Control of the Morphology of All-Silica BEA-type Zeolite Synthesized in Basic Media

All-silica BEA-type (Si-BEA) zeolite readily crystallizes in fluoride quasi-neutral media, while its synthesis under basic conditions is difficult. To compare the morphological features of BEA-type materials synthesized in quasi-neutral and basic media, a two-step nucleation procedure, which allows controlled synthesis of Si-BEA under basic conditions, was developed. The procedure consists of (i) aging of the initial system at a close to neutral pH, and (ii) raising the pH and a secondary aging in basic media. This procedure was further employed to study the effect of the synthesis formulation, especially its sodium and potassium content, on the crystal morphology of Si-BEA. With an increase in the pH of the system, a gradual decrease of the size of the pyramidal (h0l) face coupled with an increase of the pinacoidal (00l) face was observed. Thus, the morphology of zeolite crystals can be varied from well-developed truncated bipyramids to platelike crystals, where the pinacoidal face is dominant. A consequence of this morphological change is the different representation of the channel systems running along the a and c axes at the crystal surface. Potassium-containing systems provided products with bimodal crystal size distribution without changes in the crystal habit. The amount of K+ was correlated with the generation of small crystals, indicating its influence on the nucleation process. Small concentrations of sodium did not bring along substantial changes in the morphology of BEA-type crystals. At higher concentration, Na+ had a pronounced structure-breaking effect, and no crystalline product was obtained. The framework defectness and hydrophobicity of the materials synthesized in basic media were studied by 29Si NMR spectroscopy and TG analyses, which revealed that the alkali-free materials possessed a concentration of defect sites and hydrophobicity similar to that of the sample obtained under fluoride quasi-neutral conditions.