Enrique Iglesia

Design, synthesis, and use of cobalt-based Fischer-Tropsch synthesis catalysts

Catalyst productivity and selectivity to C5+ hydrocarbons are critical design criteria in the choice of Fischer-Tropsch synthesis (FTS) catalysts and reactors. Cobalt-based catalysts appear to provide the best compromise between performance and cost for the synthesis of hydrocarbons from CO/H2 mixtures. Optimum catalysts with high cobalt concentration and site density can be prepared by controlled reduction of nitrate precursors introduced via melt or aqueous impregnation methods. FTS turnover rates are independent of Co dispersion and support identity over the accessible dispersion range (0.01–0.12) at typical FTS conditions. At low reactant pressures or conversions, water increases FTS reaction rates and the selectivity to olefins and to C5+ hydrocarbons. These water effects depend on the identity of the support and lead to support effects on turnover rates at low CO conversions. Turnover rates increase when small amounts of Ru (Ru/Co<0.008 at.) are added to Co catalysts. C5+ selectivity increases with increasing Co site density because diffusion-enhanced readsorption of α-olefins reverses, β-hydrogen abstraction steps and inhibits chain termination. Severe diffusional restrictions, however, can also deplete CO within catalyst pellets and decrease chain growth probabilities. Therefore, optimum C5+ selectivities are obtained on catalysts with moderate diffusional restrictions. Diffusional constraints depend on pellet size and porosity and on the density and radial location of Co sites within catalyst pellets. Slurry bubble column reactors and the use of eggshell catalyst pellets in packed-bed reactors introduce design flexibility by decoupling the characteristic diffusion distance in catalyst pellets from pressure drop and other reactor constraints.

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.

Transport-enhanced α-olefin readsorption pathways in Ru-catalyzed hydrocarbon synthesis

Residence time and. cofeed studies show that olefins and paraffins are primary products in Rucatalyzed hydrocarbon synthesis. Olefins readsorb and initiate surface chains that are indistinguishable from those formed directly from CO/H2 and that continue to grow and ultimately desorb as higher molecular weight hydrocarbons. Transport-enhanced α-olefin readsorption leads to an increase in chain growth probability (a) and in paraffin content with increasing pore and bed residence time. Deviations from conventional (Flory) polymerization kinetics and the increasing paraffinic content of higher hydrocarbons are quantitatively described by transport effects on the residence time of intermediate olefins, without requiring the presence of several types of chain growth sites. Our transport-reaction model combines a description of diffusive and convective transport with a mechanistic kinetic model of olefin readsorption and of CO hydrogenation and chain growth. It quantitatively describes carbon number, site density, pellet size, and space velocity effects on hydrocarbon synthesis rate and product distribution. The model is consistent with the experimentally observed maximum C5+ selectivities at intermediate values of site density and pellet size. These intermediate values permit extensive readsorption of α-olefins without significant CO arrival transport limitations.

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.

Formic Acid Dehydrogenation on Au-Based Catalysts at Near-Ambient Temperatures

Selective HCOOH decomposition to H(2)/CO(2) on Au: Au species catalyze HCOOH dehydrogenation at higher rates than on Pt, previously considered the most active metal. Dehydrogenation occurs through formate decomposition limited by H(2) desorption on Au species undetectable by TEM. CO did not form (<10 ppm), making products suitable for low-temperature fuel cells.

Effects of Zn, Cu, and K Promoters on the Structure and on the Reduction, Carburization, and Catalytic Behavior of Iron-Based Fischer–Tropsch Synthesis Catalysts

Zn, K, and Cu effects on the structure and surface area and on the reduction, carburization, and catalytic behavior of Fe–Zn and Fe oxides used as precursors to Fischer–Tropsch synthesis (FTS) catalysts, were examined using X-ray diffraction, kinetic studies of their reactions with H2 or CO, and FTS reaction rate measurements. Fe2O3 precursors initially reduce to Fe3O4 and then to metallic Fe (in H2) or to a mixture of Fe2.5C and Fe3C (in CO). Zn, present as ZnFe2O4, increases the surface area of precipitated oxide precursors by inhibiting sintering during thermal treatment and during activation in H2/CO reactant mixtures, leading to higher FTS rates than on ZnO-free precursors. ZnFe2O4 species do not reduce to active FTS structures, but lead instead to the loss of active components; as a result, maximum FTS rates are achieved at intermediate Zn/Fe atomic ratios. Cu increases the rate of Fe2O3 reduction to Fe3O4 by providing H2 dissociation sites. Potassium increases CO activation rates and increases the rate of carburization of Fe3O4. In this manner, Cu and K promote the nucleation of oxygen-deficient FeOx species involved as intermediate inorganic structures in reduction and carburization of Fe2O3 and decrease the ultimate size of the Fe oxide and carbide structures formed during activation in synthesis gas. As a result, Cu and K increase FTS rates on catalysts formed from Fe–Zn oxide precursors. Cu increases CH4 and the paraffin content in FTS products, but the additional presence of K inhibits these effects. Potassium titrates residual acid and hydrogenation sites and increases the olefin content and molecular weight of FTS products. K increases the rate of secondary water–gas shift reactions, while Cu increases the relative rate of oxygen removal as CO2 instead of water after CO is dissociated in FTS elementary steps. Through these two different mechanisms, K and Cu both increase CO2 selectivities during FTS reactions on catalysts based on Fe–Zn oxide precursors.

