The oxidation of alcohols to carbonyl compounds is an important reaction in synthetic organic chemistry. While stoichiometric oxidants are effective for this transformation, they often produce large amounts of toxic waste, which renders them unacceptable from an environmental and economic perspective. Consequently, there is a strong impetus to develop catalytic processes that utilize environmentally friendly, inexpensive primary oxidants, the use of molecular oxygen being particularly attractive. Recently, hydrotalcites have attracted attention as both catalysts and catalyst supports for the selective oxidation of alcohols to ketones and aldehydes, using either oxygen or TBHP as the oxidant. This review is intended to provide a comprehensive summary of work performed in this area to date. The effects of composition and structure on catalyst properties are highlighted, and mechanistic aspects are discussed.
2HTs as catalyst
Ru containing HTsRuthenium is recognized to have excellent catalytic properties in the oxidation of a variety of organic functional groups, including alcohols. For example, rutheniumcontaining organometallic complexes are well known for their ability to catalyze alcohol oxidations using O 2 as the sole oxidant. 13 However, for ease of product work-up, heterogeneous catalysts are generally preferred to homogeneous catalysts. Recent examples of supported Ru catalysts have been provided by Liu and coworkers 14, 15 who performed the selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5diformylfuran (DFF) using carbon-supported Ru in the presence of O 2 . Similarly, Takagaki et al. 16 reported the use of Ru/HT for the aerobic oxidation of HMF to DFF. In comparison with other supported Ru catalysts such as Ru/C, Ru/Mg(OH) 2 , and Ru/Al 2 O 3 , Ru/HT had the highest activity and selectivity.While the catalytic properties of hydrotalcites (HTs) have attracted interest for many years, their use in alcohol oxidations is a relatively recent development. 12 The first such study appeared in 1988 and concerned the photocatalytic oxidation of isopropanol to acetone by polyoxometallate intercalated HT (vide infra). 17 Roughly a decade later reports were published concerning the use of isomorphically substituted HTs for the oxidation of alcohols under non-photocatalytic conditions. In the initial article by Kaneda et al. 18 , Mg-Al-HT was isomorphically substituted with Ru to obtain Ru-HTs with various interlayer anions for the aerobic oxidation of cinnamyl alcohol, CO 3 2having the highest activity of all the anions tested. The same authors indicated that they had also screened HTs isomorphically substituted with Fe, Ni, Mn, V, and Cr in addition to Ru, Ru substituted HTs having the highest activity; however, no data were presented for the other catalysts mentioned. Building on this work, Kaneda and co-workers 19 subsequently evaluated a series of transition metal containing Ru-HTs for the aerobic oxidation of cinnamyl alcohol. Divalent transition metal ions were substituted for Mg in the HT structure (...