Ex situ catalytic fast pyrolysis (CFP) is a promising route for producing fungible biofuels; however, this process requires bifunctional catalysts that favor C−O bond cleavage, activate hydrogen at near atmospheric pressure and high temperature (350−500 °C), and are stable under highsteam, low hydrogen-to-carbon environments. Recently, early transition-metal carbides have been reported to selectively cleave C−O bonds of alcohols, aldehydes, and oxygenated aromatics, yet there is limited understanding of the metal carbide surface chemistry under reaction conditions and the identity of the active sites for deoxygenation. In this paper, we evaluated molybdenum carbide (Mo 2 C) for the deoxygenation of acetic acid, an abundant component of biomass pyrolysis vapors, under ex situ CFP conditions, and we probed the Mo 2 C surface chemistry, identity of the active sites, and deoxygenation pathways using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. The Mo 2 C catalyst favored the production of acetaldehyde and ethylene from acetic acid over the temperature range of 250−400 °C, with decarbonylation pathways favored at temperatures greater than 400 °C. Little to no ethanol was observed due to the high activity of the carbide surface for alcohol dehydration. The Mo 2 C surface, which was at least partially oxidized following pretreatment and exposure to reaction conditions (possibly existing as an oxycarbide), possessed both metallic-like H-adsorption sites (i.e., exposed Mo and C) and Brønsted acidic surface hydroxyl sites, in a ratio of 1:8 metallic:acidic sites following pretreatment. The strength of the acidic sites was similar to that for H-Beta, H-Y, and H-X zeolites. Oxygen vacancy sites (exposed Mo sites) were also present under reaction conditions, inferred from DRIFTS results and calculated surface phase diagrams. It is proposed that C−O bond cleavage steps proceeded over the acidic sites or over the oxygen vacancy sites and that the deoxygenation rate may be limited by the availability of adsorbed hydrogen, due to the high surface coverage of oxygen under reaction conditions. Importantly, the reaction conditions (temperature and partial pressures of H 2 and H 2 O) had a strong effect on oxygen surface coverage, and accordingly, the relative concentrations of the different types of active sites, and could ultimately result in completely different reaction pathways under different reaction conditions.
The existence of a series of organic peroxy radical-water complexes [CH3O2.H2O (methyl peroxy); CH3CH2O2.H2O (ethyl peroxy); CH3C(O)O2.H2O (acetyl peroxy); CH3C(O)CH2O2.H2O (acetonyl peroxy); CH2(OH)O2.H2O (hydroxyl methyl peroxy); CH2(OH)CH2O2.H2O (2-hydroxy ethyl peroxy); CH2(F)O2.H2O (fluoro methyl peroxy); CH2(F)CH2O2.H2O (2-fluoro ethyl peroxy)] is evaluated using high level ab initio calculations. A wide range of binding energies is predicted for these complexes, in which the difference in binding energies can be explained by examination of the composition of the R group attached to the peroxy moiety. The general trend in binding energies has been determined to be as follows: fluorine approximately alkyl < carbonyl < alcohol. The weakest bound complex, CH3O2.H2O, is calculated to be bound by 2.3 kcal mol-1, and the strongest, the CH2(OH)O2.H2O complex, is bound by 5.1 kcal mol-1. The binding energy of the peroxy radical-water complexes which contain carbonyl and alcohol groups indicates that these complexes may perturb the kinetics and product branching ratios of reactions involving these complexes.
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.
Herein we report an extensive ab initio study on the existence of eight beta-hydroxy isoprene peroxy radical-water complexes. Binding energies calculated at the MP2(full)/6-311++G(2d,2p)//CCSD(T)/6-311++G(d,p) level of theory range between 3.85 and 5.66 kcal mol(-1). The results of natural bond orbital calculations are presented to help rationalize complex formation. Atmospheric lifetimes, equilibrium constants, heats of formation, and the relative abundance of complexed to uncomplexed peroxy radicals are also reported and discussed.
Peroxy radicals can complex with water vapor. These complexes affect tropospheric chemistry. In this study, β-HEP (hydroxyethyl peroxy radical) serves as a model system for investigating the effect of water vapor on the kinetics and product branching ratio of the self-reaction of peroxy radicals. The self-reaction rate coefficient was determined at 274-296 K with water vapor between 1.0 × 10 15 and 2.5 × 10 17 molecules cm −3 at 200 Torr total pressure by slow-flow laser flash photolysis coupled with UV time-resolved spectroscopy and long-path, wavelength modulated, diode-laser spectroscopy. The overall self-reaction rate constant expressed as the product of both a temperature-dependent and water vapor-dependent term is k o = 7.8 × 10 −14 exp((8.3 ± 2.5kJ /mol)/RT ) + {(13.2 ± 1.56) × 10 −44 × exp((79.3 ± 17.18kJ /mol)/RT ) × [H 2 O]}, suggesting formation of a β-HEP-H 2 O complex is responsible for the increase in the self-reaction rate coefficient with increasing water concentration. Complex formation is supported by computational results identifying three local energy minima for the β-HEP-H 2 O complex. As the troposphere continues to get warmer and wetter, more of the peroxy radicals present will be complexed with water. Investigating the effect of water vapor on kinetics of atmospherically relevant radicals and determining the effects of these altered kinetics on tropospheric ozone concentrations is thus important. C 2015
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