The calorific value and cetane number of C4+ long-chain higher alcohols are similar to those of diesel, blending with which can significantly reduce pollutant emissions from diesel engines. Direct upgrading of aqueous bioethanol derived from the biomass fermentation to higher alcohols faces the challenge of efficient catalysts with excellent catalytic activity and hydrothermal stability. Herein, NiSn/MgAlO catalysts prepared by loading Ni and Sn into hydrotalcite through coprecipitation and ion exchange methods were developed. Higher alcohols with a yield of 48.4 C-mol % and selectivity of 93.8% were obtained by upgrading of aqueous ethanol under hydrothermal conditions. NiSn/MgAlO catalysts exhibited excellent stability even after four recycle runs. The experiment results and catalyst characterizations revealed that the Ni­(Mg)O solid solution formed by calcination of Ni­(Mg)Al hydrotalcite promotes the dispersion and stability of Ni particles. Moreover, the high-valence Sn regulates the electronic structure and coordination environment of Ni (ratio of Ni0, Ni2+, Niδ+, and Ni–Sn alloys), both of which give the synergistic effect for the formation of C4+ long chain higher alcohols by selectively regulating the ethanol coupling reaction.

A suitable catalyst with an exact match of the acidity/basicity, ingenious geometric structure, and proper electronic environment is vital to the Guerbet reaction for the synthesis of C6+ higher alcohols. Herein, we synthesized a series of Ni/MgAlO and NiSn/MgAlO catalysts based on hydrotalcite laminate for one-pot upgrading of aqueous bioethanol to C6+ higher alcohols. Through the introduction of Sn to form NiSn alloys, the Ni–Ni interactions were attenuated and the cleavage of C–C bonds was suppressed with a yield of 21.9% of C6+ higher alcohols at 250 °C. Meanwhile, by precisely adjusting the ratio of the metal cation Mg2+ to Al3+, the acidity and basicity of the catalysts were optimized and Ni/(4-Mg)AlO with an optimal ratio of 4 presented a considerable C6+ alcohol yield (25.2%) at even 220 °C, as well as an impressive ethanol conversion of 67.7%. Such an excellent activity achieved at lower temperature was attributed to highly dispersed Ni metal with a Ni(Mg)O solid solution structure, leading to the enormously inhibited aqueous ethanol reforming process and significant increased yield of higher alcohols. The results reveal that the electronic structure and coordination environment of metal sites as well as higher moderately basic and strong acid sites were demonstrated to be beneficial for carbon-chain extension of higher alcohols and ethanol selectivity for dehydrogenation/hydrogenation. Moreover, the number of metal, acid, and base sites should be balanced to inhibit side reactions.

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