전이금속 촉매를 이용한 유기상/수상 경계 면에서의 유기반응

장 수현, 안 화승

인천광역시 남구 용현동 253, 인하대학교, 화학공학과

Organic Reactions Using Transition Metal Catalysts

at the Organic/Water Interface

S. H. Chang and W. S. Ahn

Department of Chemical Engineering, Inha Univ.

Introduction

Recent studies have demonstrated the feasibility and advantages of chemical synthesis at organic-water (hereafter O-W) interface using surface-active catalyst complexes[1-3]. Compared to conventional liquid-phase synthesis, biphasic synthesis based on surface-active catalyst complexes offers the following advantages: (1) circumvention of a solubility incompatibility between reactants or between a reactant and catalyst, (2) avoidance of the use of a toxic or environmentally-troublesome organic solvent that may otherwise be needed, (3) ease of catalyst recovery in a compact form unencumbered by the solubilities of reactants and products in either aqueous or organic phase, (4) attainment of high reactivity, selectivity and reproducibility under mild reaction conditions, (5) ease of processing and control, (6) avoidance of an adverse gelling effect on a polymerization reaction, (7) a low pressure requirement for a dissolved gas reaction, and (8) possibility of regio- and enantioselectivity control due to the directional influence of the interface on molecular orientations. In this report, we present our findings on extending the biphasic technique to the autoxidation of tetralin(or 1,2,3,4-tetrahydronaphthalene). The latter reaction is a key intermediary step in the commercial production of □-naphthol[4].

Experiment

The reaction was carried out batchwise at 1 atm in a 250-□ Morton flask reactor using tetralin-water as the biphasic mixture, a transition metal complex of a surface-active ligand as the catalyst, and dodecyl sodium sulfate(DSS) as the emulsifier. The reactor was placed in a water bath and purged with oxygen for about 10 minutes and then allowed to equilibrate to the set temperature, which was typically 60℃. The reaction was initiated by catalyst addition with the help of a needle-stemmed spoon that could be raised or lowered through a septumed joint on a side neck of the reactor. The reaction was followed by monitoring the oxygen uptake with the help of a constant-pressure manometric unit. The reaction mixture was analyzed using high performance liquid chromatography, gas chromatography and mass spectroscopy.

Result and Discussion

In the presence of a suitable surface-active catalyst complex, such as a tetramethylethylenediamine complex of manganese, chromium or nickel, the reaction took off after an induction period, as evident in Fig.1 which shows a representative oxygen uptake profile of a reaction run. Under biphasic conditions, □-tetralone and □-tetralol were the major autoxidation products, independent of the catalyst and ligand used to promote the reaction.  □-Tetralyl hydroperoxide, which has been reported as a major product in one-liquid-phase systems, was found in a very small amount and could only be detected by distilling off the bulk of the unreacted tetralin under a reduced pressure. The reaction stoppage occurred at a conversion of about 35% of the bulk tetralin phase, and it was caused by the formation of higher oxidation products that were inhibitive. Control runs with small additions of □-naphthol and 1,4-dihydroxynaphthalene confirmed the pronounced inhibiting effect of these products. It should be noted that a similar product inhibition effect has also been observed in the homogeneous reaction. Presumably, these phenolic products inhibit the reaction by scavenging free radicals. Attempts to use ESR spectroscopy to detect free-radical intermediates brought inconclusive results. No free radicals could be detected above the detection limit of the spectrometer(10-5M). However, there was a possibility of radical intermediates being tied to the catalyst complexes. Intensity variations in the metal signal were observed, along with some line broadening.

The results of a screening study on potential catalysts and ligands showed that catalysts known to promote the tetralin autoxidation reaction either homogeneously or heterogeneously - namely, manganese, nickel, and chromium - were also found to promote the biphasic autoxidation reaction, provided that they were used in conjunction with a suitable surface-active ligand or complexing agent. Managanese-TMEN complex gave the highest activity but a relatively low □-tetralone selectivity of about 60%. The latter stayed essentially constant independent of the tetralin conversion. Chromium-TMEN complex was about half as active as the manganese-TMEN complex, but it gave a much higher □-tetralone selectivity.

The variation of the reaction rate with the ligand-catalyst mole ratio is shown in Fig.2 and 3 for the manganese - and chromium - catalyzed reactions, respectively. In each case, the optimal ligand-catalyst ratio was found to be around 2:1. The variations of the rate and induction period with the catalyst concentration are shown in Figs.4 and 5, respectively, for the manganese - and chromium - catalyzed reactions. In each case, the "steady-state" rate immediately following the induction period increased with the metal concentration until a threshold metal concentration was reached, whereupon the rate leveled off with further increase in the metal concentration. The reaction order with respect to the catalyst decreased from a limiting value of 2.0 and 1.4, respectively, for the manganese - and chromium - catalyzed reactions to 1.0 and then 0 with increasing metal concentration. Two possible causes for the leveling of the rate beyond a threshold metal concentration are (1) a biphasic solubility limit on the active catalyst species and (2) a catalyst-inhibitor conversion.

The rate dependence of the biphasic autoxidation reaction on the oxygen partial pressure for chromium catalyst is shown in Fig.6. The data indicate that the reaction order with respect to oxygen decreased from first to zero order above an oxygen pressure of about 0.15 atm. The empirical finding suggests that the segregation of surface-active catalyst complex, coupled with the latter's ability to absorb the gaseous reactant, may create a localized high concentration of the dissolved gas around the O-W interface, where the reaction occurs.

Conclusions

Biphasic autoxidation of tetralin has been carried out using surface-active tetramethylethylenediamine complexes of manganese, chromium and nickel as catalysts, tetralin as the substrate and organic phase, and dodecyl sodium sulfate as emulsifier. The biphasic reaction was found to offer significant advantages over the homogeneous and heterogeneous counterparts, including avoidance of the use of a troublesome solvent, ease of catalyst recovery and substrate recycle, and attainment of high reactivity, selectivity and reproducibility under mild reaction conditions. The main reaction products were □-tetralone and □-tetralol. The selectivity for the former decreased from 95% with the chromium complex to 90% with the nickel complex and 60% with the manganese complex, and the activity varied in a reverse order. The biphasic reaction was inhibited by higher oxidation products that are radical scavengers.

The reaction order with respect to oxygen decreased from 1.0 to 0 above an oxygen partial pressure of 0.15 atm. The reaction order with respect to catalyst decreased from 2.0 or 1.4 to 1.0 and then 0 with increasing metal concentration.

References

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2. Y. Zhong, V. M. Godfrey, P. Brown, and P. K. Lim, Chem. Eng. Sci., 51, 757 (1996)

3. Y. Zhong, C. F. Abrams, and P. K. Lim, "Ind. Eng. Chem. Res., 34, 1529 (1995)

4. F. Mizukami and J. Imamura, "Bull. Chem. Soc. Jpn., 51, 1404 (1978)