Jakob Munkholt Christensen

Coupling of Alcohols over Alkali-Promoted Cobalt-Molybdenum Sulfide

The catalytic conversion of syngas into mixed alcohols is a possibility for converting coal, carbon-containing waste fractions, and biomass into synthetic fuels or fuel additives via a gasification process. The addition of higher alcohols to mixtures of methanol–gasoline or ethanol–gasoline improves the miscibility of such mixtures, as the higher alcohols act as cosolvents, and stabilize the blends. Such a mixture of alcohols can be formed directly from syngas over various catalytic systems. These systems include alkali-promoted Cu, 3] Rh, Co–Cu, and alkali-promoted MoS2 [6] catalysts. The catalytic system studied in this work is potassium carbonate-promoted cobalt– molybdenum sulfide on a carbon support. For syngas conversion, it has previously been determined that the addition of alkali salts to MoS2 shifts the selectivity from hydrocarbons to alcohols, and that addition of Co to alkali-promoted MoS2 catalysts shifts the product distribution towards the desirable higher alcohols. With the sulfide catalyst, the main products of syngas conversion are primary, linear alcohols, whereas the predominant byproducts are short-chain hydrocarbons. Experiments with isotopic labeling indicated that an important route to chain growth over MoS2-based catalysts is the addition of CO to an alkyl group (RCH2) to form an acyl species (RCH2CO). [7] The resultant acyl species can then be hydrogenated into the corresponding alcohol or to a new, longer alkyl group. Hydrocarbons are believed to form from hydrogenation of the alkyl groups. This route to formation of alcohols and hydrocarbons is illustrated in Scheme 1. Herein we show that another route to chain growth for the sulfide catalyst is the coupling of alcohols, as exemplified by the homocoupling of ethanol to form 1-butanol. To boost the production of the desirable higher alcohols, lower alcohols could be co-fed along with the syngas. Previous work has clearly shown that methanol co-fed along with syngas is converted into higher alcohols. 7] Herein we investigate the cofeeding of ethanol along with the syngas fed into the reactor. The sulfide catalyst is affected by the presence of sulfur sources, such as H2S, in the syngas. The presence of H2S in the syngas shifts the distribution of the alcohol product from methanol towards the higher alcohols, but also leads to increased production of hydrocarbons and to incorporation of sulfur species into the alcohol product. Due to the negative effects of H2S, the present experiments were conducted with sulfur-free syngas. In S-free syngas, we found that the catalyst requires a stabilization period of around 30 h on stream to achieve steady state. The present addition of ethanol to the syngas is therefore commenced after the catalyst has been given 30 h to stabilize in sulfur-free syngas. Figure 1 shows the alcohol production rates as functions of the molar fraction of ethanol in the syngas feed.

Ketene as a Reaction Intermediate in the Carbonylation of Dimethyl Ether to Methyl Acetate over Mordenite

Unprecedented insight into the carbonylation of dimethyl ether over Mordenite is provided through the identification of ketene (CH2CO) as a reaction intermediate. The formation of ketene is predicted by detailed DFT calculations and verified experimentally by the observation of doubly deuterated acetic acid (CH2DCOOD), when D2O is introduced in the feed during the carbonylation reaction.

In Situ Observation of Cu-Ni Alloy Nanoparticle Formation by X-Ray Diffraction, X-Ray Absorption Spectroscopy, and Transmission Electron Microscopy: Influence of Cu/Ni Ratio

Silica‐supported, bimetallic Cu–Ni nanomaterials were prepared with different ratios of Cu to Ni by incipient wetness impregnation without a specific calcination step before reduction. Different in situ characterization techniques, in particular transmission electron microscopy (TEM), X‐ray diffraction (XRD), and X‐ray absorption spectroscopy (XAS), were applied to follow the reduction and alloying process of Cu–Ni nanoparticles on silica. In situ reduction of Cu–Ni samples with structural characterization by combined synchrotron XRD and XAS reveals a strong interaction between Cu and Ni species, which results in improved reducibility of the Ni species compared with monometallic Ni. At high Ni concentrations silica‐supported Cu–Ni alloys form a homogeneous solid solution of Cu and Ni, whereas at lower Ni contents Cu and Ni are partly segregated and form metallic Cu and Cu–Ni alloy phases. Under the same reduction conditions, the particle sizes of reduced Cu–Ni alloys decrease with increasing Ni content. Estimates of the metal surface area from sulfur chemisorption and from the XRD particle size generally agree well on the trend across the composition range, but show some disparity in terms of the absolute magnitude of the metal area. This work provides practical synthesis guidelines towards preparation of Cu–Ni alloy nanomaterials with different Cu/Ni ratios, and insight into the application of different in situ techniques for characterization of the alloy formation.

