Isomerization Of Cyclohexane Research Articles

Mechanistic insights into positional and skeletal isomerization of cyclohexene in the H-BEA zeolite.

The isomerization of cycloalkenes via the formation of carbenium cations assisted by the Brønsted acid site (BAS) in zeolites is the vital reaction step in hydrocracking and hydroisomerization processes of the petrochemical industry. To understand the acid-catalyzed positional isomerization and skeletal isomerization of cycloalkenes via carbenium intermediates, a series of ab initio molecular dynamics (AIMD) simulations of cyclohexene within the H-BEA zeolite have been carried out. AIMD simulations combined with the enhanced sampling technique reveal that the half-chair conformer is the most stable conformation for cyclohexene within H-BEA. Free energy landscapes characterizing protonation/deprotonation, positional isomerization, and skeletal isomerization of cyclohexene have been mapped out at 413 K. The free energy barrier for the formation of carbenium is calculated to be 44 kJ mol-1. The skeletal isomerization of cyclohexene to methylcyclopentylium via the protonated cyclopropane transition state involves four stages with a total free energy barrier of 134 kJ mol-1. Further geometrical analysis provides additional information about the structural origin of free energy barriers.

The effect of the tungsten oxide concentration in Pt/WO3-ZrO2 catalysts on hydroisomerization of the heptane-benzene mixture

The effect of the tungsten oxide content in Pt/WO3-ZrO2 catalysts over the range of 10–35 wt.% on hydroisomerization of the heptanebenzene mixture (80/20, wt.%) was studied. The catalysts with the WO3 content of 15–25 wt.% were shown to have the highest fraction of the tetragonal zirconia phase (t-ZrO2, 87–96%), which allows achieving the highest yield of the sum of heptane isomers, particularly high-octane di- and trimethyl substituted isomers (DTMS), the yield of which reaches 69.4–76.7 and 24.5–27.9 wt.%, respectively. In the presence of such catalysts, the yield of methylcyclopentane is 52.1–61.3 wt.% at the 72.6–90.9 % selectivity of cyclohexane isomerization to methylcyclopentane.

Highly selective skeletal isomerization of cyclohexene over zeolite-based catalysts for high-purity methylcyclopentene production

Cyclohexene skeletal isomerization towards methylcyclopentene is an economically favorable process due to the higher added value of the product. Traditional oxide-based catalysts face the challenge of achieving both high activity and stability. In this work, cyclohexene skeletal isomerization is achieved under mild conditions over designed zeolite-based catalysts with 96.8 wt.% liquid yield, 95.8 wt.% selectivity towards methylcyclopentene and satisfactory stability for multiple runs. The favorable performance is attributed to the unique acidic, structural and morphological features of the optimized cobalt/NaUZSM-5 catalyst. Further experimental data and DFT studies suggest that a carboncationic mechanism might be followed and that the reaction mainly occurs within the internal pores of the zeolite structures.

Influence of Nitrate and Phosphate on Silica Fibrous Beta Zeolite Framework for Enhanced Cyclic and Noncyclic Alkane Isomerization.

Phosphate and nitrate were loaded on silica BEA (P/HSi@BEA and N/HSi@BEA), which is fibrously protonated by the impregnation method for n-hexane and cyclohexane isomerization. The characterization analysis specified the removal of tetrahedral aluminum atoms in the framework, which was triggered by the existence of phosphate and nitrate groups in the catalyst. The exchanged role of Si(OH)Al to P-OH as active acidic sites in the P/HSi@BEA catalyst reduced its acidic strength, which was confirmed by the FTIR results. Lewis acidic sites of P/HSi@BEA performance are a significant part in the generation of high protonic acid sites, as proven by the in situ ESR study. However, FTIR evacuation and 27Al NMR revealed that the reduction in the amount of extraframework Al (EFAl) is due to its interaction with the nitrate group on the outside of the catalyst surface. The N/HSi@BEA catalyst exhibited high acidic strength because of the existence of more Si(OH)Al, which was initiated during the nitrate-incorporation process. Of significance is that the catalytic performance of n-hexane isomerization in the presence of hydrogen reached 50.3% product isomer yield at 250 °C, which might be ascribed to the presence of P-OH active sites that are responsible for accepting electrons, forming active protonic acid sites. NO3-EFAl interaction induced the formation of Brønsted acid sites, and higher mesopore volume favors the production of cyclohexane isomers up to 48.4% at 250 °C. This fundamental study exhibits that significant interactions given by such phosphate and nitrate groups with the unique silica fibrous BEA support could enhance isomerization, which contributes to the high quality of fuel.

