Hierarchy Brings Function: Mesoporous Clinoptilolite and L Zeolite Catalysts Synthesized by Tandem Acid–Base Treatments

Abstract

Hierarchical clinoptilolite and L zeolites are prepared using optimized tandem dealumination–desilication treatments. The main challenge in the postsynthetic modification of these zeolites is the high Al content, requiring a tailored dealumination prior to the desilication step. For natural clinoptilolite, sequential acid treatments using aqueous HCl solutions were applied, while for L a controlled dealumination using ammonium hexafluorosilicate is required. Subsequent desilication by NaOH treatment yields mesopore surfaces of up to 4-fold (clinoptilolite, 64 m2 g–1; L, 135 m2 g–1) relative to the parent zeolite (clinoptilolite, 15 m2 g–1; L, 45 m2 g–1). A thorough characterization sheds light on the composition, crystallinity, porosity, morphology, coordination, and acidity of the modified clinoptilolite and L zeolites. It is elaborated that, besides the degree of dealumination, the resulting Al distribution is a critical precondition for the following mesopore formation by desilication. Adsorption experiments of Cu2+ and methylene blue from aqueous solutions and the catalytic evaluation in alkylations and Knoevenagel condensation evidence the superiority of the hierarchical zeolites, as compared to their purely microporous counterparts. Finally, the postsynthetic routes for clinoptilolite and L are generalized with other recently reported modification strategies, and presented in a comprehensive overview.