Transformation of α-olefins over Pt–M (M = Re,Sn, Ge) supported chlorinated alumina

Three bimetallic catalysts, consisting of platinum and a second metal supported on chlorinated alumina, i.e. Pt–Re/Al₂O₃, Pt–Sn/Al₂O₃, and Pt–Ge/Al₂O₃, have been used in the transformation of two α-olefins, 1-pentene and 1-hexene. Their conversion to internal and branched olefins is highly interesting for their use in reformulated gasolines and as intermediate chemicals. The catalysts characterization has been accomplished by different techniques, such as elemental and XRD analysis, N₂ adsorption, TPD of ammonia and hydrogen chemisorption. Among the three catalysts, Pt–Sn/Al₂O₃ showed the highest hydrogenation activity, whereas Pt–Ge/Al₂O₃ was the less active towards the hydrogenation of the olefins, according to their H₂ adsorption abilities. The bimetallic catalysts were compared to the monometallic one (Pt/Al₂O₃), as well as to the support. The catalyst and reaction conditions significantly influenced the products distribution. Hydrogenation was dominant at low temperatures (up to 350–400 °C) while skeletal isomerization and double bond shift prevailed at higher temperatures.     

Highly selective and stable multimetallic catalysts for propane dehydrogenation

Different mono (Pt), bi (Pt–Sn, Pt–Pb, Pt–Ga) and trimetallic (Pt–Sn–Ga) catalysts based on Pt and supported on different materials (Al₂O₃, Al₂O₃–K and ZnAl₂O₄) were tested under severe process conditions in the propane dehydrogenation reaction (both in continuous and in pulse reactors). Results show that the Pt–Sn–Ga/ZnAl₂O₄ catalyst has a better and more stable performance in propane dehydrogenation (high yield to propene and low coke deposition), than the other bi‐ and trimetallic systems and a commercial catalyst. Thus, the use of an adequate support (ZnAl₂O₄) in combination with the addition of Ga to the Pt–Sn bimetallic system enhances the catalytic performance. © 2000 Society of Chemical Industry     

Enantioselective hydrogenation of ethyl pyruvate on tin promoted Pt/Al2O3 catalysts

The enantioselective hydrogenation of ethyl pyruvate has been studied on a Pt/Al₂O₃-dihydrocinchonidine catalyst promoted with different amount of tin. The surface reaction between hydrogen adsorbed on Pt and tin tetraalkyls is used for the tin introduction. This reaction leads to the formation of surface organometallic complexes (I), with SnR_(4-x) moieties anchored to the platinum surface. The enantioselectivity of the Pt/Al₂O₃-dihydrocinchonidine catalyst is found to change only slightly upon promotion with tin, while the rate of ethyl pyruvate hydrogenation depends strongly on the amount and the form of tin introduced. The hydrogenation activity is suppressed completely at relatively low tin coverage (Sn/Pt_s < 0.06). The highest hydrogenation rate is measured over catalysts containing complex (I) (Sn/Pt_s = 0.025) on the platinum surface. On Sn-Pt alloy type active sites, which are formed after decomposition of (I) in hydrogen, the rate of hydrogation is considerably lower than on the unpromoted reference Pt/Al₂O₃ catalyst.On leave from the Central Research Institute for Chemistry of the Hungarian Academy of Sciences.     

Pt催化丙烷脱氢过程中结焦反应的粒径效应与Sn的作用

用乙二醇还原法制备了Pt颗粒平均粒径分别为2.0、4.6、12.1 nm的Pt/Al₂O₃催化剂,同时用浸渍法制备了PtSn/Al₂O₃双金属催化剂,并考察了各催化剂在丙烷脱氢过程中的结焦行为。分别用H₂化学吸附、透射电镜、热重分析、元素分析、红外光谱、拉曼光谱等手段对催化剂进行了表征。表征结果显示,催化剂金属上的结焦速率与Pt金属颗粒粒径密切相关。具有较小Pt颗粒的催化剂金属上的结焦速率明显大于具有较大Pt颗粒的催化剂。具有较小Pt颗粒的催化剂上生成的焦含有较少的氢,其石墨化程度也较高。本研究中PtSn/Al₂O₃催化剂金属上的结焦速率高于Pt/Al₂O₃催化剂,并且在双金属上生成的焦具有更高的石墨化程度。结合Pt/Al₂O₃催化剂上的结焦机理,对高性能丙烷脱氢催化剂提出了新的概念设计。     