Isomerization of Cyclohexane over Bifunctional Pt-, Au-, and PtAu-Heteropoly Acid Catalysts

Isomerization of cyclohexane was investigated in the presence of monofunctional acid and bifunctional metal–acid catalysts based on Keggin-type heteropoly acid H3PW12O40 and Pt and Au as the metal ...

Platinum and Molybdenum Oxide Supported on Mesostructured Silica Nanoparticles for n-Pentane and Cyclohexane Isomerization

Alkane isomerization into its equivalent branched isomers has gained numerous attention as a reaction to obtain high quality fuel. In this study, platinum and molybdenum oxide supported on mesostructured silica nanoparticles (Pt/MoO3/MSN) was prepared via impregnation method and tested for n-pentane and cyclohexane isomerization. The catalyst was characterized by using X-ray diffraction (XRD), nitrogen (N2) physisorption, and pyridine Fourier-transform infrared (pyridine-FTIR) spectroscopy. The IR result revealed that the addition of Pt and MoO3 into MSN formed different strength of Lewis and Brönsted acid sites. It was observed that the catalyst possessed strong acid sites and several numbers of relatively weak Lewis and Brönsted acid sites. Pt/MoO3/MSN was catalytically active towards n-pentane and cyclohexane isomerization with conversion of 63 and 87%, respectively, at 300 °C. It was proposed that the addition of Pt might assist the generation of active protonic acid sites from molecular hydrogen via the mechanism of hydrogen spillover and hence improve isomerization reaction.

Endpoint-restricted adiabatic free energy dynamics approach for the exploration of biomolecular conformational equilibria.

A method for calculating the free energy difference between two structurally defined conformational states of a chemical system is developed. A path is defined using a previously reported collective variable that interpolates between two or more conformations, and a restraint is introduced in order to keep the system close to the path. The evolution of the system along the path, which typically presents a high free energy barrier, is generated using enhanced sampling schemes. Although the formulation of the method in terms of a path is quite general, an important advance in this work is the demonstration that prior knowledge of the path is, in fact, not needed and that the free energy difference can be obtained using a simplified definition of the path collective variable that only involves the endpoints. We first validate this method on cyclohexane isomerization. The method is then tested for an extensive conformational change in a realistic molecular system by calculating the free energy difference between the α-helix and β-hairpin conformations of deca-alanine in solution. Finally, the method is applied to a biologically relevant system to calculate the free energy difference of an observed and a hypothetical conformation of an antigenic peptide bound to a major histocompatibility complex.

Modeling the catalytic process of cyclohexane isomerization to methylcyclopentane

The kinetics of the isomerization of cyclohexane to methylcyclopentane on Pd-mordenite catalyst in a flow reactor has been studied. In the experiments, parameters such as the volumetric flow rate of the feedstock, the component composition of the feed gas (cyclohexane, methylcyclopentane, hydrogen), and the temperature and pressure in the reactor have been varied. The concentrations of hydrogen, cyclohexane, and methylcyclopentane in the feed gas have been determined. A stepwise reaction mechanism of cyclohexane isomerization has been suggested. A kinetic model that contains the direct constant of the limiting stage rate and the combination of equilibrium constants of the fast stages in a set for estimating the constants has been built. An additional precision (improving) experiment has shown that the model constants are estimated with a high accuracy. A predictive ability of the model under experimental conditions has been proved to be high.

