Stuart Leon Soled

Fischer-Tropsch synthesis on cobalt and ruthenium. Metal dispersion and support effects on reaction rate and selectivity

Metal dispersion and support effects on Fischer-Tropsch synthesis rate and selectivity were studied at conditions that favor the information of C5+ hydrocarbons (> 80% selectivity). On Ru, these effects are minor for the supports (SiO2, Al2O3, TiO2) and the dispersion range (0.0009–0.60) tested. Site-time yields are similar (1.25–1.95 × 10−2 s−1) on all Ru catalysts (476 K, 560 kPa, H2/CO = 2.1). On Co, hydrocarbon synthesis rates are also proportional to metal dispersion (0.0045–0.095) and independent of the metal oxide support (SiO2, Al2O3, TiO2, and ZrO2-modified SiO2 and TiO2). Site-time yields (1.6–3.0 × 10−2 s−1) are independent of Co dispersion and support (473 K, 2000 kPa, H2/CO = 2.1). Dispersion and support influence C5+ selectivity slightly on both Co and Ru catalysts; these changes reflect transport-enhanced secondary reactions and not modifications of intrinsic chain growth kinetics. Specifically, transport restrictions imposed by the physical structure of the support and by a high site density within catalyst pellets increase the residence time and the readsorption probability of reactive a-olefins and lead to higher C5+ yields and more paraffinic products.

Selectivity Control and Catalyst Design in the Fischer-Tropsch Synthesis: Sites, Pellets, and Reactors

Publisher Summary This chapter focuses on selectivity control and catalyst design in the Fischer-Tropsch (FT) synthesis. Chain growth during the FT synthesis is controlled by surface polymerization kinetics that place severe restrictions on our ability to alter the resulting carbon number distribution. Intrinsic chain growth kinetics are not influenced strongly by the identity of the support or by the size of the metal crystallites in supported Co and Ru catalysts. Transport-limited reactant arival and product removal, however, depend on support and metal site density and affect the relative rates of primary and secondary reactions and the FT synthesis selectivity. Diffusion-limited removal of products from catalyst pellets leads to enhanced readsorption and chain initiation by reactive α-olefins. Diffusive and convective transport processes introduce flexibility in the design of catalyst pellets and in the control of FT synthesis selectivity. The model is proposed in the chapter that describes the catalytic behavior of more complex Fe based materials, where several chain termination steps and highly non-uniform and dynamic surfaces introduce additional details into the models required to describe FT synthesis selectivity models.

Selective isomerization of alkanes on supported tungsten oxide acids

Abstract Tungsten oxide species form strong acid sites on ZrO 2 supports. After calcination at 1000–1100 K and promotion with Pt, these solids catalyze C 7+ alkane isomerization at 400–500 K with much higher selectivity than sulfated oxides or zeolitic acids at similar turnover rates. Alkane isomerization proceeds via biomolecular reactions involving hydrogen transfer from alkanes or H 2 , which cause the desorption of isomeric carbocations before β-scission occurs. On Pt/SO x -ZrO 2 , carbocation desorption is slow, leading to long surface residence times and extensive cracking. On Pt/WO x -ZrO 2 , carbocation desorption is rapid and surface isomerization steps limit n-heptane isomerization turnover rates. Saturation coverage by WO x surface species inhibits ZrO 2 sintering and its tetragonal to monoclinic structural transformation. High isomerization turnover rates appear to require the presence of WO x clusters on ZrO 2 surfaces. X-ray absorption at the W-L 1 and W-L III edges suggests the predominant presence of distorted octahedral species, even after dehydration at 673 K, in all WO x -ZrO 2 samples calcined at 1073 K. Tetrahedral species, which lead to a strong pre-edge feature in the W-L 1 absorption edge, are not detectable in these samples. UV-visible spectra suggest an increase in WO x domain size with increasing loading. These distorted octahedral WO x domains on ZrO 2 differ markedly in structure, reduction rates, and alkane isomerization turnover rates and selectivities from tetrahedral WO x species on Al 2 O 3 .

Bifunctional pathways in catalysis by solid acids and bases

Abstract Chemical reactions catalyzed by solid acids and bases often require that reactants, intermediates, or activated complexes interact with several surface functions. Concerted and sequential bifunctional pathways also occur in homogeneous and enzyme catalysis. Hydrogenation and dehydration reactions require acid-base site pairs of intermediate strength, because such sites can form, stabilize, and discard adsorbed intermediates during a catalytic turnover. Deuterium exchange and H-H dissociation reactions also occur on acid-base pairs present in single-component or binary oxides and in supported oxide clusters. Hydrogenation of aromatic acids, dehydration of alkanols and methanolamine, condensation of alcohols, and deuterium exchange provide specific examples of bifunctional acid-base catalysis. Dehydration and dehydrogenation reactions of alkanols, widely used as probes of acid or base sites, probe instead the density and chemical properties of acid-base site pairs. Concerted bifunctional pathways require that sites co-exist within molecular distances. On surfaces, the inappropriate location of these sites can prevent concerted interactions, but rapid transfer of intermediates via surface or gas phase diffusion leads to kinetic coupling between distant sites and to sequential bifunctional pathways. These bifunctional sequences overcome proximity requirements by equilibration of adsorbed species throughout surface regions containing several types of sites. Diffusion of alkenes in the gas phase couples dehydrogenation and acid sites during n-alkane isomerization on bifunctional tungsten carbides modified by chemisorbed oxygen. These bifunctional surfaces form Bronsted acid sites by surface migration of H adatoms from WC to WOx sites. Acid (or base) sites and H2 dissociation sites on surfaces interact via surface diffusion of H adatoms. This leads to bifunctional alkane and alkanol reactions via kinetic coupling of C-H or O-H bond activation and hydrogen adsorption-desorption steps. Propane dehydrogenation on H-ZSM5 modified by exchanged cations, n-heptane isomerization on ZrO2 doped with WOx and Pt, and alcohol condensation on Cu-promoted Mg5CeOx oxides illustrate the role of kinetic coupling mediated by migration of hydrogen adatoms. In each example, metal clusters or isolated cations increase the rate of acid or base catalysis by providing a ‘porthole’ for hydrogen adsorption and desorption.

