Role of the Support and Reaction Conditions on the Vapor-Phase Deoxygenation of <i>m</i>‑Cresol over Pt/C and Pt/TiO₂ Catalysts

The catalytic deoxygenation of biomass fast pyrolysis vapors offers a promising route for the sustainable production of liquid transportation fuels. However, a clear understanding of the mechanistic details involved in this process has yet to be achieved, and questions remain regarding the role of the catalyst support and the influence of reaction conditions. In order to gain insight into these questions, the deoxygenation of <i>m</i>-cresol was investigated over Pt/C and Pt/TiO₂ catalysts using experimental and computational techniques. The performance of each catalyst was evaluated in a packed-bed reactor under two conditions (523 K, 2.0 MPa and 623 K, 0.5 MPa), and the energetics of the ring hydrogenation, direct deoxygenation, and tautomerization mechanisms were calculated over hydrogen-covered Pt(111) and oxygen vacancies on the surface of TiO₂(101). Over Pt(111), ring hydrogenation to 3-methylcyclohexanone and 3-methylcyclohexanol was found to be the most energetically favorable pathway. Over TiO₂(101), tautomerization and direct deoxygenation to toluene were identified as additional energetically favorable routes. These calculations are consistent with the experimental data, in which Pt/TiO₂ was more active on a metal site basis and exhibited higher selectivity to toluene at 623 K relative to Pt/C. On the basis of these results, it is likely that the reactivity of Pt/TiO₂ and Pt/C is driven by the metallic phase at 523 K, while contributions from the TiO₂ support enhance deoxygenation at 623 K. These results highlight the synergistic effects between hydrogenation catalysts and reducible metal oxide supports and provide insight into the reaction pathways responsible for their enhanced deoxygenation performance

Alkaline Pretreatment of Corn Stover: Bench-Scale Fractionation and Stream Characterization

Biomass pretreatment generally aims to increase accessibility to plant cell wall polysaccharides for carbohydrate-active enzymes to produce sugars for biological or catalytic upgrading to ethanol or advanced biofuels. Significant research has been conducted on a suite of pretreatment processes for bioethanol processes. An alternative option, which has received less attention in the biofuels community, is the use of alkaline pretreatment for the partial depolymerization of lignin from intact biomass. A known issue with alkaline pretreatment is the loss of polysaccharides from peeling reactions, but this loss can be mitigated with anthraquinone, as commonly practiced in pulping. Here, we conduct a comprehensive bench-scale evaluation of alkaline pretreatment using corn stover at temperatures of 100, 130, and 160 °C and sodium hydroxide loadings from 35 to 660 mg NaOH/g dry biomass with anthraquinone. Compositional analysis is conducted on the starting material and residual solids after pretreatment, and mass balance is inferred in the liquor by difference. The residual solids after alkaline pretreatment are characterized for crystallinity and imaged by scanning and transmission electron microscopy to reveal the physical changes in the carbohydrate portions of the biomass remaining after pretreatment, which demonstrate dramatic modifications to biomass cell wall architecture with lignin removal but rather insignificant changes in cellulose crystallinity. Our results show that alkaline pretreatment at relatively mild conditions is able to remove substantial amounts of lignin from biomass. Going forward, to be an economically feasibile process, technologies will be required to upgrade the resulting lignin-rich liquor stream

