Insights into the Ceria-Catalyzed Ketonization Reaction for Biofuels Applications

The ketonization of small organic acids is a valuable reaction for biorenewable applications. Ceria has long been used as a catalyst for this reaction; however, under both liquid and vapor phase conditions, it was found that given the right temperature regime of about 150–300 °C, cerium oxide, which was previously believed to be a stable catalyst for ketonization, can undergo bulk transformations. This result, along with other literature reports, suggest that the long held belief of two separate reaction pathways for either bulk or surface ketonization reactions are not required to explain the interaction of cerium oxide with organic acids. X-ray photon spectroscopy, scanning electron microscopy, and temperature programmed decomposition results supported the formation of metal acetates and explained the occurrence of cerium reduction as well as the formation of cerium oxide/acetate whiskers. After thermogravimetry/mass spectrometry and FT-IR experiments, a single reaction sequence is proposed that can be applied to either surface or bulk reactions with ceria

CeMO_<i>x</i>‑Promoted Ketonization of Biomass-Derived Carboxylic Acids in the Condensed Phase

Ketonization of the model bio-oil compound acetic acid was performed in a toluene-solvent condensed phase using five different CeMO_<i>x</i> catalysts. The catalysts were characterized using a number of different techniques both before and after reaction testing to gain understanding of how material traits influence their catalytic performance. A number of potentially important catalytic properties were found to be of little importance for the respective reaction. For instance, it was discovered that there was no direct correlation between prereaction surface area with activity for ketonization at high or at low temperatures. Furthermore, better reducibility of the oxide did not appear to correlate with improved ketonization rates. XRD of postreaction materials used at different temperatures demonstrated the reaction temperature regime influenced whether the crystal structure of the fresh mixed oxide was disrupted. The precise temperature regimes were different, depending on composition of the catalytic material. Catalytic activity was then found to be maximized when metal carboxylate formation and subsequent decomposition of the carboxylate were appropriately balanced

Investigation of Primary Reactions and Secondary Effects from the Pyrolysis of Different Celluloses

The primary reactions and secondary effects resulting from cellulose fast pyrolysis were investigated using a micropyrolyzer system by changing sample weight and length scale. To exclude the catalytic effects from metal ions, all cellulose samples were demineralized prior to pyrolysis. Heat transfer calculations estimated the characteristic time scale for heat transfer to be 1 order of magnitude smaller than the pyrolysis reaction time when the sample weight was less than 800 μg. It was found that mass transfer limitations existed when the sample weight of the powder cellulose was larger than 800 μg or when the cellulose particles were pyrolyzed at a larger characteristic length scale. The mass transfer limited system led to secondary reactions including secondary char and gas formation from volatile products and decomposition/dehydration of levoglucosan into low molecular weight products, furans, and dehydrated pyranose. The secondary reactions were found to be catalyzed by the char from cellulose pyrolysis. The pyrolysis of powder celluloses of differing crystallinity, degree of polymerization, and feedstock type were studied. Over 87 wt % mass balance closure was achieved for each type of cellulose. Similar product distributions were obtained for all of the different celluloses, implying that the primary products from cellulose were not influenced by these factors

Experimental and Mechanistic Modeling of Fast Pyrolysis of Neat Glucose-Based Carbohydrates. 2. Validation and Evaluation of the Mechanistic Model

A computational framework based on continuous distribution kinetics was constructed to solve the mechanistic model that was developed for fast pyrolysis of glucose-based carbohydrates in the first part of this study [Zhou et al. <i>Ind. Eng. Chem. Res.</i> <b>2014</b>, 53. DOI 10.1021/ie502259w]. Comparing modeling results with experimental yields from fast pyrolysis over a wide range of reaction conditions validates the model. Agreement between model yields of final pyrolysis products with experimental data of fast pyrolysis of cellulose at temperatures ranging from 400 to 600 °C and maltohexaose, cellobiose, and glucose at 500 °C showed that the mechanistic model was robust and extendable. In comparison to our previous model [Vinu, R.; Broadbelt, L. J. <i>Energy Environ. Sci.</i> <b>2012</b>, 5, 9808–9826], the mechanistic model presented in this work incorporating new findings from experiments and theoretical calculations showed enhanced performance in capturing experimental yields of major products such as levoglucosan-pyranose, char, H₂O, CO₂, CO, and especially glycolaldehyde and 5-hydroxymethylfurfural. The model was also able to well match the yields of pyrolysis products that our previous model did not include, such as levoglucosan-furanose, methyl glyoxal, and minor products with yields of less than 1 wt % like levoglucosenone, acetone, dihydroxyacetone, and propenal. The mechanistic model showed its versatility in providing insights that were difficult to obtain from experiments, including a time scale of 4–5 s for complete thermoconversion of cellulose at 500 °C. Analysis of the contributions of competing reaction pathways showed that decomposition of cellulosic chains played a more important role in the formation of levoglucosan and glycolaldehyde than in that of other pyrolysis products

