Reforming of C6 Hydrocarbons Over Model Pt Nanoparticle Catalysts

Size-controlled model Pt nanoparticle catalysts, synthesized by colloidal chemistry, were used to study the hydrogenative reforming of three C₆ hydrocarbons in mixtures with 5:1 excess of H₂: methylcyclopentane, n-hexane and 2-methylpentane. We found a strong particle size dependence on the distribution of different reaction products for the hydrogenolysis of methylcyclopentane. The reactions of 50?Torr methylcyclopentane in 250?Torr H₂ at 320 °C, using 1.5 and 3.0 nm Pt nanoparticles produced predominantly C₆ isomers, especially 2-methylpentane, whereas 5.2 and 11.3 nm Pt nanoparticles were more selective for the formation of benzene. For the hydrogenolysis of n-hexane and 2-methylpentane, strong particle size effects on the turnover rates were observed. Hexane and 2-methylpentane reacted up to an order of magnitude slower over 3.0 nm Pt than over the other particle sizes. At 360 °C the isomerization reactions were more selective than the other reaction pathways over 3.0?nm Pt, which also yielded relatively less benzene.     

Effect of hydrochlorination and hydrofluorination of Pt/H-ZSM-5 and Pt–Ir/H-ZSM-5 catalysts for n-hexane hydroconversion

Pt/H-ZSM-5 and Pt–Ir/H-ZSM-5 catalysts were hydrochlorinated or hydrofluorinated with 3.0 wt%HCl or HF, respectively. These mono- and bimetallic catalysts were tested for n-hexane hydroconversion in a pulsed microcatalytic reactor in a flow of H₂ gas. The catalysts were characterized via XRD, metal dispersion via H₂ chemisorption and acid site strength distribution using temperature programmed desorption of ammonia (TPD). Although metallic promoters frequently cause inhibition of the catalytic activities of Pt and since iridium is less active than platinum, fortunately, Ir was found to enhance the hydroconversion activities of the current catalysts, particularly after hydrochlorination.     

Studying the role of the state of platinum in Pt/SO4/ZrO2/Al2O3 catalysts in the isomerization of n-hexane

Samples of SO₄/ZrO₂/Al₂O₃ and Pt/Al₂O₃ Pt/Al₂O₃ catalysts and their physical mixtures are prepared, and the catalytic properties of the samples in n-hexane isomerization are studied. The considerable effect of the state of platinum on the catalytic performance of the samples is revealed. IR spectroscopy (CO_ads), oxygen chemisorption, and oxygen-hydrogen titration show that the reduced catalysts contain ionic forms of platinum capable of adsorbing up to three hydrogen atoms per each surface atom of platinum. By means of H/D isotopic exchange, it is found that specific properties of ionic platinum are apparent in the formation of the hydride form of adsorbed hydrogen. It is speculated that the activity and stability of catalysts based on sulfated zirconia in n-hexane isomerization can be attributed to the involvement of ionic and metallic platinum in the activation of hydrogen. The results can be used to develop effective catalysts for the isomerization of C₅–C₆ gasoline fractions in order to obtain the isomerizate as a high-octane additive for modern gasolines.     

The calcination temperature after platinum addition and its effect on Pt/WOx–ZrO2 properties

Platinum on tungsten oxide promoted zirconia was prepared using two different platinum precursors: hexachloroplatinic acid and tetraammine platinum nitrate. The catalysts were calcined at different temperatures after platinum addition. The materials were characterized by thermal-programmed reduction, transmission electron microscopy, hydrogen chemisorption at room temperature and they were used for n-hexane isomerization reaction. The calcination temperature after metal addition plays an important role on the catalyst properties. Calcination above 500 °C after platinum addition is not convenient because of the migration of zirconia species over Pt, thus decreasing the hydrogen dissociation capacity of Pt.     

Hydrogenative ring opening of propylcyclopropane over silica-supported Pt and Pd catalysts

The effects of temperature and hydrogen pressure on the hydrogenative ring opening of propylcyclopropane over Pt/SiO₂ and Pd/SiO₂ catalysts were studied. Temperature dependence in the 323–373 K temperature range in a pulse system was investigated, while a static recirculation reactor was used for hydrogen pressure dependence measurements. The ring-opening reactions took place exclusively at all temperatures and hydrogen pressures studied. Monotonous increase was observed for the reactivity of propylcyclopropane as a function of temperature. At constant temperature, the reaction rate vs. hydrogen pressure dependence curves passed through a maximum indicating dissociative adsorption over both Pt and Pd catalysts. The scission of the sterically less hindered direction (producing 2-methylpentane) was the major reaction pathway on both catalysts with practically identical regioselectivity values. On the basis of these results aselective mechanism was proposed for the ring-opening reaction.     

