Hierarchy
Brings Function: Mesoporous Clinoptilolite
and L Zeolite Catalysts Synthesized by Tandem Acid–Base Treatments
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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 m<sup>2</sup> g<sup>–1</sup>; L, 135 m<sup>2</sup> g<sup>–1</sup>) relative to the parent zeolite (clinoptilolite,
15 m<sup>2</sup> g<sup>–1</sup>; L, 45 m<sup>2</sup> g<sup>–1</sup>). 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 Cu<sup>2+</sup> 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