ChemInform Abstract: A New Microporous Polymorph of Silica Isomorphous to Zeolite MCM-22 (original) (raw)
Related papers
Journal of the American Chemical Society, 2012
A new small-pore germanosilicate zeolite, named as ITQ-49, has been synthesized using a new ditetraalkylphosphonium dication as an organic structuredirecting agent, and its structure has been solved by direct methods applied to the powder X-ray diffraction pattern of the calcined solid. This new zeolite crystallizes in the space group Immm with cell parameters a = 19.6007(8) Å, b = 18.3274(7) Å, and c = 16.5335(6) Å. The pore topology of ITQ-49 consists of large, nonspherical cavities that are connected to each other through small eight-memberedring windows, resulting in a unidirectional small-pore zeolite that has a relatively large adsorption capacity. Also, ITQ-49 contains double four-membered-ring units where Ge is preferentially located, and fluoride anions are placed inside these units. Z eolites are crystalline microporous materials, mainly constituted by oxides of silicon and/or other elements (e.g., Al, B, Ti, Ge, Sn) in tetrahedral coordination. The welldefined size and distribution of the structural channels in each different zeolitic framework type confer to these materials multiple applications in processes such as gas adsorption, separation, catalysis, and encapsulation or controlled release of molecules, among others, some of them with industrial applications. 1−9 This is the major force for studying novel methods for the preparation of new zeolitic structures, since it would give the possibility of obtaining tailored materials with the most appropriate channel system for each specific application. To date, up to 201 different zeolitic structures have been accepted by the International Zeolite Association, 10 and this number is still increasing.
Microporous and Mesoporous Materials, 2019
The transformation of microporous, amorphous silica membranes into b-oriented silicalite-1 (MFI) zeolite layers via in-situ crystallisation was investigated. The effect of synthesis parameters, such as the type and concentration of the silica precursor, crystallisation time and temperature, on the morphology of silicalite-1 (MFI) zeolite layers was studied. By optimizing these parameters, silicalite-1 zeolite layers were formed from the already-deposited silica layers, which promotes the crystallisation from the surface in the preferred b-orientation. The use of a monomeric silica precursor, which has slower hydrolysis kinetics than a colloidal one, resulted in the formation of zeolite crystals via heterogeneous nucleation on the surface and suppressed the formation of crystal nuclei in the liquid media via homogeneous nucleation, which then would further deposit onto the surface in a random orientation. Lastly, by optimizing the crystallisation time and temperature of the synthesis, thickness, coverage and orientation of silicalite-1 zeolite layers were controlled.
Journal of Materials Chemistry, 2001
Zeolite Beta/MCM-41 composites have been prepared with various crystallinities of zeolite Beta through a twostep crystallization process involving the combination of low crystallinity zeolite Beta synthesis gel with cetyltrimethylammonium bromide surfactant solution. The composites are characterized by XRD, SEM, 29 Si and 27 Al MAS NMR, N 2 adsorption, NH 3-TPD and catalytic cracking. The experimental results show that the relative crystallinity of Beta in the composites greatly affects the morphology and surface acidity of the materials, while exhibiting similar 29 Si, 27 Al MAS NMR spectra and N 2 adsorption-desorption isotherms. The pore structural data indicate that the composites contain bimodal mesopore systems and microporous structures of zeolite Beta. Although the number of weak acid sites on the composites with various crystallinities of Beta is comparable, composites with higher Beta crystallinity possess more medium acid sites and show higher catalytic activity for n-heptane cracking.
Closely Packed Zeolite Nanocrystals Obtained via Transformation of Porous Amorphous Silica
Chemistry of Materials, 2004
Amorphous silica grains were subjected to a hydrothermal treatment to be transformed into closely packed ZSM-5 zeolite nanocrystalline bodies. Three synthesis approaches have been developed: (I) direct hydrothermal treatment of the amorphous silica grains in a ZSM-5 precursor solution; (II) impregnation of charge-reversed amorphous silica grains with 2-10 nm preorganized units followed by a hydrothermal treatment in a silica-free precursor synthesis solution; and (III) electrostatic adsorption of 50-nm sized ZSM-5 seeds on the amorphous silica grains followed by a hydrothermal treatment with a ZSM-5 precursor solution. The synthesized solids were characterized by X-ray diffraction, Raman spectroscopy, TG analysis, N 2 adsorption measurements, and scanning electron microscopy. The resulted zeolitic bodies are built of uniform closely packed nanocrystals and retain the size and morphological features of the initial amorphous silica grains. The crystallinity, the average size of crystallites, and the mechanical properties of the nanozeolite bodies depend strongly on the synthesis procedure. It was found that procedure II provides mechanically stable bodies built of closely packed nanocrystallites with a size of about 40 nm, which are very promising for the production of self-bonded nanozeolite structures.
