1,721,156 research outputs found
Raman spectra of tetramethylammonium ion-containing molecular sieves
No full textRaman spectra of the tetramethylammonium (TMA) ions trapped inside the beta- or gmelinite cages of molecular sieves with different framework compositions and structures are presented. Most skeletal Raman bands, especially the nu(s)(C-N) and nu(as)(C-N) bands of TMA in the beta-cages of molecular sieves with SOD, LTA and FAU structures, are not sensitive to the framework composition, but are sensitive to the structure type of the molecular sieves. The nu(s)(C-N) and nu(as)(C-N) bands of TMA in the beta-cages of these zeolites give a correlation with the average intertetrahedral T-O-T angles calculated using the nu(s)(T-O-T) vibration of the corresponding zeolites. However, such a correlation is not found in the Raman spectral changes of TMA with calculated T-O-T angles for non-zeolitic molecular sieves. Raman spectra of TMA in gmelinite cages show no significant effect from either the framework composition or the structure type of molecular sieves when compared to the spectra from TMA in the beta-cages. The appearance of an intense structural Raman band at 1000-1200 cm(-1) in the Raman spectra of non-zeolitic molecular sieves suggests that the technique can be useful to prove the substitution of heteroatoms other than Al or Si in the framework of molecular sieves.1125sciescopu
Use of flexible diquaternary structure-directing agents in zeolite synthesis: Discovery of zeolites TNU-9 and TNU-10 and their catalytic properties
No full textThis review summarizes our recent work on zeolite syntheses in the presence of linear diquaternary alkylammonium ions, with the object of controlling the conformations of these flexible, hydrophilic organic structure-directing agents by varying the type and concentration of inorganic components in synthesis mixtures. After describing how we have been concerned about the search for novel zeolite structures through this synthetic strategy, we discuss the cooperative structure-directing effects of 1,4-bis(N-methylpyrrolidinium)butane and specific levels of Na+ ions and lattice negative charges introduced by Al substitution on the crystallization of new medium-pore zeolites TNU-9 and TNU-10. The physicochemical and catalytic properties of these two materials are also outlined.X1113sciescopu
Double 6-Ring as a Unique Cation Site for the Ce3+/Ce4+ Redox Couple in Zeolites
No full textLuminescence spectroscopy reveals that the intrazeolitic redox process of Ce3+ Ce4+ takes place only inside the double 6-ring (D6R) unit among the cation sites in seven different zeolite structures studied here. This unexpected phenomenon can be rationalized in terms of the spatial constraints in D6Rs imposed by the location of Ce3+ ions larger than Ce4+ ions, leading to a considerable decrease in the potential barrier of the Ce3+/Ce4+ couple.X1110sci
Mechanistic Investigations of Ethylbenzene Disproportionation over Medium-Pore Zeolites with Different Framework Topologies
No full textEthylbenzene disproportionation has long been recognized as a useful test reaction for distinguishing between medium- and large-pore zeolites because medium-pore materials show neither an induction period nor a diethylbenzene deficit, unlike large-pore ones. Hence, the mechanism of this reaction over all medium-pore zeolites has been regarded as simply monomolecular to date. Here, we demonstrate that the pore architecture of medium-pore zeolites strongly influences the type of main reaction intermediates of ethylbenzene disproportionation and, thus, that of the prevailing reaction mechanism: dual-cycle diphenylethane-mediated, bimolecular diphenylethane-mediated, or monomolecular ethyl-transfer reaction pathway. To our knowledge, our study provides more manifest experimental evidence for the transition state shape selectivity in zeolite catalysis than has ever been reported.X112219sciescopu
N,N,N,N ',N ',N '-HEXAMETHYLPENTANEDIAMMONIUM-MWW LAYERED PRECURSOR: A REACTION INTERMEDIATE IN THE SYNTHESIS OF ZEOLITES TNU-9 AND EU-1
No full textIt is found that the layered precursor of a medium-pore zeolite MCM-22, frequently referred as MCM-22(P), which was initially formed in the presence of a flexible, linear diquaternary cation, N,N,N,N',N',N'-hexamethylpentanediammonium (Met(6)-diquat-5), and Na+ is a reaction intermediate in the synthesis of another two medium-pore zeolites TNU-9 and EU-1. The overall results of this study reveal that the crystallization of the latter two zeolites under hydrothermal conditions at the expense of the Me-6-diquat-5/MCM-22(P) composite material depends not only on the concentration of alkali metal cations in synthesis mixtures, but also on their type. (C) 2009 Elsevier Inc. All rights reserved.X119sciescopu
