Isomerization Of Hydrocarbons Research Articles

Superacids Based on Zirconium Dioxide

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High surface area mesoporous SiC synthesized via nanocasting and carbothermal reduction process

Silicon carbide has excellent mechanical strength, thermal stability and chemical inertness, and avoids several of the problems inherent in the performance of commercial supports, such as alumina, silicon oxide and carbon-based materials [1]. SiC supported catalyst exhibit high performance in automotive exhaust treatment, selective isomerization of paraffinic hydrocarbons, and H2S removal by direct oxidation into elemental sulfur [1]. SiC is also a wide band gap semiconductor for high temperature, high frequency and high voltage power application and optical sensors in the ultraviolet region [2]. These unique properties of SiC are of great interest to material scientists. Several routes have been used to synthesize SiC, for instance, carbothermal reduction of silica [3], sol–gel process [4, 5], decomposition of organic silicon compounds [6], chemical vapor deposition [7], and direct combustion synthesis [8]. Pure SiC was obtained by removal of the impurities through calcination to burn away residual carbon and acid treatment to remove SiO2 and/or catalyst [5, 7]. In the above-mentioned studies, many efforts have been done on the synthesis of high surface area SiC. For instance, SiC, synthesized with activated carbon granulates loaded with small amounts of nickel through fluidized bed chemical vapor deposition, exhibits surface areas ranging between 25 and 80 m2/g [9]. SiC synthesized by SiO vapors and activated charcoal reaches a surface area of 50 m2/g by a reaction at 1200 ◦C for 15 hr and a high (Si + SiO2)/C weight ratio [10]. Recently, porous SiC has been synthesized with a surface area from 112 to 120 m2/g by a modified sol– gel method [5], chemical vapor infiltration and a carbothermal reduction process [3]. In the latter process, MCM-48 silica was filled with pyrolytic carbon using propylene as carbon precursor and treated up to 1250– 1450 ◦C in inert atmosphere leading to the formation of SiC [3].

Superacids Based on Zirconium Dioxide

The synthesis of the superacidic materials SO 4 2− /ZrO2 and WO3/ZrO2 is examined. The structure of their acidic centers and their activity in the isomerization and acylation of hydrocarbons are discussed.

Activation and isomerization of hydrocarbons over WO 3/ZrO 2 catalysts : I. Preparation, characterization, and X-ray photoelectron spectroscopy studies

Tungstated zirconia catalysts with loadings ranging from near monolayer (6 wt% W) to multilayer coverages (30 wt% W) were prepared by wet impregnation with ammonium metatungstate as precursor, characterized by X-ray diffraction, temperature-programmed reduction, and in situ X-ray photoelectron spectroscopy and compared for surface reducibility. Two catalyst preparation methods were used. On the one hand, the ammonium metatungstate precursor was deposited directly on commercial monoclinic ZrO 2, or it was deposited on amorphous ZrO x (OH) 4−2 x hydroxide, according to the Hino and Arata procedure and leading to tetragonal ZrO 2. The degree of reducibility of the WO 3/ZrO 2 systems and consequently the catalytic behavior (forthcoming Part II) strongly depend on the initial WO 3 content but not at all on the monoclinic or tetragonal structure of ZrO 2. Whatever the preparation method, the surface of the near-saturation monolayer catalyst is mainly constituted of amorphous tungstate species in which tungsten atoms (W 6+ and W 5+) are in tetrahedral coordination, in direct interaction with the support and thus difficult to reduce. The increase in tungsten content yields a progressive formation of tungstate species in which tungsten is octahedrally coordinated, more easy to reduce, and giving W 4+ surface species. Increasing the WO 3 content above 15 wt% W leads to the formation of bulk-like WO 3 crystallites, whose reduction leads to the appearance of metallic tungsten.

Chaos-genetic algorithms for optimizing the operating conditions based on RBF-PLS model

A novel genetic algorithm (GA) including chaotic variable named chaos-genetic algorithm (CGA) was proposed. Due to the nature of chaotic variable, i.e. pseudo-randomness, ergodicity and irregularity, the evolutional process of CGA makes the individuals of subgenerations distributed ergodically in the defined space and circumvents the premature of the individuals of subgenerations. The performance of CGA was demonstrated through two examples and compared with that by the traditional genetic algorithms (TGA). The results showed the superior performances of CGA over TGA, and moreover, the probability of finding the global optimal value by using CGA is larger than that by using TGA. To illustrate the performance of CGA further, it was employed to optimize the operating conditions of aromatic hydrocarbon isomerization (AHI) process modeled by the radial basis functions (RBF) coupled with partial least squares (PLS) approach. Satisfactory results were obtained. Further, a generalized methodology, which employs RBF-PLS approach to model the complex chemical process based upon practical observation data and subsequently applies CGA to find the optimal operating conditions, was suggested too.

