Al-induced orthorhombic SnO2 and oxygen vacancies synergistically promote lactic acid production from dihydroxyacetone

Orthorhombic SnO₂ is a metastable crystalline phase of SnO₂. Its synthesis is too difficult to require harsh conditions, inhibiting its further utilization and development. In this work, orthorhombic SnO₂ has been synthesized by facile co-precipitation under mild conditions, in which Al induces the formation of orthorhombic SnO₂ and stabilizes its presence. The action mechanism of orthorhombic SnO₂ in the hydrothermal catalyzed conversion of dihydroxyacetone to lactic acid was investigated. The results show that the content of orthorhombic SnO₂ is 42.3 % of the total SnO₂ in the Al-Sn composite metal oxides, and a large number of oxygen vacancies occur. Orthorhombic SnO₂ and oxygen vacancies synergistically promote the formation of weak Lewis acid sites. The weak Lewis acid sites significantly improved the lactic acid selectivity, leading to a lactic acid yield of 97.21 %. In addition, the catalyst exhibits excellent stability with 89 % lactic acid yield obtained at the fifth use. The present work provides a novel method for synthesizing orthorhombic SnO₂ and offers new insights into its application in hydrothermal catalysis.