Search

A literature survey reveals a prominent reduction in the concentration of Brønsted acid sites in hierarchically organized zeolites with increasing mesoporous or external surface area independent of the framework type or synthesis route; this suggests a common fundamental explanation. To determine the cause, nature, and impact of the underlying changes in aluminum speciation, this study combines a multitechnique analysis that integrates basic characterization, a detailed synchrotron XRD and multiple-quantum NMR spectroscopy assessment, and catalytic tests to correlate evolution of the properties with performance during successive steps in the preparation of hierarchical MFI-type zeolites by desilication. The findings, subsequently generalized to FAU- and BEA-type materials, identify the crucial impact of calcination on the protonic form, which is an integral step in the synthesis and regeneration of zeolite catalysts; on aluminum coordination; and the associated acidity trends. [ABSTRACT FROM AUTHOR]

Wax on, wax off: Bifunctional cobalt‐based catalysts on zeolite supports are applied for the valorization of biosyngas through Fischer‐Tropsch chemistry. By using these catalysts, waxes can be hydrocracked to shorter‐chain hydrocarbons, increasing the selectivity towards the C5–C11 (gasoline) fraction. The zeolite topology and the amount and strength of acid sites are key parameters to maximize the performance of these bifunctional catalysts, steering Fischer‐Tropsch product selectivity towards liquid hydrocarbons. [ABSTRACT FROM AUTHOR]

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]

[Display omitted] • Prins cyclisation from isovaleraldehyde and isoprenol to tetrahydropyranol. • Zeolites with varying acidity and hierarchical materials as catalysts. • H-Beta-300 zeolite giving 72% selectivity to the desired product at 99% conversion. • The cis/trans ratio of tetrahydropyranols depends on porous structure. • Parallel formation of tetrahydropyranol and dihydropyran confirmed by DFT. In this work, several heterogeneous micro- and mesoporous, acidic catalysts were tested for the selective synthesis of Florol®, an industrial product formed via condensation and rehydration starting from isoprenol and isovaleraldehyde in dimethylcarbonate as a solvent. The results showed that a mildly acidic, microporous H-Beta-300 with SiO 2 /Al 2 O 3 ratio of 300 was the best catalyst, giving 72% selectivity with 99% conversion at 40 °C when using the molar ratio isoprenol to isovaleraldehyde of 1:5. More acidic zeolites gave slightly lower selectivity, while the lowest selectivity (up to 52%) was obtained with mesoporous catalysts exhibiting mild acidity and no strong Brønsted acid sites. Selectivity to pyranols was nearly constant when changing temperature, while a larger excess of isovaleraldehyde promoted formation of tetrahydropyranol. H-Beta-300 catalyst was successfully after calcination at 400 °C. DFT calculations pointed out on the parallel formation of tetrahydro pyranols and dihydropyrans, which are the corresponding dehydration products. [ABSTRACT FROM AUTHOR]

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]

Hierarchical clinoptilolite and Lzeolites are prepared using optimizedtandem dealumination–desilication treatments. The main challengein the postsynthetic modification of these zeolites is the high Alcontent, requiring a tailored dealumination prior to the desilicationstep. For natural clinoptilolite, sequential acid treatments usingaqueous HCl solutions were applied, while for L a controlled dealuminationusing ammonium hexafluorosilicate is required. Subsequent desilicationby NaOH treatment yields mesopore surfaces of up to 4-fold (clinoptilolite,64 m2g–1; L, 135 m2g–1) relative to the parent zeolite (clinoptilolite,15 m2g–1; L, 45 m2g–1). A thorough characterization sheds light on thecomposition, crystallinity, porosity, morphology, coordination, andacidity of the modified clinoptilolite and L zeolites. It is elaboratedthat, besides the degree of dealumination, the resulting Al distributionis a critical precondition for the following mesopore formation bydesilication. Adsorption experiments of Cu2+and methyleneblue from aqueous solutions and the catalytic evaluation in alkylationsand Knoevenagel condensation evidence the superiority of the hierarchicalzeolites, as compared to their purely microporous counterparts. Finally,the postsynthetic routes for clinoptilolite and L are generalizedwith other recently reported modification strategies, and presentedin a comprehensive overview. [ABSTRACT FROM AUTHOR]

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]

An accessibility index (ACI) was derived from infrared spectroscopy of substituted alkylpyridines with different size (pyridine: 0.57nm, 2,6-lutidine: 0.67nm, and 2,4,6-collidine: 0.74nm) over hierarchical porous ZSM-5 crystals. The samples were prepared by selective silicon extraction of a commercial sample in NaOH (desilication), and they contained different degrees of intracrystalline mesoporosity. Our results demonstrate the enhanced accessibility of Brønsted acid sites in the hierarchical zeolites. For example, a relatively bulky molecule such as collidine, which probes practically no acid site of the parent medium-pore MFI structure, can access up to 40% of the acid sites in the mesoporous sample. The ACI is a powerful tool to standardize acid site accessibility in zeolites, and can be used to rank the effectiveness of synthetic strategies to prepare hierarchical zeolites. [Copyright &y& Elsevier]