Gas-phase Ethylene Research Articles

Effect of the amounts and proportions of active components on the activity and selectivity of a catalyst for vinyl acetate synthesis by gas-phase ethylene acetoxylation

The activity, selectivity, and service life of a palladium-, gold-, and potassium acetate-containing catalyst for vinyl acetate (VA) synthesis are reported as a function of the catalyst composition. Equations relating these performance parameters to the percentage of each component are suggested. General relation-ships between the catalytic activity and the component contents are established for optimization of the catalyst composition. These relationships have been validated by bench tests. The average activity of the 1.5% Pd + 0.75% Au + 5.0% KOAc catalyst is 700 (g VA)/(l Cat h), and its ethylene-to-VA conversion selectivity is 93–95%.

IR spectroscopy of the structural states of ethylene in aerogel nanopores

We have investigated for the first time the combination vibrational absorption bands ν 5 + ν 9 and ν 1 + ν 11 of ethylene adsorbed by nanopores of a silicon dioxide aerogel in the 5800-6300 cm -1 region. Our measurements show significant differences in the absorption spectra of gas-phase ethylene and ethylene found in the nanoporous structure of the aerogel, including a change in the shape of the absorption band contours, their frequency shift, and an increase in the absorption intensity.

CFD simulation of hydrodynamics and heat transfer in gas phase ethylene polymerization reactors

The hydrodynamics and temperature of a two-dimensional gas–solid fluidized bed of gas phase olefin polymerization reactor had been studied. A two-fluid Eularian Computational Fluid Dynamics (CFD) model with closure relationships according to the kinetic theory of granular flow has been applied in order to simulate the gas–solid flow. Fluidization regime and gas–solid flow pattern were investigated using three different drag models. Model predictions of bed pressure drop were compared with corresponding experimental data reported in the literature to validate the model. The predicted values were in reasonable agreement with the experimental data. The temperature behavior of fluidized bed with various drag models was investigated. The temperature gradient in the primary section of the bed was much larger than the gradient in other sections and the effect of all drag models on temperature gradient along the bed was approximately similar.

Autocatalytic gas-phase ethane dehydrogenation in a wall-less reactor

The thermal gas-phase pyrolysis of ethane was studied under conditions of the bulk heating of the reaction mixture with IR-laser radiation. The concentrations of ethane pyrolysis products as functions of reaction time were calculated in accordance with standard kinetic schemes; they showed that a classical radical chain mechanism corresponded to only highly dilute mixtures of ethane with an inert gas. As found by calculations, the experimental data on the kinetics of consumption of the initial substance and on the kinetics of buildup of pyrolysis products in undiluted mixtures of ethane and its conversion products were adequately described by an autocatalytic (with respect to ethylene) mechanism of ethane dehydrogenation. This mechanism involved the step of ethane interaction with ethylene to form methyl and propyl radicals.

Theoretical study on the effect of intramolecular hydrogen bonding on OH stretching overtone decay lifetime of ethylene glycol, 1,3-propanediol, and 1,4-butanediol

It is well known that the existence of hydrogen bonds causes the vibrational spectra for the donor OH bond to broaden. Most times this broadening is attributed to inhomogeneous contributions, however in this paper, we studied the homogeneous contribution coming from the decay lifetime of the OH stretching excitation. Alkane diol, which possesses an OH(b)OH(f) (b and f stand for hydrogen bonded and free, respectively) intramolecular hydrogen bond, provides us with a controllable molecule to systematically study the effect of intramolecular hydrogen bond on the decay rate of the OH vibrational excitation. In the present study we performed local mode vibrational calculation of gas phase ethylene glycol (EG), 1-3 propanediol (PD), and 1-4 butanediol (BD) using the potential energy curves calculated using B3LYP/6-31+G(d,p), MPW1PW91/6-311+G(2d,p), M06-2X/6-311+G(2df,2p), and MP2/6-311++G(3df,3pd) methods. In addition, we studied the dynamics of OH overtone excited state (Δv(OH) = 3, 4) for the two most stable conformers of the aforementioned diols using the direct dynamics calculation employing the B3LYP/6-31+G(d,p) method. From the trajectory simulations we showed that the decay lifetime from the donor OH(b) excitation decreases while that from the acceptor OH(f) excitation increases as we increase the intramolecular hydrogen bond strength in going from EG, PD, to BD. These results are consistent with the gas phase experimental results of Kjaergaard et al. where the OH(b) peaks, which are sharp for EG disappear for BD. From the detailed analysis on the trajectories, we observed that the decrease in decay lifetime for the OH(b) bond excitation originates from the increase in the energy flow to the OH(b)O torsion/bend motion, while the increase in lifetime for OH(f) bond is due to the fixing of the OH(f) geometry by accepting a donor hydrogen. Furthermore, by plotting the calculated red shift versus the decay lifetime we obtained the following relationship T(decay)(fs) = 343176(Δω(cm(-1)))(-1.36).

