Cellulosic glycols: an integrated process concept for lignocellulose pretreatment and hydrogenolysis

Abstract

Lignocellulose is the most abundant source of saccharides and it is therefore a promising feedstock for glycols, such as ethylene-glycol, via catalytic hydrogenolysis of the polysaccharides that it contains. However, this catalytic hydrogenolysis step is hampered by the presence of lignin and other biomass contaminants, such as ash, which need to be removed in a pretreatment step. We propose an organosolv-like pre-treatment that can delignify and de-ash lignocellulose to a level that allows it to be upgraded to glycol with comparable yields to pure cellulose under demanding hydrogenolysis conditions. This work identifies the main design constraints of the integrated process and provides an initial experimental validation of it. Pretreatment of biomass in water/ethanol/acetic acid solutions at 180-200 °C can reduce the lignin content of the solid residue to ≤6 wt%. The addition of an organic acid, such as acetic acid, appears important to improve the removal of the recalcitrant Ca 2+ , which is a known inhibitor of the tungstate catalyst typically used in this process. The pretreatment medium is designed to use the by-product(s) of the process as organic solvent, to reduce the need for fresh solvent input. However, the process still needs a high solvent recovery (between 93.5 and 99.9 wt%). This can be achieved by selecting volatile component as organic solvent, as it allows recovery by evaporation from the dissolved lignin and ashes.

From Woody Biomass to Ethylene Glycol: Inorganics Removal Boosts the Yield

“…Note that water leaching is rather ineffective for the removal of divalent ions such as calcium as was previously shown. 26 This mild pretreatment was more effective in removing calcium and magnesium than the high severity pretreatments PT-A (ethanol−water, 50/50, 200 °C, 3 h) and PT-B (70 wt % acetic acid in water, 180 °C, 1 h) reported in our previous work, 12 whose solid residues had a combined calcium and magnesium content of 9 and 3.2 mmol per kg of biomass, respectively. On the contrary, acid leaching only removes inorganics and water-soluble extractives, whereas lignin removal might be imperative for long-term operation.…”

“… a Quantification was performed at least in duplicate. b Data previously published in refs. , c Recalculated from untreated feedstocks. …”

“…For industrial operation, the catalyst consumption should be minimized and preferably stay below 1 kg catalyst /ton product . 22 Under the assumption that 1 mol of cation titrates:1 mol of WO x , we previously derived that the divalent cation content of the biomass feedstock can be 4 mmol of divalent ions per kg biomass 12 at max. This assumption was validated in this work as we showed that calcium precipitates with tungstate in an equimolar ratio.…”

Cellulosic Glycols: Identification and Prevention of Catalyst Deactivation

“…Note that water leaching is rather ineffective for the removal of divalent ions such as calcium [115] . Acid leaching was more effective in removing calcium and magnesium than the high severity pretreatments PT-A (Ethanol-water, 50/50, 200 • C, 3 h) and PT-B (70 wt.% Acetic acid in water, 180 • C, 1 h) reported in Chapter 7 [111] , whose solid residues had a combined calcium and magnesium content of 9 and 3.2 mmol per kg of biomass respectively. However, these high severity pretreatments also remove lignin.…”

“…For industrial operation the catalyst consumption should be minimized and preferably stay below 1 kg Catalyst / tonne product [89] . Under the assumption that one mole of cation precipitates one mole of WO x , we derived that the maximum allowable divalent cation content of the biomass feedstock can be 4 mmol per kg biomass (Chapter 7) [111] . This assumption was validated in this chapter as we showed that calcium precipitates with tungstate in an equimolar ratio.…”