Vol 59, No 3 (2019)

The review is devoted to the features of the synthesis of zeolites with nanosized crystals, which are of great interest for the processes of petroleum chemistry, gas chemistry, and organic synthesis. On the basis of analysis of published data, a classification is proposed for methods of directional control of the size of zeolite crystals. Methods for qualitative and quantitative controlling the composition of the reaction mixtures crystallized into nanoscale zeolites are considered in detail, as well as the effect of crystallization conditions on the change in the dispersity of zeolite crystals.

The review summarizes results of research work on the development of technologies for producing granular zeolites of LTA, FAU and MOR types of high degree of crystallinity of porous structures consisting of micro-, meso- and macropores and creation on their basis of highly effective adsorbents for industrial processes for drying and purification of natural and associated gases to remove sulfur compounds, as well catalysts for processing hydrocarbon feedstock.

The article presents an overview of the development in the field of zeolite chemistry at the Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, beginning from the 1960s to the present. A brief description of the current state of the problem and a number of unique approaches to solving the problems of the directed synthesis of zeolites, as well as new areas of their possible application is given.

Results of a study on the synthesis of two sets of zeolites of the MFI structural type with a crystal size of a few microns (1–5 μm) in the first set and 120–140 nm in the second set and different Si/Al molar ratios of 20–220 in each of the sets have been described. The crystal sizes have been determined by dynamic light scattering and electron microscopy; the phase, textural, and acid properties of the samples have been studied by XRD, nitrogen sorption by the BET method, and ammonia TPD, respectively. The synthesized samples have been tested in the model n-hexane catalytic cracking reaction; the catalytic properties of the samples with respect to n-hexane conversion and the protolytic cracking, dehydrogenation, and hydride transfer reactions have been compared. It has been shown that a key role in the catalyst activity is played by the combination of the strong acid site concentration and the pore hierarchy factor (HF); the maximum conversion is observed at a strong acid site concentration of 0.2 mmol/g and HF = 0.26.

The features of hydrothermal crystallization of reaction mixtures RM-I and RM-II that are similar in chemical composition and differ in the order of mixing of the reactants, have been studied in detail. It has been shown that changing the sequence of mixing the reactants during the preparation of the reaction mixture (RM) leads to the formation of aluminosilicate solid gels of different compositions and determines the formation mechanism of the BEA zeolite crystalline structure. It has been established that the addition of a source of silicon at the initial step of mixing the reactants (RM-I) leads to the formation of an aluminum-rich aluminosilicate gel, the charge of which is compensated by alkali metal cations, with the TEA⁺ cations occurring in solution. Adding a source of aluminum at the initial steps of mixing the reactants (RM-II) leads to the formation of amorphous aluminosilicate hydrogel with occluded TEA⁺ cations, the chemical composition of which is close to that of the final zeolite. It has been shown that during crystallization of RM-I the formation of nuclei apparently occurs in solution. According to the infrared spectroscopy data, during crystallization of RM-II the formation of secondary structural fragments of BEA zeolite occurs in the bulk of the solid phase. The products of separated hydrothermal transformation of solid and liquid phases isolated by centrifugation from RM-I and RM-II at the initial steps of the synthesis have been studied. It has been demonstrated that in order to obtain zeolite Beta crystals without admixtures of other phases it is necessary to have not only a high concentration of TEA⁺ cations, but also a high concentration of aluminum in the reaction mixture.