Search

Deuterium tracer studies coupled with kinetic approach were developed to accurately determine the kinetic isotopic effects (KIE) for the CO hydrogenation and the formations of CO2, CH4 and light hydrocarbons (C2-C4) over iron, cobalt and ruthenium catalysts. Pronounced inverse kinetic isotope effects (IKIE) (kco,H/kco,D = 0.64–0.66) over the Fe and Co catalysts and a moderate IKIE (kco,H/kco,D = ∼0.8) on the Ru catalysts for the CO hydrogenation were obtained under typical FTS conditions. The IKIE was also found in CH4 formation, but the Co and Ru catalysts showed stronger IKIE than the Fe catalysts (kCH4/kCD4 = 0.6 vs 0.87). Regardless of catalyst type, only a minor normal KIE for the CO2 formation was obtained (kCO2, H/kCO2, D ˜ 1.1). The IKIE was found on various light 1-olefins and n-paraffin hydrocarbons as well. All the IKIEs can be explained by thermodynamic and kinetic isotope effects. The results suggested that deuterium was involved in quasi-equilibrated and hydrogenation steps that determined KIE. However, the Fe, Co and Ru catalysts showed different deuterium isotopic effect for the hydrocarbon selectivity. Deuterium promoted chain growth, thus lowered CH4 and light hydrocarbons selectivities and enhanced C5+ selectivity on both the Fe and Co catalysts; while it enhanced the hydrogenation rate to the low hydrocarbons and suppressed the formation of heavier hydrocarbons on the Ru catalysts. Deuterium isotopic effect on the formation of 1-olefins, n-paraffins and 2-olefins in C10-C18 range (in liquid phase) was also studied in detail. Deuterium was found to enhance 1-olefins formation, but suppressed or only slightly changed n-paraffins formation over the Fe and Co catalysts. However, deuterium led to greater amount of n-paraffins and less 1-olefins on the Ru catalysts. The results suggested that the IKIE originated from different hybridization changes of carbon in C–H (sp1->sp2 and/or sp2->sp3) on the Fe, Co and Ru catalysts, which implied different reaction pathways on the Fe, Co and Ru catalysts.

Quantitative data comparing iron and cobalt based catalysts for the Fischer-Tropsch synthesis (FTS) are scarce due to the fact that these two kinds of catalysts typically utilize different process conditions. This paper focuses on studying the catalytic behavior of two highly active iron and cobalt based research catalysts at clean conditions and during poisoning with common syngas contaminants. The catalyst activity and selectivity at identical conditions, the CO conversion effect, and the effect of poisons on iron and cobalt catalysts were systematically explored in a quantitative manner. At a set of identical FTS conditions, the cobalt catalyst was 2.5 times as active as the iron catalyst with higher CH4 and C5+ selectivities but much less olefins and lower CO2 selectivity. Cobalt based catalysts are more susceptible to deactivation by oxidation at high CO conversions (e.g. > 80%) due to the high partial pressure of water (PH2O) in the reactor, while the iron catalyst can be stabilized at a high conversion level. Under clean FTS conditions, the cobalt catalysts displayed a more pronounced CO conversion effect on stability and selectivity; on the other hand, a combination of effects (i.e. from CO conversion and the nature of the catalyst) were observed for the iron catalysts. The sensitivities of the Fe and Co catalysts to the typical contaminants (i.e., H2S and NH3) present in the syngas derived from coal, natural gas or biomass were compared. Iron and cobalt catalysts exhibited similar resistance to the H2S poison (i.e. threshold levels 25–50 ppb), but the iron catalyst was found to be much more resistant to ammonia than the cobalt catalyst (i.e., threshold levels of 80 ppm and 45 ppb, respectively). Co-feeding 150–200 ppm ammonia lowered CH4 selectivity and 2-olefin content (suppressing secondary reactions of 1-olefin) on both types of catalysts. In contrast, co-feeding up to 1 ppm H2S significantly increased CH4 formation only on cobalt catalysts but had a minor effect on CH4 selectivity with iron catalysts. It increased 2-olefin content (enhanced secondary reactions of 1-olefin) regardless of catalyst type. H2S and NH3 have different impacts on H2, CO adsorption, and the formation of sulfur and nitride compounds have been proposed to explain these dissimilar effects.

The explanation for CH4 selectivity for iron based Fischer-Tropsch catalysts in the low conversion region (i.e.

As a follow-up to a previous alkali chloride poisoning study, the effect of up to 100 ppm KCl on the Fischer-Tropsch synthesis (FTS) performance of representative iron (Fe-Si-Cu doped with Rb as the alkali) and cobalt (Pt-Co/Al 2 O 3 ) catalysts was studied at 270 °C and 230 °C, respectively, by co-feeding KCl in a water/ethylene glycol (EG) solution. The used catalysts were characterized by XANES at the K and Cl K-edges; furthermore, ICP was used to analyze residual K and Cl ions possibly remaining in the FTS products. KCl was found to be a weak poison for the iron and cobalt catalysts. The addition of 20–100 ppm KCl deactivated the catalysts to only a low to moderate extent. For the cobalt catalyst, less than 25 ppm KCl was found to give negligible deactivation. The added KCl and EG-H 2 O solvent was found to slightly modify the selectivity for both catalysts, such that KCl slightly promoted light hydrocarbon formation as well as olefins and slightly suppressed C 5+ and 2-olefin formation, while the EG-H 2 O solvent was found to have a different effect on the C 1 C 4 and C 5+ selectivities. It appears that K and Cl played opposite roles in modifying hydrocarbon selectivities. The ICP results suggested 48–98% K and Cl ions were adsorbed by the iron catalyst. XANES results confirmed the presence of K and Cl ions on the used iron and cobalt catalysts and showed a structure with characteristics that were similar to bulk KCl. Two possible mechanisms, including site blocking by K and Cl ions and electronic modification impacting CO/H 2 adsorption, were proposed to explain the deactivating effect of KCl on the iron and cobalt catalysts.

