Nuclear magnetic resonance studies of zeolites

Silica speciation in aqueous fluids at high pressures and high temperatures

Inorganic nano- and mesoporous materials can be prepared with a variety of compositions, structures, and macroscopic morphologies. Due to their wide range of properties and applications, a detailed understanding of the mechanisms governing their formation under different synthesis conditions is of particular interest. Magnetic resonance spectroscopy offers a variety of powerful and complementary techniques for acquiring insights on the molecular processes and structures that underly crystallization or self-assembly of these complex heterogeneous systems. This review examines contributions from NMR and EPR spectroscopy to the recent developments and current understanding in the nucleation and growth of nanoporous molecular sieves and mesoporous inorganic solids.

Nucleation and growth of zeolites and inorganic mesoporous solids: Molecular insights from magnetic resonance spectroscopy

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.

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

A model describing the distribution of silicate sites in concentrated alkali metal silicate solutions used in the synthesis of geopolymeric gels is presented. The model is based on a free energy minimization algorithm, using free energy of formation data obtained from the literature. A simplified form of the Pitzer method (Pitzer, K. S. J. Phys. Chem. 1973, 77, 26) is used to calculate activity coefficients, with interaction parameters calculated from known values for ions of similar size and charge type. Ion pairing effects are incorporated into the model formulation by a simple approximation, whereby larger cyclic or cagelike ions form strong ion pairs with dissolved alkali metal cations, and smaller monomeric (Q0) or end-group (Q1) sites pair less strongly with the cations. The model is able to describe accurately the speciation data obtained from a systematic 29Si NMR investigation of sodium, potassium, and mixed (1:1) sodium/potassium silicate solutions. The model is also tested against a large data ...

James R. Jones

Papers

Silicon-29 NMR studies of aqueous silicate solutions: Part II. Isotopic enrichment2