The relative stability of SCM-14 germanosilicate with different distributions of germanium ions in the absence and presence of structure-directing agents

Abstract

We report a computational study of the distribution of germanium ions among the double four-membered rings (D4Rs) in SCM-14 germanosilicate and the influence of a structure-directing agent (SDA) on the stability of the as-synthesized material. For the pure zeolite structure, the calculations suggested clustering of Ge ions and the formation of D4Rs populated entirely by Ge heteroatoms, while part of the D4Rs contain only Si as T-atoms. As a general trend, structures with more Ge–O–Ge bridges are more stable. Variations in the stabilization energy of the zeolite due to the presence of the SDA are larger in magnitude than the variations due to different germanium distributions. For this reason, the presence of the SDA was found to affect the stability order of the individual structures with different germanium distributions but the structures with clustered germanium remain the most stable. The location of the fluoride anion in D4Rs composed of silicon T-atoms or outside the D4Rs results in strong destabilization of the structure. The simulated ¹⁹F NMR chemical shift of fluorine in D4Rs with different germanium contents suggested similar values of the shifts for models with different compositions and different shifts for models with the same composition. The calculated chemical shifts for most of the models with two to eight germanium atoms are in the range of −15 to −5 ppm. Thus, the ¹⁹F NMR chemical shift may not be a reliable method for the determination of the germanium content of the D4Rs in germanosilicates.