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Abstract: A two-step route comprising treatment in sodium aluminate followed by acid washing was developed to introduce secondary mesoporosity in ZSM-5 zeolites with different Si/Al ratios. The parent and treated zeolites were characterized by ICP-OES (solids and filtrates), XRD, TGA, TEM, gas adsorption (N2 at 77K and Ar at 87K), FTIR, and NH3-TPD. The contact of the zeolite with NaAlO2 solutions induced mild leaching of framework silicon, viz. desilication. However, the porosity of the resulting samples was mostly blocked by NaAlO2-derived deposits and Si-containing debris. A subsequent acid washing with HCl or oxalic acid removed the blocking species, largely regaining the native microporosity and further uncovering the extensive intracrystalline mesoporosity generated in the first step. In order to maximize the micro- and mesoporosity in the final zeolites, the conditions of the NaAlO2 and acid treatments were varied with respect to concentration, temperature, and time. The efficiency of the two-step treatment was remarkable for zeolites with Si/Al=25 and 40. The mesopore surface area of the hierarchical ZSM-5 was 3–4 times higher than in the parent zeolites, while other intrinsic properties like crystallinity and acidity were largely preserved. The physico-chemical properties of hierarchical ZSM-5 obtained by the two-step route and by conventional desilication in NaOH were compared. Both desilication variants led to equivalent mesopore surface areas, although the two-step route presented a higher solid yield and produced smaller mesopores due to the much lower amount of silicon leached compared to the treatment in sodium hydroxide. Mechanistic and kinetic aspects of silicon dissolution and porosity development in ZSM-5 were discussed on the basis of alkaline treatments in mixtures of NaOH and NaAlO2. [Copyright &y& Elsevier]

Physical gas adsorption is extensively used in the characterization of micro- and mesoporous materials and is often considered as a straightforward-to-interpret technique. However, physical phenomena like the tensile strength effect, adsorbate phase transitions, and monolayer formation in combined micro- and mesoporous materials frequently lead to extra contributions in the adsorption isotherm. Models for pore size determination mostly do not account for this, and assignment to real pores leads to improper analysis of adsorption data. In this review, common pitfalls and limitations in the analysis of pore size distributions derived from adsorption isotherms of micro- and mesoporous materials are identified and discussed based on new results and examples reported in the recent literature. [Copyright &y& Elsevier]

Abstract: Desilication of ITQ-4 (IFR code, 1-dimensional 12-MR channel system of 0.62nm×0.72nm) in aqueous NaOH solutions was practiced in order to produce the zeolite in the hierarchical form, i.e. combining the native micropores with a secondary network of intracrystalline mesopores. The parent material (Si/Al=32) was synthesized using benzylquinuclidinium as the structure-directing agent in the presence of fluoride anions. The samples were characterized by ICP-OES, XRD, TEM, N2 adsorption, and FTIR spectroscopy. In a representative treatment, the mesopore surface area of the ITQ-4 experienced a sevenfold increase (from 29 to 206m2 g−1) upon optimized NaOH post-treatment, whereas the microporosity was reduced by 70% (from 0.20 to 0.06cm3 g−1). Chemical analysis and infrared spectroscopy of adsorbed pyridine showed that after alkaline treatment the amount of Brønsted acid sites remains practically constant whereas a significant increase in Lewis acid sites was obtained. Infrared studies with adsorbed 2,4,6-collidine concluded that the accessibility of acid sites in the hierarchical zeolite increased substantially due to an increase in the available active sites at the pore mouths of the hierarchical zeolite. [Copyright &y& Elsevier]