Electronic conditions for carbon–carbon bond rearrangements on metallic surfaces

The relation between the electronic structure of platinum metallic aggregates and their selectivity is discussed. A correlation is underlined between the energy of the d-band center, ε d , and the catalytic reactions such as hydrocarbon isomerization. Similar behavior appears when CO chemisorption on a platinum surface is compared with activity in alkane isomerization. This relationship can be used to interpret the changes in the mechanisms observed in acid catalysis, with bimetallic systems, and with catalysts treated by microwaves.

Isomerization of cyclic hydrocarbons mediated by an AlCl3-based ionic liquid as catalyst

Cyclohexane and methylcyclopentane isomerization were carried out under mild conditions using an ionic liquid consisting of n-butylpyridinium chloride and aluminum chloride as catalyst. The ionic liquid showed reasonably high catalytic activity in both reactions with a selectivity to corresponding products of 100%. In the case of cyclohexane isomerization, the thermodynamic equilibrium methylcyclopentane ⇌cyclohexane was almost achieved. For the reaction of cyclohexane isomerization, the kinetic study was performed, and the rate constants and the activation energy of the process were determined.

Preparation of New Solid Bases Derived from Supported Metal Nitrates and Carbonates

A highly basic solid catalyst was obtained by dispersing nitrates or carbonates of alkali metals such as KNO3, CsCO3 over alumina and zirconia at the surface density of 10-15 × 1018 cations m-2 and heating them at 773-873 K in an inert atmosphere. The temperature is far below the decomposition temperatures of the nitrates and the carbonates. Based on the temperature-programmed decomposition (TPD) experiments, two types of decomposition process are suggested; a bulk decomposition and an anion decomposition. Generation of a strong basicity relates to the anion decomposition. The basic center may be composed of the oxygen anions liberated from the decomposition of the nitrate or carbonate anions that are dissociatively adsorbed on the support surface during the preparation. These catalysts effectively catalyzed the double bond isomerization of aliphatic hydrocarbons and bicyclic hydrocarbons at moderate temperature.

Catalysis on Pd/WO3 and Pd/WO2: II. Effect of Redox Treatments in Hexanes and Hexenes Re-forming Reactions

Skeletal rearrangements of alkanes and alkenes were investigated on Pd/WO3 and Pd/WO2 catalysts following various activation treatments. Catalysts reduced at low temperature (350°C) are active and selective for isomerization of hydrocarbons. Palladium loses its intrinsic catalytic properties for isomerization but keeps partly its (de)hydrogenation properties. Hence we postulated that an interaction between palladium and tungsten oxide takes place under an H2 stream. The catalysts exhibit high selectivity in isomerization which is linked to the presence of acidic active centers, mainly Brønsted sites, on WO3 or W20O58 phases. Catalytic properties were interpreted by the traditional bifunctional mechanism, including the formation of carbocation species. Palladium metallic function, required for alkane dehydrogenation, deactivates with time under reactants but is easily regenerated by air exposure at 400°C. Striking results were obtained after exposures under oxygen and traces of water, providing an increase in the number of Brønsted acidic sites and in the acidity strength leading to an improvement of activity and isomerization selectivity. Catalysts reduced at higher temperature (600°C) are very active and selective for isomerization. Referring to our previous paper (35), devoted to catalyst characterizations (BET, TPR, XRD, XAS, XPS), we suggested that catalytic properties are then due to the W3O phase. The catalytic behaviour of such a phase has been interpreted with a monofunctional mechanism including tungstenacyclobutane species as intermediates.

