Rostam J. Madon

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.

A study of the nickel-titanium oxide interaction

Previous studies have demonstrated that, when nickel supported on titanium oxide is reduced in hydrogen at 450 °C and higher, the system exhibits SMSI properties. We have employed several complementary experimental approaches in an attempt to gain an insight into the intimate details surrounding the nickel-titanium oxide interaction. High resolution transmission electron microscopy was used to examine the changes in morphology of nickel particles following reduction at increasing temperatures. In situ ferromagnetic resonance studies have provided characterization of the state of the nickel as a function of reduction temperature. A geometrically designed catalyst in combination with scanning Auger surface analysis was used to probe transport phenomena involving nickel and titanium oxide during treatment in hydrogen. The combined results of these studies have enabled us to develop a model which involves the migration of titanium-oxygen moieties onto the surface of the nickel particles during reduction in hydrogen. This decoration model provides a mechanism whereby SMSI properties are observed.

Fischer-Tropsch synthesis on a precipitated iron catalyst

Abstract The Fischer-Tropsch synthesis was carried out at various experimental conditions on an unpoisoned and sulfur-poisoned precipitated, alkali-promoted iron-copper catalyst. The sulfur, present in the fixed catalyst bed as a longitudinal concentration gradient, had little effect on conversion or product selectivity. However, for both poisoned and unpoisoned catalysts, there was a significant change in the CO conversion to CO2 on going from a reaction temperature of 240 to 250 °C. This suggests that there is a rapid change of our catalyst surface after 240 °C: the surface changes from one that is effective for CO2 formation to one that is not as effective. Finally, analysis of the condensed products led us to suggest that there are at least two types of sites for hydrocarbon chain formation, each with a slightly different value of the chain growth probability.

Methylcyclohexane and methylcyclohexene cracking over zeolite Y catalysts

Abstract Naphthenes are an important class of molecules in fluid catalytic cracking. The cracking behavior of the model naphthenes, methylcyclohexane and methylcyclohexene was investigated over rare earth Y and USY zeolite catalysts. Initial products from methylcyclohexane are formed by a combination of protolytic and β-scission cracking plus isomerization, H − transfer, H + transfer and dehydrogenation reactions. Methylcyclohexane is a sensitive probe for characterizing the chemistry occurring on solid acid surfaces. Methylcyclohexene is the key intermediate in the formation of aromatics from methylcyclohexane. Methylcyclohexene cracks at a slower rate than methylcyclohexane but overall conversion is higher because hydride transfer reactions are fast.

Secondary effects in the Fischer-Tropsch synthesis

Abstract α-Olefins, which are primary Fischer-Tropsch products, are known to participate in secondary reactions during the synthesis process. Thus, these α-olefins may readsorb and initiate new hydrocarbon chains, and this may be an important route that decides the overall distribution of products. The analysis for a continuous stirred tank reactor shows that with such a secondary reaction the products still follow a Flory type of distribution. For a plug flow reactor, the distribution deviates from the Flory model if most of the readsorbed α-olefins participate in growth. If, however, as in a more real situation, they also participate in hydrogenation and isomerization reactions, then the distribution rapidly becomes similar to the Flory distribution. Finally, a case where the chain growth parameter, which is normally assumed to be constant, is varied along the length of an isothermal plug-flow reactor by forcing the C 1 surface intermediate concentration to vary is analyzed. The overall product distribution, in such a case, is still quite close to a Flory distribution.

