Nima Nikbin

A First Principles-Based Microkinetic Model for the Conversion of Fructose to 5-Hydroxymethylfurfural

We have tested and discussed the accuracy of hybrid quantum mechanics/molecular mechanics molecular dynamics free energy calculations for the investigation of the mechanism of dehydration of biomass‐derived carbohydrates in solution. In this respect and taking into account earlier calculations of this type, we have developed a microkinetic model for the dehydration of fructose to 5‐hydroxymethylfurfural (HMF) in acidic water and embedded it in a reaction network that includes fructose and HMF degradation reactions. Sensitivity analysis of the kinetic model has shown the rate‐limiting step of the reaction network under consideration to be an intramolecular hydride transfer that takes place right after the first water removal from fructose. We predict the formation of two stable intermediates, one of which is structurally similar to the (4R,5R)‐4‐hydroxy‐5‐hydroxymethyl‐4,5‐dihydrofuran‐2‐carbaldehyde intermediate identified by NMR studies in pure DMSO solution. We find remarkable agreement between calculated and experimental concentration profiles over a wide range of temperatures and over the entire range of timescales considered in the kinetic study of Asghari and Yoshida. We demonstrate that the microkinetic model cannot capture the correct temperature dependence of the rates unless one uses Marcus theory rate constants for those elementary steps of the mechanism that involve hydride transfer. The computed apparent activation energy and Arrhenius frequency factor for fructose conversion to HMF are also found to be in excellent agreement with those obtained from experiments.

On the Brønsted Acid-Catalyzed Homogeneous Hydrolysis of Furans

Furan affairs: Electronic structure calculations of the homogeneous Brønsted acid-catalyzed hydrolysis of 2,5-dimethylfuran show that proton transfer to the β-position is rate-limiting and provides support that the hydrolysis follows general acid catalysis. By means of projected Fukui indices, we show this to be the case for unsubstituted, 2-, and 2,5-substituted furans with electron-donating groups.

Catalysis at the sub-nanoscale: complex CO oxidation chemistry on a few Au atoms

Au has been widely used as jewelry since ancient times due to its bulk, chemically inert properties. During the last three decades, nanoscale Au has attracted remarkable attention and has been shown to be an exceptional catalyst, especially for oxidation reactions. Herein, we elucidate a puzzle in catalysis by using multiscale computational modeling: the experimentally observed “magic number” CO oxidation catalytic behavior of sub-nanoscale Au clusters. Our results demonstrate that support effects (cluster charging), symmetry-induced electronic effects on the clusters, catalyst reconstruction, competing chemical pathways and formation of carbonate contribute to the marked differences in the observed catalytic behavior of Aun− clusters with n = 6, 8 and 10 atoms. This is the first demonstration of multiscale simulations on sub-nanoscale catalysts unraveling the magic number activity for the CO oxidation reaction on Au.