Synthesis of Dendritic ZSM-5 Zeolite through Micellar Templating Controlled by the Amphiphilic Organosilane Chain Length

The synthesis ofZSM-5 zeolites by hydrothermal crystallizationof protozeolitic nanounits functionalized with amphiphilic organosilanesof different chain length (C- n -N(CH3)(2)-(CH2)(3)-Si-(OCH3)(3), n = 10, 14, 18 and 22) has been investigated.Well-developed dendritic nanoarchitectures were achieved when usingC14 and C18 organosilanes, exhibiting a radial and branched patternof zeolitic nanounits aggregates. In contrast, although C10 and C22organosilanes led to materials with hierarchical porosity, they lack of dendritic features. These differences have been linked to theformation of an amorphous mesophase at the gel preparation stage forthe C14 and C18 samples, in which the surfactant micelles are covalentlyconnected with the protozeolitic nanounits through siloxane bonds.The presence of the dendritic nanostructure positively impacts boththe textural and catalytic properties of ZSM-5 zeolite. Thus, ZSM-5(C14) and ZSM-5 (C18) samples exhibit the largest contribution ofmesoporosity in terms of both surface area and pore volume. On theother hand, when tested as catalysts in the aldol condensation offurfural with cyclopentanone, which is an interesting reaction forthe production of sustainable jet fuels, the highest catalytic activityis attained over the dendritic ZSM-5 materials due to their remarkableaccessibility and balanced Bronsted/Lewis acidity. Hierarchical ZSM-5 zeolite nanoarchitecture can be modulatedby varying the chain length of amphiphilic organosilanes. The formationof a covalently stabilized mesophase is a necessary condition forobtaining dendritic ZSM-5 zeolites through a micellar soft-templatingmechanism. This mesophase is formed in the synthesis gel when employingorganosilanes with a suitable hydrophobicity.