Chemisorption of CO and Mechanism of CO Oxidation on Supported Platinum Nanoclusters

Kinetic, isotopic, and infrared studies on well-defined dispersed Pt clusters are combined here with first-principle theoretical methods on model cluster surfaces to probe the mechanism and structural requirements for CO oxidation catalysis at conditions typical of its industrial practice. CO oxidation turnover rates and the dynamics and thermodynamics of adsorption-desorption processes on cluster surfaces saturated with chemisorbed CO were measured on 1-20 nm Pt clusters under conditions of strict kinetic control. Turnover rates are proportional to O(2) pressure and inversely proportional to CO pressure, consistent with kinetically relevant irreversible O(2) activation steps on vacant sites present within saturated CO monolayers. These conclusions are consistent with the lack of isotopic scrambling in C(16)O-(18)O(2)-(16)O(2) reactions, and with infrared bands for chemisorbed CO that did not change within a CO pressure range that strongly influenced CO oxidation turnover rates. Density functional theory estimates of rate and equilibrium constants show that the kinetically relevant O(2) activation steps involve direct O(2)* (or O(2)) reactions with CO* to form reactive O*-O-C*=O intermediates that decompose to form CO(2) and chemisorbed O*, instead of unassisted activation steps involving molecular adsorption and subsequent dissociation of O(2). These CO-assisted O(2) dissociation pathways avoid the higher barriers imposed by the spin-forbidden transitions required for unassisted O(2) dissociation on surfaces saturated with chemisorbed CO. Measured rate parameters for CO oxidation were independent of Pt cluster size; these parameters depend on the ratio of rate constants for O(2) reactions with CO* and CO adsorption equilibrium constants, which reflect the respective activation barriers and reaction enthalpies for these two steps. Infrared spectra during isotopic displacement and thermal desorption with (12)CO-(13)CO mixtures showed that the binding, dynamics, and thermodynamics of CO chemisorbed at saturation coverages do not depend on Pt cluster size in a range that strongly affects the coordination of Pt atoms exposed at cluster surfaces. These data and their theoretical and mechanistic interpretations indicate that the remarkable structure insensitivity observed for CO oxidation reactions reflects average CO binding properties that are essentially independent of cluster size. Theoretical estimates of rate and equilibrium constants for surface reactions and CO adsorption show that both parameters increase as the coordination of exposed Pt atoms decreases in Pt(201) cluster surfaces; such compensation dampens but does not eliminate coordination and cluster size effects on measured rate constants. The structural features and intrinsic non-uniformity of cluster surfaces weaken when CO forms saturated monolayers on such surfaces, apparently because surfaces and adsorbates restructure to balance CO surface binding and CO-CO interaction energies.

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 .

Structure and function of metal cations in light alkane reactions catalyzed by modified H-ZSM5

Abstract The rate of propane dehydrocyclodimerization to form C 6 aromatics is limited by a sequence of irreversible dehydrogenation reactions leading to propene, higher alkenes, dienes, trienes, and aromatics. Quasi-equilibrated acid—catalyzed cracking, oligometization, and cyclization reactions of alkene intermediates occur in sequence with these dehydrogenation reactions. Each dehydrogenation reaction is in turn limited by the rate of elementary steps that dispose of H-atoms formed in C-H bond activation steps. The rate of C-H bond activation, recombinative hydrogen desorption, and propane chemical conversion have been measured from the rates of isotopic redistribution and chemical conversion during reactions of C 3 H 8 /C 3 /D 8 and D 2 /C 3 /H 8 mixtures on H-ZSM5, Ga/H-ZSM5, and Zn/H-ZSM5. Isotopic studies show that C-H activation steps are fast during steady-state propane dehydrocyclodimerization on H-ZSM5, Ga/H-ZSM5, and Zn/H-ZSM5. Ga and Zn species increase the rates of propane chemical conversion, recombinative hydrogen desorption, and deuterium incorporation from D 2 into reaction products. Disposal of hydrogen formed in C-H bond activation steps occurs by transfer of H-atoms to unsaturated species to form alkanes or to Ga and Zn species, which catalyze the recombinative desorption of H-atoms to form dihydrogen (H 2 ). The sequential release of several H-atoms during a propane dehydrocyclodimerization turnover limits the rate and selectivity of this reaction on H-ZSM5. In-situ X-ray absorption studies suggest that Ga and Zn species reside at cation exchange sites as monomeric cations and that recombinative desorption involve reduction—oxidation cycles of such cations during each dehydrocyclodimerization turnover. These monomeric species form directly during exchange of Zn ions from solution onto H-ZSM5. Ga 3+ species, however, do not exchange directly from solution onto H-ZSM5, but instead form extrazeolitic Ga 2 O 3 crystals. Ion exchange occurs during subsequent contact with propane or hydrogen at 700-800 K via vapor phase exchange of volatile Ga 1+ species.

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.