Studies on the partial hydrogenation of benzene

• Main Authors: Hu, Sung-Cheng, 胡松城
• Other Authors: Chen Yu-Wen
• Format: Others
• Language: zh-TW
• Online Access: http://ndltd.ncl.edu.tw/handle/20210370517117877741

博士 === 國立中央大學 === 化學工程學系 === 86 === The objective of this study was to develop a catalytic process capable ofproducing cyclohexene in an economical way. Cyclohexene is attractive in the industrial viewpoint in the sense that not only opening up a new route to nylon-66via adipic acid or to nylon-6 via cyclohexanone but also acting as a raw material for fine chemicals. It could be simply produced by partial hydrogenation ofbenzene, however, this reaction is rather difficult due to the much more unfavorable thermodynamic limitation. A kinetic way has to be found to tackle this problem. The hydrogenation capacity of the right catalyst should not be so strong that over-hydrogenation provails or so weak that reaction rate is low. Higher yield of cyclohexene could be obtained with zinc oxide or zinc-modified ruthenium catalyst under proper reaction conditions. Catalysts were prepared by co- precipitation (binary oxides, atomic ratio of La/Zn = 1/0, 2/1,1/1, 1/2, 1/5, 0/1 and A/Zn = 1/5, A = La/Zr/Cr/Ga/Ba/Mg/Sr/ Ca; nanoparticles) and incipient wetness impregnation method. The characteristics of the catalysts were examined by X-ray diffraction, -196℃ nitrogen adsorption (B.E.T method), trnasmission electron microscopy (TEM), and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The results indicate that activity is suppressed overall binary oxide- supported catalysts, however, the selectivity to cyclohexeneis greatly enhanced. Under a kinetically controlled regime, (activation energyobtained is 18.9 kcal/mol), 2% Ru catalyst with the oxide composition of La/Zn= 1/5 gave the best result of 80% iniaial selectivity and 34% yield to cyclohexene. As gallium oxide replaces lanthanum oxide as a secondary oxide composition, as high as 90% initial selectivity and 37.8% yield to cyclohexene was observed. The zinc oxide modified Ru catalyst, originally hydrophobic, is rendered hydrophilic. Water film renders a slow and competitive adsorption mode of hydrogen, cyclohexene and benzene on the catalyst surface. There is a maximum pressure that gives the highest activity, at still higher hydrogen pressure; the hydrogenation rate would decline on account of a higher surface coverage of hydrogen than that of benzene on the catalyst surface. The selectivity to cyclohexene remains approximately constant regardliss of the hydrogen pressure. This could be explained if hydrogen concentration in the rate expressions appears to anequal power. The Arrhenius plot for 2% Ru/LaZn oxide (La/Zn = 1/5) shows a concave down break at 150℃, which could be explained by decreasing surface coverage of benzene and hydrogen. Fitting the kinetic model with experimental datasuggests that a stronger inhibition of the rate of cyclohexene hydrogenation and/or a faster desorption of the cyclohexene occurs on the catalyst surface and the direct hydrogenation route is suppressed as the zinc content increases. TPR profiles show that zinc oxide retards the reduction of ruthenium oxide as evidenced by the increased reduction temperature. TPD profiles show both strongly and weakly bonded form of hydrogen adsorbed on the surface. Benzene hydrogenation activity is corrlated to the total amount of these binding sites for hydrogen available on the catalyst surface. The amount of the weakly bounded hydrogen, however, grows with the formed new bimetallic phase and might be responsible for the increased selectivity. The performance of the indirect hydroxide method- prepared Ru-Zn nanoparticles are better than that prepared by direct reduction method. These nanoparticles are nearly nonporous as evidenced by the low BET surface area found. With a proper Ru/Zn ratio and a more close packed character, the maximum yield of cyclohexene was over 40%. Cyclohexene selectivityaltered on changing the hydrogen pressure suggests a different reaction mechanism. The hydrogenation rate of benzene follows first-order kinetics in benzene,and no deactivation was observed within the scope studied.