Effect of water on silica-supported phosphotungstic acid catalysts for 1-butene double bond shift and alkane skeletal isomerization

The thermal stability, acidity, catalytic activity and regenerability of silica-supported phosphotungstic acid were evaluated in reference to the bulk solid acid. Samples were prepared with 5, 10, and 25 wt.% heteropolyacid (HPA) on fumed silica to examine the effect of loading on the properties of supported acid catalysts. X-ray absorption spectroscopy indicated that the supported heteropolyacids did not completely decompose when heated to 573 K. The catalytic activity of both bulk and supported HPAs for double bond shift and skeletal isomerization of light hydrocarbons decreased with increasing pretreatment temperatures, which suggests that the level of hydration is critical for acid catalysis. After pretreatment at 573 K, a well dispersed sample (5 wt.% HPA) catalyzed 1-butene double bond isomerization, a reaction that requires weak acid sites, whereas the sample was inactive for pentane skeletal isomerization, a reaction that requires strong acid sites. Evidently, the well dispersed HPAs did not have sufficient acidity to catalyze pentane isomerization since higher loaded samples were active for the reaction under identical conditions. Ammonia sorption microcalorimetry results correlated well with reactivity, revealing that 5 wt.% HPA on silica had the lowest heat of ammonia sorption, while a 25 wt.% sample had initial heats near that of bulk heteropolyacid (150 kJ mol −1). Treatment of the catalysts with water at mild conditions recovered activity for the bulk and supported heteropolyacids for the 1-butene and pentane isomerization reactions whereas treatment in air or N 2O was ineffective. These findings suggest that water treatment regenerates acid sites by rehydrating partially decomposed Keggin units. Results from in situ UV–VIS spectroscopy of the catalyst samples support the above conclusion.

Density functional calculations of enthalpies of hydrogenation, isomerization, and formation of small cyclic hydrocarbons

Density functional molecular orbital calculations using a high-level basis set on cyclic hydrocarbons (C≤5) make less severe demands on computer resources than the G2 family of ab initio calculations, and give results that are only slightly less accurate than G2. We present results for B3LYP/6-311+G(3df,2p) calculations at the MP2(full)/6-31G(d) geometry on 30 cyclic and eight acyclic hydrocarbons having two, three, four, or five carbon atoms. The arithmetic mean absolute deviation from 26 experimental results for Δ f H 298 is 2.1 kcal mol −1.

ChemInform Abstract: Hydrogenation and Isomerization of Hydrocarbons on Zeolite Catalysts

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Hollow ordered zirconia microcage formation by spherical micelle templating with chelating triol surfactants

By appropriate selection of surfactant head group volume and binding constant of the surfactant head group moiety to Zr, the first example of spherical micelle templating in transition metal oxides was achieved. These materials consist of well-ordered arrays of spherical zirconia units as evidenced by an X-ray diffraction peak directly related to the repeat distance of the individual units which range from 15 to 30 Å, depending on the surfactant chain length employed. Calcination of these materials leave an ordered cage structure of hollow zirconia spheroids which completely retain their XRD diffraction pattern. Small pores in the range of 6–8 Å were revealed by argon adsorption studies. These pores are attributed to either gaps between adjacent spheres or cracks in the overall structure. This approach was also extended to make Ti, Hf, and Fe analogues of this material. These materials show promise as new sorbents, materials for separation membranes, components in sensors, new hydrocarbon isomerization catalysts, substrates for nano-encapsulation and microreactors. The hollow nature of the zirconia spheres may make them an inorganic alternative to carbon-based fullerenes.

Low temperature hydrocracking of hydrocarbons on Ni-supported catalysts

Metal vapor method was employed to design a new method for preparation of nickel-containing supported catalysts on different metal oxides such as Al 2O 3, MgO, ZSM-5. The activation of carbon–carbon bonds in some n-alkanes and cycloalkanes was investigated. Results have shown that these catalysts retained very high activity, selectivity and stability toward hydrocracking and isomerization of hydrocarbons. Since metal vapor method yielded metal particles in a highly dispersed state, these particles were unique and demonstrated considerable decrease in the cleavage temperature of C–C bonds in hydrocarbons. The most significant difference was observed in the presence of bimetallic Ni–Re/ZSM-5 catalyst: it caused almost complete conversion of tested hydrocarbons at 463–493 K under normal pressure. n-Heptane could be converted virtually to isobutane and propane only. Hydrocracking of high alkanes (C 16) on these catalysts produced high liquid products and lower gas products.

