Design of hierarchical zeolite catalysts by desilication (original) (raw)
Hierarchical (or mesoporous) zeolites have received an ever-increasing attention due to their improved performance in catalysed reactions with respect to conventional (purely microporous) zeolites. Desilication in alkaline media has become a widely applied preparation method to tailor these modified zeolites, due to an optimal combination of efficiency and simplicity. This review presents recent developments that have expanded its general understanding and turned this top-down treatment highly versatile, controllable, and scalable. Design aspects of mesoporous zeolites for catalytic applications are emphasised, encircling the establishment of synthesis-property-function relationships. Alkaline treatment is a key step in strategic combinations with other post-synthesis modifications towards superior zeolite catalysts. The outlook of the field, pinpointing present needs and short-term priorities, is discussed.
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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, N 2 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.
Catalysis Today, 2012
Zeolites and related materials (including a wide range of microporous and mesoporous materials with ordered pore structure) have been one of the areas in the field of materials and catalysis with the largest impact on science, technology and industrial processes. We discuss here some recent developments in this field, with particular references how to tailor and design zeolite and related material properties to control/enhance the catalytic performances. Four main topics have been addressed. (i) The recent progress and perspectives in the field of tailored syntheses, with selected examples showing the trend and prospects to develop new structures, control the location of active sites, and the crystal size and morphology, including nanoarchitecture of the final catalysts. (ii) The development and prospects of two-dimensional zeolites presenting an extended view/concept of zeolite structures integrating the classical 3D frameworks and the various lamellar forms. (iii) The progresses in the design and synthesis of hierarchical zeolites, with discussion on the still existing challenges related to the synthesis, characterization and catalytic application. (iv) Novel opportunities and needs in terms of zeolite multifunctional design for catalytic applications, with a discussion of the critical issues related to the use in the field of fine chemicals, organic industrial syntheses and biorefinery, and the prospects for the use in two novel challenging areas of the direct conversion of CO 2 to light olefins and methane to methanol.
Hierarchical high-silica zeolites as superior base catalysts
Andrew J. deMello, Joshua B. Edel et al. Rapid cell extraction in aqueous twophase microdroplet systems PERSPECTIVE Barry M. Trost et al. Catalytic asymmetric allylic alkylation employing heteroatom nucleophiles: a powerful method for C-X bond formation 5
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