ZSM-5 Catalysts for MTO: Effect and Optimization of the Tetrapropylammonium Hydroxide Concentration on Synthesis and Performance

Light olefin production from methanol using various zeolite catalysts has industrial and economic importance considering the growth of the petrochemical market. Zeolites are generally synthesized using various organic templates as structure-directing agents (SDAs). In this study, synthesis of a series of ZSM-5 zeolites was performed systematically using the microwave-assisted crystallization method, and these samples were analyzed in detail to understand the effect of the SDA concentration. Powder diffraction, N₂ adsorption, scanning electron microscopy, ammonia adsorption desorption, and ²⁷Al and ²⁹Si NMR spectroscopies were used for the characterization. The organic SDA tetrapropyl ammonium hydroxide (TPAOH/SiO₂ mole ratio = 0.0500) is found to have an optimum concentration against the silica precursor for achieving the highest crystallinity, suitable morphology, ideal pore size, effective pore volume, and tuned microporous/mesoporous area. For samples with a template concentration ratio of 0.050 or higher, ²⁹Si and ²⁷Al NMR data revealed the presence of an intact ZSM-5 structure. Using a fixed bed reactor at 500 °C and atmospheric pressure, the catalytic performance of the selected catalysts from the series is investigated for the methanol-to-olefin conversion reaction. The sample with the highest crystallinity showed the best conversion, selectivity toward light olefins, and time on stream stability. It is also worth noting that the highest crystallinity, micropore area, and micropore volume are reached for the optimum value rather than the highest template concentration. This allows for a reduction in the template concentration and a move closer to a synthesis pathway benign to environment and economics.

Milling Activation for the Solvent-Free Synthesis of Zeolites. A Practical Guide

The paramount challenge of current chemical technology, including catalysis, is meeting the ecological sustainability requirements. The feasible production of zeolites is crucial because they constitute the principal group of heterogeneous catalysts. However, current zeolite manufacturing via hydrothermal synthesis is connected with considerable consumption of water resources, generation of wastewater, and significant equipment costs. Solvent-free strategies have attracted great attention as high-yielding methods for sustainable synthesis, particularly beneficial in terms of water consumption minimization and an outstanding increase in production due to the efficient use of synthesis space capacity. So far, the solvent-free preparation of numerous zeolite materials has been performed with the preceding grinding of reagents in a mortar. The proposed article describes recent advances in the application of automatized milling instead of manual work. Pretreatment automatization makes the entire process well-controlled, less sensitive to human factors, and is significantly more relevant to industrial scale-up. Moreover, the automatized milling activation unlocks opportunities for solvent-free synthesis from a broader range of common reagents, where the manual treatment is not efficient. Possibilities and limitations of the milling methods are discussed on the basis of a comprehensive analysis of parameter optimization, activation mechanisms, and applicability of common reagents used for zeolite production.

A Brief Review on Solvent-Free Synthesis of Zeolites

The traditional hydrothermal method to prepare zeolite will inevitably use a large amount of water as a solvent, which will lead to higher autogenous pressure, low efficiency, and wastewater pollution. The solvent-free method can be used to synthesize various types of zeolites by mechanical mixing, grinding, and heating of solid raw materials, which exhibits the apparent advantages of high yield, low pollution, and high efficiency. This review mainly introduces the development process of solvent-free synthesis, preparation of hierarchical zeolite, morphology control, synthesis mechanism and applications of solvent-free methods. It can be believed that solvent-free methods will become a research focus and have enormous industrial application potential.

State of the Art and Perspectives of Hierarchical Zeolites: Practical Overview of Synthesis Methods and Use in Catalysis

Microporous zeolites have proven to be of great importance in many chemical processes. Yet, they often suffer from diffusion limitations causing inefficient use of the available catalytically active sites. To address this problem, hierarchical zeolites have been developed, which extensively improve the catalytic performance. There is a multitude of recent literature describing synthesis of and catalysis with these hierarchical zeolites. This review attempts to organize and overview this literature (of the last 5 years), with emphasis on the most important advances with regard to synthesis and application of such zeolites. Special attention is paid to the most common and important 10- and 12-membered ring zeolites (MTT, TON, FER, MFI, MOR, FAU, and *BEA). In contrast to previous reviews, the research per zeolite topology is brought together and discussed here. This allows the reader to instantly find the best synthesis method in accordance to the desired zeolite properties. A summarizing graph is made available to enable the reader to select suitable synthesis procedures based on zeolite acidity and mesoporosity, the two most important tunable properties.