Simple One-Step Synthesis of Aromatic-Rich Materials with High Concentrations of Hydrothermally Stable Catalytic Sites, Validated by NMR

We report a facile one-step synthesis of aromatic-rich materials that contain high concentrations of Brønsted acidic or basic groups attached via alkyl linkers, which previous work has shown to be particularly hydrothermally stable. The method is based on the Maillard reaction and low-temperature (250 °C) pyrolysis of glucose with primary amines linked to acidic sulfonic or phosphonic functionalities or basic piperidine or pyridine groups. The resulting black, carbon-rich materials were characterized using one- and two-dimensional solid-state ¹³C and ¹⁵N NMR, supplemented by elemental analysis. Synthesis with ¹³C-enriched glucose enabled a selective NMR characterization of the aromatic scaffold, which is composed mostly of interlinked pyrrole, indole, and pyridine rings. The fraction of sp²-hybridized C in the matrix is 72%; the aromaticity is ∼60% for the sulfonic-acid functionalized material made from glucose and taurine in a 1:1 molar ratio, where sulfur exceeds 11 wt %. The alkyl linkers remained intact in the synthesis at 250 °C, as proved by distinctive NMR signals of CH₂ groups bonded to heteroatoms. The incorporation of the amine-derived nitrogen into the aromatic matrix was characterized by ¹⁵N, ¹⁵N–¹³C, and ¹³C­{¹⁵N} NMR of a material made from ¹⁵N-taurine with ¹³C-enriched glucose. ¹⁵N NMR shows that no significant unreacted alkyl amine groups remain in the material. Hydrothermal stability as well as catalytic activity of the materials for an esterification reaction was verified

Experimental and Mechanistic Modeling of Fast Pyrolysis of Neat Glucose-Based Carbohydrates. 1. Experiments and Development of a Detailed Mechanistic Model

Fast pyrolysis of lignocellulosic biomass, utilizing moderate temperatures ranging from 400 to 600 °C, produces a primary liquid product (pyrolytic bio-oil), which is potentially compatible with existing petroleum-based infrastructure and can be catalytically upgraded to fuels and chemicals. In this work, experiments were conducted with a micropyrolyzer coupled to a gas chromatography–mass spectrometry/flame ionization detector system to investigate fast pyrolysis of neat cellulose and other glucose-based carbohydrates. A detailed mechanistic model building on our previous work was developed for fast pyrolysis of neat glucose-based carbohydrates by integrating updated findings obtained through experiments and theoretical calculations. The model described the decomposition of cellulosic polymer chains, reactions of intermediates, and formation of a range of low molecular weight compounds at the mechanistic level and specified each elementary reaction step in terms of Arrhenius parameters. The mechanistic model for fast pyrolysis of neat cellulose included 342 reactions of 103 species, which included 96 reactions of 67 species comprising the mechanistic model of neat glucose decomposition

The Alpha–Bet(a) of Glucose Pyrolysis: Computational and Experimental Investigations of 5‑Hydroxymethylfurfural and Levoglucosan Formation Reveal Implications for Cellulose Pyrolysis

As biomass pyrolysis is a promising technology for producing renewable fuels, mechanistic descriptions of biomass thermal decomposition are of increasing interest. While previous studies have demonstrated that glucose is a key primary intermediate and have elucidated many important elementary mechanisms in its pyrolysis, key questions remain. For example, there are several proposed mechanisms for evolution of an important product and platform chemical, 5-hydroxymethylfurfural (5-HMF), but evaluation with different methodologies has hindered comparison. We evaluated a host of elementary mechanisms using a consistent quantum mechanics (QM) level of theory and reveal a mechanistic understanding of this important pyrolysis pathway. We also describe a novel route as a target for catalyst design, as it holds the promise of a more selective pathway to 5-HMF from glucose. We further demonstrate the effect of conformational and structural isomerization on dehydration reactivity. Additionally, we combined QM and experimental studies to address the question of whether only the reactions of β-d-glucose, the cellulose monomer, are relevant to biomass pyrolysis, or if α-d-glucose needs to be considered in mechanistic models of glucose and cellulose pyrolysis. QM calculations show notable differences in elementary mechanisms between the anomers, especially in levoglucosan formation, which provide a means for evaluating experimental yields of α-d-glucose and β-d-glucose pyrolysis. The combined data indicate that both anomers are accessible under pyrolysis conditions. The kinetic and mechanistic discoveries in this work will aid catalyst design and mechanistic modeling to advance renewable fuels from nonfood biomass