Analysis and Modeling of the Hydroisomerization Process for <Emphasis Type="Italic">n</Emphasis>-Hexane

A stage six-path mechanism for the reaction of n-hexane hydroisomerization on a BEA Pt-zeolite catalyst has been proposed. The mass flow rate of n-hexane in the feed stream varies in the range of 0.7?2.8 h^?1; the mole ratio of hydrogen to the n-hexane range of 2.7–14.6; the reactor temperature falls in the range from 453 to 573 K; and the gas flow pressure varies in the range of 1.0–10.0 atm. Thirty experiments have been carried out. In the product stream, the concentrations of n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane, propane, C₁-alkanes, and C₂-alkanes are analyzed by gas chromatography. A kinetic model has been constructed for the six-path mechanism of the reaction of n-hexane hydroisomerization, which contains 15 kinetic constants. Based on the results of laboratory experiments, the kinetic constants of the model have been estimated by the method of nonlinear least squares. The model has been shown to correspond with the experiment in the selected field of experimentation.     

SO2 Reactions over γ-Al2O3,Pt/γ-Al2O3,Sn/γ-Al2O3 and Pt–Sn/γ-Al2O3

Platinum and Platinum–tin bimetallic catalysts supported on alumina were prepared by co-impregnation of both metallic precursors on the support and used as catalysts for the oxidation of SO₂. Platinum dispersion was determined by means of H₂–O₂ titration. Tin addition (1 and 2 wt%) only slightly decreased the exposed platinum atoms suggesting that tin is mainly over the support. At temperatures lower than 300 °C, SO₂ did not react with oxygen. Nevertheless, when the temperature was increased, the SO₂ oxidation began. The ignition temperatures for SO₂ oxidation (taken at 50% conversion) were 345 °C for 1% Pt/Al₂O₃ and 520 °C for 1% Pt–2% Sn/Al₂O₃. The strong displacement on activity suggests that tin plays an important role as inhibitor of the SO₂ oxidation reaction.     

Hydroconversion of n-paraffins in light naphtha using Pt/Al2O3 catalysts promoted with noble metals and/or chlorine

The effect of combining 0.35 wt.% Pt with 0.35 wt.% of either Ir, Rh, Re or U on γ-Al₂O₃ support was investigated for the hydroconversion of n-pentane and n-hexane in a pulsed micro-reactor system at a temperature range of 300–500°C, except for Rh/Al₂O₃ (150–500°C). The dispersion of the metals in the catalysts under study was determined by H₂ chemisorption. The effect of chlorine addition between 1.0 and 6.0 wt.% was investigated and a content of 3.0 wt.% Cl being of optimum promotion. Highest activities for hydroisomerization, hydrocracking and hydrogenolysis were exhibited by Pt, Ir and Rh/Al₂O₃ catalysts, respectively, whether the catalysts were Cl-free or containing 3% Cl. However, Re and U catalysts were inactive. Maximum hydroisomerization selectivities using chlorinated bimetallic catalysts could be arranged in the following order: PtU/Al₂O₃>PtRe/Al₂O₃>PtIr/Al₂O₃>PtRh/Al₂O₃. However, PtRh/Al₂O₃, before and after chlorination, was the most active catalyst for hydrogenolysis. The apparent reaction rate constants as well as the apparent activation energies (E_a) for the hydroconversion of n-pentane and n-hexane were calculated and the compensation effect relationship between E_a and logarithm of the pre-exponential factor was estimated. n-hexane reaction on PtRh/Al₂O₃ catalysts deviates from this relationship for mechanistic variation.     

