Hydrogen Production from Residual Glycerol in Biodiesel Synthesis by Photocatalytic Reforming

抄録

Biodiesel (BDF) has gained much attention as new sustainable energy alternative to petroleum-based fuels. In particular, BDF market has significantly increased in Europe to adhere energy and climate policies. Therefore, the increased BDF production requires new utilization of glycerol which is by product of BDF synthesis. We have focused on photocatalytic reforming using titanium oxide (TiO₂) photocatalyst using glycerol as sacrificial agent (SaH). Recently we have found that SaH with all of the carbon attached oxygen atoms such as glycerol and methanol continued to serve as an electron source until their sacrificial ability was exhausted in photocatalyzed hydrogen evolution using a Pt-loaded TiO₂ (Pt/TiO₂) catalyst. Here we investigated the utilization of residual glycerol in BDF synthesis by photocatalytic reforming. BDF (methyl alkanoate) preparation by transesterification of commercially-available vegetable oil (136.5 g) was performed by heating with MeOH (23.8 g) in the presence of NaOH (0.485 g) at 61 ℃ for 2 h. After cooling, standard follow-up operation was performed as follows. The products were separated into a lower layer (Solution A) and an upper layer. The upper layer was washed with water and separated into a BDF layer and an aqueous washing solution (Solution B). Glycerol (10.4 g) was obtained from Solution A in 73.3% yield along with methanol (6.85 g). BDF (114.5 g) was isolated in 83.7% yield. Solution B contained methanol in 4.38 g. The residual glycerol and unreacted methanol were turned to hydrogen by TiO₂-photocatalytic reforming of Solutions A and B using Pt/TiO₂ (100 mg) in aqueous solution (150 mL) under irradiation by high-pressure mercury lamp. Hydrogen was evolved in 2.82 g and 0.28 g from Solutions A and B, respectively. The combustion energy (ΔH) of the evolved H2 corresponded to 100.8% based on the total ΔH of glycerol and methanol involved in Solutions A and B. Similar experiment was performed using pre-used vegetable oil. Hydrogen was recovered from Solutions A and B as 69.6% of ΔH. Presence of sodium alkanoate disturbed the hydrogen evolution.