Hierarchical H-ZSM5 zeolites based on natural kaolinite as a high-performance catalyst for methanol to aromatic hydrocarbons conversion

The present work introduces a good prospect for the development of hierarchical catalysts with excellent catalytic performance in the methanol to aromatic hydrocarbons conversion (MTA) process. Hierarchical H-ZSM5 zeolites, with a tailored pore size and different Si/Al ratios, were synthesized directly using natural kaolin clay as a low-cost silica and aluminium resource. Further explored for the direct synthesis of hierarchical HZSM-5 structures was the steam assisted conversion (SAC) with a cost-effective and green affordable saccharide source of high fructose corn syrup (HFCS), as a secondary mesopore agent. The fabricated zeolites exhibiting good crystallinity, 2D and 3D nanostructures, high specific surface area, tailored pore size, and tunable acidity. Finally, the catalyst performance in the conversion of methanol to aromatic hydrocarbons was tested in a fixed bed reactor. The synthesized H-ZSM5 catalysts exhibited superior methanol conversion (over 100 h up to 90%) and selectivity (over 85%) in the methanol conversion to aromatic hydrocarbon products.

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.

Morphology-Controlled Synthesis of H-type MFI Zeolites with Unique Stacked Structures through a One-Pot Solvent-Free Strategy

H-type aluminosilicate zeolites are extensively used as solid-acid catalysts and support materials in industrial catalysis. However, the conventional synthesis methods involving hydrothermal syntheses and ion-exchange processes suffer from severe water pollution and toxic gas emissions. Herein, H-type MFI zeolite catalysts with a unique stacked structure were directly synthesized in the presence of NH₄ F and with the help of zeolite confinement through a solvent-free route without further ion-exchange procedures. A range of ex situ and in situ characterization procedures were used to provide evidence of the simultaneous use of pre-made ZSM-5 and NH₄ F as a confined Al source and mineralizer, respectively. The confined zeolite framework of ZSM-5 prevented the formation of AlF_x species between NH₄ F and Al atoms, ensuring that the prepared samples had desirable acidic properties. Moreover, the resulting morphology could be controlled by using different silica substrates. The obtained H-type MFI zeolites showed excellent catalytic performance in methanol-to-gasoline reactions owing to their unique structure and directly exposed acidic sites. The developed one-pot strategy provides an alternative method for the facile synthesis of H-type zeolites with defined morphology.

Synthesis and catalytic performance of hierarchically structured MOR zeolites by a dual-functional templating approach

Novel hierarchical MOR zeolites have been successfully synthesized via a one-step dual-functional templating strategy utilizing gemini organic surfactant (C_18-2-8) through hydrothermal process. After a period of ∼96 h for crystallization, the hierarchy MOR zeolite with a larger BET (412.0 m²/g), abundant intracrystalline mesopores (average mesopore size distribution of 4.55 nm), and more accessible acid sites can be synthesized. The XRD study revealed a long range structural ordering of mesoporous and a good crystallinity of microporous structure. The results indicated that the surfactant acted as a dual-functional template for generating both micropores and mesopores simultaneously. Compared with conventional MOR zeolite, hierarchically structured MOR zeolite not only has higher activity and stability, but also can avoid side-reaction taking place in ethanol dehydration reactions. This hierarchical micro/mesostructured mordenite zeolite may be a candidate for practical industrial applications especially in those reactions where bulky molecules are involved.

Controllable fabrication of single-crystalline, ultrafine and high-silica hierarchical ZSM-5 aggregates via solid-like state conversion

Single-crystalline, ultrafine and high-silica HA-ZSM-5 aggregates were directly synthesizedviaSSC method using HTAB as the mesoporogen through a S-1 seed-induced route.

Methanol to gasoline over zeolite ZSM-5: improved catalyst performance by treatment with HF

Zeolite ZSM-5 (SiO₂/Al₂O₃ = 50) has been treated with hydrofluoric acid (HF) solutions, characterized with many techniques and studied in the conversion of methanol to gasoline (MTG).