Investigation of the Pt/MOR–Al2O3 Catalysts by IR Spectroscopy

Pt/MOR–Al2O3 catalysts, with platinum supported from H2PtCl6 and [Pt(NH3)4]Cl2 solutions are investigated in the work with the methods of IR spectroscopy. The received results testify about platinum fixing mainly in channels of a mordenite in case of supporting from an ammine, and its localization on Al2O3 surface when supporting from chloroplatinic acid. Small concentration of the absorption bands referred to bridging CO species on platinum testify about high dispersion of metal on samples. Data explain higher activity of the catalysts prepared with [Pt(NH3)4]Cl2 in reactions of a hydroisomerization of benzene and an isomerization of cyclohexane.

Niobium-Based Nickel-Tungsten Hydrotreatment Catalysts with Enhanced Isomerization Properties

A balancing act: The presence of niobium leads to a highly improved performance of nickel–tungsten-based catalysts for the isomerization of cyclohexane (CC6). Importantly, the hydrogenation activity is preserved at high nickel–tungsten ratio and tungsten content.

Tungstophosphoric Acid Embedded SBA-15 Prepared by Modified Template Micelles for Cyclohexene Isomerization

Tungstophosphoric Acid Embedded SBA-15 Prepared by Modified Template Micelles for Cyclohexene Isomerization

Water-assisted spreading of MoO3 onto SiO2–Al2O3 supports for preparation of sulfide CoMo hydrodesulfurization catalysts

Catalysts with various MoO3 and Al2O3 contents were prepared by a reaction of acidic SiO2–Al2O3 supports with aqueous slurry of MoO3. The saturated adsorption amount of MoO3 corresponded with the amount of Al2O3 in the supports. The deposited and sulfided Mo species were accessible for promotion of the activity and selectivity in benzothiophene hydrodesulfurization reaction by Co. The acidity of the SiO2–Al2O3 supports modified by dealumination by leaching with nitric acid in terms of cyclohexene isomerization and cumene cracking were preserved after deposition of the CoMo sulfide phase.

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.

Curved nanostructures of unsupported and Al2O3-supported MoS2 catalysts: Synthesis and HDS catalytic properties

The active phase of hydrodesulfurization (HDS) catalysts is usually composed of MoS2 slabs promoted by cobalt and/or nickel. Because of the anisotropic structure of molybdenum disulfide, the active sites are believed to be present only on the edges of the slabs, exhibiting dangling bonds. However, slabs of MoS2-based industrial catalysts often exhibit disordered bent morphology which might generate strain effects creating new active sites. In order to evaluate a possible effect of bending MoS2 slabs on HDS catalytic properties, synthesis of well-defined curved morphologies was carried out in the present study. Two different strategies were evaluated herein: acidification of ammonium tetrathiomolybdate, (NH4)2MoS4 (ATM) in the presence of polyethyleneglycol or decomposition of ammonium thiodimolybdate, (NH4)2Mo2S12 (ATDM). The resulting amorphous sulfides, deposited or not on γ-Al2O3, were then post-treated at 400°C or 750°C under N2 or H2 in the presence or not of H2S. Cobalt promotion was also attempted on the final post-treated solids. The results show that the nature of the sulfur-containing precursor strongly influences the final morphology. Using ATM, the as-obtained nanospheres are formed of bundles of slabs while using ATDM, fullerenic-like MoS2 particles are obtained after post-treatment at 750°C. These fullerenic-like particles exhibit non constant radius of curvature with flat regions separated by strongly localized distorted zones suggesting the formation of defect sites on basal planes. The possibility of promoting these systems by Co sustains the presence of such surface sites. XPS and TPR results also show a marked weakening of the metal–sulfur bond strength when compared to flat crystals. Finally, catalytic activities determined for the HDS of thiophene and the simultaneous hydrogenation (HYD) of toluene and isomerization of cyclohexane are rather noticeable despite the absence of conventional edge sites. Bending MoS2 slabs therefore seems to lead to the creation of new active sites on basal planes with original catalytic properties.

