Hyun-Ku Rhee

ON THE THEORY OF MULTICOMPONENT CHROMATOGRAPHY

A theoretical study of multicomponent chromatography is here presented in which the system is considered to be one-dimensional, isothermal, locally at equilibrium and to have negligible diffusion effects. The discussion starts with constant initial and entry conditions and goes on to stepwise constant data with an arbitrary number of discontinuities. The Langmuir adsorption isotherm is perfectly fitted to the exposition of the mathematical theory of quasilinear equations for it leads to explicit forms for the Riemann invariants and characteristic parameters. This paper develops the theory of simple waves and of shock waves on an independent basis and illustrates this theory by the construction of solutions and the analysis of the interaction of waves. It is shown incidentally that the entropy change across a shock is consistent with the second law of thermodynamics. The separation of solutes is discussed and brief consideration is given to the problems associated with non-uniform geometry and non-isothermal adsorption.

Partial hydrogenation of 1,3-cyclooctadiene using dendrimer-encapsulated Pd–Rh bimetallic nanoparticles

Abstract This article reports the first effort for the preparation of Pd–Rh bimetallic nanoparticles in the presence of poly(amidoamine) dendrimers with surface hydroxyl groups (fourth generation, PAMAM-OH). The resulting dendrimer-encapsulated Pd–Rh bimetallic nanoparticles show a promising catalytic activity in partial hydrogenation of 1,3-cyclooctadiene.

Characteristics of Pt/H-beta and Pt/H-mordenite catalysts for the isomerization of n-hexane

Abstract Bifunctional Pt/H-beta and Pt/H-MOR catalysts were prepared under various pretreatment conditions and applied to the isomerization of n -hexane. Pt/H-beta gave rise to the higher yield of high octane dimethylbutanes than the commercial Pt/H-MOR. To explain this result, both the dispersion and distribution of Pt clusters in the zeolite channels were investigated. The acidity and the distribution of acid sites in both catalysts were also examined by using ammonia and pyridine as probe molecules. Pt/H-beta was found to be a potential catalyst for the isomerization of n -hexane because Pt clusters could be well dispersed to give proper balance between metallic centers and acid sites in H-beta zeolite although its acidity was low compared with H-MOR.

A study of the shock layer in nonequilibrium exchange systems

Abstract Shock layer theory is extended to the nonequilibrium exchange system with axial dispersion. A moving coordinate scheme enables us to establish the existence condition and the uniqueness of the shock layer. In addition to the strategy for the numerical determination of the shock layer, an approximate solution is discussed which neglects the couple effect of the dispersion and the mass transfer resistance. This solution provides a basis for the quantitative analysis of the shock layer thickness and the departure from equilibrium. Applications are illustrated by using the Langmuir isotherm and a numerical solution of the transient equation confirms the usefulness of the shock layer theory.

Pt-Pd Bimetallic Nanoparticles Encapsulated in Dendrimer Nanoreactor

This article reports the first effort for the preparation of dendrimer-templated Pt-Pd bimetallic nanoparticles and their application to catalysis. The resulting monodispersed bimetallic nanoparticles show a promising catalytic activity in the partial hydrogenation of 1,3-cyclooctadiene.

Shock layer in two solute chromatography: effect of axial dispersion and mass transfer

Abstract A theoretical study for two solute chromatography when axial dispersion or interphase mass transfer is significant is presented. With a Langmuir isotherm the mathematical theory of the shock layer plays a key role in generating asymptotic solutions, from which one can deduce the effect of axial dispersion or mass transfer resistance. A full discussion is given for the existence and uniqueness of the asymptotic solution. If a shock layer exists, the end states satisfy the compatibility condition and it propagates at the same speed as the corresponding shock. Analytic expressions are developed for the case of equal Peclet numbers (or equal Stanton numbers) whereas for other cases the shock layer profiles are determined numerically. Comparison between transient solutions and shock layers demonstrates the validity and usefulness of the present study.

