RhCl_3/CH_3I催化醋酸甲酯羰基化制醋酐——Ⅱ.LiOAc存在下的动力学和机理

RhCl3/CH3I Catalyzed Carbonylation of Methyl Acetate to Acetic Anhydride Ⅱ. Kinetics and Mechanism in the Presence of LiOAc

1.前言 在醋酸甲酯羰基化高活性均相铑催化体系中,CH_3I和锂化合物及含氮、膦有机物是最常用的助催剂。Schrod等首先对RhCl_3—CH_3I—有机碱—络合物体系催化醋酸甲酯羰基化反应动力学及红外光谱进行了研究,发现反应对醋酸甲酯为零级;CO的反应级数在压力1.5MPa以上也为零级;铑和CH_3I反应级数为一级;膦的反应级数随浓度增加由一级变为零级;Cr(CO)_6对反应速率影响不大,同时发现存在着中间反应物Rh(CO)_2I_2^-。根据这些实验事实,他们提出了类似于甲醇羰基化制醋酸的催化反应机理,认为速率控制步骤是CH_3I与Rh(CO)l-2^-的氧化加成,但他们没有报导有关锂化物助催剂的作用。Eastman公司的研究人员Polichnowski研究了RhCl_3—CH_3I—Lil体系催化醋酸甲酯羰基化动力学和原位红外光谱,也发现当反应总压在3.0MPa以上时,反应初速与CO压力无关,但RhCl_3。

A kinetics study of the carbonylation of methyl acetate to acetic anhydride at P=4. 0 MPa, T= 414 - 458 K, catalyzed by RhCl3-CH3I-LiOAc homogeneous system, has been made. It was found that at high concentration of LiOAc, the reaction is first order with respect to CH3I as its concentration is low and the reaction order approaches zero as its concentration is increased. The reaction order with respect to LiOAc was also found to shift from first to zero as its concentration is increased. The activation parameters: Ea=31. 9 kJ/mol, ΔH++ =27. 9 kJ/mol,ΔS++ =-49 eu, were determined. From the results obtained and those in the literature it is considered that the overall rate of carbonylation involves very probably two rate-limiting steps, one is the oxidation addition of methyl iodide to Rh(CO)2I2-, and the other is the consumption of acetyl iodide by its reaction either with LiOAc or with CH3COOCH3. At high concentration of LiOAc, the consumption of acetyl iodide is rapid enough so that the rate-limiting step is the oxidation addition. As the CH3I concentration is increased, the rate of oxidation addition enhanement occurs up to the point when the rate of acetyl iodide consumption becomes slower and the rate-limiting step is shifted to the reaction of acetyl iodide with OAc- to give acetic anhydride.