Dehydrogenation Research Articles

Effect of support on selectivity and on-stream stability of surface VOx species in non-oxidative propane dehydrogenation

Al2O3, SiO2(MCM-41), and Al2O3–SiO2 (Siral®) with a SiO2 content varying from 1 to 70 wt.% were used to prepare supported catalysts with a V loading below one monolayer. Their activity, selectivity and on-stream stability were tested in non-oxidative propane dehydrogenation (DH) at 550 °C. The highest space–time yield of propene was only 25% lower than that over industrially relevant Pt–Sn/Al2O3 under the same reaction conditions. All catalysts deactivated with time on stream, but restored their initial performance after oxidative regeneration as proven in a sequence of 10 DH/regeneration cycles lasting in total over 60 h. The deactivation was related to propene-derived carbon deposits covering active VOx sites. However, depending on the catalyst, such deposits formed on bare support sites can also participate in propane dehydrogenation. Their DH activity is, however, significantly lower compared to VOx species. Acidic properties of the support were found to be crucial for the generation of such catalytically active carbon species.

Pyrolysis of <em>n</em>-Decane and Dimethylbenzene under Supercritical Pressure

The pyrolysis of n-decane and dimethylbenzene under supercritical pressure was studied using a continuous flow reactor.Samples were heated to a temperature of 650,700,or 750 ℃ under a pressure of 4 MPa without oxygen.n-Decane pyrolyzed more easily than dimethylbenzene.We analyzed gaseous products by online gas chromatography,and liquid products by gas chromatography-mass spectrometry,allowing us to calculate the cracking gas yield and cracking conversion of these systems.A quantum chemistry computation was used to evaluate the binding energies of C-C and C-H bonds in n-decane and dimethylbenzene.Both experimental and theoretical results were also used to analyze the cracking reactivity of these species.Analysis of the components in the products indicated that the main products of n-decane were C1-C3 hydrocarbons and hydrogen,whereas ethylbenzene,toluene and other aromatic compounds were the main products of dimethylbezene after pyrolysis.Binding energy calculations showed that both C-C and C-H bonds in n-decane possessed lower binding energies than those in dimethylbezene,and a C-C bond was the weakest.In dimethylbenzene,a C-H bond in the methyl groups was the weakest,and its binding energy was much smaller than those of the C-C and CH bonds in the benzene ring.Therefore,the main reactions in the cracking process of n-decane are breakage of a C-C bond and dehydrogenation.However,the cracking process in dimethylbenzene mainly involves the fracture and dehydrogenation of methyl groups.The theoretical calculations reasonably explained the experimental phenomena.

Interfacial phenomena in (de)hydrogenation reactions

Interfacial phenomena in (de)hydrogenation reactions

Comparative study of propane dehydrogenation over V-, Cr-, and Pt-based catalysts: Time on-stream behavior and origins of deactivation

Catalytic performance of VOx/MCM-41 possessing highly dispersed VOx species in non-oxidative propane dehydrogenation (DH) was compared with that of industrially relevant CrOx/MCM-41 and Pt–Sn/Al2O3 over four DH (24h on-stream) and oxidative regeneration cycles. The effect of reduction with H2 on the catalysts DH properties was also investigated. VOx/MCM-41 in both oxidized and reduced form showed superior time on-stream stability in all DH cycles and its activity could be fully recovered by oxidative regeneration. In situ time-resolved UV/Vis spectroscopy and temperature-programmed oxidation of the used catalysts enabled us to elucidate the origins of the catalysts deactivation. In a single DH cycle, the formation of carbon deposits is the main reason for the deactivation. Activity loss of CrOx/MCM-41 and Pt–Sn/Al2O3 from cycle to cycle was caused by structural changes in the catalytically active metal/metal oxide species. In contrast to these two catalysts, the structure of active VOx species on MCM-41 was not changed during the DH/regeneration cycles, which is the reason for stable performance of the latter catalyst.

