Zeolites are important as catalysts and functional materials in many applications. Our main interest is on interactions at interfaces, nanostructured materials, sol-gel processes, self-assembly phenomena, pore design. Solid-state NMR spectroscopy is employed as major tool to characterize various aspects of zeolites.
Zeolites are microporous materials with technological importance for applications in many areas, such as heterogeneous catalysis, ion exchange, separation technology and others. The classical zeolite structures consist of SiO4/2 and AlO4/2- tetrahedra that are connected to a three-dimensional framework. Charge balance for Al is achieved by extraframework cations. Since Al-O-Al connectivities are uncommon due to the Loewenstein rule, the silicon-to-aluminum ratio is Si/Al≥1. Examples with high Al content are zeolites A, X and Y, or the dense sodalite structure, all of which contain the sodalite cage. The synthesis of high-Al zeolites is typically carried out under hydrothermal conditions under basic pH conditions in the presence of alkali cations. The pores of the as-made crystals are filled with extraframework alkali cations and water. Ion exchange, for example of zeolite Y, with NH4+ and subsequent calcination generates Brønsted acid sites which are highly important for heterogeneous catalysis, for example in fluid catalytic cracking of the vacuum gas oil fraction of crude oil, hydrocracking, or the catalytic reforming of naphtha, just to name a few.
Zeolites with Si/Al≥8 are called high-silica zeolites, and the most important of these is ZSM-5 . ZSM-5 is used in oil refineries as a catalytic additive to zeolite Y in order to increase the octane number of gasoline. In addition, there is a high potential for the future supply of liquid fuels by the use of zeolites in the conversion of methanol to olefins, where SAPO-34, a silica-doped aluminophosphate material, is the most attractive candidate todate.
High-silica zeolites are typically synthesized in the presence of a quaternary ammonium cation as structure-directing agent (SDA). For example, tetrapropylammonium cations are employed as SDA for the synthesis of ZSM-5. The charge of the SDA cation is compensated for example by the AlO4/2- (or BO4/2- in the case of borosilicate zeolites) tetrahedra. Negative charge may also exist by the presence of framework defect sites or fluoride ions. The latter often forms pentacoordinated silicon in framework SiO4/2F- sites.
Structure Direction in Zeolite Synthesis
Structure direction in zeolite synthesis is a major field of current zeolite research. A breakthrough came into this topic as experimental techniques have been introduced or improved to look at early stages of zeolite crystallization. The observation of particles in the pre-crystalline situation being discussed as 'nuclei' or 'precursor particles' is an important issue carried out by several groups in the world.
We focus with our work on the characterization of interaction centers which are responsible for the structure-directing effect. Organic structure directing agents (or templates) interact with the inorganic surface. Quaternary ammonium cations as structure directing agents and negative charge centers in the zeolite framework are these crucial interaction centers. The negative framework charge can be caused by the presence of trivalent framework atoms in tetrahedral coordination, such as AlO4/2- or BO4/2-. It is expected that this interaction determines the location of catalytic sites in the activated zeolites.
The location of active sites in zeolites is one of the most important challenges to an understanding of the catalytic properties. Due to the inherent long-range disorder of the distribution of these sites in most zeolites, it is difficult to locate them by diffraction methods. In the past ten years a set of sophisticated solid state NMR tools has been developed to measure atomic distances based on the heteronuclear dipole interaction in solids. We use these triple resonance methods to measure the local neighborhood between 11B or 27Al nuclei in the zeolite framework and the 13C (or 1H) nuclei in the SDA. These methods can be applied to the crystallized zeolite as well as to the pre-crystalline phase.
Catalytic Centers and Zeolite Catalysis
Zeolites belong to the most important heterogeneous acid catalysts. One of our projects on these materials aims at the characterization of the local structure and interatomic distances of these acid sites. To this end, we have employed modern double resonance solid state NMR methods. Two of them (abbreviated TRAPDOR and REAPDOR) have been tested for use on such systems.
Double resonance methods such as REDOR, TRAPDOR, or REAPDOR are used to measure the heteronuclear dipole coupling which is a direct function of the interatomic distance. The dipole coupling is normally averaged out by magic angle spinning, but it can be reintroduced into the evolution period of the aforementioned methods by disturbing the time evolution of the dipolar Hamiltonian in the periodic rotor spinning. The spectra acquisition is then performed under high-resolution conditions.
This NMR tool is suitable to be further applied to measure the interaction between adsorbed molecules and the catalytic center. We have used it to prove that methanol is not protonated in the ground state for a 1:1 loading in ZSM-5, a problem which had been debated in the community over years.