A.D. van Langeveld

Carbon supported Ru catalysts as promising alternative for Raney-type Ni in the selective hydrogenation of D-glucose

Abstract The activity, selectivity and stability of Ru/C catalysts in d -glucose hydrogenation have been compared with those of the conventional catalysts for this process, viz. Raney-type Ni. All catalysts show high selectivity to d -sorbitol (>98%). Promoting Raney-type Ni with Mo and Cr/Fe has a positive effect on the hydrogenation rate. The Cr/Fe promoted system exhibits the highest activity but the Fe leaches from the catalyst into the reaction mixture. Moreover, this catalyst deactivates after successive runs. For all Raney-type Ni catalysts leaching of Ni in the product mixture occurs. Carbon supported ruthenium is an attractive alternative for Raney-type catalysts. The Ru/C catalysts have higher activities, while Ru does not leach. The activity is proportional to the Ru surface area, independent of the preparation method. A novel anionic deposition method renders catalysts with dispersions of 40%.

Catalysts for second-stage deep hydrodesulfurisation of gas oils

Abstract The interest for new deep hydrodesulfurisation (HDS) processes is expected to rise since more stringent legislation for the maximum sulfur concentration in automotive diesel fuel has been proposed. A realistic option is the application of a separate deep HDS reactor following the existing HDS process in which alternative catalysts may be applied. It was shown that ASA-supported Pt and PtPd catalysts are very active in model feed deep HDS reactions. Moreover, in the deep HDS of a pre-hydrotreated straight-run gas oil (P-SRGO) under relevant industrial conditions, PtPd/ASA showed a very promising performance. The applicability of ASA-supported noble metal catalysts in practice will be largely determined by the H 2 S concentration in the second-stage reactor and the price of noble metals. In addition, NiW/γ-Al 2 O 3 is also considered to be a promising catalyst for second-stage deep HDS. From the differences in the relative performance between model and real feed experiments, it is found that the suitability of a catalyst for deep HDS of gas oils cannot be evaluated by single-component model studies alone. The H 2 S concentration and the presence of other competing reactants largely determine the outcome of model experiments and should therefore be chosen carefully.

Investigation of MoS2 on γ-Al2O3 by HREM with atomic resolution

Abstract High resolution electron microscopy and image calculations are performed on MoS 2 γ- Al 2 O 3 based catalysts. It is shown that HREM allows atomic imaging of the MoS2 particles on the alumina carrier. Restrictions with respect to atomic imaging are found from image simulations. The orientation of the slabs with respect to the electron beam is important. Whereas atomic imaging can be obtained in 〈100〉 and 〈110〉 directions, 〈001〉 oriented crystals will be invisible by HREM because of the way the slabs are mounted on the alumina support. The slabs are basal bounded. Since essentially single slabs are observed, it is suggested that a strong interaction exists between the slabs and the alumina support. EDX analysis shows that the MoS2 particles are sulfur deficient. We suggest that sulfur deficiency is present in the first sulfur layer on the γ-Al2O3 support. In specimens sulfided below 673 K an intermediate phase is present, which is presumably an oxy-sulfide of molybdenum.

On the difference between gas- and liquid-phase hydrotreating test reactions

Abstract In industrial practice, hydrotreating of oil fractions is carried out in either a gas-phase process or a trickle flow process. We previously noticed that a remarkable difference exists between the relative activity of mixed sulfide catalysts in gas-phase and liquid-phase hydrodesulfurization (HDS) reactions. In the literature, however, no satisfying explanation with respect to the possible fundamental differences between these reactions can be found. In this paper, we report an elaborate investigation on the effect of reaction conditions, type of reactant and type of the catalyst on the occurrence of differences between the relative activity, i.e. ranking, of mixed sulfide catalysts in gas- and liquid-phase reactions. Striking differences were observed between the ranking of nitrilo-triacetic acid (NTA) and conventionally prepared NiMo catalysts in thiophene gas-phase HDS and liquid-phase dibenzothiophene (DBT) HDS. Importantly, these differences did not depend on the nature of the reacting sulfur-containing compound. This allows the generalisation that NTA-based Ni(Mo) catalysts are relatively more active in gas-phase HDS reactions, whereas conventionally prepared NiMo catalysts are relatively more active in liquid-phase HDS reactions. An analogous behaviour was observed for low- and high-temperature sulfided NiW/γ-Al 2 O 3 catalysts, of which the latter is much more active in gas-phase HDS reactions and the former is more active in liquid-phase HDS reactions. It is concluded that this so-called ‘gas–liquid-phase controversy’ is a generic phenomenon in hydrotreating reactions over metal sulfide catalysts. It was verified that mass transfer limitations do not play a role in this matter. The active sites of stacked slabs of the type II catalysts are more affected than those of type I catalysts, in which the active phase is in a more close interaction with the support. It is proposed that the phenomenon is related to a non-selective competitive adsorption of the a-polar solvent molecules on sites protruding from the catalyst surface. Apparently, the proximity of the ionic surface of the alumina support hinders the adsorption of the a-polar hydrocarbon molecules on the non-stacked systems, whereas the sulfur- and nitrogen-containing molecules are not so much affected in their adsorption behaviour on these active sites.

