Deactivation Coefficients Research Articles

Ca/ZSM-5 catalysts for the methanol-to-hydrocarbons reaction: Activity – Selectivity trade-off?

Establishing structure–activity relationships for the methanol-to-hydrocarbons reaction can help to identify better catalysts and optimal reaction conditions. Modification of ZSM-5 catalysts with alkaline-earth cations such as Ca increases the selectivity to C3+ olefins, which has been linked to a change in the effective pore geometry, affecting the formation of particular hydrocarbon pool (HCP) components. To further understand the catalytic properties of Ca/ZSM-5, we estimated the methanol conversion rate constants and deactivation coefficients. The kinetic experiments were carried out at different contact times with and without water in the feed. Ca/ZSM-5 is substantially less active than HZSM-5 and Na/ZSM-5 in a wide range of conditions. The decreased activity can be correlated to a different composition of the HCP due to the presence of Ca, which is supported by step-response IR experiments using various feed mixtures (methanol, methanol + water, water). Ca/ZSM-5 converts methanol significantly slower than HZSM-5 and Na/ZSM-5 but with a higher C3+ olefins selectivity, pointing to the delicate balance between activity and selectivity in these materials.

Hydrogen production from sorption enhanced steam reforming of ethanol using bifunctional Ni and Ca-based catalysts doped with Mg and Al

This work evaluated the performance of nickel-based catalysts supported on CaO and CaO–MgO–Al2O3 in the sorption enhanced steam reforming of ethanol (SESRE) aiming the production of high purity H2. The catalysts were prepared by sol-gel method and characterized by different methods: Temperature programmed reduction (TPR), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) with chemical element mapping, N2 physisorption and CO2 capture capacity determined by thermogravimetric analysis (TGA). XRD analysis showed that the predominant phases were CaO, MgO, CaCO3, Ca(OH)2 and NiO in the calcined samples and Ni0 in the reduced and passivated samples. TPR profiles indicated that all catalysts presented a high degree of reduction (Ni/CaMgAl-68 > Ni/CaMgAl-79 > Ni/Ca), although Ni/CaMgAl-X samples presented high reduction temperatures indicating the formation of NiAl2O4. The addition of MgO and Al2O3 to CaO was very beneficial since the deactivation coefficients, calculated by the TGA data modeling, decreased by a factor of 3.8 for Ni/CaMgAl-79 and by a factor of 4.3 for Ni/CaMgAl-68 when compared to the Ni/Ca catalyst. The catalytic tests in the SESRE showed that Ni/CaMgAl-79 catalyst had the best performance since it had the longest high purity hydrogen production time. In the pre-breakthrough period, the H2 mole fractions were close to 90% for all samples during all reaction cycles. After the reaction-regeneration cycles, the average crystallite size of CaO estimated by XRD increased around 38, 6 and 35% for Ni/Ca, Ni/CaMgAl-79 and Ni/CaMgAl-68, respectively. Thus, adding a dopant to the sorbent material proved to be an effective strategy to obtain a more stable catalyst capable to produce hydrogen of high purity.

Boundary conditions for fluid-dynamic parameters of a single-component gas flow with vibrational deactivation on a solid wall

Boundary conditions for fluid-dynamic parameters of a strongly non-equilibrium singlecomponent rarefied gas flow in the slip regime are obtained using kinetic-theory methods. The gas flow is described in the frame of the state-to-state approach assuming vibrational energy exchange as the slow relaxation process. The set of governing equations including conservation equations coupled with additional relaxation equations for vibrational state populations is presented. The gas-solid surface interaction is considered on the basis of the specular-diffusive model, and possible vibrational deactivation/excitation processes on the wall are taken into account. The obtained boundary conditions depend on the accommodation and deactivation coefficients along with the transport coefficients such as the multi-component vibrational energy diffusion and thermal diffusion coefficients; the thermal conductivity; the bulk and shear viscosity coefficients and the relaxation pressure. The dependence of boundary conditions on the normal mean stress has been obtained for the first time. In the particular case of the gas without internal degrees of freedom, the slip velocity and the temperature jump can be reduced to the well-known in the literature expressions. Implementation of the state-specific boundary conditions should not cause additional computational costs in numerical simulations of viscous flows in the state-to-state approach, since the slip/jump equations depend on the transport coefficients which have to be evaluated regardless of the boundary conditions used in the code.

