Martin Lucas

Supported gold nanoparticles: in-depth catalyst characterization and application in hydrogenation and oxidation reactions

Silica, titania, zirconia and alumina supported gold particles of 1–6 nm size, prepared by various synthetic routes (sol–gel technique, deposition–precipitation, metal organic-chemical vapor deposition, impregnation, dip-coating) were employed in the selective hydrogenation of acrolein, crotonaldehyde and 1,3-butadiene and in the low-temperature oxidation of carbon monoxide. In-depth characterization of their structural and electronic properties by transmission electron microscopy (TEM), electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) was aimed at disclosing the nature of the active sites controlling the hydrogenation and oxidation reactions. The structural characteristics such as mean particle size, size distribution and dispersion depend both on the synthetic method employed and the nature of the support. For extremely small gold particles on titania and zirconia (1.1 and 1.4 nm mean size), conduction electron spin resonance of the metal and paramagnetic F-centers (trapped electrons in oxygen vacancies) of the support were observed. The marked structure-sensitivity observed for hydrogenation properties with decreasing particle size may be attributed to structural and electronic properties due to the quantum-size effect of sufficiently small gold particles. Furthermore, the adaptability of gold particles in coatings is demonstrated for a microchannel reactor.

Selective hydrogenolysis of methyl and ethyl acetate in the gas phase on copper and supported group viii metal catalysts

Abstract The selective hydrogenolysis of methyl and ethyl acetate to ethanol over different copper-based and supported Group VIII metal (Pd, Rh, Pt, Co, Ni) catalysts has been studied in the gas phase at 448–623 K and 0.1–6.0 MPa. Measurements of activity and selectivity show that the copper-based catalysts CuO/MgO-SiO 2 , CuO/ZnO/MnO/Al 2 O 3 , and CuO/ZnO/Fe 2 O 3 in particular exhibit very high selectivities for ethanol at nearly complete acetate conversions under moderate reaction conditions. In the case of copper-catalyzed methyl acetate hydrogenolysis kinetic measurements indicated a complex reaction network consisting of consecutive-competitive reactions due to transesterification of methyl acetate to give ethyl acetate, followed by its subsequent hydrogenolysis to ethanol. Therefore, the kinetics of selective hydrogenolysis of ethyl acetate have been studied separately. Kinetic parameters of a power rate law were determined by fitting to conversion versus space time data and made it also possible to describe satisfactorily the complex reaction network. In contrast, supported monometallic catalysts containing Pd, Rh or Ni were found to be ineffective in the hydrogenation of the acyl group due to multiple splitting of C-O and C-C bonds forming light hydrocarbons, acetic acid and carbon oxides. Addition of Zn to Pd/Al 2 O 3 provides a major change towards higher ethanol selectivity. Furthermore, the catalytic properties of mono- and multimetallic Co/TiO 2 catalysts have been investigated. The use of an electropositive metal, such as Fe, for the preparation of a Co-Rh-Fe/TiO 2 catalyst promotes the formation of ethanol from ethyl acetate with substantially higher conversions.

Gold Catalysts for the Partial Hydrogenation of Acrolein

The manufacture of unsaturated alcohols through selective hydrogenation of α,β-unsaturated aldehydes continues to be a topical issue. The objective of this study is to investigate the use of supported gold catalysts for the partial hydrogenation of acrolein to allyl alcohol. So far, in catalyst research or even chemical engineering gold has not attracted much attention as a catalyst mainly because of its chemical inertness. The intrinsic inertness of gold, however, can be influenced when the metal is applied with a high dispersity to a suitable support.

Ruthenium‐Catalyzed Selective Hydrogenation of Benzene to Cyclohexene in the Presence of an Ionic Liquid

The hydrogenation of benzene in organic phase leads rapidly to cyclohexane. A very simple catalyst system comprising only supported ruthenium in water with the addition of the ionic liquid 1 (R=Me) in the ppm range catalyzes the extremely difficult selective hydrogenation of benzene to cyclohexene. It is not necessary to add large amounts of salt (ZnSO₄) or other metals, which is otherwise done to control selectivity.

Hydrogenolysis of cellulose to valuable chemicals over activated carbon supported mono- and bimetallic nickel/tungsten catalysts

The hydrogenolysis of cellulose was systematically investigated at 488 K and under 65 bar H2 in the absence of a catalyst and over six different catalytic systems containing nickel and/or tungsten on activated carbon (AC) in order to understand the role of individual active components (AC, W/AC, Ni/AC, a physical mixture of Ni/AC + W/AC, and two differently prepared Ni/W/AC catalysts) with respect to the product distribution wherein polyols (e.g. ethylene glycol (EG), propylene glycol, and sorbitol) are highly valuable chemicals. Without a catalyst and when using only AC, a hydrochar, due to hydrothermal carbonization of cellulose, was obtained. Although the catalyst W/AC was effective for the degradation of cellulose (high conversion of 90%) and facilitates C–C bond cleavage, selective production of any product was not possible, and the carbon efficiency (CEL) is the lowest (9.1%). Also, with highly dispersed Ni on AC the polyol yield was only 5.3%. The desired behavior showed Ni/W/AC provided its preparation occurs by a two-step incipient wetness (IW) technique. Starting with a remarkably high cellulose/catalyst ratio of 10, a cellulose conversion of 88.4%, CEL of 78.4% and EG yield of 43.7% were achieved (overall polyol yield = 62.1%). Drastically lower yields towards EG by an order of magnitude and decreased CEL were obtained by a co-impregnated Ni/W/AC catalyst and the Ni/AC + W/AC mixture. By the detailed analysis via XRD, TPR and CO chemisorption, it can be concluded that in the Ni/W/AC catalyst, after the first IW step of the activated carbon with ammonium metatungstate hydrate and the following reduction in H2 up to 1128 K, metallic tungsten was formed. This leads, in combination with the hydrogenation properties of nickel introduced in the second IW step, to a virgin bimetallic catalyst, i.e. before the hydrogenolysis starts, in which both components must be metallic. This is a prerequisite for high polyol production. Finally, varying the AC type, high space–time-yields up to 2.5 g polyols (gcatalyst h)−1 were obtained. A slight deactivation after two runs followed by a strong decrease of polyol yield in the next two runs was observed. Leaching and structural changes on the catalyst surface (formation of NiWO4) are mainly responsible for deactivation.