Biochemical Characterization of Biomphalaria arabica , the Molluscan Intermediate Host for Schistosomes in Saudi Arabia

Schistosomiasis is the most important human helminth infection in terms of morbidity and mortality. The present study provides a comparative analysis on Biomphalaria arabica , the intermediate hosts of Schistosoma mansoni from two different areas having different prevalence level of Schistosomiasis in Saudi Arabia. B. arabica snails were collected from Riyadh and Hofuf district, and the levels of various enzymes like aspartate and alanine aminotransferase (AST, ALT), Lactate dehydrogenate (LDH), acid and alkaline phosphatase (ACP,ALP), α amylase and lipases were measured in the tissue homogenate of snails, to confirm the relationship between the biochemical and endemic of the disease. Also amino acid profile of B. arabica was determined using amino acid analyzer. The result showed higher levels of AST and LDH in the sample of B. arabica from Hofuf as compared to Riyadh while ALT, ACP, ALK, α amylase and lipase enzymes were found lower in samples from Hofuf as compare to Riyadh. The amino acid profile shows that Riyadh samples have remarkably higher levels of most of the measured amino acids. The present study confirms that biochemical profile of B. arabica is critically important for the success of schistosome life cycle and that snail host definitely plays a part in prevalence of the disease.

Dehydrogenation–halogenation of a 4,5,6,7-tetrahydroisoxazolo[4,3-c]pyridin-4-one to provide a scaffold for acylpyridones

Dehydrogenation–halogenation of a 4,5,6,7-tetrahydroisoxazolo[4,3-c]pyridin-4-one to provide a scaffold for acylpyridones

Thermodynamic equilibrium analysis of ring opening of decalin

Thermodynamic equilibrium calculations were carried out in order to understand the effect of temperature and hydrogen-to-hydrocarbon ratio on the yield and selectivity of ring opening products that are obtained from hydrocracking of decalin. Thermochemical analysis of single hydrocracking reactions involving selected alkanes and naphthenes indicates that endocyclic cracking of naphthenes is less favored than acyclic and exocyclic cracking reactions due to entropy considerations. The equilibrium calculations were performed in the temperature range 298–800K at a pressure of 1bar for a mixture of compounds obtained from hydrocracking of decalin using the assumption that the reaction mixture behaves as an ideal gas. The quantum chemical method G3MP2 was used to calculate the standard Gibbs free energy of formation for compounds for which thermochemical data are so far unavailable in the open literature. The results from equilibrium calculations indicate that, as long as cracking (CR) and dehydrogenation (DH) products are allowed to form simultaneously, total ring opening (TRO) products cannot be present in the reaction mixture in appreciable concentrations. If the pathways leading to the formation of CR products are excluded from the equilibrium calculations, the yield of TRO products reaches a significant value of ∼0.75kg/kg decalin fed in the temperature range 400–500K. The highest yield of TRO products, ∼1kg/kg decalin fed, observed for a hydrogen-to-hydrocarbon ratio of 4 in the temperature range <500K could be the maximum achievable yield for TRO products of decalin. The TRO products consist mostly of saturated ring opening (SRO) products and traces of unsaturated ring opening (USRO) products.

Effects of Loaded Sodium on the Structure of Cu-ZnO/Na-SiO2 Catalysts and the Dehydrogenation of 2-Butanol

The catalyst support SiO2 was firstly pretreated with different amounts of sodium nitrate solution followed by impregnation in copper and zinc nitrates solution.The effects of the preparation method and contents of sodium on the structures and performances of Cu-ZnO/Na-SiO2 catalysts were investigated.The catalytic activities of these catalysts were evaluated using the dehydrogenation of 2-butanol and the selectivity for methyl ethyl ketone as probes.These catalysts were characterized by means of XRD and TPR.The results indicated that sodium-pretreatment can improve the dispersion of metallic copper and the interaction between the copper species and the support.

