Raman spectroscopy of network solids such as zeolites is critical for shedding light on collective vibrations of medium-range structures such as rings that exist in crystals and that form during crystallization processes. Despite this importance, assignments of Raman spectra are not completely understood, though it is often assumed that Raman bands can be assigned to individual rings. We report a systematic zeolite synthesis, spectroscopy, and periodic DFT study of several all-silica zeolites to test this assumption and to determine the fundamental structural motifs that explain Raman spectral features. We have discovered from normal-mode analysis that Raman bands can be assigned to tricyclic bridgesthree zeolite rings that share a common Si–O–Si bridge. Furthermore, we have found that the vibrational frequency of a given Raman band can be correlated to the smallest ring of its tricyclic bridge and not to the ring that is actually vibrating. Finally, we have discovered a precise anticorrelation between Raman frequency and Si–O–Si angle. These discoveries open new ways to investigate structures of network materials made of corner-sharing tetrahedra and to study crystallization from amorphous gels where structural information is limited.
Fluoride (F–) has been essential for the synthesis of low-defect siliceous zeolites. It has been hypothesized that F– balances the positive charges from organic structure-directing agents (OSDAs) and stabilizes key building units during zeolite crystallization such as the double four-membered ring (D4R). However, due to the lack of characterization techniques for investigating medium-range structures, including rings and cages formed during zeolite crystallization, the roles of F– in stabilizing building units and maintaining local charge balance during zeolite assembly are not yet fully understood. Here, the crystallization of siliceous Linde type A (LTA) zeolite in the presence of F– was investigated using Raman spectroscopy and periodic density functional theory (DFT) calculations. We have discovered that the F–-filled double four-membered ring (F–/D4R) and the empty D4R exhibit rather distinct Raman features. Both the F–/D4R and empty D4R are formed in the LTA zeolite synthesized in the presence of F– using 1,2-dimethyl-3-(4-methylbenzyl) imidazolium as the OSDA. Observed Raman bands of the F–/D4R and empty D4R, along with predictive DFT calculations on LTA supercells, reveal an ordered distribution of these two D4R units in the final as-made LTA zeolite. The discovery of these distinct Raman signatures of F–/D4R and empty D4R units opens an interesting new window for studying defects in the D4R during zeolite formation. In particular, we have observed variation in Raman intensities of F–/D4R and empty D4R bands during LTA crystallization; periodic DFT calculations indicate that the observed Raman behavior is consistent with empty D4R units containing one or two Si vacancies surrounded by Q3 Siconsistent also with solid-state nuclear magnetic resonance measurement. These defects appear to heal during further crystallization, leading to the formation of defect-free LTA zeolite crystals. Overall, our results provide deeper understanding on the roles of F– in charge balancing and stabilizing intact D4R units during zeolite formation.
Zeolites are microporous silicates with a large variety of applications as catalysts, adsorbents, and cation exchangers. Stable silica-based zeolites with increased porosity are in demand to allow adsorption and processing of large molecules but challenge our synthetic ability. We report a new, highly stable pure silica zeolite called ZEO-3, which has a multidimensional, interconnected system of extra-large pores open through windows made by 16 and 14 silicate tetrahedra, the least dense polymorph of silica known so far. This zeolite was formed by an unprecedented one-dimensional to three-dimensional (1D-to-3D) topotactic condensation of a chain silicate. With a specific surface area of more than 1000 square meters per gram, ZEO-3 showed a high performance for volatile organic compound abatement and recovery compared with other zeolites and metal-organic frameworks.
Melamine formaldehyde (MF) foam is kind of fire-retardant material and has great potential in acoustic and thermal insulation area. In this article, MF resin foam was prepared by microwave radiation. We discussed the thermal stability of MF foam and the effect of different emulsifiers on its morphology, apparent density, fire-retardancy and mechanical property. The decomposition temperature of MF foam we prepared is nearly 400℃ and the constitution of residue after combustion is made up of carbon and graphite. Emulsifier influenced the apparent density of MF foam and using coemulsifiers can get flexible foam with uniform cell size, good morphology and low apparent density. When the fire-retardant MF foam's apparent density is low of 5.53 kg/cm -3 , its value of LOI can reach 32.4. The mechanical property of foam is consistent with apparent density. Figure 3. TG curve of MF foam.
Selective production of glucose from hydrolysis of cellulose is the key step for efficient utilization of lignocellulose biomass. Crystalline cellulose can be dissolved and hydrolyzed into glucose with a high...
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