Ammonia Pretreatment of Corn Stover Enables Facile Lignin Extraction

Thermochemical pretreatment of lignocellulose is often employed to render polysaccharides more digestible by carbohydrate-active enzymes to maximize sugar yields. The fate of lignin during pretreatment, however, is highly dependent on the chemistry employed and must be considered in cases where lignin valorization is targeted alongside sugar conversionan important feature of future biorefinery development. Here, a two-step process is demonstrated in which anhydrous ammonia (AA) pretreatment is followed by mild NaOH extraction on corn stover to solubilize and fractionate lignin. As known, AA pretreatment simultaneously alters the structure of cellulose with enhanced digestibility while redistributing lignin. The AA-pretreated residue is then extracted with dilute NaOH at mild conditions to maximize lignin separation, resulting in a digestible carbohydrate-rich solid fraction and a solubilized lignin stream. Lignin removal of more than 65% with over 84% carbohydrate retention is achieved after mild NaOH extraction of AA-pretreated corn stover with 0.1 M NaOH at 25 °C. Two-dimensional nuclear magnetic resonance (2D-NMR) spectroscopy of the AA-pretreated residue shows that ammonolysis of ester bonds occurs to partially liberate hydroxycinnamic acids, and the AA-pretreated/NaOH-extracted residue exhibits a global reduction of all lignin moieties caused by reduced lignin content. A significant reduction (∼70%) in the weight-average molecular weight (<i>M</i>_w) of extracted lignin is also achieved. Imaging of AA-pretreated/NaOH extracted residues show extensive delamination and disappearance of coalesced lignin globules from within the secondary cell walls. Glycome profiling analyses demonstrates ultrastructural level cell wall modifications induced by AA pretreatment and NaOH extraction, resulting in enhanced extractability of hemicellulosic glycans, indicating enhanced polysaccharide accessibility. The glucose and xylose yields from enzymatic hydrolysis of AA-pretreated/NaOH-extracted corn stover were higher by ∼80% and ∼60%, respectively, compared to untreated corn stover at 1% solids loadings. For digestions at 20% solids, a benefit of NaOH extraction is realized in achieving ∼150 g/L of total monomeric sugars (glucose, xylose, and arabinose) in the enzymatic hydrolysates from AA-pretreated/NaOH-extracted corn stover. Overall, this process enables facile lignin extraction in tandem with a leading thermochemical pretreatment approach, demonstrating excellent retention of highly digestible polysaccharides in the solid phase and a highly depolymerized, soluble lignin-rich stream

Application of a Pyroprobe–Deuterium NMR System: Deuterium Tracing and Mechanistic Study of Upgrading Process for Lignin Model Compounds

In this study, a pyroprobe–deuterium (²H) NMR system has been used to identify isotopomer products formed during the deuteration and ring opening of lignin model compounds. Several common model compounds for lignin and its upgraded products, including guaiacol, syringol, toluene, <i>p</i>-xylene, phenol, catechol, cyclohexane, methylcyclohexane, and methylcyclopentane, have been examined for selective ring opening. Similar pathways for upgrading of toluene and <i>p</i>-xylene has been found, which will undergo hydrogenation, methyl group elimination, and ring opening process, and benzene, cyclohexane, and methylcyclohexane have been found as major intermediates before ring opening. Very interestingly, the ²H NMR analysis for the deuterium-traced ring opening of catechol on Ir/γ-Al₂O₃ is almost identical to the ring opening process for phenol. The ring opening processes for guaiacol and syringol appeared to be very complicated, as expected. Benzene, phenol, toluene, cyclohexane, and methylcyclohexane have been determined to be the major products

Evaluation of Clean Fractionation Pretreatment for the Production of Renewable Fuels and Chemicals from Corn Stover

Organosolv fractionation processes aim to separate the primary biopolymers in lignocellulosic biomass to enable more selective deconstruction and upgrading approaches for the isolated components. Clean fractionation (CF) is a particularly effective organsolv process that was originally applied to woody feedstocks. The original CF pretreatment employed methyl isobutyl ketone (MIBK), ethanol, and water with sulfuric acid as a catalyst at temperatures ranging from 120 to 160 °C. Understanding the feasibility and applicability of organosolv processes for industrial use requires mass balances on the primary polymers in biomass, detailed understanding of the physical and chemical characteristics of the fractionated components, and viable upgrading processes for each fraction. Here, we apply two CF approaches to corn stover, one with MIBK/ethanol/water and acid and the other with MIBK/acetone/water and acid, with the aim of understanding if these fractionation methods are feasible for industrial application. We quantify the full mass balances on the resulting solid, organic, and aqueous fractions and apply multiple analytical methods to characterize the three fractions. Total mass yields of the cellulose-enriched, hemicellulose-enriched, and lignin-enriched fractions are near mass closure in most cases. For corn stover, the MIBK/acetone/water CF solvent system is more effective relative to the original CF method based on the enhanced fractionation susceptibility of the aqueous and organic phases and the lower molecular weight distribution of the lignin-enriched fractions. Overall, this work reports component mass balances for the fractionation of corn stover, providing key inputs for detailed evaluation of CF processes based on bench-scale data

