Activation of methane with organopalladium complexes

Selective, direct oxidation of methane to methanol is a process of scientific interest and industrial importance. Reports have appeared in the literature describing the use of organometallic complexes to effect this transformation [1–5]. Investigation of one of these reaction schemes in our laboratory has produced interesting results. Our research effort was an extension of work reported by Sen et al. [3]. The reported reaction occurs between methane (at 800 psig 5.52 MPa) and palladium(II) acetate in trifluoroacetic acid at 80°C (Eq. (1)). The product, methyl trifluoroacetate, is readily hydrolyzed to produce methanol and trifluoroacetic acid. It is reported that methyl trifluoroacetate is produced with reported conversions, calculated on palladium metal recovery, of 60 percent.

     

Mass transfer resistances in the palladium-catalysed hydrogenation of 1-methoxy-2,4-(nitrophenyl)-ethane

Hydrogenation of 1-methoxy-2,4-(nitrophenyl)-ethane (MNPE) to 1-methoxy-2,4-(aminophenyl)-ethane (MAPE) over Pd/C catalyst was studied in a three-phase stirred slurry reactor. The reaction was performed in a kinetic region (no internal and external mass transfer resistances) at temperatures ranging from 293 to 320 K, hydrogen pressure 200 kPa using powdered pellets of 1–4% Pd on carbon support. The rates have been correlated with power law and Hougen–Watson kinetic models. The rate expression which best fit the data is

which postulates a reaction between adsorbed MNPE (marked by B) and hydrogen (marked by A).     

Reaction of NO with urea supported on activated carbons

Reaction of NO in air with urea supported on activated carbons (AC) was examined in the range of 100–1000 ppm NO at room temperature to establish a basic scheme for its reduction in open atmosphere. NO in atmosphere containing O₂ was found to be selectively reduced with urea supported on AC at the W/F of 2.5×10⁻³ to 1.5×10⁻³ ACF g min/ml and its reduction continued until the complete consumption of urea. The reduction of NO with urea supported on AC appears to proceed through the following steps.

NO₂+NO+(NH₂)₂CO→2N₂+CO₂+H₂O

Combining (1) and (2) steps results in the followed reaction equation.

Since (1) is rate determining, high NO oxidation activity is essential for AC to be active for the reaction. At the same time, activation of urea by AC is also necessary to reduce NO₂. The present scheme for NO reduction with urea on the AC is very effective to remove NO_x in the open atmosphere under ambient conditions.     

Role of acidity on the hydrolysis of dimethyl ether (DME) to methanol

The activity of dimethyl ether (DME) hydrolysis was investigated over a series of solid acid and non-acid catalysts, zeolite Y [Si/Al = 2.5 and 15: denoted Y(Si/Al)], zeolite ZSM-5 [Si/Al = 15, 25, 40, and 140: denoted Z(Si/Al)], silica, zirconia, γ-alumina, and BASF K3-110 (commercial Cu/ZnO/Al₂O₃ catalyst). Dimethyl ether hydrolysis was carried out in an isothermal packed-bed reactor at ambient pressure.

Acid catalyzed dimethyl ether hydrolysis is equilibrium limited. All solid acid catalysts, with the exception of ZrO₂, attained equilibrium-limited conversions in the temperature range of interest (125–400 °C). Z(15), Z(25), and Z(40) reached equilibrium conversions at 200 °C, while Z(140), Y(15), and Y(2.5) reached equilibrium at 275 °C. γ-Alumina, the most active non-zeolite solid acid, attained equilibrium at 350 °C. Silica and BASF K3-110 were both ineffective in converting dimethyl ether to methanol. The observed activity trend for DME hydrolysis to methanol as a function of Si–Al ratio and catalyst type was:

     

Numerical simulations of slip correction

Stoke's law

F=6πηRυ

is valid only in liquids. In order to apply this law to particles suspended in air, the slip correction is needed, especially for particles with diameters less than 1 μm.