Chemical diagnosis of DLC by ESR spectral analysis

Diamond-like carbon (DLC) films were deposited utilizing plasma enhanced chemical vapor deposition (PECVD) with four precursor gases such as methane, ethylene, acetylene and benzene in gas phase. Electron spin resonance (ESR) spectra showed that dangling-bond sites (DBSs) observed in all films were characterized by an isotropic broad single line. The DLC film with unsaturated precursor gases had the higher film growth rate and the higher DBS accumulative rate. Although the DBS in DLC films were quite stable at room temperature under anaerobic conditions, the DBS decayed rapidly to level off toward a limiting value when exposed to air. The stability and reactivity of the DBS in DLC film were assumed to depend on chemical structure of organic gas used as precursor. The detailed-ESR study on DBS of the DLC films could be one of the powerful tools for diagnosing the micro-structural properties and the quality of films.

Channelled optical fibre photoreactor for improved air quality control

An optical fibre reactor with 30 hexagonal-shaped channels distributed within the optical fibre structure was investigated as a gas-phase photocatalytic reactor. TiO 2 photocatalyst, with SiO 2 sol acting as a binder, was coated on the channel walls at a thickness of 1.5 μm. Effective light propagation lengths of 3.4 and 4.9 cm were observed for incidental angles of 81.5° and 87.1°, respectively. The TiO 2-coated channelled optical fibre reactor (COFR) was assessed for the photocatalytic degradation of gas-phase ethylene. The photocatalytic reaction rate of ethylene degradation was linear with respect to the incident photons. The reaction rate order for the incident photons was determined to be 0.93. Despite a longer effective light propagation length for an incidental angle of 87.1°, the quantum yield was independent of the incidental angle. The independence of the quantum yield on the incident photons and the angle of light incidence was attributed to the COFR design, where the propagating light was wholly confined within the reactor and, in turn, more effectively utilised by the TiO 2.

Efficient Photocatalytic Activity of PZT/TiO2 Heterojunction under Visible Light Irradiation

A Pb(Zr0.52Ti0.48)O3/TiO2 composite photocatalyst with nanostructured heterojunction was prepared by a simple sol−gel method. The catalyst was characterized by powder X-ray diffraction, nitrogen adsorption−desorption, transmission electron microscopy, UV−vis diffuse reflectance spectroscopy, electrochemistry technology, and spin-trapping electron paramagnetic resonance. The visible light-induced photocatalytic activities were evaluated by decomposing ethylene in gas phase. The result showed that the prepared composite catalyst exhibited efficient photocatalytic activities with high photochemical stability under visible light irradiation. Moreover, it also showed excellent photocatalytic performance under UV light or simulated sunlight irradiation. On the basis of the measurement of flatband potentials of the samples and the detection of active oxygen species (such as O2•− and OH•), a visible light-induced photocatalytic degradation mechanism of ethylene on PZT/TiO2 was proposed.

The Kinetics of Ethylidyne Formation from Ethylene on Pd(111)

Recent density functional theory (DFT) calculations of the activation energy for the conversion of ethylene to ethylidyne on Pd(111) (Moskaleva, L. V.; Chen, Z.-X.; Aleksandrov, H. A.; Mohammed, A. B.; Sun, Q.; Rosch, N. J. Phys. Chem. C 2009, 113, 2515) predicted an activation barrier with respect to gas-phase ethylene of ∼46 kJ/mol, substantially lower than the results of previous DFT calculations. Thus, the kinetics of formation of ethylidyne from ethylene on Pd(111) are measured as a function of sample temperature using reflection−absorption infrared spectroscopy to monitor the ethylidyne coverage as a function of time. The results yield an experimental value of the height of the activation barrier with respect to gas-phase ethylene of 49 ± 5 kJ/mol, in good agreement with the results of the DFT calculations. The agreement between experiment and theory indicates that the proposed ethylidyne formation pathway involving an initial, rate-limiting ethylene dehydrogenation step to form vinyl species that f...

