Catalytic depolymerization of lignin into monophenols over an amorphous mesoporous geopolymer monolith

A new economical, stable, and recyclable amorphous mesopomus geopolymer monolith (GM) was prepared from metakaolin and slag. The obtained GM contained a porous structure mainly with mesopores, and rich Bronsted and Lewis acid sites, which showed higher activity than Ni/MCM-41 and Pt/NiAl2O4, etc. Over 45 aromatic compounds, including guaiacol, syringol, isovanillin, 3,4-dimethoxybenzyl alcohol, etc., were found in the bio-oil. Under optimal conditions, the total yield of 10 major monophenols reached 74.69 +/- 1.00 mg/g, half of which was 4-ethyl guaiacol (36.77 +/- 1.89 mg/g). Moreover, the GM was easy to regenerate and reuse. In three consecutive runs, the catalytic activity of GM was only reduced by 6.69% per cycle. The synergism of Bronsted and Lewis acidic sites and mesopores of GM was responsible for the high catalytic activity. These indicate the GM is low-cost, easy to prepare, environmentally benign, stable, recyclable, and has great potential in depolymerizing lignin into monophenols.