Hydrogenolysis Of Cyclohexane Research Articles

A facile synthesis of 3D pyramidal hierarchical CeO2–Al2O3 nanocomposites supported nickel catalyst for cyclohexane dehydrogenation

In this study, hierarchical 10 and 20%CeO2–Al2O3 nanocomposites with 10 and 20% ceria content as well as pure γ-Al2O3 were synthesized via template-free modified co-precipitation method. The as-synthesized nanocomposites were characterized using several techniques such as BET, XRD, TEM, TGA-DTA, XPS, FE-SEM and AFM. The synthesized nanocomposites were loaded by 5% NiO via ultrasonic assisted route and successfully investigated for cyclohexane dehydrogenation in a catalytic flow system. The XRD results ensured the formation of crystalline cubic γ-alumina and cubic CeO2 with fluorite structure. Additionally, a solid-solution has been formed in all CeO2–Al2O3 nanocomposites with retaining the ceria fluorite structure. Moreover, a large fraction of dispersed CeO2 phase over the nanocomposites surface accompanied by CeAlO3 cubic phase was observed. TEM, AFM and FE-SEM technique emphasized the development of totally novel 3-dimensional hierarchical pyramidal structure nanocomposites which newly synthesized with a facile preparation procedure. Finally, the synthesized nano-composites achieved high benzene yield of 98.9 and 99%, at LHSV of 1 h−1 whereas, 100% selectivity was achieved at LHSV of 3 h−1 towards benzene production up to 350 °C of reaction temperature. At higher reaction temperature, a considerable amount of methane is clearly detected in the product gas suggesting the occurrence of cyclohexane hydrogenolysis. The obtained catalytic performance was attributed to the proposed experimental conditions, the high surface area of the synthesized hierarchical structure and the optimized ceria loading to the alumina ratios.

Ring Opening of Naphthene Hydrocarbons under Conditions of Thermal and Microwave Heating

A comparative study of the activity and selectivity of monometallic Ir-containing catalysts in ring opening reactions under conditions of thermal and microwave heating is performed using the example of cyclohexane. The hydrogenolysis of cyclohexane is shown to be possible with microwave activation. It is found that upon microwave heating, iridium-containing catalysts supported on titanium oxide and activated carbon allow 60% selectivity to be reached with respect to ring opening products at 30% conversion of the initial cyclohexane.

Iridium Catalysts for C-C and C-O Hydrogenolysis: Catalytic Consequences of Iridium Sites

The support effect on the properties of iridium catalysts for C-C and C-O hydrogenolysis was investigated. Cyclohexane conversion and glycerol hydrogenolysis were used to compare the behavior of iridium catalysts in terms of C-C and C-O cleavage. The nature of the support influenced the catalyst performance in both cyclohexane conversion and glycerol hydrogenolysis. This effect was more evident on the product selectivity. Ir/SiO2 catalyst presented the highest cyclohexane hydrogenolysis activity and the highest selectivity to minor hydrocarbons formed from hexane re-adsorption. For glycerol hydrogenolysis, all catalysts displayed a higher selectivity to products formed by C-O cleavage, mainly 1,2-propanediol (1,2-PDO). Ir/ZrO2 catalyst presented the highest activity in all reaction conditions and the lowest selectivity to minor alcohols produced by C-C cleavage like methanol, ethanol and ethylene glycol. The results were explained in terms of the requirements and the structure of the catalytic sites.

Active sites and reactive intermediates in the hydrogenolytic cleavage of C–C bonds in cyclohexane over supported iridium

Hydrogenolysis of cyclohexane has been explored over supported Ir catalysts. The kinetic data are combined with modeling results to assess the structural requirements and the nature of catalytically relevant surface intermediates for endocyclic C–C bond cleavage. The turnover frequency (TOF) for cyclohexane hydrogenolysis showed complex dependence on Ir particle size, while the selectivity to the primary ring opening product, n-hexane, decreased monotonically with decreasing Ir dispersion. The decreasing TOF as the Ir dispersion decreased from 65% to 52% originates principally from the diminishing abundance of low-coordination Ir atoms at particle surfaces. The increase of the TOF with further Ir particle growth is attributed to an increased fraction of terrace planes, or step sites, and a less unsaturated nature of the most abundant reactive intermediate. Selectivities for multiple C–C bond cleavage, yielding C<6 alkanes, varies with the relative abundance of coordinatively unsaturated Ir atoms and terrace planes. The multiple hydrogenolysis depends additionally upon H2 pressure, because single and multiple C–C bond scissions are mediated by surface intermediates with different H-deficiencies.