Hydrogen peroxide modified Mg–Al–O oxides supported Pt–Sn catalysts for paraffin dehydrogenation

In this work, a new method to prepare Mg–Al–O oxide by co-precipitation method with addition of H₂O₂ was developed. The application of Mg–Al–O as a support of Pt–Sn catalysts for paraffin dehydrogenation was investigated. Characterization results indicated that modification of H₂O₂ (i) enlarged the pore structure and decreased the density and mechanical strength of the Mg–Al–O oxide and (ii) increased the platinum dispersion, weakened the acidity of the Pt–Sn/Mg–Al–O catalysts. Dehydrogenation of n-dodecane results showed that modification of H₂O₂ (iii) improved the paraffin conversion and olefin selectivity; (iv) inhibited the coke deposition on the catalyst and (v) extended the catalyst life-time.     

Dehydrogenation of isobutane over Sn/Pt/Na-ZSM-5 catalysts: The effect of SiO<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ratio,amount and distribution of Pt nanoparticles on the catalytic behavior

Sn/Pt/Na-ZSM-5 was used as catalyst for the dehydrogenation of isobutane, and the effect of SiO₂/Al₂O₃ ratio and the dispersion of Pt nanoparticles on the conversion and product selectivity were studied under atmospheric pressure at 848 K. The catalysts were characterized by various techniques such as H₂ chemisorption, TEM, SEM, EDX, XRD, FT-IR, TG/DTG, elemental analysis by XRF and ICP techniques. Higher dispersion of Pt nanoparticles in the catalyst with SiO₂/Al₂O₃ ratio of 40 resulted in higher selectivity for isobutene.     

FTIR and XPS study of supported PtSn catalysts used for light paraffins dehydrogenation

A comparison between the characteristics of the metallic phase (studied by FTIR and XPS) of Pt and PtSn catalysts supported on Al₂O₃, K-doped Al₂O₃ and MgO (used for light paraffins dehydrogenation reactions) is reported in this paper. The beneficial effects produced by tin addition to platinum, both in the increase of the selectivity to propene and the low coke formation, would be related with the possible electronic modifications of Pt by Sn, with probable formation of alloys, mainly for Al₂O₃ and MgO supported bimetallic catalysts. On the other hand, the modification of the electronic state of Pt by Sn addition appears to be of a minor importance in bimetallic catalysts supported on K-doped Al₂O₃.     

Effect of Catalyst Supports on Water‐Gas Shift Reaction at Ultrahigh Temperatures Using Syngas

Monometallic and bimetallic catalysts (Pt, Ni, and Pt‐Ni) with single support (Al₂O₃, TiO₂) and composite support (CeO₂/Al₂O₃, CeO₂/TiO₂) were prepared and tested for water‐gas shift reaction in a tubular quartz reactor. Syngas and steam with different steam‐to‐carbon ratios served as feedstock. The operating pressure was fixed while the reaction temperature was varied. The measured results indicated that the monometallic Ni/Al₂O₃ catalyst exhibits the lowest CO conversion and H₂ yield as compared with other catalysts. About the same CO conversion can be obtained from Pt and Pt‐Ni catalysts with single or composite support. However, higher H₂ yield can be achieved from the TiO₂‐supported catalyst compared with those supported by Al₂O₃. The experimental data also indicated that good thermal stability can be reached for the Pt‐based catalysts studied.     

