DEVELOPMENT OF SODIUM ALKOXIDE CATALYSTS FROM POLYOLS

Abstract

Metal alkoxide and hydroxides are popular and inexpensive base catalysts used by industry to produce fatty acid esters. However hydroxides produce water when dissolved in an alcohol based solvent. Consequently the water molecule releases hydroxide anion, which attacks triglyceride to produce free fatty acids instead of fatty acid methyl ester. Presence of free fatty acid is highly unfavorable because it reacts with the base catalyst to produce soap and hence, inactivates the catalyst. The alternative, alkoxide, is therefore, a preferred choice over hydroxide. Nevertheless, alkoxides are more expensive, their production is hazardous and they are dangerous to transport. In this study, low cost sodium alkoxide base catalysts were synthesized from 50 wt% sodium hydroxide solution and non-volatile, non-toxic polyols using an alternative route which is less expensive and hazardous. Gravimetric analysis showed that polyols effectively aid in evaporation of 50 wt% aqueous sodium hydroxide during formation of alkoxide compounds. The resulting products are strong base compounds, which were characterized using single crystal, X-ray powder diffraction and elemental analyses. Results have shown that the polyol-derived alkoxide compounds are predominately mono-sodium substituted alkoxide that occur as adducts with sodium hydroxide.

Studies of transesterification reactions catalyzed by polyol-derived sodium alkoxide/hydroxide were conducted to evaluate reaction efficiency and kinetics. The reactions catalyzed by the polyol-derived sodium alkoxide/hydroxide successfully achieved comparable biodiesel yield with sodium methoxide (0.5 wt% of equivalent biodiesel yield under reaction conditions of 6:1 methanol to oil mole ratio at 60°C). Additionally, all polyol-derived alkoxide/hydroxide catalysts investigated in these studies were capable of achieving >95 wt% of biodiesel yield after 1.5 h in a single step transesterification reaction. Initial reaction rates (2 min) differed depending on the polyol used in producing the catalyst. The reaction rates over two minutes are in order of increasing activity: sorbitol < xylitol < sodium methoxide < 1,2-propanediol < 1,3-propanediol < glycerol. This result can be associated with release of these polyols in small quantity (<0.5%) as a result of methanol solvation to liberate the methoxide ion as catalytic agent. Presence of these polyols at the beginning of a reaction may help to stabilize the immiscible oil/methanol phase by formation of emulsifier compounds (such as glycerol esters or glycol esters), which in return facilitate the transesterification reaction. In conclusion, the polyol-derived sodium alkoxide/hydroxide catalysts have demonstrated promising qualities to the industry. These catalysts may serve as an alternative solution to lower the cost of biodiesel plant operation without compromising production efficiency.