Decane hydroconversion on bifunctional Zeogrid and nano-zeolite assembled from aluminosilicate nanoslabs of MFI framework type (original) (raw)
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Journal of Catalysis, 2006
Micrometer-size MTT-type zeolite crystals with aluminum depletion in the outer layer were synthesized. The compositional variation in individual crystals was obtained by altering the chemical composition of the gel during crystal growth of MTT-type zeolite. Chemical composition of the bulk and the surface of the crystals was determined using 27 Al MAS NMR and XPS, respectively. The zeolite with siliceous rim was converted into a bifunctional catalyst by neutralizing the cation-exchange capacity with protons and plating with a trace amount of platinum metal. The MTT zeolite with siliceous crystal termination was catalytically much less active than the isostructural ZSM-23 zeolite with homogeneous chemical composition. The catalytic activity reflects the aluminum content on the surface of the crystals determined by XPS, rather than the bulk composition. The reaction mechanism and the reaction kinetics were analyzed using a microkinetic model developed previously for the structurally related TON-type zeolite. The catalytic data are in favor of the pore mouth catalysis model of skeletal isomerization of long n-alkanes on 10-membered ring tubular pore zeolites.
Angewandte Chemie, 2021
Synthesis of ap entasil-type zeolite with ultra-small few-unit-cell crystalline domains,which we call FDP (few-unitcell crystalline domain pentasil), is reported. FDP is made using bis-1,5(tributyl ammonium) pentamethylene cations as structure directing agent (SDA). This di-quaternary ammonium SDAc ombines butyl ammonium, in place of the one commonly used for MFI synthesis,p ropyl ammonium, and af ive-carbon nitrogen-connecting chain, in place of the sixcarbon connecting chain SDAs that are knowntofit well within the MFI pores.X-ray diffraction analysis and electron microscopyi maging of FDP indicate ca. 10 nm crystalline domains organized in hierarchical micro-/meso-porous aggregates exhibiting mesoscopic order with an aggregate particle sizeu pt oc a. 5 mm. Al and Sn can be incorporated into the FDP zeolite framework to produce active and selective methanol-to-hydrocarbon and glucose isomerization catalysts, respectively.
Stable single-unit-cell nanosheets of zeolite MFI as active and long-lived catalysts
Nature, 2009
Zeolites-microporous crystalline aluminosilicates-are widely used in petrochemistry and fine-chemical synthesis 1-3 because strong acid sites within their uniform micropores enable size-and shape-selective catalysis. But the very presence of the micropores, with aperture diameters below 1 nm, often goes hand-in-hand with diffusion limitations 3-5 that adversely affect catalytic activity. The problem can be overcome by reducing the thickness of the zeolite crystals, which reduces diffusion path lengths and thus improves molecular diffusion 4,5 . This has been realized by synthesizing zeolite nanocrystals 6 , by exfoliating layered zeolites 7-9 , and by introducing mesopores in the microporous material through templating strategies 10-17 or demetallation processes . But except for the exfoliation, none of these strategies has produced 'ultrathin' zeolites with thicknesses below 5 nm. Here we show that appropriately designed bifunctional surfactants can direct the formation of zeolite structures on the mesoporous and microporous length scales simultaneously and thus yield MFI (ZSM-5, one of the most important catalysts in the petrochemical industry) zeolite nanosheets that are only 2 nm thick, which corresponds to the b-axis dimension of a single MFI unit cell. The large number of acid sites on the external surface of these zeolites renders them highly active for the catalytic conversion of large organic molecules, and the reduced crystal thickness facilitates diffusion and thereby dramatically suppresses catalyst deactivation through coke deposition during methanol-togasoline conversion. We expect that our synthesis approach could be applied to other zeolites to improve their performance in a range of important catalytic applications.