Effect of Hydrohalogenation of PtRe/H-ZSM-5 for Cyclohexene Conversion

Extended use of supported Pt catalysts causes thermal migration of Pt particles to form large agglomerates, thus decreasing the catalytic activity. The combination of Pt with Re protects Pt against migration. In addition, Cl − injection into the reactor assists Pt particles redispersion to prolong catalyst life. In this work, a PtRe/H-ZSM-5 catalyst was treated with either HCl or HF to investigate their role in activating or deactivating the catalyst. The conversion exceeded 90% in the whole temperature range with the PtRe/H-ZSM-5(HCl) catalyst, and its activity for the direct isomerization of cyclohexene to methylcyclopentenes (MCPEs) was the lowest but the activity for the hydrogenation of the MCPEs to methylcyclopentane was the highest. The reactivities of MCPEs and cyclohexadienes on the catalysts were similar because both are dehydrogenation reactions. Benzene production was significantly higher on the hydrochlorinated catalyst than on the other catalysts, and its hydrocracking activity was the lowest, which is a good characteristic for processing catalysts where cracking is undesired. Rhenium is the most prevailing promoter for platinum in the naphtha catalytic reforming catalysts used for octane number improvement. An additional promotion of acidity by HCl or HF has also been examined in cyclohexene hydroconversion as a model reaction to explore the activation of intermediate reaction steps producing isomerized as well as hydrogenated, dehydrogenated, and hydrocracked products.

An experimental and kinetic modeling study of cyclohexane pyrolysis at low pressure

The pyrolysis of cyclohexane at low pressure (40mbar) was studied in a plug flow reactor from 950 to 1520K by synchrotron VUV photoionization mass spectrometry. More than 30 species were identified by measurement of photoionization efficiency (PIE) spectra, including some radicals like methyl, propargyl, allyl and cyclopentadienyl radicals, and stable products (e.g., 1-hexene, benzene and some aromatics). Among all the products, 1-hexene is formed at the lowest temperature, indicating that the isomerization of cyclohexane to 1-hexene is the dominant initial decomposition channel under the condition of our experiment. We built a kinetic model including 148 species and 557 reactions to simulate the experimental results. The model satisfactorily reproduced the mole fraction profiles of most pyrolysis products. The rate of production (ROP) analysis at 1360 and 1520K shows that cyclohexane is consumed mainly through two reaction sequences: cyclohexane→1-hexene→allyl radical+n-propyl radical, and cyclohexane→cyclohexyl radical→hex-5-en-1-yl radical that further decomposes to 1,3-butadiene via hex-1-en-3-yl and but-3-en-1-yl radicals. Besides the stepwise dehydrogenation of cyclohexane, C3+C3 channels, i.e. C3H3+C3H3 and C3H3+aC3H5 also have important contribution to benzene formation. The simulation reveals that C3H3+C3H3=phenyl+H reaction is the key step for other aromatics formation, i.e. toluene, phenylacetylene, styrene, ethylbenzene and indene in this work.

Activity of CoMo/MSA Catalysts in Benzothiophene Hydrodesulfurization, Cumene Cracking and Cyclohexene is Omerization

The mesoporous silica-alumina (SA) was modified by acid leaching and studied as an alternative support for Mo and CoMo catalysts for hydrodesulfurization (HDS) of benzothiophene. The supports were characterized by N 2 adsorption, XRD, 27AL MAS NMR, electron microscopy (SEM, HRTEM) and their acidity were compared in cyclohexene isomerization and cumene cracking. The composition of catalysts were determined by ICP. Progressive leaching of the parent SA containing 52wt.% Al2O3 (SA52) led to decrease of Al2O3 content (33 and 19wt.% in MSA33 and MSA19 support, respectively). This decrease was accompanied with an increase of the BET surface area and exposition of strong acidity. The acid leaching positively affected the activities of the sulfided CoMo catalysts in the benzothiophene HDS, which were slightly above the activity of an industrial reference CoMo catalyst supported on gamma-Al2O3.

