Effects of Acidity on the Conversion of the Model Bio-oil Ketone Cyclopentanone on H−Y Zeolites

Deoxygenation of bio-oils on acidic zeolites is a potential method for upgrading bio-oils into energy-dense liquid fuels. One of the factors affecting the viability of zeolite-catalyzed dehydration/deoxygenation is minimization of coke formation. Here, cyclopentanone, a ketone commonly found in bio-oils, is utilized as a prototypical bio-oil ketone and its reactivity is evaluated over commercial H−Y zeolites with different acidities. 1H, 13C, and 27Al NMR studies of acidic H−Y zeolites contacted with cyclopentanone suggest that the nature of the zeolite acidity significantly affects the organic reactivity and coke formation. The conversion of cyclopentanone is shown to be preferentially initiated at lower temperatures on the zeolite with lower acid strength, higher Brønsted acid site density, and larger concentration of extraframework aluminum species (H−Y/5.2, 27.1 SiOHSupAl/uc, 12.05 EFA/uc) than over the zeolite with higher acid strength but lower acid site density (H−Y/30, 8.4 SiOHSupAl/uc, and 0.28 EFA/uc). The H−Y/5.2 initiates the three-step condensation of cyclopentanone to trindane with essentially complete deoxygenation at room temperature. In contrast, on H−Y/30, the main reaction at room temperature is a one-step condensation to produce dimer with 50% deoxygenation. These observations suggest that relatively weak Brønsted acid sites are able to initiate the conversion of cyclopentanone and that a large density of accessible acid sites may promote the reactions. The results suggest that hydride transfer, double-bond migration, and cracking reactions occur at relatively low reaction temperatures on H−Y/5.2. At temperatures higher than 400 °C, heavy coke is generated, as evidenced by 13C NMR spectra, and local strain in the zeolite framework at strong acid sites (SiOHAl), as observed in the 29Si MAS NMR spectrum

In Situ Generation of Radical Coke and the Role of Coke-Catalyst Contact on Coke Oxidation

A thermogravimetric analyzer (TGA) equipped for flowing hydrocarbon gases allowed in situ deposition of coke on catalyst and support samples with excellent coke-catalyst contact. The coke deposition on the catalysts and supports, which occurs via a gas phase radical mechanism, depends on the reaction time, temperature, hydrocarbon concentration, and sample external surface area but not on the chemical composition of the support under the conditions used. The coke samples, including in situ generated samples and an industrial coke sample, are characterized quantitatively by both deconvolution of Raman spectra and temperature-programmed oxidation (TPO) analyses. Thermal aging of coke is shown to be effective in increasing the hardness of the coke samples. Ceria dispersed on α-alumina, used as a model catalyst for coke oxidation, allows coke oxidation at lower temperatures. Using these catalysts, coke deposited in situ is shown to oxidize similarly to ground (tight contact conditions) coked catalyst samples, suggesting that in situ coke deposition in the TGA can be used to generate samples with realistic coke-catalyst contacting, as might be found in an industrial reactor or catalyst bed. In situ coking is also observed to be reproducible and reliable as compared to loose and tight contact methodologies

Anticoking Performance of Electrodeposited Mn/MnO Surface Coating on Fe–Ni–Cr Alloy during Steam Cracking

Manganese electrodeposition and anodization are performed on an Fe–Ni–Cr alloy (Incoloy 800H) to form an Mn/MnO surface coating after thermal pretreatment. The Mn/MnO-coated alloy is coked under simulated steam cracking conditions in ethylene-steam, and its anticoking performance is compared with pretreated, uncoated alloys. The mass of deposited coke during repeated coking cycles is measured by thermogravimetric analysis (TGA) and also determined from the measured CO/CO2 concentrations during decoking with air. Compared to the uncoated alloys that have Cr2O3-rich surfaces, the Mn/MnO-coated alloy shows 30–40% less deposited coke and a peak coke oxidation temperature reduced by about 100 °C. The Mn/MnO surface coating is hypothesized to reduce coke deposition by limiting the amount of Fe/Ni species on the surface and by catalyzing coke gasification/oxidation reactions via the formation of catalytically active Mn3+ species

Formation and Oxidation/Gasification of Carbonaceous Deposits: A Review

A wide variety of hydrocarbon processes, catalytic or noncatalytic, involve the formation of carbon deposits, either on catalysts or on reactor (or engine/exhaust) surfaces. Therefore, researchers have developed a large array of catalysts to aid the combustion of these deposits. Recently, the mechanism of catalytic carbon oxidation and/or gasification has been the focus of research in an attempt to design better catalysts for carbon removal. With this approach, understanding the mechanism of formation of different types of carbon deposits is desired. Efforts undertaken for studying oxidation or gasification of various forms of carbon deposits are discussed in this review, along with the techniques used to study the mechanism of oxidation/gasification. The kinetics of catalyzed and noncatalytic carbon oxidation are described in detail. The effect of reactive gases such as NO_<i>x</i>, water vapor, CO₂, and SO₂ on the gasification behavior of carbon deposits is also discussed. Reaction rates of oxidation/gasification of carbon under different operating conditions have been calculated, allowing for a comprehensive overview of carbon removal reactivity

Origins of Unusual Alcohol Selectivities over Mixed MgAl Oxide-Supported K/MoS₂ Catalysts for Higher Alcohol Synthesis from Syngas

A series of MoS₂ catalysts supported on Mg/Al hydrotalcite-derived mixed-metal oxide (MMO) supports promoted with K₂CO₃ is used for alcohol synthesis via CO hydrogenation. Alcohol selectivities are found to vary greatly when the Mo is loaded on the support at 5 wt % compared with 15 wt % Mo samples, all with a Mo/K atomic ratio of 1:1. The most striking difference between the catalysts is the comparatively low methanol and high C₃₊ alcohol selectivities and productivities achieved with the 5% Mo catalyst. This catalyst also produces more ethane than the 15% Mo catalyst, which is shown to be associated with ethanol dehydration and hydrogenation over residual acid sites on this catalyst with lower K content. A series of catalysts with common composition (5% Mo and 3% K supported on MMO) prepared in different manners all yield similar catalytic selectivities, thus showing that selectivity is predominately controlled by the MMO-to-Mo ratio rather than the synthesis method. When the Mo loading is the same, catalytic higher alcohol productivity shows some correlation to the degree of stacking of the MoS₂ layers, as assessed via X-ray diffraction and scanning transmission electron microscopy. Control reactions in which K loading is increased or the positioning of the MMO in the catalyst bed is changed via creation of multiple or mixed catalyst beds show that Mo/K/MMO domains play a significant role in alcohol-forming reactions. Higher alcohol-forming pathways are proposed to occur via CO insertion pathways or via coupling of adsorbed reaction intermediates at or near MoS₂ domains. No evidence is observed for significant alcohol-coupling pathways by adsorption of alcohols over downstream, bare MMO supports. Nitrogen physisorption, XRD, Raman, UV–vis DRS, STEM, and XANES are used to characterize the catalysts, demonstrating that the degree of stacking of the MoS₂ domains differs significantly between the low (5% Mo) and high (15% Mo) loading catalysts