Glycerol Hydrogenolysis to Propylene Glycol and Ethylene Glycol on Zirconia Supported Noble Metal Catalysts

Monoclinic zirconia (m-ZrO2) supported Ru, Rh, Pt, and Pd nanoparticles with controlled sizes were prepared and examined in glycerol hydrogenolysis to propylene glycol and ethylene glycol at similar conversions in the kinetic regime. Their activity (normalized per exposed surface metal atom, i.e., turnover rate) and selectivity depend sensitively on the nature of the noble metals and their particle size. At a similar size (ca. 2 nm), Ru exhibited a greater turnover rate than Rh, Pt, and Pd, and the rate decreased in the sequence Ru ≫ Rh > Pt > Pd by a factor of about 25 (from 0.035 to 0.0014 mol glycerol (mol surface metal·s)−1) at 473 K and 6.0 MPa H2. Following such activity sequence, Ru was more prone to catalyze excessive cleavage of C–C bonds, leading to the formation of ethylene glycol and methane, while Pd exhibited the highest selectivity to cleavage of C–O bonds to propylene glycol. Similarly, larger Ru particles possessed higher glycerol hydrogenolysis activity concurrently with higher selectivities to ethylene glycol and especially methane at the expense of propylene glycol in the range of 1.8–4.5 nm. Analysis of kinetics and thermodynamics for the proposed elementary steps involving kinetically relevant glycerol dehydrogenation to glyceraldehyde leads to expressions of glycerol hydrogenolysis rate and selectivity to cleavage of C–O bonds relative to C–C bonds. Together with different effects of reaction temperature and atmosphere of H2 and N2 on the activity and selectivity for Ru/m-ZrO2 and Pt/m-ZrO2, these results suggest that the observed difference for different noble metals and particle sizes can be attributed to the difference in the strength of adsorption of glycerol and glyceraldehyde, derived from their different availability of unoccupied d orbitals

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

Influence of Dioxygen on the Promotional Effect of Bi during Pt-Catalyzed Oxidation of 1,6-Hexanediol

A series of carbon-supported, Bi-promoted Pt catalysts with various Bi/Pt atomic ratios was prepared by selectively depositing Bi on Pt nanoparticles. The catalysts were evaluated for 1,6-hexanediol oxidation activity in aqueous solvent under different dioxygen pressures. The rate of diol oxidation on the basis of Pt loading over a Bi-promoted catalyst was 3 times faster than that of an unpromoted Pt catalyst under 0.02 MPa of O₂, whereas the unpromoted catalyst was more active than the promoted catalyst under 1 MPa of O₂. After liquid-phase catalyst pretreatment and 1,6-hexanediol oxidation, migration of Bi on the carbon support was observed. The reaction order in O₂ was 0 over Bi-promoted Pt/C in comparison to 0.75 over unpromoted Pt/C in the range of 0.02–0.2 MPa of O₂. Under low O₂ pressure, rate measurements in D₂O instead of H₂O solvent revealed a moderate kinetic isotope effect (rate_H₂O/rate_D₂O) on 1,6-hexanediol oxidation over Pt/C (KIE = 1.4), whereas a negligible effect was observed on Bi-Pt/C (KIE = 0.9), indicating that the promotional effect of Bi could be related to the formation of surface hydroxyl groups from the reaction of dioxygen and water. No significant change in product distribution or catalyst stability was observed with Bi promotion, regardless of the dioxygen pressure

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