Seed-Induced Crystallization of Nanosized Na-ZSM-5 Crystals

OSDA-Free Seeded Cu-Containing ZSM-5 Applied for NH3-SCR-DeNOx

A series of ZSM-5 zeolite materials were synthesized from organic structure-directing agent (OSDA)-free seeded systems, including nanosized silicalite-1 (12 wt % water suspension or in powder form) or nanosized ZSM-5 (powder form of ZSM-5 prepared at 100 or 170 °C). The physicochemical characterization revealed aggregated species in the samples based on silicalite-1. Contrarily, the catalysts based on ZSM-5 seeds revealed isolated copper species, and thus, higher NO conversion during the selective catalytic reduction of NOx with NH3 (NH3-SCR-DeNOx) was observed. Furthermore, a comparison of the Cu-containing ZSM-5 catalysts, conventionally prepared in the presence of OSDAs and prepared with an environmentally more benign approach (without OSDAs), revealed their comparable activity in NH3-SCR-DeNOx.

Seed-Assisted Synthesis of Hierarchically Structured Nano-Sized Ti-β Zeolites for the Efficient Epoxidation Reaction of Alkenes

The development of nano-sized titanosilicate zeolites with hierarchical structures is crucial in promoting the efficient epoxidation of alkenes. In the present work, nano-sized hierarchical Ti-β (*BEA) zeolites with high crystal yield are prepared by a one-pot nanoseed-assisted approach. The influence of seed size on the resultant Ti-β zeolites is investigated by complementary characterizations, including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), N₂ adsorption/desorption, UV-vis diffuse reflectance spectroscopy (DRS), and UV Raman spectroscopy. The possible process for the formation of hierarchical Ti-β nanocrystals with the assistance of nanoseeds in the synthesis gel is proposed. Consequentially, the nano-sized hierarchical material prepared by the nanoseed-assisted method shows excellent mass transportation and accessibility to active sites by reducing particle size and constructing hierarchical porosity, hence showing a remarkably enhanced catalytic activity and selectivity in the epoxidation reaction of alkenes. This work will shed light on the efficient preparation of nano-sized titanosilicate zeolites.

Size controllable synthesis of ZSM-5 zeolite and its catalytic performance in the reaction of methanol conversion to aromatics

ZSM-5 zeolites were hydrothermally synthesized with commercial silica sol, and the crystal size was controlled by adding silicalite-1 seed in the synthetic system. The crystal size of ZSM-5 was affected by the crystallization time of seed, seed content and seed size. ZSM-5 zeolites with controllable particle size in the range of 200-2200 nm could be obtained. The prepared samples with different particle sizes were used for the reaction of methanol conversion to aromatics (MTA). The results suggested that the HZSM-5 catalyst with small crystal size showed much longer catalyst lifetime and higher selectivity for C₅ ⁺ hydrocarbons and aromatics, especially C₉ ⁺ aromatics in the MTA reaction. Moreover, the introduction of zinc (Zn) into the HZSM-5 zeolites can considerably promote the selectivity to aromatics in the products. Zn modified HZSM-5 zeolites with different Zn loading amounts were prepared by the incipient wetness impregnation method, and the highest aromatics selectivity was obtained when the Zn loading was 1.0%. The improvement of methanol aromatization was ascribed to the synergistic effect of Brønsted acid sites and the newly formed Zn-Lewis acid sites.

Controlling Nucleation Pathways in Zeolite Crystallization: Seeding Conceptual Methodologies for Advanced Materials Design

A core objective of synthesizing zeolites for widespread applications is to produce materials with properties and corresponding performances that exceed conventional counterparts. This places an impetus on elucidating and controlling processes of crystallization where one of the most critical design criteria is the ability to prepare zeolite crystals with ultrasmall dimensions to mitigate the deleterious effects of mass transport limitations. At the most fundamental level, this requires a comprehensive understanding of nucleation to address this ubiquitous materials gap. This Perspective highlights recent methodologies to alter zeolite nucleation by using seed-assisted protocols and the exploitation of interzeolite transformations to design advanced materials. Introduction of crystalline seeds in complex growth media used to synthesize zeolites can have wide-ranging effects on the physicochemical properties of the final product. Here we discuss the diverse pathways of zeolite nucleation, recent breakthroughs in seed-assisted syntheses of nanosized and hierarchical materials, and shortcomings for developing generalized guidelines to predict synthesis outcomes. We offer a critical analysis of state-of-the-art approaches to tailor zeolite crystallization wherein we conceptualize whether parallels between network theory and zeolite synthesis can be instrumental for translating key findings of individual discoveries across a broader set of zeolite crystal structures and/or synthesis conditions.

