Theo P. M. Beelen

Imaging the Assembly Process of the Organic-Mediated Synthesis of a Zeolite

The formation and consumption of precursors during zeolite crystallisation has been followed by small-angle X-ray scattering techniques. The nanometer-scale precursors are specific for the zeolite topology formed, and their aggregation is an essential step in the nucleation process shown here.

Silicon deposition in diatoms: control by the pH inside the silicon deposition vesicle

To test the hypothesis that silicification occurs under acid conditions in the silicon deposition vesicle (SDV), the acidity of the SDV of the pennate diatoms Navicula pelliculosa (Brébisson et Kützing) Hilse, N. salinarum (Grunow) Hustedt, and Nitzschia sigma (Kützing) Smith was determined during development of new frustule valves. Cells were incubated with the weak base 3‐(2,4‐dinitroanilino)‐3′‐amino‐N‐methylpropylamine (DAMP) followed by immunocytochemical localization in whole cells and on ultrathin sections. After resupplying silicate to cells synchronized by silicon depletion, the uptake of this nutrient from the medium was the same with or without DAMP; new valves developed without morphological aberrations that could conceivably have been caused by the probe. DAMP was found in cellular compartments known to be acidic, such as vacuoles active as lysosomes, the lumen of thylakoids, and microbodies. In the nucleus and mitochondria, which are circumneutral and basic compartments, the probe did not appear. Besides its presence in acidic compartments, DAMP was specifically accumulated within the SDV during formation of new valves; during the process of valve maturation, the SDV seemed to become increasingly acidic. In control experiments using the ionophores chloroquine, valinomycin, and nigericin, the compartmental location of DAMP was clearly disturbed, resulting in a random intracellular distribution. Accumulation of the fluorescent probe rhodamine 123, which can be translocated over membranes by a reducing potential, confirmed that the SDV can translocate weak bases. The results with DAMP suggest that the pH of the SDV is important in the silicification of diatoms: It facilitates a fast nucleation and aggregation of silica particles, thus increasing the rate of formation of the mature frustules. In addition, the acidic environment might protect the newly formed valves against dissolution before completion and coverage by the organic casing prior to their secretion.

SAXS and USAXS Investigation on Nanometer-Scaled Precursors in Organic-Mediated Zeolite Crystallization from Gelating Systems

The formation of precursor particles in the crystallization of zeolites from gelating systems has been studied using X-ray scattering at small angles. The crystallization of Si−MFI using trimethylene-bis(N-hexyl, N-methyl-piperidinium) as the structure-directing agent shows the formation of two categories of precursors:  gel particles and nanometer-scaled primary units. The size of the primary units for the crystallization of Si−MFI is found to be 2.8 nm both for gelating and nongelating systems using different structure-directing agents. Zeolites Si−BEA and Si−MTW have been prepared using the same organic (trimethylene-bis(N-benzyl, N-methyl-piperidinium)) at different concentrations. Primary units with a size of 2.6 nm are found to be present in the synthesis of Si−BEA, while their size is 1.5 nm in the synthesis mixture directing to Si−MTW. Our data suggest that the nanometer-scaled primary units are specific for the zeolite topology formed.

Silica gel dissolution in aqueous alkali metal hydroxides studied by 29SiNMR

The rate of silica gel dissolution in aqueous alkaline media was investigated using 29Si nuclear magnetic resonance spectroscopy. A profound difference of alkali metal hydroxides on the dissolution rate of amorphous silica gel was observed. The dissolution rate increases in order LiOH ≈ CsaOH) < (RbOH ≈ NaOH) < KOH, as was confirmed by the β = silicomolybdate complexation method. Silica gel dissolution involved formation of monomeric silicic acid, Q0. The monomeric anions oligomerise into dimer species, which in turn form cyclic and lineartrimer species. The structure of highly polymerised silicate species depends on the alkali metal cation, i.e. low pH silicate solutions have structurally different silicate species as a function of alkali metal hydroxide, as is shown by 29Si-NMR spectroscopy. Potassium cations gave rise to more polymerised silica species compared with other alkali metal cations studied. On the contrary, when lithium hydroxide and silica gel are mixed in a molar SiO2/Li2O = 1:1, a microcrystalline phase is formed which consists of lithium silicate crystals. Crystallisation of the lithium silicate proceeds via monomeric silica being in solution.

SAXS/WAXS study on the formation of precursors and crystallization of silicalite

Abstract The formation of colloidal aggregates which can act as precursors in zeolite crystallization has been studied using small-angle scattering of X-rays (SAXS). In the same experiment the onset of crystallization and the increase of crystallinity were measured using wide-angle scattering of X-rays (WAXS). To perform in situ time-resolved combined SAXS WAXS measurements high brilliance synchrotron radiation was applied. The influence of silica, template, and OH + concentrations on the synthesis of silicalite (MFI structure) from a clear solution was studied. The alkalinity of the synthesis mixture appeared to be decisive for the formation of colloidal aggregates. At relatively low alkalinity colloidal aggregates were formed prior to the onset of the crystallization. At higher alkalinities no colloidal aggregates could be observed at any time during the synthesis, suggesting that nucleation and crystal growth occurred directly from the solution.