Platinum incorporated into the SBA-15 mesostructure via deposition-precipitation method: Pt nanoparticle size estimation and catalytic testing

We report the use of the deposition precipitation (DP) method for the functionalization of mesoporous silica SBA-15 with Pt. [Pt(NH₃)₄](OH)₂ was used as a platinum precursor. Experiments were performed at 90 °C, and urea was used to control the pH during precipitation. From the obtained pH profiles, an interaction between the support and the precipitating species is suggested. A general formula of the species is proposed to be [Pt^II(OH)_n] _s ^II-n . By Transmission electron microscopy (TEM) it is shown that the majority of Pt nanoparticles on SBA-15 reside in the range between 2 and 4 nm. However, particles in the range of 15 nm were also detected, which indicates that the precipitation does not occur exclusively within the channels of the SBA-15. The obtained material is compared to a reference catalyst, Pt/SBA-15, prepared by a conventional wet impregnation method. Here, larger Pt nanoparticles (4–6 nm) were detected. The catalysts were found to exhibit comparable activity in toluene hydrogenation in terms of turnover frequency based on CO chemisorption.     

Effect of magnesium addition on catalytic performance of PtSnK/γ-Al2O3 catalyst for isobutane dehydrogenation

The effect of magnesium addition on the catalytic properties of PtSnK/γ-Al₂O₃ catalyst for isobutane dehydrogenation has been investigated by reaction tests and some physicochemical characterizations such as nitrogen adsorption, TEM, H₂ chemisorption, TPR, H₂-TPD, and TPO. It was found that with the suitable addition of magnesium (0.2 and 0.4 wt.%), the platinum dispersion increased, while the carbon depositions decreased. The presence of Mg in the PtSnMgK/γ-Al₂O₃ catalysts could not only strengthen the Sn-Al₂O₃ interaction, but also stabilize the oxidation states of Sn species, which resulted in the increased reaction activity and stability. However, when the content of magnesium was excessive (0.6 and 0.8 wt.%), the character of platinum and the interfacial properties between the metal and the support changed evidently, which was disadvantageous to the reaction. In our experiments, addition of 0.4 wt.% Mg to PtSnK/Al₂O₃ catalyst showed the best catalytic performance. After reaction for 6 h, selectivity toward isobutene of higher than 94% was achieved with the corresponding conversion value of about 29.0%.     

n‐octane reforming over alumina‐supported Pt, Pt–Sn and Pt–W catalysts

Pt, Pt–Sn and Pt–W supported on γ‐Al₂O₃ were prepared and characterized by H₂ chemisorption, TEM, TPR, test reactions of n‐C₈ reforming (500°C), cyclohexane dehydrogenation (315°C) and n‐C₅ isomerization (500°C), and TPO of the used catalysts. Pt is completely reduced to Pt⁰, but only a small fraction of Sn and of W oxides are reduced to metal. The second element decreases the metallic properties of Pt (H₂ chemisorption and dehydrogenation activity) but increases dehydrocyclization and stability. In spite of the large decrease in dehydrogenation activity of Pt in the bimetallics, the metallic function is not the controlling function of the bifunctional mechanisms of dehydrocyclization. Pt–Sn/Al₂O₃ is the best catalyst with the highest acid to metallic functions ratio (due to its lower metallic activity) presenting a xylenes distribution different from the other catalysts. The acid function of Pt–Sn/Al₂O₃ is tuned in order to increase isomerization and cyclization and to decrease cracking, as compared to Pt and Pt–W. This revised version was published online in July 2006 with corrections to the Cover Date.     

Differences in coke burning-off from Pt–Sn/Al2O3 catalyst with oxygen or ozone

Pt–Sn/γ-Al₂O₃ catalysts with different Sn loadings were prepared by incipient wetness coimpregnation of γ-Al₂O₃ with H₂PtCl₆ and SnCl₂. The Pt–Sn interaction was tested by temperature-programmed reduction and the catalytic activity was measured by cyclohexane dehydrogenation. The catalysts were coked by cyclopentane at 500 °C and totally or partially decoked with O₂ at 450 °C or O₃ at 125 °C. Coke deposits were studied by TPO and the catalytic activity of coked catalysts, partially or totally regenerated, by cyclohexane dehydrogenation.The TPO with O₃ shows that coke combustion with O₃ starts at a low temperature and has a maximum at 150 °C, that is a compensation between the increase of the burning rate and the rate of O₃ decomposition when increasing the temperature. Meanwhile O₂ burns coke with a maximum at 500 °C. When performing partial decoking with O₃ (125 °C) the remaining coke is more oxygenated and easier to burn than the coke that remains after decoking with O₂ (450 °C).After burning with O₃ the dehydrogenation activity of the fresh catalyst is recovered, while after burning with O₂ the activity is higher than that of the fresh catalyst. The burning with O₃ practically does not change the original Pt–Sn interaction while the burning with O₂ produces a decrease in the interaction, producing free Pt sites with higher dehydrogenation capacity.The differences in coke combustion with O₃ and O₂ are due to the different form of generation of activated oxygen, the species that oxidizes the coke. O₃ is activated by the γ-Al₂O₃ support at low temperatures firstly eliminating coke from the support while O₂ is activated by Pt at temperatures higher than 450 °C and the coke removal starts on the metal. Then, the recovery of the Pt catalytic activity as a function of coke elimination is faster with O₂ than with O₃.     

