Structure and Colloidal Stability of Nanosized Zeolite Beta Precursors

During the synthesis of zeolite beta in solutions of molar composition 1 SiO2 / 80 H2O / 0.25 TEA2O / 4 CH3CH2OH / 0.06 Na2O / 0.01 Al2O3 three populations of particles are observed by small angle scattering. First, at room temperature, silica self-assembles into primary particles (< 3 nm). Upon heating the least stable primary particles aggregate into secondary particles. Some of the secondary particles are stable as spheroidal monomer particles but others are unstable and aggregate into small clusters. After 4 days of heating most silica comprising secondary particles remains amorphous (by attenuated total reflectance – Fourier transform infrared spectroscopy) but their composition is similar to zeolite beta (by scattering length density matching using small angle neutron scattering) showing that organization of the silica occurs after structure directing agents are occluded into secondary particles. After heating solutions containing Al for 6 days we start to observe Al-secondary particles aggregating into tertiary particles (zeolite beta crystals, < 200 nm, volume fraction 1.1 * 10^-4). The coherence length of the zeolite beta structure in tertiary particles increases sigmoidally over time with most of the changes occurring between 12 days and 18 days of heating when the majority of Al and Al-secondary particles are consumed by the tertiary particles. This shows that as silica reorganizes from amorphous into zeolite beta Al-secondary particles become less stable (in the colloidal sense) and aggregate with tertiary particles. The observed mechanism for zeolite beta nucleation is different from classical nucleation theory where the crystalline order fluctuates simultaneously with the density since we observe that there is a step- wise aggregation of precursor particles and a continuous evolution of their structure towards zeolite beta.