Hydroxyl-Presenting Metal-Organic Frameworks as Support Platforms for Vanadium- and Titanium-Based Oxidation Catalysts.

摘要

Heterogeneous catalysts such as supported vanadium oxides have great potential to be developed into economically viable oxidation catalysts if they can exhibit greater selectivity; however, their current syntheses often yield a mixture of species, rendering the analysis of structure-activity relationships for each possible vanadium oxide species difficult and, thus, frustrating efforts to improve catalyst activity and selectivity. To address this issue, two metal-organic framework (MOF) systems possessing well-defined and single-site catecholate supports for monomeric vanadyl species were synthesized and their catalytic activities were tested.;Through post-synthesis grafting of catechol-presenting dopamine to the unsaturated Cr centers of MIL-101, followed by metallation with vanadyl ion, vanadyl monocatecholate-functionalized V(dop)-MIL-101 was obtained. V(dop)-MIL-101 can efficiently catalyze the oxidation of an organic sulfide and exhibited tunable selectivity for the sulfoxide product. In an alternative strategy, a series PG-CatBrO (PG = protecting group) paddlewheel MOFs were constructed from ZnII ions, tetracarboxylate struts, and protected catecholate dipyridyl struts, which could be deprotected and subsequently metallated with vanadyl ions to give V-CatBrO MOF. This MOF can catalyze the benzylic oxidation of tetralin to tetralol and tetralone.;To demonstrate that MOF-based catalysts are viable for gas-phase reactions, a thermally stable hydroxyl-presenting MOF, UiO-66, was explored as a MOF-based support. V-UiO-66 proved to be an effective analogue of hydroxyl-presenting solid supports for VV species in gas-phase oxidation reactions, catalyzing the oxidative dehydrogenation of cyclohexene to benzene at high temperatures (350 °C). Generalization of the metallation of the hydroxyl-presenting nodes of UiO-66 to other redox-active metals, such as titanium, has also allowed other chemical oxidations to be accessed.