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Large-scale production of biofuels and chemicals will require cost-effective, sustainable, and rapid deconstruction of woody biomass into its constituent sugars. Here, we introduce a novel two-step liquefaction process for producing fermentable sugars from red oak using a mixture of tetrahydrofuran (THF), water and dilute sulfuric acid. THF promotes acid-catalyzed solubilization of lignin and hemicellulose in biomass achieving 61% lignin extraction and 64% xylose recovery in a mild pretreatment step. The pretreatment opens the structure of biomass through delignification and produces a cellulose-rich biomass, which is readily solubilized at low temperature giving 65% total sugar yields in a subsequent liquefaction process employing the same solvent mixture. This process achieves competitive sugar yields at high volumetric productivity compared to conventional saccharification methods. THF, which can be derived from renewable resources, has several benefits as solvent including ease of recovery from the sugar solution and relatively low toxicity and cost.

We demonstrate for the first time non-catalytic, oxidative cracking with molecular oxygen (O2) to depolymerize native lignin into oxygenated phenolic monomers. Maximum monomer yield of 10.5 wt% was achieved at 250 °C after only 10 min of reaction and included vanillin, syringaldehyde, vanillic acid, and syringic acid. High rates of oxidation are attributed to the use of perfluorodecalin as solvent. Perfluorodecalin is a perfluorocarbon (PFC), characterized by their chemical stability and exceptionally high solubility for O2. Monomer yields were typically five-fold higher in perfluorodecalin compared to solvents more commonly employed in lignin conversion, such as methanol, butanol, acetonitrile, and ethyl acetate. Phenolic monomer production in perfluorodecalin favors high temperatures and short reaction times to prevent further oxidation of the produced monomers. Lignin oil obtained under oxidative conditions in perfluorodecalin showed lower molecular weight and smaller polydispersity compared to other solvents. Increasing the reaction time further decreased the molecular weight, while increasing reaction time in an inert atmosphere increased the molecular weight of the lignin oil. High concentrations of O2 in perfluorodecalin not only increased lignin depolymerization but suppressed undesirable condensation reactions. Depolymerization is likely initiated by thermally induced homolytic cleavage of ether linkages in lignin to form phenoxy and carbon-based radicals. These radicals bind with O2 as a radical scavenger and further react to form phenolic monomeric products rather than repolymerizing to large oligomers. The PFC process was scaled from 5 mL to 250 mL without any loss of yield. Because most organic compounds are not soluble in perfluorodecalin, recycling is easily achieved via liquid–liquid separation.

Efforts to understand the reaction mechanisms of cellulose pyrolysis have been stymied by short reaction times and difficulties in probing the condensed phase of cellulose intermediate products. Using time-resolved yields of both volatile and non-volatile products of pyrolysis, we demonstrate that cracking reactions generate anhydro-oligosaccharides while subsequent reactions produce levoglucosan from these anhydro-oligosaccharides. Eventually, cracking of anhydro-oligosaccharides is eclipsed by levoglucosan-producing reactions. These reactions compete with other reactions that produce light oxygenates and non-condensable gases. The relative reaction rates in this competition limit levoglucosan yields from cellulose pyrolysis to approximately 60 wt%.

Passivation of naturally occurring AAEM in biomass enhances sugar yields from the fast pyrolysis of biomass by preventing these metals from catalyzing the fragmentation of pyranose rings in cellulose and hemicellulose. However, because AAEM also catalyzes lignin depolymerization, its passivation can be accompanied by undesirable char agglomeration. Pretreatment of biomass with ferrous sulfate both passivates AAEM and substitutes ferrous ions as lignin depolymerization catalysts. This pretreatment has been particularly successful for high ash biomass like corn stover, but of limited value for low ash biomass like wood. This study explores the reasons for this discrepancy and offers a combined pretreatment of ferrous sulfate and ferrous acetate pretreatment to overcome char agglomeration in wood. This new pretreatment increased sugar yields from 4.4 wt% to 15.5 wt% and 5.4 wt% to 19.0 wt% for hardwood and softwood biomasses, respectively. This pretreatment produces an iron-rich biochar that catalyzes oxidation of the biochar under the oxygen-rich conditions of autothermal pyrolysis, which is preferentially consumed to provide the enthalpy for pyrolysis, preserving bio-oil as a more desirable energy product. Instead of producing carbon monoxide, which dominates oxidation of biochar from untreated biomass, the iron catalyzes oxidation to carbon dioxide, producing more energy per mole of oxygen consumed. In fact, oxygen demand to support autothermal pyrolysis of red oak and southern yellow pine was reduced 15% by the presence of iron in the biochar.

Microgram-scale reactors combined with gas chromatography (GC) coupled to mass spectrometry (MS) or flame ionization detection (FID) are used widely in pyrolysis research. Whether these devices meet the expected fast heating rates and short vapor residence times of fast pyrolysis have not been verified. In this study, experiments and simulations are used to investigate heat and mass transfer in a furnace-based micropyrolyzer. Surprisingly, heating rates obtained from the temperature history of sample cups in the reactor were modest compared to the greater than 1000 K s−1 heating rates sometimes assumed for such reactors. The heating rate at 773 K, employed commonly in fast pyrolysis, was only 180 K s−1. The highest rate observed was 494 K s−1 at a furnace temperature of 1268 K, which is well above typical pyrolysis temperatures. The mass transfer of volatilized samples was studied using both an optically accessible furnace and computational fluid dynamics. The standard sample cups used with these micropyrolyzers impede the escape of vapors. The use of shallow perforated cups overcame this mass transfer limitation to lead to levoglucosan yields ≈10 % higher than usually reported for the pyrolysis of cellulose.

Fast pyrolysis of lignocellulosic biomass yields little sugar or anhydrosugars compared to pyrolysis of pure polysaccharides because naturally abundant alkali and alkaline earth metals (AAEM) in biomass catalyze the fragmentation of pyranose and furanose rings. Sugar yields can be increased dramatically by pretreating the biomass with sulfuric acid prior to pyrolysis, which passivates the catalytic activity of the metals by converting them into thermally stable salts. However, depolymerization of lignin in biomass also depends on the catalytic activity of AAEM. Thus, passivating AAEM has the unintended consequence of slowing the rate of depolymerization and volatilization of lignin, resulting in a transient melt phase that agglomerates and can foul pyrolysis reactors. To overcome this problem, various non-alkali metal sulfates were tested as replacements for sulfuric acid. Iron in the form of ferrous sulfate proved the most effective in depolymerizing lignin without fragmenting pyranose rings. Conventional nitrogen-blown pyrolysis of ferrous sulfate pretreated corn stover achieved WHSV of 4 h−1 compared to only 0.6 h−1 for acid pretreated corn stover. Autothermal (air-blown) pyrolysis of ferrous sulfate pretreated corn stover showed even more dramatic improvement, increasing WHSV from 1 h−1 to 10 h−1 compared to acid pretreated corn stover under autothermal operation. Fermentable sugar yields from the pyrolysis of corn stover increased from 0.9 wt% to 11.8 wt% on a biomass basis, a 13-fold increase as a result of the ferrous sulfate pretreatment. These advantages combine to increase volumetric sugar productivity from 62 g L−1 h−1 for conventional pyrolysis of untreated corn stover to 2041 g L−1 h−1 for autothermal pyrolysis of ferrous sulfate treated corn stover.

Correction for ‘Tetrahydrofuran-based two-step solvent liquefaction process for production of lignocellulosic sugars’ by Arpa Ghosh et al., React. Chem. Eng., 2020, DOI: 10.1039/d0re00192a.