Contents 1. Introduction 3885 2. Zeolites 3887 2.1. General Remarks 3887 2.2. Anhydrous High-Silica Zeolites 3889 2.3. Anhydrous Aluminosilicate Zeolites 3891 2.4. Zeolite Hydration and Cation Exchange 3891 3. Mesoporous Silica 3895 4. Interactions with Structure-Directing Agents 3895 5. Zeotypes 3896 6. Synthesis and Transformations of Zeolites and Related Materials 3897 7. Concluding Remarks 3899 8. References 3900

Small-angle X-ray scattering (SAXS) and microcalorimetry were used to study the dissolution of silica nanoparticles that serve as precursors in the synthesis of the pure-silica zeolite, silicalite-1. Temporal changes in nanoparticle size were monitored by SAXS to obtain radial dissolution rates on the order of 1 × 10 -2 nm/min, 10 times greater than those of silicalite-1. Nanoparticle dissolution rates are independent of solution alkalinity (above pH 11) and particle surface area, although contributions from the latter account for more than 60% of the nanoparticle enthalpy of dissolution (13.5 ( 0.1 kJ/mol SiO 2 relative to silicalite-1). We show that dissolution enthalpies and rates correlate to the molecular structure of silicates. Comparisons among amorphous silica, silicalite-1, and silica nanoparticles suggest that the latter are amorphous and therefore not simply small fragments of a crystalline zeolite. Nevertheless, they do possess a degree of ordering greater than that in dense amorphous silica. Dissolution experiments were also performed on heat-treated nanoparticles grown via Ostwald ripening. With increasing time of heat treatment, the nanoparticle dissolution rates and enthalpies decrease in magnitude toward those of silicalite-1, suggesting a structural reorganization of silica within the particles. The results offer insight on silicalite-1 nucleation as well as relevant time scales and rate-determining steps involved in zeolite crystallization.

Mesoporous silica phases, with uniform pores of dimensions in the 2-30 nm range, offer a uniquely well-defined environment for the study of the effects of two-dimensional spatial confinement on the properties of glass-forming liquids. We report observations by differential scanning calorimetry of the vitrification of o-terphenyl (OTP), salol, and glycerol in hexagonal mesoporous silica (MCM-41 and SBA-15) in a wide range of pore sizes from 2.6 to 26.4 nm. In agreement with previous studies, where a controlled porous glass is used as a solid matrix, the glass transition temperature for o-terphenyl diminishes with decreasing pore size. In contrast to OTP, glycerol shows a gradual increase in glass transition temperature, while in salol a slight reduction of glass transition temperature is observed, followed by an increase, which results in glass transition temperature indistinguishable from that of the bulk for the smallest pores. These results are discussed in terms of liquid-surface interactions in an interfacial layer, monitored by Fourier-transformed infrared spectroscopy in the study. The hydrogen bonding with silica surface silanols dominates the glass transition trends observed in salol and glycerol.

Two families of calcined highly ordered mesoporous silicas, designed as M41S (MCM-41 and MCM-48) and SBA-n (SBA-15 and SBA-16), are investigated in a wide range of pore sizes from 2.1 to 26.4 nm by high-temperature oxide melt solution calorimetry using lead borate solvent at 974 K. These data are consistent with and extend our earlier studies of zeolite microporous and of mesoporous silicas. The formation enthalpies observed are 19.0−31.4 kJ/mol less exothermic than that of quartz and correlate linearly with pore size for a given structure type. Small- and wide-angle X-ray scattering, nitrogen adsorption (BET), thermogravimetric analysis, and 29Si NMR are employed to give insight into structure and symmetry. The enthalpy differences among samples are discussed in terms of symmetry and structural (point and ring) defects in the materials.