70EO20) as a supramolecular template through a pH adjusting method. The resulting materials were characterized using X-ray diffraction, transmission electron microscopy (TEM), nitrogen adsorption/desorption, scanning electron microscopy (SEM), solid-state nuclear magnetic resonance (MAS-NMR) and ammonia temperature programmed desorption (TPD) analyses. Adjusting the synthesis pH allowed obtaining a well-crystallized ZSM-5 material with meso-channels of size of 8–10 nm as calculated from non-local density functional theory (NLDFT) and also supported by TEM images. This strategy performs the growth of zeolite crystals with a mesostructured pore lattice in a cooperative manner, avoiding both a phase separation between the surfactant and zeolite units during crystallization and the formation of a separate amorphous mesoporous material. In addition, ammonia TPD studies revealed that the acidity of the mesoporous ZSM-5 material is comparable with the microporous conventional one. It was found from NH3-TPD profiles that in the case of the mesoporous ZSM-5 samples some of the strong acid sites have been transformed into weak acid sites which might be attributed to the acid sites present on the crystal external surface in mesopores.

Zeolite Crystal Growth Analysis by Dissolution

Zeolite Crystal Growth Analysis by Dissolution

Phase Transformation Behavior of a Two-Dimensional Zeolite.

Understanding the molecular-level mechanisms of phase transformation in solids is of fundamental interest for functional materials such as zeolites. Two-dimensional (2D) zeolites, when used as shape-selective catalysts, can offer improved access to the catalytically active sites and a shortened diffusion length in comparison with their 3D analogues. However, few materials are known to maintain both their intralayer microporosity and structure during calcination for organic structure-directing agent (SDA) removal. Herein we report that PST-9, a new 2D zeolite which has been synthesized via the multiple inorganic cation approach and fulfills the requirements for true layered zeolites, can be transformed into the small-pore zeolite EU-12 under its crystallization conditions through the single-layer folding process, but not through the traditional dissolution/recrystallization route. We also show that zeolite crystal growth pathway can differ according to the type of organic SDAs employed.

Phase Transformation Behavior of a Two‐Dimensional Zeolite

AbstractUnderstanding the molecular‐level mechanisms of phase transformation in solids is of fundamental interest for functional materials such as zeolites. Two‐dimensional (2D) zeolites, when used as shape‐selective catalysts, can offer improved access to the catalytically active sites and a shortened diffusion length in comparison with their 3D analogues. However, few materials are known to maintain both their intralayer microporosity and structure during calcination for organic structure‐directing agent (SDA) removal. Herein we report that PST‐9, a new 2D zeolite which has been synthesized via the multiple inorganic cation approach and fulfills the requirements for true layered zeolites, can be transformed into the small‐pore zeolite EU‐12 under its crystallization conditions through the single‐layer folding process, but not through the traditional dissolution/recrystallization route. We also show that zeolite crystal growth pathway can differ according to the type of organic SDAs employed.

Genesis of poikilotopic zeolite in aeolianites: An example from the Parnaíba Basin, NE Brazil

