Enhanced Biodiesel synthesis by supported sodium aluminate catalysts

Fossil fuel dependence is nowadays at the heart of many international debates, ranging from environmental to geopolitical issues, renewable energy availability is of increasing importance in our society. Biodiesel, i.e. a mixture of fatty acid methyl esters (FAME) [1], is mainly produced through the transesterification of triglycerides via a homogeneously catalyzed process. Catalysts as KOH, NaOH are largely available and cheap, however, the application is restricted by high recycle cost, soap formation and consequent loss of activity and problems of product separation [2]. It follows that heterogeneous catalysts could be easily recycled and reused, overcoming many of these drawbacks. Base materials such as MgO, CaO, Na/SiO2, Ca/Mg-SBA-15 etc. have been intensively investigated [3-4]. Nevertheless, heterogeneous based technology is not a breakthrough yet mainly because it is still difficult to find catalysts with a high activity and an economic feasibility for the necessary scale up. Sodium Aluminate, NaAlO2 (SA), is a promising candidate to overcome these limits because it is a solid strongly basic catalyst, cheap and available (byproduct of Boehmite industrial process). Furthermore, it’s useful to incorporate SA on a suitable support. While sparse works have been found on the pure material [5-7], supporting it over a material with adequate morphological and textural properties is still matter of open research [8-9]. The aim of this work is to develop new Sodium Aluminate-based catalysts to produce biodiesel. SA was successfully incorporated by impregnation on Al2O3, hydrotalcite and TiO2. The so obtained materials were tested in the methanolysis reaction of commercial sunflower oil and compared with other common catalysts. SA-based catalysts showed excellent performance, outcompeting other classical catalysts in terms of biodiesel production in the same reaction conditions. Furthermore, to correlate catalytic efficiency with morphological and surface properties, all the materials were deeply characterized. Techniques as TPD and DRIFTS show that the key role, in the catalytic behaviour of SA-based materials, is played by the quantity and strength of the basic sites. Moreover, special attention was given to the effect of SA loading respect with the support textural features, revealing complex mechanism in basic sites formation and environmental poisoning by CO2. SA-Hydrotalcite based materials possess the highest number of available strong basic sites and thus catalytic activity (70% biodiesel yield with 9.4% wt SA loading)