助剂含量对CuLi/AC催化剂结构及甲醇氧化羰基化反应性能的影响

Effects of Lithium Content on the Structural Properties and Catalytic Activities of CuLi /AC Catalysts in the Oxidative Carbonylation of Methanol to Dimethyl Carbonate

采用微波辐射法制备出不同助剂含量的CuLi/AC(活性炭)催化剂,考察了其在甲醇气相氧化羰基化合成碳酸二甲酯(DMC)反应中的催化性能,使用X射线衍射、扫描电子显微镜、比表面积、H2程序升温还原、X射线光电子能谱和CO程序升温脱附对催化剂的结构进行了表征.研究结果表明,添加适量的Li有助于铜物种还原为低价态的Cu0,形成颗粒尺寸更小、分布更加均匀的铜纳米颗粒,并高度分散在活性炭载体表面.催化剂活性与表面单质铜的含量有关,即Cu0是催化剂的主要活性物种,并且Cu0的晶粒尺寸越小,催化剂活性越好.添加Li后增加了催化剂表面的CO弱吸附位,有利于CO对Cu—OCH3的插入反应,因此提高了催化活性.随着Li含量的增加,DMC的时空收率逐渐升高,当Li添加质量分数达到0.15%时,DMC的时空收率达到最高值540.6 mg·g-1·h-1,甲醇转化率为4.5%,DMC选择性为81.4%.

The CuLi /AC(active carbon) catalysts with different lithium contents were prepared by microwave heating and tested in the oxidative carbonylation of methanol to dimethyl carbonate( DMC). The bulk and surface properties of the catalysts were characterized by means of X-ray diffraction(XRD),H2-temperature program reduction( H2-TPR),X-ray photoelectron spectrum( XPS),scanning electron microscope( SEM),and CO-temperature programe desorption(CO-TPD) techniques. The results show that the addition of moderate content of lithium into the Cu / AC catalyst enhances the auto-reduction of copper species to Cu0,leading to more well-dispersed Cu nanoparticles with more uniform grain size on the surface of active carbon. The catalytic performance is related to the concentration of Cu0in the surface of CuLi / AC catalysts,indicating that Cu0species is the main active centers for the synthesis of DMC. The addition of lithium enhances CO weak adsorption centers,which favors the insertion reaction between CO molecules and Cu—OCH3intermediates,and thus effectively improve the catalytic activity of CuLi / AC catalysts. The STY( space time yield) of DMC firstly increases and then decreases with the content of lithium,and reaches the maximum of 540. 6 mg·g- 1·h- 1when the mass fraction of lithium is up to 0. 15% in the catalysts,and the corresponding MeOH conversion and DMC selectivity are 4. 5% and 81. 4%,respectively.