Influence of preparation method on supported Cu–Ni alloys and their catalytic properties in high pressure CO hydrogenation

Silica supported Cu–Ni (20 wt% Cu + Ni on silica, molar ratio of Cu/Ni = 2) alloys are prepared via impregnation, coprecipitation, and deposition–coprecipitation methods. The approach to co-precipitate the SiO2 from Na2SiO3 together with metal precursors is found to be an efficient way to prepare high surface area silica supported catalysts (BET surface area up to 322 m2 g−1, and metal area calculated from X-ray diffraction particle size up to 29 m2 g−1). The formation of bimetallic Cu–Ni alloy nanoparticles has been studied during reduction using in situ X-ray diffraction. Compared to impregnation, the coprecipitation and deposition–coprecipitation methods are more efficient for preparation of small and homogeneous Cu–Ni alloy nanoparticles. In order to examine the stability of Cu–Ni alloys in high pressure synthesis gas conversion, they have been tested for high pressure CO hydrogenation (50 bar CO and 50 bar H2). These alloy catalysts are highly selective (more than 99 mol%) and active for methanol synthesis; however, loss of Ni caused by nickel carbonyl formation is found to be a serious issue. The Ni carbonyl formation should be considered, if Ni-containing catalysts (even in alloyed form) are used under conditions with high partial pressure of CO.

Ceria Prepared by Flame Spray Pyrolysis as an Efficient Catalyst for Oxidation of Diesel Soot

Ceria has been prepared by flame spray pyrolysis and tested for activity in catalytic soot oxidation. In tight contact with soot the oxidation activity (measured in terms of the temperature of maximal oxidation rate, Tmax) of the flame made ceria is among the highest reported for CeO2. This can to a significant degree be ascribed to the large surface area achieved with the flame spray pyrolysis method. The importance of the inherent soot reactivity for the catalytic oxidation was studied using various soot samples, and the reactivity of the soot was found to have a significant impact, as the Tmax-value for oxidation in tight contact with a catalyst scaled linearly with the Tmax-value in non-catalytic soot oxidation. The Tmax-value in non-catalytic soot oxidation was in turn observed to scale linearly with the H/C ratio of the carbonaceous materials.Graphical Abstract

Reaction mechanism of dimethyl ether carbonylation to methyl acetate over mordenite – a combined DFT/experimental study

The reaction mechanism of dimethyl ether carbonylation to methyl acetate over mordenite was studied theoretically with periodic density functional theory calculations including dispersion forces and experimentally in a fixed bed flow reactor at pressures between 10 and 100 bar, dimethyl ether concentrations in CO between 0.2 and 2.0%, and at a temperature of 438 K. The theoretical study showed that the reaction of CO with surface methyl groups, the rate-limiting step, is faster in the eight-membered side pockets than in the twelve-membered main channel of the zeolite; the subsequent reaction of dimethyl ether with surface acetyl to form methyl acetate was demonstrated to occur with low energy barriers in both the side pockets and in the main channel. The present analysis has thus identified a path, where the entire reaction occurs favourably on a single site within the side pocket, in good agreement with previous experimental studies. The experimental study of the reaction kinetics was consistent with the theoretically derived mechanism and in addition revealed that the methyl acetate product inhibits the reaction – possibly by sterically hindering the attack of CO on the methyl groups in the side pockets.

Pressure Induced Effects During In Situ Characterization of Supported Metal Catalysts

Iron-based catalysts are typically used commercially for Fischer-Tropsch synthesis, as they are cheaper than their cobalt and ruthenium-based counterparts [1]. Therefore, looking at their activation process is critical for gaining further insight on the nature of the active sites in order to further enhance their performance. α-Fe2O3, used as precursor for Fischer-Tropsch catalysts, is typically activated in either H2, CO or syngas before synthesis reaction. As CO or syngas activation lead to iron carbides and iron oxides we look at the unsupported αFe2O3 precursors exposed to hydrogen to gain information about their evolution in time under in situ conditions. The activation involve interaction between the precursor, αFe2O3, and the surrounding gas constituents, in this case hydrogen, which is not only altering the morphology and surface structure but eventually also the catalytic performance.