Gas-phase selective oxidation of toluene on TiO2–sepiolite supported vanadium oxides: Influence of vanadium loading on conversion and product selectivities

Gas-phase selective oxidation of toluene has been carried out on vanadium oxide systems (5–20 wt.% of V₂O₅, equivalent to 0.4–1.7 theoretical monolayers) supported on TiO₂–sepiolite (with titania loading around the theoretical monolayer, 12 wt.%) and on sepiolite. A study has been made on both the influence of vanadia loading and of the support on the catalytic behaviour of the supported vanadium systems. The reducibility by H₂ TPR was also studied as well as the acid and basic/redox sites from the results of conversion of the 2-propanol test reaction of the solids. Benzaldehyde, benzoic acid and several coupling products were the main ones detected, attaining over 50% selectivity towards the benzaldehyde and benzoic acid products at a total conversion around 10%. The activity and selectivity to the selective products exhibited by vanadium systems supported on mixed support were superior to those exhibited by the systems supported on sepiolite and increased notably in both series with the increase in vanadium loading. The best catalytic behaviour exhibited by the vanadium systems supported on mixed support, which also exhibited the highest density of sites capable of being reduced (as well as their reducibility) and of those responsible for propanone formation, could be attributed not only to the different balance of the vanadia species existing in the two supports (monomeric + oligomeric/polymeric), but also to such other factors as the nature of the support and, concretely, its chemical composition.     

Characterization and performance of Pd-La/spinel catalyst for preparation of 2,6-diisopropylaniline

The Pd-La/spinel catalyst for the preparation of 2,6-diisopropylaniline (2,6-DIPA) by gas-phase amination of 2,6-diisopropylphenol (2,6-DIPP) has been studied. The catalysts before and after reaction were characterized by BET, XRD, differential thermal analysis (DTA)-thermo gravimetric analysis (TGA), FTIR, and NH₃-TPD techniques. The DTA study results show two kinds of coke deposited on the metal and support of Pd-La/spinel catalyst, and they are combusted at about 242 and 324 °C, respectively. The XRD and FTIR spectra of the Pd-La/spinel catalyst show that the coke contains the aromatic and aliphatic rings, alkyl groups, polynuclear aromatic system, hydroxyl groups, and amine groups. The reason for catalyst deactivation can be expressed as follows: coke formed on palladium metals may move from metal to the interface or boundaries of metal-support and acid sites of the support where further dehydrogenation and polymerization are occurring. The H₂ plays a key role in retarding coke formation, but must be in suitable amount to get relatively high selectivity. The rare earth promoter La not only promotes the activity and selectivity by retaining Pd species in the metallic state, but also decreases the formation of carbon by neutralizing the strong acid sites on the catalyst.     

预硫化加氢催化剂钝化技术研究进展

钝化是预硫化加氢催化剂的关键处理过程。预硫化催化剂经过钝化处理后,有效抑制了自热性质和硫的脱离,便于催化剂的存储和运输,并保持催化剂的活性,有利于器外预硫化催化剂的大规模生产。介绍了气相、液相和固相3种钝化方式、钝化机理及其应用和研究进展。     

Future prospect of remote Cat-CVD on the basis of the production, transportation and detection of H atoms

The future prospect of remote Cat-CVD, in which the decomposition and the deposition chambers are separated, is discussed on the basis of the absolute density measurements of H atoms. It is now well recognized that uniform deposition is possible on a large area without plasma damages by Cat-CVD. However, we may not overlook the demerits in Cat-CVD. One of the demerits is the poisoning of the catalyzer surfaces by the material gases, both temporary and permanent. One technique to overcome this problem is remote Cat-CVD. The question is how to separate the decomposition and deposition areas. If the separation is not enough, there should be back diffusion of the material gases, which will poison the catalyzers. If the separation is too tight, radicals may not effuse out from the decomposition chamber. These problems are discussed and it is shown that SiO₂ coating to reduce the radical recombination rates on walls is promising. The possibility of the polytetrafluoroethene coating by Cat-CVD is also discussed.     

