Zeolite Catalysts with Tunable Hierarchy Factor by Pore‐Growth Moderators

Abstract

The design of hierarchical zeolite catalysts is attempted through the maximization of the hierarchy factor (HF); that is, by enhancing the mesopore surface area without severe penalization of the micropore volume. For this purpose, a novel desilication variant involving NaOH treatment of ZSM‐5 in the presence of quaternary ammonium cations is developed. The organic cation (TPA+ or TBA+) acts as a pore‐growth moderator in the crystal by OH−‐assisted silicon extraction, largely protecting the zeolite crystal during the demetallation process. The protective effect is not seen when using cations that are able to enter the micropores, such as TMA+ Engineering the pore structure at the micro‐ and mesolevel is essential to optimize transport properties and catalytic performance, as demonstrated in the benzene alkylation with ethylene, a representative mass‐transfer limited reaction. The hierarchy factor is an appropriate tool to classify hierarchically structured materials. The latter point is of wide interest to the scientific community as it not only embraces mesoporous zeolites obtained by desilication methods but it also enables to quantitatively compare and correlate various materials obtained by different synthetic methodologies.

Preparation of Mesoporous Fe2O3-Supported ZSM-5 Zeolites by Carbon-Templating and their Evaluation as Photo-Fenton Catalysts to Degrade Organic Pollutant

“…However, the catalytic efficiency of ZSM-5 zeolite may be severely limited due to their microporous nature, which restricts the access of reactant molecules to active catalyst sites located inside of the zeolite crystals 11,12 . Due to this fact, the preparation of mesoporous ZSM-5 zeolites has attracted a great interest in the catalytic applications [13][14][15] . Among the several methods showed in the literature to produce mesoporous zeolites [16][17][18][19] , the carbon-templating method has demonstrated to be relatively facile and inexpensive, being that after burning of the carbon particles, zeolite crystals with intracrystalline mesoporous are obtained 16,20,21 .…”

Mesopore quality determines the lifetime of hierarchically structured zeolite catalysts

“…Porosity modification can be tuned through the inclusion of additives, such as quaternary ammonium or metal salts, to the alkaline solution. Since the size and amount of developed mesopores can be regulated by the identity and concentration of additive applied, they are commonly referred to as pore-directing agents 16 . In the case of ZSM-5 zeolites, tetrapropylammonium cations (TPA þ ) are known to provide a high level of control over the dissolution process due to their strong interaction with the crystal surface 17 .…”

Preparation of Hierarchical ZSM-5 from Indonesian Kaolin by Adding Silica

“…Some researchers reported preparation of hierarchical ZSM-5 by post synthesis as demetalisation and desilication without organic template done [7][8][9], while the other researchers published the synthesis of hierarchical ZSM-5 by direct template (supramolecular and solids template) [10][11][12][13]. The synthesis also requires silica and alumina source, in order to decrease the cost of ZSM-5, many researcher substituted the commercial chemical products by the natural ones.…”