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+/Zn-) + 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry