TUD-C: A tunable, hierarchically structured mesoporous zeolite composite

TUD-C is a new type of hierarchically structured composite with ZSM-5 nanocrystals embedded in a well-connected mesoporous matrix. It is synthesized by employing only one organic templating/scaffolding molecule (TPAOH) through a two-step procedure involving gelation and crystallization. Both the Si/Al ratio and crystallization time are important factors influencing the acidity and the micro- and mesopore structures of TUD-C. A lower Si/Al ratio results in slower formation of zeolite crystals and weaker acidity. Nevertheless, when Si/Al > 50, the mesopore size can only be tuned inside a narrower range. The tuning of the mesopore size by crystallization is identified as primarily due to a thermal effect, rather than to crystal growth. SEM reveals the intriguing formation of clusters consisting of nanoparticles during the crystallization of samples with Si/Al = 30. Micropore structure analysis by Ar sorption and TEM shows the formation of a pre-formed microporous structure before the zeolite structure appears. This sheds light on the formation mechanism of zeolite crystals through a solid-phase transformation. Compared with TUD-M, TUD-C is more hydrothermally stable, more acid, and the mesopore size is more easily tuned. However, the method to synthesize TUD-M can be applied to a wider range of zeolite types. The choice between TUD-C and TUD-M depends on the application.