New avenues for mechanochemistry in zeolite science

Abstract

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.

Fig. 1. Schematic overview of several advantageous avenues for mechanochemical methods in zeolite science.

Fig. 2. Schematic illustration of mechanochemical pre-treatment of reagents prior to crystallisation.

Fig. 3. PXRD patterns of samples prepared without milling (a, bottom, black), 25 min (b, middle, red) and 50 min ball milling (c, top, blue). The top panel shows materials after the mechanochemical treatment, the bottom panel the respective products after heating in an autoclave for various time periods, showcasing the importance of sufficient mechanical activation of the precursors prior to crystallisation at elevated temperature in the synthesis of ZSM-5. Reproduced from ref. with permission from the Royal Society of Chemistry.

Fig. 4. X-ray diffraction and scanning electron microscopy analysis of a zincosilicate zeolite with MFI framework. The top panel shows PDF analysis of the ball milled precursors, evidencing a mixed silicon-zinc oxide. PXRD patterns are shown in the middle, comparing experiments with varying amounts of zinc (middle panel). The bottom panel shows SEM images of ball milled precursor (A) as well as samples obtained after 1, 2, and 3 days of hydrothermal synthesis (B, C, and D, respectively). Reproduced from ref. with permission from Elsevier, copyright 2019.

Fig. 5. PXRD patterns and SEM images of experiments using ball milling for the treatment of reagents and zeolite seed crystals prior to steam treatment. SEM images show the precursors silica powder (a), sodium aluminate (b) and ZSM-5 seed crystals (c), as well as materials obtained after milling (d) and final products after 6 h (e), 12 h (f), and 18 h (g) of steam treatment. Reproduced from ref. with permission from Wiley-VCH, copyright 2017.

Fig. 6. Schematic illustration of the preparation of nano sized ZSM-5 zeolite crystals by ball milling and subsequent recrystallisation (top panel) and SEM images of zeolite A (LTA) crystals: pristine (a), post milling (b) and after subsequent recrystallisation (c and d). Adapted and reproduced with permission from ref. Copyright (2011) American Chemical Society.

Fig. 7. Introduction of mesoporosity into silicalite-1 (MFI) crystals by grinding the zeolite in the presence of ammonium fluoride and an organic templating agent and subsequent heat treatment. The left panel shows TEM images of a pristine zeolite crystal (a) as well as a ground example (b) and the final product (c). PXRD patterns (d), nitrogen adsorption (e), and 19F MAS NMR (f). The righthand panel depicts STEM images and elemental maps of similarly treated mesoporous silicalite-1, previously impregnated using platinum (a and b) or cobalt (c and d) precursors to produce active catalyst materials. Reproduced from ref. with permission from the Royal Society of Chemistry.

Fig. 8. Hydrolysis of zeolite UTL using a ball milling assisted method using comparatively low amounts of hydrochloric acid. PXRD patterns and d₂₀₀-spacing vs acid concentration are shown on the left-hand side. The right panel shows the known structures of the parent zeolite UTL and relevant daughter phases, as well as TEM images of materials obtained with 25 mL 12 M HCl, showing lattice fringes indicative of the PCR framework. Adapted and reproduced from ref. with permission from the Royal Society of Chemistry.

Daniel N. Rainer

Russell E. Morris