Naotsugu Itoh

An adiabatic type of palladium membrane reactor for coupling endothermic and exothermic reactions

Abstract Palladium can play an interesting role as a catalytic membrane, that is, a hydrogen separative and catalytically active wall. Utilizing this function, a palladium membrane reactor capable of working under an adiabatic condition was designed in this study for coupling two conjugated reactions. On one side of the membrane, dehydrogenation of cyclohexane as a model takes place in the catalyst-packed layer, and on the membrane surface of the other side hydrogen permeated reacts in-situ with oxygen. In the adiabatic membrane reactor, a heat compensation between the endothermic dehydrogenation and the exothermic oxidation is expected to be realized. As a result, it became obvious experimentally that the generated heat due to the oxidation refluxed to the dehydrogenation side, heated up the catalyst layer and therefore enhanced the dehydrogenation. A simple mathematical model derived for analyzing the reaction process could simulate the practical reactor performances well.

Selective hydrogenation of phenol to cyclohexanone using palladium-based membranes as catalysts

Selective hydrogenation of phenol to cyclohexanone was attempted using a catalytic palladium membrane reactor (CPMR) at atmospheric pressure and temperatures ranging from 150 to 300°C. Four kinds of catalytic membranes made from palladium, Pd93Ni7, Pd93Ru7 and Pd77Ag23 were tested. The results showed the possibility of one-step production of cyclohexanone using the CPMR. Of all the membranes tested, pure palladium showed the highest activity for the production of cyclohexanone. The hydrogen permeating through the membranes was found to have higher reactivity than that pre-mixed into phenol.

Hydrogen recovery from cyclohexane as a chemical hydrogen carrier using a palladium membrane reactor

A palladium membrane reactor was applied to recover the hydrogen from cyclohexane as one of the promising chemical hydrogen carriers. The operation conditions of the palladium membrane reactor to obtain a higher hydrogen recovery were predicted by computer simulation. As a result, it was shown that the hydrogen recovery rate becomes higher as the pressure on the hydrogen permeation side is lowered below atmospheric pressure or as the reaction pressure increases. This was confirmed experimentally. As the perm-side pressure was lowered, the conversion as well as the hydrogen recovery rate at 573 K was found to increase. About 80% of the hydrogen contained in cyclohexane, depending on the operation condition was successfully recovered.

A carbon membrane reactor

A carbon membrane reactor as one of the applications of carbon membranes was newly developed and examined using dehydrogenation of cyclohexane as a test reaction. Permeation of hydrogen, argon, cyclohexane and benzene through carbon membranes showed the so-called molecular-sieving like diffusion behavior. The performance of the carbon membrane reactor for the dehydrogenation was found to exceed that of a normal reactor, i.e., equilibrium conversion.

Preparation of pore-free disk of La1−xSrxCoO3 mixed conductor and its oxygen permeability

Abstract Pore-free disks of La 1− x Sr x CoO 3 ( x =0.4, 0.5, 0.6) as one of the mixed conductive perovskite-type oxides were successfully prepared, where a powder precursor having not yet the perovskite structure was selected as a starting material. The rate of oxygen permeation through the La 1− x Sr x CoO 3 disk was found to increase with an increment in Sr content, and to be mainly controlled by the amount of oxygen vacancy. Effects of thickness of the disk and oxygen partial pressure on the oxygen permeation rate could be elucidated by the electrochemical transport equation. In conclusion, La 1− x Sr x CoO 3 is a promising material for oxygen permeation at high temperatures.

