Preparation And Characterization Of MCM-41 Surface Titanium Oxides

The semiconductor oxides were widely used in the field of photocatalysis. The mechanisms of photocatalysis were explained by the classic semiconductor theory. Recently, some studies showed that zeolites involving transition metal ions on the zeolite surface or within the framework also exhibited evident photocatalytic activity, which was difficult to explain by the semiconductor theory. Some research workers supposed that it may be related to the surrounding of the metal ions. However, there was no direct evidence to prove that so far. In this paper, we had prepared a well-defined surface neopentyl titanium on the surface of MCM-41 mesoporous molecular sieves by surface organometallic chemistry. The Ti-MCM-41 was prepared through the hydrolysis and oxidation of the surface neopentyl titanium. The reaction of tetraneopentyltitanium with the surface hydroxyl groups was studied by in situ FTIR. The structure and component of surface neopentyl titanium were investigated by probe reaction, gas analysis and element analysis. The states of the surface Ti and the structure of Ti-MCM-41 were characterized by IR, DRS, XAFS, N2 adsorption, XRD and HRTEM. In this study, highly dispersed titanium oxides on the surface of MCM-41 were used as photocatalyst for the oxidation of C2H4.The results showed that: (1) the TiNp4 could react quantitatively with the surface hydroxyl groups. 1mol TiNp4 could react with 2mol (SiOH to produce bigrafted complexes (≡Si-O)2Ti(CH2CMe3)2; (2) The (≡Si-O)2Ti(CH2CMe3)2 readily reacted with CH3OH, H2O and O2 to produce (≡Si-O)2Ti(OCH3)2, (≡Si-O)2Ti(OH)2 and (≡Si-O)2Ti (OCH2CMe3)2 at room temperature, respectively. (3) The surface isolated Ti-O units can be prepared by the hydrolysis and oxidation of (≡Si-O)2Ti(CH2CMe3)2 and had evident photocatalytic activity for C2H4 oxidation. (4) The relations between surface complexes stucture and photocatalytic activity were studied by DRS and XAFS. We infered that the surface isolated tetrahedral coordination Ti is the active center of photocatalysis. The photocatalysis was relate to the number of active centers. The conversion of C2H4 is 72% when the Si/Ti is 11, but it decreased to 64% if the Si/Ti is 7. The main reason was that Ti was in the state of isolated tetrahedral at low content, but some might convert to hexa-coordinated Ti-O-Ti oxides with the increasing content.The innovations of this study are: (1) the surface Ti oxides which had evident photocatalytic activity were prepared by surface organomatellic chemistry for the first time and the characteristics were studied in detail; (2) the relations between isolated tetrahedral Ti on the surface and its photocatalytic activity were studied and offered some direct evidence to surpport the opinion of Anpo. The results showed that the special Ti complexes on the surface of molecular sieves could be prepared by the quantitative reaction of neopentyltitanium with the surface hydroxyl groups. This open a new door to design and explore new type of photocatalysts.