活性炭负载钯催化剂对邻硝基甲苯波相加氢制邻甲苯胺的研究

STUDY ON THE LIQUID PHASE HYDROGENATION OF o-NITROTOLUENE TO o-AMINOTOLUENE WITH ACTIVE CARBON-SUPPORTED PALLADIUM CATALYSTS

本文研究了不同载体负载的钯催化剂对邻硝基甲苯催化加氢制邻甲苯胺的性能,考察了载体的性能、钯的分散度对催化加氢速率的影响,探讨了该反应的动力学特征.结果表明,Pd/C催化剂液相催化邻硝基甲苯加氢制邻甲苯胺活性高,选择性好,寿命长.该反应为双分子吸附的表面反应.属于非择形反应.

The supported palladium catalysts were prepared by impregnating PdCl2 on different kinds of supports, such as active carbon (C) , polystyrene resin (D3520) , alumina (A12O3) and aluminium phosphate (A1PO4), respectively, and then reduced by HCHO. Activities of these catalysts in catalytic hydrogenation of o-nitrotoluene and the relation between activity and dispersion of Pd were investigated. It is shown that the activies (the initial rates of catalytic hydrogenation R0) of Pd catalysts supported on different supports decrease in the order: Pd/ C (6.7ml H2/ min.mg Pd) >Pd/ D3520(3.8) >Pd/ A12O3(1.9) > Pd/ A1PO4 (0.9). This activity pattern is probably related to the electron environment of Pd which is influenced by the surface groups on the supports. Furthermore, the activities of Pd catalysts supported on different active carbons decrease in the order: Pd/ PC-J(6.7 ml H2/ min.mgPd) >Pd/ GH(2.5) -Pd/ SX-1(2.5) >Pd/ PK-2(2.0) > Pd / PK-3 (1 .6) > Pd / HL-4 (0.9) . This difference in activity is probably related to the existence of different surface groups (such as -COOH, -OH) on the active carbon and the degree of graphitization. It can be seen from GLC analysis that the selectivity of Pd / C is 100% , i.e. all o-nitrotoluenes are converted into o-aminotoluene, and the life of this catalyst is long, i.e. 1 g of Pd converts 7692. 3g of o-nitrotoluene into o-aminotoluene. XRD results show that activities do not change with the dispersion of Pd on active carbon. It seems to infer that the hydrogenation of o-nitrotoluene on Pd is a non-structure-sensitive reaction. Influence of solvents on the catalytic hydrogenation rate is also studied. Experimental results show that the stronger the ability of the solvent to donate protons the higher the hydrogenation rate. The hydrogenation rate in different solvents decreases in the order: CH3COOH> CH3CH2OH> CH3OH> CH3COCH3> C6H6> CHCl3, Kinetics of this reaction system has been studied. On the basis of this study, it is suggested that the hydrogenation of o- nitrotoluene on Pd/ C is a bimolecular surface reaction, in which H2 and o-nitrotoluene molecules are adsorbed on different active sites of Pd. The following equation may be used to explain the experiment results: where R is the reaction rate; K is the reaction rate constant, b1, b2, b3, b4, are the adsorption coefficients of H2, o-nitrotoluene, o-aminotoluene and water respectively; PH2 is the pressure of H2in reaction system; and C_C_7 H7 NO2 'C_c_7 H7 NH2' C_H_2o are the concentrations of o-nitrotoluene, o-aminotoluene and water, respectively.