Cyclohexane transformations over metal oxide catalysts. 1. Effect of the nature of metal and support on the catalytic activity in cyclohexane ring opening

The activities of monometallic Pt-, Ru-, and Rh-containing catalysts supported on Al2O3, Al2O3—F, SiO2, WO3/ZrO2, and La2I3/ZrO2, in cyclohexane ring opening to form n-hexane were studied. The most active catalyst is Rh/Al2O3. Cyclohexane hydrogenolysis to n-hexane also occurs over the Pt/Al;>2O3 and Pt/La2I3/ZrO2 catalysts. Ring opening over the Ru catalysts proceeds at significantly lower temperatures (210—230 °C) than over the Pt and Rh catalysts (350—400 °C), but the ruthenium systems are less selective for n-hexane formation than Rh/Al2O3 catalysts. The effects of acid-basic properties of the support and the reaction conditions on the activities of the catalytic systems in cyclohexane ring opening was studied.

Hydrogenolysis of Cyclohexane over Ir/SiO2 Catalyst: A Mechanistic Study of Carbon−Carbon Bond Cleavage on Metallic Surfaces

The hydrogenolysis of cyclohexane catalyzed by supported Ir/SiO(2) has been studied to get mechanistic information on the elementary steps of C--C bond cleavage for cyclic saturated hydrocarbons. The reaction was studied under conditions in which no dehydrogenation to benzene occurs. When a mixture of cyclohexane and H(2) flows over a Ir/SiO(2) catalyst at 200 degrees C and for a H(2)/cyclohexane ratio superior to 40, methane, ethane, propane, n-butane, n-pentane, and n-hexane are identified to be primary products. The hydrogenolysis of ethane and n-hexane has also been studied to clarify several mechanistic questions. To account for the primary products in the above reactions, a mechanism is proposed in which the key step of the carbon--carbon bond cleavage occurs via concerted electronic transfer in dimetallacyclopentane intermediate. The comparison of product distributions in the hydrogenolysis of cyclohexane and that observed for n-hexane led to conclusions about the relative ease of carbon--carbon bond cleavage with respect to surface alkyl isomerization.

Effect of thermal treatment on various characteristics of nickel/aluminum phosphate catalysts

Five Ni/AlPO 4 samples containing 5–50 wt.% nickel were prepared by impregnation method, calcined at 400–1000°C and reduced at 400°C. The structural properties of the samples were determined by thermal analysis and X-ray diffraction (XRD) techniques, while the adsorption of nitrogen at –196°C was used to elucidate their textural parameters. The catalytic conversion of cyclohexane on the prepared solids was investigated using a pulse microcatalytic technique. The thermal analysis of pure AlPO 4 and 35 wt.% Ni/AlPO 4 samples demonstrated the presence of crystalline aluminum phosphate phase. The presence of nickel greatly affected the position of exothermic peak of AlPO 4 phase. The X-ray diffraction lines of the investigated samples suggest the role of both nickel content and calcination temperature on the crystallization of AlPO 4 and nickel metal phases. The results of N 2-adsorption indicated that the surface areas evaluated for various Ni/AlPO 4 samples studied were found to decrease on increasing their nickel content and calcination temperatures. The selectivity of cyclohexane was found to depend on the nickel content and thermal treatment of the solids employed. The dehydrogenation and hydrogenolysis of cyclohexane are parallel reactions and proceed on active sites of different natures.

