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Abstract:
A case study on the understanding of the formation of hierarchical Beta zeolites using gemini-type piperidine based multiammonium surfactant (N6-diphe) is reported. Complementary techniques were used to investigate N6-diphe’s structure-directing effect at the molecular level. Combining characterization of the resulting zeolite materials with the toolboxes herein developed for studying clear solutions and dense gels discloses self-assembly processes that govern the growth (and growth inhibition) of nano-Beta zeolite crystals. In clear solution, small-angle X-ray scattering and liquid-state NMR provide insights about the formation of nanoparticles and their degree of order. 14N and 1H-DOSY NMR probe the dynamics and mobility of soluble species. In a dense gel, on the other side, 27Al- and 29Si-(MAS) NMR elucidate the varying local connectivity between initial nano-objects and the final solid products. It has been found that cylindrical micelles control the transformation of solubilized silica and alumina during the formation of zeolite nuclei and guide their crystal growth to nano-Beta rods with bimodal mesoporosity. The predominant smaller mesopores (6 to 8 nm) originate from the template’s hydrophobic alkyl chains, while larger mesopores (10 to 30 nm) are supposed to result from a spinodal decomposition-type segregation of phases consisting of as-formed hydrophobic zeolite rods and an aqueous solution.
