关键词: FAU/α-Al2O3整体材料, 多级孔道, 晶种胶, 预陈化, 凝胶预处理

Abstract: Hierarchically macro-/meso-/microporous supported FAU monoliths with different micro-morphologies were synthesized using monolithic α-Al2O3 with interconnected macro-mesopores as blank carriers via gel pre-aging routes using seed gel as directing agent. Prior to the hydrothermal treatment, α-Al2O3 monoliths were subject to pre-aging in the crystallization gel at ambient temperature for 24 h, subsequently, in situ crystallization (ISC) was conducted at 373 K for 5 h. The FAU coatings in the size range of several-hundred nanometers were distributed on the wall of the macropores or precipitated as aggregates in the macro-channels of the substrates by conducting the gel pre-treating step before the pre-aging step or not. The as-synthesized samples were characterized using Fourier transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and N2 sorption techniques. It was found that the pure-phase FAU could be supported on the alumina substrate with the presence of seed gel in the synthesis system via gel pre-aging routes, whereas the phase transition of alumina could be inhibited by pre-treating α-Al2O3 substrate with seed gel. Involving the gel pre-aging routes with the first-step of seed gel pre-treatment, the crystal size, shape and loading of the supported FAU could be readily tuned by varying the crystallization gel molar ratios of Na2O/Al2O3 or SiO2/Al2O3, the addition of seed gel into the crystallization mixture, the extra pre-treating step or the position of the mixture of substrate and crystallization gel in the reactor without notably changing the structure of the α-Al2O3 support. The effects of seed gel, gel pre-treating and pre-aging step were determined, while the possible formation mechanism of alumina composites with different synthesis processes was discussed based on the experimental results. With the characteristic of simple, reproducible and practically easy to integrate treble porous structures into large-dimension monoliths, the gel pre-aging route coupled with the one step pre-treatment of seed gel proved to be very effective in affording pure FAU crystals deposited on the α-Al2O3 skeleton surface and strengthening the interfacial interaction between them. Moreover, the successful preparation of FAU/α-Al2O3 composites via gel pre-aging route with two-step of gel pre-treatments and crystallized in the form of steam-assisted conversion (SAC) suggested that a more environment-friendly and cost-effective crystallization mode could be developed with the above optimal route and might provide inspiration to the synthesis of other hierarchical zeolites.

Key words: FAU/α-Al2O3 monoliths, Hierarchical porosity, seed gel, pre-aging, gel pre-treatment