Hung-Shan Weng

Liquid-phase oxidation of cyclohexane using CoAPO-5 as the catalyst

Abstract In this study, CoAPO-5 was found to be an effective heterogeneous catalyst for the liquid-phase oxi- dation of cyclohexane with glacial acetic acid as the solvent without any promotors being added to the reaction mixture. The effects of cobalt content, BET surface area and dissolved cobalt ions and the kinetics of the reaction were investigated at 5–15 kg/cm2 and 115–135°C in a semi-batch autoclave reactor. Under these reaction conditions, no induction period was observed. The products of reaction were cyclohexanone, cyclohexanol, dibasic acids, and caprolactone, etc. The ratio of the concentration of adipic acid to succinic acid had a maximum value at the middle stage of reaction. The initial rate of reaction was found to be proportional to the 2.0 power of the cyclohexane concentration and to the 0.5 power of the catalyst loading. The effects of oxygen pressure on the reaction rate depended on the cobalt content and the reaction conditions. The cobalt ions of CoAPO-5 are mainly in the lattice positions and they can transform reversibly between the oxidizing states of CoII and CoIII. Reaction mechanism and rate expressions were proposed. Since the conversion and selectivity reported in this study were mod- erately good, and because CoAPO-5 has the advantage of being a durable heterogeneous catalyst, it could prove useful as a catalyst for the one-step liquid-phase oxidation of cyclohexane.

Liquid-phase oxidation of cyclohexane over CoAPO-5: Synergism effect of coreactant and solvent effect

Cyclohexanone was employed as a coreactant in the liquid-phase oxidation of cyclohexane using glacial acetic acid as a solvent and CoAPO-5 as a catalyst at 105-135°C and 10 kg/cm2. Addition of cyclohexanone increases the rate of cyclohexane oxidation and selectivity to adipic acid. When the cyclohexanone concentration increases, the rate will increase at 115°C but decrease at 135°C, while the cyclohexanone concentration has no obvious effect on the rate at 125°C. The apparent activation energy for cyclohexane oxidation in the cooxidation system is lower than that of cyclohexane oxidation in the system without the addition of cyclohexanone. An increase of catalyst loading in the cooxidation system, though, increases the rate of cyclohexane oxidation but decreases the yield of adipic acid. Use of carboxylic acid, except formic acid, as the solvent is necessary for the oxidation of cyclohexane. Of acetic acid, propionic acid, butyric acid and n-pentanic acid, the use of propionic acid results in the highest reaction rate when the temperature is below 135°C, while these four carboxylic acids give almost the same conversion after 2 h at 135°C.

The promoter effect and a rate expression of the catalytic incineration of (CH3)2S2 over an improved CuO–MoO3/γ-Al2O3 catalyst

The CuO-MoO3/gamma-Al2O3 catalyst, confirmed previously as having good activity in the catalytic incineration of (CH3)2S2, was employed as the principal catalyst in this study. With the aim of improving catalyst activity and resistance to deactivation by sulfur compounds, a promoter was added either before adding the precursors of Cu and Mo or together with Cu and Mo onto the gamma-Al2O3. Promoters included transition metals and elements from groups IA-VIIA in the chemical periodic table. Experimental results reveal Cr2O3 as the most effective promoter, with an optimal composition of 5 wt.% Cu, 6 wt.% Mo and 4 wt.% Cr (designated as Cu(5)-Mo(6)-Cr(4)/gamma-Al2O3). Knowing that higher acidity can improve activity, we further investigated the effect of acid treatment on the performance of the Cu(5)-Mo(6)-Cr(4)/gamma-Al2O3 catalyst. Experimental results indicate the H2SO4-treated catalyst (Cu(5)-Mo(6)-Cr(4)/sulfated-gamma-Al2O3) has a better activity and durability. A study for finding an appropriate rate expression for the catalytic incineration of (CH3)2S2 by Cu(5)-Mo(6)-Cr(4)/sulfated-gamma-Al2O3 was carried out in a differential reactor. The results show that the Mars-Van Krevelen model is applicable to this destructive oxidation reaction. Results additionally reveal that competitive adsorption of CH4 reduces conversion of (CH3)2S2.

