ZSM-5分子筛催化甲醇制汽油反应中的晶粒粒径效应研究

通过调控水热合成ZSM-5凝胶液中H_2O/Si物质的量比,实现了粒径为70、200、400和650 nm四种单分散ZSM-5的可控合成。采用XRD、TEM、BET和NH_3-TPD等多种表征对其微观结构进行分析,结合催化性能评价,考察了晶粒粒径对其催化甲醇制汽油反应性能的影响机制。结果表明,整体上随着ZSM-5晶粒粒径的增加,其外比表面积减小,结晶度提高,酸量呈现出先增加后基本不变的趋势。但外表面附着小晶粒的粒径为650 nm的分子筛体现出了大的外表面积和强的酸性。ZSM-5晶粒粒径的增加整体上降低了其催化MTG反应的寿命和最高收率。晶粒粒径为70 nm时,ZSM-5体现出了96 h的催化寿命和30.8%的最高收率。晶粒粒径为650 nm样品由于其大的外比表面积和较强的表面酸性,也体现出91 h的寿命。在大晶粒ZSM-5外表面附着生长小晶粒ZSM-5,是一种制备高性能催化剂的新方法。

Effect of ZSM-5 crystal size on its catalytic properties for conversion of methanol to gasoline

Four-sized monodispersed ZSM-5 crystals, being 70, 200, 400 and 650 nm, were hydrothermally synthesized by changing the H₂O/Si molar ratio in the synthesis gel, and characterized with XRD, TEM, BET and NH₃-TPD techniques. The crystal size effect of ZSM-5 on its catalytic performance for conversion of methanol to gasoline (MTG) was investigated. It was shown that the external surface area of the sample decreased with its crystal size, while the acid site amount firstly increased, and then kept almost constant. Nevertheless, 650 nm ZSM-5 crystals attaching small particles exhibit large external surface area and strong acidity. The catalytic stability and the liquid hydrocarbon yield decreased with increasing crystal size. The sample with a crystal size of 70 nm shows a catalytic lifetime of 96 h and a gasoline yield of 30.8%. The large external surface area and relatively strong acidity endow the sample with a crystal size of 650 nm also has a catalytic lifetime of 91 h, indicating that synthesis of large ZSM-5 crystals with small crystallites adhered to their surface could be a potential way to improve the catalytic performance.