Zeolite cement is one of the main components of the Jurassic to Lower Cretaceous fluvial-aeolian succession in the Parnaiba Basin, northeastern Brazil. Core and outcrop-based stratigraphic analyses permitted the paleo-environmental and diagenetic interpretation of aeolian sandstone and showed the influence of the volcanic substrate in the origin of the zeolite cement. The substrate is represented by an extensive volcanic plain developed in the last stages of the Central Atlantic Magmatic Province (CAMP) in West Gondwana. The siliciclastic deposits consist of dune field, sand sheet and wadi deposit facies associations. The zeolite cement of the aeolian succession forms poikilotopic crystals with anomalous up to 50% modal content. X-ray diffraction and SEM/EDS/CL analyses showed laumontite and Ca-stilbite exhibiting, respectively, massive and radial textures. During eodiagenesis, the main processes included clay infiltration linked to water table fluctuations and flash flood events, producing clay coatings and displacive stilbite cementation. Massive laumontite cementation and smectite-chlorite conversion occurred in the mesodiagenesis, while in the final stages, the zeolite was partially replaced by calcite cement. Telodiagenesis is recorded by iron exsudation and the corrosion of clay films. Several processes can be inferred to explain the origin of the zeolite cement in the Jurassic-Lower Cretaceous sandstone of the Parnaiba Basin. After the first magmatic pulse (Triassic magmatism), the zeolite-bearing basalts were submitted to chemical weathering, causing cation leaching. Afterwards, the deposition of aeolian sands related to a wet desert system occurred above this weathered substrate. Fractured basaltic substrate favored the release of ions to the surface, with fluids enriched in sodium and calcium that percolated the permeable aeolian sandstone beds during the diagenesis. Furthermore, zeolite cement is geographically associated with the volcanic substrate in the western Parnaíba Basin. However, it is absent in the eastern part, where the aeolianites overlie sedimentary rocks. We suggest that the system was thermally reactivated by the last thermal CAMP phase, with basalt emplacement in the eastern Parnaiba Basin linked to the opening of the Equatorial Atlantic Ocean (Cretaceous magmatism). The progressive heating towards the western part of the basin triggered fluids convection, and the enrichment in ions such as Ca2+, Si4+, Na+, Al3+, HCO−3 and SO42−, promoting optimal conditions for zeolite crystal growth (100 °C–300 °C). Sandstone textural aspects, as well as extensional faults and fractures allowed fluids migration resulting in the massive poikilotopic zeolite cementation, characterizing one of the most singular features of the diagenetic history of the West Gondwana record.

Synthesis of a DDR-type zeolite membrane by using dilute solutions of various alkali metal salts

First, we attempted to synthesize a DD3R zeolite membrane by using precursor solutions containing small amounts of a structure-directing agent and potassium fluoride (KF), with the aim of obtaining a membrane with high permeance toward CO2. The most dilute precursor solution (1-adamantylamine:KF:SiO2:H2O = 0.026:0.026:1:52) produced a thin and dense zeolite membrane that showed a high CO2 permeance of 12 × 10−8 mol m−2 s−1 Pa−1 and a high PCO2/PCH4 of 270 at a pressure drop of 1.0 MPa. We then studied the synthesis of DD3R membranes with various alkali metal halides (KCl, KBr, NaF, NaCl, or NaBr) as the mineralizing agent (MA). Regardless of the alkali metal halide selected, the addition of a small amount of MA permitted the preparation of DD3R membranes with a high separation selectivity of 190–380 at a pressure drop of 1.0 MPa. Addition of excessive amounts of MA caused gelling of the solution and a decrease in the separation selectivity. The presence of a few alkali metal cations promotes the growth of zeolite crystals on the support and permits the synthesis of dense DD3R zeolite membranes.

Micro-structure change of polycrystalline FAU zeolite membranes during a hydrothermal synthesis in a dilute solution

Faujasite (FAU) zeolite membranes were prepared by seed growth method. The polycrystalline zeolite membrane became less porous with the growth of zeolite crystals. The microstructure of the membrane was characterized by SEM and TEM after several rates of crystal growth. In addition, Si/Al ratio was analysed by TEM-EDS. Meso-voids existed in the zeolite membrane after 1-h synthesis, and hexagonal shaped FAU crystals were easily distinguished. Twin relations were quite often observed for the connection between FAU crystals, while they were seldom found in the seed crystals. On the other hand, crystals lost hexagonal shape with further synthesis time. Moreover, the density of twin relations became less. While voids could hardly being observed by SEM in the zeolite membrane, there existed intercrystalline boundaries formed by two crystals of different orientation. Microstructures of these boundaries were directly observed by HREM. Si/Al ratio of the zeolite membrane was close to 1, which is smaller than that of seed crystals (Si/Al = 2.5). Step growth at the twin boundary was firstly observed. This growth process is assumed to be the key growth mechanism in densification of the polycrystalline zeolite membrane. Finally, densification process of zeolite membrane is proposed based on the microscopic analysis.