Synthesis and characterization of the MCM-22 zeolite
Zeolites, 1995
MCM-22 has been synthesized as a unique phase and with good crystallinities in a range of different Si02/AI=O3 ratios using hexamethylenimine as a template. For Si02/AI=O3 ratios around 30, MCM-22 can be obtained as a pure phase and with good yields in a relatively broad range of OH/SiO2 ratios, although the efficiency in Si incorporation decreases with increasing pH. For low Al=O3 contents (SiOJAI203 /> 70) the sample becomes contaminated by ZSM-5. This contamination can be eliminated by lowering the crystallization temperature. If the synthesis is carried out under static conditions, ferrierite also appears in the crystallized sample. Upon calcination to eliminate the organic template, some dealumination occurs, while strong acid sites are formed, pyridine being accessible to all hydroxyl groups associated to framework AI. This together with the large void volume of the zeolite suggest the presence of a multidirectional system of channels where at least some of the channels are formed by large pores.
Crystallization of Zeolites from Organo-Silicic Colloids
Inorganic Chemistry, 2006
As shown recently, the networks of mesoporous high-surface-area silicates and zeolites undergo a deep depolymerization process in glycerol, near 200°C. Within 1 h, X-ray diffraction analysis amorphous gels are obtained. However, some local ordering subsists as demonstrated by a striking similarity between the silicon and aluminum high-resolution solid-state NMR spectra before and after the reaction. The residual organization could be investigated indirectly in studying the recrystallization of these gels in the presence or absence of structure-directing agents. Were this attempt successful, the way should be opened for the synthesis of molecular sieves starting from gels obtained from naturally occurring zeolites. Here, it will be shown that an amorphous gel obtained from HZSM-5 recovers the initial long-range structure of the parent material in a few hours at 85°C in the presence of an aqueous solution of tetrapropyl ammonium (TPA) or NH 3. The recrystallization of HY requires the presence of tetramethylammonium, but about 25% of the crystallization is obtained rapidly (=1day) at 80°C with ammonia. Hypotheses about the preorganized structural units are presented. The value of the Si−O−Si angle in the silica cluster seems to be of paramount importance.
2020
Microspherical ZSM-5 zeolite possesses a hierarchical porosity with high crystallinity and an adequate surface area was successfully synthesized using the self-assembling method induced by aging in the absence of seedingassistance, a mesoscale template and organosilanes. The structural properties of the hierarchically porous ZSM-5 zeolite were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis by X-ray (EDAX) thermogravimetric analyzer (TGA), Fourier transform infrared spectrophotometer (FTIR) and nitrogen adsorption/desorption isotherms. The results showed that the formation of microspherical clusters of polycrystalline nanosized ZSM-5 by a conventional hydrothermal treatment at 170 � C in the range of crystallization time 40-72 h from colloidal silica in the absence of seeds. Aging is thought to be the key factor influencing the generation of the hierarchically porous ZSM-5 with a Si/Al ratio of 12.5, surface area of (86.0302 � 2.1698 m 2 /g) and pore volume of (0.0684 cm 3 /g) via the self-assembling method in the absence of seedingassistance, a mesoscale template and organosilanes. Also, ethanol was an essential element to direct the crystallization toward a favorable product with an outstanding shape and hierarchical porosity. Investigating the order of mixing confirms the importance of adding aluminates precursor to silicates precursor to gain MFI structure.
Zeotile-2: A microporous analogue of MCM-48
Solid State Sciences, 2005
Ordered mesoporous materials with specific microporosity in the mesopore walls can be assembled by a secondary templating synthesis departing from a clear subcolloidal suspension dedicated to the tetrapropylammonium (TPA) mediated synthesis of colloidal Silicalite-1. A typical member of this material family is Zeotile-2. Zeotile-2 is mesostructurally similar to the cubic MCM-48 material with exceptional long-range order of the mesostructure. Zeotile-2 samples in which the TPA was either left or evacuated were prepared by leaching in boiling ethanolic acetic acid and calcination. The evacuation of the TPA gave rise to a substantial micropore volume revealed with nitrogen adsorption isotherms. The mesoporosity was independent of the presence of the TPA. Molecular separations of isooctane/octane mixtures illustrated the occurrence of molecular shape selectivity similar to MFI-type zeolites.