William R. Moser

The mechanism of phosphine-modified rhodium-catalyzed hydroformylation studied by CIR-FTIR

The mechanism of phosphine-modified rhodium-catalyzed hydrofonnylation of 1-hexene was studied by in situ infrared spectroscopy using high pressure autoclaves equipped with embedded cylindrical internal reflectance crystals (CIR-FTIR). A series of RhH(CO)2(PR3)2 complexes 1 were synthesized using p-substituted triarylphosphines where the electron density on rhodium was varied by using p-N(CH3)2, p-OCH3, p-H, p-F,p-Cl or p-CF3. The metal carbonyl and metal hydride infrared stretching frequencies were correlated by a standard Hammett treatment of the data. Reaction rates and selectivities for linear aldehydes both increased with increasingly electron-withdrawing phosphines. The IR spectra, measured under autogenous conditions of 70 †C and 200 psi (1.38 MPa), showed the presence of various intermediates in the catalytic cycle depending upon the phosphine modification, P/Rh ratio, total syngas pressure, and degree of olefin conversion. The rate and spectroscopic data permitted the assignment of a reaction mechanism involving CO dissociation from RhH(CO)2L2 1 as the primary selective hydroformylation pathway.

Oxygen-permeable dense membrane reactor for the oxidative coupling of methane

A perovskite material (BaCe0.8Gd0.2O3) in powder form, with both electronic and ionic conductivity, was synthesized by the ethylene glycol method. A dense membrane tube was fabricated using a plastic extrusion technique. The oxygen permeances of the dense membrane tube were measured as functions of temperature and oxygen partial pressure on the feed side. In the temperature range of 688–955°C, the oxygen flux showed an approximately exponential dependence on temperature. The oxygen flux increased proportionally to the natural logarithm of the ratio of oxygen partial pressures across the membrane. Experimental results for the oxidative coupling of methane (OCM) to C2 hydrocarbons, in the absence of additional catalyst, showed that this material has fairly good catalytic activity for the OCM reaction. The maximum yield to C2 hydrocarbons that was obtained was 16%, which compares favorably to prior dense membrane studies.

Oxidative coupling of methane using oxygen-permeable dense membrane reactors

Abstract Oxidative coupling of methane was studied with La/MgO catalyst and a distributed oxygen feed through mixed-conducting dense membrane tubes in a shell-and-tube reactor configuration. A SrFeCo0.5O3 membrane was tested, and a blank run confirmed that it acted as a total oxidation catalyst, with no C2 products. Attempts to coat the inside of the membrane tube with a non-combustion material BaCe0.6Sm0.4O3 were only partially successful, giving 7% yield to C2 products. The oxygen flux through the coated tube was reduced to 30% of its original value. A membrane tube was fabricated from a non-combustion oxygen-permeating material, BaCe0.8Gd0.2O3, and it was confirmed that this was not a total oxidation catalyst. Yields to C2 products of up to 16.5% were obtained, higher than those in comparable fixed bed studies. The C2 yield obtained is the highest reported in the literature for oxidative coupling of methane in dense membrane reactors. The flux of oxygen through both dense membranes increased under reaction conditions, by a factor of four, over the non-reaction flux measured in permeation experiments. Changes in surface morphology were observed for the side of the membrane in contact with the reducing atmosphere. Similar phenomena have been observed in previous studies.

Silicon-rich H-ZSM-5 catalyzed conversion of aqueous ethanol to ethylene

A study of the H-ZSM-5 catalysis of the conversion of aqueous ethanol solutions to ethylene was carried out using zeolites having Si/Al atom ratios of 35 to 15,000. Rates of ethanol conversion dropped sharply as expected until a Si/Al ratio of about 500; at higher ratios the activity was maintained at a nearly constant level. All of the synthesized zeolites afforded 100% conversion of 20% aqueous ethanol when examined at 400 °C and space velocities (WHSV) of 3.4 hr−1 (based on the ethanol component). The ethylene selectivities steadily increased with the Si/Al ratio and reached a maximum of 99.6% using the silicon-rich H-ZSM-5 zeolites. The unusually high catalytic activity observed for the silicon-rich materials was attributed to especially reactive clusters of hydrogen-bonded silanols and was shown to be independent of the aluminum concentration.

Cylindrical internal reflectance: A new method for high-pressure in situ catalytic studies

Abstract Cylindrical internal reflectance using crystals embedded into well-stirred high-pressure autoclaves was developed as a new technique for in situ infrared studies of a variety of catalytic reactions. The technique demonstrated several advantages for in situ analysis permitting the direct infrared observation of catalytically active intermediates which were not previously observable. Examples of a variety of high-pressure homogeneous metal-catalyzed reactions, coordination complex reactions, zeolite syntheses, and heterogeneous catalyst analyses are presented to demonstrate the unique advantages of the technique at autogeneous conditions. Spectral data are presented for reactions operating up to 1500 psi (10.3 MPa) and 150 s°C for methanol catalyzed carbonylation by cobalt and rhodium, cobalt-ruthenium-catalyzed carbonylations, and ZSM-5 zeolite synthesis. The technique was found to be useful for several other catalytic reactions and for the study of the reactions of coordination complexes used as homogeneous catalysts.