Preparation of a Well-Defined Silica-Supported Nickel-Diimine Alkyl Complex − Application for the Gas-Phase Polymerization of Ethylene

The interaction of [(α-diimine)Ni(CH2SiMe3)2] (1) (α-diimine = 2,3-dimethyl-1,4-[(2′,6′)-diisopropylphenyl]-N,N′-diazadiene) with the silanol ≡Si−OH groups of Aerosil SiO2-(700) leads to the formation of the surface complex [≡SiO−Ni(α-diimine)(CH2SiMe3)] (2) with the concomitant emission of 1 mol equiv of SiMe4 (TMS). The surface complex characterized by IR, chemical and mass balance analyses, solid-state NMR, and EXAFS has been activated by gaseous BF3 to give an active gas-phase ethylene polymerization catalyst. The polymers thus obtained present a highly branched structure. The activity is attributed to the formation of the putative surface ion pair [≡SiOBF3]−[Ni(α-diimine)(CH2SiMe3)]+.

Integral Absorption Coefficients of C−H Stretching Bands in IR Spectra of Ethane Adsorbed by Cationic Forms of Y Zeolite

An attempt to measure absolute intensities of IR C-H stretching bands of ethane specifically adsorbed by Na+, Ca2+, or Mg2+ cations in Y zeolite was carried out via simultaneous transmission IR and volumetric measurements at very low pressures using self-supporting wafers. For wafers with a thickness of 7-9 mg/cm2 quantitative measurements of absorption coefficients of C-H stretching bands were possible. More pronounced shifts of C-H bands toward lower frequencies indicated that the perturbation of the corresponding bonds in ethane upon adsorption by the bivalent Ca2+ or Mg2+ ions was much stronger than by the monovalent Na+ ions. In addition, the integral extinction coefficient of the C-H stretching bands of ethane adsorbed by Na+ ions was only about 53% of that of gas-phase ethane. For adsorption on the bivalent Ca2+ or Mg2+ ions the coefficients exceeded those of the free ethane molecule, indicating strong polarization of the C-H bonds during the vibration. Bands representing the fully symmetric C-H stretching vibrations, which are IR inactive for free ethane, were perturbed most strongly upon adsorption. Their intensities reached up to 60% of the total intensity in the C-H stretching region. In contrast, the intensities of the other C-H bands were several times lower than for free ethane. These trends indicate anisotropy in the polarizability of the adsorbed molecules. In addition, the absorption coefficients indicate a weaker adsorption and polarization of ethane by Mg2+ than by Ca2+ ions in Y zeolite. Because of their smaller ionic radius, the Mg2+ cations are more strongly shielded by the surrounding oxygen atoms of the zeolite framework than the Ca2+ cations, and therefore, their polarizing ability is decreased.

Theoretical study of the gas-phase ethane C–H and C–C bonds activation by bare niobium cation

The potential energy surfaces for the reaction of bare niobium cation with ethane, as a prototype of the C–H and C–C bonds activation in alkanes by transition metal cations, have been investigated employing the Density Functional Theory in its B3LYP formulation. All the minima and key transition states have been examined along both high- and low-spin surfaces. For both the C–H and C–C activation pathways the rate determining step is that corresponding to the insertion of the Nb cation into C–H and C–C bond, respectively. However, along the C–H activation reaction coordinate the barrier that is necessary to overcome is 0.13 eV below the energy of the ground state reactants asymptote, while in the C–C activation branch the corresponding barrier is about 0.58 eV above the energy of reactants in their ground state. The overall calculated reaction exothermicities are comparable. Since the spin of the ground state reactants is different from that of both H–Nb+–C2H5 and CH3–Nb+–CH3 insertion intermediates and products, spin multiplicity has to change along the reaction paths. All the obtained results, including Nb+–R binding energies for R fragments relevant to the examined PESs, have been compared with existing experimental and theoretical data.

