Barakov R. Influence of template synthesis conditions on structure and acidity of micro-mesoporous composites of aluminosilicates with nanocrystalline ZSM-5 zeolite

The thesis is devoted to the development of scientific approaches to obtaining of micro mesoporous aluminosilicates (MMAS), which combine the properties of zeolites and mesoporous molecular sieves (MMS), clarifying the influence of the composition of reaction mixtures and template synthesis conditions on structure, acidity and catalytic properties of the obtained aluminosilicates. The most promising approaches – dual template synthesis in the presence of molecular and micellar templates promoting the formation of zeolite and MMS, respectively; crystallization of a pre-dried gel of MMS in the presence of a molecular template; as well as the use of dual-functional templates that are able to form zeolite and mesostructure simultaneously – were used to obtain the micro-mesoporous composites of aluminosilicates with nanocrystalline ZSM-5 zeolite. It was established that the use of molecular and micellar templates, alkali metal-free cations mixtures and a decrease the synthesis temperature up to 100 °C allow to increase the mesopore specific surface area of MMAS and concentration of their acid sites. An increase in the mesopore specific surface area can be explained by the formation of ZSM-5 precursors of small size (ca. 2 nm), from which high-porous aluminosilicates are formed. An increased number of X-ray amorphous sub-nanosized aluminosilicate particles with a high content of tetrahedrally coordinated aluminum atoms determines a high concentration of acid sites in synthesized aluminosilicates. The use of the template-containing MMS SBA-15, dilute solutions of a molecular template, a decrease of the temperature of the steam-assisted thermal treatment up to 100 – 120 °C allow to carry out the partial zeolitization of SBA-15 in the direction of ZSM-5 formation, retain the hexagonal mesostructure of the initial material and significantly increase concentration and strength of the acid sites due to formation of ZSM-5 precursors and nucleus. The specific action of cetyltrimethylammonium bromide, with lower concentration in the reaction mixture than the first critical micelle concentration of micelle formation, is based on the interaction of its hydrophobic part with “tails” of dual-functional template [C8H17–N+(CH3)2–C6H12–N+(CH3)2–C8H17](Br–)2. This action promotes an increase in the mesopore specific surface area of MMAS and concentration of acid sites accessible for bulk molecules. It was shown that in the cumene cracking reaction the higher selectivity toward propylene and benzene in presence of micro-mesoporous composites of aluminosilicates with nanocrystalline zeolite, compared to ZSM-5, is associated with the presence of mainly medium strength acid sites in MMAS on which cumene is selectively converted to propylene and benzene. X-ray amorphous aluminosilicate, with the higher concentration of acid sites accessible for bulk molecules among the investigated samples, exhibits an increased catalytic activity in the reaction of α-pinene oxide isomerization (complete conversion after 2 h) and it is characterized by high selectivity toward trans-carveol.