Bimolecular Decomposition Pathways for Carboxylic Acids of Relevance to Biofuels

The bimolecular thermal reactions of carboxylic acids were studied using quantum mechanical molecular modeling. Previous work investigated the unimolecular decomposition of a variety of organic acids, including saturated, α,β-unsaturated, and β,γ-unsaturated acids, and showed that the type and position of the unsaturation resulted in unique branching ratios between dehydration and decarboxylation, [H₂O]/[CO₂]. In this work, the effect of bimolecular chemistry (water–acid and acid–acid) is considered with a representative of each acid class. In both cases, the strained 4-centered, unimolecular transition state, typical of most organic acids, is opened up to 6- or 8-centered bimolecular geometries. These larger structures lead to a reduction in the barrier heights (20–45%) of the thermal decomposition pathways for organic acids and an increase in the decomposition kinetics. In some cases, they even cause a shift in the branching ratio of the corresponding product slates

Comparison of Unimolecular Decomposition Pathways for Carboxylic Acids of Relevance to Biofuels

Quantum mechanical molecular modeling is used [M06-2X/6-311++G­(2df,p)] to compare activation energies and rate constants for unimolecular decomposition pathways of saturated and unsaturated carboxylic acids that are important in the production of biofuels and that are models for plant and algae-derived intermediates. Dehydration and decarboxylation reactions are considered. The barrier heights to decarboxylation and dehydration are similar in magnitude for saturated acids (∼71 kcal mol^–1), with an approximate 1:1 [H₂O]/[CO₂] branching ratio over the temperature range studied (500–2000 K). α,β-Unsaturation lowers the barrier to decarboxylation between 2.2 and 12.2 kcal mol^–1 while increasing the barriers to dehydration by ∼3 kcal mol^–1. The branching ratio, as a result, is an order of magnitude smaller, [H₂O]/[CO₂] = 0.07. For some α,β-unsaturated acids, six-center transition states are available for dehydration, with barrier heights of ∼35.0 kcal mol^–1. The branching ratio for these acids can be as high as 370:1. β,γ-Unsaturation results in a small lowering in the barrier height to decarboxylation (∼70.0 kcal mol^–1). β,γ-Unsaturation also leads to a lowering in the dehydration pathway from 1.7 to 5.1 kcal mol^–1. These results are discussed with respect to predicted kinetic values for acids of importance in biofuels production

Diffusion of Biomass Pyrolysis Products in H‑ZSM‑5 by Molecular Dynamics Simulations

Diffusion of biomass pyrolysis vapors and their upgraded products is an essential catalytic property of zeolites during catalytic fast pyrolysis and likely plays a critical role in the selectivity of these catalysts. Characterizing the diffusivities of representative biofuel molecules is critical to understand shape selectivity and interpret product distribution. Yet, experimental measurements on the diffusivities of oxygenated biofuel molecules at pyrolysis temperatures are very limited in the literature. As an alternative approach, we conducted MD simulations to measure the diffusion coefficients of several selected molecules that are representative of biomass pyrolysis vapors, namely water, methanol, glycolaldehyde, and toluene in H-ZSM-5 zeolite. The results show the diffusion coefficients calculated via MD simulations are consistent with available NMR measurements at room temperature. The effect of molecular weight and molecular critical diameter on the diffusivity among the chosen model compounds is also examined. Furthermore, we have characterized the diffusivities of representative biofuel molecules, namely xylene isomers, in H-ZSM-5. Our calculations determined that the ratio of the diffusion coefficients for xylene isomers is <i>p-</i>xylene:<i>o-</i>xylene:<i>m-</i>xylene ≈ 83:3:1 at 700 K. Additionally, our results also demonstrate the different diffusivity between <i>p</i>-xylene and toluene is due to the molecular orientations when the molecules diffuse along the channels in H-ZSM-5 and provide deep insight into the effect of molecular orientation on its diffusivity

Group Additivity Determination for Oxygenates, Oxonium Ions, and Oxygen-Containing Carbenium Ions