Effects of drying control chemical additive on properties of Li4Ti5O12 negative powders prepared by spray pyrolysis

High-density Li₄Ti₅O₁₂ powders comprising spherical particles are prepared by spray pyrolysis from a solution containing dimethylacetamide (drying control chemical additive) and citric acid and ethylene glycol (organic additives). The prepared powders have high discharge capacities and good cycle properties. The optimum concentration of dimethylacetamide is 0.5 M. The addition of dimethylacetamide to the polymeric spray solutions containing citric acid and ethylene glycol helps in the effective control of the morphology of the Li₄Ti₅O₁₂ powders. At a constant current density of 0.17 mA g⁻¹, the initial discharge capacities of the powders obtained from the spray solution with and without the organic additives are 171 and 167 mAh g⁻¹, respectively.     

An analytical and numerical investigation of hetero-/homogeneous combustion with deficient reactants having larger than unity Lewis numbers

The heterogeneous combustion and the combined hetero-/homogeneous combustion of deficient reactants with Lewis numbers (Le) larger than unity was investigated analytically and numerically in two geometrical configurations, the flat plate and the planar channel, under the condition of infinitely fast catalytic chemistry. Analytical results based on similarity solutions for the catalytic flat plate and on heat and mass transfer solutions for the channel were complemented by detailed 2-D numerical simulations. The larger than unity Lewis number led to the underadiabatic surface temperatures, which in turn gave rise to gas-phase regions with local energy excess. For the flat plate case, the maximum gas-phase energy excess was a non-monotonic function of the Lewis number. The peak occurred at Le = 6.5 with a corresponding energy excess 6.4% above the total energy of the fresh reactants. In channel-flow combustion, the maximum gas-phase energy excess was a monotonically increasing function of Lewis number, approaching asymptotically the considerably higher value of 20.8% as Le → ∞. For current catalytic combustion methodologies, which include the fuel-lean hydrocarbon/air combustion (Lewis numbers of deficient hydrocarbon fuels up to ∼3.2) and the fuel-rich hydrogen/air combustion (Lewis number of deficient oxygen ∼2.3), the energy excess in the gas was significant and could reach up to 14%. Hetero-/homogeneous combustion simulations have shown that, upon homogeneous ignition, the gas-phase energy excess manifested itself with superadiabatic flame temperatures. However, the superadiabaticity in the gas was confined to the channel core, such that the surface temperature did not exceed the adiabatic equilibrium temperature. This behavior had key implications for the reactor thermal management and catalyst stability. Moreover, the gas-phase superadiabaticity led to peak prompt NO_x values 30% higher than those achieved by a diffusionally neutral deficient reactant (Le = 1).     

GAS-PHASE CONTROLLED MASS TRANSFER IN A FOAM-BED REACTOR

Gas-phase controlled absorption of ammonia in foams made of solutions of sulphuric acid has been studied experimentally. Effects of gas-phase concentration of ammonia and type of surfactant on the performance of the foam-bed reactor are investigated. Gas-phase controlled absorption from a spherical bubble is anaylzed using the asymptotic value of Sherwood number (Sh = 6·58), for both negligible as well as significant changes in the volume of the bubble. The experimental data are shown to be in good agreement with the single-stage model of the foam-bed reactor using these asymptotic sub-models, as well as the diffusion-in-sphere analysis available in literature. Influence of effective diffusivity on the time dependence of fractional gas absorption has been found to be unimportant for foam columns with large times of contact. The asymptotic sub-models have been compared and use of the rigid-sphere asymptotic sub-model is recommended for foam columns of practical relevence.     

