In situ27Al NMR kinetic investigation of zeolite A crystallization

• In situ27Al NMR used for monitoring of changes in both solution and solid phase.
• Kinetic parameters n and k determined using Sharp–Hancock plots.
• Rate constant k shows increasing trend with increase of batch Na2O concentration.
• Initial Al(OSi)4 content inside gel phase is dependent on batch Na2O quantity.
• SiO2/Al2O3 ratio has pronounced influence on processes proceeding in solution phase.

In this study, in situ27Al NMR spectroscopy was used to gain detailed information concerning kinetics of zeolite A crystallization. Changes of relative intensity and width at half-height of NMR lines at 59 ppm ascribed to Al(OSi)4 solid structures units and at 79 ppm to Al(OH)4− ions in liquid phase during crystallization, were observed. Normalized curves for series of synthesis, performed under different batch molar composition, were used for kinetic analysis applying Sharp and Hancock approach. The influence of total batch Na2O concentration (H2O/Na2O was 20 and 32) for three specific batch SiO2/Al2O3 molar ratios (1.7, 2.0 and 2.5) on the kinetic parameters was investigated. Kinetic analysis of line at 79 ppm provided values of parameter n that ranged from 0.93 to 1.97 indicating contribution of both diffusion and phase boundary controlled mechanisms, depending on batch SiO2/Al2O3 molar composition. On the other hand, analysis of line at 59 ppm showed strong correlation to Na2O batch content. Values of n from 1.94 to 3.34 for syntheses performed at lower alkalinity conditions (H2O/Na2O = 32) indicate Avrami–Erofe’ev nucleation and crystal growth model. Almost dominant presence of diffusion mechanism for synthesis conducted at higher alkalinity conditions (H2O/Na2O = 20) was verified by obtained values of n gathered around 1.5.

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