不同前驱体制备的Mn-H_4SiW_(12)O_(40)/SiO_2催化剂对二甲醚催化氧化制取甲缩醛反应性能的影响

Effect of different Mn salt precursors on Mn-H_4SiW_(12)O_(40)/SiO_2 used for dimethoxymethane synthesis from dimethyl ether oxidation

采用浸渍法分别用硫酸锰、醋酸锰、氯化锰和硝酸锰为原料制备了Mn-H4SiW12O40/SiO2杂多酸催化剂。在常压连续流动固定床反应器中,考察了二甲醚选择氧化制取甲缩醛的反应活性。实验结果表明,催化剂的催化活性顺序为MnCl2-H4SiW12O40/SiO2>Mn(NO3)2-H4SiW12O40/SiO2>MnSO4-H4SiW12O40/SiO2>Mn(AC)2-H4SiW12O40/SiO2。并进一步考察了反应温度对不同锰盐前驱体催化剂性能的影响。结果表明,随温度的升高,硫酸锰修饰的H4SiW12O40/SiO2催化剂催化氧化比较剧烈,在613K时二甲醚转化率高达42.4%,但此时甲缩醛选择性仅为0.9%。采用氯化锰修饰的H4SiW12O40/SiO2催化剂,二甲醚催化氧化反应较缓和,并且甲缩醛的选择性明显高于分别采用硫酸锰、醋酸锰和硝酸锰改性的催化剂,在593K反应1h时,二甲醚转化率为8.6%,甲缩醛选择性达到37.5%。H2-TPR结果显示,硫酸锰改性的催化剂高温氧化性能明显强于另外三种催化剂,氯化锰的修饰使得催化剂的低温氧化性能变强。XRD结果表明,MnCl2-H4SiW12O40/SiO2催化剂的衍射特征峰明显强于其他三种催化剂,并且发现了MnO2衍射特征峰。

Mn-H4SiW12O40/SiO2 heteropolyacid catalysts were prepared by impregnation method from different Mn salt precursors, such as MnSO4, Mn（NO3 ）2, MnCl2 and Mn（ CH3COO）2. The catalytic oxidation reaction of dimethyl ether （DME） to dimethoxymethane （DMM） was carded out in a continuous flow type fixed-bed reactor with a ratio of DME/O2 = 1 ： 1 （ md ratio）. It is found that the sequence of catalytic activity for DMM synthesis is MnCl2-H4SiW12O40/SiO2〉 Mn（NO3）2-H4SiW12O40/SiO2 〉 MnSO4-H4SiW12O40/SiO2 〉 Mn（AC）2- H4SiW12O40/SiO2. The effects of reaction temperature （573 K - 633 K） on the catalysts were also investigated. With the increase of reaction temperature, the DME oxidation reaction is more exquisite over MnSO4-H4SiW12O40/SiO2 catalyst. 42.4% of DME conversion and 0.9% of DMM selectivity have also been obtained at 613 K. However, MnCl2 modified H4SiW12O40/SiO2 catalyst obtains higher DMM selectivity （37.5%, at 593 K） than other three catalysts at mild reaction conditions. H2-TPR profiles show that MnSO4 modification demonstrates stronger oxidative performance at high temperature than other catalysts, while MnCl2-H4SiW12O40/SiO2 catalyst exhibits better oxidative performance at low temperature. XRD patterns of the catalysts show that the diffraction peaks are strong and MnO2 diffraction peak is also found over the MnCl2 modified catalyst.