An EXAFS study of the interaction of different reactant gases over nanometer scale Pt clusters deposited on γ-Al2O3

Face to the difficulty to find an efficient catalyst in the reduction of NO by hydrocarbon in diesel exhaust gases, we have examined the interaction of some concerned reactant gases, NO, C₃H₆ and O₂, with a catalyst constituted of nanometer scale platinum particles deposited on alumina. Indeed, in order to understand the catalytic process, it seems essential to have information on the adsorption process and on the evolution of the structural parameters of the particles when submitted to such gases. This work has been performed on a 1% Pt/Al₂O₃ catalyst by using in situ EXAFS measurements. We have concentrated our study, on the one hand, on the influence of NO, C₃H₆ in the presence or not of an excess of oxygen and, on the other hand, on the influence of the total mixture. The most striking result is the growth of the platinum particles under NO. Moreover, when the catalyst is submitted either to a mixture of NO + O₂ or C₃H₆ + O₂, the particles are not fully oxidised. With the total mixture, the particles both grow and are oxidised.     

Catalytic oxidation of CO over ordered mesoporous platinum

Hexagonal phase mesoporous (H₁Pt) was recently reported to have different catalytic properties compared to conventional platinum catalysts. To further investigate this observation the catalytic activity of H₁Pt/Al₂O₃ for CO oxidation was compared with the activity of a corresponding catalyst prepared from Pt-black (Pt-black/Al₂O₃). The H₁Pt/Al₂O₃ catalyst showed ignition at lower temperatures but extinction at higher temperatures compared to Pt-black/Al₂O₃. These observations were further supported by oxygen step-response experiments at constant temperature, where the H₁Pt/Al₂O₃ catalyst showed ignition at lower oxygen concentrations when starting from a CO poisoned surface and extinction at higher O₂ concentrations when starting from the high-reactive state. Furthermore, adsorption of CO on the catalysts was studied in situ using infrared spectroscopy in absence and presence of oxygen after pre-oxidation and pre-reduction, respectively. At 150 °C the H₁Pt/Al₂O₃ sample showed activity for CO oxidation in the presence of oxygen regardless of pretreatment, whereas Pt-black/Al₂O₃ was inactive due to CO self-poisoning. The differences observed in the low reactive state are suggested to be due to structural differences of the platinum surface in the catalysts resulting in a lower sensitivity of the H₁Pt/Al₂O₃ catalyst towards CO self-poisoning and a higher capacity to activate oxygen, and thus a higher activity for CO oxidation. During the high reactive state, the observed higher sensitivity to the concentration ratio between CO and oxygen, and to the temperature is likely due to less optimal ratio between the sticking coefficients of the reactants on the H₁Pt catalyst and to higher mass-transport limitations in its narrower pores during the initial stage of the extinction.     

Influence of Oxygen Mobility over Supported Pt Catalysts on Combustion Temperature of Coke Generated in Propane Dehydrogenation

Abstract  

Five supported Pt catalysts (Pt/SBA-15, Pt/Al₂O₃, Pt/MgO, Pt/CeO₂, and Pt/Ce_0.63Zr_0.37O₂ ) were prepared to systematically investigate the influence of oxygen mobility over the support on combustion temperatures of coke produced in propane dehydrogenation. A strong correlation between the oxygen mobility of the supports and the combustion temperature of the coke was observed. The coke combustion temperatures increased following the reverse order of oxygen mobility of the supports: CeZrO₂ > CeO₂ > MgO > Al₂O₃ > SBA-15, implying that the oxygen transfer over supports was a rate-determining step in the coke combustion process.     

