Desilication Mechanism Revisited: Highly Mesoporous All‐Silica Zeolites Enabled Through Pore‐Directing Agents

Abstract

The role of pore-directing agents (PDAs) in the introduction of hierarchical porosity in silicalite-1 in alkaline medium was investigated. By incorporation of various PDAs in aqueous NaOH, homogenously distributed mesopores were introduced in 2.5 μm silicalite-1 crystals. It was proven for the first time that framework aluminum is not a prerequisite for the introduction of intracrystalline mesoporosity by desilication. The pore-directing role is not directly exerted by framework trivalent cations metals, but by species on the external surface of the zeolite. The inclusion of metal complexes (Al(OH)(4)(-), Ga(OH)(4)(-)) and tetraalkyl ammonium cations (tetramethyl ammonium (TMA(+)), tetrapropyl ammonium (TPA(+))) in the alkaline solution led to distinct mesopore surface areas (up to 286 m(2) g(-1)) and pore sizes centered in the range of 5-20 nm. In the case alkaline treatment was performed in the presence of Al(OH)(4)(-), all aluminum partially integrated in the zeolite giving rise to both Lewis and Brønsted acidity. Apart from the concentration and location, the affinity of the PDA to the zeolite surface plays a crucial role in the pore formation process. If the PDA is attracted too strongly (e.g., TMA(+)), the dissolution is reduced dramatically. When the pore-directing agent is not attracted to the zeolite's external surface, excessive dissolution occurs (standard alkaline treatment). TPA(+) proved to be the most effective PDA as its presence led to high mesopore surface areas (>200 m(2) g(-1)) over a broad range of PDA concentrations (0.003-0.1 M). Importantly, our results enable to extend the suitability of desilication for controlled mesopore formation to all-silica zeolites.

Hierarchy concepts: classification and preparation strategies for zeolite containing materials with hierarchical porosity

“…20 and 50. 35,98,99 However, it has recently been shown that the partial coverage of the outer crystal surface with organic molecules allows the controlled desilication even of aluminium-free zeolites, 98 while a combined acid-base treatment allows the desilication even of Al-rich zeolites. 100,101…”

Catalytic cracking performance of alkaline-treated zeolite Beta in the terms of acid sites properties and their accessibility

“…Alkaline leaching, performed in aqueous NaOH and tetraalkylammonium hydroxides mixtures, leads to the dissolution of both Si and smaller amounts of Al species from the framework. Nevertheless, most of these extracted Al species are able to realuminate on the mesopore surface, resulting in the lowering the Si/Al ratio of the hierarchical [9,10]. Zeolite desilication has been successfully applied to produce a large number of hierarchically structured zeolites MFI [11,12]MTW [13], MOR [14,15], FER [16,17], FAU [18], and Beta [19,20].…”

The Role of Non-Framework Lewis Acidic Al Species of Alkali-Treated HZSM-5 in Methanol Aromatization

“…In contrast, the Tmax of the Brønsted acid of HZ-DA (412 °C) was higher than those of HZ and HZ-D, and a slight increase of the Brønsted site concentration from 0.14 to 0.15 mmol/g was found. This small increase ascertains that small amounts of extracted Al species from the ZSM-5 matrix can be reincorporated into the framework [26]. Figure 6 shows the NH 3 -TPD profiles, and Table 2 lists the acid site densities.…”

“…As for HZ-DA, Al(Td) seemed to grow back from 77% to 87% with Al(Oh) that decreased from 23% to 13%. The increased Al(Td) ratio was possible caused by a re-alumination process through which some extra-framework Al could be reinserted into the ZSM-5 framework in an acidic environment [25,26], while the decrease of Al(Oh) indicates that some extra-framework Al species were removed from the outer surface. Briefly, HZ-D had the highest compositions of Al(Oh) and the lowest compositions of Al(Td) among tested catalysts.…”