Thesis for the degree of a Candidate of Chemical Sciences in specialty 02.00.04 "Physical chemistry". – L.V. Pisarzhevsky Institute of Physical Chemistry of the National Academy of Sciences of Ukraine, Kyiv, 2021.
The thesis is devoted to the study of influence of template structure and composition of reaction mixture, in particular ratio of framework-forming elements (Si4+, Al3+, Ga3+ and B3+) on crystallization direction of hierarchical Al-, Ga- and B-silicate zeolites, their morphology, adsorption and acidic properties, as well as catalytic activity in the reactions of dehydration of substituted indenols, hydroisomerization of n-hexane, reduction of nitrogen oxides by propene or carbon monoxide, and the annealing of naphthol.
Hierarchical Al-, Ga- and B-silicate zeolites of structural types MOR, BEA, MFI and MTW with morphology of nanorods, nanosheets or approximate to spherical nanoparticles and different chemical composition are obtained. Peculiarities of their porous structure are characterized with number of independent physicochemical methods.
The criteria for selection of organic surfactants with hydrophilic part of different structure as potential structural-directing agents (SDA) admissible of promoting the crystallization of hierarchical zeolites of different topology from Al-, Ga- and B-silicate reaction mixture (RM) of different composition are formulated. Depending on the structure of the hydrophilic part of the SDA and the alkyl chanes, the crystallization direction of zeolites symbatically with the content of the aluminum source in the RM varies in the range MFI → MOR or MTW → BEA → MOR. The formation of two types of crystals for zeolites BEA and MOR is possible: nanolayers or nanoparticles and nanolayers or nanorods, respectively.
Studies by SEM and HRTEM methods have shown that the morphology of zeolite crystals is determined by the type of using template, in particular for SDAs with 4 or more quaternary nitrogen atoms leads to formation of crystals with nanorods morphology (D ~ 10 nm) with channels in axial direction. In turn, MOR samples of zeolites obtained using templates containing 2 – 3 quaternary nitrogen atoms have a morphology of nanosheets with a thickness of about 1.5 – 2 nm. The using of polycationic organic compounds with alkylpiperidine chains as SDA leads to formation of lamellar zeolites with the BEA topology.
A scheme based on charge matching between the zeolite framework and the SDA molecules is proposed, which explains the influence of the Si/E ratio in the reaction mixture and the structure of the hydrophilic part of SDA (due to the formation of zeolite structure with a certain topology) on the morphology of hierarchical nanocrystallites. In case of the balance achieving the hydrophilic part of the SDA located inside the seed of nanocrystals, hydrophobic tails restricts the crystals growth in the axial direction, and leads to growth in the planar direction and forms nanosheets morphology. In charge disbalance, SDA molecules are localized outside the crystals on planes with a high density of T-atoms, resulting in the growth of crystals in the axial direction and forms nanorods morphology.
The decrease in the average value of the strength of Brønsted acid centers (increase in pKa by 0.4 – 0.9) for zeolites with nanosheets morphology compared to isostructural analogues with nanorods (MOR) or nanoparticles (BEA) morphology, which may be due to changes in local environment of acid centers, primarily a decrease in the valence angles of bridging ≡Al–O(H)–Si≡ groups due to decrease in the flexibility of the zeolite framework in the near-surface zone of nanosheets.
The obtained zeolite materials were tested as catalysts for the processes of dehydration of substituted indenols, hydroisomerization of n-hexane, and annulation of naphthol. Al-, Ga- and B-silicate hierarchical zeolites of structural types MOR, BEA, MFI and MTW with morphology of nanorods, nanolayers or nanoparticles, due to the presence of optimal strength of Brønsted acid centers are active and highly selective catalysts for dehydration in the mild conditions (yield of the target product 97 – 99%).
Hierarchical zeolites of structural types MOR, BEA, MFI of different morphology with supported nickel nanoparticles show high selectivity in the process of hydroisomerization of linear hexane (~ 70% at 275 °C, and 20% for the most valuable product 2,2-dimethylbutane), which allows us to consider them as promising catalysts for this reaction.
The priority and novelty of the proposed method of obtaining catalysts based on hierarchical zeolites for substituted indenols dehydration are protected by the patent of Ukraine.
Keywords: Hierarchical Al-, Ga and B-silicate zeolites, MOR, BEA, MFI and MTW, structure-directing agents, morphology of nanocrystals, nanoparticles, nanosheets, nanorods, porosity, acidity, indenols dehydration, n-hexane hydroisomerization, 2-naphthol annulation.
