Synthesis Of Pure Silica Zeolites Research Articles

Synthesis of Pure Silica Zeolites

Synthesis of Pure Silica Zeolites

Sustainable Synthesis of Pure Silica Zeolites from a Combined Strategy of Zeolite Seeding and Alcohol Filling

AbstractCurrently, the synthesis of pure silica zeolites always requires the presence of organic structure‐directing agents (OSDAs), which direct the assembly pathway and ultimately fill the pore space. A sustainable route is now reported for synthesizing pure silica zeolites in the absence of OSDAs from a combined strategy of zeolite seeding and alcohol filling, where the zeolite seeds direct crystallization of zeolite crystals from amorphous silica, while the alcohol is served as pore filling in the zeolites. Very importantly, the alcohol could be fully washed out from zeolite pores by water at room temperature, which completely avoids calcination at high temperature for removal of OSDAs in the synthesis of pure silica zeolites.

Sustainable Synthesis of Pure Silica Zeolites from a Combined Strategy of Zeolite Seeding and Alcohol Filling.

Currently, the synthesis of pure silica zeolites always requires the presence of organic structure-directing agents (OSDAs), which direct the assembly pathway and ultimately fill the pore space. A sustainable route is now reported for synthesizing pure silica zeolites in the absence of OSDAs from a combined strategy of zeolite seeding and alcohol filling, where the zeolite seeds direct crystallization of zeolite crystals from amorphous silica, while the alcohol is served as pore filling in the zeolites. Very importantly, the alcohol could be fully washed out from zeolite pores by water at room temperature, which completely avoids calcination at high temperature for removal of OSDAs in the synthesis of pure silica zeolites.

Study of the performance of imidazolium-derived cations as structure directing agents in the synthesis of zeolites in fluoride media

Three imidazolium cations based on the imidazolium ring, 1,2,3-triethylimidazolium (123TEI), 1,2,3-triethyl-4-methylimidazolium (123TE4MI), and 2-ethyl-1,3-dimethyl (2E13DMI), were employed as structure directing agents (SDA) in the synthesis of pure silica zeolites.

Synthesis, Structure, and Optical Activity of HPM-1, a Pure Silica Chiral Zeolite

2-Ethyl-1,3,4-trimethylimidazolium is a poor organic structure-directing agent in the synthesis of pure silica zeolites using fluoride as a mineralizer at 150 °C. Under these conditions only ill-crystallized solids are obtained after long hydrothermal treatments (several weeks). It disappoints despite its relatively large size, conformational rigidity, and intermediate hydrophilic/hydrophobic character, attributes which would qualify it as a promising structure-directing agent, according to prior investigations. By raising the crystallization temperature to 175 °C under otherwise identical conditions, crystallization is dramatically accelerated. Depending on the water/silica ratio and crystallization time, two different materials are obtained: the recently reported pure silica polymorph of the chiral STW-type zeolite, HPM-1, and the new layered organosilicate, HPM-2. Prolonged heating transforms these phases into the small-pore ITW-type zeolite, while no signs of the SOF-type zeolite (formally built from the same layers as STW) was found. A complete physicochemical and structural characterization of the as-made chiral HPM-1 zeolite is provided, and the proposed stabilization of this zeolite by polarization of the Si-O bond is supported by the observed deviation from tetrahedrality. HPM-1 is optically active, and a study of several crystallites by Mueller matrix microscopy shows that their optical activity can be individually measured and that this technique could be useful for the assessment of the enantiomeric purity of a microcrystalline powder.

Benzylimidazolium cations as zeolite structure-directing agents. Differences in performance brought about by a small change in size

Two imidazolium cations containing a benzyl group, 1-benzyl-3-methylimidazolium and 1-benzyl-2,3-dimethylimidazolium, generally produce the large pore zeolite MTW when used as structure directing agents in the synthesis of pure silica zeolites by the fluoride route. When working at low water contents only the smallest of these cations is able to crystallize MFI, a zeolite with crossing medium pore channels. In that case, MFI is metastable towards MTW, but the in situ transformation slows down as the water content is decreased. Structure-direction is discussed in terms of hydrophobicity, flexibility and size of the cations. A conformational analysis shows that the bulkier 1-benzyl-2,3-dimethylimidazolium is slightly more rigid than the smaller 1-benzyl-3-methylimidazolium, and thus could have been expected to depart more easily from crystallizing a default structure such as MTW. The different performance of both cations is investigated by molecular mechanics and the simulations show that the small increase in size brought about by a methyl group at position 2 of the imidazolium ring prevents an adequate sitting of the cation at channel intersections with the imidazolium ring close to the small cavity in which fluoride resides in MFI. The calculated interaction energies help to understand the phase selectivity and time evolution experimentally observed with both cations.

