Aggregation and Peptization Behavior of Zeolite Crystals in Sols and Suspensions

Many catalyst shaping procedures such as spray drying or extrusion involve aggregation of zeolite crystals with particles of the binder or matrix in the liquid phase. A better control of these processes should result in catalyst bodies with improved diffusional and mechanical properties. We have studied the attractive and repulsive interactions between zeolite crystals in aqueous sols and suspensions by monitoring the zeta (ζ) potential and the mean aggregate size at varying pH values. The zeolites under investigation were of the structure types MFI, *BEA, and OFF, with single-crystal diameters ranging from 65 to 3500 nm and with Si/Al ratios between 5 and >2000. The following major observations were made: (i) at the isoelectric point (IEP, defined as the pH at which ζ = 0 mV), zeolites undergo spontaneous aggregation; (ii) framework aluminum causes a shift of the IEP to lower pH values; (iii) the IEP shifts to very low values as the template of zeolites containing aluminum is removed; and (iv) aggregation and peptization are completely reversible. Among the samples under investigation, no effect of the structure type could be observed. The size of the crystals can affect the slope of the ζ-potential curve as a function of the pH, but not the position of the IEP.