Preparation of high‐performance zeolite NaA membranes in clear solution by adding SiO2 into Al2O3 hollow‐fiber precursor

Zeolite NaA membranes were prepared in a clear synthesis solution without the aid of nanoseeds. To improve the properties of the membranes formed in a clear solution, alumina hollow fibers were fabricated by adding silica powder to the conventional spinning slurry, resulting in hollow fibers with a mullite phase. Prior to the membrane synthesis, the hollow fibers were pretreated by dipping in an aged synthesis solution diluted with isopropanol. Dense zeolite NaA membranes on mullite‐containing alumina hollow fibers were successfully obtained at 100°C for 2 h without the aid of nanoseeds. The membranes have a good pervaporation performance with a high flux of 10.8 kg m−2 h−1 and a separation factor of over 10,000. The abundant mullite‐phase hydroxyl groups on the support surface promote the nucleation and growth of zeolite crystals on the support, resulting in dense membranes. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2679–2688, 2018

Hierarchical ZSM-5 zeolite synthesized via dry gel conversion-steam assisted crystallization process and its application in aromatization of methanol

Hierarchical ZSM-5 zeolites were prepared by “Dry Gel Conversion-Steam Assisted Crystallization” (DGC-SAC) procedure without the involvement of additional template. Moreover, the formation mechanism and feather of mesopores were summarized. Nanoscale Kirkendall effect was existed in the growth of zeolite crystals under high SiO2/TPA+ ratio, which was responsible for the production of intra-crystal mesopores. However, small ZSM-5 nano-crystallites, with the average diameter of <50 nm, were successfully synthesized under low SiO2/TPA+ ratio. The random assembly of these nano-crystallites gave rise to abundant inter-crystal mesopores within these nano-crystallites zeolites. Benefiting from the significantly higher concentration of TPA+ at the surface of amorphous aluminosilicate during the DGC-SAC process, the DGC-SAC procedure exhibited higher usage efficiency of template (TPA+) than that of conventional “Hydrothermal Crystallization” (HC) route, and the ZSM-5 zeolites with a low consumption of TPA+ (SiO2/TPA+ = 53.7) could be successfully synthesized by DGC-SAC method. Compared with the conventional ZSM-5 zeolites prepared by HC routes, the hierarchical ZSM-5 zeolites prepared by DGC-SAC procedure exhibited significantly higher catalytic lifetime and selectivity of light aromatics (benzene (B), toluene (T) and xylene (X)) in aromatization of methanol under same low addition of TPA+ (SiO2/TPA+ = 13.5). Moreover, the produced hierarchical ZSM-5 zeolites (SiO2/TPA+ = 13.5) also exhibited superior catalyst stability after being further hydrothermally treated at 160 °C for 72 h. TG analysis of all used catalysts confirmed that the produced hierarchical ZSM-5 zeolites had higher coke capability and lower average rate of coke formation.

Nanoscale Control of Homoepitaxial Growth on a Two-Dimensional Zeolite.

Nanoscale crystal growth control is crucial for tailoring two-dimensional (2D) zeolites (crystallites with thickness less than two unit cells) and thicker zeolite nanosheets for applications in separation membranes and as hierarchical catalysts. However, methods to control zeolite crystal growth with nanometer precision are still in their infancy. Herein, we report solution-based growth conditions leading to anisotropic epitaxial growth of 2D zeolites with rates as low as few nanometers per day. Contributions from misoriented surface nucleation and rotational intergrowths are eliminated. Growth monitoring at the single-unit-cell level reveals novel nanoscale crystal-growth phenomena associated with the lateral size and surface curvature of 2D zeolites.

Nanoscale Control of Homoepitaxial Growth on a Two‐Dimensional Zeolite

AbstractNanoscale crystal growth control is crucial for tailoring two‐dimensional (2D) zeolites (crystallites with thickness less than two unit cells) and thicker zeolite nanosheets for applications in separation membranes and as hierarchical catalysts. However, methods to control zeolite crystal growth with nanometer precision are still in their infancy. Herein, we report solution‐based growth conditions leading to anisotropic epitaxial growth of 2D zeolites with rates as low as few nanometers per day. Contributions from misoriented surface nucleation and rotational intergrowths are eliminated. Growth monitoring at the single‐unit‐cell level reveals novel nanoscale crystal‐growth phenomena associated with the lateral size and surface curvature of 2D zeolites.