Crystallization of Zeolites from Organo-Silicic Colloids

Abstract

As shown recently, the networks of mesoporous high-surface-area silicates and zeolites undergo a deep depolymerization process in glycerol, near 200 degrees C. Within 1 h, X-ray diffraction analysis amorphous gels are obtained. However, some local ordering subsists as demonstrated by a striking similarity between the silicon and aluminum high-resolution solid-state NMR spectra before and after the reaction. The residual organization could be investigated indirectly in studying the recrystallization of these gels in the presence or absence of structure-directing agents. Were this attempt successful, the way should be opened for the synthesis of molecular sieves starting from gels obtained from naturally occurring zeolites. Here, it will be shown that an amorphous gel obtained from HZSM-5 recovers the initial long-range structure of the parent material in a few hours at 85 degrees C in the presence of an aqueous solution of tetrapropyl ammonium (TPA) or NH3. The recrystallization of HY requires the presence of tetramethylammonium, but about 25% of the crystallization is obtained rapidly (approximately = 1 day) at 80 degrees C with ammonia. Hypotheses about the preorganized structural units are presented. The value of the Si-O-Si angle in the silica cluster seems to be of paramount importance.

Synthesis of silicalite-1 from organo-silicic gels

“…The t-plot micropore volume of SLR is smaller that of SLD, but both values are smaller than that reported for porous carbon template silicalite-1 [13]. The micropore surface area is much larger in SLD than in SLR.…”

“…The orthorhombic phase of silicalite-1 has twelve different (tetrahedral) sites [11]. Consequently, twelve 29 Si MAS NMR lines should be observed in well crystallized materials, this number being limited by instrumental resolution [12,13].…”

“…The similarity between the 29 Si (and 27 Al) MAS NMR spectra of the gels and those of the corresponding starting material had been interpreted as if a local ordering persisted in the gels. In particular, in a gel, obtained from a HY zeolite [13], the observation of tetrahedral aluminum has suggested that the smallest organized structural unit has a volume about 1 nm 3 .…”

Crystallization of faujasite Y from seeds dispersed on mesoporous materials

“…Several two-step syntheses have been reported in the literature [3][4][5][6][7][8][9][10][11][12]. Kaliaguine et al [4] have prepared a series of materials, called UL-ZSM-5, exhibiting micro-and mesoporosity from a mesoporous worm-like alumino-silicate precursor.…”

“…The inspiration arises from the observation that the networks of mesoporous, high surface area silicates or zeolites undergo a deep depolymerization process in glycerol [10] at 200°C. The depolymerized material is a hyperbranched silicon alkoxide which proceeds via condensation reaction between the silanol groups of the silicate and alcohol functions of glycerol; water being continuously evaporated from the reacting mixture promotes the reaction to proceed to completion [11,12].…”