(448b) Transition Metal Oxides As Catalysts in the Diels-Alder Reaction between Furan and Methyl Acrylate
We investigate catalytic effects of transition metal oxides such as HfO2 and ZrO2 on the Diels-Alder reaction of furan and methyl acrylate forming the oxanorbornene carboxylic acid methyl ester. Our theoretical and experimental studies confirm that transition metal oxides exhibit reactivity in the Diels-Alder reaction. Furthermore, we theoretically investigate the effects of surface hydroxylation on the activities for both Langmuir and Eley-Rideal mechanisms. Our calculations suggest that Eley-Rideal is the favorable mechanism for this reaction on all studied surfaces. Clean, partially hydroxylated and fully hydroxylated HfO2 and ZrO2 surfaces lower the activation barrier of the Diels-Alder reaction between furan and methyl acrylate to different degrees. While it turned out that clean oxide surfaces are weak catalysts, surprisingly partially hydroxylated oxide surfaces show higher activities than fully hydroxylated ones. Based on our extensive studies, surface hydroxylation can be used as a means to enhance catalytic effects of transition metal oxides on the Diels-Alder reaction of furan and methyl acrylate. This is quite interesting from application point of view because metal oxides tend to be hydrated. This study shows that there is no need for catalyst dehydration prior to use in a catalytic cycle. Calculated activation barriers are in very good agreement with those obtained from experiments. Although the obtained turnover frequencies are lower than Hf- and Zr-BEA zeolites, this finding paves the road for more applicable and scalable methodologies to derive powerful Diels-Alder intermediates.