Effects of reaction medium and crystallite size on Ti3+ surface defects in titanium dioxide nanoparticles prepared by solvothermal method

Abstract Titanium dioxide nanoparticles were prepared using solvothermal methods in 1,4-butanediol and toluene. Three different crystallite sizes (10 nm, 13 nm, and 17 nm) of titania as anatase crystals were produced and were then employed as catalysts in photocatalytic oxidation of ethylene in gas phase. As the crystallite size of titania increased, more Ti3+ surface defects were observed. The amounts of surface defects on titania were determined by temperature-programmed desorption using carbon dioxide as a probe molecule. Despite the smaller specific surface area, titania with larger crystallite size exhibited higher photocatalytic activity, which was attributed to more surface defects in the catalyst.

Probing reaction pathways on model catalyst surfaces: Vinyl acetate synthesis and olefin metathesis

The reaction pathway for the palladium-catalyzed synthesis of vinyl acetate from acetic acid, ethylene and oxygen is investigated using reflection–absorption infrared spectroscopy by monitoring the rate of acetate titration by gas-phase ethylene. This reveals that acetate species are removed by reaction with gas-phase ethylene resulting in vinyl acetate formation. Reaction with C 2D 4 reveals a large (∼6) isotope effect indicating that hydrogen is involved in the rate-limiting step. This also results in the appearance of an infrared feature that is assigned to an acetoxyethyl-palladium intermediate. Acetate reaction rates are different for the isotopomers, CH 2CD 2 and CHDCHD. These observations are consistent with a reaction pathway first proposed by Samanos in which ethylene reacts with an acetate species to form an acetoxyethyl-palladium intermediate, which then reacts to form vinyl acetate by a β-hydride elimination reaction. The Hérisson–Chauvin pathway for olefin metathesis, which proposes that reaction occurs between a surface carbene and alkene to form a C 3 metallacycle, which decompose by the reverse of this pathway to form metathesis products, is tested on a MoAl alloy formed by reactions with Mo(CO) 6 with an alumina thin film grown on a refractory metal substrate. This is the accepted pathway in homogeneous phase, and is demonstrated to occur on the alloy surface by grafting intermediates using iodine-containing precursors.

Development of a multi-scale, multi-phase, multi-zone dynamic model for the prediction of particle segregation in catalytic olefin polymerization FBRs

A comprehensive multi-scale, multi-phase, multi-compartment dynamic model is developed to analyze the extent of particle segregation in catalytic, gas-phase ethylene–propylene copolymerization fluidized bed reactors (FBRs). From the numerical solution of the proposed integrated model, the temporal-spatial evolution of the morphological (i.e., particle size distribution, PSD) and molecular (i.e., molecular weight distribution, MWD) polymer properties in a catalytic polymerization FBR can be predicted. In particular, the polymer molecular properties are determined by employing a generalized multi-site, Ziegler–Natta kinetic scheme. To determine the growth of a single catalyst/polymer particle, the random pore polymeric flow model (RPPFM) is utilized. The RPPFM is solved together with a dynamic discretized particle population balance equation (PBE) to calculate the dynamic evolution of PSD in the various compartments of the FBR. Moreover, overall dynamic mass and energy balances are derived in order to assess the dynamic behavior of catalytic gas-phase FBRs. The effects of various fluidized bed operating conditions (e.g., fluidization gas velocity, temperature and catalyst feed rate) on the morphological and molecular distributed polymer properties are thoroughly analyzed.

Regulation of morphology and mechanical properties of polypropylene/poly(ethylene-co-propylene) in-reactor alloys by multi-stage sequential polymerization

A series of polypropylene/poly(ethylene-co-propylene) (PP/EPR) in-reactor alloys were synthesized by the homopolymerization of propylene in the first stage, after that it switches to gas-phase ethylene–propylene copolymerization and gas-phase homopolymerization of propylene in a circular mode. The alloys were characterized by FTIR, DSC, PLM, TEM and SEM. Changing switch frequency had hardly any influence on the EPR content of the PP/EPR in-reactor alloys. However, as the switch frequency increased, the catalyst efficiency evidently increased. Measurement of mechanical properties shows that increasing the switch frequency has a positive effect on both the impact strength and flexural modulus. Although the content of EPR was almost similar to each other, as the switch frequency increased, the size of EPR phase decreases and the size distribution in the PP matrix trend to be more uniform that forms strong interfacial adhesion between PP phase and EPR phase. The simultaneous improvement of both impact strength and flexural modulus of the alloy can be mainly attributed to changes in its phase morphology as a result of fast circulation between homopolymerization and copolymerization.

