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A mixed homogeneous–heterogeneous catalytic system for the transesterification of sunflower oil with methanol

The use of heterogeneous catalysts in the transesterification of vegetable oils into biodiesel has been investigated because of easier separation of products and catalysts after reaction, no formation of soaps through free fatty acid neutralization by NaOH or triglyceride saponification, and high reaction yields (>95%). Supported fluorides have demonstrated high catalytic activities in a wide range of base-assisted reactions. However, the support must be chosen to avoid interactions with the fluoride and there may also be some dissolution of the fluoride in the methanol. In fact, the results shown here indicate that the most practical application of this catalytic system could be a process combining homogeneous (i.e. CsF dissolved in methanol) and heterogeneous (i.e. alumina) catalysts.

The apparatus used to test this homogeneous/heterogeneous concept is shown below. The reactants are recirculated through a fixed catalyst bed.

 

 

The activation of the CsF/alumina catalyst was critical, with the highest esterification activity being obtained at 120 °C

Comparison of the activity of CsF/α-Al2O3 (0.6 mmol/g) activated at different temperatures: (×) 80 °C, (○) 120 °C, (up triangle, open) 160 °C and (■) 200 °C

 

The loss of activity at higher activation temperatures could be due to solid/solid reactions between the CsF and the alumina as indicated by changes observed in a temperature-programmed XRD experiment.

 

In situ XRD patterns of CsF/α-alumina at 25, 120, 200 and 300 °C under flowing N2.

 

 

The nature of the active phase was investigated by comparing the activity of the supported CsF catalysts to that of (i) bulk CsF pre-dissolved in MeOH, (ii) α-Al2O3 and (iii) a blending of CsF (dissolved in methanol) with α-Al2O3.

Full-size image

 

Activity of (up triangle, open) bulk CsF, (×) α-Al2O3, (●) CsF/α-Al2O3 and (■) a blending of CsF (dissolved in methanol) with α-Al2O3.

 


Using CsF alone or α-Al2O3 alone resulted in no observable catalytic activity, showing that neither component could catalyse by itself the transesterification. Interestingly, the mixture of the separate constituents exhibited a threefold higher activity than that of the CsF/α-Al2O3. This observation shows that:

             1. there is a synergy between CsF and α-Al2O3 leading to an active phase active in triglyceride transesterification.
             2. the ex situ deposition of CsF on the α-Al2O3 does not reveal the full potential of the system. In other words, the    
    deposition/calcination step inhibits the sample activity.  
3. CsF, one of the active species, is most likely in solution (near to the surface site involved in the catalysis) rather than directly
                  anchored onto the alumina surface.

 

In summary, a simple biodiesel production process could comprise a fixed bed made of alumina particles and CsF continuously injected with the methanol as shown in this schematic:

Schematical representation of a biodiesel continuous production process based on the alumina–CsF catalytic system.

The alumina is located in a fixed bed reactor through which is passed the feedstock (vegetable oil or recycled oil)

and the methanol, which also contains dissolved CsF. The salt, which does not lead to any water or soap formation,

and the methanol can be recovered from reactor products and recycled.