The slip correction is included in Stoke's law by

with and the Knudsen number (ratio of the particle radius to the mean free path of the gas molecules), η the viscosity of the gas, R the particle radius and and υ the velocity of the particle. For large Knudsen numbers, this equation reduces to

In the present work a simple Monte-Carlo simulation model is used to determine slip corrections in the free molecule regime (Knudsen number Kn 1: The velocity of the air molecules are assumed to follow a Maxwellian distribution. The particle moves steadily in the gas, and the molecules impinge on its surface. The impaction points are distributed uniformly over the particle's surface. A simple criterium shows whether a molecule can in fact hit the surface at the selected point. If so, the transferred momentum is calculated. After sufficient iterations the slip correction is obtained by comparing the total transferred momentum with the expression for the Stokes drag force. Since only the free molecule regime is considered, the slipcorrection equals + β.     

The kinetics of photocatalytic degradation of trichloroethylene in gas phase over TiO2 supported on glass bead

In this investigation, a packed bed filled with coated titanium dioxide glass beads to study the kinetics of photocatalytic degradation of trichloroethylene under irradiation of 365 nm UV light. In the range of 100–500 ml/min of flowrate, the reaction rate for 6 μM TCE increased with an increasing flowrate upto 300 ml/min, while was not affected by the flowrate at the values higher than 300 ml/min. For moisture in the range of 9.4–1222.2 μM, the reaction rate of TCE was decreased with an increasing humidity. The adsorbed water on the catalyst surface could compete with the adsorption of TCE on the sites. The reaction rate of 6 μM TCE increased as the light intensity increased, and was proportional to the 0.61 order of the light intensity. Among the three L–H bimolecular form models, the experimental data had the best fit for one of models:

     

Effect of hydrothermal treatment and ammonium ion incorporation in platinum-mordenite catalysis for n-hexane hydroconversion

Hydroconversion of n-hexane was studied on catalysts containing 0.25% Pt supported on H-mordenite (H-M) and NH₄-M. The H-M containing catalysts were Pt/H-M, Pt on steamed H-M (Pt/St H-M) and steamed Pt/H-M (StPt/H-M), whereas the NH₄-M containing catalysts were Pt/NH₄-M and StPt/NH₄-M. Steam-treatment of H-M containing catalysts enhanced the hydroconversion activity, whereas such treatment decreased the activity of the Pt/NH₄-M catalyst. The diffusion resistance parameter, i.e., the Thiele modulus, Φ_L, estimated for the reaction on the catalysts under study was found to increase with the increase in the catalytic activity, and both were found to decrease in the order:

Pt dispersion in the zeolite was not comparable with the catalytic activities of the H-M containing catalysts. The higher activity of the Pt/NH₄-M catalyst could be attributed to a higher Pt dispersion in the zeolitic channels, higher strength of the acid sites (determined by temperature programmed desorption (TPD) of ammonia) and higher diffusion limitation of the reactant in the catalyst pores.     

Generating hydrogen-rich fuel-cell feeds from dimethyl ether (DME) using physical mixtures of a commercial Cu/Zn/Al2O3 catalyst and several solid–acid catalysts

Homogeneous physical mixtures containing a commercial Cu/ZnO/Al₂O₃ catalyst and a solid–acid catalyst were used to examine the acidity effects on dimethyl ether hydrolysis and their subsequent effects on dimethyl ether steam reforming (DME-SR). The acid catalysts used were zeolites Y [Si/Al = 2.5 and 15: denoted Y(Si/Al)], ZSM-5 [Si/Al = 15, 25, 40, and 140: denoted Z(Si/Al)] and other conventional catalyst supports (ZrO₂, and γ-Al₂O₃). The homogeneous physical mixtures contained equal amounts, by volume, of the solid–acid catalyst and the commercial Cu/ZnO/Al₂O₃ catalyst (BASF K3-110, denoted as K3). The steam reforming of dimethyl ether was carried out in an isothermal packed-bed reactor at ambient pressure.