Deposition of palladium inside straight mesopores of anodic alumina tube and its hydrogen permeability

Abstract A new method of preparing a composite type of palladium membrane for hydrogen separation, viz. a combined sputtering and electroplating technique, was elaborated, and its hydrogen permeability and selectivity were measured in the range 200–350°C. As a support, an anodic aluminum oxide tube, was used with uniform straight pores of several tens of nanometers in diameter, inside which palladium deposition was tried by means of electroplating. The principal conditions of the electroplating, such as the current density and time, were examined to obtain a composite membrane tube with as few defects as possible. Under optimum electroplating conditions, it was observed with SEM and FE-SEM that a thin layer of palladium formed with a thickness of several micrometers was densely packed inside the pores. The ideal separation factor for hydrogen and nitrogen was found to be more than 1000, above 300°C.

Preparation of a tubular anodic aluminum oxide membrane

Abstract An anodic aluminum oxide tube with perforated straight pores to use as a porous membrane or as a support for a composite membrane was prepared. An aluminum oxide layer with straight micropores closed by an oxide barrier layer was formed on the surface of an aluminum tube, 45 mm long, 0.5 mm thick and 6 mm in outer diameter, by anodic oxidation in an aqueous solution of oxalic acid. The pores were opened by dissolution of the inner aluminum and the subsequent barrier layer of aluminum oxide. The tubular alumina membrane obtained was 35–40 μm thick with straight micropores of 20–50 nm, and therefore showed Knudsen permselectivity for inorganic gases. It was found that the tube could withstand at least up to 4.4 atm of transmembrane pressure.

Hydrogen permeation through palladium-coated amorphous ZrMNi (M = Ti, Hf) alloy membranes☆

Abstract Two series of alloys, (Zr36Ni64)1−α (Ti39Ni61)α and (Zr36Ni64)1−α(Hf36Ni64)α (0 ≤ α ≤ 1), were rapidly quenched by a single-roller, melt-spinning technique; amorphous alloy membranes 30–40 μm thick were obtained except for Ti39Ni61. X-ray diffraction showed that the amorphous structure was almost independent of Hf content but became denser with increasing Ti. Additionally, differential scanning calorimetry showed that thermal stability was improved by Hf, but not by Ti. Palladium-coated amorphous membranes were never broken during permeation testing in the range of 473 to 623 K. The permeation rate was rather stable at 573 K or less and approximately proportional to the square-root difference of hydrogen partial pressures across the membrane, suggesting that permeation was controlled by the diffusion process of hydrogen atoms in the membrane; it was also inferred that membranes had no defect that all other gases could pass through. Permeability was found to decrease with Ti or Hf due to increased activation energy for permeation. On the other hand, as-quenched Ti39Ni61 membranes were crystalline and too brittle to be used for hydrogen separation.

Application of a membrane reactor system to thermal decomposition of CO2

Abstract An yttria stabilized zirconia (YSZ) membrane reactor system was applied to enhance the direct thermal decomposition of CO 2 at high temperature. The decomposition rate of CO 2 and the permeation rate of oxygen through the membrane were measured and their equations were established. The reaction itself and the reactions involved in the separation process were analyzed by computer simulation. It was found that the YSZ-membrane reactor system may improve the final conversion in CO 2 decomposition. The experimental results were in good agreement with the model predictions presented in this study.

Electrochemical coupling of benzene hydrogenation and water electrolysis

Abstract To establish an efficient process for hydrogenation of benzene to cyclohexane as one of the candidates for transportable, chemical hydrogen carriers, a combined scheme of water electrolysis and hydrogenation in the polymer electrolyte cell was proposed. A Rh–Pt electrode, which was found to be more active for electrochemical hydrogenation of benzene, was formed on a polymer electrolyte (Nafion, Du Pont) by means of soaking-reduction. Using an electrochemical cell, hydrogen pumping rate, electrochemical hydrogenation of benzene and water electrolysis were investigated at temperatures 25–70°C and atmospheric pressure, followed by combined electrochemical hydrogenation of benzene and water electrolysis. It became clear that benzene could be electrochemically hydrogenated on the cathode with hydrogen (proton), which was produced by water electrolysis at the anode and then pumped. The combined effects were recognized as a drop in decomposition voltage of water as well as a rise in current.