Oxidative coupling of methane in a modified γ-alumina membrane reactor

Methane oxidative coupling experiments were conducted in a porous γ-alumina membrane reactor using Mn–W–Na/SiO2 catalyst, and its performance was compared with a packed reactor operated at similar conditions. The γ-alumina membrane tube was thermally stabilized by treating the membrane tube with La(NO3)3 aqueous solution and calcining at 900°C. Non-uniform oxygen permeation was achieved by partially coating the outer surface with a high-temperature glaze. C2 yields up to 27.5% were obtained in the membrane reactor. The experimental results clearly demonstrated that it was beneficial to distribute the feed of oxygen along the reactor length for the methane oxidative coupling reactions. Although the membrane reactor showed lower methane conversion at the same reaction conditions, it gave higher C2 selectivity and C2 yield at similar methane conversions. The helium flow rate was varied in the membrane and co-feed reactors to keep the temperature, methane flow rate, and oxygen flow rate constant. At the same methane conversion the membrane reactor gave 10% higher C2 yield and 30% higher C2 selectivity than the co-feed reactor. At similar C2 yield and C2 selectivity, the methane conversion of the membrane reactor was 15% lower than that of a co-feed reactor. The oxygen flow rate was varied in the membrane and co-feed reactors to keep conversion constant at the same temperature, methane flow rate, and helium flow rates. At the same methane conversion, the membrane reactor gave about 3% higher C2 yield and selectivity than the co-feed reactor. Higher helium flow rate gave higher C2 selectivity and yield, whereas changing methane flow rate did not significantly affect the reactor performance.

MODELING AND SIMULATION OF A NONISOTHERMAL CATALYTIC MEMBRANE REACTOR

Abstract A two-dimensional nonisothermal mathematical model has been developed to simulate a tube-and-shell configuration, catalytic membrane reactor. The three-layer membrane consists of an inert large-pore support, an o2 semipermeable dense perovskite layer and a porous catalytic layer. The model is applied to the simulation of the partial oxidation or methane to syngas (oxyreforming). The membrane reactor simultaneously supplies oxygen to the catalytic reaction along the reactor length, and separates oxygen from the air feed, using a dense perovskite layer which is a mixed conductor, thus allowing rapid oxygen permeation without the use of an external circuit. Two configurations of catalytic membrane reactors are simulated, for both bench-scale and industrial-scale conditions. Comparisons are made to the conventional fixed-bed reactor, and to membrane reactors which are isothermal, adiabatic or wall-cooled. The simulation results imply that the temperature rise in exothermic partial oxidation reactions...

Analysis and optimization of cross-flow reactors with staged feed policies : isothermal operation with parallel-series, irreversible reaction systems

A general model was proposed for the simulation of cross-flow reactors with six types of feed policies. All of the six types of cross-flow reactors were analyzed for series-parallel reaction systems and their feed distributions were optimized by maximizing the desired product yield at the reactor outlet. The comparisons of reactor performance between reactors with different types of feed policies were made in terms of kinetic and operation parameters. In order for a reactor with staged feed to achieve higher yield and selectivity of the desired product than a conventional co-feed reactor operated at the same conditions, the reaction order in the distributed reactant in the desired reaction has to be lower than that in the undesired reactions. The improvement of the desired product yield by using staged feed reactors increases with the increase in residence time and increase in reaction order with respect to the distributed reactant of the undesired reactions. In addition, the dimensionless groups σi (rate constant of reaction i multiplied by the residence time) should be within certain ranges for the distributed feed reactor to achieve higher desired product yield than the conventional co-feed reactor. Optimally fed reactors give only slightly higher maximum yields than uniformly fed reactors with the same number of feed points. The modeling study provides guidance in terms of kinetic and operating parameters for the evaluation of the effectiveness of using multiple-staged feed reactors and membrane reactors to improve the reactor performance.

Oxidative coupling of methane in porous Vycor membrane reactors

Abstract Porous Vycor membrane tubes were used in shell-and-tube type membrane reactors to study the effect on the oxidative coupling of methane of metering the oxygen into the catalyst bed. Experimental studies showed that under conditions of complete oxygen conversion, Vycor membrane reactors packed with Sm 2 O 3 catalyst exhibited enhanced hydrocarbon (C 2 ) selectivity. C 2 yields were comparable to those of the conventional co-feed packed bed reactors operated under the same conditions. The higher C 2 selectivity in the membrane reactors indicated that, for methane coupling, regulating the supply of oxygen along the length of the packed bed may be beneficial to C 2 formation.

Analysis and optimization of cross-flow reactors with distributed reactant feed and product removal

Abstract A systematic and general model was proposed for the simulation of cross-flow reactors with product removal and reactant feed policies. Six types of cross-flow reactors were analyzed for reversible series-parallel reaction systems and their optimal feed distributions were determined by maximizing the desired product yield at the outlet of the reactor. The performances of reactors with different types of feed policies were compared at their optimal operating conditions. For irreversible reaction systems with lower order in distributed reactant for the desired reaction than those for undesired reactions, a higher yield and selectivity of the desired product could be achieved with the reactors with staged feed than with conventional co-feed reactors and a sufficiently high residence time was required by staged feed reactors to significantly improve the desired product yields and selectivities over those obtained by a co-feed reactor. However, for reversible reaction systems, the desired product yield always reached a maximum value, and then dropped down as the residence time increased. In addition to the kinetic order and residence time requirements, the rate constants of the reactions involved have to fall within certain ranges for the distributed feed reactor to obtain a higher maximum yield than one-stage co-feed reactors. Optimally distributed feed reactors always give higher maximum product yields than evenly distributed reactors with the same number of feed points. However, the improvement of yields is not as great as that between co-feed reactors and evenly distributed reactors. On the other hand, for reaction systems with higher order with respect to the distributed reactant in the desired reaction than the undesired reactions, co-feed reactors always give higher yield than staged feed reactors.