Formic Acid-Assisted Selective Hydrogenolysis of 5-Hydroxymethylfurfural to 2,5-Dimethylfuran over Bifunctional Pd Nanoparticles Supported on N-Doped Mesoporous Carbon.

Biomass-derived 5-hydroxymethylfurfural (HMF) is regarded as one of the most promising platform chemicals to produce 2,5-dimethylfuran (DMF) as a potential liquid transportation fuel. Pd nanoparticles supported on N-containing and N-free mesoporous carbon materials were prepared, characterized, and applied in the hydrogenolysis of HMF to DMF under mild reaction conditions. Quantitative conversion of HMF to DMF was achieved in the presence of formic acid (FA) and H2 over Pd/NMC within 2 h. The reaction mechanism, especially the multiple roles of FA, was explored through a detailed comparative study by varying hydrogen source, additive, and substrate as well as by applying in situ ATR-IR spectroscopy. The major role of FA is to shift the dominant reaction pathway from the hydrogenation of the aldehyde group to the hydrogenolysis of the hydroxymethyl group via the protonation by FA at the C-OH group, lowering the activation barrier of the C-O bond cleavage and thus significantly enhancing the reaction rate. XPS results and DFT calculations revealed that Pd2+ species interacting with pyridine-like N atoms significantly enhance the selective hydrogenolysis of the C-OH bond in the presence of FA due to their high ability for the activation of FA and the stabilization of H- .

Anchoring of palladium nanoparticles on N-doped mesoporous carbon.

Pd nanoparticles deposited on nitrogen-doped mesoporous carbon are promising catalysts for highly selective and effective catalytic hydrogenation reactions. To design and utilize these novel catalysts, it is essential to understand the effect of N doping on the metal-support interactions. A combined experimental (X-ray photoelectron spectroscopy) and computational (density functional theory) approach is used to identify preferential adsorption sites and to give detailed explanations of the corresponding metal-support interactions. Pyridinic N atoms turned out to be the preferential adsorption sites for Pd nanoparticles on nitrogen-doped mesoporous carbon, interacting through their lone pairs (LPs) with the Pd atoms via N-LP - Pd dσ and N-LP - Pd s and Pd dπ - π* charge transfer, which leads to a change in the Pd oxidation state. Our results evidence the existence of bifunctional palladium nanoparticles containing Pd0 and Pd2+ centers.

The Catalytic Effect of Iron and Alkali and Alkaline Earth Metal Sulfates Loading Series on the Conversion of Cellulose-Derived Hydrochars and Chars.

The catalytic effect of minerals on biomass conversion was studied focusing on Fe as well as alkali and alkaline earth metals as the metallic inorganic elements typically present in minerals found in biomass. A mineral-free reference hydrochar and an analogous char material based on cellulose were systematically doped with sulfates of the different metallic inorganic elements in various amounts via impregnation, thereby excluding differences originating from the counterion and the carbon matrix. Thermogravimetric reactivity measurements were performed in diluted O2 and CO2, and the derivative thermogravimetry curves were fitted using the random pore model. This procedure enabled a quantification of the apparent activation energy decrease due to doping as well as the influence of doping on the carbon structural parameter. Fe sulfate was always among the most active minerals, and alkali metal sulfates were typically more active than alkaline earth metal sulfates. The only exception was the high activity of very small Ca sulfate loadings during gasification. A saturation behavior of the kinetic parameter upon increasing the mineral loading was observed. The Langmuir-type modeling of this dependence further revealed that catalytically influenced devolatilization results in a char with higher oxidation reactivity, whereas for gasification, thermal annealing dominates. The systematically derived parameters provide a comprehensive description of catalytic effects, taking into account the type of mineral, the applied loading, the used atmosphere, and the fuel morphology. The derived activation energies can be used to include catalytic effects into combustion models.

Thermicity of the Decomposition of Oxygen Functional Groups on Cellulose-Derived Chars.

The evolution of oxygen functional groups (OFGs) and the associated thermic effects upon heat treatment up to 800 °C were investigated experimentally as well as by theoretical calculations. A synthetic carbon with a carbonaceous structure close to that of natural chars, yet mineral-free, was derived from cellulose and oxidized by HNO3 vapor at different temperatures and for varied durations in order to generate char samples with different concentrations and distributions of OFGs. The functionalized samples were subjected to calorimetric temperature-programmed desorption measurements in correlation with an extensive effluent gas analysis, thereby focusing on the specific heat effects of individual OFG evolution. Interpretation of the experimental results was aided by density functional theory (DFT) calculations which allowed one to infer the thermal stability of different OFGs and the reaction energy associated with their evolution upon heating. Results showed that, with increasing temperature, H2O was released due to the loss of physisorbed water, the decomposition of clusters bound to carboxylic acids, and condensation reactions. The associated heat uptake amounted to about 100 kJ mol-1. Contrarily, the release of CO2, attributed to the decomposition of condensed acids, carboxylic acids, anhydrides, and lactones, resulted in a heat release of about 40 kJ mol-1. The most strongly pronounced thermic effects were detected for the release of CO, comprising highly exothermic effects due to the decomposition of condensed acids and carbonyls/quinones as well as endothermic effects attributed to anhydrides and phenols/ethers. Notably, anhydrides can be formed during the oxidative treatment as well as during heating by condensation of adjacent carboxylic acids. In the latter case, the two-step decomposition is overall highly exothermic, indicating the associated occurrence of pronounced carbon matrix rearrangements. DFT investigations suggest that these rearrangements not only affect the immediate OFG proximity but also involve several carbon sheets.

Effect of Dipole Orientation in Mixed, Charge-Equilibrated Self-assembled Monolayers on Protein Adsorption and Marine Biofouling.

While zwitterionic interfaces are known for their excellent low-fouling properties, the underlying molecular principles are still under debate. In particular, the role of the zwitterion orientation at the interface has been discussed recently. For elucidation of the effect of this parameter, self-assembled monolayers (SAMs) on gold were prepared from stoichiometric mixtures of oppositely charged alkyl thiols bearing either a quaternary ammonium or a carboxylate moiety. The alkyl chain length of the cationic component (11-mercaptoundecyl)-N,N,N-trimethylammonium, which controls the distance of the positively charged end group from the substrate's surface, was kept constant. In contrast, the anionic component and, correspondingly, the distance of the negatively charged carboxylate groups from the surface was varied by changing the alkyl chain length in the thiol molecules from 7 (8-mercaptooctanoic acid) to 11 (12-mercaptododecanoic acid) to 15 (16-mercaptohexadecanoic acid). In this way, the charge neutrality of the coating was maintained, but the charged groups exposed at the interface to water were varied, and thus, the orientation of the dipoles in the SAMs was altered. In model biofouling studies, protein adsorption, diatom accumulation, and the settlement of zoospores were all affected by the altered charge distribution. This demonstrates the importance of the dipole orientation in mixed-charged SAMs for their inertness to nonspecific protein adsorption and the accumulation of marine organisms. Overall, biofouling was lowest when both the anionic and the cationic groups were placed at the same distance from the substrate's surface.