Deactivation of metals, fabrics, and soils in a liquid CO2 medium

Liquid (as well as supercritical) carbon dioxide can be used in deactivation. Solutions of TBP and phosphorus-and fluorine-containing acids in CO2 in the presence of alcohols make it possible to achieve the deactivation coefficients equal to 50 and above for surfaces polluted with weakly fixed α-, β-, and γ-radiating nuclides by two successive treatments. When performing the double treatment of metals contaminated with strongly fixed nuclides using solutions of chelating agents in CO2, it is possible to achieve deactivation coefficients equal to 2–3 for aluminum, 10–20 for brass (90–95 of radionuclides are removed), and 10–30 for stainless steel (90–97% of radionuclides are removed). Using an enlarged pilot apparatus, the possibility of deactivating protective clothes and metal products in liquid carbon dioxide down to the natural radioactive background is shown.

Airglow on Mars: Some model expectations for the OH Meinel bands and the O 2 IR atmospheric band

This work presents model calculations of the diurnal airglow emissions from the OH Meinel bands and the O 2 IR atmospheric band in the neutral atmosphere of Mars. A time-dependent photochemical model of the lower atmosphere below 80 km has been developed for this purpose. Special emphasis is placed on the nightglow emissions because of their potential to characterize the atomic oxygen profile in the 50–80 km region. Unlike on Earth, the OH Meinel emission rates are very sensitive to the details of the vibrational relaxation pathway. In the sudden death and collisional cascade limits, the maximum OH Meinel column intensities for emissions originating from a fixed upper vibrational level are calculated to be about 300 R, for transitions v ′ = 9 → v ′ ⩽ 8 , and 15,000 R, for transitions v ′ = 1 → v ′ = 0 , respectively. During the daytime the 1.27 μm emission from O 2( a Δ g 1 ), primarily formed from ozone photodissociation, is of the order of MegaRayleighs (MR). Due to the long radiative lifetime of O 2( a Δ g 1 ), a luminescent remnant of the dayglow extends to the dark side for about two hours. At night, excited molecular oxygen is expected to be produced through the three body reaction O + O + CO 2. The column emission of this nighttime component of the airglow is estimated to amount to 25 kR. Both nightglow emissions, from the OH Meinel bands and the O 2 IR atmospheric band, overlap in the 50–80 km region. Photodissociation of CO 2 in the upper atmosphere and the subsequent transport of the atomic oxygen produced to the emitting layer are revealed as key factors in the nightglow emissions from these systems. The Mars 5 upper constraint for the product [H][O 3] is revised on the basis of more recent values for the emission probabilities and collisional deactivation coefficients.

Continuous flow toluene methylation over AlPO4 and AlPO4-Al2O3 catalysts

Toluene methylation with methanol over AlPO4 (AP) and AlPO4-Al2O3 (APAl) catalysts, obtained through different methods, was carried out in a continuous down-flow fixed bed reactor. The main products were xylenes (XYL), although trimethylbenzenes (TMB) were also found over APAl catalysts. The benzene and ethylbenzene selectivities increased slightly with time on stream at the expense of XYL and TMB selectivities. Isomer distribution was approximately 50, 24 and 26 mol% foro-, m- andp-XYL, and 72, 27 and 0 mol% for 1,2,3-, 1,2,4- and 1,3,5-TMB. The initial reaction rate constants were higher on APA1 catalysts and, furthermore, APAl catalysts exhibited similar catalytic activities, although those obtained in ethylene or propylene oxide are the most active ones. The same occurs on AP catalysts. Moreover, the changes in catalytic activity are similar to the changes in the acidic characteristics measured, in gas phase, versus pyridine. Furthermore, the activity decreased with time on stream due to coke deposition according to the expressionk = k0 exp(-βt). The rate of deactivation, evaluated from the deactivation coefficients (β), was greater for APAl than for AP catalysts.

Liquid phase decomposition of t‐butyl derivatives over solid acidic catalyst

AbstractThe de‐t‐butylation of some t‐butyl‐derivatives has been carried out in the liquid phase using amorphous silica alumina and HY type zeolite. The deactivation coefficients obtained in liquid phase were about 10 to 100 times as large as those in the vapor phase de‐t‐butylation of t‐butylbenzene. The activation energies for the reaction of the phenolic derivatives were nearly the same as that for the vapor phase de‐r‐butylation. However, the activation energies for the di‐t‐butyl derivatives were almost doubled. The activity of HY type zeolite was lower than that of the silica alumina, but the shape selectivity of the former was observed in the reaction of p‐t‐butyl‐o‐phenol.

Quantum yield and total pressure effect in perfluoroglutaric anhydride photolysis in the gas phase

The quantum yield of the photolysis of perfluoroglutaric anhydride (PFGA) in the gas phase (φ CO = 0.34 ± 0.07) was determined in runs using light of wavelength 254 nm. This value is intermediate between the quantum yields of open-chain perfluorinated anhydrides and of strained cyclic perfluorinated anhydrides. The PFGA quantum yield is independent of time and total pressure. The photolysis of PFGA vapour produces CO, CO 2, C 2F 4 and c-C 3F 6. The ratio [C 2F 4]/[ c-C 3F 6] is significantly dependent on the total pressure as shown by runs with the addition of inert gases such as helium, argon, oxygen, nitrogen, CCl 2F 2 and C 2Cl 3F 3. Each of these gases produces a specific effect as can be seen from their collisional deactivation coefficients.

