Pd-SiW_(12)/SiO_2催化剂上乙烯直接氧化制乙酸的反应机理

Reaction Mechanism of Ethylene Oxidation to Acetic Acid on Pd-SiW_(12)/SiO_2 Catalyst

利用积分反应器和微分反应器对Pd SiW12 /SiO2 催化剂上乙烯直接氧化生成乙酸的反应机理进行了探讨 .乙烯在积分反应中氧化的主要产物为乙酸 (选择性为 77 6 %) ,很少生成乙醛 (选择性仅为 8 1%) ;而在微分反应中氧化的主要产物是乙醛 (选择性为 98 4 %) .在微分反应中分别以乙醛和乙醇为主反应物时 ,乙醛氧化完全生成乙酸 ,选择性为 10 0 %,而乙醇氧化生成乙酸的选择性低于 0 15 %.可以认为 ,在Pd SiW12 /SiO2 催化剂上 ,水蒸气存在下乙烯主要经由中间物乙醛而生成乙酸 .通过对含有不同组分和不同还原条件处理的催化剂活性的比较 ,认为目的反应主要发生在Pd与SiW12 相互接触的部位 ,催化剂中的Pd0 是活性Pd物种的主要形态 .

The reaction mechanism of direct oxidation of ethylene to acetic acid on Pd-SiW12/SiO2 catalyst was investigated by comparing the product distribution in integral and differential reactors. In the integral reactor, the ethylene was mainly converted to acetic acid (77.6%) and a small quantity of acetaldehyde (8.1%). However, in the differential reactor, the main product was acetaldehyde (98.4%). When an appropriate amount of acetaldehyde was pulsed with oxygen and steam into the differential reactor, it was converted to acetic acid almost entirely. On the other hand, feeding ethylene with steam (free of O-2) into the integral reactor resulted in very little amount of ethanol corresponding to 1150 of the amount of acetic acid produced under normal conditions (V(C2H4): V(O-2): V(N-2): V(H2O) = 50:7:13:36). Therefore, the acetic acid was formed mainly via the intermediate acetaldehyde. By comparing the activity of catalysts containing various components with different chemical states, it was proposed that the reaction mainly takes place on the sites where Pd and SiW12 contact each other and Pd-0 is the main state of active Pd species.