From microcrystalline cellulose to hard- and softwood-based feedstocks: their hydrogenolysis to polyols over a highly efficient ruthenium–tungsten catalyst

The utilization of cellulose and its integration in a biorefinery concept is essential even in the near future due to the growing global shortage of crude oil. Here, we report the catalyzed one-pot hydrogenolysis of cellulosic materials to valuable bio-derived molecules, especially polyols (e.g. ethylene glycol). We demonstrate how a very promising bifunctional catalyst, Ru/W/AC, converted not only 100% of microcrystalline cellulose to polyols in repeated experiments with a maximum yield of 84% and an ethylene glycol productivity of 3.7 g (gcatalyst h)−1, but also pine-, birch-, and eucalyptus-derived materials. Moreover, we systematically investigated the problem of catalyst stability with time by studying the changes in both the catalyst structure and the liquid phase, which have often been overlooked when biomass is converted to fuels and chemicals. Control of the active sites for the conversion of cellulosic feedstocks coupled with reaction engineering and strategies to prevent catalyst deactivation, is a prerequisite to understanding how high yields of platform chemicals can be achieved.

Parallel synthesis and fast screening of heterogeneous catalysts

We are presenting an effective method to prepare and test heterogeneous catalysts much faster than by the conventional way. A catalyst array was prepared via an incipient wetness method by combination of different amounts of Pt, Zr, and V on Al2O3 by means of an automatic liquid handler. For catalytic testing for methane oxidation a ceramic monolith reactor module, the channels of which contain the different catalyst compositions, was developed in which up to 250 catalyst compositions can be prepared and tested in parallel. Gas samples from each channel of the monolith were analysed sequentially by a mass spectrometer by moving the QMS inlet capillary into the channels using a three-dimensional positioning system which works at high temperatures. By comparison of the testing results with experiments carried out in flow reactors it is shown that the monolithic reactor is an efficient tool for fast screening of heterogeneous catalysts.

Simple selective hydrogenation of benzene to cyclohexene in the presence of sodium dicyanamide

A new catalyst system was used in the liquid phase hydrogenation of benzene to cyclohexene containing only an aqueous solution of Ru/La2O3 and a very small quantity of sodium dicyanamide (NaDCA). This additive considerably improves the catalyst performance compared to DCA based ionic liquids. The effects of the amount of NaDCA and metal loading of the catalyst were investigated and a high initial selectivity to cyclohexene of 70% was reached.

High throughput screening in monolith reactors for total oxidation reactions

We present here a powerful tool for high throughput screening of heterogeneous catalysts, successfully developed in our laboratory with the application focus on common monoliths as a special type of microreactors. It could be shown that it is possible to prepare in a reproducible manner catalytically active coatings on the wall of single channels of the monoliths by a channel-by-channel procedure enabling the application of these multichannel reactors, coupled with mass spectrometry or gas chromatography, for the fast screening of heterogeneous catalysts. Because time and spatially resolved sampling is needed and complex gaseous mixtures have to be analyzed, a special 3D positioning system, which allows measurements at high temperatures, is needed as the central element of the equipment. The high efficiency and reliability of the channel-by-channel preparation as well as the developed screening method is demonstrated for the total oxidation of hydrocarbons and carbon monoxide in the presence of further components like O2, H2O, CO2, NO, SO2 and inert gas over precious metal catalysts.

Implementation of the multi-channel monolith reactor in an optimisation procedure for heterogeneous oxidation catalysts based on genetic algorithms.

A multi-criteria optimisation procedure based on genetic algorithms is carried out in search of advanced heterogeneous catalysts for total oxidation. Simple but flexible software routines have been created to be applied within a search space of more then 150,000 individuals. The general catalyst design includes mono-, bi- and trimetallic compositions assembled out of 49 different metals and depleted on an Al2O3 support in up to nine amount levels. As an efficient tool for high-throughput screening and perfectly matched to the requirements of heterogeneous gas phase catalysis - especially for applications technically run in honeycomb structures - the multi-channel monolith reactor is implemented to evaluate the catalyst performances. Out of a multi-component feed-gas, the conversion rates of carbon monoxide (CO) and a model hydrocarbon (HC) are monitored in parallel. In combination with further restrictions to preparation and pre-treatment a primary screening can be conducted, promising to provide results close to technically applied catalysts. Presented are the resulting performances of the optimisation process for the first catalyst generations and the prospect of its auto-adaptation to specified optimisation goals.