Promoting Effect Of Water Research Articles

Selective oxidation of linear alcohols: the promotional effect of water and inhibiting effect of carboxylates over dilute PdAu catalysts

The promotional effect of Pd for selective alcohol oxidation over dilute Pd-in-Au bimetallic alloy catalysts strongly depends on the alkyl chain length and is characterized via a combination of reactor and in situ infrared studies.

Water accelerates and directly participates soot oxidation: An isotopic study

Water vapor is inevitable in automobile exhaust and its influence on soot oxidation is of great interests to many researchers. In this paper, we utilized heavy-oxygen water (H218O) to investigate the water participation extent and its relationship with soot oxidation activity. By comparing the quantity of labeled species in products, we noticed water participated in both bare and catalytic soot oxidation. The tighter contact between soot and catalyst, the more involvement of water. But the extent was not affected by the redox property of Mn2O3 catalysts. The promotional effect of water on activity was observed in all investigated conditions, which was probably because water could be dissociated to more active oxygen species and accelerate the regeneration of oxygen vacancies. But hydrogen atoms left by water still need to be eliminated by dioxygen. Hence in general water worked as an accelerator rather than oxidant.

Effects of water on the kinetics of acetone hydrogenation over Pt and Ru catalysts

We employed an approach combining reaction kinetics measurements at steady state conditions, electronic structure calculations employing density functional theory, and microkinetic modeling for acetone hydrogenation to provide insights into the effects of water on metal catalyst surfaces for the hydrogenation of oxygenates over a wide range of reaction conditions. Elucidation of the repulsive interactions due to adsorbed water molecules at various reaction conditions provides a basis to formulate rate expressions for heterogeneous catalytic processes of biomass oxygenates. Reaction kinetics experiments were carried out at partial pressures of H2, acetone, water and helium in the range of 0.51–0.79, 0.02–0.13, 0.08–0.23, 0–0.28 atm, respectively. We show that the addition of water enhances the hydrogenation rate at 353 K and 1 atm on oxophilic metal catalysts such as Ru/C, whereas the same promotional effect of water is not observed for Pt-based catalysts. Microkinetic model predictions for the hydrogenation of acetone on Ru in the absence and presence of water, using enthalpies and entropies obtained from DFT calculations, were in agreement with the experimentally observed reaction orders and activation barriers. The model shows that a water-assisted hydroxypropyl path is expected to be the favored path on Ru with a rate-determining step of H-OH-mediated hydrogenation of C3H6OH (i.e., the hydroxypropyl intermediate formed by H2O-mediated initial hydrogenation of acetone) to produce isopropyl alcohol (IPA). Furthermore, hydrogen, acetone, hydroxypropyl intermediate and hydroxyl species were predicted to be abundant on the Ru surface with a high coverage of nearly 85%. The combined studies of computational and experimental catalysis on hydrogenation reactions help to elucidate the mechanistic role of water on metal catalyzed reactions for producing chemical building blocks from biomass-derived oxygenates.

Recent Advances in Ruthenium-Catalyzed Hydrogenation Reactions of Renewable Biomass-Derived Levulinic Acid in Aqueous Media.

Levulinic acid (LA) is classified as a key platform chemical for the development of future biorefineries, owing to its broad spectrum of potential applications and because it is simply available from lignocellulosic biomass through inexpensive and high-yield production routes. Catalytic hydrogenation reactions of LA into the pivotal intermediate compound γ-valerolactone (GVL), and beyond GVL to yield valeric acid (VA), 1,4-pentanediol (1,4-PDO), and 2-methyltetrahydrofuran (2-MTHF) have gained considerable attention in the last decade. Among the various transition metals used as catalysts in LA hydrogenation reactions, ruthenium-based catalytic systems have been the most extensively applied by far, due to the inherent ability of ruthenium under mild conditions to hydrogenate the keto functionality of LA selectively into an alcohol group to form 4-hydroxyvaleric acid intermediate, which yields GVL spontaneously after dehydration and cyclization. This review focuses on recent advances in the field of aqueous-phase ruthenium-catalyzed hydrogenation reactions of LA toward GVL, VA, 1,4-PDO, 2-MTHF, 2-pentanol, and 2-butanol. It employs heterogeneous catalysts on solid supports, and heterogeneous water-dispersible catalytic nanoparticles or homogeneous water-soluble catalytic complexes with biphasic catalyst separation, for the inter alia production of advanced biofuels such as valeric biofuels and other classes of liquid transportation biofuels, value-added fine chemicals, solvents, additives to gasoline, and to food as well. The significance of the aqueous solvent to carry out catalytic hydrogenations of LA has been highlighted because the presence of water combines several advantages: (i) it is highly polar and thus an ideal medium to convert polar and hydrophilic substrates such as LA; (ii) water is involved as a byproduct; (iii) the presence of the aqueous solvent has a beneficial effect and enormously boosts hydrogenation rates. In sharp contrast, the use of various organic solvents gives rise to a dramatic drop in catalytic activities. The promotional effect of water was proven by numerous experimental investigations and several theoretical studies employing various types of catalytic systems; (iv) the large heat capacity of water renders it an excellent medium to perform large scale exothermic hydrogenations more safely and selectively; and (v) water is a non-toxic, safe, non-inflammable, abundantly available, ubiquitous, inexpensive, and green/sustainable solvent.