Bio-oil produced from biomass fast pyrolysis often requires catalytic upgrading to remove oxygen and acidic species over zeolite catalysts. The elementary reactions in the mechanism for this process involve carbenium and oxonium ions. In order to develop a detailed kinetic model for the catalytic upgrading of biomass, rate constants are required for these elementary reactions. The parameters in the Arrhenius equation can be related to thermodynamic properties through structure–reactivity relationships, such as the Evans–Polanyi relationship. For this relationship, enthalpies of formation of each species are required, which can be reasonably estimated using group additivity. However, the literature previously lacked group additivity values for oxygenates, oxonium ions, and oxygen-containing carbenium ions. In this work, 71 group additivity values for these types of groups were regressed, 65 of which had not been reported previously and six of which were newly estimated based on regression in the context of the 65 new groups. Heats of formation based on atomization enthalpy calculations for a set of reference molecules and isodesmic reactions for a small set of larger species for which experimental data was available were used to demonstrate the accuracy of the Gaussian-4 quantum mechanical method in estimating enthalpies of formation for species involving the moieties of interest. Isodesmic reactions for a total of 195 species were constructed from the reference molecules to calculate enthalpies of formation that were used to regress the group additivity values. The results showed an average deviation of 1.95 kcal/mol between the values calculated from Gaussian-4 and isodesmic reactions versus those calculated from the group additivity values that were newly regressed. Importantly, the new groups enhance the database for group additivity values, especially those involving oxonium ions

Predictive Model for Particle Residence Time Distributions in Riser Reactors. Part 1: Model Development and Validation

In this computational study, we model the mixing of biomass pyrolysis vapor with a solid catalyst in circulating riser reactors with a focus on the determination of solid catalyst residence time distributions (RTDs). A comprehensive set of 2D and 3D simulations were conducted for a pilot-scale riser using the Eulerian–Eulerian two-fluid modeling framework with and without subgrid-scale models for the gas–solid interaction. A validation test case was also simulated and compared to experiments, showing agreement in the pressure gradient and RTD mean and spread. For simulation cases, it was found that for accurate RTD prediction, the Johnson and Jackson partial slip solid boundary condition was required for all models, and a subgrid model is useful so that ultra high resolutions grids, which are very computationally intensive, are not required. We discovered a 2/3 scaling relation for the RTD mean and spread when comparing resolved 2D simulations to validated unresolved 3D subgrid-scale model simulations

Furan Production from Glycoaldehyde over HZSM‑5

Catalytic fast pyrolysis of biomass over zeolite catalysts results primarily in aromatic (e.g., benzene, toluene, xylene) and olefin products. However, furans are a higher value intermediate for their ability to be readily transformed into gasoline, diesel, and chemicals. Here we investigate possible mechanisms for the coupling of glycoaldehyde, a common product of cellulose pyrolysis, over HZSM-5 for the formation of furans. Experimental measurements of neat glycoaldehyde over a fixed bed of HZSM-5 confirm furans (e.g., furanone) are products of this reaction at temperatures below 300 °C with several aldol condensation products as coproducts (e.g., benzoquinone). However, under typical catalytic fast pyrolysis conditions (>400 °C), further reactions occur that lead to the usual aromatic product slate. ONIOM calculations were utilized to identify the pathway for glycoaldehyde coupling toward furanone and hydroxyfuranone products with dehydration reactions serving as the rate-determining steps with typical intrinsic reaction barriers of 40 kcal mol^–1. The reaction mechanisms for glycoaldehyde will likely be similar to that of other small oxygenates such as acetaldehyde, lactaldehyde, and hydroxyacetone. This study provides a generalizable mechanism of oxygenate coupling and furan formation over zeolite catalysts

Understanding Trends in Autoignition of Biofuels: Homologous Series of Oxygenated C5 Molecules

