EFFECT OF CESIUM ON ALCOHOL SYNTHESIS, WATER-GAS SHIFT REACTION, AND ESTER HYDROGENOLYSIS OVER COPPER - ZINC OXIDE CATALYSTS.

摘要

This study comprises three subjects: (1) The effects of cesium doping and water concentration in the synthesis gas on methanol synthesis rate and the water-gas shift (WGS) reaction; (2) The hydrogenolysis of alkyl esters; and (3) The effect of cesium on higher alcohol synthesis, as well as the investigation of the chain growth reaction in higher alcohol synthesis over the Cu/ZnO catalysts.; A two-parameter kinetic model quantitatively demonstrated that the maximum rate of methanol synthesis is determined by the balance between the promotional effect of cesium and the retarding effect stemming from the blockage of hydrogen activing sites. Another six-parameter kinetic model, in which the methanol and WGS reactions were considered simultaneously, can account for methanol activity in the water dependence study. At higher water concentrations, the cesium-doped catalysts were more active than the undoped catalyst for methanol synthesis, which was quantitatively shown to be due to the promotion of the WGS reaction by the cesium doping.; The inhibition effect that occurred at high water concentrations in methyl acetate hydrogenolysis is due to the blockage of hydrogen activating sites. In addition, the kinetics of hydrogenolysis of n-propyl acetate resulted in a -0.49 order with respect to n-propyl acetate and a +1.83 order to hydrogen, which are in fair agreement with those derived in terms of Langmuir-Hinshelwood type kinetics. The aldehydic species involved in the hydrogenolysis of esters was evidenced by a isopropylamine trapping experiment.; By injecting the potential precursors of the intermediates into the synthesis gas, it was found that in the higher alcohol synthesis (i) the most pronounced effect of the cesium doping was to promote the reaction step from C(,2) to C(,3); (ii) the limiting step was the conversion of C(,1) alcohol to C(,2) alcohol; (iii) the growth reaction was terminated after forming branched alcohols such as 2-methyl-1-propanol; (iv) the products resulting from (beta) carbon addition ((beta)(,C)), but not the products formed from CO insertion (i(,CO)) and (alpha) oxygen addition ((alpha)(,O)), were significantly promoted by cesium doping; (v) methyl esters and alcohols were formed from the same precursors; and (vi) the formation of methyl esters was constrained by equilibrium at high temperatures. Further, a modified aldol-type condensation mechanism, where adsorbed 1,3 diols are involved as the intermediates, was proposed to account for the cesium effect. The chain growth scheme in higher alcohol synthesis was constructed in terms of C(,1) ((beta)(,C), i(,CO), and (alpha)(,O)), C(,2), and C(,3) additions.