The effects of co-fed ammonia in synthesis gas on the activity and product selectivities of a typical cobalt catalyst (0.5% Pt–25% Co/Al2O3) were investigated during the Fischer–Tropsch synthesis using a continuously stirred tank reactor (CSTR). The product selectivities were compared at a similar CO conversion level for various concentrations (10–1000 ppmv) of ammonia, as well as clean (un-poisoned) conditions. The addition of 10–1000 ppmv ammonia (concentration of ammonia with respect to the syngas feed) significantly decreased activity; the percentage of deactivation was similar (∼40%) for the various concentrations of ammonia used. At similar CO conversions, the addition of ammonia caused an increase in olefin selectivity and the corresponding paraffin and alcohol selectivities were decreased compared to the ammonia free synthesis conditions. Olefin selectivity increased with increasing concentration of ammonia, and the paraffin and alcohol selectivities were decreased with increasing ammonia concentration. At similar CO conversions, ammonia addition exhibited a positive effect on hydrocarbon selectivity (i.e., lower light gas products and higher C5+) and also light gas product selectivities (C1–C4) were decreased and C5+ selectivity increased with increasing concentration of ammonia compared to ammonia free conditions.

Fischer-Tropsch synthesis was conducted in a small channel compact heat exchange reactor that was constructed of aluminum. While limited to lower temperature-pressure regions of the Fischer-Tropsch synthesis, the reactor could be operated in an isothermal mode with nearly a constant temperature along the length of the channel. The results obtained with the compact heat exchange reactor were similar to those obtained in the isothermal continuous stirred tank reactor, with respect to both activity and selectivity. Following a planned or unplanned shutdown, the reactor could be restarted to produce essentially the same catalytic activity and selectivity as before the shutdown.

A large number of kinetic data points (83 sets) was obtained over a wide range of CO conversion (7–90%), pressure (1.3–2.5 MPa) and H 2 /CO ratio (0.67–1.5) with an iron catalyst (100 Fe/5.1 Si/1.25 K). The kinetics of the catalyst in the low (X CO CO > 70%) regions were studied separately. Twenty six Fischer–Tropsch synthesis (FTS) and water gas shift (WGS) kinetic models were tested and discriminated. Water and CO 2 inhibition was evaluated. While all thirteen FTS models gave a satisfactory fit, the new FTS models that included CO 2 inhibition surpassed the others. Water inhibition of the FTS rate was insignificant over both low and high conversion ranges. For the WGS kinetics of the iron catalyst, a newly constructed empirical model and one from the literature provided the best fits of the WGS rates, while nine mechanistic models and one power law WGS model were unable to satisfactorily fit the WGS kinetic data. Water did not significantly limit the WGS rate and CO 2 only inhibited the rate at high CO conversions. The equations obtained for the low and high CO conversion ranges varied greatly. The errors for the models for 85% of the FTS and WGS data points were less than 10%, and the errors of the remaining points fell in the range of 10–15%.

The effect of phosphorus addition on silica-supported cobalt catalyst was investigated for Fischer-Tropsch synthesis (FTS). As shown by STEM images and chemisorption, the addition of phosphorus to cobalt of up to 1 wt% increased the dispersion of cobalt. Further addition of phosphorus (e.g., 3 and 5 wt.%), as demonstrated by TPR, pulse reoxidation, and XANES, significantly hindered the reduction of cobalt oxides. The cobalt FTS catalysts containing 0.5 and 1.0 wt% P exhibited greater stability in comparison with undoped and 3.0 wt% P containing cobalt catalysts. Analysis of XANES spectra at the P and Co K-edges, along with DRIFTS results of H 2 -activated cobalt catalysts, suggest that cobalt particles interact with PO 4 3− ions, indicating a role played by P in anchoring Co particles to the support, thus hindering the cobalt sintering rate. The initial selectivity to methane was slightly higher for 0.5%P-20%Co/SiO 2 and 1.0%P-20%Co/SiO 2 catalysts compared to the undoped catalyst, but at longer times differences were small. At higher loadings of P (3 wt.%), FT activity and selectivity were adversely and irreversibly affected.

The effect of co-fed hydrogen chloride (HCl) in syngas on the performance of iron and cobalt-based Fischer–Tropsch (FT) catalysts was investigated in our earlier studies (Ma et al. in ACS Catal 5:3124–3136, 2015; Davis et al., DOE final report, 2011; Gnanamani et al. in Catal Lett 144:1127–1133, 2014). For an iron catalyst, lower HCl concentrations (

Fischer–Tropsch cobalt-based catalyst deactivation is one of the major challenges facing gas-to-liquids processes. Changes in catalyst formulation and pre-treatment methods are among the promising routes to address this challenge. In this study, diluted CO was used to pre-treat a 0.5%Pt/25%Co/Al2O3 catalyst, in comparison to H2, and the resulting effects on catalyst stability, activity and product selectivity were evaluated. The investigation was performed using BET, TEM, TPR, XPS and XRD techniques, combined with catalyst evaluation in a 1 L continuously stirred tank reactor. The results reveal that microporous carbon shells encapsulate Co particles during activation by CO at higher temperatures, resulting in lower activity and higher methane selectivity compared to the H2-reduced catalyst, at early reaction times. However, the CO-activated catalyst displayed superior stability, resulting in better performance at extended time-on-stream compared to a deactivating H2-activated catalyst, which showed an increase in CH4 selectivity and a decline in C5+ hydrocarbon formation rate.

The starting solvent is essential for carrying out Fischer-Tropsch (FT) synthesis in a stirred-tank reactor. The most commonly used solvents are C28+ hydrocarbon oils, wax, heavier wax, and FT wax. In general, FT activity of Co-based catalysts was found to decrease for initial days, and one of the reasons may be related to the filling of catalyst pores with heavier wax leading to lower gas diffusions and thus lower CO conversion. In this work, therefore, solvents with varying molecular weight and polarity, including glycerol, 1-octanol, Polywax-655, and dimethyl phthalate were tested as a starting medium for 0.27%Ag-25%Co/Al2O3 catalyst. It was found that low molecular weight non-polar solvent, for example, Polywax 655 showed higher conversions. On the other hand, glycerol a high polar solvent tends to firmly bound with a catalyst surface and the associated water produced during FT synthesis could trigger catalyst deactivation from the oxidation of cobalt. This indicates the superiority of non-polar over polar solvents for FT synthesis. However, 1-octanol as a starting solvent showed the least catalytic deactivation rate and competitive FTS activity. Thus, 1-octanol has the potential as a start-up solvent for FT reaction in stirred tank reactors.