Concerted or Stepwise Hydrogen Transfer in the Transfer Hydrogenation of Acetophenone Catalyzed by Ruthenium–Acetamido Complex: A Theoretical Mechanistic Investigation

In this paper, the mechanism of transfer hydrogenation of acetophenone catalyzed by ruthenium-acetamido complex was studied using density function theory (DFT) method. The catalytic cycle of transfer hydrogenation consists of hydrogen transfer (HT) step and dehydrogenation (DH) step of isopropanol (IPA). Inner sphere mechanism (paths 1 and 7) and outer sphere mechanism (paths 2-6) in HT step are fully investigated. Calculated results indicate that DH step of IPA (from (i)1 to (i)2) is the rate-determining step in the whole catalytic cycle, which has a potential energy barrier of 16.2 kcal/mol. On the other hand, the maximum potential energy barriers of paths 1-7 in the HT step are 5.9, 12.7, 24.4, 16.8, 23.7, 7.2, and 6.1 kcal/mol, respectively. The inner sphere pathways (paths 1 and 7) are favorable hydrogen transfer modes compared with outer sphere pathways, and the proton transferred to the oxygen atom of acetophenone comes from the hydroxyl group but not from amino group of acetamido ligand. Those theoretical results are in agreement with experimental report. However, in view of this DFT study in the inner sphere mechanism of HT step, hydride transfer and proton transfer are concerted and asynchronous hydrogen transfer but not a stepwise one, and hydride transfer precedes proton transfer in this case.

Structural, textural and catalytic properties of pure and Li-doped NiO/Al2O3 and CuO/Al2O3 catalysts

Pure and Li-doped NiO/Al2O3 and CuO/Al2O3 catalysts were prepared to contain 2, 4 and 8 wt.% of Ni and Cu, respectively. The structural properties were determined using DTA, XRD and FTIR techniques, and the textural properties of the catalysts were determined from their adsorption–desorption isotherms of nitrogen at 77 K. The chemisorption of hydrogen at 473–823 K with the pre-reduced catalysts was measured. The data obtained allowed the determination of the metal surface area, S (m2/g); the percentage of metal distribution, R; and the diameter of metal crystallite, d (nm). The amount of surface acidity, measured in mmol/g, was determined from the amount of chemisorbed pyridine necessary to completely inhibit the catalytic dehydration (DHD) of isopropanol. The conversion of isopropanol at 533–623 K was investigated using the micro-catalytic pulse technique. DTA, XRD and FTIR indicated that NiO and CuO exist as separate phases with crystallite sizes too small to be detected. No evidence has been gathered to indicate the existence of an aluminate phase.With the increase of metal loading, the surface area decreased whereas the total pore volume and the mean pore radius increased. Conversion of iso-propanol to propene proceeded via (DHD) on surface acid sites, and conversion of isopropanol to acetone proceeded via dehydrogenation (DHG) on redox sites. DHD and DHG exhibited first-order kinetics, and the rates of both reactions increased with temperature, with the latter being more temperature-dependent.

ChemInform Abstract: Dehydrogenation of 5,6‐Dihydropyrido[2,3‐d]pyrimidin‐7(8H)‐ones: A Convenient Last Step for a Synthesis of Pyrido[2,3‐d]pyrimidin‐7(8H)‐ones.

AbstractTwo new protocols for the dehydrogenation of 5,6‐dihydropyrido[2,3‐d]pyrimidin‐7(8H)‐ones (I) are presented.

Synthesis and Oxidation of Acetoxy-Substituted Triene of Oleanolic Acid

Acetoxy-substituted triene(6) was synthesized from oleanolic acid(1) via methyl esterification of 17-carboxylic acid,benzoylation of 3-hydroxy,oxidation of 12-double bond by O3,dehydrogenation with Br2/HBr,enol acetylation of carbonyl group,and ring-C opening under UV light.Oxidations of compound 6 with different oxidants such as O3,meta-cholorperoxybenzoic acid(mCPBA),H2O2,t-BuOOH and Na2S2O8,yield decalin-type chiral synthons,epoxides,and/or lactone of oleanolic acid derivatives.

