New acid zeolites obtained from silicogermanates

Zeolites are microporous crystalline aluminosilicates composed of three-dimensional arrangements of SiO4 and AlO4 tetrahedra. These materials have various applications in the fields of adsorption, catalysis, separation and ion exchange. The introduction of germanium during the synthesis of these zeolites is a strategy for accessing new structures, sometimes with extra-large pores, attractive for the catalytic transformation of bulky molecules. However, a major remaining challenge is the substitution of germanium for aluminum to generate structures with compensation cations assuring the acidic activity. Also, microporous silicogermanates are often unstable in the presence of water after the removal of the organic structure directing agents, which limits their use. To stabilize these silicogermanates, recently, two experimental post-treatment approaches were developed. The first approach allows the initial structure of the parent zeolite to be maintained during the substitution of Ge atoms with other atoms like Al. In the second method elimination of the Ge atoms through hydrolysis leads to substructures that can be connected again creating this way new stable structures having smaller pores due to systematic omission of T-atoms from the original structure. Herein, a combination of theoretical calculations (DFT, Density Functional Theory) and experimental work (synthesis, characterization, catalysis) is used to explore the stabilization of silicogermanates. The ab initio study shows that all silicogermanates having structural codes attributed by the International Zeolite Association and their (alumino)silicates analogues are intrinsically stable. It also indicates that substitution of Ge for Si or Al is possible thermodynamically and is favorable using chloride precursors. As a consequence, a silicon tetrachloride treatment unit was used for the first time to substitute Ge for Si experimentally. This treatment led to the stabilization of the crystalline UTL structure of the IM-12 zeolite. Further treatments using polyaluminum chloride or trichloride solutions succeeded in incorporating aluminum in the zeolite framework. Various elemental and physicochemical techniques (XRD, N2 physisorption, XRF, ICP, FTIR and MAS NMR) were implemented to characterize the materials along the treatment procedure. DFT models of the bulk and the external surfaces of the UTL structure with different elemental composition were build and helped assigning the experimental MAS NMR spectra. Finally, the obtained materials were tested as acid phases in the bi-functional hydroisomerization of n-decane and n-hexadecane, reflecting promising catalytic activity. This work opens perspectives for the catalytic use of stable derivatives of silicogermanate zeolites.