Nanoscale uniformity of pore architecture in diatomaceous silica: a combined small and wide angle x‐ray scattering study

Combined small and wide angle X‐ray scattering (SAXS and WAXS) analysis was applied to purified biogenic silica of cultured diatom frustules and of natural populations sampled on marine tidal flats. The overall WAXS patterns did not reveal crystalline phases (WAXS domain between 0.07 to 0.5 nm) in this biogenic silica, which is in line with previous reports on the amorphous character of the SiO2 matrix of diatom frustules. One exception was the silica of the pennate species Cylindrotheca fusiformis Reimann et Lewin, which revealed wide peaks in the WAXS spectra. These peaks either indicate the presence of a yet unknown crystalline phase with a repetitive distance (d‐value ≈0.06 nm) or are caused by the ordering of the fibrous silica fragments; numerous girdle bands. The SAXS spectra revealed the size range of pores (diameter d between 3.0 and 65 nm), the presence of distinct pores (slope transitions), and structure factors (oscillation of the spectra). All slopes varied in the range of −4.0 to −2.5, with two clear common regions among species: d < 10 nm (slopes –4, denoted as region I and also called the Porod region), and 10.0 < d < 40.0 nm (slopes −2.9 to −3.8, denoted as region II). The existence of these common regions suggests the presence of comparable form (region I) and structure (region II) factors, respectively the shape of the primary building units of the silica and the geometry of the pores. Contrast variation experiments using dibromomethane to fill pores in the SiO2 matrix showed that scattering was caused by pores rather than silica particles. Electron microscopic analysis confirmed the presence of circular, elliptical, and rectangular pores ranging in size from 3 to 65 nm, determining the structure factor. The fine architecture (length/width ratio of pore diameters) and distribution of the pores, however, seemed to be influenced by environmental factors, such as the salinity of and additions of AlCl3 to the growth medium. The results indicate that diatoms deposit silica with pores <50 nm in size and are highly homologous with respect to geometry. Consequently, it is suggested that in diatoms, whether pennate or centric, the formation of silica at a nanoscale level is a uniform process.

Growth and silica content of the diatoms Thalassiosira weissflogii and Navicula salinarum at different salinities and enrichments with aluminium

The dependence of the cellular (biogenic) and frustule-associated (mineralized) silica content of the diatoms Navicula salinarum and Thalassiosira weissflogii on salinity and aluminium conditions was studied in order to make it possible to manipulate silicification in vitro and maximize it to levels required for physico-chemical frustule characterization by physisorption, X-ray scattering analysis and NMR, which is our ultimate objective. Enrichments with AlCl3 increased growth and the final cell density of the pennate N. salinarum, but not of the centric species T. weissflogii. Aluminium additions did not, however, result in a significant increase in the biogenic or mineralized silica content per cell and could not be detected in the silica matrix of the frustule. In contrast, lowering the salinity from 28 practical salinity units (PSU) to 20 and 15 resulted in a significant increase in the biogenic silica content per cell of both species, which is in line with an increase in density of the chemically de...

The structure directing effect of cations in aqueous silicate solutions. A 29Si-NMR study

Abstract Studies of the dissolution of amorphous silica gel in aqueous hydroxides by 29 Si-MAS-NMR, give information on the oligomerisation of dissolved monomeric silicic acid and on the interaction between cations and the silica gel surface. The dissolution rate increases in the order (LIOH ≈ CsOH)

The molecular basis of aging of aqueous silica gel

Abstract The influence of aging on the local structure of aqueous silica gels has been investigated using small angle X-ray scattering (SAXS), solid state nuclear magnetic resonance (NMR), and nitrogen sorption techniques (BET). From SAXS experiments it is deduced that aging of aqueous silica aggregates is a process of migration of active, dissolved silicate species (e.g., monomeric silicic acid) from the more soluble and less dense places within silica aggregates (i.e., the peripheral primary particles) to the less soluble and denser parts within the aggregates (i.e., the core of the aggregates). This process results not only in an increase in size of the scattering primary particles but also in an increase in the gradient of mass density, as such corresponding to a decrease in fractal dimensionality of the silica aggregates from D = 2.25 to D = 2.0. The rate at which aging processes occur is strongly effected by the pH value of the solution. Addition of fluorine anions to the polymerizing silicate solutions causes an increase in rate of restructuring as well. Nitrogen sorption measurements on freeze-dried samples of the gels show that a porous silica structure is only obtained in case the silica gel network has been reinforced through reorganization of the fragile aggregates. The aged silica gel consists of an ensemble of densified aggregates formed by primary particles with sizes in the nanometer range.

Nanometer scale precursors in the crystallization of Si-TPA-MFI

Abstract The crystallization of Si-TPA-MFI from a clear synthesis mixture has been studied in situ using X-ray scattering. Utilizing a combination of scattering techniques and high brilliance synchrotron X-ray radiation, we were able to study a unique range of length scales (four decades), which covers the scattering from all species present in the synthesis mixture. Combined small- and wide-angle X-ray scattering (SAXS-WAXS) results show that for cases with relatively high alkalinity the crystallization occurs while only 2.5 nm sized particles are present in the solution. In case of a lower alkalinity, additional 10 nm sized precursors are present, which act as a gel phase. These 10 nm sized particles probably play a role in the nucleation process, although their presence is not indispensable, as is shown by their absence in the high alkalinity synthesis. Applying in situ ultra-small-angle X-ray scattering (USAXS), the size of the crystals could be monitored, and when their linear growth has finished, aggregation of the discrete crystals to structures larger than 6 urn was found.