Siliceous zeolite-derived topology of amorphous silica.

The topology of amorphous materials can be affected by mechanical forces during compression or milling, which can induce material densification. Here, we show that densified amorphous silica (SiO2) fabricated by cold compression of siliceous zeolite (SZ) is permanently densified, unlike densified glassy SiO2 (GS) fabricated by cold compression although the X-ray diffraction data and density of the former are identical to those of the latter. Moreover, the topology of the densified amorphous SiO2 fabricated from SZ retains that of crystalline SZ, whereas the densified GS relaxes to pristine GS after thermal annealing. These results indicate that it is possible to design new functional amorphous materials by tuning the topology of the initial zeolitic crystalline phases.

Revealing the evolution of local structures in the formation process of alkaline earth metal cation-containing zeolites from glasses.

Alkaline earth metal cations are ubiquitously present in natural zeolites but less exploited in synthetic zeolites due to their low solubility in water, and hence it remains elusive how they contribute to zeolite formation. Herein, harmotome, a PHI-type zeolite with Ba2+, is readily synthesized from a Ba-containing aluminosilicate glass. This glass-to-zeolite transformation process, in particular the structure-regulating role of Ba2+, is investigated by anomalous X-ray scattering and high-energy X-ray total scattering techniques. The results demonstrate that the steady Ba2+-aluminosilicate interactions not only help prevent the precipitation of barium species under alkaline synthetic conditions, but also dictate the local structures with distinct interatomic distances between the Ba2+ and the surrounding aluminosilicate species throughout the transformation process, which lead to the successful formation of harmotome without detectable impurities. This study highlights the usefulness of the comprehensive X-ray scattering techniques in revealing the formation scheme of the zeolites containing specific metal species. In addition, a promising alternative approach to design and synthesize zeolites with unique compositions and topologies by using well-crafted glasses with suitable metal cation dopants is demonstrated.

Exploring Hydrothermal Synthesis of SAPO-18 under High Hydrostatic Pressure.

The effect of external hydrostatic pressure on the hydrothermal synthesis of the microporous silicoaluminophosphate SAPO-18 has been explored. The crystallization of the SAPO-18 phase is inhibited at 150 °C under high pressures (200 MPa) when using relatively diluted synthesis mixtures. On the contrary, the use of concentrated synthesis mixtures allowed SAPO-18 to be obtained in all the studied conditions. The obtained solids were characterized with XRD, SEM, ICP-AES, TG and 27Al and 31P MAS NMR spectroscopy. The results highlight the importance of the external pressure effect on the hydrothermal synthesis of molecular sieves and its influence on the interaction between the organic molecule and the silicoaluminophosphate network.

Reaction Kinetics Regulated Formation of Short-Range Order in an Amorphous Matrix during Zeolite Crystallization.

The crystallization of zeolites, a disorder-to-order transformation of aluminosilicates, has not been thoroughly understood because the nucleation events in the amorphous matrix are difficult to recognize from the diverse structural changes, especially for the dense hydrogel systems. Therefore, relationships between the synthesis conditions, the generated amorphous species, and the crystallization behavior of zeolites remain unclear. Herein, by comparatively investigating the structural evolution of the aluminosilicate matrix in a dense hydrogel system when different Si reactants (fumed silica and silicate solution) are employed, we demonstrate that the reactivity of the reactants and the kinetics of the condensation reaction is critical to the formation of short-range order in an amorphous matrix, which greatly influences the nucleation frequency of zeolites. It was revealed that an amorphous solid containing plentiful Al-rich four-membered rings and Si-rich six-membered rings could be produced when fumed silica gradually reacted with sodium aluminate solution at 80 °C. It is considered that the interaction between these rings promotes the construction of the essential building units of zeolite X (FAU). In contrast, a complex aluminosilicate matrix was formed immediately when sodium silicate solution was mixed with sodium aluminate solution due to the intense condensation reaction. Furthermore, this complex matrix became more stable when the reactant mixture was hydrothermally treated at 80 °C, which significantly impedes the crystallization process. Aging the reactant mixture at ambient temperature before heating, instead, facilitated the formation of short-range order in the amorphous matrix, which increases the nucleation frequency of zeolites.

Unique crystallization behavior in zeolite synthesis under external high pressures.

The function of pressure has long been overlooked in the hydrothermal synthesis of zeolites, which is typically carried out under an autogenous pressure (below 2 MPa). We herein report, the first of its kind, a detailed investigation on the hydrothermal synthesis of zeolites under external high pressures, where crystallization behaviors contradicting the common observations were generated.

Control of crystallization behaviour of supercooled liquid composed of lithium disilicate on platinum substrate.

Crystalline lithium disilicate (Li2Si2O5, LS2) materials, which have excellent mechanical properties with high transparency, should be obtained efficiently through the crystallization of supercooled liquid composed of LS2. However, in addition to LS2, a lithium monosilicate (Li2SiO3, LS) phase is also precipitated during the crystallization of the liquid. The precipitation of the LS phase renders it difficult to obtain a single-phase LS2 material. Here, we show that by altering the oxygen partial pressure, it is possible to change the selectivity of the precipitated phase by controlling the interfacial phenomena that occur between the liquid and platinum contact material. During cooling of the supercooled liquid, the type of precipitated phase can be controlled by optimizing the atmosphere and type of contact material. This methodology can be applied for the fabrication of other functional materials and does not require the use of other additives.

Ultrafast synthesis of high-silica erionite zeolites with improved hydrothermal stability.

High-silica erionite (ERI) zeolites are conventionally synthesised via a so-called charge density mismatch (CDM) approach, and a typical synthesis takes several days to complete. We herein demonstrate an ultrafast route to synthesise high-silica erionite zeolites in as short as 2 h at 210 °C. The fast-synthesised ERI has been proved to show higher hydrothermal stability compared with the conventionally synthesised product.

Oxygen Speciation in Multicomponent Silicate Glasses Using Through Bond Double Resonance NMR Spectroscopy.

The description of the structure of aluminosilicate glasses is more often centered on its cationic constituents, and oxygen ions determine their connectivity, directly impacting the physical properties of those disordered materials. A very powerful approach to ascertain this short- to medium-range order is to use 17O NMR, but up to now the speciation of the chemical bonds was only ambiguously achieved for multicomponent glasses. Here, we propose to directly probe the very scarcely explored through-bond correlations using 17O{27Al} and 17O{23Na} solid-state nuclear magnetic resonance (NMR) double-resonance experiments. Our approach allows quantifying the strongly overlapping components of the 17O NMR spectra of a quaternary aluminosilicate glass. We observe a cooperative location of alkali and aluminum ions in the neighborhood of bridging oxygens, which is consistent with the modified random network model where the glass structure is composed of two regions: network structure and breakage region (i.e., channel).