Dehydrogenation Of Cyclohexane Research Articles

Oxidative dehydrogenation of cyclohexene on size selected subnanometer cobalt clusters: improved catalytic performance via evolution of cluster-assembled nanostructures

The catalytic activity of oxide-supported metal nanoclusters strongly depends on their size and support. In this study, the origin of morphology transformation and chemical state changes during the oxidative dehydrogenation of cyclohexene was investigated in terms of metal-support interactions. Model catalyst systems were prepared by deposition of size selected subnanometer Co(27±4) clusters on various metal oxide supports (Al(2)O(3), ZnO and TiO(2) and MgO). The oxidation state and reactivity of the supported cobalt clusters were investigated by temperature programmed reaction (TPRx) and in situ grazing incidence X-ray absorption (GIXAS) during oxidative dehydrogenation of cyclohexene, while the sintering resistance monitored with grazing incidence small angle X-ray scattering (GISAXS). The activity and selectivity of cobalt clusters shows strong dependence on the support. GIXAS reveals that metal-support interaction plays a key role in the reaction. The most pronounced support effect is observed for MgO, where during the course of the reaction in its activity, composition and size dynamically evolving nanoassembly is formed from subnanometer cobalt clusters.

Modification of the performance of WO3-ZrO2 catalysts by metal addition in hydrocarbon reactions

A study of the different hydrocarbon reactions over Ni doped WO3-ZrO2 catalysts was performed. Ni was found as NiO at low Ni concentration while at high Ni concentrations a small fraction was present as a metal. For both cases, Ni strongly modified total acidity and concentration of strong acid sites. In the cyclohexane dehydrogenation reaction, Ni addition promotes both benzene and methyl cyclopentane production. The hydroconversion activity (n-butane and n-octane) increases with the augment of total acidity produced by Ni. The selectivity to reaction products is modified according to the acid strength distribution changes produced by Ni addition.

Preparation of carbon-supported Pt catalysts covered with microporous silica layers using organosilanes: Sintering resistance and superior catalytic performance for cyclohexane dehydrogenation

Carbon black-supported Pt metal particles (Pt/CB) were covered with microporous silica layers using different organosilanes like methyltriethoxysilane (MTES) (SiO2(MTES)/Pt/CB) and phenyltriethoxysilane (PhTES) (SiO2(PhTES)/Pt/CB). Pt metal particles in Pt/CB covered with microporous silica layers were stable at high temperature up to 973K in a hydrogen atmosphere, because the microporous silica layers that wrapped around the Pt metal particles essentially prevent particle sintering. Methyl or phenyl groups were introduced into the silica layers that covered the Pt metal particles using MTES or PhTES hydrolysis. Micropores were formed in the silica layers effectively after thermal treatment at 973K. The microporous silica-coated Pt catalysts were used as model catalysts for the dehydrogenation of cyclohexane. The microporous silica-coated Pt catalysts with thermal treatment at 973K exhibited a higher conversion of cyclohexane compared with Pt/CB. Especially, the SiO2(PhTES)/Pt/CB catalysts showed relatively higher conversion of cyclohexane compared with the other silica-coated Pt catalysts even when the SiO2 loading was high. Microporous silica layers with a larger micropore volume promoted the diffusion of cyclohexane during the catalytic reaction.

A comparative study of two different configurations for exothermic–endothermic heat exchanger reactor

The coupling of the energy intensive endothermic reaction systems with appropriate exothermic reactions reduces the size of the reactors and can improve the thermal efficiency of processes. One type of a suitable reactor for such a kind of coupling is the heat exchanger reactor. In this study, the catalytic methanol synthesis is coupled with the catalytic dehydrogenation of cyclohexane to benzene in an integrated reactor formed from two fixed beds separated by a wall where heat is transferred across the surface of the tube. A steady-state heterogeneous model of the two fixed beds predicts the performance of the two different configurations of the thermally coupled reactor. The co-current mode is investigated and the simulation results are compared with the corresponding predictions for the industrial methanol fixed bed reactor operating in the same feed conditions. The results of the study reveal that should the exothermic and endothermic reactions be located in the shell side and tube side, respectively, the methanol production rate will increase in comparison with the conventional methanol synthesis reactor as well as the case where the exothermic reaction is located in the tube side and endothermic reaction in the shell side.

