Alex D. Paulsen

Revealing pyrolysis chemistry for biofuels production: conversion of cellulose to furans and small oxygenates.

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 α-cyclodextrin as an appropriate small-molecule 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.

The chain length effect in pyrolysis: bridging the gap between glucose and cellulose

Despite the potential for biomass pyrolysis to produce renewable fuels, the governing chemical reactions are largely unknown due to the complexity of biopolymers, such as cellulose. In this work, we use isothermal pyrolysis experiments to reveal the chain length (or end-group) effect that controls the distribution of pyrolysis products from linear β-1,4-glucan polymers (e.g., cellulose). Finally, we show that a simplified end-group/interior monomer model is largely incapable of predicting product yields from cellodextrin pyrolysis.

Pyrolytic conversion of cellulose to fuels: levoglucosan deoxygenation via elimination and cyclization within molten biomass

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.

Fast Pyrolysis of Wood for Biofuels: Spatiotemporally Resolved Diffuse Reflectance In situ Spectroscopy of Particles

Fast pyrolysis of woody biomass is a promising process capable of producing renewable transportation fuels to replace gasoline, diesel, and chemicals currently derived from nonrenewable sources. However, biomass pyrolysis is not yet economically viable and requires significant optimization before it can contribute to the existing oil-based transportation system. One method of optimization uses detailed kinetic models for predicting the products of biomass fast pyrolysis, which serve as the basis for the design of pyrolysis reactors capable of producing the highest value products. The goal of this work is to improve upon current pyrolysis models, usually derived from experiments with low heating rates and temperatures, by developing models that account for both transport and pyrolysis decomposition kinetics at high heating rates and high temperatures (>400 °C). A new experimental technique is proposed herein: spatiotemporally resolved diffuse reflectance in situ spectroscopy of particles (STR-DRiSP), which is capable of measuring biomass composition during fast pyrolysis with high spatial (10 μm) and temporal (1 ms) resolution. Compositional data were compared with a comprehensive 2D single-particle model, which incorporated a multistep, semiglobal reaction mechanism, prescribed particle shrinkage, and thermophysical properties that varied with temperature, composition, and orientation. The STR-DRiSP technique can be used to determine the transport-limited kinetic parameters of biomass decomposition for a wide variety of biomass feedstocks.

Efficient mechano-catalytic depolymerization of crystalline cellulose by formation of branched glucan chains

Selective hydrolysis of cellulose into glucose is a critical step for producing value-added chemicals and materials from lignocellulosic biomass. In this study, we found that co-impregnation of crystalline cellulose with sulfuric acid and glucose can greatly reduce the time needed for ball milling compared with adding acid alone. The enhanced reaction time coincides with the rapid formation of branched α(1→6) glycosidic bonds, which have been shown to increase water solubility of β(1→4) glucan oligomers. Co-impregnation of glucose was crucial for the rapid formation of the α(1→6) branches, after which a carbon-based catalyst can rapidly hydrolyze the water-soluble glucan oligomers to 91.2% glucose yield faster than conventional approaches.

Tuning cellulose pyrolysis chemistry: selective decarbonylation via catalyst-impregnated pyrolysis

Widespread adoption of biomass pyrolysis for lignocellulosic biofuels is largely hindered by a lack of economical means to stabilize the bio-oil (or pyrolysis oil) product. In this work, impregnation of supported metal catalysts provides a new approach to selectively decarbonylate primary pyrolysis products within intermediate cellulose liquid to targeted gasoline-like molecules with enhanced energy content and stability. Selective deoxygenation of hydroxy-methylfurfural (HMF) and furfural (F) to 88% yield of stable furans occurred over carbon-supported Pd, with negligible loss in overall bio-oil yield or furanic content.

Alkaline-Earth-Metal-Catalyzed Thin-Film Pyrolysis of Cellulose

The conversion of lignocellulosic biomass to “bio‐oils” by thermochemical pyrolysis is a promising reactor technology for renewable chemicals and biofuels. Although the fundamental understanding of relevant catalysts within reacting biomass particles is only in its infancy, it is known that inorganic materials naturally present within biomass act as catalysts that limit the yield of bio‐oil and alter the product distribution. In this work, the effect of alkaline earth metals on cellulose pyrolysis chemistry was investigated to determine the catalytic effect on primary (transport‐free) and secondary (diffusion‐limited) reaction pathways. The catalytic materials included homogeneous metal ions Ca2+ and Mg2+ from their inorganic salts, Ca(NO3)2 and Mg(NO3)2, and their corresponding heterogeneous metal oxides, CaO and MgO. Although the oxides had a limited impact on cellulose pyrolysis chemistry, the metal ions altered the secondary reaction pathways of cellulose significantly under diffusion‐limited conditions common to lignocellulosic particles within industrial reactors.

Quantitative carbon detector (QCD) for calibration-free, high-resolution characterization of complex mixtures.

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.

Spontaneous Aerosol Ejection: Origin of Inorganic Particles in Biomass Pyrolysis

At high thermal flux and temperatures of approximately 500 °C, lignocellulosic biomass transforms to a reactive liquid intermediate before evaporating to condensable bio-oil for downstream upgrading to renewable fuels and chemicals. However, the existence of a fraction of nonvolatile compounds in condensed bio-oil diminishes the product quality and, in the case of inorganic materials, catalyzes undesirable aging reactions within bio-oil. In this study, ablative pyrolysis of crystalline cellulose was evaluated, with and without doped calcium, for the generation of inorganic-transporting aerosols by reactive boiling ejection from liquid intermediate cellulose. Aerosols were characterized by laser diffraction light scattering, inductively coupled plasma spectroscopy, and high-speed photography. Pyrolysis product fractionation revealed that approximately 3 % of the initial feed (both organic and inorganic) was transported to the gas phase as aerosols. Large bubble-to-aerosol size ratios and visualization of significant late-time ejections in the pyrolyzing cellulose suggest the formation of film bubbles in addition to the previously discovered jet formation mechanism.

Reactive Liftoff of Crystalline Cellulose Particles

The condition of heat transfer to lignocellulosic biomass particles during thermal processing at high temperature (>400 °C) dramatically alters the yield and quality of renewable energy and fuels. In this work, crystalline cellulose particles were discovered to lift off heated surfaces by high speed photography similar to the Leidenfrost effect in hot, volatile liquids. Order of magnitude variation in heat transfer rates and cellulose particle lifetimes was observed as intermediate liquid cellulose droplets transitioned from low temperature wetting (500–600 °C) to fully de-wetted, skittering droplets on polished surfaces (>700 °C). Introduction of macroporosity to the heated surface was shown to completely inhibit the cellulose Leidenfrost effect, providing a tunable design parameter to control particle heat transfer rates in industrial biomass reactors.