Increasing the porosity of zeolites
Zeolites are important industrial catalysts; their unique shape-selectivity is the basis of important applications, but also a pitfall limiting their efficiency. Overcoming or decreasing the diffusion limitations in zeolites is important to improve their catalytic and separation performance. The present Ph.D. thesis reports work on the preparation of zeolites with increased porosity via post-synthesis chemical etching. The work aims to create secondary porosity (mesopores) connected to the native microporosity without altering the intrinsic zeolite properties. Three zeolite types are studied: a small pore SSZ-13 (CHA), a medium pore ZSM-5 (MFI), and a large pore zeolite L (LTL). Zeolite L study compares the etching abilities of NH4F and NH4HF2 solutions in the hierarchization of zeolite L. The results show that NH4F can be replaced with NH4HF2. The etching with 1 and 2 wt/% NH4HF2 solutions yield hierarchical derivatives similar to those obtained with 20 and 40 wt/% NH4F solutions. Thus by replacing NH4F with NH4HF2 a substantial decrease in the used fluorine is achieved. SSZ-13 is etched with 40 wt/% NH4F, which generates mesopores in all prepared samples. The results reveal the mesopore generation starts from the crystal surface due to the constrained diffusion of hydrated bifluoride ions through the small pore channels. Chromic acid etching of zeolites with different pore opening (8, 10, 12 MR) reveals that this dissolution process is dependent on the size of the pore opening as 8 MR and 10 MR zeolites are more resistant to etching with chromic acid than 12 MR zeolite. In general, the chromic acid does not generate substantial mesopore formation. The number of accessible acid sites in etched derivatives is close to the parent material, although some preferential dealumination is observed.
Keywords: zeolites; hierarchical; mesoporosity; etching