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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]

Abstract: Zeolites with various levels of micro- and mesoporosity are prepared by alkaline treatment of ZSM-5 at different temperatures. Terminal silanol groups generated at the mesopore surface during desilication are reacted with aminopropyltriethoxysilane or mercaptopropyltrimethoxysilane leading to the obtainment of hierarchical zeolites with organic-functionalized hybrid mesopores. Grafting of 3-aminopropyl and 3-thiopropyl moieties is directly evidenced by elemental analysis, nitrogen adsorption, thermogravimetry, and infrared spectroscopy, and the relative extents of surface functionalization of the purely microporous and of the mesoporous ZSM-5 zeolites are compared. Based on these results the mechanism of organosilane attachment is thought to be equivalent in both cases. Organic-functionalized mesopores exhibit good hydrothermal stability. The adsorption properties of the ZSM-5 samples both prior and post silanization are assessed for the uptake of lipase enzyme. Mesoporous zeolites show increased uptakes than the parent zeolite in all cases. This demonstrates that the mesoporosity enhances the adsorption of large guest species that cannot be accommodated within the micropores of their conventional counterparts. Immobilization on surface functionalized zeolites greatly improves the retention of lipase activity when applied in the hydrolysis of p-nitrophenylbutyrate. [Copyright &y& Elsevier]

Abstract: The combination of dealumination and desilication of ZSM-5 zeolites has been investigated in order to decouple mesoporosity creation and modification of acidity. Individually, desilication by treatment in alkaline medium leads to a spectacular mesoporosity development, without distinct modification of the acidic properties, while dealumination by steam treatment mainly impacts on the acidity of the ZSM-5 zeolite. It is shown that application of the alkaline treatment first followed by steam treatment results in MFI zeolites with independently tailored pore architecture and acidic properties, with proven hydrothermal stability under severe conditions. The independent modification of porous and acidic properties cannot be accomplished if the alkaline treatment is performed after the steam treatment, since the presence of extra-framework Al species inhibit Si extraction and related mesopore formation. A promising outlook of the successive alkaline-steam treatment is expected in catalytic applications of microporous materials. [Copyright &y& Elsevier]

Abstract: The improvement of molecular transport properties of hierarchical H-ZSM-5 obtained by desilication was evidenced by studying the desorption of o-xylene and isooctane by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). This technique enabled monitoring simultaneously bands associated with the molecular probes and the zeolite, using powdered sample masses as low as 1mg. Two H-ZSM-5 samples with markedly different crystal sizes were investigated. The first sample was commercial and consisted of small crystallites (ca. 250nm). The second sample were laboratory-made large crystals with coffin-like shape (ca. 17×4×4μm3). The hierarchical derivatives of the small and large zeolite crystals displayed 250 and 120m2 g−1 of mesopore surface area, respectively, in contrast to the 62 and 5m2 g−1 of the parent counterparts. The data based on o-xylene desorption were partly disguised by site-desorption limitations. Desorption experiments using isooctane evidenced a 4-fold reduction in the characteristic diffusion path length on both mesoporous small and large zeolites with respect to their purely microporous analogues. These results confirm the substantial potential for improvement of commercial nanocrystalline zeolites in diffusion-limited reactions upon the introduction of intra-crystalline mesoporosity by post-synthesis modification. [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]

Abstract: Desilication in alkaline medium is demonstrated as a general route to synthesize hierarchical metallosilicate zeolite crystals with preserved crystallinity. The lattice trivalent cation (not only Al3+ but also Ga3+, Fe3+, and B3+) regulates the alkaline-assisted hydrolysis of silicon towards controlled mesoporosity development. This finding widens the family of modified zeolite compositions that integrate in a single crystalline material micropores and mesopores and enables better accessibility in catalytic applications compared to the purely microporous counterparts. [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]

Abstract: Zeolite beta crystals (Si/Al=35) synthesized in fluoride medium were treated in aqueous 0.2M NaOH solution for mesopore formation by selective extraction of framework silicon. A 16 parallel-batch reactor was used to study the influence of the treatment time and temperature on the physico-chemical properties of the zeolites, which were characterized by ICP-OES, XRD, N2 adsorption at 77K, SEM, TEM, DRIFTS, and in situ ATR-IR. Alkaline treatment of H-beta within the optimal window of Si/Al ratios identified for other zeolite families leads to extensive silicon extraction at mild treatment conditions. This originates substantial mesoporosity and presumably improved transport, but negatively impacts on the microporous and acidic properties of the resulting sample. Consequently, the alkaline-treated beta zeolites show lower catalytic activity in the acid-catalyzed liquid-phase benzene alkylation than the purely microporous parent material. The relatively low stability of framework aluminum in BEA, compared to MFI and MOR, is detrimental for the controlled mesopore formation by Si dissolution, since aluminum cannot optimally exert its pore-directing role. [Copyright &y& Elsevier]

Abstract: The preparation of mesoporous mordenite zeolite and its subsequent application in the liquid-phase alkylation of benzene with ethylene is discussed. Mesoporous mordenite was obtained by controlled silicon extraction on alkaline treatment of successfully synthesized high-silica mordenite zeolites with molar Si/Al ratios in the range of 20–30. Besides substantial mesoporosity development, the combined microporous and mesoporous zeolites show preserved Brønsted acidic properties, which are highly attractive when acid-catalyzed reactions are targeted. Catalytic testing of the mesoporous mordenite found superior performance in benzene alkylation due to the unique interplay between improved physical transport in the shortened micropores and the preserved high density of acid sites. [Copyright &y& Elsevier]