Nature of ≡SiOCrO2Cl and (≡SiO)2CrO2Sites Prepared by Grafting CrO2Cl2onto Silica

Abstract

The room-temperature reaction between chromyl chloride and Sylopol 952 silicas pretreated at 200, 450, and 800 °C was investigated using IR, XANES, and EXAFS spectroscopy, as well as by DFT modeling. On the silicas pretreated at 200 and 450 °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 °C some participate in hydrogen bonding with the grafted monochlorochromate sites. On the silica pretreated at 800 °C, both ≡SiOCrO2Cl and (≡SiO)2CrO2 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 °C by condensation between a pair of vicinal silanols in which one of the silanols is also a member of a geminal pair.