Isomorphous Introduction of Boron in Germanosilicate Zeolites with UTL Topology

The direct introduction of boron into the framework of germanosilicate zeolites with UTL topology using different organic spiro azo compounds as structure-directing agents (SDAs) was performed. The influence of the SDA's nature, chemical composition and pH of the reaction mixture, and duration of the synthesis on the phase selectivity, limit of isomorphous introduction of boron into the zeolite framework and its location and coordination, and adsorption and acidic properties of prepared borogermanosilicates was investigated in detail. Experimental data provided clear evidence that the properties of the SDAs control the upper limit of the B content in the reaction mixture for successful synthesis of a pure UTL phase. UTL is formed in the pH region having the highest concentration of Ge 8 O 15 (OH) 5 3– (D4R) in water. In synthesized UTL samples the contents of Si and Ge depend particularly on the pH values of the reaction mixtures with similar chemical compositions. With increasing pH of the reaction mixture, the silicon concentration increases, the germanium content predictably decreases, and the boron content shows a local minimum at pH ≈ 9. The concentration of boron in UTL samples is much less dependent on its content in the reaction mixtures. There is a sharp enhancement of the Si/Ge ratio in UTL samples synthesized from boron-rich reaction gels. A tentative assignment of 11 B magic-angle spinning NMR suggests that at lower contents of boron in the structure the boron atoms are located mainly in the T4 and T9 positions. With increasing pH above 11, a sharp increase in the Si/Ge ratio (decreasing the concentration of Ge atoms in D4R) was observed. It favors additional introduction of boron atoms into the T9 position. B-UTL zeolites contain predominantly Lewis acid sites with a small amount of Brønsted acid sites. The concentration of Brønsted acid sites sharply increases for samples prepared at pH lower than 8.