Dehydrogenation of cyclohexane on catalysts containing noble metals and their combinations with platinum on alumina support

The dehydrogenation of cyclohexane over catalysts containing 0.35 wt% of each of the following metals: Pt, Rh, Re, U, PtIr, PtRh, PtRe and PtU on γ-Al 2O 3 was carried out in a pulsed micro-reactor system at the temperature range 200–500°C. PtRh catalyst was the most active for the dehydrogenation of cyclohexane in the temperature range 250–350°C. The catalyst containing Rh was the most active for hydrogenolysis of cyclohexane to propane due to the higher percentage of d-bond character of Rh. It was found that the introduction of U inhibits the activity of Pt catalyst for the dehydrogenation. The effect of chlorine and fluorine contents (1, 3 and 6 wt%) on the activities of these catalysts were investigated. Optimum halogen content was found to be 3% for dehydrogenation enhancement which may be attributed to improved hydrogen spillover as well as to increased dispersion of the metal in the support. The catalysts containing Ir (3.0% F) and PtU (3.0% F) exhibit some hydroisomerization activity to methylcyclopentane. However, inclusion of chlorine reduces the activity of PtIr whereas both chlorine and fluorine reduce the activity of PtRh-containing catalysts.

Ni [sbnd]MgO catalyst prepared by using a melt of the nitrates and citric acid

At high Ni contents ranging from 70 to 80 wt.%, Ni MgO catalysts prepared by using a melt of the corresponding nitrates and citric acid were found to have high nickel surface area, and exhibited catalytic activities for the hydrogenolysis of cyclohexane at 350°C.

Nanostructured Fe-Co Catalysts Generated by Ultrasound

ABSTRACTBimetallic catalysts have been studied intensively because of their unique activity and selectivity. Unsupported alloy catalysts, however, are usually of limited value due to their very small surface areas. We have now developed a sonochemical synthesis of bimetallic alloys that provides both high surface areas and high catalytic activity. We have produced Fe-Co alloys by ultrasonic irradiation of mixed solutions of Fe(CO)5 and Co(CO)3(NO) in hydrocarbon solvents. The alloy composition can be controlled simply by changing the ratio of precursor concentrations. After treatment at 673K under H2 flow for 2 hours, we obtain nearly pure alloys. BET results show that the surface areas of these alloys are large (10-30 m2/g). TEM and SEM show that the alloy particles are porous agglomerates of particles with diameters of 10-20 nm. Sonochemically prepared Fe, Co, and Fe-Co powders have very high catalytic activity for dehydrogenation and hydrogenolysis of cyclohexane. Furthermore, sonochemically prepared Fe-Co alloys show high catalytic selectivity for dehydrogenation of cyclohexane to benzene; the 1:1 ratio alloy has much higher selectivity for dehydrogenation over hydrogenolysis than either pure metal.

Adsorption and catalysis on silicon-modified W(110) surfaces

The adsorption of CO, hydrogen, and benzene was studied at 120 K on silicon-covered W(110) surfaces using temperature-programmed desorption. The saturation coverage of virgin CO is reduced from 0.80 monolayer (ML) on the clean W(110) surface to 0.40 ML in the presence of 0.30 ML of silicon and to 0.21 ML in the presence of 0.56 ML of silicon. No CO adsorption occurs on W(110) covered by a monolayer of silicon. Dissociation of hydrogen, CO, and benzene is strongly inhibited by silicon. The inhibition of CO and hydrogen adsorption by silicon is discussed in terms of the proposed overlayer structures for silicon on W(110). Upon annealing silicon overlayers to 1050 K, silicon diffuses into the bulk to form tungsten silicide, and the resulting increase in the concentration of tungsten atoms at the surface increases the ability of the surface to adsorb CO. Results are compared to those from adsorption studies on silicon-covered polycrystalline tungsten and on a nickeldisilicide surface. The dehydrogenation and hydrogenolysis of cyclohexane were used to test the catalytic properties of the silicon-modified surfaces. Tungsten silicide appears to be a highly selective catalyst for the dehydrogenation of cyclohexane. Comparison with a study of cyclohexane reactions over silane-treated nickel catalysts indicates that high selectivity for dehydrogenation may be a general attribute of silicon-modified metal catalysts. The activity of tungsten silicide for cyclohexane dehydrogenation to benzene is at least as great as that of the Ru(M01) surface.

