磷钼钒杂多酸在苯直接羟化反应中的催化作用—H2O2活化机理的探讨

磷铝钒杂多酸(H_(3+n)PMo_(12-n)V_nO_(40))+H_2O_2体系在苯直接羟化反应中的催化行为和众所周知的Fenton试剂(Fe~(++)+H_2O_2)完全不同。酚是羟化的唯一产物; H_2O_2分解动力学曲线呈“S”型。电子光谱数据表明, H_2O_2分解过程形成两种中间络合物。以此为依据, 提出了磷钼钒杂多酸对H_2O_2活化和苯羟化的反应机理, 认为杂多酸阴离子上的钒离子先和H_2O_2形成过渡的金属过酸型络合物, 然后再按自催化机理转化成金属过氧络合物, 后者在反应中起活化H_2O_2和羟化苯的活性种作用。按此反应机理所得的动力学公式可以对实验结果作出比较合理的解释。

CATALYTIC ACTION OF MOLYBDOVANADOPHOSPHORIC ACIDS IN DIRECT HYDROXYLATION OF BENZENE——AN EXAMINATION OF THE MECHANISM OF H2O2 ACTIVATION

The catalytic behavior of H_(3+n)PMo(12-n)V_nO_(40) (abbreviated to HPA-n hereafter) +H_2O_2 system in the direct hydroxylation of benzene is completely different from that of well-known Fenton reagent(Fe_(2+)+H_2O_2). The comparison experiments showed that the kinetics of the decomposition of H_2O_2 in HPA-n system generally gave Sshaped curves, which is different from that in Fenton system(Fig.2). The sensitivity of active species generated in the HPA_(-n)+H_2O_2 system toward benzene is far less than that generated in Fenton system, which parallels with the response to the concentration of H~+. Phenol is the only product in the HPA-n system. Data obtained from the hydroxylation experiments under different conditions and kinetic studies of the decomposition of H_2O_2 showed that, although the substituted number (n) of vanadium ion in heteropoly anion and the concentration of the catalyst had a great effect on the rate of the decomposition, they had little effect on the product yield (based on H_2O_2). On the other hand, the initial concentration of oxidant had little to do with the initial stage rate of decomposition, but affected the induced period and product yield to certain extent. Electronic spectra showed that during the decomposition of H_2O_2, two intermdiates might formed (Fig.9). Based on these facts, the mechanism of the activation of H_2O_2 and hydroxylation of benzene by molybdovanadophosphoric acid was suggested. It seems that the vanadium ion in hte heteroply anion might associated with H_2O_2 to form the transient “peracid like” species, followed by the transformation into side-on peroxo compound by auto catalytic process. The latter complex, which acted as an active species, played a major role in the activation of H_2O_2 and hydroxylation of benzene. The kinetic equation deduced from the suggested mechanism is as follows:-(dH_2O_2])/(dt)=k_1I]+2k_4H_2O_2]{III]HPA]expHPA]k_3(t-τ)}{HPA]+III]expHPA]k_3(t-τ)}I] is the concentration of complex HPA·H_2O_2...