The sulfur limit, the relationship between the sulfur added and the surface Fe atoms lost (Fe/S), and mechanism of sulfur poisoning were studied using an iron Fischer–Tropsch synthesis (FTS) catalyst (100 Fe/5.1 Si/2.0Cu/3.0K). The FTS reaction was carried out at 230–270 °C, 1.3 MPa, H2/CO = 0.67–0.77 and 30–70% CO conversion using a 1-L slurry phase reactor. The used Fe catalysts were characterized by XRD, Mossbauer spectroscopy and XANES spectroscopy to understand the deactivation mechanism of the Fe based catalyst after adding up to 1 ppm H2S in the feed. Co-feeding of 0.1 ppm H2S in syngas for 70 h caused a very small change in the activity of the Fe catalyst, but increasing the H2S level to 0.2 ppm or above resulted in measurable deactivation of the Fe catalyst over a similar time period. The limit of sulfur level in the syngas feed (sensitivity) was determined to be 50 ppb. The added sulfur improved the selectivities of the secondary reactions of olefins and the WGS reaction even though the rates for these declined. The addition of H2S decreased CH4 selectivity and increased C5+ selectivities of the Fe catalyst. The Fe/S ratio, which can be used to define the poisoning ability of sulfur for the iron catalyst, was quantified based on the deactivation data obtained. The Fe/S ratio strongly depended on temperature and decreased remarkably with increasing temperature. At 270 °C one sulfur atom was found to eliminate ∼6 surface Fe atoms, and the ratio increased to 7.2 at 260 °C and increased further to 13.5 at 230 °C. The Fe/S relationship with increasing temperature is in good agreement with sulfur sorption theory. The changes in FTS and WGS rates of the Fe catalyst by sulfur were also studied. The decreases in rates of the two reactions were nearly the same. The results of XRD and Mossbauer spectroscopy indicated that the online addition of sulfur did not greatly alter the distributions of iron carbide and magnetite. Both data sets consistently suggest an adsorption mechanism, in line with the results of reaction testing. XANES results at the S K-edge further confirmed sulfur adsorption, and some sulfide and sulfate species, likely confined to the surface zone, were detected. In this study, the sulfur tolerances of the precipitated Fe and a supported Co catalyst were compared at an identical temperature (i.e., 230 °C), and similar M/S ratios (13.5–15.0) were obtained.

Tracer studies were performed on cobalt and iron Fischer–Tropsch catalysts using a synthesis gas containing a 20:80 mixture of 13C18O and 12C16O. The objective of the work was to investigate the antecedents of the C–O bonds in alcohols and CO2 formed during Fischer–Tropsch (FT) synthesis. It was found that chain growth proceeded by a CO insertion mechanism over both cobalt and iron catalysts. Over the cobalt catalyst, the dominant pathway for methanol synthesis involved a partial hydrogenation of CO as well as CO2 by a reaction pathway separate from the Fischer–Tropsch pathway. Over the iron catalyst, the majority of the methanol was formed by partial hydrogenation of only CO through the FT reaction pathway. Iron is active for water gas shift conversion, which produced CO2. Oxygen exchange reactions of CO2 were likely over both catalysts and complicated the interpretation of the results.

The effect of pretreatment conditions of cobalt on activity and product selectivity for the hydrogenation of carbon dioxide was studied over a 1%Na/20%Co–SiO 2 catalyst using a fixed-bed catalytic reactor operated at 220 °C and 1.89 MPa. The metallic form of cobalt was obtained from the reduction of cobalt oxide (Co 3 O 4 ) by H 2 at 350 °C and produced primarily methane and lower hydrocarbons (C 2 –C 4 ) from CO 2 . Pretreatments with pure H 2 or syngas (H 2 :CO) at 250 °C yield a fraction of partially reduced cobalt (CoO), which tends to suppress the hydrogenation activity of cobalt somewhat, reducing methane selectivity to a limited degree. After CO activation, partially reduced cobalt oxide (CoO) and cobalt carbide phases formed and produced significantly less methane (i.e., selectivity of 15.3%), and surprisingly, the selectivity for alcohols increased to 73.2%. In the absence of sodium, direct methanation is still a preferred reaction for CO 2 for CO pretreated cobalt; the XRD results of used catalysts revealed that the cobalt carbide phase converted to metallic cobalt. The results indicate that Na doping and carbide are important for low methane and high oxygenate selectivities. However, further work is needed to determine whether the role of Na is merely to stabilize the cobalt carbide phase, or whether Na is involved in promoting the catalytic cycle.

The effect of ammonia in syngas on the Fischer–Tropsch synthesis (FTS) reaction over 100Fe/5.1Si/2.0Cu/3.0K catalyst was studied at 220–270 °C and 1.3 MPa using a 1-L slurry phase reactor. The ammonia added in syngas originated from adding ammonia gas, ammonium hydroxide solution, or ammonium nitrate (AN) solution. A wide range of ammonia concentrations (i.e., 0.1–400 ppm) was examined for several hundred hours. The Fe catalysts withdrawn at different times (i.e., after activation by carburization in CO, before and after co-feeding contaminants, and at the end of run) were characterized by ICP-OES, XRD, Mossbauer spectroscopy, and synchrotron methods (e.g., XANES, EXAFS) in order to explore possible changes in the chemical structure and phases of the Fe catalyst with time; in this way, the deactivation mechanism of the Fe catalyst by poisoning could be assessed. Adding up to 200 ppmw (wt. NH 3 /av. Wt. feed) ammonia in syngas did not significantly deactivate the Fe catalyst or alter selectivities toward CH 4 , C 5+ , CO 2 , C 4 -olefin, and 1-C 4 olefin, but increasing the ammonia level (in the AN form) to 400 ppm rapidly deactivated the Fe catalyst and simultaneously changed the product selectivities. The results of ICP-OES, XRD, and Mossbauer spectroscopy did not display any evidence for the retention of a nitrogen-containing compound on the used catalyst that could explain the deactivation (e.g., adsorption, site blocking). Instead, Mossbauer spectroscopy results revealed that a significant fraction of iron carbides transformed into iron magnetite during co-feeding high concentrations of AN, suggesting that oxidation of iron carbides occurred and served as a major deactivation path in that case. Oxidation of χ-Fe 5 C 2 to magnetite during co-feeding high concentrations of AN was further confirmed by XRD analysis and by the application of synchrotron methods (e.g., XANES, EXAFS). It is postulated that AN oxidized χ-Fe 5 C 2 during FTS via its thermal dissociation product, HNO 3 . This conclusion is further supported by reaction tests with co-feeding of similar concentrations of HNO 3 . Additional oxidation routes of iron carbide to magnetite by HNO 3 and/or by its thermal decomposition products are also considered: Fe 5 C 2 + NO x (and/or HNO 3 ) → Fe 3 O 4 . In this study, ion chromatography detected that 50–80% HNO 3 directly added or dissociated from AN eventually converted to ammonia during or after its oxidation of iron carbide, resulting from the reduction of NO x (NO x + H 2 + CO → NH 3 + CO 2 + N 2 + H 2 O) by H 2 and/or CO.

