Hierarchical Zeolites by Desilication: Occurrence and Catalytic Impact of Recrystallization and Restructuring

Abstract

The manifestation of zeolite recrystallization and the formation of amorphous aluminosilicate species during desilication are examined to better understand the properties of alkaline-treated hierarchical zeolites and their catalytic performance. This is achieved using a systematic experimental strategy, starting from treating the filtrate of alkaline-treated silicalite-1 at different temperatures in the presence of various external additives. No recrystallization is evidenced upon addition of tetrapropylammonium (TPA + ) and/or aluminum hydroxide ions (Al(OH) 4 -),confirming the low probability of zeolite nucleation and/or growth during desilication.Conversely, ordered mesoporous materials (OMMs) form on addition of cetyltrimethyl ammonium (CTA + ) to the filtrate. By using other silicon sources, i.e. tetramethyl orthosilicate or the organosilane dimethyloctadecyl-[3-(trimethoxysilyl)propyl]-ammonium, we verify the facile formation of amorphous materials during alkaline treatment of USY zeolites in the presence of hydrophobic micelle-forming alkyl moieties. A systematic characterization by XRD, TEM, N 2 and Ar adsorption, ICP-OES, and FTIR spectroscopy of pyridine adsorbed demonstrates that zeolites exposed to base solutions containing CTA + display less zeolitic properties, compared to those prepared using TPA + , and should be considered as hierarchical zeolite/OMM composites.Catalytic tests in the alkylation of toluene with isopropyl alcohol or benzyl alcohol demonstrate that CTA + -derived composites do not outperform the conventional USY zeolite. Only the hierarchical USY zeolite prepared by alkaline treatment in the presence of TPA + yielded a superior catalytic performance.

Synthesis, characterisation, and catalytic evaluation of hierarchical faujasite zeolites: milestones, challenges, and future directions

“…These contribute to the solid's mesoporosity, but lower the specific micropore volume. 52 (Right) Transmission electron microscopy (TEM) images of (e) Y nanozeolites, 28 (f) MCM-41, 49 and (g and h) MCM-41 prepared by reacting alkaline filtrates derived from desilication of zeolites with CTABr. 3e).…”

“…In a recent work, Awala et al 28 demonstrated the potential of bottom-up methods by synthesizing, without organic additives, X and Y nanocrystals (Fig. 49 Table S1 (ESI †). The external surface is almost double compared to that of the hierarchical X prepared using TPHAC ( (Fig.…”

Catalyst Design by NH4OH Treatment of USY Zeolite

“…In combination with CTA + as organic micelle-forming cationic surfactant, hydrothermal NH 4 OH treatment also results in partial amorphization and signifi cant material leaching (20 wt%), while at least a part of the formed small mesopores are ordered (Figure 9 Ic,II). [ 28 ] This process thus produces a material comparable to the hierarchical USY studied here, but additionally requires organics and a hydrothermal step. On the other hand, alkaline treatment of USY with NaOH in the presence of the organic pore directing agent TPA + results in mesopore formation by extensive material leaching, while microporosity and crystallinity of the remaining material are not severely altered (Figure 9 Id,II).…”

“…Later, it was suggested that these mesostructured materials are best described as zeolite/OMM (ordered mesoporous materials) composites. [ 28 ] Recently, we treated commercially available and highly dealuminated USY zeolites with highly diluted NH 4 OH solutions at room temperature in the absence of any protecting or structure-directing agent. [ 29 ] An X-ray diffraction (XRD) and magicangle spinning nuclear magnetic resonance (MAS NMR) study has been performed on these materials, revealing the partial and gradual formation of amorphous phases containing silanol nests, strongly bound ammonium and highly structured water molecules in close interaction with each other.…”

Role of Defects in Pore Formation in MFI Zeolites

“…16,20 The modulating role of aluminum is fairly well described in the literature. However, other factors also affect the product in base treatment, such as recrystallization [21][22][23][24] and irregularities in the crystal lattice. [25][26] In particular, such irregularities exist in the form of defect sites and boundaries between smaller, fused crystallites, which build up larger aggregates.…”