Coking characteristics of reforming catalysts

Coking rates were measured for two different γ-aluminas, each with and without platinum, under near commercial conditions using a gravimetric reactor. Coke on catalyst was characterized by a Temperature-Programmed Oxidation (TPO) technique. With a naphtha feed, coke formed on both aluminas at rates related to the respective population of α-sites as measured by IR. For the corresponding Pt on alumina catalysts, coke, as measured by TPO, predominantly formed on sites associated with alumina (“alumina coke”), while coke associated with Pt (“Pt coke”), was relatively minor. With a n-heptane feed, under the same conditions, coke formation on both aluminas was much less than with the naphtha feed. However, the corresponding Pt on alumina catalysts generated comparatively more coke with a higher proportion associated with Pt. A correspondence between this proportion of “Pt coke” and the decline in reforming activity was observed. It is postulated that most of the coke produced during naphtha reforming with an active catalyst is formed by a reaction between α-sites on alumina and certain components in the feed via a polymerization mechanism. This type of coke has minimal effect on the reforming reactivity of the catalyst. However, in n-heptane reforming, about 50% of the coke also results from precursors formed from reactions with Pt. In either case, coke associated with Pt appears to be the probable cause of deactivation.     

On the role of adsorbed atomic oxygen and CO2 in copper based methanol synthesis catalysts

The reactions of cyclohexane (6.7 kPa) andn-hexane (13.3 kPa) in H₂ (450 kPa) were studied on platinum-rhenium model catalysts in the range 570–740 K. The catalysts were prepared by depositing rhenium on a platinum foil or platinum on a rhenium foil. The catalyst total area was 0.5–1.0 cm². The surfaces were pre-sulfided before reaction and thiophene added to the feed to maintain the surface sulfided under reaction conditions. Sulfur binds with rhenium forming a catalytically inactive rhenium-sulfur compound. As the concentration of rhenium-sulfur on the surface is increased and the platinum ensembles become smaller, only dehydro-genation-hydrogenation activity is observed. However, the catalyst becomes more resistant to deactivation.     

Catalysts based on ferrierite for the selective hydrocracking of <Emphasis Type="Italic">n</Emphasis>-hexane

Supported platinum catalysts based on ferrierite type zeolite with Pt content in the range of 1.3–2.8 wt % are prepared. The localization of Pt in zeolite channels is studied for the first time. It is shown that the platinum localized in them increases the yield of products from the selective hydrocracking of n-hexane. Platinum on the outer surface of the zeolite crystals participates in the transformation of n-hexane in the direction of forming hydrocarbons with isomeric structure. The catalysts can be used to improve the octane rating of reformed gasoline, owing to the selective removal of low-octane n-paraffin hydrocarbons and the increase of the portion of high-octane isoparaffin hydrocarbons in the catalytic reforming gasolines.     

Macroporous Monolithic Pt/γ-Al2O3 and K–Pt/γ-Al2O3 Catalysts Used for Preferential Oxidation of CO

Macro-porous monolithic γ-Al₂O₃ was prepared by using macro-porous polystyrene monolith foam as the template and alumina sol as the precursor. Platinum and potassium were loaded on the support by impregnation method. TG, XRD, N₂ adsorption–desorption, SEM, TEM, and TPR techniques were used for catalysts characterization, and the catalytic performance of macro-porous monolithic Pt/γ-Al₂O₃ and K–Pt/γ-Al₂O₃ catalysts were tested in hydrogen-rich stream for CO preferential oxidation (CO-PROX). SEM images show that the macropores in the macro-porous monolithic γ-Al₂O₃ are interconnected with the pore size in the range of 10 to 50 μm, and the monoliths possess hierarchical macro-meso(micro)-porous structure. The macro-porous monolithic catalysts, although they are less active intrinsically than the particle ones, exhibit higher CO conversion and higher O₂ to CO oxidation selectivity than particle catalysts at high reaction temperatures, which is proposed to be owing to its hierarchical macro-meso(micro) -porous structure. Adding potassium lead to marked improvement of the catalytic performance, owing to intrinsic activity and platinum dispersion increase resulted from K-doping. CO in hydrogen-rich gases can be removed to 10 ppm over monolithic K–Pt/γ-Al₂O₃ by CO-PROX.     

On the deactivation of Pt/L-zeolite catalysts

Pt/L-zeolite catalysts have a unique activity forn-hexane aromatization to benzene. There have been proposals which attribute this to electronic and to geometric origins of the L-zeolite. Recently, the uniqueness of the L-zeolite support has been understood to derive from the ability to stabilize very small particles in a non-acidic environment and it has been proposed that a further stabilization against deactivation (by geometric constraint of bimolecular coke precursor reactions) is what distinguishes these catalysts relative to SiO₂ supported small Pt particles. We have investigated the initial deactivation rate of four Pt/L-zeolite catalysts and a Pt/SiO₂ reference during reaction ofn-hexane, neopentane and 2-methyl-2-pentene. In all cases, the relative rates of deactivation correlate with the apparent acidity (as determined by competitive benzene/toluene hydrogenation) suggesting that the deactivation stabilization may have an electronic component.     