The current investigation was undertaken to identify limits of hydrohalic acid (HX, X = F, Cl, Br) impurities in syngas and shed light on the mechanism of HX poisoning of a 100 Fe/5.1 Si/2 Cu/3 K FTS catalyst under industrially relevant conditions using a 1-L slurry-phase reactor. Co-feeding

The reactivity of 14C-labeled ethanol (CH 3 14 CH2OH) and 1-propanol (CH3CH 2 14 CH2OH) added during Fischer–Tropsch synthesis (FTS) was studied in a continuous-flow well-stirred reactor operated at 220 °C and 18.7 atm using a supported cobalt or ruthenium catalyst. The added labeled alcohols are mostly found in the product gas and water phase (≥99 %) without being involved in any reaction over both fcc and hcp metallic cobalt. The data indicate that C–O bond of ethanol and 1-propanol remains mostly stable on cobalt under FTS conditions. However, 1-propanol is likely to be involved in a chain initiation after decarbonylation apart from being degraded into methanol on ruthenium.

The conversion of CO2 over a CoPt/Al2O3 catalyst was investigated. Single-gas adsorption studies indicated that carbon was deposited on the catalyst by exposure to both CO2 and CO in the absence of H2 cofeed. When CO2 was preadsorbed followed by H2 flow, methane was produced, as well as traces of C3–C4 hydrocarbons, but no evidence of the reverse water gas shift reaction was found. Use was made of carbon-14-labeled carbon dioxide to track CO2 conversion and selectivity during reaction of syngas mixtures with different ratios of CO, CO2, and H2. Absence of 14C in unconverted CO and the unequal molar concentration of 14C in the products from reaction at 220 °C and 2 MPa provided strong evidence that 14CO2 was not converted by the reverse water gas shift reaction. The antecedence of the carbon from CO2 mattered, and the carbon did not become part of a common carbon pool for hydrocarbon synthesis. Conversion of CO2 proceeded by a pathway separate from CO. Conclusions drawn from this experimental study were em...

The effects of activation gas and copper promoter loading on the Fischer–Tropsch synthesis performance of potassium promoted precipitated iron-based catalysts were investigated using a continuously stirred tank reactor. In this study, CO and syngas (H2/CO = 0.7) activated catalysts were tested after varying the copper promoter loading (0, 2 and 5 %, atomic ratios relative to iron). After attaining a steady-state conversion for the CO-activated catalysts, similar or slightly higher CO conversions were exhibited with increasing copper loading, and the induction period was reduced with increasing copper loading. Partial pressure of hydrogen in the activation gas influenced the resulting activity of the catalysts. For syngas activated catalysts, CO conversion was found to increase with increasing copper loading up to 2 %, and slightly decrease with further increases in copper (5 %) loading. For similar CO conversion levels, the selectivities were similar for the CO activated catalysts, whereas for the syngas activated catalysts, the selectivity varied with copper loading. With increasing copper loading, lower hydrocarbon (methane and C2–C4) selectivities decreased and the corresponding higher hydrocarbon (C5+) selectivity increased.

The kinetics of Fischer–Tropsch synthesis (FTS) over a 25%Co/Al2O3 catalyst was studied using a 1-L continuously stirred tank reactor (CSTR) under the conditions of 205–230 °C, 1.4–2.5 MPa, H2/CO = 1.0–2.5 and 3–16 NL/g-cat/h (XCO = 7–54%). Thirty-one sets of kinetic data collected at 220 °C with a low extent of deactivation were used for kinetic parameter regression. The CAER empirical kinetic model ( r F T = k P CO a P H 2 b / ( 1 + m P H 2 O / P H 2 ) ) was employed to study the kinetic effect of water. A positive kinetic water effect was first evidenced using the kinetic approach, consistent with the results of the effect of co-fed water on cobalt FTS in this work and the literature (e.g. Loegdberg et al., 2011 [21] ) for Co/Al2O3 catalysts. The current kinetic results are based on kinetic data taken following an initial catalyst induction period, where the CO conversion had stabilized. These data are different from our earlier investigations where reversible oxidation of small cobalt crystallites and/or catalyst support effects likely impacted the cobalt site densities, resulting in a negative water effect. Thus, in this study, we decoupled the effect of reversible oxidation from the kinetics, so that the effect of water on stable (i.e., presumably larger) metallic cobalt particles could be assessed. In this study, an additional eleven classical FT kinetic models for Co catalysts were tested using the kinetic data. Five of them were also found to adequately describe the kinetic data, including two mechanistic models developed based on carbide mechanisms. The CAER model containing a water effect term and the mechanistic model of Botes et al. (2009) [31] yielded comparable reaction orders for the partial pressures of H2 and CO, resulted in a better fit of the kinetic data.

The effect of halides and potassium addition on cobalt Fischer–Tropsch synthesis (FTS) was studied using 1L CSTR at 493 K, 1.99 MPa and H2/CO of 2.0. The potential catalyst poison was co-fed with syngas after reaching steady-state FTS. Halides were added in an aqueous solution of HX (HF, HCl, HBr) and potassium as aqueous potassium nitrate solution along with syngas at various concentration levels ranging from 500 ppbv to 100 ppmv. Fluoride addition did not affect FT performance of cobalt up to 1.0 ppmv level. However, the presence of other halides (Cl− and Br−) decreased the FT activity proportionally. Addition of potassium decreased cobalt FT activity considerably and increases syngas conversion to methane and CO2 at higher potassium concentrations. The observed poisoning effects for metallic cobalt could be attributed to a blocking effect of cobalt metal sites by halides and potassium in agreement with results of H2 chemisorption on used cobalt FT catalysts.

