Ejaz, Tabassum; (1997) Hydrothermal precipitation of zeolite A crystals. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Synthetic zeolite-4A, a crystalline, high-volume, speciality chemical product, is mainly used as a water softener in detergents. During its chemical synthesis an initial amorphous gel transforms into zeolite-4A crystals but as yet there is no agreement as to the mechanism. Both solution-mediated and solid-state transformation mechanisms have been postulated but conclusive evidence has proved elusive. The objectives of this study were to design experiments elucidating the transformation mechanism, investigate nucleation phenomena and examine the possibility of crystallising zeolite-4A without gel formation. Solubility measurements at synthesis pertinent conditions were undertaken using ICPAES, TVM, XRF and TGA. It was found that zeolite-4A solubility, which increases with temperature and [NaOH], is lower than the amorphous phase. Precursor solubility increases with caustic concentration but decreases with temperature. A novel technique is conceived combining an original precipitation cell, capable of withstanding hydrothermal zeolite-synthesis conditions, and digital image processing. This was used to present in-situ microscopic evidence of a solution-mediated amorphous to zeolite-4A transformation at synthesis temperatures of 50-65°C. Below 50°C no amorphous phase dissolution was observed despite noticeably increasing crystallinity. These syntheses were repeated on a 1.5-litre scale where turbidity, conductivity and pH measurements were recorded. Results from these experiments verified the microscopy findings. Decreased turbidity, pH, plus increased conductivity mark gel phase dissolution at 50-65°C. Similar changes are absent at lower temperatures. The turbidimetric data was used to calculate an activation energy of 57.6 kj/mol for zeolite crystallisation. The gel structure, its solubility characteristics and the colloidal nature of the amorphous particles were used to postulate macro- and micro-scale dissolution based solution-mediated transformation mechanisms. Macroscale dissolution is suggested for syntheses at 50-75°C, where free liquid plus that trapped within gel inclusions and pores dissolves the amorphous solid. Microscale dissolution is proposed for syntheses below 50°C where dissolution occurs within inclusions and pores of the amorphous solid only. Gel dissolution was thought to result from solution desupersaturation and disruption of the electrical double layer surrounding amorphous particles, caused by zeolite crystal formation. Nucleation phenomena were investigated using turbidimetric and conductivity-based induction time measurements. Results for zeolite A indicate homogeneous nucleation for synthesis temperatures below 50°C and heterogeneous above that. Results for the amorphous precursor are consistent with a homogeneous nucleation mechanism. Interfacial tension for zeolite-A was higher than the amorphous phase at synthesis temperatures below 50°C but found to be lower at above 50°C. Finally, microscopic evidence is presented for zeolite A synthesis from clear solution (i.e. without initial amorphous gel formation) using recipes, based upon the solubility results, undersaturated to the amorphous phase. These results may have important implications for the controlled industrial manufacture of zeolite crystals and provide new directions for zeolite crystallisation research.

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