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

- Kraft lignin was reductively depolymerized to low-Mw products with formic acid.
- Ni/zeolite catalyzed kraft lignin (KL) depolymerization at a low temp. of 200 °C.
- At 200 °C with Ni/zeolite, Mw of KL reduced from 10,000 g/mol to 3100 g/mol.
- Reductively depolymerization of KL 200-300 °C led to ∼90% yield of low Mw products.
- FHUDS-2 (W-Mo-Ni) was the most effective catalyst for removal of sulfur in KL.

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 (Ni10%/Zeolite and FHUDS-2), compared with an expensive Ru-based reference catalysts (Ru5%/C). At 200-300 °C for 1-3 h with or without catalyst, Kraft lignin (KL, Mw ∼10,000 g/mol) was effectively de-polymerized to low Mw (Mw 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 Mw 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, Ni10%/Zeolite exhibited excellent activity for de-polymerization of KL, producing DL of Mw = 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 Ni10%/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 Ni10%/Zeolite and to ∼0 wt% in DL with FHUDS-2 catalyst at 300 °C.

Reductive de-polymerization of lignin over a supported metal catalyst with in-situ hydrogen source.96