Novel composites based on zeolitic imidazolate framework (ZIF-8) and alumina nanofibers as an efficient heterogeneous catalyst for the Knoevenagel reaction

Novel composites based on zeolitic imidazolate framework (ZIF-8) and alumina nanofibers (Nafen^T1M, n-Ah03) were synthesized by a solvothermal method. ZIF-8/n-Al2O3 materials were characterized by physicochemical methods, such as XRD, HR-TEM and DRIFT spectroscopy. HR-TEM data pointed that ZIF-8 content affected its dispersity in ZIF-8/п-АЬОз. HR-TEM data pointed that n-АЮз consists of the rod-like nanofibers with 0.4-0.6 mm length. The individual alumina fibers have a diameter of ~10 nm. Modification of n-АЮз with ZIF-8 leads to exfoliation of layered material. Two regions can be revealed in sample 8.1%ZIF-8/n-Ah03. First region consists offew-layer sheets of nanofibers, where ZIF-8 crystals locate between two sheets of nanofibers. Other region consists of individual nanofibers, where ZIF-8 forms individual homogeneous distributed phase. The shape of crystal of ZIF-8 does not change, however, size of crystal decreases to 40-50 nm. Probably, a small distance between nanofibers prevents the formation of large crystals. However, the following augment of ZIF-8 content in composite promotes the increase in crystal size. Basicity of samples was investigated by DRIFT spectroscopy using CDCh as probe molecule. It was found that the amount of basic sites can be tailored by the ZIF-8 content in the solids. Basic sites of ZIF-8 can be formed by Zn-0H groups and N-moieties of nonbridging linkers, whereas bridging oxygen atom (Al-0-Al) and -0H groups can act as basic sites of Nafen. The increasing ZIF-8 content in composite raises amount of these basic sites. Catalytic properties of ZIF-8/п-АЮз composites were investigated in condensation of benzaldehyde (BA) with malononitrile (MN) (the Knoevenagel reaction). This reaction is heterogeneous, the catalyst is in a different phase from the reactants. Stopping the reaction is done after separation of ZIF-8/п-АЮз from the reaction mixture. The high activity of studied materials were achieved under mild conditions at 1/1 molar ratio of BA/MN and 45^oC. The maim product was benzylidene malononitrile (BMN) with 78-98.7% selectivity. The maximum selectivity of 98.7% was observed for 8.1 %ZIF-8/n-Ah03. It was found that ZIF-8 content in ZIF-8/n-Al03 composite affected the reaction rate and selectivity towards BMN. The increasing ZIF-8 content into ZIF-8/n-Al03 composites led to a rise in reaction rate and decreasing selectivity towards BMN, simultaneously. These trends are related to the variation of amount of basic sites.

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