Selective hydrogenation of citral to unsaturated alcohols over mesoporous Pt/Ti–Al 2O 3 catalysts. Effect of the reduction temperature and of the Ge addition

Pt/ y%Ti–Al 2O 3 ( y corresponding to the atomic percent of Ti in alumina, in the range 10–33%), and derived bimetallic Pt-Ge/10%Ti–Al 2O 3 nanocomposite catalysts were synthesized, characterized, and reduced either at 300 °C and 500 °C (this latter temperature being performed in order to generate strong metal–support interactions, i.e. a SMSI effect). The materials were characterized in detail with techniques including elemental analysis, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and nitrogen physisorption to evaluate their structural and textural properties. Due to the templating approach used, well-defined mesopore structures with high surface areas and mesopore volumes are obtained for all materials. The SMSI effect, evaluated using a structure insensitive model reaction, i.e. the cyclohexane dehydrogenation, is observed to be more pronounced on the Pt/ y%Ti–Al 2O 3 catalysts with y = 20 and 33% than on a Pt/TiO 2 P25 (Degussa P25 titania) sample, showing the beneficial effect of Ti fine dispersion through incorporation in alumina matrix on the generated metal–support interaction. During citral hydrogenation reaction, the selectivity toward unsaturated alcohols (UA: nerol and geraniol) is related to Ti and/or Ge loadings on the nanocomposite, as well as reduction temperature. Both SMSI effect and Ge addition promote the UA selectivity leading to similar values than on Pt/TiO 2 P25 sample.

Hydrogen looping approach in optimized methanol thermally coupled membrane reactor

In this study, cyclohexane and hydrogen loop approach is proposed in optimized thermally coupled dual reactors in methanol production via Differential Evolution (DE) method. This new generation of thermally coupled membrane reactors uses simultaneously the advantages of hydrogen carrier characteristic and multifunctional reactors. This configuration is named thermally coupled dual methanol reactor (TCDMR). In the first reactor, cyclohexane dehydrogenation reaction is coupled with methanol production reactions. Cyclohexane is produced in the second reactor due to carrying all produced hydrogen from the first reactor to the second reactor for benzene hydrogenation reaction. The operating conditions of TCDMR are optimized via DE method and six decision variables are considered to investigate its performance. Since cyclohexane is produced continuously in the second reactor and enters the first reactor as the endothermic feed, the external cyclohexane injection rate is minimized, too. A comparison is made between the optimized TCDMR (OTCDMR), TCDMR and Thermally coupled methanol reactor (TCMR) and conventional methanol reactor (CMR). The modeling results demonstrate the superiority of OTCDMR to all previously proposed configurations. A continuous system is achieved and slight amount of exterior cyclohexane injection rate (3.6 mol h −1) is required in this configuration. Furthermore, the hydrogen storage problem is solved by this configuration owing to simultaneous hydrogen production and utilization. In addition, produced benzene in OTCDMR is about 10% of the one in TCMR which can be appealing from the environmental viewpoint.

Start-up and Dynamic Analysis of a Novel Thermally Coupled Reactor for the Simultaneous Production of Methanol and Benzene

Coupling energy intensive endothermic reaction systems with suitable exothermic reactions improves the thermal efficiency of processes, achieving the autothermality within the reactor, reducing the size of the reactors, and achieving a multiple reactants–multiple products configuration. In this study, a dynamic heterogeneous model for a novel thermally coupled reactor—containing methanol synthesis reactions and cyclohexane dehydrogenation—is developed to consider the startup and transient response of the system. This heat-exchanger reactor consists of two fixed beds separated by a wall, where heat is transferred across the surface of tube from the exothermic side to the endothermic side. The proposed model was validated against conventional methanol synthesis reactor, and a good agreement was achieved. Dynamic simulation results for the co-current mode have been investigated to present the response of the reactor outlet temperature, methanol yield, and cyclohexane conversion in the cases of a step change in the initial molar flow rate and inlet temperature of both sides. The challenges posed by the transient operation of thermally coupled reactor are identified to avoid severe issues that can arise in the course of operating the reactor (such as reactor extinction). The results suggest that control of this coupled reactor could be feasible and beneficial.

