Local Order in Depolymerized Silicate Lattices

Abstract

In glycerol, near 200 degrees C, the silicate networks of mesoporous silicates and zeolites undergo a deep depolymerization process. In a few hours, depending on the initial concentration of the solid in glycerol and on the temperature, amorphous gels are obtained. In these gels, a fraction of the Si-O-Si bonds are transformed into Si-O-C. The constitutional aluminum remains bound to the silica network in the gel. The short range ordering is maintained to some extent: the size of the smallest structural unit in gels obtained from zeolites is in the range of the cubic nanometer, nm3.

Synthesis of silicalite-1 from organo-silicic gels

“…According to earlier investigations [7], the reaction of silicates with glycerol results in the formation of polysilanols or alkoxides and in the depolymerization of the original silica with the partial conservation of the original ordering. 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.…”

“…A broad exothermic accident, observed near 350-550 • C, is probably due to the organic matter oxidation. The procedure to obtain the C/Si ratio in the gel, excluding the carbon of the glycerol solvent has been described previously [7]. For instance, the C/Si ratio was ≈1 in the gel obtained from RHA before hydrolysis [3].…”

Crystallization of faujasite Y from seeds dispersed on mesoporous materials

“…The line at À112 ppm is probably associated to the Q 4 resonance in the amorphous silica. These resonance lines are reported in ''gels" of aluminum depleted Y zeolite or in gels of FSM-16 [10], respectively. The chemical shift of the Si(1Al) or Q 3 cluster at À101 ppm corresponds to Si-OH or Si-O-Al species found in DR materials as well as in the starting USY or RHA and FSM-16.…”

“…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].…”