Framework Stability of Heteroatom-Substituted Forms of Extra-Large-Pore Ge-Silicate Molecular Sieves: The Case of ITQ-44

The paper reports computational modeling with periodic density functional method of the relative stabilities of pure silicate, germanosilicate, aluminosilicate, titanosilicate, and zincosilicate forms of zeolite with IRR-type topology (ITQ-44). The calculated relative substitution energies of Ge in different crystallographic positions correspond very well with the experimentally observed occupation of Ge in ITQ-44 zeolite. The results also suggest that the Si form of ITQ-44 zeolite is stable without the presence of Ge in the framework. The incorporation of Al in structural positions in the silicate and germanosilicate form stabilizes the framework and aluminosilicate form of zeolite is found more stable than the pristine germanosilicate. Taking into account that, so far, ITQ-44 cannot be synthesized without Ge, one can suggest that a potential role of Ge is the shift of the equilibrium in the initial system to species that favor the formation of the IRR structure. The preference for replacement of Ge or Si in the IRR structure, which is related with possible post-synthesis isomorphic substitution in the framework, has also been studied.