Clean Fractionation Pretreatment Reduces Enzyme Loadings for Biomass Saccharification and Reveals the Mechanism of Free and Cellulosomal Enzyme Synergy

Enzymatic depolymerization of polysaccharides is often a key step in the production of fuels and chemicals from lignocellulosic biomass. Historically, model cellulose substrates have been utilized to reveal insights into enzymatic saccharification mechanisms. However, translating findings from model substrates to realistic biomass substrates is critical for evaluating enzyme performance. Here, we employ a commercial fungal enzyme cocktail, purified cellulosomes, and combinations of these two enzymatic systems to investigate saccharification mechanisms on corn stover deconstructed either via clean fractionation (CF) or deacetylated dilute sulfuric acid pretreatments. CF is an organosolv pretreatment method utilizing water, MIBK, and either acetone or ethanol with catalytic amounts of sulfuric acid to fractionate biomass components. The insoluble cellulose-enriched fraction (CEF) from CF contains mainly cellulose, with minor amounts of residual hemicellulose and lignin. Enzymatic digestions at both low and high solid loadings demonstrate that CF reduces the amount of enzyme required to depolymerize polysaccharides relative to deacetylated dilute acid-pretreated corn stover. Transmission and scanning electron microscopy of the digested biomass provides evidence for the different mechanisms of enzymatic deconstruction between free and cellulosomal enzyme systems and reveals the basis for the synergistic relationship between the two enzyme paradigms on a process-relevant substrate. These results also demonstrate that the presence of lignin is more detrimental to cellulosome action than to free fungal cellulases. As enzyme costs are a major driver for biorefineries, this study provides key inputs for evaluation of CF as a pretreatment method and synergistic mixed enzyme systems as a saccharification strategy for biomass conversion

Aqueous Stream Characterization from Biomass Fast Pyrolysis and Catalytic Fast Pyrolysis

Biomass pyrolysis offers a promising means to rapidly depolymerize lignocellulosic biomass for subsequent catalytic upgrading to renewable fuels. Substantial efforts are currently ongoing to optimize pyrolysis processes including various fast pyrolysis and catalytic fast pyrolysis schemes. In all cases, complex aqueous streams are generated containing solubilized organic compounds that are not converted to target fuels or chemicals and are often slated for wastewater treatment, in turn creating an economic burden on the biorefinery. Valorization of the species in these aqueous streams, however, offers significant potential for substantially improving the economics and sustainability of thermochemical biorefineries. To that end, here we provide a thorough characterization of the aqueous streams from four pilot-scale pyrolysis processes: namely, from fast pyrolysis, fast pyrolysis with downstream fractionation, <i>in situ</i> catalytic fast pyrolysis, and <i>ex situ</i> catalytic fast pyrolysis. These configurations and processes represent characteristic pyrolysis processes undergoing intense development currently. Using a comprehensive suite of aqueous-compatible analytical techniques, we quantitatively characterize between 12 g kg^–1 of organic carbon of a highly aqueous catalytic fast pyrolysis stream and up to 315 g kg^–1 of organic carbon present in the fast pyrolysis aqueous streams. In all cases, the analysis ranges between 75 and 100% of mass closure. The composition and stream properties closely match the nature of pyrolysis processes, with high contents of carbohydrate-derived compounds in the fast pyrolysis aqueous phase, high acid content in nearly all streams, and mostly recalcitrant phenolics in the heavily deoxygenated <i>ex situ</i> catalytic fast pyrolysis stream. Overall, this work provides a detailed compositional analysis of aqueous streams from leading thermochemical processesanalyses that are critical for subsequent development of selective valorization strategies for these waste streams

