酸性中心对复合氧化物催化苯酚羟基化反应的影响

铁基复合氧化物催化苯酚羟基化反应具有明显的诱导期,适当引入酸性中心,在氧化还原活性位的共同存在下,可以迅速缩短羟基化反应的诱导期;酸性中心弱或没有采用酸改性的复合氧化物催化剂,对苯酚羟基化反应催化作用活性低,催化剂用量大,反应诱导期不稳定。乙酸、乙酸酐等有机弱酸可以直接加入羟基化反应体系。对复合氧化物催化剂进行后处理改性,引入适当的酸性中心,有助于提高催化剂的活性,缩短甚至消除羟基化反应诱导期,避免由于未反应过氧化氢的积累增加反应器运行的风险,提高过氧化氢有效利用率和目标产物选择性。将5.3g苯酚、0.051g催化剂、过氧化氢与苯酚的物质的量比为0.33,在65℃下,于10.6g溶剂水中反应,时间为30min,反应诱导期仅5min,苯酚转化率达21.2%,苯二酚选择性为92.9%,过氧化氢有效利用率为59.6%。分析了直接加入酸性中心和对复合氧化物催化剂进行酸改性所带来的问题,推测了苯酚羟基化反应的机理。

Effects of acid site on the hydroxylation of phenol catalyized by the composite oxides

The inducement period of phenol hydroxylation could be significantly shortened by virtue of organic weak acids such as acetic acid and acetic anhydride. The absence of acidic site in the composite oxides resulted in instability of inducement period for the reaction, low catalytic capability and poor selectivity of hydrogen peroxide. The hydroxylation of phenol is also been improved by the modification of composite oxides with various ammonium solutions such as NH_(4)NO_(3), (NH_(4))_(2)SO_(4), NH_(4)Cl and (NH_(4))_(2)HPO_(4) since acidic sites were produced in the composite oxides catalysts after calcinations. The interposition of suitable acidic sites, greatly enhanced the activities of these composite oxides catalysts, increased the utilizing efficiency of hydrogen peroxide and the selectivity of goal products, and reduced operation risk of hydroxylation with the low concentration of hydrogen peroxide accumulated in the reactor because of the shortened inducement period for the hydroxylation of phenol. Using Fe-AE-the catalyst modified by phosphorus oxides as catalyst in water medium, 21.2% phenol conversion and 92.9% of dihydric phenol selectivity with 5 min inducement period could be achieved under following conditions: reaction temperature of 65 ℃, reaction time of 30 min, phenol of 5.3 g, catalyst of 0.051 g, hydrogen peroxide/phenol (molar ratio) of 0.33, and water/phenol (weight ratio) of 2.0. Finally, the mechanism for phenol hydroxylation and the problem was proposed originated in directly addition of liquid acids and in acidic modification of composite oxides catalysts.