ZSM-5分子筛酸性质对甲醛合成三聚甲醛催化性能的影响

Effect of acidity on the catalytic performance of ZSM-5 zeolites in the synthesis of trioxane from formaldehyde

ZSM-5分子筛是合成三聚甲醛的有效催化剂。本工作通过XRF、XRD、SEM、NH3-TPD、Py-FTIR和27Al MAS NMR等手段对一系列不同SiO2/Al2O3物质的量比的ZSM-5分子筛催化剂进行了表征,研究了ZSM-5分子筛中BrΦnsted酸中心和Lewis酸中心对其甲醛合成三聚甲醛催化性能的影响。结果表明,SiO2/Al2O3物质的量比为250的ZSM-5分子筛具有合适的BrΦnsted酸中心用于催化甲醛缩聚为三聚甲醛的反应,同时其Lewis酸中心量极少,可有效抑制Cannizzaro或Tishchenko等副反应,提高三聚甲醛的选择性,因而具有最佳的合成三聚甲醛催化性能。寿命实验评价结果显示,SiO2/Al2O3物质的量比为250的ZSM-5分子筛具有良好的催化稳定性,单程寿命长达114 h,并且可通过550℃焙烧再生恢复其催化活性。

ZSM-5 zeolite is considered as an effective catalyst in the synthesis of trioxane from formaldehyde.In this work,a series of ZSM-5 zeolites with different SiO2/Al2O3 molar ratios were used in the synthesis of trioxane from formaldehyde;through characterization by XRF,XRD,SEM,NH3-TPD,Py-FTIR and 27Al MAS NMR,the effect of acidity including the BrΦnsted and Lewis acid sites on the catalytic performance of ZSM-5 zeolites in the trioxane synthesis was investigated.The results indicate that the ZSM-5-250 zeolite with a SiO2/Al2O3 molar ratio of 250 exhibits excellent catalytic performance in the synthesis of trioxane.The ZSM-5-250 zeolite owns sufficient amount of BrΦnsted acid sites which are active for the synthesis of formaldehyde to trioxane;meanwhile,it has few Lewis acid sites and can then effectively inhibit various side-reactions like the Cannizzaro or Tishchenko reactions.Moreover,the ZSM-5-250 zeolite displays high stability with a single-pass lifetime of 114 h and can be regenerated easily through calcination at 550℃.