Reactions of Cyclohexane on Platinum, Palladium, or Iridium-loaded H-ZSM-5 Zeolite Hydrohalogenated Catalysts

The catalytic hydroconversion of cyclohexane using catalysts containing H-ZSM-5 zeolite loaded with 0.35 wt% platinum, palladium, or iridium was studied in a pulse-type microreactor/GC system at atmospheric pressure. These catalysts were also either doped with 3.0% of HCl or HF. The activities of these catalysts and the distribution of the products formed were found to depend on the dispersion of the metallic component as well as on the acidity, acid sites number, and strength in the catalysts. TPD of ammonia and hydrogen chemisorption were applied to evaluate acid site strength distribution and metals dispersion, respectively, in the catalysts. In case of the Pt- and Pd-containing catalysts, hydrochlorination enhanced the isomerization and dehydrogenation activities and selectivities of cyclohexane, but decreased its hydrocracking activity. However, catalyst hydrofluorination resulted in the reverse effects. Nevertheless, for the Ir-loaded catalyst, both hydrohalogenation treatments decreased the isomerization and dehydrogenation of cyclohexane. The Ir/H-ZSM-5 catalyst exhibited higher hydrogenolysis activities than did those acquired by the Pt- and Pd-containing catalysts.

Hydroisomerization of benzene-containing gasoline fractions on a Pt/SO 4 2− -ZrO2-Al2O3 catalyst: II. Effect of chemical composition on acidic and hydrogenating and the occurrence of model isomerization reactions

The acidic and hydrogenating of Pt/SO 4 2− -ZrO2-Al2O3 samples containing from 18.8 to 67.8 wt % Al2O3 as a support constituent were studied by the IR spectroscopy of adsorbed CO and pyridine, and the model reactions of n-heptane and cyclohexane isomerization on these catalysts were examined. The total catalyst activity in the conversion of n-heptane decreased with the concentration of Al2O3; this manifested itself in an increase in the temperature of 50% n-heptane conversion from 112 to 266°C and in an increase in the selectivity of isomerization to 94.2%. In this case, the maximum yield of isoheptanes was 47.1 wt %, which was reached on a sample whose support contained 67.8 wt % Al2O3. A maximum yield (69.6 wt %) and selectivity (93.7%) for methylcyclopentane formation from cyclohexane were also reached on the above catalyst sample. This can be explained by lower concentrations of Lewis and Bronsted acid sites in the Pt/SO 4 2− -ZrO2-Al2O3 system, as compared with those in Pt/SO 4 2− -ZrO2. The experimental results allowed us to make a preliminary conclusion that the Pt/SO 4 2− -ZrO2-Al2O3 catalyst whose support contains 67.8 wt % Al2O3 is promising for use in the selective hydroisomerization of benzene-containing gasoline fractions in the thermodynamically favorable process temperature range of 250–300°C.

Effect of catalysts acidity on residues hydrotreatment

The impact of hydrotreatment catalysts acid properties in residues hydrotreatment activities (hydrodemetallization (HDM), hydrodesulfurization (HDS), hydrodeasphaltenization (HDAsC7), hydrodenitrogenation (HDN) and Conradson carbon conversion (HDCCR)) was evaluated. NiMoP based catalysts carried on multimodal/bimodal alumina were used. Within this framework, the acid properties of alumina carriers were modified (in terms of nature, number and strength of acid sites) in a wide range of acidity. This was achieved by doping the carriers using different additives (fluorine, SiO2, zeolite) at different contents introduced by impregnation (for F and SiO2) or co-mixing (for SiO2 and zeolite).Acidity additives effect on sulfide active phase was followed by means of a toluene hydrogenation catalytic test (to characterize the hydrogenating function), transmission electron microscopy and infrared spectroscopy using CO as probe molecule. Sulfided catalysts acidity was quantified through a cyclohexane isomerization catalytic test. The distribution of Brønsted and Lewis acid sites were distinguished by infrared spectroscopy using respectively lutidine and pyridine as probe molecules.It was shown that only fluorine introduced after the active metals impregnation, allowed to develop the most enhanced Brønsted acidity without a major impact in the active phase.A catalytic test was performed in a lab scale batch reactor at hydrotreatment conditions and using a Safaniya vacuum residue feedstock. Hydrotreatment functions were followed showing that among the acidity additives that were used, fluorine enhanced most the asphaltenes conversion. This was attributed to the higher number of strong Brønsted acid sites.