E. van Steen

Chapter 8 – Basic studies

Publisher Summary This chapter reviews that the understanding of the fundamental processes taking place on the metal surfaces during the fischer-tropsch (FT) synthesis leads to improved catalyst design and improved macroscopic description of the FT process. Reaction parameters are selected to optimize product formation. Knowing the possible surface species on the catalyst surface during the FT synthesis and their reactivity, enables the formulation of reaction pathways. This insight leads to mechanistic descriptions for the rate of product formation in the FT synthesis. The chapter also focuses on the concept of the FT synthesis. It is a polymerization reaction, in which the monomers are being produced in situ from the gaseous reactants hydrogen and carbon monoxide. Thus, all the reaction pathways proposed in literature have three different reaction sections: generation of the chain initiator, chain growth or propagation, and chain growth termination or desorption. It discusses that huge variety of products of different chain length and different functionality is formed in FT synthesis. The actual composition/product distribution of a FT process depends on many reaction variables, such as reaction conditions, the reactor system used, as well as the catalyst formulation and physical properties of a catalyst.

Chapter 3 – Mechanistic Issues in Fischer–Tropsch Catalysis

Computational studies have recently generated important information regarding reaction intermediates and activation barriers of elementary reaction steps that are part of the Fischer–Tropsch synthesis. We use these results to analyze various mechanistic options that have been proposed for the Fischer–Tropsch synthesis. The computational results do not support the Pichler–Schulz chain-growth mechanism, which postulates chain growth by CO insertion. Rather, the results are in agreement with the Sachtler–Biloen mechanism, which postulates chain growth via adsorbed “C1” species; furthermore, the Gaube chain-growth mechanism, which closely resembles that proposed by Maitlis, is found to be preferred over the initially assumed Brady–Pettit mechanism. The various elementary steps are discussed, and the values that their relative rates must assume for successful Fischer–Tropsch chain growth are outlined. Within the Sachtler–Biloen kinetics scheme, a high chain-growth probability is obtained when chain termination is rate limiting. Consequently, CO dissociation has to be facile. The “C1” species that is incorporated into the growing chain appears to be “CH” or “CH2”; thus, these species must be present in high surface concentrations. Bronsted–Evans–Polanyi relationships are used to link activation energies to surface reactivity. The structure sensitivity of the elementary reaction steps, specifically, initiation, chain growth, and termination, is analyzed. On the basis of these considerations, one can understand why particular metals are suitable Fischer–Tropsch catalysts.

Use of TPR/TPO for characterization of supported cobalt catalysts

The extent of reduction of supported cobalt catalysts is difficult to determine using TPR due to the unknown stoichiometry of reduction and due to the dynamic nature of the measurement. A method is described where, by using a combined TPR/TPO technique, it is possible to determine the extent of cobalt reduction and obtain an estimate of the extent of cobalt-support species formation. The results showed that the extent of metal reduction following hydrogen reduction at 500°C is affected considerably by the type of metal carrier. In particular, the extent of metal reduction decreased with increasing aluminium content of the support material. Decreasing extents of metal reduction could be correlated with an increase in the temperature required for reduction of the nitrate ion during TPR. Increasing the time and temperature of hydrogen reduction results in increased extents of metal reduction.

The use of a jet loop reactor to study the effect of crystal size and the co-feeding of olefins and water on the conversion of methanol over HZSM-5

Abstract The conversion of methanol into olefins has been carried out in a gradientless quartz jet-loop reactor, thus ensuring the absence of mass and heat transfer effects and the absence of wall reactions. The catalysts used were three samples of H-ZSM-5, each with a Si:Al ratio of about 100 and with a different crystal size. A thermodynamic analysis of the methanol (MeOH)–dimethylether (DME)–[CH2]n system showed that in the jet loop reactor the MeOH–DME reaction was far removed from equilibrium, and this increased with increasing temperature. No fundamental difference was observed in the selectivities of the hydrocarbon fraction at the same conversion of oxygenates between the jet loop reactor and published data using fluidized bed and fixed bed reactors. Co-fed water reduced the conversion, probably by reducing the number of available sites due to preferential adsorption. In the jet loop reactor the alkylation of olefins with oxygenates appeared to occur to a lesser extent than that which is usually observed in a fixed bed reactor. Moreover, significantly different behaviour was observed in the case of each crystal size, with the least amount of DME forming when the largest crystals were used. It was possible to explain these differences in terms of the diffusional resistance experienced by DME inside the crystals. Pseudo rate constants were derived using a mechanistic model typical of the MTO reaction. Methanol and DME were both involved to a similar extent in the alkylation of the [CH2]n species. The model predicted that the first C–C bond formation was the slow step in the reaction sequence. Alkylation was faster than the reversible MeOH to DME reaction.

Re‐dispersion of Cobalt on a Model Fischer–Tropsch Catalyst During Reduction–Oxidation–Reduction Cycles

Reduction–oxidation–reduction treatment is employed to regenerate cobalt Fischer–Tropsch synthesis catalysts. Reduction–oxidation–reduction cycles on a model supported‐cobalt Fischer–Tropsch catalyst showed that hollow‐sphere formation occurs during the oxidation step, followed by the formation of a catalyst with an improved metal dispersion. It is deduced that the metal particles consist of both hexagonal close packed Co and face‐centered cubic Co. High‐pressure oxidation led to smaller cobalt oxide crystallites, resulting in a more facile reduction and an increase in metal surface area. Hence, oxidative regeneration is an attractive method to improve the dispersion of cobalt metal on a sintered deactivated catalyst. However, metallic cobalt crystallites may become too small after regeneration, and the intrinsic activity per unit surface area in the Fischer–Tropsch synthesis may drop.

