The influence of residue sodium ions in mixed oxide on catalytic activity in transesterification of vegetable oil

Abstract The Mg-Al mixed oxides are an attractive acid-base catalyst with various activity published even for the same reaction under similar conditions. The differences in the results can be caused by impurities remaining after synthesis, which is very often omitted in papers. The influence of remaining sodium impurities (from 5 to 345 μmol/g of hydrotalcite) on the catalysed transesterification of vegetable oil was studied. The novelty consists in the detailed description of the transformation of sodium species throughout synthesis of mixed oxides: the sodium nitrates, presented in hydrotalcites, were transformed to sodium oxide (during calcination), which reacted with water (present in oil and methanol) to sodium hydroxides. During transesterification, the sodium hydroxide leached to the liquid phase and so increased the ester yield as a homogeneous catalyst. The ester content was only 19 wt.% with the catalyst containing almost no sodium ions (5 μmol/g), but only 18 μmol/g caused increasing of ester content to 50 wt.%. Moreover, the sodium compounds rapidly increased the catalyst activity by changing the active sites. The reusability of the catalyst was caried out and the ester content decreased to only 22% after four cycles, because the sodium leached.