Preparation and properties of new acid catalysts obtained by grafting alkoxides and derivatives on the most common supports note I — grafting aluminium and zirconium alkoxides and related sulphates on silica

In this paper we will show that it is possible to modify the acid properties of a surface, rich of hydroxyl groups by grafting in suitable conditions metal alkoxides, such as aluminium and zirconium alkoxide. Depending on the amount of metal alkoxide it is possible to obtain, after steaming of the surface and calcination, different coverage degree up to the monolayer. Iteration of the grafting technique gives a multilayer, completely modifying the orginal surface. Grafting aluminium and zirconium alkoxides pre-treated in homogeneous phase with pure sulphuric acid leads to catalysts with very strong acid sites on the surface. The acidic properties of the catalysts obtained by grafting aluminium and zirconium alkoxides and their sulphate derivatives on silica have been characterized by different techniques such as: potentiometric titrations, Temperature Programmed Desorption of organic bases and differential calorimetry to evaluate the density of the active sites and their strength. Prepared catalysts were proven in test reactions such as methanol dehydration and hydrocarbon isomerization and cracking. We have shown that sulphated catalysts have very strong acid sites of both the Brönsted and Lewis type, able to promote hydrocarbon isomerization and cracking at the relatively low temperature of 250°C.

Isomerization of unsaturated hydrocarbons by a molecular sieve treatment

Isomerization of unsaturated hydrocarbons by a molecular sieve treatment

Skeletal reactions of hydrocarbons on Pt/WOx-(MoOx-)ZrO2 superacid catalysts

1%Pt/WO3–(MoO3–ZrO2 superacid catalysts (d app of Pt ~16–20 A) gave activity in hydrocarbon isomerization reactions which was much enhanced relative to more customary forms of supported platinum. This is ascribed to dual-functional action. Isomerization selectivity was particularly high for Pt/WOx–ZrO2. The dideuterium/cyclopentane exchange “chemical probe” reaction on identical catalysts (reduction temperature 548 K (10 h)) revealed the action of electron-deficient platinum sites. While there is no clear evidence from hydrocarbon hydrogenolysis activities for such sites, comparison with published literature suggests that the exposure of electron-deficient sites may now be sensitive to the choice of precursor compound used in catalyst preparation.

Density Functional and Hartree−Fock Calculations on the Cyclopropane Ring Intermediates Involved in the Zeolite-Catalyzed Skeletal Isomerization of Hydrocarbons and in the Carbon Isotope Scrambling in 2-Propyl Cation

Quantum-chemical calculations were carried out on the mechanism of the zeolite-catalyzed hydrocarbon skeletal isomerization via the cyclopropane ring intermediates. According to the B3LYP/6-31G* calculations, formation of cyclopropane from surface alkoxy species in zeolites occurs via a transition state whose hydrocarbon part resembles a corner-protonated cyclopropane (corner-PCP) ring. Two conformations of the transition state found differ in the orientation of the PCP portion with respect to the acid site. The activation energy for the cyclopropane ring closure reaction is found to be rather sensitive to the use of planar symmetry constraints and to the level of calculations and less sensitive to the level of the geometry optimization. Calculations on the mechanism of the carbon isotope scrambling in the free 2-propyl cation were also performed, at several theory levels up to the Gaussian-2 model. The relatively stable intermediates of this superacid-catalyzed reaction are carbocations, in contrast to t...

Study of the transalkylation of aromatic hydrocarbons over SAPO-5 catalysts

The transalkylation of toluene (T) with trimethylbenzenes (TMB) over SAPO-5 catalysts was investigated. These catalysts appear to be quite active for the studied reaction. It was found that the use of 1,2,4-TMB as alkylation agent gave the highest yield of xylenes. A strong competition between transalkylation, disproportionation and isomerization of aromatic hydrocarbons takes place according to the operating conditions.

Reproducibility of Turnover Rates in Heterogeneous Metal Catalysis: Compilation of Data and Guidelines for Data Analysis

The combination of turnover rate measurements and surface science techniques allows a firm quantification of rates in heterogeneous catalysis by metals. There are many examples of reactions where the turnover rates from different laboratories are the same. However, there are still problems, as in the isomerization and hydrogenolysis of hydrocarbons over noble metals, where the turnovers rates from different laboratories differ by many orders of magnitude. An explanation for this discrepancy is discussed. Guidelines for experimental work in heterogeneous catalysis that will help to minimize this wide scatter of turnover rates in the future are presented. *Current address: Department of Chemical Engineering, Worcester Polytechnic Institute, Worcerster, MA 01609-2280.