Catalytic Deoxygenation of Bio-Oil Model Compounds over Acid–Base Bifunctional Catalysts

An acid–base bifunctional catalyst was synthesized by treating a natural mixed-metal oxide, serpentine, with sulfuric acid. Catalyst characterization revealed that the number of acidic and basic sites increased after the acid treatment largely due to an increase in surface area. However, stronger acid sites were also introduced by the formation of bridged hydroxyl groups between a Si atom and a heteroatom, as inferred by H NMR and NH₃-TPD analysis. Results from SEM-EDS and ¹H NMR suggested that the acid and base sites were in close proximity. Catalytic conversions of carbohydrate-derived bio-oil model compounds were performed over different acid/base catalysts. Eight single bio-oil model compounds and two binary mixtures were used. The reactivity of the model compounds was found to be strongly correlated to the number of oxygen-containing functional groups in the reactant. The results from the binary mixtures showed that the acid–base bifunctional catalyst had the highest activity in aldol condensation reactions. The best deoxygenation performance was also observed with the bifunctional catalyst for the model compounds. Reaction pathways were proposed on the basis of an isotope labeling study. Deoxygenation reactions were found to be promoted by the cooperative catalysis between closely located acid and base sites

Production of 5-Hydroxymethylfurfural from Glucose Using a Combination of Lewis and Brønsted Acid Catalysts in Water in a Biphasic Reactor with an Alkylphenol Solvent

We report the catalytic conversion of glucose in high yields (62%) to 5-hydroxymethylfurfural (HMF), a versatile platform chemical. The reaction system consists of a Lewis acid metal chloride (e.g., AlCl₃) and a Brønsted acid (HCl) in a biphasic reactor consisting of water and an alkylphenol compound (2-<i>sec</i>-butylphenol) as the organic phase. The conversion of glucose in the presence of Lewis and Brønsted acidity proceeds through a tandem pathway involving isomerization of glucose to fructose, followed by dehydration of fructose to HMF. The organic phase extracts 97% of the HMF produced, while both acid catalysts remain in the aqueous phase

Modulating Reactivity and Selectivity of 2‑Pyrone-Derived Bicyclic Lactones through Choice of Catalyst and Solvent

2-Pyrones, such as coumalic acid, are promising biobased molecules that through Diels–Alder reactions can provide access to a wide range of biobased chemicals, including molecules with functionality that are not easily accessible via conventional petrochemical routes. A complete reaction network and kinetic parameters for three individual diversification routes that start from a single bicyclic lactone produced via the Diels–Alder cycloaddition of coumalic acid and ethylene were examined experimentally and probed through complementary first-principle density functional theory (DFT) calculations, in situ nuclear magnetic resonance (NMR) spectroscopy, and thin film solid-state NMR spectroscopy. These experiments provide insights into the routes for several molecular structures from bicyclic lactones by leveraging Lewis or Brønsted acid catalysts to selectively alter the reaction pathway. The bicyclic lactone bridge can be decarboxylated to access dihydrobenzenes at a substantially reduced activation barrier using γ-Al₂O₃ as the catalyst or selectively ring-opened via Brønsted acids to yield 1,3-diacid six membered rings. DFT computations and microkinetic modeling in combination with experimental results provide molecular insights into the catalytically active sites on γ-Al₂O₃ and provide a general mechanism for the catalyzed bicyclic lactone decarboxylation in polar aprotic solvents, which involves CO₂ extrusion as the kinetically relevant step. Solid-state NMR spectroscopy provides direct evidence of strong binding of the bicyclic lactone to the γ-Al₂O₃ surface, fully consistent with DFT simulation results and experimental reaction kinetics. In addition, the role of the solvent was examined and found to be an additional means to improve reaction rates and selectively produce alternative structures from the bicyclic intermediate. The rate of the decarboxylation reaction was increased dramatically by using water as the solvent whereas methanol acted as a nucleophile and selectively induced ring-opening, showing that both pathways are operative in the absence of catalyst. Taken together, the results demonstrate an approach for selective diversification of the coumalate platform to a range of molecules