Preparation of PtHY catalysts: Influence on the catalytic properties of the complexes used as platinum precursors

Three cation complexes: [Pt(NH₃)₄]²⁺, [Pt(NH₃)₂ (H₂O)₂]²⁺ and [Pt(H₂O)₄]²⁺ were used for the preparation of 0.6 wt.-% PtHY catalysts. The platinum dispersion depended on the platinum complex used as a precursor and, for a given complex, on the conditions of activation. The platinum dispersion of the catalysts prepared from the tetra-aquo platinum complex was very low, but this could be related to the great lability of the water ligands. It was also this lability which explained why the platinum dispersion was about twice as low for PtHY catalysts prepared from the diaquo-diammine complex than for those prepared from the tetra-ammine complex. However in the case of the two latter complexes, the activation conditions influenced the platinum dispersion in the same way: a highly marked effect of the calcination conditions and a small effect of the subsequent treatment under hydrogen flow. The behaviour of the PtHY catalysts for the bifunctional transformation of n-heptane did not depend on the platinum precursor. The activity and the selectivity of these catalysts depended only on their hydrogenating activity.     

Channel effect ofnC6 aromatization on Pt/KL catalysts

Designed Pt/KL catalysts were prepared and characterized by CO chemisorption and XRD. Pulse catalytic tests usingn-hexane as the probe were performed. The results showed that Pt particles located in the L-zeolite channels were fundamental for aromatization, and the spatial effect of the channels was essential for dehydrocyclization. Obstruction of the channels, while having no influence on the high Pt dispersion, affects the product distribution ofnC₆ in addition to a sharp reduction in reactivity.     

Comparison of n-Pentane Reforming Over Pt Supported on Amorphous and γ-Al2O3

The n-pentane reforming activity of Pt supported on nonhydrolytic amorphous Al₂O₃ (Pt/NH–Al₂O₃), was investigated and compared to the catalytic activity of Pt supported on crystalline -Al₂O₃. The Pt was introduced by (a) impregnation with either a solution of H₂PtCl₆ in water or a solution of platinum acetylacetonate (PtAcac) in toluene; (b) in situ introduction of a Pt precursor, either PtBr₄ or cis-bis(benzonitrile)platinum dichloride, before gelation of the NH alumina. The rate-controlling step in the reforming of n-pentane for both amorphous and crystalline aluminas was found to be the reaction on the alumina acidic sites. The Pt/-Al₂O₃ catalysts exhibit higher conversions of n-pentane and higher selectivity to isopentane, than the corresponding amorphous alumina samples. After 1.5 h at 400 °C, the highest conversion of the -Al₂O₃-based catalysts was 47% with 20.3% selectivity to isopentane. The highest conversion of the NH–Al₂O₃-based catalysts under the same conditions was only 26% with 13.6% selectivity to isopentane. The high intrinsic Cl content (2.6wt%) of the amorphous alumina was found to have a minor effect on the activity of the alumina, compared to the activity of the more ordered -alumina. Catalysts prepared by impregnation of the NH alumina with aqueous chloroplatinic acid, exhibited higher conversions compared to catalysts prepared by impregnation of the NH alumina with a solution of PtAcac in toluene. This result occurred in spite of the lower surface area and lower Pt dispersions of the chloroplatinic acid-impregnated catalysts, and is explained by the formation of microcrystalline surface structures and existence of surface chlorine.     

Promotion effect of Sn in alumina-supported Pt catalysts for CO-PROX

In this work we have studied the effect of the addition of Sn to alumina-supported Pt catalysts towards the catalytic performance in CO-PROX reaction. Monometallic Pt and Sn catalysts supported on alumina, and bimetallic Pt–Sn supported on alumina (with Pt/Sn atomic ratios of 1.92, 0.53 and 0.28) was prepared by successive impregnation, with high dispersion of the metal. The addition of Sn to Pt does not substantially increase the activity in CO-PROX at low temperatures; however, the temperature interval where the CO conversion is maximum was significantly increased. The optimum Pt/Sn atomic ratio was found to be 0.53. In a wide operation window with respect to temperature, the catalyst with optimum Pt to Sn ratio shows a maximum CO conversion of 78% for λ = 2 with constant selectivity (about 40%) and with 31%CO yield. In the presence of either CO₂ or H₂O the performance of Sn promoted catalyst was seen to show improved activity.     

