NMR quantification in hydrothermalin situ syntheses

The considerable development of microporous materials by hydrothermal synthesis led us to develop appropriate devices to perform NMR in situ observation of the species during the synthesis. The ‘hydrothermal NMR tube’ is a 10 mm outer diameter tube acting as an autoclave, withstanding temperatures and pressures common to these classes of syntheses and also the corrosive conditions of hydrofluoric acid containing media. During synthesis NMR chemical shifts and signal areas are monitored. It is therefore important to achieve quantitative NMR measurements while synthesis proceeds. On increasing the temperature of samples, considerable NMR signal loss occurs as a consequence of Curie's law. Dielectric constant and conductivity effects on signal loss are reflected in the evolution of the quality factor of the probe. As the effects of temperature, concentration of solutes and pH on NMR responses of aqueous samples are considerable, they were therefore carefully studied. The relations between acquisition conditions, transmission and reception and experimental NMR amplitudes were elucidated. A calibration method that does not make use of an internal reference and which is based on relative variations of the Q factor of the r.f. circuit leads to efficient spin counting. This method was successfully applied to quantifying the hydrolysis of aluminum at variable temperature, monitoring 27Al to follow hydrolysis and condensation processes.