ZSM-5分子筛上轻烃裂解性能:晶粒尺寸的影响

设计合成了纳米、亚微米和微米级晶粒尺寸ZSM-5分子筛,并研究了其在两种反应温度下(510,650℃)对正庚烷催化制低碳烯烃反应行为。结果表明,在反应初始阶段,两种反应温度下晶粒尺寸对正庚烷转化率和产物选择性影响较小。但随着反应进行,纳米和亚微米ZSM-5在510℃下反应性能(低碳烯烃选择性及反应活性稳定性)相近且均高于微米ZSM-5;而650℃下,具有更短扩散路径和更大外表面积的纳米ZSM-5则体现出更高的反应性能。微米ZSM-5在两种温度下虽具有相对较高的低碳烯烃选择性,但其活性稳定性最低。进一步研究晶粒尺寸对费-托过程中石脑油催化裂解性能的影响发现,亚微米ZSM-5表现出最高的催化反应性能,这可能与反应原料的组成及相关反应途径变化有关。

Transformation of light hydrocarbons to olefins: effect of ZSM-5 zeolites crystal size

The ZSM-5 zeolites with nano, submicron and micron crystal size, denoted as N-ZSM-5, S-ZSM-5 and M-ZSM-5, respectively, were synthesized via hydrothermal method. The effect of crystal size on the product selectivity in catalytic cracking of n-heptane to light olefins reaction was also studied at 510℃ and 650℃,respectively. The results showed that the crystal size of ZSM-5 zeolites had little effect on the n-heptane conversion and the selectivity to olefins at the initial phase at the above two temperatures. At 510℃, however, the submicron and nanometer ZSM-5 zeolites exhibited higher and almost similar selectivity to olefins and stability of reaction activity than the micron sample along with the reaction time. Nevertheless, the N-ZSM-5 zeolite displayed the best performance of selectivity to olefins and stability of reaction activity at 650℃ due to the short diffusion path length and the large external surface area compared to the S-ZSM-5 and M-ZSM-5. The micron-sized M-ZSM-5 zeolite presented the relatively higher selectivity to olefins than the other zeolites, but it also owned the lowest stability of reaction activity at both temperatures. In the catalytic cracking reaction of F-T naphtha, the S-ZSM-5 zeolite gave the best performance of selectivity to olefins and the stability of reaction activity and it may attribute to the complex components of the naphtha materials and their various reaction paths to olefins.