SiO- Defects in As-Synthesized Pure-Silica and Aluminosilicate Sodalites
No full textThe existence and distribution of Si-O- defects in pure-silica and aluminosilicate sodalites prepared from nonaqueous media containing ethylene glycol are investigated by Na-23 MAS NMR and XPS techniques. The overall results of this study demonstrate that Si-O- defects are predominantly present in the surface/near surface of pure-silica sodalite crystals and thus are not uniformly distributed. By contrast, the isomorphous substitution of Si by Al in the sodalite framework is found to produce nearly defect-free aluminosilicate sodalites.X119sciescopu
Aberration-corrected STEM analysis of the RHO family of zeolites with embedded isoreticular structures
No full textSpherical aberration (C-s) corrected STEM has proved to be an excellent approach for obtaining high resolution images of beam sensitive materials. Using this analytical method, for the first time, we were able to image the structural expansion of four members of the RHO family of embedded isoreticular zeolites along [001] orientation and thus to identify the main constituent cages, conforming their structures. (C) 2016 Elsevier Inc. All rights reserved.1155sciescopu
MSE-Type Zeolites: A Promising Catalyst for the Conversion of Ethene to Propene
No full textThe direct conversion of ethene to propene (ETP) is a potentially important route for the selective production of the latter olefin. Here we report that after some time on stream, H-UZM-35, an MSE-type large-pore zeolite, shows much better propene yield than H-SSZ-13, the best catalyst for the ETP reaction thus far. The key to this improvement is the presence of large cylindrical cages in H-UZM-35 that allows the easy formation of isopropylnaphthalene-based reaction centers for ETP catalysis, while being relatively resistant to coke formation. Mild dealumination was found to further mitigate catalyst deactivation.11129sciescopu
Formation of Paramagnetic Defect Centers in Aluminophosphate Molecular Sieves
No full textThe formation of paramagnetic defect centers in aluminophosphate (AlPO4) molecular sieves with different structures is investigated by electron spin resonance (ESR) spectroscopy. Distinct differences in the: position and number of the ESR signals generated by dehydroxylation of AlPO4 molecular sieves under a vacuum of 10(-5) Torr at 773 K clearly show that the nature of the paramagnetic defect centers formed is significantly different in the structure type of the molecular sieves. In addition, it is found that the spin concentration of these defects depends strongly on the pore size of the AlPO4 molecular sieves as well as their hydration level before dehydroxylation. The larger ring size the molecular sieve has, the higher spin concentration of paramagnetic defects it shows, Thus, the extra-large-pore VPI-5 shows the highest spin concentration value among the molecular sieves studied here, which corresponds to 6.4 defects per 1000 unit cells. The ESR results obtained from AlPO4-based molecular sieves containing heteroatoms other than aluminum and phosphorus (such as silicon or cobalt) reveal that the formation of paramagnetic defect centers is severely restricted when the amount of heteroatoms in their framework increases. On the basis of the overall ESR results of this study, a reliable mechanism of formation of paramagnetic defect centers in AIPO(4) molecular sieves is proposed.1119sciescopu
Host-Guest Interactions in Pure-Silica and Aluminosilicate Sodalites Containing Ethylene Glycol as a Guest Molecule
No full textFour sodalite materials with Si/Al ratios of infinity, 23.0, 11.0, and 5.7 are synthesized from essentially nonaqueous media in which ethylene glycol is acting both as a solvent and as a structure-directing agent. Elemental and thermal analyses, X-ray powder diffraction, multinuclear MAS NMR, and vibrational spectroscopies are employed to extensively investigate the host-guest interactions in as-synthesized materials. It is found that the hydrogen-bonding feature of the ethylene glycol molecules encapsulated within the P-cages of sodalites is strongly affected by the Al content in the framework. The overall results of this study lead us to conclude that the molecular conformation of ethylene glycol in the pure-silica beta-cage may be tGg' stabilized only by one intramolecular hydrogen bond, while that of the guest molecule in the beta-cage with a Si/Al ratio of 5 is gGg' stabilized by one strong hydrogen bond to ; the framework in addition to one intramolecular hydrogen bond.X1135sciescopu
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