碱金属阳离子对Cu+Y催化甲醇氧化羰基化性能影响的密度泛函理论研究

基于密度泛函理论方法构建并优化了CuMY(M为碱金属阳离子)分子筛的稳定构型, 采用速控步骤CO插入CH₃O形成CH₃OCO反应, 研究了碱金属阳离子对Cu⁺Y分子筛中活性中心周围电子环境及催化甲醇氧化羰基化合成碳酸二甲酯性能的影响. 计算结果表明, Li⁺, Na⁺和K⁺稳定落位于Y分子筛小笼中, 且随着金属离子半径的增大, CH₃OH, CO, CH₃O在CuMY上的吸附能和CO/CH₃O的共吸附能均逐渐增加, CO插入CH₃O反应的过渡态结构稳定性逐渐降低, 活化能逐渐上升, 相应的反应活性逐渐下降. 而落位在超笼中Ⅱ^*位的Rb⁺与Cs⁺则随着离子半径的增大, 反应过渡态的结构稳定性提高, 克服的活化能降低, 反应活性升高. 不同CuMY分子筛上催化活性顺序为CuLiY-Ⅰ'>CuCsY-Ⅱ^*>CuNaY-Ⅰ'>CuRbY-Ⅱ^*>CuKY-Ⅰ'>CuCuY-Ⅰ', 其中CuLiY-Ⅰ'分子筛克服速控反应的活化能垒(52.74 kJ/mol)最低.

Density Functional Theory Investigation on the Effect of Alkali Metal Cations on the Catalytic Performance for Cu+Y Zeolites in Oxidative Carbonylation of Methanol†

On the basis of density functional theory, influence of co-cations(M=Li⁺, Na⁺, K⁺, Rb⁺)adjacenting active center Cu⁺ on the electronic properties and catalytic activity of Cu⁺Y zeolites in oxidative carbonylation of methanol to dimethyl carbonate was investigated. CuMY stable configurations were constructed, CO insertion CH₃O species to CH₃OCO was used to evaluate the catalytic activity of CuY zeolites, due to the fact that it was the rate-limiting step of oxidative carbonylation of methanol. The calculation results indicate that Li⁺, Na⁺, K⁺ cations can locate site Ⅰ' in small cages of Y zeolites surrounding the active center Cu⁺, the adsorption energy of CH₃OH, CO, CH₃O and co-adsorbed CO/CH₃O on CuMY zeolite increases gradually with increase of cation size, the stability of transition states for CO insert into CH₃O reaction comes to decrease, increases the activation barriers and the corresponding catalytic performance declines. Whereas, Rb⁺ and Cs⁺ cations located Ⅱ^* in the supercage as increase in cation size, decreases activation energy for CO insertion reaction, improves the stability of transition states and the catalytic activity of CuY zeolites. The order of catalytic activity of CuMY zeolites is CuLiY-Ⅰ'>CuCsY-Ⅱ^*>CuNaY-Ⅰ'>CuRbY-Ⅱ^*>CuKY-Ⅰ'>CuCuY-Ⅰ'. The CuLiY-Ⅰ' catalyst exhibits the best catalytic performance with a lowest rate-limiting reaction activation barrier of 52.74 kJ/mol.