Zeolite Structure Direction by Simple Bis(methylimidazolium) Cations: The Effect of the Spacer Length on Structure Direction and of the Imidazolium Ring Orientation on the19F NMR Resonances

A series of doubly charged structure-directing agents based on two methylimidazolium moieties linked by a linear bridge of n = 3,4,5, or 6 methylene groups has been used in the synthesis of pure silica zeolites in the presence of fluoride. All of them yielded zeolite TON while only the one with n = 4 was able to produce also zeolite MFI at highly concentrated conditions. In this MFI zeolite, two distinct (19)F MAS NMR resonances with about equal intensity were observed, indicating two different chemical environments for occluded fluoride. With the singly charged 1-ethyl-3-methylimidazolium cation, which can be formally considered as the "monomer" of the bis-imidazolium cation with n = 4, TON and MFI were also obtained, and again two (19)F MAS NMR resonances now with largely dissimilar intensities were observed in MFI. Molecular mechanics simulations support a commensurate structure-direction effect for n = 4 in MFI, with each imidazolium ring, in two different orientations, sitting close to the [4(1)5(2)6(2)] cage. Periodic DFT calculations suggest that F in MFI resides always in the [4(1)5(2)6(2)] cages, with the different (19)F resonances observed being due to the different orientation of the closest imidazolium ring.

Diasteroselective Structure Directing Effect of (1S,2S)-2-Hydroxymethyl-1-benzyl-1-methylpyrrolidinium in the Synthesis of ZSM-12

The chiral cation (1S,2S)-2-hydroxymethyl-1-benzyl-1-methylpyrrolidinium (bmpm) and a 50% mixture of this with its diastereoisomer (1R,2S)-2-hydroxymethyl-1-benzyl-1-methylpyrrolidinium have been prepared and tested as structure directing agents (SDAs) in the synthesis of pure-silica zeolites in fluoride medium. The S,S isomer has been shown to efficiently direct the crystallization of zeolite ZSM-12 (MTW); in contrast, the use of a mixture of the two diastereoisomers as a SDA does not lead to the formation of the ZSM-12 structure under the same synthesis conditions, thus suggesting a lower efficiency of the R,S isomer to direct the crystallization of the structure. A computational study based on molecular mechanic simulations allowed explanation of the efficient structure directing role of the S,S isomer in terms of a high host−guest interaction due to a strong structural relationship between the MTW topology and the molecular geometry of the S,S isomer. However, the simulations revealed that the interac...

Pure Silica Large Pore Zeolite ITQ-7: Synthetic Strategies, Structure-Directing Effects, and Control and Nature of Structural Disorder

Several organic cations derived from the same rigid bicyclic amine 1,3,3-trimethyl-6-azabicyclo[3.2.1]octane have been used as structure-directing agents for the synthesis of pure silica zeolites in the presence of fluoride. The results are rationalized in terms of the varying size and rigidity of the cations. Both concepts were used in defining a large and rigid spiroammonium derivative which afforded the synthesis of the large pore pure silica zeolite ITQ-7. The existence of structural disorder in ITQ-7, which may be controlled through the water/silica ratio of the synthesis mixture, is confirmed by a number of techniques, but high-resolution electron microscopy results ruled out the existence of stacking faults. Thus, disorder in ITQ-7 does not appear to be due to intergrowths and may correspond to the existence of local defects, probably involving double four ring units, causing lattice strain mainly in the ab plane, according to X-ray diffraction simulations.

Mechanism of Structure Direction in the Synthesis of Pure-Silica Zeolites. 2. Hydrophobic Hydration and Structural Specificity

Several syntheses of zeolites that employ organic structure-directing agents are investigated via ^1H-^(29)Si CP MAS NMR spectroscopy. Inorganic-organic interactions in the tetrapropylammonium- and 1,6-hexanediamine-mediated syntheses of pure-silica ZSM-5 (Si-ZSM-5) are compared; these results are contrasted to those from the 1,6-hexanediamine-mediated synthesis of pure-silica ZSM-48 (Si-ZSM-48), in which the diamine serves as a pore-filling agent. Synthesis gels containing tetramethylammonium, tetraethylammonium, tetrabutylammonium, tetrapentylammonium, and tetraethanolammonium in lieu of tetrapropylammonium are evaluated by ^1H-^(29)Si CP MAS NMR to examine the significance of hydrophobic hydration behavior in structure direction. Finally, the effects of sodium, the silica source, and the substitution of D_2O for H_2O on the kinetics of nucleation and crystallization are discussed in terms of their mechanistic implications. A modified mechanism of structure direction in zeolite synthesis is proposed for which the formation of inorganic-organic composite structures is initiated by overlap of the hydrophobic hydration spheres of the inorganic and organic components, with subsequent release of ordered water to establish favorable intermolecular van der Waals interactions.

Mechanisms of Structure Direction in the Synthesis of Pure-Silica Zeolites. 1. Synthesis of TPA/Si-ZSM-5

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTMechanisms of Structure Direction in the Synthesis of Pure-Silica Zeolites. 1. Synthesis of TPA/Si-ZSM-5Sandra L. Burkett and Mark E. DavisCite this: Chem. Mater. 1995, 7, 5, 920–928Publication Date (Print):May 1, 1995Publication History Published online1 May 2002Published inissue 1 May 1995https://doi.org/10.1021/cm00053a017RIGHTS & PERMISSIONSArticle Views2197Altmetric-Citations267LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts