NH₄F modified Al-SBA-15 materials for esterification of valeric acid to alkyl valerates.

In the present work, SBA-15 aluminosilicate materials were modified for their use as catalysts in the valeric acid esterification. Firstly, Al-SBA-15 material were functionalised by ball milling with several niobium loadings (0.25-1 wt.%) and, eventualy, by incipient wetness impregnation with several F^- loadings, using ammonium fluoride as salt precursor. The catalyst herein synthesised were characterised by XRD, N2 porosimetry and DRIFT among others with a special emphasis to the acidic properties, which were evaluated by titration using pyridine and 2,6-dimethylpyridine. The prepared materials shown form moderate to high catalytic activities in the microwave-assisted transformation of valeric acid to ethyl valerate via esterification. Unexpectedly, the incorporation of niobic acid nanoparticles on the aluminosicates lead to a decrease in the conversion without finding any trend related with the metal loading. On the contrary, the incorporation of fluoride anions either on Al-SBA-15 or on Nb1%/Al-SBA-15 leads to a linear increase in valeric acid conversion with the F^- content. The increase on valeric acid conversion has been the same independently if the modification with fluoride anions has been performed over Al-SBA-15 or Nb1%/Al-SBA-15. Thus, it can be concluded that F^- modified mesoporous aluminosilicates are efficient catalyst for the transformation of valeric acid into alkyl valerate esters as renewable fuels.