Solid acid catalysts based on supported tungsten oxides

Supported WOx clusters are active and stable catalysts for isomerization, dehydration, and cracking reactions. Brønsted acid sites form on WOx clusters when a lower valent element replaces W6+ or when W6+ centers reduce slightly during catalytic reactions. WOx clusters of intermediate size provide the balance between reducibility and accessibility required to maximize the number of surface H+ species in WOx–ZrO2, zirconium tungstate, and oxygen-modified WC catalysts. H2 is involved in the generation and maintenance of Brønsted acid sites during catalytic reactions on WOx clusters.

Comparison of the acidities of WO3/Al2O3 and ultrastable faujasite catalysts

The acidity of WO3 on γ-alumina is compared with that of ultrastable faujasite using both base adsorption techniques and model compound conversion studies. The addition of WO3 to γ-alumina introduces Bronsted acidity, and the density of Bronsted sites is increased by high-temperature calcination. The acid sites displayed by the supported tungsten oxide catalyst are considerably weaker than those found in ultrastable faujasite.

Preparation of bulk and supported heteropolyacid salts

Abstract We describe the preparation of bulk and supported cesium and ammonium heteropolyacid and acid salts of 12-tungstophosphoric acid. Novel synthetic schemes to prepare supported forms of insoluble cesium- and ammonium-acid salts are discussed, and their behavior in model compound isomerization and alkylation reactions is described. In particular, we discuss the egg-white morphology associated with Cs-acid salts supported on silica extrudates prepared by a novel in situ-reaction/precipitation route. We also describe hydrothermal techniques developed for directly preparing the ammonium-acid salts of 12-tungstophosphoric acid and the extension of that technique to other heteropolyacid salts.

Selective synthesis of α-olefins on Fe-Zn Fischer-Tropsch catalysts

Fe/Zn oxides promoted with K and Cu selectively produce α-olefins at typical Fischer-Tropsch synthesis conditions (2/1 H2/CO, 1 MPa, and 270°C). The simultaneous presence of K and Cu introduces a synergistic activity enhancement while maintaining the high olefin selectivity obtained by alkali promotion. Structural and morphological differences in Fe-Zn oxides prepared from ammonium glycolate complexes or precipitated from nitrate solutions have only a small influence on catalytic properties. Catalyst behavior is strongly influenced by synergistic promoter effects (Cu, K) and by the controlled in situ conversion of iron oxide precursors to carbides.

Activity and selectivity control in iron catalyzed Fischer-Tropsch synthesis

We present results of a catalyst structure-function study that supports a CO hydrogenation model with α-olefins formed as the principal primary products and n-paraffins formed during secondary hydrogenation reactions. The interplay of catalyst composition and reaction environment controls the extent of secondary reactions. Catalysts that contain mainly oxidic phases or carbides with large concentrations of excess “matrix carbon” favor secondary reactions. The relative concentrations of oxide and carbide phases depends on the ease of reduction of the catalyst, which can be changed by cation substitutions. For example, cobalt substitution into Fe3O4 lowers the reduction temperature by 20 ° C. Excess matrix carbon has been intentionally introduced (by treatment in high temperature H2/CO) into model iron carbide catalysts produced by laser synthesis. Increased paraffin selectivity as matrix carbon is introduced suggests the influence of the diffusion constraints on product selectivity. Alkali promotion will affect secondary hydrogenation pathways. We illustrate how catalysts with low levels of alkali become increasingly more selective to paraffins at high conversions (and high effective H2/CO ratios).Reaction environment also controls catalyst composition and selectivity. Mossbauer spectroscopy on spent catalysts suggests that oxide/carbide phase formation in iron catalysts are sensitive to reactor configuration (extent of backmixing). In integral fixed bed reactors, catalysts partition into carbide phases in the front of the bed but show increasing amounts of oxide near the exit, whereas the catalysts in the stirred tank reactor remain all carbides. Product selectivities reflect the phase differences.Other examples illustrating secondary hydrogenation phenomena will be presented.

89 “Nebula”: A hydroprocessing catalyst with breakthrough activity

Abstract In this paper a new catalyst technology called NEBULA is presented. It has been developed by ExxonMobil, Akzo Nobel and Nippon Ketjen. NEBULA is based on a novel copound and it is several times more active than the hydroprocessing catalysts used in todays industrial units. The new catalyst has a much higher activity for desulfurization, dentrogenation and hydrogenation than the conventional CoMo and NiMo on alumina catalysts. The NEBULA catalyst currently in use in several commercial installations represents the biggest step forward in hydroprocessing over the last years. In this paper we will focus on the development of the NEBULA technology and on the applications for which it is appropriate.