Realization of rapid synthesis of H-ZSM-5 zeolite by seed-assisted method for aromatization reactions of methanol or methane

H-ZSM-5 zeolites are successfully synthesized within 1 h with an organic template free system by the seed-assisted method. The synthetic method not only reduces the production cost of H-ZSM-5 zeolite, but also decreases the environmental pollution produced compared with that of the conventional synthesis process. Characterization by SEM, XRD, N2 adsorption–desorption, NH3-TPD, and Py-IR confirms that the H-ZSM-5 product has a high crystallinity and reveals that the rapid synthesis process is in accordance with the seed surface crystallization mechanism. To improve the aromatic selectivity on the H-ZSM-5 zeolite, different concentrations of Mo and Zn catalysts were prepared by incipient wetness impregnation and the ion-exchange method. Mo-Zn/H-ZSM-5-SAS-1 h have similar catalytic lifetimes and higher selectivity of paraxylene in aromatic products compared with Mo-Zn/H-ZSM-5-seed in the reaction of methanol to aromatic. However, the catalytic lifetime of 6%Mo/H-ZSM-5-SAS-1 h is shorter than that of the 6%Mo/H-ZSM-5-seed catalyst in methane to benzene reaction under the same reaction conditions.

Efficient Synthesis of Mesoporous Nano ZSM-5 Zeolite Crystals without a Mesoscale Template

Hierarchical zeolites attract people’s attention due to their enhanced mass transfer properties in catalytic reactions. The research over obtaining these zeolites in green and efficient methods is of great significance for the current post-treatments and templating strategies tend to be costly for hierarchical zeolite synthesis. In this research, nanosized mesoporous ZSM-5 (SN-ZSM-5) zeolites have been synthesized without the addition of mesoscale templates under highly concentrated conditions. The physicochemical characteristics were systematically investigated by XRD, SEM, N2 sorption, TEM, and NMR. The SEM images showed ZSM-5 crystals with sizes between 50–110 nm were obtained. N2 sorption and high-resolution TEM images gave direct evidence for the hierarchical structure of SN-ZSM-5. The forming mechanism for the hierarchical structure was proposed that the etching effect of the highly alkaline environment for the starting gels played a critical role in the formation of hierarchical structure. Catalytic tests in methanol-to-olefins (MTO) showed the SN-ZSM-5 performed a longer catalyst lifetime and higher propylene selectivity than the conventional ZSM-5 zeolites (C-ZSM-5) obtained from a traditional hydrothermal method. The features of hierarchical structure in the SN-ZSM-5 crystals and the sustainability for synthesis method could show a promising choice for wide applications of these SN-ZSM-5 zeolites in the future.

Synthesis of hierarchical GaZSM-5 zeolites by the HCl treatment method and their catalytic performance in methanol aromatization

A series of ZSM-5 samples exhibiting isomorphic substitution with Ga and containing both micropores and mesopores were synthesized by a posttreatment method involving different concentrations of hydrochloric acid.

Engineering of zeolite BEA crystal size and morphology via seed-directed steam assisted conversion

The mechanism of seeding of zeolite BEA via steam assisted conversion has been studied using BEA seeds with different composition. The catalysts are characterized by X-ray diffraction, scanning and transmission electron microscopy, nitrogen adsorption-desorption, Hg-porosimetry, X-ray fluorescence and TPD of ammonia, and evaluated in benzene alkylation with propene. The results show that variation of the SiO₂/Al₂O₃ ratio from 25 to 250 in BEA seeds changes the mechanism of seeding from "core-shell" to a "dissolution" mechanism, which can serve as a tool for engineering the morphological, textural and catalytic properties of BEA zeolites. Al-rich seeds (SiO₂/Al₂O₃ = 25) do not dissolve during gel preparation and initiate dense oriented crystal growth on their surface resulting in the formation of large polycrystals (1-2 μm) with ordered densely intergrown nanocrystallites. In contrast, Si-rich seeds (SiO₂/Al₂O₃ = 250) dissolve into tiny fragments, which serve as individual nuclei leading to formation of tiny isolated nanocrystallites aggregated into small hierarchical aggregates with high intercrystalline mesoporosity. The decrease of particle size and formation of intercrystalline mesoporosity in hierarchical aggregates improves the accessibility of acidic sites and facilitates the diffusion of reaction products, which leads to the significant improvement of catalytic activity and reduces the deactivation resulting in higher stability with time on stream in cumene synthesis from benzene and propylene.