LiMn(BH&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;/2LiCl Composite Synthesized by Reactive Ball-Milling and Its Dehydrogenation Properties

以LiBH4和MnCl2为初始原料,采用反应球磨法制备了LiMn(BH4)3/2LiCl复合物,并系统地研究了该复合物的脱氢性能及含钛催化剂的掺杂对其脱氢性能的影响.结果表明:LiMn(BH4)3/2LiCl复合物是由非晶态的LiMn(BH4)3和晶态的LiCl组成,在135-190℃分解,分解反应的活化能为114.0 kJ·mol-1;LiMn(BH4)3/2LiCl复合物分解失重约7.0%(w).组分分析表明除H2外,释放的气体中还含有4.0%(摩尔分数,x)的B2H6.B2H6的生成是该复合物失重超过其理论储氢容量6.3%(w(的原因;进一步研究发现,含钛催化剂(TiF3、TiC、TiN和TiO2)中,仅TiF3能够催化LiMn(BH4)3/2LiCl复合物的分解反应,使其起始分解温度和分解反应活化能分别降低至125℃和104.0 kJ·mol-1.这主要归因于TiF3中的Ti原子取代了LiMn(BH4)3中的部分Li原子,并在局域形成了易于分解的Ti(BH4)3.

Mathematical Modeling and Simulation of the Dehydrogenation of Ethyl Benzene to Form Styrene Using Steady-State Fixed Bed Reactor

In this research, two models are developed to simulate the steady state fixed bed reactor used for styrene production by ethylbenzene dehydrogenation. The first is one-dimensional model, considered axial gradient only while the second is two-dimensional model considered axial and radial gradients for same variables. The developed mathematical models consisted of nonlinear simultaneous equations in multiple dependent variables. A complete description of the reactor bed involves partial, ordinary differential and algebraic equations (PDEs, ODEs and AEs) describing the temperatures, concentrations and pressure drop across the reactor was given. The model equations are solved by finite differences method. The reactor models were coded with Mat lab 6.5 program and various numerical techniques were used to obtain the desired solution. The simulation data for both models were validated with industrial reactor results with a very good concordance.

ChemInform Abstract: Dispironaphthalenones and Spironaphthalenones as Novel Dehydrogenation Reagents.

Abstract Dehydrogenation of a number of dihydroaromatic substrates has been carried out using either dispironaphthalenone 1 or spironaphthalenones 2 & 3 as dehydrogenating agents. The reaction is over in refluxing mesitylene in 1– hr and the yields of the aromatised products are fairly good (65–70%).

ChemInform Abstract: Dehydrogenations Using Benzofuroxan as Oxidant.

ChemInform Abstract: Dehydrogenations Using Benzofuroxan as Oxidant.

Oxidative dehydrogenation of propane in a perovskite membrane reactor with multi‐step oxygen insertion

AbstractA membrane reactor incorporating a hollow fiber with successive parts of oxygen permeable and passivated surface segments has been developed and was used for the oxidative dehydrogenation (DH) of propane. This membrane geometry allows a controlled oxygen feeding into the reactor over its axial length. In the oxidative DH, the thermodynamic limitation of propane DH can be overcome. By using this novel hollow fiber membrane reactor with a Pt/Sn/K DH catalyst, oxygen separation and propene formation could be established even at temperatures as low as 625°C with long‐term stability. Combining the hollow fiber membrane and the DH catalyst, the highest propene selectivity of 75% was observed at a propane conversion of 26% and 625°C whereas the best propene yield of 36% was obtained at 675°C (48% propene selectivity). The performance of this reactor is evaluated by applying various reaction conditions. © 2010 American Institute of Chemical Engineers AIChE J, 2010