The effect of potassium on oxides and carbides of iron for Fischer–Tropsch synthesis (FTS) was investigated by pretreating Fe3O4 and K-promoted Fe catalysts with different gases (H2/H2O and CO). A freshly activated sample and catalysts that were recovered from the CSTR before, during and after FT synthesis were characterized ex situ using Mossbauer spectroscopy. Iron carbide is found to be active for both FT and water gas shift (WGS) reactions. After H2/H2O activation, all three catalysts (Fe3O4, low α-Fe, and high α-Fe) exhibit a steady but low FT activity for a period of FT synthesis. However, both FT and WGS activity for Fe3O4 and low α-Fe catalysts were greatly improved after CO activation. In contrast, the high potassium containing catalyst (high α-Fe) did not show any further improvement in activity after CO activation. The difference in FT and WGS activity observed after pretreatment conditions using these catalysts may be associated to the amount of potassium and conversely the iron carbide present in the catalysts.

The sensitivity of 100Fe/5.1Si/2Cu/3K Fischer–Tropsch synthesis (FTS) catalyst to the impurities of KCl, NaCl, KHCO3 and NaHCO3 in syngas (0.1–100 ppm) was studied in a slurry phase reactor at 533.2 or 543.2 K, H2/CO = 0.67–0.77, 1.31 MPa and 10 NL/g-cat/h. The impurities were co-fed with syngas, and the influence of each contaminant concentration on Fe catalyst behavior was examined for 72–144 h. The presence of up to 40 ppmw halide compounds (NaCl and KCl) or alkali bicarbonates (NaHCO3 and KHCO3) in syngas at 543.2 K or up to 100 ppm of NaCl and KCl at 533.2 K had little impact on the Fe catalyst activity and selectivities to CH4, C5+ and C4 olefin and 1-olefin during 400 h (543.2 K) or 1400 h testing (533.2 K). CO2 selectivity slightly increased after feeding the impurity-containing solutions, which was due to enhanced water gas shift (WGS). ICP results for the Fe catalysts at the end of the FTS reaction test and of the FTS products (i.e., water, oil or wax phase) indicate that the impurity ions (K+, Na+ or Cl−) introduced were present in all phases of the FTS products, the greater part being retained in the wax. Therefore, the contaminant ions (i.e., Na, K or Cl) in the water solution injected appear to not strongly adsorb on the Fe catalyst surface at typical FTS conditions in the slurry phase reactor. This is assumed to be responsible for the lack of change in FTS behavior for the Fe catalyst using the contaminants studied.

Sulfur in the form of H2S has been added at various concentration levels during cobalt Fischer–Tropsch synthesis. A steady-state FTS conversion was established prior to adding H2S. At low levels (

The influence of zeolite structure for the alkylation of biphenyl with propylene was studied over various zeolites such as HY, HZSM-5, and dealuminated mordenite (DMOR), as well as amorphous SiO2/Al2O3, in a stirred tank reactor. Biphenyl conversion was found to increase with reaction time for HZSM-5 and DMOR zeolites and reach a leveling off in 4 h, whereas for HY and amorphous SiO2/Al2O3 a leveling off was reached within an hour. DMOR displayed the highest selectivity for 4,4′-diisopropylbiphenyl (4,4′-DIPB) even at temperatures as high as 300 °C, whereas for HY, HZSM-5 and amorphous SiO2/Al2O3 selectivities fell in the range of 10–35%; they were significantly lower than observed for DMOR. These differences in selectivity might be due to the structure and pore channels of the zeolites. DMOR was found to be an active catalyst, the selectivity for 4-isopropylbiphenyl (4-IPB) and (4,4′-DIPB) was high among isopropylbiphenyl (IPB) and diisopropylbiphenyl (DIPB) isomers, respectively, indicating DMOR possesses shape-selectivity. The selectivity of 4,4′-DIPB increased with time, while the corresponding selectivity of 4-IPB decreased for DMOR catalyst. Alkylation of biphenyl with propylene occurred with predominant formation of 4-IPB in the first step. 4-IPB is only a source in the second step of alkylation of biphenyl with propylene for the formation of 4,4′-DIPB, while 3-IPB does not participate in the formation of DIPB isomers.

The hydrogenation of CO2 using Pt promoted Co/γ-Al2O3 and doubly (Cu, K) promoted iron catalysts exhibits an inverse isotope effect (r H/r D

The effect of CO2 was studied for cobalt and iron Fischer–Tropsch (FT) synthesis. CO2 behaves differently in the presence of CO over cobalt and iron catalysts in terms of hydrogenation. A systematic increase of the CO2 mole fraction of carbon in the feed gas mixture alters the product distribution dramatically for cobalt FT synthesis with CO2 behaving like an inert gas at higher partial pressure of CO. With cobalt, CO appears to compete with CO2 for adsorption. Using an iron FT catalyst, hydrogenation of CO2 was effected due to the presence of the reverse water–gas shift activity of the catalyst which converts CO2 to hydrocarbons through the formation of CO. Unlike the cobalt catalyst, the product distribution was only slightly altered with increasing CO2 content in the feed gas mixture to the iron catalyst. This difference in behavior of CO2 over cobalt and iron could be attributed to the absence of reverse water–gas shift activity on cobalt and hydrogenation of CO2 to hydrocarbons—other than methane—will be derived through the formation of CO.

A series of mixed Hf X Zr1−X O2 oxide catalysts was prepared according to a recipe that yields the monoclinic structure. The samples were examined by EXAFS spectroscopy at the Zr K and Hf LIII edges. A fitting model was used that simultaneously fits data from both edges, and makes use of an interdependent mixing parameter X mix to take into account substitution of the complementary atom in the nearest metal-metal shell. For XPS analysis, Scofield factors were applied to estimate the relative atomic surface concentrations of Zr and Hf. EXAFS results suggested that a solid bulk solution was formed over a wide range of X for Hf X Zr1−X O2 binary oxides, and that the relative ratio was retained in the surface shell (i.e., including some subsurface layers by XPS) and the surface (e.g., by ISS). The increase in selectivity for the 1-alkene from dehydration of alcohols at high Zr content does not correlate smoothly with the tuned relative atomic concentration of Hf to Zr. The step change at high Zr content appears to be due to other indirect factors (e.g., surface defects, oxygen vacancies).

