Nature of SiOCrO2Cl and (SiO)2CrO2 sites prepared by grafting CrO2Cl2 onto silica

The room-temperature reaction between chromyl chloride and Sylopol 952 silicas pretreated at 200, 450, and 800 degrees C was investigated using IR, XANES, and EXAFS spectroscopy, as well as by DFT modeling. On the silicas pretreated at 200 and 450 degrees C, the structurally uniform sites formed by the reaction with one Surface hydroxyl group are described as SiOCrO2Cl. Unreacted silanols persist on these silicas even in the presence of excess CrO2Cl2, and on the silica pretreated at 200 degrees C some participate in hydrogen bonding with the grafted monochlorochromate sites. On the silica pretreated at 800 degrees C, both SiOCrO2Cl and ( SiO)(2)CrO2 sites are formed. The latter are produced despite the absence of hydrogen-bonded hydroxyl pairs on the support. The origin of the chromate sites is proposed to be the reaction between CrO2Cl2 and hydroxyl-substituted siloxane 2-rings. These rings are likely formed at 800 degrees C by condensation between a pair of vicinal silanols in which one of the silanols is also a member of a geminal pair.