A contribution to understanding the mechanism of crystallization of silicalite-1 in heterogeneous systems (hydrogels)

Controlled aggregation of core(amorphous silica)@shell(TPA+-polysilicate) nanoparticles at room temperature by selective removal of TPA+ ions from the nanoparticle shell

“…For these reasons, great attempts were made to elucidate the "structure" and properties of the sub-colloidal particles and their role in zeolite crystallization. 5,6,11,[23][24][25][27][28][29][30][31][32][34][35][36][37][38][39][40][41][42][43][44][45][46] On the basis of numerous studies of the formation of silica nanoparticles in homogeneous systems TEOS-TAAOH-H 2 O, 27,34,[47][48][49][50] it was concluded that silica-TAAOH solutions have well defined critical aggregation concentration (CAC) of silica for OH -/SiO 2 = TAAOH/TEOS ≈ 1. Many studies 21,27,30,34,[47][48][49][50] suggest that prior to this critical point (below the CAC; OH -/SiO 2 > 1) the solution (HmRM) contains only silicate monomers and polysilicate anions (oligomers).…”

“…In spite of the fact that the rt evolution of the nanoparticles in the presence of alkaline cations (mostly Na + ions), strongly influences the pathways of zeolite crystallization in both HmRMs 4,25,33,38 and HtRMs, 40,[43][44][45] the influence of concentration of Na + and/or other alkali cations on the rt evolution of the silica nanoparticles in homogeneous systems, is hardly studied. 47,48,58 For this reason, the objective of this work is the study of the influence of the concentration of the Na + ions, added into reaction mixture during and/or after formation of the core@shell PNPs on the processes of their aggregation and on the characteristics of the obtained aggregates.…”

Deep Insights into the Processes Occurring during Early Stages of the Formation and Room-Temperature Evolution of the Core (Amorphous SiO2)@Shell (Organocations) Nanoparticles

“…[24][25][26][27][28] These findings provided an additional significance of these investigations by the fact that hydrothermal treatment of such homogeneous systems results in the formation of different nanosized zeolites, 18,19,23,[26][27][28][29][30][31][32][33][34][35][36] and in addition, that the sub-colloidal precursor species (PPSs) have a crucial role at the early stage of crystallization of zeolites in heterogeneous systems (hydrogels). [37][38][39][40][41][42] Following Iler's original ideas on silica nanoparticles 43 and knowing the possibility of the stabilization of silica by organic molecules, [44][45][46] several research groups suggested that the PPSs have a core@shell structure with amorphous silica at the core and organic molecules at the shell. 20-22, 23, 25, 33, 47-51 On the basis of numerous studies of the formation of silica nanoparticles in homogeneous systems, TEOS-Org(OH)n-H2O 20-23, 25, 28, 35, 47-56 [Org(OH)n is organic base, most frequently TAAOH (n = 1), 20,22,50 but also other organoammonium hydroxides 23,27,28,50 were used as organic bases], it was concluded that silica-Org(OH)n solutions have well defined critical aggregation concentration (CAC) of silica for OH -/SiO2 ≈ 1.…”

Zeolite growth by synergy between solution-mediated and solid-phase transformations

“…The driving force for this socalled Ostwald ripening process is based on the increasing stability of particles with size, and results in the growth of larger, more stable particles at the dissolution of smaller, less stable particles. 5,6 In contrast, recent progress in understanding the zeolite growth has suggested that the nucleation occurs from the amorphous precursor phase, both in clear solutions [7][8][9][10][11][12][13][14][15] and in hydrogel systems [16][17][18][19][20][21][22] and the crystal growth mainly fall into one of the four categories: (i) addition of nanoparticles or of monomeric or oligomeric nutrients provided by the dissolution of colloidally stable precursor nanoparticles to a growing crystal, 12,[21][22][23][24][25] (ii) crystalline transformation with or without the crystal aggregation, leading to intergrowth or single zeolite crystals, [7][8][9][10][11][17][18][19][20] (iii) aggregation of crystalline building units [26][27][28][29][30] or of non-crystalline nanoparticles, [12][13][14][15] or (iv) self-assembly of small fragments around...…”