Mathematical simulation and optimization of methanol dehydration and cyclohexane dehydrogenation in a thermally coupled dual-membrane reactor

Thermally coupling of endothermic and exothermic reactions in a membrane reactor improves thermal efficiency and production rate in the processes, reduces the size of reactors and decreases purification cost. This paper focuses on modeling and optimization of a thermally coupled dual-membrane reactor for simultaneous production of hydrogen, dimethyl ether (DME) and benzene. A steady state heterogeneous mathematical model is developed to predict the performance of this novel configuration. The catalytic methanol dehydration reaction takes place in the exothermic side that supplies the necessary heat for the catalytic dehydrogenation of cyclohexane to benzene in the endothermic side. Selective permeation of hydrogen and water vapor through the Pd/Ag and composite membranes are achieved by co-current flow of sweep gas through the membrane wall. The differential evolution method is applied to optimize the thermally coupled dual-membrane reactor considering the summation of DME and benzene mole fractions from reaction sides and hydrogen mole fraction in the permeation side as the main objectives. The optimization results are compared with corresponding predictions for an industrial methanol dehydration adiabatic reactor operated at the same feed conditions. Methanol conversion enhances about 5.5% in the optimized thermally coupled dual-membrane reactor relative to the conventional DME reactor. The results suggest that coupling of these reactions in the proposed configuration could be feasible and beneficial.

Dehydrogenation of cyclohexane over SHS-produced Nickel-Zirconium alloy

Ni-Zr alloys were SHS-produced from Ni-Zi powder mixtures containing 4% Teflon powder as a chemical activator. In the presence of Teflon, the burning velocity increased by a factor of 15, at a combustion temperature of 1550°C. The synthesized powder was found to contain NiZr and Ni10Zr7 with a particle size of 1–5 μm and pores 100–300 μm in their size. In dehydrogenation of cyclohexane, the alloy exhibited a catalytic activity commencing from 160°C; at 500°C, the cyclohexane conversion attained a value of 78%. Our results illustrate the potential of SHS technology in production of heterogeneous catalysts active at relatively low temperatures.

Nanostructured metal–carbon membrane catalysts based on carbonized PAN

The nanostructured catalytic membranes consisted of stainless steel porous support modified with carbon layer, and catalytic layer, containing Pt–Ru alloy nanoparticles, homogeneously dispersed in carbon matrix, were synthesized for the first time by IR-pyrolysis method. The optimization of pore size of plates of porous stainless steel was carried out using carbon, prepared by IR-pyrolysis of PAN, introduced in support pores in DMFA solution. The structure and thickness of modifying layer were controlled by PAN initial concentration and IR-annealing intensity. The selectivity of helium–argon pair separation increased monotonously as carbon layer thickness increased. The catalytic layers, based on IR-PAN carbon, Pt–Ru nanoparticles and finely dispersed activated carbon SKT or detonation nanodiamonds, were deposited on carbon modified porous membranes. The selectivity of permeability for pair helium–argon increased after catalytic layer deposition, becoming equal to the ideal Knudsen value. Cyclohexane dehydrogenation on obtained composite metal–carbon membrane catalysts were carried out in plug flow catalytic membrane reactor at the temperatures from 220 to 520°C. The productivity on the unit of mass of active metal of composite membrane metal–carbon catalyst, was shown to be significantly higher than that of the same metal–carbon composites in grained form.

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.

Enhanced performance of Ca-doped Pt/γ-Al 2O 3 catalyst for cyclohexane dehydrogenation

The promotion effects of Ca on catalytic performance of Ca-doped Pt/Al 2O 3 catalyst were investigated by varying Ca content and impregnation orders. The Ca-doped Pt/Al 2O 3 catalyst with atomic rate Ca/Pt = 5 exhibited the highest catalytic activity and stability. Detailed analyses show that low Ca content (Ca/Pt ＜ 5) could not efficiently promote hydrogen spillover from Pt-Al interface to the support and reduce the strong acid sites on the support. However, high Ca content (Ca/Pt > 5) decreases the initial activity due to the coverage of Ca on the support and increases the amount of Pt strongly interacting with Ca, which inhibits Pt reduction. Furthermore, the Ca promotion effect is more pronounced when Ca is added prior to Pt due to surface modification of Ca on Al 2O 3 support. This modified catalyst possesses more dispersed Pt, lower acidity and larger amount of spilled-over hydrogen.

Lanthanum Modified Catalyst for Efficient Supply of Hydrogen through Dehydrogenation of Organic Chemical Hydrides

Dehydrogenation process of organic chemical hydrides was improved by modifying the catalyst of nickel-activated carbon (Ni/AC) with lanthanum (La). The catalysts were prepared in impregnation method with different amounts of La and Ni. The textural properties and morphology of catalyst were analyzed by nitrogen adsorption and transmission electron microscope equipped with energy dispersive spectrometer respectively. The effects such as metal content and granule size on the dehydrogenation of cyclohexane were investigated in fixed bed reactor. The results show that the metallic active components can be well dispersed on the support, and the elements analysis indicates the metal species tend to assemble on the surface layer rather than being distributed equally in the whole catalyst. The La modified catalyst LaNi/AC exhibited superior catalytic performance to Ni/AC and the conversion was 45% for LaNi/AC catalyst at 673K, while only 34 % for Ni/AC under the same conditions.

