CHARACTERIZATION OF TITANIUM-SUBSTITUTED MCM-41

다국어 초록 (Multilingual Abstract)

Titanium-substituted MCM-41 mesoporous molecular sieves (TiMCM-41) are expected to combine unique reactivity of TS-1 and pore structure of MCM-41. Hence, they could be applied to the oxidation of bulk organic substances usually employed in the synthesis of fine chemicals. We studied the effect of preparation variables on the structure and reactivity of TiMCM-41.
TiMCM-41 were synthesized hydrothermally using amorphous silica, (C_2H_5O)_4Si, and (C_4HgO)_4Ti in the presence of C_nH_(2n+1)(CH_3)_3NBr/OH(n=6-16) as templates. Si/Ti ratio, template/Si ratio, and template chain length (n) were varied in the synthesis. The resulting TiMCM-41 were characterized by TEM, XRD, N_2 adsorption for pore size distribution and surface area, and XAFS.
The uniform pore structure whose size increased with increasing template size was confirmed by TEM, XRD, and N_2 adsorption. TiMCM-41 synthesized with C_12 and C_14 templates showed the best defined x-ray crystallinity. The thickness of the wall was independent of template size and remained near 1 ㎚. Structural regularity was reduced as Ti replaces Si sites in the MCM-41 frame work. Furthermore, the d_100 XRD peak shifted to a lower angle. The TiMCM-41 structure was obtained only with template/Si mole ration of 0.5-1.5.
Above Si/Ti mole ration of 12, most of Ti replaced Si in the MCM-41 framework as evidenced by XANES and EXAFS. Below the ration, Ti formed bulk TiO_2. The coordination environment of Ti in TiMCM-41 was similar to that in TS-1.
The TiMCM-41 was stable in helium flow below 1273 K and under the gas phase reaction conditions of ethanol dehydrogenation (ethanol/air=1 ㏖/㏖, 375-725 K). Under the liquid phase reaction conditions of phenol hydroxylation (phenol/35% H_2O_2/acetone in moles =3:1:7, 333K), however, it lost MCM-41 structure and titanium was leached out of the silicalite framework.