Shape selective conversion of 1,2,4-trimethylbenzene over zeolite NU-87

The catalytic properties of zeolite NU-87 were investigated in the conversion of 1,2,4-trimethylbenzene in view of the product selectivity. To examine the catalysis by external acid sites, experiments were conducted over NU-87 with external surface poisoned by 1,1,1,3,3,3-hexamethyldisilazane. For comparison, the results obtained over MCM-22, which has a similar pore structure, are also included. Both disproportionation (to xylene and tetramethylbenzene) and isomerization (to 1,2,3- and 1,3,5-isomers) take place over NU-87. Under the present conditions, it is found that the disproportionation proceeds largely within the micropores of NU-87, while the isomerization reaction occurs mainly on the external surface. The distribution of xylene isomers is thermodynamically controlled inside the pores, whereas amongst tetramethylbenzene isomers, the 1,2,4,5-isomer is selectively produced through the channel system. The bulky product, 1,2,3,5-tetramethylbenzene (or 1,3,5-trimethylbenzene), is severely hindered from diffusing through the channel system of NU-87. However, 1,2,3,4-tetramethylbenzene and 1,2,3-trimethylbenzene diffuse out of the micropores to a certain extent. In the case of MCM-22, the conversion of 1,2,4-trimethylbenzene hardly proceeds within the micropores. It is evident that NU-87 has larger pores and thus higher accessibility to 12-MR cavities than MCM-22.

Multicomponent Adsorption in Continuous Countercurrent Exchangers

A theoretical analysis of multicomponent adsorption in continuous countercurrent exchangers is presented. The system is considered to be one-dimensional, isothermal, locally at equilibrium, and to have negligible diffusion effects. With constant initial and entry data the mathematical problem is essentially the same as for a fixed bed except for the boundary condition that suggests a free boundary analogue since it is given by the conservation law itself. Therefore, the boundary discontinuity is naturally encountered. With the Langmuir adsorption isotherm explicit forms for the Riemann invariants and characteristic parameters are available and thus the theories of simple waves and of shock waves as well as of interactions are readily established and applied to determine solutions. Dependence of the system behaviour upon the flow rate ratio is discussed and the steady-state argument proves that the number of steady states attainable in the contacting region is equal to the number of solute species present plus one. Application is illustrated.

Stability of Fe/ZSM-5 de-NOx catalyst : effects of iron loading and remaining Brønsted acid sites

The stability of Fe/ZSM‐5 de‐NOx catalyst has been investigated. The samples are prepared by sublimation of iron chloride. Substantial amount of protons are found to remain in the fresh catalyst after washing and calcination. After 10 h exposure to wet exhaust gas at 600°C, the catalyst is severely deactivated. The presence of steam induces dealumination of the ZSM‐5 matrix, because the protons provide the point of attack by water. In addition, highly reactive distorted tetrahedral iron species and tetrahedral species change to less reactive octahedral iron ions or iron agglomerates upon aging treatment. With the second sublimation of iron chloride, the iron loading is increased and thus the concentration of remaining protons is reduced. Also, the catalyst turns out to preserve more reactive iron ions after aging treatment. In conclusion, the second sublimation is believed to bring about a remarkable improvement in the stability of the Fe/ZSM‐5 catalyst although its de‐NOx activity is slightly decreased.

Improvement in hydrophobicity of Ti-MCM-41 using a new silylation agent MSTFA

A new silylation agent, N-methyl-N-(trimethylsilyl) trifluoroacetamide (MSTFA), has been employed for the trimethylsilylation of Ti-MCM-41. The FT-IR spectra revealed that the hydrophobic trimethylsilyl group, –Si(CH3)3, in MSTFA can replace the hydrogen in the silanol at room temperature. By virtue of their high volatility, MSTFA and its derivative, N-methyl trifluoroacetamide, can be readily removed after silylation. The MSTFA silylated Ti-MCM-41 is found to be more hydrophobic than that silylated by BSTFA and to bring about a significant enhancement in the yield of epoxide from cyclohexene epoxidation with aqueous H2O2.