CATALYST DEACTIVATION IN DECOMPOSITION OF p-SUBSTITUTED-t-BUTYLBENZENES OVER SILICA-ALUMINA CATALYST

The vapor phase de-t-butylation of p-alkylaled-t-butylbenzenes, p-halogenated-t-butylbenzenes and p-t-butylanisole were carried out over a commercially obtained silica-alumina catalyst. The surface area of the catalyst and the quantity of pyridine chemisorbed by the catalyst were also measured to obtain the information about the catalyst deactivation. Most of the substituted t-butylbenzenes decomposed to the corresponding substituted-benzene, but p-t-butylcumene and t-butylbromobenzene reacted to the products via complicated networks. In the decomposition of p-alkylated-t-butylbenzenes, the deactivation coefficients tended to be proportional to the reaction rates, that is, the activity of the catalyst decreased with increasing the rate constants. The introduction of halogeno group to t-butylbenzene caused to decrease the rate of de-t-butylalion. The surface area of the catalyst diminished in progress of the reaction, but the diminishment up to 8% was not responsible for the deactivation. The acid site, es...

Fluorescence efficiencies of nitrogen and air excited by neon and sodium ions at 0.3–2.0 MeV

The fluorescence efficiencies for excitation of nitrogen and air by neon and sodium ions have been measured at bombarding energies between 0.3 and 2.0 MeV. The molecular spectra studied were the first negative system of N2+ (1N) and the second positive system of N2 (2P). The atomic spectra studied were the N+ 5003 Å multiplet and the N+ 5680 Å multiplet. The fluorescence efficiencies of the 3914 Å band and the 4278 Å band of the 1N system and the 3371 Å band of the 2P system were derived directly from the fluorescence data as were the efficiencies of the 5003 Å and 5680 Å multiplets. Efficiencies relative to the 3914 Å band were estimated for the (ν′ = 0, ν″ = 0–3) bands of the 1N system and the (ν′ = 0, ν″ = 0–5), (ν′ = 1, ν″ = 0–6), and (ν′ = 2, ν″ = 2–7) bands of the 2P system. In addition, collisional deactivation coefficients for the 5003 A and 5680 A multiplets were derived from the efficiency data. The efficiencies of all spectra studied increased with increasing energy, with the efficiency of the 3914 Å band being the largest (0.48%) at 2.0 MeV for Ne+ in N2. The ratio of the intensity of the 4278 Å band to that of the 3914 Å band indicates that the Franck-Condon principle is obeyed in this particular case. Also, the ranges of neon and sodium in nitrogen and air were obtained from emission measurements.

Relaxation processes in excited CO states. II. Elastic cross relaxation

Interactions between the adjacent d 3Δν=7, e 3Σν=4−, and A 1Πν =2 levels of CO have been studied. By making d 3Δν=7 the initially excited level, it has been shown that the interconversion d 3Δν=7 → e 3Σν=4− is collisionally induced. Rate coefficients were measured, and when expressed as transition probability per gas kinetic collisions, the results are essentially the same for Ar, H2, N2, and CO—1 collision in 90. The He reaction probability per collision is much lower. Over-all deactivation coefficients for d 3Δν=7 and e 3Σν=4− were also obtained. The data are consistent with a radiative lifetime for the e 3Σν=4− level of ∞3 μsec. When the A 1Π state is pumped by resonance fluorescence, then collisions between Ar and A 1Πν =2 produce d 3Δν=7 and e 3Σν=4−, a process for which the cross section appears to be higher than for Δ3 →3Σ− conversion.

Excited atoms in decaying optically thick plasmas

With attention confined to decaying H-H+-e and He-He+-e plasmas which are optically thick towards lines ending on the ground level, the quasi-equilibrium number density of atoms in the first excited level nQ'(2) is computed using collisional deactivation coefficients K(2, 1) based on experiment and taking Penning ionization into account. This last is discussed in some detail. Quantal calculations are carried out on the rate at which the electronic transitions involved in 2H(2s-p) -> H(1s) + H+ + e occur. It is concluded that the rate is probably fast enough for almost all close collisions to lead to ionization. This also seems to be the case for 2He(2 3S) -> He(1 1S) + He+ + e provided collisions which can yield ionization only by violation of Wigner's spin conservation rule are excluded from consideration. Penning ionization is of little importance in hydrogen plasmas, but it can be significant in helium plasmas where it tends to reduce nQ'(2) and the apparent recombination coefficient.