Catalytic and molecular separation properties of Zeogrids and Zeotiles
Catalysis Today, 2011
Zeogrids and Zeotiles are hierarchical materials built from assembled MFI zeolite precursor units. Permanent secondary porosity in these materials is obtained through self assembly of nanoparticles encountered in MFI zeolite synthesis in the presence of supramolecular templates. Hereon, the aggregated species are termed nanoslabs. Zeogrids are layered materials with lateral spacings between nanoslabs creating galleries qualifying as supermicropores. Zeotiles present a diversity of tridimensional nanoslab assemblies with mesopores. Zeotile-1, -4 and -6 are hexagonal mesostructures. Zeotile-1 has triangular and hexagonal channels; Zeotile-4 has hexagonal channels interconnected via slits. Zeotile-2 has a cubic structure with gyroid type mesoporosity. The behavior of Zeogrids and Zeotiles in adsorption, membrane and chromatographic separation and catalysis has been characterized and compared with zeolites and mesoporous materials derived from unstructured silica sources. Shape selectivity was detected via adsorption of n-and iso-alkanes. The mesoporosity of Zeotiles can be exploited in chromatographic separation of biomolecules. Zeotiles present attractive separation properties relevant to CO 2 sequestration. Because of its facile synthesis procedure without hydrothermal steps Zeogrid is convenient for membrane synthesis. The performance of Zeogrid membrane in gas separation, nanofiltration and pervaporation is reported. In the Beckmann rearrangement of cyclohexanone oxime Zeogrids and Zeotiles display a catalytic activity characteristic of silicalite-1 zeolites. Introduction of acidity and redox catalytic activity can be achieved via incorporation of Al and Ti atoms in the nanoslabs during synthesis. Zeogrids are active in hydrocracking, catalytic cracking, alkylation and epoxidation reactions. Zeogrids and Zeotiles often behave differently from ordered mesoporous materials as well as from zeolites and present a valuable extension of the family of hierarchical silicate based materials. (J.A. Martens). zeolite has been successfully applied in many instances, the performance of compacted micron sized zeolite crystals in sometimes can be sub optimal because of inefficient mass and heat transfer. Over the last decade the scientific community has invested heavily in alternative structuring of zeolite matter . Limiting the size of the zeolite particles to the nanometer range is an obvious approach to shorten the intracrystalline diffusion path length. Nanosize versions of many types of zeolites have already been synthesized . However, the retention of nanozeolite in a reactor is problematic, and shaping of the nano material in larger bodies is needed for separating the nano zeolite from reaction products. While compaction and fixation of zeolite nanopowder in a random manner similar 0920-5861/$ -see front matter
The Journal of Physical Chemistry, 1996
The acidity of a series of commercial dealuminated H-Y zeolites was studied by means of MAS NMR and XPS spectroscopy. 27 Al MAS NMR has shown the presence of three types of Al in the dealuminated samples: framework tetrahedral Al, nonframework octahedral Al, and a third type of Al. As inferred from the framework Si/Al ratio obtained by 29 Si MAS NMR spectra and from the evolution of the unit cell parameter determined by XRD, this third type of Al may be localized into the zeolite framework. XPS characterization of the samples in their acid form has allowed us to establish the relative concentration of Al at the outer surface of the particles. Upon adsorption of ammonia on the dealuminated zeolites, three types of acid sites have been evidenced by XPS. The analysis of the XPS data indicates that the strength of the acid sites associated with tetrahedral framework Al atoms is not influenced by the other types of Al. The catalytic results of heptane and decane hydroisomerization have evidenced that the acid sites of certain dealuminated H-Y zeolites may be very efficient. The information provided by MAS NMR, combined with the XPS results and literature data, has allowed us to partially elucidate the origin of the high activity of some dealuminated H-Y zeolites.
Progress in zeolite synthesis promotes advanced applications
Microporous and Mesoporous Materials, 2014
This article outlines the importance of zeolite synthesis and their unique physicochemical characteristics promoting advanced applications. The main strategies for preparation of zeolites including organic-template assisted, organic-template free and alternative procedures are considered for synthesis of crystallites offering control and fine-tuning of their properties. Besides, rational design of zeolites with pre-determined structure, porosity, size, morphology, and composition are more viable by studying carefully the chemical and physical parameters controlling the zeolite synthesis and understanding the crystallization mechanism. Finally, a particular attention to the preparation of zeolites with nanosized dimensions and their utilization in innovative applications including photovoltaic, medicine and holographic sensors are presented.