1,721,010 research outputs found
MFI zeolite nanosheets with post-synthetic Ti grafting for catalytic epoxidation of bulky olefins using H2O2
No full textThe external surfaces of the surfactant-directed, 2.5 nm MFI zeolite
nanosheets were grafted with Ti using titanium(IV) butoxide. The
Ti/MFI nanosheets exhibited a remarkable catalytic performance for
epoxidation of bulky olefins using H2O2 as an oxidant, in comparison
to the mesoporous silicas with amorphous frameworks. © 2015 The Royal Society of Chemistry116161sciescopu
Dry-gel synthesis of mesoporous MFI zeolite nanosponges using a structure-directing surfactant
No full textThe synthesis of mesoporous MFI zeolite nanosponges was investigated in a dry-gel conversion process using C18H37[sbnd]N+(CH3)2[sbnd]C6H12[sbnd]N+(CH3)2[sbnd]C4H9 as a micropore-mesopore hierarchical structure-directing surfactant. The initial synthesis mixture was converted at 333 K to a mesoporous material exhibiting a structure similar to that of MCM-41. The mesoporous material was filtered, dried at 373 K, and subsequently heated at 423 K in an autoclave with controlled humidity. In this process, precise control of the relative chamber humidity and the pH of the precursor gel was necessary to achieve full crystallization of the zeolite. The final product was composed of three-dimensional networks of 2.5-nm thick MFI nanolayers and exhibited a high surface area and narrow distribution of mesopore diameters centered at 4 nm. The zeolite crystallization behavior at 423 K was investigated after various heating times with X-ray diffraction, gas adsorption, and electron microscopy. The results indicated that under controlled humidity, crystallization took place through a pseudomorphic transformation, where the initial gel morphology was maintained throughout. The overall quality of the dry-gel product was similar to that of a conventional hydrothermal product. The Si/Al ratios of the zeolite could be controlled over the range of 50 ∼ ∞. Furthermore, in a 100-g synthesis batch, the dry-gel method had the advantage of compact autoclave size for large-scale synthesis. © 2016 Elsevier Inc.1441sciescopu
Soft-to-hard consecutive templating one-pot route from metal nitrate/phenol resin/surfactant to mesoporous metal oxides with enhanced thermal stability
No full text© 2019Mesoporous materials with crystalline ZrO2, Y2O3, and CeO2 frameworks were one-pot synthesized from a clear solution of ethanol containing metal nitrates, organic surfactant (i.e., Pluronic® F-127), formaldehyde, and phloroglucinol. The solution was converted to a mesostructured nanocomposite of metal nitrate/phenol resin/surfactant via a solvent evaporation-induced self-assembly process. The obtained nanocomposite was calcined at 800 °C in a N2 atmosphere. X-ray powder diffraction and electron microscopic investigation revealed that the calcination caused amorphous-to-crystalline transformations in the metal oxide frameworks, while sintering to bulk metal oxides was prevented by the in-situ generated carbon component. Mesoporous metal oxides with a crystalline framework were obtained when the carbon skeleton was burnt off. The mesoporous metal oxides exhibited high BET surface areas, narrow pore-size distributions, and enhanced thermal stability. A practical benefit of the mesoporous metal oxides was demonstrated with Au/CeO2 exhibiting high catalytic activity in the water-gas-shift reactio
Sulfonium-based organic structure-directing agents for microporous aluminophosphate synthesis
No full textSulfur-centered phenyl, n-butyl, p-tolyl, and benzyl sulfoniums and sulfides were tested as organic structure-directing agents (SDAs) for the synthesis of crystalline microporous aluminophosphates (AlPOs) over various ranges of gel compositions, hydrothermal treatment temperatures, and crystallization times. Among the investigated compounds, triphenylsulfonium gave single-phase products of ATS-type AlPO and silicoaluminophosphate. Other sulfoniums yielded only non-porous, dense crystalline AlPOs. On the other hand, diaryl sulfides gave a tiny amount of AFI-type AlPO amid tridymite and amorphous phases. Based on the results, it was found to be reasonable that weakly basic sulfide could hardly act as a zeotype SDA due to the low protonation to sulfonium at the present synthesis pH. © 2019 Elsevier Inc. All rights reserved.11sciescopu
Understanding and identifying the use of emotes in toxic chat on Twitch
No full textThe latest advances in NLP (natural language processing) have led to the launch of the much needed machine-driven toxic chat detection. Nevertheless, people continuously find new forms of hateful expressions that are easily identified by humans, but not by machines. One such common expression is the mix of text and emotes, a type of visual toxic chat that is increasingly used to evade algorithmic moderation and a trend that is an under-studied aspect of the problem of online toxicity. This research analyzes chat conversations from the popular streaming platform Twitch to understand the varied types of visual toxic chat. Emotes were sometimes used to replace a letter, seek attention, or for emotional expression. We created a labeled dataset that contains 29,721 cases of emotes replacing letters. Based on the dataset, we built a neural network classifier and identified visual toxic chat that would otherwise be undetected through traditional methods and caught an additional 1.3% examples of toxic chat out of 15 million chat utterances.11Nscopu
Bulk crystal seeding on the generation of mesopores by organosilane surfactants in zeolite synthesis