Catalytic reaction rates in thermodynamically non-ideal systems

Chemical reactions reflect the universal tendency of systems to approach equilibrium. The dynamics towards equilibrium, reflected in rates of chemical reactions, are therefore influenced only by thermodynamic properties, such as reaction affinity and the chemical potential, activity, or fugacity of reactants and products. Reaction rates depend on concentrations only in ideal reaction mixtures, because here, concentration appears in the defining equations for all relevant thermodynamic properties. Catalytic reactions in gas–liquid–solid systems involve molecules solvated in a non-ideal environment and reacting on surfaces. Transition state treatments show that such reactions on surfaces detect the presence and identity of a liquid phase only when a liquid solvates kinetically relevant adsorbed intermediates and activated complexes or when its presence prevents gas–liquid equilibrium by imposing transport restrictions. Chemisorption energies are much larger than typical intermolecular interactions in liquids; therefore, inert liquids rarely influence the structure or reactivity of chemisorbed reactants and activated complexes. However, solvent effects become possible on heterogeneous catalysts when adsorption or desorption steps are rate-determining or kinetically relevant. Here, the reaction coordinate involves molecules in solution, and the corresponding activated complexes can become solvated by the surrounding fluid phase. In the special case of identical solvation of a reactant and an activated complex, a fortuitous cancellation of activity coefficients leads to reaction rates that depend on concentration rather than the thermodynamic activity of reactants. This stringent requirement makes concentration-driven reaction rates unusual exceptions to the general case of chemical reaction rates that depend on the thermodynamic activity of reactants and products. We have used transition state treatments of reaction rates in non-ideal systems to explain observed solvent effects for cyclohexene hydrogenation on Pt and Pd catalysts. A dihydrogen dissociative-adsorption rate-determining step on Pt leads to solvation of the kinetically relevant activated complex. Its activity coefficient and that for dissolved H2 cancel. As a result, the hydrogenation rate on Pt depends on H2 concentration in the liquid phase. On Pd, the rate-determining step involves chemisorbed species that are not influenced by the solvent; the reaction rate depends only on H2 partial pressure in the gas phase and not on the nature of the liquid. A similar treatment shows that the presence of liquid products in three-phase Fischer–Tropsch synthesis reactors cannot increase the rate of olefin readsorption, unless the liquid introduces transport restrictions that prevent rapid removal of olefins from catalyst pores. Higher solubility of larger olefins cannot account for enhanced readsorption. In fact, increasing solubility either has no effect or, under certain circumstances, increases the propensity for desorption rather than readsorption. Finally, we show mechanistic implications of the dependence of paraffin cracking rates on intrazeolite paraffin activities or concentrations. For example, for dependence on the latter, we involve a molecule in a precursor state that is solvated by the environment within zeolite channels to the same extent as a paraffin absorbed within the structure. Since zeolites introduce non-ideality to a reaction system, experimental observations need to be rationalized using the approach we have proposed.

Role of ZSM-5 and ultrastable Y zeolites for increasing gasoline octane number

Octane number of gasoline made in a fluid catalytic cracker can be catalytically enhanced by using an ultrastable HY zeolite instead of one containing rare earth cations or by adding small amounts of ZSM-5 to a Y zeolite catalyst. ZSM-5 addition catalyzes both normal and branched olefin cracking to give mainly propylene, butenes, 2-methyl 1-butene, and 2-methyl 2-butene. Increase in octane number is mainly due to an increase in yields of CS hydrocarbons, an increase in aromatic concentration, and a decrease in yields of C7+ paraffins and straight olefins. The decrease in paraffin yield is due to the removal of olefins which would otherwise undergo secondary hydrogen addition. The USY zeolite gives a lighter gasoline with higher yields of C5 and C6 branched olefins than the REY zeolite which gives higher yields of C7+ branched paraffins. However, the USY catalyst, unlike the ZSM-5 addition case, does not decrease the yields of C7+ olefins. Reaction pathways that affect gasoline composition are different, for the two cases; yet, in both cases, increases in C5 hydrocarbons and decreases in C7+ paraffins contribute significantly to the increase in octane number.

Acidity studies of fluid catalytic cracking catalysts by microcalorimetry and infrared spectroscopy

Abstract The acidic properties of a USY-based fluid catalytic cracking catalyst steamed at various severities and amorphous silica-alumina were investigated by microcalorimetry and infrared spectroscopy using pyridine adsorption at 473 K. Microcalorimetric measurements of the differential heat of pyridine adsorption versus adsorbate coverage revealed a heterogeneous acid site distribution for the catalysts. Besides showing the expected progressive decrease in the number of acid sites for pyridine adsorption, our measurements showed that the strength of Bronsted acid sites decreased with increasing severity of steam treatment. Infrared spectra of adsorbed pyridine revealed a significant decrease in the ratio of Bronsted to Lewis acid sites upon steaming. Amorphous silica-alumina had a relatively large number of acid sites of which a large proportion were Bronsted acid sites. However, the strength of these Bronsted sites was lower than that of the mildly steamed USY catalysts. This lower Bronsted acid strength, we believe, is related to lower activity for gas-oil cracking over silica-alumina.

2-Propanamine adsorption on a fluid catalytic cracking catalyst

Abstract We have examined temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA) of 2-propanamine on a fluid catalytic cracking catalyst as a function of steam deactivation time and compared the results to the activity for gas-oil cracking. The TPD-TGA results for the FCC catalyst are similar to results obtained on high-silica zeolites and show ammonia and propene desorbing in a well-defined feature between 575 and 650 K. The sites responsible for this desorption feature appear to be acid sites associated with framework Al in the zeolite phase. The concentration of these sites decreases steadily with steam deactivation and correlates very well with the activities for gas-oil cracking.