Solvent-Free Synthesis of Zeolites: Mechanism and Utility

Abstract

Zeolites have been extensively studied for years in different areas of chemical industry, such as shape selective catalysis, ion-exchange, and gas adsorption and separation. Generally, zeolites are prepared from solvothermal synthesis in the presence of a large amounts of solvents such as water and alcohols in sealed autoclaves under autogenous pressure. Water has been regarded as essential to synthesize zeolites for fast mass transfer of reactants, but it occupies a large space in autoclaves, which greatly reduces the yield of zeolite products. Furthermore, polluted wastes and relatively high pressure due to the presence of water solvent in the synthesis also leads to environmental and safety issues. Recently, inspired by great benefits of solvent-free synthesis, including the environmental concerns, energy consumption, safety, and economic cost, researchers continually challenge the rationale of the solvent and reconsider the age-old question "Do we actually need solvents at all in zeolite synthesis?" In this Account, we briefly summarize our efforts to rationally synthesize zeolites via a solvent-free route. Our research demonstrates that a series of silica, aluminosilicate, and aluminophosphate-based zeolites can be successfully prepared by mixing, grinding, and heating starting solid materials under solvent-free conditions. Combining an organotemplate-free synthesis with a solvent-free approach maximizes the advantages resulting in a more sustainable synthetic route, which avoids using toxic and costly organic templates and the formation of harmful gases by calcination of organic templates at high temperature. Furthermore, new insights into the solvent-free crystallization process of zeolites have been provided by modern techniques such as NMR and UV-Raman spectroscopy, which should be helpful in designing new zeolite structures and developing novel routes for synthesis of zeolites. The role of water and the vital intermediates during the crystallization of zeolites have been proposed and verified. In addition to a significant reduction in liquid wastes and a remarkable increase in zeolite yields, the solvent-free synthesis of zeolites exhibits more unprecedented benefits, including (i) the formation of hierarchical micro-, meso-, and macrostructures, which benefit the mass transfer in the reactions, (ii) rapid synthesis at higher temperatures, which greatly improve the space-time yields of zeolites, and (iii) construction of a novel catalytic system for encapsulation of metal nanoparticles and metal oxide particles within zeolite crystals synergistically combining the advantages of catalytic metal nanoparticles and metal oxide particles (high activity) and zeolites (shape selectivity). We believe that the concept of "solvent-free synthesis of zeolites" would open a door for deep understanding of zeolite crystallization and the design of efficient zeolitic catalysts.

Solvent‐Free Synthesis of Zeolitic Imidazolate Frameworks and the Catalytic Properties of Their Carbon Materials

“…[5,22,23] Recently, there was encouragingp rogress for the synthesis of zeolitest hroughasolvent-freer oute, where solvents are fully avoided. [24] Similart ot he solvent-free zeolite synthesis, as olvent-free preparation of aZ IF-8 film from the solid interfaceb etween deposited zinc oxide precursors and 2methylimidazole ligand was reported. [25,26] The deposition of zinc oxide precursors was relatively complex.…”

Protective dissolution: generating secondary pores in zeolite by mechanochemical reaction

“…However, transformation from TPABr to NH 4 Br was not detected in the solvent-free crystallization. 31,32 This is probably because in crystallization process the TPA + is pre-surrounded by silica precursor; while in dissolution process TPA + is kept outside the framework due to diffusion limit rather than being surrounded by zeolitic framework as a template. Qin has demonstrated the strong etching effect of NH 4 F solution to form cleavages and hollow mosaic structures inside MFI zeolites, 22 but when NH 4 F was used alone in solvent-free condition, dissolution only happens on the surface even at elevated temperatures, with the resultant zeolite becoming roughly rounded or even "melted" to merge with each other (Fig.…”

Mechanochemical Pretreatment for Efficient Solvent‐Free Synthesis of SSZ‐13 Zeolite

“…[13] In particular, high-silica CHA (SSZ-13) was preparedb yi nter-zeolite transformation of FAUs tructure in the presence of seeds and the bromide form of OSDA, without addition of solvent, but with water molecules occluded in the pores of the startingz eolite FAU. [14] The current advances in solvent-free synthesis of zeolites are summarized in the work by Wu et al, [15] in which the authorsp oint out that it is not clear how the scale-up of the described processesm ight be achieved for industrial catalyst production.T he possible solution would be an automatization of the pretreatment. As ar eplacement for manualg rinding, the application of milling with the planetary mill makesi tp ossible to keep as et of the reproducible parameters and minimizes the humanf actors connected with manual procedures on al aboratory scale.…”