Understanding Zeolite Growth Mechanism And Designing Rational Synthesis Strategies Of Hierarchical Zeolites

How to promote the economic, social and human development while ensuring ecosystem sustainability are the emerging tasks and challenges we faced. Characterized by a regular channel system of interconnected micropores with molecular dimensions below2nm and tailorability of pore size and surface functional groups, as well as framework stability, zeolites have been widely applied in industry as catalysts, adsorbents and ion exchangers. To meet the growing demand for ecologically sustainable solutions to global issues such as enhancing energy demand, reduction of resources, environmental protection, and human health improvement, new zeolite architectures such as hierarchically porous structures, nanosized crystals, self-standing monoliths, membranes and hollow microspheres have become one of new trends in the synthesis of zeolite materials. Although there have been many routes suitable to obtain the new architectures with novel advanced properties, rational synthesis strategies still remain a great challenge due to unclear understanding their formation mechanism.Understanding of zeolite crystal growth mechanism is essential for designing and producing better zeolite materials with desired physical and chemical properties, but still remains elusive. To uncover the zeolite nucleation and growth mechanisms, we chose a sodium-rich dense hydrogel system, used the synthesis of ZSM-5zeolite monolith mentioned above, as a model system and conducted SEM and TEM studies in combination with AFM, XRD,27Al-MAS NMR, N2and Ar adsorption measures and element analysis of the samples collected at different synthesis times. Experimental findings of zeolite crystal evolution from sodium-rich hydrogel revealed that the zeolite nucleation occurs at the equilibrated gel phase of the condensed primary aggregates precipitated from the dissolved (alumio)silicate species. The nuclei produced from the nucleation could be diffused into the liquid-solid interface of the equilibrated gel phase and the liquid phase. The zeolite growth therefore occurs through a synergistic mechanism of two growth processes:a solution-mediated process and a solid-state transformation. In the liquid phase and the liquid-gel (equilibrated gel) interface, the oriented aggregation governs in early stages of the zeolite growth. The major driving force for the aggregation is electrostatic force between the positively charged active Na+and the negative charges of the (T)O groups on the surface of the nuclei and growing nanocrystals. In the last steps the crystal growth by the coalescence and the Ostwald rule becomes predominant.Although assemblies of molecular and supramolecular templates have proven very successful for fabricating both zeolites and mesoporous molecular sieves, the combination of crystalline zeolite micropores into an ordered mesopore network by cooperative assembly of two different templating systems is challenging because of the phase separation between the mesoporous material and zeolite phases. Here, we designed a simple and general alkali-assisted template strategy which allows to solve this problem. With this strategy, we have successfully synthesized the highly ordered mesoporous zeolite with the2D hexagonal symmetry mesospores and MFI zeolitic framework walls. This strategy also allows to prepare the oriented mesoporous ZSM-5as well as self-standing hierarchical zeolite monoliths composed of the MFI nanosheet. The materials are believed to have promising catalytic activities for organic reactions involving bulky molecules.An efficient way to overcoming the diffusion and mass transfer limitations of the zeolite microporous network system for bulky molecules is to assemble nanosized zeolites into hierarchical materials. The existing strategies for the synthesis of such hierarchical materials require the use of at least one organic structure-directing agent. Designing and developing new more environmentally-friendly and more economical synthesis routes to assemble the nanocrystals into mechanically stable, hierarchical monoliths are thus significantly important for the hierarchical zeolite materials to be applied in heterogeneous catalysis and separation processes on an industrial scale. On the basis of our proposed synergistic mechanism, we designed successfully an organic-template-free, zeolite-seed-free direct route for the first synthesis of self-standing hierarchical zeolite monoliths composed of nanorod assembled mordenite (MOR) zeolites and hollow nanorod MOR zeolite assembled spheres. This route also is used successfully for the synthesis of the self-standing hierarchical ZSM-5zeolite monoliths.In order to effectively utilizezeolite micropores for the special applications as host materials for guest molecules in optical, magnetic, and electronic devices as well as fundamental studies such as precise crystal structure determination and the measurements of intracrystalline adsorption, diffusion and reactivity of guest molecules, the synthesis of large single crystals is required. To date, most existing methods for fabricating large zeolite crystals are based on repressing the zeolite nucleation and controlling the slow growing rate using toxic fluorine species. These require longer crystallization time from days to weeks and difficultly avoids the crystal growth of the second nuclei resulting in the uniformity of the crystals is difficultly obtained. Furthermore the negative effects of the use of fluorine on environment is unavoidably. We have proposed a new simple, rapid, fluorine-free, organic-template-free strategy for the synthesis of the large and uniform crystals of Na-P1(GIS), MOR and analcime (ANA) zeolites. This strategy is based on catalyzing simultaneously both the zeolite nucleation and crystal growth processes by NaOH with a high content. This rout allows saving time and energy consumption and avoiding the use of toxic fluorine species as well as organic templates. In addition, we found that the formation of ANA zeolite is result from the transformation of GIS zeolite and the mechanism of the transformation is the combination of the solid-state and solution-mediated transformations. The mechanism allows us design the hierarchical hollow or yolk@shell structures without template.Zeolite materials with ordered macroporous and/or mesoporous structures hold promise not only for advanced catalysis and separation but also advanced applications such as medicines, photonics, photoelectronics, and other emerging nanotechnologies. The existing methods of synthesis of ordered mesoporous zeolites are based on assembly of structure-directing agents. Here we demonstrated a disassembly route to generating the oriented macropores in zeolite crystals. Our route is based on the direct disassembly of zeolitic building blocks formed during the crystal growth process by NaOH. The orientation of the ordered mesopores in the crystals of fabricated MOR and Na-P1(GIS) zeolites with their good hydrothermal stability could make them interesting material platforms for fundamental studies of the structure-properties correlation and the special applications.