Numerical Modeling of Hydroperoxyl-Mediated Oxidative Dehydrogenation of Formic Acid under SCR-Relevant Conditions

Formic acid decomposition to carbon dioxide is an important step during the decomposition of alternative formate-based ammonia precursor compounds, such as ammonium formate, guanidinium formate, and methanamide, in the selective catalytic reduction (SCR) process. At oxygen and water concentrations prevalent in a typical SCR feed, an oxidative dehydrogenation (ODH) type pathway of formic acid decomposition proceeds on titania-supported gold catalysts. Using the surface perfectly stirred reactor (SPSR) model, the ODH mechanism is demonstrated to satisfactorily predict the experimentally observed conversions. The single-site mechanistic model captured the negative trend in conversion with increasing formic acid concentrations, stemming from an extensive coverage of the active sites by formates. This in turn rendered the active sites unavailable for the formation of the active hydroperoxyls required for the rate-determining step of C–H bond scission of formates. The positive order dependency on oxygen concentration and the promotional effect of water are qualitatively and semi-quantitatively described by the model. Predicted trends in the relative surface coverages of different reaction intermediates are in agreement with previously reported kinetic and spectroscopic measurements. These results are important fundamentally for the understanding of formic acid decomposition chemistry and in general catalysis by gold, as well as for the design of dedicated hydrolysis catalysts for applications in the SCR process.

Overcoming the limitations of gold catalysts in hydrogenation: enhanced activity with full hydrogen utilization

Application of nano-scale supported Au in catalytic hydrogenation delivers high chemoselectivity but low activity using (pressurized) H2 far in excess of stoichiometric requirements. We have tackled the issues of low reaction rate and inefficient hydrogen utilization through a series of approaches taking Au/CeO2 (mean Au size = 2.8 nm) as a test catalyst. Increased spillover hydrogen (using physical mixtures of Au/CeO2 with CeO2 and SiO2) and the promotional effect of water (via catalytic dissociation on surface oxygen vacancies) resulted in a fourfold increase in the selective rate of furfural hydrogenation to furfuryl alcohol. In contrast, Pd/CeO2 and Ni/CeO2 promoted decarbonylation (to furan), hydrogenolysis (to 2-methylfuran) and ring reduction (to tetrahydrofurfuryl alcohol). Coupling (2-butanol → 2-butanone) dehydrogenation over Cu/SiO2 (mean Cu size = 7.8 nm) with furfural hydrogenation over Au/CeO2 further increased rate with full utilization of the hydrogen generated in dehydrogenation. The coupling strategy allows “hydrogen free” hydrogenation that circumvents the limitation of Au in standard catalytic hydrogenation. This can open new avenues to exploit the ultra-selectivity of Au for continuous production of high value commodity products.

Enhancement of photocatalytic decarboxylation on TiO2 by water-induced change in adsorption-mode

Decarboxylation is an important process in the photocatalytic degradation of organic pollutants. In this study, the adsorption and photocatalytic decarboxylation of acetic acid and trichloroacetic acid were investigated by in-situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), which can distinguish carboxylate groups with different adsorption modes. We found that water molecules promote photocatalytic decarboxylation reaction and the promotional effect of water is attributed to the changed adsorption mode of the carboxylate group by the co-adsorbed water molecules. The IR study shows that degradation of the monodentate-coordinated acetic acids is much faster than that of the bidentate-coordinated one. The kinetic isotope effect studies and analysis of the intermediate products indicates that the degradation of acetic acids originates from the direct oxidation by holes even in the presence of water, rather than from the reaction of OH radicals as generally believed. DFT-based molecular dynamics calculations revealed that the formation of the monodentate-coordinated carboxylate group in the presence of water is attributed to competition between the O atom of the carboxylate group and water for the surface Ti sites. Because of its higher electron density, the monodentate carboxylate group is easier to be directly oxidized by holes than the bidentate group.