ChemInform Abstract: Surface‐Characterized Dispersed Metal Catalysts

The Single Turnover (STO) procedure has been used to characterize the surface of a number of different dispersed metal catalysts. With this technique it is possible to identify and measure the relative densities of five different types of surface sites. This technique has been used to evaluate those factors in catalyst preparation and pretreatment which influence catalyst activity as well as the nature of the solvent effect and some specific types of metal support interactions. The use of these STO characterized catalysts in other reactions has made it possible to identify the types of sites on the metal surface responsible for mono- and polydeuterium exchange on alkanes and the dehydrogenation and hydrogenolysis of cyclohexane. This later work provides a direct correlation between single crystal studies and reactions run on dispersed metal catalysts. A combination of the STO technique with the apopinene catalyst characterization of Smith has provided a method by which it is possible to determine the relative rates at which these different types of sites react. 32 refs., 20 figs., 3 tabs.

Novel Effects of K-Addition to Ru/SiO2 on Conversion of Cyclohexane

Abstract The effects of the addition of alkalis to a Ru/SiO2 catalyst upon the conversion (hydrogenolysis and dehydrogenation) of cyclohexane, hydrogenolysis of butane, and hydrogenations of benzene and CO were studied. The addition of K or Cs slightly enhanced the catalytic activity for the hydrogenolysis of cyclohexane, whereas dehydrogenation to benzene was markedly suppressed. This result is contrary to the effect of the addition of Cu or Au to group-VIII metals, as reported in the literature. Changes in the activities became less significant as the atomic number of the added alkali decreased. From a systematic examination by varying the alkali content, it was revealed that dehydrogenation and CO hydrogenation were very sensitively suppressed by an electronic effect imposed by the alkalis, whereas changes in the activities of hydrogenolysis and benzene hydrogenation were not great.

Selective Acceleration Effects of K-Addition in Hydrogenolysis of Cyclohexane over Ruthenium Supported on Silica

Abstract By addition of K to Ru/SiO2 catalyst (K/(K+Ru)=0.2), catalytic activity for hydrogenolysis of cyclohexane was enhanced, whereas those for dehydrogenation of cyclohexane and hydrogenation of CO were markedly suppressed.

Metal-Oxide Interaction in Alumina-Promoted Rh/SiO2 Catalyst. Effect of H2 Treatment on H2 Chemisorption and Cyclohexane Hydrogenolysis

Abstract An Al2O3-promoted Rh catalyst, containing Al2O3 deposited onto a Rh/SiO2 catalyst, exhibited an SMSI behavior: a significant suppression of both the H2 chemisorption capacity and the cyclohexane hydrogenolysis activity by high-temperature reduction and their recovery by O2 treatment at 673 K followed by low-temperature reduction.

Crystallite size effects in nickel catalysts: Cyclohexane dehydrogenation and hydrogenolysis

Crystallite size dependence of cyclohexane dehydrogenation and hydrogenolysis has been studied over silica-supported nickel catalysts. The samples were freshly prepared and sintered catalysts, produced by homogeneous precipitation-deposition and ranging from 25 to 40 wt% Ni on SiO 2. Crystallite size distributions were determined from magnetization data. Other experimental measurements, such as hydrogen chemisorption and catalytic kinetics, were made in the same cell in order to provide in situ precision. Average crystallite sizes varied from 2 to 4 nm and critical differences were found in this region. Areal rates for dehydrogenation more than doubled, whereas cyclohexane hydrogenolysis decreased by a factor of 5. These findings were shown to be consistent with a model wherein dehydrogenation occurs on crystal face sites and hydrogenolysis on edge positions. Benzene hydrogenolysis followed the same pattern as dehydrogenation, suggesting that common intermediates are involved.