Mixed oxides, prepared via the thermal decomposition of layered double hydroxides (LDHs), were screened gravimetrically for their ability to adsorb carbonyl sulfide (COS). Based on promising results obtained for Ni/Mg/Al, Ni/Mg/Fe and Co/Mg/Al mixed oxides, a study was undertaken to optimize the composition of these materials for COS adsorption. To investigate the effect of the M(II):M(III) ratio, LDHs of the type [MzMgyAlx(OH)2](CO3)x/2·0.5H2O (where M = Ni or Co, and x + y + z = 1) were prepared at values of x corresponding to 0.33 and 0.20. Simultaneously, the elemental ratio of transition metal to magnesium (z/y) was varied. Mixed oxides obtained from the resulting LDHs were tested in fixed bed mode with a feed of 100 ppm COS in N2 to determine breakthrough capacity. In general Ni/Mg/Al mixed oxides showed the best performance, a composition with Ni/Mg/Al = 0.32/0.48/0.20 showing the best adsorption capacity. Treatment of the spent adsorbent under an atmosphere of 5% H2 in N2 at 450 °C was found to provide an effective means of restoring the adsorption capacity over two cycles of adsorption and regeneration, although after three such cycles, adsorption capacity decreased.

Bi2O3/Al2O3 catalysts were prepared by sol–gel and impregnation methods, and their properties in the selective catalytic reduction of NOx with propene and propane investigated. Under simulated diesel exhaust gas conditions (500 ppm NO, 10% O2, 6% H2O, GHSV of ca. 30,000 h−1) with 525 ppm propene as reductant, maximum conversion levels to N2 reached 62% for NO and 47% for NO2, while maximum NOx reduction was observed at 550 °C. Increasing the propene concentration to 1315 ppm (corresponding to a [C1]/[NO] ratio of 8) resulted in a maximum NO reduction level of 96%, while the temperature corresponding to the maximum shifted to 525 °C. Propane was also found to function as an efficient reductant; maximum NO conversion levels were only slightly lower than for propene, although the temperature window for NO conversion was shifted by ∼25 °C to higher temperature relative to propene. While the presence of oxygen was found to be essential for SCR activity, water significantly inhibited NOx reduction by competing for adsorption sites with the propene and/or NOx reactants. Optimum SCR activity was observed for sol–gel catalysts at bismuth loadings of 3–7 wt%, XRD and XPS measurements showing that the bismuth oxide in these catalysts was present as a highly dispersed phase. The results of steady-state and transient DRIFTS measurements suggested a reaction scheme in which surface nitrate and acetate groups react to afford N-containing intermediates, including organo-nitrite and/or nitrate species, as well as isocyanate, which react further to form ultimately N2.

Bi2O3/Al2O3 materials were prepared by impregnation of a γ-Al2O3 support with bismuth citrate and tested as catalysts for the selective catalytic reduction of NO with propene. Under simulated diesel exhaust gas conditions (500 ppm NO, 10% O2, 6% H2O) with 600 ppm propene as reductant, a maximum in NO conversion of 56% was attained at 525 °C and a GHSV of ca. 30,000 h−1. Water significantly inhibited NO reduction by competing for adsorption sites with the propene and/or NO reactants. To correlate NO reduction activity with catalyst composition, the catalysts were characterized using XRD, XPS and N2 physisorption methods.

In situ DRIFTS measurements on unpromoted and Pt promoted MgO and ZrO 2 (both tetragonal and monoclinic) indicate that at high H 2 O/CO ratios, where the reaction rate has been reported to be zero order in H 2 O and first order in CO, the mechanism involved in the catalysis of water-gas shift is likely a surface formate mechanism, in agreement with Shido and Iwasawa. Pt was found to catalyze the removal of surface carbonates and to facilitate the generation of active OH groups relative to the unpromoted catalyst. Comparison with Pt/ceria revealed that the OH groups involved in the catalysis of magnesia and zirconia may be those of the bridging variety which occur at defect sites. That is, water dissociated over vacancies to produce bridging OH groups, as observed by infrared spectroscopy. The existence of such an adsorbed species is implied in the zero reaction order for water, where kinetics suggests that the surface should be saturated by an adsorbed water species. The lower extent of vacancy formation for magnesia and zirconia-based materials in comparison with ceria could explain a lower surface population of active bridging OH groups. CO was used as a probe molecule of the reduced centers, as it reacts with bridging OH groups to generate surface formates, a proposed WGS intermediate, and the decomposition of which is proposed to be the rate-limiting step. The trends in formate intensity by CO adsorption and CO conversion in WGS catalytic testing both followed the order: Pt/ceria > Pt/m-zirconia > Pt/t-zirconia > Pt/magnesia. In all cases, a normal kinetic isotope effect was observed in switching from H 2 O to D 2 O, consistent with a link between the rate-limiting step and the decomposition of surface formates, as noted previously by Shido and Iwasawa for Rh/ceria, MgO, and ZnO.

Under a pressure tuned supercritical fluid mixture of pentane/hexane as the media and using Co/A12O3 as the catalyst, the Fischer–Tropsch synthesis (FTS) reaction was conducted in a fixed bed reactor. 14C-labeled decene, tetradecene, and nonadecene were chosen as probe compounds to study the chemical behavior of the added mid-carbon number olefin under FTS supercritical reaction conditions. In the case of the run with decene, 21.8% of the added decene was reduced to decane, 73.5% remained unchanged, and only 4.7% of the added decene was incorporated into higher carbon number compounds. For the tetradecene and nonadecene runs, the percentages of reduction of the added olefin are 23.4 and 2.5%, while the percentages of incorporation are 2.6 and 1.5%, respectively. The molar radioactivity of the incorporated compounds decreases with increasing molecular size, indicating the presence of some accumulated products under these reaction conditions. The α value calculated based on the relative radioactivity of the incorporated compounds is 0.62, which is smaller than the value obtained from normal FTS under the same reaction conditions. These results indicate that the added mid-carbon number olefin cannot significantly alter the product distribution in favoring the heavier products.

In this investigation, isotope switching from H2O to D2O was used to validate the possibility that the decomposition of formate could be the rate limiting step for the water gas shift mechanism over metal promoted ceria, as was proposed earlier by Shido and Iwasawa. The surface species were followed stepwise by in-situ DRIFTS, while the conversion was monitored by GC to quantify the conversions. A normal kinetic isotope effect was observed, consistent with a link between the activation energy barrier of the rate limiting step and the breaking of the formate C–H bond.

In previous work, steady-state isotope switching was conducted on Pt/ceria with H2O and D2O, and a normal kinetic isotope effect was observed. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicated that the surface coverage of H-formate was more limited by the WGS rate than that of D-formate, linking the rate limiting step to the breaking of the formate C–H bond. In this work, the transient response of the decomposition of H and D formates was followed thermally and in the presence of H2O and D2O. A normal kinetic isotope effect was again obtained, in agreement with our previous findings, and those reported by Shido and Iwasawa. Consistent with their work, water was found to assist in promoting the forward decomposition of surface formates to hydrogen and unidentate carbonate, the precursor to CO2.