The most promising physical mixtures for the low-temperature production of hydrogen from DME contained ZSM-5 as the solid–acid catalyst, with hydrogen yields exceeding 90% (T = 275 °C, S/C = 1.5, τ = 1.0 s and P = 0.78 atm) and hydrogen selectivities exceeding 94%, comparable to those observed for methanol steam reforming (MeOH-SR) over BASF K3-110, with values equaling 95% and 99%, respectively (T = 225 °C, S/C = 1.0, τ = 1.0 s and P = 0.78 atm). Large production rates of hydrogen were directly related to the type of acid catalyst used. The hydrogen production activity trend as a function of physical mixture was

     

Attrition resistance of cobalt F–T catalysts for slurry bubble column reactor use

There exists much current interest in the use of supported Co catalysts and slurry bubble column reactors (SBCR) for the conversion of natural gas to higher hydrocarbons via the Fischer–Tropsch (F–T) synthesis. Catalyst attrition resistance is extremely important in the operation of slurry-phase reactor systems because of potential problems with plugging of system filters and/or contamination of the liquid products. This paper addresses the effects of different supports, promoters, and preparation methods on the attrition resistance of Co F–T catalysts for SBCR use.

The calcined supports had attrition resistances (inversely related to % fines <11 μm generated during attrition testing) as follows:

−Al₂O₃>TiO₂(rutile)SiO₂

Loading of Co onto the supports improved the attrition resistances of both alumina and silica significantly. It has essentially no effect on titania. The resulting catalysts had attrition resistances in the order

Co/Al₂O₃>Co/SiO₂>Co/TiO₂(rutile)>Co/TiO₂(anatase)

The addition of small amounts of metal (Ru, Cu) and oxide (La, Zr, K, Cr) promoters had mainly small effects on the attrition resistance of the supported Co catalysts. However, it would appear that the addition of Zr to Co/alumina had a negative impact on its attrition resistance. The different preparation methods used in this study (aqueous impregnation, non-aqueous impregnation, and kneading) did not appear to have a significant effect on catalyst attrition resistance.     

Photocatalytic reactors: Reaction kinetics in a flat plate solar simulator

The kinetics of the photocatalytic decomposition of low concentrations of trichloroethylene (TCE) in water was modeled and the reaction parameters have been evaluated for different catalyst loadings. The employed reactor is a flat plate configuration irradiated by tubular lamps that have emission in the 300–400 nm wavelength range.

The mass conservation model is two-dimensional while the developed radiation model is two-dimensional in space and two directional in radiation propagation. The performance of the photoreactor with this reaction can be properly represented employing only two lumped kinetic constants that can be derived from a 12 steps, complete reaction sequence. The deduced kinetic model has explicit functional dependencies for the local volumetric rate of photon absorption (LVRPA) and the effect of the catalyst concentration:

Full-size image

. Values of the kinetic constants are: =1.94×10⁻⁹ mol g^1/2 cm⁻² s^−1/2 einstein^−1/2 and ₃=5.52×10⁶ cm³ mol⁻¹. As derived from the reaction sequence and validated with experiments, it was observed that the reaction rate is proportional to the square root of the LVRPA. The dependence on the catalyst loading, well described by the model, is more complex due to its characteristic effect on the light distribution inside the reaction space.     

Relative rates of various steps of NO–CH4–O2 reaction catalyzed by Pd/H-ZSM-5

Reaction mechanism of the reduction of nitrogen monoxide by methane in an oxygen excess atmosphere (NO–CH₄–O₂ reaction) catalyzed by Pd/H-ZSM-5 has been studied at 623–703 K in the absence of water vapor, in comparison with the mechanism for Co-ZSM-5. Kinetic isotope effect for the N₂ formation in NO–CH₄–O₂ vs. NO–CD₄–O₂ reactions was 1.65 at 673 K and decreased with a decrease in the reaction temperature. In addition, H–D isotopic exchange took place significantly in NO–(CH₄+CD₄)–O₂ reaction. These results are in marked contrast with the case of Co-ZSM-5, for which the C–H dissociation of methane is the only rate-determining step, and show that the C–H dissociation is slow but not the only rate-determining step in the case of Pd/H-ZSM-5.

A reaction scheme was proposed, in which the relative rates of the three steps ((i)–(iii) below) vary depending on the reaction conditions.