Investigation of synthesis time and type of seed along with reduction of template consumption in the preparation of SAPO-34 catalyst and its performance in the MTO reaction

SAPO-34 catalysts were synthesized through the seeding approach under different seed conditions. The different seed synthesis times (6 h, 12 h, and 24 h) and three types of seeds were evaluated: the dried seed, the calcined seed, and the mother liquor from an unseeded synthesis, called the solution seed. Pure SAPO-34 was obtained using 12 h and 24 h solution seeds, in which a 40% reduction of template consumption was achieved simultaneously. All seeding induced samples represented higher catalytic performance in the MTO process than conventional SAPO-34 due to the smaller crystallite/particle sizes and larger external surface areas and mesopore volume. Furthermore, the changes in the acidity of samples affect their performance. The maximum olefin selectivity under industrial feed conditions (72 wt% methanol in water) was 91.79% for the sample prepared from the 12 h solution seed, which was 14.43% higher than the unseeded sample. Although this sample did not have the longest lifetime, it showed a 330 min lifespan, which was at least twice more than that of the conventional one (150 min). The sample prepared from the 6 h solution seed showed the longest lifetime of more than 500 min among all catalysts, although it was contaminated with a little SAPO-5.

SAPO-34 catalysts were synthesized through the seeding approach under different seed conditions.

Lower olefins from methane: recent advances

Modern methods for methane conversion to lower olefins having from 2 to 4 carbon atoms per molecule are generalized. Multistage processing of methane into ethylene and propylene via syngas or methyl chloride and methods for direct conversion of CH4 to ethylene are described. Direct conversion of syngas to olefins as well as indirect routes of the process via methanol or dimethyl ether are considered. Particular attention is paid to innovative methods of olefin synthesis. Recent achievements in the design of catalysts and development of new techniques for efficient implementation of oxidative coupling of methane and methanol conversion to olefins are analyzed and systematized. Advances in commercializing these processes are pointed out. Novel catalysts for Fischer – Tropsch synthesis of lower olefins from syngas and for innovative technique using oxide – zeolite hybrid catalytic systems are described. The promise of a new route to lower olefins by methane conversion via dimethyl ether is shown. Prospects for the synthesis of lower olefins via methyl chloride and using non-oxidative coupling of methane are discussed. The most efficient processes used for processing of methane to lower olefins are compared on the basis of degree of conversion of carbonaceous feed, possibility to integrate with available full-scale production, number of reaction stages and thermal load distribution.

The bibliography includes 346 references.

Revisiting the seed-assisted synthesis of zeolites without organic structure-directing agents: insights from the CHA case

Herein, the crystallization behaviour of the CHA zeolite synthesized via the seed-assisted method and in the absence of an organic structure-directing agent has been revisited.

Mechanochemically-assisted solvent-free and template-free synthesis of zeolites ZSM-5 and mordenite

Aluminosilicate-based zeolite materials, such as ZSM-5 and mordenite, are well-studied as catalysts. Typical approaches to synthesize these zeolites require either templates or seeds to direct ordered crystal growth and both of these are expensive and add to the complexity of zeolite synthesis. In this paper, we describe a solvent-free and template-free method to synthesize crystalline ZSM-5 and mordenite zeolites without any added seed crystals. Key to the success of this approach is a mechanochemical precursor pre-reaction step. High-energy ball-milling is used to initiate a solid-state metathesis (exchange) reaction between Na₂SiO₃ and Al₂(SO₄)₃ reagents, forming crystalline Na₂SO₄ and well-mixed aluminosilicate precursor. The solid precursor mixture is thermally converted to crystalline ZSM-5 or mordenite at moderate 180 °C temperatures without solvents or an organic amine structure directing template. Variations in Si/Al ratios in the precursor mixture and additions of solid NaOH to the mechanochemical reaction were found to influence the subsequent growth of either crystalline ZSM-5 or mordenite zeolites. The crystalline zeolites from this solvent-free and template free method have high ∼300 m² g^-1 surface areas directly from the synthesis without requiring high-temperature calcination. These materials are also comparably active to their commercial counterparts in cellulose and glucose biomass catalytic conversion to hydroxymethylfurfural.

Direct Synthesis of Aluminosilicate SSZ-39 Zeolite Using Colloidal Silica as a Starting Source

For the first time, SSZ-39 zeolite has been directly prepared using conventional colloidal silica and sodium aluminate instead of using FAU zeolite as the raw material in the alkaline media. The adjustment of the Si/Al ratios in the starting materials to the suitable values is a key factor to prepare the aluminosilicate SSZ-39 zeolite. Various characterizations (for instance, X-ray diffraction, scanning electron microscopy, nitrogen sorption, solid ²⁷Al NMR, and NH₃-temperature-programmed desorption) display that the aluminosilicate SSZ-39 zeolite owns high crystallinity, uniform cuboid morphology, large surface area, four-coordinated aluminum species, and strong acidic sites. Inductively coupled plasma analysis shows that the SiO₂/Al₂O₃ ratios of the SSZ-39 products are ranged from 12.8 to 16.8. Considering the special framework of the SSZ-39 zeolite, the yield of this synthesis is not higher than 21.3%. Moreover, the catalytic performance of Cu-SSZ-39 catalyst synthesized from this route is excellent in the selective catalytic reduction of NO _x with NH₃ (NH₃-SCR).

