Developing Organic Free Crystallization Pathways for the Optimization of Zeolite Catalysts



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Zeolites are an essential class of crystalline porous materials with a wide range of applications. A core objective of optimizing zeolites is to produce materials with physicochemical properties and corresponding performances that exceed conventional counterparts. This places an impetus on elucidating and controlling processes of crystallization where one of the most critical design criteria is the ability to prepare zeolite crystals with ultrasmall dimensions to mitigate the deleterious effects of mass transport limitations. Zeolite crystallization predominantly occurs by nonclassical pathways involving the attachment of complex (alumino)silicate precursors to crystal surfaces, yet recurrent images of fully crystalline materials with layered surfaces comprised of nanometer-sized steps are evidence that growth also occurs by a classical route of molecule (monomer) attachment. Recent studies have shown that a controlled switch from nonclassical to classical pathways can alter the anisotropic rates of crystallization with concomitant impact on material properties that affect their performance in commercial applications; however, few studies identify conditions under which zeolites grow by a purely classical mechanism. Seed-assisted approaches in zeolite synthesis differ from classical processes in that the seeds tend to dissolve, giving rise to an unknown memory of the parent crystal structure that facilitates the nucleation of the daughter. It has been hypothesized in literature that a shared structural feature, such as a composite building unit, between the parent and the crystals produced from a non-seeded growth solution results in identical parent-daughter framework types. This Thesis focuses on how seed-assisted syntheses impact zeolite properties such as size, morphology, structure, and defects. We observe that the molar composition of the growth mixture and the properties of the seed crystals play a significant role in controlling the kinetics of nucleation and the trajectory of interzeolite transformations. Furthermore, we observe that seeds offer unique routes to achieve small crystal sizes and distinct morphologies in comparison to many conventional syntheses. Advantages of seeding include shorter synthesis time and the ability to reduce or eliminate the need for organic structure-directing agents, thereby providing a facile and efficient route to design zeolites for various industrial applications. The fundamental mechanisms underlying zeolite seed-assisted crystallization are complex and elusive; however, our study provides new insight into these processes and highlights the important role of kinetics in governing parent-daughter (or seed-product) relationships. Furthermore, we use high temperature atomic force microscopy (AFM) to image zeolite crystal surface growth in situ. We report time-resolved images of 2-dimensional growth demonstrating layer generation by three distinct mechanisms, including nucleation from the edges of surface defects. Our findings reveal that silica nanoparticles in the growth medium incorporate into advancing steps on crystal surfaces to generate defects (i.e., amorphous silica occlusions) that largely go undetected in literature. In situ AFM measurements also show the dominance of gel mediated crystal growth in the case of faujasite zeolite syntheses.



Zeolites, crystal engineering, heterogeneous catalysts


Portions of this document appear in: Jain, Rishabh, and Jeffrey D. Rimer. "Seed-Assisted zeolite synthesis: The impact of seeding conditions and interzeolite transformations on crystal structure and morphology." Microporous and Mesoporous Materials 300 (2020): 110174; and in: Qin, Wei, et al. "Organic‐free interzeolite transformation in the absence of common building units." Chemistry–A European Journal 25.23 (2019): 5893-5898; and in: Jain, Rishabh, et al. "Spontaneous Pillaring of Pentasil Zeolites." Advanced Materials 33.22 (2021): 2100897; and in: Choudhary, Madhuresh K., Rishabh Jain, and Jeffrey D. Rimer. "In situ imaging of two-dimensional surface growth reveals the prevalence and role of defects in zeolite crystallization." Proceedings of the National Academy of Sciences 117.46 (2020): 28632-28639.