The nature of the oxidation states of gold on ZnO

The interaction between gold in the 0, i, ii and iii oxidation states and the zinc-terminated ZnO(0001) surface is studied via the QM/MM electronic embedding method using density functional theory. The surface sites considered are the vacant zinc interstitial surface site (VZISS) and the bulk-terminated island site (BTIS). We find that on the VZISS, only Au(0) and Au(i) are stable oxidation states. However, all clusters of i to iii oxidation states are stable as substitutionals for Zn2+ in the bulk terminated island site. Au(OH)(x) complexes (x= 1-3) can adsorb exothermically onto the VZISS, indicating that higher oxidation states of gold can be stabilised at this site in the presence of hydroxyl groups. CO is used as a probe molecule to study the reactivity of Au in different oxidation states in VZISS and BTIS. In all cases, we find that the strongest binding of CO is to surface Au(i). Furthermore, CO binding onto Au(0) is stronger when the gold atom is adsorbed onto the VZISS compared to CO binding onto a gas phase neutral gold atom. These results indicate that the nature of the oxidation states of Au on ZnO(0001) will depend on the type of adsorption site. The role of ZnO in Au/ZnO catalysts is not, therefore, merely to disperse gold atoms/particles, but to also modify their electronic properties.

Some evidence refuting the alkenyl mechanism for chain growth in iron-based Fischer–Tropsch synthesis

Abstract Recently, Maitlis et al. [J. Catal. 167 (1997) 172] proposed an alternative reaction pathway for chain growth in the Fischer–Tropsch synthesis. In this mechanism, chain growth is assumed to occur by methylene insertion into a metal–vinyl bond, forming an allyl species that will subsequently isomerise to a vinyl species. Organo-metallic allyl complexes, Fe{[η 5 -C 5 H 5 ](CO) 2 CH 2 CHCH 2 } and Fe{[η 5 -C 5 (CH 3 ) 5 ](CO) 2 CH 2 CHCH 2 } were synthesised. Under thermal treatment, the decomposition of these complexes was observed, instead of the isomerisation. In a hydrogen atmosphere, the reduction of the iron–carbon bonds and the hydrogenation yielding iron–alkyl species was observed. This clearly shows that the proposed vinyl–allyl isomerisation is unlikely to occur in mono-nuclear iron complexes. Hence, it might be expected that the reaction mechanism proposed by Maitlis et al. [J. Catal. 167 (1997) 172] is unlikely to be the main route for chain growth in the Fischer–Tropsch synthesis.

Synthesis of 1,2,3,4‐tetrahydrocarbazole over zeolite catalysts

Abstract1,2,3,4‐tetrahydrocarbazole (CAR) has been synthesized over H‐ZSM‐12, H‐beta, H‐mordenite, H‐Y, H‐ZSM‐22, H‐EU‐1, H‐ZSM‐5 and acetic acid by Fishers method using phenylhydrazine and cyclohexanone. H‐Y is more active than the other zeolites studied for the synthesis of CAR. The influence of different parameters such as the duration of the run, catalyst concentration, reaction temperature and molar ratios of the reactants in the synthesis of CAR are also studied. A number of arylhydrazines such as o-tolylhydrazine, p-tolylhydrazine and 1,1‐diphenylhydrazine and cyclic ketones such as cyclopentanone, cyclohexanone and 3‐methylcyclohexanone have also been taken to study the effect of substitution in phenylhydrazine and the size of the cyclic ketones in the reaction. The yields of indoles correlated with the degree of substitution of the reactants and products, and the limitations imposed on diffusion through zeolite pores.

Metal Support Interactions in Co 3 O 4 /Al 2 O 3 Catalysts Prepared from w/o Microemulsions

To obtain nano-sized metal and metal salt crystallites with a narrow size distribution synthesis methods utilizing water in oil (w/o) microemulsions, i.e. reverse micelles, have been widely applied and reported in literature. In this study we show the effect of support addition at different stages of the reverse micelle based preparation of cobalt oxide on alumina model catalysts. All catalysts were characterized with X-ray powder diffraction and Raman spectroscopy indicating the presence of Co3O4 on the Al2O3 support. Studies of the reduction behaviour and X-ray photoelectron spectroscopy however revealed the presence of difficult to reduce cobalt aluminate species in the samples where the support was added during or shortly after the precipitation step in the synthesis process. It can therefore be assumed that if the alumina support is added to the reverse micelle solution unprecipitated Co2+ ions and partially dissolved Al3+ combine and form cobalt aluminates. In the preparations where the solid cobalt precipitates are recovered from the microemulsion and then supported on the carrier, no metal-aluminate formation could be observed. This study therefore gives important information how metal-support interaction can be affected during catalyst preparation using reverse micelles.Graphical Abstract

Internal and external transport effects during the oxidative reforming of methane on a commercial steam reforming catalyst

Abstract In order to establish an isothermal catalyst bed it was necessary to dilute both the catalyst particles (1:10 in α-Al 2 O 3 ) and the feed mixture (2% reactants in He). Under these conditions, a linear velocity of about 6 cm/s was sufficient to remove the effect of film diffusion at 600°C. Experiments with particle sizes in the range 125–710 μm revealed that internal diffusion did not play a role in methane conversion at a fixed space velocity. No change in the apparent activation energy was found over the range 575–625°C which confirmed the absence of diffusion limitations; at 650°C, however, a slight decline in E a was observed which may be indicative of the presence of concentration gradients. Correlations taken from the literature and which are normally used to test the absence of internal and external temperature and concentration gradients, were applied to our operating conditions and confirmed our conclusions.