Further, in contrast to Co-ZSM-5, NO_x–CH₄–O₂ reaction was much slower than CH₄–O₂ reaction for Pd/H-ZSM-5; the presence of NO_x retards the reaction of CH₄ over the latter catalyst, while it accelerates the reaction over the former. It is suggested that CH₄ is activated directly by the Pd atoms in the case of Pd/H-ZSM-5, but by NO₂ strongly adsorbed on Co ion for Co-ZSM-5. The reaction order of the NO–CH₄–O₂ reaction with respect to NO pressure was consistent with this mechanism; 1.05 for Pd/H-ZSM-5 and 0.11 for Co-ZSM-5.     

Catalytic dehydrofluorination of CF3CH3(HFC143a) into CF2CH2(HFC1132a)

The catalytic dehydrofluorination of CF₃CH₃ was studied over various metal phosphate catalysts in a fixed-bed reactor. The Mg₂P₂O₇ catalyst exhibited the moderate activity and high selectivity of CF₂CH₂, and it is the most suitable catalyst for the dehydrofluorination of CF₃CH₃. Deactivation did not take place during the 100 h reaction over the Mg₂P₂O₇ catalyst, and XRD patterns of the catalyst were unchanged after 100 h reaction. However, small amounts of F⁻ ions were present on the surface of the catalyst from results of XPS. The active sites for CF₂CH₂ formation are weak acid sites of the catalysts, and carbon deposition and/or polymerization take place on strong acid sites. Results of CF₃CH₃-TPD indicated that the dehydrofluorination proceeds through a

Full-size image

carbonium-ion mechanism over Mg₂P₂O₇ catalyst, and the rate-determining step is the cleavage of the C–F bond.     

Cathodic behaviour of samarium(III) in LiF–CaF2 media on molybdenum and nickel electrodes

The electrochemical behaviour of SmF₃ is examined in molten LiF–CaF₂ medium on molybdenum and nickel electrodes. A previous thermodynamic analysis suggests that the reduction of SmF₃ into Sm proceeds according to a two-step mechanism:

Sm^III + e⁻ = Sm^II

Sm^II + 2e⁻ = Sm

The second step occurs at a potential lower than the reduction potential of Li⁺ ions.

Cyclic voltammetry, chronopotentiometry and square-wave voltammetry were used to confirm this mechanism and the results show that it was not possible to produce samarium metal in molten fluorides on an inert cathode (molybdenum) without discharging the solvent. The electrochemical reduction of SmF₃ is limited by the diffusion of SmF₃ in the solution. The diffusion coefficient was calculated at different temperatures and the values obtained obey Arrhenius’ law.

For the extraction of the samarium from fluoride media, the use of a reactive cathode made of nickel leading to samarium–nickel alloys is shown to be a pertinent route. Cyclic voltammetry and open-circuit chronopotentiometry were used to identify and to characterise the formation of three alloys: liquid Sm₃Ni and a compact layer made of SmNi₃ and SmNi₂.     

Diffusional deposition from a fluid flowing radially between concentric, parallel, circular plates

We have made a theoretical study of the diffusional losses of particles from a fluid flowing radially inward between concentric, parallel, circular plates of radius, R_o. The relative number of particles remaining in the fluid at a distance, r, from the axis of the plates is

where γ = 4πD (Ro² − r²)/3Qh, D is the diffusion coefficient of the particles, Q is the volumetric flow rate, and 2h is the separation between the plates. The practical application of the system is discussed.

The same equation applies to parallel flow through a duct of rectangular cross-section if γ = 4DBl/3Qh, where B and l are respectively the duct width and length. It is more accurate than equations now in use to describe this process.     

Sulfonic acid functionalized periodic mesostructured organosilica as heterogeneous catalyst

Periodic mesostructured organosilicas (PMO) were first synthesized using 1,2-bis(triethoxysilyl)ethylene (BTENE) under acidic conditions using Pluronic 123 as surfactant. The ethylene bridges were then arylated with benzene using AlCl₃ as catalyst. These materials were further treated with sulfuric acid for the sulfonation of the phenyl moieties yielding a new preparation of sulfonic acid functionalized PMO. Ordered hexagonal mesostructures with surface areas up to 440 m²/g and narrow pore size distribution (around 5.3 nm) were obtained. This work thus provides a new example of chemical modification for the conception of functionalized PMO catalysts. Liquid phase self-condensation of heptanal was performed at 75 °C in the presence of these catalysts and the results were compared with those obtained with several other heterogeneous acid catalysts.