Synthetic opportunities of gas-liquid phase-transfer catalysis

A concise survey of gas-liquid phase-transfer catalysis is reported, showing the main features of the method, and, in particular, discussing the influence of the temperature and pressure, catalyst selection and nature of the solid bed on the outcome of the reaction. Some new results concerning the malonate synthesis and the halide-exchange on hydrocarbons are reported: the former shows that the monoalkylated ester can be selectively obtained by using the polyethyleneglycol “Carbowax 6000” as a catalyst adsorbed over potassium carbonate, whilst the latter refers to a true catalytic process, which can proceed indefinitely without renewing the catalytic bed. The influence of the catalyst percentage on the reaction equilibrium is also discussed. The advantages and the problems of gas-liquid phase-transfer catalysis are reviewed, together with its analogies to classic phase-transfer catalysis and gas chromatography.     

Gas-phase synthesis of l-leucine-coated micrometer-sized salbutamol sulphate and sodium chloride particles

Coating of micrometer-sized particles of salbutamol sulphate or sodium chloride with the amino acid l-leucine in the gas phase is described. A novel method to synthesize core particles and coat them with l-leucine simultaneously was carried out in an aerosol flow reactor. The coating was prepared via temperature-induced heterogeneous nucleation of l-leucine vapor on the 0.6-1.0 µm core particles, and subsequent growth of l-leucine crystals by physical vapor deposition. The core salbutamol particles were amorphous, whereas the NaCl core particles were crystalline. The l-leucine sublimation that took place at 140-195 °C depended on the identity of the core material due to (i) molecular interaction and (ii) phase mixing. The former was dominant with salbutamol/l-leucine particles and the latter was dominant with NaCl/l-leucine particles. During the vapor deposition, l-leucine formed a discontinuous coating layer of leafy-looking crystallites, with sizes from a few nanometers to hundreds of nanometers, pointing out from the core particle surface. The l-leucine deposition properties depended on the core morphology: l-leucine crystallites were distributed more evenly on salbutamol core surfaces than on salt core surfaces, where the crystallites were localized mainly on edges. The stability of coated salbutamol particles was retained during storage under humid conditions.     

Moisture effects on greenhouse gases generation in nitrifying gas-phase compost biofilters

Gas-phase compost biofilters are extensively used in concentrated animal feeding operations to remove odors and, in some cases, ammonia from air sources. The expected biochemical pathway for these predominantly aerobic systems is nitrification. However, non-uniform media with low oxygen levels can shift biofilter microbial pathways to denitrification, a source of greenhouse gases. Several factors contribute to the formation of anoxic/anaerobic zones: media aging, media and particle structure, air velocity distribution, compaction, biofilm thickness, and moisture content (MC) distribution. The present work studies the effects of media moisture conditions on ammonia (NH₃) removal and greenhouse gas generation (nitrous oxide, N₂O and methane, CH₄) for gas-phase compost biofilters subject to a 100-day controlled drying process. Continuous recordings were made for the three gases and water vapor (2.21-h sampling cycle, each cycle consisted of three gas species, and water vapor, for a total of 10,050 data points). Media moisture conditions were classified into three corresponding media drying rate (DR) stages: Constant DR (wetter media), falling DR, and stable-dry system. The first-half of the constant DR period (0-750 h; MC = 65-52%, w.b.) facilitated high NH₃ removal rates, but higher N₂O generation and no CH₄ generation. At the drier stages of the constant DR (750-950 h; MC = 52-48%, w.b.) NH₃ removal remained high but N₂O net generation decreased to near zero. In the falling DR stage (1200-1480 h; MC = 44-13%) N₂O generation decreased, CH₄ increased, and NH₃ was no longer removed. No ammonia removal or greenhouse gas generation was observed in the stable-dry system (1500-2500 h; MC = 13%). These results indicate that media should remain toward the drier region of the constant DR (in close proximity to the falling DR stage; MC = 50%, approx.), to maintain high levels of NH₃ removal, reduced levels of N₂O generation, and nullify levels of CH₄ generation.