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Graphical abstract Highlights • Base-free transfer hydrogenation of menthone by primary and secondary alcohols as a hydrogen donor under sub- and sc conditions. • The similar H -donor activity of all alcohols under non-catalytic sc conditions. • Only secondary alcohols demonstrate a high H -donor activity in the presence of Raney® Ni under subcritical conditions. • A quasi-equilibrium of isomeric menthone/isomenthone mixture reached via enolization under sc conditions. Abstract The base-free transfer hydrogenation (THR) of menthone with six aliphatic alcohols as hydrogen source has been developed under sub- and supercritical conditions. Under sc conditions (at 350 °C and 78–210 atm) without any catalyst, all alcohols demonstrated the similar H -donor activity, which was high enough. The menthone conversion was about 64 ± 5% for 5 h of the reaction, whereas the selectivity on menthol reached 92–99% for all alcohols except MeOH (73%). The non-catalytic THR at 250 °C was very slow and the conversion in the most active 2-PrOH was only 7.0–7.5% for 9 h of the reaction. Under subcritical conditions at 150 °C, the non-catalytic menthone reduction was negligible in all alcohols. But in secondary 2-PrOH and 2-BuOH, the base-free THR catalyzed by Raney® nickel results in excellent conversion and selectivity of menthol at 150 °C, whereas no conversion of menthone was observed in primary alcohols under the same reaction conditions. A quasi-equilibrium of isomeric menthone/isomenthone mixture reached via enolization was evaluated under sub- and supercritical conditions and the Gibbs energies of the isomerization in each alcohol were calculated. [ABSTRACT FROM AUTHOR]

• The Al-Si oxides were synthesized for the first time via supercritical antisolvent precipitation of Al-Si-containing sols. • The alumina and silicon-doped alumina obtained by the SAS method were shown to be mesoporous and thermal stable materials. • The alumina composed of X-ray amorphous particles (2-5 nm) forming a narrow pores size distribution (3-5 nm). A method of alumina and silica sols precipitation in supercritical CO 2 has been developed for the first time to prepare alumina and silica doped alumina. The silicon content of samples is varied within the range from 0 to 30 mol. %. The approach suggested can be considered as a development of supercritical antisolvent (SAS) method that is widely used to prepare powders from solutions but not sols. The alumina and silicon-doped alumina obtained are shown to be mesoporous materials with narrow size distribution in the range of 3–5 nm. The high-temperature treatment of the sample containing 10 mol. % of Si results in the formation of γ-Al 2 O 3 phase followed by its transition into the δ -Al 2 O 3 phase at 1200 °C without α-Al 2 O 3 phase formation. It is demonstrated that the gradual introduction of silica into the alumina allows one to fine-tune the strength and amount of acid sites existing in oxides. [ABSTRACT FROM AUTHOR]

Sub- and supercritical alcohols are known as effective hydrogen donors. In this work, transfer hydrogenation (TH) of furfural with C 1 -C 4 alcohols has been studied under sub- and supercritical conditions. In the range of 230–275 °C, the observed activation energy of catalyst-free transformations of furfural into furfuryl alcohol and some minor byproducts has been shown to be 98 ± 8, 64 ± 4 and 79 ± 6 kJ/mol for EtOH, 1-PrOH and 2-PrOH, respectively. The use of a Ni/γ-Al 2 O 3 catalyst results in higher rate constants compared to the catalyst-free TH. Nevertheless, the selectivity to furfuryl alcohol in the presence of the catalyst does not exceed 50% vs 70–80% for the catalyst-free transformations. Thus, this study shows the high donor potential of alcohols in TH under both catalytic and catalyst-free conditions. A special attention is paid to primary alcohols, because they are highly demanded as H-donors due to economic and environmental reasons. [Display omitted] • Primary C 1 -C 4 alcohols demonstrate high donor activity in furfural TH. • Catalyst-free TH of furfural and 1-PrOH has lower activation energy than 2-PrOH. • Selectivity to furfuryl alcohol of catalyst-free furfural TH reaches 70–80%. • Use of Ni/γ-Al 2 O 3 results in higher rate constants but the lower selectivity. [ABSTRACT FROM AUTHOR]