No full textZeolites of MOR, CHA, and FAU-X topologies were conventionally synthesized using Na+ or an organic structure-directing agent, except that organosilane surfactants were added as a mesopore-generating agent. The organosilanes were represented by the structural formula (CH3O)(3)Si-C3H6-N+(CH3)(2)CnH2n+1, where n was varied from 12 to 16 and 18. Each zeolite synthesis was seeded with a small amount of bulk zeolites of the same structure type. The synthesis result indicated that the bulk zeolite seeds disappeared completely from the zeolite products, indicating their disintegration into tiny fragments that were undetectable by high-resolution transmission electron microscopy. Nevertheless, the zeolite seeding caused a dramatic decrease in synthesis time. More importantly, crystal seeding, in comparison to unseeded synthesis, was highly effective in extending the organosilane-directed mesopore-generating strategy to a wide range of mesoporous and zeolite structures.117191sciescopu
MFI zeolite nanosponges possessing uniform mesopores generated by bulk crystal seeding in the hierarchical surfactant-directed synthesis
No full textThe synthesis of a mesoporous material with uniform mesopore diameters and crystalline MFI zeolite walls has been achieved, simply by seeding the multiammonium surfactant-directed synthesis with bulk zeolite crystals. The bulk seeds disappeared in the final product. As a result of seeding, the mesoporous zeolites could be generated rapidly even at high Al content. © the Partner Organisations 2014.133361sciescopu
Facile synthesis of mesoporous zeolite Y using seed gel and amphiphilic organosilane
No full text© 2019 Elsevier Inc.Zeolite Y is widely used as an industrial catalyst for catalytic cracking of heavy feedstocks to lighter olefins. However, incorporation of uniform mesoporosity in zeolite Y without compromising the microporous characteristics is required to improve the catalyst efficiency and minimize catalyst deactivation. Here, we report a facile synthesis method for mesoporous zeolite Y (MPYsg) using seed gel and amphiphilic organosilane, that results in mesoporous zeolite showing a narrow mesopore size distribution (3–4 nm) with high external surface area (110 m2g-1) and high micropore surface area (654 m2g-1). During the crystallization of MPYsg, the organosilane acted as a mesopore-generating agent, and the seed gel provided secondary nuclei that created a uniform mesoporous structure in the zeolite Y nanocrystals. Analysis of aluminosilicate precursors collected during the crystallization revealed that the crystallization process starts with a long nucleation period, followed by rapid crystal growth. Due to the highly accessible strong acid sites present on its external surface, MPYsg shows catalytic activity in the decalin cracking reaction higher than that of conventional microporous zeolite Y11sciescopu
Supporting Nickel To Replace Platinum on Zeolite Nanosponges for Catalytic Hydroisomerization of <i>n</i>‑Dodecane
No full textHydroisomerization
of linear paraffins to branched isomers is an
important petrochemical process, which commonly uses a microporous
zeolite catalyst supported with Pt metal nanoparticles. Although attempts
were made in the past to find an alternative to costly Pt, replacement
with low-cost metals was not successful for conventional bulk zeolites,
due to the low metal dispersion resulting in poor catalytic performance.
Here, we demonstrate that Ni can replace Pt when it is supported on
zeolites with uniform mesopores. Ni-supported MFI-, *MRE-, and TON-type
zeolites possessing mesopores showed the same or even better catalytic
performance in comparison to that of Pt-supported conventional MFI,
*MRE, and TON zeolites, respectively. The Ni-supported zeolites exhibited
even better branched isomer yields, despite low Ni loading. This result
is ascribed to the confinement or sandwiching effect of Ni nanoparticles
by the mesopore walls in the zeolites, which stabilizes the highly
dispersed Ni nanoparticles
Mesopore-selective incorporation of strong Brønsted acid catalytic sites via aluminium grafting on hierarchically porous siliceous MFI zeolite
No full text© 2020 Elsevier Inc. Post-synthetic Al grafting onto the surface silanols in mesoporous MFI zeolite has been investigated in an attempt to obtain a model catalytic material that possesses strong Brønsted acid sites at mesopore surfaces of the zeolite, but not inside the microporous framework. To this end, a siliceous MFI zeolite with a mesopore/micropore hierarchy was synthesized using a meso-micro dual structure-directing diammonium surfactant. The hierarchically porous zeolite was treated with anhydrous AlCl3 for the formation of Si–O–Al bonding with silanols on the mesopore surfaces. According to solid-state 31P NMR spectroscopy of the surface-adsorbed phosphine oxides and catalytic activity for decalin cracking, the Al-grafting treatment performed in the conventional manner resulted in the formation of weak Brønsted acid sites, similar to Al grafting on SBA-15 mesoporous silica. To obtain strong Brønsted acid sites, we have developed a method of heating the AlCl3-grafted zeolite in a controlled-humidity chamber before calcination. The resultant zeolite was catalytically active for cracking of hydrocarbons requiring strong acid sites, but not for methanol-to-hydrocarbon conversion. The results support that the moisture treatment caused a local reconstruction of the initially grafted Al–O–Si framework portion to a strong Brønsted acid site. In addition, our model catalytic zeolite material unveiled the catalytic activity of Brønsted acid sites at the mesopore surfaces in hierarchically porous zeolite in n-octane cracking. Furthermore, we confirmed that this method was effective for other mesoporous MFI zeolites obtained through a post-synthetic desilication route11sciescopu
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