The Structural Directing Role of Water and Hydroxyl Groups in the Synthesis of Beta Zeolite Polymorphs

The investigation of stability of as-made Beta zeolite polymorphs A, B, and C (BEA, BEB, and BEC) was carried out on the basis of lattice energy minimization and molecular dynamic atomistic simulations. The force field employed provided an excellent agreement between calculated and experimental enthalpies of formation of zeolites with respect to quartz. BEA and BEB are obtained at low water and KOH concentration in the initial mixture, while BEC forms at higher concentrations of both water and KOH. The four-component systems of unit cell composition (C12NH20+)6/zeolite/F−6/(H2O)l for BEA and BEB polymorphs and five-component systems (C12NH20+)3/BEC/F−2/OH−/(H2O)l, (C18N2H302+)2/BEC/F−2/(OH−)2/(H2O)l were simulated for different water content, l. It was shown that the presence of water molecules stabilized the systems but to a different extent, in a way such that BEC is favored at higher water contents. Water amounts are rationalized in terms of the free volume available in the micropore, not totally filled by the organic. Defects have been found to form in BEC through interaction of hydroxyl anions with Si−O−Si linkages. The number and location of defects (silanol and siloxy groups) are in agreement with 29Si MAS NMR signals due to Q3 (Si) atoms.