Oxidative Dehydrogenation of Cyclohexane over Mg-V-O Catalysts Prepared via Citriate Complexation

Oxidative dehydrogenation of cyclohexane was studied over three pure Mg-V-O catalysts, which are Mg3(VO4)2, Mg2V2O7 and MgV2O6, respectively. Catalysts were prepared via citric acid complexation and characterized by N2-adsorption, XRD, FT-IR, NH3-TPD and H2-TPR techniques. Among the pure magnesium vanadates, Mg3(VO4)2 has the isolated active sites, weakly basic surface and lower reducibility of the metal cations, and could be recognized as the catalytic active phase. Mg3(VO4)2 catalyst exhibited a better catalytic performance, on which a cyclohexene selectivity of 41.5% at cyclohexane conversion of 15.5% was obtained.

Simultaneous utilization of two different membranes for intensification of ultrapure hydrogen production from recuperative coupling autothermal multitubular reactor

The potential of simultaneous hydrogen production and in situ water removal in a thermally coupled multitubular two-membrane reactor (TCTMR) were studied numerically. Methanol synthesis is carried out in exothermic side with H-SOD membrane and supplies the necessary heat for the endothermic side. Dehydrogenation of cyclohexane is carried out in endothermic side with hydrogen-permselective Pd/Ag membrane wall. Therefore, the proposed reactor consists of two membranes, one for separation of pure hydrogen from endothermic side and another one for separation of water from exothermic side. The motivation for in situ H 2O removal during methanol synthesis by using H-SOD membranes is to displace the water–gas shift equilibrium to enhance conversion of CO 2 to improve methanol productivity. A steady-state heterogeneous model is developed to analyze the operation of the coupled methanol synthesis. The proposed model has been used to compare the performance of a TCTMR with conventional reactor (CR) and thermally coupled membrane reactor (TCMR) at identical process conditions. This comparison shows that TCTMR in addition to possessing advantages of a TCMR has a more favorable profile of temperature and increased productivity compared with other reactors. Furthermore, lower water production rate in TCTMR reduces catalyst re-crystallization.

Catalytic activity of Pt-Re-Pb/Al2O3 naphtha reforming catalysts

BACKGROUND: The main purpose of the naphtha reforming process is to obtain high octane naphtha, aromatic compounds and hydrogen. The catalysts are bifunctional in nature, having both acid and metal sites. The metal function is supplied by metal particles (Pt with other promoters like Re, Ge, Sn, etc.) deposited on the support. The influence of the addition of Pb to Pt-Re/Al2O3 naphtha reforming catalysts was studied in this work. The catalysts were prepared by co-impregnation and they were characterized by means of temperature programmed reduction, thermal programmed desorption of pyridine and several test reactions such as cyclohexane dehydrogenation, cyclopentane hydrogenolysis and n-heptane reforming. RESULTS: It was found that Pb interacts strongly with the (Pt-Re) active phase producing decay in the metal function activity. Hydrogenolysis is more affected than dehydrogenation. Part of the Pb is deposited over the support decreasing the acidity and the strength of the most acidic sites. CONCLUSION: The n-heptane reforming reaction shows that Pb modifies the stability and selectivity of the Pt-Re catalysts. Small Pb additions increase the stability and greatly improve the selectivity to C7 isomers and aromatics while they decrease the formation of low value products such as methane and gases. Copyright © 2011 Society of Chemical Industry

Production of ultrapure hydrogen via utilizing fluidization concept from coupling of methanol and benzene synthesis in a hydrogen-permselective membrane reactor

