*3.1. Solid-Alkali Catalyst Preparation*

A sample of bauxite was converted into bauxite ash by heat-treating them at 500 ◦C for 4 h in a mu ffle furnace. After washing the bauxite ash using deionized water and filtration, the bauxite ash was dried at 120 ◦C for 16 h. The catalyst was prepared by grinding bauxite ash and Li2CO3 (Shimakyu's Pure Chemicals, Osaka, Japan), thoroughly mixing them in a crucible for calcination for 4 h in a mu ffle furnace, and then cooling them to room temperature.