On the Role of Calcination Temperature in Pt-SO 4 2− /ZrO2−Al2O3 Preparation and Catalytic Behaviors During the n-Hexane Hydroisomerization

Effects of calcination temperature for Pt-SO ₄ ^2? /ZrO₂?Al₂O₃ (PSZA) catalysts in n-hexane hydroisomerization were investigated by N₂-adsorption, XRD, TG-DTA, FTIR, XPS and H₂-TPR. An optimum calcination temperature is helpful to complete the crystallization process, resulting in better distribution of alumina into zirconia crystal and producing new acid centers responsible for higher catalytic activity.     

Sintering‐induced aromatization during n‐heptane reforming on Pt/Al2O3 catalysts

A platinum/alumina catalyst was sintered in oxygen and hydrogen atmospheres using two metal loadings of the catalyst: 0.3% Pt and 0.6% Pt. After sintering, the aromatization selectivity was investigated with the reforming of n‐heptane as the model reaction at a temperature of 500 °C and a pressure of 391.8 kPa. The primary products of n‐heptane reforming on the fresh platinum catalysts were methane and toluene, with subsequent conversion of benzene from toluene demethylation. To induce sintering, the catalysts were treated with oxygen at a flow rate of 60 mL min^?1, pressure of 195.9 kPa and temperatures between 500 and 800 °C. The 0.3% Pt/Al₂O₃ catalyst exhibited enhanced aromatization selectivity at various sintering temperatures while the 0.6% Pt/Al₂O₃ catalyst was inherently hydrogenolytic. The fact that aromatization was absent on the 0.6% Pt/Al₂O₃ catalyst was attributed to the presence of surface structures with dimensionality between two and three as opposed to essentially 2‐D structures on the 0.3% Pt/Al₂O₃ catalyst surface. On the 0.3% Pt/Al₂O₃ catalyst, the reaction product ranged from only toluene at a 500 °C sintering temperature to predominantly cracked product at a sintering temperature of 650 °C and no reaction at 800 °C. For sintering at about 650 °C, subsequent conversion of n‐heptane was complete and dropped thereafter. The turnover number was observed to change from 0.07 to 2.26 s^?1 as the dispersion changed from 0.33 to 0.09. The Koros–Nowark (K–N) test was used to check for the presence of internal diffusional incursions and Boudart's criterion was used for structural sensitivity determination. The K–N test indicated the absence of diffusional resistances while n‐heptane reforming was found to be structure sensitive on the Pt/Al₂O₃ catalyst. Copyright © 2006 Society of Chemical Industry     

CO Poisoning of Ethylene Hydrogenation over Pt Catalysts: A Comparison of Pt(111) Single Crystal and Pt Nanoparticle Activities

The ethylene hydrogenation reaction was studied on two platinum model catalyst systems in the presence of carbon monoxide to examine poisoning effects. The catalysts were a Pt(111) single crystal and lithographically fabricated platinum nanoparticles deposited on alumina. Gas chromatographic results for Pt(111) show that CO adsorption reduces the turnover rate from 10¹ to 10^-2 molecules/Pt site/s at 413 K, and the activation energy for hydrogenation on the poisoned surface becomes 20.2 ± 0.1 kcal/mol. The activation energy for ethylene hydrogenation over Pt(111) in the absence of CO is 10.8 kcal/mol. The Pt nanoparticle system shows the same rate for the reaction as over Pt(111) in the absence of CO. When CO is adsorbed on the Pt nanoparticle array, the rate of the reaction is reduced from 10² to 10⁰ nmol/s at 413 K. However, the activation energy remains largely unchanged. The Pt nanoparticles show an apparent activation energy for ethylene hydrogenation of 10.2 ± 0.2 kcal/mol in the absence of CO and 11.4 ± 0.6 kcal/mol on the CO-poisoned nanoparticle array. This is the first observation of a significant difference in catalytic behavior between Pt(111) and the Pt nanoparticle arrays. It is proposed that the active sites at the oxide--metal interface are responsible for the difference in activation energies for the hydrogenation reaction over the two model platinum catalysts.