Oxygenated biofuels provide a renewable, domestic source of energy that can enable adoption of advanced, high-efficiency internal combustion engines, such as those based on homogeneously charged compression ignition (HCCI). Of key importance to such engines is the cetane number (CN) of the fuel, which is determined by the autoignition of the fuel under compression at relatively low temperatures (550–800 K). For the plethora of oxygenated biofuels possible, it is desirable to know the ignition delay times and the CN of these fuels to help guide conversion strategies so as to focus efforts on the most desirable fuels. For alkanes, the chemical pathways leading to radical chain-branching reactions giving rise to low-temperature autoignition are well-known and are highly coincident with the buildup of reactive radicals such as OH. Key in the mechanisms leading to chain branching are the addition of molecular oxygen to alkyl radicals and the rearrangement and dissociation of the resulting peroxy radials. Prediction of the temperature and pressure dependence of reactions that lead to the buildup of reactive radicals requires a detailed understanding of the potential energy surfaces (PESs) of these reactions. In this study, we used quantum mechanical modeling to systematically compare the effects of oxygen functionalities on these PESs and associated kinetics so as to understand how they affect experimental trends in autoignition and CN. The molecules studied here include pentane, pentanol, pentanal, 2-heptanone, methylpentyl ether, methyl hexanoate, and pentyl acetate. All have a saturated five-carbon alkyl chain with an oxygen functional group attached to the terminal carbon atom. The results of our systematic comparison may be summarized as follows: (1) Oxygen functionalities activate C–H bonds by lowering the bond dissociation energy (BDE) relative to alkanes. (2) The R–OO bonds in peroxy radicals adjacent to carbonyl groups are weaker than corresponding alkyl systems, leading to dissociation of ROO^• radicals and reducing reactivity and hence CN. (3) Hydrogen atom transfer in peroxy radicals is important in autoignition, and low barriers for ethers and aldehydes lead to high CN. (4) Peroxy radicals formed from alcohols have low barriers to form aldehydes, which reduce the reactivity of the alkyl radical. These findings for the formation and reaction of alkyl radicals with molecular oxygen explain the trend in CN for these common biofuel functional groups

Ab Initio Surface Phase Diagrams for Coadsorption of Aromatics and Hydrogen on the Pt(111) Surface

Supported metal catalysts are commonly used for the hydrogenation and deoxygenation of biomass-derived aromatic compounds in catalytic fast pyrolysis. To date, the substrate–adsorbate interactions under reaction conditions crucial to these processes remain poorly understood, yet understanding this is critical to constructing detailed mechanistic models of the reactions important to catalytic fast pyrolysis. Density functional theory (DFT) has been used in identifying mechanistic details, but many of these works assume surface models that are not representative of realistic conditions, for example, under which the surface is covered with some concentration of hydrogen and aromatic compounds. In this study, we investigate hydrogen-guaiacol coadsorption on Pt(111) using van der Waals-corrected DFT and ab initio thermodynamics over a range of temperatures and pressures relevant to bio-oil upgrading. We find that relative coverage of hydrogen and guaiacol is strongly dependent on the temperature and pressure of the system. Under conditions relevant to ex situ catalytic fast pyrolysis (CFP; 620–730 K, 1–10 bar), guaiacol and hydrogen chemisorb to the surface with a submonolayer hydrogen (∼0.44 ML H), while under conditions relevant to hydrotreating (470–580 K, 10–200 bar), the surface exhibits a full-monolayer hydrogen coverage with guaiacol physisorbed to the surface. These results correlate with experimentally observed selectivities, which show ring saturation to methoxycyclohexanol at hydrotreating conditions and deoxygenation to phenol at CFP-relevant conditions. Additionally, the vibrational energy of the adsorbates on the surface significantly contributes to surface energy at higher coverage. Ignoring this contribution results in not only quantitatively, but also qualitatively incorrect interpretation of coadsorption, shifting the phase boundaries by more than 200 K and ∼10–20 bar and predicting no guaiacol adsorption under CFP and hydrotreating conditions. The implications of this work are discussed in the context of modeling hydrogenation and deoxygenation reactions on Pt(111), and we find that only the models representative of equilibrium surface coverage can capture the hydrogenation kinetics correctly. Last, as a major outcome of this work, we introduce a freely available web-based tool, dubbed the Surface Phase Explorer (SPE), which allows researchers to conveniently determine surface composition for any one- or two-component system at thermodynamic equilibrium over a wide range of temperatures and pressures on any crystalline surface using standard DFT output