The chemisorption and catalytic properties of nickel intermetallic compounds: Studies of single crystalline and high surface area, supported materials

We summarize here our detailed studies on the complex chemisorption properties of the single crystalline (111) surface of nickel disilicide (NiSi 2). This material is characterized by an unusual structure in which coordinately unsaturated nickel surface atoms are bonded only to silicon nearest neighbors. These studies clearly establish that the surface chemistry of nickel in NiSi 2 differs substantially from that observed on the surface of the bulk metal. The trends exhibited by these differences are particularly interesting in that they simultaneously represent both a decreased and an increased reactivity of nickel. These unique chemisorption properties prompted our development of a general procedure for preparing supported, high surface area intermetallic materials. Using model hydrocarbon reactions, such as the competitive dehydrogenation and hydrogenolysis of cyclohexane, the effects of intermetallic compound formation on catalytic activity are described. Of particular interest is the finding that the dramatic increase in the selectivity of Group VIII intermetallic compounds for activating C-H versus C-C bonds parallel a corresponding decrease in their activity for dissociatively chemisorbing hydrogen.

Derivatized, high surface area, supported nickel catalysts

A new and general procedure for preparing high surface area, supported intermetallic compound and alloy catalysts is described. In this technique, the volatile Group IIIA and IVA organometallic compounds SiH 4, Me 6Si 2, Me 6Ge 2, BH 3 · Et 3N, and Et 3Al are decomposed thermally on high surface area nickel, supported on either SiO 2 or Al 2O 3, to yield, directly, mixed metal catalysts of varied composition and structure. Using the competitive dehydrogenation and hydrogenolysis of cyclohexane as model reactions, the effects of intermetallic compound formation on catalytic activity are described.

Dehydrogenation of cyclohexane and isoamylenes on nickel and calcium-nickel phosphate catalysts

In the dehydrogenation of C/sub 4/ and C/sub 5/ olefins on calcium-nickel-phosphate catalysts cracking and coke-formation take place, in addition to the main reaction. Reduction of the catalyst by the reaction medium increases the rate of secondary reactions and reduces diene yield in dehydrogenation of butylenes, which is due to the formation of zero-valent nickel on the surface of calcium-nickel phosphate catalyst. It was found possible in a number of studies to effect transition of Ni/sup 2 +/ ..-->.. Ni/sup 0/ during treatment of the catalyst with hydrogen. It may be assumed from these studies that to suppress cracking and coke-formation, it is necessary to introduce into the catalyst modifying additives, which prevent the reduction of bivalent nickel or block the reduced nickel. When studying dehydrogenation of cyclohexane on nickel catalysts it was found that the addition to the catalyst of zinc oxide and non-transition metal oxides results in the suppression of hydrogenolysis of cyclohexane. This paper presents results of an investigation of the transformation of cyclohexane and isoamylenes on calcium-nickel-phosphate catalysts modified by strontium and zinc cations. The effect of these modifiers on the reduction of calcium-nickel-phosphate catalysts was also studied. To compare the efficiency of modifiers, cyclohexane wasmore » dehydrogenated on alumino-nickel catalysts containing strontium and zinc cations.« less

Infrared spectra of adsorbed CO and catalytic conversion of cyclohexane on [formula omitted] and [formula omitted] catalysts

Pt Al 2O 3 and PtPb Al 2O 3 catalysts have been investigated by infrared (ir) spectroscopy of adsorbed carbon monoxide and selectivity for cyclohexane conversion. Addition of lead to Pt Al 2O 3 catalysts decreases the frequency of the CO stretching vibration from adsorbed CO and affects the dependence of the respective band intensities upon the CO surface coverage. In catalysis the addition of lead to Pt Al 2O 3 strongly suppresses fragmentation and deactivation in the cyclohexane conversion. The spectroscopic results are consistent with an s-electron transfer from lead to platinum. This modification of platinum has catalytic consequences. Especially the hydrogenolysis of cyclohexane is inhibited. The ir and catalytic data are consistent with the conclusion that alumina-supported PtPb catalysts exhibit both the ligand and ensemble effects.