Mechanochemically assisted hydrothermal synthesis of Sn-substituted MFI-type silicates

Synthesis strategies to control the Al distribution in zeolites: thermodynamic and kinetic aspects

The activity and selectivity of acid-catalyzed chemistry is highly dependent on the Brønsted and Lewis acid sites generated by Al substitutions in a zeolite framework with the desired pore architecture. The siting of two Al atoms in close proximity in the framework of high-silica zeolites can also play a decisive role in improving the performance of redox catalysts by producing exchangeable positions for extra-framework multivalent cations. Thus, considerable attention has been devoted to controlling the Al incorporation through direct synthesis approaches and post-synthesis treatments to optimize the performance as (industrial) solid catalysts and to develop new acid- and redox-catalyzed reactions. This Feature Article highlights bottom-up synthetic strategies to fine-tune the Al incorporation in zeolites, interpreted with respect to thermodynamic and kinetic aspects. They include (i) variation in extra-framework components in zeolite synthesis, (ii) isomorphous substitution of other heteroatoms in the zeolite framework, and (iii) control over the (alumino)silicate network in the initial synthesis mixture via in situ and ex situ methods. Most synthetic approaches introduced here tentatively showed that the energy barriers associated with Al incorporation in zeolites can be variable during zeolite crystallization processes, occurring in complex media with multiple chemical interactions. Although the generic interpretation of each strategy and underlying crystallization mechanism remains largely unknown (and often limited to a specific framework), this review will provide guidance on more efficient methods to prepare fine-tuned zeolites with desired chemical properties.

Zr-doped SAPO-34 with enhanced Lewis acidity

Zr-doped SAPO-34 has enhanced Lewis acidity, leading to high catalytic activity for LDPE cracking.

Transcription-induced formation of paired Al sites in high-silica CHA-type zeolite framework using Al-rich amorphous aluminosilicate

The paired Al species pre-formed in Al-rich amorphous aluminosilicates were transcribed into high-silica CHA-type zeolite frameworks under hydrothermal conditions, which offers a new approach to creating paired Al sites in zeolite frameworks. This Al-pair-rich CHA exhibited a higher Sr²⁺ uptake than the control CHA zeolite synthesized by the conventional procedure.

New avenues for mechanochemistry in zeolite science

Zeolites are a class of microporous materials with tremendous value for large scale industrial applications such as catalysis, ion exchange, or gas separation. In addition to naturally ocurring variants, zeolites are made synthetically using hydrothermal synthesis, requiring temperatures beyond 100 °C and long reaction times up to weeks. Furthermore, specific applications may require more sophisticated synthesis conditions, expensive reagents, or post-synthetic modifications. Some of these issues can be tackled by using the reemerged technique of mechanochemistry. In 2014, Majano et al. reviewed the space and outlined several possibilities for the usage of mechanical forces in zeolite chemistry. Since then the field has seen many more publications employing mechanochemical methodology to further and improve the synthesis and properties of zeolite materials. The usage ranges from the activation of raw materials, rendering the synthesis of the widely used catalysts much more economical in terms of duration, atom efficiency, and production of waste, to post-synthetic modification of the materials leading to improved properties for target aplications. We present a short review of the advances that have been reported recently, highlight promising work and important studies, and give a perspective of potential future endeavours.

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.

Control of location and distribution of heteroatoms substituted isomorphously in framework of zeolites and zeotype materials

Well-controlled incorporation of heteroatoms in frameworks of zeolites and zeotype materials has been achieved by a variety of new synthetic approaches, generating outstanding catalysts compared to uncontrolled materials.