Testing and characterisation of Pt/ASA for deep HDS reactions

Abstract In the search for active catalysts for the conversion of refractory sulfur compounds in diesel fuel, the activity, the role of the support, and the nature of the active sites on Pt/ASA catalysts in deep HDS reactions were studied and compared to Pt/γ-Al 2 O 3 and Pt/XVUSY (stabilised Y zeolite). Pt/ASA appears to be much more active than Pt/γ-Al 2 O 3 but is initially less active than Pt/XVUSY. The latter however, showed strong deactivation after short reaction times. It is concluded that an appropriate tuning of the support acidity is crucial for this reaction. In contrast to the activity, the H 2 S sensitivity of the tested Pt based catalysts is hardly influenced by acidity of the support. Adsorption of H 2 S on these sulfur vacancies leads to strong competitive adsorption with the reacting sulfur compound. It is proposed that the stabilisation of small platinum clusters in the presence of H 2 S is an important effect of acidic supports. In addition, the strength and nature of the acidic sites on the support may affect the Pt–S bond strength of the active sites on small platinum particles. It is concluded that no sulfur-free platinum metal sites are present under the applied reaction conditions. It is therefore proposed that the conversion of 4-E,6-MDBT over Pt/ASA proceeds over sulfur vacancies on small platinum particles. The creation of sulfur vacancies on these small platinum particles may be related to their electron-deficient character on acidic supports.

The composition of the (111) and (100) surfaces of a Pt0.25-Rh0.75 single crystal

Abstract The surface compositions of Pt-Rh(100) and Pt-Rh(111) single-crystal surfaces were studied as a function of annealing temperature. It was found that both surfaces were enriched in platinum compared with the bulk platinum concentration. Relatively long annealing periods are required to reach the equilibrium concentration at low temperatures (

The temperature dependence of the surface composition of Pt−Rh alloys

AES has been used to investigate the variation of the surface composition of a polycrystalline Pt 0.62 −Rh 0.38 foil with the equilibration temperature. The results show that an ultraclean surface exhibits a Pt enrichment that increases with increasing equilibration temperature from 80 to 89 at% Pt in the 800–1000 K range. In the range 1000–1500 K an almost constant surface concentration of 89 at% Pt was found. The influence of small amounts of impurities such as P, C and Sn is discussed. The surface enrichment in Pt is in agreement with a number of literature data, but it cannot be understood with the classical models of surface segregation. The Pt enrichment in the surface and its temperature dependence are qualitatively in line with a model based on vibrational entropy contributions in addition to the enthalpy contributions used in classical surface segregation models.

Role of the support nature in chemisorption of Ni(acac)2 on the surface of silica and alumina

Abstract Nickel acetylacetonate surface species which are formed on silica and alumina supports in the result of chemisorption of Ni(acac)2 from the gas phase were characterized by FTIR, TG/DSC and chemical analysis. Nickel acetylacetonate is found to be covelently bound to the supports since one acetylacetonate ligand in the initial Ni(acac)2 molecule undergoes substitution with the oxygen atom of surface hydroxyl group. In the case of alumina, acetylacetone which is evolved reacts with coordinatively unsaturated Al3+ ions of the support surface that results in the appearance of the aluminum acetylacetonate surface species. Oxidation of acetylacetonate ligands coordinated to both nickel and aluminum atoms in the surface species was observed at 300–350°C.

Enhancing the start-up of pyrolysis gasoline hydrogenation reactors by applying tailored ex situ presulfided Ni/Al2O3 catalysts

Abstract Convenient and safe start-up procedures were assessed for Al 2 O 3 -supported nickel catalysts used in pyrolysis gasoline hydrogenation. A model feed was used consisting of styrene, 1,3-pentadiene and 1-octene. The Ni surface of the catalysts is partially sulfided ex situ to various degrees of sulfur loading. Temperature Programmed Reduction showed that the ex situ presulfided oxidic catalysts can be activated by reduction at temperatures lower than the non-sulfided oxidic catalyst ( Δ T=150 K ). This was confirmed under reaction conditions. This characteristic allows elimination of the presently applied time- and chemicals-consuming intermediate reduction and passivation procedure, resulting in more efficient catalyst activation procedures.

Mass transfer and kinetics of the three-phase hydrogenation of a dinitrile over a Raney-type nickel catalyst

The hydrogenation kinetics of a dinitrile over a Raney-type nickel catalyst was evaluated from experiments performed in a fed-batch operating autoclave at 320–355K and 2–7MPa hydrogen pressure. This complex catalytic reaction consists of two main parts: almost 100% selective hydrogenation of the dinitrile to the corresponding aminonitrile and consecutive hydrogenation to either the desired primary diamine or to pyrrolidine via ring formation. An extensive study has been made on the effects of mass transfer in the applied slurry-type reactor for this reaction. The gas–liquid mass transfer is enhanced by the presence of catalyst particles, and at typical hydrogenation conditions, kLa values up to 1.0s−1 can be reached. A Sherwood correlation for the three-phase reactor showed that important parameters in the gas–liquid mass transfer are stirrer speed and the density and viscosity of the solvent. The kinetic experiments were performed in absence of mass and heat transfer limitations. The kinetic data were modeled using two rate models based on Langmuir–Hinshelwood kinetics, assuming the reaction of dissociatively adsorbed hydrogen and nitrile compound as rate-limiting step. The first model involved competitive adsorption between hydrogen and organic compound and the second model was based on non-competitive adsorption. Both models successfully described both reaction parts. The reaction of dinitrile to aminonitrile is nearly 100% selective due to the relatively strong adsorption of the dinitriles as compared to the aminonitriles. By increasing the hydrogen partial pressure, higher yields of primary amine can be obtained. The models predict that operating in the mass-transfer regime at relatively high temperatures reduces the formation of the primary diamine.