DFT insights into the oxygen-assisted selective oxidation of benzyl alcohol on manganese dioxide catalysts

The reactivity pattern of the MnO2 catalyst in the selective aerobic oxidation of benzyl alcohol is assessed by density functional theory (DFT) analysis of adsorption energies and activation barriers on a model Mn4O8 cluster. DFT calculations predict high reactivity of defective Mn(IV) sites ruling a surface redox mechanism, L-H type, involving gas-phase oxygen. Bare and promoted (i.e., CeOx and FeOx) MnOx materials with high surface exposure of Mn(IV) sites were synthesized to assess kinetic and mechanistic issues of the selective aerobic oxidation of benzyl alcohol on real catalysts (T, 333- 363K). According to DFT predictions, the experimental study shows: i) comparable activity of bare and promoted catalysts due to surface Mn(IV) sites; ii) the catalytic role of oxygen-atoms in the neighboring of active Mn(IV) sites; and iii) a 0th-order dependence on alcohol concentration, diagnostic of remarkable influence of adsorption phenomena on the reactivity pattern. Evidences of catalyst deactivation due to the over-oxidation of benzyl alcohol to benzoic acid, acting as poison of the active sites, are discussed

Fermented Honey and Manna Ash Products: Novel Ecological Niches of Wine Yeasts

The selection of novel strains of yeasts is still relevant to improve flavour of wines produced around the word. Several food niches have not been microbiologically investigated and they might represent important sources of microorganisms with technological aptitudes, e.g. in wine industry. To this purpose, two novel yeast communities associated with matrices rich in carbohydrates and characterized by low levels of water activity (aw), such as fermented honey by-products (FHP) and “Manna” ash products (MAP) extracted from Fraxinus angustifolia (Oleaceae), were investigated. FHP contain mainly fructose and glucose, while MAP ash is mainly characterized by high concentrations of mannitol, fructose and mannotriose. The values of aw of both matrices is around 0.5-0.6. Yeasts were isolated, subjected to the genotypic identification and then technologically characterized to evaluate their oenological potential. The species Lachancea fermentati, Pichia anomala, Pichia kudriavzevii, Saccharomyces cerevisiae, Wickerhamomyces anomalus, Zygosaccharomyces bailii and Zygosaccharomyces rouxii were genetically identified from FHP samples. During the spontaneous alcoholic fermentation, the dominating species were S. cerevisiae, Z. bailii and Z. rouxii whose feed conversion ratio of sugars into ethanol was about 53%. On the other hand, MAP was characterized by the presence of Candida aaseri, Candida lactis-condensi, Citeromyces matritensis, Lachancea thermotolerans, Saccharomyces cerevisiae and Zygosaccharomyces bailii. Interestingly, both matrices showed the presence of S. cerevisiae at consistent levels and a high number of L. thermotolerans strains were isolated from MAP. Both species have interesting eonological potential. Six strains of S. cerevisiae were tested as starters to ferment grape must from Grillo, Catarratto and Chardonnay cultivars at industrial winery-scale over two consecutive years. Interestingly, four strains isolated from FHP showed a fructophilic potential and the experimental wines were characterized by an intense floral flavour. Among non-Saccharomyces yeasts, L. thermotolerans strains showed resistance to ethanol up to 12-13 % (v/v) and ability to ferment grape must with a feed conversion ratio of sugars into ethanol of about 45-55%. Eonological potential was showed by C. lactis-condensi and C. aaseri since a high content of glycerol was produced at the end of alcoholic fermentation and a fructophilic aptitude was found. In conclusions, the present research provided novel microbiological and physicochemical insights on the alcoholic fermentation conducted by novel starters belonging to S. cerevisiae and L. thermotolerans species and, for the first time, the species C. lactis-condensi and C. aaseri were found to be of relevance for wine application

A novel microbiological approach to impact the aromatic composition of sour loquat beer

The growing interest in novel beer development determined the exploitation of unconventional yeasts isolated from novel ecological niches to generate unexplored sensory profiles. In recent years, there is an increasing interest in generating beers brewed with the addition of fruits. For the first time, Lachancea thermotolerans MNF105 and Saccharomyces cerevisiae MN113 isolated from manna, were tested as starter cultures to process loquat beer to improve the sensory profile. Innovatively, the yeast species L. thermotolerans was investigated for the production of sour fruit beer. Sour fruit beers produced with L. thermotolerans MNF105 were more balanced than the respective control, especially in terms of perceived acidity during sensory analysis. This could be due to the lower lactic acid production (0.49 g/L) compared to the respective control (1.74 g/L). The overall organoleptic investigation showed a preference for S. cerevisiae MN113 (TF1) isolated from manna. Experimental trials conducted with the selected strains demonstrated the absence of off-odour and off-flavour and improved aroma perception. Aldehydes and alcohols were the most abundant compounds emitted from the beers. S. cerevisiae MN113 and L. thermotolerans MNF105, manna related yeasts, showed great technological properties, representing promising starters for the production of fruit beer and sour fruit beer

H2 hitting on graphene supported palladium cluster: molecular dynamics simulations