New trends in tailoring active sites in zeolite-based catalysts

This review discusses approaches for tailoring active sites in extra-large pore, nanocrystalline, and hierarchical zeolites and their performance in emerging catalytic applications.

Zeolitic intralayer microchannels of magadiite, a natural layered silicate, to boost green organic synthesis

Despite the considerable attention given to the applications of magadiite in previous research, the properties of this natural layered silicate have remained mysterious due to the lack of crystal structure information. On the other hand, no one has doubted the intercalation capability between the layers. Here we succeed in determining the structure of magadiite using X-ray pair distribution functions and synchrotron powder diffractometry. We discover unexpected zeolitic microchannels within the layers. We describe efficient synthesis of 100% pure benzoic acid from toluene by using magadiite as an additive in a TiO2 photocatalytic system oxidizing toluene. Based on the uncovered structure of magadiite, we clarify the mechanism of this unique photocatalytic system: the microchannels of magadiite not only separate/accommodate the desired partially oxidized product formed on TiO2 but also prevent the accumulation of the overoxidized products on the TiO2 surface that deactivates the photocatalytic activity.

From 3D to 2D zeolite catalytic materials

Research activities and recent developments in the area of three-dimensional zeolites and their two-dimensional analogues are reviewed. Zeolites are the most important industrial heterogeneous catalysts with numerous applications. However, they suffer from limited pore sizes not allowing penetration of sterically demanding molecules to their channel systems and to active sites. We briefly highlight here the synthesis, properties and catalytic potential of three-dimensional zeolites followed by a discussion of hierarchical zeolites combining micro- and mesoporosity. The final part is devoted to two-dimensional analogues developed recently. Novel bottom-up and top-down synthetic approaches for two-dimensional zeolites, their properties, and catalytic performances are thoroughly discussed in this review.

Anti-cancer activity of hierarchical ZSM-5 zeolites synthesized from rice-based waste materials

We report the organic template-free synthesis of hierarchical ZSM-5 zeolite using waste materials, by a simple hydrothermal method and their excellent anti-cancer activity against A549 human lung epithelial cell lines.

Hierarchical ZSM-5 nanocrystal aggregates: seed-induced green synthesis and its application in alkylation of phenol with tert-butanol

Hierarchical ZSM-5 zeolite aggregates were synthesized in an organic-template-free system via seed-induced crystallization. The obtained samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N₂ adsorption–desorption, NH₃ temperature-programmed desorption (NH₃-TPD) and inductively coupled plasma atomic emission spectrometry (ICP-AES). The prepared ZSM-5 nanocrystalline aggregates possessed open/accessible hierarchical pores and active sites, showing significant advantages in the catalytic alkylation of phenol with tert-butanol. The obtained materials could maintain the activities of the nanocrystal zeolites and meanwhile could be easily separated or recovered during the preparation and reactions. This approach was simple and also overcame the commonly-seen drawbacks such as the exceeded use of specific templates or secondary templates during the synthesis of the hierarchical zeolites.

Hierarchical ZSM-5 zeolite aggregates were synthesized in an organic-template-free system via seed-induced crystallization.

Effect of different seed sources on the hydrothermal crystallization of MCM-22 zeolite catalysts

Ball-milled MCM-22 and dried mother liquor seeds proved to be effective in the synthesis of the MCM-22P lamellar precursor with a low Si/Al ratio. Crystallization times were reduced from 25 to 2–4 days with a considerable decrease in crystallite sizes.

Vapor-phase transport synthesis of microfibrous-structured SS-fiber@ZSM-5 catalyst with improved selectivity and stability for methanol-to-propylene

Microfibrous-structured SS-fiber@HZSM-5 catalyst prepared by cost-effective and high-efficiency VPT method delivers remarkable improvement in selectivity and stability for the MTP reaction due to the improved diffusion in zeolite shell.

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.

Direct synthesis of hierarchical ZSM-5 zeolite using cetyltrimethylammonium as structure directing agent for methanol-to-hydrocarbons conversion

KOH and RbOH are suitable bases to crystallize hierarchical ZSM-5 in one step using cetyltrimethylammonium as the only structure-directing agent.