Time-resolved dissolution elucidates the mechanism of zeolite MFI crystallization

Zeolite crystal growth mechanisms are not fully elucidated owing to their complexity wherein the formation of a particular zeolite can occur by more than one crystallization pathway. Here, we have conducted time-resolved dissolution experiments of MFI-type zeolite crystals in ammonium fluoride medium where detailed structural analysis allowed us to extrapolate and elucidate the possible mechanism of nucleation and crystal growth. A combination of electron and scanning probe microscopy shows that dissolution initiates preferentially at lattice defects and progressively removes defect zones to reveal a mosaic structure of crystalline domains within each zeolite crystal. This mosaic architecture evolves during the growth process, reflecting the changing conditions of zeolite formation that can be retroactively assessed during zeolite crystal dissolution. Moreover, a more general implication of this study is the establishment that dissolution can be used successfully as an ex situ technique to uncover details about crystal growth features inaccessible by other methods

Insights into the chemical composition of Equisetum hyemale by high resolution Raman imaging

Equisetaceae has been of research interest for decades, as it is one of the oldest living plant families, and also due to its high accumulation of silica up to 25% dry wt. Aspects of silica deposition, its association with other biomolecules, as well as the chemical composition of the outer strengthening tissue still remain unclear. These questions were addressed by using high resolution (<1 μm) Confocal Raman microscopy. Two-dimensional spectral maps were acquired on cross sections of Equisetum hyemale and Raman images calculated by integrating over the intensity of characteristic spectral regions. This enabled direct visualization of differences in chemical composition and extraction of average spectra from defined regions for detailed analyses, including principal component analysis (PCA) and basis analysis (partial least square fit based on model spectra). Accumulation of silica was imaged in the knobs and in a thin layer below the cuticula. In the spectrum extracted from the knob region as main contributions, a broad band below 500 cm−1 attributed to amorphous silica, and a band at 976 cm−1 assigned to silanol groups, were found. From this, we concluded that these protrusions were almost pure amorphous, hydrated silica. No silanol group vibration was detected in the silicified epidermal layer below and association with pectin and hemicelluloses indicated. Pectin and hemicelluloses (glucomannan) were found in high levels in the epidermal layer and in a clearly distinguished outer part of the hypodermal sterome fibers. The inner part of the two-layered cells revealed as almost pure cellulose, oriented parallel along the fiber

Submicrometer Zeolite A Crystals Formation: Low-Temperature Crystallization Versus Vapor Phase Gel Transformation

International audienceIn the present Study, low-temperature hydrothermal crystallization and vapor phase gel transformation have been employed to synthesis LTA-type zeolite Crystals of submicrometer size. The crystal growth curves under hydrothermal conditions at 35, 50, and 65 degrees C were obtained. From these curves, the induction periods and the times for reaching maximum crystallinity for respective temperatures were determined. A set of characterization methods, including XRD, FTIR, SEM, TEM, and DLS, was employed to obtain complementary information. The inspection of the crystalline solids obtained under hydrothermal conditions showed that the size of the crystals is a function of the crystallization temperature. The largest crystals (300 x 300 nm) were obtained at 65 degrees C, while much smaller particles were synthesized at lower temperatures. Second approach involved vapor-phase transformation of the solid part of initial gel. This approach has also yielded submicrometer-sized zeolite crystallites. The characteristics of crystalline particles synthesized by two methods were compared Both synthetic methods showed relatively high conversion (70-80%) of the initial gel into LTA type zeolite, which is substantial advantage in respect to nanozeolite synthesis from clear solutions