A renewable route to p-xylene from biomass-derived dimethylfuran and ethylene is investigated with zeolite catalysts. Cycloaddition of ethylene and 2,5-dimethylfuran and subsequent dehydration to p-xylene has been achieved with 75% selectivity using H–Y zeolite and an aliphatic solvent at 300 °C. Competitive side reactions include hydrolysis of dimethylfuran to 2,5-hexanedione, alkylation of p-xylene, and polymerization of 2,5-hexanedione. The observed reaction rates and computed energy barriers are consistent with a two-step reaction that proceeds through a bicyclic adduct prior to dehydration to p-xylene. Cycloaddition of ethylene and dimethylfuran occurs without a catalytic active site, but the reaction is promoted by confinement within microporous materials. The presence of Brønsted acid sites catalyzes dehydration of the Diels–Alder cycloadduct (to produce p-xylene and water), and this ultimately causes the rate-determining step to be the initial cycloaddition.

No abstract

Diels–Alder cycloaddition with furans as dienes and subsequent dehydrative aromatization are potentially valuable processes for sustainable conversion of biomass-derived furans to aromatics. We have performed electronic structure calculations to investigate the catalytic activity of HY and of alkali-exchanged Y zeolites in connection with the conversion of 2,5-dimethylfuran and ethylene to p-xylene. We have used two active site settings: an active site cluster model on which we have carried out density functional theory calculations and a mechanically embedded active site cluster model on which we have performed hybrid quantum mechanics/molecular mechanics calculations with the ONIOM scheme. Even though Lewis catalyzed Diels–Alder cycloaddition has received considerable attention over the years, we show that confinement and charge transfer in zeolite catalysts play a significant role in catalysis. Both HY and alkali-Y can catalyze the aromatization of the cycloadduct through dehydration but HY is found to be far more effective. Our analysis shows that the electron withdrawing ability of the cations and the catalytic activity of alkali-Y as Lewis acids are diminished by substrate binding-induced electron density shift from the framework oxygen atoms to the cations. On account of these inductive phenomena, we show that the DMF–ethylene cycloaddition follows a bidirectional instead of normal electron flow mechanism.

Renewable production of p-xylene from [4 + 2] Diels− Alder cycloaddition of 2,5-dimethylfuran (DMF) and ethylene with H−Y zeolite catalyst in n-heptane solvent is investigated. Experimental studies varying the solid acid catalyst concentration reveal two kinetic regimes for the p-xylene production rate: (i) a linear regime at low acid site concentrations with activation energy E a = 10.8 kcal/mol and (ii) a catalyst-independent kinetic regime at high acid site concentrations with activation energy E a = 20.1 kcal/mol. We carry out hybrid QM/MM calculations with a three-layer embedded cluster ONIOM model to compute the energetics along the main reaction pathway, and a microkinetic model is constructed for the interpretation of the experimental kinetic data. At high solid acid concentrations, p-xylene production is limited by the homogeneous Diels−Alder reaction, whereas at low acid concentrations, the overall rate is limited by the heterogeneously catalyzed dehydration of the Diels−Alder cycloadduct of DMF and ethylene because of an insufficient number of acid sites, despite the dehydration reaction requiring significantly less activation energy. A reduced kinetic model reveals that the production of p-xylene follows the general kinetics of tandem reactions in which the first step is uncatalyzed and the second step is heterogeneously catalyzed. Reaction orders and apparent activation energies of quantum mechanical and microkinetic simulations are in agreement with experimental values.

No abstract

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.

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.

The CO oxidation reaction pathways on the negatively charged Au10 –1 cluster are investigated using density functional theory calculations. Pre-exponential factors and reaction rate constants of each elementary step are calculated using statistical mechanics. Our results demonstrate that the reaction of CO and O2 on Au10 –1 preferably proceeds through the formation of a 4-center intermediate rather than through O2 dissociation. The oxygen atom produced then reacts with adsorbed CO to form CO2. The rate-determining step of the CO oxidation appears to be the CO2 release from the 4-center intermediate. Interestingly, it is shown that gaseous CO2, which binds near an adsorbed atomic oxygen, forms easily a bidentate CO3 species that can poison the catalyst. The reverse reaction (carbonate decomposition) occurs with a rate constant comparable to the rate-determining step. A rate constant comparison for various reactant arrangements suggests a site-dependent reactivity, even for this subnanometer Au10 –1 catalyst.