Biomass pyrolysis utilizes high temperatures to produce an economically renewable intermediate (pyrolysis oil) that can be integrated with the existing petroleum infrastructure to produce biofuels. The initial chemical reactions in pyrolysis convert solid biopolymers, such as cellulose (up to 60% of biomass), to a short-lived (less than 0.1 s) liquid phase, which subsequently reacts to produce volatile products. In this work, we develop a novel thin-film pyrolysis technique to overcome typical experimental limitations in biopolymer pyrolysis and identify a-cyclodextrin as an appropriate smallmolecule surrogate of cellulose. Ab initio molecular dynamics simulations are performed with this surrogate to reveal the long-debated pathways of cellulose pyrolysis and indicate homolytic cleavage of glycosidic linkages and furan formation directly from cellulose without any small-molecule (e.g., glucose) intermediates. Our strategy combines novel experiments and first-principles simulations to allow detailed chemical mechanisms to be constructed for biomass pyrolysis and enable the optimization of next-generation biorefineries.

Fast pyrolysis of biomass is a next-generation biofuels production process that is capable of converting solid lignocellulosic materials (in their raw form) to a transportable liquid (bio-oil) which can be catalytically hydrogenated to fuels and chemicals. While biomass fast pyrolysis has enormous potential to produce renewable fuels, an understanding of the fundamental chemistry that converts biomass components, such as cellulose, to bio-oil is not available in the literature. In this work, we use thin-film pyrolysis to reveal the effect of temperature under transport-free reaction conditions and then evaluate the effect of sample dimension (i.e., characteristic length scale) by comparing product distributions of conventional powders and thin films. In the first part of the work, we show that the yield of total furan rings (i.e., all products containing a five-membered furan ring) does not change significantly with increased reaction temperature compared to other pyrolysis products, such as light oxygenates and anhydrosugars. However, we find that the functional groups bound to the furan ring (e.g., alcohols and aldehydes) are easily cleaved to produce smaller furans. In the second part of the work, we show that sample dimension is a key descriptor for product yields. For example, levoglucosan (the most abundant product of cellulose pyrolysis) yield differs significantly between conventional powder (millimeter-sized samples which are transport-limited) pyrolysis and thin-film (micrometer-scale thin-films which are isothermal) pyrolysis (49% for powder; 27% for thin-film at 500 °C).

Fast pyrolysis of biomass thermally cracks solid biopolymers to generate a transportable liquid (bio-oil) which can be upgraded and integrated with the existing petroleum infrastructure. Understanding how the components of biomass, such as cellulose, break down to form bio-oil constituents is critical to developing successful biofuels technologies. In this work, we use a novel co-pyrolysis technique and isotopically labeled starting materials to show that levoglucosan, the most abundant product of cellulose pyrolysis (60% of total), is deoxygenated within molten biomass to form products with higher energy content (pyrans and light oxygenates). The yield of these products can be increased by a factor of six under certain reaction conditions, e.g., using long condensed-phase residence times encountered in powder pyrolysis. Finally, co-pyrolysis experiments with deuterated glucose reveal that hydrogen exchange is a critical component of levoglucosan deoxygenation.

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Formation of branched glucan chains by co-impregnation with glucose can greatly improve efficiency of mechano-catalytic depolymerization of crystalline cellulose.

Approximately 15 million dry tons of food waste is produced annually in the United States (USA), and 92% of this waste is disposed of in landfills where it decomposes to produce greenhouse gases and water pollution. Hydrothermal liquefaction (HTL) is an attractive technology capable of converting a broad range of organic compounds, especially those with substantial water content, into energy products. The HTL process produces a bio-oil precursor that can be further upgraded to transportation fuels and an aqueous phase containing water-soluble organic impurities. Converting small oxygenated compounds that partition into the water phase into larger, hydrophobic compounds can reduce aqueous phase remediation costs and improve energy yields. HTL was investigated at 300 • C and a reaction time of 1 h for conversion of an institutional food waste to bio-oil, using either homogeneous Na 2 CO 3 or heterogeneous CeZrO x to promote in situ conversion of water-soluble organic compounds into less oxygenated, oil-soluble products. Results with food waste indicate that CeZrO x improves both bio-oil higher heating value (HHV) and energy recovery when compared both to non-catalytic and Na 2 CO 3-catalyzed HTL. The aqueous phase obtained using CeZrO x as an HTL catalyst contained approximately half the total organic carbon compared to that obtained using Na 2 CO 3-suggesting reduced water treatment costs using the heterogeneous catalyst. Experiments with model compounds indicated that the primary mechanism of action was condensation of aldehydes, a reaction which simultaneously increases molecular weight and oxygen-to-carbon ratio-consistent with the improvements in bio-oil yield and HHV observed with institutional food waste. The catalyst was stable under hydrothermal conditions (≥16 h at 300 • C) and could be reused at least three times for conversion of model aldehydes to water insoluble products. Energy and economic analysis suggested favorable performance for the heterogeneous catalyst compared either to non-catalytic HTL or Na 2 CO 3-catalyzed HTL, especially once catalyst lifetime differences were considered. The results of this study establish the potential of heterogeneous catalysts to improve HTL economics and energetics.

Industrial wastes and natural mixed oxide materials were evaluated as inexpensive heterogeneous catalysts for catalytic hydrothermal liquefaction (CHTL) of food wastes. Red mud and red clay achieved biocrude carbon yields of 47.0 and 39.5% with higher heating values (HHVs) of 40.2 and 37.7 MJ kg–1, respectively, which were much greater than those without the catalyst (biocrude carbon yield of 19.7% and HHV of 36.1 MJ kg–1). Biocrude characterization revealed that similar families of molecules were formed in the presence and absence of catalysts, implying that the main role of the catalyst is to promote rates of thermal reactions, leading to biocrude production without opening new pathways. The crystalline structures of inexpensive mixed oxides were stable under hydrothermal conditions, with modest calcium leaching (7.5%) and trace leaching of other metals. Using red clay or red mud resulted in >40% recovery of the energy in food waste as biocrude, greater than that obtained under noncatalytic conditions (18%) or from any individual constituent oxide (19–27%). The improved CHTL performance of the mixed metal oxides compared with single-metal oxides was attributed to the synergistic effects of base and acid sites present on catalyst surfaces; mixed oxides presented balanced densities of acids and bases, whereas the constituent oxides were either primarily acidic or primarily basic. The percent of energy recovered as biocrude oil was strongly correlated with the base-to-acid site density ratio, providing an important performance predictor for CHTL conversion of food waste to bioenergy.

Current research of complex chemical systems, including biomass pyrolysis, petroleum refining, and wastewater remediation requires analysis of large analyte mixtures (>100 compounds). Quantification of each carbon-containing analyte by existing methods (flame ionization detection) requires extensive identification and calibration. In this work, we describe an integrated microreactor system called the Quantitative Carbon Detector (QCD) for use with current gas chromatography techniques for calibration-free quantitation of analyte mixtures. Combined heating, catalytic combustion, methanation and gas co-reactant mixing within a single modular reactor fully converts all analytes to methane (>99.9%) within a thermodynamic operable regime. Residence time distribution of the QCD reveals negligible loss in chromatographic resolution consistent with fine separation of complex mixtures including cellulose pyrolysis products.