Dual-Site Cooperation for High Benzyl Alcohol Oxidation Activity of MnO₂ in Biphasic MnO_<i>x</i>–CeO₂ Catalyst Using Aerial O₂ in the Vapor Phase

Abstract

Selective oxidation of benzyl alcohol to benzaldehyde in the vapor phase has drawn growing interest recently. In this work, MnO&lt;sub&gt;&lt;em&gt;x&lt;/em&gt;&lt;/sub&gt;-CeO&lt;sub&gt;2&lt;/sub&gt; mixed oxide compositions have been prepared by a coprecipitation method and tested for their oxidation activities of benzyl alcohol to benzaldehyde in the vapor phase. Detailed structural analyses have indicated that MnO&lt;sub&gt;&lt;em&gt;x&lt;/em&gt;&lt;/sub&gt;-CeO&lt;sub&gt;2&lt;/sub&gt; catalysts contain two phases, namely, &lt;em&gt;α&lt;/em&gt;-MnO&lt;sub&gt;2&lt;/sub&gt; and fluorite CeO&lt;sub&gt;2&lt;/sub&gt; phases. The benzyl alcohol oxidation activity of pure MnO&lt;sub&gt;2&lt;/sub&gt; is more than 7 times higher compared to that of CeO&lt;sub&gt;2&lt;/sub&gt;, indicating a much higher intrinsic oxidation ability of the MnO&lt;sub&gt;2&lt;/sub&gt; phase. Further, enhancement of the benzyl alcohol oxidation rate over MnO&lt;sub&gt;2&lt;/sub&gt; in 10% MnO&lt;sub&gt;&lt;em&gt;x&lt;/em&gt;&lt;/sub&gt;-CeO&lt;sub&gt;2&lt;/sub&gt; catalyst by 13 times is observed in relation to pure MnO&lt;sub&gt;2&lt;/sub&gt;. The role of CeO&lt;sub&gt;2&lt;/sub&gt; in the MnO&lt;sub&gt;&lt;em&gt;x&lt;/em&gt;&lt;/sub&gt;-CeO&lt;sub&gt;2&lt;/sub&gt; catalyst has also been investigated, which indicates that the oxidation activity is almost independent of CeO&lt;sub&gt;2&lt;/sub&gt;. However, stronger adsorption of benzyl alcohol over the MnO&lt;sub&gt;&lt;em&gt;x&lt;/em&gt;&lt;/sub&gt;-CeO&lt;sub&gt;2&lt;/sub&gt; catalysts compared to that of MnO&lt;sub&gt;2&lt;/sub&gt; points to the role of CeO&lt;sub&gt;2&lt;/sub&gt; in adsorption. Thus, both the CeO&lt;sub&gt;2&lt;/sub&gt; and the MnO&lt;sub&gt;2&lt;/sub&gt; components have different roles in the catalytic process-adsorption of benzyl alcohol on the CeO&lt;sub&gt;2&lt;/sub&gt; surface and its oxidation on MnO&lt;sub&gt;2&lt;/sub&gt; at the interface. The cooperation between the two sites toward oxidation could happen due to jumping of adsorbed benzyl alcohol from the surface of the CeO&lt;sub&gt;2&lt;/sub&gt; phase to the closest Mn&lt;sup&gt;4+&lt;/sup&gt; site in the MnO&lt;sub&gt;2&lt;/sub&gt; phase at the contact surface with MnO&lt;sub&gt;2&lt;/sub&gt; by thermal motion.

Topics & Concepts