Reactions of C5 and C6-sugars, cellulose, and lignocellulose under hot compressed water (HCW) in the presence of heterogeneous acid catalysts

The benefit of TiO2, ZrO2 and SO4–ZrO2 on the reactions of C5-sugar (xylose), C6-sugar (glucose), cellulose, and lignocellulose was studied in hot compressed water (HCW) at 473–673 K with an aim to produce furfural and 5-hydroxymethylfurfural (HMF). TiO2 and SO4–ZrO2 were found to active for hydrolysis and dehydration reactions producing high furfural and HMF yields with less by-products (i.e. glucose, fructose, xylose, and 1,6-anhydroglucose (AHG)) formation, whereas ZrO2 was highly active for isomerization reaction; thus significant amount of fructose was observed in the liquid product.Importantly, it was also found that the starting salt precursor, the sulfur-doping content (for SO4–ZrO2) and the calcination temperature strongly affected the catalyst reactivity. Catalysts prepared from the chloride-based precursors (i.e. ZrOCl2 and TiCl4) gained higher reactivity compared to those prepared from nitrate-based precursors (i.e. ZrO(NO3)2 and TiO(NO3)2) due to their greater acidity, according to the NH3- and CO2-TPD studies. For SO4–ZrO2, among the catalyst with sulfur contents of 0.75%, 1.8% and 2.5%, SO4–ZrO2 with 1.8% sulfur content presented the highest acidity and reactivity toward hydrolysis and dehydration reactions. It is noted that the suitable calcination temperature for all catalysts was at 773 K; the XRD patterns revealed that different portions of phase formation was observed over catalysts with different calcination temperatures i.e. anatase/rutile for TiO2 and monoclinic/tetragonal for ZrO2 and SO4–ZrO2; the portion of these phase formations obviously affected the acidity–basicity of catalyst and thus the catalyst reactivity.