Dissociative adsorption of impinging gas-phase molecular hydrogen (H2) on a palladium cluster (Pd4) supported on defective graphene (C47) was studied by periodic density functional theory molecular dynamics simulations. The H2 on Pd4/C47 collision dynamics were investigated without any particular constraint, except for the Born–Oppenheimer approximation. The study, which had mostly method-testing aims, provided, anyway, valuable information about the collision kinetics of gas-phase molecular hydrogen. This was treated as an impacting projectile having different kinetic energy values. At lower kinetic energies, sticking was ruled by steering effects imputable to the Pd cluster that easily reoriented the incoming hydrogen molecule to the metallic surface sites, favoring fragmentation hence adsorption. Sticking decreased at higher kinetic energies for which the very fast corresponding projectile acquired a kind of elusiveness toward the metallic surface that for this was not able to steer and adsorb the gas-phase hydrogen molecule. Interestingly, altered steering effects, in this case, ruled by already adsorbed surface hydrogen, seemed to play a decisive role in refocusing both H2 hitting and sticking on the supported palladium surface, irrespective of the energy characterizing the impinging gas-phase molecules

Propan-2-ol dehydration on H-ZSM-5 and H-Y zeolite: a DFT study

The catalytic dehydration of propan-2-ol over H-Y and H-ZMS-5 aluminated zeolite models, mimicking both internal cavities and external surfaces, was studied by DFT calculations to investigate the reaction mechanism. After the adsorption of propan-2-ol on the zeolite, the dehydration mechanism starts with alcohol protonation, occurring by one acidic –OH group of the zeolite fragment, followed by a concerted b-elimination to give propene. The catalytic activity is affected by the size of the zeolite cavity, which is larger in the H-Y than in the H-ZMS-5 zeolite. The adsorption energy of the reagent, as an example, decreases in the order: H-Y cavity = H-ZMS-5 > surface H-ZMS-5 cavity, pointing that the adsorption process should preferentially occur either on open surface or inside larger cavity. More interestingly, confinement effects play a twofold role in driving the reaction pathway, resulting in two different effects on the reaction outcomes. The thermodynamic stability, evaluated by the standard free energy difference of the products (water and propene) with respect to the reactant (propan-2-ol), would indeed suggest that the reaction more smoothly could occur for the systems: H-ZMS-5 surface > non-catalyzed > H-Y cavity > H-ZMS-5 cavity. The activation standard free energy of the process conversely decreases in the order: non-catalyzed > H-ZMS-5 surface > H-ZMS-5 cavity > H-Y cavity, suggesting that the reaction is faster inside zeolite cavities. Experimental and computational results are in agreement, giving confidence on the atomistic-level insights provided

DFT calculations on subnanometric metal catalysts: a short review on new supported materials

Metal clusters have been used in catalysis for a long time, even in industrial production protocols, and a large number of theoretical and experimental studies aimed at characterizing their structure and reactivity, either when supported or not, are already present in the literature. Nevertheless, in the last years the advances made in the control of the synthesis and stabilization of subnanometric clusters promoted a renewed interest in the field. The shape and size of sub-nanometer clusters are crucial in determining their catalytic activity and selectivity. Moreover, if supported, subnanometric clusters could be highly influenced by the interactions with the support that could affect geometric and electronic properties of the catalyst. These effects also present in the case of metal nanoparticles assume an even more prominent role in the “subnano world.” DFT-based simulations are nowadays essential in elucidating and unraveling reaction mechanisms. The outstanding position of this corner of science will be highlighted through a selected number of examples present in the literature, concerning the growth and reactivity of subnanometric supported metal catalysts

H2 Transformations on Graphene Supported Palladium Cluster: DFT-MD Simulations and NEB Calculations

Molecular dynamics simulations based on density functional theory were employed to investigate the fate of a hydrogen molecule shot with different kinetic energy toward a hydrogenated palladium cluster anchored on the vacant site of a defective graphene sheet. Hits resulting in H2 adsorption occur until the cluster is fully saturated. The influence of H content over Pd with respect to atomic hydrogen spillover onto graphene was investigated. Calculated energy barriers of ca. 1.6 eV for H-spillover suggest that the investigated Pd/graphene system is a good candidate for hydrogen storage

Alkane dehydrogenation on defective BN quasi-molecular nanoflakes: DFT studies

Lower alkanes are feedstocks readily available but relatively inert. The con- version of low cost alkanes to industrially relevant alkenes is usually carried out on metal-based heterogeneous catalysts. Considering both the cost and the potential harmfulness of the metal involved in the dehydrogenation cat- alysts (typically, platinum or chromium), the study of metal-free processes represents an important challenge for the industrial chemistry in order to address more sustainable protocols and different routes either to activate or transform alkanes. Framed in this context, it was investigated, using a den- sity functional theory approach, the potential dehydrogenation activity of de- fective and partially hydrogenated boron nitride quasi-molecular nanoflakes, on ethane, chloroethane and phenylethane. Concerted and not concerted reactions were considered, resulting the former always favored with respect to the latter and being the mechanisms and the corresponding energy char- acterizing the different substrates, in any case, quite similar. Along the dehydrogenation, given a certain hydrocarbon adsorption constellation, the defective nitrogen sites were generally more active than the boron ones, being the energy barriers, in any case, smaller or at least comparable with those al- ready observed for the metal catalyzed dehydrogenation processes. After the dehydrogenation processes, the hydrogenated boron nitride quasi-molecular nanoflakes resulted highly stabilized, suggesting that specific strategies are needed to employ these materials as catalysts