Hierarchical Auto-Ignition and Structure-Reactivity Trends of C2–C4 1-Alkenes

Ignition delay times of small alkenes are a valuable constraint for the refinement of the core kinetic mechanism of hydrocarbons used in representing combustion properties of real fuels. Moreover, the chemical reactivity comparison of those small alkenes provides a reference in object-oriented fuel design and logical combustion utilization. In this study, the ignition delay times of C2–C4 alkenes (ethylene, propene and 1-butene) were measured behind reflected shock waves first, with a fixed oxygen concentration (XO2 = 6%) and equivalence ratio (φ = 1.0) at various pressures of 1.2, 4.0 and 16.0 atm, in order to facilitate the comparison. Three chemical-based-Arrhenius-type correlations covering a wide range of temperature, pressure, equivalence ratio, and dilution were proposed. The simplified reaction network for pyrolysis and oxidation of 1-alkenes was depicted relying on the reaction classes of alkenes. Nine generally accepted mechanisms were used to simulate the ignition delay times measured by this study as well as literature. All the kinetic models show reasonable structure-reactivity trends for all of the three alkenes, but only NUIGMech 1.1 is capable of representing quantificationally the chemical reactivity at all tested conditions. Generally, ethylene exhibits the highest reactivity while propene presents the lowest at high temperatures. Analyses of sensitivity and flux indicate that the main oxidation pathway of ethylene is chain-branching, which accelerates the accumulation of free radical pools, especially for the Ḣ atom, Ȯ atom and ȮH radical, which results in the highest reactivity of ethylene. For propene and 1-butene, due to the presence of the allylic site, consumption of allylic radicals becomes the decisive step of oxidation and allylic radicals are mostly consumed by the HȮ2 radical. However, there are no such efficient reaction pathways for the formation of HȮ2 radicals during the propene oxidation process, while reaction pathways for HȮ2 formation in 1-butene are efficient. Thus, 1-butene presents higher reactivity compared to propene.

Promoting Effect of Palladium on ZnAl2O4-Supported Catalysts Based on Cobalt or Copper Oxide on the Activity for the Total Propene Oxidation

Palladium-modified Co-ZnAland Cu-ZnAl materials were used and found active for the catalytic oxidation of propene and propane. According to the results obtained by XRD, TPR and XPS, the zinc aluminate-supported phases are oxide phases, Co3O4, CuO and PdOx for Co-ZnAl, Cu-ZnAl and Pd-ZnAl catalysts, respectively. These reducible oxide species present good catalytic activity for the oxidation reactions. The addition of palladium to Co-ZnAl or Cu-ZnAl samples promoted the reducibility of the system and, consequently, produced a synergic effect which enhanced the activity for the propene oxidation. The PdCo-ZnAl sample was the most active and exhibited highly dispersed PdOx particles and surface structural defects. In addition, it exhibited good catalytic stability. The H2 pre-treated PdCu-ZnAl, PdCo-ZnAl and Pd-ZnAl samples showed higher activity than the original oxide catalysts, evidencing the important role of the oxidation state of the species, mainly of the palladium species, on the catalytic activity for the propene combustion. The synergic effect between metal transition oxides and PdOx could not be observed for the propane oxidation.

Chemisorbed Oxygen or Surface Oxides Steer the Selectivity in Pd Electrocatalytic Propene Oxidation Observed by Operando Pd Ledge X‐ray Absorption Spectroscopy

Controlled electrochemical oxidation of hydrocarbons to desired products is an attractive approach in catalysis. Here we study the electrochemical propene oxidation under operando conditions using Pd L‐edge X‐ray absorption spectroscopy...

Catalytic and Electrochemical Properties of Ag Infiltrated Perovskite Coatings for Propene Deep Oxidation

This study reports the catalytic properties of Ag nanoparticles dispersed on mixed ionic and electronic conducting layers of LSCF (La0.6Sr0.4Co0.2Fe0.8O3) for propene combustion. A commercial and a synthesized LSCF powder were deposited by screen-printing or spin-coating on dense yttria-stabilized zirconia (YSZ) substrates, an oxygen ion conductor. Equal loadings (50 µg) of Ag nanoparticles were dispersed via drop-casting on the LSCF layers. Electrochemical and catalytic properties have been investigated up to 300 °C with and without Ag in a propene/oxygen feed. The Ag nanoparticles do not influence the electrochemical reduction of oxygen, suggesting that the rate-determining step is the charge transfer at the triple phase boundaries YSZ/LSCF/gas. The anodic electrochemical performances correlate well with the catalytic activity for propene oxidation. This suggests that the diffusion of promoting oxygen ions from YSZ via LSCF grains can take place toward Ag nanoparticles and promote their catalytic activity. The best specific catalytic activity, achieved for a LSCF catalytic layer prepared by screen-printing from the commercial powder, is 800 times higher than that of a pure Ag screen-printed film.

Aplicação de óxidos mistos de molibdênio, vanádio, tungstênio e cobre como catalisadores na produção do ácido acrílico

Acrylic acid is a product with several applications in the chemical industry, the main one is the production of sodium polyacrylate, a superabsorbent material used in the toiletries manufacture. Currently acrylic acid is obtained from propene oxidation using heterogeneous Mo/Bi and Mo/V oxide-based catalysts. In this process, propene is first oxidized to acrolein, which is then oxidized to acrylic acid. Although this is already a consolidated process, propylene comes from petrochemical sources and thus there is a concern to search for alternative routes to the use of this raw material and one of the possibilities is to synthesize acrolein from glycerol dehydration using specific catalysts. For the project, heterogeneous catalysts were prepared to obtain the acrylic acid, first evaluated in the oxidation of acrolein and later in the glycerol oxideshydration. Three types of samples were synthesized with different compositions B1- Mo12V4,8W2,4Cu2,2Si8,4; B2-Mo12V2W0,5Si6,2 and B3-Mo12V2,7Si6,2) by four preparation methods, namely by evaporation, evaporation followed by hydrothermal treatment, hydrothermal treatment (TH) and using a block copolymer. For the last two methods a more detailed study was performed to determine the best synthesis conditions (Phase I), and it was found that the total dissolution of the reagents in the mixture before TH resulted in samples with higher crystallinity and less active phase loss in the liquid and the use of a cold dissolved block copolymer contributed to an increase in pore volume. In the second stage, the materials synthesized by the four proposed methods were characterized and evaluated in reactor in acrylic acid production. The samples B1 showed different crystalline phase formation depending on the preparation method used, and in samples B2 and B3 the main phase was identified as a-MoO3, regardless of the method used. The samples synthesized by evaporation followed by TH showed the highest selectivity for acrylic acid formation from acrolein for the same catalyst composition, which may be related to the higher vanadium oxide content present in samples identified by FRX and the formation of the crystalline phase V0.35Mo4,65O14. The best performance was observed in sample B1-EV+TH with selectivity of 50.59% and 3.61% for acrylic acid in the processes from acrolein and glycerol, respectively