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Three new 5,6,7,8-tetrahydro-2-(2-phenylethyl)chromones and one new dimeric 2-(2-phenylethyl)chromone were isolated from the agarwood of Aquilaria crassna Pierre ex Lecomte in Laos. The structures of the isolates were elucidated by spectroscopic methods, and their configurations were determined via ROESY correlations, 3JH-H coupling constants analyses, comparisons of chemical shifts and specific rotations with known compounds, and ECD calculation in the case of 1. Compounds 1–4 were evaluated for their cytotoxicity. Compounds 1 and 2 exhibited weak cytotoxicity toward HeLa cell line (IC50: 49.8 ± 1.2 μM) and K562 cell line (IC50: 42.66 ± 0.47 μM), respectively. [ABSTRACT FROM AUTHOR]

• The effects of four types of MnO 2 (α-, β-, γ- and δ-MnO 2) on iron-based FTS catalysts were investigated. • These four types of MnO 2 exhibit distinct roles in the structure evolution and catalytic performances. • The α- and δ-MnO 2 modifying remarkably improves the selectivity of LAOs and heavy hydrocarbon. • The modification of β- and γ-MnO 2 dramatically increases the catalytic activity. Manganese oxide additive is an effective promoter for modulating the catalytic performance of iron-based Fischer-Tropsch synthesis (FTS) catalysts and studied extensively, however, the promotion effects of MnO 2 crystal structures on iron-based FTS catalysts have never been reported. In this work, four types of MnO 2 with crystal phases corresponding to α-, β-, γ- and δ-MnO 2 were prepared, and their promotion functions on iron-based FTS catalysts were studied. These four types of MnO 2 exhibit dramatically distinct promotion effects on the structure evolution and catalytic performances. The ferrite MnFe 2 O 4 only forms in the α-MnO 2 modified catalysts, while the β-, γ- and δ-MnO 2 also present different existing forms in the catalysts. The presence of abundant oxygen vacancies on the surface of α- and δ-MnO 2 notably facilitates the cleavage of Fe-O bonds and the dissociation of CO, leading to the enhancing formation of iron carbides in the activated and spent catalysts. Meanwhile, the strong Mn-Fe electronical interactions formed in α- and δ-MnO 2 modified catalysts lead to high density of surface carbon species with a long residence time, dramatically improving the selectivity of linear high carbon α-olefins and heavy hydrocarbons. In contrast, the β- and γ-MnO 2 modifying dramatically increases the catalytic activity, but suppresses the formation of olefins and long chain hydrocarbons, due to their relatively few amounts of surface oxygen vacancies and moderate Mn-Fe electronical interactions. This work provides new insights and in-depth understanding of the promotion roles of Mn additive, benefiting for the rational design of high performance iron-based FTS catalysts. [ABSTRACT FROM AUTHOR]