Theoretical analysis of the thermodynamics of ZSM-11 zeolite synthesis

Theoretical methods are used to analyze the thermodynamics of ZSM-11 synthesis from amorphous silica and an aqueous solution of tetraalkylammonium hydroxide (TAAOH). The overall process is represented by the reaction 96SiO₂(a) + n(TAA⁺/OH^-/200H₂O) = (nTAA⁺/Z^n-) + 200nH₂O. Both tetrapropylammonium (TPA⁺) and tetrabutylammonium (TBA⁺) cations are considered as the structure-directing agents, and calculations are performed for occlusion of either three or four TAA⁺ cations per unit cell of the zeolite. Both estimates of the change in internal energy and Gibbs free energy reveal that the synthesis of ZSM-11 should be favored by the occlusion of three TBA⁺ cations per unit cell, consistent with experimental observation. The present analysis also demonstrates the importance of energy and entropy changes associated with the dehydration of TAA⁺ and OH^- ions and with the occlusion of TAA⁺ cations into the zeolite. The interactions of OH^- anions with the zeolite framework to form defects in the form of siloxy (≡SiO^-) groups are also considered.