Lignin Depolymerization with Nitrate-Intercalated Hydrotalcite Catalysts

Hydrotalcites (HTCs) exhibit multiple adjustable parameters to tune catalytic activity, including interlayer anion composition, metal hydroxide layer composition, and catalyst preparation methods. Here, we report the influence of several of these parameters on β-O-4 bond scission in a lignin model dimer, 2-phenoxy-1-phenethanol (PE), to yield phenol and acetophenone. We find that the presence of both basic and NO₃^– anions in the interlayer increases the catalyst activity by 2–3-fold. In contrast, other anions or transition metals do not enhance catalytic activity in comparison to blank HTC. The catalyst is not active for C–C bond cleavage on lignin model dimers and has no effect on dimers without an α-OH group. Most importantly, the catalyst is highly active in the depolymerization of two process-relevant lignin substrates, producing a significant amount of low-molecular-weight aromatic species. The catalyst can be recycled until the NO₃^– anions are depleted, after which the activity can be restored by replenishing the NO₃^– reservoir and regenerating the hydrated HTC structure. These results demonstrate a route to selective lignin depolymerization in a heterogeneous system with an inexpensive, earth-abundant, commercially relevant, and easily regenerated catalyst

The Techno-Economic Basis for Coproduct Manufacturing To Enable Hydrocarbon Fuel Production from Lignocellulosic Biomass

Biorefinery process development relies on techno-economic analysis (TEA) to identify primary cost drivers, prioritize research directions, and mitigate technical risk for scale-up through development of detailed process designs. Here, we conduct TEA of a model 2000 dry metric ton-per-day lignocellulosic biorefinery that employs a two-step pretreatment and enzymatic hydrolysis to produce biomass-derived sugars, followed by biological lipid production, lipid recovery, and catalytic hydrotreating to produce renewable diesel blendstock (RDB). On the basis of projected near-term technical feasibility of these steps, we predict that RDB could be produced at a minimum fuel selling price (MFSP) of USD $9.55/gasoline-gallon-equivalent (GGE), predicated on the need for improvements in the lipid productivity and yield beyond current benchmark performance. This cost is significant given the limitations in scale and high costs for aerobic cultivation of oleaginous microbes and subsequent lipid extraction/recovery. In light of this predicted cost, we developed an alternative pathway which demonstrates that RDB costs could be substantially reduced in the near term if upgradeable fractions of biomass, in this case hemicellulose-derived sugars, are diverted to coproducts of sufficient value and market size; here, we use succinic acid as an example coproduct. The coproduction model predicts an MFSP of USD$5.28/GGE when leaving conversion and yield parameters unchanged for the fuel production pathway, leading to a change in biorefinery RDB capacity from 24 to 15 MM GGE/year and 0.13 MM tons of succinic acid per year. Additional analysis demonstrates that beyond the near-term projections assumed in the models here, further reductions in the MFSP toward $2–3/GGE (which would be competitive with fossil-based hydrocarbon fuels) are possible with additional transformational improvements in the fuel and coproduct trains, especially in terms of carbon efficiency to both fuels and coproducts, recovery and purification of fuels and coproducts, and coproduct selection and price. Overall, this analysis documents potential economics for both a hydrocarbon fuel and bioproduct process pathway and highlights prioritized research directions beyond the current benchmark to enable hydrocarbon fuel production via an oleaginous microbial platform with simultaneous coproduct manufacturing from lignocellulosic biomass