Hydrogen Production from Residual Glycerol in Biodiesel Synthesis by Photocatalytic Reforming

Abstract

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.

Glycerol as a Superior Electron Source in Sacrificial H2 Production over TiO2 Photocatalyst

“…Vegetable oil (150 mL, 136.5 g, 0.154 mol) was set in a reaction vessel. Since usual optimal amount for transesterification of neutral lipid is known to be 3.55 g/kg [11], the amount of NaOH necessary to the transesterification was determined to be 0.485 g (=0 + 0.485 g) by the sum of a g/kg and 3.55 g/kg. Usually, 20% of weight of 1b to vegetable oil is used for BDF synthesis.…”

“…The results are summarized in The values were the theoretical amounts (P) obtained from Eqs. (11) and (12). © 2019 The Author(s).…”

“…Reforming of glycerol has been extensively investigated through pyrolysis [5,6], steam gasification [7,8], and biological reforming [9,10]. We have focused on photocatalytic reforming over titanium dioxide (TiO 2 ) [11]:…”

Photocatalytic Reforming of Lignocelluloses, Glycerol, and Chlorella to Hydrogen

“…Also methanol can produce three equivalents of H 2 . Hydrogen transformation of glycerol and unreacted methanol isolated from the BDF synthesis was performed by sacrificial H 2 production over a Pt/TiO 2 [30].…”

Sacrificial Hydrogen Production from Enzymatic Hydrolyzed Chlorella over a Pt-loaded TiO₂ Photocatalyst

“…It is well known that the use of electron-donating sacrificial agents remarkably accelerates the TiO2-photocatalyzed H2 production since the hydroxyl radical is consumed by the sacrificial agents 7) . We have applied sacrificial H2 production over Pt/TiO2 to produce H2 from saccharides derived from lignocelluloses 8) ～ 10) and glycerol which is a byproduct in biodiesel production 11) .…”