Adsorption of Biomass-Derived Products on MoO3: Hydrogen Bonding Interactions under the Spotlight.

We performed a computational study on the interaction of O-containing compounds coming from biomass with a catalytic surface of MoO3. The addition of H atoms on the metal oxide surface mimics different scenarios of its exposure to the ambient or protons coming from biomass. Representative compounds from fatty acids (from triacylglycerides) and aromatics (from lignin) were adsorbed on the metal oxide surfaces. We covered the complete H surface coverage, and the adsorbed molecules showed structural changes due to the interactions in turn. The driven force interactions in this process is hydrogen bonding, which reveals the complexity in biomass processing. H-bonds were fully characterized by the electron density and its Laplacian where bond critical points are present. These topological properties allow us to understand the correlation between the adsorption energies and the strength on each adsorption site. We also computed the relative Gibbs energies and harmonic oscillator model of aromaticity index of the adsorbed molecules to get more insights into their stability.

Electronic structure and mesoscopic simulations of nonylphenol ethoxylate surfactants. a combined DFT and DPD study.

The aim of this work was to gain insight into the effect of ethylene oxide (EO) chains on the properties of a series of nonylphenol ethoxylate (NPE) surfactants. We performed a theoretical study of NPE surfactants by means of density functional theory (DFT) and dissipative particle dynamics (DPD). Both approximations were used separately to obtain different properties. Four NPEs were selected for this purpose (EO = 4, 7, 11 and 15 length chains). DFT methods provided some electronic properties that are related to the EO units. One of them is the solvation Gibbs energy, which exhibited a linear trend with EO chain length. DPD calculations allow us to observe the dynamic behavior in water of the NPE surfactants. We propose a coarse-grained model which properly simulates the mesophases of each surfactant. This model can be used in other NPEs applications.

Theoretical determination of the rate constant for OH hydrogen abstraction from toluene.

The OH abstraction of a hydrogen atom from both the side chain and the ring of toluene has been studied in the range 275-1000 K using quantum chemistry methods. It is found that the best method of calculation is to perform geometry optimization and frequency calculations at the BHandHLYP/6-311++G(d,p) level, followed by CCSD(T) calculations of the optimized structures with the same basis set. Four different reaction paths are considered, corresponding to the side chain and three possible ring hydrogen abstractions, and the branching ratio is determined as a function of temperature. Although negligible at low temperatures, at 1000 K ring-H abstraction is found to contribute 11% to the total abstraction reaction. The calculated rate coefficients agree very well with experimental results. Side chain abstraction is shown to occur through a complex mechanism that includes the reversible formation of a collisionally stabilized reactant complex.

Ab-initio study of initial atmospheric oxidation reactions of C3 and C4 alkanes.

Second-order, Møller-Plesset (MP2)-unrestricted Hartree-Fock calculations with full geometry optimization in the 6-31G(d, p) basis set were carried out to study the initial atmospheric oxidation reactions of alkanes. All structures in the initial hydrogen abstraction reaction by an OH radical and the subsequent addition of molecular oxygen to the alkyl radical were characterized for alkanes with three and four carbon atoms. The reaction paths for the formation of the peroxyl radicals were obtained and discussed in the light of similarities along series involving primary, secondary, and tertiary hydrogens. A 0.999 correlation was found between the height of our barriers for the OH abstraction of a primary hydrogen atom from alkanes containing one to four carbon atoms and the optimally estimated activation energies for this reaction recently presented. From the slope and the intersection at zero activation energy an equation was obtained that yields scaled values of the activation energies to account for the tunnel effect and for the error due to the basis set and the method employed. We present new results for the abstraction of the less favored primary hydrogens in propane, butane, and isobutane, which should be important at high temperatures. Negative net activation energies were obtained for the addition of molecular oxygen to all the alkyl radicals formed in the first reaction. The structure of the peroxyl radicals is discussed; and very good correlations are observed for similar compounds, regardless of the length of the carbon chain. A revision of some experimental values is suggested. Single point density functional calculations at the MP2 geometries were also performed with the B3LYP functional for comparison. The observed trends are exactly the same for the two methods. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 845-856, 1999.