Full-size image (28K)

     

Two experiments for the introductory chemical reaction engineering course

This paper describes a pair of chemical reaction experiments developed for Rowan University's introductory course in chemical reaction engineering: an esterification reaction carried out in a packed bed, and a competitive reaction in which the kinetics were influenced by micromixing.

The first experiment is the esterification of ethanol and acetic acid to form ethyl acetate. Students first examine this reaction in their organic chemistry class. The experiment developed in this project re-examines this reaction from a chemical engineering perspective. For example, the reaction is reversible and equilibrium-limited, but in the organic chemistry lab, there is no examination of the kinetics. The complementary chemical engineering experiment examines the relationship between residence time and conversion.

The second experiment is a competitive system involving two reactions:

H₂BO₃⁻ + H⁺ ↔ H₃BO₃

5I⁻ + IO₃⁻ + 6H⁺ → 3I₂ + 3H₂O

The first reaction is essentially instantaneous. Thus, when H⁺ is added as the limiting reagent, a perfectly mixed system would produce essentially no I₂. Production of a significant quantity of I₂ is attributed to a local excess of H⁺; a condition in which all H₂BO₃⁻ in a region is consumed and H⁺ remains to react with I⁻ and IO₃⁻.

In the spring of 2005, for the first time, both experiments were integrated into the undergraduate chemical reaction engineering course. This paper describes the use of the experiments in the classroom and compares the performance of the 2005 students to the 2004 cohort, for whom the course included no wet labs at all.     

Desulphurization of coals and chars by treatment in various atmospheres between 400 and 600 °C

Desulphurization of fifteen US coals of rank ranging from anthracite to sub-bituminous B and five high-temperature chars by carbon monoxide and other gases and gas mixtures between 400 and 600 °C has been studied. The sulphur content of the parent coals ranged between 3.0 and 7.3% and that of the chars between 1.3 and 3.8% by weight. A comparison between air, nitrogen, carbon monoxide, and steam-carbon monoxide mixtures as desulphurizing gases shows the order of desulphurizing ability as

Full-size image

.     

The sorption and reduction of nitrogen oxides by 12-tungstophosphoric acid and its ammonium salt

Nitrogen dioxide is removed from the gas phase by 12-tungstophosphoric (HPW), 12-tungstosilicic and 12-molybdophosphoric acids, with the two solid acids containing tungsten sorbing significantly larger quantities of NO₂ than that containing molybdenum, indicating the dependence on acidic strength. NO₂ interacts with the water contained on and in HPW to form HNO₃ which desorbs. On depletion of the water additional NO₂ remains held on HPW as

Full-size image

.

In contrast the ammonium salt of HPW reduces NO₂ to N₂ , NO and N₂O at an optimal temperature of 175°C for the reduction to N₂. The interaction process occurs between NO₂ and the reductant NH₃ while it is bound as

Full-size image

on the solid. Thus NH₃ is not lost to the gas phase during the reduction process, and ammonia slip is effectively eliminated. The ammonium salt can be regenerated by exposure of HPW to gaseous NH₃ or by precipitation from aqueous solution.     

Palladium(0) allylic alkylation in a two-phase system or with a supported aqueous phase catalyst

The reaction of allylic carbonates with various acyclic and cyclic carbonucleophiles is catalyzed by the system Pd(OAc)₂ and P(C₆H₄-m-SO₃Na)₃ (or tppts) in a two-phase liquid medium H₂O-nitrile, the activity of the catalyst depending mainly on the nature of the nitrile, the temperature of the reaction and the ratio palladium/tppts. The same system Pd(OAc)₂ and P(C₆H₄-m-SO₃Na)₃ supported on silica catalyzes also this reaction. The formation of the active palladium species in the two cases is followed by

Full-size image

NMR spectroscopy and discussed.