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 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 upon addition of cetyltrimethylammonium (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 X-ray diffraction, transmission electron microscopy, N2 and Ar adsorption, inductively coupled plasma optical emission spectroscopy, and Fourier transform infrared spectroscopy of pyridine adsorbed, demonstrates that zeolites exposed to base solutions containing CTA+ display weaker 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 evidence 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.