Valorization of olive mill waste fractions and production of furfural and 5-hydroxymethylfurfural by heterogeneous catalysis over Nb2O5, NbOPO4 and TiO2-PO4 catalysts

The production of furans from the valorization of olive mill biomass waste streams including olive mill wastewater (OMW), olive virgin pomace (OVP), dry pelletized pomace (DPP), and olive dry leaves (DL) was investigated using acidic Nb2O5, NbOPO4 and TiO2-PO4 heterogeneous catalysts. Batch reactions were performed in an autoclave at 210 °C in the presence of the suspended catalysts and the yield of furfural and 5-hydroxymethylfurfural (5-HMF) was determined. The valorization of OMW was found more effective in the presence of NbOPO4 as a result of its high acidity nature. The amount and strength of the acid sites of NbOPO4 were much higher than the other catalysts. The highest yield of hexoses sugars in OMW to 5-HMF was 44 %, while furfural was a minor product. The 5-HMF yield further increased to 91 % after pre-extraction of the polyphenolic species from the OMW. Recycling tests of NbOPO4 catalyst, the most performant one, indicated the very good stability of this material. As result of the highest xylose content in the olive mill solids fractions (olive pomace, dry pellet pomace and dry leaves), furfural's yields of 100 %, 57 % and 31 %, respectively, were observed, while 5-HMF in contrast to OMW was a minor product. This work demonstrates the feasibility of using acidic catalysts for the valorization of olive mill waste streams targeting the desired furan product.