Reductive de-polymerization of kraft lignin with formic acid at low temperatures using inexpensive supported Ni-based catalysts

Abstract This work aimed to explore inexpensive supported Ni-based catalysts for de-polymerization of kraft lignin with formic acid at lower temperatures. In this study, kraft lignin was de-polymerized in water–ethanol mixture 50/50 (v/v) with formic acid as an in-situ hydrogen source in the presence of Ni-based catalysts (Ni 10% /Zeolite and FHUDS-2), compared with an expensive Ru-based reference catalysts (Ru 5% /C). At 200–300 °C for 1–3 h with or without catalyst, Kraft lignin (KL, M w ∼10,000 g/mol) was effectively de-polymerized to low M w ( M w 1000–3000 g/mol) de-polymerized lignin (DL) products at a very high yield (85–95 wt%). Generally, at all temperatures tested, the presence of a supported metal catalyst could effectively reduce the M w of the DL product although it did not yield significant effects on the product yield. The effects of catalyst were more evident at a lower temperature. For instance, Ni 10% /Zeolite exhibited excellent activity for de-polymerization of KL, producing DL of M w  = 3150 g/mol at 93.5(±4.1) wt.% yield at a low temperature (200 °C) for 1 h. Among all catalysts examined, the two inexpensive Ni-based catalysts (although at a loading double that of the expensive Ru catalysts), i.e., FHUDS-2 (a W-Mo-Ni industrial catalyst) and Ni 10% /Zeolite demonstrated to be the most effective catalysts for sulfur-removal, reducing the sulfur content from 1.52 wt% in KL to 0.47 wt% in DL with Ni 10% /Zeolite and to ∼0 wt% in DL with FHUDS-2 catalyst at 300 °C.