Kinetics Study Of Isophorone Hydrogenation Over Ni/?-Al₂O₃

Isophorone(IP)and its hydrogenation products have wide range of industrial applications,especially in fine chemicals and pharmaceutical intermediates.It is of practical value to study the catalytic hydrogenation reaction kinetics for understanding the hydrogenation reaction mechanism.In this paper,the effect of reaction parameters on the IP hydrogenation catalyzed by Ni/?-Al2O3 were investigated,and the related catalytic reaction kinetic model was established.The results shown that IP hydrogenation catalyzed by Ni/?-Al2O3 were typical consecutive reactions,namely:IP(l)Cat?TMOL(l).The effects of various factors,such as reactant concentration,reaction temperature,hydrogen pressure,catalyst dosage and reaction solvent,on the rate of IP hydrogenation were investigated.It was found that the hydrogenation rate of alkenyl groups in IP molecular was significantly higher than that of carbonyl.With the increase of reaction temperature,the reaction rates of alkenyl and carbonyl hydrogenation increased significantly,but the carbonyl hydrogenation reaction was more sensitive to the reaction temperature.Both the reaction rate of alkenyl and carbonyl groups hydrogenation increased with the increase of H2 pressure,however,the H2 pressure has a greater influence on the rate of carbonyl hydrogenation.With the increase of IP initial concentration,the alkenyl hydrogenation reaction rate increased significantly;while the carbonyl hydrogenation reaction rate was greatly affected by the initial IP concentration.The rate equation for the alkenyl hydrogenation is:R1=k1[IP]1.04[H2]0.55,and for carbonyl hydrogenation is:R2=k2[TMCH]n[H2]1.00,the intermediate product TMCH has a reaction order of 1.02 when the initial IP concentration is low,while the reaction order tends to 0 at higher IP concentrationThe kinetic model based on the Langmuir-Hinshelwood-Hougen-Watson(LHHW)reaction mechanism was established.this model predicted the concentration versus time relationship of substrate and products,and explained the trend of IP conversion-TMOL selectivity,indicating that the established kinetic model is consistent with IP heterogeneous catalytic hydrogenation system.The adsorption equilibrium constants of organics and H2,and the adsorption heat of each substance on the catalyst surface were obtained.The results shown that TMOL has the highest adsorption heat,and the IP adsorption heat is relatively small;The activation energy of the alkenyl hydrogenation reaction was calculated to be 22.72 kJ.mol-1,and for the carbonyl hydrogenation reaction was 58.69 kJ.mol-1,this result is consistent with that calculated from the macroscopic kinetic experimental data,the consistency further explains that the alkenyl hydrogenation is a fast reaction,and the high temperature is more favorable for the carbonyl hydrogenation reaction.