Reactions of C 5 and C 6-sugars, cellulose, and lignocellulose under hot compressed water (HCW) in the presence of heterogeneous acid catalysts

Abstract

The benefit of TiO 2, ZrO 2 and SO 4–ZrO 2 on the reactions of C 5-sugar (xylose), C 6-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). TiO 2 and SO 4–ZrO 2 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 ZrO 2 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 SO 4–ZrO 2) and the calcination temperature strongly affected the catalyst reactivity. Catalysts prepared from the chloride-based precursors (i.e. ZrOCl 2 and TiCl 4) gained higher reactivity compared to those prepared from nitrate-based precursors (i.e. ZrO(NO 3) 2 and TiO(NO 3) 2) due to their greater acidity, according to the NH 3- and CO 2-TPD studies. For SO 4–ZrO 2, among the catalyst with sulfur contents of 0.75%, 1.8% and 2.5%, SO 4–ZrO 2 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 TiO 2 and monoclinic/tetragonal for ZrO 2 and SO 4–ZrO 2; the portion of these phase formations obviously affected the acidity–basicity of catalyst and thus the catalyst reactivity.