Time-Dependent Density Functional Theory Study of the X-ray Absorption Spectroscopy of Acetylene, Ethylene, and Benzene on Si(100)

The carbon 1s X-ray absorption spectroscopy (XAS) of acetylene, ethylene, and benzene in gas phase and adsorbed on the Si(100) surface is studied using time-dependent density functional theory (TDDFT) with hybrid exchange correlation functionals. The computed spectra are sensitive to the proportion of Hartree−Fock exchange in the exchange-correlation functional. The fraction of Hartree−Fock exchange is optimized to provide accurate predictions of the 1s → π* excitation energies. In gas phase, the spectra of acetylene, ethylene and benzene are dominated by 1s → π* excitations with further weaker bands at higher energies below the ionization threshold arising predominantly from Rydberg excitations. In the spectrum for ethane, bands arising from excitation to Rydberg and excitations are observed. For the molecules adsorbed on Si(100), excitations to Rydberg states are less evident. The calculations indicate that acetylene adsorbed on the Si(100) surface has an intense π* band, which lies 1.5 eV lower than in...

Influence of various crosslinking systems on the mechanical properties of gas phase EPDM/PP thermoplastic vulcanizates

AbstractDynamically cured thermoplastic elastomers or thermoplastic vulcanizates (TPVs) are widely used nowadays for their unique characteristics. In this paper, gas phase ethylene–propylene–diene terpolymer (GEPDM)/Polypropylene (PP) TPVs with various crosslinking systems have been extensively studied to optimize the curative level in each crosslinking system with special reference to their mechanical properties. Optimized systems were compared for heat aging, recyclability, crosslink density, morphology studies, and dynamic mechanical analysis. Crosslinking by peroxide in the presence of triallyl cyanurate as a coagent gives best overall performance with reference to excellent heat aging behavior, tension set, and compatibility between GEPDM and PP. Conventional EPDM/PP system was also compared with GEPDM/PP system. GEPDM/PP system was found to exhibit better behavior in all respects. Significant correlations were obtained between delta torque values obtained from Moving Die Rheometer and modulus or cross link density of TPVs irrespective of the nature of crosslinking agent. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5463–5471, 2006

Heterogenized Methylaluminoxane and Isobutylaluminoxane as activators for metallocene catalysts

Heterogenized alkylaluminoxanes prepared in situ on the montmorillonite (MMT) surface by the partial hydrolysis of AIR3 with water of the support are effective activators for metallocenes. The thermal destruction of isobutylaluminoxane molecules in the MMT-H2O/Al(i-Bu)3 system has been studied by the temperature-programmed desorption method coupled with mass spectrometry (TPD-MS). The process begins at a lower temperature and is more complicated than the destruction of methylaluminoxane (MAO) in MMT-H2O/AlMe3. Isobutyl-substituted aluminoxane and the ansa-metallocene Me2Si(Ind)2ZrCl2 form metal-alkyl complexes that are more active in propylene polymerization than methyl-substituted aluminoxane. The TPD-MS study of the initial stages of gas-phase ethylene and propylene polymerization shows that the nature of the metallocene in the heterogenized metallocene catalysts is an essential factor in the distribution of active sites by the activation energy of the thermal destruction of active Zr-C bonds.

Semi-batch reactor for kinetic measurements of catalyzed olefin co-polymerizations in gas and slurry phase

A multi-purpose setup is presented for homo- and co-polymerizations of ethylene and 1-hexene in slurry and gas phase. Polymerization experiments have been executed semi-batch with a heterogeneous metallocene catalyst under industrial conditions. During the gas phase polymerization, the monomer concentrations have been measured and controlled online. The activities for both monomers have been measured with flow meter technique. The polymerization kinetics presented have been measured in a very accurate and reproducible way in isothermal (within 0.2 ∘ C ) and isobaric mode (within 0.15 bar). The concentrations of monomer and co-monomer have been kept constant within 5% of the set point. The activity for ethylene was 50% higher in the presence of 1-hexene compared to a homo-polymerization.