Recently, the role of ceria vacancies on water–gas shift activity has been explained in terms of a redox mechanism, whereby CO adsorbed on a metal reduces the ceria surface to generate CO2, and water reoxidizes the ceria surface to CeO2, liberating hydrogen in the process. In this study, we examine the possibility of a ceria-mediated redox mechanism by examining more closely the evolution of carbonate and formate bands under different controlled treatment environments, and utilizing different reduction procedures. Earlier it was claimed that the decomposition of carbonates by water was consistent with a redox process, whereby the CO2 product could spillover to the support. We found that the observation of carbonate formation and decomposition by water was a result of the treatment procedure used in the earlier work, and that, once bridging OH groups are produced in the presence of water, the reaction more likely proceeds via a formate intermediate, which is produced by reaction of CO with the active bridging OH groups. However, the vacancies appear to play an important role in generating these active sites. Possible pathways to active site generation are discussed.

In situ, steady-state diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements for adsorption of CO and for water-gas shift (WGS) reaction conditions indicate that formates are present on the surface of reduced ceria, being formed by reaction with geminal OH groups that are present after reduction of the ceria surface shell. The process of surface shell reduction was strongly catalyzed by the presence of metal, while changing very little if at all the catalysis of bulk reduction. Gold was found to reduce the surface ceria at a lower temperature than that of platinum, but platinum gave a slightly higher degree of surface shell reduction. Under steady-state WGS at a high H 2 O/CO ratio, the concentrations of surface formates are strongly limited at high CO conversions, while metalCO was not. Since under these conditions, CO exhibits a first order rate dependency, the active site should move to sparser coverages of CO, indicating that a formate mechanism is more likely the correct one. At low temperatures and conversions, the formates were close to the equilibrium adsorption/desorption coverages obtained from only CO adsorption. In situ X-ray absorption near edge spectroscopy (XANES) directly links the metal to its ability to aid in catalyzing reduction of the surface shell of ceria. After surface shell reduction of ceria by hydrogen, addition of water to the hydrogen stream gave no indication of reoxidation whatsoever, as would be necessary under a ceria-mediated redox process. The reoxidation of ceria by water under helium alone was very slow, and only slight changes were recorded at 350 °C. Therefore, the results strongly favor a formate mechanistic scheme for low temperature water-gas shift. To date, most researchers have claimed a ceria-mediated redox process operating to describe the mechanism. Both mechanisms require reduction of the ceria surface.

In situ diffuse reflectance mode (DRIFTS) measurements for adsorption of CO and under the water-gas shift (WGS) reaction revealed that formates emerge on the surface of reduced ceria after the reaction of CO with geminal OH groups. These groups are formed after reduction of the ceria surface shell. X-ray absorption near edge spectroscopy (XANES) results demonstrated that the process of surface shell reduction was strongly catalyzed by the presence of metal, while changing very little, if at all, the catalysis of bulk ceria reduction. For 1% Pt/ceria, under steady state WGS at a high H2O/CO ratio, surface formate concentrations were strongly limited at high CO conversions, while Pt-CO was affected only slightly. Under high H2O/CO ratios, CO exhibits a first order rate dependency, and therefore, the active site is expected to move to sparser coverages of CO, which suggested that the WGS mechanism likely proceeded via formates. Later, XANES work gave no evidence for the reoxidation of ceria surface by water under a hydrogen environment, which would be necessary to substantiate an alternate mechanism, referred to as the ceria-mediated redox process. Later, isotope switching from H2O to D2O was carried out to validate the possibility that decomposition of surface formates could be the rate limiting step for the mechanism, as was proposed and demonstrated earlier by Shido and Iwasawa. In agreement with their findings, we also observed a normal isotope effect, consistent with a link between the activation energy barrier of the rate limiting step to the breaking of the CH bond of the formate. In this study, a variety of metals were screened to try to gain further insight into the role played by the metal in the catalysis of metal/ceria systems for WGS. One group of metals was selected on the basis of reduction temperature, since spillover from reduced metal likely catalyzes the reduction of ceria surface. Therefore, we tested the following metals, moving from lowest to highest reduction temperature: Pt

Steady state infrared (IR) measurements for adsorption of only CO and under water–gas shift (WGS) reaction conditions indicate that formates are present on the surface of reduced ceria, and that their concentrations vary with surface area of partially reduced ceria. Under steady state WGS, the concentrations of surface formates are strongly limited at high CO conversions. However, at low temperatures and conversions, the formates are close to the equilibrium adsorption/desorption coverages obtained from only CO adsorption. Comparisons at constant temperature indicate that formate bands from IR may provide an indication of the number of active sites present on the catalyst surface, as the rates varied accordingly. The IR results favor a formate intermediate mechanism to explain WGS. However, more kinetic studies are required, and over a broad range of temperatures, to verify this conclusion. Previous low temperature kinetic studies at a relatively high CO/H2O ratios have produced a zero-order dependency for CO and the authors related this to a mechanistic scheme involving reaction of Pt-CO with CeO2 to yield CO2, followed by reoxidation of Ce2O3 by H2O, with liberation of H2. The zero-order was suggested to be due to saturation of noble metal surface with CO during WGS. Saturation of ceria with carbonates was also reported. In this study, a high H2O/CO ratio was used where the CO rate dependency was first-order. This criteria requires that the surface coverage of the adsorbed CO intermediate should be reaction rate limited. Therefore, the formates are suggested to be the intermediates.

Modeling of the supercritical fluid mixture indicated that an important increase in density occurs above a threshold of approximately 4 MPa for the reaction temperature of 220 °C studied. While transport parameters of the fluid are largely retained, the observed improvement in wax solubility was noteable. A cobalt catalyst (25%Co/γ-Al 2 O 3 ) was used in a fixed bed reactor under a pressure/density tuned supercritical fluid mixture of n -pentane/ n -hexane. By using inert gas as a balancing gas to maintain a constant pressure, the density of the supercritical fluid could be tuned near the supercritical point while maintaining constant space velocity within the reactor. The benefits of the mixture allowed for optimization of transport and solubility properties at an optimum reaction temperature for Fischer–Tropsch synthesis with a cobalt catalyst. Indeed, above 4 MPa, increases in wax yields from sampling and carefully controlled gas measurements using an internal standard demonstrated an important increase in conversion due to greater accessibility to active sites after extraction of heavy wax from the catalyst. Additional benefits included decreased methane and carbon dioxide selectivities. Decreased paraffin/(olefin+paraffin) selectivities with increasing carbon number were also observed, in line with extraction of the hydrocarbon from the pore. Faster diffusion rates of wax products resulted in lower residence times in the catalyst pores, and therefore, decreased probability for readsorption and reaction to the hydrogenated product. Even so, there was not an observable increase in the alpha value for higher carbon number products over that obtained with just the inert gas.