Catalytic Upgrading of Biomass-Derived Compounds via C–C Coupling Reactions: Computational and Experimental Studies of Acetaldehyde and Furan Reactions in HZSM‑5

Catalytic C–C coupling and deoxygenation reactions are essential for upgrading of biomass-derived oxygenates to fuel-range hydrocarbons. Detailed understanding of mechanistic and energetic aspects of these reactions is crucial to enabling and improving the catalytic upgrading of small oxygenates to useful chemicals and fuels. Using periodic density functional theory (DFT) calculations, we have investigated the reactions of furan and acetaldehyde in an HZSM-5 zeolite catalyst, a representative system associated with the catalytic upgrading of pyrolysis vapors. Comprehensive energy profiles were computed for self-reactions (i.e., acetaldehyde coupling and furan coupling) and cross-reactions (i.e., acetaldehyde + furan) of this representative mixture. Major products proposed from the computations are further confirmed using temperature controlled mass spectra measurements. The computational results show that furan interacts with acetaldehyde in HZSM-5 via an alkylation mechanism, which is more favorable than the self-reactions, indicating that mixing furans with aldehydes could be a promising approach to maximize effective C–C coupling and dehydration while reducing the catalyst deactivation (e.g., coke formation) from aldehyde condensation

Polarized Matrix Infrared Spectra of Cyclopentadienone: Observations, Calculations, and Assignment for an Important Intermediate in Combustion and Biomass Pyrolysis

A detailed vibrational analysis of the infrared spectra of cyclopentadienone (C₅H₄O) in rare gas matrices has been carried out. <i>Ab initio</i> coupled-cluster anharmonic force field calculations were used to guide the assignments. Flash pyrolysis of <i>o</i>-phenylene sulfite (C₆H₄O₂SO) was used to provide a molecular beam of C₅H₄O entrained in a rare gas carrier. The beam was interrogated with time-of-flight photoionization mass spectrometry (PIMS), confirming the clean, intense production of C₅H₄O. Matrix isolation infrared spectroscopy coupled with 355 nm polarized UV for photoorientation and linear dichroism experiments was used to determine the symmetries of the vibrations. Cyclopentadienone has 24 fundamental vibrational modes, Γ_vib = 9a₁ ⊕ 3a₂ ⊕ 4b₁ ⊕ 8b₂. Using vibrational perturbation theory and a deperturbation–diagonalization method, we report assignments of the following fundamental modes (cm^–1) in a 4 K neon matrix: the a₁ modes of X̃ ¹A₁ C₅H₄O are found to be ν₁ = 3107, ν₂ = (3100, 3099), ν₃ = 1735, ν₅ = 1333, ν₇ = 952, ν₈ = 843, and ν₉ = 651; the inferred a₂ modes are ν₁₀ = 933, and ν₁₁ = 722; the b₁ modes are ν₁₃ = 932, ν₁₄ = 822, and ν₁₅ = 629; the b₂ fundamentals are ν₁₇ = 3143, ν₁₈ = (3078, 3076) ν₁₉ = (1601 or 1595), ν₂₀ = 1283, ν₂₁ = 1138, ν₂₂ = 1066, ν₂₃ = 738, and ν₂₄ = 458. The modes ν₄ and ν₆ were too weak to be detected, ν₁₂ is dipole-forbidden and its position cannot be inferred from combination and overtone bands, and ν₁₆ is below our detection range (<400 cm^–1). Additional features were observed and compared to anharmonic calculations, assigned as two quantum transitions, and used to assign some of the weak and infrared inactive fundamental vibrations