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.     

Treatment of Volatile Organic Compounds with a Pt/Co3O4‐CeO2 Catalyst

For emission control of volatile organic compounds (VOC), e.g., in the painting and printing industries, conventional Pt/Al₂O₃ and Co₃O₄‐CeO₂ catalysts are used. On the Pt/Al₂O₃ catalyst, aromatic hydrocarbons containing a benzene ring such as toluene can be oxidized at a lower complete oxidation temperature than on Co₃O₄‐CeO₂, under typical treatment conditions. However, ethyl acetate and isopropyl alcohol can be oxidized at a lower complete oxidation temperature on Co₃O₄‐CeO₂ than on Pt/Al₂O₃. In this study, platinum was directly supported on Co₃O₄‐CeO₂. Using chloroplatinic acid, the platinum cohered and the catalytic activity did not improve. But when the platinum was supported using platinum colloid coated with dispersant, high‐dispersion support of the platinum on the Co₃O₄‐CeO₂ surface was achieved, and toluene, ethyl acetate, and isopropyl alcohol could be oxidized at less than 250 °C.     

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     

Catalytic Oxidation of Methanol by Aqueous POM on Al2O3 Supported Catalysts and Electrochemical Performance of POM

Phosphomolybdic acid (H₃PMo₁₂O₄₀, POM) was attempted to be used as the energy‐storage agent in this paper to avoid some problems of the direct methanol fuel cell (DMFC), such as catalyst poisoning and methanol permeation. Catalytic oxidation of methanol by aqueous POM on Al₂O₃ supported catalysts with Pt and Ru active metal was evaluated in the presence of liquid water. The process takes advantage of the high catalytic activities of platinum for methanol oxidation. The effects of temperature, reaction time, and methanol concentration on activity were observed. The catalytic activity of Pt/Al₂O₃ is better than that of Ru/Al₂O₃ for the oxidation of methanol by POM. The methanol conversion rate reached 93.55% on the Pt/Al₂O₃ at 80 °C after reaction for 1 h. The electrochemical experiments indicate that POM shows a larger current density in redox processes on an Au electrode than methanol. The redox process of reduced POM is a reversible multi‐electron transfer process.     

Benzene hydroisomerization over Pt/MOR/Al2O3 catalysts

Benzene hydroisomerization is among the promising processes converting benzene into methylcyclopentane (MCP), which is an environmentally friendlier, octane boosting component of motor fuels. Benzene hydroisomerization into MCP over the Pt/MOR/Al₂O₃ (MOR = mordenite) catalytic system is reported here. The dependence of the yield of the target product on the acidic properties of the support and platinum precursor ([Pt(NH₃)₄]Cl₂ or H₂PtCl₆) have been investigated in order to optimize the catalyst composition. The acidic properties of the surface have been altered by introducing 30–95 wt % alumina into the support. Catalytic activity has been measured in the hydroisomerization of cyclohexane and a benzene (20 wt %) + n-heptane (80 wt %) mixture in a flow reactor at 250–350°C, 1.5 MPa, H₂: CH = 3: 1, a cyclohexane LHSV of 6 h^?1, a mixed feedstock LHSV of 2 h^?1, a catalyst bed volume of 2 cm³, and catalyst pellet sizes of 0.25–0.75 mm. The most efficient catalyst for cyclohexane and n-heptane isomerization and benzene hydroisomerization is the platinum-containing catalyst (0.3 wt % Pt) whose support consists of 30 wt % MOR and 70 wt % Al2O3. The highest yield of the target products of isomerization in the presence of this catalyst is attained in the temperature range from 280 to 310°C, which is thermodynamically favorable for MCP formation from benzene. This indicates that this catalyst is promising for the hydroisomerization of benzene-containing gasoline fractions. Use of H₂PtCl₆, a readily available chemical, as the platinum precursor is favorable for commercialization of the catalyst and ensures price attractiveness in its industrial-scale manufacturing.