Promotional effect of water in the clean continuous production of carvacrol from carvone

We have studied the effect of water on the selective conversion of biomass-derived carvone to carvacrol over Pd/Al2O3 (mean Pd size=3nm). Full selectivity to carvacrol was observed in H2O (H2O:Carvone=1:12-6:1) without H2 or at an inlet H2:Carvone=1:6. Carvacrol generation from carvone involves double bond migration with keto-enol tauromerisation. The formation of hydrogenation products (carvotanacetone and carvomenthone) was a feature of reaction at higher H2 content. The incorporation of water (H2O:Carvone≤1:3) with hydrogen (H2:Carvone=1:6-2:1) increased carvacrol production rate to deliver 100% yield. A higher H2O content lowered rate due to competition with carvone for surface active sites. Our results demonstrate the benefits of carvacrol production in continuous flow relative to conventional batch processes allowing full selective transformation under mild reaction conditions.

Promotional Effect of Water on Direct Dimethyl Ether Synthesis from Carbon Monoxide and Hydrogen Catalyzed by Cu–Zn/Al2O3

Cu–Zn/Al2O3 was prepared by using the sol–gel method and employed for the direct synthesis of dimethyl ether (DME) from synthesis gas (syngas) in a continuous-flow reactor. We studied the effect of water concentration in the feed on the formation rates of various products. With an increase in the amount of water, the formation rate of DME initially increased and then decreased. Co-feeding of 0.09% water was co-fed resulting in the highest formation rate of DME, 486 μmol gcat–1 h–1 (8.0% yield of DME), at 220 °C under 0.9 MPaG. Structural studies by XANES, EXAFS, XPS, and infrared spectroscopies showed that the treatment of the catalyst with 0.66% water at 240 °C under atmospheric pressure resulted in oxidation of the metallic copper species and increased the number of surface Cu+ sites. Combined with the catalytic results, we conclude that the increase in the surface Cu+ sites is responsible for the promotion effect of steam on the DME production.

Factors in gold nanocatalysis: oxidation of CO in the non-scalable size regime

Focusing on size-selected gold clusters consisting of up to 20 atoms, that is, in the size regime where properties cannot be obtained from those of the bulk material through scaling considerations, we discuss the current state of understanding pertaining to various factors that control the reactivity and catalytic activity of such nanostructures, using the CO oxidation reaction catalyzed by the gold nanoclusters adsorbed on MgO as a paradigm. These factors include the role of the metal-oxide support and its defects, the charge state of the cluster, structural fluxionality of the clusters, electronic size effects, the effect of an underlying metal support on the dimensionality, charging and chemical reactivity of gold nanoclusters adsorbed on the metal-supported metal-oxide, and the promotional effect of water. We show that through joined experimental and first-principles quantum mechanical calculations and simulations, a detailed picture of the reaction mechanism emerges.

CO Combustion on Supported Gold Clusters

Recent progress in the understanding of the fascinating catalysis of CO combustion by supported gold particles is summarized. Focusing on size-selected gold clusters consisting of only a few atoms, that is, the size regime with properties nonscalable from the bulk properties, we discuss the current knowledge of the different factors controlling the reactivity at the molecular level. These factors include the role of the oxide support, its defects, cluster charging as well as the structural fluxionality of clusters, the cluster size dependency, and the promotional effect of water. By combining experimental results with quantum mechanical ab initio calculations, a detailed picture of the reaction mechanism emerges. While similar mechanisms might be active for gold nanoparticles in the scalable size regime, it is shown that for different systems (defined by the cluster size, the support, experimental conditions, etc.) the reaction mechanism differs and, hence, no generalized explanation for the catalytic driving force of small gold particles can be given.

The promotional effect of water in hydrocracking

The effects of water on the hydrocracking of n-hexadecane and of a certain hydrocracking feed using a rare-earth containing X zeolite and an acid Y catalyst have been examined. The results indicate that the partial pressure of water in the reaction system exerts a marked influence on both activity and selectivity which are quite different between the two catalysts tested. The effects manifest themselves quickly and are reversible by drying with hydrogen at 600 °F. To explain the observations, three concurrently operating mechanisms are proposed.