(96c) Synthesis of Sn-MFI Zeolite with Use of Mechanochemical Reaction

Introduction

Since the introduction of Sn(IV) into zeolite framework in MFI-type silicate leads to introduction of Lewis acidic property, it can be utilized as a catalyst on biomass conversion. However, the higher content of Sn into the framework seems too difficult to be obtained by the conventional hydrothermal synthesis method. The objective of the present study is to synthesize the MFI-type Sn-substitute silicate with higher Sn content. To introduce Sn into zeolite framework with higher Sn concentration,¹⁾ it is considerably important to create Sn-O-Si bond in the initial step of Sn-silicate particle formation. Hence, we have applied the mechanochemical treatment²⁾ in the synthesis process of titano-silicate, reported in our laboratory, to prepare Sn-silicate precursors with atomic-level mixing of Sn and Si, and to synthesize Sn-MFI particles by the successive hydrothermal procedure. The remarkable increase in Sn content in the particles is attempted with the control in the synthesis conditions carefully.

Experimental

SiO₂ and amorphous SnO₂ was charged in Si₃N₄ vessels with Si₃N₄ balls. The mechanochemical reaction was carried out with 600 rpm to obtain SiO₂-SnO₂ precursors with their well mixing in atomic-level. Next, as-prepared precursors, SiOw (Caplex), H₂O, a 2.0 mol/dm^-3 HCl aqueous solution, and a 2.0 mol/dm^-3 tetrapropylammonium hydroxide (TPAOH) aqueous solution was put in Teflon-lined pressure‐resistant vessels. The molar ration of the concentration was 1 SiO₂: 0.02 SnO₂: 30 H₂O: 0.5 TPAOH: 0-0.4 HCl. After the mixed solution was agitated well for 48 hours at room temperature, then, it was aged at 160 °C for 120 hours. Precipitates were recovered from concentrates after centrifugation of aged sample and then treated thermally at 540 °C for 12 hours so as to obtain Sn-MFI particles.

Results and Discussion

UV spectroscopic measurements of mechanochemically treated samples for 0-24 hours and Sn-MFI particles synthesized with HCl/Si = 0-0.3 revealed that the absorption peak of 260-300 nm corresponding to hexagonally coordinated Sn species was disappeared by the mechanochemical reaction for 24 hours. On the other hand, an absorbance at ca 210 nm corresponding to tetragonally coordinated Sn was increased. It can be explained by the introduction of Sn into SiO₂ framework in atomic level, leading to change in fine structure surrounding Sn.

Sn-MFI particles obtained by use of SiO₂-SnO₂ precursor formed in mechanochemical treatment for 24 hours was well characterized. Their XRD patterns showed MFI-type structure for HCl/Si = 0-0.3, but amorphous for HCl/Si = 0.4. The UV measurements revealed that tetragonally coordinated Sn species were confirmed for HCl/Si = 0-0.3 samples, so that Sn was undoubtedly located in the Sn-O-Si framework. The Sn content in Sn-MFI particles were determined by ICP-AES measurements. The Sn content was increased from 1.45 to 2.83 wt% with the increase in the ratio of HCl/Si from 0 to 0.3. Namely, the Sn content can be controlled by the HCl concentration. The particle shape was changed to sphere with increase in the ratio of HCl/Si from 0 to 0.3. The results suggested that mechanochemical pre-treatment is a promising procedure for the dense-doping of transition metals into zeolite frameworks.

1) N. K. Mal et al., Microporous Mater., 12, 331 (1997).

2) K. Yamamoto et al., Micropor. Mesopor. Mater., 101, 90 (2007).