In this work, a novel fluidized-bed thermally coupled membrane reactor has been proposed for simultaneous hydrogen, methanol and benzene production. Methanol synthesis is carried out in exothermic side which is a fluidized-bed reactor and supplies the necessary heat for the endothermic side. Dehydrogenation of cyclohexane is carried out in endothermic side with hydrogen-permselective Pd/Ag membrane wall. Selective permeation of hydrogen through the membrane in endothermic side is achieved by co-current flow of sweep gas through the permeation side. A steady-state fixed-bed heterogeneous model for dehydrogenation reactor and two-phase theory in bubbling regime of fluidization for methanol synthesis reactor is used to model and simulate the integrated proposed system. This reactor configuration solves some observed drawbacks of new thermally coupled membrane reactor such as internal mass transfer limitations, pressure drop, radial gradient of concentration and temperature in both sides. The proposed model has been used to compare the performance of a fluidized-bed thermally coupled membrane reactor (FTCMR) with thermally coupled membrane reactor (TCMR) and conventional methanol reactor (CR) at identical process conditions. This comparison demonstrates that fluidizing the catalytic bed in the exothermic side of reactor caused a favorable temperature profile along the FTCMR. Furthermore, the simulation results represent 5.6% enhancement in the yield of hydrogen recovery in comparison with TCMR.

The effect of flow type patterns in a novel thermally coupled reactor for simultaneous direct dimethyl ether (DME) and hydrogen production

Dimethyl ether (DME) has gained wide interest in chemical industry regarding its use as a multi-source, multi-purpose fuel either for diesel engines or as a clean alternative for liquefied petroleum gas (LPG). The direct synthesis of DME from syngas would be more economical and beneficial in comparison to the indirect process via methanol dehydration. In this study, one type of the multifunctional auto-thermal reactors (the recuperative one) is selected in which the direct synthesis of dimethyl ether (DME) is coupled with the catalytic dehydrogenation of cyclohexane to benzene in a two fixed bed reactor separated by a solid wall, where heat is transferred across the surface of tube. Steady-state, heterogeneous, one-dimensional model has been used to describe the performance of this novel configuration. Both co-current and counter-current operating modes are investigated and the simulation results are compared with the available data of a pipe-shell fixed bed reactor for direct DME synthesis which operates at the same feed conditions. In addition, the influence of the molar flow rate of exothermic and endothermic stream on the reactor performance is also investigated. The results suggest that coupling of these reactions could be feasible and beneficial and the co-current mode has got better performance in DME and hydrogen production. In order to establish the validity and safety handling of the new concept, an experimental proof is required.

Catalytic dehydrogenation of cyclohexene over MoO3/γ-Al2O3 catalysts

A series of MoO3/γ-Al2O3 catalysts with different Mo surface densities (Mo atoms/nm2) has been prepared by incipient wetness impregnation method. Structural characteristics of the prepared catalysts were investigated by atomic absorption spectroscopy, X-ray diffraction, Fourier Transform Infrared spectroscopy, N2 adsorption at −196 °C, and temperature-programmed reduction (TPR). The catalytic activities of the prepared catalysts were tested by cyclohexene conversion between 200 and 400 °C. XRD results indicated that molybdenum oxide species were dispersed as a monolayer on the support up to 4.04 Mo atoms/nm2, and the formation of crystalline MoO3 was observed above this loading. FTIR and TPR results showed that molybdenum oxide species were present predominantly in tetrahedral form at lower loading, and polymeric octahedral forms were dominant at higher loading. Cyclohexene conversion reaction proceeded mainly through the simple dehydrogenation pathway in the studied temperature range 200–400 °C and was found to be highly dependent on MoO3 dispersion.

Influences of calcination temperature on the efficiency of CaO promotion over CaO modified Pt/γ-Al 2O 3 catalyst

The cyclohexane dehydrogenation on CaO modified Pt/γ-Al 2O 3 was investigated with the aim of understanding the influences of calcination temperature of CaO modified γ-Al 2O 3 support (Ca/Al) on the efficiency of CaO promotion. The results showed that the catalyst calcined at 600 °C exhibited the worst, while the one calcined at 800 °C showed the best catalytic performance during the variation of Ca/Al calcination temperature from 500 to 900 °C. The catalyst pretreated at 800 °C exhibited an excellent hydrogen productivity of up to 4.45 L/(g cat h) and high stability during 80 h continuous investigation of cyclohexane dehydrogenation in the absence of co-feed hydrogen at 300 °C and 8000 h −1 space velocity. That was attributed to weaker Pt–Al interaction and higher dispersed Pt, deriving from larger CaO particles. According to XRD spectra of model catalysts and CO 2-TPD of Ca–Pt/Al catalysts, the efficiency of CaO promotion depended on the Ca–Al interaction strength and particle size instead of quantity of free CaO particles. The main calcium compound was free CaO in catalysts with low CaO content and low calcination temperature (below 800 °C). Complex CaO with high Ca–Al interaction strength and less CaO properties are formed in Ca900–Pt/Al catalyst.