Catalytic reduction of SO2 over supported transition-metal oxide catalysts with C2H4 as a reducing agent

Abstract This work investigates performances of supported transition-metal oxide catalysts for the catalytic reduction of SO2 with C2H4 as a reducing agent. Experimental results indicate that the active species, the support, the feed ratio of C2H4/SO2, and pretreatment are all important factors affecting catalyst activity. Fe2O3/γ-Al2O3 was found to be the most active catalyst among six γ-Al2O3-supported metal oxide catalysts tested. With Fe2O3 as the active species, of the supports tested, CeO2 is the most suitable one. Using this Fe2O3/CeO2 catalyst, we found that the optimal Fe content is 10xa0wt.%, the optimal feed ratio of C2H4/SO2 is 1:1, and the catalyst presulfidized by H2+H2S exhibits a higher performance than those pretreated with H2 or He. Although the feed concentrations of C2H4:SO2 being 3000:3000xa0ppm provide a higher conversion of SO2, the sulfur yield decreases drastically at temperatures above 300xa0°C. With higher feed concentrations, maximum yield appears at higher temperatures. The C2H4 temperature-programmed desorption (C2H4-TPD) and SO2-TPD desorption patterns illustrate that Fe2O3/CeO2 can adsorb and desorb C2H4 and SO2 more easily than can Fe2O3/γ-Al2O3. Moreover, the SO2-TPD patterns further show that Fe2O3/γ-Al2O3 is more seriously inhibited by SO2. These findings may properly explain why Fe2O3/CeO2 has a higher activity for the reduction of SO2.

THE KINETICS OF CATALYTIC INCINERATION OF (CH3)2S2 OVER THE CuO–MoO3/γ-Al2O3 CATALYST

ABSTRACT In this study, by varying reaction conditions including particle size, space velocity, reactant concentration and reaction temperature, the kinetics of catalytic incineration of (CH3)2S2 catalyzed by the CuO–MoO3/γ-Al2O3 catalyst was investigated. Three kinetic models, i.e., the power-rate law model, Langmuir-Hinshelwood model and Mars-Van Krevelen model, were applied to best fit the experimental results. It was shown that the Mars-Van Krevelen model was more appropriate than the other two models for describing the mechanism of catalytic incineration of (CH3)2S2 on the CuO–MoO3/γ-Al2O3 catalyst. The reaction expression of the Mars-Van Krevelen model was as follows: where α is 5.5 and C R and C O represent concentrations of (CH3)2S2 and O2, respectively. The enlarged difference between experimental and predicted data was observed at higher operating temperatures. This might be due to the dominating mechanism at this temperature region was different.

A LIMITATION OF REUSING THE CATALYST IN TRI-LIQUID-PHASE CATALYTIC SYSTEMS

This work demonstrates important factor influencing the reusability of the phase transfer catalyst in the third liquid phase in addition to the role of the possible loss of catalyst due to the dissolution of the catalyst into the aqueous and organic phases. When the catalyst might react with the byproducts, in addition to reacting with the organic substrate and aqueous nucleophile, it would lose its catalytic activity. The substitution reaction between the organic substrate and an aqueous nucleophile (sodium phenolate) with tetra-n-butylammonium bromide as a phase-transfer catalyst was employed as a model reaction and was performed in a batch reactor. Three organic substrates, including allyl bromide, n-butyl bromide, and ethyl 2-bromoisobutyrate, were tested. Each of the third liquid phases formed in these tri-liquid-phase catalytic systems was utilized three times to observe the change in the activity of the catalyst. The catalyst in the third liquid phase can be reused without any loss of its catalytic activity when allyl bromide or n-butyl bromide is utilized as the organic substrate; however, the catalytic activity declines when ethyl 2-bromoisobutyrate is the organic reactant. Therefore, the organic reactant plays a crucial role in determining whether the catalyst can be reused or not.

ANALYSIS OF FACTORS AFFECTING THE SYNTHESIS OF ALLYL PHENYL ETHER BY TRI-LIQUID-PHASE CATALYSIS

In order to improve the selectivity of allyl phenyl ether (ROPh), the main product, in the etherification of allyl bromide (RBr) and sodium phenolate (NaOPh) with tetra-n-butylammonium bromide (QBr) as a phase-transfer catalyst, the technique of tri-liquid-phase phase-transfer catalysis, instead of the liquid-liquid one, was employed. The reaction was performed in a batch reactor, and the factors affecting the conversion and selectivity were investigated. The possibility of reusing the phase-transfer catalyst was also evaluated. Experimental results indicate that the addition of a small amount of Na2CO3 will benefit the formation of a third liquid phase and enhances both the conversion of RBr and the overall yield of ROPh. Both the conversion and the overall yield are maximal when the mole fraction of QBr in the mixture of NaOPh and QBr is about 0.3. A high reaction temperature enhances the conversion and the overall yield. Under optimal conditions, complete conversion and near 100% yield can be obtained within 10 minutes. Although the reaction rate by the tri-liquid-phase catalysis is slightly lower than that observed with the same amount of catalyst by conventional liquid-liquid phase-transfer catalysis, the selectivity of ROPh is higher and the catalyst can be easily reused by the reuse of the third liquid phase without any loss of its catalytic activity in the former case. Because the reuse of catalyst was found to be feasible, the production of ROPh with a continuous-flow reactor becomes possible.