Active coke: Carbonaceous materials as catalysts for alkane dehydrogenation

The catalytic dehydrogenation (DH) and oxidative dehydrogenation (ODH) of light alkanes are of significant industrial importance. In this work both carbonaceous material deposited on VO x /Al 2O 3 catalysts during reaction and unsupported carbon nanofibres (CNFs) are shown to be active for the dehydrogenation of butane in the absence of gas-phase oxygen. Their activity in these reactions is shown to be dependent upon their structure, with different reaction temperatures yielding structurally different coke deposits. Terahertz time-domain spectroscopy (THz-TDS), among other techniques, has been applied to the characterisation of these deposits – the first time this technique has been employed in coke studies. TEM and other techniques show that coke encapsulates the catalyst, preventing access to VO x sites, without a loss of activity. Studies on CNFs confirm that carbonaceous materials act as catalysts in this reaction. Carbon-based catalysts represent an important new class of potential catalysts for DH and ODH reactions.

Ethane dehydrogenation over pore-expanded mesoporous silica-supported chromium oxide: 2. Catalytic properties and nature of active sites

Pore-expanded mesoporous MCM-41 silica-supported chromium oxide catalysts with different chromia loadings were synthesized using three different methods. The catalysts were thoroughly characterized and used for ethane dehydrogenation (DH). Reaction studies revealed that the ethane conversion and coke formation on the catalyst depend on the chromia loading and the catalyst preparation method. All catalysts were highly selective toward ethylene. However, among the three catalysts containing 5 wt% chromia, the 5 wt% Cr/PE-MCM-41 catalyst exhibited the best performance with an ethane conversion of 23% and ∼99% ethylene selectivity at 650 °C (C 2H 6/N 2 = 0.176, total contact time = 0.0032 g min/mL). The chromia catalysts prepared using as-synthesized pore-expanded mesoporous silica (Cr/PE-MCM-41) also exhibited the highest stability. XPS analysis revealed the presence of three different chromium, namely Cr(II), Cr(III) and Cr(VI), whose overall surface concentration as well as relative content varied with the reaction time-on-stream (TOS). Furthermore, the nature and relative amounts of surface chromium species were monitored by in situ XPS measurements as a function of TOS. Correlation of the XPS findings with ethane DH data showed clearly that the Cr(III) species are the main active sites.

Ethane dehydrogenation over pore-expanded mesoporous silica supported chromium oxide: 1. Catalysts preparation and characterization

Pore-expanded MCM-41 silica was prepared in two steps, namely synthesis of MCM-41 in the presence of cetyltrimethylammonium bromide (CTAB) at 100 °C followed by a hydrothermal pore expansion treatment in the presence of dimethyldecylamine (DMDA) at 120 °C for 72 h. The as-synthesized material (PE-MCM-41), the material obtained after selective ethanol extraction of DMDA (PE-MCM-41E) and the organic-free material obtained after calcination of PE-MCM-41 (PE-MCM-41C) were used as supports for chromium oxide up to a chromium loading of 7 wt.%. All catalysts were thoroughly characterized by X-ray diffraction, N 2 adsorption, temperature programmed reduction (TPR), Raman and X-ray photoelectron spectroscopy (XPS), and used for ethane dehydrogenation (DH). N 2 adsorption measurements showed that the pore and surface characteristics strongly depend on the catalyst preparation method and relatively independent of chromia loading. The structural properties of the chromium-loaded materials were associated with the effect of water on the support, which is dependent on how well the silica framework is protected by the occluded surfactant(s) against direct contact with water. XPS analysis showed that in most cases, the surface chromium contained significant amounts of Cr(III) and Cr(VI) and minor amounts of Cr(II). Furthermore, H 2-TPR results revealed that the extent of reduction of the catalysts depend on the chromia loading and the preparation method.