The conversion of CO/H2, CO2/H2 and (CO+CO2)/H2 mixtures using cobalt catalysts under typical Fischer–Tropsch synthesis conditions has been carried out. The results show that in the presence of CO, CO2 hydrogenation is slow. For the cases of only CO or only CO2 hydrogenation, similar catalytic activities were obtained but the selectivities were very different. For CO hydrogenation, normal Fischer–Tropsch synthesis product distributions were observed with an α of about 0.80; in contrast, the CO2 hydrogenation products contained about 70% or more of methane. Thus, CO2 and CO hydrogenation appears to follow different reaction pathways. The catalyst deactivates more rapidly for the conversion of CO than for CO2 even though the H2O/H2 ratio is at least two times larger for the conversion of CO2. Since the catalyst ages more slowly in the presence of the higher H2O/H2 conditions, it is concluded that water alone does not account for the deactivation and that there is a deactivation pathway that involves the assistance of CO.

Different regeneration strategies for a Pt-SO42−–ZrO2 catalyst were investigated. Two of the regeneration methods tested on the same deactivated catalyst included sulfur addition via a SO2 step followed by a reactivation in situ step. These methods produced regenerated catalysts, which were active for the conversion of n-hexadecane. However, their high activity was not adequately sustained when compared to a fresh catalyst. Two oxidation/regeneration methods were also investigated. One oxidation method was successful in producing a catalyst that could undergo multiple regenerations with the restoration of the activity to the level of the fresh catalyst and the retention of that activity over a period of time that was comparable to, or exceeded, that of the fresh catalyst.

A sample of Fe–Mn–SO 4 2− –ZrO 2 has been heated at 500°C in air for 98 h. At intervals, the sample was evacuated and transferred without atmospheric exposure to an XPS chamber. As noted with Pt–SO 4 2− –ZrO 2 , the O 1s peak resolved to a doublet; one of these peaks is interpreted to result by dehydration of the sulfate group. Following the 98 h air treatment, the sample was treated at 150°C at 1 atm in flowing hydrogen for a total of 78 h. The XPS spectra, obtained at intervals during the heating in hydrogen, showed that both Fe and Mn remained in an oxidized state.

Sulfur was added to hydrous zirconia using a number of methods. Sulfur was added using ammonium sulfate, H 2 S, SO 2 as well as H 2 SO 4 . The impact of the method of sulfur addition on the properties of the catalysts is reported. The activity was evaluated for all catalysts prepared by measuring the conversion of n-hexadecane in batch reactors. The H 2 SO 4 , ammonium sulfate and SO 2 methods of sulfur addition produced the most active catalysts. These catalysts were then studied in a trickle bed reactor. The hydrocracking and hydroisomerization yields were independent of the method of sulfur addition and strictly a function of conversion. One of the methods using SO 2 produced a catalyst which showed high activity for over 500 h on stream.

Platinum is present in a metallic state following activation in air at 725‡C of both 5 wt% Pt/ZrO2 and 5 wt% Pt/SO 4 2− /ZrO2. Reduction of either catalyst at 725‡C produces a Pt-Zr alloy, and these reduced catalysts, upon recalcination in air at 725‡C, form metallic Pt crystallites. Likewise, reduction of these uncalcined catalysts at 725‡C in H2 leads to a Pt-Zr alloy formation. However, treatment of these uncalcined catalysts in H2 at 450‡C does not produce Pt crystallites large enough to detect by XRD.

The hydroisomerization and hydrocracking of n-hexadecane were studied using a sulfated zirconia catalyst containing 0.5 wt % Pt. The conversions were obtained using both batch microautoclaves and continuous trickle bed reactors. Various process conditions were studied in both reactors using a reaction temperature of 150 °C. Hexadecane conversions varied from 5 to 96 wt % depending on the process conditions. At lower conversions ( 80 %), cracking products accounted for most of the total conversion. The asymmetric distribution of the cracked products obtained with this catalyst is probably due to secondary cracking

Pt monofunctional and bifunctional catalysts produce a common set of products apart from the C 8 -aromatic and i-C 8 -alkane distributions at low conversions. The difference between the two types of catalysts therefore is essentially that of the rate and selectivity of producing aromatics vs. C 1 C 7 products. Overall, the bifunctional pathway provides both the most rapid and the most selective pathway for the formation of aromatics from n -alkanes. The bifunctional pathway provides a nearly equilibrium distribution of methyl heptanes whereas the monofunctional platinum pathway produces predominantly the isomers allowed from hydrogenolysis of alkylcyclopentanes formed by the five-carbon ring cyclization of the n -alkane. The non-acidic Pt catalyst produces aromatics by direct six-carbon ring formation. The acidic catalyst produces essentially a mixture of C 8 -aromatics that is near the thermodynamic equilibrium mixture, presumably because of isomerization reactions that proceed by metal catalyzed cyclization and by acid catalysed cyclization.

The data presented in this study illustrate the similarities and differences in the yield and selectivities obtained from different types of coal liquefaction reactors. The results suggest that the comparison of data in the literature obtained from different reactors should be done with careful consideration. The differences and similarities in the yields obtained depend not only on the reactor type but also on the feedstock employed and the residence time in the reactors. Data generated using microreactors are adequate for the selection of operating conditions for conversion in larger scale reactors.

The n -octane conversion data at 100 psig conditions show that the presence of tin in a Pt-alumina catalyst increases conversion when a non-acidic support is used but decreases the activity of a catalyst with an acidic alumina support. The above observation applies for catalysts prepared by coimpregnation of Sn and Pt onto the support, but not for a catalyst prepared by coprecipitation of a tin-aluminum oxide support which is then impregnated with Pt. For the catalysts prepared by coimpregnation, tin decreases the activity by poisoning acid sites that are needed for the bifunctional cyclization pathway but this is not the case for the coprecipitated catalyst for a similar Sn/Pt ratio. The role of tin in altering the activity and selectivity of Pt catalysts is correlated to catalyst characterization data.